Ligament Evolution in Nanoporous Cu Films Prepared by Dealloying

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Ligament Evolution in Nanoporous Cu Films Prepared by Dealloying Burkhard Hecker, Carsten Dosche, and Mehtap Oezaslan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06801 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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The Journal of Physical Chemistry

Ligament Evolution in Nanoporous Cu Films Prepared by Dealloying

Burkhard Hecker, Carsten Dosche, Mehtap Oezaslan* Department of Chemistry, University of Oldenburg, 26129 Oldenburg, Germany corresponding author: Mehtap Oezaslan ([email protected])

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The Journal of Physical Chemistry 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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: A facile synthetic route to form nanoporous metallic materials is the (electro)chemical dissolution of less noble metal from an alloy referred to as dealloying. In this study, we investigated the dynamic formation of nanoporous copper (np-Cu) films prepared from Zn-Cu alloys. The obtained np-Cu films were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDX), Grazing Incidence X-ray Diffraction (GI-XRD) and X-ray Photoelectron Spectroscopy (XPS) depth profiles. We show a clear correlation between the dealloying conditions (reaction time and electrolyte solution, 0.1 M HCl (pH 1) and 1.3 M NaOH (pH14)) and structural parameters (ligament size and chemical composition) for the np-Cu films. The dealloying process in 0.1 M HCl results in the formation of uniform ligaments in the np-Cu. However, the ligament size grows with decreasing Zn content, signifying a strong relation between Zn content and ligament size. Unlike, dealloying in NaOH leads to ligament structures which are independent of the Zn content. This observation is likely based on the reduced surface mobility of the Cu (hydr)oxide species and the completive reaction like re-deposition of soluble Cu(I) species to form Cu(I) oxide crystals. This knowledge enables the development of synthetic guidelines for the preparation of tunable ligament size and content of less noble metal, which is highly critical for systematic studies.


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1.

INTRODUCTION

Nanoporous materials have attracted great attention for many applications such as catalysis, energy storage, electrochemical sensors and fuel cells.1-6 They possess unique chemical, physical and catalytic properties due to their high surface area, large number of lowcoordinated surface atoms which might improve the intrinsic kinetics of heterogeneous and chemical reactions, tunable content of additional metal to modify the electronic and geometric properties of noble metal-rich materials and finally improved mass transport properties by adjusting the ligament size and pore size.7-8 A promising synthetic route to prepare nanoporous materials is the dealloying of the less noble metals from uniform binary or ternary alloys. In particular, during the dissolution of less noble metal the surface diffusion process of the remaining noble metal leads to the formation of nano-sized ligaments and pores. Thus, the dealloying processes depend on the kinetics of the dissolution of less noble metal and of the surface diffusion of remaining noble metal as well as the mass transport processes at the interface between electrolyte and alloyed surface.9-10 In addition, the pH value, kind of electrolyte and applied potential can be adjusted the structure of sponge-like materials by controlling the surface diffusion of the remaining noble metal in the presence of adsorbing ions or oxide-based passivation layers.11-12 This enables to tailor the characteristic properties like ligament size, pore size and chemical composition of nanoporous materials over a broad length scale.13-15 Although numerous research works have already performed, there is still a need for understanding of the evolution of porous materials by dealloying. In particular, the formation of nanoporous copper (np-Cu) is rarely studied to date, because numerous works have been focused on nanoporous noble metals like gold, platinum or silver.7,

16-18

Np-Cu promises

interesting application in different areas where massive copper is used such as



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(electro)catalysis, sensors and batteries.19-21 Furthermore, copper is relatively cheap compared to other noble metals. In the past, various Cu-based alloys like Al-Cu, Mn-Cu or Zn-Cu have already been investigated which are forming np-Cu structures during dealloying process.3, 12, 14, 22 Different methods are applied to produce these master alloys, for example melt spinning or quenching of a molten metal solution.22-23 A more facile technique under very mild conditions has recently been developed by Jia et al..13 They prepared Zn-rich Zn-Cu alloy films by Zn electrodeposition onto a Cu surface and subsequent thermal annealing at around 150 °C in inert atmosphere for the alloying process. The advantages of this synthetic approach are that the alloy formation is achieved without applying high temperatures and the Zn-Cu alloy film can be obtained on different shapes of Cu-based substrate like foil, plate, disk, etc. Thus, this synthetic route enables us to produce facile alloyed films with high reproducibility, less effort and cost as well as gentle resources. In this paper, we studied the dynamic formation of nanoporous copper (np-Cu) films prepared from Zn-Cu alloys. The Zn-Cu alloys were produced by two steps: electrodeposition of Zn and mild thermal annealing at 150 °C for 120 minutes. We used the alloyed films for our dealloying studies and show a clear correlation between the dealloying conditions (acidic or alkaline electrolyte and reaction time) and structural parameters (ligament size and chemical composition) for the np-Cu films. This knowledge enables us to provide deeper insights to the dynamic dealloying processes to control the ligament sizes and content of the less noble metal inside the np-Cu films over a broad length range.


