(1) The NiO/[In.sub.2][O.sub.3] with higher specific surface area than [In.sub.2][O.sub.3] substrate could afford more active sites to react with methanal gas molecules [41].
Lower temperature might not excite the adsorbed methanal and free electrons in NiO/[In.sub.2][O.sub.3] effectively, while, at higher temperature, methanal gas molecules were hard to be adsorbed, and oxygen molecules might be also desorbed, causing less influence on the resistance of the film [43].
The response and recovery time, which were also important parameters for a gas sensor, were generally defined as the times to reach 90% variation in resistance upon exposure to methanal and air were defined as the 90% response time and 90% recovery time, respectively.
A known weight (2 g) of Ti[O.sub.2] (Hombikat UV 100, Sachtleben, Germany) was suspended under continuous stirring at 250 rpm in a solution containing 40 [cm.sup.3] of 40% aqueous methanal (Chemanol), 10 [cm.sup.3] of methanol (Hayman), and the appropriate volume of hexachloroplatinic acid (Riedel-De-Haen AG) dissolved in HCl.
The concentration of formed methanal was determined spectrophotometrically at 412 nm following Nash method [42, 43] using UV-visible spectrophotometer (T80+, PG Instruments Limited, England).
Hence the rate of the methanal (HCHO) formation will be slower while the conversion of methanal to formic acid (HCOOH) is a faster process.
