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

Tantalum-oxy-nitride nanotubes (TaO_xN_y 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 unconventional 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 µm, and the inner tube diameter from 20 to 100 nm, in good correlation with the synthesis parameters. The N-content (from 2.7 % to 19.1 %) was dependent on the nitridation temperature (from 500° to 900°C), and the conversion of bare Ta₂O₅ into TaO_xN_y 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 TaO_xN_y NTs film-type electrode obtained by nitriding at 800 °C showed the highest photocurrent value (0.1 mA∙cm^-2), and the highest rates of MB degradation (k₁ of 0.0189 min^-1) and H₂ photo-production.