Molecules versus Nanoparticles: Identifying a Reactive Molecular Intermediate in the Synthesis of Ternary Coinage Metal Chalcogenides

Article abstract of DOI:10.1021/acs.inorgchem.0c00758

The identification of reactive intermediates during molecule-to-nanoparticle (NP) transformation has great significance in comprehending the mechanism of NP formation and, therefore, optimizing the synthetic conditions and properties of the formed products. We report here the room temperature (RT) synthesis of AgCuSe NPs from the reaction of di-tert-butyl selenide with trifluoroacetates (TFA) of silver(I) and copper(II). The isolation and characterization of a molecular species during the course of this reaction, [Ag₂Cu(TFA)₄(^tBu₂Se)₄] (1), which shows extraordinary reactivity and interesting thermochromic behavior (blue at 0 °C and green at RT), confirmed that ternary metal selenide NPs are formed via this intermediate species. Similar reactions with related dialkyl chalcogenide R₂E resulted in the isolation of molecular species of similar composition, [Ag₂Cu(TFA)₄(R₂E)₄] [R = ^tBu, E = S (2); R = Me, E = Se (3); R = Me, E = S (4)], which are stable at RT but can be converted to ternary metal chalcogenides at elevated temperature. Density functional theory calculations confirm the kinetic instability of 1 and throw light on its thermochromic properties.

Full text of DOI:10.1021/acs.inorgchem.0c00758

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Article
Molecules versus Nanoparticles: Identifying a Reactive Molecular
Intermediate in the Synthesis of Ternary Coinage Metal
Chalcogenides
Sweta Gahlot, Erwann Jeanneau, Deobrat Singh, Pritam Kumar Panda, Yogendra Kumar Mishra,
Rajeev Ahuja, Gilles Ledoux, and Shashank Mishra*
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ABSTRACT: The identification of reactive intermediates during
molecule-to-nanoparticle (NP) transformation has great signifi-
cance in comprehending the mechanism of NP formation and,
therefore, optimizing the synthetic conditions and properties of the
formed products. We report here the room temperature (RT)
synthesis of AgCuSe NPs from the reaction of di-tert-butyl selenide
with trifluoroacetates (TFA) of silver(I) and copper(II). The
isolation and characterization of a molecular species during the
course of this reaction, [Ag2Cu(TFA)4(tBu2Se)4] (1), which shows
extraordinary reactivity and interesting thermochromic behavior
(blue at 0 °C and green at RT), confirmed that ternary metal
selenide NPs are formed via this intermediate species. Similar
reactions with related dialkyl chalcogenide R2E resulted in the isolation of molecular species of similar composition,
t
[Ag2Cu(TFA)4(R2E)4] [R = Bu, E = S (2); R = Me, E = Se (3); R = Me, E = S (4)], which are stable at RT but can be
converted to ternary metal chalcogenides at elevated temperature. Density functional theory calculations confirm the kinetic
instability of 1 and throw light on its thermochromic properties.
the direct reaction of copper and silver salts with elemental
selenium in the presence of NaBH4,4 transforming Ag2Se or
Cu2‑xSe NPs into ternary selenides under suitable conditions,5,8
or thermolysis of suitable metal precursors at high temper-
ature,9,10 the difficulty in controlling the stoichiometry of the
constituent elements often hinders the reproducibility in their
synthesis.
Recently, dialkyl dichalcogenides (R2E2, where R = alkyl or
aryl and E = S, Se, or Te) have been used as low-temperature
chalcogen sources for the solution-phase synthesis of metal
chalcogenide nanocrystals.11,12 Not only do these ligands
possess relatively weak E−E bonds that can be readily cleaved
under mild thermolytic or photolytic conditions, but also their
reactivity can be altered by varying organic substituents R on
them. The above syntheses under mild conditions have enabled
the isolation of metastable nanocrystalline phases with unusual
composition and morphology. It has been reported that neat
INTRODUCTION
■
The coinage metal chalcogenide nanomaterials are currently
under intense investigations for their various physical properties
(low band gaps of 0.15−2.0 eV, low toxicity compared to lead-
and cadmium-containing chalcogenides, high absorption
coefficients, near-IR emission, etc.) and widespread applications
(thermoelectrics, photovoltaic solar cells, photocatalysis,
memory devices, nonlinear optics, energy storage, etc.).1,2 In
particular, the ternary silver−copper chalcogenides, which have
structures that favor high mobility of the ions, are fast emerging
as interesting materials for applications in thermoelectricity,
photocatalysis, and electrochemical devices (batteries, fuel cells,
gas sensors, etc.).3−6 Among these, AgCuSe exists in two phases:
a room-temperature (RT) β-AgCuSe phase having an
orthorhombic structure and a high-temperature α-AgCuSe
phase with a cubic structure.3,4 The structure of β-phase consists
of alternating layers of Ag and CuSe, whereas the high-
temperature α-phase is composed of Ag+ and Cu+ cations
randomly distributed at tetrahedral sites of the face-centered-
cubic unit cell of selenium atoms. The RT structure of Ag3CuS2
is similar to that of Ag2S and contains silver ions in two different
environments, i.e., octahedral and highly distorted tetrahedral.
The copper atoms are linearly coordinated by two sulfur atoms
and bridge the channels.7 Although the nanometric forms of
ternary metal chalcogenides have previously been synthesized by
Received: March 12, 2020
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Figure 1. Characterization of AgCuSe NPs obtained at RT from the reaction of Ag(TFA), Cu(TFA)2, and tBu2Se: (a) XRD pattern; (b) TEM; (c)
HRTEM, with FFT analysis given in the inset.
t
dialkyl dichalcogenides could be thermally or photolytically
decomposed to give elemental chalcogen, dialkylchalcogenide
(R2E), and/or organic byproducts via radical scission of the E−E
and E−R bonds.13−15 These observations indicate that the
dialkylchalcogenides R2E can be an interesting alternative
chalcogen source for the synthesis of metal chalcogenide
nanocrystals under milder conditions because, in addition to
all of the advantages that R2E2 possess (such as solubility,
commercial availability, property optimization by varying R,
etc.), the dialkyl chalcogenides R2E have an additional advantage
in that it bypasses the cleavage step of E−E in R2E2.
