Letter pubs.acs.org/JPCL
Developing Monolithic Nanoporous Gold with Hierarchical Bicontinuity Using Colloidal Bijels Matthew N. Lee,*,† Miguel A. Santiago-Cordoba,† Christopher E. Hamilton,† Navaneetha K. Subbaiyan,‡ Juan G. Duque,‡ and Kimberly A. D. Obrey*,† †
Polymers and Coatings, Materials Science & Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ‡ Physical Chemistry and Applied Spectroscopy, Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States S Supporting Information *
ABSTRACT: We report a universal platform for the synthesis of monolithic porous gold materials with hierarchical bicontinuous morphology and combined macro- and mesoporosity using a synergistic combination of nanocasting and chemical dealloying. This robust and accessible approach offers a new design paradigm for the parallel optimization of active surface area and mass transport in porous metal electrodes.
SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
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small pores and high surface area comes at the cost of restricted mass transport. To this end, a recent collection of materials research has centered on the development of hierarchically porous gold with combined macro- and mesoporosity. Emerging synthesis technologies based on multiple dealloying steps,15,16 electrodeposition,17−19 and dynamic templates20 have previously been used to synthesize hierarchical gold materials and rapidly advance this field. While several of these techniques facilitate the direct manipulation of pore size and morphology across disparate length scales, their implementation in the laboratory often requires access to expensive, elaborate, or hazardous equipment (for example, an arc-melting furnace, supercritical dryer, electrochemical cells, and poisonous compounds such as hydrazine and cyanide). Moreover, these methods can be restricted to structures with a narrow range of accessible pore sizes, ambiguous pore connectivity, or a thin-film geometry. It appears that many of the unique properties of dealloyed nanoporous gold are informed by its 3D morphology (i.e., a bicontinuous structure formed by spinodal decomposition). We therefore reasoned that a powerful design for next-generation gold electrodes would incorporate this same morphology in a hierarchical manner, with a means to control the pore sizes
orous gold is a widely researched class of materials given the diverse nature of its applications in catalysis,1 biosensing,2 energy systems,3 medicine,4 actuators,5 and next-generation nanocomposites.6 Dramatic progress in these areas has been aided by the recent development of nanoporous gold materials with high surface area and a bicontinuous arrangement of mesoscale ligaments and interconnected void space.7 These materials are formed by the selective dissolution of silver from silver/gold alloys through a free-corrosion process known as dealloying.8 Through this process, bicontinuous porous gold materials assemble as a result of spinodal decomposition at the alloy/etchant interface, where the characteristic pore size may be controlled from ∼2 nm to ∼1 μm by tailoring the dealloying method, time, and temperature.9,10 This elegant form of materials synthesis has surged in popularity due to its flexibility, simplicity, and scalability. In addition, the explicit morphological signatures of nanoporous gold electrodes are intimately linked to their strong performance across an array of electrochemical applications. For example, the remarkable catalytic properties of nanoporous gold have been partially attributed to its high surface-to-volume ratio (lower coordination of surface atoms), large interfacial area, and open pore volume.11,12 Similarly, performance metrics for chemical sensors such as the detection limit and response time are enhanced by the fine porosity and surface irregularities in dealloyed gold electrodes.13,14 Despite these advances, there exists a point of diminishing returns wherein the use of electrode materials with exceedingly © 2014 American Chemical Society
Received: January 28, 2014 Accepted: February 11, 2014 Published: February 11, 2014 809
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independently. Such a design paradigm could allow for the simultaneous optimization of active surface area and mass transport to fit wide catalytic and electrochemical applications while retaining the remarkable properties of nanoporous gold films. To achieve this, we based our materials synthesis platform on a relatively new class of soft materials known as bicontinuous interfacially jammed emulsion gels, or bijels. These complex fluids are formed by arrested spinodal decomposition of partially miscible liquids by colloidal particles that sequester to the fluid interface and kinetically trap the interpenetrating liquid domains against coarsening. An informative description of bijels and their physicochemical properties can be found elsewhere.21 Given their underlying bicontinuous morphology and the thermodynamic incompatibility of their constituents, bijels are versatile scaffolds for the synthesis of porous and composite materials with bicontinuous morphology.22 Here the characteristic size of the macropores may be controlled over a wide range simply by adjusting the colloidal volume fraction and wetting properties.23 In addition, post-processing bijels to form porous polymeric templates with tunable cross-link density allows for simple chemical transformations by the nanocasting method, whereby