New organometallic single-source precursors for CuGaS2 - Polytypism in gallite nanocrystals obtained by thermolysis

Article abstract of DOI:10.1039/c2dt30904a

The complex [(ⁱPr₃PCu)₂(Me ₂Ga)₂(SCH₂CH₂S)₂] (4) was synthesized from trimethylgallium, [(ⁱPr₃PCu) ₄(SCH₂CH₂S)₂] (1) and ethanedithiol by elimination of methane. The related monomethyl compound [(ⁱPr ₃PCu)₂(MeGaSPh)₂(SCH₂CH ₂S)₂] (5) has been prepared from [(ⁱPr ₃PCuSPh)₃] (2) and [(MeGaSCH₂CH ₂S)₂] (3) by a ligand exchange reaction in tetrahydrofuran solution. The molecular structures of 1 and 3-5 were determined by single crystal X-ray diffraction. Thermolysis of 4 and 5 results in the formation of the ternary semiconductor CuGaS₂, gallite. The residue of 5 was characterized using X-ray powder diffraction, energy dispersive X-ray spectroscopy and transmission electron microscopy. The CuGaS₂ crystals obtained are mainly hexagonal plates of around 200 to 300 nm diameter and 10 to 30 nm thickness, exhibiting an unusual metastable hexagonal crystal structure, related to wurtzite. Partially, the usual tetragonal chalcopyrite structure or its disordered cubic zinc-blende analogue is realized by stacking faults, resulting in an overall similarity to the zinc-blende-wurtzite polytypism in ZnS and related compounds. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2dt30904a

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PAPER
New organometallic single-source precursors for CuGaS2 – polytypism in
gallite nanocrystals obtained by thermolysis†
Oliver Kluge,a Dirk Friedrich,a Gerald Wagnerb and Harald Krautscheid*a
Received 25th April 2012, Accepted 28th May 2012
DOI: 10.1039/c2dt30904a
The complex [(iPr3PCu)2(Me2Ga)2(SCH2CH2S)2] (4) was synthesized from trimethylgallium,
[(iPr3PCu)4(SCH2CH2S)2] (1) and ethanedithiol by elimination of methane. The related monomethyl
compound [(iPr3PCu)2(MeGaSPh)2(SCH2CH2S)2] (5) has been prepared from [(iPr3PCuSPh)3] (2) and
[(MeGaSCH2CH2S)2] (3) by a ligand exchange reaction in tetrahydrofuran solution. The molecular
structures of 1 and 3–5 were determined by single crystal X-ray diffraction. Thermolysis of 4 and 5 results
in the formation of the ternary semiconductor CuGaS2, gallite. The residue of 5 was characterized using
X-ray powder diffraction, energy dispersive X-ray spectroscopy and transmission electron microscopy.
The CuGaS2 crystals obtained are mainly hexagonal plates of around 200 to 300 nm diameter and
10 to 30 nm thickness, exhibiting an unusual metastable hexagonal crystal structure, related to wurtzite.
Partially, the usual tetragonal chalcopyrite structure or its disordered cubic zinc-blende analogue is
realized by stacking faults, resulting in an overall similarity to the zinc-blende–wurtzite polytypism in
ZnS and related compounds.
Introduction
The ternary compound semiconductors CuME2 (M = Ga, In;
E = S, Se) have attracted considerable attention due to their appli-
cability as light absorbing materials in high efficiency thin film
solar cells. In the industrial fabrication process, the thin films are
usually deposited by co-evaporation of the elements or by sulfur-
ization/selenization of thin metal films.1 Efforts have been made
to synthesize molecular single-source precursors (SSPs) for these
semiconductors,2 because a solvent-assisted spray pyrolysis
process could simplify and improve the thin film processing in
view of the deposition parameters requested, e.g. lower tempera-
ture, atmospheric pressure and reliable stoichiometry control.3
Here we report on the synthesis, characterization and thermolysis
of new organometallic SSPs for CuGaS2.
