Reducing CO₂ as a harmful greenhouse gas for global warming has become a urgent issue in the last few years. Besides, CO₂ as a carbon source can be used as alternative to fossil-derived carbon, thus enabling a balanced carbon cycle. Catalytic reduction of CO₂ is a promising route toward sustainably producing valuable products; however, the tendency to produce a mixture of chemicals and the large energy demand present challenges to practical implementation of CO₂ utilization. Photoelectrochemical (PEC) CO₂ reduction (CO2R) is an attractive pathway to produce useful chemicals from inputs of only sunlight, water, and CO₂. Semiconductor photocathode required for this approach should have a conduction band edge (CBE) more negative than the reduction reaction potential of CO₂ to produce photo-generated electrons with sufficient energy for inducing the reaction of interest. Finding a suitable photocathode with high CBE, stability under cathodic conditions, and p-type conductivity is the decisive challenge for the realization of PEC-CO2R devices. Among a few known semiconductors with sufficiently high CBE for CO₂R, some compound semiconductors such as phosphides, arsenides, and nitrides have shown successful PEC-CO2R. However, a methodical investigation of the roles of morphology and electronic structure has not yet been reported.

GaN photocathodes are promising candidates for PEC-CO2R due to their high CBE and band gap tunability by In incorporation to form (In,Ga)N. Furthermore, the surface recombination velocity in group-III-nitrides is orders of magnitude lower than that in other semiconductors. In CO2R, rection pathways and selectivity towards specific products are dependent on parameters such as catalyst potential and electrode morphology. There are some reports on utilizing GaN and GaInN photocatalysts for CO2RR, however, PEC CO2RR has not been thoroughly investigated using GaN nanowire arrays. Herein, we investigated the effect of morphology and catalyst functionalization on the activity and selectivity of nanowire GaN cathode. Nanowire arrays were prepared by top-down fabrication approach via mask self-assembly and selective-area sublimation. The activity and stability of the electrodes were investigated by photoelectrochemical characterization together with detailed analysis of gaseous and liquid products. The impacts of tunable nanowire array morphology and dynamic interfacial processes on the selective reduction of CO₂ was investigated. Comparisons between p-type photoelectrodes and n-type dark analogues enable us to distinguish the phenomena uniquely occurring on light-driven PEC-CO2R devices.