Introduction

Nanoporous gold (NPG) is formed by the selective dissolution of Ag from a binary solid solution of Ag and Au1,2,3. Dealloying produces a nanoporous structure, which has a high surface area-to-volume ratio, making it a candidate material for surface area-driven energy applications4,5,6. The functionality of NPG, however, is limited by the stability of its nanostructure, especially if used at elevated temperatures.

NPG-Pt, made from AgAuPt precursors with systematic variation of Pt content, was shown by Vega and Newman to exhibit limited coarsening of ligaments during the dealloying process, making them as small as 3 nm7. 3D Atom probe tomography (APT) analysis of the ligaments by the present authors have revealed Pt clustering on nanoligament surfaces, to which the feature size refinement is attributed8,9. The use of APT had been considered difficult for NPG and NPG-Pt owing to the high porosity. To address that, a electrodeposition method of Cu was developed to infiltrate the porosity and enable successful APT analysis10.

NPG-Pt was shown to coarsen significantly in reductive atmospheres, similar to NPG, at temperatures of 250 °C and above11. The coarsening of NPG-Pt by Ostwald ripening (OR) eventually leads to the complete annihilation of porosity (i.e., densification), especially when reaching temperatures of 400 °C and above11. This is mainly owing to the desegregation of Pt away from the surfaces of nanoligaments as shown by previous studies on Au-Pt nanosystems12,13.

An interesting property investigated by Vega and Newman in a previous study14 is that the thermal coarsening of NPG-Pt in air is impeded in the range of 200–500 °C. Oxygen-induced segregation of Pt acts to inhibit the surface diffusion of Au, which is considered as the main facilitator for coarsening to occur at temperatures as low as 200 °C15. The favorable interaction of Pt with O (relative to that of Au with O), induces the segregation of Pt atoms to the surface, replacing Au atoms16.

Changes in the nanoscale chemical composition of as-dealloyed ligaments might not be limited to Pt and Au. Several theoretical studies on Au-Ag nanoparticles have shown the tendency of Ag to segregate to the surface at elevated temperatures4. Large segregates of Pt are observed. In the bigger nanoligament, Pt clusters are located inside the ligaments, consistent with the chemical profile in Fig. 3. Yet other Pt segregates, on the relatively thinner ligaments seem to be at the surfaces. This observation might appear counterintuitive, as a non-favorable interaction with the reducing atmosphere is expected to drive Pt desegregation from the surface12,13. However, considering coarsening kinetics, we postulate that these Pt segregates found at the surfaces of thinner nanoligaments are a result of Pt having a much lower surface mobility than Au27,28. This causes the Pt to be left behind at the surfaces of very thin nanoligaments that are shrinking and on the verge of collapsing.

Fig. 4: Formation of subsurface large Pt clusters.
figure 4

Concentration maps taken from 2 nm-thick slices of APT volume of NPG-Pt heated at 300 °C in Ar-H2. Scale bar is 20 nm.

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Although it is expected that Pt has a high mobility when moving on a surface of Au29, it is important to note that Pt is already clustered in as-dealloyed NPG-Pt, making Pt rearrangement even slower30. Clusters of several Pt atoms would also have a higher activation energy for moving around as a cluster31. Meanwhile, the extremely immobile Pt segregates are increasing in size, due to the tendency to minimize interaction surface area with the reducing atmosphere32,33,34,35, especially at areas of the nanoligaments with high curvature (saddle points)36.

Inhibited thermal coarsening in oxidative atmosphere

In contrast to NPG-Pt annealed in Ar-H2, the ligament/pore sizes in the air-annealed NPG-Pt do not show the same coarsening. This can be seen in both the SEM images of the NPG-Pt (Fig. 5) and the APT data (Fig. 6). The cross-sectional image in Fig. 5c reveals a thicker layer of NPG-Pt, compared with H2-annealed NPG-Pt, and with small feature size, not dissimilar to that observed in as-dealloyed NPG-Pt9. This is further supported by a direct comparison of the APT data for the two respective ligament structures, as shown in Fig. 7. Clearly, the thermal coarsening of ligaments was inhibited owing to the oxidative environment, such that the ligament network remains visibly similar in size and scale to the as-dealloyed material.

