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The GaN-based multiple quantum wells (MQWs) are etched in NaNO 3 solution under room light and are transformed into free-standing nanoporous (NP) GaN-based MQWs embedded on NP GaN reflectors. Then, the free-standing sample is transferred onto the quartz substrate, which acts as a photoanode during photoelectrochemical water splitting. Compared with the as-grown MQW sample, the etched and freestanding samples present the enhanced separation efficiency of photogenerated electron−hole pairs and faster interfacial chargetransfer ability. Importantly, the free-standing sample exhibits the lowest onset voltage (−0.80 V vs Ag/AgCl) and the maximum photoconversion efficiency (3.10%) among the three samples, which can be put down to the decreased polarization effect, as well as the large specific surface area, the improved visible light utilization capacity associated with the light reflection effect of the bottom NP GaN reflectors, and an increased internal quantum efficiency due to the improved crystalline quality of MQWs.

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