Visualization of Active Sites for Plasmon-Induced Electron Transfer

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Visualization of Active Sites for Plasmon-Induced Electron Transfer Reactions using Photoelectrochemical Polymerization of Pyrrole Hiro Minamimoto, Takahiro Toda, Ryo Futashima, Xiaowei Li, Kentaro Suzuki, Satoshi Yasuda, and Kei Murakoshi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12727 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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The Journal of Physical Chemistry

Visualization of Active Sites for Plasmon-Induced Electron Transfer Reactions using Photoelectrochemical Polymerization of Pyrrole Hiro Minamimoto, Takahiro Toda, Ryo Futashima, Xiaowei Li, Kentaro Suzuki, Satoshi Yasuda, and Kei Murakoshi* Department of Chemistry, Faculty of Science, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan

Abstract Spatially selective deposition of conductive polymer was observed at the Au nanostructures supported on TiO2 electrodes via plasmon-induced photo-polymerization of pyrrole monomers. The reactions were triggered by the excitation of localized surface plasmon resonance under near-infrared light illumination to the plasmon-active Au nanostructures. The morphological characteristics of the deposited polypyrrole prove the localization of the reactionactive sites in the plasmon-induced oxidation-reaction system. In addition, the estimation of reaction characteristics provides information on the spatial distribution and the electrochemical potential of the holes to contribute the reaction. The unique polymer-growing process observed

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in the present system provides the information on the mechanism of plasmon-induced oxidation reaction occurring at the active sites.

Introduction To improve the usability of light beyond the intrinsic limits of the materials for photochemical reactions, a combination of plasmon-active metal nanostructures and semiconductors has been proposed.1–7 One example of this is effective water splitting, which has been regarded as an important target for the realization of hydrogen economy. Although semiconductor electrodes such as titanium dioxide (TiO2) are promising photo-energy conversion materials, further improvement to the process requires a tailored response of sunlight in various wavelengths. TiO2 is known to be a superior candidate because of its very stable, its harmless properties, its conduction, and its valence band edge potential for water splitting.8–11 Even with TiO2, improving sensitivity to visible, near-infrared light is necessary because of its limited response in the ultraviolet region, which corresponds to about 6% of the solar radiation reaching the surface of the earth. Under broadband-visible illumination (λ > 400 nm), the application of plasmon-active metal nanostructures to TiO2 electrodes realizes significantly higher efficiency compared to bare TiO2 electrodes in semiconductor solar cells12–14 or in the oxygen-evolution reaction from water.15–20 These examples prove that a plasmonic photo-energy conversion system would be a useful tool, extending the ability to effectively use the energy of photons. Excitation of localized surface plasmon resonance (LSPR), which is the resonant photoninduced collective oscillation of free electrons in metals, leads to the formation of excited states in metals.2,21–23 The excited states produced by LSPR promote surface chemical reactions. After

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the 1981 report of an innovative theoretical proposal of efficient photochemistry induced on rough metal surfaces by the interaction between adsorbed molecules and a localized plasmon,24,25 various experimental approaches have proved the validity of LSPR-enhanced chemical reactions, such as metal depositions,26–29 molecule oxidation/reduction,30,31 and two-photon polymerization of resist materials.32,33 Recently, this idea has been extended to the formation of chemical bonds.34,35 Currently, it is recognized that plasmon-active materials can enhance the photoresponse to solar water-splitting systems.15–20,36–39 Several mechanisms for the LSPR enhancement have been proposed by considering the localized field-enhanced energy transfer,40,41 the photothermal heating42,43 and the exciton formation.44,45 Although the origin of plasmon-enhanced electron transfer reactions is still under discussion,46–49 unique controllability, efficiency, and selectivity for plasmonic photo-energy conversions have been receiving significant attention,3–5,50–54 and intense effort is being expended to establish composite plasmonic-metal/semiconductor systems.1–7,55–57 However, there is still room to develop the model of the electron transfer mechanisms. Previously documented results of the systematic experimental studies have already clarified the relationship between the metal nanostructures and excited plasmon mode.56–58According to previous reports, the induced plasmon mode strongly depends on not only the size and anisotropy, but also on the crystallinity, shape, and uniformity of the structures.58–61 It is recognized that nanostructures that have sharp shapes or that are differently coupled, such as bowtie structures, can generate very strong electric fields showing a wide variety of plasmon responses in molecules.62–65 In recent studies, theoretical calculations, such as the finite-difference time-domain (FDTD) method, have supported the tailored plasmon-induced phenomenon.66–69 For the application for photo-energy conversion via the LSPR excitation, unique catalytic activities in artificial photosynthesis, such

