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Letter

Rapid Microwave-Assisted Synthesis of Silver Nanoparticles in a Halide-Free Deep Eutectic Solvent Laxmi Adhikari, Nathaniel E. Larm, Nakara Bhawawet, and Gary A. Baker ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00050 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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ACS Sustainable Chemistry & Engineering

Rapid Microwave-Assisted Synthesis of Silver Nanoparticles in a Halide-Free Deep Eutectic Solvent

Laxmi Adhikari,† Nathaniel E. Larm,† Nakara Bhawawet,† and Gary A. Baker*,† †

Department of Chemistry, University of Missouri-Columbia, 601 S. College Avenue, Columbia MO 65211 *Email: [email protected]

ABSTRACT: Organosoluble silver nanoparticles (AgNPs) have been synthesized for the first time in a task-specific, halide-free deep eutectic solvent (DES) using a simple and convenient wet chemical reduction route involving microwave (MW) heating with oleylamine (OAm) acting as a surfactant and reducing agent. Nanoparticle formation is extremely rapid and occurs within 30 s of microwave heating at 100 °C. The effects of various reaction parameters (e.g., synthesis temperature, MW irradiation time, maximal MW power, water content of the medium) on the size and uniformity of the prepared AgNPs have been elucidated in this study. The produced colloidal AgNPs were characterized using UV-Vis spectroscopy and transmission electron microscopy (TEM), with the aim of identifying reaction parameters simultaneously achieving optimal particle yield and colloid uniformity. This work illustrates how the versatile nature of DESs can be exploited to create unconventional DESs designed for nanoscale tasks for which conventional (e.g., halide-containing) DESs may be poorly suited, further expanding the repertoire of these solvents as sustainable media for various nano-applications.

KEYWORDS: Deep eutectic solvent, Silver nanoparticles, Microwave-assisted, Oleylamine, Ionic liquid

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INTRODUCTION Deep eutectic solvents (DESs) currently attract immense academic and industrial interest as cost-effective and environmentally-benign media.1-3 A DES is composed of two or more components that self-associate through a complex, dynamical, and highly correlated hydrogenbonding network to produce a mixture with a melting point lower than its individual parent components.4 Typically prepared by mixing a quaternary ammonium salt (choline chloride is the most popular choice in this regard) with a Lewis or Brønsted acid (i.e., a hydrogen bond donor, HBD) in certain molar ratios, DESs share many attributes with ionic liquids (ILs), with some added benefits such as ease of synthesis, compositional versatility, and the use of cheap and biodegradable commodity chemicals as components.1-3, 5 Several properties of DESs (e.g., high polarities and charge densities, useful electrochemical potential windows, low cost, complexometric/coordinating ability) make them highly relevant to applications involving metal species, including the synthesis of tailored metal nanostructures, metal ion extraction, and metal film electrodeposition.6-11 In particular, the growth of nanocrystals bounded by high-index facets (i.e., associated with high surface energy and high catalytic activity) as a result of isotropic growth by employing a DES as the reaction medium and/or shape-controlling agent has gained considerable traction. For example, using reline (molar ratio of choline chloride:urea = 1:2) as the DES medium, Sun et al. devised an electrochemically shape-controlled synthesis of monodispersed concave tetrahexahedral (THH) Pt nanocrystals mainly enclosed by {910} and {10, 1, 0} facets, together with some other vicinal high-index facets; these THH Pt nanocrystals possessed catalytic activity and stability superior to commercial Pt black toward ethanol electrooxidation.12-13 Various single-crystalline ZnO nanostructures (e.g., twinned-cones, nanorods) have also been synthesized in reline following an 2 ACS Paragon Plus Environment

