- Email: [email protected]
TiO2 double-layered films
Materials Research Bulletin 42 (2007) 1402–1406 www.elsevier.com/locate/matresbu
Photoelectrochemical property of ZnFe2O4/TiO2 double-layered films Jing Yin, Li-Jian Bie, Zhi-Hao Yuan * Tianjin University of Technology, Tianjin 300191, China Received 27 March 2006; received in revised form 30 September 2006; accepted 7 November 2006 Available online 15 December 2006
Abstract ZnFe2O4/TiO2 double-layered films on indium-tin oxide (ITO) substrate were prepared by a dip-coating method, and the optical absorption and photocurrent of the as-prepared films were measured. In the double-layered films, the onset of fundamental absorption edge shifts to a longer wavelength, and even shifts to a longer wavelength than that of ZnFe2O4-only film as the ZnFe2O4 layer thickness increases. Application of the coupled photoanodes double-layered films composed of ZnFe2O4 and TiO2 can obviously increase the photocurrent. It was found that the photocurrent density of ZnFe2O4/TiO2 double-layered films first increased and then decreased with increasing the ZnFe2O4 layer thickness. A five-fold increase in the photocurrent density was obtained compared with TiO2-only films under optimum condition. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Multilayers; A. Nanostructures; B. Electrochemical properties
1. Introduction The applications of TiO2-based nanomaterials in photocatalysis [1,2] and photoelectrochemical conversion [3,4] have been extensively studied in the past decades owing to its excellent (photo)chemical stability, low cost and nontoxicity [1,2,5,6]. Titanium oxide, with a wide band-gap (anatase 3.2 eV, rutile 3.0 eV), is a UV absorber, utilizes only a very small fraction of the solar spectrum (<5%). There have been some successful efforts in improving the efficiency of utilizing solar energy by organic dye sensitization [7–9], especially a trimeric ruthenium complex dye, for which an efficiency of 12% was reported by Graetzel and co-worker [7]. However, such practical applications are limited by their thermal and photochemical stability [10,11]. There have also been some attempts [12–15] to extending the photoresponse of TiO2 toward visible range by using a coupled semiconductor system consisting of TiO2 and another semiconductor with relatively small band-gap energy, i.e. CdS, CdSe, FeS2, RuS2. Unfortunately, these sulfides and selenides are both sensitive to photoanodic corrosion. Recently, an inorganic semiconductor with a relatively small band-gap [16] (ca. 1.86 eV), ZnFe2O4, has attracted attention in photoelectric conversion and photochemical hydrogen production from water due to its utilizing visible light and good photochemical stability [17–21]. A nanocomposite composed of ZnFe2O4 and TiO2 might exhibit some useful characteristics of suitable applications in photocatalysis and photoelectric conversion. In previous publication,
* Corresponding author. Tel.: +86 22 23677503; fax: +86 22 23677503. E-mail address: [email protected] (Z.-H. Yuan). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.11.009
J. Yin et al. / Materials Research Bulletin 42 (2007) 1402–1406
1403
we reported that the ZnFe2O4/TiO2 nanocomposite is a photocatalyst more effective than TiO2-only nanomaterials [22,23]. In this letter, we demonstrate that the photocurrent is greatly enhanced by using double layered films composed of ZnFe2O4 and TiO2 as a coupled photoanode. 2. Experimental A dip-coating technique from colloids was adopted to prepare the ZnFe2O4/TiO2 double-layered films. First, the ZnFe2O4 and TiO2 nanoparticles were prepared by the co-precipitation and controlled hydrolysis methods, respectively, details of the methods can be found elsewhere [24,25]. Briefly, ZnFe2O4 was coprecipitated from a mixed solution of 0.1 mol dm 3 Zn(NO3)2 and 0.2 mol dm 3 Fe(NO3)3 at a pH value of 13 and at a temperature of 100 8C, and TiO2 was precipitated from a mixed solution with a volume ratio of Ti(OC4H9)4:C2H5OH:H2O = 1:10:100 at pH 2 under vigorous stirring. Both the products were filtered and washed with deionized water in