Band Engineered Epitaxial 3D GaN-InGaN Core–Shell Rod Arrays as an Advanced Photoanode for Visible-Light-Driven Water Splitting

3D single-crystalline, well-aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal–organic vapor phase epitaxy (MOVPE) for visible-light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ∼ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows us to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface, and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current–voltage, and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-nitride heterojunctions

Insights into Interfacial Changes and Photoelectrochemical Stability of In_<i>x</i>Ga_1–<i>x</i>N (0001) Photoanode Surfaces in Liquid Environments

The long-term stability of InGaN photoanodes in liquid environments is an essential requirement for their use in photoelectrochemistry. In this paper, we investigate the relationships between the compositional changes at the surface of n-type In_<i>x</i>Ga_1–<i>x</i>N (<i>x</i> ∼ 0.10) and its photoelectrochemical stability in phosphate buffer solutions with pH 7.4 and 11.3. Surface analyses reveal that InGaN undergoes oxidation under photoelectrochemical operation conditions (i.e., under solar light illumination and constant bias of 0.5 V_RHE), forming a thin amorphous oxide layer having a pH-dependent chemical composition. We found that the formed oxide is mainly composed of Ga–O bonds at pH 7.4, whereas at pH 11.3 the In–O bonds are dominant. The photoelectrical properties of InGaN photoanodes are intimately related to the chemical composition of their surface oxides. For instance, after the formation of the oxide layer (mainly Ga–O bonds) at pH 7.4, no photocurrent flow was observed, whereas the oxide layer (mainly In–O bonds) at pH 11.3 contributes to enhance the photocurrent, possibly because of its reported high photocatalytic activity. Once a critical oxide thickness was reached, especially at pH 7.4, no significant changes in the photoelectrical properties were observed for the rest of the test duration. This study provides new insights into the oxidation processes occurring at the InGaN/liquid interface, which can be exploited to improve InGaN stability and enhance photoanode performance for biosensing and water-splitting applications