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as hydrogen and oxygen evolutions and nitrogen fixation, are attracting a great deal of attention.15,70,71 Meanwhile, the application of localized plasmon to photochemical reactions is improving, as mentioned above; however, details about the initial process, especially regarding the localization as well as the electrochemical potential of the excited electrons and holes at the active sites for the photoelectrochemical reaction have not been clarified yet. Understanding those details could be important to further accelerate the developments of plasmon-enhanced photochemistry. The present study visualizes the active site of plasmon-induced chemical reaction via site-selective polymerization of a conductive polymer, polypyrrole, on the Au nanoparticles loaded on TiO2 electrodes. The study observed the plasmon-enhanced oxidative polymerization reaction of pyrrole monomer on the surface of the electrodes. Because of the irreversible oxidation properties of pyrrole that form insoluble polypyrrole in water, deposited polymers indicate the reaction-active site of plasmon-induced oxidation reactions.72,73 Well-established properties of polypyrrole, such as its oxidation potential of +0.7 V vs. Ag/AgCl and its conductive behavior as a p-type semiconductor,74–78 could provide information on not only the spatial distribution of the active site, but also on the oxidation potential of holes to contribute the plasmon-induced oxidation. The information could be valuable for the construction of tailored, systems for effective plasmon-induced photo-chemical reactions.

Experimental Section Preparation of Au/TiO2 electrode Single crystalline TiO2 (110) (10 × 10 × 0.5 mm3; rutile, 0.05 wt% niobium doped; Furuuchi Chemical Co.) was rinsed with acetone and water in an ultrasonic bath for 10 min and then

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immersed into 20% HF solution for 10 min, followed by rinsing with water and drying under nitrogen flow. An atomically smooth surface was prepared by annealing at 600 °C in air. A plasmon-active Au nanostructured TiO2 substrate, Au/TiO2, was prepared by evaporating 10 nm of Au at a deposition rate of 0.2 nm s−1 followed by annealing at 500 °C for 30 min. The average particle size of Au island was about 100 nm. For the control experiments, the same Au nanostructures were also fabricated onto transparent electrode (Au/FTO). The surface of the substrate was characterized by an atomic-force microscope (AFM; Nanoscope-IIIa; Digital Instruments). Plasmon-induced oxidative polymerization of pyrrole Photocurrent measurements for Au/TiO2 were performed with a commercial potentio-stat and potential programmer (HZ-5000; Hokuto Denko) using Pt wire and Ag/AgCl (satd KCl) electrodes as the counter and the reference electrodes, respectively. The Au/TiO2 substrate was used as the working electrode, which contacted a stainless plate with an In-Ga ohmic contact on its back side. Under light illumination, in 0.5 M Na2SO4 aqueous solution with or without 10 mM of pyrrole, photocurrent was observed in the wavelength range of 540 - 900 nm. Figure S1 shows the schematic illustration of the cell. The plasmon-induced oxidative polymerization of the pyrrole monomer was performed in the electrochemical cell and observed using a confocal Raman microscope (×100 objective lens, spot diameter of ca. 3 µm) with the illumination of a near-infrared laser light (λex = 785 nm (1.58 eV), Iex = 1 mW). In-situ Raman measurements were carried out using normal backscattering geometry. Water used in the study was prepared by Milli-Q integral (18 MΩ cm−1). The concentration of the electrolyte solution was 0.5 M Na2SO4aq with and without 10 mM of pyrrole. The structures deposited on the surface were

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analyzed by an AFM (Nanoscope-IIIa; Digital Instruments) and a scanning electron microscope (SEM; JSM-6700FT; JEOL Ltd.).

