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Local Electric Field-Enhanced Plasmonic Photocatalyst : Formation of Ag Cluster-Incorporated AgBr Nanoparticles on TiO

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Yoshihiro Hayashido, Shin-ichi Naya, and Hiroaki Tada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04894 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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The Journal of Physical Chemistry

Local Electric Field-Enhanced Plasmonic Photocatalyst : Formation of Ag Cluster-Incorporated AgBr Nanoparticles on TiO2 Yoshihiro Hayashido,a Shin-ichi Naya,b Hiroaki Tada a,b * Department of Applied Chemistry, School of Science and Engineering, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan b Environmental Research Laboratory, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan a

Supporting Information Placeholder ABSTRACT: A successive ionic layer adsorption and reaction-photoreduction (SILAR-PR) technique forms Ag clusters mainly in the interior of AgBr nanoparticles (NPs) on the TiO2 surface (Ag@AgBr/mp-TiO2). The localized surface plasmon resonance (LSPR) of Ag NP-loaded TiO2 (Ag/mp-TiO2) rapidly weakens in the air at room temperature. In contrast, the LSPR intensity of Ag@AgBr/mp-TiO2 is preserved in the air at least for several days. Ag@AgBr/mp-TiO2 stably exhibits a high level of photocatalytic activity for the decomposition of 2-naphthol under irradiation of visible-light (λ > 400 nm), whereas Ag(core)-SiO2(shell) NPincorporated TiO2 (TiO2/Ag@SiO2) is only active near UV region (J. Am. Chem. Soc. 2008, 130, 1676). Spectroscopic and photoelectrochemical measurements suggested that the high photocatalytic activity of Ag@AgBr/mp-TiO2 stems from the enhancement of the local electric field near Ag clusters increasing the rate of the photocharge carrier generation in AgBr in a manner similar to the TiO2/Ag@SiO2 system. Eventually, AgBr plays the role of not only a visible-light photocatalyt but also an oxidation inhibitor of Ag clusters, while Ag cluster acts as an amplifier for the local electric field.

AgX (X = Cl, Br, I)-based two-21-24 and three-component25-27 nanocomposites have been reported for the degradation of organic dyes. In the same manner as the Au/TiO2 system, the

INTRODUCTION Plasmonic metal nanoparticle (NP)-loaded TiO2 (PM/TiO2) represented by Au/TiO2 has recently emerged as a new class of visible-light photocatalysts.1,2 Two plausible action mechanisms on the so-called “plasmonic photocatalyst” have been proposed (Scheme 1). In the interfacial electron transfer (IFET) mechanism (A), surface redox reactions are initiated from the localized surface plasmon resonance (LSPR)-induced electron transfer from PM NP to TiO2.3 In the local electric field enhancement (LEFE) mechanism (B), the LEF near PM NP promotes the excitation of TiO2 near UV region.4 The IFET mechanism explains the visible-light activity of Au/TiO2,3 but the origin for the IFET is not entirely understood yet. The physical picture of the LEFE mechanism is articulate, but it only works under UV-light irradiation.4 While the detailed action mechanism of the plasmonic photocatalysts remains disputable, they have been applied for environmental remediation5-7 and several oxidative transformations including alcohol to carbonyl compounds,8-11 thiol to disulfide,12 benzene to phenol,13,14 and amine to imine,15,16 and reductive chemical transformations such as nitrobenzene to azobenzene,17 epoxide to olefin, and ketone to alcohol.18 On the other hand, we previously reported that the formation of Ag(core)AgCl(shell) particles on metal oxides such as TiO2 and αFe2O3 (Ag@AgCl/MOs) greatly activates the photo-reactivity of the MO supports.19,20 However, in these systems, the Ag@AgCl particles only act as a built-in sacrificing agent, and Ag@AgCl/MOs do not work as a photocatalyst. Subsequently, many studies on the visible-light-induced reactivity of Ag-

Scheme 1. Interfacial electron transfer (IFET) mechanism (A) and local electric field enhancement (LEFE) mechanism (B) proposed for the visible-light photocatalyst consisting of noble metal nanoparticle-loaded TiO2.

