Growth of High-Density Gold Nanoparticles on an Indium Tin Oxide

Growth of High-Density Gold Nanoparticles on an Indium Tin Oxide Surface Prepared Using a “Touch” Seed-Mediated Growth Technique Akrajas Ali Umar† and Munetaka Oyama*,‡

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 2 599-607

Venture Business Laboratory, Kyoto University, Sakyo-ku, Kyoto, 606-8501 Japan, and Division of Research Initiatives, International Innovation Center, Kyoto University, Sakyo-ku, Kyoto, 606-8501 Japan Received August 19, 2004;

Revised Manuscript Received September 28, 2004

ABSTRACT: This paper reports our novel approach in an attempt to grow high-density gold nanoparticles on an indium tin oxide (ITO) surface using an improved “touch” seeding technique, instead of the “normal” seeding approach, in a seed-mediated growth procedure as we reported previously [Kambayashi et al., Cryst. Growth Des. 2005, 5, 81-84]. The present approach provides a simple and useful strategy to promote the growth of gold nanoparticles on ITO surfaces by simply touching the surface that has already been covered with a drop of gold nanoparticle seed solution with a tissue paper. The FE-SEM characterization of the growth of gold nanoparticles on two different surface structures, i.e., rough and smooth structures, has confirmed that this approach is very effective and prospective in fostering the growth of high-density gold nanoparticles with a relatively small size (ca. 10-30 nm) of sphericallike structure. An optical properties study confirmed that the modified ITO system could be used as a functionalized optically transparent electrode for spectro- and photoelectrochemical applications. Introduction Nanostructured materials feature many interesting characteristics, i.e., optical,1-4 electronic,5,6 and catalytic,7 that greatly depend on the size and the shape of the crystal growth as an effect of the quantum confinement of electrons.8,9 These whole characteristics have turned out to be a driving force for exploring any opportunity to synthesize many materials of interest that range from metals and semiconductors10 to organic chemicals11 and to exploit their unusual properties for use as promising functional materials in nanoelectronics,12 photonics,13 nanobioelectronics, and sensors.14 Gold nanoparticles are among the most widely studied and have become promising candidates in these areas. Their relatively well-established chemical synthesis procedures15-17 and capability of coordinating with large ranges of organic compounds,18,19 particularly, organothiolates, allow them to be extensively employed in catalytic processes,20 biosensors,21 and photoelectrochemistry.22-24 For particular applications, such as electrochemistry and photoelectrochemistry, the immobilization of gold nanoparticles from a colloidal solution onto a solid support, such as an indium tin oxide (ITO) substrate, is necessary. Several approaches have been employed to immobilize the gold nanoparticles onto a substrate. They are self-assembly, layer-by-layer self-assembly, Langmuir-Blodgett, and electrochemical approaches. However, self-assembly through a surface-functionalized approach to bind the gold nanoparticles, such as using trialkhoxysilane25 or thiols modified surfaces,26 is preferred. This technique is straightforward such that the gold colloid that has been stabilized with a particular * To whom correspondence should be addressed. Tel +81-75-7539152. Fax +81-75-753-9145. E-mail: [email protected]. † Venture Business Laboratory, Kyoto University. ‡ Division of Research Initiatives, International Innovation Center, Kyoto University.

ionic molecule, such as citrate-stabilized gold, may be easily adsorbed on the functionalized surface through covalent26 or electrostatic27 binding. However, for a specific purpose, such as in electrochemistry, the use of a definite binding agent to immobilize the gold nanoparticles onto an electrode substrate is not favored because it might increase the nature of its resistance. For that reason, a new approach to attach the gold nanoparticles onto a particular substrate without using an organic binder is required. In our previous study, we have succeeded in attaching the gold nanoparticles onto an ITO substrate without the use of a certain binding agent using a seed-mediated growth method.28,29 Our investigation of the electrochemical properties of the gold nanoparticle-modified ITO systems prepared using this approach and using a specific linker molecule, i.e., 3-mercaptopropyl-trimethoxysilane, provided evidence that the modified ITO system prepared using the seed-mediated growth technique showed a remarkable decrease in its chargetransfer resistance compared to those prepared using the counterpart approach.29 From the electrochemical viewpoint, such evidence could be more beneficial and prospective because this could increase the sensitivity of the system in an electrochemical process. The seed-mediated growth technique is a simple approach to grow gold nanoparticles on a surface, in which a piece of substrate is simply immersed in the seed solution that contains 4-nm gold nanoparticle seeds for 2 h (see ref 15 for further details on the seed and growth solutions). The substrate is then removed, rinsed thoroughly with pure water, and dried with a nitrogen flow. After that, the substrate is immersed in a growth solution that contains a gold ion and a weak reductant, typically ascorbic acid, for a period of time from minutes to hours. In the growth solution, the gold seeds that

