size, shape, support and adsorbate effect

311 Pages·2012·10.7 MB·English

by  Farzad Behafarid

STRUCTURE, STABILITY, THERMODYNAMIC, AND CATALYTIC
PROPERTIES OF METAL NANOSTRUCTURES: SIZE, SHAPE,
SUPPORT AND ADSORBATE EFFECTS

by

FARZAD BEHAFARID

B.S. Sharif University of Technology, 2004
M.Sc. University of Tehran, 2007
M.Sc. University of Central Florida, 2010

A dissertation submitted in partial fulfillment of the requirements
for the degree of Doctor of Philosophy
in the Department of Physics
in the College of Science
at the University of Central Florida
Orlando, Florida

Fall Term
2012

Major Professor: Beatriz Roldan Cuenya
© 2012 Farzad Behafarid

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ABSTRACT

Recent advances in nanoscience and nanotechnology have provided the scientific
community with exciting new opportunities to rationally design and fabricate materials at
the nanometer scale with drastically different properties as compared to their bulk
counterparts. In this dissertation, several challenges have been tackled in aspects
related to nanoparticle (NP) synthesis and characterization, allowing us to make
homogenous, size- and shape-selected NPs via the use of colloidal chemistry, and to
gain in depth understanding of their distinct physical and chemical properties via the
synergistic use of a variety of ex situ, in situ, and operando experimental tools. A variety
of phenomena relevant to nanosized materials were investigated, including the role of
the NP size and shape in the thermodynamic and electronic properties of NPs, their
thermal stability, NP-support interactions, coarsening phenomena, and the evolution of
the NP structure and chemical state under different environments and reaction
conditions.

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OUTLINE

Recent advances in nanoscience and technology have provided the scientific
community with new exciting opportunities to rationally design and fabricate materials at
the nanometer scale with drastically different properties as compared to their bulk
counterparts. A variety of challenges related to nanoparticle (NP) synthesis and
materials characterization have been tackled , allowing us to make more homogenous,
well defined, size- and shape-selected NPs, and to probe deeper and more
comprehensively into their distinct properties. In this dissertation, a variety of
phenomena relevant to nanosized materials are investigated, including the thermal
stability of NPs and coarsening phenomena in different environments, the experimental
determination of NP shapes, gaining insight into NP-support interactions, epitaxial
relationships, and unusual thermodynamic and electronic properties of NPs, including
the effect of adsorbates on the electron density of states of small clusters, and the
chemical, and structural evolution of NPs under reaction conditions.
In chapter 2, a general description of different characterization tools that are used
in this dissertation is provided. In chapter 3, the details of two different methods used for
NP synthesis, namely inverse micelle encapsulation and physical vapor deposition
(PVD) are described.
Chapter 4 describes the thermal stability and coarsening behavior of Pt NPs
supported on TiO (110) and γ-Al O as a function of the synthesis method, support pre-
2 2 3
treatment, and annealing environment. For the Pt/TiO (110) system, micelle-
2
synthesized NPs showed remarkable stability against coarsening for annealing
temperatures up to 1060°C in vacuum, in contrast to PVD-grown NPs. When comparing
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different annealing environments (H , O , H O), Pt NPs on γ-Al O annealed in O were
2 2 2 2 3 2
found to be the least affected by coarsening, followed by those heated in H O vapor.
2
The largest NP growth was observed for the sample annealed in H . The role of the
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PtO species formed under oxidizing conditions will be discussed.
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In chapter 5, the shape of Pt and Au NPs and their epitaxial relationship with the
TiO (110) support was extracted from scanning tunneling microscopy (STM)
2
measurements. Three main categories of NP shapes were identified, and through
shape modeling, the contribution of facets with different orientations was obtained as a
function of the number of atoms in each NP. It was also shown that the micelle-
synthesized Pt and Au NPs have an epitaxial relationship with the support, which is
evident from the fact that they always have one symmetry axis parallel to TiO (110)
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atomic rows in [001] directions.
Chapter 6 describes how the presence of NPs on TiO (110) surface affects its
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reconstruction upon high temperature annealing in vacuum. In contrast to NP-free
TiO (110) substrates, long and narrow TiO stripes are observed for Pt NP-decorated
2 2
surfaces. This phenomenon is explained based on the stabilization of TiO , induced by
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Pt NPs, which hinders the desorption of oxygen atoms in TiO to vacuum.
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In chapter 7, a systematic investigation of the thermodynamic properties of γ-
Al O -supported Pt NPs and their evolution with decreasing NP size is presented. A
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combination of in situ extended x-ray absorption fine structure spectroscopy (EXAFS),
ex situ transmission electron microscopy (TEM) measurements, and NP shape
modeling is used to obtain the NPs shape, thermal expansion coefficient, and Debye
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temperature. The unusual thermodynamic behavior of these NPs such as their negative
thermal expansion and enhanced Debye temperature are discussed in detail.
Chapter 8 presents an investigation of the electronic properties of size-controlled
γ-Al O -supported Pt NPs and their evolution with decreasing NP size and adsorbate
2 3
(H ) coverage. The hydrogen coverage of Pt NPs at different temperatures was
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estimated based on XANES data and was found to be influenced by the NP size, and
shape. In addition, correlations between the shift in the center of the unoccupied d-band
density of states (theory) and energy shifts of the XANES spectra (experiment) upon
hydrogen chemisorption as well as upon modification of the NP structure were
established.
Chapter 9 is dedicated to an operando study, describing the evolution of the
structure and oxidation state of ZrO -supported Pd nanocatalysts during the in-situ
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selective reduction of NO in H via EXAFS and XANES measurements.
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To my wife, Roxana.
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ACKNOWLEDGEMENT

