Synthesis and characterization of silver nanoparticles from (bis)alkylamine silver carboxylate precursors

Octanoic acid (99%) (OctAc), lauric acid (99%) (LAc), acetic acid (AcAc), 1-octylamine (99%) (OA), 1-dodecylamine (99%) (DDA), silver nitrate (99%), silver acetate (99%) (AgOAc), and sodium hydroxide (NaOH) were purchased from Aldrich or Alfa Aesar and used as received. Solvents (acetone, cyclohexane, and methanol) were distilled prior to use; water was triply distilled. Silver laureate (AgL) was prepared by a one-pot synthesis by adding aqueous solution of NaOH (0.99 equiv. in 2 mL) to the 14-mmol dispersion of lauric acid (2.8 g/20 mL) in hot water (80 °C). The molar amount of NaOH was 1% lower than that of the acid in order to avoid the reaction of excess alkali with AgNO₃. Then, an aqueous solution of AgNO₃ (0.56 g in 20 mL of water) was added to the vigorously stirred solution. The resulting silver carboxylate in the form of white precipitate was collected, washed with water (3×), and dried at 50 °C overnight to give white solid in quantitative yield. In a similar way, silver octanoate (AgOct) was prepared.

The adduct was synthesized using ligand insertion by reacting silver dodecanoate (AgL) with 1-dodecylamine in 1:2 stoichiometry. In AgL suspension (0.4 mmol, 0.1228 g) in dry cyclohexane, 0.8 mmol (0.148 g) of DDA dissolved in 5 mL of dry cyclohexane was added. The reaction mixture was heated at 30 °C for 15 min with stirring. On the completion of reaction, a white powder was isolated and dried under vacuum to give the product with quantitative yield. Yield: 181.94 mg (0.332 mmol, 83%), mp. 69.1 °C. Analysis calc. (%) for (C₁₂H₂₅NH₂)₂AgCO₂(C₁₁H₂₃): C, 63.79; H, 11.45; N, 4.13; found (%) C, 65.24; H, 13.24; N, 4.55.

Other adducts such as [AgOAc-2DDA], [AgOct-2OA], [LAc-2DDA], [LAc-2OA], [HOAc-2DDA], and [OctAc-2OA] were prepared in a similar way using the same stoichiometric ratio.

[AgOAc-2OA]. ¹H NMR 500 MHz (benzene-d6); δ: 0.92 (t 7.2 Hz, 3H, −CH₃), 1.2–1.36 (m, CH₂), 1.40 (s), 1.55 (q, 2H), 2.31 (s, 6H, −O₂CH₃), 2.74 (t, H), 3.52 (s, b). ¹³C NMR (benzene-d6); δ: 14.73, 23.50, 25.47, 27.85, 30.25, 30.47, 32.70, 34.85, 44.68, 177.36.

[AgOct-2OA]. ¹H NMR 500 MHz (benzene-d6); δ: 0.927 (m, 9H, −CH₃), 1.295 (m, 20H), 1.349 (m, 4H), 1.452 (m, 2H), 1.591 (m, 6H), 1.965 (q, 2H, −C_βH₂-CH₂–COOAg), 2.606 (t, 2H, −C_αH₂–COOAg), 2.707 (t, 4H, −C_αH₂–NH₂), 3.16 (s, 4H, −NH₂). ¹³C NMR 125.77 MHz (benzene-d6); δ: 14.72 (OA), 14.75 (AgOct), 23.49 (OA), 23.58 (AgOct), 27.79 (OA), 28.31 (AgOct), 30.24 (OA), 30.43 (OA), 30.45 (AgOct), 31.05 (AgOct), 32.71 (OA), 32.87 (AgOct), 34.79 (OA), 38.89 (AgOct), 44.56 (OA), 180.09 (AgOct).

