(1)
Department of Chemistry and Biochemistry, University of Bern
Freiestrasse 3, CH-3012 Bern, Switzerland
Abstract
Silver chloride has been investigated intensively since the invention of
photography on paper by Henry Fox Talbot around 1834. His light sensitive papers
were made by first soaking in sodium chloride and then brushing with an excess of
silver nitrate solution. The key to success was the observation that the paper was
more sensitive to light if there was a deficiency of NaCl. Other essential
observations made by Talbot were that the paper could be stabilized by washing it
with a large excess of NaCl and that it could be sensitized by moderate illumination.
We have made similar observations about 160 years later when studying the ability
of Ag+ containing systems to photo-oxidize water to oxygen under
conditions of a Cl-
deficiency. There are good reasons to assume that the first photographic papers of
Talbot were based on the ability of silver chloride to oxidize water to
O2 in presence of an excess of Ag+. We recently
reported sustained photo catalytic oxidation of water to O2 on AgCl
layers on SnO2:F-coated glass plates modified with a very thin gold
layer.[] The AgCl layer acts as a photo catalyst in the presence of a small excess of
Ag+ ions with a maximum O2 evolution rate at pH
4-5.5. The light sensitivity is due to self-sensitation caused by reduced silver species.
A system that has never been exposed to light is insensitive to the visible part of the
spectrum: After it has once been irradiated in the near UV it becomes sensitive and is
capable to photo-oxidize water in the whole visible range with a lower limit at 600 to
700 nm. The photo catalytic water oxidation can be expressed as follows:
(1)
(2)
We show in Figure 1 an experimental result obtained in a flow system where the produced oxygen was continuously removed by a nitrogen stream as an example. The turnover number for O2 evolution with respect to the total amount of AgCl after 44h when the experiment was stopped was 11 and it is obvious that much larger turnover numbers can be achieved. The photo current (2) in these experiments is of reverse sign with respect to what has been known as Bequerel type photo current. It is well known that very compact or single crystal AgCl materials are hardly light sensitive. Pronounced light sensitivity is observed, however, for AgCl materials with a large surface. Such materials can e.g. be prepared by precipitation or by electrochemical techniques.[1]
Scheme 1
While the electronic structure of AgCl crystals has been investigated in detail
and can be considered as being well understood, little is known about the electronic
structure of AgCl layers, clusters and surfaces. A theoretical study of AgCl clusters
at relevant size is still missing. A reason for this is the fast growing number of atoms
in such clusters as illustrated in Scheme 1 for cubic (AgCl)n clusters.
Nevertheless calculating such systems and especially trying to extract relevant
information from the resulting vast amount of numbers is challenging.
Questions relevant for improving our understanding of the chemical and
photochemical properties of silver chloride nano- and microcrystals are: i) How large
must a cluster be that the inner atoms can be regarded as bulk atoms? ii) How do
the surface atoms differ from the inner atoms? iii) What is the difference between
atoms on the corner, the edge and the plain? iv) what happens if a water molecule
or a solvated Ag+ ion is adsorbed on the AgCl cluster surface? v) 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?
In order to find an answer to these questions we have carried out molecular
orbital calculation of EHMO type on the cubic clusters shown in Scheme 1. The
surface states were analyzed by comparison with an extended AgCl crystal. It turned
out that (AgCl)108 was sufficiently large for studying adsorption of a
H2O molecule and
of an Ag+(H2O)2 complex. The results
presented provide an understanding for the observations that small AgCl crystals
with their correspondingly large surface area behave so differently with respect to
very compact or single crystal AgCl materials.
Scheme 2
Manuscript submitted for publication.
| [1] | Lanz, M.; Schürch, D.; Calzaferri, G. J. Photochem. Photobiol. A: Chem. in press |
