A simple route to complex materials: The synthesis of alkaline earth-transition metal sulfides

Article abstract of DOI:10.1039/c7cc05643e

A simple, low-temperature synthesis of a family of alkaline earth metal chalcogenide thin films is reported. These materials have previously only been produced from demanding, high temperature, high pressure reactions. The decomposition of calcium, barium and copper xanthates leads to the clean formation of CaS, BaS, CaCu₂S₂, β-BaCu₂S₂ and β-BaCu₄S₃.

Full text of DOI:10.1039/c7cc05643e

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DOI: 10.1039/C7CC05643E
Journal Name
ARTICLE
A Simple Route to Complex Materials: the Synthesis of Alkaline
Earth – Transition Metal Sulfides
dReceived 00th January 20xx,
Accepted 00th January 20xx
a
b
a,b*
Mundher Al-Shakban, Peter D. Matthews and Paul O’Brien
DOI: 10.1039/x0xx00000x
A simple, low-temperature synthesis of a family of alkaline earth metal chalcogenide thin films is
reported. These materials have previously only been produced from demanding, high temperature,
high pressure reactions. The decomposition of calcium, barium and copper xanthates leads to the
www.rsc.org/
2 2 2 2 4 3
clean formation of CaS, BaS, CaCu S , β-BaCu S and β-BaCu S .
The synthesis of potentially useful complex materials is a key film. This method results in high quality films,25–27 but also
area for research, and one class of these, metal chalcogenides, relies extensively on toxic hydrazine, limiting its usefulness.7
are of particular interest to the semiconductor industry. The method that we have applied here is simpler in its
Transition metal chalcogenides have been shown to make elegance – we have spin coated single source precursors onto
promising photovoltaic devices, energy storage, electronics a substrate (a method that can be extended to any type of
1
–8
and lubricants. The introduction of an alkaline earth metal material, or size of substrate), and thermally decomposed it to
2
8,29
to make a mixed metal chalcogenides produces a broad variety generate clean metal sulfides.
of structures with distinctive properties.9–15
We, and others, have previously described the use of metal
Metal chalcogenides often provide significant synthetic xanthates [M(S COR) ] (M = transition metal, R = alkyl chain) to
challenges and syntheses often require very high temperatures generate nanostructured metal sufides via AA-CVD, melt
2
x
2
9–39
and/or pressures. Synthetic routes that involve more benign reactions and hot-injection syntheses.
conditions will represent a major step forward. The mixed a good choice as single-source precursors to metal sulfides, as
alkaline earth/transition metal compound CaCu has they decompose cleanly at low temperature. One potential
previously only been synthesized by challenging decomposition pathway is the Chugaev elimination mechanism
has been reported (Scheme 1), from which the only by-products are gases that
Metal xanthates are
2 2
S
a
1
6
ammonothermal process, whilst BaCu
2 2
S
4
0
via a hydrothermal method or in a flux of potassium are readily removed from the reaction system.
1
7,18
thiocyanate.
making use of metal xanthates in melt reactions.
There are numerous ways to deposit thin films of metal [Ca(S
chalcogenides, though it is undoubtedly true that the simpler [(PPh
the better if industrial applications are to be realised. We have xanthates were synthesized according to literature
Here we report a much simpler approach
We report here the simple, low-temperature synthesis of
AS, ACu
2
i
S
2
and ACu
4
S
3
(A = Ca or Ba; Figure 1) from
i
i
2
CO Pr)
2
( PrOH)
3
]
(1)
[Ba(S
2
CO Pr)
2
]
(2)
and
3 2 2 2 2
) Cu(S CO(CH ) OMe)] (3). The calcium and copper
1
9,20
41,42
previously discussed chemical bath deposition (CBD),
as methods,
whilst we report a novel barium xanthate (see
well as aerosol-assisted chemical vapour deposition (AA- Supporting Information for full details).
2
1,22
CVD)
as being general than metal-organic (MO-CVD) or
other CVD techniques, as the precursor does not have to be
2
3,24
volatile, which is a requirement for MO-CVD.
Mitzi has
previously proposed ‘dimensional reduction’, which involves
dismantling an extended metal-anion framework by reacting it
with an ionic reagent resulting in dissolution. The resulting
solution may be deposited and annealed to generate a thin
