Enhanced Photocatalytic Degradation of RhB Driven by Visible Light

Jun 6, 2008 - Andrea Santiago Portillo , Herme G. Baldoví , Maria Teresa García ... Visible Light Absorption of SrTiO3 and NaTaO3 (Metal = Mn, Fe, a...
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J. Phys. Chem. C 2008, 112, 9753–9759

9753

Enhanced Photocatalytic Degradation of RhB Driven by Visible Light-Induced MMCT of Ti(IV)-O-Fe(II) Formed in Fe-Doped SrTiO3 Tai-Hua Xie, Xiaoyan Sun, and Jun Lin* Department of Chemistry, Renmin UniVersity of China, Beijing 100872, People’s Republic of China ReceiVed: December 16, 2007; ReVised Manuscript ReceiVed: April 6, 2008

It is important to reveal the origin of the visible light-induced photocatalytic activity of transition metaldoped SrTiO3 in which an isolated energy level is created by the dopant in the forbidden band gap. In this particle, the photocatalytic and photophysical properties of pure SrTiO3 and Fe-doped SrTiO3 were investigated comparatively. The Fe-doped SrTiO3 has been shown to have a much higher photocatalytic activity than pure SrTiO3 for the degradation of RhB under visible light irradiation. It was showed that doping Fe into SrTiO3 exhibited an absorption extending up to the visible region in the optical absorption spectrum and established the TiIV-O-FeII heterobimetallic linkages in the host. Difference diffuse reflectance spectra revealed that the absorption in the visible region is partly attributed to the metal-to-metal charge-transfer (MMCT) excitation of TiIV-O-FeII linkage formed in the Fe-doped SrTiO3. The visible light excitation of TiIV-O-FeII linkage was demonstrated to be the cause of the enhanced degradation of RhB. Furthermore, it was noted that the TiIV-O-FeII linkages only onto and nearest the surface of the Fe-doped SrTiO3 behavior as active sites for the visible light-induced photocatalysis. 1. Introduction Photocatalysis with a primary focus on several semiconductors such as TiO2, SrTiO3, and ZnO as efficient photocatalysts is widely considered as an emerging technology for solving environmental problems, in particular, for the removal of organic contaminants.1,2 A main drawback of the lack of visible light utilization hinders its practical application because all these efficient photocatalysts are generally wide band gap semiconductors and are active only under UV irradiation. Therefore, a great effort has been undertaken to extend their absorption and conversion capacities into visible range of the solar spectrum. Recently, it was discovered that substitutional doping with anion elements such as N, S, and C into semiconductor photocatalysts such as TiO2 leads to an extension of the optical absorption to visible light region.3–6 N-doped TiO2, a typical visible light photocatalyst for air and water purification, has been well developed according to this strategy.3 N, S, or C-doped SrTiO3 was also reported to be photoactive at visible light range.7–9 Hence, doping of anionic species such as N, S, and C for O (2p) in either TiO2 or SrTiO3 has been proven to be an effective method for extending the photoactive region to visible light via narrowing the band gap with respect to both theoretical and experimental results. The influence of doping various transition metals into a semiconductor with wide band gap such as TiO2 and SrTiO3 on the photoactivity is a frequent topic of investigations with an aim of extending the photoactivity to visible light.10–15 It was reported that doping of some transition metals such as Ru3+, Fe3+, Cr3+, Mn3+, and Pb2+ into TiO2 or SrTiO3 to create a localized level in the forbidden band is one candidate method for generating an optical absorption shift to visible light. These transition metal-doped semiconductors, especially metal-doped SrTiO3, have been intensely studied as efficient * To whom correspondence should be addressed. Phone: (+8610)62516222. Fax: (+8610)-62516444. E-mail: [email protected].

