Surface chemistry of TiO2 nanoparticles: influence on electrical and gas sensing properties

The modification of the surface chemistry of semiconducting nanoparticles is often required for optimising their performance. For example, surface modifications of semiconductor-based sensors can be envisaged to tailor the device selectivity. However, surface chemical modifications should deteriorate neither the bulk characteristics nor the electrical properties of the material. This becomes critical for nanoparticles due to their high surface-to-bulk ratio. In this work, surface modifications of titanium oxide nanoparticles by grafting hexamethyldisilazane (HMDS) are monitored in situ by Fourier transform infrared spectroscopy. The HMDS grafting decreases the density of the hydroxyl groups at the titanium oxide surface and, therefore modifies the surface affinity to water molecules. The consequences of these surface modifications on the gas sensing properties of the nanomaterial are discussed. In particular, it is shown how moisture adsorption subsequently alters these new grafted chemical species, resulting in a decrease of the cross-sensitivity to humidity. The variations of the infrared background absorption versus gas exposures are demonstrated to follow a λ2 dependence in agreement with the Drude–Zener theory, thus indicating that they are essentially due to the free carrier absorption. Therefore, the variations of the infrared absorption versus gas exposures can be directly correlated to the electrical conductivity variations.