Preparation and structure elucidation of multifunctional porous TiO2 surfaces by means of plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) is an established electrochemical process to produce of stable, compact, ceramic-like and porous oxide layers and was increasingly used during the 20th century for the structuring of metal surfaces such as aluminum, magnesium and titanium. On an industrial scale, especially in the fields of thermal protection and corrosion protection, the applications of the process have increased significantly in recent years, such as its use for decorative aspects. Likewise, PEO has become interesting for medical technology and implantology due to the diversity of its varying surface properties such as porosity, layer thickness and surface composition. The PEO-process makes it possible, especially on titanium surfaces, to produce crystalline oxides through the formation of high-energy plasma discharges. These titanium dioxides can in turn enable the photocatalytic activity of the surfaces and significantly increase their wear resistance. However, the ongoing physical and chemical processes are very complex and for some metals, as here for titanium, are not fully understood. One of the most important aspects to understanding the involved process events is the investigation into used parameters, such as electrolyte composition, applied voltage, and the resulting structure of the oxide layer. These influencing factors were analyzed in the present work, and both the structures and the surface properties were examined in detail. In this work, pure titanium materials were treated with various electrolytic systems to enable a decoding of differences in structure and the resulting properties. Distinct differences between the oxide layers in terms of phase formation and structure could be demonstrated by x-ray diffraction and microscopic examination. By increasing the voltage and choosing the right electrolyte, the structure of the oxide layers could be varied with regard to pore size and distribution as well as in layer thickness and the degree of crystallinity of the titanium dioxide phases. Using this method, the proportion of crystalline oxides in the PEO-layers could be adjusted through the right electrolyte composition and an increase in the applied voltage. By determining the surface properties and compositions of the oxide layers, it was possible to investigate these in more depth regarding their structural design, thus gaining a better understanding of the effect of the plasma discharges. Using Raman spectroscopy and the EBSD technique, the crystalline constituents could be detected and identified within the entire oxide layer. Analogical to the XRD measurements, an increase in crystalline TiO2 phases was found from the lower part of the oxide layer to the surface of the layer. Furthermore, the structure of the PEO oxide layers could be decrypted because of a detailed analysis using the STEM method, whereby large crystals in the upper area of the oxide layer and smaller crystals at the boundary layer to the titanium substrate could be visualized. For the first case, these areas were created by the high energies of the discharges in the later course of the oxidation, whereas the smaller crystals at the lower part of the oxide layer could be explained by the effects of the discharges as far as the bottom of the oxide layer. Amorphous TiO2 was detected around the generated pore structures of the oxide layer. These amorphous regions led to the conclusion that the resulting TiO2 can be converted into the liquid and gaseous phases during the process. The reduced conductivity in the gaseous phase and the surrounding colder electrolyte led to a faster cooling of the TiO2 in the area of the pore structures and thus to a reduced formation of crystalline structures. The results presented in this work demonstrate the possibility of adapting the surface properties, such as morphology, crystallinity, and photocatalytic activity of PEO oxidized titanium dioxide layers for a variety of applications. The crystallinity of the titanium dioxides can be selectively controlled and helps to adjust the stability as well as the photocatalytic activity of the layers. The transfer of thin PEO-layers to polymeric substrates, as well as the improvement of the adhesion of titanium to polymeric substrates with a special adhesive layer, has thus been successfully achieved. In addition to titanium, polymer substrates are also used as an implant material in medicine, but they often cannot withstand the biocompatibility requirements. The improvement of cell adhesion to pure polymers by applying a PEO-layer was successfully achieved. Further intensive investigations into the structure of the PEO oxide layers led to a better understanding of the process and the effects of the plasma species on the entire layer. This helped to expand the model of plasma electrolytic oxidation on titanium materials

Präparation und Strukturaufklärung multifunktionaler poröser TiO2-Oberflächen mittels plasmaelektrolytischer Oxidation

