Electrochemical investigation of TiO2-nanotubes formation and its application on energy convertion nanostructured systems

Abstract

The production of nanostructured electrochemical devices energy converters has shown significant results, due to its intrinsic properties of low dimensionality. The nature of the semiconductor TiO2 nanotubes makes it a promising candidate for electrolysis of water, energy converter device, and photocatalytic self-cleaning applications. Therefore, this project proposes to evaluate the electrochemical formation of TiO2-nanotubes (TiO2NT) and its application as electrodes for oxidation of small molecules in fuel cells. The manufacture of TiO2NT was first reported by Zwilling et al. in 1999. The first generation of TiO2NT synthesized using an electrochemical route, was limited to nanotubes with short lengths of up to 600nm. The second generation of TiO2NT, in which aqueous electrolytes were replaced by organic medium, led to new morphologies. In this sense, the use of glycerol allowed to obtain the TiO2NT with walls of low roughness and length of about 7.0 micrometers. Finally, the third generation of TiO2NT materials were obtained in ionic liquids medium and these resulted in TiO2NT with double walls.TiO2 nanotubes present as main crystalline phases a mixture of rutile and anatase phases. Several factors influenced the morphology and microstructure, such as: pH and electrolyte composition, concentration of F-, the existence of convective flow in the solution, initial state of the metal to be anodized, anodizing temperature, voltage (when anodizing at constant potential), water content in the electrolyte, temperature and atmosphere of heat treatment after the electrochemical synthesis. The efficiency of photo-electrocatalytic TiO2NT is associated with recombination rate of e-/h+ once that the photocurrent magnitude is related to the efficiency of transport agent through the base of the nanotubes, and h+ on the TiO2-electrolyte interface. Thus, thick-walled nanotubes had lower rates of recombination and e-/h+ and, consequently, an increase in photocatalytic activity. Moreover, TiO2NT with larger diameters allow better diffusion of small organic molecules such as methanol and ethanol inside the tubes to be eletrooxidized.Another approach, already used, was the electrodeposition of Pt inside the nanotubes which affects significantly the electrocatalytic performance of these materials. In this sense, one of the goals of this project is to understand precisely the effect of metal nanoparticles, for example, Pt and PtRu alloy on the mechanism of electrooxidation using in situ FTIR electrochemical. This technique allows analyzing the vibrational modes of the species in solution and/or adsorbed on the surface of TiO2NT and identifies the intermediate and final products of reaction. It is clear that a thorough knowledge of electrochemical properties of the electrode, considering its structure is also necessary to achieve this goal. For this reason, a second objective of this project is to use impedance spectroscopy to study this behaviour. As it is a porous electrode, it is already discussed in the literature the necessity to describe the system using models of transmission lines, which will be also used in this project. The methods proposed, using in situ FTIR and electrochemical impedance spectroscopy were rarely used in the literature. Thus, this project is proposed to contribute to the elucidation of mechanistic processes, which directly affect the phenomena related to energy conversion. (AU)