In this project, a novel carbon-oxynitride coupled artificial photosynthesis system for solar water splitting beyond 600 nm was developed by setting and achieving a set of scientific objectives: (i) Synthesis of doped BaTaO2N (to address scientific objective 1): first, a novel approach was developed to reduce the synthesis time and defect density in BaTaO2N by localizing an NH3 delivery system directly to the starting materials. Using such localized NH3 delivery system, we succeeded to synthesize the BaTaO2N crystals, within 4 hours, with less defect density, resulting in an enhanced photoelectrochemical performance. Then, the band structure and band gap of BaTaO2N were engineered by a B-site doping via involving various aliovalent cations and changing the dopant amount up to 15%. Particularly, Al was found to be the most efficient in improving the photocatalytic H2 and O2 evolution reactions compared to other dopants. Also, the flux method was found to be efficient over hydrothermal and solid state reaction for synthesizing the BaTaO2N crystals with less defect density. (ii) Development of carbon-BaTaO2N composite (to address scientific objective 2). To develop carbon-oxynitride composites for solar water splitting, BaTaO2N and various carbon allotropes with different dimensions were combined. Particularly, the concentration (0-30 wt%) of carbon allotropes on water oxidation activity of BaTaO2N was studied, and 20 wt% carbon allotrope was found to be the most efficient amount for enhancing the water oxidation activity. In electrode system, carbon allotrope was mixed with BaTaO2N particles and deposited on conductive FTO substrate, and their photoelectrochemical performance was studied. It was found that graphene-BaTaO2N composite exhibited the highest activity in comparison to BaTaO2N combined with other carbon allotropes. (iii) Evaluation of solar water splitting activity (to address scientific objective 3). The photoelectrochemical performance was characterized in a typical three-electrode electrochemical cell setup using the fabricated photoelectrode, an Ag/AgCl electrode in saturated KCl, and a Pt wire connected to a potentiostat as working, reference, and counter electrodes, respectively. The effect of various O2 and H2 evolution cocatalysts was investigated. It was found that CoOx and Pt cocatalysts were the most efficient for O2 and H2 evolution reactions, respectively. The kinetics and mechanisms of water splitting over the developed artificial photosynthesis systems were also studied by conducting time-dependent experiments and by using time-resolved absorption spectroscopy, respectively. (iv) Design of photoelectrochemical device (to address scientific objective 4). A novel carbon-oxynitride coupled artificial photosynthesis system in the form of panel was fabricated and its stability, efficiency and scalability was tested. Various characterizations and testing revealed that the developed carbon-BaTaO2N artificial photosynthesis system has good stability, efficiency and scalability. In order to provide further application opportunities, the stability and efficiency of the carbon-BaTaO2N for solar water splitting need to be further improved. The researcher will continue his active collaboration with academic and industrial partners for further enhancing the stability and efficiency and testing newly developed materials in the future. The project findings have been widely disseminated in the form of invited and oral presentations at nine international conferences and in the form of scientific articles in seven international peer-reviewed journals.
