Control and Characterization of Titanium

Dioxide Morphology: Applications in

Surface Organometallic Chemistry

By

Gabriel Jeantelot

In Partial Fulfilment of the Requirements

For the Degree of

Masters of Science

KAUST Catalysis Center (KCC) King Abdullah University of Science and Technology

Thuwal, Kingdom of Saudi Arabia May 2014

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EXAMINATION COMMITTEE APPROVALS FORM This thesis of Gabriel Jeantelot is approved by the examination committee:

Committee Chairperson Dr Jean Marie Basset

Committee Co-chairperson Dr Samy Ould-Chikh

Committee member Dr Kazuhiro Takanabe

Committee member Dr Aurélien Manchon

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© May 2014 Gabriel Jeantelot

All Rights Reserved

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Abstract

Control and Characterization of Titanium Dioxide Morphology: Applications in Surface Organometallic

Chemistry

Surface Organometallic Chemistry leads to the combination of the high activity

and specificity of homogeneous catalysts with the recoverability and practicality of

heterogeneous catalysts. Most metal complexes used in this chemistry are grafted on

metal oxide supports such as amorphous silica (SiO2) and γ-alumina (Al2O3). In this

thesis, we sought to enable the use of titania (TiO2) as a new support for single-site

well-defined grafting of metal complexes. This was achieved by synthesizing a special

type of anatase-TiO2, bearing a high density of identical hydroxyl groups, through

hydrothermal synthesis then post-treatment under high vacuum followed by oxygen

flow, and characterized by several analytical techniques including X-ray diffraction,

transmission electron microscopy, infrared spectroscopy and nuclear magnetic

resonance. Finally, as a proof of concept, the grafting of vanadium oxychloride (VOCl3)

was successfully attempted.

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Acknowledgements

I would like to thank Prof. Jean-Marie Basset and Dr. Samy Ould-Chikh for

their advisory along my thesis, as well as Dr. Julien Sofack-Kreuzer for his precious

help with labwork in general and the use of high vacuum lines in particular, Dr. Edy

Abouhamad for performing the nuclear magnetic resonance analyses we required, and

the general KAUST Catalysis Center community for welcoming me as one of its

members.

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Table of Contents

Copyright…………………………………………………………………………...3

Abstract……………………………………………………………………………..4

List of Abbreviations……………………………………………………………..7 List of Figures……………………………………………..……………………….8 I – Context…………………………………………………………………....……10 1) Uses of TiO2 in conventional heterogeneous catalysis……...…………….10 2) Uses of TiO2 in photocatalysis……………………………………………..15

3) Targeted catalysts and photocatalysts……………………………………...18 4) Application of Surface Organometallic Chemistry to Titanium Dioxide….20

II - Properties of Titanium Dioxide………………………………....…….….23 1) General properties……………………………………………………….…23 2) Surface chemistry of the different crystalline facets exposed on anatase….24 3) Synthesis of anatase nanocrystals with morphology control………….…...29

III - Objectives and Strategy……………………………………………….….35

IV - Experimental Section……………………………………………………...36 1) Hydrothermal syntheses…………………………………………………....36 2) Dehydroxylation protocol…………………………...………………….….38 3) Grafting protocol………...………………………………………………...38 4) Analytical techniques……………………………………………………....39

V - Results and Discussion……………………………………………………..41 1) Morphology and bulk properties…………………………………………..41

2) Textural properties………………………………………………………....43 3) Sample stability during dehydroxylation post-treatment…………………..44 4) Characterization of surface chemistry……………………………………..48 5) Fluorine removal…………………………………………………………...57 6) Grafting…………………………………………………………………….61

VI – Conclusion…………………………………………………………………..66

Bibliography………………………………………………………………………67

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List of Abbreviations

BET: Brunauer-Emett-Teller physisorbtion theory DFT: Density Functionnal Theory DRIFTS: Diffuse Reflectance Infrared Fourier Transform Spectroscopy FT-IR: Fourier Transform Infrared Spectroscopy HETCOR: Heteronuclear Correlation HOMO: Highest Occupied Molecular Orbital LUMO: Lowest Unoccupied Molecular Orbital MAS: Magic Angle Spinning NMR: Nuclear Magnetic Resonnance ODH: Oxydative Dehydrogenation SOMC: Surface Organometallic Chemistry TGA: Thermogravimetric Analysis XRD: X-Ray Diffraction XRF: X-Ray Fluorescence

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List of Figures

Figure 1: General scheme of propane oxidative dehydrogenation and side reactions.

Figure 2: General scheme of photocatalytic water splitting.

Figure 3: Surface vanadium species of interest.

Figure 4: Surface chromium species of interest.

Figure 5: General scheme of surface-bound metal complexes.

Figure 6: Infrared spectra of SBA silica dehydroxylated at 200°C and 700°C under

high vacuum

Figure 7: Clean anatase {101} surface

Figure 8: Clean anatase {001} surface

Figure 9: Hydroxylated anatase {001} surface

Figure 10: Wulff construction of an anatase crystal

Figure 11: Titanium dioxide morphology variation in hydrothermal syntheses

Figure 12: X-ray diffractograms of samples {001}-anatase and {101}-anatase

Figure 13: Transmission electron micrographs of {001}-anatase and {101}-anatase

Figure 14: N2 physisorbtion isotherms and BJH plots of {001}-anatase and {101}-

anatase

Figure 15: X-ray diffractograms of {001}-anatase-NH4OH in function of

dehydroxylation temperature

Figure 16: Average (001) and (100) coherent domains of {001}-anatase-NH4OH in

function of dehydroxylation temperature

Figure 17: Transmission electron micrographs of {001}-anatase-NH4OH in function

of dehydroxylation temperature

Figure 18: BET surface area of {001}-anatase-NH4OH in function of dehydroxylation

temperature

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Figure 19: Thermogravimetric analysis of {001}-anatase-NH4OH

Figure 20: 19F NMR of {001}-anatase-NH4OH in function of dehydroxylation

temperature

Figure 21: 1H NMR of {001}-anatase-NH4OH in function of dehydroxylation

temperature

Figure 22: Double-quanta 1H NMR of {001}-anatase-NH4OH in function of

dehydroxylation temperature

Figure 23: Double-quanta and triple-quanta 1H NMR of {001}-anatase-NH4OH

dehydroxylated at 200°C

Figure 24: 1H-19F heteronuclear correlation NMR of {001}-anatase-NH4OH

dehydroxylated at 300°C

Figure 25: DRIFTS spectra of {001}-anatase-NH4OH in function of dehydroxylation

temperature

Figure 26: DRIFTS spectra of {001}-anatase-NH4OH before and after isotopic

exchange with D2O

Figure 27: XPS spectra of {001}-anatase and {001}-anatase-NH4OH

Figure 28: IR spectra of {001}-anatase depending on surface fluoration

Figure 29: IR spectra of {001}-anatase before and after VOCl3 grafting

Figure 30: 1H NMR spectra of {001}-anatase before and after VOCl3 grafting

Figure 31: RAMAN spectra of {001}-anatase before and after VOCl3 grafting

Figure 32: 51V NMR spectra of {001}-anatase after VOCl3 grafting

Figure 33: proposed structure for the surface vanadium species obtained by grafting

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I - Context

Titania, or titanium dioxide, is a widely used material, produced industrially in

scales of several million tons each year,1 mostly for its optical properties: it is indeed

the most widely used white pigment, owing to its very high refractive index (2.496 for

rutile vs. 2.419 for diamond) and high reflectivity in the visible range. Its current

applications are mainly as a pigment for paints and coatings, plastics and papers.

In the recent years, an increasing interest has been brought towards its chemical

properties: the oxygen atoms in titanium dioxide are labile, their removal leading to

oxygen vacancies and reduced Ti3+.2 As a result, it can take part in a number of oxidative

processes used in heterogeneous catalysis. Furthermore, being a semiconductor, it can

also be used in photocatalytic processes.

1) Uses of TiO2 in conventional heterogeneous catalysis

Several heterogeneous catalytic systems use titanium dioxide as support. Examples of

such systems include:

a) Common reactions

-Nitrogen oxide selective catalytic reduction by ammonia, catalysed by TiO2-

supported metal oxides, is used for the reduction of NO emissions from fixed sources,

by reducing it with ammonia to gaseous N2, according to the following equation:

NO+NH3+1/4 O2 → N2+1.5 H2O

Several catalytic systems are available for this reaction, many of them using

metal oxides supported on TiO2, such as Vanadium oxide and mixed Vanadium-

Tungsten oxides,3–7 or Manganese and Manganese-Cerium mixed oxides.6,8–10

11

- Gas phase and liquid phase Wacker process consist of the oxidation of

ethylene to acetaldehyde or terminal alkenes to ketones, according to the following

equation:

Several catalytic systems are available for this reaction, most using Pd nanoclusters

supported on V2O5-impregnated metal oxides, such as silica, alumina and titania.86-88

Among those supports, titania has been shown to provide higher activities, as well as

slower deactivation, owing to the high resistance to sintering of the vanadium oxide

monolayer on titania88.

