Photoinduced Structural Changes of Silver Nanoparticles on Glass

Feb 28, 2002 - The rate of change in the structure was dependent on both the irradiated light intensity and the existence of an external electric fiel...
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J. Phys. Chem. B 2002, 106, 3041-3045

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Photoinduced Structural Changes of Silver Nanoparticles on Glass Substrate in Solution under an Electric Field Kei Murakoshi,*,†,‡ Hiroyuki Tanaka,† Yoshitaka Sawai,† and Yoshihiro Nakato† Department of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan, and Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Corporation, Toyonaka, Osaka 560-8531, Japan ReceiVed: June 26, 2001; In Final Form: NoVember 30, 2001

Ag nanoparticles (d ) ca. 20 nm) adsorbed on the surface of a glass plate immersed in pure water dissolved when white light was irradiated under application of an external electric field. Dissolution was monitored via change in absorption spectrum. Structural characterization revealed that the number of Ag nanoparticles on the glass decreased as a consequence of the treatment. The size of the particles, however, increased to ca. 80 nm following the treatment. The rate of change in the structure was dependent on both the irradiated light intensity and the existence of an external electric field. These results demonstrate that photoinduced structural change of Ag nanoparticles on an insulating material is accelerated by an external electric field in solution.

Introduction Controlling small structures in a size range of less than one hundred nm has gained importance in various advanced technologies.1,2 We have demonstrated that selective excitation of a specific surface plasmon mode of metal nanostructures can be employed as a perturbation for effective structural control. Useful effects on the particle-interaction and highly localized electrochemical reaction were found in several nanostructured metal systems.3-7 Generally, metal nanoparticles, such as Au, Ag, and Cu, show characteristic surface plasmon absorption in the visible-near-infrared wavelength region. This absorption is known to cause a highly localized electric field at the interface between the metal surface and surrounding medium.8-10 Recent surface-enhanced Raman scattering measurements for single nanoparticles revealed that specific nanoparticles displayed large enhancement factors.11,12 These results suggest that the field excited by photoillumination is highly localized at the metal nanostructure. Thus, if such a localized field could be utilized to induce electrochemical reactions, the effect can be applied to change and/or control the structure of the metal nanostructure. In the field of structural control of metal nanoparticles, several important topics remain unresolved. One of these areas involves the application of an external electric field for structural control. It is desirable to achieve control of metal nanostructures on surfaces of insulating materials, as metal nanostructures should be constructed for use as conducting materials. In this case, structural control of metals must be conducted via electrochemical dissolution and deposition without donation of charge from the substrate electrode. Numerous reports exist describing sophisticated techniques pertaining to construction of metal nanostructure on electrodes of conducting materials,13-15 as well as photoinduced structural changes of dispersed metal nanoclusters in solution.3,16-22 However, little documentation occurs with respect to induction of the electrochemical reaction of metal * To whom correspondence should be addressed. Department of Chemistry, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan. Phone: +81-(0)66850-6237. Fax: +81-(0)6-6850-6237/6236. E-mail: [email protected]. † Department of Chemistry. ‡ Precursory Research for Embryonic Science and Technology (PRESTO).

dissolution/deposition to control the metal nanostructure adsorbed on insulating materials in the absence of an external electric circuit. An interesting approach was described by Bradley et al.23,24 In this method, application of a static electric field to copper particles in solution exhibiting a diameter of 1 mm was shown to elongate the metal micro-wire to the direction of the electric field. Copper particles demonstrate a localized electrochemical dissolution and deposition reaction upon polarization by an electric field on the order of 30 V/cm. The formation of a metal wire (size of a few tenths micrometers) was successfully achieved.23 The technique employing an external electric field can be applied to the construction of a metal nanostructure on an insulating material. The required electric field (in the range of nano), however, could become too large compared to the former case of the structure in the range of micron.23 Therefore, to induce the electrochemical reaction in a metal nanostructure polarized by an external electric field, an additional cooperative perturbation would be required. The present investigation clarified the effect of photoirradiation of Ag nanoparticles adsorbed on a glass substrate under an electric field in solution. Structural change in the metal nanoparticles adsorbed on the surface of a glass plate in solution was induced by photoirradiation; moreover, this change was accelerated by application of an external electric field. The rate of the observed structural changes depended both on the strength of the applied electric field and intensity of the irradiated light. The findings proved that application of an electric field to metal nanoparticles under photoirradiation resulted in alteration of nanostructure. Experimental Section Ag nanoparticles on a glass plate were prepared by the method of sputter deposition. A cleaned glass slide (Shibata Co.) (approximate size, 1 cm2) was placed in a sputtering (Sanyu Denshi Co. Ltd., SC-701) chamber. Sputtering of Ag in argon gas at a current of 4 mA for 20 s resulted in the formation of a discrete Ag film consisting of an island structure. Specific resistance of the film was greater than 10-5 Ω cm, indicating that the islands of silver nanoparticles did not electrically contact one another. The size of the hemispherical island was approximately 20 nm in diameter (Figure 1). The absorption