sequence. The particle sizes of ZnFe2O4 and TiO2 are 2–3 and 3–5 nm, respectively. Then, the ZnFe2O4 or TiO2 nanoparticles were introduced into 0.01 mol dm 3 of dodecyl benzene sulfonic acid (DBS) under stirring, extracted into toluene, refluxed for 1 h, washed with deionized water several times, and distilled to remove the residual water in order to obtain the ZnFe2O4 or TiO2 colloidal toluene sol [22,26,27]. The TiO2 coating was prepared on ITO substrate by dip-coating from its colloidal toluene sol at 30 8C. The dipping was performed in an equipment of Type 3033 X-Y Recorder with a dip rate of 2.5 mm/min. The freshly coated films were dried at 100 8C, followed by heat-treating at 400 8C in air to remove the organic residues. This cycle was repeated until a desired thickness was obtained. The resultant TiO2 film was then dip-coated in the ZnFe2O4 colloidal toluene sol, repeating the process similar to TiO2 film preparation, to form ZnFe2O4/TiO2 multilayer films on ITO substrate. For a comparison, the ZnFe2O4-only and TiO2-only films were also prepared by repeating the process described above. Finally, the all coated films were annealed at 450 8C in nitrogen ambience. The optical absorption of the ZnFe2O4-only, TiO2-only and ZnFe2O4/TiO2 films were measured on Carry-5E UV– vis spectrometer. The film thickness was measured from scanning electron micrograph (JSM-6301F) section observation. The ZnFe2O4/TiO2 double-layered films were scraped out from the coated substrates, dispersed in ethanol and characterized by using a JEOL JEM-200CX transmission electron microscope (TEM). The photoelectrochemical measurement was performed in a three-electrode-photoelectrochemical cell with the ZnFe2O4-only, TiO2-only or ZnFe2O4/TiO2 films as photoanode, platinum electrode as counter-electrode, and saturated calomel electrode as reference electrode, using a 200 W Xenon lamp as light source and an electrolyte of 0.l mol l 1 NaOH solution. Current–voltage (I–V) measurement was carried out using a HPD-A potentiostat with a scan voltage ranging from 2.0 to 2.0 V. 3. Results and discussion The optical absorption spectra is shown in Fig. 1, curve I and II correspond to the 5-times TiO2-only (dip-coating in the sol five times, using 5X for abbreviation) and ZnFe2O4-only films (the corresponding film thickness are 0.52 and 0.43 mm, respectively), a, b and c are curves of the ZnFe2O4/TiO2 double-layered films with 5-times TiO2 plus 1 to 3times ZnFe2O4 films (the corresponding film thickness are 0.63, 0.71 and 0.78 mm, respectively), respectively. As can be seen from Fig. 1 that the onsets of fundamental absorption edges of the ZnFe2O4/TiO2 double-layered films all lie in visible region, which may be beneficial to extend the photoresponse of TiO2 toward visible range, and present a significant red-shift with the increase of ZnFe2O4 layer thickness in the double-layered films. It is also noted that, when the layer-number of ZnFe2O4 is more than 2, the onset of fundamental absorption edge shifts to a longer wavelength than that of ZnFe2O4-only film. This might be attributed to surface and interface effect, which can induce a red-shift, or to a new phase with a relatively small band-gap energy resulted from reaction between TiO2 and ZnFe2O4 particles. As can be seen from TEM in Fig. 2, The double-layered film is composed of tiny particles with a particle size of 3–5 nm in diameter, some particles prefer an oriented congregation. The corresponding selected area diffraction (SAD) shows that the double-layered film contains two crystal phases: spinel structure of ZnFe2O4 and anatase structure of TiO2. The I–V characteristics of theTiO2-only and ZnFe2O4/TiO2 double-layered films are shown in Fig. 3. In the dark, only cathodic current is seen. Under light excitation, theTiO2-only and ZnFe2O4/TiO2 double-layered films all exhibit an anodic photocurrent, which is a characteristic behaviour of an n-type semiconductor. The photocurrent increases
1404
J. Yin et al. / Materials Research Bulletin 42 (2007) 1402–1406
Fig. 1. Optical absorption spectra of TiO2, ZnFe2O4 and ZnFe2O4/TiO2 films.
