Structure and thermal behavior of a layered silver carboxylate

Article abstract of DOI:10.1021/jp0132937

We have investigated the structure and thermal behavior of nonmolecularly layered silver stearate by using various analytical tools, i.e., X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray photoelectron sp

Full text of DOI:10.1021/jp0132937

2892
J. Phys. Chem. B 2002, 106, 2892-2900
Structure and Thermal Behavior of a Layered Silver Carboxylate
Seung Joon Lee, Sang Woo Han, Hyouk Jin Choi, and Kwan Kim*
Laboratory of Intelligent Interfaces, School of Chemistry and Molecular Engineering and
Center for Molecular Catalysis, Seoul National UniVersity, Seoul 151-742, Korea
ReceiVed: August 25, 2001; In Final Form: December 11, 2001
We have investigated the structure and thermal behavior of nonmolecularly layered silver stearate by using
various analytical tools, i.e., X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform (DRIFT)
spectroscopy, X-ray photoelectron spectroscopy (XPS), thermal analysis, UV/VIS spectroscopy, and
transmission electron microscopy (TEM). The XRD pattern was composed of a series of peaks that could be
indexed to (0k0) reflections of a layered structure. The alkyl chains in silver stearate as prepared were in an
all-trans conformational state with little or no significant gauche population. Upon heating the sample, structural
changes took place particularly at two temperatures. The binding state of carboxylate group changed from
bridging to unidentate along with the disordering of alkyl chains at ∼380 K. The layered structural motif
was, however, sustained in that temperature region, indicative of the overall structural change to be partially
irreversible. A second dramatic structural change that must be associated with the decomposition of silver
stearate, and thus a totally irreversible process, took place at ∼500 K. The major decomposition products
appeared to be metallic silver and stearic acid, but surprisingly, both species seemed finally to produce stearate-
derivatized silver nanoparticles with a size of ∼4 nm.
1
. Introduction
in recent years, not only as a result of their scientific value but
also due to their prospects for potential applications. Specific
material properties, e.g., stiffness, strength, weight, nonlinear
optical behavior, electrical conductivity, photochemical charge
transfer, and ferromagnetism, can be manipulated by systematic
variation of the structure and properties of the organic and
Both the alkane thiols and carboxylic acids form close-packed
monolayers on Ag, specifically, when the number of methylene
1,2
groups in these molecules exceeds ∼10. Alkyl chains of fatty
acid self-assembled on Ag are also known to have a more close-
packed structure than those prepared by the Langmuir-Blodgett
1
5
inorganic constituents at the molecular level.
3
(LB) method. It is known that alkyl chains in the thiol-
To develop technologically relevant organic/inorganic hybrid
materials, detailed structural information is needed for a series
of layered compounds. In this respect, we have thoroughly
investigated the structure and thermal behavior of the prototype
AgCO2R, silver stearate (AgCO2(CH2)16CH3), by using various
analytical techniques, i.e., X-ray diffraction (XRD), diffuse
reflectance infrared Fourier transform (DRIFT) spectroscopy,
X-ray photoelectron spectroscopy (XPS), differential scanning
calorimetry (DSC), thermogravimetric analysis (TGA), UV/VIS
spectroscopy, and transmission electron microscopy (TEM).
Contrary to expectation, the thermal behavior of AgCO2R is
found to be totally different from that of AgSR.
derivatized Ag nanoparticles assume a close-packed structure
similar to the case of self-assembled monolayers (SAMs) of
alkanethiols on Ag.4
As an analogue of 2D SAMs of thiols on Ag, considerable
research interest has been focused recently on silver alkanethi-
olate (AgSR) that consists of an infinite-sheet, two-dimensional,
nonmolecular layered structure.5-11 The alkyl chains in these
systems possess fully extended all-trans conformation. The S-S
spacing in 3D AgSR is very similar to that observed in close-
packed 2D organothiol SAMs on Ag, even though the thiol
occupancy (above an Ag layer) in the 3D material is only 50%
7
of that in the 2D SAMs. One intriguing characteristic of layered
AgSR species is that the material shows liquid crystalline
behavior upon melting.6 This phenomenon is associated with
the two structural motifs that the coordination of Ag to thiolates
changes from trigonal to diagonal and that the interlayer CH3-
2
. Experimental Section
.1. Preparation of Silver Stearate. Stearic acid (99+%)
,8
2
and silver nitrate (99+%) were purchased from Aldrich and
used as received. Unless specified, other chemicals were reagent
grade, and triply distilled water (resistivity greater than 18 MΩ
cm) was used throughout. Silver stearate was prepared by the
two-phase method. An aqueous solution of AgNO3 was added
dropwise to stearic acid solution dissolved in toluene. After 3
h of vigorous stirring, the resulting yellowish-white solid was
filtered, washed subsequently with ethanol, toluene, and cold
water in order, and finally dried in a vacuum.
6
CH3 contacts disrupt to form stacked-disk micellar structures.
Similarly to AgSR, silver alkane carboxylate (AgCO2R) has also
been reported to exist in a layered structure.1
2-14
Although its
detailed properties have not yet been thoroughly elucidated, the
properties of AgCO2R would be comparable to those of AgSR
due to their similar structures.
In fact, the above classes of organic-inorganic hetero-
structures that exhibit alternating 2D molecular assemblies of
organic and inorganic constituents have expanded considerably
2
.2. Characterization. XRD patterns were obtained on a
Philips X′PERT MPD diffractometer for a 2θ range of 5° to
0° at an angular resolution of 0.05° using Cu KR (1.5419 Å)
radiation. The samples were spread on antireflection glass slides
5
*
Author to whom all correspondence should be addressed. Tel: +82-
2
-8806651. Fax: +82-2-8743704. E-Mail: kwankim@plaza.snu.ac.kr.
1
0.1021/jp0132937 CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/27/2002

A Layered Silver Carboxylate
J. Phys. Chem. B, Vol. 106, No. 11, 2002 2893
to give uniform films. Variable temperature XRD measurement
was also carried out using the same diffractometer. The sample
spread on a Cr-coated Cu plate was heated to 598 K in a
temperature interval of 10 K.
