In Situ Observation of Au Nanostructure Evolution in Liquid Cell TEM

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In-Situ Observation of Au Nanostructures Evolution in Liquid Cell TEM Ying -Chen Chen, Jui-Yuan Chen, and Wen-Wei Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07956 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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The Journal of Physical Chemistry

In-Situ Observation of Au Nanostructures Evolution in Liquid Cell TEM Ying -Chen Chen†, Jui-Yuan Chen†, and Wen-Wei Wu* Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan ABSTRACT: Gold nanostructures (NSs) have been widely investigated due to their unique properties. Understanding their growth behaviors during synthesis will be beneficial to designing and applying many functional nanodevices. It’s important to enrich the fundamental science and technology in the synthesis and characterization through real time evolution In this work, we observed the dynamic growth of Au NSs by using liquid in situ transmission electron microscopy (TEM).The solution was sealed in a liquid cell, and the results indicated that the thicker solution layer tended to form multi-twinned decahedral NSs; in contrast, nanoplates easily formed in the thinner solution layer. The silver halide model, relying on side-face structures, and the Wulff Construction can be used to explain the formation of NSs. Additionally, we analyzed the growth rate of different morphologies to elucidate their growth behaviors. The growth mechanism and formation kinetics of different shapes of Au NSs were systematically studied, which provided direct evidence toward and extended the study of reaction kinetics for modifying the morphology of NSs.

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37 38 Depending on the different synthesis methods or via adding 39 surfactant to solution, different morphologies of gold 40 nanostructures can be formed, such as multi-branch struc- 41 tures1, nanoplates2, and nanorods3. Among the many 42 nanostructures, gold nanoparticles(NPs)have been used exten- 43 sively in electronics4-5, sensors6-9, catalysis10-11, and biologi- 44 cal12-13applications because of their unique size- and shape- 45 dependent properties14, such as surface plasmon resonance, 46 surface modification, and surface effects. Typically, most gold 47 NPs are synthesized through seed-mediated growth15, electro- 48 chemical methods16-17, sonic chemical preparation18, and salt 49 19-22 ■INTRODUCTION

reduction Therefore, we select the simple and inexpensive process, salt reduction, without adding surfactant, to synthe- 50 size our gold NPs. If the growth mechanisms of NPs could be 51 fully understood, their properties could be easily controlled, 52 thus improving the ability to design and apply them to more functional nanodevices. Though some studies have thoroughly 53 investigated the variation of parameters nanoparticle synthesis, 54 the growth pathways are not fully explored and the intra- 55 synthesis process has not quantified from most exploration. In 56 this work, in situ TEM is chosen due to its real-time observa- 57 tion and extremely high resolution for nanostructures. In addi- 58 tion, in situ TEM is an important technique for observing the 59 thermodynamics and electrochemistry of nanomaterials 23-28. 60 Because of the technical limitations of TEM, previous stud- 61 ies have mainly analyzed the structure of NPs in the solid state 62 by drying and transforming them; this process may result in 63 changed structures or morphologies that differ from the 64 nanostructure suspended in liquid. To break through these 65 barriers, Parsons et al. improved vacuum systems and solved 66 the problem of the vacuum environment29. Williamson et al. 67 developed an electrochemical liquid cell with an electron- 68 transparent silicon nitride membrane window, which enabled 69 the study of reaction kinetics30. In recent years, the liquid cell 70 has been extensively used in many studies because it enables

nanomaterials to be observed in the liquid phase through TEM. It has been reported that liquid cells provide a better method to understand the synthesis process of nanostructures, such as shape control mechanisms(Au-Pd31), surfactant and steric hindrance effects (Pt-Fe32), and the variation of surface atoms (Pt33). The above studies allow us to determine liquidsolid interfaces with complex phenomena and elucidate systematic growth mechanisms. In this study, we focus on observing the dynamic growth process of gold NPs by salt reduction via in situ liquid cell TEM to verify and understand how NPs grow and deepen our understanding of the growth mechanisms. ■EXPERIMENTAL DETAILS Synthesis of Au NPs: The salt reduction method was chosen for preparation due to its low cost. Here, the precursor concentrations for synthesizing Au NPs with better dispersity were 2.5 10-4M HAuCl4 reduced by10-3M citrate acid (Figure S1). During the experiment, the precursor would be observed through in situ TEM, where the electron beam would generate heat to induce the synthesis process34. In addition, it would dissociate the solution, creating solvated electrons(eaq-) and free radicals(H- and C6H5O73-) to reduce HAuCl4. Thus, growth mechanisms of Au NPs can be investigated for designing advanced nanomaterials. Preparation and observation of the in situ TEM sample: The liquid cell for in situ observation was composed of top and bottom chips, as shown in Figure S2. A 60 nm-thick Si3N4 membrane in the middle of each chip served as an observation window, which allowed the electron beam to pass through. The bottom chip had 150 nm of gold deposited as a spacer on its surface to control the volume of the precursor solution. Subsequently, the bottom and top chips were hydrophilically treated by oxygen plasma to improve the absorbability of the solution.

