Charge transfer reductive doping of nanostructured TiO 2 thin films as a way to improve their photoelectrocatalytic performance

Electrochemistry Communications (Impact Factor: 4.29). 10/2006; 8:1713-1718. DOI: 10.1016/j.elecom.2006.08.006

ABSTRACT Nanostructured TiO 2 films can be reversibly doped by applying a potential of À0.6 V Ag/AgCl in aqueous 0.1 M HClO 4 for several min-utes. This charge transfer reductive doping causes significant changes of the cyclic voltammograms in the dark. Furthermore, an up to 6-fold increase of the efficiency for water photooxidation is observed upon electrochemical reductive doping. Both observations are explained by the build-up of a space charge layer through agglomerates of appropriately sized and shaped nanoparticles.

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    ABSTRACT: A native oxide film on the biphasic alloy was studied with scanning electrochemical microscopy (SECM). This alloy is commonly used for biomedical applications due to its biocompatibility and mechanical properties. The heterogeneously composed, n-semiconducting oxide film is of particular interest as biological systems are in contact only with these oxides and immunological rejection mechanisms may be connected to their electrochemical properties. Auger electron spectroscopy showed that the elemental composition of the oxide films on the α and β phase differ from each other. Approach curves were recorded above individual grains of the α phase in the feedback mode with several redox mediators. They were selected to cover a wide range of redox potentials for a better understanding of the surface kinetics of the oxide layer. The electron-transfer kinetics changed strongly depending on the redox potential of the mediator with respect to the energetic position of the bandgap of the oxide film. Predictions about the value of the flatband potential on an individual phase were derived from these experiments. Furthermore, SECM images were recorded to laterally resolve different electrochemical properties of the oxide film originating from the heterogeneous composition of the oxide on both phases.
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    ABSTRACT: Nanoparticle-covered electrodes have altered properties as compared to conventional electrodes with same chemical composition. The changes originate from the large surface area and enhanced conduction. To test the mineralization capacity of such materials, TiO2 nanoparticles were deposited on titanium and gold substrates. The electrochemical properties were investigated using cyclic voltammetry and impedance spectroscopy while the mineralization was tested by immersion in simulated body fluid. Two types of nucleation and growth behaviours were observed. For smooth nanoparticle surfaces, the initial nucleation is fast with the formation of few small nuclei of hydroxyapatite. With time, an amorphous 2D film develops with a Ca/P ratio close to 1.5. For the rougher surfaces, the nucleation is delayed but once it starts, thick layers are formed. Also the electronic properties of the oxides were shown to be important. Both density of states (DOS) in the bandgap of TiO2 and the active area were determined. The maximum in DOS was found to correlate with the donor density (N d ) and the active surface area. The results clearly show that a rough surface with high conductivity is beneficial for formation of thick apatite layers, while the nanoparticle covered electrodes show early nucleation but limited apatite formation.
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