The crystal structure of CuGaS2 has first been investigated in
a systematic study of 20 compounds with the general formula
MIMIIIE2 (MI = Cu, Ag; MIII = Al, Ga, In, Tl; E = S, Se, Te); all
of these “Harry Hahn phases” were found to crystallize in the
4
ˉ
tetragonal chalcopyrite structure (space group I42d, Fig. 1a). If
the metal ions are statistically disordered in this tetragonal
aInstitut für Anorganische Chemie, Universität Leipzig, Johannisallee
29, 04103 Leipzig, Germany. E-mail: krautscheid@rz.uni-leipzig.de;
Fax: +341-9736199; Tel: +341-9736172
bInstitut für Mineralogie, Kristallographie und Materialwissenschaft,
Universität Leipzig, Scharnhorststr. 20, 04275 Leipzig, Germany
†Electronic supplementary information (ESI) available: Rietveld refine-
(b) cation-disordered cubic zinc-blende modification, (c) hypothetical
ment plots of powder patterns obtained from thermolysis residues of 4
and 5, additional TEM images and EDX measurements for the residue
of 5. CCDC 879009–879012. For ESI and crystallographic data in CIF
Fig. 1 Four structures of CuGaS2: (a) tetragonal chalcopyrite structure,
ordered orthorhombic phase and (d) cation-disordered hexagonal wurt-
zite-CuGaS2. The different anion layer stacking is indicated; the unit
cells are marked.
or other electronic format see DOI: 10.1039/c2dt30904a
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structure, the c-axis is halved, leading to the cubic zinc-blende
[Cu(SON(CNiPr2)2)2] and [In(SON(CNiPr2)2)3] in a mixture of
dodecanethiol and oleylamine.7g
ˉ
structure (space group F43m, Fig. 1b). Both structures exhibit a
repeated ABC-stacking of identical anion layers. Similar to hexa-
gonal ZnS, wurtzite (space group P63mc), a cation-disordered
phase with AB-stacking of identical layers can be constructed
for CuGaS2 (Fig. 1d). Ordering of the cations in this wurtzite-
CuGaS2 may result in an orthorhombic modification (space
group Pna21, Fig. 1c), which is known to exist as the high-temp-
erature phase of AgInS2.5 The tetrahedral coordination of the
atoms is retained in all cases, therefore they are energetically
nearly equivalent.
Shifting of the layers, and thereby the zinc-blende–wurtzite
phase transition requires an intermediate trigonal planar coordi-
nation sphere.5 The zinc-blende–wurtzite polytypism is well
known from binary semiconductors, for which it was shown that
the zinc-blende structure is monotonically stabilized over the
wurtzite structure as the anion size increases (except for first-row
cations).6
As it turned out, the bulk thermolysis of the new SSPs pre-
sented herein produces CuGaS2 nanocrystals with hexagonal
domains, which were thoroughly characterized by X-ray powder
diffraction, energy dispersive X-ray spectroscopy (EDX) and
transmission electron microscopy (TEM).
Results and discussion
Synthesis and molecular structures of 1–5
The copper complex [(iPr3PCu)4(SCH2CH2S)2] (1) can be pre-
pared from triisopropylphosphine, copper(I)-oxide and ethane-
dithiol in toluene by elimination of water. The complex is
tetranuclear and contains an eight-membered Cu4S4-ring
(Fig. 2a), as revealed by single crystal X-ray structure determi-
nation (space group P21/c).‡ The molecule is centrosymmetric,
the two independent copper atoms are coordinated in a trigonal
planar fashion by two sulphur atoms and one phosphine ligand,
each. Atom Cu1 is chelated by the ethanedithiolate ligand,
whereas Cu2 connects the ethanedithiolate ligand and its sym-
metry equivalent. The Cu–S bond lengths range from 225 to 230
pm, which are typical values for threefold coordinated copper.
Because of the relatively rigid bite angle of the ethanedithiolate
ligand, the angle S1–Cu1–S2 (97°) is much smaller than S1–
Cu2–S2′ (122°). The resulting difference in the steric strain at
the two independent copper atoms manifests in two different
Cu–P bond lengths (219 and 225 pm).
Recent attempts to produce ternary semiconductor nanoparti-
cles led to the formation of hexagonal CuInS2,7 CuInSe2 and
8
CuIn1−xGaxS2 (0 ≤ x ≤ 1).9 In the latter case, wurtzite-CuGaS2
(x = 1) was obtained as ‘tadpole’-shaped crystals with typical
lengths of about 30 to 50 nm.9 To our knowledge, this recent
work is the only report about hexagonal wurtzite-CuGaS2 so far.
