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Photophysical and photocatalytic properties of new photocatalysts MCrO4 (M=Sr, Ba)
Chemical Physics Letters 378 (2003) 24–28 www.elsevier.com/locate/cplett
Photophysical and photocatalytic properties of new photocatalysts MCrO4 (M ¼ Sr, Ba) Jiang Yin b
a,*
, Zhigang Zou b, Jinhua Ye
a,c,*
a Ecomaterials Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Photoreaction Control Research Center (PCRC), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan c Precursory Research for Embryonic Science and Technology, Japan Science and Technology Corporation (JST), Japan
Received 5 December 2002; in final form 14 July 2003 Published online: 8 August 2003
Abstract BaCrO4 and SrCrO4 photocatalyst powders were prepared by the solid state reaction method. They were characterized by powder X-ray diffraction, UV–Vis diffuse reflection spectroscopy and photocatalytic activity measurements in case of sacrificial reagents CH3 OH and AgNO3 under UV and visible light irradiation (k > 420 nm), respectively. The band gaps of BaCrO4 and SrCrO4 were determined as 2.63 and 2.44 eV. Due to the decrease of the ionic radius of the cation, the photocatalyst SrCrO4 showed much lower photocatalytic activity than BaCrO4 in evolving H2 from CH3 OH/H2 O solution. The difference in their photocatalytic activity is ascribed to the special electronic structures of BaCrO4 and SrCrO4 . Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction In the past three decades, the photocatalytic reaction has attracted much interest due to energy and environmental issues [1–3]. Most of the photocatalysts as found only respond to UV light irradiation. Of them, TiO2 is the most prominent photocatalyst with high photocatalytic activity. Thus, research efforts have been mainly focused on understanding fundamental processes and enhancing the photocatalytic efficiency of TiO2 [4,5]. *
Corresponding authors. Fax: +81-29-859-2601 (J. Yin). E-mail addresses: [email protected] (J. Yin), jinhua. [email protected] (J. Ye).
In the view of using natural energy, solar energy, there is an urgent need to develop new types of photocatalysts responding to visible light irradiation. Recently, transition metal tantalum oxides splitting water into H2 and O2 directly under visible light irradiation were found [3], showing a way to extend the light absorption of photocatalysts into the visible region of the solar spectrum, by using transition metal oxides. Up to now, only a few of transition metal oxides have been shown to be responsive to visible light irradiation [3,6]. In this Letter, we report new photocatalysts MCrO4 (M ¼ Ba, Sr) with d0 electronic configuration, which showed a response to visible light irradiation for the first time.
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0009-2614(03)01238-7
J. Yin et al. / Chemical Physics Letters 378 (2003) 24–28
MCrO4 compounds (M ¼ Ba, Sr) with barite structure have been studied mainly for their fundamental physics and effects on ecological systems [7,8], but their photocatalytic properties have never been reported.
2. Experimental Polycrystalline MCrO4 (M ¼ Ba, Sr) ceramic powders were synthesized by the conventional solid state reaction method, using powders of the high purity starting chemicals: BaCO3 , SrCO3 , and Cr2 O3 . The fully mixed chemical powders in stoichiometric ratios were heated at 850 °C for 16 h, and then reground. Finally, the mixed powders were sintered at 950 °C for 36 h for BaCrO4 and 1200 °C for 36 h for SrCrO4 . The crystal structures of these samples were confirmed by X-ray powder diffraction (JEOL JDX3500, Tokyo, Japan). UV–Vis diffuse reflectance spectra of the photocatalysts were measured by using a UV–Vis spectrometer (Shimadzu UV-2500PC, Tokyo, Japan). The photocatalytic reactions were examined using a closed gas circulation system. The gases evolved were determined with a TCD gas chromatograph (Shimadzu GC8A, Tokyo, Japan), which was connected to the system with a circulating line. Under UV light irradiation, an inner irradiation type quartz cell with a 400 W high-pressure Hg lamp was employed, and in case of visible light irradiation, a 300 W Xe arc lamp was focused through a window, and a 420 nm cutoff filter was placed onto the window face of the cell. Just like photocatalytic reaction for TiO2 , Pt was loaded on the surface of the photocatalyst powders in order to obtain high activity [9,10]. The H2 evolution reaction was performed in CH3 OH/H2 O solution with Pt co-catalyst (0.5 g of 0.2 wt% Pt-loaded photocatalyst powder, 50 ml CH3 OH, 220 ml pure water (Vis) or 320 ml pure water (UV)). The O2 evolution reaction was performed in AgNO3 solution (0.5 g photocatalyst powder, 5 mmol AgNO3 , 270 ml pure water (Vis) or 370 ml pure water (UV)). The surface area of the powder samples was measured by using the BET method.
