Improvement of Plasmon-Enhanced Photocurrent Generation by

Article pubs.acs.org/JPCC

Improvement of Plasmon-Enhanced Photocurrent Generation by Interference of TiO2 Thin Film Xu Shi,† Kosei Ueno,†,‡ Tomoya Oshikiri,† and Hiroaki Misawa*,† †

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan

‡

S Supporting Information *

ABSTRACT: In this study, we demonstrated an improvement in the plasmonenhanced photocurrent generation due to Fabry−Pérot interference of titanium dioxide (TiO2) thin-film electrodes loaded with Au nanoislands (Au-NIs). TiO2 thin films with thicknesses of 215−274 nm, which show obvious Fabry−Pérot interference, were deposited via atomic layer deposition (ALD) onto silica glass and were modified by the deposition of Au-NIs onto their surface. Anodic plasmon-enhanced photocurrent generation was observed over the Au-NIsloaded TiO2 thin-film photoelectrodes. The incident photon to current efficiency (IPCE) action spectra correlated strongly with the Au-NIs plasmon resonance and exhibited a strong dependence on the thickness of the TiO2 thin film. The photocurrent conversion efficiency increased when the transmission constructive interference wavelength overlapped with the Au-NIs plasmon resonance band. This work provides a simple and applicable approach for the further design of low-cost and lightweight plasmon-enhanced energy conversion devices.

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INTRODUCTION Titanium dioxide has been the focus of a large amount of research interest since the discovery by Fujishima and Honda of photochemical water splitting on TiO2 photoelectrodes under irradiation with UV light.1,2 It has been employed in numerous research areas, such as photovoltaics,3−5 photochemical water splitting,6−8 and organic degradation,9,10 for its excellent photocatalytic properties and low cost, low toxicity, and longterm stability. Despite the advantages of TiO2, only UV light, which accounts for ∼4% of the entire solar spectrum, can be used to directly drive the photovoltaic and photochemical effects for the wide band gap of the most common forms of TiO2 (3.0 eV for rutile and 3.2 eV for anatase).11 Efficiency improvements in the photovoltaic and photochemical responses of TiO2 through the use of visible-light illumination are therefore possible. Significant efforts have been made to extend the cutoff wavelength of TiO2 by making use of doped anions and cations,12−15 defect creation,16,17 and organic dye sensitization.18,19 Recently, the plasmon-assisted photoreaction improvement of TiO2 in the visible region has gained increased interest.20−28 Plasmon resonances that arise from the collective oscillations of the electrons close to the surface of the noble metal have the ability to induce electron/hole generation, and the plasmonexcited electrons can be transferred to the conduction band of TiO2.29−31 Transient absorption measurements have been used to show an efficient electron transfer from gold nanodots to TiO2 with a yield of approximately 40%.32 Plasmon-enhanced photocurrent generation and water oxidation in aqueous supporting electrolytes, even in the near-infrared wavelength © 2013 American Chemical Society

region, using gold-nanorods-loaded TiO2 single-crystal photoelectrodes have been demonstrated by our group.28,33 We also demonstrated that the contact between gold nanoparticles and TiO2 plays an important role in photocurrent enhancement and water oxidation.34 The plasmon-induced charge separation mechanism of Au nanoparticles/TiO2 system is believed to be electron transfer from plasmon-excited Au nanoparticles to the conduction band of TiO2.29−35 Therefore, to obtain higher photocurrent generation efficiency, it is important to increase the probability of interaction between Au nanoparticles and the incident photons to increase the creation of plasmon-excited electrons. In most Au/TiO2 heterogeneous systems, TiO2 nanocrystal particles or TiO2 single crystals are employed. However, the use of lightweight and reliable low-cost thin films for solar energy conversion is promising. Titanium dioxide thin films are transparent in the visible and infrared wavelength regions, with a high refractive index (n ∼ 2.7). When a TiO2 thin film with a thickness of several hundred nanometers deposited on lowrefractive-index silica glass (n ∼ 1.46) is used, the probability of plasmon excitation of Au nanoparticles on the TiO2 thin films should be strongly enhanced by Fabry−Pérot interference when the transmission constructive interference wavelength of the TiO2 thin film overlaps the plasmon band of the Au nanoparticles (Scheme 1). On the basis of this expectation, we here report on the spectral properties and the photocurrent Received: August 24, 2013 Revised: November 2, 2013 Published: November 6, 2013 24733

