Article pubs.acs.org/JPCC
On the Microstructure, Chemical Composition, and Porosity Evolution of Nanoporous Alloy through Successive Dealloying of Ternary Al−Pd−Au Precursor Xiaoguang Wang,†,‡ Junzhe Sun,† Chi Zhang,† Tianyi Kou,† and Zhonghua Zhang*,† †
Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials (MOE), School of Materials Science and Engineering, Shandong University, Jingshi Road 17923, Jinan 250061, P. R. China ‡ Research Institute of Surface Engineering, Taiyuan University of Technology, Yingze West Road 79, Taiyuan 030024, P. R. China ABSTRACT: In the present Article, we have investigated the microstructure, composition, and porosity evolution of nanoporous alloy during successive dealloying of ternary Al75Pd17.5Au7.5 precursor in NaOH/HCl and HNO3 aqueous solutions. The results show that the selective dissolution of Al through the first-step dealloying contributes to the formation of Pd−Au nanoporous composites, which are composed of two distinct structures: the finer nanoporous AuPd dendrites derived from the dealloying of Al2(Au,Pd) and the coarser nanoporous Pd(Au) derived from the dealloying of Al3(Pd,Au). Moreover, compared with those dealloyed in the 20 wt % NaOH solution, the Pd−Au nanocomposites exhibit a coarser length scale of ligaments/channels in the 5 wt % HCl solution. After the second-step dealloying in the 65 wt % HNO3 solution, the nanoporous Pd(Au) dissolves away, and the Pd dissolution results in the formation of Au-rich nanoporous alloy with bimodal channel size distributions. Nanoporous alloys with unique structures and compositions can be fabricated by dealloying based on alloy design and control over selective dissolution of elements in suitable electrolytes.
1. INTRODUCTION Nanoporous metals have recently attracted considerable interest in a wide variety of applications including catalysis, sensors, actuators, fuel cells, and so forth.1−4 Template methods are commonly used to fabricate these materials through replication of porous alumina or liquid-crystal templates.5−7 Template methods have advantages of precise control over the pore size and microstructure periodicity but normally result in materials with 1-D porosity, such as an array of tubes.8 In comparison, dealloying is another efficient route to produce porous materials via selective dissolution of less noble element(s) from suitable precursor alloys. Selective dissolution enables the reorganization of more noble element(s) into a 3-D nanoporous structure with bicontinuous ligaments/channels at nanosized scales.9−11 Until now, dealloying has been found in many alloy systems such as Ag−Au, Cu−Au, Cu−Pt, Al−Au, Al−Pd, Al−Ag, Zn− Ag, and so on,12−19 and most attention has been paid to the dealloying of Ag−Au along with the formation of nanoporous Au (NPG). Compared with binary alloy systems, only recently has the research on dealloying of ternary or multicomponent precursors been conducted. Following the dissolution of active element, two or more released elements undergo complex diffusion along electrolyte/solid interfaces. The diffusion and reorganization process will be greatly affected by the interaction between these released elements, which can further influence the microstructure of resultant nanoporous alloys.20 © 2012 American Chemical Society
In the past few years, several representative researches have been executed on dealloying of ternary alloys. Snyder et al.20 and Jin et al.21 have investigated electrochemical dealloying of Ag−Au−Pt alloys and the formation of an ultrafine nanoporous structure with ligaments/channels of ∼5 nm. Simultaneously, it has been confirmed that the addition of Pt with lower surface diffusivity can slow down the diffusion of Au atoms and result in the formation of the ultrafine nanoporous structure. Xu et al.22 have fabricated ultrafine nanoporous Au−Pt alloys through dealloying ternary Cu−Au−Pt precursors under potential control. Moreover, these nanoporous Au/Pt alloys exhibit excellent electrocatalytic activities toward methanol and formic acid electro-oxidation. Because of the single-phase characteristic of the ternary precursors, these as-dealloyed samples reveal a homogeneous 3-D porous structure. In addition, some amorphous alloys can be dealloyed to form nanoporous structures. Yu et al.23 have reported the formation of hierarchical porous Pd through electrochemically dealloying Pd30Ni50P20 amorphous ribbons. Lang et al.24 have synthesized a homogeneous nanoporous Au−Pd structure with a ligaments/channels size of ∼50 nm by electrochemically dealloying Au30Si20Cu33Ag7Pd10 amorphous ribbons. It has been argued that the existence of 3−5 at % Pd can significantly Received: April 13, 2012 Revised: May 31, 2012 Published: June 1, 2012 13271
dx.doi.org/10.1021/jp3035677 | J. Phys. Chem. C 2012, 116, 13271−13280
The Journal of Physical Chemistry C
Article
Figure 1. (a,b) Backscattered SEM images showing the microstructure of the RS Al75Pd17.5Au7.5 alloy and (c,d) typical EDX spectra.
