Thermal and photochemical characteristics of silver 4-(4-nitrophenyl)butyrate revealed by infrared and Raman spectroscopy

Vibrational Spectroscopy 44 (2007) 308–315 www.elsevier.com/locate/vibspec

Thermal and photochemical characteristics of silver 4-(4-nitrophenyl)butyrate revealed by infrared and Raman spectroscopy Kwan Kim *, Hyun Sook Lee, Hyun Min Kim Laboratory of Intelligent Interfaces, Department of Chemistry, Seoul National University, Seoul 151-742, Republic of Korea Received 16 November 2006; received in revised form 29 January 2007; accepted 7 February 2007 Available online 13 February 2007

Abstract We have investigated the thermal and photochemical characteristics of silver 4-nitrophenylbutyrate (Ag-4NPB) by means of infrared and Raman spectroscopy, coupled with X-ray diffraction (XRD) and thermogravimetry. XRD analysis indicated that Ag-4NPB consisted of a layered structure. Upon heating the sample, structural changes took place, particularly at two specific temperatures. The binding state of the carboxylate group changed from bridging to unidentate at 363 K. A second dramatic change occurred at 536 K, producing 4NPB-capped Ag nanoparticles. The latter transition temperature for Ag-4NPB is about 18 K higher than that for silver stearate (Ag-STA), but nearly 57 K lower than that for silver 4nitrobenzoate (Ag-4NBA). The thermal stability of aromatic silver carboxylate is thus lowered by the incorporation of a short alkane chain between the benzene ring and the carboxylate group. On the other hand, Ag-4NPB was readily subjected to photolysis even by the 632.8-nm radiation. Initially, silver nanoparticles appeared to be generated, followed by the nitro-to-amine group conversion. Although silver particles could also be generated from Ag-4NBA, the nitro-to-amine group conversion did not take place by the 632.8-nm light. The photochemical stability thus seemed to be lowered, parallel to the thermal stability, by the incorporation of a short alkane chain between the benzene ring and the carboxylate group. # 2007 Elsevier B.V. All rights reserved. Keywords: Silver carboxylate; Thermal decomposition; Photodecomposition; Ag nanoparticle

1. Introduction We have recently found that the surface-enhanced Raman scattering (SERS) features of 4-nitrobenzoic acid (4-NBA) and 4-nitrobenzenethiol (4-NBT) on silver are surprisingly coincident with those of 4-aminobenzoic acid (4-ABA) and 4aminobenzenethiol (4-ABT) on Ag, respectively [1–3]. This implies that the nitro-to-amine conversion occurs quite readily for 4-NBA and 4-NBT assembled on Ag by the irradiation of an argon ion laser at 514.5 nm. Much the same conclusion could be derived from X-ray photoelectron spectroscopy measurements as well as from a coupling reaction forming amide bonds [1]. The surface-induced photoreaction thereby allowed us to prepare patterned binary monolayers on Ag that would show different chemical reactivities. Using the binary monolayers as

* Corresponding author. Tel.: +82 2 8806651; fax: +82 2 8891568. E-mail address: [email protected] (K. Kim). 0924-2031/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2007.02.001

a lithographic template, we could also conduct site-specific chemical reactions [1]. Recently, we have also investigated the thermal and photochemical characteristics of silver 4-nitrobenzoate (Ag4NBA) [4]. Its X-ray diffraction pattern was composed of a series of peaks that could be indexed to (0 k 0) reflections of a layered structure. Upon heating the sample, structural changes took place particularly at two specific temperatures. The binding state of the carboxylate group changed from bridging to unidentate at 380 K. A second dramatic structural change occurred at 590 K, producing 4NBA-capped Ag nanoparticles. The latter transition temperature for Ag-4NBA is about 80 K higher than that for silver alkanecarboxylates [5,6], and such enhanced thermal stability of Ag-4NBA must be associated with the conjugation of the carboxylate group to the aromatic ring. On the other hand, Ag-4NBA was readily subjected to photolysis even by visible radiation [4]. Under irradiation by a He/Ne laser at 632.8-nm, 4-NBA-capped Ag nanoparticles were produced from Ag-4NBA. Furthermore, upon irradiation by an Ar+ laser at

