A Novel Z-Scheme Heterojunction for Efficient Photoelectrochemical Water Splitting

Abstract

Photoelectrochemical (PEC) water splitting is a promising technology to produce renewable hydrogen, which still faces problems of low photoelectric efficiency and instability. This work introduces a novel Z-scheme heterojunction consisting of BiVO4 thin film and diamond-like carbon (DLC) to the PEC system as a photoanode and achieves an increased PEC water splitting activity. Compared to the pure BiVO4, the BiVO4/DLC heterojunction has an increased photocurrent density from 0.79 mA cm-2 to 2.39 mA cm-2 at 1.23 V vs. RHE, and the IPCE from 4.8% to 23.4%. After 120 mins (180 cycles) of PEC water splitting, the BiVO4/DLC heterojunction retains up to 82.3% of the initial photocurrent density value, while the pure BiVO4 has only 33.99% remaining under the same conditions. Furthermore, the PEC system of BiVO4/DLC reaches the evolution rate of hydrogen of over 0.32 mmol h-1 cm-2, which is much higher than that of its counterparts. We ascribe the enhancement in PEC performance to the Z-scheme heterojunction that greatly promotes carrier mobility and reduces the recombination rate of electron-hole pairs. We also identify that the band bending and depletion layer are the main reasons for the decrease in charge transfer and interface resistance. It is demonstrated that the sp2 carbon phase in DLC acts as a conductive channel to improve the separation and migration efficiency of free carriers, while the sp3 carbon phase provides stable support and passivation effect to the photoanode, thereby leading to an enhancement for both PEC water splitting performance and the long-time stability. This work provides an effective way to improve the BiVO4 photoanode-based PEC water-splitting systems.

Keywords: bismuth vanadate, diamond-like carbon, Z-scheme heterojunction, photoelectrochemical, Water splitting