ribbons were further dealloyed in a 65 wt % HNO3 solution for 7 h at room temperature (the second-step dealloying). Finally, all as-dealloyed samples were rinsed using distilled water and dehydrated alcohol. The microstructure of the Al75Pd17.5Au7.5 precursor and asdealloyed samples was observed using a scanning electron microscope (SEM, LEO 1530VP), equipped with an energydispersive X-ray (EDX) analyzer. Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were obtained using a Philips CM 20 microscope. TEM specimens were prepared using a Gatan ion mill at 5 kV. The phase constitution of the Al75Pd17.5Au7.5 precursor and as-dealloyed samples was identified using an Xray diffractometer (XRD, Hitachi Rigaku D/max-RB) with Cu Kα radiation. In addition, scanning transmission electron microscopy (STEM) images and nanobeam-EDX (NB-EDX) spectra were obtained by an FEI Tecnai G2 microscope under the high-angle annular dark field (HAADF) mode. For one composition, at least three to five spots were detected in NBEDX experiments, and an average value was given in the results.
improve the stability and electro-catalytic activities of nanoporous Au.24 Up to now, little information has been available on selective dissolution of alloying elements one by one from a given multicomponent precursor. Here we used a ternary Al−Pd−Au alloy with a two-phase structure as the precursor, and a successive two-step dealloying strategy was applied to remove active elements (Al and Pd) selectively one by one from the precursor. The successive dealloying treatment was based on the discrepancy in chemical reactivity between alloying elements considering the influence of electrolytes. The microstructure, chemical composition, and porosity evolution have been investigated before and after the first and second dealloying. Nanoporous alloys with a nanocomposite (NC) structure and with bimodal channel size distributions have been obtained after the first-step and second-step dealloying, respectively. In addition, the dealloying mechanism has also been discussed.
2. EXPERIMENTAL SECTION The ternary Al75Pd17.5Au7.5 (at %) precursor was prepared from elemental Al (purity, 99.95 wt %), Pd (purity, 99.9 wt %), and Au (purity, 99.9 wt %) in a quartz crucible using a highfrequency induction furnace. Using a single roller melt spinning apparatus, the prealloyed ingots were remelted by highfrequency induction heating in a quartz tube and then meltspun onto a copper roller with a diameter of 0.35 m at a speed of 1000 rpm (rpm) in a controlled argon atmosphere. The ribbons obtained were typically 20−50 μm in thickness, 2−5 mm in width, and several centimeters in length. The first-step dealloying of the rapidly solidified (RS) Al75Pd17.5Au7.5 alloy was performed in a 20 wt % NaOH aqueous solution first at room temperature and then at 90 ± 5 °C until no obvious bubbles emerged. The dealloying was also performed in a 5 wt % HCl aqueous solution to investigate the influence of Cl− on the dealloying process. After that, a portion of as-dealloyed
3. RESULTS AND DISCUSSION Figure 1 shows the section-view SEM (backscattered mode) microstructure of the RS Al75Pd17.5Au7.5 precursor. Dendriteshaped grains can be observed throughout the whole section of the precursor ribbons (Figure 1a). The dendrites can be clearly seen at higher magnification and are several micrometers in size (Figure 1b). One equiaxed dendrite is highlighted by a dashed circle in Figure 1b. Moreover, there exist two different zones in the microstructure: light grey zones (highlighted by a downward arrow) and dark grey zones (highlighted by an upward arrow). The EDX results demonstrate that the composition of the light grey zones is 74.1 at % Al, 12.1 at % Pd, and 13.8 at % Au, whereas that of the dark grey zones is 76.5 at % Al, 22.1 at % Pd. and 1.4 at % Au. Typical EDX spectra are shown in Figure 1c,d. In addition, the XRD results 13272
dx.doi.org/10.1021/jp3035677 | J. Phys. Chem. C 2012, 116, 13271−13280
The Journal of Physical Chemistry C
Article
Figure 2. (a−d) SEM images showing the microstructure of the as-dealloyed sample by dealloying the RS Al75Pd17.5Au7.5 alloy in the 20 wt % NaOH solution and (e) a typical EDX spectrum.
EDX result (Figure 2e), it can be found that most of Al has been selectively removed owing to the following reaction.
(not shown here) show that the RS Al75Pd17.5Au7.5 precursor is mainly composed of two phases: Al2Au-type (PDF No. 170877) and Al3Pd-type (PDF No. 44-1021) intermetallic compounds. According to the atomic ratio of constituent elements, the dark grey zones have the Al3Pd-type structure and can be denoted as Al3(Pd,Au). However, the atomic ratio of Al/ (Pd+Au) in the light grey zones is ∼2.9, different from the stoichiometric ratio (Al/Au = 2) of Al2Au. The phase in the light grey zones can be designated as Al2(Au,Pd), but it is reasonable to assume that there exist vacancies in this phase. Eventually, similar phenomenon has been reported in the literature, where the RS Al75Pt15Au10 precursor is composed of a single Al2(Pt,Au) phase.25 Figure 2 shows the section-view SEM microstructure of the as-dealloyed sample in the 20 wt % NaOH solution (after the first-step dealloying). The microstructure consists of irregular granules and rods stacking together (Figure 2a,b). This morphological characteristic may be inherited from the RS Al75Pd17.5Au7.5 precursor. At higher magnification, it is clear that the as-dealloyed sample exhibits an open bicontinuous interpenetrating ligament-channel structure with a length scale of