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anti-solvent procedure developed by Dong et al.14 Similarly, polycrystalline gold nanostructures of various morphologies (e.g., Au nanowire networks with abundant {311} facets, penta-twinned star-shaped Au nanoparticles bounded with {331} and vicinal high-index facets) showing enhanced catalytic activities have been prepared in the chloride-containing DESs reline and ethaline.3, 15-16 Although DESs are being explored for synthesizing a range of noble metal nanostructures, their usefulness for silver nanoparticle (AgNP) synthesis in DESs has been limited so far to studies involving electrodeposition to form particles, nanosheets, or porous nanofilms on metallic or carbon supports (i.e., deposited and not dispersed).17-23 While various wet chemical routes to AgNPs are known in ILs,24-25 the ubiquity of halide incorporation in typical DESs results in silver halide precipitation (Ksp ≈ 10–10 for AgCl), severely limiting the prospects for well-controlled AgNP synthesis in DESs.26-27 While it is indeed true that the most studied DESs are composed of choline chloride paired with various HBDs such as glycerol or urea, the belief that every DES reported to date contains a halide ion26 is erroneous. In fact, Zhao and co-workers have already reported investigations of halide-free DESs based on choline acetate in Candida antarctica lipase B-catalyzed biodiesel synthesis,28 cross-linked protease (i.e., subtilisin and α-chymotrypsin) transesterification activation,29 and lignocellulose pretreatment toward rapid saccharification.30 Oseguera-Galindo et al. recently reported on the “top-down” synthesis of small (~4.5 nm) AgNPs based on laser ablation of a silver foil target immersed in unstirred reline using a Nd:YAG laser (1064 nm, 7 ns pulses, 113 mJ/pulse).31 The AgNPs particles formed were reportedly stable in reline for several days, but eventually precipitated and formed dendrite-shaped particles that evoke the phenomenon of cold welding.32 Although this earlier work holds fundamental interest, laser ablation is impractical for preparing useful 3 ACS Paragon Plus Environment


quantities of nanomaterials and appears to offer limited control over colloid uniformity (this is evident from the reported UV-Vis spectrum which shows broad absorbance from 500–800 nm in addition to the expected ~400 nm surface plasmon resonance feature expected for AgNPs), making a convenient “bottom-up” (i.e., wet chemical) route to long-term stable AgNPs necessary. In this Letter, we demonstrate that oleylamine-ligated AgNPs can be conveniently synthesized by the microwave (MW)-assisted reaction of silver nitrate with oleylamine in a taskspecific DES. The OAm-stabilized AgNPs are quite uniform in size and can be stably dispersed in organic solvents such as toluene. This successful outcome relies on the use of a halide-free DES comprising a 1:2 molar ratio of choline nitrate to glycerol, hereafter referred to as DES1 (see Figure S1 of the Supporting Information). Notably, this work represents the first example of the wet chemical synthesis of colloidally-stable AgNPs in a DES, highlighting the designer (problem-solving) aspect of this rapidly-developing class of solvent. Oleylamine (OAm) has previously been shown to be a versatile and sustainable reagent for the formation of various metallic, metal oxide, or semiconductor nanostructures,33-35 but has not yet been used for nanoparticle synthesis within a DES.

RESULTS AND DISCUSSION The effective wet chemical synthesis of silver colloids in a DES first requires the preparation of a halide-free DES. We initially considered a DES comprising a 1:2 molar ratio of choline nitrate (ChNO3) to glycerol (i.e., DES1 in Figure S1). Three different routes were pursued to prepare the necessary ChNO3, as summarized in the Supporting Information. The first route (Route 1; Figure S2) entailed the aqueous reaction between equimolar silver nitrate (AgNO3) and choline chloride (ChCl). Following reaction, filtration was used to remove 4 ACS Paragon Plus Environment

