Three-dimensional Cu2O nanorods modified by hydrogen treated Ti3C2TX MXene with enriched oxygen vacancies as a photocathode and a tandem cell for unassisted solar water splitting

Cuprous oxide (Cu 2 O) has been envisioned as a promising photocathode material for photoelectrochemical (PEC) water reduction due to its abundance, scalability and nontoxicity. Herein, a three-dimensional (3D) Cu 2 O nanorods (NRs) photocathode is prepared by in-situ growing Cu 2 O NRs on a copper foam substrate. The bare 3D Cu 2 O NRs photocathode exhibits a high photocurrent density of −4.02 mA cm −2 at 0 V versus the reversible hydrogen electrode (RHE) under AM 1.5 G simulated sunlight. The oxygen vacancies enriched Ti 3 C 2 T X flakes are obtained by reducing Ti 3 C 2 T X with H 2 /Ar gas (denoted as H:Ti 3 C 2 T X ). The H:Ti 3 C 2 T X /Cu 2 O, Ti 3 C 2 T X /Cu 2 O as well as Au/Cu 2 O composite photocathodes are fabricated through a facile dip-coating method. The H:Ti 3 C 2 T X /Cu 2 O and Ti 3 C 2 T X /Cu 2 O yield a photocurrent density of −5.41 and −4.45 mA cm −2 respectively, which is higher than the bare Cu 2 O and Au/Cu 2 O (−4.31 mA cm −2 ). The results suggest that the Ti 3 C 2 T X is more effective than noble metal Au for improving the PEC performance of Cu 2 O photocathode. The superior performance of H:Ti 3 C 2 T X /Cu 2 O is attributed to enriched oxygen vacancies in the H:Ti 3 C 2 T X which can enhance the conductivity, light-harvesting efficiency and charge-transfer ability of fabricated photocathode. A PEC tandem cell with H:Ti 3 C 2 T X /Cu 2 O as the photocathode and BiVO 4 as the photoanode is designed for unassisted solar water splitting. Due to the high porosity and optical transmittance of the 3D copper foam substrate, we illuminate the cell from the H:Ti 3 C 2 T X /Cu 2 O side rather than from the BiVO 4 side in usual fashion. The present configuration exhibits a 1.6 times higher solar-to-hydrogen (STH) conversion efficiency of 0.55% than the conventional design with a STH conversion efficiency of 0.34%. The enhanced STH conversion efficiency in our configuration is due to the higher light-harvesting efficiency.