Infrared spectra were measured using a Bruker IFS 113v FT-
IR spectrometer equipped with a globar light source and a liquid
N2-cooled wide-band mercury cadmium telluride detector. To
record the DRIFT spectra, a diffuse reflection attachment
(Harrick model DRA-2CO) designed to use 6:1, 90° off-axis
ellipsoidal mirrors subtending 20% of the 4π-solid angle was
fitted to the sampling compartment of the FT-IR spectrometer.
The pure powdered sample was transferred to a 4-mm diameter
cup without compression, and leveled by a gentle tap. A reaction
chamber, made of stainless steel (Harrick model HVC-DR2)
and loaded with the powdered sample, was located inside the
reflection attachment. CaF2 crystals were used as the infrared
transparent windows. The temperature of the sampling cup was
regulated using a homemade temperature controller, and the
chamber was flushed continuously with dry nitrogen (ca. 10
mL/min) during the measurement of DRIFT spectra. A total of
Figure 1. XRD pattern of silver stearate. The interlayer spacings
derived from different reflections are listed in the inset.
-
1
3
2 scans was measured in the range 3500-1000 cm at a
-1
resolution of 4 cm using previously scanned pure KBr as the
background. The temperature of the sampling cup was raised
at a rate of 10 K/min and kept for 5 min at each specified
temperature for the acquisition of the DRIFT spectra. The Happ-
Genzel appodization function was used in Fourier transforming
all the interferograms. The DRIFT spectra are reported as -log-
(R/Ro), where R and Ro are the reflectance of the sample and of
the pure KBr, respectively.
XPS measurements were made using a VG Scientific ES-
CALAB MKII spectrometer. Mg KR X-ray at 1253.6 eV was
used as a light source, and peak positions were internally
referenced to the C 1s peak at 284.6 eV. DSC and TGA data
were obtained with a TA Instrument 2010 differential scanning
calorimeter and a TA Instrument 2050 thermogravimetric
analyzer, respectively, in a nitrogen atmosphere at a heating
rate of 5 K/min. A UV/VIS spectrum was obtained using a
SCINCO S-2130 spectrophotometer. TEM was acquired for
thermally decomposed silver stearate using a JEM-200CX
transmission electron microscope at 160 kV after placing a drop
of toluene solution on carbon-coated copper grids (150 mesh).
Figure 2. Schematic depiction of the layered structure of silver stearate.
The latter value is 4-5 times larger than that of the Ag-S slab
in silver alkanethiolate. Considering that the averaged inter-
layer spacing derived from Figure 1 was 48.36 Å, the thickness
of the alkyl chain layer, i.e., 2t2 in Figure 2, should then be
5
,7
4
3.53 Å.
Assuming that the alkyl chains in silver stearate are fully
extended and all-trans (vide infra), the chain length from the
carboxylate carbon atom to the terminal methyl group should
3
. Results and Discussion
.1. Structure of Silver Stearate. 3.1.1. Room-Temperature
1
0
be 22.99 Å: 21.301 Å (1.253 Å per CH2 group × 17) plus
5
3
1.69 Å (vdW radius of CH3 group ); the C-C bond length in
X-ray Diffraction. The XRD pattern for silver stearate is shown
-OOC-CH2- is known to be the same as that in -H2C-CH2-
in Figure 1. As was known since 1949,1
2-14
the compound
16
(and -H2C-CH3). Taking into account that the stearate chain
14
shows a well-developed progression 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 neighboring layer 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 Figure 1. In the inset
of Figure 1, we list the interlayer spacings derived from different
reflections. The averaged interlayer spacing is 48.36 ( 0.22 Å.
The layered structure of silver stearate is schematically drawn
in Figure 2.
According to the Ag K-EXAFS study of the silver stearate,14
the silver atoms are bridged by the carboxylate in the form of
dimers in an eight-membered ring, and the dimers are further
bonded to each other by longer Ag-O bonds forming four-
membered rings. Using the reported bond distances and angles
of these four- and eight-membered rings, the thickness of the
silver carboxylate slab, i.e., 2t1 In Figure 2, should be 4.83 Å.
is tilted by 4.3° from the normal to the Ag plane, the thickness
of one stearate layer is estimated to be 22.93 Å. Twice this
thickness is 45.86 Å, which is 2.33 Å longer than that derived
from the measured XRD data, i.e., 43.53 Å. The extent of
interpenetration has been reported to be as small as ∼0.5 Å for
1
0
silver alkanethiolate, but the present estimate indicates that
alkyl chains interpenetrate to a fair extent between the adjacent
layers in silver stearate. Although we cannot derive the intralayer
structure due to the insufficient signal-to-noise ratio for high-
angle XRD peaks, the overall structure of silver stearate as
drawn in Figure 2 is presumably similar to that of silver
alkanethiolate.
3.1.2. Room-Temperature DRIFT Spectrum of SilVer Stearate.
Figure 3 shows the DRIFT spectrum of silver stearate at room
-1
temperature. The high-frequency region from 2800 to 3000 cm
reveals the C-H stretching modes of the methyl and the
methylene groups of stearate while the low-frequency region
-
1
from 650 to 1650 cm displays the stretching modes of the

2894 J. Phys. Chem. B, Vol. 106, No. 11, 2002
Lee et al.
Figure 3. DRIFT spectrum of silver stearate at room temperature.
-
1
carboxylate group as well as the scissoring, rocking, wagging,
and twisting modes of the methylene groups. More specifically,
(full width at half-maximum (fwhm): ∼10 cm ) is character-
istic of a crystalline chain packing in a triclinic or hexagonal
structure. The introduction of internal kink defects, which serve
to lower the number of correlated methylene units, will shift
the peak to higher values between 723 and 738 cm , as well
-
1
20
the two intense bands at 2848 and 2916 cm are assigned,
+
respectively, to the symmetric (νs(CH2), d ) and the antisym-
-
-1
metric (νas(CH2), d ) stretching vibrations of the methylene
-
1
21
groups. Two peaks observed at 2871 and 2954 cm are
as increase the fwhm. On this basis, we conclude that the alkyl
+
assigned to the symmetric (νs(CH3), r ) and antisymmetric (νas-
chains in silver stearate are quite highly organized. Regarding
the chain packing, the scissoring vibration of the methylene
group (δ(CH2)) can provide additional information. In fact, the
exact shape of the δ(CH2) band including the peak position,
width, and the number of components is well appreciated to
-
(
CH3), r ) stretching vibrations of the methyl group, respec-
tively. In the high frequency region, two shoulder peaks are
-
1
additionally identified at ca. 2895 and 2964 cm . The former
peak can be attributed to the Fermi resonance absorption due
+
17
-
to the d mode. The latter peak has to be attributed to the r
reflect the packing arrangement of the alkyl chain assem-
-
1
20,22,23
mode. We already assigned the distinct band at 2954 cm to
r . It is well-known that for the two antisymmetric stretching
blies.
