Surface Modification of Metal Oxide Semiconductors with Improved Photoelectrochemical Water Splitting Performance

Abstract

The world's heavy dependence on conventional fossil fuels has caused serious problems including global warming, environmental concerns and energy shortage issue. Solar energy is recognized as one of the most promising and cost-competitive alternatives for the replacement of fossil fuels, for achieving a carbon-neutral and sustainable society. Photoelectrochemical (PEC) water splitting is regarded as an attractive approach to convert renewable light energy into chemical energy. So far, various metal oxide photoelectrodes such as TiO₂, α-Fe₂O₃, and BiVO₄ have been developed. In spite of the uplifting progress in the field of solar water splitting, it is still challenging to fabricate robust photo-responsive materials with high performance in a cost-effective way, owing to the low charge carrier mobility, unfavourable carrier recombination and sluggish interfacial charge transfer properties of metal oxides. A series of projects related to metal oxide semiconducting photoanodes (α-Fe₂O₃ and BiVO₄) has been comprehensively investigated to nurture the understanding of these materials for practical PEC application, which are presented in three main sections.

In the first project, the fabrication of Ti-doped α-Fe₂O₃ of improved PEC activity using an ion implantation technique is proved for the first time. The nanoporous titanium (Ti)-doped hematite (α-Fe₂O₃) thin films were fabricated by a facile hydrothermal reaction, subsequently utilizing energetic plasma ion implantation technique with a post-annealing process. On the basis of materials characterization and electrochemical analysis, the optimized Ti-doped Fe₂O₃, i.e., Ti-4-Fe₂O₃, exhibits improved photocurrents of 0.55 and 1.07 mA cm^-2 at 1.23 and 1.5 V versus RHE respectively under illumination of 100 mW cm^-2 with AM 1.5G spectrum, showing approximately 1.6-fold increases compared to pristine Fe₂O₃. The increases are attributed to the improved charge carrier transport induced by Ti doping that reduces the recombination of light-driven charge carriers.

In the second project, an effective chemical reduction treatment using sodium sulfite is employed to improve the PEC performance of BiVO₄ photoanodes. The optimal treatment condition was explored by adjusting the treating time and temperature, and the concentration of sulfite. The as-prepared surface-engineered BiVO₄ exhibits a photocurrent density of 2.2 mA cm^-2 at 1.23 V vs. reversible hydrogen electrode (RHE) under 1-sun illumination, which is 1.7 times higher than that of pristine BiVO₄. By coating amorphous FeOOH cocatalysts, the photocurrent density can be further improved to 2.8 mA cm^-2. The post-synthetic treatment on the BiVO₄ photoanodes can substantially enhance interfacial charge transfer efficiency because of decreased charge carrier recombination arising from both surface oxygen vacancies (O_vac) and surface disordered layers, as characterized by XPS, EPR and TEM techniques.

In the last project, the synthesis of surface-modified BiVO₄ photoanodes by sandwiching an O_vac layer between photoanode BiVO₄ and cocatalyst NiOOH is demonstrated. The surface O_vac layer was introduced on BiVO₄ by a chemical reduction treatment using another mild reducing agent sodium hypophosphite. The induced O_vac have the ability to alleviate the interfacial charge carrier recombination loss as proved by the calculation of charge separation and transfer efficiencies, while an outer NiOOH layer was coated to prevent the O_vac layer from diminishing. As a result, the as-prepared NiOOH-P-BiVO₄ photoanode exhibits a high photocurrent density of 3.2 mA cm^-2 at 1.23 V vs. RHE under the irradiation of 100 mW cm^-2 AM 1.5G sunlight, in comparison to 1.1, 2.1 and 2.3 mA cm^-2 of bare BiVO₄, P-BiVO₄ and NiOOH-BiVO₄ photoanodes, respectively. In addition to the superior photoactivity, the 5-h amperometric measurements also indicate an improved stability of the surface-engineered NiOOH-P-BiVO₄ photoanode.

The works presented in this thesis provide new insights into the surface engineering of photo-responsive metal oxide semiconductors, by cutting-edge plasma ion implantation techniques or facile chemical treatments, which enhances the development of emerging hydrogen technologies.

Date of Award	11 Aug 2021
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Sam Hsien-Yi HSU (Supervisor)