Formation of persistent ordered lamellar mesophases in azobenzene- containing silver thiolates and their application in the controlled synthesis of silver nanomaterials

Article abstract of DOI:10.1039/c3tc32380c

Metal thiolates have aroused intensive interest mainly due to their precursor-based preparation of nanostructured metal or metal chalcogenides. In this paper, a series of azobenzene-containing thiol ligands with different length alkoxy tails and their corresponding silver thiolates AgS-C ₁₀H₂₀-Ph-NN-Ph-OC_nH_2n+1 with n = 1, 6, 8, 12, have been successfully synthesized, and their thermal properties and phase behavior have been systematically investigated by differential scanning calorimetry (DSC), variable-temperature SAXS/WAXS and temperature dependent FTIR. By introducing azobenzene mesogen for the first time into the silver thiolates, ordered lamellar liquid crystalline mesophases persisting throughout a higher temperature have been achieved derived from their specific orthorhombic crystalline structures, which are in sharp contrast to the micellar or hexagonal columnar mesophases reported for silver alkane thiolates with a longer aliphatic alkyl chain AgSC_mH_2m+1 (m ≥ 12) owing to the interplay of azobenzene mesogen π-π stacking and the inorganic skeleton binding of a Ag-S slab. Furthermore, the intermediate nanoparticles formation and silver nanodisks preparation through an in situ thermolytic reaction based on such mesogenic precursors have been demonstrated in principle. As a kind of functional metallomesogen precursor the silver mesogenic thiolates with persistent ordered lamellar mesophases provide an ideal two-dimensional (2D) confined environment for the investigation of a layered-precursor-to-lamellar- nanomaterial (LPLM) mechanism of solventless thermolysis and the fascinating controlled preparation of variant 2D shaped metal or metal sulfide nanomaterials.

Full text of DOI:10.1039/c3tc32380c

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Materials Chemistry C
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PAPER
Formation of persistent ordered lamellar
mesophases in azobenzene-containing silver
thiolates and their application in the controlled
synthesis of silver nanomaterials†
Cite this: J. Mater. Chem. C, 2014, 2,
2375
a
a
a
b
a
Junfei Duan,ac Jie Ma, Bin Wu, Qian Li, Jianglin Fang and Dongzhong Chen
*
Metal thiolates have aroused intensive interest mainly due to their precursor-based preparation of
nanostructured metal or metal chalcogenides. In this paper, a series of azobenzene-containing thiol
ligands with different length alkoxy tails and their corresponding silver thiolates AgS–C10H20–Ph–N]N–
Ph–OCnH2n+1 with n ¼ 1, 6, 8, 12, have been successfully synthesized, and their thermal properties and
phase behavior have been systematically investigated by differential scanning calorimetry (DSC), variable-
temperature SAXS/WAXS and temperature dependent FTIR. By introducing azobenzene mesogen for the
first time into the silver thiolates, ordered lamellar liquid crystalline mesophases persisting throughout a
higher temperature have been achieved derived from their specific orthorhombic crystalline structures,
which are in sharp contrast to the micellar or hexagonal columnar mesophases reported for silver alkane
thiolates with a longer aliphatic alkyl chain AgSCmH2m+1 (m $ 12) owing to the interplay of azobenzene
mesogen p–p stacking and the inorganic skeleton binding of a Ag–S slab. Furthermore, the intermediate
nanoparticles formation and silver nanodisks preparation through an in situ thermolytic reaction based
on such mesogenic precursors have been demonstrated in principle. As
a kind of functional
Received 2nd December 2013
Accepted 6th January 2014
metallomesogen precursor the silver mesogenic thiolates with persistent ordered lamellar mesophases
provide an ideal two-dimensional (2D) confined environment for the investigation of a layered-
precursor-to-lamellar-nanomaterial (LPLM) mechanism of solventless thermolysis and the fascinating
controlled preparation of variant 2D shaped metal or metal sulfide nanomaterials.
DOI: 10.1039/c3tc32380c
www.rsc.org/MaterialsC
nanostructured metal or metal chalcogenides. Especially silver
alkane thiolates with a highly ordered layered quasihexagonal
crystalline structure7 exhibited rich thermotropic phase
1. Introduction
Metal thiolates combining the characteristic properties of metal
atoms with the design exibility and uidity of organic mole-
cules have attracted considerable experimental and theoretical
study during the past decades.1 Examples such as copper thio-
lates2,3 and gold(I) thiolates4–6 with highly ordered assembled
structures facilitated the precursor based preparation of
behavior.8 Their thermotropic mesophases varied with the
length of alkyl chain, but the melting points kept almost
constant independent of the chain length.9 More importantly,
various metal or metal sulde nanomaterials with controllable
size and morphology have been prepared from the corre-
sponding metal thiolate precursors through solvothermal10 or
solventless thermolysis.11–18 Solventless thermolysis can be
regarded as a quasi-solid-state process with the nucleation and
subsequent growth being relatively controllable and adjust-
able18 and serves as an intriguing method for nanostructured
materials preparation. Korgel and co-workers have reported the
synthesis of Cu2S,11,12 NiS13 and Bi2S3 (ref. 14) by using corre-
sponding thiolate precursors through a solventless thermolysis
approach. Wu and co-workers have prepared Cu2S,15 Bi,16 PbS17
and Ag18 nanoproducts by this same method based on alkane
thiolate precursors and, consequently, they proposed a possible
mechanism of the metal thiolates based layered-precusor-to-
lamellar-nanoproduct.15–18 Some intermediates, such as small
nanoparticle aggregations for Cu2S nanodisks15b or bismuth
aDepartment of Polymer Science and Engineering, Key Laboratory of Mesoscopic
Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, P. R. China. E-mail: cdz@nju.edu.cn; Fax: +86
25-8331-7761
bCenter for Materials Analysis, Nanjing University, Nanjing, 210093, P. R. China
cDepartment of Physics and Electronic Science, Changsha University of Science and
Technology, Changsha, Hunan 410076, P. R. China
† Electronic supplementary information (ESI) available: Experimental procedures,
synthesis and characterization of thiol ligands and some intermediates; DSC,
POM and XRD characterization for thiol ligands 1a–1d; TGA thermograms of
thiolates 2a–2d, and some additional DSC curves and variable-temperature
SAXS/WAXS proles for 2a, 2b and 2d; FTIR, EDX and XRD proles of produced
silver nanomaterials; NMR analysis of thermolytic reaction byproducts
compared to thiol ligands and disulde; AFM images and additional TEM
micrographs. See DOI: 10.1039/c3tc32380c
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nanolms16, have been reported, however, no intermediate state times, then ltered and dried under vacuum overnight to
for a silver thiolate based system has been observed, which may produce a brown–yellow solid powder of 60.3 mg at 91% yield.