[AcAc-2OA]. ¹H NMR 200 MHz (benzene-d6); δ: 0.902 (t, 6H, −CH₃), 1.220 (m, 20H), 1.506 (m, 4H), 2.215 (s, 3H, CH₃–COO⁻), 2.645 (t, 4H, −C_αH₂-NH₂), 5.585 (s, 4H, −NH₂); ¹³C NMR: 14.71, 23.43, 25.66 (CH₃–COO⁻), 27.55, 30.05, 30.12, 31.86, 32.58, 41.54, 178.88.

[OctAc-2OA]. ¹H NMR 500 MHz (benzene-d6); δ: 0.907 (m, 9H, −CH₃), 1.226 (m, 18H), 1.31 (m, 6H), 1.55 (m, 8H), 1.904 (q, 2H, −C_βH₂–CH₂–COOH), 2.51 (t, 2H, −C_αH₂–COOH), 2.64 (t, 4H, −C_αH₂–NH₂), 5.58 (s, 4H, −NH₂); ¹³C NMR: 14.72 (OA, OctAc), 23.44 (OA), 23.54 (OctAc), 27.57 (OA), 27.75 (OctAc), 30.06 (OA), 30.16 (OA), 30.36 (OctAc), 30.83 (OctAc), 31.94 (OA), 32.6 (OA), 32.78 (OctAc), 39.46 (OctAc), 41.59 (OA), 181.49 (OctAc).

In a typical synthesis of silver nanoparticles, (bis)amine silver adduct, either from AgOAc ([AgOAc-2DDA], 0.4 mmol, 0.2160 g) or AgL ([AgL-2DDA], 0.4 mmol, 0.2708 g), was introduced in a 10-mL round-bottom flask connected to a nitrogen vacuum line and purged with three vacuum/nitrogen cycles. The reactor was placed in a heating jacket equipped with a temperature controller and a magnetic stirrer. The silver carboxylate-(bis)amine complex was heated to 180 °C at a rate of 3 °C/min and kept at this temperature for 20 min. The reaction was cooled down to room temperature by immersing the flask in a cold water bath. The color of the melt changed from light yellow to dark brown as the reaction proceeded. The formed particles were washed out from the row solid product using MeOH and gentle bath sonication. The dispersion process was repeated three times. The final sticky precipitate was dried with a nitrogen flow and collected for further analysis. When dissolved into benzene or cyclohexane, it resulted in a stable dark yellow solution. The supernatant was colorless and had no free carboxylic acid or amine derived from started reactant, as will be discussed later. The main component of supernatant was n-dodecyldodecanamide (white solid). ¹H NMR 500 MHz (CDCl₃) δ: 5.49 (b s, 1H), 3.22 (q, J = 6.0 Hz, 2H), 2.136 (t, J = 7.69 Hz, 2H), 1.603 (q, J = 7.28 Hz, 2H), 1.47 (q, J = 7.0 Hz, 2H), 1.42–1.19 (m, 34H), 0.95–0.84 (t, J = 7.03 Hz, 6H). ¹³C NMR 125.77 MHz (CDCl₃) δ: 173.42, 39.82, 37.27, 32.24, 30.01, 29.98, 29.95, 29.92, 29.89, 29.84, 29.71, 29.67, 29.65, 27.26, 26.2, 23.01, 14.14.

As control sample, Ag NPs were prepared from AgL precursor (0.4 mmol, 0.1228 g) at 250 °C for 10 min (Abe et al. 1998).

FTIR spectra of the studied samples were obtained using a Jasco 6200 FT-IR spectrophotometer in transmission or attenuated total reflectance (ATR) mode by spreading a drop of the sol in cyclohexane on a substrate (Si plate or Ge ATR crystal) and letting it dry. The spectra were obtained by averaging 64 interferograms with resolution of 4 cm⁻¹. Extinction UV–Vis spectra of Ag NPs were collected with a resolution of 1 nm in a quartz cell (d = 2 mm) in cyclohexane using a HP UV 8543 diode array spectrometer.

