TiO2 nanotubes modification by photodeposition with noble metals: Characterization, optimization, photocatalytic activity, and by-products analysis

Self-organized TiO2 nanotubes (TNT) with 4µm length were obtained by anodization method and calcined to obtain an anatase crystallite phase. The photocatalysts were further decorated with Au, Ag, Pt or Pd nanoparticles (NPs), varying the metal loads (0.50, 0.75, 1, 2 and 5mol%) by photodeposition. The material characterization confirmed the presence of nanoparticles in the TNTs surface, signs of the localized surface plasmon resonance (LSPR) effect for some samples and a bandgap energy of 3.1-3.2eV. Photoluminescence measurements also confirmed that most of the samples with metallic nanoparticles had the recombination of photogenerated carriers lowered due to the trapping effect of the deposited metal NPs.The photoactivity was evaluated by phenol degradation using UV-Vis or visible radiation. Under UV-Vis, the best samples (2 and 5mol% of Au-TNTs and 0.75, 1, and 2mol% of Pd-TNTs) led to 97% phenol removal in 60min while pristine TNTs reached 92% removal. Under visible radiation, the pristine and the Au-TNTs led to about 3% removal, with the best sample being 2mol% Pd-TNTs with 14% removal in 60min. Similar results were obtained for 1 and 5mol% Pd-TNTs (12% and 13% of phenol removal, respectively). The by-products analysis showed that different degradation mechanisms and kinetic rates occur when different metal types, metal loads or types of radiation are considered.