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Photocatalytic and photophysical properties of a novel series of solid photocatalysts, BiTa1−xNbxO4 (0⩽x⩽1)
3 August 2001
Chemical Physics Letters 343 (2001) 303±308
www.elsevier.com/locate/cplett
Photocatalytic and photophysical properties of a novel series of solid photocatalysts, BiTa1 x Nbx O4 0 6 x 6 1 Zhigang Zou a, Jinhua Ye b, Kazuhiro Sayama a, Hironori Arakawa a,* a
Photoreaction Control Research Center (PCRC), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8563, Japan b Materials Engineering Laboratory (MEL), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Received 1 March 2001; in ®nal form 25 May 2001
Abstract BiTa1 x Nbx O4 0 6 x 6 1 solid photocatalysts were prepared by solid-state reaction and characterized by powder X-ray diraction and Rietveld structure re®nement. The structure of BiTa1 x Nbx O4 (x 0:0 and 0.5) is triclinic. However, the structures of x 0:2, 0.8 and 1.0 are orthorhombic. The H2 evolution was obtained from an aqueous CH3 OH=H2 O solution and from pure H2 O with BiTa1 x Nbx O4 0 6 x 6 1 under UV irradiation. The orthorhombic photocatalysts exhibit much higher activity than that of triclinic photocatalysts. The orthorhombic photocatalyst at x 0:2 showed the highest activity. An UV±vis diuse re¯ectance spectroscopy measurement revealed that the band gap of orthorhombic photocatalysts is more narrow than that of triclinic photocatalysts. Ó 2001 Published by Elsevier Science B.V.
1. Introduction The photocatalytic water splitting with an oxide semiconductor has received the most attention because the attempt is aimed at producing hydrogen of clean-energy from water utilizing solar energy. Up to now, the research eorts mainly focused on understanding fundamental processes and in enhancing the photocatalytic eciency of TiO2 [1±4]. Later advances in materials fabrication led to the discovery that metal oxide semiconductors with tunnel or layered perovskite structures have the possibility to decompose water with higher activity [5,6]. Recently, a number of reports have shown that the compounds consisting of
*
Corresponding author. Fax: +81-298-61-4750. E-mail address: [email protected] (H. Arakawa).
three linear TaO6 chains could decompose water into H2 or O2 [7,8]. Unfortunately, the majority of photocatalysts needs co-catalyst to decompose water even though, the number of photocatalyst materials is limited. It is an urgent need to develop new types of photocatalyst materials with higher activity. In this Letter, we report the preparation and characterization of a new series of photocatalysts, BiTa1 x Nbx O4 0 6 x 6 1. A comparison of the photocatalytic properties of BiTa1 x Nbx O4 0 6 x 6 1 to that of the TiO2 photocatalyst is presented. 2. Experimental The polycrystalline samples of BiTa1 x Nbx O4 0 6 x 6 1 were prepared by a solid-state reaction method. The high-purity chemicals of
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 7 0 2 - 3
304
Z. Zou et al. / Chemical Physics Letters 343 (2001) 303±308
Ta2 O5 ; Bi2 CO3 3 and Nb2 O5 were used as starting materials. Ta2 O5 was dried at 700°C and Nb2 O5 was dried at 600°C before syntheses. The stoichiometric amounts of precursors were mixed and pressed into small pellets. The small pellets were sintered in an alumina crucible using an electric furnace in air three times. In the ®nal process, the pellet samples were reacted for two days at 1100°C. The chemical composition of the samples before and after photocatalytic reaction was determined by scanning electron microscope-X-ray energy dispersion spectrum (SEM±EDS) with an accelerating voltage of 25 kV. The crystal structure of samples before and after photocatalytic reaction was determined by powder X-ray diraction method (Rigaku RINT-2000 diractometer using UV±vis diuse CuKa radiation k 1:54178 A). re¯ectance spectrum of BiTa1 x Nbx O4 0 6 x 6 1 was measured by using an UV±vis spectrometer (MPS-2000, Shimadzu, Japan). The surface area was determined by BET measurement (Micrometritics, Shimadzu, FlowPrep 060). The photocatalytic reaction was examined using a gas closed circulation system and an inner-irradiation type reactor. A light source (400 W highpressure Hg lamp, Riko Kagaku, Japan) was covered with a water jacket (quartz glass; cuto k < 200 nm) to keep the reactor temperature constant at 20°C by cooling water. The gases evolved were determined with TCD gas chromatograph, which was connected with a circulating line. The photocatalytic reaction was performed in an aqueous CH3 OH=H2 O solution (1.0 g powder catalyst, 50 ml CH3 OH, 350 ml H2 O and 0.1 wt% Pt (Pt-loading instead of a H2 PtCl6 )) and in pure water (1.0 g powder catalyst, 400 ml H2 O) under UV irradiation. It is known that addition of noble metals or metal oxides to the surface of a semiconductor can change its surface properties and hence, its photocatalytic behavior [8,9]. Pt has been shown to be eective for the TiO2 photocatalyst [8,9], hence, we loaded Pt onto the catalyst surface for the photocatalytic reaction in an aqueous CH3 OH=H2 O solution to obtain higher activity. However, for the photocatalytic reaction in pure water we did not load any cocatalyst such as Pt onto the catalyst surface.