visible light-driven photocatalysts for the photocatalytic water splitting with the aid of sacrificial agents,13,15 but there are only limited reports on the photocatalytic decomposition of organic compounds. This is because only in a few cases can the localized level created by metal dopants in the forbidden band mix or overlap sufficiently with the valence band top formed by O2p or the conduction band bottom formed by Ti3d in TiO2 and SrTiO3 to continuously transfer photoexcited carriers to reactive sites. This was well demonstrated by the visible light-induced photocatalytic performances of Ag+- and Pb2+-doped SrTiO3 and other work.14,16 However, it was still frequently reported that TiO2 doped with V, Cr, Mn, Fe, Ni, and other transition metals via high energy ion implantation,11,17,18 flame spray pyrolysis technique,19 and sol-gel method20 can generate effective photocatalytic reactivity for the decomposition of organic compounds and nitrogen monoxide under visible light irradiation although no reasonable and acceptable explanation could be offered to support their photocatalytic performances. Generally, there is still a considerable gap between the traditional semiconducting photocatalysis theories and the photocatalytic behaviors of these metal-doped semiconductors. Metal-to-metal charge transfer (MMCT) for a complex with more than one type of metal is defined to be an excitation transition of an electron from one metal to the other. Recently, Frei et al. demonstrated MMCT of heterobimetallic assemblies such as Ti(IV)-O-Cu(I) and Ti(IV)-O-Sn(II) as a new class of visible light absorbing chromophores.21 Hashimoto et al. successfully developed Ti(IV)-O-Ce(III) bimetallic assemblies on mesoporous silica to be a visible light photocatalyst for the photo-oxidation of 2-propanol.22 Considering the presence of Ti(IV)-O-M linkages onto and inside the metal-doped TiO2 or SrTiO3 systems, it would be of great interest and importance for understanding if the metal-to-metal charge-transfer excitation of these linkages also occurs and helps promote a photocatalytic process under visible light. Moreover, to the best of our knowledge there

10.1021/jp711797a CCC: $40.75  2008 American Chemical Society Published on Web 06/06/2008

9754 J. Phys. Chem. C, Vol. 112, No. 26, 2008 has been no report about the investigation of the photocatalytic performance of metal-doped semiconductor photocatalyst by introducing the insight of MMCT. With such a goal, in the present work we chose SrTiO3, a typical perovskite oxide, as a host structure and doped Fe cation in it using a modified sol-gel method. The synthesized Fe-doped SrTiO3 was demonstrated to have much higher photocatalytic activity than pure SrTiO3 for the purification of dye wastewater under visible light irradiation, where Rhodamine B (RhB) was selected as a model dye contaminant. A systematic investigation on the physicochemical properties of pure SrTiO3 and the Fe-doped SrTiO3 was performed. The effects of visible light excitation of Ti(IV)-O-Fe linkages formed onto and inside the Fe-doped SrTiO3 on the photocatalytic degradation of RhB was also investigated in details. Finally, a possible photocatalytic mechanism was suggested on the basis of the understandings of the photocatalytic and various physicochemical properties of the Fe-doped SrTiO3. 2. Experimental Section 2.1. Photocatalyst Preparation. Polycrystalline powder samples of pure SrTiO3 and Fe-doped SrTiO3 (5 atm % at Ti4+ site) were synthesized by a modified sol-gel method similar to that reported by Subramanian et al.16 A solution of titanium isopropoxide (0.5 M) precursor in acidic acid was added into an aqueous solution of strontium nitrate (0.5 M) and citric acid to make the molar ratio of all metal to citric acid at 2. The resulting mixture was stirred until the solution became transparent and clear. Ethylene glycol and glycerol were dropped into this sol solution, respectively. For Fe-doped SrTiO3 perovskite synthesis, a solution of the ferric ions, prepared from the corresponding ferric nitrate in water, was added into the aqueous solution of strontium nitrate and citric acid. The molar ratio of Sr to (Ti + Fe) was controlled stoichiometrically at 1:1. The amount of ferric ion added was 5 mol % of the strontium. All precursor solutions were then stirred for another 12 h to remove water at room temperature and gradually heated first at 50 °C over 1 day to evaporate most of the water and again reheated at 120 °C. Finally, a dark brown sticky paste and solid was obtained upon heating the clear and transparent sol solution. This dark brown solid was moved into a ceramic crucible and was calcined for 2 h at 900 °C in a stove. The obtained solids were ground into fine powder prior to characterization and photocatalytic activity examination. For comparison experiments below, some of finally obtained powder was suspended in an aqueous solution of HCl (∼1 M). The suspension then was stirred for 8 h at room temperature, followed by centrifugation and rinsing with distilled water. The procedure of centrifugation and washing was repeated several times until the pH of the washing was natural and no Cl- in the washing was detected by reacting with an AgNO3 solution. The resulting precipitate was dried in an oven for 2 h before characterization and photocatalytic activity examination. 2.2. Photocatalyst Characterization. The crystal phases of as-prepared samples were identified at room temperature with an X-ray diffractometer (Rigaku D/max-2500) using Cu KR radiation under 40 kV and 200 mA. The scanned range was 2θ ) 20∼80° with a step of 2θ ) 0.02° and 0.5 s/step. The element compositions of the samples and the valence state of the doped Fe were analyzed on an X-ray photoelectron spectroscope (ESCALab220i-XL) using 300W Al KR radiation. The base pressure was about 3 × 10-9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. UV-vis diffuse reflectance spectra of the samples were recorded on a Hitachi U-3310 spectrophotometer equipped with a diffuse

Xie et al.