The appearance of a single narrow peak at 1473 or
-
-1
1467 cm has been attributed to triclinic or hexagonal subcell
packing, respectively. The appearance of a well-resolved doublet
with two distinct components is known to occur either as a result
of intermolecular vibrational coupling due to a crystal-field
splitting in orthorhombic or monoclinic packing or as a result
of the coexistence of triclinic and hexagonal packing in the
material. In addition, the peak is known to be broad when the
alkyl chains assume a disordered conformation. On these
grounds, the fact that only a single narrow band is observed in
vibrations of a CH3 group to become degenerate and appear as
a single broad peak, the CH3 group must be in at least C3
1
8
symmetry. However, this symmetry is lifted in the present
case probably due to the interpenetration of alkyl chains. The
two antisymmetric vibrations will then no longer be equivalent,
splitting into two peaks.
It has been well established that the d and d- modes are
+
+
-
strong indicators of the chain conformation. The d and d
modes usually lie in the narrow ranges of 2846-2850 and
-
1
Figure 3 at 1471 cm suggests that silver stearate exists in
triclinic packing with only one type of alkyl chain per unit
-
1
2
915-2918 cm , respectively, for all-trans extended chains
-
1
and in the distinctly different ranges of 2854-2856 and 2924-
928 cm for disordered chains characterized by a significant
presence of gauche conformers.
subcell; the small fwhm (∼5.9 cm ) is indicative of the
-
1
2
presence of highly ordered, all-trans chains. These infrared
18,19
On this basis, the observed
spectral observations are consistent with the reported X-ray
-
1
12,14
peak frequencies of 2848 and 2916 cm suggest that the alkyl
chains in silver stearate are in an all-trans conformational state
with little or no significant gauche population as in silver
alkanethiolate.8
analyses of a number of silver carboxylates.
In contrast to
silver stearate, hexagonal subcell packing with a single chain
per unit cell has been reported to occur for the case of silver
-11
10
alkanethiolate.
-
1
The low-frequency region (650-1650 cm ) in Figure 3
provides additional structural information regarding silver
stearate. As mentioned previously, peaks appearing in this region
are associated with the stretching vibration of the carboxylate
group as well as the scissoring, rocking, wagging, and twisting
modes of the methylene group. The two strong peaks at 1421
The well-resolved progression bands in the region from 1150
-
1
to 1400 cm in Figure 3 can be attributed to the wagging
vibration (Wx) of the CH2 groups. They also indicate that the
alkyl chains assume all-trans conformation as revealed by the
More specifically,
the number and the inter-band separation are known to depend
on the average number of trans conformers in the chains; the
+
-
22,24
peak positions of the d and d modes.
-
1
and 1518 cm can be assigned, respectively, to the symmetric
νs(COO )) and antisymmetric (νas(COO )) stretching vibrations
-
-
-1
(
inter-band spacing ∆ν in Wx modes below ∼1350 cm is
of the carboxylate group. No peak attributable to ν(CdO) is
related to the number of trans-units m by the equation ∆ν )
-1
25
identified around 1700 cm , indicating that the obtained sample
is not contaminated with free acid.
326/(m +1). Since the average value of ∆ν measured for silver
-
1
stearate is 18.3 cm , the number of trans methylene units is
calculated to be 16.8. Considering the errors in the frequency
estimates, this number is remarkably consistent with the actual
-
1
The peak at 717 cm is assigned to the CH2 rocking
vibration. Its appearance as a sharp singlet at this frequency

A Layered Silver Carboxylate
J. Phys. Chem. B, Vol. 106, No. 11, 2002 2895
Figure 5. DRIFT spectra of silver stearate in the temperature range
-
1
2
98-598 K: (a) high-frequency (3025-2775 cm ) and (b) low-
1
-
frequency (1800-1000 cm ) regions. The arrows denote the occur-
rence of distinct spectral changes.
5
31.3 eV. A single, symmetric peak must arise from two
-
comparable oxygen atoms in the carboxylate (-COO )
moiety.
.2. Thermal Behavior of Silver Stearate. 3.2.1. Temper-
2
8-30
3
ature-Dependent DRIFT Spectral Pattern of SilVer Stearate.
Figure 5 shows a series of DRIFT spectra obtained as a function
of temperature for silver stearate. All spectra were measured at
the temperatures indicated, with the temperature held constant
to (1 K for 5 min while the spectra were being recorded. The
temperature was raised from 298 to 598 K in 10 K steps.
Information on the molecular motions associated with the
thermal treatment of the material can be obtained from the
3
1,32
analysis of the DRIFT spectral features.
Figure 4. High-resolution XP spectra of silver stearate in (a) Ag 3d,
Silver carboxylate provides a rare opportunity to observe the
effect of temperature on the headgroup structure. Regarding this
matter, it is informative that the νas(COO ) peak at 1518 cm
(b) C 1s, and (c) O 1s spectral regions.
-
-1
number of methylene units in silver stearate. Hence, all of the
alkyl chains should be in all-trans conformation with negligible
gauche defect.
is rather invariant up to 378 K and then becomes weak and
disappears above 448 K. Instead, a new peak develops at ca.
-
1
1560 cm from 388 K and is sustained up to 598 K. On the
-
3.1.3. XPS Measurements. To provide further information on
other hand, the νs(COO ) band decreases gradually from room
the structure and chemical state of silver stearate, we obtained
the XP spectra of silver stearate. The expected peaks from Ag
temperature up to 598 K and the ν(CdO) band develops around
510 K. The present observation suggests that a certain structural
change occurs distinctly at about 380 K. Along with the peak
3
d, C 1s, and O 1s core levels were clearly detected in XP
-
1
-1
spectra; no trace of contamination was found. Figure 4 shows
the high-resolution XP spectra in the (a) Ag 3d, (b) C 1s, and
at 1518 cm , the peak developed at 1560 cm at 388 K is
-
also attributed to νas(COO ) by presuming that the type of
(
c) O 1s spectral regions. The narrow widths of the peaks suggest
bonding of carboxylate to silver changes around 380 K.