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514.5-nm, nitro-to-amine group conversion took place consecutively, finally producing 4-aminobenzoate-capped Ag nanoparticles. Accordingly, the Raman spectrum of Ag-4NBA taken using the 514.5-nm line as the excitation source became identical to the SERS spectrum of 4-NBA adsorbed on Ag. In this work, we report the structure and thermal behavior, as well as the photo-response, of silver 4-(4-nitrophenyl)butyrate (Ag-4NPB) revealed by infrared and Raman spectroscopy. Ag4NPB is chosen as one of the model silver carboxylates possessing both the aliphatic and aromatic moieties. The first concern in this work is, of course, to confirm whether a layered structure is also appropriate to Ag-4NPB, as seems likely for the aromatic carboxylate, i.e. silver 4-nitrobenzoate, and the aliphatic carboxylate, i.e. silver stearate. The second concern is to determine, if it consists of a layered structure, how much the thermal characteristics of those hybrid materials are affected by the co-presence of aliphatic chains and aromatic rings. In any case, nitro group-terminated Ag nanoparticles would be produced by the thermal decomposition of Ag-4NPB. The third concern is to know whether a photoreaction involving the nitroto-amine conversion also occurs for Ag-4NPB. The photolysis of Ag-4NPB is certainly expected to produce Ag nanoparticles. Supposing that the nitro-to-amine conversion were to take place by the through-bond flow of the photoelectrons emitted by Ag nanoparticles, the reduction efficiency of Ag-4NPB might then be far less than that of Ag-4NBA due to the three additional methylene units in Ag-4NPB. The photo-induced nitro-to-amine conversion is observed, however, to occur more readily for Ag4NPB than Ag-4NBA, and this is attributed to the lower structural stability of Ag-4NPB as compared to Ag-4NBA, as well as to the through-space e-tunneling effect. 2. Experimental Silver nitrate (99+%), 4-(4-nitrophenyl)butyric acid (4NPB, 98%), and triethylamine (99+%) were purchased from Aldrich and used as received. Unless specified, other chemicals were reagent grade, and triply-distilled water (resistivity greater than 18.0 MV cm) was used throughout. To prepare silver 4-(4-nitrophenyl)butyrate (Ag-4NPB), a methanolic solution of AgNO3 (0.1 M, 50 mL) was added dropwise to an equimolar 1:1 mixture of 4-NPB and triethylamine in methanol (0.1 M, 50 mL). After 12 h of vigorous stirring, the resulting white solid was filtered, washed thoroughly with water and methanol, and finally dried in a vacuum. X-ray diffraction (XRD) patterns were obtained on a Bruker Model D5005 powder diffractometer for a 2u range of 38–408 at ˚) an angular resolution of 0.028 using Cu Ka (1.5419 A radiation. The samples were spread on anti-reflection glass slides to give uniform films. Scanning-electron microscope (SEM) images were measured using a JSM-6700F FE-SEM operated at 5.0 kV. Transmission electron microscope (TEM) images were acquired after thermal and radiation treatment on a JEM-2000EXII transmission electron microscope at 200 kV after placing a drop of chloroform/methanol (9:1) mixed solution on carbon-coated copper grids (200 mesh). Thermogravimetric analysis (TGA) data was obtained with a TA

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Instrument 2050 thermogravimetric analyzer in a nitrogen atmosphere at a heating rate of 10 K/min. Element analyses of Ag-4NPB were conducted using an EA1110 Elemental Analyzer. 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 diffuse reflectance infrared Fourier transform (DRIFT) spectra, a diffuse reflection attachment (Harrick Model DRA-2CO) designed to use 6:1, 908 off-axis ellipsoidal mirrors subtending 20% of the 4p-solid angle was fitted to the sampling compartment of the FT-IR spectrometer. The pure powdered sample was transferred to a 4mm 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 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 were 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 Happ-Genzel apodization function was used in Fourier transforming all the interferograms. The DRIFT spectra are reported as log(R/R0), where R and R0 are the reflectance of the sample and of the pure KBr, respectively. Raman spectra were obtained using a Renishaw Raman system model 2000 spectrometer equipped with an integral microscope (Olympus BH2-UMA). The 514.5-nm radiation from a 20 mW air-cooled Ar+ laser (Melles-Griot Model 351MA520) or the 632.8-nm radiation from a 17 mW air-cooled He/Ne laser (Spectra Physics Model 127) was used as the excitation source. Raman scattering was detected with 1808 geometry using a peltier-cooled ( 70 8C) CCD camera (400  600 pixels). A glass capillary (KIMAX-51) with an outer diameter of 1.5–1.8 mm was used as a sampling device, but when it was needed to reduce photolysis, samples made into pellets were spun at 3000 rpm during the Raman spectral measurement. The holographic grating (1800 grooves/mm) and the slit permitted a spectral resolution of 1 cm 1. The Raman band of a silicon wafer at 520 cm 1 was used to calibrate the spectrometer, and the accuracy of the spectral measurement was estimated to be better than 1 cm 1. The Raman spectrometer was interfaced to an IBM-compatible PC, and the spectral data were analyzed using Renishaw WiRE software, version 1.2, based on the GRAMS/32C suite program (Galactic Industries). 3. Results and discussion 3.1. Structure of silver 4-(4-nitrophenyl)butyrate (Ag4NPB) According to the element analyses of Ag-4NPB, the atomic compositions of N, C, H, O and Ag were determined to be

