h2s sensors based on tungsten oxide nanostructures

A vailable online at www.sciencedirect.com Sensors and Actuators B 128 (2008) 488–493 H 2 S sensors based on tungsten oxide nanostructures 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 WO 3 and nanowires of WO 2.72 have been investigated for their H 2 S-sensing characteristics over the 1–1000 ppm concentration range at 40–250 ◦ C. The nanoparticles and nanoplatelets of WO 3 exhibit response values of 757 and 1852, respec- tively to 1000 ppm H 2 S at 250 ◦ C, respectively, compared to the response of 3313 of the nanowires of WO 2.72 . Interestingly, the response of the nanowires is satisfactory (121) to 10 ppm H 2 S at 250 ◦ C, while a large response (240) is observed to 1000 ppm H 2 Sevenat40 ◦ C. The WO 2.72 nanowires emerge as a good candidate for H 2 S 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 oxideshave been widely usedfor 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],H 2 [9],C 2 H 5 OH [10],NH 3 and CO [11] has been demon- strated using metal oxide nanostructures. We were interested in developing sensors for H 2 S using metal oxide nanostruc- tures, since H 2 S is a toxic gas used in chemical laboratories and industries. H 2 S 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 WO 3 have been used for sensing H 2 S at ppm level with the response values vary- ing between 3 and 10 4 depending on the temperature and the gas concentration [12–17]. Nanoparticulate WO 3 films show a response of 3–5 to 1 ppm of H 2 S at 200 ◦ C [15]. Active lay- ers of pure and Pt-doped WO 3 films deposited by rf magnetron sputtering were able to sense 100 ppb of H 2 S at 200 ◦ C [16]. Rf sputtered WO 3 films and films doped with Pt, Au, Ag, Ti, SnO 2 , ZnO and ITO have been examined; the response was improved by Au to H 2 S [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 H 2 S-sensing. Response values of 9.9 and 9.7 to 100 ppm H 2 S were achieved with 7.7 wt% Pt-doped nanocrystalline WO 3 at 220 ◦ C and 7.2 wt% Pd-doped WO 3 at 170 ◦ C, respectively [18]. Nanocrystalline WO 3 powders annealed at 400 and 700 ◦ C have been studied for sensing 20 ppm H 2 S 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 WO 3 films made by advanced reactive gas deposition were investigated for H 2 S-sensing. WO 3 nanoparticle-based sensors were sensitive to H 2 S 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 H 2 S at a working temperature of 300 ◦ C [21]. Thin films of SnO 2 exhibit a response of ∼100 to 5 ppm H 2 S at 200 ◦ C [22], while SnO 2 films impregnated with CuO show a low response to 10–500 ppm H 2 S in the100–200 ◦ C range[23–26].Fe 2 (MoO 4 ) 3 powders are reported to show a response of ∼31 to 10 ppm of H 2 Sat250 ◦ C [27]. LnFeO 3 (Ln = Eu or Gd) shows a response of ∼12 to 50 ppm H 2 S at 350 ◦ C [28]. We have investigated the sensing characteristics of WO 3 nanoparticles and nanoplatelets and of WO 2.72 nanowires towards H 2 S in the 1–1000 ppm range at working temperatures of the range of 40–250 ◦ C. Our study shows that WO 2.72 nanowires are good candidates for sensing H 2 S 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 C.S. Rout et al. / Sensors and Actuators B 128 (2008) 488–493 489 2. Experimental The procedure for preparing the WO 3 nanoparticles was as follows [29]. 0.2 g of WCl 6 (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 for1 h at a heating rate of 1 ◦ C min −1 .WO 3 nanoplatelets were obtained by following the method [30]. 0.5 g of WCl 6 and 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 WCl 6 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 Keithley 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 the R air /R H 2 S ratio, where R air is the resistance of the thick film sensor in dry air and R H 2 S is the resistance in the differ- ent concentration of H 2 S. 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 H 2 S 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 in Fig. 1. The XRD patterns of the nanoparticles and nanoplatelets corre- spond to the monoclinic structure of WO 3 (lattice parameters: a = 7.285 ˚ A, b = 7.517 ˚ A, c = 3.835 ˚ A, β = 90.15 ◦ , JCPDS no: 05- 0363). The reflections of WO 3 nanoplatelets 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. In Fig. 2a, we show a FESEM image of WO 3 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 WO 3 nanoplatelets 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 WO 3 platelets are very thin and it gets destroyed very fast by the electron beam. In Fig. 2c, we show a TEM image of the WO 2.72 nanowires. The average diameter of the nanowires is in the 5–15 nm range. The inset in Fig. 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. In Fig. 