SYNERGISTIC EFFECT OF TWO FOREIGN METAL IONS ON SHAPE-SELECTIVE SYNTHESIS OF GOLD NANOCRYSTALS TRAN TRONG TOAN NATIONAL UNIVERSITY OF SINGAPORE 2011 SYNERGISTIC EFFECT OF TWO FOREIGN METAL IONS ON SHAPE-SELECTIVE SYNTHESIS OF GOLD NANOCRYSTALS TRAN TRONG TOAN (B.Sc. (Hons.), University of Science Ho Chi Minh City) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 ACKNOWLEDGEMENTS First of all, I would like to express my deep gratitude towards the following people who have helped me complete the thesis. A special thank to my research supervisor, Assistant Professor Lu Xianmao, for offering an opportunity to me to be a part of his research group. I want to thank him for his invaluable support and all the guidance throughout the course of study. I would also like to thank my thesis examiners, Professor Zeng Hua Chun and Assistant Professor Saif A. Khan for their advice, guidance and encouragement throughout my MEng study. All the professional officers and lab technologists, Mr. Chia Phai Ann, Dr. Yuan Ze Liang, Ms. Lee Chai Keng, Ms. Li Xiang, Dr. Yang Liming, Ms. Li Fengmei, and other staffs who have unconditionally helped me in many administrative works as well as experiments and have willingly shared their knowledge and expertise to further enhance my studying process. My colleagues, Dr. Sun Zhipeng, Ms. Zhang Weiqing, Mr. Shaik Firdoz, and all the final year students for all their kind supports they provided. Finally, I want to specially thank my parents who have given me all what they have for their unconditional support and their love. I also want to thank my girlfriend for her non-stop support during my study. i Table of Contents Acknowledgement .................................................................................................. i Table of Contents ................................................................................................. ii Summary.............................................................................................................. iv Nomenclature ....................................................................................................... vi List of Figures..................................................................................................... vii List of Tables ....................................................................................................... ix Chapter 1. Introduction........................................................................................ 1 1.1. Background ................................................................................................. 1 1.2. Research objectives ..................................................................................... 2 1.3. References ....................................................................................................3 Chapter 2. Literature Review .............................................................................. 4 2.1. Shape-controlled synthesis of noble metal nanocrystals............................. 4 2.1.1. Nucleation and growth of metal nanocrystals ..................................... 4 2.1.2. Chemical methods for synthesis of metal nanocrystals with controlled shapes ................................................................................... 6 2.1.2.1. Seeded-growth route.................................................................. 6 2.1.2.2. Hydrothermal route.................................................................... 8 2.1.2.3. Electrochemical route ................................................................ 8 2.1.2.4. Photochemical route .................................................................. 9 2.1.2.5. Polyol route.............................................................................. 10 ii 2.2. Synthesis and catalytic properties of metal nanocrystals with high-index facets ..................................................................................... 12 2.3. References ..................................................................................................14 Chapter 3. Shape-controlled Synthesis of Au Nanocrystals with High-index Facets .......................................................................................17 3.1. Shape-selective growth of polyhedral gold nanocrystals with high-index facets .......................................................................................17 3.1.1. Introduction ..................................................................................... 17 3.1.2. Experimental Section ...................................................................... 19 3.1.3. Results & discussion ....................................................................... 22 3.1.4. Conclusion....................................................................................... 39 3.2. References ..................................................................................................41 Chapter 4. Conclusions and Recommendations for Future Work................. 43 4.1. Conclusions ............................................................................................... 43 4.2. Recommendations for Future work........................................................... 44 4.3. References ................................................................................................. 48 iii Summary Shape-controlled synthesis of metal nanocrystals has been widely investigated for the last several decades because of its ability to tailor the morphology of metal nanocrystals, and therefore, their physical and chemical properties. These properties, which greatly differ from their bulk counterparts, are highly dependent on the size and the shape of the nanocrystals. Metal nanocrystals with many shapes such as cubes, octahedra, cubotahedra, icosahedra, plates, rods, and wires in various sizes have been synthesized. However, these nanocrystals are mainly enclosed by low Miller-index facets (i.e. {111}, {100}, and {110}). Recently, much focus has been given to metal nanocrystals with high-index facets due to their superior catalytic properties to those bounded by low-index facets. The metal nanocrystals with high-index facets are, however, difficult to be prepared due to the fact that high-index facets are not as stable as those low-index ones during the synthetic period. In this work, we present the facile PDDA-mediated polyol route for synthesis of a series of novel Au nanocrystals, namely, truncated octahedra bounded by both {111} and {310} facets, truncated ditetragonal prisms exclusively enclosed by {310} facets, and bipyramids with exposed {117} facets by simply varying the ratio of Ag and Pd ions. The synergistic effect of Ag and Pd ions on the formation of the novel Au nanocrystals was studied. In our experimental conditions, the underpotential deposition (UPD) of Ag on Au surface was believed to inhibit the growth along directions, therefore lead to the formation of {110} facets on Au nanocrystals. Palladium ions could, on the other hand, also take part in the deposition on Au surface and stabilize {100} facets. Together, Ag and Pd ions enabled the growth of {310} facets on the Au nanocrystals as {310} facets are composed of {110} and {100} iv subfacets. Since the Au nanocrystals obtained in this report possess high-index facets, they are expected to be promising candidates for many catalytic applications. v Nomenclature CTAB Cetyltrimethylammonium bromide CTAC Cetyltrimethylammonium chloride EDX Energy dispersive X-ray spectroscopy EG Ethylene glycol FESEM Field-emission scanning electron microscopy ICP-MS Inductive coupling plasma mass spectrometry PDDA Poly(diallyldimethylamonium chloride) PEG Polyethyleneglycol PVP Polyvinylpyrrolidone SAED Selected area electron diffraction SEM Scanning electron microscopy TEM Transmission electron microscopy vi List of Figures Figure 3.1. (A) Low and (B) high magnification SEM images of Au truncated ditetragonal prisms showing well-defined structures with sharp edges and apexes. (C) HRSEM of a group of Au truncated ditetragonal prisms. (D) TEM images of Au truncated ditetragonal prisms showing its cross-section. (E) High magnification of a truncated ditetragonal prism (inset) exhibiting (200) d-spacing of fcc Au. (F) The schematic drawings at different views of an Au nanoprism ....................................... 23 Figure 3.2. Determination of facets of Au truncated ditetragonal prisms from different views (A) top view (cross-section) and (B) side view. The result indicates that Au truncated ditetragonal prisms are bound by 12 {310} facets. Note that image (A) and (B) were taken from different truncated ditetragonal prisms. (C), (D) Schematic drawing of truncated ditetragonal prisms with their theoretical angles. (E) Atomic model of Au (310) facet including (110) and (100) subfacets....................... 24 Figure 3.3. (A), (B), (C) and (D) Schematic models for Au truncated ditetragonal prisms at different views illustrating for (E), (F), (G) and (H) the corresponding TEM images. (I), (J), (K) and (L) the ED patterns that consistently show all [310] zone axes. Note that the SAED patterns were taken from different Au nanoprisms .......... 25 Figure 3.4. Schematic models for other configurations of Au truncated ditetragonal prisms which differ from the Au nanoprisms in Figure 3.1. (A) one sloping face pair (at one end) rotated 90° around the principle axis, (B) one vertical half rotated 90° so that two side faces become two new sloping faces at two ends, and (C) one sloping face pair (at one end) rotated 90° around the principle axis and one vertical half rotated 90° so that two side faces become two new sloping faces at two ends (i.e. combining (A) and (B)) .............................................................................................. 25 Figure 3.5. (A) Low and (B) high magnification SEM images of Au bipyramids. Inset of Figure 3B clearly shows a pentagonal cross-section of an exceptionally big bipyramid. Inset scale bar is 100 nm. (C) TEM image of Au bipyramids. (D) HRTEM image of a bipyramid (inset) describes the (111) d-spacing. Inset scale bar is 20 nm. (E) The corresponding ED pattern showing the superposition of [110] and [111] zones of fcc structure. (F) Schematic drawing of a bipyramid ............................................. 27 Figure 3.6. (A) TEM image of an Au nanobipyramid with defined width base (W, yellow line) and height of half (Hhf, red line). (B) Model of half of pentagonal bipyramid and formula that exhibits the relationship between morphological measurements (i.e. W and Hhf) and Miller index of the bipyramidal facets. By measuring few tens of Au bipyramids in TEM images, we could determine the average Hhf/W ratio of 2.18 which means that the Au bipyramids obtained in this work enclosed by the high-index {117} facets........................................................... 