The XRD patterns of the ATO and ATO-H nanotube films are shown in Figure  1c. Except for the peaks at 40.25°, 53.06°, and 70.71° that originated from the Ti metal, all other peaks are coincident with each other and can be indexed to anatase TiO2 (JCPDF no. 21–1272). The average crystallite size variation from 31.9 nm (ATO) to 31.3 nm (ATO-H), estimated from the major diffraction peak (2θ = 25.17°) using Scherrer’s equation [25], is less than 2%. After scraping the ATO nanotube powders off the Ti foil substrates with a razor blade, a distinct color evolution is revealed

from white (ATO powder) to blue-black (ATO-H-10) (inset of Figure  1c). The evolution of optical properties could be ascribed to the increased defect density [11] on tube surface as disclosed by the Raman spectroscopy CB-839 in vitro analysis. Figure 1 The morphology and structure characterization of ATO and ATO-H. (a) A side view of ATO nanotube film after second-step anodization. Inset of (a) shows an enlarged image indicating a smooth tube wall. (b) A TEM image of ATO

nanotubes. (c) XRD patterns of pristine ATO and ATO-H-10 films. Inset of (c) shows the photographs of ATO and ATO-H nanotube powders. Selleckchem GDC-973 (d) Raman spectra of the pristine ATO and ATO-H nanotubes with different processing time (5, 10, and 30 s). Figure  1d displays the Raman spectra of ATO nanotubes treated with different reductive processing times (denoted as ATO-H-5, ATO-H-10, and ATO-H-30 for 5-, 10-, and 30-s treatments, respectively). The six Raman vibrational mode of anatase TiO2 very samples [26] can be found at 148.4 cm-1 (E g(1)), 200.5 cm-1 (E g(2)), 399.1 cm-1 (B 1g(1)), 641.2 cm-1 (E g(3)), 520.6 cm-1 (A 1g), and 519 cm-1 (B 1g(2) superimposed with

520.6 cm-1), which is in agreement with the above XRD results. A slight blueshift and broadening of E g(1) and E g(2) peaks are observed in the ATO-H-10 sample, suggesting increased surface disorder due to the introduced oxygen vacancies [10]. According to the above analysis, the possibly introduced defect states originate from the formation of oxygen vacancies on ATO nanotubes. The photocurrent densities of ATO-H photoanodes at a constant potential of 0 V (vs Ag/AgCl) under the standard AM 1.5G solar light illumination are subsequently recorded as a function of reductive doping duration with respect to pristine ATO electrode (Figure  2a). Each duration is measured in at least three samples to average out the experimental fluctuation. The photocurrent densities increase gradually with the processing time, yielding a maximum value of 0.65 mA/cm2 for a 10-s treatment. Further prolonged processing time leads to a depressed performance, which could be ascribed to increased surface defect density and corresponding recombination rate. Thus, ATO-H electrodes with a 10-s doping duration (ATO-H-10) are employed in the following experiments unless otherwise specified.