Figure 1. Polycrystalline X-ray diffraction patterns of pure SrTiO3 and Fe-doped SrTiO3. The insert shows the (110) diffraction peaks of two samples.

reflectance accessory. Scanning electron microscopy (SEM) observation was carried out on a JEOL JSM-7401F field emission scanning electron microscope. The induction couple plasma-atomic emission spectrometry (ICP-AES) was employed for chemical composition analysis of the Fe-doped SrTiO3. 2.3. Visible Light-Induced Photocatalytic Activity Evaluation. The photocatalytic activity for the degradation of RhB was performed in a Pyrex reactor in which 100 mg of the photocatalyst was suspended in 100 mL of RhB aqueous solution (∼10-5 M). A 300 W halogen lamp (Institute of Electric Light Source, Beijing) as a light source was positioned inside a cylindrical Pyrex vessel. This vessel was surrounded by a recirculating water jacket (Pyrex) to effectively preclude the IR part of the light and cool the lamp. An appropriate cutoff filter was placed outside the Pyrex jacket to completely remove wavelengths shorter than 420 nm and to ensure that irradiation was achieved by the visible light wavelengths only. Prior to irradiation, the suspension of the photocatalyst in RhB aqueous solution was stirred in the dark for more than 1.5 h to secure an adsorption/desorption equilibrium. At given irradiation time intervals, ca. 3 mL of reaction suspension was sampled and separated by centrifugation. The absorption spectrum of the centrifuged reaction solution was measured on a Hitachi U-3310 spectrophotometer. The concentration of RhB was determined by monitoring the change in the absorbance at 554 nm. 3. Results and Discussion 3.1. Photocatalyst Characterization. (A) Crystalline Structure Analysis. The ICP-AES analysis results show the molar ratio of Sr/Ti/Fe in the synthesized Fe-doped SrTiO3 is about 1:0.946:0.049, indicating that the chemical compositions of the Fe-doped SrTiO3 are basically in agreement with the nominal compositions. Polycrystalline X-ray diffraction (XRD) was used to identify the crystalline structures of pure SrTiO3 and the Fedoped SrTiO3 at room temperature. As shown in Figure 1, two XRD patterns of both samples prepared by a modified sol-gel method followed by calcination at 900 °C could be identified as a single phase SrTiO3 with a perovskite-type structure of cubic symmetry based on JCPDS file (35-734) and literature results.23 Besides, there are not any measurable Fe2O3 and TiO2 diffraction peaks in the sample of Fe-doped SrTiO3. This indicates that doping Fe cation could be carried out to the extent of 5 mol % (at Ti4+ sites) in the host lattice using our synthesis method. A careful comparison of the (110) diffraction peaks (in the range of 2θ ) 32-33°) of two samples reveals that the diffraction peak position of the Fe-doped SrTiO3 is almost

