Water adsorption on Ti-doped silicon clusters

Ab initio calculations have been performed on adsorption of H₂O molecules on Ti-doped silicon clusters TiSi_n using the ultrasoft pseudopotential method within the generalized gradient approximation. Our results show that for n=13 and larger clusters adsorption of H₂O on TiSi_n could be difficult due to low binding energies. All these clusters have cage structures with the metal atom surrounded by the silicon atoms. On the other hand, smaller clusters with n < 13 have the metal atom partially covered by Si atoms in a basket structure so that it is available for reaction with a water molecule. This leads to significantly higher binding energies of a water molecule on such clusters. These results are in excellent agreement with the available experimental data, which show significant decrease of H₂O adsorption on clusters with n > 12.