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Effect of annealing temperature on the photocatalytic activity of WO3 for O2 evolution
Applied Surface Science 256 (2009) 165–169
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of annealing temperature on the photocatalytic activity of WO3 for O2 evolution Gang Xin *, Wei Guo, Tingli Ma Dalian University of Technology, School of Chemical Engineering, State Key Laboratory of Fine Chemicals, Dalian 116012, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 April 2009 Received in revised form 28 July 2009 Accepted 28 July 2009 Available online 4 August 2009
Commercial WO3 powder was annealed in air at four different temperatures and characterized by XRD and BET. The samples were used for the photooxidation of H2O to O2 under visible light irradiation (l > 420 nm) in the presence of IO3 and the evolved gases were analyzed by gas chromatography. The results showed that the WO3 photocatalyst of monoclinic phase, which was obtained by annealing at 750 8C for 4 h, displayed the best activity in terms of O2 evolution among all the samples. Moreover, the activity was also found to be slightly affected by the grain size of the WO3 samples. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Photocatalytic reactions Oxygen evolution Tungsten oxide
1. Introduction The hydrogen produced by the decomposition of water using solar energy has been considered as the ultimate technology to solve both energy requirements and the environmental problems resulting from the current use of fossil fuels. Tungsten (VI) oxide has been used as the photocatalyst for oxygen evolution in most of the hitherto reported two-step water-splitting systems [1–4]. The band-gap energy of tungsten (VI) oxide has been estimated as 2.8 eV [5,6], and tungsten oxide has the potential ability to promote photocatalytic reactions under visible light (l > 440 nm) irradiation. The WO3 powder split water for oxygen evolution with iron (III) ions (or silver ions) as the electron acceptor under visible light [7–10]. However, only commercially available WO3 powders have been used in these studies, and the key features of WO3 responsible for the photocatalytic activity in oxygen evolution have yet to be clarified. Therefore, the visible light-induced evolution of O2 on WO3 for the solar energy conversion systems was investigated here. Tungsten trioxide, given its simple stoichiometry, adopts a surprising number of distinct crystalline forms. There have been a number of studies on the crystallography of WO3, which have been motivated, at least in part, by the interesting physical properties and potential technological applications of this material. WO3 is singlephase monoclinic at room temperature, and four phase transitions are observed upon heating it to 1000 8C. The way in which the structure changes with temperature is indicated in Fig. 1 [11].
* Corresponding author. Tel.: +86 13841130165; fax: +86 411 39893820. E-mail address: [email protected] (G. Xin). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.07.102
It is well known that the photocatalytic activity of TiO2 depends on the phase structure, and the photocatalytic activity of BiVO4 strongly depends on the crystalline form and the calcination temperature [12]. Monoclinic BiVO4 shows a higher photocatalytic activity than the tetragonal form. Analogously, WO3 with an appropriate structure may potentially exhibit higher activity in the O2 evolution reaction. In the work described herein, commercial WO3 powder has been annealed at various temperatures, and the photocatalytic activities of the respective WO3 samples have been examined. A correlation between photocatalytic activity and physical properties has been established. 2. Experimental 2.1. Annealing of tungsten oxide WO3 powder (99.99 wt %) was provided by the Japan Pure Chemical Co. This commercial WO3 powder was annealed in air at different temperatures: 650, 750, 850, and 950 8C. The samples were calcined in a box furnace. Each sample was placed in an alumina crucible, heated to the desired temperature at a rate of 10 K min1, and kept at that temperature for 4 h. After the heating, the resulting powder was allowed to cool naturally to room temperature. 2.2. Characterization of the catalysts Powder X-ray diffraction (XRD) analysis with Cu-Ka radiation was carried out on a Rigaku RINT 2500HRPC diffractometer equipped with a carbon monochromator. Diffuse reflectance spectra were obtained on a Jasco (DRS, V-560) UV/vis spectrophotometer
G. Xin et al. / Applied Surface Science 256 (2009) 165–169
166
Fig. 1. Scheme of the WO3 phase transition.
