Scanning Transmission Electron Microscopy Characterization of Nanostructured Palladium Film Formed by Dealloying with Citric Acid from Al–N–Pd Mother Alloy Film
2019 Volume 60 Issue 4 Pages 525-530
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2019 Volume 60 Issue 4 Pages 525-530
A high-purity palladium film with a three-dimensional nanoporous structure was fabricated from a reactive sputtered Al–N–Pd alloy film by a dealloying method that used citric acid chelation. Its characteristic porous structure could be controlled by the concentration of nitrogen gas in the Ar sputtering gas. The added nitrogen gas inhibited the formation of the intermetallic Al4Pd phase in the as-deposited film, thereby improving the purity of Pd in the dealloyed nanoporous Pd film up to 99 at%. Furthermore, the formed structure of the dealloyed film changed with the nitrogen gas concentration during initial sputtering, i.e., the structure of the film could be controlled from a three-dimensional nano-network to an aggregated nanoparticle-like structure with increasing N2 gas concentration.
Noble catalytic metals with porous structures have large specific surface areas and can help achieve high catalytic activity1–3) as well as save resources. Nanoporous thin films of noble metals, in particular, ensure homogeneity and can be applied to gas sensing devices4,5) and/or electrode materials.6,7) The traditional method to form a porous metal is dealloying (selective dissolution),8–10) in which the base metal is selectively dissolved from the mother alloy with a noble metal. The idea of a confirmation of metal elements to the mother alloy is that each metal has different standard electrical potentials; further, the base metal preferentially dissolves into the electrolytic solution while leaving the noble metal, and does not form an intermetallic compound to form a solid solution in a wide composition range. For example, Fe, Co, and Mn are selected as the base metals for noble metals Au and Pt. The conventional dealloying reaction is carried out in a strong acid or strong alkaline solution, and it is necessary to apply an electric field. It is also possible to control the microstructure, such as the pore or ligament, after treatment by this anodic treatment. However, this method produces heavy-metal-containing strong acid and/or strong alkaline waste liquid.
Palladium is among the platinum group elements and has high catalytic activity11) and hydrogen occlusion ability.12) Nanostructured Pd has various applications, e.g., as a hydrogen occlusion material,13,14) hydrogen sensing material,15–17) and catalyst material.3,18) Harumoto et al. fabricated a nanoporous Pd–Al thin film by the citric acid-assisted hot-water treatment of an Al–Pd alloy film.19) This method does not require strong acid and anodic treatment, i.e., the dealloying reaction occurs spontaneously. According to the Pourbaix diagram (E-pH diagram),20) metallic aluminum is inactive in the neutral region. However, Ube et al. revealed that aluminum dissolves in ultrapure water and is precipitated as aluminum hydroxide (AlO(OH)) at temperatures above 335 K by following scheme (1)–(3).21)
| \begin{equation} \textit{Al}\to \textit{Al}^{3+} + 3e^{-} \end{equation} | (1) |
| \begin{equation} \textit{Al}^{3+} + 3H_{2}O\to \textit{Al}(\textit{OH})_{3} + 3H^{+} \end{equation} | (2) |
| \begin{equation} \textit{Al}(\textit{OH})_{3}\to\text{AlO(OH)} + H_{2}O \end{equation} | (3) |
| \begin{equation} \textit{Al}^{3+} + \textit{Cit}^{3-}\rightleftarrows \textit{AlCit} \end{equation} | (4) |
Unfortunately, the purity of Pd in this case not enough. In this study, we successfully fabricated a high-purity nanostructured Pd film by using an Al–Pd–N film as a mother alloy film instead of an Al–Pd-film. Further, we investigated the effect of adding N2 gas during sputtering for preparing an initial Al–Pd alloy. N2 addition affected the purity and morphological features of Pd in the fabricated porous Pd film.
The Al–Pd initial mother alloy film was prepared using the rf sputtering method. The composite target combining Pd sectors (99.995 mass% purity) and an Al target (3 in., 99.9995 mass% purity) was sputtered at 30 W in Ar–N2 sputtering gas (>99.9999 vol% (Ar) and 99.99995 vol% (N2) purity). The composition of Al–Pd was controlled by the areal ratio of the Pd sectors. As a sputtering gas, a mixture gas of Ar and N2 with a total pressure of 1.33 Pa was used, and the composition was changed from 0 vol%N2 to 4 vol%N2. An Eagle-XG glass (Corning) substrate with a size of 20 mm × 15 mm, and an elastic carbon transmission electron microscopy (TEM) grid film (Okenshoji Co., Ltd.) of diameter 3 mm were used. The film composition was evaluated by EDX analysis. The distance between the target and substrate was 44 mm. The initial film thickness was 70 nm. The sputtering chamber was evacuated up to 3.8 × 10−5 Pa.
