Photocatalytic Reaction Mechanism of Fe(III)-Grafted TiO2 Studied by

Article pubs.acs.org/JPCC

Photocatalytic Reaction Mechanism of Fe(III)-Grafted TiO2 Studied by Means of ESR Spectroscopy and Chemiluminescence Photometry Masami Nishikawa, Yasufumi Mitani, and Yoshio Nosaka* Department of Materials Science and Technology, Nagaoka University of Technology, Nagaoka 940-2188, Japan S Supporting Information *

ABSTRACT: We successfully clarified the mechanisms of visible-light-driven photocatalytic reactions of Fe(III)-grafted TiO2 (Fe/TiO2) and Fe(III)-grafted Ru-doped TiO2 (Fe/ Ru:TiO2). ESR spectroscopy revealed that the visible-light response of the Fe/TiO2 photocatalyst resulted in the direct charge transfer from the valence band of TiO2 to the grafted Fe ions. For the Fe/Ru:TiO2 photocatalyst, acceptor levels were formed by doping Ru ions in the lattice of TiO2, and the electrons at the acceptor levels excited on visible-light irradiation readily transfer to Fe ions. Since a longer wavelength light generated the conduction band electrons, we also proposed a two-step electron excitation from valence band to the conduction band through defect levels such as oxygen vacancy. As a result, a part of photogenerated electrons in the conduction band transfer to the grafted Fe ions. Therefore, the Fe/Ru:TiO2 photocatalyst showed a higher activity because such two kinds of indirect charge transfer to the grafted Fe ions occurred in addition to the direct interfacial charge transfer observed for Fe/TiO2. Moreover, chemiluminescence photometry confirmed that the grafted Fe ions function as a promoter to reduce O2 into H2O2 via two-electron reduction. Therefore, the acceleration in the reduction of O2 with doping Ru and grafting Fe ions allows a larger number of holes to oxidize organic compounds, resulting in the higher photocatalytic activity.

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INTRODUCTION Photocatalytic reactions at the surface of semiconductor solid have been attracting much attention in view of their practical applications to environment-cleaning materials such as selfcleaning tiles, glasses, and windows.1,2 Among many kinds of materials reported as photocatalysts so far, TiO2 is most attractive one because it is inexpensive, easily available, and nontoxic. But its band gaps are 3.2 and 3.0 eV in anatase and rutile forms, respectively, resulting in only the UV-light response. Since visible-light driven photocatalysts have been required to effectively utilize indoor light for cleanup the environments, various modification processes of TiO2 have been reported to develop visible-light response. For example, doping of N and S ions in TiO2 showed visible-light response because levels ascribed to the element were formed above the valence band of TiO2.1b,3−5 Metal ions are also doped into TiO2 for developing visible-light response.1b For example, Fe ions was doped in TiO2 showing a increased visible-light response,6 where the Fe ions were reported to form a dope level above the valence band of TiO2 like N and S.7 TiO2 sensitized with a metal complex was also reported to show visible-light response because the photoexcited metal complex gives an electron to the conduction band of TiO2 by visiblelight irradiation.8 Recently, Cu(II)- and Fe(III)-grafted TiO2 (Cu/TiO2 and Fe/TiO2) have been reported as the visible-light-driven photocatalyst showing higher activities than the N- and Sdoped TiO2 and the sensitized TiO2.9,10 In the case of Cu/ TiO2, the Cu2+ was grafted as a distorted amorphous CuO-like © 2012 American Chemical Society

