Abstract
This multi-scale and multi-technique work investigates the adsorption of phosphated species on the TiO2 anatase surface. Our original approach declines for the first time the progressive ethyl substitution of phosphates to identify the structure of complexes formed upon adsorption of these molecules on anatase in aqueous dispersions, under various pH conditions. To quantify the adsorbed amount of these molecules on TiO2, adsorption isotherms were recorded as a function of pH. In parallel, zeta potential measurements were performed to screen the evolution of the TiO2 surface charge in the presence of the phosphated compounds. Lastly, surface complexes structure was characterized using spectroscopic methods: solid-state 31P Nuclear Magnetic Resonance, Attenuated Total Reflectance Fourier Transform Infrared, and Diffuse Reflectance Infrared Fourier Transform Spectroscopy in the near infrared spectral range. Upon decreasing pH, the amount of adsorbed species increases, reaching a maximum of 1.5 phosphorus atom per nm2 at pH 2. Monoethyl-phosphate remains adsorbed in similar amounts to orthophosphate, but di- and tri-ethyl substitutions lead to a sharp decrease of adsorption. Spectroscopic analyses reveal the affinity of othophosphate and monoethyl-phosphate for the anatase surface, with formation of bridging or chelating bidentate complexes, more or less protonated according to pH values.
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