Solution routes to metal chalcogenide nanoparticles (NPs)
are vastly appealing not only because they are less energy-
intensive than vacuum techniques and have the potential for
scalability but also because of the fact that one can exploit the
advantage of dialkyl chalcogenides as low-temperature chalc-
ogen sources in the solution phase and, therefore, may
synthesize interesting metastable phases of metal chalcoge-
nides.11,12 Indeed, the silylated chalcogenoethers (Me3Si)2E (E
= S, Se, Te) have been used in the solution-phase synthesis of
metal chalcogenide nanomaterials,16 although the utilization of
relatively less reactive nonsilylated dialkyl chalcogenoethers R2E
has been restricted to the elaboration of thin films in a chemical
vapor deposition technique that requires high temperature.17−22
The use of nonsilylated R2E ligands, which have high
coordinating ability compared to R2E2,23−26 may potentially
lead to the isolation of intermediate molecular species, which
could throw light on the possible mechanism of molecule-to-NP
transformation.27,28 Starting with well-defined precursors may
also overcome the problem of controlling the stoichiometry of
the constituent elements, which often hinders the reproduci-
bility in the synthesis of ternary metal chalcogenides. In this
work, we sought to explore the divergent reactivity of R2E (E =
Se, S: R = Me, Bu) with Cu(TFA)2 and Ag(TFA) (TFA =
trifluoroacetate) as a means to establish a well-defined chemical
route for the mild synthesis of ternary copper−silver
chalcogenide NPs by attempting to isolate and characterize
molecular intermediates on the way to materials in the solution
phase.
RESULTS AND DISCUSSION
■
a. Divergent Reactivity of R2E (R = tBu, Me; E = Se, S)
with Coinage Metal Reagents: Formation of Ternary
Metal Chalcogenide NPs via a Reactive Silver−Copper
Molecular Intermediate versus Stable Molecular Com-
t
plexes. The direct reaction of Bu2Se with Ag(TFA) and
Cu(TFA)2 at RT resulted in a gradual change in the color of the
solution from blue to brown and finally precipitation of a black
powder (Figure S1). Warming the reaction mixture at 80 °C
increased the yield. The powder X-ray diffraction (XRD) pattern
of this black precipitate matched well with those of the
previously published XRD results showing a mixture of two
phases of ternary silver−copper selenide, AgCuSe, i.e., PDF 010-
0451 and 025-1180.3−5 It also showed a small amount of Ag2Se
NPs [PDF 01-080-7685; ∼5% as quantified using the reference
intensity ratio method (Figure 1a]. The scanning electron
microscopy (SEM) and transmission electron microscopy
(TEM) images show nanometric but somewhat irregular
CuAgSe particles (Figures 1b and S2 and S3a), which is not
surprising given that synthesis was achieved in the absence of
any capping ligand. The high-resolution TEM (HRTEM)
images clearly show an interplanar crystal lattice (Figures 1c and
S3b). The fast Fourier transform (FFT) image shows the
diffraction spots from the (1 0 0), (1 1 0), (1 1 1), (0 1 2), and (1
1 2) crystal planes (Figure 1c, inset). The absence of other
elements apart from carbon, copper, silver, and selenium in the
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Figure 2. (a) Perspective view of the molecular structure of 1 with 50% probability ellipsoids (hydrogen atoms omitted for clarity). Selected bond
lengths (Å) and angles (deg): Ag2−O6 2.436(6), Ag2−O8 2.412(6), Cu1−O2 1.933(6), Cu1−O4 1.944(6), Ag1−Se2 2.581(1), Ag2−Se4 2.601(1);
O1−Ag1−O3 93.7(2), Se1−Ag1−O1 102.1(2), Se2−Ag1−O1 109.9(1), Se1−Ag1−Se2 130.1(3), O2−Cu1−O5 87.4(3), O5−Cu1−O7 93.5(3).
(b) Extended structure of 1 showing the discrete nature of Ag2Cu trinuclear units (the shortest intermolecular Ag···Ag interaction being 8.5 Å).
Figure 3. TG−DTG curves of 1 (a), XRD pattern of the powder obtained after decomposition of 2 in refluxing toluene (b), and TG−DTG curves of 2
(c).
energy-dispersive X-ray (EDX) analysis indicates the high purity
of the NPs (Figure S4). The EDX analysis of several randomly
selected area gives a slightly silver-rich stoichiometry due to the
presence of an additional Ag2Se phase, as indicated by XRD. The
NPs were further studied by X-ray photoelectron spectroscopy
(XPS) analysis, which shows that the NPs are mainly composed
of Cu+, Ag+, and Se2− with very small amounts of Cu2+ and Se4+,
most probably because of the surface oxidation of NPs (Figure
S5).29 The Cu 2p3/2 state shows the presence of two electronic
states located at 932.4 and 934 eV.27 Because the difference in
the binding energies of Cu0 and Cu+ is very small (only few
millielectronvolts), Auger analysis was carried out by using the
C
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Figure 4. (a) Perspective view of the molecular structure of 4 with 50% probability ellipsoids (hydrogen atoms omitted for clarity). (b) Extended
structure with a short Ag···Ag interaction (3.23 Å) among Ag2Cu trinuclear units to afford a 1D chain. Selected bond lengths (Å) and angles (deg):
Ag1−S1 2.458(2), Ag1−S2 2.476(2), Ag1−O1 2.511(6), Ag1−O3 2.528(6), Ag2−S4 2.486(2), Ag2−S3 2.461(2), Ag2−O8 2.501(7), Ag2−O6
2.438(7), Cu1−O2 1.933(5), Cu1−O4 1.946(5), Cu1−O7 1.932(6), Cu1−O5 1.934(6); S1−Ag1−S2 153.63(7), S1−Ag1−O1 107.61(18), S1−
Ag1−O3 97.38(16), S2−Ag1−O1 92.93(17), S2−Ag1−O3 94.14(15), O1−Ag1−O3 102.1(2), S4−Ag2−O8 92.99(15), S3−Ag2−S4 148.58(7),
S3−Ag2−O8 95.82(15), O6−Ag2−S4 95.93(19), O6−Ag2−S3 108.6(2), O6−Ag2−O8 111.6(3), O2−Cu1−O4 179.2(2), O2−Cu1−O5 88.7(2),
O7−Cu1−O2 92.1(2), O7−Cu1−O4 87.9(2), O7−Cu1−O5 178.6(2), O5−Cu1−O4 91.3(2).