Photocatalytic Degradation of RhB unchanged as compared with that of pure SrTiO3, as shown in the insert of Figure 1. This result could confirm the substitution of Fe cations for Ti4+ rather than for Sr2+ in the Fe-doped SrTiO3 because the ionic radius of Fe3+ (0.79 Å) is very close to that of Ti4+ (0.75 Å) but much smaller than that of Sr2+ (1.18 Å).24,25 The structure of perovskite type SrTiO3 is stabilized by the 6-fold coordination of the Ti cation (octahedron) and 12-fold coordination of the Sr cation.26 Thus, the doped Fe cations can be thought of as the ions occupying the octachedrally coordinated positions like Ti4+ in the Fe-doped SrTiO3. The crystallite size (D) was also determined using the diffraction peak of the sample (110) crystalline plane with Scherrer equation D ) Kλ/(βc - βs) cos θ, where λ is the wavelength of the X-ray radiation, θ is the diffraction angle, and βc and βs are the full widths at half-maximum height of the sample and standard (single-crystal silicon). On the basis of the determination results, the crystalline sizes of pure SrTiO3 and Fe-doped SrTiO3 are 45.1 and 44.7 nm, respectively, suggesting that doping a small amount of Fe cation has no negative effects on the crystallite growth of the host structure. In addition, we also investigated the effect of HCl treatment on the structure and crystallinity of the host. The XRD pattern for the Fe-doped SrTiO3 after HCl treatment (Figure S1, Supporting Information) shows that all diffraction peaks are identified and indexed as a single phase SrTiO3, and no significant change in the crystallinity is observed as compared to that of the Fe-doped SrTiO3 before HCl treatment. (B) SEM ObserWation on the Morphologies of Pure SrTiO3 and the Fe-Doped SrTiO3. Parts A and B of Figure 2 show the SEM morphologies of pure SrTiO3 and the Fe-doped SrTiO3 synthesized under the same condition, respectively. As seen in Figure 2, we can notice that there is no big difference between two samples in particle size and morphologies. A careful observation finds that the particle in the Fe-doped SrTiO3 looks a little bit smoother and smaller than that of pure SrTiO3. It was reported that the particle with small size is favorable for the quick transportation of photogenerated carriers to the surface,27 whereas a rough surface might provide more effective reaction sites for photocatalysis.15 Furthermore, the particle size and surface roughness differences between two samples are very little. Thus, the micromorphological differences of two samples should not be responsible for their different photocatalytic activities. (C) X-ray Photoelectron Spectroscopy (XPS) Analysis. The XPS survey spectra (Figure S2, Supporting Information) for pure SrTiO3 and the Fe-doped SrTiO3 before and after HCl treatment illustrate that no appreciable shifts of Ti2p and Sr3s peaks are observed when Fe cations substitute for octachedrally coordinated Ti4+ in the SrTiO3 lattice and after the Fe-doped SrTiO3 is treated with HCl. These results indirectly indicate that doping Fe cation and HCl treatment do not cause a significant change in the host structure. Fe element is found to be present in the Fe-doped SrTiO3 before and after HCl treatment. The photoelectron peak of Fe2p3/2 appearing at a binding energy (Eb) of 710.3∼710.5 eV indicates that some amount of Fe is in the +2 oxidation state. Figure 3 shows the high-resolution XPS spectra of the Fe2p3/2 of the Fe-doped SrTiO3 before and after HCl treatment. From Figure 3, it can be found that the peak of binding energy for Fe2p3/2 is composed of 711.0 and 709.1 eV, which are assigned to Fe3+ and Fe2+ ions, respectively.28–30 The formation of Fe2+ in the Fe-doped SrTiO3 would be ascribed to the presence of residual carbon originating from the decomposition of the organic precursors. During the calcination at high temperature, the oxidation of carbon would draw oxygen from the surrounding atmosphere and Fe-O network, resulting in

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Figure 2. Surface SEM micrographs of pure SrTiO3 (A) and Fe-doped SrTiO3 (B).

Figure 3. High-resolution XPS spectra of Fe2p3/2 region for the Fedoped SrTiO3 (A) before and (B) after HCl treatment.

the reduction of Fe3+ to Fe2+.31,32 Part of Fe2+ would be kept in the Fe-doped SrTiO3 due to the lack of oxygen in a closed stove at high temperature. The relative dispersion of the doped Fe cations in the host structure SrTiO3 can be estimated from the XPS measurement. The surface atomic ratios of Fe2p/Ti2p can be considered as the relative dispersion of the doped Fe cations in the host structure SrTiO3. Surface atomic ratios of Fe/Ti and atomic percentages of Fe3+ and Fe2+ in the Fe-doped SrTiO3 with and without HCl treatment are given in Table 1.

