One-Step Synthesis of Tunable-Size Gold Nanoplates on Graphene

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One-Step Synthesis of Tunable-Size Gold Nanoplates on Graphene Multilayers Wenbo Xin, Joseph Severino, Igor De Rosa, Dian Yu, Jeffrey Mckay, Peiyi Ye, Xunqian Yin, Jenn-Ming Yang, Larry Carlson, and Suneel Kodambaka Nano Lett., Just Accepted Manuscript • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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Nano Letters

One-Step Synthesis of Tunable-Size Gold Nanoplates on Graphene Multilayers

Wenbo Xin,a Joseph Severino,a Igor M. De Rosa,a,b,* Dian Yu,a Jeffrey Mckay,a Peiyi Ye,a Xunqian Yin,c Jenn-Ming Yang,a Larry Carlson,b and Suneel Kodambakaa,*

a

Department of Materials Science and Engineering, University of California, Los Angeles,

410 Westwood Plaza, Los Angeles, California 90095 USA

b

Institute for Technology Advancement, University of California, Los Angeles, 410

Westwood Plaza, Los Angeles, California 90095 USA

c

School of Materials Science and Engineering, Shandong University of Science and

Technology, 579 Qianwangang Rd., Economic & Technological Development Zones, Qingdao, Shandong 26650 China

* Corresponding Authors

ABSTRACT

Au nanoplates – quasi two-dimensional single-crystals – are most commonly synthesized using a mixture of Au precursors via approaches involving multiple processing steps and/or the use of seed crystals. Here, we report the synthesis of truncated-hexagonal {111}-oriented micrometer-scale Au nanoplates on graphene multilayers using only

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potassium tetrabromoaurate (KAuBr4) as the precursor. We demonstrate that the nanoplate sizes can be controllably varied from tens of nanometers up to a few micrometers by introducing desired concentrations of chloroauric acid (HAuCl4) to KAuBr4 and their thicknesses from ~ 13 nm to ~ 46 nm with the synthesis time. Through a series of experiments carried out as a function of synthesis time and precursor composition [mixtures of HAuCl4 and KAuBr4, KBr, or ionic liquid 1-butyl-3methylimidazolium bromide ([Bmim]Br)]: we identify the optimal HAuCl4 and KAuBr4 concentrations and synthesis times that yield the largest and the thinnest size nanoplates; we show that the nanoplates are kinetically-limited morphologies resulting from preferential growth of {111} facets facilitated by bromide ions in KAuBr4 solutions; we suggest that the presence of chloride ions enhances the rate of Au deposition and the relative concentration of chloride and bromide ions determines the shape anisotropy of resulting crystals. Our results provide new insights into the kinetics of nanoplate formation and show that a single precursor containing both Au and Br is sufficient to crystallize nanoplates on graphitic layers, which serve as reducing agent while enabling the nucleating and growth of Au nanoplates. We suggest that a similar approach may be used for the synthesis of nanoplates of other metals on weakly interacting van der Waals layers for potentially a variety of new applications. KEYWORDS: Gold nanoplates, one-step synthesis, KAuBr4, tunable size, graphene

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Over the past decade, two-dimensional (2D) layered graphene has attracted a lot of attention due to its exceptional properties, including high optical transparency,1 excellent mechanical properties,2 and superior thermal conductivity.3 Owing to its 2D structure, high specific surface area (~2630 m2/g),4 and chemical stability, graphene is also considered as an ideal platform for building multi-functional heterostructures and nanocomposites

