Silicon/Electrolyte Interface - Electrochemistry of Silicon and Its Oxide - page 50

The occurrence of two different boundaries as shown in Fig. 2.4 is due to
the change of the predominant fluoride species from the un-ionized HF molecule to the
ion. 100 Thus, at the low pH end,

stability is determined by

On the other hand, at the high pH end, the stability condition is

In Figs. 2.3 and 2.4, is considered as the predominant species. These diagrams
may vary somewhat when other silicon complexes, such as SiF and are also con-
sidered. 709 Further, in real systems, the boundaries of the stability domains in the
pH-potential diagrams depend as well on the type of which may have different
crystallinity and structures and thus different solubilities, as shown in Fig. 2.5. 304

Solubilities

In the absence of fluoride ions, silicon oxide has low solubility in water and
acidic solutions but it has high solubility in alkaline solutions as shown in Fig. 2.5. 304
In acidic and weak alkaline solutions, the solubility of silicon is independent of pH.
When the total dissolved silicon activity is relatively low silica tends to dis-
solve to form the soluble species which is a weak acid that dissociates appre-
ciably only about 3 pH units above neutrality. On the other hand, at relatively high total
silicon activity silica as a solid is the stable form in nonalkaline solu-
tions. The solubility of silica also depends on the specific type of silica as shown in
Fig. 2.5.