Stabilization of the thermodynamically favored polymorph of cadmium chalcogenide nanoparticles CdX (X = S, Se, Te) in the polar mesopores of SBA-15 silica

Article abstract of DOI:10.1021/ic050517h

A versatile synthetic approach to cadmium chalcogenide nanoparticles in the mesopores of SBA-15 silica as a host matrix was developed. The use of cadmium organochalcogenolates of the type Cd(XPh)₂·TMEDA (X = S, Se, Te) allowed the preparation of nanoparticles of all three cadmium chalcogenides following the same experimental protocol. Particles of CdS, CdSe, and CdTe with a particle size of 7 nm were prepared from this class of single-source precursors. The incorporation of the precursor molecules into the pores was achieved by melt infiltration at a temperature of 140°C. Subsequent pyrolysis of the precursors in the mesopores yielded the semiconductor particles. Owing to the high polarity of the silanol-covered pore walls, which lower the surface energy of the particles to a large extent, the dimorphic cadmium chalcogenides are obtained in their thermodynamically favored modifications; e.g., CdS particles crystallize in the wurtzite type, CdTe particles are obtained in the zinc blende structure, and CdSe (where no unambiguous preference exists) crystallizes as a mixture of both structures with a rather random stacking sequence.

Full text of DOI:10.1021/ic050517h

Inorg. Chem. 2005, 44, 5890−5896
Stabilization of the Thermodynamically Favored Polymorph of Cadmium
Chalcogenide Nanoparticles CdX (X
Mesopores of SBA-15 Silica
) S, Se, Te) in the Polar
Maekele Yosef,† Andreas K. Schaper,‡ Michael Froba,§ and Sabine Schlecht*,†
1
Fachbereich Chemie, Philipps-UniVersita¨t Marburg, 35032 Marburg, Germany, Wissenschaftliches
Zentrum fu¨r Materialwissenschaften, Philipps-UniVersita¨t Marburg,
35032 Marburg, Germany, and Institut fu¨r Anorganische und Analytische Chemie,
Justus-Liebig-UniVersita¨t Giessen, 35392 Giessen, Germany
Received April 6, 2005
A versatile synthetic approach to cadmium chalcogenide nanoparticles in the mesopores of SBA-15 silica as a host
matrix was developed. The use of cadmium organochalcogenolates of the type Cd(XPh)2 TMEDA (X S, Se, Te)
‚
)
allowed the preparation of nanoparticles of all three cadmium chalcogenides following the same experimental protocol.
Particles of CdS, CdSe, and CdTe with a particle size of 7 nm were prepared from this class of single-source
precursors. The incorporation of the precursor molecules into the pores was achieved by melt infiltration at a
temperature of 140 °C. Subsequent pyrolysis of the precursors in the mesopores yielded the semiconductor particles.
Owing to the high polarity of the silanol-covered pore walls, which lower the surface energy of the particles to a
large extent, the dimorphic cadmium chalcogenides are obtained in their thermodynamically favored modifications;
e.g., CdS particles crystallize in the wurtzite type, CdTe particles are obtained in the zinc blende structure, and
CdSe (where no unambiguous preference exists) crystallizes as a “mixture” of both structures with a rather random
stacking sequence.
Introduction
pore array can act as a spatially confined reaction vessel. In
addition, the polar protic walls of mesoporous silica6 provide
the interesting opportunity to control not only the particle
size but also the volume structure of the particle. Similar to
the structure-determining effect of a protic polar reagent such
as water on the volume structure of II/VI semiconductor
nanoparticles,7,8 the silanol groups of the walls of mesoporous
silica can also be expected to induce the formation of the
thermodynamically favored bulk phase of the respective
semiconductor by an efficient reduction of the surface energy
and of the disorder phenomena related to the surface of the
nanoparticles (Scheme 1).
Thus, the aim of our investigations was the elucidation of
the phase stabilities of the nanoscale cadmium chalcogenides
CdS, CdSe, and CdTe in this polar environment of the pores
of mesoporous silica SBA-15. It can be expected that
cadmium chalcogenide particles in the mesopores of SBA-
One of the major challenges in nanoparticle synthesis is
the precise control over the size and shape of the produced
nanoscale materials. Thus, a large number of syntheses in
aqueous and nonaqueous media involving surface-capping
additives have been developed, which can provide mono-
disperse nanoparticles of metals,1,2 ceramics,3 or semiconduc-
tors.4 A different kind of concept for size control by steric
barriers that physically limit the growth of the product
particles is the use of ordered porous templates5 where the
* To whom correspondence should be addressed. E-mail:
schlecht@chemie.uni-marburg.de. Fax: +49-(0)6421-2825653.
