PRAMANA c© Indian Academy of Sciences Vol. 65, No. 4— journal of October 2005

physics pp. 647–652

Influence of CuO catalyst in the nanoscale rangeon SnO2 surface for H2S gas sensing applications

VINAY GUPTA1, S MOZUMDAR2, ARIJIT CHOWDHURI1 and K SREENIVAS1

1Department of Physics and Astrophysics, University of Delhi, Delhi 110 007, India2Department of Chemistry, University of Delhi, Delhi 110 007, IndiaE-mail: subho@bol.net.in

Abstract. The dispersal of CuO catalyst on the surface of the semiconducting SnO2

film is found to be of vital importance for improving the sensitivity and the responsespeed of a SnO2 gas sensor for H2S gas detection. Ultra-thin CuO islands (8 nm thin and0.6 mm diameter) prepared by evaporating Cu through a mesh and subsequent oxidationyield a fast response speed and recovery. Ultimately nanoparticles of Cu (average size =15 nm) prepared by a chemical technique using a reverse micelle method involving thereduction of Cu(NO3)2 by NaBH4 exhibited significant improvement in the gas sensingcharacteristics of SnO2 films. A fast response speed of ∼14 s and a recovery time of ∼60s for trace level ∼20 ppm H2S gas detection have been recorded. The sensor operatingtemperature (130◦C) is low and the sensitivity (S = 2.06× 103) is high. It is found thatthe spreading over of CuO catalyst in the nanoscale range on the surface of SnO2 allowseffective removal of excess adsorbed oxygen from the uncovered SnO2 surface due to spillover of hydrogen dissociated from the H2S–CuO interaction.

Keywords. H2S gas sensors; thin films; CuO–SnO2; nanoparticles.

PACS Nos 07.07.Df; 61.46.+w; 68.45.Da; 81.15.Cd

1. Introduction

Gas sensing applications demand materials that offer a fast response speed, a fastrecovery time, and high sensitivity for trace level detection of various gases. Semi-conducting tin oxide is found useful for various gas sensing applications, and effortsare on to improve its sensitivity and selectivity with appropriate catalysts. Maekawaet al [1] first reported the enhanced sensitivity of SnO2 with CuO dopant for H2Sgas detection. Subsequently, several investigations using thick sintered pastes [2],Cu/SnO2 bi-layers [3] and CuO–SnO2 hetero-contacts [4] have been reported.

In our earlier work [5] we reported significant improvements using a novel struc-ture consisting of uniformly distributed CuO islands (0.6 mm dia. and 10 nm thick).CuO and SnO2−x which are p- and n-type semiconductors respectively were foundto exhibit a strong electronic interaction in the presence of H2S gas, and signifi-cantly improved the sensitivity and the response speed. In this paper we reportfurther improvements using nanoparticles of the catalyst material.

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Figure 1. Sensitivity of SnO2 sensor covered with oxidized catalysts in dot-ted form.

2. Experimental

SnO2 films (90 nm) were deposited by a RF reactive sputtering process on borosil-icate glass substrates, and platinum (Pt) interdigital electrodes underneath theSnO2 film were used to measure the change in the electrical conductivity. Dottedislands of catalyst materials (oxides) on SnO2 film surface were obtained by evap-oration/sputtering of Ag, Ni and Cu dots through a mesh and a post-oxidationtreatment at 300◦C ensured the formation of the respective oxide. Alternately, Cunanoparticles were chemically derived using a reverse micelle method involving thereduction of Cu(NO3)2 by NaBH4. The Cu nanoparticles were dispersed in dis-tilled water, and after brief sonication they were loaded onto the sputtered SnO2

film surface. Annealing at 300◦C in air evaporated the water and converted theCu nanoparticles to CuO. Sensitivity to H2S gas was measured in the temperaturerange 60–250◦C. At each temperature the sensor was stabilized in air to a steadyresistance value (Ra), and the decrease in the sensor resistance (Rg) in the presenceof H2S (20 ppm) was recorded. The sensitivity factor (S) is defined as S = Ra/Rg,and the response characteristics were studied at the temperature where the sensorexhibited the maximum sensitivity.

