Supporting Information
© Wiley-VCH 2007
69451 Weinheim, Germany
Covalently functionalized cobalt nanoparticles as a platform for
inexpensive magnetic separations in organic synthesis
Robert N. Grass, Evagelos K. Athanassiou and Wendelin J. Stark*
Dipl. Chem.-Ing. R. N. Grass, Dipl. Chem.-Ing. E. K.
Athanassiou, Prof. Dr. Wendelin J. Stark
Institute for Chemical and Bioengineering
Department of Chemistry and Applied Biosciences
ETH Zurich
Wolfang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland
Fax: (+) 41 44 633 10 83
E-mail: [email protected]
Homepage: www.fml.ethz.ch
S1
Experimental:
Carbon coated cobalt nanoparticles. Flame spray pyrolysis proceeds by the combustion of a suitable
combustable metal containing organic precursor.[1, 2] The precursor (e.g. cobalt carboxylate) is dispersed
by an oxygen jet forming a spray, which is subsequently ignited by a premixed flame. In a conventional
spray reactor (Fig S1, left), the precursor combusts to H2O, CO2 and metal oxide nanoparticles. In order
to prepare metallic nanoparticles, the flame may be operated in a nitrogen filled glove box under oxygen
limitation (Fig S3, reducing flame synthesis).[3-5] In order to ensure stable combustion conditions, the
flame was encased in a double walled tube, allowing the addition of reactive gases (Fig. S1, right and Fig.
S3). Under oxygen limitation the combustion reaction yielded CO and H2 instead of CO2 and H2O and the
metal oxide nanoparticles were simultaneously reduced to the metal (See Fig S2). Addition of acetylene
through the side walls of the porous tube (see Fig. S3) allowed the controlled coating of the nanoparticles
by depositing carbon.
Figure S1. Photograph of a burning spray flame producing oxidic nanoparticles (left). In order to
produce metallic nanoparticles (right) the flame was operated in a glove box and encased in a doublewalled tube (reducing flame synthesis, Fig. S4).
S2
Figure S2. Sketch of subsequent transformations from precursor to oxide, metal and carbon coated metal
nanoparticles during reducing flame synthesis: The precursor evaporates and combusts as extensively
investigated by Heine and Pratsinis[6, 7] yielding oxide nanoparticles. These can be successively reduced
by hydrogen and CO to metallic nanoparticles.[3, 4, 8] During the whole process the nanoparticles grow by
aggregation and sintering.[9, 10] The metal nanoparticles can be further coated by carbon layers through
addition of acetylene.[11]
S3
Applied experimental set-up: A spray nozzle[12] was placed in a glove-box (2 m3, Fig. S3)[4] filled with
nitrogen (PanGas, 99.999 %). The glove box atmosphere was constantly recycled (recycle stream ~20
m3/h, Busch, Seco SV1040CV), removing H2O and CO2, which was formed during combustion from the
recycle stream, using 2 zeolite columns (zeolite 4A and 13X, Zeochem). While fresh nitrogen (PanGas,
99,9999 %, ~2 m3/h) was added, a purge stream (see Fig. S1) avoided the accumulation other combustion
related species (NOx, H2, CO) in the glove-box atmosphere. Cobalt (II) 2-ethylhexanoate in mineral spirit
(Aldrich) was diluted 2:1 (weight/weight) with tetrahydrofurane (Fluka, tech.) and filtered (Satorius,
fluted filter type 288) prior to use. The cobalt carboxylate precursor was delivered to the spray nozzle by
a micro annular gear pump (6 ml min-1, HNP Mikrosysteme, mzr-2900) where it was dispersed by oxygen
(5 l min-1; PanGas tech.) and ignited by a premixed pilot flame (CH4: 1.2 l / min, O2: 2.2 l / min, PanGas
tech.). The flame was encased in a porous tube (Fig. S1 right and S3) allowing optimal combustion
conditions[3], the inflow of additional nitrogen (PanGas, 99.999 %, 45 l / min) and the addition of
acetylene gas (PanGas, 5 l /min) for the formation of carbon shells. The product particles were separated
from the off-gas using glass fiber filters (Schleicher & Schuell, GF6).
Figure S3. Sketch of the experimental set-up for the fabrication of carbon coated cobalt nanoparticles at
production rates of up to 30 g / hr. The flame is encased in a porous tube enabling the addition of inert
cooling gases and acetylene for the deposition of the carbon layers. The flame is operated in a glove box
in an N2 atmosphere at an oxygen concentration of below 100 ppm (vol/vol).
