Role of Na+ ion removal on the photocatalytic properties of hydrothermally-prepared TiO2 nanotubes

Nanostructured TiO2 enjoys wide patronage for the remediation of water sources that have been contaminated with organic pollutants due to its excellent photocatalytic properties. This study investigated the role of removal of Na+ ions from hydrothermally-prepared TiO2 nanotubes (TNTs) by washing with dilute hydrochloric acid. The photocatalytic activity of TNTs was tested on the degradation of modelled paraquat dichloride-contaminated water. It was found that the amount of residual Na+ ions after acid washing greatly influenced the photocatalytic properties of TNTs. The Na+ ions had significant effect on the crystal structure of TNTs and the crystal structure varied with the annealing temperature. Hence, the effect of the residual Na+ ions was observed at different annealing temperatures of 500, 700 and 800 °C. It was discovered that TNTs containing negligible Na+ ions demonstrated high photocatalytic activity at 500 °C annealing temperature because it consisted of active crystalline anatase species at this temperature. On the other hand, TNTs with high Na+ ion content showed poor performance at 500 °C due to the presence of amorphous sodium titanate species which could result in rapid electron-hole pair recombination. So, it showed highest photocatalytic activity at 800 °C when the crystallinity had increased. Generally, it can be concluded that TNTs with negligible Na+ ion content demonstrated excellent photocatalytic activity by achieving 77.1% degradation of paraquat dichloride compared to those with high Na+ ion content which achieved 61.1% degradation of paraquat dichloride within 5 h