Formation of Gold Nanoplates on Indium Tin Oxide Surface: Two-Dimensional Crystal Growth from Gold Nanoseed Particles in the Presence of Poly(vinylpyrrolidone) Akrajas Ali Umar and Munetaka Oyama* DiVision of Research InitiatiVes, International InnoVation Center, Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8520, Japan
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 4 818-821
ReceiVed October 14, 2005; ReVised Manuscript ReceiVed February 3, 2006
ABSTRACT: The crystal growth of a two-dimensional plate structure of Au that covers the indium tin oxide (ITO) surface has been realized for the first time through liquid-phase reduction from Au nanoseed particles attached to the ITO surface in the presence of poly(vinylpyrrolidone) (PVP). By control of the concentration of PVP in the growth process, the formation of Au nanoplates was possible with surface coverage as high as 30%, although various shaped Au nanocrystals were concurrently formed on ITO. The Au nanoplates were single crystalline in nature with the (111) basal plane and the edge length of up to ca. ∼2 µm, growing parallel to the surface of ITO. The concentration of PVP in the growth solution was a key factor for the formation of Au nanoplates, because at higher or lower concentrations of PVP the purely spherical or irregular-shaped nanoparticles were formed. The absorption spectra implied anisotropic and specific optical characteristics of the Au nanoplate-attached ITO. Metal nanoparticles have attracted active attention in recent years due to their unique optical, electronic, magnetic, and catalytic properties that are different from those of the bulk states, e.g., as summarized in a review for Au nanoparticles.1 For synthesizing metal nanoparticles, a colloid-based wet chemical approach is a typical method that has been widely used for various metals. One of the recent developments of the chemical synthesis has been devoted to the formation of various shapes of metal nanoparticles in solution, as represented by the polyol approach by Xia and coworkers.2-4 As for the fabrication of various shaped Au and Ag nanoparticles, the history and references have been well summarized in recent papers.4-7 Among shape-controlled syntheses, the formation of triangular or hexagonal Au nanoplates, via two-dimensional (2D) crystal growth, is one of the recent active areas. The formation of Au nanoplates should be interesting, e.g., from the view of the controlled fabrication of anisotropic nanostructures. However, up to now, all of the studies have been devoted to the preparation of the colloidal solutions containing Au nanoplates.5-13 In the present communication, we report a new approach to grow Au nanoplates on indium tin oxide (ITO) surfaces, which should be applicable to novel optical and electrode devices utilizing the anisotropic properties of nanomaterials. For preparing the colloid solutions of Au nanoplates, various synthetic methods are now available. Zhou et al. reported the formation of Au triangular and hexagonal nanoparticles of less than ca. 50 nm using an ultraviolet irradiation technique in the presence of poly(vinyl alcohol) as the polymer capping material.8 Photoreduction was also utilized to form the larger Au hexagonal nanoplates (∼500 nm) in the presence of dimyristoyl-L-R-phosphatidyl-DLglycerol as a protecting agent.9 A simple reduction with salicylic acid was reported to be able to form small Au triangular and hexagonal nanoplates of ca. 100 nm.10 Recent developments permitted the formation of Au nanoplates larger than 1 µm, or microplates. Shao et al. reported a simple reduction method with aspartate without using a capping reagent.11 A polyamine process has been demonstrated for the high-yield preparation of Au nanoplates of several 10 µm in size,6 as well as the reduction with o-phenylenediamine.12 A block copolymer has been reported to be valid to form Au nano- and microplates with a capping agent such as cetyltrimethylammonium bromide (CTAB),7 or with thermal reduction.13 Microwave dielectric heating is also effective to form the Au nanoplate in the presence of poly(vinylpyrrolidone) (PVP).5 * To whom correspondence should be addressed. E-mail: oyama@ iic.kyoto-u.ac.jp.
