Improvement of Photocatalytic and Nanomechanical Properties of GaN Nanowires via Core-Shell Surface Modification

The practical implementation of photocatalytic water splitting for sustainable hydrogen production is largely limited by the efficiency of the oxygen evolution reaction at the photoanode. Gallium nitride is a promising material for photoanode fabrication, provided that suitable nanostructuring strategies are applied to enhance its efficiency and stability. Here, we report the functionalization of gallium nitride nanowires with a 5 nm-thick titanium dioxide layer of and a 10 nm biomimetic polydopamine coating, resulting in a 1.9-fold (190%) increase in the anodic photocurrent, as demonstrated by chronoamperometry. We provide a detailed explanation of this enhancement based on electrochemical impedance spectroscopy and a series of optical studies. The observed decrease in photoluminescence intensity (60-90%) and shortening of carrier lifetimes from 0.17 to 0.07 ns correlate with the improved photocatalytic performance. Moreover, the 50% reduction in the band-to-band to defect emission ratio and the analysis of exciton recombination pathways show the role of defect-related and surface recombination processes in determining the photocatalytic activity of the nanowires. The enhanced photocatalytic performance is further attributed to shell deposition, which induces surface-state passivation, band realignment, and the quantum-confined Stark effect. Concurrently, changes in strain distribution are confirmed by increased heterogeneity of the GaN E2high phonon mode frequency distribution, expressed by an increase in the standard deviation from 0.09 up to 0.17 cm-1 and linewidth broadening from 3.2 up to 3.4 cm-1, as observed by Raman spectroscopy. Additionally, we show that the nanomechanical properties of the nanowires are significantly improved after shell deposition, by the elimination of pop-in events, enhanced elastic recovery, and an increase in Young's modulus from 12 to 37 GPa. Finally, we point out opportunities for further optimization of similarly functionalized nanowire systems to achieve even higher efficiencies and support emerging applications.