† Fachbereich Chemie, Philipps-Universita¨t Marburg.
‡ Wissenschaftliches Zentrum fu¨r Materialwissenschaften, Philipps-
Universita¨t Marburg.
§ Justus-Liebig-Universita¨t Giessen.
(1) Puntes, V. F.; Krishnan, K.; Alivisatos, A. P. Top. Catal. 2002, 19,
145.
(2) Schmid, G. In Nanoscale Materials in Chemistry; Klabunde, K., Ed.;
Wiley: New York, 2001; pp 15-59.
(3) Niederberger, M.; Pinna, N.; Polleux, J.; Antonietti, M. Angew Chem.,
Int. Ed. 2004, 43, 2270.
(4) Green, M.; O’Brien, P. Chem. Commun. 1999, 2235.
(5) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartulli, J. C.; Beck,
J. S. Nature 1992, 359, 710.
(6) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950.
(7) Zhang, H.; Gilbert, B.; Huang, F.; Banfield, J. F. Nature 2003, 424,
1025.
(8) Zhang, H.; Huang, F.; Gilbert, B.; Banfield, J. F. J. Phys. Chem. B
2003, 107, 13051.
5890 Inorganic Chemistry, Vol. 44, No. 16, 2005
10.1021/ic050517h CCC: $30.25
© 2005 American Chemical Society
Published on Web 07/08/2005

Polymorph of Cadmium Chalcogenide Nanoparticles
Scheme 1. (a) Phase Transition into the Thermodynamically Favored
Polymorph, Induced by Water as a Polar Medium; (b) Formation of
Nanoparticles of the Thermodynamically Stable Phase by Precursor
Thermolysis in the Polar Pores of SBA-15 Silica
41 are covered by terminal silanol groups ≡SiOH, which
make them a highly polar and fairly acidic host compound
for any guest to be incorporated. On the other hand, the polar
character of a pore wall containing a large number of ≡SiOH
functionalities is highly advantageous for the minimization
of the surface energy of an incorporated nanoparticle of, e.g.,
a cadmium chalcogenide.
The choice of the precursor system was governed by two
principal considerations:
(i) Can the same type of compound with the same
molecular structure and the same decomposition pathway be
used for all chalcogenides CdS, CdSe, and CdTe?
(ii) Can the precursor easily be incorporated into the pores
by simple melting?
One class of compounds that is accessible for all chalco-
gens and that is known to allow melting without decomposi-
tion are organochalcogenolates of transition metals.17 In the
case of the zinc chalcogenides ZnSe and ZnTe, the respective
chalcogenolate precursors Zn(XPh)2‚TMEDA (X ) Se, Te)
were successfully applied for solution-phase syntheses of
capped nanoscale particles in trioctylphosphine oxide.18
Surprisingly, the analogous cadmium compounds have not
been described so far, although similar properties in decom-
position reactions leading to cadmium chalcogenide nano-
particles can be expected. Starting from this consideration,
a straightforward synthesis for pure compounds of the series
Cd(XPh)2‚TMEDA (X ) S, Se, Te) was worked out. First,
it became apparent that the preparation of the zinc com-
pounds starting from LiXPh (X ) Se, Te) and ZnCl2 could
not be transferred to cadmium chemistry in an analogous
manner because the cadmium halides and the cadmium
organochalcogenolates differ too strongly from their zinc
counterparts with respect to their solubilities and no pure
products could be obtained. Hence, a synthesis that avoids
the precipitation of an alkali-metal halide from the cadmium
salt proved to be the better approach. As already described
for an alternative synthesis of Zn(SePh)2‚TMEDA,19 group
12 halides react very cleanly with silylated chalcogenolates
under the elimination of trimethylsilyl chloride as a volatile
byproduct (eq 1). The silylated derivatives of phenylthi-
olate,20 -selenolate,21 and -tellurolate22 were used to prepare
15 silica follow the phase stabilities of the bulk chalcogenides
CdS, CdSe, and CdTe, where the wurtzite polymorph is
found for CdS and CdSe whereas the cubic zinc blende
structure is thermodynamically favored for CdTe.9
To make the synthetic approach versatile and equally
valuable for all three cadmium chalcogenides, the incorpora-
tion of the reactands into the pores should not require any
specific chemical binding interaction of the pore wall with
the precursor system. If a generally applicable physical
incorporation is chosen, a complete class of compounds can
be produced in nanocrystalline form following the same
experimental protocol. This last aspect is highly advantageous
for a comparative study of thermodymanic control on the
volume structure of the product particles.
