Surface Engineering of GaN Photoelectrode by NH3 Treatment for Solar Water Oxidation

Abstract

<p>Photoelectrochemical (PEC) water splitting is a vital source of clean and sustainable hydrogen energy. Moreover, the largescale H<sub>2</sub> production is currently necessary, while long-term stability and high PEC activity still remain important issues. In this study, a GaN-based photoelectrode was modified by an additional NH<sub>3</sub> treatment (900°C for 10 min) and its PEC behavior was monitored. The bare GaN exhibited a highly crystalline wurtzite structure with the (002) plane and the optical bandgap was approximately 3.2 eV. In comparison, the NH<sub>3</sub>-treated GaN film exhibited slightly reduced crystallinity and a small improvement in light absorption, resulting from the lattice stress or cracks induced by the excessive N supply. The minor surface nanotexturing created more surface area, providing electroactive reacting sites. From the surface XPS analysis, the formation of an N-Ga-O phase on the surface region of the GaN film was confirmed, which suppressed the charge recombination process and the positive shift of E<sub>FB</sub>. Therefore, these effects boosted the PEC activity of the NH<sub>3</sub>-treated GaN film, with <italic>J</italic> values of approximately 0.35 and 0.78 mA·cm<sup>−2</sup> at 0.0 and 1.23 V<sub>RHE</sub>, respectively, and an onset potential (V<sub>on</sub>) of −0.24 V<sub>RHE</sub>. In addition, there was an approximate 50% improvement in the <italic>J</italic> value within the highly applied potential region with a positive shift of V<sub>on</sub>. This result could be explained by the increased nanotexturing on the surface structure, the newly formed defect/trap states correlated to the positive V<sub>on</sub> shift, and the formation of a GaO<sub>x</sub>N<sub>1-x</sub> phase, which partially blocked the charge recombination reaction.</p>