An Efficient Visible-Light-Sensitive Fe(III)-Grafted TiO2 Photocatalyst

Download PDF
0 downloads 0 Views 386KB Size Report
Comment
Sep 1, 2010 - of the analogy to Cu(II)-grafted TiO2 photocatalyst (Irie, H.; et al. Chem. Phys. Lett. 2008, 457 .... of 16.7 µL of HCl (6 mol/L) under stirring to adjust the pH to. ∼2. ... Following the injection of ... for CO2 generation could be calculated by using the equation ... Debye-Waller factor, C2) of Fe2O3 and Fe(III)/TiO2.
J. Phys. Chem. C 2010, 114, 16481–16487

16481

An Efficient Visible-Light-Sensitive Fe(III)-Grafted TiO2 Photocatalyst Huogen Yu,† Hiroshi Irie,*,‡ Yoshiki Shimodaira,§ Yasuhiro Hosogi,| Yasushi Kuroda,| Masahiro Miyauchi,⊥ and Kazuhito Hashimoto*,†,# Research Center for AdVanced Science and Technology (RCAST), The UniVersity of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan, Clean Energy Research Center, UniVersity of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8511, Japan, Corporate R&D Center, Showa Denko K. K., 1-1-1 Ohnodai, Midori-ku, Chiba, Chiba 267-0056, Japan, Showa Titanium Co., Ltd, 3-1 Nishinomiya-machi, Toyama, Toyama 931-8577, Japan, National Institute of AdVanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Department of Applied Chemistry, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: May 14, 2010; ReVised Manuscript ReceiVed: August 15, 2010

We have prepared a TiO2-based novel visible-light-sensitive photocatalyst, in which Fe(III) species were grafted on a rutile TiO2 surface (denoted as Fe(III)/TiO2). With use of X-ray absorption fine structure analysis, the grafted iron species were determined to be in the 3+ state and adopt an amorphous FeO(OH)-like structure. Fe(III)/TiO2 displayed optical absorption in the visible light range over 400 nm, which was assigned to the interfacial charge transfer from the valence band of TiO2 to the surface Fe(III) species. Its photocatalytic activity was evaluated by the decomposition of gaseous 2-propanol under visible light (400-530 nm), which revealed a high quantum efficiency (QE) of 22%. Monochromatic light experiments indicated that the effective wavelength region was extended as far as 580 nm while maintaining a QE of greater than 10%. On the basis of the analogy to Cu(II)-grafted TiO2 photocatalyst (Irie, H.; et al. Chem. Phys. Lett. 2008, 457, 202), we speculate that the high performance of the present photocatalyst is derived from the photoproduced holes that are generated in the valence band of TiO2 and contribute to the oxidative decomposition of 2-propanol, and the catalytic reduction of oxygen (presumably multielectron reduction) by photoproduced Fe(II) species on TiO2. Introduction TiO2 photocatalysts have been investigated for many years with the aim of increasing their sensitivity to visible light in order to utilize incoming light energy more efficiently. The doping of various anions (N, S, C, etc.)1,2 and cations (Cr, V, Fe, Mn, Co, Ni, etc.)3 into the lattice of TiO2 has been found to extend its absorption into the visible light region. Most doped TiO2 photocatalysts are sensitive to visible light due to the formation of a localized narrow band above the valence band (VB) of TiO2 that originates from the dopant cations and anions. However, these photocatalysts typically exhibit a lower activity under visible light than under UV light because the oxidation power and mobility of photogenerated holes in the localized narrow band are less than those in the VB of TiO2.2a On the basis of these properties, we assumed that the doping of foreign elements into TiO2 is not an effective strategy for developing highly efficient TiO2 photocatalysts sensitive to visible light, and other approaches are therefore necessary. One of the promising approaches toward increasing the efficiency of TiO2 as a photocatalyst is so-called dyesensitization. For example, it is well-known that Ru complex* To whom correspondence should be addressed. H.I.: phone +8155-220-8092, fax +81-55-220-8092, e-mail [email protected]. K.H.: phone +81-3-5452-5080, fax +81-3-5452-5084, e-mail hashimoto@ light.t.u-tokyo.ac.jp. † Clean Energy Research Center, The University of Tokyo. ‡ University of Yamanashi. § Corporate R&D Center, Showa Denko K. K. | Showa Titanium Co., Ltd. ⊥ National Institute of Advanced Industrial Science and Technology. # Department of Applied Chemistry, The University of Tokyo.

modified TiO2 shows visible light absorption. In this system, upon excitation of the Ru complex, an electron in the excited state is subsequently injected into the conduction band (CB) of TiO2.4a-c Another example of dye-sensitization is the H2[PtCl6] (or PtCl4)-modified TiO2 system,4d,e reported by Kisch et al.,4d that involves visible light absorption by Pt(IV)chloride to generate two new redox centers, an oxidative Cl/ Cl- pair and a reductive Pt(IV)/Pt(III) pair. Pt(III) returns to Pt(IV) by injecting electrons into the CB in TiO2, while Cl returns to Cl- through the oxidization of organic compounds. In both cases, the electron transfer to TiO2 proceeds via the excited state of the adsorbed molecules. With the exception of TiO2-based materials, some reports have examined oxide materials that are sensitive to visible light for the oxidative decomposition of organic compounds,5 among which Ag3PO4 was reported very recently.5e Recently, we reported that TiO2 powders with grafted metal ions (Cu(II), Cr(III), and Ce(III))6-8 were capable of serving as photocatalysts sensitive to visible light. As thermal treatment was not performed, these ions may exist only on the surface of TiO2 and would not be doped into the lattices. The grafting of Cu(II) to form Cu(II)/TiO2 resulted in a quantum efficiency (QE) for 2-propanol decomposition under visible light (>450 nm) that was rather high (QE ) 8.8%).7 In this system, it is assumed that visible light initiates interfacial charge transfer (IFCT), i.e., electrons in the VB of TiO2 directly transfer to Cu(II), not via the excited state, to form Cu(I) while holes remain in the VB of TiO2 based on the suggestions by Creutz et al.9 relating to photoinduced IFCT between the continuous and discrete energy levels of solids and surface-associated molecular species, respectively. The holes produced in the VB of TiO2 decompose