In the present work, the feasability of this concept for the
preparation of nanoscale cadmium chalcogenides CdX (X
) S, Se, Te) from an easily accessible family of molecular
precursors was investigated. According to the principle of a
general physical incorporation of the precursor into the pores,
a class of precursor molecules with rather uniform physical
properties (e.g., solubility, melting point) and decomposition
characteristics was chosen and base adducts of cadmium
organochalcogenolates of the type Cd(XPh)2‚TMEDA (X )
S, Se, Te; TMEDA ) tetramethylethylenediamine) proved
to be suitable candidates for this synthetic strategy.
(10) Leon, R.; Margolese, D.; Stucky, G.; Petroff, P. M. Phys. ReV. B 1995,
52, R2285. Schlecht, S.; Yosef, M.; Fro¨ba, M. Z. Anorg. Allg. Chem.
2004, 630, 864.
(11) Tang, Y. S.; Cai, S.; Jin, G.; Duan, J.; Wang, K. L.; Soyez, H. M.;
Dunn, B. S. Appl. Phys. Lett. 1997, 71, 2449.
(12) Chen, L.; Klar, P. J.; Heimbrodt, W.; Brieler, F.; Fro¨ba, M. Appl.
Phys. Lett. 2000, 76, 3531. Brieler, F.; Fro¨ba, M.; Chen, L.; Klar, P.
J.; Heimbrodt, W. Chem. Eur. J. 2002, 8, 185.
(13) Chen, L.; Falk, H.; Klar, P. J.; Heimbrodt, W.; Brieler, F.; Fro¨ba, M.;
Krug von Nidda, H.-A.; Loidl, A.; Chen, Z.; Oka, Y. Phys. Status
Solidi B 2002, 229, 31.
(14) Brieler, F.; Grundmann, P.; Fro¨ba, M.; Chen, L.; Klar, P. J.; Heimbrodt,
W.; Krug von Nidda, H.-A.; Kurz, T.; Loidl, A. J. Am. Chem. Soc.
2004, 126, 797.
(15) Parala, H.; Winkler, H.; Kolbe, M.; Wohlfahrt, A.; Fischer, R. A.;
Schmechel, R.; von Seggern, H. AdV. Mater. 2000, 12, 1050.
(16) Kowalchuk, C. M.; Schmid, G.; Meyer-Zaika, W.; Huang, Y.;
Corrigan, J. F. Inorg. Chem. 2004, 43, 173.
Results and Discussion
Since the introduction of mesoporous materials in 1992,5
these materials have provided a number of new synthetic
pathways to nanostructured composites with a second
compound of choice in the ordered pore system of the silica.6
Different mesoporous silicas with a hexagonal pore structure
such as MCM-41 or SBA-15 have mostly been used as hosts
to accommodate semiconductor nanostructures such as Ge,10
SiGe,11 (Cd,Mn)S,12 (Cd,Mn)Se13 (Zn,Mn)S,14 CdSe,15 or
Cu2Te.16 The pore walls of mesoporous SBA-15 or MCM-
(17) Arnold, J. Prog. Inorg. Chem. 1995, 43, 353.
(18) Jun, Y.; Koo, J.-E.; Cheon, J. Chem. Commun. 2000, 1243. Jun, Y.;
Choi, C.-S.; Cheon, J. Chem. Commun. 2001, 101.
(19) Pfistner, H.; Fenske, D. Z. Anorg. Allg. Chem. 2001, 627, 575.
(9) Holleman, A.; Wiberg, E. Lehrbuch der Anorganischen Chemie, 101st
ed.; de Gruyter: Berlin, 1995.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5891

Yosef et al.
Table 1. Melting Points and Decomposition Characteristics of
Cd(SPh)2‚TMEDA, Cd(SePh)2‚TMEDA, and Cd(TePh)2‚TMEDA
Cd(SPh)2‚
TMEDA
Cd(SePh)2‚
TMEDA
Cd(TePh)2‚
TMEDA
melting point (°C)
first decomp. step (°C)
second decomp. step (°C)
106
314
387
127
305
351
101
196
263
the whole series of cadmium phenylchalcogenolates (eqs
2-4). To make CdCl2 more soluble in toluene, 2.5 equiv of
TMEDA was added beforehand, leading to the formation of
the base adduct CdCl2‚2TMEDA as a more reactive inter-
mediate for the reaction with the silylated organochalco-
genolates. The solution of CdCl2‚2TMEDA was held at room
Figure 1. DSC diagram of the thermolysis of Cd(SePh)2‚TMEDA,
indicating two principal decomposition steps of the precursor.
