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
Trang 1Preparation of TiO 2 nanowire gas nanosensor by AFM anode oxidation
Zhen Li, Minghong Wu, Tiebing Liu, Chao Wu, Zheng Jiao , Bing Zhao
Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200072, China
a r t i c l e i n f o
PACS:
85.65.+h
73.61.Ph
73.40.Gk
Keywords:
TiO 2
Nanowire
Gas nanosensor
AFM anode oxidation
a b s t r a c t
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 TiO2
nanowire are carried out The sensitive characteristic of such TiO2 nanowires to hydrogen is investigated
&2008 Elsevier 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
NH3 and NO2 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 nanonano-tubes,
which could pose as problems in development of sensors based on
them[6]
TiO2 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 TiO2 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 TiO2nanowires to hydrogen
at low temperature is also demonstrated
2 Experiment
In this work, the p-type Si (1 0 0) wafers were used
as substrates First, they were cleaned by standard RCA
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0304-3991/$ - see front matter & 2008 Elsevier B.V All rights reserved.
Corresponding author Tel.: +86 21 69982487; fax: +86 21 69982749.
E-mail address: zjiao@shu.edu.cn (Z Jiao).
Trang 2procedure, on which a 0.3-mm thick SiO2 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 Si3N4 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 inFig 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 : 2nHþþ2ne!nH2
Reaction on the sample : M þ nH2O ! MOnþ2nHþþ2ne
To validate our proposal, the AFM observations of the fabrication of a 70-nm wide TiO2 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 TiO2 wire is successfully fabricated
The resistance of the quantum wire can be expressed as
R ¼ rsd L
d a where a, d, and L are the width, thickness, and length of the nanowire, respectively, and rsis 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 rsd L
dða0ntÞrsd
L
d a0 where a0is the width before oxidation, L is the length, and n is the scanning rate
The resistance change during the AFM oxidation process is shown inFig 4 The curve is fitted according to the above formula The parameter is listed as follow: rs¼33.1 O/area, d ¼ 5 nm,
a0¼691 nm, n ¼ 3.98 nm/s, and L ¼ 1000 nm
The prepared TiO2 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 TiO2nanowire to hydrogen is investigated, as shown inFig 5 The TiO2nanowire is linearly sensitive to hydrogen at low tempera-ture Our results indicate that AFM fabricated TiO2nanowire can
be used to develop nanosensor for detecting hydrogen at low temperature
4 Conclusion Adopting AFM fabrication technique, a 70-nm wide TiO2
nanowire has been fabricated on Ti film By monitoring
Fig 1 Scheme of the fabrication of TiO 2 nanowire.
Trang 3Fig 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.
Trang 4the resistance feedback, the width of AFM fabricated
nanowire can be precisely controlled The semiconductor
TiO2 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
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Fig 4 The resistance change with the AFM oxidation process.
Fig 5 The sensitive characteristic of TiO 2 nanowire to hydrogen gas.