Gas-sensing properties of SnO2–TiO2-based sensor for volatile organic compound gas and its sensing mechanism
Introduction
The evaluation of indoor air quality has become an extremely urgent environmental issue. Indoor pollutants, which mainly consist of volatile organic compounds (VOCs) such as formaldehyde, benzene, toluene and xylene, may cause the environmental illnesses known as building-related sickness [1], [2], [3], [4].
Semiconductor metal oxide gas sensor can be promising candidates for monitoring VOCs due to its many advantages such as simple manufacture technique, low cost, rapid response and recovery time. [5], [6], [7], [8], [9], [10] However, one of the critical issues currently limiting the wide use of these oxides is lack of selectivity towards different VOCs. It is difficult to distinguish each VOCs species owing to their similar composition elements and molecular structure. Although great deal of effects have been out into improving the selectivity of the sensing material to VOCs, the selectivity is remain the most difficult issue for VOCs sensor research [11].
Therefore, in this work, we propose a mixed oxide of SnO2–TiO2 doped with Ag ion, in order to improve the selectivity to VOCs. As expected, we find that this mixed oxide exhibits special selective to each VOCs at different operating temperature, thereby holding technological promise for monitoring VOCs. Furthermore, based on quantum chemistry calculation, we examined the orbital energy of different VOCs molecule and qualitative explained the present mechanism of selectivity.
Section snippets
Preparation of sensing materials
Colloidal solution of TiO2 was prepared by first mixing 10 ml of tetra butyl titanium (Wako Pure Chemical, 95%) with 4 ml isopropyl alcohol. The mixture was then gradually added to 150 ml deionized water. The solution was well stirred several hours until the sol was formed. The sol was subsequently transferred into a well-sealed autoclave vessel containing 0.5 M teramethy-ammonium hydroxide solution. Peptization occurred after heating at 120 °C for 6 h. The second step, as for the preparation of SnO2
Characteristic of sensing material
Fig. 2 shows the XRD pattern of TiO2–SnO2 sample, we can observe that the sample included anatase TiO2 phase and rutile SnO2 phase. The TiO2 peaks emerge in the XRD spectrum after doping, meaning that we have successfully incorporated TiO2 into SnO2. The particle sizes D are measured from XRD peaks based on the Scherrer equation [12]:where D is the mean size of particle, λ is the X-ray wavelength (Cu-0.154056 nm), β is the full-width at half-maximum of XRD peaks and
Conclusion
We have applied a sol–gel method to fabricate the sensing material SnO2–TiO2 doped with Ag ion powder and investigated their microstructures and gas-sensing properties. We have found that the sensor exhibited remarkable selectivity to each VOCs such as ethanol, methanol, acetone and formaldehyde at different operating temperature. From the discussion based on quantum chemistry calculation, we can deduce that LUMO energy may be a qualitative factor to affect the selectivity of the sensor. These
Acknowledgments
The authors thank to Dr. ZC. Wang, Prof. S.Tsukimoto and M.Saito for editorial help with the English manuscript and useful discussion. This work was funded in part by the Chinese Scholarship Council (CSC) project (LJC20093012).
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