3. Results and discussion The chemical composition of the BiTa1 x Nbx O4 0 6 x 6 1 photocatalysts before and after photocatalytic reaction was determined using characteristic X-rays of TaLa; BiMa, and NbLa. The composition content was determined using the ZAF quanti®cation method. Oxygen content was calculated from the EDS results [10]. The SEM±EDS analysis showed that the photocatalysts have a homogenous atomic distribution with no other additional elements. The chemical composition of these samples was con®rmed to be the same before and after reaction. The structure of the photocatalysts before and after reaction was investigated using X-ray powder diraction. The result is shown in Fig. 1. The step interval was 0. 024° and scan speed, 1° min 1 . The powder X-ray diraction analysis showed that BiTa1 x Nbx O4 0 6 x 6 1 are single phase. This is consistent with the observation from SEM±EDS. Full-pro®le structure re®nement of the collected powder diraction data for BiTa1 x Nbx O4 0 6 x 6 1 was performed using the Rietveld program RE I T A N [11]. The outcome of the ®nal re®nement indicated that BiTa1 x Nbx O4 (x 0:0
Fig. 1. X-ray powder diraction patterns of BiTa1 x Nbx O4 0 6 x 6 1 before and after reaction.
Z. Zou et al. / Chemical Physics Letters 343 (2001) 303±308
and 0.5) are triclinic systems with space group P1. However, BiTa1 x Nbx O4 (x 0:2, 0.8 and 1.0) are orthorhombic systems with space group Pnna [12,13]. The results are shown in Table 1. The details of the investigations on crystal structure and phase transformation will be published elsewhere [14]. From powder X-ray diraction and SEM±EDS results, we con®rmed that these photocatalysts have not changed in both the crystal structure and the chemical composition after reaction. Fig. 2 shows the H2 evolution from CH3 OH=H2 O solution with BiTa1 x Nbx O4 0 6 x 6 1 under UV irradiation. The reaction stopped when the light was turned o in this experiment, showing the obvious light response. The result shows that the photocatalytic reaction is induced by the absorption of UV irradiation. The triclinic BiTa1 x Nbx O4 (x 0:0 and 0.5) showed much lower activity than that of orthorhombic BiTa1 x Nbx O4 (x 0:2, 0.8 and 1.0). This means that BiTa1 x Nbx O4 with orthorhombic structure has higher activity than that with triclinic structure. The orthorhombic BiTa0:8 Nb0:2 O4 x 0:2 photocatalyst showed the highest activity (see Fig. 1). The rate of H2 evolution was estimated to be 600 lmol h 1 in the ®rst 10 h (see Table 1). The total amount of H2 /catalyst(mol) was beyond 1.0 at 2 h, indicating that the reaction seems to occur catalytically. Total volume of evolved H2 was attained to 9000 lmol when this reaction achieved 24 h, the
305
Fig. 2. Photocatalytic H2 evolution on Pt=BiTa1 x Nbx O4 0 6 x 6 1 from CH3 OH=H2 O solution under UV irradiation. Cat.: 1.0 g, CH3 OH: 50 ml, H2 O: 350 ml, 400 W highpressure Hg lamp.