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IFET and LEFE mechanisms seem to be possible also in these systems. Further, a synergistic mechanism of the IFETplasmonic photocatalysis of Ag NPs and the semiconductor photocatalysis of AgBr22,25,26,28 was proposed. However, it remains unclear even whether the Ag/AgX work as a photocatalyst or not. Also, the dye-sensitization mechanism can not be excluded in the reaction systems using dyes as a reaction substrate.29 Importantly, the detailed mechanism could depend on the architecture of the hybrid photocatalyst besides the reaction substrate as shown in Scheme 1. In this study, Ag cluster-incorporated AgBr NPs have been formed on a mesoporous TiO2 nanocrystalline film (Ag@AgBr/mp-TiO2) by a successive ionic layer adsorption and reaction-photoreduction (SILAR-PR) technique. As a model water pollutant, we used 2-naphthol, which is the starting compound for azo-dyes and transparent in the visible region.30 The reactivity of Ag@AgBr/mp-TiO2 for the decomposition of 2-naphthol under irradiation of visible-light (λ > 400 nm) and the basic action mechanism were studied.

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Photoelectrochemical Measurements. The three electrodes type photoelectrochemical cell was fabricated by Ag@AgBr/mp-TiO2, Ag/AgCl, and glassy carbon as working, reference, and counter electrode, respectively. The electrolyte solution is consisting of 0.2 M aqueous solution of NaClO4 (KISHIDA CHEMICAL, > 99.5%) with 1 mM 2-naphthol. The active area of the cell was 4.0 × 2.5 cm2. Under illumination by a 300 W Xe lamp (HX-500, Wacom) with various cut off filter, the current density (A cm-2) was measured at the dark rest potential by using a galvanostat/potentiostat (HZ5000, Hokuto Denko). The light intensity integrated from 420 to 485 nm (I420–485) through a Y-45 optical filter was adjusted to 0.70 mW cm-2. The simultaneous irradiation of two lights was carried out by using the similar cell. LED (530 ± 40 nm) and Xe lamp with a band pass filter (442 ± 34 nm) were used as the light sources, and the light intensity integrated from 420 to 485 nm (I420–485) through a Y-45 optical filter was adjusted to 0.16 mW cm-2.

RESULTS AND DISCUSSION EXPERIMENTAL SECTION Catalyst preparation and characterization. A paste containing anatase TiO2 particles with mean size of 20 nm (PST-18NR, Nikki Syokubai Kasei) was coated on fluorinedoped SnO2 coated glass (FTO, sheet resistance = 10 Ω/square) by doctor blade technique. The sample was heated at 773 K for 1 h to form mp-TiO2 with sample area = 4 × 2.5 cm2. AgBr was formed on the TiO2 surface by the SILAR method. The mp-TiO2 film was dipped in a solution of 0.1 M AgNO3 (KANTO CHEMICAL, > 99.7%) in methanol (MeOH, KISHIDA CHEMICAL, >99.8%) for 1 min, and rinsed with MeOH. Then the sample was immersed in a solution of 0.1 M KBr (KANTO CHEMICAL, > 99.0%) in MeOH for 1 min, and rinsed with MeOH. The loading amount of AgBr could be controlled by the number of the cycles. After immersing AgBr/mp-TiO2 in MeOH, UV-light (Hg lamp, λ > 320 nm, I310-420 = 4 mWcm-2) was irradiated under anaerobic conditions to obtain Ag@AgBr/mp-TiO2. The sample morphology was determined by transmission electron microscopy at an applied voltage of 300 kV (JEM3010, JEOL). The loading amount of Ag was quantified by inductively coupled plasma spectroscopy (ICPS-7500, Shimadzu). X-ray diffraction (XRD) measurements were performed by a Rigaku Mini Flex X-ray diffractometer operating at 40 kV and 100 mA. The data were collected in the range from 10 to 90 o (2θ) by the use of Cu Kα radiation (λ= 1.545 Å). Photocatalytic decomposition of 2-naphthol. 2Naphthol was used as received (KANTO CHEMICAL, > 99.0%). Ag@AgBr/mp-TiO2 was immersed in an aqueous solution of 2-naphthol (1 mM or 10 µM, 90 mL), and visiblelight (I420–485 = 2.5 mW cm-2) was irradiated under aerobic conditions by using a Xe lamp (HX-500, Wacom) and a long path optical filter L-42 (λcut-off = 400 nm, AGC TECHNO GLASS). Then, samples were obtained at each period by filtration of the reaction solution. The concentration of 2naphthol and products were determined by Shimadzu HPLC [conditions : column = Shim-pack CLC-ODS (φ 4.6 mm × length 150 mm); mobile phase = MeOH : H2O (7:3 v/v); flow rate = 1.0 mL min-1; λ = 224 nm].

