Biomedical Engineering Reference
In-Depth Information
Fig. 4 (a) Chemical structure of tetra-ethyl-ortho-silicate (TEOS). (b) Removal of two oxygen
atoms from TEOS. (c) Mechanism depicting how TEOS chemisorbs onto silanol groups (Si-OH)
on surface. (d) Formation of Si-O-Si bridges by elimination reactions with neighboring
molecules. (Figure adapted from [
40
])
Fig.
4a
. TEOS is liquid at room temperature and slowly hydrolyzes into silicon
dioxide and ethanol when in contact with ambient moisture. In TEOS, the silicon
atom is already oxidized; the conversion of TEOS to SiO
2
is essentially a rear-
rangement rather than an oxidation reaction. The overall reaction for the SiO
2
matrix requires the removal of two oxygen atoms from TEOS as shown in Fig.
4b
.
In this case, formation of SiO
2
matrix was probably the result of TEOS surface
reaction. TEOS chemisorbs onto silanol groups (Si-OH) at the surface, as well as at
strained surface bonds. The mechanism is depicted in Fig.
4c
. TEOS will not adsorb
onto the resulting alkyl-covered surface during intermediate reaction steps, so SiO
2
matrix formation was probably limited by removal of these intermediate surface
alkyl groups. As shown in Fig.
4d
, these groups can undergo elimination reactions
with neighboring molecules to form Si-O-Si bridges. This process quickly occurs
in the ambient reaction conditions: TEOS can be its own oxygen source, and SiO
2
can be deposited in the form of a matrix from TEOS. However, additional oxygen
atoms provided by ZnO increases the matrix formation rate, presumably this is the
cause behind the formation of symmetrically dispersed ZnO QDs embedded in the
SiO
2
matrix.