structure on the TiO2 surface and electrons in the valence band of TiO2 are directly excited by the visible-light irradiation to the Cu2+ as interfacial charge transfer (IFCT).9 Actually, the direct charge transfer to Cu2+ and the formation of trapped holes were confirmed by means of electron spin resonance (ESR) spectroscopy.11 In addition, Cu ions grafted on TiO2 surface was reported to be the active species for CO2 photoreduction under UV irradiation, which also supported that charge transfer to Cu ions occur.12 In the N-doped TiO2, the photocatalytic oxidation activity became low because the potential of valence band ascribing to the doped N ions, which is origin of visiblelight response, was more negative than that of the bare TiO2. Therefore, it was concluded that the high photocatalytic activity for Cu/TiO2 is ascribed to the direct charge transfer from the valence band of TiO2 to the grafted Cu2+. In the case of the Fe/TiO2, it is considered that the direct IFCT from the valence band of TiO2 to Fe3+ would also occur, resulting in a higher photocatalytic activity than Cu/TiO2.10 In contrast, it was also reported that, in the case of Fe(III)-grafted S-doped TiO2, Fe3+ would give an electron to conduction band by the excitation to form Fe4+.13 Therefore, to elucidate which charge transfer occurs between grafted Fe ions and TiO2, it is necessary to directly observe the charge transfer process in the Fe/TiO2 by using ESR spectroscopy. Recently, Fe(III) grafting on Fe(III) doped TiO2 (Fe/Fe:TiO2) has been reported to Received: March 2, 2012 Revised: June 19, 2012 Published: June 25, 2012 14900

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show a higher activity than Fe/TiO2.14 When doping metal ion is the same as the grafting metal ion, the effect of doping could not be distinguished by ESR spectroscopy. Therefore, to investigate the effect of metal ion doping, Ru ions have been employed as a doping ion in the present study. It is important to elucidate the mechanism of the charge transfer processes for designing of highly visible-lightresponsive catalysts. In this work, we successfully clarified the charge transfer in the Fe/TiO2 under light irradiation by means of ESR spectroscopy. Furthermore, the effect of the doped Ru ions on the charge transfer was investigated in Fe(III)-grafted Ru-doped TiO2 (Fe/Ru:TiO2) which had higher activity than the Fe/TiO2, and the origin of the high activity of the Fe/ Ru:TiO2 was revealed in conjunction with the results of chemiluminescence detections for O2− and H2O2.

frequency was in the range of 9.420−9.440 GHz (X-band), and the microwave power was fixed at 10 mW. The field modulation was in the range of 2−10 G. To utilize the emission lines at about 405, 440, 550, and 580 nm from the mercury lamp, three kinds of sharp-cut filters of 500 nm in vacuum. These results show that under visible-light irradiation the signal of holes was increased by grafting Fe ions, and the signal of Fe3+ was decreased by the irradiation. If electrons in Fe3+ were excited to the conduction band of TiO2 to form Fe4+, the signal of electrons (g = 1.98) should be increased with decreasing the Fe3+ signal. Therefore, it is clarified that the valence band electrons transferred to Fe3+ ions by visible-light irradiation. Since the redox potential for (FeO2−,H+/HFeO2−) is +0.14 V and Fe3+/Fe2+ is +0.77 V vs SHE at pH = 0,21 the potential energy of the grafted Fe ions locate lower than that of the conduction band bottom of TiO2. Therefore, the difference in the potential energies suggests that the direct IFCT (interfacial charge transfer) from the valence band to the grafted Fe ions is induced by visible-light irradiation and it becomes the origin of the formation of valence band holes and Fe2+. In the presence of 2-propanol, the ESR spectra for Fe/TiO2 and the bare TiO2 are shown in Figures 4a and 4b, respectively. The signal intensities of photogenerated electrons were increased for both photocatalysts compared to those in vacuum (Figure 3a,b). This observation is attributable to the stabilization of electrons by lowering the recombination probability due to the consumption of holes by 2-propanol. By decreasing the wavelengths of the cutoff filter from 500 to 380 nm, the signal intensity of electrons was monotonically increased, indicating the formation of Ti3+ with the irradiation at the wavelength longer than 500 nm. This formation of trapped electron or Ti3+ is attributable to the visible-light absorption at structural defects such as oxygen vacancy, which are reported to be generated under the conduction level of TiO2 by 0.75−1.18 eV.22 Furthermore, the DFT calculation suggested that oxygen vacancy locates at 0.7 V below the conduction band.23 Thus, the visible light of longer wavelength can slightly excite electrons from such defect levels to the conduction band of TiO2. But as the defect levels would not have sufficient oxidation power given those redox potentials, photogenerated holes would not be consumed by 2-propanol. Therefore, we propose a two-step electron excitation through the defect level; that is, after the electron excitation from the defect levels to the conduction band, electrons in valence band would be excited to the holes of the defect level. When light contains UV region, the amount of photogenerated electrons in the bare TiO2 was largely increased owing to the electron excitation from the valence band to the conduction band. In contrast, in the case of Fe/TiO2, when light contains the UV region, the increase was suppressed because the photoexcited to conduction band electrons with the UV irradiation were partly transferred to the grafted Fe3+. On the basis of these results, it was revealed that the direct charge transfer to Fe ions was main origin of the visible-light photocatalytic activity for the Fe/TiO2 represented in Figure 1, and to a certain extent, two-step electron excitation through the defect level was also suggested. ESR Analysis for Fe/Ru:TiO2. Figure 5 shows ESR spectra of the Fe/Ru:TiO2 at low magnetic field in vacuum as the effect of light irradiation. The signal at g = 4.29 is ascribed to Fe3+ in distorted environment as described in the case of Fe/TiO2. The ESR spectra of the Fe/Ru:TiO2 at high magnetic field are shown in Figure 6a. Since no signal at around g = 2.00 was observed in dark, Fe ions were grafted on the surface of TiO2 and not doped in TiO2 similarly to the case of the Fe/TiO2.