10.1021/jp1071956  2010 American Chemical Society Published on Web 09/01/2010

16482

J. Phys. Chem. C, Vol. 114, No. 39, 2010

organic substances resulting in the high activity of this photocatalyst, while Cu(I) produced by electron transfer is presumed to reduce adsorbed O2 and thereby consume electrons.7 Although the QE of the Cu(II)/TiO2 under visible light irradiation was higher than that of typical N-doped TiO2, a photocatalyst currently being intensively investigated by several research groups, there is a need to further develop and improve visible light-active TiO2 systems for use in practical applications. Here, we attempted to identify a new metal ion that could be used in place of Cu(II) toward improving the photocatalytic activity of TiO2. We found that Fe(III)-grafted TiO2 exhibits a very high activity under visible light after optimization of the preparation conditions, which included the pretreatment of TiO2 and adjusting the pH of the starting solution used for the grafting of Fe(III) ions. Experimental Section Metal Ion Selection. Our strategy for identifying a new metal ion suitable for use in the visible light-active TiO2 system was as follows: (1) On the basis of the knowledge established for the Cu(II)/TiO2 system, we hypothesized that certain electroninjected metal ions could act as oxygen reduction catalysts, presumably through multielectron reactions. (2) We attempted to select a metal ion with a more positive redox potential than Cu(II)/Cu(I) (0.16 V vs SHE, pH 0) as it was expected that such metal ion-grafted TiO2 could absorb visible light in a longer wavelength region (up to 500 nm) and utilize incoming light energy more efficiently than Cu(II)/TiO2. (3) The metal ion was required to be nontoxic and abundant in natural resources to facilitate its use in industrial and practical applications. In addition to the above desired characteristics, the selection of the metal ion was influenced by the redox reactions which are active in most natural enzyme systems. We focused on the iron ion (Fe(III)) as it widely exists in natural systems, such as Cytochrome c oxidase, and acts as an effective redox catalyst for numerous enzymatic reactions. For example, Cytochrome c oxidase can effectively reduce oxygen to water via a fourelectron oxygen reduction.10 As it has also been reported that Fe(II) is unstable and easily becomes Fe(III) through the reduction of oxygen under ambient conditions (4Fe2+ + O2 + 4H+ f 4Fe3+ + 2H2O or 4Fe2+ + O2 + 2H2O f 4Fe3+ + 4OH-),11 we hypothesized that Fe(II) may also serve as an oxygen reduction catalyst. Moreover, Fe(III)/Fe(II) has a redox potential of 0.771 V (vs SHE, pH 0),12 which is more positive than that of Cu(II)/Cu(I). Iron is also a suitable candidate as it is the fourth most abundant element as determined by the Clarke number and is nontoxic. These characteristics inspired us to examine the properties of a TiO2 photocatalyst prepared with Fe(III), namely, Fe(III) grafted-rutile TiO2. Preparations. Commercial MT-150A TiO2 nanoparticles (rutile form, 15 nm grain size, specific surface area of 90 m2/g, TAYCA Corp.) (referred to as TiO2 (R-com)) were calcined at 950 °C for 3 h (TiO2 (R-950)) before use. After calcination, the specific surface area of TiO2 decreased to 3.7 m2/g. FeCl3 · 6H2O (Wako, 99.9%) was used as the Fe(III) precursor. To simplify the sample name of Fe(III)-grafted TiO2 prepared under different conditions, it will be referred to as Fe(III)/TiO2 (X-Y-Z), with X representing the TiO2 phases (A for anatase or R for rutile), Y indicating the calcination temperature (com for commercial sample without calcination, 500 for calcination at 500 °C for 3 h, or 950 for calcination at 950 °C for 3 h), and Z representing the pH, either 2 or 7, with adjusting to pH 2 or without adjusting the pH during impregnation, respectively.

