Chemistry Reference
In-Depth Information
thus a slow process, in agreement with the fact that very few materials yield a high
exchange current density for the redox couple.
1155
The exchange current density for hydrogen reduction is in the range of
depending on the specific conditions of the silicon/electrolyte system. For
example, it is on the order of
on
n
-Si in 5% HF solution.
254
In 2% HF,
depending on doping levels.
146
The
it varies from
to
exchange current of illuminated -Si appears to be much higher than that of -Si in the
on illuminated
p-
Si in 0.1M NaCl solution and
changes very little with light intensity.
869
Similar values are found on illuminated
p-
Si
in 2% HF.
146
For comparison, the exchange current of hydrogen evolution in acidic
solutions on Pt is on the order of
p
n
dark. It is about
representing a fast reaction, and it is on
1155
on Pb, representing a very slow reaction.
the order of
The rate of hydrogen evolution varies with solution composition and in particu-
lar with pH. It is higher at neutral pH values than at acidic pH, which is not consistent
with that observed on Pt.
812
The kinetics may also greatly change due to the adsorption
of solution species or presence of solid phases on the surface. For example, hydrogen
evolution in
solution due to the chemisorption of HCOOH on the electrode surface.
688
on illuminated
p-
Si is inhibited by addition of formic acid in the
The reaction kinetics is particularly sensitive to trace amounts of noble metal
impurities in the solution such as Cu and Au, which tend to deposit on the silicon
surface. For example, an increase of the exchange current density from
to is measured with 15 ppb copper ions as contaminant in the solution.
254
On the other hand, deposition of metals such as Pb and Cd, which have lower exchange
currents than silicon, inhibits the hydrogen evolution.
688
Hydrogen evolution can be catalyzed through deposition of a layer of metal or
polymer on the silicon surface or by addition of a redox couple in the solution.
553,848,875
