(Invited) Improving Charge Transport in TiO2 nanoparticle Based Hybrid Photoanodes through Spatial Control of Graphene Nanoribbon Assembly

Two-dimensional (2-D) graphene nanomaterials have spurred immense research interests ranging from water splitting solar cells to other energy storage devices because of their tantalizing morphological, nano-mechanical, and electrical properties. However, they tend to be delegated to secondary roles for establishing conductive pathways, as unwanted aggregation detrimentally decreases the fill factor (FF). In this study, we demonstrate the co-assembly of geometrically engineered graphene nanoribbons (GNRs) with TiO₂ nanoparticles (T-GNR) at an optimized weight ratio effectively alleviates the aforementioned constraints while synergistically improving junctions (V_OC), electrical contact (J_SC) and FF in photoelectrochemical (PEC) cells. Uniform and controllable-sized GNRs help define the domain of TiO₂ nanoparticles well below the upper limit of charge carrier transport length (300 nm), thus effectively eliminating unwanted grain boundaries that are known to act as electron traps. Tip sonication is used to systematically decrease the size of individual GNRs in order to determine a geometric conformation that favors charge transport around TiO₂ nanoparticles. Finally, we present the increasing temperature dependence on photocurrent density based on the elimination of defect and surface oxidation from annealing. Through this study, we identified a large gain in photocurrent density of a 3-layered (3L) tip-sonicated T-GNR assembly at 200°C, 72.6% higher than pure TiO₂ nanoparticles while simultaneously improving the junction properties and open circuit potential. It also introduced a FF of 66.6% as compared with a pure layer of TiO₂ FF of 52.4%.