Periodic Mesoporous Organosilica and Silica · Periodic Mesoporous Organosilica and Silica Wendong...

Periodic Mesoporous Organosilica and Silica

by

Wendong Wang

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy (Ph.D.)

Department of Chemistry University of Toronto

© Copyright by Wendong Wang 2011

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Periodic Mesoporous Organosilica and Silica

Wendong Wang

Doctor of Philosophy

Department of Chemistry University of Toronto

2011

Abstract

Periodic mesoporous material is a class of solids that possess periodically ordered pores

with sizes of 2–50 nm. After a brief introduction to the synthesis, structure, property and

function of periodic mesoporous materials in general in Chapter 1, a specific type of

periodic mesoporous material, periodic mesoporous organosilica (PMO), is examined in

detail in Chapter 2. Chapter 3 and Chapter 4 focus on the application of periodic

mesoporous organosilica as low-dielectric-constant (low-k) insulating materials on

semiconductor microprocessors. Specifically, Chapter 3 introduces a vapor-phase

delivery technique, vacuum-assisted aerosol deposition, for the synthesis of PMO thin

films; Chapter 4 studies one property crucial for the application of low-k PMO in detail—

hydrophobicity. The focus of Chapter 5 turns to a novel sandwich-structured

nanocomposite made of periodic mesoporous silica and graphene oxide. In Chapter 6,

progress towards the synthesis of periodic mesoporous quartz is summarized. A

conclusion and an outlook are given in Chapter 7.

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“Curiouser and curiouser!” — Alice, in Alice in Wonderland.

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Table of Contents

Prologue..………………………………………………………………………………….1

Chapter 1 Introduction to Ordered Mesoporous Materials ............................................ 3

1.1. Classification of Pore Size ................................................................................... 3

1.2. Template-Directed Synthesis ............................................................................... 3

1.3. Mesostructure ....................................................................................................... 8

1.4. Meso-composition ................................................................................................ 8

1.5. References .......................................................................................................... 16

Chapter 2 Why PMO? Towards Functionality and Utility of Periodic Mesoporous Organosilica ……………………………………………………………………………..31

2.1. Organic Chemistry of the Pore Wall .................................................................. 35

2.2. PMO and Guest Molecules ................................................................................ 38

2.3. Physical Functions: Optical, Electric, and Dielectric ......................................... 42

2.4. PMO Future ........................................................................................................ 45

2.5. References .......................................................................................................... 47

Chapter 3 Vacuum-Assisted Aerosol Deposition of a Low-Dielectric-Constant Periodic Mesoporous Organosilica Film .……………………………………………….52

3.1. Results and Discussion ....................................................................................... 55

3.2. Methods .............................................................................................................. 60

3.3. Conclusion .......................................................................................................... 62

3.4. Appendix ............................................................................................................ 63

3.5. References .......................................................................................................... 65

Chapter 4 Water Repellent Periodic Mesoporous Organosilicas ..................................... 67


4.2. Methods .............................................................................................................. 82

4.3. Conclusion .......................................................................................................... 84

4.4. Appendix ............................................................................................................ 85

4.5. References .......................................................................................................... 88

Chapter 5 Reduced Graphene Oxide–Periodic Mesoporous Silica Sandwich Nanocomposites with Vertically Oriented Channels ........................................................ 91


5.2. Methods ............................................................................................................ 105

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5.3. Conclusion ........................................................................................................ 107

5.4. References ........................................................................................................ 108

Chapter 6 Meso-Quartz ............................................................................................. 109

6.1. Methods ............................................................................................................ 110

6.2. Results .............................................................................................................. 112

6.2.1. Crystallization Temperature ...................................................................... 112

6.2.2. Keatite as an Intermediate Phase .............................................................. 117

6.2.3. Possible Epitaxial Growth ......................................................................... 121

6.2.4. Crystallization of Carbon-Filled PMS ...................................................... 123

6.3. Conclusion and Future Directions .................................................................... 129

6.4. References ........................................................................................................ 130

Chapter 7 Summary and Outlook .............................................................................. 132

Epilogue………….......…………………………………………………………………134

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List of Figures

Figure 1-1 Classification of porous materials ..................................................................... 3

Figure 1-2 Number of papers citing the first report on the synthesis of ordered mesoporous silica ................................................................................................................ 4

Figure 1-3 Generic scheme of template-directed synthesis ................................................ 4

Figure 1-4 A cooperative self-assembly route in which surfactant counterions are exchanged by charged inorganic species. ........................................................................... 5

Figure 1-5 A cooperative self-assembly route in which the interaction between surfactant chains and inorganic species is mediated by counterions. .................................................. 6

Figure 1-6 Liquid crystal templating route ......................................................................... 7

Figure 1-7 Block copolymer templating route .................................................................... 7

Figure 1-8 Four major mesostructures ................................................................................ 8

Figure 1-9 Mesoporous elements ........................................................................................ 9

Figure 1-10 Mesoporous metal oxides .............................................................................. 12

Figure 2-1 (a) Illustration of a typical PMO structure, and (b) citation of the at least one of the first three PMO papers per year .............................................................................. 33

Figure 2-2 Scheme of the Diels-Alder reaction on ethene PMO and subsequent attachment of a sulfonic acid group on the benzene ring (the cis- configuration of the double bond is drawn for the convenience of illustration). .............................................. 35

Figure 2-3 Reaction scheme of the synthesis of bifunctional PMO with acidic pore framework and basic pore surface .................................................................................... 36

Figure 2-4 Lineage of bromobenzene PMO family .......................................................... 37

Figure 2-5 Chromatograms of the mixture of four aromatic compounds separated in n-hexane at a flow rate of 1 mL·min-1 on the Nucleosil 50–10 column or at a flow rate of 2 mL·min-1 on the spherical benzene PMO column ............................................................ 39

Figure 2-6 Release Profile of tetracycline carried by solid PMS spheres, solid ethane PMO spheres, and hollow ethane PMO spheres ............................................................... 40

Figure 2-7 Amounts of released D-lysozyme as a function of poly(ethylene glycol) concentration, and scheme of refolding protein process ................................................... 41

Figure 2-8 Fluorescence spectra of biphenyl PMO samples containing 0–2.35 mol% coumarin 1 excited at 270 nm and normalized by absorption rate, and illustration of the energy transfer in coumarin-1-doped biphenyl PMO. ...................................................... 42

Figure 2-9 Double logarithmic plots of transient photocurrent measured on a 5 µm-thick film sample as a function of time at applied voltages of 150 V and 350 V, and chemical structure of three-armed phenylenevinylene ..................................................................... 43

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Figure 2-10 Dielectric constants as a function of the molar fraction of ring-shaped precursor to pure silica precursors, and the chemical structure of three-ring PMO ......... 44

Figure 3-1 Calculated gate and interconnect delay as a function of technology node according to the National Technology Roadmap for Semiconductors (NTRS) in 1997. ■ gate delay; ▲ interconnect delay (Al and SiO2); ● sum of delays (Al and SiO2). ........ 54

Figure 3-2 Scheme of vacuum-assisted aerosol deposition .............................................. 55

Figure 3-3 Scheme of de Laval nozzles and a nozzle assembly: (a) five designs of de Laval nozzles, (b) neck of a nozzle, (c) a nozzle assembly, and (d) an example of the dimensions of a bottom plate in a nozzle assembly. ......................................................... 56

Figure 3-4 X-ray diffraction patterns of (A) as-deposited films and (B) calcined films; the insets showing the corresponding grazing-incidence small angle X-ray scattering patterns........................................................................................................................................... 57

Figure 3-5 STEM images of films deposited (a, b) without spinning and (c, d) with spinning; scale bars are 10nm ........................................................................................... 58

Figure 3-6 (a) Scheme of mesostructured domains in a PMO film with channels running parallel to substrate surface; (b) empty pores, and (c) pores filled with water, being compressed by capillary forces in the X direction during an ellipsometric porosimetry measurement. .................................................................................................................... 60

Figure 3-7 Scheme of the relationship among the contact angle, the radius of the pore, and the radius of the curvature of air–water interface ...................................................... 64

Figure 4-1 XRD patterns of (a) ethane PMO, (b) methane PMO, (c) 3-ring PMO, and (d) ethene PMO, with their corresponding structural formula ............................................... 70

Figure 4-2 Representative TEM images of (a) ethane PMO, (b) methane PMO, (c) 3-ring PMO, and (d) ethene PMO ............................................................................................... 71

Figure 4-3 Isotherms of (a) ethane PMO, (b) methane PMO, (c) 3-ring PMO, and (d) ethene PMO treated at 350 °C for 2 h. In each plot, filled symbols represent an adsorption branch, and empty symbols represent a desorption branch. On each sample, two adsorption and desorption measurements are performed. The circular shape corresponds to the first, and the triangular shape corresponds to the second. ...................................... 72

Figure 4-4 Plots of thin film thickness versus relative humidity of (a) ethane, (b) methane, (c) 3-ring, and (d) ethene PMO. ........................................................................................ 74

Figure 4-5 Isotherms of (a) ethane, (b) methane, (c) 3-ring, and (d) ethene PMO treated at 300 °C, 400 °C and 500 °C for 2 h. The refractive indexes of samples treated at 400 °C and 500 °C are offset by 0.25 and 0.5, respectively. The refractive indexes of 3-ring PMO treated at 500°C are offset by 0.55. .................................................................................. 75

Figure 4-6 Isotherms of 3-ring, methane, and ethane PMOs treated at 350 °C without washing away surfactants before the thermal treatment. The refractive indexes of methane PMO are offset by 0.1, and those of ethane PMO are offset by 0.2. .................. 77

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Figure 4-7 Experimental and fitted X-ray reflectivity patterns of –CH2CH2– bridged, –CH2– bridged, 3-ring and –CH=CH– bridged polysilsesquioxane film (non-porous) ..... 87

Figure 5-1 Comparison of GO and mGO solutions: (a) photos of the solutions, (b) UV-vis spectra, and (c) Raman spectra. ........................................................................................ 93

Figure 5-2 Low angle XRD results of mGO–PMS and rGO–PMS samples synthesized at (a) pH = 11.7 and (c) pH = 12.7, and the corresponding control PMS samples in (b) and (d), respectively. ................................................................................................................ 94

Figure 5-3 SEM images of rGO–PMS (0.27) synthesized at pH = 11.7. Gradual zoom-in from (a) to (d).................................................................................................................... 95

Figure 5-4 TEM images of microtomed sample of rGO–PMS (0.27) synthesized at pH = 11.7.................................................................................................................................... 97

Figure 5-5 Electron microscope images of rGO–PMS (0.27) synthesized at pH = 12.7. . 98

Figure 5-6 Nitrogen sorption isotherms of (a) rGO–PMS synthesized at pH = 11.7, (b) the control PMS synthesized at pH = 11.7, (c) rGO–PMS synthesized at pH = 12.7, and (d) the control PMS synthesized at pH = 12.7. ....................................................................... 99

Figure 5-7 Zeta potential of (a) GO and mGO solution as a function of pH, (b) mGO solution with 57 mM CTACl as a function of pH at 298 K and 348 K, (c) mGO solution with different concentrations of CTACl, and (d) pure CTACl solutions with concentrations of 6 mM, 56 mM, and 1020 mM. ........................................................... 101

Figure 5-8 Proposed mode of formation for rGO–PMS with channels oriented vertically with respect to the rGO sheets ........................................................................................ 102

Figure 5-9 Electrical conductivity of rGO-PMS as a function of carbon content. The number above the data point is the concentration of mGO content in the starting dispersion. ....................................................................................................................... 103

Figure 5-10 Isotherms of hexane and water sorption measurement on rGO–PMS (0.27) at 303 K. The adsorption volume were normalized against the total pore volume obtained from the nitrogen sorption measurement ........................................................................ 104

Figure 5-11 Relative electrical resistivity of rGO-PMS (0.27), rGO-PMS (4), and rGO film as a function of the relative pressure of hexane; and relative electrical resistivity of rGO-PMS (0.27) as a function of the relative humidity. ................................................ 104

Figure 6-1 Comparison between (a) wet-impregnation and (b) vapor-diffusion experiments. In the case of wet–impregnation, a catalyst was directly mixed with PMS. In the case of vapor–diffusion, a catalyst was placed inside a small crucible, physically separated from PMS, and was delivered after being vaporized at high temperatures through vapor diffusion................................................................................................... 110

Figure 6-2 XRD pattern of a wet-impregnation experiment at 600 °C for 15 h in air ... 112

Figure 6-3 XRD results of a wet-impregnation experiment at 700 °C for 15 h in air. (a) XRD pattern, and (b) the peak lists of the sample, α-quartz (PDF83-0539, space group P3121), keatite (PDF76-0912, space group P43212), and Li2Si2O5 (PDF72-0102, space group Cc). ....................................................................................................................... 113

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Figure 6-6 XRD results of vapor-diffusion experiments at 800 °C for 40 h in air. (a) XRD pattern, and (b) the peak list of the sample and α-quartz (PDF83-0539, space group P3121) ............................................................................................................................. 116

Figure 6-7 XRD pattern of a control experiment at 800 °C for 40 h in air without any lithium catalyst ................................................................................................................ 117

Figure 6-8 XRD pattern of mesoporous silica mixed with lithium oxide and treated at 550 °C for 2h. ......................................................................................................................... 118

Figure 6-9 XRD results of mesoporous silica mixed with lithium oxide and treated at 650 °C for 2 h. (a) XRD patterns, and (b) the peak lists of the sample, keatite (PDF# 76-0912, space group P43212), and Li2Si2O5 (PDF# 24-0651, orthorhombic). ............................. 119

Figure 6-10 XRD results of mesoporous silica mixed with lithium oxide and treated at 750 °C for 2h. (a) XRD pattern, and (b) the peak lists of the sample, α-quartz (PDF83-0539, space group P3121), and Li2Si2O5 (PDF40-0376, space group Cc) ...................... 120

Figure 6-11 XRD results of a control experiment performed in the absence of water (a) XRD pattern of mesoporous silica treated at 800 °C for 24 h under nitrogen flow, and (b) the peak lists of the sample, α-quartz (PDF83-0539, space group P3121), and Li2SiO3 (PDF74-2145, space group Cmc21) ................................................................................ 122

Figure 6-12 Filling materials to support porous structure during crystallization ........... 123

Figure 6-13 XRD results of a carbon filling experiment. (a) the XRD pattern of mesoporous silica filled with carbon and treated at 850 °C for 5 h, and (b) peak lists of the sample, α-quartz (PDF83-0539, space group P3121), Li2Si2O5 (PDF40-0376, space group Cc), and Li2SiO3 (PDF70-0330, space group Cmc21) .......................................... 125

Figure 6-14 Low angle XRD patterns of a carbon filling experiment. Carbon-filled mesoporous silica (a) before the crystallization, and (b) after the crystallization. ......... 126

Figure 6-15 TEM images of porous quartz ..................................................................... 127

Figure 6-16 Electron diffraction patterns of selected regions in porous quartz .............. 128

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List of Tables

Table 1-1 Mesoporous Elements ...................................................................................... 10

Table 1-2 Mesoporous Metal Oxides ................................................................................ 12

Table 3-1 Properties of PMO Thin Films ......................................................................... 59

Table 4-1 Comparison of Contact Angles Measured Macroscopically on Bridged Polysilsesquioxane Thin films (Non-porous) and Contact Angles Measured by Ellipsometric Porosimetry (EP) on Corresponding PMO Thin films ............................... 78

Table 4-2 Structural and Physical Properties of Four PMOs ............................................ 80

Table 4-3 d-spacing (Å) of PMOs Post-Treated at 300–500 °C ........................................ 85

Table 4-4 Pore Sizes Derived from the Second Isotherms of Four PMOs ....................... 85

Table 4-5 Densities and Young’s Moduli of Bridged Polysilsesquioxane Film (Non-porous) .............................................................................................................................. 85

Table 4-6 Densities of PMO Film Used for Fitting SAWS Data ..................................... 86

Table 5-1 Surface Area, Pore Volume, and Pore Size of rGO–PMS and Control PMS Samples ............................................................................................................................. 99

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List of Abbreviations

ATO: Antimony-doped tin oxide

CTACl: cetyltrimethylammonium chloride

EP: Ellipsometric porosimetry

ITO: Indium tin oxide

GO: graphene oxide

rGO: reduced graphene oxide

mGO: modified graphene oxide

PMS: Periodic mesoporous silica

PMO: Periodic mesoporous organosilica

SAWS: Surface acoustic wave spectroscopy

TEM: Transmission Electron Microscopy

XRD: X-ray diffraction

XRR: X-ray reflectivity

1

Prologue

Seven days before flying to Toronto for graduate studies, I attended Asian

Technology Initiative Entrepreneurial Conference 2006 in Shanghai. One theme of the

conference was Web 2.0, the key feature of which was user-generated content.

Wikipedia, Facebook, blogging websites, YouTube, and Twitter all fell into this category.

Although those social media websites were just fledging at that time, time has proved that

they are not only surviving but are thriving now. In the meantime, I have turned from a

skeptic to an active user of most of these sites. They have provided me with a means to

stay connected with distant friends and family members, a platform to share thoughts and

memories, and most importantly a way to express my individual identity. My experience

mirrors a large portion of the society: at the most basic level, Web 2.0 gives a voice to

everyone. However, because everyone has a voice, the web becomes increasingly

democratic and messy.

Science publishers have also adopted Web 2.0 quickly. Not only have they

digitized their journals and made them available online, they have also built their

websites as the central platforms to communicate with authors and readers. Nature

publishing group, ACS, and Wiley all publish RSS feeds (developed initially for blogs)

of their papers; and Nature now even allows every reader to comment on all published

articles online. This immediate integration of science publishing with Web 2.0 reflects

the key similarities of the two: both are democratic because every researcher publishes,

and once a paper is published, everyone can comment on it; both are messy because

given the large volume of data published, finding what one wants can be daunting.

My experience with social media websites and with science publishing leads me

to the question—why thesis? Scientific journals are like traditional media such as New

York Times and BBC; a thesis is like an individual’s blog. In much the same way that

most people read news from traditional media outlets but not from an individual’s blog,

most people will know about my work through my papers instead of my thesis. Since my

work has been communicated to the scientific community through papers, why am I

writing a big thick book that only a handful of people in the world will actually (have to)

2

read? Or from the perspective of my thesis committee, why reading a lengthy thesis

instead of more compact papers when I (committee members) am already overloaded

with information? Without adding much new to what is already known, my thesis will

probably contribute to the messiness of information, in much the same way individuals’

blogs do these days.

I want to try something different.

At University of Toronto, I have attended courses, workshops, and seminars on

academic writings, but I have not found any official training on scientific illustration.

Given the importance and increasing use of graphics to present ideas and data in

scientific publishing—almost all journals require a table-of-content image accompanying

a paper and every journal encourages authors to submit images to be considered for front

covers—it will be beneficial for a graduate student to have some formal training on

scientific illustration. Since none exists, I take every opportunity to train and practice

myself, including this thesis. I shall therefore present key ideas in graphic form and keep

words to a minimum level. And in doing so, I hope to make it compact and reader-

friendly.

3

Chapter 1 Introduction to Ordered Mesoporous Materials

1.1. Classification of Pore Size

Figure 1-1 Classification of porous materials

According to International Union of Pure and Applied Chemistry, micropores

have diameters less than 2 nm; mesopores have diameters between 2 nm and 50 nm;

macropores have diameters more than 50 nm.1

1.2. Template-Directed Synthesis

A breakthrough in the synthesis of porous materials came in 1992, when

researchers at Mobil discovered the template-directed synthesis of ordered mesoporous

materials.2, 3 This work has inspired an enormous amount of research around the globe

(Figure 1-2). The term template-directed in this context indicates the use of a sacrificial

template for creating space, organizing structure, and balancing charges in the formation

of porous materials (Figure 1-3).

4

Figure 1-2 Number of papers citing the first report on the synthesis of ordered mesoporous silica

Figure 1-3 Generic scheme of template-directed synthesis

This scheme (Figure 1-3) emphasizes the central role of template in creating

pores: (a) surfactant micelles, (b) inorganic–surfactant mesostructure, and (c)

mesoporous structure after surfactant removal.

The details of template-directed synthesis are still a subject of ongoing

investigation and debate, with new routes being discovered every few years. Four routes

that describe the overwhelming majority of experiments are illustrated in the following

schemes.

5

Figure 1-4 A cooperative self-assembly route in which surfactant counterions are exchanged by charged inorganic species.

A typical example of this route (Figure 1-4) is the synthesis of periodic

mesoporous silica using CTACl in a basic solution, with deprotonated silicate species

bearing a negative charge and interacting with positively charged CTA+.4-6 (a) Surfactant

micelles in an equilibrium with the monomers, and inorganic species bearing charges of

the same sign as those of the surfactant counterions, (b) inorganic species replacing

surfactant counterions, (c) the newly formed inorganic—surfactant micelles, and (d)

inorganic—surfactant mesostructure. The term surfactant is used loosely in this instance

to mean the organic part of a surfactant molecule: for example, C16H33(CH3)3N+ (CTA+)

in C16H33(CH3)3NCl (CTACl).

It is a deliberate choice to change the shape of the micelle from spheres in (a) to

cylinders in (c) in order to illustrate a possible phase transformation during the self-

assembly process. This transformation is due to the fact that the inorganic species

effectively function as polydentate (polycharged) counterions and that an increase of the

charge density of the counterions results in a change of the micellar shape.

6

Figure 1-5 A cooperative self-assembly route in which the interaction between surfactant chains and inorganic species is mediated by counterions.

A typical example of this route (Figure 1-5) is the synthesis of periodic

mesoporous silica using CTACl as the surfactant in a solution with pH<2, where

positively charged protonated silicate species are surrounded by counterions Cl-

throughout the self-assembly process.6 (a) Surfactant micelles in equilibrium with

surfactant monomers, and inorganic species bearing charges of the opposite sign to those

of the surfactant counterions, (b) inorganic species interacting with surfactant headgroups

through surfactant counterions, (c) the newly formed inorganic-surfactant structure, and

(d) inorganic-surfactant mesostructure.

7

Figure 1-6 Liquid crystal templating route

A typical example of this route (Figure 1-6) is the deposition of platinum in

concentrated solution of nonionic oligomeric alkyl polyethylene-oxide surfactant such as

C16EO8, which is a liquid crystal before the addition of platinum precursors.7 This route

distinguishes itself from the two previous cooperative self-assembly routes in that the

final solid is a direct cast or replica of liquid crystalline phase of the surfactant.

