Influence of Copper Addition on Optical Properties of TiO2

Abstract

Cu-doped TiO~ could be considered as a better absorber of sunlight for solar energy conversion. The Cu-doped TiO2 has been prepared by anodic oxidation of a Ti-2.5% Cu alloy. The structural properties of these anodic oxides are examined by electron microdiffraction, XPS, and nuclear microanalysis. The optical properties of both the Ti-Cu alloy and the anodic films have been determined as functions of the thickness of the oxides. It is concluded that Cu doping o~ TiO2 creates an extra state in the forbidden gap of TiO2 which gives two extra peaks in the absorption spectrum of the oxide. But for solar energy applications, the preparation of the doped oxide by anodic oxidation is not suitable because one does not control the level of impurity.Titanium dioxide electrodes created a great interest in the field of semiconductor-electrolyte interfaces when it was shown by Fijishima and Honda (1). that water photoelectrolysis was possible without degradation or decomposition of the semiconductor electrode. For solar applications, however, TiO2 is not suitable because the bandgap of this oxide is 3 eV (410 nm) too far away in the u.v. region to significantly absorb the solar energy. A great amount of work has been done to find the right semiconductor electrode with a bandgap around 2 eV (620 nm) and stability comparable to the TiO2 electrode; two review articles (2, 3) summarize these works. Among the numerous materials or combinations of materials which have been tested to do this water photoe]ectrolysis, TiO2 remains high in the list since it is the most stable material for a long period of time. So the problem is to decrease the TiO~ bandgap or increase the optical absorption in the visible region by increasing the number of defects (holes, vacancies, etc.). Both approaches have been tested on specific systems; Ghosh and Maruska (4) have reported water photoelectrolysis using visible light on a Cr-doped TiO2 electrode, Augustinski et al. (5) have also reported results for TiO~ doped with 6% A120~.Keeping in mind the possible applications of this study we have chosen Cu to fill the bandgap of TiO2. The preparation of Cu-doped TiO2 was made by anodic oxidation of a Ti-Cu alloy which is commercially available and has good mechanical properties. Our purpose is also to show the importance of the knowledge of the optical indexes of both the substrate and the semiconductor film to determine the bandgap shift and the absorption variations due to the incorporation of copper.

Raman Spectra of the Anodic Oxide Film on Titanium in Acidic Sulfate and Neutral Phosphate Solutions

“…Here, we notice that the potential where the Raman bands begins to appear is closely related to a potential where the crater-like morphology begins to be observed, and that the oxide film that exhibits the ripple-like morphology provides the strong Raman bands of the anatase type of TiO2. This identification of the film composition is in agreement with the results of electron diffraction measurements conducted by Blondeau et al (8), although the potential region is different from our potential region. They detected a diffraction pattern corresponding to the anatase TiO~ for the anodic oxide film formed at potentials more positive than 50V for a much shorter time than that in our experiments and a diffraction pattern corresponding to a quasiamorphous structure at more negative potentials.…”

Photoelectrochemical Investigations of Passive Films on Titanium Electrodes

“…This offers the possibility of testing photoelectrochemistry as an in situ method in a system such as passive titanium where thickness and properties of the passive film can be changed by changing parameters of the passive film formation. A transition from the amorphous to the crystalline state of passive films on titanium electrodes with increasing formation potential has been found from electron diffraction (6). The same authors have also described the optical indexes of the passive film (7).…”

Electrochemical and ellipsometric data related to the formation of soluble silver (I) during the potentiodynamic polarization of polycrystalline silver in basic solutions

“…Ellipsometric data are frequently interpreted on the basis that the optical constants remain nearly unchanged within a certain range of the layer thickness [27] and the reliability in the calculation of the different parameters can be increased by increasing the number of data and by decreasing the number of the unknown parameters. Thus, a single point in the P us A plane is ambiguously defined by a set of n,, kJ and d values.…”

“…Ellipsometric data furnish reasonable values of the optical constants and a satisfactory estimation of the anodic layer average thickness. For this purpose, the freedom of interpretation of the ellipsometric results is greatly reduced if the series of measurements are evaluated taking the thickness as a changing parameter with a common set of optical indexes which are at least valid within a certain range of thickness [26,27].…”