Growing TiO2 nanowires on the surface of graphene sheets in supercritical CO2: characterization and photoefficiency

Tremendous interest exists towards synthesizing nanoassemblies for dye-sensitized solar cells (DSSCs) using earth-abundant and -friendly materials with green synthetic approaches. In this work, high surface area TiO2 nanowire arrays were grown on the surface of functionalized graphene sheets (FGSs) containing -COOH functionalities acting as a template by using a sol-gel method in the green solvent, supercritical carbon dioxide (scCO(2)). The effect of scCO(2) pressure (1500, 3000 and 5000 psi), temperature (40, 60 and 80 degrees C), acetic acid/titanium isopropoxide monomer ratios (HAc/TIP = 2, 4 and 6), functionalized graphene sheets. FGSs/TIP weight ratios (1: 20, 1: 40 and 1: 60 w/w) and solvents (EtOH, hexane) were investigated. Increasing the HAc/TIPweight ratio from 4 to 6 in scCO(2) resulted in increasing the TiO2 nanowire diameter from 10 to 40 nm. Raman and high resolution XPS showed the interaction of TiO2 with the -COOH groups on the surface of the graphene sheets, indicating that graphene acted as a template for polycondensation growth. UV-vis diffuse reflectance and photoluminescence spectroscopy showed a reduction in titania's bandgap and also a significant reduction in electron-hole recombination compared to bare TiO2 nanowires. Photocurrent measurements showed that the TiO2 nanowire/graphene composites prepared in scCO(2) gave a 5 x enhancement in photoefficiency compared to bare TiO2 nanowires.