hydrothermal synthesis and photoluminescence of tio2 nanowires

Hydrothermal synthesis and photoluminescence of TiO 2 nanowires Y.X. Zhang, G.H. Li * , Y.X. Jin, Y. Zhang, J. Zhang, L.D. Zhang Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, China Received 29 July 2002; in final form 20 September 2002 Abstract Anatase TiO 2 single crystalline nanowires have been successfully synthesized using a simple hydrothermal synthesis method from TiO 2 nanoparticles. X-ray diffraction, transmission electron microscopy and high-resolution electron microscopy investigations show the TiO 2 nanowires have high crystallinity with diameter range from 30 to 45 nm and length in several micrometers. The TiO 2 nanowires can emit blue–green light peaked at 487 nm under excitation at 413 nm. Ó 2002 Published by Elsevier Science B.V. 1. Introduction One-dimensional (1D) nanostructured materi- als have attracted considerable attention due to their unusual electronic, optical, mechanical properties and potential applications [1,2]. Over the past few years, many methods have been suc- cessfully developed for the fabrication of these nanowires, including vapor–liquid–solid (VLS) [2], solution–liquid–solid (SLS) [3], template-based synthetic approaches [4], arc discharge [5], and laser ablation [6], which have all been proved to be very effective methods. However, almost all of the methods used either catalyst materials or physical template, which unavoidably brought some con- tamination to the products. Therefore, it is very interesting to explore a new approach to synthesize 1D nanomaterials without using preformed tem- plates or catalyst materials. Among a large amount of nanomaterials, the nanostructured titania materials are of great in- terest for possible application to photovoltaic cells [7], semiconductor photo-catalyst [8], catalyst support [9], and gas and humidity sensor [10]. To date, a few methods have been developed to syn- thesize TiO 2 nanowires. Li et al. [11] successfully prepared rutile TiO 2 nanowhiskers by annealing a precursor powder in which NaCl and TiðOHÞ 4 were homogeneously mixed. Kobayashi et al. [12] employed supramolecular assemblies to synthesize TiO 2 hollow-fibres. Recently, Lei et al. [13] and Zhang et al. [14] have fabricated TiO 2 nanowires in anodic alumina membranes. In this Letter, we adopt a simple chemical approach to synthesize single crystalline anatase TiO 2 nanowires. Chemical Physics Letters 365 (2002) 300–304 www.elsevier.com/locate/cplett * Corresponding author. Fax: +86-551-5591434. E-mail address: ghli@mail.issp.ac.cn (G.H. Li). 0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 0 09-2614(02)01 4 9 9 - 9 2. Experimental TiO 2 nanowires were prepared using a chemical process similar to that described by Kasuga and co-workers [15]. In a typical preparation proce- dure, 1 g anatase TiO 2 white powders were placed into a Teflon-lined autoclave of 50 ml capacity. Then, the autoclave was filled with 10 M NaOH aqueous solution up to 80% of the total volume, sealed into a stainless steel tank and maintained at 200 °C for 24 h without shaking or stirring during the heating. After the autoclave was naturally cooled to room temperature, the obtained sample was sequentially washed with dilute HCl aqueous solution, distilled deionized water and absolute ethanol for several times. The samples were dried at 70 °C for 6 h. Finally, soft fibrous powders with white color were obtained. The composition of the sample was examined by a Japan Rigaku Dmax c A X-ray diffractometer with Cu Ka radiation (k ¼ 1:54056  AA). The mor- phologies of the sample were analyzed with scanning electron microscopy (SEM) (JEOL JSM- 6300), transmission electron microscopy (TEM) (JEM-200CX) and high-resolution electron mi- croscopy (HRTEM) (JEM-2010). Samples for SEM observation were presputtered with a layer of conducting Pt metal. Samples for TEM observa- tion were prepared by 10 min ultrasonic dispersion of a small amount of sample in absolute ethanol; a drop of the solution was then dipped onto a cop- per microgrid or carbon film and dried in air be- fore performance. Photoluminescence (PL) spectra were measured in an Edinburgh FLS 920 spec- trophotometer with an Xe lamp as the excitation light source. 