- Ethane oxidation to acetic acid can be performed using titanium dioxide

supported molybdenum and vanadium, either simply impregnated on the support 90,91,

or as Keggin-structured heteropolyacids containing both Mo and V atoms89

- Methane oxidation to methanol has been carried out in hydrothermal

conditions, using hydrogen peroxide as the oxidizer, and a catalytic system consisting

of Au-Pd nanoparticles supported on titanium dioxide.21,22

- Steam reforming of methane can also be achieved, yielding hydrogen and

carbon monoxide through the following equation :

CH4 + H2O � CO + 3H2

Most catalytic systems used for this reaction are constituted of nickel nanoparticles on

a metal oxide support, and are progressively deactivated by coke deposition93. Recent

works have shown that the use of titanium dioxide as a support strongly reduces coke

deposition during the reaction.92

R C ½ O2

CH2

H

R CCH3

O

+

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b) Focus on alkane oxidative dehydrogenation

Alkane oxidative dehydrogenation is the conversion of alkanes or alkyl groups

into alkenes, using an oxidizer such as O211–16 or CO2

17,18. The general scheme of the

reaction in the presence of dioxygen is the following:

CxHy + ½O2 → CxHy-2 +H2O

i) Thermodynamics

Currently, alkanes conversion to alkenes is done by catalytic dehydrogenation,

an endothermic process (ΔH°=124 kJ.mol-1 for propane conversion to propylene and

hydrogen) that has to be carried out at high temperatures (~600°C), in which an alkane

molecule will dissociate into an alkene and a dihydrogen molecule.

The use of an oxidizer to generate H2O instead of H2 makes the reaction

exothermic (ΔH°=-161.6 kJ.mol-1 for propane and oxygen conversion to propylene and

water), allowing it to be performed at lower temperatures,

Figure 1 : general scheme of propane oxidative dehydrogenation and side reactions

H2C CH3 H2C CH3

CO, CO2

k1

k3k2

CH3 CH3

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The main challenges in achieving that reaction are both overcoming the low

reactivity of the C-H bond (ΔH298=411 kJ/mol for propane weakest (terminal) C-H

bond94) and avoiding C-C bond activation (ΔH298=372 kJ/mol for propane C-C94),

which would lead to complete oxidation to CO and CO2.

This reaction has mostly been studied for two alkanes: ethane and propane,

yielding respectively ethylene and propylene. Three families of catalytic systems are

available for each.

ii) Catalysts

-Noble metal catalysts

Noble metal catalysts generally consist of platinum, alone113,115,116 or in

association with other metals (Pt/Sn99,110,112,115,116, Pt/Rh111, Pt/Ga112), deposited on a

ceramic monolith (alumina, silica). Other metals such as Pd114, Rh113 and Ru113 have

also been used. These catalysts have achieved yields up to yields up to 20% for propane

conversion to propylene, and up to 50% for ethane to ethylene.110

-Non reducible metal oxides

These catalysts generally consist of an alkali earth metal oxide, such as CaO or

MgO97,117,118, promoted by alkali metal oxides (e.g Li)117,118, halogens (e.g Cl)117 or rare

earths (e.g Dy)117. They generally operate at temperatures above 600°C.

Yields up to 23.8%117 have been reported for propane oxidative

dehydrogenation to propylene with these catalysts.

Alkali-earth doped rare earth oxides such as SrxLaNdOy have also shown

activity for ethane oxidative dehydrogenation. 96

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-Reducible transition metal oxides

It is on this family of catalysts that most alkane oxidative dehydrogenation

studies have been conducted. They include:

-Supported reducible oxides (Vanadium11,12,14,19,20, Molybdenum15,20,

Chromium13,16 supported on TiO2 or Al2O3)

-Metal vanadates and metal molybdates (such as Mg2V2O798)

-Mixed oxides (such as Mo1V0.6Nb0.12Ox95 or TiO2-ZrO2

18)

They work at generally lower temperatures than noble metal and non-reducible

oxide catalysts (<600°C). V/TiO2 is therefore a system of high interest for alkane ODH.

iii) Mechanism and active sites on V/MOx

A Mars Van Krevelen mechanism is generally proposed as the dominant

mechanism on those catalysts.119 In such a mechanism, propane would adsorb on the

metal oxide surface. The H-abstraction is done by a lattice oxygen, and followed by a

β-H elimination with an adjacent oxygen leading to propylene. The two hydroxyl

groups generated in the previous steps will then condense, eliminating a water molecule,

and the reduced catalyst surface is finally reoxidized by oxygen from the gas feed.

In the case of supported vanadium oxide catalysts, different oxygen species are

proposed as active sites for alkane oxidative dehydrogenation: V=O isolated oxygens19,

V-O-V bridging oxygens109,137 or V-O-Support bridging oxygens138.

+ O* + HO*R-CH2-CH2-O*

R-CH=CH2

HO* + HO* H2O

+ O*R-CH2-CH3 R-CH2-CH3 O*

R-CH2-CH3 O*

R-CH2-CH2-O*

+ O*

½ O2O*

+ HO*

+ *

+ *

+ *

* active site

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2) Uses of TiO2 in heterogeneous photocatalysis

a) Photocatalytic oxidation of organic compounds

Due to its semiconductor properties, TiO2 can also take part in a number of

photocatalytic processes. One of them is the complete photooxidation of organic

compounds by dioxygen, breaking them down to water, CO2 and HCl for those

containing chlorine. This reaction is of high interest for water and air depollution and

the elaboration of self-cleaning surfaces. It has been shown to occur with aromatic

compounds (benzene, toluene, xylenes, phenol, salicylic acid)23–31 ; alcohols (methanol,

ethanol, 2-propanol)25,29,32 ; carboxylic acids (formic, acetic),29,33–35 chloroalkanes

(chloroform, dichloroethane, trichlorethylene)23,36–38 and alkanes (methane, ethane,

propane, isobutane, cyclohexane)39–41. They can be achieved on TiO2 alone,28,29,31,33–41

or in the presence of cocatalysts, such as surface bound halides,23–25 silver

nanoparticles,34 and supported vanadium oxide26,27,30,32 or molybdenum oxide32.

When only a limited amount of oxygen is present in the reaction medium, the

selective oxidation of hydrocarbons to ketones and aldehydes can be achieved. The

photocatalytic conversions of toluene to benzaldehyde42,43, styrene to benzaldehyde43

and cyclohexane to cyclohexanone44,45 have been reported. All those syntheses used

pure titanium dioxide, with no cocatalyst.

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b) Photocatalytic water splitting

Finally, a photocatalytic reaction

of high interest in which titanium dioxide

can take part is the photocatalytic splitting

of water into hydrogen and oxygen46. Its

slightly negative conduction band

potential (approx. ̠ 0.52 V vs SHE)47

makes its valence band electrons more

reducing than the H+/H2 couple (0.00V vs

SHE), allowing it to perform proton

reduction to dihydrogen when a proper cocatalyst is present on its surface. Its high

valence band potential (approx. +2,53 V vs SHE)47 makes conduction band holes more

oxydizing than the O2/H2O couple (+1.229 V vs SHE) allows it to perform water

oxydation to dioxygen with other cocatalysts. There are three main limiting factors to

titanium dioxide’s efficiency in photocatalytic water splitting :

1) Its excessively high bandgap, only allowing it to absorb light in the UV

region of the spectrum (with a 3-3.1 eV bandgap, only 3.3% of sunlight photons can

theoretically be absorbed, placing a theoretical upper limit on dihydrogen production

to 6.5 mol.m-2.h-1 in direct sunlight).

2) Direct recombination of photogenerated electron-hole pairs, without taking

part in water oxidation or reduction, therefore lowering the efficiency with which

absorbed photons are used. 100

3) Fast backwards reaction100 : water splitting into hydrogen and oxygen being

Figure 2 : general scheme of photocatalytic water splitting.

Potential (V vs SHE)

-1

0

1

2

3

H+

½ H2

½ H2O

¼ O2

TiO2 Conduction band

TiO2 Valence band

H+/H2

H2O/O

2

TiO2

H2O

e-

e-hν

Water reductioncatalyst

Water oxydationcatalyst

e-

H2O H

2 + ½O

2

hν

Overall splitting equation :

17

thermodynamically unflavoured without energy from an external source, the opposite

reaction can spontaneously proceed, consuming the obtained reaction products.

To overcome those limiting factors, different strategies are available:

-Limited absorption of solar light by TiO2 is enhanced by dye sensitization. It is

performed by grafting a stable dye on the surface of titanium dioxide with a higher

HOMO, slightly higher LUMO, and a lower overall HOMO-LUMO gap, allowing an

increased light absorption by the photocatalyst, and potentially lowering electron-hole

recombination. The absorption of a photon by the dye generates an electron-hole pair.

