h2s sensors based on tungsten oxide nanostructures

Rao∗ Chemistry and Physics of Materials Unit, DST Unit on Nanoscience and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Ba

Sensors and Actuators B 128 (2008) 488–493

Chandra Sekhar Rout, Manu Hegde, C.N.R Rao∗

Chemistry and Physics of Materials Unit, DST Unit on Nanoscience and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India

Received 12 March 2007; received in revised form 28 June 2007; accepted 4 July 2007

Available online 10 July 2007

Abstract

Nanoparticles and nanoplatelets of WO3 and nanowires of WO2.72 have been investigated for their H2S-sensing characteristics over the 1–1000 ppm concentration range at 40–250◦C The nanoparticles and nanoplatelets of WO3 exhibit response values of 757 and 1852, respec-tively to 1000 ppm H2S at 250◦C, respectively, compared to the response of 3313 of the nanowires of WO2.72 Interestingly, the response of the nanowires is satisfactory (121) to 10 ppm H2S at 250◦C, while a large response (240) is observed to 1000 ppm H2S even at 40◦C The WO2.72 nanowires emerge as a good candidate for H2S sensors, with little effect of humidity up to 60% relative humidity as well as satisfactory response and recovery times

© 2007 Elsevier B.V All rights reserved

Keywords: Nanostructures; Nanoparticles; Nanowires; Chemical sensors

1 Introduction

Semiconducting metal oxides have been widely used for

sens-ing gases and vapors In the last few years, nanostructures of

metal oxides have been found to be effective as gas-sensing

materials[1–6] Detection of nitrogen oxides[7], hydrocarbons

[8], H2[9], C2H5OH[10], NH3and CO[11]has been

demon-strated using metal oxide nanostructures We were interested

in developing sensors for H2S using metal oxide

nanostruc-tures, since H2S is a toxic gas used in chemical laboratories

and industries H2S is also liberated in nature due to biological

processes and also from mines and petroleum fields In the

lit-erature, there are reports where films of WO3have been used

for sensing H2S at ppm level with the response values

vary-ing between 3 and 104 depending on the temperature and the

gas concentration[12–17] Nanoparticulate WO3films show a

response of 3–5 to 1 ppm of H2S at 200◦C [15] Active

lay-ers of pure and Pt-doped WO3films deposited by rf magnetron

sputtering were able to sense 100 ppb of H2S at 200◦C[16].

Rf sputtered WO3films and films doped with Pt, Au, Ag, Ti,

SnO2, ZnO and ITO have been examined; the response was

improved by Au to H2S[17] Tungsten oxide nanocrystalline

∗Corresponding author Tel.: +91 80 2208 2761; fax: +91 80 2208 2760.

E-mail address:cnrrao@jncasr.ac.in (C.N.R Rao).

films[18–20] and nanowire networks [21]have been studied for H2S-sensing Response values of 9.9 and 9.7 to 100 ppm

H2S were achieved with 7.7 wt% Pt-doped nanocrystalline WO3

at 220◦C and 7.2 wt% Pd-doped WO3at 170◦C, respectively [18] Nanocrystalline WO3powders annealed at 400 and 700◦C have been studied for sensing 20 ppm H2S in the 200–300◦C range Samples annealed at 400◦C show a higher response (∼10) compared to those annealed at 700◦C [19] Pure and Al- or Au-doped nanocrystalline WO3films made by advanced reactive gas deposition were investigated for H2S-sensing WO3 nanoparticle-based sensors were sensitive to H2S at room tem-perature, but the response times were of several minutes and recovery times were of several hours[20] Three-dimensional tungsten oxide nanowire networks show a response of∼100 to

10 ppm H2S at a working temperature of 300◦C[21] Thin films

of SnO2exhibit a response of∼100 to 5 ppm H2S at 200◦C[22], while SnO2films impregnated with CuO show a low response to 10–500 ppm H2S in the 100–200◦C range[23–26] Fe2(MoO4)3 powders are reported to show a response of∼31 to 10 ppm of

H2S at 250◦C[27] LnFeO

3(Ln = Eu or Gd) shows a response

of∼12 to 50 ppm H2S at 350◦C[28] We have investigated the sensing characteristics of WO3nanoparticles and nanoplatelets and of WO2.72nanowires towards H2S in the 1–1000 ppm range

at working temperatures of the range of 40–250◦C Our study shows that WO2.72nanowires are good candidates for sensing

H2S in the 10–1000 ppm range at 250◦C.