for

applications

such

as

electrochemical

sensors,5

ultra-fast

photodectors,6 highly efficient catalysts,7 and Raman signal enhancers.8 One such heterostructure system is graphene (and graphene oxide) supported Au nanocrystals.9-11 Among a variety of graphene-supported nanocrystal morphologies – particles,12 dendritic flowers,13 rods,14,15 stars,16 and bipyramids17 – {111}-oriented Au nanoplates [thin (nanometer-scale) crystals with relatively larger {111} facets] on the graphitic support have been found to exhibit superior catalytic activity,10 super-lubricating behavior,11 and tip- and surface- enhanced Raman scattering.18,19 Given that the chemical, optical, and electronic properties are sensitive to the shape and size of the nanocrystals,20-23 development of synthesis approaches that enable control over the morphologies at the nanoscale are highly desirable.24 For the synthesis of Au nanoplates, chloroauric acid (HAuCl4) is the most commonly used Au precursor and solutions containing iodide or bromide ions are added to facilitate anisotropic morphologies;19,25-30 these approaches typically involve multiple processing steps, the use of seed crystals, and/or irradiation using energetic electron beams.31 Graphene and graphene oxide supported Au and Ag nanoplates32-34 have been prepared via annealing of gold thin films35 and nanoparticles.36 Studies have shown that spontaneous redox reactions involving galvanic displacement between Au3+ in the

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precursor and sp2–bonded C in carbon structures32,37-39 and Au-graphene interfacial interactions, while weak, are both expected to influence the morphologies of Au crystals on graphene.10,32 The graphene layers not only provide nucleation sites and support for the Au nanoplates, but also act as reduction agent. The latter function of graphene removes the need for an addition reduction step and thus simplifies the synthesis process. We take advantage of these aspects and propose the use of potassium tetrabromoaurate (KAuBr4), as the sole precursor containing both Au and Br, for the formation of Au nanoplates on graphene multilayers. Here, we present a direct one-step approach for synthesis of {111}-oriented Au nanoplates with truncated hexagonal shapes on graphene layers using KAuBr4. We demonstrate that lateral sizes of the nanoplates can be tuned from ~ 27 nm up to a maximum of ~ 4.9 µm by adding desired concentrations of HAuCl4 to KAuBr4. We find that their thicknesses can be increased from ~ 13 nm to ~ 46 nm by increasing the synthesis time t from 2 h to 48 h. Furthermore, our studies indicate that these highly anisotropic nanoplate morphologies are not stable during synthesis. Based upon our results, we identify the optimal HAuCl4 and KAuBr4 concentrations and synthesis times that yield the largest and the thinnest size nanoplates. By comparing our synthesis approach with the Au nanocrystals obtained using HAuCl4 and ionic liquid 1-butyl-3methylimidazolium bromide ([Bmim]Br) mixtures, we suggest that the nanoplate morphologies observed with KAuBr4 are due to the presence of bromine in the precursor. The presence of chloride ions enhances the rate of Au deposition and the relative concentration of chloride and bromide ions determines the shape anisotropy of resulting crystals. Our results, which provide new insights into the kinetics of nanoplate formation,

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show that a single precursor containing both Au and Br is sufficient to crystallize nanoplates on graphitic layers. All of our experiments are carried out using the following commercially available materials:

potassium

tetrabromoaurate

hydrate

(KAuBr4·xH2O,

99.9%

purity),

chloroauric acid (HAuCl4·xH2O, 99.999% purity), potassium bromide (KBr, 99.99%), and 1-butyl-3-methylimidazolium bromide ([Bmim]Br, > 97.0% purity) are purchased from Sigma-Aldrich, USA; multilayer graphene flakes with average thickness ~ 8 nm and average size ~ 5 µm (commercially referred to as Grade AO-2 graphene nanopowder) are purchased from Graphene Supermarket (Calverton, New York). This particular grade of graphene flakes is chosen because they are available in large quantities (50 g samples), possess high surface area (≥ 15 m2/g), and hence are ideal platforms for large-scale synthesis of Au nanoplates. The as-received graphene flakes and the graphene – Au nanoplate heterostructures synthesized in our experiments are characterized using combination of scanning electron microscopy (SEM) coupled with energy dispersive xray spectroscopy (EDX), transmission electron microscopy (TEM), Raman spectroscopy, atomic force microscopy (AFM), and x-ray diffraction (XRD) techniques. All the details of the material characterization procedures are presented in the Supplemental Information (SI). The as-received graphene flakes are characterized using scanning and transmission electron microscopies (SEM and TEM) and Raman spectroscopy and the representative data are presented in Figure S1. In the Raman spectrum, we observe peaks characteristic of graphene layers (and graphite) at ~ 1575 cm-1 and ~ 2639 cm-1,