Differential scanning calorimetry (DSC) and thermogravimetry (TG) analyses were recorded in the presence of nitrogen on a DSC 2920 (TA Instruments) and TGA 2950 (TA Instruments), respectively. Approximately 3–4 mg of the material was used, and the heating process was recorded at the rate of 10 °C/min. Microanalysis was performed with a Euro-Vector model 3018 instrument.

1D and 2D NMR spectra were recorded using either a Bruker Advance 200 or DRX 500 spectrometer equipped with a 5-mm triple-resonance inverse Z-gradient probe. All diffusion measurements were made by using the stimulated echo pulse sequence with bipolar gradient pulses. The 2D ROESY measurements were performed with a mixing time of 100 ms. Solution NMR samples were prepared in benzene-d6 and were done at 25 °C. The solid-state cross-polarization magic angle spinning NMR measurements were performed on a 400-MHz Bruker Avance III spectrometer and equipped with a MAS probe head and a 4-mm ZrO₂ rotor. The spectra were recorded with a proton 90° pulse length of 4 ms, contact time of 2 ms, repetition delay of 4 s, and 8 kHz MAS rotation rate.

Scanning electron microscope (SEM) images were obtained using a Hitachi S-4700 FE-SEM operating between 8 and 12 keV. The substrate for SEM was carbon tape (Agar Scientific). Samples were prepared by deposition of colloidal solution on a substrate.

Surface chemical characterization of NPs thin film was conducted using AXIS Ultra photoelectron spectrometer (XPS, Kratos Analytical Ltd.) equipped with monochromatic Al–K-α X-ray source (1486.6 eV). The power of anode was set at 150 W, and the hemispherical electron energy analyzer was operated at pass energy 20 eV for all high-resolution measurements. The sample area subjected to analysis was 300 × 700 μm in size.

Powder X-ray diffraction patterns were collected using a Panalytical X′PERT MPD diffractometer for a 2θ range of 5° to 120° at an angular resolution of 0.05° using Co-Kα (1.7890 Å) radiation.

The thermal behavior of the silver carboxylates in the complex with DDA has been explored using a combination of DSC, thermogravimetric analysis (TGA), and mass spectrometry (MS). During thermal scanning (Fig. 1), the samples undergo clear endothermic transformations, temperature position, and enthalpy of which can be related to the composition and the amount of the coordinated ligands. DSC traces of pure dodecylamine, dodecanoic acid, and silver acetate and laureate with the transition temperature T_m at 32.1, 45.6, 51.9, and 101.3/111.4 °C, respectively, were substantially modified. In the complexes with amine, these broad peaks show one component sharp endothermic transitions that occur between 60 and 80 °C (Fig. 1 and Table 1). Thus, the melting temperatures are evidently changed compared to those corresponding to pure ligands, especially for AgL which exhibits a series of phase changes at heating above 100 °C. Silver laureate at around 110 °C undergoes a first-phase transition connected with the formation of a new crystalline structure. DSC results suggest a hypothesis that all passivating ligands are coordinated to the silver ion, which in turn imposes the packing of the aliphatic chains and their cooperative arrangements. The well-defined phase transitions occurring upon heating cannot be assigned to pure melting processes of aliphatic chains, and structural transformation of these materials has to be taken into account. The details of this transition are currently being studied using temperature-dependent FTIR and XRD.

For both [AgOAc-2DDA] and [AgL-2DDA] complexes, thermogravimetric analysis was also performed under nitrogen atmosphere (Fig. 2) to examine the composition and thermal stability. From the first derivative curve, it follows that for (bis)amine silver laureate adduct, the weight loss is a three-step decomposition with decomposition maximum temperatures at T = 156, 197, and 224 °C. The weight loss between 80 and 180 °C can correspond to the loss of DDA molecules weakly coordinated in the complex with silver salt. Between 180 and 280 °C, the weight loss is assigned to the thermal decomposition of the silver laureate. This is supported by the molar ratio of Ag to laureate and dodecylamine estimated from the relative weight loss of inorganic to organic fragments (Ag/laureate/amine = 19.4:33.8:46.8). Pure silver laureate exhibits weight loss between 150 and 300 °C (inset in Fig. 2a). The decomposition events of (bis)amine silver acetate start at lower temperature due to the low decomposition temperature of pure silver acetate and run similarly through a three-step decomposition at around 145, 179, and 205 °C (Fig. 2b). Thus, silver acetate/(bis)amine complex could be well suited for material processing purposes.

