C.M. Expt. 1
Synthesis of Amine Capped Cadmium Selenide Nanoparticles (temperature dependent
growth)
Developed by Karen S. Quaal, Department of Chemistry and Biochemistry, Siena
College, Loudonville, NY. quaal@siena.edu
Synthesis was as reported in the literature with the following modifications 1,2:
Precursor and Single-Source Inorganic Cluster are synthesized by instructor (see
below).
Temperature: Hexadecylamine were degassed at 60°C (as opposed to 120°C).
Time: Hexadecylamine was degassed for 30 minutes (as opposed to an unspecified
time).
Temperature controller: 5°C/minute temperature interval.
Sample aliquots: Quenched in 1 mL of room temperature toluene.
Bulk sample isolation by precipitation in methanol and stripped of ligands for x-ray
diffraction for Physics module.
Preparation of Li2[Cd4(SPh)10], the precursor for the single source inorganic cluster
(Li)4[Cd10Se4(SPh)16]: (Instructor prep)
The precursor synthesis is a modification of procedures in the literature1,2
Authors in the Dance et al paper report the structure of the precursor as Li2[Cd4(SPh)10],
while the authors of the Cumberland et al paper report the structure as Li2[Cd4(SPh)12].
For the purposes of this experiment, the structure reported by Dance et al will be used.
Due to the stench and toxicity of thiophenol, it is strongly recommended that the
Li2[Cd4(SPh)10] precursor be prepared in a hood, by an instructor or graduate student. All
glassware and equipment (including gloves) should be thoroughly rinsed in a bleach bath
prior to removal from the hood.
The formation of [Cd4(SPh)10]2- is accomplished by the addition of a stoichiometric
amount of Cd(NO3)2 to a solution of methanol containing a 1:1 mole ratio of
triethylamine and thiophenol under ambient conditions. Triethylamine is added to
deprotonate the thiophenol. Addition of LiNO3, instead of tetramethylammonium
chloride, allows for the isolation of the crystalline product of Li2[Cd4(SPh)10] instead of
the TMA2[Cd4(SPh)10].
Conversion of Li2[Cd4(SPh)10] to Li4[Cd10Se4(SPh)16] is accomplished by treatment of a
room-temperature acetonitrile solution of Li2[Cd4(SPh)10 with Selenium under nitrogen.
Single crystals can be generated by recrystalization from hot acetonitrile.
Procedure for preparation of the purcursor: Li2[Cd4(SPh)10]
At room temperature, a solution of Cd(NO3)2. 4H2O (10.5g, 34 mmol) in methanol (30
mL) was added to a well-stirred solution of benzenethiol (10g, 91mmol) and
triethylamine (6.71 mL, 9.25 g, 91mmol) in methanol (20 mL). A solution of LiNO3
(2.8g, 40 mmol) in methanol (20 mL) was added with continuous stirring to dissolve any
precipitate. The mixture was cooled to 0C and toluene was added until a white oil paste
formed. The lithium salt of the precursor is not very soluble in the recommended
recrystalization solvent, hot acetonitrile.
A mixed solvent of acetonitrile and
Dimethylformamide (DMF) proved to be a better solvent for recrystalization. The
crystals were dried under vacuum overnight and used as is in the preparation of
(Li)4[Se4Cd10(SPh)16].
Procedure for preparation of the single source inorganic cluster:
(Li)4[Cd10Se4(SPh)16]
Black selenium powder (0.35 g, 4.45 mmol) was added in one portion to a solution of
Li2Cd4(SPh)10] (7.0 g, 4.45 mmol) in acetonitrile (10mL) and allowed to stir for two
hours at room temperature. Most of the selenium dissolved and a creamy white
precipitate formed. The mixture was warmed to ~75ºC and acetonitrile was added until
all the precipitate dissolved. The solution was allow to cool slowly and creamy white
crystals formed which were washed with acetonitrile and vacuum dried. The resulting
crystalline solid appears stable for at least a year when stored dry and shielded from light.
Preparation of the amine-Capped CdSe Nanoparticles from a single source
precursor: Temperature-dependent growth. (Student prep)
Hexadecylamine (5.5 grams) was degassed under vacuum at 60ºC for 30 minutes and
then switched to a nitrogen atmosphere. While under nitrogen, the solution was cooled to
below 40ºC at which point the septum was removed and 0.1 g (0.028 mmol) of
(Li)4[Cd10Se4(SPh)16] was added to the top of the solidified H.D.A. Under vacuum, the
temperature was raised until a melt forms (~40-50ºC). Heating was continued under
vacuum until 120ºC, at which point, the system was switched to nitrogen atmosphere and
heating continued. Starting at 130ºC, small aliquots were removed with a syringe every
10-15 degrees and quenched in 1 mL of toluene. Note: Solutions are very hot and the
room temperature syringe clogs easily if too much time passes in the quenching stage. A
second syringe should always be available in case the syringe becomes clogged. The
obvious color change is observed as the CdSe nanoparticles nucleate and grow. When
the solution reached 220ºC, the reaction mixture was allowed to cool to 60C, at which
point the solution was transferred to glass centrifuge tubes. The CdSe was isolated by
precipitation through the addition of methanol to each centrifuge tube. The tubes were
placed in an ice bath (15-20 minutes), centrifuged (10 minutes), and the methanol was
then removed. A minimum amount of toluene was added to each tube to dissolve the
CdSe nanoparticles.
The aliquots were analyzed by visible and fluorescence spectroscopy. Spectral data was
used in the quantum mechanical modeling experiment.
and bubbler
Figure 1. Apparatus for the Synthesis of Quantum Dots. Glassware consists of a 14/20
three-neck 25 mL RB flask, two “piggy-backed” 14/20-air condensers, a 14/20-tee
connector, 2 stopcocks to control the system’s atmosphere, and a 14/20-glass insert,
referred to as a thermowell which is of sufficient length to rest in the melt. The
temperature probe is inserted in the thermowell and connected to a temperature controller,
which controls the heating mantle.
Note: Two “piggy-backed” 14/20-air condensers are used to minimize the amount of
H.D.A. or TOPO that condenses on the inside of the glassware even though the reactions
are performed at temperatures lower than suggested in the literature. Due care must be
taken to insure the system is not clogged, and that nitrogen is free flowing so the risk of
heating a “closed system” is avoided. Finally, the two-stopcock system, although less
sophisticated than a manifold, is much easier to clean.
References:
1. Dance, I. G., Choy, A., Scudder, M. L., Synthesis, Properties and Molecular and
Crystal Structures of (Me4N)4 [E4M10(SPh)16] (E= S, Se; M=Zn, Cd) Molecular
Supertetrahedral Fragments of the Cubic Metal Chalcogenide Lattice, J. Am. Chem. Soc.
1984, 106, 6285-6295.
2. Cumberland, S. L., Hanif, K. M., Javier; A., Khitrov, G. A., Strouse, G. F., Woessner,
S. M., Yun, C. S., Inorganic Clusters as Single-Source Precursors for Preparation of
CdSe, ZnSe, and CdSe/ZnS Nanomaterials, Chem. Mater. 2002, 14, 1576-1584.