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2.

EXPERIMENTAL 2.1

Electrodeposition of Zn on Cu Substrates and Formation of Zn-Cu Alloy

Films. Pure polycrystalline Cu disks with a diameter of 5 mm and height of 4 mm (Alfa Aesar, Cu 99.999%) were used as substrate. The Cu disks were polished with 9, 6 and 3 µm of diamond dispersions (Buehler, Germany) and Microcloth polishing clothes (Buehler, Germany). The polished Cu disks were then cleaned in 2-propanol and purified water by using an ultra-sonication bath. The general procedure for the electrodeposition process of Zinc (Zn) films was taken from Jia et al.13 In our case, we used a rotating disk electrode (RDE) set-up (PINE Research Instrumentation, U.S.A.) with a three-electrode configuration to electrodeposit a uniform Zn film. Ag/AgCl (4M KCl) and Zn foil were used as reference and counter electrodes, respectively. The Zn plating bath contained a mixture of 30 g L-1 ZnCl2 (anhydrous, Alfa Aesar, 98+%), 20 g L-1 H3BO3 (Alfa Aesar, 99+%), and 150 g L-1 NH4Cl (Roth, ≥99,5%). To avoid the successive oxidation of the Cu surface, the Cu disk as working electrode was immersed into the electrolyte solution by holding the potential at -0.9 V vs. Ag/AgCl (4M KCl). After a constant current had been reached, the potential was immediately changed to 1.4 V vs. Ag/AgCl (4M KCl) to obtain a current density of around 150 mA cmgeo-2. The electrodeposition process of Zn was stopped as an electric charge of 0.23 mA h was obtained, corresponding to a film thickness of 2 µm. The electrode was removed, cleaned with highly purified water (20 MΩ cm, ElgaPurelab Classic) and then dried in an Argon atmosphere (Air liquid, 99.999%). Afterward, the samples were thermally treated to form an alloy film by using a tube furnace at 150 °C for 120 min in Argon atmosphere (150 ml min-1, Air liquid, 99.999%). 2.2

Chemical Dealloying of Zn-Cu Alloy Films. Dealloying solutions were

prepared by diluting of 37% HCl (VWR, Reag. PhEur) or by dissolving NaOH pellets (Alfa Aesar, 99.99%) in highly purified water for the preparation of 0.1 M HCl and 1.3 M NaOH, respectively. All Zn-Cu alloy films were chemically treated in single 25 ml vials under different 5 ACS Paragon Plus Environment


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experimental conditions. After 3.5, 8, 24, 48, and 72 hours of dealloying time, the treated samples were taken out, cleaned with purified water and then dried in Argon atmosphere. All experiments were repeated at least three times on different days to estimate the reproducibility and standard deviations of our results. 2.3

Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray

Spectroscopy (EDX). A Helios NanoLAB 600i scanning electron microscope (SEM) equipped with an Apollo 10 EDX detector (EDAX AMETEK) was employed to investigate the structure, morphology and chemical composition of as-prepared, annealed and dealloyed samples. An accelerating voltage of 10 kV and beam current of 0.34 nA were used for all SEM measurements. The SEM micrographs were evaluated by using ImageJ Software to analyze the average ligament sizes and the respective standard deviations of different np-Cu films. For the elemental quantification via EDX, the characteristic energy intensities of the K lines for Cu, Zn and O were analyzed. 2.4

Grazing Incidence X-Ray Diffraction (GI-XRD). To analyze the crystal

phases of bimetallic films at different preparation steps, an X-ray diffractometer Empyrean (PANalytical) equipped with Cu K tube, Bragg-Brentano unit and parallel plate collector (tolerance of 0.27°) was employed. All XRD measurements were carried out at an incident angle (Θ) of 2°, step size of 0.026° and acquisition time of 15.7 s in grazing incidence mode. Data processing and evaluation were performed by using HighScore 4.5 Software. 2.5

X-Ray Photoelectron Spectroscopy (XPS). An ESCALAB 250 Xi

spectrometer (Thermo Fisher) with monochromatized Al K radiation (h = 1486.6 eV) was employed to evaluate the chemical composition of samples at the surface as well as in the deeper layers up to 300 nm by using a sequential sputter process. A pass energy of 10 or 20 eV was used to record high resolution spectra for the Zn 2p and Cu 2p signals. The binding energy was referred to the C 1s peak of ubiquitous aliphatic hydrocarbon contaminations at 284.8 eV.