with the applied bias at low potentials until saturation. When the applied bias exceeds 1.3 V, the photocurrent increase dramatically, which is due to the electrolysis of water. It is seen from Fig. 3 that application of double-layered films composed of ZnFe2O4 and TiO2 as a coupled photoanode obviously increases the photocurrent compared with TiO2-only and ZnFe2O4-only films. For example, when the surface of the TiO2 nanoparticles is covered with the 2-times ZnFe2O4 film, the photocurrent density in the saturation region increases to about 0.1 mA cm 2 from 0.02 mA cm 2, which almost is a five-fold increase (see Fig. 3b and e). However, it is also noted from Fig. 3d, e and h, that a further increase of the dip-coating times of ZnFe2O4 film gives a decrease photocurrent. Thus, to achieve a high photocurrent density, it is necessary to control the film thickness. Then, what should be responsible for the improved photocurrent? It is well-known that the TiO2 (3.2 eV) can only be excited by UV light, while the ZnFe2O4 nanomaterials with a relatively smaller band-gap energy (ca. 1.86 eV) can be done by visible light, which should allow it to utilize the main part of the irradiation used in our experiment (Xenon lamp). In the double-layered films, a coupled effect might exist between the energy bands of ZnFe2O4 and TiO2 due to
Fig. 2. TEM micrograph of ZnFe2O4/TiO2 double-layered films with 5XTiO2 + 3XZnFe2O4 (represent that this film was prepared by dip-coating the substrate from TiO2 colloidal toluene sol 5 times, then coating with ZnFe2O4 sol 3 times), inserted SAD pattern shows that the double-layered film contains two crystal phases: spinel structure of ZnFe2O4 and anatase structure of TiO2.
J. Yin et al. / Materials Research Bulletin 42 (2007) 1402–1406
1405
Fig. 3. Photocurrent density vs. applied bias (vs. SCE) for the TiO2 and ZnFe2O4/TiO2 double-layered films.
Fig. 4. Schematic diagram of energy-band coupling between ZnFe2O4 and TiO2 nanoparticles.
some differences in their band-gap positions [22]. The photogenerated electrons produced by the ZnFe2O4 nanoparticles can migrate to the TiO2 nanoparticle (as shown in Fig. 4), which is similar to CdS/TiO2 system [28]. Such a coupled semiconductor system can improve the efficiency of light energy utilizing and augment the concentration of the photogenerated carriers, and thus greatly increase the photocurrent. This is further supported by the fact that the photocatalytic activity of TiO2 can be greatly improved by adding of ZnFe2O4 or AlFeO3 nanoparticles to TiO2 matrix [22,29]. As far as the decrease effect on the photocurrent of TiO2 with the dip-coating of ZnFe2O4 excess two layers is concerned, it may be attributed to a competition between gain in absorbance and photogenerated electron/hole recombination. Although an increase of the ZnFe2O4 layer thickness can improve the absorbance of the irradiation produced by the ZnFe2O4, it can increase the distance that the photogenerated carries migrate to film surface, and thus enhance the recombination of the photogenerated carries, which is detrimental to the improved photocurrent. 4. Conclusion A coupled semiconductor system composed of ZnFe2O4 and TiO2 double-layered film is prepared by dip-coating technique from their colloidal solutions, and a remarkable red-shift of the fundamental absorption edge is observed with the increase of the ZnFe2O4 layer thickness in the ZnFe2O4/TiO2 multilayered films. Application of such a coupled ZnFe2O4/TiO2 double-layered film as photoanode can extend the photoresponse of TiO2 toward visible light