value corresponded to 4.3 mol ratio of H2 evolution to catalyst. The durability of the catalyst is given in Fig. 3 using the BiTa0:5 Nb0:5 O4 x 0:5 photocatalyst with triclinic structure. Although the rate of H2 evolution decreased after 10 h, the H2 production continued even after 260 h. The total volume of
Table 1 Rate of H2 evolution and physical properties of photocatalystsa Catalyst
Crystal structureb
Surface area (m2 g 1 )
Rate of gas evolutions (lmol h 1 ) CH3 OH=H2 Oc
BiTaO4 BiTa0:8 Nb0:2 O4 BiTa0:5 Nb0:5 O4 BiTa0:2 Nb0:8 O4 BiNbO4 TiO2 (P25) a
Triclinic Orthorhombic Triclinic Orthorhombic Orthorhombic
0.49 0.42 0.41 0.46 0.47 53.8
Band gap (eV) Pure H2 Od
H2
CO
H2
90 600 90 190 180 550
3 35 5 10 8 15
4 41 4 15 8 1
2.7 2.3 2.7 2.5 2.6 3.2
1.0 g powder catalyst under UV irradiation using 400 W high-pressure Hg lamp. The triclinic structure with space group P1 and the orthorhombic with space group Pnna were obtained by Rietveld re®nement. c 0.1% Pt was loaded on the surface of powder catalyst by inside photodeposition method. d Co-catalysts were not used such as Pt. b
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Z. Zou et al. / Chemical Physics Letters 343 (2001) 303±308
evolved H2 was attained to 6000 lmol/catalyst when this reaction achieved 260 h. This result contains important evidence for indicating that reaction occurs catalytically. The CO evolution was observed as the oxidation product in this reaction (see Table 1). The CO evolution increases with illumination time as does H2 evolution. However, the rate of CO evolution is much lower than that of H2 evolution. The CO is probably from decomposition of the formaldehyde, which decomposes slower than methanol dehydrogenates [8]. On the other hand, the ratios of non-stationary and non-stoichiometry evolutions between H2 and CO might result from generation of CO2 and other oxidation products and intermediates. It is well known that when CH3 OH is added to a Pt/TiO2 aqueous suspension, sustained H2 production is observed under UV irradiation and the alcohol molecules are oxidized to ®nal productions of CO2 ; CO; CH4 and etc. [8,15]. The presence of oxygen vacancy defects strongly enhances such interaction due to electron backdonation from surface Ti3 into p orbital of molecular CO [15]. The rate of CO evolution with
Fig. 3. Behavior of H2 evolution on Pt=BiTa0:5 Nb0:5 O4 x 0:5 from CH3 OH=H2 O solution under UV irradiation for 260 h. Cat.: 1.0 g, CH3 OH: 50 ml, H2 O: 350 ml, 400 W highpressure Hg lamp.
orthorhombic photocatalysts is larger than that of triclinic photocatalysts, showing the same tendency as observed in H2 evolutions. The rate of CO evolution for the BiTa0:8 Nb0:2 O4 x 0:2 photocatalyst is also the largest. It is known that the TiO2 photocatalyst has very high-photocatalytic activity under UV light irradiation. In order to compare BiTa1 x Nbx O4 0 6 x 6 1 to TiO2 photocatalyst, TiO2 photocatalyst (TiO2 -P25) was tested by the same method. The result is shown in Fig. 2. The formation rate of H2 evolution was estimated to be 550 lmol h 1 in the ®rst 10 h from CH3 OH=H2 O solution under UV light irradiation (see Table 1). It is noted that the rate of H2 evolution using BiTa0:8 Nb0:2 O4 x 0:2 is slightly larger than that of the TiO2 photocatalyst (TiO2 -P25). The rate of CO evolution is also larger than that of the TiO2 photocatalyst (see Table 1). This means that the activity of BiTa0:8 Nb0:2 O4 x 0:2 is higher than that of the TiO2 photocatalyst. BET measurement showed that the surface area of BiTa0:8 Nb0:2 O4 x 0:2 is 0:42 m2 g 1 (see Table 1). This is about 1% of the TiO2 photocatalyst (53.8 m2 g 1 ). This means that surface area of BiTa0:8 Nb0:2 O4 x 0:2 is much smaller than that of the TiO2 photocatalyst. Since this is only about 1% of the surface area of the TiO2 photocatalyst, it demonstrates the much higher eciency of the new photocatalyst. It is evident that further increase in activity might be expected from increasing the surface area. Fig. 4 shows the H2 evolution from pure water with photocatalyst suspension under UV irradiation without co-catalyst such as Pt. The H2 evolution increases with illumination time as does H2 evolution from an aqueous CH3 OH=H2 O solution. The formation rate of H2 from pure water showed the same tendency as observed from an aqueous CH3 OH=H2 O solution. The photocatalyst with orthorhombic structure has higher activity than that with triclinic structure. The photocatalyst at x 0:2 showed the highest activity. The rate of H2 evolution from pure water was estimated to be about 41 lmol h 1 in the ®rst 10 h for x 0:2 (see Table 1). Although the rate of H2 evolution decreased after 10 h, the total volume of evolved H2 was attained to 900 lmol when this reaction achieved 24 h. The rate of H2 evolution
Z. Zou et al. / Chemical Physics Letters 343 (2001) 303±308
was estimated to be about 37.5 lmol h 1 divided by 24 h. The TiO2 photocatalyst was tested by the same method. The result is shown in Fig. 4. The rate of H2 evolution from pure water was about 1:0 lmol h 1 in the ®rst 10 h (see Table 1). It is interesting to notice that the TiO2 photocatalyst shows much lower activity than that of BiTa1 x Nbx O4 0 6 x 6 1. This means that although the activity of the TiO2 photocatalyst in an aqueous CH3 OH=H2 O solution is very high under UV light irradiation, the activity is very low in pure water. Oxygen evolution was not observed from pure water in this experiment using both of BiTa1 x Nbx O4 0 6 x 6 1 and TiO2 . It is commonly accepted that there are both physisorbed and chemisorbed oxygen molecules in TiO2 surface by low-energy photon irradiation [5,7,8]. We speculate that the phenomenon as observed in TiO2 takes place on the surface of the photocatalysts. The UV±vis diuse re¯ectance spectra of the BiTa1 x Nbx O4 0 6 x 6 1 photocatalysts are shown in Fig. 5. The onset of diuse re¯ection spectra of the photocatalysts (x 0:2, 0.8 and 1.0) with orthorhombic structure showed a shift to
Fig. 4. Photocatalytic H2 evolution on BiTa1 x Nbx O4 0 6 x 6 1 from pure water without co-catalyst such as Pt under UV irradiation. Cat.: 1.0 g, H2 O: 400 ml, 400 W high-pressure Hg lamp.
