The nonpolar a-plane and m-plane of GaN are anisotropic due to the fact that in-plane stresses of nonpolar GaN along different directions are varied, which is considered to be the reason why they are chosen for the preparation of polarization-sensitive ultraviolet (UV) photo-detectors (PDs), etc., when compared to polar c-plane GaN-based UV PDs. However, the nonpolar GaN-based UV PDs suffer from poor-quality GaN epitaxial films, which limit their performance enhancement. Herein, we report the fabrication of high responsivity and low dark current nonpolar a-plane GaN-based metal-semiconductor–metal (MSM) UV PDs, by using high-quality a-plane GaN epitaxial films grown on r-plane sapphire substrates through control of the dislocation density. The ∼2.5 μm-thick GaN epitaxial films were grown by the combination of low-temperature pulsed laser deposition and high-temperature metal–organic chemical deposition technologies and were then fabricated into MSM UV PDs. The MSM UV PDs were revealed to have a high responsivity of 0.74 A W⁻¹ and a low dark current of 1.3 nA at an applied bias of 2 V, as well as a very small full-width at half-maximum for GaN(11-20) of 360 arcsec. The performance is better than the results for all of the nonpolar GaN-based MSM UV PDs ever reported. These UV PDs shed light on the potential for wide applications.

Bi₄Ti₃O_12−x nanosheet photocatalysts with abundant oxygen vacancies are fabricated by a facile solid-state chemical reduction method for the first time. This method is simple in operation, has short reaction time, and can be conducted at mild temperatures (300~400 °C). The electron paramagnetic resonance, thermogravimetric analysis, X-ray photoelectron spectrometer, and positron annihilation lifetime spectra results indicate that oxygen vacancies are produced in Bi₄Ti₃O_12−x, and they can be adjusted by tuning the reduction reaction conditions. Control experiments show that the reduction time and temperature have great influences on the photocatalytic activities of Bi₄Ti₃O_12−x. The optimal Bi₄Ti₃O_12−x is the sample undergoing the reduction treatment at 350 °C for 60 min and it affords a hydrogen evolution rate of 129 μmol·g⁻¹·h⁻¹ under visible-light irradiation, which is about 3.4 times that of the pristine Bi₄Ti₃O₁₂. The Bi₄Ti₃O_12−x photocatalysts have good reusability and storage stability and can be used to decompose formaldehyde and formic acid for hydrogen production. The surface oxygen vacancies states result in the broadening of the valence band and the narrowing of the band gap. Such energy level structure variation helps promote the separation of photo-generated electron-hole pairs thus leading to enhancement in the visible-light photocatalytic hydrogen evolution. Meanwhile, the narrowing of the band gap leads to a broader visible light absorption of Bi₄Ti₃O_12−x.