SHAPE-CONTROLLED SYNTHESIS OF MONODISPERSE GOLD NANOCRYSTALS AND GOLD-BASED HYBRID NANOCRYSTALS YU YUE (B. Eng., Nanyang Technological University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL and BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 Acknowledgement  ACKNOWLEDGEMENT I am sincerely grateful to every individual who has helped me in one way or the other in this Ph.D project. This thesis would not have been possibly written and completed without their support and guidance in my research. First and foremost, I am very grateful for having Prof. Lee Jim Yang as my advisor. I would like to thank him for the latitude and trust that he has given me this research, while providing visionary directions. I appreciate the candor and the many sessions of in-depth discussions which have sharpened my thought process. I am indebted to his incisive but constructive criticisms on my manuscripts which have significantly increased the scientific content and possibly the impact of this work. I would also like to express my gratitude to Prof. Xie Jianping and Prof. Lu Xianmao for their insightful comments and ample inspirations and motivations. I have the good fortune to work with a group of wonderful and delightful colleagues in the laboratory, in particular, Dr. Zhang Qingbo, Dr. Liu Bo, Dr. Zhang Chao, Dr. Zhou Weijiang, Dr. Fu Rongqiang, Ms. Fang Chunliu, Ms. Ji Ge, Ms. Lv Meihua, Ms. Xue Yanhong, Mr. David Julius, Mr. Yang Jinhua, Mr. Yao Qiaofeng, Mr. Chia Zhi Wen, Mr. Cheng Chin Hsien, Mr. Bao Ji, Mr. Ma Yue and Mr. Ding Bo. I thank them for their valuable suggestions and stimulating discussions. I also thank Dr. Zhang Jixuan in the Department of Materials Science and Engineering for her invaluable input on TEM measurements. I am indebted to the technical staff in the department especially Mr. Boey Kok Hong, Mr. Chia Phai Ann, Dr. Yuan Zeliang, i Acknowledgement  Mr. Mao Ning, Ms. Lee Chai Keng, Mr. Liu Zhicheng, Ms. Samantha Fam, Mr. Rajamogan Suppiah and Mr. Evan Tan. Their superb technical service and support are essential for the timely completion of this study. This thesis work will not be possible without the generous research scholarship from the National University of Singapore throughout my Ph.D candidature. Last but not least, I would like to dedicate this thesis to all my family members. Without their constant love, encouragement and inexhaustible emotional and spiritual support, this endeavor of mine will perhaps be a dream. ii Table of content TABLE OF CONTENT ACKNOWLEDGEMENT . i TABLE OF CONTENT iii SUMMARY . viii LIST OF TABLES . xi LIST OF FIGURES xii LIST OF SCHEMES . xix LIST OF ABBREVIATIONS . xxi CHAPTER INTRODUCTION 1. Background 1. Objectives and scope CHAPTER LITERATURE REVIEW . 2. Fundamentals of the formation of polyhedral NCs 2.1.1 Shape evolution under thermodynamic control . 2.1.2 Shape evolution under kinetic control . 2.1.2.2 Kinetic control through surface adsorbents 2.1.2.3 Kinetic control through metal ion reduction rate variations . 11 2.1.3 2. Effect of twinning on shape evolution 12 Colloidal chemistry synthesis 13 2.2.1 Homogeneous nucleation methods 14 2.2.2 Seed-mediated growth methods 15 2. Polyhedral high-index NCs 16 2.3.1 High-index planes and terrace-step notations 16 2.3.2 Polyhedral high-index NCs and surface structures . 18 iii Table of content 2.3.3 Relationships between Miller indices and the geometry of high-index NCs 20 2.3.4 Synthesis of polyhedral high-index NCs . 21 2.3.4.1 Electrochemical synthesis of high-index NCs 22 2.3.4.2 Colloidal chemistry synthesis of high-index NCs 23 2. Hybrid nanostructures 25 2.4.1 Classification of hybrid NCs . 25 2.4.2 Synthesis methods for hybrid nanostructures 26 2.4.3 Factors that determine the configuration of hybrid nanostructures . 28 2.4.4 Shape controlled synthesis of noble metal hybrid nanostructures . 30 CHAPTER SEED-MEDIATED SYNTHESIS OF MONODISPERSE CONCAVE TRISOCTAHEDRAL GOLD NANOCRYSTALS WITH CONTROLLABLE SIZES . 32 3. Introduction 32 3. Experimental section 35 3.2.1 Materials 35 3.2.2 Preparation of Au seeds . 35 3.2.3 Synthesis of 55-nm TOH Au NCs . 36 3.2.4 Seed-mediated growth of larger TOH Au NCs . 36 3.2.5 Materials characterizations 37 3.2.6 Electrochemical measurements . 37 3. Results and discussion . 38 3.3.1 Structure characterization of TOH Au NCs 38 3.3.2 Size control 40 3.3.3 Growth mechanisms 42 iv Table of content 3.3.3.1 Selective “face-blocking” . 43 3.3.3.2 Regulating the reduction rate 45 3.3.4 Surface Plasmon resonance (SPR) spectra 46 3.3.5 Electrochemical measurements . 47 1.3.6 Self-assembly of TOH Au NCs . 49 3. Conclusion . 51 CHAPTER SYNTHESIS OF SHIELD-LIKE SINGLY TWINNED HIGH-INDEX GOLD NANOCRYSTALS . 53 4. Introduction 53 4. Experimental Details 55 4.2.1 Materials 55 4.2.2 Preparation of Au seeds . 55 4.2.3 Synthesis of shield-like Au NCs . 55 4.2.4 Materials characterizations 56 4.2.5 Electrochemical measurements . 56 4. Results and discussion . 57 4.3.1 Structural characterization of shield-like Au NCs . 57 4.3.2 Formation mechanisms of shield-like NCs . 62 4.3.3 SPR spectra and electrochemical measurements . 