Effects of the fabrication temperature and oxygen flux on the properties and nitrogen dioxide sensitivity of the tin oxi
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e 1 nm tin oxides–tin (SnOx–Sn) compound films were thermally evaporated onto the chemical vapor deposition (CVD)-grown graphene films for the improved nitrogen dioxide (NO2) gas sensitivity, and the effects of the fabrication temperature and oxygen (O2) flux on the properties of the SnOx–Sn/graphene hybrid sensors including their composition, morphology, and microstructure as well as NO2 sensitivity were investigated. The composition of the SnOx–Sn compound films exhibited strong dependence on the fabrication temperature and O2 flux which could be ascribed to the hybrid effect of the desorption of the oxygen functional groups on the graphene and oxidation of the graphene and Sn. Such combining effects also demonstrated tremendous influence on the SnOx–Sn film morphology, in which the enhanced desorption of the oxygen functional groups on the graphene together with the oxidation of Sn with increasing fabrication temperature would facilitate the formation of large grain-sized and discontinuous films while the increasing O2 flux showed the opposite effects. Meanwhile, the crystallization of the SnOx–Sn compound films was promoted and deteriorated with the increasing temperature and O2 flux, respectively. The SnOx–Sn film morphology played vital role in NO2 gas sensitivity at room temperature, and the mechanism responsible for that was also discussed.
I. INTRODUCTION 1–4
Tin dioxide (SnO2 ), as a typical metal oxide semiconductor like zinc oxide (ZnO),5–11 was widely applied as excellent gas sensing materials in detecting hazardous gases like formaldehyde (HCHO),1,3,6,7 nitrogen dioxide (NO2),5,12–16 volatile organic compounds,2,16–25 carbon oxides,26–29 etc. With the increasing concern regarding the environmental protection, the development of mild cost, room temperature, and highly sensitive NO2 gas sensors in mass production scale was highly desirable. For the semiconductor gas sensing materials, it was widely recognized that the mechanism responsible for such sensitivity could be explained by the reaction of target gas and adsorbed oxygen species at the metal oxide surface, which caused the change of the surface conductance and subsequent sensor electrical resistance through the transducer function.30 Therefore, the possession of the optimized sensor structure for obtaining not only severe reaction between target gas and oxygen adsorbates on the metal oxide surface, but also the high conductivity of the semiconductor materials for carriers transporting was crucial for the superior gas sensor performance. Contributing Editor: Gary Messing a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.242 J. Mater. Res., Vol. 31, No. 14, Jul 28, 2016
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However, the metal oxide gas sensing materials usually exhibited relatively inferior conductivity due to their semiconductor nature. Graphene, on the other hand, as a new promising gas sensing material, possessed specific advantages such as high surface-to-volume ratio (specific surface area), hig
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