The motivation for the synthesis of these semiconductor nano-
particles is that their colloidal dispersions may serve as inks for
high-throughput printing techniques applicable for thin film
deposition. Usually, the synthesis of the nanoparticles is
achieved by mixing metal salts with a chalcogen source in high
boiling coordinating solvents, and heating this mixture to elev-
ated temperatures of 100 to 350 °C. A SSP has been used to
produce CuInS2 nanoparticles in a mixture of dodecanethiol and
trioctylphosphine.7e The ratio of these surfactants as well as
the reaction temperature were found to influence the crystal
structure (wurtzite or zinc-blende) of the product.7e Stacking
faults in the nanocrystals were reported for CuInS2 obtained
from metal chlorides and thiourea in oleylamine,7d and from
The
compounds
[(iPr3PCuSPh)3]
(2)
and
[(MeGaSCH2CH2S)2] (3) have been described in the literature
already.10,11 For 2 the synthesis as well as the crystal structure
were reported. The complex can be obtained from copper(I)-
acetate, triisopropylphosphine and Me3SiSPh. The crystal struc-
ture is built up from nearly planar six-membered Cu3S3-rings,
with one phosphine ligand bound to each copper atom and a
rarely observed planar environment for some of the sulfur
atoms.10 Compound 3 was synthesized from Me3Ga and ethane-
dithiol, and characterized spectroscopically. Its low solubility in
common organic solvents was attributed to association in the
solid state, and from mass spectra it was concluded that it might
be trimeric.11 According to the structural determination by single
crystal X-ray diffraction reported herein, the compound is
‡Crystallographic data: 1 (C40H92Cu4P4S4), M = 1079.42, monoclinic, a
= 11.4153(6), b = 21.140(1), c = 11.0770(6) Å, β = 100.105(5)°, V =
2631.6(2) Å3, T = 180(2) K, space group P21/c (no. 14), Z = 2, 15 748
reflections measured, 5733 unique (Rint = 0.0235), R1 = 0.0197 (I >
2σ(I)), wR2 = 0.0439 (all data). 3 (C6H14Ga2S4), M = 353.85, triclinic, a
= 6.603(1), b = 6.761(1), c = 7.116(1) Å, α = 73.93(1)°, β = 80.80(1)°, γ
3
ˉ
= 83.69(1)°, V = 300.57(8) Å , T = 180(2) K, space group P1 (no. 2), Z
= 1, 3134 reflections measured, 1452 unique (Rint = 0.0288), R1
=
0.0161 (I > 2σ(I)), wR2 = 0.0397 (all data). 4 (C26H62Cu2Ga2P2S4), M =
831.46, monoclinic, a = 17.437(1), b = 15.7295(7), c = 15.365(1) Å, β
= 116.444(5)°, V = 3773.3(4) Å3, T = 105(2) K, space group C2/c (no.
15), Z = 4, 12 527 reflections measured, 4106 unique (Rint = 0.0339), R1
= 0.0188 (I > 2σ(I)), wR2 = 0.0464 (all data). 5·2 THF (C44H82Cu2-
Ga2O2P2S6), M = 1163.92, triclinic, a = 10.600(1), b = 11.507(1), c =
13.693(2) Å, α = 67.290(7)°, β = 72.432(8)°, γ = 65.161(7)°, V =
1378.6(2) Å , T = 180(2) K, space group P1 (no. 2), Z = 1, 11 622
reflections measured, 5374 unique (Rint = 0.0796), R1 = 0.0535 (I >
2σ(I)), wR2 = 0.1409 (all data).
Fig. 2 Molecular structures of (a) [(iPr3PCu)4(SCH2CH2S)2] (1) and
(b) [(MeGaSCH2CH2S)2] (3). Thermal ellipsoids of C, Cu, Ga, P and S
are drawn at 50% probability level. Selected bond lengths [pm] and
angles [°] for 1: Cu1–S1 225.78(4), Cu1–S2 227.31(4), Cu2–S1 225.97
(4), Cu2–S2′ 229.65(4), Cu1–P1 219.46(4), Cu2–P2 224.74(4), P1–
Cu1–S1 138.89(1), P1–Cu1–S2 122.90(2), S1–Cu1–S2 97.05(1), P2–
Cu2–S1 117.29(2), P2–Cu2–S2′ 120.95(2), S1–Cu2–S2′ 121.51(1),
Cu1–S1–Cu2 105.05(2), Cu1–S2–Cu2′ 113.19(2).
3
ˉ
8636 | Dalton Trans., 2012, 41, 8635–8642
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dimeric in the solid state, but this may be different in solution
and the gas phase, as it is well known that organometallic chal-
cogenolates of group 13 metals undergo exchange reactions
involving different aggregates, when dissolved.12 The molecular
structure in the crystal is shown in Fig. 2b. The central Ga2S2-
ring and the chair-like structure is generated out of the asym-
metric MeGaSCH2CH2S-unit by the crystallographic inversion
C1–Ga1–C2 (124°), whereas the angle S1–Ga1–S2′ is nearly
ideal for tetrahedral coordination.