Referring to the empirical relationship between the frequency
difference, ∆ν ) νas(COO ) - νs(COO ), and types of
that each kind of element is present in similar conditions. The
Ag 3d3/2 and Ag 3d5/2 peaks are identified at 374.3 and 368.3
eV, respectively. In fact, these values are comparable to those
of pure silver and n-alkanethiol-adsorbed Ag surfaces. Owing
to the smaller binding energy difference between Ag(0) and
Ag(I), it is difficult to specify the actual oxidation state of silver.
-
-
3
2,33
bonding,
380 K.
the bridging state must turn into unidentate around
26
27
Regarding the d+ and d- modes, the peak positions are
invariant from 298 to 378 K. This implies that the initial all-
trans conformational order of the alkyl chains is preserved up
Two C 1s peaks are identified at 284.6 and 288.4 eV. Consulting
the data in the literature,2
8,29
these peaks can be attributed to
to ∼380 K. Upon increasing the temperature, the d and d
+
-
the carbon atoms in the aliphatic chain (C-C) and carboxylate
(
modes shift gradually toward higher frequencies reaching 2853
-
-1
-COO ) moieties, respectively. The absence of the C 1s peak
and 2923 cm , respectively, at 418 K. These upward shifts
due to the carboxylic carbon (-COOH) indicates that the sample
is not contaminated with free acid. It also suggests that only a
single type of carboxylate exists in silver stearate. This may be
further confirmed from the spectral feature of the O 1s peak at
can be attributed to a premelting event characterized by the
formation of gauche conformers. The values of 2853 and 2923
-
1
cm are sustained up to 498 K. It is very interesting that the
+
-
d and d modes then sharply decrease in frequency reaching

2896 J. Phys. Chem. B, Vol. 106, No. 11, 2002
Lee et al.
Figure 7. High-resolution XP spectra in the O 1s spectral region for
silver stearate (a) as prepared and (b) after annealing to 458 K.
Figure 6. DRIFT spectra of silver stearate subjected to cyclic thermal
treatments; the couples of a, b, and c traces correspond to a cycle of
the spectra taken (a) initially at room temperature, (b) after
heating to 458 K, and (c) after cooling back to room temperature.
Certain spectral changes take place in an obvious manner upon
2
2
98 to 458 to 298 K and the couples of a, d, and e traces to a cycle of
98 to 598 to 298 K. The dotted lines are drawn only as a guide to the
-
annealing at 458 K. In particular, the νas(COO ) band appears
eye.
-
1
-1
at 1560 cm rather than at 1518 cm after the thermal cycling;
-
-1
the ν (COO ) mode is also subjected to change by annealing.
2
850 and 2917 cm , respectively, at 548 K. These values are
s
The irreversible change in the binding state of the carboxylate
group can also be proven using the XPS measurement. Figure
in fact comparable to those at room temperature. This implies
that the alkyl chains assume fully extended all-trans conforma-
tion at higher temperatures. Such a stable state around ∼550 K
has not been observed in silver alkanethiolate systems. As will
be discussed later, the stability must result from the formation
of stearic acid-derivatized silver nanoparticles; we are currently
conducting a separate experiment for the structure and thermal
behavior of stearate-capped silver nanoparticles.
Much the same observations are made for the low-frequency
bands. The δ(CH2) band shows a noticeable red-shift above 378
K, and then a blue-shift at ∼550 K. The band is also
considerably broadened above 378 K, suggesting that the
triclinic chain packing is disrupted along the transition. On the
other hand, the Wx progression bands remain constant to within
7
b shows the high-resolution O 1s XP spectrum for silver
stearate annealed previously at 458 K. For comparison, the O
1
7
s XP spectrum taken before annealing is also given in Figure
a (the same one in Figure 4c). Contrary to the latter case, two
peaks are identified at 531.1 and 533.0 eV (via deconvolution)
for the annealed sample, indicative of the presence of two
different oxygen atoms. These observations may not be unrea-
sonable if we recall the previous presumption that the bridging
state of the carboxylate group becomes unidentate around 380
K. We have to mention, however, that the conformationally
sensitive methylene stretching and scissoring bands as well as
the Wx progression bands are completely restored even after
the thermal treatment up to 458 K. The first transition at ∼380
K (see section 3.2.1) is thus a partially irreversible process.
Figures 6d and 6e summarize other cases in which two spectra
are taken (d) after heating silver stearate to 598 K and (e)
subsequent cooling back to room temperature. The spectrum at
598 K is significantly different from that at room temperature
before heating (see Figure 6a), and furthermore the original
spectral feature is never restored after cooling back to room
temperature. The present observation clearly indicates that any
structural change occurring around ∼500 K is a totally irrevers-
ible one. Such an irreversible change is surely associated with
the thermal degradation of the sample material at higher
temperature.
-
1
(
1 cm up to ∼380 K. Then, the bands weaken abruptly, being
absent up to ∼500 K. However, the Wx peaks begin to reappear,
although weak and somewhat irregular, above 510 K. These
+
thermal characteristics are consistent with those of the d and
d peaks.
-
Temperature-dependent infrared analysis of several related
systems have been reported. For instance, silver dodecanethiolate
has been reported to have two major transitions in the range
from 298 to 523 K. The first transition occurring at ∼400 K is
characterized by an abrupt, but fully reversible, change in the
chain conformational order from the initial all-trans state to the
one characterized by mixed or partial chain disorder. This is
consistent with the previous prediction of a rapid and drastic
change in the structural motif from an initial bilayer to the final
The relative intensity ratio I(νas(CH2))/I(νs(CH2)) is frequently
adopted as a measure of disorder; increasing in conformational
6
micellar state. The second transition at ∼460 K is irreversible
1
8b
and represents thermal degradation of the material. On the other
hand, copper(I) alkanethiolates have been reported to show only
one phase transition at 423 K in the range from 298 to 473
order, the ratio decreases. On this basis, we evaluate the
intensity ratios for the two cases, one using the DRIFT spectra
taken at increasing temperature and the other from those taken
at room temperature after cyclic thermal treatments. Shown as
open circles in Figure 8, the intensity ratio I(νas(CH2))/I(νs(CH2))
steadily increases with rising temperature as one would expect.