The dealloying process was conducted in ultrapure water (18.2 MΩ·cm) or an aqueous solution (1 L) of citric acid (100 µmol/L, 99.7 mass% purity, Wako Pure Chemical Co.). The temperature during the dealloying process was 368 K. The time required to complete dealloying was determined from the light transmittance of the thin film.
The surface morphologies of the specimens were observed using a scanning transmission electron microscope (STEM; HD-2300C, Hitach High-Technologies Ltd.) operating at 200 kV and equipped with an energy dispersive X-ray spectrometer (R-TEM, EDAX Inc.). The images acquired by the STEM at approximately the same position on the specimen were secondary electron (SE), high-angle annular dark field (HAADF), and bright-field (STEM-BF) images. Furthermore, the selected area electron diffraction (SAED) pattern of the film specimen was recorded by conventional TEM (JEM-2000FX, JEOL Co. Ltd.), and the images were captured as 16-bit grayscale data by a film scanner (GT-X970, SEIKO EPSON Co.) with a linear gamma correction.
Figure 1 shows the dealloyed Al–Pd film in hot ultrapure water. The SE image (Fig. 1(a)) shows that the surface of the film is covered with complex precipitated plates. According to our previous studies, boehmite (AlOOH),21,23,24) which is an aluminum hydroxide, is present. Compared to the SE image, the HAADF and STEM-BF images relatively show the transmission projection structure of the film. Figures 1(b) and 1(c) show the porous or network structure, which is formed under the boehmite layer. This boehmite layer could not be removed by post-processing using a citric acid solution.
STEM images of hot-water-treated Al0.8Pd0.2 film. (a) SE image, (b) HAADF image, and (c) STEM-BF image.
Figure 2 shows the nitrogen gas concentration ($c_{N_{2}}$ (vol%N2)) dependence of the film structure before and after dealloying the citric acid solution. The in-plane size of the undulation on the surface of the as-deposited film decreases as $c_{N_{2}}$ increases (Figs. 2(A-SE))–2(D-SE). There is no internal structure corresponding to the surface morphology in the as-deposited film at $c_{N_{2}} = 0$ (Figs. 2(A-SE), 2(A-ADF), and 2(A-BF). On the other hand, in the films with $c_{N_{2}} = 2$ and 3, a composition distribution similar to the surface undulation is observed (Figs. 2(B-ADF) and 2(B-BF), Figs. 2(C-ADF) and (C-BF)). In the HAADF image (Fig. 2(B-ADF)), we considered that the bright spot contrast corresponds to aggregated Pd and the dark contrast corresponds to Al. This compositional fluctuation was not revealed by EDX analysis at this time owing to the lack of spatial resolution and detection sensitivity; therefore, this phenomenon will be discussed in our next publication.
STEM images of specimen. Capital letters (A)–(D) indicate as-deposited specimen, and small letters (a)–(d) indicate dealloyed specimen. SE: secondary electron image, ADF: high-angle annular dark field image, and BF: STEM-BF image.
As shown in Figs. 2(a-SE), (b-SE), and (c-SE), a nanoporous structure is confirmed up to $c_{N_{2}} = 3$ in the film after dealloying, and the pore size becomes smaller with increasing $c_{N_{2}}$. Comparing the film structure before and after dealloying, the pore size seems to correspond to the size of the initial surface modulation (Figs. 2(A-SE), (B-SE), and (C-SE)), and/or the size of the concentration distribution of the as-deposited film. Moreover, the ligament size is small as compared with that reported in literature.3,25,26)
3.2 Crystallographic characterizationFigure 3 shows the SAED pattern of each specimen. The as-deposited alloy film (Fig. 3(A)) is composed of a Pd–Al solid solution (PdxAl(1−x), 0.82 < x < 1) (space group no. 255, $\text{Fm}\bar{3}m$, a0 = 3.891 − 3.868),27,28) and Al4Pd (λ-phase) (space group no. 182, P6322, a0 = 1.3086, and c0 = 0.9631).29) However, individual diffraction lines of a simple substance Al (Space group No. 255, $\text{Fm}\bar{3}m$, a0 = 0.40494)30) could not be distinguished in all SAED patterns, because of the small atomic scattering factor of Al compared with Pd. Due to this, Al has the same crystal structure and nearly similar lattice constant as Pd. However, this Al4Pd alloy was not observed in the N2 gas-added specimen. Nevertheless, at more than $c_{N_{2}} = 3$ in the N2 gas-added specimen (Figs. 3(C) and (D)), the diffraction lines of aluminum nitride (AlN; space group no. 186, P63mc, a0 = 0.31117, and c0 = 0.49788)31) were observed.