1406
J. Yin et al. / Materials Research Bulletin 42 (2007) 1402–1406
range, and greatly increase the photocurrent density. Furthermore, the thickness of ZnFe2O4 layer coated on the TiO2 has a complicated influence on the photocurrent. When the ZnFe2O4 layer increases from 1 to 2, the photocurrent density rises rapidly. However, a further increase the layer-number of ZnFe2O4 results in decreasing the photocurrent. The present results show that the ZnFe2O4/TiO2 multilayered films may be a promising solar energy materials for the applications in solar energy cell and photoelectrochemical cell as well as in photocatalytic decomposition of organic contaminants. Acknowledgement Thanks for the financial support from the National Natural Science Foundation of China under Grant No. 20671070 and 20571055. Reference [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
A. Mills, R.H. Davies, D. Worsley, Chem. Soc. Rev. 22 (1993) 417. N. Serpone, Solar Energy Mater. Solar Cells 38 (1995) 369. A. Fujishima, K. Honda, Nature 238 (1972) 37. R. Gruwald, H. Tributsch, J. Phys. Chem. B 101 (1997) 2564. D.F. Olis, E. Pelizzetti, N. Serpone, Environ. Soc. Technol. 25 (1991) 1523. T. Asahi, T. Morikawa, K. Ohwaki, Y. Aoki, Science 293 (2001) 269. B. O’Regan, M. Graetzel, Nature 353 (1991) 737. H. Ross, J. Bending, S. Hecht, Solar Energy Mater. Solar Cells 33 (1994) 475. R. Grunwald, H. Tributsch, J. Phys. Chem. B 101 (1997) 5552. Q. Qu, J.C. Zhao, T. Sheu, J. Mol. Catal. A: Chem. 129 (1998) 257. K. Vinodgopal, D.E. Wynkop, P.V. Kamat, Environ. Sci. Technol. 30 (1996) 1660. R. Vogel, P. Hoye, H. Weller, Chem. Phys. Lett. 174 (1990) 241. D. Liu, P.V. Kamat, J. Phys. Chem. 97 (1993) 10769. A. Ennaoui, S. Fiechter, H. Tributsch, M. Giersig, et al. J. Electrochem. Soc. 139 (1992) 2514. M. Ashokkumar, A. Kndo, N. Saito, T. Sakata, Chem. Phys. Lett. 299 (1994) 383. Z.H. Yuan, W. You, J.H. Jia, L.D. Zhang, Chin. Phys. Lett. 15 (1998) 535. J. Qiu, C. Wang, M. Gu, Mater. Sci. Eng. B 112 (2004) 1. G.X. Lu, S.B. Li, Int. Hydrogen Energy 17 (1992) 767. M.A. Valenzuela, P. Bosch, J. Jime´nez-Becerrill, O. Quiroz, A.I. Pa´ez, J. Photochem. Photobio. A: Chem. 148 (2002) 177. J.J. Liu, G.X. Lu, H.L. He, H. Tan, T. Xu, et al. Mater. Res. Bull. 31 (1996) 1049. X.Y. Li, S.B. Li, G.X. Lie, J. Mol. Catal. 31 (1996) 187 (in Chinese). Z.H. Yuan, L.D. Zhang, J. Mater. Chem. 11 (2001) 1256. P. Cheng, W. Li, T. Zhou, Y. Jin, M. Gu, J. Photochem. Photobiol. A: Chem. 168 (2004) 97. T. Sato, K. Haneda, M. Seki, T. Iijima, Appl. Phys. A 50 (1990) 13. D. Bahnemann, A. Henglein, J. Lilie, L. Spanhel, J. Phys. Chem. 88 (1984) 709. Z.H. Yuan, L.D. Zhang, Mater. Res. Bull. 33 (1998) 1587. B.S. Zhou, L.Z. Xiao, T.J. Li, J.L. Zhao, et al. Appl. Phys. Lett. 59 (1991) 1826. A. Schafani, M.N. Mozzanaga, P. Pichat, J. Photochem. Photobiol. A: Chem. 59 (1991) 181. Z.H. Yuan, Y.H. Wang, Y.C. Sun, J. Wang, L.J. Bie, Y.Q. Duan, Sci. China B: Chem. 49 (2006) 67.