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precipitated AgCl(s) and the solution then subjected to unfiltered output from a 450 W Xe arc lamp in an attempt to photoreduce any Ag+ remaining in solution to Ag0. The resulting brown solution was centrifuged (8500 rpm, 15 min) to sediment out AgCl/Ag species, yielding a clear supernatant containing the desired ChNO3 which was subsequently recovered as a white powder by rotary evaporation. The ChNO3 made via Route 1 was combined with glycerol in a 1:2 molar ratio to produce a room temperature liquid DES. Upon addition of oleylamine (OAm) to this DES, however, a yellow solution immediately resulted, indicating the presence of silver residues in the ChNO3 prepared by this route. Subsequent tests confirmed the presence of silver in the millimolar regime in ChNO3 prepared by Route 1. Accordingly, given the unreliability in the residual silver content of the ChNO3, we discontinued this strategy, seeking a silver-free route that would not compromise our ability to subsequently perform nanosynthesis in a reproducible manner. This end result should also serve as a caution to others using metathesis of organic halides with silver salts of the desired anion (e.g., AgBF4, AgTfO) to generate ILs. Our original attempt at a silver-free route to ChNO3 (Route 2; Figure S2) involved neutralization of a commercial solution of choline hydroxide (ChOH; Sigma Aldrich, St. Louis, MO) using nitric acid. Unfortunately, nuclear magnetic resonance (1H NMR) analysis of the purchased ChOH solution revealed it to be partially degraded (6–7%) as received from the vendor, blocking this pathway as well. Consequently, we decided to freshly prepare ChOH by ion exchange, rather than rely on a commercial source. In this tactic, aqueous choline chloride (ChCl) was treated with basic hydroxide anion-exchange resin (Amberlite® IRA-402 OH), followed by neutralization with nitric acid to produce the desired silver-free and non-degraded ChNO3 (Route 3; Figure S2). The clean preparation of ChNO3 via Route 3 was verified by 1H NMR analysis (Figure S3). Using this ChNO3, we now prepared DES1 using a 1:2 molar ratio of 5 ACS Paragon Plus Environment


ChNO3 to glycerol. The 1H NMR and thermogravimetric analysis (TGA) of the resulting DES1 are provided in Figures S4 and S5, respectively, of the Supporting Information. For comparison, the conventional halide-containing DES glyceline, comprising a 1:2 ChCl:glycerol mixture (denoted DES2 herein), was also prepared alongside as a control medium with 1H NMR analysis results for DES2 given in Figure S6. With a halide-free DES safely in hand, we were now in a position to test the suitability of DES1 as a medium for the synthesis of AgNPs using the MW-assisted reaction of silver nitrate with oleylamine. Detailed experimental procedures can be found in the Supporting Information. Our preliminary experiments using DES1 demonstrated that MW heating to 100 °C resulted in rapid AgNP formation in a matter of seconds. The resulting AgNPs can be readily dispersed in nonpolar solvents such as toluene and hexane, consistent with OAm acting as a surface capping ligand. It is well known that MW irradiation provides rapid and homogeneous “inside-out” heating, favoring uniform particle nucleation and growth, leading to improved synthetic control and product crystallinity.36-37 Encouraged by the initial results, we sought to examine the influence of several relevant factors such as MW reaction temperature (TMW), maximal MW power (Pmax), and reaction time (hold time). The Pmax refers to a user-determined maximum amount of microwave power that can be applied when heating to the set TMW. We also note that the reaction time refers to the duration of time the reaction is actually under MW irradiation at the programmed temperature and thus excludes the ramping time (during which the MW reactor magnetron is also activated) required to achieve the target temperature. UV-Vis extinction profiles measured for toluene dispersions of AgNPs prepared in DES1 for 30 s reactions showed a clear dependence upon the temperature range reached in MW experiments (Figure 1A). 6 ACS Paragon Plus Environment

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Although MW-assisted AgNP formation by OAm in DES1 occurs within 30 s for MW heating to 100 °C, UV-Vis profiles reveal that this may not be the optimal reaction temperature. AgNP synthesis conducted at a higher temperature of 150 °C is associated with a more intense localized surface plasmon resonance (LSPR) band, whereas even higher reaction temperatures of 200 and 230 °C yield noticeably narrower LSPR profiles. The corresponding full width at half maximum (FWHM) values determined for the measured LSPR bands are compiled within Table 1. Transmission electron microscopy (TEM) analysis (Figure 2) indicates that reaction temperatures of 100, 150 200, and 230 °C yield AgNPs with average particle sizes of 7.6 ± 2.8, 10.3 ± 4.9, 11.4 ± 3.8, and 11.5 ± 3.7 nm, respectively. Interestingly, the 150 °C sample associated with the most intense LSPR band also shows the poorest AgNP size uniformity. Meanwhile, 30 s MW reactions conducted at a reaction temperature of 200 or 230 °C produced AgNPs with slightly larger but more monodispersed particles, consistent with the smaller FWHM values observed. Control experiments carried out using the chloride-containing DES glyceline (DES2 = 1:2 ChCl:glycerol) or neat OAm under mild MW conditions (30 s, 100 °C) gave no evidence for AgNP formation. In fact, extensive MW heating periods (5 min for DES2 and 30 min for neat OAm, respectively) at temperatures up to 200 °C were required for apparent AgNP formation to occur within these control media and, even then, the UV-Vis profiles showed significantly lower extinction values and/or broader LSPR profiles (Figure S7). TEM analysis of the AgNPs produced in DES2 after a 5 min MW reaction at 200 °C revealed a population of heterogeneous particles 11.4 ± 6.1 nm in size (Figure S8) as well as much larger particles, some with diameters in excess of 100 nm (see insert of Figure S8A). Attempts to prepare AgNPs using the MWassisted reaction of AgNO3 with OAm in DES2 produced significant amounts of a white precipitate which was rinsed with aqueous methanol, dried, and then recovered for powder X-ray 7 ACS Paragon Plus Environment