Figure 1-7 Block copolymer templating route

This last route (Figure 1-7) extends the principles in the three previous routes to

creating pores with diameters up to 30 nm by using triblock copolymer. A typical

8

example is the synthesis of large pore mesoporous silica using triblock copolymer

poly(ethylene oxide)—poly(propylene oxide)—poly(ethylene oxide), Pluronic P123

EO20PO70EO20 and Pluronic F127 EO106PO70EO106 being the two most popular choices.8,

9

1.3. Mesostructure

Figure 1-8 Four major mesostructures

The mesoporous structures formed by surfactant and amphiphilic block

copolymer templating often belong to high symmetry space groups; Figure 1-8 shows

four of the most common symmetries, each annotated with the plane/space group

symbols.10-12 From left to right, they are 2D hexagonal, body-centered cubic, face-

centered cubic, and gyroid structures. Of particular interest is the gyroid structure. It

belongs mathematically to the family of periodic minimal surface, formulated in 1970

and later observed in lipid, surfactant and block-copolymer systems.13, 14 The mesoporous

material is perhaps the only inorganic solid representation of this structure.

1.4. Meso-composition

The pore wall of the mesoporous materials can possess a large number of

chemical compositions. Briefly summarized below are two major categories: elements

and oxides.

9

Figure 1-9 Mesoporous elements

Mesoporous metals are usually synthesized through electrochemical deposition;

mesoporous carbon is synthesized through the block-copolymer templating route or

through casting a replica of mesoporous silica; mesoporous germanium is synthesized

through judicious choice of inorganic starting precursors such as Ge4- and Ge94-,

mesoporous silicon is synthesized by reducing mesoporous silica with magnesium.

Looking at this figure, one cannot help but ask the question why no mesoporous copper,

silver, gold and zinc? Similarly, why no mesoporous aluminum, phosphorus or sulfur?

Their syntheses may be just a matter of time, as no known fundamental law precludes

their existence.

Selected references are listed in Table 1-1. The table attempts to provide a holistic

view of each material by linking composition with structure, property, function, and

utility.

10

Table 1-1 Mesoporous Elements

Composition Meso-structure

Property/function Utility

Osmium15 36 /P mmc

Catalytic Catalysis(oxidative cleavage and dihydroxylation)15

Cobalt16 6p mm Magnetic (size dependent coercivity)

Rhodium17, 18 6p mm Electrode Electrochemical reduction/detection of nitrate (nuclear waste cleaning, water purification)18

Nickel19-22 6p mm Electrode Amperometric sensor (glucose detection)21, Supercapacitor22

Palladium17, 22,

23 6p mm Electrode Supercapacitor22, pH

sensor23

Platinum7, 24-26 6p mm Electrode, catalytic Electrocatalysis (anode for methanol membrane fuel cell)25, hydrogen peroxide detection (glucose sensing, water purification, and bleaching)26

Cadmium27 unreported

11

Carbon28-35 6p mm ,

3Ia d

Conductive (electrode material)32, catalysis support31, membrane35

water and air purification, gas separation35, catalysis31, supercapacitor33, biofuel cell electrode32, biosensor34

Silicon36, 37 6p mm ,

3Im m ,

3Fm m

Photoluminescence, semiconducting, nontoxic

Sensor, photovoltaics, photocatalysis, drug delivery37

Germanium38-41 6p mm ,

3Ia d

Semiconducting, tunable bandgap energy,40 photoluminescent40

Catalysis, photovoltaics, sensors,

Tin42 6p mm Electrode Lithium ion battery42

Selenium43 6p mm Semi-metallic, potential for the synthesis of mesoporous CdSe

Tellurium44 6p mm Potential to for mesoporous II-VI semiconductors

Platinum-ruthenium45-47

6p mm Electrode Catalysis,46 fuel cell47

Platinum-nickel48

6p mm Electrode Catalysis

12

Figure 1-10 Mesoporous metal oxides

Mesoporous silicon oxides are the first mesoporous materials reported. And the

synthesis of mesoporous metal oxides has been the center of mesoporous materials. Even

so, there are still unexplored lands on the periodic table for mesoporous metal oxides

(Figure 1-10). The corresponding selected references are listed in Table 1-2.

Table 1-2 Mesoporous Metal Oxides

Composition Meso-structure

Property/function Utility

Magnesium oxide49, 50

6p mm Basic Catalysis, CO2 absorption50

Calcium oxide51

wormhole Basic CO2 absorption51

13

Titanium oxide52-65

6p mm ,

3Im m ,

3R m ,

3Fm m

Pseudocapacitance,56 semiconducting, Li insertion,63, 64

Supercapacitor,56 electrochromics, photocatalyst,62 solar cells,61, 65, 66

Battery

Zirconium oxide67-70

6p mm Affinity to phosphate69, 70 Catalysis,68 phosphoproteomics,69 water purification

Hafnium oxide70, 71

Affinity to phosphate70 Phosphoproteomics70

Vanadium oxide72-74

Wormhole, Electrode, Li insertion72, 74, electrochromic73, chemochromic73

Battery and supercapacitor72, 74

Niobium oxide53-55, 75-87

6p mm ,

36 /P mmc,

3Im m

Red-ox of pore wall,79, 80 guest inclusion (superparamagnetic cobaltocene,79 semiconducting bis-arene chromium,81 superconducting potassium fulleride83) pseudocapacitive charge storage85

Catalysis86 (N2 activation82), battery,85 fast ion conductor

Tantalum oxide54, 55, 84, 86,

88-92

6p mm ,

3Im m

Decomposition of water under UV light,84

Water splitting,84, 91, 92 catalysis86

Chromium oxide93, 94

6p mm ,

3Im m

Oxidizing,94 photocatalytic, VOC elimination,94 catalysis

14

Molybdenum oxide75, 95-99

6p mm ,

3Pm m ,

3Fm m ,

3Ia d

Magnetic (mixed valence),96 pseudocapacitance(MoO3),

97 metallic (MoO2)

98

Catalysis, electrochemical capacitor,97 lbattery98

Tungsten Oxide54, 55, 99-104

6p mm ,

3Fm m ,

3Ia d ,

3Im m

Band-gap corresponding to visible light,100 conductive (mixed valence),101, 102 electrochromic103, 104

Photocatalysis,100 supercapacitor,101 fuel cell,102 electrochromic device103, 104

Manganese oxide105

6p mm Semiconducting, catalytic, mixed valence

Catalysis,105 supercapacitor

Iron oxide106-110 (α-Fe2O3

106, 107, γ-Fe2O3

108, Fe3O4

108)

6p mm ,

,

long range magnetic order (near single crystal),106 ferromagnetic,106 superparamagnetic,106 and elevated magnetic freezing temperature108

Catalysis,110 data storage, battery

Ruthenium oxide111

wormhole Electrode,111 ferroelectric Battery and supercapacitor111

Cobalt oxide112,

113 3Ia d ,

6p mm

ferromagnetic71 Battery113

Nickel/nickel oxide22, 114

6p mm Electrode Battery and supercapacitor22, 114

Copper oxide115, 116

Catalytic, semiconducting (Cu2O) Catalysis, battery

3Ia d

3Fm m

6p mm

15

Zinc oxide116,

117 6p mm CO and NO2 sensing,92

semiconducting116 Sensing92

Cerium oxide118, 119

6p mm ,

3Im m

Catalytic Catalysis119

Aluminum oxide120-123

6p mm Thermally stable Catalyst Support

Indium oxide124 6p mm ,

3Ia d

Transparent, conductive Solar cell

Tin oxide125, 126 6p mm Conductive, electroluminescence sensor, electronics, solar cell

Bismuth oxide127

Distorted cubic

Photocatalytic, semiconducting Catalysis

Indium tin oxide128, 129

Wormhole Transparent, conducting Solar cells, transparent electrode

ATO130 6p mm Transparent, 130conducting130 Solar cell, transparent electrode130

16

1.5. References

1. Rouquerol, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. H.;

Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K., Recommendations for the

Characterization of Porous Solids. Pure Appl. Chem. 1994, 66, 1739-1758.

2. Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S., Ordered

Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Template Mechanism.

Nature 1992, 359, 710-712.

3. Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt,

K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W., A New Family of Mesoporous

Molecular Sieves Prepared with Liquid Crystal Templates. J. Am. Chem. Soc. 1992, 114,

10834-10843.

4. Monnier, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.;

Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F.,

Cooperative Formation of Inorganic-Organic Interfaces in the Synthesis of Silicate

Mesostructures. Science 1993, 261, 1299-1303.

5. Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.;

Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F.,

Cooperative Organization of Inorganic-Surfactant and Biomimetic Assemblies. Science

1995, 267, 1138-1143.

6. Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.;

Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schut, F.; Stucky, G. D., Organization of Organic

Molecules with Inorganic Molecular Species into Nanocomposite Biphase Arrays. Chem.

Mater. 1994, 6, 1176-1191.

7. Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.;

Wang, J. H., Mesoporous Platinum Films from Lyotropic Liquid Crystalline Phases.

Science 1997, 278, 838-840.

17

8. Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.;

Stucky, G. D., Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to

300 Angstrom Pores. Science 1998, 279, 548-552.

9. Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D., Nonionic Triblock

and Star Diblock Copolymer and Oligomeric Sufactant Syntheses of Highly Ordered,

Hydrothermally Stable, Mesoporous Silica Structures. J. Am. Chem. Soc. 1998, 120,

6024-6036.

10. Sakamoto, Y.; Kaneda, M.; Terasaki, O.; Zhao, D. Y.; Kim, J. M.; Stucky, G.;

Shin, H. J.; Ryoo, R., Direct Imaging of the Pores and Cages of Three-Dimensional

Mesoporous Materials. Nature 2000, 408, 449-453.

11. Kaneda, M.; Tsubakiyama, T.; Carlsson, A.; Sakamoto, Y.; Ohsuna, T.; Terasaki,

O.; Joo, S. H.; Ryoo, R., Structural Study of Mesoporous Mcm-48 and Carbon Networks

Synthesized in the Spaces of Mcm-48 by Electron Crystallography. Journal of Physical

Chemistry B 2002, 106, 1256-1266.

12. Yu, T.; Zhang, H.; Yan, X.; Chen, Z.; Zou, X.; Oleynikov, P.; Zhao, D., Pore

Structures of Ordered Large Cage-Type Mesoporous Silica Fdu-12s. Journal of Physical

Chemistry B 2006, 110, 21467-21472.

13. Mariani, P.; Luzzati, V.; Delacroix, H., Cubic Phases of Lipid-Containing

Systems. Structure Analysis and Biological Implications. Journal of Molecular Biology

1988, 204, 165-189.

14. Hajduk, D. A.; Harper, P. E.; Gruner, S. M.; Honeker, C. C.; Kim, G.; Thomas, E.

L.; Fetters, L. J., The Gyroid: A New Equilibrium Morphology in Weakly Segregated

Diblock Copolymers. Macromolecules 1994, 27, 4063-4075.

15. Lee, K.; Kim, Y. H.; Han, S. B.; Kang, H.; Park, S.; Seo, W. S.; Park, J. T.; Kim,

B.; Chang, S., Osmium Replica of Mesoporous Silicate Mcm-48: Efficient and Reusable

Catalyst for Oxidative Cleavage and Dihydroxylation Reactions. J. Am. Chem. Soc. 2003,

125, 6844-6845.

18

16. Bartlett, P. N.; Birkin, P. N.; Ghanem, M. A.; De Groot, P.; Sawicki, M., The

Electrochemical Deposition of Nanostructured Cobalt Films from Lyotropic Liquid

Crystalline Media. Journal of the Electrochemical Society 2001, 148.

17. Bartlett, P. N.; Marwan, J., Electrochemical Deposition of Nanostructured (H1-E)

Layers of Two Metals in Which Pores within the Two Layers Interconnect. Chem. Mater.

2003, 15, 2962-2968.

18. Bartlett, P. N.; Marwan, J., Preparation and Characterization of H1-E Rhodium

Films. Microporous Mesoporous Mater. 2003, 62, 73-79.

19. Yamauchi, Y.; Yokoshima, T.; Momma, T.; Osaka, T.; Kuroda, K., Direct

Physical Casting of the Mesostructure in Lyotropic Liquid Crystalline Media by

Electroless Deposition Confirmation by Tem. Electrochem. Solid-State Lett. 2005, 8.

20. Yamauchi, Y.; Momma, T.; Yokoshima, T.; Kuroda, K.; Osaka, T., Highly

Ordered Mesostructured Ni Particles Prepared from Lyotropic Liquid Crystals by

Electroless Deposition: The Effect of Reducing Agents on the Ordering of Mesostructure.

J. Mater. Chem. 2005, 15, 1987-1994.

21. Ling, T. R.; Li, C. S.; Jow, J. J.; Lee, J. F., Mesoporous Nickel Electrodes Plated

with Gold for the Detection of Glucose. Electrochimica Acta 2011, 56, 1043-1050.

22. Nelson, P. A.; Owen, J. R., A High-Performance Supercapacitor/Battery Hybrid

Incorporating Templated Mesoporous Electrodes. Journal of the Electrochemical Society

2003, 150.

23. Imokawa, T.; Williams, K. J.; Denuault, G., Fabrication and Characterization of

Nanostructured Pd Hydride Ph Microelectrodes. Analytical Chemistry 2006, 78, 265-271.

24. Attard, G. S.; Goltner, C. G.; Corker, J. M.; Henke, S.; Templer, R. H., Liquid-

Crystal Templates for Nanostructured Metals. Angew. Chem. Int. Ed. Engl. 1997, 36,

1315-1317.

19

25. Franceschini, E. A.; Planes, G. A.; Williams, F. J.; Soler-Illia, G. J. A. A.; Corti,

H. R., Mesoporous Pt and Pt/Ru Alloy Electrocatalysts for Methanol Oxidation. Journal

of Power Sources 2011, 196, 1723-1729.

26. Evans, S. A. G.; Elliott, J. M.; Andrews, L. M.; Bartlett, P. N.; Doyle, P. J.;

Denuault, G., Detection of Hydrogen Peroxide at Mesoporous Platinum Microelectrodes.

Analytical Chemistry 2002, 74, 1322-1326.

27. Bender, F.; Mankelow, R. K.; Hibbert, D. B.; Gooding, J. J., Lyotropic Liquid

Crystal Templating of Groups 11 and 12 Metal Films. Electroanalysis 2006, 18, 1558-

1563.

28. Ryoo, R.; Joo, S. H.; Jun, S., Synthesis of Highly Ordered Carbon Molecular

Sieves Via Template-Mediated Structural Transformation. Journal of Physical Chemistry

B 1999, 103, 7745-7746.

29. Joo, S. H.; Jun, S.; Ryoo, R., Synthesis of Ordered Mesoporous Carbon Molecular

Sieves Cmk-1. Microporous Mesoporous Mater. 2001, 44-45, 153-158.

30. Ryoo, R.; Joo, S. H.; Kruk, M.; Jaroniec, M., Ordered Mesoporous Carbons. Adv.

Mater. 2001, 13, 677-681.

31. Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R., Ordered

Nanoporous Arrays of Carbon Supporting High Dispersions of Platinum Nanoparticles.

Nature 2001, 412, 169-172.

32. Guo, C. X.; Hu, F. P.; Lou, X. W.; Li, C. M., High-Performance Biofuel Cell

Made with Hydrophilic Ordered Mesoporous Carbon as Electrode Material. Journal of

Power Sources 2010, 195, 4090-4097.

33. Li, Q.; Jiang, R.; Dou, Y.; Wu, Z.; Huang, T.; Feng, D.; Yang, J.; Yu, A.; Zhao,

D., Synthesis of Mesoporous Carbon Spheres with a Hierarchical Pore Structure for the

Electrochemical Double-Layer Capacitor. Carbon 2011, 49, 1248-1257.

20

34. Li, F.; Song, J.; Shan, C.; Gao, D.; Xu, X.; Niu, L., Electrochemical

Determination of Morphine at Ordered Mesoporous Carbon Modified Glassy Carbon

Electrode. Biosensors and Bioelectronics 2010, 25, 1408-1413.

35. Mahurin, S. M.; Lee, J. S.; Wang, X.; Dai, S., Ammonia-Activated Mesoporous

Carbon Membranes for Gas Separations. Journal of Membrane Science 2011, 368, 41-47.

36. Richman, E. K.; Kang, C. B.; Brezesinski, T.; Tolbert, S. H., Ordered Mesoporous

Silicon through Magnesium Reduction of Polymer Templated Silica Thin Films. Nano

Lett. 2008.

37. Guo, M.; Zou, X.; Ren, H.; Muhammad, F.; Huang, C.; Qiu, S.; Zhu, G.,

Fabrication of High Surface Area Mesoporous Silicon Via Magnesiothermic Reduction

for Drug Delivery. Microporous Mesoporous Mater. 2011, In Press, Corrected Proof.

38. Armatas, G. S.; Kanatzidis, M. G., Mesostructured Germanium with Cubic Pore

Symmetry. Nature 2006, 441, 1122-1125.

39. Sun, D.; Riley, A. E.; Cadby, A. J.; Richman, E. K.; Korlann, S. D.; Tolbert, S.

H., Hexagonal Nanoporous Germanium through Surfactant-Driven Self-Assembly of

Zintl Clusters. Nature 2006, 441, 1126-1130.

40. Armatas, G. S.; Kanatzidis, M. G., Size Dependence in Hexagonal Mesoporous

Germanium: Pore Wall Thickness Versus Energy Gap and Photoluminescence. Nano Lett

2010, 10, 3330-3336.

41. Armatas, G. S.; Kanatzidis, M. G., Hexagonal Mesoporous Germanium. Science

2006, 313, 817-820.

42. Whitehead, A. H.; Elliott, J. M.; Owen, J. R.; Attard, G. S., Electrodeposition of

Mesoporous Tin Films. Chem. Comm. 1999, 331-332.

43. Nandhakumar, I.; Elliott, J. M.; Attard, G. S., Electrodeposition of Nanostructured

Mesoporous Selenium Films (Hi-Ese). Chem. Mater. 2001, 13, 3840-3842.

21

44. Gabriel, T.; Nandhakumar, I. S.; Attard, G. S., Electrochemical Synthesis of

Nanostructured Tellurium Films. Electrochemistry Communications 2002, 4, 610-612.

45. Attard, G. S.; Leclerc, S. A. A.; Maniguet, S.; Russell, A. E.; Nandhakumar, I.;

Bartlett, P. N., Mesoporous Pt/Ru Alloy from the Hexagonal Lyotropic Liquid

Crystalline Phase of a Nonionic Surfactant. Chem. Mater. 2001, 13, 1444-1446.

46. Jiang, J.; Kucernak, A., Electrooxidation of Small Organic Molecules on

Mesoporous Precious Metal Catalysts Ii: Co and Methanol on Platinum-Ruthenium

Alloy. Journal of Electroanalytical Chemistry 2003, 543, 187-199.

47. Jiang, J.; Kucernak, A., Mesoporous Microspheres Composed of Ptru Alloy.

Chem. Mater. 2004, 16, 1362-1367.

48. Yamauchi, Y.; Sadasivan Nair, S.; Momma, T.; Ohsuna, T.; Osaka, T.; Kuroda,

K., Synthesis and Characterization of Mesoporous Pt-Ni (Hi-Pt/Ni) Alloy Particles

Prepared from Lyotropic Liquid Crystalline Media. J. Mater. Chem. 2006, 16, 2229-

2234.

49. Roggenbuck, J.; Tiemann, M., Ordered Mesoporous Magnesium Oxide with High

Thermal Stability Synthesized by Exotemplating Using Cmk-3 Carbon. J. Am. Chem.

Soc. 2005, 127, 1096-1097.

50. Bhagiyalakshmi, M.; Lee, J. Y.; Jang, H. T., Synthesis of Mesoporous

Magnesium Oxide: Its Application to CO2 Chemisorption. International Journal of

Greenhouse Gas Control 2010, 4, 51-56.

51. Caixin, L.; Lei, Z.; Jiguang, D.; Qing, M.; Hongxing, D.; Hong, H., Surfactant-

Aided Hydrothermal Synthesis and Carbon Dioxide Adsorption Behavior of Three-

Dimensionally Mesoporous Calcium Oxide Single-Crystallites with Tri-, Tetra-, and

Hexagonal Morphologies. Journal of Physical Chemistry C 2008, 112, 19248-19256.

52. Antonelli, D. M.; Ying, J. Y., Synthesis of Hexagonally Packed Mesoporous TiO2

by a Modified Sol-Gel Method. Angew. Chem. Int. Ed. Engl. 1995, 34, 2014-2017.

22

53. Lee, J.; Christopher Orilall, M.; Warren, S. C.; Kamperman, M.; Disalvo, F. J.;

Wiesner, U., Direct Access to Thermally Stable and Highly Crystalline Mesoporous

Transition-Metal Oxides with Uniform Pores. Nat. Mater. 2008, 7, 222-228.

54. Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D., Generalized

Syntheses of Large-Pore Mesoporous Metal Oxides with Semicrystalline Frameworks.

Nature 1998, 396, 152-155.

55. Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D., Block

Copolymer Templating Syntheses of Mesoporous Metal Oxides with Large Ordering

Lengths and Semicrystalline Framework. Chem. Mater. 1999, 11, 2813-2826.

56. Brezesinski, T.; Wang, J.; Polleux, J.; Dunn, B.; Tolbert, S. H., Templated

Nanocrystal-Based Porous TiO2 Films for Next-Generation Electrochemical Capacitors.

J. Am. Chem. Soc. 2009, 131, 1802-1809.

57. Smarsly, B.; Grosso, D.; Brezesinski, T.; Pinna, N.; Boissière, C.; Antonietti, M.;

Sanchez, C., Highly Crystalline Cubic Mesoporous TiO2 with 10-Nm Pore Diameter

Made with a New Block Copolymer Template. Chem. Mater. 2004, 16, 2948-2952.

58. Grosso, D.; Soler-Illia, G. J. D. A. A.; Crepaldi, E. L.; Cagnol, F.; Sinturel, C.;

Bourgeois, A.; Brunet-Bruneau, A.; Amenitsch, H.; Albouy, P. A.; Sanchez, C., Highly

Porous TiO2 Anatase Optical Thin Films with Cubic Mesostructure Stabilized at 700°C.

Chem. Mater. 2003, 15, 4562-4570.

59. Choi, S. Y.; Lee, B.; Carew, D. B.; Mamak, M.; Peiris, F. C.; Speakman, S.;

Chopra, N.; Ozin, G. A., 3d Hexagonal (R-3m) Mesostructured Nanocrystalline Titania

Thin Films: Synthesis and Characterization. Adv. Funct. Mater. 2006, 16, 1731-1738.

60. Choi, S. Y.; Mamak, M.; Coombs, N.; Chopra, N.; Ozin, G. A., Thermally Stable

Two-Dimensional Hexagonal Mesoporous Nanocrystalline Anatase, Meso-Nc-TiO2:

Bulk and Crack-Free Thin Film Morphologies. Adv. Funct. Mater. 2004, 14, 335-344.

23

61. Docampo, P.; Guldin, S.; Stefik, M.; Tiwana, P.; Orilall, M. C.; Hüttner, S.; Sai,

H.; Wiesner, U.; Steiner, U.; Snaith, H. J., Control of Solid-State Dye-Sensitized Solar

Cell Performance by Block-Copolymer-Directed Tio2 Synthesis. Adv. Funct. Mater.

2010, 20, 1787-1796.

