Crystal Growth of Gold Nanoparticles on Indium Tin Oxides in the Absence and Presence of 3-Mercaptopropyl-trimethoxysilane Miyako Kambayashi, Jingdong Zhang,† and Munetaka Oyama* Division of Research Initiatives, International Innovation Center, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 81-84
Received March 3, 2004
ABSTRACT: By applying a seed-mediated growth method, which was developed for chemical synthesis of gold nanorods in aqueous solution, we observed the changes in the crystal growth of gold nanoparticles on the surfaces of indium tin oxides (ITO) in the presence and absence of 3-mercaptopropyl-trimethoxysilane (MPTMS). While flat and amorphous-like growth of gold nanoparticles were observed on the MPTMS layer-modified ITO due to the interaction with the thiol group, in the absence of the MPTMS layer, gold nanoparticles were found to grow relatively freely to form the crystal-like structures in the solution phase at the ITO surface. The gold nanoparticle-modified ITO surfaces prepared under the latter conditions are promising as new functional interfaces, because gold nanoparticles could be attached on the surfaces without using binder molecules, such as MPTMS. Gold nanoparticles have been attracting much attention in recent years because of their unique optical, electronic, magnetic, and catalytic properties. The assembly, supermolecular chemistry, quantum size-related property, and applications toward biology, catalysis, and nanotechnology of gold nanoparticles have been summarized in a recent review by Daniel and Astuc.1 While bottom-up type synthetic methods of gold nanoparticles in solution are well-known and established in the field of chemistry,2-5 if we would like to use the formed nanoparticles as functional materials for devices with solid support, the gold nanoparticles formed in solution have to be fixed on or into the substrates. Because interesting characteristics of nanoparticles would diminish when they gather to form larger clusters, it is necessary to fix or attach the nanoparticles keeping some dispersed states. Thus, the development of such attachment methods with an appropriate dispersion seems to be very important to make the best use of the characteristic features of gold nanoparticles for nanodevices, such as sensing and optical devices. As a method to attach gold nanoparticles on the surfaces, peculiar binding molecules suitable for connecting gold with substrates have been normally used. For the case of the glass surfaces, 3-mercaptopropyl-trimethoxysilane (MPTMS) or 3-aminoptopropyl-trimethoxysilane were adopted utilizing the bonding ability of the silanol group to the glass surfaces and the affinity of the -SH or -NH2 group toward the gold particles.6-8 The gold-attached surfaces thus formed were successfully applied to the surface-enhanced Raman spectroscopic measurements.6-8 On the other hand, to attach the gold nanoparticles onto the glassy carbon surfaces, 4-mercaptobenzenediazonium salt was used to bind the 4-mercaptophenyl groups onto the GC surfaces forming the C-C bond and then to attach gold nanoparticles with the -SH group.9 At present, to use peculiar binder molecules seems to have become a standard strategy to attach gold nanoparticles onto substrate surfaces, so that numerous applications of this method can be found in the literature nowadays.1,10-13 In addition to such uses of mercapto or thiol groups to attach gold nanoparticles on the surfaces, it is also well-known that the modifications of gold nano* To whom correspondence should be addressed. E-mail:
[email protected]. † On leave from Huazhong University of Science and Technology, People’s Republic of China.
particles themselves with chemicals having thiol groups are effective for the size control4,5 and functionalization of gold nanoparticles.1 However, it is expected that the characteristics of gold nanoparticles, such as conductivity and catalytic ability, are strongly affected by the chemical reagents surrounding or binding to them. For example, the electron transfer reactions are affected by the MPMTS layer on the indium tin oxide (ITO) surfaces.10,11 Thus, although the use of thiols is undoubtedly effective in some respects, such as size control and fictionalizations, these reagents might become interferences to use the peculiar characteristics of gold itself, such as effectiveness in the electron transfer and catalytic reactions. If we can attach gold nanoparticles on the conducting substrates without using peculiar binder molecules, it is expected that we can fabricate novel conducting monodispersed materials with unique electrochemical properties involving both the gold nanoparticles and the conducting substrates. In addition, such surfaces can be applicable, and very promising, to use for the further functionalizations or modifications of the surfaces of gold nanoparticles attached on the substrates and for further nanofabrications on the nanoparticle-attached surfaces for nanodevices. In the present work, we apply a seed-mediated growth method, which was developed by Murphy and co-workers for synthesizing gold nanorods by the chemical reduction of HAuCl4 in aqueous solution,14,15 to the formation of gold nanoparticles on the ITO surfaces. By observing the changes in the growth structure of gold nanoparticles on the ITO surfaces in the presence and absence of MPTMS, the significant differences in the crystal growth of gold nanoparticles are revealed. In particular, for the case of the absence MPTMS, we would like to propose the validity of the present approach as a new method to attach gold nanoparticles on the ITO surfaces without using the binder molecules. As actual procedures, a piece of the ITO film-coated glass (Ashahi Beer Optical Ltd., resistance 80 Ω/0, size ca. 9 mm × 9 mm) was washed with sonication in acetone and then pure water (purified with Autopure WR600A, Millipore Ltd., resistivity > 18 MΩ). The washed ITO glass was dried in the air and then immersed in the seed solution, which was prepared by adding 0.5 mL of cooled pure water solution of 0.1 M NaBH4 into 19.5 mL of pure water solution containing 0.25 mM HAuCl4 and 0.25 mM trisodium citrate with stirring and left for 2 h.14 In this procedure, it has been reported that the gold seed particles
