Synergic effect of ZnO nanostructures and cobalt phosphate co-catalyst on photoelectrochemical properties of GaN

Novel PEC cell design based on GaN/ZnO/CoPi heterojunction cascade was synthesized using united metal organic chemical vapor deposition (MOCVD), hydrothermal, and photo-electrodeposition strategy. Two different type of ZnO architectures viz., honeycomb (HON) and nanorods (NRs) were grown on the GaN layer by executing structure directing agents of multiwall carbon nanotubes, and ZnO buffer layer, enhancing photoelectrocatalytic (PEC) activity toward oxygen evolution reaction (OER). In an expansion of such approach, the PEC activity was further improved by incorporating photo-electrodeposited cobalt phosphate (CoPi) in synthesis system accompanying negative shift of the photocurrent onset potential. A steady-state and highest photocurrent density (Jph), of 4.81 ± 0.02 mA/cm2 was obtained with GaN/ZnO NRs/CoPi heterojunction cascade electrode at 1.23 V verses reversible hydrogen electrode (VRHE) under simulated sunlight illumination. PL result shows quenching of photoluminescence with CoPi modification acts as co-catalyst and preferably captures hole from ZnO NRs which likely to facilitate the electron-hole (e−/h+) pair separation and OER reaction kinetics. However, CoPi modified GaN/ZnO HON and pristine GaN photoelectrodes exhibited relatively lesser photocurrent densities. Experimental evidences by electrochemical impedance (EIS) and incident photon to current conversion efficiency (IPCE) confirmed superior electrical conductivity, light absorption capability and charge transfer across GaN/ZnO NRs/CoPi heterojunction interfaces attributable to type II band alignment together with dominant synergistic effect of ZnO NRs nanostructures and CoPi oxidation co-catalyst which could augment PEC water splitting. This work encourages futuristic insights into integrated synthesis methods toward development of high performance photoelectrodes based on underexplored GaN and other heterostructures.