Artificial photosynthesis can potentially address the global energy challenges and environmental issues caused by fossil fuels. Photoelectrochemical heterojunction structures of new photonic structures have been developed for efficient sunlight absorption, charge generation and separation and transport, and selective reduction of CO₂ and water splitting. In this review, an overview of several recently developed heterojunction model systems comprised of low-cost photonic materials such as transition metal dichalcogenides (TMDs), perovskite semiconductor nanocrystals, and plasmonic nanostructures is presented to rationalize the potential benefits of utilizing heterojunction structures for efficient and selective CO₂ reduction with renewable energy resources. Recent advances in electroanalytical methods for CO₂ reduction such as scanning electrochemical microscopy (SECM) are reviewed. These techniques can potentially resolve local CO₂ reduction kinetics and their spatial heterogeneities of a heterojunction photoelectrochemical structure.

Close-spaced vapor transport is a plausibly low-cost, high-rate method to grow III–V materials for photovoltaic and photoelectrochemical device applications. We report the first homoepitaxial growth of GaAs microstructures on (100)- and (111)B-oriented GaAs substrates using patterned SiO_x and Al₂O₃ masks and show that the resulting microstructured GaAs is an efficient semiconductor absorber for photovoltaic and photoelectrochemical applications. Cross-sectional transmission electron microscopy reveals an unusually low density of twin-plane defects in the (111)-oriented microstructures and the occurrence of stacked twin-plane defects in the (100)-oriented microstructures. Nonaqueous photoelectrochemical measurements show similar short-circuit currents of 9.7 and 9.1 mA cm^–2 for (100)- and (111)-oriented microstructures, respectively, with promising external quantum efficiencies. Together, the low twin density and good electronic properties indicate that micro- or nanostructures grown by selective area epitaxy in close-spaced vapor transport are promising for device applications that take advantage of their three-dimensional structure.