Potential-Induced Shape Evolution of Gold Nanoparticles Prepared on

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2009, 113, 15796–15800 Published on Web 08/12/2009

Potential-Induced Shape Evolution of Gold Nanoparticles Prepared on ITO Substrate Dafeng Zhang, Peng Diao,* and Qi Zhang Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang UniVersity, Beijing 100191, P. R. China ReceiVed: July 10, 2009; ReVised Manuscript ReceiVed: August 1, 2009

A well-defined potential gradient was introduced to the surface of indium tin oxide (ITO) during the seedmediated synthesis of anisotropic gold nanoparticles (AuNPs) on substrates. Benefiting from this potential gradient, we found an evolution of particle morphology as a function of the position potential on ITO substrates. With increasing potential, gold triangular nanoprisms and triangular, hexagonal, and polygonal nanoplates were successively produced as the main products. The results obtained in a traditional three-electrode cell demonstrated a similar morphology evolution trend, confirming that substrate potential is one of the key factors controlling the morphology of nanoparticles. The different adsorption behavior of capping agents on gold facets under different potentials was believed to be the cause of the potential-induced particle morphology evolution. This work offers opportunities for potential-assisted shape-selective synthesis of nanoparticles on conducting substrates. Introduction The synthesis of monodisperse and shape-selective gold nanoparticles (AuNPs) is currently attracting considerable attention for their distinct shape-induced physical1,2 and chemical3 properties.Goldnanostructures,includingrods,4-9 triangles,10-14 hexagons,15,16 and polyhedrons,17,18 have already been synthesized. Among the present methods, the aqueous approach, employing gold nanoseeds and shape-directing reagents, is one of the most efficient strategies for the fabrication of anisotropic AuNPs. It has been suggested7,19 that the variation of protection effects of the capping molecules, coming from different adsorption behaviors on different gold facets, can greatly alter the growth rate along certain facet directions, and then results in an anisotropic growth of the nanoseeds. To develop functional nanodevices, nanostructures are required to be constructed or assembled on substrates. Though many efforts have been made to understand and control the surface growth of nanoparticles,20-24 the preparation of nanoparticles on substrates with well-defined shape and size is still a challenge for nanostructure construction. Electrochemistry is an efficient method for directly producing and patterning metal nanocrystals on conducting substrates.13,25-27 The potentialinduced morphological evolution of Pt mesostructures has been reported using anodic alumina oxide templates.28 In the surfactant-assisted shape-controlled synthesis, the potential applied to substrates can greatly influence the capping behavior of the shape-directing surfactants on different facets, and then may result in the formation of anisotropic nanoparticles. In this work, we introduced a well-defined potential gradient29,30 to the surface of a seed-modified indium tin oxide (ITO) substrate to explore the potential effect on the growth of AuNPs. Benefiting from this potential gradient, we have found a clear dependence of * To whom correspondence should be addressed. Phone: +86-1082339562. E-mail: [email protected].

10.1021/jp906530e CCC: $40.75

particle shape on the substrate potential, which can be used in shape-controlled synthesis of AuNPs. Experimental Section Preparation of Gold Nanoseeds. In a typical experiment, 2.5 mL of 1 mM HAuCl4 was mixed with 10 mL of 0.1 M cetyltrimethlyammonium bromide (CTAB) at 30 °C. Then, 0.12 mL of a freshly prepared ice-cold 50 mM NaBH4 was quickly injected into the mixed solution under vigorous stirring. The resulting brownish yellow gold colloid was stirred for at least 10 min to completely decompose the excess NaBH4. The obtained gold nanoseeds were capped with a cetyltrimethlyammonium (CTA+) bilayer,7,12,31,32 which was positively charged due to the protonation of amine groups in a neutral aqueous solution. Assembly of Gold Nanoseeds on ITO Substrates. The ITO substrates were successively sonicated in acetone, water, and methanol. Then, they were immersed in a 2 mM 11-aminoundecanoic acid (AUA) solution of absolute methanol for 48 h to form a self-assembled monolayer of AUA on the ITO surface.33,34 The resulting film-coated ITO substrates were rinsed with absolute methanol and copious water, respectively, and then dried in a stream of high-purity N2. The AUA monolayer has a terminal carboxylic acid group, which will be negatively charged due to the disassociation of the -COOH groups in a neutral solution. The strong electrostatic interaction between the negatively charged AUA monolayer and the positively charged gold nanoseeds leads to the immobilization of large numbers of gold nanoseeds on ITO substrates. Synthesis of Anisotropic AuNPs. The potential gradient on the ITO surface was introduced according to a literature protocol.30 Figure 1 shows the schematic experimental setup we used to generate a well-defined surface potential gradient on the ITO substrate. Briefly, after being modified with gold nanoseeds, the ITO substrate was connected to a conventional  2009 American Chemical Society

