Syntheses and structural understanding of a Ti–Ta alloy-based nanotubular oxide photocatalyst

• Published in: CrystEngComm 2018, Vol.20 (37), p.5583-5591
• Main Authors: Soares, Thiago A. S., Holanda, Lilian C., Galvão, Rhauane A., Gonçalves, Renato V., Bestetti, Massiliano, Kinast, Éder J., Teixeira-Neto, Érico, Teixeira-Neto, Ângela A., Khan, Sherdil, Teixeira, Sérgio R., Almeida, Luciano C., Machado, Giovanna
• Format: Article
• Language: English
• Subjects: Anatase; Anodizing; Catalytic activity; Crystal structure; Crystallinity; Dislocations; Heat treatment; Hydrogen production; Irradiation; Migration; Mixed oxides; Morphology; Nanotubes; Optical properties; Oxidation; Phase transitions; Phases; Photocatalysis; Photocatalysts; Surface properties; Synthesis; Tantalum base alloys; Tantalum oxides; Titanium base alloys; Titanium dioxide
• ISSN: 1466-8033; 1466-8033
• DOI: 10.1039/C8CE00743H

Herein, we have synthesized Ti–Ta based mixed oxide vertically oriented and highly ordered freestanding nanotubes having an average length of 60 μm by anodic oxidation of a homemade Ti–Ta (50–50 at%) alloy. The effect of heat treatment on the morphology, phase transformation to TiTa 2 O 7 nanostructures, crystalline structures, optical and surface properties has been investigated by a variety of characterization techniques. The samples remained mainly amorphous at temperatures up to 700 °C, and according to the Rietveld refinement analyses the bulk formation of crystalline Ta 2 O 5 and TiO 2 -anatase phases occurred only at 800 °C in the tubular matrix followed by a slight solid to solid transformation of TiTa 2 O 7 and TiO 2 -rutile phases starting from 850 °C. At 1000 °C, the content of TiTa 2 O 7 increased followed by a decrease in TiO 2 and Ta 2 O 5 content; however, the nanotubes appeared to be interconnected particles imitating the verticality of the nanotubes. Compared to the literature (∼1350 °C), in this work the transformation of TiTa 2 O 7 occurred at a lower temperature which is related to the amorphous nature of the starting nanotubular samples. The HAADF-STEM images and localized EDS mapping showed the distribution of Ti and Ta elements along the tubular matrix demonstrating that the as-anodized nanotubes consist of a uniform mixture of TiO 2 and Ta 2 O 5 whereas for the calcined samples random distribution of Ti was observed indicating the migration of Ti inside the tubular matrix for the formation of mixed oxide and TiTa 2 O 7 phases. X-ray photoelectron spectroscopy (XPS) analysis identified the dislocation in the binding energies of Ti 2p and Ta 4f regions for the heat treated samples as compared to the as-anodized nanotubes; related to the formation of TiTa 2 O 7 . The optical band gap of TiTa 2 O 7 was found to be 3.08 eV. These oxide mixtures were applied in photocatalytic H 2 generation under solar irradiation; where the sample heat treated at 1000 °C has resulted in a production rate of 41.58 μmol h −1 g −1 which was higher than all the other samples synthesized in this work. For continuous 48 h irradiation the photocatalytic activity of the sample remained highly stable. This result is related to the higher content and lower bandgap energy of TiTa 2 O 7 in the mixed oxide matrix indicating that TiTa 2 O 7 is a promising photocatalyst.