Cathodic Behavior and Redox Couples - Electrochemistry of Silicon and Its Oxide - page 272

Figure 6.29 shows, as an example, the effect of surface modification on the sta-
bility of silicon electrode in an aqueous solution. 679 The bare silicon surface is quickly
passivated in aqueous solution under illumination. Coating the electrode with a layer
of either ferrocene or polypyrrole gives an improvement in the stability. The stability
is further improved by a two-layer coating of ferrocene/polypyrrole as shown in
Fig. 6.29.

6.7.1. Metal Deposits

Deposition of a small amount of noble metals such as Cu, Pt, and Au increases
the kinetics of redox reactions on silicon electrodes as shown in Fig. 6.3. 829 Deposition
of equivalent of 1 to 10 monolayers of Pt on silicon surface results in a shift of about
0.2V of the onset potential for hydrogen evolution to the positive direction. 875 Because
the flatband potential does not change with the Pt deposition, the enhanced hydrogen
reaction kinetics is due to the catalytic effect of the deposited metal. The energy levels
of the deposited metal grains are considered to lie in the middle of the band gap and
communicate favorably to the surface states both energetically and spatially. The pho-
tovoltage of -Si coated with sparsely scattered Pt islands has been found to increase

proportionally to the increase in the potential of the redox couple. 178 Noble metal islands
effectively collect photoelectrons and thus prevent the oxidation of the silicon surface
by the photoelectrons.
The surface coverage by a deposited metal is critical for the catalytic effect on
photoactivity. The photoelectrochemical behavior of p- Si coated with a very thin layer
of Pt or Au can greatly change depending on the structure of the metal layer. 413,680 It
has been found that with a uniform coating of Au or Pt, the electrode loses its pho-
toactivity and behaves just like a simple metal electrode. Only when the electrode
is coated with very minute metal islands (5nm wide and separated by 20 nm) does it
cause hydrogen photoevolution at potentials much more positive than those for the

n