64 4. Conclusion . 66 CHAPTER SYNTHESIS OF NANOCRYSTALS WITH VARIABLE HIGHINDEX PALLADIUM FACETS THROUGH THE CONTROLLED HETEROEPITAXIAL GROWTH OF TRISOCTAHEDRAL GOLD TEMPLATES . 67 5. Introduction 67 v Table of content 5. Experimental Section . 69 5.2.1 Materials 69 5.2.2 Preparation of Au TOH seeds . 70 5.2.3 Synthesis of TOH, HOH and THH Au@Pd NCs 71 5.2.4 Materials characterizations 71 5.2.5 Electrochemical measurements . 72 5. Results and Discussion . 72 5.3.1 Synthesis of polyhedral NCs with high-index facets of variable classes 72 5.3.2 Synthesis of polyhedral NCs with high-index facets of variable Miller indices 79 5.3.3 Mechanisms . 83 5.3.4 Electrochemical measurements . 86 5. Conclusion . 88 CHAPTER ARTIFICIAL METALLIC MOLECULES AND THEIR MORPHOLOGY DIVERSITY 90 6. Introduction 90 6. Experimental section 92 6.2.1 Materials 92 6.2.2 Synthesis of corner-satellite Au(AgPd) artificial molecules. 93 6.2.3 Synthesis of edge-satellite Au(AgPd) artificial molecules 95 6.2.4 Materials characterizations 95 6. Results and discussion . 96 6.3.1 Synthesis of corner-satellite Au(AgPd) artificial molecules . 96 6.3.2 Synthesis of edge-satellite Au(AgPd) artificial molecules 99 6.3.3 Tuning the exposed facets of the satellite artificial atoms 101 vi Table of content 6.3.4 Tailoring the size of the artificial metallic molecules . 104 6.3.5 Confirmations of composition distribution . 105 6.3.6 Other artificial metallic molecules 106 6. Conclusion . 111 CHAPTER MECHANISTIC STUDY OF THE FORMATION OF ARTIFICIAL METALLIC MOLECULES . 112 7. Introduction 112 7. Results and discussion . 113 7.2.1 Formation of bimetallic satellite NCs . 113 7.2.2 The site-selective growth of satellite NCs . 115 7.2.2.1 Precursor addition sequence and aging of Pd precursor . 115 7.2.2.2 Evolution of corner- and edge-satellite growth with time 116 7.2.2.3 A proposed mechanism for site-selective growth . 121 7.2.3 The shape-selective growth of satellite NCs . 125 7.2.3.1 Effect of AgNO3 concentration on the satellite NC shape 125 7.2.3.2 Effect of reduction rates on the shape of the satellite NCs . 132 7. Conclusion . 134 CHAPTER CONCLUSION AND RECOMMENDATIONS 136 8. Conclusion . 136 8. Suggestions for future work . 140 REFERENCES . 143 APPENDIX A . 154 APPENDIX B . 157 PUBLICATIONS . 163 vii Summary SUMMARY At the nanoscale, the physicochemical properties of metals are strongly dependent on their shape and size, a finding that has generated tremendous interest and considerable efforts in the morphology-controlled synthesis (“morphosynthesis”) of nanometals. The ultimate goal is to develop a rational approach to the design and synthesis of nanometals in the desired morphology for the desired functions. The efforts to date have led to some advances although many of the successes are limited to the creation of relatively simple shapes (e.g. platonic nanocrystals (NCs) and their truncated forms). A gap still exists in the controlled synthesis of complex nanostructures where properties and functionalities may be “programmed” through morphological diversifications. This thesis study is an attempt to fill some of the void by using a directed evolution approach for the controlled synthesis of complex metal nanostructures. The approach will be demonstrated by the synthesis of two particular types of nanostructures: polyhedral NCs bound by high-index facets and hybrid metal NCs with complex but well-defined geometries. High quality polyhedral high-index NCs with customizable particle attributes such as size, crystallinity and exposed facets were demonstrated first. Specifically monodisperse concave trisoctahedral (TOH) gold NCs with high-index {hhl} facets and in various sizes were formed by seed-mediated growth under kinetically controlled conditions. The particle size could be increased stepwise by applying the seed-mediated growth method successively. Favorable metal precursor reduction rates and preferential adsorption of cetyltrimethylammonium cations (CTA+) on high-index facets created the favorable conditions for the development of high-index NCs. Through a slight modification of the preparation procedure, a new Au nanostructure viii Summary shield-like Au NCs with a single twin plane and high-index {hhl} facets could also be grown. The single twin planes in the NCs were formed by the coalescence of NCs in growth solutions using NaCl to screen out the repulsive interaction between CTA+capped Au NCs. These NCs were then used as templates to guide the evolution (“directed evolution”) of high-index facets on a different metal by a heteroepitaxial growth method. This was demonstrated by the epitaxial growth of a Pd shell on concave TOH Au NC seeds under carefully controlled growth conditions. By this method, polyhedral Au@Pd NCs with three different classes of high-index facets, namely concave TOH NCs with {hhl} facets, concave hexoctahedral NCs with {hkl} facets and tetrahexahedral NCs with {hk0} facets; could be synthesized in high yield. The miller indices of NCs were also modifiable. Hybrid NCs with exotic but designable morphologies were also