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The precursors (2.5 10-4 M HAuCl4and10-3 M citrate acid) were sealed inside the cell and then subsequently loaded into a Protochips Poseidon 510 holder and the in situ TEM (JEOL JEM-2100F) under 10-5Torr with a video recorder. The synthesis process was observed at 200 kV at abeam current density of 105 A/m2. Structural and compositional analysis were performed via JEOL JEM-ARM 200F spherical aberration corrected transmission electron microscope (Cs-corrected TEM) and energy dispersive spectroscopy (EDS). ■RESULTS AND DISCUSSION

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In this work, the growth of different shapes of Au NPs and 48 aggregation of NPs were monitored via in situ observation, and will be discussed sequentially. In the nucleation process, 49 the NPs would first nucleate. Apart from using citrate acid as a 50 reductant, the electron beam irradiation would create hydrated 51 52 electrons and hydrogen radicals, with reactions as follows:

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53 54 55 Au3+ + 3 eaqAu 56 57 3+ Au + 3 H• Au 58 Au3+ + C6H5O73Au 59 60 Figure 1a−d is a series of in situ TEM images of the Au NP 61 evolution, which was taken from Movie S1. At the beginning, 62 the locally high-concentration region exhibited dark contrast 63 (Figure 1a), resulting from the difficulty of the electron beam 64 penetrating. Au NPs gradually nucleated and continued to 65 grow with different shapes by monomer attachment, as shown 66 in Figure 1b-d. Figure 1e shows the schematic diagram of 67 nucleation of the LaMer model35, which can explain the 68 growth behaviors. When the concentration of Au3+monomers 69 increased (part I) up to the critical supersaturation point(Ccrit), 70 particles quickly nucleated, and then the concentration would 71 decrease below Ccrit because of the continuous precipitation (part II). The nucleation would not continue while the remain- 72 H 2O

eaq- + H• + OH• + H2O2 + H2

ing monomers diffused to the original nuclear surface by at73 tachment for growth (part III). 74 It was reported that the electron beam would result in the 75 formation of bubbles; detailed information on this bubble for76 mation will be discussed later. These bubbles affected the 77 thickness of the liquid space, which resulted in different mor-

78 79 80 81 82 83 84 85 86 Figure 1. In situ-synthesis process of Au NPs and schematic dia87 gram of the nucleation mechanism (a−b) Incubation period and 88 gathering precursor concentration to form gold NPs. (c−d) Differ- 89 ent shapes of NPs nucleated and grown by monomer attach- 90 ment.(e)Schematic diagram of the nucleation process explained 91 by the La Mer model, indicating that the concentration of mono92 mers affected the transition in the nucleation mechanism. 93 94 95

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Figure 2. The growth process of the decahedral NPs. (a−f) A series of in situ TEM images. During the synthesis, the nanoparticles marked by the yellow arrow have isotropic growth behavior with the same morphology. (g) HRTEM image and FFT diffraction pattern with five cyclic twinning structures. Twin planes are highlighted by white lines. (h, i) The enlarged TEM image corresponds to the blue and red regions in Figure 2g. (j, k) Projective and 3D schematic diagram of the decahedral NPs consists of five tetrahedral units with {111} twinned interfaces.