Auger parameter.30 In this case, the Auger parameter is
approximately 1850, attributable to Cu+ for the first chemical
state at 932.4 eV. The state located at 934 eV corresponds to
Cu2+, as further indicated by the presence of satellite states
center. The tendency of TFA to act as an assembling ligand to
afford heterometallics has previously been highlighted.31 Two
terminal tBu2Se ligands present on each of the silver atoms then
complete a distorted tetrahedral O2Se2 environment around
metal centers [if short AgI···CuII interactions (3.589−3.679 Å)
are not taken into account], as revealed in the range of angles
around the silver center (93.72−131.21°). The Ag−O
[2.412(6)−2.436(6) Å] and Ag−Se [2.581(1)−2.601(1) Å]
bond lengths compare well with the literature values on the
bridging TFA and terminally bonded selenium-containing
ligands, respectively.27,28 The four oxygen atoms around Cu1
are essentially in the same plane and at almost the same distance
from the copper [Cu−O = 1.941(6)−1.944(6) Å].32 These
trinuclear Ag2Cu species are discrete in nature, with the shortest
intermolecular Ag···Ag interaction being 8.5 Å (Figure 2b).
The TG studies of 1 confirmed its low thermal stability and
high reactivity. The TG curve, recorded under a nitrogen
atmosphere, indicates a two-step decomposition in the temper-
ature range 60−160 °C (Figure 3a), with two DTG peaks at 82.5
and 91.4 °C. A residual mass of 28% at 200 °C is consistent with
the formation of 1 equiv of AgCuSe + 0.5 equiv of Ag2Se
(calculated value 26.5%) as the end product. The high reactivity
of 1 is further evident from the fact that it is transformed to a
mixture of AgCuSe and Ag2Se upon being left in open air for a
few hours (Figure S6). It reacts differently with water and gives
mainly Ag2Se, Cu2Se, and metallic silver (Figure S7 and S8).
A similar reaction with tBu2S in toluene did not lead to either a
change in color or any precipitation even after stirring for several
hours at RT but yielded a black precipitate immediately upon
reflux. XRD of this precipitate showed ternary silver−copper
sulfide Ag3CuS2 (PDF 04-016-6112) as the major phase, along
around 943 eV. The spin−orbit states for Ag 3d5/2 and 3d3/2
,
which appear at 368 and 374 eV, respectively, correspond to
Ag+.28 The Se 3d5/2 state from Se2− appears at 53.6 eV.27,28 The
state visible at 58.5 eV can be attributed to Se4+.
Identification of reactive intermediate species operating at the
interface of stable molecular complexes and NPs is an important
aspect and a key factor to (i) comprehend the mechanism of
molecule-to-NP formation and (ii) achieve fine control over
their chemical composition and reactivity to optimize the
synthetic conditions and properties of the NPs.27,28 Therefore,
attempts were made to identify the species present in the
solution before the precipitation of AgCuSe NPs. We succeeded
in isolating and characterizing an intermediate molecular
species, [Ag2Cu(TFA)4(tBu2Se)4] (1), in good yield from the
reaction mixture. Although 1 is highly reactive and turns black in
a few days even at low temperature and in an inert atmosphere,
apparently because of the formation of metal selenide NPs, it is
stable for a sufficient duration to be characterized by single-
crystal XRD, Fourier transform infrared (FT-IR), thermog-
ravimetry−differential thermal gravimetry (TG−DTG), and
thermochromic studies. The molecular structure of 1 is based on
a spirocyclic metal−oxygen framework. It crystallizes in the
orthorhombic space group Pbca, and its structure can be
conceptually seen as a bidentate interaction of the two
monoanionic {Ag(μ-TFA)2(tBu2Se)2}− moieties with an
electrophilic Cu2+ center (Figure 2a). The two six-membered
boat-shaped “AgO4Cu” rings are fused at a common Cu2+
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with a small amount of Ag2S (PDF 00-068-0300; Figure S9).
Blue crystals of the composition [Ag2Cu(TFA)4(tBu2S)4] (2)
were readily obtained in good yield from the reaction mixture of
Ag(TFA), Cu(TFA)2, and tBu2S at RT. In the FT-IR spectrum
of 2, the presence of only one strong band at 1715 cm−1 due to
νas(CO2) (Figure S10)33,34 suggests a structure similar to that of
1, where TFA ligands show only one bonding mode. The
structure of 2 is indeed similar to that of 1 and is given in Figure
S11. Unlike 1, which gradually turns black even when kept in the
dark at low temperature and under an inert atmosphere,
complex 2 is stable at RT for several weeks. However, it
decomposes in refluxing toluene to give a mixture of Ag3CuS2
(PDF 04-016-6112) and Ag2S (PDF 00-068-0300), with the
former being the major phase (Figure 3b). This thermal
instability can be explained by the TG studies of 2, which
indicate a single-step decomposition with only one DTG peak at
121 °C (Figure 3c). The presence of a small amount of binary
silver chalcogenide Ag2Se/Ag2S in the ternary AgCuSe/
Ag3CuS2 is not always a drawback and may actually be beneficial
for certain applications. For example, we have recently shown
that the presence of two phases of ternary metal chalcogenide in
AgCuSe/TiO2 composites enhances their photocatalytic activity
because of a synergic effect between the two phases.5
metal reagents at RT, we have been able to isolate reactive
intermediates [Cu2(TFA)2(tBu2Se)3] and [Ag(TFA)-
(tBu2Se)2], which confirmed that Cu2‑xSe and Ag2Se NPs are
formed via these intermediates.27,28 The synthesis of ternary
metal chalcogenide materials is even more challenging, and very
few single-source precursors (SSPs) are reported so far for them.
