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Theses, Dissertations, and Student Research: Department of Physics and Astronomy Physics and Astronomy, Department of

5-2020

Symmetry and Interface Considerations for Interactions on MoSSymmetry and Interface Considerations for Interactions on MoS2 2

Prescott E. Evans University of Nebraska–Lincoln, prescott.evans@huskers.unl.edu

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SYMMETRY AND INTERFACE CONSIDERATIONS FOR INTERACTIONS ON

MoS2

by

Prescott E. Evans

A DISSERTATION

Presented to the Faculty of

The Graduate College at the University of Nebraska

In Partial Fulfilment of Requirements

For the Degree of Doctor of Philosophy

Major: Physics and Astronomy

Under the Supervision of Peter A. Dowben

Lincoln, Nebraska

May, 2020

SYMMETRY AND INTERFACE CONSIDERATIONS FOR INTERACTIONS ON

MoS2

Prescott E. Evans, Ph.D.

University of Nebraska, 2020

Advisor: Peter A. Dowben

The critical role of symmetry, in adsorbate-MoS2 interactions, has been

demonstrated through a variety of electronic structure, topology, and catalytic studies of

MoS2 and MoS2 composites. A combination of density functional theory and experiment

exhibiting diiodobenzene isomer dependent adsorption rates highlight frontier orbital

symmetry as key to adsorption on MoS2. It is clear that the geometry and symmetry of

MoS2 influences the creation and stability of surface defects, that in turn affect catalytic

activity and a myriad of other applications. We have shown that surface reactions such

the methanol to methoxy reaction can create defects on MoS2. From experiment, it is

clear moving forward, if there is to be a better understanding of reactions and catalysis,

theory must include further considerations of frontier orbital symmetry and input from

band structure. Extensive electronic band structure studies have shown bulk MoS2 differs

from the monolayer material, as observed with other transition metal dichalcogenides

(TMDs). Further differences in the electronic behavior of monolayer and bulk MoS2 are

illustrated by variations in behavior that occurs with adsorbed metals om MoS2.

Experiments show metal to bulk TMD interactions exhibit electronic behavior fitting the

expected conventional models while monolayer to metal interactions show a distinctly

opposite trend. Undoubtedly, more complete interaction theory on MoS2 and other TMDs

will require a better detailing of the difference in band structure between monolayer and

bulk. Added to the differences in electronic structure, between monolayer and bulk, bulk

MoS2 has the volume reservoir that permits defect removal by annealing, while defects

will persist for monolayer MoS2. Monolayer MoS2 and other monolayer material are

generally accompanied by an underlying substrate. The influence of these underlying

substrates on the monolayer systems must now be taken into account along with band

bending and interface charge transfer effects. More complete theory which includes

underlying mechanisms of frontier orbital symmetry in addition to monolayer and bulk

electronic properties is necessary for accurately predicting the catalytic behavior and

application of TMDs.

iv

ACKNOWLEDGEMENTS

Foremost, I would like to express my gratitude to my advisor, Dr. Peter A.

Dowben, for his constant support, guidance, and advocacy of my work. Over the course

of my graduate career, Dr. Dowben has encouraged and provided an exceptional research

environment with ample opportunity for scientific exploration, platforms in which to

present my work, and international collaboration. I would also like to acknowledge Dr.

Dowben’s continual fight to secure funding for mine, and my group members’ research.

It is through great effort that Dr. Dowben has consistently made sure that our group has

had the resources to ensure that the science comes first.

I would also like to express my gratitude to Dr. Hae-Kyung Jeong and Dr.

Takashi Komesu for their monumental help in mentoring and teaching me the

fundamental experimental UHV and photoemission skills that form the basis of this

thesis. I must also thank Dr. Andrew J. Yost for his guidance and knowledge in scanning

tunneling microscopy. Additionally, I would also like to thank all of my collaborators for

their providing of experimental data, theoretical modeling, and high-quality samples.

I would like to thank my committee members: Dr. Christian Binek, Dr. Rebecca

Lai, and Dr. Xiaoshan Xu. I am thankful for their time, effort, and insight in reviewing

my work and dissertation.

I would also like to express my deepest gratitude to the mentors that I have had in

my life, those who have consistently pushed me to do my best. I would like to thank my

undergraduate advisor at UMKC, Dr. Fred Leibsle, who inspired me to study physics and

greatly encouraged me to pursue a graduate career. I started undergrad as a chemist, and

v

through Fred’s guidance and motivation I graduated with degrees in both chemistry and

physics. I would also like to thank my long-time mentor, Lee Waits III, who believed I

could do anything I set my mind to. As a CASA volunteer, Lee supported me on my

journey through the Missouri Foster Care System and has remained a friend and mentor

ever since.

Finally, I would like to thank my wife Caitlin Evans for her years of believing in

me and motivating me throughout my undergraduate and graduate studies. I would also

like to thank Caitlin’s very supportive and accepting family: Kim & Kendall Buller and

Marjorie & Gary Embry.

Thank you all.

Prescott Evans

vi

PREFACE

The theoretical description of photoemission covered within Chapter 2, Section 1 has

been submitted for publication in:

Yost, A. J.; Evans, P. E.; Tanabe, I.; Hao, G.; Gilbert, S.; Komesu, T. Integrated

Experimental Methods for the Investigation of the Electronic Structure of Molecules on

Surfaces. In Springer Handbook for Surface Science; Springer, 2020.

Material contained within Chapter 3, is in preparation to be submitted for publication in:

Evans, P. E.; Le. D.; Komesu, T.; Rahman, T. S.; Dowben P. A.; Bartels L. The

Interaction of Gold and Other Metals with MoS2 Single Crystals and Thin Films

Chapter 4, Section 1 has been published in part in:

Niyitanga, T.; Evans, P. E.; Ekanayake, T.; Dowben, P. A.; Jeong, H. K. Carbon

Nanotubes-Molybdenum Disulfide Composite for Enhanced Hydrogen Evolution

Reaction. J. Electroanal. Chem. 2019, 845 (February), 39–47.

https://doi.org/10.1016/j.jelechem.2019.05.041.

Chapter 4 Section 2 has been published in part in:

Ibukun, O.; Evans, P. E.; Dowben, P. A.; Jeong, H. K. Titanium Dioxide-Molybdenum

Disulfide for Photocatalytic Degradation of Methylene Blue. Chem. Phys. 2019, 525

(February), 110419. https://doi.org/10.1016/j.chemphys.2019.110419.

vii

Chapter 5 Section 1 has been published in part in:

Evans, P. E.; Jeong, H. K.; Hooshmand, Z.; Le, D.; Rawal, T. B.; Naghibi Alvillar, S.;

Bartels, L.; Rahman, T. S.; Dowben, P. A. Methoxy Formation Induced Defects on MoS2.

J. Phys. Chem. C 2018, 122 (18). https://doi.org/10.1021/acs.jpcc.8b02053.

Chapter 5 Section 2 has been published in part in:

Evans, P. E.; Jeong, H. K.; Dowben, P. A. Sulfur Segregation and Surface Site Vacancy

Compensation During Methanol to Methoxy Reactions on MoS2. MRS Adv. 2019, 4 (15),

873–878. https://doi.org/10.1557/adv.2018.634.

Material contained within Chapter 6 has been submitted for publication to Surface

Science:

Evans, P. E; Hooshmand, Z.; Rahman, T. S.; Dowben, P. A. The Importance of Frontier

Orbital Symmetry in the Adsorption of Diiodobenzene on MoS2(0001). Submitted to

Surface Science

viii

Table of Contents

INTRODUCTION ................................................................................................................... 1

References ............................................................................................................... 5

EXPERIMENTAL TECHNIQUES ............................................................................................ 6

2.1 Photoemission ................................................................................................... 6 2.1.1 The Theory of Photoemission ............................................................ 7

2.1.2 X-ray Photoemission Spectroscopy (XPS) ...................................... 12

2.1.3 Ultraviolet Photoemission Spectroscopy (UPS) .............................. 13

2.1.4 Synchrotron Radiation ..................................................................... 14 2.1.5 Inverse Photoemission Spectroscopy (IPES) ................................... 14

2.2 Low Energy Electron Diffraction (LEED) ..................................................... 17

2.3 Scanning Tunneling Microscopy (STM) ........................................................ 18

2.4 Sample Preparation ......................................................................................... 20

References ............................................................................................................. 22

METAL NANOCLUSTERS ON MOS2 FOR CATALYSIS ........................................................ 23

3.1 Introduction ..................................................................................................... 23

3.2 Gold Growth on MoS2 .................................................................................... 24

3.3 Charge Transfer in the Gold-MoS2 system ..................................................... 29

3.4 Expectations for Gold Nanocluster Theory .................................................... 32

3.5 Metal – Transition Metal Dichalcogenides Trends ......................................... 34

3.6 Summary ......................................................................................................... 36

References ............................................................................................................. 38

CHARGE TRANSFER IN CATALYSIS .................................................................................. 43

4.1 Carbon Nanotubes–MoS2 for Enhanced Hydrogen Evolution Reaction ........ 43 4.1.1 Introduction ...................................................................................... 43 4.1.2 Synergetic Interaction of CNT Conductivity and MoS2 Active Sites

................................................................................................................... 45

ix

4.1.3 Summary .......................................................................................... 52

4.2 Titanium Dioxide-MoS2 for Photocatalytic Degradation of Methylene Blue 53

4.2.1 Introduction ...................................................................................... 53 4.2.2 Enhanced Charge Transfer and Exciton Lifetime ............................ 55 4.2.3 Summary .......................................................................................... 61

References ............................................................................................................. 62

CREATION AND ANNIHILATION OF SULFUR VACANCIES ................................................ 70

5.1 Methanol Oxidation and Methoxy Formation ................................................ 73

5.2 Sulfur Vacancies and Induced Defects ........................................................... 76

5.3 Sulfur Segregation .......................................................................................... 87

5.3.1 Surface Sulfur Loss .......................................................................... 87 5.3.2 Surface Sulfur Vacancy Compensation ........................................... 88

5.4 Summary ......................................................................................................... 90

References ............................................................................................................. 92

FRONTIER ORBITAL SYMMETRY ..................................................................................... 97

6.1 Relative Rate of Diiodobenzene Adsorption ................................................ 100

6.2 Heats of Diiodobenzene Adsorption ............................................................. 105

6.3 Frontier Orbital Interactions ......................................................................... 115

6.4 Summary ....................................................................................................... 119

References ........................................................................................................... 120

CONCLUSIONS AND FUTURE STUDY ............................................................................... 122

References ........................................................................................................... 125

1

CHAPTER 1

INTRODUCTION

The studies, covered within this thesis, focus primarily on the fundamental

physics and surface science of adsorbate interactions on MoS2. There may not always be

a strict practical application for the systems studied, but in a wider context, the research

covered in this thesis on MoS2 relates to catalysis issues associated to alternative fuel

production and environmental waste management needs. MoS2 is a much studied 2D

material and well-recognized to be a valuable catalyst. Both MoS2 single crystals and

MoS2 in composite materials were investigated in the context of catalytic application to

methanol synthesis, hydrogen evolution reactions, and photocatalytic degradation of

process dyes. Other investigations focused on the catalysis context of catalytic promoters,

through the investigation of MoS2 single crystals and monolayers decorated with metals.

Metals on MoS2 are sometimes views as alternatives to current noble metal catalysts.

The symmetry and interface effects on the interaction and properties of MoS2 with

adsorbates were intensely studied through a variety of spectroscopic techniques. To

understand the chemical properties of the MoS2 systems within this thesis, the band

structure (both the occupied and unoccupied electronic states) of materials were studied

through use of photoemission and inverse photoemission techniques. Morphological

studies of adsorbate to MoS2 reactions were also carried out using scanning tunneling

microscopy. From extensive studies of the electronic band structure, it is now understood

that, like the other transition metal dichalcogenides,1,2 the electronic structure of

monolayer MoS2 differs from bulk MoS2(0001). The role of symmetry in adsorbate-MoS2

2

interactions is nonetheless often overlooked. Yet in this thesis I explore how symmetry

plays a critical part in influencing interactions on MoS2.

The role of adsorbate frontier orbital symmetry and its influence on interactions

at the MoS2 surface has been demonstrated by comparing the adsorption of the isomers of

diiodobenzene on MoS2. Experiments described herein illustrate that the adsorption and

uptake rate of 1,3-diiodobenzene on MoS2(0001) is much greater as compared to other

studied isomers, contrary to negligible differences in calculated adsorption energies and

electron affinities which indicate adsorption should be isomerically insensitive. The

combination of density functional theory and experiment highlights frontier orbital

symmetry as key to understanding the role of diiodobenzene isomer in the adsorption

behavior on MoS2(0001). From these adsorption studies of 1,2- 1,3- and 1,4-

diiodobenzene on MoS2 it is concluded that frontier orbital symmetry, or specifically a

requirement of reduction, accompanying rehybridization, has major influence on

adsorption. This rehybridization requirement provides a framework of understanding for

the isomeric dependent adsorption of diiodobenzene on MoS2.

It is clear that notions of increased surface to bulk ratios and defects or vacancies

can enhance surface reactions.3,4 It is also clear that the geometry and symmetry of the

MoS2 have a hand in the creation and stability of defects in the surface.5 Surface reactions

on MoS2 can be enhanced by defects,3,4 but we have shown that surface reactions can also

create defects, as in the example of methanol to methoxy reactions. These defects, and

the stability of these defects within a larger 2D framework have great utility in increasing

catalytic activity as shown in studies of hydrogen evolution reactions and photocatalytic

dye degradation. The difference in electronic structure and application of the monolayer

3

from bulk is emphasized by the ability to anneal out defects for bulk MoS2(0001),

through bulk sulfur segregation to the surface, a possible catalytic inhibitor, that will not

occur in monolayer MoS2. From studies of methanol to methoxy reactions on MoS2, the

production of sulfur vacancies is apparent under scanning tunneling microscopy (STM)

images. This production of surface sulfur vacancies as both line and patch defects is

interesting in two ways. First, the experiment provides validation of the influence of

symmetry in vacancy creation and stability. Second, the creation of sulfur defects is

apparent both from photoluminescence of the monolayer and STM of the bulk, meaning

that though the band structure may be dissimilar for the monolayer and bulk, this reaction

appears indifferent. However, further exploration in the healing of these sulfur vacancies,

through annealing, show bulk sulfur segregation to the surface and diminishing of the

number of vacancies. While this may seem problematic with the high temperatures often

utilized with catalytic reactions, the monolayer, lacking a reservoir of bulk sulfur, should

not face the same issues. All of the studies investigating defect creation and suppression,

as a result of the methanol to methoxy reaction, provide a deeper insight into aspects not

considered in the wider goals of finding catalysts for syngas and higher alcohol

production.

While increasing the surface to bulk ratio is paramount to MoS2 active site

accessibility, from the above methanol studies, the importance of defect production, and

hence symmetry, on bulk MoS2 and its relation to catalysis is evident. In light of the

constraints that accompany the use of bulk MoS2, the use of a dispersed MoS2 particles

for catalysis is attractive. Yet, dispersed form, MoS2 lacks an efficient pathway for charge

mobility for oxidation and reduction reactions involved for catalysis. Hence exploration

4

of MoS2 composite materials has been undertaken. In addition to the increase in MoS2

active sites, due to the geometry within these composite systems, the electronic structure

modification stemming from MoS2 interactions in a matrix provides unique charge

transfer characteristics that can enhance catalysis. Composites of carbon nanotubes and

MoS2 have been shown to increase catalytic activity for the hydrogen evolution reaction

as compared to MoS2 alone. With a contribution of the aforementioned catalytic active

sites from the MoS2 and a conductive network provided by CNTs, which reduce charge

saturation, the composite has catalytic HER activity comparable to platinum. In

photocatalytic dye degradation studies of methylene blue using TiO2 and MoS2, it was

found that dye degradation efficiency was improved from TiO2, by itself. The charge

transfer from the MoS2 increases the photogenerated charge carrier recombination time.

These composite systems, using micron scale MoS2, exploited MoS2 behavior not

regularly found in the bulk. Along with the other studies covered within this thesis, a

picture emerges that the differences between the bulk and monolayer behavior of MoS2

can be profound and that local site symmetry and charge matters in reaction chemistry.

In certain cases, such as the methanol synthesis reactions as covered within

chapter 5, properties of both the monolayer and bulk MoS2 are calculated to have similar

properties which are substantiated by experimental results. However, great differences in

the electronic behavior of monolayer and bulk MoS2 were found in the interactions of

adsorbed and decorated metals with MoS2. Experiments demonstrate a wide variety of

metals on bulk MoS2 exhibit interactions resulting in electronic behaviors that better fit

conventional long-established models, making the predicting and extension of properties

for these metal adsorbate on bulk MoS2 systems widely applicable. However, metal

5

interactions with monolayer MoS2 show a distinctly opposite trends and do not follow the

conventional models. While the monolayer MoS2 limit is very attractive to catalytic

applications, from experiment it is clear moving forward theory must include further

considerations of bulk and monolayer properties such as band bending, interface charge

transfer, and the influence of supporting substrates to gain a more accurate insight into

reaction mechanisms on MoS2. From several of the studies covered within this thesis,

there appears to be some limitations to Density Functional Theory in predicting the

properties and chemistry of MoS2.

References

(1) Tanabe, I.; Komesu, T.; Le, D.; Rawal, T. B.; Schwier, E. F.; Zheng, M.; Kojima,

Y.; Iwasawa, H.; Shimada, K.; Rahman, T. S.; et al. The Symmetry-Resolved

Electronic Structure of 2H-WSe2(0001). J. Phys. Condens. Matter 2016, 28 (34).

https://doi.org/10.1088/0953-8984/28/34/345503.