1. Preparation and characterization of Ag@AgBr/mp-TiO2 At the first step, AgBr was grown on the mp-TiO2 surface by the SILAR method. Figure 1A shows the X-ray diffraction (XRD) patterns for the samples with varying SLAR cycle number (N), and AgBr (ICDD card number 01-079-0149) for comparison. At N ≥ 5, clear diffraction peaks are observed at 2θ = 30.9, 44.3, and 55.0°, which are indexed as the diffrac-

(A)

(B)

Figure 1. (A) X-ray diffraction patterns for AgBr/mp-TiO2 prepared by the SILAR method with the cycle number (N) varied, and the pattern at the top is for AgBr/mp-TiO2 irradiated in MeOH for 5 min. (B) UVvisible absorption spectra of AgBr(N)/mp-TiO2.


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tion from the (200), (220), and (222) planes of AgBr crystal, respectively, and the intensities increase with an increase in N. Clearly, AgBr particles grow on the surface of mp-TiO2 (AgBr/mp-TiO2) by the SILAR process. The mp-TiO2 films consisting of anatase TiO2 nanocrystals has only absorption at λ < 390 nm. Figure 1B shows the UV-visible absorption spectra for AgBr(N)/mp-TiO2. Loading AgBr induces absorption due to its interband transition in the visible region, and the absorption intensifies with increasing N. Whereas the absorption edge calculated from the indirect bandgap of AgBr (2.6 eV) is ~480 nm,31 the absorption of AgBr/mp-TiO2 extends to ~600 nm. At the second step, AgBr/mp-TiO2 was irradiated by UV-light in MeOH. Figure 2A shows the absorption spectral change of AgBr/mp-TiO2. UV-light irradiation causes a strong absorption around 500 nm, of which intensity increases with irradiation time (tp) to be saturated at tp > 5 min. In the XRD pattern for AgBr/mp-TiO2 irradiated for 5 min in MeOH (Figure 1A), a weak signal from the diffraction of the Ag (111) plane appears. The absorption around 500 nm can be assigned to the LSPR of Ag clusters. Clearly, Ag cluster-incorporated AgBr NPs are formed on the mp-TiO2 surface by the present SILAR-PR method. To check the stability of the UV-irradiated AgBr/mp-TiO2, the change of the LSPR intensity was traced after the sample was exposed to the air in the dark. Figure 2B shows the change in the absorbance at λ = 500 nm for the UVirradiated AgBr/mp-TiO2, and Ag/mp-TiO2 for comparison. The LSPR of Ag/mp-TiO2 significantly weakens due to the oxidation of Ag NPs by O2 in the air. Surprisingly, the absorption intensity is almost maintained for the UV-irradiated AgBr/mp-TiO2 at least for 4 days. This fact indicates that Ag clusters are mainly generated not on the surface but in the interior of the AgBr NPs (Ag@AgBr/mp-TiO2). The LSPR-