25
3. Results and discussion The crystal structures of the photocatalysts MCrO4 (M ¼ Ba, Sr) were determined by powder X-ray diffraction, and all of the photocatalyst powders are of single phase. BaCrO4 and SrCrO4 have barite structure. BaCrO4 shows orthorhombic symmetry with space group Pbnm and lattice parameters a ¼ 0:9105 nm, b ¼ 0:5541 nm and c ¼ 0:7343 nm, but SrCrO4 shows monoclinic symmetry with space group P21 /n and lattice parameters a ¼ 0:7083 nm, b ¼ 0:7388 nm, c ¼ 0:6771 nm, and b ¼ 103.4°. In the unit cell, Cr6þ and O2 ions form the CrO4 tetrahedron, in which the four vertex corners are occupied by O2 ions and the central position is occupied by a Cr6þ ion. Each cell contains four MCrO4 molecules, with every Ba2þ (or Sr2þ ) ion surrounded by 12 O2 ions. The schematic drawing of the crystal structure for BaCrO4 is shown in Fig. 1. The structure parameters of BaCrO4 and SrCrO4 are listed in Table 1. The average surface area of both powder samples is about 2.0 m2 /g.
Fig. 1. Schematic showing the crystal structure for BaCrO4 .
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J. Yin et al. / Chemical Physics Letters 378 (2003) 24–28
Table 1 Physical and photocatalytic properties of the photocatalysts BaCrO4 , SrCrO4 Photocatalyst
BaCrO4
Structure symmetry
Lattice parameter (nm)
Band gap (eV)
Co-catalyst
Orthorhombic
a ¼ 0:9105 b ¼ 0:5541 c ¼ 0:7343
2.63
a ¼ 0:7083 b ¼ 0:7388 c ¼ 0:6771 b ¼ 103:4
2.44
Monoclinic
SrCrO4
Fig. 2 shows UV–Vis diffuse reflectance spectra of BaCrO4 and SrCrO4 powders. We determined the band gaps of BaCrO4 and SrCrO4 with the following equation, as in our previous work [11]: n=2
a¼A
ðhm Eg Þ hm
;
ð1Þ
where a, m, Eg , A and n are absorption coefficient, light frequency, band gap energy, a constant, and an integer, respectively. The value of n depends on the characteristics of the optical transition (n ¼ 1; 2; 4, and 6). The value of n is 2 for BaCrO4 and SrCrO4 , indicating that the optical transitions as shown in Fig. 2 for BaCrO4 and SrCrO4 are
Fig. 2. UV–Vis diffuse reflectance spectra of the photocatalysts BaCrO4 and SrCrO4 , and the wavelength dependence of H2 evolution from CH3 OH/H2 O solution with Pt co-catalyst (0.2 wt%) in 10 h for 0.5 g BaCrO4 .
