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 ii 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. iii 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 iv 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 2 PtO species formed under oxidizing conditions will be discussed. x 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) 2 atomic rows in [001] directions. Chapter 6 describes how the presence of NPs on TiO (110) surface affects its 2 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 2 Pt NPs, which hinders the desorption of oxygen atoms in TiO to vacuum. 2 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 2 3 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 v 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 2 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 2 selective reduction of NO in H via EXAFS and XANES measurements. 2 vi To my wife, Roxana. vii 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. viii 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 ix 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 x
Description: