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Visible-Light Activation of Strontium Titanate by the Surface Modification with Iron(III) Oxide Nanoclusters Keigo Fujiwara, Ryo Negishi, Musashi Fujishima, and Hiroaki Tada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08058 • Publication Date (Web): 20 Oct 2016 Downloaded from http://pubs.acs.org on October 27, 2016

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The Journal of Physical Chemistry

Visible-Light Activation of Strontium Titanate by the Surface Modification with Iron(III) Oxide Nanoclusters Keigo Fujiwara, Ryo Negishi, Musashi Fujishima, Hiroaki Tada * Department of Applied Chemistry, School of Science and Engineering, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan

Supporting Information Placeholder ABSTRACT: Tris(acetylacetonato)iron(III) (Fe(acac)3) is chemisorbed on strontium titanate (SrTiO3) via partial ligand-

exchange between the acac-ligand and the surface Ti-OH group. Post-heating at 773 K in the air yields extremely small iron(III) oxide clusters on SrTiO3 (Fe2O3/SrTiO3). The Fe2O3 loading amount per unit surface area of SrTiO3 (Γ/Fe-ions nm2 ) was controlled by the Fe(acac)3 concentration. The surface modification gives rise to visible-light activity for the oxidation of 2-naphthol used as a model water pollutant simultaneously with the UV-light activity significantly boosted. The visiblelight activity is sensitive to Γ to reach a maximum at Γ ≈ 0.36. Valence band-X-ray photoelectron spectroscopy (VB-XPS) and electrochemical (EC) measurements indicated that the surface modification by the Fe2O3 nanoclusters (NCs) generates new vacant surface levels below the conduction band (CB) minimum of SrTiO3, while the VB maximum level is invariant. The band energy diagram of Fe2O3/SrTiO3 suggested that the visible-light activity can be induced by the photo-excitation of the electrons in the VB of SrTiO3 to the vacant surface levels or the bulk-to-surface interfacial electron transfer (IFET) in contrast to the visible light-driven surface-to-bulk IFET in Fe2O3/TiO2. Eventually, the high visible-light activity of Fe2O3/SrTiO3 stems from the effective charge separation and the electrocatalytic activity of the Fe2O3 NCs for the oxygen reduction reaction (ORR).

also a representative wide gap semiconductor photocatalyst. SrTiO3 has a flatband potential ~0.2 V more negative than TiO2.8 We can expect stronger reducing ability of the CBelectrons for SrTiO3, while its VB-holes have powerful oxidizing ability. Owing to the compatibility of the reducing and oxidizing ability, SrTiO3 has been investigated as a photocatalyst mainly for water splitting.9-11 Like TiO2, SrTiO3 with the bandgap of 3.25 eV12 only responds to UV-light. Very recently, Miyauchi and co-workers have reported that CuxO/SrTiO3 exhibits UV-light activity for the reduction of CO2 to CO.13 The deep understanding of the electronic state of MOs/SrTiO3 allows us to expect to underpin its further development as a visible-light photocatalyst.

INTRODUCTION The environmental issue should be globally tackled to be solved because of its borderless character. The environmental purification processes usually do not yield any useful products but need a large quantity of energy. Thus, the development of “solar environmental catalysts (SECs)” utilizing the huge and indispensable sunlight energy is crucial.1 A promising approach is the visible-light activation of TiO2 simultaneously with the high UV-light activity maintained or further increased. Recently, surface modified TiO2 by transition metal ions (Mn+/TiO2)2 and transition metal oxide NCs (MO/TiO2) 3,4 has emerged as a new SEC. The surface modification has unique features.3 First, a high level of visible-light activity can be induced without generation of recombination centers in the bulk. Second, the transition metal ions and metal oxide NCs can act as an electrocatalyst for the surface redox reactions. These properties can further increase the UV-light activity of TiO2. Third, the band edge energy (or the redox ability of the electrons and holes) can be tuned by controlling the loading amount. This feature also paves the application of Mn+/TiO2 and MOs/TiO2 to the selective organic synthesis using the sunlight as an energy source.5,6 In view of the climate change issue, the reduction and fixation of CO2 is also a very important subject. A basic study on the CO2 reduction has recently been reported for the MnOx/TiO2 system under thermally and UV-light activated conditions.7 Although various transition-metal surface modifiers have been studied, the semiconductor is almost limited to TiO2. On the other hand, SrTiO3 is

Here we report the preparation of iron(III) oxide NCsurface modified SrTiO3 (Fe2O3/SrTiO3) by the chemisorptioncalcination (CC) technique,14 and the photocatalytic (UV- and visible-light) activities for the oxidation of 2-naphthol used as a model water pollutant. Moreover, the action mechanism of Fe2O3/SrTiO3 is discussed on the basis of the band energy diagram determined by VB-XPS and EC measurements. To our knowledge, this is the first report on the photocatalysis of Fe2O3/SrTiO3.

EXPERIMENTAL SECTION Catalyst preparation. SrTiO3 particles (product # 517011, Sigma-Aldrich, specific surface area = 22 m2·g-1) were used as

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received. After SrTiO3 particles (1 g) had been added to 100 mL of a Fe(acac)3 methanol solution, they were allowed to for referencestand for 24 h at 298 K. The Fe(acac)3 concentration was changed from 6.5×10-4 M to 1×10-2 M. The resulting samples were washed repeatedly with the solvent and then dried under vacuum at room temperature, followed by heating in air at 773 K for 1 h. The adsorption process of Fe(acac)3 on SrTiO3 particles was examined by following the change of the electronic absorption spectra with SrTiO3 particles (10 g) dispersed in 6.5 × 10-4 M Fe(acac)3 methanol solution (100 mL) at 298 K for 24 h. Adsorption isotherms of Fe3+ ions were obtained by exposing SrTiO3 particles (1 g) to methanol solutions with different concentrations of Fe(acac)3 (100 mL). The concentrations of Fe(acac)3 in the solutions were determined from the absorbance at 274.5 nm with a Shimadzu UV-1800 spectrophotometer.

carbon and an Ag/AgCl electrode (TOA-DKK) were used as a counter electrode and a reference electrode, respectively. Photocatalytic reaction. Photocatalytic activity of the Fe2O3/SrTiO3 was examined as follows: Fe2O3/SrTiO3 particles (0.1 g) were placed in 50 mL of 1.0 × 10-5 M solution of 2-naphthol (solvent, acetonitrile : water = 1 : 9999 v/v) in a borosilicate glass container. The reaction cell was irradiated with a Xe lamp (HX-500, Wacom) through a band-pass filter (D33S, AGC Techno Glass) superposed on a piece of FTOcoated glass for the UV-light photocatalytic activity evaluation (I320-420 = 0.5 mW cm-2), and a high-pass filter (L-42, Toshiba) to cut off the UV-light for the visible-light-induced activity test (I420-485 = 1.0 mW cm-2). Three mL of the solution was sampled at a given time and the electronic absorption spectra of the reaction solutions were measured using a spectrometer (UV-1800, Shimadzu) to determine 2-naphthol concentration from the absorption peak at 224 nm.

Catalyst characterization. The Fe loading amount was determined by inductively coupled plasma spectroscopy (ICPS7510, Shimadzu). X-ray diffraction measurements were performed using Rigaku RINT2500. Transmission electron microscopic (TEM) observation images were obtained with a JEOL JEM-2100F at an applied voltage of 200 kV. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of the samples were obtained with a JASCO FT/IR-470Plus spectrometer equipped with a diffuse reflectance attachment. UV/Vis diffuse reflectance spectra of Fe2O3/SrTiO3 prepared by the CC technique14 were recorded on a Hitachi U-4000 spectrophotometer. The spectra were converted into the absorption spectra by using the Kubelka–Munk function. The photoluminescence (PL) spectra were measured with an excitation wavelength of 320 nm at 77 K using a JASCO FP-6000 spectrofluorometer.

RESULTS AND DISCUSSION 1. Preparation of Fe2O3/SrTiO3 The solid particles used were identified by XRD measurements (Figure S1). Diffraction peaks are observed at 2θ = 32.32°, 39.9°, 46.44° and 57.76°, which can be assigned to the diffraction from the (110), (111), (200) and (211) crystal planes of SrTiO3 with a cubic perovskite structure, respectively (ICDD card No. 10805334). The morphology of SrTiO3 particles was observed by TEM. Figure 1A shows the TEM image for unmodified SrTiO3 particles. Cubic-like particles are observed, and the mean particle size was determined to be 20.4 nm. The high resolution-TEM image in Figure 1B shows a fringe with a period of 0.26 nm, which is in agreement with the distance between the (110) crystal planes of SrTiO3. The specific surface area of the SrTiO3 particles was determined to be 22.0 m2 g-1 by the Brunauer-Emmett-Teller (BET) method.

(Photo)electrochemical measurements. For electrochemical measurements, SrTiO3 film electrodes were used. A paste containing SrTiO3 particles was coated on fluorine-doped SnO2 (FTO)-film coated glass substrates by a squeegee method, and the samples were heated in air at 773 K to form mesoporous SrTiO3 nanocrystalline films (mp-SrTiO3/FTO). Surface modification with iron(III) oxide cluster was performed on mpSrTiO3 with the same procedure for the particulate samples. Dark current–potential curves of the Fe2O3/mp-SrTiO3/FTO electrodes were measured in a 0.1 M NaClO4 electrolyte solution in a regular three-electrode electrochemical cell using a galvanostat/potentiostat (HZ-5000, Hokuto Denko). Glassy (A)

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The photocatalytic activity of MOs/TiO2 is sensitive to the loading amount of metal oxide NCs.3 Figure 2 shows the adsorption isotherm of Fe(acac)3 on SrTiO3 from the methanol solution at 298 K. The Γ value steeply increases with increasing equilibrium concentration (Ceq). The Langmuir plot shows good linearity, of which slope and intercept yield the saturated adsorption amount (Γ∞) and equilibrium constant (Kad) to be (C)

(B)

Figure. 1. TEM image (A), HR-TEM image (B) of pristine SrTiO3, and TEM image of Fe oxide-surface modified SrTiO3 with Γ = 0.21 (C).


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0.37 ions nm-2 and 3.2 × 103 M-1, respectively. These values are close to those for the adsorption of Fe(acac)3 on anatase TiO2 (Γ∞ = 0.35 ions nm-2, Kad = 1.52 × 103 mol-1 dm3). The Gibbs energy for the adsorption of Fe(acac)3 (∆adG0) on SrTiO3 and TiO2 at 298 K were calculated to be -20.0 and 18.2 kJ mol-1, respectively.

Figure 1C shows the TEM image for the Fe oxide-surface modified sample. The loading amount of Fe was quantified by inductively coupled plasma spectroscopy, being expressed by the number of Fe ions per unit SrTiO3 surface area (Γ/Fe-ions nm-2). In the TEM image of the sample (Γ = 0.21), no deposits is observed on the SrTiO3 surface. Previously, the oxidation number of Fe in the oxide NCs formed on TiO2 by the CC technique was confirmed to be +3 by X-ray absorption nearedge structure spectra.14 These results indicate that Fe(III) oxide species, formally written as Fe2O3, are generated on the SrTiO3 surface (Fe2O3/SrTiO3), and the loading amount can be precisely controlled by the initial concentration of Fe(acac)3 in a fashion similar to the Fe2O3/TiO2 system.15 The chemisorption of the complex on SrTiO3 at the first step would suppress the growth of Fe2O3 particles to yield the molecular scale NCs on the surface at the second step.

2. Optical properties of Fe2O3/SrTiO3 Bulk-state SrTiO3 and α-Fe2O3 possess the band gaps of 3.25 eV12 and 2.1 eV,18 respectively. The optical property of Fe2O3/SrTiO3 was studied in connection with the photocatalytic activity. Figure 3 shows UV-visible absorption spectra of Fe2O3/SrTiO3. As shown in the inset, white SrTiO3 turns pale brown by the surface modification with the Fe2O3 NCs. The surface modification causes visible-light absorption below 550 nm, of which intensity increases with an increase in Γ. The sample with Γ = 0.68 has the absorption due to the d-d transition around λ = 470 nm, which is characteristic of bulk Fe2O3. This fact also supports the above assumption that the oxidation number of Fe in the NCs is +3. However, the absorption is very weak at Γ ≤ 0.36. In the Fe2O3/TiO2 system, the absorption at λ ≤ 440 nm was attributed to the interfacial charge transfer (IFCT) from the surface Fe2O3 cluster levels to the CB of TiO2.14,19 The assignment of the IFCT band in the Fe2O3/SrTiO3 system is discussed later.