Fig. 5: Cross-sectional analysis and APT specimen preparation of NPG-Pt thermally coarsened in oxidative atmosphere.
figure 5

SEM images showing FIB sample preparation of an APT sample for Cu-filled NPG-Pt, heated at 300 °C in air. a Sample surface (scale bar is 20 μm). b Cut out showing the Cu-filled NPG layer, between the Cu deposition and bulk alloy (scale bar is 2 μm). c A high magnification image showing the fine ligament-pore structure (scale bar is 200 nm). d The APT needle following annular milling (scale bar is 500 nm). e The final APT specimen after low-kV sharpening (scale bar is 200 nm).

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Fig. 6: Atom map of NPG-Pt layer along with the parent alloy (AgAuPt) after coarsening in air.
figure 6

3D APT atom maps of Cu-filled dealloyed layer on NPG-Pt, air-annealed at 300 °C for 2 h showing Au, Ag, Pt, and Cu atoms. Scale bar is 50 nm. Refer to supplementary video for 3D reconstruction.

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Fig. 7: Retained ligament-pore structure in NPG-Pt after annealing in air, owing to impeded thermal coarsening.
figure 7

a 3D atom maps showing ligaments of a NPG-Pt air-annealed at 300 °C for 2 h, and b as-dealloyed NPG-Pt (full methodology for spatially-determining the ligament boundaries and data for as-dealloyed NPG-Pt from ref. 8). Scale bar is 20 nm.

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To understand the effect of the oxidizing environment on the chemical distribution across nanoligaments, chemical map**s are examined. These are shown for a vertical cross-section in Fig. 8. Significant Pt enrichment is observed at ligament surfaces. Compared with the segregates observed in the Ar-H2-annealed NPG-Pt, the Pt segregates in this case are higher in number, yet considerably smaller. These observations agree with the anticipated adsorbate-induced segregation behavior of Pt which, owing to its favorable interaction with O, is expected to segregate to the ligament surfaces. As a result, Au atom movement on nanoligament surfaces is impeded, leading to mitigated thermal coarsening. Figure 8 also reveals a rather homogenized nanoligament composition, where the as-dealloyed core-shell structure appears to be annealed out. This suggests that while the movement of Au atoms on nanoligament surfaces is impeded due to the high surface area coverage of Pt segregates, the Au (and Ag) atoms within the ligament are sufficiently mobile as to eliminate the non-equilibrium core-shell structure.

Fig. 8: Formation of nanoscale Pt surface segregates and elimination of core-shell distribution of AgAu.
figure 8

Concentration maps taken from 5 nm-thick slices of APT volumes of NPG-Pt air-annealed at 300 °C for 2 h, along the longitudinal axis. Scale bar is 30 nm.

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A precise indication on the degree of coarsening for NPG-Pt in air is obtained by comparing its measured surface area-to-volume ratio with previously measured values for as-dealloyed NPG-Pt. As shown in Table 1, the volume fraction of the nanoligaments remains basically unchanged by air-annealing. However, a decrease of ~17% in the ligament surface-area-to-volume ratio suggests that some coarsening does occur. It is likely that the coarsening is not totally inhibited until the surfaces of the ligaments are saturated with enough Pt-O clusters to completely immobilize the Au at that temperature.

Table 1 Limited degradation of surface area-to-volume ratio after annealing in oxidative environment.
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A final observation made, requiring investigation in future work, is that the total Pt content in the dealloyed layer appeared to increase by as much as 4 at.% after heating in air. Composition profiles from the APT data, excluding Cu, that spans across the dealloying interface are presented in Fig. 9. These show the Pt content increasing moving across the dealloying interface towards the dealloyed layer. This allows us to suggest, for now, that the source of the extra Pt content might be the bulk layer of non-dealloyed AgAuPt3, which could be considered as a reservoir of Pt atoms that, at high enough temperature, could diffuse across the non-uniform dealloying interface.

Fig. 9: Overall Pt enrichment in NPG-Pt after coarsening in air.
figure 9

1D concentration profiles for NPG-Pt air-annealed at 300 °C for 2 h and as-dealloyed NPG crossing the dealloying interface.

