- Email: [email protected]
Enhancement of the visible light photocatalytic performance of C-doped TiO2 by loading with V2O5
Catalysis Communications 11 (2009) 82–86
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Enhancement of the visible light photocatalytic performance of C-doped TiO2 by loading with V2O5 Zhongbiao Wu, Fan Dong, Yue Liu *, Haiqiang Wang Key Laboratory of Polluted Environment Remediation and Ecological Health of Ministry of Education, and College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 9 July 2009 Received in revised form 26 August 2009 Accepted 27 August 2009 Available online 9 September 2009 Keywords: TiO2 Carbon doping Visible light Photocatalyst Heterojunction V2O5
a b s t r a c t V2O5 was loaded on the surface of C-doped TiO2 (C-TiO2) by incipient wetness impregnation in order to enhance the visible light photocatalytic performance. The physicochemical properties of the C-TiO2/ V2O5 composite were characterized by XRD, Raman, TEM, XPS, UV–vis diffuse reflectance spectra, and PL in detail. The result indicated that a heterojunction between C-TiO2 and V2O5 was formed and the separation of excited electron–hole pairs on C-TiO2/V2O5 is greatly promoted. Thus, this composite photocatalyst exhibited enhanced visible light photocatalytic activity in degradation of gas-phase toluene compared with the pristine C-TiO2. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor photocatalysis is developing into one of the most promising technologies for solar energy conversion and environmental applications [1,2]. The leading TiO2 photocatalyst requires ultraviolet (about 3% of solar radiation) irradiation due to its wide band gap (ca. 3.2 eV for anatase TiO2), which seriously restricts its solar efficiency. Nonmetal doping, such as C and N doping, has displayed promising results in shifting the light absorption of TiO2 into visible light region [3–8]. As to the photocatalytic activity, C-doped TiO2 was found more active than N-doped TiO2 under visible light irradiation [3]. However, nonmetal doping intrinsically brings the serious problem of massive charge carrier recombination, which largely limits the visible light photocatalytic activity of nonmetal-doped TiO2 [8]. From the viewpoint of practical application, higher photocatalytic reaction efficiency is required since the photocatalytic efficiency of nonmetal-doped TiO2 under visible light is still low [9,10]. Loading of metal oxide semiconductor onto photocatalyst has been extensively studied as it is proved to enhance the photocatalytic activity due to promoted separation of charge carrier [11–13]. Thus, modification of C-doped TiO2 by further loading metal oxide semiconductor can be considered as an effective approach to * Corresponding author. Tel./fax: +86 571 87953088. E-mail address: [email protected] (Y. Liu). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.08.015
promote the visible light activity. To the best of our knowledge, there are few reports on modification of C-doped TiO2 for the purpose of enhancing the visible light photocatalytic performance. In this research, the C-doped TiO2 prepared by partial oxidation of TiC was further loaded with vanadium oxide through incipient wetness impregnation method followed by thermal treatment. The resulted composite photocatalysts were systematically investigated by various techniques. Based on the characterization results, the effect of vanadium oxide loading on the photocatalytic performance of C-doped TiO2 was discussed. 2. Experimental 2.1. Preparation of catalysts TiC powder (3.0 g) was loaded in a ceramic crucible, and then placed in muffle furnace open to the atmosphere. The temperature was ramped up to 400 °C at a rate of 2 °C/min and kept for 2 h to obtain C-doped TiO2 (C-TiO2). Vanadium oxide loading was performed by incipient wetness impregnation of C-TiO2 with aqueous solutions of NH4VO3, followed by stirring for 1 h and heating at 150 °C for water evaporation. Finally, the catalysts were obtained by treating at 300 °C for 1 h. The amount of loaded vanadium was controlled at 0, 0.01, 0.05, 0.2, 0.5, and 1.0 wt.%. The as-prepared samples were labeled as C-TiO2/V2O5-x, where x represented the content of vanadium. For comparison, V2O5 (vanadium: 0.5%) was loaded on SiO2 by the same process. The resulted sample
Z. Wu et al. / Catalysis Communications 11 (2009) 82–86
was labeled as SiO2/V2O5-0.5%. Pure V2O5 was prepared accordingly in the absence of C-TiO2 or SiO2. 2.2. Characterization The crystal phase was analyzed by X-ray diffraction with Cu Ka radiation (XRD: model D/max RA, Rigaku Co., Japan). Raman spectra were recorded using a micro-Raman spectrometer (Raman: RAMANLOG 6, USA) with a 514.5 nm Ar+ laser as the excitation source. X-ray photoelectron spectroscopy with Al Ka X-rays (hm = 1486.6 eV) radiation (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties. The UV–vis diffuse reflection spectra were obtained for the dry-pressed disk samples using a Scan UV–vis spectrophotometer (UV–vis DRS: TU-1901, China). The photoluminescence spectra were measured with a fluorospectrophotometer (PL: Fluorolog-3-Tau, France) using a Xe lamp as excitation source with optical filter.
