User:Memcbr/sandbox/Reductive Dissolution (draft)

Reductive dissolution
Reductive dissolution results from a redox event in solution which reduces a cation in a transition metal oxide or hydroxide. Dissolution occurs when the reduced cation is unstable in the solid material. In minerals such as ferric oxides, reduction may be caused by electron transfer from organic molecules or bacteria in anoxic waters or soils. Charge carriers responsible for reductive dissolution may also be introduced by photoexcitation or by electrochemical poising at negative potentials. Reductive dissolution is integral to natural geochemical phenomena such as the iron cycle.

Using ferric oxide (Fe2O3) as an example, the fundamental formula of reductive dissolution is:
 * {Fe^3+_(s)} + {e^-} -> {Fe^2+_(aq)}

Here, an {Fe^3+} cation at the oxide surface captures an electron ({e^-}), converting the cation to {Fe^2+}. However, {Fe^2+} is unstable in the oxide lattice relative to the solution and is subsequently solvated.

Reductants causing reductive dissolution include natural electron donors such as ascorbic acid and {Fe2+_(aq)}. Chelating species such as oxalate accelerate the process by detaching surface-bound {Fe^2+}, opening surface sites for further attack by reductants. Reductive dissolution is also promoted by light.

Reductive dissolution does not necessarily occur at the site of reductant adsorption, particularly for conductive specimens. Excess electrons injected into a hematite particle during a redox event can travel through the particle, causing reductive dissolution elsewhere on the particle. The transport of charge across a hematite particle is driven by differences in the surface potential of different crystal terminations.

Photocorrosion
Photocorrosion is the light-induced degradation of semiconductor materials used as electrodes in photoelectrochemical cells. This can occur when photoexcited charge carriers change the oxidation state of surface atoms or ions, destabilizing the material. Materials with smaller bandgaps which can absorb larger regions of the solar spectrum are more susceptible to photocorrosion. In photocatalytic water splitting using a cadmium sulfide photoelectrode, for example, it is desired that holes (<ce>{h^+}</ce>) generated in <ce>{CdS}</ce> by absorption of photons will oxidize hydroxyl species in solution:
 * <ce>2{h^+}+{OH^-}->{1/2O_2}+{H^+}</ce>

However, in a competing pathway, holes may instead degrade <ce>{CdS}</ce>:
 * <ce>2{h^+}+{CdS}->{Cd^2+}+{S}</ce>

The photocorrosion of some photo-absorbing electrodes can be mitigated by using protective thin film coatings.