27 Figure 3.7. (A) Low magnification SEM image of Au truncated octahedra. Inset shows schematic model of a truncated octahedron that exposes both {111} and {310} facets. (B) High magnification SEM image of truncated octahedra with a superposed drawing frame on single truncated octahedra shows the consistency with the schematic model. (C) TEM image of Au truncated octahedra with the inset showing vii (200) d-spacing of Au fcc. (D), (E) ED patterns of Au truncated octahedra clearly show [310] and [111] zone axes. (F) Schematic drawing showing the morphological relationship between an octahedron and a truncated octahedron................................ 28 Figure 3.8. EDX analyses of Au nanostructures: (A) truncated ditetragonal prisms, (B) bipyramids and (C) truncated octahedra............................................................... 30 Figure 3.9. XPS analyses of Au nanostructures: (A) truncated ditetragonal prisms, (B) bipyramids and (C) truncated octahedra............................................................... 31 Figure 3.10. XRD patterns of Au nanostructures: (A) truncated ditetragonal prisms, (B) bipyramids and (C) truncated octahedra............................................................... 33 Figure 3.11. UV-vis spectra of Au truncated ditetragonal prisms, bipyramids and truncated octahedra ..................................................................................................... 34 Figure 3.12. Au nanostructures synthesized at different temperature: (A, C and E) at 140 °C and (B, D and F) at 170 °C. The procedures were similar to those used for the syntheses of Au truncated ditetragonal prisms, bipyramids and truncated octahedra except that no NaCl was used for the truncated ditetragonal prism synthesis. (A, B) Truncated ditetragonal prisms with the longest lengths of 52 and 30 nm, (C, D) bipyramids with lengths of 53 and 40 nm and (E, F) truncated octahedra with diameters of 75 and 32 nm.......................................................................................... 35 Figure 3.13. Au nanostructures obtained without the addition of Pd2+. The concentration of AuCl4- in these experiments was kept the same as previously. The concentrations of Ag+ are as follows: (A) 0.024 mM; (B) 0.096 mM; (C) 0.476 mM ..................................................................................................................................... 37 Figure 3.14. Au nanoparticles synthesized without the addition of Ag+. (A) [Pd2+] = 0 mM, 195 °C, 30 min; (B) [Pd2+] = 0.06 mM, 120 °C, 12 h; (C) [Pd2+] = 0.12 mM, 120 °C, 12 h ....................................................................................................................... 39 viii vii List of Tables Table 3.1. Atomic composition based on EDX, ICP-MS and XPS of Au truncated ditetragonal prisms, bipyramids and {310} truncated octahedra.................................32 ix Chapter 1. Introduction 1.1. Background Noble metal nanoparticles are excellent catalysts for many chemical transformations due to their much higher surface-to-volume ratio than the bulk materials.1-3 Since the catalytic properties of metal nanoparticles are highly dependent on the morphology,3-8 control of their shape and size holds great promise for the preparation of catalysts with improved performance.3,9,10 Noble metal nanocrystals with high-index facets are known to provide high catalytic activities because of their high density of low-coordinated surface atoms that can serve as active sites for breaking chemical bonds.11,12 Therefore, synthesis of metal nanocrystals with high-index facets has been of much interest to numerous investigators during the past decade. However, it still remains challenging to fabricate such nanocrystals because of their high surface energy and thus low stability. Recently, metal nanocrystals bounded by high-index facets such as Pt and Pd tetrahexahedral (THH) nanocrystals have been synthesized using electrochemical method.13,14 The Pt and Pd THH particles have exhibited 2-6 times higher catalytic activity per unit surface area than the commercial catalysts toward ethanol electrooxidation. These works, therefore, shed new light to the synthesis of metal nanocrystals enclosed by high-index facets for catalysis, although the electrochemical approach is limited to small-scale production. Wet chemical synthesis is promising for large-scale preparation of nanocrystals.15,16 However, the current wet chemical routes still lack the ability to simultaneously control over the shape and size of metal nanocrystals bounded by high-index facets. 1 1.2. Research objectives The synthesis of metal nanocrystals with high-index facets using wet chemical methods is currently of intensive focus. Moreover, the study of catalytic properties of metal nanocrystals bounded by high-index facets as well as the use of these nanocrystals as the building blocks for more complex heterometallic nanostructures are promising topics in nanoscience and nanotechnology. So far, by using a modified polyol process in combined with the use of Ag(I) and Pd(II) as foreign ions, we have successfully synthesized Au nanocrystals with exposed high-index facets including truncated octahedra enclosed by both {111} and {310} facets, truncated ditetragonal prisms bounded by twelve {310} facets, and bipyramids with {117} facets. 2 1.3. References (1) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 2, 2385. (2) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (3) Wang, D. S.; Xie, T.; Li, Y. D. Nano Res. 2009, 2, 30. (4) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (5) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (6) Tao, A. R.; Habas, S.; Yang, P. Small 2008, 4, 310. (7) Xu, R.; Wang, D. S.; Zhang, J. T.; Li, Y. D. Chemistry Asian J. 2006, 1, 888. (8) Schmidt, E.; Vargas, A.; Mallat, T.; Baiker, A. J. Am. Chem. Soc. 2009, 131, 12358. (9) Xia, Y. N.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem. Int. Ed. 2009, 48, 60. (10) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (11) Somorjai, G. A.; Blakely, D. W. Nature 1975, 258, 580. (12) Somorjai, G. A. Science 1985, 227, 902. (13) Tian, N.; Zhou, Z.-Y.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (14) Tian, N.; Zhou, Z.-Y.; Yu, N.-F.; Wang, L.-Y.; Sun, S.-G. J. Am. Chem. Soc. 2010, 132, 7580. (15) Ma, Y.; Kuang, Q.; Jiang, Z.; Xie, Z.; Huang, R.; Zheng, L. Angew. Chem. Int. Ed. 2008, 47, 8901. (16) Ming, T.; Feng, W.; Tang, Q.; Wang, F.; Sun, L.; Wang, J.; Yan, C. J. Am. Chem. Soc. 2009, 131, 16350. 3 Chapter 2. Literature Review 2.1. Shape-control synthesis of noble metal nanocrystals In order to control the shape and size of metal nanocrystals, one should know how they are created and grown. From these understandings, one can basically choose the appropriate synthetic method to selectively fabricate the desired shapes and sizes of the metal nanocrystals. Thus, in this part, a brief discussion of the growth mechanism of metal nanocrystals is introduced, followed by a review of various chemical methods in shape-controlled synthesis of noble metal nanocrystals. 2.1.1. Nucleation and growth of metal nanocrystals Chemical synthesis of nanoparticles involves either decomposition or reduction of metal precursors. For the decomposition route, nucleation stage is considered to follow the LaMer diagram.1 Briefly, under suitable conditions the number of metal atoms increases with time. As this concentration reaches supersaturation stage, the nucleation events start to happen and precursor concentration drops accordingly. In case the atomic concentration sinking too fast, no more homogeneous nucleation can occur, leading to uniform size of the nuclei. With the non-stop addition of new metal atoms from the bulk solution, the nuclei develop into nanocrystals and then cease to grow when the equilibrium state is achieved between surface atoms and the atoms remaining in the bulk solution.2 For the reduction route, the chemical precursors are to be reduced into atoms before these atoms agglomerate with each other to form nuclei. Afterwards, these nuclei keep growing in size through an autocatalytic process in which the newly born 4 atoms are continuously added onto the nuclei surfaces. Finally, these nuclei grow into nanoparticles with much bigger sizes.2 During the growth from nuclei to nanocrystals, firstly, the nuclei grow and form seed with the presence of facets due to the fact that thermal fluctuation is no longer energetically sufficient to randomly change the morphology of the nuclei.2 The seeds, at this stage, must take their own configurations either single-crystals, singly twinned or multiply twinned structures. This stage can be considered as the most important stage to define the final shape and structure of the resultant nanocrystals because the configuration (i.e., single-crystalline, singly twinned or multiply twinned) taken by the seed will also be the resultant configuration for the nanocrystals later on. With single-crystalline seeds, the final nanocrystals would accept either polyhedral or anisotropic structures. For polyhedral shapes, the seeds will take the octahedral forms if R (ratio of growth rate along to directions.) is equal to 1.73, cuboctahedral forms if R = 0.87 and cubic shapes if R = 0.58. Therefore, for fcc nanocrystals, R value is a very important parameter to control if one expects to exclusively produce one of the three polyhedrons. For anisotropic structures which are the consequences of symmetry breaking effect, octagonal rod and bar can be formed from cuboctahedron and cube, respectively, through the so-called surface passivation. With singly twinned seeds, the resultant nanocrystals could be either right bipyramids or beams which are favorable shapes for nanocrystals with one twinned plane located in the middle. With multiply twinned (usually penta-twinned) seeds, three possible shapes have been obtained, namely, decahedron, icosahedron and pentagonal rod. While 5 decahedron and icosahedron are composed of certain numbers of identical tetrahedra subunits, pentagonal rod is formed by five elongated tetrahedra which share one common edge. Finally, with plate-like seeds having stacking faults, the resultant nanocrystals will take the hexagonal or triangular plate shape. 