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TABLE 1: Surface Atomic Ratios of Fe2p/Ti2p and Atomic Percentages of Fe (Fe3+ and Fe2+) at Ti4+ Site in the Fe-Doped SrTiO3 before and after HCl Treatment sample

Fe2p/Ti2p

Fe2p (Fe3+)

Fe2p (Fe2+)

Fe-doped SrTiO3 before HCl treatment Fe-doped SrTiO3 after HCl treatment

∼0.075

3.40 %

3.50 %

∼0.053

2.54 %

2.46 %

From Table 1, it can be found in the Fe-doped SrTiO3 that about half of the doped Fe cations are reduced to form Fe2+, and that the surface atomic ratio of Fe2p/Ti2p is estimated to be about 0.075. This means that 6.9% of Ti4+ sites in the host are occupied by doped Fe cations, which is higher than the nominal value (0.05). XPS is a surface technique whose analysis depth is about 4∼5 nm. This indicates that more doped Fe cations substitute for the octachedrally coordinated Ti4+ located near or onto the surface of the host SrTiO3. From the point of crystallography view,33 it is not a surprise to obtain this result. A solid solution first crystallizes in its host composition at the nucleation and initial growth stages, then more and more foreign atoms are involved to form a solid solution with the growth of the crystal grain. The surface atomic ratio of Fe2p/Ti2p is decreased by about 0.053 close to the nominal value after HCl treatment, suggesting some of Fe cations substituting Ti4+ onto or near surface is removed by HCl. According to the discussion above, the Fe-doped SrTiO3 perovskite structure is depicted in Figure 4. As seen in Figure 4, all TiO6 including small number of FeO6 octahedrons link through corners (O) to form a threedimensional cubic lattice, and Sr2+ cation is in the center of the cube. According to XPS measurement, in the Fe-doped SrTiO3 more FeO6 octahedrons are dispersed near or onto the surface than in the bulk of the host structure. Certainly, for a charge balance, there should be small amount of oxygen vacancy present in the structure, which is caused by Fe3+ and Fe2+ substitution for Ti4+. Herein, it is obvious that the interaction between host ion Ti4+ and foreign ion Fe (Fe3+ and Fe2+) can be considered to be in the form of the linkage (Ti-O-Fe) in two neighboring octahedrons. (D) UV-Wis Diffuse Reflectance Spectra. Figure 5A displays UV-vis diffuse reflectance spectra of pure SrTiO3 and the Fedoped SrTiO3. As can be seen from Figure 5A, doping Fe cations into SrTiO3 lattice exhibits an absorption tail extending from the UV to about 650 nm. It was reported that doping Fe cations into TiO2 lattice leads to the formation of impurity energy level between the conduction band and valence band of

Figure 4. Ideal Fe-doped SrTiO3 perovskite structure. The TiO6 including small amount of FeO6 octahedrons link through O to form a three-dimensional cubic lattice, and Sr2+ cation is in the center of the cube.

TiO2.12,34,35 The energy state of 3d electrons from the Fe impurity energy level could be excited to the conduction band of TiO2, resulting in an absorption in visible light region in the spectrum. SrTiO3 has an electronic band structure similar to that of TiO2. Morin et al. demonstrated the presence of the isolated energy levels created by dopant Fe3+ and Fe2+ in the forbidden band of SrTiO3.36 Thus, the absorption tail extending to 650 nm, observed in Figure 5A, should be partly attributed to the electron excitation from two isolated energy levels to the conduction band of SrTiO3. Such a photoinduced electrontransfer process where an electron is promoted from an occupied level of the electron donor (reductant, Red) to a vacant level of the semiconductor conduction band is referred to as reductantto-band charge transfer (RBCT).37 As discussed above, according to XRD and XPS measurements there exist (TiIV-O-FeII) linkages in the Fe-doped SrTiO3. Oxo-bridged MMCT moieties of mixed metal oxides are known to absorb visible light and even near-IR light.38 Frei et al. demonstrated the use of metalto-metal (TiIV-O-CuI f TiIII-O-CuII, and TiIV-O-SnIIf TiIII-O-SnIII) charge transfer as a new class of visible light absorbing chromophores.21 Therefore, it may be concluded that the absorption extending to visible light region observed in Figure 5A also originates from the visible light excitation of TiIV-O-FeII f TiIII-O-FeIII in addition to the above RBCT. The presence of the MMCT of TiIV-O-FeII is more clearly revealed by the optical difference spectroscopy following an exposure of the Fe-doped SrTiO3 to 420 nm irradiation for 4 h in the presence of O2, as shown in Figure 5B; an absorption loss in the 250-550 nm is obviously observed. Although the electron excitation from the two isolated energy levels (created by Fe3+ and Fe2+ doping) to the conduction band of SrTiO3 (RBCT) is also in this spectral region, the trapping of O2 by the photoexcited electron hardly occurs due to the substantial recombination of the excited electron and the hole left on the isolated energy levels39 and the presence of oxygen vacancy.40,41 As a result, the absorption loss is only assigned to the MMCT transition TiIV-O-FeII f TiIII-O-FeIII. Thus, it is conceivable that the absorption profile in visible light region observed in Figure 5A contains contributions from both RBCT and MMCT. The absorption loss is found to be significantly reduced after the surface linkage (Ti-O-Fe) is destroyed via the removal of the Fe cation onto or near the surface by HCl treatment, as shown in Figure 5C. This result to a great extent confirms that the excited electron from the Fe cation left inside the bulk to the conduction band of the host could not be trapped by the adsorbed oxygen molecules, causing an absorption loss. It is to be noted that the excited MMCT band could go back to the original form of TiIV-O-FeII after exposed to a reduction atmosphere H2, as can be seen from the difference optical spectrum before and after irradiation in O2 and then in H2 in Figure 5D. 3.2. Photocatalytic Performance and Mechanism. (A) Photocatalytic Performance. Prior to visible light irradiation, the adsorption properties of RhB over pure SrTiO3 and the Fe-doped SrTiO3 were examined in the dark, respectively. The adsorption/desorption equilibriums of RhB over two samples are totally established within 1.5 h. With respect to the same initial RhB concentration, only 21 and 19% RhB are adsorbed over pure SrTiO3 and the Fe-doped SrTiO3, respectively, indicating that two samples have a similar ability to adsorb RhB in aqueous solution. The photocatalytic degradation of RhB in aqueous solution over various samples under visible light irradiation (λ g 420 nm) is depicted in Figure 6. As found in Figure 6, the Fe-doped SrTiO3 exhibits

