Thermochromism and piezochromism of an atomically precise high-nuclearity silver sulfide nanocluster

Article abstract of DOI:10.1039/d0cc07085h

A novel high-nuclearity silver sulfide nanocluster [Ag₅₀S₇(SC₆H₄F)₃₆(dppp)₆]·4DMI, (hereafter abbreviated as1?4DMI) was synthesised. Solvent-free crystals of 1 displayed a completely reversible narrowing and broadening of the optical band gap that was accompanied by visual thermochromism and piezochromism changeovers, when stimulated by varying temperatures between 113 and 413 K or by changing the pressure from 1 atm to 7.5 GPa.

Full text of DOI:10.1039/d0cc07085h

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Thermochromism and piezochromism of an
atomically precise high-nuclearity silver sulfide
nanocluster†
Cite this: Chem. Commun., 2021,
57, 2372
Received 26th October 2020,
Accepted 26th January 2021
Qiao-Qiao Sun,a Qian Li,b Hai-Yang Li,*a Miao-Miao Zhang,a Meng-En Sun,a Si Li,a
a
Zewei Quan *b and Shuang-Quan Zang
DOI: 10.1039/d0cc07085h
rsc.li/chemcomm
A novel high-nuclearity silver sulfide nanocluster [Ag50S7(SC6H4F)36(dppp)6]Á variety of fields, including chemical anticounterfeiting, optical
4DMI, (hereafter abbreviated as 1Á4DMI) was synthesised. Solvent- sensors, thermal energy storage materials, and smart windows.15
free crystals of 1 displayed a completely reversible narrowing and Many silver sulfide nanocluster crystals display thermochromic
broadening of the optical band gap that was accompanied by visual luminescence in a range of cryogenic and room temperatures.16
thermochromism and piezochromism changeovers, when stimulated However, thermally induced color change and energy-gap mod-
by varying temperatures between 113 and 413 K or by changing the ulation in a high-temperature range have not been reported. In
pressure from 1 atm to 7.5 GPa.
addition, piezochromism is a new type of force response of
intelligent materials that depicts luminescence or color varia-
Silver sulfide Ag2S has been intensively studied as the most tion under the application of pressure.17,18 Because the applied
in-demand semiconductor, which has potential application in pressure appears as an independent thermodynamic parameter
optoelectronics, biosensorics and catalysis.1–3 Among them, that can both compress and distort the structure of materials,
nanosized metal clusters have gained wide research interest pressure-induced optical color-changing materials have
since their well-defined structures, attractive size distribution attracted wide attention in crystalline materials including
and inherent properties can be applied in various areas, such as halide perovskites and metal–organic frameworks.19,20 To date,
chemical sensing, biolabeling, drug delivery and medical only a few research studies have been conducted on the bulk
therapy.1–3 In previous studies, significant progress has been crystals of atomically precise metal-cluster at high pressure.21
made in the chemical synthesis and well-defined structural
Herein, we prepared single crystals of a novel high-nuclearity
characterization of high-nuclear silver sulfide nanoclusters silver sulfide nanocluster (1Á4DMI) via a reaction between a
protected by thiolate or co-protected by thiolate and phosphine silver-SC6H4F precursor and dppp in a mixture of DMI and
ligands.4–10 However, for most silver sulfide nanoclusters, their trichloromethane at room temperature, (DMI = 1,3-dimethyl-2-
inherent instability, especially thermal instability, has hampered imidazolidinone, dppp = 1,3-bis(diphenylphosphino)propane).
their potential application. To date, only a few low-nuclearity Solvent-free crystals of [Ag50S7(SC6H4F)36(dppp)6] (1) demon-
silver sulfide nanoclusters can resist high temperatures in strate unusual thermal stability in air up to 413 K. When the
air,11–13 and no high-nuclearity case has been reported.