ZnCl2 + 2Me3SiSePh +TMEDA8
which is related to the loss of TMEDA ligand, and found
X-ray amorphous products that still contained a significant
amount of organic material, which could be verified by
elemental analysis. No reflections of already formed grains
of cadmium chalcogenides were detected at this stage of
thermolysis. However, a fully developed set of reflections
of the respective cadmium chalcogenide were present in all
cases after the second decomposition step, which comprises
the elimination of arylchalcogenide Ph2E, was completely
finished. These experiments made clear that the formation
of grains of the semiconductor is limited to this second event.
Immediately following the second endothermic signal, a
weaker exothermic peak at Tcryst ) 382 °C, which belongs
to this decomposition step, is observed and can be attributed
to the crystallization of the primary grains of the semicon-
ductor into nanoscale-ordered particles. The differential
scanning calorimetry (DSC) analysis of Cd(SePh)2‚TMEDA
in the temperature range where the decomposition occurs is
shown as one representative example in Figure 1. The
melting points of the chalcogenolate precursors proved to
be a second important aspect (Table 1). All three compounds
show melting points below 150 °C and can thus easily be
incorporated into the pores of SBA-15 in the liquid state
without decomposition.
The infiltration of the molten precursors into SBA-15 silica
was conducted in the following way: calcined SBA-15 and
an equal amount of the chalcogenolate precursor were slowly
heated to ∼140 °C. Gradually, the precursor melt was
incorporated into the pores of SBA-15 silica. The samples
were held at a final temperature of pyrolysis of 350 °C for
14-16 h. This prolonged heating also leads to complete
decomposition of the thiolate complex that has a nominally
higher decomposition temperature (Table 1). During the
pyrolysis of the precursors, the condensation of the volatile
byproduct Ph2S, Ph2Se, or Ph2Te at the cold top part of the
Schlenk tube was observed, and the obtained diphenylchal-
cogenides could be identified by NMR spectroscopy. After
pyrolysis had been completed, the samples were washed with
hexane and tetrahydrofuran to remove all organic residues
potentially present on the surface of the composites. The
remaining powders were characterized by energy-dispersive
X-ray (EDX) analysis, powder diffractometry, and transmis-
sion electron microscopy (TEM). For all three nanocompos-
ites CdS@SBA-15, CdSe@SBA-15, and CdTe@SBA-15,
Zn(SePh)2*TMEDA + 2Me3SiCl (1)
CdCl2 + 2Me3SiSPh +TMEDA8
Cd(SPh)2*TMEDA + 2Me3SiCl (2)
CdCl2 + 2Me3SiSePh +TMEDA8
Cd(SePh)2*TMEDA + 2Me3SiCl (3)
CdCl2 + 2Me3SiTePh +TMEDA8
Cd(TePh)2*TMEDA + 2Me3SiCl (4)
temperature while 2 equiv of Me3SiSPh, Me3SiSePh, or Me3-
SiTePh was added. The reaction mixtures were stirred
overnight, the solvent was removed under vacuum, and the
resulting solids were recrystallized from a toluene solution
at -24 °C. Very pure organochalcogenolates Cd(XPh)2‚
TMEDA were produced in good yield by this method, and
suitable crystals for X-ray crystal structure analysis were
obtained for Cd(SPh)2‚TMEDA and for Cd(SePh)2‚TMEDA.
Crystal structure determinations revealed that both com-
pounds are isotypic and crystallize in the monoclinic system
in the space group P21/c with four molecules in the unit cell.
Monomeric complexes with a rather unusual arrangement
of two nitrogen atoms and two chalcogen atoms attached to
the cadmium center are found in the crystal structures. Details
on the crystal structures and on the crystal structure deter-
mination are given in the Supporting Information.
To find the most suitable thermal treatment of the cadmium
organochalcogenolate precursors for their decomposition into
cadmium chalcogenides, calorimetric investigations of the
decomposition procedure were performed prior to the ther-
molyses in the silica host. The investigations showed that
all three precursor compounds degraded through two distinct
endothermal events. The individual decomposition charac-
teristics for the different organochalcogenolates of cadmium
are given in Table 1. In all cases, we recorded X-ray powder
diffraction patterns after the first step of the decomposition,
(20) Kuwajima, I.; Abe, T. Bull. Chem. Soc. Jpn. 1978, 51, 2183.
