Synthesis of cadmium sulfide and tin sulfide-sensitized cadmiumnanorods and effect of nickel doping for photoelectrochemical applications

Zinc oxide (ZnO) is a promising semiconducting material for photoelectrochemical (PEC) applications such as water splitting due to its high electron mobility, stability, and favorable band-edge for water oxidation. However, bulk ZnO exhibits high recombination rate of photogenerated electrons and holes and limited visible light absorption owing to its wide band gap. To overcome these barriers, this study developed low-dimensional nanostructures of ZnO, in form of nanoparticles (NPs) and nanorods (NRs). Heterostructures comprised of cadmium sulfide/zinc oxide (CdS/ZnO), tin sulfide/zinc oxide (SnS/ZnO), and tin sulfide/cadmium sulfide/zinc oxide (SnS/CdS/ZnO) extend the absorption range in the solar spectrum, prolong electron lifetime, and enhance PEC performance of ZnO-based photoanodes. The study also examined the effect of nickel (Ni) doping on SnS/CdS/ZnO heterostructures. Synthesis approaches such as sol-gel spin coating, hydrothermal growth, and successive ionic layer adsorption and reaction (SILAR) were utilized to fabricate the proposed nanoheterostructures, followed by parameter optimization. Vertically aligned ZnO NRs demonstrated superior optical absorption and photocurrent generation compared to NPs due to their high aspect ratio and larger surface area. Optimized ZnO NR arrays were then sensitized with CdS forming a core-shell heterostructure. Deposition of CdS resulted in the ultraviolet-visible absorption band edge shifting to a higher wavelength, indicating enhanced visible light harvesting. The type-II interband alignment in the CdS/ZnO heterostructure facilitated electron-hole separation and transfer, boosting the incident light-to-current generation up to 5.44 mA/cm2. Additionally, polycrystalline tin sulfide (SnS) was also successfully deposited on ZnO NRs, showing an improved photoconversion efficiency (η) from 0.56 % to 1.33 % and photocurrent density (Jph) from 0.48 mA/cm2 to 1.63 mA/cm2 due to increased carrier lifetime and efficient charge transfer across the p-SnS/n-ZnO junction. Co-sensitization with a SnS/CdS bilayer further enhanced PEC performance of ZnO, achieving a Jph of 7.50 mA/cm2. Ni incorporation into the ternary SnS/CdS/ZnO NRs improved film uniformity, crystallinity, and charge transfer kinetics without significantly altering structural properties and absorption behavior. This improvement is attributed to reduced crystallographic defects and electronic structure modulation by Ni doping. In conclusion, the Ni-doped SnS/CdS/ZnO NRs photoanode demonstrates significant potential for enhancing visible light absorption, charge carrier generation and separation, and suppression of electron-hole recombination, thereby significantly improved PEC cell performance.

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