adapted for diffuse reflectance measurement and recorded after Kubelka–Munk analysis. The surface morphology of the annealed WO3 was investigated by field-emission scanning electron microscopy (FESEM, S-4700, Hitachi). Specific surface areas were determined by the BET single-point method using nitrogen uptake at 77 K. 2.3. Photocatalytic reactions Reactions were typically carried out in a Pyrex top-irradiationtype reaction vessel connected to a gas-tight glass circulation system. Pt-loaded WO3 was prepared by an impregnation method using an aqueous solution of H2PtCl6 followed by calcination in air at 500 8C for 1 h. Photooxidation of H2O to O2 in the presence of an electron acceptor IO3 was examined as a test photoreaction under visible light irradiation. A WO3 powder sample (0.2 g), loaded with 0.5 wt% Pt, was suspended in an aqueous NaIO3 solution (5 mmol L1, 200 mL) under magnetic stirring and the reaction vessel was evacuated several times to completely remove the air. The mixture was then irradiated with light from a 300 W Xe lamp equipped with cut-off filters (Asahi Techno Glass, L-42). The temperature of the reaction medium was maintained at 288 K by a flow of cooling water during the reaction. Photogenerated electrons reacted with the electron acceptor IO3 to produce iodide ions, while photogenerated holes on the WO3 particles reacted with water to produce oxygen. The gases evolved in the reaction were analyzed by gas chromatography (GC-8A, TCD, Shimadzu; Ar as carrier). 3. Results and discussion 3.1. Crystal structures and photophysical properties X-ray powder diffraction patterns of annealed WO3 are shown in Fig. 2. Three distinct peaks in the range 238 < 2u < 258 are clearly discernible in these XRD patterns. These three peaks, together with those at 2u = 33–34.58 and 50–568, indicated that the respective samples of tungsten trioxide had similar phase structure
compositions. Tungsten trioxide exists in several polymorph phases. All of the polymorphs are three-dimensional networks of corner-linked WO6 octahedra, and the majority, those comprising four-membered rings of WO6 octahedral, can be described as distorted variants of the cubic ReO3 crystal structure. Characteristic XRD patterns are obtained, depending on the different crystal structures. WO3 has been found to adopt a wide variety of crystal structures: orthorhombic [13,14], triclinic [15], monoclinic [16– 18], tetragonal, and hexagonal [19]. The monoclinic WO3 phase characteristically exhibits three intense peaks at 2u = 23.38, 23.88, and 24.68 and is the most stable phase at room temperature [12,13]. The diffraction lines in Fig. 2a and b resemble those of the orthorhombic phase of WO3 based on the JCPD files (JCPDS card 201324), from which it could be concluded that the commercial tungsten trioxide consisted of an orthorhombic phase. Due to reversibility of the phase transition, the WO3 annealed at 650 8C was mainly orthorhombic with only a small amount of monoclinic phase (P21/n). The twin peaks seen in the ranges 28–308, 35–368, and 40–438 in Fig. 2c–e may be attributed to monoclinic (P21/n) phase WO3. On the basis of Fig. 1, the irreversible phase transition changed at 720 8C, and a reversible phase transition took place between 720 and 950 8C. WO3 annealed at 750 8C was of the monoclinic (P21/n) phase, whereas samples annealed at 850 and 950 8C were of monoclinic (P21/n) and tetragonal phases, respectively. The XRD results in Fig. 2 also reveal changes in the peak intensities of the obtained powders with temperature. A distinct sharpening resolution of peaks can be observed after annealing. This probably resulted from the crystallite aggregation process, leading to a bigger crystallite dimensions in the solid, and from an increase in the grain size with annealing temperature up. An effect of the calcination temperature on the crystallite dimensions of WO3 was thus detected. The crystal size of WO3 (120) was determined using the Scherrer equation: D¼
Kl bcosu