(21) Clarembeau, M.; Cravador, A.; Dumont, W.; Haresi, L.; Krief, A.;
Lucchetti, J.; van Ende, D. Tetrahedron 1985, 41, 4793.
(22) Sasaki, K.; Aso, Y.; Otsubo, T.; Ogura, F. Tetrahedron Lett. 1985,
26, 453.
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Polymorph of Cadmium Chalcogenide Nanoparticles
Figure 2. X-ray powder diffraction patterns of the nanocomposites
CdS@SBA-15, CdSe@SBA-15, and CdTe@SBA-15.
EDX investigations confirmed a 1:1 ratio of cadmium and
the respective chalcogen in the products. The close similarity
of the preparations of the different nanocomposites directly
in the template pores provides an optimum comparability of
the formation conditions of the different cadmium chalco-
genides. Thus, the same influence of the polar pore walls
and the same sequence of pyrolytic steps can be expected to
lead to the same degree of thermodynamic control over the
different cadmium chalcogenide products.
The obtained nanocomposite products CdS@SBA-15,
CdSe@SBA-15, and CdTe@SBA-15 were characterized by
X-ray powder diffraction. The diffraction patterns depicted
in Figure 2 show the reflections of the cadmium chalco-
genides in the amorphous matrix. All samples exhibit high
crystallinity of the nanoparticles incorporated into the me-
sopores of the silica host. The different cadmium chalco-
genides are formed in different modifications. Cadmium
telluride was obtained in the zinc blende structure, cadmium
sulfide in the wurtzite structure, and cadmium selenide as a
mixture of the two polymorphs. Using Scherrer’s equation,23
a particle size of 8 ( 2 nm was found for CdS and CdTe,
which is in accordance with the particle size determined by
TEM investigations. No particle size for CdSe was calculated
from the half-width of the reflection for the reasons given
below. It was crucial to use recrystallized, very pure
precursors in these experiments because the presence of
impurities in the chalcogenolates always leads to the forma-
tion of the wurtzite phase only.
Figure 3. Cross section of partially filled pores in CdTe@SBA-15. Top
view of an array of empty pores of SBA-15 (left inset) and of an array of
pores filled with CdTe (right inset).
CdTe outside the template. Corresponding TEM micrographs
of CdSe@SBA-15 and of CdS@SBA-15 are shown in Figure
4. In these cross-sectional views, partially filled pores with
incorporated CdSe and CdS can be identified. To identify
the material within the pores without ambiguity, we con-
ducted nanobeam diffraction experiments on filled regions
of CdSe@SBA-15 grains (Figure 5). The diffraction patterns
were taken from two circular regions of 5 nm diameter as
indicated in Figure 5. After appropriate tilting, nanobeam
diffraction patterns for zone axis [100] of the cubic modifica-
tion of CdSe and for zone axis [111] of the cubic phase (or
[001] of the wurtzite phase) in the filled pores of SBA-15
silica could be obtained. These nanobeam diffraction experi-
ments indicate that the filling of the pores of the mesoporous
host is unambiguously CdSe.
To obtain free-standing particles of the cadmium chalco-
genides and to investigate the influence of the template walls
on the structure of the particles, the template was removed
by dissolution in a strong base such as aqueous NaOH or
KOH. As-obtained nanoparticles of CdS were yellow in
color, whereas brownish CdSe and gray CdTe were obtained.
The remaining pure particles of the respective cadmium
chalcogenides were investigated by TEM to study their
morphology and their structural identity. In Figure 6, high-
resolution TEM (HRTEM) micrographs of as-obtained
unprotected particles of CdTe and CdSe are shown. It can
be seen from the HRTEM pictures and from electron
diffraction experiments (see inset) that the free particles show
very good crystallinity and that CdTe is still in the cubic
sphalerite structure. In the case of CdSe, the strong reflections
of the cubic phase are dominant, whereas the reflections of
the wurtzite polymorph can only vaguely be identified by
our electron diffraction experiments because they are intrin-
sically of lower intensity. Because of the loss of the protective
surface of the template, the nanoparticles immediately form
higher aggregates of individual particles to minimize their
surface energy. Because the overall polarity and the capability
The cadmium chalcogenide nanocomposites were further
investigated by TEM to gain insight into the degree of filling
of the pores and to get additional information about the
possible presence of nonnanoscale material outside the pores
and on the surface of the template. As can be seen from
Figure 3, showing bright-field images of an array of empty
pores of SBA-15 and an array of filled pores of CdTe@SBA-
15 as well as a cross section of the partially filled pores in
this composite, no additional material outside the channels
was found. Overview images of larger assemblies of
CdTe@SBA-15 also showed no separate phase of macroscale
(23) Guinier, A. X-ray Diffraction in Crystals, Imperfect Crystals and
Amorphous Bodies; Dover: New York, 1994; pp 121-130.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5893

Yosef et al.