phologies of NPs. The thicker solution layer where no bubbles were present tended to form multi-twinned nanostructures because the particles could grow without limitation; however, nanoplates were easily formed in the thinner solution layer where steric hindrance restricted growth, as schematically illustrated in Figure S3. Face-centered cubic (FCC) NPs prefer to minimize their free energy by developing{111}twin planes with the lowest surface energy. In addition, gold NPs have low stacking fault energy to form twins easily; therefore, the particles mainly grow with twin structures. According to the orientation of the twin planes, nanoplates would form as {111} twin planes parallel to each other, while multi-twinned nanostructures would be generated when the twin planes were not parallel36. multi--twinned nanostructures Formation of multi The formation behaviors of multi-twinned NPs are shown in Figure 2a−f. From the in situ observation, the particle isotropically grew by monomer attachment. Moreover, it maintained the same shape during the synthesis reaction (Movie S2). Figure 2g is an HRTEM image demonstrating that the particle was composed of a pentagonal cyclic twinning structure (multi-twinned) and (11 ) twin facets as a rotational symmetry axis. Additionally, Figures 2h and 2i are the enlarged TEM images of the twin nanostructures corresponding to the red and blue regions in Figure 2g. The pentagonal cyclic twinning crystal is a decahedral NP, consisting of two pentagonal cyclic twinning structures on the top and bottom sides and five tetrahedral units with {111} facets for each side. Calculations indicate that the dihedral angle between twin planes is 70.53° or 0°; here, 70.53° is consistent with our experimental results, and there is a gap resulting from the 1.47°([360-(70.53x5)]/5) angle difference between two tetrahedral units37, as shown in the projected plane view (Figure 2j). To fill these gaps to increase the stability, the lattice would expand slightly, leading to the formation of round shapes, as observed in the TEM. The decahedral NP comprised ten {111} planes, as shown in Figure 2k, which showed the lowest surface energy; therefore, it is thermodynamically favorable to form decahedral NPs.


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ble and that because of their low coordination number, their

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To summarize, as the twin planes are not parallel to each other, with anintra-plane angle of 70.53°, the 3D multitwinned NPs can be observed in the thicker solution layer. Formation of nanoplates

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In the thinner solution layer, the twin planes are parallel, 48 leading to the formation of nanoplates. Lofton et al. proposed 49 that the crystal formed a single twin plane with a concave- and 50 convex-side-face structure38, which resulted in the formation 51 of a triangular nanoplate. According to the Young − Laplace 52 formula: 53

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Figure 3. The evolution of a triangular nanoplate. (a) The thermodynamically stable truncated octahedron nucleus at the initial stage. (b) A series of in situ TEM images revealed that the nucleus grows to a triangular nanoplate. Schematic of the formation of the triangular nanoplate owing to the growth rate difference in the concave (faster growth) and convex (slower growth) side. (c) The in situ TEM images and corresponding illustration showing that the triangular plate would transform to a hexagonal nanoplate.

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54 55 the curvature of the concave side is negative, and its chemical potential is lower than the convex side, causing the con- 56 cave surfaces to be more attractive than the convex for mono- 57 mers, leading to rapid grow. The results demonstrated that a 58 single twin plane formed at the growth stage, leading to the 59 different growth kinetics for the side faces. Additionally, gold 60 nuclei were stably formed in a truncated octahedron of which 61 the 2D projection was hexagonal, as shown in Figure 3a. The 62 three concave surfaces would rapidly disappear because of 63 their faster growth rate39. Subsequently, the hexagonal nuclei 64 transformed into triangular nanoplates, as shown in Figure 65 3b.When the growth rate of a triangular nanoplate slowed due 66 to the lack of monomers, it started to change its morphology. 67 There were two main steps of Au reduction, and the ionic re- 68 69 actions are as follows40: 70 AuCl4- 2e- AuCl2- 2Cl- E = 0.926 V (1) 71 72 AuCl2- e- Au0 2Cl- E = 1.154 V (2) 73 Because the redox potential is positive, the oxidation of Au does not occur under normal conditions. However, the vertex- 74 = Ω (1/R1 1/R2)

Figure 4.The growth process of the hexagonal nanoplate.(a−f) A series of in situ TEM images. The nanoparticles marked by the orange arrow have isotropic growth behavior with the same shape. (g) Schematic diagram of the formation of the hexagonal nanoplate. Each of the faces contained concave and convex structures, leading to isotropic growth. (h) TEM images of hexagonal nanoplates and the corresponding FFT diffraction pattern.