a liquid precursor is drawn into the nanoporous polymer phase via capillary action and calcined to a monolithic body.24,25 Here we introduce a novel synthesis platform for 3D gold monoliths possessing interconnected porosity across widely separated length scales based on colloidal bijel templates and a simple combination of nanocasting and chemical dealloying. Our work is motivated by the growing diversity of applications for hierarchically porous solids reported in the literature and the urgent need to provide broader access of these materials to the research community through flexible, benchtop materials synthesis techniques. Therefore, we specifically designed our experiments to eliminate the need for sophisticated laboratory equipment and enable the synthesis of hierarchical gold materials on the benchtop. These hierarchically porous structures should find immediate applications in catalysis, separations, and renewable energy systems, where rapid mass transport and high surface area are often simultaneously required.26,27 We prepared bijels of 2,6-lutidine/water stabilized by colloidal silica microspheres and selectively polymerized the lutidine-rich phase by partially exchanging that liquid with ethoxylated trimethylolpropane triacrylate monomer (Sartomer SR454) mixed with a photoinitiator (Darocur 1173). After polymerization, the remaining water-rich phase was drained to form continuous macropores and excess lutidine leached to form textural pores on the submicrometer scale within the crosslinked polymer phase (Supporting Information). The monolayer of colloidal silica was etched from the polymer interface using sodium hydroxide, and the resultant monolith was used as a template to synthesize porous gold via nanocasting and chemical dealloying (Figure 1). To access a wide range of pore sizes in the hierarchical structure, we employed one of two experimental methods. In the simplest approach used to generate gold materials with submicrometer pores and larger (Method 1), the nanoporous phase of the bijel template was impregnated with a concentrated solution of chloroauric acid dissolved in anhydrous ethanol. The composite was then dried at 70 °C to evaporate the solvent and then further heated to decompose the salt to elemental gold and simultaneously pyrolyze the polymer template. This resulted in a metallic gold monolith with a bicontinuous macroporous structure inherited from the bijel template (Figure 2). At higher
Figure 1. Processing routes to hierarchically porous gold monoliths using bijels as templates. Method 1: A nanocasting approach. Method 2: A nanocasting and dealloying approach.
Figure 2. Digital and SEM images of hierarchically porous gold monoliths synthesized using a nanocasting approach (Method 1). The samples were annealed in air at (a) 550 and (b) 420 °C.
magnification, a hierarchical pore network is clearly visible. The release of volatile organics upon thermal decomposition of the polymer and concomitant sintering of gold particles at this high temperature generate a network of smaller pores that comprise each solid ligament in the macroporous structure. Here the annealing temperature provides a simple means to tailor the size of the smaller pores from the micrometer to submicrometer scale, where a reduction in temperature limits coarsening of gold ligaments and produces smaller voids in the terminal microstructure. Estimates of the average pore and ligament size at the larger and smaller length scales (DP, DL, dP, and dL, respectively) as measured by SEM found DP = 38 ± 4 μm, DL = 42 ± 5 μm, dP = 1.1 ± 0.9 μm, and dL = 2.3 ± 1.8 μm for a monolith calcined at 550 °C for 2 h (Figure 2a). We found that the minimum temperature required to completely decompose the metal precursor and form a mechanically robust monolith was ∼420 °C, which produced hierarchical pores of several hundred nanometers (DP = 34 ± 5 μm, DL = 48 ± 6 μm, dP = 0.48 ± 0.19 μm, and dL = 0.41 ± 0.23 μm) (Figure 2b). Elemental analysis confirmed the material chemistry as elemental gold, with no detectable levels of residual carbon (Supporting Information). 810
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To access significantly smaller length scales in the hierarchical structure, we refined our method to produce bicontinuous macroporous structures with ligaments composed of dealloyed nanoporous gold (Method 2). Here the polymerized bijel was soaked in a concentrated solution of silver nitrate dissolved in water, dimethylformamide (DMF), and a small proportion of isopropyl alcohol as a wetting agent. The impregnated monolith was then sealed in a vial containing excess water/DMF and heated to 90 °C to reduce the silver cations to metal.28 Next, the sample was infiltrated with chloroauric acid as previously described and annealed at 800 °C to decompose the gold precursor, remove the polymer template, and alloy the two metals. Here the final alloy composition is controlled simply by adjusting the concentration of the metal salt precursors, where between roughly 15 and 40 atom % gold is necessary for the formation of continuous mesopores (Supporting Information).15 Finally, the alloyed monolith was dealloyed in nitric acid to generate a hierarchical structure on the mesoscale (Figure 3). Two bicontinuous pore networks formed by
Figure 4. Cyclic voltammogram of ferrocene in 0.5 M tetra butyl ammonium perchlorate in acetonitrile using bijel-templated gold (red) and bulk gold (black). Scan rate −200 mV/s, 0.079 M ferrocene.
