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Dealloying and morphology evolution of ordered and disordered Cu3Au
Scripta Materialia 176 (2020) 112–116
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Dealloying and morphology evolution of ordered and disordered Cu3 Au Ariana Y. Tse, Erin K. Karasz, Karl Sieradzki∗ Ira A. Fulton School of Engineering, Arizona State University, Tempe AZ 85287, United States
a r t i c l e
i n f o
Article history: Received 10 August 2019 Revised 6 September 2019 Accepted 7 September 2019
Keywords: Nanoporous gold Copper alloys Order-disorder phenomena Electrochemistry Porous material
a b s t r a c t This research compares the electrochemical behavior and morphology evolution of nanoporous gold made from dealloying parent-phase ordered and disordered Cu3 Au. Using electron backscatter diffraction, we found that dealloying of both sets of Cu-Au samples leaves the grain orientations and grain shapes essentially unaltered with respect to that of the parent phase alloys. The mean ligament diameter, characterized by digital image analysis, made from the disordered alloy was 124 nm and that from the ordered alloy was 135 nm. A statistical T-test showed that this difference was not significant. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Nanoporous gold (NPG) has a bi-continuous solid/void morphology that is made by a selective electrochemical dissolution process. These morphologies are finding applications in diverse applications including bio-sensing [1–3], catalysis/electrocatalysis [4–8], drug delivery [9] and actuation [10–12]. The prototypical alloy used to produce this structure is Ag-Au within a composition range of ∼ 60–80 at. % Ag [13]. The bi-continuous structure forms by what is generally referred to as a self-organization process involving Ag dissolution and surface diffusion [14–15]. Interestingly, the crystal orientation of the parent phase is generally retained in the NPG structure [16,17], although in the case of gold leaf, an enhancement in crystallographic texture following dealloying has been reported [18]. Retention of the crystal orientation following dealloying is not surprising; as Ag dissolves, the remaining gold on the surface diffuses and clusters in a manner similar to an epitaxial vapor deposition process. The parent phase substrate serves as an effective template for the clustering of Au atoms. The NPG structure is typically characterized by a mean ligament diameter/length and similar measures of the void phase. Often, depending on electrochemical processing conditions, the porous structure can retain Ag in the range of ∼1–50 at.%. The aim of this manuscript is to gain a deeper understanding of the self-organization process by comparing various aspects of the electrochemical behavior and morphology evolution of ordered and disordered Cu3 Au. Henceforth, we will use
∗
Corresponding author. E-mail address: [email protected] (K. Sieradzki).
https://doi.org/10.1016/j.scriptamat.2019.09.008 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
the designations Cu3 Au and Cu0.75 Au0.25 respectively, when referring to the ordered and disordered alloy. Cu3 Au has the L12 structure in which the Au atoms occupy the corner cube positions and the Cu atoms occupy face-centered positions. Cu3 Au has a lattice parameter of 0.3743 nm [19]. Cu0.75 Au0.25 has a random face-centered cubic (FCC) structure and has a lattice parameter of 0.3754 nm [19]. This alloy composition is a model system for our particular study, as we are able to compare and contrast the electrochemical dealloying process and resultant structure in a binary alloy that involves a phase change as a result of dealloying. For example, as described above, in the case of Ag-Au alloys, electron backscatter diffraction (EBSD) has shown that the parent phase and the resultant NPG morphology have very similar grain orientations [16–18]. Since the lattice misfit between Ag and Au is only 0.17% this result is not surprising. On the other hand, the lattice misfit between Cu and Au is 11.35% so it is unclear whether or not crystal orientations will be maintained in the resultant NPG structures following parent phase dealloying of the FCC Cu0.75 Au0.25 alloy. In the case of the Cu3 Au alloy which necessarily involves a phase change upon dealloying, we expect to see differences in crystallite orientation and possibly grain shapes. One might also expect that the surface mobilities on the ordered and disordered alloys will be different enough to result in morphology differences (e.g., mean ligament diameter/length) in the NPG structures derived from the parent phase. 100 μm thick sheet of 75 at.% copper and 25 at.% gold alloy purchased from ESPI Metals was cut into 0.5 cm by 1.2 cm samples, polished with 0.05 μm alumina suspension, and encapsulated in quartz ampules and backfilled to 1/2 atm pressure with a mixture
A.Y. Tse, E.K. Karasz and K. Sieradzki / Scripta Materialia 176 (2020) 112–116
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Fig. 1. (A) XRD results for Cu0.75 Au0.25 and Cu3 Au samples (B) Linear sweep voltammetry of Cu3 Au and Cu0.75 Au0.25 alloys (sweep rate 0.1 mVs-1 ) and (C) chronopotentiometry for the galvanostatic electrochemical dealloying of Cu3 Au and Cu0.75 Au0.25 alloys (1 mAcm-2 ). Electrolyte: 0.5 M Na2 SO4 + 0.005 M H2 SO4 .