The ternary complex [(iPr3PCu)2(MeGaSPh)2(SCH2CH2S)2]
(5) can be obtained by mixing 2 and 3 in tetrahydrofuran
(Scheme 1). The complex co-crystallizes with solvent molecules
ˉ
in space group P1, but these can be easily removed in vacuum,
resulting in a solvent-free powder of 5. Obviously, ligand
exchange reactions take place in solution, since the SPh−-ligand
initially bound only to copper atoms in 2 is found as a terminal
ligand at the gallium atom in 5 (Fig. 4). It is noteworthy that
attempts to synthesize 5 from 4 and HSPh were not successful.
In 4, the methyl groups at the gallium atom do not show any
reactivity towards thiols. Although synthesized in a different
manner, it is useful to compare the structures of 4 and 5 by
assuming a substitution of one of the methyl groups at the
gallium atom by a phenylthiolate ligand. The molecule of 5
again contains a crystallographic inversion centre. The bond
lengths and angles in the (iPr3PCuSCH2CH2S)−-units containing
the chelated copper atoms are similar to those in 1 and 4. The
Ga–S bonds in the Cu2Ga2S4-ring system are around 232 pm,
and are therefore somewhat shorter than in 4, which is the
expected effect of the substitution of the more covalently bound
methyl group by a partially datively bound thiolate ligand.13 As
expected, the terminal bond Ga1–S3 (227 pm) is shorter than the
bridging Ga–S bonds in the ring.
ˉ
centre (space group P1). The Ga–S bond lengths in this ring are
237.18(5) pm for Ga1–S2 and 239.69(5) pm for Ga1–S2′, which
are typical values for Ga⋯Ga-μ2-bridging thiolate ligands. The
terminal bond Ga1–S1 is relatively short (222.93(5) pm). The
Ga–C bond length is 194.5(2) pm. As can be expected from the
different covalent vs. dative character of Ga–C and Ga–S
bonds,13 the angles C–Ga–S (112° to 125°) are larger than the
angles S–Ga–S (93° to 106°). The smallest bond angle of 86.02
(2)° is Ga1–S2–Ga1′.
The ternary complex [(iPr3PCu)2(Me2Ga)2(SCH2CH2S)2] (4)
can be obtained from the binary complex 1, trimethylgallium
and ethanedithiol in toluene solution (Scheme 1). The molecular
structure (Fig. 3) in the crystal (space group C2/c) is related to
that of 1, again with a crystallographic inversion center in the
molecule. The two chelated copper units are retained, but the
other copper atoms in 1 are replaced by gallium atoms. There-
fore, the eight-membered Cu4S4-ring system in 1 changes into
an eight-membered Cu2Ga2S4-ring system in 4. The bond
lengths and angles in the (iPr3PCuSCH2CH2S)−-units containing
the chelated copper atoms are nearly the same as in 1. One of
i
Thermolysis of 4 and 5
the Pr-groups in the phosphine ligand is slightly disordered; the
disorder could be resolved by measuring the crystal at lower
temperature (105 K). The Ga–S bond lengths are around 235
pm. As expected,13 the largest bond angle at the gallium atom is
The thermolysis of 4 and 5 was investigated by thermogravime-
try (TG), differential scanning calorimetry (DSC) and simul-
taneous mass spectrometry. The curves recorded are shown in
Fig. 5. According to the TG curve of compound 4, the thermoly-
sis starts at a temperature of 201 °C and ends at 320 °C. The first
derivative of this TG curve (DTG) shows two distinct minima,
Scheme 1 Synthesis of 4 and 5 from 1–3.
Fig. 3 Molecular structure of [(iPr3PCu)2(Me2Ga)2(SCH2CH2S)2] (4).
Thermal ellipsoids of C, Cu, Ga, P and S are drawn at 50% probability
level. Selected bond lengths [pm] and angles [°]: Cu1–S1 227.01(4),
Cu1–S2 226.04(4), Cu1–P1 220.24(4), Ga1–S1 235.65(4), Ga1–S2′
234.49(4), Ga1–C1 197.2(2), Ga1–C2 197.4(2), P1–Cu1–S1 126.88(2),
P1–Cu1–S2 134.76(2), S1–Cu1–S2 94.42(2), Cu1–S1–Ga1 118.68(2),
Cu1–S2–Ga1′ 116.04(2), S1–Ga1–S2′ 109.42(1), C1–Ga1–C2 123.84
(9).