further confirming the chemical makeup of our materials as elemental gold. Cycling from 1.0 to 0.0 V, the hierarchical sample exhibits a different CV shape, where a broad reduction wave is observed for the bijel-based material when compared with the bulk electrode. This behavior can be attributed to confinement of the oxidized species within the hierarchical microstructure of the bijel electrode (demonstrated here by SEM). A systematic evaluation of the electrochemical properties of these materials in catalysis and sensing experiments will be the focus of our future work; however, this simple experiment illustrates, for the first time, the rational design of a f unctional porous material derived from a colloidal bijel. While the use of advanced materials based on bijels has been proposed for a number of engineering applications, evidence of their intended chemical functionality has not been shown until now. On the basis of these results, we believe that the use of bijels can offer a low-cost and accessible synthesis platform for hierarchically porous metal electrodes with enhanced performance for current technologies such as supercapacitors, rechargeable batteries, and catalysis. We have outlined a straightforward and flexible materials synthesis platform for hierarchically porous gold materials using bijels as templates. A synergistic combination of nanocasting and chemical dealloying allows for the benchtop fabrication of monolithic gold with hierarchical bicontinuity on widely separated length scales, which we believe can pave the way to a new generation of porous metal electrodes for applications in catalysis, sensing, and renewable energy systems.
Figure 3. SEM images of monolithic porous gold with hierarchical bicontinuous pore structures synthesized using a nanocasting/dealloying approach (Method 2).
spinodal decomposition characterize the morphology: the macroporous structure from the original bijel template and the mesoporous structure from the dealloying process (DP = 26 ± 4 μm, DL = 29 ± 7 μm, dP = 34 ± 6 nm, and dL = 39 ± 11 nm). In these materials, the walls of the macroporous structure are composed of micrometer-sized grains of nanoporous gold that extend throughout the 3D monolith. While we observed that the hierarchical pore network formed by dealloying (Method 2) is more uniform when compared with nanocasting (Method 1), simply annealing the dealloyed material at moderate temperatures permits access to much larger pores while preserving the bicontinuous morphology (Supporting Information). 10 Although we consider Method 1 noteworthy given its simplicity, the generation of true hierarchical bicontinuity on the submicrometer scale is easily accomplished by Method 2. It may be possible to generate ultrafine porosity (dP < 10 nm) through the use of low-temperature or electrochemical dealloying to enable further control of surface area,10 and although we have focused here on silver/gold alloys, we believe our approach can be generalized to other alloy systems whose constituents are deposited from solution (e.g., Nix/Cu1−x) and therefore open up new areas for future research.29 To verify the intended functionality of our materials, we electrochemically characterized the gold monolith of Figure 2b using cyclic voltammetry (Figure 4). One-electron organometallic ferrocene redox behavior is well known,30 and here we simply compared the behavior of our bijel-templated gold and bulk metallic gold as working electrodes under identical conditions. For both electrodes, we observe the same oxidation potential for ferrocene (0.31 vs Ag pseudoreference electrode),
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ASSOCIATED CONTENT
* Supporting Information S
Experimental methods, additional characterization of bijel templates, and porous gold materials. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*M.N.L.: E-mail:
[email protected]. *K.A.D.O.: E-mail:
[email protected]. Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. 811
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