Surface property control of semiconducting metal oxide nanoparticles

The comprehension of the underlying mechanism attributing either a reducing or oxidizing effect to humidity on the metal oxides used in gas sensors is still a challenge as the literature on this subject shows contradictory results. The high surface-to-bulk ratio of nanosized powders conferring them a much higher surface reactivity compared to their micronsized counterpart, clearly helps in a better understanding of the gas-surface interactions. A thorough FTIR surface study of a nanosized titanium dioxide powder subjected to various sequences of carbon monoxide and mixtures of carbon monoxide and water vapor is reported. The effect of acetic acid and hexamethyldisilazane grafting on TiO₂ is studied under the same conditions. It is found that water molecules have a reducing or oxidizing effect, depending on the acido-basicity of the metal oxide. Our study clearly shows that a thorough knowledge of the surface reactivity of metal oxides is a prerequisite for further major gas sensing optimization.