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rutile in mixed-phase TiO2: Formation and photo-generated charge carriers properties
Chemical Physics Letters 504 (2011) 71–75
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Interface junction at anatase/rutile in mixed-phase TiO2: Formation and photo-generated charge carriers properties Xiaoru Zhang, Yanhong Lin, Dongqing He, Jianfu Zhang, Zhiyong Fan, Tengfeng Xie ⇑ State Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 24 July 2010 In final form 23 January 2011 Available online 26 January 2011
a b s t r a c t Nanosized TiO2 photocatalysts of anatase, rutile and mixed-phase were synthesized through hydrolysis method. The influence of the interface junction between anatase and rutile TiO2 on the photo-generated charge carriers properties were studied by Kelvin probe (KP), surface photovoltage (SPV) and transient photovoltage (TPV) techniques. The KP results revealed that the difference of the surface work function between anatase and rutile might cause the formation of built-in field at the interface in mixed-phase TiO2. The role of the interface junction was further testified through the characteristics of photo-generated charge carriers by SPV and TPV. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2) has been widely studied as one of the most promising semiconductor photocatalyst for its various applications in photocatalysis and photoelectrochemistry, such as solar energy conversion [1], environmental applications [2], photocatalysis [3], sensor [4], photochromic devices [5], self-cleaning and photoinduced hydrophilicity [6,7], etc. The performance of TiO2 in these applications mostly depends on the crystalline forms [8], which are usually used as anantase and rutile, with band gaps (Eg) of 3.2 and 3.0 eV, respectively. However, the mixed-phase TiO2 has been reported to exhibit higher activity than single-phase systems [9], such as Degussa P25, which involves about 80% anatase and 20% rutile. The high photocatalytic activity of mixedphase TiO2 is considered as the phase junction that photoexcited charges migrated between the two phases and then enhances the charges separation [10,11]. In recent years, several efforts related to the interface between anatase and rutile have been carried out, with the aim of investigating the synergetic effect of mixed-phase TiO2. The research groups of Can Li et al. have prepared mixed-phase TiO2 by calcining Ti (OH)4 in air, and they show that the surface-phase structure can be visualized by HRTEM, and the photocatalytic activity of TiO2 nanoparticles is directly related to the surface-phase structure on the basis of UV Raman spectroscopy [12]. Many studies show that the electrons and holes can be separated effectively and directionally under the influence of heterostructure between two different materials [13]. For example, based on the model of P25, Kawahara and co-workers have prepared a patterned bilayer-type TiO2 photocatalyst consisting of anatase and rutile phases, and they ⇑ Corresponding author. E-mail address: [email protected] (T. Xie). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.01.060
consider that the high photocatalytic activity is caused mainly by the increase in charge-separation efficiency resulting from interfacial electron transfer from anatase to rutile [14]. While Gray and Rajh have the different idea about the direction of electrons transfer. They have probed the charge separation characteristics of P25 by EPR spectroscopy, and suggest that within mixed-phase titania there is small rutile crystallites interweave with anatase crystallites, the transition points between these two phases allow for rapid electron transfer from rutile to anatase, and the structural arrangement creates catalytic ‘hot spots’ at the rutile–anatase interface [15,16]. The photoelectric properties, such as photocatalytic activity, are closely related to the behaviors of the photo-induced charge carriers. We could estimate the photoelectric properties by analyzing the photo-induced charge carriers transfer behaviors. However, the transport process of photo-induced charge carriers at the interface junction between anatase and rutile phase of TiO2 remains ambiguous. Few of them gave the reports involving the formation of interface junction and the photovoltaic properties of the interface, which would be very significant for developing efficient photocatalyst. Therefore, the techniques that can detect the surface electronic potentials and the behavior of photoinduced charges in mixed-phase TiO2 are desirable. Kelvin probe (KP), Surface photovoltage (SPV) spectroscopy and Transient photovoltage (TPV) techniques are useful in investigating the behavior of photo-generated electron–hole pairs. The KP is used to measure the surface work function of TiO2 samples, which would provide the information of electronic properties on surface [17]. Based on the work function, we are able to determine the existence and orientation of the built-in electric field at the interfaces. The SPV method is a well-established technique for characterizing the photoelectronic and electronic properties of inorganic semiconductors [18,19], and the corresponding phase value of the photovoltage signal shows the statistic kinetic
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characteristics of the SPV generating process, which has been successfully applied for characterizing photo-induced charge carries [20]. The TPV technique is a very promising method for the investigation of dynamic properties (including separation, transport and recombination) of the photo-induced charge carriers in semiconductor materials [21–28]. In this study, we synthesized TiO2 nanoparticles by calcining the TiO2 precursor obtained by hydrolysis of tetrabutyl titanate (TBOT) [29]. Photocatalytic degradation of Rhodamine B (RhB) under UV light irradiation was adopted to evaluate the photocatalytic properties of the TiO2 nanoparticles. The investigation of the surface electronic potentials of the TiO2 nanoparticles was carried out by Kelvin probe. Furthermore, the dynamic behavior of photoinduced charge carriers in the anatase, rutile and mixed-phase TiO2 were also studied via the SPV and TPV techniques. 2. Experimental section 2.1. Catalyst preparation All the chemicals are of analytical grade and used as received without further purification. In a typical synthesis, 10 mL of TBOT (Sigma–Aldrich, 97%) was added into 50 mL of anhydrous ethanol (Beijing Fine Chemicals, P96%) under vigorously stirring at ambient temperature, named as solution A. Then a mixture solution of deionized water and 50 mL of anhydrous ethanol named solution B. The solution B was added to the solution A under vigorous stirring. The molar ratio of the water/TBOT was 75 [29]. The system was kept stirring for 24 h. Next, the white precipitate was obtained by centrifugation, followed by washing with deionized water and anhydrous ethanol four times. After that, the precursor was dried under vacuum overnight before characterization. The precursor was calcined in air at 580 °C, 680 °C, and 850 °C for 3 h, respectively, then cooled to room temperature. The as-prepared samples were labeled as T580, T680, and T850, respectively. 