Al2O3-passivated TiO2 nanorods for solid–liquid heterojunction ultraviolet photodetectors

The aim of this study was to improve the performance of a TiO2-nanorod solid–liquid heterojunction (SLHJ) ultraviolet photodetector covered with Al2O3-thin-film passivation layer through liquid phase deposition. First, we prepared a TTIP-HCl solution to grow TiO2 nanorods on a fluorine-doped tin oxide glass substrate through hydrothermal synthesis. During growth, HCl etching caused the TiO2 nanorods to transform from a two-dimensional film to a unidimensional nanorod structure. Because the etching effect strengthened with the HCl concentration, excessively high HCl concentrations resulted in over-etching and thin nanorods, which could easily collapse. By contrast, excessively low concentrations did not result in the formation of nanorod structures. In annealed TiO2 nanorods, the internal impedance of the SLHJ device is reduced, but oxygen vacancies are not reduced. Therefore, we coated the nanorods with a passivated Al2O3 layer though LPD; thus, the abundant oxygen atoms in the solution filled the original oxygen vacancies in TiO2. X-ray photoelectron spectroscopy analysis revealed that the proportions of defective Ti3+ and Ti2+ were substantially decreased, whereas the proportion of stable Ti4+ was increased considerably. Moreover, the responsivity of the photodetector was enhanced by 31% (from 303 to 397 mA/W), yielding a 5-order difference between the photo- and dark currents (from 10–3 to 10–8 A). After passivation, the rise and decay times of the photodetector were 0.5 s and 16.5 s, respectively, and the turn-off time was reduced by 10 s. The analysis results demonstrated that a passivated Al2O3 layer prepared through LPD can reduce oxygen vacancies in oxide semiconductors and improve device performance.