electronic structure of titanium oxide nanotubules

Electronic structure of titanium oxide nanotubules Jian Hong, Jian Cao, Jingzhi Sun * , Hanying Li, Hongzheng Chen, Mang Wang * Department of Polymer Science and Engineering, State Key Laboratory of Silicon Materials, Institute of Polymer Composites, Zhejiang University, Hangzhou 310027, PR China Received 19 May 2003; in final form 18 August 2003 Published online: 2 October 2003 Abstract We investigated the electronic states of titanium oxide nanotubules (TiO x -NTs) by using field induced surface photovoltage spectroscopy (FISPS). Compared with common TiO 2 P25 nanocrystals, new surface photovoltage re- sponse bands extending to 550 nm were found under the effect of external electric field. Based on the principle of FISPS, these responses were ascribed to the surface state transitions. X-ray diffraction (XRD) indicated that the crystalline structure changed remarkable during the hydrothermal synthesis process. The existence of oxygen vacancies contrib- uting to the surface states was further confirmed by the sub-band gap photoluminescence. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction Recently, one-dimensional (1D) nanostructured TiO 2 materials such as nanotubules, nanowires have gained considerable attention for their bril- liant prospects in photocatalyst, environment pu- rification, solar cell and gas and humidity sensor [1–4]. Various methods have been introduced to acquire the 1D nanostructured TiO 2 . Using sol–gel strategy, Hoyer [5] prepared TiO 2 nanotubes with diameters of 70–100 nm. Applying porous alumina template, Imai et al. [6] successfully fabricated TiO 2 nanotubes in a tunable way. The simple method of hydrothermal synthesis, first developed by Kasuga et al. [7], was also extensively employed to synthesize TiO 2 nanotubes, nanoribbons and nanowires. Among these researches, however, the efforts focus on the characterization of crystallo- graphic structures and microscopic morphologies, only little attentions have been paid to the elec- tronic properties of these 1D nanostructured materials. In fact, the electronic properties play a key role in determining their photo/electronic performance. Considering that the surface photovoltage spectrum (SPS) an d electric field-induced surface photovoltage spectrum (FISPS) are highly sensi- tive tools to study the photophysics of the photo- generated species or excited states without any sample-contamination and de struction [8,9], we have investigated the surface electronic properties of the titanium oxide nanotubules using the SPS, FISPS techniques and photoluminescence Chemical Physics Letters 380 (2003) 366–371 www.elsevier.com/locate/cplett * Corresponding authors. Fax: +86-571-8795-1635. E-mail addresses: sunjz@zju.edu.cn (J. Sun), mwang@zju. edu.cn (M. Wang). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.09.037 (PL) spectroscopy. In this Letter, we report our experimental results, besides photovoltaic re- sponse corresponding to band-to- band transition; new surface photovoltage responses extended to 550 nm have been observed under the effect of the external electric field. Based on the principle of FISPS and the feature of PL, the new responses can be assigned to the surface states, which are resulted from oxygen vacancies formed in the structural evolution of the raw materials TiO 2 P25 to the nanotubules. 2. Experimental The method of hydrothermal synthesis, as de- scribed by Kasuga et al. [7]. was employed to prepare titanium oxide nanotubules. In a typical experiment, 500 micr ogramme titanium dioxide power (Degussa P25) and 50 ml 5–10 M NaOH aqueous solutions was put together into a Teflon- lined autoclave. The autoclave was sealed into a stainless tank, and maintained the temperature at 180 °C for 24 h, without shaking or stirring. And after it was cooled to room temperature, a white tousy power was obtained by centrifugation. The precipitate was washed with hydrochloric acid and distilled water several times until the pH was 7, and dried at 60 °C for 12 h, then annealed at 350 °C for 2 h, finally a soft white powder was obtained. SPS is a measurement of the relation between the surface photovoltage vs. the light wavelength. That is to say, the signal detected by SPS is equivalent to the change in the surface potential barrier on illumination: dV ¼ V s À V 0 s , where V s and V 0 s are the surface potential height with and without illumination, respectively. The surface photovoltaic spectra of the as-synthesized prod- ucts were measured with a Metal–Insulator– Semiconductor (MIS) approach, using steady state chopped light source monochromator-lock-in de- tection technique [8]. Periodic excess carriers gen- eration and subsequent redistribution changes the surface potential; this change is picked up by a transparent ITO glass electrode in a capacitor ge- ometry (Fig. 3b inset). Monochromatic light was obtained by passing light from a 500 W xenon lamp through a grating monochromator. A lock-in