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4.32(0.2), 37.82(0.3), 3.16(0.1), 20.50(0.20) and 34.20(0.3)%, respectively. These measured values agree well with the theoretical values, i.e., 4.43, 38.00, 3.19, 20.25 and 34.13%, illustrating the successful synthesis of Ag-4NPB. The SEM image in Fig. 1(a) reveals that Ag-4NPB has a fibrous morphology. These fibrous samples were identified as possessing a layered structure from the XRD data shown in Fig. 1(b). The XRD pattern shows a well-developed progression of intense reflections. These reflections can be interpreted in

Fig. 1. (a) SEM image and (b) XRD pattern of Ag-4NPB. The interlayer spacings derived from different reflections are listed in the inset. (c) Schematic depiction of layered Ag-4NPB.

terms of three-dimensionally stacked silver carboxylate layers with an interlayer lattice dimension [5–10]. This implies that, as in silver alkanecarboxylates, each layer of Ag-4NPB is separated from the neighboring layer by twice the length of the nitrophenylbutyric moiety. In this sense, all intense reflections can be indexed as (0 k 0), and the reflections are assigned in Fig. 1(b). In the inset of Fig. 1(b), we list the interlayer spacings derived from different reflections. The ˚ . The layered averaged interlayer spacing is 19.69  0.89 A structure of Ag-4NPB is schematically drawn in Fig. 1(c). The XRD pattern of Ag-4NPB is quite comparable to that of silver stearate (Ag-STA) [5], as well as to that of silver 4nitrobenzoate (Ag-4NBA) [4]. The arrangement of silver and carboxylate may then be similar to one another in these compounds. As likely in Ag-STA, it is thus supposed that the silver ions in Ag-4NPB are bridged by the carboxylate in the form of dimers in an eight-membered ring and that the dimers are further bonded to each other by longer Ag–O bonds forming four-membered rings [7]. In these configuration, the reported ˚ , r(Ag– bond lengths in Ag-STA are r(Ag–Ag) = 2.90 A ˚ ˚ ˚. O) = 2.25 A, r(C–O) = 1.20 A, and r(–OOC–CH2–) = 1.54 A Similar bond lengths were reported for silver perfluorobutyrate ˚ , r(Ag– by Blakeslee and Hoard [11], viz r(Ag–Ag) = 2.90 A ˚ ˚ ˚. O) = 2.24 A, r(C–O) = 1.25 A, and r(–OOC–CF2–) = 1.54 A When these values are used, the thickness of the Ag–COO slab, ˚ . Considering that is, 2t1 in Fig. 1(c), is estimated to be 4.83 A that the averaged interlayer spacing derived from Fig. 1(b) was ˚ , the thickness of the nitrophenylpropane layer, that is, 19.69 A ˚. 2t2 in Fig. 1(c), should be 14.86 A By a similar calculation, the thickness of the nitrobenzene ˚ [4]. It is very layer in Ag-4NBA was determined to be 14.58 A surprising that the latter value is almost the same as that of the nitrophenylpropane layer in Ag-4NPB. The differernce is at ˚ . In principle, the presence of three additional best 0.3 A ˚ methylene groups can increase the 2t2 value by up to 7.52 A ˚ (1.253 A per CH2 group [12]) more than that in Ag-4NBA if the alkyl chains assume a perpendicular orientation with respect to the Ag–O slabs. The comparable 2t2 values in Ag-4NBA and Ag-4NPB, therefore, dictate that the nitrophenyl moiety must take a strongly tilted orientation in Ag-4NPB. The tilted orientation is surely attributed to the presence of three flexible methylene groups in Ag-4NPB. Due to the latter characteristics, the prepared Ag-4NPB might also be comprised of several different layered structures. The XRD peak at 9.88 in Fig. 1(b) would then be assgined to either the (0 2 0) reflection or (0 3 0) reflection, corresponding to the interlayer spacing of 18.1 and ˚ , respectively. 27.1 A Room temperature DRIFT spectral features indicate that the as-prepared Ag-4NPB is not contaminated with free acid (vide infra). In addition, referring to the empirical relationship between the frequency difference of the carboxylate stretching bands [13–15], that is, Dn (170 cm 1) = nas(COO )– ns(COO ) and the types of bonding, the binding state of the carboxylate group to silver in Ag-4NPB is presumed to be a bridging one. Previously, a bridging bond was also assumed for Ag-STA and Ag-4NBA, but the Dn value for Ag-4NBA was 190 cm 1, while the value for Ag-STA was at best 100 cm 1.

