Room Temperature Synthesis and Thermal Evolution of Porous Nanocrystalline TiO₂ Anatase

Abstract

TiO 2 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 m 2 /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.

Effect of Anatase Synthesis on the Performance of Dye-Sensitized Solar Cells

“…Following the results of Enright, Hegasy was able to follow the evolution of the indirect energy band gap as a function of the anatase crystal size and observed an energy shift of that band Δ Eg(eV) = 0.15 eV. By applying TiO 2 m with the anatase phase to the development of DSSC solar cells, Hegasy and collaborators reported an efficiency η  = 0.68 % [27]. …”

Influence of surface disorder, oxygen defects and bandgap in TiO2 nanostructures on the photovoltaic properties of dye sensitized solar cells

“…Further the synthesis method play significant influence on the values with the wet chemical methods leading to lower E g . Recently Prouzet's [59] group has shown that the indirect bandgap model gives better agreement on the E g values of anatase nanoparticles. Further, Uddin et al's [60] report on the bandgap of TiO 2 show that the indirect bandgap model gives lower E g , close to the reported bulk values (3.2 eV), than the estimates obtained using direct bandgap model.…”

Light-activated self-propelled colloids

“…We use an iron oxide, (α-Fe 2 O 3 ), named haematite (figure 1a), with a band gap of about 2.2 eV, corresponding to a visible wavelength λ Fe 2 O 3 ∼ 560 nm [23] and previously studied in [24,25]. We moreover present a new colloidal particle taking advantage of an alternate photoactive material: titania (TiO 2 ) in the anatase phase (figure 1b), with a bandgap of 3.1 eV, corresponding to the wavelength λ TiO 2 = 400 nm [26]. The particles are conveniently observed with an inverted optical microscope equipped with high magnification objectives, typically 60× or 100× and a conventional brightfield diascopic illumination.…”