of 5% H2 /Ar. The samples were heat treated in a tube furnace. Samples to be structurally ordered were heat treated at 850 °C for 72 h, lowered to 380 °C for 72 h, and then slowly annealed by lowering the temperature 10 °C every 24 h to room temperature [1]. The samples to have a disordered crystal structure were heat treated at 850 °C for 72 h and water quenched. After heat treatment, electron dispersive spectroscopy (EDS), using a Philips XL30 ESEM-FEG (Cu Kα ), verified the composition of the samples. X-ray diffraction (XRD) on a Bruker D8 multipurpose Powder X-ray Diffractometer confirmed that the desired structures were obtained. Electron backscatter diffraction (EBSD) using a Zeiss Auriga FIB-SEM instrument determined the initial grain size and crystal orientations. Linear sweep voltammetry was conducted in a three-electrode cell using a Gamry PCI4/300 potentiostat to determine the critical potential of the ordered and disordered Cu-Au alloys. The electrolyte was 0.5 M Na2 SO4 + 0.005 M H2 SO4 and the potential was swept from 384 to 1394 mV vs. SHE at 0.1 mV per second. The reference electrode was saturated calomel (+241 mV vs. SHE) and a platinum mesh was used as the counter electrode. All potentials in this manuscript are converted to SHE. The ordered and disordered samples were dealloyed galvanostatically in 0.5 M Na2 SO4 + 0.005 M H2 SO4 at a current density of 1 mAcm−2 . This process took approximately 3 days to fully dealloy samples which was indicated by a rapid increase in voltage as a result of oxygen evolution. Following dealloying, the samples were rinsed with nanopure water allowed to dry in laboratory air and mounted for SEM examination. Following this dealloying procedure, the composition of the remaining copper in the NPG structures was determined using EDS. EBSD was conducted again to determine changes in grain size and crystal orientation. The dealloyed samples were cross-sectioned with a razor blade and digital image analysis was performed over multiple areas over the interior of the NPG structure. The NPG morphology was characterized using a Nova 200 NanoLabUHRFEG SEM system. Digital image analysis was performed using AQUAMI open source software. [20]. We confirmed the parent phase composition of the alloy sheet using EDS. The post heat treatment XRD data are shown in Fig. 1A.