62. Stone Jr, V. F.; Davis, R. J., Synthesis, Characterization, and Photocatalytic

Activity of Titania and Niobia Mesoporous Molecular Sieves. Chem. Mater. 1998, 10,

1468-1474.

63. Kavan, L.; Rathouský, J.; Grätzel, M.; Shklover, V.; Zukal, A., Mesoporous Thin

Film TiO2 Electrodes. Microporous Mesoporous Mater. 2001, 44-45, 653-659.

64. Szeifert, J. M.; Feckl, J. M.; Fattakhova-Rohlfing, D.; Liu, Y.; Kalousek, V.;

Rathousky, J.; Bein, T., Ultrasmall Titania Nanocrystals and Their Direct Assembly into

Mesoporous Structures Showing Fast Lithium Insertion. J. Am. Chem. Soc. 2010, 132,

12605-12611.

65. Zukalovà, M.; Zukal, A.; Kavan, L.; Nazeeruddin, M. K.; Liska, P.; Grätzel, M.,

Organized Mesoporous TiO2 Films Exhibiting Greatly Enhanced Performance in Dye-

Sensitized Solar Cells. Nano Lett 2005, 5, 1789-1792.

66. Grätzel, M., Mesoporous Oxide Junctions and Nanostructured Solar Cells.

Current Opinion in Colloid and Interface Science 1999, 4, 314-321.

67. Hudson, M. J.; Knowles, J. A., Preparation and Characterisation of Mesoporous,

High-Surface-Area Zirconium (Iv) Oxide. J. Mater. Chem. 1996, 6, 89-95.

68. Pârvulescu, V. I.; Bonnemann, H.; Pârvulescu, V.; Endruschat, U.; Rufinska, A.;

Lehmann, C. W.; Tesche, B.; Poncelet, G., Preparation and Characterisation of

Mesoporous Zirconium Oxide. Applied Catalysis A: General 2001, 214, 273-287.

69. Nelson, C. A.; Szczech, J. R.; Xu, Q.; Lawrence, M. J.; Jin, S.; Ge, Y.,

Mesoporous Zirconium Oxide Nanomaterials Effectively Enrich Phosphopeptides for

Mass Spectrometry-Based Phosphoproteomics. Chem. Comm. 2009, 6607-6609.

24

70. Nelson, C. A.; Szczech, J. R.; Dooley, C. J.; Xu, Q.; Lawrence, M. J.; Zhu, H.;

Jin, S.; Ge, Y., Effective Enrichment and Mass Spectrometry Analysis of

Phosphopeptides Using Mesoporous Metal Oxide Nanomaterials. Analytical Chemistry

2010, 82, 7193-7201.

71. Brezesinski, T.; Smarsly, B.; Iimura, K. I.; Grosso, D.; Boissière, C.; Amenitsch,

H.; Antonietti, M.; Sanchez, C., Self-Assembly and Crystallization Behavior of

Mesoporous, Crystalline HfO2 Thin Films: A Model System for the Generation of

Mesostructured Transition-Metal Oxides. Small 2005, 1, 889-898.

72. Lee, J. K.; Kim, G. P.; Song, I. K.; Baeck, S. H., Electrodeposition of Mesoporous

V2O5 with Enhanced Lithium-Ion Intercalation Property. Electrochemistry

Communications 2009, 11, 1571-1574.

73. Liu, P.; Lee, S. H.; Edwin Tracy, C.; Turner, J. A.; Pitts, J. R.; Deb, S. K.,

Electrochromic and Chemochromic Performance of Mesoporous Thin-Film Vanadium

Oxide. Solid State Ionics 2003, 165, 223-228.

74. Liu, P.; Lee, S. H.; Tracy, C. E.; Yan, Y.; Turner, J. A., Preparation and Lithium

Insertion Properties of Mesoporous Vanadium Oxide. Adv. Mater. 2002, 14, 27-30.

75. Brezesinski, T.; Groenewolt, M.; Pinna, N.; Amenitsch, H.; Antonietti, M.;

Smarsly, B. M., Surfactant-Mediated Generation of Iso-Oriented Dense and Mesoporous

Crystalline Metal-Oxide Layers. Adv. Mater. 2006, 18, 1827-1831.

76. Antonelli, D. M.; Ying, J. Y., Synthesis of a Stable Hexagonally Packed

Mesoporous Niobium Oxide Molecular Sieve through a Novel Ligand-Assisted

Templating Mechanism. Angew. Chem. Int. Ed. Engl. 1996, 35, 426-430.

77. Antonelli, D. M.; Nakahira, A.; Ying, J. Y., Ligand-Assisted Liquid Crystal

Templating in Mesoporous Niobium Oxide Molecular Sieves. Inorganic Chemistry 1996,

35, 3126-3136.

25

78. Lee, B.; Lu, D.; Kondo, J. N.; Domen, K., Three-Dimensionally Ordered

Mesoporous Niobium Oxide. J. Am. Chem. Soc. 2002, 124, 11256-11257.

79. Murray, S.; Trudeau, M.; Antonelli, D. M., Synthesis and Magnetic Tuning in

Superparamagnetic Cobaltocene-Mesoporous Niobium Oxide Composites. Adv. Mater.

2000, 12, 1339-1342.

80. Vettraino, M.; Trudeau, M. L.; Antonelli, D. M., Synthesis and Electronic

Properties of Reduced Mesoporous Sodium Niobium Oxides. Adv. Mater. 2000, 12, 337-

341.

81. He, X.; Trudeau, M.; Antonelli, D.; Trudeau, M. L., Electronic Properties of

Novel Mixed Oxidation-State Bis-Arene Chromium Nanowires Supported by a

Mesoporous Niobium Oxide Host. Adv. Mater. 2000, 12, 1036-1040.

82. Vettraino, M.; He, X.; Trudeau, M.; Drake, J. E.; Antonelli, D. M., Synthesis of a

Stable Metallic Niobium Oxide Molecular Sieve and Subsequent Room Temperature

Activation of Dinitrogen. Adv. Funct. Mater. 2002, 12, 174-178.

83. Ye, B.; Trudeau, M.; Antonelli, D., Synthesis and Electronic Properties of

Potassium Fulleride Nanowires in a Mesoporous Niobium Oxide Host. Adv. Mater. 2001,

13, 29-33.

84. Takahara, Y.; Kondo, J. N.; Takata, T.; Lu, D.; Domen, K., Mesoporous Tantalum

Oxide. 1. Characterization and Photocatalytic Activity for the Overall Water

Decomposition. Chem. Mater. 2001, 13, 1194-1199.

85. Brezesinski, K.; Wang, J.; Haetge, J.; Reitz, C.; Steinmueller, S. O.; Tolbert, S.

H.; Smarsly, B. M.; Dunn, B.; Brezesinski, T., Pseudocapacitive Contributions to Charge

Storage in Highly Ordered Mesoporous Group V Transition Metal Oxides with Iso-

Oriented Layered Nanocrystalline Domains. J. Am. Chem. Soc. 2010, 132, 6982-6990.

86. Rao, Y.; Antonelli, D. M., Mesoporous Transition Metal Oxides: Characterization

and Applications in Heterogeneous Catalysis. J. Mater. Chem. 2009, 19, 1937-1944.

26

87. Khushalani, D.; Ozin, G. A.; Kuperman, A., Glycometallate Surfactants Part 2:

Non-Aqueous Synthesis of Mesoporous Titanium, Zirconium and Niobium Oxides. J.

Mater. Chem. 1999, 9, 1491-1500.

88. Antonelli, D. M.; Ying, J. Y., Synthesis and Characterization of Hexagonally

Packed Mesoporous Tantalum Oxide Molecular Sieves. Chem. Mater. 1996, 8, 874-881.

89. Shirokura, N.; Nakajima, K.; Nakabayashi, A.; Lu, D.; Hara, M.; Domen, K.;

Tatsumi, T.; Kondo, J. N., Synthesis of Crystallized Mesoporous Transition Metal Oxides

by Silicone Treatment of the Oxide Precursor. Chem. Comm. 2006, 2188-2190.

90. Kondo, J. N.; Domen, K., Crystallization of Mesoporous Metal Oxides. Chem.

Mater. 2008, 20, 835-847.

91. Stodolny, M.; Laniecki, M., Synthesis and Characterization of Mesoporous

Ta2o5-Tio2 Photocatalysts for Water Splitting. Catalysis Today 2009, 142, 314-319.

92. Noda, Y.; Lee, B.; Domen, K.; Kondo, J. N., Synthesis of Crystallized

Mesoporous Tantalum Oxide and Its Photocatalytic Activity for Overall Water Splitting

under Ultraviolet Light Irradiation. Chem. Mater. 2008, 20, 5361-5367.

93. Zhu, K.; Yue, B.; Zhou, W.; He, H., Preparation of Three-Dimensional Chromium

Oxide Porous Single Crystals Templated by Sba-15. Chem. Comm. 2003, 9, 98-99.

94. Sinha, A. K.; Suzuki, K., Novel Mesoporous Chromium Oxide for Vocs

Elimination. Applied Catalysis B: Environmental 2007, 70, 417-422.

95. Antonelli, D. M.; Trudeau, M., Phase Changes and Electronic Properties in

Toroidal Mesoporous Molybdenum Oxides. Angew. Chem. Int. Ed. 1999, 38, 1471-1475.

96. Chen, J.; Burger, C.; Krishnan, C. V.; Chu, B., Morphogenesis of Highly Ordered

Mixed-Valent Mesoporous Molybdenum Oxides. J. Am. Chem. Soc. 2005, 127, 14140-

14141.

27

97. Brezesinski, T.; Wang, J.; Tolbert, S. H.; Dunn, B., Ordered Mesoporous Alpha-

MoO3 with Iso-Oriented Nanocrystalline Walls for Thin-Film Pseudocapacitors. Nat.

Mater. 2010, 9, 146-151.

98. Shi, Y.; Guo, B.; Corr, S. A.; Shi, Q.; Hu, Y. S.; Heier, K. R.; Chen, L.; Seshadri,

R.; Stucky, G. D., Ordered Mesoporous Metallic MoO2 Materials with Highly Reversible

Lithium Storage Capacity. Nano Lett 2009, 9, 4215-4220.

99. Brezesinski, T.; Groenewolt, M.; Gibaud, A.; Pinna, N.; Antonietti, M.; Smarsly,

B. M., Evaporation-Induced Self-Assembly (Eisa) at Its Limit: Ultrathin, Crystalline

Patterns by Templating of Micellar Monolayers. Adv. Mater. 2006, 18, 2260-2263.

100. Li, L.; Krissanasaeranee, M.; Pattinson, S. W.; Stefik, M.; Wiesner, U.; Steiner,

U.; Eder, D., Enhanced Photocatalytic Properties in Well-Ordered Mesoporous WO3.

Chem. Comm. 2010, 46, 7620-7622.

101. Yoon, S.; Kang, E.; Kim, J. K.; Lee, C. W.; Lee, J., Development of High-

Performance Supercapacitor Electrodes Using Novel Ordered Mesoporous Tungsten

Oxide Materials with High Electrical Conductivity. Chem. Comm. 2011, 47, 1021-1023.

102. Kang, E.; An, S.; Yoon, S.; Kim, J. K.; Lee, J., Ordered Mesoporous WO3-X

Possessing Electronically Conductive Framework Comparable to Carbon Framework

toward Long-Term Stable Cathode Supports for Fuel Cells. J. Mater. Chem. 2010, 20,

7416-7421.

103. Brezesinski, T.; Rohlfing, D. F.; Sallard, S.; Antonietti, M.; Smarsly, B. M.,

Highly Crystalline WO3 Thin Films with Ordered 3d Mesoporosity and Improved

Electrochromic Performance. Small 2006, 2, 1203-1211.

104. Sallard, S.; Brezesinski, T.; Smarsly, B. M., Electrochromic Stability of Wo3

Thin Films with Nanometer-Scale Periodicity and Varying Degrees of Crystallinity.

Journal of Physical Chemistry C 2007, 111, 7200-7206.

28

105. Tian, Z. R.; Tong, W.; Wang, J. Y.; Duan, N. G.; Krishnan, V. V.; Suib, S. L.,

Manganese Oxide Mesoporous Structures: Mixed-Valent Semiconducting Catalysts.

Science 1997, 276, 926-930.

106. Jiao, F.; Harrison, A.; Jumas, J. C.; Chadwick, A. V.; Kockelmann, W.; Bruce, P.

G., Ordered Mesoporous Fe2o3 with Crystalline Walls. J. Am. Chem. Soc. 2006, 128,

5468-5474.

107. Jiao, F.; Bruce, P. G., Two- and Three-Dimensional Mesoporous Iron Oxides with

Microporous Walls. Angew. Chem. Int. Ed. 2004, 43, 5958-5961.

108. Jiao, F.; Jumas, J. C.; Womes, M.; Chadwick, A. V.; Harrison, A.; Bruce, P. G.,

Synthesis of Ordered Mesoporous Fe3o4 and Gamma-Fe2O3 with Crystalline Walls

Using Post-Template Reduction/Oxidation. J. Am. Chem. Soc. 2006, 128, 12905-12909.

109. Brezesinski, T.; Groenewolt, M.; Antonietti, M.; Smarsly, B., Crystal-to-Crystal

Phase Transition in Self-Assembled Mesoporous Iron Oxide Films. Angew. Chem. Int.

Ed. 2006, 45, 781-784.

110. Srivastava, D. N.; Perkas, N.; Gedanken, A.; Felner, I., Sonochemical Synthesis

of Mesoporous Iron Oxide and Accounts of Its Magnetic and Catalytic Properties.

Journal of Physical Chemistry B 2002, 106, 1878-1883.

111. Sassoye, C.; Laberty, C.; Khanh, H. L.; Cassaignon, S.; Boissière, C.; Antonietti,

M.; Sanchez, C., Block-Copolymer-Templated Synthesis of Electroactive Ruo2-Based

Mesoporous Thin Films. Adv. Funct. Mater. 2009, 19, 1922-1929.

112. Wang, Y.; Yang, C. M.; Schmidt, W.; Spliethoff, B.; Bill, E.; Schüth, F., Weakly

Ferromagnetic Ordered Mesoporous Co3O4 Synthesized by Nanocasting from Vinyl-

Functionalized Cubic Ia3d Mesoporous Silica. Adv. Mater. 2005, 17, 53-56.

113. Jiao, F.; Shaju, K. M.; Bruce, P. G., Synthesis of Nanowire and Mesoporous Low-

Temperature LiCoO2 by a Post-Templating Reaction. Angew. Chem. Int. Ed. 2005, 44,

6550-6553.

29

114. Nelson, P. A.; Elliott, J. M.; Attard, G. S.; Owen, J. R., Mesoporous

Nickel/Nickel Oxide - a Nanoarchitectured Electrode. Chem. Mater. 2002, 14, 524-529.

115. Lai, X.; Li, X.; Geng, W.; Tu, J.; Li, J.; Qiu, S., Ordered Mesoporous Copper

Oxide with Crystalline Walls. Angew. Chem. Int. Ed. 2007, 46, 738-741.

116. Luo, H.; Zhang, J.; Yan, Y., Electrochemical Deposition of Mesoporous

Crystalline Oxide Semiconductor Films from Lyotropic Liquid Crystalline Phases. Chem.

Mater. 2003, 15, 3769-3773.

117. Wagner, T.; Waitz, T.; Roggenbuck, J.; Fröba, M.; Kohl, C. D.; Tiemann, M.,

Ordered Mesoporous Zno for Gas Sensing. Thin Solid Films 2007, 515, 8360-8363.

118. Brezesinski, T.; Erpen, C.; Iimura, K. I.; Smarsly, B., Mesostructured Crystalline

Ceria with a Bimodal Pore System Using Block Copolymers and Ionic Liquids as

Rational Templates. Chem. Mater. 2005, 17, 1683-1690.

119. Roggenbuck, J.; Schäfer, H.; Tsoncheva, T.; Minchev, C.; Hanss, J.; Tiemann,

M., Mesoporous Ceo2: Synthesis by Nanocasting, Characterisation and Catalytic

Properties. Microporous Mesoporous Mater. 2007, 101, 335-341.

120. Yuan, X.; Xu, S.; Lü, J.; Yan, X.; Hu, L.; Xue, Q., Facile Synthesis of Ordered

Mesoporous Gamma-Alumina Monoliths Via Polymerization-Based Gel-Casting.

Microporous Mesoporous Mater. 2011, 138, 40-44.

121. Liu, Q.; Wang, A.; Wang, X.; Zhang, T., Ordered Crystalline Alumina Molecular

Sieves Synthesized Via a Nanocasting Route. Chem. Mater. 2006, 18, 5153-5155.

122. Niesz, K.; Yang, P.; Somorjai, G. A., Sol-Gel Synthesis of Ordered Mesoporous

Alumina. Chem. Comm. 2005, 1986-1987.

123. Yuan, Q.; Yin, A. X.; Luo, C.; Sun, L. D.; Zhang, Y. W.; Duan, W. T.; Liu, H. C.;

Yan, C. H., Facile Synthesis for Ordered Mesoporous Gamma-Aluminas with High

Thermal Stability. J. Am. Chem. Soc. 2008, 130, 3465-3472.

30

124. Yang, H.; Shi, Q.; Tian, B.; Lu, Q.; Gao, F.; Xie, S.; Fan, J.; Yu, C.; Tu, B.; Zhao,

D., One-Step Nanocasting Synthesis of Highly Ordered Single Crystalline Indium Oxide

Nanowire Arrays from Mesostructured Frameworks. J. Am. Chem. Soc. 2003, 125, 4724-

4725.

125. Che, H.; Han, S.; Hou, W.; Liu, A.; Yu, X.; Sun, Y.; Wang, S., Ordered

Mesoporous Tin Oxide with Crystalline Pore Walls: Preparation and Thermal Stability.


126. Aprile, C.; Teruel, L.; Alvaro, M.; Garcia, H., Structured Mesoporous Tin Oxide

with Electrical Conductivity. Application in Electroluminescence. J. Am. Chem. Soc.

2009, 131, 1342-1343.

127. Brezesinski, K.; Ostermann, R.; Hartmann, P.; Perlich, J.; Brezesinski, T.,

Exceptional Photocatalytic Activity of Ordered Mesoporous Beta-Bi2o3 Thin Films and

Electrospun Nanofiber Mats. Chem. Mater. 2010, 22, 3079-3085.

128. Emons, T. T.; Li, J.; Nazar, L. F., Synthesis and Characterization of Mesoporous

Indium Tin Oxide Possessing an Electronically Conductive Framework. J. Am. Chem.

Soc. 2002, 124, 8516-8517.

129. Pohl, A.; Dunn, B., Mesoporous Indium Tin Oxide (ITO) Films. Thin Solid Films

2006, 515, 790-792.

130. Hou, K.; Puzzo, D.; Helander, M. G.; Lo, S. S.; Bonifacio, L. D.; Wang, W. D.;

Lu, Z.-H.; Scholes, G. D.; Ozin, G. A., Dye-Anchored Mesoporous Antimony-Doped Tin

Oxide Electrochemiluminescence Cell. Adv. Mater. 2009, 21, 2492-2496.

31

Chapter 2 Why PMO? Towards Functionality and Utility of

Periodic Mesoporous Organosilica

This image titled “Writing on the Pore Wall with a Synthetic Quill” symbolizes the synthetic feat of periodic mesoporous organosilica

32

The first chapter introduced materials with periodically ordered mesopores,

summarized three basic routes for their synthesis, and listed non-exhaustively the

chemical composition of pore walls reported in the literature. This chapter will focus on

one specific class of mesoporous materials: periodic mesoporous organosilica (PMO).

PMO is distinguished from other ordered mesoporous materials by the chemical

composition of its pore wall, which is made of organosilica in which each individual

organic group is covalently bonded to two or more silicon atoms (Figure 2-1). Three

groups independently reported the first examples of PMO in 1999.1-3 Within a few years,

PMOs with a large number of organic bridging groups, pore sizes, and pore geometries

have been synthesized and characterized. As the synthetic techniques of PMOs mature,

the focus of the field is moving from simple synthesis and structural determination to the

investigation of function, often within the context of an intended application. In these

new endeavors, researchers often compared PMO with the current material of choice in a

given application or with other material candidates, and asked the question, “Why

PMO?”4 The answer is often traced back to two features that have given PMO its name:

the inorganic-organic hybrid composition and ordered porous structure.

33

Figure 2-1 (a) Illustration of a typical PMO structure, and (b) citation of the at least one of the first three PMO papers per year

The first distinguishing feature of PMO is its chemical composition: organic-

inorganic hybrids, most of which have a generic formula O1.5Si-R-SiO1.5, the so-called

silsesquioxanes (Figure 2-1). The term silsesquioxane, meaning one silicon and one and a

half oxygens, reflects a theoretical stoichiometry when all three silanol groups connected

to one silicon atom (-Si(OH)3) are condensed to form Si-O-Si bonds so that every oxygen

is shared between two silicon atoms. The study of bridged polysilsesquioxanes long

precedes the invention of PMO.5 When prepared in the form of xerogels and aerogels,

34

bridged polysilsesquioxanes are highly porous, often transparent and lightweight. They

can be used as a matrix for growing metal and semiconductor nanoclusters or used for

optical applications such as waveguides, lasers, and nonlinear optics.5 However, their

random porous network and broad pore size distributions severely limit the size and

shape selectivity in any kind applications involving host-guest interaction. This limitation

is overcome by narrow pore size distribution and regular pore geometry in PMO.

The second distinguishing feature of PMO is its ordered porous structure. In this

regard, PMO’s two closest cousins are zeolite and periodic mesoporous silica (PMS).

Zeolite possesses micropores with diameters less than two nanometers. The small pore

size of zeolite limits its use in areas when large guest species need to be introduced. PMS,

on the other hand, possesses the same porous structure as PMO; however, the pore wall

of PMS is composed entirely of silica, which limits its use in areas when hydrophobicity

is desired or when the interaction between porous host and guest species needs to be

designed and tuned. To overcome this limit, researchers first added organosilanes to the

silica matrix through two approaches: (1) grafting monofunctional organosilanes such as

R-Si(OCH2CH3)3 on the pore surface of PMS, and (2) co-condensing organosilanes with

pristine silica precursors such as tetraethyl orthosilicate (TEOS). The former approach

usually suffers from incomplete surface coverage and pore blockage, while the latter

suffers from inhomogeneous distribution of organic moieties and a limiting 25% loading

of organosilanes. (This loading limit is due to the intrinsic instability of an interrupted

framework in which more than a quarter of SiO4 are replaced by RSiO3 building blocks.)

Compared with materials made by two previous approaches, PMO is composed entirely

of organosilica and has a uniform distribution of organic groups in the pore wall at the

molecular level, advantageous in applications where high loading of organic groups or

uniform composition of pore wall is needed.

