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Article Cite This: ACS Omega 2019, 4, 7096−7102
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One-Pot Synthesis of Reactive Base Metal Nanoparticles in Multifunctional Pyridine Alexander Egeberg,† Tim P. Seifert,† Peter W. Roesky,† Dagmar Gerthsen,‡ and Claus Feldmann*,† †
Institute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstraße 15, 76131 Karlsruhe, Germany Laboratory for Electron Microscopy, Karlsruhe Institute of Technology (KIT), Engesserstrasse 7, 76131 Karlsruhe, Germany
‡
ACS Omega 2019.4:7096-7102. Downloaded from pubs.acs.org by 46.148.120.214 on 04/19/19. For personal use only.
S Supporting Information *
ABSTRACT: A pyridine-mediated, one-pot synthesis of Zn0, Sn0, V0, and Mn0 nanoparticles is presented for the first time. Herein, pyridine is multifunctional, serving as a solvent, a reducing agent, and a surface-stabilizing agent. All metals were obtained as crystalline, spherical nanoparticles with very small sizes of 5.5 ± 0.6 nm (Zn0), 5.1 ± 0.7 nm (Sn0), 5.3 ± 0.6 nm (V0), and 6.9 ± 0.8 nm (Mn0). Simple metal chlorides (ZnCl2, SnCl2, VCl3, MnCl2) were used as the starting materials and heated in pyridine (autoclave, 210−300 °C). In addition to detailed particle characterization (high-resolution transmission electron microscopy, scanning transmission electron microscopy, Fourier-transformed infrared (FT-IR), X-ray powder diffraction, elemental analysis), the mechanism of reaction was explored (MS, NMR, FT-IR), indicating pyridine as the reducing agent that was oxidized to 2,2′bipyridine. As a reliable and simple synthesis strategy, the novel pyridine-mediated approach can be highly relevant to obtain high-purity base-metal nanoparticles, and as a next step, to address widespread applications such as surface finishing of steel, catalysis, high-power batteries, or hydrogen storage.
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INTRODUCTION Metal nanoparticles are considerably more reactive than the respective bulk metals due to the great number of surface atoms. Accordingly, the synthesis of metal nanoparticles is generally more challenging if the electrochemical potential of the metal is lower and the particle diameter is smaller.1 In this regard, iron is an illustrative example; whereas bulk iron is oxidized in humid air on a time scale of months to years, iron nanoparticles are pyrophoric and show spontaneous ignition when in contact with air.2 Highly oxophilic metals are even more reactive, which is due to the high stability of the resulting metal oxides. This holds, for instance, for vanadium, tin, or manganese that form oxides (VO2/V2O5, SnO2, MnO2) with very high lattice energy.2 Although the synthesis of base metals in the form of high-quality nanoparticles (i.e., uniform size and shape, high purity) is challenging, they are highly requested in regard of their material properties and potential application, which can comprise surface finishing of steel, selective catalysis, high-power batteries, or metal-hydride-driven hydrogen storage.3−6 Zinc, tin, vanadium, and manganese, used as examples in the following, are currently prepared by gas-phase methods (e.g., plasma deposition, chemical vapor deposition, laser ablation)7−9 or milling of bulk metals.10 Liquid-phase synthesis typically requires elaborate, expensive, and highly sensitive starting materials (e.g., organometallic compounds, carbonyls).11−13 Especially for zinc, vanadium, and manganese, powerful reducing agents are needed (e.g., Na[C10], K[BEt3H], BuLi).14,15 Moreover, strong-binding stabilizers (e.g., poly(vinylpyrrolidone), long-chained alkyl amines) are often necessary to control the particle diameter and to suppress particle agglomeration.16−19 These limitations result in © 2019 American Chemical Society
advanced, multistep synthesis and time-consuming, partly less reliable approaches. Especially for the most reactive metals Zn0 and V0, only few synthesis strategies have been reported until today. In many cases, moreover, the particle quality in terms of size distribution and the absence of metal-oxide impurities is limited. Aiming at new options for liquid-phase synthesis of metal nitride nanoparticles,14,20−22 we discovered a novel straightforward pyridine-mediated access to high-quality base-metal nanoparticles such as Zn0, Sn0, V0, and Mn0, which is reported here for the first time. Pyridine turned out to be multifunctional, serving as a solvent, a reducing agent, and a surfacestabilizing agent. The resulting one-pot approach is easy to perform and uses simple metal chlorides (ZnCl2, SnCl2, VCl3, MnCl2) as the starting materials.
