Charge separation versus charge utilization in TiO2/rGO photocatalysts: opposite effects on phenol degradation and hydrogen evolution

TiO2-reduced graphene oxide (rGO) hybrids were investigated to clarify how TiO2 morphology and interfacial charge transfer govern photocatalytic activity in oxidative and hydrogen-evolution pathways. Three anatase TiO2 morphologies were compared: bipyramids predominantly exposing {101} facets and two nanosheetlike samples enriched in {001} facets, either fluorinated or thermally de-fluorinated. TiO2/rGO composites were prepared by introducing a constant nominal GO content of 2 wt% followed by in situ hydrazine reduction in the presence of TiO2. Photocatalytic tests showed that rGO systematically enhanced phenol degradation for all TiO2 morphologies, whereas H2 evolution from formic acid photoreforming was consistently suppressed. Photoelectrochemical measurements revealed that rGO generally promotes charge extraction and electron transport; however, in the presence of formic acid and without O2 as an electron scavenger, its effect becomes morphology-dependent, with {101}-rich bipyramids showing reduced photocurrent upon rGO incorporation. These results demonstrate that improved charge separation does not necessarily translate into enhanced photoactivity. Instead, photocatalytic performance depends on whether the extracted charges can be productively utilized by the target surface reaction. The study highlights the need for reaction-oriented design of TiO2-carbon photocatalysts, where charge separation, surface faceting, and electron-utilization pathways must be considered together.