Facile preparation of defective black TiO2 through the solution plasma process: Effect of parametric changes for plasma discharge on its structural and optical properties

Defective TiO2 or black TiO2 nanoparticles (NPs) are remarkably attractive for photocatalytic applications, since the defect-disordered structure crucially affects the optical properties. In this work, high visible light-active black TiO2 NPs were synthesized by an innovative solution plasma process (SPP) with a titanium (Ti) electrode in HNO3 solution. The effects of the plasma operating parameters, in terms of the pulse frequency (20-80 kHz) and pulse width (0.5-2.0 mu s) under two different HNO3 concentrations (0.3 mM and 3.0 mM) on the morphology, structural and optical properties of the obtained black TiO2 NPs were explored. Increasing the pulse frequency, pulse width and concentration of HNO3 all increased the input energy per pulse and energy per second, which induced an increased volume-based particle size, synthesis rate and Ti3+/Ti4+ ratio, but decreased the rutile mass fraction, the Ti-OH/TiO ratio and the band gap energy of the obtained black TiO2 NPs. Among all the prepared black TiO2 NP samples, those prepared at a pulse frequency of 20 kHz, pulse width of 2 mu s (energy input per second of 17.3 J) in 3.0 mM HNO3 exhibited the highest visible-light absorption capability (the lowest band gap energy of -2.44 eV). The variation in the band gap energy was more strongly dependent on the Ti3+/Ti4+ ratio than the Ti-OH/ TiO ratio in the structure of black TiO2 NPs. (C) 2017 Elsevier B.V. All rights reserved.