Sacrificial reactant
Activity (lmol/h)
0.2 wt% Pt 0.2 wt% Pt None
CH3 OH CH3 OH 5 mmol AgNO3
34.2 (UV) 0.67 (Vis)
0.2 wt% Pt 0.2 wt% Pt None
CH3 OH CH3 OH 5 mmol AgNO3
2.1 (UV) 0.09 (Vis)
H2
O2
16.2 (UV)
19.5 (UV)
directly forbidden [12]. The band gaps as determined are 2.63 and 2.44 eV for BaCrO4 and SrCrO4 , respectively. It means that all of these semiconductors can be excited by visible light irradiation. The band structures of transition metal oxides are generally defined by the d-level of the transition metal and O-2p level of the ligand O atom [13], when the d orbitals of the transition metal are empty. Here, it is suggested the conduction bands of the photocatalysts BaCrO4 and SrCrO4 are composed of the Cr6þ 3d orbital, and the valence bands are composed of the O 2p orbital. The optical absorptions around 340 nm for both BaCrO4 and SrCrO4 in Fig. 2 may be ascribed to the electronic excitation from the O 2p state to the Cr6þ 4s state or the Ba2þ 6s state. The definite assignment of these optical transitions will be made by use of X-ray absorption spectroscopy and the ultraviolet photoemission spectroscopy later. The suggested band structures of BaCrO4 and SrCrO4 are schematically drawn in Fig. 3. Due to the decrease of the crystal symmetry from BaCrO4 to SrCrO4 , the band gap of SrCrO4 becomes narrower, thus leading to the lowering of the conduction band and the elevation of the valence band. The electrochemical requirements for watersplitting are as follows: if the potential at the bottom of the conduction band is more negative than the redox potential of Hþ /H2 (0 V vs. SHE, pH ¼ 0), and the potential at the top of the valence band is more positive than the redox potential of O2 /H2 O (1.23 V vs. SHE, pH ¼ 0), the photogenerated electrons and holes will move to the surface of the photocatalysts, and cause redox reactions.
J. Yin et al. / Chemical Physics Letters 378 (2003) 24–28
27
Fig. 3. Suggested band structures for the photocatalysts BaCrO4 and SrCrO4 .
Water molecules will be reduced by the electrons to form hydrogen and oxidized by the holes to form oxygen for overall water-splitting. In order to evaluate the photocatalytic activity of the photocatalysts BaCrO4 and SrCrO4 , reducing reagent CH3 OH and electron acceptor AgNO3 were employed in our experiments. In the case of CH3 OH, the photogenerated holes irreversibly oxidize CH3 OH instead of water. In case of AgNO3 solution, the photogenerated electrons will be consumed by Agþ ions and O2 evolution reactions will be enhanced. Fig. 4 shows the formation rates of H2 evolution from CH3 OH/H2 O solution with 0.2 wt% Pt co-catalyst under UV light irradiation for the photocatalysts BaCrO4 and SrCrO4 . The formation rates of H2 evolution for BaCrO4 and SrCrO4 in the first 15 h are about 34.2 and 2.1 lmol/h, respectively. BaCrO4 shows much higher photocatalytic activity than SrCrO4 , which can be ascribed to its more negative potential of the conduction band than SrCrO4 , since both powder samples almost show the same surface area. Generally, it is believed that for a semiconductor the higher position of the conduction band means the stronger reducing potential, and the lower position of the valence band means the stronger oxidizing potential [14]. After H2 evolution reactions, the crystal structures of BaCrO4 and SrCrO4 were checked again by X-ray diffraction. It was found that before and after H2 evolution reactions, BaCrO4 and SrCrO4 keep the same crystal struc-
Fig. 4. Photocatalytic H2 evolution from CH3 OH/H2 O solution for BaCrO4 and SrCrO4 with 0.2 wt% Pt co-catalyst under UV light irradiation (Cat.: 0.5 g, CH3 OH: 50 ml, H2 O: 320 ml, 400 W Hg lamp), and photocatalytic O2 evolution from AgNO3 solution for BaCrO4 and SrCrO4 under UV light irradiation (Cat.: 0.5 g, 5 mmol AgNO3 , H2 O: 370 ml, 400 W Hg lamp).
