Shape Control of Gold Nanoparticles by Silver Underpotential

Jul 1, 2011 - Illinois 60208, United States. bS Supporting Information. The controlled synthesis of noble metal nanoparticles is an important area of ...
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Shape Control of Gold Nanoparticles by Silver Underpotential Deposition Michelle L. Personick, Mark R. Langille, Jian Zhang, and Chad A. Mirkin* Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States

bS Supporting Information ABSTRACT: Four different gold nanostructures: octahedra, rhombic dodecahedra, truncated ditetragonal prisms, and concave cubes, have been synthesized using a seed-mediated growth method by strategically varying the Ag+ concentration in the reaction solution. Using X-ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectroscopy, we provide quantitative evidence that Ag underpotential deposition is responsible for stabilizing the various surface facets that enclose the above nanoparticles. Increasing concentrations of Ag+ in the growth solution stabilize more open surface facets, and experimental values for Ag coverage on the surface of the particles fit well with a calculated monolayer coverage of Ag, as expected via underpotential deposition. KEYWORDS: Anisotropic nanostructures, gold, underpotential deposition, silver, seed-mediated synthesis

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he controlled synthesis of noble metal nanoparticles is an important area of study because of the unique shape-dependent properties of noble metals on the nanoscale. These shape-tailorable physical properties have potential applications in areas such as optics, spectroscopy, biological labeling, and catalysis.1 8 As a result, a number of syntheses, which include thermal,9 12 photochemical,2,13 18 electrochemical,19,20 and templated methods,1,21 have been developed to achieve control over the shape of metal nanoparticles. Among the noble metals, gold has dominated the literature with regard to the number of unique shapes that have been realized through bottom-up solution-based synthesis. In particular, the seed-mediated thermal synthesis of Au nanostructures has proven to be especially versatile and is useful for preparing particles with shapes ranging from platonic solids, such as cubes and octahedra,12,22,23 to structures with high-index facets, such as tetrahexahedra, concave cubes, and trisoctahedra.24 30 This method has a wide variety of variables, including pH, surfactants, and additives, which can be used to direct the growth and final morphology of the particles.12,25,31 34 However, the large number of components in these syntheses greatly complicates mechanistic studies of particle growth, since the numerous reactants often work in cooperation or in competition with one another. Therefore, the chemical reasons for why certain reaction conditions and additives cause the growth of a particular nanostructure are not well understood. One cationic additive that is commonly employed in the seedmediated synthesis of Au nanostructures is Ag+. High-index {720}-faceted concave cubic gold nanocrystals have been synthesized via the reduction of HAuCl4 with ascorbic acid in a chloridecontaining surfactant in the presence of a small amount of Ag+.26 Ag+ has also been used in the synthesis of {730}-faceted tetrahexahedra,24,35 {711}-faceted bipyramids,25,36 and high-index nanorods.29,30,33 Several suggest that these shape-directing effects are due to the underpotential deposition (UPD) of Ag onto the r 2011 American Chemical Society

surface of the Au particles,24 26,37 a process which involves the reduction of up to a monolayer of Ag onto an existing Au surface. It is also possible that the Au particles serve as a catalyst to overcome kinetic barriers to Ag+ reduction (rather than thermodynamic barriers, as in UPD), however, since literature suggests that the catalysis of Ag+ reduction by Au particles is unlikely,38,39 we will refer to this process of Ag deposition on the gold surface as Ag UPD. Thus far, there has not been an in-depth study on the specific role of Ag+ in directing Au particle shape. Herein, we show how Ag UPD can be used to stabilize three different nanocrystal facets, {110}, {310}, and {720}, by strategically varying the amount of Ag+ in the seed-mediated synthesis. The ability to do this allows us to deliberately control particle shape using UPD to direct the growth of four different particle morphologies: octahedra with {111} facets, rhombic dodecahedra with {110} facets, truncated ditetragonal prisms with {310} facets, and concave cubes with {720} facets (Figure 1). The four different shaped particles were synthesized using a seed-mediated, Ag-assisted growth procedure. Briefly, a solution of 7 nm Au seed particles was prepared by the rapid reduction of HAuCl4 by NaBH4 in the presence of cetyltrimethylammonium chloride (CTAC). Growth solutions were then prepared by sequentially adding HAuCl4, variable amounts of AgNO3, HCl, and ascorbic acid to an aqueous solution of CTAC. The reactions were initiated by the addition of Au seed particles to the growth solutions, and then they were mixed gently and allowed to sit undisturbed overnight (see Supporting Information). By varying the concentration of Ag+ in the particle growth solutions, four different particle morphologies can be Received: May 26, 2011 Revised: June 30, 2011 Published: July 01, 2011 3394

