NaTaO3 photocatalysts of different crystalline structures for water

APPLIED PHYSICS LETTERS 89, 211904 共2006兲

NaTaO3 photocatalysts of different crystalline structures for water splitting into H2 and O2 Wan-Hsien Lin Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan

Ching Cheng Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan

Che-Chia Hu and Hsisheng Tenga兲 Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan

共Received 28 July 2006; accepted 10 October 2006; published online 20 November 2006兲 Perovskite-type NaTaO3 derived from a sol-gel synthesis exhibited a larger surface area and a remarkably higher photocatalytic activity in water splitting than the solid-state synthesized NaTaO3. The sol-gel and solid-state NaTaO3 had different crystalline structures of monoclinic P2 / m and orthorhombic Pcmn, respectively. Diffuse reflectance spectra showed that the sol-gel specimen had a slightly larger band gap. The band structure analysis revealed an indirect band gap for the sol-gel NaTaO3, contrary to the direct band gap of the solid-state one. The difference in the electronic structure and surface area explained the higher photocatalytic activity of the sol-gel NaTaO3. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2396930兴 Photocatalytic water splitting with catalyst particles suspended in water has been intensively studied because the process presents a clean and renewable source for hydrogen energy.1–9 Metal-oxide semiconductors containing tantalum have shown promising stability and activity for water splitting into H2 and O2 under irradiation.1–6 Among these catalysts perovskite-type NaTaO3 shows a remarkable water splitting rate.4 Solid-state method has been generally employed to prepare NaTaO3.4,6 This method requires a high temperature and long duration in calcination and leads to a small surface area of the products. Doping with La was shown to reduce the particle size of NaTaO3 and create nanosteps on the surface.6 As a result, the photocatalytic activity of NaTaO3 was significantly enhanced by the La doping. In view of the influence received from the morphology variation, a sol-gel synthesis, an alternative to the conventional solid-state, is employed to prepare NaTaO3 nanoparticles,10 in an attempt to elucidate further the correlation between the catalyst structure and activity. Reagent-grade CH3COONa 共Nihon Shiyaku兲, TaCl5 共Alfa Aesar兲, and citric acid 共C6H8O7 · H2O; Riedel-deHaën兲 were used in the sol-gel synthesis. Solutions of CH3COONa 共0.9M兲, TaCl5 共0.8M兲, and citric acid 共4.6M兲 were mixed to form a sol solution of NaTaO3. The molar ratio of Na/ Ta/ citric acid was 1 / 1 / 5. The solution was continuously stirred at 90 ° C until the sol became a gel. The gel was then calcined at 350 ° C for 1 h and 500 ° C for 3 h to give the NaTaO3 product. The solid-state method was also employed to synthesize NaTaO3 for comparison purposes. The synthesis procedure was analogous to those reported.4,6 In brief, a mixture of Na2CO3 共Nihon Shiyaku兲 and Ta2O5 共Alfa Aesar兲 was calcined in air at 1200 ° C for 10 h. This calcination was conducted for three times with intermediate grinding at the ama兲