diffraction (PXRD) analysis. PXRD analysis was performed on a Scintag X2 diffractometer equipped with a monochromatic Cu-kα source (λ = 1.5406 Å) operated at 45 kV and 40 mA over a scanning range of 0 to 70° 2ϴ. As shown in Figure S9, PXRD peaks at 27.9°, 32.2°, 46.2°, 54.8°, 57.5°, and 67.5° can be clearly assigned to (111), (200), (220), (311), (222), and (400) planes of the cubic phase of AgCl (JCPDS No. 31-1238). The remaining peaks at 38.1°, 44.4° and 64.6° we attribute to (111), (200), and (220) for cubic Ag (JCPDS No. 65-2871). Thus, the PXRD pattern clearly indicates that a predominant cubic phase of AgCl coexists with a minor amount of metallic Ag. The failure to reliably generate AgNPs in the chloride-containing DES2 is likely associated with the inability of OAm to reduce the resulting AgCl. For example, whereas the electropositive reduction potential for Ag+ in water is relatively large (for Ag+ + e– ↔ Ag(s), E0 = +0.799 V) reduction of AgCl(s) is much less favorable (E0 = +0.222 for AgCl(s) + e– ↔ Ag(s) + Cl–). TEM analysis of the AgNPs produced during the course of a 30 min MW-assisted reaction at 150 °C in neat OAm reveals a predominant fraction of relatively small but inconsistently-sized particles with a mean size of 6.2 ± 4.5 nm, as well as some larger AgNPs in the 10 to 25 nm range (Figure S10). Overall, AgNP synthesis in the halide-free DES is shown to be far more expeditious and less energy intensive while producing more uniform nanoparticles compared with the use of conventional choline chloride-derived DESs or neat oleylamine as reaction solvent. In continuation of our efforts to elucidate the effects of MW reaction parameters on AgNP synthesis, we studied the effects of the programmed maximum MW power (Pmax) for a fixed reaction temperature of 100 °C with a hold time of 30 s (Table 2, Figure S11). We discovered that a Pmax of 20 W was insufficient for AgNP production. At a Pmax limit of 50 W or 8 ACS Paragon Plus Environment

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100 W, rapid AgNP formation was evident, with an increase to 200 W yielding even more efficient reaction on the basis of the maximum extinction value obtained. A further increase in Pmax to 250 W, however, proved detrimental to both particle yield and uniformity, ostensibly due to the rapid introduction of excessive energy into the system. Indeed, the use of a 250 W limit is associated with the formation of fused, asymmetric, or agglomerated AgNPs as shown in Figure S12 (panel C). We note the appearance of a secondary applied power spike in the MW powertime profile (Figure S11, panel B) for Pmax settings of 100, 200 or 250 W, although the consequences of this ancillary power delivery step (which is related to the feedback control program of the microwave reactor) for particle nucleation and growth remains to be elucidated. We next assessed the effects of MW reaction time on AgNP synthesis in DES1 at a reaction temperature of 200 °C, setting the Pmax limit to 200 W. Both UV-Vis (Figure 3) and TEM (Figure 4) analysis reveal that longer heating periods are necessary for the formation of uniform particles. The FWHM for the LSPR band of the synthesized AgNPs decreases monotonically from 123 nm to 80 nm as the reaction time increases from 30 to 240 s (see Table 1). TEM analysis indicates that a brief reaction period of 30 s yields AgNPs with an average size of 10.1 ± 4.7 nm. The particle size uniformity improved to some extent to 11.3 ± 3.2 nm for a 240 s MW reaction time. Nadagouda and Varma similarly observed that, in order to obtain uniform Ag nanorods, it was necessary to carry out MW reactions for an extended period of time.38 We next examined the stability of as-prepared, concentrated ([Ag] ≈ 5.9 mM) toluene dispersions of the AgNPs formed in DES1 during ambient storage within a laboratory drawer (i.e., shielded from room light but with no additional precautions). UV-Vis analysis of AgNPs formed in rigorously dry DES1 (333 ppm water as determined by coulometric Karl Fischer 9 ACS Paragon Plus Environment


