Formation of High-Yield Gold Nanoplates on the Surface: Effective Two-Dimensional Crystal Growth of Nanoseed in the Presence of Poly(vinylpyrrolidone) and Cetyltrimethylammonium Bromide Akrajas Ali Umar,*,† Munetaka Oyama,‡ Muhamad Mat Salleh,† and Burhanuddin Yeop Majlis†
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2835–2840
Institute of Microengineering and Nanoelectronics, UniVersiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia, and Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto, 615-8520 Japan ReceiVed January 30, 2009; ReVised Manuscript ReceiVed March 25, 2009
ABSTRACT: This paper reports a simple technique to grow high-yield gold nanoplates directly on the surface via an effective two-dimensional growth promotion of the attached nanoseeds in the presence of a binary surfactant mixture, namely, poly(vinylpyrrolidone) (PVP) and cetyltrimethylammonium bromide (CTAB). The gold nanoplates formation strongly depended on the concentration of PVP used in the solution, while the nanoplate size depended on the CTAB concentration. In a typical process with optimum PVP and CTAB concentrations, 60% of the nanocrystal product was nanoplates. Triangular nanoplates were found to be the major shape of the nanoplates with a yield of up to ca. 50%, while hexagonal or truncated-hexagonal and rounded-nanoplates shared up to ca. 30 and 20% of the nanoplates product, respectively. The nanoplates were characterized by a very thin structure with a thickness of less than 10 nm. The edge-length size of the nanoplates was found to be up to ca. 1 µm. At optimum growth conditions, ca. 70% of the surface area was covered by nanoplates. X-ray diffraction results on the surface modified nanoplates samples indicated exceedingly high Au(111) peaks of gold nanocrystal without the presence of other peaks, such as (200) and (220), in the diffraction spectrum. The present approach may be used to produce a surface that contains unique nanostructured Au(111) crystallographic plane characteristics, which should find potential applications in catalysis, surface-enhanced Raman scattering, sensors and photonics.
1. Introduction Metals nanoparticles have been actively studied as active components in widespread applications including catalysis,1 optoelectronics,2 surface enhance Raman spectroscopy (SERS),3 and optics4,5 due to their unique properties that are different from their bulk structure counterpart. Current growing interest in nanoparticles synthesis is to prepare platelets anisotropic structures because of their peculiar properties resulting from retaining unique geometry with a single facet of surface atoms that is exposed to the outside.6-11 Gold nanoplates are key in this issue because of their special characteristics, such as possessing exceptionally high biocompatibility and having an intrinsic characteristic to easily form a variable geometry including triangular and hexagonal nanoplates with single-crystalline (111) planes. A broad range of techniques is available for the synthesis of gold nanoplates. Some are presented in the currently existing literature.6-11 However, all of these techniques grew the gold nanoplates in solution and none of them grew on solid support, which limits them for further use in applications. We recently reported a wet-chemical process for direct growth of gold nanoplates on an indium tin oxide (ITO) surface via a two-dimensional (2D) crystal growth of the attached gold nanoseeds particles.12 The nanoplates growth was obtained by simply immersing the nanoseeds attached-surface into a growth solution that contains a mixed aqueous solution of HAuCl4 and poly(vinylpyrrolidone) (PVP) without the presence of certain reducing agents. Large-size (111) planes bounded-gold nanoplates (edge length ca. ∼0.5-2 µm) with surface coverage up to ca. ∼30% could be obtained using this method. Despite the * To whom correspondence should be addressed. E-mail:
[email protected] (A.A.U.),
[email protected] (M.O.). † Universiti Kebangsaan Malaysia. ‡ Kyoto University.
interesting 2D crystal growth obtained, the nanoplates yield was still relatively low. Considering the prominent application of nanoplates modified surface in the currently existing application, efforts for growing high-yield gold nanoplates directly on the surface are required. In this paper, we demonstrated a new procedure for producing a high-yield gold nanoplates modified-ITO surface by using a new growth solution which is a modification of our previous growth solution.12 In our previous approach, the formation of nanoplates was basically as an effective 2D crystal growth of nanoseed in the presence of PVP, which functions as both the reducing and the capping agent.12 On this basis, we hypothesized that if the capping function of the PVP is maximized by introducing a certain reducing agent into the growth solution, such as ascorbic acid, the yield of the nanoplates could be improved. However, according to our preliminary studies, this condition is contradictive to the promotion of 2D crystal growth as the gold ions immediately reduced into Au(0) in solution instead of on the seed particles surface upon addition of reductant due to a weak protecting nature of PVP molecules. Consequently, the attached gold nanoseeds particles did not grow. Here, we proposed the use of cetyltrimethylammonium bromide (CTAB) together with PVP and in the presence of ascorbic acid as a reductant in the growth solution to control the reduction of gold ions as to occur only on the seed particles. It is enabled by the excellent protecting capability of CTAB on gold ions that prevents the reduction of gold ions in the bulk solution but promotes catalytic reduction on the surface of gold nanoseed only. Hence, high-yield gold nanoplates formation on the surface could be obtained. Our X-ray diffraction (XRD) result on the grown gold nanostructures on the ITO surface showed an exceedingly high Au(111) peak without the presence of other gold nanocrystals planes, confirming the formation of large-scale nanoplates on the surface prepared using the present
10.1021/cg900109x CCC: $40.75 2009 American Chemical Society Published on Web 04/10/2009
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procedure. Our field emission scanning electron microscopy (FESEM) images further revealed that the gold nanoplates grew massively and covered almost 70% of the surface area. The experimental procedures and the mechanism that is responsible for the formation of gold nanoplates in the present method will be discussed.