10.1021/jp012443a CCC: $22.00 © 2002 American Chemical Society Published on Web 02/28/2002

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Figure 1. Absorption spectrum (top) and an atomic force microscopic image (bottom) of evaporated Ag nanoparticles on a glass substrate.

Figure 2. Two-electrode cell; distance between the two platinum electrodes is 2.5 cm.

spectrum of the film showed a plasmon absorption peak at 480 nm in air (Figure 1).25 Application of an electric field to the Ag nanoparticles on glass immersed in solution was conducted employing a twoelectrode cell (Figure 2). Two platinum wires (d ) 0.5 mm, 99.99%) separated by 2.5 cm were fixed onto a Teflon plate. The glass substrate of the Ag nanoparticles was fixed in a square hole on the Teflon plate between the platinum wires. The surface plane of the glass substrate was adjusted to be parallel to the wires and the Teflon plate. The Ag-sputtered glass substrate and the platinum wires were immersed in Mill-Q water. Following immersion, an external electronic bias voltage derived from a power supply (Hewlett-Packard, 6200B) was applied. The bias voltage could be adjusted in the range of 0-50 V.

Figure 3. Time-dependent absorption spectral change of the evaporated Ag nanoparticles on a glass substrate in pure water during irradiation at (a) 0.06 mW/cm2, (b) 0.26 mW/cm2, and (c) 0.90 mW/cm2. Applied electric field was 20 V/cm. Measurements were performed nine times at intervals of 15 s.

Results were obtained at 50 V, providing an expected electric field of 20 V/cm. During application of the voltage, timedependent changes in the absorption spectrum of the Ag films in the visible-near-infrared region were recorded utilizing a multichannel spectrometer (MCPD-2000, Ohtsuka Electronics) for a period of 15 s. A 300-W halogen lamp was employed as the light source for spectral measurements. Irradiated light in the wavelength region from 300 to 1000 nm was also used for excitation of the Ag nanoparticles. Irradiated light intensity was altered via utility of three types of neutral density filters. Light intensities irradiated on the Ag nanoparticles in this experiment, as estimated by a thermopile, were 0.06, 0.26, and 0.91 mW/

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Figure 4. Time-course of absorption change of the evaporated Ag nanoparticles on a glass substrate during irradiation at (a) 0.06 mW/cm2 and (b) 0.90 mW/cm, in the presence (closed circle) and absence (open circle) of an electric field of 20 V/cm. Monitoring wavelength was 480 nm.

cm2. The structure of the Ag nanoparticles adsorbed on the glass substrate was inspected utilizing an atomic force microscope (AFM, Nanoscope-IIIa, Digital Instruments) in air. Results and Discussion Absorption spectra of the Ag nanoparticles changed due to photoirradiation under an external electric field in water. Figure 3 shows the time-dependent changes in the absorption spectra of the Ag nanoparticles on the glass plate during photoirradiation at various intensities (0.06, 0.26 and 0.90 mW/cm2) under applying an electric field of 20 V/cm. Absorption around the spectral peak wavelength (approximately 500 nm) decreased due to the photoirradiation. Slight changes in the absorption were observed in the wavelength region below 420 nm. The rate of the changes increased linearly as the intensity of the irradiated light increased (Figure 3a-c). Contribution of the electric field to the production of spectral change under photoirradiation was confirmed by the following experiment. Figure 4 displays the time-course of the absorption change during photoirradiation at 0.06 and 0.90 mW/cm2, with and without an applied electric field. For irradiation at 0.06 mW/ cm2 without an electric field (dotted line in Figure 4a), the rate of decrease in the absorption is relatively slow, possibly reflecting the intrinsic dissolution rate of the Ag nanoparticles in pure water.26 It should be noted that simply applying an electric field in darkness also resulted in findings comparable to those obtained in the case of weak light illumination in the absence of an electric field, i.e., slight decrease in absorption. This relatively slow dissolution of the film suggests that the contribution of an external electric field to the structural change of the nanoparticles is negligible under the dark condition. When the electric field was applied to the particles during irradiation, dissolution appeared to accelerate. As depicted by the solid line in Figure 4a, the apparent acceleration in the change was observed after an induction period of approximately 50 s. Upon irradiation with a light source of greater intensity (0.90 mW/ cm2), acceleration by the electric field became much more evident (solid line in Figure 4b) than that of the weak light excitation. The induction period of the absorption change became shorter (ca. 30 s) under intense irradiation. The findings shown in Figure 4 proved that the observed change in the absorption could be accelerated by the electric field when particles were irradiated by light. Results presented in Figures 3 and 4 indicate that both photoirradiation and application of an external electric field contribute to the spectral change in Ag nanoparticles on a glass substrate.