Yu et al. Fe(III)/TiO2 (R-Y-Z). Fe(III)/TiO2 photocatalyst was prepared by an impregnation technique. In a typical preparation, 10 mL of distilled water was added to a mixture of 1 g of TiO2 (R950) and FeCl3 · 6H2O (0.00244 g, Fe/TiO2 ) 0.05 wt %, we preliminary determined the optimal amount of Fe(III) ions, see the Supporting Information (S. I. 1)), followed by the addition of 16.7 µL of HCl (6 mol/L) under stirring to adjust the pH to ∼2. After heating at 90 °C for 1 h in a sealed vial reactor, the suspension was filtered with a 0.025 µm membrane filter (Millipore Corp.) and washed twice with sufficient amounts (∼600 mL) of distilled water. The resulting residue was dried at 110 °C for 24 h and subsequently ground into a powder with an agate mortar to obtain Fe(III)/TiO2 (R-950-2). For comparison, commercial TiO2 (R-com) was used as the TiO2 precursor to prepare Fe(III)/TiO2 (R-com-2) under the identical experimental conditions. The photocatalyst Fe(III)/TiO2 (R-950-7) was also prepared without adjusting the pH of FeCl3 solution. Cu(II)/TiO2 (R-Y-Z). Cu(II)/TiO2 (R-950-7) photocatalyst was prepared by an identical method as previously reported,7 with the only difference being the use of calcined TiO2 (R950). Fe(III)/TiO2 (A-Y-Z). Commercial anatase TiO2 (ST01, Ishihara Sangyo Kaisha Ltd.) was calcined at 500 °C for 3 h before it was used for the preparation of Fe(III)/TiO2 (A-5002) photocatalyst. The preparation procedure was identical with that described for Fe(III)/TiO2 (R-950-2). Fe(III)/WO3 (pH ∼2). Commercial WO3 (average grain size 0.25 µm, surface area 9.5 m2/g, Kojundo Chemical Lab. Co., Ltd.) was filtrated with a mesh filter (1 µm, Kiriyama glass Co.) and then calcined at 650 °C for 3 h before it was used for the preparation of Fe(III)/WO3 photocatalyst at pH ∼2. The preparation procedure was identical with that described for Fe(III)/ TiO2 (R-950-2). Characterizations. Elemental analysis of the prepared photocatalyst (Fe(III)/TiO2 (R-950-2)) was performed using an inductively coupled plasma atomic emission spectrometer (ICPAES, ICPS-7500, Shimadzu Co., Ltd.) for Ti and Fe. Surface composition was evaluated by X-ray photoelectron spectroscopy (XPS; Axis Ultra, Kratos Analytical Co.). UV-visible spectra were obtained by a diffuse reflection method with a spectrometer (UV-2550, Shimadzu Ltd.) and BaSO4 as a reflectance standard. The structural features of the grafted Fe ions in the prepared Fe(III)/TiO2 (R-950-2) were characterized by X-ray absorption fine structure (XAFS) analysis, in addition to X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses. For XAFS measurements, a portion of the Fe/TiO2 photocatalyst (20 mg) was mixed with 80 mg of boron nitride (Kishida Chemical; 99.5%) in a mortar, and the mixture was then pressed into a 10-mm-diameter disk. The XAFS spectra for the Fe K-edges were recorded on a beamline BL14B2 at the SPring-8 facility, which is administrated by the Japan Synchrotron Radiation Research Institute (JASRI). Transmission (Fe foil, FeO, and Fe2O3) and fluorescence yield (photocatalyst) spectra were acquired by using a double-crystal Si(111) monochromator, ion chambers, and a 19element germanium solid state detector (SSD) equipped with a nickel filter. The XAFS data were analyzed with the Ifeffit software package13 with theoretical standards given by the FEFF.14 The photocatalytic oxidation activities of the photocatalysts were evaluated by the decomposition of gaseous 2-propanol under visible light illumination (400-530 nm, 1 mW/cm2) from a Xe lamp (LA-251Xe, Hayashi Tokei) equipped with glass filters (L-42, B-47, C-40C, Asahi Techno-glass). For the

Fe(III)-Grafted TiO2 analysis, 300 mg of the photocatalyst was uniformly spread over a 5.5-cm2 irradiation area in a 500-mL quartz vessel. Before injecting 6 µmol (∼300 ppm) of gaseous 2-propanol, the organic compounds (originating from the air) absorbed on the surface of catalysts were first photo-oxidized into CO2 and the gas in the quartz vessel was then replaced with pure synthetic air (containing no CO2 or organic compounds). Following the injection of 2-propanol, the reaction vessel was kept in the dark for 12 h and was then subjected to visible light irradiation to initiate the photocatalytic reactions. The concentrations of acetone and CO2 produced were monitored by using a gas chromatograph (model GC-8A, Shimadzu Co., Ltd.). Additionally, 2-propanol decomposition in the presence of Fe(III)/TiO2 (R-950-2) was measured under monochromatic light to observe the wavelength dependence of the QE using a Czerny-Turner type monochromator (Ritsu Ouyou Kogaku, MC-10N) and a Xe lamp (as described above). Higher order diffracted light was cut off with the appropriate glass filter. The full width at halfmaximum (fwhm) value of the monochromatic light was ∼5-20 nm and the absorption photon number of the catalyst was controlled to be ∼1.1 × 1014 quanta/s. The irradiation area of the sample was 1.65 cm2 and the weight of the photocatalyst was 89 mg. The QE was calculated on the assumption that six photons were required to produce one CO2 molecule from 2-propanol: C3H8O + 5H2O + 18 h+ f 3CO2 + 18H+. Therefore, the QE for CO2 generation could be calculated by using the equation QE ) 6 × CO2 generation rate/absorption rate of incident photons.

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16483

Figure 1. (A) Fe K-edge XANES spectra of Fe metal, FeO, Fe2O3, and Fe(III)/TiO2 (R-950-2) photocatalyst. (B) Enlargement of part A, in which all of the spectra are superimposed.