Not only do the heterometallics [Ag2Cu(TFA)4(R2E)4] (R =
Me, tBu; E = S, Se) described here fill this clear void of SSPs for
such ternary chalcogenide materials, but, more importantly,
some of them were isolated as intermediates with tailored
reactivity, which can be used to investigate the fundamentals of
the nucleation and growth of these ternary materials.
t
The high reactivity of Bu2E (E = Se, S) to give metal
chalcogenide NPs under mild conditions can be attributed to the
availability of a decomposition path via β-hydrogen elimination,
which first leads to the formation of tert-butylchalcogenol and
then a chalcogenide ligand (eqs S1−S3).39 This mechanism
finds support from the above-mentioned isolation of chalcoge-
nolate complexes [(NHC)Cu-ESiMe3] (E = S, Se, Te) and the
proposed sulfide-bridged intermediate [(NHC)2Ag2(μ-S)]
during the reaction of (Me3Si)2S with (NHC)Cu(OAc) and
(NHC)AgX, respectively.37,38 A higher reactivity of selenide
precursors in comparison to analogous sulfide precursors has
previously been demonstrated and explained on the basis of
density functional theory (DFT) calculations.12 The coordi-
nated tBu2E (E = Se, S) ligands play a dual role in the formation
of metal chalcogenide NPs, i.e., a facile source of chalcogens and
a reducing reagent to get the desirable 1+ oxidation state of the
copper center.
b. DFT Calculations. DFT calculations were performed on
[Ag2Cu(TFA)4(R2E)4] [R = tBu, E = Se (1), S (2); R = Me, E =
Se (3), S (4)] to gain information about their electronic
structures and, consequently, their reactivity and properties.
Popular quantum-mechanical descriptors, e.g., the highest
occupied molecular orbital (HOMO)−lowest unoccupied
molecular orbital (LUMO) energies, play a major role in
governing a wide range of chemical interactions. The frontier
molecular orbital gives insight into the reactivity of the molecule,
and the active site can be demonstrated by the distribution of
frontier orbitals. The HOMO−LUMO energy gap generally
implies the kinetic energies and chemical reactivity rate. The
energy gap between the HOMO and LUMO electronic levels is
a critical parameter that corresponds to the energy difference
between the ionization potential and electron affinity of a
molecular species or material and determines its electronic,
optical, redox, and transport (electrical) properties. The band
gap is also referred to as the transport gap because it represents
the minimum energy necessary to create a positive charge carrier
somewhere in the material minus the energy gained by adding a
negative charge carrier. The HOMO and LUMO energies as
well as the HOMO−LUMO energy gaps of 1−4 are presented
in Table 1. The HOMO−LUMO gaps of 0.25, 3.13, 2.09, and
3.06 eV are calculated for 1−4, respectively, which confirm the
We then attempted to mimic the above reaction with the
Me2E (E = Se, S) ligands, for which there is no possibility of
decomposition via the β-hydrogen elimination path-
way.27,28,35,36 As expected, these reactions yielded molecular
complexes of similar composition, [Ag2Cu(TFA)4(Me2E)4] [E
= Se (3), S (4)], which are kinetically as well as thermally stable.
The isostructural 3 and 4 essentially have a structure that is
t
similar to their Bu2E (E = Se, S) analogues 1 and 2 at the
trinuclear level (Figures 4a and S13a). However, unlike the
discrete nature of the Ag2Cu trinuclear units in 1 and 2, these
units have strong enough Ag···Ag interaction (2.5−3.23 Å) in 3
and 4 to give 1D chain (Figures 4b and S13b). As a result, the
Ag−O bond lengths are slightly longer here [2.438(7)−
2.528(6) Å in 4 vs 2.391(4)−2.436(6) Å in 1 and 2]. The
four oxygen atoms around Cu1 are arranged in the same plane,
with the Cu−O distances [1.932(6)−1.946(5) Å] being
comparable to those found in 1 and 2. The Ag−S distances
are spread in the range 2.458(2)−2.486(2) Å. The polymeric
nature of 3 and 4 is reflected in their TG−DTG curves, which
show a multistep thermal decomposition that lasts well beyond
300 °C (Figure S14). Upon decomposition under an inert
atmosphere at 350 °C, 3 is transformed to a mixture of AgCuSe
and Ag2Se (and a very small impurity of the metallic silver;
Figure S15).
The silylated chalcogenoethers (Me3Si)2E (E = S, Se, Te) are
important reagents for the solution-phase synthesis of metal
chalcogenides under mild conditions.16 However, their
mechanism has been the subject of many speculations.
Formation of a sulfide-bridged intermediate, [(NHC)2Ag2(μ-
S)] (NHC = N-heterocyclic carbene), has been suggested
during the formation of Ag2S NPs from the RT reaction of
(Me3Si)2S with (NHC)AgX (X = halide), although no structural
evidence was presented.37 On the other hand, the reaction of
(Me3Si)2E with the related (NHC)Cu(OAc) was shown to
afford thermally stable complexes [(NHC)Cu-ESiMe3] (E = S,
Se, Te) at low temperature,38 suggesting the conversion of
chalcogenoethers to chalcogenolates, before affording metal
chalcogenide NPs. Using the bulkier nonsilylated dialkylselenide
tBu2Se, which is slightly less reactive than its silylated
counterpart but still gives metal selenide NPs with group 11
Table 1. HOMO and LUMO Energies and the HOMO−
LUMO Band Gap for 1−4
molecular species
HOMO
LUMO
ΔE
Ag2Cu(TFA)4(tBu2Se)4 (1)
Ag2Cu(TFA)4(tBu2S)4 (2)
Ag2Cu(TFA)4(Me2Se)4 (3)
Ag2Cu(TFA)4(Me2S)4 (4)
−3.68176
−0.15735
−0.43356
−0.63741
−3.43226
2.9695
1.659036
2.42129
0.25
3.13
2.09
3.06
E
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Figure 5. Fluctuation in energy of the system as a function of the optimization steps for 1−4 at 353 (a) and 393 K (b) (molecular dynamics
simulations).
