Structure and Thermal Behavior of a Layered Silver Carboxylate - The

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 stearatederivatized silver nanoparticles with a size of ∼4 nm.

1. Introduction Both the alkane thiols and carboxylic acids form close-packed monolayers on Ag, specifically, when the number of methylene groups in these molecules exceeds ∼10.1,2 Alkyl chains of fatty acid self-assembled on Ag are also known to have a more closepacked structure than those prepared by the Langmuir-Blodgett (LB) method.3 It is known that alkyl chains in the thiolderivatized 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 alkanethiolate (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 closepacked 2D organothiol SAMs on Ag, even though the thiol occupancy (above an Ag layer) in the 3D material is only 50% of that in the 2D SAMs.7 One intriguing characteristic of layered AgSR species is that the material shows liquid crystalline behavior upon melting.6,8 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 CH3CH3 contacts disrupt to form stacked-disk micellar structures.6 Similarly to AgSR, silver alkane carboxylate (AgCO2R) has also been reported to exist in a layered structure.12-14 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 heterostructures that exhibit alternating 2D molecular assemblies of organic and inorganic constituents have expanded considerably * Author to whom all correspondence should be addressed. Tel: +822-8806651. Fax: +82-2-8743704. E-Mail: [email protected].

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 inorganic constituents at the molecular level.15 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. 2. Experimental Section 2.1. Preparation of Silver Stearate. Stearic acid (99+%) 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. 2.2. Characterization. XRD patterns were obtained on a Philips X′PERT MPD diffractometer for a 2θ range of 5° to 50° at an angular resolution of 0.05° using Cu KR (1.5419 Å) radiation. The samples were spread on antireflection glass slides

10.1021/jp0132937 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/27/2002

A Layered Silver Carboxylate 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 FTIR 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 32 scans was measured in the range 3500-1000 cm-1 at a resolution of 4 cm-1 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 HappGenzel 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 ESCALAB 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). 3. Results and Discussion 3.1. Structure of Silver Stearate. 3.1.1. Room-Temperature X-ray Diffraction. The XRD pattern for silver stearate is shown in Figure 1. As was known since 1949,12-14 the compound shows a well-developed progression of intense reflections. These intense reflections can be interpreted in terms of threedimensionally 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 fourmembered 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.

J. Phys. Chem. B, Vol. 106, No. 11, 2002 2893

Figure 1. XRD pattern of silver stearate. The interlayer spacings derived from different reflections are listed in the inset.

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.5,7 Considering that the averaged interlayer 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 43.53 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 be 22.99 Å: 21.301 Å (1.253 Å per CH2 group10 × 17) plus 1.69 Å (vdW radius of CH3 group5); the C-C bond length in -OOC-CH2- is known to be the same as that in -H2C-CH2(and -H2C-CH3).16 Taking into account that the stearate chain is tilted by 4.3° from the normal to the Ag plane,14 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 silver alkanethiolate,10 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 highangle 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 temperature. The high-frequency region from 2800 to 3000 cm-1 reveals the C-H stretching modes of the methyl and the methylene groups of stearate while the low-frequency region from 650 to 1650 cm-1 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.