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RESULTS AND DISCUSSION Zn0 Nanoparticles (Two-Step lq-NH3 Approach). Primarily, our synthesis approach intended to obtain highpurity Zn3N2 instantaneously in the liquid phase via thermal decomposition of zinc amide as an intermediate. Such a strategy has already been reported in 1938 by Juza et al. to obtain bulk-Zn3N2 by thermal decomposition of Zn(NH2)2 in vacuum.23 Aiming at nanoparticles, in principle, the course of the reaction, i.e., the formation of high-purity metal nitrides via lq-NH3-made intermediate amides, recently also turned out to be successful in the case of CoN, Ni3N, and Cu3N.20 With the intention of preparing Zn3N2 in pyridine, to our surprise, we Received: January 15, 2019 Accepted: March 28, 2019 Published: April 19, 2019 7096
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observed the formation of deep black suspensions containing nanoparticles of elemental zinc (Zn0) (Figure 1). Since facile
Figure 1. Two-step, lq-NH3 synthesis of Zn0 nanoparticles: (a) photo of a solution of ZnCl2 in pyridine; (b) formation of a colorless Zn(NH2)2 suspension after condensation of lq-NH3 and addition of KNH2; and (c) formation of a black suspension of Zn0 nanoparticles in pyridine after heating.
and reliable liquid-phase syntheses of reactive base-metal nanoparticles are interesting as well, we have explored the underlying mechanism and focused the synthesis on the realization of Zn0 nanoparticles. The as-prepared Zn0 nanoparticles were obtained in a twostep process by dissolving ZnCl2 and KNH2 in a mixture of pyridine and lq-NH3 at −35 °C to form colorless Zn(NH2)2 as a first step (Figure 1a,b). Herein, lq-NH3 serves as an excellent solvent for KNH2. In the second step, the obtained suspension was warmed to room temperature to evaporate ammonia and thereafter heated in an autoclave to 240 °C. To avoid uncontrolled particle growth and particle agglomeration, heating at 240° was kept as short as possible. Thereafter, a dark black suspension of Zn0 nanoparticles was obtained (Figure 1c). The Zn0 nanoparticles were purified by centrifugation and redispersion in methanol (three times). KCl as a byproduct, which is insoluble in pyridine, was removed by the dissolution in methanol (Figure S2). It should be noticed that any centrifugation needs to be performed at low rotation (12.000 rpm/12.700g) to avoid uncontrolled particle agglomeration and fusing. This behavior can be rationalized based on the low melting point of zinc (Tmp(Zn0) = 419 °C),24 which is known to decrease even further for small particle sizes and supports the fusion of particles at high-speed centrifugation. Particle size, size distribution, crystallinity, and phase purity of the as-prepared Zn0 nanoparticles were determined by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). The TEM overview images show very small, uniform nanoparticles with a low degree of agglomeration (Figure 2a,b). A statistical evaluation of at least 100 particles on the TEM images results in a mean diameter of 5.5 ± 0.6 nm (Figure 2c). High-resolution transmission electron microscopy (HRTEM) images validate the crystallinity of the as-prepared Zn0 nanoparticles (Figure 2b, inset). Parallel lattice fringes are observed throughout the marked particles with a distance of 2.1 Å. This value is compatible with bulk zinc (d101 with 2.14 Å)25 and confirms the crystallinity and phase purity of the as-prepared Zn0 nanoparticles. Furthermore, X-ray powder diffraction patterns confirm the crystallinity and purity of the as-prepared Zn0 nanoparticles as well (Figure 2d). Here, it needs to be noticed that the Zn0 nanoparticles were pestled with silica spheres (9−13 μm) and filled into glass capillaries (0.3 mm in diameter) under argon to perform XRD measurements in inert conditions. Pestling with silica spheres, however, causes merging of the
Figure 2. Characterization of Zn0 nanoparticles: (a, b) TEM overview at different magnifications and HRTEM images (inset); (c) particle size distribution (according to statistical evaluation of >100 particles); (d) XRD of the as-prepared Zn0 nanoparticles (after pestling with amorphous silica spheres, filling in glass capillaries and sealing under argon; bulk-Zn: ICDD-No. 00-004-0831 shown as a reference).
