Synthesis of Nanostructured and Nanoporous TiO 2 -AgO Mixed Oxide Derived from a Particulate Sol-Gel Route: Physical and
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TRANSITION metal oxide films have found wide applications as gas sensors,[1] as catalysts,[2] and in optical electronics.[3] TiO2 is a common single metal oxide semiconductor used as gas sensors, because its electric conductivity changes when exposed to gases such as trimethylamine, ethanol, oxygen (O2), hydrogen (H2), carbon monoxide (CO), and petrol vapors.[4–7] Recently, many efforts have been aimed at improving the gas sensing performance by improvements in selectivity, sensitivity, and durability. In order to improve these properties, microstructure control by preparing porous, high specific surface area films and doping with hetero components (such as Ag, Sn, V, Cr, W, Co, Cu, Fe, Nb, Ta, Ga, and Mo) are known to be effective, because active sites for particular gas species can be produced.[8–12] The gas sensing mechanism involves the excitation of valence electrons to the conduction band by exposure to target gas, resulting in the formation of holes in the valence band. It is known that doped particles can facilitate the electron excitation by creating
M.R. MOHAMMADI, Assistant Professor, is with the Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran. Contact e-mail: [email protected]; mohammadi@ sharif.edu D.J. FRAY, Professor, is with the Department of Materials Science and Metallurgy, University of Cambridge. Manuscript submitted November 18, 2010. Article published online March 24, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
a local electric field and enhance the electron-hole pair separation, therefore improving gas sensing performance. Another method to improve gas sensing performance of metal oxide semiconductors is to employ binary metal oxide semiconductors. This novel alternative has the potential to form tailored film morphologies, which facilitates gas-film interaction by altering the atomic ratio of each element. Furthermore, it is possible to increase the current single metal oxide surface-tovolume ratio and to fabricate stable nanosized grain morphologies for high-performance gas sensing thin films.[13] Sensing properties of binary oxides based on TiO2 such as TiO2-MoO3,[13] TiO2-WO3,[14] TiO2-Cr2O3,[15] TiO2-V2O5,[11] TiO2-CeO2,[16] and TiO2-Fe2O3[7] were reported previously. The major disadvantage of TiO2based gas sensors is their high power consumption, which is required to operate the sensors at the required elevated temperature (>673 K (400 C)).[17,18] Recently, many efforts have been aimed at producing high performance gas sensors with low power consumption.[19–21] The empirical exploration of mixing TiO2 and AgO may lead to new gas sensing properties or may simply lead to a material composed of characteristics similar to TiO2 and AgO. Ruiz et al.[10] studied the sensing performance of Ag-doped–TiO2 film, synthesized by the hydrothermal method, toward CO. The sensor showed a response magnitude of 3.5 toward 500 ppm CO at 773 K (500 C) operating temperature. Moreover, it was found that the response magnitude of Ag-doped–TiO2 was higher than that
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