S-doped copper selenide thin films synthesized by chemical bath deposition for photoelectrochemical water splitting

Abstract

The utilization of fossil fuels to meet ever-increasing energy demands is causing a significant threat to the Earth's climate and its inhabitants. To address this issue, photoelectrochemical (PEC) water splitting has emerged as a promising solution to produce oxygen (O2) and hydrogen (H2) in a cost-effective, environmentally friendly, clean, and renewable manner. In this study, we present the development of cost-effective Cu3Se2 and S-doped Cu3Se2 thin films on Fluorine-doped Tin Oxide (FTO) substrates using a chemical bath deposition (CBD) process to enhance the efficiency of PEC water oxidation to produce oxygen. The X-Ray Diffraction (XRD) pattern confirms the presence of a single-phase tetragonal Cu3Se2 crystal structure with a noticeable shift in the characteristic (211) reflection towards a higher 2θ value, from 38.1° to 38.4°, indicating the successful doping of S in the Cu3Se2 thin films. To further validate successful S doping in the Cu3Se2 crystal lattice, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray (EDX), Fourier transform infrared (FTIR), and Raman Spectroscopy were employed. TEM reveals a granular structure with an average particle size of 7–8 nm, which is consistent with the crystallite size determined by XRD. Furthermore, the valence band and conduction band edge positions are found to be 1.56 and −0.57 eV, respectively. The presence of S increased the band gap of Cu3Se2, from 1.97 to 2.15 eV, thus confirming the bending of the band edge upon doping. The specific surface areas (SSA) of pristine and S-doped Cu3Se2 were determined to be 5.03 and 111.48 m2g−1, respectively, showing a twenty-two-fold increase upon doping. The PEC performance of the materials was evaluated through chopped chronoamperometry (CA), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and Mott-Schottky (M-S) analysis. The S-doped Cu3Se2, prepared under optimum conditions, exhibited a photocurrent density difference of 880 µA cm−2, which was 6 times greater than for the pristine Cu3Se2 thin film. The results of S-doped Cu3Se2 indicate a negative slope, implying p-type conductivity, and a lower flat band potential value of 0.33 V vs. RHE. Our approach provides a green and cost-effective solution to fabricate S-doped Cu3Se2 thin films for producing hydrogen and oxygen using CBD.