Ion-exchangeable cobalt polysulfide chalcogel

Article abstract of DOI:10.1021/ja1089028

We present a promising approach in synthetic chalcogel chemistry that is extendable to a broad variety of inorganic spacers. Polychalcogenide aerogels with ion-exchange properties are demonstrated in cobalt polysulfide. The new materials show a broad range of pore sizes and high surface area of 483 m ²/g.

Full text of DOI:10.1021/ja1089028

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Ion-Exchangeable Cobalt Polysulfide Chalcogel
Maryam Shafaei-Fallah, Jiaqing He, Alexander Rothenberger, and Mercouri G. Kanatzidis*
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
S Supporting Information
b
ABSTRACT: We present a promising approach in synthetic
chalcogel chemistry that is extendable to a broad variety of
inorganic spacers. Polychalcogenide aerogels with ion-
exchange properties are demonstrated in cobalt polysulfide.
The new materials show a broad range of pore sizes and high
surface area of 483 m2/g.
Figure 1. Precursor solutions in formamide/water (a), combinations
of the solutions and immediate reaction (b), and the rigid wet gel of
KCo6S21 (c).
n addition to the well-established aerogels made from metal
oxides,1 low-density porous materials composed of metal
I
gels, we wanted to alter the anions and focus on the use of ditopic
linear polychalcogenido anions. The long polychalcogenide ligands
would act as struts and bridge metal centers and introduce
openness and porosity into metal polychalcogenide networks. Only a
few metal-polyselenide open networks have been described, but
they lack porosity.10
In the present study, we report the preparation and character-
ization of a porous cobalt polysulfide that was obtained from the
reaction of K2S5 with cobalt acetate. K2S5 was prepared by
heating a stoichiometric mixture of K2S and S for 4 h at 500 °C in
an evacuated sealed quartz tube.11 A total of 0.05 g (0.2 mmol) of
chalcogenides have recently come into focus.2 Such systems could
be capable of combining the electronic properties of chalcogenides
with internal porosity. By replacement of O2- in porous materi-
als with S2-, Se2-, or Te2-, a logical progression in the research
of aerogels is made.3 In particular, there is a distinct attraction in
moving from oxides to chalcogenides because of the increased
covalency (and decreased band gap) in the semiconducting
phases.2b Aerogels based on aggregated binary nanocrystals (e.g.,
CdS, CdSe) and on amorphous GeS2 have been reported.2b,4
These chalcogenide gels by virtue of their more polarizable metal-
chalcogenide surface promise new applications beyond those of
conventional aerogels, including remediation of heavy metals,
energy conversion, chemoselective absorption of molecules, and
catalysis.2a,5
[Co(OAc)2 4H2O] was dissolved in 1 mL of H2O, and 1 mL of
3
formamide was added to it. The pink solution of cobalt acetate
was slowly added to an orange solution of 0.048 g (0.2 mmol)
K2S5 in 1.5 mL of formamide (Figure 1a). After keeping the reaction
undisturbed for 1 week, a black gel was obtained (Figure 1c) and
solvent exchanged with 50:50 ethanol,water 5-6 times. Treat-
ment of the gel then involved 4-5 washings with ethanol over a
week followed by supercritical CO2 drying. This process removes
the majority of the solvent from the pores retaining the gel
network. After critical point drying, the obtained aerogel consists
of very fluffy particles and larger chunks of up to 0.5 cm in diameter.
The scanning electron micrograph (Supporting Information,
Figure S1) of the obtained chalcogel showed the spongy nature
The preparation of chalcogenide gels has been achieved by
three different synthetic routes: Thiolysis (e.g., LaSx, WSx, ZnS,
and GeSx),6 nanoparticle condensation (e.g., ZnS, CdS, CdSe,
PbS),7 and metathesis reactions between soluble chalcogenide
clusters and salts of linking metal ions (e.g., Pt2[M4Q10], M = Ge,
Sn, Q = S, Se).8 Thiolysis of alkoxides, thiolates, silylamides, and
metal alkyl precursors in nonaerobic conditions in the presence
of hydrogen sulfide have been shown to yield metal sulfide gels or
precipitates. The use of the thiolysis route is limited due to the
scarcity of suitable precursors and difficulties in handling and
syntheses. Nanoparticle based aerogels have been prepared by
applying the oxidative condensation of preformed metal chalco-
genide nanoparticles to three-dimensional networks. Also the
sol-gel assembly of nanoparticles into metal chalcogenide aero-
gels is limited to nanoparticles with established syntheses and to
compositions that are robust to gelation conditions. For the
metathesis approach, up until now alkali metal salts of molecular
building blocks, such as adamantane [Ge4Q10]4- or [Sn2Q6]4-
clusters (Q = S).8 In the case of the reaction of thiomolybdate with
cobalt salts, polymeric networks (chalcogels) are constructed from
[MoS4]2- units.9
of the gel for which a skeletal density of 3.02 g cm-3 was determined.
3
The obtained gel was initially characterized by energy dis-
persive X-ray spectroscopy (EDS) which confirmed that most of
the potassium ions were washed out during solvent exchange. In
the dried gel, the elements Co, S, and K are present with an
average ratio Co/S/K of 1:3.5:0.17 (S1). The average composi-
tion of the aerogel can be written as “KCo6S21” (Figure S1 ). This
indicates that the Co2þ ions do not fully neutralize the polysulfide
chains and the Co/Sx network has a residual anionic charge which is
balanced by Kþ ions. The Kþ ions are thus part of the materials