low-melting Zn0 nanoparticles (see the Supporting Information (SI)). Finally, the surface functionalization of the as-prepared nanoparticles was examined by Fourier-transformed infrared (FT-IR) spectroscopy. By comparing the nanoparticles with pure pyridine as a reference, FT-IR spectra indicate the presence of pyridine attached to the particle surfaces (Figure 3a and SI Figure S1). Thus, pyridine not only serves as the solvent but also as a stabilizing agent. However, pyridine adhered on the particle surface can be easily removed by heating (70 °C) in a solution of hydrazine in tetrahydrofuran (THF) (Figure 3b). Accordingly, all pyridine-related vibrations vanish and the surface functionalization was modified from pyridine to THF, which is now clearly visible in the FT-IR spectra. Generally, suspensions of the as-prepared Zn0 nanoparticles in both pyridine or THF are colloidally highly stable and do not show agglomeration or sedimentation over periods of several weeks. Reaction Mechanism. Because of the unexpected reduction with the formation of Zn0 nanoparticles, we were naturally motivated to explore the mechanism of the reaction. In principle, two types of reaction sequences could be relevant. On the one hand, zinc nitride could have been obtained as an intermediate that thermally decomposes into the elements. In principal, such thermal decomposition is well-known for transition-metal nitrides.26,27 On the other hand, pyridine could serve as a reducing agent for obtaining elemental zinc. To obtain deeper insights regarding the reaction mechanism, we have involved mass spectrometry (MS), NMR, and XRD to identify potential reaction products. First, the Zn0 nanoparticles were completely separated by high-speed centrifugation (25.000 rpm/55.200g) over 60 min. Mass spectra of the transparent supernatant point to a bipyridine isomer as a product. Thus, spectra show a dominating peak at m/z = 157.076 (in addition to pyridine at m/z = 80), which corresponds to a protonated bipyridine isomer (Figure 4a). Pure pyridine as the solvent, in comparison, shows a peak at m/z = 80, referring to protonated 7097
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Figure 3. Surface functionalization of the Zn0 nanoparticles: (a) after pyridine-mediated synthesis and (b) after hydrazine treatment in THF (pure pyridine and THF as an inset with transmittance different from the y-axis).
Figure 4. Mass spectra of (a) the supernatant (after complete separation of the Zn0 nanoparticles) and (b) pure pyridine as a reference.
points to the generation of gaseous reaction products. Already due to its characteristic odor, this gas could be identified as ammonia. A pH indicator paper confirmed the formation of NH3 as the alkaline gas (SI Figure S4). Control experiments with pure pyridine, heated at identical conditions (240 °C), did not result in any formation of 2,2′-bipyridine or NH3. In principle, amines as reducing agents for obtaining metal nanoparticles are well known. A prominent example, for instance, relates to the formation of Fe0 nanoparticles by the reduction of Fe(II) precursors as reported by Chaudret et al.28 Here, hexadecylamine was used as a surface-active agent and a reducing agent, with the amine being oxidized to imine by dehydrogenation. Similar syntheses of metal nanoparticles (e.g., Ni, Cu, Pd, Pt, Au) were also widely reported with oleylamine as a reducing agent.29 Hydrogen elimination as a reduction process, however, requires the presence of primary or secondary amines. For pyridine, such a reduction process is not possible due to the absence of N−H groups and adjacent aliphatic C−H groups. Pyridine as a reducing agent, especially in regard to base metals such as zinc, is even more surprising, since the aromatic system is already electron deficient. In fact, the reduction and synthesis of metal nanoparticles by pyridine, to the best of our knowledge, is here reported for the first time. As a key to understand the reactivity of pyridine, Hein et al. in 1928 already described the dehydrogenation of pyridine with the formation of 2,2′-bipyridine in a solvothermal reaction of pyridine and FeCl3 according to the equation: 2C5H5N +
pyridine (Figure 4b). In contrast, not any noticeable peak at m/z = 157 occurred, indicating that bipyridine is not an impurity of the solvent or a product of the ionization in the MS experiment. Consequently, bipyridine was formed together with Zn0. The presence of a bipyridine isomer (i.e., 2,2′bipyridine, 2,4′-bipyridine, 4,4′-bipyridine) was also confirmed by NMR spectroscopy of the colorless residue obtained after
Figure 5. NMR analysis of the supernatant (a, after complete separation of the Zn0 nanoparticles) and vacuum distillation of pyridine (b) with 1H NMR spectrum of the residual colorless solid (c); (d) 2,2′-bipyridine with assignment of protons according to 1H NMR spectra.