Fig. 4 Molecular structure of [(iPr3PCu)2(MeGaSPh)2(SCH2CH2S)2]
(5). Thermal ellipsoids of C, Cu, Ga, P and S are drawn at 50% prob-
ability level. Selected bond lengths [pm] and angles [°]: Cu1–S1 227.6
(1), Cu1–S2 229.0(1), Cu1–P1 221.4(1), Ga1–S1 231.6(1), Ga1–S2′
233.3(1), Ga1–S3 226.7(1), Ga1–C1 196.8(5), P1–Cu1–S1 131.97(5),
P1–Cu1–S2 133.36(5), S1–Cu1–S2 93.06(4), Cu1–S1–Ga1 120.23(6),
Cu1–S2–Ga1′ 116.35(6), S1–Ga1–S2′ 112.04(5), S3–Ga1–S1 98.90(5),
S3–Ga1–S2′ 107.89(5), C1–Ga1–S3 117.7(2).
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corresponding to a two-step decomposition process. The first
step involves the release of the phosphine ligands, as can be seen
from the m/z curves (iPr3P+, m/z = 160). This first step is com-
pleted at 255 °C and corresponds to the partial mass change of
38.4%, in good agreement with the mass proportion of iPr3P in 4
(38.5%). The mass loss proceeds slower in the second step, and
some n-butane (C4H10+, m/z = 58) is detected, which may be a
recombination product from methyl groups at the gallium atom
and the ethylene bridges of the ethanedithiolate ligand. The m/z
agreement with the calculated value (61.3%) for two formula
units CuGaS2 out of one molecule of 5.
The greenish brown residues of several thermolysis exper-
iments at different temperatures were investigated by X-ray
powder diffraction (XRPD). The determination of the unit cell
constants and an estimation of the fractions of the phases present
were achieved by Rietveld refinement (see Fig. 6 and ESI†). The
powder patterns obtained from residues of 4 always display the
reflection peaks expected for tetragonal CuGaS2. Additionally,
peaks corresponding to hexagonal CuGaS2 and Cu2−xS, digenite,
are observed. The presence of digenite indicates that some
Me3Ga is released during the thermolysis. The phase fraction of
the cation-disordered hexagonal modification of CuGaS2 is
higher when the maximum temperature of the thermolysis is
lower, and the respective reflections completely vanish when the
maximum temperature is 900 °C. Depending on thermolysis
conditions, the phase fraction of digenite ranges from 9% to
24%.
In contrast, no binary copper sulfides are observed in powder
patterns obtained from residues of 5. The reflection peaks are
generally broader, indicating smaller crystal sizes. The reflections
expected for the usual ordered tetragonal phase of CuGaS2 do
not show the required tetragonal splitting, which can be assigned
to cation disorder, i.e. cubic CuGaS2. When a sample of 5 is
heated slightly above the end temperature of the thermal
decomposition process, the residue consists of approximately
61% hexagonal and 39% cubic CuGaS2 (Fig. 6). The hexagonal
unit cell axes were determined to be a = 374.28(5) pm and c =
621.6(2) pm, the cubic unit cell axis was determined to be a =
531.67(5) pm. The resulting volume of the hexagonal unit cell is
75.41(3) Å3 (Z = 1), and compares well with the volume of the
cubic unit cell, which is 150.29(5) Å3 (Z = 2).
+
curve for methyl fragments (CH3 , m/z = 15) shows a maximum
for each decomposition step. The overall mass loss of 60.7% is
higher than expected (52.5%) for the formation of two formula
units CuGaS2 out of one molecule of 4, indicating the loss of
fragments which contain Cu, Ga and/or S atoms.
