preparation of tio2 nanowire gas nanosensor by afm anode oxidation

Preparation of TiO 2 nanowire gas nanosensor by AFM anode oxidation Zhen Li, Minghong Wu, Tiebing Liu, Chao Wu, Zheng Jiao à , Bing Zhao Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200072, China article info PACS: 85.65.+h 73.61.Ph 73.40.Gk Keywords: TiO 2 Nanowire Gas nanosensor AFM anode oxidation abstract Applications of atomic force microscopy (AFM) to the fabrication of chemical nanosensors are presented in this paper. Using AFM cantilever as cathode, the surface of Ti thin film is oxidized to form a few tens of nanometers wide oxidized metal semiconductor wire, which works as a nanowire-based hydrogen sensor. The reaction mechanism is proposed. The AFM observations of fabrication of a TiO 2 nanowire are carried out. The sensitive characteristic of such TiO 2 nanowires to hydrogen is investigated. & 2008 Elsevie r B.V. All rights reserved. 1. Introduction Presently, central to detection is the signal transduction associated with selective recognition of a biological or chemical species of interest. Nanostructures, such as nanowires, offer new and sometimes unique opportunities in this interesting and interdisciplinary field of science and technology [1,2]. The diameters of these nanowires are comparable to the sizes of biological and chemical species being sensed, and thus intuitively represent excellent primary transducers for producing signals that ultimately interface with macroscopic instruments. For instance, inorganic nanowires have exhibited unique electrical and optical properties that can be exploited for sensing. It has been reported that the properties of gas sensors could be great improved by adopting nanoscale semiconducting oxide powders [3,4]. Kong et al. [5] have reported the high sensitivity of the individual semiconducting single-walled carbon nanotubes to NH 3 and NO 2 at room temperature. However, there are great difficulties in fabricating the pure semiconducting carbon nano- tubes, as well as modifying the surface of the carbon nanotubes, which could pose as problems in development of sensors based on them [6]. TiO 2 has been found as an ideal alternative in assembly of humidity or gas sensors [7] as well as in catalyst support [8] due to their unique dielectric and chemical properties. Furthermore, their photocatalyst activities [9] could also result in some potential applications, such as environmental purification, decomposition of carbonic acid gas, and generation of hydrogen gas. During the past several years, a variety of methods have been developed to synthesize TiO 2 nanoparticles [10], nanowhiskers [11], nanobelts [12], and nanowires [13], respectively. It is notable that various types of scanning probe microscopy (SPM), such as scanning tunnel microscopy (STM) and atomic force microscopy (AFM), have been used in nanoscale fabrication. A typical use of SPM is to measure the topo- graphy of a surface by bringing a cantilever beam into contact with a sample and observing the deflection of the cantilever when it is scanning across the surface. Moreover, a voltage will be introduced between the probe and the sample to investigate the topography. Since both position of the probe and distance between it and the sample can be possibly manipulated, SPM is considered as one of the best ways to execute nanoscale fabrication. Accordingly, many related works have been reported since Dagata et al. [14] presented the STM direct writing oxidation process using the oxide as a mask for a pattern transfer. STM-based anodic oxidation on metallic substrates, such as Ti, has been also reported [15–17]. So far, although AFM has been proved effective in generation of several kinds of oxide patterns on Ti [18–22], assembly of the corresponding patterns of nanowires by this means is still a challenge. In this paper, we report on the oxide nanowires on Ti thin film surface using contact mode AFM in ambient atmosphere. Mean- while, high sensitivity of the resulted TiO 2 nanowires to hydrogen at low temperature is also demonstrated. 