The electron is injected in the conduction band of TiO2, while the hole is be filled by

water oxidation on the dye.100

Currently, this is done using organic dyes (such as [Ru(dcbpy)2dpq]2+ 101 where dcpy is

2,2’-bipyridine-4,4’-dicarboxylate and dpq is 2,3-bis-(2 pirydyl quinoxaline).)

However, when U.V light is present, titanium dioxide is capable of photocatalytically

decompose organic compounds to CO2 and H2O, which will in this case result in the

degradation of the dye.108 A way of avoiding it would be through the use of inorganic

dyes, such as supported chromium oxides120,121.

-Depositing a water reduction cocatalyst on the surface of Titanium dioxide can

improve the energetic efficiency by increasing the water H2 formation rate compared to

recombination kinetics. Such a catalyst must allow the quick transfer of electrons from

the titanium conduction band, and as rapidly use them for water reduction, since

increasing water splitting rate will favour it over electron-hole recombination.

Examples of such catalysts are nanoparticles of gold, palladium, platinum, rhodium, tin,

chromium oxide or molybdenum disulfide48-50.

-Similarly, water oxidation cocatalysts will allow a faster use of holes in the

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valence band for oxygen production. Such catalysts include such as ruthenium oxide,

iridium oxide, manganese oxide, or cobalt oxide.51–54

3) Targeted catalysts and photocatalysts

a) Alkane oxidative dehydrogenation catalysts

Surface vanadium species have

shown on multiple times both high activities

and selectivities in alkane oxidative

dehydrogenation11,12,14,19,20,94,95,98,99. Similar

efficiencies have been reported with

molybdenum species15,20,98,95. Achieving the

deposition of identical well defined vanadium or molybdenum sites on the surface of

TiO2 could provide useful insight into the exact nature of active sites involving these

elements, and lead to catalysts with improved activity and selectivity. In particular,

tetrahedral vanadium(V) oxide dimers and isolated tripodal vanadium V monomers

have been cited as possible active species.19,105,109,137

b) Photocatalytic water splitting inorganic dyes

Surface octahedral chromium (III)

species have been shown to allow titanium

dioxide to perform photocatalysis in the

visible light domain.120,121 However, this

gain of activity in the visible light was also

accompanied by a loss of activity in the UV range, that could be due to the formation

Figure 3 : targeted surface vanadium species. Left : isolated tripodal tetrahedral vanadium (V) oxide. Right : tetrahedral vanadium (V) oxide dimer.

O O O

V

O

O O

OV

O

O O

V

TiO2

Figure 4 : examples of surface chromium species of interest.

O O O

Cr

O O

TiO2

OH2

OH2H

2O

Cr

OH2

H2O

OHH2O

19

of chromium recombination centers. The single-site surface grafting of chromium could

allow the exclusive obtention of visible light sensitizing species while avoiding the

generation of surface recombination sites.

A known way of obtaining such identical and well defined grafted surface

species on a metal oxide surface is Surface Organometallic Chemistry (SOMC).

4) Application of Surface Organometallic Chemistry to titanium

dioxide

a) General principles of Surface Organometallic Chemistry (SOMC)

Surface Organometallic Chemistry

(SOMC) is a field of chemistry that studies

metal complexes bound to the surface of a

metal oxide material via oxygen atoms

bridging between the metal complex and

metal oxide support.56,57 Therefore, said

complexes are immobilized on the surface

of the material, enabling the use of

organometallic complexes in heterogeneous catalysis, especially highly electron-

deficient metal complexes that would otherwise cluster together.57,58

In order to acquire a good comprehension of catalytic reaction mechanisms

involving these complexes and achieve high selectivity, one must perform single-site

catalysis, meaning all metal complexes must be chemically identical.

Figure 5 : general scheme of a surface-bound metal complex

MS

O

Mc

R

X

OO

O

Reactive species

Active site: Metal center and ligands

Surface ligand : the surface is considered as a standard organometallic chemisttry ligand

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b) SOMC requirements

Surface hydroxyl groups are the main grafting sites used for SOMC: graftings

are performed through a nucleophilic substitution reaction between the –OH group and

a ligand of a metal complex, such as a metal alkyl, alkoxide or halide ; with departure

of an alkane, alcohol or hydrogen halide. Therefore, the presence of a single type of

OH group on the surface of a material often means a single type of surface-bound

complex will be obtained, achieving single-site grafting.

In order to be suitable for surface grafting of organometallic complexes, a metal

oxide must therefore bear surface OH groups (preferably all identical), and no other

available nucleophile, such as physisorbed water. Amorphous silica is a suitable

material for SOMC57, and is currently the most used metal oxide for SOMC. Although

this material presents different hydroxyl groups, the selectivity for isolated silanols (Fig.

Figure 6 : Infrared spectrum of SBA silica dehydroxylated under dynamic vacuum (10-8 bar) at 200°C

(black) and 700°C (blue).

The sharp peak at 3740 cm-1 is due to O-H stretching from isolated silanols. The broad peak centered

around 3600 cm-1 is due to O-H stretching from vicinal and geminal silanols.The absorbance has been

normalized to match the isolated silanol peaks intensities

21

3) is controlled by temperature and water partial pressure. At room temperature, silica’s

surface bears mostly vicinal and geminal hydroxyls, along with physisorbed water. By

heating it up under dynamic vacuum (~10-8 bar), one can selectively remove these

species : physisorbed water is removed at 120°C, vicinal and geminal silanols are

mostly present up to 500-600°C59, and at 700 °C, isolated silanols become the largely

majoritary species, as shown by the infrared spectrum in figure 3. Surface hydroxyl

coverage is then of 1.1 OH.nm-2.

If the natures of grafting sites are well known, the amorphous nature of the

support leads to a random disposition of hydroxyl groups, and therefore varying

interactions between grafted complexes, or even complexes bound to the silica surface

by a different number of oxygen atoms (monpodal, bipodal or tripodal). A way to

control the arrangement and distances between grafted complexes as well as their

podality would be through the use of crystalline materials, owing to their fixed and

repetitive geometry. However, the surface of crystalline materials usually displays

varying surface chemical properties, depending on the exposed crystalline facet60:

atoms present on different facets display different geometrical arrangements and

degrees of coordination.

Another metal oxide used for SOMC is γ-alumina. However, this material often

bears different available crystalline facets for which few means of morphological

control are known122. Also, different hydroxyl groups are present on a same facet,

preventing grafting in a well-defined manner.102,103 It is, however, one of the only

crystalline materials on which surface organometallic graftings have been achieved.123

Presently, ways of controlling the morphology of several titanium dioxide

allotropes are known, and knowledge of the surface hydroxyl groups present on their

facets is available61–65. These studies show titanium dioxide could be a suitable

22

candidate for the preparation of a crystalline material on which single-site surface

organometallic chemistry can be performed.

II – Properties of titanium dioxide

1) General properties

Titanium dioxide is a transition metal oxide of which three different crystalline

allotropes are known : anatase, rutile and brookite. All three can be obtained pure by

sol-gel synthesis. Rutile is the most thermodynamically stable allotrope, whereas

anatase is the kinetically favored allotrope using sol-gel syntheses at basic pH. Brookite

is a relatively uncommon allotrope. In all cases, bulk oxygens are bound to three

titanium atoms, and bulk titanium atoms are hexacoordinated. Titanium centers of rutile

show a D2h symmetry, and oxygens show a C2v symmetry. Similarly, Titanium centers

of anatase show D2d symmetry, oxygens also show a C2v symmetry. Brookite is far less

symmetrical, with all Ti-O bonds around a same Ti atom having different lengths, which

confers it a Cs symmetry. Two different types of oxygen atoms are present in its

structure, both showing a Cs symmetry.

rutile anatase brookite

Crystalline

system : Tetragonal Tetragonal Orthorhombic

Unit cell :

Table 1 : crystal structures of titanium dioxide allotropes

23

To the best of our knowledge, hydrothermally synthesized rutile samples only bear {110}

and {111} facets.31,68–70 Of those two facets, water chemisorption has only been studied

on the first one. Most of those studies seem to show water does not dissociate on that

facet, or is in an equilibrium between dissociative and non-dissociative

chemisorption.71 Therefore, this allotrope does not seem fit for our purposes. In the case

of brookite, no means of morphology control are known. It will therefore not be studied

here.

Regarding anatase, three different facets are frequently seen: the {101}, {100}

and {001} facets. Several theoretical studies investigating the chemisorption of water

on each of those facets are available. There are known means of efficiently controlling

the morphology of anatase using hydrothermal syntheses, and therefore the proportions

in which each facet is present. For these reasons, the focus of the following work will

be on the surface chemistry of anatase allotrope.

