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Reduction-induced decomposition: spontaneous formation of monolithic nanoporous metals of tunable structural hierarchy and porosity Congcheng Wang, and Qing Chen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01431 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018
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Chemistry of Materials
Reduction-induced decomposition: spontaneous formation of monolithic nanoporous metals of tunable structural hierarchy and porosity Congcheng Wang,1 Qing Chen1,2,3* 1
Department of Mechanical and Aerospace Engineering, 2Department of Chemistry, and 3the Energy Institute, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. *Correspondence should be addressed to
[email protected].
Abstract The beauty of dealloying, i.e. the selective dissolution of an alloy, lies in the spontaneous formation of bicontinuous nanoporous metal, whose robustness, conductivity, and high specific area are otherwise difficult to achieve in a monolithic form. Yet a uniform structure of nanoporous metal requires a homogeneous alloy precursor, whose laborious fabrication has been limiting the application of dealloying and dealloyed materials. Here we replace the alloy precursor with a compound, and design another type of selective dissolution, reduction-induced decomposition (RID). Using the RID of AgCl as an example, we chemically reduced bulk AgCl samples to create bicontinuous nanoporous Ag that resembles dealloyed structures. The monolithic material possesses a uniform ligament width of 72 nm and a specific area of 7.57 m2/g. The ligament width can be tuned in a range from 30 nm to 1 µm through coarsening, and the porosity from 57% to 87% by replacing silver cations in the compound with sodium cations. The RID of this multi-cation compound can lead to a hierarchical structure, which evolves because of two simultaneous percolation dissolutions. The hierarchical nanoporous Ag delivered a stable performance as a high-capacity Ag/AgxO electrode owing to its micron-sized pores for fast mass transfer. RID not only provides an inexpensive alternative to dealloying, it also expands the design space of nanoporous materials for meeting diverse needs in electrochemical applications.
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Dealloying, the selective dissolution of alloys, creates bicontinuous metallic monoliths comprising nanometer-wide metal ligaments and pores.1,2 The structure retains the geometry of the alloy precursor, which can be designed beforehand as the form factor of the nanoporous monolith to fit directly into a device. For example, we can dealloy white gold leaf (a Ag-rich AgAu alloy) for a thin sheet of nanoporous Au that can fit in between a gas-diffusional layer and a proton-exchange membrane in a fuel cell as its catalyst layer.3 Surface functionalization of this nanoporous Au with another noble metal or alloy has led to outstanding performance without a high loading of precious metal.4,5 This ease of fabrication, along with their large specific area and high electrical conductivity, has enabled nanoporous metals to serve various needs in the fields of electrocatalysis,6,7 actuation,8,9 sensing,10,11 and energy storage.3,12,13 However, the ease of dealloying should not dwarf the difficulty of selecting and fabricating an alloy precursor. To achieve a bicontinuous structure, the two components in the alloy first have to be far apart in their oxidation potentials, and miscible in a suitable compositional range.14 After choosing an alloy, we face the challenge of its fabrication. The alloy has to be homogeneous so that the resulted morphology will be uniform enough to maximize desirable structural characteristics while minimizing mechanically or chemically weak spots. Hence, the preparation of an alloy often ends up as the most laborious and demanding step, and it could eventually become a cost barrier to the development of a dealloying-based technology. New methods of dealloying, including liquid metal dealloying,15,16 vapor phase dealloying,17 and Gapaint dealloying,18 have strived to circumvent the limits of alloy precursors, but structures from these methods deviate from the typical morphology or length scale of nanoporous metals, let alone that they still require careful preparation of alloys. To eliminate alloys from the recipe, we turn to an alternative type of selective dissolution, in which the dissolving metal component is replaced by anions. The reaction is thus decomposition of a compound instead of dealloying. In dealloying, the bicontinuous porosity evolves because of the mechanism of percolation dissolution;14 the dissolution proceeds through the percolating clusters of the reactive component, which is spontaneously turned into the continuous pores, while the percolating noble component stays as the continuous solid phase. This mechanism puts no constrains on the type of components involved, and thus it should also apply to decomposition.19 Take the decomposition of AgCl solid for example. We can reduce the compound electrochemically via a half-reaction of AgCl + e − → Ag + Cl − to selectively dissolve Cl- and render a metal product. This type of reaction, termed herein as reduction-induced decomposition (RID), mechanistically resembles dealloying. The dissolving component, chloride ions, leaves behind percolating pores that sustains the dissolution, while the silver cation-turned silver atoms stay as the continuous metal phase. Such a mechanistic resemblance promises a morphological resemblance between the products of dealloying and RID.19 RID-based fabrication removes the challenging step of alloy preparation, and enables the use of ample precursors that are easily accessible in a range of compositions and structures. In this work, we use the RID of AgCl to show that it can indeed create monolithic nanoporous Ag that not only shares the morphology of dealloyed materials but also provides high tunability in its porosity and length scales.