Photocatalytic Degradation of RhB

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Figure 5. (A) UV-vis diffuse reflectance spectra of pure SrTiO3 and Fe-doped SrTiO3. (B) Difference UV-vis spectrum before and after exposure to 420 nm irradiation for 4 h in the presence of O2 for Fe-doped SrTiO3. (C) Difference UV-vis spectrum before and after exposure to 420 nm irradiation for 4 h in the presence of O2 for Fe-doped SrTiO3 treated with HCl. (D) Difference UV-vis spectrum before and after exposure to 420 nm irradiation for 4 h in the presence of O2, then H2 for Fe-doped SrTiO3.

Figure 6. Photocatalytic degradation of RhB in aqueous solution over pure SrTiO3, the Fe-doped SrTiO3, and the Fe-doped SrTiO3 after HCl treatment under visible light irradiation.

much higher photocatalytic activity than pure SrTiO3 for the degradation of RhB. Less than 50% RhB is photocatalytically degraded over pure SrTiO3 after visible light irradiation for 6 h, whereas more than 80% RhB degradation over the Fedoped SrTiO3 is observed within the same time. Control experiments show that no RhB degradation is found under visible light irradiation in the absence of catalysts. The results demonstrate that doping Fe cation in SrTiO3 significantly enhances the photocatalytic activity of SrTiO3 for the degradation of RhB. Meanwhile, the photocatalytic activity of the Fe-doped SrTiO3 is greatly reduced to be close to that of pure SrTiO3 after the Ti-O-Fe linkages onto and near the surface of the Fe-doped SrTiO3 are destroyed by HCl treatment, suggesting that the Ti-O-Fe linkages onto and near surface rather than inside the bulk of the host perovskite SrTiO3 play a significant role in the enhanced photocatalytic

