Exploration of the adsorption of caffeine molecule on the TiO2 nanostructures: A density functional theory study

The first principles were calculated to study the adsorption behaviors of caffeine molecules on the pristine and N-doped TiO2 anatase nanoparticles. Both oxygen and nitrogen in the caffeine molecule can react strongly with TiO2 nanoparticle. Thus, the binding sites were located on the oxygen or nitrogen atom of the caffeine, while the binding site of the TiO2 nanoparticle occurs on the fivefold coordinated titanium atoms. Counting van der Waals (vdW) interactions showed that adsorption on the N-doped TiO2 is more favorable in energy than the adsorption on the undoped one that indicates the high sensitivity of N-doped TiO2 nanoparticles towards caffeine molecules. This condition refers to a dominant effect of nitrogen doping on the adsorption properties of pristine TiO2. The existence of large overlaps in the PDOS spectra of the oxygen and nitrogen atoms of the caffeine and titanium atom of TiO2 represent forming Ti-O and Ti-N bonds between them. The results of molecular orbital calculation demonstrate that the HOMOs are strongly localized on the caffeine. Charge analysis based on Mulliken charges reveals a considerable charge transfer from the caffeine to the TiO2 nanoparticle.