Figure 6. Steps of optimized structures as a function of the total energy for 1−4 at 353 and 393 K.
kinetic instability and higher chemical reactivity of 1. This is
further confirmed by the temperature-dependent simulation of
1−4 at 353 and 393 K, performed for 200 ps with a time step of 1
fs (Video S1). Figure S16 gives some intrinsic insight into the
structural properties and stabilities of 1−4 simulated under
constant pressure and varying temperatures. Compared to 2−4,
the reactive intermediate 1 shows the highest fluctuation in the
total energy difference ΔE variation and takes the highest steps
to reach the equilibrium (Figures 5 and 6). The Langevin
thermostat and production with a Nose−Hoover (NVT)
ensemble for 200 ps for 1−4, where the number of atoms
(N), volume (V), and temperature (T) were kept constant,
represent the stability of the structures as a function of the total
energy (Figure 7). The high total energy variation can only be
observed for 1 at both 353 and 393 K, which is in accordance
with its structural deformation, as depicted previously in Figure
S16. The isothermal and isobaric (NPT) ensemble for 200 ps as
a function of the total energy for 1−4 at 353 and 393 K
temperature, where the number of atoms (N), pressure (P), and
temperature (T) were conserved, is depicted in Figure S17. The
observed large volumetric expansion for 1 at 353 K and less
expansion of volume at 393 K indicates its instability at 353 K
compared to 2−4. Figure S18 shows the pair distribution
function g(r), which accounts for the number of neighbors for
each atom that are within a given cutoff range (in our case, at r =
3.5) around its position by analysis of the structural stability at
different temperatures. g(r) measures the probability of finding a
particle at distance r given that there is a particle at position 0; it
is essentially a histogram of interparticle distances. The pair
distribution function is normalized by the number density of the
particles (i.e., total number of particles divided by the simulation
cell volume).
c. Thermochromic Behavior of 1. Interestingly, when
taken in a coordinating solvent (or even in contact with the
vapor of this solvent), the color of 1 changes from blue (273 K
and below) to green at RT (Figures 8, inset, and S19 and Video
S2). Figure 8 shows the evolution of the peaks at 463 nm (blue)
and 543 nm (green) in the absorption spectrum of 1 in
tetrahydrofuran (THF) in terms of a blue-to-green (B/G) ratio
as a function of the temperature. As the temperature increases,
the B/G ratio continuously decreases until the complex
decomposes at about 330 K. The simulated UV−visible
absorption spectra of 1−4 at RT are presented in Figure S20.
While 1 has relatively higher intensity of the absorption bands
F
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Figure 7. Langevin thermostat and production with the Nose−Hoover (NVT) ensemble for 200 ps as a function of the total energy for 1−4 at 353 and
393 K.
Because 1 is not thermochromic in a noncoordinating solvent
such as toluene (Figure S21), it is very probable that the above
reversible phenomenon is due to coordination/discoordination
of the THF molecule with the copper atom. In the X-ray
structure of 1 at 100 K, the copper center has weak
intramolecular interactions with silver (3.59−3.68 Å) and
selenium atoms (3.87−3.95 Å). At RT, these interactions are
expected to further weaken, thus allowing solvent THF
molecules to get weakly coordinated with the copper center,
which would, by virtue of a dynamic pseudo-Jahn−Teller effect,
cause a red shift in the absorption (blue to green).40−42 This
theory is further supported by the fact that complex 2, which has
stronger Cu···S interaction (3.55 Å), or 3 and 4, which are
structurally more rigid because of additional intermolecular Ag···
Ag interaction (2.5−3.23 Å) among the Ag2Cu trinuclear units,
do not demonstrate any visible thermochromism and the B/G
ratio does not change significantly with the temperature (Figure
8 and Video S3). Unfortunately, our efforts to corroborate
further the above theory by measuring the single-crystal X-ray
structure of 1 at RT were not successful because of the instability
of this compound, which decomposed after a few frames of data
collection.
Figure 8. Change in the absorption intensity of the blue (463 nm) and
green (543 nm) peaks of 1 and 3 taken in THF, in terms of the B/G
ratio, as a function of the temperature. The photographs at the bottom
show two different colors of 1 at two different temperatures.
and shows the most dominating peak at around 550 nm,
compounds 2−4 have the most dominant absorption bands in
the region 430−460 nm. As expected, the absorption spectrum
of the kinetically and thermally instable 1 shows significant
changes at 353 K (Figure 9a), whereas 4, which is thermally
stable, presents similar absorption peaks at high temperature
(although of slightly different intensity; Figure 9b).
CONCLUSIONS
■
Mild syntheses of ternary coinage metal chalcogenides AgCuSe
and Ag3CuS2 from the reaction of Ag(TFA) and Cu(TFA)2 with
t
tBu2Se or Bu2S are presented. The successful isolation and
G
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Figure 9. Simulated UV−visible absorption spectra of 1 (a) and 4 (b) at variable temperature.
from LINKAM Ltd. DSC600, which allowed one to vary the
temperature using liquid nitrogen.
characterization of intermediate species during the course of
these reactions, [Ag2Cu(TFA)4(tBu2E)4] [E = Se (1), S (2)],
establishes a precursor-mediated pathway for the formation of
ternary metal chalcogenides. The highly reactive intermediate 1
is thermally and kinetically instable, as confirmed by TG studies
and DFT calculations, and shows an interesting thermochromic
behavior (green at 20 °C and blue at 0 °C). In contrast, the
Synthesis of CuAgSe NPs and Isolation of a Silver−Copper
Heterometallic Molecular Intermediate. A reaction mixture of
Cu(TFA)2(THF) (0.05 g, 0.14 mmol), Ag(TFA) (0.03 g, 0.14 mmol),
t
and Bu2Se (0.1 mL, 0.56 mmol) was stirred in the absence of any
solvent at RT, which led to a gradual change in the color of the solution
from blue to brown, and finally a black powder was precipitated. After
the reaction mixture was stirred for 1.5 h, the black precipitate was
isolated, washed with ethanol several times, and dried at RT. The
powder XRD of this black precipitate confirmed it to be CuAgSe NPs
containing also ∼5% of Ag2Se NPs.
t
analogue complexes with Bu2S and Me2E ligands, i.e.,
t
[Ag2Cu(TFA)4(R2E)4] [R = Bu, E = S (2); R = Me, E = Se
(3), S (4)], are kinetically stable and nonthermochromic in
nature. In addition to the utility of these heterometallic
precursors in the solution-phase synthesis of ternary metal
chalcogenide NPs, their tailored reactivity can also be used to
study the fundamentals of nucleation and growth of ternary
materials.