Figure 5. ESR spectra of Fe3+ for Fe/Ru:TiO2 measured at 77 K in a vacuum. The inset shows these integral ESR spectra.

Figure 6. ESR spectra of electrons and holes for (a) Fe/Ru:TiO2 and (b) Ru:TiO2 measured at 77 K in a vacuum. The insets show ESR spectra obtained by integration of the spectra under light irradiation after subtracting the spectrum in dark.

The signal intensity of Fe3+ in Fe/Ru:TiO2 was decreased by light irradiation as seen in Figure 5. When the decrease by visible-light irradiation of λ > 500 nm is compared with that for Fe/TiO2 (Figure 2), a large fraction of Fe3+ was reduced to Fe2+ than the Fe/TiO2, even though the Fe content in the catalysts was larger. This indicates that the charge transfer to Fe3+ by visible-light irradiation was enhanced by doping Ru ions into TiO2. To elucidate the reason for the enhancement of the charge transfer to Fe3+, we examined the behavior of photogenerated holes and electrons. In Figure 6, it is seen that the signals of electrons (g = 1.98) were observed even by visible light (λ > 500 nm) for the Fe/Ru:TiO2 and the Ru:TiO2. Figure 7 shows the effect of a hole scavenger, 2-propanol, on the ESR spectra for Fe/Ru:TiO2 and Ru:TiO2 photocatalysts. The large increase of electrons was observed in both photocatalysts owing to the hole scavenging. The values of full width at half-maximum (fwhm) of the integral ESR spectra of photogenerated electrons 14903

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from the unpaired electron at oxygen vacancy which cannot oxidize 2-propanol. Ruthenium ions did not show any distinct signal in the ESR spectrum of the Ru:TiO2 in dark as shown in Figure 6b. Since its ionic radius and valence are compatible to Ti4+, Ru ions are probably doped as the form of Ru4+ in TiO2, which may not be detected by ESR because of its d4 electron configuration. When the Ru:TiO2 was irradiated by light, the signal at 3010 G (g = 2.23) was observed for Ru:TiO2 (Figure 8b). The signal at g =

Figure 7. ESR spectra of electrons and holes for (a) Fe/Ru:TiO2 and (b) Ru:TiO2 measured at 77 K in 2-propanol. The insets show ESR spectra obtained by integration of the spectra under light irradiation after subtracting the spectrum in the dark.

measured in 2-propanol are summarized in Table 1. The fwhm values for the Fe/Ru:TiO2 and the Ru:TiO2 were larger than Table 1. Fwhm Values of ESR Signals for the Trapped Electrons in the Photocatalysts Measured in 2-Propanol fwhm values (G) λ (nm)

Fe/TiO2

TiO2

Fe/Ru:TiO2

Ru:TiO2

>500 >420 >380

101 106 109

98 101 86

129 128 132

117 111 112

Figure 8. ESR spectra of Ru3+ for (a) Fe/Ru:TiO2 and (b) Ru:TiO2, measured at 77 K in a vacuum.

