Silver chloride clusters and surface states

Single-crystal or very compact AgCl materials are hardly Light sensitive. I n presence of adsorbed Ag+ ions, AgCl precipitates with their corresponding ly large surface area, however, lead to the discovery of photography on pap er by Henry Fox Talbot in 1834 and were recently found to act as a catalyst for sustained photocatalytic oxidation of water to O-2. How large must a c luster be so that the inner atoms can be regarded as bulk? How do the surfa ce atoms differ from inner ones? What is the difference between atoms at th e corner, the edge, and the plane? What happens upon adsorbing water molecu les and solvated Ag+ ions on the AgCl cluster surface? Of what type are the first electronic transitions of such clusters, how large is their oscillat or 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 infinit e AgCl crystal. The Ag-Cl distance was found to increase by 0-35 Angstrom f rom AgCl to (AgCl)(4) and by 0.13 Angstrom to (AgCl)(32), but then the chan ges become small, 0.02 Angstrom from (AgCl)(108) to (AgCl)(256), despite th e fact that the latter still contains 58% surface atoms. The HOMO is made u p of Cl lone pairs. It changes, little from AgCl to (AgCl)(4), then increas es smoothly until no significant change is observed after(AgCl)(108). The l owest unoccupied orbitals are of the Ss(Ag) type and can be identified as s urface state levels (SURS) mainly localized at the corners. The next higher levels extend over the whole cluster. They correlate with the lower edge o f the conduction band of the crystal. The charges of the innermost (AgCl)(1 08) 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 coordina ted to Ag+ at a corner site, which is slightly favored with respect to an A g+ site at the plane. Water coordinated to Ag+ in the plane and on, the edg e 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 hig her energy. Although the [Ag(H2O)(n)](+) (n = 2, 4, 6) species have been in vestigated, the most direct way to study the interaction of solvated silver ions with an (AgCl)(n) cluster is to choose [Ag(H2O)(2)](+); We distinguis h 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 Sp(Ag) which derives from the cluster LUMO region and lies more than 1 eV below the SUR S of (AgCl)(108) thus forming a new low-lying surface State. Investigating the frontier orbital electronic dipole-allowed transitions, we found that f or (AgCl)(108) the HOMO-to-SURS transition is very weak and that transition s to the next higher levels are forbidden. In the case of [Ag(OH2)(2)](+) adsorbed on (AgCl)(108) electronic dipole al lowed transitions, corresponding to a charge transfer from the Sp(Cl)-type HOMO of the cluster to the Ss(Ag) level of the aquocomplex, were found to b e responsible for the increased photochemical activity observed for such sy stems.