2) Surface chemistry of the different crystalline facets exposed

on anatase

The four exposed facets of anatase obtained by hydrothermal synthesis are the {101},

{001}, the relatively unusual {100}, and the {110}, which, to the best of our knowledge,

has only been reported once.72

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a) Surface chemistry of the {101} facet

― Nature and density of surface groups on dehydroxylated surface

The {101} facet is the most

common (~90% of thermodynamic

anatase surface). It exhibits both TiV and

TiVI (5 atoms.nm-2 for each) as well as µ2-

O (5 atoms.nm-2) and µ3-O (10 atoms.nm-

2) sites. It has the smallest density of

undercoordinated atoms of all facets, and

consequently the lowest surface energy, calculated around 0.4 - 0.49 J.m-2 when fully

dehydroxylated.65,73,74

According to DFT calculations, water molecules are chemisorbed on this

surface, most likely without dissociation, even though dissociated and non-dissociated

states could compete within less than 7 kJ.mol-1.61,73

― Thermal desorption of water

This facet would be the easiest to dehydroxylate, losing all water at 180°C under

1 atm of water, and being already dehydroxylated at ambient temperature under 10-2

atm of water vapor.61,73

― Nature and density of surface groups on hydroxylated surface

For coverage up to 5 H2O.nm-2, oxygen free doublets from the water molecules

Figure 7 : Clean anatase {101} surface. TiVI and TiV are respectively light and dark blue, μ3O and μ2O are respectively light and dark red. Dashed lines are bonds towards the bulk

25

molecularly adsorb non dissociatively on undercoordinated TiV atoms, leading to

surface TiV-OH2 groups. For higher coverages, these sites being saturated, water

molecules develop hydrogen bonds with the surface µ2-O sites . Saturation coverage is

then reached at 10.1 H2O.nm-2 61,73

It has also been shown by DFT that water could absorb dissociatively on defect

sites of this facet, generated by missing surface oxygen atoms when the material is

partly reduced.106,107

― Experimental evidences

The aforementioned DFT calculations predicts that the O-H stretching

vibrational motions of chemisorbed water on this surface should cause two infrared

absorption bands, respectively at 3665 and 3646 cm-1, since both hydrogen atoms of a

chemisorbed water molecule are in different positions relative to surface atoms. The H-

O-H bending mode is predicted around 1565-1646 cm−1 .61 Experimentally, two bands

at 3670-3676 and 3640-3642 cm-1 have been observed on anatase samples73, as well as

a third one at 1620 cm-1, showing similar wavenumbers to those calculated by DFT.

Furthermore, the intensity of these bands has been shown to increase relatively to other

O-H stretching bands on samples bearing a higher proportion of {101} facet. 73

Thermally programmed desorption studies conducted on thermodynamic

anatase103,104 have shown two desorption peaks, one at 370 K for coverages above 2.2

H2O.nm-2, and one at 470 K at all coverages. This can be explained by physisorbed

water desorption from the 101 facet at 370 K, followed by chemisorbed water

desorption at 470 K

Scanning Tunneling Microscope observations of hydroxylated {101} anatase

surface have shown non-dissociated water molecules adsorbed next to each other, along

TiV rows124.

26

b) Surface chemistry of the {001} facet

― Nature and density of surface groups on dehydroxylated surface

The 001 facet seems to be by far

the most interesting for our purposes. It

shows a high density of TiV sites (6,74

atoms.nm-2), along with µ2-O and µ3-O

sites (also 6,74 atoms.nm-2 for each). Its

surface energy is the highest of all three considered surfaces, at 0.88-0.98 J.m-2,65,73,74

due to its high density of insaturated atoms.

― Thermal desorption of water

This facet would retain water at much higher temperatures than the {101} : at

pH2O/P0 =10-3 , it would still bear 3.5 H2O.nm-2 at 550 K (277 °C), and would not be

dehydroxylated before 700 K (427°C). A 3.5 H2O.nm-2 coverage should in theory be

achieved at temperatures between 120 and 400°C at 1 atm water

― Nature and density of surface groups on hydroxylated surface

For coverages up to 3.5 H2O.nm-2, it bears

exclusively µ1-OH groups from

dissociative adsorption followed by lattice

Ti-O bond breaking : The formed OH

groups are in pairs, bonded by a strong

hydrogen bond, and replacing a surface

Figure 9 : partly hydroxylated (001) surface at 3.5 H2O/nm². Dashed lines are bonds towards bulk atoms. Titanium atoms are in blue, oxygen atoms are in red.

Figure 8 : Clean anatase {001} surface. Dashed lines are bonds towards bulk atoms

27

µ2-O.

― Experimental evidences

The predicted O-H vibrational wavenumbers for the surface hydroxyls

described above are 3746-3751 cm-1 for the isolated proton, and around 2300 cm-1 for

the hydrogen-bonded proton, although doubts exist regarding that last value. IR spectras

of anatase bearing some {001} facets have shown a peak measured at 3736 cm-1, related

to free O-H stretching, that increases with the proportion of {001} facet. The 2300 cm-

1 band has, however, never been reported. 61,73

Therefore, this surface seems ideal for organometallic grafting, especially

bipodal : all chemisorbed water is dissociated, surface hydroxyls would be grouped

in pairs, and the distance between the two titanium atoms beneath an OH pair is

of 3.8 Å, versus 3.1 Å for the Si-Si distance on silica, which should allow the

grafting of slightly larger metal atoms. The hydroxyl coverage is also very high, at

7 OH/nm2.

28

3) Synthesis of anatase nanocrystals with morphology control

a) General principle of hydrothermal synthesis

The sol-gel process being the only method through which efficient control of

the morphology of anatase has been achieved, all samples will be synthesised through

this method.

The way hydrothermal precipitation proceeds from metal alkoxydes is through

their hydrolysis followed by the condensation, nucleation and growth of the resulting

hydrolysis products to lead to the desired oxides.

-Hydrolysis

When a metal alkoxide M(OR)x is placed in the presence of water, the

nucleophillic character of water will lead to substitutions between the alkoxide ligands,

in ways depending on the acidity of the solution : At very low pH, aquo ligands (M-

OH2) will be present in the coordination sphere, and will be increasingly deprotonated

into hydroxo (M-OH) ligands at higher pH. Aquo or hydroxo ligand addition can also

lead to an increase in metal coordinance when it is thermodynamically favoured.

Ti OR H2O+ Ti OH HO+ R

29

-Condensation

Complexes bearing hydroxo ligands can condense through two different

mechanisms :

Olation occurs when a hydroxo ligand of a metal centre will form a bridging

OH group between two metals with the departure of a highly labile aquo group :

Oxolation occurs in the absence of aquo ligands being initially present : a

hydroxo ligand can still form a bridging OH group between two metal centres. The

proton present on the bridge is then transferred to another non-bridging hydroxo ligand,

which departs the metal as a water molecule:

-Nucleation

Once a hydroxyl group bearing metal complex able to polymerize are present

in the solution, whether the growth of a crystallite from an oligomeric specie is

thermodynamically favoured depends on two factors :

-Its bulk energy : species in the bulk of a crystalline phase are more stabilized

than in solution. Generating more bulk species by condensation is therefore

thermodynamically favoured.

-Its interfacial energy : undercoordinated species on the surface of a crystal are

less stabilized than species in solution. The increase in particle surface that

Ti TiOH H2O+ Ti TiOH

H2O+

Ti TiOH HO+ Ti TiOH

H2O+

OH

Ti TiO

30

condensation causes is therefore thermodynamically unfavoured.

Consequentlly, the condensation of dissolved species into small particles with a

high surface/volume ratio is thermodynamically unfavoured, until the particle reaches

a critical size where its bulk energy overcomes its surface energy and growth becomes

favorable. The formation of such particles, or nuclei, is called nucleation. It takes place

once the metal concentration in solution is high enough above saturation. From then on,

growth happens by addition of metal complexes from the solution on the metal oxide

surface.

b) Equilibrium shape of a crystal

Particle growth favors the most stable facets :

the crystal’s morphology will tend towards the shape

offering the lowest overall surface energy possible.

Said surface energy is given by the following

equation :

∆Gi=∑(j)γjA j

With ∆Gi (J) the difference between the

energy of a crystal made of i molecules and the energy

of the same number of molecules in the bulk of an

infinite crystal ; γj (J.m-2) the surface energy of the jth

facet of the crystal, and Aj (m2) the area of that facet.

