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2009, 113, 19386–19388 Published on Web 10/19/2009
Defect Engineering of Photocatalysts by Doping of Aliovalent Metal Cations for Efficient Water Splitting Tsuyoshi Takata and Kazunari Domen* Department of Chemical System Engineering, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: September 7, 2009; ReVised Manuscript ReceiVed: October 10, 2009
Doping of aliovalent metal cations to SrTiO3, a photocatalyst for overall water splitting, was variously examined for the control of defect structure as well as the photocatalysis. Doping of a cation with valence lower than that of the parent cation effectively enhanced photocatalytic activity, while the doping of a higher valence cation led to the suppressed photocatalysis. It was demonstrated that the doping of a lower valence cation to intentionally introduce oxygen vacancies and decrease Ti3+ is effective to enhance photocatalytic activity. Introduction The demand for energy is increasing, and there is an urgent need to develop a new supply of clean, sustainable energy. Solar energy, which is available in abundance, is an ideal alternative to fossil fuel. Photocatalysis of water on a particulate semiconductor is one possible method of conversion and storage of light energy. Numerous studies have been performed on the fundamental and practical aspects of photocatalysis, aiming at excellent photocatalytic performance. There are two key requirements for efficient solar energy conversion. One is the extension of the usable wavelength region of light, and the other is the enhancement of the quantum efficiency at each wavelength. In the former case, the usable wavelength region of light depends on the intrinsic optical absorption properties of the photocatalyst materials. Therefore, the development of novel photocatalyst materials, based on the design of electronic structure, has been examined in detail. With respect to the latter case, enhancement of quantum efficiency has been attempted by improving the methods of modification and synthesis. It is generally believed that the recombination of photoexcited electrons and holes is one of the most detrimental factors negatively impacting the photocatalytic activity, and that lattice defects work as the recombination center.1-3 Therefore, much effort has been spent for understanding and control of this defect structure. However, direct observation and specification of defect species and their relation to recombination is difficult by instrumental analysis, and remains unclear. As a result, only limited control of the defect structure and photocatalytic performance is available. In the present work, the doping of aliovalent metal cations was examined as defect engineering of a photocatalyst in an attempt to improve photocatalytic performance. SrTiO3 was employed as a model photocatalyst for defect engineering by aliovalent doping. SrTiO3 has a band gap of 3.2 eV, and is known to decompose water into H2 and O2 under UV * To whom correspondence should be addressed. Phone: +81-3-58411148. Fax: +81-3-5841-8838. E-mail:
[email protected].
10.1021/jp908621e CCC: $40.75
irradiation.4,5 SrTiO3 has a perovskite structure with the general formula of ABO3. There are two distinct cationic sites in a perovskite structure, and a variety of doping combinations are possible. The A-site cation is coordinated by 12 O2-, and is preferentially occupied by relatively lager cations. The B-site cation is occupied with relatively smaller cations, and the coordination number of this site is 6. Site-selective doping is possible by the selection of a suitable-sized dopant according to the Goldschmidt tolerance factor, an indicator of the stability of the perovskite structure. In this study, a unique but quite basic approach was clearly demonstrated for the understanding and control of the defect structure to enhance photocatalysis. Considering the thermodynamic behavior of the defect species in the lattice, a strategy for defect control was obtained. Various types of lattice defects are known to exist, but oxygen vacancies appear to be the majority defect in most oxide photocatalysts. SrTiO3 contains oxygen vacancies because it is an intrinsic nonstoichiometric compound, slightly deficient in oxygen from its ideal composition. A small amount of lattice oxygen is released to the gas phase due to the dissociation of Ti-O bonds, which leads to the creation of oxygen vacancies along with the release of free electrons in the lattice, as expressed by eq 1.
SrTiO3 ) SrTiO3-x + 1/2xO2 + 2xe- + xVo
(1)
SrTiO3 ) SrTi(IV)1-2xTi(III)2xO3-x + 1/2xO2 + xVo
(2) Then, the free electron reduces Ti4+ to Ti3+, as denoted in eq 2, so that the relevant defect species are Ti3+ and the oxygen vacancy, Vo. With respect to the energy structure of SrTiO3, the conduction and valence bands predominantly consist of Ti3d orbitals and O2p orbitals, respectively. The Ti3+ defect forms a donor level just below the bottom of the conduction band, which is the origin of the n-type semiconductivity. This raises the question of which defects, Ti3+, Vo, or both, are responsible for the deactivation of photocatalysis. The 2009 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 113, No. 45, 2009 19387 Photocatalytic water splitting was carried out in a conventional apparatus. Photocatalyst powder was dispersed in distilled water by magnetic stirring, and the suspension was irradiated by a high pressure Hg lamp (450 W) from the inside of a tubular reactor through a Pyrex water cooling jacket. The produced H2 and O2 were accumulated in a directly connected closed gas circulation system and analyzed by gas chromatography (TCD, MS-5A column, Ar carrier). The synthesized photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscropy (SEM), and UV-visible spectroscopy. Results and Discussion
Figure 1. Schematic illustration of aliovalent-doped SrTO3. Doping of trivalent cation (a) and pentavalent cation (b).
