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

Abstract

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