Efficient photoelectrodes from anatase TiO2 nanotube arrays decorated with particles/rods/3D microflower rutile crystals for photoelectrochemical water splitting

Efficient photoanodes are designed of vertically aligned anatase TiO2 nanotube arrays (anatase TNTAs) decorated with different shaped rutile TiO2 structures (particles, 1D nanorods, 3D microflowers) to improve the photoelectrochemical water oxidation performance of pristine TNTAs. Anatase TNTAs were prepared by anodic oxidation of Ti substrate in NH4F electrolyte, and the rutile percent and shape are controlled by tuning the TiCl4 treatment time (20-120 min). The effects of treatment time on the morphology, crystal structure, band gap, photocurrent, and photoconversion efficiency are characterized by FESEM, HRTEM, XRD, UV-Vis diffuse reflectance spectroscopy, and photoelectrochemical measurements. XRD data confirmed that TiCl4 treatment induced the formation of the rutile phase of TiO2 over anatase TNTAs and the rutile amount in the TNTAs was increased upon increasing the treatment time. Additionally, the band gap of TNTAs was gradually decreased by increasing the rutile percent. The optimum treatment time is 80 min, which produces TiO2 photoanode array that possess the following characteristics: rutile/anatase mixed phase, nanorode/nanotube mixed morphology, 3.09 eV band gap, two times increase in photocurrent, and almost more than twofold enhancement in the photoconversion efficiency relative to pure TNTA.