On the Origin of the Volume Contraction during Nanoporous Gold

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On the Origin of the Volume Contraction during Nanoporous Gold Formation by Dealloying for High-Performance Electrochemical Applications Ke Ma, John S. Corsi, Jintao Fu, and Eric Detsi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00055 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Nano Materials

On the Origin of the Volume Contraction during Nanoporous Gold Formation by Dealloying for High-Performance Electrochemical Applications Ke Ma†, John S. Corsi†, Jintao Fu† and Eric Detsi †,*

†

Department of Materials Science & Engineering, University of Pennsylvania, Philadelphia PA

19104-6272, USA. *Correspondence should be addressed to E.D. ([email protected])

ABSTRACT Nanoporous metals used in various electrochemical applications including electrochemical actuators, electrocatalysts, supercapacitors and batteries exhibit an irreversible volume shrinkage during their formation by dealloying, the origin of which remains obscure. Here we use dilatometry techniques to measure the irreversible shrinkage in nanoporous Au in situ during electrochemical dealloying. A linear contraction up to ~9 % was recorded. To identify the origin of this dimensional change, we borrow the time-dependent isothermal shrinkage model from sintering theory, which we use to fit the dimensional changes measured in our nanoporous Au during dealloying. This shrinkage model suggests that bulk transport through plastic flow is the primary mass transport mechanism responsible for the material contraction in dealloying. Based on the current understanding of the mechanism of porosity formation in dealloying, mass transport through surface diffusion of undissolved materials is critical in the process. The present work sheds new light in the sense that bulk transport through plastic flow seems also to play an important role in dealloying.

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Keywords: In situ dilatometry, volume shrinkage, electrochemically dealloying, nanoporous gold, sintering theory, plastic flow, surface diffusion

1. Introduction

Nanoporous metals and their composites have gained considerable interest due to their exceptional materials properties,1,2,3,4,5,6,7,8,9,10,11 and their broad range of potential applications as catalysts,12,13 electrocatalysts14,15,16,17,18,19,20 Li-ion battery electrodes,21,22,23,24 Na-ion battery electrodes25, electrochemical actuators,2,26,27,2829,30 sensors,4,31,32,33 and plasmonic devices.34,35,36,37 Among these, nanoporous gold (NP-Au) is the most studied system. Even though the mechanism of NP-Au formation by selective removal of silver from gold-silver alloys has been widely investigated,38 some aspects of that dealloying process are still obscure. For instance, a large irreversible volume shrinkage has been reported during the formation of NP-Au by dealloying,39,40 however the origin of such a volume contraction remains unknown.26 Weissmüller and co-workers found an unusually high number of lattice defects and local plastic deformation sites from electron micrographs of dealloyed NP-Au, and suggested that these defects were responsible for the volume shrinkage in dealloying.39 To date, their findings have not been further investigated for validation. In this work, experimental and analytical methods are used to investigate the origin of the volume contraction in dealloyed NP-Au. First, we use dilatometry techniques26,27,

29

to probe the linear shrinkage in NP-Au in situ during

electrochemical dealloying. Next, we borrow two models from sintering theory, namely the relation between macroscopic shrinkage in sintered materials and the corresponding microscopic neck size ratio on one hand,41 and the time-dependent isothermal macroscopic shrinkage model


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on the other hand,41 to identify the primary mass transport mechanism responsible for the contraction in NP-Au. Our results suggest that shrinkage during dealloying is driven by bulk transport through plastic flow.

2. Experimental and Analytical Methods

2.1. Sample preparation A gold-silver master alloy with composition Au25Ag75 at. % was made by melting ~3.1 g of pure gold (99.9 %, Abington Reldan Metals, LLC) and ~5.0 g of pure silver (99.9 %, Alfa Asear) following the procedure in reference.26 The resulting gold-silver alloy was cold-rolled down to a uniform thickness of 25 µm.26,42 Disks with diameter ~6.0 mm and thickness ~25 µm were then cut from this gold-silver alloy using a high-precision disk cutter (MSK-T-09, MTI Corp). Prior to dealloying, these disks were smoothly polished using silicon carbide polishing papers, and the final thickness was accurately measured using an optical microscope. Note that the thickness of the starting alloy was also verified using scanning electron microscopy (see supporting figure S1). However, since we are dealing with macroscopic bulk samples (i.e. 6 mm in diameter), scanning electron microscopy only shows a very small fraction of the 6000 µm sample diameter. Therefore, optical microscopy was preferred.