The potential at which bulk nanoporosity forms, referred to in the corrosion literature as the critical potential for dealloying, is defined by the rapid upturn in current density for each of the alloys. As shown in Fig. 1B, the critical potential takes on values of 830 and 1080 mV vs. SHE for the disordered and ordered alloy respectively. These results showing a ∼250 mV difference in critical potentials are in excellent agreement with earlier work of Parks et al. [21]. It is important to point out that the surfaces of our alloys were not polished in any manner following heat treatment. Previous work that involved polishing sample surfaces after heat treatment resulted in some confusion regarding the magnitude of the critical potential for the ordered and disordered alloys [22]. Post heat treatment polishing tends to destroy the nearsurface ordering of Cu3 Au and results in a critical potential measurement for the ordered alloy close to that of the disordered phase. The difference in critical potentials for Cu3 Au and Cu0.75 Au0.25 results from two effects. One is the ordering energy and the other is the difference in size of the injected vacancy cluster length scale, ξ , resulting from terrace dissolution. Accordingly, this critical potential difference is given by [23],
V = Vordered − Vdisordered = + order ing ener gy
4γAu/elec nq
1
ξordered
−
1
ξdisordered (1)
where γ Au is the interfacial free energy of the Au/electrolyte interface (1.27 Jm−2 ), is the atomic volume of Cu (1.2 × 10−29 m3 ), n (2) is the number of electrons transferred in the dissolution process and q is the elementary charge. For the disordered alloy, percolation concepts are used to define the length scale, ξdisordered = (1 + p)a/(1 − p) = 7a, where p is the atom fraction of Cu in the alloy and a (0.2654 nm) is the nearest neighbor spacing. Percolation arguments cannot be used to evaluate ξ ordered . Instead, we examined the low index surfaces and normal cross-sections of the L12 structure to estimate ξ ordered to be ∼ =2a. We evaluate the first term in Eq. (1) to be 256 mV. The order/disorder temperature of Cu3 Au is 663 K corresponding to kB T = 57 mV [19]. First principles-based calculations for the heat of formation of the ordered alloy range from 37–71 mV [24] while calorimetry results for the difference
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Fig. 2. EBSD grain orientation maps of the ordered and disordered phases over the same region of the samples prior to and post dealloying. (A) Cu3 Au parent phase and (B) resultant NPG following dealloying. The blue-colored region in the upper left region of the image resulted from sample handling. (C) Cu0.75 Au0.25 parent phase and (D) resultant NPG following dealloying. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Statistical analysis of NPG morphology of the ordered and disordered phases. Cu3 Au: (A) Representative SEM image of the NPG morphology resulting from galvanostatic dealloying at 1 mAcm-2 . Cumulative histograms of the ligament size (B) and ligament length (C) obtained from DIA of 5 images on 4 different samples. Cu0.75 Au 0.25 : (D) Representative SEM image of the NPG morphology resulting from galvanostatic dealloying at 1 mAcm-2 . Cumulative histograms of the ligament size (E) and ligament length (F) obtained from DIA of 5 images on 4 different samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
A.Y. Tse, E.K. Karasz and K. Sieradzki / Scripta Materialia 176 (2020) 112–116
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Fig. 4. Representations of the small differences in ligament diameter statistics of the NPG formed from the parent phase ordered and disordered alloys. (A) Superimposed ligament diameter histograms and (B) Box and whisker plots of the same data. Parent phase ordered alloy (black), Parent phase disordered alloy (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