be mainly due to the complex micellar phase formation for Anal. calcd (%) for 2c: C 59.50, H 7.49, N 4.63; found: C 60.14,
longer chain alkane thiolates at higher temperatures,8 further- H 7.93, N 4.24%. Similarly, for 2a: yield 93.6%, Anal. calcd (%)
more, the reactivity rate of the precursors has remarkable C 54.44, H 6.16, N 5.52; found: C 54.31, H 5.89, N 5.61%;
inuence on the synthetic processes of nanomaterials.19
for 2b: yield 91.8%, Anal. calcd (%) C 58.23, H 7.16, N 4.85;
By introducing azobenzene mesogen into silver thiolates, found: C 58.40, H 7.22, N 4.80%; and for 2d: yield 87.3%,
unusual lamellar liquid crystalline (LC) phases over a wide Anal. calcd (%) C 60.65, H 7.79, N 4.42; found: C 61.54, H 7.92,
temperature range can be attained, which are quite different to N 4.21%.
those based on simple alkane metal thiolates under higher
temperatures, such as micellar mesophases from silver alkane
thiolates and the hexagonal columnar phase from copper
2.3. Instruments and measurements
alkane thiolates.2,8 Such persistent lamellar LC mesophases 1H NMR spectra were obtained on 300 MHz (Bruker AMX300)
provide an opportunity for the controlled preparation of metal instruments with TMS as the internal standard. Mass spectra were
or metal sulde nanomaterials and investigation of the mech- recorded on a LCQ Fleet ion trap mass spectrometer by electro-
anism of the layered-precusor-to-lamellar-nanoproduct of spray ionization. Elemental analyses (EA) were carried out with an
solventless thermolysis within a real constrained layered Elementar Vario Micro. Fourier transform infrared spectra (FTIR)
precursor system.
were recorded on a NICOLET NEXUS 870 infrared spectrometer
Herein in this paper, we introduce azobenzene mesogen for equipped with a variable temperature control unit with a preci-
ꢁ
the rst time into this organic–inorganic hybrid system to sion of ꢀ1 C using pressed potassium bromide pellet samples.
investigate the phase behavior of this novel series of silver thi- Differential scanning calorimetry (DSC) traces were recorded on a
olates and the preparation of silver nanodisks based on such Perkin-Elmer Pyris I calorimeter equipped with a cooling acces-
ꢂ1
ꢁ
mesogenic precursors. A series of thiols introducing azo- sory with a 10 C min heating or cooling rate under nitrogen
benzene mesogen with different length alkoxy tails and thus atmosphere. Thermal gravimetric analysis (TGA) was performed
their silver thiolates were designed and synthesized. Then a using a Perkin-Elmer Pyris I instrument at a heating rate of 20 ꢁC
systematic study was conducted on their thermal behavior and minꢂ1 under a nitrogen atmosphere. Polarized optical microscopy
phase structures by differential scanning calorimetry (DSC), (POM) texture investigation was conducted with melt-pressed
variable-temperature simultaneous small- and wide-angle X-ray samples sandwiched between two glass plates on a PM6000
scattering (SAXS/WAXS) and temperature dependent Fourier optical microscope under cross-polarizers equipped with a Leitz-
transform infrared spectroscopy (FTIR). Furthermore, a two- 350 heating stage and an associated Nikon (D3100) digital
dimensional (2D) disk-shaped silver nanomaterials preparation camera. Variable-temperature X-ray scattering experiments were
has been preliminarily explored based on the mesogenic silver performed with a high-ux small-angle X-ray scattering instru-
thiolate precursors.
ment (SAXSess mc2, Anton Paar) equipped with Kratky block-
collimation system and a temperature control unit (Anton Paar
TCS300). Both small-angle X-ray scattering (SAXS) and wide-angle
X-ray scattering (WAXS) were simultaneously recorded on an
imaging-plate (IP) which extended to the high-angle range (the q
range covered by the IP was from 0.06 to 29 nmꢂ1, q ¼ (4p sin q)/l,
where the wavelength l is 0.1542 nm of Cu Ka radiation and 2q is
the scattering angle) at 40 kV and 50 mA for 5 min. Typically, the
powder sample was encapsulated with aluminum foil during the
measurements and the obtained X-ray analysis data was pro-
cessed with the associated SAXSquant soware 3.80. Trans-
mission electron microscopy (TEM) studies were performed on a
JEM-2100 (JEOL) at 200 kV. Each sample was prepared by
depositing a drop of a dilute suspension of the corresponding
nanocrystals in THF onto a carbon-lm-coated Cu grid. For
2. Experimental section
2.1. Chemical reagents
Silver nitrate (99.8%), potassium thioacetate (98%) and sodium
borohydride (98%) were purchased from Alfa Aesar and used as
received without purication. Anhydrous tetrahydrofuran (THF)
was distilled over sodium–benzophenone under nitrogen prior
to use. All other chemical reagents were puried according to
standard procedures before use.