Results and Discussion Characterizations of Fe(III)/TiO2. The elemental analysis indicated that the weight fraction of Fe relative to TiO2 in Fe(III)/TiO2 (R-950-2) was 3.5 × 10-4, a value that was somewhat smaller than the starting ratio used in the preparation. In Figure 1A, the Fe K-edge XANES spectra for Fe metal, FeO, Fe2O3, and Fe(III)/TiO2 (R-950-2) photocatalyst are shown. It is clear that the spectrum for Fe(III)/TiO2 (R-950-2) resembles that of Fe2O3 and is qualitatively different from the spectra obtained for Fe and FeO, indicating that the Fe species are grafted in the 3+ oxidation state, Fe(III), as was predicted. The Fourier transforms (FTs) of the k3χ(k) EXAFS for the Fe2O3 reference and Fe(III)/TiO2 (R-950-2) photocatalyst are shown in Figure 2 without phase shift correction. The structural parameters (bond length (R) and Debye-Waller factor (C2) of different shells) determined by the FEFF simulation are summarized in Table 1, which also includes the FeO(OH) structural data obtained by ICSD no. 163341. Upon comparison of the structural parameters, it is clear that those of the photocatalyst more closely resemble the parameters of FeO(OH) than those of Fe2O3. Thus, through analogy to the Cu(II)/TiO2 photocatalyst, in which Cu(II) ions are grafted in amorphous form as clusters,7 the Fe(III) ions in this system appear to form FeO(OH)-like amorphous clusters on the TiO2 surface. The Fe(III)/TiO2 (R-950-2) photocatalyst was also subjected to XPS analysis, which revealed the presence of both Fe2p3/2 and Fe2p1/2 peaks in the resulting spectrum (Figure 3A). It was reported that the Fe2p3/2 XPS peaks at 710.8, 711.3, and 711.5 eV correspond to Fe(III) in Fe2O3, FeCl3, and FeOOH, respectively, whereas those at 709.4 and 710.7 eV correspond to Fe(II) in FeO and FeCl2.15 Thus, the peak observed at ∼710 eV in Fe(III)/TiO2 (R-950-2) is likely to have been derived from Fe(II). Although this result contradicts the XANES measurement that indicated the Fe ions were grafted as Fe(III), the presence

Figure 2. Fourier transforms (FTs) of the k3χ(k) EXAFS for the Fe2O3 reference and Fe(III)/TiO2 (R-950-2) photocatalyst.

TABLE 1: Structural Parameters (bond length, R and Debye-Waller factor, C2) of Fe2O3 and Fe(III)/TiO2 (R-950-2), As Derived from EXAFS and FeO(OH) Fe(III)/TiO2 (R-950-2) FeO(OH)a

Fe2O3 shell

CN

Fe-O1 Fe-O2 Fe-O3 Fe-Fe1 Fe-Fe2 Fe-Fe3 R-fac.

3 3 3 1 3 3

a

R (Å)

C2 (10-2 Å2) CN

1.919 2.088 3.353 2.858 2.931 3.319 7.5%

0.31 0.25 0.76 0.16 0.45 0.27

3 3 1 2 2 4

R (Å)

C2 (10-2 Å2) CN

1.912 2.084 3.339 2.919 3.410 3.670 17.6%

0.39 0.48 1.13 0.98 0.91 0.31

3 3 1 2 2 4

R (Å) 1.97 2.14 3.20 3.02 3.29 3.44

ICSD no. 163341.

of Fe(II) ions may be plausible considering that the XPS measurement was performed under high vacuum conditions (10-10-10-9 Torr). Upon introduction of the photocatalyst into the high vacuum XPS chamber, the grafted Fe(III) may have been reduced to Fe(II). This conclusion was supported by the observation that if the Fe(III) was crystallized prior to grafting (e.g., either crystalline Fe2O3 or FeOOH was grafted on TiO2), the reduction of Fe(III) to Fe(II) scarcely proceeded, and the

16484

J. Phys. Chem. C, Vol. 114, No. 39, 2010

Figure 3. XPS spectra for (A) Fe 2p and (B) Cl 2p of (a) rutile TiO2, (b) a mechanical mixture of TiO2 and FeCl3 · 6H2O, and (c) Fe(III)/ TiO2 (R-950-2).

Yu et al.

Figure 5. Schematic diagram illustrating the possible photocatalytic mechanism of Fe(III)/TiO2, involving interfacial charge transfer (IFCT) (arrow 1) and multielectron reduction processes. The band gap excitation is indicated by arrow 2.

Figure 4. UV-vis diffuse reflectance spectra of (a) TiO2 (R-950) and (b) Fe(III)/TiO2 (R-950-2). The inset shows the difference UV-vis spectrum of TiO2 (R-950) and Fe(III)/TiO2 (R-950-2).

