Article pubs.acs.org/JPCC
Impact of Structure and Composition on the Dealloying of AuxAg(1−x) Alloys on the Nanoscale M. Kamundi, L. Bromberg, E. Fey, C. Mitchell, M. Fayette, and N. Dimitrov* Department of Chemistry, SUNY at Binghamton, P.O. Box 6000, Binghamton, New York 13902-6000, United States S Supporting Information *
ABSTRACT: This work discusses the impact of structural and compositional factors on the dealloying behavior of different AuxAg(1−x) alloys. It has been found that the dealloying critical potential, Ec, of (111) oriented alloys is systematically more positive than the one for polycrystalline alloys with identical composition. Results also indicate that thin alloy films (thickness of 20−100 nm) exhibit almost identical Ec to the bulk samples, whereas spherical particles (diameters ∼100 nm) feature consistently lower Ec (by about 0.050 to 0.100 V). A trend toward even lower Ec (0.300 to 0.450 V) is illustrated by the dealloying of AuxAg(1−x) nanoparticles (NPs, diameter 10−15 nm). It has been shown for the first time that no dealloying threshold applies to AuxAg(1−x) NPs. Surface area increase has been registered upon dealloying of all samples, except the NPs that exhibit an opposite trend. SEM characterization of dealloyed samples reveals (i) finer structures with increase of the original Au content, (ii) inherent ordering for Au-rich (111) alloys, (iii) size- and shape-independent porosity length scale, and (iv) remarkable structural stability down to critical sizes of 15−20 nm. The results of this study demonstrate the viability of dealloyed frameworks as universal carriers of potent catalysts for various applications.
1. INTRODUCTION The fuel cell industry largely relies on catalysts that include Pt. A way to lower the total cost would be to use an inert support and immobilize a small amount of Pt-based catalyst on the support. Presently, most catalysts have been synthesized and used in practice as nanoparticles (NPs), naturally featuring large surface-to-volume ratio and physical and chemical properties that are often different from their bulk counterparts.1,2 A key step in catalyst preparation is the establishment of reliable contact between NPs and a support electrode (usually C-based, e.g., glassy carbon (GC)) depending on some mechanism to provide adhesion. Issues associated with NPbased catalysts include incomplete capping agent removal, aggregation of capping agent-free NPs,3,4 and enhanced curvature-driven corrosion.5 Addressing these issues is an important aspect of catalyst development because they all eventually result in a loss of electrochemically active surface area (ECASA).6,7 Use of Nafion helps to hold particles strongly to the support but still leads to reduced overall catalytic activity.8 Recently, following reports of the application of Ptcoated dealloyed bulk Ag−Au alloys in catalysis,9−11 our group introduced an all-electrochemical approach as an alternative to NPs for developing catalysts for fuel-cell applications.12 In this approach, a highly stable nanoporous Au (NPG) framework that can be further functionalized13,14 is generated by electrodeposition of a AuxAg(1−x) thin film (TF), followed by selective electrochemical dissolution (dealloying) of Ag.15 A similar approach has been used by Strasser et al.,16−18 where dealloyed Cu3Pt TFs are studied and critically compared with dealloyed NP counterparts as catalysts for the oxygen reduction reaction (ORR). © 2012 American Chemical Society
Ideally, the greatest benefits of the implementation of a continuous porous film as an alternative to its NP counterpart would be the improved control of thickness and porosity length scale, along with enhanced mechanical stability provided by better adhesion to the supporting substrate.19 However, our initial work revealed some limitations and size constraints that generally impact the outcome of the proposed synthetic route. For instance, attempts to electrodeposit continuous layers of alloys 20 to 50 nm thick on GC have been found to produce isolated and somewhat polydisperse spherical particles instead of the continuous films grown at similar thicknesses on metal surfaces (Au, Cu).12 Also, whereas both types of deposit display dealloying behavior typical for bulk single-phase alloys, the critical potential, Ec, associated with the onset of Ag dissolution, occurs at more negative potentials for the spherical shaped particles.12 In addition to that, despite the almost equal surface area developed by the dealloying process, the TFs have been seen to develop substantial cracks unlike the spherical particles that seem to better accommodate the loss of material by shrinking isotropically.12 Cracks have also been observed in the dealloyed Cu3Pt films that substantially impact their catalytic activity versus ORR depending upon the film thickness and level of compressive strain in the resulting structure.16 Overall, the understanding that small porosity dimensions and ultrathin size are desired features for an ideal catalyst raises concerns about the long-term stability of the nanoscale structures generated by dealloying. It is therefore clear that Received: February 17, 2012 Revised: June 11, 2012 Published: June 11, 2012 14123
dx.doi.org/10.1021/jp301603t | J. Phys. Chem. C 2012, 116, 14123−14133
The Journal of Physical Chemistry C