Therefore, the crystal’s shape is calculated from the energies of its different

possible facets through what is known as the Wulff construction: for each possible facet,

a vector is drawn, starting from the center of the crystal and normal to that facet, with

Figure 10 : Projection along the [010] axis of the Wulff construction of an Anatase crystal. Red : (001) plane (0.98 J.m-2), Green : (101) plane (0.44 J.m-2), Blue : (100) plane (0.53 J.m-2)

{001}

{100}

0.9

8 J

.m-2

0.53 J.m-2

31

a length proportional to the facet’s energy. The remaining volume, enclosed within all

planes, is the ideal shape of the crystal. Consequently, morphology control is achieved

by selective stabilization of the desired facet. This can be done through the addition of

species in solution with the ability to selectively adsorb on the Lewis acidic and Lewis

basic sites that surface undercoordinated atoms constitute, changing its surface energy.

Changing the temperature at which the particle grows can also affect the chemisorption

equilibriums of species on its surface, thereby affecting its morphology.65

In the case of the anatase polymorph, The three lowest energy known facets are

the {101}, {001} and {100} facets, with calculated respective energies of 0.44 ; 0.53

and 0.98 J.m-2 when dehydrated with no species coordinated to them.65,73 These values

compare to the densities of undercoordinated atoms on each facet (respectively 5,0 ; 5,5

and 6,74 TiV.nm-2), and the Wulff construction obtained from those values also seems

to be in reasonable agreement with experimentally measured shapes of thermodynamic

anatase, as it would predict crystals with 98% (101) surface, 2% (001) surface, and no

(100) surface (shown in fig. 9).

32

c) Methods for the controlled synthesis of faceted anatase

nanocrystals.

A way of controlling the

morphology of hydrothermally prepared

anatase crystals is by addition of

fluorides in solution. Fluorides can be

absorbed on any facet’s TiV atoms,

thereby lowering their energy. However,

their stabilizing effect is much more

important on the {001} facet, and, at low

temperatures, the {100} facet. It is

therefore possible, by using high enough

fluoride concentrations, to obtain anatase with majoritary {001} facets.42,63,65,72,75–78

Aminoacids have also been successfully used for controlling the morphology of

anatase : Histidine has shown to slightly favour the {001} facet, while Glutamine and

Asparginin lead to almost exclusively {101} facets. 79

Methods for increasing the proportion of {001} facet are summarized in table 2.

Figure 11 : titania morphology variation after hydrothermal synthesis depending on synthesis temperature and fluoride concentration. Adapted from J.Mater.Chem., 22, (2012) 23906-23912

33

Table 2: Hydrothermal and solvothermal synthesis techniques for {001} majoritary anatase

Fluorides being by far the most documented and efficient way to control the

morphology of titanium dioxide in a way that favours the desired 001 facet, our

synthesis method will be the hydrolysis of tetrabutyltitanate by HF/H2O.This process

has previously been determined to lead to around 89% of 001 facet on the obtained

anatase.75

Reference Starting material

Solvent Morphology control agent

Other chemicals present

Hydrolysis temperature

Highest {001} surface

proportion

42 TiF4 tBuOH, BzOH

(F- from TiF4) / 160°C 65%

72 Ti metal H2O HF H2O2 180°C ND 75

128

129 Ti(OBu)4 / HF / 180°C 89%

65 TiCl4 H2O NaBF4 HCl 160°C 55% 76 TiF4 H2O HF / 180°C 47% 77 TiF4 iPrOH HF HCl 180°C 64%

78 TiF4 1-

octadecene 1-octadecanol (F- from TiF4)

Oleic acid 290°C ~100%

80 TiF4 BuOH HF HCl 210°C 98.7%

130 TiF4 Diethylene

glycol (F- from TiF4) 180°C 90%

131 Ti(OiPr)4 Acetic acid

1-butyl-3-methylimidaz

olium tetrafluoro

borate

H2O 25°C 31.4%

132 TiF4 H2O (F- from TiF4) Disodium

EDTA 200°C 22.8%

133 Ti metal H2O HF 180°C ND

79 TiCl4 H2O Histidine HCl,

NaOH 60°C ND

34

III – Objectives and Strategy

Our long term objective is to achieve stable, well-defined and uniform grafting

of metal complexes on the surface of titania. We must therefore obtain a specific titania,

with a high surface area and bearing an important density of identical TiV-μ1OH surface

hydroxyl groups.

As the {001} facet of anatase seems to possess the desired surface chemistry

towards water chemisorption, showing exclusively dissociative chemisorption and high

coverage and stability of the resulting hydroxyl groups, it will be the main focus of our

efforts.

In order to minimize the influence of other facets and obtain a high density of

surface hydroxyls, we will first attempt to synthesize pure anatase, with a high surface

area, dominated by the {001} facet.

We will then apply different pre-treatments to the obtained anatase samples in

order to selectively remove its surface physisorbed water without causing sintering or

morphology reconstruction. Sample stability and surface chemistry will be studied

depending on the pre-treatment conditions. For comparison, thermodynamic anatase

will be studied in the same conditions. Finally, as a proof of concept, we will attempt

the grafting of metal complexes, such as VOCl5.

35

IV – Experimental section

1) Hydrothermal syntheses

a) {001}-anatase

Anatase with majoritary {001} facets exposed was successfully synthesised

through the following method75: tetrabutyl orthotitanante (Ti(OBu)4), 560 mL, was hy-

drolysed with 90 mL 47% hydrofluoric acid in water. Hydrolysis ratio was 2 H2O : 1

Ti : 1.2 F. Product then underwent hydrothermal treatment in a 1 l autoclave at 180°C

for 24h for the crystallization to occur. Once the hydrothermal phase was finished, the

product was rinsed three times with 500 mL water, to remove the large excess of HF.

This sample will be referred to as “{001}-anatase”.

In order to remove remaining surface fluorides, sample {001}-anatase-H2O was

prepared by repeatedly rinsing sample {001}-anatase with milli-Q water, until the con-

ductivity of the rinsing solution became lower than 10 µS.cm-1.

Sample {001}-anatase-NH4OH was prepared by rinsing sample {001}-anatase

ten times with a 1M aqueous solution of ammonia, followed by repeated rinsing with

milli-Q water, until the conductivity of the rinsing solution became lower than 10

µS.cm-1.

Samples {001}-anatase-base-100°C were prepared by autoclaving 5 g of sam-

ple {001}-anatase-NH4OH in 500ml of a 1M aqueous base at 100°C for 24h, followed

by repeated rinsing with milli-Q water, until the conductivity of the rinsing solution

became lower than 10 µS.cm-1.The base was chosen among NH4OH, KOH and NaOH.

They produced respectively the samples {001}-anatase-NH4OH-100°C, {001}-ana-

tase-KOH-100°C and {001}-anatase-NaOH-100°C

36

b) {101}-anatase

Thermodynamic anatase (with majoritary {101} facets) was successfully syn-

thesized through the following method75 :

A 55 mL titanium oxychloride starting solution was made by hydrolyzing dropwise 20

ml TiCl4 with 25 ml H2O in an ice bath (H2O:Ti 2.5:1). Separately, a 545 mL solution

of 8.4 M NH4OH and 0.9 M NH4Cl was prepared. Both solution were then mixed under

vigorous stirring. Precipitation immediately occurred. Final pH was measured at 10.36

The precipitate was then centrifuged out, and placed back in suspension in 600

mL milli-Q water. Measured pH was 10.002, conductivity was 18.2 mS.cm-1. The sus-

pension was then autoclaved at 180°C for 24h for the crystallization to occur. The prod-

uct was then repeatedly rinsed and centrifuged until the conductivity of the rinsing so-

lution was below 10 µS.cm-1. This sample will be referred to as “{101}-anatase”.

2) Dehydroxylation protocol

Dehydroxylations were performed by placing the sample in a quartz tube

with an adapter on each end and a frit in the middle. A dynamic vacuum of 10-8 bar is

then applied using a high vacuum line, and the sample is heated at the desired

temperature using a tubular oven. The tube is then opened in a glove box, where some

of its content can be separated for further characterization.

Due to the labile nature of titanium dioxide’s lattice oxygens, samples

obtained after dehydroxylation were partly reduced. Reoxidations were therefore

performed by applying to the same setup a flow of 20%O2 and 80% Ar mix, dried over

a molecular sieve.

37

3) Grafting protocol

Grafting was performed by placing 0.3g of the titanium dioxide support in

suspension in 10 ml of dry pentane under an inert atmosphere. An excess of vanadium

oxychloride (5 ml) is then added to the suspension. The reaction medium is left under

agitation for 24 hours. The pentane containing the excess metal is then filtered out using

a cannula, and the support is washed five times using additional dry pentane. The

sample is then dried at room temperature under dynamic vacuum (10-8 bar) for 12 hours,

and finally collected and kept under inert atmosphere and protected from UV light.

4) Analytical techniques

DRIFTS infrared spectras were acquired using a Nicolet 6700 FT-IR spectrometer with

an airtight argon-filled IR cell with ZnSe windows.

