Ž . Sensors and Actuators B 56 1999 98–105 SnO :Bi O based CO sensor: Laser-Raman, temperature programmed 2 2 3 desorption and X-ray photoelectron spectroscopic studies 1 G. Sarala Devi, S.V. Manorama ) , V.J. Rao Materials Science Group, Indian Institute Of Chemical Technology, Hyderabad 500 007, India Received 26 March 1998; received in revised form 25 February 1999; accepted 26 February 1999 Abstract Ž . Ž . The sensitivity and selectivity of tin oxide SnO based gas sensors towards carbon monoxide CO was improved by doping the base 2 Ž . material with Bi O and Sb O . Laser-Raman studies of the compound confirm the formation of bismuth stannate Bi Sn O above 2 3 2 3 2 2 7 8008C, which seems to be acting as a molecular sieve allowing only CO gas to react with the sensor surface, thereby imparting selectivity to the sensor. The chemisorption of oxygen on SnO :Bi O was investigated over a wide range of temperatures from room temperature to 2 2 3 Ž . Ž . 8008C by means of Temperature Programmed Desorption TPD and X-ray Photoelectron Spectroscopic XPS studies which were carried out to establish the exact chemical species present on the sensor surface before and after the reaction. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Carbon monoxide sensor; XPS studies; Valence band spectra; SnO :Bi O 2 2 3 1. Introduction Amorphous and polycrystalline tin dioxide have widespread technological applications. Extensive research into devices for the detection of inflammable or toxic gas Ž . inclusions in air has been centered on tin IV oxide, SnO 2 w x based thick film sensors 1–4 . The operation of such transducers relies on the conductivity changes experienced by the n-type semiconducting materials when surface chemisorbed oxygen reacts with reducing gases at elevated temperatures. Their unusual electrical and optical proper- ties are intimately related to their chemical composition and microstructure. These properties are exploited for the production of miniaturized gas detectors. The mechanism that governs such devices is still not clear and has stimu- wx lated intensive research 5 . For example, the behaviour of thin films has been extrapolated from that of bulk materi- als assuming that the chemiresistivity of SnO films is 2 determined by the chemisorption of oxygen ions on the surface which can trap or scatter free carriers, generate potential barriers, or severely deplete the grains. However, there exists another possibility, the chemiresistance for thin ) Corresponding author. Tel.: q91-40-717-0921; Fax: q91-40-7173- 757, q91-40-7173-387; E-mail: manorama@iict.ap.nic.in 1 qIICT Communication No.: 3931. films could be dominated by oxygen vacancies produced by the reduction process, which take part in the gas detection. 2. Experimental details Tin oxide powder was prepared by two methods. First is the hydrolysis of SnCl with ammonium hydroxide 4 wx Ž . solution 6 . The obtained tin oxide hydrate stannic acid was dried at room temperature and subjected to Differen- Ž . tial Thermal Analysis DTA studies to know the dehydra- tion temperature. The tin oxide hydrate was subsequently dried at 1108C for 12 h and calcined in temperatures of 500–10008C to obtain SnO crystallites of various sizes in 2 the range 113–302 A8. The resulting SnO powder was 2 impregnated with 10 wt.% Bi O and calcined at different 2 3 temperatures ranging from 500–10008C in order to follow the evolution of the new phase subsequent to heat treat- ment. These samples were subjected to X-ray diffraction Ž . Philips PW 1140r90 CuK a radiation , Laser-Raman Ž . Jobin Yvon J 64000, 514–532 nm , temperature pro- Ž . grammed desorption G 80-TCD and X-ray photoelectron Ž . spectroscopic studies SCIENTA ESCA-300 . The second method was the co-precipitation of Bi O 2 3 Ž . along with SnO . Calculated amount of Bi NO .5HO 2 3 3 2 0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0925-4005 99 00164-1