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TiO2
Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 15–19
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Enhanced photovoltaic effect of TiO2 -based composite ZnFe2 O4 /TiO2 Z.L. Zhang a , M. Wan c , Y.L. Mao a,b,∗ a
School of Physics and Electronics, Henan University, Kaifeng 475004, China Key Laboratory of Photovoltaic Materials of Henan Province, Kaifeng 475004, China c Computer Center, Henan University, Kaifeng 475004, China b
a r t i c l e
i n f o
Article history: Received 27 August 2011 Received in revised form 16 January 2012 Accepted 6 February 2012 Available online 16 February 2012 Keywords: Composite TiO2 ZnFe2 O4 Enhanced photovoltaic effect
a b s t r a c t A TiO2 -based composite, ZnFe2 O4 /TiO2 , was prepared with a commercial TiO2 (Degussa P25) and ZnFe2 O4 nanopowders using a simple method. The composite was characterized by X-ray diffraction (XRD), UV–vis absorption spectrum, surface photovoltage spectroscopy (SPS) and SEM. The composite presents an enhanced photovoltaic effect compared with the pure TiO2 . The trap distributions of TiO2 and ZnFe2 O4 /TiO2 composite were investigated and the energy levels of TiO2 and ZnFe2 O4 were analyzed. The results indicate that the enhancement of photovoltaic effect is mainly due to the increase of photo-generated carrier concentration and improvement of the separation efficiency of photo-generated carriers by the addition of ZnFe2 O4 . © 2012 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is considered as one of the most promising materials for the applications of photocatalysis and dye sensitized solar cells due to its chemical stability, low cost, non-toxic nature, long-term stability and high activity [1–4]. While the disadvantages of TiO2 limit its wide applications. One is that TiO2 with a wide band-gap (anatase ca.3.2 eV, rutile ca. 3.0 eV) only absorbs UV light which is a very small fraction of solar spectrum (<5%). The other is that the photoactivity of TiO2 is low mainly due to the fast recombination of photo-generated electrons and holes [5,6]. To improve the low photoactivity, TiO2 was doped with some transition metal ions [7–9] and modified with some metal oxides through composite, core/shell structure and doping [10–15]. To improve the solar energy utilizing efficiency, one of the approaches is to prepare a coupled system consisting of TiO2 and another semiconductor with a relatively small band-gap, such as CdS, CdSe, or Fe2 O3 [16–18], which can extend the photoresponse of TiO2 to the visible range. Recently, ZnFe2 O4 with a relatively small band-gap [19] (ca. 1.92 eV) has attracted attentions in photoelectric conversion and photocatalysis due to its absorption in visible range and good photochemical stability [20,21]. It was reported that ZnFe2 O4 /TiO2 coupling system presents an enhanced photoactivity compared with pure TiO2 [22–24]. The enhancement of photoactivity was explained only by energy level matching based on some
∗ Corresponding author at: School of Physics and Electronics, Henan University, Kaifeng 475004, China. Tel.: +86 0378 3881602. E-mail address: [email protected] (Y.L. Mao). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2012.02.009
assumptions. Some authors thought that the photo-generated electrons transferred from TiO2 to ZnFe2 O4 [22], while others argued that the photo-generated electrons transferred from ZnFe2 O4 to TiO2 [24]. At present, there are few reports on the photovoltaic characteristics of ZnFe2 O4 /TiO2 nanocomposite. Herein, we prepared the ZnFe2 O4 /TiO2 nanocomposite with commercial TiO2 (Degussa P25) and ZnFe2 O4 nanopowders using a simple method and investigated its photovoltaic characteristics using surface photovoltage spectrum (SPS). The ZnFe2 O4 /TiO2 nanocomposite presents an enhanced photovoltaic effect compared with the pure TiO2 . The mechanisms of the enhanced photovoltaic effect were investigated by measuring the trap densities of TiO2 and ZnFe2 O4 /TiO2 composite, and analyzing the energy level matching of TiO2 and ZnFe2 O4 based on the reported data.
