Communication pubs.acs.org/cm
Ultrafast Syntheses of Silver Foams from Ag2NCN: Combustion Synthesis versus Chemical Reduction Debora Ressnig* and Markus Antonietti Max Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, 14476 Potsdam, Germany S Supporting Information *
fficient energy conversion catalysts, electrodes, or gas storage materials and their economic syntheses are essential to meet the rising demands with ubiquitous, tailored technological concepts. Metal foams with meso- (50 nm) certainly have the potential to contribute to this development owing to the beneficial combination of metallic characteristics with nanostructural properties.1 This calls for innovative, simple, and fast syntheses, in particular for those that minimize the use of secondary waste chemicals, i.e., surface modifiers, expensive sacrificial templates, and reagents needed for their preparation or removal. The controlled combustion of metal bistetrazolamine (MBTA) complexes meets some of the above criteria, given that the semiorganic, solid precursors convert within seconds to nanostructured metal foams along a self-sustained reaction front.2 In stark contrast to the elegance and simplicity of the synthesis, finding energetically appropriate metal−ligand combinations is challenging. “Too hot” materials, such as silver azide, detonate violently, readily also upon friction or impact. Others are “too cold” and cannot sustain the reaction.3 The preparation of energetic materials can therefore be very elaborative and nonsafe. Ag foams are attractive materials, owing to the wide spectrum of possible applications. Silver is used as electrocatalyst in fuel cells,4 for selective CO2 reduction,5 in electrochemical sensors,6 as electrode support for many active species, also directly as catalyst,7 and as a plasmonic8,9 or antimicrobial10 material. Application-oriented foams should have tunable shape and be stable and easy to process for flexible use. Very successful wetchemical routes for the preparation of Ag foams are established, which include dealloying,8 the reducing of nonzero-valent nanostructures11,12 (NZVNs), or templating,13 among others. These methods have intrinsic limitations to the fabrication of small monoliths or thin films.1 In the NZVNs approach, the reductantbeing dissolved in solution, a gas, or an electrochemical sourcepropagates through the solid.11,12,14 The precursor nanostructures act as sacrificial templates; hence, the bicontinuous foams are maximum a few micrometers in size. We propose a method to overcome these limitations by drawing parallels to chemical dealloying. Here, an alloy with flexible shape is prepared first. The less noble metal is then removed through etching with strong acids or bases. These foams are mostly confined to thin films because diffusion properties of the etchants are highly sensitive to thickness and porosity patterns. Limited diffusion through the nanochannels might equally hamper postfunctionalization of the metal foams, e.g., for electrocatalysis.
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© 2014 American Chemical Society
A key challenge is therefore the combined optimization of active surface area (nanostructure) and flow-through (macrostructure), i.e., the formation of a continuous, hierarchical material. Here, we present silver cyanamide as bifunctional precursor for the ultrafast formation of Ag foams from compressed powders either as a new type “energetic material” via combustion synthesis or by a wet-chemical reduction process. For the latter we develop a cost-effective, versatile method, which allowed us to synthesize Ag foams with predefined shapes, tunable density, and in situ functionalization in a onepot reaction. Silver cyanamide powders were prepared via precipitation reaction, after immersing Ag(OAc) in a cyanamide-containing aqueous solution (1/1 molar ratio). This procedure allows full conversion to Ag2NCN (c.f. Supporting Information Figure S1) and is scalable to larger quantities in the gram scale. Combustion of compressed pellets was ignited with a flame, and the Ag foam immediately grew along a gradual propagating reaction front. The powder X-ray diffraction (PXRD) pattern confirms the formation of elemental Ag (Supporting Information Figure S2). The causal dependency of the foam shape to the initial