10.1021/cg049711p CCC: $30.25 © 2005 American Chemical Society Published on Web 11/13/2004

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Ali Umar and Oyama

Table 1. ITO Substrate Samples for Growth of the Gold Nanoparticles label

types

surface structure

surface resistivity (Ω/square)

manufacturer

ITO-A ITO-B ITO-C

ITO on glass ITO on plastic ITO on glass

rugged, high roughness comparatively very smooth smooth

50 ( 0.31 500 ( 6.18 4.2 ( 0.12

Asahi Beer Optical, Ltd., Japan OIKE & Co., Ltd., Japan Kuramoto Seisakusho Co., Ltd., Japan

already exist on the surface will grow due to the reduction of gold ions by ascorbic acid in the presence of gold seed. There is a crucial consequence arising from this approach that the number of gold nanoparticles that could grow on the surface depends greatly on the number of gold particle seeds that were successfully adsorbed on the surface during immersion in the seed solution. On the basis of our understanding of the seedmediated growth method, we concluded that highdensity gold nanoparticle growth on the surface could be achieved if we could seed high-density gold particle seeds on the surface prior to immersion into the growth solution. In this paper, we report our new approach to promote the growth of gold nanoparticle seeds on the ITO surface to obtain a high-density gold nanoparticle growth. This approach is called a “touch” seed-mediated growth. This technique is a physically promoted seeding method that grows the gold nanoparticle seeds by simply touching the surface that has already been covered with the gold nanoparticle seed solution using a clean tissue paper. The gold seed particles were instantaneously transferred onto the surface every time the surface was touched with the tissue paper. To confirm the effectiveness of this technique in promoting the growth of gold nanoparticles, this procedure was applied to several ITO samples that possessed a different surface structure, namely, rough and smooth surfaces. Another physically promoted seeding approach, i.e., a sonication-promoted seeding technique, was also used to acquire strong verification of the effectiveness of the touch seeding approach in promoting the growth of gold nanoparticles. Experimental Section Materials. Hydrogen tetrachloroaurate (HAuCl4‚3H2O) and cetyltrimethylamonium bromide (CTAB) were purchased from Sigma Aldrich. Sodium tetraborohydride (NaBH4), trisodium citrate, ascorbic acid, and 0.1 M NaOH were purchased from WAKO Chemical Company. All of the chemicals were used as received. The solutions of these chemicals were prepared using ultrapure water, which was obtained from a water purification system Autopure WR600A, Yamato Co., Ltd, with a resistivity higher than 18.2 MΩ. Three types of ITO substrates were used in this work. They were two types of ITO film-coated glass substrates and an ITO film-coated plastic substrate. Table 1 displays the characteristics of the substrates. A clean laboratory tissue paper, WIPERS S-200, was obtained from CRECIA Co., which belongs to Kimberly-Clark Worldwide, Inc., and used as received. Nanoparticle Growth. Prior to the growth of gold nanoparticles onto the ITO surface, two kinds of solutions, i.e., seed and growth, were prepared. The seed solution, which contained ca. 4-nm gold nanoparticle seeds,15 was prepared by adding 0.5 mL of 0.1 M NaBH4 into a solution that contained 0.5 mL of 0.01 M HAuCl4, 0.5 mL of 0.01 M trisodium citrate, and 18 mL of pure water. This solution was aged undisturbed for 2 h prior to use. The growth solution was prepared by adding 0.1 mL of 0.1 M ascorbic acid and 0.1 mL of 0.1 M NaOH into a solution that contained 0.5 mL of 0.01 M HAuCl4 and 18 mL of 0.1 M CTAB. The ITO substrates with dimensions of ca. 1 × 1 cm2 were cleaned by sonication in acetone and in ethanol

