Shape controlling of silver and gold nanocrystals

SHAPE CONTROLLING OF SILVER AND GOLD NANOCRYSTALS ZHOU XUEDONG NATIONAL UNIVERSITY OF SINGAPORE 2008 SHAPE CONTROLLING OF SILVER AND GOLD NANOCRYSTALS ZHOU XUEDONG A THESIS SUMBITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2008 ACKNOWLEDGE First I would like to acknowledge the guides of my two supervisors, Professor Chan Sze On, Hardy and Professor Xu Guo Qin. This thesis is the product of their expertise in synthetic field and surface science. I am grateful to their continuing support, patience, and trust. It is difficult for me to imagine how I would complete this task without the support of a number of people. I am grateful to Mr. Liu Binghai, Madam Loy Gek Luan and Tang Chui Ngoh for their help in Electron Microscope experiments, Madam Toh Soh Lian and Miss Tan Geok Kheng for powdered and thin film XRD experiments. Special thanks also go to Dr. Zhang Xinhuai (SVU) for his effort in getting license of Material Studio. I’ve a happy time to work with my colleagues Mr. Gu feng and Mrs. Zhao AiQing. They are always ready to help in the lab. I also would like to thank Miss De Witt Shivani Cassandra for her kindness to proofreading my manuscript. I thank National University of Singapore for providing me the research scholarship and the opportunity of my pursuit of the degree. Last but not least, I would like to thank my family: my parents and my wife for their support during the long hours associated with a Ph. D study. Without their assistance, I would never have completed this project. i TABLE OF CONTENTS ACKNOWLEDGEMENT i TABLE OF CONTENTS ii SUMMARY viii ABBREVIATION LISTS xi LISTS OF FIGURES xviii LISTS OF TABLES xix CHAPTER INTRODUCTION 1.1 Background of nanoscience and nanotechnology 1.2 Synthesis of free standing metal nanocrystals of different shapes 1.2.1 Condensation methods 1.2.2 Solution methods 1.3 1.2.2.1 Template synthesis 1.2.2.2 Chemical reduction 1.2.2.3 Reduction by energy beams 1.2.2.4 Pyrolysis 1.2.2.5 Solvothermal synthesis 1.2.2.6 Micelles method 11 Critical parameters for controlling morphology of metal nanocrystals 1.3.1 13 Surface energy effect: intrinsic crystallographic surface energies and their adsorption energy modification 14 ii 1.3.2 Kinetic control 15 1.4. Research objectives 16 1.5 20 References Chapter Experimental and computational Methodology 28 2.1 28 Instrumental methods 2.1.1 2.2 Ultraviolet-visible (UV-Vis) spectroscopy observation of SPR of metal Nanocrystals 28 2.1.2 Electron microscopy 30 2.1.2.1 Scanning electron microscopy (SEM) 32 2.1.2.2 Transmission electron microscopy (TEM) 34 2.1.3 X-ray diffraction 39 2.1.4 Atomic force microscopy 39 Analysis of UV-Vis spectrum by computation of Surface plasma Resonance (SPR) 2.2.2 41 Other shapes treated by numerical methods 42 2.2.2.1 T-matrix method 42 2.2.2.2 Discrete dipole approximation (DDA) 42 2.3 Analysis of planar defects by Structural Modeling 2.4 Surface energy, Adsorption energy, surface reaction and atomic processes on 2.5 44 Noble metal surfaces by ab initio modeling 47 References 52 iii Chapter Simulation of Surface Plasma Resonance of Nanoplates 55 3.1 Introduction 55 3.2 Results and Discussion 56 3.2.1 Triangular nanoprisms 58 3.2.2 Hexagonal nanoprisms 64 3.2.3 Compressed cylinders 67 3.2.4 Thickness determination from SPR spectrum 68 3.3 Conclusion 75 3.4 References 76 Chapter Synthesis and Formation Mechanims of Silver Nanoprisms with Hydrogen Peroxide as Shape Control Agent 78 4.1 Introduction 78 4.2 Experimental Section 81 4.3 Experimental results and discussion 81 4.3.1 Absence of Citrate 82 4.3.1.1 Ethanol providing active hydrogen 82 4.3.1.2 PVP as reducing agent and capping agent 87 4.3.2 Presence of citrate ions 91 4.3.2.1 Aging Effect 91 4.3.2.2 Dynamics of nanoprisms formation 94 iv 4.3.2.3 The role of potassium borohydride in nanoprisms formation 101 4.3.2.4 The role of PVP in nanoprisms formation 105 4.3.2.5 The role of citrate and Reaction Mechanism discussion 106 4.4 Conclusion 110 4.5 References 112 Chapter Chemical Synthesis of Silver Nanoprisms and Investigation of Formation Mechanism 118 5.1 Introduction 118 5.2. Experimental Section and Results 120 5.2.1 MTP silver seed mediated growth of nanopris 120 5.2.2 5.2.1.1 Experimental details 120 5.2.1.2 Results 121 5.2.1.2.1 pH effect 121 