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The bonding characteristics of the carboxylate group to silver in Ag-4NPB thus seem to be more comparable to that in Ag4NBA than Ag-STA. This may indicate that the phenyl ring in Ag-4NPB must affect, in one way or the other, the binding state of the carboxylate group to silver. 3.2. Thermal behavior of silver 4-(4-nitrophenyl)butyrate One aim of this work was to determine how much the thermal characteristics of aromatic silver carboxylate would be affected by the incorporation of a short alkane chain between the benzene ring and the carboxylate group. In this regard, we have conducted a thermal gravimmetric analysis (TGA) along with a temperature-dependent DRIFT spectroscopy study. 3.2.1. TGA measurement Fig. 2 shows the TGA and its first derivative traces. It is evident that no mass loss occurs up to 470 K. This suggests that any transition, if occurring below 470 K, would be associated with the conformational change in Ag-4NPB. Above 470 K, two distinct mass loss features are observed in Fig. 2. Most of the mass loss occurs around 536 K, and then a small amount is subsequently lost around 740 K. The actual mass loss after 536 and 740 K amounts to 57% and 62%, respectively. The mass loss around 536 K is presumably associated with the formation of 4NPB-capped silver nanoparticles caused by the thermal decomposition of Ag-4NPB (vide infra), while the additional mass loss around 740 K is due to the desorption of 4NPB from the nanoparticles as well as the decomposition of residual free acid. If the organic moiety is completely lost, the mass loss should amount to 66% in total, theoretically leaving free silver above 800 K. We have to note that the first mass loss occurs for Ag-STA at 518 K [5], while a corresponding mass loss occurs for Ag4NBA at 593 K [4]. The transition temperature observed for Ag-4NPB, i.e. 536 K, is thus about 18 K higher than that for Ag-STA, but nearly 57 K lower than that for Ag-4NBA. Because the first mass loss event is associated with the formation of Ag nanoparticles, the lower temperature must be a result of the weaker Ag–COO bond in Ag-4NPB than Ag-

Fig. 2. TGA (full line) and its first derivative traces (dotted line) of Ag-4NPB.

