Home
Search
Collections
Journals
About
Contact us
My IOPscience
The detection of H2S at room temperature by using individual indium oxide nanowire
transistors
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2009 Nanotechnology 20 045503
(http://iopscience.iop.org/0957-4484/20/4/045503)
View the table of contents for this issue, or go to the journal homepage for more
Download details:
IP Address: 140.113.39.244
The article was downloaded on 31/12/2011 at 19:21
Please note that terms and conditions apply.
IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 20 (2009) 045503 (4pp)
doi:10.1088/0957-4484/20/4/045503
The detection of H2S at room temperature
by using individual indium oxide nanowire
transistors
Zhongming Zeng, Kai Wang, Zengxing Zhang,
Jiajun Chen and Weilie Zhou1
Advanced Materials Research Institute, University of New Orleans, New Orleans,
LA 70148, USA
E-mail: [email protected]
Received 25 September 2008, in final form 29 October 2008
Published 18 December 2008
Online at stacks.iop.org/Nano/20/045503
Abstract
In2 O3 nanotransistors for gas sensor applications were fabricated using individual In2 O3
nanowires prepared by chemical vapor deposition. The nanosensors demonstrate characteristics
of high sensitivity to H2 S, and fast response and recovery, with the detection limit at 1 ppm at
room temperature. The high sensitivity might be attributed to the strong electron accepting
capability of H2 S to the nanowires and the high surface-to-volume ratio of the nanowires. In
addition, the nanosensors show a good selective detection of H2 S under exposure to NH3 and
CO even at 1000 ppm; they are highly promising for practical applications in detection of low
concentration H2 S at room temperature.
(Some figures in this article are in colour only in the electronic version)
capable of achieving sensitivity down to ppb levels at room
temperatures [12, 16].
Among the chemicals studied, hydrogen sulfide (H2 S)
is one of the most toxic chemical gases. Even very low
concentration of H2 S can lead to losing consciousness or even
death. Therefore, the detection and test of H2 S gas are of
importance for industry and environmental safety. In present
work, single crystalline In2 O3 nanowires prepared by chemical
vapor deposition were used to fabricate H2 S gas sensors.
The sensors exhibited excellent performance, such as high
sensitivity and selectivity as well as fast recovery response at
room temperature.
1. Introduction
Chemical sensors based on semiconducting oxide materials
have been extensively studied due to their advantageous
features, such as high sensitivity, low cost and simplicity
in fabrication [1–3]. Among them, In2 O3 is a natively ntype semiconductor and the carrier concentration is primarily
determined by the amount of equilibrium oxygen vacancies [4].
In2 O3 materials have been studied as various chemical sensors,
such as O3 , H2 , Cl2 , NH3 , NO2 , and other chemical
species [5–13]. However, most of the earlier works are focused
on In2 O3 thin-films, and some critical limitations are difficult
to be overcome by using thin-film based sensors. For instance,
this kind of sensors usually works at high temperature and
has a limited maximum sensitivity [14, 15], which brings
impossibility for practical applications at lower concentration
and room temperature. One promising method to overcome
those drawbacks is to explore one-dimensional nanostructures,
such as nanowires, for chemical sensors because nanostructure
based sensors have great potential to enhance the sensitivities
due to their large surface-to-volume ratios [11, 12]. The recent
experiments demonstrated that nanostructure based sensors are
2. Experiment
The synthesis of In2 O3 nanowires by chemical vapor
deposition is described as the following. The mixture of
indium oxide and graphite with the weight ration 1:1 was
loaded in an alumina boat, which was placed at the center
of a quartz tube in a horizontal furnace. A piece of (100)
oriented silicon wafer with a 3 nm sputtered gold thin-film
was placed in the downstream to collect the products. The
furnace was heated to 1000 ◦ C and kept for 60 min. During
1 Author to whom any correspondence should be addressed.
0957-4484/09/045503+04$30.00
1
© 2009 IOP Publishing Ltd Printed in the UK
Nanotechnology 20 (2009) 045503
Z Zeng et al
Figure 2. Typical current–voltage ( I –V ) curves of the sensor
recorded at different gate voltages ( Vg = from −16 to +16 V
with a step of 8 V). The insets in top left and bottom right show
corresponding SEM image (scale bar is 2 µm) of a typical In2 O3
nanosensor and its corresponding Ids –Vg curve at a Vds of 2.0 V,
respectively.
Figure 1. (a) Representative SEM image and (b) bright-field TEM
image of In2 O3 nanowires. The inset shows the corresponding
SAED pattern.
(SAED) pattern, which was identified as single crystalline
grown along (100). It can be seen in figure 1(b) that there is
a gold catalyst at the head of the nanowire tip directing In2 O3
nanowire growth, implying that the nanowire growth follows
vapor–liquid–solid (VLS) mechanism [17].