dx.doi.org/10.1021/jp408472g | J. Phys. Chem. C 2013, 117, 24733−24739

The Journal of Physical Chemistry C

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diffraction spectra of the TiO2 thin films were recorded with a RIGAKU RINT-2000/PC using Cu Kα radiation and a scanning speed of 2° (2θ)/min within the 2θ range from 20 to 60°. Photoelectrochemical Measurement. An In−Ga alloy (4:1 in weight ratio) film was pasted onto the backside and the side walls of the TiO2-film-coated silica substrate to make ohmic contact and was connected to an electrochemical analyzer (ALS/CH Instruments 852C, ALS) with a copper lead wire. The conductivity between the TiO2 and the In−Ga alloy was dominated by the ohmic contact of the side walls. The normal incident illumination was always used in our experiment, and it is important to maintain the normal incident due to the sensitive interference resonance wavelength dependence on the incident angle. The method to ensure the normal incident during the photoelectrochemical measurement is to align the optical circuit and the Au-NIs/TiO2 electrode to make sure the incident and reflected light of the Au-NIs/TiO2 go through the same pinhole that located about 10 cm in front of the Au-NIs/TiO2 electrode; the pinhole size is approximately 2.5 mm. For the measurement of the IPCE action spectra and I−V curves, a platinum wire and a saturated calomel electrode (SCE) were employed as the counter electrode and reference electrode, respectively. To obtain the IPCE action spectra, band-pass filters with a bandwidth less than 15 nm full-width at half-maximum (fwhm) were used. An Ar-gas-bubbled KClO4 (0.1 mol/dm3) aqueous solution was used as a supporting electrolyte solution without a specific electron donor. The AuNIs/TiO2 working electrode potential was set at +0.3 V versus SCE when the I−t curve was measured.

Scheme 1. Fabry−Pérot Interference in the Au-NIs/TiO2 Thin Films Deposited onto a Silica Substratea

a

Normal incident light illumination was used in the present experiment.

generation efficiency of Au-NIs-loaded TiO2 thin-film electrodes and demonstrate the dependence of these properties on the thickness of the TiO2 thin films.

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EXPERIMENTAL SECTION Au-NIs/TiO2 Substrate Preparation and Characterization. Before ALD deposition, silica glass with a size of 10 × 10 × 0.5 mm3 (Pier Optics, Japan) was rinsed with acetone, methanol, and deionized water in an ultrasonic bath for 5 min and was then immersed into a solution containing concentrated sulfuric acid and hydrogen peroxide (30 wt % in H2O) in a volume ratio of 1:1 for 1 h. The glass was rinsed with deionized water and dried with a pure nitrogen flow. Titanium dioxide thin films with different thicknesses were deposited onto the silica glass substrate using a commercial hot-wall flow-type ALD reactor (SUNALETM R series, Picosun, Finland). The deposition procedure involved alternating exposure of TiCl4 and deionized water vapor at a process temperature of 300 °C with N2 as a precursor carrier and purge gas at a pressure of 1.6 kPa. The pulse and purge times for the precursors were 0.1 and 4 s, respectively. The deposition rate of TiO2 on silica glass was estimated to be 0.43 Å per cycle. To fabricate uniform, onesided deposition TiO2 films, the silica glass was tightly attached to a smooth cover glass, which was washed with deionized water in an ultrasonic bath for 5 min. The TiO2 film thickness was estimated from the interference oscillations in the visible spectra using an optical spectroscopic reflectometer (F20, Filmetrics, U.S.A.) with a lower bound accuracy of ±2 nm, and the difference between film thicknesses is ±4 nm. A 3 nm Au thin film was evaporated by a thermal evaporator with a deposition rate of 0.2 Å per second. All of the depositions were performed under a pressure less than 5.0 × 10−3 Pa, and the thicknesses were monitored with a quartz-crystal oscillator placed near the substrates. After evaporation of the 3 nm Au film, the samples were annealed in H2 (3.9% in Ar) at 250 °C for 2 h, and the Au nanoislands appeared on the TiO2 film surfaces. The optical reflectance and transmittance spectra were obtained using a photonic multichannel analyzer (PMA C7473; Hamamatsu Photonics) system operated within the wavelength range of 400−900 nm. The surface morphology of the ALDdeposited TiO2 and the Au-NIs on the TiO2 surface were observed by field-emission scanning electron microscopy (FESEM, JSM-6700FT, JEOL), whose maximum resolution at an electron acceleration voltage of 15 kV was 1 nm. X-ray