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bient temperature. The amount of Na was in 5% excess to compensate the volatilization loss. The crystalline structure of the NaTaO3 specimens was characterized by powder x-ray diffraction 共XRD, Rigaku RINT2000兲. Figure 1 shows the XRD patterns of the specimens obtained from the sol-gel 共SG兲 and solid-state 共SS兲 methods. Both specimens exhibited nearly identical 2-theta positions for the principal peaks. By a detailed inspection on the patterns, some principal peaks, such as those designated as A, B and C, were identified to comprise several subpeaks, as shown in the insets of Fig. 1. The subpeaks can only be identified at an XRD scan rate as low as 0.03° / min. By comparing with the NaTaO3 patterns documented in the powder diffraction files of the JCPDS, the SG and SS NaTaO3 materials prepared in the present work should be assigned to the monoclinic 关P2 / m with a = 3.8995 Å, b = 3.8965 Å, and c = 3.8995 Å 共Ref. 11兲 and orthorhombic 关Pcmn with a = 5.5213 Å, b = 7.7952 Å, and c = 5.4842 Å 共Ref. 12兲 phases, respectively. The crystalline planes responsible for the diffraction peaks are labeled in Fig. 1 for the two different phases. The differences in the XRD peaks observed might result from the difference in the preparation temperature because the crystalline structure of NaTaO3 is sensitive to temperature.13 The N2 Brunauer-Emmett-Teller surface area was determined to be 23 and 0.60 m2g−1 for the SG and SS NaTaO3, respectively, reflecting that the SG method synthesized specimens of smaller particle sizes. The photocatalytic reactions over these catalysts were carried out in a gas-closed system. 1 g of each NaTaO3 specimen was suspended in 900 ml pure water by a magnetic stirrer in an inner irradiation cell made of quartz. The light source was a 400 W high-pressure mercury lamp 共SEN HL400EH-5兲. The amounts of H2 and O2 evolved were determined by using a gas chromatograph 共Hewlett Packard HP6890; molecular sieve 5A column, thermal conductivity detector, Ar carrier兲. Figure 2 shows H2 and O2 evolutions from the photocatalytic reaction system. In principle, H2 and O2 were

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Appl. Phys. Lett. 89, 211904 共2006兲

FIG. 3. 共Color online兲 Diffuse reflectance spectra of the SG and SS NaTaO3.

FIG. 1. 共Color online兲 XRD patterns of NaTaO3 specimens prepared from the sol-gel 共SG兲 and solid-sate 共SS兲 methods. The crystalline planes labeled on the diffraction peaks correspond to the monoclinic and orthorhombic phases for SG and SS, respectively. The insets show the magnification of peaks A, B, and C in the patterns.

evolved in the ratio of approximately 2 / 1. The evolution was slow initially and accelerated to reach a stable rate after 50 min of reaction. The mean rates of H2 formation were estimated to be approximately 1940 and 13 ␮mol h−1 g−1 for the SG and SS NaTaO3, respectively. The photocatalytic activity of the SG NaTaO3 was high compared to that of the SS. The difference in the surface area must have contributed to this activity difference. The difference in the activity was approximately 150 times greater for the SG while that in the surface area is only approximately 38 times, however. There should be other factors, in addition to the difference in the surface area, causing the high activity of the SG specimen.

Diffuse reflectance ultraviolet-visible spectroscopic analysis was conducted for the NaTaO3 specimens. Figure 3 shows the absorbance spectra that were converted from reflection by the Kubelka-Munk method. Band gaps of the SG and SS were estimated to be 4.1 and 4.0 eV, respectively, from onsets of the absorption. There was no surprise to observe the difference in the optical property because these two specimens had different constituting crystalline phases. Figure 4 shows the band structure and density of states that were calculated on the basis of the density functional theory14 with the proposed generalized gradient approximation by Perdew et al.15 for the nonlocal correction to a purely local treatment of the exchange-correlation potential and energy. The single-particle Kohn-Sham equations16 were solved using the plane-wave-based Vienna ab initio simulation program 共VASP兲.17,18 The interactions between the ions and valence electrons were described by the projector augmented-wave method19 in the implementation of Kresse

