Chemistry Reference
In-Depth Information
the high-temperature solvent ODE and heated to
ca.
70
C, followed by
a single injection of precursor followed by heating for up to 5 hours (or longer
if a lower annealing temperature was used). The particles were isolated by
size-selective precipitation, into either chloroform or pyridine. Rods isolated
a
er a single hours growth were found to be
ca.
20 nm long and 5 nm wide,
although the aspect ratio could be increased further by multiple injections.
Prolonged heating times also increased the anisotropy but also generated
cubes and rectangular shaped particles. Interestingly, the shape anisotropy
was not found to signi
d
n
1
y
4
n
g
|
2
ect the magnetic properties of MnP.
A related material, MnAs, has been prepared using the same manganese
precursor, and C
6
H
5
AsO as an arsenic precursor.
95
The use of this precursor
is somewhat surprising, as the related TOPO molecule has generally been
shown to be ine
cantly a
ective as a phosphorus precursor. The reaction can be
carried out at both high and low temperatures, yielding particles with
di
erent crystalline structures. In a typical high-temperature reaction, both
precursors were dissolved in ODE and injected into TOPO at 330
C, followed
by 18 hours growth and isolation by solvent/non-solvent interactions. The
resulting particles exhibited the
b
-MnAs structure, usually unstable at room
temperature. Alternatively, slow heating of the precursors up to 200
Cthen
increasing the reaction temperature to 250
C resulted in
a
-MnAs (hot
injection at 250
C did not yield an isolatable product). Both sets of particles
were
ca.
25 nm in diameter, ferromagnetic and consisted of a crystalline core
and an amorphous shell, clearly visible by electron microscopy.
The synthesis of FeP does not use TOP or P(SiMe
3
)
3
exclusively as phos-
phorus precursors. Single-source precursors, as detailed in Chapter 7, have
also been used in the preparation of the related Fe
2
P.
96
In this case, no
separate pnictide precursor was required, as the compound H
2
Fe
3
(CO)
9
P
t
Bu
possessed both anionic and cationic constituents. The precursor was heated
in trioctyl amine (TOA) and oleic acid to
ca.
315
C followed by 20 minutes
further heating. The precursor underwent a rearrangement to
[Fe
3
(CO)
9
P
t
Bu]
2
and decomposed at
ca.
140
C, forming Fe
2
P particles,
which were precipitated with ethanol a
.
er cooling. The particles exhibited
varying morphologies, depending on experimental conditions, forming rods,
bundles, split rods, X-shaped and T-shaped structures.
A signi
cant number of reports describe the synthesis of the nickel
phosphide system. The rapid injection of bis(1,5-cyclooctadiene)nickel and
TOP in TOPO at 345
C followed by 24 hours stirring resulted in mono-
dispersed Ni
2
P particles,
ca.
11 nm in diameter, which could be isolated
using pyridine and hexane as solvent and non-solvent respectively.
97
Unlike
other reports, the formation of spherical particles was attributed to the rapid
injection of precursors, resulting in the conversion of all reactive monomer
into particles, leaving none for anisotropic particle growth. Several related
reports also describe the synthesis of hollow NiP particles, attributable to the
Kirkendall e
ect.
86,98
In a typical example,
99
Ni(CH
3
COCHCOCH
3
)
2
, OAm,
TOP and ODE were mixed together and heated to 320
C over a 15 minute
period, and le
for 1 hour, followed by cooling and precipitation of the
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