2. Experimental Procedures Materials. Hydrogen tetrachloroaurate (HAuCl4 3H2O), CTAB, and PVP were purchased from Sigma Aldrich. NaBH4 and trisodium citrate were purchased from Wako Pure Chemicals, Ltd. All chemicals were used as received. The solutions were prepared using pure water, which was obtained from a water purification system Autopure WR600A, Yamato Co., Ltd., with a resistivity higher than 18.2 MΩ. The ITO on glass substrate (surface resistance ca.∼3 Ω/0 (square)) was purchased from CBC Ings Co. Ltd. Growth of Gold Nanoplates on ITO Surface. The seed-mediated growth method was used to grow gold nanoplates on an ITO surface.13 Prior to a growth process, two kinds of solutions, namely, seed and growth solution, were prepared. The seed solution, that is, the gold colloids solution containing ca. 4 nm gold nanoseed particles, were prepared using a Murphy procedure,14 namely, by adding ice-cold 0.5 mL of 0.1 M NaBH4 into a solution that contains 0.5 mL of 0.01 M HAuCl4, 2 mL of 0.01 M trisodium citrate, and 18 mL of pure water. The seed solution was left undisturbed for 2 h after the reaction before further use. The growth solution was prepared by adding 0.1 mL of 0.1 M ascorbic acid (WAKO Chemicals) into a solution that contained 0.5 mL of 0.01 M HAuCl4, 10 mL of 1 mM PVP (Aldrich), 8 mL of 0.1 M CTAB (Aldrich), and 2 mL of pure water. This solution was used thoroughly in the experiment, otherwise mentioned later. The effect of PVP and CTAB concentration in the growth solution was examined by changing the concentration of PVP and CTAB from 0.2 to 1.2 mM and 10 mM to 50 mM for PVP and CTAB, respectively. If we used 1 mM PVP and 0.1 M CTAB, the final concentrations of PVP and CTAB are 0.5 and 40 mM, respectively. The attachment of gold nanoseeeds onto ITO surface was performed by immersing the ITO samples (CBC Ings Optics Ltd.) with a dimension of ca. ∼1 × 1 cm2, which has been consecutively cleaned in acetone, ethanol, and pure water while sonicating for 15 min each, into a glass tube that contained 5 mL of the as-prepared seed solution for 2 h. After that, the samples were removed and gently washed with pure water, dried with a flow of nitrogen gas, and finally transferred into a regular laboratory oven for an annealing process at 250 °C for at least 1 h. After this treatment, the gold nanoseeds of size in the range of 3-10 nm were strongly attached onto the surface with the absence of citrate molecules of which is removed during the annealing process. Finally, the samples were transferred into a glass vial that already contained 10 mL of the new prepared growth solution, for a growth process, for a period of 0.5-18 h. After finishing the growth process, the samples were cleaned with a copious amount of pure water, dried with a flow of nitrogen gas, and then transferred into an oven for an annealing at 100 °C for 1 h. The effect of the preannealling process of gold nanoseed attached-ITO surface on the nanocrystal growth was also examined. Characterizations. The structural growth of the Au nanocrystals on ITO surface was examined using an X-ray diffraction technique using an RINT 2500 X-ray diffraction instrument with Cu KR irradiation operated at 50 kV and 300 mA and a scan rate as low as 2 degrees per minute. The morphology of the Au nanocrystals growth was characterized using a field-emission scanning electron microscopy (FESEM, JSM-7400F JEOL, Japan). Visible-near-infrared (NIR) optical absorption spectroscopy was performed using Ocean Optics S2000 Optical Fiber spectrophotometer.
3. Results and Discussion Growth of High-Yield Gold Nanoplates. Figure 1 shows the FESEM image of the as prepared gold nanocrystals growth on the ITO surface that was prepared using the present method with a growth solution, namely, mixed aqueous solution of 0.5 mL of 0.01 M HAuCl4, 10 mL of 1 mM PVP (Aldrich), 8 mL of 0.1 M CTAB (Aldrich), and 2 mL of pure water. The growth
Figure 1. FESEM images of gold nanoplates growth on ITO surface using a growth solution of 0.5 mL of 0.01 M HAuCl4, 10 mL of 1 mM PVP, 8 mL of 0.1 M CTAB and 2 mL of pure water. Growth time was 18 h. (A, B) Low and high-magnification images (scale bar: 1 µm). (C-G) Typical shapes of nanoplates product that includes triangular (C), hexagonal (D), truncated hexagonal (E-G). Scale bar (C-G) is 100 nm.
time and temperature were 18 h and 28 °C, respectively. It was found that large number of variable shape gold nanoplates, such as triangular, hexagonal, and truncated hexagonal (see Figure 1C-G), of edge-length size in the range of 50 nm to 1 µm were observed to favorably grow on the surface. Our result shows that the nanoplates yield can be estimated to be 60% of the product. Among the nanoplates, triangular was found to be the major shape of the product with a yield of up to ca. 50%, while hexagonal and truncated hexagonal morphologies shared up to ca. 30% of the product, respectively. The nanoplates are mainly flat nanoprisms rather than polyhedral structures, which grow parallel to the substrate surface forming an atomically flat area. The surface covered by the nanoplates was calculated to be as high as 70% of the surface area, a value that is much higher compared to those obtained in our previous study,12 inferring the effectiveness of the present approach in facilitating a 2D crystal growth of the attached gold nanoseeds. In general, the shape of the nanoplates product is actually consistent with those obtained previously.12 However, in the present technique, a rounded-shape structure without the presence of clear vertices was also obtained with a relatively high yield, namely, ca. 20%. This could be due to the oxidation process of gold atoms at the nanoplates vertex (the high-energy part) in the presence of CTAB, which causes the reshaping of the nanocrystal.16 Additionally, the nanoplates had a thinner structure, ca. ∼