between the heats of formation of the ordered and disordered alloy is 23 mV [25]. Taking the mean value of the first principlesbased calculations, we obtain calculate V = 273 ± 17 mV . The greatest uncertainty in this estimate relates to the value of ξ ordered . In order to compare the morphologies of the ordered and disordered samples, dealloying was performed galvanostatically at a current density of 1 mAcm−2 . This was done to ensure that the dealloying rate of the alloys remained nominally identical and constant through the entire process, so that whatever morphology differences observed in the alloys could be connected to surface diffusion processes. EDS was used to determine the compositions of the resultant morphologies. Following the lengthy galvanostatic dealloying protocol, the resultant NPG structures for both the disordered and ordered alloys contained between 1–2 at.% Cu. Fig. 1C shows typical chronopotentiometry results for the galvanostatic dealloying. We observe that the 250 mV difference in the dealloying critical potentials discussed above is approximately maintained for the first ∼ 1.3 × 104 s after which the difference is only ∼30 mV. We believe that this small difference in potential may reflect a difference in surface mobility of the alloys. Fig. 2 shows electron back-scattered diffraction (EBSD) results of the grain boundary orientations before and after dealloying within the same location on sample surfaces. Fig. 2A and B show the results for the ordered alloy. We observe subtle changes in grain color indicating that some of the grain orientations may
have changed. Fig. 2C and D show similar results for the disordered alloy. Even though significant precautions were taken in handling the NPG samples, it is possible that these samples may have been slightly deformed in preparing them for examination. Even so, there are not significant differences between grain orientations of parent phase and the corresponding NPG grain orientations in either the disordered or the ordered samples. Dealloying of both sets of Cu-Au samples leaves the grain orientations and shapes essentially unaltered with respect to that of the parent phase alloys. Fig. 3 shows digital image analysis (DIA) results of the NPG formed as a result of dealloying the ordered and disordered alloys. The ligament diameter histograms (Fig. 3B and E) have a normal distribution, while the ligament length histograms (Fig. 3C and F) are asymmetric. However, in both cases the histograms are similar. Fig. 4A shows the ligament diameter histograms superimposed in order to more clearly illustrate the differences. For the NPG made from the disordered samples the mean and median ligament diameters are equal (124 nm) while for the ordered alloys the mean and median ligament diameters are 135 and 133 nm respectively. Fig. 4B which shows the corresponding box and whisker plot (with the histograms embedded) illustrate these small differences in the statistical data more clearly. We conducted a statistical T-test in order to determine whether the difference in the diameter mean values is significant. We used a pooled variance estimate for the difference in the standard error and a confidence value of 95%. Our
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results indicate that there is not sufficient evidence to conclude that there is a difference in the mean values of the NPG ligament diameters produced from the parent phase ordered and disordered samples. In summary we investigated the electrochemical dealloying and morphology evolution of ordered and disordered Cu3 Au. We found that the difference in the electrochemical critical potential for the formation of bulk nanoporous gold is ∼ =250 mV in agreement with earlier work [21]. Electron back-scattered diffraction was used to determine whether the grain orientations changed following the formation of nanoporous gold. For each of the alloys, we found no significant change in grain orientations following dealloying. For the ordered alloy, that involves a phase change from the L12 parent phase structure to FCC, we believe that the general cubic structure of the parent phase enforces the retention of the grain orientations in the nanoporous gold. Digital image analysis was used to characterize the statistics for ligament diameter and length in the nanoporous gold formed from the parent phase alloys. In total 20 different images were analyzed for each of these alloys. For the constant current dealloying conditions (1 mAcm−2 ) used in this study, we found that the mean ligament diameter for the ordered alloy was 135 nm and that for the disordered alloy was 124 nm. A statistical T-test was used to determine whether or not this difference in mean ligament diameter is significant with the result that there is insufficient evidence to make such a claim. Since the morphology length scales (ligament diameter, ligament length) in these structures depend on surface diffusion in the electrolyte, our results suggest that there is not a large difference in surface diffusion of gold on the ordered and disordered surfaces.
Acknowledgment The authors are grateful for the support of this work by the US DOE Basic Energy Sciences under award DE-SC0 0 08677. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
