From Flat Films to Granules: Seed-Assisted Hydrothermal Growth of TiO2 Nanorods on FTO and their Photocatalytic Behavior

Seed-assisted hydrothermal growth of TiO2 nanorods (TiO2 NRs) on three types of fluorine-doped tin oxide (FTO) substrates was comparatively examined to investigate the role of FTO seed morphology and surface chemistry in directing TiO2 NRs growth, and its impact on photocatalytic activity. A commercial flat FTO-coated substrate was used as the planar case, a dip-coated FTO seed layer served as the hemispherical geometry, and spray-dried spherical FTO granules were used as simple 3D granular model substrates. An identical hydrothermal process was applied for the preparation of all seeds. NR morphology and alignment were investigated by optical microscopy, SEM, and TEM. Microscopy of dip-coated FTO seeds revealed that the molar ratio of NH4F or the decrease in solvent content in the precursor solutions changed the morphology of the seeds from hemispherical to polygonal islands. Two of the precursor solutions differing in F− content were further spray-dried to obtain spherical FTO granules. Temperature-dependent (450 °C - 850 °C) XRD and FTIR measurements of spray-dried granules revealed the effect of annealing on the crystallization of FTO seeds. The porous nature of the granules led to the formation of spiderweb-like TiO2 NRs, as well as urchin-like structures associated with surface convexity during hydrothermal growth. XPS analysis confirmed higher Cl− and F− concentrations in the TiO2 NRs grown on the spray-dried granules than on the flat FTO. Comparative methyl orange decoloration tests confirmed the effects of annealing, halide doping, seed morphology, and BET-estimated surface area on the photocatalytic performance of TiO2 NRs.