C.M. Expt. 2
Synthesis of Amine Capped Zinc 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 modifications1, 2:
Precursor and the Single Source Inorganic Cluster were synthesized by instructor
(see below).
Temperature: Hexadecylamine was degassed under vacuum at 60°C (as opposed
to 120°C).
Time: Hexadecylamine was degassed for 30 minutes (as opposed to 2 hours).
Temperature controller: 5°/minute temperature interval.
Sample aliquots: Quenched in 1 mL room temperature toluene.
Preparation of Li2[Zn4(SPh)10], the precursor for the single source inorganic cluster
(Li)4[Zn10Se4(SPh)16]: (Instructor prep)
Authors in the Dance et al paper report the structure of the precursor as Li2[Zn4(SPh)10],
while the authors of the Cumberland et al paper report the structure as Li2[Zn4(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[Zn4(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 [Zn4(SPh)10]2- is accomplished by the addition of a stoichiometric
amount of Zn(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[Zn4(SPh)10] instead of
the TMA2[Zn4(SPh)10].
Conversion of Li2[Zn4(SPh)10] to Li4[Zn10Se4(SPh)16] and (TMA)2[Zn4(SPh)10] to
(TMA)4[Zn10Se4(SPh)16] is accomplished by treatment of a room-temperature acetonitrile
solution of [Zn4(SPh)102- with Selenium under nitrogen. Product can be recrystalized
from specified solvents.
Procedure for preparation of the precursor: Li2[Zn4(SPh)10]:
At room temperature, a solution of Zn(NO3)2.6H2O (10.5 g, 35 mmol) in methanol (35
mL) was added to a well stirred solution of benzenethiol (9.28mL, 10 g, 91 mmol) and
triethylamine (6.71mL, 9.25 g, 91 mmol) in methanol (20 mL). A solution of LiNO3
(2.74g, 40 mmol) in methanol (20mL) was added with stirring to dissolve any precipitate.
The mixture was allowed to crystallize at 0C ,yielding a pasty oil. Numerous attempts
to crystallize the product from methanol, diethyl ether, acetoniltrile and
dimethylfromamide failed. The oil was dried under vacuum overnight and yielded ~9 g
of oil. The oil, Li2 Zn4(SPh)10, was used without further purification in the next
synthesis.
Procedure for preparation of the single source inorganic cluster: Li4[Zn10Se4(SPh)16]
Black selenium powder (0.8217 g, 10.4mmol) was added to a solution of
(Li)2[Zn4(SPh)10] (8.5 g, 6.5 mmol) in DMF (20mL) and allowed to stir at room
temperature for four hours. Most of the selenium dissolved, and the mixture turned a
gray-white color. The solution was filtered and methanol was added until cloudy, and the
solution was cooled to 0C yielding white crystals of (Li)4[Zn10Se4(SPh)16] (yielded 3.8
g). The crystals were filtered, washed with acetonitrile, and vacuum dried. Water was
added to the mother liquid to precipitate more of the product. The resulting crystalline
solid appears stable for at least a year when stored dry and shielded from light.
Preparation of (TMA)2[Zn4(SPh)10], the precursor for the single source inorganic
cluster (TMA)4[Zn10Se4(SPh)16]: (Instructor prep)
Procedure for preparation of the precursor: (TMA)2[Zn4(SPh)10]
At room temperature, a solution of Zn(NO3)2.6H2O (10.5 g, 35 mmol) in methanol (35
mL) was added to a well stirred solution of benzenethiol (9.28mL, 10 g, 91 mmol) and
triethylamine (6.71mL, 9.25 g, 91 mmol) in methanol (20 mL). A solution of
Tetramethylammonium chloride (TMAC) (4.4 g, 40 mmol) in methanol (20mL) was
added with stirring in an attempt to dissolve all of the precipitate. The undissolved
material was separated by filtration and dissolved in hot acetonitrile. Cool methanol was
added until cloudy and the solution was allowed to cool slowly. White crystals of
(TMA)2[Zn4(SPh)10 were obtained and dried under vacuum overnight (first crop 1.9g,
second crop with the addition of more methanol, 5.1 g).
Procedure for preparation of the single source inorganic cluster:
(TMA)4[Zn10Se4(SPh)16]
Black selenium powder (0.2 g, 2.53mmol) was added to a solution of
(TMA)2[Zn4(SPh)10] (1.9 g, 1.27 mmol) in DMF (20mL) and allowed to stir at room
temperature for one hour. Some of the selenium dissolved, and the mixture turned a
yellow-orange color. The solution was filtered to remove the unreacted Selenium, and
the solution was cooled to 0C without any formation of crystals or precipitate. Methanol
was added until cloudy, yielding white crystals of (TMA)4[Zn10Se4(SPh)16] (yielded 0.9
g). The crystals were filtered, washed with methanol, and vacuum dried. Water was
added to the mother liquid to precipitate more of the product. The resulting crystalline
solid appears stable for at least a year when stored dry and shielded from light.
Preparation of the amine-Capped ZnSe Nanoparticles: (Student prep)
Apparatus set up: The same apparatus as was used in the CdSe nanoparticle synthesis,
which allows the system to be placed under vacuum or nitrogen at different times.
Hexadecylamine (6 grams) was degassed under vacuum at 100C for 30 minutes and then
switched to nitrogen atmosphere. While under nitrogen, the solution was cooled to below
40C, at which point the septum was removed and (TMA)4Zn10Se4(SPh)16 (0.3 g, 0.009
mmol) was added to the top of the solidified hexadecylamine. The septum was replaced
and the system was placed under vacuum, while the temperature was raised to 120C. At
120C, the system was returned to a nitrogen atmosphere, and the heating was continued
until 240C at constant rate of about 5C per minute. At 240C, the first aliquot (five to
six drops) was removed and quenched in 1 mL of toluene. Subsequent aliquots were
removed every 5 minutes until the wavelength of the first excitation peak of the aliquots
remained constant. When the reaction mixture reached a constant state, the solution was
allowed to cool to 60C, at which point the solution was transferred to glass centrifuge
tubes. The ZnSe 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 ZnSe 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:
The precursor synthesis is a modification of procedures in the literature 1,2:
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.