One of the patterned In2 O3 nanowire sensors is shown
in the top left inset of figure 2, where a single nanowire
with diameter around 80 nm bridges the source and drain
electrodes with a width of around 6 µm. The conductance
of the In2 O3 nanowire is supposed to be directly related to
the carrier concentration inside, which in turn can be altered
by applying gate voltage or the adsorbed H2 S molecules due
to its electron accepting capability. In order to confirm the
back-gate effect of the In2 O3 nanowire sensor, current–voltage
(Ids –Vds ) curves of the sensor, as plotted in figure 2, were
recorded by applying different gate voltages (Vg ) while varying
the gate voltage from −16 to +16 V with a step of 8 V. It was
found that the sensor is significantly sensitive to electrostatic
gating, which can be clearly observed in the current Ids –Vg
curve shown in the low right inset of figure 2, where a change
up to 300% in sensor conductance was observed when Vg was
swept from 0 to +16 V. These results indicate that the sensor
is supposed to be very sensitive to dilute molecules [12].
Figure 3 represents room-temperature current–voltage
(Ids –Vds ) characteristics (at Vg = 0 V) recorded in the
five different H2 S concentrations after exposure for 5 min.
The conductance of the sensor was monotonically enhanced
with increasing H2 S concentration, from 42.5 nS before the
exposure to 321.6 nS with 80 ppm H2 S presence. These curves
are found to be rather linear as shown in figure 2, which
indicates good ohmic contacts formed between the nanowire
and the meal electrodes. This ohmic behavior is of importance
to the sensing properties because the sensitivity of the gas
sensor or the ratio of electrical resistance in H2 S gas to dry
air can be maximized.
the synthesis of nanowires, a 50 sccm Ar was introduced
in the tube as a carrier gas and the system pressure was
maintained about 200 Torr. After the system was cooled
down to room temperature, white cotton-like products were
harvested on the Si substrate. Then a Carl Zeiss 1530 VP field
emission scanning electron microscope (FESEM), and a JEOL
2010 transmission electron microscope (TEM) were used to
characterize morphologies and structures of the products.
To fabricate gas sensor, the In2 O3 nanowires were firstly
removed from the Si wafer and suspended in 1–2 ml ethanol
by ultrasonication for 5–10 s. Then the nanowires were
deposited onto doped Si wafers with 600 nm-thick thermal
oxide film. The Ti/Au electrodes were patterned on the ends
of the nanowires through electron beam lithography, metal
evaporation, and lift-off process. The silicon substrate was
employed as a back-gate while the metal contacts worked as the
source and drain electrodes. For the gas sensing measurement,
the sensors were mounted in a small chamber with electrical
feed-through. The system was pumped to vacuum first to clean
the surface of the nanowires, and then the conductance of the
sensors was monitored by a source-meter unit (Keithley 2400)
while the flowing gases (H2 S, NH3 or CO) in dry air were
introduced.
3. Results and discussion
A typical SEM image of In2 O3 nanowires was shown in
figure 1(a), revealing In2 O3 nanowires have a diameter of
30–100 nm and a length up to several tens of micrometers.
Figure 1(b) shows a TEM image of a single In2 O3 nanowire
and its corresponding selected-area electron diffraction
2
Nanotechnology 20 (2009) 045503
Z Zeng et al
Figure 3. Current–voltage ( I –V ) curves of the sensor recorded after
exposure to H2 S of successively increasing concentrations.
Figures 4(a) and (b) show the response of the In2 O3
nanowire gas sensor exposed to different concentrations
for H2 S, NH3 and CO at room temperature and 120 ◦ C,
respectively. The experiments were carried out while the
applied voltage (Vds ) was 3.0 V. It can be seen that the sensors
show high selectivity to H2 S both at room temperature and high
temperature, but no response to NH3 and CO. The sensitivity
to H2 S can reach down to 1ppm level at room temperature,
higher than other In2 O3 nanosensors [18], and the response
and recovery are both very fast, taking only 48 s and 56 s,
respectively, for 98% full response and recovery at 1 ppm of
H2 S.
Figure 5 demonstrates current change as a function of gas
concentration. It can be found that the In2 O3 nanowire gas
sensors are distinctly sensitive to H2 S at room temperature
and the elevated temperature, and increases towards linear
with increasing concentration of H2 S at both room temperature
and the elevated temperatures. It is noted that the sensitivity
(defined as Ig /Ia , where Ig and Ia are the current between
the source and drain in H2 S gas and dry air, respectively)
significantly enhances by increasing testing temperature,
which are from 2 at room temperature to 13.8 at 120 ◦ C under
exposure to 20 ppm of H2 S. However, in the case of NH3
gas, we did not observe any response below 160 ppm, even
at 1000 ppm, only a very weak response was observed, which
is extremely small compared to that of H2 S. A detection of
the response to CO was also performed. No response was
observed for CO both at room temperature and the elevated
temperature for concentration up to 1000 ppm, indicating the
sensor is totally insensitive to CO.
