Silver Chloride Clusters and Surface States

Stephan Glaus and Gion Calzaferri
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland

Abstract

Single crystal or very compact AgCl materials are hardly light sensitive. In presence of adsorbed Ag+ ions AgCl precipitates with their correspondingly large surface area, however, lead to the discovery of the photography on paper by Henry Fox Talbot in 1834 and were recently found to act as catalyst for sustained photocatalytic oxidation of water to O2. How large must a cluster be so that the inner atoms can be regarded as bulk? How do the surface atoms differ from inner ones? What is the difference between atoms at the corner, the edge and the plane? What happens upon adsorbing water molecules and solvated Ag+ ions on the AgCl cluster surface? Of what type are the first electronic transitions of such clusters, how large is their oscillator strength and how are they influenced by adsorbed silver cations? Cubic (AgCl)n clusters with n = 4, 32, 108 and 256 have been studied by means of MO calculations and compared with the AgCl molecule and with the infinite AgCl crystal. The Ag-Cl distance was found to increase by 0.35 from AgCl to (AgCl)4, by 0.13 to (AgCl)32 but then the changes become small, 0.02 from (AgCl)108 to (AgCl)256, despite of the fact that the latter still contains 58% surface atoms. The HOMO is made up by Cl lone pairs. It changes little from AgCl to (AgCl)4, then increases smoothly until no significant change is observed after the (AgCl)108. The lowest unoccupied orbitals are of 5s(Ag) type and can be identified as surface state levels (SURS) mainly localized at the corners. The next higher levels extend over the whole cluster. They correlate with the lower edge of the conduction band of the crystal. The charges of the innermost (AgCl)108 species are almost the same as those of the innermost (AgCl)256. These results lead to the conclusion that the (AgCl)108 is sufficiently large for studying the influence of adsorption of an H2O and of Ag+(H2O)2. The largest stabilization of H2O on (AgCl)108 is observed when it is coordinated to Ag+ at a corner site, which is slightly favored with respect to an Ag+ site at the plane. Water coordinated to Ag+ in the plane and on the edge has only minor influence on the SURS and no influence on the HOMO region. However, coordination at the corner shifts the SURS by about 0.5 eV to higher energy. Although the [Ag(H2O)n]+ (n = 2, 4, 6) species have been investigated, the most direct way to study the interaction of solvated silver ions with an (AgCl)n cluster is to choose [Ag(H2O)2]+. We distinguish between a silver site, a chloride site an interstitial site and points in between. The position with the Ag+ of the aquocomplex directly on top of an Ag+ of the cluster was found to be the most stable. The frontier orbital region of the (AgCl)108 is little affected by the adsorbed aquocomplex. However, the 5s(Ag) level shows a bonding interaction with the surface at the most stable position. It is stabilized by interacting with 5p(Ag) which derives from the cluster LUMO region and lies more than 1 eV below the SURS of (AgCl)108, thus forming a new low lying surface state. Investigating the frontier orbital electronic dipole-allowed transitions we found that for (AgCl)108 the HOMO to SURS transition is very weak and that those to the next higher levels are forbidden. In case of [Ag(OH2)2]+ adsorbed on (AgCl)108 electronic dipole allowed transitions, corresponding to a charge transfer from the 3p(Cl) type HOMO of the cluster to the 5s(Ag) level of the aquocomplex, were found to be responsible for the increased photochemical activity observed for such systems.

published in: J. Phys. Chem. B 1999, 103, 5622.


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