83
was conducted with a GC-FID (FULI 9790, China). The procedure of the test has been reported elsewhere [7]. The photocatalytic oxidation of toluene is a pseudo-first-order reaction and its kinetics can be expressed as follows: ln(C0/C) = kt [15], C0 and C are the initial concentration and reaction concentration of toluene, respectively. 3. Results and discussion 3.1. Structure and morphology
Photocatalytic degradation of toluene in air is chosen as the probe reaction to test the activities of the as-prepared sample [14]. The photocatalytic activity tests were performed at room temperature using a 1.8 L photo-reactor. A 150 W Xe lamp with a UVcut optical filter (k < 425 nm) and an IR cutter (k > 800 nm) was placed above the reactor. The analysis of toluene concentration
The XRD patterns of as-prepared samples are shown in Fig. 1a. The phase structure in C-TiO2 and C-TiO2/V2O5 samples consists of anatase phase (JCPDS file no. 21-1272) and rutile (JCPDS, file no. 77-442). In the absence of C-TiO2, pure orthorhombic V2O5 (JSPD file no. 72-433) was generated during catalyst preparation. However, no V2O5 diffraction peaks could be observed in C-TiO2/V2O5, indicating that V2O5 was uniformly dispersed on the surface of CTiO2. Fig. 1a also shows that V2O5 loading has almost no influence on the phase structure of C-TiO2. By using the Debye–Scherrer equation, the crystallite sizes of anatase and rutile phase are calculated to be 19.9 and 30.3 nm, respectively. Raman spectra of C-TiO2, selected C-TiO2/V2O5 and V2O5 samples are shown in Fig. 1b. The observed characteristic Raman bands at 144, 196, 395, 515, and 638 cm 1, assigned to the Eg, B1g, A1g, B2g, and Eg vibrational modes of anatase TiO2 [16]. The Raman bands
Fig. 1. (a) XRD patterns and (b) Raman spectra of C-TiO2, C-TiO2/V2O5 and V2O5 samples (A: anatase, R: rutile).
Fig. 2. (a) TEM images and (b) XPS spectra for V2p of C-TiO2/V2O5-1.0%.
2.3. Tests of photocatalytic activities
84
Z. Wu et al. / Catalysis Communications 11 (2009) 82–86
due to rutile phase appear at 143 (overlapped by 144 cm 1 band of anatase phase), 235, 447, and 612 cm 1, which can be ascribed to the B1g, two-phonon scattering, Eg, and A1g modes of rutile phase, respectively [17]. Raman mode of V2O5 is also shown in Fig. 1b. No Raman bands relevant to V2O5 are observed [18], which is consistent with XRD result. TEM observation in Fig. 2a reveals that the C-TiO2/V2O5-1.0% sample consists of agglomerates of primary particles of 20– 30 nm in diameter. Some V2O5 clusters are found around C-TiO2 and dispersed on the surface of C-TiO2. The HRTEM images (inset) shows that a V2O5 particle is deposited on the surface of C-TiO2. The intimate contact between V2O5 and C-TiO2 favors the formation of heterojunction between the two components. Therefore, the C-TiO2/V2O5 heterojunction is expected to improve the charge separation and thereby the photocatalytic activity [19,20]. Fig. 2b shows the binding energy for V2p3/2 of C-TiO2/V2O51.0%. The V2p3/2 peak can be fitted into two peaks, located at 517.4 and 516.3 eV. They can be assigned to V5+ and V4+, respectively [21], which indicates the oxidation state of vanadium is variable. In XPS measurement, electrons in C-TiO2/V2O5 can be exited by X-rays (hm = 1486.6 eV). The V5+ can accept the generated electrons to produce V4+, indicating the ability of V2O5 to accept photo induced electrons. Besides, the carbon content in C-doped TiO2 is estimated to be 0.56 at.% and have little change after loading with V2O5 according to the XPS result.