(2) Tanabe, I.; Gomez, M.; Coley, W. C.; Le, D.; Echeverria, E. M.; Stecklein, G.;

Kandyba, V.; Balijepalli, S. K.; Klee, V.; Nguyen, A. E.; et al. Band Structure

Characterization of WS2 Grown by Chemical Vapor Deposition. Appl. Phys. Lett.

2016, 108 (25), 10–14. https://doi.org/10.1063/1.4954278.

(3) Rawal, T. B.; Le, D.; Rahman, T. S. MoS2-Supported Gold Nanoparticle for CO

Hydrogenation. J. Phys. Condens. Matter 2017, 29 (41), 415201.

https://doi.org/10.1088/1361-648X/aa8314.

(4) Rawal, T. B.; Le, D.; Rahman, T. S. Effect of Single-Layer MoS2 on the

Geometry, Electronic Structure, and Reactivity of Transition Metal Nanoparticles.

J. Phys. Chem. C 2017, 121 (13), 7282–7293.

https://doi.org/10.1021/acs.jpcc.7b00036.

(5) Le, D.; Rawal, T. B.; Rahman, T. S. Single-Layer MoS2 with Sulfur Vacancies:

Structure and Catalytic Application. J. Phys. Chem. C 2014, 118 (10), 5346–5351.

https://doi.org/10.1021/jp411256g.

6

CHAPTER 2

EXPERIMENTAL TECHNIQUES

To understand the chemical and electronic structure properties of the MoS2

systems, investigated as part of this thesis, both the occupied and unoccupied partial

density electronic states of a variety of materials, containing MoS2, were studied

primarily through use of photoemission and inverse photoemission. In addition to

providing great insight into the charge state of systems under question, photoemission,

especially x-ray photoemission spectroscopy, was extensively used in several cases. The

additional benefits were the determination and quantification of the chemical composition

of interacting systems. This quantification became essential in several studies in

ascertaining the presence of adsorbed/desorbed atoms and molecules on the MoS2 surface

and indications of interaction strength. From the above techniques, the effects of the

MoS2 interface chemistry was explored. Other techniques to confirm surface topology,

and structure were utilized: these techniques include low energy electron diffraction and

scanning tunneling microscopy. These techniques confirmed influence of surface

symmetry in ways not directly observable from photoemission.

2.1 Photoemission

Photoemission and photoemission spectroscopy, especially x-ray photoemission

spectroscopy, are a widely utilized technique in the determination, and identification of

atomic composition of materials. Beyond simple identification of atomic composition,

photoemission is exceedingly useful in the exploration and analysis of the electronic or

chemical environment of atoms within a material. This examination of changes in

7

chemical environment are extremely helpful in studies of charge transfer as well as

bonding configurations between adsorbates and substrates. Additionally, photoemission,

principally, when using a polarized light source, can reveal an incredible amount of

useful information about atomic orbital symmetries of materials. However, the use of

polarized light sources was beyond the scope of this study and is better discussed

elsewhere.1 Furthermore, photoemission, and inverse photoemission, where the latter will

be discussed later, are used in concert in the mapping of the valence band and conduction

band of materials, often giving information on the band gap of a material, or the

difference in energy between the highest occupied molecular orbital (HOMO), given by

photoemission and the lowest unoccupied molecular orbital (LUMO), as provided by

inverse photoemission. Further history and much more detailed information on

photoemission may be found elsewhere.2 The following theory was reviewed by us

elswhere.1

2.1.1 The Theory of Photoemission

Fundamentally, photoemission is the probing of the occupied electronic states of a

material via light, i.e. a photon. Photoemission is a direct result of the application of the

photoelectric effect, where upon impinges on a material with a photon of energy above

the work function of the material and we may observe the ionization of the material and

the ejection of a photoelectron whose kinetic energy (𝐸𝐾𝑖𝑛𝑒𝑡𝑖𝑐) obeys the conservation of

energy, as outlined by the photoelectric effect, where ℎ𝜈 is the energy of the incident

photon and 𝜙 is the work function of the material.

𝐸𝑘𝑖𝑛𝑒𝑡𝑖𝑐 = ℎ𝜈 − 𝜙 (2.1)

8

In the case of solids, it is common to reference the energy of states in terms of the

Fermi level which is the difference of the vacuum energy and the work function, we may

think of the work function as the energy required to access the highest occupied state

within the material. If we are to access more deeply bound states, within the material,

this requires energy more than purely the work function and is referred to as binding

energy (𝐸𝐵𝑖𝑛𝑑𝑖𝑛𝑔).

𝐸𝑘𝑖𝑛𝑒𝑡𝑖𝑐 = ℎ𝜈 − 𝜙 − 𝐸𝐵𝑖𝑛𝑑𝑖𝑛𝑔 (2.2)

Within photoemission spectroscopy, it is the kinetic energy of the photoelectron which is

directly measured through use of a hemispherical electron energy analyzer. From

knowledge of the light sourced used, the work function of the material, and the kinetic

energy of the electron, the binding energy of the accessed state is identified, and this

information is usually what is presented as a function of binding energy.

9

Figure 2.1 a) A schematic diagram of a typical configuration for photoemission

spectroscopy, with an incident photon of energy hν and incident angle θi impinging on

the sample surface and producing a photoelectron with energy Ekin and emission angle of

θf, b) The energetic representation of the photoemission process with a photoelectron

being produced from the ionization of an occupied state. Reused and modified with

permission from P. A. Dowben taken from.3

For photoemission to occur, we must consider the transition probability per unit time

between the initial state of a bound occupied state electron 𝜓𝑖 and the final state of the

unbound photoelectron 𝜓𝑓, given by time-dependent perturbation theory. Assuming a

small perturbation 𝐻′(𝑡) due to the incident photon, the photoemission transition rate 𝜔

can be given by Fermi’s Golden Rule:

𝜔 ∝ 2𝜋

ħ|⟨𝜓𝑓|𝐻′|𝜓𝑖⟩|

2𝛿(𝐸𝑓 − 𝐸𝑖 − ℎ𝜈) (2.3)

10

The Hamiltonian of an electron in an external electromagnetic field is given by the

following:

𝐻 = 𝐻𝑜 + 𝐻′ =𝒑2

2𝑚− 𝑒𝑉 +

𝑒

2𝑚𝑐(𝑨 ∙ 𝒑 + 𝒑 ∙ 𝑨) +

𝑒2

2𝑚𝑐2𝑨 ∙ 𝑨 − 𝑒𝜙 (2.4)

Where 𝑨 is the vector potential and 𝜙 is the scalar potential, and 𝒑 is the momentum

operator 𝒑 = 𝑖ħ𝛁. Thus, the perturbation Hamiltonian in question is given as:

𝐻′ = 𝑒

2𝑚𝑐(𝑨 ∙ 𝒑 + 𝒑 ∙ 𝑨) +

𝑒2

2𝑚𝑐2𝑨 ∙ 𝑨 − 𝑒𝜙 (2.5)

Here by choosing the gauge 𝜙 = 0, and disregarding the negligibly small, statistically

unlikely two photon process, 𝑨 ∙ 𝑨, contribution, the Eq. 2.5 becomes:

𝐻′ = 𝑒

2𝑚𝑐(𝑨 ∙ 𝒑 + 𝒑 ∙ 𝑨) (2.6)

Now if we use the commutation relation 𝑨 ∙ 𝒑 + 𝒑 ∙ 𝑨 = 2𝑨 ∙ 𝒑 + 𝑖ħ(𝛁 ∙ 𝐀) while

arguing 𝛁 ∙ 𝐀 = 0 due to translational invariance with the photon wavelength much larger

than the given atomic dimensions, Eq. 2.6 becomes:

𝐻′ = 𝑒

𝑚𝑐𝑨 ∙ 𝒑 (2.7)

From Eq 2.7, our photoemission transition rate becomes the following:

𝜔 ∝ 2𝜋

ħ

𝑒

𝑚𝑐|⟨𝜓𝑓|𝑨 ∙ 𝒑|𝜓𝑖⟩|

2𝛿(𝐸𝑓 − 𝐸𝑖 − ℎ𝜈) (2.8)

However, we should note that unless the system in question is a hydrogen atom, systems

under study will have many electrons with many degrees of freedom with many body

interactions. Yet, we may adopt an assumption of a one-electron view for the initial state,

11

𝜓𝑖, and final state, 𝜓𝑓, wave function. When photoelectrons are created by photoionizing

orbital electrons of quantum number, 𝑘, the initial wave function, 𝜓𝑖(𝑁), is described as

the product of a wave function of a single electron which is to be photoemitted, 𝜑𝑖,𝑘 , and

the wave function of the remaining (𝑁 − 1)-electrons, 𝜓𝑖𝑘(𝑁 − 1) , provided that the

system has 𝑁 number of electrons. Thus, the 𝑁-electron initial state,|𝜓𝑖(𝑁)⟩ , is

|𝜓𝑖(𝑁)⟩ = |𝜑𝑖,𝑘⟩|𝜓𝑖𝑘(𝑁 − 1)⟩ (2.9)

Similarly, the final wave function, 𝜓𝑓(𝑁), is written as the product of a wave function of

the emitted electron, 𝜑𝑓,𝐸𝑘𝑖𝑛,with kinetic energy equal to 𝐸𝐾𝑖𝑛 and the wave function of

the rest of the electrons, 𝜓𝑓,𝑠𝑘 (𝑁 − 1) . Here, 𝑠 denotes the number of excited states the

remaining (𝑁 − 1)-electrons have since the final (𝑁 − 1)-electron states should be, in

general, different from the initial one, due to energy relaxation. Thus, the 𝑁-electron final

state |𝜓𝑓(𝑁)⟩ is

|𝜓𝑓(𝑁)⟩ = |𝜑𝑓, 𝐸𝑘𝑖𝑛⟩|𝜓𝑓,𝑠

𝑘 (𝑁 − 1)⟩ (2.10)

From this, Eq. 2.8, our photoemission transition rate becomes:

𝜔 ∝ 2𝜋

ħ

𝑒

𝑚𝑐|⟨𝜑𝑓, 𝐸𝑘𝑖𝑛

|𝑨 ∙ 𝒑|𝜑𝑖,𝑘⟩|2

∑ |⟨𝜓𝑓,𝑠𝑘 (𝑁 − 1)|𝜓𝑖

𝑘(𝑁 − 1)⟩|2

𝑠

× 𝛿(𝐸𝐾𝑖𝑛 − 𝐸𝑠(𝑁 − 1) − 𝐸0(𝑁) − ℎ𝜈) (2.11)

Where 𝐸0(𝑁)is the ground state energy of the 𝑁-electron system. Here the vital take

away is, assuming energy is conserved, 𝛿(𝐸𝑓 − 𝐸𝑖 − ℎ𝜈), and if the final state, 𝜓𝑓, of the

free electron, is of the same irreducible representation to the state cast by the effect of the

perturbation Hamiltonian on the initial state, 𝑨 ∙ 𝒑|𝜓𝑖⟩, then 𝜔 ≠ 0, and photoemission

will occur, with the final state electron our observable as measured within experiment.

12

2.1.2 X-ray Photoemission Spectroscopy (XPS)

As discussed within the prior photoemission theory section, photoemission is used

in the probing of occupied electronic states, here X-ray photoemission spectroscopy

(XPS), is used in the probing of core level electronic states due to the energy of incident

photons. All XPS spectra shown within the following studies were performed in situ in an

ultra-high vacuum chamber. X-rays were produced via a SPECS X-ray source (Model

XR50) with either an Al anode (hν=1486.6 eV) or Mg anode (hν=1253.6 eV).

Photoelectron energy was determined through use of a Phi hemispherical electron

analyzer (PHI Model 10-360) with angular acceptance of ±10° or more. Typical XPS

experimental geometry is shown in figure 2.2.

Figure 2.2 Schematic of XPS experimental setup, including X-ray source, sample, and

hemispherical analyzer.

13

2.1.3 Ultraviolet Photoemission Spectroscopy (UPS)

Much like XPS, ultraviolet photoemission spectroscopy (UPS) is used to probe

occupied electronic states, however here UPS is used to probe the valence band of

materials giving information on the structure and chemical bonding of the highest

occupied electronic states. All UPS spectra shown within the following studies were

similarly performed in situ within the same ultra-high vacuum chamber. Ultraviolet light

was produced via a VSW He discharge lamp (Model VSW UV10) with the He I line

(hν=21.2 eV) being produced. Photoelectron energy was again determined through use of

a Phi hemispherical electron analyzer (PHI Model 10-360) with angular acceptance of

±10° or more. Typical UPS experimental geometry is shown in figure 2.3.

Figure 2.3 A schematic of UPS experimental setup, including ultraviolet source, sample,

and hemispherical analyzer.

14

2.1.4 Synchrotron Radiation

High-resolution photoemission spectroscopy measurements covered within

studies herein were performed on the linear undulator beamline (BL-1) of the Hiroshima

Synchrotron Radiation Center (HiSOR), Hiroshima University, Japan utilizing a variety

of incident photon energies. In context of the covered studies, quantitative shifts in band

structure are provided, this is however a very small portion of the data possible to collect

provided by HiSOR facilities. The usage of sample geometry to allow for p-polarized and

s-polarized photoemission geometry were essential in other studies for band mapping,

providing the basis of other, fundamental studies, as not covered within this thesis.

2.1.5 Inverse Photoemission Spectroscopy (IPES)

Essentially, photoemission spectroscopy and inverse photoemission spectroscopy

(IPES) share a similar derivation as outlined in section 2.1.1, however IPES entails the

probing of the unoccupied electronic states of a material (conduction band) via

impingement with an electron. Here a free electron of energy 𝐸𝑖 impinges the sample

surface, decaying in energy to an unoccupied bound state with energy 𝐸𝑓, while emitting

a photon of energy ℎ𝜈 = 𝐸𝑖 − 𝐸𝑓 as shown in figure 2.4 below.

15

Figure 2.4 A schematic diagram of inverse photoemission spectroscopy. (a) An electron

with energy Ei with the incident angle θi with respect to the surface normal impinges on

the surface of the sample. Then, a photon with energy Ef is emitted with an angle θf. (b)

an energetic representation of the inverse photoemission process with a photon being

produced from the decay of the incident electron to an unoccupied state. Reused and

modified with permission from P.A. Dowben taken from.3

Starting from Eq. 2.11, for the photoemission transition rate, while the classical vector

potential 𝑨 for the photoemission process generates mostly the same results as when

treated quantum mechanically, the inverse photoemission process requires the vector

potential 𝑨 to be quantized as:

𝑨(𝒙, 𝑡) = ∑ ∑ (ħc2

2𝜈𝜔𝒒)

2

𝜺𝒓(𝒒)[𝑎𝑟(𝒒, 𝑡)𝑒𝑖𝒒∙𝒙 + 𝑎𝑟†(𝒒, 𝑡)𝑒−𝑖𝒒∙𝒙]

𝑟𝒒(2.12)

Where 𝜔𝒒 = 𝑐|𝒒|, and 𝑎𝑟(𝒒, 𝑡), and 𝑎𝑟† (𝒒, 𝑡) are the creation and annihilation operators

respectively, and 𝜺𝒓(𝒒) being the polarization vector.4,5 This is due to the inverse

photoemission process being an excited electron system without an electromagnetic field,

which causes the transition matrix elements of the dipole operator to become zero in the

16

semiclassical expression.3 Similar to the initial and final states of photoemission, for

inverse photoemission, the initial state comprises an electron within a continuum state

with no photons and is expressed as |𝜓𝑖 , 𝑛ℎ𝜈 = 0⟩ , and the final state comprises an

electron in a bound state and a photon with wave vector 𝒒 and is expressed as

|𝜓𝑓 , 𝑛ℎ𝜈(𝒒) = 1⟩.3

All IPES spectra shown within the following studies were performed in situ within the

same ultra-high vacuum chamber as used with XPS and UPS capabilities. A Kimball

Physics Inc. electron source and iodine-helium Geiger-Müller detector with a fixed

photon energy acceptance of 9.7 eV and instrumental linewidth of approximately 400

meV were utilized in data collection. Typical IPES experimental geometry is shown in

figure 2.5.

Figure 2.5 The schematic of IPES experimental setup, including electron gun source,

sample, and Geiger-Müller detector.

17

2.2 Low Energy Electron Diffraction (LEED)

Low energy electron diffraction (LEED), within the scope of the following

studies, was used in qualitative analysis of sample surface crystallinity, symmetry, and

symmetry of systems in which an adsorbate was present on the sample surface. While

LEED may be used in more quantitative analysis methods, such as material temperature

studies providing insight into phase transitions,6 these are outside the realm of the studies

outlined in this thesis. The principle of LEED is to directly image the reciprocal lattice of

a material via the scattering/diffraction of low energy (< 200 eV within the following

studies) electrons which are fired from an electron gun at the sample surface. Here, if the

sample surface in question is sufficiently crystalline, back scattering and diffraction of

elections will be observable, in our case on a phosphor screen, providing a diffraction

pattern. Due to the energy of the incident electrons, the maximal mean free path of the

electron tends to be ~1 nm, meaning provided diffraction patterns are only probing the

topmost atoms of a surface and thus is extremely surface sensitive. In the context of the

following studies, LEED was used in preliminary investigations in determining quality of

MoS2(0001) single crystals to be used in studies. Here polycrystalline MoS2 samples

were commonly rejected by the presence of diffuse diffraction patterns and multiple

diffraction patterns. Within some of the adsorption studies performed, LEED was taken

on the adsorbate-MoS2 systems to determine if any change of surface crystallinity or

symmetry occurred following the adsorbate-MoS2 interaction. Low energy electron

diffraction (LEED) patterns were taken in situ within the same UHV system containing

XPS, UPS, and IPES capabilities using an Omicron SPECTALEED rear-view LEED

optics system.