crystals of 1 were subjected to continuously varying temperatures
Thermochromism is a phenomenon in which the luminescence between 113 and 413 K and varying hydrostatic pressures
or color of materials changes as the temperature increases or between 1 atm and 7.5 GPa in a diamond anvil cell (DAC),
decreases.14 Thermochromic materials have been applied in a continuously reversing thermochromism and piezochromism
was observed. Temperature-dependent powder X-ray diffraction
(PXRD) and pressure-dependent Raman spectra demonstrated
that no obvious phase transformation occurred during the
a Green Catalysis Center and College of Chemistry, Zhengzhou University,
Zhengzhou 450001, China. E-mail: lihaiyang@zzu.edu.cn
b Department of Chemistry, Academy for Advanced Interdisciplinary Studies,
Shenzhen Engineering Research Center for Frontier Materials Synthesis at High
Pressures, Southern University of Science and Technology (SUSTech), Shenzhen,
Guangdong 518055, China. E-mail: quanzw@sustech.edu.cn
thermochromism and piezochromism processes. This work
shows that temperature and pressure can significantly tailor
the electronic structure of metal clusters, resulting in the
potential discovery of novel physical phenomena and new cluster
materials under high pressure.
† Electronic supplementary information (ESI) available: Experimental details,
powder X-ray diffraction, thermal analysis, and additional figures. CCDC
1992657–1992659. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d0cc07085h
Single-crystal structural analysis revealed that 1Á4DMI crys-
tallized in the monoclinic space group P21/c. Cluster 1 was
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Fig. 2 In situ variable-temperature PXRD of 1Á4DMI during both heating
and cooling cycles.
temperature was changed using temperature-dependent PXRD
(Fig. 2). In the first cycle, the structure of 1Á4DMI nearly did not
change between 113 K and 393 K since the diffraction patterns
remain invariable. However, when the temperature increases
from 393 K to 413 K, the diffraction peaks below a 2y of 51
gradually disappear, suggesting the loss of the DMI solvent
molecules, which corresponds to the TG results. During the
cooling process when the temperature decreases from 413 K to
113 K, the diffraction patterns no longer change until the
second cycle of temperature variation was finished. Such high
thermal stability is very rare in atomically-precise silver sulfide
Fig. 1 (a) Ag6S7 core in the space-filling mode. (b) Ag6S7 encapsulated in
the Ag32S22 shell in the stick mode. (c) Ag6S7@Ag32S22 surrounded by the
Ag12S14 motif around the waist. (d) Top view of Ag6S7@Ag32S22@Ag12S14
protected by dppp ligands. (e) Side view of Ag6S7@Ag32S22@Ag12S14 by the
–SC6H4F ligand. (f) Stacking of the [Ag50S7(SC6H4F)36(dppp)6] cluster in a
unit cell. Color code: green, Ag in the core and shell; turquoise, Ag in the
waist; yellow, S; bright green, F; purple, P; gray, C.
comprised of 50 silver(I) ions bridged by 36-SC6H4F ligands and nanoclusters.11–13
seven S2À ions together with six dppp ligands (Fig. S1, ESI†);
Subsequently, we collected the crystal data of 1Á4DMI at
structurally, Cluster 1 can be depicted as an Ag6S7 core encap- 100 K, 150 K and 200 K (Table S2, ESI†). 1Á4DMI exhibits a
sulated in an Ag32S22 shell (Fig. 1a and b). The Ag6S7@Ag32S22 phenomenon of volume expansion in the unchanged space
core–shell structure is further surrounded by an Ag12S14 motif group in the tested temperature range (Fig. S5 and Table S3,
around the waist (Fig. 1c and d). The Ag6S7 core is composed of ESI†). Notably, at 200 K, the DMI molecules become disordered,
six Ag+ and seven S2À ions. From the coordination modes of and cannot be solved; at 298 K, many of the silver atoms
S2À ions, one of the seven S2À ions is in the center and adopts a become even more disordered. Thus, we could not obtain
m4-bridging mode that is coordinated to Ag+ ions; the remaining useful crystallographic data of solvent-free 1. Instead, we
S2À ions function as m5- and m6-bridges binding both the core plotted the function of the unit-cell parameters versus tempera-
and Ag32S22-shell Ag atoms (Fig. S2, ESI†). Each P atom of dppp ture in a temperature range of 100–260 K (Fig. S6, ESI†), which
ligands coordinates to one Ag atom. The bond lengths including suggested that thermal expansion should occur along the three
Ag–Ag, Ag–S and Ag–P are listed in Table S1 (ESI†).