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longer wavelength than that with triclinic structure. This is consistent with the observation from photocatalytic reaction (see Figs. 2 and 4). Since the onset of diuse re¯ection spectrum shifted to longer wavelength means that the photocatalyst can utilize more irradiation light energy. The band gaps of the BiTa1 x Nbx O4 0 6 x 6 1 photocatalysts were estimated from onset of diuse re¯ection spectra and is shown in Table 1. The band gap of the photocatalyst at x 0:2 was estimated to be about 2.6 eV, showing the narrowest band gap. It is known that the process for photocatalysis of semiconductors is the direct absorption of photon by band gap of the materials and generates electron±hole pairs in the semiconductor particles. The excitation of an electron from the valence band to the conduction band is initiated by light absorption with energy equal to or greater than the band gap of the semiconductor. Upon excitation by a photon the separated electron and hole can follow surface of solid. This suggests that the more narrow band gap is the easier it is to excite an electron from the valence band to the conduction band. The dierence in the band gap leads to dierent photocatalytic behavior. The UV± vis diuse re¯ectance spectra are in good agreement with the observed photocatalytic activity, which show that the appearance of electrons from the valence band to the conduction band initiated by direct light absorption results in hydrogen evolution, as can be seen from Table 1. Also, it is evident from Table 1 that the dierent materials have very similar surface areas; hence the dierences in photocatalytic activity cannot be attributed to variations in surface area.
Fig. 5. UV±vis diuse re¯ectance spectra of BiTa1 x Nbx O4 0 6 x 6 1.
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Z. Zou et al. / Chemical Physics Letters 343 (2001) 303±308
It is interesting to note that BiTa1 x Nbx O4 0 6 x 6 1 show photoabsorption in the visible light region (k > 420 nm), but the photoabsorption is weak. This means that the photocatalysts have ability to respond to wavelength of visible light region. However, these photocatalysts do not work under visible light irradiation (k > 420 nm). Alig et al. [16,17] have shown that direct absorption of photons by the band gap of oxides can generate electron±hole pairs in the solid. However, the energy requirement is generally higher than the band gap of the oxides. In order to increase the activity of these catalysts, two approaches are possible. One way would be to further modify the catalyst surface to increase the range of wavelengths at which the catalyst is active. Another would be to increase the irradiation energy. In summary, we have presented the evidence for photocatalytic decomposition of an aqueous solution and pure water with the BiTa1 x Nbx O4 0 6 x 6 1 photocatalysts. The change of x in BiTa1 x Nbx O4 0 6 x 6 1 could cause the change of structure. This dierence of structure might cause the dierence in the band levels and the corresponding dierence in the band gaps, leading to dierent photocatalytic behavior. Although the BiTa1 x Nbx O4 0 6 x 6 1 photocatalysts have a suitable band, the photocatalysts do not work under visible light irradiation to directly decompose pure water, even CH3 OH=H2 O solution. Modi®cation of
the surface of the compound may be needed to increase responding wavelength range. References [1] K. Honda, A. Fujishima, Nature 238 (1972) 37. [2] T. Kawai, T. Sakata, Nature 286 (1980) 474. [3] B.S. Georey, E.M. Thomas, J. Phys. Chem. B 101 (1997) 2508. [4] K. Yeong, S. Samer, J.H. Munir, E.M. Thomas, J. Am. Chem. Soc. 113 (1991) 9561. [5] K. Sayama, K. Yase, H. Arakawa, K. Asakura, K. Tanaka, K. Domen, T. Onishi, J. Photochem. Photobiol. A 114 (1998) 125. [6] T. Takata, A. Tanaka, M. Hara, J. Kondo, K. Domen, Catalysis Today 44 (1998) 17. [7] A. Kudo, H. Kato, Chem. Lett. (1997) 867. [8] Z. Zou, J. Ye, H. Arakawa, Catalysis Lett. 68 (2000) 235. [9] E.A. Meulenkamp, A.R. Wrr, Electrochimica Acta 41 (1996) 109. [10] Z. Zou, J. Ye, K. Oka, Y. Nishihara, Phys. Rev. Lett. 80 (1998) 1074. [11] F. Izumi, J. Crystallogr. Assoc. Jpn. 27 (1985) 23. [12] E.T. Keve, A.C. Skapski, J. Solid State Chem. 8 (1973) 159. [13] M.A. Subramanian, J.C. Calabrese, Mater. Res. Bull. 28 (1993) 523. [14] Z. Zou, J. Ye, H. Arakawa, to be published in Solid State Comm. [15] L.L. Amy, L. Guangqan, T. John, Jr. Yates, Chem. Rev. 95 (1995) 735. [16] R.C. Alig, S.W. Bloom, Phys. Rev. B 22 (1980) 5565. [17] A.L. Linsebigler, G. Lu, J.T.Jr. Yates, Chem. Rev. 95 (1995) 735.
