Novel functionalized elongated titanate nanostructures for emergent pollutants photocatalytic degradation

Abstract

The primary goal of this work was the preparation of new elongated titanate nanoparticles, like nanotubes (TNT) and nanowires (TNW), with improved ability for pollutants adsorption and photo(electro)catalytic degradation. TNT and TNW were prepared using a hydrothermal approach in alkaline medium, starting from TiO2 nanoparticles and amorphous precursors, respectively. The TNT/TNW were modified by ion-exchange and doping with metals (Co, Ru, and Fe/Mn), and by sensitization with Ag nanoparticles and ethylenediamine. The catalytic ability of these new materials for the pollutants photodegradation, including phenol, caffeine, theophylline, and dyes, was also investigated. The results showed that all modified TNT/TNW demonstrated excellent photocatalytic activity for the degradation processes studied. The sensitization process can improve the light absorption on the visible range and the ability for pollutants adsorption and photocatalytic degradation, due to the change imposed on the TNT/TNW surface. The metal modification can impose a light absorption shift to the visible range and/or the introduction of intermediary levels in the forbidden band, reducing the electron-hole (e-/h+) recombination. For the ethylenediamine sensitized samples, it was demonstrated that the N-species improves the photocatalytic activity. The results revealed h+ was mainly responsible for the hydroxyl radical formation, and the production of nitrogen oxidant species was proposed. The results for sensitized samples with Ag nanoparticles, Ag-HTNW, revealed the presence of Ag+ in the interlayers and Ag nanoparticles in the HTNW surface, and the h+ action with highly oxidant species enhances the photocatalytic performance. For the cobalt modified powders, depending on the synthesis methodology and Co/Ti ratio, the dopant can replace Na+ in the interlayers and/or substitute Ti4+ in lattice positions or sit in interstitial sites. The structure and optical behavior are dependent on the metal ions either substituting Ti4+ or replacing Na+. For RuTNW sample, Run+ was detected in the crystalline structure replacing Ti4+ and in the interlayers replacing Na+, and for RuTNT it was only replacing Ti4+. The photocatalytic improvement of these materials was principally to recombination rate reduction, by metal incorporation. The utmost difference between these samples was related to the pollutants’ photodegradation mechanism and intermediates formation/degradation. In this work, the influence of slight metal contaminations on the TNW structural, optical and photocatalytic behavior was also studied. FeMnTNW were obtained due to a reactants’ vestigial contamination. The modification occurred by Ti4+/Fe3+-Mn3+ substitution and by Na+ replacement in the interlayers. The TNW/TNT immobilization in conductive films to be used in pollutants photo(electro)degradation, was also studied. Ru- and Co-modified TNW/TNT were immobilized on a conductive substrate by drop-casting method followed by 1,8- diaminocarbazole electropolymerization and were remarkably effective catalysts in pollutants photo(electro)degradation. The films’ reutilization showed that these are stable and can be used in successive degradation without performance loss.