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4NBA; for silver ions to be reduced to form nanoparticles, the Ag–COO bonds have to be weakened. The stronger Ag–COO bond in Ag-4NBA must be attributed to the conjugation effect of the carboxylate group to the aromatic ring as well as to the inter-ring p–p interaction. Upon incorporating alkyl chains between the carboxylate and the phenyl ring, the Ag–COO bond becomes weaker, but it is still stronger than that of AgSTA. The thermal stability of aromatic silver carboxylate thus seems to be lowered by the incorporation of a short alkane chain between the benzene ring and the carboxylate group. 3.2.2. Temperature-dependent drift spectral pattern of Ag4NPB Fig. 3(a) shows a series of DRIFT spectra obtained as a function of temperature for Ag-4NPB. 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 293 to 623 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. The initial spectral change occurring upon heat treatment in Fig. 3(a) is associated with the headgroup structure. The nas(COO ) peak at 1573 cm 1 is invariant up to 353 K and then becomes weak, disappearing above 443 K. Instead, a new peak develops at 1583 cm 1 at 363 K and is sustained up to 623 K. On the other hand, the ns(COO ) band gradually shifts from 1405 to 1371 cm 1 as the temperature is increased from room temperature to 623 K. Its intensity is nonetheless invariant up to 403 K and then starts to decrease above 413 K. We also notice that a band assignable to n(C O) develops at 1700 cm 1 around 413 K; the band shifts to 1677 cm 1 at 443 K, however. The NO2 group related peaks also exhibit temperature dependent spectral changes. Both the nas(NO2) and ns(NO2) bands at 1515 and 1353 cm 1, respectively, in Fig. 3(a) are invariant up to 433 K, but gradually weaken from 443 to 623 K. Concomitantly, the ring vibration at 1596 cm 1, that is, 8a, is also invariant up to 433 K and then gradually becomes weak. All these spectral features suggest that at least three conformational changes take place below 470 K, i.e. at 363, 413, and 443 K; in the TGA analyses, no mass loss occurs up to 470 K. It is also suggested that the 4-NPB moiety is sustained, although in part, up to 623 K without decomposition. Assigning the peak at 1583 cm 1 to nas(COO ), the binding state of the carboxylate group must then change from bridging to unidentate at 363 K. Considering the melting point of pure 4NPB (363–366 K) [16], the observed spectral change at 363 K can also be associated with the disordering of 4NPB moieties. On the other hand, free acid, although small, produced at 413 K must derive from the thermal decomposition of Ag-4NPB. The main structural change accompanying a substantial mass loss occurs at 443 K. The latter temperature is 20–30 K lower than that observed in the TGA analyses. This may reflect that a minute, microscopic structural change is detectable by infrared spectroscopy prior to identifying the macroscopic structural change by TGA.

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Fig. 3(b) shows a typical TEM image of the sample. The image reveals that silver nanoparticles are indeed formed by the thermal decomposition of Ag-4NPB. The sizes of the nanoparticles are quite uniform with an average diameter of 3.9  1.1 nm. The formation of silver nanoparticles could also be confirmed from the XRD pattern. As indicated in Fig. 4(a), all the XRD peaks could be attributed to the reflections of facecentered cubic Ag [17]. From the DRIFT spectrum shown in Fig. 4(b), we could also identify peaks due to ns(COO ), ns(NO2), and ns(CH2), nas(CH2), and ring 8a modes at 1408, 1342, 2854, 2920 and 1595 cm 1, respectively, supporting our contention that the silver nanoparticles were derivatized with 4NPB. 3.3. Photochemical behavior of Ag-4NPB Bokhonov et al. [18,19] reported that long-chain silver carboxylates were photochemically decomposed into silver nanoparticles under UV irradiation. Yonezawa et al. [20,21] showed that a silver film composed of silver clusters could be obtained from the photolysis of silver alginate under UV irradiation. These reports suggest that silver carboxylates can

Fig. 3. (a) DRIFT spectra of Ag-4NPB in the temperature range 293–623 K. The arrows denote the occurrence of distinct spectral changes. (b) TEM image of silver nanoparticles obtained from the thermal decomposition of Ag-4NPB at 573 K in N2 atmosphere.

Referring to the earlier reports on Ag-4NBA [4] and Ag-STA [5], a sample heated to 573 K is presumed to consist of 4NPBcapped silver nanoparticles. To verify the presumption, a minute amount of Ag-4NPB was preheated to 573 K for 5 min in N2 atmosphere, and then rinsed thoroughly with ethanol to remove free acids or possible impurities. The remaining solid was soluble in a mixed solvent of chloroform and methanol.

Fig. 4. (a) XRD pattern and (b) DRIFT spectrum taken after the thermal decomposition of Ag-4NPB salt at 573 K in N2 atmosphere. All the XRD peaks in (a) can be attributed to the reflections of face-centered cubic Ag and all the DRIFT peaks in (b) can be attributed to 4-NPB adsorbed on Ag nanoparticles.

