The W18O49nanowires of high surface areas exhibit a concentration-dependent response when exposed to ammonia.. The response of W18O49nanowires to ammonia transited from n-type to p-type
Trang 1Sensors and Actuators B 137 (2009) 27–31
Contents lists available atScienceDirect Sensors and Actuators B: Chemical
j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / s n b
Y.M Zhao, Y.Q Zhu∗
Nanotubes Laboratory, Division of Materials, Mechanics and Structures, Faculty of Engineering, The University of Nottingham, University Park,
Nottingham NG7 2RD, UK
a r t i c l e i n f o
Article history:
Received 10 July 2008
Received in revised form
22 December 2008
Accepted 3 January 2009
Available online 19 January 2009
Keywords:
Gas sensor
Tungsten oxide nanowire
Ammonia
a b s t r a c t
This paper describes the room temperature ammonia sensing properties of ultra-thin W18O49nanowire (diameter less than 5 nm) bundles prepared by a solvothermal technique The W18O49nanowires of high surface areas exhibit a concentration-dependent response when exposed to ammonia An abnormal behavior of resistance increases first and followed by a decrease was observed when ammonia concentra-tion is above 1 ppm The response of W18O49nanowires to ammonia transited from n-type to p-type when the concentration decreased to sub-ppm level The W18O49nanowires are highly sensitive to sub-ppm and ppb level ammonia at room temperature, which is attributed to the small diameters, high surface areas and non-stoichiometric crystal structure
© 2009 Elsevier B.V All rights reserved
1 Introduction
Metal semiconductor oxides, such as SnO2, TiO2 and WO3,
etc., have been widely explored for application in detecting toxic
gases[1–4] The principle for the applications of these materials
in gas sensing is based on the fact that the electrical resistance
of the materials will be changed when they are exposed to the
gases Typically, semiconducting oxide type gas sensors are
clas-sified to p-type (resistance increases, e.g CuO and Cr2O3) and
n-type (resistance decreases, e.g SnO2 and ZnO) when exposed
to reducing gas (e.g NH3and CO)[4,5] Reducing the crystal size
and increasing the specific surface area of the sensing
materi-als are an effective way to enhance the sensing signmateri-als, since
the reduction/oxidation reactions are mainly affected by the
sur-faces of the oxide [6–9] Gas sensors based on single nanotube
or nanowire have been reported to exhibit high sensitivity, fast
response time, and operable ability at room temperature that are
unattainable for conventional materials[10–13] At present,
inves-tigations on single nanowire sensor are rather rare because of
the challenge in manipulation, fabrication, reproducibility and low
mechanical stability of a single nanowire Most of the studies are
based on films sensors comprising of nanowires Room
temper-ature detections of low concentration level (sub-ppm) ammonia
are of great importance in clinical diagnosis and food safety We
here report the ammonia detection properties of ultra-thin
tung-sten oxide nanowires with a diameter less than 5 nm at room
temperature
∗ Corresponding author.
E-mail address:yanqiu.zhu@nottingham.ac.uk (Y.Q Zhu).
2 Experimental
Tungsten oxide nanowires were prepared by a solvothermal technique 100 mg of WCl6(Sigma–Aldrich, 99.99%) was slowly dis-solved in 50 ml cyclohexanol to obtain a uniform solution The solution was transferred to a 125 ml Teflon-lined stainless steel pressure vessel for heating at 200◦C for 5 h After reaction, a blue precipitate was centrifuged and washed with deionized water and acetone for several times The resulting powder was pure tung-sten oxide nanowires (WONWs), which was subject to structural and morphological characterization by using scanning electron microscopy, transmission electron microscopy and powder X-rays diffraction utilizing a Cu K␣ radiation source having a wavelength
of 0.154 nm
An image of the sample assembly for the gas sensing mea-surement is shown in Fig 1 In order to assembly the tungsten oxide nanowires gas sensors, two gold electrodes with a gap of about 80m were first fabricated by cold sputtering of a thin layer of gold onto a masked quartz plate Then, a layer of tungsten oxide nanowire film was casted onto the substrate spinning across the two gold electrodes using a casting suspension of WONWs in ethanol The sensor sample was heated at 200◦C for 2 h to evap-orate the organic species and to improve the contact between the nanowires and the gold electrodes After the heat treatment, two conducting wires were attached to the two gold electrodes by sil-ver paste or by conductive tape The electrical resistance of the nanowire samples bridging the gap between the two gold elec-trodes at room temperature will be measured as a response to the gas The sensor sample was placed in a quartz test chamber (about
50 ml) A continuous flow of mixed example gas passes through the test chamber Gases were applied through two separate lines 0925-4005/$ – see front matter © 2009 Elsevier B.V All rights reserved.