18

2.3 Scanning Tunneling Microscopy (STM)

In the context of data provided within this thesis, the utilization of a scanning

tunneling microscopy (STM) provided surface mapping of samples in question with a

great deal of focus on the qualitative structure of samples before and after interaction

with an adsorbate. While STM has the capability to provide a great number of

spectroscopic data, including but not limited to measurement of band gap and density of

states, these techniques were not employed.

In principle, scanning tunneling microscopy relies on the quantum mechanical

principle of the tunneling of electrons from the sample surface towards the STM tip, or

vice versa. Under proper conditions, this steady, net, tunneling of electrons produces a

measurable current, which is interpolated into STM images. In experiment, an STM tip

made from a metal, in our case tungsten, is brought incredibly close to the sample (<10

Å). In such close proximity, there still exists a barrier, the vacuum, between sample and

tip preventing a meaningful net tunneling of electrons to occur. Here the wave function of

the electron experiences and exponential decay as a function of distance within the

barrier, with spontaneous transmission through the vacuum unlikely. If, however, when a

positive bias voltage is applied between tip and sample, the Fermi level of tip can be

raised providing an effective path for electrons between electronically occupied tip states

to unoccupied electronic states in the sample producing a tunneling current. Inversely, if

a negative bias is applied between tip and sample, the Fermi level of the tip will be

lowered, providing an effective path for electrons between occupied electronic sample

states to unoccupied electronic tip states, producing a tunneling current in the opposite

direction as outlined above.

19

Within completed studies, it is the tunneling current, or by extension the use of a

constant tunneling current feedback loop used during surface scanning used to create

STM images. Here, once the STM tip is brought into close contact with the sample

surface, with an appropriate bias applied, a suitable tunneling current is achieved. If a

tunneling current feedback loop mode is employed, as the tip is moved across the sample

surface in the XY plane, the tip’s Z position, through use of fine piezo-motor control, is

automatically manipulated to maintain the desired tunneling current. From this

information of the tip’s Z position, and the XY position throughout the scan, the local

density of states (LDOS) of the sample is imaged and output in topographical form.

Herein lies the need for use of UHV as well as an “atomically” sharp tip, one as the

tunneling barrier of vacuum is less than open air, and provides less possibility for

interaction with gas molecules between sample and surface, and secondly a sharp tip

allows for a greater resolution as tunneling is localized to the utmost atoms of the STM

tip.

The UHV system used for scanning tunneling microscopy is separate from the

UHV chamber used for UPS, XPS, and IPES, as outlined in earlier sections. The STM

used for characterization was a variable temperature scanning tunneling microscope as

part of an Omicron modular UHV system with capabilities for atomic force microscopy

(AFM) and Kelvin probe force microscopy (KPFM). In the presented cases, for the

methanol interaction on MoS2 molecular deposition and subsequent STM measurements

were completed in situ, however for the interaction of gold on MoS2 samples were moved

between chambers as the Omicron system lacked a suitable gold source at the time of the

experiment.

20

2.4 Sample Preparation

The majority of studies covered within involve the use of bulk MoS2(0001) single

crystals. While other forms of MoS2 in combination with other materials were studied,

these were prepared via other methods such as chemical vapor deposition (CVD) by

collaborators or purchased powdered MoS2. In general samples of MoS2 single crystal

were acquired through SPI-Structure Probe Inc. or 2DSemiconductors. These bulk MoS2

samples had the top surface layers removed through exfoliation through application of

adhesive tape, exposing a fresh layer of MoS2, before being loaded into a UHV chamber.

As mention in the previous methods sections, LEED and XPS were commonly used to

determine the crystallinity of the sample, as well as surface contamination. Surface

contamination in the form of C1s and O1s peaks present in survey XPS spectra were

often occurring. If samples were found to be polycrystalline in LEED, samples were

removed, exfoliated, and reinserted into UHV. If surface contamination in the form of

organics were found on MoS2 samples, samples were subjected to gentle annealing cycles

(~450 – 550 K, resistive heating) with subsequent XPS until contaminant outgassing

plateaued.

Several of the studies pertain to the interaction of a molecular adsorbate with the

MoS2(0001) surface. Here samples of MoS2 were cooled to low temperatures (~85 – 100

K) through use of liquid nitrogen and cold finger reservoir in contact with the sample

plate. Deposition of the adsorbates (methanol and isomers of diiodobenzene) was

accomplished through the use of a home built molecular evaporator as shown below,

wherein after outgassing of the molecule in question, molecule vapor was admitted into

the UHV chamber onto a cooled sample through controlled use of a leak valve. Varying

21

the exposure pressure and time allowed for differing amounts of molecule to interact with

the sample surface. A schematic of the molecular evaporator is shown in figure 2.6.

Figure 2.6 Schematic of the molecular evaporator used in several experiments, including

molecule source, a roughing pump path to degas molecules, a pirani gauge to measure

pressure and leak valve to admit molecules to the UHV chamber.

For studies of the interaction of metals, mainly gold, on MoS2, gold was deposited

onto the sample in UHV through use of a resistively heated tungsten basket containing a

small amount of high purity gold. While evaporating, the deposition rate was calculated

using an Inficon XMT/2 deposition monitor. When a suitable deposition rate was

achieved, samples were moved into position and desired gold cover completed via direct

exposure of the sample to the gold source for varying amounts of time.

22

References

(1) Yost, A. J.; Evans, P. E.; Tanabe, I.; Hao, G.; Gilbert, S.; Komesu, T. Integrated

Experimental Methods for the Investigation of the Electronic Structure of

Molecules on Surfaces. In Springer Handbook for Surface Science; Springer,

2020.

(2) Hüfner, S. Photoelectron Spectroscopy, 3rd ed.; Advanced Texts in Physics;

Springer Berlin Heidelberg: Berlin, Heidelberg, 2003. https://doi.org/10.1007/978-

3-662-09280-4.

(3) Dowben, P. A.; Xu, B.; Choi, J.; Morikawa, E. Band Structure and Orientation of

Molecular Adsorbates on Surfaces by Angle-Resolved Electron Spectroscopies. In

Handbook of Thin Films; Elsevier, 2002; pp 61–114. https://doi.org/10.1016/b978-

012512908-4/50021-7.

(4) Sakurai, J. J. Advanced Quantum Mechanics; Pearson Education and Dorling

Kindersley: New Delhi, 1967.

(5) Mandl, F.; Shaw, G. Quantum Field Theory; Wiley: West Sussex, 2010.

(6) Matsumoto, M.; Fukutani, K.; Okano, T. Low-Energy Electron Diffraction Study

of the Phase Transition of Si(001) Surface below 40 K. Phys. Rev. Lett. 2003, 90

(10), 4. https://doi.org/10.1103/PhysRevLett.90.106103.

23

CHAPTER 3

METAL NANOCLUSTERS ON MOS2 FOR CATALYSIS

Within this chapter, after demonstrating that gold nanoclusters on MoS2 may

exhibit properties favorable for catalysis, we show that there exists stark difference in the

electronic behavior of metal on monolayer and bulk transition metal dichalcogenides.

These differences in the details of the perturbations to the electronic structure, due to the

adsorption of metals on monolayer and bulk MoS2 are not predicted by conventional

theory, which calculates no difference between the behavior of metal adsorption on

monolayer, bilayer, trilayer, and bulk MoS2. From a review of several metal-TMD

studies, the electronic behavior of the bulk transition metal dichalcogenide systems fits a

long-held model, whereas the monolayer express opposite behavior. In light of the studies

covered within this chapter, it is of little surprise that methanol carbonylation to

acetaldehyde reactions were observed for gold on mololayer MoS2 but were not observed

for gold on bulk MoS2.1

3.1 Introduction

Nanoparticle deposition of gold, in addition to thin films on metal oxide substrates

such as cerium oxide and titanium oxide have been observed to be catalytically active.2–4

It has also been observed that adsorption of gold on graphene and on hexagonal boron

nitride nanomesh results in noncatalytic gold behavior, or metallic gold behavior in

nature.5–8 With desirable nonmetallic gold features apparent with the adsorption on metal

oxides and some behavior known with similar structured species, the metal dichalcogenide,

MoS2 appears to be a desirable material to study the behavior of adsorbed nano-gold.9–17

24

Much effort has been spent on the investigations of metals on MoS2,18,19 and other

transition metal dichalcogenides, primarily for addressing contact issues associated with

devices where the transition metal dichalcogenides is the semiconductor channel. Yet in

addition to the investigation of Au nanoparticle combinations with MoS2 as catalyst,9–17

other transition metal nanoparticles combinations, including Cu,20 Ag,21–23 Pd,23,24 Pt13,23,24

have also been explored.

While, in general, the interaction of metals on monolayer dichalcogenides and the

bulk material should differ little, as only the interface should matter, there are

semiconductors, and band bending will be influenced by a metal adlayer. The band shifts

of various metals on MoS2(0001) and WeSe2(001) have been explored,25 and the response

of these different chalcogenides is found to be very different. Band bending might be

implicated in these differences because MoS2 is typically n-type26–29 and WSe2 is typically

p-type,30,31 but another test for band bending as affecting the electronic structure of metals

deposited on metal dichalcogenides is to compare monolayer films with bulk single

crystals. This study is in comparison with other studies of low coverage gold nanoclusters

and films on graphene and hexagonal boron nitride, as well as other known catalytic metal

– MoS2 systems.

3.2 Gold Growth on MoS2

Comparison of low energy electron diffraction (LEED) imaging of clean MoS2 to

that of 0.3 ML gold deposited on MoS2 (Figure 3.1) supports a gold clustering deposition

pathway. This gold clustering behavior is apparent in the LEED imaging as there is no

apparent change to surface symmetry while there is signal intensity reduction, meaning a

heterogeneous gold cluster and MoS2 surface. A homogeneous gold growth surface would

25

produce Moiré patterns, further diffraction patterns, within LEED from a lattice mismatch

of the MoS2 sublayer and gold over layer. Deposited gold clustering is further supported

through scanning tunneling microscopy (STM) imaging (Figure 3.2) where an affinity for

deposited gold on the MoS2 substrate to cluster heterogeneously at lower coverages, 0.3

ML gold on MoS2, is apparent. A topological slice of the STM image (Figure 3.3)

illustrates the bilayer nature of deposited gold on the MoS2 surface at low coverage.

Surface heterogeneity, i.e. gold clustering, is further illustrated through compiled gold

coverage dependent valance and conduction band spectra on MoS2 (Figure 3.4). Gold 5d

and metallic states found within the gold-graphene-ruthenium system16 are not as present

(possible hybridization) within gold on monolayer MoS216 nor the Au-MoS2 under study

here. Here it is apparent that there is strong hybridization of the gold to the MoS2 substrate.

The lack of characteristic gold 5d features in combination with a distinct lack of a density

of states near the Fermi level, reject homogenous gold growth and metallic character in the

Au-MoS2 system. From other studies, where the gold 5d structure is apparent, as shown

gold on graphene16 there is a distinct lack of catalytic behavior6 whereas with the Au-MoS2

system there is catalytic behavior in the catalytic production of acetaldehyde.1

26

Figure 3.1: LEED images of clean MoS2 and 0.3 ML Au on MoS2 at 86 eV and 110 eV

beam energy respectively.

Figure 3.2: The STM image of 0.3 ML Au on MoS2. Note that the deposited gold formed

islands nominally 5 nm in diameter.

27

Figure 3.3: Height trace along bar inset to the lower left corner of Figure 3.2, note that

the gold islands are approximately 1.8 nm in height

28

Figure 3.4: The spectra of occupied and unoccupied states of increasing thickness of Au

on MoS2 single crystal with comparison to Au on graphene on ruthenium16, note the

similarity in occupied states at -6 eV at increasing Gold coverage compared to states

present with gold on graphene. Also note the suppression of the MoS2 states at -2.5 eV

and -4 eV at higher cover while the Gold 5d state is not present as it is in the graphene

system. Finally note the suppression of the unoccupied Mo state at higher coverage.

29

3.3 Charge Transfer in the Gold-MoS2 system

Key to the catalytic character of the gold-MoS2 system,9–17 along with other gold

nanocluster systems,4–8 can be found with the interaction and electron density transfer

between the supporting MoS2 with the deposited gold, and importantly a shift of the gold

5d band towards Fermi level,8,16 resulting in electron density availability for interaction

with adsorbates. In addition to valence band measurements of the gold-MoS2 system

(Figure 3.4) which showed no prominent gold 5d structures and hence cannot speak to a

movement of charge, X-ray spectroscopy of the gold 4f peaks (Figure 3.5) show an

indication of a small degree of charge transfer to the gold of about 0.1 eV shift to lower

binding energy between the 0.3 ML deposition and 1.0 ML gold deposition pushing the

gold peaks in line with the bulk peak positions of 4f7/2 = 84 eV.

30

Figure 3.5: The coverage dependent XPS spectra of the gold-MoS2 system centered on

the Au 4f states, included reference lines are to the standard 4f7/2 Au peak at 84 eV and

4f5/2 with Δ =3.67 eV

Indications as to the source of the charge transfer are provided by the X-ray

spectra of the supporting MoS2 substrate. One indicator being the 3d peak of

molybdenum and the 2p of sulfur. The 3d molybdenum spectrum (Figure 3.6) also shows

a small binding energy shift, here of 0.1 eV but now towards higher binding energy,

when comparing the peak positions of clean MoS2 to that of MoS2 following deposition

of 1.0 ML of gold. This charge transfer is indicative that there is electron density transfer

from the metal center molybdenum on the substrate to the adsorbed gold.

31

Figure 3.6: The coverage dependent XPS spectra of the gold-MoS2 system centered on

the molybdenum 3d states, where there is a slight shift towards higher binding energy,

included reference lines are to clean MoS2 Mo3d peak positions.

Further support into to the nature of the charge transfer is apparent when looking at the

sulfur 2p spectra (Figure 3.7) where there is indeed a small energy shift to higher binding

energy similar to that found with the molybdenum 3d peak.

32

Figure 3.7: The coverage dependent XPS spectra of the gold MoS2 system centered on

the sulfur 2p states with fitting providing insight on sulfur chemical environments at

greater gold coverages. included reference lines are to clean MoS2 S2p peak positions.

3.4 Expectations for Gold Nanocluster Theory

To illustrate the expectation that the electronic structure of Au on the basal plane

of single layer (SL) MoS2 and on the MoS2(0001) surface should be similar, Duy Le of

Talat Rahman’s research group calculated and then compared the density of states (DOS)

few atom clusters, 1 layer and two layer of Au on single layer, bilayer, and trilayer MoS2.

The results are shown in Figure 3.8. Our calculations show that there is hardly any

difference between density of states of Au on different thickness of MoS2. Even though we

33

did not consider thicker MoS2 to mimic the MoS2(0001) surface, the results from 1 to 3

layers can be extrapolated to a general conclusion for the case of the MoS2(0001) surface,

i.e. no significant difference in the effect of single layer or (0001) surface of MoS2 on

electronic density of states of adsorbed Au. Here there is no clear indication of the

experimentally observed strong hybridization of the gold-MoS2 as apparent within the band

structure in Figure 3.4. While the shortcomings of standard DFT theory in predicting

behavior are further discussed in the following section in terms of monolayer versus bulk

properties, we may immediately observe the short coming of DFT in prediction of the

unoccupied states at ~1 eV in Figure 3.4 and Figure 3.8.

Figure 3.8: The calculated density of states for gold on single layer, bilayer and trilayer

MoS2 (binding energy are given with respect to the Fermi level E-EF=0) both predicting a

large density of state near the Fermi level. Courtesy of Duy Le.

34

3.5 Metal – Transition Metal Dichalcogenides Trends

In consideration of foundational Gurney modeling32 of adsorbate interaction on metals, a

collection of binding energy shifts seen for multiple metals deposited on MoS2 and WSe2

are plotted with respect to a modified Gurney model for bulk and monolayer substrates,

i.e. in terms of the work function of the metal versus the work function of the transition

metal dichalcogenide, as shown in Figure 3.9 and Figure 3.10, vastly different trend lines

are apparent. From Figure 3.9, it is clear that for these bulk transition metal

dichalcogenides, the Gurney model is an incomplete description. Following the Gurney

model, for bulk substrates, the trend line apparent within Figure 3.9 should have opposite

correlation. There should be a reduction of the substrate work function with the addition

of a lower work function metal, based upon a donation of electron density to the

substrate. Similarly, for the addition of a higher work function metal, there should be

observed increase in work function. However, due to band bending within the substrate

volume, this does not occur. Yet, as apparent with the metal-monolayer interactions

within Figure 3.10, with some offset, we see a trend line in general agreement with the

Gurney model. While the inclusion of more systems would greatly help illustrate the

trends shown in Figure 3.10, the general difference in trend lines between the bulk and

monolayer systems point to a failure of current standard predictive modeling. If we are to

uses standard rudimentary forms of DFT which are not including of an accounting for the

difference in band structure between monolayer and bulk MoS2, including substrate

interactions and band bending.33–36 Monolayer MoS2 and any other monolayer material is

usually accompanied and thus perturbed by an underlying substrate. These substrates can

influence the monolayer systems with the addition of band bending and interface charge

35

transfer effects. A better, more accurate, predictive theory requires a more complete

picture and understanding of the multitudes of underlying mechanisms.