a.
School of Materials, University of Manchester, Oxford Road, Manchester M13
9
+
PL, UK. E-mail: paul.o’brien@manchester.ac.uk; Fax: +44 (0)161 275 4616; Tel:
44 (0)161 275 4653
b.
School of Chemistry, University of Manchester, Oxford Road, Manchester, M13
PL, UK.
Electronic Supplementary Information (ESI) available: materials and methods,
2 2 2 2
Figure 1. Schematic drawing of the unit cells of (a) CaCu S , (b) β-BaCu S and (c)
β-BaCu S . Teal = Ca, brown = Ba, blue = Cu, yellow = S.
4 3
9
†
TGA, GIXRD, Raman and SEM/EDX data. See DOI: 10.1039/c000000x/
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Journal Name
The Ca and Ba xanthates were synthesized from the pattern JCPDS 04-001-3579, (c) hexagonal CaCu
2 2
S and reference pattern generated
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from Purdy’s work,16 (d) β-BaCu
orthorhombic β-BaCu and reference pattern generated from Iglesias’s work.
S with reference pattern JCPDS 04-008-2860 and (e)
2 2
DOI: 10.1039/C7CC05643E
insertion of CS
procedure of Bezougli et al. (Equation 1). The Cu xanthate
was prepared from CuCl, PPh and potassium 2-
methoxyethylxanthate by a method that we have previously
2
into the metal-alkoxide bond, according to the
43
4 3
S
4
1
3
1
decomposes cleanly at 300 °C to form cubic CaS, with the
grazing incidence X-ray diffraction (GIXRD) pattern agreeing
well with phase formed being that of oldhamite (JCPDS 00-08-
0464, Figure 2a). The unit cell of a = 5.699(2) Å is a close match
to the literature (a = 5.694 Å). The decomposition of the
barium xanthate 2 requires higher temperatures to form a
pure phase. At 550 °C, cubic BaS is formed (Figure 2b) - the
unit cell is a = 6.392(6) Å, which is a good match with the
literature (a = 6.388 Å). CaS and BaS both adopt the cubic
4
2
reported.
–
H2
CS2
i
i
i
i
i
A + PrOH → A(O Pr) ∙푥PrOH → A(S CO Pr) ∙푥PrOH (1)
2
2
2
An assessment of the thermogravimetric profile of 1, 2 and 3
ESI Figure S1) suggests that the complexes breakdown cleanly
in the region of 225-250 °C and 100-125 °C respectively. The
final weight of the residue is in good agreement with the
formation of CaS, BaS and CuS respectively. The suggested
breakdown pathway is shown in Scheme 1.
(
structure of NaCl. 3 forms orthorhombic chalcocite Cu1.73
JCPDS 00-009-0328, ESI Figure S3) at 300 °C, with a unit cell of
a = 11.810(1) Å, b = 27.020(7) Å and c = 13.435(1) Å.
S
(
In order to generate the mixed alkaline earth/transition
metal compounds the appropriate molar ratios of 1/2 and 3
were dissolved in dry THF and spin coated onto glass slides and
annealed in a N
2
atmosphere. For the Ca-Cu system an
annealing temperature of 300 °C was used. The GIXRD pattern
of the obtained films matches that of hexagonal CaCu
reported by Purdy (Figure 2c) and the unit cell parameters of
a = 3.949(3) Å, c = 6.520(3) Å are consistent with that data.16
2
S
2
as
Scheme 1. A generalised breakdown mechanism proposal for the xanthate precursors.
i
i
16
M = Ca, Ba or Cu, L = PrOH or PPh
3
, R = Pr or 2-methoxyethyl.
Thin films of the metal sulfides were prepared by spin-
coating a solution of the precursor xanthates in dry THF onto a
clean glass slide, followed by thermolysis in a tube furnace
under an inert nitrogen atmosphere (ESI Table S1).
Comparing the Raman spectra of this film with that of Cu1.73
the structure produced from the decomposition of 3)
indicates that there is no chalcocite contamination of the
calcium copper sulfide film (ESI Figure S10). In the CaCu
structure, planes of double layer, puckered six member Cu-S
S
(
2 2
S
2
+
rings are separated by Ca (Figure 1a). Energy-dispersive X-ray
spectroscopy (EDX) analysis gives a formula consistent with
that of CaCu
dispersion of the elements (ESI Figure S5 and Table S4).
Unlike CaCu , which exists as only one phase, there are
two phases for BaCu : an α (orthorhombic) and a β
tetragonal) phase. We find at 550 °C, the temperature
required to breakdown 2, that we form a mixture of α- and β-
BaCu from a 2:1 ratio of 3:2 (ESI Figure S2). Increasing the
temperature to 650 °C leads to a loss of the orthorhombic
phase, leaving pure tetragonal -BaCu (Figure 2d, JCPDS 04-
08-2860) with a unit cell of a = 3.907(3) Å, c = 12.648(3)
literature values: a = 3.907 Å, c = 12.640 Å). Note that in this
2 2
S and EDX mapping reveals a homogenous
2 2
S
2 2
S
(
2 2
S

