Effects of impurities containing phosphorus on the surface properties and catalytic activity of TiO2 nanotube arrays

TiO2 nanotube arrays were prepared by titanium anodic oxidation with either HF or H3PO4/NH4F aqueous electrolyte solutions. The samples were characterized by means of X-ray diffraction (XRD), infrared spectroscopy (IR), Raman spectroscope, photoluminescence spectra (PL) and photocurrent response. Aqueous solutions of methylene blue or Cr(VI) ions were used as the target pollutants to compare catalytic activities of the two nanotube array types. The amorphous impurities containing phosphorus were confirmed by XRD and IR, for the sample synthesized with H3PO4/NH4F electrolytes. They closed a portion of the active sites, acted as recombination centers of photo-generated charges, and were also involved in the negative reactions of competing photo-generated holes or OH radicals. The TiO2 nanotube arrays formed in the H3PO4/NH4F electrolytes exhibited a stronger fluorescence spectrum, a weaker photocurrent and a lower catalytic activity than the sample fabricated with HF electrolyte without phosphorus impurities.