2.2. Characterization The crystalline phase was examined by X-ray diffraction (XRD) using a Rigaku D/Max-2550 diffractometer with Cu Ka radiation (k = 1.54056 Å) at 40 kV and 30 mA in the range of 20–80° (2h) at a scanning rate of 5 min1. The UV–vis experiments were carried out on a Shimadzu UV-2450 spectrophotometer. The UV–visible data were recorded on a Shimadzu UV-2450 spectrophotometer with a labsphere diffuse reflectance accessory using BaSO4 as the background, and the samples were pressed pellets of a mixture of 2 g of BaSO4 with 50 mg of the powder. The samples for transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analysis were observed by a high-resolution transmission electron microscope (TECNAIG2 FEI Company). 2.3. Kelvin probe, surface photovoltage spectroscopy and transient photovoltage measurements The work function of samples was measured with a commercial Kelvin probe system (SKP5050, KP Technology, Ltd.). The width of the gold reference probe is 1.8 mm, whose work function is 5.1 eV. The contact potential difference (CPD) between the sample and the probe was measured in dark. The CPD of each sample is measured with the same probe distance from the sample, and the probe distance was controlled by the software of Kelvin probe system. CPD is defined as the work function of the sample minus the work function of the reference plate connected to the preamplifier. The surface photovoltage (SPV) measurement system consisted of a source of monochromatic light, a lock-in amplifier (SR830-
DSP) with a light chopper (SR540), a photovoltaic cell, and a computer. A 500 W xenon lamp (CHFXQ500 W, Global Xenon Lamp Power) and a double-prism monochromator (Hilger and Watts, D300) provided monochromatic light. A low chopping frequency of approximately 23 Hz was used. The contact between samples and the indium tin oxide (ITO) electrode was not ohmic when we carried out the measurement of surface photovoltage. The construction of the photovoltaic cell is a sandwich-like structure of ITO–sample–ITO. TPV measurements were carried out in a device which was described in our previous paper [19]. A sample chamber like a parallel capacitor consisted of the TiO2 powders on the FTO substrate, a piece mica (about a few microns) and a platinum wire gauze electrode. A laser radiation pulse (wavelength of 355 nm and pulse width of 5 ns) from a third harmonic Nd:YAG laser (Polaris II, New Wave Research, Inc.) was used to excite the TPV. The signals were registered by a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix) with a preamplifier. The energy of the laser beam was adjusted by modulating the flash lamp voltage and absorption filter and measured by a joulemeter (Molectron Detector, Inc., EM500). Photocatalytic tests: Photodegradation studies of RhB were carried out on a quartz photochemical reactor. Before light irradiation, the TiO2 photocatalyst (0.025 g) was suspended in 25 mL of RhB aqueous solution (10 mg L1). The mixture was first sonicated for 5 min and then kept in the dark for 1 h with stirring to reach the adsorption–desorpion equilibrium. The UV irradiation source was a high-pressure mercury lamp (500 W). At the given time intervals (10 min), the analytical samples were taken from the suspension and immediately centrifuged at 10 000 rpm for 5 min. The concentration analysis of RhB was determined by using a visible spectrophotometer (723PC). The maximum absorptive wavelength of RhB is 553 nm. 3. Results and discussion Figure 1a shows the XRD patterns of different TiO2 nanoparticles. From the XRD results we know that well-crystallized TiO2 nanoparticles have been obtained. The results demonstrate that the crystal phases of T580, T680, and T850 are anatase, mixedphase, and rutile, respectively. The crystal structures of the samples are greatly changed with the increasing calcine temperature. The weight fraction of the rutile phase in the T680 is 17.6% estimated from the formula [30]: WR = 1/[1 + 0.884(Aana/Arut)], where Aana and Arut represent the X-ray integrated intensities of anatase (1 0 1) and rutile (1 1 0) diffraction peaks, respectively. The microstructure of the TiO2 samples was characterized by TEM and HRTEM. Figure 2 shows the TEM images of the three particles. Most particles in the sample T580 exhibit diameters in a range from 10 to 30 nm (Figure 2a), and the particle size increases after calcination at 680 °C (Figure 2b). The particle size of T680 increased to about 50–70 nm. When the calcination temperature was increased to 850 °C, TiO2 particles further grew and the particle size could be as large as about 100–200 nm. (Figure 2c). The corresponding surface areas of T580, T680, and T850 are 30.8, 24.2, and 12.2 m2/g, respectively. With the increase of particle size, the BET surfaces are decreased. According to the results of XRD, the sample undergoes the phase transformation from anatase to rutile. To visualize the mixed-phase junction, the T680 sample was investigated by HRTEM (Figure 2d), which reveals that two sets of different lattice images are observed with d spaces of 0.351 and 0.218 nm, corresponding to the (1 0 1) plane of anatase TiO2 and the (1 1 1) plane of rutile TiO2, respectively, which are in good accordance with the results of the XRD patterns shown in Figure 1. The HRTEM analysis gives the direct evidence for the interface junction between anatase and rutile phase formed in TiO2 particles.
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Figure 1. (a) XRD patterns of TiO2 powder samples calcined at 580 °C, 680 °C, and 850 °C. (b) UV–vis diffuse reflectance spectra of TiO2 calcined at 580 °C, 680 °C, and 850 °C.
Figure 2. TEM images of TiO2 powder samples calcined at (a) 580 °C, (b) 680 °C, and (c) 850 °C. (d) HRTEM image of a TiO2 sample calcined at 680 °C, (e) the enlarge part of rutile.
Figure 1b presents the UV–visible spectra of TiO2 nanoparticles. It can be observed from Figure 1b that the absorption edges of pure anatase and rutile are located at about 380 nm and 430 nm, respectively. While the absorption edge of mixed-phase TiO2 shifts to 600 nm and a weak shoulder in the range of 425–600 nm unlike the other samples. It is deduced that there might be more defect, surface states and interface states in the sample T680, which promoted extended light absorption range. The photocatalytic activity results of the samples are shown in Figure 3, the T680 sample shows much higher activity than the others in the presence of UV light, and the RhB was completely degraded within 50 min. The highest overall and specific photocatalytic activities were observed for the TiO2 sample calcined at 680 °C (Figure S1 in the Supporting Information), that is, the surface area was not the key factor different photocatalytic activities. This result is similar to previous reports [12,31], and suggests that the intimate physical interaction between the two phases enhances the photocatalytic activity significantly.