for 15 min each. They were then rinsed thoroughly with a copious amount of pure water and dried with a nitrogen flow before use. The growth of the gold nanoparticles on the ITO surface was achieved using the seed-mediated growth approach adopted from the procedure for gold nanoparticle growth that was introduced by Murphy and co-worker.15 In our approach, the gold nanoparticles were grown on a surface by simply immersing the substrate into the seed solution for 2 h to “seed” the nanoparticles without any external physical treatment (This procedure will be called the “normal” seed-mediated growth technique later). After that, the substrate was removed, rinsed with pure water, and dried with a nitrogen flow. The sample was then immersed in the growth solution for a period of time. The gold nanoparticle growth can be observed after completion of the immersion process in the growth solution. However, in the present study, a new seeding approach, namely, a touch seeding technique instead of the normal seeding procedure, was introduced. The touch seeding technique is straightforward: a small amount of seed solution was dropped onto the ITO surface. After that, the surface was scrupulously touched several times throughout the surface using a clean tissue paper. This procedure was repeated at least three times to ascertain that a large number of gold particle seeds resided on the surface. The substrate was then rinsed with a copious amount of water and was dried cautiously with a nitrogen flow. At this stage, the substrate is ready to be immersed in the growth solution to promote the growth of gold seeds that have already remained on the surface. Four different immersion times in the growth solution, i.e., 15 min, 30 min, 1 h, and 24 h, were used to study the structural growth of gold crystals with the variation in immersion time in the growth solution. In this study, the normal seed-mediated growth approach was also used to confirm the effectiveness of the novel introduced touch seed-mediated growth technique in fostering the growth of gold nanoparticles on the ITO surface. The verification of the effectiveness of the touch seeding technique was also performed by applying this approach to grow the gold particle seeds on a different surface structure of the ITO films, namely, on rough and smooth structures. On the basis of our understanding of the mechanisms that might play a part in the adsorption of the gold nanoparticles seed on the surface using the touch seeding approach, i.e., a physically promoted adsorption-like process, another physical treatment for the promotion of gold nanoparticle seeding, namely, a “sonicationpromoted” seeding technique was also used. This technique was carried out by immersing an ITO substrate into the seed solution for 5, 15, 30, and 60 min while sonicating in an ultrasonic instrument prior to immersion into the growth solution. The substrate was kept for 1 h in the growth solution. In addition, the effect of surface pretreatments was also investigated mainly for ITO-A. As a trial, the surface of ITO sample was touched using a tissue that was treated with ultrapure water prior to pursuing a normal seed-mediated growth procedure. As another pretreatment, a clean ITO sample was immersed into the piranha solution, solution of sulfuric acid and 30% of hydrogen peroxide with a ratio of 70: 30, for 2 h followed by normal and touch seed-mediated growth of gold nanoparticles. The JEOL JSM-7400F field emission scanning electron microscopy (FE-SEM) instrument was used to characterize the growth of the gold nanoparticles on the ITO surface. The optical property of the gold nanoparticle-modified ITO surface was characterized using the U-4100 spectrophotometer, Hitachi, Ltd. The absorption spectra of the samples were recorded by blanks against air and the substrates. The surface resistiv-

Growth of High-Density Gold Nanoparticles

Figure 1. FE-SEM images of the gold nanoparticle growth on the surface of ITO-A prepared using the touch seedmediated growth (A) and the normal seed-mediated growth procedure (B). (C) an extended image of (A). The immersion time in the growth solution for both approaches was 1 h. ity of the ITO samples before and after modification with gold nanoparticles was measured using a four-point probe system, an SRM-232-1000 Surface Resistivity meter, Guardian Manufacturing.

Results and Discussion FE-SEM Characterization. Gold nanoparticles have been grown on the surface of ITO substrates using a touch seed-mediated growth approach. Figure 1A shows a typical FE-SEM image of gold nanoparticles grown on an ITO of high surface roughness (ITO-A substrate, with a rugged surface structure), which was prepared using a touch seeding technique and was followed by immersion into the growth solution for 1 h. It was observed that the gold nanoparticles have been successfully assembled on the ITO surface. The growth characteristic of the gold nanoparticles was found to feature a relatively small size (ca. 20-30 nm) of spherical-like structure with a quite narrow distribution. They were