3.2. UV–vis DRS and PL Fig. 3a shows UV–vis DRS of V2O5, C-TiO2 and C-TiO2/V2O5, and P25. When the amount of vanadium was less than 0.20 wt.%, no obvious change in visible light absorption was observed compared with C-TiO2. An obvious increase in absorbance of C-TiO2 in the visible light region was observed when the content of vanadium was greater than 0.50 wt.%. The tailing of C-TiO2/V2O5 absorption curves is different from that for pure V2O5. The relation between absorption coefficient (a) and incident photon energy (hm) can be written as ahm = Bd(hm Eg)n for allowed transitions (n = 2 for indirect transition, n = 1/2 direct transition), where Bd is the absorption constants. Plot of (ahm)1/2 and (ahm)2 versus hm from the spectra data of C-TiO2 and V2O5 in Fig. 3a are presented in Fig. 3b and c. The Eg estimated from the intercept of the tangents to the plots is 2.76 and 2.24 eV for C-TiO2 and V2O5, respectively [22,23]. Carbon doping reduces the band gap of TiO2 (3.0 eV for P25) by modifying the valence band [5,7]. The estimated Eg for V2O5 is consistent with literature [23]. Fig. 3d shows the room temperature PL spectra of C-TiO2 and CTiO2/V2O5-0.5%. The PL emission is the result of the recombination of excited electron–hole pairs. The lower PL intensity indicates the decrease in recombination rate, thus higher photocatalytic activity [24]. It can be seen from Fig. 3d that the PL intensity of C-TiO2/ V2O5-0.5% is lower than that of C-TiO2, indicating that V2O5 loaded
Fig. 3. (a) UV–vis DRS of V2O5, C-TiO2 and C-TiO2/V2O5 samples and P25, (b) plot of (ahm)1/2 vs. photon energy of C-TiO2 (indirect semiconductor) and (c) plot of (ahm)2 vs. photon energy of V2O5 (direct semiconductor), (d) PL spectra of C-TiO2 and C-TiO2/V2O5-0.5% (excitation light: 300 nm).
Z. Wu et al. / Catalysis Communications 11 (2009) 82–86
85
Table 1 Absolute electronegativity, calculated CB edge, calculated VB position and band gap energy for P25, C-doped TiO2 and V2O5 at the point of zero charge. Semiconductors
Absolute electronegativity (X) (eV)
Calculated CB position (eV)
Calculated VB position (eV)
Band gap energy Eg (eV)
P25 V2O5 C-doped TiO2
5.81 6.10 –
0.19 0.48 0.19
2.81 2.72 2.57
3.0 2.24 2.76
Fig. 5. The apparent rate constant (k) of as-prepared samples with varying weight ratio of V% under visible light irradiation, (a) SiO2/V2O5 sample, (b) C-TiO2, (c–g) C-TiO2/V2O5 samples.
Fig. 4. Schematic electronic band structure of C-TiO2 (2.76 eV) loaded with V2O5 (2.44 eV) and charge transfer process under visible light irradiation.
on C-TiO2 surface (inset in Fig. 2a) can enhance the separation of electron–hole pairs. 3.3. Photocatalyic activities and mechanism To elucidate the effect of V2O5 loading on the activity of C-doped TiO2, visible light photocatalytic degradation of gaseous toluene were performed. The band edge positions of conduction band (CB) and valence band (VB) of semiconductor can be determined by the following approach. The CB edge (E0CB ) of a semiconductor at the point of zero charge (pHZPC) can be predicted by the equation E0CB = X EC 1/2Eg [12,23], where X is the absolute electronegativity of the semiconductor (for V2O5, X is 6.10 eV [25]; for P25 TiO2, X is 5.81 eV [25]; X is unknown for C-doped TiO2). EC is the energy of free electrons on the hydrogen scale (4.5 eV). Eg is the band gap energy of the semiconductor. The calculated positions of CB and VB of V2O5 and P25 are listed in Table 1. It is well known that nonmetal doping doesn’t change the CB position of TiO2 [5,7]. The VB position of C-doped TiO2 can be calculated based on the calculated CB position of TiO2 and Eg of C-doped TiO2, as also shown in Table 1. The schematic electronic band structure, electron–hole generation and separation pathways under visible light irradiation are illustrated in Fig. 4. It can be seen that a heterojunction