Photoelectrochemical property of ZnFe2O4/TiO2 double-layered films Jing Yin, Li-Jian Bie, Zhi-Hao Yuan * Tianjin University of Technology, Tianjin 300191, China Received 27 March 2006; received in revised form 30 September 2006; accepted 7 November 2006 Available online 15 December 2006
Abstract ZnFe2O4/TiO2 double-layered films on indium-tin oxide (ITO) substrate were prepared by a dip-coating method, and the optical absorption and photocurrent of the as-prepared films were measured. In the double-layered films, the onset of fundamental absorption edge shifts to a longer wavelength, and even shifts to a longer wavelength than that of ZnFe2O4-only film as the ZnFe2O4 layer thickness increases. Application of the coupled photoanodes double-layered films composed of ZnFe2O4 and TiO2 can obviously increase the photocurrent. It was found that the photocurrent density of ZnFe2O4/TiO2 double-layered films first increased and then decreased with increasing the ZnFe2O4 layer thickness. A five-fold increase in the photocurrent density was obtained compared with TiO2-only films under optimum condition. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Multilayers; A. Nanostructures; B. Electrochemical properties
1. Introduction The applications of TiO2-based nanomaterials in photocatalysis [1,2] and photoelectrochemical conversion [3,4] have been extensively studied in the past decades owing to its excellent (photo)chemical stability, low cost and nontoxicity [1,2,5,6]. Titanium oxide, with a wide band-gap (anatase 3.2 eV, rutile 3.0 eV), is a UV absorber, utilizes only a very small fraction of the solar spectrum (<5%). There have been some successful efforts in improving the efficiency of utilizing solar energy by organic dye sensitization [7–9], especially a trimeric ruthenium complex dye, for which an efficiency of 12% was reported by Graetzel and co-worker [7]. However, such practical applications are limited by their thermal and photochemical stability [10,11]. There have also been some attempts [12–15] to extending the photoresponse of TiO2 toward visible range by using a coupled semiconductor system consisting of TiO2 and another semiconductor with relatively small band-gap energy, i.e. CdS, CdSe, FeS2, RuS2. Unfortunately, these sulfides and selenides are both sensitive to photoanodic corrosion. Recently, an inorganic semiconductor with a relatively small band-gap [16] (ca. 1.86 eV), ZnFe2O4, has attracted attention in photoelectric conversion and photochemical hydrogen production from water due to its utilizing visible light and good photochemical stability [17–21]. A nanocomposite composed of ZnFe2O4 and TiO2 might exhibit some useful characteristics of suitable applications in photocatalysis and photoelectric conversion. In previous publication,
* Corresponding author. Tel.: +86 22 23677503; fax: +86 22 23677503. E-mail address: [email protected] (Z.-H. Yuan). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.11.009
J. Yin et al. / Materials Research Bulletin 42 (2007) 1402–1406
1403
we reported that the ZnFe2O4/TiO2 nanocomposite is a photocatalyst more effective than TiO2-only nanomaterials [22,23]. In this letter, we demonstrate that the photocurrent is greatly enhanced by using double layered films composed of ZnFe2O4 and TiO2 as a coupled photoanode. 2. Experimental A dip-coating technique from colloids was adopted to prepare the ZnFe2O4/TiO2 double-layered films. First, the ZnFe2O4 and TiO2 nanoparticles were prepared by the co-precipitation and controlled hydrolysis methods, respectively, details of the methods can be found elsewhere [24,25]. Briefly, ZnFe2O4 was coprecipitated from a mixed solution of 0.1 mol dm 3 Zn(NO3)2 and 0.2 mol dm 3 Fe(NO3)3 at a pH value of 13 and at a temperature of 100 8C, and TiO2 was precipitated from a mixed solution with a volume ratio of Ti(OC4H9)4:C2H5OH:H2O = 1:10:100 at pH 2 under vigorous stirring. Both the products were filtered and washed with deionized water in sequence. The particle sizes of ZnFe2O4 and TiO2 are 2–3 and 3–5 