Mechanochemical synthesis of functionalized silicon nanoparticles
with terminal chlorine groups
Steffen Hallmann
Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, Louisiana 70118
Mark J. Fink
Department of Chemistry, Tulane University, New Orleans, Louisiana 70118
Brian S. Mitchella)
Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, Louisiana 70118
(Received 29 September 2010; accepted 17 January 2011)
A facile and efficient, one step method using high-energy ball milling (HEBM) to produce
chloroalkyl-functionalized silicon nanoparticles is described. HEBM causes silicon wafers to
fracture and exposes reactive silicon surfaces. Nanometer-sized, functionalized particles with
alkyl-linked chloro groups are synthesized by milling the silicon precursor in presence of an
x-chloroalkyne in either hexene or hexyne. This process allows tuning of the concentration of the
exposed, alkyl-linked chloro groups, simply by varying the relative amounts of the coreactants. The
silicon nanoparticles formed serve as a starting point for a wide variety of chemical reactions, which
may be used to alter the surface properties of the functionalized nanoparticles.
I. INTRODUCTION
Address all correspondence to this author.
e-mail: [email protected]
DOI: 10.1557/jmr.2011.31
report the utilization of this process to form chloroalkylfunctionalized silicon nanoparticles in a high yield, facile,
and efficient one-step procedure. To our knowledge, this
is the first time that a one-step process to produce
chloroalkyl group–functionalized silicon nanoparticles
has been reported.
HEBM has been shown to form alkyl-passivated
silicon nanoparticles through fragmentation events followed by chemical reaction of the exposed surface.34,35
The alkyl passivation of the silicon surface is obtained
through milling in a reactive liquid medium like alkynes
and alkenes under an inert atmosphere in a stainless steel
milling vial. The reaction of the terminal triple or double
bond with the reactive Si5Si and silicon surface radicals
results in the formation of a covalent Si–C bond, which
prevents further oxidation of the silicon surface.35–38 This
cycloaddition of unsaturated hydrocarbons results in
organic passivated silicon nanoparticles. The milling in
monofunctionalized organic liquids forms silicon nanoparticles that are soluble exclusively in organic solvents.
A high-yield mechanochemical production of chloroalkyl-functionalized silicon nanoparticles accessible for
secondary chemical reactions is presented here. For this
approach, we used 6-chloro-1-hexyne, which has a reactive triple bond and a terminal chloro group. The triple
bond has been shown to react strongly with the reactive
silicon sites during the milling process,34 whereas the
chloro group has been shown to be nonreactive, confirmed
by milling experiments with ω-chloro-hexane. Thus, the
reaction introduces a terminal chloro group linked via an
alkyl chain to the surface of the silicon nanoparticles. The
chloroalkyne can be milled with either hexyne or hexene
1052
Ó Materials Research Society 2011
Silicon nanoparticles attract growing interest because
of their use in optoelectronic devices,1–3 solar cells,4 and
nanoparticle lasers5,6 and as fluorescent biomarkers7–9
with low cytotoxicity. Quantum confinement of electrons
in silicon nanoparticles with a crystallite radius less than
the Bohr exciton radius (5 nm for silicon) results in sizedependent bandgaps10–12 and thus a Stokes shift with high
quantum yield photoluminescence.13–15 A wide range of
methods to produce silicon nanoclusters have been published and can be separated into bottom-up processes such
as inverse micelles,16 plasma,17 metallorganic precursor
decomposition,18–20 solution precursor reduction,21–24 or
electro-chemical reduction25 and top–down procedures
like wafer etching26–30 and laser ablation.31,32 These
methods utilize either multistep reactions or hazardous
chemicals like hydrofluoric acid to functionalize the
surface with reactive alkyl-linked groups. The functionalization of nanoparticles with highly reactive chloroalkyl
groups using these methods remains a challenging task.
Linford et al. previously reported the successful functionalization of porous silicon surfaces with chloroalkyl
groups using a secondary hydrosilylation reaction33 However, this nondirect approach utilizes high temperatures
and strong acids. Previously, we reported that high-energy
ball milling (HEBM) is an alternative route to produce
functionalized silicon nanoparticles.34 In this study, we
a)
J. Mater. Res., Vol. 26, No. 8, Apr 28, 2011
S. Hallmann et al.: Mechanochemical synthesis of functionalized silicon nanoparticles with terminal chlorine groups
as a coreactant. These two a-unsaturated carbon chains
differ in their reactivity because of different heats of
reaction39 and thus have different capabilities to compete
with the triple bond of the terminal chloro-alkyne for the
silicon-binding sites. Here, the statistically favorable
formation of the Si–C bond with an alkyne results in
silicon nanoparticles with different chloroalkyl coverage
when milled with either alkene or alkyne as coreactant.