3. Results and discussion The X-ray diffraction (XRD) pattern (Fig. 1) revealed the overall crystalline structure and phase purity of the nanowires. All the relatively sharp peaks could be indexed as anatase TiO 2 with crystalline cell constants a ¼ 3:7806, c ¼ 9:4977  AA, which are basically in agreement with the reported values (JCPDS No. 21-1272). Although the dif- fraction peak of brookite (denoted as B in Fig. 1) can also be found, it is much lower than those of anatase phase. No characteristic peaks of other impurities, such as NaCl and Na 2 TiO 3 , were ob- served, which indicates that the product has high purity. Fig. 2 showed a typical SEM image of the as- prepared TiO 2 nanowires. The SEM image indi- cated the nanowires are very copious in quantity and quite clean with no contamination attached to their surface. On the other hand, some of the nanowires aggregated into bundles in the solution Fig. 1. XRD pattern of the as-prepared nanowires (A and B represent anatase and brookite, respectively). Fig. 2. A typical SEM image of anatase TiO 2 nanowires. Y.X. Zhang et al. / Chemical Physics Letters 365 (2002) 300–304 301 or during the preparation of SEM sample. This might explain why some of the nanowires looked wider than others. TEM and HRTEM were used to study the fine structure of the nanowires. Fig. 3a showed a typ- ical TEM image of an isolated TiO 2 nanowire. The electron diffraction (the inset in Fig. 3a) recorded perpendicular to the long axis of this nanowire determines the anatase phase of the obtained sample, which was consistent with the XRD pat- tern. The diffraction spots were indexed as (0 0 4), (2 0 0), and (2 0 4) diffraction of anatase TiO 2 , be- longing to the [0 1 0] zone axis. The single-crys- talline structure was further confirmed by HRTEM images. Fig. 3b shown the corresponding HRTEM image of the nanowire shown in Fig. 3a. The clear lattice stripes showed that the nanowire has high crystallinity with fewer defects such as microtwins. The plane intervals, measured as 0.35 nm, represented the stripe image of the (1 0 1) plane of anatase TiO 2 , which showed the forma- tion of single crystalline anatase TiO 2 nanowires in our experiments. The growth of single-crystalline TiO 2 nanowires in the hydrothermal condition is achieved for the first time. Although the exact growth mechanism of the TiO 2 nanowires is not very clear, we believe that NaOH plays an important role similar to the so-called ÔsoftÕ template. In addition, the temper- ature was definitely also very important for the growth. At a low temperature, for example at 110 °C [15], because of the limited growth of TiO 2 particles, the layered structures were very thin, which could easily be rolled up into tubular structures. While in our experiments, due to the rapid growth of particles, the layered structures were very thick, naturally decomposed into wires after washing with HCl, which could induce a structural rearrangement, i.e., a morphological transformation from the layered structures into the fibrous materials. Fig. 4 shows the PL spectra of the TiO 2 nanowires at room temperature together with that for TiO 2 nanocrystals. The excitation wave- length for curves (1) and (2) is 413 nm, and that for curves (3) and (4) is 473 nm. A very strong blue–green PL band can be observed which con- sists of two PL peaks situated at 487 nm (2.55 eV) and 492 nm (2.27 eV), respectively, under excitation at 413 nm. The PL peaks position and intensity are obviously different under different excitation wavelengths. The main peak is respec- tively located at about 487 and 545 nm under excitation at 413 and 473 nm. The PL intensity of TiO 2 nanowires excited at 413 nm is relatively Fig. 3. (a) A TEM image of a single anatase TiO 2 nanowire with a diameter of 40 nm. The inset shows a [0 1 0] SAED re- corded perpendicular to the long axis of the wire. (b) The corresponding HRTEM image of the nanowire showing lattice planes. The space of 0.35 nm corresponds to the distance be- tween two (1 0 1) planes. 302 Y.X. Zhang et al. / Chemical Physics Letters 365 (2002) 300–304 weak, while that excited at 473 nm is stable and strong. These results indicate that the optimal excitation wavelength for TiO 2 nanowires is 473 nm. The PL intensity of TiO 2 nanowires is higher than that of TiO 2 nanocrystals under all the cir- cumstances which indicates that the nanowires might have higher activity than nanocrystals. While the PL peaks shape and position of TiO 2 nanowires and nanocrystals are basically identical under the same excitation wavelength, which indicates that the PL mechanism of TiO 2 nano- wires might be the same as that of TiO 2 nanocrystals. The PL mechanisms of TiO 2 materials have been intensively studied in the past few years. De Hart et al. [16] observed a sharp emission line at 412 nm together with two other lines at 419 and 427 nm for TiO 2 anatase single crystalline. They assigned 412 nm line to free-exciton and the latter two lines to phonon repetitions