peaks of Ag NPs (diameter ≈ 50 nm) in the aqueous solution and SiO2-coated Ag NP (diameter ≈ 20 nm) are located at 424 nm32 and 410 nm,4 respectively. The LSPR-peak wavelength in the UV-irradiated AgBr/mp-TiO2 (~500 nm) longer than these values also supports that the conclusion since the LSPRpeak redshifts with an increase in the refractive index (or dielectric constant) of the surrounding medium.10,33 A similar technique has recently been reported for the formation of Ag/AgBr composite particles on the surface of TiO2 nanotube array (TiO2-NTA).28 Although the authors claimed that the Ag particles are formed on the surface of AgBr, it should be noted in the TEM observation of AgBr that Ag particles are generated by the electron-beam irradiation, and they can not be distinguished from the ones formed by the SILAR-PR technique. Our TEM observation confirmed that the surface of mp-TiO2 is partially covered with AgBr particles. UV-light irradiation of TiO2 excites the electrons from the valence band (VB) to the conduction band (CB). While the VB-holes oxidize MeOH, the CB-electrons reduce Ag+ ions to yield Ag in AgBr. It is known that AgBr usually involves interstitial Ag+ ions in the crystal, and the trap levels below the CB edge.34 The cessation of the Ag formation at tp > 5 min suggests that not the lattice Ag+ ions but the interstitial Ag+ ions are reduced by the CB-electrons in TiO2. The resulting Ag nuclei would grow to Ag cluster due to the large mobility of Ag+ ions in AgBr.35 This scheme rationalizes the formation of Ag@AgBr/mp-TiO2 by the SILAR-PR technique. Also, the long tail of the as-prepared AgBr/mp-TiO2 in Figure 1A is ascribable to Ag clusters slightly generated in the AgBr NPs because AgBr is sensitive even to weak ambient light. According to this mechanism, AgBr crystal restricts the growth of Ag clusters to yield small size ones, and consequently, the LSPR

(A)

(A)

(B)

(B)

Figure 3. (A) Time courses for the degradation of 2-naphtholhthol under visible-light irradiation (λ > 400 nm, light intensity I420–485 = 2.5 mW cm-2) of Ag@AgBr(N = 25/mp-TiO2: [2-naphthol]0 = 1 mM. (B) The amount of 2-naphthol (2-NAP) degraded as a function of irradiation time.

Figure 2. (A) UV-visible absorption spectral change of AgBr/mp-TiO2 with UV-light irradiation in MeOH(tp / min = 0.5 , 1.0 , 1.5 , 2.0 , 2,5 , 3.0 , 3.5 , 4.0 , 4.5, and 5.0). (B) Change in the absorbance at 500 nm for the AgBr/mp-TiO2 in the air and Ag/mp-TiO2 for comparison.



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should be weak. Unexpectedly, Ag@AgBr/mp-TiO2 has strong LSPR around 500 nm as shown in Figure 2A. When metal particles much smaller than the wavelength of light are placed in a medium with dielectric constant of εm, the absorption cross-section of metal NPs is proportional to εm3/2.36 The large dielectric constant of AgBr (ε = 12.50)37 can increase the LSPR intensity of small Ag clusters. Thus, this result is also not inconsistent with the conclusion that most Ag clusters are formed in the interior of AgBr. 2. Photocatalytic performances Firstly, the reactivity of Ag@AgBr/mp-TiO2 for the degradation of 2-naphthol was examined at the initial concentration ([2-naphthol]0) of 1 mM under visible-light irradiation (λ > 400 nm). The pH of the reaction solution was 6.73. The adsorption amount of 2-naphthol was negligibly small, and the dedegradation of 2-naphthol did not occur with the catalyst in the dark. Also, no new absorption appeared in the visible region with the adsorption on Ag@AgBr/mp-TiO2, while catechol analog-adsorbed TiO2 is known to have the chargetransfer absorption because of the surface complex formation.38 Figure 3A shows time courses for the 2-naphthol degradation in the presence of Ag@AgBr/mp-TiO2 under visible-light irradiation. The degradation proceeds under aerated conditions, whereas it hardly occurs under de-aerated conditions. Clearly, visible-light irradiation and oxygen in addition to the catalyst are necessary for the 2-naphthol degradation to take place. Further, the photoreaction time was prolonged with Ag@AgBr/mp-TiO2 under aerated conditions. Figure 3B shows the amount of 2-naphthol degraded as a function of tp. The broken line exhibits the mole number (n) of AgBr loaded on mp-TiO2. At tp = 40 h, the turnover number (= n(2-napthol decomposed) / n(AgBr)) reaches 2.3. The generation of CO2 was also confirmed by gas chromatography. The decomposed 2-naphthol amount increases with an increase in tp up to 40 h. Consequently, Ag@AgBr/mp-TiO2 stably acts as a photocatalyst in this reaction, and 2-naphthol is finally decomposed to CO2. Further to check the stability of Ag@AgBr(N = 25)/mpTiO2, the photocatalytic reaction (tp = 4 h) was repeated five times (Figure S1). As a result, the photocatalytic activity hardly changes, confirming the stability of Ag@AgBr/mp-TiO2. A two-electrode PEC cell employing the AgBr/Ag nanocomposite-loaded TiO2 nanotube array (TiO2-NTA) as the photoanode has recently been reported to show a higher reactivity for the degradation of methyl orange than the AgBr/TiO2-NTA

