In-depth study of organophosphonic acid modification on titanium dioxide
23 August 2017
Campus Drie Eiken, Promotiezaal Q0.02 - Universiteitsplein 1 - 2610 Antwerpen-Wilrijk (route: UAntwerpen, Campus Drie Eiken
Vera Meynen, Frank Blockhuys
PhD defence Annelore Roevens - Faculty of Science, Department of Chemistry
In several applications, materials’ surface properties play a key role in their performance (e.g. separation, sorption, sensors). Through surface modification a (sub)monolayer is grafted on the surface, altering the surface properties without changing the structural material properties. Although many applications for these modified materials have been described, a lack of knowledge about their physico-chemical properties remains. To design materials with properties adjusted to the applications, thorough understanding of the modification conditions’ influence on the surface characteristics is necessary. Hence, the topic of this thesis focusses on methods to control the organophosphonic acids (PA) modification on TiO2.
TiO2 is modified with PA under different synthesis conditions to study the solute-solvent-surface interactions occurring during the modification. The samples are characterised with DRIFT, TGA and 31P-CP/MAS NMR and a correlation between the characteristic signals is made. It was found that for modification in toluene compared to water, a lower propylphosphonic acid (3PA) concentration is required to obtain closed packed monolayers. However, in water also titanium organophosphonates (TiPs) are formed. Furthermore, for 3PA, high grafting densities are obtained in toluene, while water is the preferred solvent for phenylphosphonic acid modification. Sorption and calorimetric measurements after modification show that random grafting of the PAs occurs in toluene while in water patches of grafted PA and patches of hydroxyl groups are formed.
By combining adsorption experiments and isothermal titration calorimetry (ITC), the initial adsorption of PA during modification is investigated. Small changes in binding types are observed after several adsorption points of PA. Furthermore, in this case, the concentration and type of PA does not influence the modification degree, with the maximal adsorbed amount being 1.6 groups/nm². Pyrophosphates and TiPs are observed when analyzing the materials obtained via the stepwise adsorption, while only surface grafting occurs when the synthesis is executed at once, pointing to an influence of pre-adsorbed species on the obtained structural properties.
This Ph.D. demonstrated that the choice of synthesis conditions applied, has a large impact on the final surface properties and the sorption behavior of the obtained materials. Additionally, combining ITC with adsorption experiments is a powerful methodology to study surface grafting.