2.2. In situ dilatometry during dealloying The Au25Ag75 disks were electrochemically dealloyed under either potentiostatic or galvanostatic control. Potentiostatic controlled electrochemical dealloying is a process in which a constant 3 ACS Paragon Plus Environment


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voltage is maintained between the working and reference electrode in order to dissolve Ag, and the corresponding Ag dissolution current between the working and counter electrode is measured.43 Galvanostatic controlled electrochemical dealloying is a process in which a constant Ag dissolution current is maintained between the working and counter electrode and the corresponding change in overpotential between the working and reference electrode is measured. A high-precision dilatometer (ECD-3-nano) with a resolution of ~5 nm was used to measure the dimensional changes (specifically changes in thickness) in the disk in situ during electrochemical dealloying. The dilatometer cell which we refer here to as “E-Cell”, was configured as a Ag halfcell, in analogy to Li half-cells in Li-ion batteries.21,24 In this configuration, the Au25Ag75 disk to dealloy was used as the working electrode, a pure Ag disk was used as the counter and reference electrodes and 1.0 M AgNO3 made by dissolving an appropriate amount of AgNO3 (99+%, Acros Organics) in DI water was used as the Ag-ion electrolyte.44 The dealloying process was performed under ambient conditions (room temperature and atmospheric pressure). Note that although acidic electrolytes are commonly used for electrochemical dealloying, benign routes to fabricate nanostructured materials are needed and the use of a neutral pH electrolyte as in this work is desirable. 2.3. Characterization Scanning electron microscopy (SEM) JEOL 7500 and transmission electron microscopy (TEM) JEOL 2100 were used to analyze the microstructure of the dealloyed samples. 2.4. Analytical methods On the one hand, it is known that porosity formation during dealloying involves surface diffusion of undissolved Au adatoms to form Au clusters, which further coalesce into isostructural porous 4 ACS Paragon Plus Environment

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networks.38 On the other hand, it is also known that during sintering a material in the form of powder (i.e. fine grains) is compacted into a monolithic structure, which is commonly porous.41 This thermally-driven (or pressure-driven) process results in a significant volume shrinkage in the sintered material.41 As assumption in this work, we state that the mechanism of coalescence of Au clusters during dealloying can be assimilated with coalescence of particles during sintering. The mechanism of sintering has been widely investigated and mass transport through bulk processes such as plastic flow, grain boundary diffusion and volume diffusion are known to induce large volume shrinkage in sintered materials.41 Interestingly, most sintering models do not predict materials shrinkage when mass transport is driven by surface processes such as surface diffusion and evaporation-condensation.41 This suggests in the case of dealloying that even though surface 38

diffusion plays a key role in porosity formation,

the large

volume shrinkage in dealloyed NP-Au may not be caused by

Figure 1. (a-c) Illustration of neck formation during coalescence of two particles. X is the neck size, D is the particle diameter.

surface diffusion processes. Eq. 1 below relates the macroscopic shrinkage in a sintered material to the microscopic neck size ratio in the underlying grains, and the illustration in Figure 1 shows how the neck size ratio is defined.41 ∆

Eq. 1



In Eq.1,

∆

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represents the relative sintering shrinkage (i.e. change in dimension normalized by

the initial length), D and X correspond to the particle and neck sizes, respectively. X/D represents the neck size ratio (see Figure 1). The empirical factor b is reported as 3.6 in literature.41 The empirical value b is associated with bulk transport processes in sintering, irrespective of the type of material.41 In the present work, the relative shrinkage

∆

in NP-Au was

experimentally measured in situ during dealloying, and the relation in Eq. 1 was used to deduce the characteristic neck size ratio in dealloyed NP-Au, and compare it with the actual value directly measured from electron micrographs of NP-Au. In addition to Eq.1, the time-dependent isothermal shrinkage model for sintering given by Eq. 2 below is commonly used to predict the contraction behavior of porous materials as a results of sintering:41

∆

Here also,

∆

Eq. 2

and D represent the relative sintering shrinkage and particle size, respectively. B is

a term made up of material and geometric constants and t is the sintering time.41 The parameter m corresponds to the “Herring scaling law exponent” and its value depends on the specific mass transport mechanism taking place during sintering.41,45 However, since the derivation of m is based on the assumption that changes in the geometry of sintered particles remain the same (i.e. geometrically similar microstructural changes),45 which is obviously not the case during porosity formation in dealloying, the value of m cannot be used to identify the mass transport mechanism in dealloying. Finally, in Eq. 2, n is another exponent which value also depends on the mass transport mechanism in sintering.41 Typical values of n are shown in Table 1 below for four 6 ACS Paragon Plus Environment

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common mass transport mechanisms. In the present work, knowing the relative shrinkage