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Figure 2. (A) TEM of a truncated ditetragonal prism oriented along the [100] zone axis with measured angles between surface facets and the {100} planes. Insets: model and electron diffraction pattern. Highmagnification SEM image (B) and model (C) of truncated ditetragonal prisms in various orientations. Scale bars: 50 nm.

Figure 1. SEM images of (A) octahedra, (B) rhombic dodecahedra, (C) truncated ditetragonal prisms, and (D) concave cubes synthesized from reaction solutions containing Ag+/Au3+ ratios of 1:500, 1:50, 1:12.5, and 1:5, respectively. Scale bars: 200 nm. Note that the octahedra form concomitantly with {111}-faceted twinned truncated bitetrahedra, which are larger in size.

synthesized: octahedra, rhombic dodecahedra, truncated ditetragonal prisms, and concave cubes, with solution Ag+/Au3+ ratios of 1:500, 1:50, 1:12.5, and 1:5, respectively (Figure 1). The octahedra are bound by eight identical {111} facets while the rhombic dodecahedra are bound by twelve identical {110} facets, as confirmed by transmission electron microscopy (TEM) and electron diffraction experiments (Supporting Information Figure S1). Note that the octahedra and rhombic dodecahedra form concomitantly with their planar-twinned analogues: {111}-faceted truncated bitetrahedra and {110}-faceted bipyramids,40 respectively. While the octahedra solution was studied as synthesized, a pure sample of rhombic dodecahedra was obtained by separating the much larger bipyramids by a filtration procedure (see Supporting Information). The concave cubes are a highindex faceted structure bound by twenty-four identical {720} facets, as determined by a previously published structural characterization study.26 The facets of the truncated ditetragonal prisms were determined via TEM by tilting a representative particle to the [100] zone axis and measuring the angle between the facets of the truncated ditetragonal prism particles and the {100} planes (Figure 2A and Supporting Information Figure S2). The average angle measured using this method was 19 ( 2°, which is close to published values for the angle between a {100} facet and a {310} facet (18.4°). As further structural confirmation, high-magnification scanning electron microscopy (SEM) images of the truncated ditetragonal prisms in various orientations match well with models of a particle bounded by twelve {310} facets (Figures 2B,C). This facet assignment is also in agreement with a recently published report of similar Au truncated ditetragonal prisms synthesized by a polyol method.37 While most previously published Ag-assisted syntheses utilize Ag+/Au3+ ratios of 1:5,24 26,33,35 we find that by strategically