Figure 4. Top view of partially filled pores of CdSe@SBA-15 (top). Cross-
sectional view of partially filled pores of CdS@SBA-15 (bottom).
Figure 6. HRTEM micrograph and electron diffraction pattern of isolated
particles of CdTe (top) and CdSe (bottom).
to dissolve the template, no structural changes were observed,
not even for CdSe, where a phase transition to the thermo-
dynamically favored pure wurtzite structure could be imag-
ined. The effect of a polar capping, e.g., by water, on
nanoparticles of the II/VI semiconductor family such as CdSe
or ZnS is well established in the literature and was
investigated theoretically as well as experimentally.7,8,24,25 In
all of these cases, the wurtzite structure is considered
thermodynamically favored26 and the occurrence of sphalerite
particles was assigned to an additional stabilization of the
isotropic modification by polar surface interactions and the
rather spherical shape of very small grains.7,8,25
Phase transitions and phase stability in cadmium chalco-
genide nanoparticles of 3-10 nm size have been the subject
of continuing investigations for more than a decade.27-29 In
a number of cases, not only particles with the crystal structure
Figure 5. Overview of CdSe@SBA-15 grains with partially filled pores.
White circles indicate the two regions where nanobeam diffraction patterns
(beam diameter d ) 5 nm) were taken along zone axis [100] (top) and
zone axis [111] (bottom) of the zinc blende modification.
(24) Rabani, E. J. Chem. Phys. 2001, 115, 1493.
(25) Leung, K.; Whaley, K. B. J. Chem. Phys. 1999, 110, 11012.
(26) On a possible thermodynamic preference of the zinc blende struc-
ture: Yeh, C. Y.; Lu, Z. W.; Froyen, S.; Zunger, A. Phys. ReV. B
1992, 46, 10086.
to compensate the extra surface energy of the nanoparticles
are similar for silanol groups and for the aqueous bases used
5894 Inorganic Chemistry, Vol. 44, No. 16, 2005

Polymorph of Cadmium Chalcogenide Nanoparticles
1H NMR (CDCl3): δ 7.33-7.31 (m, 4H), 6.97-6.88 (m, 6H),
2.50 (s, 4H), 2.35 (s, 12H). 13C NMR (CDCl3): δ 133.3 (m-C),
129.1 (o-C), 128.8 (p-C), 124.5 (ipso-C), 57.7 (-N-CH2), 47.7
(N-CH3). Anal. Calcd for C18H26CdN2S2: C, 48.37; H, 5.86; N,
6.27. Found: C, 48.51; H, 6.02; N, 6.13.
of one of the two common polytypes (zinc blende or wurtzite)
but also nanoparticles with domains of zinc blende and
wurtzite within the same particle were observed.27-29 It was
shown that such intergrowth, caused by either stacking faults
or twin borders in the nanoscale crystallite, leads to diffrac-
tion patterns with the same kind of overlapping reflections
as those of a mixture of separate crystallites of the same
size.27 Thus, the “mixture” of zinc blende and wurtzite
obtained for the CdSe particles can stem from (i) a real
mixture of separate particles of the cubic and hexagonal
polymorphs, (ii) from particles consisting of domains of a
polytype with a stacking sequence different from strict
ABCABC and strict ABABAB (e.g., ABABCBCABCB with
a longer translational period),27,30 or (iii) random stacking
faults in a zinc blende type, leading to layers of wurtzite
type plane stacking (e.g., ABCABCABAB). All three pos-
sibilities have already been observed in cadmium chalco-
genide nanocrystals. Especially in the case of CdSe, there is
apparently no preference for a certain polytype in small
nanoparticles,27 and the stacking along [111] of the cubic
phase seems to be almost random. In light of these observa-
tions, it becomes much more obvious why there is no driving
force for a room-temperature phase transition of the particles
into pure wurtzite either in the silanol covered pores or in
aqueous solution.