2.1.2. Chemical methods for synthesis of metal nanocrystals with controlled shapes Current wet chemical methods for shape-controlled synthesis of metal nanocrystals mainly include seeded-growth, hydrothermal, electrochemical, photochemical and polyol routes. Each method has its advantages and disadvantages and can find applications in different areas. 2.1.2.1. Seeded-growth route Seeded-growth method is a two- or multi-stage chemical process. At the first stage, metal precursor is quickly reduced in aqueous solution with high surfactant concentration by using a strong reducing agent (usually NaBH4). Under such a concentrated-surfactant condition, metal seeds formed are very small, about 3-5 nm in diameter.3-5 These preformed-seeds are subsequently added into the so-called “growth solution” that contains suitable concentrations of the metal precursor, surfactant and a mild reducing agent. The ability to control the shape and size of the resultant nanocrystals relies on the rational input ratio between seeds, precursor, and surfactant. This method has been widely used to control the shape and size of metal nanocrystals as it can separate the nucleation stage from the growth stage. 6 Seeded-growth method has been reported by the Murphy’s group in the synthesis of spherical and rod-like Au nanoparticles.3 This method has been employed to synthesize various shapes of Au nanoparticles including cubes, octahedra, rods, and multipods.4 Seeded-growth has also been adopted and modified by other groups to further improve its ability to produce various shapes of gold and other noble metal nanoparticles with uniform sizes. For example, El-Sayed and co-workers modified the synthesis of Au nanorods with the use of CTAB-capped seeds and the addition of trace AgNO3 that could boost the yield of single-crystalline Au nanorods up to 99%.6 Guyot-Sionnest et al. reported the growth of either Au nanorods or bipyramids by using single-crystalline or multiply twinned seeds.5 Recently, Huang et al. presented a facile seed-mediated growth with the use of Cu UPD on Pd nanocrystals to synthesize monodisperse, long Pd nanorods.7 Very recently, Xu and co-workers performed the growth of uniform Pd polyhedral nanoparticles, namely, cubes, octahedra, rhombic cuboctahedra and their intermediate forms by controlling KI concentration and reaction temperature.8 Although seeded-growth has been considered as one of the most powerful methods for synthesizing metal nanoparticles, it strictly requires the very accurate conditions for making seeds such as pH value and concentration of the strong reducing agent. Additionally, metal nanoparticles synthesized by this method are usually very difficult to be stored for a long time. 67 8 2.1.2.2. Hydrothermal route Hydrothermal method involves a process in which metal precursor, surfactant and solvent (normally water) are first mixed together at room temperature. The whole reaction solution is then transferred into a Teflon vessel that is closely sealed by the external metal shell. The system is subsequently heated up to a high temperature which is usually higher than the boiling point of the solvent in the system. The nucleation and growth stage are to be one after another to finally produce the metal nanocrystals. The hydrothermal pathway has attracted much attention due to its simple one-step reaction but can provide a wide range of shapes of metal nanocrystals. For example, Quian et al. reported the procedure for synthesis of Ag nanowires by using a simple hydrothermal method.9 Dong et al. used PDDA-mediated hydrothermal route to obtain Ag nanocubes, Au nanoplates, Pd and Pt nanopolyhedra.10 Recently, monodisperse Au octahedra with different sizes have been synthesized by using sodium citrate as mild reducing agent.11 Zheng and co-workers, for the first time, have presented a new hydrothermal route to synthesize uniform Pd nanowires.12 Although this method is facile, it is usually time- and energy-consuming, and it needs to be done under highly safe conditions. 2.1.2.3. Electrochemical route Electrochemical method relies on the trigger of redox chemical reactions by using an external applied voltage. This method can be applied with or without nanoporous template (i.e., hard template) such as anodized-aluminum oxides13. For the electrochemical method with hard template, deposition on one face of the membrane 8 with a metal layer is first prepared so that this layer can serve as a cathode for electrocoating. Subsequently, desired metal precursors are to be reduced and delivered inside the pore channels of the membrane. The shape and size of metal nanoparticles can be rationally controlled by varying the potential, deposition time, and surfactant during the electrochemical process13. This method has a main advantage that it can be applied to fabricate nanostructures of most of metals. Electrochemical method has been developed by many research groups.13-16 Despite of its wide range of synthesis of metal nanoparticles, electrochemical route cannot be considered for large-scale applications due to its high cost and low yield of product. 2.1.2.4. Photochemical route Photochemical route is the chemical process in which irradiation of light is used to reduce metal ions into metal atoms with or without pre-formed nanopaticles in the presence of suitable surfactant in the solution. This method has been known to be very effective in the synthesis of Ag and Au nanocrystals. For instance, Mirkin et al. reported that Ag nanoprisms could be obtained via the transformation of Ag nanospheres under irradiation of fluorescent light.17 Using this method, they obtained Ag nanoprism with sizes ranging from 40 to 120 nm.18 Au nanorods was also fabricated using UV irradiation by Yang and co-workers.19 Recently, transformation of Ag nanoplates into rounded Ag nanoplates with increased thickness has been observed by using UV light irradition.20 Very recently, the Mirkin’s group has synthesized Ag right bipyramids with a very high yield (>95%) by using halogen lamp irradiation.21 9 Photochemical synthesis can be considered as an effective and green method (i.e., without the use of strong reducing agent, low reaction temperature). However, this method usually gives rise to low yield of product, and it can be only performed in the syntheses of Ag and Au nanocrystals. 2.1.2.5. Polyol route Polyol route has been known as a powerful method to control the shape and size of metal nanocrystals. In this method, either ethylene glycol (EG) or other polyols such as 1,5-pentadiol and polyethyleneglycol (PEG) are used to serve as both the solvent and reducing agent in the reaction. Polyvinylpyrrolidone (PVP) or its copolymer with different molecular weight is used as both the capping agent and stabilizer where PVP and metal precursor are able to form complex compounds. The reduction power in the polyol method can be easily tuned by adjusting the reaction temperature since EG becomes easier to be oxidized at higher temperature. The method was first introduced by Fievet et al. in the late 90’s.22 Great enhancement has been made by the Xia’s group who discovered the so-called “oxidative etching process” and “surface passivation” on Pd and Ag nanocrystals. By using these strategies, Xia et al. have successfully controlled the shape and size of Ag, Pd and Pt nanoparticles. Silver nanostructures, namely, nanocubes,23-27 nanowires,28-31 nanobipyramids,27,32 nanobeams,33 nanorices and nanobars27 have been obtained by rational control of foreign ions such as Cl¯, Br¯ and Fe3+. Additionally, a series of Pd nanocrystals have been synthesized with the similar strategies, namely, nanocubes,34,35 nanoboxes and nanocages,36 nanoplates,37 nanowires, nanobipyramids,38 and nanobars and nanorods.39 10 Though more difficult to control the shape, Pt nanocrystals have also been prepared in different morphologies such as Pt nanowires, nanooctahedra, nanoplates, nanomultipods.40 Moreover, Au nanocrystals with various shapes have been also fabricated based on the modified polyol syntheses in which a trace amount of Ag+ is used. For example, Yang et al. reported the synthesis of Au nanocrystals by using PVP as surface-regulating agent, EG as a solvent heated up at 280 °C.41 While Au nanotetrahedra and nanoicosahedra were obtained without the absence of Ag ions, Au nanocubes were synthesized by adding a trace amount of Ag ions. Song and coworkers prepared Au nanooctahedra, nanocuboctahedra and nanocubes by simply adjusting the ratio of Ag to Au ions in the 1,5-pentadiol.42 The polyol method has a drawback that the PVP bounded on the surface of assynthesized nanoparticles is difficult to completely remove. This limitation inhibits some applications of metal nanocrystals synthesized by the polyol synthesis, especially in biomedical applications. Therefore, the prominent post-treatment of those metal nanocrystals is of great necessity for this method to be promising for biomedical applications. 11 2.2. Synthesis and catalytic properties of metal nanocrystals with high-index facets High-index facets are facets composed of periodic combination of two or more microfacets of low Miller-indices (i.e., {111}, {100} and {110}). The high-index facets of noble metal crystals can serve as active sites for breaking chemical bonds due to their high density of ledges, steps and kinks.43,44 Synthesis of metal nanostructures with high-index facets has become an increasingly important research topic due to the fact that high-index surfaces usually exhibit superior catalytic properties in many chemical reactions. Noble metal nanostructures bounded by high-index facets have been mainly synthesized using two methods: electrochemical approach45,46 and seeded-growth synthesis.47,48 In electrochemical method, Pt nanoparticles with high-index facets (i.e., tetrahexahedra or THH) were synthesized through the adsorption and desorption of oxygen onto Pt surface inspired by the square wave potential45. THH Pt nanocrystal is composed of twenty-four (730) high-index faces which comprise (310) and (210) subfacets. The THH nanocrystals are surprisingly stable even under strict condition such as 800 °C. The reason for this high stability can be explained that the adsorption and desorption of oxygen on the Pt surface can stabilize the high-index facets. Although such nanocrystals bounded by high-index facets can be formed by using this method, the feasibility of up-scaling and ease of processing should be further improved to make those nanocrystals useful in catalytic applications. Seeded-growth synthesis of Au nanocrystals with high-index facets has been reported by Xie et al.47 By reducing HAuCl4 in the presence cetyltrimethylammonium chloride (CTAC) and ascorbic acid (AA), THH Au nanocrystals bounded by 24 12 11 {211} facets were obtained in high yield. On the same trend, Wang et al. successfully synthesized elongated THH Au nanocrystals in high yield (~95%).48 In their synthesis, the amount of seed and pH adjustment were claimed to be the crucial factors responsible for the formation of these Au nanocrystals. Inspired by these two works on the synthesis of nanocrytals with high-index facets, several reports have recently been introduced to further improve the yield and size range.49-51 Very recently, Mirkin et al. have prepared gold nanocrystals with a unique shape called “concave cube”52. This structure can be described as a cube with six concave square pyramids on its faces (in contrast to tetrahexohedron that possesses six convex square pyramids on six faces). Metal nanocrystals bounded by high-index facets have been renowned for their superior catalytic activities to those of the nanocrystals with low-index facets. For example, THH Pt nanocrystals exhibit the 200% and 400% higher catalytic activities of electro-oxidation compared with that of 3.2 nm Pt/C commercialized catalyst for ethanol and formic acid, respectively.45 In addition, the trisoctahedra Au (TOH) nanocrystals displayed different electrochemical behavior from that of polycrystalline Au and Au nanocrystals with low-index facets.47 13 12 2.3. References (1) Lamer, V. K.; Dinegar, R. H. J. Am. Chem. Soc. 1950, 72, 4847. (2) Xia, Y. N.