Figure 2. The adsorption isotherm of Fe(acac)3 on SrTiO3 from the methanol solution at 298 K (red), and the Langmuir plot (blue).

To clarify the adsorption mechanism, the absorption spectral change in the Fe(acac)3 solution with the adsorption was pursued (Figure S2). In the spectra, there present a weak and broad absorption around 430 nm and a strong absorption centered at 272 nm assignable to the d-d transition and the π- π* transition in the acac-ligand, respectively, while free acetylacetone (AcacH) only has an absorption peak at 272 nm. The absorption coefficient of Fe(acac)3 was approximately 3.0 times the value of AcacH. Although both the absorption intensities weaken after adsorption, the reducing degree at adsorption time = 24 h for the former (-46.7%) is significantly larger than that for the latter (-29%). This fact suggests that Fe(acac)3 is chemisorbed on SrTiO3 via the ligand-exchange between the acac-ligand of Fe(acac)3 and the surface Ti-OH group.15 Further to examine the states of the surface species on SrTiO3, diffuse reflectance Fourier transform infrared (DRIFT) spectra were measured (Figure S3). The difference DRIFT spectra for SrTiO3 before and after adsorption of Fe(acac)3 (a), and the complex-adsorbed SrTiO3 before and after postheating (b). In the difference DRIFT spectra for SrTiO3 before and after adsorption of Fe(acac)3 (a), a negative peak is observed at 3696 cm-1 due to the stretching vibration of the surface Ti-OH groups on the surface ν(Tis-OH).16 Also, three positive peaks are present at 1590 cm-1, 1507 cm-1, and 1380 cm-1, which can be assigned to the combination of ν(C-C) + ν(C-O), the combination of ν(C-O) + ν(C-C), and νs(CH3), respectively.17 The difference DRIFT spectra for the complexadsorbed SrTiO3 before and after post-heating (b) have three negative signals at 1590, 1525 and 1368 cm-1, which correspond to the positive peaks in spectrum (a). Evidently, at the first step, Fe(acac)3 is chemisorbed on SrTiO3 due to the ligand-exchange between the acac-ligand of Fe(acac)3 and the surface Ti-OH. At the second step, the post-heating completely pyrolyzes the acac-ligands left after the complex adsorption to yield Fe oxide species on SrTiO3.

Figure 3. UV-visible absorption spectra of Fe2O3/SrTiO3 with varying Γ. The inset exhibits the photographs for the samples.

3. Photocatalytic performances of Fe2O3/SrTiO3 2-Naphthol, the starting material for the azo dyes, has an n-π* absorption band centered at 224 nm, and is transparent in the



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visible region. In this study, 2-naphthol was used as a model water pollutant.20 Benzylalcohol analog-adsorbed TiO2 has the CT absorption in the visible region due to the surface complex formation, showing visible-light activity for the selective oxidation of benzylalcohol analogs to the corresponding aldehydes.21 However, no new absorption appeared in the visible region with the 2-naphthol adsorption on SrTiO3. The photocatalytic degradation of 2-naphthol was carried out under illumination of visible-light and UV-light at 298 K: initial 2naphthol concentration ([2-naphthol]0) = 10 µM. The pH of the solution was 5.72. Figure 4A shows the fraction of 2naphthol degraded at irradiation time (tp) = 1 h under visiblelight irradiation (λ > 400 nm, I420-480 nm = 1.0 mW cm-2) in the presence of Fe2O3/SrTiO3 with varying Γ at 298 K. Unmodified SrTiO3 is almost inactive. Interestingly, the surface modification of SrTiO3 by the Fe2O3 NCs induces visible-light activity. The relation between the activity and Γ exhibits a volcano-shaped curve with a maximum activity at Γ = 0.36. The 2-naphthol concentration decreases almost linearly with an increase in tp (Figure S4). Also, the reaction was conducted with the [2-naphthol]0 increased to 1 mM. The turnover number at tp = 12 h was calculated to be 30 by assuming that the Fe2O3 are the photocatalytically active sites (post infra). These results indicate that this reaction proceeds photocatalytically. Further, gas chromatography analysis confirmed that the generation of CO2 as the final product even under visible-light irradiation. Figure 4B shows the fraction of 2-naphthol degraded at tp = 16 min under UV-light irradiation (330 < λ < 400 nm, I 320-420 nm = 0.5 mW cm-2) in the presence of Fe2O3/SrTiO3 with varying Γ at 298 K: [2-naphthol]0 = 10 µM.

