Nanoporous metals characterized by nanoscale pores and open-cell structures have attracted much attention due to their potential applications as sensors [1,2], actuators [3,4], catalysts [5], fuel cells [6], battery electrodes [7], super-capacitors [8], and optical and medical devices [9,10]. The method of dealloying has been demonstrated as a promising way to prepare the nanoporous metals. By selectively dissolving the less noble component of a precursor alloy, atoms of noble component will be subjected to diffusion, aggregation, undercutting, furcation and coarsening, which eventually leads to the formation of a well-connected nanoscale porous structure consisting of nanoscale ligaments and pores [11].

Starting with a representative nanoporus Au prepared by dealloying Au-Ag alloys as a model system [11,12], novel nanoporous metals have been extensively designed in recent years, such as nanoporous Pt [13], Pd [14,15], Ag [16], Cu [17], Ni [18], and Au-Ag alloy [19]. Particularly, the nanoporous Pd has attracted enough attention due to its excellent performance for catalysis [20] and potential applications for hydrogen sensing and storage [2,[21], [22], [23]].

The properties of nanoporous metals are influenced by the morphologies and scales of the nanoporous structures. Despite the great potentials of the nanoporous Pd in various applications, the reports on controlling and tuning its nanostructure are limited. It has been known that the dealloying process, normally realized by free corrosion or electrochemical corrosion, is a competition between dissolution of less noble component induced by surface roughening and diffusion of noble component induced by surface smoothing [11,24]. Therefore, it is believed that the nanoporous structure of nanoporous metals can be tailored by controlling the relevant dissolution or diffusion rate, via adjusting the composition of the precursor alloy [19,25], etchant concentration [12,26,27], corrosion potential [28,29] and temperature [30,31]. Stable microstructures will be achieved if the dissolution rate and the diffusion rate are well balanced [12,32]. On the other hand, a post-fabrication heat treatment can lead to coarsening of the microstructures to obtain the desired ligament and pore sizes [[33], [34], [35]]. However, the underlying coarsening mechanisms are still under debated [30,32,[36], [37], [38], [39], [40], [41],51]. In this work, a systematic investigation on the effects of the compositions of precursor alloys, the dealloying parameters, and the post-fabrication heat treatment on the nanostructure of the nanoporous Pd prepared by dealloying was carried out in order to clarify the factors and mechanisms in controlling the formation and evolution of the nanoporous structure of the nanoporous Pd.

Nanoporous Pd samples were prepared by dealloying Pd-Co precursor alloys with the compositions of Pd₁₅Co₈₅, Pd₂₀Co₈₀ and Pd₂₅Co₇₅, respectively. The precursor alloys were prepared by melting high purity Pd (99.99%) and Co (99.99%) in a vacuum induction furnace. Afterwards, the as-prepared precursor alloy samples were homogenized at 1173 K for 48 h in a vacuum electric resistance furnace and subsequently quenched into water. Dealloying of the precursor alloys was conducted in a 0.1 mol/L H₂SO₄ electrolyte in a three-electrode electrochemical cell. The precursor alloy samples, a platinum net, and a saturated calomel electrode (SCE) were set as the work electrode, the counter electrode, and the reference electrode, respectively. Before dealloying, the electrolyte was purged by argon gas for 4 h to remove oxygen.

Two approaches were adopted to tube the ligament size of nanoporous Pd upon dealloying: 1) the Pd₂₀Co₈₀ precursor alloy was dealloyed by adjusting the dealloying potentials ranging from +0.16 V to +0.5 V (versus SCE), and 2) the precursor alloys with different compositions, i.e., Pd₁₅Co₈₅, Pd₂₀Co₈₀ and Pd₂₅Co₇₅, were dealloyed under a constant dealloying potential of +0.3 V (versus SCE). In order to ensure a complete dissolution of Co, the dealloying processes have been conducted for about 96 h until the detected corrosion current was below 0.025 mA. In order to investigate the coarsening of nanoporous structure of the nanoporous Pd, heat treatment at 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C and 800 °C were performed for 10 min, 30 min, 60 min, 90 min, and 180 min in a Perkin Elmer DSC 8500 apparatus under the protection of Ar gas with a flow speed of 25 ml/min. This minimized the oxidation of the samples during heating. The EDS analyses on the heat treated nanoporous samples indicated that the oxidation of the samples is negligible. Microstructures of the samples were observed by an FEI Quanta 650 F scanning electron microscope.

No nanoporous structures were observed when the dealloying process of the Pd₂₀Co₈₀ precursor alloy is conducted at the potentials of +0.16 V or +0.5 V (versus SCE). At +0.16 V, no apparent changes occur in the precursor alloy samples, while at +0.5 V, macrocracks appear in the samples. The former can be ascribed to the fact that +0.16 V is lower than the critical value required to separating surface passivation and porosity formation behaviors [42,43], while the latter can be attributed to a higher potential than the threshold potential separating porosity formation and fracture failure behaviors [31]. The nanoporous structures appear when the Pd₂₀Co₈₀ precursor alloy was dealloyed at +0.2 V, +0.25 V and +0.3 V (versus SCE; see Fig. 1(a) and (b)). The average pore diameters d_p are comparable to the average ligament diameters d_l. Both d_p and d_l are found to decrease with increasing dealloying potential, see Fig. 1(c). The evolution of d_l of the nanoporous Pd as a function of dealloying potential is similar to that of the nanoporous Au [28,29]. A higher dealloying potential leads to a faster dissolution of the less noble component (Co) which will limit the long-range diffusion of the noble component (Pd) along the alloy-electrolyte interface. This effect will result in the reduced diameters of both ligaments and pores, as suggested in Refs. [11,42].