5.2.1.2.2 Effect of oxygen 125 Electrochemical growth of silver nanoplates under ambient conditions 128 5.2.2.1 Electrodepositing of silver nanoplates 128 5.2.2.2 Results 128 5.3 Discussion 134 5.4 Conclusion 137 5.5 References 138 v Chapter Perfect single crystal of clean gold nanoprisms with tunable size 141 6.1 Introduction 141 6.2 Experimental section 142 6.3 Experimental Results and discussion 143 6.3.1 Tunable size 143 6.3.2 151 Structure investigation 6.4 Conclusions 161 6.5 References 162 Chapter Ab initio modeling of formation mechanism of gold nanoprisms 165 7.1 Introduction 165 7.2 Computational Methods 168 7.3 Computational results and discussions 169 7.3.1 Surface free energy 169 7.3.2 Citric acid on gold surfaces 172 7.3.3 Reaction Pathway in the presence of hydroxyl radicals 174 7.3.3.1 The reaction in solution 175 7.3.3.2 Reaction on gold surfaces 177 7.3.4 7.3.2.2.1 Pathway (Bidentate Æ Product) 178 7.3.2.2.2 Pathway (bidentate Æunidentate Æproduct) 181 Gold adtoms self diffusion on terrace 187 7.3.4.1 Au (111) Terrace 188 7.3.4.2 190 Au (110) terrace vi 7.3.5 7.4 Gold adatoms diffuse over edge 192 7.3.5.1 Edge 111/1-10 (900) 192 7.3.5.2 Edge 111/110 (1320) 194 Conclusion and prospect 199 7.5 References 201 Chapter Conclusion and prospect 206 vii SUMMARY First the SPR of silver nanoprisms are simulated by DDA method in chapter 3. The UV-Vis spectrum is intensively used to monitor the formula screening and formation process in thereafter experimental chapters. In this chapter, a scheme is proposed to derive the thickness of nanoprisms with round forms from TEM and simulation data. In chapter 4, hydrogen peroxide is used to activate the reductant precursors such as citrate, ethanol and PVP on the surfaces of in situ formed silver seeds. It is found that the silver nanoprisms can be successfully produced. In chapter 5, Silver nanoprisms with different sizes have been successfully produced by simply tuning the pH value of growth bath in the seed mediated crystal growth method. The role of oxygen is investigated in formation mechanism of silver nanoprisms. It is found that no silver nanoprisms can be produced under anaerobic condition. This view is further supported by electrodepositing of silver nanoplates. It is found that the silver nanoplates can be successfully electrochemically prepared under the 2e- pathway of reducing oxygen. In chapter 6, single crystal gold nanoprisms with high purity are also successfully prepared at ambient condition under light irradiation of 254 nm. The size of these nanoprisms can be tuned by simply adjusting the pH value of growth bath. The XRD data disapproves the popular view viii Figure 7-13. View of reactant, transition state and product for gold adatoms diffusing across Au(111)/Au(110) along , and direction. 195 Figure 7-14. The reaction pathway for gold atom hopping through 111/110 edge, the structure is view along direction. The gold atoms from Au (111) could diffuse easily to Au (110), but not true for the reversed way. There are two reasons behind this phenomenon. First, when Au (110) and Au (111) surfaces are in contact, the gold atoms thermodynamically incline to move to the more open surface such as 110 which is more unsaturated in bonding pair. Secondly, the gold atoms are more difficult to diffuse on perfect unreconstructed Au (110) terrace compared to the Au (111) surface. Furthermore, the barrier of overcoming the edge Au (111)/Au (110) is near to the energy barrier of gold adatoms diffusing on Au (111) terrace. With this fact in mind, Au (110) surface is not stable because the growth rate of Au (110) depends on the reaction rate of Reaction (1) on Au (110) and the diffusing rate of gold atom crossing the edge of Au (111)/Au (110). Furthermore, the growth rate of Au (110) is mainly determined by the diffusing rate of gold atom crossing the edge of Au (111)/Au (110) because the small barrier energy is needed to overcome compared to large barrier energy of the reaction (1) (0.121eV vs. 0.653 eV). In other words, Au (110) is not stable and will thermally converted into Au (111). 