Formation of soluble and quite stable complexes of AgOAc and AgL in nonpolar solvents in the presence of dodecylamine at stoichiometric ratio 1:2 was evidenced in FTIR studies (Fig. 3 and Table 1). Before discussing the spectra, it would be informative to analyze analogous complexes formed between LAc and DDA in the mixture of 1 equiv. of both LAc and DDA [LAc-DDA] (ammonium carboxylate salt) and in the mixture of 1 equiv. of LAc and 2 equiv. of DDA [LAc-2DDA] (Fig. 3c). The high-frequency region of infrared spectra for aliphatic LAc, DAA, [LAc⁻-DDA⁺], and DDA[LAc⁻-DDA⁺] mixture is characterized by two strong bands assigned to the antisymmetric ν_as(CH₂) and symmetric ν_s(CH₂) methylene C–H stretching modes at 2918 and 2850 cm⁻¹, respectively, and two weak bands at 2952 and 2871 cm⁻¹ assigned to the asymmetric ν_as(CH₃) and symmetric ν_s(CH₃) stretching modes of the methyl in the alkyl chain, respectively (Lee et al. 2002; Pelletier et al. 2003; Wu et al. 2004). These bands for LAc overlap stretching vibrations of hydroxyl ν(OH) ranging from 3400 to 2200 cm⁻¹ and originating from H-bonded dimers. The specified position of CH₂ stretching peaks indicates a high percentage of all-trans conformations. In DDA spectrum, amine asymmetrical (ν_as(NH₂) = 3330 cm⁻¹) and symmetrical (ν_s(NH₂) = 3253 cm⁻¹) N–H stretching modes are also observed. In turn, for ion-paired ammonium carboxylate [LAc⁻-DDA⁺] and its mixture with amine DDA[LAc⁻-DDA⁺], the observed broad band between 3200 and 2100 cm⁻¹ originates from the antisymmetric and symmetric N–H stretching of the resultant ammonium anion −NH₃ ⁺ of the ion-pair [LAc⁻-DDA⁺] formed in apolar cyclohexane with the well-developed combinational band at 2202 cm⁻¹. Moreover, characteristic N–H amine stretching vibrations fade for primary amine salts (Goodreid et al. 2014) though not completely for the [LAc-2DDA] mixture. The infrared low-frequency region of LAc contains bands from the C═O stretching vibration of the carboxylic group at 1699 cm⁻¹ in the H-bonded dimer, methylene bending mode δ(CH₂) at 1468 cm⁻¹, and methylene rocking ρ(CH₂) at 722 cm⁻¹. The spectrum also clearly shows several bands at 1299, 1410, and 937 cm⁻¹ due to C–O stretching vibration and in-plane and out-of plane bending of the C–OH group, respectively. All bands from the carboxylic acid groups disappear from the spectra of ion-pair [LAc⁻-DDA⁺] and DDA[LAc⁻-DDA⁺]. Instead, new bands at 1510 and 1407 cm⁻¹, ascribed to carboxylate asymmetric and symmetric vibrations, respectively, are observable. The amine bond deformations δ(NH₂) present in dodecylamine solid as composite band at 1606 cm⁻¹ are also observed as one intensive band at 1642 cm⁻¹ for both [LAc⁻- DDA⁺] and DDA[LAc⁻-DDA⁺] ion pair. This indicates that the ion pair formed in the solution is stable in the solid. Three bands observed for DDA[LAc⁻-DDA⁺] at 3333, 1561, and 1386 cm⁻¹, which are not presented in [LAc⁻-DDA⁺] ion pair, are ascribed to DDA in the mixture of amine, ammonium, and amine in interaction with the ion pair.