Chemical Physics Letters 343 (2001) 303±308
www.elsevier.com/locate/cplett
Photocatalytic and photophysical properties of a novel series of solid photocatalysts, BiTa1 x Nbx O4 0 6 x 6 1 Zhigang Zou a, Jinhua Ye b, Kazuhiro Sayama a, Hironori Arakawa a,* a
Photoreaction Control Research Center (PCRC), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8563, Japan b Materials Engineering Laboratory (MEL), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Received 1 March 2001; in ®nal form 25 May 2001
Abstract BiTa1 x Nbx O4 0 6 x 6 1 solid photocatalysts were prepared by solid-state reaction and characterized by powder X-ray diraction and Rietveld structure re®nement. The structure of BiTa1 x Nbx O4 (x 0:0 and 0.5) is triclinic. However, the structures of x 0:2, 0.8 and 1.0 are orthorhombic. The H2 evolution was obtained from an aqueous CH3 OH=H2 O solution and from pure H2 O with BiTa1 x Nbx O4 0 6 x 6 1 under UV irradiation. The orthorhombic photocatalysts exhibit much higher activity than that of triclinic photocatalysts. The orthorhombic photocatalyst at x 0:2 showed the highest activity. An UV±vis diuse re¯ectance spectroscopy measurement revealed that the band gap of orthorhombic photocatalysts is more narrow than that of triclinic photocatalysts. Ó 2001 Published by Elsevier Science B.V.
1. Introduction The photocatalytic water splitting with an oxide semiconductor has received the most attention because the attempt is aimed at producing hydrogen of clean-energy from water utilizing solar energy. Up to now, the research eorts mainly focused on understanding fundamental processes and in enhancing the photocatalytic eciency of TiO2 [1±4]. Later advances in materials fabrication led to the discovery that metal oxide semiconductors with tunnel or layered perovskite structures have the possibility to decompose water with higher activity [5,6]. Recently, a number of reports have shown that the compounds consisting of
*
Corresponding author. Fax: +81-298-61-4750. E-mail address: [email protected] (H. Arakawa).
three linear TaO6 chains could decompose water into H2 or O2 [7,8]. Unfortunately, the majority of photocatalysts needs co-catalyst to decompose water even though, the number of photocatalyst materials is limited. It is an urgent need to develop new types of photocatalyst materials with higher activity. In this Letter, we report the preparation and characterization of a new series of photocatalysts, BiTa1 x Nbx O4 0 6 x 6 1. A comparison of the photocatalytic properties of BiTa1 x Nbx O4 0 6 x 6 1 to that of the TiO2 photocatalyst is presented. 2. Experimental The polycrystalline samples of BiTa1 x Nbx O4 0 6 x 6 1 were prepared by a solid-state reaction method. The high-purity chemicals of
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 7 0 2 - 3
304
Z. Zou et al. / Chemical Physics Letters 343 (2001) 303±308
Ta2 O5 ; Bi2 CO3 3 and Nb2 O5 were used as starting materials. Ta2 O5 was dried at 700°C and Nb2 O5 was dried at 600°C before syntheses. The stoichiometric amounts of precursors were mixed and pressed into small pellets. The small pellets were sintered in an alumina crucible using an electric furnace in air three times. In the ®nal process, the pellet samples were reacted for two days at 1100°C. The chemical composition of the samples before and after photocatalytic reaction was determined by scanning electron microscope-X-ray energy dispersion spectrum (SEM±EDS) with an accelerating voltage of 25 kV. The crystal structure of samples before and after photocatalytic reaction was determined by powder X-ray diraction method (Rigaku RINT-2000 diractometer using UV±vis diuse CuKa radiation k 1:54178 A). re¯ectance spectrum of BiTa1 x Nbx O4 0 6 x 6 1 was measured by using an UV±vis spectrometer (MPS-2000, Shimadzu, Japan). The surface area was determined by BET measurement (Micrometritics, Shimadzu, FlowPrep 060). The photocatalytic reaction was examined using a gas closed circulation system and an