Chemistry Reference
In-Depth Information
Reaction (15) competes with
At high light intensities, reaction (5.15) dominates, resulting in hydrogen evolution
and the effective dissolution valence of 2. On the other hand, at low light intensities,
reaction (5.16) is the dominant reaction and there is little hydrogen evolution, result-
ing in the effective dissolution valence of 4. However, this reaction scheme does not
explain why transition occurs from (5.15) to (5.16) when the light intensity changes
from high to low.
An explanation for the transition of the dissolution reaction from valence 4 to
valence 2 with increasing light intensity was given in a model proposed by Kooij and
Vanmaekelbergh.
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They assume the presence of an intermediate Si(I), which is an
electron-deficient surface silicon atom and has a catalytic effect on the hydrogen reduc-
tion reaction. The essential steps in the reaction scheme are described by the follow-
ing equations:
The chemical detail of reaction (5.18) is
Reaction (5.17) involves one hole from the valence band and the injection of three
electrons into the conduction band with no hydrogen evolution. Reaction (5.18) requires
the participation of radical Si(I) which is mobile within the layer of surface back
bonds and depends on the availability of holes from the valence band,
in the form of is not stable and will further react to form and
as described by reaction (5.19). The formation rate of Si(I) is proportional to the product
of the surface hole concentration and hole capturing rate constant whereas
electron injection is a thermally activated process, depending only on the electron
injection rate constant At low light intensities, the reaction via electron injection
dominates as the product is smaller than but as the number of photogenerated
holes increases at high light intensities, and thus the concentration of Si(I) increase,
which facilitates reaction (5.18), so that hydrogen evolution becomes the predominant
process.
