Adsorption of carbon monoxide on Si_xGe_4–x(x = 0–4) nano-clusters: a hybrid meta density functional study

Abstract

Theoretical study of carbon monoxide adsorption on Si _x Ge_{4 − x} (x = 0–4) nano-clusters has been carried out using advanced hybrid meta density functional method of Truhlar (MPW1B95). MG3 semi-diffuse (MG3S) and correlation consistent valence basis sets with relativistic core potential were employed to improve the results. The agreement of the calculated ionization and dissociation energies with experimental values validates the reported structures of nano-clusters and justifies the use of hybrid meta density functional method. The geometry, adsorption energy, charge distribution, and vibrational frequency of CO adsorption on all possible structures were investigated. The maximum vibrational frequency changes occur in the bridge structures while the most stable structures occur when CO adsorbs on one silicon atom in a flat surface. The changes of spin densities arising through bridged structures with higher spin multiplicities were rationalized. Adsorption energies of CO on one Si atom are by far more negative than the corresponding value for on Ge atom, at the highest being nearly −77 and −35 kJ mol⁻¹. Comparison was made of adsorbed CO bridging neighbouring and diagonal Si atoms and the former was more stable, having adsorption energy of nearly −77 kJ mol⁻¹. Flat surfaces adsorb CO more favourably. Exhaustive vibrational frequency analyses were performed to confirm the local minima energy of all optimized structures.

Keywords:

• cluster
• HMDFT
• adsorption
• MPW1B95
• silicon

Acknowledgement

We thank Sharif University of Technology for financial support.