carboxylate group as well as the scissoring, rocking, wagging, and twisting modes of the methylene groups. More specifically, the two intense bands at 2848 and 2916 cm-1 are assigned, respectively, to the symmetric (νs(CH2), d+) and the antisymmetric (νas(CH2), d-) stretching vibrations of the methylene groups. Two peaks observed at 2871 and 2954 cm-1 are assigned to the symmetric (νs(CH3), r+) and antisymmetric (νas(CH3), r-) stretching vibrations of the methyl group, respectively. In the high frequency region, two shoulder peaks are additionally identified at ca. 2895 and 2964 cm-1. The former peak can be attributed to the Fermi resonance absorption due to the d+ mode.17 The latter peak has to be attributed to the rmode. We already assigned the distinct band at 2954 cm-1 to r-. It is well-known that for the two antisymmetric stretching vibrations of a CH3 group to become degenerate and appear as a single broad peak, the CH3 group must be in at least C3 symmetry.18 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 dmodes usually lie in the narrow ranges of 2846-2850 and 2915-2918 cm-1, respectively, for all-trans extended chains and in the distinctly different ranges of 2854-2856 and 29242928 cm-1 for disordered chains characterized by a significant presence of gauche conformers.18,19 On this basis, the observed peak frequencies of 2848 and 2916 cm-1 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.8-11 The low-frequency region (650-1650 cm-1) 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 and 1518 cm-1 can be assigned, respectively, to the symmetric (νs(COO-)) and antisymmetric (νas(COO-)) stretching vibrations of the carboxylate group. No peak attributable to ν(CdO) is identified around 1700 cm-1, indicating that the obtained sample is not contaminated with free acid. The peak at 717 cm-1 is assigned to the CH2 rocking vibration. Its appearance as a sharp singlet at this frequency

(full width at half-maximum (fwhm): ∼10 cm-1) is characteristic of a crystalline chain packing in a triclinic or hexagonal structure.20 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-1, as well as increase the fwhm.21 On this basis, we conclude that the alkyl 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 reflect the packing arrangement of the alkyl chain assemblies.20,22,23 The appearance of a single narrow peak at 1473 or 1467 cm-1 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 Figure 3 at 1471 cm-1 suggests that silver stearate exists in triclinic packing with only one type of alkyl chain per unit subcell; the small fwhm (∼5.9 cm-1) is indicative of the presence of highly ordered, all-trans chains. These infrared spectral observations are consistent with the reported X-ray analyses of a number of silver carboxylates.12,14 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 alkanethiolate.10 The well-resolved progression bands in the region from 1150 to 1400 cm-1 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 peak positions of the d+ and d- modes.22,24 More specifically, the number and the inter-band separation are known to depend on the average number of trans conformers in the chains; the inter-band spacing ∆ν in Wx modes below ∼1350 cm-1 is related to the number of trans-units m by the equation ∆ν ) 326/(m +1).25 Since the average value of ∆ν measured for silver stearate is 18.3 cm-1, 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

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 298-598 K: (a) high-frequency (3025-2775 cm-1) and (b) lowfrequency (1800-1000 cm-1) regions. The arrows denote the occurrence of distinct spectral changes.

Figure 4. High-resolution XP spectra of silver stearate in (a) Ag 3d, (b) C 1s, and (c) O 1s spectral regions.

number of methylene units in silver stearate. Hence, all of the alkyl chains should be in all-trans conformation with negligible gauche defect. 3.1.3. XPS Measurements. To provide further information on the structure and chemical state of silver stearate, we obtained the XP spectra of silver stearate. The expected peaks from Ag 3d, C 1s, and O 1s core levels were clearly detected in XP spectra; no trace of contamination was found. Figure 4 shows the high-resolution XP spectra in the (a) Ag 3d, (b) C 1s, and (c) O 1s spectral regions. The narrow widths of the peaks suggest 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 silver26 and n-alkanethiol-adsorbed Ag surfaces.27 Owing to the smaller binding energy difference between Ag(0) and Ag(I), it is difficult to specify the actual oxidation state of silver. Two C 1s peaks are identified at 284.6 and 288.4 eV. Consulting the data in the literature,28,29 these peaks can be attributed to the carbon atoms in the aliphatic chain (C-C) and carboxylate (-COO-) moieties, respectively. The absence of the C 1s peak 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

