Room Temperature Synthesis and Thermal Evolution of Porous Nanocrystalline TiO2 Anatase

Abstract

TiO2 nanoparticles are a major component in many areas, and especially for dye-sensitized solar cells (DSSC) as a result of their electronic structure that allows them to collect the electrons transferred from the dye molecules after sunlight irradiation, as well as of their semiconducting properties, which provide the surface transport of these electrons up to the collecting electrode. However, for this application or others, the optimization of both structural and electronic properties of titanium oxide is still a challenge because it depends on both crystalline structure and material nano/mesostructure. We report how small (<6 nm) titanium oxide nanoparticles were synthesized by a single step method, with the anatase crystalline phase obtained at room temperature, and an opened nanostructure. A mixture design method was required to identify the precise composition that led to the suitable material. Mesoporous materials made of pure anatase nanocrystals were obtained with the suitable porosity (5 nm pore diameter, 190 m2/g, 0.3 mL/g porous volume) without any surfactant agent. Both the evolution of the crystal size and nature of the phases were studied as a function of heating temperatures ranging from 20 to 800 °C. These materials display a good thermal stability up to 400 °C, in term of crystal size, and up to 700 °C, regarding the crystalline phase. Finally, the study of their semiconducting properties as a function of the crystal size allowed us to confirm the previous theoretical models regarding the crystal size-dependence of band gap and to set the limit of the size quantum confinement effect around 7 nm.