2. Experimental 2.1. Preparation of ZnFe2 O4 /TiO2 nanocomposite TiO2 (Degussa P25) and ZnFe2 O4 nanopowders were provided by Degussa (German) and Aladdin Reagent Database Inc. (Shanghai, China), respectively. The ZnFe2 O4 /TiO2 composite was prepared by the following procedures. 0.5 g of TiO2 and 0.015 g of ZnFe2 O4 powders were mixed with 30 ml ethanol by magnetic stirring for 6 h, followed by evaporation at 60 ◦ C for overnight. Then the mixture was sintered at 400 ◦ C for 2 h. The ZnFe2 O4 /TiO2 (Zn:Ti = 0.01:1, mole ratio) composite was obtained after being ground with an agate molar for 20 min. For precise comparison, the samples named TiO2 and ZnFe2 O4 in the text were obtained by the same
16
Z.L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 15–19
Fig. 1. XRD patterns of TiO2 (a), ZnFe2 O4 (b), and ZnFe2 O4 /TiO2 composite (Zn:Ti = 0.01:1, mole ratio) (c).
preparation method of ZnFe2 O4 /TiO2 composite using the TiO2 (Degussa P25) and ZnFe2 O4 as delivered, respectively. 2.2. Preparation of nanostructured TiO2 and ZnFe2 O4 /TiO2 films TiO2 or ZnFe2 O4 /TiO2 nanocomposite powders with the mass of 1.2 g was added to 5.4 ml of ethanol and mixed for 24 h. Then 2 g of 10% ethyl cellulose ethanol solution, 0.4 g of terpineol, and 0.12 g of polyethyleneglycol 20000 (PEG20000) were added to the paste. The mixture was ground with an agate molar for 20 min. TiO2 or ZnFe2 O4 /TiO2 film was deposited on a conducting glass substrate (F-doped SnO2 ; sheet resistance 15 /) using a screen printing technique. The film was dried at room temperature and sintered at 450 ◦ C for 30 min.
Fig. 2. UV–vis absorption spectra of TiO2 (a), ZnFe2 O4 (b), and ZnFe2 O4 /TiO2 composite (Zn:Ti = 0.01:1, mole ratio) (c).
ZnFe2 O4 /TiO2 composite. In the XRD pattern of TiO2 , the peaks at 2 = 25.28◦ and 2 = 27.24◦ are usually taken as the characteristic peaks of anatase (1 0 1) and rutile (1 1 0) crystal phase, respectively [25,26]. The mass fraction of rutile in the sample can be calculated using the following equation [27,28], fr =
1.26Ir Ia + 1.26Ir
(1)
where fr is the mass fraction of rutile, Ir and Ia are the intensities of the strongest (1 1 0) and (1 0 1) diffraction peaks of rutile and antase in the XRD pattern, respectively. It can be calculated from Fig. 1a that TiO2 contains a mixed phase with 86% anatase and 14% rutile. The crystallite size of anatase phase can be determined from the width of the strongest peak (1 0 1) with Scherrer’s equation [26], 0.9 ˇ cos
2.3. Characterization
D=
X-ray diffraction (XRD) measurements were performed on a DX-2500 diffractometer (Fangyuan, Dandong) with Cu K␣ ( = 0.1542 nm) radiation. The absorption spectra were collected on a UV-vis spectrophotometer (Varian Cary 5000). Surface photovoltage spectroscopy (SPS) was used to evaluate the photoelectric property of the samples with a home-built apparatus which consists of a 500 W xenon lamp (CHF XQ500W, China, Beijing Trusttech Co. Ltd., China), a double-grating monochromator (Zolix SP500), a lock-in amplifier (SR830-DSP), and a light chopper (SR540). Morphologies were observed using scanning electron microscopy (SEM, JSM-5600LV, JEOL). Electrochemical experiments were performed on an electrochemical analyzer (CHI660B, CH Instrument). The electrochemical measurements were carried out in a typical three electrodes system. The TiO2 or ZnFe2 O4 /TiO2 film electrode, a platinum wire, and a saturated Ag/AgCl electrode acted as working, counter, and reference electrodes, respectively. All potentials are hereafter given with reference to the saturated Ag/AgCl electrode. TiO2 or ZnFe2 O4 /TiO2 film electrode was 0.64 cm2 with a thickness of 3 m. The supporting electrolyte solution was prepared in deionized water with LiClO4 (0.1 M, pH 7.0). Chronoamperometry was used to record the current response (i) versus time (t). Before the scanning, the electrode was stabilized at 0.8 V for 5 min.