synthesized. These hybrid nanostructures may be regarded as artificial metallic molecules since they were constructed from metal NCs in different configurations (which served as the artificial (metallic) atoms) by direct metallic bonds. A diverse range of artificial metallic molecules with complex but well-defined geometries were formed by precise and independent control of the size and shape of the artificial atoms; and their spatial organization. This was exemplified by artificial metallic molecules consisting of monometallic Au NCs in the centre surrounded by bimetallic AgPd satellite NCs. The central NC was enclosed by {111} or {100} facets (or both) upon which satellite artificial atoms with exposed {111} or {100} facets (or both) were deposited. The distribution of the satellite NCs on the central NC could be varied, i.e. they could be located selectively at the corners or along the edges of the central NCs to increase the morphological diversity of the artificial molecules. The mechanism of formation of ix References Min, M., Kim, C., Yang, Y.I., Yi, J. and Lee, H. Surface-specific overgrowth of platinum on shaped gold nanocrystals. PCCP 11, 9759-9765 2009. Ming, T., Feng, W., Tang, Q., Wang, F., Sun, L.D., Wang, J.F. and Yan, C.H. Growth of Tetrahexahedral Gold Nanocrystals with High-index Facets. J. Am. Chem. Soc. 131, 16350-16351 2009. Mulvihill, M.J., Ling, X.Y., Henzie, J. and Yang, P.D. Anisotropic Etching of Silver Nanoparticles for Plasmonic Structures Capable of Single-Particle SERS. J. Am. Chem. Soc. 132, 268-274 2010. Murphy, C.J., San, T.K., Gole, A.M., Orendorff, C.J., Gao, J.X., Gou, L., Hunyadi, S.E. and Li, T. Anisotropic metal nanoparticles: Synthesis, assembly, and optical applications. J. Phys. Chem. B 109, 13857-13870 2005. Nikoobakht, B. and El-Sayed, M.A. Evidence for bilayer assembly of cationic surfactants on the surface of gold nanorods. Langmuir 17, 6368-6374 2001. Nikoobakht, B. and El-Sayed, M.A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method. Chem. Mater. 15, 1957-1962 2003. Niu, W.X. and Xu, G.B. Crystallographic control of noble metal nanocrystals. Nano Today 6, 265-285 2011. Niu, W.X., Zheng, S.L., Wang, D.W., Liu, X.Q., Li, H.J., Han, S.A., Chen, J., Tang, Z.Y. and Xu, G.B. Selective Synthesis of Single-Crystalline Rhombic Dodecahedral, Octahedral, and Cubic Gold Nanocrystals. J. Am. Chem. Soc. 131, 697-703 2009. Njoki, P.N., Wu, W.J., Zhao, H., Hutter, L., Schiff, E.A. and Maye, M.M. Layer-byLayer Processing and Optical Properties of Core/Alloy Nanostructures. J. Am. Chem. Soc. 133, 5224-5227 2011. Pal, T., De, S., Jana, N.R., Pradhan, N., Mandal, R., Pal, A., Beezer, A.E. and Mitchell, J.C. Organized media as redox catalysts. Langmuir 14, 4724-4730 1998. Pastoriza-Santos, I., Sanchez-Iglesias, A., de Abajo, F.J.G. and Liz-Marzan, L.M. Environmental optical sensitivity of gold nanodecahedra. Adv. Funct. Mater. 17, 14431450 2007. Pazos-Perez, N., Baranov, D., Irsen, S., Hilgendorff, M., Liz-Marzan, L.A. and Giersig, M. Synthesis of flexible, ultrathin gold nanowires in organic media. Langmuir 24, 9855-9860 2008. Perez-Juste, J., Liz-Marzan, L.M., Carnie, S., Chan, D.Y.C. and Mulvaney, P. Electric-Field-Directed Growth of Gold Nanorods in Aqueous Surfactant Solutions. Adv. Funct. Mater. 14, 571-579 2004. Personick, M.L., Langille, M.R., Zhang, J. and Mirkin, C.A. Shape Control of Gold Nanoparticles by Silver Underpotential Deposition. Nano Lett., DOI: 10.1021/ nl201796s. 148    References Pietrobon, B., McEachran, M. and Kitaev, V. Synthesis of Size-Controlled Faceted Pentagonal Silver Nanorods with Tunable Plasmonic Properties and Self-Assembly of These Nanorods. ACS Nano 3, 21-26 2009. Sanchez-Iglesias, A., Pastoriza-Santos, I., Perez-Juste, J., Rodriguez-Gonzalez, B., de Abajo, F.J.G. and Liz-Marzan, L.M. Synthesis and optical properties of gold nanodecahedra with size control. Adv. Mater. 18, 2529-2534 2006. Sau, T.K. and Murphy, C.J. Room temperature, high-yield synthesis of multiple shapes of gold nanoparticles in aqueous solution. J. Am. Chem. Soc. 126, 8648-8649 2004. Sau, T.K. and Murphy, C.J. Seeded high yield synthesis of short Au nanorods in aqueous solution. Langmuir 20, 6414-6420 2004. Sau, T.K. and Rogach, A.L. Nonspherical Noble Metal Nanoparticles: ColloidChemical Synthesis and Morphology Control. Adv. Mater. 22, 1781-1804 2010. Seo, D., Il Yoo, C., Chung, I.S., Park, S.M., Ryu, S. and Song, H. Shape adjustment between multiply twinned and single-crystalline polyhedral gold nanocrystals: Decahedra, icosahedra, and truncated tetrahedra. J. Phys. Chem. C 112, 2469-2475 2008. Seo, D., Il Yoo, C., Jung, J. and Song, H. Ag-Au-Ag heterometallic nanorods formed through directed anisotropic growth. J. Am. Chem. Soc. 130, 2940-2941 2008. Seo, D., Park, J.C. and Song, H. Polyhedral gold nanocrystals with O-h symmetry: From octahedra to cubes. J. Am. Chem. Soc. 128, 14863-14870 2006. Seo, D., Park, J.H., Jung, J., Park, S.M., Ryu, S., Kwak, J. and Song, H. OneDimensional Gold Nanostructures through Directed Anisotropic Overgrowth from Gold Decahedrons. J. Phys. Chem. C 113, 3449-3454 