I have prepared nanocrystalline TiO2 powder and I am looking for gas sensor applications.

Firstly, most of the gas sensors are specified only for a certain temperature. Since cooling is more difficult, the heating temperature is set well above room temperature in order to be sure that the simple Joule-heating does the job.

Secondly, the gas sensor itself needs to be water-free. Usually, all objects at room temperature (or lower) around us are covered with a multi-molecular H2O-film, coming from the air humidity and sticking on the surface due to the high polarity of the H2O-molecule. In heating, you get rid of that, like baking of ultrahigh-vacuum chambers.

The key process in the response of the semiconductor to a reducing gas involves the modulation of the concentration of adsorbed oxygen species. By withdrawing electron density from the semiconductor surface, adsorbed oxygen gives rise to Schottky potential barriers at grain boundaries, and thus increases the resistance of the sensor surface.

Reducing gases decrease the surface oxygen concentration and thus decrease the sensor resistance. The temperature dependence of this process arises in part from the differing stabilities of the surface oxygen species over different temperature ranges. The different gases have characteristic optimum oxidation temperatures, and therefore give rise to characteristic conductance temperature profiles, which can be modified by doping the semiconductor with noble metals or other catalytic materials.

most gas sensor is built from materials and these materials have maximum working temperature to detect the gases, so increasing the temperature of detecting materials to the best sensitivity like ZnO is has very good sensitivity at 200C for methane gas so they using heater to increase the ZnO temperature to 200 C to be always at maximum sensitivity. this type of work cause the draw back the sensitivity of sensor with time and can effect on the accuracy of detector

Generally metal oxide based gas sensors operate at relatively higher temperatures to facilitate the chemical reaction to produce the sensor response (as endothermic chemical reaction requires some activation energy that can be provided in form of heat) and to reduce the effects of humidity.