Trang 2Fig 1 The sample assembly (the two gold electrodes with a gap of about 80m
deposited on glass by cold sputtering, top; with W 18 O 49 nanowires deposition
bridg-ing the gold gap, and silver paste electrode connections, bottom).
The first one passes dry air (compressed air, zero grade, BOC gases)
which is used to dilute the target toxic gas The other one
car-ried calibration gas of known concentration (Ammonia, 1000 ppm
balanced with air or 5 ppm balanced with nitrogen, BOC gases)
The desired gas concentration was obtained by varying the flow
rates of the target gases while the flow rate of dry compressed was
fixed The large flow rate dry compressed air, air usually at least ten
times faster than that of the target gas, was introduced into the
test chamber continuously throughout the whole measurement
The electrical current of the sensor sample was monitored when
a fixed potential was applied between the two gold electrodes by a
computer controlled CHI 650 potentiostat
3 Results and discussion
The SEM images of the WONWs are shown inFig 2a, exhibiting
the highly pure and uniform nanowires with a high aspect ratio
Fur-ther characterization by TEM (Fig 2b) reveals that the nanowires
shown in the SEM image (Fig 2a) are actually WONW bundles
consisting of ultra-thin nanowires with diameters of 2–5 nm, with
orientation parallel to each other The electron diffraction (ED)
pat-tern collected from an isolated bundle displays elongated spots in
a direction vertical to the nanowire growth direction attributed
to the alignment of the ultra-thin nanowires comprising the
bun-dle, as shown in the inset ofFig 1b The ED pattern suggests that
the nanowires grow along [0 1 0].Fig 3shows the XRD profile of
the WONWs, and all of the peaks can be indexed to the
mon-oclinic W18O49 (JCPDF No.71-2450) The specific surface area of
the WONW samples based on Brunauer–Emmett–Teller (BET)
gas-sorption measurements have been calculated to be 151 m2/g, which
is higher than that of the mesoporous WO3thin films (143 m2/g)
[14], about one hundred times of the best for commercial WO3
particles (1.7 m2/g)[15], and five times of the reported value for
nano-sized monoclinic WO3powders (less than 25 m2/g)[15]
Pho-toluminescence characterization of the tungsten oxide nanowires
by the solvothermal treatment of tungsten chloride in
cyclohex-anol, as we published previously[16], showed a ultraviolet (UV)
Fig 2 (a) SEM and (b) TEM images of the WONWs synthesized by solvothermal
process; the inset is the ED pattern.