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RESULTS AND DISCUSSION Characteristics of ALD-Deposited TiO2 Thin Films. To study the relationship between the thickness and the photocurrent generation of Au-NI-modified TiO2 thin films, we deposited a series of TiO2 thin films onto silica glass substrates by atomic layer deposition (ALD), which is a method for depositing thin films with precise thickness control and excellent step coverage; this method is known for its unique self-limiting growth and layer-by-layer deposition mechanism.36−38 However, for the deposition of crystalline TiO2 thin film with thickness larger than 200 nm, the obvious aggregation of TiO2 crystalline were formed, which makes the ultraprecise thickness control difficult in experiment. The TiO2 film thicknesses were estimated from interference oscillations using an optical spectroscopic reflectometer and were confirmed by SEM cross-section observations (Figure S1 of Supporting Information). Color photographs of the TiO2 thin films on silica glass substrates are shown in Figure 1. The TiO2 film thicknesses ranged from 215 to 274 nm and exhibited a characteristic color due to the Fabry−Pérot interference. The colors of the sample edges differed from those of the centers, where the colors were nonuniform because the center part of the silica glass only had TiO2 deposited on one side, whereas both sides were covered near the edge. One-sided TiO2 deposition is favorable for optical spectrum measurements compared to two-sided TiO2 deposition, which results in a nonuniform film on the back side. Only the center area with a diameter of 2 mm was used for the investigation of the photoelectrochemical (PEC) properties. The XRD investigation, as shown in Figure 2, indicated that the crystallographic phase of the TiO2 films prepared in the present work, with thicknesses that ranged from 215 to 274 nm, 24734



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thin film in thickness of 215, 228, 243, and 274 nm, respectively (Figure S3 of Supporting Information); these roughness factors correspond to approximately λ/40. The Au-NIs loaded on all of the TiO2 thin films exhibited similar particle size distributions and nearly round shapes when viewed from the top, and the statistical analysis of the particles for all samples revealed an average size distribution of 11.5 nm in diameter with a standard deviation of approximately 6 nm (Figure S4 of Supporting Information). The PEC properties of the Au-NIs/TiO2 were studied under normal light illumination, as shown in Scheme 1. Figure 3c,d shows the I−t and I−V curves, respectively, of the Au-NIs/TiO2 photoelectrodes with TiO2 thicknesses of 215 and 243 nm under illumination by 600 nm monochromatic light with an intensity of 1.68 mW/cm2. The Au-NIs/TiO2, with a thickness of 215 nm, exhibited a much higher photocurrent than the 243 nm film under irradiation with 600 nm light. To obtain the plasmon resonance band of the Au-NIs and eliminate the interference effects, we fabricated Au-NIs on an ALD-deposited TiO2 thin film with a thickness of 30 nm. The extinction spectrum shown in Figure 4, which was measured in a 0.1 M KClO4 solution, represents the plasmon resonance band of the Au-NIs. The Au-NIs plasmon resonance band is centered at 603 nm, and the wavelength region (from 540 to 675 nm) marked with light-blue depicts the full-width at halfmaximum (fwhm) of the plasmon band. Figure 5a−d shows the transmission and reflection spectra of the TiO2 films with thicknesses of 215 and 243 nm with and without Au-NIs loaded; the spectra reveal the interference wave due to the high refractive index of the TiO2 films and the large refractive index difference from the surrounding materials. The light-blue wavelength region indicates the fwhm of the Au-NIs plasmon resonance band, as shown in Figure 4. The wavelengths at maximum or minimum reflectance and transmittance were correlated with the thickness and the refractive index of the thin film: thicker films exhibited a redshift in the maximum or minimum. The reflectance and transmittance of the TiO2 films show symmetric interference waves with maximum values that correspond to the minimum values of each other. The 1-T-R of the TiO2 thin film without Au-NIs increased as the wavelength decreased, as shown by the green lines in Figure 5a,c. The ALD-deposited TiO2 films showed obvious agglomerated grains on their surfaces, as seen in the SEM micrographs in Figure 3a,b; these films exhibited significant Rayleigh scattering and resulted in the nonzero and wavelength-dependent 1-T-R spectra. A key step in improving the efficiency of the photoreaction is to increase the optical absorption. Our investigations were thus primarily based on the correlation of the TiO2 thin film interference and the Au-NIs plasmon resonance. When the transmittance constructive interference of the TiO2 thin films overlaps with the plasmon resonance of the Au-NIs, the possibility of incident light interacting with the Au-NIs is increased, and an increasing number of photons are captured by the Au-NIs, which results in a large difference between the transmittance spectra with and without Au-NIs-loaded TiO2 films. For the 215 nm thick TiO2 films, the transmission constructive interference wavelength was approximately 600 nm, which is in the wavelength region marked with light-blue and total overlaps the Au-NIs plasmon band. This overlapping led to a wavelength mismatch of the constructive interference of the transmission and reflection at approximately 680 nm (as the black and red lines show in Figure 5b) and resulted in a larger difference in the 1-T-R

