Periodic Reporting for period 2 - SECANS (Solar-to-Chemical Energy Conversion with Advanced Nitride Semiconductors)
Okres sprawozdawczy: 2022-01-01 do 2023-06-30
As photovoltaic technologies gain prominence, an outstanding challenge is the development of systems that can robustly store solar energy with high density. Within this context, the capture of sunlight and its direct conversion to chemical fuels in artificial photosystems provides a promising route for sustainably meeting future energy demands. However, a central challenge is the lack of systems that can efficiently direct light excitations towards desired chemical products with high efficiency and stability under harsh reaction conditions. In recent years, advances in using thin films to protect chemically sensitive light absorbers have relaxed the requirement for intrinsically stable semiconductors and motivate an entirely new perspective for rational design of materials and interfaces. Within this context, SECANS aims to create the scientific basis for solar-to-chemical devices that achieve unprecedented combinations of efficiency and stability, enabled by targeted exploration and rational optimization of an underexplored class of materials – transition metal nitride semiconductors. The electronic structures of these materials can enable a favourable balance between large charge carrier mobility and high defect tolerance. However, due to the high stability of molecular nitrogen, the expansive range of possible nitrides is largely unexplored and their basic properties poorly understood. SECANS overcomes these challenges through an interdisciplinary approach that couples non-equilibrium semiconductor deposition and newly developed interface engineering methods, supported by advanced operando spectroscopies, to enable high efficiency light harvesting systems with self-healing interfaces. This project will develop novel nitride semiconductors optimized for solar-to-chemical energy conversion, elucidate the roles of defects and disorder on competitive kinetics within photochemical reaction cycles, and provide new insights into the science of photochemical stability.
SECANS research to date has focused on the synthesis and characterization of binary and ternary transition metal nitrides based primarily on Ta, Zr, and Zn. A common focus of this work was on the role of oxygen in stabilizing the compounds and simultaneously affecting their optoelectronic properties. This work resulted in the first two publications from SECANS, both devoted to nitrogen-rich phases of tantalum nitride. In the first of these publications, we used reactive sputter deposition and took advantage of the strong electronegativity of oxygen to inductively stabilize high Ta oxidation states, thereby allowing synthetic access to the metastable and rarely reported Ta2N3 phase. While the inclusion of oxygen was critical to Ta2N3 formation, the resulting oxygen incorporation in structural vacancies drastically modified the free electron concentration in the as-grown material, thus leading to a semiconducting character with a 1.9 eV bandgap. Reducing the oxygen impurity concentration via post-synthetic ammonia annealing increased the conductivity by seven orders of magnitude and yielded the metallic characteristics of a degenerate semiconductor. Such dual role of oxygen, in both facilitating the formation of metastable nitrides and tuning their functional characteristics, provides broad tunability of material characteristics and opens avenues of developing the previously underexplored nitride chemical space.
In a second publication, we turned to the more commonly explored nitrogen-rich phase of Ta3N5 and investigated the impact of substitutional oxygen impurities on its optoelectronic and photoelectrochemical characteristics. Using our reactive sputtering approach combined with ammonia annealing, we were able to generate some of the lowest oxygen content films reported to date, which enabled fundamental studies of the semiconducting properties of this compound. Our results enabled a resolution to a long-standing uncertainty regarding the nature of the fundamental bandgap, indicating conclusively that it is indirect. The commonly-observed photoluminescence from the material is disorder-activated and can be suppressed in films possessing high compositional purity and structural quality. The photoelectrochemical performance of resulting Ta3N5 photoelectrodes could be understood in terms of the characteristics of native and impurity defects.
Considering the propensity of nitride surfaces to oxidize under photoelectrochemical conditions, we have developed atomic layer deposition processes for formation of phase-tunable water oxidation catalysts, which resulted in a third publication. Ongoing work is devoted to applying these materials to nitride photoanode surfaces and to studying photocarrier and reaction dynamics on their surfaces.
Though still in progress, studies of Zr- and Zn-based nitrides have provided additional new insights into the role of oxygen on stabilizing different phases of material, affecting the electronic transport characteristics, and defining the interfacial reactivity for energy conversion. Overall, the compact morphology, low defect content, and high optoelectronic quality of these films provide a basis for further optimization of photoanodes and may open up further application opportunities beyond photoelectrochemical energy conversion.
In a second publication, we turned to the more commonly explored nitrogen-rich phase of Ta3N5 and investigated the impact of substitutional oxygen impurities on its optoelectronic and photoelectrochemical characteristics. Using our reactive sputtering approach combined with ammonia annealing, we were able to generate some of the lowest oxygen content films reported to date, which enabled fundamental studies of the semiconducting properties of this compound. Our results enabled a resolution to a long-standing uncertainty regarding the nature of the fundamental bandgap, indicating conclusively that it is indirect. The commonly-observed photoluminescence from the material is disorder-activated and can be suppressed in films possessing high compositional purity and structural quality. The photoelectrochemical performance of resulting Ta3N5 photoelectrodes could be understood in terms of the characteristics of native and impurity defects.
Considering the propensity of nitride surfaces to oxidize under photoelectrochemical conditions, we have developed atomic layer deposition processes for formation of phase-tunable water oxidation catalysts, which resulted in a third publication. Ongoing work is devoted to applying these materials to nitride photoanode surfaces and to studying photocarrier and reaction dynamics on their surfaces.
Though still in progress, studies of Zr- and Zn-based nitrides have provided additional new insights into the role of oxygen on stabilizing different phases of material, affecting the electronic transport characteristics, and defining the interfacial reactivity for energy conversion. Overall, the compact morphology, low defect content, and high optoelectronic quality of these films provide a basis for further optimization of photoanodes and may open up further application opportunities beyond photoelectrochemical energy conversion.
SECANS will continue on the path of discovery and development of functional nitride semiconductors with properties tailored for conversion of solar energy to chemical fuel. Expanding beyond the initial binary nitrides space, the next phase of research will focus on studies of advanced ternary semiconductors and their their interactions in operating photoelectrochemical environments. The results from SECANS will provide advanced nitride semiconductors with tunable optoelectronic properties specially tailored for solar energy conversion.