The rest of this chapter will discuss in detail nine examples, divided into three

categories, to illustrate the unique functions and emerging applications of PMOs. The

first category illustrates how chemical modification of bridging organic groups can

introduce function in PMOs. The next category examines the role of organic groups

played in host-guest chemistry and related applications. The last category shows that

35

PMOs can have a range of physical functions due to their distinctive optical, electric and

dielectric properties, which are in large part governed by the choice of bridging organic

groups. For the convenience of discussion, I identify each PMO by the parent compound

of its bridging organic group. For example, a C6H4-bridged PMO is called benzene PMO,

and a CH2CH2-bridged PMO is called ethane PMO.

2.1. Organic Chemistry of the Pore Wall

Figure 2-2 Scheme of the Diels-Alder reaction on ethene PMO and subsequent attachment of a sulfonic acid group on the benzene ring (the cis- configuration of the

double bond is drawn for the convenience of illustration).

The bridging organic groups in PMO may contain functional groups that can be

readily modified according to well-known reactions in organic chemistry. Because these

reactions involve one reagent anchored in a solid matrix, they can be designed to take

advantages of this unique feature. Four examples are given here: (1) if the reaction is

heterogeneous, the product can be separated from the reaction solution easily through

filtration or centrifugation; (2) varying pore size and pore geometry enable control over

reaction kinetics through diffusion; (3) the siloxane network (–Si–O–Si–) in a PMO is

highly electron withdrawing, so the organic bridging groups become significantly more

susceptible to nucleophiles; (4) the geometric constraints set by the solid siloxane

network on the bridging organic groups can be used to enhance the selectivity of organic

reactions. Indeed, a key area of PMO development lies in the chemistry of the pore walls.

Simple reactions such as sulfonation to yield acidic functional groups can convert a PMO

into a solid-state acid. This acid can serve as a heterogeneous catalyst in various key

industrial reactions, such as the synthesis of monoglycerol, nylon fibers and resins.6 More

involved chemistry has also been explored. In 2005, Kondo and coworkers successfully

36

attached an arenesulfonic acid group to an ethene PMO through a two-step process to

yield a solid-state acid (Figure 2-2).7 First, pendant phenylene groups were attached to

the double bond through a Diels-Alder reaction with benzocyclobutene. Then the pendant

phenylene groups were exposed to concentrated sulfuric acid to produce sulfonic acid

groups on the pore surface. The catalytic activity of the resulting PMO in the

esterification of acetic acid and ethanol was found to be comparable to sulfonated

polystyrene resin (Ambelyst-15) and perfluorinated sulfonic acid resin (Nafion).

Additionally, the PMO’s catalytic activity in the pinacol rearrangement, which requires a

highly acidic environment, was much higher than both commercially available

heterogeneous acid, and was comparable instead to concentrated sulfuric acid. The

authors attributed the strong acidity of the PMO to the phenyl groups that disperse and

stabilize the negative charges of the conjugate base.

Figure 2-3 Reaction scheme of the synthesis of bifunctional PMO with acidic pore framework and basic pore surface

Acidic and basic molecules are antagonistic and will react quickly when

combined in solution. In solid state, however, the movement of molecules is restricted, so

it is possible to place both acidic and basic groups in one solid material. In 2006, Ahmad

Mehdi and colleagues successfully incorporated both groups into one single PMO (Figure

2-3).8 Building upon their previous work on PMO with a basic pore surface,9 they

synthesized this bifunctional PMO by taking advantage of different hydrophilic–

hydrophobic character of two organosilanes: the one with a bridging disulfide bond

tended to stay in the hydrophilic head group region of the templating micelles, whereas

the other with a protected aminopropyl group preferred the hydrophilic–hydrophobic

interface of template micelles, with its hydrophobic arm extending into the cores of

micelles. As a result, the final product had two types of functional groups distributed in

two different regions: the sulfonic acid groups in the framework and the basic amine

37

groups on the pore surface. To demonstrate the effect of proton transfer from sulfonic

acid groups to amine groups, the authors performed Michael addition reactions between

acrylamide (CH2=CH-CONH2) and the amine groups in anhydrous tetrahydrofuran, an

aprotic solvent, and in ethanol, a protic solvent. They found that the yield was 82% in

anhydrous tetrahydrofuran, but only 10% in ethanol.

Figure 2-4 Lineage of bromobenzene PMO family

Biologically important molecules such as amino acids, proteins, sugars and DNA

are chiral, and processes that preferentially interact with one enantiomer require a chiral

medium, be it a solvent or a surface. Efforts to incorporate chirality inside mesoporous

materials often aim at producing a heterogeneous chiral catalyst.10 However, such

incorporations are challenging due to the difficulty in synthesizing enantiomerically pure

precursors. In 2008, Polarz and coworkers reported a generic synthesis that incorporated

enantiomerically pure amino acids inside PMO. They started with a bromobenzene PMO

and transformed it to a series of PMOs (Figure 2-4).11, 12 Of particular interest was the

PMO with enantiomerically pure amino acid groups. The molecular chirality of the

38

amino acid groups were translated into a surface property, as demonstrated by different

amounts of two enantiomers of propylene oxide absorbed inside PMO.12

The above three examples illustrate that the organic chemistry of the pore walls

adds diverse functionalities into otherwise simple PMOs. These added functionalities

give rise to many potential applications, catalysis being the first explored.13 Compared

with microporous zeolites, the larger mesopores in PMOs can accommodate larger

reactant molecules and facilitate fast diffusion into and out of the material; compared

with organosilica aerogels or xerogels, the uniform mesopore diameter makes possible

size selectivity and easy access to catalytically active sites; compared with PMS, PMOs

further offer a tunable hydrophobic–hydrophilic microenvironment around catalytically

active sites,14 excellent hydrothermal stability due to their less easily hydrolyzed Si-O-Si

bonds15 and mechanical stability under compression;16 and finally, compared with the

materials produced by grafting organic functional groups onto the PMS pore walls, PMO

may reduce catalyst leaching, pore blockage, and inhomogeneous distribution of

functional groups.17 Besides providing novel catalysts, the organic chemistry of the pore

wall is in essence a synthetic route to new PMOs. It circumvents the difficulty of self-

assembling precursors with bulky organic groups, and gives access to a vast array of new

PMOs with highly tunable functionalities.

2.2. PMO and Guest Molecules

39

Figure 2-5 Chromatograms of the mixture of four aromatic compounds separated in n-hexane at a flow rate of 1 mL·min-1 on the Nucleosil 50–10 column or at a flow rate of 2

mL·min-1 on the spherical benzene PMO column

Separation is a typical application that requires tuning host–guest interactions.

High-performance liquid chromatography (HPLC) is a well-known technique for

separating compounds based on their polarity. In normal phase HPLC, a nonpolar liquid

mobile phase carries a mixture of compounds through a column packed with a solid

stationary phase with a polar surface, whereas reverse phase HPLC uses a polar mobile

phase and a nonpolar stationary phase. The duration of each compound retained inside

the column depends on the strength of the interaction between the compound and the

solid phase. The investigation of PMS microspheres as a stationary phase showed that

their high surface area and narrow pore size distribution led to better separations.18 In

2006, Fröba and coworkers used benzene PMO microspheres as the stationary phase and

showed that the π-π interaction between the aromatic analytes and phenylene bridges led

to stronger retention and better separation.19 The benzene PMO spheres achieved clear

peak-to-peak baseline separation in a mixture of similar aromatic compounds (Figure

2-5), a feat not achieved by the best commercially available stationary phase materials, or

by its PMS counterpart.18 The combination of silanol groups and variable organic bridges

in the pore wall consolidated the properties of normal and reverse stationary phases,

endowing PMO with a versatility ideal for separating a wide range of analyte systems.

40

Figure 2-6 Release Profile of tetracycline carried by solid PMS spheres, solid ethane PMO spheres, and hollow ethane PMO spheres

While micron-sized spheres are best for HPLC, hollow spheres offer an advantage

in drug delivery. In traditional therapies, drugs are delivered in measured doses at regular

intervals, which results in a peak–shape profile of drug concentration during a short

period of time.20, 21 Research on drug delivery system focuses on developing drug carriers

with a release profile that both minimizes drug toxicity at high doses and maximizes drug

activity at low doses. Studies on PMS as drug carriers indicated that grafting organic

groups on the pore wall was necessary to achieve optimal release profiles.22 In 2009, Lu

and coworkers extended these studies to PMO.23 They compared the loading capacities

and release profiles of tetracycline (TC), a common broad-spectrum antibiotic, in PMS

solid spheres (PMS-SS), ethane PMO solid spheres (PMO-SS), and ethane PMO hollow

spheres (PMO-HS). They found that PMO-HS had the highest loading of TC and the

most desirable release profile in simulated body fluid with pH 7.4 (Figure 2-6). Further

modeling study confirmed that the hydrophobic pore wall of ethane PMO determined the

slow release of TC in PMO-HS. This report highlights the vital role of the chemical

nature of PMO, which can be tailored by the choice of organic bridging groups, in

making PMOs promising candidates as drug carriers for drug delivery applications.

41

Figure 2-7 Amounts of released D-lysozyme as a function of poly(ethylene glycol) concentration, and scheme of refolding protein process

Crucial to protein function is folding.24 Synthetic proteins often form insoluble

misfolded aggregates called inclusion bodies.25 Refolding inclusion bodies is a major

challenge in bioengineering.25 In ex vivo refolding, the most common approach is batch

dilution of inclusion bodies in a refolding solution. To minimize protein aggregation, the

dilution is carried out at protein concentrations below 0.1 mg/mL, which is too low for

preparative work. In 2007, Feng and coworkers overcome this limit by using ethane PMO

to assist protein refolding and achieved over 80% refolding yield at ~0.6 mg/mL.26 The

protein, denatured hen egg white lysozyme, was loaded into spheres of two ethane PMOs

with different pore sizes and a PMS, and was then released from the porous hosts by

poly(ethylene glycol) (PEG) to a refolding buffer (Figure 2-7). The authors found that

although the maximum loading capacity of PMS (280 mg/g) is larger than that of ethane

PMO (168 mg/g), more protein is released from PMO than from PMS (Figure 2-7). They

attributed this effect to a higher concentration of silanol groups in PMS than in PMO and

to the strong binding of silanol groups with proteins. Although strong binding contributed

to higher loading, it decreased the release rate. Consequently, ethane PMO possessed a

surface character best suited for refolding the protein.

In the three examples above, the interactions between a porous host and guest

molecules determine the choice of the host. These interactions are shaped by the

properties of both the host and the guest: for the host, pore size, pore volume, pore

geometry, and pore surface character; for the guest, the size, the shape, and the

42

hydrophobic–hydrophilic character. Similar to the application in catalysis, PMOs will

excel at areas where regular pore geometry, narrow pore size distribution, and large pore

size are desired. Moreover, the tunable pore surface character in PMOs offers them a

competitive edge over PMS. The natural extension of this tunable surface character is the

design of guest–specific interactions that includes possible biomolecule recognition in

PMOs, similar to the interaction between an antibody and an antigen. We expect more of

such designs to come as PMOs enter their second decade of development.

2.3. Physical Functions: Optical, Electric, and Dielectric

Figure 2-8 Fluorescence spectra of biphenyl PMO samples containing 0–2.35 mol% coumarin 1 excited at 270 nm and normalized by absorption rate, and illustration of the

energy transfer in coumarin-1-doped biphenyl PMO.

Optical functions of mesostructured materials are usually obtained by

incorporating photoactive species such as dyes into the pores.27 The solid porous

framework separates individual dye molecules from each other by confining them inside

porous channels and hence prevents them from dense-packing and self-quenching.

Consequently, mesoporous materials may have a higher loading of dye molecules while

maintaining high quantum efficiency than sol-gel glasses doped with dyes. In PMO, the

organic bridging group offers a possible second photoactive species, thereby enabling

energy transfer within one material. Building upon their previous discovery of an

unusually high quantum yield of biphenyl PMO,28 Inagaki and coworkers reported an

efficient light harvesting system using energy transfer from biphenylene groups in the

pore wall to coumarin 1 in the channels.29 As the concentration of dye inside biphenyl

43

PMO increases, the emission spectrum gradually shifts from the emission of biphenyl at

380nm to the emission of coumarin 1 at 440 – 450 nm (Figure 2-8). Based on the

measurements of fluorescence quantum yield, the authors estimated that at 0.8 mol%

coumarin 1/biphenyl in PMO powders or at 1.8 mol% coumarin 1/biphenyl in PMO

films, biphenyl groups pump coumarin 1 with near 100% energy transfer efficiency. The

same group successfully extended the optical absorption to the visible wavelength range

by synthesizing acridone PMO that absorbs at 450 nm,30 extending potential applications

to lighting, solar cells, and displays.

Figure 2-9 Double logarithmic plots of transient photocurrent measured on a 5 µm-thick film sample as a function of time at applied voltages of 150 V and 350 V, and chemical

structure of three-armed phenylenevinylene

PMS is an insulator, and the predominance of localized Si–O and Si–C σ bonds in

the pore walls of PMO indicates that it should be an insulator as well. But by creating a

large conjugated π-system in the PMO matrix can in principle make PMO conductive. In

2009, Inagaki and coworkers investigated the charge transport properties of a PMO with

a large conjugated π-system in the pore wall (Figure 2-9).31 Under the application of a

static electric field and upon irradiation at 337 nm using a laser pulse, a transient

photocurrent was generated and measured at the positive electrode. Carriers were shown

to be holes with mobility on the order of 10-5 cm2V-1s-1. Though the value was

significantly lower than well-stacked oligo(phenylenevinylene), this report is the first

electrically conductive PMO. These results pave the way for innovative material design

that takes advantage of the high surface area, periodic mesopores and organosilica pore

walls, and the energy and electron transfer potential of PMOs.

44

Figure 2-10 Dielectric constants as a function of the molar fraction of ring-shaped precursor to pure silica precursors, and the chemical structure of three-ring PMO

The ever-shrinking dimensions of microelectronic chips demand that insulating

dielectric materials have a low dielectric constant, k, in order to reduce cross-talk and

signal delays from interlayer capacitive coupling.32 The dielectric constant of a material

is related to the polarizability and electron density of its constituent atoms and bonds.33

Two primary approaches to achieve low k values are reducing electron density and

avoiding polar bonds. The former is done by introducing porosity and by substituting

lighter atoms for heavier ones, while the latter demands reducing polar bonds such as –

OH groups and moisture repellency. Compared with the traditional dielectric material

silicon dioxide (k = 3.9–4.2), PMOs possess significantly lower k (k = 2.0) due to their

high porosity and the high molar ratio of carbon to silicon atoms in the framework.34, 35

Our group reported a three-ring PMO composed of interconnected [Si(CH2)]3 ring

building blocks that offered even higher carbon content than traditional PMOs because

each silicon was connected to two organic groups instead of one (Figure 2-10).34

Moreover, at 400oC, trace amount of residual silanol groups could be eliminated by

quantitatively transforming bridging methylenes adjacent to the silanols into terminal

methyl groups,36 making the material even more humidity-resistant without sacrificing

mechanical strength.

The above three examples explore functions beyond traditional chemical

functions. The successes of these endeavors depend on multidisciplinary expertise, which

usually necessitates collaborations among different disciplines within universities and

between universities and industry. These collaborations not only have provided

45

momentum for PMO research by inventing new precursors and by developing new

synthetic protocols, but also have enabled a transfer of knowledge and valuable tools

from other fields to study PMOs. We expect the future of the PMO field will rely heavily

on and benefit most significantly from the collaborative efforts of creative minds and

hands-on expertise in physical sciences, engineering, and life sciences.

2.4. PMO Future

Many of the applications of PMOs are natural extensions from those of PMS. For

these applications, we have indicated where PMOs may offer a clear advantage. In

particular, while both materials offer tunable pore size, controlled pore geometry and

command over morphology, PMO offers organic functionalities within the pore wall,

increased hydrothermal stability, higher mechanical stability, variable hydrophilic–

hydrophobic microenvironments, and myriad pore surface compositions that fine-tune

interactions with guest molecules.

In addition to applications projected from PMS, a completely new application

portfolio is made possible because of the intelligent designs of PMO precursors.

Examples involving the tunable physical properties of PMO illustrate this potential, but

we are certain that great effort will follow to pursue more ambitious goals. In particular,

we anticipate innovations that will extend the current advantages of PMO and apply them

to areas such as heterojunction solar cells, photodetectors, light emitting diodes, as well

as contaminant sequestration and drug detoxification through molecular imprinting in the

pore walls. These novel functional PMOs often contain large and complex precursors,

whose synthesis and self-assembly may pose a significant challenge. However,

researchers have demonstrated that dendrimers,37 polyhedral oligomeric silsesquioxanes

(POSS),38 and even C60 molecules39 can be incorporated into PMO pore walls. These

examples continue to provide us with insight into the key interactions between templates

and organosilanes, and the mechanism behind self-assembly. The knowledge acquired

through investigating bulky precursors will not only guide the PMO field, but also shed

light on the controlled self-assembly of other nanoscale objects such as nanocrystals,

nanorods and nanowires.

46

The intense activity in and wide range of applications of PMO is mainly due to its

versatile chemical compositions, a feat achieved by materials chemists. However, we

have witnessed exciting transfer of knowledge on PMOs from chemistry to life science,

physics, materials science, and engineering. This transfer is not unidirectional, and

continued feedback from collaborators in other fields has resulted in greater innovation

and more creative chemistry; indeed these partnerships have been incredibly fruitful in

directing PMO research for specific applications. To solve significant materials

challenges of our time, and to take PMO research to the next level, the synergy between

different disciplines of materials science and between academic and industrial research

will continue to play a crucial role.

47

2.5. References

1. Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O., Novel

Mesoporous Materials with a Uniform Distribution of Organic Groups and Inorganic

Oxide in Their Frameworks. J. Am. Chem. Soc. 1999, 121, 9611-9614.

2. Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A., Mesoporous Sieves with

Unified Hybrid Inorganic/Organic Frameworks. Chem. Mater. 1999, 11, 3302-3308.

3. Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A., Periodic Mesoporous

Organosilicas with Organic Groups inside the Channel Walls. Nature 1999, 402, 867-

871.

4. Wang, W. D.; Lofgreen, J. E.; Ozin, G. A., Why PMO? Towards Functionality

and Utility of Periodic Mesoporous Organosilicas. Small 2010, 6, 2634-2642.

5. Loy, D. A.; Shea, K. J., Bridged Polysilsesquioxanes. Highly Porous Hybrid

Organic-Inorganic Materials. Chem. Rev. 1995, 95, 1431-1442.

6. Melero, J. A.; van Grieken, R.; Morales, G., Advances in the Synthesis and

Catalytic Applications of Organosulfonic-Functionalized Mesostructured Materials.

Chem. Rev. 2006, 106, 3790-3812.

7. Nakajima, K.; Tomita, I.; Hara, M.; Hayashi, S.; Domen, K.; Kondo, J. N., A

Stable and Highly Active Hybrid Mesoporous Solid Acid Catalyst. Adv. Mater. 2005, 17,

1839-1842.

8. Alauzun, J.; Mehdi, A.; Reye, C.; Corriu, R. J. P., Mesoporous Materials with an

Acidic Framework and Basic Pores. A Successful Cohabitation. J. Am. Chem. Soc. 2006,

128, 8718-8719.

9. Mehdi, A.; Reye, C.; Brandes, S.; Guilard, R.; Corriu, R. J. P., Synthesis of

Large-Pore Ordered Mesoporous Silicas Containing Aminopropyl Groups. New Journal

of Chemistry 2005, 29, 965-968.

48

10. Li, C.; Zhang, H.; Jiang, D.; Yang, Q., Chiral Catalysis in Nanopores of

Mesoporous Materials. Chem. Comm. 2007, 547-558.

11. Kuschel, A.; Polarz, S., Organosilica Materials with Bridging Phenyl Derivatives

Incorporated into the Surfaces of Mesoporous Solids. Adv. Funct. Mater. 2008, 18, 1272-

1280.

12. Kuschel, A.; Sievers, H.; Polarz, S., Amino Acid Silica Hybrid Materials with

Mesoporous Structure and Enantiopure Surfaces. Angew. Chem. Int. Ed. 2008, 47, 9513-

9517.

13. Yang, Q.; Liu, J.; Zhang, L.; Li, C., Functionalized Periodic Mesoporous

Organosilicas for Catalysis. J. Mater. Chem. 2009, 19, 1945-1955.

14. Melero, J. A.; Iglesias, J.; Arsuaga, J. M.; Sainz-Pardo, J.; Frutos, P. d.; Blazquez,

S., Synthesis and Catalytic Activity of Organic-Inorganic Hybrid Ti-Sba-15 Materials. J.

Mater. Chem. 2007, 17, 377-385.

15. Guo, W.; Li, X.; Zhao, X. S., Understanding the Hydrothermal Stability of Large-

Pore Periodic Mesoporous Organosilicas and Pure Silicas. Microporous Mesoporous

Mater. 2006, 93, 285-293.

16. Burleigh, M. C.; Markowitz, M. A.; Jayasundera, S.; Spector, M. S.; Thomas, C.

W.; Gaber, B. P., Mechanical and Hydrothermal Stabilities of Aged Periodic Mesoporous

Organosilicas. The Journal of Physical Chemistry B 2003, 107, 12628-12634.

17. Dufaud, V.; Beauchesne, F.; Bonneviot, L., Organometallic Chemistry inside the

Pore Walls of Mesostructured Silica Materials13. Angew. Chem. Int. Ed. 2005, 44, 3475-

3477.

18. Boissiere, C.; Kummel, M.; Persin, M.; Larbot, A.; Prouzet, E., Spherical Msu-1

Mesoporous Silica Particles Tuned for HPLC. Adv. Funct. Mater. 2001, 11, 129-135.

49

19. Rebbin, V.; Schmidt, R.; Fröba, M., Spherical Particles of Phenylene-Bridged

Periodic Mesoporous Organosilica for High-Performance Liquid Chromatography.

Angew. Chem. Int. Ed. 2006, 45, 5210-5214.

20. Langer, R., New Methods of Drug Delivery. Science 1990, 249, 1527-1533.

21. Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff, K. M., Polymeric

Systems for Controlled Drug Release. Chem. Rev. 1999, 99, 3181-3198.

22. Maria, V.-R.; Francisco, B.; Daniel, A., Mesoporous Materials for Drug Delivery.

Angew. Chem. Int. Ed. 2007, 46, 7548-7558.

23. Lin, C. X.; Qiao, S. Z.; Yu, C. Z.; Ismadji, S.; Lu, G. Q., Periodic Mesoporous

Silica and Organosilica with Controlled Morphologies as Carriers for Drug Release.


24. Clark, E. D. B., Protein Refolding for Industrial Processes. Current Opinion in

Biotechnology 2001, 12, 202-207.

25. Middelberg, A. P. J., Preparative Protein Refolding. Trends in Biotechnology

2002, 20, 437-443.

26. Wang, X.; Lu, D.; Austin, R.; Agarwal, A.; Mueller, L. J.; Liu, Z.; Wu, J.; Feng,

P., Protein Refolding Assisted by Periodic Mesoporous Organosilicas. Langmuir 2007,

23, 5735-5739.