Figure 1. Photographs of the TiO2 films deposited onto silica glass by atomic layer deposition at 300 °C. The TiO2 films thicknesses are (a) 215, (b) 228, (c) 243, and (d) 274 nm.

Figure 2. XRD measurements of the TiO2 thin films with thicknesses that range from 215 to 274 nm; the films were deposited by atomic layer deposition at 300 °C.

were anatase-type titanium dioxide. The anatase (101) reflections, at 2θ = 25.3°, exhibited the highest intensities in the XRD spectra. In particular, the XRD intensity of the TiO2 films gradually increased with increasing TiO2 film thickness. Anatase-phase TiO2 has previously been shown to exhibit substantially greater photocatalytic activity than rutile-phase TiO2 because the conduction band of the anatase phase is 0.2 eV more negative than that of the rutile phase and because the anatase phase is more hydrophilic than the rutile phase.39 Thus, anatase TiO2 is valuable for the investigation of the PEC properties. PEC and Optical Properties of Au-NIs/TiO2 Thin Films with Thicknesses of 215 and 243 nm. Pure anatase TiO2 absorbs light within the UV region due to its large bandgap.11 To improve and extend the activity in the visible region, we have deposited small Au nanoparticles onto TiO2 thin films. The Au nanoparticles can increase the incident light-absorption cross-section and enhance the generation of electron/hole pairs due to the localized surface plasmon resonance. In our study, we fabricated Au-NIs by thermal evaporated a 3 nm Au thin film onto a TiO2 film surface and annealed the film under H2 to increase the charge-carrier density of the TiO2 by creating a high concentration of oxygen vacancies.16 Top-view SEM micrographs of the Au-NI-loaded TiO2 thin films with thicknesses of 215 and 243 nm are shown in Figure 3a and b, respectively. The TiO2 surface morphology indicated significant surface roughening. The agglomerate formation can be attributed to the growth of anatase-type TiO2 crystals during the ALD deposition process. The root-mean-square roughness factor Rrms of the TiO2 films, measured by atomic force microscopy, are 13.7, 13.5, 13.8, and 13.9 nm for TiO2 24735



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Figure 3. SEM images of Au-NIs on TiO2 films with thicknesses of 215 nm (a) and 243 nm (b), and the I−t plots (c) and I−V curves (d) of AuNIs/TiO2 with thicknesses of 215 and 243 nm under illumination with 600 nm monochromatic light with an intensity of 1.68 mW/cm2. The scale bars in the SEM images represent 200 nm. The potential for the I−t measurement was 0.3 V vs SCE, and the scanning speed for the I−V measurement was 5 mV/s.

films with and without Au-NIs, as indicated by the red and green lines in Figure 5c,d. To provide details of our investigation of the 1-T-R difference between the TiO2 films with thicknesses of 215 and 243 nm, the Δ(1-T-R) spectra with and without Au-NIs with thicknesses of 30, 215, and 243 nm are plotted in Figure 6a. The Δ(1-T-R) spectrum reveals the Au-NIs optical absorption and scattering, and, in the case of small Au nanoparticles (∼10 nm), it is dominated by the absorption of the optical absorption cross-section, which is far larger than the scattering cross-section.40 As previously discussed, the Au-NIs plasmon resonance highly depend on the TiO2 thin-film interference. The Δ(1-T-R) spectrum of the TiO2 thin film at 215 nm shows an obvious Au-NIs plasmon resonance band at approximately 600 nm, and the Δ(1-T-R) value is increased compared to that of the Au-NIs on the 30 nm TiO2 thin film due to the overlap of the transmission constructive interference and the Au-NIs plasmon resonance band. In the Δ(1-T-R) spectrum of the 243 nm thick TiO2 film, an unobvious Au-NIs Δ(1-T-R) band was observed at approximately 600 nm that exhibited a low Δ(1-T-R) value due to the negligible overlapping of the transmission constructive interference and the Au-NIs plasmon resonance band. The TiO2 photoelectrode loaded with Au-NIs has already been demonstrated to achieve plasmon-enhanced photocurrent at visible wavelengths.34 The monochromatic efficiency is indicated by the incident photon to current efficiency (IPCE), which is simply the number of electrons measured under shortcircuit conditions divided by the number of incident photons. The IPCE value is calculated from the formula