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also observed to have a high tendency to efficiently fill the lower part of the surface, i.e., the region between two adjacent ITO crystals. This case was quite different compared to the growth of gold nanoparticles prepared using the normal seed and growth technique with a similar immersion time in the growth solution (see Figure 1B), in which the gold nanoparticles were found to be sparsely built up on the surface with a low particle density and a relatively broad size distribution. Observation on an extended area of the image of gold nanoparticle growth prepared using the touch seeding technique indicated that the gold nanoparticles have been effectively developed on the surface by homogeneously covering the surface with a high particle density (see Figure 1C). On the basis of this experimental result, it could be worth mentioning that the touch seeding approach is effective in fostering the growth of highdensity gold nanoparticles on the ITO surface. In our previous study on attaching the gold nanoparticles onto the ITO surface, using a normal seed and growth technique,28,29 the immersion time in the growth solution showed a great effect on the structural growth (shape and size) of the gold crystals on the surface. For a particular longer immersion time, typically 24 h, the gold crystal growth displayed a great tendency to form rodlike structures. On this basis, by using the touch seeding approach, we also investigated the effect of the time elongation in the growth solution on the structural growth of the gold crystals. This was carried out by varying the immersion time in the growth solution, namely, for 15 min, 30 min, 1 h, and 24 h. Figure 2 displays typical FE-SEM images of gold nanoparticles which were grown using the touch seeding procedure and immersed into the growth solution for various period of times. It was observed that, for a short time regime, i.e., 15 min, the gold nanoparticles of spherical structure had already grown with a particle size as small as 5-10 nm. From the image, it can be clearly seen that the nanoparticle size and density increase with increasing in the immersion time. However, there is an interesting phenomenon that can be pointed out here: there is an absence of nanorod-like structures on the surface even though the immersion time in the growth solution was elongated (see Figure 2C). This case is in contrast to those prepared using the normal seedmediated growth procedure, in which they showed the presence of nanorod-like structures on the surface.29 Conversely, the aggregation-like structure was observed on the image instead. This could be due to a merging of two or more adjacent gold nanoparticles as a result of interparticle growth while being immersed in the growth solution. To verify the effectiveness of the touch seeding technique in promoting the growth of gold nanoparticles on the surface, this technique was also applied on an ITO surface with a comparably much smoother surface structure, i.e., ITO-B and ITO-C. These types of substrates feature a much smoother structure that is constituted of very small close-packed ITO crystals, particularly, ITO-B, compared to the previous ITO samples, namely, ITO-A. Figure 3 shows the FE-SEM images of gold crystal growth on the ITO-B prepared using the touch and normal seeding approaches. It was clearly observed that the number of gold nanoparticles

602


Figure 2. FE-SEM images of the gold nanoparticle growth on the ITO-A surface prepared using the touch seed-mediated growth approach for different immersion times in the growth solution: (A) 15 min, (B) 1 h, and (C) 24 h.

built up on the surface prepared using the touch seeding approach (Figure 3A) is much larger than those prepared using the normal procedure (Figure 3B). Also, as indicated by the images, for a similar immersion time in the growth solution, the size of the nanoparticles prepared using the touch seeding approach was found to be much smaller, namely, ca. 10-20 nm, compared to those prepared using its counterpart, ca. 40-60 nm. Observation of an extended area of the sample that was prepared using the touch seeding approach confirmed that the present technique could also grow remarkably high-density nanoparticles on such a surface (Figure 3C). Similar to what has been performed on a roughrugged surface ITO sample, for such a surface structure, a study of the effect of a time variation in the growth solution on the structural growth of gold nanoparticles on the surface was also carried out. Figure 4 shows the FE-SEM images of gold nanoparticle growth on the surface of ITO-B prepared using the touch seeding approach and immersed in the growth solution for 15

Ali Umar and Oyama

Figure 3. FE-SEM images of the gold nanoparticle growth on a smooth surface structure, i.e., the surface of ITO-B prepared using (A) the touch seed-mediated growth and (B) the normal seed-mediated growth technique. (C) an extended image of (A). The immersion time in the growth solution was 1 h.

min, 1 h, and 24 h. It was found that the gold nanoparticles were impressively assembled on the surface even for a short period of time in the growth solution and were homogeneously distributed on the entire surface. As shown for the gold nanoparticle growth on the ITO-A surface, a variation in the immersion time in the growth solution also causes a variation in the structural growth of the gold crystals, particularly, the particle size. It was observed that the nanoparticles size increased with an increase in the immersion time in the growth solution. Also, in this case, rodlike structures were not observed on the surface. A similar effect was also obtained for the other ITO sample, namely, ITOC. As shown in the images (see Figure 5), in the case of the touch seeding approach, the gold nanoparticles were built up on such a surface with a surprisingly high particle density and a small particle size (ca. 10-30 nm) compared to those prepared using its counterpart (ca. 20-40 nm) on the same surface structure. As had been


