Role of temperature and NiO addition in improving photocatalytic properties of TiO2 nanotubes

Self-organized TiO2 nanotubes (TiNTs) have been synthesized by electrochemical anodization in an electrolyte containing ethylene glycol, ammonium fluoride, and de-ionized water at 40 V for 3 h. TiNTs were then annealed at different temperatures in air (that is 340 °C, 450 °C and 670 °C). All of the annealed TiNTs were then loaded with NiO using 0.2 M precursor concentration. Furthermore, TiNTs annealed at 450 °C were additionally loaded with NiO using 1.6 M precursor concentration. The XRD data revealed the formation of low-temperature anatase phase at 340 °C, high-temperature anatase at 450 °C and dominantly rutile phase of TiNTs at 670 °C. Addition of NiO to the TiNTs was also confirmed by XRD. SEM images revealed the morphological evolution of TiNTs at different annealing temperatures as well as loading of NiO. Raman spectroscopy results further confirmed the formation of different phases of TiNTs and the peak shifting as a result of NiO addition. The photoelectrochemical current (PEC) densities were recorded from zero to 1 V (vs. Ag/AgCl/3.5 M KCl) under 1 sun illumination in water without incorporating additional ions in it. TiNTs annealed at 450 °C showing anatase phase were found to be most promising for the PEC measurements in water. The current densities for the aforementioned TiNTs were enhanced by ~ 80 times with respect to dark measurement conditions. NiO addition using 0.2 M precursor concentration for 450 °C annealed anatase TiNTs produced a further fourfold increase in PEC densities under light with respect to its unmodified form. This is in contrast to ~ 1.6-fold and ~ 1.9-fold increase in PEC densities with respect to their unmodified forms for NiO-modified low-temperature anatase TiNTs and predominantly rutile-phase TiNTs, respectively. The NiO loading using 1.6 M precursor concentration in anatase TiNTs annealed at 450 °C exhibit PEC densities ~ sevenfold higher with respect to their unloaded form. This work highlights the roles of annealing temperature and NiO addition in TiNTs (particularly anatase phase) for achieving enhanced PEC densities for water splitting.