∆

in

NP-Au as a function of the dealloying time, we use the relation in Eq. 2 to deduce the parameter n. This can be done without knowing B, D and m, simply by plotting the logarithm of the relative ∆

shrinkage i.e. log( ) as a function of the logarithm of the time i.e. log(). The corresponding

slope, which is

can be used to derived n and deduce from Table 141 the primary mass transport

mechanism responsible for materials shrinkage in dealloying. Note that, although given as integer, the value of n can vary slightly with the degree of sintering. For example, n values up to 7.5 are obtained in some models for surface diffusion sintering.41

Table 1. Values of n for different mass transport mechanisms41 Mass Transport Mechanism Surface diffusion Ground boundary diffusion Volume diffusion Plastic flow

n 7 6 5 2

3. Results and discussions 3.1. Electrochemical dealloying in Ag half-cell



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Figure 2a shows three successive cyclic voltammograms obtained from the Au25Ag75 alloy in the Ag half-cell configuration. The scans were carried out in the voltage range between -200 mV to 400 mV vs Ag/Ag+ at the sweep rate of 5 mV/s. The shape of these voltammograms is typical for electrochemical plating/stripping processes.46 The oxidation peak centered around 200 mV overpotential is associated with Ag removal from the Au25Ag75 working

electrode

through

oxidation44,46

following Eq. 3, and plating of Ag onto the Ag metal counter electrode through reduction following Eq. 4. → →

Eq. 3 Eq. 4

The negative current at potentials below 0 V vs Ag/Ag+ is simply the reverse process, i.e. plating of Ag on the Au25Ag75 working electrode through reduction following Eq. 4 Figure 2. (a) Cyclic voltammograms obtained

during stripping of Ag from Au-Ag alloy working and removal of Ag from the Ag metal counter electrode and plating of Ag onto pure Ag counter electrode. (b) Current vs. time recorded during potentiostatic dealloying. (c) Voltage vs time electrode through oxidation following in Eq. 3. recorded during galvanostatic dealloying.


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Since no acid is used in this experiment, the present result shows that NP-Au formation by dealloying can take place in neutral pH as previously reported by Snyder et al.44 Once the half-cell signature was verified as depicted in Figure 2a, electrochemical dealloying was carried out in an appropriate voltage window in either potentiostatic or galvanostatic mode. Figures 2b and 2c show the typical curves obtained during electrochemical dealloying in potentiostatic mode (applied overpotential: 500 mV, dealloying time: 3 h) and galvanostatic mode (applied current: 450 µA, dealloying time: 3.5 h), respectively. The total equivalent electric charge associated with Ag dissolution was ~7.1 C and 6.6 C for the dealloying in potentiostatic and galvanostatic mode, respectively. This difference in charge can be associated with small differences in the mass of the disks after polishing.

Microstructural characterization Figure 3. (a) Au-Ag alloy before (shining silvery)

Fig. 3a shows a Au25Ag75 disk before (silvery and after (brown) dealloying. (b) SEM, (c, d) TEM appearance)

and

after

(brown

of NP-Au. The blue and red markers in (d) show

color) how the neck size X and diameter D are measured.

dealloying. SEM and TEM of the dealloyed sample are shown in Fig. 3b and Fig. 3c, 3d, respectively. The blue and red arrow from the TEM image of Fig. 3d are aimed at showing how 9 ACS Paragon Plus Environment


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the “neck size” and “grain diameter” as defined in Figure 1 can be obtained from dealloyed nanoporous gold (see also supporting Figure S2). From analyzing several TEM micrographs, the characteristic ligament size was found to range between ~10 to 15 nm and the average neck size ratio

was found to be ~0.7. Figure S2 from the Online Supplementary Information shows

a typical distribution of X and D for the TEM micrograph of Fig. 3c. According to Eq. 1, the overall macroscopic shrinkage

∆

in NP-Au should be directly proportional to the neck size ratio

41

. This will be verified in the next sub-sections.

3.2. In situ dilatometry Fig. 4 shows the linear strain (relative change in thickness) measured as a function of charge transferred during Ag dissolution (bottom x-axis) and mass of Ag dissolved (top x-axis). The red curve corresponds to dealloying at constant overpotential (500 mV), while the blue curve is associated with dealloying under Figure 4. Linear strains versus charge transferred (bottom

constant current (450 µA). Both curves x-axis) and mass of dissolved silver (top x-axis) during dealloying in potentiostatic (red) and galvanostatic (blue)

show

a

strong

dependence

of

the modes.

shrinkage on the charge transferred. A similar behavior was reported by Weissmüller and coworkers.39 The initial thickness of 25.00 µm shrinks to ~23.00 µm for the blue curve 10 ACS Paragon Plus Environment