For compound 5 a distinct endothermic signal is observed in
the DSC curve at 107 °C, which can be assigned to the melting
of the compound. The mass change and thereby the decompo-
sition starts at 216 °C (extrapolated from the TG curve). The first
derivative of the TG curve displays three minima, corresponding
to a three-step decomposition process. From the m/z curves it
can be concluded that the first step involves the release of phos-
phine ligands (iPr3P+, m/z = 160), and in the second step
n-butane (C4H10+, m/z = 58) is detected. In the third step the
+
phenyl groups (C6H6 , m/z = 78) are released, presumably as
SPh2. The end temperature extrapolated from the TG curve is
395 °C. The overall mass change of 58.9% is in reasonable
In order to characterize the unusual hexagonal modification of
CuGaS2 in more detail, the residue of 5 obtained at a maximum
temperature of 400 °C was investigated by temperature depen-
dent XRPD, TEM and EDX. From temperature dependent
XRPD (Fig. 7) it can be seen that the metastable cation-
Fig. 5 Thermolysis of compounds (a) 4 and (b) 5. The TG curve, its
first derivative (DTG), the DSC curve and selected ion current curves
are shown. (a) m/z = 58 multiplied by factor 20, m/z = 160 multiplied by
factor 2000; (b) m/z = 78 multiplied by factor 10, m/z = 58 multiplied by
factor 100, m/z = 160 multiplied by factor 500.
Fig. 6 Rietveld refinement (wRp = 0.0946, Rp = 0.0734, R(F2) =
0.0387) of the powder pattern obtained from the residue of 5 heated in a
quartz tube to 400 °C. Observed (+) and calculated intensities, reflection
positions of cubic (first row) and hexagonal (second row) CuGaS2 and
the difference curve are displayed.
8638 | Dalton Trans., 2012, 41, 8635–8642
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Fig. 7 Temperature dependent XRPD patterns of the residue of 5. The
112, 220 and 024 reflections from the tetragonal phase are indexed at
T = 700 °C.
disordered hexagonal phase transforms slowly into the usual
tetragonal phase at temperatures above approximately 600 °C,
where the relative intensities start to shift, respectively. The
phase transition is accompanied by ordering of the metal ions;
the tetragonal reflection splitting is well resolved at 700 °C, but
the peaks of the hexagonal phase are still present at this tempera-
ture, because completion of the phase transition takes several
hours of time. Similar results have been obtained for thermally
treated samples of hexagonal CuInS2, although there were sig-
nificant intensity shifts at 500 °C. Four hours at 600 °C resulted
in complete transformation.7b Therefore, the gallium homologue
seems to be somewhat more stable, kinetically.
Fig. 8 TEM bright-field image of CuGaS2 nanocrystals sedimented by
drop-casting. Crystals 1, 2 and 3 in (0001) orientation, where in 3 two
crystals overlap. Inset: electron diffraction pattern of crystal 1.
According to several scanning TEM images in combination
with EDX mapping, the residue consists of hexagonal crystal
plates with 200 to 300 nm diameter and 10 to 30 nm thickness,
with Cu, Ga and S homogeneously present (various images are
included in the ESI†). Nano-probe EDX measurements of the
three crystals shown in Fig. 8 give, in agreement with the
formula CuGaS2, an average of 24.7 at% Cu, 25.0 at% Ga and
50.3 at% S. The selected area electron diffraction (SAED)
patterns of these crystals show sixfold symmetry (Fig. 8), con-
sistent with the hexagonal crystal system. It should be noted here
that a tetragonal or cubic CuGaS2 crystal would also display
sixfold symmetry along the zone axis [221] or [111],
respectively.
Fig. 9 HRTEM lattice image of a CuGaS2 crystal. Beam direction par-
allel to [0001]. The inset represents the FFT of the image shown.
A high resolution TEM (HRTEM) image of a hexagonal
CuGaS2 crystal with beam direction parallel to the [0001] zone
axis is shown in Fig. 9. The d(100) spacing is in agreement with
the value a/2 × √3 = 324 pm, which has to be expected from
the hexagonal unit cell determined by Rietveld refinement. The
indexing of the fast Fourier transform (FFT) of the HRTEM
image is consistent with a hexagonal modification of CuGaS2; in
the tetragonal and cubic phases these reflections would be
extinguished.
TEM images with beam direction perpendicular to the [0001]
zone axis of the nanocrystals are shown in Fig. 10 and 11. It can
be seen that the crystals are friable along the c-axis, the smeared
reflections (streaks) in the SAED pattern indicate the presence of
stacking faults. The AB-stacking of the layers in the hexagonal
domains and the ABC-stacking in the cubic domains are clearly
distinguishable in the HRTEM image shown in Fig. 11, and are
consistent with simulated HRTEM images (see ESI†). The cubic
domains are preferably located in the surface region, while the
hexagonal domains build up most of the bulk of the
nanocrystals.