Figure 5. Atomic microscopic images of the evaporated Ag nanoparticles on a glass substrate, as-deposited ((a) and (b)), following irradiation treatment at 0.90 mW/cm ((c) and (d)) in the absence of an electric field. Dimensions indicated on the images.

The observed change in the absorption can be due to changes in the structure and number of Ag nanoparticles on the glass substrate. Figure 5 exhibits AFM images of the as-deposited Ag nanoparticles on the glass substrate and those following photoirradiation treatments in the absence of an external electric field. These images show that photoirradiation without an applied field caused a slight decrease in the number of nanoparticles on the substrate. Observed absorption change presented in Figure 4 reflected this structural change in the number of Ag nanoparticles. On the other hand, photoirradiation under the applied field led to a much more remarkable change in the structure of the Ag nanoparticles. Figure 6 presents AFM images of nanoparticles after photoirradiation under an electric field. The images show that the treatment caused not only a decrease in the number of nanoparticles, as in the case of the photoirradiation in the absence of an applied electric field, but also a change in the structure of the nanoparticles (Figures 6a,b). Formation of larger sized particles as a result of this treatment was observed. These changes in the size of the nanoparticles were more apparent when irradiated light intensity became stronger (Figure 6c-f).

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Figure 6. Atomic microscopic images of Ag nanoparticles after treatment during irradiation at 0.06 mW/cm2 ((a) and (b)), 0.26 mW/cm2 ((c) and (d)) and 0.90 mW/cm2 ((e) and (f)) with an electric field of 20 V/cm. Dimensions indicated on the images.

In addition to the size increment, formation of an interconnected structure of the residual nanoparticles was also evident, forming networks on the glass substrate. Anisotropy in the interconnected structure, depending on the direction of the electric field, was not apparent in the present system. These results regarding changes in nanoparticle structure proved that photoirradiation of Ag nanoparticles, which were polarized by an external electric field, caused accelerated dissolution of the nanoparticles. Furthermore, the reaction induced under the present conditions was accompanied by not only dissolution but also size growth in some nanoparticles. The observed behavior of the spectral change in Ag nanoparticles suggests that both photoirradiation and electric polarization are indispensable for acceleration of the reaction of nanoparticles on the glass substrate under the present conditions. Generally, application of an electric field to metal particles in solution leads to localized electrochemical reactions, such as the deposition and/or dissolution of each particle, only when the field causes a sufficiently high electrochemical potential so as to induce reaction. An interesting example was previously shown in a system of copper particles.23,24 When copper particles with a diameter of approximately 1 mm were placed in solution under an electric field on the order of 20-50 V/cm, localized electrochemical dissolution and deposition induced growth of a metal wire directed to the electric field. The reaction was induced by electric polarization of the particles in solution; the estimated difference in the electrochemical potential in one particle should be several eV, which appears to be sufficient to induce localized electrochemical reaction. In comparison to the relatively large sized metal particles, the present electric polarization of the Ag nanoparticles approximately 20 nm in size should be much smaller. If polarization is assumed to be dependent on particle size in a linear fashion, the estimated value would be on the order of 10-5 V, which appears to be insufficient to induce an electrochemical reaction. In the present system, photoirradiation may assist the increase in the polarization of the nanoparticles by photoexcitation. When resonant electronic excitation of nanoparticles is induced via plasmon excitation,27-29 the resultant polarization corresponding to several eV can generate electrochemical reactions such as metal dissolution and deposition. It should be noted that photoirradiation alone failed to produce any apparent changes in the spectra and structure of the nanoparticles (Figures 4 and 5c,d). The observed reaction was

