Photoelectrochemical (PEC) water splitting is a promising path for sustainable energy production, but long-term stability in practical two-electrode configurations has been a significant challenge. While three-electrode configurations are commonly used for stability studies, they don't reflect the performance and stability of the entire PEC system, including the counter electrode. Previous high-efficiency photoelectrodes, often incorporating protective schemes and extrinsic catalysts, show poor stability in two-electrode systems due to factors like charge build-up, solution resistance, and inherent instability. This paper focuses on addressing this stability bottleneck by utilizing GaN nanowires on Si, two widely produced semiconductors. Previous work showed GaN's self-improving property in three-electrode configurations, attributed to oxynitride formation on nonpolar surfaces. However, whether this self-improvement could be maintained and enhanced in practical two-electrode configurations with GaN morphologies dominated by active nonpolar surfaces remained unknown. This study aims to investigate this, focusing on the mechanism of oxynitride formation, its catalytic properties, and long-term stability in a two-electrode system. The integration of GaN and Si is particularly attractive due to their proven manufacturability, scalability, and low cost.

Extensive research has been conducted on metal-oxide, Si, and III-V semiconductor photoelectrodes for PEC water splitting. Recently, metal nitrides, particularly III-nitrides like InGaN, have attracted attention due to their tunable bandgaps. GaN, in particular, shows promise due to its potential for seamless integration with Si. Previous studies demonstrated GaN's self-improving PEC performance in three-electrode configurations, linked to the formation of oxynitride on nonpolar and semipolar surfaces. However, these studies used quasi-film GaN morphologies, limiting the active surface area. The long-term stability of GaN in two-electrode configurations and the detailed mechanism behind its self-improvement remained largely unexplored before this work. Existing strategies for enhancing stability, such as protective layers, often lead to a trade-off between efficiency and stability because these layers are typically not catalytically active.

This study involved the synthesis and characterization of GaN nanowires/Si photocathodes. n-GaN nanowires were grown on p-type Si wafers using plasma-assisted molecular beam epitaxy (PAMBE). The nanowires have an average length of ~600 nm and diameter of ~100 nm, with nonpolar sidewalls dominating. The photocathodes were evaluated in both three- and two-electrode configurations. Three-electrode measurements used an IrOx counter electrode and a Ag/AgCl reference electrode under AM 1.5 G illumination. Chronoamperometry (CA), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) were employed to assess performance and stability. X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDS), and inductively coupled plasma mass spectroscopy (ICP-MS) were used for material characterization. To understand the self-improvement mechanism, density functional theory (DFT) calculations were performed to investigate the formation, electronic structure, and catalytic properties of Ga-O-N species on the GaN surface. A thin Al2O3 passivation layer was also applied to some samples to investigate the role of surface modification in the self-improvement process. Long-term stability tests in two-electrode configurations were conducted under AM 1.5 G one-sun illumination for over 3000 hours.

Three-electrode characterization showed a significant self-improvement in the GaN NW/Si photocathode, with a rapid increase in photocurrent density from 0.6 to -35 mA/cm² over 40-50 h. LSV curves indicated improvements in onset potential and photocurrent density. The self-improvement was attributed to the *in situ* formation of gallium oxynitride on the nonpolar m-plane of GaN nanowires. XPS confirmed the formation of Ga-O-N species. The self-improvement was accelerated using concentrated sunlight, achieving high hydrogen evolution activity within 15 minutes. Two-electrode tests showed remarkable stability exceeding 3000 h with no performance degradation, an order of magnitude improvement over previous reports. The photocathode maintained a photocurrent density >25 mA/cm² and a Faradaic efficiency of ~100%. DFT calculations revealed that the *in situ* formation of atomic-scale GaON nanoclusters on N-terminated GaN nanowires involves oxygen replacing nitrogen atoms. This process reduced surface band bending and created atomic-scale localized nanoclusters of semiconductor surface metallization, acting as reduction reaction sites. The GaN nanowires remained structurally stable throughout the long-term tests, with negligible Ga dissolution in the electrolyte. The high stability is attributed to the material properties of GaN nanowires, including strong ionic bonds, lack of dislocations, and unique N-termination.

The findings demonstrate that the *in situ* formation of Ga-O-N oxynitride on the surface of GaN nanowires significantly enhances the stability and efficiency of PEC water splitting. This approach overcomes the limitations of traditional methods that rely on extrinsic cocatalysts or protective layers, which often compromise either stability or efficiency. The superior stability observed over 3000 hours in a practical two-electrode configuration represents a major advancement in the field. The atomic-scale understanding of the self-improvement mechanism, provided by DFT calculations, offers valuable insights for the design of future, highly stable PEC devices. The in-situ formation of catalytically active GaON nanoclusters provides a compelling solution to the long-standing stability bottleneck of semiconductor photoelectrodes. The use of GaN nanowires with their unique surface properties is crucial to this success. This work opens avenues for the practical application of PEC systems for clean energy production.

This study demonstrates unprecedented long-term stability (3000 hours) for a GaN NW/Si photocathode in a two-electrode configuration for PEC water splitting. The *in situ* formation of Ga-O-N oxynitride species enhances both stability and efficiency, solving a major challenge in the field. The atomic-scale mechanism was elucidated using DFT calculations. Future work will focus on exploring the limits of durability under harsher conditions (high temperature, concentrated sunlight) and expanding the application to other materials and PEC systems.

While the study demonstrated exceptional long-term stability, further investigation is needed to determine the exact limits of the photocathode's durability. The experiments were performed under specific conditions (AM 1.5 G one-sun illumination, 0.5 M H2SO4 electrolyte). The generalizability of these findings to different electrolytes or operating conditions requires further investigation. The scale-up of this technology to practical, large-scale applications also warrants further research.