inner-irradiation type reactor. A light source (400 W highpressure Hg lamp, Riko Kagaku, Japan) was covered with a water jacket (quartz glass; cuto k < 200 nm) to keep the reactor temperature constant at 20°C by cooling water. The gases evolved were determined with TCD gas chromatograph, which was connected with a circulating line. The photocatalytic reaction was performed in an aqueous CH3 OH=H2 O solution (1.0 g powder catalyst, 50 ml CH3 OH, 350 ml H2 O and 0.1 wt% Pt (Pt-loading instead of a H2 PtCl6 )) and in pure water (1.0 g powder catalyst, 400 ml H2 O) under UV irradiation. It is known that addition of noble metals or metal oxides to the surface of a semiconductor can change its surface properties and hence, its photocatalytic behavior [8,9]. Pt has been shown to be eective for the TiO2 photocatalyst [8,9], hence, we loaded Pt onto the catalyst surface for the photocatalytic reaction in an aqueous CH3 OH=H2 O solution to obtain higher activity. However, for the photocatalytic reaction in pure water we did not load any cocatalyst such as Pt onto the catalyst surface.
3. Results and discussion The chemical composition of the BiTa1 x Nbx O4 0 6 x 6 1 photocatalysts before and after photocatalytic reaction was determined using characteristic X-rays of TaLa; BiMa, and NbLa. The composition content was determined using the ZAF quanti®cation method. Oxygen content was calculated from the EDS results [10]. The SEM±EDS analysis showed that the photocatalysts have a homogenous atomic distribution with no other additional elements. The chemical composition of these samples was con®rmed to be the same before and after reaction. The structure of the photocatalysts before and after reaction was investigated using X-ray powder diraction. The result is shown in Fig. 1. The step interval was 0. 024° and scan speed, 1° min 1 . The powder X-ray diraction analysis showed that BiTa1 x Nbx O4 0 6 x 6 1 are single phase. This is consistent with the observation from SEM±EDS. Full-pro®le structure re®nement of the collected powder diraction data for BiTa1 x Nbx O4 0 6 x 6 1 was performed using the Rietveld program RE I T A N [11]. The outcome of the ®nal re®nement indicated that BiTa1 x Nbx O4 (x 0:0
Fig. 1. X-ray powder diraction patterns of BiTa1 x Nbx O4 0 6 x 6 1 before and after reaction.
Z. Zou et al. / Chemical Physics Letters 343 (2001) 303±308
and 0.5) are triclinic systems with space group P1. However, BiTa1 x Nbx O4 (x 0:2, 0.8 and 1.0) are orthorhombic systems with space group Pnna [12,13]. The results are shown in Table 1. The details of the investigations on crystal structure and phase transformation will be published elsewhere [14]. From powder X-ray diraction and SEM±EDS results, we con®rmed that these photocatalysts have not changed in both the crystal structure and the chemical composition after reaction. Fig. 2 shows the H2 evolution from CH3 OH=H2 O solution with BiTa1 x Nbx O4 0 6 x 6 1 under UV irradiation. The reaction stopped when the light was turned o in this experiment, showing the obvious light response. The result shows that the photocatalytic reaction is induced by the absorption of UV irradiation. The triclinic BiTa1 x Nbx O4 (x 0:0 and 0.5) showed much lower activity than that of orthorhombic BiTa1 x Nbx O4 (x 0:2, 0.8 and 1.0). This means that BiTa1 x Nbx O4 with orthorhombic structure has higher activity than that with triclinic structure. The orthorhombic BiTa0:8 Nb0:2 O4 x 0:2 photocatalyst showed the highest activity (see Fig. 1). The rate of H2 evolution was estimated to be 600 lmol h 1 in the ®rst 10 h (see Table 1). The total amount of H2 /catalyst(mol) was beyond 1.0 at 2 h, indicating that the reaction seems to occur catalytically. Total volume of evolved H2 was attained to 9000 lmol when this reaction achieved 24 h, the
305
Fig. 2. Photocatalytic H2 evolution on Pt=BiTa1 x Nbx O4 0 6 x 6 1 from CH3 OH=H2 O solution under UV irradiation. Cat.: 1.0 g, CH3 OH: 50 ml, H2 O: 350 ml, 400 W highpressure Hg lamp.
