Phase behavior of organic-inorganic crystal: Temperature-dependent diffuse reflectance infrared spectroscopy of silver stearate

Article abstract of DOI:10.1007/s100530170113

We have investigated the structure and phase behavior of nonmolecularly layered silver stearate by means of temperature-dependent diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Upon heating the sample, remarkable spectral changes took place. The first phase transition took place that might be associated with a premelting event characterized by the formation of gauche conformers at 390-420 K. A second phase transition took place in which silver nanoparticles with a size of ～4 nm were formed by thermal decomposition of silver stearate at 520-550 K. These silver nanoparticles, derivatized by stearate, were readily spread as a monolayer at air/water interface, and could be packed in 3-D assemblies by the Langmuir-Blodgett method.

Full text of DOI:10.1007/s100530170113

Eur. Phys. J. D 16, 293–296 (2001)
THE EUROPEAN
PHYSICAL JOURNAL D
EDP Sciences
Societa` Italiana di Fisica
Springer-Verlag 2001
c
ꢀ
Phase behavior of organic-inorganic crystal
Temperature-dependent diffuse reflectance infrared spectroscopy of silver stearate
S.J. Lee, S.W. Han, H.J. Choi, and K. Kima
School of Chemistry and Molecular Engineering and Center for Molecular Catalysis, Seoul National University,
Seoul 151-742, Korea
Received 29 November 2000
Abstract. We have investigated the structure and phase behavior of nonmolecularly layered silver stearate
by means of temperature-dependent diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy.
Upon heating the sample, remarkable spectral changes took place. The first phase transition took place
that might be associated with a premelting event characterized by the formation of gauche conformers at
390-420 K. A second phase transition took place in which silver nanoparticles with a size of ∼4 nm were
formed by thermal decomposition of silver stearate at 520-550 K. These silver nanoparticles, derivatized by
stearate, were readily spread as a monolayer at air/water interface, and could be packed in 3-D assemblies
by the Langmuir-Blodgett method.
PACS. 81.07.Pr Organic-inorganic hybrid nanostructures – 61.46.+w Nanoscale materials: clusters,
nanoparticles, nanotubes, and nanocrystals
1 Introduction
been attempted to differentiate similarity as well as dis-
similarity of the structural properties between AgCO2R
and AgSR.
It has been found recently that silver alkane thiolate
salt (AgSR) consists of an infinite-sheet, two-dimensional
(2-D), nonmolecular layered structure [1–4]. The alkyl
chains even in AgS(CH2)4CH3 possess fully-extended all-
trans conformation [1]. Silver alkane carboxylate salt
(AgCO2R) was also reported to consist in a layered struc-
ture [5]. Its detailed structural characteristics have not yet
been thoroughly elucidated, however. Owing to their sim-
ilar structures, the properties of AgCO2R are expected to
be similar to those of AgSR. The layered AgSR species
are known to show liquid crystalline behavior upon melt-
ing [2]. This phenomenon is associated with the two struc-
tural motifs that the coordination of Ag to thiolates
changes from trigonal to diagonal and that the interlayer
CH3-CH3 contacts disrupt to form stacked-disk micellar
structures. These organic-inorganic heterostructures have
attracted interest since technologically relevant materials
with specific properties should be readily prepared by sys-
tematic variation in the structure and properties of the
organic and inorganic constituents at the molecular level
[6–8]. In particular, AgCO2R is known to be decomposed
into silver nanoparticles [9]. In this respect, we have in-
vestigated the structure and phase behavior of the proto-
type AgCO2R, silver stearate (AgCO2(CH2)16CH3), by
means of temperature-dependent diffuse reflectance in-
frared Fourier transform (DRIFT) spectroscopy. It has
2 Experimental
2.1 Preparation of silver stearate
Stearic acid (99+%) and silver nitrate (99+%) were pur-
chased from Aldrich and used as received. Unless specified,
other chemicals were reagent grade, and triply-distilled
water (resistivity greater than 18 M cm) was used through-
out. Silver stearate was prepared by two-phase method.
An aqueous solution of AgNO3 was added dropwise to
stearic acid solution dissolved in toluene. After 3 hrs of
vigorous stirring, the resulting bright-yellowish solid was
filtered, washed subsequently with ethanol, toluene and
cold water in order, and finally dried under vacuum.
2.2 Characterization
X-ray diffraction (XRD) patterns were obtained with a
Rigaku Dmax-3C diffractometer for a 2θ range of 5◦ to 50◦
◦
˚
at an angular resolution of 0.05 using Cu Kα (1.5418 A)
radiation. Infrared spectra were measured using a Bruker
IFS 113v FT-IR spectrometer. The method for obtain-
ing the room temperature and temperature-dependent
a
e-mail: kwankim@plaza.snu.ac.kr

