Surface reactions at nanoparticulate TiO2 inverse opals

MENDIVE C.; CURTI M.; GRELA M.A.; IVANOVA I.; SCHNEIDER J.; BAHNEMANN D.

Mainz

Conferencia; Ostwald-Kolloquium "Particles@Interfaces"; 2014

Max Planck Institute of Colloid Sciences

Water and air treatment by means of Advanced Oxidation Processes (AOP) using wide band-gap semiconductors is a research topic which has been extensively addressed to optimize the photocatalytic performance of the materials for technological applications. Because such approaches have advanced leaving slightly behind fundamental explorations, the promising prospects become nowadays in areal need of a breakthrough to fulfill the expectations. In this sense is that the new and creative projects investigating the variety of interesting open questions are mostly focused on fundamental and mechanistic aspects underlying the surface and photocatalytic reactions taking place at the semiconductor-liquid or -gas interface.In the particular case of TiO2, one of the most exciting challenges is to extend its absorption to the visible region in order to efficiently take advantage of the solar light. One interesting strategy is to structure the semiconductor as photonic crystals in order to use slow photons within the spectral range where the semiconductor absorbs poorly. Of special interest are systems of compact packed air spheres in a semiconductor particles matrix, e.g., TiO2nanoparticles electronically well coupled by suitable interfacial (surface)interactions.In this work, mechanistic aspects concerning the photocatalytic activity of TiO2 are studied ranging from the strong interactions within the most primary steps, between the incoming photons and the semiconductor, e.g.,the electron-hole pair creation and recombination, to the weak surface-surface interactions among the nanoparticles, contributing to the all-in-all photocatalytic transformation of the pollutant at the interface. Within this mechanistic framework, the enhanced photocatalytic activity of nanoparticulate TiO2inverse opals can be explained as the resultant of at least four mechanisms promoting the photocatalytic activity in the systems. Methylene blue, acetaldehyde, and TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl) were chosen for the photocatalytic tests as model compounds with different degrees of complexity regarding the molecular structure and adsorption possibilities.The results suggest that for a given TiO2 particle size, while retarded electron-hole recombination favors the AOP, the capability of the system to encapsulate an amount of molecules thus hiding them from the action of light could compensate, on average, the observed photonic efficiency. At the same time, the antenna and deaggregation mechanisms operating in the system can promote a partial loss of the photonic structure hindering the enhancement of the photocatalytic activity.