The intensity ratio varies differently, however, when subjected
to the cyclic thermal treatment (see the filled circles in Figure
8). In particular, the ratio abruptly decreases around ∼380 K.
This implies that alkyl chains are more ordered upon annealing;
however, the bridging state of the carboxylate group becomes
3
4
K. The transition is a partially irreversible one, resulting in a
mesophase state. Contrary to silver stearate, a thermally stable
region over ∼500 K was not identified for both the silver and
copper alkanethiolate systems, however.
3
.2.2. Cyclic Thermal Treatment. The reversibility of the
structural changes in silver stearate was also examined with
DRIFT spectroscopy through a cyclic thermal treatment. Some
of the results are shown in Figure 6. Traces a, b, and c show

A Layered Silver Carboxylate
J. Phys. Chem. B, Vol. 106, No. 11, 2002 2897
constant to (1 K. The temperature was raised from 298 to 598
K in 10 K steps. It is noticeable that a well-developed
progression of intense reflections is invariably seen up to 488
K. (The interlayer spacing increased slightly upon heating,
however. For instance, the value at 458 K is larger than that at
2
98 K by ∼0.5 Å. This small increase in the interlayer spacing
can be ascribed to the change in the binding state of the
carboxylate group around 380 K (vide supra).) The presence of
progressional reflections up to 488 K indicates that the bilayer
structural motif is sustained up to the temperature. Accordingly,
the lamellar structure should be retained at least in the first
transition region, i.e., around ∼380 K. This is in sharp contrast
to the case of silver or copper alkanethiolates. As mentioned
previously, silver or copper alkanethiolates undergo mesogenic
phase transitions that are prompted by the temperature-induced
6
,35
restructuring of the Ag-S (or Cu-S) lattice.
More specif-
ically, the initial bilayer motif is abruptly converted at ∼400 K
Figure 8. DRIFT intensity ratio of the antisymmetric and symmetric
to a micellar (columnar hexagonal) motif.
2 s 2
methylene stretching bands, I(νas(CH ))/I(ν (CH )), obtained during the
As mentioned in the Introduction, we expected at the
beginning that the thermal characteristics of AgCO2R are quite
comparable to those of AgSR because of their similar layered
structures. However, the actual observed data indicates that the
thermal behavior of organic/inorganic hybrid material is highly
dependent on the nature of the headgroup anchored to metal
atoms. The specific structural constraint imposed by the
carboxylate group seems thus to be associated with the different
thermal behavior of silver stearate with respect to that of silver
alkanethiolate. This is the reason the layered structure is
sustained for silver stearate even though the binding state of
tc oa r ub no ixd ye l na tt ea t ies .subjected to change around ∼380 K from bridging
increasing temperature phase (open circles) and at room temperature
after heating to the specified temperatures (filled circles).
It can also be noticed from Figure 9 that the XRD peaks
indexed as (0k0) abruptly decrease from 498 K and cannot be
identified at all above 538 K. Instead, two new peaks are
identified at 37.7 and 43.9° when the sample is heated to above
5
38 K. These peaks can be assigned to the (111) and (200)
3
6
reflections of metallic silver, however. The temperatures at
which any XRD structural changes occur are thus overall quite
close to those observed in the DRIFT measurements. Hence,
the totally irreversible structural change that is identified around
5
00 K from the DRIFT spectroscopy must be caused by the
thermal degradation of the sample to produce metallic silver.
.2.4. TGA and DSC Measurements. Figures 10a and 10b
3
show, respectively, the TGA and its first derivative traces for
silver stearate. Figure 10b illustrates that most of the mass loss
occurs around 516 K, and then a small amount is subsequently
lost around 673 K. Figure 10a shows on the other hand that the
major loss commences at ∼450 K and is complete around ∼610
K. During the interval, the actual mass loss amounts to 66%.
On the basis of the formula weight of silver stearate (AgCO2-
(CH2)16CH3), if the organic moiety is completely lost, the mass
loss has to amount to 72% in total. There is thus 6% mass
difference, and this is attributed to the formation of stearate-
capped silver nanoparticles. As described in the footnote,37 the
residual stearate (6%) can cover ∼90% of the silver nanopar-
ticles with a size of ∼4 nm (see section 3.3.2). This implies
that the thermal decomposition product is mostly stearate-
derivatized silver nanoparticles; if free silver is also present, it
must be a minimal amount. As can be seen in Figure 10a, the
stearate-derivatized silver nanoparticles are stable up to ∼650
K. The particles are then subjected to further mass loss around
670 K, and the eventual mass loss amounts to 72% in total. It
is remarkable that this value is exactly the same as that predicted
for the case when the organic moiety is completely lost from
Figure 9. Variable temperature XRD patterns of silver stearate.
Reflections from the metallic silver are marked in the topmost
diffractogram. The peak labeled with asterisk (*) is due to the base
plate.
unidentate. In turn, the intensity ratio gradually increases as the
temperature is increased up to 498 K, implying the continuous
disordering of the alkyl chains. More significant disordering
takes place in the temperature region from 498 to 548 K, and
this must be associated with the thermal decomposition of silver
stearate. Above 550 K, the intensity ratio decreases once again,
however, indicating that the decomposition product(s) possess
rather ordered alkyl chains (vide infra).
3.2.3. Temperature-Dependent XRD Pattern of SilVer Stear-
ate. To obtain further information on the structural changes of
silver stearate, we have performed a variable temperature XRD
measurement. Figure 9 shows a series of X-ray diffractograms
taken as a function of temperature. All diffractograms were
obtained at the temperatures indicated, with the temperature held

2898 J. Phys. Chem. B, Vol. 106, No. 11, 2002
Lee et al.
unreasonable, and it instead indicates that the first transition
occurring at ∼400 K is a partially irreversible one, as revealed
by DRIFT and XPS measurements.