2009. Seo, D., Yoo, C.I., Park, J.C., Park, S.M., Ryu, S. and Song, H. Directed surface overgrowth and morphology control of polyhedral gold nanocrystals. Angew. Chem. Int. Ed. 47, 763-767 2008. Shi, W.L., Zeng, H., Sahoo, Y., Ohulchanskyy, T.Y., Ding, Y., Wang, Z.L., Swihart, M. and Prasad, P.N. A general approach to binary and ternary hybrid nanocrystals. Nano Lett. 6, 875-881 2006. Skrabalak, S.E., Chen, J.Y., Sun, Y.G., Lu, X.M., Au, L., Cobley, C.M. and Xia, Y.N. Gold Nanocages: Synthesis, Properties, and Applications. Acc. Chem. Res. 41, 15871595 2008. Skrabalak, S.E. and Xia, Y. Pushing Nanomanufacturing. ACS Nano 3, 10-15 2009. Nanocrystal Synthesis toward Smith, D.K., Miller, N.R. and Korgel, B.A. Iodide in CTAB Prevents Gold Nanorod Formation. Langmuir 25, 9518-9524 2009. 149    References Sohn, K., Kim, F., Pradel, K.C., Wu, J.S., Peng, Y., Zhou, F.M. and Huang, J.X. Construction of Evolutionary Tree for Morphological Engineering of Nanoparticles. Acs Nano 3, 2191-2198 2009. Somorjai, G.A. and Blakely, D.W. Mechanism of Catalysis of Hydrocarbon Reactions by Platinum Surfaces. Nature 258, 580-583 1975. Song, H., Kim, F., Connor, S., Somorjai, G.A. and Yang, P.D. Pt nanocrystals: Shape control and Langmuir-Blodgett monolayer formation. J. Phys. Chem. B 109, 188-193 2005. Sun, S.G., Chen, A.C., Huang, T.S., Li, J.B. and Tian, Z.W. Electrocatalytic properties of Pt(111), Pt(332), Pt(331) and Pt(110) single crystal electrodes towards ethylene glycol oxidation in sulphuric acid solutions. J. Electroanal. Chem. 340, 213-226 1992. Sun, Y.G. and Xia, Y.N. Shape-controlled synthesis of gold and silver nanoparticles. Science 298, 2176-2179 2002. Tang, Y. and Ouyang, M. Tailoring properties and functionalities of metal nanoparticles through crystallinity engineering. Nat Mater 6, 754-759 2007. Tao, A.R., Habas, S. and Yang, P.D. Shape control of colloidal metal nanocrystals. Small 4, 310-325 2008. Tian, N., Zhou, Z.Y. and Sun, S.G. Platinum Metal Catalysts of High-Index Surfaces: From Single-Crystal Planes to Electrochemically Shape-Controlled Nanoparticles. J. Phys. Chem. C 112, 19801-19817 2008. Tian, N., Zhou, Z.Y. and Sun, S.G. Electrochemical preparation of Pd nanorods with high-index facets. Chem. Commun., 1502-1504 2009. Tian, N., Zhou, Z.Y., Sun, S.G., Ding, Y. and Wang, Z.L. Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High ElectroOxidation Activity. Science 316, 732-735 2007. Tian, N., Zhou, Z.Y., Yu, N.F., Wang, L.Y. and Sun, S.G. Direct Electrodeposition of Tetrahexahedral Pd Nanocrystals with High-Index Facets and High Catalytic Activity for Ethanol Electrooxidation. J. Am. Chem. Soc. 132, 7580-7581 2010. Tsuji, M., Matsuo, R., Jiang, P., Miyamae, N., Ueyama, D., Nishio, M., Hikino, S., Kumagae, H., Kamarudin, K.S.N. and Tang, X.L. Shape-dependent evolution of Au@Ag core-shell nanocrystals by PVP-assisted N,N-dimethylformamide reduction. Cryst. Growth Des. 8, 2528-2536 2008. Tsuji, M., Miyamae, N., Lim, S., Kimura, K., Zhang, X., Hikino, S. and Nishio, M. Crystal Structures and Growth Mechanisms of Au@Ag Core-Shell Nanoparticles Prepared by the Microwave-Polyol Method. Cryst. Growth Des. 6, 1801-1807 2006. Tsuji, M., Ogino, M., Matsuo, R., Kumagae, H., Hikino, S., Kim, T. and Yoon, S.H. Stepwise Growth of Decahedral and Icosahedral Silver Nanocrystals in DMF. Cryst. Growth Des. 10, 296-301 2010. 150    References Vanhove, M.A. and Somorjai, G.A. New Microfacet Notation for High-Miller-Index Surfaces of Cubic Materials with Terrace, Step and Kink Structures. Surf. Sci. 92, 489518 1980. Viswanath, B., Kundu, P., HaIder, A. and Ravishankar, N. Mechanistic Aspects of Shape Selection and Symmetry Breaking during Nanostructure Growth by Wet Chemical Methods. J. Phys. Chem. C 113, 16866-16883 2009. Wang, C., Tian, W.D., Ding, Y., Ma, Y.Q., Wang, Z.L., Markovic, N.M., Stamenkovic, V.R., Daimon, H. and Sun, S.H. Rational Synthesis of Heterostructured Nanoparticles with Morphology Control. J. Am. Chem. Soc. 132, 6524-6529 2010. Wang, D.S. and Li, Y.D. One-Pot Protocol for Au-Based Hybrid Magnetic Nanostructures via a Noble-Metal-Induced Reduction Process. J. Am. Chem. Soc. 132, 6280-6281 2010. Wang, F., Li, C., Sun, L.-D., Wu, H., Ming, T., Wang, J., Yu, J.C. and Yan, C.-H. Heteroepitaxial Growth of High-Index-Faceted Palladium Nanoshells and Their Catalytic Performance. J. Am. Chem. Soc., DOI: 10.1021/ja1095733 2010. Wang, F., Li, C.H., Sun, L.D., Wu, H.S., Ming, T.A., Wang, J.F., Yu, J.C. and Yan, C.H. Heteroepitaxial Growth of High-Index-Faceted Palladium Nanoshells and Their Catalytic Performance. J. Am. Chem. Soc. 133, 1106-1111 2010. Wang, H. and Halas, N.J. Mesoscopic Au "Meatball" particles. Adv. Mater. 20, 820825 2008. Wang, Z.L. Transmission electron microscopy of shape-controlled nanocrystals and their assemblies. J. Phys. Chem. B 104, 1153-1175 2000. Watt, J., Cheong, S., Toney, M.F., Ingham, B., Cookson, J., Bishop, P.T. and Tilley, R.D. Ultrafast Growth of Highly Branched Palladium Nanostructures for Catalysis. ACS Nano 4, 396-402 2010. Watt, J., Young, N., Haigh, S., Kirkland, A. and Tilley, R.D. Synthesis and Structural Characterization of Branched Palladium Nanostructures. Adv. Mater. 