Figure 4. FE-SEM images of the gold nanoparticle growth on the surface of the ITO-B substrate prepared using the touch seed-mediated growth approach for different immersion times in the growth solution: (A) 15 min, (B) 1 h, and (C) 24 h.

obtained for the previous two ITO samples, i.e., ITO-A and ITO-B, a substantially homogeneous gold nanoparticle distribution on the surface with a spherical structure was also observed on this ITO sample. An investigation of the effect of immersion time in the growth solution on the structural growth of gold crystals on such a surface also found the presence of structures identical with what have been seen for the gold crystal growth on ITO-A and ITO-B (see Figure 6). On the basis of these experimental results, it can be concluded that the touch seed-mediated growth approach is extremely effective in growing high-density gold nanoparticles on various types of surfaces. The touch seeding technique is straightforward. A small volume of seed solution was dropped onto a surface that was horizontally positioned. After the drop of seed solution was touched with a tissue paper, the aqueous solvent was absorbed into the paper leaving a great number of particle seeds on the surface of the paper. By applying slight pressure, the paper was


Figure 5. FE-SEM images of the gold nanoparticle growth on a smooth surface structure, i.e., the surface of ITO-C, prepared using (A) the touch seed-mediated growth and (B) the normal seed-mediated growth technique. (C) an extended image of (A). The immersion time in the growth solution was 1 h.

cautiously pressed onto the surface, and the seed particles were instantaneously transferred onto the surface. The seed particles could adhere strongly on the surface even after being washed with a copious amount of water. Although the exact mechanism that causes the promotion of the gold seeds onto the surface is not clearly understood, there is one possible mechanism that may underlie the adsorption of gold nanoparticle seeds onto the surface, that is, the van der Waals dispersion force between negatively charged-gold particles with the ITO crystal surface. The presence of the charged-gold particles in the vicinity of the ITO surface may cause a fluctuation in the electronic charge distribution in the ITO crystal. This in turn will result in the region near the surface of the ITO crystal becoming slightly positively charged. Hence, the van der Waals bond has occurred. van der Waals force is a relatively weak force occurring between atoms or molecules, in which the

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Ali Umar and Oyama

Figure 7. FE-SEM images of the gold nanoparticles growth on the surface of ITO-A prepared using the sonicationpromoted seed-mediated growth technique. The substrates were immersed into the seed solution under sonication for 5 min (A) and 1 h (B) followed by immersion into the growth solution for 1 h each.

Figure 6. FE-SEM images of the gold nanoparticle growth on the surface of ITO-C substrate prepared using the touch seed-mediated growth approach for different immersion times in the growth solution: (A) 15 min, (B) 1 h, and (C) 24 h.

magnitude is determined by their distant to each other. However, in the case of the touch seeding approach, the strength of such force is more immense compared to the van der Waals force occurring between the gold nanoparticle seeds and ITO surface in the normal seeding process. This is because of this technique allows the gold nanoparticles to be situated very close to, even in touch with, the ITO surface, which does not occur in the normal seeding process. Hence, the van der Waals force strength increases. On the basis of our understanding of the adsorption phenomenon that occurred in the touch seeding approach, we extended our study using another physical promoted seeding approach, namely, a sonicationpromoted seeding technique, to grow the gold nanoparticles. The sonication-promoted seeding technique was carried out by simply immersing the substrate into the seed solution while being sonicated for several periods of time prior to immersion into the growth solution. Figure 7 shows the FE-SEM images of gold nanoparticle

growth on the surface of ITO-A prepared by the sonication-promoted seeding technique for different sonication times and followed by an immersion process into the growth solution for 1 h each. It was observed that the number of gold nanoparticles appearing on the surface was determined to be very much lower compared to those prepared using the previous approach, i.e., touch seed-mediated growth approach. These results gave clear evidence that the sonication treatment had no distinct effect on the improvement of gold nanoparticle growth. This evidence became more noticeable when a low density of gold nanoparticle growth was obtained even though the sonication time was elongated. Additionally, it could be worth noting that the gold nanoparticle growth prepared using this procedure is very similar to those prepared using a normal seedmediated growth technique. Effect of Surface Pretreatment. The effect of chemical impurities that may be yielded by the tissue itself in the promotion of the growth of the gold nanoparticle seeds on the substrate surface in the touch seed-mediated growth approach was also investigated. The study was performed by treating a clean substrate surface using an ultrapure water treated-tissue (wet tissue) paper prior to being subjected to a seeding process. A tissue paper that has been treated with pure water was touched several times onto a clean substrate surface to allow the chemicals of the paper to be adsorbed onto the surface. The substrate was then rinsed with the pure water and dried with a stream of nitrogen gas. The normal seed-mediated growth tech-