answer can be obtained by investigating the correlation between defect density and photocatalytic activity. The densities of the two defects are determined by the thermodynamic equilibrium, and the equilibrium constant, K, can be expressed by the multiplication of the square of Ti3+ density, Vo density, and the square root of the partial pressure of O2, regarding the behaviors of point defects as that of ideal gas.
K ) [Ti3+]2[Vo]P(O2)1/2
(3)
Equation 3 provides a prediction of tunable defect densities. According to this equation, one defect density varies inversely to the change of the other defect density, and an increase in the partial pressure of O2, P(O2), reduces both defect densities. The tuning of the individual defect density was performed by aliovalent doping, which is a useful method to modify the defect density, as schematically illustrated in Figure 1. The doping of a cation with a valence lower than that of the parent cation extrinsically introduced Vo, inhibiting the formation of Ti3+.6 On the other hand, the doping of a higher valence cation did not lead to the opposite state, the introduction of cation vacancies, unless an extremely strong oxidative atmosphere was maintained. In this case, the higher valence cation stabilized the Ti3+ without producing Vo.7,8 Control of the individual defect density is possible by these doping methods. The former method is well-known and commonly employed in O2- conductors, and the latter method corresponds to n-doping. The synthesis and photocatalysis of various aliovalent-doped SrTiO3 samples were examined. Experimental Methods Preparation of the photocatalyst was performed according to conventional methods. Various doped SrTiO3 samples were prepared by the solid-state reaction. SrCO3, TiO2, and oxides of dopant metal, except for the case of Na doping with Na2CO3, were thoroughly mixed in a stoichiometric ratio by mechanical grinding in an agate mortar. Then, the mixture was heated in air at 1373 K for 20 h to give the product. The synthesized powder was modified with a cocatalyst using an impregnation method.9,10 SrTiO3 powder was immersed in a small amount of aqueous solution containing Na3RhCl6 and Cr(NO3)3 as precursors. The suspension was evaporated to dryness on a boiling water bath and then heated in air at 623 K for 1 h to disperse fine particles of Rh-Cr mixed oxides on the surface of the SrTiO3.
The crystal structures of SrTiO3 samples with Ga and La dopings were identified by XRD. The diffraction patterns of the products were analogous to those of SrTiO3, with no additional diffraction lines from other crystal phases (Figure S1 in the Supporting Information). This confirmed that dopants were introduced to the perovskite slab, as expected. The surface morphologies of SrTiO3 particles with various Ga and La dopings were observed by SEM, and the images were shown in Figure S2 in the Supporting Information. The primary particle size of the undoped sample was of submicrometer order, and several particles aggregated by sintering to form a secondary particle. A minor change in morphology was observed in the Ga-doped sample, in which grain boundaries appeared in the parent particles. Vo was generated in the crystal structure by Ga doping, but the part of Vo exceeding the doping capacity was probably excluded from the bulk to form a grain boundary. However, the doping examined here did not significantly change the surface morphology of the catalyst particles. The optical absorption edges were also unchanged, because of the small amount of doping (Figure S3 in the Supporting Information). Characterization determined that the crystal structure, particle size, and band gap energy, which would be factors possibly affecting photocatalysis, were not significantly changed by doping. A series of Ga- and La-doped SrTiO3 was examined. The dopant was selected with consideration of invariable valence, so that any change to compensate for charge balance should occur in the host material. The formulas of the doped samples are denoted as SrTi(IV)0.95Ga0.05O2.975, Sr0.95La0.05Ti(IV)0.95Ga0.05O3, Sr0.95La0.05Ti(IV)0.95Ti(III)0.05O3, etc. Ga3+ occupies the Ti4+ site as a lower valence cation, and La3+ occupies the Sr2+ site as a higher valence cation. Figure 2a-e shows the photocatalytic performance of a series of Ga- and La-doped SrTiO3, with the amount of produced H2 and O2 plotted against irradiation time. Undoped SrTiO3 modified with Rh-Cr oxides evolved H2 and O2 in a stoichiometric ratio, although the activity was low. In the second case, a 5% Ga-doped sample, remarkably enhanced photocatalytic activity of water splitting was observed. The rate of H2 and O2 evolution on the Ga-doped sample was higher than that of the undoped sample by about 10 times. The activity decreased with codoping of equimolar amounts of Ga and La compared with the case of the Ga-doped sample but was still higher than that of the undoped sample. Codoping of La and Ga with double the amount of La, or doping of only La, led to a decrease in activity. These results demonstrated that the doping of a lower valence cation can effectively enhance photocatalytic activity, while the doping of a higher valence cation leads to the suppressed photocatalysis. This means that the doping of a lower valence cation to intentionally introduce oxygen vacancies and decrease Ti3+ is effective to enhance photocatalytic activity. For further confirmation, a series of SrTiO3 with Na and Ta doping was investigated as a second example.
