TiO₂Nanotubes Formed in Aqueous Media: Relationship between Morphology, Electrochemical Properties and Photoelectrochemical Performance for Water Oxidation

Abstract

This paper studies the effect of voltage and electrolyte composition on the morphology of TiO 2 nanotube films, their semiconductor properties and photoelectrochemical performance for water oxidation. TiO 2 nanotube films were formed by potentiostatic anodization in fluoride containing electrolytes (using either HF or NH 4 F) during 1 h at four different voltages and were heat treated at 450 • C to obtain anatase polymorph. SEM images showed a more significant chemical attack when NH 4 F was employed, which was evidenced by larger tube diameters and thinner films. UV-vis reflectance spectra showed that the TiO 2 films formed in aqueous electrolyte were highly defective, presenting various broad absorption peaks in the visible region attributed to the sub-bandgap energy states. The semiconducting properties were modified by the nature of the counter-ion utilized during film growth, resulting in films with a higher donor density and a lower flatband potential in NH 4 F electrolytes. The films formed in NH 4 F showed higher photopotentials and photocurrents related to the modification of their morphology and semiconducting properties, attaining a maximum at a formation voltage of 15 V.

A review of growth mechanism, structure and crystallinity of anodized TiO2 nanotubes

“…In a recent review, Paramasivam et al [64] highlight the need to establish a more quantitative analysis on the influence of tube morphology and composition and the need to investigate approaches to modify the electronic, optical and chemical properties in order to maximize the outcome on the applied research on anodized TiO 2 NTs. While the morphology and composition can be varied by operating either in aqueous or organic electrolytes [284,285], a straightforward way to modify and enhance the electronic and optical properties of the NTs is certainly represented by post-growth annealing treatments. The annealing and the resulting crystal phase(s) have a significant impact on the electrochemical properties of the NTs [286].…”

Titania Nanotubes by Electrochemical Anodization for Solar Energy Conversion

“…75,[84][85][86] A higher doping density was observed using NH 4 F-based anodization electrolytes versus HF electrolytes; increasing the anodization voltage also resulted in increased doping density of the TiO 2 nanotubes. 85,87 Two-slope Mott-Schottky plots obtained by Muñoz indicated the presence of multiple defect levels identified by scanning the potential in the anodic direction. 86 The influence of water content on the density of trap states was probed by low frequency EIS measurements, and it was found that a 2 vol% water content anodization electrolyte produced three times as many trap states as an 11 vol% water electrolyte, affecting the photon-to-current conversion efficiency.…”

Should TiO2 nanostructures doped with Li+ be used as photoanodes for photoelectrochemical water splitting applications?

“…Titanium dioxide (TiO2) is a wide band-gap semiconductor (Eg ≈ 3.2 eV in the anatase crystalline form) which has stirred up the interest of the scientific community over the last decades because of its interesting chemical and electronic properties. Specifically, its good photocatalytic properties make TiO2 suitable for several energy and environmental applications, such as photocatalytic degradation of organic pollutants and hydrogen generation via water photoelectrolysis [1][2][3][4][5][6][7][8][9][10].…”