structure is formed at the interface between V2O5 and C-TiO2. Under visible light irradiation, the charge transfer process includes (A) photoexcitation of an electron from the VB to the CB of C-TiO2 and generation of a hole (B) transition of an electron from CB of C-TiO2 to the CB of V2O5, (C) photoexcitation of an electron from the VB to the CB of V2O5 and generation of a hole, (D) hole transfer from the VB of V2O5 to VB of C-TiO2. Thus, the separation of excited electron–hole pairs on C-TiO2/V2O5 composite is promoted (Fig. 3d), which will greatly enhance the visible light photocatalytic activity [19,20,23]. Fig. 5 shows the apparent reaction rate constant (k) of SiO2/ V2O5, C-TiO2 and C-TiO2/V2O5 with different content of V2O5. SiO2/V2O5 exhibits very low activity, which rules out the possible thermal catalytic activity of V2O5. When the V2O5 content is less than 0.5 wt.%, the activities increase with the increasing of V2O5
content. When the V2O5 content is more than 0.5 wt.%, the activities of samples decrease with further increased of V2O5 content. The optimized loading content of V2O5 is found to be about 0.5 wt.%. The optimal catalyst exhibited about 5 times higher activity than that of C-TiO2. The improved separation of electron–hole pairs on C-TiO2/V2O5 heterojunction is directly responsible for the dramatically enhanced visible light photocatalytic activity [19,20]. Excessive V2O5 contents are detrimental to the photocatalytic degradation efficiency, because the excessive V2O5 particles may cover active sites on C-TiO2 surface, thus decreasing the photocatalytic activity. 4. Conclusion A new C-TiO2/V2O5 composite photocatalyst was prepared by surface loading C-TiO2 with V2O5 through incipient wetness impregnation. The composite photocatalyst exhibited enhanced visible light photocatalytic activity in degradation of gaseous toluene. The heterojunction structure between C-TiO2 and V2O5 was formed. The proposed mechanism showed that the heterojunction could greatly promote the separation of electron–hole pairs, resulting in the improvement of the photocatalytic degradation efficiency. The C-TiO2/V2O5-0.5 wt.% showed the best activity, which was 5 times higher than that of unmodified C-TiO2. Acknowledgement This research was financially supported by the New Century Excellent Scholar Program of Ministry of Education of China (NCET-04-0549) and Hangzhou Science & Technology Development Program (20061133B27). References [1] J.H. Mo, Y.P. Zhang, Q.J. Xu, J.J. Lamson, R.Y. Zhao, Atmos. Environ. 43 (2009) 2229. [2] Z.H. Ai, W.K. Ho, S.C. Lee, L.Z. Zhang, Environ. Sci. Technol. 43 (2009) 4143. [3] S. Sakthivel, H. Kisch, Angew. Chem., Int. Ed. 42 (2003) 4908. [4] X.B. Chen, C. Burda, J. Am. Chem. Soc. 130 (2008) 5018. [5] Y. Huang, W.K. Ho, S.C. Lee, L.Z. Zhang, G.S. Li, J.C. Yu, Langmuir 24 (2008) 3510. [6] Q. Li, R.C. Xie, Y.W. Li, E.A. Mintz, J.K. Shang, Environ. Sci. Technol. 41 (2007) 5050. [7] Z.B. Wu, F. Dong, W.R. Zhao, H.Q. Wang, Y. Liu, B.H. Guan, Nanotechnology 20 (2009) 235701. [8] Q. Li, Y.W. Li, P.G. Wu, R.C. Xie, J.K. Shang, Adv. Mater. 20 (2008) 3717. [9] H. Ozaki, S. Iwamoto, M. Inoue, J. Phys. Chem. C 111 (2007) 17061.
86
Z. Wu et al. / Catalysis Communications 11 (2009) 82–86
[10] T. Morikawa, T. Ohwaki, K.I. Suzuki, S. Moribe, S. Tero-Kubota, Appl. Catal. B 83 (2008) 56. [11] M.C. Long, W.M. Cai, J. Cai, B.X. Zhou, X.Y. Chai, Y.H. Wu, J. Phys. Chem. B 110 (2006) 20211. [12] Q. Xiao, J. Zhang, C. Xiao, X.K. Tan, Catal. Commun. 9 (2008) 1247. [13] T. Arai, M. Yanagida, Y. Konishi, Y. Iwasaki, H. Sugihara, K. Sayama, Catal. Commun. 9 (2008) 1254. [14] J.H. Mo, Y.P. Zhang, Q.J. Xu, Y.F. Zhu, J.J. Lamson, R.Y. Zhao, Appl. Catal. B 89 (2009) 570. [15] J.G. Yu, S.W. Liu, H.G. Yu, J. Catal. 249 (2007) 59. [16] Y.R. Su, J.G. Yu, J. Lin, J. Solid State Chem. 180 (2007) 2080.