Figure 3.9: The trends of binding energy binding energy shifts of occupied and

unoccupied states for deposited metals on bulk MoS2 and WeS2 crystals. Bulk

interactions of sodium and cobalt data were taken from earlier works.25

36

Figure 3.10: The trends of binding energy shifts of occupied states for deposited metals

on monolayer MoS2, note the opposite slope as seen from Figure 3.8. Monolayer data

was taken by the Bartels group, with gold data taken from earlier work.16

3.6 Summary

The combined results of spectrographic and imaging point to the possibility that the Au-

MoS2 system may possess catalytic activity properties. Most prominent support of this is

the clustering seen within STM and LEED that match theory and other experimental

findings that nano-particles of gold produce catalytic activity3,17,37. Additionally within

similar systems5,7,8 obvious density of states near the fermi level within compiled spectra

point to the metallic behavior of these non-catalytic systems; Au-graphene and Au-h-BN.

37

What we may conclude from this experimentation is the following: The Au-MoS2 system

experiences clustering that aligns with possible catalytic behavior, and secondly the Au-

MoS2 system behaves differently than gold deposited in similar symmetry 2-D

materials5,7,8 which lacked catalytic activity while expressing metallic like behavior with

density of states near the Fermi level while also expressing shifts in Au 5d state. Herein

we determine that the Au-MoS2 system may exhibit possible catalytic activity and as

such testing for the surface oxidation of carbon monoxide or other processes may provide

further insight to catalytic behavior.

From the compiled data, for a variety of metals on monolayer and bulk transition

metal dichalcogenides, there is apparent discrepancy in behavior bulk crystal versus

monolayer behavior not predicted by theory. As presented, the source of this discrepancy

cannot be simply identified. Certainly band bending and charge transfer effects can be

significant in the surface region of a bulk semiconductor crystal, so the comparison bulk

semiconductor crystals to equivalent monolayer systems is difficult, especially for

MoS2(0001), where the top of the valence band is at the Brillouin zone edge for the

monolayer38–40 and at the Brillouin zone center for the bulk crystal.39,41–55 For

experiments, the monolayer MoS2 and WSe2 are complicated in the fact that there is an

underlying support of often SiO2 or other suitable substrate that influence the surface

behavior of the attached TMD monolayer with a further degree of band bending that is no

often taken into consideration with theoretical calculations16. Herein lies the issue, to use

theoretical modeling as a prediction of catalytic behavior on metal-MoS2 systems more

appropriately, influence, such as that provided by a supporting substrate, must be

accounted in calculation.

38

References

(1) Almeida, K.; Chagoya, K. L.; Felix, A.; Jiang, T.; Le, D.; Rawal, T. B.; Evans, P.

E.; Wurch, M.; Yamaguchi, 1 Koichi; Dowben, P. A.; et al. Towards Higher

Alcohol Formation Using a Single-Layer MoS2 Activated Au on Silica: Methanol

Carbonylation to Acetaldehyde. 2019.

https://doi.org/10.26434/CHEMRXIV.8044607.V1.

(2) Rodriguez, J. A.; Si, R.; Evans, J.; Xu, W.; Hanson, J. C.; Tao, J.; Zhu, Y. Active

Gold-Ceria and Gold-Ceria/Titania Catalysts for CO Oxidation: From Single-

Crystal Model Catalysts to Powder Catalysts. Catal. Today 2015, 240 (PB), 229–

235. https://doi.org/10.1016/j.cattod.2014.06.033.

(3) Hong, S.; Rahman, T. S. Rationale for the Higher Reactivity of Interfacial Sites in

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43

CHAPTER 4

CHARGE TRANSFER IN CATALYSIS

Indications for a role played by charge transfer were provided in the previous

chapter. There the indicators of charge transfer were very small. In this chapter, they are

not. This chapter focuses on composite materials of MoS2 for use in catalysis. Here we

introduce composite materials of MoS2 where the electronic properties, and structure of

MoS2 aid in increasing catalytic efficiency over single constituent systems. In the case of

carbon nanotube and MoS2 composites, MoS2 provides large surface area and active Mo

sites for catalytic activity, while CNT contributes a highly conductive framework

allowing for charge transfer from MoS2 active sites increasing efficiency over pure MoS2

in hydrogen evolution reactions. In the case of TiO2 and MoS2 composite materials, we

also see an influence of charge transfer, where the MoS2 electron density donation to

TiO2 allows for MoS2 to act as an electron sink, extending the photogenerated charge

carrier recombination time.

4.1 Carbon Nanotubes–MoS2 for Enhanced Hydrogen Evolution

Reaction

4.1.1 Introduction

Hydrogen is a potentially renewable and eco-friendly candidate with high energy

density to replace fossil energy.1–4 Hydrogen produced, however, by steam reforming of

fossil fuel hydrocarbons is not as desirable due to carbon monoxide by-products.5 The

hydrogen evolution reaction (HER), i.e. water splitting via electrolysis, is a far more

promising way to produce hydrogen.6 Yet an electrocatalyst with low overpotential is

44

required to obtain hydrogen efficiently. The most well-known catalyst is platinum (Pt),

which has exhibited high catalytic activity at nearly zero overpotential,7,8 but its high cost

and scarcity diminish its application. Hence, a cheap and earth abundant electrocatalyst is

still needed as an alternative for Pt.

Theoretical and experimental analysis has shown that transition metal

dichalcogenides, such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2),

could be good candidates to replace Pt as electrocatalysts for HER.9–14 The low

conductivity and insufficient number of active sites of the transition metal

dichalcogenides limits their practical applications.15,16 Various strategies have been

developed to increase applications, and the latest strategy is to support MoS2 using

carbon based conducting materials such as graphene, activated carbon, and carbon

nanotubes.13,17–19 We have demonstrated that MoS2 supported by thermally reduced

graphene oxide improved the catalytic activity based on good electrical conductivity,

with the number of active sites of MoS2 increased in methanol solvent due to the defects

formed on MoS2 surface.20,21 Indeed, it has been demonstrated that methanol adsorption

can be responsible for defect creation on MoS2 .22 Carbon nanotube (CNT) also has

excellent properties such as high electrical conductivity, high surface area, and strong

mechanical strength.23 Utilizing these advantageous properties, CNT has been also used

in catalyst applications, such as methanol fuel cells.23–25

Herein, MoS2 supported by CNT was successfully synthesized by the

hydrothermal method. Based on low overpotential, low Tafel slope, and low resistance

with excellent stability, we conclude that the synthesized CNT-MoS2 composite has high

HER activity. The high HER activity is due to the synergetic effects of both CNT and

45

MoS2 which improve composite conductivity in addition to contributing to the large

surface area leading to improved catalytic activity.

Molybdenum disulfide, supported by a carbon nanotube matrix fabricated through

the hydrothermal method, is found to be an alternative catalyst for enhanced hydrogen

evolution reactions. The improved performance of carbon nanotube and molybdenum

disulfide (CNT–MoS2) composites is attributed to the synergic effects of CNT and MoS2

that provide a good conducting network as well as a large surface area, increasing

catalytic activity for the hydrogen evolution reaction. The synthesized carbon nanotube

and molybdenum disulfide (CNT–MoS2) composite provides a low overpotential (-0.14

V), much lower than MoS2 itself (-0.29 V), at a current density of 10 mAcm-2. The CNT–

MoS2 composite shows lower resistance (24 Ω) compared to the MoS2 precursor (1000

Ω) at a frequency of 3000 Hz, demonstrating an improved composite conductivity. The

CNT–MoS2 composite also exhibits high stability and excellent durability.

4.1.2 Synergetic Interaction of CNT Conductivity and MoS2 Active Sites

Key to emphasizing the synergetic interactions of conductivity and active sites

between CNT and MoS2, we compare CNT–MoS2 samples prepared from commercially

available MoS2 (< 2 μm particle) denoted as CNT–MoS2–S and those prepared with

MoS2 prepared through the hydrothermal method, denoted as CNT–MoS2. Figure 4.1a-e

shows XPS survey spectra of the composites, proving existence of O, C, Mo, and S based

on O 1s (532 eV), C 1s (285 eV), Mo 3d (230 eV), and S 2p (163 eV) peaks without any

contaminants.26–30 The high resolution XPS C 1s peak of the composites was

deconvoluted to two peaks which are C=C (285.1 eV) and C–C (286.9 eV),

corresponding to sp2 and sp3 states of carbon. The CNT–MoS2 composite showed

46

indications of more sp2 structure and less sp3, in the electronic structure, compared to

CNT–MoS2–S, providing an expectation of a bit higher conductivity of CNT–MoS2 due

to the sp2 structure. The O 1s peak was also deconvoluted to three peaks; O–C=O (534.4

eV), C=O (533.2 eV), and C–O (531.6 eV). Figure 4.1d shows Mo core level 3d spectra

of the composites, and was deconvoluted to six peaks, such as Mo4+ 3d5/2 (230 eV), Mo4+

3d3/2 (233 eV), Mo5+ 3d5/2 (230.6 eV), Mo5+ 3d3/2 (233.7 eV), Mo6+ 3d5/2 (232.5 eV), and

Mo6+ 3d3/2 (236 eV) in CNT–MoS2–S. The presence of additional oxidation states of Mo

other than Mo4+, in the CNT–MoS2–S composite, is consistent with results reported

elsewhere for small MoS2 clusters undergoing redox reactions,31 while remaining also

with the Raman results as both the Raman and these XPS results indicate charge transfer.

However, for the CNT–MoS2 composite, the features are less easily discerned which

might well be the result of highly desirable more heterogeneous nature of the smaller

MoS2 clusters,13,32 within the CNT–MoS2 composite. The differences between the CNT–

MoS2 composite and the CNT–MoS2–S composite are reflected in S 2p core level

photoemission spectra (Figure 4.1e), not just the Mo 3d core level features. The spin-orbit

coupling peaks at 162.6 eV (S 2p3/2) and 163.5 eV (S 2p1/2) in CNT–MoS2–S are merged,

weak, and broad in CNT–MoS2.

47

Figure 4.1 The XPS core level spectra of the CNT–MoS2 and CNT–MoS2–S samples in

(a) wide energy scan, and for the (b) C 1s, (c) O 1s, (d) Mo 3d, and (e) S 2p core levels.

The PL results (taken by Thilini Ekanayake) for the composite are plotted as well (f).

Figure 4.1f shows photoluminescence (PL) spectra of the composites. The two

main PL spectral peaks originated from the direct and indirect bandgap,33 denoted by D

and I (respectively) in Figure 4.1f. The D feature is present at 622 nm, and is the same for

the both the CNT-MoS2 composite and the CNT–MoS2–S composite, and corresponds to

1.9 eV direct band gap of bulk MoS2.34 However, the indirect bandgap transition (I) were

located at 754 and 715 nm, corresponding to 1.64 and 1.73 eV for CNT–MoS2–S and

CNT–MoS2, respectively. It was concluded that CNT–MoS2 synthesized by the

hydrothermal method dramatically changed its chemical states of MoS2 as well as

indirect bandgap by 0.1 eV compared to CNT–MoS2–S.

From the morphology, we would expect better conductivity and higher catalytic

properties CNT–MoS2–S and CNT–MoS2, but especially the latter. To further investigate

48

the catalytic activity of the samples, linear sweep voltammetry (Figure 4.2a) was

performed in 0.5 M H2SO4. CNT–MoS2 exhibited the lowest overpotential (-0.14 V)

followed by CNT–MoS2–S (-0.19 V), MoS2 (-0.23 V), MoS2–S (0.29 V), respectively, at

10 mAcm-2, and the results are summarized in Figure 4.2b. The CNT–MoS2 composite

exhibited the lowest overpotential among the MoS2 samples, meaning good catalytic

behavior for HER in comparison with the others. Tafel plots (Figure 4.2c) were used to

examine the efficiency of the catalytic reaction as it describes the potential difference

necessary to increase or decrease the current density. The Tafel equation;

𝜂 = 𝑏 𝑙𝑜𝑔 (𝑗

𝑗𝑜) (4.1)

(where 𝑗 is the current density, 𝑗𝑜 is the exchange current density, and 𝑏 is the Tafel

slope) was used to demonstrate a relationship between the current and potential at a

certain overpotential (𝜂), as described elsewhere.35,36 The CNT, by itself, exhibited the

highest Tafel slope of 173 mVdec-1 followed by MoS2-S, MoS2, CNT–MoS2–S, and

CNT–MoS2 with Tafel slopes of 86, 72, 41 and 39 mVdec-1, respectively. The CNT–

MoS2 exhibited the lowest Tafel slope which is comparable to that (30 mVdec-1) of

commercial platinum (Pt/C), demonstrating that CNT–MoS2 by the hydrothermal method

improved catalytic behavior due to the synergistic effects of CNT and MoS2. Previous

studies of MoS2 for HER also showed comparable results with a Tafel slope of 38,37 43,38

47,39 and 32 mVdec-1 has been obtained.40,41 Electrochemical impedance spectroscopy

(EIS) of the samples was performed (Figure 4.2d), and CNT–MoS2 exhibited the lowest

resistance compared to the others throughout the frequency range, confirming that CNT

49

acts as a conducting network. It is noteworthy that MoS2 synthesized by the hydrothermal

method has lower resistance compared to MoS2–S.

Figure 4.2 (a) the linear sweep voltammetry results of various sample with (b) the

corresponding overpotential at 10 mVcm-2 and (c) Tafel plot. The electrochemical

impedance spectroscopy results of the various samples are also plotted (d). Measurement

and characterization by Theophile Niyitanga.

Several weight ratios of CNT to MoS2 but of the same total weight were

investigated to obtain the polarization curves. The linear sweep voltammetry results of

the CNT–MoS2 composites in which the weight ratio of CNT to MoS2 are 15:15, 10:20,

5:25, and 3:27, are plotted in Figure 4.3a. The overpotential increases with the quantity of

MoS2 in the composites. Similar behavior was also shown in Figure 4.3b in which the

50

quantity of CNT increased. The corresponding Tafel plots are shown in Figure 4.3c and

Figure 4.3d. The ratio of 15:15 composite exhibited the best catalytic activity. The

corresponding electrochemical impedance spectroscopy results are displayed in Figures

4.4a-d, where the resistance was also increased with the quantity of CNT or MoS2.

Capacitance was also calculated from cyclic voltammetry at various scan rates from 5 to

120 mV s-1, results showing that the ratio of 15:15 composite has the highest capacitance,

as shown in Fig. 4.4c. The surface area of the samples also showed that the ratio of 15:15

composite has the highest surface area, as shown in Fig. 4.4d. Therefore, proper quantity

of CNT or MoS2 is needed for the highest catalytic and capacitive performance of the

composites with the largest surface area.

51

Figure 4.3 Linear sweep voltammetry (a and b), the Tafel plots (c and d) results of the

various CNT–MoS2 composite samples, with different ratio of CNT and MoS2.

Measurement and characterization by Theophile Niyitanga.

52

Figure 4.4 Electrochemical impedance spectroscopy (a and b), capacitance (c), and

surface area (d) results of the various CNT–MoS2 composite samples, with different ratio

of CNT and MoS2. Measurement and characterization by Theophile Niyitanga.

4.1.3 Summary

The CNT-MoS2 composite, synthesized via the hydrothermal method, appears to

be an extremely valid low-cost catalyst for the enhanced hydrogen evolution reaction

(HER). The composite demonstrated a low overpotential of -0.14V at 10 mAcm-2 and

small Tafel slope of 39 mVdec-1, proving there is enhanced HER activity of the CNT–

MoS2 composite. The CNT–MoS2 composite also exhibited the lowest resistance in all

frequency ranges investigated. The higher conductivity of the composite material is

attributed to a highly conductive CNT network. The enhanced catalytic performance of

the composite, comparable to expensive Pt, is attributed to the synergic effects of high

53

conductivity stemming from the CNT network and the large active sites originating from

the MoS2. The composite, therefore, could be a promising catalyst candidate to replace

the existing noble metal for the HER catalyst.

4.2 Titanium Dioxide-MoS2 for Photocatalytic Degradation of

Methylene Blue

4.2.1 Introduction

Environmental contamination and degradation due to the release of large amounts

of wastewater (containing azo-dyes) into the environment, pose a great threat to

surrounding eco-systems.42 Hence, there is an urgent need to detoxify harmful

compounds in the wastewater. In this context, semiconductor photocatalysts have

attracted much interest due to their potential application in energy conversion and

environmental purification. Certainly, photocatalysts have been used for the reduction of

organic contaminants both in water and air.43–45

Among photocatalytic semiconductors, titanium dioxide (TiO2) has attracted

considerable interest due to its ability to readily breakdown organic pollutants, strong

oxidizing capabilities, low toxicity, chemical stability, low cost, and availability.46–49

Photocatalytic performance of TiO2 is mainly determined by the lifetime of

photogenerated electron-hole pairs, but the fast recombination rate of electron-hole pairs

in TiO2 has limited its application in photocatalysis.50,51 In order to extend the lifetime of

photogenerated electron-hole pairs, hybrid photocatalysts consisting of semiconductor

hetero-junctions are necessary to suppress the fast recombination rate of photogenerated

carriers.52,53

54

While TiO2 has long been established as a photocatalyst for the removal of

pollutants,54–56 studies have also shown that molybdenum disulfide (MoS2) acts as good

co-catalysts for the degradation of pollutants by coupling with other semiconductor

photocatalysts.57–59 Furthermore, MoS2 is non-toxic, a strong oxidizing agent with high

stability, and a relatively low-cost material.60 Due to the large surface area of MoS2, it

could act as a good co-catalyst for TiO2. The band gap of TiO2 is 3.0 eV and 3.2 eV for

the rutile and anatase structures, respectively, whereas the bandgap of MoS2 has been

reported to be of ~1.9 eV.34 Efficient charge separation might be obtained due to the

difference in bandgap,61,62 thus the TiO2 and MoS2 combination may have a much lower

threshold for photocatalysis and possibly greater efficiency at lower photon energies.