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J. Phys. Chem. C, Vol. 113, No. 45, 2009
Letters In summary, drastic enhancement of photocatalytic activity for water splitting was achieved by the doping of lower valence cations. We determined that the defect species most responsible for the deactivation of photocatalysis was not Vo but Ti3+. Vo defects play the important role of scavenging Ti3+ which suppresses photocatalytic activity. The concept of defect engineering by aliovalent doping to enhance photocatalysis was demonstrated, which provides an example of the design of active photocatalysts. The present doping method is applicable to various photocatalysts other than SrTiO3, and further examination is in progress. Using the methodology demonstrated in the present study, further investigation is required to better understand the deactivation mechanism in the presence of Ti3+. Such investigation is already underway, and the results will be reported in a future publication. Acknowledgment. This work was financially supported by Research and Development in a New Interdisciplinary Field Based on Nanotechnology and Materials, Science Program of the Ministry of Education and Culture, Sports, Science, and Technology (MEXT) of Japan, and a Grant-in-Aid for Young Scientists (B) No. 20760526, Japan Society for the Promotion of Science (JSPS).
Figure 2. Photocatalytic water splitting on various aliovalent-doped SrTiO3. A series of La and Ga dopings: (a) undoped, (b) Ga 5%, (c) Ga 5%-La 5%, (d) Ga 5%-La 10%, and (e) La 5%. A series of Na and Ta dopings: (f) undoped, (g) Na 5%, (h) Na 5%-Ta 5%, and (i) Ta 5%. Filled circles, H2; filled squares, O2; catalyst, 0.3 g; cocatalyst, Rh2O3 + Cr2O3 (0.5 + 0.5 wt %) loading; reaction solution, 400 mL of distilled water; light source, high pressure Hg lamp (450 W). +
2+
In this case, Na occupied Sr sites as a lower valence cation and Ta5+ occupied Ti4+ sites as a higher valence cation, respectively. As shown in Figure 2f-i, the 5% Na-doped sample showed a drastically higher photocatalytic activity for water splitting than the undoped sample. The rate of H2 and O2 evolution increased by about 18 times with Na doping. The photocatalytic activity decreased with codoping of equimolar amounts of Na and Ta, or doping with only Ta. These results were similar in tendency to the case of Ga and La doping, and verified that the doping of lower valence cations is effective at enhancing photocatalysis, and led to the conclusion that Ti3+ is responsible for the deactivation of photocatalysis.
Supporting Information Available: XRD patterns, SEM images, and UV-visible DR spectra of various doped SrTiO3 samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shockley, W.; Read, W. T., Jr. Phys. ReV. 1952, 87, 835–842. (2) Luo, Z.; Gao, Q. H. J. Photochem. Photobiol., A 1992, 63, 367– 375. (3) Serpone, N.; Lawless, D.; Disdier, J.; Herrmann, J. M. Langumuir 1994, 10, 643–652. (4) Domen, K.; Naito, S.; Soma, M.; Onishi, T.; Tamaru, K. J. Chem. Soc., Chem. Commun. 1980, 543–544. (5) Domen, K.; Naito, S.; Onishi, T.; Tamaru, K. Chem. Phys. Lett. 1982, 92, 433–438. (6) Etsell, T. H.; Flengas, S. N. Chem. ReV. 1970, 70, 339–376. (7) Tufte, O. N.; Chapman, P. W. Phys. ReV. 1967, 155, 796–802. (8) Frederiks, H. P. R.; Hosler, W. R. Phys. ReV. 1967, 161, 822–827. (9) Maeda, K.; Teramura, K.; Masuda, H.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. J. Phys. Chem. B 2006, 110, 13107–13112. (10) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. J. Phys. Chem. B 2006, 110, 13753–13758.
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