E. Seker, W.-.C. Shih, K.J. Stine, MRS Bull. 48 (2018) 49–56. K. Hu, D. Lan, X. Li, S. Zhang, J. Electroanal. Chem. 80 (2008) 49–56. Y. Zheng, et al., J. Electroanal. Chem. 840 (2019) 165–173. A. Wittstock, V. Zielasek, J. Biener, C.M. Friend, M. Baumer, Science 327 (2010) 319–322. K.J. Stowers, R.J. Madix, C.M. Friend, J. Catal. 308 (2013) 131–141. W. Dononelli, et al., ACS Catal. 9 (2019) 5201–5216. C. Reece, M. Luneau, R.J. Madix, ACS Catal. 9 (2019) 4477–4484. T. Fugita, et al., Nat. Mat. 11 (2012) 775–780. V. Garcia-Gradilla, et al., Small 10 (2014) 4154–4159. H.-.J. Jin, X.-.L. Wang, S. Parida, K. Wang, M. Seo, J. Weissmüller, Nano Lett. 10 (2010) 187–194. H.-.J. Jin, J. Weissmüller, Science 332 (2011) 1179–1182. E. Detsi, Z.G. Chen, W.P. Vellinga, P.R. Onck, J.T.M. De Hosson, J. Nanosci, . Nanotechnol 12 (2012) 4951–4955. J. Weissmüller, K. Sieradzki, MRS Bull. 48 (2018) 14–19. K. Sieradzki, J. Electrochem. Soc. 140 (1993) 2868–2872. J. Erlebacher, et al., Nature 410 (2001) 450–453. H. Rösner, S. Parida, D. Kramer, C. Volkert, J. Weissmüller, Adv. Eng. Mater. 9 (2019) 535–541. S.V. Petegem, et al., Nano Lett. 9 (2009) 1158–1163. L.T.H. De Jeer, et al., Microsc. Microanal. 21 (2015) 1387–1397. H. Okamoto, D.J. Chakrabarti, D. Laughlin, T.B. Massalski, Bull. Alloy Phase Diagr. 8 (1987) 454–473. J.A. Stuckner, et al., Comp. Mat. Sci. 139 (2017) 320–329. W.B. Parks, J.D. Fritz, H.W. Pickering, Scripta Metall. 23 (1989) 951–956. T.P. Moffat, F-R F. Ren, A.J. Bard, J. Electrochem. Soc. 138 (1991) 3224–3235. R. Rugolo, J. Erlebacher, K. Sieradzki, Nat. Mat. 5 (2006) 946–949. Y. Zhang, G. Gresse, C. Wolverton, Phys. Rev. Lett. 112 (2014) 075502. R.L. Orr, Acta Metall. 8 (1960) 489–493.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Dealloying and morphology evolution of ordered and disordered Cu3 Au Ariana Y. Tse, Erin K. Karasz, Karl Sieradzki∗ Ira A. Fulton School of Engineering, Arizona State University, Tempe AZ 85287, United States
a r t i c l e
i n f o
Article history: Received 10 August 2019 Revised 6 September 2019 Accepted 7 September 2019
Keywords: Nanoporous gold Copper alloys Order-disorder phenomena Electrochemistry Porous material
a b s t r a c t This research compares the electrochemical behavior and morphology evolution of nanoporous gold made from dealloying parent-phase ordered and disordered Cu3 Au. Using electron backscatter diffraction, we found that dealloying of both sets of Cu-Au samples leaves the grain orientations and grain shapes essentially unaltered with respect to that of the parent phase alloys. The mean ligament diameter, characterized by digital image analysis, made from the disordered alloy was 124 nm and that from the ordered alloy was 135 nm. A statistical T-test showed that this difference was not significant. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Nanoporous gold (NPG) has a bi-continuous solid/void morphology that is made by a selective electrochemical dissolution process. These morphologies are finding applications in diverse applications including bio-sensing [1–3], catalysis/electrocatalysis [4–8], drug delivery [9] and actuation [10–12]. The prototypical alloy used to produce this structure is Ag-Au within a composition range of ∼ 60–80 at. % Ag [13]. The bi-continuous structure forms by what is generally referred to as a self-organization process involving Ag dissolution and surface diffusion [14–15]. Interestingly, the crystal orientation of the parent phase is generally retained in the NPG structure [16,17], although in the case of gold leaf, an enhancement in crystallographic texture following dealloying has been reported [18]. Retention of the crystal orientation following dealloying is not surprising; as Ag dissolves, the remaining gold on the surface diffuses and clusters in a manner similar to an epitaxial vapor deposition process. The parent phase substrate serves as an effective template for the clustering of Au atoms. The NPG structure is typically characterized by a mean ligament diameter/length and similar measures of the void phase. Often, depending on electrochemical processing conditions, the porous structure can retain Ag in the range of ∼1–50 at.%. The aim of this manuscript is to gain a deeper understanding of the self-organization process by comparing various aspects of the electrochemical behavior and morphology evolution of ordered and disordered Cu3 Au. Henceforth, we will use
∗
Corresponding author. E-mail address: [email protected] (K. Sieradzki).