Article
Au mole fractions between 0.1 and 0.9 were synthesized. All glassware and magnetic stir bars used in these syntheses were thoroughly cleaned in aqua regia (HCl/HNO3 3:1 v/v). 2.2. Electrodeposition of AuxAg(1−x) Thin Films. AuxAg(1−x) TF alloys were electrodeposited on Au and GC working electrodes following a modified process developed and discussed in our previous study12 from solutions containing Na3Au(S2O3)2 + AgClO4 of varying molar ratios of Au and Ag in 0.1 M Na2S2O3. The deposition was carried out at constant potential −0.400 V versus Ag+/Ag. 2.3. UV−Visible Spectroscopy. The Ag and Au NPs were diluted 2.5 and 2 times, respectively, and their UV−vis absorption spectra were recorded on a HP 8453 diode array spectrophotometer over the range of 200−1100 nm. The components’ mixing in NPs with different alloy composition was confirmed by UV−vis spectroscopy. 2.4. Transmission Electron Microscopy. Transmission electron microscopy (TEM) was carried out to characterize the NPs for size. Some analysis was performed at an acceleration voltage of 200 kV using a JEOL JEM 2100F, and the rest was performed at an acceleration voltage of 75 kV using a Hitachi H-7000. 2.5. Scanning Electron Microscopy and EnergyDispersive X-ray Spectroscopy. Scanning Electron Microscopy (SEM) was preformed for morphological characterization, and the alloy composition was confirmed by energy-dispersive X-ray spectroscopy (EDS) using an FEG-SEM Zeiss Supra 55 VP. The AuxAg(1−x) TFs subjected to EDS analysis were electrodeposited on Cu electrodes from solutions with prescribed concentration of Ag(I) and Au(I) complexes at a constant potential of −0.600 V versus Ag+/Ag. The NP samples were centrifuged twice at 10 000 rpm for 20 min in 2.0 mL centrifuge tubes, using an Eppendorf MiniSpin Plus centrifuge, to remove any unreacted species prior to EDS and TEM analysis. For TEM sample preparation, the pellet was diluted to its original volume, redispersed via sonication, and further diluted. The NPs were dropcast on a C-coated Cu grid sample holder, followed by evaporation at room temperature. NP samples for electrochemical measurements were prepared by redispersion of the pellets through sonication and loading the NPs with a micropipet onto the GC working electrodes in small increments while air drying at room temperature in between each load until the NP load on the GC electrodes reached 200 μL. 2.6. Electrode Preparation. Single-crystalline (111) cylindrical Au and Ag (99.999+%) working electrodes of diameter 6 and 8.5 mm, respectively, and (111) cylindrical AuxAg(1−x) (99.999+%) alloy working electrodes of diameter 6 mm were purchased from Monocrystals Company. These electrodes were mechanically polished down to 1 μm with alumina slurry on a Buehler polishing pad. They were rinsed and sonicated for 5 min in Barnstead Nanopure water. The Ag electrode was then placed in a 50% H2SO4 solution while the Au electrode was placed in concentrated HNO3 for 5 min. The Ag electrode was chemically polished following a procedure previously described.28 The Ag and Au electrodes were rinsed with Barnstead Nanopure water and dried under nitrogen, annealed with a flame, and protected with a drop of deoxygenated water. Bulk polycrystalline AuxAg(1−x) alloy strips (x in the range 0.05 to 0.50) were synthesized through a protocol described in a previous study.29 Rectangular portions of the polycrystalline alloy strips were cut, and one face of the strip was coated with nail polish while the other face had 0.5 ×
further investigation of the parameters that impact the outcome of the dealloying process such as structure, size, and shape would provide more insight into the way these parameters affect the porosity dimensions, surface area development, and structural stability of accordingly generated architectures on the nanoscale level. Experimental effort for understanding the dealloying process at the atomistic level using scanning tunneling microscopy was made for Ag80Au20 (111),20 Cu3Au (hkl),21,22 and Cu3Pt (111).16 Also, a recent kinetic Monte Carlo (KMC) simulation provided more insight into the immediate mechanism of AuxAg(1−x) NP dealloying and stability of the resulting structure.23 In this work, contributing to the study of dealloying on the nanoscale, we compare the dealloying of nanosized AuxAg(1−x) TFs deposited on Au and GC substrates with that of AuxAg(1−x) single-crystalline (111) and polycrystalline bulk alloys (Poly). Those results are also compared with the dealloying behavior of a set of AuxAg(1−x) NPs with diameters of 10−15 nm and atomic fraction, x, varying from 0 to 1. More specifically, we report on the electrochemical dealloying of the abovementioned materials with focus on Ec, the presence or absence of parting limits (dealloying threshold)24 characteristic for bulk AuxAg(1−x) alloys, the surface area evolution after dealloying, and the structural stability of the resulting nanoarchitectures with interconnected porosity. Finally, a critical analysis of these results and semiquantitative estimates of associated trends facilitates a discussion on the factors controlling the dealloying behavior of these types of alloys. The outcome of this work outlines future directions in the development of ultrathin and continuous nanoporous catalysts for immediate fuel cell applications.