2. Experiment In this work, the p-type Si (10 0) wafers were used as substrates. First, they were cleaned by standard RCA ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ultramic Ultramicroscopy 0304-3991/$ -see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2008.04.059 à Corresponding author. Tel.: +86 2169982487; fax: +86 2169982749. E-mail address: zjiao@shu.edu.cn (Z. Jiao). Ultramicroscopy 108 (2008) 1334– 1337 procedure, on which a 0.3-mm thick SiO 2 surface layer was then grown. Subsequently, a 12-nm thick Ti layer was sputtered on the Si substrates with a deposition rate of 0.1 nm/s. The specimen were glued onto the holder by silver paste, and then placed in a SEIKO’s SPI3700/SPA300 AFM, which provides anodic oxidation functions. By scanning the cantilever with the applied bias, negative to the cantilever and positive to the metal thin film, the surface of the metal was oxidized to form the metal oxide nanostructure. The scheme of the fabrication titanium oxide nanowire is shown in Fig. 1. The AFM worked in contact mode and relative humidity was 60%. The cantilevers were Au coated Si 3 N 4 tips available from Olympus Company. 3. Results and discussion For AFM anode oxidation, electrochemical reaction might take place to form nanoscale oxide pattern on the sample surface, since a thin adsorbed water film exists on surface of the sample. As shown in Fig. 2, the sample is covered with a thin layer of water under the moist atmosphere. When the AFM tip moves near to the sample surface, the surface molecular cohesive forces between the surface water layer both on AFM tip and sample surface will lead to a water contact between AFM tip and sample surface. Provided bias is introduced between AFM tip and sample surface through water contact, then the electrochemical reaction takes place. If the sample acts as anode, the sample surface under AFM tip will be oxidized. At the same time the deoxidization reaction is carried out on the AFM tip, resulting in the faraday current through AFM tip and sample. Reaction on AFM tip : 2nH þ þ 2ne À ! nH 2 Reaction on the sample : M þ nH 2 O ! MO n þ 2nH þ þ 2ne À To validate our proposal, the AFM observations of the fabrication of a 70-nm wide TiO 2 wire are shown in Figs. 3(a)–(e) in sequence, during which the bias is 18 V and the scanning rate is 1 mm/s. The Ti film will be completely oxidized along vertical direction, and as such becomes insulate. Following the above process, a 70-nm wide TiO 2 wire is successfully fabricated. The resistance of the quantum wire can be expressed as R ¼ r s Á d Á L d Á a where a, d, and L are the width, thickness, and length of the nanowire, respectively, and r s is the resistance per area. According to the above formula, resistance R is in inverse proportion to width a, and a decreases with the increase of the resistance. The time dependence of the resistance can be expressed by the formula as DR ¼ r s d  L dða 0 À ntÞ À r s d  L d Á a 0 where a 0 is the width before oxidation, L is the length, and n is the scanning rate. The resistance change during the AFM oxidation process is shown in Fig. 4. The curve is fitted according to the above formula. The parameter is listed as follow: r s ¼ 33.1 O/area, d ¼ 5 nm, a 0 ¼ 691 nm, n ¼ 3.98 nm/s, and L ¼ 1000 nm. The prepared TiO 2 nanowire were placed in a small sealed glass chamber with a certain concentration of hydrogen inside, and Ti were used as electrode. The whole chamber was placed on an oven and heated to 80 1C, and the sensitive characteristic of TiO 2 nanowire to hydrogen is investigated, as shown in Fig. 5. The TiO 2 nanowire is linearly sensitive to hydrogen at low tempera- ture. Our results indicate that AFM fabricated TiO 2 nanowire can be used to develop nanosensor for detecting hydrogen at low temperature. 4. Conclusion Adopting AFM fabrication technique, a 70-nm wide TiO 2 nanowire has been fabricated on Ti film. By monitoring ARTICLE IN PRESS Fig. 1. Scheme of the fabrication of TiO 2 nanowire. Fig. 2. Mechanism of AFM anode oxidation. Z. Li et al. / Ultramicroscopy 108 (2008) 1334–1337 1335 ARTICLE IN PRESS Fig. 3. The AFM image of the fabrication sequence of a 70-nm wide TiO 2 nanowire from (a)–(e). (a) Oxidization in 5 mm  20 mm area, (b) oxidization in 5 mm  10 mm area, (c) oxidization in 4 mm  4 mm area, (d) oxidization in 1 mm  1 mm area, and (e) oxidization in 70 nm  1 mm area. Z. Li et al. / Ultramicroscopy 108 (2008) 1334–13371336 the resistance feedback, the width of AFM fabricated nanowire can be precisely controlled. The semiconductor TiO 2 nanowire displays high sensitivity to hydrogen at low temperature. Acknowledgments This work was financially supported by Shuguang project (07SG46), 973 program (2006CB705604) and Program for New Century Excellent Talents in Universities (NCET 05-0434), China. References [1] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [2] T. Sasaki, S. Nakano, S. Yamauchi, M. Watanabe, Chem. Mater. 9 (1997) 602. [3] G. Martinelli, M.C. Carotta, E. Traversa, G. Ghiotti, MRS Bull. 24 (1999) 30. [4] Y. Lei, L.D. Zhang, J.C. Fan, Chem. Phys. Lett. 338 (2001) 231. [5] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. 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