Facile one-step process for synthesis of vertically aligned cobalt oxide doped TiO2 nanotube arrays for solar energy conversion

Vertically aligned cobalt oxide doped TiO2 nanotube arrays (Co.-oxide TNTAs) were synthesized via one pot-anodic oxidation of pure titanium substrate in the presence of ammonium fluoride and cobalt salt. After subsequent annealing in air the produced arrays were in the tubular structure and doped with Co.-oxide. The designed TNTAs and Co.-oxide TNTAs were tested as photoanode electrodes in a photoelectrochemical cell. Energy dispersive X-ray (EDX) spectroscopy confirms the incorporation of Co. in the doped TNTAs. The Tauc plots estimated from UV-Vis diffuse reflectance spectra displayed that the insertion of the optimum amount of Co.-oxide leads to a decrease in the band gap of TiO2 from 3.2 to 2.9 eV. The influences of various cobalt salt concentrations in the electrolyte solution (5, 10, 15, 20 and 25 mM) on the morphology were studied. The evaluation of the photocurrent and photoconversion efficiency was performed for all the fabricated electrodes. Morphological studies illustrated that the addition of cobalt salt with small concentration has no an obvious effect on the ordered tubular structure of TNTAs, whereas, at higher concentrations the tubular structure was partially collapsed. Co.-oxide enhanced the photoconversion efficiency of TNTA electrode by 30 % at optimum concentration under 110 mW/cm(2) solar simulator illumination.