Tantalum oxynitride nanotube film arrays for unconventional nanostructured photo-electrodes active with visible light

Tantalum-oxy-nitride nanotubes (TaOxNy NTs) ordered in 1D were successfully synthesized through controlled anodic oxidation of metallic Ta foils followed by high-temperature annealing under NH3 flow to prepare un-conventional nanostructured photo-electrodes active with visible light. For the synthesis, a two-step anodization process was adopted: i) the first step in ethylene glycol with 0.33 % NH4F and 3 % H2O, ii) the second step in H2SO4 (98 %) with 1 % HF and 4 % H2O, by applying different voltages and anodization times. Scanning Electron Microscopy integrated with Energy Dispersive X-ray spectroscopy (SEM-EDX) revealed the NTs length ranging from 2 to 5 mu m, and the inner tube diameter from 20 to 100 nm, in good correlation with the synthesis pa-rameters. The N-content (from 2.7 % to 19.1 %) was dependent on the nitridation temperature (from 500 degrees to 900 degrees C), and the conversion of bare Ta2O5 into TaOxNy strongly influenced the absorption in the visible region with a bandgap shift from 4.0 to 2.1 eV, respectively. Photo-electrochemical and catalytic performances were evaluated with chronoamperometric measurements and tests in different photo-reactions using AM 1.5 G simulated sunlight: (i) degradation of methylene blue (MB), (ii) ethanol gas-phase photo-reforming and (iii) bias -assisted photoelectrochemical water splitting. The TaOxNy NTs film-type electrode obtained by nitriding at 800 degrees C showed the highest photocurrent value (0.1 mA center dot cm-2), and the highest rates of MB degradation (k1 of 0.0189 min-1) and H2 photo-production.

Automated summary

Tantalum-oxy-nitride nanotubes (TaOxNy NTs) were synthesized through controlled anodic oxidation of metallic Ta foils followed by high-temperature annealing under NH3 flow. The length and inner tube diameter of the NTs were in correlation with the synthesis parameters. The conversion of Ta2O5 into TaOxNy influenced the absorption in the visible region, resulting in a bandgap shift. The TaOxNy NTs film-type electrode showed excellent photo-electrochemical and catalytic performances in various photo-reactions.