In this study, we synthesized TiO2–MoS2 hetero-photocatalyst using the sol-gel

method. The sol-gel method has some advantages compared to chemical vapor

condensation and micro-emulsion, such as low temperature synthesis, cost effectiveness

and easy control of reaction kinetics by varying the composition of chemicals.63–65 The

photocatalytic activities of the synthesized materials were investigated by measuring the

degradation rate of aqueous methylene blue (MB) under ultraviolet (UV) light irradiation.

It could be expected that the loading of TiO2 with MoS2 varies the lifetime of

photogenerated charge carriers, and that the addition of the smaller band gap MoS2 could

result in improved photocatalytic performance.

Photocatalytic degradation of methylene blue (MB) was performed using titanium

dioxide and molybdenum disulfide (TiO2–MoS2) composites. TiO2–MoS2 composites

were synthesized using the sol-gel method. By weight, the ratio of MoS2 to TiO2 was

varied from 3:5 to 9:5. The composites showed better photocatalytic degradation of MB

55

compared to pure TiO2 under irradiation of ultraviolet light, and the 7:5 ratio (which was

labelled S3) provides the best photodegradation efficiency for MB. TiO2 degraded MB

under 180 min while S3 degraded MB under 80 min. The photocurrent response of pure

TiO2 was 0.55 μA but was increased to 20.08 μA (S3) after the addition of MoS2. The

electrochemical results reveal that the introduction of MoS2 into TiO2 improves the

electronic conductivity and the lifetime of the photogenerated charge carriers, resulting in

higher photocatalytic behavior in the decomposition of methylene blue.

4.2.2 Enhanced Charge Transfer and Exciton Lifetime

Core level X-ray photoemission spectroscopy (XPS) was used to characterize the

chemical states of TiO2 and TiO2–MoS2 (S3 sample). Figure 4.5 shows the high

resolution XPS spectra of the O 1s, S 2p, Ti 2p, and Mo 3d core levels. The O 1s state of

TiO2 can be deconvoluted into two peaks at 530.45 eV and 531.59 eV, as shown in

Figure 4.5a. The O 1s core level peak at a binding energy of 530.45 eV corresponds to

oxygen bonded to Ti (Ti–O–Ti peak) and the binding energy at 531.45 eV corresponds to

hydroxyl OH− groups on the surface.66 The addition of MoS2 shifted Ti–O–Ti binding

energy peak from 530.45 eV to 529.05 eV while the OH− peak also decreased in binding

energy from 531.59 eV to 530.52 eV. Figure 4.5b shows the S 2p core level spectra of the

samples. Using a Gaussian fitting to the XPS features, the S 2p spectra was deconvoluted

into two peaks located at 161.40 eV and 162.55 eV which can be assigned to the S 2p3/2

and S 2p1/2 of divalent sulfide ions (S2−) respectively.67 When MoS2 is combined with

TiO2, the S 2p3/2 and S 2p1/2 core level peaks of MoS2 increased in binding energy to

161.55 eV and 162.68 eV, respectively. The S 2p core level peak shift was not as

dramatic as the O 1s shift. Figure 4.5c shows Ti 2p core level spectra of the samples. The

56

two peaks of TiO2 at binding energies 466.65 eV and 460.88 eV can be assigned to the Ti

2p1/2 and Ti 2p3/2 respectively. These peaks indicate the presence of Ti4+ in TiO2.68

Figure 4.5 XPS spectra of (a) O 1s of TiO2, (b) S 2p of MoS2, (c) O 1s and (d) S 2p of

TiO2–MoS2, and (e) Ti 2p and (f) Mo 3d of the two samples.

Upon the addition of MoS2, there was a decrease in the Ti 2p1/2 and Ti 2p3/2 spin orbit

doublet binding energies from 466.65 eV to 465.53 eV and 460.88 eV to 459.42 eV, for

Ti 2p1/2 and Ti 2p3/2 respectively. The Mo 3d3/2 and Mo 3d5/2 peaks of the Mo 3d state are

57

located at binding energies of 232.95 eV and 229.82 eV respectively, as shown in Figure

4.5d. These core level peaks of Mo 3d3/2 and Mo 3d5/2 shifted to higher binding energies

by 0.2 eV and 0.12 eV, respectively, when MoS2 is combined with TiO2. The shift of the

core level binding energies of TiO2 and MoS2 is due to the electronic interaction between

TiO2 and MoS2, confirming the formation of heterostructures between them.69 The

decrease in the core level binding energies of TiO2 when MoS2 is added, and the

corresponding increase in MoS2 core level binding energies confirm that charge transfer

can occur at the interface of TiO2 and MoS2.70 In this case, transfer of an electron density

from MoS2 to TiO2.

The photocatalytic activities of the samples were examined by degrading MB

under UV-light irradiation. Figure 4.6 shows the UV–Vis spectra of the samples after

photocatalytic activity with MB in 20 min increments. The typical absorption peak of MB

was observed near 600 nm,71 and the peak intensity decreased with UV irradiation time.

This decrease in the MB optical absorption peak is due to the photocatalytic activity of

the samples (S1-S4 which are 3:5, 5:5, 7:5, and 9:5 compositions by weight ratios of

MoS2 to TiO2 respectively). The decrease in the peak at 600 nm implies that MB is being

photodegraded by the sample.72 After 180 min of UV irradiation, no MB optical

absorption peak was observed for the TiO2 sample (Figure 4.6a). By way of comparison,

the 600 nm MB optical absorption peak almost completely vanishes after irradiation

times of 140, 100, 80, and 120 min for the composite samples S1, S2, S3 and S4,

respectively. The photocatalytic activity of MoS2 is shown in Figure 4.6f. Though MoS2

has a large surface area, but because of the low bandgap ~1.9 eV,34 the photogenerated

charges recombined very fast. This leds to a bad photocatalytic activity of MoS2. All the

58

TiO2–MoS2 composites degraded MB faster compared to pure TiO2. The variations of the

photocatalysis performance with varying MoS2 to TiO2 compositions indicate that the

photodegradation is sensitive to the catalyst compositions. This is consistent with the

expectations. The different energy bandgaps of MoS2 and TiO2 delayed the

recombination time of excitons,73,74 resulting in the enhancement of the photocatalytic

activity of the composites. MoS2 is also known to have a large specific surface area,

which can serve as an active adsorption sites for the degradation of pollutants and

improve the photocatalytic activities,73 so that MoS2–TiO2 mixtures is better for the

charge transfer compared to pure TiO2.

Figure 4.6 UV–Vis spectra of the degradation of MB in the presence of the samples as a

function of time interval of 20 min. (a) TiO2, (b) S1, (c) S2, (d) S3, (e) S4, and (f) MoS2.

Measurement and characterization by Olaniyan Ibukun.

59

The percentage of photodegradation of MB can be estimated using the equation

below:75

𝐷(%) =𝐶0 − 𝐶𝑡

𝐶0

(4.2)

where 𝐶0 is the initial concentration of MB, and 𝐶𝑡 is the concentration of MB after

photo-irradiation at a given time 𝑡. Figure 4.7 shows the photodegradation percentage of

the TiO2, S1, S2, S3, and S4 samples. The photodegradation efficiency of the TiO2 and

MoS2 composite samples are significantly better than pure TiO2, as it is evident from the

decrease in the MB optical absorption peak as discussed above. The photodegradation

efficiency at 40 min was 40, 59, 63, 80, and 62% for TiO2, S1, S2, S3, and S4,

respectively (i.e. 3:5, 5:5, 7:5, and 9:5 compositions by weight ratios of MoS2 to TiO2).

The improved photocatalytic efficiencies are the result of the MoS2 layered sheets that

favor photogenerated charge carrier transfer.76,77 In addition, the MoS2 bandgap is smaller

thus more amendable to photoexcitations while the TiO2 and MoS2 interactions favor

electron transfer from MoS2 to TiO2, as is evident in XPS. The best photocatalytic

degradation efficiency was displayed in the S3 sample, finding that excessive loading of

MoS2 compared to TiO2 can reduce the photocatalytic activity in the composite samples.

60

Figure 4.7 (a) Photocatalytic degradation of MB of the samples. Measurement and

characterization by Olaniyan Ibukun.

The transient photocurrent response under the UV light irradiation which was

conducted at the bias potential of 1 V is shown in Figure 4.8a. The photocurrent response

of pure TiO2 (0.55 μA) was the lowest due to the fast recombination of the electron-hole

pair.78 As loading of MoS2 increased, the photocurrent increased until the S3 sample due

to the extension of the lifetime of the photogenerated charge carriers under UV

illumination which helps to increase the photodegradation efficiency.79,80 However,

further loading of MoS2 in the case of the S4 sample reduces the photocurrent due to the

less absorption of UV light. Figure 4.8b shows the electrochemical (absolute) impedance

of the samples as a function of frequency under the UV light irradiation. It is evident that

the composite samples had lower impedances as compared to TiO2, implying faster

transfer of electrons which further related with the faster rate constant of the degradation

of methylene blue.

61

Figure 4.8 (a) Transient photocurrent responses of the samples with light on and off

cycles, and (b) Bode plot of the impedance as a function of frequency. S1, S2, S3, and S4

denotes 3:5, 5:5, 7:5, and 9:5 compositions, by weight ratios, of MoS2 to TiO2.

Measurement and characterization by Olaniyan Ibukun.

4.2.3 Summary

TiO2–MoS2 photocatalyst composites were successfully synthesized using the

sol–gel method. The TiO2–MoS2 composite samples exhibited better performance in the

photocatalytic degradation of methylene blue (MB) under UV-light irradiation compared

to pure TiO2. The best degradation efficiency was achieved with the optimal loading of

MoS2 in the S3 sample (7:5 of weight ratio of MoS2 to TiO2), where 100% of MB was

decomposed in 80 min. Photocurrent measurements reveal that the recombination time of

the photogenerated charge carriers increased in the composite samples as compared with

pure TiO2. These results suggest that the introduction of MoS2 into TiO2 increases the

recombination time of the charge carriers, as expected from our hypotheses, as well as

MB degradation efficiency which will be of great help in the treatment of wastewater.

The presence of MoS2 in the composite acted as a sink for the excited electrons thus

62

elongating the recombination time of photogenerated charge carriers. In addition, MoS2

loaded samples favored fast charge transfer due to its layered structure. More

modification and reformation of the composite, such as precursor size effects, surface

pretreatments, and so on, could maximize the photocatalytic performance in the near

future.

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CHAPTER 5

CREATION AND ANNIHILATION OF SULFUR VACANCIES

Temperature and methanol exposure dependent chemistry and topographical

studies on MoS2(0001) are presented within this chapter. From X-ray photoemission

spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), inverse

photoemission spectroscopy (IPES), and scanning tunneling microscopy (STM) we show

that the methanol to surface methoxy reaction on the MoS2 produces surface sulfur

vacancies. We also show that these surface sulfur vacancies may be healed through

moderate temperature annealing. This creation and annihilation of sulfur vacancies on the

MoS2(0001) surface, driven by surface symmetry stabilization, has significant

implication on the catalytic application and behavior of MoS2 in the production of

alcohols as an alternative fuel.

The possible application of transition-metal dichalcogenides, such as MoS2, for

catalytic conversion of syngas (CO, CO2, and H2) to methanol (CH3OH),1–7 rests upon a

strong foundation identifying MoS2 as a catalyst for production of methanol and higher

alcohols.8–16 Thus, there is an excellent motivation to study some of the simpler observed

reaction steps predicted in theoretical calculations. The last key reaction step for

methanol formation is the conversion of a surface methoxy species and hydrogen to

methanol (CH3O* + H* → CH3OH).1,3 While this final intermediate reaction would be

difficult to observe under standard ultrahigh vacuum surface science techniques, the

reverse reaction using methanol to produce the more stable intermediate methoxy (CH3O)

species on the MoS2 substrate is amenable to experimental investigation. The adsorption

71

and subsequent dissociative reaction of methanol at the edge sites of MoSx clusters has

certainly been predicted using density functional theory (DFT), with the methanol

dissociation via O–H bond scission predicted to be the most favorable.17 On the

MoS2(0001) surface, however, methanol dissociation, via O–H bond scission, should be

less facile, unless accompanied by defect creation.

Significant efforts have been made by the several theoretical18–21 and

experimental22–26 studies in understanding the mechanism and energetics for methanol

dissociation on the metal-oxide surfaces. None of these studies, however, put forward

suggestions that the mechanism for methanol dissociation includes the reaction-induced

anion defect (vacancy) creation on the surface, which are critical for facile methanol

dissociation. Defects not only offer a favorable site for methanol adsorption.18,27

Methanol may also migrate to a defect site, which may enhance catalytic activity.

It is clear that defects remain an integral part of MoS2-based catalysis,1,28–30 and

this is no less true with the methanol to methoxy reaction. A previous DFT study1

indicated that there are barriers of 1.40 eV and 1.19 eV between the methoxy and

methanol moieties on MoS2, at a sulfur vacancy row and patch, respectively. Another

DFT study17 suggested that the most favorable pathway for methanol dissociation is via

O-H bond scission with the barrier of 1.0 eV at the 50% Mo-edge, and that the dominant

species, after the MoS2 surface is exposed to methanol, is methoxy (CH3O). Here, we

attempt to gain a better understanding the energetics of the methanol to methoxy reaction

process on MoS2. We propose a mechanism for methoxy formation and induced S

vacancy formation on MoS2, based on the scanning tunneling microscopy, core level

photoemission, photoluminescence measurements, and DFT simulations.

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Sulfur loss from the surface of MoS2(0001), as described above, follows the

adsorption of methanol on MoS2 at 86 K and subsequent annealing of MoS2 near 300 K.

This sulfur loss, at the MoS2 surface, leads to suppression of inverse photoemission

features characteristic of the unoccupied states associated with MoS2. This sulfur loss is

counteracted by further annealing to 350 K, as is evident in the temperature dependent

sulfur to molybdenum integrated X-ray photoemission intensity ratios near 300 to 350 K.

Upon further annealing to 350 K, inverse photoemission additionally indicates a

reestablishment of characteristic features associated with the unoccupied states of MoS2.

These results are indicative of sulfur segregation to the surface and compensation of

surface vacancy sites.

We find that exposure of the MoS2 basal face i.e. (0001) plane, to methanol leads

to the formation of adsorbed methoxy and coincides with sulfur vacancy generation. The

conversion of methanol to methoxy on MoS2 is temperature dependent. Density

functional theory simulations and experiment indicate that the methoxy moieties are

bound to molybdenum, not sulfur, while some adsorbed methanol is readily desorbed

near or slightly above room temperature. Supporting calculations also suggest that the

dissociation of methanol via O-H bond scission occurs at the defect site (sulfur vacancy),

followed subsequently by formation of a weakly bound H2S species that promptly

desorbs from the surface with creation of sulfur vacancy. Photoluminescence and

scanning tunneling microscopy show clear evidence of the sulfur vacancy creation on the

MoS2 surface, after exposure to methanol.

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Figure 5.1 XPS of the C 1s core level after 1 × 10−6 Torr·s methanol exposure on the left

and 6 × 10−6 Torr·s exposure on the right. Note that at low temperature there are two

contributions to C 1s core level, indicated by shading of area under XPS spectra,

indicative carbon in both methanol (blue) and methoxy (red) moieties.

5.1 Methanol Oxidation and Methoxy Formation

The formation of an adsorbed methoxy species from the exposure of MoS2 to

methanol is evident from the presence of both methanol reactants and a separate methoxy

product apparent on the surface after the exposure of MoS2 to methanol. Methanol is

predicted to be only weakly adsorbed on MoS2(0001)1 and is expected to be a minority

species and easily desorbed from the MoS2(0001) surface. In fact, present experiments

confirm this. The C 1s XPS core level spectra, after both short and long exposures of

MoS2 to methanol at low temperature (roughly at a MoS2(0001) substrate temperature of

100 K), show two carbon species with different chemical environments on the

MoS2(0001) substrate, as indicated in Figure 5.1. The methanol C 1s XPS core level

peak, seen in Figure 5.1, that appears at binding energies greater than 287 eV is

74

consistent with the methanol C 1s binding energy of 288.0 ± 0.3 eV31 and 286.6 eV32

reported for methanol adsorbed on CuO2, 287.07 eV33 and 286.734 for methanol on

Cu(111), 287.6 eV for methanol on TiO2,35 and the methanol C 1s binding energy of

286.5 eV.36 This XPS C 1s photoemission component is quickly quenched as temperature

increases above room temperature. Methanol, both volatile and weakly bound to the

substrate,11 is driven off with increased temperature leaving a strongly bound species

with smaller binding energy of 285.0 ± 0.2 eV, corresponding to methoxy, the dominant

surface species after methanol exposure to MoS2(0001). Such a behavior has been seen

for methanol decomposition to methoxy on CuO2, where again, with increasing

temperature, methanol is lost from the surface and a methoxy C 1s XPS feature is

observed at lower binding energies of 285.9 eV,31 286.0 eV,32 and 285.5 eV.33 Similar is

the example of methanol decomposition to methoxy on TiO2, where again, with

increasing temperature, methanol is lost from the surface and a methoxy C 1s XPS

feature is observed at lower binding energies of 286.2 eV, as opposed to the core level

binding energy of adsorbed methanol at 287.6 eV.35 This behavior is also true for

methanol decomposition to methoxy on Cu(111),34 where methanol is lost from the

surface well below room temperature and a methoxy C 1s XPS feature is observed at

lower binding energies of 285.8 eV, as opposed to the core level binding energy of

adsorbed methanol at 286.7 eV. Indeed, C 1s core level binding energies for adsorbed

methoxy in the range of 285.2−286.2 eV31–35,37–41 are pretty typical. The much lower

bonding energy for methoxy, of 285.0 ± 0.2 eV, is indicative of methoxy bonding to the

surface at a MoS2 defect, as discussed below, where charge donation from the substrate to

the methoxy moiety is more facile.