0925-4005/$ – see front matter © 2007 Elsevier B.V All rights reserved.

doi: 10.1016/j.snb.2007.07.013

2 Experimental

The procedure for preparing the WO3nanoparticles was as

follows[29] 0.2 g of WCl6 (Aldrich, 98% pure) was taken in

80 ml of a mixture of water and ethanol mixture (3:1 ratio) and

kept in an autoclave for 6 h at 150◦C The obtained product was

washed with deionized water and ethanol Then it was heated at

400◦C for 1 h at a heating rate of 1◦C min−1 WO3nanoplatelets

were obtained by following the method[30] 0.5 g of WCl6and

20 ml of benzyl alcohol were taken in a beaker After vigorous

stirring for 1 h, the solution was transferred to a 25 ml autoclave

and kept at 100◦C for 48 h The obtained product was washed

with ethanol several times and dried in vacuum at 60◦C To

remove water, the product was heated at 400◦C for 1 h at a

heat-ing rate of 1◦C min−1 Tungsten oxide (WO

2.72) nanowires were prepared by solvothermal synthesis[29] One gram of WCl6

was taken in a 25 ml autoclave filled with ethanol up to 90%

of its volume Solvothermal synthesis was carried out at 200◦C

for 24 h The product obtained by centrifugation was washed

with ethanol The various tungsten oxide nanostructures were

characterized by X-ray diffraction (Cu K␣ radiation), scanning

electron microscopy (SEM, LEICA S440i), field emission

scan-ning electron microscopy (FESEM with a NOVA NANOSEM

600), transmission electron microscopy (JEOL JEM 3010) and

micro-Raman spectroscopy (LABRAMAN-HR) using He-Ne

laser (632.81 nm) in the back scattering geometry

To fabricate thick film sensors, an appropriate quantity of

diethyleneglycol was added to the desired nanostructure of

tung-sten oxide to obtain a paste The paste was coated on to an

alumina substrate (5 mm× 20 mm, 0.5 mm thick) attached with

a comb-type Pt electrode on one side, the other side having a

heater The films were dried and annealed at 300◦C for 1 h at

a heating rate of 1◦C min−1 Gas sensing properties were

mea-sured using a home-built computer-controlled characterization

system consisting of a test chamber, a sensor holder, a

Keith-ley multimeter-2700, a KeithKeith-ley electrometer-6517A, mass flow

controllers and a data acquisition system The test gas was mixed

with dry air to achieve the desired concentration and the flow

rate was maintained at 200 sccm using mass flow controllers

The current flowing through the samples was measured using

a Keithley multimeter-2700 The working temperature of the

sensors was adjusted by changing the voltage across the heater

side By monitoring the output voltage across the sensor, the

resistance of the sensor in dry air or in test gas can be measured

The gas response magnitude of the sensor, S, was determined

as theRair/RH2S ratio, where Rairis the resistance of the thick

film sensor in dry air andRH2S is the resistance in the

differ-ent concdiffer-entration of H2S The resistance of the sensors based

on nanostructures of tungsten oxides was in the 200–1 M in

dry air in the 40–250◦C range Resistance of the nanoparticles

and nanoplatelets films was higher than that of the nanowires

The response time is defined as the time required for the

con-ductance to reach 90% of the equilibrium value after the test

gas is injected The recovery time is the time necessary for the

sensor to attain a conductance 10% above the original value in

air The H2S response of thick film sensors was also measured

in atmospheres with different relative humidities

Fig 1 XRD patterns of tungsten oxide nanoparticles, nanoplatelets and nanowires.