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 −1 which confirm the monoclinic structure of tungsten oxide [31,32]. Fig. 4a shows the sensing characteristics of WO 3 nanopar- ticles towards 1000 ppm of H 2 S 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 WO 3 nanoparticles with the concentration (1–1000 ppm) of H 2 Sat250 ◦ C is shown in Fig. 4b. The nanoparticles show a response of 19 to 1 ppm 490 C.S. Rout et al. / Sensors and Actuators B 128 (2008) 488–493 Fig. 2. FESEM images of (a)tungstenoxide 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 WO 2.72 nanowires with the inset showing a HREM image. Fig. 3. Raman spectra of tungsten oxide nanoparticles, nanoplatelets and nanowires. of H 2 S at 250 ◦ C. The response and recovery times of the WO 3 nanoparticles are 132 and 19 s, respectively, to 1000 ppm H 2 S at 250 ◦ C. Fig. 5 shows the sensing characteristics of the WO 3 nanoplatelets. The nanoplatelets show the highest response of 1852 to 1000 ppm of H 2 S at 250 ◦ C. The response is ∼180 at 40 ◦ C. The variation in response of the WO 3 nanoplatelets with the concentration of H 2 S (1–1000 ppm) at 250 ◦ C is shown in Fig. 5b. A response of 35 is obtained to 1 ppm of H 2 S. The response and recovery times of the WO 3 platelets are 91 and 20 s, respectively, to 1000 ppm H 2 Sat250 ◦ C. In Fig. 6a, we show the H 2 S-sensing characteristics ofWO 2.72 nanowires, while Fig. 6b shows the variation in response with concentration in the 1–1000 ppm range. The response of WO 2.72 nanowires varies between 3313 and 236 to 1000 ppm H 2 S over the temperature rangeof 250–40 ◦ C. To 1 ppmof H 2 S, a response Fig. 4. (a) Gas sensing characteristics of tungsten oxide nanoparticles to 1000 ppm H 2 S, and (b) variations in response with concentration of H 2 Sat 250 ◦ C. C.S. Rout et al. / Sensors and Actuators B 128 (2008) 488–493 491 Fig. 5. (a) Gas sensing characteristics of tungsten oxide nanoplatelets to 1000 ppm H 2 S, and (b) variations in response with concentration of H 2 Sat 250 ◦ C. of 48 is found at 250 ◦ C. The response and recovery times of the WO 2.72 nanowires are 83 and 18 s, respectively, to 1000 ppm H 2 Sat250 ◦ 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 H 2 S. We see that the WO 2.72 nanowires show the highest values of response towards H 2 S while the WO 3 nanoparticles show the least response at all the Fig. 6. (a) Gas sensing characteristics of WO 2.72 nanowires to 1000 ppm H 2 S, and (b) variations in response with concentration of H 2 Sat250 ◦ C. Fig. 7. A comparison of the response values of tungsten oxide nanostructures with (a) temperature (to 1000 ppm H 2 S) and (b) H 2 S concentration (at 250 ◦ C). temperatures studied. All the nanostructures, however, show a response of ∼150 at 50 ◦ C to 1000 ppm of H 2 S, 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 in Fig. 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 H 2 S at 250 ◦ Cin the case of the WO 2.72 nanowires. The response of 121 of the nanowires to 10 ppm of H 2 S is significant since the bad odour of H 2 S 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. Theresponse times vary in the 55–100 srange 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 allthe nanostructures arein the 18–40 s range depending on the temperature. We have studied the effect of humidity on the H 2 S-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 WO 2.72 nanowires at 250 ◦ C to 1000 ppm of H 2 SinFig. 9a, and of WO 3 nanoplatelets in Fig. 9b. There isa slight decrease in the response withan 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- 492 C.S. Rout et al. / Sensors and Actuators B 128 (2008) 488–493 Fig. 8. Temperature variation of (a) response and (b) recovery times (to 1000 ppm H 2 S) 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 − . O 2 (g) + e − → O 2(ads) − (1) O 2(ads) − + e − → 2O (ads) − (2) Fig. 9. Effect of humidity on the response of tungsten oxide (a) nanowires and (b) nanoplatelets at 250 ◦ C to 1000 ppm H 2 S. The adsorbed oxygen species play a crucial role in H 2 S-sensing. The reaction for H 2 S-sensing is given by H 2 S + 3O (ads) − → SO 2 + H 2 O + 3e − (3) As expected from Eq. (3), the resistance of the nanostructured oxide decreases on contact with H 2 S. It is expected that the resistance change upon the exposure to H 2 S is mainly due to the resistance change of tungsten oxide. According to Eqs. 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Fang, Fabrication and structural charac- terization of porous tungsten oxide nanowires, Nanotechnology 16 (2005) 2647–2650. [33] A. Rothschild, Y. Komem, The effect of grain size on the sensitivity of nanocrystalline metal-oxide gas sensors, J. Appl. Phys. 95 (2004) 6374– 6380. [34] M.E. Franke, T.J. Koplin, U. Simon, Metal and metal oxide nanoparticles in chemiresistors: does the nanoscale matter? Small 2 (2006) 36–50. 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. . the conduction band according to Eq. (3). 4. Conclusions Tungsten oxide nanostructures exhibit good sensing charac- teristics to H 2 S in the concentration. resistance in the differ- ent concentration of H 2 S. The resistance of the sensors based on nanostructures of tungsten oxides was in the 200–1 M in dry