ture. For identifying whether the samples were a little dissolved in CH3 OH/H2 O solution due to possible photo-corrosion, further atomic absorption measurement looking for Ba2þ , Sr2þ , or Cr6þ will be performed in the future work. The formation rates of O2 evolution from AgNO3 solution under UV light irradiation for the photocatalysts BaCrO4 and SrCrO4 are shown in Fig. 4 by the inset. The photocatalysts show negligible difference in their activity in evolving O2 . Fig. 5 shows the formation rates of H2 evolution from CH3 OH/H2 O solution with 0.2 wt% Pt co-catalyst under visible light irradiation (k > 420 nm) for BaCrO4 and SrCrO4 . The formation rates of H2 evolution for BaCrO4 and SrCrO4 in the first 20 h are estimated about 0.67 and 0.09 lmol/h, respectively. This result is in agreement with that shown in Fig. 4. In order to identify whether H2 evolution reactions under visible light irradiation for these compounds are photocatalytic, the following experiments were also performed. Firstly, we checked H2 evolution from CH3 OH/H2 O solution with 0.2 wt% Pt co-catalyst and without any
28
J. Yin et al. / Chemical Physics Letters 378 (2003) 24–28
In summary, the visible light driven photocatalysts MCrO4 (M ¼ Ba, Sr) with single phase were synthesized. From UV–Vis diffuse reflectance spectra of the photocatalysts BaCrO4 and SrCrO4 , it was determined that the optical transitions in these compounds are directly forbidden. With the decrease of the ionic radius of the M cation, these photocatalysts undergo symmetric variations from orthorhombic to monoclinic, and the band gaps become narrower, thus leading to the decrease of the photocatalytic activity. Although the photocatalytic activities of BaCrO4 and SrCrO4 under visible light irradiation are not very high, it may be a possible way to develop new photocatalysts active under visible light irradiation by synthesizing new materials including the CrO4 tetrahedron. Fig. 5. Photocatalytic H2 evolution from CH3 OH/H2 O solution for BaCrO4 and SrCrO4 with 0.2 wt% Pt co-catalyst under visible light irradiation (Cat.: 0.5 g, CH3 OH: 50 ml, H2 O: 220 ml, 300 W Xe lamp (k > 420 nm)).
photocatalyst powder under visible light irradiation (k > 420 nm). No H2 evolution could be observed in this blank experiment, indicating that H2 evolved in case of the sacrificial reagent CH3 OH is contributed by the photocatalyst BaCrO4 and SrCrO4 . Secondly, the wavelength dependence of H2 evolution from CH3 OH/H2 O solution with 0.2 wt% Pt co-catalyst in 10 h for BaCrO4 was investigated with a quartz cell by using different cut-off filters, as shown in Fig. 2. The number of the incident photons was increased as the wavelength of the cut-off filter was shortened. When the wavelength is larger than 500 nm, almost no H2 evolution could be obtained. This is in accordance with the UV–Vis diffuse reflectance spectrum of BaCrO4 . So, it is reasonably concluded that the photocatalytic activities of the photocatalysts BaCrO4 and SrCrO4 , as observed in our experiments under visible light irradiation, are ascribed to the visible light absorption. O2 evolution reactions were also performed in AgNO3 solution under visible light irradiation for both photocatalysts, and the amounts of O2 evolved in each case are very low. From Figs. 3 and 4, it is obvious that the potentials of the conduction band and the valence band for BaCrO4 and SrCrO4 meet the electrochemical requirements for water-splitting.
Acknowledgements One of the authors would like to thank JSPS fellowship for financial support.