Figure 3. Kinetics of photodegradation of RhB by using TiO2 calcined at 580 °C, 680 °C, and 850 °C as photocatalysts under UV light.
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To further understand the cause of the high photo-activity of mixed-phase TiO2, we used the KP technique first to investigate the surface electronic potentials and consequently infer the direction of electron transfer. Figure 4a shows the CPDs of the three samples obtained on the KP. We can obtain that the surface work functions of anatase, mixed-phase and rutile TiO2 are 5.41 eV, 5.37 eV and 5.19 eV, respectively. The surface of the grains is a variety of complex mixture of crystal faces, and in our Kelvin probe system, the width of the gold reference probe is 1.8 mm, which is much larger than the size of crystal face of the grains, so the surface work function means the average work function. Base on the surface work functions, we can deduce that the Fermi level of rutile is 0.22 eV higher than that of anatase. Thus the electrons would transfer from rutile to anatase through their interface when the nanoparticles with different phases come to contact. It is just consistent with the KP results that the work function of mixed-phase TiO2 is between the pure anatase and rutile. This result implies that there may be a built-in electric field formed with the orientation from rutile to anatase TiO2 in mixed-phase TiO2, and a downwards band bending from anatase to rutile TiO2 (as shown in Figure 5a and b). So when the T680 is illuminated under the UV light, the transfer process of photo-generated electrons from anatse TiO2 to rutile TiO2 would occur, which was driven by the electric field at the interface, and the probability of the recombination of the photo-generation charge carriers would significantly reduced, thus enhancing the photocatalytic activity. We used SPV and TPV measurements to confirm the formation of interface junction and probe the dynamic behavior of the photoinduced charge carriers between anatase and rutile TiO2. Figure 6 shows surface photovoltage spectra (SPS) of the three samples. Note that similar to the results obtained from UV–visible measurements, the SPV response positions for all the samples are about 425 nm and the inset of Figure 6 shows that T580 commences generating photovoltage from less than 425 nm. Because the band gap (Eg) of anatase is larger than that of rutile, so the onset of T580 (anatase) is at shorter wavelength. While the difference with UV–visible spectra is that the photovoltage response of T680 is not also extended to this higher wavelength. This demonstrates that the generated electron–hole pairs could be separated effectively only under the irradiation of UV light. The SPS peaks of T580, T680, and T850 located at 328 nm, 360 nm and 370 nm, respectively. A red shift of photovoltage response peaks occurs. And the SPV response related to band-to-band transition of the TiO2 powders in the mixed-phase TiO2 is apparently enhanced for the spectral region of 350–425 nm. This shows that both excitation of anatase and rutile contribute to the SPV signal and result in a larger total charges in mixed-phase TiO2. Then the excess
Figure 5. Schematic band diagrams of the anatase TiO2 and the rutile TiO2. (a) Before the anatase TiO2 and the rutile TiO2 contact, the work function values of anatase TiO2 and rutile TiO2 are 5.41 eV and 5.19 eV, respectively. (b) After contact. E0F is the Fermi level at equilibrium conditions. The arrows indicate the orientation of the built-in electric fields.
Figure 6. Surface photovoltage spectra (SPS) of TiO2 calcined at 580 °C, 680 °C, and 850 °C. Inset: the corresponding phase spectra.
carriers diffuse toward the interface and after separation the charge carriers diffuse away from the interface. So we can deduce that the interface junction might promote the photo-generated electrons transfer from anatase to rutile TiO2 and excess carriers are effectively separated. According with the KP results, the SPS conclusion may be one of the reasons why the sample T680 exhibits superior photocatalytic activity under UV light. And the photocatalytic experimental results are also in line with the above discussion (Figure 6).
Figure 4. (a) CPDs of TiO2 calcined at 580 °C, 680 °C, and 850 °C related to the Au reference probe. (b) Transient photovoltage (TPV) measurements of TiO2 calcined at 580 °C, 680 °C, and 850 °C. The wavelength and the intensity of the laser are 355 nm and 50 lJ.
X. Zhang et al. / Chemical Physics Letters 504 (2011) 71–75
The above discussion is also supported by the SPV phase spectra (Figure 6, inset). For the sample T580, the phase value is in the response range of 0–90° from 300 nm to 425 nm, While the phase values of the T680 and T850 are in the response range of 90– 180°. This observation further supports our interpretation that the photo-generated electrons transfer from anatase to rutile, which highlight the statistic kinetic characteristics that T680 is similar to T850. So we conclude that the interface junction would affect the transport of the photo-generated charges. The TPV measurements were carried out to investigate the kinetics of the photo generation of excess carriers in the nanostructure. Figure 4b shows the TPV responses of the samples under the excitation wavelength of 355 nm, when the intensity of the laser pulse (excitation level) was 50 lJ. According to the TPV results, we can obtain the information as follows: (1) positive photovoltage transients are observed for all the samples, when the samples are exposed to the laser pulse; (2) the TPV response of T680 is the strongest one of the three samples; (3) the curves clearly displayed two TPV response processes, at the response time 107–103 s and 103–101 s; (4) the TPV response peak of the mixed-phase sample moves to the long timescale, namely, the recombination of charge carriers in the mixed-phase sample is much slower than the others. Based on the above results, the direction in which the photogenerated carriers move during the transient measurements can be deduced from the signs of the PV response (positive or negative). According to previous reports [21], a positive TPV response implies that the electric potential of the top electrode is positive with respect to the back electrode. The mechanism may exist that positive charges transfer towards the top electrode and accumulate at the area nearby. Therefore, under top illumination the positive charges are closer to the top electrodes in our results. That is, the excess electrons move towards the back electrode. There are two response components in the TPV curves (Figure 4b). Such as T680, the photovoltage signal reaches about 0.7 mV in 3 107 s, which is the fast process, and then it reaches about 2.5 mV in 2 103 s, which is the slow process. According to the result of Zhang et al. [24], the fast process had resulted from the separation of the photo-generated electron–hole pairs by the action of the built-in electric field in TiO2. And the slow process should be attributed to the diffusion of the excess electrons and holes for which the diffusion coefficients are different. For T680, large numbers of charges could generate on the surface of the particles in the fast process, which is similar to T580 and T850. However, in the slow process, the delay for TPV peak of T680 maybe mean the time retardation of the maximum separation, which indicates the different charge transfer processes from T580 to T850. Taking into account the microstructure, there are the interface junctions between anatase and rutile in T680 shown in HRTEM image, which may be able to promote the separation of photo-generated electron–hole pairs. The complete recombination time of T680 is about 0.1 s, while the others are about 0.02 s. This implies that the interface junctions could reduce the recombination of electron–hole pairs and extend the lifetime of carriers. Consequently, we suggest that the interface may have the major effect on the photovoltage formation. According to the above results, the role of the interface junction between anatase and rutile is confirmed. The behavior of photo-induced charge carriers obtained via the SPV and TPV measurements further proves the existence of interface junction. The built-in electric field that exists at the interface of the mixed-phase sample promotes the charge separation, thus the recombination of electrons and holes is restrained. Consequently, there would be more well-separated photo-induced electrons and holes to degrade dye molecules. The oxygen molecule could be combined with electrons to form superoxide anions ð O 2 Þ, and simultaneously, the excess holes trapped by OH, forming hydroxyl radicals. Therefore, the
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RhB would be oxidized with high efficiency on the surface of mixed-phase TiO2 by photocatalysis. 4. Conclusion Better understanding of the effects of the interface junction between anatase and rutile TiO2 on the photo-activity has been obtained via comprehensive KP, SPV and TPV analysis. We have qualitatively investigated the photovoltaic properties of the mixed-phase TiO2 by SPV and TPV techniques. The revealed dynamic behavior of the photo-generated charge carriers provided more favorable evidence for the effect of the interface junction. The most valuable results of the research should be the surface photovoltage response has a close relationship to the performance of the photocatalytic activities. And we can obtain the complementary information about the nanostructured (such as interface junction) materials from the SPV and TPV measurement. Acknowledgments This work was supported by National Basic Research Program of China (973 Program) (No. 2007CB613303), and the National Natural Science Foundation of China (No. 20703020, 20873053). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2011.01.060. References [1] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 6 (2006) 215. [2] I.K. Konstantinau, T.A. Albanis, Appl. Catal. B 42 (2003) 319. [3] S. Sakthivel, H. Kisch, Angew. Chem., Int. Ed. 42 (2003) 4908. [4] R.K. Sharma, M.C. Bhatnagar, G.L. Sharma, Sens. Actuators B: Chem. 46 (1998) 194. [5] I.P. Parkin, R.G. Palgrave, J. Mater. Chem. 15 (2005) 1689. [6] J. Tang, H. Quan, J. Ye, Chem. Mater. 19 (2007) 116. [7] K.I. Iuchi, Y. Ohko, T. Tatsuma, A. Fujishima, Chem. Mater. 16 (2004) 1165. [8] Z. Ding, G.Q. Lu, P.F. Greenfield, J. Phys. Chem. B 104 (2000) 4815. [9] T. Ohno, K. Sarukawa, M. Matsumura, J. Phys. Chem. B 105 (2001) 2417. [10] G.H. Li, K.A. Gray, Chem. Phys. 339 (2007) 173. [11] A. DiPaola, M. Bellardita, R. Ceccato, L. Palmisano, Parrino, J. Phys. Chem. C 113 (2009) 15166. [12] J. Zhang, Q. Xu, Z.C. Feng, M.J. Li, C. Li, Angew. Chem. Int. Ed. 47 (2008) 1766. [13] K.Y. Song, M.K. Park, Y.T. Kwon, H.W. Lee, W.J. Chung, W.I. Lee, Chem. Mater. 13 (2001) 2349. [14] T. Kawahara, Y. Konishi, H. Tada, N. Tohge, J. Nishii, S. Ito, Angew. Chem. Int. Ed. 41 (2002) 2811. [15] D.C. Hurum, A.G. Agrios, K.A. Gray, T. Rajh, M.C. Thurnauer, J. Phys. Chem. B 107 (2003) 4545. [16] D.C. Hurum, A.G. Agrios, S.E. Crist, K.A. Gray, T. Rajh, M.C. Thurnauer, J. Electron. Spectrosc. Relat. Phenom. 150 (2006) 155. [17] R. Cohen et al., J. Am. Chem. Soc. 121 (1999) 10545. [18] L. Kronik, Y. Shapira, Surf. Sci. Rep. 37 (1999) 1. [19] Q. Zhang, D. Wang, X. Wei, T.F. Xie, Z. Li, Y.H. Lin, M. Yang, Thin Solid Films 491 (2005) 242. [20] Q.D. Zhao, D.J. Wang, L.L. Peng, Y.H. Lin, M. Yang, T.F. Xie, Chem. Phys. Lett. 434 (2007) 96. [21] B. Mahrov, G. Boschloo, A. Hagfeldt, L. Dloczik, Th. Dittrich, Appl. Phys. Lett. 84 (2004) 5455. [22] D. Moses, C. Soci, X. Chi, A.P. Ramirez, Phys. Rev. Lett. 97 (2006) 067401. [23] C. Soci, D. Moses, Q.H. Xu, A. Heeger, J. Phys. Rev. B 72 (2005) 245204. [24] Q.L. Zhang, D.J. Wang, X. Wei, Q.D. Zhao, Y.H. Lin, M. Yang, Chem. J. Chinese U 27 (2006) 550. [25] S. Nakade, Y. Saito, W. Kubo, T. Kanzaki, Y. Wada, S. Yanagida, J. Phys. Chem. B 108 (2004) 1628. [26] B. Levy, W. Liu, S.E. Gilbert, J. Phys. Chem. B 101 (1997) 1810. [27] I. Mora-Serò, Th. Dittrich, A. Belaidi, G. Garcia-Belmonte, J. Bisquert, J. Phys. Chem. B 109 (2005) 14932. [28] V. Duzhko, V.Y. Timoshenko, F. Koch, Th. Dittrich, Phys. Rev. B 64 (2001) 075204. [29] J. Zhang, M.J. Li, Z.C. Feng, J. Chen, C. Li, J. Phys. Chem. B 110 (2006) 927. [30] A.A. Gribb, J.F. Banfield, Am. Mineral. 82 (1997) 717. [31] S. Bakardjieva, J. Šubrt, V. Štengl, M.J. Dianez, M.J. Sayagues, Appl. Catal., B: Environ. 58 (2005) 193.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Interface junction at anatase/rutile in mixed-phase TiO2: Formation and photo-generated charge carriers properties Xiaoru Zhang, Yanhong Lin, Dongqing He, Jianfu Zhang, Zhiyong Fan, Tengfeng Xie ⇑ State Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 24 July 2010 In final form 23 January 2011 Available online 26 January 2011