It has been found that the sensing properties for metal
oxides depend on several factors, such as surface area and
surface states, as well as the efficiency with the test gas
molecules adsorbed on the surface [19–21]. In our case, the
nature of H2 S with strong electron accepting capability may
contribute to the highly selectivity and sensitivity. When
the In2 O3 nanowire was exposed to H2 S gas, the strong
reducing gas might react with oxygen ions on the surface and
put back the electrons into In2 O3 semiconductor, resulting
Figure 4. Response of the In2 O3 sensor under exposure to variant
concentration of H2 S at (a) room temperature (RT) and (b) 120 ◦ C.
The inset in (a) shows the response and recovery behaviors of the
sensor at 1 ppm.
Figure 5. The sensitivities of the gas sensor as a function of H2 S gas
concentration at RT and 120 ◦ C.
in the increase of conductance of In2 O3 nanowire. On the
other hand, high surface-to-volume ratio of the nanowires
also contribute to the high sensitivity, which depends on
3
Nanotechnology 20 (2009) 045503
Z Zeng et al
the diameter of the nanowire at a given condition. The
smaller diameter of the nanowires is, the more H2 S molecules
will be absorbed on the surface. As the diameter of the
nanowire used in our experiment is quite small around 80 nm,
it is reasonable that more H2 S molecules absorbed on the
surface of the nanowire generate more electrons into the
In2 O3 semiconductor nanowire, and further enhanced the
sensitivity of our nanosensor. Further studies are still needed
to fully understand the underlying mechanism that governs the
sensitivity and selectivity of our devices.
References
[1] Williams D E 1999 Sensors Actuators B 57 1
[2] Bendahan M, Boulmani R, Seguin J L and Aguir K 2004
Sensors Actuators B 100 320
[3] Uoefer U, Frank J and Fleischer M 2001 Sensors Actuators B
78 6
[4] Bellingham J R, Mackenzie A P and Phillips W A 1991
Appl. Phys. Lett. 58 2506
[5] Gurlo A, Barsan N, Weimar U, Ivanovskaya M, Taurino A and
Siciliano P 2003 Chem. Mater. 15 4377
[6] Chung W Y 2001 J. Mater. Sci. Mater. Electron. 12 591
[7] Chung W Y, Sakai G, Shimanoe K, Miura N, Lee D D and
Yamazoe N 1998 Sensors Actuators B 46 139
[8] Tamaki J, Naruo C, Yamanoto Y and Matsuoka M 2002
Sensors Actuators B 83 190
[9] Kiriakids G, Bender M, Kataarakis N, Gagaoudakis E,
Hourdakis E, Douloufakis E and Cimalla V 2001
Phys. Status Solidi a 185 27
[10] Jiao Z, Wu M H, Gu J Z and Sun X L 2003 Sensors Actuators
B 94 216
[11] Li C, Zhang D, Liu X, Han S, Tang T, Han J and Zhou C 2003
Appl. Phys. Lett. 82 1613
[12] Zhang D H, Liu Z Q, Li C, Tang T, Liu X L, Han S, Lei B and
Zhou C 2004 Nano. Lett. 4 1919
[13] Xua J Q, Wang X H and Shen J N 2006 Sensors Actuators B
115 642
[14] Winter R, Scharnagl K, Fuchs A, Doll T and Eisele I 2000
Sensors Actuators B 66 85
[15] Xie D, Jiang Y, Pan W, Li D, Wu Z and Li Y 2003 Sensors
Actuators B 90 163
[16] Comini E, Faglia G, Sberveglieri G, Pan Z W and Wang Z L
2002 Appl. Phys. Lett. 81 1869
[17] Chen Y X, Compbell L R and Zhou W L 2004 J. Cryst. Growth
270 505
[18] Xu J Q, Chen Y P, Pan Q Y, Xiang Q, Cheng Z X and
Dong X W 2007 Nanotechnology 18 115615
[19] Franke M E, Koplin T J and Simon U 2006 Small 2 36
[20] Chen Y, Zhu C L and Xiao G 2006 Nanotechnology 17 4537
[21] Feng P and Wang T H 2005 Appl. Phys. Lett. 87 213111
4. Conclusions
In summary, we have demonstrated a highly sensitive and
selective H2 S nanosensor by using a single In2 O3 nanowire
transistor. The nanosensor works at room temperature and
exhibits a limited detection of 1 ppm for H2 S, and the
sensitivity increases with increasing the concentration and
operating temperature. The response and recovery are both
very fast. Moreover, the nanosensor demonstrates extremely
weak response to NH3 and totally insensitive to CO, which
is highly promising for practical application for detecting low
concentration of H2 S.
Acknowledgments
This work was supported by the DARPA Grant No. HR001107-1-0032 and research grants from Louisiana Board of
Regents Contract Nos. LEQSF(2007-12)-ENH-PKSFI-PRS04 and LEQSF(2008-11)-RD-B-10. W L Zhou acknowledges
the partial support from the Research Fund of Key Laboratory
for Nanomaterials, Ministry of Education (No. 2007-1).
4