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 Å from AgCl to (AgCl)4, by
0.13 Å to (AgCl)32 but then the changes become small, 0.02 Å 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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