21, 2288-2289 2009. Wetz, T., Soulantica, K., Talqui, A., Respaud, M., Snoeck, E. and Chaudret, B. Hybrid Co-Au nanorods: Controlling Au nucleation and location. Angew. Chem. Int. Ed. 46, 7079-7081 2007. Wiley, B., Herricks, T., Sun, Y.G. and Xia, Y.N. Polyol synthesis of silver nanoparticles: Use of chloride and oxygen to promote the formation of single-crystal, truncated cubes and tetrahedrons (vol 4, pg 1734, 2004). Nano Lett. 4, 2057-2057 2004. Wiley, B.J., Xiong, Y.J., Li, Z.Y., Yin, Y.D. and Xia, Y.A. Right bipyramids of silver: A new shape derived from single twinned seeds. Nano Lett. 6, 765-768 2006. 151    References Wu, H.L., Kuo, C.H. and Huang, M.H. Seed-Mediated Synthesis of Gold Nanocrystals with Systematic Shape Evolution from Cubic to Trisoctahedral and Rhombic Dodecahedral Structures. Langmuir 26, 12307-12313 2010. Xia, Y., Xiong, Y., Lim, B. and Skrabalak, S.E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem. Int. Ed. 48, 60-103 2009. Xiang, Y.J., Wu, X.C., Liu, D.F., Jiang, X.Y., Chu, W.G., Li, Z.Y., Ma, Y., Zhou, W.Y. and Xie, S.S. Formation of rectangularly shaped Pd/Au bimetallic nanorods: Evidence for competing growth of the Pd shell between the {110} and {100} side facets of Au nanorods. Nano Lett. 6, 2290-2294 2006. Xie, J.P., Zhang, Q.B., Lee, J.Y. and Wang, D.I.C. The Synthesis of SERS-Active Gold Nanoflower Tags for In Vivo Applications. ACS Nano 2, 2473-2480 2008. Xiong, Y.J., Cai, H.G., Wiley, B.J., Wang, J.G., Kim, M.J. and Xia, Y.N. Synthesis and mechanistic study of palladium nanobars and nanorods. J. Am. Chem. Soc. 129, 3665-3675 2007. Xiong, Y.J., Cai, H.G., Yin, Y.D. and Xia, Y.N. Synthesis and characterization of fivefold twinned nanorods and right bipyramids of palladium. Chem. Phys. Lett. 440, 273-278 2007. Xiong, Y.J., McLellan, J.M., Chen, J.Y., Yin, Y.D., Li, Z.Y. and Xia, Y.N. Kinetically controlled synthesis of triangular and hexagonal nanoplates of palladium and their SPR/SERS properties. J. Am. Chem. Soc. 127, 17118-17127 2005. Xiong, Y.J., Wiley, B. and Xia, Y.N. Nanocrystals with unconventional shapes - A class of promising catalysts. Angew. Chem. Int. Ed. 46, 7157-7159 2007. Xu, J., Li, S.Y., Weng, J., Wang, X.F., Zhou, Z.M., Yang, K., Litt, M., Chen, X., Cui, Q., Cao, M.Y. and Zhang, Q.Q. Hydrothermal syntheses of gold nanocrystals: From icosahedral to its truncated form. Adv. Funct. Mater. 18, 277-284 2008. Yin, Y. and Alivisatos, A.P. Colloidal nanocrystal synthesis and the organic-inorganic interface. Nature 437, 664-670 2005. Yu, T., Kim, D.Y., Zhang, H. and Xia, Y.N. Platinum Concave Nanocubes with HighIndex Facets and Their Enhanced Activity for Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 50, 2773-2777 2011. Yu, Y., Zhang, Q., Liu, B. and Lee, J.Y. Synthesis of Nanocrystals with Variable High-Index Pd Facets through the Controlled Heteroepitaxial Growth of Trisoctahedral Au Templates. J. Am. Chem. Soc. 132, 18258-18265 2010. Yu, Y., Zhang, Q.B., Lu, X.M. and Lee, J.Y. Seed-Mediated Synthesis of Monodisperse Concave Trisoctahedral Gold Nanocrystals with Controllable Sizes. J. Phys. Chem. C 114, 11119-11126 2010. 152    References Zhang, J., Li, S.Z., Wu, J.S., Schatz, G.C. and Mirkin, C.A. Plasmon-Mediated Synthesis of Silver Triangular Bipyramids. Angew. Chem. Int. Ed. 48, 7787-7791 2009. Zhang, J.A., Langille, M.R., Personick, M.L., Zhang, K., Li, S.Y. and Mirkin, C.A. Concave Cubic Gold Nanocrystals with High-Index Facets. J. Am. Chem. Soc. 132, 14012-14014 2010. Zhang, J.H., Liu, H.Y., Wang, Z.L. and Ming, N.B. Shape-selective synthesis of gold nanoparticles with controlled sizes, shapes, and plasmon Resonances. Adv. Funct. Mater. 17, 3295-3303 2007. Zhang, Q.B., Xie, J.P., Lee, J.Y., Zhang, J.X. and Boothroyd, C. Synthesis of Ag@AgAu metal core/alloy shell bimetallic nanoparticles with tunable shell compositions by a galvanic replacement reaction. Small 4, 1067-1071 2008. Zhang, Q.B., Xie, J.P., Yang, J.H. and Lee, J.Y. Monodisperse Icosahedral Ag, Au, and Pd Nanoparticles: Size Control Strategy and Superlattice Formation. ACS Nano 3, 139-148 2009. Zhang, Q.B., Xie, J.P., Yu, Y. and Lee, J.Y. Monodispersity control in the synthesis of monometallic and bimetallic quasi-spherical gold and silver nanoparticles. Nanoscale 2, 1962-1975 2010. Zhang, Q.B., Xie, J.P., Yu, Y., Yang, J.H. and Lee, J.Y. Tuning the Crystallinity of Au Nanoparticles. Small 6, 523-527 2010. Zhang, X., Tsuji, M., Lim, S., Miyamae, N., Nishio, M., Hikino, S. and Umezu, M. Synthesis and growth mechanism of pentagonal bipyramid-shaped gold-rich Au/Ag alloy nanoparticles. Langmuir 23, 6372-6376 2007. Zhao, N., Wei, Y., Sun, N.J., Chen, Q.J., Bai, J.W., Zhou, L.P., Qin, Y., Li, M.X. and Qi, L.M. Controlled synthesis of gold nanobelts and nanocombs in aqueous mixed surfactant solutions. Langmuir 24, 991-998 2008. Zheng, H.M., Smith, R.K., Jun, Y.W., Kisielowski, C., Dahmen, U. and Alivisatos, A.P. Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories. Science 324, 1309-1312 2009. Zhou, Z.Y., Huang, Z.Z., Chen, D.J., Wang, Q., Tian, N. and Sun, S.G. High-Index Faceted Platinum Nanocrystals Supported on Carbon Black as Highly Efficient Catalysts for Ethanol Electrooxidation. Angew. Chem. Int. Ed. 49, 411-414 2010. Zhou, Z.Y., Tian, N., Huang, Z.Z., Chen, D.J. and Sun, S.G. Nanoparticle catalysts with high energy surfaces and enhanced activity synthesized by electrochemical method. Faraday Discuss. 140, 81-92 2008. Zhou, Z.Y., Tian, N., Li, J.T., Broadwell, I. and Sun, S.G. Nanomaterials of high surface energy with exceptional properties in catalysis and energy storage. Chem. Soc. Rev. 40, 4167-4185 2011. 153    Appendix A APPENDIX A Synthesis of low-index polyhedral gold nanocrystals Experimental details for the preparation of low-index polyhedral Au nanocrystals (NCs) Synthesis of Au octahedral seed NCs. Au octahedral NCs were first prepared and used as seeds for the preparation of larger octahedral or other types polyhedral NCs. The synthesis of Au octahedral seed NCs was based on a seed-mediated growth method with small Au NCs as seeds. For the preparation of small Au seed NCs, mL of 75 mM CTAB solution was first prepared by heating at 30 oC with stirring to dissolve the CTAB. 87.5 µL of 20 mM HAuCl4 solution was added to the CTAB solution. 0.6 mL of an ice-cold NaBH4 solution (10 mM) was then injected quickly into the mixture under vigorous mixing to form a brown seed solution. Stirring continued gently at 30 oC for to hours to decompose the excess NaBH4. The seed solution was then diluted 100 fold with ultrapure water. A growth solution was prepared by adding 25 µL of 20 mM HAuCl4 solution and 0.387 mL of 38.8 mM ascorbic acid (in that order) into 12.1 mL of 16.5 mM CTAB solution in a clean test tube at 28 oC with thorough mixing after each addition. 0.15 mL of the diluted seed solution was added to the growth solution and thoroughly mixed. The mixture was left unperturbed at 28 oC overnight. The color of the solution changed to pink indicating the formation of Au NCs. 154    Appendix A Synthesis of low-index polyhedral Au NCs. For the preparation of octahedral Au NCs, mL of the octahedral Au seed solution was added to 12.5 mL of a growth mixture containing 16 mM CTAB, 0.04 mM HAuCl4 and 1.2 mM ascorbic acid to enlarge the octahedral Au NCs. The mixture was thoroughly mixed and left unperturbed overnight. For growth of larger octahedral NCs, mL of the octahedral Au solution was added to 12.5 mL of a growth mixture containing 16 mM CTAB, 0.04 mM HAuCl4 and 1.2 mM ascorbic acid to enlarge the octahedral Au NCs. For the preparation of truncated octahedral Au NCs, mL of the octahedral Au seed solution was added to 12.5 mL of growth mixture. The growth solution for smaller truncated octahedral Au NCs contained 16 mM CTAB, 0.04 mM HAuCl4 and mM ascorbic acid. For larger truncated octahedral Au NCs, the growth solution contained 16 mM CTAB, 0.08 mM HAuCl4 and mM ascorbic acid. The mixture was thoroughly mixed and left unperturbed overnight. Cubic Au NCs were prepared in a similar manner as the TOH Au NCs (Chapter 3) except with the substitution of CTAC by CTAB at the same concentration. The growth solution for cubic NCs contained 16 mM CTAB, 0.2 mM HAuCl4 and 9.5 mM ascorbic acid. 6.5 mL of the octahedral seed solution was added to 12.5 mL of the growth solution to initiate the growth of cubic NCs. The mixture was thoroughly mixed and left unperturbed overnight. These polyhedral Au NCs was used as central NCs for the preparation of artificial metallic molecules in Chapter 6. 155    Appendix A Characterization for the low-index polyhedral Au NCs Figure 1. TEM and SEM images of (A)-(B) octahedral, (C)-(D) truncated octahedral Au NCs with small truncation, (E)-(F) truncated octahedral Au NCs with large truncation and (G)-(H) cubic Au NCs. 156    Appendix B APPENDIX B Synthesis of cubic and octahedral Au@Pd nanocrystals and the electrochemical measurement towards formic acid oxidation Experimental details for the preparation of cubic and octahedral Au@Pd NCs For the synthesis of cubic NCs, the cubic Au NC solution was centrifuged twice to remove excess of ascorbic acid and then redispersed in same volume of a solution containing 16 mM CTAB and mM ascorbic acid. 50 L of 10 mM H2PdCl4 was then added to mL of the cubic Au NC solution. The mixture was mixed and left overnight. For the synthesis of octahedral Au@Pd NCs, 0.258 mL of 38.8 mM ascorbic acid and 10 L of 10 mM of H2PdCl4 were added sequentially to 5mL of the Au octahedral NC solution. The solution was well shaken and left overnight. Characterization of cubic and octahedral Au@Pd NCs with low-index facets Figure shows the TEM and SEM images of the cubic Au@Pd NCs with different amounts of deposited Pd. After the epitaxial growth of Pd shells on the Au cubes, the NCs adopted a perfect cubic shape. The contrast in the TEM images indicates the core-shell structure of the NCs. The HRTEM images taken from the corner of the NC revealed that the exposed facets are low-index {100} facets. The Au@Pd NCs easily assembled into ordered square arrays on the substrate with their facets in close