2.2. Synthesis and characterization for the series of silver
thiolates 2
The synthesis of AgS–C10H20–Ph–N]N–Ph–OCnH2n+1 with an atomic force microscopy (AFM) investigation, the samples were
octyloxy tail of n ¼ 8 (2c) is representatively shown here as an prepared by drop-casting the dilute silver nanoparticles (nano-
example, all other analogues were prepared by the same disks) suspension (0.1 mg mLꢂ1) onto a silicon substrate, aer
protocol using thiol ligands with various tails. A solution of evaporation the AFM imaging and measurements were performed
silver nitrate (17.0 mg, 0.1 mmol) in THF–H2O (5 mL : 1 mL) under ambient conditions using a JPK NanoWizard II system (JPK
mixture was added dropwise into a solution of 4-(10-mercap- Instruments AG, Germany) working in tapping-mode (TM). The X-
todecyl)-40-octyloxy-azobenzene (1c, 150 mg, 0.3 mmol) in THF ray powder diffraction (XRD) patterns of inorganic samples were
(15 mL) at room temperature. A light yellow precipitate recorded on a Shimadzu XRD-6000 X-ray diffractometer by
formed immediately, and the mixture was stirred for 2 h. The weighing about 10 mg of solid in a standard cavity mount with Cu
product was puried by washing with THF and ethanol several Ka radiation at room temperature.
2376 | J. Mater. Chem. C, 2014, 2, 2375–2386
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and Table S1 in the ESI†). Such thermal behavior was attributed
to the thermolysis as indicated by the small exothermic peaks in
the rst heating run and aer experiencing this in situ thermal
reducing reaction, the sequential DSC traces displayed the
thermal properties of the corresponding thermolytic byprod-
ucts, which will be conrmed in the next part to be mainly
composed of azobenzene-containing alkyl sulde and disulde.
It is worth noting that thermolysis (thermal reduction) rather
than decomposition occurred here, as indicated by TGA analysis
that for all of the series 2 samples there was no detectable
weight loss before 280 ꢁC (Table 1, also see Fig. S3 in the ESI†).
As shown in Fig. 1A, before the thermolysis occurs, all of the
thiolate samples exhibited enantiotropic mesophases, except
the silver thiolate 2a with the shortest methoxy tail, the melting
temperatures of the series silver thiolates decreased with the
alkoxy tail length ꢁincreasing from methoxy to dodecyloxy and
showed about 10 C supercooling in the cooling cycles, while
the clearing temperatures increased from 2b to 2d thus leading
to an ever broadening liquid crystalline temperature range as
listed in Table 1. The monotonically decreasing tendency of the
melting temperatures of the azobenzene mesogen-containing
series of silver thiolates was in sharp contrast with the behavior
of simple alkane silver thiolates (AgSCmH2m+1) which showed
almost unaltered melting temperatures around 130 ꢁC inde-
pendent of the chain-length.9 Based on the understanding that
silver thiolates possess a bilayered structure as rstly proposed
by Dance et al. with a rigid inorganic central slab through the
trigonal-planar coordination of Ag and three SR moieties, with
the R groups extending almost perpendicular to both sides of
the slab.7 For the inorganic skeleton structure is not signi-
cantly perturbed by the organization of aliphatic alkyl chains
originated from weaker van der Waals interactions, thus the
melting temperature is almost unchanged for simple alkane
thiolates AgSCmH2m+1 of variant alkyl length.21 Whereas for the
studied series of thiolates 2a–2d, owing to the introduction of
the mesogenic chromophore azobenzene exerting p–p inter-
actions substantially stronger than van der Waals forces, their
melting points are obviously higher than those of aliphatic
alkane thiolates and diminish with the decrease of proportion
and extent of p–p stacking as the alkoxy tails lengthen.
Furthermore, it is interesting to notice that liquid crystalline
mesophases, which will be conrmed to be highly ordered
smectic phases by SAXS/WAXS analysis, POM investigation and
temperature-dependent FTIR study as presented in the
following context, were formed in the silver thiolates 2b–2d
derived from the mesogenic thiol ligands 1b–1d, while no LC
phase was observed in the thiolate 2a originated from the
crystalline thiol ligand 1a.
3. Results and discussion
3.1 Synthesis
The synthetic route of the series of azobenzene-containing thiol
ligands 4-(10-mercaptodecyloxy)-40-alkoxy-azobenzene (1) and
their corresponding silver thiolates
2 are illustrated in
Scheme 1. The detailed synthetic procedures for thiols 1 and the
characterization data by 1H NMR, FTIR, and MS are provided in
the ESI.† The preparation of the silver thiolates 2 was according
to a modied procedure of a literature method,20 as described in
the Experimental section.