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(corresponding to ~8 % linear contraction) and to the ~22.75 µm for the red curve (corresponding to ~9 % linear contraction). These linear strain values are comparable to those reported in reference.39 Note that since the material is isotropic, the value of the linear strain should be the same for any given direction, so that the relative change in volume (i.e. volume strain) can be deduced from the linear strain measured in a single direction.39

3.3. Analytical models 3.3.1 Shrinkage and neck size ratio The neck size ratio X/D was deduced from the time dependent linear strain data

∆

measured

during dealloying, as depicted in Figure 5a and 5b, using Eq.1.41 At the end of the dealloying process, the ~9 % relative shrinkage for the sample dealloyed at constant voltage gives a neck size ratio of ~0.6. Interestingly, this value is comparable to the one (i.e. 0.7) directly measured from electron micrograph images (see Figure 3d and supporting Figure S2). Knowing that in sintering Eq.1 is only valid for mass transport through bulk processes rather than surface diffusion, we can conclude that since Eq.1 seems to predict the contraction in dealloying, the shrinkage mechanism in NP-Au should also involve bulk processes such as volume diffusion, grain boundary diffusion, plastic flow. In the next sub-section, we will identify the specific bulk transport mechanism.



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Figure 5. (a) Measured macroscopic linear strain as a function of the dealloying time for the potentiostatic dealloying mode. (b) Corresponding microscopic neck size ratio as a function of time and deduced using Eq.1.

3.3.2 Time-dependent isothermal shrinkage Knowing the relative shrinkage in NP-Au as a function of time from our experiment (see Figure 5a), Eq. 2 was used to deduce the exponent n, which value depends on the type of mass transport mechanism involved in the process. This was done by plotting the experimentally measured relative shrinkage as a function of time in a double logarithmic scale as shown in Figure 6, ∆

where the black curve represents the log( ) versus log() and the red line is the corresponding

linear fit. Note that a certain threshold amount of silver needs to be dissolved before a uniform change in dimension during dealloying can be observed. In order words, the strain in the material at the early stage of dealloying is not considered in our fit because it is not representative of the all shrinkage process. The slope of the linear fit can be used to derive the value of n in Eq. 2 by considering the logarithm value of that equation: ∆

log( ) log( ) log()

(Eq. 5)


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The slope is

from Eq. 5 and ~0.8 from the linear fit (Figure 6). This gives a value of ~2.5 for

n. Therefore, it can be concluded from Table 1 that plastic flow, the motion of dislocation under stress,41 is most likely the primary mass transport mechanism responsible for the measured shrinkage. This result is in agreement

with

previous

findings

of

Weissmüller and co-workers,39 who reported on a high density of lattice defects and local plastic deformation sites in dealloyed NP-

Figure 6. Log-log plot of the linear strain as a function of time (black). Corresponding linear fit (red).

Au, and suggested therefore that these defects were responsible for the volume shrinkage in dealloying.39 It should be emphasized that a mixed-mode mass transport cannot be excluded. In particular, since a pure plastic flow mechanism is associated with a value of 2 for “n”, one can speculate that the above value of 2.5 obtained from our experiments is a combination of “plastic flow” and “surface diffusion”. However, experimental verification of such an argument is not straightforward and could be addressed through computational studies. It is worth mentioning that the value of n does not seem to depend to the dealloying method or the rate of silver dissolution. Indeed, the “constant potential” and “constant current” dealloying methods used in the present work are associated with two different silver dissolution rates, but they yield the same value for n.

Conclusion



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In summary, dilatometry techniques were used to measure the irreversible shrinkage in NP-Au in situ during electrochemical dealloying. Dimensional changes (i.e. linear strains) up to ~9 % were measured. Further analysis of these dimensional changes using shrinkage models from sintering reveals that the measured strains are induced by plastic flow, the movement of dislocations under stress. Our finding is in agreement with the pioneering work in the field reporting on a high number of lattice defects and local plastic deformation sites in dealloyed NP-Au. Although surface diffusion is known to be a critical mass transport mechanism in dealloying, the present work suggests that bulk diffusion through plastic flow also plays an important role during porosity formation in dealloying. A better understanding of the origin of the volume shrinkage in dealloying and its control may lead to the design of high-performance monolithic nanoporous metal structures for advanced electrochemical applications.

Acknowledgement The authors are thankful to Penn Engineering for the financial support through the PI startup.

Supporting Information Supporting Information available: SEM image showing the typical cross section thickness of a starting Au-Ag alloy; typical TEM image of a dealloyed sample with a set of 10 pairs of markers used to evaluate the average neck size and particle diameter.

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On the Origin of the Volume Contraction during Nanoporous Gold

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