High efficiency artificial photosynthesis, that can convert solar energy directly into chemical fuels, has been extensively investigated. To date, however, success in finding abundant visible-light active photocatalyst has been very limited. Recently, metal-nitrides (e.g. InGaN) have attracted considerable attention for applications in artificial photosynthesis, due to their excellent stability and tunable energy bandgap across nearly the entire solar spectrum. Moreover, InGaN is the only known material whose energy bandgap can straddle the redox potential of water under deep visible and near-infrared light irradiation. In this context, we have investigated the design, fabrication, and performance characterization of multi-band InGaN/GaN nanowire photocatalysts and photoelectrodes for solar water splitting and carbon dioxide reduction. In this work, InGaN nanowires are grown on Si substrate by molecular beam epitaxy. The water splitting reaction takes place on the nonpolar surface (m-planes) of InGaN nanowire photocatalysts. By optimizing the surface charge properties through controlled Mg dopant incorporation, the efficiency for solar-to-hydrogen conversion is enhanced by nearly two orders of magnitude. The absorbed photon conversion efficiency reaches ~90% with optimum Mg doping concentration. The significantly enhanced efficiency is directly related to the optimized surface electronic properties that lead to both efficient water oxidation and proton reduction. A solar-to-hydrogen conversion efficiency of 3.3% and stability >500 hours was demonstrated in photocatalytic overall water splitting. High stability of these nanowires is attributed to the N-rich surfaces of GaN nanowire structures, which protects against photocorrosion and oxidation. We have further demonstrated multi-band InGaN/GaN nanowire photoelectrodes monolithically integrated on a Si solar cell wafer. The tandem PEC device consists of a planar n⁺-p Si solar cell wafer and p-InGaN nanowire segments. The p-InGaN nanowire arrays are designed to absorb the ultraviolet and visible solar spectrum. The remaining photons with wavelengths up to 1.1 µm are absorbed by the underlying planar Si p-n junction. Such a monolithically integrated photocathode promises solar-to-hydrogen conversion efficiency >20%. With the use of such a photoelectrode, we have also demonstrated that syngas, a key feedstock to produce methanol and liquid fuels in industry, can be produced from a CO₂ and H₂O with a benchmark turnover number of 1330 and a desirable CO/H₂ ratio of 1:2. Work is currently in progress to achieve high efficiency syngas and methanol generation in an aqueous photoelectrochemical cell.