I would like to thank my supervisor and mentor, Prof. Beatriz Roldan Cuenya for
her unwavering support and insightful guidance throughout my endeavor as a PhD
candidate, and for giving me numerous research opportunities that have allowed me to
achieve my goals as a researcher. Her enthusiasm and hard work, and her honest
pursuit of scientific knowledge, have set a great example for me as a successful
scientist. I feel lucky to be among her students and I am hopeful that our scientific
collaborations will be continued in my entire professional carrier, in the same way that I
am confident our friendship will be lasting through my personal life.
I want to also thank my lab members, Dr. Luis Ono, Dr, Jason Croy, Dr. Ahmed
Naitabdi, Dr, Lindsay Merte, Dr. Estephania Lira, , Simon Mostafa, Jeronimo Matos,
Mahdi Ahmadi, Hemma Mistry, and Sudeep Pandey for continuous scientific
discussions and collaborations and above all for their life lasting friendship. Thanks also
to our collaborators, Prof. Talat Rahman, Prof. Werner Keune, Prof. Anatoly Frenkel,
and Prof. Judith Yang for providing me with their support and scientific insight.
Thanks also to my committee members Prof. Abdelkader Kara, Prof. Helge
Heinrich, Prof. Lee Chow, and Prof. Winston Schoenfeld, for taking time to evaluate my
work and for their constructive comments and discussions.
Special thanks to my wife and my best friend Roxana, who has been a great
source of hope, joy and support, without whom this work would not have been
accomplished.