a b s t r a c t Nanosized TiO2 photocatalysts of anatase, rutile and mixed-phase were synthesized through hydrolysis method. The influence of the interface junction between anatase and rutile TiO2 on the photo-generated charge carriers properties were studied by Kelvin probe (KP), surface photovoltage (SPV) and transient photovoltage (TPV) techniques. The KP results revealed that the difference of the surface work function between anatase and rutile might cause the formation of built-in field at the interface in mixed-phase TiO2. The role of the interface junction was further testified through the characteristics of photo-generated charge carriers by SPV and TPV. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2) has been widely studied as one of the most promising semiconductor photocatalyst for its various applications in photocatalysis and photoelectrochemistry, such as solar energy conversion [1], environmental applications [2], photocatalysis [3], sensor [4], photochromic devices [5], self-cleaning and photoinduced hydrophilicity [6,7], etc. The performance of TiO2 in these applications mostly depends on the crystalline forms [8], which are usually used as anantase and rutile, with band gaps (Eg) of 3.2 and 3.0 eV, respectively. However, the mixed-phase TiO2 has been reported to exhibit higher activity than single-phase systems [9], such as Degussa P25, which involves about 80% anatase and 20% rutile. The high photocatalytic activity of mixedphase TiO2 is considered as the phase junction that photoexcited charges migrated between the two phases and then enhances the charges separation [10,11]. In recent years, several efforts related to the interface between anatase and rutile have been carried out, with the aim of investigating the synergetic effect of mixed-phase TiO2. The research groups of Can Li et al. have prepared mixed-phase TiO2 by calcining Ti (OH)4 in air, and they show that the surface-phase structure can be visualized by HRTEM, and the photocatalytic activity of TiO2 nanoparticles is directly related to the surface-phase structure on the basis of UV Raman spectroscopy [12]. Many studies show that the electrons and holes can be separated effectively and directionally under the influence of heterostructure between two different materials [13]. For example, based on the model of P25, Kawahara and co-workers have prepared a patterned bilayer-type TiO2 photocatalyst consisting of anatase and rutile phases, and they ⇑ Corresponding author. E-mail address: [email protected] (T. Xie). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.01.060
consider that the high photocatalytic activity is caused mainly by the increase in charge-separation efficiency resulting from interfacial electron transfer from anatase to rutile [14]. While Gray and Rajh have the different idea about the direction of electrons transfer. They have probed the charge separation characteristics of P25 by EPR spectroscopy, and suggest that within mixed-phase titania there is small rutile crystallites interweave with anatase crystallites, the transition points between these two phases allow for rapid electron transfer from rutile to anatase, and the structural arrangement creates catalytic ‘hot spots’ at the rutile–anatase interface [15,16]. The photoelectric properties, such as photocatalytic activity, are closely related to the behaviors of the photo-induced charge carriers. We could estimate the photoelectric properties by analyzing the photo-induced charge carriers transfer behaviors. However, the transport process of photo-induced charge carriers at the interface junction between anatase and rutile phase of TiO2 remains ambiguous. Few of them gave the reports involving the formation of interface junction and the photovoltaic properties of the interface, which would be very significant for developing efficient photocatalyst. Therefore, the techniques that can detect the surface electronic potentials and the behavior of photoinduced charges in mixed-phase TiO2 are desirable. Kelvin probe (KP), Surface photovoltage (SPV) spectroscopy and Transient photovoltage (TPV) techniques are useful in investigating the behavior of photo-generated electron–hole pairs. The KP is used to measure the surface work function of TiO2 samples, which would provide the information of electronic properties on surface [17]. Based on the work function, we are able to determine the existence and orientation of the built-in electric field at the interfaces. The SPV method is a well-established technique for characterizing the photoelectronic and electronic properties of inorganic semiconductors [18,19], and the corresponding phase value of the photovoltage signal shows the statistic kinetic
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characteristics of the SPV generating process, which has been successfully applied for characterizing photo-induced charge carries [20]. The TPV technique is a very promising method for the investigation of dynamic properties (including separation, transport and recombination) of the photo-induced charge carriers in semiconductor materials [21–28]. In this study, we synthesized TiO2 nanoparticles by calcining the TiO2 precursor obtained by hydrolysis of tetrabutyl titanate (TBOT) [29]. Photocatalytic degradation of Rhodamine B (RhB) under UV light irradiation was adopted to evaluate the photocatalytic properties of the TiO2 nanoparticles. The investigation of the surface electronic potentials of the TiO2 nanoparticles was carried out by Kelvin probe. Furthermore, the dynamic behavior of photoinduced charge carriers in the anatase, rutile and mixed-phase TiO2 were also studied via the SPV and TPV techniques. 2. Experimental section 2.1. Catalyst preparation All the chemicals are of analytical grade and used as received without further purification. In a typical synthesis, 10 mL of TBOT (Sigma–Aldrich, 97%) was added into 50 mL of anhydrous ethanol (Beijing Fine Chemicals, P96%) under vigorously stirring at ambient temperature, named as solution A. Then a mixture solution of deionized water and 50 mL of anhydrous ethanol named solution B. The solution B was added to the solution A under vigorous stirring. The molar ratio of the water/TBOT was 75 [29]. The system was kept stirring for 24 h. Next, the white precipitate was obtained by centrifugation, followed by washing with deionized water and anhydrous ethanol four times. After that, the precursor was dried under vacuum overnight before characterization. The precursor was calcined in air at 580 °C, 680 °C, and 850 °C for 3 h, respectively, then cooled to room temperature. The as-prepared samples were labeled as T580, T680, and T850, respectively. 