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commonly referred to as G and G' (or 2D), respectively. The presence of an additional peak labeled D at ~ 1314 cm-1 is attributed to defects in the graphene layers.40 Figure S2 is a schematic of our procedure for the synthesis of multilayer graphene – Au nanoplate heterostructures using either pure KAuBr4 solution or a mixture of KAuBr4 and HAuCl4 solutions. Our approach relies on the spontaneous redox reaction between gold precursor(s) and graphene.32 We note that in the absence of graphene layers, we do not observe any Au deposition. First, 10 mg of AO-2 graphene powder is added to 40 mL of deionized water and probe ultrasonicated (for details, please see SI) to obtain a homogenous dispersion of graphene powder in water. 1 g of the as-prepared multilayer graphene flake suspension is injected into a glass vial containing 4 mL of deionized water. 240 µL of 1 mM Au precursor (either pure KAuBr4 or KAuBr4 + HAuCl4 mixture) solution is then added and the solution is gently shaken for about 1 minute, after which the glass vial is sealed, heated to 80 oC in an oven, and held at the same temperature for times t up to 48 h. The final product is collected by centrifuge, washed thoroughly with water, and subsequently dried overnight in the oven at 100 oC. In order to obtain Au nanoplates with different sizes, precursor solutions are prepared with varying amounts of 1 mM HAuCl4 and 1 mM KAuBr4 solutions. For comparison across the samples, the total concentration of gold is maintained constant in the mixtures. In our experiments, the total volume of KAuBr4 (1 mM) and HAuCl4 (1 mM) solutions is always 240 µL, and the relative concentrations R of KAuBr4 and HAuCl4 in the precursor solutions are 1:15, 1:5, 1:2, 1:1, 2:1, and 5:1. The relative amounts of solutions used to prepare these mixtures are listed in Table S1. The synthesis time t of all samples presented here is 5 h, unless otherwise stated.

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The synthesis of Au nanocrystals using HAuCl4 and ionic liquid [Bmim]Br or KBr mixtures is carried out as follows. 1 g of multilayer graphene flake suspension is added to a glass vial containing 4 mL of deionized water. Varying amounts of [Bmim]Br or KBr (please see Table S1 for details) are injected into the vials, followed by 20 s probe ultrasonication. After the ultrasonication, 240 µL of HAuCl4 (1 mM) are added to each vial, and the mixtures are maintained at 80 oC for t = 5 h. The resulting samples are collected following a similar procedure as mentioned above. We first demonstrate the synthesis of Au nanoplates using only KAuBr4 as the gold precursor. Figures 1(a) and 1(b) are representative SEM images of the assynthesized multilayer graphene - Au nanoplate assemblies obtained after t = 5 h. We observe well-dispersed Au nanoplates with truncated hexagonal and triangular shapes [see Figure 1(a)]. Additional SEM images of individual and stacks of Au nanoplates are shown in Figures S3(a) and S3(b). In this experiment, the nanoplate yield (defined as the number fraction of nanoplates compared to other morphologies) is 85 %, the average size is 1.7 ± 0.1 µm, and the average thickness is 19 ± 3 nm. (We define the term "size" as the normal distance from the base to the apex in the triangular plates, and the normal distance between any two parallel edges in hexagonal plates.) We show later that the yield of nanoplates depend on the synthesis time t. Figure 1(b) is a higher magnification SEM image of the nanoplates. We find that these nanoplates are electron-transparent, indicative of their nanoscale thickness. Figure 1(c) is an AFM image of one of the truncated hexagonal Au nanoplates shown in Figure 1(a). From the surface height profile plotted in Figure 1(d), we determine the Au nanoplate thickness as ~15 nm, consistent with the thickness values measured from the SEM images. Furthermore, the surface