3 Results and discussion

The X-ray diffraction (XRD) patterns of tungsten oxide nanoparticles, nanoplatelets and nanowires are shown inFig 1 The XRD patterns of the nanoparticles and nanoplatelets corre-spond to the monoclinic structure of WO3(lattice parameters:

a = 7.285 ˚ A, b = 7.517 ˚ A, c = 3.835 ˚A,β = 90.15◦, JCPDS no: 05-0363) The reflections of WO3nanoplatelets are broader than those of the nanoparticles, because of the smaller crystal size The average diameter of the nanoparticles calculated from the XRD line broadening is∼20 nm The XRD pattern of the tung-sten oxide nanowires (Fig 1) corresponds to the monoclinic

structure (lattice parameters: a = 18.33 ˚ A, b = 3.78 ˚ A, c = 14.03

˚

A,β = 115.2◦, JCPDS no: 36-101) characteristic of WO

2.72 The XRD peak intensity of the (0 1 0) reflection is relatively higher than that of other reflections This implies that the nanowires grow along the (0 1 0) direction InFig 2a, we show a FESEM image of WO3 nanoparticles, with the inset showing a TEM image and the selected area electron diffraction (SAED) pattern The SAED pattern indicates the particles to be single crys-talline Fig 2b shows a FESEM image of WO3nanoplatelets with a TEM image as the inset The TEM image reveals that the platelets are of 60± 20 nm long and 1–5 nm thick During TEM analysis it is observed that the thickness of the WO3platelets are very thin and it gets destroyed very fast by the electron beam In Fig 2c, we show a TEM image of the WO2.72nanowires The average diameter of the nanowires is in the 5–15 nm range The inset inFig 2c shows a high-resolution image of a nanowire The single crystalline nature of the nanowire is seen from the HREM image, with a lattice spacing of 3.78 ˚A corresponding to the (0 1 0) planes InFig 3, we show the Raman spectra of tung-sten oxide nanoparticles, nanoplatelets and nanowires Raman bands occur at 130, 265, 328, 710 and 805 cm−1which confirm the monoclinic structure of tungsten oxide[31,32]

Fig 4a shows the sensing characteristics of WO3 nanopar-ticles towards 1000 ppm of H2S at working temperatures of 40–250◦C The highest response found is 757 at 250◦C, and

29 at 40◦C The variation in response of the WO3nanoparticles with the concentration (1–1000 ppm) of H2S at 250◦C is shown

inFig 4b The nanoparticles show a response of 19 to 1 ppm

Fig 2 FESEM images of (a) tungsten oxide nanoparticles with the inset showing

a TEM image and electron diffraction and (b) tungsten oxide nanoplatelets with

the inset showing a TEM image (c) A TEM image of WO2.72 nanowires with

the inset showing a HREM image.

Fig 3 Raman spectra of tungsten oxide nanoparticles, nanoplatelets and nanowires.

of H2S at 250◦C The response and recovery times of the WO3 nanoparticles are 132 and 19 s, respectively, to 1000 ppm H2S

at 250◦C.Fig 5shows the sensing characteristics of the WO3 nanoplatelets The nanoplatelets show the highest response of

1852 to 1000 ppm of H2S at 250◦C The response is∼180 at

40◦C The variation in response of the WO3nanoplatelets with the concentration of H2S (1–1000 ppm) at 250◦C is shown in Fig 5b A response of 35 is obtained to 1 ppm of H2S The response and recovery times of the WO3 platelets are 91 and

20 s, respectively, to 1000 ppm H2S at 250◦C.

InFig 6a, we show the H2S-sensing characteristics of WO2.72 nanowires, whileFig 6b shows the variation in response with concentration in the 1–1000 ppm range The response of WO2.72 nanowires varies between 3313 and 236 to 1000 ppm H2S over the temperature range of 250–40◦C To 1 ppm of H2S, a response

Fig 4 (a) Gas sensing characteristics of tungsten oxide nanoparticles to

1000 ppm H2S, and (b) variations in response with concentration of H2S at

250 ◦C.

Fig 5 (a) Gas sensing characteristics of tungsten oxide nanoplatelets to

1000 ppm H2 S, and (b) variations in response with concentration of H2 S at

250 ◦C.

of 48 is found at 250◦C The response and recovery times of the

WO2.72 nanowires are 83 and 18 s, respectively, to 1000 ppm

H2S at 250◦C.

Fig 7a shows the effect of working temperature in the range

of 40–250◦C, on the sensor response of the tungsten oxide

nanostructures towards 1000 ppm H2S We see that the WO2.72

nanowires show the highest values of response towards H2S

while the WO3nanoparticles show the least response at all the

Fig 6 (a) Gas sensing characteristics of WO2.72 nanowires to 1000 ppm H2S,

and (b) variations in response with concentration of H2S at 250 ◦C.