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem. Int. Ed. 2009, 48, 60. (3) Jana, N. R.; Gearheart, L.; Murphy, C. J. Adv. Mater. 2001, 13, 1389. (4) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (5) Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B 2005, 109, 22192. (6) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (7) Chen, Y. H.; Hung, H. H.; Huang, M. H. J. Am. Chem. Soc. 2009, 131, 9114. (8) Niu, W.; Zhang, L.; Xu, G. ACS Nano 2010, 4, 1987. (9) Wang, Z. H.; Liu, J. W.; Chen, X. Y.; Wan, J. X.; Qian, Y. T. Chem. Eur. J. 2005, 11, 160. (10) Chen, H.; Wang, Y.; Dong, S. Inorg. Chem. 2007, 46, 10587. (11) Chang, C.-C.; Wu, H.-L.; Kuo, C.-H.; Huang, M. H. Chem. Mater. 2008, 20, 7570. (12) Huang, X. Q.; Zheng, N. F. J. Am. Chem. Soc. 2009, 131, 4602. (13) Sau, T. K.; Rogach, A. L. Adv. Mater. 2010, 22, 1781. (14) Martin, C. R. Science 1994, 266, 1961. (15) Martin, C. R. Acc. Chem. Res. 1995, 28, 61. (16) Chen, A. C.; Holt-Hindle, P. Chem. Rev. 2010, 110, 3767. (17) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (18) Jin, R. C.; Cao, Y. C.; Hao, E. C.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (19) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (20) Zhang, Q.; Ge, J. P.; Pham, T.; Goebl, J.; Hu, Y. X.; Lu, Z.; Yin, Y. D. Angew. Chem. Int. Ed. 2009, 48, 3516. (21) Zhang, J.; Li, S. Z.; Wu, J. S.; Schatz, G. C.; Mirkin, C. A. Angew. Chem. Int. Ed. 2009, 48, 7787. 14 (22) Fievet, F.; Lagier, J. P.; Figlarz, M. MRS Bull. 1989, 14, 29. (23) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (24) Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y. N. Angew. Chem. Int. Ed. 2005, 44, 2154. (25) Siekkinen, A. R.; McLellan, J. M.; Chen, J. Y.; Xia, Y. N. Chem. Phys. Lett. 2006, 432, 491. (26) Skrabalak, S. E.; Au, L.; Li, X. D.; Xia, Y. N. Nature Protocols 2007, 2, 2182. (27) Wiley, B.; Sun, Y. G.; Xia, Y. N. Acc. Chem. Res 2007, 40, 1067. (28) Sun, Y. G.; Xia, Y. N. Adv. Mater. 2002, 14, 833. (29) Sun, Y. G.; Yin, Y. D.; Mayers, B. T.; Herricks, T.; Xia, Y. N. Chem. Mater. 2002, 14, 4736. (30) Sun, Y. G.; Mayers, B.; Herricks, T.; Xia, Y. N. Nano Lett. 2003, 3, 955. (31) Wiley, B.; Sun, Y. G.; Xia, Y. N. Langmuir 2005, 21, 8077. (32) Wiley, B. J.; Xiong, Y. J.; Li, Z. Y.; Yin, Y. D.; Xia, Y. N. Nano Lett. 2006, 6, 765. (33) Wiley, B. J.; Wang, Z. H.; Wei, J.; Yin, Y. D.; Cobden, D. H.; Xia, Y. N. Nano Lett. 2006, 6, 2273. (34) Xiong, Y. J.; Chen, J. Y.; Wiley, B.; Xia, Y. N.; Yin, Y. D.; Li, Z. Y. Nano Lett. 2005, 5, 1237. (35) Xiong, Y. J.; Chen, J. Y.; Wiley, B.; Xia, Y. N. J. Am. Chem. Soc. 2005, 127, 7332. (36) Xiong, Y. J.; Wiley, B.; Chen, J. Y.; Li, Z. Y.; Yin, Y. D.; Xia, Y. N. Angew. Chem. Int. Ed. 2005, 44, 7913. (37) Xiong, Y. J.; McLellan, J. M.; Chen, J. Y.; Yin, Y. D.; Li, Z. Y.; Xia, Y. N. J. Am. Chem. Soc. 2005, 127, 17118. (38) Xiong, Y. J.; Cai, H. G.; Yin, Y. D.; Xia, Y. N. Chem. Phys. Lett. 2007, 440, 273. (39) Xiong, Y. J.; Cai, H. G.; Wiley, B. J.; Wang, J. G.; Kim, M. J.; Xia, Y. N. J. Am. Chem. Soc. 2007, 129, 3665. (40) Chen, J. Y.; Lim, B.; Lee, E. P.; Xia, Y. N. Nano Today 2009, 4, 81. (41) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem. Int. Ed. 2004, 43, 3673. (42) Seo, D.; Park, J. C.; Song, H. J. Am. Chem. Soc. 2006, 128, 14863. 15 (43) Somorjai, G. A.; Blakely, D. W. Nature 1975, 258, 580. (44) Somorjai, G. A. Science 1985, 227, 902. (45) Tian, N.; Zhou, Z.-Y.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (46) Tian, N.; Zhou, Z.-Y.; Yu, N.-F.; Wang, L.-Y.; Sun, S.-G. J. Am. Chem. Soc. 2010, 132, 7580. (47) Ma, Y.; Kuang, Q.; Jiang, Z.; Xie, Z.; Huang, R.; Zheng, L. Angew. Chem. Int. Ed. 2008, 47, 8901. (48) Ming, T.; Feng, W.; Tang, Q.; Wang, F.; Sun, L.; Wang, J.; Yan, C. J. Am. Chem. Soc. 2009, 131, 16350. (49) Li, J.; Wang, L. H.; Liu, L.; Guo, L.; Han, X. D.; Zhang, Z. Chem. Commun. 2010, 46, 5109. (50) Wu, H.-L.; Kuo, C.-H.; Huang, M. H. Langmuir 2010, 12307. (51) Yu, Y.; Zhang, Q.; Lu, X.; Lee, J. Y. J. Phys. Chem. C 2010, 114, 11119. (52) Zhang, J.; Langille, M. R.; Personick, M. L.; Zhang, K.; Li, S.; Mirkin, C. A. J. Am. Chem. Soc. 2010, 132, 14012. 16 Chapter 3. Shape-controlled synthesis of Au Nanocrystals with High-index Facets 3.1. Shape-selective growth of polyhedral gold nanocrystals with high-index facets 3.1.1. Introduction As discussed previously, shape-controlled synthesis of noble metal nanocrystals in solution-phase have relied on the flexibility of choosing reaction parameters such as precursor, solvent, surfactant and foreign ions. Among these strategies, introduction of foreign metal ions, especially silver ions, in the synthesis of gold nanocrystals has shown drastic morphology-selection effect. Au nanoparticles with tailored shapes including cube,1-4 octahedron,2-4 nanorod,2,5-7 bipyramid,8-11 and plate12 have been successfully synthesized in aqueous or polyol solvents in the presence of trace amount of Ag ions. The influence of silver ions has been recognized for nearly a decade in the growth of Au nanorods and polyhedral nanocrystals. Murphy and co-workers firstly discovered that adding Ag ions to the growth solution of Au in a seed-mediated approach can significantly improve the yield of single crystalline Au nanorods; while for the same method but without Ag+, only polycrystalline nanorods can be obtained.8 Later, Yang et al. extended the use of Ag+ to preparation of Au nanocubes of high yield in ethylene glycol.1 This method was further developed by Song and co-workers to generate Au octahedra and cuboctahedra.3,4 Recently, the use of Ag+ in N-alkylmidazole was reported in which Au octahedra, cubes, rhombic dodecahedra and high-index tetrahexahedra (THH) 17 were successfully synthesized.13 The Ag underpotential deposition (UPD) on Au surface refers to a process in which the Ag layer can be deposited on the Au surface at the potential much positive than the Nernst potential for the reduction of Ag. This deposition of Ag usually appears to be one or two atomic layers on Au surface that are able to adjust the growing rate of different facets of Au (i.e., {110}, {100} and {111}). In addition, the presence of Cl- are consider to further boost the Ag UPD shift by some hundreds mV which is critically necessary for the morphological control of Au nanocrystals.14 Compared to silver, palladium has received much less attention in shape-selective growth of Au nanocrystals. Although Pd and Au have been both used in some reactions, focus has been mainly on the formation of Au-Pd bi-metallic structures especially core-shell nanoparticles.15-19 For example, Yacaman and co-workers conducted successive reduction of PdCl2 and HAuCl4 in ethylene glycol using PVP as the protective agent. The 5-nm particles formed this way show a three-layer core-shell structure with a Au-Pd alloy inner core, an intermediate layer rich of Au, and a third layer of Pd-rich alloy.20 A one-step aqueous synthesis was reported by Han et al. who found that Au@Pd core-shell particles with an octahedral shape were formed because Au(III) was preferentially reduced over Pd(II) in the presence of CTAC, thus Au octahedral were formed first followed by deposition of Pd on the surface to give the core-shell structure.17 Recently, Krichevski and Markovich found that Pd doping may induce growth of Au nanowires, where small Pd nuclei formed in situ can reduce the intermediate Au+ species and the incorporation of Pd into the growing Au nanostructures induced nanowire formation in high yield.21 Due to the different behavior of Ag and Pd ions when they are involved in the Au nanocrystal synthesis, one would expect that combining these two foreign metal ions 18 in the synthesis of Au nanocrystals would show synergistic effect on controlling the shapes of the resultant particles. Indeed, although scarcely reported, controlled growth of nanocrystals in tri-metallic nanocrystal systems has been noticed recently. LizMarzan et al. examined the influence of Ag ions on the growth of Pt on Au nanorods and found that in the presence of Ag+, the deposition of Pt takes place on the rod tips; while without Ag+, homogeneous coating of Pt on rod surface are obtained. This was attributed to the UPD of Ag on Au(110) which causes slower growth of Pt on {110} faces compared to those on {100} and {111} faces.22 Here, we report the shapeselective synthesis of Au nanocrytals in the presence of two foreign metal ions – Ag and Pd. A facile one-pot polyol synthesis was employed with poly(diallyldimethylammonium chloride) (PDDA) as the capping agent. For the first time, a series of Au nanostructures, namely, Au truncated ditetragonal nanoprisms bounded by twelve {310} facets, bipyramids enclosed with {117} high-index facets, and truncated octahedra with exposed {111} and {310} facets, were synthesized by simply varying the ratio of Ag and Pd ions. 3.1.2. Experimental Section Ethylene glycol (EG, Sigma-Aldrich), chloroauric acid trihydrate (HAuCl4⋅3H2O, Alfa Aesa), silver nitrate (AgNO3, Sigma-Aldrich), palladium(II) chloride (PdCl2, Alfa Aesar), poly(diallyldimethylammonium chloride) solution (PDDA, 20%, MW = 200 000-350 000, Aldrich), poly(vinyl pyrrolidone) (PVP, MW = 55 000, Aldrich), hydrochloric acid (HCl, 37%, Merck) and sodium chloride (NaCl, Sigma-Aldrich) were used as adopted without any further purification. 