Crystallographically preferred oriented porous Ta 3 N 5 nanotubes (NTs) were synthesized by thermal nitridation of vertically oriented, thick-walled Ta 2 O 5 NTs, strongly adhered to the substrate. The adherence on the substrate and the wall thickness of the Ta 2 O 5 NTs were finetuned by anodization, thereby helping to preserve their tubular morphology for nitridation at higher temperatures. Samples were studied by scanning electron microscopy, high-resolution electron microscopy, X-ray diffraction, Rietveld refinements, ultraviolet−visible spectrophotometry, X-ray photoelectron spectroscopy, photoluminescence spectra, and electrochemical techniques. Oxygen content in the structure of porous Ta 3 N 5 NTs strongly influenced their photoelectrochemical activity. Structural analyses revealed that the nitridation temperature has crystallographically controlled the preferential orientation along the (110) direction, reduced the oxygen content in the crystalline structure and the tubular matrix, and increased the grain size. The preferred oriented porous Ta 3 N 5 NTs optimized by the nitridation temperature presented an enhanced photocurrent of 7.4 mA cm −2 at 1.23 V vs RHE under AM 1.5 (1 Sun) illumination. Hydrogen production was evaluated by gas chromatography, resulting in 32.8 μmol of H 2 in 1 h from the pristine porous Ta 3 N 5 NTs. Electrochemical impedance spectroscopy has shown an effect of nitridation temperature on the interfacial charge transport resistance at the semiconductor−liquid interface; however, the flat band of Ta 3 N 5 NTs remained unchanged.

Monoclinic Ta3N5 thin films were synthesized by thermal nitridation of amorphous Ta2O5 films directly sputtered by radio frequency magnetron sputtering. The samples were studied by high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-Vis-NIR spectrophotometry, rietveld refinements, spectroscopic ellipsometry and electrochemical techniques. The surface composition of Ta3N5 thin film was found to be different than the underlying film, affecting the optical properties of the material. Rietveld refinement has confirmed that the nitridation process results in Schottky and oxygen substitutional defects within the crystalline structure of monoclinic Ta3N5 thin film. The optical constants of the film were obtained by spectroscopic ellipsometry within a spectral range of 4.60-0.54 eV, i.e. 270-2300 nm. The suitable parameterization was found to consist of three Tauc-Lorentz and one Lorentz oscillators. The conduction band, valence band and the flat band positions were determined by photoelectrochemical techniques, presenting a strong dependence on pH of the eletrolyte. Improved photocurrent was obtained in alkaline conditions and attributed to the shorter depletion region width measured by Mott-Schottky and the lower recombination life time measured by open circuit potential decay analyses.

The use of metal/oxide nanoparticles (NPs) as cocatalysts in heterogeneous photocatalysis is an important strategy to improve the photocatalytic activity of semiconductors for hydrogen generation. This article reports the use of a modified sputtering deposition method to prepare ultrafine NiO NPs cocatalysts dispersed on anodic Ta2O5 nanotubes (NTs). In situ X-ray absorption near-edge spectroscopy (XANES) measurements revealed that after exposing the as-prepared Ni NPs to air atmosphere a mixture of 68% of Ni and 32% of NiO was formed. Pure phase NiO NPs was successfully obtained after a controlled thermal oxidation at 500 °C which was confirmed by in situ XANES and ex situ XPS analyses. The photocatalytic hydrogen production activity was evaluated using ethanol as a sacrificial agent. Ta2O5 NTs with 0.16 wt % of NiO showed superior photocatalytic activity (up to 7.7 ± 0.3 mmol h–1 g–1) as compared to pure Ta2O5 NTs (4.9 ± 0.3 mmol h–1 g–1) The observed higher photocatalytic activity suggests that NiO/Ta2O5 NTs is a promising material for photocatalytic hydrogen evolution.

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Highly ordered TiO2 NT arrays were easily decorated with CdSe via RF magnetron sputtering. After deposition thermal annealing at different temperatures was performed to obtain an improved TiO2/CdSe interface. The heterostructures were characterized by RBS, SEM, XRD, HRTEM, UV-Vis, EIS, IPCE and current versus voltage curves. The sensitized semiconducting electrodes display an enhanced photocurrent density of ca. 2 mA cm(-2) at 0.6 V (vs. Ag/AgCl) under visible light (λ > 400 nm). The sensitized photoelectrodes displayed 3 and 535-fold enhanced photocurrent when compared to bare TiO2 NTs under 1 sun and under visible light illumination, respectively. IES results confirmed the improved charge transfer across the TiO2/CdSe/electrolyte interface after annealing at 400 °C. Incident photon-to-electron conversion efficiency measurements confirmed the efficient sensitization by allowing photoresponse in the visible range.