The role of water in the photoinduced deaggregation mechanism for TiO2 nanoparticles

MENDIVE C.; GRANONE L. I.; CURTI M.

Sofia

Conferencia; The 11th Annual Conference on the Physics, Chemistry and Biology of Water; 2016

Nanoparticulate TiO2 is one of the most used semiconductorsin aqueous phase photocatalysis. It is a non-toxic, inexpensive, chemically,and photochemically stable material, that finds many applications, not only forenvironmental remediation, but for disinfection, self-cleaning surfaces, or inthe field of alternative energy sources: for H2 production fromwater splitting1. Since the principle of photocatalysis is basedon the primary step that involves the electron-hole pair generation uponabsorption of supra band-gap photons, the fate of those species is crucial toensure the oxidation and reduction reactions at the surface of thesemiconductor. But because this result is rather scarce as compared to theelectron-hole recombination, investigations of the consequences of that processis highly relevant, as it is accompanied by a release of energy. In particular,if at least part of that energy can be absorbed and used for overcomingenergetic barriers to separate agglomerates, a deaggregation mechanism takesplace. Experimental evidence shows that TiO2 nanoparticles, eitherimmobilized as layers, or dispersed in suspensions, but always immersed inwater, i.e., in intimate chemical contact with liquid water, disaggregate uponirradiation2. Careful thermodynamic measurements of suchprocess yield energies which are in the order of one hydrogen bond3. Assuming that the size of the attaching facesbelonging to two nanoparticles within an aggregate exceeds the extent of oneatom, the fact that both nanoparticles maintained together via only onehydrogen bond is rather improbable. A realistic picture that describes theinteraction between the nanoparticles and a mechanism for deaggregation must includeall actors taking place in the process. Thus, the role of water becomescentral, firstly because the water molecules are chemically absorbed at thesurface of the semiconductor4 and are able to participate in the lightabsorption step through the orbital contribution to the electronic structure ofthe solid5, and secondly, because several layers ofadsorbed water molecules can be built at the surface in an organized manner6 resembling a crystallization/solidification process(the water solidification enthalpy is -6.008 kJ/mol). A fully mechanicalseparation of attached nanoparticles shall be therefore described not only froma solely thermodynamic point of view by considering the energetic requirementsfor deaggregation arising from all energy sources, i.e., the electron-holerecombination and the exothermic formation of structured water, but by thecrystal buildup of water at the surface wedging into the space in between both particles.(1)         Schneider, J.; Matsuoka, M.;Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. UnderstandingTiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014,114 (19), 9919?9986.(2)          Mendive,C. B.; Hansmann, D.; Bredow, T.; Bahnemann, D. New Insights into the Mechanismof TiO2 Photocatalysis: Thermal Processes beyond the Electron - HoleCreation. J. Phys. Chem. C 2011, 115 (40), 19676?19685.(3)          Wang,C.-Y.; Pagel, R.; Bahnemann, D. W. Quantum Yield of Formaldehyde Formation inthe Presence of Colloidal TiO2 -Based Photocatalysts: Effect ofIntermittent Illumination, Platinization, and Deoxygenation. Quantum 2004,108 (37), 14082?14092.(4)          Mendive,C. B.; Bredow, T.; Schneider, J.; Blesa, M.; Bahnemann, D. Oxalic Acid at theTiO2/water Interface under UV(A) Illumination: Surface ReactionMechanisms. J. Catal. 2015, 322, 60?72.(5)          Shapovalov,V.; Stefanovich, E. V.; Truong, T. N. Nature of the Excited States of theRutile TiO2 (110) Surface with Adsorbed Water. Surf. Sci. 2002, 498,L103?L108.(6)          Das, R.; Pollack, G. H. Charge-Based Forces at theNafion?Water Interface. Langmuir 2013, 29 (8), 2651?2658.