Room temperature gas sensing of p -type Te O2 nanowires

Article abstract of DOI:10.1063/1.2732818

Tellurium dioxide (Te O2) nanowires with a tetragonal structure have been grown by thermally evaporating tellurium metal at 400 °C in air. The nanowires produced have diameters ranging from 30 to 200 nm and have lengths of several tens of micrometers. Gas sensors were fabricated using the obtained Te O2 nanowires. The sensing behavior to N O2, N H3, and H2 S gases at room temperature showed typical characteristics of a p -type semiconductor. The results demonstrate the potential to develop Te O2 nanowire based gas sensors with low power consumption.

Full text of DOI:10.1063/1.2732818

Room temperature gas sensing of p -type Te O 2 nanowires
Zhifu Liu, Toshinai Yamazaki, Yanbai Shen, Toshio Kikuta, Noriyuki Nakatani, and Tokimasa Kawabata
Citation: Applied Physics Letters 90, 173119 (2007); doi: 10.1063/1.2732818
View online: http://dx.doi.org/10.1063/1.2732818
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APPLIED PHYSICS LETTERS 90, 173119 ͑2007͒
Room temperature gas sensing of p-type TeO nanowires
2
a͒
Zhifu Liu, Toshinai Yamazaki, Yanbai Shen, Toshio Kikuta,
Noriyuki Nakatani, and Tokimasa Kawabata
School of Engineering, University of Toyama, Toyama 930-8555, Japan
͑
Received 13 February 2007; accepted 29 March 2007; published online 25 April 2007͒
Tellurium dioxide ͑TeO ͒ nanowires with a tetragonal structure have been grown by thermally
2
evaporating tellurium metal at 400 °C in air. The nanowires produced have diameters ranging from
30 to 200 nm and have lengths of several tens of micrometers. Gas sensors were fabricated using
the obtained TeO2 nanowires. The sensing behavior to NO , NH , and H S gases at room
2
3
2
temperature showed typical characteristics of a p-type semiconductor. The results demonstrate the
potential to develop TeO nanowire based gas sensors with low power consumption. © 2007
2
American Institute of Physics. ͓DOI: 10.1063/1.2732818͔
Oxide semiconductor gas sensors are increasingly de-
manded for industrial safety, environmental monitoring, and
process control because of their small size, low cost, and
the tellurium metal to collect the products. Then, the crucible
was covered and heated in a muffle furnace at 400 °C for
2 h. A white layer was deposited on the lower surface of the
Si wafer and was confirmed to be single crystal TeO2 nano-
wires via x-ray diffraction ͑XRD͒ ͑Shimadzu, XRD-6100͒,
scanning electron microscopy ͑SEM͒ ͑JEOL, JSM-6700F͒,
and transmission electron microscopy ͑TEM͒ ͑JEOL, EM-
002B͒.
1
,2
compatibility with microelectronic processing. To achieve
high performance oxide semiconductor gas sensors, large ef-
fective surface area and small grain size materials are
3
preferred. Therefore, a large surface-to-volume ratio and the
size effect make quasi-one-dimensional oxide semiconductor
nanomaterials as ideal candidates for gas sensing
A typical SEM image of the products is shown in
Fig. 1͑a͒. The XRD measurement ͓Fig. 1͑b͔͒ indicated that
4
applications. The quasi-one-dimensional nanostructure of
5
,6
the TeO nanowires had a well-crystallized tetragonal
well-established gas sensing materials such as SnO ,
2
2
1
9
7
8,9
10
structure. Figure 2͑a͒ is the TEM image of a single TeO2
nanowire. A high resolution TEM image of the selected
nanowire is shown in Fig. 2͑b͒, in which the lattice planes
separated by 0.42 nm can be clearly seen, indicating a single
crystal structure. The selected area electron diffraction
ZnO, WO , Ga O , and In O ͑Ref. 11͒ have higher
3
2
3
2
3