FIG. 2. Photocatalytic H2 共empty兲 and O2 共full兲 evolution from 900 ml pure FIG. 4. 共Color online兲 Calculated band structure 共top兲 and density of states water suspended with 1 g photocatalysts of SG and SS NaTaO3. Light source: 400 W high-pressure mercury lamp. 共bottom兲 for the SG and SS NaTaO3 from first-principles methods. Downloaded 19 Oct 2009 to 140.116.208.56. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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and Joubert.20 The numbers of treated valence electrons were 7, 11, and 6 for Na, Ta, and O atoms, respectively. In Fig. 4 the SG NaTaO3 exhibits a slightly larger band gap, in agreement with the data shown in the diffuse reflectance spectra. Thus, the photoinduced charge carriers could have a larger energy on the SG NaTaO3. This would have resulted in the higher activity of the SG. A second distinction in electronic structures found between the SG and SS NaTaO3 from the calculations is the indirect and direct band gaps for SG and SS, respectively, as shown by the band structures of Fig. 4. The recombination rate for electron-hole pairs is usually much lowered in the systems with indirect band gap. The longer lifetime of the free electrons and holes is expected to increase the probability of their participations in water splitting. This can therefore contribute to the higher photocatalytic activity of the SG NaTaO3. One can see why the orthorhombic phase of NaTaO3 has a direct band gap providing that the indirect-band gap structure of the monoclinic phase is already established. The orthorhombic phase of NaTaO3 can be constructed from the monoclinic one by taking a 冑2 ⫻ 冑 2 cell of the xz plane and twice the cell alone the y axis of the monoclinic structure 共called the “ideal ortho” hereafter兲, and finally completed with slightly adjusting the lattice parameters as well as the atomic positions to optimize the structure. Our investigations using ab initio calculations found that the dispersion of the band structures for the ideal ortho and the orthorhombic phase are similar. The band structure of the ideal ortho can be obtained from folding the band structure of the monoclinic phase, which has its top of valence band around 共0.5 0.5 0.5兲 and the bottom of conduction band around 共0 0 0兲 of the reciprocal space. When folding back to form the Brillouin zone for the ideal ortho, the 共0.5 0.5 0.5兲 in the monoclinic phase becomes the 共0 0 0兲 of the ideal ortho, and therefore resulting in a direct band gap.

In conclusion, the present work has demonstrated an unsophisticated sol-gel route to prepare NaTaO3 applicable for efficient water splitting. The remarkably high photocatalytic activity of the sol-gel NaTaO3, in comparison with that of the solid-state one, can be adequately explained by some specific features exhibited on the material electronic structure, in addition to the large surface area. This research was supported by the National Science Council of Taiwan 共NSC 94-212-M-006-006 and 95-2221-E006-408-MY3兲. The computer resources were partially provided by the National Center for High Performance Computing in Hsin-Chu, Taiwan. A. Kudo, H. Kato, and S. Nakagawa, J. Phys. Chem. B 104, 571 共2000兲. Z. Zou, J. Ye, and H. Arakawa, Chem. Phys. Lett. 332, 271 共2000兲. Z. Zou, J. Ye, K. Sayama, and H. Arakawa, Nature 共London兲 414, 625 共2001兲. 4 H. Kato and A. Kudo, J. Phys. Chem. B 105, 4285 共2001兲. 5 J. Ye, Z. Zou and A. Matsushita, Int. J. Hydrogen Energy 28, 651 共2003兲. 6 H. Kato, K. Asakura, and A. Kudo, J. Am. Chem. Soc. 125, 3082 共2003兲. 7 J. Sato, N. Saito, H. Nishiyama, and Y. Inoue, J. Phys. Chem. B 107, 7965 共2003兲. 8 H. Luo, T. Takata, Y. Lee, J. Zhao, K. Domen, and Y. Yan, Chem. Mater. 16, 846 共2004兲. 9 T. Sreethawong, Y. Suzuki, and S. Yoshikawa, Catal. Commun. 6, 119 共2005兲. 10 C. C. Tsai and H. Teng, J. Am. Ceram. Soc. 87, 2080 共2004兲. 11 JCPDS-International Center for Diffraction Data, Card No. 74-2479 共2001兲. 12 JCPDS-International Center for Diffraction Data, Card No. 73-0878 共2001兲. 13 M. Ahtee and C. N. W. Darlington, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 36, 1007 共1980兲. 14 P. Hohenberg and W. Kohn, Phys. Rev. 136, B864 共1964兲. 15 J. P. Perdew, K. Burke, and Y. Wang, Phys. Rev. B 54, 016533 共1996兲. 16 W. Kohn and L. J. Sham, Phys. Rev. 140, 1133 共1965兲. 17 G. Kresse and J. Hafner, Phys. Rev. B 47, 558 共1993兲. 18 G. Kresse and J. Furthmuller, Phys. Rev. B 54, 11169 共1996兲. 19 P. E. Blochl, Phys. Rev. B 50, 17953 共1994兲. 20 G. Kresse and D. Joubert, Phys. Rev. B 59, 1758 共1999兲. 1 2 3

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