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height profile in Figure 1(d) reveals that the nanoplate surface is not flat. Similar bent morphologies are observed in the SEM images of the nanoplates, see for example Figure S3(a). The observed deformation of the nanoplates is likely due to their ultrathin geometry and may occur during or upon transfer of the sample from the solution to the substrate. We determined the composition of Au nanoplates using EDX. Figure S4 shows typical EDX data obtained from the multilayer graphene – Au nanoplate heterostructures samples using pure KAuBr4 solution. Elemental composition maps acquired from the heterostructures reveal the presence of Au and carbon uniformly distributed within and outside the nanoplate, respectively. Similar EDX data (not shown) obtained from all the nanoplate samples prepared using different precursor ratios R indicated that the nanoplates are made of Au. The absence of any peaks due to Br or Cl suggest that their concentrations in our samples are lower than the detection limits in our EDX data. Crystal structure and crystallinity of the Au nanoplates are determined using XRD and TEM. Figure S5 is a typical 2θ-ω XRD scan obtained from multilayer graphene – Au nanoplate powder sample obtained using KAuBr4 as the only Au precursor. We observe peaks at 2θ = 38.2o, 44.5o, 77.8o, and 82.1o due to {111}, {200}, {311}, and {222} reflections, respectively due to face-centered cubic (fcc) Au. (A shoulder peak at 2θ around 37o may correspond to close-packed hexagonal (hcp) structured Au.41 However, we did not observe hcp-structured nanocrystals in our TEM studies, suggesting that the yield of hcp-Au nanocrystals, if any, is considerably lower than the fcc-Au nanoplates.) We find that the {111} peak has the highest intensity and the ratio of {200} and {111} peak intensities is 0.06, significantly lower than 0.52, expected for a polycrystalline bulk

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Au (from JCPDS #04-0784). Intensities of peaks due to other reflections are either weak (e.g., {311}) or absent (e.g., {220}). All of these results indicate that {111}-oriented Au crystals are predominant in our samples. We attribute the observation of all the reflections other than {111} to off-normal orientations of nanoplates with respect to the incident x-rays and the presence of a small fraction of fcc-Au crystallites with other morphologies. This is plausible since the XRD data is acquired from a fairly large (mmscale) powder sample. We provide direct evidence below that the Au nanoplates are fccstructured {111}-oriented single-crystals. In order to determine the crystallinity of individual Au nanoplates, we used TEM. Figure 2 shows representative TEM data obtained from nanoplates grown in pure KAuBr4 [top panel, Figures 2(a)-(c)] and using KAuBr4 and HAuCl4 mixtures at R = 1:1 [bottom panel, Figures 2(d)-(f)]. Figures 2(a) and 2(d) are typical bright field TEM images of the Au nanoplates. Contrast variations and bend contours visible within the nanoplates are due to bending of the ultrathin crystals. SAED pattern in Figure 2(b), acquired from the nanoplate in Figure 2(a), reveals two sets of six-fold symmetric spots. From the measurements of spot spacings and their relative orientations, we identify the stronger and weaker intensity spots as {220} and 1/3{422} Bragg reflections, respectively in {111}-oriented fcc Au crystal. The fact that all other reflections are absent in the SAED pattern indicates that the nanoplate is a single-crystal. Figure 2(c) is a HRTEM image of the nanoplate and the inset in Figure 2(c) is a Fourier transform of the image. Figures 2(e) and 2(f) show similar data obtained from the nanoplates grown using R = 1:1. In the Fourier transforms shown as insets in Figures 2(c) and 2(e), we find only those spots corresponding to {220} and 1/3{422} reflections of fcc-Au, indicating that