We then made attempts to isolate intermediate molecular species
present in the solution during the course of the above reaction. We
carried out the above reaction under varied conditions (different
reaction times and solvents such as toluene, THF, diethyl ether, etc.) in
order to avoid black precipitation and to isolate molecular species. In
one such optimized reaction, 0.20 mL of tBu2Se (1.10 mmol) was added
to a blue toluene solution (20 mL) containing Cu(TFA)2(THF) (0.1 g,
0.28 mmol) and Ag(TFA) (0.06 g, 0.28 mmol), and the resulting
solution was stirred for 15 min at RT. After that, the solution was
concentrated under vacuum and layered with n-pentane (10 mL) to
obtain dark-blue crystals of [Ag2Cu(TFA)4(tBu2Se)4] (1) along with a
few colorless crystals of the composition [Cu2(TFA)2(tBu2Se)3] (a).
The mother liquor was separated through a cannula and concentrated
to give more of compound a. The formation of compound a, which was
previously reported by us,27 can be explained by the fact that only half of
Cu(TFA)2 is consumed during formation of the heterometallic 1, which
has a 2:1 ratio of the silver and copper atoms. The blue crystals of 1 were
isolated and washed with cold n-pentane. Yield: 0.17 g (52%). FT-IR
(Nujol, cm−1): 1712s, 1460s,1381s, 1200m, 1139m, 729m, 842w,
793w, 525w.
Alternatively, compound 1 can also be synthesized in a better yield
and without having any contamination of the copper species (a) by
reacting [Ag2Cu(TFA)4(Me2Se)4] (3) (see the synthetic details of 3
below) with tBu2Se in toluene. In this reaction, tBu2Se (0.07 mL, 0.37
mmol) was dropwise added to a solution of 3 (0.11 g, 0.09 mmol) in
toluene (3 mL), which led to a change in the color of the solution from
blue to green (Figure S22). This mixture was stirred at RT for 20 min
and then layered with n-pentane (7 mL). Upon diffusion, blue solution
was given, from which blue crystals of 1 were obtained at 0 °C. The
mother liquor was removed through a cannula, and the crystals were
washed with cold n-pentane. Yield: 0.08 g (71%). 1 is highly reactive
and unstable and turns black in 1−2 days even when kept at low
temperature and under an inert atmosphere. This precluded us to
perform its elemental analysis. FT-IR (Nujol, cm−1): 1714s,
1463s,1382s, 1205m, 1144m, 733m, 845w, 798w, 526w.
EXPERIMENTAL SECTION
■
General Procedures. We performed all of the manipulations under
an argon atmosphere using standard Schlenk and glovebox techniques.
Solvents were dried using MB SPS-800 and used without further
purification. Silver trifluoroacetate (Aldrich), di-tert-butyl selenide
(SAFC Hitech), di-tert-butyl sulfide and dimethyl sulfide (TCI Europe
NV), and dimethyl selenide and copper(II) trifluoroacetate hydrate
(Alfa Aesar) were purchased and used without further purification. A
copper(II) trifluoroacetate/tetrahydrofuran adduct was synthesized in
the laboratory from the reaction of Cu2O and TFAH in toluene and
then extracted from THF. The IR spectra were obtained as Nujol mulls
on a Bruker Vector 22 FT-IR spectrometer at RT and registered from
4000 to 400 cm−1. Scanning electron microscopy (SEM) measure-
ments were performed on a Hitachi S800 system. Transmission
electron microcsopy (TEM) experiments were performed using a JEM-
2100F system with 200 kV field emission (FE) and a JEOL 2010 LaB6
system with 200 kV FE. We used a Bruker D8 Advance A25 system with
Cu Kα1 + 2 (λ = 0.154184 nm) radiation at 50 kV and 35 mA to
measure the X-ray diffraction (XRD) patterns of the NPs. The copper,
silver, sulfur, and selenium contents were determined by inductively
coupled plasma optical emission spectroscopy (ICP-OES). Thermog-
ravimetric analyses (TGA) were performed with a TGA/differential
scanning calorimetry 1 STARe system from Mettler Toledo. Around 8
mg of the sample was sealed in a 100 μL aluminum crucible in the
glovebox and heated under an argon atmosphere at a heating rate of 5
°C/min. Diffusion-reflectance measurements were performed on a
homemade apparatus. The sample was illuminated by an eq 99X laser-
driven point source of the lamp, and it was reimaged on the sample by
two 100-m-focal-length, 2-in.-diameter MgF2 lenses. The emitted light
from the sample was collected by an optical fiber connected to a Jobin-
Yvon TRIAX320 monochromator equipped with a cooled charge-
coupled detector (CCD). The resolution of the detection system was 2
nm. For temperature variation, the sample was placed on a cooling stage
Synthesis of Ag3CuS2 NPs and Isolation of a Silver−Copper
Heterometallic Molecular Intermediate. In contrast to the above
reaction, the reaction of Cu(TFA)2(H2O) (0.3 g, 0.96 mmol),
t
Ag(TFA) (0.21 g, 0.96 mmol), and Bu2S (0.85 mL, 4.80 mmol) in
H
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Table 2. Crystallographic and Refinement Data for 1−4
1
2
3
4
empirical formula
fw
C40H72Ag2CuF12O8Se4
1504.1
C40H72Ag2CuF12O8S4
1316.5
C48H72Ag6Cu3F36O24Se12
3502.4
C16H24Ag2CuF12O8S4
979.9
cryst syst
space group
a (Å)
b (Å)
c (Å)
orthorhombic
Pbca
17.9685(15)
22.642(4)
27.703(3)
90
90
90
11270(2)
8
3.73
monoclinic
C2/c
17.4632(11)
16.5079(9)
20.0811(14)
90
101.451(7)
90
5673.8(6)
4
monoclinic
P21/n
17.0247(11)
31.466(3)
19.6823(19)
90
104.751(8)
90
10196.4(15)
4
triclinic
P1
̅
10.4336(10)
11.0465(9)
16.3292(10)
75.224(6)
86.689(6)
64.143(9)
1634.5(3)
2
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
μ (mm−1
)
1.28
6.15
2.19
temp (K)
100
150
150
150
measd reflns
48269
38823
79599
14879
indep/obsd [I > 2σ(I)] reflns
Rint
13493/7579
0.086
7289/5967
0.041
24441/13442
0.096
14879/9923
0.09
restrains/param
GOF
69/656
1.03
533/428
1.10
1080/1150
1.61
0/396
1.07
R [F2 > 2σ(F2)]
wR (F2)
0.069
0.204
0.046
0.119
0.180
0.507
0.071
0.224
residual electron density (e/Å)
CCDC
−1.67 to 2.55
1903337
−1.07 to 0.68
1968807
−3.24 to 13.92
1903338
−1.53 to 3.47
1968808
toluene (30 mL) did not lead to any change in color at RT. However,
under reflux, the color of the solution changed from dark blue to dark
green, and after some time, black powder was precipitated. After 3 h of
reflux, the precipitates were collected and washed thoroughly with
ethanol and then dried at RT. The final product, when characterized by
a powder XRD technique, was found to be a mixture of Ag3CuS2 (70%)
and Ag2S (30%). Yield: 0.09 g (42% by weight Ag).