3
.3. Identity of the Thermal Decomposition Product of
Silver Stearate. 3.3.1. Consideration of the Possible Product-
s). Although the presence of silver metals can be confirmed
(
from XRD measurement, the actual composition and the
characteristics of the thermal decomposition product(s) of silver
stearate are a matter of conjecture. In fact, the composition of
the thermal decomposition products of silver carboxylates is
known to depend on the detailed reaction conditions, including
such factors as the surrounding atmosphere, heating rate, and
temperature. Hence, there is no generally accepted mechanism
for the thermal decomposition of silver carboxylates, therefore
the composition of the products is not entirely clear. One scheme
proposed by Fields and Meyerson and by Fiorucci et al. for the
thermal decomposition of silver carboxylates is as follows:41
CH (CH ) COOAg f
3
2 n
Ag + CO + CH (CH ) CH (n ) 0, 2, 4) (1)
2
3
2 n
3
in which the primary products are the corresponding paraffins
and metallic silver. A similar reaction scheme (with additional
formation of H2) is cited elsewhere by Judd et al.42 for silver
acetate:
2
CH COOAg f 2Ag + CH CO H + CO + H + C
3 3 2 2 2
(2)
For long silver carboxylates, including silver stearate, Andreev
Figure 10. (a) TGA and (b) its first derivative traces of silver stearate.
43
et al. reported that metallic silver, CO2, the corresponding acid,
and products of the organic radical reaction were produced upon
heating in a vacuum; the organic radicals were supposed to
recombine with the formation of paraffins, or to undergo further
44
reactions. On the other hand, Uvarov et al. reported that silver
stearate showed phase transitions at 397 and 426 K in air. The
former transition was claimed to be irreversible leading to
decreased conductivity while the latter, accompanied by the
thermal decomposition, resulted in increased conductivity. They
explained the decomposition as hydrolysis of the silver car-
boxylate that proceeds in humid air (their experimental condi-
4
4
tion) as follows:
(
CH (CH ) COOAg) + H O f
3
2 n
2
2
2
CH (CH ) CO H + Ag O (3)
3 2 n 2 2
In the present work, the thermal decomposition of silver
stearate was observed to occur above 500 K. The CdO
Figure 11. DSC trace of silver stearate. The inset shows traces for
heating (lower one) and cooling (upper one) cycles.
-1
stretching peak was clearly identified at ∼1700 cm above 508
K, which must have arisen from a free acid. (To form a free
acid, a hydrogen source is required. Its source is not yet clear,
but we presume that hydrogen atoms are provided by the
dehydrogenation reaction that may occur during the thermal
decomposition of the organic moiety of silver stearate. Regard-
ing this matter, it would be informative to recall the representa-
tive type of thermal decomposition reaction occurring for
polyethylene-containing materials. In fact, the dehydrogenation
of the hydrocarbon chains has been reported to be the
silver stearate (vide supra). This implies that free silver remains
exclusively above 670 K.
The DSC trace of silver stearate shown in Figure 11 reveals
two endothermic peaks at 397 and 500 K. The first peak can
be assigned to the structural transformation associated mainly
with the disordering of the alkyl chains. The observed temper-
ature (397 K) is in fact comparable to that of a related system
such as stearic acid SAMs on Ag31 or silver alkanethiolate,11,40
but is higher than that of octadecanethiol-capped Ag nanopar-
ticles.40 The second endothermic peak at 500 K should be
attributed to the thermal decomposition of silver stearate. On
the other hand, as shown in the inset of Figure 11, an exothermic
peak is observed at 388 K on cooling after heating to 410 K. It
is noticed that the exothermic transition enthalpy (∆Hexo) is only
about 55% of the endothermic enthalpy (∆Hendo). This is not
4
5
representative reaction of such materials. ) Metallic silver was
also found to form upon decomposition of silver stearate.
However, we could not find any evidence that would indicate
the presence of Ag2O and CO2 as the constituents of the
decomposition products. We thus suppose that the primary
decomposition products should be metallic silver and stearic
acid.

A Layered Silver Carboxylate
J. Phys. Chem. B, Vol. 106, No. 11, 2002 2899
surroundings. The IR spectrum of the silver nanoparticles shown
in Figure 13 is almost the same as that of SAMs of stearic acid
3
,31
on silver surfaces.
Along with the appearance of the Wx
+
-
progression bands, the positions of the d and d modes dictate
that the alkyl chains assume fully extended all-trans conforma-
tion. As observed in the DRIFT spectrum of stearic acid SAMs
on silver, the intense peak at 1396 cm in Figure 13 can be
assigned to the νs(COO ) band. The presence of the band
3
1
-1
-
supports the contention that the silver nanoparticles are deriva-
-
tized with stearate. The νas(COO ) band was completely absent
Figure 12. Typical TEM image of silver nanoparticles obtained from
the thermal decomposition of silver stearate.
in the DRIFT spectrum of nanoparticles. Considering that the
corresponding band is the most intense one in the DRIFT
spectrum of silver stearate (see Figure 3), its absence in the
nanoparticle spectrum is intriguing. However, this can be
understood by recalling our previous finding that the usual
infrared surface selection rule is applicable even to the surface
3
.3.2. EVentual Product: Stearate-DeriVatized Ag Nanopar-
46
ticles. Abe et al. reported recently that thermal decomposition
of silver stearate at 523 K in an atmosphere of N2 should
produce silver nanoparticles with a size of 5∼20 nm. The latter
temperature corresponds to that of the second transition in the
present work. On these grounds, we have more thoroughly
analyzed a sample of silver stearate preheated to 598 K for 10
min in N2 environment. The heated sample was rinsed thor-
oughly with methanol to remove free acids or possible impuri-
ties. It is noticeable that the rinsed sample was readily dispersed
in a nonpolar medium such as toluene, benzene, and hexane.
Figure 12 shows a typical TEM image of the sample taken after
vaporizing the solvent on the surface of the 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
3
1,32,49
of fine metal particles.
In light of this, the absence of the
-
νas(COO ) band in the DRIFT spectrum of nanoparticles can
be thought to indicate that the carboxylate group is bound to
the surfaces of silver nanoparticles symmetrically via its two
oxygen atoms. All of these observations suggest that upon
heating silver stearate in N2, metallic silver and stearic acid are
produced, but the metallic silver is immediately stabilized as
nanosized particles by derivatizing with stearate.
