A low-cost portable electrical sensor for hydroxyl ions based on amorphous InGaZnO4 thin film at room temperature

The measurement and control of hydroxide ion (OH−) concentration in solution are essential in industrial processes. However, no portable sensing method directly targeting OH− ion with low-cost has been reported till date. Herein, we demonstrate an electrical detection method for OH− concentration in solution based on impedance spectroscopy of hydroxyl ions (OH−) attached to amorphous InGaZnO4 (aIGZO) film surfaces. The systematic examination of impedance response reveals that the resistance component of impedance is sensitive to the OH− ions interaction with the film surface. Results of X-ray photoemission spectroscopy confirm that the change of the impedance property is directly attributed to the amount of hydroxyl radical on the film surface originated from OH− ions in the solution. The impedance behavior of the film upon interaction with OH− was reasonably described by the theoretical analysis of optical measurements based on a vacancy-dependent model. Developed by applying this mechanism as a reference application, an easy-to-use aIGZO thin film based resistance OH− sensor at room temperature shows superior sensitivity, reproducibility, and linearity in the alkali range. This study extends the understanding and usage of aIGZO thin film regarding surface-sensing for the detection of surface interaction and process involving chemical ions and species.