Figure 5. IPCE action spectrum for the Ag@AgBr/mp-TiO2photocatalyzed degradation of 2-naphthol. For comparison, the absorption spectra for mp-TiO2, AgBr/TiO2 and Ag@AgBr/mp-TiO2 are shown.

photoanode cell under visible-light irradiation.28 Secondly, the N-dependence of the photocatalytic activity of Ag@AgBr(N)/mp-TiO2 for the 2-naphthol decomposition ([2-naphthol]0 = 10 µM) at tp = 5 min was studied under irradiation of visible light (λ > 400 nm). Figure 4 compares the photocatalytic activity of Ag@AgBr/mp-TiO2 with varying N. The unmodified mp-TiO2 was inactive for the 2-naphthol degradation. The activity increases with an increase in N, going through a maximum around N = 15. The origin for this intriguing volcao-shaped relation between the photocatalytic activity and N is discussed later.

(A)

(B)

Figure 4. Photocatalytic activity of Ag@AgBr(N)/mp-TiO2 for the 2naphthol degradation under irradiation of visible light (λ > 400 nm, light intensity I420–485 = 2.5 mW cm-2) at tp = 5 min: [2-NAP]0 = 10 µM.

Figure 6. (A) The excitation band for the degradation of 2-naphthol. (B) The comparison of the photocurrent under different excitation conditions.


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3. Action mechanism of Ag@AgBr/mp-TiO2 under visiblelight irradiation To gain the information about the action mechanism of the Ag@AgBr/mp-TiO2 photocatalyst, the incident photon-tocurrent efficiency (IPCE) was measured in a three-electrode photoelectrochemical (PEC) cell with a structure of Ag@AgBr/mp-TiO2 (working electrode) | Ag/AgCl (reference electrode) | 0.2 M NaClO4 aqueous solution with 1 mM 2naphthol | glassy carbon (counter electrode). Visible-light irradiation of the PEC cell generated an anodic current, of which magnitude depended on the wavelength of incident light. Figure 5 shows the action spectrum of IPCE, and for comparison, the absorption spectra of mp-TiO2, AgBr/mp-TiO2 and Ag@AgBr/mp-TiO2. The onset wavelength of the IPCE is in agreement with the absorption edge of AgBr. Importantly, the IPCE steeply increases at λ < 500 nm without exhibiting a clear peak corresponding to the LSPR, which is frequently observed in the Au/TiO2 plasmonic photocatalytic reactions.6,810,15,16 It is also worth noting that the IPCE exceeds ~10% at 400 < λ < 450 nm, where the absorption of AgBr is weak but effectively overlapped with the LSPR of the Ag clusters. These results strongly suggest that the LEF near Ag clusters enhances the bandgap excitation of AgBr to give rise to the high visible-light activity of Ag@AgBr/mp-TiO2.

Scheme 2. Action mechanism of the Ag@AgBr/mp-TiO2 photocatalyst under visible-light irradiation.

trapping and photoluminescence techniques have confirmed the formation of ·OH radicals by visible-light-irradiation (λ = 532 nm) of Ag/AgBr/TiO2-NTA in aqueous solution.42,43 There is also a possibility that 2-naphthol is indirectly oxidized by ·OH radicals. The oxidation route can depend on the reaction substrate,44 and further work is necessary to clarify the detailed mechanism. In this manner, the Ag clusters act as the electric field amplifier for the excitation of AgBr, while AgBr works as a visible-light photocatalyst and an inhibitor of the Ag NP oxidation. A similar plasmon-enhanced mechanism was proposed for the coupling system between CdS(core)SiO2(shell) and Au(core)-SiO2(shell).45 Since the visible-light activity of Ag@AgBr/mp-TiO2 originates from Ag@AgBr, the photocatalytic activity of increases with increasing N at N ≤ 15. However, at N > 20, the high coverage by Ag@AgBr particles would retard the O2 reduction on the TiO2 surface. Another possibility is the blocking of the mp-TiO2 pores by Ag@AgBr particles incurring a decrease in the effective reaction area of Ag@AgBr/mp-TiO2.46 As a result, the optimum SILAR cycle number would be present.