3.2 Thermal properties and phase behaviors by DSC
measurements
The thermal properties of the silver thiolates 2 and their cor-
responding thiol ligands 1 were rst investigated by DSC and
TGA. For the series of thiolates 2, it is interesting to note that
when the highest temperature reached upon heating was lower
than the exothermic transition temperature around 245 ꢁC,
such as below 210 ꢁC, the DSC thermograms of the heating and
cooling cycles were almost reversible, as shown in panel A of
Fig. 1, The thermal transition peak temperatures and associated
enthalpy changes, and also the phase assignments in combi-
nation with other characterization techniques as described next
for the silver thiolates 2 and their corresponding thiol ligands 1
(see Fig. S1 in the ESI†) are summarized in Table 1. As shown in
panel B of Fig. 1, when the maximum temperature in the rst
heating run reached 250 ꢁC, which was beyond the temperature
of phase transition peaks with small exothermic enthalpy
changes of 2–6 J gꢂ1 at 237, 241, 245, and 247 C for 2a to 2d,
ꢁ
respectively, the thermogram traces in the cooling run were
totally different with the curves displayed in Fig. 1A. Moreover,
for all the series 2 samples aer going through a higher
temperature cycle at 250 ꢁC, other almost reversible endo-
thermic transitions lower than 150 ꢁC were observed in the
following heating runs and essentially reversible exothermic
peaks were exhibited in all succeeding cooling runs (see Fig. S2
3.3 Variable temperature SAXS/WAXS investigation
SAXS is a powerful tool and can oen provide some structural
details, simultaneous SAXS and WAXS investigations were per-
formed in situ using a high-ux SAXSess X-ray scattering
instrument equipped with a temperature control accessory for
probing into the organized structure and phase characteristics
of silver thiolates. As representatively shown in Fig. 2 for 2c, all
Scheme 1 Synthetic route of the series of thiol ligands 1 and their
corresponding silver thiolates 2.
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Fig. 1 DSC thermograms of tꢂh1e series of silver thiolates 2a–2d during (A) the 2nd heating and 1st cooling scans in the temperature rangeꢂo1f 50–
210 ꢁC at a rate of 10 ꢁC min ; and (B) the 1st heating and 1st cooling scans in the range of 50–250 ꢁC at the same rate of 10 ꢁC min
.
of the series of thiolates investigated displayed layered struc- interactions (Fig. 2A). As shown in Fig. 2B, another two peaks
tures in their crystalline solid with more than ten orders of with q values of 17.20 and 21.15 nmꢂ1 (corresponding to 0.37
reection fringes in the low-angle region and lamellar meso- and 0.30 nm, respectively), were ascribed to the azobenzene
phases with ve equidistant SAXS peaks aer melting for 2b to moiety of p–p stacking and the intralayer structure of the Ag–S
2d with longer alkoxy tails (Fig. 2A and also see Fig. S4 in the slab, respectively,7,9,21 their temperature dependent shis are
ESI†). The background peaks from the packing aluminum foil discussed later in this paper. Upon heating into the mesophase
as marked by an asterisk located in the wide-angle region have a state as at 190 ꢁC, with the disappearance of the crystalline
negligible disturbance to the interested sample peaks, thus no fringes ve new reections appeared in the small-angle range
complete subtraction was conducted so as to avoid distortion of with a q ratio of 1 : 2 : 3 : 4 : 5 characterizing a highly ordered
the peak shape and smoothing over some signicant weak lamellar liquid crystalline mesophase of spacing 5.51 nm,
signals. Furthermore, with the introduction of mesogenic azo- which was 1.24 nm shorter than the 6.75 nm observed in the
benzene, besides the SAXS higher order reections of lamellar crystalline state implying tilt or disorder of the melted alkyl
structure, three characteristic reections in the WAXS region chains as demonstrated by the following temperature-depen-
were exhibited in the crystalline state, the band at around dent FTIR investigation. Highly ordered smectic like thin lms
14.80 nmꢂ1 (0.42 nm distance) superimposed by some weak from azobenzenes showing up to eight Bragg reections were
higher order peaks of the layered structure was attributed to the reported by the thermal evaporation method or Langmuir–
average alkyl interchain spacing due to van der Waals Blodgett (LB) technique.22 Moreover, a shoulder reection of
Table 1 Thermal properties and phase assignments for the thiol ligands 1 and corresponding silver thiolates 2 in the temperature range of 50–
210 ꢁC for the 1st cooling and 2nd heating runs
Transition Tbc (ꢁC) [DH (J gꢂ1)]
LC range (ꢁC)
a
Ligand or thiolate
Tdecomp (ꢁC)
Thermal cycle
1a
1b
1c
1d
2a
2b
2c
2d
225
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
I 91.5 [71.5] Cr
Cr 101.4 [87.5] I
—
—
(10) + 21
9 + (10)
(2) + 26
7
18
13
—
—
14
6
232
I 99.1 [3.55] N 88.6 [3.26] SmA 66.9 [66.7] Cr
Cr 82.3 [87.5] SmA 91.4 [3.50] N 101.1 [3.3] I
I 99.9 [1.98] N 97.6 [2.26] SmA 72.0 [74.8] Cr
Cr 82.9 [4.2] 94.9 [104.8] SmA 102.0 [12.7] I
I 100.5 [14.8] SmA 82.3 [71.5] Cr
Cr 90.7 [75.0] SmA 103.4 [16.7] I
I 195.9 [65.2] Cr
238
245
333
Cr 207.1 [88.6] I
323
I 191.7 [8.4] SmX 177.9 [64.0] Cr
Cr 189.5 [56.4] SmX 195.0 [3.4] I
I 199.4 [8.5] SmX 174.0 [62.0] Cr
Cr 183.8 [67.4] SmX 200.6 [9.8] I
I 198.8 [7.4] SmX 167.5 [51.7] Cr
Cr 177.1 [78.9] SmX 200.9 [10.9] I
330
25
17
31
24
328
a
b
The onset of decomposition temperatures determined from TGA at a heating rate of 20 ꢁC minꢂ1 under a nitrogen atmosphere. The phase
transition temperatures and enthalpy changes (in square brackets) obtained from DSC at 10 ꢁC minꢂ1
smectic A, SmX ¼ highly ordered smectic phase with unidentied intralamellar organization details, N ¼ nematic, I ¼ isotropic.
.