https://doi.org/10.1016/j.scriptamat.2019.09.008 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
the designations Cu3 Au and Cu0.75 Au0.25 respectively, when referring to the ordered and disordered alloy. Cu3 Au has the L12 structure in which the Au atoms occupy the corner cube positions and the Cu atoms occupy face-centered positions. Cu3 Au has a lattice parameter of 0.3743 nm [19]. Cu0.75 Au0.25 has a random face-centered cubic (FCC) structure and has a lattice parameter of 0.3754 nm [19]. This alloy composition is a model system for our particular study, as we are able to compare and contrast the electrochemical dealloying process and resultant structure in a binary alloy that involves a phase change as a result of dealloying. For example, as described above, in the case of Ag-Au alloys, electron backscatter diffraction (EBSD) has shown that the parent phase and the resultant NPG morphology have very similar grain orientations [16–18]. Since the lattice misfit between Ag and Au is only 0.17% this result is not surprising. On the other hand, the lattice misfit between Cu and Au is 11.35% so it is unclear whether or not crystal orientations will be maintained in the resultant NPG structures following parent phase dealloying of the FCC Cu0.75 Au0.25 alloy. In the case of the Cu3 Au alloy which necessarily involves a phase change upon dealloying, we expect to see differences in crystallite orientation and possibly grain shapes. One might also expect that the surface mobilities on the ordered and disordered alloys will be different enough to result in morphology differences (e.g., mean ligament diameter/length) in the NPG structures derived from the parent phase. 100 μm thick sheet of 75 at.% copper and 25 at.% gold alloy purchased from ESPI Metals was cut into 0.5 cm by 1.2 cm samples, polished with 0.05 μm alumina suspension, and encapsulated in quartz ampules and backfilled to 1/2 atm pressure with a mixture
A.Y. Tse, E.K. Karasz and K. Sieradzki / Scripta Materialia 176 (2020) 112–116
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Fig. 1. (A) XRD results for Cu0.75 Au0.25 and Cu3 Au samples (B) Linear sweep voltammetry of Cu3 Au and Cu0.75 Au0.25 alloys (sweep rate 0.1 mVs-1 ) and (C) chronopotentiometry for the galvanostatic electrochemical dealloying of Cu3 Au and Cu0.75 Au0.25 alloys (1 mAcm-2 ). Electrolyte: 0.5 M Na2 SO4 + 0.005 M H2 SO4 .
of 5% H2 /Ar. The samples were heat treated in a tube furnace. Samples to be structurally ordered were heat treated at 850 °C for 72 h, lowered to 380 °C for 72 h, and then slowly annealed by lowering the temperature 10 °C every 24 h to room temperature [1]. The samples to have a disordered crystal structure were heat treated at 850 °C for 72 h and water quenched. After heat treatment, electron dispersive spectroscopy (EDS), using a Philips XL30 ESEM-FEG (Cu Kα ), verified the composition of the samples. X-ray diffraction (XRD) on a Bruker D8 multipurpose Powder X-ray Diffractometer confirmed that the desired structures were obtained. Electron backscatter diffraction (EBSD) using a Zeiss Auriga FIB-SEM instrument determined the initial grain size and crystal orientations. Linear sweep voltammetry was conducted in a three-electrode cell using a Gamry PCI4/300 potentiostat to determine the critical potential of the ordered and disordered Cu-Au alloys. The electrolyte was 0.5 M Na2 SO4 + 0.005 M H2 SO4 and the potential was swept from 384 to 1394 mV vs. SHE at 0.1 mV per second. The reference electrode was saturated calomel (+241 mV vs. SHE) and a platinum mesh was used as the counter electrode. All potentials in this manuscript are converted to SHE. The ordered and disordered samples were dealloyed galvanostatically in 0.5 M Na2 SO4 + 0.005 M H2 SO4 at a current density of 1 mAcm−2 . This process took approximately 3 days to fully dealloy samples which was indicated by a rapid increase in voltage as a result of oxygen evolution. Following dealloying, the samples were rinsed with nanopure water allowed to dry in laboratory air and mounted for SEM examination. Following this dealloying procedure, the composition of the remaining copper in the NPG structures was determined using EDS. EBSD was conducted again to determine changes in grain size and crystal orientation. The dealloyed samples were cross-sectioned with a razor blade and digital image analysis was performed over multiple areas over the interior of the NPG structure. The NPG morphology was characterized using a Nova 200 NanoLabUHRFEG SEM system. Digital image analysis was performed using AQUAMI open source software. [20]. We confirmed the parent phase composition of the alloy sheet using EDS. The post heat treatment XRD data are shown in Fig. 1A.
