Morphology Evolution in Titanium Dioxide Nanotubes Arrays Prepared By Electrochemical Anodization
Authors: Tirano Joaquin , Zea Hugo , Luhrs Claudia 
Published: February 2020

The production of hydrogen by electrochemical dissociation of water has been known for minimal environmental impact involved. This process consumes more electrical energy than electricity generated by combustion of produced hydrogen, but it is still interesting in order to find sustainable future sources of clean energy. One way to address this problem is to investigate options for reducing inefficiencies on the electrochemical cell, these are voltage consuming, it is also possible to take advantage of cheaper energy sources. Photocatalysis has established as viable option in the development of processes for the treatment of pollutants and clean energy production. This option is based on the ability of semiconductors to generate an electron flow by means of the interaction with solar radiation.
Owing to its electronic structure, TiO2 is the most frequently used semiconductors in photocatalysis, although it has a high recombination of photogenerated charges and low solar energy absorption. Due to TiO2 can produce electricity from solar energy, it can be used to satisfice the electrical energy requirements on the operation of electrochemical cells. During this process pairs hole – electron are photogenerated, the efficiency can be improved if charge recombination is avoided, also if the energy absorption into the visible spectrum is increased and its specific surface area is increased. An alternative to reduce these limitations is the use of nanostructured morphologies which can be produced during the synthesis of TiO2 nanotubes (TNTs). Therefore, if possible to produce oriented nanostructures it will be possible to generate a greater contact area with electrolyte and better charge transfer. At present, however, the development of these innovative structures still presents an important challenge for the development of competitive photoelectrochemical devices.
This research focuses on established correlations between synthesis variables and nanostructure morphology which has a direct effect on the photocatalytic performance. TNTs with controlled morphology were synthesized by potentiostatic anodization of titanium foil. The anodization was carried out at room temperature in an electrolyte composed of ammonium fluoride, deionized water and ethylene glycol. Consequent thermal annealing of as-prepared TNTs was conducted in the air at 450 °C. Morphology and crystalline phase of the TNTs were analyzed by SEM, EDS and XRD. As results, the synthesis conditions were established to produce nanostructures with specific morphological characteristics. Anatase was the predominant phase of TNTs after thermal treatment. Nanotubes longer than 7 μm and pore diameter around 50 nm with different surface-volume ratio are important in photoelectrochemical applications based on TiO2 due to their dimmensional characteristics, surface-volume ratio, reduced radial dimensions and high oxide/electrolyte interface. Finally, this knowledge can be used to improve the photocatalytic activity of TNTs by making additional surface modifications with dopants that improve their efficiency.