contact with each other. This was made possible because of a high monodispersivity in shape and size. Figure shows the octahedral Au@Pd NCs prepared by the seeded growth of 157    Appendix B different amounts of Pd on octahedral Au seeds. The fringes and the contrast in the TEM images confirm the successful deposition of Pd onto the Au cores. The growth of the Pd layer on octahedral Au seeds resulted in slightly truncated corners. The facets exposed by the truncation are {100} facets. Figure 1. (A)-(C) TEM images of cubic Au@Pd NCs prepared with Pd:Au atomic ratios of 1:4, 1:2 and 1:1 respectively. Below each TEM image is the corresponding HRTEM image (D)-(F) of the corner of a single cubic Au@Pd NC viewed from the directions; showing that the NC is enclosed by {100} facets. The inset is the corresponding FFT pattern. Representative SEM images at (G) high and (H) low magnifications. 158    Appendix B Figure 2. TEM images of octahedral Au@Pd NCs with Pd:Au atomic ratios of (A) 1:8, (B) 1:4, and (C) 1:2 respectively. (Column 1) TEM images showing the majority of NCs as octahedrons and the successful growth of Pd on the Au cores. (Column 2) A single octahedral Au@Pd NC viewed from the directions showing the increase in thickness with Pd amount. (Column 3) HRTEM images of the corner of a Au@Pd NC showing that the NC is enclosed by {111} facets with {100} truncation. The corresponding FFT pattern of the HRTEM image is provided as an inset. (D) and (E) are representative SEM images of octahedral Au@Pd NCs at high and low magnifications. 159    Appendix B Electrochemical measurement The effects of Pd shell thickness on the electrochemical behavior of cubic and octahedral Au@Pd NCs were investigated and shown in Figures and 4. The oxidation of formic acid on cubic Au@Pd NCs showed a slight positive shift of the anodic peak potential from 0.44 V to 0.49 V with the increase in Pd/Au atomic ratio from 1/4 to 1. The peak current density was around 20 mA/cm2 and did not change much with the Pd amount. The cathodic scans showed sudden increase at around 0.5 V corresponding to the reduction of the oxide layer on Pd. The activity for formic acid oxidation recovered after the oxide film was reduced. For formic acid oxidation on octahedral Au@Pd NCs enclosed by {111} facets, the peak current increased from 2.57 to 5.75 mA/cm2 with increasing Pd amount. The peak current density of octahedral NCs with {111} facets was only one fifth of that of cubes with {100} facets and occurred at a lower potential of ~0.2 V. The results agree well with measurements on bulk single crystalline electrodes and NCs with well-faceted cubic and octahedral shapes. For single crystalline Pd surfaces, the anodic peak current on Pd (100) surface is approximately four times higher than that on the Pd (111) surface while the peak potential on the (111) surface is about 0.35 V lower than that on the (100) surface. (Baldauf and Kolb, 1996; Hoshi et al, 2006) For NCs with well-defined Pd facets, the cubes showed a peak current which was five times higher than that of octahedrons; whereas the latter showed a peak potential lower than that of the cubes by 0.21 V. (Habas et al, 2007) The surface structure affected the electrocatalytic properties more significantly than the thickness of the Pd layer in this study since both cubic and octahedral NCs displayed characteristics of the polyhedral shape regardless of the thickness of the Pd layer used. 160    Current density (mA/cm ) Current density (mA/cm ) Current density (mA/cm ) Appendix B 20 Pd:Au=1:4 15 10 -0.2 20 0.0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 Pd:Au=1:2 15 10 -0.2 20 0.0 Pd:Au=1:1 15 10 -0.2 0.0 Potential (V vs Ag|AgCl) Figure 3. Cyclic voltammograms of formic acid oxidation in 0.1 M HClO4 and M HCOOOH catalyzed by cubic Au@Pd NCs with different Pd:Au atomic ratios. Scan rate: 10 mV/s. 161    Current density (mA/cm ) Appendix B Pd:Au=1:8 0.0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 Current density (mA/cm ) -0.2 Pd:Au=1:4 0.0 Current density (mA/cm ) -0.2 Pd:Au=1:2 -0.2 0.0 Potential (V vs Ag|AgCl) Figure 4. Cyclic voltammograms of formic acid oxidation in 0.1 M HClO4 and M HCOOOH catalyzed by octahedral Au@Pd NCs with different Pd:Au atomic ratios. Scan rate: 10 mV/s. Table 1. Peak potential, peak current density and current density at 0V (vs Ag|AgCl) for Au@Pd NCs with different polyhedral shapes Shape of the Au@Pd NCs Cube Cube Cube Octahedron Octahedron Octahedron Pd/Au atomic ratio 1/4 1/2 1/1 1/8 1/4 1/2 Peak potential (V vs Ag|AgCl) 0.44 0.47 0.49 0.18 0.21 0.21 Peak current density (mA/cm2) 20.38 20.59 20.19 2.57 3.76 5.15 Current density at V (mA/cm2) 0.97 0.78 0.92 0.72 1.12 1.48 162    Publications PUBLICATIONS Yu, Y., Zhang, Q.B., Xie, J.P., Lu, X.M. & Lee, J.Y. Synthesis of shield-like singly twinned high-index Au nanoparticles. Nanoscale 3, 1497-1500 2011. Yu, Y., Zhang, Q.B., Liu, B. & Lee, J.Y. Synthesis of Nanocrystals with Variable High-Index Pd Facets through the Controlled Heteroepitaxial Growth of Trisoctahedral Au Templates. J. Am. Chem. Soc. 132, 18258-18265 2010. Yu, Y., Zhang, Q.B., Lu, X.M. & Lee, J.Y. Seed-Mediated Synthesis of Monodisperse Concave Trisoctahedral Gold Nanocrystals with Controllable Sizes. J. Phys. Chem. C 114, 11119-11126 2010. Zhang, Q.B., Xie, J.P., Yu, Y. & Lee, J.Y. Monodispersity control in the synthesis of monometallic and bimetallic quasi-spherical gold and silver nanoparticles. Nanoscale 2, 1962-1975 2010. Zhang, Q.B., Xie, J.P., Yu, Y., Yang, J.H. & Lee, J.Y. Tuning the Crystallinity of Au Nanoparticles. Small 6, 523-527 2010. 