Monodisperse Colloidal Mg NPs by DigestiWe Ripening
Preparation of Mg-HDA-THF-Toluene Colloid by the
SMAD Method. About 1.5 g of HDA was taken at the bottom of
the reactor. A two-way bridge equipped with THF and toluene
containing Schlenk tubes were connected at the top of the reactor.
Initially, about 40 mL of toluene was condensed on the walls of
the reactor which was maintained at 77 K. Next, the liquid nitrogen
dewar was removed and the toluene was then allowed to melt. The
dewar was placed back after the toluene melted completely and
fell to the bottom of the reactor. The rest of the experiment was
carried out similarly to that of Mg-THF SMAD experiment. The
ratio of Mg:HDA maintained in this experiment is ∼1:2 (molar
ratio). The siphoned colloid was brownish black in color. THF was
removed from this colloid under vacuum to yield a stable transparent
colloid. The stability of the final colloid largely depends on the
rate of evaporation of magnesium. The typical evaporation rate to
realize stable colloids is around 30-35 mg/h.
Room-Temperature Digestive Ripening of Mg-THF Nano-
particles. From a Mg-THF colloid (30 mg of Mg) in a Schlenk
tube, THF was removed partially under a vacuum. To this was
added 30 mL of degassed toluene under argon. A fixed molar ratio
of hexadecylamine (1:0.5 or 1:1 or 1:2 Mg:HDA) was added to
this solution under vigorous stirring (18 h). Stable colloids were
obtained in case of higher Mg:HDA molar ratios.
Isolation of Mg Nanopowders. A large excess (1:20 toluene:
THF) of degassed THF was added to Mg-HDA-toluene colloid.
The colloid was then centrifuged at 3000 rpm inside a glovebox
for 2-3 h. The black precipitate that resulted was washed several
times with THF to remove the excess HDA. Finally, the powders
were dried and stored inside the glovebox. These powders were
found to be extremely pyrophoric in nature.
Hydrogen Storage Studies. MgH2 was synthesized from
Mg-HDA nanopowder (120 mg) by reacting with hydrogen (33
bar pressure) at 118 °C for 3 h. H2 desorption studies were carried
out using a laboratory made temperature programmed desorption
equipment. About 55 mg of MgH2 powder was loaded in the sample
tube between ceramic wool inside the glovebox. The desorption of
H2 was monitored from 30 to 450 °C at a ramp rate of 5 °C/min
under argon flow. The H2 gas was detected using a TCD detector
which was calibrated against uptake of hydrogen for a known
quantity of CuO.
in size and a dynamic equilibrium is established. This
treatment was successfully used for gold,9 silver,10 copper,
and zinc11 nanoparticles. Using this methodology, we also
recently synthesized highly monodisperse colloids of copper,
and zinc and Cu@ZnO11 and Au@Pd12 core shell nanopar-
ticles in gram quantities. Motivated by the advantages that
the SMAD method offers such as easy scale up, high
reproducibility, and avoidance of tedious purification pro-
cedures and the strength of the digestive ripening process,
we attempted the synthesis of Mg colloids in an effort to
realize smaller sized particles.
Experimental Section
Materials. Mg turnings (99%) were purchased from Aldrich.
Hexadecylamine (HDA) was dried and degassed for 12 h at 100
°C. Tetrahydrofuran (THF) and toluene were dried over sodium-
benzophenone. The solvents were degassed by several freeze-
pump-thaw cycles. All the glassware was dried in a hot air oven
around 120 °C and evacuated in hot condition just before the use
to remove trace quantities of moisture.
Instrumentation. Transmission electron microscope (TEM)
bright-field images and high-resolution TEM (HRTEM) images
were obtained using TECHNAI F30 electron microscope operating
at 200 kV. All the TEM samples were prepared inside a N2 filled
glovebox by placing 2 µL of sample on carbon coated copper grids.
The powder X-ray diffraction measurements were carried out on
samples placed in 0.5 mm diameter capillaries flame-sealed under
N2 atmosphere using Philips powder X-ray diffractometer and
Bruker ADVANCE X-ray diffractometer using Cu KR radiation.
FTIR spectra were obtained using Perkin-Elmer Spectrum One
instrument.
Preparation of Mg-THF Nanopowders by the SMAD
Method. The SMAD setup is described in detail in ref 7. The
tungsten crucible coated with alumina cement was connected
between two water-cooled copper electrodes and a 3000 mL reactor
vessel was connected to a reactor head that is equipped with copper
electrodes. The entire setup was evacuated to 2-3 × 10-3 mbar.
The crucible was heated in steps and at each step the pressure was
allowed to come down to 2-3 × 10-3 mbar. The crucible was
kept under gentle heating overnight. This curing process ensures
removal of moisture and other volatile impurities from the crucible.
In a typical experiment, about 100 mg of Mg turnings was loaded
in a tungsten crucible and the crucible was resistively heated by
applying appropriate voltage between two water-cooled copper
electrodes. The reactor walls were maintained at 77 K using a liquid
nitrogen bath and precoated with 20 mL of THF before the metal
vaporization. The voltage was increased stepwise until metal
vaporization began, which was apparent by the appearance of
brownish-yellow color on the white matrix. Voltage was maintained
at this point for 3 h and about 80-90 mL of THF was co-condensed
on the walls of reactor. During this period, the matrix turned dark
brown in color. When the vaporization was complete, the liquid
nitrogen dewar was removed and the matrix was warmed to room
temperature under an argon atmosphere. Upon warm up, precipita-
tion of Mg-THF powder took place. The product was stirred using
a magnetic stirrer and siphoned into a Schlenk tube under argon.
Caution: Magnesium nanopowders are highly pyrophoric! They
must be handled with extreme caution, taking due care not to expose
the samples to air. Safety shields must be used while operating
high-pressure equipment.
Results and Discusssion
(a) Synthesis and Characterization of Mg Nano-
particles. Micron-sized magnesium particles have been
prepared by Klabunde and co-workers using the SMAD
method.13 Imamura et al. synthesized micrometer-sized
Mg-THF powders and studied the hydrogen absorption
properties and found that they are more active compared to
bulk Mg.14 In these two reports, the metal to THF ratios
were quite low (5 mL of THF for 1 g of Mg) and the rate of
evaporation of Mg was quite fast. We prepared Mg nano-
particles using the SMAD method under carefully controlled
conditions. By maintaining a higher metal to THF ratio (1
(9) Stoeva, S.; Klabunde, K. J.; Sorensen, C. M.; Dragieva, I. J. Am. Chem.
Soc. 2002, 124, 2305.
(10) Smetana, A. B.; Klabunde, K. J.; Sorensen, C. M. J. Colloid Interface
Sci. 2005, 284, 521.
(11) Kalidindi, S. B.; Jagirdar, B. R. J. Phys. Chem. C 2008, 112, 4042.
(12) Jose, D.; Jagirdar, B. R. J. Phys. Chem. C 2008, 112, 10089.
(13) Klabunde, K. J.; Efner, H.-F.; Satek, L.; Donley, W. J. Organomet.
Chem. 1974, 71, 309.
(14) Imamura, H.; Nobunaga, T.; Kawahigashi, M.; Tsuchiya, S. Inorg.
Chem. 1984, 23, 2509.
Inorganic Chemistry, Vol. 48, No. 10, 2009 4525