Pd-decorated CdS nanostructures with controlled morphologies for efficient photocatalytic hydrogen evolution by water splitting.

Photocatalytic water splitting has emerged as a promising strategy for sustainable hydrogen production under visible-light irradiation. In this study, CdS photocatalysts with different crystalline phases and morphologies were synthesised via a hydrothermal method using various sulphur precursors. Detailed structural and morphological characterisations using XRD, SEM, and TEM were carried out to investigate the influence of the sulphur precursors on the formation of different crystalline phases, including hexagonal and cubic CdS, as well as distinct morphologies such as spherical, leaf-like, and flower-like structures. UV-vis DRS analysis revealed that all the synthesised catalysts had band gaps within the visible-light region. Among them, the nano-spherical CdS photocatalyst exhibited the highest hydrogen evolution rate of 1009 µmol g-1 under visible-light irradiation, which was attributed to its higher surface area, as determined by BET analysis, along with its narrower band gap and enhanced light-absorption capability. To further enhance the photocatalytic performance, Pd was impregnated onto the CdS surface to form an efficient metal-semiconductor (M-S) junction, resulting in a two-fold increase in hydrogen production. The improved activity was associated with reduced band gap energy and suppressed charge-carrier recombination, as confirmed by PL spectroscopy. Furthermore, the effect of Pd loading (1-5 wt%) on the photocatalytic activity was investigated, while metal dispersion was evaluated using CO pulse chemisorption. All the photocatalysts exhibited good stability and sustained activity throughout 12 hours of photocatalytic testing. This study highlights the crucial role of crystalline phases, morphology, and metal loading in the design of highly efficient CdS-based photocatalysts for visible-light-driven hydrogen production.