1H MAS solid-state NMR spectra were acquired on a 600 MHZ NMR spectrometer

with 2.5mm rotor under 20 KHz MAS spinning frequency, number of scans=8,

repetition delay = 5s

2D 1H-1H double quantum and 1H-1H triple quantum NMR spectras were acquired on

a 600 MHz NMR spectrometer under 22KHz MAS spinning frequency with a back-to-

back recoupling sequence, number of scans =32, repletion delay =5 s number of t1

increments =128, with the increment set equal to one rotor period of 45.45 μs

19F MAS solid-state NMR spectra were acquired using direct polarization (DP) on a

38

600 MHZ NMR spectrometer with 2.5 mm rotor under 20 KHz MAS spinning

frequency, number of scans=2000, repetition delay = 15s

2D CP/MAS 1H-19F HETCOR NMR spectra were acquired with contact times of 0.2

ms under 20 kHz MAS, number of scan per increment = 2000, repetition delay = 4 s,

number of t1 increments = 32, line broadening = 80 Hz

51V MAS NMR as determined by variation of the spinning frequency was acquired on

900 MHz NMR spectrometer under a 20 KHz and 18 KHz MAS spinning frequency,

repetition delay =0.25s

X-ray diffractograms were acquired on a Bruker D8-Advance diffractometers

Thermogravimetric Analysis was performed using a Mettler Toledo TGA/DSC1

STARe System. Ramp rate : 10°C/min, air flow : 50 ml/min

BET surface area and nitrogen adsorption measurements were done using a

Micromeritics ASAP 2420 surface area and porosity analyser.

39

V – Results and discussion

1) Morphology and bulk properties

The XRD patterns of {101}-anatase and {001}-anatase-NH4OH are diepicted

on figure 12. For both samples, XRD showed exclusively anatase diffraction peaks

(Fig.1) . The particles dimensions, measured along the [004] and [200] directions, were

respectively 12 nm and 69 nm for sample {001}-anatase-NH4OH, against 35 and 24

nm for sample {101}-anatase, indicating the expected anisotropy.

20 40 60 80

36.0 36.5 37.0 37.5 38.0 38.5 39.0 39.5 46.5 47.0 47.5 48.0 48.5 49.0 49.5 50.0

{103} {112}

{101}-anatase {001}-anatase-NH

4OH

Inte

nsity

(A

.U)

2θ (deg)

{004} {200}

Figure 13: X -ray diffractograms of the as-synthesised samples

Sample {001}-anatase {101}-anatase

ε[001] (nm) 12 35

ε[100] (nm) 69 28

Table 3: Particle dimensions along the [001] and [100] axis of the particles, determined by XRD.

40

Transmission Electron Microscopy showed for both samples that the {001} and

{101} facets were the only facets exposed, as shown in figure 2 : Miller indexes of

different spots on fast Fourrier transforms can be deduced from the distances between

atom planes. The only facets present on the particles are perpendicular to the [001] and

[101] directions. Particle maximum width and length measured along the [100] and

[001] axes are respectively 20 and 29 for sample {101}-anatase, and 4.7 and 43 nm for

sample {001}-anatase-NH4OH, showing 81% {001} surface has been obtained.

The difference between the dimensions of sample {001}-anatase measured on

TEM micrographs and those determined by X-ray diffraction can be explained by the

observed stacking of particles with their atom planes aligned, increasing the observed

coherent domain on X-ray diffraction.

Figure 14 : a and b : TEM images of respectively {101}-anatase and {001}-anatase,

with facet attributions based on Fast Fourier Transforms.

Sample {101}-anatase {001}-anatase

Avg. {100} width (TEM) (nm) 20 43

Avg. {001} width (TEM) (nm) 29 4.7

Theoretical surface area (TEM) (m²/g) 54 130

Table 4 : Particle dimensions measured by TEM and calculated surface area

41

2) Textural properties

Textural properties of the samples were investigated by nitrogen physisorption.

The surface areas of {001}-anatase and {101}-anatase were determined to be respec-

tively 92 m2.g-1 and 58 m2.g-1. Pore volumes were respectively of 0.54 and 0.34 cm3g-1

The measured surface area of {001}-anatase, 30% lower than that calculated

from crystallite dimensions, can be explained by the fact that particle aggregation ren-

ders part of the surface inaccessible to nitrogen molecules.

BJH plots show that most pores on sample{001}-anatase are approxiamtely 40

nm wide, corresponding with the length of anatase platelets, while the H1 hysteresis

shows no slit-like pores. The porous volume would therefore be located between plate-

let aggregates.

Most pores of sample {101}-anatase are within 20 to 30 nm, corresponding with

particle dimensions. The porous volume would therefore be located between individual

crystallites.

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500 {001}-anatase {101}-anatase

Qty

ads

orbe

d (c

m3 /g

)

P/P0

0 200 400 600 800 10000.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

dV/d

D (

cm3 /g

)

Pore diameter (Å)

{001}-anatase {101}-anatase

Figure 15 : N2 physisorption isotherms (left) and BJH plots on desorption branch (right) of {001}-anatase and {101}-anatase

42

3) Sample stability during dehydroxylation post-treatment

In order to remove physisorbed water from the obtained samples, a dehydrox-

ylation post-treatment has to be applied. Similarly to what is done to silica, samples

were heated under dynamic vacuum (10-8 bar). The stability of sample {001}-anatase-

NH4OH under those conditions is studied here.

X-ray diffractograms showed no changes in the coherent domains of the sam-

ples for dehydroxylation temperatures up to 400°C, followed by a morphology recon-

struction starting between 400°C and 500°C. This is shown by an increase in the co-

herent domains on both the {100} and {001} directions, as well as an increase in the

e(001)/e(100) ratio, indicating an evolution of morphology towards that of thermody-

namic anatase. No rutile formation could be seen.

This was confirmed by TEM images, on which anatase crystallites show distinct

octahedral geometries after dehydroxylation at 500°C and reoxidation. In addition to

that, sample surface reconstruction can be seen starting between 350°C and 400°C, as

indicated by the presence of clear patches on the crystallites. No changes can be seen

below 350°C.

43

20 40 60 80

500°C 400°C 350°C 300°C 200°C 100°C

Inte

nsity

(A

.U)

2θ

Dehydroxilation temperature :

Figure 16 : X-ray diffractograms of {001}-anatase-NH4OH after heating under dynamic vacuum (10-8 bar) for 20h at varying temperature, and reoxidation for 10h in 20% O2 in Ar at 200°C

0 100 200 300 400 50014

16

18

20

22

24

26

(004

) co

here

nt d

omai

n [n

m]

e(004) e(200) e(004)/e(200) ratio

Post-treatment temperature

48

50

52

54

56

58

60(1

00) co

here

nt d

omai

n [n

m]

Stable morphologyMorphologyreconstruction

0.0

0.1

0.2

0.3

0.4

Rat

io

Figure 17 : Average coherent domains along the (001) and (101) directions of {001}-anatase-NH4OH (measured from XRD data) in function of dehydroxylation temperature

44

Figure 18: TEM images of {001}-anatase-NH4OH a) at 25°C, and after

dehydroxylation at b) 200°C, c) 300°C, d) 350°C, e) 400°C and f) 500°C

a) b)

c) d)

e) f)

45

Nitrogen adsorbtion shows a decrease in sample surface area starting between

350°C and 400°C, followed by a decrease in total porous volume starting between 400

and 500°C. This can be explained by an increase in particle aggregation between 350°C

and 400°C, followed by the morphology reconstruction previously shown by TEM and

XRD between 400°C and 500°C.

0 100 200 300 400 5000

10

20

30

40

50

60

70

80

90

100

BET surface area Total porous volume

Temperature (°C)

SB

ET (m

2 /g)

0.0

0.2

0.4

0.6

0.8

1.0

Vto

t (cm

3 /g)

Figure 19 : Evolution of sample {001}-anatase-NH4OH surface area and porous volume in function of dehydroxylation temperature under vacuum (20h, 10-5 bar)

Thermogravimetric analysis shows an important weight loss between room tem-

perature and 120°C, which can be explained by the departure of physisorbed water. It

is followed by a slower weight loss between 120°C and ~550°C, which we attribute to

water elimination from the condensation of surface hydroxyl groups. A small increase

in weight loss slope is seen between 450°C and 500°C. This can be attributed to mor-

phology reconstruction, causing a decrease in the number of surface hydroxyl groups.

46

200 400 600

-4

-2

0W

eigh

t los

s (%

)

Temperature (°C)

Phy

siso

rbed

w

ater

loss

Water loss from surfacehydroxyl condensation

Figure 20 : Thermogravimetric analysis of sample {001}-anatase-NH4OH (ramp rate : 10°C/min, flowing air 10ml/min)

4) Characterization of surface chemistry

a) NMR spectroscopy

Several NMR techniques were used to determine the nature of the different

surface species present on the {001}-anatase-NH4OH sample : 19F NMR, 1H NMR,

1H double and triple quantum NMR, and 1H-19F heteronuclear correlation NMR.

