IR and quantum-chemical studies of carboxylic acid and glycine adsorption on rutile TiO2 nanoparticles.

Nanocrystalline TiO2 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, CH3COOH and HOC(COOH)(CH2COOH)2--but not NH2CH2COOH--adsorbed onto TiO2. The adsorption of HCOOH, CH3COOH and NH2CH2COOH 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 TiO2 are compared and match well with the experimental IR measurements, supporting the bridge-binding geometry of carboxylic-acid adsorption on the TiO2 nanoparticles.