Available online at www.sciencedirect.com
Sensors and Actuators B 130 (2008) 589–593
NO
x
sensing properties of In
2
O
3
nanoparticles prepared by metal
organic chemical vapor deposition
Ch.Y. Wang
∗
, M. Ali, Th. Kups, C.-C. R ¨ ohlig, V. Cimalla, Th. Stauden, O. Ambacher
Institute of Micro- and Nanotechnologies, Technical University Ilmenau, P.O. Box 100565, 98684 Ilmenau, Germany
Received 30 July 2007; received in revised form 10 October 2007; accepted 11 October 2007
Available online 18 October 2007
Abstract
In
2
O
3
nanoparticles were deposited by low-temperature metal organic chemical vapor deposition. The response of 10-nm thick In
2
O
3
particle
containing layers to NO
x
and O
2
gases is investigated. The lowest detectable NO
x
concentration is ∼200 ppb and the sensor performance is strongly
dependent on the gas partial pressure as well as on the operating temperature. The sensor response towards 200 ppm of NO
x
is found to be above
10
4
. Furthermore, the cross-sensitivity against O
2
is very low, demonstrating that the In
2
O
3
nanoparticles are very suitable for the selective NO
x
detection.
© 2007 Elsevier B.V. All rights reserved.
Keywords: MOCVD; In
2
O
3
nanoparticles; NO
x
1. Introduction
Demands for monitoring of nitrogen oxide gases (NO
x
),
which are air pollutants, released from the combustion exhaust
of automobile engines, home heaters, furnaces and plants, have
become more serious all over the world in recent years. In par-
ticular the detection of NO
x
gas at low concentrations, from
sub-ppm to few ppm, has recently attracted much attention [1,2],
since NO
x
of a concentration at sub-ppm levels is already con-
sidered to be harmful to human health.
In order to detect a low concentration of NO
x
gases, many
efforts on metal oxide based sensors have been made due to
their structural simplicity, high sensitivity, short response time,
small size, low cost and good compatibility with the fabrica-
tion process for microelectronic devices [3–5]. Furthermore,
metal oxides, like cuprous oxide [1], tungsten oxide [6] and
indium oxide (In
2
O
3
) [2,7], with differing structural properties
(e.g. amorphous, polycrystalline and nanostructured) have been
found to be sensitive to NO
x
gases at sub-ppm levels. Among
them, In
2
O
3
in particular In
2
O
3
nanostructures exhibit a very
high sensitivity to oxidizing gases like NO
x
[8] and O
3
[9,10].
Li et al. examined the NO
2
-sensitivity of In
2
O
3
nanowires and
∗
Corresponding author. Tel.: +49 3677 693409; fax: +49 3677 693499.
E-mail address: chunyu.wang@hotmail.com (Ch.Y. Wang).
found that the lowest detectable concentration was 5 ppb [7].
Therefore, In
2
O
3
nanostructures are very suitable to be used as
an active material in NO
x
detectors, especially at sub-ppm levels.
Common deposition methods of In
2
O
3
films are evapora-
tion [11], magnetron sputtering [12], and sol–gel processing
[13]. Using these technologies, polycrystalline In
2
O
3
films can
be grown. Although In
2
O
3
, having a variety of electrical [14]
and structural properties (i.e. single crystalline [15], polycrys-
talline [16], and nanostructured [17]), can be obtained by metal
organic chemical vapor deposition (MOCVD), the growth of
In
2
O
3
films by means of MOCVD is rarely reported in available
literature.
In our previous work, highly textured In
2
O
3
(1 0 0) films
grown by MOCVD [18] were tested and found to be suitable
for a NO
x
sensing material [19]. Highly textured In
2
O
3
(1 0 0)
films were grown at a substrate temperature of 600
◦
C by means
of the MOCVD method. The mean diameter of In
2
O
3
grains was
determined to be ∼270 nm. The lowest detectable NO
x
concen-
tration was determined to be 2 ppm with the help of In
2
O
3
(1 0 0)
film as the NO
x
sensing layer operated between 150 and 200
◦
C.
The present work focuses on In
2
O
3
nanoparticle containing lay-
ers, prepared by low-temperature MOCVD, for the detection of
NO
x
gases. In
2
O
3
was deposited at a low substrate temperature
(200
◦
C), forming a 10-nm thick In
2
O
3
nanoparticle containing
layer with a mean crystallite diameter of ∼7 nm. The main goal
of this work is to determine the response to low-concentration
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2007.10.015