10 mM H2PdCl4 solution was prepared by dissolving 35.6 mg of PdCl2 in 20 ml of 20 mM HCl at 100 °C till a 19 transparent solution was obtained. The water used throughout this work was 18.2 MΩ ultrapure deionized water. In a typical synthesis of Au truncated ditetragonal prisms, 0.2 ml of PDDA (20% in H2O) solution and 10 ml of EG were mixed in a glass vial using magnetic stirrer at room temperature. To this solution, 18.8 μl of 0.5 M HAuCl4, 2 μl of 500 mM AgNO3, 62.5 μl of 10 mM H2PdCl4, and 430 μl of de-ionized H2O were added under stirring. The volumes of AgNO3 and H2PdCl4 aqueous solutions were controlled so that the final concentrations of Au, Ag and Pd were 0.895 mM, 0.024 mM, and 0.06 mM, respectively. It was found that for the synthesis of nanoprisms, adding 10 μl of 1 M NaCl aqueous solutions to the reactions could further improve the yield. This solution (in EG) was then kept mixing for 5 min before closely sealed with the cap. The vial was then heated up in an oil bath at 120 °C for 12 h with stirring. The solution color changed from yellow to colorless gradually after heated up at 120 °C for 2 h. After 3.5 h, the solution became pale pink and finally stopped at reddish brown at 12 h. The product was harvested by centrifugation and washed with acetone once and with water five times to remove excess PDDA on the surface of the particles. The as-synthesized Au nanocrystals were finally store in water for further uses. The synthetic procedure for gold bipyramids was similar to the Au nanoprisms, except that the Ag+ concentration was increased to 0.476 mM (10 μl of 500 mM AgNO3). For the reactions of truncated octahedra, Ag+ concentration was reduced to 0.024 mM (25 μl of 10 mM AgNO3). Note that the purification was carried out in case of Au truncated octahedra to remove minor amount of big Au octahedra and the washing with saturated NaCl was applied in case of Au bipyramids to remove the 20 AgCl cluster in the resultant solution for more accurate elemental analysis and better imaging. Scanning Electron Microscopy (SEM) images were taken using a JEOL JSM6700F operating at 10 kV. Transmission electron microscopy (TEM) images, electron diffraction (ED) patterns and energy dispersive X-ray spectroscopy (EDX) spectra were acquired on a JEOL JEM-2010F operating at 200 kV. High-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM-2100F operating at 200 kV. Samples for TEM and SEM were prepared by drop casting the sample solutions onto carbon-coated copper grids and silicon substrates, respectively. The TEM and SEM samples were then rinsed for a few hours with deionized water to remove excess polymers, followed by drying at 60 °C in air. The X-ray diffraction (XRD) spectra were acquired using a SHIMADZU XRD-6000 diffractometer equipped with a Cu Kα radiation source (λ = 1.5418 Å) from samples deposited onto a plastic substrate. The scan rate and step size used were 1 (deg/min) and 0.02 (deg). UV-visible spectra of the Au nanocrystals were recorded using a SHIMADZU UV-1601 spectrometer with plastic cuvettes of 1-cm path length at room temperature. X-ray photoelectron spectroscopy (XPS) analyses were performed using a Kratos AXIS Ultra DLD spectrometer equipped with an Al Kα monochromatized X-ray source with a penetration depth of ~3 nm. Inductively coupled plasma mass spectrometry (ICP-MS) measurements were carried out on an Agilent 7500 ICP-MS instrument. Samples were prepared by dissolving nanocrystals using freshly made aqua regia followed by diluting with DI-H2O. 21 3.1.3. Result & discussion Figure 3.1A-D are representative SEM and TEM images of the Au nanocrystals obtained from reactions with molar ratio of Au:Pd:Ag = 15:1:1.6. Most of particles (>95%) exhibit well-defined facets with average longest edge length of ~195 nm, respectively. SEM images at high magnifications (Figure 3.1B, C) revealed that the majority of the particles exhibit a ditetragonal prism shape with truncated ends. The particles with this shape are enclosed with twelve faces – eight side-faces parallel to the principle axis and four terminating faces located at both ends of the prism. A schematic drawing of this truncated ditetragonal prism is presented in Figure 3.1F, which illustrates the projections of such a particle at different viewing angles. The TEM projection along the principle axis parallel to the prism side-faces show a ditetragonal cross-section (Figure 3.1D), with measured inner angles alternating between 143° and 127°. The measured angles match well with those calculated from a ditetragonal prism with {310} facets, which alternate between 143.1 and 126.9 degree (Figure 3.2).23,24 This indicates that the facets of the Au nanocrystals are {310}. To further confirm the facets of the truncated prisms, we recorded electron diffraction (ED) patterns of a number of particles. Figure 3.3 shows four representative ED patterns and the corresponding TEM images at different project angels. We found that all the patterns can be indexed with a zone axis of [310]. 22 24 Figure 3.1. (A) Low- and (B) high-magnification SEM images of Au truncated ditetragonal prisms showing well-defined structures with sharp edges and apexes. (C) HRSEM of a group of Au truncated ditetragonal prisms. (D) TEM images of Au truncated ditetragonal prisms showing its cross-section. (E) High magnification of a truncated ditetragonal prism (inset) exhibiting (200) d-spacing of fcc Au. (F) The schematic drawings at different views of an Au nanoprism. 23 Figure 3.2. Determination of facets of Au truncated ditetragonal prisms from different views (A) top view (cross-section) and (B) side view. The result indicates that Au truncated ditetragonal prisms are bounded by 12 {310} facets. Note that image (A) and (B) were taken from different truncated ditetragonal prisms. (C), (D) Schematic drawing of truncated ditetragonal prisms with their theoretical angles. (E) Atomic model of Au (310) facet including (110) and (100) subfacets. In addition to the typical shape illustrated in Figure 3.1F, the truncated ditetragonal prisms also exhibited a few other forms, but all with 12 exposed {310} faces. Based on SEM and TEM images, we carefully established the models of the particles with different arrangement of the faces (Figure 3.4). The three additional configurations that can be described based upon the deviations from the one described in Figure 3.1. 24 Figure 3.3. (A), (B), (C) and (D) Schematic models for Au truncated ditetragonal prisms at different views illustrating for (E), (F), (G) and (H) the corresponding TEM images. (I), (J), (K) and (L) the ED patterns that consistently show all [310] zone axes. Note that the SAED patterns were taken from different Au nanoprisms. Figure 3.4. Schematic models for other configurations of Au truncated ditetragonal prisms which differ from the Au nanoprisms in Figure 3.1. In addition to truncated nanoprism particles, we also synthesized Au bipyramids in high yield following the same reaction procedure but with higher concentration of silver ions. Figure 3.5 shows the morphology of the Au bipyramids obtained at a ratio of Au:Pd:Ag = 15:1:8 with a reaction temperature of 120 °C (same as for the case of 25 Au prisms). FESEM images (Figure 3.5A, B) indicate that the Au bipyramids have smooth side faces with truncated tips. The truncated tips of the bipyramid showed a pentagonal shape (Figure 3.5B, inset). The average base length and height of the bipyramids are 50 and 127 nm, respectively. TEM image of the Au bipyramids (Figure 3.5C) displayed twinned planes, indicating that the bipyramid-shaped Au particles have a five-fold twinned structure. This 5-fold twinned structure has been previously observed from nanorods and bipyramids by various research groups.6,9-11,25 High-resolution TEM (HRTEM) of an Au bipyramid (Figure 3.5D) revealed lattice fringes with planar distances of 0.23 nm which matches well with the d-spacing of (111) lattice plane of gold. Selected-area electron diffraction pattern (Figure 3.5E) of the same particle showed the superposition of two sets of patterns characteristic of [111] and [110] zone diffraction of fcc gold. This result not only confirmed the 5-fold twinned structures of Au bipyramids but also revealed the growth direction of as described by previous works on Au bipyramids.9-11 A schematic model for a Au bipyramid is showed in Figure 3.5F. Additionally, based on the formula for indexing pentagonal bipyramid facets established by Sun et al.,26 we proposed that the Au bipyramids are bounded by {117} facets (Figure 3.6). The same result was also reported previously by Guyot-Sionnest and co-workers.9 26 Figure 3.5. (A) Low and (B) high magnification SEM images of Au bipyramids. Inset of Figure 3B clearly shows a pentagonal cross-section of an exceptionally big bipyramid. Inset scale bar is 100 nm. (C) TEM image of Au bipyramids. (D) HRTEM image of a bipyramid (inset) describes the (111) d-spacing. Inset scale bar is 20 nm. (E) The corresponding ED pattern showing the superposition of [110] and [111] zones of fcc structure. (F) Schematic drawing of a bipyramid. Figure 3.6. (A) TEM image of an Au nanobipyramid with defined width base (W, yellow line) and height of half (Hhf, red line). (B) Model of half of pentagonal bipyramid and formula that exhibits the relationship between morphological measurements (i.e., W and Hhf) and Miller index of the bipyramidal facets. By measuring few tens of Au bipyramids in TEM images, we determined the average Hhf/W ratio of 2.18 corresponding to high-index {117} facets. In recognition of the effect of silver ions on controlling the shape of the Au nanocrystals, we also lowered the concentration of the Ag+ in the reactions, which led 27 to the formation of truncated octahedra enclosed by 8 {111} and 24 {310} facets. Figure 3.7A shows the truncated octahedral nanocrystals prepared at a ratio of Au:Pd:Ag = 15:1:0.4. SEM at higher magnification (Figure 3.7B) revealed that the exposed faces of the nanocrystals are pentagons and hexagons, with each hexagonal face surrounded by 6 pentagonal ones. The homogeneous contrast under TEM (Figure 3.7C) is attributed to the single-crystallinity of the particles. Electron diffraction patterns taken from a number of Au nanocrystals showed either [310] or [111] zone axes (Figure 3.7D-E), indicating that the exposed faces include both {310} and {111} facets. Based on the careful analysis of SEM images and diffraction patterns, we proposed that each truncated Au octahedral nanocrystal contains 8 {111} facets in hexagonal shape and 24 {310} facets in pentagonal shape. While the {111} facets are created from