The Influence of a Surface Treatment of Metallic Titanium on the Photocatalytic Properties of TiO2 Nanotubes Grown by Anodic Oxidation

Abstract

Titanium dioxide (TiO2) nanotubes obtained by the anodic oxidation of titanium metal foils can be used for the photocatalytic degradation of organic pollutants. The aim of our study was to determine the influence of the titanium foil’s surface treatment on the final morphology of the TiO2 nanotubes and their photocatalytic activity. In our experiments, we used two different titanium foils that were electropolished or untreated prior to the anodic oxidation. The morphologies of the starting titanium foils and the resulting TiO2 nanotube layers were investigated and the photocatalytic activities measured by the decomposition of caffeine under UV irradiation. Our results showed that electropolishing of the starting foils produced a more uniform and smoother TiO2 nanotubes surface. In contrast, the TiO2 nanotube surfaces from untreated titanium foils mimic the initial surface roughness of the titanium foil. A comparison of the photocatalytic properties of the TiO2 nanotube layers obtained from the untreated and electropolished titanium foils showed that electropolishing does not necessarily improve the photocatalytic properties of the resulting TiO2 nanotube layer. It was found that the determining factors influencing the photocatalytic activity are the chemical impurities (Ti-nitride) on the surface of the titanium foils and the surface roughness of the TiO2 nanotube layer. The highest photocatalytic activity was achieved with the anodized untreated foil with the minimal presence of Ti-nitride and a relatively high roughness of the TiO2 nanotubes.

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“…• pH pH affects the surface charge or isoelectric point of the photocatalyst directly as well as the catalyst particles, size of catalyst aggregates, and the positions of conductance and valence bands [75,76]. The point of zero charge (PZC) of TiO 2 is the main parameter used to study the effect of pH in photocatalyst reactions.…”

“…The point of zero charge (PZC) of TiO 2 is the main parameter used to study the effect of pH in photocatalyst reactions. PZC is a condition where the surface charge of TiO 2 is zero or neutral in the range of 4.5-7.0 [69,75]. The catalyst surface will be negatively charged and repel the anionic compounds in water, which can affect the charge densities [75].…”

“…PZC is a condition where the surface charge of TiO 2 is zero or neutral in the range of 4.5-7.0 [69,75]. The catalyst surface will be negatively charged and repel the anionic compounds in water, which can affect the charge densities [75]. At pH = PZC, the neutral surface charge of the catalyst particles will not be able to produce the interactive rejection necessary for the separation of the solid liquid.…”

“…One of the key factors affecting photodegradation efficiency and overall cost is the amount of catalyst used. Increasing the TiO 2 load does not only increase the number of active sites for pollutant adsorption, but it also increases the number of e−/h+ pairs, which speeds up the reaction rate [18,19,75]. However, if the amount of TiO 2 is increased above a saturation level, the catalyst may agglomerate, resulting in a light screening deficiency [69,75].…”

“…Increasing the TiO 2 load does not only increase the number of active sites for pollutant adsorption, but it also increases the number of e−/h+ pairs, which speeds up the reaction rate [18,19,75]. However, if the amount of TiO 2 is increased above a saturation level, the catalyst may agglomerate, resulting in a light screening deficiency [69,75]. This makes the excess TiO 2 particles reduce the active surface area of the TiO 2 exposed to illumination and consequently reduces the photocatalytic efficiency.…”