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the nanoplate is {111}-oriented Au single-crystal. Both the HRTEM images reveal sixfold symmetric structure with an interplanar spacing of ~ 2.50 Å, which is the same as that of 1/3{422} planes in fcc-Au. We note that the {422} reflections are forbidden in fcc structure but have been observed in electron diffraction patterns of ultrathin fcc crystals.42,43 These results are typical of all the truncated hexagonal and triangular nanoplates (see, for example, Figure S6) obtained using pure KAuBr4 or KAuBr4 + HAuCl4 mixtures at R ≥ 1:5 as the Au precursors, and indicate that the crystallinities of Au nanoplates are all qualitatively similar, irrespective of R. We now focus on tuning the size of the Au nanoplates. To this purpose, we introduce a second gold precursor, HAuCl4, and synthesize Au nanoplates using different relative concentrations of the two gold precursors, KAuBr4 and HAuCl4 (see Table S1 for details). In these experiments, the total volume of the resulting nanoplates, a measure of the Au deposition rate, estimated from SEM images of nanoplates is, within the measurement uncertainties, nearly the same (see Table S2). Figures 3(a)-(f) are representative SEM images of the samples obtained using solutions with varying KAuBr4:HAuCl4 ratios R. For R = 1:15, the resulting product is primarily composed of Au nanoparticles as seen in Figure 3(a). TEM characterization [see Figures S7(a) and (b)] of this sample, however, revealed that about 20% of the sample is truncated triangularshaped Au nanoplates with sizes between 10 nm and 40 nm. (One notable difference between these smaller-size nanoplates and all the other larger nanoplates is the absence of any evidence of 1/3{422} planes in the TEM data, which we attribute to increased thickness in the nanoplates grown using R = 1:15.) While the identification of these nanoplates obtained with R = 1:15 in the SEM images is difficult due to their small-size,

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the nanoplate morphologies are clearly evident in the SEM images [Figures 3(b)-(f)] of samples obtained using R = 1:5 and higher. Figure 3(g) shows size distributions of the nanoplates for four different values of R. We find that the nanoplate sizes vary from tens of nanometers at R = 1:15 up to a few micrometers at R = 1:2 and higher. Figure 3(h) is a plot of the average size of nanoplates as a function of R, which shows that the average sizes increase initially with increasing R from 27 ± 9 nm at R = 1:15 up to a maximum of 4.9 ± 0.6 µm at R = 1:1. Increasing R beyond 1:1 leads to a decrease in the nanoplate sizes to 2.6 ± 0.4 µm at R = 2:1 and 1.8 ± 0.3 µm at R = 5:1 [see Figures 3(e) and (f)]. We know that the average size of Au nanoplates obtained with only KAuBr4 as the Au precursor, i.e. R = ∞, is 1.7 ± 0.1 µm (see Figure 1). Therefore, we expect that synthesis of Au nanoplates using R > 5:1 will yield Au nanoplates of sizes ≳ 1.7 µm. These results clearly demonstrate that Au nanoplate sizes can be controllably tuned over two orders of magnitude, from tens of nanometers up to a few micrometers, by using desired ratio R of the two Au precursors, KAuBr4 and HAuCl4. We also note that the nanoplates synthesized using KAuBr4 + HAuCl4 mixtures are all considerably thicker with average thicknesses between ~ 39 nm (R = 2:1) and ~ 56 nm (R = 1:5) than the nanoplates (thickness ~ 19 nm) obtained using pure KAuBr4; we did not find any correlation between the thickness and R (see Table S2). Since the nanoplate thicknesses appear to be independent of R, the variation in average volumes [∝ thickness×(size)2] of the nanoplates with R will be qualitatively similar to the lateral sizes vs. R behavior observed in Figure 3(h). That is, the nanoplate volume increases with increasing R up to 1:1 and then decreases to an R-independent value at some R ≥ 1:5.

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Using the lateral size data in Figure 3 and the thickness values in Table S2, we estimate that the volumes of Au nanoplates synthesized using R values between 1:5 and 5:1 are all higher than the volume of nanoplates obtained with pure KAuBr4. These results indicate that the addition of HAuCl4 to KAuBr4 increases the amount of Au in the nanoplates, and the deposited amount depends on the ratio of KAuBr4 and HAuCl4 precursors. Based upon prior literature,39 we expect that gold is directly reduced on the graphene surface, and the freshly deposited Au atoms nucleate and grow into nanoplates on the graphene layer. However, the role of defects in the graphene layers on the nucleation and growth of Au nanoplates is not clear and is beyond the scope of this manuscript. We now focus on understanding the role of KAuBr4 on the evolution of nanoplate morphology. Previous studies30,44,45 have shown that surfactants and the presence of bromide ions facilitate anisotropic growth of noble metal crystals such as ultrathin Au nanoplates, similar to those observed in our experiments. Therefore, it is plausible that KAuBr4 serves as both the source of Au and as a promoter of anisotropic morphologies. As a first step toward understanding the role of KAuBr4, we investigated Au nanocrystal synthesis using only HAuCl4 as the Au precursor without any KAuBr4, i.e. R = 0. We obtain one-dimensional nanowire-like crystals [see Figure S8(a)]. We did not observe any 2D nanoplate morphologies in these samples. Clearly, in our experiments, pure HAuCl4 solution and graphene multilayers are not sufficient to form large nanoplates. In order to determine whether potassium ions or bromide ions promote nanoplate synthesis, we carried out synthesis experiments using HAuCl4 as the Au precursor