faces of an octahedron, the {310} facets are formed at the 6 vertexes of the octahedron (Figure 3.7F). Figure 3.7. (A) Low magnification SEM image of Au truncated octahedra. Inset shows schematic model of a truncated octahedron that exposes both {111} and {310} facets. (B) High magnification SEM image of truncated octahedra with a superposed drawing frame on single truncated octahedra shows the consistency with the 28 schematic model. (C) TEM image of Au truncated octahedra with the inset showing (200) d-spacing of Au fcc. (D), (E) ED patterns of Au truncated octahedra clearly show [310] and [111] zone axes. (F) Schematic drawing showing the morphological relationship between an octahedron and a truncated octahedron. The compositions of the nanocrystals were analyzed using EDX on SEM. It was found that the nanoprisms are mainly composed of Au (>98 at%), with a very small amount of Ag (~0.4%) and Pd (~1.2%) (Figure 3.8). This is consistent with the ICPMS measurement, which gives 98.3%, 1.12%, and 0.58% for Au, Ag and Pd, respectively. However, XPS analysis showed much higher atomic percentages of Ag and Pd, which account for ~11% and ~19%, respectively (Figure 3.9). Since the penetration depth of XPS is ~3 nm, the much higher concentrations of Ag and Pd from XPS measurements indicate that Ag and Pd are mainly located at the surface of the nanocrystals. This result agrees well with previous works where UPD of Ag was used to control the shape of Au nanoparticles.3,27 The EDX, ICP-MS and XPS analyses of the bipyramids and truncated octahedra showed similar composition profiles - the nanocrystals are mainly composed of Au with trace amount of Pd and Ag on the surface. Table 3.1 summarizes the elemental analyses of the Au nanocrystals with three different shapes. 29 Figure 3.8. EDX analyses of Au nanostructures: (A) truncated ditetragonal prisms, (B) bipyramids and (C) truncated octahedra. 30 Figure 3.9. XPS analyses of Au nanostructures: (A) truncated ditetragonal prisms, (B) bipyramids and (C) truncated octahedra. 31 Table 3.1. Atomic composition based on EDX, ICP-MS and XPS of Au truncated ditetragonal prisms, bipyramids and {310} truncated octahedra. EDX ICP-MS XPS Au 98.48 98.30 70.18 Ag 0.40 1.12 10.79 Pd 1.12 0.58 19.03 EDX ICP-MS XPS Au 96.61 92.93 58.93 Ag 1.50 5.97 18.47 Pd 1.89 1.10 22.60 EDX ICP-MS XPS Au 99.19 97.56 74.71 Ag 0.24 1.94 10.35 Pd 0.57 0.50 14.94 Atomic % Atomic % Atomic % Figure 3.10 shows the XRD patterns of the Au ditetragonal prisms, bipyramids, and truncated octahedra, respectively. The peaks positions of all three XRD patterns match well with those of gold with fcc crystal structure. The ditetragonal prisms exhibited a much stronger (200) reflection peak than the bipyramids and truncated 32 octahedral particles. The intensity ratios of (111) to (200) and (111) to (220) for ditetragonal prisms are 0.37 and 1.33, respectively, which are much lower than those of standard powder sample of fcc gold (1.92 and 3.12, respectively). The relatively stronger (200) and (220) reflections than (111) may be attributed to the abundance of (310) facets of the nanocrystals since the high-index (310) facet is composed of (100) terrace and (110) step denoted by 3(100)x(110) (Figure 3.2). The XRD pattern of Au bipyramids (Figure 3.10B) clearly matches the standard pattern of fcc Au. The polycrystallinity of these nanoparticles were revealed by the peaks splitting.28 The strong (111) peak could be attributed to the abundance of {111} planes derived from this structures. Same result was reported by Wu et al. for Au nanobipyramids.29 The very strong (111) peak for the truncated octahedra could be explained that the particles were preferentially seated on their {111} facets parallel to the substrate. Figure 3.10. XRD patterns of Au nanostructures: (A) truncated ditetragonal prisms, (B) bipyramids and (C) truncated octahedra. 33 35 Figure 3.11. UV-vis spectra of Au truncated ditetragonal prisms, bipyramids and truncated octahedra. UV-vis spectra of the Au nanocrystals with three different shapes were presented in Figure 3.11. The ditetragonal prisms exhibit a strong surface plasmon resonance (SPR) peak at 600 nm and another broader peak at 836 nm corresponding to the transversal and longitudinal vibrations of the particles, respectively. The Au bipyramids show a sharp, strong peak at 775 nm and another broad peak at 533 nm. Unlike the ditetragonal prisms and bipyramids, Au truncated octahedra showed only one peak at 578 nm due to the more symmetric configuration. Au nanostructures including Au ditetragonal prisms, bipyramids and truncated octahedra can be synthesized with different sizes by simply adjusting reaction temperature. By increasing reaction temperature from 120 °C to 140 °C and 170 °C, we have obtained Au ditetragonal prisms with longest edge lengths of 52 (Figure 3.12A) and 30 nm (Figure 3.12B), bipyramids with lengths of 53 (Figure 3.12C) and 40 nm (Figure 3.12D), and truncated octahedra with diameters of 75 (Figure 3.12E) and 32 nm (Figure 3.12F). 34 Figure 3.12. Au nanostructures synthesized at different temperature: (A, C and E) at 1400C and (B, D and F) at 1700C. The procedures were similar to those used for the syntheses of Au truncated ditetragonal prisms, bipyramids and truncated octahedra except that no NaCl was used for the truncated ditetragonal prism synthesis. (A, B) Nanobars with the longest lengths of 52 and 30 nm, (C, D) bipyramids with lengths of 53 and 40 nm and (E, F) truncated octahedra with diameters of 75 and 32 nm. It is clear that at higher temperatures, smaller Au nanostructures were formed since higher temperatures favor the formation of larger number of nuclei that causes smaller extent of nanoparticle growth. The effect of Ag ions on controlling the shape of Au nanocrystals has been studied extensively.14 The role of silver ions in the growth of Au nanocrystals with different shapes can be multi-faceted. One possibility is that the adsorption of silver halide monolayer on Au {110} faces could inhibit or slow down the growth of Au in the direction perpendicular to these facets.14 Another explanation is because of the 35 different UPD shifts of silver on gold surfaces which decrease following the order {110}>{100}>{111}.9 The different UPD shifts indicate that adatom deposition of Ag on Au is easier for {110} and {100} that {111} facets, which causes symmetry breaking effect and thus the appearance of {110} and {100} facets. In order to interpret the role of Ag+ and Pd2+ in the process, we firstly conducted experiments with the same conditions as described previously but in the absence of Pd2+. Figure 3.13 shows the Au nanocrystal formed at [Pd2+] = 0 for three different Ag concentrations. Without Pd2+ addition, truncated octahedra, mixtures of multiplefacets crystals, mixtures of faceted particles and bipyramids in a wide range of sizes appeared (Figure 3.13A-C, respectively). It is worth noting that with low and medium concentrations of Ag ions added in the reactions, edge-truncated octahedra (Figure 3.13A) and edge-truncated faceted-nanocrystals (Figure 3.13B) were obtained. These structures appeared to be exposed with {110} facets. The growth mechanism of the Au nanocrystals in the absence of Pd2+ can be described as follows. At first, Au3+, Ag+ and PDDA can form complex at room temperature upon mixing. As the reactant solution is heated up, Au3+ and Ag+ are reduced into Au0 and Ag0 atoms with the protection of PDDA. Since the reduction potential of Au3+/Au is much higher than that of Ag+/Ag, the resulting Ag atoms can be re-oxidized by Au3+. Consequently, only Au atoms exist in the solution during this period. Due to the extremely high surface energy of the as-formed Au atoms, they tend to aggregate to form the so-called nuclei with a size of a few nanometers under the protection of PDDA macromolecules. At this period, the PDDA-protected Au nuclei keep growing in the presence of underpotential deposition (UPD) of Ag, and their shape profile may vary randomly. As the Au nuclei grow to certain sizes, they begin to take their own shape since the coverage of PDDA macromolecules and the 36 UPD of Ag may become more energetically favorable than the thermal effect. Because the PDDA macromolecules contain Cl- anions which could form AgCl precipitate in the presence of Ag+ cations, it is believed that PDDA could influence the UPD shift of Ag, leading to the preferential deposition on {110} facets of the Au nanoparticles. For this reason, the growth of Au nanoparticles perpendicular to the {110} facets is strongly inhibited. As a result, the Au nanocrystals with {110} facets remain in the final products. Therefore, we believe that the UPD of Ag on the surface of Au nanocrystals inhibits the growth along directions, leading to the formation of the {110} facets. At the very high Ag+ concentration (0.476 mM), there was the formation of Au nanobipyramids with fivefold twinned structure (Figure 3.13C). This multiply twinned structure could be due to the interference of AgCl species which is mostly insoluble in ethylene glycol30 leading to the formation of Au mutiply twinned nuclei. These multiply twinned nuclei were further developed into Au nanobipyramids via a continuous deposition of Au adatoms on the surface of the pre-formed nuclei. Figure 3.13. Au nanostructures obtained without the addition of Pd2+. The concentration of AuCl4- in these experiments was kept the same as previously. The concentrations of Ag+ are as follows: (A) 0.024 mM; (B) 0.096 mM; (C) 0.476 mM. Pd deposition (either under- or over-potential) may also take place on the surface of Au crystals to form epitaxial layer. It has been found that the tendency of alloy formation of Pd on Au decrease following the order of {110}>{100}>{111}, which is 37 the same as that of Ag. The role of Pd2+ can be concluded based on another set of experiments without adding Ag ions. It is clear that in the absence of Pd2+ only Au octahedra with uniform size can be obtained (at 195 °C) (Figure 3.14A). At [Pd2+] = 0.06 mM, the mixture of vertex-truncated nanocrystals were harvested, namely, octahedra, tetrahedra and twinned plates (Figure 3.14B). Higher concentration of Pd2+ only led to the formation of similar mixture of Au nanocrystals obtained previously (Figure 3.14C). The truncation at vertices of those Au nanocrystals is, in fact, the indication for the formation of {100} facets of the nanocrystals which might be attributed to the deposition of Pd on Au nanocrystal surface. The growth mechanism of the Au nanocrystals in the absence of Ag+ can be interpreted similar to the above process (i.e. in the absence of Pd2+). In