ion was detected as Fe(III) by XPS even under high vacuum conditions. A similar phenomenon was observed in the previously reported Cu(II)/TiO2 system.7 When a mechanical mixture of TiO2 and FeCl3 · 6H2O with a weight fraction of Fe to TiO2 of 5.0 × 10-4 was examined by XPS, a peak at 711.3 eV was observed (Figure 3A), which corresponded to the peak from Fe(III) in FeCl3. Significantly, the peak at ∼198 eV, which was derived from Cl (Cl 2p3/2),15 was also observed in the mechanical mixture of TiO2 and FeCl3 · 6H2O; however, this peak was not observed in the photocatalyst (Figure 3B). This result clearly indicates that the prepared photocatalyst is free of Cl ions. The Cl- ions were assumed to be released when the FeCl3 · 6H2O powder was added to the aqueous TiO2 suspension, which was subsequently heated at 90 °C with stirring for 1 h. During this treatment, the hydrated Fe(III) were grafted onto the TiO2 particles and the Cl- ions were removed when the particles were subsequently washed with copious amounts of distilled water. The UV-vis diffuse reflectance spectra of bare TiO2 (R-950) and Fe(III)/TiO2 (R-950-2) were also measured and compared (Figure 4). For TiO2 (R-950), no absorption could be observed other than the band gap excitation of rutile TiO2 at wavelengths shorter than 410 nm. After grafting Fe(III) ions on the surface of TiO2, a new absorption between 410 and 580 nm was observed. The difference UV-vis spectrum of bare TiO2 (R950) and Fe(III)/TiO2 (R-950-2) is shown in the inset of Figure 4. It is assumed that the new absorption could be attributed to the IFCT from the VB of TiO2 to the Fe(III) ions (arrow 1 in Figure 5). The absorption derived from IFCT in the presence of Fe(III)/TiO2 (R-950-2) extended to a longer wavelength region (∼580 nm) than that observed for Cu(II)/TiO2 (∼500 nm).7 It should be noted that the present photocatalyst is not Fe(III)doped TiO2, i.e., Fe(III) ions are not introduced into TiO2 lattice; however, Fe(III)-grafted TiO2, i.e., Fe(III) ions, are attached onto the TiO2 surface in the form of FeO(OH)-like amorphous clusters (Supporting Information, S. I. 2).

Figure 6. Changes in acetone and CO2 concentrations as a function of irradiation time in the presence of TiO2 (R-950) and Fe(III)/TiO2 (R-950-2) under visible light irradiation.

TABLE 2: CO2 Generation Rate (k), Quantum Efficiency (QE), and Absorbed Photon Number (N) of Various Photocatalysts (visible light source: 400-530 nm) samples

N QE k (µmol/h) (quanta/s) (%)

1.6 × 10-1 N-doped TiO2(HP-N08)a b Pt chloride-modified TiO2 (MPT-623) 1.5 × 10-1 TiO2 (R-com) 2.1 × 10-2 Fe(III)/TiO2 (R-com-2) 8.8 × 10-2 TiO2 (R-950) 1.1 × 10-2 Fe(III)/TiO2 (R-950-7) 7.1 × 10-2 Fe(III)/TiO2 (R-950-2) 2.3 × 10-1 Cu(II)/TiO2 (R-950-7) 1.7 × 10-1 3.6 × 10-2 Fe(III)/TiO2 (A-500-2) Fe(III)/WO3 (pH ∼2) 1.3 × 10-1

4.1 1.6 3.7 1.4 7.6 1.5 1.1 1.1 1.5 4.1

× × × × × × × × × ×

1015 3.9 1015 9.8 1014 5.7 1015 6.1 1014 1.4 1015 4.6 1015 22 1015 15 1015 2.5 1015 3.1

a N-doped TiO2 (HP-N08, Showa Denko, K. K., specific surface area: 95 m2/g) is a commercial anatase TiO2 photocatalyst with high photocatalytic activity under visible light irradiation. b Pt chloridemodified TiO2 (MPT-623, Ishihara Sangyo Kaisha Ltd.) is a commercial photocatalyst with high photocatalytic activity under visible light irradiation.

Photocatalytic Activity of Fe(III)/TiO2. The rate of 2-propanol decomposition in the presence of Fe(III)/TiO2 (R-950-2) was determined as a function of irradiation time under visible light irradiation (400-530 nm, 1 mW cm-2) (Figure 6). For comparison, bare TiO2 (R-950) was also tested under the identical experimental conditions. In the presence of photocatalysts, 2-propanol was decomposed into the final product CO2 via the intermediary product acetone, and the QEs were then calculated from the slopes of the CO2 concentration profiles (k values are shown as broken lines in Figure 6 and are summarized in Table 2). For bare TiO2 (R-950), 2-propanol was gradually oxidized into CO2 (k ) 1.1 × 10-2 µmol/h) and the calculated QE was 1.4%. As the band gap of rutile TiO2 is ∼3.0 eV, it can partially absorb incident light (400-530 nm) and the

Fe(III)-Grafted TiO2

Figure 7. Dependence of the quantum efficiency (QE) on the wavelength of monochromatic light for the decomposition of 2-propanol by Fe(III)/TiO2 (R-950-2) photocatalyst. For each experiment, the absorption photon number was kept constant at 1.1 × 1014 quanta s-1.