In contrast to such increasing attention for the 2D structural growth of Au to form nanoplates in solution, to our best knowledge, there are no studies on the 2D nanostructural growth of Au, or even for Ag, on different materials’ surfaces utilizing liquid-phase synthesis. This is probably because if Au or Ag nanoplate-attached surfaces are to be fabricated, normally nanoplates are synthesized in solution and then they are attempted to be attached on the substrate surfaces. Our group is interested in the solution-phase nanostructural growth from nanoseed particles attached onto substrate surfaces, in particular, onto the surface of ITO.14-16 By applying the seedmediated growth approach, which was originally developed for the formation of Au or Ag nanorods in solution,17,18 to the surface modification of ITO, we have succeeded in attaching Au grown nanocrystals,14 Au nanoparticles with high density,16 and Ag nanoparticles and nanowires15 onto ITO surfaces. It was characteristic that direct attachment of Au or Ag nanoparticles significantly reduced the electron-transfer resistivity compared with conventional attachment using bridging reagents,15,16,19 so that the thus fabricated surface was utilized for electrochemical measurements.19-23 In addition, interesting optical properties of Au or Ag nanoparticleattached ITO were reported by the collaboration with Kityk.24-26 While these achievements are some examples of the uses of our metal nanoparticle-attached surfaces, direct attachment without using bridging reagents followed by structural growth should be universally promising to form the nanoparticles’ function-modified surfaces, or interfaces, for the fabrication of the nanoparticle-based devices. This is because the presence of bridging reagents should change the nature of the nanoparticles, e.g., hindering the electrontransfer ability of metal nanoparticles. In addition, if the 2D nanostructures can be grown parallel to the different material’s surface, e.g., Au nanoplates on ITO, so as to cover the hetero junction, further unique characteristics are expected to be brought about by (i) the effective coverage by thin nanoplates, (ii) the anisotropic electronic and optical properties of the nanoplates, and (iii) the exposure of peculiar crystal planes of nanoplates to the outside. Thus, in this communication, we report on the formation of Au nanoplates on the ITO surface by applying the seed-mediated growth approach. By simply changing the contents of the growth solution from the previous contents (HAuCl4, CTAB, ascorbic acid, and NaOH)15 to the present one (HAuCl4 and PVP), we found that the 2D crystal growth proceeded to form Au nanoplates on the ITO surfaces. Interestingly, although no reduction reagent was added in the present growth conditions, the Au nanoseed particles could be grown in the presence of PVP to form Au nanoplates bigger than 1 µm.
10.1021/cg050548n CCC: $33.50 © 2006 American Chemical Society Published on Web 03/10/2006
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Figure 1. (A-G) FE-SEM images of Au nanocrystals grown on the ITO surface prepared by the present seed-mediated growth method. The growth solution contained 0.32 mM HAuCl4 and 0.32 mM PVP. The growth time was 15 h at a controlled temperature, 28 °C. High magnification image (A) shows the formation of Au nanoplates with a size in the several micron range; (B) the low magnification image of (A). Variable structures Au nanocrystals, such as truncated hexagonal plate (C), overlapped growth Au nanoplates (D), decahedron (E), spear-head (F), and icosahedron particles (G), were found as byproducts. (H) X-ray diffraction pattern of Au nanocrystals on the ITO surface.
The Au nanoplates on the ITO surface were grown by adopting the seed-mediated growth approach. First, the gold seed particles, size in the range of ca. 3-4 nm, were prepared by reducing the gold salt (HAuCl4) using sodium tetraborohydride (NaBH4) (WAKO Chemicals) in the presence of trisodium citrate (WAKO) as the capping agent.17,18 Briefly, in typical procedures, 0.5 mL of 0.1 M NaBH4 was added to a solution that contains 0.5 mL of 10 mM HAuCl4, 2 mL of 10 mM trisodium citrate, and 18 mL of pure water (resistivity > 18.2 MΩ). The solution was aged undisturbed at room temperature for 2 h prior to further use. Second, the growth solution was prepared by adding 0.5 mL of 10 mM HAuCl4 into 5 mL of aqueous solution of poly(vinylpyrrolidone) (PVP) (MW ∼ 55 000, Aldrich) with stirring, and then 1 min later 10 mL of pure water was added with another 1 min of stirring. The effect of PVP was examined by changing the concentration from 0.1 to 10 mM. If we use 1 mM PVP solution, the final concentration of PVP in the growth solution is 0.32 mM. The attachment of Au nanoseed particles on the ITO substrate was carried out by immersing an ITO sample (CBC Ings Optics Ltd.) with dimensions of ca. 1 × 1 cm2, which was cleaned by a consecutive sonication process in pure water, acetone, ethanol, and finally again in pure water, into the seed solution for 2 h at a controlled temperature of 28 °C. After that, the substrate was removed from the seed solution, rinsed cautiously by flushing with a copious amount of pure water, and followed by a drying process using a flow of nitrogen gas. After this treatment, the Au nanoseed