vacuum distillation of the supernatant (Figure 5). According to 1 H NMR spectra, the characteristic peaks of 2,2′-bipyridine can be identified. This finding is further validated by XRD, which also indicates the colorless residue is 2,2′-bipyridine (SI Figure S3). Beside bipyridine as the reaction product, certain overpressure prevailing in the autoclave after the reaction
Figure 6. Proposed reaction mechanism for the pyridine-mediated formation of Zn0 nanoparticles: (a) ZnCl2 dissolved in pyridine; (b) Zn(NH2)2 formed after addition of KNH2; (c) formation of bis(2-pyridinyl)zinc intermediate after the elimination of NH3; and (d) coupling reaction generating 2,2′-bipyridine and elemental zinc. 7098
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2FeCl3 → C10H8N2 + 2FeCl2 + 2HCl.30 In fact, this reaction still is the most efficient synthesis of 2,2′-bipyridine. The exclusive formation of the 2,2′-isomer instead of the 4,4′isomer, moreover, excludes a radical-based mechanism, since the 4,4′-isomer would dominate in the case of radical intermediates.31 Taking all findings into consideration, the pyridine-mediated reduction of ZnCl2 can be rationalized by the reaction mechanism shown in Figure 6. After the formation of Zn(NH2)2 (Figure 6a), deprotonation of the most acidic H atoms in the ortho position result in the generation of NH3 (Figure 6b). Bis(2-pyridinyl)zinc as a highly reactive intermediate (Figure 6c), thereafter, decomposes with the formation of 2,2′-bipyridine and elemental zinc (Figure 6d). In sum, pyridine has a triple function of a solvent, a reducing agent, and a surface-active stabilizing agent. Zn0, Sn0, V0, and Mn0 Nanoparticles (One-Pot Approach). On the basis of the unexpected synthesis of Zn0 nanoparticles and the multifunctional role of pyridine, we intended to transfer the strategy to other metals. With the knowhow of the reaction mechanism, we also aimed at simplifying the approach even further. Thus, we skipped the two-step process, especially including the low-temperature first step and the dissolution of the starting materials in lq-NH3 (see Figure 1). The synthesis of Zn0 nanoparticles was yet conducted as one-pot reaction by heating ZnCl2 and KNH2 as the base in pure pyridine (Figure 7). Again, the as-prepared
Figure 8. Size of the as-prepared Sn0 nanoparticles: (a, b) TEM overview images (at different magnifications); (c) HRTEM image with lattice fringes; and (d) particle size distribution (according to statistical evaluation of >100 particles).
Figure 7. Scheme illustrating the pyridine-mediated one-pot synthesis of Zn0, Sn0, V0, and Mn0 nanoparticles using ZnCl2, SnCl2, VCl3, and MnCl2 as starting materials.