2. EXPERIMENTAL SECTION The following chemicals were used for the experiments: Silver perchlorate AgClO4·H2O (Aldrich 99.999%), sodium perchlorate NaClO4·H2O (Aldrich 99.99% metal basis), lead perchlorate Pb(ClO4)2·2H2O (Aldrich ≥99.995%), perchloric acid HClO4 (GFS Chemicals 70%, veritas redistilled), nitric acid HNO3 (Fisher Scientific, Certified ACS Plus), phosphoric acid H3PO4 ((GFS Chemicals, 99.999%), 85% solution), ethylene glycol (Fisher Scientific, Certified), sodium thiosulfate Na2S2O3·5H2O (Alfa Aesar (99 +%)), gold(I) sodium thiosulfate hydrate Na3Au(S2O3)2·XH2O (Alfa Aesar, 99.9% metal basis), ethanol C2H6O (Pharmco-AAPER, 200 proof ACS/USP grade), sodium citrate Na3C6H5O7·2H2O (J.T. Baker, 99.5%), silver nitrate AgNO3 (Fisher Scientific, 99.98%), and sodium tetrachloroaurate(III) NaAuCl4·2H2O (Alfa Aesar, 99.99%). Barnstead Nanopure water (R > 18.2 M Ω cm) was used for the preparation of all solutions for electrodeposition, NP synthesis, and rinsing of all glassware. Unless stated otherwise, electrochemical processing was done with an EG&G Princeton Applied Research model 273 Potentiostat/Galvanostat and model 270/250 Research Electrochemistry Software 4.00. 2.1. Pure Ag, Au, and AuxAg(1−x) Alloy Nanoparticles Synthesis. Ag NPs were synthesized according to a protocol by Gopchandran et al.25 via reduction of AgNO3 under reflux with Na3C6H5O7. Au NPs were synthesized following a procedure introduced by Turkevich et al.26 via reduction of NaAuCl4 under reflux by Na3C6H5O7. AuxAg(1−x) alloy NPs of varying mole fractions were synthesized according to a modified protocol27 in aqueous solution by the coreduction of NaAuCl4 and AgNO3 with Na3C6H5O7. Mixed particles with 14124
dx.doi.org/10.1021/jp301603t | J. Phys. Chem. C 2012, 116, 14123−14133
The Journal of Physical Chemistry C
Article
0.5 cm square spots left exposed. A cotton applicator, 1 μm alumina powder, and water were used to polish the polycrystalline alloy strips. The strips were rinsed, sonicated in Barnstead Nanopure water for 5 min, and rinsed again. Cylindrical Cu working electrodes of diameter 6 mm were mechanically polished on 1200 grit silicon carbide paper, followed by 0.05 μm alumina powder on a Buehler polishing pad. The electrodes were then electrochemically polished in a 5:3:2 phosphoric acid/glycol/water solution at 3.84 V dc with a Pt ring functioning as the cathode. Cylindrical GC working electrodes with GC of diameter 5 mm bought from Goodfellow were first mechanically polished down to 1 μm with alumina slurry on a Buehler polishing pad, followed by rinsing and sonication for 5 min in a solution containing ethanol and Barnstead Nanopure water. They were then mechanically polished down to 0.05 μm with diamond suspension on a Buehler polishing pad, rinsed, and dried under ultrahigh purity N2. A pseudoreference (PRE) Ag/Ag+ electrode etched in 50% HNO3 at 50 °C was used in the electrodeposition of TFs and in the dealloying processes. A PRE Pb/Pb2+ electrode etched in 50% HNO3 at 50 °C was used in the Pb UPD surface area measurements. In all experiments, the Pt wire used as a CE was pretreated by submersion in 50% HNO3 at 50 °C, followed by thorough flame annealing. 2.7. Electrochemical Processing - UPD Measurements. The AuxAg(1−x) NP alloys supported on GC electrodes, AuxAg(1−x) (111) alloys, bulk Poly AuxAg(1−x) alloys, and AuxAg(1−x) TFs were first subjected to UPD-assisted surface area measurements29 by cyclic voltammetry (CV) in a solution containing 3 mM Pb(ClO4)2 + 0.1 M NaClO4 at a pH 2 adjusted with HClO4. The solution was deoxygenated with ultrahigh purity N2 for 2 h prior to Pb UPD and N2 passed over the solution surface during the scans. A Pb/Pb2+ PRE electrode and Pt wire CE were used, and the CV was controlled by a PC potentiostat/galvanostat (EC Epsilon, BASi). The potential was scanned from 0.600 to 0.010 V for six cycles at sweep rates of 20, 10, and 5 mVs−1 for Ag and Ag-rich alloys, whereas for Au and nanoporous gold (NPG), the potential was scanned from 0.700 to −0.010 V. The average charge from the forward and reverse scans was integrated from the curves of alloy samples and dealloyed samples. 