Abstract
The global imperative to transition from conventional energy sources to green and renewable alternatives has become increasingly evident in the face of escalating environmental challenges. The key motivation for this transition includes the urgent need to mitigate climate change by reducing greenhouse gas emissions, obtaining energy security and independence, fostering economic growth and increasing employment opportunities. Hence, there is a necessity for ongoing advancements and innovations in green energy technologies to drive global collaborations towards sustainable developments. One example of such advancement is the use of the vast solar energy available on the surface of our planet. Solar technologies, characterised by their renewable and inexhaustible nature, present a compelling solution to the challenges posed by conventional energy sources. One promising avenue within solar technology is photoelectrochemical water splitting, which holds a unique potential in directly converting solar energy into storable chemical energy by splitting water into hydrogen and oxygen. This process provides a clean and abundant source of hydrogen fuel and addresses the issues associated with the intermittent nature of solar power by enabling energy storage. The current challenge surrounding photoelectrochemical water splitting includes searching for abundant and low-cost semiconductor materials with efficiencies and stability comparable to those containing precious metals, the benchmark materials currently used in solar technologies. Antimony selenide, Sb2Se3, as a semiconductor material, is a promising candidate for a wide range of applications in photovoltaics and photoelectrochemical water-splitting. This is due to its favourable properties, including high electrical conductivity, optimum optical characteristics, reasonable cost and abundance. Scientists have dedicated over six decades to transforming silicon into the commercially prevalent solar cells available today, and with time, Sb2Se3 can achieve comparable strides in solar-to-hydrogen (STH) efficiencies. This thesis focuses on enhancing the efficiency of Sb2Se3 photocathode for photoelectrochemical water-splitting purposes. It employs a simple and cost-effective synthesis technique with the objective of mitigating the photovoltage deficiency inherent to this material, a fundamental impediment constraining its overall efficiency.The first publication within this thesis endeavours to address the surface defects found in Sb2Se3. This study delves into innovative approaches for enhancing the performance of this photoabsorber material by subjecting it to treatments involving (NH4)2S etching and CuCl2 passivation. These treatments yielded a substantial increase in onset potential and photocurrent, surpassing the untreated Sb2Se3 films. The treatments induced morphological changes, eliminated surface Sb2O3 layers, and improved charge separation at the interface. The second publication extends upon the preceding one by conducting a comprehensive screening to identify other viable surface treatment candidates capable of augmenting efficiency. The study sought to elucidate the influence of metal treatments and ascertain whether the enhancement could be attributed to specific metal characteristics, including oxidation state, ionic radius, or electronegativity. Furthermore, given the prior performance improvements due to bulk passivation via sulphurisation, this investigation aimed to explore the combined impact of surface and bulk treatments. Interestingly, the most efficacious post-synthetic treatment proved to be the combination of silver nitrate and sulphurisation. These treatments substantially increased the photovoltage, leading to a remarkable onset potential increase of more than 200 mV. However, it was intriguingly revealed that the enhancement did not stem from the initially hypothesised causes. Instead, the treatments were found to eliminate amorphous Se and metallic Sb from the surface, forming a high-quality Sb2(S1–x, Sex)3 surface layer. These improvements instigate favourable morphological alterations, ultimately enhancing light absorption and scattering. The final publication centres on substituting the noble and costly platinum catalyst with a more abundant MoSx catalyst. While prior research explored the amorphous variant of this catalyst, it was hypothesised that MoSx clusters, such as [Mo3S4]4+ and [Mo3S13]2–, would offer enhanced selectivity and reactivity due to the catalyst's increased reaction sites. Overall, this thesis has successfully addressed several factors hindering the performance of Sb2Se3, leading to an enhancement in both photovoltage and photocurrent. This study serves as a robust foundation that can be further refined to increase the practical potential of Sb2Se3 for large-scale water-splitting applications.
Adams, Pardis