nm, respectively. Then, the ZnFe2O4 or TiO2 nanoparticles were introduced into 0.01 mol dm 3 of dodecyl benzene sulfonic acid (DBS) under stirring, extracted into toluene, refluxed for 1 h, washed with deionized water several times, and distilled to remove the residual water in order to obtain the ZnFe2O4 or TiO2 colloidal toluene sol [22,26,27]. The TiO2 coating was prepared on ITO substrate by dip-coating from its colloidal toluene sol at 30 8C. The dipping was performed in an equipment of Type 3033 X-Y Recorder with a dip rate of 2.5 mm/min. The freshly coated films were dried at 100 8C, followed by heat-treating at 400 8C in air to remove the organic residues. This cycle was repeated until a desired thickness was obtained. The resultant TiO2 film was then dip-coated in the ZnFe2O4 colloidal toluene sol, repeating the process similar to TiO2 film preparation, to form ZnFe2O4/TiO2 multilayer films on ITO substrate. For a comparison, the ZnFe2O4-only and TiO2-only films were also prepared by repeating the process described above. Finally, the all coated films were annealed at 450 8C in nitrogen ambience. The optical absorption of the ZnFe2O4-only, TiO2-only and ZnFe2O4/TiO2 films were measured on Carry-5E UV– vis spectrometer. The film thickness was measured from scanning electron micrograph (JSM-6301F) section observation. The ZnFe2O4/TiO2 double-layered films were scraped out from the coated substrates, dispersed in ethanol and characterized by using a JEOL JEM-200CX transmission electron microscope (TEM). The photoelectrochemical measurement was performed in a three-electrode-photoelectrochemical cell with the ZnFe2O4-only, TiO2-only or ZnFe2O4/TiO2 films as photoanode, platinum electrode as counter-electrode, and saturated calomel electrode as reference electrode, using a 200 W Xenon lamp as light source and an electrolyte of 0.l mol l 1 NaOH solution. Current–voltage (I–V) measurement was carried out using a HPD-A potentiostat with a scan voltage ranging from 2.0 to 2.0 V. 3. Results and discussion The optical absorption spectra is shown in Fig. 1, curve I and II correspond to the 5-times TiO2-only (dip-coating in the sol five times, using 5X for abbreviation) and ZnFe2O4-only films (the corresponding film thickness are 0.52 and 0.43 mm, respectively), a, b and c are curves of the ZnFe2O4/TiO2 double-layered films with 5-times TiO2 plus 1 to 3times ZnFe2O4 films (the corresponding film thickness are 0.63, 0.71 and 0.78 mm, respectively), respectively. As can be seen from Fig. 1 that the onsets of fundamental absorption edges of the ZnFe2O4/TiO2 double-layered films all lie in visible region, which may be beneficial to extend the photoresponse of TiO2 toward visible range, and present a significant red-shift with the increase of ZnFe2O4 layer thickness in the double-layered films. It is also noted that, when the layer-number of ZnFe2O4 is more than 2, the onset of fundamental absorption edge shifts to a longer wavelength than that of ZnFe2O4-only film. This might be attributed to surface and interface effect, which can induce a red-shift, or to a new phase with a relatively small band-gap energy resulted from reaction between TiO2 and ZnFe2O4 particles. As can be seen from TEM in Fig. 2, The double-layered film is composed of tiny particles with a particle size of 3–5 nm in diameter, some particles prefer an oriented congregation. The corresponding selected area diffraction (SAD) shows that the double-layered film contains two crystal phases: spinel structure of ZnFe2O4 and anatase structure of TiO2. The I–V characteristics of theTiO2-only and ZnFe2O4/TiO2 double-layered films are shown in Fig. 3. In the dark, only cathodic current is seen. Under light excitation, theTiO2-only and ZnFe2O4/TiO2 double-layered films all exhibit an anodic photocurrent, which is a characteristic behaviour of an n-type semiconductor. The photocurrent increases
1404
J. Yin et al. / Materials Research Bulletin 42 (2007) 1402–1406
Fig. 1. Optical absorption spectra of TiO2, ZnFe2O4 and ZnFe2O4/TiO2 films.