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the surface modification of SrTiO3 by the Fe2O3 NCs causes a noticeable visible-light activity for the 2-naphthol degradation simultaneously with the UV-light activity significantly increased. It is worth noting that UV-light activity dominates the overall photocatalytic activity of Fe2O3/SrTiO3 under filter-free light irradiation since the UV-light activity is greater than the visible-light activity by a factor of approximately one-order of magnitude. The stability is also essential for the photocatalysts. Recycle test of the Fe2O3/SrTiO3 photocatalyst was carried out for the 2-naphthol degradation under visible-light irradiation at 298 K. The repeated use of Fe2O3/SrTiO3 hardly changes the photocatalytic activity at least four times (Figure S5).

4. Action mechanism of Fe2O3 NCs on SrTiO3 We measured dark current density (J/mA cm-2)-potential (E/V) curves for the mp-SrTiO3/FTO electrodes without and with the surface modification by the Fe2O3 NCs. Figure 5 shows the cyclic voltammograms (CV) in a 0.1 M NaClO4 aqueous solution. In the CV curves for the unmodified mp-SrTiO3/FTO (black broken line) and Fe2O3/mp-SrTiO3/FTO (red broken line) electrodes in the deaerated electrolyte solution, small currents are only observed at E < -0.1 V (vs. standard hydrogen electrode, SHE). In the aerated electrolyte solution, the cathodic current for the mp-SrTiO3/FTO electrode (black solid line) somewhat increases, because the ORR occurs. Importantly, as shown by the red solid curve, the surface modification by the Fe2O3 NCs drastically increases the current with the onset-potential shifted towards the positive direction. Clearly, the ORR is remarkably enhanced owing to the electrocatalytic activity of the Fe2O3 NCs on SrTiO3. In the Cu2+/WO3 system, two-electron ORR has been shown to occur under visible-light irradiation by chemiluminescence photometry.22 In the present Fe2O3/mp-SrTiO3 system, no H2O2 was detected from the solution after the reaction. The current onset potential of +0.1 V is more negative than the thermodynamic potential for the twoelectron ORR (E(O2/H2O2) = + 0.358 V at pH 5.72).23 However, the possibility of the two-electron ORR can not be excluded since H2O2 may be adsorbed on SrTiO3 as well as TiO2.24 The current density for Fe2O3/mp-SrTiO3/FTO is greater than Fe2O3/mp-TiO2/FTO.3 Since the CB minimum of SrTiO3 is

(A)

(B)

Figure 4. Photocatalytic activities of Fe2O3(Γ)/SrTiO3 for the degradation of 2-naphthol under visible (λ > 400 nm, I420-485 nm = 1.0 mW cm-2) (A) and UV (330 < λ < 400 nm, I 320-420 nm = 0.5 mW cm-2) light irradiation (B) at 298 K. The initial concentration of 2-naphthol is 10 µM. Figure 5. Dark current (J)- potential (E/V vs. standard hydrogen electrode, SHE) curves for unmodified mp-SrTiO3/FTO (black) and Fe2O3/mp-SrTiO3/FTO (red) electrodes measured in 0.1 M NaClO4 solutions in the absence (broken line) and presence (solid line) of O2.