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Diffusion across the dealloying interface at 300 °C, where minimal lattice diffusion is expected, could be assisted by excessive point defects37 injected at the dealloying front, as per previous discussions on the topic by Rösner and Weissmüller20. Nevertheless, this is yet to be validated through future in-depth studies that could reveal an interesting substrate effect on the functionality of NPG-Pt at high temperatures.

The findings presented in this paper highlight the different roles played by Ag, Au, and Pt during thermal coarsening and how such roles are closely dependent on chemical environment of coarsening. In reducing environments, nanoligaments increase in size due to the surface diffusion of Au. Ag segregates to nanoligament surfaces, highlighting the possible contribution of Ag to the coarsening of nanoligaments in nanoporous layers with residual Ag. Meanwhile, large clusters of Pt are formed on subsurfaces of coarsened ligament. Nanoligaments on the verge of collapsing had immobile Pt segregates at the surface, left behind by the more mobile Ag and Au.

In oxidative environments, thermal coarsening is inhibited owing to the enrichment of Pt at nanoligament surfaces, via the positive interaction between O and Pt. The nanoligament size showed little change as a result. The core-shell structure of as-dealloyed ligaments is also eliminated in oxidative air, even though the change in surface area-to-volume ratio is minimal and surface diffusion of Ag, Au is practically inhibited. Compositional changes across the entire dealloyed layer remain to be further investigated, with particular focus on local chemistry at the dealloying interface.

Methods

Dealloying

Two rectangular samples with nominal surface areas of ~0.8 cm2 were cut out of 200 μm foil of Ag77Au20Pt3 (at.%) procured from Goodfellow Metals, Cambridge, UK. All annealing was done at 975 °C for 15 h in a 2.5% H2-Ar atmosphere. The samples were then soldered to a copper electrical wire from one end (taking care not to alter the active surface area to be dealloyed). A fine coating of Microshield TM lacquer supplied by SPI was applied to the solder-copper-sample junction to insulate it from the solution used. Dealloying solution was prepared with 18 MΩ cm de-ionized water and de-aerated by high-purity N2 purging (min purity: 99.998%). The samples were then dealloyed potentiostatically, at room temperature, in a solution of 0.5 M HClO4 from Analar grade HClO4 (Alfa-Aesar, 62%), at a potential of 550 mV vs. MSE (Mercury/Mercurous Sulfate Electrode/sat. K2SO4), until the removal of ~0.5 C cm−2 was attained. The dealloyed samples were then immersed in de-ionized water for ~1 h to remove residual acid.

After cutting the NPG-Pt samples to detach them from the lacquered wire, they were annealed in two separate conditions, one for 2 h in a reducing atmosphere of Ar-2.5% H2 at 300 °C followed by quenching in furnace by purging Ar-2.5% H2, and the other for 2 h in oxidizing laboratory air at 300 °C followed by air quenching. Then the annealed samples were reattached to a copper electrical wire from one end and coated with an insulating layer of lacquer, as explained above, for pore infiltration by Cu deposition. Pore infiltration was then achieved by stepped potentiostatic deposition of Cu in CuSO4 + H2SO4 solution, following the deposition method explained in a previous APT analysis published recently10.

Atom probe tomography

APT samples were prepared from the Cu-filled dealloyed layer of NPG-Pt3 using a Zeiss NVision40 FIB and standard liftout procedures38. Target areas in the sample, with well-filled dealloyed layers, were identified by cross-sectional FIB cuts, and then extracted with a micro-manipulator. Specimens were then mounted to pre-sharpened Si posts using deposition of W. The samples were then sharpened into needles of suitable scale for APT using the FIB. Initial annular milling was conducted at 30 kV, with final cleaning/sharpening at 10 kV.

APT analysis was conducted using a Cameca local electrode atom probe (LEAP) ×4000 HR (Cameca Instruments, USA). Data were acquired while operating in laser-pulsing mode (λ = 355 nm) with a pulse of 40–60 pJ and a pulse rate of 100–125 kHz. The target evaporation rate was set to 0.003 or 0.005 ions per pulse (0.3 or 0.5%) by adjusting an applied DC voltage (typically ranging from ~2 to 5 kV). The base temperature for the specimen stage during analysis was ~60 K, and the chamber pressure was approximately 10−8 Pa. Reconstruction and analysis were performed using IVAS 3.8 software, using SEM images of the tips to assist with spatial calibration of the reconstructions.