where is the wavelength of the radiation, ˇ is the corrected peak width at full width at half maximum (FWHM), and is the peak position. The anatase crystallite size of TiO2 is calculated to be about 20.1 nm. The characteristic peaks at 2 = 30.4◦ , 35.7◦ , and 57.1◦ in XRD pattern of ZnFe2 O4 indicates that ZnFe2 O4 has a spinel structure (JCPDS file: No. 79-1150). The crystallite size of ZnFe2 O4 is calculated to be about 25 nm. The XRD pattern of ZnFe2 O4 /TiO2 composite shows the presence of TiO2 peaks with strong intensities and ZnFe2 O4 peaks with weak intensities. This could be due to the small amount of ZnFe2 O4 in the composite. No appearance of new peaks in the XRD pattern of the composite indicates that TiO2 and ZnFe2 O4 crystalline phase separate from each other. The anatase crystallite size of TiO2 in the composite is calculated to be about 19.8 nm. The size change of anatase TiO2 in pure TiO2 and in composite is very small. Fig. 2 shows the UV–vis absorption spectra of TiO2 , ZnFe2 O4 , and ZnFe2 O4 /TiO2 composite, respectively. A strong absorption in UV range is observed for TiO2 . While ZnFe2 O4 presents an intensive absorption in a wide wavelength range from UV to visible light. The absorption spectrum of ZnFe2 O4 /TiO2 composite contains the absorptions of TiO2 in UV range and ZnFe2 O4 in visible range. This indicates that the addition of ZnFe2 O4 to TiO2 can expand the photoresponse of TiO2 to the visible range. In our experiments, the absorption of ZnFe2 O4 /TiO2 composite in visible range becomes stronger with the increase of ZnFe2 O4 quantity in the composite. While the absorption of the composite in UV range becomes weaker firstly, then stronger again with the increase of ZnFe2 O4 quantity in the composite (Fig. SI1). This could be due to the coupling between ZnFe2 O4 and TiO2 in the composite. The composite ZnFe2 O4 /TiO2
3. Results and discussion 3.1. XRD, UV–vis absorption, SPS and SEM XRD can give some detailed information on crystalline structure characteristics. Fig. 1 shows the XRD patterns of TiO2 , ZnFe2 O4 , and
(2)
Z.L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 15–19
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3.2. Transient currents and trap state distribution
Fig. 3. SPS of TiO2 (a), ZnFe2 O4 (b), and ZnFe2 O4 /TiO2 composite (Zn:Ti = 0.01:1, mole ratio) (c).
(Zn:Ti = 0.01:1, mole ratio) exhibits a slightly decreased UV absorption and an obvious absorption in visible range. Fig. 3 shows the surface photovoltage spectroscopy (SPS) of TiO2 , ZnFe2 O4 , and ZnFe2 O4 /TiO2 composite, respectively. Compared with TiO2 and ZnFe2 O4 , ZnFe2 O4 /TiO2 composite presents an enhanced photovoltaic response. The SPS intensity of the composite is largely increased. To better understand the SPS enhancement of ZnFe2 O4 /TiO2 composite, the trap distributions of TiO2 and ZnFe2 O4 /TiO2 composite, and the energy levels of TiO2 and ZnFe2 O4 are discussed in the following sections. Fig. 4 illustrates the SEM images of TiO2 and ZnFe2 O4 /TiO2 films. It indicates that the morphology of TiO2 /ZnFe2 O4 composite film did not change compared with that of TiO2 film.
The traps in semiconductor are energetically located below the conduction band (CB). The trap distribution of a semiconductor can be investigated by means of transient current decay during the trap-filling process [29,30]. Fig. 5 shows the transient currents of TiO2 and ZnFe2 O4 /TiO2 electrodes at varied potentials in 0.1 M LiClO4 solution at pH 7.0. It indicates that the transient current is significantly influenced by the applied potentials. For the two electrodes, at potential 0 to −0.3 V, the transient current decreases to almost zero within a few seconds. At −0.4 V, the transient current decreases slowly, and this behavior is observed in all the potentials negative of −0.4 V. The accumulated charge (Q) was calculated based on the current–time curves shown in Fig. 5. Some interesting features are observed. Fig. 6 shows the accumulated charge dependence on the applied potential. For TiO2 and ZnFe2 O4 /TiO2 electrodes, at potential positive of −0.3 V, the accumulated charge is very small. Between −0.3 and −0.7 V, the accumulated charge increases sharply. At potential negative of −0.7 V, the accumulated charge increases slowly. Fig. 6 displays that the accumulated charge on ZnFe2 O4 /TiO2 composite electrode is less than that of TiO2 electrode until the accumulated charge saturates. The trap-filling process reflects the trap density, which can be expressed as [29,30], Ntrap (U) =
1 dQ q dU
where Q is the accumulated charge, Ntrap (U) is the density of trap states at potential U, and q is electron charge. Eq. (3) indicates that the trap density is directly proportional to dQ/dU, which provides a direct measurement of trap distribution. A plot of dQ/dU against U can be obtained by differentiating the accumulated charge to the
Fig. 4. SEM of TiO2 (a) and ZnFe2 O4 /TiO2 (b) electrode film.
a
(3)
b
Fig. 5. Current–time curves of a TiO2 electrode (a) and ZnFe2 O4 /TiO2 electrode (b) in 0.1 M LiClO4 (in water) at pH 7.0.