The advantages of mechanochemical synthesis of silicon
nanoparticles using HEBM include the rapid production of
large quantities in a one-step process and the possibility to
scale up. Furthermore, by varying the relative amounts of
6-chloro-1-hexyne to hexene or hexyne the final concentration of the alkyl linked, terminal chloro group can be
tuned. This is significant for replacement reactions with
biocompatible compounds and functional groups like
amines, which have been shown to induce cytotoxicity
at a high charge density.40 Additionally, by comparing the
relative surface coverage of chloro alkyl groups to methyl
groups from milling experiments using a constant concentration of 6-chloro-1-hexyne in a coreactant mixture
with hexyne or hexane, a preliminary statement about
the reactivity of the alkyne and alkene groups toward the
reactive silicon surface can be made.
Fourier transform infrared (FTIR) spectroscopy was
performed using a Thermo Nicolet NEXUS 670 FTIR
(Thermo Fisher Scientific Inc., Waltham, MA). FTIR was
performed on films of functionalized silicon nanoparticles
deposited on a KBr plate. Nuclear magnetic resonance
(NMR) spectroscopy was conducted in chloroform-d1
using a Bruker Avance 300 MHz high resolution NMR
spectrometer (Bruker Corporation, Billerica, MA). The chemical shifts are referenced to residual peaks from chloroform
present in the used chloroform-d1 solvent at 7.24 ppm. Transmission electron microscopy (TEM) images were taken
with a JOEL 2011 TEM using an accelerating voltage of
200 kV (JOEL Ltd., Tokyo, Japan). Energy dispersive
spectroscopic (EDS) data were obtained in the TEM using
an Oxford Inca attachment (Oxford Instrument, Tubney
Woods, UK) with a 3 nm beam spot on a copper grid.
The photoluminescence data from the nanoparticles in
dichloromethane were obtained using a Varian Cary Eclipse
spectrofluorimeter (Agilent Technologies Inc., Santa Clara,
CA). UV–vis absorbance characteristics in dichloromethane were acquired with a Cary 50 spectrophotometer.
Quantum yields were measured using a multiple concentration method with 9,10-diphenylanthracene as standard.
III. RESULTS AND DISCUSSION
II. EXPERIMENTAL
All chemicals used were purchased from Sigma-Aldrich
(St. Louis, MO) and TCI America (Portland, OR) and used
without further purifications unless specified. Hexyne (97%
assay) was purified by atmospheric distillation. To produce
the chloroalkyl group–functionalized silicon nanoparticles,
0.75 g of crystalline silicon wafer (as obtained from
Silrac, Lexington, KY; undoped, mirror finish, orientation
[111], one to two pieces of 1–4 cm2 surface area) was milled
in various concentrations of 6-chloro-1-hexyne in hexyne or
hexene with a total liquid volume of 25 mL. The stainless
steel milling vial was loaded with the appropriate chemical
components and three stainless steel milling balls (d 5
12 mm) under inert nitrogen atmosphere to prevent oxidative side reaction during the milling process. The milling
was performed for 12 h in a SPEX 8000D high-energy ball
mill in a cold room at 2 °C with an oscillation frequency of
18 Hz (SPEX CertiPrep Ltd., Metuchen, NJ). After the
milling was completed, the vial contents were transferred
to a plastic centrifugation tube and centrifuged for 30 min
at 511 G. The clear supernatant was transferred to a glass
vial, and the solvent was removed under reduced pressure
in a vacuum oven at 25 °C. The oily residue containing the
nanoparticles was then dissolved in organic solvents such
as dichloromethane. The nanoparticle yield of this process
varies between 60 and 120 mg for a 0.75 g batch,
depending on the chemical composition of the organic
liquids and thus reactivity. This equals a nanoparticle yield
of 8–16% to the silicon precursor mass.
The milling in reactive liquids results in a slurry of
silicon particles dispersed in the applied organic milling
medium, with a size distribution in the nanometer and
micrometer range. Purification of this suspension, using
centrifugation, yields a clear, light yellow solution that
contains functionalized silicon nanoparticles in the size
range between 0.5 –and 100 nm with the overwhelming
majority centered at a size of 2.0 nm (Fig. 1).