generation of CO2 is therefore derived from the bandgap excitation (indicated by arrow 2 in Figure 5). After grafting Fe(III) on the surface of TiO2, a surprising increase in the CO2 generation rate (k ) 2.3 × 10-1 µmol/h) was observed, and the corresponding calculated QE increased to 22% for the Fe(III)/ TiO2 (R-950-2). Compared with bare TiO2 (R-950), the obvious increase of QE for Fe(III)/TiO2 (R-950-2) is thought to originate from the new absorption in the 410-580 nm range (this is discussed in more detail below). After 200 h of visible-light irradiation, ∼18 µmol of CO2 was formed in the presence of Fe(III)/TiO2 (R-950-2), indicating the complete oxidation of 2-propanol into CO2 (18 µmol of CO2 is produced when 6 µmol of 2-propanol is completely degraded). The turnover number of Fe(III) catalyst on TiO2 was therefore estimated to be ∼70 for this complete oxidation process. Note that both the CO2 generation rate (k ) 3.4 × 10-2 µmol/h) and the corresponding QE (4.1%) markedly decreased for Fe(III)-doped TiO2, prepared by the calcinations of the Fe(III)-grafted TiO2 (R-9502) (Supporting Information, S. I. 2). To reveal the photocatalytic mechanism of the visible-lightsensitive Fe(III)/TiO2 (R-950-2) photocatalyst, the wavelengthdependence of the QE in the range of 360-650 nm was plotted (Figure 7). For comparison, the QE of bare TiO2 (R-950) at 380 nm (monochromatic light) was determined under the identical experimental conditions (maintaining the same absorbed photon number as Fe(III)/TiO2 (R-950-2)). From this analysis, it was found that the QE of bare TiO2 (R-950) irradiated with monochromatic light at 380 nm was 1.7%, which was similar to the value (1.4%) obtained after irradiation with 400-530 nm light (Figure 6 and Table 2). This result further confirms that the photocatalytic activity of bare TiO2 (R-950) is derived from its band gap excitation. With respect to Fe(III)/TiO2 (R-950-2), it is interesting that this photocatalyst displayed a high QE value (10-24%) in a wide visible-light range of up to 580 nm. Under such monochromatic visible-light illumination, the only possible way for electron transfer to occur in Fe(III)/TiO2 (R-950-2) is by IFCT, as indicated by arrow 1 in Figure 5. In this IFCT mechanism, photogenerated electrons are directly transferred to Fe(III) ions, producing Fe(II) ions capable of reducing oxygen, while photogenerated holes remain in the VB of TiO2 and oxidize organic compounds, such as 2-propanol and acetone. It was also found that at 360 nm, Fe(III)/TiO2 (R-950-2) displayed a low QE value of 1.6% (Figure 7), which was very close to that of bare TiO2 (R-950) at 380 nm (QE ) 1.7%). This result suggests that under UV light irradiation at 360 nm, the Fe(III) clusters have almost no contribution to the photocatalytic oxidation of 2-propanol and indicates that electron transfer from the CB of

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16485 TiO2 to the Fe(III) clusters cannot effectively proceed, even though the potential of the TiO2 CB (∼0 V, vs SHE, pH 0) is lower than that of Fe(III)/Fe(II) (E0 ) 0.771 V, vs SHE, pH 0).16 In this case (360 nm excitation), only one-electron oxygen reduction may occur in Fe(III)/TiO2 (R-950-2), as shown in Figure 5, and therefore exhibits the identical electron-transfer process as bare TiO2. Although the actual mechanism is presently unknown, these results suggest that Fe(III)/TiO2 (R-9502) is capable of the multielectron reduction of oxygen, resulting in an obvious enhancement of photocatalytic activity in the visible-light region compared with the one-electron reduction of oxygen by bare TiO2 (R-950). The use of other multielectron reduction catalysts for oxygen, such as Pt,16 Cu(I),7 and WC,17 has also been shown to enhance the photocatalytic performance of either WO3 or TiO2 (Table 2). Compared with the well-known, visible light-sensitive Ndoped TiO2 photocatalyst (QE ) 3.9%, HP-NO8, Showa Denko, K. K.), which has a specific surface area of 95 m2/g, the QE value (22%) of Fe(III)/TiO2 (R-950-2) was significantly higher, even though the TiO2 (R-950) in Fe(III)/TiO2 (R-950-2) has a much lower specific surface area of only 3.7 m2/g. For the higher QE for the photodegradation of organic compounds, there are several obvious advantages of using Fe(III)/TiO2 (R-950-2) compared with N-doped TiO2.1,2 First, the photogenerated holes in the VB of Fe(III)/TiO2 (R-950-2) have a stronger oxidation power (∼3.0 V vs SHE, pH 0) to oxidize organic compounds than those in the localized N 2p narrow band of N-doped TiO2 (∼2.3 V vs SHE, pH 0). Second, the photogenerated holes in consecutive O2p energy levels (VBs) of Fe(III)/TiO2 (R-9502) have a higher mobility than those in the localized N2p narrow band of N-doped TiO2. In general, the “overall” photocatalytic activity under visible light (CO2 generation rate, k value) is determined by the multiplication of the visible-light absorption capability (N value) and the visible-light utilization efficiency (QE value). The visible-light absorption capability of N-doped TiO2 is superior to that of Fe(III)/TiO2 (R-950-2). Thus, although the QE of Fe(III)/TiO2 (R-950-2) was up to 5.6 times higher than that of N-doped TiO2, the k value was 1.4 times higher. However, the approach in the present study to increase in the QE is very important in practical applications because the approach to increase in the visible-light absorption capability produces the colored TiO2 (in most cases, yellow TiO2). This should be an obstacle to practical applications such as interior self-cleaning glasses and tiles, and antifogging mirrors, because the appearance of such substrates is drastically changed when they are coated with the colored TiO2. In contrast, when keeping the TiO2 almost white (actually, whitish pale yellow) while increasing the QE, the appearance of substrates is nearly unchanged, which will expand the possibility of TiO2 toward indoor usages. Pt chloride-modified TiO2 (MPT-623, Ishihara Sangyo Kaisha Ltd.) is also commercially available as a visible light-sensitive photocatalyst and it has been already applied to practical indoor applications. Compared with this photocatalyst (QE ) 9.8%), the QE value (22%) of Fe(III)/TiO2 (R-950-2) was higher. To prepare the efficient visible-light-sensitive Fe(III)/TiO2 photocatalyst, two factors are of key importance: the crystallinity of TiO2 and the pH of the FeCl3 solution. In a typical preparation of Fe(III)/TiO2 (R-950-2) (k ) 2.3 × 10-1 µmol/h, QE ) 22%), the as-received commercial TiO2 (rutile phase, 15-nm particle size, 90-m2/g specific surface area) was calcined at 950 °C for 3 h to obtain highly crystalline TiO2 (rutile phase, 3.7-m2/g specific surface area) before use, while the pH of the FeCl3 aqueous solution was adjusted to ∼2. Prior to the grafting of