sensitivity, faster response, and/or enhanced capability to de-
tect low concentration gases compared with the correspond-
ing thin film materials. However, most of these gas sensors
made from nanomaterials are effective at only temperatures
above 200 °C resulting in high power consumption and
complexities in integration. Thus there is a need to develop
nanomaterials for gas sensors that have good performance at
a low temperature.
͑
SAED͒ pattern in the inset of Fig. 2͑b͒ also supports the
formation of single crystal tetragonal TeO .
2
Gas sensors were fabricated by dispersing TeO nano-
2
wires on oxidized Si substrates with an interdigitated Pt elec-
trode. Figure 3͑a͒ illustrates the diagram of the gas sensor
Tellurium dioxide ͑TeO ͒ is a versatile wide band gap
2
device based on TeO nanowires. The gap between two Pt
2
semiconductor material. Single crystal TeO has wide appli-
2
1
2
fingers is 0.04 mm. Figure 3͑b͒ is a photograph of TeO2
nanowire gas sensor ready for measurements. The gas sens-
ing properties of the gas sensors were measured in a tubular
facility. Dry synthetic air mixed with the desired concentra-
tion of test gases was flowed at 200 ml/min through the
quartz tube kept at room temperature ͑26 °C͒ and at ambient
pressure. The electrical measurement was done by a volt-
amperometric method at a constant bias of 10 V, and a mul-
timeter ͑Agilent 34970A͒ was used to monitor the change of
electrical resistance upon turning the test gases on and off.
Figure 4͑a͒ shows the resistance response of the TeO2
nanowire derived gas sensor as the ambient gas switched
cations in active optical devices such as deflectors,
modulators, and tunable filters because of its remarkable
acousto-optical and electro-optical properties. Thin film
1
3
14
1
5
TeO has been studied as an optical storage material. TeO2
2
based glasses find applications in laser devices and nonlinear
1
6
optics. TeO is also an important component of propane
2
1
7
oxidation catalysts. We are unaware of the use of TeO as
2
a gas sensing material. In addition there are only limited
1
8
reports on tellurium oxide nanomaterials. In this letter, the
authors describe the synthesis and properties of single crys-
talline TeO2 nanowires. Our TeO2 nanowires are about
30–200 nm in diameter and have lengths of several tens of
from synthetic air to 10, 50, and 100 ppm NO gas at room
2
micrometers. Gas sensors made using TeO2 nanowires
showed good sensing performance to NO , NH , and H S
temperature ͑26 °C͒. The sensor resistance decreases upon
2
3
2
the introduction of NO gas. The initial response to NO gas
2
2
gases at room temperature.
is rapid and the resistance can reach 90% of the maximum
The TeO nanowires were synthesized by the thermal
2
change in about 2 min after the introduction of NO gas. The
2
evaporation of tellurium metal in air in the absence of any
catalyst. High purity tellurium metal ͑99.999%͒ was put into
an alumina crucible of 30 mm height and 20 mm diameter. A
piece of silicon wafer was put in the crucible of 2 mm above
response has no obvious decay at each concentration during
three air-NO -air cycles. Figure 4͑b͒ shows the change in
2
resistance upon exposure to NH at 100, 200, and 500 ppm
3
at room temperature. The response to NH is relatively slow.
3
The resistance is still increasing after a 10 min exposure to
a͒Electronic mail: zhifuគliu@yahoo.com
NH at each concentration and the resistance cannot return to
3
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003-6951/2007/90͑17͒/173119/3/$23.00 90, 173119-1 © 2007 American Institute of Physics
28.114.34.22 On: Wed, 03 Dec 2014 08:17:46
0
1