The EDS of nanoparticles show that these particles
have a high content of silicon [Fig. 1(b)]. The oxygen is
attributed primarily to Si–O species from the precursor
material and the serendipitous reaction of silicon with trace
concentrations of water in the milling medium. The EDS
also indicates that chlorine atoms are associated with these
nanoparticles. NMR spectroscopy from the oily residue
obtained after solvent removal of silicon milled in the
indicated organic media was used to further characterize the
organic monolayer of the nanoparticles (Fig. 2). The
measurements reveal that these chlorine atoms are linked
to the surface of the silicon nanoparticles via the alkyl
chain.
1
This can be seen
in
the
NMR
peaks
at
3.55
ppm
(
H
NMR)
13
and 45 ppm ( C{1H} NMR) for a –CH2– of a primary
chloro group when hexyne or hexene is milled in the
presence of 6-chloro-1-hexyne. This peak is not detectable
in milling experiments without the chloroalkyne.
The NMR spectra indicate a successful reaction of the
triple and double bond of the coreactants to the nanoparticle surface. In the case of alkenes, the 1 lack of
characteristic peaks in the olefinic region in the H NMR
J. Mater. Res., Vol. 26, No. 8, Apr 28, 2011
1053
S. Hallmann et al.: Mechanochemical synthesis of functionalized silicon nanoparticles with terminal chlorine groups
13
FIG. 2. (a) 1H nuclear magnetic resonance (NMR) and (b) C{1H} NMR
spectra of silicon milled in 25 mL hexyne, in 10% 6-chloro-1-hexyne in
hexyne, in 25 mL hexene, and in 3.5% 6-chloro-1-hexyne in hexene. The
1
H NMR spectra are normalized to the alkyl –CH2– at around 1.3 ppm.
1
FIG. 1. (a) Transmission electron microscopy images, (b) size distribution, and (c) energy dispersive spectroscopy analysis of silicon
nanoparticles produced by milling silicon wafer in 20% 6-chloro-1hexyne in hexyne after centrifugation and vacuum drying.
13
and C{1H} NMR are observed for the resultant nanoparticles. Similarly,
there is a lack of alkyne peaks in the
1
13
H NMR and C {1H} NMR spectra of the alkyne-reacted
nanoparticles. The spectra also show the anticipated peaks
1054
for
the alkyl chain at 1–2 ppm ( H NMR) and 20–30 ppm
13
( C{1H} NMR) and for the methyl group at 0.9 and
26 ppm, respectively. From these and previous results,34,41
we conclude two different Si–C linkage modes as illustrated
in Fig. 3.
1
Peaks at around 7–813 ppm in all H NMR spectra and
around 135 ppm in the C{1H} NMR spectra of Fig. 2 can
be attributed to 1,3,5-butyl- and 1,2,4-butyl- and/or
chlorobutyl-substituted benzene isomers, which are
formed from the alkyne components in the organic
medium (cyclotrimerization of alkynes results in aromatic
impurities, possibly through catalytic processes because of
metallic erosion in the milling vial. Results will be
published in a separate paper). These specific impurities
can be minimized by using an alkene instead of an alkyne
coreactant, which results in a decreasing peak area in the
J. Mater. Res., Vol. 26, No. 8, Apr 28, 2011
S. Hallmann et al.: Mechanochemical synthesis of functionalized silicon nanoparticles with terminal chlorine groups
FIG. 3. Idealized schematic illustration of silicon carbon linkage
modes formed by cyclo and aliphatic addition processes during highenergy ball milling by milling in a mixture of 6-chloro-1-hexyne in
hexyne and 6-chloro-1-hexyne in hexene.
1
1
H NMR at ~7–8 ppm in the normalized H NMR spectra
of Fig. 3.
1
Integration of H NMR peaks attributable to α-hydrogen
atoms of the terminal chloro groups and the methyl hydrogens from the alkyne or alkene linked to the silicon nanoparticles were used to further prove that the triple bond of
the organic solvents and not the chloro groups react with the
silicon surface during the fracturing of the particles. If the
chloro groups were favored in the reaction with the silicon,
then the integration area of the primary chloro group peaks
should be smaller than the initial molar concentration of
6-chloro-1-hexyne. In the investigated experiments, silicon
was milled in 6-chloro-1-hexyne in hexyne, which resulted
indeed in nanoparticles with a ratio of the primary chloro
group to the methyl group equivalent to the initial molar
concentration. However, milling in 6-chloro-1-hexyne, this
time in 1-hexene, showed to yield a three times higher ratio
of primary chloro groups to methyl. These results not only
indicate a favorable reaction of the unsaturated carbon chain
over the halogen group but also show the higher reactivity of
the triple bond compared with the double bond.