Figure 8. FE-SEM images of the gold nanoparticles growth on the pretreated surfaces of ITO-A. (A) Normal seeding technique on wet tissue touching-treated surface, (B) normal seeding technique on piranha-treated surface, and (C) touch seeding technique on piranha-treated surface. Each sample was immersed into the growth solution for 1 h.

nique with 1 h in the growth solution was used to grow the gold nanoparticles on the wet tissue-treated surface. The effect of another surface pretreatment, i.e., piranha solution-treated surface, on the promotion of the gold nanoparticle seeds attachment was also investigated. This treatment was meant for studying the effect of surface activation on the promotion of the growth of the gold nanoparticle seeds on the surface. A clean substrate was immersed into the piranha solution for 2 h. After being rinsed and dried, the treated surface was subjected to the growth of gold nanoparticles using the normal and touch seed-mediated growth approaches with 1 h in the growth solution. Figure 8 shows the FESEM images of the gold nanoparticles growth on two kinds of pretreated substrate, i.e., wet tissue touching and piranha-treated surfaces. For the case of the wet tissue-treated surface, it was observed that the gold nanoparticle growth on


such surface exhibited a relatively low particle density (Figure 8A) compared with those prepared using the touch seeding approach. It was also noted that the gold nanoparticle growth on such a pretreated surface was very similar to that prepared using the same technique for the untreated surface (see Figure 1B). This result indicated that the effect of chemical impurities probably yielded by the tissue itself on the promotion of highdensity of gold nanoparticle seed growth on the surface was not noticed and can be neglected. For the case of the piranha-treated surface, using the normal seeding technique, it was found that very few gold nanoparticles were observed on the surface (Figure 8B). This could be directly related to the unsuccessful adsorption process of the gold nanoparticle seeds onto the surface due to the presence of a repulsion force among the negatively charged surface and negative citrate-stabilized gold nanoparticle. It is well understood that the ITO surface when treated using piranha solution may contain a rich hydroxylated end surface. When such a surface is immersed into the aqueous gold nanoparticles solution, the hydroxyl functionalized surface becomes negatively charged due to the easy releasing of the hydrogen ion into the solution. This may absolutely cause a hindrance to the adsorption of negatively charged gold nanoparticles onto the surface. However, the touch seeding technique was found to be valid for attaching the gold nanoparticles densely even for the piranha-treated surface as shown in Figure 8C, which is quite in contrast to the result of the normal seeding (Figure 8C). Thus, while the pretreatment of the substrate surface using piranha solution followed by the normal seeding decreased the attachment of the gold nanoparticles on the surface significantly, the FESEM image of high-density gold nanoparticles grown on the surface of Figure 8C clearly showed that the results of the “touch” seeding was independent of the surface pretreatment (see Figure 1C for comparison). These results absolutely confirmed that the touch seedmediated growth technique provides a promising strategy for the purpose of the growth of high-density gold nanoparticles on various kinds of surfaces. Optical and Surface Resistivity Properties. ITO glass is an optically transparent conductive electrode material that has been widely used in many applications including photoelectrochemical applications.30,31 In photoelectrochemical, in particular, or in electrochemistry applications in general, the ITO glass was favored for use as an electrode because of its high conductivity and high chemical stability. However, for a special case such as in photoelectrochemistry, modification of the ITO surface by immobilization of organic or inorganic materials onto the surface was required to enhance the catalytic properties and the charge-transfer processes.20 Consequently, the optical and surface conductivity properties of the ITO system changed. For that reason, at the same time, one has to be aware and take this into proper consideration to maintain the conductivity and transparency properties at an acceptable level. From this point of view, it was essential also to study the surface conductivity as well as the optical transparency of the ITO surface systems after being modified with gold nanoparticles. A study of the effect of gold nanoparticle growth on