2.2. Characterization The crystalline phase was examined by X-ray diffraction (XRD) using a Rigaku D/Max-2550 diffractometer with Cu Ka radiation (k = 1.54056 Å) at 40 kV and 30 mA in the range of 20–80° (2h) at a scanning rate of 5 min1. The UV–vis experiments were carried out on a Shimadzu UV-2450 spectrophotometer. The UV–visible data were recorded on a Shimadzu UV-2450 spectrophotometer with a labsphere diffuse reflectance accessory using BaSO4 as the background, and the samples were pressed pellets of a mixture of 2 g of BaSO4 with 50 mg of the powder. The samples for transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analysis were observed by a high-resolution transmission electron microscope (TECNAIG2 FEI Company). 2.3. Kelvin probe, surface photovoltage spectroscopy and transient photovoltage measurements The work function of samples was measured with a commercial Kelvin probe system (SKP5050, KP Technology, Ltd.). The width of the gold reference probe is 1.8 mm, whose work function is 5.1 eV. The contact potential difference (CPD) between the sample and the probe was measured in dark. The CPD of each sample is measured with the same probe distance from the sample, and the probe distance was controlled by the software of Kelvin probe system. CPD is defined as the work function of the sample minus the work function of the reference plate connected to the preamplifier. The surface photovoltage (SPV) measurement system consisted of a source of monochromatic light, a lock-in amplifier (SR830-
DSP) with a light chopper (SR540), a photovoltaic cell, and a computer. A 500 W xenon lamp (CHFXQ500 W, Global Xenon Lamp Power) and a double-prism monochromator (Hilger and Watts, D300) provided monochromatic light. A low chopping frequency of approximately 23 Hz was used. The contact between samples and the indium tin oxide (ITO) electrode was not ohmic when we carried out the measurement of surface photovoltage. The construction of the photovoltaic cell is a sandwich-like structure of ITO–sample–ITO. TPV measurements were carried out in a device which was described in our previous paper [19]. A sample chamber like a parallel capacitor consisted of the TiO2 powders on the FTO substrate, a piece mica (about a few microns) and a platinum wire gauze electrode. A laser radiation pulse (wavelength of 355 nm and pulse width of 5 ns) from a third harmonic Nd:YAG laser (Polaris II, New Wave Research, Inc.) was used to excite the TPV. The signals were registered by a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix) with a preamplifier. The energy of the laser beam was adjusted by modulating the flash lamp voltage and absorption filter and measured by a joulemeter (Molectron Detector, Inc., EM500). Photocatalytic tests: Photodegradation studies of RhB were carried out on a quartz photochemical reactor. Before light irradiation, the TiO2 photocatalyst (0.025 g) was suspended in 25 mL of RhB aqueous solution (10 mg L1). The mixture was first sonicated for 5 min and then kept in the dark for 1 h with stirring to reach the adsorption–desorpion equilibrium. The UV irradiation source was a high-pressure mercury lamp (500 W). At the given time intervals (10 min), the analytical samples were taken from the suspension and immediately centrifuged at 10 000 rpm for 5 min. The concentration analysis of RhB was determined by using a visible spectrophotometer (723PC). The maximum absorptive wavelength of RhB is 553 nm. 3. Results and discussion Figure 1a shows the XRD patterns of different TiO2 nanoparticles. From the XRD results we know that well-crystallized TiO2 nanoparticles have been obtained. The results demonstrate that the crystal phases of T580, T680, and T850 are anatase, mixedphase, and rutile, respectively. The crystal structures of the samples are greatly changed with the increasing calcine temperature. The weight fraction of the rutile phase in the T680 is 17.6% estimated from the formula [30]: WR = 1/[1 + 0.884(Aana/Arut)], where Aana and Arut represent the X-ray integrated intensities of anatase (1 0 1) and rutile (1 1 0) diffraction peaks, respectively. The microstructure of the TiO2 samples was characterized by TEM and HRTEM. Figure 2 shows the TEM images of the three particles. Most particles in the sample T580 exhibit diameters in a range from 10 to 30 nm (Figure 2a), and the particle size increases after calcination at 680 °C (Figure 2b). The particle size of T680 increased to about 50–70 nm. When the calcination temperature was increased to 850 °C, TiO2 particles further grew and the particle size could be as large as about 100–200 nm. (Figure 2c). The corresponding surface areas of T580, T680, and T850 are 30.8, 24.2, and 12.2 m2/g, respectively. With the increase of particle size, the BET surfaces are decreased. According to the results of XRD, the sample undergoes the phase transformation from anatase to rutile. To visualize the mixed-phase junction, the T680 sample was investigated by HRTEM (Figure 2d), which reveals that two sets of different lattice images are observed with d spaces of 0.351 and 0.218 nm, corresponding to the (1 0 1) plane of anatase TiO2 and the (1 1 1) plane of rutile TiO2, respectively, which are in good accordance with the results of the XRD patterns shown in Figure 1. The HRTEM analysis gives the direct evidence for the interface junction between anatase and rutile phase formed in TiO2 particles.
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Figure 1. (a) XRD patterns of TiO2 powder samples calcined at 580 °C, 680 °C, and 850 °C. (b) UV–vis diffuse reflectance spectra of TiO2 calcined at 580 °C, 680 °C, and 850 °C.
Figure 2. TEM images of TiO2 powder samples calcined at (a) 580 °C, (b) 680 °C, and (c) 850 °C. (d) HRTEM image of a TiO2 sample calcined at 680 °C, (e) the enlarge part of rutile.
Figure 1b presents the UV–visible spectra of TiO2 nanoparticles. It can be observed from Figure 1b that the absorption edges of pure anatase and rutile are located at about 380 nm and 430 nm, respectively. While the absorption edge of mixed-phase TiO2 shifts to 600 nm and a weak shoulder in the range of 425–600 nm unlike the other samples. It is deduced that there might be more defect, surface states and interface states in the sample T680, which promoted extended light absorption range. The photocatalytic activity results of the samples are shown in Figure 3, the T680 sample shows much higher activity than the others in the presence of UV light, and the RhB was completely degraded within 50 min. The highest overall and specific photocatalytic activities were observed for the TiO2 sample calcined at 680 °C (Figure S1 in the Supporting Information), that is, the surface area was not the key factor different photocatalytic activities. This result is similar to previous reports [12,31], and suggests that the intimate physical interaction between the two phases enhances the photocatalytic activity significantly.