2.8. Electrochemical Processing - Dealloying/NPG Preparation. Dealloying to remove Ag selectively from the alloys and measure their dealloying Ec was carried out in a 0.1 M HClO4 + 0.1 mM AgClO4 solution at a constant current density of 1 mA cm−2. Linear sweep voltammetry (LSV) with potential scanned from 0 to 0.800 V at a sweep rate of 1 mV s−1 versus Ag/Ag+ PRE electrode and Pt wire used as CE was performed on single-crystalline and polycrystalline AuxAg(1−x) alloys as well as on the AuxAg(1−x) NP alloys supported on GC electrodes. LSV with potential scanned from −0.010 to 0.800 V at a sweep rate of 0.2 mV s−1 versus Ag/Ag+ PRE electrode, and Pt wire CE was used for AuxAg(1−x) TF alloys. The samples imaged by SEM were dealloyed at a constant potential that was chosen to be 0.100 V more positive than the Ec for the respective ratios until the nominal current decayed to zero.
Figure 1. CV curves showing Pb UPD on AuxAg(1−x) (111) alloys (scan rate 20 mV s−1): (A) prior to dealloying and (B) after dealloying. (C) Anodic polarization curves illustrating the dealloying behavior of AuxAg(1−x) (111) alloys (scan rate 1 mV s−1).
(Figure 1B). Whereas Pb UPD CV curves are recorded initially on all alloys (Figure 1A), it is clear that only those with Ag content above the recently calculated parting limits (55 at % Ag)24 feature dealloying behavior with the alloy containing 35 at % Ag remaining passive (Figure 1C). An increase in surface area and substantial enrichment in Au29 as a result of the dealloying process are evidenced by the set of Pb UPD curves in Figure 1B. A top view of the morphology developed as a result of the dealloying process is presented in Figure 2. It can be seen that the structure and length scale of porosity depend substantially on the alloy composition of the samples prior to dealloying with the finest structure developing at 35 at % initial Au content. A similar set of experimental results is presented in
3. RESULTS 3.1. AuxAg(1−x) (111) and Polycrystalline AuxAg(1−x) Alloys. Figure 1 represents the electrochemical characterization of a set of AuxAg(1−x) (111) alloys by Pb UPD before (Figure 1A) and after (Figure 1B) the dealloying process. Figure 1C depicts specifically the dealloying behavior of that set 14125
dx.doi.org/10.1021/jp301603t | J. Phys. Chem. C 2012, 116, 14123−14133
The Journal of Physical Chemistry C
Article
Figure 2. FE SEM images showing the morphology of dealloyed (111) AuxAg(1−x) alloys with different initial Au content (x): (A) x = 10 at %, (B) x = 20 at %, and (C) x = 35 at %.
Figure 3 for a set of AuxAg(1−x) polycrystalline samples with Ag content in the range of 5 to 50 at % presumably encompassing the parting limits. As seen in Figure 3C, all samples in the set except those with atomic fraction x = 0.45 and 0.50 undergo dealloying (with the Au0.40Ag0.60 featuring borderline dissolution/passivation behavior) and develop surface area proportional to the amount of Ag dissolved/oxidized during dealloying.29 The inset in Figure 3C helps to visualize better the transition from dealloying to passive behavior that is a manifestation of the presence of parting limits contained within a compositional difference of ∼10 at %. The Pb UPD CVs registered before and after dealloying presented in Figure 3A,B, respectively, suggest also an increase in surface area for all dealloyed samples with up to 40 at % of Au. SEM images, obtained after dealloying of AuxAg(1−x) (Poly) samples with relevant composition and previously published in ref 29, suggest dependence between porosity length scale and original
Figure 3. CV curves showing Pb UPD on AuxAg(1−x) Poly alloys (scan rate 20 mV s−1): (A) prior to dealloying and (B) after dealloying. (C) Anodic polarization curves illustrating the dealloying behavior of AuxAg(1−x) Poly alloys (scan rate 1 mV s−1). Inset in panel C: Narrow potential range emphasizing the transition from dealloying (Au0.60Ag0.40) to passive behavior (Au0.55Ag0.45) upon anodization.
alloy composition that follows closely the trend seen in Figure 2. It is noteworthy that Pb UPD CV curves for alloys with Ag content