amplifier (Stanford SR 830), synchronized with a light chopper (Stanford SR 540) was employed to amplify the photovoltage signal. FISPS is a tech- nique that combines the field-effect principle with SPS. With an external voltage applied to the two sides of the sample, the mobile direction and the diffusion length of the photogenerated charge carriers can be altered. Moreover, the space charge density and the electronic state of the molecular can be changed. So the two factors will have direct effects on the SPV intensity and the photovoltaic characteristics. The principle and the illustration of the FISPS were discussed in detail by Zhang et al. [9]. The morphology of the nanotubules was ob- served on a JEM-200CX transmission electron microscopy (TEM). Samples for observation were prepared by ultrasonic dispersion of a small amount of sample in absolute ethanol; then a drop of the solution was dipped onto a copper micro- grid with carbon film. X-ray diffraction (XRD) patterns were obtained using 98-XD. UV-visible absorption spectra of the samples were recorded on a Varian Cary bio100 spectrometer. PL spectra were measured on a Hitachi 800 spectrometer with a Xe lamp as the excitation light source. Colloid solutions in absolute ethanol were prepared ul- trasonically for the UV–Visible and the PL measurements. 3. Results and discussion A typical TEM image of the as-synthesized TiO 2 nanostructured materials is shown in Fig. 1. It is apparent that the products are consisted of uniform tubules of about twenty nanometers in diameter and several hundred in length. Further- more, the titanium oxide nanotubules are quite pure without raw materials on the surface of the tubules. XRD measurements (Fig. 2) show that the as- prepared titanium oxide nanotubules have a new crystalline structure. The diffraction patterns are not only different from well-defined anatase and rutile phase, but also different from the report ed phases of known titanate [10,11]. Based on the J. Hong et al. / Chemical Physics Letters 380 (2003) 366–371 367 data of literature. [11], we mark the obtained na- notubules as a mixture of anatase TiO 2 and tita- nate that has not been completely transformed into anatase phase in the thermal process. There- fore, ignoring the possible existence of light ele- ments such as H, the obtained nanotubules can in general be referred as TiO x [12]. UV–Visible spectra of the raw materials TiO 2 P25 and the obtained TiO x nanotubules (TiO x - NTs) are depicted in Fig. 3a. The spectral lines for both samples exhibit only one characteristic ab- sorption band, which is assigned to the intrinsic transition from the valence band (VB) to the conduction band (CB); the correspondent thresh- old values are 375 nm for TiO 2 P25 powders while 295 nm for the TiO x -NTs. The 80 nm blue shift of absorption maximum can be attributed to the nano-size effect, because the average diameter of the nanotubules (as shown by the TEM image in Fig. 1) is 12 nm smaller than that of TiO 2 P25 particles. The SPS of TiO 2 P25 and TiO x -NTs without external field are illustrated in Fig. 3b. The profiles of the SPS and the UV–Visible absorption spec- trum resemble each other. The symbolic feature between the absorption and surface photovoltaic action spectrum suggests that the band-to-band transition is the major contribution to the surface photovoltage [8]. In the framework of band the- ory, electron–hole pairs are generated in the TiO 2 under the illumination. Driven by the built-in field, 10 20 30 40 50 60 0 1000 2000 3000 4000 (1) TiO 2 P25 powders (2) TiO x -NTs (2) (1) R R R A A A Intensity (a. u.) Fig. 2. XRD of TiO 2 P25 and TiO x -NTs (peaks resulting from the anatase and rutile phase are denoted by A and R, respec- tively). 300400500600700 0 2 4 6 h ν To lock-in Amplifier Bias R ITO Sample Optical glass (2) (1) Photovoltage (a. u.) Wavelength (nm) (1) TiO x -NTs (2) TiO 2 P25 powders 200 300 400 500 600 700 0.0 0.4 0.8 1.2 1.6 Absorbance (a.u.) Wavelength (nm) (1) TiO x -NTs (2) TiO 2 P25 powders (2) (1) (a) (b) Fig. 3. (a) UV–Visible spectra of TiO 2 P25 and TiO x -NTs. (b) SPS of TiO 2 P25 and TiO x -NTs without an external electric field (inset shows the structure of the photovoltaic cell for measuring the SPS and FISPS). Fig. 1. A typical TEM image of TiO x -NTs. 368 J. Hong et al. / Chemical Physics Letters 380 (2003) 366–371 the photogenerated holes move in the valance and the electrons in the CB. The displacement of the photogenerated electrons and holes leads to the change of the surface net charges thereby the surface photovoltage is prod uced [13]. Interestingly, the FISPS of the TiO 2 P25 and TiO x -NTs are distinctly different. Figs. 4a,b show the discrepancy of the TiO 2 P25 and TiO x -NTs under the effect of a series external electric field. For TiO 2 P25 (Fig. 4a), the intensity of surface photovoltage response systematically enhanced whereas no new photovoltage responses appeared when the applied bias varied from 0 to 1 V. For TiO x -NTs, in contrast, distinct scenery emerged when bias were applied to the samples. As shown in Fig. 4b, the intensity of intrinsic photovoltage response greatly increases with the increase of bi- ases from 0 to 1 V. At the same time, the low energy part of the photovoltage response extends to 550 nm, and a new feature is found at about 388 nm. According to the principle of FISPS [9], the SPV response of the surface states transition is sensitive to external field, while the intrinsic band- to-band transition is insensitive. Therefor e the new photovoltage response could be rationally attr ib- uted to the extrinsic sub-band gap or surface state transitions. In generally, the sub-band gap energy levels localize between the CB and VB, the charge carriers populating in these levels are bounded and the electronic transitions of local states are for- bidden. Consequently, the SPV response is weak even undetectable without the induction of the external field. But if an external field is applied, the surface states energy band tilts and its optical constant changes [14]. These two effects enlarge the transition momentum of local states and increase the probability of electronic transitions. As a re- sult, the enhanced surface photovoltage responses can be observed under the effect of the external field. It is reasonable to associate the new feature of photovoltage response with the surface state transitions of the TiO x -NTs. It is noted that we have not observed the SPV response associated with the subgap surface states in the TiO 2 P25. Although many investigations have de monstrated that the anatase, rutile, their single nanocrystal and doped TiO 2 have subgap [15], it is also found that the subgap states are close related to the di- mension of the nanocrystals and the preparation conditions. In fact, TiO 2 P25 has surface state, too (it can be seen from the PL spectroscope); but because of the synthesis condition and the sensi- tivity of our SPS instrument, we havenÕt observed the response of subgap in TiO 2 P25. To further testify the existence of the surface states, the PL spectra of the TiO x -NTs and TiO 2 P25 powders were compared at room temperature, the excitation wavelength was 310 nm. From the spectral line (1) and (2) in Fig. 5a, it is noted that the PL intensity of the TiO x -NTs is much stronger than that of TiO 2 P25 powders; a quantitative comparison demonstrates that the luminescence quantum efficiency for TiO x -NTs is up to 40 times of the TiO 2 P25 counterpart. The evidentFig. 4. FISPS of TiO 2 P25 and TiO x -NTs. J. Hong et al. / Chemical Physics Letters 380 (2003) 366–371 369 enhancement in PL efficiency implies the more multitudinous existence of surface states in TiO x - NTs. On the other hand, the investigations done by other groups [16–19] suggested that the traps were the main contribution to the PL of TiO 2 nanomaterials. Furthermore the oxygen vacancies had been considered to be the origin of the ob- served PL of TiO 2 nanowires [16]. Therefore, it is reasonable to assign the new band of photovoltage response under the external electric field to the local surface states caused by oxygen vacancies in the TiO x -NTs. The schema tic energy level diagra m showing the position of the surface state inside the band gap in relation to the VB is illustrated in Fig. 5b. The energy space between the VB and the surface state was calculated to be $0.08 eV, which was equal to the energy difference of band-to-band and local state transitions. 4. Conclusions In summary, TiO x -NTs have electronic prop- erties different from common TiO 2 nanocrystals, the photovoltage response band expands to 550 nm and new feature exhibits under the effect of the external electric field. Based on the experimental data of FISPS and PL, in the framework of band theory, we ascribe the new photovoltage responses to the electronic transitions of surface states, which are mainly caused by the oxygen vacancies at the surface. These results together with previous studies on TiO 2 nanomaterials indicate the simul- taneous evolutions of crystallographic and elec- tronic structures in the formation of TiO x -NTs using a hydrothermal synthesis method. It can be expected the repopulation of surface electronic states may provide new opportunities for the ap- plication of TiO x -NTs in the fields of photo/elec- tronic devices. Acknowledgements The authors would like to thank the financial support of the National Natural Science Founda- tion of China with granted number of 90101008. References [1] Y.N. Xia, P.D. Yang, Adv. Mater. 15 (2003) 351. [2] A. Hagfeldt, M. Gr € aatzel, Chem. Rev. 95 (1995) 49. [3] M. Adachi, I. Okada, S. Ngamsinlapasathian, Y. Murata, S. Yoshikawa, Electrochemistry 70 (2002) 449. [4] S.L. Zhang, J.F. Zhou, Z.J. Zhang, Z.L. Du, A.V. Vorontsov, Z.S. Jin, Chin. Sci. Bull. 45 (2000) 1533. [5] P. Hoyer, Langmuir 12 (1996) 1411. [6] H. Imai, Y. Takei, M. Matsuda, H. Hirashima, J. Mater. 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