Plasma electrolytic oxidation (PEO) is an established electrochemical process to produce of stable, compact, ceramic-like and porous oxide layers and was increasingly used during the 20th century for the structuring of metal surfaces such as aluminum, magnesium and titanium. On an industrial scale, especially in the fields of thermal protection and corrosion protection, the applications of the process have increased significantly in recent years, such as its use for decorative aspects. Likewise, PEO has become interesting for medical technology and implantology due to the diversity of its varying surface properties such as porosity, layer thickness and surface composition. The PEO-process makes it possible, especially on titanium surfaces, to produce crystalline oxides through the formation of high-energy plasma discharges. These titanium dioxides can in turn enable the photocatalytic activity of the surfaces and significantly increase their wear resistance. However, the ongoing physical and chemical processes are very complex and for some metals, as here for titanium, are not fully understood. One of the most important aspects to understanding the involved process events is the investigation into used parameters, such as electrolyte composition, applied voltage, and the resulting structure of the oxide layer. These influencing factors were analyzed in the present work, and both the structures and the surface properties were examined in detail. In this work, pure titanium materials were treated with various electrolytic systems to enable a decoding of differences in structure and the resulting properties. Distinct differences between the oxide layers in terms of phase formation and structure could be demonstrated by x-ray diffraction and microscopic examination. By increasing the voltage and choosing the right electrolyte, the structure of the oxide layers could be varied with regard to pore size and distribution as well as in layer thickness and the degree of crystallinity of the titanium dioxide phases. Using this method, the proportion of crystalline oxides in the PEO-layers could be adjusted through the right electrolyte composition and an increase in the applied voltage. By determining the surface properties and compositions of the oxide layers, it was possible to investigate these in more depth regarding their structural design, thus gaining a better understanding of the effect of the plasma discharges. Using Raman spectroscopy and the EBSD technique, the crystalline constituents could be detected and identified within the entire oxide layer. Analogical to the XRD measurements, an increase in crystalline TiO2 phases was found from the lower part of the oxide layer to the surface of the layer. Furthermore, the structure of the PEO oxide layers could be decrypted because of a detailed analysis using the STEM method, whereby large crystals in the upper area of the oxide layer and smaller crystals at the boundary layer to the titanium substrate could be visualized. For the first case, these areas were created by the high energies of the discharges in the later course of the oxidation, whereas the smaller crystals at the lower part of the oxide layer could be explained by the effects of the discharges as far as the bottom of the oxide layer. Amorphous TiO2 was detected around the generated pore structures of the oxide layer. These amorphous regions led to the conclusion that the resulting TiO2 can be converted into the liquid and gaseous phases during the process. The reduced conductivity in the gaseous phase and the surrounding colder electrolyte led to a faster cooling of the TiO2 in the area of the pore structures and thus to a reduced formation of crystalline structures. The results presented in this work demonstrate the possibility of adapting the surface properties, such as morphology, crystallinity, and photocatalytic activity of PEO oxidized titanium dioxide layers for a variety of applications. The crystallinity of the titanium dioxides can be selectively controlled and helps to adjust the stability as well as the photocatalytic activity of the layers. The transfer of thin PEO-layers to polymeric substrates, as well as the improvement of the adhesion of titanium to polymeric substrates with a special adhesive layer, has thus been successfully achieved. In addition to titanium, polymer substrates are also used as an implant material in medicine, but they often cannot withstand the biocompatibility requirements. The improvement of cell adhesion to pure polymers by applying a PEO-layer was successfully achieved. Further intensive investigations into the structure of the PEO oxide layers led to a better understanding of the process and the effects of the plasma species on the entire layer. This helped to expand the model of plasma electrolytic oxidation on titanium materials

Electrochemical rutile and anatase formation on PEO surfaces

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 50 V and 300 V 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.13914

Photocatalytic activity of TiO2 layers produced with plasma electrolytic oxidation

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 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.71072