Peculiar adsorption induced by strong hydrogen bonds on perfect anatase (001) surface

Titanium dioxide is a material with a wide range of applications in several fields, and understanding its interaction with molecules is fundamentally important for improvements of the various performance of materials. One important point not always sufficiently considered is the surface coverage of adsorbates on TiO2 surfaces. In this work, we studied NH3 adsorption at different coverages and cell sizes on the anatase (001) surface by means of density functional theory (DFT) calculations. The structural differences for given coverages of 0.25 at 1/4 and 4/16 ML are very significant depending on the cell size. Upon adsorption, there are Ti-O bond breaking, and enhanced hydrogen bondings between NH3 and surface oxygen at only 4/16 ML coverage. The result shows that the use of a small cell cannot reproduce the surface relaxation due to the large displacement of underlying O atoms and surface Ti-O bonds breaking. Bond breaking and reformation of Ti-O bonds, and desorption of NH3 have been observed in ab initio molecular dynamics simulations. Similar structural relaxation was also confirmed with H2O and H2S adsorption. It was revealed that the adsorptions are induced by the high reactivity of the anatase surface and hydrogen bonding with adsorbates. The strong hydrogen bonding effects causing chemical bond breaking challenge conventional surface chemical bonding mechanisms.