of the free-exciton line. These emission lines were observed to be superimposed on a broad emission band centered at 485 nm. The broad PL band was ascribed to bound-exciton emission due to the trapping of free excitons by titanate groups near defects. Saraf et al. [17] also observed PL peak of TiO 2 nanoparticles at 412 nm. They assigned these PL bands to self-trapped excitons localized on TiO 6 octahedra. Serpone et al. [18] reported the PL bands of anatase TiO 2 nanocrystals at the long wavelength range (465 and 520 nm band) and attributed them to the oxygen vacancies. Jin et al. [19] observed two peaks at 488 and 510 nm from anatase TiO 2 film with the addition of ZnFe 2 O 4 and ascribed them to impurities and defects. In our case, since the excitation wavelength is far deviated from the absorption edge, the excitons tend to be unstable, and self-trapped excitons as the source of the PL is basically ruled out. Stre- kalovsky et al. [20] reported that the principal intrinsic effects in the powdered zirconia are an- ion vacancies. Emeline et al. [21] considered that the PL in colloidal ZrO 2 began with trapping of free electrons by anion vacancies accompanied by photon emission to yield F-type color centers. Since zirconia and titania are similar in crystalline structure and both are wide bandgap metal oxide, such PL in TiO 2 nanowires may originate from defect sites, especially from anion vacancies through the reaction e þ v a ! F þ hm; where v a is the anion vacancy and F is the color center. Of course, the exact photoluminescence mechanism of TiO 2 nanowires needs further in- vestigation. 4. Conclusions In conclusion, we have successfully fabricated single-crystalline anatase TiO 2 nanowires using a simple chemical approach. This method produced a large quantity of single-crystalline nanowires at relatively high purity and very low cost. The TiO 2 nanowires have a very strong PL band at blue– green wavelength range. The nanowires might have many potential applications in photocatalysts and photoelectronics. Acknowledgements The authors thank Professor Y. Qin for his help in HRTEM observations. The financial support of this work by the Key Project of National Funda- mental Research of China is gratefully acknowl- edged. Fig. 4. PL spectra of the TiO 2 samples under different excita- tion wavelengths at room temperature. (1) Nanowires and (2) nanocrystals under excitation at 413 nm; (3) nanowires and (4) nanocrystals under excitation at 473 nm. Y.X. Zhang et al. / Chemical Physics Letters 365 (2002) 300–304 303 References [1] S. Iijima, Nature 354 (1991) 56. [2] A.M. Morales, C.M. Liber, Science 279 (1998) 208. [3] T.J. Trantler, K.M. Hickman, S.C. Goel, A.M. Viano, P.C. Gibbons, W.E. Buhro, Science 270 (1995) 1791. [4] C.R. Martin, Science 266 (1994) 1961. [5] Y.C. Choi, W.S. Kim, Y.S. Park, S.M. Lee, D.J. Bae, Y.H. Lee, G S. Park, W.B. Choi, N.S. Lee, J.M. Kim, Adv. Mater. 12 (2000) 746. [6] X.F. Duan, C.M. Lieber, Adv. Mater. 279 (2000) 208. [7] A. Hagfeldt, M. Gr € aatzel, Chem. Rev. 95 (1995) 49. [8] Z R. Tian, W. Tong, J Y. Wang, N G. Duan, V.V. Krishnan, S.L. Suib, Science 276 (1997) 926. [9] S. Matsuda, Appl. Catal. 8 (1983) 149. [10] Y.C. Yeh, T.T. Tseng, D.A. Chang, J. Am. Ceram. Soc. 73 (1990) 1992. [11] G.L. Li, G.H. Wang, J.M. Hong, J. Mater. Res. 14 (1999) 3346. [12] S. Kobayashi, K. Hanabusa, N. Hamasaki, M. Kimura, H. Shirai, Chem. Mater. 12 (2000) 1523. [13] Y. Lei, L.D. Zhang, G.W. Meng, G.H. Li, X.Y. Zhang, C.H. Liang, W. Chen, S.X. Wang, Appl. Phys. Lett. 78 (2001) 1125. [14] X.Y. Zhang, L.D. Zhang, W. Chen, G.W. Meng, M.J. Zheng, L.X. Zhao, Chem. Mater. 13 (2001) 2511. [15] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 15 (1999) 1307. [16] L.G.J. De Harrt, G. Blasse, J. Sol. Stat. Chem. 61 (1986) 135. [17] L.V. Saraf, S.I. Patil, S.B. Ogale, S.R. Sainkar, S.T. Kshirsager, Int. J. Mod. Phys. B 12 (1998) 2653. [18] N. Serpone, D. Lawless, R. Khairutdinov, J. Phys. Chem. 99 (1995) 16646. [19] Y.X. Jin, G.H. Li, Y. Zhang, Y.X. Zhang, L.D. Zhang, J. Phys.: Condens. Matter. 13 (2001) 913. [20] V.N. Strekalovsky, Yu.M. Polejaev, S.V. Palguev, Oxides with Extrinsic Disorder, Nauka, Moscow, Russia, 1987. [21] A.V. Emeline, N. Serpone, Chem. Phys. Lett. 345 (2001) 105. 304 Y.X. Zhang et al. / Chemical Physics Letters 365 (2002) 300–304 . Hydrothermal synthesis and photoluminescence of TiO 2 nanowires Y.X. Zhang, G.H. Li * , Y.X. Jin, Y. Zhang, J. Zhang, L.D. Zhang Institute of Solid. XRD pattern of the as-prepared nanowires (A and B represent anatase and brookite, respectively). Fig. 2. A typical SEM image of anatase TiO 2 nanowires. Y.X.