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Z.L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 15–19
-2 CB
E vs Ag/AgCl (eV)
-1 CB
Eg=1.92 eV
0 hv Eg=3.0 eV
+1
VB
+ ZnFe2O4
+2 VB Fig. 6. Cathodic charges accumulated at varied potentials as derived by integrating the current–time curves in Fig. 5a and b.
applied potential, which is shown in Fig. 7. This plot reflects the trap density distribution. For TiO2 and ZnFe2 O4 /TiO2 electrodes, at potential positive of −0.3 V, the trap density is low, thus the trap-filling time is short. This results in fast decay of the transient current [32]. At potential −0.4 to −0.5 V, the trap density increases, and a longer time is required to fill these traps. Between −0.5 and −0.7 V, the trap density decreases again. Fig. 7 indicates that most traps are located at positive of −0.7 V for ZnFe2 O4 /TiO2 and TiO2 electrodes, and the trap density of ZnFe2 O4 /TiO2 composite electrode is smaller than that of TiO2 electrode. The totally trapped electrons for ZnFe2 O4 /TiO2 and TiO2 electrodes were calculated to be 8.75 × 1016 cm−2 and 1.21 × 1017 cm−2 , respectively. Compared with TiO2 , ZnFe2 O4 /TiO2 composite has a smaller trap density. Then, what should be responsible for the decreased trap density for the composite? It is reported that structural defects associated with oxygen vacancies can create some energy levels in the midgap of TiO2 , and these oxygen vacancies work as recombination center for photo-generated electrons and holes [33]. Zhang et al. reported that the number of oxygen vacancies could be decreased by the addition of ZnFe2 O4 [34]. Thus the decreased trap density for the composite could be due to the decrease of recombination center resulting from the reduction of oxygen vacancies of TiO2 by addition of ZnFe2 O4 . More information on the mechanism of the decreased trap density will be investigated in the future work. 3.3. Energy levels of ZnFe2 O4 and TiO2 The flat band of TiO2 electrode was determined to be −0.88 V vs. Ag/AgCl at pH 7.0 using electrochemical method (Fig. SI3), which is close to the reported value, and the band gap of TiO2 is 3.0 eV [31].
+3
+ TiO2 Electron
+ Hole
Fig. 8. Schematic of energy levels of ZnFe2 O4 and TiO2 . (CB: conduction band; VB: valence band; Eg: energy band gap; hv: photon energy.)
The conduction band of ZnFe2 O4 can be calculated to be −1.11 V vs Ag/AgCl at pH 7.0 based on the reported value and the band gap of ZnFe2 O4 is 1.92 eV [19]. The conduction band of semiconductor is very close to the flat band. The energy levels of ZnFe2 O4 and TiO2 are schematically shown in Fig. 8. Under UV light irradiation, the photogenerated electrons and holes are produced in TiO2 and ZnFe2 O4 by electron excitation from valence band to conduction band. In the ZnFe2 O4 /TiO2 coupled system, the conduction and valence bands of TiO2 are lower than those of ZnFe2 O4 , thus the photo-generated electrons can migrate from ZnFe2 O4 to TiO2 , and photo-generated holes can transfer from TiO2 to ZnFe2 O4 . Such a coupled system improves the separation efficiency of photo-generated carriers and increases the concentration of photo-generated carriers [22].
4. Conclusion The composite ZnFe2 O4 /TiO2 was prepared with TiO2 and ZnFe2 O4 nanopowders using a simple method. The composite presents an enhanced photovoltaic effect compared with pure TiO2 . The trap distributions of TiO2 and ZnFe2 O4 /TiO2 composite were investigated and the energy levels of TiO2 and ZnFe2 O4 were analyzed. These results indicate that the enhancement of ZnFe2 O4 /TiO2 composite is mainly due to the improvement of the separation efficiency of photo-generated carriers and the increase of photogenerated carrier concentration by the addition of ZnFe2 O4 . The ZnFe2 O4 /TiO2 composite with the enhanced photovoltaic effect is expected to be a promising electrode material for dye sensitized solar cells.
Acknowledgements This work is supported by the Project of Cooperation between Province and School in Hean Province (No. 092106000033) and the Natural Science Research Project of Education Department of Henan Province (No. 2011B480002).
Appendix A. Supplementary data
Fig. 7. Plot of dQ/dU distribution against potential.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jphotochem.2012.02.009.