27. Scott, B. J.; Wirnsberger, G.; Stucky, G. D., Mesoporous and Mesostructured

Materials for Optical Applications. Chem. Mater. 2001, 13, 3140-3150.

28. Goto, Y.; Mizoshita, N.; Ohtani, O.; Okada, T.; Shimada, T.; Tani, T.; Inagaki, S.,

Synthesis of Mesoporous Aromatic Silica Thin Films and Their Optical Properties.

Chem. Mater. 2008, 20, 4495-4498.

50

29. Inagaki, S.; Ohtani, O.; Goto, Y.; Okamoto, K.; Ikai, M.; Yamanaka, K.-i.; Tani,

T.; Okada, T., Light Harvesting by a Periodic Mesoporous Organosilica Chromophore.

Angew. Chem. Int. Ed. 2009, 48, 4042-4046.

30. Takeda, H.; Goto, Y.; Maegawa, Y.; Ohsuna, T.; Tani, T.; Matsumoto, K.;

Shimada, T.; Inagaki, S., Visible-Light-Harvesting Periodic Mesoporous Organosilica.

Chem. Comm. 2009, 6032-6034.

31. Mizoshita, N.; Ikai, M.; Tani, T.; Inagaki, S., Hole-Transporting Periodic

Mesostructured Organosilica. J. Am. Chem. Soc. 2009, 131, 14225-14227.

32. Volksen, W.; Miller, R. D.; Dubois, G., Low Dielectric Constant Materials.

Chem. Rev. 2010, 110, 56-110.

33. Maex, K.; Baklanov, M. R.; Shamiryan, D.; lacopi, F.; Brongersma, S. H.;

Yanovitskaya, Z. S., Low Dielectric Constant Materials for Microelectronics. J. Appl.

Phys. 2003, 93, 8793-8841.

34. Landskron, K.; Hatton, B. D.; Perovic, D. D.; Ozin, G. A., Periodic Mesoporous

Organosilicas Containing Interconnected [Si(Ch2)]3 Rings. Science 2003, 302, 266-269.

35. Hatton, B. D.; Landskron, K.; Whitnall, W.; Perovic, D. D.; Ozin, G. A., Spin-

Coated Periodic Mesoporous Organosilica Thin Films - Towards a New Generation of

Low-Dielectric-Constant Materials. Adv. Funct. Mater. 2005, 15, 823-829.

36. Asefa, T.; MacLachlan, M. J.; Grondey, H.; Coombs, N.; Ozin, G. A.,

Metamorphic Channels in Periodic Mesoporous Methylenesilica. Angew. Chem. Int. Ed.

2000, 39, 1808-1811.

37. Landskron, K.; Ozin, G. A., Periodic Mesoporous Dendrisilicas. Science 2004,

306, 1529-1532.

38. Lei, Z.; Hendrikus C.L., A.; Qihua, Y.; Yi-Meng, W.; Pieter C.M.M., M.; Brahim,

M.; Rutger A., v. a.; Can, L., Mesoporous Organic-Inorganic Hybrid Materials Built

51

Using Polyhedral Oligomeric Silsesquioxane Blocks. Angew. Chem. Int. Ed. 2007, 46,

5003-5006.

39. Whitnall, W.; Cademartiri, L.; Ozin, G. A., C60 - Pmo: Periodic Mesoporous

Buckyballsilica. J. Am. Chem. Soc. 2007, 129, 15644-15649.

52

Chapter 3 Vacuum-Assisted Aerosol Deposition of a Low-

Dielectric-Constant Periodic Mesoporous Organosilica Film

This image presents the main idea of aerosol depostion: after passing thorugh a nozzle, aerosol

droplets impinge on the substrate and coalesce to form a film.

53

Chapter one introduced periodic mesoporous materials in general. Chapter two

discussed in detail one specific class of periodic mesoporous materials, periodic

mesoporous organosilica (PMO), and their applications in catalysis, separation,

biomedicine, optics, and microelectronics. This chapter will further expand one topic—

PMOs as low-dielectric-constant (low-k) materials in microelectronics. Specifically, it

will introduce vacuum-assisted aerosol deposition for the synthesis of PMO thin films

and examine the film structure and physical properties relevant to the low-k application.

Low-k materials are currently the insulating materials in computer microprocessor

chips. The name low-k denotes a material challenge posed by decreasing chip

dimensions: as the minimum dimension of individual device decreases below 250 nm, the

speed of a chip is limited less by the gate delay of transistors and more by the delay

arising from transmitting signals between transistors, the so-called interconnect delay

(Figure 3-1);1 to reduce interconnect delay, the dielectric constant k of the insulating

material on a chip must decrease.1 The traditional insulating material silicon dioxide (k =

3.9 – 4.5) has been progressively replaced by fluorinated silica (k = 3.8 – 3.6), carbon-

doped silica (k = 3.3 – 2.7), and porous carbon-added silica (k = 2.7 – 2.2).1 The physical

principle behind these material choices is summarized in the Debye Equation

(Equation(3-1)), where k is the dielectric constant; N the density of dipoles; αe the

electronic polarization (the displacement of electron cloud with respect to the nuclei); αd

the distortion polarization (the distortion of the position of the nuclei); μ the orientation

polarizability (re-orientation of permanent dipoles).2 The Debye equation indicates that

decreasing one or more of the four variables on the right-hand-side of the equation will

decrease k. Therefore, the addition of carbon (decreasing αe) and the introduction of

porosity (decreasing N) in PMOs are mainly responsible for their low k values.

54

Figure 3-1 Calculated gate and interconnect delay as a function of technology node according to the National Technology Roadmap for Semiconductors (NTRS) in 1997. ■

gate delay; ▲ interconnect delay (Al and SiO2); ● sum of delays (Al and SiO2).

2

0

1

2 3 3e d

k N

k kT

(3‐1)

To be integrated in the manufacturing process in the industry, low-k materials

must be fabricated in thin film morphology. Techniques to synthesize thin films include

vapor-phase delivery techniques, such as chemical vapor deposition, or liquid-phase

delivery techniques, such as spin-coating or dip-coating. Given the predominance of

chemical vapor deposition in the microelectronics industry, a vapor-phase delivery

technique will be more compatible with the existing infrastructure in the industry than a

liquid-phase delivery technique. Here I extended our previous work on spin-on low-k

PMO3, 4 to a vapor-phase delivery technique—vacuum-assisted aerosol deposition

(VAAD).5

55

3.1. Results and Discussion

Figure 3-2 Scheme of vacuum-assisted aerosol deposition

The VAAD system (Figure 3-2) consisted of an atomizer (TSI Model 3076), a

transport tube, a deposition chamber with a spinning stage, and a vacuum system. The

aerosol was generated in the atomizer, carried by nitrogen gas through the transport tube,

accelerated by a de Laval nozzle (Figure 3-3), and deposited onto a substrate on the

spinning stage. The de Laval nozzle, also called converging–diverging nozzle, increased

the impact of aerosol on the substrate and hence increase the efficiency of deposition.

Higher aerosol flow rates and shorter nozzle-to-substrate distances (0.5 – 1 mm)

significantly increased deposition rates6 and decreased deposition times from a few

minutes to 10 – 20 seconds. The solution for generating the aerosol was prepared by

increasing the amount of ethanol in our previous spin-on recipe4 in order to extend the

window of optimum sol-aging time.

56

Figure 3-3 Scheme of de Laval nozzles and a nozzle assembly: (a) five designs of de Laval nozzles, (b) neck of a nozzle, (c) a nozzle assembly, and (d) an example of the

dimensions of a bottom plate in a nozzle assembly.

VAAD improved an aerosol deposition method reported in 20037 in three crucial

aspects: (1) the use of a de Laval nozzle to maximize deposition efficiency, (2) the use of

vacuum to assist the evaporation of solvent, and (3) room temperature deposition. The de

Laval nozzle consisted of a converging part and a diverging part, joined by a neck (Figure

3-3b). When the aerosol passed through the nozzle, it was compressed at its neck,

increasing the pressure of the gas. This increased pressure, together with the vacuum at

the exit of the system, expanded the pressure drop across the nozzle, thereby accelerating

the aerosol flow. The vacuum not only created a pressure gradient to facilitate the

function of the de Laval nozzle, but it also helped evaporate the solvent in aerosol

57

droplets, assisted the self-assembly of mesostructures,8, 9 and hence eliminated the need

to heat the aerosol during its transport.

Figure 3-4 X-ray diffraction patterns of (A) as-deposited films and (B) calcined films; the insets showing the corresponding grazing-incidence small angle X-ray scattering patterns

The highly ordered mesostructures in the PMO films were probed by grazing-

incidence small angle X-ray scattering (GISAXS) (Figure 3-4). Since X-ray diffraction

produced an almost undistorted image in the reciprocal space,10 the GISAXS patterns of

PMO thin films indicated that the films possessed hexagonally ordered mesostructures

with porous channels running parallel to the substrate surface.

58

Figure 3-5 STEM images of films deposited (a, b) without spinning and (c, d) with spinning; scale bars are 10nm

The ordered mesostructures were further confirmed by scanning transmission

electron microscope (STEM) (Figure 3-5). A comparison of films deposited with and

without a spinning substrate showed that the spinning elongated the mesostructured

domains in one direction and shortened it in other directions. These individual

mesostructured domains could be attributed to the distortion of submicron aerosol

droplets by spinning-induced shear when the droplets hit the substrate. Because the

submicron-sized aerosol droplets contained only 15 – 20% of its original weight after the

solvent within was evaporated, their sizes were on the order of ten to a few hundred

nanometers, the same as the size of each domain observed in STEM. Most importantly,

spinning a substrate during the deposition generated a film surface smooth enough for

further spectroscopic characterizations, the results of which are summarized in Table 3-1.

59

Table 3-1 Properties of PMO Thin Films

No. Method CTACl/Si ratio

Porosity [%] [a]

Pore Diameter [Å] [b]

d(100) [Å] [a]

wall thickness [Å] [c]

k [d] E (EP) [GPa]

[a]

E (SAWS)

[GPa] [e]

1 VAAD 0.10 39±3 20±2 41±2 27±3 1.9±0.1 5.4±0.4 6.0±0.4

2 VAAD 0.14 47±3 20±2 36±1 22±2 1.7±0.1 2.5±0.2 4.4±0.3

3 Spin-coating

0.10 38±2 20±2 40±1 26±2 1.9±0.1 4.4±0.3 6.0±0.4

4 Spin-coating

0.14 46±2 20±2 36±1 22±2 1.7±0.1 1.9±0.1 4.4±0.3

[a] The Standard Deviations (SD) of porosity, d(100), E values (from EP) were obtained from six or more samples.

[b] Pore diameter was measured from TEM images, with an estimated 10% error.

[c] Wall thickness was calculated using 2 (100) / 3 pored d , and its SD was estimated by

error propagation.

[d] The SD of k was obtained from measurements on four to six pads on one film.

[e] The SD of E (SAWS) was obtained from measurements at different laser-to-detector distances on one sample.

The results in Table 3-1 indicate that films produced from VAAD and spin-

coating had similar properties. Porosities were varied by using different molar ratios of

the surfactant cetyltrimethylammonium chloride (CTACl) and the precursor (EtO)3Si–

CH2CH2–Si(OEt)3 in the starting solution. For the brevity of expression, this ratio was

expressed in terms of moles of CTACl verses moles of silicon atoms, CTACl/Si. Smaller

CTACl/Si value meant fewer templating molecules, and hence thicker pore wall and a

denser material. Consequently, the dielectric constant k and Young’s modulus E

increased.

60

The difference in E (EP) and E (SAWS) values was attributed to the preferential

orientation of porous channels inside the films. Because the channels were running

parallel to the substrate, and because E (EP) was derived from the thickness change

during the measurement, only lateral directions of channels (X direction in Figure 3-6)

contributed to E (EP) values, whereas both lateral and axial directions of channels (Y and

Z directions in Figure 3-6) contributed to E (SAWS) values. The results confirmed an

intuitive picture that the Young’s modulus of a porous channel was higher in the axial (Z)

direction than in lateral directions (X,Y).

Figure 3-6 (a) Scheme of mesostructured domains in a PMO film with channels running parallel to substrate surface; (b) empty pores, and (c) pores filled with water, being

compressed by capillary forces in the X direction during an ellipsometric porosimetry measurement.

3.2. Methods

Synthesis: 1,2-bis(triethoxysilyl)ethane (96%), cetyltrimethylammonium chloride

(CTACl) (25wt% aqueous solution) were purchased from Aldrich and used without

further purification. The atomizer was TSI Model 3076. In a typical experiment, CTACl

solution (1.426g), 10-3M hydrochloride acid solution (1.8g), and ethanol (4.54g) were

mixed together before 1,2-bis(triethoxysilyl)ethane (1.745g) was added. The molar ratio

of the components was Si : HCl : CTACl : H2O : EtOH = 1 : 1.45 x 10-4: 0.143 : 15.6 :

61

10. The other recipe had the molar ratio Si : HCl : CTACl : H2O : ethanol = 1 : 2x10-4:

0.10 : 16.4 : 15. The solution had an optimum deposition time from 30 min to 2 h. Before

deposition, the wall of the transport tube was wetted with a small amount of water.

During the deposition, the pressure of the chamber was set to be 100 – 300 mmHg below

the atmospheric pressure. The time for the aerosol to travel from the atomizer to the

deposition nozzle was less than 5 s. The as-deposited films were aged at room

temperature for one day before being calcined at 300 – 325°C for 6 h to remove the

templates.

X-ray diffraction (XRD) and scanning transmission electron microscope

(STEM): The GISAXS patterns were collected on a Bruker D8 Discovery machine. The

detector-to-sample distance was 30 cm, and the detector was positioned at 10.2° (2θ) to

avoid direct beam exposure. The collection time was 60 s. The grazing angle, 0.2°–0.4°,

was chosen based on a previous report.11 θ–2θ geometry XRD measurements were

performed on a Siemens D5000 diffractometer. STEM imaging was performed on a

Hitachi HD-2000 microscope with an acceleration voltage of 200 kV. Film fragments

were scratched off from the substrate and deposited on carbon-coated copper grid for

STEM imaging.

Dielectric-constant measurement: Capacitance was measured using parallel

plate method at 1 MHz on 4280A Hewlett-Packard C meter. The top electrodes were

100-nm-thick gold pads sputtered through a shadow mask on thin films, and the bottom

electrodes were heavily doped N+ type silicon wafer (100) with a resistivity of 0.001–

0.020 ohms·cm. The area of each gold pad was measured using an Olympus PME3

optical microscope and its accompanying software Clemex Vision PE 5.0. Film thickness

was measured by ellipsometry. The dielectric constant values were calculated according

to the equation k = C x d / (A x ε0), with C being capacitance, d being the film thickness,

A being the area of a gold pad. The k values were smaller than our previous report4

because the edge effect (known to introduce positive errors) was reduced by increasing

the area of gold pads from ~0.6 mm2 to ~0.9 mm2.

62

Ellipsometric Porosimetry (EP): EP measurements were performed on a Sopra

GES-5E EP-A machine. Porosity was calculated by applying the Clausius–Mossotti

equation to the indexes of films with empty pores and with completely filled pores.12

Young’s modulus was calculated from the change of film thickness during desorption

using the Young–Laplace and the Kelvin equtions.13 Contrary to a previous report,14

Young’s modulus does not depend on the shape of the pores or the shape of the menisci

(see 3.4. Appendix).

Surface Acoustic Wave Spectroscopy: SAWS measurements were performed on

LAwave system at Fraunhofer USA in Michigan. A nitrogen pulse laser was used to

introduce a 500ps laser pulse focused on a line with a length of about 5mm. This laser

pulse generated a surface acoustic wave package that showed phase velocity dispersion as

it propagated along the thin film surface and was detected by a piezoelectric transducer.

Phase spectra are obtained by Fourier-transforming the signals, and are fitted to

theoretical dispersion curve to deduce Young’s Modulus. Poisson’s ratio was set to be

0.25. The influence of Poisson’s ratio on Young’s modulus was small: 4.5 GPa for

Poisson’s ratio 0.25 and 4.8 GPa for 0.165.

3.3. Conclusion

I used ethane PMO as an archetype to demonstrate the synthesis of PMO thin film

through VAAD, a vapor-phase delivery technique that I envision will be favored by the

semiconductor industry. I provided detailed characterizations of the structural and

physical properties of PMO VAAD thin films, in particular key information on the

mechanical and dielectric properties. Specifically, films synthesized using a solution with

CTACl/Si = 0.1 possessed a k value less than 2 and Young’s modulus (both in-plane and

out-of-plane) higher than 4 GPa, a combination of properties that satisfied the immediate

need of the semiconductor industry for low-k materials.

From a chemistry perspective, not only the synthesis–structure–property–utility

relationship of a low-k material of great technological importance was revealed, but also

the synergistic effect of the science of materials chemistry and the technology of

63

semiconductor fabrication was demonstrated by leveraging the command of PMO

properties beyond the traditional boundaries of chemical functionality.

3.4. Appendix

To obtain Young’s Modulus from ellipsometric porosimetry, I follow the

derivation published by Mogilnikov and Baklanov.15

In the Kelvin equation

0

2ln LP V

P r RT

,

(3‐2)

P/P0 is relative humidity, γ the surface tension of water, VL the molar volume of water,

and r the radius of curvature of the air–water interface inside a pore (assuming water is

the solvent).

In the Young–Laplace equation

2

c r

, (3‐3)

πc is the pressure across the air–water interface, γ the surface tension of water, and r the

radius of curvature of the air–water interface. Combining equation (3-2) and equation

(3-3) gives the expression of the pressure across the air–water interface expressed in

terms of relative humidity:

0ln /c

L

RT P P

V (3‐4)

Insert equation (3-4) into the definition of Young’s Modulus to obtain the

relationship between relative humidity and the thickness during desorption:

0 0( ) /

cEd d d

(3‐5)

00

0

ln( )L

RTd Pd d

EV P (3‐6)

64

Young’s modulus E is obtained from the slope of a linear fit of thickness with

respect to natural logarithm of relative humidity:

0 0

L L

RTd RT dslope E

EV V slope (3‐7)

In the calculation, the molar volume of water under pressure is obtained from

published tables,16 and the initial thickness d0 is measured. This expression has no contact

angle term, and therefore Young’s Modulus calculated from EP does not depend on the

contact angle values.

The pore radius (or more accurately Kelvin pore radius) rk is linked to the radius

of the curvature of air–water interface r through the contact angle θ:

coskr r (3‐8)

Figure 3-7 Scheme of the relationship among the contact angle, the radius of the pore, and the radius of the curvature of air–water interface

In summary, to calculate pore size distribution, one needs to take into account the

effect of a contact angle. However, to calculate Young’s Modulus, one does not take into

account the effect of a contact angle.

65

3.5. References


Chem. Rev. 2010, 110, 56-110.

2. Maex, K.; Baklanov, M. R.; Shamiryan, D.; lacopi, F.; Brongersma, S. H.;

Yanovitskaya, Z. S., Low Dielectric Constant Materials for Microelectronics. J. Appl.

Phys. 2003, 93, 8793-8841.

3. Landskron, K.; Hatton, B. D.; Perovic, D. D.; Ozin, G. A., Periodic Mesoporous

Organosilicas Containing Interconnected [Si(Ch2)]3 Rings. Science 2003, 302, 266-269.




5. Wang, W. D.; Grozea, D.; Kim, A.; Perovic, D. D.; Ozin, G. A., Vacuum-

Assisted Aerosol Deposition of a Low-Dielectric-Constant Periodic Mesoporous

Organosilica Film. Adv. Mater. 2010, 22, 99-102.

6. Kodas, T. T.; Hampden-Smith, M. J., Aerosol Processing of Materials. Wiley-

VCH: 1999.

7. Lu, Y. F.; McCaughey, B. F.; Wang, D. H.; Hampsey, J. E.; Doke, N.; Yang, Z.

Z.; Brinker, C. J., Aerosol-Assisted Formation of Mesostructured Thin Films. Adv.

Mater. 2003, 15, 1733-+.

8. Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.;

Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I., Continuous Formation of

Supported Cubic and Hexagonal Mesoporous Films by Sol-Gel Dip-Coating. Nature

1997, 389, 364-368.

9. Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J., Aerosol-

Assisted Self-Assembly of Mesostructured Spherical Nanoparticles. Nature 1999, 398,

223-226.

66

10. Gibaud, A.; Dourdain, S.; Vignaud, G., Analysis of Mesoporous Thin Films by X-

Ray Reflectivity, Optical Reflectivity and Grazing Incidence Small Angle X-Ray

Scattering. Appl. Surf. Sci. 2006, 253, 3-11.

11. Klotz, M.; Albouy, P. A.; Ayral, A.; Menager, C.; Grosso, D.; Van der Lee, A.;

Cabuil, V.; Babonneau, F.; Guizard, C., The True Structure of Hexagonal Mesophase-

Templated Silica Films as Revealed by X-Ray Scattering: Effects of Thermal Treatments

and of Nanoparticle Seeding. Chem. Mater. 2000, 12, 1721-1728.

12. Kobler, J.; Bein, T., Porous Thin Films of Functionalized Mesoporous Silica

Nanoparticles. ACS Nano 2008, 2, 2324-2330.

13. Mogilnikov, K. P.; Baklanov, M. R., Determination of Young's Modulus of

Porous Low-K Films by Ellipsometric Porosimetry. Electrochem. Solid-State Lett. 2002,

5, F29-F31.

14. Boissiere, C.; Grosso, D.; Lepoutre, S.; Nicole, L.; Bruneau, A. B.; Sanchez, C.,

Porosity and Mechanical Properties of Mesoporous Thin Films Assessed by

Environmental Ellipsometric Porosimetry. Langmuir 2005, 21, 12362-12371.

15. Dultsev, F. N.; Baklanov, M. R., Nondestructive Determination of Pore Size

Distribution in Thin Films Deposited on Solid Substrates. Electrochem. Solid-State Lett.

1999, 2, 192-194.

16. Bridgman, P. W., The Pressure-Volume-Temperature Relations of the Liquid, and

the Phase Diagram of Heavy Water. J. Chem. Phys. 1935, 3, 597-605.

67

Chapter 4 Water Repellent Periodic Mesoporous

Organosilicas

Periodic mesoporous organosilica prevents water molecules from penetrating into a digital circuit.

68

The previous chapter introduced periodic mesoporous organosilica (PMO) as a

promising low-k material and elaborated on a vapor-phase delivery technique, vacuum-

assisted aerosol deposition, which we envision that the microelectronics industry will

favor. This chapter will further analyze in detail one material property crucial to low-k

application—hydrophobicity.

As explained in the previous chapter, porosity in low-k material is necessary to

reduce k value below 2.2. The introduction of porosity, however, has created two

additional material challenges.1 First, the mechanical properties of a porous material

degrade as its porosity increases. Second, moisture in the ambient environment can

diffuse into and condense inside the pores, modifying the pore wall composition and

degrading its dielectric characteristics.