196 Figure 7-15. The construction of gold nanoprism with R110/R111 equal to 55. In summary, the symmetrically equivalent surfaces of Au (110) and Au (1-10) is not degenerate in crystal growth. The reason lies in that gold adatoms can move easily from Au (111) to Au (110) across the edge of Au (111)/Au (110) while they are difficult to overcome the edge Au (111)/Au (1-10). In addition, Au (1-10) is perpendicular to Au (111). This phenomenon is called as dynamic breaking of symmetry. Thus, only subgroup (-3m or 6/mm) of point group (m-3m) can be observed in the crystal habits. From similar analysis, Au (-110), Au (10-1), Au (-1 1), Au (01-1) and Au (0-11) are degenerate crystal planes in crystal growth. According to the relative growth rate of Au (111) and Au (1-10), the hexagonal plate is constructed as shown in Figure 7-15. Interestingly, the aspect ratio between diameter/thickness is about 1000 nm/25 nm=40 in the prepared gold nanoprism as shown in Figure 6-2a in last chapter. The experimental relative growth rate (R110/R111)exp should be equal to 17 ( /4 of aspect ratio), which is about 1/3 as the calculated (R110/R111)cal (55). This discrepancy could come from citric acid used instead of formic acid. For the case of formic acid as shown in 197 Figure 7-16, the aspect ratio is about 180 and (R110/R111)exp is about 72 which is near to (R110/R111)cal. Figure 7-16. The morphology of gold nanoprisms prepared by formic acid. The other condition is same as Figure 6-2(a) in last chapter. The dynamical breaking of symmetry is natural for f.c.c. metals in the temperature region above the roughening temperature. The only requirement is that 110 surfaces are exposed or stabilized. This dynamical symmetry breaking has important consequence in controlling the morphology of f.c.c metals such as gold and silver which are famous in their small surface energy differences and high tendency to approaching isotropic crystal habits. For example, silver and gold microplates70-74 were observed with MTPs as major product in evaporation method in inert gas in some zone and special environment. In the presence of the impurities of O2, O2 can be used as surfactant75-83 which will modulate the homoepitaxial 198 growth of silver and gold. It is known that the sticking coefficient of O2 on Ag (110) or Au (110) is higher than other low miller index surfaces. This stabilizing Ag (110) or Au (110) will result in dynamical symmetry breaking and plate form of silver or gold crystal will be formed. 7.4 Conclusion and prospect In this study, detailed formation mechanism of gold nanoprisms prepared in last chapter has been investigated in terms of first principle DFT calculation. We found that the surface energies can not be employed to account for the nanoprism formation. Our DFT result of citric acid adsorption on gold surfaces also shows that the adsorption energy is too small to support the popular view of blocking effect due to selective adsorption of citric acid on specific surface. On the other hand, the Reaction (1) was found to be surface sensitive, resulting in different growth rates for Au (111), Au (110) and Au (100). Importantly, crystal equivalent surfaces can have different growth rates under suitable temperature. This phenomenon is termed as dynamical breaking of symmetry. Reducing m-3m symmetry to a lower symmetry is the key reason of formation of plate form. The general ideas of dynamical breaking may also be extended to the formation mechanism of silver nanoprisms. The perspective in this study can lend a hand to explain other shapes of f.c.c. nanocrystals. For example, Ag (100) is the important surface for the preparation of silver nanowires and nanocubes by polyol process. Currently, we not understand why the Ag (100) is exposed. Thermodynamically, Ag (100) can be exposed by specially adsorbed species on Ag (100) if the adsorption energy on Ag (100) can compensate the surface energy difference between Ag (111) and Ag (100). Kinetically, Ag (100) can be also exposed if the surface reaction on Ag (100) is slow enough compared to on Ag (111) and Ag (110). Xia’s group 199 realized that O2/Cl- plays important role in the selective etching of silver MTPs. The oxidation of polyol on surfaces of silver nanocrystals was lately discovered by his group as well. Ag (100) can also be exposed too if its etching rate is fastest on compared to Ag (111) and Ag (110). The detailed growth mechanism is yet to be explored experimentally and theoretically. 200 7.5 References: 1. R. Jin, Y. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schutz, J. G. 