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of 4 nm were formed in the seed solution.14 Next, we immersed the ITO glass into this seed solution to attach the seed particles on the surface; actually, the glass sample was just left in the seed solution without particular treatments for typically 2 h. Then, the ITO glass was taken out from the seed solution, and the surface was washed carefully by flushing pure water over the surface for several times. The water that remained on the surface was removed using tissue paper by just touching the edges of the glass. After the surface of the ITO was blown with N2 gas until dry, it was immersed next in the growth solution, which was prepared by the addition of 2.5 mL of pure water solution of 0.01 M HAuCl4, 0.5 mL of pure water solution of 0.1 M ascorbic acid, and 0.5 mL of pure water solution of 0.1 M NaOH into 90 mL of pure water solution of 0.1 M cetyltrimethylammonium bromide (CTAB). Again, the ITO glass was just kept or left in the growth solution for typically 24 h to promote the growth of gold nanoparticles from the attached seeds. Finally, the sample was washed by flushing pure water again similarly. To observe the surface structures formed with such procedures, field emission scanning electron microscopy (FE-SEM; JSM7400F, JEOL) was used. Figure 1A shows the FE-SEM image of the ITO surface after immersing it into the seed solution for 2 h. On some parts (e.g., marked by white circles in this figure), small seed nanoparticles are found to attach, although the size of the seed particles is nearly the resolution limit of FESEM. Figure 1B shows the FE-SEM image of the ITO surface after immersing the ITO treated in the seed solution into the growth solution for 24 h. As shown in this image, gold sphere nanoparticles were observed to grow up to ca. 80 nm on the ITO surface. Here, because of the differences in the work functions of gold and ITO, gold particles were observed as white images as compared with the background image of ITO crystals. Figure 1C shows the expanded FE-SEM image of the ITO after treating it in the growth solution (the sample was identical to that in Figure 1B). As shown in this figure, gold nanoparticles were confirmed to be present on the ITO crystals keeping a moderate dispersion. Thus, as compared with the images obtained just after immersing in the seed solution (e.g., Figure 1A), it was clarified that gold nanoparticles could be grown on the ITO surfaces by immersing into the growth solution, i.e., by applying the seed-mediated growth method. On the strength of the attachment of gold particles on the ITO crystals, we tried to remove them by flushing pure water on the surface several times or sonicating in pure water for 1 min. However, the FE-SEM measurements after such treatments showed that gold nanoparticles, once grown and attached on the ITO surface, could not be removed by such procedures. This means that the attachment of gold nanoparticles onto the surface is strong enough to be practically usable for further applications, although no special binder molecules were used in the present procedure. However, after the sonication of 15 min, some of the particles of the surface were found to be removed. To observe the differences of attached states and grown structures of gold nanoparticles in the absence and presence of MPTMS on the ITO surfaces, we actually modified MPTMS, and then, the seed-mediated growth procedure was carried out on the MPTMS layer. After the MPTMSmodified ITO surface was immersed into the seed solution, the observed FE-SEM images were almost identical to Figure 1A; i.e., just small seed particles were observed to attach on the ITO crystals (Figure 2A). In contrast, after the ITO was next immersed in the growth solution, we could confirm the growth of gold nanoparticles even when
Communications
Figure 1. FE-SEM images of the ITO surfaces. (A) After immersing ITO into the seed solution for 2 h. The attachment of small seed particles was found, for example, in the while circles. (B) After immersing the ITO, which was treated in the seed solution, into the growth solution for 24 h. (C) An expanded image of the ITO surface of panel B.
the MPTMS layer is present on the ITO surfaces as shown in Figure 2B. However, as clearly shown in this image, the shapes of gold nanoparticles are quite different from those in Figure 1B. That is, some flat and amorphous shapes were observed in the presence of MPTMS, while those in the absence of MPTMS have more crystalloid shapes as shown in Figure 1B. For this difference, the effect of the surface thiol groups on the growing process of gold around the seed particles should be a probable reason to cause the changes in the formed structures and morphology. It is expected that the thiol groups on the surface interact with Au(0) formed during the reduction of AuCl4- when gathering and growing around the seed particles in the growth solution. Thus, it is expected that the growing parts of gold can attach or connect with the thiol groups on the surface. On the other
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Figure 2. FE-SEM images of the MPTMS-modified ITO surfaces. (A) After immersing ITO into the seed solution for 2 h. (B) After immersing the ITO, which was treated in the seed solution, into the growth solution for 24 h. (C) An expanded image of the ITO surface of panel B.
