Physical origin of diminishing photocatalytic efficiency for recycled TiO2 nanotubes and ag-loaded TiO2 nanotubes in organic aqueous solution

Abstract

Arrays of titania nanotubes (TiO2 NTs) were developed by electrochemical anodization and doped with silver on their surface by photodeposition to achieve TiO2 NTs/Ag. It is found that only anatase TiO2 NTs were formed, with the preferential growth direction perpendicular to the titanium substrate, and with the length and diameter of ~2 µm and 90–120 nm, respectively. The presence of Ag on the surface of TiO2 NTs was also confirmed. The TiO2 NTs and TiO2 NTs/Ag were used as photocatalysts to decolorize the methylene blue (MB) aqueous solution. The photodegradation efficiency (PDE) is as high as 83% for TiO2 NTs and 98% for TiO2 NTs/Ag photocatalysts. This work focused on the investigation of the stability and recyclability of these photocatalysts in terms of efficiency and its physical origin by surface analysis using X-ray photoelectron spectroscopy (XPS). It is found that PDE diminishes from 83% to 76% in TiO2 NTs upon eight recycling runs and from 98% to 80% in TiO2 NTs/Ag upon six recycling runs. The XPS analysis revealed that the physical origin of diminishing efficiency is the carbon contamination on the surface of recycled TiO2 NTs and a combination of carbon contamination and Ag leaching in recycled TiO2 NTs/Ag.