Similar interactions occur between silver alkanoate and dodecylamine. Figure 3 shows FTIR ATR spectra of AgOAc and AgL and the spectra of their complexes with DDA. The selected vibrational mode assignments and frequencies are collected in Table 1. When 2 equiv. of DDA is added to 1 equiv. of AgL or AgOAc in cyclohexane at room temperature, insoluble silver salt forms (bis)amine silver carboxylate adduct. These complexes are stable in the solid state as was checked by thermal characterization (vide supra), and their presence manifests in the changes of FTIR spectra of both silver carboxylate and amine groups (Fig. 3a, b). For example, peaks for carboxylate asymmetric stretching ν_as(COO⁻) and symmetric stretching ν_s(COO⁻) from carboxylate group in AgL appear at 1519 and 1422 cm⁻¹, respectively. They replace two bands typically presented in dimeric form of carboxylic acid due to the presence of carbonyl C═O at 1699 cm⁻¹ and hydroxyl C–OH group at 1299 cm⁻¹. It is worth mentioning here that analogous frequency shift due to electron delocalization is observed for [LAc⁻-DDA⁺] ion pair (Table 2). For silver n-alkanoate interacting with amine, the corresponding peaks were found to be respectively in high frequency (1551 cm⁻¹) and low frequency (1392 cm⁻¹) shifted from the original peak position. Amine stretching and bending vibrations are also low wavenumber shifted with an intensity increase due to complex formation as compared to bulk amine.

Interaction of amine with silver carboxylate can be considered taking into account changes in the oscillation energy of the carboxyl group (Nelson and Taylor 2014). The type of coordination of carboxylate to metal cations is a consequence of changes in the CO bond lengths and the OCO angle. Thus, it would be useful to discuss the position changes of carboxylate anion COO⁻ stretching frequencies in the presence of amine as the wavenumber separation between COO⁻ antisymmetric and symmetric vibrations Δν(=ν_as-ν_s), in a similar way, as was empirically established for the interaction of metal ions with the carboxylate group itself (Gericke and Hühnerfuss 1994; Nara et al. 1996). The frequency difference Δν allows the identification of the type of coordination of the carboxylate group to metal cations (Scheme 1a).

It was found that for bivalent metal cation coordination ability, Δν weakens in the order of monodentate, bridging/ionic bidentate to chelating bidentate form (Ellis et al. 2005; Ohe et al. 1999). The larger band shift for the complex indicates a larger unsymmetrical interaction between the metal ion and the carboxylate group. Monodentate mode of coordination, where only one carbonyl oxygen atom interacts with the metal, due to its lower symmetry than the free ion shows similarities with the spectra of the monomeric form of carboxylic acids. Thus, for monodentate coordination, the asymmetric COO⁻ stretching frequency increases while the symmetric COO⁻ stretch decreases relative to the values observed for free carboxylate ion (Ohe et al. 1999). For monovalent silver carboxylates, an opposite effect is observed with a significant lowering of the asymmetric COO⁻ stretching position and generation of Δν as low as 103 cm⁻¹. Based on recent powder XRD studies of the homologous series of metal n-alkanoates which revealed that the silver atoms form symmetric dimers in an eight-membered ring (Aret et al. 2006; Tolochko et al. 1998), we propose to correlate the observed stretching separation Δν for AgL with bridging bidentate coordination (Scheme 1). According to previous correlation (Nelson et al. 2015), the frequency difference below 110 cm⁻¹ would have suggested a dimeric monodentate coordination mode for silver carboxylates. The opposite effect was observed for (bis)amine-silver carboxylate complex. Due to coordination of two amine ligands to silver ion, carboxylate group is slightly separated and becomes more ionic similarly to highly electropositive sodium carboxylate (Δν = 135 cm⁻¹) (Nelson et al. 2015; Wulandari et al. 2015). The larger band shift for the complex with amine suggests chelating character of coordination (Scheme 1). The measured frequencies of the antisymmetric and symmetric COO⁻ stretches for the two silver salts studied, AgL and AgOAc, and for their complexes with amine and silver NPs are shown in Table 2.