where K related to the crystallite shape, l and u are the radiation wavelength and Bragg’s angle, respectively, b = B0 b0, B0 and b0 is peak half-width of samples and standard separately. The average grain size of commercial Tungsten oxide is 55 nm and after annealing 4 h at 650 8C, the average grain size became larger, which is 220 nm. In the case of higher annealing temperature, the crystal size could not be accurately calculated using the Scherrer equation for the larger crystal size. The SEM photographs in Fig. 3 also proved this. The commercial Tungsten oxide grain size is 50– 150 nm, the crystallites aggregated and grew up with heattreatment temperature increasing, after annealing 4 h at 950 8C, the grain size is 1–3 mm. The UV–vis spectra of the WO3 samples are shown in Fig. 4. The absorption onset wavelengths of the obtained powders were 455, 460, 462, 464 and 468 nm corresponding to different heattreatment temperature, Eg was calculated as following formula: Eg(eV) = 1240/lg, which is respectively equivalents to 2.72, 2.70, 2.68, 2.68 and 2.65 eV. As the annealing temperature was increased, the absorbing edge was red-shifted slightly from the curves in the short-wavelength region. We speculate on the absorption wavelength of the red shift may be due to WO3 phase changes, but also an increase in oxygen vacancy-related defects with a corresponding increase in light absorption in the longwavelength region above 470 nm. 3.2. Photocatalytic properties
Fig. 2. XRD patterns of WO3, (a) the commercial sample, and after annealing for 4 h at (b) 650 8C, (c) 750 8C, (d) 850 8C, and (e) 950 8C.
Fig. 5 shows the courses of O2 evolution on the annealed tungsten trioxide samples under visible light irradiation
G. Xin et al. / Applied Surface Science 256 (2009) 165–169
167
Fig. 3. SEM photographs of WO3, (a) the commercial sample, and after annealing for 4 h at (b) 650 8C, (c) 750 8C, (d) 850 8C, and (e) 950 8C.
(l > 420 nm) in a 0.005 M aqueous NaIO3 solution without pH adjustment. Tungsten trioxide samples annealed at 750 and 850 8C for 4 h exhibited the highest activity for O2 evolution (Fig. 5c and d). A loss of activity for samples annealed at T = 950 8C is evident from Fig. 5e. The phase of the WO3 is evidently the key influencing factor for photocatalytic activity. In Table 1, it is clear that all WO3 samples of monoclinic (P21/c) phase exhibited good photocatalytic activity. We supposed that the monoclinic (P21/c) phase of WO3 is responsible for the oxygen evolution reaction. WO3 samples annealed at 650 8C having the orthorhombic phase, and at 950 8C having the tetragonal phase mainly, showed lower photocatalytic activities for water splitting. The activities of the WO3 versus their specific surface areas can be compared by inspecting the fourth and fifth columns of Table 1. As can be seen, no systematic trend was apparent between the activity and the specific surface area. Thus, increasing the specific surface area did not significantly affect the photocatalytic performance; the results suggest that the WO3 annealed at 750 and 850 8C, having a low specific surface
area, showed the better photocatalytic activity. As regards the WO3 sample annealed at 650 8C and the commercial WO3 sample, the specific surface areas of these two samples were higher, but their photocatalytic activities were lower, hence the specific surface area of WO3 is not a decisive factor for activity in O2 evolution. Another possible explanation may lie in crystallites aggregates impact on recombination of the photo-generated electrons. It can be observed from the SEM images in Fig. 3 that the grain size of the tungsten oxide increased with the annealing temperature up because of the gradual formation of larger WO3 crystallites and the consequent coalescing at high temperature. The quantity and life of the photo-generated electrons are the determinant factors in photocatalysis reactions, photo-generated electrons in the crystallites are transferred to the WO3 surface and aggregated on Pt nano-particles, and then the photocatalysis reaction could take place quickly on it. So the recombination of the photo-generated electrons in the WO3 crystallites affect on photocatalytic activity in the WO3/Pt system. In the case of bigger crystallites, the longer transferred
G. Xin et al. / Applied Surface Science 256 (2009) 165–169
168
Table 1 Amounts of O2 evolved in dependence on the annealing conditions of tungsten trioxide. T/8C
t/h
Phase
SBET/m2 g1
O2/mmol
650 750 850 950 750 750 750 750 Commercial WO3
4 4 4 4 1 2 8 16
Monoclinic (P21/n) + orthorhombic Monoclinic (P21/c) Monoclinic (P21/c) + tetragonal Monoclinic (P21/c) + tetragonal Monoclinic (P21/c) Monoclinic (P21/c) Monoclinic (P21/c) Monoclinic (P21/c) Orthorhombic
1.83 1.34 0.29 0.26 1.65 1.59 1.26 1.27 3.44
3.1 45.5 36.4 2.6 10.4 19.1 37.8 26.1 10.6
Thermal treatment was carried out at T/8C for t hours. The right-hand column shows the amount of O2 evolved on the annealed tungsten trioxide samples under visible light irradiation (l > 420 nm) in a 0.005 M aqueous NaIO3 solution without pH adjustment.