The FTIR spectra taken from thin films of the same
silicon nanoparticles (Fig. 4) are in agreement with the
results from the obtained NMR spectra (Fig. 2).
All the FTIR spectra indicate the formation of an
organic surface layer on the silicon nanoparticle by strong
ts(C–H) stretching vibrations in the 2800–3000 cm 1
range and weaker scissoring vibrations (d, C–H) around
1460 cm 1. The lack of alkyne C–H and C[C stretching
vibrations and the appearance of both weak C5C stretching vibrations (~1600 cm 1) and a weak vibration for
ts(C–H) vinylic hydrogens (~3500 cm 1) in samples
milled under the presence of alkynes provide further proof
of a reaction of the alkyne group with the silicon surface
and their reduction to a vinylic group. The reaction of the
silicon results in the formation of ts(Si–CH2–) stretching
vibrations 1259 cm 1.42 The presence of a vinylic group is
further supported by the appearance of the out-of-plane
deformation vibration for Si–C5C–Si at 628 cm 1 [Figs.
4(a) and 4(b)] and the out-of-plane CH deformation
vibration at 958 cm 1, which are more intense in samples
milled in higher concentrations of alkynes.43 The existence of a primary chloro group is corroborated by the
characteristic stretching vibration ts(C–Cl) in the samples
milled with a chloroalkyne at around 650 cm 1. Furthermore, the FTIR spectra show bands for Si–H stretching at
2119 cm 1 and Si–O stretching modes including Si–O–Si
at 1048 cm 1 and Si–OH at 849 cm 1 with a moderate
intensity.44–47 The Si–O and Si–H species are primarily
attributed to silicon oxide from the precursor material and
reaction of the reactive silicon surface with trace amounts
of water in the milling medium.48 However, the strong
hydrophobic character, in particular, the excellent solubility in nonpolar solvents, of the obtained sample suggests
a rather negligible influence of these groups on the overall
surface characteristics of the silicon nanoparticles.
The nanoparticles show a blue luminescence under UV
radiation with a quantum yield of 1.25% as an average
value for a polydisperse sample. The samples also exhibit
the characteristic bathochromic shift of the emission
wave length with increasing excitation wavelength
(Fig. 5), typical of a polydisperse samples of nanoparticles.49 The polydisperse nature of the nanoparticle
distribution is also indicated by the TEM image (Fig. 1).
The origin of the photoluminescence typically arises
from particle size–dependent core states, but can be also
influenced by surface states, oxidation, and crystal defects
of the silicon nanoparticles.50–52 The photoluminescence
spectra are in good agreement with previously published
photoluminescence spectra for alkyl-functionalized silicon nanoparticles produced by methods including inverse
micelle procedures,53 solution precursor reduction,54 and
laser and etching derived synthesis, which show a strong
blue luminescence for small silicon nanoparticles of sizes
up to 5 nm.55–58 Milling in the coreactant hexyne [Figs. 5(a)
and 5(b)] instead of hexene [Figs. 5(c) and 5(d)] results in
a red shift of more than 90 nm, whereas the terminal
chloroalkyl group does not seem to have a strong influence
on the luminescence properties. The origin of this shift
J. Mater. Res., Vol. 26, No. 8, Apr 28, 2011
1055
S. Hallmann et al.: Mechanochemical synthesis of functionalized silicon nanoparticles with terminal chlorine groups
FIG. 4. Fourier transform infrared (FTIR) spectra of silicon milled (a) in 10% 6-chloro-1-hexyne in hexyne, (b) in 25 mL hexyne, (c) in 3.5%
6-chloro-1-hexyne in hexane, and (d) in 25 mL hexene. The FTIR spectra are normalized to the –CH2– stretching vibration at 2919 cm 1.
might be caused by electronic interactions because of the
close proximity of the (C5C) double bond and a particle
size distribution variation owing to the different reactivity of
the coreactant. Oxygen states on the surface of the silicon
nanoparticles seem to be an unlikely cause since the FTIR
shows no correlating trend for the Si–O absorbance band at
~1048 cm 1 (Fig. 5); however, they cannot be excluded at
this point.