varying the ratio over a range from 1:500 to 1:5, we can stabilize different shaped nanoparticles. We hypothesized that the deposition of Ag onto the surface of a growing Au nanoparticle via the process of UPD can significantly affect the growth of that particle; by depositing onto an Au surface facet, Ag hinders the further deposition of Au on that same facet and stabilizes it, leading to its slowed growth and subsequent retention in the final nanoparticle structure.25 The phenomenon of UPD occurs because while Ag+ cannot be reduced in bulk by ascorbic acid under the conditions of the seed-mediated synthesis, Ag+ can be reduced onto an existing metal surface, in this case Au, because the surface enables reduction at a potential less negative than the Nernst potential, an effect known as the “underpotential shift.”25,41 44 Because the UPD of Ag on Au results in a monolayer or submonolayer coverage of Ag,41 44 quantification of the amount of Ag on the surface of the Au particles can provide evidence of the role of Ag UPD in controlling particle growth. Using a combination of X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES), it is possible to quantitatively determine the extent of the Ag coverage on the surface facets that enclose each of the four particle shapes. Samples of each particle type were repeatedly drop-cast onto oxidized silicon substrates to ensure full coverage for XPS analysis. In agreement with expected results, the XPS survey scan has peaks indicating the presence of Ag and Au from the particles, C and Cl from the surfactant (CTAC), as well as Si and O from the silicon substrate (Figure 3A). For quantitation of the Ag surface coverage, high-resolution scans were performed in the binding energy range of the highest intensity Ag and Au peaks: Ag 3d and Au 4f (Figure 3A, inset). These measurements were conducted in triplicate for each of the four particle shapes, and by integrating the area under the high-resolution peaks we determined the ratio of Ag/Au for each of the facet types (Figure 3B). The XPS data show that the concave cubes have the most Ag on their surface, followed by truncated ditetragonal prisms, rhombic dodecahedra, and octahedra, respectively. We would predict that a higher Ag coverage is necessary to stabilize a higher energy surface facet because these facets are the most reactive toward Au deposition and more Ag would be necessary to inhibit gold 3395

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Figure 4. Maximum number of surface atoms per unit area, side view, and top view models for each of the four reported surface facets. Blue spheres represent surface atoms.

Figure 3. (A) XPS survey scan of concave cube particles and highresolution XPS spectra of Ag 3d (top inset) and Au 4f peaks (bottom inset). (B) Ag/Au ratio for each particle type, as obtained from XPS data (black), and theoretical values for monolayer coverage of Ag (red). These results show the correlation between experimental surface Ag coverage on the particles and monolayer coverage of Ag, suggesting that the octahedra are a thermodynamic product while the other shapes are stabilized by Ag UPD.

reduction onto these surfaces. For Au, which is a face-centered cubic metal, the surface energy of different facets generally increases in the order γ{111} < γ{100} < γ{110} < γ{hkl},45 and thus we expect that the energies of the facets enclosing the particles and their corresponding Ag coverage would increase in the following order: octahedra < rhombic dodecahedra < truncated ditetragonal prisms < concave cubes. The XPS data follow this trend, suggesting that the facets are being stabilized by Ag UPD. Further validation of the claim that Ag UPD plays a major role in directing particle shape can be obtained by comparing the XPS data to theoretically predicted Ag/Au ratios for each facet. However, these values are difficult to calculate, as the amount of signal generated by the sample decays with increasing X-ray penetration depth.46 While the maximum penetration depth for XPS is approximately 10 nm, the majority of the signal comes from the top few nanometers of the sample.46 Thus, to calculate the expected trend of Ag/Au ratios, we assumed that all of the signal originated uniformly from the top 1 nm of each sample. Models of the four facets, {111}, {110}, {310}, and {720}, were used to determine the number of surface atoms for a given surface area (Figure 4). Because the lattice parameters of Ag and Au are similar (4.09 and 4.08 Å, respectively), we assumed that the number of Ag atoms that can deposit onto a specific facet is