particles were stably attached onto the ITO surface as shown in our previous work.14 Although the Au nanoseed particles were not fully visible in the field emission scanning electron microscopy (FE-SEM) images due to the restricted resolution, the grown Au nanoparticles showed that the ITO surface was covered by the nanoseed particles with moderate dispersion.14 The ITO samples that have been treated with gold seed solution were, then, immersed into the growth solution and kept undisturbed for 15 h at 28 °C to allow crystal growth from the nanoseed particles that already adsorbed onto the surface. After that, the samples were removed, rinsed with flushing pure water, and dried with the flow of nitrogen gas for further characterization. FE-SEM and optical absorption studies on the nanocrystals growth on the surface were carried out using a JSF 7400F JEOL FESEM machine and Ocean Optics S2000 optical spectrophotometer, respectively. The X-ray diffraction pattern of the nanocrystals growth was taken using a RIGAKU RINT 2500 X-ray diffraction instrument with Cu KR irradiation operated at 50 kV and 300 mA, whose beam was irradiated through a slit of 0.4 mm, and a scan rate as low as 2 deg/min. Figure 1A shows the FE-SEM image of the Au nanocrystals that were grown on the ITO surface using the present seed-mediated growth approach. Actually, after treating the sample in the seed solution, the ITO was immersed into the growth solution containing 0.32 mM HAuCl4 and 0.32 mM PVP. With this procedure, hexagonal-shaped nanoplates with an edge length from 0.5 to 2
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Figure 2. (A) FE-SEM image of Au nanocrystal-attached ITO surface prepared from the growth solution, which contained 0.32 mM HAuCl4 and 3.23 mM PVP. (B) Low magnification image of (A).
µm were formed on the ITO surface as shown in Figure 1A, together with equilateral and truncated triangular Au nanoplates. The observed angles of 120° and 60° of the corners of Au nanoplates indicated that the characteristic 2D crystal growth proceeded parallel to the ITO surface. Interestingly, as shown in Figure 1A, some overlapping parts were observed between the grown nanoplates, which should imply that the Au nanoplates did not grow by sticking to the ITO surface. It is inferred that the contact between the nanoplates and the ITO surface is not very rigid but allows slightly tilted crystal growth and that the Au nanoplates are thin enough to allow the overlapped growth. However, concerning the contact with ITO, the adhesion of Au nanoplates is inferred to be strong enough, because the FE-SEM image of the sample was observed after washing the sample with flushing water. The low magnified image is shown in Figure 1B. Although many Au nanosphere-like particles were concurrently formed on the surface as shown in this image and also in Figure 1A, it was found that the Au nanoplates were grown on the whole ITO surface with the surface coverage over 30% in the present conditions. In the zoom-in observations of nanosphere-like particles, some different crystal growth shapes of Au nanoparticles were observed, e.g., spear-head, icosahedron-shaped nanoparticles, and decahedron as shown in Figure 1C-G. However, these particles are minor byproducts, and the characteristics of the surface crystal growth would be mentioned as the 2D crystal growth parallel to the surface. Figure 1H shows the X-ray diffraction (XRD) pattern of the Au nanoplates with Au nanoparticles on the ITO surface prepared using the present method. In the recorded XRD spectrum, the diffraction peaks from Au{111}, {200}, {220}, and {311} planes of the face centered-cubic (fcc) gold were observed together with small peaks from the crystals of ITO. Thus, we could recognize that the nanoplates and nanocrystals are single crystalline in nature. The diffraction intensity of {111} plane was dominant compared with those of the other planes, implying the preferential orientation of {111} on the surface. However, referring the previous results for pure Au nanoplates dispersed on the substrates,6 the intensity ratios of {200} or {220} versus {111} were not very small. This would be due to the coexistence of Au nanoparticles on ITO in the present case. In our experimental trials, the concentration of PVP in the growth solution was found to be the key factor to allow the 2D crystal growth. The Au nanoplates were similarly formed on the ITO surface when the concentration of PVP in the growth solution was increased from 0.32 to 0.81 mM. However, over 1.62 mM, almost no formation of Au nanoplates was observed on the ITO surface. Figure 2 shows the FE-SEM image of ITO when the concentration of PVP in the growth solution was 3.23 mM. As clearly recognized from the FE-SEM image, no nanoplates were formed on the ITO surface, but nanospheres of ca. 40 nm were formed. This result means that the higher concentration of PVP did not allow 2D growth. On the other hand, at lower concentrations from 0.03 to 0.16 mM, no Au nanoplates were formed, and only relatively small nanospherical particles of ca. 30 nm were formed. These results would be in relation to two different functions of PVP, i.e., protection and reduction. While the function of PVP for protection
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Figure 3. Absorption spectra of (curve a) Au nanoplate-attached ITO (whose surface image corresponds to Figure 1A,B) and (curve b) Au nanosphere-attached ITO (whose surface corresponds to Figure 2).