value corresponded to 4.3 mol ratio of H2 evolution to catalyst. The durability of the catalyst is given in Fig. 3 using the BiTa0:5 Nb0:5 O4 x 0:5 photocatalyst with triclinic structure. Although the rate of H2 evolution decreased after 10 h, the H2 production continued even after 260 h. The total volume of
Table 1 Rate of H2 evolution and physical properties of photocatalystsa Catalyst
Crystal structureb
Surface area (m2 g 1 )
Rate of gas evolutions (lmol h 1 ) CH3 OH=H2 Oc
BiTaO4 BiTa0:8 Nb0:2 O4 BiTa0:5 Nb0:5 O4 BiTa0:2 Nb0:8 O4 BiNbO4 TiO2 (P25) a
Triclinic Orthorhombic Triclinic Orthorhombic Orthorhombic
0.49 0.42 0.41 0.46 0.47 53.8
Band gap (eV) Pure H2 Od
H2
CO
H2
90 600 90 190 180 550
3 35 5 10 8 15
4 41 4 15 8 1
2.7 2.3 2.7 2.5 2.6 3.2
1.0 g powder catalyst under UV irradiation using 400 W high-pressure Hg lamp. The triclinic structure with space group P1 and the orthorhombic with space group Pnna were obtained by Rietveld re®nement. c 0.1% Pt was loaded on the surface of powder catalyst by inside photodeposition method. d Co-catalysts were not used such as Pt. b
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evolved H2 was attained to 6000 lmol/catalyst when this reaction achieved 260 h. This result contains important evidence for indicating that reaction occurs catalytically. The CO evolution was observed as the oxidation product in this reaction (see Table 1). The CO evolution increases with illumination time as does H2 evolution. However, the rate of CO evolution is much lower than that of H2 evolution. The CO is probably from decomposition of the formaldehyde, which decomposes slower than methanol dehydrogenates [8]. On the other hand, the ratios of non-stationary and non-stoichiometry evolutions between H2 and CO might result from generation of CO2 and other oxidation products and intermediates. It is well known that when CH3 OH is added to a Pt/TiO2 aqueous suspension, sustained H2 production is observed under UV irradiation and the alcohol molecules are oxidized to ®nal productions of CO2 ; CO; CH4 and etc. [8,15]. The presence of oxygen vacancy defects strongly enhances such interaction due to electron backdonation from surface Ti3 into p orbital of molecular CO [15]. The rate of CO evolution with
Fig. 3. Behavior of H2 evolution on Pt=BiTa0:5 Nb0:5 O4 x 0:5 from CH3 OH=H2 O solution under UV irradiation for 260 h. Cat.: 1.0 g, CH3 OH: 50 ml, H2 O: 350 ml, 400 W highpressure Hg lamp.
orthorhombic photocatalysts is larger than that of triclinic photocatalysts, showing the same tendency as observed in H2 evolutions. The rate of CO evolution for the BiTa0:8 Nb0:2 O4 x 0:2 photocatalyst is also the largest. It is known that the TiO2 photocatalyst has very high-photocatalytic activity under UV light irradiation. In order to compare BiTa1 x Nbx O4 0 6 x 6 1 to TiO2 photocatalyst, TiO2 photocatalyst (TiO2 -P25) was tested by the same method. The result is shown in Fig. 2. The formation rate of H2 evolution was estimated to be 550 lmol h 1 in the ®rst 10 h from CH3 OH=H2 O solution under UV light irradiation (see Table 1). It is noted that the rate of H2 evolution using BiTa0:8 Nb0:2 O4 x 0:2 is slightly larger than that of the TiO2 photocatalyst (TiO2 -P25). The rate of CO evolution is also larger than that of the TiO2 photocatalyst (see Table 1). This means that the activity of BiTa0:8 Nb0:2 O4 x 0:2 is higher than that of the TiO2 photocatalyst. BET measurement showed that the surface area of BiTa0:8 Nb0:2 O4 x 0:2 is 0:42 m2 g 1 (see Table 1). This is about 1% of the TiO2 photocatalyst (53.8 m2 g 1 ). This means that surface area of BiTa0:8 Nb0:2 O4 x 0:2 is much smaller than that of the TiO2 photocatalyst. Since this is only about 1% of the surface area of the TiO2 photocatalyst, it demonstrates the much higher eciency of the new photocatalyst. It is evident that further increase in activity might be expected from increasing the surface area. Fig. 4 shows the H2 evolution from pure water with photocatalyst suspension under UV irradiation without co-catalyst such as Pt. The H2 evolution increases with illumination time as does H2 evolution from an aqueous CH3 OH=H2 O solution. The formation rate of H2 from pure water showed the same tendency as observed from an aqueous CH3 OH=H2 O solution. The photocatalyst with orthorhombic structure has higher activity than that with triclinic structure. The photocatalyst at x 0:2 showed the highest activity. The rate of H2 evolution from pure water was estimated to be about 41 lmol h 1 in the ®rst 10 h for x 0:2 (see Table 1). Although the rate of H2 evolution decreased after 10 h, the total volume of evolved H2 was attained to 900 lmol when this reaction achieved 24 h. The rate of H2 evolution