Abbreviations: Cr ¼ crystalline, SmA ¼
c
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Fig. 2 SAXS/WAXS profiles of silver thiolate 2c at various temperatures upon heating or cooling cycles: (A) q ¼ 0.32–16.5 nmꢂ1; (B) q ¼ 16.0–24
nmꢂ1. The peaks labelled by asterisks are due to reflections of the packing aluminum foil background.
lower intensity, as indicated by an arrow emerged, and as the patterns superimposed (Fig. 3e), and 2d exhibited a pseudo
ꢁ
temperature further increased to 210 C two broad reections focal conical texture (Fig. 3f), all of these textures could be
appeared at q ¼ 1.35 and 2.80 nmꢂ1 presumably attributed to attributed to some kind of highly ordered smectic phases,24–27
the mixture of residual lamellar structure of the precursor and while they might also be displayed by some typical columnar
the silver nanoparticles formed due to partial thermolysis mesophases as generated from discotic mesogens.28,29 There-
within the time period of measurement at high temperature fore, here attention is prompted to avoid a columnar phase
which will be further discussed. Similar to 2c, 2b and 2d also misjudgment based only on POM textures, in fact from the
exhibited lamellar mesophases, while with the methoxy tail 2a temperature-dependent SAXS proles as discussed above
directly melted into the isotropic state in agreement with the showing more than four order reections, in combination with
DSC analysis results. The interlayer spacings of 2b and 2d in their thermal properties and POM observations, the LC meso-
their mesophases were shortened to 5.34, 6.10 nm compared phases can be assigned affirmatively to be some kind of highly
with 6.28 and 7.66 nm in their corresponding crystalline pha- ordered smectic phase SmX, which is also quite reasonable as a
ses, respectively. The dependence of the interlayer spacing d successive mesophase derived from the orthorhombic crystal-
against the alkoxy tail carbon number n in the crystalline phase line precursor phase as demonstrated next by the temperature
obeyed the strictly linear relationship of d ¼ 4.96 + 0.22n (with dependent FTIR investigation, while at the moment we cannot
correlation coefficient R2 ¼ 0.999, see Fig. S5 in the ESI†), which determine their detailed intralamellar arrangement due to the
was similar to the trendline of simple alkane silver thiolates lack of sufficient signals to characterize the lateral structure
with a much larger intercept value of 4.96 nm compared with within the lamellar.
ꢃ0.8 nm (ref. 23) due to the contribution of the azobenzene and
decyloxy spacer.
3.5 Temperature dependent FTIR study
Fourier transform infrared spectroscopy (FTIR) can provide a
highly sensitive probe of the molecular structure and dynamics
3.4 Texture observation by POM
The phase transition and mesomorphic properties of the silver of alkyl chains,23,30,31 thus variable-temperature FTIR was
thiolates together with their corresponding thiol ligands were employed to explore the silver thiolates for better under-
further conrmed by polarized optical microscopy (POM) standing their phase structure and transition at a molecular
investigation. As representatively shown in Fig. 3a and b for level. As representatively shown in Fig. 4 for 2c, 2954 and
ligand 1b, a typical focal conical texture of smectic A phase 2873 cmꢂ1 vibrations were assigned to asymmetric (rꢂ) and
(SmA) and schlieren texture of nematic phase (N) were observed symmetric (r+) stretching vibrations of the terminal methyl
largely in agreement with the temperature ranges determined groups, respectively. A remarkable shoulder at 2935 cmꢂ1 was
by DSC for thiol ligands 1b to 1d (Table 1, see also Fig. S1 and S6 attributed to the Fermi resonances of the zigzag conformer of
in the ESI†), which were also conrmed by the SAXS/WAXS the polymethylene chain (r+FR).23,31 The rꢂ and r+ bands of the
study (see Fig. S7 in the ESI†). For silver thiolate 2a, only terminal methyl groups became diminished and were gradually
spherulitic crystalline texture is observed upon cooling from more difficult to identify from the second to third phase tran-
isotropic phase corroborating its crystalline character without sition (Fig. 4a). The stronger bands at 2918 and 2849 cmꢂ1 were
any mesophase formation (Fig. 3c). It is interesting that for the assigned to the asymmetric (dꢂ) and symmetric (d+) stretching
longer tail mesogenic silver thiolates, 2b showed a mosaic like vibrations of the methylene group, respectively. The dꢂ and d+
texture with some extinction area (Fig. 3d), the texture of 2c was ranges are known to be strong indicators of chain conformation
quite similar to that of 2b with some small focal conical with low wavenumbers 2916–2918 and 2846–2849 cmꢂ1
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Fig. 3 Representative POM texture images under crossed polarizers of silver thiolates and a typical thiol ligand: (a) ligand 1b at 85 ꢁC, SmA; (b) 1b
at 98 ꢁC, N; (c) thiolate 2a at 193 ꢁC, Cr; (d) 2b at 191 ꢁC; (e) 2c at 188 ꢁC; and (f) 2d at 190 ꢁC, SmX.