5.40; Se, 26.90. FT-IR (Nujol, cm−1): 1715s, 1456s, 1377s, 725s, 970m,
928m, 846m, 793m, 521m, 601w, 427w.
Using the above method, compound [Ag2Cu(TFA)4(Me2S)4] (4)
was synthesized from Cu(TFA)2(H2O) (0.10 g, 0.32 mmol), Ag(TFA)
(0.14 g, 0.64 mmol), and Me2S (0.1 mL, 1.47 mmol) in Et2O (30 mL)
and crystallized as dark-blue crystals by layering the concentrated
solution with n-hexane. Yield: 0.22 g (68%). Calcd for
C16H24Ag2CuF12O8S4 (979.87): C, 19.60; H, 2.45; Ag, 22.02; Cu,
6.48; S, 13.09. Found: C, 19.43; H, 2.34; Ag, 21.79; Cu, 6.43; S, 12.89.
FT-IR (Nujol, cm−1): 1682s, 1457s, 1377s, 1198s, 1151s, 726s, 1036m,
987m, 843m, 792m, 679w, 668w, 606w, 523w.
Decomposition of 1 in Air. Upon exposure to air, the crystals of 1
(0.05 g) turned black in a few hours. The powder XRD on these black
crystals after 3 days showed the presence of CuAgSe and Ag2Se phases
in approximately equal proportions (Figure S6).
Hydrolysis of 1. The reaction of water (0.5 mL) with 1 (0.05 g taken
in 5 mL toluene) at an ambient atmosphere resulted in a light-gray
solution within the first few minutes, which upon further stirring for 3 h
gave black precipitates consisting mainly of Cu2Se, Ag2Se, and metallic
silver, as revealed by powder XRD (Figures S7 and S8).
Thermal Decomposition of 2. A total of 0.14 g of dark-blue crystals
of 2 was dissolved in toluene (30 mL). After the addition of 0.5 mL of
tBu2S, the solution was refluxed for 1 h to obtain black precipitates.
Powder XRD results of this sample confirmed the presence of Ag3CuS2
as a major phase along with Ag2S (Figure 3b).
Because the reaction medium was stable at RT, an attempt to isolate
intermediate molecular species present in the solution during the
course of the above reaction was relatively straightforward. The green
toluene solution (30 mL) containing Cu(TFA)2(H2O) (0.32 g, 1
mmol), Ag(TFA) (0.22 g, 1 mmol), and tBu2S (0.7 mL, 4 mmol), after
stirring for 1 h, was concentrated and layered with n-hexane to obtain
dark-blue crystals of [Ag2Cu(TFA)4(tBu2S)4] (2) along with a few
green crystals later identified as [Cu(TFA)2(tBu2S)] (b) by elemental
analysis. The formation of compound b can be explained by the fact that
only half of Cu(TFA)2 is consumed during formation of the
heterometallic 2, which has a 2:1 ratio of the silver and copper
atoms. The blue crystals of 2 were isolated and washed with cold n-
pentane. Yield: 0.58 g (62%). FT-IR (Nujol, cm−1): 1694s, 1465s,
1367s, 1200s, 842m, 790m, 724m, 668w, 615w, 592w, 522w. The
reaction in the right stoichiometry [Cu(TFA)2(H2O) (0.2 g, 0.64
mmol), Ag(TFA) (0.28 g, 1.29 mmol), and tBu2S (0.46 mL, 2.58 mol)]
improved the yield of the product (78%) and avoided contamination of
the copper species (b). Calcd for C40H72Ag2CuF12O8S4 (1316.5): C,
36.46; H, 5.47; Ag, 16.38; Cu, 4.83; S, 9.74. Found: C, 36.21; H, 5.29;
Ag, 16.23; Cu, 4.73; S, 9.65. FT-IR (Nujol, cm−1): 1694s, 1465s, 1367s,
1200s, 842m, 790m, 724m, 668w, 615w, 592w, 522w. Unlike 1, 2 is
stable for several days when kept at low temperature/RT and under an
inert atmosphere.
Thermal Decomposition of 3. A total of 0.15 g of 3 was decomposed
in the solid state at 350 °C for 2 h in an inert atmosphere to obtain black
precipitates, the powder XRD of which showed the presence of CuAgSe
as a major phase along with small amounts of Ag2Se and metallic silver
(Figure S15).
X-ray Crystallography. Crystals of 1−4 were obtained as
described in the synthetic procedure. Crystal structures were
determined using molybdenum radiation (λ = 0.71073 Å) on an
Oxford Diffraction Gemini diffractometer equipped with an Atlas CCD.
Intensities were collected at 100 K by means of CrysAlisPro software.43
Reflection indexing, unit-cell parameter refinement, Lorentz-polar-
ization correction, peak integration, and background determination
were carried out with CrysAlisPro software.43 An analytical absorption
correction was applied using the modeled faces of the crystal.44 The
resulting sets of hkl were used for structure solutions and refinements.
Synthesis of Stable [Ag2Cu(TFA)4(Me2Se)4] (E = Se, S).
[Ag2Cu(TFA)4(Me2Se)4] (3). After the dropwise addition of Me2Se
(0.1 mL, 1.30 mmol) to a blue solution of Cu(TFA)2(THF) (0.1 g, 0.28
mmol) and Ag(TFA) (0.12 g, 0.54 mmol) in THF (20 mL), the
resulting solution was stirred for 2 h at RT. After the solution was
concentrated, it was layered with n-hexane (10 mL) to obtain dark-blue
crystals of 3. The mother liquor was removed through a cannula, and
the crystals were isolated and washed with cold n-pentane. Yield: 0.19 g
(56%). Calcd for C16H24Ag2CuF12O8Se4 (1167.5): C, 16.44; H, 2.06;
Ag, 18.48; Cu, 5.44; Se, 27.06. Found: C, 16.40; H, 1.98; Ag, 18.45; Cu,
I
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The structures were solved with the ShelXT45 structure solution
program using intrinsic phasing and by using Olex246 as the graphical
interface. The model was refined with version 2018/3 of ShelXL47 using
least-squares minimization. Some selected crystallographic and refine-
ment data of 1−4 are listed in Table 2.