Results and Discussion Figure 1 shows the photocurrent responses under the illumination of visible light onto Au/TiO2 kept at +0.5 V in electrolyte solutions both with and without pyrrole. Apparently, the photocurrent increased by adding pyrrole to the electrolyte solution. This increment reflects accelerated hole consumption under light illumination by the addition of pyrrole. The study also examined the surface structure of the Au/TiO2 by SEM observation after light illumination. After the illumination, the formation of deposits on the Au/TiO2 substrate was confirmed at the lightspot area. Figure 2 shows that the amount deposited increased with the intensity of the light and the exposure time. It is noteworthy that in the absence of Au nanostructures on TiO2 (110), neither photocurrent nor deposits were observed under comparable conditions. The Au/TiO2 can be used for plasmon-induced photo-oxidation reactions. The extinction spectrum of Au/TiO2 (Fig. S1) shows that the extinction maximum due to the plasmon-resonance wavelength at 810 nm, indicating that near-infrared light used in this experiment (785 nm, 1.58 eV) can excite LSPR of the substrate. As shown in Fig. 1, increments in the photocurrent due to the addition of pyrrole suggest that plasmon-induced oxidative polymerization of pyrrole proceeds in the present system, accompanied by water oxidation, under near-infrared light illumination.15 Deposits observed as conductive material in SEM images in Fig. 2 could be attributable to the formation of polypyrrole on the surface of the Au/TiO2. To confirm the formation of polypyrrole, Raman spectroscopic measurement was conducted. Typical Raman spectrum observed at +0.5 V is shown in the upper column of Fig

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3(a). For comparison, Raman spectrum obtained at potentio-static polymerization of pyrrole onto Au/FTO was also shown in the bottom column of Fig. 3(a). Characteristic Raman bands at 1600, 1323, and 930 cm−1 which are assigned to the C=C, C−C, and C−H stretching of polypyrrole were clearly observed in both spectra.79–81 The vibration mode at 930 cm−1, corresponding to C−H out of the plane deformation of the quinoid form, is characteristics to the formation of oxidized polypyrrole.81,82 The broad band of the 1200 to 1500 cm−1 region is derived from the backbone stretching band and other ring stretching bands.81 Moreover, we also carried out the XPS measurement. In the XPS spectra of the deposit, we confirmed the peaks of N1s assigned to polypyrrole (see supplementary information).83–85 The results prove that the polymerization of pyrrole takes place in the vicinity of Au nanostructures on Au/TiO2 under near-infrared light illumination to form the deposits. It was found that Raman scattering intensity depends on the electrochemical potential. Raman scattering from polypyrrole were not observed at −0.9 V. As the potential was polarized positive than −0.8 V, the evolution of Raman bands was observed (see supplementary information). Figure 3(b) depicts the electrochemical potential dependence of the Raman intensity of 930 cm−1 band assigned to the quinoid form in oxidized polypyrrole. It is shown that the evolution of the Raman band intensity increased at −0.8 V, and saturated at more positive than −0.4 V. The results indicate that the plasmon-induced polymerization oxidative reaction of pyrrole is triggered by generated holes under illumination of near-infrared light with the energy of 1.58 eV (785 nm) at polarizations more positive than −0.9 V (Fig. 3(c)). The potential dependence of the Raman band evolution provides the estimation on the electrochemical potential of the holes contribute to the oxidation of pyrrole. Generally, it is known that the relaxation of the excited holes in the contentious electronic states of the band