smaller grain size of the WO3 annealed at 850 8C show the better photocatalytic activity than at 950 8C. Similar arguments are applicable to the effect of the annealing time. To determine whether the annealing time has an influence on the activity of the powder, similar experiments were repeated for precursors annealed at 750 8C for 1–16 h in static air. In Table 1, it can clearly be seen that the activity depended on the annealing time and increased as this parameter was increased up to 4 h. The low activity of the starting WO3 was improved with longer annealing times, since defects in the lattice were eliminated. However, the activity of the WO3 reached an optimal level after annealing for 4 h, as shown in Table 1 the specific surface area was seen to decrease and in Fig. 3c the surface of the WO3 crystallite was smooth; we guess that the grain size must be bigger when the specific surface area of WO3 is lower, somewhat with longer annealing time, and the photocatalytic activity showed the same downward trend. It is concluded that the grain size of the catalyst had a slight effect on the activity. Fig. 4. UV/vis absorption spectra of WO3, (a) the commercial sample, and after annealing for 4 h at (b) 650 8C, (c) 750 8C, (d) 850 8C, and (e) 950 8C.
4. Conclusions The results reported herein have shown that annealing at 750 8C for 4 h is effective for improving the activity of WO3 for O2 evolution under irradiation by visible light. Annealed WO3 samples have three phases, namely orthorhombic, monoclinic, and tetragonal. The samples of monoclinic phase have been found to exhibit higher photocatalytic activity than those of the other phases, indicating that the phase of the WO3 is a decisive factor with regard to activity in O2 evolution. The photocatalytic activity has also been found to be slightly affected by the crystallites aggregates of samples; long-term annealing increased the grain size of the sample, which resulted in reduced activity. Acknowledgments We thank Prof. Kazunari Domen of Tokyo University for assistance with photocatalytic reactions. The project sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
Fig. 5. Amounts of O2 evolved under visible light irradiation (l > 420 nm) in a 0.005 M aqueous NaIO3 solution without pH adjustment, in the presence of: (a) commercial WO3, and after annealing for 4 h at (b) 650 8C, (c) 750 8C, (d) 850 8C, and (e) 950 8C.
distance results in the more recombination chance of photogenerated electrons, and increasing oxygen vacancy-related defect also consumed the photo-generated electrons after annealing at 950 8C. Hence the photocatalytic activity was decreased accordingly, in Fig. 3 improved crystallites with
References [1] G.R. Bamwenda, H. Arakawa, Solar Energy Mater. Solar Cells 70 (2001) 1. [2] G.R. Bamwenda, K. Sayama, H. Arakawa, J. Photochem. Photobiol. A: Chem. 122 (1999) 175. [3] H. Kominami, K. Yabutani, T. Yamamoto, Y. Kera, B. Ohtani, J. Mater. Chem. 11 (2001) 3222. [4] G.R. Bamwenda, H. Arakawa, Appl. Catal. A: General 210 (2001) 181. [5] D.W. Hwang, J. Kim, T.J. Park, J.S. Lee, Catal. Lett. 80 (2002) 53. [6] M. Spichiger-Ulmann, J. Augustynski, J. Appl. Phys. 54 (1983) 6061. [7] J.R. Darwent, A. Mills, J. Chem. Soc. 78 (1982) 359. [8] W. Erbs, J. Desilvestro, E. Borgarello, M. Gra¨tzel, J. Phys. Chem. 88 (1984) 4001. [9] K. Sayama, H. Arakawa, J. Phys. Chem. 97 (1993) 531.