Figure 3. Kinetics of photodegradation of RhB by using TiO2 calcined at 580 °C, 680 °C, and 850 °C as photocatalysts under UV light.
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To further understand the cause of the high photo-activity of mixed-phase TiO2, we used the KP technique first to investigate the surface electronic potentials and consequently infer the direction of electron transfer. Figure 4a shows the CPDs of the three samples obtained on the KP. We can obtain that the surface work functions of anatase, mixed-phase and rutile TiO2 are 5.41 eV, 5.37 eV and 5.19 eV, respectively. The surface of the grains is a variety of complex mixture of crystal faces, and in our Kelvin probe system, the width of the gold reference probe is 1.8 mm, which is much larger than the size of crystal face of the grains, so the surface work function means the average work function. Base on the surface work functions, we can deduce that the Fermi level of rutile is 0.22 eV higher than that of anatase. Thus the electrons would transfer from rutile to anatase through their interface when the nanoparticles with different phases come to contact. It is just consistent with the KP results that the work function of mixed-phase TiO2 is between the pure anatase and rutile. This result implies that there may be a built-in electric field formed with the orientation from rutile to anatase TiO2 in mixed-phase TiO2, and a downwards band bending from anatase to rutile TiO2 (as shown in Figure 5a and b). So when the T680 is illuminated under the UV light, the transfer process of photo-generated electrons from anatse TiO2 to rutile TiO2 would occur, which was driven by the electric field at the interface, and the probability of the recombination of the photo-generation charge carriers would significantly reduced, thus enhancing the photocatalytic activity. We used SPV and TPV measurements to confirm the formation of interface junction and probe the dynamic behavior of the photoinduced charge carriers between anatase and rutile TiO2. Figure 6 shows surface photovoltage spectra (SPS) of the three samples. Note that similar to the results obtained from UV–visible measurements, the SPV response positions for all the samples are about 425 nm and the inset of Figure 6 shows that T580 commences generating photovoltage from less than 425 nm. Because the band gap (Eg) of anatase is larger than that of rutile, so the onset of T580 (anatase) is at shorter wavelength. While the difference with UV–visible spectra is that the photovoltage response of T680 is not also extended to this higher wavelength. This demonstrates that the generated electron–hole pairs could be separated effectively only under the irradiation of UV light. The SPS peaks of T580, T680, and T850 located at 328 nm, 360 nm and 370 nm, respectively. A red shift of photovoltage response peaks occurs. And the SPV response related to band-to-band transition of the TiO2 powders in the mixed-phase TiO2 is apparently enhanced for the spectral region of 350–425 nm. This shows that both excitation of anatase and rutile contribute to the SPV signal and result in a larger total charges in mixed-phase TiO2. Then the excess
Figure 5. Schematic band diagrams of the anatase TiO2 and the rutile TiO2. (a) Before the anatase TiO2 and the rutile TiO2 contact, the work function values of anatase TiO2 and rutile TiO2 are 5.41 eV and 5.19 eV, respectively. (b) After contact. E0F is the Fermi level at equilibrium conditions. The arrows indicate the orientation of the built-in electric fields.
Figure 6. Surface photovoltage spectra (SPS) of TiO2 calcined at 580 °C, 680 °C, and 850 °C. Inset: the corresponding phase spectra.
carriers diffuse toward the interface and after separation the charge carriers diffuse away from the interface. So we can deduce that the interface junction might promote the photo-generated electrons transfer from anatase to rutile TiO2 and excess carriers are effectively separated. According with the KP results, the SPS conclusion may be one of the reasons why the sample T680 exhibits superior photocatalytic activity under UV light. And the photocatalytic experimental results are also in line with the above discussion (Figure 6).
Figure 4. (a) CPDs of TiO2 calcined at 580 °C, 680 °C, and 850 °C related to the Au reference probe. (b) Transient photovoltage (TPV) measurements of TiO2 calcined at 580 °C, 680 °C, and 850 °C. The wavelength and the intensity of the laser are 355 nm and 50 lJ.
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The above discussion is also supported by the SPV phase spectra (Figure 6, inset). For the sample T580, the phase value is in the response range of 0–90° from 300 nm to 425 nm, While the phase values of the T680 and T850 are in the response range of 90– 180°. This observation further supports our interpretation that the photo-generated electrons transfer from anatase to rutile, which highlight the statistic kinetic characteristics that T680 is similar to T850. So we conclude that the interface junction would affect the transport of the photo-generated charges. The TPV measurements were carried out to investigate the kinetics of the photo generation of excess carriers in the nanostructure. Figure 4b shows the TPV responses of the samples under the excitation wavelength of 355 nm, when the intensity of the laser pulse (excitation level) was 50 lJ. According to the TPV results, we can obtain the information as follows: (1) positive photovoltage transients are observed for all the samples, when the samples are exposed to the laser pulse; (2) the TPV response of T680 is the strongest one of the three samples; (3) the curves clearly displayed two TPV response processes, at the response time 107–103 s and 103–101 s; (4) the TPV response peak of the mixed-phase sample moves to the long timescale, namely, the recombination of charge carriers in the mixed-phase sample is much slower than the others. Based on the above results, the direction in which the photogenerated carriers move during the transient measurements can be deduced from the signs of the PV response (positive or negative). According to previous reports [21], a positive TPV response implies that the electric potential of the top electrode is positive