Photoelectric conversion efficiency of TiO2 nanotube with different amounts of fluoride ion residues

Titania nanotube arrays (TNTs) were fabricated via two-step anodization of titanium in an ethanediol electrolyte containing 0.5 wt% hydrofluoric acid (HF) and then characterized by SEM, XPS and UV-vis spectrometry. In order to investigate the photoelectric conversion efficiencies of TNTs, the nanotube with different amounts of fluoride ion residues on the surface were served as a model photoelectrode. The degradation efficiency of methylene blue attains 65% when the percentage of fluoride ion residues in the TNTs falls from 4.38% to 2.46% after 150 min. In addition, the open circuit voltage of the sample that was immersed in deionized water seven days is approximately 0.6 eV (vs. SCE), which is almost 1.5 times as high as the value of untreated sample. These results reveal that a decrease in fluoride ion residues that is on nanotube surface could increase the photoelectric conversion efficiency mainly because the fluoride ions can enhance the recombination efficiency of photoinduced electron-hole pairs.