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It is further found that the dipole in-plane SPR peaks red shift as the aspect ratio increases. This result is used to estimate the thickness of rounded forms of silver nanoplates including hexagonal nanoprisms and compressed cylinders from TEM and simulation data. In chapter 4, hydrogen peroxide is used to activate the reductant precursors such as citric acid (citrate), ethanol and PVP on the surfaces of in situ formed silver seeds. It is found that the silver nanoprisms can be successfully produced. In chapter 5, Silver nanoprisms with different sizes have been successfully produced by simply tuning the pH value of growth bath in the seed mediated crystal growth method. The role of oxygen is investigated in formation mechanism of silver nanoprisms. It is found that no silver nanoprisms can be produced under anaerobic condition, more importantly the pH effect will vanish in absence of oxygen. This view is further supported by electrodepositing of silver nanoplates. It is found that the silver nanoplates can be successfully electrochemically prepared under the 2e- pathway of reducing oxygen. In chapter 6, single crystal gold nanoprisms with high purity are also successfully prepared under 254 nm irradiation in presence of H2O2. Under our experimental condition, hydrogen peroxide is decomposed into hydroxyl radicals in presence of 254 nm irradiation. 206 These in situ produced active hydroxyl radicals are surface sensitively adsorbed onto the gold surface bringing about the surface sensitive reaction on gold surface which will be ab initio modelled in chapter in detail. The size of these nanoprisms can be tuned by simply adjusting the pH value of growth bath. The XRD data disapproves the popular view of stacking faults existing in gold nanoprisms. The result further confronts the role of stacking faults in formation of plate form of gold. In theoretical part of this study, the study about surface energy and adsorption of target molecules shows that only surface energy is not enough to explain why nanoplates are formed. The theoretical study on gold nanoprisms forms shows one possible pathway of formation of gold nanoprism. DFT modelling shows that the adsorbed hydroxyl radical can abstract the hydrogen in the nearby formate ions, and the electron is simultaneously transferred to the ligand aurous ion, and the aurous ion is reduced. More importantly, this reaction is also surface sensitive. The growth of Au (110) is slower than Au (100) and faster than Au (111). In the room temperature which is far away from the melting point of gold, it is found that the equilibrium condition cannot be achieved. Microscopically, two atomic processes are investigated in this study. First, the atomic diffusion on terrace is studied. It is found that the activation barrier energy of gold atom diffusion on Au (111) is about 0.103 eV. Secondly, the atomic diffusion over edge is further considered. We found that the activation barrier energies over Au (111)/Au (110) and Au (111)/Au (1-10) differentiate much. The dynamical symmetry is termed in this case. Physically speaking, the crystal equilibrium surfaces such as Au (110) and Au (1-10) have a different growth rate when they are in contact with Au (111). The six vertically aligned Au (1-10) surface and Au (111) and Au (-1-1-1) will make the hexagonal gold nanoprisms. Furthermore, the calculated aspect ratio (55) coincides well with the 207 experimentally measured aspect ratio (72). This result further proves the validity of DFT modelling in this work. In summary, two methods are experimentally practiced in controlling the shapes of nanocrystal in solution growth of silver and gold nanocrystals. In both methods, the ROS plays important role in formation of silver and gold nanoprisms. Another key factor in producing silver and gold nanoprisms is using weak reductants. For example, in chapter 5, the ascorbic acid is used. In chapter and 6, the reductants are more diversified. For example, citric acid or citrate is usually used. Beside citric acid, even ethanol and PVP without reducing ability usual are used to producing the nanoprisms too. The common feature of these compounds is that all of them should be activated first to reduce silver ions. These compounds are termed