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Figure 9. The optical absorption of the gold nanoparticle-modified ITO that is referenced to the air (left column) and to the substrate (right column) for (A) ITO-A, (B) ITO-B, and (C) ITO-C. (a ) untreated ITO, b ) touch seed-mediated and immersed in growth solution for 15 min, c ) touch seed, in growth solution for 30 min, d ) touch seed, in growth solution for 1 h and e ) touch seed, in growth solution for 24 h). Table 2. Charge Transfer Resistance (Rct) of the Gold Nanoparticle-Modified ITO System in 1 mM of [Fe(CN)6]3-/[Fe(CN)6]4- in 0.1 M PBS (pH 7.4) Measured Using 220 mV DC Bias and Ag/AgCl Reference Electrode Rct/kΩ untreated touch seed technique normal seed technique

ITO-A

ITO-B

ITO-C

35 12 28

90 11 24

14 10 12

the surface conductivity of all ITO samples has been carried out using a four-point probe system, an SRM232-1000 Surface Resistivity meter. No significant change was found in the bulk surface conductivity of the ITO system after being modified with the gold nanoparticles. This condition remained unaltered even though the density and the size of the gold nanoparticles attached on the surface were increased. This circumstance was quite noticeable in the case of the ITO with a relatively high surface conductivity (ITO-C), on which no change in the surface conductivity could be observed. These results were in total contrast to the electrochemical impedance properties of the gold nanoparticlemodified ITO system. Our investigation of the electrochemical impedance spectroscopy on the modified ITO system, using a redox probe of 1 mM of [Fe(CN)6]3-/ [Fe(CN)6]4- in 0.1 M PBS (pH. 7.4), showed that the

charge transfer resistance of the ITO electrode was substantially reduced after the growth of the gold nanoparticles on the surface. The decrease in the charge transfer resistance of the modified ITO system was totally observable even at a low density of gold nanoparticles present on the ITO surface. Table 2 shows a typical charge transfer resistance data of the ITO electrodes after being modified by the growth of the gold nanoparticles prepared using touch and normal seed approaches that followed by immersion in the growth solution for 1 h. From the electrochemical impedance results, we could observe that the change in the charge transfer resistance of the ITO system after being modified with the gold nanoparticles was very high, particularly for the ITO with a low surface conductivity. For instance, for the ITO-B samples, it was found that the charge transfer resistance value surprisingly decreased from ca. 90 kΩ for the untreated ITO to ca. 24 kΩ and ca. 11 kΩ for the gold nanoparticle-modified ITO system prepared using normal and touch seeding techniques, respectively. On the basis of these results, it can be concluded that the presence of gold nanoparticles on the ITO surface did not change the bulk surface conductivity of the ITO but did affect the charge-transfer resistance property. Optical absorption spectroscopy has also been carried


out on the gold nanoparticle-modified surfaces using a U-4100 spectrophotometer to obtain the transparency properties of the modified surface and to observe their changes due to structural growth changes in the gold crystals on the surface. The optical absorption was measured using two different procedures, namely, referenced against air and the substrate. Figure 9 shows the optical absorption of the gold nanoparticle-modified ITO surface, referenced against air and the substrate, for gold nanoparticle-modified ITO-A, ITO-B, and ITO-C surfaces, respectively. For the case of the air-referenced spectra, an increase in the optical absorption of the ITO surface upon modification with the gold nanoparticles was observed, particularly in the region of >450 nm. As shown in the spectra, for all of the ITO samples, the absorbance of the modified system increases with the increase in the size and density of attached gold nanoparticles on the ITO surface. We also can see that there is the presence of particular absorption peaks in the region of 500 to 550 nm, for the modified ITO-A and ITO-B, and an absorption shoulder, for the modified ITO-C, of the spectra. These bands could be straightforwardly associated with the contribution of the plasmonic band of gold nanoparticles to the spectra. This fact was obviously confirmed by the measurement of the absorption spectrum of the modified ITO that referenced against their substrate (see the right-hand side of the corresponding graph of Figure 9), which indicated the presence of a plasmonic band of gold in that region. Despite the increase in the optical absorption of ITO systems in the whole spectrum, after being modified with the gold nanoparticles, it could be concluded that the gold nanoparticle-modified ITO surface features an optically transparent characteristic and can be used as a functionalized optically transparent electrode (OTE) in spectro- or photoelectrochemical applications. Conclusions High-density gold nanoparticle growth on the surface of ITO films has been achieved by using a touch seedmediated growth approach. The gold nanoparticles grown using this technique exhibited a distinct smaller size compared to those prepared using its counterpart approach, namely, the normal seed-mediated growth. The touch seeding techniques was found to be applicable to various types of surfaces, i.e., rough and smooth surface structures, and provides a simple strategy for surface modification. The attachment of gold nanoparticles on the ITO surface was observed to have little effect on their surface conductivity and optical absorption characteristics. Hence, this surface may have potential as a functionalized optically transparent electrode with novel characteristics for use in spectro- or photoelectrochemical applications. Acknowledgment. The authors would like to thank the Kyoto Nanotechnology Cluster Project, a grant for