accelerated by an applied electric field in the present system. Contributions of the electric field to the structural change may be considered as follows. Possible electrochemical reactions in a single particle during the initial stage may include the oxidation reaction of Ag metal dissolution, the reductive reaction of hydrogen evolution, dissolved oxygen, and/or native oxide partially covering the surface of the Ag nanoparticles.30 As the reaction proceeds, a Ag+ ion is likely to be generated by dissolution, and diffuse to neighboring particles. When a Ag+ ion arrives at a neighboring particle, Ag deposition could occur as a reductive reaction in lieu of the other electrochemical reactions at the particles, leading to changes in the size of the Ag nanoparticles. The applied electric field can assist the migration of the Ag+ ions; consequently, the electric field accelerates the present reactions. Feasibility of the contribution of dissolved Ag+ ions in solution and the electric field is presumably supported by the observation of the induction period in the spectral changes at the initiation of dissolution. As shown in Figure 4, a photoinduced decrease in the absorption under polarization began following a specific induction time of 2040 s, depending on the irradiated light intensity. This period should reflect the point at which the concentration of dissolved Ag+ ions surrounding the nanoparticles attains an appropriate level so as to accelerate dissolution. After a specific concentration is achieved, switching of the reductive reaction to Ag deposition on the particles occurs, presumably leading to changes in the rate of the overall reactions in the present system.31 The importance of the electric field in the solution in order to induce structural change in nanoparticles was also supported by the effect of addition of electrolytes to the solution. Spectral change slowed in a manner similar to the case involving the absence of an electric field upon addition of 10 mM sulfuric acid to the system. Even after a few minutes, spectral change during irradiation with the applied electric field employing the system containing 10 mM sulfuric acid was negligible, which was similar to that observed in the absence of an applied electric field. Addition of electrolytes to pure water resulted in a decrease in the electric field between the two platinum electrodes in solution. This observation was a consequence of the change in ionic conductivity of the solution. As a result, acceleration of the dissolution could be reduced. Contribution of several electrically connected particles, as well as some other portion of the aggregation, can also be possible

Letters in the present sample. However, high specific resistance and AFM images suggest the existence of discrete island structures of the particles. Larger size of the metal structure should be substantially more sensitive in order for the electric polarization to cause structural change. Nonlinear time-dependent changes in absorption can occur due to the structural evolution of the particles. However, lack of size growth upon application of an electric field under conditions of darkness supports the fact that the observed structural change is initiated by the combined effects of photoillumination and application of electric field. Generally, small Ag nanoparticles readily react with oxygen, forming Ag2O and AgO upon exposure to air.32 Thus, specific amounts of oxidized Ag material contributed to the response of the structural change in the present experiment. In particular, Ag2O is known to be a photoactive material displaying a band gap in the visible region (2.25 eV or 550 nm).33 These oxides, however, should be readily photoreduced to yield metallic Ag clusters.32 Therefore, the dominant chemical process responsible for the induction of structural change in the present system could be attributed to metallic Ag dissolution and deposition. A structural change from discrete nanoparticles to the interconnected larger particles may change the electric field on the glass plate. Dissolved Ag+ ion in solution can also lead to a change in the field. A correlative study between the evolved structure and the electric field, which would be dynamically altered during photoirradiation and electric field treatment, is currently in progress. Conclusions The absorption spectrum of Ag nanoparticles on a glass substrate in pure water was altered when white light was irradiated under an external electric field. The change was attributed to the change in the size and number of Ag nanoparticles. The rate of change depended on illuminated light intensities. The system lacking an applied electric field demonstrated very small changes in the spectra and structure of Ag nanoparticles. The addition of the electrolyte, sulfuric acid, to the system also led to decreased acceleration. These results demonstrate that photoinduced structural change of Ag nanoparticles on an insulating material is accelerated by an external electric field in solution. The change is likely to be attributable to the highly localized electrochemical reactions of metal dissolution/deposition on the surface of the Ag nanoparticles. This phenomenon can be utilized for structural control of Ag nanoparticles adsorbed on the surface of an insulating material, circumventing the need for an external electric circuit. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References and Notes (1) Timp, G. Nanotechnology; Springer-Verlag: New York, 1999.

