Photoelectrochemical study of carbon-modified p-type Cu2O nanoneedles and n-type TiO2−x nanorods for Z-scheme solar water splitting in a tandem cell configuration

Abstract

Nanostructured photoelectrodes with a high surface area and tunable optical and electrical properties can potentially benefit a photoelectrochemical (PEC) water splitting system. The PEC performance of a nanostructured photoelectrode is usually quantified in a standard three-electrode configuration under potential-assisted conditions because of the additional overpotentials for the two half-reactions of water splitting. However, it is a necessity to fully recognize their potential to split water under unassisted conditions by designing a tandem cell that can provide sufficient voltage to split water. Herein, we present a tandem cell consisting of carbon-modified cuprous oxide (C/Cu₂O) nanoneedles and oxygen-deficient titanium dioxide (TiO_2−x) nanorods for unassisted solar water splitting. The synthesized photoelectrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and electrochemical impedance spectroscopy (EIS) techniques. The tandem cell performance was analyzed by measuring the current–voltage responses in various photoelectrode configurations to validate the collective contributions of both photoelectrodes to unassisted solar water splitting. The PEC properties of C/Cu₂O nanoneedles coupled with TiO_2−x nanorods in a tandem configuration exhibited a photocurrent density of 64.7 μA cm⁻² in the absence of any redox mediator and external bias. This photocurrent density can be further enhanced with an application of external bias. Moreover, the heterojunction formed by the above-mentioned nanostructured photoelectrodes in intimate contact and in the absence of water exhibited 2 μA cm⁻² UV photoresponsivity at 1.5 V with promising rectifying characteristics of a diode.