At the beginning of DynNano, we synthesized ternary oxynitride photoelectrodes with an initial focus on Ta-based oxynitride semiconductors. One research direction has centered on controlling oxygen incorporation and understanding the impact of oxygen on optoelectronic properties and photoelectrochemical performance. We established a highly controlled synthesis approach using sputtered precursor thin films with different oxygen content, followed by high-temperature annealing. The synthesis yields films with tailored concentrations of both shallow and deep-level defects. Unlike conventional studies that focus on bulk properties, we analyzed and optimized the surface properties, which are equally important, or even dominant, in defining photoelectrochemical characteristics. Depth-sensitive structural, compositional, and (opto)electronic analyses reveal that surfaces consistently exhibit oxygen enrichment, increased structural disorder, and elevated deep-level defect densities relative to the bulk. Removing the surface layer results in improved surface crystallinity, hydrophilicity, and reduced oxygen content, leading to enhanced photoelectrochemical performance and stability. Overall, we demonstrate that independent optimization of surface and bulk properties is essential for advancing the efficiency and stability of semiconductor-based photoelectrodes.
Considering the propensity of photoelectrodes to degrade under photoelectrochemical conditions, we have developed atomic layer deposition processes for forming multifunctional TiO2 coatings with catalytic Pt layers and nanoparticles. As a major result, we established the synthesis of coatings that simultaneously enhance durability, facilitate efficient charge transfer, and improve catalytic activity. Interestingly, Pt nanoparticle catalysts simultaneously improve the saturation current density and onset potential despite decreased Pt loadings compared to continuous Pt layers. This behavior is attributed to their morphologies, which increase the active area, reduce catalytic overpotentials, and decrease parasitic absorption losses.
In parallel with material synthesis, DynNano initiated the development of the proposed nanoscale multimodal research program. Over the past few years, we have laid the foundation by implementing the necessary modes in the atomic force microscope and establishing procedures for correlative measurements using various atomic force microscopy techniques.
