Preparation of TiO₂ spheres with hierarchical pores via grafting polymerization and sol-gel process for dye-sensitized solar cells

Titania (TiO₂) nanoparticles were surface-modified via atom transfer radical polymerization (ATRP) with hydrophilic poly(oxyethylene) methacrylate (POEM), which can coordinate to the TiO₂ precursor, titanium(IV) isopropoxide (TTIP). Following application of a sol-gel process and calcination at 450 °C, TiO₂ nanospheres with hierarchical pores were generated, as confirmed by the shifting of conduction bands in TiO ₂ using UV-visible spectroscopy and X-ray photoelectron spectroscopy (XPS). The particle size and morphology of TiO₂ were characterized using wide angle X-ray scattering (WAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Brunauer-Emmett-Teller (BET) analysis revealed bimodal distribution of TiO ₂ pore sizes with peaks at 6 nm and 50 nm to afford better penetration of polymer electrolyte, as confirmed by electrochemical impedance spectroscopy (EIS). Dye-sensitized solar cells (DSSC) made from TiO₂ nanospheres with hierarchical pores exhibited improved photovoltaic efficiency (3.3% for low molecular weight (M_w) and 2.5% for high M_w polymer electrolytes), as compared to those from neat TiO₂ nanoparticles (2.4% for low M_w and 1.3% for high M_w) at 100 mW/cm², owing to the increased surface areas and light scattering.