Low temperature CO and CH4 dual selective gas sensor using SnO2 quantum dots prepared by sonochemical method

In this study, a low temperature sensor with a dual function property is fabricated by using 3–4 nm SnO2 quantum dots (QDs) prepared by sonication-assisted precipitation. The sensor shows high selectivity to CO in the presence of methane below 375 °C. SnCl4 aqueous solution was precipitated by ammonia under sonication, which continued for 1, 2 and 3 h. A part of the sample was then dried and calcined at 400 °C for 1.5 h and characterized by XRD and BET. UV–vis analyses were carried out for band gap measurements. The average particle size and the specific surface area of the SnO2 QDs as well as their sensing properties were compared with the SnO2 nano-particles prepared by conventional sol–gel method. The BET surface area of sonochemically as-prepared product after 2 h sonication and the one calcined at 400 °C after 1.5 h are 257 and 212 m2/g, respectively while the specific surface area for SnO2 prepared by conventional sol–gel method is about 80 m2/g. XRD spectra revealed pure crystalline phase of SnO2 is formed for both as-prepared and calcined samples of SnO2 QDs. However, for the sample prepared by sol–gel method and calcined at 400 °C, SnO crystals are detected along with the SnO2 crystals. Band gap measurements for the sample fabricated by sonochemical method and calcined at 400 °C indicated band gap energy of 5.7 eV which shows 2.1 eV blue-shift (shift into smaller wave lengths) from that of the bulk SnO2 which was reported earlier by others.Quantum dots of SnO2 show exceedingly high response to different concentrations of 100, 300 and 1000 ppm of CO in a temperature range of 25–350 °C. At about 50 °C a response of 27 was obtained for 1000 ppm CO, which increases to a maximum of 147 at 225 °C and then decreases whereas the maximum of 47.2 was detected for the SnO2 sample prepared by the sol–gel method occurred at about 300 °C. At the same time no response to methane is observed in the whole range of temperatures for SnO2 QDs. On the other hand the response to methane is higher than that to CO at above 375 °C.