activity. Moreover, the results also indicate that the presence of Fe cation substitution for Ti4+ inside the bulk of the host does not make contributions to the degradation of RhB. (B) Photocatalytic Mechanism. SrTiO3 is a wide band gap semiconductor (Eg ) 3.2 eV), and it alone is active only under UV irradiation. Thus, RhB degradation over pure SrTiO3 upon visible light irradiation is a self-photosensitized process. According to the discussion above, the visible light absorption bands in the diffuse reflectance spectrum (Figure 5A) originates from the electron excitation from two isolated energy levels created by Fe3+ and Fe2+ dopants to the conduction band of SrTiO3 (RBCT transition) and the excitation of TiIV-O-FeII MMCT band, as confirmed by the optical difference spectroscopy (Figure 5B). Because of the formation of the isolated energy levels and the oxygen vacancy caused by Fe3+ and Fe2+ substituting for Ti4+ in SrTiO3, there is a substantial recombination between the photoexcited electron and hole left in the isolated energy levels. Thus, we could rule out the possibility that the oxygen molecule is scavenged by the excited electron via the reductant-to-band charge transfer (RBCT) to form superoxide anion radical, which acts as an additional driving force for the degradation of RhB. Therefore, the excitation of TiIV-O-FeII f TiIII-O-FeIII should be responsible for the enhanced degradation of RhB. Frei et al. demonstrated the excitation of TiIV-O-SnII linkage leads to the formation of Ti3+ under visible light irradiation, as was confirmed by observation of an identical EPR signal.21 Hashimoto et al. reported that the photogenerated Ti3+ via the excitation of TiIV-O-CeIIIf TiIII-O-CeIV reduced O2 to O2- · . The formed O2- · was regarded to be the cause of the oxidation of 2-propanol.22 In our current case, there are (Ti-O-Fe) linkages present in the Fe-doped SrTiO3. A possible photocatalytic

9758 J. Phys. Chem. C, Vol. 112, No. 26, 2008 SCHEME 1: Schematic illustration of the photocatalytic process driven by the visible light-induced MMCT of Ti(IV)-O-Fe(II) in Fe-doped SrTiO3

Xie et al. Natural Science Foundation of China (20673145) and National Basic Research Program of China (973 Program, No. 2007CB613302). Supporting Information Available: Polycrystalline X-ray diffraction patterns of the Fe-doped SrTiO3 before and after HCl treatment (Figure S1), and XPS survey spectra for pure SrTiO3, Fe-doped SrTiO3, and Fe-doped SrTiO3 treated with HCl (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

process driven by the visible light-induced excitation of (Ti-O-Fe) linkages is proposed in Scheme 1. The excitation of TiIV-O–FeII f TiIII-O–FeIII occurs under visible light irradiation, as is evidenced in Figure 5(B). Subsequently, the oxygen molecule adsorbed could be reduced by the generated TiIII to form superoxide anion radical. Meantime, the reduction of FeIII to FeII for further excitation of MMCT could be achieved by the oxidation of the excited RhB* because the redox potential of E°(Fe3+/Fe2+) (+0.771 V versus NHE) is more positive than that of RhB* (-0.559 V versus NHE).42,43 Our optical difference spectroscopy (Figure 5D) also demonstrates that the original form of TiIV-O-FeII could be recovered after exposed to a reduction atmosphere H2. In such a recycle process of electron transfer, the superoxide anion radical could be generated continuously. Finally, the RhB, RhB*, or RhB+ · is oxidized by the superoxide anion radical to form degraded or mineralized products.44 We believe that the superoxide anion radical formed via this channel would be responsible for the enhanced degradation of RhB. Of course, an alternative mechanism that the visible light excitation of TiIV-O-FeIII f TiIII-O-FeIV drives the photocatalytic degradation of RhB might not be excluded completely, although (Ti-O-Fe) linkages are more preferable on the interfacial than in the bulk of the Fe-doped SrTiO3 based on the XPS measurement. HCl treatment significantly diminishes the excitation of TiIV-O-FeII f TiIII-O-FeIII through destroying the interfacial (Ti-O-Fe) linkages (Figure 5C). This suggests that the MMCT of the interfacial TiIV-O-FeII is indispensable for the enhanced visible light photocatalytic process. 4. Conclusions The visible light photocatalytic properties of pure SrTiO3 and Fe-doped SrTiO3 were investigated. Remarkably higher visible light photocatalytic activity for the degradation of RhB was observed over the Fe-doped SrTiO3 than over pure SrTiO3. The visible light-induced MMCT of TiIV-O-FeII linkages formed in the Fe-doped SrTiO3 was demonstrated to be the origin of the enhanced activity. The present study introduced an insight of oxo-bridged MMCT into the understanding of the photocatalytic behaviors of the Fe-doped SrTiO3 under visible light irradiation. Furthermore, it was also revealed that the MMCT of only interfacial TiIV-O-FeII is responsible for the enhanced degradation of RhB under visible light irradiation. We believe this study would open a door for extending the traditional photocatalysis theories and offer a promising strategy for developing new visible light photocatalysts. Acknowledgment. The authors genuinely appreciate the generous financial support of this work from the National

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