axes of solvent-free 1; furthermore, the unit-cell volume should
The –SC6H4F groups which are located at the top and bottom expand, and thus the crystal size should increase (Fig. S7 and
of the discus-like clusters, point upward and downward (Fig. 1e). Table S4, S5, ESI†).
The weak intercluster C–HÁÁÁF bonding and van der Waals inter-
Interestingly, a synchronous temperature-dependent color
actions consolidate the stacking of the cluster in all three dimen- change in the solvent-free crystals of 1 from yellow to dark red
sions. A 2-fold screw axis appears along the [0,1,0] direction at 1/4, is observed from 113 to 413 K (Fig. 3a). The reversibility of the
y, 1/4, and the glide plane is perpendicular to the b axis with a color change is demonstrated by imaging for at least four
glide component of [1/2, 0, 1/2]. Thus, two cluster 1 molecules are heating and cooling cycles (Fig. 3b). UV-vis absorption spectra
located on the surface of the (001) planes, and four clusters pass from 113–413 K varies in the range of 525–565 nm (Fig. 3c).
through the c axis in the unit cell (Fig. 1f). The phase purity of Correspondingly, Tauc plots of the UV-vis spectra indicate that
1Á4DMI was confirmed by PXRD (Fig. S3, ESI†). The thermogravi- the band gap and temperature change are negatively correlated,
metry (TG) curve shows a weight loss that occurs below 413 K, and the former decreases from 2.28 to 2.10 eV with increasing
which corresponds to the 4 DMI molecules (Fig. S4, ESI†).
temperature (Fig. 3e). The large band gap may be caused by the
The thermal stability observed in the TG experiments quantum confinement effect as the dimensions of the Ag50
encouraged us to examine the structural stability when the cluster is close to the exciton Bohr radius of about 2 nm for
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Fig. 4 (a) Optical micrographs of
1
in
a
DAC upon compression.
(b) Selected high-pressure absorption spectra of 1 from ambient condi-
tions to 7.5 GPa. (c) Direct band gap evolution of 1 as a function of
pressure, as calculated from Tauc plots. The inset shows the band gap
Tauc plots at 1 atm, and the red line represents the linearly fitted curve.
The evolution of the optical band gap can be effectively evaluated
according to the movement of the absorption edge. Thus, Tauc
plots are implemented to determine the band gap variations.
Fig. 4c depicts the relationship between the direct band gap and
pressure, which presents an approximately linear relationship
that can be described by the following equation, Eg = 2.200 À
(2.609 Â 10À2) Â P. The band gap of 1 undergoes a continuously
noticeable decrease from ambient conditions to 7.5 GPa, indicating
Fig. 3 (a) Alternating colors in a single crystal of 1 from 113 to 413 K.
(b) Varying colors of 1 in a four-cycle test. (c) Temperature-dependent
solid state UV-vis absorption spectra and (e) the corresponding Tauc plots
recorded from 113 to 413 K. (d) Unit-cell volume of 1 versus temperature.
(f) Plots displaying the linear relationship between the band gap and
temperature.