and are located within the pores of the network. These ions are
Because metathesis reactions have been demonstrated to be
particularly useful for the preparation of new compositions of
Received: October 2, 2010
Published: January 7, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja1089028 J. Am. Chem. Soc. 2011, 133, 1200–1202
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for polychalcogenide anions of the type (S2)-, (S3)2-, and S4
(Figure S4 ).13 In the thermogravimetric analysis (TGA), con-
tinuous weight loss is observed upon heating (Figure S8 ). EDS
analysis of the residue after TGA gave an average composition of
Co:S:K = 6:1:1 (S5). From the electronic absorption spectrum
the bandgap of the gel was determined by diffuse reflectance
showing a value of ca. 0.7-1.1 eV (S8).
2-
The nitrogen adsorption-desorption isotherm of the aerogel
exhibits a type-IV adsorption branch with a combination of H1
and H3 hysteresis loops characteristic for a predominantly
mesoporous system (Figure 2c).14 The Barrett-Joynes-
Halenda (BJH) plot of pore size distribution shows a character-
istic broad range of pores (1.8-138 nm) with an average pore
diameter of 12 nm (Figure 2d). The intraparticle microporosity
of 15% was determined using the Dubinin-Radushkevich model
(Table S6). The BET surface area (SBET) of 483 m2/g, to the best
of our knowledge, is one of the largest reported so far for chalcogenide
gels. When compared on a per mole basis, the silica equivalence is
810 m2/g which is comparable to porous silica prepared by wet
methods.15
The K/Co/Sx gel is the first example of an ion-exchangeable
chalcogel. This opens the path for functionalizing these materials
for a variety of uses and enables efforts directed toward the
synthesis of functional chalcogels that have already been shown
to have interesting applications, for example, in the catalysis
of hydrodesulfurization reactions.9 The preparation of the
K/Co/Sx gel by the reaction of an alkali metal polysulfide
(inorganic spacer) with a transition metal salt expands the series
of gel preparations. Introducing porosity in such a straightfor-
ward manner marks the end of the paradigm that surfactants,
bulky organic cations, templates, or preformed clusters or nano-
particles are necessary for the preparation of porous chalcogen-
ide frameworks.
Figure 2. Characterization and properties of the K/CoSx gel. (a) Low
magnification image showing amorphous porosity in most regions,
(b) high magnification image depicting some of the few crystalline
nanoparticles present. They were identified as Co3S4 by electron
diffraction together with simulation (inset). (c) Nitrogen adsorption-
desorption isotherm. (d) Pore-size (V-D) distribution plot calculated
from the adsorption isotherm by the BJH method.
accessible and can be exchanged fully and reversibly with Csþ
ions. (Supporting Information Figure S2, eq 1). Confirmation for
the K/Cs exchange and back again comes from EDS analysis of
the materials.
KCo6S21 þ CsþhCsCo6S21 þ Kþ
ð1Þ
KCo6S21 is the first example of an ion-exchangeable chalcogel
and underscores a new functional property for this class of materials.
This finding suggests that a Co/Sx network has formed with an
overall negative charge that is likely to be the result of polysulfide
ligands with a terminal negative charge and a Kþ counterion
[Co-(S)x-S-Kþ, (x = 0-4)]. Polysulfide ligands can also
chelate the metal atoms in KCo6S21 similar to S2-, (S3)2-, and
(S4)2- ligands in the molecular at-complex (NH4)2[Re2S16].12
The lower S/Co ratio in KCo6S21, however, suggests that here
the majority of polysulfide ions coordinate Co atoms in bridging
mode. Polysulfide ions are in fact the essential ingredient for the
high-porosity aerogel to form.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details and char-
b
acterization of the KCo6S21 gel. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
m-kanatzidis@northwestern.edu
The phase and crystallinity of the xerogel and the aerogel were
investigated by powder diffraction. Whereas the xerogel was
amorphous, the final aerogel features a poorly resolved diffrac-
tion pattern with high noise level and nonlinear background due
to the amorphous character of the gel. The presence of weak
broad crystalline peaks is due to the presence of the cobalt sulfides
Cattierite CoS2 and Linnaeite Co3S4 (Figure S3 ). The presence
of tiny amounts of crystalline Co3S4 in the amorphous frame-
work was also confirmed by transmission electron microscopy
(TEM) (Figure 2a,b). The morphology of the aerogel was
studied by TEM showing a broad range of pores extending from
the meso- (2-50 nm) into the macro- (>50 nm) porous region
(Figure 2a). The pores are arranged irregularly and do not seem
to form channels throughout the aerogel. Instead, a random
porous interconnected network is observed that is typical for
aerogel structures. The infrared spectrum of the final gel showed
some remaining adsorbed solvent after critical point drying. It
also confirmed the presence of polysulfide chains with observed
ν(S-S) bands at 570 (br) and 461 cm-1 that are characteristic
’ ACKNOWLEDGMENT
This work was supported by a grant from the DOW Chemical
Company. A.R. thanks the Deutsche Forschungs Gemeinshaft
DFG (RO 3069/4-1) for a fellowship. This work made use of the
J. B. Cohen X-ray Diffraction Facility supported by the MRSEC
program of the National Science Foundation (DMR-0520513)
at the Materials Research Center of Northwestern University.
TEM work was performed in the EPIC facility of the NUANCE
at Northwestern University.
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