The UV-light activity for the 2-naphthol degradation increases with the surface modification by a factor of ~3, and the Γdependence of the UV-light activity is rather weak. Clearly,


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~0.2 eV higher in energy than that of anatase TiO2,25 the larger ORR current in the Fe2O3/mp-SrTiO3/FTO system can result from the larger driving force for the CB-electron transfer to O 2.

cathodic current flew immidiately after the potential shift. The accumulated charge (Q) was calculated by integrating the current with respect to time. The number of unoccupied states at a potential (E(Nuos(E)) can be assumed to be propotional to dQ/dE by Eq (1).

5. Band energy diagram of Fe2O3/SrTiO3 For the deep understanding of the surface modification effect, the density of states (DOS) of the Fe2O3 NC-derived surface levels near the band edges is crucial. The near-top of the VB and the near-bottom of the CB of pure SrTiO3 primarily consist of O2p and Ti3d orbitals, respectively.12 To gain the information about the filled energy levels of Fe2O3/SrTiO3, we performed the VB-XPS measurements. Figure 6A shows the VB-XPS spectra for Fe2O3/SrTiO3 with varying Γ. The emission from the VB of SrTiO3 extends from 2 to 8 eV. The VB maximum level of SrTiO3 hardly changes by the surface modification with the Fe2O3 NCs. This is in contrast to the Fe2O3/TiO2 system, in which the VB maximum gradually rises up with increasing Γ.3 On the other hand, the DOS of the unoccupied states can be investigated by means of transient

Nuos(E) = q-1 dQ/dE (1) where q is the electron charge. Figure 6B shows the plots of DOS as a function of E. The DOS for SrTiO3/FTO steeply increases at E < -1.1 V by the electron-filling of the CB of SrTiO3 with a weak shoulder below the CB minimum. The CB-edge potential of SrTiO3 is close to the literature value for the flatband potential of SrTiO3 at pH 13 (-1.12 V).8 A peak is observed in the mid-gap in the DOSs of SrTiO3/FTO and Fe2O3/SrTiO3/FTO around -0.5 V. In the photoluminescence (PL) spectrum of SrTiO3/FTO (Figure S6), a broad signal is observed around 530 nm. This signal significantly weakens after heating at 773 K. As a result of the surface modification by the Fe2O3 NCs, the PL peak remains with a slight blueshift. This fact is consistent with the slight upwardshift in the midgap energy levels in Figure 6B. The mid-gap levels would result from surface oxygen vacancy levels.27 It is worth noting that the surface modification also leads to the significant increase in the DOS below the CB minimum (-1.1 < E < -0.95 V). These levels can be attributed to the Fe2O3 NC levels, and further, the IFCT absorption at λ < 440 nm is assignable to the excitation of the electrons in the VB of SrTiO3 to the vacant surface Fe2O3 levels.

(A)

500 Intensity (cps)

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400 300 200 Γ= 0 Γ= 0.20 Γ= 0.21 Γ= 0.28 Γ= 0.34

100 0 8

6 4 2 Binding Energy (eV)

6. Photocatalytic reaction mechanism In the bulk-state, the Fe2O3/SrTiO3 coupling system does not work as a SEC under visible-light irradiation28 because the CB minimum of Fe2O3 (+0.24 V) is too low for the excited electrons to reduce O2 (standard reduction potential of O2, E0(O2/O2-) = -0.284 V). The contrastive surface modification effect by the Fe2O3 NCs is discussed below (Scheme 1). The surface modification of SrTiO3 by Fe2O3 NCs remarkably increases the surface levels below the CB minimum to induce the absorption in the visible region. Visible-light irradiation of Fe2O3/SrTiO3 gives rise to the IFET from the VB of SrTiO3 to

0

(B)

Figure 6. (A) Valence band (VB)-XPS spectra for Fe2O3/SrTiO3 with varying Γ. (B) Plots of dQ/dE as a function of the electrode potential (E) for the SrTiO3/FTO and Fe2O3/SrTiO3/FTO electrodes.

current during the state-filling process.26 The transient dark current was measured for the SrTiO3/FTO and Fe2O3/SrTiO3/FTO electrodes by shifting the electrode potential (E) step-by-step towards the negative direction in an electrolyte solution containing 0.1 M NaOH and 0.1 M LiClO4 (pH 13). In every chronoamperometry curve, a transient

Scheme 1. A proposed action mechanism of the Fe2O3/SrTiO3 photocatalyst.



