Analysis of Photocatalytic Properties of Titanium Dioxide Electrode Supported By Hydroxyapatite Co-Catalyst in a Marine Solar Cell
Authors: Wunn Htoo , Motoda Shinichi , Morita Motoaki , Itakura Haruki 
Published: February 2024

Titanium dioxide (TiO2), as an n-type semiconductor photocatalyst, has been widely studied and used in many environmental and energy applications such as solar cells, wastewater treatment, air purification, water splitting for hydrogen production and CO2 conversion due to its superior electrical and optical properties, stability, and biocompatibility. Using its photocatalytic and other properties, we have been studying a floating photoelectrochemical cell that can be applicable to the marine environments. This cell is designed to utilize titanium dioxide as its photoanode and copper oxides photocathode, and seawater is used as an electrolyte between electrodes. The drawbacks of using titanium dioxide as photocatalysts are its high electron-holes recombination rates and lower absorption efficiency in visible light regions. To improve against these drawbacks, we studied hydroxyapatite (HAp, Ca10(PO4)6(OH)2) as co-catalyst for titanium dioxide electrode. Since HAp possesses biocompatibility, it is suitable to be used with TiO2 electrode in marine environments. In previous studies, it was found that HAp improved and boosted the current density produced by the titanium dioxide electrode. However, removal of HAp occurred from the surface of titanium dioxide during the photoelectrochemical measurements. Therefore, in this study, we have optimized the content of HAp paste that would be applied to the surface of titanium dioxide electrode by screen printing method.
For preparation of titanium dioxide electrode, type 329J4L stainless steel was selected as the base substrate. Firstly, surface of the substrate was grinded to form the grid pattern and cleaned with ethanol in ultrasonic cleaner and then with distilled water. After that, the substrate was passivated with 10 % HNO3 at 60 ºC for 30 minutes. The passivated substrate was then printed and heat treated with TiO2 paste twice at 150 ºC for 1 hour and 550 ºC for 30 minutes respectively. Double layered titanium dioxide electrode was then printed with HAp paste and heat treated at 150 ºC for 1 hour. To check the characteristics of photocatalytic effect of electrode, photopotential and potentiodynamic polarization measurements were performed to the electrode. For illumination test, xenon light with an intensity of 10.5 mW/cm2 and a calibrated wavelength of 250-800 nm was used as the light source. The measurement was conducted in artificial seawater as the controlled electrolyte. Standard calomel electrode (SCE) was used as a reference electrode and platinum electrode as a counter electrode. The measurement was performed both with and without irradiated conditions. The microstructures and surface morphology of electrode before and after electrochemical measurements were analyzed and examined by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD).
The results show optimized HAp also improve the photocatalytic activities of n-type semiconductor titanium dioxide. In previous study, removal of HAp was considered due to the addition of carboxymethyl cellulose (CMC) as a viscosity modifier to HAp paste. CMC possess higher hydration rate which may cause rapid agglomeration when it is introduced to water. Although it has satisfactory quality to control the viscosity of the paste, rapid hydration cause removal of HAp after the electrode was heat treated and measurement was performed in artificial seawater. In this study, polyethylene glycol (PEG) was used as the viscosity modifier for HAp paste and Triton-X-100 was used as the surfactant to control the surface tension of the paste. As a result, HAp paste was produced to be screen-printable onto the surface of TiO2 electrode and removal of HAp was minimized compared to the previous study. Moreover, HAp became a good sorbent and co-catalyst for titanium dioxide electrode in its photocatalytic activities.