with the applied bias at low potentials until saturation. When the applied bias exceeds 1.3 V, the photocurrent increase dramatically, which is due to the electrolysis of water. It is seen from Fig. 3 that application of double-layered films composed of ZnFe2O4 and TiO2 as a coupled photoanode obviously increases the photocurrent compared with TiO2-only and ZnFe2O4-only films. For example, when the surface of the TiO2 nanoparticles is covered with the 2-times ZnFe2O4 film, the photocurrent density in the saturation region increases to about 0.1 mA cm 2 from 0.02 mA cm 2, which almost is a five-fold increase (see Fig. 3b and e). However, it is also noted from Fig. 3d, e and h, that a further increase of the dip-coating times of ZnFe2O4 film gives a decrease photocurrent. Thus, to achieve a high photocurrent density, it is necessary to control the film thickness. Then, what should be responsible for the improved photocurrent? It is well-known that the TiO2 (3.2 eV) can only be excited by UV light, while the ZnFe2O4 nanomaterials with a relatively smaller band-gap energy (ca. 1.86 eV) can be done by visible light, which should allow it to utilize the main part of the irradiation used in our experiment (Xenon lamp). In the double-layered films, a coupled effect might exist between the energy bands of ZnFe2O4 and TiO2 due to
Fig. 2. TEM micrograph of ZnFe2O4/TiO2 double-layered films with 5XTiO2 + 3XZnFe2O4 (represent that this film was prepared by dip-coating the substrate from TiO2 colloidal toluene sol 5 times, then coating with ZnFe2O4 sol 3 times), inserted SAD pattern shows that the double-layered film contains two crystal phases: spinel structure of ZnFe2O4 and anatase structure of TiO2.
J. Yin et al. / Materials Research Bulletin 42 (2007) 1402–1406
1405
Fig. 3. Photocurrent density vs. applied bias (vs. SCE) for the TiO2 and ZnFe2O4/TiO2 double-layered films.
Fig. 4. Schematic diagram of energy-band coupling between ZnFe2O4 and TiO2 nanoparticles.
some differences in their band-gap positions [22]. The photogenerated electrons produced by the ZnFe2O4 nanoparticles can migrate to the TiO2 nanoparticle (as shown in Fig. 4), which is similar to CdS/TiO2 system [28]. Such a coupled semiconductor system can improve the efficiency of light energy utilizing and augment the concentration of the photogenerated carriers, and thus greatly increase the photocurrent. This is further supported by the fact that the photocatalytic activity of TiO2 can be greatly improved by adding of ZnFe2O4 or AlFeO3 nanoparticles to TiO2 matrix [22,29]. As far as the decrease effect on the photocurrent of TiO2 with the dip-coating of ZnFe2O4 excess two layers is concerned, it may be attributed to a competition between gain in absorbance and photogenerated electron/hole recombination. Although an increase of the ZnFe2O4 layer thickness can improve the absorbance of the irradiation produced by the ZnFe2O4, it can increase the distance that the photogenerated carries migrate to film surface, and thus enhance the recombination of the photogenerated carries, which is detrimental to the improved photocurrent. 4. Conclusion A coupled semiconductor system composed of ZnFe2O4 and TiO2 double-layered film is prepared by dip-coating technique from their colloidal solutions, and a remarkable red-shift of the fundamental absorption edge is observed with the increase of the ZnFe2O4 layer thickness in the ZnFe2O4/TiO2 multilayered films. Application of such a coupled ZnFe2O4/TiO2 double-layered film as photoanode can extend the photoresponse of TiO2 toward visible light