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169
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of annealing temperature on the photocatalytic activity of WO3 for O2 evolution Gang Xin *, Wei Guo, Tingli Ma Dalian University of Technology, School of Chemical Engineering, State Key Laboratory of Fine Chemicals, Dalian 116012, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 April 2009 Received in revised form 28 July 2009 Accepted 28 July 2009 Available online 4 August 2009
Commercial WO3 powder was annealed in air at four different temperatures and characterized by XRD and BET. The samples were used for the photooxidation of H2O to O2 under visible light irradiation (l > 420 nm) in the presence of IO3 and the evolved gases were analyzed by gas chromatography. The results showed that the WO3 photocatalyst of monoclinic phase, which was obtained by annealing at 750 8C for 4 h, displayed the best activity in terms of O2 evolution among all the samples. Moreover, the activity was also found to be slightly affected by the grain size of the WO3 samples. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Photocatalytic reactions Oxygen evolution Tungsten oxide
1. Introduction The hydrogen produced by the decomposition of water using solar energy has been considered as the ultimate technology to solve both energy requirements and the environmental problems resulting from the current use of fossil fuels. Tungsten (VI) oxide has been used as the photocatalyst for oxygen evolution in most of the hitherto reported two-step water-splitting systems [1–4]. The band-gap energy of tungsten (VI) oxide has been estimated as 2.8 eV [5,6], and tungsten oxide has the potential ability to promote photocatalytic reactions under visible light (l > 440 nm) irradiation. The WO3 powder split water for oxygen evolution with iron (III) ions (or silver ions) as the electron acceptor under visible light [7–10]. However, only commercially available WO3 powders have been used in these studies, and the key features of WO3 responsible for the photocatalytic activity in oxygen evolution have yet to be clarified. Therefore, the visible light-induced evolution of O2 on WO3 for the solar energy conversion systems was investigated here. Tungsten trioxide, given its simple stoichiometry, adopts a surprising number of distinct crystalline forms. There have been a number of studies on the crystallography of WO3, which have been motivated, at least in part, by the interesting physical properties and potential technological applications of this material. WO3 is singlephase monoclinic at room temperature, and four phase transitions are observed upon heating it to 1000 8C. The way in which the structure changes with temperature is indicated in Fig. 1 [11].
* Corresponding author. Tel.: +86 13841130165; fax: +86 411 39893820. E-mail address: [email protected] (G. Xin). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.07.102
It is well known that the photocatalytic activity of TiO2 depends on the phase structure, and the photocatalytic activity of BiVO4 strongly depends on the crystalline form and the calcination temperature [12]. Monoclinic BiVO4 shows a higher photocatalytic activity than the tetragonal form. Analogously, WO3 with an appropriate structure may potentially exhibit higher activity in the O2 evolution reaction. In the work described herein, commercial WO3 powder has been annealed at various temperatures, and the photocatalytic activities of the respective WO3 samples have been examined. A correlation between photocatalytic activity and physical properties has been established. 2. Experimental 2.1. Annealing of tungsten oxide WO3 powder (99.99 wt %) was provided by the Japan Pure Chemical Co. This commercial WO3 powder was annealed in air at different temperatures: 650, 750, 850, and 950 8C. The samples were calcined in a box furnace. Each sample was placed in an alumina crucible, heated to the desired temperature at a rate of 10 K min1, and kept at that temperature for 4 h. After the heating, the resulting powder was allowed to cool naturally to room temperature. 2.2. Characterization of the catalysts Powder X-ray diffraction (XRD) analysis with Cu-Ka radiation was carried out on a Rigaku RINT 2500HRPC diffractometer equipped with a carbon monochromator. Diffuse reflectance spectra were obtained on a Jasco (DRS, V-560) UV/vis spectrophotometer
G. Xin et al. / Applied Surface Science 256 (2009) 165–169
166
Fig. 1. Scheme of the WO3 phase transition.