Further, the photocurrent (J/µA cm-2) for the threeelectrode PEC cell was measured under different excitation conditions. As shown in Figure 6A, two excitation wavelengths were used: one is 442 ± 34 nm (EX1) and another is 530 ± 40 nm (EX2). EX1 excites both the bandgap of AgBr and the LSPR of the Ag clusters, while EX2 almost selectively excites the LSPR of the Ag clusters. Figure 6B compares the photocurrent (J/µA cm-2) under the EX1, EX2 and EX1 + EX2 excitation. While the photocurrent for the EX2 excitation (J(EX2)) is only ~1.1 µA cm-2. Under the EX1 excitation, the photocurrent (J(EX1)) increases to ~7.2 µA cm-2. Interestingly, simultaneous excitation (EX1 + EX2) further increases the photocurrent (J(EX1 + EX2)) up to ~ 14.2 µA cm-2, which is significantly larger than the sum of J(EX1) and J(EX2). A similar accelerating effect by the simultaneous excitation by blue and green light-emitting diode has recently been reported in the Ag deposited-AgI system.39 Clearly, the LSPR-excitation of the Ag clusters in the AgBr NPs boosts the visible-light activity of AgBr/mp-TiO2 in this Ag@AgBr photocatalyst.

A synergistic mechanism of the plasmonic photocatalysis of Ag NPs involving the IFET from Ag NP to AgBr and the semiconductor photocatalysis of AgBr was proposed for Ag NPsurface modified AgBr (Ag/AgBr) system22 and Ag/AgI system.39 In these cases, Ag NPs should be located on the surface of AgX for the Ag/AgX-based photocatalysts to effectively work as the IFET-plasmonic photocatalyst as many researchers reported.22,24,28,39,43 In the Ag@AgBr/mp-TiO2 system, this mechanism could be excluded from the major route because such a small molecule as O2, and thus, 2-naphthol are inaccessible to the Ag clusters enveloped by AgBr crystal. Further, even if the LSPR-induced hot charge carriers in Ag clusters can be injected into AgBr, the hole transfer to the AgBr surface seems to be difficult because of the small mobility in AgBr.47 However, the IFET mechanism by the infinitesimal Ag clusters on the AgBr surface can not be ruled out as a minor route. The weak shoulder around 500 nm in the IPCE curve may arise from the oxidation of 2-naphthol via the IFET route.

On the basis of these results, the basic reaction mechanism can be summarized as follows (Scheme 2). Visible-light irradiation of Ag@AgBr/mp-TiO2 excites the electrons from the VB of AgBr to the CB. The LSPR of Ag clusters can intensively increase the LEF near the Ag/AgBr interface to enhance the photocatalytic activity of AgBr, because the light absorption or the rate of the photocharge carrier generation is proportional to the electric field squared |E|2.40 The CB minimum of AgBr was estimated to be -3.4 eV versus vacuum level by the Mott-Schottky plot,31 and the value for anatase TiO2 is -3.91eV at pH 6.73.41 Due to a large gap in the CB minima of AgBr and TiO2 (~ 0.5 eV) and the assistance by the intense LSPR at the Ag/AgBr interface, the CB-electrons in AgBr could be efficiently transferred to the CB of TiO2. The CB-electrons in TiO2 reduce O2 on the Ag@AgBr-uncovered TiO2 surface. Meanwhile the successive attack by the VBholes would directly oxidize 2-naphthol finally to CO2. Recent electron spin resonance spectroscopic study using the spin-

CONCLUSIONS



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(7) Naya, S.; Kume, T.; Okumura, N.; Tada, H. Bi-overlayer Type Plasmonic Photocatalyst Consisting of Mesoporous Au/TiO2 and CuO/SnO2 Films Separately Coated on FTO. Phys. Chem. Chem. Phys. 2015, 17, 18004-18010. (8) Kowalska, E.; Abe, R.; Ohtani, B. Visible Light-Induced Photocatalytic Reaction of Gold-Modified Titanium(IV) Oxide Particles: Action Spectrum Analysis. Chem. Commun. 2009, 45, 241243.