with

1)

a

K-free,

Br-containing

12

precursor,


namely

1-Butyl-3-

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methylimidazolium bromide ([Bmim]Br), a commonly used water-soluble ionic liquid that readily releases Br¯ ions in the solution,46,47 and 2) KBr. Table S1 lists the relative concentrations of HAuCl4, KBr, and [Bmim]Br used in our experiments. Figures S8(b) and (c) show the morphologies of Au nanocrystals synthesized using HAuCl4 with [Bmim]Br and KBr, respectively. Both these samples reveal Au nanoplates. These results are qualitatively similar to the effect of KAuBr4, which when added to HAuCl4 (e.g., R = 1:15 and higher) leads to plate-like shapes seen in Figures 3 and 4. Our results provide further evidence in support of the existing idea19,26-30 and clearly demonstrate that Brions, irrespective of the source, whether introduced as an additive ([Bmim]Br), KBr, or as part of the Au precursor (KAuBr4), promote the growth of Au nanoplates. In order to better understand the kinetics of Au nanoplate growth using KAuBr4 as the Au precursor, we studied their morphological evolution as a function of synthesis time t. Figures 4(a)-(d) are SEM images acquired from samples grown for t = (a) 2 h, (b) 5 h, (c) 20 h, and (d) 48 h. (Additional images are shown in Figures S9(a)-(d).) The insets in Figure 4 are cross-sectional views of representative nanoplates obtained in these experiments, which show thicknesses of the nanoplate We observe both nanoplate and nanoparticle morphologies in samples obtained for t as short as 2 h. The fact that nanoparticles are observed even at shorter synthesis times suggest that they are stable and that they can co-exist with nanoplates. While the mechanisms leading to nanoparticle formation, stability, and time-dependent morphological evolution are not clear, we find that the yield of nanoplates, calculated based on their areal coverage in the SEM images increases with increasing t, see Figure S9(e). Interestingly, nanoplates with corrugated edges are observed at shorter t as shown in Figure 4(a); more regular shaped nanoplates

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with smoother edges are observed in longer t samples. The t-dependent changes in nanoplate sizes and thicknesses are plotted in Figure 4(e). We find that the thinnest (~ 13 nm) and the largest (~ 1.9 µm) nanoplates are obtained for t ≤ 2 h. With increasing t from 2 h to 48 h, the nanoplate sizes decrease by ~ 20 % to ~ 1.5 µm while their thicknesses increase over three-fold to ~ 46 nm. The decrease in lateral size of the nanoplates with increasing t implies that the nanoplates are not stable during synthesis and can happen due to one or both of the following processes: 1) loss of Au atoms from the nanoplates to the solution, i.e. dissolution of the nanoplates and 2) diffusion of the Au atoms from the nanoplate edges to the {111} facets resulting in relatively more compact shapes. The dissolution of nanoplates is expected if the chemical potential of Au in the nanoplate is larger than that of Au in the solution. However, the average volume of nanoplates monotonically increases ≈ 2.8× with increasing t from 2 h to 48 h, i.e. the rate of deposition of Au onto the nanoplates is more than the rate of dissolution. Therefore, we rule out nanoplate dissolution as a dominant factor and suggest that the observed decrease in lateral size of the nanoplates is primarily due to rearrangement of Au atoms in the nanoplates. Moreover, the observed non-linear t-dependent change in the sizes is consistent with the curvature-driven mass transport associated with surface morphological evolution as a means to minimize surface energy.48 (This is possible since the nanoplates exhibit regular three-fold symmetric triangular or truncated hexagonal shapes, indicative of sufficiently high mobilities of Au atoms along the edge facets during the synthesis.) We also note that both the thickness and volume of the nanoplates increase at nearly the same rate with increasing t. That is, the increase in nanoplate thickness is predominantly due to direct