J. Phys. Chem. B, Vol. 106, No. 12, 2002 3045 (2) Nalwa, H. S., Ed. Nanostructured Materials and Nanotechnology; Academic Press: New York, 2000. (3) Murakoshi, K.; Nakato, Y. AdV. Mater. 2000, 12, 791. (4) Murakoshi, K.; Kitamura, T.; Nakato, Y. Jpn. J. Appl. Phys. 2001, 40, 1918. (5) Nakato, Y.; Murakoshi, K.; Imanishi, A.; Morisawa, K. Electrochemistry 2000, 68, 556. (6) Murakoshi, K.; Kitamura, T.; Nakato, Y. Phys. Chem. Chem. Phys. 2001, 3, 4572. (7) Murakoshi, K.; Nakato, Y. Jpn. J. Appl. Phys. 2000, 39, 4633. (8) Kreibig, U. In Handbook of Opitcal Properties; Hummel, R. E., Wismann, P., Eds.; CRC press: New York, 1997; Vol. 2, Chapter 7. (9) Gonella, F.; Mazzoldi, P. In Handbook of Nanostructured Materials and Nanotechnology; Nalwa, H., Ed.; Academic Press: New York, 2000; Vol. 4, Chapter 2. (10) Quinten, M.; Kreibig, U. Surf. Sci. 1986, 172, 557. (11) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (12) Michaels, A. M.; Nirmal, M.; Brus, L. E. J. Am. Chem. Soc. 1999, 121, 9932. (13) Engelmann, G. E.; Ziegler, J. C.; Kolb, D. M. Surf. Sci. 1998, 401, L420. (14) Kolb, D. M.; Ullmann, R.; Will, T. Science 1997, 275, 1097. (15) Schuster, R.; Kirchner, V.; Allongue, P.; Ertl, G. Science 2000, 289, 98. (16) Eckstein, H.; Kreibig, U. Z. Phys. D 1993, 26, 239. (17) Satoh, N.; Hasegawa, H.; Tsujii, K.; Kimura, K. J. Phys. Chem. 1994, 98, 2143. (18) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. 1999, 103, 3073. (19) Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.-C.; Wang, C. R. C. Langmuir 1999, 15, 701. (20) Kamat, P. V.; Flumiani, M.; Hartland, G. J. Phys. Chem. B 1998, 102, 3123. (21) Hodak, J. H.; Henglein, A.; Hartland, G. V. J. Phys. Chem. B 2000, 104, 9954. (22) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (23) Bradley, J.-C.; Chen, H.-M.; Crawford, J.; Eckert, J.; Ernazarova, K.; Kurzeja, T.; Lin, M.; McGee, M.; Nadler, W.; Stephens, S. G. Nature 1997, 389, 268. (24) Bradley, J.-C.; Crawford, J.; McGee, M.; Stephens, S. G. J. Electrochem. Soc. 1998, 145, L45. (25) Contributions of oxidized Ag that should be considered will be discussed in forthcoming papers. (26) Even in the case without an electric field, the rate of the change in the absorption also depends on the intensity of the photoirradiation (dotted lines in Figure 5a,b). This fact implies that slight amounts of the Ag nanoparticles dissolve by the photoirradiation under the present condition. (27) Kidd, T. R.; Meech, S. R.; Lemmon, D. Chem. Phys. Lett. 1996, 262, 142. (28) Fedurco, M.; Shklover, V.; Augustynski, J. J. Phys. Chem. B 1997, 101, 5158. (29) Kidd, R. T.; Lennon, D.; Meech, S. J. Chem. Phys. 2000, 113, 8276. (30) Deaerated treatment of the solution by bubbling argon gas to remove dissolved oxygen did not change the rate of the structural change significantly. The result suggests that the contribution of dissolved oxygen for reduction process might be minor. (31) Addition of 10 µM Ag+ did not show apparent effects on the rate of the absorption change and the induction period (20 V/cm under 0.9 mW/ cm2 illumination). Experiment using higher concentration of Ag+ in solution (in the order of mM or higher) was rather difficult due to deposition of Ag metal on the platinum electrode. Thus, at this moment, the contribution of Ag+ to induce the structural change is one of the possibilities in the present system. (32) Peyser, L. A.; Vinson, A. E.; Bartko, A. P.; Dickson, R. M. Science 2001, 291, 103. (33) Varkey, A. J.; Fort, A. F. Sol. Energy Mater. Sol. Cells 1993, 29, 253.