Figure 4. Plasmon resonance band of Au nanoislands loaded on a 30nm-thick ALD deposited TiO2 thin film surface; the measurement was performed with the sample in 0.1 M KClO4 solution. The particle size of the Au nanoislands is approximately 11.5 nm with a standard deviation of approximately 6 nm.

between the TiO2 films with and without Au-NIs at a wavelength of approximately 600 nm (as shown by the green lines in Figure 5a,b). However, for the 243 nm thick TiO2 film, the transmittance constructive wavelengths were approximately 480 and 680 nm, which are far from the Au-NIs plasmon resonance band, and a transmittance destructive interference wavelength was present at approximately 560 nm, which led to negligible overlapping of the interference and Au-NIs plasmon resonance. The negligible overlapping resulted in a small difference in the transmittance and the 1-T-R between the TiO2 24736



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Figure 5. Reflection, transmission, and 1-T-R spectra of TiO2 films with a thickness of 215 nm without Au-NIs (a) and with Au-NIs (b) and those of films with a thickness of 243 nm without Au-NIs (c) and with Au-NIs (d).

of the 243 nm TiO2 film shows only a small hump without large enhancement at 620 nm. Δ(1-T-R) and IPCE Spectra of Au-NIs/TiO2 Thin-Film Electrodes. The transmission and reflection spectra of TiO2 thin film with vary thickness are shown in the Figure S6 (Supporting Information). Transmission constructive (reflection destructive) wavelength of 215, 228, and 243 nm TiO2 are totally, partially, and negligibly overlapping with the Au-NIs plasmon band in the wavelength region longer than 603 nm, respectively. The 274 nm shows a transmission constructive (reflection destructive) interference partially overlapping with Au-NIs plasmon band in the wavelength region shorter than 603 nm. These totally, partially, and negligibly overlap between the interference of TiO2 thin film and Au-NIs plasmon band lead to the difference of Δ(1-T-R) spectra. The Δ(1-T-R) spectra and the IPCE action spectra of Au-NIs/TiO 2 photoelectrodes with various TiO2 film thicknesses are shown in Figure 7a and b, respectively. The IPCE action spectra correlate with the Δ(1-T-R) spectra for all the Au-NIs/TiO2 photoelectrodes. The IPCE of bare TiO2 (without Au-NIs) annealed in H2 is shown in Figure 7b for reference. A greater Δ(1-T-R) intensity indicates a greater number of incident photons interacting with the Au-NIs, which results in a higher photocurrent conversion efficiency. The two higher Δ(1-T-R) values for the Au-NIs on 215 and 228 nm thick TiO2 films exhibit higher IPCE values, and both the Δ(1-T-R) and IPCE bands are centered at approximately 610 nm. The 215 nm TiO2 exhibited the highest Δ(1-T-R) value, although its IPCE is similar to that of the 228 nm film, probably due to an improvement of the photocatalytic properties of TiO2 with increased film thickness.41,42 For the TiO2 film with thicknesses of 274 nm, the IPCE spectrum shows blue shift that correspond to the blue shift of the Au-NIs Δ(1-T-R) band. The IPCE value decreases as the Δ(1-T-R) values decreases. For the TiO2 film thickness of 243 nm, the IPCE value sharply decreases and

Figure 6. (a) Δ(1-T-R) spectra, with and without Au-NIs, of TiO2 thin films with thicknesses of 30, 215, and 243 nm. (b) The IPCE action spectra of Au-NIs/TiO2 with TiO2 thicknesses of 215 and 243 nm.

