Nanostructured surfaces
Outer- and intraspheric complexes of phosphoric acid on hydrated anatase surface
O.V. Filonenko, E.M. Demianenko, A.G. Grebenyuk, V.V. Lobanov
Chuiko Institute of Surface Chemistry, NAS of Ukraine,
17 General Naumov Str., Kyiv 03164, Ukraine
E-mail: oksana.filonenko@isc.gov.ua

Nanoscale forms of anatase (TiO2) are widely used in many fields, including medicine, pharmacology and photocatalysis, due to their properties such as chemical and optical stability, non-toxicity and low cost. It is extremely important to elucidate the nature of the interaction of surface groups of anatase with functional groups of biological objects. Due to the high content of phosphate groups in the composition of various biological objects, it is important to study the interaction of phosphoric acid molecules with the hydrated surface of anatase.
Quantum chemical modeling has been carried out of orthophosphoric acid adsorption centers on the hydrated surface of anatase by the method of density functional theory (exchange-correlation functional PBE0, basis set 6-31 G (d, p)). The effect of the aqueous medium was taken into account within the continual model of the solvent. The cluster approach is applied in the work. The surface of anatase is represented by a neutral cluster of Ti(OH)4(H2O)2.
The results of an analysis of the geometry and energy characteristics of all the calculated extraspheric complexes show that the greatest interaction energy is inherent in the intermolecular complex of orthophosphoric acid and hydrated anatase surface, where the oxygen atom of the phosphoryl group (O=P≡) forms a hydrogen bond with the hydrogen atom of coordinated water molecule of the Ti(OH)4(H2O)2 cluster whereas two hydrogen atoms of the hydroxyl groups of the orthophosphoric acid molecule form two hydrogen bonds with two oxygen atoms of the titanol groups (binding energy Eb = -134.0 kJ/mol). The formation of intermolecular charge-separated complexes due to proton transfer from the molecule H3PO4 to a cluster of Ti(OH)4(H2O)2 with the formation of dihydrogen phosphate anion and the protonated form of the titanol group is less energy efficient (Eb = -131.1 kJ/mol).
Two cases of the interaction between titanium and phosphorus oxospecies have been examined: the first one involves an attack of a phosphoryl group on a titanium atom, another case deals with a P-OH group reacting with a Ti-OH one. Among the considered intraspherical complexes, the most energetically advantageous is the covalently bound complex with a Ti–O–P link formed due to cleavage of a water molecule (Eb = -215.67 kJ/mol).