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Figure 5.2 Comparison and visualization of critical temperature for methanol to methoxy

conversion in 1 × 10-6 Torr·s methanol exposure on the left and 6 × 10-6 Torr·s exposure

on the right, note the increase in critical conversion temperature at higher methanol

exposures (300 K versus 400 K)

Figure 5.2 shows the binding energies of the various contributions to the C 1s

core level spectra as a function of temperature, following 1 × 10−6 Torr·s methanol

exposure on the left and 6 × 10−6 Torr·s exposure to MoS2(0001) at 100 K. At the low

exposure of methanol two peaks from methanol and methoxy coexist until 300 K, as

noted above, but evolve into a single C 1s feature corresponding to methoxy, as shown in

Figure 5.2a. However, methanol remains resident on MoS2 surface until 400 K, following

higher exposures to methanol, as shown in Figure 5.2b. We note that the higher

desorption temperature of methanol, at the longer methanol exposures, as summarized in

Figure 5.2, may be the result of a much higher sulfur vacancy defect concentration and

the greater possible coordination and thus stronger interaction with the substrate for

adsorbed methanol.1

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5.2 Sulfur Vacancies and Induced Defects

The effects of the methanol to methoxy reaction, on the MoS2 surface, are directly

observed in scanning tunneling microscopy (STM) and photoluminescence (PL). Under

multiple low temperature methanol exposure and annealing cycles (Figure 5.3) the

substrate begins to show point defects, as shown in Figure 5.3b, consistent with sulfur

vacancy creation. Under further methanol exposure and annealing, line vacancies and

larger multipoint defects appeared and became abundant with larger multipoint defects

nominally 2 nm in diameter as shown in Figure 5.3c. The scale of the line defects and

larger multipoint defects, nominally 2 nm in diameter, is highlighted in the inset to Figure

5.3.

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Figure 5.3 STM images validating the creation of point defects and larger defects on

MoS2(0001) with methanol exposure: (a) the clean MoS2(0001) surface (0.900 V gap

voltage, 800 pA constant current), (b) areas of point defects after 1 × 10−6 Torr·s

methanol exposure at 350 K (0.900 V gap voltage, 800 pA constant current), and (c)

larger 1 nm divots on sample after 2 × 10−6 Torr·s methanol exposure to MoS2(0001) at

350 K, followed by 1 × 10−6 Torr·s methanol exposure to MoS2(0001) on 110 K sample

and then annealing at 350 K (2.00 V gap voltage, 400 pA constant current). The inset

gives a larger view of defects within (c) with dotted lines tracing line defects and circled

multipoint defects.

There are experimental indications of the mechanisms for sulfur vacancy creation.

The pristine MoS2(0001) surface contains very few defects (Figure 5.3a). Repeated

exposure of MoS2(0001) to methanol at 100 K has little to no effect on the defect density.

However, exposure of the MoS2(0001) surface to methanol vapor at low temperatures,

followed by annealing to room temperature or above, or direct exposure at elevated

temperatures results in an increased number of point defects, some of which have grown

78

into larger, isolated defects. It is important to note that defect creation was not clearly

obvious on the MoS2 surface until a multiple sequence of methanol exposure, followed

by annealing of the sample was performed, thus providing a greater defect density in the

STM measurement. The formation of line defects is consistent with the sulfur-vacancy

row formation as predicted elsewhere.1 As discussed below, we need prior sulfur

vacancies to create other sulfur vacancies on MoS2. Without a sulfur vacancy, methanol

does not chemisorb on pristine MoS2. So, it is probable that methoxy formation initially

starts from a small concentration of isolated sulfur vacancies. The density functional

theory results indicate that the process of methanol dissociation on pristine MoS2 is

endothermic. There must be the enthalpy penalty for this process to be feasible.

We performed complementary experiments, at the micrometer scale, using MoS2

single-layer islands deposited by chemical vapor deposition.42 Pristine single-layer MoS2

has a pronounced photoluminescence response at 1.87 eV,43,44 which decays with

increased defect density of the material.45 We have characterized the material from our

chemical vapor deposition process in detail with regards to photoluminescence yield,46

and the results are indicative of a very low native defect density prior to methanol

exposure.

Rather than using argon ion beam based desulfurization, as in Ref. (45), here we

find that exposure to methanol leads to the same bleaching of the MoS2

photoluminescence. Figure 5.4 shows two single-layer MoS2 islands on 300 nm SiO2/Si

before and after boiling them in methanol under reflux for 4 h. While this procedure

leaves the overall structure of the islands intact, we find a significant reduction of the

photoluminescence yield indicative of defect formation. Notably, the photoluminescence

79

bleaching is not confined to nor particularly strong at the island edges, but rather is more

complete on the basal face. This clearly reveals that the basal plane is directly activated

by the methanol species and a methanol to methoxy reaction at the material edges is not

required. Although these results are for monolayer MoS2, they are consistent with the

above STM results.

Figure 5.4 The photoluminescence mapping of CVD-grown single-layer molybdenum

disulfide (MoS2) islands (triangles), performed before (left) and after (right) exposure to

methanol at 338 K, over a period of 4 hours. Decrease in photoluminescence indicates

defect formation and/or inhomogeneity creation in the MoS2 lattice. Defect formation is

not limited to edges but occurs throughout the basal plane. From the Ludwig Bartels

research group.

DFT simulations indicate that the weak chemisorption and dissociation of

methanol on basal plane of pristine MoS2 is unrealistic. Methanol was found to only

80

weakly adsorb on MoS2, with a binding energy of -0.24 eV while that of a co-adsorption

of a methoxy and an atomic H is +4.02 eV. It is worth mentioning that the presence of

atomic H, on pristine MoS2 does not substantially affect the binding energy of the

methanol moieties, which is found to be -0.23 or -0.31 eV for methanol adsorption on top

of or adjacent to S atom on which an atomic H binds. On the other hand, as interpreted

from results shown in Ref. (1), co-adsorption of a methoxy and an atomic H on sulfur

vacancies structures, such as vacancy row and vacancy "patch", as is experimentally seen

in Figure 5.3c, is energetically favorable over the weakly bound adsorption or

chemisorption of methanol. This leads us to a conclusion that the adsorption and

conversion to methoxy of methanol occurs only at defected sites on the basal plane of

MoS2. These calculations also show methoxy and atomic H prefer to adsorb at defect

sites. Based on these results, we suggest a mechanism for the creation of vacancy on

MoS2 under the exposure of methanol: Step 1: methanol adsorbs and dissociates at sulfur

vacancy sites to form methoxy and H atoms occupying the vacancy sites; Step 2: Once

the sites are populated, a migration of atomic H onto the pristine terrace of MoS2 occurs.

Step 3: atomic H diffuses on the pristine terrace of MoS2 to meet with other H to form

H2S, which, in turn, desorbs from the MoS2 surface to create S vacancy. The schematic

representation of these steps is shown in Figure 5.5.

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Figure 5.5 Schematic representation of the methoxy-formation-induced sulfur vacancy

on MoS2 with the various reaction steps: (1) adsorption of methanol, (2) dissociation of

methanol into methoxy and hydrogen, (3) diffusion of atomic hydrogen, (4) coalescence

of two hydrogen atoms with surface sulfur atom, and (5) desorption of hydrogen sulfide

with the formation of sulfur vacancy. Blue, yellow, black, pink, and red balls represent

the Mo, S, C, H, and O, atoms respectively. Image by Duy Le, with permission.

To validate the proposed mechanism, Duy Le and Talat Rahman investigated the

decomposition of methanol on MoS2 with a single sulfur vacancy site within DFT. They

find, as illustrated in Figure 5.6, that methanol physically adsorbs at the defect site with a

binding energy of -0.3 eV. It then needs to overcome a barrier of 0.26 eV to become

chemisorbed, via a Mo-O bond. Next, the molecule dissociates to methoxy and bound

atomic H with a barrier of 0.2 eV occupying the vacancy site. The resultant H atom, in

turn, can migrate toward a neighboring S atom with a barrier of 0.8 eV and start to move

away with a barrier of 0.12 eV, leaving methoxy adsorbed at the vacancy site.

82

Figure 5.6 Chain reaction of methanol on basal plane of MoS2 with a sulfur vacancy.

Horizontal bars and values (in eV) printed on it represent each intermediate state and

corresponding potential energy (zero potential corresponds to a state in which methanol

and MoS2 with a S vacancy are far apart) in this chain reaction. Potential of transition

states (TS) are noted. Values (in eV) along the arrows are the barrier for the

corresponding reaction. Blue, yellow, black, pink, and red balls represent Mo, S, C, H,

and O atoms, respectively. Image by Duy Le, with permission.

The potential energy of each state of the above chain reaction is summarized in

Figure 5.6. Even though the potential energy profile indicates an uphill chain reaction

with a noticeably high barrier, the values are in a reasonable range that does not require

extremely high temperatures to occur, suggesting that such a proposed mechanism for H

atoms to diffuse from vacancy sites to pristine terrace of MoS2 is feasible.

83

Figure 5.7 The core level O 1s XPS peak after 6 × 10-6 Torr·s exposure of methanol to

MoS2(0001) at 150 K (bottom) and 350 K (top) note the shift in binding energy.

Confirmation that the methoxy is bound to molybdenum, not sulfur, is evident in

Figure 5.7. The O 1s core level binding energy is far less at elevated temperatures, where

sulfur vacancy formation is more apparent, while the higher O 1s binding energies are

observed for the adsorbed methanol/methoxy species upon initial adsorption at 150 K, as

seen in the O 1s XPS spectra of Figure 5.7. At 150 K, the core level O 1s binding energy

is higher, this can be attributed to the presence of both methanol and methoxy species

coexisting at low temperature as suggested by the C 1s XPS spectra Figures 5.1a and

5.2a. The shift of the O 1s feature to lower binding energies at higher temperatures may

be attributed solely to methoxy bound to the MoS2 substrate, as the methanol is no longer

present at 350 K (Figure 5.2). The higher binding energy at low temperature coincides

with a more electronegative chemical environment requiring greater energy for the

ejection of an electron, such as would occur with weak binding of methanol and methoxy

84

species to the surface. The O 1s binding energy of 533.8±0.2 eV for methoxy adsorbed

on MoS2 (Figure 5.7) is much larger than the 530.1 eV to 530.9 eV binding energies

reported for methoxy bound to copper surface,37–40 but more in line with the 533.1 eV

binding energy reported for methoxy on Cu(111)33 and the binding energy of 532.8 eV

observed for methanol condensed on ZnO.47 Yet MoS2, even with sulfur vacancies, the

molybdenum atoms are still well coordinated to sulfur and a higher binding energy is

expected than for a methoxy species on a bare metal surface. We may additionally

conclude support of methoxy-molybdenum bonding, through an additional

electronegativity argument, as molybdenum will contribute electron density through the

Mo-O bond, decreasing the O 1s binding energy. The O 1s binding energy of 532.1 eV is

close to the binding energy of hydroxylated molybdenum oxide48 while the methoxy-

sulfur bonding seen at lower temperatures is larger, as expected, this remains a smaller

binding energy than uncoordinated methoxy of 533.7 eV.36

Figure 5.8 Reaction of atomic H forming H2 and H2S. The nomenclature "TS" indicates

transition state. Blue, yellow, and pink balls represent Mo, S, and H atoms respectively.

Image by Duy Le, with permission.

85

From our DFT simulations, we also find that on the basal plane of MoS2, without

defects, the barrier for H to rotate about one S atom is 0.19 eV and to hop from one S

atom to the other is 0.29 eV. These indicate the high mobility of bound atomic H on basal

plane of MoS2, suggesting that eventually two H atoms may land on two neighboring S

atoms from which either H2 or H2S is formed. The barrier for the formation of the former

product is found to be 0.42 eV (red reaction in Figure 5.8) and that of the latter product is

found to be 0.38 eV, suggesting a slight preference for the formation of H2S complex.

Once the H2S complex formed, it can desorb, with a barrier of 0.53 eV, creating S

vacancy on the MoS2 surface. As summarized in Figure 5.8, the formation and desorption

of H2S are downhill reactions (exothermic). This indicates that should H atoms exist on

the basal plane of MoS2, a sulfur vacancy will be formed.

Clearly, the conversion of the adsorbed methanol to methoxy consumes sulfur

atoms of MoS2, leaving the sulfur vacancies in the lattice, especially at the surface. This

is evident in experiment (Figures 5.3 and 5.4). In the conversion process, sulfur is

removed from the surface into a mobile state, likely producing H2S. As both methanol

and methoxy moieties are present on the MoS2 surface at low temperature (in the vicinity

of 100 to 200 K), and the conversion process is sulfur eliminating, at temperatures where

H2S desorbs, additional methanol exposure and conversion to methoxy is required to

further increase the sulfur vacancy density. The process, for methanol to methoxy

conversion, must be accompanied by an activation barrier, as is evident from the

energetics just discussed. From theory,17 the activation barrier has been estimated to be in

the vicinity of 0.45 eV on the bald Mo-edge and about 1.0 eV on the 50% Mo-edge and

86

50% S-edge of MoS2, where methanol adsorption is more facile, and must be much more

significant than on the basal face, as just discussed.

Figure 5.9 The oxygen to molybdenum XPS intensity ratios, for exposure of 1 × 10-6

Torr·s exposure of methanol, plotted as a temperature dependent rate (R), for given

temperature. The line indicates the best fit to the data and the shaded region, the range of

fits based on a statistical analysis of the data (using a jackknife indicator).

An activation barrier of the reaction is apparent from ratio of the O 1s to Mo 2p

core level intensities as a function of time and temperature. The rate of increase in the O

1s to Mo 2p core level intensity ratio, as a function of temperature has been plotted as an

Arrhenius plot in Figure 5.9. The measured activation energy is small, 12±6

meV/molecule, but experimentally, subsumes a number of different process, as indicated.

Nonetheless, an activation barrier for methoxy formation is both anticipated and

observed.

87

5.3 Sulfur Segregation

Figure 5.10 The Mo 3d5/2 and S 2p3/2 XPS intensity ratios, over the temperature range

from 86 K to 550 K. after 5 × 10-7 Torr·s methanol exposure on the MoS2(0001) at 86 K.

5.3.1 Surface Sulfur Loss

The methanol to methoxy conversion on MoS2(0001) following methanol

exposure (1 × 10-6 Torr·s exposure), with subsequent sulfur vacancy creation, occurs at a

critical temperature of ~300 K, as described in first discussed study in this chapter. To

corroborate the decrease in the surface sulfur, expected as a result of sulfur vacancy

creation as previously observed, the sulfur to molybdenum ratio, as derived from S 2p3/2

and Mo 3d5/2 core level photoemission intensities, are plotted in Figure 5.10. It is evident

that at 300 K, there is a sharp drop in the sulfur to molybdenum ratio. As X-ray

photoemission spectroscopy (XPS) is limited by the photoelectron mean free path, the

observed decrease in S/Mo XPS intensity must be confined to surface and near surface

region, thus indicative of a loss of surface sulfur. This drop in surface sulfur

88

concentration is consistent with the production of sulfur vacancies at the MoS2 surface, as

a result of the methanol to methoxy conversion on MoS2(0001). As the XPS intensities

are characteristic of composition, the changes in the sulfur to molybdenum intensity ratio

indicates a loss of 4.86 ± 0.12% in surface sulfur due to the methanol to methoxy

conversion process on MoS2(0001) 300 K. This is a significant amount of surface sulfur

loss not suggested by prior scanning tunnelling microscopy (STM) experiments, except

after multiple methanol exposure and annealing cycles.

5.3.2 Surface Sulfur Vacancy Compensation

Of particular interest is the restoration of the sulfur to molybdenum ratio derived

from XPS, as seen in Figure 5.10. The increase of the S/Mo intensity above 300 K, in

Figure 5.10, follows sulfur vacancy production, and thus a sulfur vacancy compensation

process. Such a sulfur vacancy compensation process can only result from sulfur

segregation from the bulk of MoS2 to the (0001) surface. The methanol to methoxy

reaction, and successive vacancy creation occurs as sulfur departs the surface as a volatile

H2S. Given that the H2S surface sticking coefficient will decrease with increasing

substrate temperatures and that the ambient H2S partial pressure will be low, sulfur

adsorption from the gas phase is unlikely.

89

Figure 5.11: Inverse photoemission spectra for the same a) clean MoS2(0001) at room

temperature, b) MoS2(0001) at room temperature after 1.2 × 10-6 Torr·s methanol

exposure at 86 K, and c) methanol-exposed MoS2(0001) at room temperature after further

annealing at 350 K. Binding energies are labeled with respect to the Fermi level.