Trang 3Y.M Zhao, Y.Q Zhu / Sensors and Actuators B 137 (2009) 27–31 29
Fig 4 Electrical response of the WONWs film to 200 ppm NH3
emission peak at 369 nm (3.36 eV) assigned to band–band
transi-tion and a strong blue emission peak at 423 nm (2.94 eV) attributed
to large amounts of oxygen vacancies that are often implicit in the
preparation of oxide semiconductors
The room temperature electrical response of the nanowire
sen-sor to a pulse of 200 ppm (balance air) NH3is shown inFig 4 The
current drops sharply when NH3was introduced into the
cham-ber, and it is then increased slowly to reach a stable state When the
NH3flow was cut off, the electrical current recovered and increased
quickly Heat treatment at elevate temperatures usually is used to
stabilize the current However, heat treatment at 400◦C and above
will significantly change the morphology and reduce the surface
areas of the ultra-thin WONWs, as we have reported elsewhere[17]
Therefore, in this work the sensor sample was annealed at 200◦C
for 2 h The baseline drift of the currents occurred during the
mea-surement due to the low annealing temperature and low operation
temperature[15,18,19] It is well-known that for a typical n-type
metal oxide semiconducting material, the electrical resistance
normally decreases when exposed to a reducing gas, such as
ammo-nia[5] However, the electrical response of the present WONW
sensor exhibits something unusual, as shown inFig 4, and a
resis-tance increase was observed Non-stoichiometric tungsten oxide
nanowires has been reported to experience a conductivity-type
change from p-type at room temperature to n-type at temperature
above 150◦C when exposed to 100 ppm ammonia[6] Abnormal
behavior of an abrupt decrease followed by a slow increase of the
electrical resistance of a nanocrystalline WO3sensor has also been
recorded[20]
Fig 5shows the electrical response of the WONWs to 45 and
400 ppm NH3 When exposed to 45 ppm NH3, a similar behavior
of sharp decrease and then slow increase in the electrical current
was observed; when a pulse of 400 ppm NH3was introduced, the
current decreased sharply again and followed by a fast increase
The stabilized current is higher than that in the synthetic dry air
The dynamic electrical response of the WONWs sensor to different
concentrations of NH3ranging from 0.1 ppm to 10 ppm was shown
inFig 6 In the range of 0.1–1 ppm NH3, the sensor exhibits a good
p-type response (the current decreases fast and then maintains at
a stable value) and the signal intensity increases with increasing
gas concentration, as shown in Fig 6 Above 5 ppm, the
abnor-mal behavior again appears (the current decreases suddenly and
then followed by an increase) The responses of the WONWs to
ammonia are dependent on the concentration of the ammonia and
experience a conductivity-type change with the increasing of the
ammonia concentration The electrical resistance changes of the
Fig 5 Electrical response of the WONWs to 45 ppm and 400 ppm NH3
W18O49nanowires when exposed to ammonia at room tempera-ture is believed to be caused by the variation of the surface acceptor states density related to the chemisorbed oxygen The surfaces of the ultra-thin diameter (less than 5 nm) of the W18O49nanowires are more active than the conventional bulk fully oxidized tung-sten oxide materials The large amounts of oxygen vacancies in the reduced tungsten oxide W18O49can serve as adsorption site The small diameter and large amounts of oxygen vacancies in the WONWs will facilitate the chemisorption of oxygen at a low temper-ature William has predicted that if the grain size of the materials is smaller than the depletion layer thickness (Debye-length), the grain could be considered as completely depleted so that the conductiv-ity would become surface-trap limited[5] Taking into account of the equilibrium between gaseous oxygen and surface oxygen ions, the Debye-length can be defined as
d =
2εε0KT
where ε is the material’s relative dielectric constant and ε0 is
dielectric constant of vacuum, K is the Boltzmann constant, T
is the absolute temperature, e is the electron charge, and N D is bulk donor density The conductivity of such materials can be expressed in terms of the surface acceptor states density formulated
Fig 6 Electrical response of WONWs to NH of different concentrations.
Trang 4Fig 7 Electrical responses of WONWs to (a) 100 ppb and (b) 10 ppb NH3 ; (c) and (d) after the subtraction of the baseline in (a) and (b).