Ag cluster-incorporated AgBr NPs were formed on the mesoporous TiO2 nanocrystalline film (Ag@AgBr/mp-TiO2) by the SILAR-PR technique. Ag@AgBr/mp-TiO2 stably exhibits a high level of photocatalytic activity for the decomposition of 2-naphthol under visible-light irradiation (λ > 400 nm). Spectroscopic and photoelectrochemical measurements indicated that the local electric field near the Ag/AgBr interface enhances the excitation of AgBr to increase the rate of the photocharge carrier generation, and simultaneously, AgBr works as an oxidation inhibitor of Ag clusters. We anticipate that this study presents basic and useful information about the design for the silver halide-based visible-light photocatalysts.

(9) Naya, S.; Inoue, A.; Tada, H. Self-Assembled Heterosupramolecular Visible Light Photocatalyst Consisting of Gold Nanoparticle-Loaded Titanium(IV) Dioxide and Surfactant. J. Am. Chem. Soc. 2010, 132, 6292-6293. (10) Kimura, K. Naya, S. Jin-nouchi, Y. Tada, H. TiO2 Crystal FormDependence of the Au/TiO2 Plasmon Photocatalyst’s Activity. J. Phys. Chem. C 2012, 116, 7111-7117.

ASSOCIATED CONTENT

(11) Tsukamoto, D.; Shiraishi, Y.; Sugano, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. Gold Nanoparticles Located at the Interface of Anatase/Rutile TiO2 Particles as Active Plasmonic Photocatalysts for Aerobic Oxidation. J. Am. Chem. Soc. 2012, 134, 6309-6315.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website: Repeated test of the Ag@AgBr/TiO2-photocatalyzed 2-naphthol decomposition (Figure S1).

(12) Naya, S.; Teranishi, M.; Isobe, T.; Tada, H. Light WavelengthSwitchable Photocatalytic Reaction by Gold Nanoparticle-Loaded Titanium(IV) Dioxide. Chem. Commun. 2010, 46, 815-817. (13) Ide, Y.; Matsuoka, M.; Ogawa, M. Efficient Visible-LightInduced Photocatalytic Activity on Gold-Nanoparticle-Supported Layered Titanate. J. Am. Chem. Soc. 2010, 132, 16762-16764.

AUTHOR INFORMATION Corresponding Author Tel +81-6-6721-2332; Fax +81-6-6727-2024; E-mail: [email protected] (H.T.).

(14) Zheng, Z.; Huang, B.; Qin, X.; Zhang, X.; Dai, Y.; Wei, J.; Whangbo, M.-H. Facile in Situ Synthesis of Visible-light Plasmonic Photocatalysts M@TiO2 (M . Au, Pt, Ag) and Evaluation of Their Photocatalytic Oxidation of Benzene to Phenol. J. Mater. Chem. 2011, 21, 9079-9087.

ACKNOWLEDGMENT

(15) Naya, S.; Kimura, K.; Tada, H. One-Step Selective Aerobic Oxidation of Amines to Imines by Gold Nanoparticle-Loaded Rutile Titanium(IV) Oxide Plasmon Photocatalyst. ACS Catal. 2013, 3, 1013.

This work was partially supported by a Grant-in-Aid for Scientific Research (C) No. 15K05654 and MEXT-Supported Program for the Strategic Research Foundation at Private Universities.

(16) Sato, Y.; Naya, S.; Tada, H. A New Bimetallic Plasmonic Photocatalyst Consisting of Gold(core)-Copper(shell) Nanoparticle and Titanium(IV) Oxide Support. APL Mater. 2015, 3, 104502.

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