References [1] A. Fujishima, K. Honda, Nature (Lond.) 238 (1972) 37. [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [3] Zhigang Zou, Jinhua Ye, Kazuhiro Sayama, Hironori Arakawa, Nature (Lond.) 414 (2001) 625. [4] B.S. Geoffrey, E.M. Thomas, J. Phys. Chem. B 101 (1997) 2508. [5] K. Yeong, S. Samer, J.H. Munir, E.M. Thomas, J. Am. Chem. Soc. 113 (1991) 9561. [6] A. Kudo, K. Ueda, H. Kato, I. Mikami, Catal. Lett. 53 (1998) 229. [7] J.M. Alia, H.G.M. Edwards, A. Fernandez, M. Prieto, J. Raman Spectrosc. 30 (1999) 105. [8] B.R. James, J.C. Petura, R.J. Vitale, G.R. Mussoline, Environ. Sci. Technol. 29 (1995) 2377. [9] K. Sayama, K. Yase, H. Arakawa, K. Asakura, T. Tanaka, K. Domen, T. Onishi, J. Photochem. Photobiol. A 114 (1998) 125. [10] H.G. Kim, D.W. Hwang, J. Kim, Y.G. Kim, J. Lee, J. Chem. Commun. 1077 (1999). [11] J. Yin, Z. Zou, J. Ye, J. Mater. Res.: Rapid Commun. 17 (9) (2002) 2201. [12] M.A. Butler, J. Appl. Phys. 48 (1977) 1914. [13] D.E. Scaife, Solar Energy 25 (1980) 41. [14] Y. Oosawa, R. Takahashi, M. Yonemura, T. Sekine, Y. Goto, New J. Chem. 13 (1989) 435.
Photophysical and photocatalytic properties of new photocatalysts MCrO4 (M ¼ Sr, Ba) Jiang Yin b
a,*
, Zhigang Zou b, Jinhua Ye
a,c,*
a Ecomaterials Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Photoreaction Control Research Center (PCRC), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan c Precursory Research for Embryonic Science and Technology, Japan Science and Technology Corporation (JST), Japan
Received 5 December 2002; in final form 14 July 2003 Published online: 8 August 2003
Abstract BaCrO4 and SrCrO4 photocatalyst powders were prepared by the solid state reaction method. They were characterized by powder X-ray diffraction, UV–Vis diffuse reflection spectroscopy and photocatalytic activity measurements in case of sacrificial reagents CH3 OH and AgNO3 under UV and visible light irradiation (k > 420 nm), respectively. The band gaps of BaCrO4 and SrCrO4 were determined as 2.63 and 2.44 eV. Due to the decrease of the ionic radius of the cation, the photocatalyst SrCrO4 showed much lower photocatalytic activity than BaCrO4 in evolving H2 from CH3 OH/H2 O solution. The difference in their photocatalytic activity is ascribed to the special electronic structures of BaCrO4 and SrCrO4 . Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction In the past three decades, the photocatalytic reaction has attracted much interest due to energy and environmental issues [1–3]. Most of the photocatalysts as found only respond to UV light irradiation. Of them, TiO2 is the most prominent photocatalyst with high photocatalytic activity. Thus, research efforts have been mainly focused on understanding fundamental processes and enhancing the photocatalytic efficiency of TiO2 [4,5]. *
Corresponding authors. Fax: +81-29-859-2601 (J. Yin). E-mail addresses: [email protected] (J. Yin), jinhua. [email protected] (J. Ye).
In the view of using natural energy, solar energy, there is an urgent need to develop new types of photocatalysts responding to visible light irradiation. Recently, transition metal tantalum oxides splitting water into H2 and O2 directly under visible light irradiation were found [3], showing a way to extend the light absorption of photocatalysts into the visible region of the solar spectrum, by using transition metal oxides. Up to now, only a few of transition metal oxides have been shown to be responsive to visible light irradiation [3,6]. In this Letter, we report new photocatalysts MCrO4 (M ¼ Ba, Sr) with d0 electronic configuration, which showed a response to visible light irradiation for the first time.
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0009-2614(03)01238-7
J. Yin et al. / Chemical Physics Letters 378 (2003) 24–28
MCrO4 compounds (M ¼ Ba, Sr) with barite structure have been studied mainly for their fundamental physics and effects on ecological systems [7,8], but their photocatalytic properties have never been reported.