as chemisorbed oxygen species:
e = eKIND
NA +pKIINA
KIND
(2)
where K I = n(1 − f A )/f A and K II = pn, where e and p are electron
and hole mobility respectively, p and n are concentration of hole
and electron respectively, f Ais fraction of acceptors with trapped
electrons The interaction of the surface with the gas presented
in air will cause the surface acceptor state density change From
the above equation it can be seen that the conductivity is not
lin-early dependent on the surface acceptor state The conductivity will
decrease pass through a minimum and then increase again with
the increases of N A The minimum occurs when N Ais satisfying the
following condition:
NA= ND
eKI
pKII
(3)
If the bulk donor density N Dis too large or too small the
achiev-able surface state density N Acould not reach the minimum and the
material will exhibit a pure n-type or p-type response to the gas
pre-sented However, if the N D is in a proper range that N Acould reach
the minimum with variation of gas concentrations and a change
of the sign in conductivity variation will be observed The
abnor-mal behavior of the response of the WONWs to ammonia can be
explained if the diameter of the WONWs (less than 5 nm) is less
than the Debye-length at room temperature when the bulk donor
density of the WONWs is in a proper range By adopting the typical
values of√
ε = 2.29 and N = 4–6× 1015cm3for typically sputtered
tungsten oxide thin films[21,22], the Debye-length at room tem-perature is calculated to be about 45–50 nm We cannot use this value directly as the Debye-length for the WONWs since the donor density of the WONWs is affected by the oxygen vacancies in the nanowires However, for a reference, the ultra-small diameter of the WONWs and the conductive sign change observed in the experi-mental suggest that the diameter of the nanowires may be smaller than the Debye-length of the WONWs at room temperature Nevertheless, the electrical response of the WONWs inFig 6
is indicative that the WONWs sensor is particularly suitable for
NH3detection at sub-ppm level In fact, low concentration ammo-nia detection is actually desired for the development of highly sensitive sensor Electrical responses of the sensor to low concen-tration of NH3down to 10 ppb were studied.Fig 7a and b shows the electrical response to 100 ppb and 10 ppb NH3 Good signals were recorded at low concentration of 10 ppb NH3, even with the general baseline drift at room temperature.Fig 7c and d displays the electrical response of the WONW sensor to 100 ppb and 10 ppb
NH3, after the subtraction of the baseline. It is clear that stable and repeatable signals were obtained for both low concentrations
of 100 ppb and 10 ppb NH3.These results show that this type of WONWs sensor is indeed capable of NH3 detection at ppb level, which is still a big challenge for conventional sensing materials [23–25] A simple comparison with other forms of tungsten oxide
in NH3detection reveals the advantages of the present WONW sen-sors It has been reported that pure WO3do not respond to 100 ppm ammonia at room temperature[26] W18O49nanowires have been found to be responsive to 10 ppm ammonia at room temperature
Trang 5Y.M Zhao, Y.Q Zhu / Sensors and Actuators B 137 (2009) 27–31 31
[6] Fully oxidized WO3only have several active sites, however the
large amounts of oxygen vacancies in the reduced tungsten oxide
W18O49can serve as adsorption sites of gas molecular [27] The
current high specific surface areas of the oxide nanowires may be
due in part to a combination of the small diameters of individual
nanowires comprising the bundles and the unique packing
char-acteristic of the bundles themselves Barret–Joyner–Halenda (BJH)
analysis for the pore size distribution reveal an apex centred at
about 2.2 nm The pores are considered to arise from two possible
mechanisms: (1) direct formation within nanowires during the low
temperature solvothermal preparation technique and (2) indirect
formation via the inter-nanowire spaces within a bundle The high
sensitivity of the W18O49nanowires sensor should be attributed to
the small diameter (less than 5 nm), the high surface area and the
non-stoichiometric crystal structure
4 Conclusion
We have investigated the ammonia sensing properties of
W18O49 nanowires prepared by a low temperature
solvother-mal technique The electrical resistance of W18O49 nanowires is
increased when exposed to sub-ppm level ammonia, exhibiting
a p-type behavior The electrical resistance of W18O49nanowires
increases first and then decreases when exposed to ammonia with
a concentration more than 1 ppm The abnormal behavior of the
W18O49nanowires to ammonia is related with the ultra-thin
diam-eter (less than 5 nm) The high sensitivity of the W18O49nanowires
in the detection of sub-ppm to ppb level ammonia at room
tem-perature is attributed to the small diameter, high surface area and
non-stoichiometric crystal structure
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Biographies Y.M Zhao has just received her PhD title from The University of Nottingham, UK She
received her first degree and master degree in Materials Science and Engineering in University of Science and Technology (Beijing, China) in 1999 and 2002, respectively Her research interests focus on the preparation, characterization and application of novel inorganic one-dimensional nanomaterials.
Yanqiu Zhu obtained his BSc and MSc degree at Harbin Institute of Technology
(Harbin, China) in 1989 and 1992, and his PhD degree from Tsinghua University (Beijing China) in 1996 His research covers carbon nanotubes and a variety of inorganic nanomaterials He is a Reader in Nanomaterials at the University of Not-tingham (UK) with a research interest focusing on the synthesis and application of nanomaterials.