Regional Science and Technology Promotion, the Ministry of Education, Culture, Sports, Science and Technology, Japan, for support of this work. A.A.U. is grateful for the postdoctoral fellowship from the Venture Business Laboratory, Kyoto University (KU-VBL). They also acknowledge the KU-VBL Project, and OIKE & Co., Ltd. for the kind gift of ITO-B. References (1) Mulvaney, P. Langmuir 1996, 12, 788. (2) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Wheten, R. L. J. Phys. Chem. B 1997, 101, 3706. (3) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (4) Brus, L. E. Appl. Phys. A 1991, 53, 465. (5) Khairutdinov, R. F. Colloid J. 1997, 59, 535. (6) Klein, D. L.; Roth, R.; Kim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (7) Okahata, Y.; Aziga, K.; Seki, T. J. Am. Chem. Soc. 1988, 110, 2495. (8) Alivisatos, A. P. Science 1996, 271, 933. (9) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (10) Kamat, P. V.; Shanghavi, B. J. J. Phys. Chem. B 1997, 101, 7675. (11) Imahori, H.; Fukuzumi, S. Adv. Funct. Matter. 2004, 14, 526. (12) Sato, T.; Ahmed, H. Appl. Phys. Lett. 1997, 70, 2759. (13) McConnell, W. P.; Novak, J. P.; Brousseau, L. C.; Fuierer, R. R.; Tenent, R. C.; Feldheim, D. L. J. Phys. Chem. B 2000, 104, 8925. (14) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18. (15) Busbee, B. D.; Obare, S. O.; Murphy, C. J. Adv. Matter. 2003, 15, 414. (16) Jana, N. R.; Gearhearth, L.; Murphy, C. J. J. Phys. Chem. 2001, 105, 4065. (17) Jana, N. R.; Gearhearth, L.; Murphy, C. J. Adv. Matter. 2001, 13, 1389. (18) Buining, P. A.; Humbel, B. M.; Philipse, A. P.; Verkleij, A. J. Langmuir 1997, 13, 3921. (19) Johnson, S. R.; Evans, S. D.; Brydson, R. Langmuir 1998, 14, 6639. (20) Shipway, A. N.; Lahav, M.; Blonder, R.; Willner, I. Chem. Matter. 1999, 11, 13. (21) Liu, S.; Ju, H. Electroanalysis 2003, 15, 1488. (22) Thomas, K. G.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888. (23) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (24) Kamat, P. V. Pure Appl. Chem. 2002, 74, 1693. (25) Doron, A.; Katz, I.; Willner, I. Langmuir 1995, 11, 1313. (26) Xu, J.; Li, H-. L. J. Colloid Interface Sci. 1995, 176, 138. (27) Lvov, Y. M.; Rusling, J. F.; Thomsen, D. T.; Papadimitrakopolous, F.; Kawakami, T.; Kunitake, T. Chem. Commun. 1998, 1229. (28) Kambayashi, M.; Zhang, J.; Oyama, M. Cryst. Growth Des. 2005, 5, 81-84. (29) Zhang, J.; Kambayashi, M.; Oyama, M. Electrochem. Commun. 2004, 6, 683. (30) Zudans, I.; Paddock, J. R.; Kuramitz, H.; Magashi, A. T.; Wansapura, C. M.; Conklin, S. D.; Kaval, N.; Shtoyko, T.; Monk, D. J.; Bryan, S. A.; Hubler, T. L.; Richardson, J. N.; Seliskar, C. J.; Heineman, W. R. J. Electroanal. Chem. 2004, 565, 311. (31) Richardson, J. N.; Aguilar, Z.; Kaval, N.; Andria, S. E.; Shtoyko, T.; Seliskar, C. J.; Heineman, W. R. Electrochim. Acta 2003, 48, 4291.

CG049711P

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