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deposition of Au from the solution, rather than diffusion of Au atoms from the nanoplate edges, onto {111} facets. Based upon our observations, we propose the following mechanism for the synthesis of Au nanoplates using KAuBr4. We know that Br¯ ions released during Au3+ → Au0 transformation preferentially adsorb on the {111} facets of as-nucleated Au crystallites and prevent growth along 〈111〉.44,45 We assume that sticking coefficient of Br¯ ions is highest on {111} and lowest along the edge facets bounding the nanoplates; we further assume that the nucleation rate of Au crystallites on Br-passivated {111} is significantly smaller than on Br-free edge facets. Consequently, at the early stages of synthesis, i.e. short t, growth occurs predominantly along directions other than 〈111〉 and result in ultrathin nanoplates with large {111} facets. With increasing t, the likelihood of Br-passivation of the edge facets, nucleation of Au islands on the {111} surfaces, and morphological changes associated with surface energy minimization will also increase. The consequence of these three processes is the suppression of lateral growth and enhanced growth normal to {111}, i.e. growth of thicker nanoplates with similar or reduced sizes as observed in Figure 4(e). Finally, we focus on the role of Cl¯ ions during the synthesis of Au nanoplates using KAuBr4 solutions. As mentioned earlier, the nanoplates synthesized in KAuBr4 + HAuCl4 precursor mixtures are ~2× to ~3× thicker and contain more Au than those obtained using pure KAuBr4 for the same synthesis times. Clearly, more Au is being deposited onto the nanoplates in KAuBr4 + HAuCl4 precursor mixtures. Therefore, we suggest that the presence of Cl¯ ions enhances the rate of Au deposition, preferentially on {111} facets. The extent of Au deposition along the nanoplate edges vs. on the surfaces is,

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however, determined by the relative concentrations of Br¯ and Cl¯ ions. In solutions with R = 1:15, the presence of Br¯ ions promotes the formation of nanoplates but the relatively higher concentration of Cl¯ ions leads to near-isotropic growth. As a result, we obtain a higher fraction of compact shapes and a lower yield of nanoplates that are thicker and smaller than those obtained at higher R. With increasing R ≥ 1:5, Br¯ ion concentration is sufficient to facilitate preferential growth of Au{111} facets resulting in larger nanoplates, while the presence of Cl¯ ions leads to increased thickness compared to the nanoplates grown using pure KAuBr4. Our data suggest that there is an optimal concentration of Br¯ and Cl¯ ions required for the growth of largest size nanoplates, which in our experiments is achieved with R = 1:1. Increasing the Br¯ ion concentration beyond this value likely affects the stability of Au0 in favor of [AuBr2]¯ radicals due to which we observe relatively smaller size nanoplates. In summary, we report a facile, one-step approach for the synthesis of Au nanoplates with tunable sizes on multilayer graphene sheets with dual gold precursors (KAuBr4 and HAuCl4). We demonstrate that KAuBr4 is an appropriate single precursor for the growth of large, {111}-oriented, single-crystalline Au nanoplates. In addition, we show that sizes of the Au nanoplates can be controllably increased from tens of nanometers up to a few micrometers by varying the relative concentrations of the two precursors. Through a series of synthesis experiments carried out by systematically changing the precursor chemistry and reaction time, we suggest that Br¯ ions promote the development of nanoplate morphologies while the Cl¯ ions increase the Au deposition rate and suppress anisotropic growth. We expect that our approach can be extended for the synthesis of a variety of metallic nanoplates and other highly anisotropic shapes with

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desired sizes on graphene and other two-dimensional layered substrates with potential applications in catalysis, sensing, tribology, and plasmonics.

Conflict of Interest The authors declare no competing financial interest.