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3 Result and discussion
3.1 Structure of silver stearate
Fig. 1(a) shows the XRD data for silver stearate. Known
from 1950 [5], the compound shows a well-developed pro-
gression of intense reflections. These intense reflections
can be interpreted in terms of three dimensionally stacked
silver carboxylate layers with a large interlayer lattice
dimension. Each layer of silver carboxylate is separated
from the other by twice the length of the alkyl chain. In
this sense, all intense reflections can be indexed as (0k0)
and the reflections are assigned in Fig. 1(a). In the in-
set of Fig. 1(a), we list the interlayer spacings derived
from different reflections. The averaged interlayer spac-
˚
ing is 49.47 A. Following the Ag K-EXAFS studies of
Tolochko et al. [11], the thickness of the Ag-O slab, i.e.,
˚
2t1 in Fig. 1(b), could be estimated to be 4.91 A. Con-
sidering that the averaged interlayer spacing derived from
˚
Fig. 1(a) was 49.47 A, the thickness of the alkyl chain
˚
layer, i.e., 2t2 in Fig. 1(b), should then be 44.56 A. As-
suming that the alkyl chains in silver stearate are fully ex-
tended and all-trans (vide infra) and invoking the known
bond lengths and vdW radii, the chain length from the
carboxylate carbon atom to the terminal methyl group
˚
˚
should be 22.99 A. Twice of the latter is 45.98 A, which is
˚
1.42 A longer than that derived from the measured XRD
˚
data, i.e., 44.56 A. The estimated difference indicates that
the extent of overlap between the adjacent layers, if exists,
is small. A similar conclusion is made for silver alkanethi-
olate; the extent of interpenetration has been reported to
˚
be 0.5 A [4]. The overall structure of silver stearate as
drawn in Fig. 1(b) is supposed to be quite similar to that
of silver alkanethiolate.
3.2 DRIFT spectral features of silver stearate
Fig. 2 shows a series of DRIFT spectra obtained as a
function of temperature for silver stearate. The high fre-
quency region of 2750-3050 cm−1 reveals the C-H stretch-
ing modes of the methyl and the methylene groups of
stearate while the low frequency region of 1000-1800 cm−1
displays the stretching modes of the carboxylate group
as well as the scissoring, rocking, wagging, and twisting
modes of the methylene groups. It can be evidenced from
the room temperature spectrum that the obtained sample
is not contaminated with free acid and the alkyl chains
in silver stearate are in an all-trans conformational state
Fig. 1. (a) XRD pattern of silver stearate. (b) Structure of
silver stearate.
DRIFT spectra has been reported previously [10]. Trans- with little or no significant gauche population as in silver
mission electron micrograph of the thermally decomposed alkanethiolate. (The two strong peaks observed at 2848
sample was obtained with a JEM-200CX transmission and 2916 cm−1 in Fig. 2(a) are assigned to the sym-
electron microscope (TEM) at 160 kV after placing a metric (ν (CH2), d+) and the asymmetric (ν (CH2), d−)
s
as
drop of toluene solution on carbon-coated copper grids stretching vibrations of the methylene groups, respec-
(150 mesh). The UV/Vis spectrum of thermally decom- tively. These modes usually lie in the narrow ranges of
posed sample was taken in toluene solvent with a SCINCO 2846-2850 and 2915-2918 cm−1, respectively, for all-trans
S-2130 spectrophotometer. Monolayers of the thermally extended chains [12] (vide supra) and in the distinctly dif-
decomposed sample at air/water interface were character- ferent ranges of 2854-2856 and 2924-2928 cm−1 for disor-
ized by a KSV 3000 Langmuir balance.
dered chains [13].) The fact that only a single narrow band

S.J. Lee et al.: Phase behavior of organic-inorganic crystal
295
Fig. 2. DRIFT spectra of silver stearate taken as a function of temperature: (a) high and (b) low frequency regions.
(fwhm ∼ 5.9 cm−1), which can be attributed to the scissor-
ing vibration of the methylene group (δ(CH2)), is observed
at 1471 cm−1 also suggests that silver stearate consists in
triclinic packing with two chains per unit cell [14]. This in-
frared data is consistent with the reported X-ray analysis
on a number of silver carboxylates [5]. Shown in Fig. 2(a),
the thermotropic upwards shifts of methylene stretching
bands from 390 to 420 K can be attributed to a premelting
event characterized by the formation of gauche conform-
ers. The frequency change of δ(CH2) is abrupt around
420 K appearing at 1463 cm−1 as shown in Fig. 2(b). The
thermal variation of δ(CH2) band observed herein suggests
that a triclinic chain packing disrupts around 420 K to
hold a hexagonal packing [14]. Interestingly, the frequen-
cies of d+ and d− modes sharply decrease from 2853 and
2923 cm−1 at 520 K to 2848 and 2916 cm−1 at 550 K;
the latter values are nearly the same as those at room
temperature. As to be discussed later, this results from
the formation of stearic acid-derivatized silver nanoparti-
cles. Above 550 K, the relative intensities of all infrared
bands gradually decrease, and the intensities at 600 K are
observed to be below half of the initial values at 298 K.
Such lowering of the intensities is attributed to the ther-
mal decomposition of silver stearate to produce various
products including metallic silver, CO2, stearic acid, and
Fig. 3. TEM image of stearate-derivatized Ag nanoparticles.
paraffins [15].
3.3 Thermal decomposition of silver stearate
sphere, heating rate, and temperature. Andreev et al. [15]
reported that thermal decomposition of silver stearate led
The composition of the products of the thermal decom- to metallic silver, CO2, stearic acid, and paraffins. On the
position of silver carboxylates depends on reaction con- other hand, Uvarov et al. [16] reported that silver stearate
ditions, including such factors as the surrounding atmo- showed phase transitions at 397 K and 426 K in air. The