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structure of metals is very fast, typically a few tens fs.86,87 The relaxation to the Fermi level of excited holes after the formation of the exciton via the LSPR excitation could be much faster than the electron transfer of pyrrole oxidation. Thus, the present observation of the pyrrole oxidation could be attributed to the shift of the Fermi level to positive potential, or the formation of the active sites to accumulate holes after the relaxation of the excited holes as well as the injection of the excited electrons to the conduction band of TiO2 electrode. The onset potential of TiO2 (φTiO2), −0.8 V, for the polymerization suggests that the electrochemical potential of the trapped holes (φtrap) at the active site or the Fermi level is ≥ 0.68 V, because of the energy of illuminated light (hν = 1.58 eV) and the relation (hν ≥ −e(φTiO2 − φtrap)) (Fig. 3(c)). The validity of the estimated value of the electrochemical potential of the active site is proved by the experimentally observed the polymerization of pyrrole whose oxidation potential is 0.7 V.74 In the inverse potential region more negative than −0.9 V, the recombination of excited holes and hot electrons is promoted, resulting in the inhibition of polymerization (Fig. 3(d)). The relatively positive potential of the onset of polymerization (−0.8 V) vs. the potential of the flat band (−0.6 V) 88 could be attributable to the intense light intensity of confocal in-situ Raman measurements with relatively high photon density (1 mW at ca. 3 µm diameter spot corresponds to 3.5 × 104 W cm−2). To characterize the spatial localization of plasmon-induced polymerization reactions, the SEM measurements were carried out to obtain the images of polypyrrole deposits at the initial stage prepared by polarized light illumination for 10 s on an Au-isolated single structure (such as an anisotropic, rod-like structure) and on a series of multiple structures (such as dimer or trimer structures) (Fig. 4). In the case of a single Au-isolated structure (Fig. 4(a)), polypyrrole formed on both sides of the structure, parallel to the direction of polarization. In the case of a series of

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multiple structures, polypyrrole formed selectively at the gaps between the Au dimer- or trimerstructures (Fig. 4(b)). Previously documented results on theoretical FDTD calculations and experimental microscopic LSPR spectroscopies depict the localization of the electromagnetic field generated by LSPR of metal nanostructures as being at the sides of anisotropic particles and the gap of dimers.37,66,67,89,90 The present results on spatially selected formation of those polypyrrole smaller in size than a few tens of nm are consistent with the localization of the electromagnetic field by LSPR. The present result proves the important role of the spatial localization of the LSPR field in the plasmon-induced electron transfer reaction. The structural change of polypyrrole deposits during growth provides information on the reaction process promoted by LSPR. This study examined the time-dependence of morphological changes of polypyrrole. The SEM and AFM images in Fig. 5 show light illumination on the surface of the Au/TiO2 for 0, 10, 100, and 180 s. After illumination for 10 s, a very small amount of deposit was observed inside the illuminated area, and polypyrrole continued growing for 100 s. Finally, polypyrrole deposited throughout the entire irradiated area. These results suggest that morphologies change depending on the illumination time. To estimate semi-quantitatively the amount of polypyrrole formed by plasmon-induced polymerization, the study used AFM measurement to examine the morphological changes of a single Au nanostructure on Au/TiO2. Figure 6 shows the AFM images and cross-sectional views at various illumination times. After 10 s of illumination, a small amount of deposit less than 10 nm thick appeared at the edge of the Au nanostructures on Au/TiO2 (Fig. 6(b)). However, after 10 s of illumination, the height of the Au nanostructure was comparable to what it had been before illumination (see Fig. 6(a), which shows 0 s), indicating that the plasmon-induced reactions proceeded at the interface of the Au nanostructures and TiO2 the same as they did in the