19F NMR showed a single peak at -123.45 ppm that does not seem to be

affected in any way by post-treatment temperature. It is believed, from XPS results

shown in figure 27 and for reasons detailed below, that this peak corresponds to

surface fluorides.

47

Figure 21 : 19F NMR of sample {001}-anatase-NH4OH after dehydroxylation (20h,

10-8bar, varying temperature) and reoxydation (20h, flowing 20% O2 in Ar, 200°C)

Proton NMR, on the other hand, shows a main peak which intensity decreases

as post-treatment temperature increases. It is, however, still present after the sample

has been dehydroxylated at 500°C. Its chemical shift, starting at 5.67 ppm,

progressively increases up to 6.98 ppm as the post-treatment temperature is increased.

A smaller peak is also present at 0.8 ppm, and shows no noticeable change in

shift or intensity.

48

Figure 22: 1H NMR of {001}-anatase-NH4OH after dehydroxylation (20h, 10-8bar,

varying temperature) and reoxydation (20h, flowing 20% O2 in Ar, 200°C)

In order to get more informations on the nature of those peaks, 2D 1H-1H

correlation (double quantum and triple quantum) NMR spectras were acquired (fig. 11).

They show that on the samples where post-treatment temperature did not exceed 100°C,

protons on the 5.67 ppm peak are within correlation distance from each other (<5Å),

indicating the contribution of species bearing two protons in close proximity. The

correlation dissapears when the post-treatment temperature is of 200°C or above. The

0.8 ppm peak also appears in double quantum NMR, regardless of post-treatment

temperature.

49

Triple quantum NMR of the {001}-anatase-NH4OH sample after post-treatment

at 200°C shows that the peak at 0.8 ppm is due to species bearing at least three protons.

Remaining butanol from the hydrolysis of Ti(OBu)4 could explain its presence.

Figure 23 : Double quantum 1H NMR of {001}-anatase-NH4OH: a) Room temperature

sample, and after dehydroxylation at b) 100°C, c) 200°C, d) 300°C followed by

reoxydation at 200°C in flowing 20% O2 in Ar.

Figure 24 : 1H NMR of {001}-anatase-NH4OH dehydroxylated at 200°C under vacuum

and reoxidized :double quantum (left) and triple quantum (right)

a) 25°C b) 100°C

c) 200°C d) 300°C

50

To determine whether surface hydroxyl groups were in physical proximity with

fluorine groups, proton-fluorine heteronuclear correlation spectras were acquired.

No correlation was observed on {001}-anatase-NH4OH after post-treatment at

200°C. However, the more fluorine-rich sample {001}-anatase-H2O showed a 19F-1H

correlation with protons exhibiting a 5.8 ppm shift, while its 19F spectrum still showed

the same single peak at -123.45 ppm. (fig. 13)

The value of the 1H NMR main peak’s chemical shift (5.67 ppm) falls in the

range of what is expected for physisorbed water. Indeed, no additionnal peak is

observed when no dehydroxylation post-treatment above 100°C was applied to the

sample, and water is therefore present, as confirmed by double quantum NMR.

However, the peak is still present after the sample was dehydroxylated at temperatures

from 200°C and up to 500°C, where no physisorbed water can be present, as confirmed

by double quantum NMR. Therefore, this peak should be constituted of at least two

contributions : that of physisorbed water (when post-treatment temperature are below

200°C), and that of chemisorbed water.

19F-1H correlation NMR of the {001}-anatase-H2O after dehydroxylation at

200°C shows that protons of the 5.8 ppm peak of that sample are within correlation

distance (5 Å) of the fluorine atoms. Since no noticeable difference in the nature of the

fluorine and hydrogen atoms between the dehydroxylated {001}-anatase-H2O and

{001}-anatase-NH4OH can be seen from either their 1H and 19F NMR spectras, as well

as their IR spectras (shown below), it can be supposed that the absence of 1H-19F

correlation observed on {001}-anatase-NH4OH could be caused by an excessively

weak signal from the fluorine atoms rather than excessive distance between them. The

51

increase in number of fluorine atoms on the surface of {001}-anatase-H2O would then

make the correlation strong enough to be visible.

Figure 25: 1H-19F heteronuclear correlation of {001}-anatase-H2O after

dehydroxylation (20h, 10-8bar, 300°C) and reoxidation (10h, flowing 20%O2 in Ar,

200°C)

52

b) Infrared spectroscopy

Infrared spectras of sample {001}-anatase-NH4OHwere made after the samples

were dehydroxylated at 100, 200, 300, 350, 400 and 500°C ; and then reoxidized at

200°C in a flow of 20% oxygen in argon.

The main features of these spectras are one peak at 3666 cm-1 that can be at-

tributed to surface O-H stretching,61,73 and a broad peak at 2388 cm-1. The baseline

change of the sample dehydroxylated at 500°C is due to incomplete reoxidation of the

oxygen vacancies

Other features are a minor O-H stretching peak at 3717 cm-1 and, two small

peaks at 3487 and 3397 cm-1 that can be attributed to N-H stretching from residual

ammonia,127 a peak at 1618 cm-1, that can be attributed to H-O-H bending from phy-

sisorbed water (below 200°C) or non-dissociatively chemisorbed water 61,73. All peaks

below 1600 cm-1 do not appear on sample {001}-anatase-H2O, which was not rinsed

with ammonia. They can therefore be attributed to nitrogen species: a peak at 1560 cm-

1 can correspond to surface NH2 bending from residual ammonia125, and a peak at 1352

cm-1 could be explained by the presence of nitrate ions from the oxidation of said am-

monia126.

The breadth of the 3150 cm-1 would indicate protons engaged in hydrogen bonds

with other nearby atoms. That could be due to surface hydroxyl groups bonding with

surface oxygen atoms, with surface fluorines, or with each other. Another explanation

would be the presence of titanium vacancies in the bulk and on the surface of TiO2,

charge-compensated by protons.

53

4000 3500 3000 2500 2000 1500 1000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

500°C 400°C 350°C 300°C 200°C 100°C

Abs

orba

nce

Wavenumber (cm-1)

Dehydroxylation temperature :

4000 3500 3000 2500 2000 1500 10000.0

0.5

1.0

1.5

1353 cm-1

1620 cm-1

300°C

3152 cm-1

3397 cm-1

3487 cm-1

Abs

orba

nce

Wavenumber (cm-1)

3666 cm-13717 cm-1

Figure 26 : Top : Infrared spectras of {001}-anatase-NH4OH after dehydroxylation (20h, 10-8bar, varying temperature) and reoxydation (10h, flowing 20%O2 in Ar, 200°C)

Bottom : Infrared spectrum of {001}-anatase-NH4OH after dehydroxylation (20h, 10-8bar, 300°C) and reoxydation (10h, flowing 20%O2 in Ar, 200°C)

To determine the accessibility of protons constituting each IR peak, an

isotopic exchange experiment was conducted: the sample {001}-anatase-NH4OH was

first dehydroxylated at 300°C in a dry flow of 20% Oxygen in Argon. A first infrared

spectrum was acquired. We then proceeded to the isotopic exchange with D2O: the

54

sample temperature was brought down to 150°C, and the oxygen/argon flow was first

bubbled in deuterium oxide before passing through the sample. The exchange was al-

lowed to occur for 24 hours. The sample was then dehydroxylated again at 300°C in a

dry flow of 20% oxygen in argon. A second infrared spectrum was acquired. Both spec-

tras are reported in figure 15.

The results show that every peak previously attributed to O-H stretching (seen

here at 3715 ; 3661 and 3170 cm-1), as well as the peak attributed to H-O-H bending

(seen here at 1622 cm-1) all shifted simultaneously at respectively 2737, 2701, 2388 and

1569 cm-1. This would indicate that all protons on the sample are accessible to deuter-

ium cations.

4000 3500 3000 2500 2000 15000

1

2

3

3715 cm-1

1266 cm-1

1432 cm-1

1510 cm-1

1569 cm-1

2388 cm-1

2701 cm-1

1560 cm-1

1622 cm-1

3170 cm-1

Before isotopic exchange After isotopic exchange

Abs

orba

nce

(offs

et)

Wavenumber (cm-1)

3661 cm-1

3715 cm-1

2737 cm-1

Figure 27 : In-situ DRIFTS spectrum of {001}-anatase-NH4OH after dehydroxylation at 300°C under flowing 20% O2 in Ar (black) ; followed by isotopic exchange of with D2O, and dehydroxylated again at 300°C under flowing 20% O2 in Ar (red)

55

5) Fluorine removal

As the synthesis was carried out in a fluorinated medium, it is expected that

remaining fluorine atoms will still be present on the surface

Particle dimensions Width (along (001) axis) : 4,68 nm

Length (along (100) axis) : 42,8 nm

Surface area

Total surface area 134 m2/g

001 surface area 109 m²/g (81%)

101 surface area 25 m²/g (19%)

Surface fluorides

Maximum surface fluorides (One F on each TiV)

1,47 mmol.g-1 / 2,8 %wt.