titration) at 200 °C for a 120 s reaction time demonstrate excellent colloidal stability for at least a month (Figure 5). Similarly, we also evaluated a 40-day-old sample of AgNPs prepared in comparatively wet DES1 (3600 ppm water). As Figure 6 illustrates, the AgNPs aged for 40 days in toluene dispersion show a mean particle size of 11.1 ± 3.4 nm, essentially identical to the value of 11.3 ± 3.2 nm measured for the freshly made AgNPs (Figure 4, panels G and H). Additional studies substatiate the significant influence of the water content in the DES medium on the uniformity of the resulting AgNPs (Figure 7). For AgNPs generated by MW reaction for a period of 120 s at 200 °C, striking differences were observed for moderately wet (3600 ppm water) versus rigorously dry (333 ppm water) DES1. Not only is the resulting amplitude of the LSPR band nearly 3-fold higher for AgNPs made in a DES containing 333 ppm water, the LSPR spectrum is also blue shifted by 11 nm and the spectral FWHM is roughly twofold smaller than the corresponding sample made in a DES containing 3600 ppm water (Table 3). The UV-Vis screening results are in accord with the respective TEM results. The AgNPs produced in extremely dry DES1 (333 ppm water) have a fairly narrow size distribution (9.1 ± 2.9 nm; Figure 7C,D) and are notably smaller and more uniform than AgNPs made using identical MW parameters in the same DES containing 3600 ppm water (11.6 ± 4.3 nm; Figure 4E,F). In this MW-assisted reaction, oleyamine (an unsaturated fatty amine) serves a multifunctional role, acting as both the reducing agent and a capping ligand. The resulting AgNPs are soluble and stable in relatively nonpolar solvents like hexane and toluene due to the long unsaturated alkyl chain of OAm. As shown in Figure S13, 1H NMR studies performed on AgNPs dispersed in deuterated chloroform signify the presence of intact OAm. The notion that OAm acts as the capping ligand is further substatiated by the fact that the water level in the 10 ACS Paragon Plus Environment

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toluene (dispersing phase) exerts significant control over the resulting colloidal stability of dilute samples. Indeed, dilute AgNP dispersions (58-fold diluted relative to as-prepared samples) stored in “dry” toluene (60 ppm water) show excellent colloidal stability over 24 h, while dilutions made with “wet” toluene (403 ppm water) produce marked aggregation within 12 h, as evidenced by significant spectral broadening and diminished absorbance (Figure S14). Finally, we point to the generality of halide-free DESs for nanoparticle synthesis. This is demonstrated by the successful synthesis of AgNPs in 1:2 molar ratio of choline acetate to glycerol (DES3; Figure S15). In future work, the designer nature of DESs will be exploited to develop alternative non-halide media for nanoparticle preparation by wet chemistry.