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spatially selective deposition observed by the SEM measurement shown in Fig. 4. After illumination for 100 s, the thickness of the deposits increased to about 20 nm (Fig. 6(c)). It is noteworthy that the height of the Au was still comparable to that both before and after 10 s of illumination (Fig. S3). The AFM image showing 100 s of illumination indicates that polypyrrole grows selectively at the edge of Au nanostructures on TiO2 rather than covering entire Au surfaces (Fig. 6(b) and (c)). After 1000 s of illumination, Au particles were covered with polypyrrole, showing lateral growth, with a thickness of more than 40 nm (Fig. 6(d)). The estimated thickness of polypyrrole may provide the information on the rate of the plasmon-induced polymerization of pyrrole. In the present system, the reaction rate of the pyrrole molecules could be limited by the diffusion of pyrrole molecules to the active site in nano-size. From the AFM images, experimentally determined values of the volume of polypyrrole after 10, 100, and 1000 s of illumination are 2.0 × 10−4, 8.0 × 10−4, and 2.0 × 10−3 µm3, and gives the values of the deposition rate, 2.0 × 10−5, 8.0 × 10−6 and 2.0 × 10−6 µm3 s-1, respectively. These values are comparable with that roughly estimated from the diffusion-limited rate of the reaction, 2.3 ×10−5 µm3 s−1 (see supplementary information). Because of enough holes generated by LSPR excitation with relatively intense light illumination, the plasmon-induced polymerization of pyrrole in the present system proceeds effectively, but limited by the diffusion. The present observation is to investigate semi-quantitatively the rate of the plasmon-induced photo-electrochemical reaction taking place at highly localized areas of metal-semiconductor interfaces. In addition, the morphological characterizations of the present study (Figs. 4-6) provide interesting characteristics of the plasmon-induced reaction system. Highly localized deposition of polypyrrole at the initial stage visualizes the active site for the polymerization reaction of

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pyrrole. Usually, plasmon-induced electron transfer reactions are described by the charge in the electrochemical potential of the Fermi level of the system after the charge separation.7,21 According to this model, the polypyrrole should be deposited onto the surface of the Au nanostructures uniformly from the initial stage of polymerization. The present observation of the spatially selective deposition of polypyrrole proves the localization of the active sites in the plasmon-induced reaction system. Electrons and holes excited by LSPR excitation could highly localize to initiate the electron transfer reaction at the active site. In the present system, the edge of an Au nanostructure on the surface of TiO2 contributes to localizing holes to oxidize pyrrole molecules. In addition, the present study proves that the electrochemical potential of the Fermi level of the Au does not shift positively enough to induce the polymerization, even under conditions of LSPR excitation by intense near-infrared light illumination. The lateral growth of polypyrrole after 100 s of illumination is due to the nature of polypyrrole, which has been known as the conductive property by the doping with anions from surrounding electrolyte solutions. This anion-doped polymer shows typical semiconductor properties with high conductivity, leading to continuous growth via successive oxidative deposition onto the surface. In the present system, effective generation of holes at the active site may positively polarize the electrochemical potential of polypyrrole to induce continuous growth of polypyrrole films in the lateral direction. During the growth of polypyrrole on the surface of an Au nanostructure for 1000 s, this process should also contribute to the formation. As proof of this, Au nanostructures covered with deposited polypyrrole grew in height, as can be seen in Fig. 6(d) as well as the height histogram shown in Fig. S3. After its formation, direct excitation of polypyrrole by near-infrared light could also contribute to its growth because polypyrrole shows absorption of light in the regions from visible to near-infrared. However, the contribution of

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photo-absorption due to the polypyrrole itself, without LSPR excitation, could be minor, because of the decrease in the reaction rate for 1000 s of illumination. The spatial localization and the electrochemical potential dependence of the deposition clarify the contribution of LSPR rather than direct electronic photoexcitation of electrons. Generally, the contribution of the inter band transition from occupied d-bands to unoccupied spbands in the bulk band structure of Au can be considered under the photo-illumination. The energies for excitation of the inter-band transition are about 2.3 eV.91 Taking into account the energy of the incident laser (1.58 eV), the contribution of the inter band transition could be minor, because the energy of the incident laser is smaller than that of the inter band transition. On the other hand, the intra band excitation could contribute to the electronic excitation under the present illumination condition. However, this contribution to the polymerization reaction could be excluded. Specially localized deposition under controlled polarization direction of the illuminated light shown in Fig. 4 proves the importance of the LSPR excitation because the intra band excitation may not depend on the polarization direction. Among the contributions the LSPR, the localized field-enhanced energy transfer and the photothermal heating can also be considered to the enhancement of the chemical reactions.40–43 In the present system, however, highly localized deposition depending on the electrode potential can exclude these contributions. The energy transfer as well as photothermal heating cannot be controlled by the electrode potential. The contributions from the localized field-enhanced energy transfer could be very important to explain the highly spatial localization of the LSPR enhanced chemical reaction as shown in the previously documented results.41,90,92 However, the electrochemical potential dependence shown in the present results proves that the present polymerizations reactions were triggered by the holes accumulated at the active site. Addition to