The loss of sulfur from the surface means significant changes to the surface

electronic structure. Inverse photoemission is extremely surface sensitive,49 far more so

than photoemission, making inverse photoemission especially sensitive to changes in

electronic structure changes driven by changes in surface stoichiometry. Inverse

photoemission spectra, as illustrated by Figure 5.11, indicate a suppression of the MoS2

lowest unoccupied molecular orbital (LUMO) and other unoccupied features at roughly

1.7, 2.6 and 5.7 eV above the Fermi level. These features present in the clean MoS2

sample (Figure 5.11a), and observed in other studies,50 are almost entirely suppressed in

the inverse photoemission spectra after exposure to methanol and annealing at room

temperature (Figure 5.11b). After further annealing of the methanol-exposed MoS2

sample to 350 K (Figure 5.11c), these features associated with the MoS2 unoccupied

90

states are partly restored. This suggests that the sulfur vacancies that accompany methoxy

species formation on the MoS2 surface are reduced in number by an increasing amount of

surface sulfur with increasing temperature. From the inverse photoemission it is evident

that this sulfur, at the surface, is placed not randomly but at the MoS2 sulfur sites at

annealing temperatures at or above 350 K. Complete restoration of the MoS2 unoccupied

states would not be expected as the surface methoxy species on the MoS2 surface should

remain present well above annealing temperatures of 350 K, as noted in previous study.

The persistence of a surface methoxy species would contribute an unoccupied density of

states in the inverse photoemission spectra (Figure 5.11b and 5.11c).

5.4 Summary

The coverage and temperature dependent XPS measurements confirm the

existence of the methoxy on MoS2 after methanol exposure. The STM and PL

measurements indicate that sulfur vacancies are generated during methoxy formation via

O-H scission of methanol, resulting in defects on the MoS2(0001) surface. While

methoxy binds to vacancy sites, on the MoS2 surface, the detached hydrogen atoms spill

over onto the pristine terrace of MoS2. The coalescence of these hydrogen atoms with

surface S atom results in a volatile sulfur product in the form of H2S, as suggested by

density functional theory. The continual of such process leads to the higher concentration

of the S vacancies, which ultimately give rise to larger defect patches or line defects.

Thus, the methanol to methoxy conversion is most facile on MoS2 surface in the presence

of defects, and defect creation sequentially takes place.

Surface sulfur site vacancy compensation, as observed, is indicative of sulfur

segregation to the MoS2(0001) at 350 K and above. The surface sulfur site vacancy

91

compensation follows sulfur vacancy creation that accompanies the conversion of surface

methanol to methoxy reactions at the MoS2(0001) surface. Sulfur vacancy compensation

is apparent through a return in sulfur to molybdenum XPS intensity ratios to pre-vacancy

formation conditions after annealing of the MoS2 sample. A similar reappearance of key

unoccupied electronic structure state features, characteristics of the MoS2(0001) surface,

after annealing gives further support to sulfur segregation to the sulfur vacancy sites. This

sulfur loss, resulting from the conversion of surface methanol to methoxy reactions at the

MoS2(0001) surface, is significant and in the region of 5%.

Sulfur vacancy compensation, and inherently the compensation of catalytic active

sites, from the sulfur segregation processes, may hinder catalysis of higher alcohol

synthesis on the MoS2(0001) basal plane, where larger patch vacancies aid in catalytic

efficiency.1 In light of the sulfur segregation findings and with the knowledge of the

significant role of surface electronic structure and topology in catalysis, great attention

must be given to avoid these catalytically reductive processes in the application of MoS2

as a catalyst.

92

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B At. Spectrosc. 1985, 40 (5–6), 725–737. https://doi.org/10.1016/0584-

8547(85)80122-1.

96

(48) Barr, T. L. An ESCA Study of the Termination of the Passivation of Elemental

Metals. J. Phys. Chem. 1978, 82 (16), 1801–1810.

https://doi.org/10.1021/j100505a006.

(49) Smith, N. V. Inverse Photoemission. Reports Prog. Phys. 1988, 51 (9), 1227–

1294. https://doi.org/10.1088/0034-4885/51/9/003.

(50) Komesu, T.; Le, D.; Zhang, X.; Ma, Q.; Schwier, E. F.; Kojima, Y.; Zheng, M.;

Iwasawa, H.; Shimada, K.; Taniguchi, M.; et al. Occupied and Unoccupied

Electronic Structure of Na Doped MoS2(0001). Appl. Phys. Lett. 2014, 105 (24).

https://doi.org/10.1063/1.4903824.

97

CHAPTER 6

FRONTIER ORBITAL SYMMETRY

This chapter provides a deeper context for the need for consideration of symmetry

in interactions on MoS2 beyond geometric and structural as provided by earlier chapters.

While vacancy and active site, symmetry driven, creation and stability, are vital in earlier

catalysis applications as shown, a more profound level of orbital symmetry is exemplified

in this chapter. As will be discussed, the symmetry of the interacting frontier orbitals, that

of the diiodobenzene adsorbate and MoS2 surface, have serious impact interaction

character. In catalysis, the analogues to diiodobenzene are the isomers of benzenedithiol,

which have applicability in hydrosulfurization reactions in the petrochemical sector. Due

to the available spectroscopic techniques, discerning MoS2 sulfur contribution to that of

sulfur from benzenedithiol isomers would prove difficult, hence the iodine containing

molecule was used. Of great note in this chapter is the limitation of conventional

theoretical methods in predicting the consequences of frontier orbital symmetry

considerations. From the following discussion of diiodobenzene isomer dependent

adsorption it will be clear that theoretical calculations predict no isomeric difference in

adsorption based on a multitude of calculations contrary to experimental findings. Only

an examination of frontier orbital symmetry provides serious explanation as to the

isomeric dependent adsorption of diiodobenzene on MoS2.

The role of frontier orbital symmetry, in the adsorption process of diiodobenzene,

appears in an activation energy barrier to adsorption, evident in the huge differences in

the rate of adsorption between 1,3-diiodobenzene, 1,2-diiodobenzene and 1,4-

98

diiodobenzene isomers on MoS2. Experiments indicate that the rate of adsorption of 1,3-

diiodobenzene on MoS2(0001) is much greater than that of the 1,2-diodobenzene and 1,4-

diiodbenzene isomers. In the absence of a significant difference in the heat of adsorption

between diiodobenzene isomers, this result suggests that a rehybridization requirement

forced by frontier orbital symmetry comes at an energy cost. With the introduction of

defects, which lower the adsorption site symmetry at the MoS2(0001) surface, the rate of

1,3-diiodobenzene adsorption on MoS2(0001) decreases, while the rates of 1,2-

diodobenzene adsorption significantly increases. The similarities in the observed

molecular desorption, without fragmentation on surfaces with few defects, indicates

adsorption is molecular and is consistent with the negligible differences in calculated

diiodobenzene isomer-MoS2 system adsorption energies and electron affinities. Theory

highlights frontier orbital symmetry as key to diiodobenzene adsorption on MoS2(0001)

as, upon adsorption, 1,3-diiodobenzene on MoS2(0001) has no frontier orbital symmetry

requirement of rehybridization, whereas the 1,2-diodobenzene and 1,4-diiodobenzene

isomers each must undergo symmetry reduction induced rehybridization for frontier

orbital interaction.

Both the magnitude of the molecular electrostatic dipole and the frontier orbital

symmetry could potentially play a dominant role in the adsorption process, but prior work

with diiodobenzene adsorption1,2 indicates that frontier orbital symmetry dominates over

dipolar interactions,2 at a heteromolecular interface. On p-benzoquinonemonoimine

zwitterion molecular films of (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-

dien-1-olate C6H2(NHR)2(O)2, where R = n-C4H9, 1,3-diiodobenzene was observed to

be strongly favored in adsorption, at 150 K, over other isomers of diiodobenzene.1

99

Diiodobenzene adsorption on films of the molecular ferroelectric copolymer

poly(vinylidene fluoride) with trifluoroethylene (70:30) was found to be very sensitive to

the diiodobenzene isomer and to the ferroelectric domain orientation.2 The adsorption of

1,2-diiodobenzene on poly(vinylidene fluoride) with trifluoroethylene, at 150 K, was

much more favorable with the polarization direction towards the vacuum, rather than into

the film, while the adsorption of 1,4-diiodobenzene, at 150 K, was more favorable with

the polarization into the film.2 For diiodobenzene adsorption on both p-

benzoquinonemonoimine zwitterion1 and poly(vinylidene fluoride with

trifluoroethylene)2 molecular films, the heat of adsorption was not excluded as

contributing to isomer dependent the adsorption mechanism. Additionally, isomeric

effects have also been seen for the adsorption of diiodobenzene on graphite.3

While diiodobenzene adsorption studies, both experimental4–8 and theoretical,4,7,9

on various copper surfaces have been common, other studies of diiodobenzene on

Pt(111)10,11 indicate that there was a significant deformation of the diiodobenzene, as the

molecule approached the surface, leading to a transient activation barrier to adsorption.

This means that for diiodobenzene on Pt(111), the initial physisorption state is separated

from the chemisorption state by a potential energy barrier.10,11 In this context, the goal

here is to clearly establish that frontier orbital symmetry (the symmetry of the highest

occupied orbitals) not heat of adsorption, drives favorable absorption. Through

comparison of the adsorption of diiodobenzene isomers on MoS2(0001), where the bond

strength of each isomer to the surface is nearly identical, variation in adsorption rate

points to the key role of symmetry.

100

6.1 Relative Rate of Diiodobenzene Adsorption

Consistent with prior diiodobenzene absorption experiments, 1,3-diiodobenzene is

readily adsorbed on the MoS2(0001) surface, at 100 K, as seen from the increase in the

iodine core level signal in Figure 6.1. The rate of adsorption on the MoS2(0001) surface

at 100 K, is significant for 1,3-diiodobenzene, and very low for the other diiodobenzene

isomers, as indicated by the coverage dependent XPS iodine to molybdenum core level

intensity ratios plotted in Figure 6.2.

While it is evident that 1,3-diiodobenzene readily adsorbs onto the MoS2(0001)

surface at 100 K, resulting in molecular monolayer saturation at roughly 25 Langmuirs

exposure, there is only slight adsorption of the 1,2-diiodobenzene isomer evident at 30

Langmuirs exposure. The experimental studies of adsorption make clear that adsorption

of 1,4-diiodobenzene requires exposures greater than 60 Langmuirs to achieve even a

very small coverage on MoS2(0001) at 100 K. This translates into 1,3-diiodobenzene

having an adsorption rate 15 times greater than 1,2-diiodobenzene and more than 150

greater than 1,4-diiodobenzene on the MoS2(0001) surface at 100 K.

101

Figure 6.1: The compiled XPS I 3d peak intensity as a function of 1,3-diiododbenzene

exposure to MoS2(0001), at 100 K. The top inset shows the layered MoS2(0001) crystal,

where sulfur, molybdenum are yellow, and dark blue, respectively. Exposure is denoted

in Langmuirs (L) equivalent to 1 × 10-6 Torr⋅s of 1,3-diiododbenzene.

102

Figure 6.2: The compiled XPS I 3d5/2/Mo 3d5/2 peak core level photoemission intensities,

as a function of exposure MoS2(0001), at 100 K, for 1,2-diiodobenzene (red circles), 1,3-

diiododbenzene (black squares), and 1,4-diiodobenzene (blue triangles), as schematically

indicated at the right. Exposure is denoted in Langmuirs (L) equivalent to 1 × 10-6 Torr⋅s.

The inset shows the optimal adsorption configuration for 1,3-diiododbenzene on

MoS2(0001), as indicated by density functional theory. Here sulfur, molybdenum, iodine,

carbon and hydrogen are yellow, dark blue, purple, dark grey and grey, respectively.

103

Figure 6.3: The compiled XPS I 3d5/2/Mo 3d5/2 peak core level photoemission intensities,

as a function of exposure MoS2(0001), at 85 K, for 1,2-diiodobenzene (red circles), 1,3-

diiododbenzene (black squares), and 1,4-diiodobenzene (blue triangles). Exposure is

denoted in Langmuirs (L) equivalent to 1 × 10-6 Torr⋅s. Data connected, with a dashed

line, indicates adsorption of diiodobenzene on MoS2(0001) rich in sulfur vacancies. The

solid line (surface with few sulfur vacancies) and dashed lines (a surface rich in sulfur

vacancies) connecting the adsorption data for 1,3-diiododbenzene (black squares)

indicates that defects actually decrease the rate of adsorption. The right are the calculated

charge redistributions for 1,2-diiodobenzene on MoS2(0001), where charge accumulation

is red and charge depletion is blue, for a charge isosurface = 0.002 e/Bohr3. The latter

provided by Zahra Hooshmand.

104

This disparity in observed diiodobenzene isomer adsorption rates has been

observed before, with 1,3-diiodobenzene having an adsorption rate 3-5 times larger than

1,2-diiodobenzene and 15 times larger than 1,4-diiodobenzene on zwitterion p-

benzoquinonemonoimine molecular films of (6Z)-4-(butylamino)-6-(butyliminio)-3-

oxocyclohexa-1,4-dien-1-olate C6H2(NHR)2(O)2, at 150 K.1 However, diiodobenzene

adsorption on graphite3 presents very differently, initial isomer adsorption rates on

graphite at 110 K, were found to be similar, but at high diiodobenzene coverages,

adsorption rates became isomer dependent due to steric effects between adsorbed

molecules. The diiodobenzene isomer adsorption results from the investigation on

graphite3 provides a compelling proof that the isomeric dependence does not come from

the molecule alone, but the interactions with the substrates. We note the initial condition

of the surface plays a big role in the rate of diiodobenzene adsorption. As seen in Figure

6.3, after introducing defects into the surface, there is an appreciable rate of adsorption

for both the 1,2-diiodobenzene and 1,4-diiodobenzene isomers, for exposures less than 15

Langmuirs (L) (equivalent to 15 × 10-6 Torr⋅s exposures). While some isomeric

dependence is retained in the rate of adsorption, we also find that the rate of adsorption,

for the 1,3-diiodobenzene isomer, decreases with the introduction of surface defects.

105

6.2 Heats of Diiodobenzene Adsorption

The asymmetry in rate of adsorption of the various diiodobenzene isomers is

clearly dependent on the initial condition of the surface. For MoS2(0001) surfaces with

few sulfur vacancies the isomeric dependence in adsorption, as observed in Figure 6.2, is

not due to differences in the heat of adsorption. An examination of all the possible

adsorption configurations of diiodobenzene isomers on MoS2 was carried out using DFT

calculations. These configurations are shown in Figure 6.3 for 1,3-diiodobenzene on

MoS2. Similar configurations were used for 1,2- and 1,4-diiodobenzene. The most stable

configuration was obtained when diiodobenzene isomers were adsorbed horizontally on

MoS2 with the center of carbon ring on the S atom of the surface and a vertical distance

of about 3Å between the center of benzene ring and S of the surface but with differing

registrations of the iodine to MoS2 surface sites, as shown in Figure 6.4. The results of

these calculations are summarized in table 6.1 for all three isomers and predict that these

isomers adsorb with almost the same strength on MoS2. Our calculations also show that

all three isomers adsorb with no adsorption barriers on MoS2. To calculate an absorption

barrier, the adsorption energy of each isomer for the most stable adsorption configuration

was calculated while moving the isomer away from its stable position by steps of 0.5Å

with fixed coordinates at each separation to avoid the relaxation of isomers back to their

original minimum energy. The results of these calculations are presented in Figure 6.5.

106

Figure 6.4:(a-f) Model adsorption configurations of 1,3-diiodobenzene on MoS2. Similar

configurations were used for adsorption of 1,2-diiodobenzene and 1,4-diiodobenzene on

MoS2. Color code of atoms: Mo: Dark Blue, S: yellow, C: Dark Grey, H: grey, I: Purple.

Courtesy of Zahra Hooshmand and Talat Rahman’s research group.

107

Isolated Molecules

1,2-C6H4I2 1,3-C6H4I2 1,4-C6H4I2

Electron Affinity (eV) 2.46 2.38 2.13

Coupled Structures (Pristine MoS2)

1,2-C6H4I2/MoS2 1,3- C6H4I2/MoS2 1,4- C6H4I2/MoS2

Eads (eV) -0.81 -0.80 -0.80

ΔΦ (eV) -0.16 -0.12 -0.02

Bader Charge (e) -0.07 -0.07 -0.07

Coupled Structures (Defect-Laden MoS2)

Eads (eV) -0.89 -0.87 -0.89

Bader Charge (e) -0.07 -0.07 -0.09

Table 6.1: Calculated quantities for diiodobenzene isomers in isolated gas phase and on

MoS2. Courtesy of Zahra Hooshmand and Talat Rahman’s research group.

108

Figure 6.5:(a-c) The most stable adsorption configurations of 1,2-, 1,3-, and 1,4-

diiodobenzene isomers on MoS2, respectively. The upper and lower rows show the top

and side view of each isomers after adsorption on MoS2. Color code of atoms: Mo: Dark

Blue, S: yellow, C: Dark Grey, H: grey, I: Purple. Courtesy of Zahra Hooshmand and

Talat Rahman’s research group.

109

Figure 6.6: Charge density redistribution in diiodobenzene/MoS2 systems. 1,2-, 1,3- and

1,4-diiodobenzene on MoS2 as displayed in Figures 6.5(a-c), respectively. The charge

accumulation and depletion are shown in dark red and dark blue, respectively with the

isosurface set to 0.0015 e/Bohr3. Courtesy of Zahra Hooshmand and Talat Rahman’s

research group.

