IR and quantum-chemical studies of carboxylic acid and glycine adsorption on rutile TiO 2 nanoparticles

Abstract

Nanocrystalline TiO 2 powders of the rutile polymorph, synthesized by a sol–gel method, were treated with water solutions containing, respectively, formic, acetic, and citric acid and glycine in order to study the adsorption properties of these organic species. The samples were characterized by FTIR, Raman, powder XRD, and TEM. It was found that HCOOH, CH 3COOH and HOC(COOH)(CH 2COOH) 2—but not NH 2CH 2COOH—adsorbed onto TiO 2. The adsorption of HCOOH, CH 3COOH and NH 2CH 2COOH onto the (110) surface of rutile was also studied by quantum-chemical periodic density functional theory (DFT) calculations. The organic molecules were from the computations found to adsorb strongly to the surfaces in a bridge-coordinating mode, where the two oxygens of the deprotonated carboxylic acid bind to two surface titanium ions. Surface relaxation is found to influence adsorption geometries and energies significantly. The results from DFT calculations and ab initio molecular-dynamics simulations of formic acid adsorption onto TiO 2 are compared and match well with the experimental IR measurements, supporting the bridge-binding geometry of carboxylic-acid adsorption on the TiO 2 nanoparticles.