binding force was weak, resulting in the vertex sites easily collapsing, diffusing to edge sites or reverse reacting. Unlike the Au surface atoms, the vertex sites were unstable; therefore, these sites slowly disappeared as a function time, as shown in Figure 3c. At 17 seconds, the gold monomers selectively adsorbed on the most stable {111} flat surface to grow and finally form the hexagonal nanoplate. The twin plane formation with the concave and convex side-face structure led to the different growth kinetics and then generated the triangular nanoplates. The kinetic Wulff Construction [hi(t) = λ(t)vi], where growth velocities (v) determine the shape, is consistent with the above behavior41, which means that kinetics dominate the growth. Finally, when the triangular nanoplates grew to a certain size, the vertexes began to disappear and the monomers adsorbed on the {111}c with the lowest surface energy to form hexagonal nanoplates, indicating that this behavior is thermodynamics dominant, which behavior could be explained by the Wulff Construction [hi(t) = λ(t) i],where surface free energies ( ) determine the thermostable shape.

es of triangular nanoplates disappeared over time and became round after 3seconds, as shown in the second image in Figure 75 On the other hand, if several twin planes are parallel to each other, 3c. This demonstrated that the vertex atoms were more unsta- 76 all the sides of the hexagonal nuclei contained similar grooves,

77

resulting in each face having the same growth rate, as



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52 53 54 55 56 57 58 Figure 5. The variation of diameter (nm) and growth time (s) of 59 Au nanoparticles with decahedral, triangular, hexagonal shapes. 60 The colors in each curve correspond to the marked NPs in the 61 TEM images. 62 shown in Figure 4g. We also confirmed from Figure 4a−f and 63 Movie S3that the hexagonal nuclei grew into hexagonal nano- 64 plates over time with isotropic growth. In Figure 4h, the irreg- 65

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higher current density would generate an abundance of the reducing agent, driving the reaction to the right and reducing more Au atoms. Therefore, the interfacial reaction rate is faster than the diffusion rate, which is classified as a diffusionlimited regime. When the solutes were depleted and the diffusion rate was slow to replenish them from the vicinity, the growth rate slowed over time, which was consistent with our experimental results in Figure 5. The growth kinetics equations were as follows: (r-r0) = k

(1)

log(r-r0) = log k + n log t (4) where k is the growth rate constant and n is the time exponent, which can be obtained by using linear regression as in equation (4). The value of n in different shapes is approximately equal to 0.5, also indicating that the growth behavior of Au NPs is a diffusion-limited system. Aggregation of gold nanoparticles Not only the small volume in the liquid cell but also the electrostatic attraction between the Si3N4 membrane and the aqueous solution would cause the Au NPs to compactly arrange with each other, resulting in increased compressive stress that led adjacent NPs to tend to aggregation. Figure 6 shows in situ observation of Au NP aggregation when two NPs are close to each other and then reconstruct to form a new particle. Additionally, we calculated the change of free energy in this behavior associated with thermodynamic stability, as shown in Figure S5. The particles were prone to minimize the free energy for stable states by aggregation. Even if we can observe NP growth or interactions with each other, the hydrogen bubbles generated by electron beam irradiation restrict the observing time. (Figure S6)

ular hexagonal nanoplates formed because the Au3+ monomers were insufficient or the space created a hindrance. These NPs were also analyzed by EDS to confirm their chemical composition (Figure S4). According to the report, twin structures were only observed in the lateral sites of nanoplates. In addition, the diffraction pattern verified that there were no twin spots in the top view, which is consistent with the NPs being hexagonal nanoplate structures. Growth kinetics from statistical statistical results To understand the growth kinetics, we analyzed the growth of three differently shaped NPs (decahedral, triangular and hexagonal)observed by in situ TEM. The diameter (nm) versus time (s) was plotted to investigate their growth behavior, as shown in Figure 5. It revealed that the initial growth rate was higher for each shape (part I); however, after a few seconds the growth gradually decreased, resulting in the NPs ceasing to grow (part II). The NP growth mainly followed two reactions. The nearby solutes diffused to the solid/liquid interface, and 66 then the reduction reaction caused the NP growth. The interfa67 Figure 6. (a−e) In situ TEM images of Au NP aggregation. They cial reaction was as follows:

68 69 70 According to previous literature, low beam current resulted 71 in reaction-limited growth; however, the high beam current 72 42 +3 e-

(3)

that we used in our study led to diffusion-limited growth . A

revealed that the two NPs marked by the yellow arrows were close to each other due to compressive stress, and then aggregated together leading to crystal reconstruction (red dashed line). Finally, a new shape formed. (f) HRTEM image of NP aggregation showing the atomic arrangement.


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H 2O

-

eaq + H• + OH• + H2O2 + H2

H• + H•

H2

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W.W.W. acknowledge the support by Ministry of Science and Technology through grants 104-2221-E-009 -050 -MY4.