4
. Summary and Conclusion
We confirmed by XRD analysis that silver stearate consists
of an infinite-sheet, 2D, nonmolecular layered structure. Along
with the appearance of the Wx progression bands, the DRIFT
∼
4 nm. The distance between nanoparticles is estimated to be
+
-
peak frequencies of d and d modes suggested that the alkyl
chains in silver stearate were in an all-trans conformational state
with little or no significant gauche population as in silver
alkanethiolate. The appearance of single narrow CH2 rocking
and δ(CH2) bands suggested further that silver stearate exists
in triclinic packing with only one type of alkyl chain per unit
subcell. The temperature-dependent DRIFT, XRD, and XPS
measurements indicated that there are two distinct transitions,
resulting in dramatic structural changes for silver stearate. In
the first transition at ∼380 K, the binding state of carboxylate
is converted from bridging to unidentate. Although the alkyl
chains are also subjected to considerable disordering, the layered
structural motif is nonetheless sustained; hence, the overall
∼3.5 nm. Recalling that the chain length of stearic acid is ∼2.3
nm, silver nanoparticles are thus supposed to be surrounded by
stearates with their alkyl chains between neighboring particles
47
interdigitated. The formation of silver nanoparticles could also
be confirmed from the UV/Vis spectroscopy. A distinct peak
is observed at 417 nm for a sample dispersed in toluene. This
must arise from the surface plasmon absorption of silver
nanoparticles.4 The fact that the peak shape is nearly Gaussian-
like suggests that the nanoparticles are uniformly distributed
as evidenced by the TEM image.
8
We could confirm from DRIFT spectroscopy that the silver
nanoparticles are indeed passivated by the uniform stearate
Figure 13. DRIFT spectrum of silver nanoparticles obtained from the thermal decomposition of silver stearate.

2900 J. Phys. Chem. B, Vol. 106, No. 11, 2002
Lee et al.
structural change is partially irreversible. This is in sharp contrast
to the case of silver or copper alkanethiolates for which
mesogenic phase transition exclusively takes place under similar
temperature conditions. In the second transition at ∼500 K, a
totally irreversible structural change takes place for silver
stearate, that is, the decomposition of the sample material. The
primary products must be metallic silver and free stearic acid,
both of which finally become stabilized by forming stearate-
derivatized Ag nanoparticles.
(24) (a) Senak, L.; Moore, D.; Mendelsohn, R. J. Phys. Chem. 1992,
6, 2749. (b) Naselli, C.; Rabe, J. P.; Rabolt, J. F.; Swalen, J. D. Thin
9
Solid Films 1985, 134, 173.
25) Sndyer, R. G.; Schaachtschneider, J. H. Spectrochim. Acta A 1963,
19, 117.
(
(
26) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D.
Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden
Prairie, MN, 1992.
(27) Benseba, F.; Zhou, Y.; Deslandes, Y.; Kruus, E.; Ellis, T. H. Surf.
Sci. 1998, 405, L472.
(28) (a) Marshbanks, T. L.; Jugduth, H. K.; Delgass, W. N.; Franses,
E. I. Thin Solid Films 1993, 232, 126. (b) Frydman, E.; Cohen, H.; Maoz,
R.; Sagiv, J. Langmuir 1997, 13, 5089. (c) Ohta, T.; Yamada, M.; Kuroda,
H. Bull. Chem. Soc. Jpn. 1974, 47, 1158.
(
29) Han, S. W.; Joo, S. W.; Ha, T. H.; Kim, Y.; Kim, K. J. Phys. Chem.
B 2000, 104, 11987.
30) Lindberg, B.; Berndtsson, A.; Nilsson, R.; Nyholm, R.; Exner, O.
Acta Chem. Scand. A 1978, 32, 353.
Acknowledgment. This work was supported in part by the
Korea Research Foundation (KRF, 042-D00073) and the Korea
Science and Engineering Foundation (KOSEF, 1999-2-121-001-
(
(
31) Lee, S. J.; Kim, K. Vib. Spectrosc. 1998, 18, 187.
5
). K.K. also acknowledges KOSEF for providing a leading-
(32) Lee, S. J.; Han, S. W.; Yoon, M.; Kim, K. Vib. Spectrosc. 2000,
scientist grant (KOSEF, R03-2001-00021). S.W.H. was sup-
ported by KOSEF through the Center for Molecular Catalysis
at Seoul National University. S.J.L. and H.J.C. are recipients
of the BK21 fellowship.
2
4, 265.
(33) (a) Gericke, A.; H u¨ hnerfuss, H. Thin Solid Films 1994, 245, 74.
(
b) Ohe, C.; Ando, H.; Sato, N.; Urai, Y.; Yamamoto, M.; Itoh, K. J. Phys.
Chem. B 1999, 103, 435.
(
(
34) Sandhyarani, N.; Pradeep, T. J. Mater. Chem. 2001, 11, 1924.
35) Espinet, P.; Lequerica, M. C.; Mart ´ı n-Alvarez, J. M. Chem. Eur.
J. 1999, 5, 1982.
36) JCPDS ICDD PDF No. 02-1098, 03-0921, 03-0931.
References and Notes
(
(
1) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350.
(37) The number of stearate required for the monolayer coverage on a
silver nanoparticle is calculated as follows. Using the average diameter of
silver nanoparticles, 4 nm, determined from TEM image (see Figure 12),
the mean surface area and volume of a single particle are estimated to be
(2) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12,
3
604.
3) Ahn, S. J.; Son, D. H.; Kim, K. J. Mol. Struct. 1994, 324,
(
2
3
2
23.
(
50.27 nm and 33.51 nm , respectively. Assuming then that each stearate
occupies an area of 0.20 nm2 on silver surface, 251 stearates can be
anchored on each nanoparticle. On the other hand, referring to the atomic
radius of silver, 0.1445 nm,39 each nanoparticle should consist of 2651 Ag
atoms. In the actual TGA experiment, we used 10.0 mg of silver stearate
so that there was 2.8 mg of Ag in the sample (28 wt %). In fact, 2.8 mg of
38
4) Kang, S. Y.; Kim, K. Langmuir 1998, 14, 226.
(5) Dance, I. G.; Fisher, K. J.; Bamda, R. M. H.; Scudder, M. L. Inorg.