SAED pattern of specimen. Capital letters (A)–(D) indicate as-deposited specimen, and small letters (a)–(d) indicate as-dealloyed specimen. The corresponding intensity profile is shown at the bottom of each SAED pattern.
After the dealloying process, the Al in the initial alloy film dissolved into the electrolyte and residual Pd remained with a relatively high crystallinity (Fig. 3(a)) in the specimen without added N2 gas (Fig. 3(a)). However, the λ-phase intermetallic compound still remained in the specimen (see following EDS analysis). On the other hand, in the N2 gas-added specimen, the λ-phase was not observed after dealloying. Furthermore, the AlN which was contained at initial alloy film, was not observed in the dealloyed specimen (Figs. 3(c) and (d)).
The crystallinity of Pd after dealloying the specimen was improved by increasing $c_{N_{2}}$, except in the specimen without added N2 gas.
3.3 Characterization of composition by energy dispersive X-ray spectroscopyTable 1 shows the EDX analysis results for each specimen, quantified by thin-film approximation with the calculated K factors using the Zaluzec model32) via the EDAX TEM Quant Materials software (AMETEK Co., Ltd.). All composition ratios of Al and Pd in the initial alloy thin film were approximately 8:2. This composition ratio corresponds to that of stoichiometric Al4Pd (λ-phase) in the binary phase diagram of Al and Pd.33)
The fabricated porous Pd specimen without added N2 gas has 15 at% of residual Al, clearly indicating that the Al from the initial starting alloy is not completely removed by dealloying using the citric acid solution. This residual Al is also observed in the above SAED pattern as an intermetallic compound of Al4Pd (Fig. 3(A) and (a)). On the other hand, the purity of Pd in the dealloyed specimen improved by the addition of N2 gas during rf sputtering, reaching 99 at% for the addition of more than $c_{N_{2}} = 2$.
In a previous study on porous materials, the pore diameter and ligament size were discussed to characterize their structural properties. However, our fabricated Pd film shows a three-dimensional network structure rather than a nanoporous structure, and we could not determine the unique value of threshold at the binary image operations without arbitrariness. Thus, we performed two-dimensional fast Fourier transform (2D-FFT) for obtaining SE images in Fig. 2 from a 2048 × 2048 pixels by 656 × 656 nm area using Digital Micrograph software (Gatan. Inc.). The horizontal intensity profiles of the 2D-FFT images are shown in Fig. 4, and each SE 2D-FFT image indicates that our fabricated porous Pd films are approximately isotropic in structure. Further, the maximum spatial frequency in the 2D-FFT profile shifted to a high-frequency side with increasing N2 gas concentration for the as-deposited specimen as well as dealloyed specimen. The peak of spatial frequency qualitatively corresponds to the size of the island-like structure before dealloying the specimen and the mesh size of the network structure after dealloying the specimen. Therefore, we considered that the initial surface structure is the key factor to decide the characteristic structure of a dealloyed nanoporous Pd film.
Horizontal 2D-FFT profiles of SE images (from Fig. 2) (a): as-deposited film, (b): dealloyed film. Inset images are the original 2D-FFT images.