pellet size and the ignition point enables the synthesis of differently shaped monoliths, e.g., sheets and cylinders (Figure 1). The transformation to the Ag foam goes in hand with a drastic lateral volume expansion which accounts roughly for a factor of 4.6 (2D-sheets) and 1.5 (3D-monoliths). The resulting foam is a continuous, macroporous network with ligaments in the micrometer scale, according to scanning electron microscopy images (SEM) (Figure 1). The surface texture of the foam is irregular through partially calcined Ag particles that are embedded within a polymeric matrix. The thermal decomposition of Ag2NCN has been previously described as an exothermic process that liberates N2 and (CN)2 as decomposition gases while the metal is reduced.15 Both gases can act as integrated blowing agents to drive the reaction front forward, mediate continuous ligament growth, and ensure a reductive atmosphere at the reaction front, allowing the synthesis to take place in air. Whereas N2 is solely a foaming agent, (CN)2 may fuse to a polymeric, amorphous CxNy byproduct that can restructure, for example, to C3N4.16,17 The contribution of the organic residues to the overall composition accounts for N 4.1 wt % and C 2.6 wt % (for comparison Ag2NCN: N 10.9 wt %, C 4.7 wt %) Received: April 23, 2014 Revised: June 27, 2014 Published: July 8, 2014 4064
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ligament size could further be tuned trough postannealing (Supporting Information Figure S4). The shape of the compressed monolith is accurately preserved during the reduction, even if complex, while the density decreases (Figure 2). Hence, the Ag2NCN pellet acts in its entire entity as continuous sacrificial template independent of the initial crystallite morphology. The electrical resistivity of 0.05 Ω is less compared to the above foams, due to better long-range interconnectivity of the Ag ligaments, favored by the absence of CxNy. The structural similarity of these Ag foams with porous films synthesized with the dealloying technique is striking. The formation mechanisms of metal nanofoams through dealloying has been intensively debated in the past.1 A step forward was made by Erlenbacher et al., who presented a theoretical model fully consistent with the experiment, by taking short lengthscale diffusion/fluctuations of atoms at the surface and in the electrolyte into account.19 In brief, the less noble metal is dissolved through acidic/basic etching in a layer-by-layer manner while the nobler metal agglomerates by diffusion or spinodal separation into islands at the alloy surface. In our case, the alloy is replaced by the semiorganic crystal from which the anion is dissolved as the metal is reduced. A fast, solid/solid reduction process with a strong reductant such as NaBH4 and a readily reducible metal cation are therefore prerequisites to this scenario. Consequently, a weaker reductant (ascorbic acid) did not lead to the formation of the bicontinuous Ag foam but of colloidal assemblies instead (Supporting Information Figure S5). The dominance of short-range chemo-physical events over diffusion or capillary processes is further evidenced by the fact that solvent variations (EtOH, MeOH, isopropanol) did not significantly affect the Ag foam structure (Supporting Information Figure S6). However, if the pellet heights are increased to 2−3 mm the inside appears to be denser (Supporting Information Figure S7) after the reduction. This is a result of diffusion limitations that affect the kinetics at the reaction front. As the pathways get longer, the probability of adsorbing BH4− to the Ag foam surface increases. Significant amounts of H2 gas are evolved which in turn blocks the channel system from fresh reductant. To address these limitations, we searched for a second sacrificial spacer for the formation of a hierarchical material. The latter must be a compound which, like the anion, is removed during the reduction, allowing the reaction front to travel through the monolith without being impeded by the spacer. NaCl turned out to be very suitable, as it is not only very economical, ecologically friendly, and highly
Figure 1. (a) Ag2NCN monoliths and the corresponding Ag foams after combustion synthesis. The red point indicates the ignition point. (b) SEM images of the Ag foam.