Preparation of Cd(SePh)2‚TMEDA. An amount of 0.340 g
(0.75 mmol) of Me3SiSePh in 20 mL of toluene was added to a
mixture of 4 mL of TMEDA and 0.137 g (0.75 mmol) of CdCl2 in
30 mL of toluene. The reaction mixture was stirred at room
temperature for 8 h and subsequently filtered to remove a small
amount of a white solid.. The amount of solvent was reduced to
half of its volume under reduced pressure, and the solution was
stored at -24 °C. Colorless crystals of Cd(SePh)2‚TMEDA were
obtained (mp 127 °C, 0.285 g, 70% yield).
1H NMR (CDCl3): δ 7.49-7.47 (m, 4H), 6.95-6.86 (m, 6H),
2.53 (s, 4H), 2.34 (s, 12H). 13C NMR (CDCl3): δ 136.1 (m-C),
128.8 (o-C, p-C), 124.3 (ipso-C), 57.0 (-N-CH2), 47.3 (N-CH3).
Anal. Calcd for C18H26CdN2Se2: C, 39.98; H, 4.85; N, 5.18.
Found: C, 39.14; H, 4.92; N, 5.41.
Preparation of Cd(TePh)2. A sample of 0.818 g (2.0 mmol) of
diphenyl ditelluride was dissolved in 30 mL of THF, and the
solution was cooled to -78 °C. An amount of 4.2 mL of a 1 M
solution of Li[Et3BH] was added, and the reaction mixture was
stirred for 2 h. An amount of 0.51 mL (4.0 mmol) of Me3SiCl was
added to the solution, and the reaction mixture was stirred for 2 h
and allowed to warm to room temperature. The resulting lithium
chloride was removed by filtration. A colorless solution was
obtained. A solution of 0.366 mg (2.0 mmol) of CdCl2 in THF
was added at -78 °C. The reaction was allowed to warm to room
temperature within 12 h, and the solvent was removed under
reduced pressure. An orange solid of Cd(TePh)2 was obtained (0.345
g, 32% yield).
Experimental Section
All synthetic manipulations were carried out under exclusion of
air and moisture in an argon atmosphere. All solvents were dried
and freshly distilled under argon prior to use. CdCl2, Me3SiCl, Ph2-
Te2, Ph2Se2, Ph2S2, and Li[Et3BH], 1 M in THF, were purchased
from Aldrich and used as obtained. Powder diffraction patterns were
recorded on a Philips X′Pert diffractometer using Cu KR radiation.
Recording times of 48 h were applied for the nanocomposites. Me3-
SiTePh, Me3SiSePh, and Me3SiSPh and SBA-15 silica with a pore
diameter of d ) 7 nm were synthesized according to literature
procedures.14,20-22
TEM was performed using a JEOL JEM 3010 electron micro-
scope (300 kV; LaB6 cathode). The samples were finely dispersed
in ethanol. One drop of the dispersion was placed on a carbon-
coated copper grid and allowed to dry, leaving the crystallites in a
random orientation on the support film.
Thermal analyses were carried out under a dynamic argon
atmosphere with a Netzsch Pegasus 404 DSC calorimeter.
Preparation of Cd(SPh)2‚TMEDA. In a Schlenk flask, 0.137
g (0.75 mmol) of CdCl2 and 6 mL of TMEDA were dissolved in
30 mL of toluene. An amount of 0.273 g (1.5 mmol) of Me3SiSPh
in 20 mL of toluene was added at room temperature, and the
resulting reaction mixture was stirred for 8 h. The mixture was
filtered to remove a small quantity of a white solid, and the filtrate
was evaporated to about half of its volume under reduced pressure.
The resulting light yellow solution was stored at -24 °C to give
colorless crystals of Cd(SPh)2‚TMEDA (mp 106 °C, 0.423 g, 0.95
mmol, 63% yield).
Preparation of Cd(TePh)2‚TMEDA. A suspension of 1.80 g
(3.45 mmol) of Cd(TePh)2 in 30 mL of toluene was stirred at room
temperature, 2 mL of TMEDA was added, and the mixture was
stirred for an additional 24 h. An amount of 15 mL of pyridine/
octane (1:2, v/v) was added to the light yellow solution, and the
solution was filtered. The filtrate was evaporated to dryness, and
the residue was dissolved in 20 mL of toluene. The solution was
filtered again, the toluene was removed under vacuum, and a light
yellowish solid was obtained (mp 101 °C, 60% yield).