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Figure 1. Schematic representation of the experimental setup used to generate a well-defined potential gradient and to synthesize AuNPs. The potential drop on the ITO substrate was clearly labeled.

DC power supply. A 3.8 V voltage was applied between two ends of the ITO substrate (the ITO substrate was stable at the applied potentials in the growth solution). A constant current flow of 100 mA was applied along the length direction of the ITO substrate to generate a potential gradient of 2.4 V · cm-1. The surface growth of AuNPs was then carried out at 28 °C for 3 h in a growth solution consisting of 20 mM CTAB, 0.2 mM HAuCl4, 0.3 mM ascorbic acid, and 0.08 mM KI. Results and Discussion The morphology of the produced AuNPs was examined using scanning electron microscopy (SEM). Figure 2 shows the typical SEM images taken from the positions labeled with corresponding letters in Figure 1. An evolution of particle morphology as a function of the position on ITO substrate is clearly seen in Figure 2, though the particles obtained at each position are not monodispersed in shape. With the photographed position moving toward the high potential end, the main products gradually change from nanosphere to triangular nanoprism, to triangular nanoplate, then to hexagonal nanoplate, and finally to quasicircular nanoplate. The yield of different nanocrystals at each position was obtained by calculating their appearance ratios in SEM images. The results are shown in Figure 3. Moreover, the particle shape evolution is accompanied with a size increase and a coverage decrease, as can be seen from Figure 2 and the Supporting Information (Figure S1), respectively. To make a clear description of the surface potential at each position, the potential of the negative end is designated as 0 V, and the potentials at all other sites are reported with respect to this value. At the negative potential end, only very small AuNPs with isotropic morphology were formed in high density (Figure S2, Supporting Information). When the observation position was moved from the negative potential end to site A, the surface potential increased 0.2 V, which induced the formation of some small nanopyramids with an edge length of ca. 20 nm (Figure 2A). At position B, triangular nanoprisms appear in large numbers (Figure 2B). The yield of the triangular nanoprisms is ca. 17%, as can be seen in Figure 3. The average edge length of the nanoprisms is ca. 100 nm, and the average thickness obtained by atomic force microscopy (AFM) is ca. 35 nm (Figure 4A). Moreover, at position B, we also found sporadic triangular nanoplates, which are much larger in size and smaller in thickness compared with the nanoprisms. The typical edge length and the thickness of the nanoplates obtained by SEM and AFM are ca. 180 and 13 nm (Figure 4A), respectively. Both triangular nanoprisms and nanoplates appear in large numbers at position C (Figures 2C and 4B). However, compared with position B, the yield of triangular nanoprisms decreases greatly to ca. 12% and the triangular nanoplates become another main product with a yield of ca. 10% (Figure 3). As the