The potential at which bulk nanoporosity forms, referred to in the corrosion literature as the critical potential for dealloying, is defined by the rapid upturn in current density for each of the alloys. As shown in Fig. 1B, the critical potential takes on values of 830 and 1080 mV vs. SHE for the disordered and ordered alloy respectively. These results showing a ∼250 mV difference in critical potentials are in excellent agreement with earlier work of Parks et al. [21]. It is important to point out that the surfaces of our alloys were not polished in any manner following heat treatment. Previous work that involved polishing sample surfaces after heat treatment resulted in some confusion regarding the magnitude of the critical potential for the ordered and disordered alloys [22]. Post heat treatment polishing tends to destroy the nearsurface ordering of Cu3 Au and results in a critical potential measurement for the ordered alloy close to that of the disordered phase. The difference in critical potentials for Cu3 Au and Cu0.75 Au0.25 results from two effects. One is the ordering energy and the other is the difference in size of the injected vacancy cluster length scale, ξ , resulting from terrace dissolution. Accordingly, this critical potential difference is given by [23],
V = Vordered − Vdisordered = + order ing ener gy
4γAu/elec nq
1
ξordered
−
1
ξdisordered (1)
where γ Au is the interfacial free energy of the Au/electrolyte interface (1.27 Jm−2 ), is the atomic volume of Cu (1.2 × 10−29 m3 ), n (2) is the number of electrons transferred in the dissolution process and q is the elementary charge. For the disordered alloy, percolation concepts are used to define the length scale, ξdisordered = (1 + p)a/(1 − p) = 7a, where p is the atom fraction of Cu in the alloy and a (0.2654 nm) is the nearest neighbor spacing. Percolation arguments cannot be used to evaluate ξ ordered . Instead, we examined the low index surfaces and normal cross-sections of the L12 structure to estimate ξ ordered to be ∼ =2a. We evaluate the first term in Eq. (1) to be 256 mV. The order/disorder temperature of Cu3 Au is 663 K corresponding to kB T = 57 mV [19]. First principles-based calculations for the heat of formation of the ordered alloy range from 37–71 mV [24] while calorimetry results for the difference
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Fig. 2. EBSD grain orientation maps of the ordered and disordered phases over the same region of the samples prior to and post dealloying. (A) Cu3 Au parent phase and (B) resultant NPG following dealloying. The blue-colored region in the upper left region of the image resulted from sample handling. (C) Cu0.75 Au0.25 parent phase and (D) resultant NPG following dealloying. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Statistical analysis of NPG morphology of the ordered and disordered phases. Cu3 Au: (A) Representative SEM image of the NPG morphology resulting from galvanostatic dealloying at 1 mAcm-2 . Cumulative histograms of the ligament size (B) and ligament length (C) obtained from DIA of 5 images on 4 different samples. Cu0.75 Au 0.25 : (D) Representative SEM image of the NPG morphology resulting from galvanostatic dealloying at 1 mAcm-2 . Cumulative histograms of the ligament size (E) and ligament length (F) obtained from DIA of 5 images on 4 different samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
A.Y. Tse, E.K. Karasz and K. Sieradzki / Scripta Materialia 176 (2020) 112–116
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Fig. 4. Representations of the small differences in ligament diameter statistics of the NPG formed from the parent phase ordered and disordered alloys. (A) Superimposed ligament diameter histograms and (B) Box and whisker plots of the same data. Parent phase ordered alloy (black), Parent phase disordered alloy (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
between the heats of formation of the ordered and disordered alloy is 23 mV [25]. Taking the mean value of the first principlesbased calculations, we obtain calculate V = 273 ± 17 mV . The greatest uncertainty in this estimate relates to the value of ξ ordered . In order to compare the morphologies of the ordered and disordered samples, dealloying was performed galvanostatically at a current density of 1 mAcm−2 . This was done to ensure that the dealloying rate of the alloys remained nominally identical and constant through the entire process, so that whatever morphology differences observed in the alloys could be connected to surface diffusion processes. EDS was used to determine the compositions of the resultant morphologies. Following the lengthy galvanostatic dealloying protocol, the resultant NPG structures for both the disordered and ordered alloys contained between 1–2 at.