did not allow 2D structural growth at the higher concentration, the reduction power of PVP became weaker at the lower concentrations so as not to promote 2D crystal growth. Although the function of PVP as a reductant is not known explicitly, PVP should have a function to reduce Au3+, at least in the vicinity of the Au nanoseeds, judging from the fact that Au nanoplates were formed at the moderate concentration of PVP. Actually, we tried similar Au crystal growth from nanoseeds on ITO by changing the contents of the growth solution from (HAuCl4 and PVP) to (HAuCl4 and CTAB) or (HAuCl4 and sodium dodecyl sulfate). However, the crystal growth from Au nanoseed particles did not proceed presumably due to the absence of reductants. Thus, we believe PVP has a function as the reductant, but details are now under investigation. The presence of Au nanoseeds on ITO is essential for Au nanoplate formation, which was experimentally confirmed on the basis of the following two results. (1) Without seed treatment, i.e., the direct insertion of ITO into the growth solution, irregular small nanoparticles were mainly formed accompanying very rare formation of Au nanoplates. (2) Although the seed solution of 0.5 mL was dropped into the growth solution of 20 mL, i.e., the normal seed-mediated growth experiment, no Au nanoplates were formed in solution. Thus, the formation of Au nanoplates is reasonably expected to proceed via 2D crystal growth from Au nanoseeds on ITO. Next, to evaluate the optical characteristics of Au nanoplates grown on ITO, absorption spectra of Au nanoplate-attached ITO (AuNP/ITO, whose surface image corresponds to Figure 1A,B) and Au nanosphere-attached ITO (AuNS/ITO, whose surface corresponds to Figure 2) were recorded. The ITO sample was placed so as to face the surface perpendicular to the light beam, and the reference light was recorded using unmodified ITO. As a result, quite different absorption spectra were recorded for AuNP/ITO and AuNS/ITO as shown in Figure 3. To mention the characteristics of the absorption spectrum of the AuNP/ITO, first, a single absorption maximum was observed at 572 nm, and no peak was observed in the longer wavelength region. This is different from the absorption spectra of Au nanoplates in solution,7,12 for which broad absorption spectra have been reported due to the longitudinal (in-plane) mode in addition to the transverse (out-of plane) mode and due to interactions with the byproduct. The present result would be evidence that the Au nanoplates locate parallel to the ITO surface, although of course it could be suggested from the FE-SEM images. On the AuNP/ITO, the transverse mode is inferred to be only responsible for the surface plasmon resonance. As another feature, at the observed wavelength region between 350 and 950 nm, a uniform increase of absorbance was observed in curve a, as if light scattering independent of the wavelength occurred. This similar tendency was noted also in curve b (Figure 3) of AuNS/ITO, although the magnitude is smaller. Because no such increase was observed for the dense Au nanoparticles-attached ITO substrates,16 this increase may arise from the PVP coverage or interaction with byproducts. However, the degree of the increase is so extensive for AuNP/ITO that it may be due to the specific
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interaction between the Au nanoplates and the ITO surface. Although the details are now under investigation, the present absorption result supports the possibility that the flat coverage of Au nanoplates exhibits unique optical properties. In conclusion, we propose a new synthetic method of Au nanoplates directly attached to ITO surfaces. Interestingly, by simply using growth solution containing a moderate concentration of PVP and HAuCl4, 2D crystal growth was possible from Au seed particles to cover the surface. The concentration of PVP was the key factor in promoting 2D crystal growth. At present, although byproducts were concurrently attached to the ITO surface, the AuNP/ITO was clarified to have unique optical features. To elucidate the 2D growth mechanism and to suppress the formation of byproducts, more detailed studies on the effect of PVP are now in progress. Also, utilizing the specific flat surfaces exposed to the outside, studies of the electrochemical applications of AuNP/ITO are now underway. In preliminary trials, it has been confirmed that Au nanoplates formed on ITO are physically stable for use as a working electrode. Acknowledgment. This work was supported by Kyoto Nanotechnology Cluster Project, a Grant for Regional Science and Technology Promotion from the Ministry of Education, Culture, Sports, Science and Technology, Japan. A.A.U. thanks the Japan Society for the Promotion of Science (JSPS) Post-Doctoral Fellowship for Foreign Researchers. We thank Mr. Gang Chang for assisting us with an XRD characterization.
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