3. Influence of different oxide catalysts (Ag, Ni and Cu)

Oxides of Ni, Ag and Cu were investigated in the dotted island form and figure 1shows the sensitivity variation of the three SnO2 sensors. The maximum sensitivityis exhibited by the CuO dotted islands (S ∼ 7.3 × 103 at 150◦C) followed by NiO(S ∼ 9.6×102 at 140◦C) and SnO2 sensors with Ag2O dotted islands exhibited thelowest sensitivity due to the incomplete oxidation to Ag2O at the low annealingtemperature of 300◦C in air [Lee (1994)].

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Figure 2. CuO–SnO2 gas sensor. Figure 3. Sensitivity with varying CuO

dotted island thickness at 150◦C.

Figure 4. Response speed with varying CuO dotted island thickness.

4. Optimization of CuO dotted islands

Figure 3 shows the variation of the sensitivity as a function of temperature forSnO2 with different CuO island thickness values (2.5 to 20 nm). The maximumsensitivity is noted to be 8.065×103 at a low operating temperature of 150◦C for 8nm thick CuO dotted islands.

Figure 4 shows the response speed of the SnO2–CuO dotted sensor as a functionof CuO thickness. The values of response speed at different thickness values ofCuO are reported in table 1. It is noted that the response speed decreases as thethickness of the CuO dotted island increases.

The increase in time taken by the sensor to respond with the increasing thicknessof CuO catalyst (table 1) is attributed to the presence of excess oxygen and slowconversion of thick CuO to CuS during the detection of H2S gas. It may be noted

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Table 1. Response speed characteristics for 20 ppm H2Sgas with varying CuO dotted island thickness on SnO2.

Thickness Response Recovery(nm) speed (s) time (s)

2.5 8 2795 9 3028 12 366

10 18 34715 29 31320 42 294

Table 2. Sensitivity for different gases measured with 8 nmthick CuO dotted islands on SnO2.

Operating temperatureGas Sensitivity (◦C)

H2S 8.065 × 103 150H2 618.86 130SO2 288.22 170LPG 70.19 220CH3OH 15.24 190C2H5OH 11.60 190

that the SnO2 sensor with 8 nm thick CuO dotted island exhibited the highest sen-sitivity (figure 3), hence its cross selectivity to other gases was tested and the resultsare presented in table 2. As the observed improvements associated with the size(thickness of the CuO dotted islands) were promising, further enhancement couldbe envisaged with the use of CuO nanoparticles for increased oxygen adsorptionand interaction with the H2S gas.

A chemical method involving the reduction of Cu2+ ions to Cu in a reverse-micellar system was employed to prepare copper nanoparticles. Aerosol OT(AOT), the commercial name of the sodium salt of bis(2-ethylhexyl) sulfosucci-nate [Na(DEHSS)] was used as the surfactant in the process. To a reverse-micellarCu(NO3)2 solution another reverse-micellar solution of NaBH4 solution was addeddrop-wise with constant stirring. In the presence of inert nitrogen the solutionwas allowed to stand for 4 h and Cu nanoparticles were extracted by adding dryacetone and centrifugation. The Cu nanoparticles were dispersed in distilled waterand evenly spread onto the SnO2 film surface using a micropipette. Figure 5 showsthe surface of the SnO2 film loaded with CuO nanoparticles as seen with an atomicforce microscope (AFM). The nanoparticles are round in shape with an averagesize of 15 nm. A post-deposition annealing treatment of the Cu nanoparticles at300◦C in air was considered sufficient to transform them to CuO.

The response speed and the recovery characteristics of the SnO2 sensor withCuO nanoparticles is shown in figure 6, and is compared with the earlier dataobtained with the coarse distribution of large diameter (0.6 mm) CuO dots having

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Figure 5. AFM image of SnO2 film surface loaded with CuO nanoparticles.

Figure 6. Comparison of response characteristics with 8 nm thick CuOdotted islands and CuO nanoparticles.

an optimum thickness of 8 nm. It is interesting to note that the recovery timeof the sensor with CuO nanoparticles is reduced to 61 s, and the sensor is ableto completely recover its initial high resistance value. The reproducibility of thesensor was good for consecutive back-to-back runs as shown in the inset of figure 6.In summary, SnO2 films dispersed with chemically derived CuO nanoparticles onthe surface exhibited high sensitivity with a fast response speed and quick recoverytime.

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Acknowledgements

This work is supported by the Ministry of Science and Technology, Government ofIndia.

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A M Gaskov, Mater. Sci. Eng. B57, 241 (1999)[5] Arijit Chowdhuri, P Sharma, V Gupta, K Sreenivas and K V Rao, J. Appl. Phys. 92,

2172 (2002)

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