Z.L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 15–19
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Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Enhanced photovoltaic effect of TiO2 -based composite ZnFe2 O4 /TiO2 Z.L. Zhang a , M. Wan c , Y.L. Mao a,b,∗ a
School of Physics and Electronics, Henan University, Kaifeng 475004, China Key Laboratory of Photovoltaic Materials of Henan Province, Kaifeng 475004, China c Computer Center, Henan University, Kaifeng 475004, China b
a r t i c l e
i n f o
Article history: Received 27 August 2011 Received in revised form 16 January 2012 Accepted 6 February 2012 Available online 16 February 2012 Keywords: Composite TiO2 ZnFe2 O4 Enhanced photovoltaic effect
a b s t r a c t A TiO2 -based composite, ZnFe2 O4 /TiO2 , was prepared with a commercial TiO2 (Degussa P25) and ZnFe2 O4 nanopowders using a simple method. The composite was characterized by X-ray diffraction (XRD), UV–vis absorption spectrum, surface photovoltage spectroscopy (SPS) and SEM. The composite presents an enhanced photovoltaic effect compared with the pure TiO2 . The trap distributions of TiO2 and ZnFe2 O4 /TiO2 composite were investigated and the energy levels of TiO2 and ZnFe2 O4 were analyzed. The results indicate that the enhancement of photovoltaic effect is mainly due to the increase of photo-generated carrier concentration and improvement of the separation efficiency of photo-generated carriers by the addition of ZnFe2 O4 . © 2012 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is considered as one of the most promising materials for the applications of photocatalysis and dye sensitized solar cells due to its chemical stability, low cost, non-toxic nature, long-term stability and high activity [1–4]. While the disadvantages of TiO2 limit its wide applications. One is that TiO2 with a wide band-gap (anatase ca.3.2 eV, rutile ca. 3.0 eV) only absorbs UV light which is a very small fraction of solar spectrum (<5%). The other is that the photoactivity of TiO2 is low mainly due to the fast recombination of photo-generated electrons and holes [5,6]. To improve the low photoactivity, TiO2 was doped with some transition metal ions [7–9] and modified with some metal oxides through composite, core/shell structure and doping [10–15]. To improve the solar energy utilizing efficiency, one of the approaches is to prepare a coupled system consisting of TiO2 and another semiconductor with a relatively small band-gap, such as CdS, CdSe, or Fe2 O3 [16–18], which can extend the photoresponse of TiO2 to the visible range. Recently, ZnFe2 O4 with a relatively small band-gap [19] (ca. 1.92 eV) has attracted attentions in photoelectric conversion and photocatalysis due to its absorption in visible range and good photochemical stability [20,21]. It was reported that ZnFe2 O4 /TiO2 coupling system presents an enhanced photoactivity compared with pure TiO2 [22–24]. The enhancement of photoactivity was explained only by energy level matching based on some
∗ Corresponding author at: School of Physics and Electronics, Henan University, Kaifeng 475004, China. Tel.: +86 0378 3881602. E-mail address: [email protected] (Y.L. Mao). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2012.02.009
assumptions. Some authors thought that the photo-generated electrons transferred from TiO2 to ZnFe2 O4 [22], while others argued that the photo-generated electrons transferred from ZnFe2 O4 to TiO2 [24]. At present, there are few reports on the photovoltaic characteristics of ZnFe2 O4 /TiO2 nanocomposite. Herein, we prepared the ZnFe2 O4 /TiO2 nanocomposite with commercial TiO2 (Degussa P25) and ZnFe2 O4 nanopowders using a simple method and investigated its photovoltaic characteristics using surface photovoltage spectrum (SPS). The ZnFe2 O4 /TiO2 nanocomposite presents an enhanced photovoltaic effect compared with the pure TiO2 . The mechanisms of the enhanced photovoltaic effect were investigated by measuring the trap densities of TiO2 and ZnFe2 O4 /TiO2 composite, and analyzing the energy level matching of TiO2 and ZnFe2 O4 based on the reported data.
