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 degrees C, TiO2 nanospheres with hierarchical pores were generated, as confirmed by the shifting of conduction bands in TiO2 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 TiO2 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 Mw 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/cm(2), owing to the increased surface areas and light scattering.