Chemistry Reference
In-Depth Information
The role of thiols in other systems, such as the III
-
V materials is much
more bene
cial. As-synthesised InP QDs capped with TOPO have a low
quantum yield (<1% at room temperature), which can be greatly improved by
treatment with octanethiol, increasing to
ca.
30%.
105
d
n
1
y
4
n
g
|
6
6.5 Carboxylic Acids
The use of carboxylic acids as capping agents and surfactants in colloids
predates the organometallic route by over 20 years; oleic acid was used as
a capping agent for Fe
3
O
4
as far back as 1971.
106
Oleic acid, CH
3
(CH
2
)
7
CH
]
CH(CH
2
)
7
COOH, is the standard carboxylic acid to be used as a surfactant,
the double bond and associated
'
'
in the alkyl chain is found to be an
essential feature for imparting colloidal stability;
107
the closely related stearic
acid does not stabilise magnetic colloids, but is o
kink
en used in the capping of
QDs. Long-chain fatty acids have also been described as inhibitors in
nanoparticles reactions, slowing down the synthesis.
108,109
What is notable
about the use of carboxylic acids is the fact that they are Lewis acids, unlike
the majority of other capping agents which are generally Lewis bases. One
might arguably state that oleic acid was
rst used as a stabiliser in an
organometallic route in the preparation of an iron colloid, where a solution
of Fe(CO)
5
was sonicated with oleic acid, giving particles of iron
ca.
8nmin
diameter with a narrow size distribution (notably narrower than when the
polymer polyvinylpyrolidone (PVP) was used instead).
110
Oleic acid was then
used in the seminal paper describing the preparation of cobalt nanoparticles,
where it was found to be an extremely tightly binding ligand, and required
the use of a weaker binding co-ligand (an alkylphosphine) to control the
particle size.
111
The use of oleic acid alone inhibited particle growth. X-ray
photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR)
spectroscopy studies concluded that oleic acid chemisorbed to cobalt
nanoparticles in a symmetrical bidentate manner onto a single cobalt atom,
forming Co
.
O bonds.
112
In a similar study on oleic acid-capped FePt parti-
cles, two bonding modes were observed; monodentate (RC(
-
]
O)
-
O
-
M) and
bidentate (RCO
2
-
M).
72
A change in the ligand stereochemistry, from the oleyl
(
cis
) to the elaidyl (
trans
) isomer, was also observed, attributed to the high-
temperature synthesis, and is important when self-assembly of the nano-
particles is considered. Interestingly, oleic acid appears to be able, in some
cases, to coordinate to a nanoparticle surface through the double bond,
making the nanoparticle soluble in polar rather than non-polar solvents, and
allowing phase-transfer reaction without changing the surfactant.
113
This
feature, having an interchangeable binding point, exposing either the alkyl
group or the alkyl group and carboxylic acid group, makes oleic acid
extremely unusual among standard capping agents. The formation of bila-
yers of oleic acid on a particle surface has been reported, where the ligand
has been used to cap the particles, and use as a phase-transfer reagent has
shown that the ligand is extremely versatile and can be used e
ectively with
magnetic and luminescent materials, and is, to date almost unique in its use
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