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 The oxygen electrochemical reduction at nitrogen treated TiO2 nanotubes electrode shows distinctive and well-separated 2-electrons reduction peaks. The oxygen reduction was investigated in alkaline solution at titanium oxide nanotube (TON) arrays fabricated by anodizing of titanium foil in hydrogen fluoride solution followed by annealing at 450 C in nitrogen (TON-N2) atmosphere. The morphology of the arrays was characterized by SEM and the electrochemical behavior was studied by cyclic voltammetry in potassium ferrocyanide and alkaline solutions. The treated TON arrays showed a significant enhancement in conductivity and in the activity for reduction of O2 in alkaline media in comparison to the data of plain TiO2. TON arrays annealed in N2 showed a unique electrochemical behavior of two well-resolved 2-electrons O2 reduction peaks in contrast to TON arrays annealed air, which only showed a single reduction peak at more negative potential. At TON-N2 arrays, hydrogen peroxide was detected as the main product during the first reduction. The switch in mechanism is proposed to be due to the presence of oxygen vacancies at the surface of titania nanotube arrays. The finding is of a high importance for the selective production of hydrogen peroxide by electrochemical reduction of oxygen in alkaline media using TiO2 nanotubes electrode.TiO2 nanotube (TON) arrays self-assembled via anodization have attracted considerable research interest due to unique nanoscale features and electronic properties . TON offers interesting semiconducting properties, chemical inertness and stability, cost effectiveness and corrosion resistance , and is used in many fields, such as photocatalysis , solar cells, electronic devices, and environmental cleaning and protection . Several approaches have been established to obtain highly ordered TON arrays including the use of a template alumina membrane , sol–gel processes , seeded growth methods , hydrothermal techniques , and the anodization of titanium foil in a fluoride-based electrolyte solution . However, TON arrays prepared by anodization usually exhibit relatively low electrical conductivity , which limits appli-cations in electro-catalysis and as catalyst support. Therefore, several methods for generating of oxygen vacancies, e.g. metal or non-metal doping, are often adopted to improve the electrical conductivity and reactivity of TON structures to satisfy the requirements for effective electrode materials. The oxygen reduction reaction (ORR) in aqueous solutions occurs mainly by two pathways: the direct 4-electron reduction pathway from O2 to H2O, and the 2-electron reduction pathway from O2 to hydrogen peroxide (H2O2) . In non-aqueous aprotic solvents and/or in alkaline solutions, the 1-electron reduction pathway from O2 to superoxide (O2 ) can also occur . The electrochemical reduction of oxygen (ORR) was found to occur on various forms of titanium dioxide at much higher overvoltage in acid and alkaline media. Clark et al. reported that the overvoltage for ORR is significantly reduced after activation of TiO2 layer by cyclic polarization. Clark et al. and Parkinson et al. , using a ring-disk electrode, established a close to 4e pathway in alkaline solutions. Mentus observed on anodically formed TiO2/Ti layers the transition from 2e to 4e path with the transition from acidic to alkaline solutions. An effective production of hydrogen peroxide on TON materials in alkaline media has so far not been reported. The synthesis of TON arrays by Ti foil adonization in different electrolytes and annealing at different temperatures has been widely studied. However, only few reports addressed the signifi-cant effect of the annealing treatment and corresponding change in chemical composition (e.g. the formation of oxygen vacancies) on the electrochemical reduction of oxygen in alkaline solution. In this work, highly conducting titanium oxide nanotube (TON) arrays were fabricated by anodizing and annealing at 450 C in air (TON-air) or in nitrogen (TON-N2) atmospheres. The influence of annealing gases on the surface chemistry of TON array electrodes was investigated by cyclic voltammetry and by scanning electron microscopy (SEM). Oxygen and hydrogen peroxide reduction in alkaline media was studied at annealed TON arrays by cyclic voltammetry and by generator-collector electrode experiments and new insights into the mechanism were obtained. TON-N2 electrodes are shown to be highly effective and selective for the production of hydrogen peroxide at low over-potential. |
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