IPCE(%) = (1240 × Jsc )/(λ × Iinc) × 100

(1)

where Jsc is the short-circuit photocurrent (A·cm−2), Iinc is the incident light power (W·cm−2), and λ is the wavelength (nm). The IPCE action spectra of the Au-NIs/TiO2 with thicknesses of 215 and 243 nm are shown in Figure 6b. The IPCE spectrum of the 215 nm TiO2 film reveals a clear enhancement band at approximately 620 nm (shown in black). In contrast, the IPCE 24737



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CONCLUSIONS In summary, we demonstrated the dependence of the plasmonenhanced photocurrent generation of Au-NIs-loaded anataseTiO2 thin films prepared on the silica substrate by ALD deposition. Anodic photocurrent enhancement that depended on the thickness of a TiO2 thin film was observed using AuNIs/TiO2 electrodes under illumination with visible light, and the IPCE action spectra of the Au-NIs/TiO2 electrodes closely correlated to the Au-NIs plasmon band. This result indicates that the photocurrent originated from the plasmon-excited AuNIs. We easily controlled the plasmon-enhanced photocurrent conversion efficiency by controlling the TiO2 thin film thickness within several tens of nanometers. Higher photocurrent conversion efficiency was obtained when the TiO2 thin film transmittance constructive interference overlapped with the plasmon resonance of the Au-NIs. We believe that this thinfilm-thickness-dependent photocurrent generation is functional for the further design of low-cost and lightweight plasmonenhanced energy conversion devices.

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ASSOCIATED CONTENT

* Supporting Information

Figure 7. Δ(1-T-R) spectra (a) and IPCE spectra (b) of the Au-NIs/ TiO2 thin-film electrodes. The IPCE spectrum of bare TiO2 with a thickness of 270 nm is plotted as the black, hollow-square scatter line shown in the lower panel. The data for TiO2 films with thicknesses of 215 and 243 nm were obtained from Figure 6

S

Cross-section of Au-NIs/TiO2 thin film on silica glass, XPS characterization of the back of ALD TiO2 thin film, AFM characterization of surface roughness of ALD deposited TiO2 thin film, statistical analysis of the Au-NIs particle size distribution, and calculation of interference reflection spectrum of TiO2 thin film on silica glass. This material is available free of charge via the Internet at http://pubs.acs.org.

shows the lowest value, which results from the lowest Δ(1-TR) of its Au-NIs. The IPCE action spectra of the all the AuNIs/TiO2 and the Δ(1-T-R) spectra coincide with the overlap between TiO2 thin film interference and Au-NIs plasmon band, which indicates that the photocurrent enhancement results from the Au-NIs plasmon resonance that is enhanced by coupling with the TiO2 thin-film interference. For the Au-NIs/ TiO2 films with thicknesses of 228 and 243 nm, the difference in their thicknesses is about 15 ± 4 nm, yet their PEC properties differ dramatically. This interference effect in PEC properties provides a facile approach for tailoring the plasmonenhanced photocurrent generation. The mechanism for the anodic photocurrent enhancement of Au-NIs/TiO2 electrodes is direct electron transfer29−35,43−45 from the photon-excited Au-NIs to the conduction band of TiO2 at the Au-NIs/TiO2 heterogeneous interface; the holes in the Au-NIs with active oxidation ability are immediately captured by the TiO2 surface state and, consequently, by the oxidation of water to evolve oxygen.33,34 For wavelengths shorter than 500 nm, the electron transfer from the Au-NIs to the conduction band of TiO2 is based on direct excitation of the electron interband transition of the Au-NIs and shows that the IPCE value increases sharply as the wavelength decreases. For wavelengths longer than 500 nm, the electron transfer is dominated by plasmon-excited electrons of the Au-NIs and their transfer to the conduction band of TiO2; this process results in a plasmon-enhanced IPCE band in visible region. The dependence of the IPCE action spectra on the Au-NIs plasmon band enables us to confirm the plasmon resonance origin of the observed photocurrent enhancements. The plasmon-enhanced optical and PEC activities could be facilely engineered through control of the thickness of the TiO2 thin film.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-11-706-9358. Fax: +81-11-706-9359. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by funding from the Ministry of Education, Culture, Sports, Science, and Technology of Japan: KAKENHI Grant-in-Aid for Scientific Research No. 23225006, the Nanotechnology Platform (Hokkaido University), and the Low-Carbon Research Network of Japan. This study was also supported by the China Scholarship Council (CSC).

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