% Cu. Fig. 1C shows typical chronopotentiometry results for the galvanostatic dealloying. We observe that the 250 mV difference in the dealloying critical potentials discussed above is approximately maintained for the first ∼ 1.3 × 104 s after which the difference is only ∼30 mV. We believe that this small difference in potential may reflect a difference in surface mobility of the alloys. Fig. 2 shows electron back-scattered diffraction (EBSD) results of the grain boundary orientations before and after dealloying within the same location on sample surfaces. Fig. 2A and B show the results for the ordered alloy. We observe subtle changes in grain color indicating that some of the grain orientations may
have changed. Fig. 2C and D show similar results for the disordered alloy. Even though significant precautions were taken in handling the NPG samples, it is possible that these samples may have been slightly deformed in preparing them for examination. Even so, there are not significant differences between grain orientations of parent phase and the corresponding NPG grain orientations in either the disordered or the ordered samples. Dealloying of both sets of Cu-Au samples leaves the grain orientations and shapes essentially unaltered with respect to that of the parent phase alloys. Fig. 3 shows digital image analysis (DIA) results of the NPG formed as a result of dealloying the ordered and disordered alloys. The ligament diameter histograms (Fig. 3B and E) have a normal distribution, while the ligament length histograms (Fig. 3C and F) are asymmetric. However, in both cases the histograms are similar. Fig. 4A shows the ligament diameter histograms superimposed in order to more clearly illustrate the differences. For the NPG made from the disordered samples the mean and median ligament diameters are equal (124 nm) while for the ordered alloys the mean and median ligament diameters are 135 and 133 nm respectively. Fig. 4B which shows the corresponding box and whisker plot (with the histograms embedded) illustrate these small differences in the statistical data more clearly. We conducted a statistical T-test in order to determine whether the difference in the diameter mean values is significant. We used a pooled variance estimate for the difference in the standard error and a confidence value of 95%. Our
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results indicate that there is not sufficient evidence to conclude that there is a difference in the mean values of the NPG ligament diameters produced from the parent phase ordered and disordered samples. In summary we investigated the electrochemical dealloying and morphology evolution of ordered and disordered Cu3 Au. We found that the difference in the electrochemical critical potential for the formation of bulk nanoporous gold is ∼ =250 mV in agreement with earlier work [21]. Electron back-scattered diffraction was used to determine whether the grain orientations changed following the formation of nanoporous gold. For each of the alloys, we found no significant change in grain orientations following dealloying. For the ordered alloy, that involves a phase change from the L12 parent phase structure to FCC, we believe that the general cubic structure of the parent phase enforces the retention of the grain orientations in the nanoporous gold. Digital image analysis was used to characterize the statistics for ligament diameter and length in the nanoporous gold formed from the parent phase alloys. In total 20 different images were analyzed for each of these alloys. For the constant current dealloying conditions (1 mAcm−2 ) used in this study, we found that the mean ligament diameter for the ordered alloy was 135 nm and that for the disordered alloy was 124 nm. A statistical T-test was used to determine whether or not this difference in mean ligament diameter is significant with the result that there is insufficient evidence to make such a claim. Since the morphology length scales (ligament diameter, ligament length) in these structures depend on surface diffusion in the electrolyte, our results suggest that there is not a large difference in surface diffusion of gold on the ordered and disordered surfaces.
Acknowledgment The authors are grateful for the support of this work by the US DOE Basic Energy Sciences under award DE-SC0 0 08677. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
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