Role of Reduced Graphene Oxide in the Critical Components of a CdS-Sensitized TiO2-Based Photoelectrochemical Cell

Nitrogen (N)-doped reduced graphene oxide (nRGO) is systematically incorporated into a TiO2-CdS photoelectrochemical (PEC) cell and its role is examined in the three main components of the cell: 1) the CdS-sensitized TiO2 photoanode, 2) the cathode, and 3) the S2-/S center dot- aqueous redox electrolyte. The nRGO layer is sandwiched between TiO2 nanorods (deposited by using a solvothermal method) and CdS (deposited by using the successive ionic-layer-adsorption and -reaction method). Scanning electron microscopy with energy dispersive X-ray analysis (EDS) reveals the spatial distribution of CdS and nRGO, whereas nRGO formation is evident from Mott Schottky analysis. Chronoamperometry and PEC analysis indicate that upon incorporation of nRGO, a photocurrent density that is at least 27 times higher than that of pristine TiO2 is achieved; this increase is attributable to the ability of the nRGO to efficiently separate and transport charges. Stability analysis performed by continuous photoillumination over -3 h indicates a 26% and 42% reduction in the photocurrent in the presence and absence of the nRGO respectively. Formation of SO42- is identified as the cause for this photocurrent reduction by using X-ray photoelectron spectroscopy. It is also shown that nRGO-coated glass is as effective as a Pt counter electrode in the PEC cell. Unlike the benefits offered by nRGO at the anode and cathode, introducing it in the redox electrolyte is detrimental. Systematic and complementary electrolyte and film-based studies on this aspect reveal evidence of the capacitive behavior of nRGO. Competition between the nRGO and the oxidized electrolyte is identified, based on linear-sweep voltammetry analysis, as the limiting step to efficient charge transport in the electrolyte.