Nanostructured Group-III Nitrides for Photoelectrocatalytic Applications and Renewable Energy Harvesting
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AbstractGroup-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.