equivalent to the number of surface atoms for that facet, and thus it is possible to calculate the Ag/Au ratio for a monolayer of Ag on an Au surface, given an X-ray penetration depth of 1 nm. Assuming a greater or lesser penetration depth would slightly change the absolute values of the calculated Ag/Au ratios but the overall trend, which is more important for our data analysis, would remain the same. When these values are plotted against the experimental XPS data, they exhibit an excellent correlation with experimental results for the rhombic dodecahedra, truncated ditetragonal prisms, and concave cubes, indicating it is likely that these particles have monolayer or submonolayer coverage of Ag on their surface (Figure 3B). However, the theoretical Ag/Au ratio for the octahedra is much higher than the experimental ratio, suggesting that Ag is not a major factor in the stabilization of this particular particle shape, since there is not a sufficient amount of Ag on the octahedron surface to substantially cover {111} facets.47 Because the {111} facet is generally the most thermodynamically favorable,45 it is most likely that the octahedra are a thermodynamic product and not a result of stabilization due to Ag UPD. We note that the oxidation state of the Ag cannot be determined by XPS because the binding energies of Ag0 and AgCl are identical (368.1 eV for 3d5/2).48 In addition, the adsorption of a Cl adlayer on top of Ag can affect the final oxidation state of the Ag, which may not reflect the oxidation state of the Ag during particle growth.49 In conjunction with XPS, ICP-AES was also used to quantify the amount of Ag on the surface of the Au particles. Because ICPAES is a bulk measurement, rather than a surface measurement, it is important to account for the shape and size of the nanoparticles in the analysis and interpretation of the ICP-AES data, since it is the Ag/Au ratio of each surface facet which is informative rather than that of a particular size or shaped particle. For a constant volume, less spherical shapes will have a greater surface area and thus higher apparent Ag coverage than more spherical shapes. In addition, the surface area-to-volume ratio of a particle decreases at larger sizes, so the apparent Ag/Au ratio of particles of a specific shape should decrease as a function of size simply because of this geometric trend. This prediction is confirmed by ICP-AES analysis of concave cubes of different edge lengths (Supporting Information Figure S3A). Both size and shape effects can be corrected for by dividing the ICP-AES data by the surface area to volume ratio of the specific particle shape and size being analyzed, as determined by SEM. Indeed, after dividing the measured Ag/Au ratios (Supporting Information Figure S4) by the surface area to volume ratio of the particles, the Ag coverage on the surface of the concave cubes, which are all composed of 3396

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Figure 5. Ag/Au ratio for each particle type, as obtained from ICP-AES data (black), and theoretical values for monolayer coverage of Ag (red). All Ag/Au values have been adjusted for size and shape effects (see text for a discussion of this adjustment). These results show the correlation between experimental surface Ag coverage on the particles and monolayer coverage of Ag, suggesting that the octahedra are a thermodynamic product while the other shapes are stabilized by Ag UPD.

identical {720} facets, is constant for all particle sizes (Supporting Information Figure S3B). For the four different-shaped particles, the adjusted ICP-AES results correlate well with the predicted trend: an increasing Ag+ concentration in the particle growth solution leads to more Ag being deposited onto the particle surface, and consequently higher energy facets are stabilized (Figure 5). A comparison of the experimental data with the calculated Ag/Au ratio for each facet type again provides strong evidence that the particles have a submonolayer surface coverage of Ag, as the experimental Ag/Au values fall just below those of the theoretical monolayer with the exception of the octahedra, which have coverage well below a monolayer (Figure 5).47 This is further confirmation that the octahedra are simply a thermodynamically favored structure, while the underpotentially deposited Ag stabilizes the rhombic dodecahedra, truncated ditetragonal prisms, and concave cubes. Together, XPS and ICP-AES results provide quantitative evidence that Ag UPD is responsible for controlling the growth of {110}, {310}, and {720} facets in this seed-mediated synthetic system. The coverage of Ag on the surface of the Au particles reported here matches well with what is theoretically predicted for UPD of Ag on the facets which enclose the rhombic dodecahedra, truncated ditetragonal prisms, and concave cubes. Deposition of Ag in this manner blocks surface sites on higher energy facets, slowing the growth of the highest energy facet for which there is sufficient Ag to cover, and ensuring that those specific facets are retained in the final nanostructure. We believe that at low concentrations of Ag+, the preference of the reaction involving the displacement of Ag by Au on the particle surface shifts to favor Au deposition because Au is significantly more abundant in the reaction solution. Because Au also preferentially deposits on more open facets, and since Ag cannot completely cover these facets at such low concentrations, Ag which initially deposits on high energy facets is more rapidly displaced. Ag then redeposits onto a lower-energy facet where it can achieve complete and uniform coverage and thus more effectively prevent Au deposition. When a higher concentration of Ag+ is present in the reaction solution, the greater availability of Ag+ ions allows for the rapid and uniform coverage of more open surfaces, thus inhibiting the displacement of Ag by Au. Recent literature has suggested that varying the ratio of Ag+/Pd2+ additives in a polyol synthesis can stabilize different Au particle morphologies via UPD.37 However, because most of the resulting

