Chemistry Reference
In-Depth Information
shows the case when HF reacts directly with the surface leading to breaking of Si-O
bonds. On the other hand, if the adsorption of hydrogen on the oxygen to form hydroxyl
groups is the first step, the following step to break the Si-O bond may have two pos-
sible paths. In the first path, as shown in Fig. 4.35II, formation of the hydroxyl group
causes polarization and weakening of the underlying Si-O bond, thus facilitating the
attack by a fluoride species.
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In the second path, as shown in Fig. 4.35III, the hydroxyl
group is replaced by the fluoride ion, which results in weakening and breaking of the
underlying Si-O bond.
Osseo-Asare
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proposed an etching model based on the effect of surface charge
on the absorption of solution species. It was postulated that the interface
contains neutral hydroxylated surface groups (Si-OH) which can protonate to give pos-
itively charged sites or deprotonate to give negatively charged sites
depending on the pH. The adsorption of the fluoride ion, as a surface ligand exchange
reaction results in the polarization of the underlying Si-O
bonds. The subsequent detachment of the surface Si-F complex constitutes the disso-
lution event.
The basic idea in Osseo-Asare's model is that when protons, hydroxyl ions,
ligands, or water molecules bind to the surface sites, they cause a polarization of the
underlying silicon-oxygen bonds. With a sufficient number of bound species, the
Si-O-Si bonds become sufficiently weakened to permit detachment of the surface
silicon ion. In the absence of ligands, the dissolution rate is determined by the surface
concentration of protonated species at pH < pzc, of water at and
of deprotonated species at pH > pzc. The surface charge is modulated by the
protons in the solution immediately adjacent to the solid surface according to the
reactions
The specific adsorption of an anion at the oxide/electrolyte surface, which changes the
surface charge, may be viewed as a surface complexation reaction. Thus, fluoride ions
that are adsorbed at the silicon oxide surface centers form Si-F complex:
where is the surface complexation constant. The Si-F bond polarizes the underly-
ing Si-O bonds and this leads to the detachment of the surface silicon center:
4.7.2. Rate Equations
The dissolution in neutral water is proportional to the interfacial area between
silica and water. Since silica has low solubility in neutral water, the dissolution can be