Z. Zou et al. / Chemical Physics Letters 343 (2001) 303±308
was estimated to be about 37.5 lmol h 1 divided by 24 h. The TiO2 photocatalyst was tested by the same method. The result is shown in Fig. 4. The rate of H2 evolution from pure water was about 1:0 lmol h 1 in the ®rst 10 h (see Table 1). It is interesting to notice that the TiO2 photocatalyst shows much lower activity than that of BiTa1 x Nbx O4 0 6 x 6 1. This means that although the activity of the TiO2 photocatalyst in an aqueous CH3 OH=H2 O solution is very high under UV light irradiation, the activity is very low in pure water. Oxygen evolution was not observed from pure water in this experiment using both of BiTa1 x Nbx O4 0 6 x 6 1 and TiO2 . It is commonly accepted that there are both physisorbed and chemisorbed oxygen molecules in TiO2 surface by low-energy photon irradiation [5,7,8]. We speculate that the phenomenon as observed in TiO2 takes place on the surface of the photocatalysts. The UV±vis diuse re¯ectance spectra of the BiTa1 x Nbx O4 0 6 x 6 1 photocatalysts are shown in Fig. 5. The onset of diuse re¯ection spectra of the photocatalysts (x 0:2, 0.8 and 1.0) with orthorhombic structure showed a shift to
Fig. 4. Photocatalytic H2 evolution on BiTa1 x Nbx O4 0 6 x 6 1 from pure water without co-catalyst such as Pt under UV irradiation. Cat.: 1.0 g, H2 O: 400 ml, 400 W high-pressure Hg lamp.
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longer wavelength than that with triclinic structure. This is consistent with the observation from photocatalytic reaction (see Figs. 2 and 4). Since the onset of diuse re¯ection spectrum shifted to longer wavelength means that the photocatalyst can utilize more irradiation light energy. The band gaps of the BiTa1 x Nbx O4 0 6 x 6 1 photocatalysts were estimated from onset of diuse re¯ection spectra and is shown in Table 1. The band gap of the photocatalyst at x 0:2 was estimated to be about 2.6 eV, showing the narrowest band gap. It is known that the process for photocatalysis of semiconductors is the direct absorption of photon by band gap of the materials and generates electron±hole pairs in the semiconductor particles. The excitation of an electron from the valence band to the conduction band is initiated by light absorption with energy equal to or greater than the band gap of the semiconductor. Upon excitation by a photon the separated electron and hole can follow surface of solid. This suggests that the more narrow band gap is the easier it is to excite an electron from the valence band to the conduction band. The dierence in the band gap leads to dierent photocatalytic behavior. The UV± vis diuse re¯ectance spectra are in good agreement with the observed photocatalytic activity, which show that the appearance of electrons from the valence band to the conduction band initiated by direct light absorption results in hydrogen evolution, as can be seen from Table 1. Also, it is evident from Table 1 that the dierent materials have very similar surface areas; hence the dierences in photocatalytic activity cannot be attributed to variations in surface area.
Fig. 5. UV±vis diuse re¯ectance spectra of BiTa1 x Nbx O4 0 6 x 6 1.
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It is interesting to note that BiTa1 x Nbx O4 0 6 x 6 1 show photoabsorption in the visible light region (k > 420 nm), but the photoabsorption is weak. This means that the photocatalysts have ability to respond to wavelength of visible light region. However, these photocatalysts do not work under visible light irradiation (k > 420 nm). Alig et al. [16,17] have shown that direct absorption of photons by the band gap of oxides can generate electron±hole pairs in the solid. However, the energy requirement is generally higher than the band gap of the oxides. In order to increase the activity of these catalysts, two approaches are possible. One way would be to further modify the catalyst surface to increase the range of wavelengths at which the catalyst is active. Another would be to increase the irradiation energy. In summary, we have presented the evidence for photocatalytic decomposition of an aqueous solution and pure water with the BiTa1 x Nbx O4 0 6 x 6 1 photocatalysts. The change of x in BiTa1 x Nbx O4 0 6 x 6 1 could cause the change of structure. This dierence of structure might cause the dierence in the band levels and the corresponding dierence in the band gaps, leading to dierent photocatalytic behavior. Although the BiTa1 x Nbx O4 0 6 x 6 1 photocatalysts have a suitable band, the photocatalysts do not work under visible light irradiation to directly decompose pure water, even CH3 OH=H2 O solution. Modi®cation of
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