2 2
S
0
Å
(
case we switched to a silicon substrate to reflect the high
temperature of deposition. Compositional analysis by EDX
indicates an appropriate ratio of Ba:Cu:S, which is evenly
distributed across the sample (ESI Figure S6 and Table S4).
Figure 2. The grazing incidence X-ray diffraction (GIXRD) patterns of the thin films. (a)
cubic CaS and reference pattern JCPDS 00-008-0464, (b) cubic BaS and reference
2
| J. Name., 2012, 00, 1-3
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ARTICLE
images of the films (ESI Table S1) indicate that VtiehweArCticaleS/OBnlainSe
DOI: 10.1039/C7CC05643E
films are a lighter brown colour, whilst the others are a dark
black.
The TGA data (ESI Figure S1) indicates that there is unlikely
to be much carbon residue left in the films, as the final
decomposition percentage is in good agreement with the clean
formation of CaS, BaS and CuS. We probed the samples for C
inclusion through EDX by depositing the films on a Si substrate
(
as glass is known to contain carbon) and coated them with an
Au/Pt target. The binary systems (CaS, BaS and CuS) all contain
~
~
0.1 at.% C, whereas the ternary barium-copper films contain
0.3 at.% C (ESI Table S5 and ESI Figure S9). The use of EDX to
determine carbon content quantitatively is notoriously
difficult, however, these values suggest that only a very small
amount of C is included in the final film.
In summary, we have presented a simple, efficient route to
three complex alkaline earth metal-copper sulfides; CaCu
β-BaCu and β-BaCu . This synthesis has been achieved by
a melt reaction of calcium isopropylxanthate and the novel
compounds barium isopropylxanthate and
2 2
S ,
2
S
2
4 3
S
bis(triphenylphosphine)copper 2-methoxyethylxanthate.
Acknowledgements
The authors would like to acknowledge the Iraqi Culture
Attaché in London for financial support (M.A.S.) and the EPSRC
Figure 3. SEM images of thin films of (a) CaS, (b) BaS, (c) hexagonal CaCu
BaCu and (e) β-BaCu
2 2
S (d) β-
2
S
2
4 3
S .
CaCu
the barium family there is another: BaCu
and β phase, though in this case they are both orthorhombic.
The structure consists of BaS
triangular face sharing trigonal Capability in Chemistry, EPSRC grant number EP/K039547/1).
prisms (Figure 1c). The key difference between the α and β We thank Prof. D. Michael P. Mingos (University of Oxford) for
phases is that S caps a rectangular face in α-BaCu
and a helpful discussions.
Previous work has focused on
2
S
2
is the only known calcium copper sulfide, but for (Doctoral Prize for P.D.M., grant EP/M507969/1). Some of the
. This also has an α equipment used in this study were provided by the
Engineering and Physical Sciences Research Council (Core
4 3
S
4
4
6
4 3
S
43,44
4 3
triangular face in β-BaCu S .
the synthesis of these materials through a vapour transport
method, and Iglesias et al. noted an α to β phase transition at
Conflicts of Interest
There are no conflicts of interest to declare.
4
3,44
6
40
temperature phase, β-BaCu
The unit cell of a = 4.053(1) Å, b =13.848(3) Å and c = 10.377(2)
±
10°C.
We have successfully synthesized the high
4 3
S
, at 580 °C on glass (Figure 2e).
Å agrees with the literature values of a = 4.058 Å, b = 13.863 Å Notes and references
and c = 10.373 Å and the EDX analysis is consistent with the
target formula (ESI Figure S7 and Table S4).
It is clear from the GIXRD patterns that there is a broad
2 2
hump in the CaS, BaS and CaCu S spectra in the 25-30°
region. This is due to the glass signal showing through from
beneath the film. We have calculated the penetration
depth, which is the thickness of the sample contributing to
1
2
3
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