16486

J. Phys. Chem. C, Vol. 114, No. 39, 2010

Fe(III) ions, the as-received TiO2 (R-com) (k ) 2.1 × 10-2 µmol/h, QE ) 5.7%) displayed a higher photocatalytic activity than calcined TiO2 (R-950) (k ) 1.1 × 10-2 µmol/h, QE ) 1.4%) under visible light irradiation (400-530 nm) owing to the larger specific surface area of the former. However, after grafting Fe(III) ions, the calcined TiO2 (Fe(III)/TiO2 (R-9502), k ) 2.3 × 10-1 µmol/h, QE ) 22%) exhibited a much higher photocatalytic performance than the as-received commercial TiO2 (Fe(III)/TiO2 (R-com-2), k ) 8.8 × 10-2 µmol/h, QE ) 6.1%). Without adjusting the pH of the FeCl3 solution (pH ∼7) during the photocatalyst preparation, the k and QE values of Fe(III)/TiO2 (R-950-7) decreased to 7.1 × 10-2 µmol/h and 4.6%, respectively. The main reason may be due to the fact that Fe(III) in aqueous solution is unstable and easily forms Fe2O3 particles during impregnation at 90 °C.18 Additional controlled experiments indicated that Fe2O3 cannot enhance the photocatalytic activity of TiO2 by the physical mixture of TiO2 and Fe2O3 alone (Fe/TiO2 ) 0.05 wt %). At lower pH (∼2) the formation of Fe2O3 particles was effectively inhibited (as indicated by EXAFS results), resulting in a more homogeneous grafting of Fe(III) clusters (amorphous FeO(OH)-like structure) on the surface of TiO2. Therefore, it is considered that the interaction between highly crystalline TiO2 and Fe(III) clusters contributes to more efficient IFCT, and the formation of homogeneous Fe(III) clusters results in the effective multielectron reduction of oxygen. In the presently prepared Fe(III)/TiO2 (R-950-2), the Fe(III) clusters were randomly grafted on the surface of rutile TiO2 that had a small specific surface area of 3.7 m2/g. Therefore, the increased performance of Fe(III)/TiO2 can be further explained by the conditions used in its preparation: (1) highly crystalline TiO2 nanoparticles with a large specific surface area (or small particle size) were prepared, (2) more Fe(III) clusters with the proper size were homogeneously dispersed on the surface of TiO2, and (3) the interaction between Fe(III) clusters and TiO2 was properly modified during the post-treatment after the preparation of Fe(III)/TiO2, thereby yielding more efficient electron transfer from the VB of TiO2 to the Fe(III) clusters. The visible light sensitization mechanism, which involves IFCT and presumably multielectron reduction processes, is not only restricted to Fe(III)-grafted rutile TiO2. In addition to rutile TiO2, both anatase TiO2 and WO3 can be used to prepare visiblelight-sensitive Fe(III)/TiO2 (A-500-2) and Fe(III)/WO3 (pH ∼2) photocatalysts (as shown in Table 2). In our previous study, we reported the visible-light-sensitive Cu(II)/WO3 (pH ∼7) with a QE of 17%.7 In the case of WO3, the activity of Cu(II)/WO3 was superior to that of Fe(III)/WO3. At present, we do not know the reason exactly, but the electron transfer from the CB of WO3 to the Fe(III) clusters cannot effectively proceed, even though the potential of the WO3 CB, ECBM ) 0.3-0.5 V (vs SHE, pH 0), is lower than that of Fe(III)/Fe(II) (E0 ) 0.771 V, vs SHE, pH 0). A similar phenomenon was also observed in Fe(III)/ TiO2 under UV light irradiation. We also reported the visible-light-sensitive Cu(II)/TiO2 (Rcom-7) with a QE of 8.8%.7 The present investigation demonstrates that the QE of Cu(II)/TiO2 (R-com-7) can be further improved to 15% (Table 2) by using calcined TiO2 (Cu(II)/ TiO2 (R-950-7)). These results indicate that the visible-light photosensitization strategy presented here is a simple and effective method for the design of novel and efficient photocatalysts which respond to visible light.