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■REFERENCES

In liquid cell TEM, the aqueous solution would generated the gaseous products, such as H3O+,H+, OH-, H2, and H2O2. 60 The H2 maybe the main component for bubble formation. The 61 byproducts, H2(g), not only affect the Au3+ monomer diffusion, 62 63 leading to particles growing in a smaller liquid space,but also 64 restrict the observation time and resolution during the reac- 65 tion.21,43 66

67 68 ■CONCLUSIONS 69 70 In this study, we successfully observed the dynamic growth 71 process of Au NPs via liquid in situ TEM. The nucleation of 72 Au NPs followed the LaMer model, where monomers of 73 Au3+would reduce to Au0. Moreover, the growth of single 74 nanoparticles tended to form twin structures or aggregates 75 with nearby nanoparticles to minimize their free energy. In the 76 77 growth process, the multi-twinned decahedral and nanoplate 78 morphologies of NPs were analyzed from atomic structural 79 images, which were composed of different directions of twin 80 planes. The thickness of the liquid space was one of the pa- 81 rameters to affect the NP growth and resulted from either the 82 original liquid volume or was hindered by bubbles. The for- 83 mation of multi-twinned decahedral NPs would occur in the 84 thicker solution layer were no bubbles were present; because 85 of the large growth space, the dihedral angle between twin 86 87 planes was 70.53°. In contrast, nanoplates were easily formed 88 in the thinner solution layer because steric hindrance restricted 89 growth such that the twin planes were parallel to each other, as 90 explained by silver halide model. The shape of the nanoplates 91 was directly related to the (kinetic) Wulff Constructiontheory. 92 We revealed the synthesis process of Au NPs relying on diffu- 93 sion-limited growth from thermodynamic and kinetic view- 94 points, and provided direct evidence of the synthesis process 95 96 as associated with a theoretical mechanism. These experi- 97 mental results shed light on the salt reduction method. 98 99 100 ■ASSOCIATED CONTENT 101 102 Supporting Information 103 The Supporting Information is available free of charge on the104 105 ACS Publications website. SEM image and EDS analysis of Au nanoparticles, schematic of106 chips and liquid cell construction, free energy calculation of na-107 noparticles in liquid cell. In addition, three in-situ TEM videos108 (AVI) are included. This material is available free of charge via109 110 the Internet at http://pubs.acs.org. 111 ■AUTHOR INFORMATION 112 113 Corresponding Author 114 115 * Email: [email protected] 116 Author Contributions 117 † 118 These authors Y.C. Chen and J.Y. Chen contributed equally. 119 Notes 120 The authors declare no competing financial interest. 121 ■ACKNOWLEDGMENT 122 123

