The photocatalytic activity of titanium dioxide (TiO2) results from its crystalline phase's anatase and rutile. In this regard, plasma electrolytic oxidation (PEO) is a promising process for producing highly porous surfaces with a high proportion of crystalline phases into the oxide layer on pure titanium. PEO-coatings were produced under different conditions in various electrolytes in order to identify the crystalline fractions of the surfaces and to examine the associated photocatalytic activity. The composition of the PEO electrolyte was varied to optimize the polymorphic composition of the TiO(2 )comparable to the photocatalytic active TiO2 material AEROXIDE (R) P25. X-ray powder diffraction (XRD) was selected to identify the produced crystal structures of anatase and rutile on the surface material depending on the electrolytic system. In order to establish the expected band gap of the TiO2 on the surfaces, the samples were subjected to a diffuse reflectance measurement, which detected direct transitions for all samples using the TAUC and DASF methods. The acceptance of the photocatalytic reaction by the crystalline PEO-samples was further confirmed by the degradation of two typical dyes (methylene blue MB, rhodamine B RB) under UV-light irradiation. Both a high proportion of anatase and the presence of rutile on the PEO-layers had a targeted effect on the catalytic efficiency. However, the average crystallite sizes also played an important role in the samples produced in an optimum range of 30-40 nm. Both effects support the photocatalytic properties of PEO-surfaces.

Plasma electrolytic oxidation (PEO) is a type of high-voltage anodic oxidation process capable of producing a thick oxide layer with a wide variety of structural and chemical properties influenced by the electrolytic system. This process enables the combined adjustment of various characteristics, i.e. the morphology and chemical composition. The procedure facilitates the possibility of generating an individual structure as well as forming a crystalline surface in a single step. A highly porous surface with a high crystalline content consisting of titanium dioxide phases is ensured through the process of plasma electrolytic oxidizing pure titanium. In the present study plasma electrolytic oxidized TiO2-layers were investigated regarding their crystallinity through the layer thickness. The layers were prepared with a high applied voltage of 280 V to obtain a PEO-layer with highly crystalline anatase and rutile amounts. Raman spectroscopy and electron backscatter diffraction (EBSD) were selected to clarify the structure of the oxide layer with regard to its crystallinity and phase composition. The composition of the TiO2-phases is more or less irregularly distributed as a result of the higher energy input on the uppermost side of the layer. Scanning transmission electron microscopy (STEM) provided a deeper understanding of the structure and the effects of plasma discharges on the layer. It was observed that the plasma discharges have a strong influence on crystallite formation on top of the oxide layer and also at the boundary layer to the titanium substrate. Therefore, small crystallites of TiO2 could be detected in these regions. In addition, it was shown that amorphous TiO2 phases are formed around the characteristic pore structures, which allows the conclusion to be drawn that a rapid cooling from the gas phase had to take place in these areas. (C) 2018 Elsevier B.V. All rights reserved.

A highly porous surface with a high crystalline content and resultant photocatalytic activity is ensured through the process of plasma electrolytic oxidation on pure titanium. In the present study the morphology, crystallinity and photocatalytic activity of plasma electrolytic oxidized TiO2-surfaces were investigated. The surfaces were prepared in acidic and alkaline electrolytes over an applied voltage range between 50V and 300V to optimize the crystalline and photocatalytic properties. Scanning electron microscopy (SEM) and X-ray powder diffraction (XRD) were selected to determine the morphologies which differ according to the type of electrolyte as well as the crystal structures of anatase and rutile on the surface material, which increase with the applied voltage. The oxide surfaces did not show morphological differences compared to typical PEO surfaces with the exception of oxide films obtained in H2SO4-solution which also exhibited an astounding amount of rutile even with low applied voltages. The increased parts of anatase and rutile on the surfaces resulted in photocatalytic activity, which was investigated under UV-light using methylene blue, while the PEO surfaces showed degradation activity. There is an indication that a high proportion of anatase and small amounts of rutile in the PEO layers positively influence photocatalytic activity.

The photocatalytic activity of titanium dioxide (TiO2) results from its crystalline phase’s anatase and rutile. In this regard, plasma electrolytic oxidation (PEO) is a promising process for producing highly porous surfaces with a high proportion of crystalline phases into the oxide layer on pure titanium. PEO-coatings were produced under different conditions in various electrolytes in order to identify the crystalline fractions of the surfaces and to examine the associated photocatalytic activity. The composition of the PEO electrolyte was varied to optimize the polymorphic composition of the TiO2 comparable to the photocatalytic active TiO2 material AEROXIDE® P25. X-ray powder diffraction (XRD) was selected to identify the produced crystal structures of anatase and rutile on the surface material depending on the electrolytic system. In order to establish the expected band gap of the titanium dioxides on the surfaces, the samples were subjected to a diffuse reflectance measurement, which detected direct transitions for all samples using the TAUC and DASF methods. The acceptance of the photocatalytic reaction by the crystalline PEO-samples was further confirmed by the degradation of two typical dyes (methylene blue MB, rhodamine B RB) under UV-light irradiation. Both a high proportion of anatase and the presence of rutile on the PEO-layers had a targeted effect on the catalytic efficiency. However, the average crystallite sizes also played an important role in the samples produced in an optimum range of 30 40 nm. Both effects support the photocatalytic properties of PEO-surfaces.