531.3 eV. A single, symmetric peak must arise from two comparable oxygen atoms in the carboxylate (-COO-) moiety.28-30 3.2. Thermal Behavior of Silver Stearate. 3.2.1. Temperature-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 analysis of the DRIFT spectral features.31,32 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-1 is rather invariant up to 378 K and then becomes weak and disappears above 448 K. Instead, a new peak develops at ca. 1560 cm-1 from 388 K and is sustained up to 598 K. On the other hand, the νs(COO-) band decreases gradually from room 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 at 1518 cm-1, the peak developed at 1560 cm-1 at 388 K is also attributed to νas(COO-) by presuming that the type of 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 bonding,32,33 the bridging state must turn into unidentate around 380 K. Regarding the d+ and d- modes, the peak positions are invariant from 298 to 378 K. This implies that the initial alltrans conformational order of the alkyl chains is preserved up to ∼380 K. Upon increasing the temperature, the d+ and dmodes shift gradually toward higher frequencies reaching 2853 and 2923 cm-1, respectively, at 418 K. These upward shifts can be attributed to a premelting event characterized by the formation of gauche conformers. The values of 2853 and 2923 cm-1 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 298 to 458 to 298 K and the couples of a, d, and e traces to a cycle of 298 to 598 to 298 K. The dotted lines are drawn only as a guide to the eye.

2850 and 2917 cm-1, respectively, at 548 K. These values are in fact comparable to those at room temperature. This implies that the alkyl chains assume fully extended all-trans conformation 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 (1 cm-1 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 micellar state.6 The second transition at ∼460 K is irreversible 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 K.34 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

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 annealing at 458 K. In particular, the νas(COO-) band appears at 1560 cm-1 rather than at 1518 cm-1 after the thermal cycling; the νs(COO-) mode is also subjected to change by annealing. The irreversible change in the binding state of the carboxylate group can also be proven using the XPS measurement. Figure 7b shows the high-resolution O 1s XP spectrum for silver stearate annealed previously at 458 K. For comparison, the O 1s XP spectrum taken before annealing is also given in Figure 7a (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 unreasonable 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 irreversible one. Such an irreversible change is surely associated with the thermal degradation of the sample material at higher temperature. The relative intensity ratio I(νas(CH2))/I(νs(CH2)) is frequently adopted as a measure of disorder; increasing in conformational order, the ratio decreases.18b 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

A Layered Silver Carboxylate

Figure 8. DRIFT intensity ratio of the antisymmetric and symmetric methylene stretching bands, I(νas(CH2))/I(νs(CH2)), obtained during the increasing temperature phase (open circles) and at room temperature after heating to the specified temperatures (filled circles).

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 Stearate. 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

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 298 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 restructuring of the Ag-S (or Cu-S) lattice.6,35 More specifically, the initial bilayer motif is abruptly converted at ∼400 K to a micellar (columnar hexagonal) motif. 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 carboxylate is subjected to change around ∼380 K from bridging to unidentate. 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 538 K. These peaks can be assigned to the (111) and (200) reflections of metallic silver, however.36 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 500 K from the DRIFT spectroscopy must be caused by the thermal degradation of the sample to produce metallic silver. 3.2.4. TGA and DSC Measurements. Figures 10a and 10b 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 stearatecapped silver nanoparticles. As described in the footnote,37 the residual stearate (6%) can cover ∼90% of the silver nanoparticles with a size of ∼4 nm (see section 3.3.2). This implies that the thermal decomposition product is mostly stearatederivatized 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

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

CH3(CH2)nCOOAg f Ag + CO2 + CH3(CH2)nCH3 (n ) 0, 2, 4) (1) 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:

2CH3COOAg f 2Ag + CH3CO2H + CO2 + H2 + C Figure 10. (a) TGA and (b) its first derivative traces of silver stearate.

(2)

For long silver carboxylates, including silver stearate, Andreev et al.43 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 reactions. On the other hand, Uvarov et al.44 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 carboxylate that proceeds in humid air (their experimental condition) as follows:44

(CH3(CH2)nCOOAg)2 + H2O f 2CH3(CH2)nCO2H + Ag2O (3) Figure 11. DSC trace of silver stearate. The inset shows traces for heating (lower one) and cooling (upper one) cycles.