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particles are not enclosed by identical facets and due to the simultaneous use of additives such as Pd2+ and Ag+, quantitative data is difficult to obtain and analyze, and questions about the specific role of Ag+ in directing Au particle shape have remained unanswered.37 Our work provides the first conclusive quantitative support for Ag UPD as the major factor in directing Au nanoparticle shape in Ag-assisted synthetic methods. Previously published Ag-mediated syntheses have shown successful stabilization of specific high-index facets in cetyltrimethylammonium bromide (CTAB) at high Ag+/Au3+ ratios,24,25,33,35,36 but at lower Ag+ concentrations, a variety of particles with ill-defined facets are formed.40 The ability to control Au nanoparticle shape in this particular synthesis with a low Ag+/Au3+ ratio is due in part to the use of the Cl -containing surfactant, CTAC, rather than a Br -containing surfactant, such as CTAB. We hypothesize that various lower energy facets can be stabilized with a reduced concentration of Ag+ in CTAC, as opposed to CTAB, because Cl is a weaker adsorbate on Au surfaces than Br .50,51 As a result, with CTAC, there is less competition between Ag+ and the halide anion for binding to the Au surface, and less Ag+ can be added to the growth solution while still providing sufficient Ag coverage on the growing nanoparticle to cause stabilizing effects. It is the combination of the relatively low binding affinity and low specificity of Cl for Au surfaces50,51 which enables Ag UPD to control crystal growth rather than surfactant effects, even at a low Ag+ concentration, with this CTAC-based, seed-mediated growth method. Thus, the preservation of surfaces such as {110}, which require less Ag deposition to obtain monolayer coverage, can be achieved without the addition of enough Ag+ to stabilize a higher index facet. In conclusion, we have provided quantitative support for the claim that Ag underpotential deposition is responsible for selectively stabilizing {110}, {310}, or {720} facets through XPS and ICP-AES experiments showing that the Ag coverage on the facets of the Au nanoparticles correlates strongly with what would be expected for UPD. In addition, four nanoparticle shapes: octahedra, rhombic dodecahedra, truncated ditetragonal prisms, and concave cubes, have been synthesized by systematically adjusting the concentration of Ag+ in the particle growth solution. This improved understanding of the role of Ag+ in controlling the shape of anisotropic Au nanoparticles enables the rational design of nanostructures by using a tailored concentration of Ag+ to generate a particle with desired surface facets and shape.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details; TEM, electron diffraction, and models of octahedra and rhombic dodecahedra; additional TEM of a truncated ditetragonal prism with angle measurements; ICP-AES data for concave cubes of different edge lengths; unadjusted Ag/Au ratios from ICP-AES. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT C.A.M. is grateful for an NSSEF Fellowship from the DoD and support from the AFOSR. This work was supported in part by 3397

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Nano Letters the MRSEC program of the National Science Foundation at the Materials Research Center of Northwestern University (DMR-0520513) and supported as part of the Non-Equilibrium Energy Research Center (NERC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-SC0000989). The electron microscopy work was performed in the EPIC facility of NUANCE Center at Northwestern University. NUANCE Center is supported by NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University. M.L.P. gratefully acknowledges support from the DoD through the National Defense Science & Engineering Graduate (NDSEG) Fellowship Program (32 CFR 168a).

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