Zn0 nanoparticles indeed exhibit a particle quality in terms of size, size distribution, crystallinity, and purity (SI Figures S5 and S6) similar to the aforementioned two-step lq-NH3 approach (see Figure 2). Rapid and short heating (240 °C) remained essential to avoid any uncontrolled particle growth and particle agglomeration of the low-melting zinc. Beside the simple and reliable synthesis of Zn0 nanoparticles, we could also successfully transfer the one-pot approach to Sn0, V0, and Mn0 nanoparticles (Figures 7−11). Similar to Zn0, tin, vanadium, and manganese nanoparticles were prepared using the metal chlorides SnCl2, VCl3, and MnCl2 as the starting materials and KNH2 as the base. The respective solutions in pyridine were heated in an autoclave at temperatures of 210−300 °C. The formation of 2,2′-bipyridine was evidenced by 1H NMR spectra in all these cases, suggesting a similar reaction mechanism as proposed for zinc (see Figure 5). The low melting tin (Tmp(Sn0) = 232 °C),24 similar to zinc, requires fast and short heating to 210 °C to avoid any uncontrolled particle agglomeration and particle fusion. In contrast, the much more basic and high-melting vanadium (Tmp(V0) = 1910 °C)24 and manganese (Tmp(Mn0) = 1246 °C)24 need long-term heating at high temperatures (8 h, 300 °C). For all metals, deep black and colloidally highly stable suspensions were obtained that do not show agglomeration or sedimentation over periods of several weeks. TEM overview images and HRTEM detail images show uniform, spherical nanoparticles with a low degree of
Figure 9. Size of the as-prepared V0 nanoparticles: (a, b) TEM overview images (at different magnifications); (c) HRTEM image with lattice fringes; and (d) particle size distribution (according to statistical evaluation of >100 particles).
agglomeration (Figures 8−10). According to statistical evaluation of >100 nanoparticles on TEM images, mean particle diameters of 5.1 ± 0.7 nm for Sn0, 5.3 ± 0.6 nm for V0, and 6.9 ± 0.8 nm for Mn0 were determined. The HRTEM images show parallel lattice fringes, indicating the crystallinity of all the as-prepared metal nanoparticles (Figures 8−10). The observed lattice distances of 2.0 Å (Sn0), 2.1 Å (V0), and 2.1 Å (Mn0) are compatible with those of the respective bulk metals (d211 of bulk-Sn0: 2.02 Å;32 d110 of bulk-V0: 2.14 Å;33 d411 of bulk-Mn0: 2.10 Å).34 XRD patterns of the as-prepared Zn0 and Sn0 nanoparticles show distinct Bragg peaks directly after synthesis, which fit well with the respective bulk-metal references (Figures 2d and 11a). On the contrary, the XRD patterns of the as-prepared V0 and Mn0 nanoparticles do not show any Bragg peaks (Figure 7099
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CONCLUSIONS Starting with the unexpected formation of Zn0 nanoparticles upon reacting ZnCl2 and KNH2 in a two-step approach, we could develop a novel pyridine-mediated one-pot synthesis of Zn0 nanoparticles. The examination of the mechanism of the reaction reveals pyridine as the reducing agent, which is oxidized to 2,2′-bipyridine. With the knowhow of the course of the reaction, the synthesis strategy, on the one hand, could be facilitated from the two-step liquid-ammonia-based reaction to a simple one-pot approach in pyridine. In addition, the synthesis strategy could be transferred to further metal nanoparticles such as Sn0, V0, and Mn0. Based on the one-pot synthesis, Zn0, Sn0, V0, and Mn0 nanoparticles were prepared by heating of simple metal chlorides (ZnCl2, SnCl2, VCl3, and MnCl2) in pyridine (autoclave 210−300 °C). All metals were obtained with uniform shape and very small sizes of 5.5 ± 0.6 nm (Zn0), 5.1 ± 0.7 nm (Sn0), 5.3 ± 0.6 nm (V0), and 6.9 ± 0.8 nm (Mn0). According to TEM, furthermore, all metal nanoparticles are crystalline. Pyridine turned out to have multifunctional roles and served as a solvent, a reducing agent, and a surfacestabilizing agent. Such a synthesis strategy, and especially the synthesis of metal nanoparticles by pyridine-mediated reduction, is here shown for the first time. In general, a one-pot liquid-phase synthesis of reactive base metals can be highly relevant in view of widespread opportunities of application, including surface finishing of steel, catalysis, high-power batteries, or hydrogen storage. Using low-cost metal chlorides to obtain high-purity metals in the absence of any oxygen/oxide source (e.g., starting materials, solvents, reducing agents) is also very promising with regard to advanced material properties. Finally, the synthesis strategy can be optionally transferred to other transition metals.