A. Effect of the process time
The synthesis of x-chloroalkyl-functionalized silicon
nanoparticles using HEBM is performed as a batch process and thus is highly influenced by the milling time. A
mixture of 3.5% 6-chloro-1-hexyne was milled in hexene
for various times, and the resulting composition of nanoparticles and impurities was analyzed. The integration
values for the peaks attributed to the terminal chloroalkyl
group
at 3.55 ppm and the alkyl silicon group at 0.2 ppm in
1
the H NMR spectra performed of resuspended nanoparticles show a steady increase of the peak area with
milling time, whereas the peak areas of the molecular
impurities at ~7–8 ppm decrease, indicating different
formation kinetics (Fig. 6).
1056
The peak areas of the molecular impurities at ~7–8
ppm, however, decrease with milling time. In particular, it
can be observed that, with longer milling times, the
relative concentrations of the impurities decrease further
to an almost undetectable level. We, therefore, conclude
that the formation of the molecular impurities is relatively
rapid and occurs at the beginning of the milling process,
whereas the nanoparticle formation and functionalization
are slower and continue with time. This conclusion is
supported by FTIR measurements, where the elevated
concentration of silicon nanoparticles manifests itself in an
increased Si–C absorbance at around 850 cm 1 compared to
impurity associated absorbance bands at around 1700 cm 1
at longer milling times (Fig. 7).
It can also be seen that the extended silicon surface
area, due to fracturing into the nanometer size regime,
expresses itself in an increased Si–C (~850 cm 1) and
Si–H (~2100 cm 1) absorbance compared with the Si–O
band at around 1050 cm 1 because of the prevention of
further
oxygen passivation. The Si–H can also be found in
1
all H NMR spectra at 4.3 ppm and originates as a byproduct on the surface of the silicon nanoparticles. However,
the concentration of the detected H atoms participating at the
Si–H bond is extremely low and account for only 0.1% of
J. Mater. Res., Vol. 26, No. 8, Apr 28, 2011
S. Hallmann et al.: Mechanochemical synthesis of functionalized silicon nanoparticles with terminal chlorine groups
FIG. 5. Photoluminescence and UV–vis absorption spectra of functionalized silicon nanoparticles obtained from milling (a) silicon in hexyne,
(b) 6-chloro-1-hexyne in hexyne, (c) silicon in hexane, and (d) 6-chloro-1-hexyne in hexene.
1
FIG. 6. Normalized H NMR spectra of the silicon nanoparticles
obtained from milling silicon in 3.5% 6-chloro-1-hexyne in hexene
1
for the indicated times. The H NMR spectra are normalized to the alkyl
–CH2– at around 1.3 ppm.
the overall hydrogens detected. The Si–H species are
persistent because of the protection by the surrounding alkyl
chains, which prevents access to this highly reactive moiety
and impedes reaction with oxygen and water.
The NMR and FTIR spectra indicate an increase of the
nanoparticle concentration with milling time. This observation is in agreement with the measurements of the
overall mass of the dried nanoparticles accumulated in
the organic milling medium (Fig. 8).
FIG. 7. Normalized FTIR spectra of the silicon nanoparticles obtained
from milling silicon in 3.5% 6-chloro-1-hexyne in hexene for the
indicated times. The FTIR spectra are normalized to the –CH2–
stretching vibration at 2919 cm 1.
The data show a linear increase in the mass of the
silicon nanoparticles up to a milling time of 12 h. The
slope of the nanoparticle formation after this time starts to
decrease and an asymptotic approach to the maximal
J. Mater. Res., Vol. 26, No. 8, Apr 28, 2011
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S. Hallmann et al.: Mechanochemical synthesis of functionalized silicon nanoparticles with terminal chlorine groups
mass of silicon nanoparticle soluble in the given volume
of organic medium can be observed. These results
indicate that the optimal milling time for the introduced
process is 12 h because at this time a high yield in
a reasonable time can be obtained with a minimal amount
of impurities.
B. Tuning the chloroalkyl surface coverage
The mechanochemical synthesis of chloroalkylfunctionalized silicon nanoparticles also has the advantage of fine tuning the percent coverage of the terminal
alkyl-linked chloro groups of the silicon nanoparticles.
This can be achieved by the variation of the concentration
of x-chloroalkyne as the coreactant.