1406
J. Yin et al. / Materials Research Bulletin 42 (2007) 1402–1406
range, and greatly increase the photocurrent density. Furthermore, the thickness of ZnFe2O4 layer coated on the TiO2 has a complicated influence on the photocurrent. When the ZnFe2O4 layer increases from 1 to 2, the photocurrent density rises rapidly. However, a further increase the layer-number of ZnFe2O4 results in decreasing the photocurrent. The present results show that the ZnFe2O4/TiO2 multilayered films may be a promising solar energy materials for the applications in solar energy cell and photoelectrochemical cell as well as in photocatalytic decomposition of organic contaminants. Acknowledgement Thanks for the financial support from the National Natural Science Foundation of China under Grant No. 20671070 and 20571055. Reference [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
A. Mills, R.H. Davies, D. Worsley, Chem. Soc. Rev. 22 (1993) 417. N. Serpone, Solar Energy Mater. Solar Cells 38 (1995) 369. A. Fujishima, K. Honda, Nature 238 (1972) 37. R. Gruwald, H. Tributsch, J. Phys. Chem. B 101 (1997) 2564. D.F. Olis, E. Pelizzetti, N. Serpone, Environ. Soc. Technol. 25 (1991) 1523. T. Asahi, T. Morikawa, K. Ohwaki, Y. Aoki, Science 293 (2001) 269. B. O’Regan, M. Graetzel, Nature 353 (1991) 737. H. Ross, J. Bending, S. Hecht, Solar Energy Mater. Solar Cells 33 (1994) 475. R. Grunwald, H. Tributsch, J. Phys. Chem. B 101 (1997) 5552. Q. Qu, J.C. Zhao, T. Sheu, J. Mol. Catal. A: Chem. 129 (1998) 257. K. Vinodgopal, D.E. Wynkop, P.V. Kamat, Environ. Sci. Technol. 30 (1996) 1660. R. Vogel, P. Hoye, H. Weller, Chem. Phys. Lett. 174 (1990) 241. D. Liu, P.V. Kamat, J. Phys. Chem. 97 (1993) 10769. A. Ennaoui, S. Fiechter, H. Tributsch, M. Giersig, et al. J. Electrochem. Soc. 139 (1992) 2514. M. Ashokkumar, A. Kndo, N. Saito, T. Sakata, Chem. Phys. Lett. 299 (1994) 383. Z.H. Yuan, W. You, J.H. Jia, L.D. Zhang, Chin. Phys. Lett. 15 (1998) 535. J. Qiu, C. Wang, M. Gu, Mater. Sci. Eng. B 112 (2004) 1. G.X. Lu, S.B. Li, Int. Hydrogen Energy 17 (1992) 767. M.A. Valenzuela, P. Bosch, J. Jime´nez-Becerrill, O. Quiroz, A.I. Pa´ez, J. Photochem. Photobio. A: Chem. 148 (2002) 177. J.J. Liu, G.X. Lu, H.L. He, H. Tan, T. Xu, et al. Mater. Res. Bull. 31 (1996) 1049. X.Y. Li, S.B. Li, G.X. Lie, J. Mol. Catal. 31 (1996) 187 (in Chinese). Z.H. Yuan, L.D. Zhang, J. Mater. Chem. 11 (2001) 1256. P. Cheng, W. Li, T. Zhou, Y. Jin, M. Gu, J. Photochem. Photobiol. A: Chem. 168 (2004) 97. T. Sato, K. Haneda, M. Seki, T. Iijima, Appl. Phys. A 50 (1990) 13. D. Bahnemann, A. Henglein, J. Lilie, L. Spanhel, J. Phys. Chem. 88 (1984) 709. Z.H. Yuan, L.D. Zhang, Mater. Res. Bull. 33 (1998) 1587. B.S. Zhou, L.Z. Xiao, T.J. Li, J.L. Zhao, et al. Appl. Phys. Lett. 59 (1991) 1826. A. Schafani, M.N. Mozzanaga, P. Pichat, J. Photochem. Photobiol. A: Chem. 59 (1991) 181. Z.H. Yuan, Y.H. Wang, Y.C. Sun, J. Wang, L.J. Bie, Y.Q. Duan, Sci. China B: Chem. 49 (2006) 67.