Supporting Information Details of experimental and characterization procedures; SEM and TEM images and Raman spectrum of as-received multilayer graphene (AO-2); Schematic of the synthesis of Au nanoplates on multilayer graphene sheets with tunable sizes; SEM images of multilayer graphene - Au nanoplate assemblies; Compositional analysis of Au nanoplates on graphene; XRD spectrum of Au nanoplates on graphene multilayers. TEM images of a triangular Au nanoplate on multilayer graphene synthesized using KAuBr4 as the Au precursor; TEM characterization of multilayer graphene - Au nanostructures obtained using the two precursors, KAuBr4 and HAuCl4, at a ratio R = 1:15; Typical SEM images acquired from multilayer graphene - Au nanostructure samples prepared with HAuCl4 + ionic liquid [Bmim]Br and HAuCl4 + KBr mixtures; SEM images and plot of Au nanoplate yield in KAuBr4 as the Au precursor as a function of synthesis time; Table showing a list of KAuBr4, HAuCl4, KBr, and [Bmim]Br concentrations used in the preparation of Au nanoplate samples. Table showing the procedure used for the estimation of Au nanoplate yield as a function of KAuBr4/HAuCl4 ratio R.

Acknowledgments

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We gratefully thank the Department of Defense (Grant # 000-16-C-0081) for generous financial support. We would like to acknowledge Electron Imaging Center for NanoMachines at California Nanosystem Institute for microscopy analysis. We particularly thank Ivo Atanasov, Noah Bodzin, and Sergey Prikhodko for helpful discussions regarding TEM and SEM characterizations.

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List of Figures

Figure 1. Morphological characterization of Au nanoplates on multilayer graphene grown with KAuBr4 as the gold precursor. The synthesis time t is 5 h. (a, b) Representative scanning electron microscopy (SEM) images of Au nanoplates on multilayer graphene. (c) Typical atomic force microscopy (AFM) image of a Au nanoplate supported on an oxidized Si(001) substrate. The nanoplate edge contrast is enhanced for clarity. (d) Surface height profile measured along the red dashed line in (c).

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Figure 2. Transmission electron microscopy (TEM) characterization of the multilayer graphene – Au nanoplate heterostructures obtained synthesized using (top panel) pure KAuBr4 and (bottom panel) a mixture of KAuBr4 and HAuCl4 at ratio R = 1:1 with t = 5 h. Top panel: (a) Representative bright field TEM image of a Au nanoplate from the sample shown in Figure 1. (b) Selected area electron diffraction (SAED) pattern of the nanoplate in (a). (c) High-resolution TEM (HRTEM) image of the region highlighted by a yellow box in (a). Inset is a Fourier transform of the image in (c). Bottom panel: (d) Bright-field TEM image of an individual Au nanoplate. (e) A higher magnification TEM image of the region within the yellow box in (d). Inset in (e) is a Fourier transform and (f) a HRTEM image of the region within the blue box. Spots corresponding to (220) and forbidden reflections (422) in face-centered cubic (fcc) Au lattice are labeled as shown. The measured spacing within any two white parallel lines is ~2.5 Å, which corresponds to the 1/3{422} interplanar spacing in fcc-Au.

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Figure 3. Effect of precursor composition on the size of the Au nanoplates. (a-d) Typical SEM images of Au nanoplates on multilayer graphene synthesized using KAuBr4 and HAuCl4 precursors at KAuBr4:HAuCl4 concentration ratios R = (a) 1:15, (b) 1:5, (c) 1:2, (d) 1:1, (e) 2:1, and (f) 5:1. (g) Plot showing size distributions of Au nanoplates obtained using four different concentration ratios R of the KAuBr4 and HAuCl4 precursors. (h) Average size of the Au nanoplates plotted as a function of R.

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Figure 4. Effect of synthesis time t on the Au nanoplate morphologies obtained using only KAuBr4 as the precursor. (a-d) Plan-view SEM images of Au nanoplates obtained with a synthesis time t = (a) 2 h, (b) 5 h, (c) 20 h, and (d) 48 h. Insets are cross-sectional views of individual nanoplates in each of the as-synthesized samples, whose thicknesses are labeled as shown. (e) Plot showing Au nanoplate size and thickness vs. t.

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Table of Content (TOC) graphic 328x151mm (96 x 96 DPI)


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One-Step Synthesis of Tunable-Size Gold Nanoplates on Graphene

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