Fig. 2(a)-(c) shows the morphologies of nanoporous Pd prepared by respectively dealloying the Pd₁₅Co₈₅, Pd₂₀Co₈₀ and Pd₂₅Co₇₅ precursor alloys at a constant dealloying potential of +0.3 V (versus SCE). The measured d_p and d_l are found to decrease with increasing the Pd concentration (Fig. 2(d)). Subjected to dealloying at an identical dealloying potential, the precursor alloy with a higher concentration of noble component is expected to cause a higher local concentration of noble component and in turn a shorter diffusion length of the noble component atoms. Thus, ligaments and pores with smaller diameters can be resultant.

Fig. 3 shows typical SEM images of nanoporous Pd annealed for 3 h at various temperatures (T_ann) (Fig. 3(a) and (b)) and the measured d_l as a function of T_ann (Fig. 3(c)). Below 300 °C, no apparent coarsening of the nanoporous structure happens, while above 300 °C, d_l rapidly increases as T_ann increases. The observed coarsening behavior of nanoporous Pd is similar to that reported for nanoporous Au where a significant coarsening of ligaments occurs at T_ann > 200 °C [34,44]. To further investigate kinetics of microstructure coarsening, the nanoporous Pd samples were respectively annealed at 200 °C, 400 °C and 600 °C for various time durations. Continuous coarsening of the nanoporous structures with increasing time are observed for all the three temperatures (Fig. 4, Fig. 5). In the following, we analyze the coarsening mechanism of the nanoporous structure during heat treatment.

The coarsening of nanoporous structures is driven by reduction of the overall surface energy, grain boundary energy and strain energy [38,45,46]. Different from the coarsening behavior of ligaments during dealloying process which is dominated by atomic surface diffusion [30,47] due to the largely surface diffusivity along the alloy/electrolyte interface [41], the coarsening of ligaments during heat treatment can be multiple-mechanism controlled. The mechanisms of ligament collapse [37] and pinch-off [32], ligament coalescence [48] and the combination of surface diffusion and bulk diffusion [[38], [39], [40],49,52] have been reported to be responsible for the coarsening of ligaments under the effect of heat treatment.

Here, we analyze the coarsening process using the following equation [50]:

where d(t) is the time dependent ligament size, d₀ is the initial ligament size, k₀ is constant, t is the time, Q is the diffusion activation energy, R is the ideal gas constant, T is the annealing temperature, and n is the coarsening exponent. A value of n = 3 suggests that bulk diffusion controls the coarsening process, while n = 4 means that surface diffusion dominates the process.

For the nanoporous Pd being heat-treated at 200 °C, the ligaments of the nanoporous structures do not exhibit significant coarsening compared to the initial state (Fig. 5(a)). Therefore, d₀ⁿ in the left hand side of Eq. (1) cannot be neglected. A linear fit of the experimental data of ln(d(t)ⁿ-d₀ⁿ) versus lnt can be obtained by adjusting the values of n and k₀. A best fit of the experimental data is achieved when n = 4.5 and k₀ = 0.8 (Fig. 6(a)). The fitted value of n is quite close to 4, suggesting that surface diffusion dominates the coarsening process.

At 400 °C and 600 °C, the ligaments coarsen rather fast and become much larger than the initial state (Fig. 5(b) and (c)), i.e. d(t)>> d₀. Thus, Eq. (1) can be simplified as:

Therefore, the coarsening exponent n can be determined by linearly fitting the experimental data of lnd(t) versus lnt. The fittings yield n = 1.03 for 400 °C (Fig. 6(b)) and n = 1.42 for 600 °C (Fig. 6(c)), respectively, which are much smaller than either surface diffusion or bulk diffusion controlled coarsening processes. This suggests that the ligament coarsening at these two temperatures is dominated by a mechanism or several combined mechanisms which exhibit much faster coarsening kinetics. Probably ligament collapse and ligament coalescence substantially contribute to the coarsening process. These two mechanisms have recently been directly observed by in situ scanning electron microscopy and are demonstrated to significantly enhance the kinetics of ligament coarsening [4].

The similar transition of coarsening mechanisms for the nanoporous metals prepared by dealloying method has been reported in nanoporous Au [35,44]. Concerning the different activation energies required for metal types, primary ligament sizes, and annealing durations, the critical temperatures for the transition of coarsening mechanisms should also depend on these parameters. This issue needs to be confirmed by more rigorous works.

Nanoporous Pd were prepared by dealloying Pd-Co solid solution alloys electrochemically at room temperature. It is shown that the sizes of ligaments and pores of nanoporous Pd can be tuned by adjusting the dealloying potential and alloy composition. The increase of potential and/or content of noble component in precursor alloy leads to a simultaneous decrease in sizes of ligaments and pores. Heat treatment causes ligament coarsening of the nanoporous Pd. At a temperature lower than 300 °C, the ligament coarsening is not significant, while above 300 °C, pronounced ligament coarsening is observed. At relatively low temperatures (e.g. 200 °C), the ligament coarsening may be caused by surface diffusion-controlled mechanism. At high temperatures (e.g. 400 °C and 600 °C), ligament collapse and ligament coalescence probably contribute substantially to the coarsening. This work shows that the nanostructures of nanoporous Pd can be tuned by controlling dealloying and heat treatment parameters.

This work was supported financially by the National Natural Science Foundation of China (Nos.51771153, 51371147, 51790481and51431008), and the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (No. CX201825). The authors would also like to thank the Analytical & Testing Center of Northwestern Polytechnical University for providing essential experimental apparatuses.