To assess the potential morphological effects of molybdenum catalysts on the Sb2Se3 surface, scanning electron microscopy (SEM) images were obtained (Figure ). Differences observed between bare Sb2Se3 and catalyst-treated samples were attributed to the etching treatment before catalyst soaking, consistent with prior studies. SEM images after PEC measurements showed no noticeable differences in the morphology of the films. As different films were measured for each image, the differences in grain size can be attributed to sample-to-sample or even region-to-region variation. Atomic force microscopy (AFM) studies (Figure 3). Raman measurements with 488 and 532 nm excitation wavelengths reveal clear patterns belonging to [Mo3S4] 4+ and [Mo3S13] 2-phases, as shown in Figures and, respectively. This behavior is expected, as both 488 nm and 532 nm lasers have penetration depths corresponding to about 50 and 100 nm, respectively. This reduces the ratio of Sb2Se3/[Mo3S4] 4+ , and Sb2Se3/[Mo3S13] 2-probed volumes, rendering these laser excitations more sensitive to detecting clusters on the surface of Sb2Se3. In the spectra of Sb2Se3 with [Mo3S4] 4+ clusters, intense Raman peaks at 354, 440, 456 and 493 cm -1 are observed, which are not featured in the Raman spectra measured on the reference Sb2Se3 with both 488 nm and 532 nm excitations. These peak positions are in good agreement with the Raman peak positions of [Mo3S4] 4+ reported in the literature. Furthermore, in the Raman spectra of Sb2Se3 with [Mo3S13] 2-clusters, peaks at 285, 329, 360, 385, 453, 518 and 552 cm -1 in addition to peaks belonging to the Sb2Se3 phase were identified. These peaks are in good agreement with the Raman peak positions of [Mo3S13] 2-phase reported in the literature. The comparison of Raman spectra measured with 785 nm does not reveal any significant differences in the spectral features, including intensity, width and position of Raman peaks among the three samples. Only Raman peaks corresponding to the Sb2Se3 phase are observed at this wavelength, as shown in Figure . While the mechanism of action of molybdenum sulphide catalysts is still debated, there are currently two fundamentally different theories of how HER occurs on Mo3Sx. One is a "molybdenum based" catalysis whilst the other is "sulphur based". In the molybdenum-based catalysis, unsaturated Mo sites (originating from the loss of terminal disulphides in [Mo3S13] 2-) serve as both a redox-active element and a site for substrate binding. In this system, molybdenum hydride is generated which is then protonated to liberate H2. Meanwhile, in the sulphur-based system, disulfides play the dual role of proton binder and redox-active component. In both theories, a molecular system would mean that there are more active sites per unit area for H2 evolution as the concentration of unsaturated Mo sites in both [Mo3S4] 4+ and [Mo3S13] 2-and terminal disulphides in [Mo3S13] 2-are higher compared to MoS2 thin films. Based on the electrochemical and X-ray photoelectron spectroscopy (XPS) results in this study, it is hypothesised that both H2 evolution mechanisms are in action in conjunction as there are no terminal disulphides in the [Mo3S4] 4+ clusters, however, the higher activity of [Mo3S13] 2-is likely due to the contributions from the disulphide based hydrogen evolution leading to a better performance.