those of the Fe/TiO2 and the bare TiO2. It was previously reported that the peak of the integral ESR spectrum of the photogenerated electrons in amorphous TiO2 shifted to high magnetic field, and its fwhm value was large.20 Based on the report, the reason that fwhm values for the Fe/Ru:TiO2 and Ru:TiO2 were large in the present work is considered to be due to low crystallinity. Thus, by doping Ru ions, the structural defects such as oxygen vacancy were introduced in the lattice of TiO2. Therefore, it is probable that the above-mentioned twostep electron excitation through the defect level would be increased by the Ru-doping. The signal of trapped electrons (g = 1.98) for Fe/Ru:TiO2 was smaller than that for Ru:TiO2. This decrease of the trapped electrons indicates that the electrons transfer to the grafted Fe3+ as shown by the decrease of the ESR signal of Fe3+ in Figure 5. If one compare the signals of valence band holes at g = 2.01 for the Fe/Ru:TiO2 in 2propanol (Figure 7a) and in vacuum (Figure 6a), the signal was decreased with 2-propanol, indicating the consumption of holes by the oxidation of 2-propanol. This observation agrees well with the higher photocatalytic activity of the Fe/Ru:TiO2 photocatalyst as seen in Figure 1. On the other hand, the signal at g = 2.01 was observed by visible-light irradiation for Ru:TiO2 in 2-propanol (Figure 7b). This signal may originate

2.23 is ascribed to Ru3+ substituted for Ti4+.24 If Ru ions were grafted on TiO2 such as Ru(OH)x, the signal should be observed at around 3250 G (g = 2.0).25 Such a signal, however, was not observed in Figure 6b. Therefore, it was confirmed that Ru was doped as the form of Ru4+ into the lattice of TiO2. Based on these results, the Ru4+ ions doped in the lattice of the TiO2 were considered to play a role as acceptor level. The potential of the Ru 4d orbitals in rutile RuO2 are reported to be more positive than that of the Ti 3d orbitals in rutile TiO2.26 In addition, the work function of Ru(IV)-doped CeO2 is theoretically calculated to be 4.92 eV,27 which corresponds to the electronic potential of ca. +0.4 V vs SHE. Then, the excitation energy from valence band of TiO2 (+3.0 V) to Ru4+ level is estimated to be ca. 2.6 eV or 480 nm in wavelength. Therefore, on visible-light irradiation, the Ru4+ can receive an electron from the valence band of TiO2 to form Ru3+ in the Ru:TiO2, as seen in Figure 8b. In contrast, the formation of Ru3+ on visible-light irradiation was not observed for the Fe/ Ru:TiO2, as seen in Figure 8a. This means that the Ru4+ would receive an electron by visible-light excitation, and then the resultant Ru3+ would give an electron to Fe3+, returning to the Ru4+ rapidly. The efficiency of this electron transfer should be 14904