characteristic of the ordered all-trans extended alkane chains, vibration and phenyl-N stretching mode, respectively.31 The
and upward shiing to 2924–2928 and 2854–2856 cmꢂ1 indic- vibrational band assigned to the phenyl ring was more sensitive
ative of the signicant increase in gauche population or and became broader accompanied with a downward shiing
switching to a high-temperature liquid-like disorder state.23,30
from 1603 to 1599 cmꢂ1 as the temperature only rose to around
Based on this cognition, the observed peak frequencies of 160 ꢁC, followed by broadening and further shiing to 1597
2919 and 2849 cmꢂ1 before melting indicated the predomi- cmꢂ1 as the temperature passed through the second to the third
nantly all-trans crystalline state, while obviously shiing to phase transition (Fig. 4b). The most interesting spectral indi-
higher wavenumbers and peak broadening exhibited when the cations for the current examined azobenzene based thiolate
ꢁ
temperature was increased to around 170 C (Fig. 4a), slightly system were revealed by some characteristic low frequency
lower than the melting transition temperature determined by bands. The bands within the range of 1465 to 1473 cmꢂ1 cor-
DSC measurements, indicating increased disorder of the alkyl responding to CH2 scissoring bending modes are widely used to
chain ensembles and
a pre-melting transition to some diagnose alkyl chains packing, the occurrence of a single
extent.23,30 The peaks became much broader when entering the narrow peak at 1473 cmꢂ1 is ascribed to the triclinic subcell
isotropic amorphous phase, all these spectral behaviors were packing of alkyl chains, while a single band at 1467 cmꢂ1
quite similar to that of the simple alkane silver thiolates.23,30 indicates a hexagonal subcell, both subcells are the crystal ones
Some more specic characters exhibited in the lower frequency of a single chain in an individual cell.23,30,31 In the present case,
region for the mesogenic silver thiolates were examined. The the slightly split peaks at 1473 and 1465 cmꢂ1 (Fig. 4b) were
characteristic vibrational peaks located at 1602, 1581, and presumably derived from orthorhombic arrangements of the
1498 cmꢂ1 were from the aromatic ring, and the strong bands at alkyl tail chains by interchain interactions between the contig-
1247 cmꢂ1 and 1150 cmꢂ1 were due to the phenyl ring-O uous CH2 groups of the two chains constituting the crystal
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Fig. 4 Variable-temperature FTIR spectra of 2c over the temperature range 30–203 ꢁC at an (a) high-frequency region (3000–2800 cmꢂ1); and
low-frequency region: (b) 1650–1400 cmꢂ1; (c) 1400–1000 cmꢂ1; and (d) 950–600 cmꢂ1
.
sub-cell.23,32 This kind of splitting, referred to as factor-group around q ¼ 21.15 nmꢂ1 (0.30 nm, labeled by a red dashed line as
splitting, is specic to orthorhombic sub-cells.23 Upon heating a guide for the eye in Fig. 2b) characteristic of the Ag–S inor-
to above 170 ꢁC the split bands at 1473 and 1465 cmꢂ1 merged ganic slab were indicative of the structural adjustment or
into a single wider band at 1467 cmꢂ1. Moreover, as shown in coordination symmetry change with temperature, especially
Fig. 4c and d in the crystalline solid state below 160 ꢁC, the split around the phase transition. As shown in Fig. 2b, the diffraction
bands at 1394 and 1385 cmꢂ1 were assigned to the CH3 peak shied to a lower angle at a temperature higher than
symmetric deformation or bending vibration, the slightly split 120 ꢁC and disappeared at around 180 ꢁC approaching the
bands at 1026 and 1016 cmꢂ1 attributed to the skeletal modes of mesophase transition temperature, which was due to the
the hydrocarbon chains or C–C–C backbone stretching modes, elongation or fracture of some Ag–S connection inside the
and the spilt bands at 721 and 730 cmꢂ1 assigned to the inorganic slab upon heating, resulting in the coordination
methylene C–H rocking mode, which are all very sensitive to the symmetry change from a regular trigonal planar structure in the
environment and to interlamellar interactions, further con- crystalline state to a mixture of digonal and trigonal coordina-
rming the orthorhombic packing feature.23,30 This highly tion at higher temperatures, as observed in the alkane silver
ordered orthorhombic organization is in sharp contrast to the thiolates.7 It is worth noting that the reections characteristic of
usual hexagonal packing structure observed in simple alkane the main layered structure, such as the crystalline fringes due to
silver thiolate23,30 or to the mainly hexagonal or mixed the self-assembly of the alkyl chain and p–p stacking of the
arrangements exhibited in the complex clusters of azobenzene- azobenzene, were essentially reversible upon cooling from their
based cationic surfactants and polyoxometalates,31 which is mesophases, while the diffraction pattern due to the Ag–S
ascribed to both the stronger p–p stacking interactions intro- inorganic slab was largely irreversible, especially when the silver
duced by azobenzene mesogenic units and the skeleton binding thiolates were held at higher temperatures for a prolonged time.
of the inorganic Ag–S slab as will be illustrated later in this Such as for dodecyloxy tail thiolate 2d, which also showed a
paper.
weak shoulder superimposed on the (001) peak besides the
multiple evenly spaced reections of up to six orders at a normal
measuring run upon heating to 190 ꢁC indicating a mesophase
lamellar structure similar to that of 2c (Fig. 2A), while an obvi-
ously enhanced scattering peak at 1.30 nmꢂ1 exhibited with the
3.6 The preparation of 2D shaped Ag nanodisks through
thermolysis within or near the lamellar LC mesophases
Based on some reections other than the layered crystalline lamellar structure reections of slightly downward shied q
fringes Dance et al. proposed a general lattice geometry for values aer isothermal annealing for 30 minutes at 190 ꢁC (see
some short chain alkyl and para-substituted phenyl silver thi- Fig. S8 in the ESI†). Such SAXS signal evolution could be
olates.7 Although no suitable thiolate single crystals were reasonably interpreted as the changes of size and morphology
available for the full analysis of the two dimensional intra- of intermediate thiol capped silver nanoparticles reduced from
lamellar organization, the shis of the diffraction peak at thiolate precursors, as will be demonstrated in the following
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paragraphs from the 2c-based system for example, which exhibited for the 2 series samples in the normal thermal cycles
implied that for the mesogenic silver thiolates (2b–2d) the below 210 ꢁC, obvious thermolysis occurred when annealing in
process of thermolysis could be triggered in the temperature their LC mesophases for a prolonged time, such as isothermal
range of LC mesophases in sharp contrast to the suggested annealing at 190 ꢁC for 2 h resulting in most of the thermolytic
reversible rearrangement of the m3-bridged solid structure into a products (thermogram curves group a in Fig. 5A), which was
cyclic m2-bridged micellar mesophase structure reported for further conrmed by the direct TEM observation of the result-
8
simple alkane thiolates AgSCmH2m+1
.