163    [...]... basic concepts and surveys of current progress in the shapecontrolled synthesis of high-index and hybrid noble metal NCs in two consecutive sections 2 1 Fundamentals of the formation of polyhedral NCs One of the major goals of current nanomaterials research is to understand the shape evolution processes in NCs This section is a summary of the current understanding on shape evolution and the major factors... needs of the application, is also predicated upon the ability of shapecontrolled synthesis to create the desired shape or to generate a sufficient number of shape variants for explorations (Tao et al, 2008; Xia et al, 2009; Guo and Wang, 1 Chapter 1 2011) A good understanding of the growth mechanisms can reduce the number of trial -and- error in shape controlled synthesis; thereby allowing the tuning of. .. images of edge-satellite growth of hybrid NCs at AgNO3 concentration of (A) 5 µM, and (B) 40 µM All other experimental conditions were the same as in the preparation of edge-satellite hybrid NCs with octahedral central NCs 132 Figure 7.8 TEM images of hybrid NCs obtained with HCl concentration of (A)-(B) 0 mM and (C)-(D) 5 mM at two AgNO3 concentrations namely (A) and (C) 10 µM and (B) and (D)... shape and size of individual components but also on the spatial organization of these components within each NC The synthesis of hybrid NCs with programmable morphology is a significant challenge as it requires the development of synthetic protocols capable of the independent control of shape, size and crystallinity of the constituent building blocks, and the precision organization of these units at... simple size-tuning of spherical NCs could be realized by tuning the NC shape Morphology programming can also be used to enhance the NC properties or generate new functionalities (Guo and Wang, 2011) The premise for all these is the availability of capable shape- controlled synthesis methods The development of a fundamental understanding of shape- dependent properties, through which shape of nanomaterials... approach to the overall synthesis is still lacking at this time A better understanding of the mechanisms of formation of hybrid 3 Chapter 1 nanostructures is also needed for the development of synthetic protocols of general utility that can be applied to most nanomaterials 1 2 Objectives and scope The primary objective of this thesis study is to develop synthesis methods capable of producing noble metal... with programmable diversity and complexity by a rational, evolutionary approach This was accomplished by systematic tuning of parameters such as the shape and size of polyhedral NCs The approach will be demonstrated by the successful synthesis of Au -based high-index NCs and hybrid NCs in a few configurations Au was chosen not only because of its application potentials but also of the vast literature that... the synthesis of Au NCs; and the ease of nanogold characterization (Eustis and El-Sayed, 2006) The polyhedral NCs in this study are mostly nearly spherical and hence may be regarded as zero-dimensional NCs The following is a list of completed research activities: 1 Synthesis of high-index NCs with customizable sizes and good monodispersity control Trisoctahedral (TOH) Au NCs in the size range of 55... provides a succinct but up-to-date account of major topics relevant to the shape controlled synthesis of high-index NCs and hybrid NCs These topics are presented in four sections The first section introduces the basic principles and the factors in the development of polyhedral NCs This is followed by a short overview of the colloidal chemistry methods of NC synthesis especially for the noble metal NCs... 118 Figure 7.5 (A) Schematic illustrations of the main stages in the formation of the edge-satellite hybrid NCs (B-F) TEM images of hybrid NCs formed after addition of H2PdCl4 to the growth solution and aged for 10 min; and 20 min, 1 hour, 3 hours and 6 hours after the introduction of AgNO3 to the growth solution and (G-K) the corresponding HRTEM xvii List of figures images Note: AgNO3 was introduced . SHAPE-CONTROLLED SYNTHESIS OF MONODISPERSE GOLD NANOCRYSTALS AND GOLD- BASED HYBRID NANOCRYSTALS YU YUE (B. Eng., Nanyang Technological. Classification of hybrid NCs 25 2.4.2 Synthesis methods for hybrid nanostructures 26 2.4.3 Factors that determine the configuration of hybrid nanostructures 28 2.4.4 Shape controlled synthesis of noble. illustrations of the main stages in the formation of the edge-satellite hybrid NCs. (B-F) TEM images of hybrid NCs formed after addition of H 2 PdCl 4 to the growth solution and aged for 10 min; and