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 temperature (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 nanoparticles.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

In the present work, the thermal decomposition of silver stearate was observed to occur above 500 K. The CdO stretching peak was clearly identified at ∼1700 cm-1 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. Regarding this matter, it would be informative to recall the representative type of thermal decomposition reaction occurring for polyethylene-containing materials. In fact, the dehydrogenation of the hydrocarbon chains has been reported to be the representative reaction of such materials.45) 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

Figure 12. Typical TEM image of silver nanoparticles obtained from the thermal decomposition of silver stearate.

3.3.2. EVentual Product: Stearate-DeriVatized Ag Nanoparticles. Abe et al.46 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 thoroughly with methanol to remove free acids or possible impurities. 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 ∼4 nm. The distance between nanoparticles is estimated to be ∼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 interdigitated.47 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.48 The fact that the peak shape is nearly Gaussianlike suggests that the nanoparticles are uniformly distributed as evidenced by the TEM image. We could confirm from DRIFT spectroscopy that the silver nanoparticles are indeed passivated by the uniform stearate

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 on silver surfaces.3,31 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 conformation. As observed in the DRIFT spectrum of stearic acid SAMs on silver,31 the intense peak at 1396 cm-1 in Figure 13 can be assigned to the νs(COO-) band. The presence of the band supports the contention that the silver nanoparticles are derivatized with stearate. The νas(COO-) band was completely absent 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 of fine metal particles.31,32,49 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 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

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 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 stearatederivatized Ag nanoparticles. Acknowledgment. This work was supported in part by the Korea Research Foundation (KRF, 042-D00073) and the Korea Science and Engineering Foundation (KOSEF, 1999-2-121-0015). K.K. also acknowledges KOSEF for providing a leadingscientist grant (KOSEF, R03-2001-00021). S.W.H. was supported 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. References and Notes (1) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350. (2) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (3) Ahn, S. J.; Son, D. H.; Kim, K. J. Mol. Struct. 1994, 324, 223. (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 1998, 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, 398. (13) Matthews, F. W.; Warren, G. G.; Michell, J. H. Anal. Chem. 1950, 22, 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, 892. (b) Lacroix, P. G.; Clement, R.; Nakatani, K.; Zyss, J.; Ledoux, I. Science 1994, 263, 658. (c) Vermeulen L. A.; Thompson, M. E. Nature 1992, 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, 34, 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. 1982, 86, 5145. (b) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623. (c) MacPhail, R. A.; Snyder, R. G.; Strauss, H. L. J. Chem. Phys. 1982, 77, 1118. (19) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1982, 86, 334. (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., 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.

Lee et al. (24) (a) Senak, L.; Moore, D.; Mendelsohn, R. J. Phys. Chem. 1992, 96, 2749. (b) Naselli, C.; Rabe, J. P.; Rabolt, J. F.; Swalen, J. D. Thin 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. (31) Lee, S. J.; Kim, K. Vib. Spectrosc. 1998, 18, 187. (32) Lee, S. J.; Han, S. W.; Yoon, M.; Kim, K. Vib. Spectrosc. 2000, 24, 265. (33) (a) Gericke, A.; Hu¨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.; Martı´n-Alvarez, J. M. Chem. Eur. J. 1999, 5, 1982. (36) JCPDS ICDD PDF No. 02-1098, 03-0921, 03-0931. (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 50.27 nm2 and 33.51 nm3, respectively. Assuming then that each stearate occupies an area of 0.20 nm2 on silver surface,38 251 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 Ag can be converted to 5.7 × 1015 nanoparticles with a size of 4 nm; [(2.8 mg)(10-3 g mg-1)/(107.868 g mol-1)][(6.02 × 1023 mol-1)/(2651)] ) 5.7 × 1015. These nanoparticles will then be covered in total by 0.67 mg of stearate; (5.7 × 1015)(251)(283.473 g mol-1)(103 mg g)/(6.02 × 1023 mol-1) ) 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. (39) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (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. (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. (47) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (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.