[17] [18] [19] [20] [21] [22] [23] [24] [25]
J. Zhang, M.J. Li, Z.C. Feng, J. Chen, C. Li, J. Phys. Chem. B 110 (2006) 927. F. Chen, J. Wang, J.Q. Xu, X.P. Zhou, Appl. Catal. A 348 (2008) 54. H. Wang, X. Quan, H.T. Yu, S. Chen, Carbon 46 (2008) 1126. Z.Y. Wang, B.B. Huang, Y. Dai, X.Y. Qin, X.Y. Zhang, P. Wang, H.X. Liu, J.X. Yu, J. Phys. Chem. C 113 (2009) 4612. M. Heber, W. Gru1nert, J. Phys. Chem. B 104 (2000) 5288. F. Dong, W.R. Zhao, Z.B. Wu, Nanotechnology 19 (2008) 365607. H.Q. Jiang, M. Nagai, K. Kobayashi, J. Alloys Compd. 479 (2009) 821. Y. Cong, J.L. Zhang, F. Chen, M. Anpo, D.N. He, J. Phys. Chem. C 111 (2007) 10618. Y. Xu, M.A.A. Schoonen, Am. Miner. 85 (2000) 543.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Enhancement of the visible light photocatalytic performance of C-doped TiO2 by loading with V2O5 Zhongbiao Wu, Fan Dong, Yue Liu *, Haiqiang Wang Key Laboratory of Polluted Environment Remediation and Ecological Health of Ministry of Education, and College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 9 July 2009 Received in revised form 26 August 2009 Accepted 27 August 2009 Available online 9 September 2009 Keywords: TiO2 Carbon doping Visible light Photocatalyst Heterojunction V2O5
a b s t r a c t V2O5 was loaded on the surface of C-doped TiO2 (C-TiO2) by incipient wetness impregnation in order to enhance the visible light photocatalytic performance. The physicochemical properties of the C-TiO2/ V2O5 composite were characterized by XRD, Raman, TEM, XPS, UV–vis diffuse reflectance spectra, and PL in detail. The result indicated that a heterojunction between C-TiO2 and V2O5 was formed and the separation of excited electron–hole pairs on C-TiO2/V2O5 is greatly promoted. Thus, this composite photocatalyst exhibited enhanced visible light photocatalytic activity in degradation of gas-phase toluene compared with the pristine C-TiO2. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor photocatalysis is developing into one of the most promising technologies for solar energy conversion and environmental applications [1,2]. The leading TiO2 photocatalyst requires ultraviolet (about 3% of solar radiation) irradiation due to its wide band gap (ca. 3.2 eV for anatase TiO2), which seriously restricts its solar efficiency. Nonmetal doping, such as C and N doping, has displayed promising results in shifting the light absorption of TiO2 into visible light region [3–8]. As to the photocatalytic activity, C-doped TiO2 was found more active than N-doped TiO2 under visible light irradiation [3]. However, nonmetal doping intrinsically brings the serious problem of massive charge carrier recombination, which largely limits the visible light photocatalytic activity of nonmetal-doped TiO2 [8]. From the viewpoint of practical application, higher photocatalytic reaction efficiency is required since the photocatalytic efficiency of nonmetal-doped TiO2 under visible light is still low [9,10]. Loading of metal oxide semiconductor onto photocatalyst has been extensively studied as it is proved to enhance the photocatalytic activity due to promoted separation of charge carrier [11–13]. Thus, modification of C-doped TiO2 by further loading metal oxide semiconductor can be considered as an effective approach to * Corresponding author. Tel./fax: +86 571 87953088. E-mail address: [email protected] (Y. Liu). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.08.015
promote the visible light activity. To the best of our knowledge, there are few reports on modification of C-doped TiO2 for the purpose of enhancing the visible light photocatalytic performance. In this research, the C-doped TiO2 prepared by partial oxidation of TiC was further loaded with vanadium oxide through incipient wetness impregnation method followed by thermal treatment. The resulted composite photocatalysts were systematically investigated by various techniques. Based on the characterization results, the effect of vanadium oxide loading on the photocatalytic performance of C-doped TiO2 was discussed. 2. Experimental 2.1. Preparation of catalysts TiC powder (3.0 g) was loaded in a ceramic crucible, and then placed in muffle furnace open to the atmosphere. The temperature was ramped up to 400 °C at a rate of 2 °C/min and kept for 2 h to obtain C-doped TiO2 (C-TiO2). Vanadium oxide loading was performed by incipient wetness impregnation of C-TiO2 with aqueous solutions of NH4VO3, followed by stirring for 1 h and heating at 150 °C for water evaporation. Finally, the catalysts were obtained by treating at 300 °C for 1 h. The amount of loaded vanadium was controlled at 0, 0.01, 0.05, 0.2, 0.5, and 1.0 wt.%. The as-prepared samples were labeled as C-TiO2/V2O5-x, where x represented the content of vanadium. For comparison, V2O5 (vanadium: 0.5%) was loaded on SiO2 by the same process. The resulted sample
Z. Wu et al. / Catalysis Communications 11 (2009) 82–86
was labeled as SiO2/V2O5-0.5%. Pure V2O5 was prepared accordingly in the absence of C-TiO2 or SiO2. 2.2. Characterization The crystal phase was analyzed by X-ray diffraction with Cu Ka radiation (XRD: model D/max RA, Rigaku Co., Japan). Raman spectra were recorded using a micro-Raman spectrometer (Raman: RAMANLOG 6, USA) with a 514.5 nm Ar+ laser as the excitation source. X-ray photoelectron spectroscopy with Al Ka X-rays (hm = 1486.6 eV) radiation (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties. The UV–vis diffuse reflection spectra were obtained for the dry-pressed disk samples using a Scan UV–vis spectrophotometer (UV–vis DRS: TU-1901, China). The photoluminescence spectra were measured with a fluorospectrophotometer (PL: Fluorolog-3-Tau, France) using a Xe lamp as excitation source with optical filter.