CONCLUSIONS In summary, we describe for the first time a simple, eco-friendly, and convenient wet chemical route for the rapid synthesis of organosoluble AgNPs within a halide-free DES using efficient MW heating. This approach employs a nitrate-based DES to overcome the innate incompatability of halide-based DESs (e.g., popular choline chloride-derived DESs such as glyceline) with the colloidal synthesis of dispersed AgNPs. Experimental parameters (i.e., reaction temperature, heating duration, maximal power setting, water content) exert significant control over particle yield and uniformity. Our findings also indicate that the water content in both the DES reaction medium as well as the dispersion medium (i.e., toluene) greatly impact AgNP growth and storage stability. Overall, this strategy highlights how the intrinsic designer nature of DESs can be used to advantage in nanosynthesis, providing incentive for consideration of other, less conventional DESs, particularly other halide-free examples such as those based on the acetate, triflate, trifluoroactetate, or methylsulfate anions. We are currently expanding this synthetic approach to target anisotropic growth toward shape-controlled AgNP morphologies. 11 ACS Paragon Plus Environment


ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.???. Experimental details, 1H NMR spectra of choline nitrate, 1:2 choline nitrate:glycerol, and 1:2 choline chloride:glycerol, TGA profiles of choline nitrate, glycerol, and 1:2 choline nitrate:glycerol, UV-Vis spectra of AgNPs obtained in neat OAm, DES2, and DES3, UVVis spectra showing the relative stabilities of AgNPs in dry versus wet toluene, supplemental TEM images and their corresponding particle size histograms (PDF).

■ AUTHOR INFORMATION Corresponding Author * Gary A. Baker. E-mail: [email protected].

ORCID Laxmi Adhikari: 0000-0001-9023-8191 Nathaniel E. Larm: 0000-0002-3369-4980 Nakara Bhawawet: 0000-0001-7381-4847 Gary A. Baker: 0000-0002-3052-7730

Notes The authors declare no competing financial interest.


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Table 1. Plasmonic Parameters Determined for AgNPs Prepared in DES1 under Various Microwave Reaction Conditionsa Water content (ppm)

TMW (°C)

Reaction time (s)b

FWHM (nm)

LSPR Max (nm)

Extinctionc

4967 100 30 149 427 0.435 4967 150 30 131 423 0.539 4967 200 30 115 423 0.414 4967 230 30 105 421 0.372 3600 200 30 123 422 0.562 3600 200 60 101 422 0.564 3600 200 120 92 420 0.686 3600 200 240 80 421 0.513 a Pmax was set to 200 W. b This denotes the time held at the specified temperature (TMW) and will be shorter than the actual time the magnetron is turned on by an amount equal to the ramp time. c Maximum extinction (optical density) measured for AgNPs in toluene for a sample 58-fold diluted relative to as-prepared AgNPs (i.e., [Ag] ≈ 0.10 mM).

Table 2. Plasmonic Parameters Determined for AgNPs Prepared in DES1 for a 30 s Reaction Time at Different Maximum Microwave Power Limits Reaction FWHM LSPR max Extinctiona time (s) (nm) (nm) 50 100 30 136 425 0.296 100 100 30 136 425 0.305 200 100 30 138 425 0.595 250 100 30 244 429 0.107 a Determined for a sample 58-fold diluted in toluene relative to as-prepared AgNPs (i.e., [Ag] ≈ 0.10 mM). Pmax (W)

TMW (°C)

Table 3. Plasmonic Parameters Determined for AgNPs Prepared in DES1 Containing Different Water Contentsa Water content TMW (°C) Reaction FWHM LSPR max Extinctionb (ppm) in DES1 time (s) (nm) (nm) 3600 200 120 92 420 0.686 2700 200 30 101 419 0.614 333 200 120 44 409 1.951 a Pmax was limited to 200 W. b Determined for a sample 58-fold diluted in toluene relative to asprepared AgNPs (i.e., [Ag] ≈ 0.10 mM).


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Figure 1. (A) UV-Vis spectra of toluene-dispersed AgNPs formed by the MW-assisted reduction of AgNO3 in DES1 (1:2 choline nitrate:glycerol) using OAm as a dual reducing/capping agent. The DES1 had a water content of 4967 ppm, based on Karl Fischer titration. Samples were diluted 58-fold in toluene in relation to the as-made AgNPs ([Ag] ≈ 0.10 mM for all samples shown). Conditions are identical (reaction time of 30 s, Pmax = 200 W) except for the heating temperature indicated. (B) Power-time profiles for the MW reactions. (C) Corresponding normalized UV-Vis spectra. (D) Temperature-time profiles measured for the MW reactions. The legend in (A) corresponds to all plots.