The degree of interaction between carboxylic acid and amine measured with the magnitude of splitting Δν between asymmetric and symmetric stretching indicates that the bond order of both CO groups is similar to the silver carboxylate complex; thus, the ion pair can be modeled like two amine molecules interacting symmetrically with two oxygen atoms. Indeed, as can be seen from Table 2 for (bis)amine-carboxylic acid ion pair, the carboxylic stretching frequency at 1696 cm⁻¹ (C═O bond) shifts to 1510 cm⁻¹ while the C–O vibration is high frequency shifted from 1299 to 1407 cm⁻¹. Diminishing the separation frequency between the two bands from Δν = 397 cm⁻¹ for LAc to Δν = 103 cm⁻¹ for DDA[LAc⁻-DDA⁺], the ion pair can be ascribed to the bridging coordination character of the carboxylic group. As a matter of fact, it is not a strict electrostatic ion pair, rather H-bonded charge-assisted associate (Scheme 1).

All X-ray diffractograms of the studied complexes demonstrate a well-developed progression of intense reflections, which can be interpreted in terms of stacked layers. Similar diffraction patterns are observed for layered structures of silver carboxylates (Blanton et al. 2011; Lee et al. 2002) where the closely spaced peaks were attributed to the (00 l) indexes. The silver atoms in carboxylate salts 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, in turn, four-membered rings (Aret et al. 2006; Tolochko et al. 1998). The crystal morphology is led by the stacking of the carbon chains (Aret et al. 2006; Binnemans et al. 2004). The experimental d-interlayer spacing of the silver laureate obtained by determining the average position of the first six measured reflections, (002) to (007), from the small angle region was 34.659 Å. Similar X-ray powder diffractogram was recorded for the 1:2 complex of silver laureate and dodecylamine [AgL-2DDA]. The well-defined reflection peaks forming the d-spacing values are in the ratio 1:½:¹/₃...¹/₆. Such a diffraction pattern of reflection peaks, as discussed earlier, is consistent with a layered structure. The calculated value of the average d-spacing is 33.490 Å, slightly lower than that for the pure AgL material.

Figure 4 shows the XRD patterns of (bis)dodecylamine silver acetate complex. In the inset of Fig. 4, the interlayer distances derived from different reflections are listed. The index of the first diffraction peak assumed to be (003) (the (001) and (002) reflections are outside the detector range) and corresponds to a d-interlayer spacing of ~29.57 Å. The fully extended dodecylamine molecule can be estimated to be about 18 Å long. The values indicate that each layer of [AgOAc-2DDA] is separated from the neighboring layer by less than twice the length of the amine alkyl chain and suggests interpenetrated aliphatic chains in the bilayer.

Silver NPs were synthesized at 180 °C from [AgL-2DDA] adduct (2 mmol, without solvent) under nitrogen atmosphere. The powder was heated by immersing a Schlenk flask into silicon oil bath and allowed to react for 20 min, during which it gradually turned yellow–brown. The (bis)amine silver carboxylate complex slowly melted and finally afforded homogeneous dispersion containing silver nanoparticles. The reaction was quenched by immersing the flask in a cold water bath. The reaction products are composed of lauric acid/amine, n-dodecyldodecanamide, and carboxylate/amine-capped silver NPs. Crude nanoparticles were cleaned and separated by washing out twice with methanol (5 mL) and air dried. Purified dark blue waxy material was easily soluble in organic solvents. The silver fraction for the pure (bis)amine carboxylate sample exactly corresponds to that for nanoparticles obtained by the thermal decomposition under hydrogen pressure (Uznanski and Bryszewska 2010).