2. Experimental Polycrystalline MCrO4 (M ¼ Ba, Sr) ceramic powders were synthesized by the conventional solid state reaction method, using powders of the high purity starting chemicals: BaCO3 , SrCO3 , and Cr2 O3 . The fully mixed chemical powders in stoichiometric ratios were heated at 850 °C for 16 h, and then reground. Finally, the mixed powders were sintered at 950 °C for 36 h for BaCrO4 and 1200 °C for 36 h for SrCrO4 . The crystal structures of these samples were confirmed by X-ray powder diffraction (JEOL JDX3500, Tokyo, Japan). UV–Vis diffuse reflectance spectra of the photocatalysts were measured by using a UV–Vis spectrometer (Shimadzu UV-2500PC, Tokyo, Japan). The photocatalytic reactions were examined using a closed gas circulation system. The gases evolved were determined with a TCD gas chromatograph (Shimadzu GC8A, Tokyo, Japan), which was connected to the system with a circulating line. Under UV light irradiation, an inner irradiation type quartz cell with a 400 W high-pressure Hg lamp was employed, and in case of visible light irradiation, a 300 W Xe arc lamp was focused through a window, and a 420 nm cutoff filter was placed onto the window face of the cell. Just like photocatalytic reaction for TiO2 , Pt was loaded on the surface of the photocatalyst powders in order to obtain high activity [9,10]. The H2 evolution reaction was performed in CH3 OH/H2 O solution with Pt co-catalyst (0.5 g of 0.2 wt% Pt-loaded photocatalyst powder, 50 ml CH3 OH, 220 ml pure water (Vis) or 320 ml pure water (UV)). The O2 evolution reaction was performed in AgNO3 solution (0.5 g photocatalyst powder, 5 mmol AgNO3 , 270 ml pure water (Vis) or 370 ml pure water (UV)). The surface area of the powder samples was measured by using the BET method.
25
3. Results and discussion The crystal structures of the photocatalysts MCrO4 (M ¼ Ba, Sr) were determined by powder X-ray diffraction, and all of the photocatalyst powders are of single phase. BaCrO4 and SrCrO4 have barite structure. BaCrO4 shows orthorhombic symmetry with space group Pbnm and lattice parameters a ¼ 0:9105 nm, b ¼ 0:5541 nm and c ¼ 0:7343 nm, but SrCrO4 shows monoclinic symmetry with space group P21 /n and lattice parameters a ¼ 0:7083 nm, b ¼ 0:7388 nm, c ¼ 0:6771 nm, and b ¼ 103.4°. In the unit cell, Cr6þ and O2 ions form the CrO4 tetrahedron, in which the four vertex corners are occupied by O2 ions and the central position is occupied by a Cr6þ ion. Each cell contains four MCrO4 molecules, with every Ba2þ (or Sr2þ ) ion surrounded by 12 O2 ions. The schematic drawing of the crystal structure for BaCrO4 is shown in Fig. 1. The structure parameters of BaCrO4 and SrCrO4 are listed in Table 1. The average surface area of both powder samples is about 2.0 m2 /g.
Fig. 1. Schematic showing the crystal structure for BaCrO4 .
26
J. Yin et al. / Chemical Physics Letters 378 (2003) 24–28
Table 1 Physical and photocatalytic properties of the photocatalysts BaCrO4 , SrCrO4 Photocatalyst
BaCrO4
Structure symmetry
Lattice parameter (nm)
Band gap (eV)
Co-catalyst
Orthorhombic
a ¼ 0:9105 b ¼ 0:5541 c ¼ 0:7343
2.63
a ¼ 0:7083 b ¼ 0:7388 c ¼ 0:6771 b ¼ 103:4
2.44
Monoclinic
SrCrO4
Fig. 2 shows UV–Vis diffuse reflectance spectra of BaCrO4 and SrCrO4 powders. We determined the band gaps of BaCrO4 and SrCrO4 with the following equation, as in our previous work [11]: n=2
a¼A
ðhm Eg Þ hm
;
ð1Þ
where a, m, Eg , A and n are absorption coefficient, light frequency, band gap energy, a constant, and an integer, respectively. The value of n depends on the characteristics of the optical transition (n ¼ 1; 2; 4, and 6). The value of n is 2 for BaCrO4 and SrCrO4 , indicating that the optical transitions as shown in Fig. 2 for BaCrO4 and SrCrO4 are
Fig. 2. UV–Vis diffuse reflectance spectra of the photocatalysts BaCrO4 and SrCrO4 , and the wavelength dependence of H2 evolution from CH3 OH/H2 O solution with Pt co-catalyst (0.2 wt%) in 10 h for 0.5 g BaCrO4 .