83
was conducted with a GC-FID (FULI 9790, China). The procedure of the test has been reported elsewhere [7]. The photocatalytic oxidation of toluene is a pseudo-first-order reaction and its kinetics can be expressed as follows: ln(C0/C) = kt [15], C0 and C are the initial concentration and reaction concentration of toluene, respectively. 3. Results and discussion 3.1. Structure and morphology
Photocatalytic degradation of toluene in air is chosen as the probe reaction to test the activities of the as-prepared sample [14]. The photocatalytic activity tests were performed at room temperature using a 1.8 L photo-reactor. A 150 W Xe lamp with a UVcut optical filter (k < 425 nm) and an IR cutter (k > 800 nm) was placed above the reactor. The analysis of toluene concentration
The XRD patterns of as-prepared samples are shown in Fig. 1a. The phase structure in C-TiO2 and C-TiO2/V2O5 samples consists of anatase phase (JCPDS file no. 21-1272) and rutile (JCPDS, file no. 77-442). In the absence of C-TiO2, pure orthorhombic V2O5 (JSPD file no. 72-433) was generated during catalyst preparation. However, no V2O5 diffraction peaks could be observed in C-TiO2/V2O5, indicating that V2O5 was uniformly dispersed on the surface of CTiO2. Fig. 1a also shows that V2O5 loading has almost no influence on the phase structure of C-TiO2. By using the Debye–Scherrer equation, the crystallite sizes of anatase and rutile phase are calculated to be 19.9 and 30.3 nm, respectively. Raman spectra of C-TiO2, selected C-TiO2/V2O5 and V2O5 samples are shown in Fig. 1b. The observed characteristic Raman bands at 144, 196, 395, 515, and 638 cm 1, assigned to the Eg, B1g, A1g, B2g, and Eg vibrational modes of anatase TiO2 [16]. The Raman bands
Fig. 1. (a) XRD patterns and (b) Raman spectra of C-TiO2, C-TiO2/V2O5 and V2O5 samples (A: anatase, R: rutile).
Fig. 2. (a) TEM images and (b) XPS spectra for V2p of C-TiO2/V2O5-1.0%.
2.3. Tests of photocatalytic activities
84
Z. Wu et al. / Catalysis Communications 11 (2009) 82–86
due to rutile phase appear at 143 (overlapped by 144 cm 1 band of anatase phase), 235, 447, and 612 cm 1, which can be ascribed to the B1g, two-phonon scattering, Eg, and A1g modes of rutile phase, respectively [17]. Raman mode of V2O5 is also shown in Fig. 1b. No Raman bands relevant to V2O5 are observed [18], which is consistent with XRD result. TEM observation in Fig. 2a reveals that the C-TiO2/V2O5-1.0% sample consists of agglomerates of primary particles of 20– 30 nm in diameter. Some V2O5 clusters are found around C-TiO2 and dispersed on the surface of C-TiO2. The HRTEM images (inset) shows that a V2O5 particle is deposited on the surface of C-TiO2. The intimate contact between V2O5 and C-TiO2 favors the formation of heterojunction between the two components. Therefore, the C-TiO2/V2O5 heterojunction is expected to improve the charge separation and thereby the photocatalytic activity [19,20]. Fig. 2b shows the binding energy for V2p3/2 of C-TiO2/V2O51.0%. The V2p3/2 peak can be fitted into two peaks, located at 517.4 and 516.3 eV. They can be assigned to V5+ and V4+, respectively [21], which indicates the oxidation state of vanadium is variable. In XPS measurement, electrons in C-TiO2/V2O5 can be exited by X-rays (hm = 1486.6 eV). The V5+ can accept the generated electrons to produce V4+, indicating the ability of V2O5 to accept photo induced electrons. Besides, the carbon content in C-doped TiO2 is estimated to be 0.56 at.% and have little change after loading with V2O5 according to the XPS result.