Figure 10 shows a SEM image of AgL NPs, the average diameter of which is 5.0 ± 0.2 nm. Their polydispersity, small size, and regular shape are reflected itself in the formation of 2D and 3D supramolecular structures and in almost symmetrical UV–Vis spectrum of plasmon resonance with a maximum at 413 nm (inset in Fig. 10), which is slightly blue shifted as compared to NPs obtained in the presence of tertiary amine (Yamamoto et al. 2006).

Products from thermal decomposition of [AgL-2DDA] complex are composed of three main components: lauric aid/amine ion pair, n-dodecyldodecanamide (Goodreid et al. 2014), and Ag NPs capped with laureate/amine ligands (Fig. 11). As confirmed by FTIR, laureate appears to be the dominant capping agent as it was not replaced by the long chain amine (Fig. 11c). The characteristic vibrational modes for the coordinative carboxylate group when bound to a metal surface are significantly modified as compared to [AgL-2DDA] precursor (Figs. 3b and 11c). The symmetric COO⁻ stretching mode is red-shifted and shows an intense band at 1397 cm⁻¹. A low-intensity band at 1528 cm⁻¹ is ascribed as a relict vibration of the asymmetrical COO⁻ stretch. Such IR band suppression is often observed at metallic surfaces, and results here from the parallel alignment of the induced dipole moment to the silver surface. Very weak bands at 1644, 3250, and 3303 cm⁻¹ are δ(NH₂) bond deformation and symmetric and asymmetric ν(NH₂) stretching, respectively, observed for the neat primary amine. The spectrum on Fig. 11b shows very sharp peaks at 3315, 1636, and 1546 cm⁻¹ characteristic for amide moiety which was formed during high-temperature synthesis of Ag NPs (Goodreid et al. 2014). Ag NPs from [AgOAc-DDA] are not stable and are not stabilized by acetate and/or amine ligands.

Figure 12 presents the typical diffraction peaks from Ag crystal structure. Observed by powder XRD diffraction peaks at 2θ = 38.65, 43.62, 64.70, and 77.20° can be indexed to the (111), (200), (220), and (311) planes of face-centered cubic lattice of metallic silver, respectively (Kashiwagi et al. 2006). The main source of the line broadening of the powder specimen XRD peaks arises from the small particle size and the presence of even smaller crystallites inside the nanoparticles. A specimen broadening can be further widened by inhomogeneous strain and/or defects in single crystalline domains. The mean size of crystallites calculated from the Williamson-Hall plot obtained from the diffraction pattern was 1.9 nm, i.e., smaller than that obtained from the SEM image, as expected.

XPS analysis enabled further insight into the composition of the surface of the Ag NPs. The XPS spectrum and the high-resolution XPS window of the core level atoms comprising the Ag nanoparticles capped with carboxylate/DDA are presented in Fig. 13. The surface scan spectra showed the presence of Ag, C, O, and N atoms according to their binding energies. The most prominent signal in the XPS spectrum is the Ag 3s consisting of two spin-orbit components at 368.8 (Ag3d_5/2) and 374.8 (Ag3d_3/2) eV and separated by 6.0 eV (Fig. 13b). Moreover, the deconvolution of Ag (3d) doublet revealed asymmetric peak shape. These two characteristics indicate that the Ag exists in the metallic form. Energy loss features at 371.9 and 378.0 eV are observed at the higher binding energy side of each spin-orbit component for Ag3d metal. These results are in good agreement with the XRD characterization. XPS high-resolution scan for the C1s core level (Fig. 13c) showed the presence of four different peaks. The main peak centered at 285.56 eV was attributed to C–C (sp3), while the peaks at 286.3, 288.0, and 288.8 eV were attributed to C–O, C═O, and C–O–Ag, respectively, in the Ag NPs structure. The doublet for O1s at 531.97 and 530.33 eV was assigned to metal carbonates and AgO species, respectively. Therefore, XPS studies supported results obtained from IR studies on coordination mode of carboxylate moiety.