Fig 7 A comparison of the response values of tungsten oxide nanostructures with (a) temperature (to 1000 ppm H2S) and (b) H2S concentration (at 250 ◦C). temperatures studied All the nanostructures, however, show

a response of ∼150 at 50◦C to 1000 ppm of H2S, but we found a reasonably good response value even at 50–100◦C The concentration-variation of response of the tungsten oxide nanos-tructures at 250◦C is shown inFig 7b In the 50–100 ppm range, the response is generally satisfactory The values of response are 392, 121 and 50 to 50, 10 and 1 ppm of H2S at 250◦C in the case of the WO2.72 nanowires The response of 121 of the nanowires to 10 ppm of H2S is significant since the bad odour

of H2S manifests above this concentration

Fig 8 shows the response and recovery time curves of the tungsten oxide nanoparticles, nanoplatelets and nanowires at 40–250◦C The response times vary in the 55–100 s range for the nanoplatelets and nanowires, whereas for the nanoparticles the response time is 80–130 s Thus, the nanoparticles show slower response compared to the nanowires and platelets The recovery times of all the nanostructures are in the 18–40 s range depending

on the temperature

We have studied the effect of humidity on the H2S-sensing characteristics of the tungsten oxide nanostructure sensors in the range of 35–90% relative humidity We illustrate the effect of humidity on the response of the WO2.72 nanowires at 250◦C

to 1000 ppm of H2S in Fig 9a, and of WO3 nanoplatelets in Fig 9b There is a slight decrease in the response with an increase

in humidity above a relative humidity of 60%, but there is not much change in the response and recovery times There was

no change in the response as well as the response and recovery times even after 2000 cycles

It is known that the sensing mechanism of the oxide materials

is surface controlled in which the grain size, surface states and oxygen adsorption play an important role[33,34] The larger

sur-Fig 8 Temperature variation of (a) response and (b) recovery times (to

1000 ppm H2S) of tungsten oxide nanoparticles, nanoplatelets and nanowires.

face area generally provides more adsorption–desorption sites

and thus the higher sensitivity Atmospheric oxygen adsorbs

electrons from the conduction band of the sensing metal oxide

and occurs on the surface in the form of O−and O

2 −.

Fig 9 Effect of humidity on the response of tungsten oxide (a) nanowires and

(b) nanoplatelets at 250 ◦C to 1000 ppm H2S.

The adsorbed oxygen species play a crucial role in H2S-sensing The reaction for H2S-sensing is given by

As expected from Eq.(3), the resistance of the nanostructured oxide decreases on contact with H2S It is expected that the resistance change upon the exposure to H2S is mainly due to the resistance change of tungsten oxide According to Eqs.(1)and (2), oxygen adsorption reaction occurs prior to H2S-sensing, creating a thin electron-depleted layer at the surface of tung-sten oxide As H2S is adsorbed, electrons are released into the conduction band according to Eq.(3)

4 Conclusions

Tungsten oxide nanostructures exhibit good sensing charac-teristics to H2S in the concentration range of 1–1000 ppm over the temperature range 40–250◦C The best results are obtained with the WO2.72nanowires at 250◦C where the response value reaches 3313 to 1000 ppm of H2S and 121 for 10 ppm of H2S The response is satisfactory at 40◦C The recovery and response times are generally satisfactory It is noteworthy that the response

is not affected significantly by humidity up to 60% relative humidity

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Biographies Chandra Sekhar Rout obtained his master degree in physics from Utkal

Uni-versity in 2003 He is working as a PhD student at the Jawaharlal Nehru Centre for Advanced Scientific Research His current research interests include devel-opment of gas sensors using different nanomaterials and supercapacitors based

on different nanostructured carbon materials.

Manu Hegde obtained his master degree in physics from Mangalore

Univer-sity in 2005 Currently he is working in Prof C.N.R Rao’s group as a project assistant.

C.N.R Rao obtained his PhD degree from Purdue University and DSc degree

from the University of Mysore He is the National Research Professor of India, Linus Pauling Research Professor at the Jawaharlal Nehru Centre for Advanced Scientific Research and Honorary Professor at the Indian Institute of Science (both at Bangalore) His research interests are in the chemistry of materials.