The first challenge on the mechanical properties of PMO has been briefly

addressed in the previous chapter. In essence, PMO benefits from the fact that its organic

groups are bridged between two silicon atoms instead of being terminal groups, so the

structural connectivity of PMO is higher, and hence its mechanical properties are better,

than the existing carbon-added silica low-k materials, which are synthesized from

organosilane precursors with terminal organic groups.1

The second challenge on moisture absorption in PMO, the focus of this chapter,

has been addressed in our previous reports qualitatively:2, 3 the bridging organic groups in

PMO can react with trace amount of residual silanol groups to form Si–O–Si and Si–R

bonds at a temperature higher than 400 °C, thereby self-hydrophobizing the pore wall.2

Here, I will use ellipsometric porosimetry, a tool first developed by the semiconductor

industry around the year of 2000, to quantify the self-hydrophobization of PMO.4 This

method can be readily extended to other porous thin film materials, whose

hydrophobicity is critical for their respective applications.

Ellipsometric porosimetry (EP) is an analytical technique that combines thin-film

analysis by spectroscopic ellipsometry and porous-material analysis by adsorption

porosimetry. It was first introduced in response to the semiconductor industry’s need for

69

a nondestructive method to determine pore size, pore volume5, 6 and Young’s modulus7 of

porous low-k thin films. The adsorbates used in these early studies were volatile organic

solvents such as toluene, heptanes and carbon tetrachloride. And the analysis used a

vacuum chamber with vapor pressure control system. Sanchez and coworkers improved

the EP system by using water as the adsorbate so that the analysis could be performed at

ambient temperature and pressure.8 Water does not wet most of the solid perfectly, so a

contact angle needs to be included in the analysis of experimental data using the Kelvin

equation:

0

2 cosln

P V

P rRT

(4‐1)

where P/P0 is the relative humidity, γ the surface tension of water, V the molar volume of

water, the contact angle of water on the surface of an adsorbent, r the pore radius, R

the gas constant, and T the temperature. Although the Kelvin equation is usually used for

calculating pore size distribution with the knowledge of contact angle values, it can be

used to deduce the value of a contact angle if pore size is known. And this value provides

information of the hydrophilic–hydrophobic character of a porous material, which is the

basis for the following analysis.

70


Figure 4-1 XRD patterns of (a) ethane PMO, (b) methane PMO, (c) 3-ring PMO, and (d) ethene PMO, with their corresponding structural formula

The structures of the four PMOs (Figure 4-1) we choose to investigate have

relatively simple bridging groups. Because their precursors are commercially available,

they are suitable for volume manufacturing. In fact, they have been recognized by the

semiconductor industry as ideal candidates for the next generation of ultra-low-k

materials.1 The meso-ordering of these films were probed by X-ray diffraction (XRD)

(Figure 4-1) and confirmed by scanning transmission electron microscopy (STEM)

(Figure 4-2). The pore size estimated from STEM images is about 2 nm.

71

Figure 4-2 Representative TEM images of (a) ethane PMO, (b) methane PMO, (c) 3-ring PMO, and (d) ethene PMO. Scale bar is 10 nm.

72

Figure 4-3 Isotherms of (a) ethane PMO, (b) methane PMO, (c) 3-ring PMO, and (d) ethene PMO treated at 350 °C for 2 h. In each plot, filled symbols represent an adsorption

branch, and empty symbols represent a desorption branch. On each sample, two adsorption and desorption measurements are performed. The circular shape corresponds

to the first, and the triangular shape corresponds to the second.

A typical EP measurement produces two plots: (1) refractive indexes versus

relative humidity (RH), and (2) thickness versus RH. The first plot relates RH

quantitatively to the amount of water adsorbed inside mesopores, so the term isotherm

was introduced to refer to this plot.6 Figure 4-3 shows typical isotherms of four PMOs,

each being measured twice. The sharp increase of the refractive index in an adsorption

branch signaled pore condensation (moisture condensing inside pores) and occurred at a

higher relative humidity in the first isotherm than in the second one. This difference is

due to water rehydrolyzing pore wall surface during the measurements. The sharp

decrease of the refractive index in a desorption branch signaled pore evaporation

(condensed water evaporating from the pores) and occurred at the same relative humidity

73

in the first and the second isotherm, suggesting the rehydrolysis was completed before the

pore evaporation in the first measurement.

According to the Kelvin equation, the higher the relative humidity of the pore

condensation is, the more hydrophobic the pore wall is. The sequence of hydrophobicity

of all four PMOs treated at 350 °C were 3-ring (83% RH) > ethane (78% RH) > methane

(73% RH) > ethene (61% RH). A subtle feature also related to the hydrophilic–

hydrophobic character of the pore wall was the slope of an isotherm before pore

condensation. This slope was higher in the second isotherm than in the first isotherm for

all four PMOs. This increase in the slope is also due to the rehydrolysis that occurred in

the first absorption–desorption circle. Of all PMOs, this difference in the slopes of two

isotherms was most pronounced in ethene PMO, indicating that ethene PMO was most

susceptible to rehydrolysis. In addition, ethene PMO had the highest slope among all four

PMOs, suggesting again that the pore wall of ethene PMO was the most hydrophilic.

74

Figure 4-4 Plots of thin film thickness versus relative humidity of (a) ethane, (b) methane, (c) 3-ring, and (d) ethene PMO.

In plots of film thickness versus RH, decreases in thickness were observed in all

four PMOs (Figure 4-4). This decrease was due to capillary pressure produced from

condensed water inside mesopores. The decrease in methane PMO was the largest, at

about 10 percent of its original thickness. Moreover, at the RH of the highest

compression in thickness, the corresponding refractive index was the highest, even higher

than the refractive index at maximum relative humidity. This peculiar feature of methane

PMO suggests an elastic character in its mechanical response. A similar feature is also

discernable in ethane PMO, albeit less pronounced.

75

Figure 4-5 Isotherms of (a) ethane, (b) methane, (c) 3-ring, and (d) ethene PMO treated at 300 °C, 400 °C and 500 °C for 2 h. The refractive indexes of samples treated at 400 °C

and 500 °C are offset by 0.25 and 0.5, respectively. The refractive indexes of 3-ring PMO treated at 500°C are offset by 0.55.

To investigate the effect of thermal treatment on the hydrophobicity, I performed

EP measurements on PMO thin films treated at 300 – 500 °C (Figure 4-5). In all four

PMOs, pore condensation shifted to higher relative humidity with increasing

temperatures. These shifts indicated that the pore surface became progressively more

hydrophobic as the temperature of thermal treatment increased. At 500 °C, methane and

3-ring PMOs showed no pore condensation, which suggested a contact angle equal to or

greater than 90°. The chemical nature of this hydrophobization has been reported

previously:2, 9 at temperatures equal to or greater than 400 °C, trace amount of silanol

groups at the vicinity of bridging organic groups were eliminated by transforming into

Si–O–Si bonds and producing terminal organic groups.

76

The porosities of these water repellent PMOs remained after the thermal

treatment, indicating that the porous structures were not collapsed. The refractive indexes

at 633 nm of the samples treated at 500 °C were 1.2244 for methane PMO and 1.2445 for

3-ring PMO, which correspond to a porosity of 51% for methane PMO and 46% for 3-

ring PMO.

To demonstrate the accessibility of these pores, spectroscopic ellipsometric

measurements were performed on these samples in an atmosphere saturated with ethanol

vapor. Ethanol wets the pore wall perfectly, so pore condensation should occur in all

accessible pores. The refractive indexes at 633 nm measured under this condition were

1.4240 for methane PMO and 1.4449 for 3-ring PMO, which corresponded to a porosity

of 50.9% for methane PMO and 50.4% for 3-ring PMO, indicating that all pores were

still accessible. The slightly higher porosities obtained from the measurements under

ethanol vapor can be attributed to the small amount of ethanol absorbed at the surface of

the thin film. In these calculations, Lorentz–Lorenz equation10 was used with the

assumption that the refractive index of ethanol in confined nanopores was the same as in

the bulk.

77

Figure 4-6 Isotherms of 3-ring, methane, and ethane PMOs treated at 350 °C without washing away surfactants before the thermal treatment. The refractive indexes of

methane PMO are offset by 0.1, and those of ethane PMO are offset by 0.2.

Water repellency can also be achieved in ethane, methane and 3-ring PMOs at

lower temperatures if the surfactants were not washed away by HCl–ethanol solution

before the thermal treatment (Figure 4-6). The films heated at 350 °C showed little water

adsorption before 80% RH, and adsorbed only a small amount beyond 80% RH,

evidenced by a small increase in the refractive indexes. Quantitatively, this increase was

0.03 for 3-ring PMO, 0.05 for methane PMO, and 0.01 for ethane PMO. These values

were small compared to the usual increase of 0.16–0.18 when normal pore condensation

occurred. This enhanced hydrophobicity could be attributed to residual carbon left from

the thermal decomposition of surfactants.

So far in this chapter, the hydrophobicity has been discussed based on the

occurrence of pore condensation, and the Kelvin equation has been used to deduce

differences in hydrophobic-hydrophilic characters. This approach warrants some

justification, because it has been long noted that the very concept of a meniscus, as well

78

as the concept of a contact angle, becomes questionable as the pore size decreases to the

width of a few molecules.11 In thin films of PMOs, the pore size is about 2 nm, which

roughly corresponds to the width of 20 water molecules.12 Although small, this number is

sufficient for forming a curved surface from a purely geometric point of view. To further

investigate this proposition, I measured the macroscopic contact angles on thin films of

bridged polysilsesquioxane (non-porous) and compared them with the values derived by

applying the Kelvin equation to the adsorption branch of the isotherms of corresponding

PMOs (Table 4-1). The values obtained from two methods were generally in good

agreement with each other, which suggested that the concept of a meniscus, as well as a

contact angle, was still valid in mesopores.

Table 4-1 Comparison of Contact Angles Measured Macroscopically on Bridged Polysilsesquioxane Thin films (Non-porous) and Contact Angles Measured by

Ellipsometric Porosimetry (EP) on Corresponding PMO Thin films

300 °C 350 °C 400 °C 450 °C 500 °C

–CH2CH2– bridged

polysilsesquioxane 66.0±0.6 76.1±0.4 82.3±0.2 81.4±0.2 78.0±0.5

Ethane PMO (EP) 69±2 76±1 79±1 79±1 83±0.5

–CH2– bridged


Methane PMO (EP) 62±2 73±1 82±0.5 >90 >90

3‐ring


3‐ring PMO (EP) 71±2 80±1 79±1 >90 >90

–CH=CH– bridged


Ethene PMO (EP) 63±3 63±3 63±3 65±3 69±2

Footnotes: Standard deviations of macroscopically measured contact angles were

obtained from at least 6 advancing angles on the same sample. Standard deviations of

contact angels derived from EP measurements were obtained by propagating the errors in

the estimated pore size of PMOs.

Quantitative discrepancies were attributed mainly to two assumptions. First, it

was assumed that the 2 nm pore size was independent of the temperature of thermal

treatment and was independent of the type of PMOs. This assumption was partially

79

justified because the values of d-spacing vary only slightly below 500 °C (Appendix

Table 4-3). And also since the template used was the same for all PMOs, the pore size

was expected to be about the same value for all PMOs. Strictly speaking, 2 nm was the

length of the short axis of an elliptical cross-section of the pores, as suggested by

previous studies13, 14 on thin films of mesoporous silica that showed anisotropic

contraction along the out-of-plane direction. Second, the thickness of adsorbed water

layer inside a pore was assumed to be zero. The radius term in the Kelvin equation was

more accurately described as the radius of a meniscus, and the true radius should be the

sum of the radius of the meniscus and the thickness of an adsorbed water layer before

pore condensation. The assumption that the thickness of this adsorbed water layer was

zero introduce little errors in ethane, methane, and 3-ring PMOs because the slopes of the

isotherms before pore condensation were almost flat, indicating little water adsorption

before pore condensation. However in ethene PMO, this assumption could introduce

significant errors because the observed slopes were not flat. However, it was difficult to

measure the thickness of this absorbed water layer, so for easy comparison and the

convenience in discussion, I still assumed the adsorbed layer was zero in ethene PMO.

In the preceding discussion, I also tacitly assumed that the contact angle was

derived by applying the Kelvin equation to the adsorption branch, but not to the

desorption branch. However, the existence of hystereses in all EP isotherms raised the

question why not the latter. To quantitatively explain this question, it would be necessary

to take into account different models of hysteresis,11 the effect of capillary pressure in

nanopores on the molar volume of water,15 the decrease of surface tension in nanometer-

sized fluid,16 and the possible effect of one dimensional nanopore confinement on the

behavior of phase transition from gas to condensed fluid.17 Addressing all these issues is

out of the scope of this study. Here I only briefly explain my choice from an

experimentalist’s point of view. First, during an EP measurement, the surface of the pore

wall was rehydrolyzed. This rehydrolysis was most likely to occur after pore

condensation and before pore evaporation because condensed water was readily available

for the reaction. Therefore, the chemical nature of the pore surface reflected by the pore

evaporation in desorption branches did not reflect the chemical nature of the pore surface

80

right after thermal treatment. Second, I did not observe a strict 2 to 1 ratio of pore sizes

derived from adsorption and desorption branches of the isotherms (Appendix Table 4-4),

which should be expected if the model of cylindrical pores with two open ends applies,11

so the existence of a hysteresis is most likely related to the ink-bottle model,11 in which a

“neck” having a smaller diameter than the size of a mesopore regulated the flow of vapor

in and out of the mesopores. And according to this ink-bottle model, pore evaporation in

the desorption branch is related to the size of the neck, whereas pore condensation in the

adsorption branch is related to the size of the mesopore. This model was used in a

previous report,18 although no justification was given.

Table 4-2 Structural and Physical Properties of Four PMOs

Name Porosity (E) /%

Porosity (EP)/%

d‐spacing

/Å

Pore diameter

/Å

Pore wall thickness

/Å

E (SAWS) /GPa

E (EP) /GPa

k

Ethane PMO

45±2 45±2 34.8±0.2 20±2 20.4±2 10.2±0.4 2.2±0.4 1.67±0.06

Methane PMO

53±2 51±2 35.1±0.4 20±2 20.5±2 10.1±0.9 2.3±0.2 1.60±0.03

3‐ring PMO

50±2 49±2 38.4±0.7 20±2 24.3±2 14.4±0.4 2.2±0.5 1.63±0.07

Ethene PMO

44±2 40±3 35.4±1.4 20±2 20.1±2 9.1±0.5 3.2±0.5 1.70±0.06

Footnote: Standard deviations (STDs) of porosities (E) and (EP), d-spacing and E (EP)

were obtained from values of 4–6 samples. STDs of pore wall thickness were obtained by

propagating the errors in estimated pore size. STDs of k were obtained from 4–6 pads on

one thin film sample. STDs of SAWS were obtained from measurements at 6 different

laser-to-detector distances on one sample.

Key structural, mechanical and dielectric properties of four PMOs are

summarized in Table 4-2. The difference between Young’s moduli obtained by EP and

by surface acoustic wave spectroscopy (SAWS) could be attributed to the difference in

81

out-of-plane and in-plane Young’s moduli. Both techniques exclude the effect of a

substrate, and SAWS in particular has been the choice for studying Young’s modulus of

low-k thin films in the semiconductor industry.19-21 It has been pointed out that Young’s

modulus (SAWS) higher than 4 GPa is sufficient for a low-k material to survive chemical

mechanical planarization.19 Since reducing structural connectivity decreases the Young’s

modulus of a sol-gel film,21, 22 increasing the degree of polycondensation should better

the mechanical properties. The Young’s moduli (SAWS) in Table 4-2 were obtained

from samples heated with a slow ramping rate 1 °C/min and at 350 °C for 2–6 h (a

temperature before self-hydrophobization occurs), so they represented possibly the

highest values these PMOs can achieve, higher than the values reported in literature with

same densities (~7 GPa for both ethane and methane PMO19, 20). In addition, a collective

effect of periodically ordered structure in PMOs might also contribute to the enhanced

mechanical properties.

Insights into the hydrophilic–hydrophobic sequence of the four PMOs can help

guide the future design of PMO precursors in application areas not limited to low-k

materials. Here I offer some thoughts from a chemistry perspective. Since silanol groups

are most attractive to water molecules, a reduction of the number of silanol groups per

unit surface area will increase hydrophobicity. In 3-ring PMO, each silicon atom is

bonded to only two oxygen atoms (and thus maximum two silanol groups per silicon

atom), whereas in the rest of PMOs each silicon atom is bonded to three oxygen atoms,

so 3-ring PMO is expected to have the least number of silanol groups per unit surface

area. In addition, the symmetrical ring-shape of 3-ring PMO removes the contribution of

any polarity of Si–C bonds, rendering the overall structure more non-polar and less

attractive to water molecules. Therefore, 3-ring PMO is the most hydrophobic of all four

PMOs. A similar argument based on the number of silanol groups per unit surface area

can be made on the difference between ethane and methane PMO: Because ethane

contains one more carbon in the bridge, it has a slightly lower concentration of silanol

groups than methane PMO. In ethene PMO, the electrons of π-bond are more polarizable

by the dipoles of water molecules than σ-bond electrons in the other three PMOs, making

ethene PMO more attractive to water molecules. In addition, the precursor of ethene

82

PMO contains both cis and trans isomers, and cis isomer is expected to contribute

additional dipole moments that attract water molecules. The double bonds also delocalize

the charge of any transition state involved in the rehydrolysis reaction and hence facilitate

the rehydrolysis. Therefore, ethene PMO is the least hydrophobic of all PMOs.

4.2. Methods

Synthesis of PMO thin films: 1,2-bis(triethoxysilyl))ethane or 1,2-

bis(trimethoxysilyl)ethane (ethane PMO precursor) and cetyltrimethylammonium

chloride (CTACl) 25 wt% solution in water were purchased from Aldrich.

Bis(triethoxysilyl)methane (methane PMO precursor), 1,1,3,3,5,5-hexaethyoxy-1,3,5-

trisilacyclohexane (3-ring PMO precursor), and bis(triethoxysilyl)ethylene (ethene PMO

precursor) were purchased from Gelest. In a typical synthesis, PMO precursor was added

into a solution of CTACl, HCl, water and ethanol with moderate stirring. The molar ratio

of the reactants was Si : CTACl : HCl : H2O : EtOH = 1 : 0.15 : 2x10-4 : 19.1 : 15 for

ethane, methane and 3-ring PMO, and Si : CTACl : HCl : H2O : EtOH = 1 : 0.15 : 2x10-3

: 19.1 : 15 for ethene PMO. The optimum solution aging time to obtain well-ordered

mesostructure (the time from adding precursors to spin-coating) was 80–100 min for

ethane PMO, 70–90 min for methane PMO, 40–60 min for 3-ring PMO, and 25–40 min

for ethene PMO. These as-deposited thin films were heated under nitrogen at 150 °C for

2 h (ramping 1 °C/min) to increase the degree of polycondensation of bridged

polysilsesquioxane inside the pore wall, before being washed by HCl 10 vol% solution in

ethanol to remove surfactants. The thin films were then heated again under nitrogen at a

temperature between 300 °C and 500 °C for 2 h with a ramping rate of 1 °C/min.

Organic contaminants during the washing and heating steps were carefully

avoided because those contaminants would be deposited inside the pores and alter the

hydrophilic–hydrophobic character of the pore surface. Moisture in air could also react

with the pore surface, particularly at elevated temperatures. To reduce the effect of

moisture in the processing of thin films, the furnace tube was first evacuated and then

refilled with dry nitrogen before any thermal treatment.

83

X-ray diffraction (XRD) and Scanning transmission electron microscope

(STEM): XRD patterns were collected on a Siemens D5000 diffractometer using Cu Kα

radiation operated at 50 kV and 35 mA with a Kevex solid-state detector. STEM imaging

was performed on a Hitachi HD-2000 at acceleration voltage of 200 kV on thin film

fragments scratched from the substrate and deposited onto carbon-coated copper grids.

Dielectric-constant measurement: Capacitance was measured using parallel

plate method at 1 MHz on 4280A Hewlett-Packard C meter. The top electrodes were

100-nm-thick gold pads with an area of ~0.9 mm2, sputtered through a shadow mask on

thin films, and the bottom electrodes were heavily doped N+ type silicon wafer (100)

with a resistivity of 0.001–0.020 ohms·cm. The area of each gold pad was measured

using an Olympus PME3 optical microscope and its accompanying software Clemex

Vision PE 5.0. Film thickness was measured by ellipsometry. The dielectric constant

values were calculated according to the equation k = C x d / (A x ε0), with C being

capacitance, d being the film thickness, A being the area of a gold pad.

Surface Acoustic Wave Spectroscopy (SAWS): SAWS measurements were

performed on LAwave system at Fraunhofer USA in Michigan. A nitrogen pulse laser

was used to introduce a 500ps laser pulse focused on a line with a length of about 5mm.

This laser pulse generated a surface acoustic wave package that showed phase velocity

dispersion as it propagated along the thin film surface and was detected by a piezoelectric

transducer. Phase spectra are obtained by Fourier-transforming the signals, and are fitted

to theoretical dispersion curve to deduce Young’s Modulus. (Poisson’s ratio is set to be

0.25.19)

Ellipsometric porosimetry (EP): EP measurements were performed on a Sopra

GES-5E EP-A machine. A thin film was placed inside a chamber where a combination of

dry and wet air flow was introduced to vary the relative humidity inside the chamber.

Each step of relative humidity was allowed to stabilize for about 30–60 s before a

spectrum was taken. Contact angles were derived by adjusting the peak of pore size

distribution derived from the adsorption branch of the first adsorption–desorption circle

84

to be at 2 nm. Molar volume of water used is 18mL/mol, and the surface tension of water

used is 0.07203 N/m.23

Contact angle: Contact angle measurements were performed according to the

procedures described in the literature.24 Briefly, solid surface was leveled using a bubble

level. Water was delivered onto the surface of a thin film through a stainless needle

connected to a syringe. The speed of advancing and receding drop front was controlled

by a stepper motor connected to the syringe. The drop was illuminated from behind, and

the image was taken by a charge-coupled device camera. Contact angles were determined

from the images by axisymmetric drop shape analysis-no apex. Advancing angles were

used to compare the contact angles derived from EP. Thin films of dense bridged

polysilsesquioxane (non-porous) were prepared by dip-coating a solution of PMO

precursor, HCl, water and ethanol or tetrahydrofuran. Films were thermally treated in the

same way as thin films of PMOs.

4.3. Conclusion

I have demonstrated the gradual hydrophobization of PMOs and monitored this

change quantitatively by ellipsometric porosimetry. In particular, ethane, methane and 3-

ring PMOs exhibited moisture resistance at temperature as low as 350 °C, with

corresponding Young’s modulus values measured by SAWS greater than 10 GPa and k

values smaller than 2. This combination of dielectric, mechanical and humidity resistant

properties satisfies the immediate demand of the semiconductor industry for low-k

materials, and I believe these PMOs will play a role in solving the major roadblock in

integrating porous low-k materials on semiconductor chips.25 From the perspective of

materials chemistry, by borrowing a tool invented in the semiconductor industry, I have

demonstrated the fine-tuning of hydrophilic–hydrophobic properties of PMOs, and I

believe this flexibility in hydrophobization will benefit potential areas of applications

beyond low-k materials.