this process, the as-formed Au nanoparticles were affected by the UPD of Pd in combination with the coverage of PDDA macromolecules leading to the exclusive inhibition of the growth perpendicular to the {100} facets. Therefore, the resultant Au nanocrystals appeared to have certain exposure of {100} facets. These results clearly indicate that the deposition of Pd on Au nanocrystal surface induces the growth of {100} facets. Overall, Ag and Pd ions, which stabilize the {110} and {100} facets, respectively, together are able to inhibit the growth along directions that results in the growth of {310} facets existing in the Au nanocrystals obtained in this work. 38 Figure 3.14. Au nanoparticles synthesized without the addition of Ag+. (A) [Pd2+] = 0 mM, 195 °C, 30 min; (B) [Pd2+] = 0.06 mM, 120 °C, 12 h; (C) [Pd2+] = 0.12 mM, 120 °C, 12 h. 3.1.4. Conclusion We have presented a new route to Au nanoparticles with high-index facets including Au truncated ditetragonal prisms enclosed by 12 {310} high-index facets, bipyramids bounded by {117} stepped facets and truncated octahedra with exposed of 8 {111} and 24 {310} facets. By using a facile one-pot PDDA-mediated polyol process combined with the synergistic effect of Ag and Pd ions, we have obtained those Au nanocrytals in high yield, monodispersity and wide range of sizes. In this process, UPD of Ag serve to inhibit the growth along direction that leads to the formation of {110} facets of Au nanocrystals. The presence of Pd2+, on the other hand, stabilizes the {100} facets of Au nanocrystals. Therefore, together, Ag and Pd ions enable the growth of {310} facets that lead to the formation of Au truncated octahedra partially enclosed by {310} facets and truncated ditetragonal prisms totally bounded by {310} facets. Very high Ag ion concentration which could induce the existence of AgCl species was appeared to interfere with the construction of Au adatoms on the Au nuclei leading to the formation of multiply twinned nuclei. These nuclei could finally grow further into the Au nanobipyramids with penta-twinned structures. The Au truncated ditetragonal prisms and truncated octahedra with the {310} high-index facets could be excellent candidates for catalytic applications due to 39 their abundance in unsaturated atomic steps, ledges and kinks. The Au bipyramids with {117} stepped faces were synthesized in the highest yield ever using our method. Because of their high yield and monodispersity, these bipyramids could not only serve as catalysts for chemical reactions but also act as good substrate for surface-enhanced Raman scattering (SERS) and contrast agent for biomedical applications. 40 3.2. References (1) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem. Int. Ed. 2004, 43, 3673. (2) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (3) Seo, D.; Park, J. C.; Song, H. J. Am. Chem. Soc. 2006, 128, 14863. (4) Seo, D.; Yoo, C. I.; Park, J. C.; Park, S. M.; Ryu, S.; Song, H. Angew. Chem. Int. Ed. 2008, 47, 763. (5) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (6) Wu, H.-Y.; Huang, W.-L.; Huang, M. H. Cryst. Growth Des. 2007, 7, 831. (7) Huang, X.; Neretina, S.; El-Sayed, M. A. Adv. Mater. 2009, 21, 4880. (8) Jana, N. R.; Gearheart, L.; Murphy, C. J. Adv. Mater. 2001, 13, 1389. (9) Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B 2005, 109, 22192. (10) Kou, X.; Zhang, S.; Tsung, C.-K.; Yeung, M. H.; Shi, Q.; Stucky, G. D.; Sun, L.; Wang, J.; Yan, C. J. Phys. Chem. B 2006, 110, 16377. (11) Kou, X.; Ni, W.; Tsung, C.-K.; Chan, K.; Lin, H.-Q.; Stucky, G. D.; Wang, J. Small 2007, 3, 2103. (12) Lofton, C.; Sigmund, W. Adv. Funct. Mater. 2005, 15, 1197. (13) Hsu, S. J.; Su, P. Y. S.; Jian, L. Y.; Chang, A. H. H.; Lin, I. J. B. Inorg. Chem. 2010, 49, 4149. (14) Grzelczak, M.; Pérez-Juste, J.; Mulvaney, P.; Liz-Marzán, L. M. Chem. Soc. Rev. 2008, 37, 1783. (15) Xiang, Y.; Wu, X.; Liu, D.; Jiang, X.; Chu, W.; Li, Z.; Ma, Y.; Zhou, W.; Xie, S. Nano Lett. 2006, 6, 2290. (16) Fan, F.-R.; Liu, D.-Y.; Wu, Y.-F.; Duan, S.; Xie, Z.-X.; Jiang, Z.-Y.; Tian, Z.Q. J. Am. Chem. Soc. 2008, 130, 6949. (17) Lee, Y. W.; Kim, M.; Kim, Z. H.; Han, S. W. J. Am. Chem. Soc. 2009, 131, 17036. (18) Ding, Y.; Fan, F.; Tian, Z.; Wang, Z. L. J. Am. Chem. Soc. 2010. (19) Lim, B.; Kobayashi, H.; Yu, T.; Wang, J.; Kim, M. J.; Li, Z.-Y.; Rycenga, M.; Xia, Y. J. Am. Chem. Soc. 2010, 132, 2506. 41 (20) Ferrer, D.; Torres-Castro, A.; Gao, X.; Sepúlveda-Guzmán, S.; Ortiz-Méndez, U.; José-Yacamán, M. Nano Lett. 2007, 7, 1701. (21) Krichevski, O.; Markovich, G. Langmuir 2007, 23, 1496. (22) Grzelczak, M.; Pérez-Juste, J.; Rodríguez-González, B.; Liz-Marzán , L. M. J. Mater. Chem. 2006, 16, 3946. (23) Tian, N.; Zhou, Z.-Y.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (24) Tian, N.; Zhou, Z.-Y.; Yu, N.-F.; Wang, L.-Y.; Sun, S.-G. J. Am. Chem. Soc. 2010, 132, 7580. (25) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765. (26) Tian, N.; Zhou, Z.-Y.; Sun, S.-G. Chem. Commun. 2009, 1502. (27) Seo, D.; Park, J. H.; Jung, J.; Park, S. M.; Ryu, S.; Kwak, J.; Song, H. J. Phys. Chem. C 2009, 113, 3449. (28) Li, C. C.; Sato, R.; Kanehara, M.; Zeng, H. B.; Bando, Y.; Teranishi, T. Angew. Chem. Int. Ed. 2009, 48, 6883. (29) Wu, H.-L.; Chen, C.-H.; Huang, M. H. Chem. Mater. 2009, 21, 110. (30) An, C. H.; Peng, S. N.; Sun, Y. G. Adv. Mater. 2010, 22, 2570. (31) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P. Nat. Mater. 2007, 6, 692. (32) Lim, B.; Wang, J. G.; Camargo, P. H. C.; Jiang, M. J.; Kim, M. J.; Xia, Y. N. Nano Lett. 2008, 8, 2535. (33) Seo, D.; Il Yoo, C.; Jung, J.; Song, H. J. Am. Chem. Soc. 2008, 130, 2940. (34) Chen, Y. H.; Hung, H. H.; Huang, M. H. J. Am. Chem. Soc. 2009, 131, 9114. 42 Chapter 4. Conclusions and Recommendation for Future Work 4.1 Conclusions The main focus of this thesis is to devise a new approach for synthesis of Au nanocrystals with novel shapes and with high-index facets, and novel heterometallic nanostructures based on the Au nanocrystals obtained. First, by using the modified PDDA-mediated polyol process, we have successfully synthesized Au nanocrystals with high-index facets, including truncated octahedra bounded by both {111} and {310} facets, truncated ditetragonal prisms enclosed by {310} facets, and bipyramids with {117} facets. In this process, UPD of Ag serve to inhibit the growth along direction that leads to the formation of {110} facets of Au nanocrystals. The presence of Pd2+, on the other hand, stabilizes the {100} facets of Au nanocrystals. Therefore, together, Ag and Pd ions enable the growth of {310} facets that lead to the formation of Au truncated octahedra partially enclosed by {310} facets and truncated ditetragonal prisms totally bounded by {310} facets. Very high Ag ion concentration which could induce the existence of AgCl species was appeared to interfere with the construction of Au adatoms on the Au nuclei leading to the formation of multiply twinned nuclei. These nuclei could finally grow further into the Au nanobipyramids with penta-twinned structures. The Au truncated ditetragonal prisms and truncated octahedra with the {310} high-index facets as well as the Au bipyramids could be excellent candidates for catalytic applications due to their abundance in unsaturated atomic steps, ledges and kinks. 43 4.2 Recommendations for Future work Gold has recently been recognized as the very good catalyst for many reactions such as hydrogenation of alkenes, dienes and alkynes, selective oxidations of alcohols and alkenes and activation of carbonyl groups.1-4 Gold nanocrystals with high-index facets have electrochemically proved to be more active than Au nanocrystals enclosed by low-index facet (i.e., {111}, {100} and {110}).5,6 However, to date, the applications of these Au nanocrystals with high-index facets in chemical synthesis have not been reported yet. Besides, bimetallic nanostructures, made of two different metal elements, have long been known as excellent catalysts owing to their synergistic effects between the two different metal components.7 Among bimetallic structures, gold-palladium core-shell (Au@Pd) has proved to be one of the most attractive catalysts due to synergistic effect from the Au core and highly-strained Pd shell leading to the high catalytic activity for many chemical syntheses.8 The application of novel nanocrystals with high-index facets or controlled bimetallic structures in catalysis are therefore of technical and scientific interest. In this work, we have successfully synthesized a series of high-index Au nanocrystals, namely, truncated ditetragonal prisms bounded by 12 {310} facets, bipyramids enclosed by {117} facets, and {310} and {111} truncated octahedra that can theoretically provide superior catalytic properties. In addition, we have also prepared Au@Pd nanorods which contain the high density of grooves and defects. These nanocrystals can be applied as catalysts for reactions such as direct formation of hydrogen peroxide from the reaction of H2 and O2. Hydrogen peroxide has been considered as the significantly important commodity chemicals for fine chemical industry and household uses. Approximately a few 44 million tons of H2O2 is produced every year. The production of H2O2 is of economic importance for pharmaceutical industry and daily uses. Previously, H2O2 was synthesized by using the indirect anthraquinone which was first invented by Riedl and co-workers in 1939. The process includes the hydrogenation of a substituted anthraquinone using metal catalyst such as Ni or Pd to form the diol9. The successive oxidation of anthraquinol using oxygen-enriched air recovers the initial anthraquinone and produces H2O2. This process is still used for commercial H2O2 supply even though it mainly depends on the effective recycle of anthraquinone which is a highly expensive chemical. In addition, the production of H2O2 must be carried out on large scale for lowering the manufacturing expenses and the transportation of concentrated H2O2 can be hazardously dangerous that could end up with unexpected explosion. Therefore, there has been a strong need for a small-scale and safe process for producing H2O2 in which H2O2 is directly produced from H2 and O2. The direct synthesis of H2O2 has been pursued by many research groups.9-20 The synthesis of hydrogen peroxide was first catalyzed by Pd colloids in the presence of H2 and O2.10 Afterwards, Hutchings et al. discovered that Al2O3 supported Au catalyst could produce much higher selectivity as compared to Al2O3 supported Pd catalyst.13,14 Interestingly, in the same work, the authors also showed that the supported Au:Pd (1:1 by wt) catalysts gave rise to the significantly