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As for the Fe/Ru:TiO2 photocatalyst, a fast formation of O2− was also observed as well as H2O2, indicating the presence of direct one electron reduction of O2. As seen for the case of Fe/ TiO2, the potential of the photogenerated Fe2+ is not enough for O2 to be reduced into O2− directly. Since oxygen vacancy was increased by doping with Ru(IV), the excitation of the electrons from the vacancy level to a middle of the conduction band causes the conduction band electrons, which allows the one-electron reduction of O2 leading to the fast formation of O2−. As shown in Figure 9a, with increasing the irradiation time after 5 s, the amount of O2− was decreased, while the amount of H2O2 in Figure 9b increased up to 40 s. This observation indicates that the generated O2− was converted into H2O2 by further reactions by photogenerated Fe2+. On the irradiation after 40 s, the amount of H2O2 became a steady state. Since H2O2 increase independently to that of O2−, we concluded that H2O2 was mainly generated by two-electron reduction process of O2 with a catalytic action of the grafted Fe ions. Therefore, it was confirmed that the large amount of electrons transfer to the Fe ions in the Fe/Ru:TiO2, which is in good agreement with the results of ESR measurement. For the Ru:TiO2, both O2− and H2O2 were not detected as well though the Ru:TiO2 showed the ESR signals of holes as Ti4+−O• species and electrons as Ru3+ and Ti3+ by the visiblelight irradiation at 77 K. The reduction potential for Ru3+ (> +0.38 V at pH = 0) is not enough for the one-electron reduction of O2. Moreover, though H2O2 can be generated energetically at the Ru ions, H2O2 was not detected. This indicated that doped Ru ions might have no function as a catalyst and then act as a recombination center because they locate middle of the TiO2 lattice. Reaction Mechanism of Fe/TiO2 and Fe/Ru:TiO 2 Photocatalyst. In the present study, we employed ESR spectroscopy to investigate the behavior of electrons and holes produced by irradiating visible and UV light on the photocatalysts at 77 K because, at room temperature, the lifetime of these radicals was too short to be observed. In the ESR measurements that can investigate the initial process of charge transfer in the photocatalysts, reactant (2-propanol) was added to see the effect on the signal intensities of trapped electrons. Therefore, some of the electrons and holes observed by ESR measurement at 77 K may not be relevant to the reaction at room temperature. However, photocatalytic activity measurements at room temperature indicated that visible-light response appeared in the Fe/TiO2 and photocatalytic activity of Fe/ Ru:TiO2 under visible-light irradiation was further increased. In addition, chemiluminescence photometry measurement at room temperature indicated that the amount of H2O2 was increased by grafting Fe ions. These results were consistent with the charge transfer to the Fe ions observed by the ESR measurements. In the test of the photocatalytic activity, a gas phase photocatalytic reaction was performed for acetaldehyde because it is one of the representative pollutant molecules. The mechanism of the photocatalytic acetaldehyde decomposition was known1b to be that it is oxidized by holes and then decomposed starting from the reaction with the reduced O2. Since the surface of TiO2 is covered with several layers of water molecules under environmental atmosphere,28 the reaction mechanism at the surface of the photocatalyst in air is not so much different from that in aqueous suspension, except for the difference in the rate of diffusion from bulk to the surface.11

very high because the signal of Ru3+ in the Fe/Ru:TiO2 was not observed at all by any type of light irradiation. Chemiluminescence Detections for O2− and H2O2. Photocatalytic reactions cannot proceed without consumption of photogenerated electrons, even though the redox potential of the photogenerated holes is positive enough to decompose organic compounds. Molecular oxygen in air is usually the only molecule to be reduced in ambient atmosphere. Then, it is important to confirm the reduction of O2 into O2− or H2O2 by one-electron or two-electron reductions, respectively.11 Figure 9 shows the concentrations of (a) O2− and (b) H2O2 produced by irradiation of visible light (470 nm LED) for the TiO2, Fe/ TiO2, Ru:TiO2, and Fe/Ru:TiO2 photocatalysts.

Figure 9. Concentrations of O2− and H2O2 formed in aqueous suspension of photocatalysts as a function of irradiation time of 470 nm light.

The bare TiO2 did not produce detectable amounts of O2− and H2O2 because visible light could not almost be absorbed and then the excitation was negligible. In the two Fe-grafted photocatalysts, some amounts of O2− and H2O2 were detected as seen in Figures 9a and 9b, respectively. In the case of Fe/TiO2, the rapid formation of H2O2 and slow growth of O2− indicate that the formed H2O2 was oxidized into O2− with photocatalytic reaction. These results indicate that the photogenerated Fe2+ can energetically generate H2O2 by two-electron reduction of O2 but not generate O2− by the one-electron reduction of O2. The electrons having a potential energy of +0.38 V (vs SHE at pH = 0) can reduce O2 into O2− at the experimental condition, and those of +0.695 V can reduce O2 by two electrons into H2O2 as discussed in the previous report.11 Therefore, the redox potential of Fe3+/Fe2+ in the grafted Fe ions may be near to or more negative than +0.695 V at pH = 0. 14905

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Figure 10. Suggested reaction mechanism of visible-light-responsive photocatalysis for Fe(III)-deposited TiO2 and Fe(III)-deposited Ru(IV)-doped TiO2. Vo represents oxygen vacancy.