ing products showing some Ag nanodisks and nanoparticles
Solventless thermolysis is an environmentally friendly and with a diameter #6.0 nm (Fig. 5B).
intriguing method for the synthesis of various metal or metal
When isothermal annealing was conducted at a higher
sulde nanomaterials from their metal thiolate precursors.11–18 temperature just above the LC clearing temperature with some
The structure and composition of the precursors played a residual order as revealed by temperature dependent SAXS and
ꢁ
crucial role in determining the size and morphology of the thus FTIR studies, such as 210 C for 2c as shown in Fig. 5A, much
produced reducing nanoproducts.19 Wu et al. reported an remarkable thermolysis took place within a shorter reaction
example of Ag nanodisks prepared from long chain alkane time 10–60 min, and no peak was detected for the precursor
thiolate AgSCmH2m+1 under certain carefully controlled condi- silver thiolate at around 167 ꢁC in the cooling scan aer only 30
tions, and they proposed a layered-precursor-to-lamellar-nano- min isothermal annealing, indicating almost completion of the
product mechanism for the size and shape control of the thus thermolytic reaction into reduced products. The reaction
produced nanomaterials.18 While for simple alkane thiolate evolution was also veried by TEM investigation directly. As
precursors the layered structure cannot be preserved under a shown in Fig. 5C, the nanoparticles were mainly obtained with a
temperature just a little higher than their melting points due to somewhat regular alignment by annealing for 15 min at 210 ꢁC
the complex phase evolution,8 so the reproducible production (average diameter less than 3.0 nm). Aer annealing for 30 min,
of certain shaped nanomaterials is not easy to realize and no the nanoparticles became obviously bigger, and a nice amount
intermediates supporting the suggested mechanism have ever of nanodisks emerged, with some disks showing obvious
been observed due to their rather high reaction rate. In the nanoparticles aggregation as labeled by arrows indicating
investigated 2 series of thiolates, the SAXS/WAXS analysis strongly that the nanodisks were formed by the fusion of
results implied that the triggering times of Ag nanoparticles nanoparticles (Fig. 5D). As the annealing time was extended to 1
formation at the high temperature end of the mesophase ranges h, most of the small particles conglomerated into quite uniform
were different for various analogues (Fig. 2, also see Fig. S4 and nanodisks with an average diameter of 22.6 ꢀ 2.2 nm (Fig. 5E,
S8 in the ESI†), which were consistent with their instability also see Fig. S10 in the ESI†). AFM can offer sub-nanometer
performance, as indicated by DSC measurements (Fig. 1, and precision for height measurements,33 and the measured heights
see Fig. S2 in the ESI†). The thiolates showing LC mesophases of the nanodisks by AFM were around 2–4 nm (see Fig. S11 in
exhibited stabler during the repetitive heating cycles or the ESI†), which were slightly larger than the disk thickness
isothermal annealing compared with 2a (Fig. S9 in the ESI†). A reported for the alkane silver thiolate precursor by Wu and co-
mixture state of undissociated lamellar structure and reduced workers18 and presumably due to the longer azobenzene
silver nanoparticles coexisted for LC thiolates 2b–2d, while mesogenic capping agents and without optimization of the
crystalline 2a quickly reached almost completion of thermolysis experimental conditions for materials preparation. The much
which implied controllable, much slower thermolytic reaction diminished and controllable reaction rate of the LC silver thi-
rates for the LC thiolates compared with the non-LC analogues olates facilitated the rst observation of the intermediate
such as 2a and in particular the simple alkane silver thiolates nanoparticles in this kind of system in contrast with that of
AgSCmH2m+1
.
AgSCmH2m+1.
XRD analysis conrmed the fcc character of the thus
produced silver nanodisks (as compared with standard fcc Ag
pattern JCPDS 4-783, see Fig. S12 in the ESI†). Furthermore, the
3.7 LPLM Thermolytic reaction mechanism
The much lower reaction rate in the LC thiolate system offered silver nanoproducts were validated to be capped with the azo-
an opportunity to monitor the reaction process during ther- benzene thiol ligands by FTIR and energy dispersive X-ray (EDX)
molysis and better understand the reaction mechanism. More analyses. The main characteristic peaks such as 2919 and 2849
importantly, the persistent real layered mesophases may cmꢂ1 of the methylene stretching vibrations, 1603 and 1581
provide an ideal conned environment according to a layered- cmꢂ1 of the typical vibration absorption of the benzene ring,
precursor-to-lamellar-nanomaterial
(LPLM)
thermolysis and 1247 cmꢂ1 and 1150 cmꢂ1 of the phenyl ring-O vibration
pathway to prepare 2D constrained nanoproducts. Taking 2c as and phenyl-N stretching mode were clearly observed in the FTIR
an example, based on a batch of comparison experiments spectrum of the reduced Ag nanoproduct (Fig. S13a in the ESI†).
loaded with similar amounts of reagents, the effects of the Its EDX prole indicated that the S, O and N elements co-existed
thermolysis reaction temperature and time on the product size with the Ag (Fig. S13b in the ESI†) thus also proving the char-
and morphology were investigated by DSC and TEM. Here DSC acter of the azobenzene thiol capped Ag nanoproduct. For better
served as a sensitive and very effective method to monitor the understanding of the thermolysis process, the byproducts of the
thermolytic reaction progress, although as discussed previously thermolytic reaction were also briey analyzed using 1H NMR in
with the essentially almost reversible thermal behavior comparison with the correspondingly synthesized alkyl
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Fig. 5 (A) DSC thermograms of 2c experiencing different thermal conditions (a) heating to 190 ꢁC and isothermal annealing for 2 h then cooling;
(b–e) first heating to 210 ꢁC and isothermally annealing for 10, 15, 30, and 60 min then cooling, respectively, with all of the heating and cooling
scans at the same rate of 10 ꢁC minꢂ1. TEM micrographs of the Ag nanoproducts produced from 2c after the in situ thermolytic reaction as
accordingly described in panel A after DSC thermal cycles a–e of annealing at 190 ꢁC for 2 h (B) or 210 ꢁC for 15 min (C), 30 min (D), and 60 min
(E), respectively.