110

To find the origin of the difference in diiodobenzene isomeric dependent adsorption on

MoS2, the charge redistribution for the systems of diiodobenzene isomers on MoS2 was

also calculated. The results of these calculations are presented in Figure 6.6 and show that

the charge redistribution occurs mainly within molecules and MoS2 rather than in the

region between them. No significant difference among three isomer systems is observed.

The small to negligible charge transfer between molecule and MoS2 obtained via charge

density analysis, is substantiated by similar results from Bader charge analysis, which are

summarized in table 6.1. The calculated electron affinities of the various diiodobenzene

isomers suggests that a difference of electron affinity is not the origin to the observed

differences in various diiodobenzene isomer-MoS2 adsorption rates.

111

Figure 6.7: The temperature dependence of the XPS I 3d peak intensity, (a) following 6

Langmuir exposure of 1,3-diiodobenzene to MoS2(0001) at 100 K, (b) following 14

Langmuir exposure of 1,3-diiodobenzene to the MoS2(0001) surface rich in sulfur

vacancies, at 85 K and (c) following 14 Langmuir exposure of 1,2-diiodobenzene to the

MoS2(0001) surface rich in sulfur vacancies, at 85 K, (a) The spectra are taken with

increasing annealing temperatures, as indicated.

112

The nearly identical calculated maximal adsorption energies of the various

diiodobenzene isomers, on the MoS2(0001) basal plane, is consistent with the observation

that all three isomers have molecular desorption temperatures of ~225 K from a

MoS2(0001) surfaces with few defects, as illustrated by Figure 6.7 for 1,3-diiodobenzene.

The similarities in adsorption energy, electron affinity and charge transfer between

isomer systems does not mean that the bonding of the diiodobenzene isomers to

MoS2(0001) are identical. The complete removal of any iodine core level photoemission

signature is also, we note, entirely consistent with complete molecular adsorption and

desorption. This last point is particularly important as dehalogenation as seen with

halogenated benzene adsorption, occurs on many substrates,4,6–8,12–16 sometimes

accompanied by arene ring polymerization,4 although dehalogenation does not occur with

initial halogenated benzene adsorption on some select nonmetallic substrates.17–20

113

Figure 6.8: The most stable adsorption configurations of diiodobenzene isomers on

defect-laden MoS2. The upper and lower rows show the top and side view of each

isomers after adsorption on defect-laden MoS2. Color code of atoms: Mo: Dark Blue, S:

yellow, C: Dark Grey, H: grey, I: Purple. Courtesy of Zahra Hooshmand and Talat

Rahman’s research group.

Not surprisingly, with the addition of a sulfur vacancies the magnitude of the

charge redistribution, upon adsorption of diiodobenzene increases, as summarized in

table 6.1. All three isomers of diiodobenzene prefer to adsorb on the defect-laden

MoS2(0001) with center of carbon ring on top of the sulfur vacancy site and with a tilted

configuration as displayed in Figure 6.8. The results of charge redistribution for isomer

adsorption on defect-laden MoS2 are plotted in Figure 6.9. Compared to the results

obtained for isomer adsorption on pristine MoS2 (Figure 6.6), an increase in the amount

of charge redistribution can be observed. This is most significant for the 1,2-

diiodobenzene isomer. Nonetheless, the Bader charge on MoS2(0001) remains the same

for 1,2-diiodobenzene adsorbed on MoS2(0001) with and without a sulfur vacancy (-0.07

114

electrons). 1,2-diiodobenzene is also coincidently where the most noticeable

experimental increase in the adsorption rate, after the introduction of defects occurs.

Figure 6.9: Charge density redistribution in diiodobenzene/MoS2 systems. 1,2-, 1,3- and

1,4-diiodobenzene on defect-laden MoS2 as displayed in Figures 6.8(a-c), respectively.

The charge accumulation and depletion are shown in dark red and dark blue, respectively

with the isosurface set to 0.0015 e/Bohr3. Courtesy of Zahra Hooshmand and Talat

Rahman’s research group.

With diiodobenzene adsorption on defect laden MoS2(0001), not only are the rates

of adsorption perturbed, as noted above and as seem from a comparison of the adsorption

profiles in Figures 6.2 and 6.3, the molecular desorption temperature is no longer ~225 K

but now extends to well above room temperature ~300 K, for some adsorbed molecules,

as seen in Figure 6.7. With the addition of defects on the MoS2(0001) surface, much of

the diiodobenzene is lost, upon annealing the surface to 300 K, however some

115

diiodobenzene remains and potentially fragments, so that there is a persistent iodine

signal in XPS that cannot be removed without preparing a whole new surface.

6.3 Frontier Orbital Interactions

Frontier orbital analysis of the diiodobenzene isomers, and their interaction with

MoS2(0001) surface provide confirmation that while the heats of adsorption of all three

isomers to MoS2(0001) are similar, the adsorbate-substrate orbitals interaction are not

identical. As illustrated in Figure 6.10, each diiodobenzene isomer has π-conjugated

electron density within the benzene ring contributing to the HOMO, in addition to

electron density surrounding each iodine by pz orbitals. Comparison of the frontier

orbitals of three systems reveals that the symmetry of interacting orbitals from surface

with HOMO of each isomer is the key component in different adsorption rates of these

isomers.

116

Figure 6.10: (a-c) The highest occupied molecular orbitals (HOMO) of the 1,2-, 1.3-, and

1,4-diiodobenzene molecules in gas phase. (d-f) The highest occupied orbitals of coupled

systems of 1,2-, 1,3- and 1,4-diiodobenzene isomers on MoS2, respectively. Green (red)

represent the positive (negative) phase of orbitals. Color code of atoms: Black: C, Grey:

H, Dark Blue: Mo, Yellow: S and Purple: I. The isosurface is set to 0.002 eV/Bohr3.

Courtesy of Zahra Hooshmand and Talat Rahman’s research group.

For 1,2-diiodobenzene, in vacuo (Figure 6.10a), the symmetry of the frontier

orbital is assigned Cs point group symmetry, with only a mirror plane of symmetry which

bisects the I pz orbitals and benzene ring, note that the I pz orbitals are not symmetric

about the dihedral plane due to phase. Upon 1,2-diiodobenzene adsorption on the MoS2,

C3v, surface, the π-conjugated electron density and pz orbital of one of iodine atoms

interacts with surface Mo dyz and dxz HOMO orbitals. The second iodine atom lies above a

hollow site of MoS2, which results in reduction of interaction strength. In interacting, the

symmetry of the system must reduce to the lowest symmetry contribution. If we map the

117

higher Cs symmetry of the I pz, of A’ irreducible representation, to the C3v symmetry for

the interacting dyz and dxz of the surface, of E irreducible representation, we must reduce

the symmetry of the I pz meaning rehybridization is required for interaction and thus a

barrier to adsorption. For 1,4-diiodobenzene, in vacuo, (Figure 6.10c), the symmetry of

the frontier orbital is again assigned Cs point group symmetry, with only a mirror plane of

symmetry which bisects the I pz orbitals and benzene ring, note that the I pz orbitals are

not symmetric about the dihedral plane due to phase. Upon 1,4-diiodobenzne adsorption,

both the π-conjugated electron density and pz orbital of one of iodine atoms interact with

the dx2-y

2 orbitals of Mo atoms with the second iodine above an MoS2 hollow site. Similar

to the above symmetry argument, the surface C3v dx2-y

2 orbitals are of E irreducible

representation with the interacting I pz orbital of A’ irreducible representation requiring a

reduction in symmetry and rehybridization in the interaction. Thus, it would seem that the

interaction of the 1,4-diiodobenzene should be similar to that of the 1,2-diiodobenzene in

terms of this barrier in rehybridization, which contradicts experimental findings.

However, as calculated and expressed within Figure 6.10(f), it is clear from other

studies21 that the stable interaction configuration is not of the 1,4-diiodobenzene with the

MoS2 surface HOMO but the HOMO-1 as the Mo dyz and dxz are higher in the valence

band than the dx2-y

2 at 𝛤, thus this interaction has an even higher barrier to adsorption as

compared to the 1,2-diiodobenzene, substantiating experimental findings. For adsorption

of either 1,2-diiodobenzene or 1,4-diiodobenzene, a key reduction in symmetry and

rehybridization of the I pz orbital must occur in adsorption lending to a barrier in

adsorption, and in the case of 1,4-diiodobenzene, stable bonding requires interaction with

deeper MoS2 orbitals. For 1,3-diiodobenzene, in vacuo, (Figure 6.10c), the symmetry of

118

the frontier orbital is assigned the much lower symmetry C1 point group symmetry, with

no mirror planes of symmetry, note that the I pz orbitals are not symmetric about the

dihedral plane due to phase. The interaction among the molecular orbitals and surface Mo

d orbitals is somewhat similar between the 1,2- and 1,3-diiodobenzene isomers, here the

π-conjugated electron density and pz orbital of both of iodine atoms interacts with surface

Mo dyz and dxz HOMO orbitals (Figure 6.10e). This interaction of both iodine atoms

serves to increase the strength of molecular interactions. Following similar symmetry

arguments as outlined above, the I pz orbitals within the C1 point group are of the A

irreducible representation, which have no inherent symmetry, and map onto the C3v

surface Mo dyz E irreducible representation without a reduction in symmetry and hence in

this interaction no rehybridization is required for interaction. With frontier orbital

symmetry in mind, 1,3-diiodobenzene does not have the same requirement of

rehybridization, an energetic barrier, upon adsorption on MoS2(0001) compared to the

other diiodobenzene isomers. Furthermore, for 1,3-diiodobenzene, both iodine atoms are

above the Mo atoms (Figure 6.10(e)), which increases the strength of interactions.

As noted in the introduction, for diiodobenzene on Pt(111), the initial

physisorption state is separated from the chemisorption state by a potential energy barrier

10,11. A similar barrier between the physisorption state and chemisorption state could be

mediated by symmetry. In the absence of a significant difference in the heat of adsorption

between diiodobenzene isomers, this suggests that requirement for the reduction in

symmetry and rehybridization comes at an energy cost. This barrier needed in

rehybridization in the 1,2-diiodobenzene and 1,4-diiodobenzene isomers upon adsorption

is reflected in the observed lower rate of adsorption on MoS2(0001) compared to 1,3-

119

diiodobenzene. This unparalleled behavior in interaction is in spite of the similar

adsorption energies, electron affinities, and desorption temperature of all three

diiodobenzene isomers.

6.4 Summary

The similarities in the calculated maximal adsorption energies and electron

affinities for all diiodobenzene isomers, upon adsorption on the MoS2(0001) surface,

support the contention that it is not the interaction itself that leads to variations in the

isomer dependent adsorption rates, but an activation energy barrier associated with

symmetry reduction induced rehybridization effects in the adsorption process for 1,2-

diiodobenzene and 1,4-diiodobenzene, not evident in the case of 1,3-diiodobenzene.

These differences in surface interaction are supported through frontier orbitals analysis

which shows the strongest interaction between the molecular orbitals of 1,3-

diiodobenzene with d orbitals of Mo. Therefore, we must conclude that differences in

frontier orbital symmetry matter in the determining the adsorption rates for the various

diiodobenzene isomers on MoS2(0001).

120

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122

CHAPTER 7

CONCLUSIONS AND FUTURE STUDY

The role and influence of both symmetry and interface effects on adsorption on

MoS2 has been explored and substantiated through a combination of photoemission,

inverse photoemission, and scanning tunneling microscopy studies. As MoS2 exhibits

promise for a wide variety of device and catalytic application, the need for a more

complete understanding to predict better combinations of materials for a variety of

applications becomes increasingly essential. Providing explanations, after the

experimental picture is known, is useful, but not an efficient approach to material

development. While some of the experiments show in general good agreement with

theory, many others have exhibited properties and results not completely encompassed by

standard DFT. The inclusion of the influence of the dynamics of symmetry retention and

symmetry reduction, of adsorbate frontier orbital symmetry may be necessary for better

predictive theory of catalysis. Furthermore, more complete interaction theory on MoS2

requires a better accounting for the difference in band structure between monolayer and

bulk MoS2, including substrate interface interactions and band bending. Monolayer MoS2

and any other monolayer material are usually accompanied by an underlying substrate.

These substrates can influence the monolayer systems with the addition of band bending

and interface charge transfer effects. The conclusion here is that better, more accurate

predictive modeling of MoS2 and other transition metal dichalcogenides, requires a more

complete theoretical basis, which includes both frontier orbital symmetry and interface

effects such as band bending and interface charge transfer.

123

From studies of gold nanoclusters on MoS2 and other metal-transition metal

dichalcogenide systems, the interaction of metal to bulk and metal to monolayer was

found to exhibit drastically different behavior from each other. Conventional DFT

modeling, as it stands, predicts the electronic properties of metal adsorption on

monolayer and bulk MoS2 to be alike, independent of MoS2 thickness. Interface effects

stemming from a supporting substrate for the monolayer such as charge transfer must be

influencing the electronic behavior resulting in possible differences in catalytic behavior.

This difference in monolayer and bulk catalysis has been explored in other works where

gold nanoparticles on monolayer MoS2 on silica allows for the catalytic carbonylation of

methanol to acetaldehyde whereas gold nanoparticles on bulk MoS2 do not form

acetaldehyde.1

Charge transfer effects and influence in catalysis extend beyond the metal-

monolayer transition metal dichalcogenide studies. In composite studies of both carbon

nanotubes-MoS2 and TiO2-MoS2 composites, charge transfer effects in addition to the

large bulk to surface active sites of MoS2 were found to aid and increase catalytic

effectivity. Within the CNT-MoS2 system study, it was found that the large catalytic

active sites from the MoS2 and a conductive network provided by CNTs, which reduce

charge saturation, produce a composite material that has catalytic hydrogen evolution

reaction activity comparable to platinum. Within the TiO2-MoS2 system study it was

established that the photocatalytic degradation of methylene blue, a waste dye product,

was improved from lone TiO2 due to charge transfer from the MoS2 elongating

photogenerated charge carrier recombination time.

124

From studies related to catalytic methanol synthesis on MoS2, it is established

that surface symmetry is integral to the creation and stability of surface defects which

verifies earlier theoretical works.2 While within these methanol to methoxy reaction

studies on MoS2, the monolayer and bulk behaved similar in the creation of sulfur

vacancies, further studies exploring the healing of these sulfur vacancies through

annealing give further credence to difference in catalytic application. Under moderate

annealing conditions, the defect and active site rich, bulk MoS2 was diminished in the

number of sulfur vacancies through surface sulfur segregation. As monolayer MoS2 lacks

a bulk sulfur reservoir for sulfur segregation, the same temperature catalytically

diminishing conditions for the bulk will leave the monolayer unaffected.

Finally, studies of adsorption of 1,2- 1,3- and 1,4-diiodobenzene on MoS2 have

revealed that there is an apparent energy barrier associated with the reduction of local

symmetry that is required for rehybridization creating a marked difference in isomeric

adsorption. From these studies there is proof that frontier orbital symmetry an integral

pathway towards adsorption that is lacking from density functional theory modeling.

While density DFT calculations show negligible difference in adsorption energy and

electron affinities between isomer and substrate, meaning there should be no isomeric

dependence in adsorption. From experiment, there is greater uptake of the 1,3-

diiodobenzene isomer on the MoS2 surface as compared to other isomers due to a lack of

need to reduce the frontier orbital symmetry in adsorption.

While not performed within the context of this thesis, a possible extension of the

diiodobenzene study could provide evidence of the preservation of local symmetry in the

MoS2(0001) band structure as found in other studies.3 Within the diiodobenzene isomer

125

adsorption study as covered in chapter 6, it was found that the 1,4-diiodobenzene isomer

bonds through the surface Mo 4dx2-y

2 or the HOMO-1 and not the highest occupied

molecular orbital at 𝛤, compared to the 1,2- and 1,3- which interact with the Mo 4dxz or

4dyz which are degenerately the HOMO at 𝛤. Here if we were to preform ARPES along

the 𝛤 − 𝐾 direction for MoS2(0001) with the adsorbed diiodobenzene isomers, and

observed that the 1,2- and 1,3-diiodobenzene isomers interact at the high symmetry 𝛤

point as expected, but the 1,4-diiodobenzene interacts with the 4dx2-y

2 at 𝐾, where it is in

fact the HOMO, we could conclude that local symmetry in the MoS2 band structure exists

away from the high symmetry of the 𝛤 point. Thus, there would be additional support for

the need to include frontier orbital symmetry and other considerations into theory.

References

(1) Almeida, K.; Chagoya, K. L.; Felix, A.; Jiang, T.; Le, D.; Rawal, T. B.; Evans, P.

E.; Wurch, M.; Yamaguchi, 1 Koichi; Dowben, P. A.; et al. Towards Higher

Alcohol Formation Using a Single-Layer MoS2 Activated Au on Silica: Methanol

Carbonylation to Acetaldehyde. 2019.

https://doi.org/10.26434/CHEMRXIV.8044607.V1.

(2) Le, D.; Rawal, T. B.; Rahman, T. S. Single-Layer MoS2 with Sulfur Vacancies:

Structure and Catalytic Application. J. Phys. Chem. C 2014, 118 (10), 5346–5351.

https://doi.org/10.1021/jp411256g.

(3) Komesu, T.; Le, D.; Ma, Q.; Schwier, E. F.; Kojima, Y.; Zheng, M.; Iwasawa, H.;

Shimada, K.; Taniguchi, M.; Bartels, L.; et al. Symmetry-Resolved Surface-

Derived Electronic Structure of MoS2(0001). J. Phys. Condens. Matter 2014, 26

(45), 0–8. https://doi.org/10.1088/0953-8984/26/45/455501.