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Group-III-nitrides have been intensively investigated for optoelectronics and power electronics and are uniquely suitable for energy-related applications, such as solar hydrogen generation and nanogenerators. Compared to planar group-III-nitrides, their nanostructures offer a high surface-to-volume ratio, increased light absorption cross-section, and improved carrier transportation behavior. This thesis focuses on molecular-beam-epitaxy-grown group-III-nitrides, specifically nanowires and membranes, and applications in renewable energy harvesting and conversion. A Mo2C-decorated (In,Ga)N nanowire-based photocathode was demonstrated for nitrogen fixation. The conventional Haber-Bosch method demands high reaction pressure and temperature while releasing a considerable amount of greenhouse gas. The proposed photoelectrocatalytic method can utilize solar energy to generate ammonia without carbon emissions. The proposed photocathodes can achieve maximum faradaic efficiency of 12 %, ammonia yield of 8.9 µg/h/cm2, and excellent stability for over 12 hrs. Moreover, group-III-nitrides were fabricated into a freestanding membrane through a novel method combining electrochemical porosification and controlled spalling. The novel method is reproducible and scalable, which can significantly reduce the consumption of sacrificial substrates compared to existing nitride membrane exfoliation techniques, thus promising a scalable platform. The as-fabricated GaN membranes were demonstrated for photoelectrocatalytic methylene blue degradation. Through laboratory tests and rooftop field tests, we proved the feasibility of our wafer-scale GaN membranes in achieving a dye degradation efficiency of 92%, a total organic carbon removal rate of 50.2%, and extraordinary stability for ~ 50 hours under solar illumination. The membrane can also degrade ~87% of MB under visible-light illumination. Furthermore, the (Al,Ga)N membranes were fabricated into flexible transparent piezoelectric devices. The devices can sense compression pressure and bending strain while giving a comparable compression sensitivity to other thin film piezotronics devices of ~ 2.41 mV/kPa and 42.36 pA/kPa, a maximum bending gauge factor of ~ 1271, and an output power density of ~ 5.38 nW/cm2. The sensors can withstand over 35000 cycles of operation and can be utilized for sensing and harvesting mechanical energies from human motions and environmental signals. This research utilized nanowires and membrane-based group-III-nitrides for different photoelectrocatalytic reactions and piezotronics devices, from material preparation and characterizations, and demonstrated practical devices for clean energy-related applications.

III-nitride nanowires (NWs) have been recognized as efficient photoelectrochemical (PEC) devices due to their large surface-to-volume ratio, tunable bandgap, and chemical stability. Doping engineering can help to enhance the PEC performance further. Therefore, addressing the effects of Si and Mg doping on the III-nitride NW photoelectrodes is of great interest. In this study, doping levels of NWs were tuned by the dopant effusion cell temperature of the molecular beam epitaxy (MBE) growth. The successful doping of the III-nitride NWs was confirmed using photoluminescence (PL), Raman spectroscopy, and open circuit potential (OCP) measurements. The ionized dopant concentrations of Si-doped InGaN/GaN NWs were systematically quantified by electrochemical impedance studies (EIS). Due to the three dimensional surfaces of NWs, modified Mott-Schottky formulas were induced to improve the accuracy of ionized dopant concentrations. The highest dopant concentration of Si-doped InGaN NWs can reach 2.1x1018 cm-3 at Tsi = 1120 oC. Accordingly, the estimated band edge potentials of the tested NWs straddled the redox potential of water splitting. The PEC performance of these devices was investigated by linear scan voltammetry (LSV), chronoamperometry tests, and gas evolution measurements. The results were consistent with the quantified dopant concentrations. The current density of n-InGaN NWs doped at TSi = 1120 oC was nine times higher than the undoped NWs. Additionally, the doped NWs exhibited stoichiometric hydrogen and oxygen evolution. By doping Mg into InGaN and GaN segments separately, the p-InGaN/p-GaN NWs demonstrated improved PEC performance, compared with undoped-InGaN/p-GaN and n-InGaN/n-GaN NWs. The p-InGaN/p-GaN NWs exhibited a highly stable current density at ~-9.4 mA/cm2 for over ten hours with steady gas evolution rates (~107 μmol/cm2/hr for H2) at near a stoichiometric ratio (H2: O2~ 1.8:1). This study demonstrated that optimizing the doping level and appropriate band engineering of III-nitride NWs is crucial for enhancing their PEC water splitting performance.