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this potential dependence, we have proved that the laser power coefficients of the temperature rise of the Au nanostructure about 40 K per 1 mW−1.93 Based on this fact, it can be considered that the contribution of thermal effects would also be negligible in the present system. The findings of the present study make it clear that the initial stage of plasmon-induced photochemical reaction takes place at the specific sites localized at the interface. It can be considered that the holes generated by LSPR excitation are trapped on the electronic states at the interface between Au and TiO2. Because of the extremely short lifetime of the excited holes in the continuum electronic states of Au, importance of the active site for plasmon-induced chemical reaction has been discussed.3,15,37,90 The results of the present study are the experimental proof of the localization of the active sites in plasmon-induced photo-electrochemical reactions. Although there is still a margin for discussion, our proposed method could be a good candidate to explore the electron transfer process in the plasmonic chemical reaction system. More detailed investigations would clarify the dependence of the reaction on the Au structures, incident light wavelength, the oxidation potential of the reactants, and the energy of LSPR.

Conclusions The present technique using polymerization of pyrrole monomers with a plasmon-active Au/TiO2 substrate can be a powerful tool for visualizing a plasmon-induced reaction at an active site. For this system, an appropriate choice of electro-deposited polymer, polypyrrole, with suitable redox potential, high conductivity, and insolubility in electrolyte solutions, is key to realizing spatially selective deposition at the active site. In addition, based on the characterization of the morphology of the deposits, the reaction efficiency of plasmon-induced chemical reactions can be discussed. Furthermore, this method provides the ability to estimate

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the oxidation potential of holes because of the well-established redox properties of both monomers and polymers of the pyrrole molecule. The results of the present study imply the need for more detailed investigation of the reaction mechanism of plasmon-enhanced chemical reactions. By adapting various electro-deposited conducting polymers with distinct redox potentials and reaction rates, future studies could explore applications using a wide variety of LSPR modes in the UV-to-IR energy ranges to extend the possibility of plasmon-enhanced chemical reactions for more effective use of photon energies.

Acknowledgements This work was partially supported by Grants-in-Aid for scientific research from the Ministry of Education, Science and Culture, Japan (Nos. 26248001, 26620188, and 25620003).

Supporting Information. Brief statement in nonsentence format listing the contents of the material supplied as Supporting Information.

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FIGURES

Figure 1. Photocurrent measurements of Au/TiO2 photo-electrodes. The concentration of electrolyte solution was 0.5 M Na2SO4aq (a) with and (b) without 10 mM of pyrrole. The electrode potential was kept at +0.5 V.

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Figure 2. SEM images of the surface of Au/TiO2 substrate after illumination onto electrolyte solutions containing 10 mM of pyrrole monomer. Laser intensities were (a, b) 0.1 and (c, d) 1 mW. The wavelength of the incident laser was 785 nm, and the electrode potential was +0.5 V. The red, dashed-lines enclose the laser-exposure areas for the illumination time of (a) 400, (b) 1000, (c) 40, and (d) 100 s, respectively.

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Figure 3. (a) Raman spectra of deposited. The upper spectrum was obtained from the deposit on Au/TiO2 prepared at + 0.5 V. The bottom spectrum was for the polymer synthesized with the standard electrochemical method on Au/FTO. The upper and bottom spectra were obtained with

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exposure time of 600 s and 15 s. respectively. (b) Electrochemical potential dependence of the Raman intensity at 1600 cm−1 spectrum recorded at various potentials. (c, d) The energy diagram of Au/TiO2 electrodes at the applied electrode potentials were (c) +0.5 and (d) −0.9 V, respectively.