85

4.4. Appendix

Table 4-3 d-spacing (Å) of PMOs Post-Treated at 300–500 °C

300 °C 350 °C 400 °C 450 °C 500 °C

Ethane PMO 34.8 33.8 32.8 32.8 27.1 Methane PMO 35.1 34.5 33.1 31.1 30.1

3‐ring PMO 38.4 38.7 37.4 36.5 23.9

Ethene PMO 35.4 34.4 34.4 30.8 30.0

Table 4-4 Pore Sizes Derived from the Second Isotherms of Four PMOs

Pore size from desorption branch (nm) Pore size from adsorption branch/nm

Ethane PMO 1.24 2.00

Methane PMO 1.21 2.00

3-ring PMO 1.11 2.00

Ethene PMO 1.16 2.00

Footnote: The second isotherm instead of the first isotherm was chosen for each PMO

because the chemical nature of the pore wall was the same for both adsorption and

desorption branches in the second adsorption and desorption circle. These samples were

post-treated at 300 °C for 2 h.

Table 4-5 Densities and Young’s Moduli of Bridged Polysilsesquioxane Film (Non-porous)

Density by XRR (g/cm3) E from SAWS (GPa)

–CH2CH2–bridged polysilsesquioxane 1.685 28±1

–CH2–bridged polysilsesquioxane 1.864 40±1

3-ring polysilsesquioxane 1.864 45±3

–CH=CH– bridged polysilsesquioxane 1.776 36±4

86

Table 4-6 Densities of PMO Film Used for Fitting SAWS Data

Porosity (%) Density (g/cm3)

Ethane PMO 44.0 0.944

Methane PMO 52.5 0.885

3-ring PMO 50.0 0.932

Ethene PMO 44.0 0.995

Footnote: Because of the surface roughness inherent in the porous structure of PMO thin

films, and issues with the diffraction peaks at low angles resulted from the ordered

periodic structure in PMO, as well as issues with density contrasts between films and

substrate, I chose to measure the density of bridged polysilsesquioxane (non-porous)

films with thickness less than 100 nm and then calculated the density of each PMO thin

film using porosity obtained from spectroscopic ellipsometry.

87

Figure 4-7 Experimental and fitted X-ray reflectivity patterns of –CH2CH2– bridged, –CH2– bridged, 3-ring and –CH=CH– bridged polysilsesquioxane film (non-porous)

X-ray reflectivity (XRR) measurements were performed with using Bruker D-8

Discover X-ray diffractometer (Cu X-ray source, line focus) with a goniometer having

seven axes of motion. A Göbel mirror was placed on the primary beam side and a

scintillation detector on the diffracted beam side during the XRR measurements. A knife-

edge collimator (KEC) was used to ensure the optimum collimation of the primary beam.

Multi-range measurements were set up to have a minimum of 1500 counts over the

measurement range (0.2° 2 4°). The multi-range data were normalized and the

results were modeled using the Bruker Leptos software. In fitting of all the experimental

data (Figure 4-7), an interface layer was needed to produce the best-fit results. Thickness,

roughness and density of layers were fitted to get the best-fit XRR curves.

88

4.5. References


Chem. Rev. 2010, 110, 56-110.

2. Asefa, T.; MacLachlan, M. J.; Grondey, H.; Coombs, N.; Ozin, G. A.,

Metamorphic Channels in Periodic Mesoporous Methylenesilica. Angew. Chem. Int. Ed.

2000, 39, 1808-1811.




4. Wang, W. D.; Grozea, D.; Kohli, S.; Perovic, D. D.; Ozin, G. A., Water Repellent

Periodic Mesoporous Organosilicas. ACS Nano 2011, 5, 1267-1275.

5. Dultsev, F. N.; Baklanov, M. R., Nondestructive Determination of Pore Size

Distribution in Thin Films Deposited on Solid Substrates. Electrochem. Solid-State Lett.

1999, 2, 192-194.

6. Baklanov, M. R.; Mogilnikov, K. P.; Polovinkin, V. G.; Dultsev, F. N.,

Determination of Pore Size Distribution in Thin Films by Ellipsometric Porosimetry. J.

Vac. Sci. Technol., B 2000, 18, 1385-1391.

7. Mogilnikov, K. P.; Baklanov, M. R., Determination of Young's Modulus of

Porous Low-K Films by Ellipsometric Porosimetry. Electrochem. Solid-State Lett. 2002,

5, F29-F31.

8. Boissiere, C.; Grosso, D.; Lepoutre, S.; Nicole, L.; Bruneau, A. B.; Sanchez, C.,

Porosity and Mechanical Properties of Mesoporous Thin Films Assessed by

Environmental Ellipsometric Porosimetry. Langmuir 2005, 21, 12362-12371.

9. Hatton, B. D.; Landskron, K.; Hunks, W. J.; Bennett, M. R.; Shukaris, D.;

Perovic, D. D.; Ozin, G. A., Materials Chemistry for Low-K Materials. Mater. Today

2006, 9, 22-31.

89

10. Gerald E. Jellison, J., Data Analysis for Spectroscopic Ellipsometry. In Handbook

of Ellipsometry, Tompkins, H. G.; Irene, E. A., Eds. William Andrew Publishing and

Springer-Verlag GmbH & Co. KG: Norwich and Heidelberg, 2005.

11. Gregg, S. J.; Sing, K. S. W., Adsorption, Surface Area, and Porosity. 2nd ed.;

Academic Press: London, 1982.

12. Lu, H. M.; Jiang, Q., Size-Dependent Surface Tension and Tolman's Length of

Droplets. Langmuir 2005, 21, 779-781.

13. Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A., Synthesis

of Oriented Films of Mesoporous Silica on Mica. Nature 1996, 379, 703-705.

14. Klotz, M.; Albouy, P. A.; Ayral, A.; Menager, C.; Grosso, D.; Van der Lee, A.;

Cabuil, V.; Babonneau, F.; Guizard, C., The True Structure of Hexagonal Mesophase-

Templated Silica Films as Revealed by X-Ray Scattering: Effects of Thermal Treatments

and of Nanoparticle Seeding. Chem. Mater. 2000, 12, 1721-1728.

15. Bridgman, P. W., The Pressure-Volume-Temperature Relations of the Liquid, and

the Phase Diagram of Heavy Water. J. Chem. Phys. 1935, 3, 597-605.

16. Tolman, R. C., The Effect of Droplet Size on Surface Tension. J. Chem. Phys.

1949, 17, 333-337.

17. Neimark, A. V.; Ravikovitch, P. I., Capillary Condensation in Mms and Pore

Structure Characterization. Microporous Mesoporous Mater. 2001, 44-45, 697-707.

18. Kuemmel, M.; Grosso, D.; Boissière, C.; Smarsly, B.; Brezesinski, T.; Albouy, P.

A.; Amenitsch, H.; Sanchez, C., Thermally Stable Nanocrystalline Gamma-Alumina

Layers with Highly Ordered 3d Mesoporosity. Angew. Chem. Int. Ed. 2005, 44, 4589-

4592.

90

19. Dubois, G.; Volksen, W.; Magbitang, T.; Miller, R. D.; Gage, D. M.; Dauskardt,

R. H., Molecular Network Reinforcement of Sol-Gel Glasses. Adv. Mater. 2007, 19,

3989-3994.

20. Dubois, G.; Volksen, W.; Magbitang, T.; Sherwood, M. H.; Miller, R. D.; Gage,

D. M.; Dauskardt, R. H., Superior Mechanical Properties of Dense and Porous

Organic/Inorganic Hybrid Thin Films. J. Sol--Gel Sci. Technol. 2008, 48, 187-193.

21. Oliver, M. S.; Dubois, G.; Sherwood, M.; Gage, D. M.; Dauskardt, R. H.,

Molecular Origins of the Mechanical Behavior of Hybrid Glasses. Adv. Funct. Mater.

2010, 20, 2884-2892.

22. Ro, H. W.; Char, K.; Jeon, E. C.; Kim, H. J.; Kwon, D.; Lee, H. J.; Lee, J. K.;

Rhee, H. W.; Soles, C. L.; Yoon, D. Y., High-Modulus Spin-on Organosilicate Glasses

for Nanoporous Applications. Adv. Mater. 2007, 19, 705-710.

23. Kayser, W. V., Temperature Dependence of the Surface Tension of Water in

Contact with Its Saturated Vapor. J. Colloid Interface Sci. 1976, 56, 622-627.

24. Kalantarian, A.; David, R.; Neumann, A. W., Methodology for High Accuracy

Contact Angle Measurement. Langmuir 2009, 25, 14146-14154.

25. International Technology Roadmap for Semiconductors - Interconnect; 2009.

91

Chapter 5 Reduced Graphene Oxide–Periodic

Mesoporous Silica Sandwich Nanocomposites with Vertically Oriented

Channels

A sandwich structure consisting of vertically-aligned periodic mesoporous silica and reduced graphene oxide sheet

92

This chapter will describe the synthesis, the structure and the electronic properties

of reduced graphene oxide –periodic mesoporous silica (rGO–PMS) nanocomposite. The

most remarkable feature of this nanocomposite is the vertical orientation of the channels

of PMS with respect to rGO sheets. This key feature distinguishes this nanocomposite

from what have been previously reported on the growth of PMS on mica1 or graphite2, 3

substrates, where the channels were lying parallel with respect to the substrate surface. In

the previous studies on mica or graphite surface, the parallel orientation of the porous

channels was attributed to the orientation of surfactant micelles adhered to the substrate.

In the case of mica, a hydrophilic substrate, surfactant head groups preferentially

interacted with the substrate, leading to the formation of micelles orientated parallel to

the substrate. In the case of graphite, a hydrophobic substrate, surfactant tails

preferentially interacted with the substrate, leading to the formation of hemi-micelles

oriented parallel to the substrate. In this context, the vertical orientation of mesopores

came as a surprise finding in our exploration of nanocomposite materials composed of

graphene oxide and periodic mesoporous materials.4


Mild heating (50–90 °C) in a basic solution, reported to partially remove oxygen-

containing function groups from graphene oxide (GO),5 was used to produce modified

graphene oxide (mGO) (Figure 5-1). The peak of absorption, assigned to the *

transition of aromatic –C=C– network, shifted from 230 nm to 245 nm after the thermal

treatment, suggesting a partial restoration of the conjugated system. This partial

restoration of conjugated system may be accompanied by an increase in defect sites, as

evidenced by a relative increase of the intensity of D band (disorder-induced6) with

respect to the intensity of G band (sp2 carbon bond stretching6) in Raman spectra.

93

Figure 5-1 Comparison of GO and mGO solutions: (a) photos of the solutions, (b) UV-vis spectra, and (c) Raman spectra.

94

Figure 5-2 Low angle XRD results of mGO–PMS and rGO–PMS samples synthesized at (a) pH = 11.7 and (c) pH = 12.7, and the corresponding control PMS samples in (b) and

(d), respectively.

Modified graphene oxide–periodic mesoporous silica (mGO–PMS)

nanocomposite was synthesized by stirring a mixture of silica precursor, surfactant,

catalyst, and an mGO dispersion at specific temperatures, followed by filtration to obtain

powder samples. The as-synthesized power samples contained surfactants and was

denoted as mGO–PMS (x), x being the GO content in the starting mGO dispersion and

having a value of 0.1–4 mg/ml. mGO–PMS samples were then heated under nitrogen

flow to remove surfactant and obtain reduced graphene oxide–periodic mesoporous silica

(rGO–PMS). rGO–PMS synthesized at pH = 11.7 using an mGO dispersion with 0.27

mg/ml GO content showed the best XRD results (Figure 5-2a). A control PMS sample

synthesized at pH = 11.7 (without mGO) showed a weak bump, indicating poor meso-

ordering (Figure 5-2b). As a comparison, mGO–PMS and a second control PMS were

95

synthesized at pH = 12.7. Although the as-synthesized mGO–PMS samples showed well-

defined peaks, the XRD peak of rGO–PMS had a lower intensity compared with mGO–

PMS, suggesting a partial loss of meso-ordering (Figure 5-2c). The same trend was also

observed in the control PMS sample (Figure 5-2d). In contrast, surfactant removal in

mGO–PMS (0.27) synthesized at pH = 11.7 increased the intensity of the diffraction

peak, as expected because of an increase of the electron density contrast between pore

wall and pore after surfactant removal, suggesting that the meso-ordering of mGO–PMS

was preserved in rGO–PMS.

Figure 5-3 SEM images of rGO–PMS (0.27) synthesized at pH = 11.7. Gradual zoom-in from (a) to (d).

The proposed sandwich structure of the nanocomposite was confirmed by electron

microscopy (Figure 5-3 and Figure 5-4). The pore opening of PMS was observed over

large areas, and a stacking of PMS platelet suggested a layered structure (Figure 5-3).

The direct proof of the sandwich structure was the high resolution TEM on microtomed

96

sample of rGO–PMS (0.27) (Figure 5-4). In contrast, rGO–PMS (0.27) synthesized at pH

= 12.7 only showed worm-like porous structure (Figure 5-5).

97

Figure 5-4 TEM images of microtomed sample of rGO–PMS (0.27) synthesized at pH = 11.7.

98

Figure 5-5 Electron microscope images of rGO–PMS (0.27) synthesized at pH = 12.7.

The nitrogen sorption isotherm of rGO–PMS (0.27) belonged to type IV,7 with a

characteristic hysteresis associated with mesopores (Figure 5-6a). The hysteresis could be

attributed to type H3, typical of aggregates of plate-like particles.7 In contrast, control

PMS sample synthesized at pH = 11.7 showed no hysteresis, and had less amount of

nitrogen adsorption (Figure 5-6b), indicating smaller pore size and less pore volume. As a

comparison, sorption measurements were also performed on samples synthesized at pH =

12.7 (Figure 5-6c and Figure 5-6d). The results of the sorption measurements are

summarized in Table 5-1.

99

Figure 5-6 Nitrogen sorption isotherms of (a) rGO–PMS synthesized at pH = 11.7, (b) the control PMS synthesized at pH = 11.7, (c) rGO–PMS synthesized at pH = 12.7, and (d)

the control PMS synthesized at pH = 12.7.

Table 5-1 Surface Area, Pore Volume, and Pore Size of rGO–PMS and Control PMS Samples

Sample SBET (m2/g) a SBET

(m2/g) b SDFT (m2/g) V0,DFT (cm3/g)

DDFT (nm)

rGO–PMS (0.27) (pH = 11.7)

925 770 740 0.762 4.2

rGO–PMS (0.27) (pH = 12.7)

760 630 560 0.509 3.2

Control PMS (pH = 12.5)

1035 860 725 0.566 2.9

a using cross-section area of nitrogen molecule σN2 = 0.162 nm2; b using σN2 = 0.135 nm2;

100

Zeta potential measurements were performed to investigate the interaction

between surfactants and mGO (Figure 5-7). mGO dispersion showed a negative zeta

potential (-37 to -40 mV) in pH = 5–12 (Figure 5-7a), corresponding to the pH range for

forming stable mGO solution. The addition of CTACl surfactant inverted the sign of the

charge (Figure 5-7b). Because the zeta potential of pure CTACl surfactant solution of

similar concentration (56 mM) was close to zero (Figure 5-7d), the inversion of the sign

of the charge suggested surfactant micelles adhering to the mGO sheets and stabilizing

the mGO sheets. The larger potential observed at higher temperature (Figure 5-7b) could

be attributed to greater amount of CTACl adhering to the mGO sheets or increased

dissociation of chlorine counter ions from CTACl micelles. The adsorption of micelles on

mGO sheets reached saturation at 20–100 mM (Figure 5-7c). The decrease in zeta

potential beyond 100 mM could be attributed to a decrease in CTACl adsorption on mGO

layers either due to the “salting-out” effect that unstabilizes the colloidal solution or an

increased tendency for CTACl molecules to form liquid crystalline phase.

Based on the results of zeta potential measurements, a route for the formation of

rGO–PMS with porous channels oriented vertically with respect to rGO sheet was

proposed (Figure 5-8). The central idea was that a mixture of hydrophilic and

hydrophobic patches of mGO averaged out any preferential interaction between head

groups or tails of surfactant molecules and the mGO sheets, and as a result the micelles

grew vertically to the mGO sheets. The surface roughness of the mGO sheet might play a

role in this vertical orientation as well.

101

Figure 5-7 Zeta potential of (a) GO and mGO solution as a function of pH, (b) mGO solution with 57 mM CTACl as a function of pH at 298 K and 348 K, (c) mGO solution

with different concentrations of CTACl, and (d) pure CTACl solutions with concentrations of 6 mM, 56 mM, and 1020 mM.

102

Figure 5-8 Proposed mode of formation for self-assembly of rGO–PMS with channels oriented vertically with respect to rGO sheets

Electrical conductivity of rGO–PMS increased as GO content increased (Figure

5-9). The values 0.04–4 Sm-1 were much smaller than those of pure rGO (2400 Sm-1),8

because rGO sheets were sandwiched by the insulating PMS on both sides in rGO–PMS

(0.27). Although small, these conductivity values were in the range of a typical

semiconductor and suggested possible sensing functions. To perform a proof-of-concept

experiment, the adsorption ability of hexane and water on rGO–PMS (0.27) were first

demonstrated by respective sorption measurement at 303 K (Figure 5-10), and then the

vapor sensing function of rGO–PMS (0.27) was demonstrated (Figure 5-11). In the vapor

sensing experiment, the introduction of hexane or water vapor decreased the resistivity of

the nanocomposite by as much as 30%. As a comparison, rGO–PMS (4), which did not

possess a porous channels oriented perpendicular to the rGO sheets, showed much

smaller reduction in its resistivity. No significant change was observed in pure rGO film

either. These results demonstrate a clear advantage of the sandwich structure in rGO–

PMS (0.27): the vertical orientation of porous channels made possible direct contact

103

between rGO sheets and analyte molecules, and adsorption of analyte molecules onto the

rGO sheets modified the electrical properties of rGO and hence the whole

nanocomposite.

Figure 5-9 Electrical conductivity of rGO-PMS as a function of carbon content. The numbers above the data points are the concentrations of mGO content in the starting

dispersion.

104

Figure 5-10 Isotherms of hexane and water sorption measurement on rGO–PMS (0.27) at 303 K. The adsorption volume were normalized against the total pore volume obtained

from the nitrogen sorption measurement

Figure 5-11 Relative electrical resistivity of rGO-PMS (0.27), rGO-PMS (4), and rGO film as a function of the relative pressure of hexane; and relative electrical resistivity of

rGO-PMS (0.27) as a function of the relative humidity.

105

5.2. Methods

Synthesis of graphene oxide (GO) and modified graphene oxide (mGO): GO

was prepared from natural graphite by the Hummer–Offeman’s method.9 For the

synthesis of mGO. GO powder was dispersed in a NaOH solution (0.014–0.05 M) in a

propylene (PP) bottle (GO content: 0.1–4 mg/ml, pH = 11.8–12.8). After being sonicated

for 1 h, the mixture turned to a brown dispersion. This dispersion was then heated in an

oil bath at 353 K for 12 h. The PP bottle was tightly sealed during the sonication and the

thermal treatment to prevent solvent loss. After the thermal treatment, the dispersion

turned dark grey and was stable for months.

Synthesis of reduced graphene oxide–periodic mesoporous silica (rGO–PMS)

nanocomposite: In a typical synthesis, cetyltrimethylammonium chloride (CTACl, 25

wt% aqueous solution) (0.102 mol) was dispersed in an mGO solution with a GO content

of 0.27 mg/ml. The pH of the dispersion was adjusted by adding droplets of NaOH

solution (1 M) or HCl solution (diluted 10-fold from concentrated HCl). The dispersion

was stirred first at room temperature (RT) and then at 353 K for 80 min. After the

dispersion was cooled to RT, tetraethyl orthosilicate (0.128 mol) was added drop-wise.

The resulting mixture was heated at 353 K for 24 h. The precipitate was collected by

centrifugation and was aged at RT overnight and at 353 K for 24 h. Throughout the

synthesis, the container PP bottle was tightly sealed except when reagents were added.

The as-synthesized dry powder was labeled as mGO–PMS(x), x being the initial GO

content in mGO dispersion. mGO–PMS was then heated under N2 flow at 773 K for 3 h

(ramp rate 1 K/min) to give the black rGO–PMS. The pH after CTACl addition, CTACl

concentration, the amount of TEOS, mGO concentration, and reaction temperature were

varied. The optimum synthetic conditions for obtaining the sandwich structure were pH =

11.7, mGO concentration = 0.27 mg/ml, CTACl concentration = 57 mM, a molar ratio of

TEOS : CTACl = 1.18:1, and aging temperature 353 K.

Powder X-ray diffraction (XRD): XRD patterns were collected on an automated

Bruker/Siemens AXS D5000 diffractometer in a /2 Bragg-Brentano reflection

geometry with fixed slits. The system was equipped with a high power line focus Cu-K

106

source operating at 50 kV and 35 mA. A solid-state Si/Li Kevex detector was used for

removing unwanted K lines. The scanning range was 2 = 0.8o–5.0o with a step size of

0.02o and a counting time 3.0 s per step. Slits were set up according to the scanned range

in order to ensure maximum peak-to-noise ratio.

Scanning Transmission electronic microscopy (STEM): STEM was performed

on Hitachi HD-2000 with an acceleration voltage of 200 kV. Microtomed samples had a

thickness about 20–100 nm.

Ultraviolet–visible spectroscopy (UV–Vis): UV–vis spectra were collected on a

Varian CARY 100 BIO UV–vis spectrophotometer. GO and mGO dispersion in 0.05 M

NaOH aqueous solution were diluted with distilled water to an appropriate absorbance

range. Their spectra were compared based on absorbance normalized against the

maximum values.

Fourier transform infrared spectroscopy (FT-IR): FT-IR spectra were

collected on a PerkinElmer spectrometer. Solution dispersions or water slurries of

samples were drop-casted onto non-doped Si wafers. Data collections were carried out in

the 400–4000 cm-1 range with a step size of 2 cm-1. A non-doped Si wafer was used as

the background.

Raman Spectroscopy: Raman spectra were collected on a Horiba Jobin Yvon

LabRam Raman microscope equipped with a 532 nm laser excitation source and a 100×

(0.8 NA) microscope objective. Spectrum step size was 2 cm-1. Samples were drop-casted

on non-doped Si wafers.

Sorption measurements: Nitrogen adsorption measurements were performed on

Quantachrome Autosorp-1 at 77 K. Samples were heated under vacuum at 393 K for 3 h

before measurements.