J. Phys. Chem. C, Vol. 113, No. 36, 2009 15797 observation position was moved from C to D, we found that many triangular nanoplates are truncated and presented themselves as subregular hexagonal AuNPs (Figure 2D). At position E, the potential further increased 500 mV and large hexagonal plates became the major products (Figures 2E and 3). Interestingly, still further increase of the potential directly leads to the polygonization of the nanoplates. As a result, gold nanoseeds preferred to grow into polygonal nanoplates with morphologies as truncated hexagons or triangles (Figure S3, Supporting Information). The substrate potential was increased to 2.27 V at position F. Under such a high potential, the large and quasicircular gold plates with an average diameter of ca. 500 nm were obtained as the primary products with a yield of nearly 40% (Figures 2F and 3). No particles can be observed at the positions with potentials higher than 2.3 V. This is due to the fact that the potentials at the substrate regions right to site F are too high for the gold precursors to be reduced. The obtained results clearly demonstrate an influence of the potential on the final morphology of the produced AuNPs. That is, within a certain potential range, preassembled gold nanoseeds prefer to grow into nanocrystals with specific shapes. In order to confirm this potential effect on the nanocrystal morphology, a conventional three-electrode cell was employed to prepare AuNPs. By applying several selected potentials to the ITO substrates that were modified with gold nanoseeds, we have found a morphology evolution trend (Figure S4, Supporting Information) similar to the one shown in Figure 2. Therefore, it is safe to conclude that potential can direct the shape-selective growth of AuNPs on conducting substrates. Theoretical predictions of the growth of two-dimensional nanostructures indicate that high driving force (for example, the low values of Gibbs free energy) results in isotropic structures, whereas low driving force leads to plate-shaped structures.35,36 The observed “nanosphere-nanopyramidnanoprism-nanoplate” transition in this work is in good agreement with the predictions because low potentials imply high driving force for the reduction of HAuCl4 in an electrochemical system. However, the potential-dependent shape evolution of nanoplates (from triangular to hexagonal and then to polygonal nanoplates) observed in this work can not be well explained by the predictions. This may be because the predictions were made on the basis of a surfactant-free system. Therefore, we believe the adsorption and the capping behaviors of the shape-directing additives (Br- and I-) and the capping agent (CTAB) on a gold surface at different potentials may play a key role in the morphology evolution of AuNPs. Halide ions have a strong adsorption affinity on gold surfaces, and the adsorbed halide ions induce an adlayer of positive charged surfactant on the gold surfaces, such as the protonized cetyltrimethlyammonium (CTA+).37,38 The interaction between the hydrophobic chains of the surfactant leads to a bilayer structure which is critical to the growth of anisotropic AuNPs.7,12,31,32 Accordingly, with CTA+ as the main shapedirecting agent, Br- and I- ions were usually used as shapedirecting additives during the synthesis of anisotropic AuNPs. In Br- added solution, the strong and specific adsorption of Bron Au {100} and {110} facets induced the preferential binding of a CTA+ bilayer on these facets, and then led to the formation of Au nanorods with {100} or {110} as the remaining side faces.6,7,12,31,32 I- ions can strongly bind to the Au {111} facet and the induced CTA+ bilayer greatly hinders the growth of this facet, resulting in the formation of nanotriangles.11,12 Moreover, it has been proved that a very small amount of Icould greatly promote the yield of gold nanotriangles.11,12

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Figure 2. Typical SEM images of AuNPs taken from the positions labeled with corresponding letters A, B, C, D, E, and F in Figure 1.

Figure 3. Variation of the yield of different AuNPs with the position of ITO substrate. The letters on the abscissa correspond to the positions marked in Figure 1.

In this work, substrate potential plays a dual role in the synthesis of AuNPs. On one hand, the potential can directly influence the reduction rate of gold precursors. The higher the potential, the lower the reduction rate. The decline of the particle coverage with position (Figure 2 and the Supporting Information (Figure S1)) is believed to result from the increase of substrate potential. On the other hand, the potential also greatly influences the adsorption behavior of I- and Br-,39-41 and then changes the induced CTA+ bilayer structure on the surface of gold nanoseeds. Generally, the adsorption of halide ions will be strengthened by increasing potential within a certain range in which the adsorbed anions are stable. Outside the stable range, the halide ions will completely desorb at very low potentials, or be immediately oxidized at very high potentials. Figure 5 shows a schematic mechanism for the potentialinduced anisotropic growth of AuNPs. In general, gold nanoseeds containing stacking faults prefer to show a plate-like morphology with {111} facets exposed on top.42 At very low potentials, both I- and Br- cannot be adsorbed on any facet, and therefore, no CTA+ bilayer is presented on gold surfaces to protect them. Meanwhile, the reduction of AuCl4- is very fast at low potentials. All of these result in the formation of a large amount of isotropic AuNPs, as shown in Figure S2 (Supporting Information). It was reported that I- starts to bind with gold at such a low potential that the surface of gold is negatively charged.43 Moreover, the coverage of I- will significantly increase as the potential is higher than the potential of zero charge.43 Therefore,