Figure 4. SEM images of (a) single rod-like and (b) dimer Au structures after the illumination. The black lines and double-headed arrows indicate the scale bar of 100 nm and the polarization direction of the incident light, respectively. The black shadows inside the red, dashed-line circles correspond to the position of deposits.

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Figure 5. SEM and AFM images of the Au/TiO2 substrate after the illumination. Laser exposure times were (a) 0, (b) 10, (c) 100, and (d) 180 s. Red and white dashed-line circles indicate the laser-illumination position. The wavelength and the light intensity were 785 nm and 1mW, respectively.

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Figure 6. AFM images of the Au island on the TiO2 substrate after laser illumination, and expected cross-sectional views of the structures with the height line obtained by AFM. The red parts indicate the morphology of the deposited polypyrrole. Laser exposure times were (a) 0, (b) 10, (c) 100, and (d) 1000 s, respectively.

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Table of Contents Graphic

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions K.M., and S.Y. designed the research; T.T., R.F., X. L. and K.S. prepared the samples and performed experiments; T.T., R.F., X.L., K.S. and H.M. analyzed and interpreted the results; H.M., T.T. and K.M. developed the model and prepared this manuscript.

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Figure 1. Photocurrent measurements of Au/TiO2 photo-electrodes. The concentration of electrolyte solution was 0.5 M Na2SO4aq (a) with and (b) without 10 mM of pyrrole. The electrode potential was kept at +0.5 V. 107x124mm (150 x 150 DPI)


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Figure 2. SEM images of the surface of Au/TiO2 substrate after illumination onto electrolyte solutions containing 10 mM of pyrrole monomer. Laser intensities were (a, b) 0.1 and (c, d) 1 mW. The wavelength of the incident laser was 785 nm, and the electrode potential was +0.5 V. The red, dashed-lines enclose the laser-exposure areas for the illumination time of (a) 400, (b) 1000, (c) 40, and (d) 100 s, respectively. 147x128mm (150 x 150 DPI)



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(a) Raman spectra of deposited. The upper spectrum was obtained from the deposit on Au/TiO2 prepared at + 0.5 V. The bottom spectrum was for the polymer synthesized with the standard electrochemical method on Au/FTO. The upper and bottom spectra were obtained with exposure time of 600 s and 15 s. respectively. (b) Electrochemical potential dependence of the Raman intensity at 1600 cm−1 spectrum recorded at various potentials. (c, d) The energy diagram of Au/TiO2 electrodes at the applied electrode potentials were (c) +0.5 and (d) −0.9 V, respectively. 127x145mm (300 x 300 DPI)


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Figure 4. SEM images of (a) single rod-like and (b) dimer Au structures after the illumination. The black lines and double-headed arrows indicate the scale bar of 100 nm and the polarization direction of the incident light, respectively. The black shadows inside the red, dashed-line circles correspond to the position of deposits. 160x73mm (150 x 150 DPI)



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Figure 5. SEM images of (a) single rod-like and (b) dimer Au structures after the illumination. The black lines and double-headed arrows indicate the scale bar of 100 nm and the polarization direction of the incident light, respectively. The black shadows inside the red, dashed-line circles correspond to the position of deposits. SEM and AFM images of the Au/TiO2 substrate after the illumination. Laser exposure times were (a) 0, (b) 10, (c) 100, and (d) 180 s. Red and white dashed-line circles indicate the laser-illumination position. The wavelength and the light intensity were 785 nm and 1mW, respectively. 276x117mm (150 x 150 DPI)


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Figure 6. AFM images of the Au island on the TiO2 substrate after laser illumination, and expected crosssectional views of the structures with the height line obtained by AFM. The red parts indicate the morphology of the deposited polypyrrole. Laser exposure times were (a) 0, (b) 10, (c) 100, and (d) 1000 s, respectively. 212x112mm (150 x 150 DPI)



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TOC 222x158mm (150 x 150 DPI)


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Visualization of Active Sites for Plasmon-Induced Electron Transfer

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