2. Experimental 2.1. Preparation of ZnFe2 O4 /TiO2 nanocomposite TiO2 (Degussa P25) and ZnFe2 O4 nanopowders were provided by Degussa (German) and Aladdin Reagent Database Inc. (Shanghai, China), respectively. The ZnFe2 O4 /TiO2 composite was prepared by the following procedures. 0.5 g of TiO2 and 0.015 g of ZnFe2 O4 powders were mixed with 30 ml ethanol by magnetic stirring for 6 h, followed by evaporation at 60 ◦ C for overnight. Then the mixture was sintered at 400 ◦ C for 2 h. The ZnFe2 O4 /TiO2 (Zn:Ti = 0.01:1, mole ratio) composite was obtained after being ground with an agate molar for 20 min. For precise comparison, the samples named TiO2 and ZnFe2 O4 in the text were obtained by the same
16
Z.L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 15–19
Fig. 1. XRD patterns of TiO2 (a), ZnFe2 O4 (b), and ZnFe2 O4 /TiO2 composite (Zn:Ti = 0.01:1, mole ratio) (c).
preparation method of ZnFe2 O4 /TiO2 composite using the TiO2 (Degussa P25) and ZnFe2 O4 as delivered, respectively. 2.2. Preparation of nanostructured TiO2 and ZnFe2 O4 /TiO2 films TiO2 or ZnFe2 O4 /TiO2 nanocomposite powders with the mass of 1.2 g was added to 5.4 ml of ethanol and mixed for 24 h. Then 2 g of 10% ethyl cellulose ethanol solution, 0.4 g of terpineol, and 0.12 g of polyethyleneglycol 20000 (PEG20000) were added to the paste. The mixture was ground with an agate molar for 20 min. TiO2 or ZnFe2 O4 /TiO2 film was deposited on a conducting glass substrate (F-doped SnO2 ; sheet resistance 15 /) using a screen printing technique. The film was dried at room temperature and sintered at 450 ◦ C for 30 min.
Fig. 2. UV–vis absorption spectra of TiO2 (a), ZnFe2 O4 (b), and ZnFe2 O4 /TiO2 composite (Zn:Ti = 0.01:1, mole ratio) (c).
ZnFe2 O4 /TiO2 composite. In the XRD pattern of TiO2 , the peaks at 2 = 25.28◦ and 2 = 27.24◦ are usually taken as the characteristic peaks of anatase (1 0 1) and rutile (1 1 0) crystal phase, respectively [25,26]. The mass fraction of rutile in the sample can be calculated using the following equation [27,28], fr =
1.26Ir Ia + 1.26Ir
(1)
where fr is the mass fraction of rutile, Ir and Ia are the intensities of the strongest (1 1 0) and (1 0 1) diffraction peaks of rutile and antase in the XRD pattern, respectively. It can be calculated from Fig. 1a that TiO2 contains a mixed phase with 86% anatase and 14% rutile. The crystallite size of anatase phase can be determined from the width of the strongest peak (1 0 1) with Scherrer’s equation [26], 0.9 ˇ cos
2.3. Characterization
D=
X-ray diffraction (XRD) measurements were performed on a DX-2500 diffractometer (Fangyuan, Dandong) with Cu K␣ ( = 0.1542 nm) radiation. The absorption spectra were collected on a UV-vis spectrophotometer (Varian Cary 5000). Surface photovoltage spectroscopy (SPS) was used to evaluate the photoelectric property of the samples with a home-built apparatus which consists of a 500 W xenon lamp (CHF XQ500W, China, Beijing Trusttech Co. Ltd., China), a double-grating monochromator (Zolix SP500), a lock-in amplifier (SR830-DSP), and a light chopper (SR540). Morphologies were observed using scanning electron microscopy (SEM, JSM-5600LV, JEOL). Electrochemical experiments were performed on an electrochemical analyzer (CHI660B, CH Instrument). The electrochemical measurements were carried out in a typical three electrodes system. The TiO2 or ZnFe2 O4 /TiO2 film electrode, a platinum wire, and a saturated Ag/AgCl electrode acted as working, counter, and reference electrodes, respectively. All potentials are hereafter given with reference to the saturated Ag/AgCl electrode. TiO2 or ZnFe2 O4 /TiO2 film electrode was 0.64 cm2 with a thickness of 3 m. The supporting electrolyte solution was prepared in deionized water with LiClO4 (0.1 M, pH 7.0). Chronoamperometry was used to record the current response (i) versus time (t). Before the scanning, the electrode was stabilized at 0.8 V for 5 min.