Figure 10. Size of the as-prepared Mn0 nanoparticles: (a, b) TEM overview images (at different magnifications); (c) HRTEM image with lattice fringes; and (d) particle size distribution (according to statistical evaluation of >100 particles).
11b,c). To rationalize this observation, again, the sample pretreatment for XRD analysis needs to be noticed. Thus, the highly moisture- and oxygen-sensitive metal nanoparticles were pestled with dried silica spheres in a glovebox, filled in glass capillaries, and sealed under argon (see the SI). Due to pestling, the low-melting zinc and tin fuse into a crystalline bulk metal, which explains the occurrence of intense and narrow-lined Bragg peaks (Figures 2d and 11a). In contrast, the high-melting vanadium and manganese (Tmp(V0) = 1910 °C, Tmp(Mn0) = 1246 °C)24 do not merge upon pestling and do not show any Bragg peaks due to the low scattering power of the small-sized particles (Figure 11b,c). After sintering and crystallization of the as-prepared nanoparticles, however, the obtained Bragg peaks are well in agreement with bulk-V0 and bulk-Mn0 (Figure 11b,c). Although, of course, no nanoparticles were available subsequent to sintering, the absence of any impurity compound (e.g., metal oxides, metal hydroxides, metal carbonates) validates the purity of the as-prepared V0 and Mn0 nanoparticles. Finally, elemental analysis shows low carbon contents for all four as-prepared metal nanoparticles (see SI Table S1). Upon comparison with pure pyridine, both the carbon and the nitrogen content can be ascribed to the surface-adhered pyridine on the metal nanoparticles.
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EXPERIMENTAL SECTION General Aspects. All experiments and purification procedures were performed under inert gas (argon), using standard Schlenk techniques and gloveboxes. This also includes all centrifugation and washing procedures. Moreover, sample preparation and sample transfer for analytical characterization were strictly performed under inert conditions, e.g., by using specific transfer modules (see the SI for details). Pyridine (ABCR, 99%) was refluxed for 3 days and freshly distilled over CaH2. Methanol (Sigma-Aldrich, 99.5%) was refluxed over Mg for 3 days. KNH2 was synthesized by reacting potassium (Riedel-de-Haen, 99%) in liquid ammonia (Air Liquide, 99.98%) at −50 °C using Fe2O3 as a catalyst, followed by filtering and drying in vacuum. Tin(II)chloride (Sigma-
Figure 11. Crystallinity and purity of the Sn0, V0, and Mn0 nanoparticles: (a) as-prepared Sn0 (reference bulk-Sn: ICDD-No. 00-004-0673); (b) asprepared and sintered V0 (reference bulk-V: ICDD-No. 00-022-1058); and (c) as-prepared and sintered Mn0 (reference bulk-Mn: ICDD-No. 01089-2412). All samples after pestling with silica spheres, filling in glass capillaries, and sealing under argon (see the SI). 7100
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room temperature. Finally, a deep black suspension was obtained with V0 nanoparticles that can be purified as described above. Synthesis of Mn0 Nanoparticles (One-Pot Approach). Fifty milligrams of MnCl2, 43.6 mg of KNH2, and 20 mL of pyridine were filled into an autoclave. The autoclave was treated in an ultrasonic bath for 2 min to guarantee intimate mixing of all starting materials. Thereafter, the autoclave was positioned in a preheated heating jacket (300 °C). This temperature was kept for additional 8 h before the autoclave was allowed to cool to room temperature. Finally, a deep black suspension was obtained with Mn0 nanoparticles that can be purified as described above. Further detailed information regarding sample handling and analytical techniques can be found in the Supporting Information.