Experimental section
All reactions were carried out in an atmosphere of dry nitrogen
by standard Schlenk line and dry box techniques. THF and
toluene were dried over sodium–benzophenone and n-heptane
was dried over CaH2. All solvents were distilled under a nitrogen
atmosphere prior to use. Ethanedithiol and Cu2O were purchased
i
commercially. Me3Ga–OEt2 and Pr3P were prepared according
to the literature.14,15 Elemental analyses were carried out with a
Vario EL-Heraeus microanalyzer. 1H (400 MHz) and 31P
(161.9 MHz) NMR spectra were recorded with C6D6 as the
solvent on a Bruker Avance DRX-400 instrument using SiMe4
as the internal standard and H3PO4 as the external standard,
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ensure completion of the reaction and to obtain single crystals of
appropriate size for structure determination by X-ray diffraction.
Synthesis of [(iPr3PCu)2(Me2Ga)2(SCH2CH2S)2] (4). In a
mixture of 10 ml toluene and 1 ml THF, 0.65 g (0.6 mmol) of 1
were dissolved, and 0.45 ml (2.4 mmol) of Me3Ga–OEt2 and
1.2 ml of a 1 molar solution of ethanedithiol (1.2 mmol) in
toluene were successively added via a syringe, resulting in a
yellow solution. The solution was stirred for 6 h at ambient
temperature, while methane gas evolution was observed, and the
colour of the reaction mixture faded to colourless. The solution
was layered with 10 ml of n-heptane, kept overnight at 2 °C and
then for three days at −20 °C. Colourless crystals of 4 (0.75 g,
78%) formed, while n-heptane slowly diffused into the toluene
solution. Elemental analysis found: C 37.6, H 7.8, S 15.4; calc.
1
for 4: C 37.6, H 7.5, S 15.4. H NMR: 3.30 (m, 4HCH2), 2.80
3
3
(m, 4HCH2), 1.73 (m, 6HCH-iPr), 1.03 (dd, JHH = 7, JPH = 14,
36HCH3-iPr), 0.35 (s, 12HMe). 31P{1H} NMR: 34.4 (br s).
Synthesis of [(iPr3PCu)2(Me2GaSPh)2(SCH2CH2S)2] (5). A
total of 0.25 g (0.7 mmol; 1.4 mmol Ga) of 2 was dissolved in
20 ml THF and 0.48 g (0.5 mmol; 1.4 mmol Cu) of 1 were
added, while stirring at room temperature. Half of the solvent
was removed at reduced pressure and the colourless and clear
solution was stored at −20 °C, resulting in crystallisation of
5·2 THF (0.43 g, 52%) as colourless plates. The yield may be
increased by successive removal of solvent and/or further
cooling of the reaction mixture. The crystal plates of 5·2 THF
lose solvent when removed from the mother liquor, and thereby
decompose to a colourless powder. The powder was dried in
vacuum to give 5 nearly solvent free. Mp 107 °C. Elemental
analysis found: C 42.8, H 6.9, S 18.6; calc. for 5: C 42.4, H 6.5,
Fig. 10 TEM bright-field image of CuGaS2 nanocrystals embedded in
epoxy. Inset: electron diffraction pattern of the crystal encircled.
1
S 18.9; calc. for 5·2 THF: C 45.4, H 7.1, S 16.5. H NMR: 7.83
(br s, 4Hortho-Ph), 7.06 (br s, 4Hmeta-Ph), 6.94 (t, 3JHH = 7, 2Hpara-
Ph), 3.21 (br s, 4HCH2), 2.80 (br s, 4HCH2), 1.72 (m, 6HCH-iPr),
3
3
1.02 (dd, JHH = 7, JPH = 14, 36HCH3-iPr), 0.35 (br s, 6HMe).
31P{1H} NMR: 33.3 (br s).
X-ray crystallographic data of single crystals were collected
on a STOE IPDS 2T diffractometer. Data reduction and numeri-
cal absorption correction were performed with STOE X-RED.16
Structure solutions were carried out with direct methods (SIR
92)17 and refinement with SHELXL-97.18 Hydrogen atoms were
calculated on idealized positions and all non-hydrogen atoms
were refined with anisotropic thermal parameters. Figures were
generated with Diamond 2.1b.19
The thermogravimetric measurements were performed with
the thermobalance NETZSCH STA 409 CD in a stream of N2 at
a heat rate of 10 K min−1. The DSC curve was measured with an
empty Al2O3 crucible as the reference. Thermal analysis was
carried out with the NETZSCH Proteus 5.0.1 software.20
X-ray powder diffraction patterns were measured on a STOE
Stadi P diffractometer in Debye–Scherrer geometry with Cu Kα1
radiation (λ = 1.540598 Å). Room temperature data were
recorded with a linear PSD, high temperature measurements
were performed with a STOE oven type 0.65.3 and a curved
image plate detector. For an improved signal-to-noise ratio,
several single measurements were added. Rietveld refinement
was performed with GSAS.21 Simultaneously refined parameters
included the diffractometer zero point, the fractions and cell con-
stants of present phases, the background function (Chebyschev
polynomial, GSAS background function 1) and profile
Fig. 11 HRTEM lattice image of a CuGaS2 crystal. Beam direction
perpendicular to [0001], in particular parallel to [11–20] of the hexago-
nal phase. Insets: FFT and Bragg-filtered portion of the crystal.