chemiluminescence photometry for the reduction ability at the catalyst surface. It was confirmed that the direct charge transfer from the valence band of TiO2 to the Fe ions grafted on the TiO2 surface is the origin of visible light response for Fe/TiO2 photocatalyst. In the case of the Fe/Ru:TiO2, the Ru ions doped into the lattice of TiO2 played a role as an acceptor level and the photogenerated electrons at the acceptor level readily transfer to the Fe ions. On visible-light irradiation with a longer wavelength, electrons in oxygen vacancy or defect levels were excited to the conduction band, generating the electrons which provided fast formation of O2− and reduction of the grafted Fe. Since such two kinds of indirect charge transfers to the Fe ions occurred in addition to the direct charge transfer (IFCT) in the Fe/Ru:TiO2, a larger number of holes and electrons were generated, leading to the highly visible-lightdriven photocatalytic activity compared to that of the Fe/TiO2. Furthermore, it was revealed that the reduced Fe ions can reduce O2 into H2O2 via the two-electron process. Owing to the consumption of electrons, the oxidation activity for decomposition of organic compounds would be enhanced. We expect that these findings give valuable information for designing highly active visible-light-driven photocatalysts for applying to environmental cleanup.

Therefore, plausible reaction mechanisms for the Fe/TiO2 and the Fe/Ru:TiO2 photocatalyst could be deduced from the measurements of ESR and chemiluminescence as summarized in Figure 10. When the Fe/TiO2 is irradiated by visible light, electrons in the valence band of TiO2 are directly transferred to the grafted Fe3+ as IFCT process. In the case of UV-including light, a part of the electrons excited to the conduction band of TiO2 transfer to Fe3+. In the case of Fe/Ru:TiO2 photocatalyst, the IFCT process to the grafted Fe ions takes place by visible-light irradiation as well as the case of Fe/TiO2. In addition, by doping Ru ions into the TiO2, the defect levels and acceptor level due to the oxygen vacancy and the doped Ru ions were formed, respectively. Owing to the defect levels, the two-step of charge transfer from the valence band of TiO2 to the conduction band of the TiO2 through the level was developed, as evidenced by the fast formation of O2− with the longer wavelength light. Then some photoexcited electrons in the conduction band and in the acceptor level of Ru ions readily transferred to the grafted Fe3+. Therefore, it was concluded that the enhanced charge transfer to Fe3+ in the Fe/Ru:TiO2 by visible-light irradiation was caused by the two kinds of indirect charge transfers in addition to the direct IFCT process, while the direct IFCT occurred alone in the Fe/TiO2 by visible-light irradiation. Next, we can discuss the difference in the photocatalytic activities between Fe/Ru:TiO2 and Ru:TiO2. In the case of Ru:TiO2, the photocatalytic activity and reduction ability of O2 under visible-light irradiation were low as seen in Figures 1 and 9 because Ru ions would act as a recombination center. In contrast, in the case of the Fe/Ru:TiO2, the photoexcited electrons in Ru ions readily transfer to the grafted Fe3+ located at the surface of TiO2 to prevent the recombination. Furthermore, the reduced Fe2+ at the surface allows two electron reduction of O2 to H2O2 as shown in Figure 9. Owing to the reduction ability of the grafted Fe ions, holes in valence band would efficiently generated in the Fe/Ru:TiO2 photocatalyst under visible-light irradiation, leading to the high photocatalytic activity compared to the Ru:TiO2.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the evaluation of photocatalytic activity with Figure S1 of outline of the equipment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +81-258-47-9315. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mr. Hiroshi Suizu and Mr. Hideyuki Nagai of Mitsui Chemical Ltd. and Mr. Yasuhiro Hosogi and Dr. Yasushi Kuroda of Showa Titanium Co., Ltd., for supplying the samples. This work was performed under the management of the Project to Create Photocatalyst Industry for Recycling-

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CONCLUSIONS The photocatalytic reaction mechanisms of the Fe/TiO2 and the Fe/Ru:TiO2 were successfully clarified by means of ESR spectroscopy for the carriers in the photocatalyst and 14906

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Oriented Society, supported by the New Energy and Industrial Technology Development Organization (NEDO) in Japan.

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