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be transformed to nanosized silver particles upon irradiation by UV light. As described in the Introduction, we found recently that Ag nanoparticles are produced even by the irradiation of visible light onto Ag-4NBA [4]. Specifically, upon irradiation by a He/Ne laser at 632.8 nm, 4-NBA-capped Ag nanoparticles were produced from Ag-4NBA. Furthermore, upon irradiation by an Ar+ laser at 514.5 nm nitro-to-amine group conversion took place consecutively, finally producing 4-aminobenzoatecapped Ag nanoparticles. Based on these observations, we have examined the visible-light response of Ag-4NPB by Raman spectroscopy. Fig. 5(a) and (b) show the normal Raman (NR) spectra of pure 4-NPB and Ag-4NPB, respectively, taken using the 632.8nm line of a He-Ne laser as the excitation source; the NR spectral pattern in Fig. 5(b) is the same as that taken by spinning a pelletized Ag-4NPB at 3000 rpm in order to minimize any possible photoreaction. In the acid spectrum (Fig. 5(a)), the n(C O) and n(C–OH) bands derived from the H-bonded carboxylic group are clearly identified at 1648 and 1288 cm 1, respectively. The corresponding bands are completely absent in the Ag-4NPB spectrum. Instead, in Fig. 5(b), the ns(COO ) and d(COO ) bands derived from the carboxylate group are observed at 1408 and 861 cm 1, respectively, in agreement with the DRIFT peaks at 1405 and 862 cm 1. Fig. 6(a) shows the Raman spectra of Ag-4NPB taken under exposure of a He-Ne laser at 632.8 nm for 5, 455, 465, 470, 540, 600, 1200 and 1800 s; the power at the sampling position was 17 mW. As a control experiment, we confirmed separately that no spectral change occurs at all for the free acid, that is, 4-NPB, even after prolonged exposure to a He-Ne laser. In contrast, dramatic spectral changes occur for Ag-4NPB under laser irradiation. We can notice from Fig. 6(a) that the peak at 1596 cm 1 gradually intensifies, reaching a steady state after 540 s of laser irradiation (see also Fig. 6(b)). As indicated by an arrow in Fig. 6(a), the peak at 1342 cm 1 also increases up to 540 s, but after that it becomes weaker. On the other hand, a new peak at 1454 cm 1, completely absent initially, grows continuously upon increasing the laser exposure time. In fact,

Fig. 6. (a) A series of Raman spectra taken for Ag-4NPB under irradiation of a He/Ne laser at 632.8-nm line; the power was 17 mW at the sampling position. (b) The 8a mode intensity vs. the duration of laser irradiation in (a); the dashed line is drawn only as a guide to the eye. (c) TEM image of silver nanoparticles obtained from the photolysis of Ag-4NPB using the 632.8-nm line of a He/Ne laser.

Fig. 5. Normal Raman spectra of (a) pure 4-NPB and (b) Ag-4NPB taken using the 632.8-nm line of a He-Ne laser as the excitation source: the power at the sampling position was 17 mW, and the spectral acquisition time was 30 s.

the peaks at 1596 and 1342 cm 1 can be assigned to the ring 8a mode and the symmetric NO2 stretching vibration (ns(NO2)) of 4-NPB, respectively. Presumably, the growth of these two bands is associated with the formation of Ag nanoparticles that can induce the occurrence of SERS for the bands (vide infra). The eventual decrease in intensity of the ns(NO2) band suggests

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further that another photoreaction occurs later at the nitro group of 4-NPB adsorbed on Ag nanoparticles. The identity of the 1454 cm 1 peak is not certain, but it is attributed to the photoconversion of the nitro group to an amine group. To confirm the production of nanosized silver particles, a piece of pelletized Ag-4NPB that had been exposed to the 632.8-nm line of a 17 mW He/Ne for 10 min was immersed in a chloroform/methanol mixture, and then the mixture was shaken gently after which its decanted solution was dropped on a copper grid for the TEM measurement. As can be seen in Fig. 6(c), nanosized silver particles are clearly observed. The sizes are polydisperse and the shapes are quite irregular. Some particles are also seen to be present in an aggregated state. These data confirm that the spectra in Fig. 6(a) must be SERS spectra that have been derived from the photolysis of Ag-4NPB to give aggregated Ag nanoparticles followed by nitro-to-amine photoconversion on these particles (vide supra). We also have to mention that there is hardly any NR spectrum obtained for silver 4-aminophenylbutyrate (Ag4APB), as well as for neat 4-aminophenylbutyric acid. These samples became dark immediately after the irradiation of a He/ Ne laser. However, Raman peaks began to appear for Ag-4APB upon the laser irradiation for longer times. In the latter case, we separately confirmed the formation of Ag nanoparticles (TEM images not shown). The appearance of stronger peaks at 1454 and 1596 cm 1 was thus attributed to SERS induced by those nanoparticles. The peak at 1454 cm 1 in Fig. 6(a) may then be a characteristic SERS feature of aromatic amines, as surmised above. Although not shown here, photoreaction occurs faster using an Ar+ laser at 514.5 nm. As we have noticed above, the photoreaction of Ag-4NPB also occurs quite noticeably by the irradiation of a He/Ne laser at 632.8 nm. The initial photoreaction is the production of Ag nanoparticles, followed by the nitro-to-amine group conversion. According to Fig. 6(b), the initial generation of Ag nanoparticles is completed within 10 min. We should also mention that the photoreaction of Ag4NBA occurs far less effectively than that of Ag-4NPB. Under the same experimental condition (using a He/Ne laser), the generation of Ag nanoparticles in Ag-4NBA is completed after the exposure of laser light for 80 min. In addition, the nitro-toamine group conversion hardly takes place at all, even after a prolonged exposure to the 632.8-nm light. The photoreducing capability of 4-NPB to generate Ag nanoparticles has thus to be far greater than that of 4-NBA. Considering the local heating by the laser line, the inferior photochemical stability of Ag-4NPB is presumed at least in part to be associated with its inferior thermal stability when compared to Ag-4NBA. Nonetheless, it is still questioned why 4-NPB-capped Ag nanoparticles formed are immediately subjected to surface-induced photoreaction to form amine-group derivatized Ag nanoparticles. It is also questioned what the hydrogen source is. The detailed mechanism of the nitro-to-amine group conversion on Ag is a matter of conjecture, but photoelectrons are known to be readily ejected from Ag even by a visible laser [22]. It has been observed by Fedurco et al. [23] that the surface roughening of Ag results in a drastic increase of the