disulde samples and literature results.5 The double triplet without LC mesophase as 2a, a much quicker formation of the
peaks at around 2.7 and 2.5 ppm of the 1H NMR spectrum relative irregular nanoproducts was observed due to the rapid
characteristic of the methylene protons adjacent to sulfur in thermolytic reaction (Fig. S15 in the ESI†). The DSC investiga-
disulde –CH2–S–S–CH2– (Fig. S14A in the ESI†) and sulde tion and the direct TEM observation of the thus produced
–CH2–S–CH2–, respectively, rather than the quartet peak of alkyl nanoproducts combined with the spectral characterization and
thiol (Fig. S14B in the ESI†) veried the presence of both byproducts analysis, help one to better understand the ther-
disulde and sulde in the byproducts.5 So the thermolytic molytic reaction process and phase transition and to elucidate
reaction was suggested as schematically shown below:
the LPLM mechanism. Mainly small Ag nanoparticles were
produced under a relatively low temperature or short reaction
time such as for 2c annealing at 210 ꢁC for 10 min, then the
small nanoparticles restrained by the layered structure of the
precursor aggregated and merged into nanodisks during the
subsequent thermal reaction. The growth in thickness direction
constrained and hindered by the long organic chain bilayers
was much slower than in the diameter dimension controlled by
intralamellar diffusion. By the way, the thermolytic reaction
performance of series 2 samples at high temperature 250 ꢁC
above their exothermic transitions far from the LC clearing
For other mesogenic thiolates examined such as 2b and 2d,
their phase behavior and reduced nanoproducts were quite
similar to that of 2c, the nanoparticles were rst formed and
conglomerated into 2D nanodisks slowly. While for the thiolate
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points may also serve as a circumstantial evidence for the between the p–p stacking of azobenzene mesogen and the
constraint character within or very near the LC mesophases. inorganic framework of the Ag–S slab, which provided an ideal
Mainly three dimensional Ag nanospheres of monotonically 2D conned environment to explore the thermolytic reaction
decreased in size from 59.3 ꢀ 2.7 nm for 2a to 16.1 ꢀ 1.4 nm for mechanism and prepare 2D shaped nanomaterials through
2d with the increased alkoxy tail lengths obtained under 250 ꢁC solventless thermolysis.
thermolysis, demonstrating the almost loss of 2D restraint at
Herein the mesogenic ligands are not limited to the azo-
high temperature and another alternative control mechanism benzene chromophore and the thermally reduced nano-
taking effect principally on the volume ratio of reactive inor- products not circumscribed to silver, other systems based on
ganic moieties to a nonpolar hydrocarbon matrix as will be such biphenyl mesogenic units and metal chalcogenide nano-
reported elsewhere in detail.
materials, e.g. cuprous sulde ultrathin nanoribbons and
The suggested structures of the azobenzene-based silver nanolms have been similarly prepared in our lab with excel-
thiolates in their crystalline solid and the lamellar liquid crys- lent controllability and will be reported separately soon. The
talline mesophase, and also the intermediate nanoparticle silver thiolates with mesogenic ligands are a kind of metal-
aggregates and disk-shaped nanoproducts from the thermolytic containing liquid crystal and belong to the well-known metal-
reaction imaged by TEM have been schematically depicted in lomesogens, which combine the intrinsic characteristics of
Fig. 6. Herein, as sketched in the insets we speculated that the liquid crystals with the unique properties associated with metal
inorganic skeleton of Ag–S still maintained a relatively incom- atoms.34 While unlike the usually reported metallomesogens
pact stratum though experiencing bond changes from strictly focusing on the study of the structure and phase behavior aer
regular trigonal symmetry into a structure of partial elongation introducing different metals and their comparison with the
or fracture of Ag–S connection as proposed in the simple alkane precursor liquid crystals without metal,35 the silver mesogenic
silver thiolate system.7 The adoption of azobenzene mesogenic thiolates mainly serve as a kind of functional metallomesogen
ligands led to the silver thiolates with quite a broad range of precursor with LC mesophases to provide the constrained
ordered lamellar LC mesophases due to the cooperativity environment for the controlled preparation of variant
Fig. 6 Schematic representation of the proposed structures of azobenzene-based silver thiolates in the crystalline solid state and the ordered
smectic liquid crystalline mesophase, and the formation of intermediate nanoparticle aggregates and controlled preparation of 2D disk-shaped
nanoproducts imaged by TEM from the thermolytic reaction through a layered-precursor-to-lamellar-nanomaterial (LPLM) mechanism (with
insets showing the suggested coordination change from regular trigonal planar symmetry in the crystalline state to the mixture of digonal and
trigonal coordination due to partial thermolysis inside the inorganic Ag–S slab in the smectic phase).
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nanomaterials, especially the persistent ordered lamellar mes-
ophases provide an intrinsic 2D restrained space for the facile
controllable production of 2D shaped nanomaterials rather
than the dependence on the ne control of the preparation
conditions from the system based on simple alkane silver thi-
late precursors.
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Acknowledgements
This work was supported by the National Natural Science 22 R. Jones, R. H. Tredgold, Z. Ali-Adib and A. P. L. Dawes, Thin
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