Indirect bandgap, optoelectronic properties, and photoelectrochemical characteristics of high-purity Ta3N5 photoelectrodes

Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting

Transition metal oxynitrides are a promising class of functional materials for photoelectrochemical (PEC) applications. Although these compounds are most commonly synthesized via ammonolysis of oxide precursors, such synthetic routes often lead to poorly controlled oxygen-to-nitrogen anion ratios, and the harsh nitridation conditions are incompatible with many substrates, including transparent conductive oxides. Here, we report direct reactive sputter deposition of a family of zirconium oxynitride thin films and the comprehensive characterization of their tunable structural, optical, and functional PEC properties. Systematic increases of the oxygen content in the reactive sputter gas mixture enable access to different crystalline structures within the zirconium oxynitride family. Increasing oxygen contents lead to a transition from metallic to semiconducting to insulating phases. In particular, crystalline Zr2ON2-like films have band gaps in the UV-visible range and are n-type semiconductors. These properties, together with a valence band maximum position located favorably relative to the water oxidation potential, make them viable photoanode candidates. Using chopped linear sweep voltammetry, we indeed confirm that our Zr2ON2 films are PEC-active for the oxygen evolution reaction in alkaline electrolytes. We further show that high-vacuum annealing boosts their PEC performance characteristics. Although the observed photocurrents are low compared to state-of-the-art photoanodes, these dense and planar thin films can offer a valuable platform for studying oxynitride photoelectrodes, as well as for future nanostructuring, band gap engineering, and defect engineering efforts.

Enhanced Spatial Charge Separation in a Niobium and Tantalum Nitride Core-Shell Photoanode: In Situ Interface Bonding for Efficient Solar Water Splitting

Tantalum nitride (Ta3 N5 ) has emerged as a promising photoanode material for photoelectrochemical (PEC) water splitting. However, the inefficient electron-hole separation remains a bottleneck that impedes its solar-to-hydrogen conversion efficiency. Herein, we demonstrate that a core-shell nanoarray photoanode of NbNx -nanorod@Ta3 N5 ultrathin layer enhances light harvesting and forms a spatial charge-transfer channel, which leads to the efficient generation and extraction of charge carriers. Consequently, an impressive photocurrent density of 7 mA cm-2 at 1.23 VRHE is obtained with an ultrathin Ta3 N5 shell thickness of less than 30 nm, accompanied by excellent stability and a low onset potential (0.46 VRHE ). Mechanistic studies reveal the enhanced performance is attributed to the high-conductivity NbNx core, high-crystalline Ta3 N5 mono-grain shell, and the intimate Ta-N-Nb interface bonds, which accelerate the charge-separation capability of the core-shell photoanode. This study demonstrates the key roles of nanostructure design in improving the efficiency of PEC devices.

Crystal structure engineering of metal halide perovskites for photocatalytic organic synthesis

Engineering crystal structure of Cs₃BiBr₆ and Cs₃Bi₂Br₉ is theoretically and experimentally demonstrated to modulate their photocatalytic performance. This work offers insights into the structure-photoactivity relationships of metal halide perovskites (MHPs) and provides a guideline for exploiting MHPs toward efficient photocatalytic organic synthesis.

Characterizing Density and Spatial Distribution of Trap States in Ta3N5 Thin Films for Rational Defect Passivation

Tantalum nitride (Ta₃N₅) has gained significant attention as a potential photoanode material, yet it has been challenged by material quality issues. Defect-induced trap states are detrimental to the performance of any semiconductor material. Beyond influencing the performance of Ta₃N₅ films, defects can also accelerate the degradation in water during desired electrochemical applications. Defect passivation has provided an enormous boost to the development of many semiconductor materials but is currently in its infancy for Ta₃N₅. This is in part due to a lack of experimental understanding regarding the spatial and energetic distribution of trap states throughout Ta₃N₅ thin films. Here, we employ drive-level capacitance profiling (DLCP) to experimentally resolve the spatial and energetic distribution of trap states throughout Ta₃N₅ thin films. The density of deeper energetic traps is found to reach ∼2.5 to 6 × 10²² cm^-3 at the interfaces of neat Ta₃N₅ thin films, over an order of magnitude greater than the bulk. In addition to the spatial profile of deep trap states, we report neat Ta₃N₅ thin films to be highly n-type in nature, owning a free carrier density of ∼9.74 × 10¹⁷ cm^-3. This information, coupled with the present understanding of native oxide layers on Ta₃N₅, has facilitated the rational design of a targeted passivation strategy that simultaneously provides a means for catalyst immobilization. Loading catalyst via silatrane moieties suppresses the density of defects at the surface of Ta₃N₅ thin films by two orders of magnitude, while also reducing the free carrier density of films by over one order of magnitude, effectively dedoping the films to ∼2.40 × 10¹⁶ cm^-3. The surface passivation of Ta₃N₅ films translates to suppressed defect-induced trapping and recombination of photoexcited carriers, as determined through absorption, photoluminescence, and transient photovoltage. This illustrates how developing a deeper understanding of the distribution and influence of defects in Ta₃N₅ thin films has the potential to guide future works and ultimately accelerate the integration and development of high-performance Ta₃N₅ thin film devices.