Formic acid adsorption and photodecomposition on rutile TiO2 (110) : An in situ infrared reflection absorption spectroscopy study

Abstract: TiO2 based photocatalysis is an emerging green nanotechnology that can be used forremoval of pollutants from water and air. It has had an increased research interest, bothfrom an application and fundamental point of view, during the last decades. Despite thismany elementary processes that occur on the photocatalyst surface are not fullyunderstood and are thus limiting our ability to purposefully manufacture more efficientphotocatalytic materials.In this licentiate thesis, the adsorption geometry and photodecomposition of formicacid on differently prepared rutile TiO2 (110) surfaces were investigated. The surface wasprepared by repeated cycles of Argon ion sputtering and annealing. By modifying thisprocedure either reduced, stoichiometric or oxylated surfaces have been obtained. Thesedifferent surfaces are all well-ordered as evident from the obtained low energy electrondiffraction pattern. In addition, a totally disordered surface was also prepared by Argonsputtering alone. Grazing incidence infrared reflection-absorption spectroscopy (IRRAS)employing polarized light with different azimuthal orientations of the TiO2 single crystalwas used to investigate the binding geometry of formic acid (HCOOH) on the surface.Upon adsorption of formic acid on the TiO2 surface, the molecule is deprotonatedresulting in a formate (HCOO-) and a hydrogen (H+) molecule on the surface. The formatemolecules are mainly bridge-bonded to the Ti5c surface atoms with their molecular axisalong the [001] direction. A minority of the formate species was found to adsorb throughhydroxylated oxygen vacancies (or protonated oxygen atoms) and therefore have differentorientations on the surface. For the disordered surface, it was found that the orientation ofthe formate adsorbates are more or less random since no changes in the IRRAS spectraare seen for the different directions of the single crystal. The adsorption geometry for thedisordered surface is also changed, as seen in the shift of the peak positions in the IRRASspectra. This changed adsorption geometry is attributed to exposures of Ti3+ atoms on thesurface, and is a result of the Ar ion sputtering.Irradiation of the HCOO/TiO2 systems by UV light (365 nm, 2 mW/cm2) showed onlya small change in formate coverage after 100 minutes of illumination. The decrease waslargest on the disordered surface and miniscule on the ordered surface. These results werecompared with those obtained on rutile nanoparticles at ambient conditions. Thecomparison shows that the adsorption geometry for the nanoparticles is similar to that ofthe ordered single crystal surfaces and that the photodecomposition rate is about a factorof 30 higher on the nanoparticles than on the disordered surface. This difference isexpected as the single crystal experiments were performed in vacuum, where the supplyof O2 electron acceptors and OH/H2O donors from the gas phase is limited.These results shows that the rutile TiO2 (110) surface is a good model system forfundamental studies of nanoparticle systems and that the presence of hydroxylated oxygenvacancies and protonated oxygen atoms are important for the reactivity of the TiO2surface and must be included in the description of surface reactions on rutile surfaces.