Effect of Different Hydroxyapatite Coating on Photocatalytic Characteristics of Titanium Dioxide Photoanode

Abstract

The photocatalytic n-type semiconductor characteristics of titanium dioxide (TiO2) has been under study for decades. We have studied the photocatalytic effect of titanium dioxide electrode such as the effect of impurities on TiO2, and the effect of heat treatment on TiO2. We also have studied the effect of dye sensitizing on TiO2 electrode with seawater as the electrolyte. In that study, we found that the hydroxyapatite (HAp) coating used for the purpose to increase the binding ability between dye and TiO2 improved the capability of TiO2 electrode. In order to clearly know the mechanism of HAp coating and to improve the effect of HAp coating on TiO2 electrode, this study is conducted by preparing different types of HAp paste for coating on TiO2 electrode. Screen printing method was used for TiO2 layer and both manual and screen printings were used for HAp (Ca10(PO4)6(OH)2) coating for comparison. Type 329J4L stainless steel was used as a substrate for TiO2 electrode. The surface of the substrate was cleaned with ultrasonic cleaner and then the surface was passivated with HNO3 solution for 30 minutes. Double layers of TiO2 were screen printed on the substrate. The first layer was screen printed and heat treated at 150 ºC for 60 minutes. The second one after printed was heat treated at 550 ºC for 30 minutes. HAp was coated on TiO2 doubled layer. HAp paste was heat treated after printed. HAp paste was prepared from HAp powder. The powder was mixed with carboxymethyl cellulose (CMC) in various ratios. Water and alcohols are added to the mixture and well stirred to get a paste. Different types of HAp paste were prepared by changing the ratios of the constituents of the paste. The finishing was performed to get the electrodes ready for measurements in artificial seawater. The potential measurement, polarization and power density measurements against platinum counter electrode were performed by a potentiostat in dark and irradiated conditions. The irradiation used for the experiments is a xenon lamp with 150 W and calibrated wavelength range from 250 nm to 800 nm. The lamp was calibrated so that the light intensity produced became approximately 10.5 mW/cm2. The surfaces of the electrodes before and after measurement were analysed and the microstructures of the electrodes were compared by scanning electron microscope (SEM). The photocatalytic effects of HAp were observed but the cause is still unclear. The study showed that screen printing of HAp on TiO2 electrode showed higher photopotentials than manual printing or without printing. The variations in HAp paste constituents also affected the performance of TiO2 electrodes. The power density output of TiO2 can be increased more than 2 times with HAp coating. Removal of HAp particles from the electrode surfaces were observed. Further study on new techniques for HAp paste preparation and film forming process is suggested.