where is the wavelength of the radiation, ˇ is the corrected peak width at full width at half maximum (FWHM), and is the peak position. The anatase crystallite size of TiO2 is calculated to be about 20.1 nm. The characteristic peaks at 2 = 30.4◦ , 35.7◦ , and 57.1◦ in XRD pattern of ZnFe2 O4 indicates that ZnFe2 O4 has a spinel structure (JCPDS file: No. 79-1150). The crystallite size of ZnFe2 O4 is calculated to be about 25 nm. The XRD pattern of ZnFe2 O4 /TiO2 composite shows the presence of TiO2 peaks with strong intensities and ZnFe2 O4 peaks with weak intensities. This could be due to the small amount of ZnFe2 O4 in the composite. No appearance of new peaks in the XRD pattern of the composite indicates that TiO2 and ZnFe2 O4 crystalline phase separate from each other. The anatase crystallite size of TiO2 in the composite is calculated to be about 19.8 nm. The size change of anatase TiO2 in pure TiO2 and in composite is very small. Fig. 2 shows the UV–vis absorption spectra of TiO2 , ZnFe2 O4 , and ZnFe2 O4 /TiO2 composite, respectively. A strong absorption in UV range is observed for TiO2 . While ZnFe2 O4 presents an intensive absorption in a wide wavelength range from UV to visible light. The absorption spectrum of ZnFe2 O4 /TiO2 composite contains the absorptions of TiO2 in UV range and ZnFe2 O4 in visible range. This indicates that the addition of ZnFe2 O4 to TiO2 can expand the photoresponse of TiO2 to the visible range. In our experiments, the absorption of ZnFe2 O4 /TiO2 composite in visible range becomes stronger with the increase of ZnFe2 O4 quantity in the composite. While the absorption of the composite in UV range becomes weaker firstly, then stronger again with the increase of ZnFe2 O4 quantity in the composite (Fig. SI1). This could be due to the coupling between ZnFe2 O4 and TiO2 in the composite. The composite ZnFe2 O4 /TiO2
3. Results and discussion 3.1. XRD, UV–vis absorption, SPS and SEM XRD can give some detailed information on crystalline structure characteristics. Fig. 1 shows the XRD patterns of TiO2 , ZnFe2 O4 , and
(2)
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3.2. Transient currents and trap state distribution
Fig. 3. SPS of TiO2 (a), ZnFe2 O4 (b), and ZnFe2 O4 /TiO2 composite (Zn:Ti = 0.01:1, mole ratio) (c).
(Zn:Ti = 0.01:1, mole ratio) exhibits a slightly decreased UV absorption and an obvious absorption in visible range. Fig. 3 shows the surface photovoltage spectroscopy (SPS) of TiO2 , ZnFe2 O4 , and ZnFe2 O4 /TiO2 composite, respectively. Compared with TiO2 and ZnFe2 O4 , ZnFe2 O4 /TiO2 composite presents an enhanced photovoltaic response. The SPS intensity of the composite is largely increased. To better understand the SPS enhancement of ZnFe2 O4 /TiO2 composite, the trap distributions of TiO2 and ZnFe2 O4 /TiO2 composite, and the energy levels of TiO2 and ZnFe2 O4 are discussed in the following sections. Fig. 4 illustrates the SEM images of TiO2 and ZnFe2 O4 /TiO2 films. It indicates that the morphology of TiO2 /ZnFe2 O4 composite film did not change compared with that of TiO2 film.
The traps in semiconductor are energetically located below the conduction band (CB). The trap distribution of a semiconductor can be investigated by means of transient current decay during the trap-filling process [29,30]. Fig. 5 shows the transient currents of TiO2 and ZnFe2 O4 /TiO2 electrodes at varied potentials in 0.1 M LiClO4 solution at pH 7.0. It indicates that the transient current is significantly influenced by the applied potentials. For the two electrodes, at potential 0 to −0.3 V, the transient current decreases to almost zero within a few seconds. At −0.4 V, the transient current decreases slowly, and this behavior is observed in all the potentials negative of −0.4 V. The accumulated charge (Q) was calculated based on the current–time curves shown in Fig. 5. Some interesting features are observed. Fig. 6 shows the accumulated charge dependence on the applied potential. For TiO2 and ZnFe2 O4 /TiO2 electrodes, at potential positive of −0.3 V, the accumulated charge is very small. Between −0.3 and −0.7 V, the accumulated charge increases sharply. At potential negative of −0.7 V, the accumulated charge increases slowly. Fig. 6 displays that the accumulated charge on ZnFe2 O4 /TiO2 composite electrode is less than that of TiO2 electrode until the accumulated charge saturates. The trap-filling process reflects the trap density, which can be expressed as [29,30], Ntrap (U) =
1 dQ q dU
where Q is the accumulated charge, Ntrap (U) is the density of trap states at potential U, and q is electron charge. Eq. (3) indicates that the trap density is directly proportional to dQ/dU, which provides a direct measurement of trap distribution. A plot of dQ/dU against U can be obtained by differentiating the accumulated charge to the
Fig. 4. SEM of TiO2 (a) and ZnFe2 O4 /TiO2 (b) electrode film.
a
(3)
b
Fig. 5. Current–time curves of a TiO2 electrode (a) and ZnFe2 O4 /TiO2 electrode (b) in 0.1 M LiClO4 (in water) at pH 7.0.
