Effect of Oxygen Content on the Photoelectrochemical Activity of Crystallographically Preferred Oriented Porous Ta3N5 Nanotubes

Abstract

Crystallographically preferred oriented porous Ta3N5 nanotubes (NTs) were synthesized by thermal nitridation of vertically oriented, thick-walled Ta2O5 NTs, strongly adhered to the substrate. The adherence on the substrate and the wall thickness of the Ta2O5 NTs were fine-tuned 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 Ta3N5 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 Ta3N5 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 H2 in 1 h from the pristine porous Ta3N5 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 Ta3N5 NTs remained unchanged.