Carbon Impurity Entrapping and Charge Localization within TiO₂ Nanoparticle Films

Titanium dioxide (TiO₂) particle systems are well-established photocatalysts with high performance under UV irradiation. They are often used as supported nanostructured thin films composed of interconnected TiO₂ nanoparticles. During the film preparation, a variety of defects can be introduced, which can have a significant influence on the material performance. This can be used for defect engineering to enhance charge generation and separation within photocatalysts. We report here a study of the paramagnetic properties of four different TiO₂ nanoparticle architectures. The spin concentrations measured on supported films and free-standing nanoparticles, in the presence or absence of dense TiO₂ thin films prepared via sputtering, are compared. Organic additives are typically used for the immobilization of powdered photocatalyst materials or the production of photoelectrodes. Despite extensive cleaning and oxidative treatment for all cases where nanoparticle aggregation can occur or interfaces can form between the particles and the silicon substrate, paramagnetic carbon-related defects appear and become part of the lattice. In the concentration range of a few parts per million, underlying carbonecks act as electron traps and represent a previously overlooked defect type that may determine the photoelectronic properties of TiO₂-based nanostructures.