with respect to the back electrode. The mechanism may exist that positive charges transfer towards the top electrode and accumulate at the area nearby. Therefore, under top illumination the positive charges are closer to the top electrodes in our results. That is, the excess electrons move towards the back electrode. There are two response components in the TPV curves (Figure 4b). Such as T680, the photovoltage signal reaches about 0.7 mV in 3 107 s, which is the fast process, and then it reaches about 2.5 mV in 2 103 s, which is the slow process. According to the result of Zhang et al. [24], the fast process had resulted from the separation of the photo-generated electron–hole pairs by the action of the built-in electric field in TiO2. And the slow process should be attributed to the diffusion of the excess electrons and holes for which the diffusion coefficients are different. For T680, large numbers of charges could generate on the surface of the particles in the fast process, which is similar to T580 and T850. However, in the slow process, the delay for TPV peak of T680 maybe mean the time retardation of the maximum separation, which indicates the different charge transfer processes from T580 to T850. Taking into account the microstructure, there are the interface junctions between anatase and rutile in T680 shown in HRTEM image, which may be able to promote the separation of photo-generated electron–hole pairs. The complete recombination time of T680 is about 0.1 s, while the others are about 0.02 s. This implies that the interface junctions could reduce the recombination of electron–hole pairs and extend the lifetime of carriers. Consequently, we suggest that the interface may have the major effect on the photovoltage formation. According to the above results, the role of the interface junction between anatase and rutile is confirmed. The behavior of photo-induced charge carriers obtained via the SPV and TPV measurements further proves the existence of interface junction. The built-in electric field that exists at the interface of the mixed-phase sample promotes the charge separation, thus the recombination of electrons and holes is restrained. Consequently, there would be more well-separated photo-induced electrons and holes to degrade dye molecules. The oxygen molecule could be combined with electrons to form superoxide anions ð O 2 Þ, and simultaneously, the excess holes trapped by OH, forming hydroxyl radicals. Therefore, the
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RhB would be oxidized with high efficiency on the surface of mixed-phase TiO2 by photocatalysis. 4. Conclusion Better understanding of the effects of the interface junction between anatase and rutile TiO2 on the photo-activity has been obtained via comprehensive KP, SPV and TPV analysis. We have qualitatively investigated the photovoltaic properties of the mixed-phase TiO2 by SPV and TPV techniques. The revealed dynamic behavior of the photo-generated charge carriers provided more favorable evidence for the effect of the interface junction. The most valuable results of the research should be the surface photovoltage response has a close relationship to the performance of the photocatalytic activities. And we can obtain the complementary information about the nanostructured (such as interface junction) materials from the SPV and TPV measurement. Acknowledgments This work was supported by National Basic Research Program of China (973 Program) (No. 2007CB613303), and the National Natural Science Foundation of China (No. 20703020, 20873053). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2011.01.060. References [1] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 6 (2006) 215. [2] I.K. Konstantinau, T.A. Albanis, Appl. Catal. B 42 (2003) 319. [3] S. Sakthivel, H. Kisch, Angew. Chem., Int. Ed. 42 (2003) 4908. [4] R.K. Sharma, M.C. Bhatnagar, G.L. Sharma, Sens. Actuators B: Chem. 46 (1998) 194. [5] I.P. Parkin, R.G. Palgrave, J. Mater. Chem. 15 (2005) 1689. [6] J. Tang, H. Quan, J. Ye, Chem. Mater. 19 (2007) 116. [7] K.I. Iuchi, Y. Ohko, T. Tatsuma, A. Fujishima, Chem. Mater. 16 (2004) 1165. [8] Z. Ding, G.Q. Lu, P.F. Greenfield, J. Phys. Chem. B 104 (2000) 4815. [9] T. Ohno, K. Sarukawa, M. Matsumura, J. Phys. Chem. B 105 (2001) 2417. [10] G.H. Li, K.A. Gray, Chem. Phys. 339 (2007) 173. [11] A. DiPaola, M. Bellardita, R. Ceccato, L. Palmisano, Parrino, J. Phys. Chem. C 113 (2009) 15166. [12] J. Zhang, Q. Xu, Z.C. Feng, M.J. Li, C. Li, Angew. Chem. Int. Ed. 47 (2008) 1766. [13] K.Y. Song, M.K. Park, Y.T. Kwon, H.W. Lee, W.J. Chung, W.I. Lee, Chem. Mater. 13 (2001) 2349. [14] T. Kawahara, Y. Konishi, H. Tada, N. Tohge, J. Nishii, S. Ito, Angew. Chem. Int. Ed. 41 (2002) 2811. [15] D.C. Hurum, A.G. Agrios, K.A. Gray, T. Rajh, M.C. Thurnauer, J. Phys. Chem. B 107 (2003) 4545. [16] D.C. Hurum, A.G. Agrios, S.E. Crist, K.A. Gray, T. Rajh, M.C. Thurnauer, J. Electron. Spectrosc. Relat. Phenom. 150 (2006) 155. [17] R. Cohen et al., J. Am. Chem. Soc. 121 (1999) 10545. [18] L. Kronik, Y. Shapira, Surf. Sci. Rep. 37 (1999) 1. [19] Q. Zhang, D. Wang, X. Wei, T.F. Xie, Z. Li, Y.H. Lin, M. Yang, Thin Solid Films 491 (2005) 242. [20] Q.D. Zhao, D.J. Wang, L.L. Peng, Y.H. Lin, M. Yang, T.F. Xie, Chem. Phys. Lett. 434 (2007) 96. [21] B. Mahrov, G. Boschloo, A. Hagfeldt, L. Dloczik, Th. Dittrich, Appl. Phys. Lett. 84 (2004) 5455. [22] D. Moses, C. Soci, X. Chi, A.P. Ramirez, Phys. Rev. Lett. 97 (2006) 067401. [23] C. Soci, D. Moses, Q.H. Xu, A. Heeger, J. Phys. Rev. B 72 (2005) 245204. [24] Q.L. Zhang, D.J. Wang, X. Wei, Q.D. Zhao, Y.H. Lin, M. Yang, Chem. J. Chinese U 27 (2006) 550. [25] S. Nakade, Y. Saito, W. Kubo, T. Kanzaki, Y. Wada, S. Yanagida, J. Phys. Chem. B 108 (2004) 1628. [26] B. Levy, W. Liu, S.E. Gilbert, J. Phys. Chem. B 101 (1997) 1810. [27] I. Mora-Serò, Th. Dittrich, A. Belaidi, G. Garcia-Belmonte, J. Bisquert, J. Phys. Chem. B 109 (2005) 14932. [28] V. Duzhko, V.Y. Timoshenko, F. Koch, Th. Dittrich, Phys. Rev. B 64 (2001) 075204. [29] J. Zhang, M.J. Li, Z.C. Feng, J. Chen, C. Li, J. Phys. Chem. B 110 (2006) 927. [30] A.A. Gribb, J.F. Banfield, Am. Mineral. 82 (1997) 717. [31] S. Bakardjieva, J. Šubrt, V. Štengl, M.J. Dianez, M.J. Sayagues, Appl. Catal., B: Environ. 58 (2005) 193.