reductant precursors. If the activation process takes place on the crystal surfaces, it will help in controlling the shapes. This process has been discussed in chapter experimentally and chapter theoretically. In chapter 7, we have demonstrated one possible pathway to produce nanoprisms theoretically. In fact, reductant precursors such as PVP or DMF have been successfully used to produce noble metal nanocrystals with different shapes although the underlying principles are not well understood for all existing method. Surface sensitivity reaction is believed to play the most important role in shaping the nanocrystals. For silver and gold nanoprisms, oxygen or ROS are convenient ways to tuning the reaction pathway of producing nanoprisms. These ROS are open shell species which are strong chemically adsorbed on silver and gold surface. Furthermore, these active ROS can initialize the reaction which is Langmuir-Hinshelwood mechanism in nature. Therefore, the growth rate of low miller index surface is surface sensitive. From DFT modelling in chapter 7, it is known that the symmetry breaking will take place in the room temperature only if growth rate of (110) 208 surface is slower than (100) and faster than (111). Generally, the surface reaction rate is usually in the sequence (110) > (100) > (111). This is why we usually cannot produce nanoprisms. This sequence is just in reverse of the surface energy sequence due to the surface properties of these three low miller index surfaces. For nanoprism case, experimental condition should be found to reverse the growth rate of (110) and (100). For cubic case, it is more critical to found a condition where growth rate of (100) is slowest. It is estimated that the growth rate sequence is (100) < (111) < (110) due to the appearance of truncated cubic in the growth of cubic nanocrystals. Experimentally, cubic form of silver, gold has been successfully prepared by polyol process invented by Xia Younan. However, there is no theoretical work in this respect. In this study, we only studied the nanoprism case. However, the general idea can be extended to formation mechanism of all single crystal nanocrystals although the detailed ways to exposed interesting surfaces should be depend upon the actual chemistry. For example, Ag (100) is the important surfaces for the preparation of silver nanowires and nanocubes by polyol process. For the time being, we not know why the Ag (100) is exposed. Thermodynamically, Ag (100) can be exposed by specially adsorbed species adsorption on Ag (100) if the adsorption energy on Ag (100) can compensate the surface energy difference between Ag (111) and Ag (100). Kinetically, Ag (100) can be also exposed if the surface reaction on Ag (100) can be slow enough compared to on Ag (111) and Ag (110). Surface reactions on this system have been investigated by Xia’s group. Xia’s group has realized that O2/Cl- plays important role in the selective etching of silver MTPs. The oxidation of polyol was lately discovered by his group too. Unfortunely, we not know the rate determining step of deposition silver atoms on the silver surfaces. There is another possibility, if the etching rate is fastest on Ag (100) 209 compared to Ag (111) and Ag (110), Ag (100) can be exposed too. The detailed mechanism is waiting to be explored experimentally and theoretically. However, there are some limitations in this thesis. First, the difference between triangle and hexagonal nanoprisms lie in the symmetry difference. We not touch this problem yet in this thesis. Secondly, for the nanostructures with defects such as MTPs and five twinned nanowires and nanorods, the elastic energy should be incorporated to explain the formation mechanism to geometrically construct the morphology. Lastly, exchange mechanism (gold atom exchange through solution) is not considered in this thesis. I believe this mechanism is not surface sensitive, however this mechanism could lower the overall activation barrier when the gold atom diffusion on the terrace or over edges. 210 [...]