3.2. UV–vis DRS and PL Fig. 3a shows UV–vis DRS of V2O5, C-TiO2 and C-TiO2/V2O5, and P25. When the amount of vanadium was less than 0.20 wt.%, no obvious change in visible light absorption was observed compared with C-TiO2. An obvious increase in absorbance of C-TiO2 in the visible light region was observed when the content of vanadium was greater than 0.50 wt.%. The tailing of C-TiO2/V2O5 absorption curves is different from that for pure V2O5. The relation between absorption coefficient (a) and incident photon energy (hm) can be written as ahm = Bd(hm Eg)n for allowed transitions (n = 2 for indirect transition, n = 1/2 direct transition), where Bd is the absorption constants. Plot of (ahm)1/2 and (ahm)2 versus hm from the spectra data of C-TiO2 and V2O5 in Fig. 3a are presented in Fig. 3b and c. The Eg estimated from the intercept of the tangents to the plots is 2.76 and 2.24 eV for C-TiO2 and V2O5, respectively [22,23]. Carbon doping reduces the band gap of TiO2 (3.0 eV for P25) by modifying the valence band [5,7]. The estimated Eg for V2O5 is consistent with literature [23]. Fig. 3d shows the room temperature PL spectra of C-TiO2 and CTiO2/V2O5-0.5%. The PL emission is the result of the recombination of excited electron–hole pairs. The lower PL intensity indicates the decrease in recombination rate, thus higher photocatalytic activity [24]. It can be seen from Fig. 3d that the PL intensity of C-TiO2/ V2O5-0.5% is lower than that of C-TiO2, indicating that V2O5 loaded
Fig. 3. (a) UV–vis DRS of V2O5, C-TiO2 and C-TiO2/V2O5 samples and P25, (b) plot of (ahm)1/2 vs. photon energy of C-TiO2 (indirect semiconductor) and (c) plot of (ahm)2 vs. photon energy of V2O5 (direct semiconductor), (d) PL spectra of C-TiO2 and C-TiO2/V2O5-0.5% (excitation light: 300 nm).
Z. Wu et al. / Catalysis Communications 11 (2009) 82–86
85
Table 1 Absolute electronegativity, calculated CB edge, calculated VB position and band gap energy for P25, C-doped TiO2 and V2O5 at the point of zero charge. Semiconductors
Absolute electronegativity (X) (eV)
Calculated CB position (eV)
Calculated VB position (eV)
Band gap energy Eg (eV)
P25 V2O5 C-doped TiO2
5.81 6.10 –
0.19 0.48 0.19
2.81 2.72 2.57
3.0 2.24 2.76
Fig. 5. The apparent rate constant (k) of as-prepared samples with varying weight ratio of V% under visible light irradiation, (a) SiO2/V2O5 sample, (b) C-TiO2, (c–g) C-TiO2/V2O5 samples.
Fig. 4. Schematic electronic band structure of C-TiO2 (2.76 eV) loaded with V2O5 (2.44 eV) and charge transfer process under visible light irradiation.
on C-TiO2 surface (inset in Fig. 2a) can enhance the separation of electron–hole pairs. 3.3. Photocatalyic activities and mechanism To elucidate the effect of V2O5 loading on the activity of C-doped TiO2, visible light photocatalytic degradation of gaseous toluene were performed. The band edge positions of conduction band (CB) and valence band (VB) of semiconductor can be determined by the following approach. The CB edge (E0CB ) of a semiconductor at the point of zero charge (pHZPC) can be predicted by the equation E0CB = X EC 1/2Eg [12,23], where X is the absolute electronegativity of the semiconductor (for V2O5, X is 6.10 eV [25]; for P25 TiO2, X is 5.81 eV [25]; X is unknown for C-doped TiO2). EC is the energy of free electrons on the hydrogen scale (4.5 eV). Eg is the band gap energy of the semiconductor. The calculated positions of CB and VB of V2O5 and P25 are listed in Table 1. It is well known that nonmetal doping doesn’t change the CB position of TiO2 [5,7]. The VB position of C-doped TiO2 can be calculated based on the calculated CB position of TiO2 and Eg of C-doped TiO2, as also shown in Table 1. The schematic electronic band structure, electron–hole generation and separation pathways under visible light irradiation are illustrated in Fig. 4. It can be seen that a heterojunction structure is formed at the interface between V2O5 and C-TiO2. Under visible light irradiation, the charge transfer process includes (A) photoexcitation of an electron from the VB to the CB of C-TiO2 and generation of a hole (B) transition of an electron from CB of C-TiO2 to the CB of V2O5, (C) photoexcitation of an electron from the VB to the CB of V2O5 and generation of a hole, (D) hole transfer from the VB of V2O5 to VB of C-TiO2. Thus, the separation of excited electron–hole pairs on C-TiO2/V2O5 composite is promoted (Fig. 3d), which will greatly enhance the visible light photocatalytic activity [19,20,23]. Fig. 5 shows the apparent reaction rate constant (k) of SiO2/ V2O5, C-TiO2 and C-TiO2/V2O5 with different content of V2O5. SiO2/V2O5 exhibits very low activity, which rules out the possible thermal catalytic activity of V2O5. When the V2O5 content is less than 0.5 wt.%, the activities increase with the increasing of V2O5
