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Abstract: We used a new synthesis of TiO2 anatase 6nm nanocrystals prepared at room temperature (Hegazy and Prouzet, 2012 [10]) to explore the influence of different physical-chemical parameters on photocatalysis, and bench-tested the material against two commercial powders made of either pure anatase (Sigma™), or composite anatase–rutile particles (P25 Degussa™). The initial as-synthesised material demonstrates a low photocatalytic activity, which is greatly improved after thermal activation as a result of improved crystallinity without any drastic change in crystal size. The influence of several other parameters was studied, the resulting tests being compared with commercial products. The cumulative improvement provided by these different parameters led finally to a material that exhibits a higher photocatalysis compared to commercial anatase, and similar to the commercial material usually used for reference (P25). This study, which can apply to other titania materials, illustrates how the post-treatment and process adaptation can help to optimise an initial material. [Copyright &y& Elsevier]

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. The parameters for the synthesis of mesoporous nanocrystalline anatase made of 4 nm nanocrystals were identified via a mixture design method. This surfactant-free room temperature synthesis gives mesoporous materials (5 nm pore size, 190 m2/g, 0.3 mL/g porous volume), which display band gap size quantum confinement, that were tested more specifically for dye-sensitized solar cell applications. [ABSTRACT FROM AUTHOR]

Dye-sensitized solar cells (DSSCs) are still attracting great attention as a potential alternative to conventional silicon solar cells, and a lot of current research is focusing on enhancing their efficiency through the individual improvement of the DSSCs components, as well as their integration. Among all components, mesoporous TiO 2 plays a major role, because it is the charge collector for the dye molecules or perovskite in liquid and solid DSSCs, respectively. Specificities of TiO 2 are now well-known: an anatase crystalline structure for an optimized bandgap, small particle size to shorten the charge diffusion path, reduced grain boundaries to limit current resistivity, significant porosity to enhance integration and interaction between the photon absorbers and the charge-collecting anode. We reported previously how well-crystallized small nanoparticles were successfully synthesized via a room temperature synthesis by Hegazy and Prouzet (2012) [ 10 ], leading to very small crystallites (4–10 nm) of pure TiO 2 anatase. Since as-synthesized materials still contain hydration water, a major drawback in DSSCs, we report here how dehydration and crystallinity can be improved by annealing at a moderate temperature, without any sintering or significant particle coarsening. The resulting impact on properties is confirmed by testing the resulting annealed TiO 2 nanopowder in photovoltaic liquid DSSCs. In addition, we describe a comprehensive study of the preparation and test of TiO 2 based photoanodes, including UV exposure and HCl treatment. Photovoltaic tests were conducted on a large active surfaces (5 cm 2 ) cells to give a more realistic picture of the influence of each parameter involved in the cell performance, characterized under AM 1.5 illumination with 100 mWcm −2 light intensity. The specific contribution of each parameter on the cell performance is discussed. We demonstrate why the TiO 2 nanopowder developed in our previous synthesis, in connection with the suitable treatments during the photoanode preparation, can replace commercial powders, with a greater success in cell photoefficiency. [ABSTRACT FROM AUTHOR]