adapted for diffuse reflectance measurement and recorded after Kubelka–Munk analysis. The surface morphology of the annealed WO3 was investigated by field-emission scanning electron microscopy (FESEM, S-4700, Hitachi). Specific surface areas were determined by the BET single-point method using nitrogen uptake at 77 K. 2.3. Photocatalytic reactions Reactions were typically carried out in a Pyrex top-irradiationtype reaction vessel connected to a gas-tight glass circulation system. Pt-loaded WO3 was prepared by an impregnation method using an aqueous solution of H2PtCl6 followed by calcination in air at 500 8C for 1 h. Photooxidation of H2O to O2 in the presence of an electron acceptor IO3 was examined as a test photoreaction under visible light irradiation. A WO3 powder sample (0.2 g), loaded with 0.5 wt% Pt, was suspended in an aqueous NaIO3 solution (5 mmol L1, 200 mL) under magnetic stirring and the reaction vessel was evacuated several times to completely remove the air. The mixture was then irradiated with light from a 300 W Xe lamp equipped with cut-off filters (Asahi Techno Glass, L-42). The temperature of the reaction medium was maintained at 288 K by a flow of cooling water during the reaction. Photogenerated electrons reacted with the electron acceptor IO3 to produce iodide ions, while photogenerated holes on the WO3 particles reacted with water to produce oxygen. The gases evolved in the reaction were analyzed by gas chromatography (GC-8A, TCD, Shimadzu; Ar as carrier). 3. Results and discussion 3.1. Crystal structures and photophysical properties X-ray powder diffraction patterns of annealed WO3 are shown in Fig. 2. Three distinct peaks in the range 238 < 2u < 258 are clearly discernible in these XRD patterns. These three peaks, together with those at 2u = 33–34.58 and 50–568, indicated that the respective samples of tungsten trioxide had similar phase structure
compositions. Tungsten trioxide exists in several polymorph phases. All of the polymorphs are three-dimensional networks of corner-linked WO6 octahedra, and the majority, those comprising four-membered rings of WO6 octahedral, can be described as distorted variants of the cubic ReO3 crystal structure. Characteristic XRD patterns are obtained, depending on the different crystal structures. WO3 has been found to adopt a wide variety of crystal structures: orthorhombic [13,14], triclinic [15], monoclinic [16– 18], tetragonal, and hexagonal [19]. The monoclinic WO3 phase characteristically exhibits three intense peaks at 2u = 23.38, 23.88, and 24.68 and is the most stable phase at room temperature [12,13]. The diffraction lines in Fig. 2a and b resemble those of the orthorhombic phase of WO3 based on the JCPD files (JCPDS card 201324), from which it could be concluded that the commercial tungsten trioxide consisted of an orthorhombic phase. Due to reversibility of the phase transition, the WO3 annealed at 650 8C was mainly orthorhombic with only a small amount of monoclinic phase (P21/n). The twin peaks seen in the ranges 28–308, 35–368, and 40–438 in Fig. 2c–e may be attributed to monoclinic (P21/n) phase WO3. On the basis of Fig. 1, the irreversible phase transition changed at 720 8C, and a reversible phase transition took place between 720 and 950 8C. WO3 annealed at 750 8C was of the monoclinic (P21/n) phase, whereas samples annealed at 850 and 950 8C were of monoclinic (P21/n) and tetragonal phases, respectively. The XRD results in Fig. 2 also reveal changes in the peak intensities of the obtained powders with temperature. A distinct sharpening resolution of peaks can be observed after annealing. This probably resulted from the crystallite aggregation process, leading to a bigger crystallite dimensions in the solid, and from an increase in the grain size with annealing temperature up. An effect of the calcination temperature on the crystallite dimensions of WO3 was thus detected. The crystal size of WO3 (120) was determined using the Scherrer equation: D¼