content. When the V2O5 content is more than 0.5 wt.%, the activities of samples decrease with further increased of V2O5 content. The optimized loading content of V2O5 is found to be about 0.5 wt.%. The optimal catalyst exhibited about 5 times higher activity than that of C-TiO2. The improved separation of electron–hole pairs on C-TiO2/V2O5 heterojunction is directly responsible for the dramatically enhanced visible light photocatalytic activity [19,20]. Excessive V2O5 contents are detrimental to the photocatalytic degradation efficiency, because the excessive V2O5 particles may cover active sites on C-TiO2 surface, thus decreasing the photocatalytic activity. 4. Conclusion A new C-TiO2/V2O5 composite photocatalyst was prepared by surface loading C-TiO2 with V2O5 through incipient wetness impregnation. The composite photocatalyst exhibited enhanced visible light photocatalytic activity in degradation of gaseous toluene. The heterojunction structure between C-TiO2 and V2O5 was formed. The proposed mechanism showed that the heterojunction could greatly promote the separation of electron–hole pairs, resulting in the improvement of the photocatalytic degradation efficiency. The C-TiO2/V2O5-0.5 wt.% showed the best activity, which was 5 times higher than that of unmodified C-TiO2. Acknowledgement This research was financially supported by the New Century Excellent Scholar Program of Ministry of Education of China (NCET-04-0549) and Hangzhou Science & Technology Development Program (20061133B27). References [1] J.H. Mo, Y.P. Zhang, Q.J. Xu, J.J. Lamson, R.Y. Zhao, Atmos. Environ. 43 (2009) 2229. [2] Z.H. Ai, W.K. Ho, S.C. Lee, L.Z. Zhang, Environ. Sci. Technol. 43 (2009) 4143. [3] S. Sakthivel, H. Kisch, Angew. Chem., Int. Ed. 42 (2003) 4908. [4] X.B. Chen, C. Burda, J. Am. Chem. Soc. 130 (2008) 5018. [5] Y. Huang, W.K. Ho, S.C. Lee, L.Z. Zhang, G.S. Li, J.C. Yu, Langmuir 24 (2008) 3510. [6] Q. Li, R.C. Xie, Y.W. Li, E.A. Mintz, J.K. Shang, Environ. Sci. Technol. 41 (2007) 5050. [7] Z.B. Wu, F. Dong, W.R. Zhao, H.Q. Wang, Y. Liu, B.H. Guan, Nanotechnology 20 (2009) 235701. [8] Q. Li, Y.W. Li, P.G. Wu, R.C. Xie, J.K. Shang, Adv. Mater. 20 (2008) 3717. [9] H. Ozaki, S. Iwamoto, M. Inoue, J. Phys. Chem. C 111 (2007) 17061.
86
Z. Wu et al. / Catalysis Communications 11 (2009) 82–86
[10] T. Morikawa, T. Ohwaki, K.I. Suzuki, S. Moribe, S. Tero-Kubota, Appl. Catal. B 83 (2008) 56. [11] M.C. Long, W.M. Cai, J. Cai, B.X. Zhou, X.Y. Chai, Y.H. Wu, J. Phys. Chem. B 110 (2006) 20211. [12] Q. Xiao, J. Zhang, C. Xiao, X.K. Tan, Catal. Commun. 9 (2008) 1247. [13] T. Arai, M. Yanagida, Y. Konishi, Y. Iwasaki, H. Sugihara, K. Sayama, Catal. Commun. 9 (2008) 1254. [14] J.H. Mo, Y.P. Zhang, Q.J. Xu, Y.F. Zhu, J.J. Lamson, R.Y. Zhao, Appl. Catal. B 89 (2009) 570. [15] J.G. Yu, S.W. Liu, H.G. Yu, J. Catal. 249 (2007) 59. [16] Y.R. Su, J.G. Yu, J. Lin, J. Solid State Chem. 180 (2007) 2080.
[17] [18] [19] [20] [21] [22] [23] [24] [25]
J. Zhang, M.J. Li, Z.C. Feng, J. Chen, C. Li, J. Phys. Chem. B 110 (2006) 927. F. Chen, J. Wang, J.Q. Xu, X.P. Zhou, Appl. Catal. A 348 (2008) 54. H. Wang, X. Quan, H.T. Yu, S. Chen, Carbon 46 (2008) 1126. Z.Y. Wang, B.B. Huang, Y. Dai, X.Y. Qin, X.Y. Zhang, P. Wang, H.X. Liu, J.X. Yu, J. Phys. Chem. C 113 (2009) 4612. M. Heber, W. Gru1nert, J. Phys. Chem. B 104 (2000) 5288. F. Dong, W.R. Zhao, Z.B. Wu, Nanotechnology 19 (2008) 365607. H.Q. Jiang, M. Nagai, K. Kobayashi, J. Alloys Compd. 479 (2009) 821. Y. Cong, J.L. Zhang, F. Chen, M. Anpo, D.N. He, J. Phys. Chem. C 111 (2007) 10618. Y. Xu, M.A.A. Schoonen, Am. Miner. 85 (2000) 543.