Yu et al. Conclusions In this study, we produced a visible-light-driven Fe(III)/TiO2 (R-950-2) photocatalyst with a QE value of 22%, which is the highest value previously reported for a TiO2-based photocatalyst, including N-doped TiO2 and Cu(II)/TiO2 systems. The key factors for generating our highly active photocatalyst are the grafting of Fe(III) ions and the use of TiO2 with high crystallinity, which served to enhance the mobility of holes in the VB of TiO2 and allow the IFCT and presumed multielectron reduction processes to proceed effectively. We conclude that the present system represents an effective strategy for the development of efficient visible-light-sensitive photocatalysts. It should also be stressed that our system is promising for industrial applications, as all of the materials are abundant, chemically stable, and nontoxic, and in addition, the Fe(III)/ TiO2 photocatalyst can be synthesized by a facile impregnation method. We expect that the visible-light-sensitive Fe(III)/TiO2 system can be applied to various products requiring airpurification, self-cleaning, antibacterial, and antiviral properties, particularly those used indoors. Acknowledgment. The X-ray absorption fine structure (XAFS) measurements were performed at the SPring-8 facility with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2009B1006). We are grateful to Dr. T. Honma for help on the measurements of XAFS spectra. We are also grateful to Ms. M. Iyonaga for measuring the photocatalytic activity. This work was performed under the management of the Project to Create Photocatalyst Industry for Recycling-oriented Society supported by NEDO (New Energy and Industrial Technology Development Organization). We express gratitude to Mr. G. Newton for the careful reading of the manuscript. Supporting Information Available: Dependence of Fe(III) amounts on the photocatalytic activity and comparison of Fe(III)-grafted TiO2 and Fe(III)-doped TiO2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (2) (a) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (b) Irie, H.; Watanabe, Y.; Hashimoto, K. Chem. Lett. 2003, 32, 772. (c) Li, Q.; Li, Y. W.; Wu, P.; Xie, R.; Shang, J. K. AdV. Mater. 2008, 20, 3717. (d) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffmann, M. R. J. Phys. Chem. B 2004, 108, 17269. (e) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2004, 108, 10617. (3) (a) Anpo, M.; Takeuchi, M. Int. J. Photoenergy 2001, 3, 89. (b) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. J. Phys. Chem. Solids 2002, 63, 1909. (c) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (4) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (b) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (c) Cho, Y.; Choi, W.; Lee, C.H.; Hyeon, T.; Lee, H.-I. EnViron. Sci. Technol. 2001, 35, 966. (d) Kisch, H.; Zang, L.; Lange, C.; Maier, W. F.; Antonius, C.; Meissner, D. Angew. Chem., Int. Ed. 1998, 37, 3034. (e) Ishibai, Y.; Sato, J.; Akita, S.; Nishikawa, T.; Miyagishi, S. J. Photochem. Photobiol., A 2007, 188, 106. (5) (a) Tang, J.; Zou, Z.; Ye, J. Angew. Chem. 2004, 116, 4563. (b) Tang, J. W.; Zou, Z. G.; Ye, J. Chem. Mater. 2004, 16, 1644. (c) Maruyama, Y.; Irie, H.; Hashimoto, K. J. Phys. Chem. C 2006, 110, 23274. (d) Amao, F.; Nogami, K.; Abe, R.; Ohtani, B. J. Phys. Chem. C 2008, 112, 9320. (e) Yi, Z.; Ye, J.; Kikugawa, N.; Kako, T.; Ouyang, S.; Stuart-Williams, H.; Yang, H.; Cao, J.; Luo, W.; Li, Z.; Liu, Y.; Withers, R. L. Nat. Mater. 2010, 9, 559. (6) Irie, H.; Miura, S.; Nakamura, R.; Hashimoto, K. Chem. Lett. 2008, 37, 252. (7) (a) Irie, H.; Miura, S.; Kamiya, K.; Hashimoto, K. Chem. Phys. Lett. 2008, 457, 202. (b) Irie, H.; Kamiya, K.; Shibanuma, T.; Miura, S.; Tryk, D. A.; Yokoyama, T.; Hashimoto, K. J. Phys. Chem. C 2009, 113, 10761. (8) Nakamura, R.; Okamoto, A.; Osawa, H.; Irie, H.; Hashimoto, K. J. Am. Chem. Soc. 2007, 129, 9596.

Fe(III)-Grafted TiO2 (9) (a) Creutz, C.; Brunschwig, B. S.; Sutin, N. J. Phys. Chem. B 2005, 109, 10251. (b) Creutz, C.; Brunschwig, B. S.; Sutin, N. J. Phys. Chem. B 2006, 110, 25181–25190. (10) Hofacker, I.; Schulten, K. Proteins: Struct., Funct., and Genet. 1998, 30, 100. (11) McBain, J. W. J. Phys. Chem. 1901, 5, 623. (12) Bard, A. J.; Parsons, R.; Jordan, J., Eds. Standard Potentials in Aqueous Solution; Marcel Dekker: New York, 1985. (13) Newville, M. J. Synchrotron Radiat. 2001, 8, 322. (14) Zabinsky, I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. ReV. B 1995, 52, 2995.

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16487 (15) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D.; Chastain, J.; King, R. C., Jr., Eds. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Eden Prairie, MN, 1995. (16) Abe, R.; Takami, H.; Murakami, N.; Ohtani, B. J. Am. Chem. Soc. 2008, 130, 7780. (17) Kim, Y. H.; Irie, H.; Hashimoto, K. Appl. Phys. Lett. 2008, 92, 182107. (18) Xiong, Y.; Shi, L.; Chen, B.; Mayer, M. U.; Lower, B. H.; Londer, Y. J. Am. Chem. Soc. 2006, 128, 13978.

JP1071956