... thesis, H2O2 and O2 are employed in controlling the shapes of nanocrystals in solution growth of silver and gold nanocrystals To produce silver and gold nanoprisms, weak reductants or precursor of reducing agent should be used For example, in chapter 5, the ascorbic acid is used In chapter 4 and 6, the reductants are more diversified For example, citric acid or citrate in Chapter 4 and 6, ethanol and PVP... Calculated geometries and bonding energies of reactants and transition state and product of Reaction (1) by DMol3 Table 7-5 Calculated geometries and bonding energies of reactants and transition states and products of Reaction (1) by DMol3 Table 7-6 Summary of the barrier energy of Reaction (1) in solution and on gold surfaces Table 7-7 Attaching energies of gold atoms at different positions of Au (111) Table... 2010, 200KV, scale bar in a and b is 100 nm) image of sample A1 (a) and E1 (b) without centrifuge separation and SAED of sample A1(c) Figure 5-3 UV-Vis spectrum of silver nanocrystals The shapes of silver nanocrystals are mediated by the pH values of L-ascorbic acid Red line represents the original ascorbic acid(formula as sample E1), black line represents the pH value of ascorbic acid is adjusted... times of Samples A-E at about 30 0C Table 6-2 The information of related peaks of Fig 6.7 Table 6-3 Simulation* of SAED of sample A with planar defects** Chapter 7 Table 7-1 Summary of the calculated energy (eV) of by slab method Table 7-2 Adsorption energy of citric acid on different gold surfaces (3x3) Table 7-3 Calculated geometry and bonding energy of reactants and transition state and product of. . .of stacking faults existing in gold nanoprisms The result further confronts the role of stacking faults in formation of plate form of gold In theoretical part of this study, surface energy and adsorption energy of citric acid shows that they are not enough to explain why nanoplates are formed Ab initio modeling on formation mechanism of gold nanoprisms shows one possible pathway of formation of gold. .. that the structure of micelles control the shapes of the resulting nanocrystals 11 Cetyltrimethylammonium bromide (CTAB) is a good shape controller and has been extensively used to produce silver and gold nanocrystals with specific shapes Generally, freshly prepared small seeds of nanocrystals are added into the growth solution which contains mild reductants such as ascorbic acid and saturated CTAB... nanoprisms24-46, nanocubes47-50, nanobars51 and tetrahedron52 In the following section, a review on the synthetic methods for shape- controlled metal nanocrystals is given 1.2 Synthesis of free standing metal nanocrystals of different shapes A number of physical and chemical methods have been developed to produce free standing metal nanocrystals. 25, 27, 58, 92, 95 Noble metal nanocrystals can be fabricated by... and Au (100) Figure 7-6 Reaction pathway from bidentate reactant to product on Au (100), Au (110) and Au (111) Figure 7-7 The calculated crystal shape of gold by keep the symmetry of point group (m-3m) with R111=1.0, R110=55 and R100=233 (left), R110/R111 equal to 1.0 (right) Figure 7-8 The summary of the pathways of gold adatoms diffusing on Au (111)(upper); lower graph represents energy profile of. .. with surface science in UHV and cluster science In order to explain the morphology 3 selection of the nanocrystals prepared in colloidal systems in this work, theoretical concepts in surface science and cluster science are employed It is important to control the morphology of silver and gold nanocrystals because of application of noble nanoparticles in the required forms Silver has the strongest Surface... final shapes of nanocrystals during the subsequent growth stages One of the critical parameters influencing the growth patterns of nanocrystals is the surface energy of the crystallographic faces of the seeds It is therefore important to examine the surface energy related effects on the anisotropic growth of nanocrystals Due to the lack of any reported example in which the equilibrium crystal shape . SHAPE CONTROLLING OF SILVER AND GOLD NANOCRYSTALS ZHOU XUEDONG NATIONAL UNIVERSITY OF SINGAPORE 2008 SHAPE CONTROLLING OF SILVER AND GOLD NANOCRYSTALS. in a and b is 100 nm) image of sample A1 (a) and E1 (b) without centrifuge separation and SAED of sample A1(c). Figure 5-3. UV-Vis spectrum of silver nanocrystals. The shapes of silver nanocrystals. gold nanoplates are formed even in physical evaporating process in vacuum. In this thesis, H 2 O 2 and O 2 are employed in controlling the shapes of nanocrystals in solution growth of silver