Sacrificial reactant
Activity (lmol/h)
0.2 wt% Pt 0.2 wt% Pt None
CH3 OH CH3 OH 5 mmol AgNO3
34.2 (UV) 0.67 (Vis)
0.2 wt% Pt 0.2 wt% Pt None
CH3 OH CH3 OH 5 mmol AgNO3
2.1 (UV) 0.09 (Vis)
H2
O2
16.2 (UV)
19.5 (UV)
directly forbidden [12]. The band gaps as determined are 2.63 and 2.44 eV for BaCrO4 and SrCrO4 , respectively. It means that all of these semiconductors can be excited by visible light irradiation. The band structures of transition metal oxides are generally defined by the d-level of the transition metal and O-2p level of the ligand O atom [13], when the d orbitals of the transition metal are empty. Here, it is suggested the conduction bands of the photocatalysts BaCrO4 and SrCrO4 are composed of the Cr6þ 3d orbital, and the valence bands are composed of the O 2p orbital. The optical absorptions around 340 nm for both BaCrO4 and SrCrO4 in Fig. 2 may be ascribed to the electronic excitation from the O 2p state to the Cr6þ 4s state or the Ba2þ 6s state. The definite assignment of these optical transitions will be made by use of X-ray absorption spectroscopy and the ultraviolet photoemission spectroscopy later. The suggested band structures of BaCrO4 and SrCrO4 are schematically drawn in Fig. 3. Due to the decrease of the crystal symmetry from BaCrO4 to SrCrO4 , the band gap of SrCrO4 becomes narrower, thus leading to the lowering of the conduction band and the elevation of the valence band. The electrochemical requirements for watersplitting are as follows: if the potential at the bottom of the conduction band is more negative than the redox potential of Hþ /H2 (0 V vs. SHE, pH ¼ 0), and the potential at the top of the valence band is more positive than the redox potential of O2 /H2 O (1.23 V vs. SHE, pH ¼ 0), the photogenerated electrons and holes will move to the surface of the photocatalysts, and cause redox reactions.
J. Yin et al. / Chemical Physics Letters 378 (2003) 24–28
27
Fig. 3. Suggested band structures for the photocatalysts BaCrO4 and SrCrO4 .
Water molecules will be reduced by the electrons to form hydrogen and oxidized by the holes to form oxygen for overall water-splitting. In order to evaluate the photocatalytic activity of the photocatalysts BaCrO4 and SrCrO4 , reducing reagent CH3 OH and electron acceptor AgNO3 were employed in our experiments. In the case of CH3 OH, the photogenerated holes irreversibly oxidize CH3 OH instead of water. In case of AgNO3 solution, the photogenerated electrons will be consumed by Agþ ions and O2 evolution reactions will be enhanced. Fig. 4 shows the formation rates of H2 evolution from CH3 OH/H2 O solution with 0.2 wt% Pt co-catalyst under UV light irradiation for the photocatalysts BaCrO4 and SrCrO4 . The formation rates of H2 evolution for BaCrO4 and SrCrO4 in the first 15 h are about 34.2 and 2.1 lmol/h, respectively. BaCrO4 shows much higher photocatalytic activity than SrCrO4 , which can be ascribed to its more negative potential of the conduction band than SrCrO4 , since both powder samples almost show the same surface area. Generally, it is believed that for a semiconductor the higher position of the conduction band means the stronger reducing potential, and the lower position of the valence band means the stronger oxidizing potential [14]. After H2 evolution reactions, the crystal structures of BaCrO4 and SrCrO4 were checked again by X-ray diffraction. It was found that before and after H2 evolution reactions, BaCrO4 and SrCrO4 keep the same crystal struc-
Fig. 4. Photocatalytic H2 evolution from CH3 OH/H2 O solution for BaCrO4 and SrCrO4 with 0.2 wt% Pt co-catalyst under UV light irradiation (Cat.: 0.5 g, CH3 OH: 50 ml, H2 O: 320 ml, 400 W Hg lamp), and photocatalytic O2 evolution from AgNO3 solution for BaCrO4 and SrCrO4 under UV light irradiation (Cat.: 0.5 g, 5 mmol AgNO3 , H2 O: 370 ml, 400 W Hg lamp).