S4
Powder Analysis. The nanoparticles were analyzed by X-ray diffraction (Stoe STADI-P2, Ge
monochromator, CuKα1, PSD detector), nitrogen adsorption (Tristar Micromeritics Instruments),
magnetic hysteresis susceptibility (Quantum Design, Physical Property Measurement System), thermal
gravimetric analysis (Linseis TG/STA-PT1600, 25-500°C, 10°C/min, air), FTIR spectroscopy (1% in
KBr using a Tensor 27 Spectrometer, Bruker Optics equipped with a diffuse reflectance accessory,
DiffusIR™, Pike Technologies), element microanalysis (LECO CHN-900) and transmission electron
microscopy (CM30 ST-Philips, LaB6 cathode, operated at 300kV point resolution ~ 4 Å). The average
particle size was calculated from the specific surface area and the density as:
6
d BET =
(1)
SSAN 2 ρ
Chloro functionalization. The as-prepared carbon coated cobalt nanobeads (1 g) were suspended in 5 ml
H2O by the use of an ultrasonic bath (Sonorex RK 106, Bandelin). The 4-chlorobenzene diazonium ion
was prepared by adding a cool solution of sodium nitrate (2.3 mmol, in 12 ml H2O) to a cooled (ice bath)
solution of 4-chloroaniline (1.5 mmol) and HCl (0.6 ml, concentrated) in 20ml H2O.[13] After addition of
the carbon coated nanobeads, the reaction mixture was sonicated for 15 minutes. The nanobeads were
recovered from the reaction mixture with the aid of a neodymium based magnet (N48, W-12-N, Webcraft
GmbH, side length 12 mm) and washed 3x with water, 3x with hexane and 3x with ethylacetate. Each
washing step consisted of suspending the particles in the solvent, 5 minutes ultrasonication and retracting
the particles from the solvent by the aid of the magnet. After the last washing step the particles were dried
in vacuo at 60°C.
Nitro functionalization. As-prepared carbon coated cobalt nanoparticles (150 mg) were suspended in a
solution of 1 wt% SDS in water (10 ml) by the use of a ultrasonic bath (7 minutes). 4nitrobenzenediazonium tetrafluoroborate (0.3 mmol) was added to the suspension and the mixture was
left to react in the ultrasonic bath for 15 minutes.[14] The reaction was stopped by removing the magnetic
nanobeads from the reaction mixture by the use of a magnet. Washing was preformed similarly to the
preparation of chloro functionalized nanobeads (water 3x, acetone 3x) and the particles were
subsequently dried in vacuo at 60°C.
Amine functionalization. Sulfur (30 mmol), NaHCO3 (30 mmol) and nitro functionalized carbon coated
cobalt nanoparticles (100 mg) were suspended in DMF (20 ml). The mixture was kept at 130°C under N2
bubbling for 5 hours. After cooling the functionalized nanobeads were washed with DMF, water and
acetone and dried in vacuo.
References:
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
W. J. Stark, L. Madler, M. Maciejewski, S. E. Pratsinis, A. Baiker, Chem. Commun. 2003, 588.
W. J. Stark, L. Mädler, S. E. Pratsinis, WO 2004/005184, 2004.
R. N. Grass, W. J. Stark, J. Mater. Chem. 2006, 16, 1825.
R. N. Grass, W. J. Stark, J. Nanopart. Res. 2006, 8, 729 .
R. N. Grass, E. K. Athanassiou, W. J. Stark, PCT Patent application, 2005.
M. C. Heine, L. Madler, R. Jossen, S. E. Pratsinis, Combust. Flame 2006, 144, 809.
M. C. Heine, S. E. Pratsinis, Ind. Eng. Chem. Res. 2005, 44, 6222.
R. N. Grass, T. F. Albrecht, F. Krumeich, W. J. Stark, J. Mater. Chem. 2007,
DOI:10.1039/B614317B.
S. E. Pratsinis, Prog. Energy Combust. Sci. 1998, 24, 197.
S. E. Pratsinis, S. V. R. Mastrangelo, Chem. Eng. Prog. 1989, 85, 62.
E. K. Athanassiou, R. N. Grass, W. J. Stark, Nanotechnology 2006, 17, 1668.
L. Madler, H. K. Kammler, R. Mueller, S. E. Pratsinis, J. Aerosol. Sci. 2002, 33, 369.
S5
[13]
[14]
J. A. Belmont, US 5554739, 1996.
C. A. Dyke, J. M. Tour, Nano Lett. 2003, 3, 1215.
S6
Additional figures:
Figure S4. X-ray diffraction pattern of as-prepared carbon coated cobalt nanoparticles. The two peaks
match with the reference peaks of face centered cubic cobalt and no indication of any oxides or
crystalline carbon species can be observed.
S7
Figure S5. Magnetic hysteresis of as-prepared carbon coated cobalt nanobeads (solid line). The material
exhibited a saturation magnetization of 158 emu / g and a coercivity of ~ 200 Oersted. The magnetic
properties were strongly enhanced due to the carbon layers if compared with a carbon free reference
material (broken line).[3]
S8
Figure S6. Transmission electron images of chloro functionalized carbon coated nanoparticles. The
structure of the nanoparticles is unaffected by the functionalization chemistry.
S9