There is an intrinsic difference in the reactivity of an
alkyne and an alkene to the surface of a silicon nanoparticle. This difference manifests itself in the surface
coverage of terminal chloroalkyl groups milled in different concentrations of 6-chloro-1-hexyne in hexene or
hexyne. The lower reactivity of the double bond from the
hexene compared with the triple bond of the chloroalkyne
results in a higher coverage of chloroalkyl groups on the
nanoparticle compared to the starting molar ratio of the
two reactants. Conversely, the coverage of chloroalkyl
groups should be nearly equal to the initial molar ratio of
the chloroalkyne when milled in a hexyne-based system,
assuming a statistical reactivity. This is indeed observed
for a variety of different reaction conditions, in particular,
various silicon precursor masses, milling times, and
concentration of 6-chloro-1-hexyne. The milling of silicon in a mixture of various 6-chloro-1-hexyne concentrations in hexene for 4 h resulted in silicon nanoparticles
with different degrees
of chloroalkyl group coverage as
1
determined by H NMR (Fig. 9). This increasing coverage
is indicated by the increasing chloroalkyl peak at around
3.55 ppm and a decreasing methyl peak at around 0.90 ppm,
which is due to the suppression of the hexene moiety by the
higher 6-chloro-1-hexyne concentration.
To evaluate the surface coverage, the integration
values of the chloroalkyl group at 3.55 ppm to the
methyl group at 0.89 ppm of silicon nanoparticles formed
in different concentrations of chloroalkyne were compared (Fig. 10). For this approach, the values obtained
from silicon milled in various mixtures of 6-chloro-1hexyne in hexene for 4 h and additionally the values from
silicon milled in the same mixtures for 12 h and in
mixtures of 6-chloro-1-hexyne in hexyne for 12 h were
compared.
Figure 10 shows that milling silicon in a mixture of 6chloro-1-hexyne in hexene (.) yields nanoparticles with
a chloroalkyl group coverage, which exceeds the initial
molar ratio indicated by the straight line by 7.5–10%. This
increased coverage of chloroalkyl groups is due to the
higher reactivity of the triple bond compared with the
double bond of the coreactant. Milling silicon in mixtures of
6-chloro-1-hexyne in hexyne (j), however, results in
a chloroalkyl group coverage equivalent to the starting
concentration, as both coreactants are alkynes. These results
further prove that milling in an alkene-based system not
only reduces the formation of tributyl benzene impurities
but also increases the surface coverage of the compound
with the higher reactivity. Figure 10(b) also shows that the
initial mass of the silicon wafer starting material has no
influence on the resulting surface coverage of primary
chloroalkyl groups. This observation suggests that the rate
of silicon surface exposure is the rate-limiting step.
1
FIG. 8. Total yield of silicon nanoparticles obtained from milling
silicon in 3.5% 6-chloro-1-hexyne in hexene for various times. The
standard deviation of the yields is approximately 63%.
1058
FIG. 9. Normalized H NMR spectra of the silicon nanoparticles
obtained from milling silicon in the indicated molar concentration of
1
6-chloro-1-hexyne in hexene for 4 h. The H NMR spectra are
normalized to the alkyl –CH2– at around 1.3 ppm.
J. Mater. Res., Vol. 26, No. 8, Apr 28, 2011
S. Hallmann et al.: Mechanochemical synthesis of functionalized silicon nanoparticles with terminal chlorine groups
ACKNOWLEDGMENTS
This work is supported by the National Science
Foundation (NSF Grant CMMI-0726943) and by the
Tulane Institute for Macro-Molecular Engineering and
Sciences (TIMES, NASA Grant NNX08AP04A).
REFERENCES
FIG. 10. 1H NMR study of the chloroalkyl group coverage at the
nanoparticle surface produced by (a) milling 0.75 g of silicon in various
concentration of 6-chloro-1-hexyne in hexyne (j) and in hexene (.)
and (b) milling 2.5 g of silicon in various concentrations of 6-chloro-1hexyne in hexene (m). The straight line indicates the coverage equal to
the starting molar ratio.
IV. CONCLUSION
We have demonstrated that mechanochemical synthesis via HEBM is a facile and fast method to simultaneously produce and functionalize silicon nanoparticles
with reactive chloroalkyl groups. Furthermore, we have
shown that not only the amount of molecular impurities
can be supressed, but also an increased surface coverage
of the silicon-bound chloroalkyl groups of the nanoparticles can be obtained by variation of the coreactants.
This is particularly important for the synthesis of
chloroalkyl-functionalized silicon nanoparticles because
bifunctional organic molecules are often costly. The terminal chloro groups at the termini of alkyl chains on the
silicon nanoparticles are highly reactive and thus suitable
for a wide range of organic reactions.
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