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Chemistry of Materials
Results and discussion Nanoporous Ag from reduction-induced decomposition Fig. 1a shows the steps of fabrication that start with AgCl powder. The powder was first melted at 500 ̊C to form larger AgCl crystals. We then cut the solidified melt into cubes (Fig 1b) or rolled it into a thin sheet (Fig. 1c) by taking advantage of the softness of AgCl. We used relatively large AgCl samples so that we can measure their dimensional changes accurately and demonstrate the fabrication of monolithic samples. The melting process requires no inert gas or vacuum as the metal component is in its oxidized state, which is a general advantage of RID over dealloying. We then selectively dissolved the chloride from the AgCl samples by reducing them with 0.1 M NaBH4 aqueous solution. Upon immersion into the solution, the translucent samples quickly turned dark gray in color. The RID of a 40 µm thick AgCl sheet finished in 2 minutes, corresponding to an average reaction rate of 0.062 A/cm2. The rapid decomposition overwhelms the slow dissolution of AgCl, and confines the reaction to the interface, ensuring the mechanistic resemblance to dealloying. For both the cube and the sheet, the reaction led to freestanding monolithic products (Fig. 1d and e) with little changes in their shapes and dimensions. At the cross-section (Fig. 1f), we see highly curved ligaments well connected to their neighbors. The average width of the ligaments is about 72 nm, similar to the size of the pores. This structure will be referred to as the base case throughout this work. Unlike nanoparticles formed by rapidly reducing solvated metal cations, the nanoporous monolith formed by the rapid RID possesses a uniform, reproducible length scale independent of the precursors. We attribute the uniformity to the surface-diffusion controlled morphology evolution and simple stoichiometry of the compound. Surface diffusion is relatively insensitive to reaction rates or precursor sizes,1 which gives little chance to electrolytic processes to reshape the structure. The simple stoichiometry of AgCl eliminates the issue of phase separation even when the compound is prepared in simple steps. This contrasts the preparation of Ag-Al alloys20,21, the most common precursors for nanoporous Ag via dealloying, which often encounters phase separation that results in non-uniform porous structures. The length scale of nanoporous Ag can be controlled by changing the temperature of RID or post-annealing. When we lowered the reaction temperature to 0 ̊C with an ice-bath, the ligament width decreased to ~30 nm (Fig 2a). As neither this sample nor the base case experienced much coarsening after being taken out of the electrolyte, the resulted difference in their length scales manifests coarsening in the electrolyte. When we annealed the base-case structure at 200 ̊C for an hour, the length scale increased to ~500 nm (Fig. 2b). The ligament width after annealing depends on the temperature and the time as shown in Fig. 2c and d. The dependence can be analyzed by an equation derived for particle sintering,22 i.e. d (t ) n = d 0 n + A × t × D0 exp(
− Ea ) , kT
(1)
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where d is the ligament width, n is the scaling exponent, A is a constant, t is the annealing time, D0 is the maximal surface diffusion coefficient, Ea is the activation energy for coarsening, k is the Boltzmann constant, and T is the annealing temperature. Given the base-case ligament width as d0, we estimate the value of n to be ~3.3 from Fig. 2c, and the value of Ea ~78 kJ/mol from Fig. 2d. Although the scaling exponent gives no strong indication for either surface or bulk diffusion as the coarsening mechanism, bulk self-diffusion of Ag at the examined temperature is too slow to be accountable. The activation energy falls into the typical range quoted for surface diffusion.23,24 The observation appears similar to those reported for dealloying.24,25 Table 1 compares structures from RID and dealloying side-by-side. The dealloyed structure was prepared from a Ag0.75Zn0.25 alloy (Fig. S1). Besides the base case, we include the RID sample annealed at 150 ̊C as it shares the length scale with the dealloyed structure. For decomposed samples, porosity (ε) can be estimated using
ε = 1−
VAgm V
m AgCl
1 , 1−δ
(2)
where Vm is the molar volume and δ is the percentage of volume shrinkage in RID. A careful measurement of the dimensional changes in Fig. 1b-e returns a value of δ ~7.34%, similar to those measured in dealloying.26 The porosity is thus 56.9% for the base case. After annealing, the value decreased to 51.3%, likely due to the pinch-off of ligaments necessary in the coarsening process. We also measured specific area via the underpotential deposition (UPD) of Pb on Ag27 (Fig. S2). This electrochemical method, unlike gas adsorption, takes place at room temperature and only requires a small amount of material. Similar methods have been successfully applied to measure the specific area of nanoporous Au.28,29 The UPD experiment of the base case estimates a specific area of 7.57 m2/g, which agrees with literature on nanoporous Au of a similar ligament size.29,30 Comparing the second and the third rows in Table 1, we see a quantitative resemblance between the structures from RID and dealloying. Table 1. A comparison between nanoporous Ag structures from RID and dealloying. The first row corresponds to the base case, and the second row is the sample annealed at 150 ̊C for an hour, which shares the length scale of the dealloyed structure in the third row. Ligament size (nm)
Specific area (m2/g)
Volume change (%)
Porosity (%)
RID
72 ± 20
7.57
7.34%
56.9%
RID + Annealing
213 ± 50
1.92
18.0%
51.3%
Dealloyed
192 ± 45
1.84
11.7%
68.7%
Porosity and structural hierarchy
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Chemistry of Materials
Besides the resemblance, RID can offer structural tunability not yet attained by dealloying. Porosity is one characteristic not easily tunable via dealloying. For a higher porosity, we usually dealloy an alloy richer in the reactive component, whose miscibility with the noble component, however, limits the range of attainable porosity. For a similar reason, one may not expect RID to afford tunable porosity due to stoichiometry. Yet we can circumvent this constrain by forming a multi-cationic compound, for example, AgxNa1-xCl (0< x