Figure 2. TEM micrographs (A, C, E, G) of representative AgNPs formed in DES1 at 100, 150, 200, and 230 °C, respectively, alongside their respective particle size distribution histograms (B, D, F, H). These samples correspond to those in Figure 1 (30 s reaction time; Pmax = 200 W; DES1 water content: 4967 ppm).


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Figure 3. (A) UV-Vis spectra of toluene-dispersed AgNPs formed by the MW-assisted reduction of AgNO3 in DES1 (1:2 choline nitrate:glycerol) using OAm as a dual reducing/capping agent. The DES1 had a water content of 3600 ppm, based on Karl Fischer titration. Conditions are identical (200 °C, Pmax = 200 W), except for the MW reaction time indicated. Samples were dispersed in toluene and diluted 58-fold relative to as-made AgNPs. (B) Power-time profile for the MW reaction. (C) The corresponding normalized UV-Vis spectra. (D) Temperature-time profiles for the MW reactions. The legend in (A) applies to all panels.



Figure 4. TEM micrographs (A, C, E, G) of representative AgNPs synthesized in DES1 at 200 °C for 30, 60, 120, and 240 s reaction times, respectively, alongside their respective particle size distribution histograms (B, D, F, H). These samples correspond to those in Figure 3 (200 °C, Pmax = 200 W; DES1 water content: 3600 ppm). The mean AgNP size resulting from a 30 s MW reaction is slightly smaller than for those for more prolonged heating, however, particle morphology appears to become a bit more monodisperse as reaction time increases. 20 ACS Paragon Plus Environment

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Figure 5. (A) Aging time-dependent UV-Vis spectra of toluene-dispersed AgNPs formed by the MW-assisted reduction of AgNO3 in DES1 (333 ppm water content) at 200 °C for 120 s using OAm as the reductant (Pmax = 200 W). Aliquots of the aging toluene AgNP stock were periodically withdrawn and diluted (58-fold) into toluene having a Karl Fischer-determined water content of 60 ppm to assess the apparent storage stability of toluene dispersions of the AgNPs. A trivial decrease in the absorbance was observed after 12 h, at which point the extinction and peak width values stabilized. (B) The corresponding normalized UV-Vis spectra are provided for facility of spectral shape comparison. These results substantiate that toluene dispersions of the as-made AgNPs remain fully stable for at least a month.



Figure 6. (A) Representative TEM micrograph of AgNPs formed in DES1 (200 °C, 240 s reaction, Pmax setting of 200 W, DES water content: 3600 ppm) after 40 days of storage in toluene at room temperature. (B) The respective particle size distribution histogram reveals that the mean particle size of 11.1 ± 3.4 nm is statistically equivalent to that of freshly-made particles (11.3 ± 3.2 nm; see Figure 4, panels G and H), demonstrating the full retention of AgNP size and morphology during ambient storage as a toluene dispersion.


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Figure 7. (A) Effects of the DES1 reaction water content on measured UV-Vis profiles for toluene dispersions of the AgNPs resulting from the OAm-mediated reduction of AgNO3 in DES1 for a 120 s MW reaction at 200 °C (Pmax = 200 W). The toluene used for dispersion was rigorously dry (50 ppm water). The absorbance intensity and spectral FWHM were comparable for DES water contents of 2700 and 3600 ppm while, in contrast, AgNPs made in rigorously dry DES1 (333 ppm water) exhibited a strikingly narrow FWHM concurrent with a higher extinction value. All samples were diluted to the same level (58-fold) such that [Ag] ≈ 0.10 mM. (B) The corresponding normalized spectra from panel (A) are shown. (C) Representative TEM micrograph and (D) the corresponding particle size distribution histogram for of AgNPs formed in rigorously dry DES1 containing 333 ppm water.



TOC Graphic:

Synopsis Organosoluble silver nanoparticles were prepared for the first time within a non-halide deep eutectic solvent by rapid microwave-assisted wet chemical reduction.


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Rapid Microwave-Assisted Synthesis of Silver ... - ACS Publications

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