which is less than that obtained from the atom-richer BTA ligand rendering thermal post-treatments in reductive atmosphere (H2) dispensable.3 The Ag foam composite has a high apparent surface area (SBET) of 20 m2 g−1. The electrical resistivity of the Ag foam was measured with a commercial monometer and is, with 0.2 Ω, relatively low despite the presence of the carbonaceous material. Still, it exceeds the resistivity of bulk silver by several orders, which is a typical consequence of the nanostructuration; e.g., for nanoparticlebased copper monoliths, the resistivity was found to be 0.4 Ω.18 While the combustion synthesis allows structuring macroscopic Ag foams, enhanced modulation of the nanostructure can be expected from a chemical reduction process. Chemical reduction was performed under optimized conditions through immersion of the Ag2NCN pellets (250 mg) to an aqueous NaBH4-containing solution (0.04 M). The immediate color change to black is indicative for the fast transition of AgI to Ag0. The successful conversion to Ag, which is nearly quantitative, is inferred from PXRD patterns and the absence of NCN2− absorption bands in the Fourier transform infrared (FTIR) spectrum (Supporting Information Figure S3). SEM images show that the transformation results in the formation of a bicontinuous network. The Ag ligaments feature sizes in the range of 50−100 nm enclosing voids of approximately 30−80 nm range up to a maximum of 200 nm (Figure 2). The
Figure 2. (a) Photographs of an Ag2NCN monolith before and after reduction with NaBH4 (0.04 M) illustrate the accurate retention of even complex shapes. The corresponding SEM image shows the porous Ag network. (b) SEM images of Ag foams prepared from different Ag2NCN/ NaCl (w/w: 1/0; 1/3; 1/5, left to right). The inset shows a pellet (1/3) before and after the chemical reduction. 4065
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carbodiimides (Au, Pt). The wet-chemical transformation with a strong reducing agent induces the formation of spinodal networks similar to metal foams produced by dealloying. The herein presented “reductive dealloying” of macroscopic NZVN pellets comes with the advantages that it (1) is independent from the precursor crystallite morphology and size, (2) minimizes strongly acidic/alkaline noble metal wastes, (3) minimizes impurities in the targeted metal foam, and (4) offers tuning options through anion variations. This suggests the method to be a cheap alternative to dealloying techniques with a wide scope of design options, from which density tuning with NaCl and in situ functionalization are just examples.
soluble in water but also not affected by the reductant. Different Ag2NCN/NaCl mixtures were finely crushed with a mortar before pressing and reducing the pellets. The dissolution of NaCl during the reduction was fast enough to maintain conservation of the macroscopic shape (Figure 2). SEM images show that microscopic cavities are present now in between nanostructured Ag sheets, resulting in a combination of two distinct different pore types. Variation of the Ag2NCN/NaCl weight ratio allowed the concentration of these macro-cavities (Figure 2, Supporting Information Figure S8) to be adjusted. Stable monoliths could be produced from ratios up to (1/3) to maximal (1/5). The newly introduced hierarchy leads to an increase of the apparent surface area (SBET) from 6−7 m2 g−1 (674−755 m2 mol−1, 1/0) to 14−15 m2 g−1 (1510−1618 m2 mol−1, 1/5) owing to better accessibility of voids and density effects. This value is exceptionally high for self-standing Ag foams (reference values range from 0.9 to 8 m2 g−1).11,20−22 The density of the Ag foams accounts for 0.45−0.55 ± 0.1 g cm−3 among the series, which corresponds to a relative density (δfoam/δbulk) of 4−5% or a porosity of 95%. The Ag foams are mechanically stable at low ratios (≤1/3); i.e., they can be easily processed and handled, for example, with tweezers but become brittle at higher ratios (say 1/5). The same strategy as with the NaCl sacrificial template can be followed for the in situ deposition of other heterogeneous compounds to the Ag foam (e.g., nanocatalysts). As a proof of concept, this is demonstrated for TiO2 nanoparticles. Mixing, pressing, and reduction lead to a uniform dispersion of TiO2 nanoparticles through the entire Ag foam (Figure 3, Supporting
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ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures and characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +49 331 567 9515. E-mail: debora.ressnig@mpikg. mpg.de. Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 3. PXRD pattern and SEM of the Ag foam/TiO2 nanocomposite. Reference patterns: Ag [ICDD 04-003-2941], TiO2 [ICDD 04-014-5764], scale bar: 200 nm.
Information Figure S9). PXRD analysis proves the stability of TiO2 during the reduction (Figure 3). The route is flexible with regard to the species to be incorporated and was also applicable to, e.g., carbon structures (Supporting Information Figure S10). Ag/TiO2 and Ag/C composites show application in (photo/ electro)catalysis or surface enhanced Raman scattering.23,24 The only factors to be fulfilled by the deposit are water insolubility and chemical inertness against NaBH4. All in all, Ag2NCN is easily accessible, is safe to handle, and fulfills the requirements for an energetic material, which are (1) sustaining combustion and (2) ensuring reduction of the metal. The favorable metal/ligand atomic ratio and its high nitrogen content reduce the amount of CxNy byproducts compared to other energetic materials. These findings are promising for the extension of the concept to other potential noble metal 4066
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(24) Yang, L.; Jiang, X.; Ruan, W.; Yang, J.; Zhao, B.; Su, W.; Lombardi, J. R. J. Phys. Chem. C 2009, 113, 16226.
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