Max. (001) surface fluo-rides

1,25 mmol.g-1 / 2,4 %wt.

Max. (101) surface fluo-rides

0,22 mmol.g-1 / 0,4% wt.

Elementary analysis showed that the sample {001}-anatase, on which no wash-

ing was applied initially contained 6.5% wt. fluorine. This very high value could be

explained by the possible presence of physisorbed HF on the surface of the sample, or

by the presence of fluorides in the bulk of the material.

To determine whether all fluorides are on the surface or some are in the bulk,

an XPS analysis was performed. According to several publications, XPS of fluorinated

TiO2 allows to differentiate surface fluorides from bulk fluorides. The first give an F 1s

signal at 684.1 eV, while the second give an F 1s signal at 689.6 eV.65,76,77,81–83

56

The spectrum obtained from the as-synthesised sample showed a single peak at

684.1 eV, indicating only surface fluorides are present, in amounts large enough to

completely saturate the surface.

Two different methods for removing fluorides from the surface of titanium di-

oxide have been reported. The first one is calcination at 600°C for 90 minutes63,77,84,85.

It was attempted by spreading the {001}-anatase in an aluminum oxide crucible,

and heating it in a muffle furnace at 600°C for 90 minutes, under static air, with a ramp-

ing rate of 5°C/min, then letting it cool down in open air. XRD analysis showed the

morphology was highly altered by the treatment, making it inapplicable to the samples

studied here.

The second method is rinsing with an aqueous base24,75,82, to substitute fluoride

ions by hydroxyl ions. Here, aqueous ammonia was chosen as the base, as it is easier to

remove from metal oxide surfaces than alkali metals. The sample was rinsed 10 times

with a 1M solution of ammonia, then with water until σ < 10 μS.cm-1, to remove the

ammonia. As a comparison standpoint, an other sample was thoroughly rinsed with

pure water only, until σ < 10 μS.cm-1.

Elemental analysis showed {001}-anatase-H2O, rinsed exclusively with water

contained 2.6% wt. fluorine, a value in accordance with the one predicted in table 2 for

fully fluorinated TiO2. The ammonia-rinsed sample {001}-anatase-NH4OH contained

1% wt. fluorine. This value could fit with the fluorination of half of the titanium atoms

present on the 001 face. XPS again showed a single peak at 684.1 eV, though of lower

intensity than previously.

57

695 690 685 680 675

6000

8000

{001}-anatase {001}-anatase-NH4OH

Inte

nsity

(A

.U)

Binding energy (eV)

684.1 eV F 1s

Figure 28 : XPS spectra of {001}-anatase and {001}-anatase-NH4OH

Further defluoration could be achieved by autoclaving the obtained {001}-ana-

tase-NH4OH at 100°C in solutions of other bases. The effect of those treatments on

fluoride concentration are reported below, in table 3.

58

Sample name Rinsing method Fluorine content (wt. %)

Other ele-ments (wt.%)

{001}-anatase No rinsing 6.5

{001}-anatase-H2O

Rinsing with water until σ<10 µS.cm-

1 2.6

{001}-anatase-NH4OH

Rinsing with 1M ammonia (10x), then water until σ<10 µS.cm-1

1.0 N : 0.14

{001}-anatase-NH4OH-100°C

Rinsing with 1M ammonia (10x), then autoclaving with ammonia (100°C, 24h), then rinsing with water until σ<10 µS.cm-1

0.69 N : 0.10

{001}-anatase-KOH-100°C

Rinsing with 1M ammonia (10x), then autoclaving with 1M KOH (100°C, 24h), then rinsing with water until σ<10 µS.cm-1

0.61 K : 0.33

{001}-anatase-NaOH-100°C

Rinsing with 1M ammonia (10x), then autoclaving with 1M NaOH (100°C, 24h), then rinsing with water until σ<10µS.cm-1

0.42 Na : 0.15

To determine the effect of surface fluorination on surface hydroxyls, the infrared

spectra of sample {001}-anatase-H2O ; {001}-anatase-NH4OH and {001}-anatase-

NaOH-100°C were recorded after dehydroxylation at 300°C and reoxydation.

The results, reported in figure 17, show that lowering the fluorine concentration

first increases the intensity of the 3666 cm-1 O-H stretching band. The 3717 cm-1

stretching band then starts appearing at ~1% wt. fluorine, and increases in intensity as

the fluorine concentration is lowered to 0.42% wt.

59

4000 3500 3000 25000.0

0.5

1.0

1.5

3152 cm-13674 cm-1

{001}-anatase-NaOH-100°C (0.42% F) {001}-anatase-NH4OH (1.0 % F)

{001}-anatase-H2O (2.6 % F)A

bsor

banc

e

Wavenumber (cm-1)

3717 cm-1

Figure 29 : Infrared spectras of {001}-anatase- NaOH-100°C, {001}-anatase-NH4OH and {001}-anatase-H2O after dehydroxylation (20h, 10-8 bar, 300°C) and reoxydation (10h, flowing 20%O2 in Ar, 200°C)

6) Grafting of VOCl 3

Metal grafting on the surface of {001}-Anatase was done using vanadium (V)

oxychloride (VOCl3), following the protocol described above, in part IV.3.

Elemental analysis shows a 1.81% wt. vanadium loading, with 2.4 wt.% chlo-

rine. This corresponds to a 1.9 Cl:V molar ratio, which could indicate an exclusively

monopodal grafting of vanadium.

Infrared spectroscopy (DRIFTS) of the {001}-anatase-NH4OH sample shows

that grafting exclusively occurred on the OH groups constituting the 3666 cm-1 peak,

60

which disappears completely. For reasons yet to be explained, the intensity of the broad

peak at 3152 cm-1 increases after grafting.

4000 3500 3000 2500 2000 1500 10000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5 After Grafting Before Grafting

Abs

orba

nce

(offs

et)

Wavenumber (cm-1)

Figure 30 : Infrared spectras of {001}-anatase-NH4OH before and after VOCl3 grafting

1H NMR of the sample shows a decrease and shift in the main peak, going from

5.7 to 6.8 ppm. This would indicate that the protons from the 3666 cm-1 IR peak are

contributing to the 5.7 ppm NMR peak, without being the only contribution to it. The

0.8 ppm peak remains unchanged.

61

Figure 31 : 1H NMR of {001}-anatase-NH4OH before and after VOCl3 grafting

To further characterize the nature of the grafted vanadium species, Raman and

51V NMR spectras of the sample were acquired.

Raman spectroscopy shows the apparition of a single peak at 1026 cm-1, which

can be attributed isolated V=O stretching135.

51V NMR shows a single peak at -581 ppm with multiple rotation bands. This

chemical shift is within the expected range for inorganic vanadium V(V) compounds

(for comparaison, [VO4]3- and [V2O7]4- complexes show shifts of respectively 535 ppm

and 559 ppm, while VOCl3 is the standard defined as a 0 ppm shift.).136

62

300 400 500 600 700 800 900 1000 1100 1200

Eg

B1g

+A1g

After grafting Before grafting

Inte

nsity

(A

.U, s

moo

thed

)

Wavenumber (cm-1)

1026 cm-1

B1g

Figure 32 : Raman spectrum of {001}-anatase-NH4OH before and after VOCl3 grafting

Figure 33 :Magic Angle Sample Spinning 51V NMR of {001}-anatase-NH4OH after

VOCl3 grafting at 20 and 18 KHz spinning rate

63

Figure 34: Proposed structure for the surface vanadium species obtained by grafting

O

V

O

O

Cl

Cl

Tis

OO

O

O

64

VI – Conclusion

The long term objective of this research is to achieve stable, well-defined and

uniform grafting of metal complexes on the surface of well-defined planes on titania.

We have first obtained a specific isotrope of titania (anatase), with a high surface area

(92 m2/g ) dominated by the {001} facet (81%) and bearing a relatively important

density of TiV-μ1OH surface hydroxyl groups (according to IR data).

We then applied different pre-treatments to the obtained anatase samples in

order to selectively remove its surface physisorbed water without causing sintering or

morphology reconstruction. Sample stability and surface chemistry have been studied

depending on the pre-treatment conditions. For comparison, thermodynamic anatase

has been studied in the same conditions. Finally, as a proof of concept, we have

attempted successfully the grafting of VOCl3. The surface structure, according to

analyses, is a monopodal Ti-O-[V(V)(O)Cl2].

The research will continue in several directions:

Improvement of the fluorine removal process to obtain clean fully defluorinated

samples

Study of the reactivity of this supported in propane oxidative dehydrogenation.

Study of the photocatalytic activity of this material for water splitting with several co-

catalysts.

65

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