hand, in the absence of a MPTMS layer, it is considered that gold particles grow around the seed particles relatively freely to form the crystal-like structures as in Figure 1B in the solution phase at the interface. This also can be inferred from the differences in the expanded FE-SEM images between Figure 1C and Figure 2C. While some formation of gold nanorods was observed in the absence of MPTMS as in Figure 1C, whose formation is reasonably expected by the previous work in solution,14,15 no formation of gold nanorods was observed on the MPTMS surface as in Figure 2C. Actually, although we tried to observe the wide ranges of the MPTMS surfaces of several samples, nanorods were scarcely found on the MPTMSmodified surfaces after the seed-mediated growth procedure. This difference also supports the conclusions that the crystal growth without MPTMS is similar to that in solutions but that the growing process is quite different
Crystal Growth & Design, Vol. 5, No. 1, 2005 83 on the MPMTS layer, as suggested by the changes in the shapes and morphology of gold nanoparticles. Next, to confirm the actual occurrence of the seedmediated growth of gold nanoparticles, we observed the FESEM images of the ITO surfaces after immersing them only into the growth solution without treating them in the seed solution. As a result, the dispersed nanoparticles as in Figure 1C were not observed on the ITO surfaces, but the grown gold amorphous crystals over ca. 500 nm were observed on very localized parts of the surface sparsely. Thus, it can be concluded that the treatments in the seed solution are essential to form the dispersed structures of gold nanoparticles as shown in Figure 1C. This was also true for the case that gold nanoparticles were grown on the MPTMS layer. For controlling the density and sizes of attached and grown gold nanoparticles, we carried out many experiments by changing the immersion time in the seed solution and the growth solution. Consequently, while the time to immerse in the seed solution was changed from 5 min to 2 h, significant changes were not found in the density or the dispersion of the gold nanoparticles finally formed. This result indicates that the attachments of the seed particles were completed rapidly in the seed solution. On the other hand, the time to immerse into the growth solution was found to affect the size of the grown gold nanoparticles. Typically, the approximate sizes of sphere particles were 9 nm for the immersion of 1 min, 18 nm for 10 min, and 30 nm for 60 min. At present, the mechanism or the driving force of the attachment of the seed particles is unclear, although the seeding process was clarified to be essential in the present method. However, practically, by just dipping the ITO into the seed solution, small nanoseed particles are expected to attach easily onto the surface. While the seed particles are surrounded by the citrates as the capping reagent in the seed solution, the citrate molecules should not have strong specific properties to bind with the ITO crystals. Despite the absence of binder molecules, our observation showed that the seed particles attach on the surface actually, and moreover, the attaching strength of grown gold molecules is strong enough for further uses and applications. In conclusion, in the present work, we succeeded in attaching gold nanoparticles on the ITO surfaces in the presence and absence of MPTMS by applying the seedmediated growth method. It was found that the gold seed particles of ca. 4 nm surrounded by the citrate molecules attached on the ITO surfaces by just immersion of the ITO glass into the solution. Then, by immersing next into the growth solution containing both HAuCl4 and CTAB, crystal growth of the gold sphere particle was confirmed on the surface. As a result of the observation of the grown structures of gold nanoparticles on the ITO surfaces in the presence and absence of MPTMS, gold nanoparticles were found to grow to form the crystal-like structures in the absence of the MPTMS layer, while flat and amorphouslike growth of gold nanoparticles was observed on the MPTMS layer due to the interaction with the thiol group. While the attaching mechanism is unclear at present, the binding is strong enough to maintain the nanoparticles on the surface for water-flushing or short-time sonication even for the case of the absence of MPTMS. Although the attachment and growth of gold nanoparticles10,11 and nanorods12 have been recently studied on the MPTMS layers, the present approach would be the first example to attach gold nanoparticles on the surface without using peculiar binder molecules and to grow them using the chemical bottom-up technique.
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The gold nanoparticle-modified ITO surfaces prepared in the absence of MPTMS can be used as a new electrode material because the electron transfer reactions should not be interfered by chemical reagents, while the obtained electrochemical responses had to be considered including the insulating property of the MPTMS layer in the previous works.10,11 Actually, we confirmed that the surface impedance value of the gold nanoparticles attached to the ITO electrode formed by the proposed method is reduced to ca. one-third of that of the gold nanoparticles attached to the MPTMS-modified ITO electrode. The detailed electrochemical studies using the gold nanoparticles attached to the ITO electrode are now in progress. 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. We also thank the Kyoto University Venture Business Laboratory (VBL) Project.
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