Room Temperature Synthesis and Thermal Evolution of Porous Nanocrystalline TiO 2 Anatase Aiat Hegazy † and Eric Prouzet* ,‡ † Chemical Engineering and ‡ Chemistry and Institute of Nanotechnology, University of Waterloo, 200 University Avenue West, Ontario N2L 3G1, Canada * S Supporting Information 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. KEYWORDS: TiO 2 , DSSC, photovoltaic, nanoparticle, photoanode, anatase, mixture design, integrative synthesis, photocatalysis, semiconductor, band gap ■ INTRODUCTION Nanocrystalline phases of titanium oxides are required in many domains that use the specific semiconducting properties of this metal oxide. For example, dye-sensitized solar cells (DSSCs) 1 have attracted interest since the pioneering work of Grä tzel and coll., 2-4 and DSSC thin film-based solar cell technologies are currently considered one of the most promising candidates, 5 due to the low cost of raw materials and potential high conversion efficiency. 1,6 However, results have been demon- strated only at the laboratory scale, and a lot of development and processing have still to be developed. DSSCs are multifunctional systems where photons are initially collected by dye molecules that host photoconversion. 4 The electrons generated by the dye molecule are extracted via an oxidation mechanism and injected into the conduction band of a semiconducting material, named the photoanode, before being transferred to the electrical circuit through a charge migration into a small surface region (1.5 nm) of the photoanode, 7 as a result of a surface charge depletion induced by the surrounding electrolyte. 8 In parallel, the dye molecules are regenerated by reduction with a I 3 - /I - based electrolyte, itself regenerated by the electrons coming from the external electrical circuit. One of the most important challenges in DSSC devices is achieving synergy between all cell components, and the nature, morphology, and interfacial properties of the photoanode are master keys to achieving high solar conversion efficiency. First, the material must possess a wide bandgap to allow for the electron transfer from the dye molecule and prevent any electron-hole recombination. Second, its nanostructure must allow for a high dye loading and good connectivity for easy electron transport to the current collectors, a requirement that implies the existence of a hierarchical nanostructure. 9 There- fore, TiO 2 is one of the most commonly used photoanodes for DSSC. 10 Among the different crystalline phases of TiO 2 crystalline structures, anatase has a more opened structure and a larger bandgap (3.2 eV) than rutile (3.0 eV). This wider bandgap not only slows down the electron-hole recombination but also allows the material to be transparent to most of the visible light spectrum, a compulsory requirement to the electrode, 4 even though the porous structure of the electrode will allow it to reduce transparency and improve light scattering within, for a better photon capture by the dye molecules. Received: June 7, 2011 Revised: December 14, 2011 Published: December 14, 2011 Article pubs.acs.org/cm © 2011 American Chemical Society 245 dx.doi.org/10.1021/cm201602a | Chem. Mater. 2012, 24, 245-254