Fabrication of large diameter TiO2 nanotubes for improved photoelectrochemical performance

Anodic TiO2 nanotubes (ATNTs) have been investigated for many years. Most of the studies were performed under low voltage and low water content. In this work, large diameter ATNTs have been successfully fabricated in ethylene glycol electrolyte containing 12 vol% H2O and n wt% NH4F (n = 0.2, 0.3, 0.4, 0.5, 0.7) at a high applied voltage of 150 V. The results show that fluoride concentration has little effect on outer diameter (∼450–520 nm) and pore size (∼100 nm) of ATNTs. Besides, a simple method was used to enlarge the pore size of the ATNTs. The pore size increases gradually with the corrosion processing time. The increase of pore size of ATNTs is favorable to their photoelectrochemical (PEC) performance. The ATNTs with large pore size (∼310 nm) show the best PEC performance on water splitting, which can be ascribed to the ordered structure and the proper pore size. However, the surface defects on ATNTs caused by excessive corrosion depress the PEC performance.