Min-Seok Lee

Vertical gallium nitride (GaN) nanowire core and few‐layer graphene oxide shell (GO/GaN CS NWs) represent a novel core‐shell structure with unique double‐bond phenomena, enabling dual energy band mechanisms that enhance direct seawater‐splitting efficiency. At the GO/GaN interface, atomic interactions form configurations such as C–O–N–Ga (N‐polar van der Waals interactions) and C–O–Ga–N (Ga‐polar covalent interactions), which facilitate efficient photon absorption and charge migration into the graphene oxide shell. These mechanisms promote effective electron–hole separation and rapid interfacial charge transfer, crucial for photocatalytic processes. Two energy band probability mechanisms, both type‐II (Ga‐polar) and Z‐scheme (N‐polar), are expected to play a key role in enhancing photocatalytic processes, aiding in the oxidation of seawater through the combined effects of thermal energy and photon absorption. Blockage of active sites and corrosion induced by chloride ions are common issues in direct seawater splitting, constituting a significant challenge in this process. We suggest that N‐polar GaN and the hydroxide ions in GO act as promising photoanode electrodes by repelling chloride ions during direct seawater splitting. The study highlights the significance of core–shell architectures, specific chemical bonds, double‐bond phenomena, dual energy bands, theoretical analysis, temperature‐dependent OER conditions, photocatalytic activity, and chloride‐ion repelling ability in advancing sustainable energy technologies.

While solar‐driven photoelectrochemical (PEC) water splitting offers a promising route for sustainable hydrogen production, sluggish interfacial charge transfer and long‐term instability remain critical bottlenecks. To address these limitations, a chemically integrated Ti‐squarate metal–organic framework (IEF‐11)/ n ‐GaN nanowire (NW) hybrid photoanode is designed via a (3‐aminopropyl) triethoxysilane (APTES) interlayer for efficient PEC water splitting. Density functional theory (DFT) calculations support energetically favorable interfacial bonding and the associated charge redistribution across the APTES‐mediated interface, highlighting the dual role of APTES in chemical anchoring and interfacial electronic modulation. This interfacial modification is accompanied by a reduced work function and a cathodic shift in the apparent flat‐band potential. Consequently, the hybrid photoanode demonstrates accelerated charge‐transfer kinetics, featuring reduced charge‐transfer resistance and a 10.4‐fold enhancement in applied‐bias photon‐to‐current efficiency (ABPE) compared to pristine NWs. Furthermore, this chemical anchoring supports the retention of principal structural and chemical features after 100 h of continuous operation, thereby promoting durable solar‐driven hydrogen production.

Programmable photonic computing requires nanoscale cavities that can be driven between distinct optical states using simple and physically controllable optical inputs. Here, we demonstrate logic-compatible all-optical modal switching in GaN nanorod cavities through artificial refractive-index engineering under ultraviolet excitation. At room temperature, the nanorods exhibit lasing at 369.4 nm. As the pump fluence increases above the threshold, the initial single lasing line evolves into reproducibly split modes around the original emission wavelength. Power-dependent micro-photoluminescence measurements, laser-heating simulations, and temperature-dependent finite-difference time-domain calculations collectively show that localized optical heating increases the GaN refractive index and reorganizes the cavity modes. The resulting thermo-optic perturbation detunes the initial triangle-like whispering-gallery mode and promotes linearly polarized-like modes at higher excitation, producing reversible modal switching during repeated pump cycling without progressive spectral drift. These results identify the physical origin of pump-induced peak splitting in GaN ultraviolet nanorod cavities and establish a compact III-nitride nanolaser platform whose cavity states can be optically programmed. Such controllable modal-state engineering offers a promising route toward ultraviolet nanophotonic switches, optical-state encoders, and prospective building blocks for all-optical logic and next-generation photonic computing.

Direct photoelectrochemical splitting of natural seawater offers a scalable route to green hydrogen but is constrained by an interfacial paradox: alkaline conditions that accelerate hydrogen evolution also induce Mg/Ca hydroxide scaling, rapidly blocking semiconductor surfaces. Here, we report a membrane-defined III–nitride photocathode that stabilizes the seawater interface and enables catalyst-free hydrogen evolution directly in Yellow Sea seawater. N-polar InGaN quantum pyramids integrated on vertical GaN nanowires provide broadband (UV–NIR) absorption and a composition-graded “quantum-sliding” band profile, funneling photogenerated electrons toward the pyramid apex and the catalytically active N-polar surface. A conformal graphene-oxide/polylactic-acid (GO/PLA) nano-membrane laminated on the nanowire forest serves as an ion-sieving transport layer, preserving water access while suppressing multivalent-ion-driven precipitation and electrode blockage. X-ray photoelectron spectroscopy quantifies heterointerfacial band offsets, while Raman spectroscopy and electron microscopy confirm conformal encapsulation and mitigated deposits after operation. The photocathode sustains ~0.5 mA cm−2 at −0.45 V versus RHE for over 200 h in untreated Yellow Sea seawater (pH 7) and reaches ~−16.51 mA cm−2 in Yellow Sea water containing 0.5 M NaOH, with onset potentials of 0.26–0.29 V versus RHE. This membrane–band co-engineering strategy decouples semiconductor photophysics from electrolyte complexity, enabling practical solar hydrogen production from unprocessed seawater.