Oxygen Sensors Based on Titanium Oxide Layers Produced By Plasma Electrolytic Oxidation
Authors: Engelkamp Bernd , Schierbaum Klaus Dieter 
Published: May 2020

Introduction

Plasma electrolytic oxidation (PEO) of titanium is a powerful technique for creating key materials, for instance, in photocatalytic devices, automotive components, biomedical prostheses and recently in gas sensors [1,2]. Graphite-platinum electrodes on top of PEO oxide layers on titanium foils led to gas sensors for H2, H2O and CO, depending on the applied detection principle [2]. In our study, we investigate the potential of PEO produced titanium oxide layers for detecting O2.
In this context, the porous microstructure, layer thickness and crystal titanium dioxide phase composition is essential. The porous microstructure provides a high surface-to-bulk ratio which is beneficial for gas interaction. The thickness and phase composition affect the electronic resistivity. Recent studies show, that the titanium oxide layer may vary in its porosity, thickness and phase composition, depending on the fabrication parameters in PEO [3, 4]. For instance, the crystal phase, either anatase or rutile, can be adjusted intentionally by customizing the electrolyte in the process. Based on recent findings [3, 4], we optimize the titanium oxide layer and apply it as sensor material for detecting oxygen.

Method

Oxide layers on titanium foils were produced by galvanostatically controlled plasma electrolytic oxidation in concentrated sulfuric acid with small additions of phosphoric acid up to 3% mole fraction. Before and after PEO, the samples were analyzed for weight gain with a microbalance. Oxide surfaces and cross sections were systematically studied in different phases of the process by scanning electron microscopy and x-ray diffraction. From confocal Raman microscopy on cross sections, we derived some information about the phase distribution in the oxide layer. Elemental composition was investigated by energy dispersive x-ray spectroscopy.
The top electrode on the oxide layers consists of platinum and was applied by sputter deposition. Electrical connection between substrate as well as top electrode and socket pins was achieved with either silver epoxy glue or gold wire bonding. The gas sensing experiments were performed in a gas-flow apparatus with two pressurized gas bottles (oxygen and nitrogen). Mass flow controllers adjust the overall gas flow and the concentration of oxygen.

Results and Conclusions

Oxide coatings on titanium were produced in a PEO process with constant current density and were applied as gas sensing materials. The electrolyte in the PEO process was varied by different amounts of sulfuric and phosphoric acid. Therefore, the intensity of characteristic breakdowns in the PEO process and their impact on the oxide layer changes. In pure sulfuric acid, rough surfaces with high pore densities and mainly rutile evolve. With small additions of phosphoric acid, the destructive reforming is suppressed and mainly anatase is formed. Our findings explain the drastic change by a preferential adsorption of H3PO4 anions on the anodic area, which block the surface for more reactive H2SO4 anions [3]. Our findings from PEO were exploited to customize the oxide layers for application as sensor material for oxygen detection. The sensor performance varies, depending on the structure of the oxide layer. The signal was investigated regarding the sensitivity, stability and reproducibility. The performance was explained in a simple model of oxygen interaction.

References

[1] Diamanti M. V., Ormellese, M. and Pedeferri, M., Application-wise nanostructuring of anodic films on titanium: a review, Journal of Experimental Nanoscience
10 (2015) 1285-1308
[2] El Achhab M. and Schierbaum K., Gas sensors based on plasma-electrochemically oxidized titanium foils, Journal of Sensors and Sensor Systems
5 (2016) 273-281
[3] Engelkamp B., Fischer B. and Schierbaum K., Plasma Electrolytic Oxidation of Titanium in Concentrated H2SO4 and the Impact of Additional H3PO4, to be published (2019)
[4] Engelkamp B., El Achhab M., Fischer B., Kökcam-Demir Ü. and Schierbaum K., Combined Galvanostatic and Potentiostatic Plasma Electrolytic Oxidation of Titanium in Different Concentrations of H2SO4, Metals
8 (6) (2018) 386