Figure 4. AFM (A) and SEM (B) images of typical triangular nanoprisms and triangular nanoplates. The vertical distance reveals that the thickness of the nanoprism and the nanoplate is 33 and 13 nm, respectively. The former is over 2-fold thicker than the latter.

with increasing potential, I- first begins to specifically adsorb on the Au {111} facet, though the potential may still be in a negative region. The loosely packed I- structure at low potentials generates a loose CTA+ bilayer on the {111} facet, which slows down the growth rate of this facet. As a result, gold nanopyramids or triangular nanoprisms are obtained as the main

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J. Phys. Chem. C, Vol. 113, No. 36, 2009 15799 Conclusion In this work, we have demonstrated that potential is a key factor controlling the morphology of nanoparticles prepared on substrates. The potential-dependent morphology evolution of the AuNPs offers opportunities for shape-selective synthesis of nanoparticles on conducting substrates. Acknowledgment. We gratefully acknowledge the financial support of this work by National Natural Science Foundation of China (NSFC 20773007), Program for New Century Excellent Talents in University (NCET-08-0034), Program for Changjiang Scholars and Innovative Research Team in University (IRT 0805), and Innovation Foundation of BUAA for PhD Graduates.

Figure 5. Schematic representation of potential-induced anisotropic growth of gold nanoparticles. (a) At potentials negative to the potential of zero charge, the loosely adsorbed I- on the Au {111} facet induces a loosely packed CTA+ bilayer, which decreases the growth rate of this facet to a certain extent, and then leads to the formation of a nanopyramid or triangular nanoprisms with {111} as terminal facets. (b) At potentials positive to the potential of zero charge, the densely adsorbed I- induces a densely packed CTA+ bilayer on the {111} facet, which greatly decreases and almost completely stops the growth of the edge and the flat {111} facets, respectively. This leads to the formation of triangular nanoplates. (c and d) At potentials that Brbegins to adsorb on Au {100} facets, the Br--induced CTA+ bilayer protects these crystal faces, and leads to the formation of truncated triangular and hexagonal nanoplates. (e) At higher potentials that Bradsorbs on both {100} and {110} facets, the induced CTA+ bilayer makes these two facets well-protected, resulting in the formation of polygonal nanoplates.

products with the slowest growing {111} facet as the remaining crystal faces. Further increase of the potential, especially to a value higher than the potential of zero charge, will cause a steep rise of the surface coverage of I- ions on Au {111} facets. Under this condition, the flat {111} facets are well-protected by a densely packed CTA+ bilayer, which almost completely blocks the mass transfer of gold precursors to the surface and then stops the growth of the flat {111} planes, resulting in the formation of thin plates, whereas, on the edge {111} facets, the CTA+ bilayer is packed not so densely as that on the flat {111} facets due to the high orientation freedom of the CTA+ chain. The gold precursors can still penetrate the protection bilayer and be reduced at the edge facets. As a result, the nanoseeds can grow slowly along the edge [111] direction and finally form a triangular nanoplate.44 The adsorption of Br- on a gold surface starts at a relatively higher potential compared to that of I-.43 As the potential is further increased to the region in which Br- can strongly and specifically adsorb on gold, a compact Br- adsorption layer will be formed on the edge {100} facets,6,7,37 considering the concentration of Br- in solution is over 2 orders of magnitude higher than that of I-. These facets will be protected by the Br--induced CTA+ bilayer, and their growth rate is greatly reduced. Therefore, these side {100} facets, together with the side and flat {111} facets protected by the I--induced CTA+ bilayer, remain, leading to the formation of truncated triangles, and then hexagons at a relative high potential. At a still higher potential, the side {110} facets begin to be capped by Br- and I-. Then, three low-index edge facets {100}, {110}, and {111} are protected during the seed growth process, leading to the formation of polygonal nanoplates.44

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