Kl bcosu
where K related to the crystallite shape, l and u are the radiation wavelength and Bragg’s angle, respectively, b = B0 b0, B0 and b0 is peak half-width of samples and standard separately. The average grain size of commercial Tungsten oxide is 55 nm and after annealing 4 h at 650 8C, the average grain size became larger, which is 220 nm. In the case of higher annealing temperature, the crystal size could not be accurately calculated using the Scherrer equation for the larger crystal size. The SEM photographs in Fig. 3 also proved this. The commercial Tungsten oxide grain size is 50– 150 nm, the crystallites aggregated and grew up with heattreatment temperature increasing, after annealing 4 h at 950 8C, the grain size is 1–3 mm. The UV–vis spectra of the WO3 samples are shown in Fig. 4. The absorption onset wavelengths of the obtained powders were 455, 460, 462, 464 and 468 nm corresponding to different heattreatment temperature, Eg was calculated as following formula: Eg(eV) = 1240/lg, which is respectively equivalents to 2.72, 2.70, 2.68, 2.68 and 2.65 eV. As the annealing temperature was increased, the absorbing edge was red-shifted slightly from the curves in the short-wavelength region. We speculate on the absorption wavelength of the red shift may be due to WO3 phase changes, but also an increase in oxygen vacancy-related defects with a corresponding increase in light absorption in the longwavelength region above 470 nm. 3.2. Photocatalytic properties
Fig. 2. XRD patterns of WO3, (a) the commercial sample, and after annealing for 4 h at (b) 650 8C, (c) 750 8C, (d) 850 8C, and (e) 950 8C.
Fig. 5 shows the courses of O2 evolution on the annealed tungsten trioxide samples under visible light irradiation
G. Xin et al. / Applied Surface Science 256 (2009) 165–169
167
Fig. 3. SEM photographs of WO3, (a) the commercial sample, and after annealing for 4 h at (b) 650 8C, (c) 750 8C, (d) 850 8C, and (e) 950 8C.
(l > 420 nm) in a 0.005 M aqueous NaIO3 solution without pH adjustment. Tungsten trioxide samples annealed at 750 and 850 8C for 4 h exhibited the highest activity for O2 evolution (Fig. 5c and d). A loss of activity for samples annealed at T = 950 8C is evident from Fig. 5e. The phase of the WO3 is evidently the key influencing factor for photocatalytic activity. In Table 1, it is clear that all WO3 samples of monoclinic (P21/c) phase exhibited good photocatalytic activity. We supposed that the monoclinic (P21/c) phase of WO3 is responsible for the oxygen evolution reaction. WO3 samples annealed at 650 8C having the orthorhombic phase, and at 950 8C having the tetragonal phase mainly, showed lower photocatalytic activities for water splitting. The activities of the WO3 versus their specific surface areas can be compared by inspecting the fourth and fifth columns of Table 1. As can be seen, no systematic trend was apparent between the activity and the specific surface area. Thus, increasing the specific surface area did not significantly affect the photocatalytic performance; the results suggest that the WO3 annealed at 750 and 850 8C, having a low specific surface
area, showed the better photocatalytic activity. As regards the WO3 sample annealed at 650 8C and the commercial WO3 sample, the specific surface areas of these two samples were higher, but their photocatalytic activities were lower, hence the specific surface area of WO3 is not a decisive factor for activity in O2 evolution. Another possible explanation may lie in crystallites aggregates impact on recombination of the photo-generated electrons. It can be observed from the SEM images in Fig. 3 that the grain size of the tungsten oxide increased with the annealing temperature up because of the gradual formation of larger WO3 crystallites and the consequent coalescing at high temperature. The quantity and life of the photo-generated electrons are the determinant factors in photocatalysis reactions, photo-generated electrons in the crystallites are transferred to the WO3 surface and aggregated on Pt nano-particles, and then the photocatalysis reaction could take place quickly on it. So the recombination of the photo-generated electrons in the WO3 crystallites affect on photocatalytic activity in the WO3/Pt system. In the case of bigger crystallites, the longer transferred