The process of addition of N2 gas in a sputtering atmosphere in this study is none other than the reactive sputtering method itself. Typically, there are two modes of reactive sputtering. One is the metallic mode, in which sputtering progresses while the surface of the target remains in a metallic state. The other is the reactive mode, in which during sputtering, the target surface has been “poisoned” by the reactive gas. Furthermore, the deposition rate was significantly different between the two modes, and their threshold value exhibited hysteresis behavior with the concentration of reactive gas.34) Figure 5 shows the measured deposition rate of the Al–Pd alloy film using a composite target with increasing concentration of N2 gas. To ensure reproducibility, all experiments were performed for 20 min pre-sputtering time without adding N2 to refresh the target surface as a metallic state. Comparing $c_{N_{2}} = 0$ and 4, there is an approximately two-fold difference in the deposition rate. Bordo et al. reported a positive correlation between the surface roughness and deposition rate for a 100 nm-thick Al film by electron beam evaporation.35) Ishiguro et al. also reported that the surface roughness in an Al–N film was reduced by increasing the N2 gas concentration.36)
Dependence of N2 gas concentration on the deposition rate of the Al–Pd alloy film.
One of the effects of adding nitrogen gas is on the size of the island-like structure at the surface of as-deposited Al–Pd initial alloy film; this size determines the nanoporous structure, i.e., the mesh size of the network structure.
4.2 Effect of addition of nitrogen gas on the purity and crystallinity of the nanoporous Pd after dealloyingThe Pd purity of the dealloyed film reached 99 at% at the more than $c_{N_{2}} = 2$. The effect of the addition of N2 gas can be considered similar to that on reactive sputtering. The formation of palladium nitride requires extreme conditions such as high pressures;37) therefore, AlN31) was mainly formed when a small amount of N2 gas was added in the sputtering atmosphere. The formed AlN did not form an intermetallic compound with palladium. Therefore, Al4Pd was not formed in specimens that contained nitrogen before and after dealloying. Furthermore, the AlN in the initial alloy film decomposed into aluminum hydroxide and ammonia by water.38) However, the most stable structure of aluminum in an acidic condition induced by citric acid is Al3+, and this metallic ion was rapidly chelated by citric acid. Therefore, we considered that the dissolution of Al proceeded effectively. The co-produced ammonia quickly evaporated to the atmosphere due to the high-temperature condition (369 K). However, the remaining ammonia affected the dealloying rate, and the saturation time of light transmittance was approximately 4 times longer than that of the Al–Pd alloy film that did not contain nitrogen. The local pH at the interface between the specimen and solution would increase by the produced ammonia; hence, the AlN decomposition rate decreased. The macroscopic dealloying rate is also considered to be related to the structural feature of the dealloyed specimen (Fig. 2). This problem will be addressed in our next publication by controlling the pH to control the dealloying rate.
The crystallinity of Pd after dealloying the specimen was improved by increasing the N2 gas concentration. As mentioned above, the dealloying rate decreased with increasing N2 gas concentration due to the production of ammonia by the decomposition of AlN. Thus, the treatment time of dealloying in hot water ranged between 30 min and 120 min. Experimental results indicated that the grain growth of Pd increased with the process time except for the $c_{N_{2}} = 0$ specimen. Guisbiers et al. reported the size-dependent melting temperature of palladium.39) According to their report, the melting point of palladium particles reduced rapidly for a diameter of 5 nm, regardless of the shape. The ligament size in our fabricated porous Pd film i.e., near the maximum diameter of the Pd nano-crystal, was under 5 nm. Therefore, we considered that atomic migration progressed even at a low temperature of 368 K, and improvement in the crystallinity was observed. Thus, this migration phenomenon in the nanoscale would metamorphose the porous structure from a three-dimensional network to an aggregated nanoparticle structure (Fig. 2).
In this work, the suppressing effect during the formation of an intermetallic compound of an Al–Pd alloy, and the controlling ability of the characteristic structure of a porous Pd film were identified by adding nitrogen gas during the co-sputtering of Al and Pd. The addition of more than 2 vol% N2 yielded a high purity of Pd (>99 at%) in the dealloyed porous Pd film. Furthermore, the addition of N2 allowed control of the characteristic structure of the porous specimen after dealloying; the size of the nano-network structure decreased with increasing N2 concentration during sputtering. Further, owing to the decreasing dealloying rate with increasing N2 concentration, the characteristic structure changed from a three-dimensional network to aggregated nanoparticles by the thermal migration of Pd atoms.
The citric acid dealloying method used herein to fabricate nanoporous Pd from Al–Pd alloy has simple and environmental friendly features, and is therefore promising for practical applications to fabricate large-area nanoporous Pd films with the desired shape.