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-2 CB
E vs Ag/AgCl (eV)
-1 CB
Eg=1.92 eV
0 hv Eg=3.0 eV
+1
VB
+ ZnFe2O4
+2 VB Fig. 6. Cathodic charges accumulated at varied potentials as derived by integrating the current–time curves in Fig. 5a and b.
applied potential, which is shown in Fig. 7. This plot reflects the trap density distribution. For TiO2 and ZnFe2 O4 /TiO2 electrodes, at potential positive of −0.3 V, the trap density is low, thus the trap-filling time is short. This results in fast decay of the transient current [32]. At potential −0.4 to −0.5 V, the trap density increases, and a longer time is required to fill these traps. Between −0.5 and −0.7 V, the trap density decreases again. Fig. 7 indicates that most traps are located at positive of −0.7 V for ZnFe2 O4 /TiO2 and TiO2 electrodes, and the trap density of ZnFe2 O4 /TiO2 composite electrode is smaller than that of TiO2 electrode. The totally trapped electrons for ZnFe2 O4 /TiO2 and TiO2 electrodes were calculated to be 8.75 × 1016 cm−2 and 1.21 × 1017 cm−2 , respectively. Compared with TiO2 , ZnFe2 O4 /TiO2 composite has a smaller trap density. Then, what should be responsible for the decreased trap density for the composite? It is reported that structural defects associated with oxygen vacancies can create some energy levels in the midgap of TiO2 , and these oxygen vacancies work as recombination center for photo-generated electrons and holes [33]. Zhang et al. reported that the number of oxygen vacancies could be decreased by the addition of ZnFe2 O4 [34]. Thus the decreased trap density for the composite could be due to the decrease of recombination center resulting from the reduction of oxygen vacancies of TiO2 by addition of ZnFe2 O4 . More information on the mechanism of the decreased trap density will be investigated in the future work. 3.3. Energy levels of ZnFe2 O4 and TiO2 The flat band of TiO2 electrode was determined to be −0.88 V vs. Ag/AgCl at pH 7.0 using electrochemical method (Fig. SI3), which is close to the reported value, and the band gap of TiO2 is 3.0 eV [31].
+3
+ TiO2 Electron
+ Hole
Fig. 8. Schematic of energy levels of ZnFe2 O4 and TiO2 . (CB: conduction band; VB: valence band; Eg: energy band gap; hv: photon energy.)
The conduction band of ZnFe2 O4 can be calculated to be −1.11 V vs Ag/AgCl at pH 7.0 based on the reported value and the band gap of ZnFe2 O4 is 1.92 eV [19]. The conduction band of semiconductor is very close to the flat band. The energy levels of ZnFe2 O4 and TiO2 are schematically shown in Fig. 8. Under UV light irradiation, the photogenerated electrons and holes are produced in TiO2 and ZnFe2 O4 by electron excitation from valence band to conduction band. In the ZnFe2 O4 /TiO2 coupled system, the conduction and valence bands of TiO2 are lower than those of ZnFe2 O4 , thus the photo-generated electrons can migrate from ZnFe2 O4 to TiO2 , and photo-generated holes can transfer from TiO2 to ZnFe2 O4 . Such a coupled system improves the separation efficiency of photo-generated carriers and increases the concentration of photo-generated carriers [22].
4. Conclusion The composite ZnFe2 O4 /TiO2 was prepared with TiO2 and ZnFe2 O4 nanopowders using a simple method. The composite presents an enhanced photovoltaic effect compared with pure TiO2 . The trap distributions of TiO2 and ZnFe2 O4 /TiO2 composite were investigated and the energy levels of TiO2 and ZnFe2 O4 were analyzed. These results indicate that the enhancement of ZnFe2 O4 /TiO2 composite is mainly due to the improvement of the separation efficiency of photo-generated carriers and the increase of photogenerated carrier concentration by the addition of ZnFe2 O4 . The ZnFe2 O4 /TiO2 composite with the enhanced photovoltaic effect is expected to be a promising electrode material for dye sensitized solar cells.
Acknowledgements This work is supported by the Project of Cooperation between Province and School in Hean Province (No. 092106000033) and the Natural Science Research Project of Education Department of Henan Province (No. 2011B480002).
Appendix A. Supplementary data
Fig. 7. Plot of dQ/dU distribution against potential.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jphotochem.2012.02.009.
Z.L. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 233 (2012) 15–19
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