Zeta potential: Zeta potential measurements were performed on Malvern

ZETASIZER 3000 HAS at 298 K and 348 K. Dispersions of GO (1 mg/ml) and mGO (1

mg/ml) were diluted with either NaOH solution or distilled water with a volume ratio of

107

1:40. The pH of the diluted solution was adjusted by adding droplets of NaOH (0.05 M)

solution or HCl solution (diluted from concentrated HCl with a volume ratio of 1:20).

Thermal gravimetric analysis (TGA): TGA was measured by SDT Q600 (TA

Instrument) under N2 flow (200–250 ml/min) at a ramp rate of 1 K/min. -Al2O3 pan was

used as the reference.

Elemental analysis (EA): EA was performed on PerkinElmer 2400 Series II

CHNS Analyzer.

Electrical conductivity: Electrical conductivity measurements were performed

by the four probe method on rectangular pellets of pressed samples with a density of 1.2–

1.5 gcm-3.

Vapor sensing tests: The vapor sensing were performed at room temperature on a

home-made apparatus that allowed the real-time measurements of vapor pressure and

electrical conductivity while the analyte vapor were introduced or evacuated.

5.3. Conclusion

In summary, rGO–PMS nanocomposite with channels oriented vertically to the

mGO sheets was synthesized. The structure of this novel nanocomposite, determined by

XRD and electron microscopy, suggested that rGO sheets were sandwiched between

PMS platelets. The optimum synthetic conditions were pH = 11.7, mGO concentration =

0.27 mg/ml, CTACl concentration = 57 mM, a molar ratio of TEOS : CTACl = 1.18:1,

and aging temperature = 353 K. Zeta potential measurements indicated the key role of

adsorption of surfactant micelles onto mGO sheets in directing the growth of vertically

oriented channels. This novel rGO–PMS nanocomposite possessed electrical conductivity

in the semiconductor range, and its vapor sensing functions were demonstrated. This new

class of nanocomposites bodes well for many basic and applied research opportunities

that take advantage of the synergistic integration of GO and mesostructures into reduced

graphene oxide–mesoporous materials sandwich structures.

108

5.4. References

1. Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A., Synthesis

of Oriented Films of Mesoporous Silica on Mica. Nature 1996, 379, 703-705.

2. Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A., Registered Growth of

Mesoporous Silica Films on Graphite. J. Mater. Chem. 1997, 7, 1285-1290.

3. Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.;

Eisenberger, P. M.; Gruner, S. M., Biomimetic Pathways for Assembling Inorganic Thin

Films. Science 1996, 273, 892-898.

4. Wang, Z.-M.; Wang, W.; Coombs, N.; Soheilnia, N.; Ozin, G. A., Graphene

Oxide-Periodic Mesoporous Silica Sandwich Nanocomposites with Vertically Oriented

Channels. ACS Nano 2010, 4, 7437-7450.

5. Fan, X.; Peng, W.; Li, Y.; Li, X.; Wang, S.; Zhang, G.; Zhang, F., Deoxygenation

of Exfoliated Graphite Oxide under Alkaline Conditions: A Green Route to Graphene

Preparation. Adv. Mater. 2008, 20, 4490-4493.

6. Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R.,

Perspectives on Carbon Nanotubes and Graphene Raman Spectroscopy. Nano Lett 2010,

10, 751-758.

7. Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.;

Rouquerol, J.; Siemieniewska, T., Reporting Physisorption Data for Gas/Solid Systems

with Special Reference to the Determination of Surface Area and Porosity

(Recommendations 1984). Pure Appl. Chem. 1985, 57, 603-619.

8. Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S., The Chemistry of

Graphene Oxide. Chemical Society reviews 2010, 39, 228-240.

9. Hummers Jr, W. S.; Offeman, R. E., Preparation of Graphitic Oxide. J. Am.

Chem. Soc. 1958, 80, 1339.

109

Chapter 6 Meso-Quartz

Transforming periodic mesoporous silica to periodic mesoporous quartz

110

This chapter will summarize the progress made in the synthesis of periodic

mesoporous quartz. Periodic mesoporous silica (PMS), synthesized through the sol–gel

route, have been proved to have amorphous pore wall by powder X-ray diffraction,

Raman spectroscopy and NMR studies.1 The synthesis of mesoporous crystalline silica,

particularly mesoporous quartz, remained a challenge.1 Efforts to resolve this long-

standing challenge carry scientific and technological importance. Scientifically, these

efforts necessitate the investigation of the thermochemistry of PMS,2, 3 the details of the

crystallization process,4 and the effect of catalysts. These investigations will increase our

understanding of the chemistry in nanometer-confined environment and further

strengthen our ability to control the compositions and structures of all periodic

mesoporous materials. Technologically, crystalline PMS, particularly mesoporous quartz,

will have a broad range of new properties arising from its crystalline nature, such as

enhanced mechanical properties, thermal stability, non-linear optical properties, and

piezoelectric properties. These properties will benefit applications such as catalysis,

biomedicine, microelectronics, random-motion energy harvesting, and

telecommunication.

6.1. Methods

Figure 6-1 Comparison between (a) wet-impregnation and (b) vapor-diffusion experiments. In the case of wet–impregnation, a catalyst was directly mixed with PMS. In

the case of vapor–diffusion, a catalyst was placed inside a small crucible, physically separated from PMS, and was delivered after being vaporized at high temperatures

through vapor diffusion.

Two approaches have been reported in the literature to crystallize amorphous

silica;5, 6 they differ in the way by which catalysts were delivered (Figure 6-1). In this

study, both approaches were used. In wet-impregnation experiments (Figure 6-1a),

111

mesoporous silica and catalysts were physically mixed together inside one crucible

before thermal treatment. In vapor-diffusion experiments (Figure 6-1b), catalysts were

placed inside a small crucible, which was at the center of a larger crucible; mesoporous

silica was placed around the small crucible. In both approaches, the sample-containing

crucible was then heated in a tube furnace at 600–900 °C in air, under nitrogen flow, or

under vacuum.

The reported catalysts for crystallizing amorphous silica include alkali metal ions

and alkaline earth metal ions (Li+, Na+, K+, Cs+, Mg2+, and Ca2+)5, 6, noble metals (Au and

Ag),7, 8 carbon,9, 10 and water.11 Among them, the lithium ion has been shown to generate

the highest perturbation of the amorphous silica framework,12 and favors the formation of

quartz.5 I will focus on the use of lithium ethoxide (1M in ethanol solution) and lithium

oxide as catalysts in this chapter, and will briefly discuss the effect of water and carbon in

the crystallization process.

112

6.2. Results

6.2.1. Crystallization Temperature

Wet-impregnation experiments using lithium ethoxide as the catalyst showed that

the crystallization occurred between 600–700 °C. At 600 °C, no crystal was formed

(Figure 6-2); at 700 °C, a mixture of quartz, keatite, and lithium silicate (Li2Si2O5) was

formed (Figure 6-3); at 800 °C, quartz becomes the most dominant phase in the mixture

(Figure 6-4).

Figure 6-2 XRD pattern of a wet-impregnation experiment at 600 °C for 15 h in air

113

Figure 6-3 XRD results of a wet-impregnation experiment with PMS at 700 °C for 15 h in air. (a) XRD pattern, and (b) the peak lists of the sample, α-quartz (PDF83-0539, space

group P3121), keatite (PDF76-0912, space group P43212), and Li2Si2O5 (PDF72-0102, space group Cc).

114



The XRD results of samples treated at 800 °C showed a bump centered at around

23°, indicative of residual amorphous silica. Extending the duration of thermal treatment

from 15 h to 40 h decreases the amorphous content (Figure 6-5).

115



While wet-impregnation experiments usually produced a mixture of crystalline

phases, vapor-diffusion experiments produced quartz phase only (Figure 6-6).

116

Figure 6-6 XRD results of vapor-diffusion experiments with PMS at 800 °C for 40 h in air. (a) XRD pattern, and (b) the peak list of the sample and α-quartz (PDF83-0539, space

group P3121)

A small amount of water (below 5 mol% H2O/Si) was added into the mixture to

increase the rate of the crystallization. This effect can be attributed to water re-

hydrolyzing silica and breaking Si–O–Si bonds at high temperatures, thereby facilitating

the rearrangement of atoms.13 However, water alone did not catalyze the crystallization at

800 °C (Figure 6-7).

117

Figure 6-7 XRD pattern of a control experiment with PMS at 800 °C for 40 h in air without any lithium catalyst

6.2.2. Keatite as an Intermediate Phase

Keatite is a metastable phase first synthesized over a pressure range of 345–1241

bar and a temperature range of 380–585 °C.13, 14 It has not been found in nature.15 Reports

on its synthesis are rare,16, 17 probably as a result of its metastable nature. It was observed

to be an intermediate between cristobalite and quartz in crystallizing amorphous silica

under high pressures.16 In the past decade, a renewed interest in keatite focused on its

solid solution of lithium oxide and aluminum oxide, which possesses low thermal

expansion coefficient, high resistance to temperature difference, adjustable translucent

and opaque appearance, and colourability.18

The results of the wet-impregnation experiments suggested that keatite was an

intermediate between amorphous silica and quartz, although its amount compared to

quartz was relatively small. A new set of experiments were conducted to synthesize

keatite in large quantities. At 550 °C, no crystal was formed (Figure 6-8); at 650 °C,

keatite phase dominated; at 750 °C (Figure 6-9), quartz became the dominant phase again

(Figure 6-10).

118

Figure 6-8 XRD pattern of mesoporous silica mixed with lithium oxide and treated at 550 °C for 2h.

119

Figure 6-9 XRD results of mesoporous silica mixed with lithium oxide and treated at 650 °C for 2 h. (a) XRD patterns, and (b) the peak lists of the sample, keatite (PDF# 76-0912,

space group P43212), and Li2Si2O5 (PDF# 24-0651, orthorhombic).

120

Figure 6-10 XRD results of mesoporous silica mixed with lithium oxide and treated at 750 °C for 2h. (a) XRD pattern, and (b) the peak lists of the sample, α-quartz (PDF83-

0539, space group P3121), and Li2Si2O5 (PDF40-0376, space group Cc)

121

6.2.3. Possible Epitaxial Growth

Water has been postulated in section 6.2.1 to re-hydrolyze silica, break Si–O–Si

bonds, and hence facilitate the rearrangement of atoms during the crystallization process.

The catalytic effect of lithium was postulated to destabilize the amorphous silica

framework, enter into the crystalline phase and act as a nucleation center.5 To examine

the catalytic effect of lithium alone, a control experiment performed with care to remove

all possible sources of water, including removing potential water produced from

polycondensation of residual silanol groups in the internal surfaces of PMS by calcining

PMS at 800 °C for 6h, showed a new Li2SiO3 phase, which possesses a peak ((111)

plane) very close to the strongest quartz peak ((101) plane) at around 27° (Figure 6-11).

The closeness of two peaks might suggest a possible epitaxial growth from Li2SiO3 phase

to quartz through these two planes.

122

Figure 6-11 XRD results of a control experiment performed in the absence of water (a) XRD pattern of mesoporous silica treated at 800 °C for 24 h under nitrogen flow, and (b)

the peak lists of the sample, α-quartz (PDF83-0539, space group P3121), and Li2SiO3 (PDF74-2145, space group Cmc21)

123

6.2.4. Crystallization of Carbon-Filled PMS

Figure 6-12 Filling materials to support porous structure during crystallization

In the experiments presented so far in this chapter, crystallization was always

accompanied by a collapse of mesoporous structure, as evidenced by the disappearance

of low angle XRD peaks around 1°. Therefore, a filling material to support the

framework seemed necessary to prevent the collapse of mesoporous structure during

crystallization (Figure 6-12). Carbon was used as the filling materials because it can be

conveniently produced by carbonizing the templating block copolymer used in the

synthesis of mesoporous silica.19 Carbon source is either block copolymer itself, such as

P123 or F127, or additional carbon sources such as aromatic compounds or carbon pitch.

XRD results of carbon filling experiments suggested that quartz was formed

during the crystallization (Figure 6-13), and that meso-ordering was partially retained

(Figure 6-14). However, attempts to remove residual carbon after crystallization by

calcining the sample in air or by plasma treatment failed. As a result, the surface area

(below 50 m2/g) and pore volume (below 0.1 cm3/g) of the sample were one order of

magnitude less than those of mesoporous silica.

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TEM inspection (Figure 6-15 and Figure 6-16) of the sample revealed that it

contained both macropores and mesopores, but periodicity was absent in regions where

ED spots were clear.

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Figure 6-13 XRD results of a carbon filling experiment. (a) the XRD pattern of mesoporous silica filled with carbon and treated at 850 °C for 5 h, and (b) peak lists of the sample, α-quartz (PDF83-0539, space group P3121), Li2Si2O5 (PDF40-0376, space

group Cc), and Li2SiO3 (PDF70-0330, space group Cmc21)

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Figure 6-14 Low angle XRD patterns of a carbon filling experiment. Carbon-filled mesoporous silica (a) before the crystallization, and (b) after the crystallization.

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Figure 6-15 TEM images of porous quartz

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Figure 6-16 Electron diffraction patterns of selected regions in porous quartz

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6.3. Conclusion and Future Directions

In summary, lithium ions were found to catalyze the crystallization of PMS into

porous quartz at temperature as low as 700 °C. It was postulated that lithium acted as a

nucleation center for possible epitaxial growth of quartz from Li2SiO3. Water was found

to increase the rate of this crystallization process by promoting the breaking of Si–O–Si

bonds by re-hydrolysis. Keatite phase was found to be an intermediate phase between

amorphous silica and quartz. Crystallization of carbon-filled mesoporous silica partially

retained the meso-ordering, but the carbon was difficult to remove completely after

crystallization

One direction for future efforts on synthesizing mesoporous quartz will be on

finding a proper filling material to support the mesostructure during the crystallization.

Criteria for a proper filling material include not reacting with silica, low-temperature

preparation (below 800 °C), and high temperature stability (800–900 °C). Possible

candidates include lithium–inert metal alloy, iron oxide, and chromium oxide. The

choices of iron oxide and chromium oxide may carry additional benefits because they can

consume lithium ions in regions where lithium ions were in excess by forming lithium

iron oxide or lithium chromium oxide, thereby averaging the crystallization speed

throughout the sample and maintaining porosity uniformity.

The other direction for future effort is to synthesize mesoporous silica with large

pore size and thick pore wall for the crystallization experiment. Experiments on

macroporous quartz6 and mesoporous silicon20 suggested that a large pore size and a

thick pore wall may also prevent the collapse of mesostructures during crystallization. A

systematic study to identify the critical pore and wall dimensions for preserving the

mesostructure will be necessary to fully understand the effect of nano-confinement on the

crystallization process.

130

6.4. References

1. Davis, M. E., Ordered Porous Materials for Emerging Applications. Nature 2002,

417, 813-821.

2. Trofymluk, O.; Levchenko, A. A.; Tolbert, S. H.; Navrotsky, A., Energetics of

Mesoporous Silica: Investigation into Pore Size and Symmetry. Chem. Mater. 2005, 17,

3772-3783.

3. Navrotsky, A.; Trofyrnluk, O.; Levchenko, A. A., Thermochemistry of

Microporous and Mesoporous Materials. Chem. Rev. 2009, 109, 3885-3902.

4. Epping, J. D.; Chmelka, B. F., Nucleation and Growth of Zeolites and Inorganic

Mesoporous Solids: Molecular Insights from Magnetic Resonance Spectroscopy. Current

Opinion in Colloid and Interface Science 2006, 11, 81-117.

5. Venezia, A. M.; La Parola, V.; Longo, A.; Martorana, A., Effect of Alkali Ions on

the Amorphous to Crystalline Phase Transition of Silica. J. Solid State Chem. 2001, 161,

373-378.

6. L. Zhao, N. L., A. Langner, M. Steinhart, T. Y. Tan, E. Pippel, H. Hofmeister, K.-

N. Tu, U. Gösele Crystallization of Amorphous Sio2 Microtubes Catalyzed by Lithium.

Adv. Funct. Mater. 2007, 17, 1952-1957.

7. Pol, V. G.; Gedanken, A.; Calderon-Moreno, J., Deposition of Gold Nanoparticles

on Silica Spheres: A Sonochemical Approach. Chem. Mater. 2003, 15, 1111-1118.

8. Garníca-Romo, M. G.; González-Hernández, J.; Hernández-Landaverde, M. A.;

Vorobiev, Y. V.; Ruiz, F.; Martínez, J. R., Structure of Heat-Treated Sol-Gel Sio2

Glasses Containing Silver. Journal of Materials Research 2001, 16, 2007-2012.

9. Deepak, F. L.; Gundiah, G.; Seikh, M. M.; Govindaraj, A.; Rao, C. N. R.,

Crystalline Silica Nanowires. Journal of Materials Research 2004, 19, 2216-2220.

131

10. Okabe, A.; Niki, M.; Fukushima, T.; Aida, T., Amorphous Carbon-Promoted

Low-Temperature Crystallization of Silica. Chemistry Letters 2006, 35, 228-229.

11. Wagstaff, F. E.; Richards, K. J., Kinetics of Crystallization of Stoichiometric Sio2

Glass in H2o Atmospheres. J. Am. Ceram. Soc. 1966, 49, 118-&.

12. Navrotsky, A.; Geisinger, K. L.; McMillan, P.; Gibbs, G. V., The Tetrahedral

Framework in Glasses and Melts - Inferences from Molecular Orbital Calculations and

Implications for Structure, Thermodynamics, and Physical Properties. Physics and

Chemistry of Minerals 1985, 11, 284-298.

13. Iler, R. K., The Chemistry of Silica. John Wiley & Sons: 1979.

14. Keat, P. P., A New Crystalline Silica. Science 1954, 120, 328-330.

15. Keatite. In Encyclopædia Britannica. Encyclopædia Britannica Online. , 2011.

16. Carr, R. M.; Fyfe, W. S., Some Observations on the Crystallization of Amorphous

Silica. Am. Miner. 1958, 43, 908-916.

17. Ferreira da Silva, M. G.; Fernández Navarro, J. M., Formation of Keatite from

Li2o-Sio2 Gels in the Presence of Chromium Oxide. Journal of Non-Crystalline Solids

1989, 109, 191-197.

18. Roos, C.; Becker, O.; Siebers, F., Microstructure and Stresses in a Keatite Solid-

Solution Glass-Ceramic. J. Mater. Sci. 2007, 42, 50-58.

19. Yan, X.; Song, H.; Chen, X., Synthesis of Spherical Ordered Mesoporous

Carbons from Direct Carbonization of Silica/Triblock-Copolymer Composites. J. Mater.

Chem. 2009, 19, 4491-4494.

20. Richman, E. K.; Kang, C. B.; Brezesinski, T.; Tolbert, S. H., Ordered Mesoporous

Silicon through Magnesium Reduction of Polymer Templated Silica Thin Films. Nano

Lett. 2008.

132

Chapter 7 Summary and Outlook

Starting with a broad overview on periodic mesoporous materials, the thesis

focused on two specific types of periodic mesoporous materials—periodic mesoporous

organosicilica (PMO) and periodic mesoporous silica (PMS).

My research on PMO focused on its use as a low-k material, and has been

narrated from the perspective of materials chemistry. I presented a novel synthetic

method, vacuum-assisted aerosol deposition, and characterized pertinent dielectric,

mechanical and hydrophobic properties of PMO for low-k application. Three PMOs—

ethane, methane, and 3-ring PMOs—possess k < 2, E (SAWS) > 4 GPa, and moisture

resistance: a combination of properties that make them ideal as low-k materials. Given

PMO’s promising properties, the next logical step is to fine-tune their synthesis based on

feedback from further materials testing. Possible modifications of the current synthesis

that may benefit the low-k application include (1) making cubic or 3d hexagonal

mesostructures, (2) making smaller pores, and (3) designing volatile precursors and

templates for the synthesis of PMOs by chemical vapor depositions. I hope this work may

inspire future materials engineering efforts to bring the low-k PMO materials to a higher

level.

Compared with PMO as low-k materials, the chapters on PMS have been more

exploratory and focused mainly on the syntheses and structures. A novel sandwich-

structured reduced graphene oxide (rGO)–periodic mesoporous silica (PMS)

nanocomposite with porous channels oriented vertically to the rGO sheets has been

discovered. The key feature of this nanocomposite is its vertically oriented porous

channels, which permit guest molecules in direct contact with rGO sheets. The

experiment on its vapor sensing ability illustrated the potential of rGO–PMS, as well as

rGO–periodic mesoporous material composites in general, when the design of material

functions was made possible by combining the best of two materials: the conductivity of

rGO and the large surface area, large pore volume, and narrow pore size distribution of

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periodic mesoporous materials. Future exploration will follow an approach by

incorporating rational design of function into materials synthesis. Examples include

rGO–periodic mesoporous transition metal oxides for energy storage and solar energy

harvesting.

The other chapter on PMS summarized the progress in synthesizing periodic

meosoporous quartz, a long-standing challenge in the field of periodic mesoporous

materials since its inception in early 1990s. The understanding of the effect of catalysts,

crystallization temperatures, intermediate phases, and crystal growth mechanism will

guide future efforts in this solving this challenge.

Finally, to put my work in the larger context of nanoscience and nanotechnology,

it represents an example of a general research direction in the past five years of applying

the expertise accumulated on the synthesis of nanomaterials to solving practical

problems, either through exploring the intrinsic properties of a particular nanomaterial, or

through rational design of nanomaterial functionality. Valuable experience gained in this

work will guide me in identifying future research opportunities by combining personal

curiosity with greater needs of humankind.

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Epilogue

I remember five years ago, when I first entered Geoff’s office, he gave me a big

firm handshake and a big welcoming smile. Later when we talked about the group and

projects, Geoff made an analogy that being in a mentor–student relationship is like being

married for five years. Although I found that analogy a bit weird, I got the idea that

getting a perfect match between a mentor and a student is difficult. After five years, I

think that this mentor–student pair is as perfect as any student can get. Geoff personifies

what Samuel Ullman wrote about Youth: “Youth is not a time of life; it is a state of mind;

it is not a matter of rosy cheeks, red lips and supple knees; it is a matter of the will, a

quality of the imagination, a vigor of the emotions; it is the freshness of the deep springs

of life.” His unlimited enthusiasm, knowledge, wisdom, and unyielding support have

guided me through what have been the best years in my life. And for that, I am forever

grateful.

My PhD experience also reminds that it requires much more than the efforts of

two persons. My co-supervisor Professor Doug Perovic has always been another source

of light in guiding my journey. The lab manager Sue Mamiche, with the most cheerful

personality I have ever seen, has always been a great source of support in the lab. Dr.

Daniel Grozea has been not only a great collaborator in all of my low-k projects, but also

a helpful advisor on professional development. My day-to-day lab experience could never

have been so enjoyable if it were not with a group of the most fantastic students, post-

docs, and senior researchers I have met. The list is too long to write. To accord with the

style of this thesis, I post here a recent group photo.

Periodic Mesoporous Organosilica and Silica · Periodic Mesoporous Organosilica and Silica Wendong...

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