1H NMR (CDCl3): δ 7.74-7.72 (m, 4H), 7.15-7.07 (m, 6H),
2.41 (s, 4H), 2.29 (s, 12H). 13C NMR (CDCl3): δ 137.7 (m-C),
129.4 (o-C), 128.0 (p-C), not detected (ipso-C), 57.1 (-N-CH2),
47.4 (N-CH3). Anal. Calcd for C18H26CdN2Te2: C, 33.88; H, 4.11;
N, 4.38. Found: C, 33.47; H, 4.37; N, 4.59.
Synthesis of the Nanocomposites. A mixture of 20 mg of Cd-
(EPh)2‚TMEDA (E ) S, Se, Te) and 50 mg of calcined SBA-15
silica was heated to 140 °C for 4 h. The temperature was raised to
350 °C at a rate of 1 K/min, and the sample was kept at this
temperature for 16 h. The product was cooled to room temperature
under a constant flow of argon. It was washed twice with 20 mL
of hexane to remove potentially remaining organic residues and
dried under vacuum. A yellowish product was obtained for
CdS@SBA-15, whereas gray powders were obtained for CdSe@
SBA-15 and CdTe@SBA-15.
(27) Bawendi, M. G.; Kortan, A. R.; Steigerwald, M. L.; Brus, L. E. J.
Chem. Phys. 1989, 91, 7282.
(28) Ricolleau, C.; Audinet, L.; Gandais, M.; Gacoin, T. Eur. Phys. J. D
1999, 9, 565.
(29) Ricolleau, C.; Audinet, L.; Gandais, M.; Gacoin, T.; Boilot, J.-P.;
Chamarro, M. J. Cryst. Growth 1996, 159, 861.
(30) Lincot, D.; Mokili, B.; Froment, M.; Corte`s, R.; Bernard, J.-P.; Witz,
C.; Lafait, J. J. Phys. Chem. B 1997, 101, 2174.
Calorimetric Measurements. An amount of 30 mg of Cd(EPh)2‚
TMEDA (E ) S, Se, Te) was placed in a DSC pan consisting of
a Pt/Rh mantle and a Al2O3 inlay and covered with a Pt/Rh lid.
The calorimeter was evacuated twice and filled with argon before
the measurements. The measurements were carried out under a
constant gentle flow of argon at a heating rate of 10 K/min.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5895

Yosef et al.
Summary
this tendency is observed for the particles in the mesopores
of SBA-15 just as in aqueous solution, it can be concluded
that the walls of this template provide an environment of
sufficient polarity to allow the small incorporated particles
investigated here to adopt their thermodynamically controlled
minimum. This new role of the silica template, not only to
define the particle size by an appropriate pore size but also
to control the volume crystal structure of the produced
particles, opens a generally new and different aspect of
tailoring particle properties by template-assisted syntheses
of nanoparticles.
Organochalcogenides of the composition Cd(XPh)2‚
TMEDA proved to be useful and versatile single-source
precursors for the synthesis of nanoscale particles of the
whole family of cadmium chalcogenides CdX (X ) S, Se,
Te). Nanocomposites of the type CdX@SBA-15 were
synthesized by melt infiltration of the mesopores of SBA-
15 silica with the organochalcogenolate precursors and
subsequent thermolysis of the precursor in the template.
Following this protocol, we obtained CdS in the wurtzite
structure, CdTe in the sphalerite structure, and CdSe as a
polytype involving both. The nanocomposites were charac-
terized by X-ray powder diffraction and by TEM, showing
highly crystalline particles in the mesopores of the template.
After removal of the silica host with an aqueous base, the
crystallinity and morphology of the cadmium chalcogenide
particles remained unchanged. These findings regarding the
structural chemistry of the three cadmium chalcogenides
nicely reflect the thermodynamic stability of these dimorphic
compound semiconductors with wurtzite as the stable phase
for CdS, zinc blende as the stable phase for CdTe, and a
mixed polymorph for CdSe, where the difference in the
energy content of the two dimorphs is negligible. Because
Acknowledgment. Financial support by the DFG (SCHL
529/2-1 and FR 1372/10-1, SPP 1165 “Nanowires and
Nanotubes”), the Fonds der Chemischen Industrie, and the
Dr. Otto-Ro¨hm Geda¨chtnisstiftung is gratefully acknowl-
edged.
Supporting Information Available: Crystal structures of Cd-
(SPh)2‚TMEDA and Cd(SePh)2‚TMEDA and details of the crystal
structure determinations. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC050517H
5896 Inorganic Chemistry, Vol. 44, No. 16, 2005