G. Xin et al. / Applied Surface Science 256 (2009) 165–169
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Table 1 Amounts of O2 evolved in dependence on the annealing conditions of tungsten trioxide. T/8C
t/h
Phase
SBET/m2 g1
O2/mmol
650 750 850 950 750 750 750 750 Commercial WO3
4 4 4 4 1 2 8 16
Monoclinic (P21/n) + orthorhombic Monoclinic (P21/c) Monoclinic (P21/c) + tetragonal Monoclinic (P21/c) + tetragonal Monoclinic (P21/c) Monoclinic (P21/c) Monoclinic (P21/c) Monoclinic (P21/c) Orthorhombic
1.83 1.34 0.29 0.26 1.65 1.59 1.26 1.27 3.44
3.1 45.5 36.4 2.6 10.4 19.1 37.8 26.1 10.6
Thermal treatment was carried out at T/8C for t hours. The right-hand column shows the amount of O2 evolved on the annealed tungsten trioxide samples under visible light irradiation (l > 420 nm) in a 0.005 M aqueous NaIO3 solution without pH adjustment.
smaller grain size of the WO3 annealed at 850 8C show the better photocatalytic activity than at 950 8C. Similar arguments are applicable to the effect of the annealing time. To determine whether the annealing time has an influence on the activity of the powder, similar experiments were repeated for precursors annealed at 750 8C for 1–16 h in static air. In Table 1, it can clearly be seen that the activity depended on the annealing time and increased as this parameter was increased up to 4 h. The low activity of the starting WO3 was improved with longer annealing times, since defects in the lattice were eliminated. However, the activity of the WO3 reached an optimal level after annealing for 4 h, as shown in Table 1 the specific surface area was seen to decrease and in Fig. 3c the surface of the WO3 crystallite was smooth; we guess that the grain size must be bigger when the specific surface area of WO3 is lower, somewhat with longer annealing time, and the photocatalytic activity showed the same downward trend. It is concluded that the grain size of the catalyst had a slight effect on the activity. Fig. 4. UV/vis absorption spectra of WO3, (a) the commercial sample, and after annealing for 4 h at (b) 650 8C, (c) 750 8C, (d) 850 8C, and (e) 950 8C.
4. Conclusions The results reported herein have shown that annealing at 750 8C for 4 h is effective for improving the activity of WO3 for O2 evolution under irradiation by visible light. Annealed WO3 samples have three phases, namely orthorhombic, monoclinic, and tetragonal. The samples of monoclinic phase have been found to exhibit higher photocatalytic activity than those of the other phases, indicating that the phase of the WO3 is a decisive factor with regard to activity in O2 evolution. The photocatalytic activity has also been found to be slightly affected by the crystallites aggregates of samples; long-term annealing increased the grain size of the sample, which resulted in reduced activity. Acknowledgments We thank Prof. Kazunari Domen of Tokyo University for assistance with photocatalytic reactions. The project sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
Fig. 5. Amounts of O2 evolved under visible light irradiation (l > 420 nm) in a 0.005 M aqueous NaIO3 solution without pH adjustment, in the presence of: (a) commercial WO3, and after annealing for 4 h at (b) 650 8C, (c) 750 8C, (d) 850 8C, and (e) 950 8C.
distance results in the more recombination chance of photogenerated electrons, and increasing oxygen vacancy-related defect also consumed the photo-generated electrons after annealing at 950 8C. Hence the photocatalytic activity was decreased accordingly, in Fig. 3 improved crystallites with
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