photocurrent for the wavelength close to the surface plasmon frequency. Then, it is well conceivable that for Ag-4NPB the condition of surface plasmon resonance could be satisfied even with the 632-nm excitation due to the presence of enough Ag nanoparticles to form nanoaggregates, while for Ag-4NBA only a limited number of Ag nanoparticles were available. On the other hand, it is known that when photoreaction occurs for aromatic nitro molecules in a solution phase, a chemical species from which hydrogen atom(s) can be abstracted is needed [24]. For a similar photoreaction to occur in air, water has been claimed to act as a hydrogen source [25]. The photoreaction of 4-nitrobenzenethiol on Ag was observed to occur several times faster in water or ethanol medium than in air [1]. The source of hydrogen atoms in the surface-induced photoreaction of 4-NPB on Ag to amine molecules would then be water or solvent molecules trapped inside the Ag-4NPB salt rather than 4-NPB itself, i.e. hydrogen atoms attached to phenyl ring and methylene groups. 4. Summary and conclusions We confirmed by XRD analysis that silver 4-nitrophenylbutyrate (Ag-4NPB) consisted of a layered structure. The temperature-dependent DRIFT and TGA measurements indicated that Ag-4NPB was subjected to three distinct transitions, as happened in the cases of silver stearate (Ag-STA) and silver 4nitrobenzoate (Ag-4NBA). In the first transition at 363 K, the binding state of carboxylate is converted from bridging to unidentate. In the second transition at 536 K, Ag-4NPB is decomposed to produce 4NPB-capped Ag nanoparticles. This second transition temperature for Ag-4NPB is about 18 K higher than that for Ag-STA, but nearly 57 K lower than that for Ag4NBA. The thermal stability of aromatic silver carboxylate is thus degraded by the incorporation of a short alkane chain between the benzene ring and the carboxylate group. In the third transition at 740 K, the organic moieties are totally lost from the Ag nanoparticles. Separately, SERS-active, nanosized silver particles or aggregates were produced from Ag-4NPB by visiblelaser irradiation at 514.5 nm and 632.8 nm. Silver nanoparticles can also be produced from Ag-4NBA using both laser light. However, the photoreaction occurs far more effectively for Ag4NPB than for Ag-4NBA. Under the same experimental conditions, Ag nanoparticles are generated within a shorter time for Ag-4NPB than for Ag-4NBA. In addition, even the nitroto-amine group conversion can take place for Ag-4NPB by the 632.8-nm light, whereas such a reaction is not feasible for Ag4NBA. The photochemical stability seems thus to be degraded by the incorporation of a short alkane chain between the benzene ring and the carboxylate group. The inferior photochemical stability of Ag-4NPB is presumed to be deeply associated with its inferior thermal stability when compared to Ag-4NBA. Acknowledgement This work was supported by the Ministry of Commerce, Industry and Energy of the Republic of Korea (Nano Project, M10213240001-02B1524-00210).

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