Aldrich, 99.99%), zinc(II)chloride (Sigma-Aldrich, 99.999%), vanadium(III)chloride (Sigma-Aldrich, 97%), and manganese(II)chloride (Sigma-Aldrich, 99.99%) were used as purchased. Safety Advice. The as-prepared base-metal nanoparticles are highly reactive, especially when in contact with air or moisture. Powder samples are pyrophoric and show spontaneous ignition in the presence of oxygen. Suspensions of the base-metal nanoparticles also show decolorization due to the formation of metal oxides and/or metal hydroxides on a time scale of some minutes when in contact with air or moisture. Suspension and powders of the base-metal nanoparticles are highly stable when strictly kept under inert conditions (e.g., vacuum, argon, nitrogen). Suitable laboratory equipment and personal safety equipment are required for handling of reactive metals. Synthesis of Zn0 Nanoparticles (Two-Step LiquidAmmonia Approach). Fifty milligrams of ZnCl2 was dissolved in 20 mL of pyridine. At −35 °C, about 5 mL of liquid ammonia (lq-NH3) was condensed to the colorless solution. Thereafter, 40.2 mg of KNH2 were rapidly added with vigorous stirring. Instantaneously, a colorless suspension was obtained and stirred for additional 10 min. The suspension was then naturally warmed to room temperature and transferred into a titanium autoclave (Parr Instruments). This autoclave was positioned in a preheated heating jacket (240 °C). As soon as the autoclave too reached 240 °C, the heater was immediately switched off and the autoclave was allowed to cool to room temperature. Finally, a deep black suspension was obtained. The Zn0 nanoparticles were purified by centrifugation and redispersion in methanol (three times). They can be easily redispersed in pyridine or dried in vacuo to obtain powder samples. Removal of Surface-Adhered Pyridine. The as-prepared Zn0 nanoparticles were stirred in 5 mL of hydrazine solution in THF (1.0 M) for 1 h at 70 °C to remove surface-adhered pyridine. Synthesis of Zn0 Nanoparticles (One-Pot Approach). Fifty milligrams of ZnCl2, 40.2 mg of KNH2, and 20 mL of pyridine were put into an autoclave, which was treated in an ultrasonic bath for 2 min to guarantee intimate mixing of all starting materials. Thereafter, the autoclave was positioned in a preheated heating jacket (240 °C). As soon as the autoclave too reached 240 °C, the heater was immediately switched off and the autoclave was allowed to cool to room temperature. Finally, a deep black suspension was obtained with Zn0 nanoparticles that can be purified as described above. Synthesis of Sn0 Nanoparticles (One-Pot Approach). Seventy-five milligrams of SnCl2, 43.5 mg of KNH2, and 20 mL of pyridine were filled into an autoclave, which was treated in an ultrasonic bath for 2 min to guarantee intimate mixing of all starting materials. Thereafter, the autoclave was positioned in a preheated heating jacket (210 °C). As soon as the autoclave too reached 210 °C, the heater was immediately switched off and the autoclave was allowed to cool to room temperature. Finally, a deep black suspension was obtained with Sn0 nanoparticles that can be purified as described above. Synthesis of V0 Nanoparticles (One-Pot Approach). Forty milligrams of VCl3, 42.0 mg of KNH2, and 20 mL of pyridine were filled into an autoclave, which was treated in an ultrasonic bath for 2 min to guarantee intimate mixing of all starting materials. Thereafter, the autoclave was positioned in a preheated heating jacket (300 °C). This temperature was kept for additional 8 h before the autoclave was allowed to cool to
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00136.
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Analytical equipment, surface conditioning of the nanoparticles, and the mechanism of the reaction (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Claus Feldmann: 0000-0003-2426-9461 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.E., D.G., and C.F. are grateful to the Deutsche Forschungsgemeinschaft (DFG) for funding of personnel (NanoMet: FE911/11-1, GE 841/29-1) and TEM equipment (INST 121384/33-1 FUGG). Moreover, the authors thank the Helmholtz-Program Science and Technology of Nanosystems (STN), subtopic Nanocatalysis, for support. Finally, they acknowledge H. Berberich for performing the 1H NMR spectra.
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DOI: 10.1021/acsomega.9b00136 ACS Omega 2019, 4, 7096−7102