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the vacant surface Fe2O3 levels. The visible-light-induced IFET between TiO2 and the surface-grafted Fe(III) ion2 or loaded Fe2O3 NC3 has been well established. On UV-light irradiation, the electrons in the VB of SrTiO3 are excited to the CB, which can be further transferred to the surface Fe2O3 NC levels. In each case, the IFET from SrTiO3 to Fe2O3 NC enhances the charge separation. The electrons collected in the Fe2O3 NCs with a potential of E = -0.95 V effectively reduces O2 with the assistance of the electrocatalysis for the ORR. On the other hand, 2-naphthol is oxidized finally to CO2 even under visible-light irradiation by the VB-holes in SrTiO3. This can result from the strong oxidation ability of the VB-holes since the VB maximum of SrTiO3 is invariant with the surface modification by Fe2O3 NCs. This scheme rationalizes the visible-light activation and the improvement in the UV-light activity of SrTiO3 by the surface modification with the Fe2O3 NCs. A similar mechanism was reported in the Cu2+/TiO229 and CuO/TiO2 systems,4,30 where the vacant surface levels near the CB minimum of TiO2 are generated by the surface modification. The increase in Γ increases the number of the surface Fe2O3 levels to intensify the absorption in the visible region, concurrently enhancing the charge separation and the subsequent ORR. At Γ ≥ 0.68, the electronic state of the Fe2O3 clusters approaches that in the bulk-state, and the situation in the bulk Fe2O3/SrTiO3 coupling system is valid also for this system. Consequently, an optimum loading amount is present in the plot of visible-light activity vs. Γ (Figure 4A). Meanwhile, the dependence of the UV-light activity on Γ is much weaker at 0.25 ≤ Γ ≤ 0.68 (Figure 4B). Recently, the bulk coupling system consisting of SrTiO3 nanocube and Fe2O3 nanowires has been reported to exhibit visible-light activity for the degradation of tetracycline due to the two-electron ORR on the Fe2O3 surface.31 As suggested in Section 4, there is a possibility that the two-electron ORR occurs also in the present system particularly under UV-light irradiation because the electron density in the Fe2O3 NCs would be greater than that under visible-light irradiation.2 Thermodynamically, the two-electron ORR can occur much more easily than the one-electron ORR, and thus, explain the weak dependence of the UV-light activity on Γ, although further work is necessary to confirm this hypothesis.

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ASSOCIATED CONTENT AUTHOR INFORMATION Supporting Information XRD patterns of SrTiO3; Absorption spectral change of the Fe(acac)3 solution with the adsorption on SrTiO3; difference DRIFT spectrum of SrTiO3 before and after Fe(acac)3 adsorption; DRIFT spectra for Fe(acac)3/SrTiO3 and Fe2O3/SrTiO3; Chronoamperometry curve; Recycle test of the photocatalytic reaction; PL spectra; This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author TEL: +81-6-6721-2332, FAX: +81-6-6727-2024 E-mail: [email protected]

ACKNOWLEDGMENT The authors acknowledge Dr. Qiling Jin, Hiroaki Nishijima, and Kouichi Takayama for experimental assistance. 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.

REFERENCES

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CONCLUSIONS The surface modification of SrTiO3 with the Fe2O3 NCs (Fe2O3/SrTiO3) causes a remarkable visible-light activity simultaneously with the UV-light activity significantly increased. The band energy diagram of Fe2O3/SrTiO3 determined by the VB-XPS and EC measurements suggested that its visible-light activity is initiated by the photo-excitation of the electrons in the VB of SrTiO3 to the vacant surface levels. The high visible-light activity of Fe2O3/SrTiO3 can stem from the effective charge separation due to the bulk-to-surface IFET and the electrocatalytic activity of the Fe2O3 NCs for the ORR. We anticipate that Fe2O3/SrTiO3 can be applied as the “solar catalyst” for not only oxidative but also reductive chemical transformations.


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Visible-Light Activation of Strontium Titanate by the ... - ACS Publications

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