Abstract
This work investigates the production of hydrogen (H(2)) and formic acid (HCOOH) through a photoelectrochemical (PEC) approach. Nickel oxide nanofilms prepared by sputtering capped on n-GaN photoelectrodes were employed to achieve simultaneous water photoelectrolysis and CO(2) reduction. The study delves into the role of the nickel oxide layer, examining its potential as a catalyst and/or a protective layer. Furthermore, the influence of nickel oxide layer thickness on the performance of the photoelectrodes is explored. In essence, appropriate nickel oxide thickness is beneficial in increasing the photocurrent of the PEC reaction. The observed improvements in photocurrents and, hence, the production rates can be attributed to the functionality of the nickel oxide nanofilm: mitigating the negative influence of surface defects on n-GaN and facilitating the separation of photogenerated electron-hole pairs at the electrolyte/n-GaN interface. Specifically, PEC cells utilizing the 4 nm-thick nickel oxide nanofilms deposited on n-gallium nitride (n-GaN) electrodes demonstrate a significant enhancement in hydrogen and formic acid production rates. These rates were at least 45% higher compared to PECs using bare n-GaN electrodes.