173119-2
Liu et al.
Appl. Phys. Lett. 90, 173119 ͑2007͒
FIG. 3. ͑a͒ Schematic illustration of the TeO nanowire gas sensor device.
2
The gap between two Pt fingers is 0.04 mm; ͑b͒ photograph of the TeO2
nanowire gas sensor. Gas sensor was connected to the measurement circuit
using gold wires.
relation between sensor response and gas concentration. The
resistance increase of TeO2 nanowire sensor exposed to
5
0 ppm H S is only 30% of that of 100 ppm H S. There is
2
2
no obvious response when exposed to 10 ppm H S gas. Con-
2
sidering the three kinds of gases NO , NH , and H S, it is
2
3
2
important to note that the resistance of the TeO nanowire
2
sensor increases upon exposure to oxidative gas NO and
2
tends to decrease to reductive gases such as NH and H S.
3
2
A widely accepted model to explain the oxide semicon-
ductor gas sensing mechanism suggests that, upon exposure
to reductive gas, chemisorbed oxygen on the semiconductor
surface reacts with the reductive gas species and the elec-
trons trapped by oxygen are released into the semiconductor,
2
0
changing the resistance. For n-type semiconductor, the re-
sistance decreases due to the increase of electron concentra-
tion in the semiconductor. For p-type semiconductor, the re-
sistance increases because of the combination of holes with
electrons released from surface reaction. Upon exposure to
oxidative gases, the gas species act as acceptors, leading to
resistance increase for n-type semiconductor and decrease
for p-type semiconductor. In this investigation, the resistance
of the sensor increases upon NO and decreases upon NH
FIG. 1. ͑a͒ Typical SEM image of TeO2 nanowires grown on Si substrate
and ͑b͒ XRD pattern of TeO nanowires grown on Si substrate. The nano-
2
wires show a tetragonal structure ͑Ref. 19͒.
the initial value even in 30 min after switching to air flow,
implying a slow reaction process on the TeO2 surface.
Figure 4͑c͒ illustrates the resistance response of the TeO2
2
3
and H S, suggesting that TeO nanowires have a p-type
2
2
conduction.
nanowire gas sensor to air-H S-air cycles. The resistance
2
It is known that the remarkable acousto-optical and
change shows good reversibility during the gas switch cycles
electro-optical properties make TeO an important material
2
for 50 and 100 ppm H S gas. However, the response reduces
2
for nonlinear optoelectronic devices. Its electrical properties
much quicker with decreasing H S concentration compared
21–28
2
also attract much attention.
N-type conduction of TeO2
with that of NO and NH gases, which show a nearly linear
2
3
material was observed at high temperature ͑above 400 °C͒
or low oxygen partial pressure, which was explained by the
2
3
presence of oxygen vacancies. However, more evidences of
2
4
22
p-type conduction of TeO single crystal, thin film, or
2
polycrystalline TeO ͑Ref. 28͒ have been obtained via See-
2
beck coefficient or photoconductivity measurements at nor-
mal condition. The p-type conduction mechanism has been
2
8
described using an oxygen-interstitial model, which indi-
cated that oxygen interstitials ͑O ͒ dissolved in TeO lattice
i
2
act as electron trapping centers and, as a result, holes are
majority carriers of conduction. The gas sensing of TeO2
nanowires should reflect the response of hole concentration
change induced by the interaction between the surface states
of TeO and the target gas molecules.
2
With exposure to reductive gases such as NH and H S,
3
2
the oxygen species on the surface of TeO nanowires are
2
consumed by reaction with the reductive gas molecules, and
the electrons trapped by oxygen are released and combined
FIG. 2. ͑a͒ TEM image of a single TeO nanowire and ͑b͒ high resolution
2
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TEM image of the selected TeO nanowire and SAED.
with the holes in the valence band. As a result, the concen-
2
1
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173119-3
Liu et al.
Appl. Phys. Lett. 90, 173119 ͑2007͒
chemical interaction mechanism of NO2 molecules with
TeO nanowires can be described as
2
+
TeO + NO ⇔ TeO − NO
⇔ TeO2
2
2͑g͒
2
2͑ad͒
NO −2 ⇔ TeO − NO
−
2͑ad͒
+ h.
͑1͒
−
͑ad͒
2
Therefore, the concentration of the holes as majority carriers
in TeO nanowires increases and the resistance decreases.
2
Further investigation on the electrical properties and gas re-
sponse of TeO nanowires is underway.
2
In conclusion, single crystal TeO nanowires were syn-
2
thesized by thermal evaporation of high purity tellurium
metal in air. The TeO nanowires have a tetragonal phase
2
structure and when employed as gas sensors showed typical
p-type response and were sensitive to toxic gases such as
NO , NH , and H S at room temperature. These results dem-
2
3
2
onstrated the possibility of making low power consumption
gas sensors using TeO nanowires.
2
The authors are grateful for the support of Venture Busi-
ness Laboratory of the University of Toyama.
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