Adsorption of β-diketones on a surface of ZnO nanopowder: Dependence of the adsorbate on the diketone structure

Surface modification has been established to control chemical, mechanical, and electronic properties of oxide surfaces. Surface chemistry of β-diketones on ZnO nanomaterials presents an opportunity to investigate the dependence of the adsorbate structure on the type of diketone and, specifically, on the presence of electron-donating and electron-withdrawing functional groups. This work compares the adsorption of 1,1,1-trifluoro-2,4-pentane-dione (trifluoroacetylacetone, tfacH) and 1,1,1,5,5,5-hexafluoro-2,4-pentane-dione (hexafluoroacetylacetone, hfacH) on ZnO nanopowder by interrogating the molecular structure of adsorbates with spectroscopic and computational methods. Despite the fact that in the gas phase the enol structure dominates for hfacH and the diketone has substantial presence for tfacH, once these compounds are adsorbed on ZnO, the diketonate is the majority of surface species for hfacH and dissociated enolate is dominant for tfacH. Moreover, given the amphoteric nature of ZnO, it is proposed that on a surface of basic oxide, the O-H dissociation of the enol form could be driven to completion for hfacH, and this proposal is confirmed by comparing chemistry of hfacH on ZnO and MgO surfaces.