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(20) Bönnemann, H.; Brijoux, W.; Brinkmann, R.; Joußen, T.; 70 Korall, B.; Dinjus, E., Formation of Colloidal Transition Metals in 71 Organic Phases and Their Application in Catalysis. Angewandte 72 Chemie International Edition in English 1991, 30, 1312-1314. 73 (21) Ahmad, N.; Wang, G.; Nelayah, J.; Ricolleau, C.; 74 Alloyeau, D., Exploring the Formation of Symmetric Gold Nanostars 75 by Liquid-Cell Transmission Electron Microscopy. Nano Letters 76 2017, 17, 4194-4201. 77 (22) Park, J. H.; Schneider, N. M.; Grogan, J. M.; Reuter, M. C.; 78 Bau, H. H.; Kodambaka, S.; Ross, F. M., Control of Electron Beam- 79 Induced Au Nanocrystal Growth Kinetics through Solution 80 Chemistry. Nano Letters 2015, 15, 5314-5320. 81 (23) Lee, W.-C.; Chen, J.-Y.; Huang, C.-W.; Chiu, C.-H.; Lin, 82 T.-Y.; Wu, W.-W., Phosphorus-Doped P–N Homojunction Zno 83 Nanowires: Growth Kinetics in Liquid and Their Optoelectronic 84 Properties. Chemistry of Materials 2015, 27, 4216-4221. 85 (24) Chen, J.-Y.; Huang, C.-W.; Chiu, C.-H.; Huang, Y.-T.; Wu, 86 W.-W., Switching Kinetic of Vcm-Based Memristor: Evolution and 87 Positioning of Nanofilament. Advanced Materials 2015, 27, 5028- 88 5033. 89 (25) Hsieh, T.-H.; Chen, J.-Y.; Huang, C.-W.; Wu, W.-W., 90 Observing Growth of Nanostructured Zno in Liquid. Chemistry of Materials 2016, 28, 4507-4511. (26) Huang, C.-W.; Chen, J.-Y.; Chiu, C.-H.; Hsin, C.-L.; Tseng, T.-Y.; Wu, W.-W., Observing the Evolution of Graphene Layers at High Current Density. Nano Research 2016, 1-8. (27) Sutter, E. A.; Sutter, P. W., Determination of Redox Reaction Rates and Orders by in Situ Liquid Cell Electron Microscopy of Pd and Au Solution Growth. Journal of the American Chemical Society 2014, 136, 16865-16870. (28) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E., ShapeControlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angewandte Chemie International Edition 2009, 48, 60-103. (29) Parsons, D., Structure of Wet Specimens in Electron Microscopy. Science 1974, 186, 407-414. (30) Williamson, M. J.; Tromp, R. M.; Vereecken, P. M.; Hull, R.; Ross, F. M., Dynamic Microscopy of Nanoscale Cluster Growth at the Solid-Liquid Interface. Nat Mater 2003, 2, 532-536. (31) Jungjohann, K. L.; Bliznakov, S.; Sutter, P. W.; Stach, E. A.; Sutter, E. A., In Situ Liquid Cell Electron Microscopy of the Solution Growth of Au–Pd Core–Shell Nanostructures. Nano Letters 2013, 13, 2964-2970. (32) Liao, H.-G.; Zheng, H., Liquid Cell Transmission Electron Microscopy Study of Platinum Iron Nanocrystal Growth and Shape Evolution. Journal of the American Chemical Society 2013, 135, 5038-5043. (33) Jeong, M.; Yuk, J. M.; Lee, J. Y., Observation of Surface Atoms During Platinum Nanocrystal Growth by Monomer Attachment. Chemistry of Materials 2015, 27, 3200-3202. (34) Liang, W.-I.; Zhang, X.; Bustillo, K.; Chiu, C.-H.; Wu, W.W.; Xu, J.; Chu, Y.-H.; Zheng, H., In Situ Study of Spinel Ferrite Nanocrystal Growth Using Liquid Cell Transmission Electron Microscopy. Chemistry of Materials 2015, 27, 8146-8152. (35) Pong, B.-K.; Elim, H. I.; Chong, J.-X.; Ji, W.; Trout, B. L.; Lee, J.-Y., New Insights on the Nanoparticle Growth Mechanism in the Citrate Reduction of Gold(Iii) Salt: Formation of the Au Nanowire Intermediate and Its Nonlinear Optical Properties. The Journal of Physical Chemistry C 2007, 111, 6281-6287. (36) Alloyeau, D.; Dachraoui, W.; Javed, Y.; Belkahla, H.; Wang, G.; Lecoq, H.; Ammar, S.; Ersen, O.; Wisnet, A.; Gazeau, F., Unravelling Kinetic and Thermodynamic Effects on the Growth of Gold Nanoplates by Liquid Transmission Electron Microscopy. Nano letters 2015, 15, 2574-2581. (37) Bao, S.; Zhang, J.; Jiang, Z.; Zhou, X.; Xie, Z., Understanding the Formation of Pentagonal Cyclic Twinned Crystal from the Solvent Dependent Assembly of Au Nanocrystals into Their Colloidal Crystals. The Journal of Physical Chemistry Letters 2013, 4, 3440-3444.

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For TOC only


7


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TOC 327x138mm (150 x 150 DPI)


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Figure 1. In situ-synthesis process of Au NPs and schematic dia-gram of the nucleation mechanism 293x135mm (150 x 150 DPI)



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Figure 2. The growth process of the decahedral NPs. 217x132mm (150 x 150 DPI)


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Figure 3. The evolution of a triangular nanoplate. 149x96mm (150 x 150 DPI)



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Figure 4. The growth process of the hexagonal nanoplate. 125x151mm (150 x 150 DPI)


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Figure 5. The variation of diameter (nm) and growth time (s) of Au nanoparticles with decahedral, triangular, hexagonal shapes. 121x155mm (150 x 150 DPI)



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Figure 6. In situ TEM images of Au NP aggregation. 167x164mm (150 x 150 DPI)


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In Situ Observation of Au Nanostructure Evolution in Liquid Cell TEM

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