ture. For identifying whether the samples were a little dissolved in CH3 OH/H2 O solution due to possible photo-corrosion, further atomic absorption measurement looking for Ba2þ , Sr2þ , or Cr6þ will be performed in the future work. The formation rates of O2 evolution from AgNO3 solution under UV light irradiation for the photocatalysts BaCrO4 and SrCrO4 are shown in Fig. 4 by the inset. The photocatalysts show negligible difference in their activity in evolving O2 . Fig. 5 shows the formation rates of H2 evolution from CH3 OH/H2 O solution with 0.2 wt% Pt co-catalyst under visible light irradiation (k > 420 nm) for BaCrO4 and SrCrO4 . The formation rates of H2 evolution for BaCrO4 and SrCrO4 in the first 20 h are estimated about 0.67 and 0.09 lmol/h, respectively. This result is in agreement with that shown in Fig. 4. In order to identify whether H2 evolution reactions under visible light irradiation for these compounds are photocatalytic, the following experiments were also performed. Firstly, we checked H2 evolution from CH3 OH/H2 O solution with 0.2 wt% Pt co-catalyst and without any
28
J. Yin et al. / Chemical Physics Letters 378 (2003) 24–28
In summary, the visible light driven photocatalysts MCrO4 (M ¼ Ba, Sr) with single phase were synthesized. From UV–Vis diffuse reflectance spectra of the photocatalysts BaCrO4 and SrCrO4 , it was determined that the optical transitions in these compounds are directly forbidden. With the decrease of the ionic radius of the M cation, these photocatalysts undergo symmetric variations from orthorhombic to monoclinic, and the band gaps become narrower, thus leading to the decrease of the photocatalytic activity. Although the photocatalytic activities of BaCrO4 and SrCrO4 under visible light irradiation are not very high, it may be a possible way to develop new photocatalysts active under visible light irradiation by synthesizing new materials including the CrO4 tetrahedron. Fig. 5. Photocatalytic H2 evolution from CH3 OH/H2 O solution for BaCrO4 and SrCrO4 with 0.2 wt% Pt co-catalyst under visible light irradiation (Cat.: 0.5 g, CH3 OH: 50 ml, H2 O: 220 ml, 300 W Xe lamp (k > 420 nm)).
photocatalyst powder under visible light irradiation (k > 420 nm). No H2 evolution could be observed in this blank experiment, indicating that H2 evolved in case of the sacrificial reagent CH3 OH is contributed by the photocatalyst BaCrO4 and SrCrO4 . Secondly, the wavelength dependence of H2 evolution from CH3 OH/H2 O solution with 0.2 wt% Pt co-catalyst in 10 h for BaCrO4 was investigated with a quartz cell by using different cut-off filters, as shown in Fig. 2. The number of the incident photons was increased as the wavelength of the cut-off filter was shortened. When the wavelength is larger than 500 nm, almost no H2 evolution could be obtained. This is in accordance with the UV–Vis diffuse reflectance spectrum of BaCrO4 . So, it is reasonably concluded that the photocatalytic activities of the photocatalysts BaCrO4 and SrCrO4 , as observed in our experiments under visible light irradiation, are ascribed to the visible light absorption. O2 evolution reactions were also performed in AgNO3 solution under visible light irradiation for both photocatalysts, and the amounts of O2 evolved in each case are very low. From Figs. 3 and 4, it is obvious that the potentials of the conduction band and the valence band for BaCrO4 and SrCrO4 meet the electrochemical requirements for water-splitting.
Acknowledgements One of the authors would like to thank JSPS fellowship for financial support.
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