Chem. 1991, 30, 183.
6) Baena, M. J.; Espinet, P.; Lequercia, M. C.; Levelut, A. M. J. Am.
Chem. Soc. 1992, 114, 4182.
7) Fijolek, H. G.; Grohal, J. R.; Sample, J. L.; Natan, M. J. Inorg.
Chem. 1997, 36, 622.
8) Bensebaa, F.; Ellis, T. H.; Kruss, E.; Voicu, R.; Zhou, Y. Can. J.
Chem. 1998, 76, 1654.
9) Bensebaa, F.; Ellis, T. H.; Kruus, E.; Voicu, R.; Zhou, Y. Langmuir
998, 14, 6579.
10) Parikh, A. N.; Gillmor, S. D.; Beers, J. D.; Beardmore, K. M.; Cutts,
R. W.; Swanson, B. I. J. Phys. Chem. B 1999, 103, 2850.
11) Bardeau, J. F.; Parikh, A. N.; Beers, J. D.; Swanson, B. I. J. Phys.
Chem. B 2000, 104, 627.
12) Vand, A.; Atkins, A.; Cambell, R. K. Acta Crystallogr. 1949, 2,
98.
(
15
(
Ag can be converted to 5.7 × 10 nanoparticles with a size of 4 nm; [(2.8
-3 -1 -1 23 -1
mg)(10 g mg )/(107.868 g mol )][(6.02 × 10 mol )/(2651)] ) 5.7
15
(
× 10 . These nanoparticles will then be covered in total by 0.67 mg of
15 -1 3 23 -1
stearate; (5.7 × 10 )(251)(283.473 g mol )(10 mg g)/(6.02 × 10 mol
)
(
) 0.67 mg. The latter amount corresponds to 6.7% mass loss in the TGA
experiment. In this light, 0.6 mg of stearate which corresponds to 6% mass
loss in TGA can be presumed to cover 90% of the nanoparticles; (0.60/
0.67)(100) ) 90. This implies in turn that 90% of metallic silver can be
stabilized as nanoparticles by only 6% of stearate present after the thermal
decomposition of silver stearate between 450 and 610 K.
(38) Ulman, A. An Introduction to Ultrathin Organic Films; Academic
Press: San Diego, New York, 1991.
1
(
(
(
3
(
13) Matthews, F. W.; Warren, G. G.; Michell, J. H. Anal. Chem. 1950,
(39) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf.
Sci. 1988, 200, 45.
2
2, 514.
(
14) Tolochko, B. P.; Chernov, S. V.; Nikitenko, S. G.; Whitcomb, D.
R. Nucl. Instrum. Methods A 1998, 405, 428.
15) (a) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou,
L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273,
92. (b) Lacroix, P. G.; Clement, R.; Nakatani, K.; Zyss, J.; Ledoux, I.
Science 1994, 263, 658. (c) Vermeulen L. A.; Thompson, M. E. Nature
992, 358, 656. (d) Laget, V.; Hornick, C.; Rabu, P.; Drillon, M.; Turek,
P.; Ziessel, R. N. AdV. Mater. 1998, 10, 1024.
16) CRC Handbook of Chemistry and Physics, 71st ed.; Lide, D. R.,
Ed.; CRC Press: Boca Raton, FL, 1990.
17) (a) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta A 1978,
4, 395. (b) Hill, I. R.; Levin, I. W. J. Chem. Phys. 1979, 70, 842.
18) (a) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem.
982, 86, 5145. (b) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark,
(40) Sandhyarani, N.; Antony, M. P.; Selvam, G.; Pradeep, T. J. Chem.
Phys. 2000, 113, 9794.
(41) (a) Fields, E. K.; Meyerson, S. J. Org. Chem. 1976, 41, 916. (b)
Fiorucci, A. R.; Saran, L. M.; Cavalheiro, E. T. G.; Neves, E. A.
Thermochim. Acta 2000, 356, 71.
(42) Judd, M. D.; Plankett, B. A.; Pope, M. I. J. Therm. Anal. 1974, 6,
555.
(43) Andreev, V. M.; Burleva, L. P.; Boldyrev, V. V.; Mikhaliv, Y. I.
IzV. SO AN USSR, SerKhim. Nauk 1983, 2, 58.
(44) Uvarov, N. F.; Burleva, L. P.; Mizen, M. B.; Whitcomb, D. R.;
Zou, C. Solid State Ionics 1998, 107, 31.
(45) (a) Blazso, M. J.; Zelei, B.; Jakab, E. J. Anal. Appl. Pyrolysis 1995,
35, 221. (b) Choi, H. J.; Han, S. W.; Lee, S. J.; Kim, K. Appl. Spectrosc.
2001, 55, 1085.
(
8
1
(
(
3
(
1
V. M. J. Phys. Chem. 1986, 90, 5623. (c) MacPhail, R. A.; Snyder, R. G.;
Strauss, H. L. J. Chem. Phys. 1982, 77, 1118.
(46) Abe, K.; Hanada, T.; Yoshida, Y.; Tanigaki, N.; Takiguchi, H.;
Nagasawa, H.; Nakamoto, M.; Yamaguchi, T.; Yase, K. Thin Solid Films
1998, 327-329, 524.
(19) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J.
Phys. Chem. 1982, 86, 334.
(47) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem.
Res. 2000, 33, 27.
(
(
(
20) Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 116.
21) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316.
22) Snyder, R. G. In Methods of Experimental Physics; Marton, L.,
(48) Henglein, A. J. J. Phys. Chem. 1993, 97, 5457.
(49) (a) Han, H. S.; Kim, C. H.; Kim, K. Appl. Spectrosc. 1998, 52,
1047. (b) Han, S. W.; Han, H. S.; Kim, K. Vib. Spectrosc. 1999, 21,
133. (c) Han, H. S.; Han, S. W.; Kim, C. H.; Kim, K. Langmuir 2000, 16,
1149.
Marton, C., Eds.; Academic Press: New York, 1980.
23) (a) Borja, M.; Dutta, P. K. J. Phys. Chem. 1992, 96, 5434. (b)
Almirante, C.; Minoni, G.; Zerbi, G. J. Phys. Chem. 1986, 90, 852.
(