higher rate of H2O2 formation than the pure Au catalyst due to the synergistic effect of Pd serving as a promoter for Au catalyst. Since then, many reports of direct synthesis of H2O2 using supported Au:Pd catalyst have been published.9,16-19,21 Among those, supported Au- 45 50 49 core Pd-shell catalyst appears to be very efficient for catalyzing the formation of H2O2 with high production rate and selectivity.18,19 While most reports on direct synthesis of H2O2 focus on in situ preparation of supported Au, Pd, and Au:Pd catalysts (i.e., impregnation or co-precipitation), few is on ex situ preparation of these catalysts. Very recently, Lundsford and co-workers has devised a novel route to synthesize the supported Pd nanoparticles for catalyzing the direct formation of H2O2.22 In their work, the as-prepared Pd nanoparticles were immobilized on the XC-72 carbon black support to use as catalysts for the synthesis of H2O2. By making a comparison between the supported Pd nanoparticles prepared ex situ and in situ (i.e., impregnation), they have proved that the specific activity and selectivity of the supported Pd nanoparticles prepared ex situ are significantly higher than that of the conventionally prepared Pd catalyst. In view of the above findings, we propose the direct synthesis of H2O2 using our Au nanocrystals and Au@Pd nanorods immobilized ex situ on support as the catalyst. In this proposed work, the Au nanocrystals including truncated ditetragonal prisms and bipyramids, and Au@Pd nanorods with the smallest sizes (30 nm, 40 nm and 40 nm for Au prisms, bipyramids, and Au@Pd nanorods, respectively) will be used as the active components of the supported catalysts. The immobilization of the Au and Au-Pd catalysts will follow the procedure developed by Lundsford et al.22 Briefly, the as-synthesized Au and Au-Pd nanoparticles and carbon black will be mixed in toluene followed by drying in oven to obtain supported catalyst. The synthesis of H2O2 can be performed in a stainless steel autoclave with a H2/O2 ratio of 1:2. Gas analysis for H2 and O2 will be recorded by gas chromatography. The conversion of H2 will be calculated based on the gas analyses before and after the reaction. The yield of H2O2 can be estimated by titration of the final solution with acidified Ce(SO4)2. 46 Subsequently, Ce(SO4)2 solution is standardized against (NH4)2Fe(SO4)2.6H2O by using ferroin as indicator.14 The performance of the supported Au catalyst can be evaluated based upon their productivity and selectivity for H2O2 formation. The productivity of the catalysts is defined as the mole of H2O2 generated per kg of catalysts per hour. The selectivity of the catalyst is defined as the mole of H2O2 generated per hour divided by the mole of H2 consumed per hour. Although the Au and Au-Pd nanoparticles used as the active components of the supported catalyst have larger sizes than the conventional Au or Pd nanoparicles used in previous reports in this field, it is expected that the supported catalyst will provide high productivity and selectivity because of the following two reasons: i. The Au nanocrystals are bounded by high-index facets, namely, {310} or {117} which are composed of low-coordinated atoms at the steps, ledges and kinks that serve as highly active sites for breaking and forming of chemical bonds. The surface of the Au nanocrystals including ditetragonal prisms and bipyramids are constituted of Au and Pd that can synergistically give rise to superior catalytic activities for H2O2 synthesis. ii. The Au@Pd nanorods which contain Au-core and Pd-highly strained shell may exhibit synergistic effect enhancing the catalytic activities. In addition, the Au@Pd nanorods possess the high density of grooves and defects which may act as highly active sites for chemical reactions. 47 51 4.3 References (1) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem. Int. Ed. 2006, 45, 7896. (2) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180. (3) Arcadi, A. Chem. Rev. 2008, 108, 3266. (4) Li, Z. G.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239. (5) Ma, Y.; Kuang, Q.; Jiang, Z.; Xie, Z.; Huang, R.; Zheng, L. Angew. Chem. Int. Ed. 2008, 47, 8901. (6) Ming, T.; Feng, W.; Tang, Q.; Wang, F.; Sun, L.; Wang, J.; Yan, C. J. Am. Chem. Soc. 2009, 131, 16350. (7) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 22, 1179. (8) Ferrando, R.; Jellinek, J.; Johnston, R. L. Chem. Rev. 2008, 108, 845. (9) Edwards, J. K.; Hutchings, G. J. Angew. Chem. Int. Ed. 2008, 47, 9192. (10) Krishnan, V. V.; Dokoutchaev, A. G.; Thompson, M. E. J. Catal. 2000, 196, 366. (11) Choudhary, V. R.; Gaikwad, A. G.; Sansare, S. D. Catal. Lett. 2002, 83, 235. (12) Choudhary, V. R.; Sansare, S. D.; Gaikwad, A. G. Catal. Lett. 2002, 84, 81. (13) Landon, P.; Collier, P. J.; Papworth, A. J.; Kiely, C. J.; Hutchings, G. J. Chem. Commun. 2002, 2058. (14) Landon, P.; Collier, P. J.; Carley, A. F.; Chadwick, D.; Papworth, A. J.; Burrows, A.; Kiely, C. J.; Hutchings, G. J. Phys. Chem. Chem. Phys. 2003, 5, 1917. (15) Chinta, S.; Lunsford, J. H. J. Catal. 2004, 225, 249. (16) Edwards, J. K.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Faraday Discuss. 2008, 138, 225. (17) Edwards, J. K.; Thomas, A.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Green Chem. 2008, 10, 388. (18) Edwards, J. K.; Ntainjua, E.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Angew. Chem. Int. Ed. 2009, 48, 8512. (19) Edwards, J. K.; Solsona, B.; N, E. N.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Science 2009, 323, 1037. 48 56 57 (20) Piccinini, M.; Ntainjua, E.; Edwards, J. K.; Carley, A. F.; Moulijn, J. A.; Hutchings, G. J. Physical Chemistry Chemical Physics 2010, 12, 2488. (21) Edwin, N. N.; Edwards, J. K.; Carley, A. F.; Lopez-Sanchez, J. A.; Moulijn, J. A.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Green Chem. 2008, 10, 1162. (22) Liu, Q. S.; Bauer, J. C.; Schaak, R. E.; Lunsford, J. H. Angew. Chem. Int. Ed. 2008, 47, 6221. 49 [...]... followed by a review of various chemical methods in shape- controlled synthesis of noble metal nanocrystals 2.1.1 Nucleation and growth of metal nanocrystals Chemical synthesis of nanoparticles involves either decomposition or reduction of metal precursors For the decomposition route, nucleation stage is considered to follow the LaMer diagram.1 Briefly, under suitable conditions the number of metal atoms increases... 2.1 Shape- control synthesis of noble metal nanocrystals In order to control the shape and size of metal nanocrystals, one should know how they are created and grown From these understandings, one can basically choose the appropriate synthetic method to selectively fabricate the desired shapes and sizes of the metal nanocrystals Thus, in this part, a brief discussion of the growth mechanism of metal nanocrystals. .. R.; Personick, M L.; Zhang, K.; Li, S.; Mirkin, C A J Am Chem Soc 2010, 132, 14012 16 Chapter 3 Shape- controlled synthesis of Au Nanocrystals with High-index Facets 3.1 Shape- selective growth of polyhedral gold nanocrystals with high-index facets 3.1.1 Introduction As discussed previously, shape- controlled synthesis of noble metal nanocrystals in solution-phase have relied on the flexibility of choosing... have relied on the flexibility of choosing reaction parameters such as precursor, solvent, surfactant and foreign ions Among these strategies, introduction of foreign metal ions, especially silver ions, in the synthesis of gold nanocrystals has shown drastic morphology-selection effect Au nanoparticles with tailored shapes including cube,1-4 octahedron,2-4 nanorod,2,5-7 bipyramid,8-11 and plate12 have... applications of metal nanocrystals synthesized by the polyol synthesis, especially in biomedical applications Therefore, the prominent post-treatment of those metal nanocrystals is of great necessity for this method to be promising for biomedical applications 11 2.2 Synthesis and catalytic properties of metal nanocrystals with high-index facets High-index facets are facets composed of periodic combination of two. .. nanocrystals would show synergistic effect on controlling the shapes of the resultant particles Indeed, although scarcely reported, controlled growth of nanocrystals in tri-metallic nanocrystal systems has been noticed recently LizMarzan et al examined the influence of Ag ions on the growth of Pt on Au nanorods and found that in the presence of Ag+, the deposition of Pt takes place on the rod tips; while... homogeneous coating of Pt on rod surface are obtained This was attributed to the UPD of Ag on Au(110) which causes slower growth of Pt on {110} faces compared to those on {100} and {111} faces.22 Here, we report the shapeselective synthesis of Au nanocrytals in the presence of two foreign metal ions – Ag and Pd A facile one-pot polyol synthesis was employed with poly(diallyldimethylammonium chloride) (PDDA)... surfactant concentration by using a strong reducing agent (usually NaBH4) Under such a concentrated-surfactant condition, metal seeds formed are very small, about 3-5 nm in diameter.3-5 These preformed-seeds are subsequently added into the so-called “growth solution” that contains suitable concentrations of the metal precursor, surfactant and a mild reducing agent The ability to control the shape and size of. .. cuboctahedra and their intermediate forms by controlling KI concentration and reaction temperature.8 Although seeded-growth has been considered as one of the most powerful methods for synthesizing metal nanoparticles, it strictly requires the very accurate conditions for making seeds such as pH value and concentration of the strong reducing agent Additionally, metal nanoparticles synthesized by this method... morphological control of Au nanocrystals. 14 Compared to silver, palladium has received much less attention in shape- selective growth of Au nanocrystals Although Pd and Au have been both used in some reactions, focus has been mainly on the formation of Au-Pd bi-metallic structures especially core-shell nanoparticles.15-19 For example, Yacaman and co-workers conducted successive reduction of PdCl2 and .. .SYNERGISTIC EFFECT OF TWO FOREIGN METAL IONS ON SHAPE- SELECTIVE SYNTHESIS OF GOLD NANOCRYSTALS TRAN TRONG TOAN (B.Sc (Hons.), University of Science Ho Chi Minh City)... ratio of Ag and Pd ions The synergistic effect of Ag and Pd ions on the formation of the novel Au nanocrystals was studied In our experimental conditions, the underpotential deposition (UPD) of. .. reaction parameters such as precursor, solvent, surfactant and foreign ions Among these strategies, introduction of foreign metal ions, especially silver ions, in the synthesis of gold nanocrystals