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former transition was claimed to be irreversible leading tion as in silver alkanethiolate. Upon heating the sam-
to decreased conductivity while the latter was accompa- ple, remarkable spectral changes took place. The first
nied by the thermal decomposition leading to increased phase transition took place that might be associated
conductivity. Free stearic acid was presumed to be one with a premelting event characterized by the formation
of the decomposition products. In the present work, the of gauche conformers. A second phase transition took
first phase transition is observed to occur in the range place in which silver nanoparticles with a size of ∼ 4 nm
from 390 K to 420 K. Above 510 K, the C=O stretching were formed by thermal decomposition of silver stearate.
peak is clearly identified at ∼ 1700 cm−1; the peak must These nanoparticles were readily spread as a monolayer
arise from a free acid as suggested by Uvarov et al. Re- at air/water interface. Considering that organic-inorganic
cently, Abe et al. [9] reported that thermal decomposition hetero-structures have recently been attracting immense
of silver stearate at 523 K in an atmosphere of N2 should interest, the present observations should prove useful in
produce silver nanoparticles with a size of 5 nm. In fact, developing technologically relevant materials with specific
the latter temperature corresponds to that of the second properties by systematic variation of the structure and
phase transition in the present work, i.e., 520-550 K. On properties of organic and inorganic constituents at the
these grounds, we have conducted TEM measurement for molecular level.
a sample of silver stearate heated to 520 K for 10 min
and then rinsed in methanol; the rinsed sample was read-
KK was supported by the Korea Research Foundation (KRF,
ily dispersed in non-polar medium such as toluene. Fig. 3
042-D00073) and by the Korea Science and Engineering Foun-
shows the TEM image of the sample taken after vaporiz-
dation (KOSEF, 1999-2-121-001-5). SWH was supported by
KOSEF through the Center for Molecular Catalysis at Seoul
National University. SJL and HJC acknowledge the KRF for
providing the BK21 fellowship.
ing the toluene solvent on the surface of copper grid. The
image reveals that silver nanoparticles are indeed formed
by the thermal decomposition of silver stearate. The sizes
of the nanoparticles are quite uniform with an average
diameter of ∼ 4 nm. The distance between nanoparti-
cles is estimated to be 3-3.5 nm. Recalling that the chain
length of stearic acid is ∼ 1.7 nm, silver nanoparticles
are thus supposed to be surrounded by stearate. A dis-
tinct peak was observed at 417 nm in the UV/Vis spec-
trum, which must arise from the surface plasmon absorp-
tion of silver nanoparticles [17]. We can confirm from
DRIFT spectroscopy that the silver nanoparticles are in-
deed passivated by uniform surroundings of stearate of
which alkyl chains assume fully-extended all-trans confor-
mation. From the TEM image, it is evident that stearate-
derivatized silver nanoparticles are readily condensed to
form two-dimensional arrays. On these grounds, we firstly
spread silver nanoparticles on the water subphase of a LB
film balance and recorded the surface pressure as a func-
tion of surface area. The isotherm appears to consist of
liquid expanded, liquid condensed, and their composite
plateau regions. It is remarkable that the collapse pres-
sure reaches up to 59 mN/m; the collapse pressure for
stearic acid on pure water (at pH ∼ 6) at 293 K is only
∼ 50 mN/m and it increases to ∼ 59 mN/m at pH 9.0 [18].
This may indicate that energetically favorable interdigi-
tated structure is formed to maximize the hydrophobic
interaction between the interdigitated chains. A more de-
tailed study on the structure and the intermolecular inter-
action of the nanoparticles at an air/water interface and
in 3-D assemblies is under progress.
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4 Summary and conclusion
The structure and phase behavior of silver stearate have
been investigated by DRIFT spectroscopy. We confirmed
by XRD analysis that silver stearate consisted of an infi-
nite sheet, 2-D, nonmolecular layered structure. The alkyl
chains in silver stearate were in an all-trans conforma-
tional state with little or no significant gauche popula-