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TABLE OF CONTENTS

ACKNOWLEDGEMENT ................................................................................................ viii
LIST OF FIGURES ......................................................................................................... xii
LIST OF TABLES ........................................................................................................ xxiv
LIST OF ACRONYMS ................................................................................................ xxvii
CHAPTER 1: INTRODUCTION ....................................................................................... 1
CHAPTER 2: MEASUREMENT TECHNIQUES .............................................................. 6
2.1 Scanning tunneling microscope (STM) ........................................................... 6
2.1.1 Fundamentals ....................................................................................... 6
2.1.2 Instrumentation ..................................................................................... 8
2.1.3 Tip preparation .................................................................................... 12
2.1.4 STM images ........................................................................................ 13
2.2 Atomic force microscope (AFM) .................................................................... 14
2.2.1 Fundamentals ..................................................................................... 14
2.2.2 Instrumentation ................................................................................... 18
2.2.3 AFM images ........................................................................................ 20
2.3 X ray photoelectron spectroscopy (XPS) ...................................................... 21
2.3.1 Fundamentals ..................................................................................... 21
2.3.2 Instrumentation ................................................................................... 24
2.3.3 XPS spectra ........................................................................................ 26
2.4 X-ray absorption spectroscopy (XAS): extended X-ray absorption fine-
structure (EXAFS) and X-ray absorption near-edge structure (XANES)
spectroscopy ....................................................................................................... 28
2.4.1 Fundamentals ..................................................................................... 28
2.4.2 Instrumentation ................................................................................... 32
2.4.3 XAS data ............................................................................................. 33
CHAPTER 3: NANOPARTICLE SYNTHESIS METHODS ............................................ 37
3.1 Inverse micelle encapsulation ....................................................................... 37
3.2 Physical vapor deposition via electron beam evaporation ............................. 40
CHAPTER 4: THERMAL STABILITY AND COARSENING PHENOMENA OF
METAL NANOPARTICLES AND THE INFLUENCE OF THE ENVIRONMENT
AND SUPPORT PRE-TREATMENT: Pt/TiO (110) AND Pt/γ-Al O ....................... 43
2 2 3
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4.1 Introduction ................................................................................................... 43
4.2 Sample preparation methods and experimental ............................................ 48
4.3 Theoretical and simulation methods .............................................................. 53
4.3.1 Ostwald ripening ................................................................................. 54
4.3.2 Diffusion-coalescence ......................................................................... 57
4.4 Results .......................................................................................................... 60
4.4.1 Pt NPs evaporated on pristine TiO (110) (STM) ................................. 60
2
4.4.2 Pt NPs evaporated on polymer-modified TiO (110) (STM) ................ 64
2
4.4.3 Micellar Pt NPs/TiO (110) (STM) ........................................................ 68
2
4.4.4 Micellar Pt NPs/ γ-Al O (EXAFS) ...................................................... 73
2 3
4.5 Discussion ..................................................................................................... 81
4.5.1 STM observations ............................................................................... 81
4.5.2 Simulation of coarsening mechanisms ................................................ 88
4.5.3 In situ investigation of coarsening phenomena: environmental
effects ........................................................................................................... 98
4.6 Conclusions ................................................................................................. 102
CHAPTER 5: SHAPE DETERMINATION OF NANOPARTICLES AND EPITAXIAL
RELATION WITH THE UNDERLYING SUPPORT ............................................... 104
5.1 Introduction ................................................................................................. 104
5.2 Experimental ............................................................................................... 106
5.3 Results and discussion ............................................................................... 108
5.4 Conclusions ................................................................................................. 128
CHAPTER 6: NANOPARTICLE-SUPPORT INTERACTIONS .................................... 130
6.1 Introduction ................................................................................................. 130
6.2 Experimental ............................................................................................... 131
6.3 Results and discussion ............................................................................... 132
6.4 Conclusions ................................................................................................. 144
CHAPTER 7: THERMODYNAMIC PROPERTIES OF γ-Al O SUPPORTED Pt
2 3
NANOPARTICLES: SIZE, SHAPE, SUPPORT, AND ADSORBATE EFFECTS .. 146
7.1 Introduction ................................................................................................. 146
7.2 Experimental and theoretical methods ........................................................ 150
7.2.1 Sample preparation ........................................................................... 150
7.2.2 Morphological characterization (TEM) ............................................... 151
7.2.3 Structural and vibrational characterization (EXAFS) ......................... 153
7.2.4 Nanoparticle shape modeling ............................................................ 159
7.3 Results ........................................................................................................ 163
7.4 Discussion ................................................................................................... 175
7.4.1 Anomalous lattice dynamics and thermal properties of supported,
size-and shape-selected Pt nanoparticles ................................................... 175
7.4.2 Debye temperature ........................................................................... 180
7.5 Conclusions ................................................................................................. 187
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