Ag2S.22 The relationship between the band gap and temperature a negative correlation between the band gap and pressure. In
presents an approximately linear relationship that can be described addition, the functional relationship between the indirect band
by the following equation, Eg = 2.354 À (5.956 Â 10À4) Â T (Fig. 3f). gap and pressure shows a similar trend (Fig. S11, ESI†). Thus, the
Since no phase transformation occurs, and the transitions of the band gap transition regardless of its direct or indirect nature is
thiolated silver(I) clusters generally originate from metal-to-ligand modulated by a pressure-induced shrinkage of bond;25 thus, an
charge-transfer (MLCT) that are modified by Ag–Ag interactions,23 increase in pressure can promote the overlapping of elemental
the synchronous expansion of crystals and the increase in orbital orbitals.25 In addition, the decreases in the band gap induced by
hybrids may be induced by the increase in the degree of disorder of high temperature and high pressure seem to be contradictory. We
the metal atoms. The above observations may collectively induce tentatively propose that high temperature may cause an increase in
a decrease in the energy gap of the crystals as the temperature both atomic distances and electron sharing owing to a weakened
increases (Fig. 3d). The negative temperature effects are char- binding force, and the latter is probably the dominant source of the
acteristic of typical semiconductors, and they tend to decrease gap narrowing. In addition, the number of atoms in the unit cell is
with increasing temperature.24
The bond length and volume-dependent energy gap during which will be overcome in the future.
too large to calculate the details for this type of semiconductor,
the thermochromic phenomenon further inspired us to inves-
The pressure-dependent Raman spectra of 1 were recorded to
tigate the pressure response of this cluster crystal, which is a investigate the possibility of a structural change. At ambient
newly developed research field in atomically-precise metal pressure, the Raman spectra consist of ten strong vibrational
clusters. Subsequently, the pressure-dependent optical spectra modes with a frequency distribution from 50 cmÀ1 to 1200 cmÀ1
of 1 were measured using a DAC. The extraordinary piezo- (Fig. S12 and S13, ESI†).26 With increasing pressure, most
chromic transition of 1 is seen in the optical micrographs. vibrational modes show normal blueshifts, which may be attrib-
Upon compression, the original orange-red gradually changes uted to the reduction in the interatomic distance. The blueshift
to translucent dark red at 7.5 GPa, and the color reverses after of the lattice vibration of n1 is the fastest among all vibrational
decompression (Fig. 4a). The reversible color change phenomenon modes27 (Fig. S14, ESI†), suggesting the shrinkage of Ag–Ag
can be maintained for at least two cycles (Fig. S8, ESI†). Corre- bonds. The vibrational modes are no longer observed when the
spondingly, the absorption spectra also return to the initial state pressure reaches 4.6 GPa, probably because of masking that is
(Fig. S9 and S10, ESI†). Selected UV-vis absorption spectra of 1 induced by a high fluorescence background. Pressure-induced
upon compression are depicted in Fig. 4b. Under ambient condi- fluorescence background is commonly observed in a variety of
tions, the absorption edge is observed at approximately 586 nm, materials, especially organic materials.28 Such fluorescence back-
and the absorption edge gradually redshifts to 670 nm at 7.5 GPa. ground is too weak to be clearly observed in photoluminescence
2374 | Chem. Commun., 2021, 57, 2372À2375
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features a multilayer core–shell structure and unusual stability
at a high temperature in air. The temperature and pressure can
modulate the optical band gap of the cluster crystals, leading to
reversible thermochromism and piezochromism in the visible
light range that is unprecedented among metal clusters. This
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temperature or pressure probes and represents a breakthrough
in the field of metal nanoclusters.
We acknowledge the support for this work from the National
Science Fund for Distinguished Young Scholars (21825106), the
National Natural Science Foundation of China (No. 21671175
and 21801227), the Program for Innovative Research Team (in
Science and Technology) in Universities of Henan Province
(19IRTSTHN022), Zhengzhou University and the Shenzhen
Engineering Research Center for Frontier Materials Synthesis
at High Pressures.
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Conflicts of interest
There are no conflicts to declare.
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