respectively. Chemical shifts are given in ppm, coupling con-
stants are denoted in Hz.
Synthesis of [(iPr3PCu)4(SCH2CH2S)2] (1). To a suspension
of 0.70 g (4.9 mmol) Cu2O in 10 ml toluene, 0.45 ml
i
(5.4 mmol) ethanedithiol and 3.80 ml (19.9 mmol) Pr3P were
added. The reaction mixture was refluxed for 1 hour, resulting in
a clear yellow solution. Storage of this solution for three days at
−20 °C yields colourless crystals of 1 (2.64 g, 100%). Excess of
iPr3P is necessary, but can be recovered by distillation. Elemental
analysis found: C 44.3, H 8.6, S 12.2; calc. for 1: C 44.5, H 8.6,
1
S 11.9. H NMR: 3.10 (br s, 8HCH2), 1.84 (m, 12HCH-iPr), 1.17
3
3
(dd, JHH = 7, JPH = 14, 72HCH3-iPr). 31P{1H} NMR: 30.6
(br s), 19.5 (br s).
Compounds 2 and 3 have been prepared according to the lit-
erature.10,11 For 3 a slightly modified method has been
employed: toluene was used as the solvent instead of benzene,
and the reaction mixture was heated up to 80 °C in order to
8640 | Dalton Trans., 2012, 41, 8635–8642
This journal is © The Royal Society of Chemistry 2012

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parameters (GSAS profile function 4). Asymmetry parameters
for the reflections were taken from the actual diffractometer geo-
metry.22 The cubic crystal structure of CuGaS2 was modelled by
substituting the Zn position in zinc-blende by half Ga and Cu,
each. In the same manner, hexagonal CuGaS2 was modelled by
substituting the Zn position in wurtzite. Because of the crystal
shape and polytypism of the CuGaS2 nanocrystals, as confirmed
by TEM, the micro-strain function of Stephens23 was used to
account for the obvious anisotropic reflection line broadening.
The TEM examinations have been carried out in a Philips CM
200 STEM equipped with a super twin objective lens (U0 = 200
kV; point resolution 0.23 nm). Simulations of electron diffraction
patterns and HRTEM images were done by the JEMS software
package.24 The chemical composition of samples was measured
by a calibrated EDX system adapted to the STEM used (spot
diameter in nano-probe mode 5–10 nm). Nanocrystalline
CuGaS2 samples for transmission electron microscopy (TEM)
were prepared in two ways.
some Me3Ga is released during the thermolysis of 4, but not
from 5. In addition to the usual tetragonal or cation-disordered
cubic modification of CuGaS2, an unusual metastable hexagonal
modification is observed in the residues of 4 and 5 by powder
diffraction. As shown by electron microscopy, the residue of 5
consists of polytypic CuGaS2 nanocrystals. To our knowledge,
this is the first report on hexagonal CuGaS2 obtained from
single-source precursors, and also the first report on polytypism
in CuGaS2.
Acknowledgements
Assistance in synthesis by Jeremias Zill and Martin Welke and
financial support by the University of Leipzig (PbF-1), ESF-LIP
and the graduate school BuildMoNa are gratefully acknowl-
edged. Funded by the European Union and the Free State of
Saxony.
(1) CuGaS2 powder was dispersed in ethanol by ultrasonic
treatment followed by drop-casting onto carbon-coated molyb-
denum and/or nickel TEM grids. In this way the samples are
ready for TEM inspection.
Notes and references
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Conclusions
From Me3Ga, ethanedithiol and the new copper complex [(iPr3P-
Cu)4(SCH2CH2S)2] (1), the organometallic copper dimethyl
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This journal is © The Royal Society of Chemistry 2012