Accession Codes
CCDC 1903337−1903338 and 1968807−1968808 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
Computational Details. In the present work, we have performed
electronic structure calculations within DFT,48,49 which is implemented
in the Vienna Ab Initio Simulation Package (VASP) code.50 The
projector-augmented-wave potentials were adopted to describe the
difference between cores and valence electrons.51 The generalized
gradient approximation (GGA) functional in the form of the Perdew,
Burke, and Ernzerhof functional was adopted to describe the exchange
and correlation potentials.52 We have chosen k-point sampling (5 × 4 ×
3), (5 × 5 × 5), (5 × 2 × 5), and (7 × 7 × 5) for 1−4, respectively, for
Brillouin zone integration in k space. Here, the k-point mesh is
generated by the Monkhorst−Pack scheme.53 The kinetic energy cutoff
for the plane-wave basis set is 600 eV. Atoms are optimized until the
force per atom in the unit cell was converged within 10−3 eV/Å. The
energy convergence criterion per self-consistent field is 1 × 10−6 eV. For
simplistic visualization of the structural configurations at various
temperatures, e.g., 353 and 393 K, we have used54 a classical molecular
dynamic with a focus on material modeling. Visual molecular
dynamics55 and OVITO56 were used to visualize the molecular
dynamics trajectories of various structural configurations. The
structural minimization was performed using the Polak−Ribiere
version of the conjugate gradient algorithm coupled with NPT57
(isothermal and isobaric) and NVT58 (Langevin thermostat and
production with Nose−Hoover) ensembles using the universal force
field59 for all of the structural configurations. The simulation was
performed for 200 ps with a time step of 1 fs (Video S1). Moreover, we
have also analyzed the pair correlation function g(r), which accounts for
the number of neighbors for each particle that are within a given cutoff
range (in our case at r = 3.2) around its position by analysis of the
structural stability at different temperatures.
AUTHOR INFORMATION
■
Corresponding Author
Shashank Mishra − Institut de Recherches sur la Catalyse et
l’Environnement de Lyon (IRCELYON), Universite Lyon,
́
́
Universite Claude Bernard Lyon 1, CNRS, UMR 5256, 69626
Villeurbanne, France; orcid.org/0000-0003-2846-4221;
Email: shashank.mishra@ircelyon.univ-lyon1.fr; Fax: (+33)
472445399
Authors
Sweta Gahlot − Institut de Recherches sur la Catalyse et
́
l’Environnement de Lyon (IRCELYON), Universite Lyon,
́
Universite Claude Bernard Lyon 1, CNRS, UMR 5256, 69626
Villeurbanne, France
́
Erwann Jeanneau − Centre de Diffractometrie Henri
Longchambon, Universite Lyon, Universite Claude Bernard Lyon
1, 69100 Villeurbanne, France
́
́
Deobrat Singh − Condensed Matter Theory Group, Department
of Physics and Astronomy, Uppsala University, 75120 Uppsala,
Sweden
Pritam Kumar Panda − Condensed Matter Theory Group,
Department of Physics and Astronomy, Uppsala University,
75120 Uppsala, Sweden; orcid.org/0000-0003-4879-2302
Yogendra Kumar Mishra − Mads Clausen Institute, NanoSYD,
University of Southern Denmark, Alsion 2, Denmark;
orcid.org/0000-0002-8786-9379
The electronic properties of materials with different configurations
were calculated by employing the GGA functional. The HOMO and
LUMO energies of all compounds are presented in Table 1. The
electronic absorption relates to the transition from the ground state to
the first excited state and is mainly described by one-electron excitation
from HOMO to LUMO.60 The optical absorption coefficient has been
extracted from the imaginary part of the complex dielectric function,
ε(ω) = εr(ω) + εi(ω), where εr(ω) is the real part and εi(ω) is the
imaginary part of the complex dielectric function.61 The optical
absorption coefficient is given as
Rajeev Ahuja − Condensed Matter Theory Group, Department of
Physics and Astronomy, Uppsala University, 75120 Uppsala,
Sweden; orcid.org/0000-0003-1231-9994
̀
̀
́
Gilles Ledoux − Institut Lumiere Matiere, Universite Lyon,
́
Universite Claude Bernard Lyon 1, CNRS, 69626 Villeurbanne,
α = 2 ω |ε(ω)| − εr(ω)
(1)
France; orcid.org/0000-0002-0867-1285
where |ε(ω)| = εr2(ω) + εi2(ω) is the relative dielectric constant.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00758
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00758.
■
Author Contributions
sı
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Digital images of the synthesis, reactivity, and thermo-
chromic behavior of 1, SEM, TEM, HRTEM, size
distribution curve, EDX, and XPS analysis of AgCuSe
NPs, additional XRD patterns of AgCuSe and Ag3CuS2,
FT-IR of 1−4, X-ray structures of 2 and 3, TG-DTG
curves of 3 and 4, additional figures related to DFT
calculations and simulated UV−visible spectra of 1−4,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.G. thanks the French ministry of higher education for her
Ph.D. grant (doctoral school of chemistry, Lyon). Authors also
thank L. Burel (TEM), Y. Aizac (powder XRD), N. Bonnet (ICP
elemental analysis), and Dr. L. Cardenas (XPS) of IRCELYON.
D.S., P.K.P., and R.A. thank Olle Engkvists Stiftelse, Carl
Tryggers Stiftelse for Vetenskaplig Forskning (CTS), and the
Swedish Research Council (VR) for financial support. SNIC and
HPC2N are acknowledged for providing the computing
facilities.
t
and decomposition pathway of Bu2Se via β-hydrogen
elimination (PDF)
Video of the temperature-dependent simulation of 1−4 at
353 and 393 K (MP4)
Video of thermochromic behavior of 1 (MOV)
Video of nonthermochromic behavior of 3 (MP4)
J
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