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Data analysis was performed by using the Avantage Software (version 5.952). For the XPS depth profiles a geometric surface area of around A ≈ 2.25 mm2 was step-wise removed by using an Argon ion source (MAGCIS, Thermo Fisher) at three different sputter rates. The depth profiles consist of 9 sputtering cycles using 2 keV for 30 s, 9 sputtering cycles using 4 keV for 120 s and finally one sputtering cycle with 4 keV for 300 s, respectively. Based on the different sublimation temperatures for Cu (2595 °C at 1013 mbar) and Zn (907 °C at 1013 mbar), the sputter processes occurred inhomogeneous, resulting in a stronger removal process of Zn compared to that of Cu. To consider the inhomogeneity of the sputter process, the results from the XPS depth profiles were corrected with respect to the standard derivations which we established from commercially available Zn-Cu alloys as reference materials.

3.

RESULTS 3.1

Characterization of Electrodeposited Zn Films and Zn-Cu alloy Films. The

morphology and crystal structure of Zn-electrodeposited and thermally treated alloy films were investigated by using SEM, GI-XRD and XPS techniques. Figure 1a shows a SEM image of the electrodeposited Zn film with grain sizes of few micrometers, which seems to be dense and cover the entire surface of the Cu substrate. After thermal annealing, the morphology of the metallic film strongly changed as shown in Figure 1b. The surface of the metallic film seems to be smooth. In addition, we observed the formation of surface cracks after annealing, indicating mechanical stress inside the annealed film. The mechanical stress is very likely induced by the mixture of dissimilar atoms and resulting change of crystal structure (Cu: cubic / Zn: hexagonal). Figure 1c shows the GI-XRD profiles of the Zn-electrodeposited and annealed films. For the electrodeposited Zn film, the reflexes at 35.2°, 39.0°, 43.3°, 54.4°, 70.3°, and 70.7° correspond to the hexagonal Zn phase (P63/mmc) (COD24, pattern ID: 9012435). The GI-XRD 7 ACS Paragon Plus Environment


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profile for the annealed film shows reflexes at 2Θ = 38.1°,42.4°, 43.8°, 58.2°, 68.6°, and 78.2°, related to a Zn-rich hexagonal CuZn4 phase with a space group of P63/mmc (COD24, pattern ID: 1524894). This indicates a mixture of Zn and Cu after annealing to produce an alloy. Moreover, we observed additional small reflexes at 2Θ = 35.4°,48.6°, 63.4°, and 79.8° which are assigned to the Cu5Zn8 phase (I-43m) (COD24, pattern ID: 1100057). Since the intensities of the reflexes for the Cu5Zn8 phase appear very weakly, we conclude that the thermal annealing process mainly forms hexagonal CuZn4 phase. It is noted that further reflexes at 2Θ = 43.5°, 50.5°, and 74.4° are visible in both GI-XRD profiles which correspond to the crystal structure of face-centered cubic (fcc) Cu phase (space group of Fm-3m) (COD24, pattern ID: 4313211). Since the penetration depth of X-rays in Zn-Cu or pure Zn layers at an incident angle of 2° is roughly 2.5 µm, we assume that these reflections are stemmed from the polycrystalline Cu substrate.


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Figure 1. Structural characterization of the electrodeposited Zn and Zn-Cu alloy films on polycrystalline Cu substrate after thermal annealing at 150 °C for 2 h in N2 atmosphere: (a) SEM image of the obtained Zn film with µm-sized grains, (b) SEM image of the thermally treated Zn-Cu alloy film, (c) GI-XRD profiles of the pure Zn and Zn-Cu alloy films including the reference patterns from the COD database24, (d) XPS depth profile up to 300 nm showing the chemical distribution of Zn and Cu within the bimetallic film. The hachured areas represent the derivations of the composition due to the inhomogeneous sputter process. In Figure 1d, a XPS depth profile reveals the distribution of Zn and Cu atoms in the thermally treated metallic films. It is noted that after few mild sputter processes (2keV and 30s) the contents of oxygen and carbon were almost negligible and thus not illustrated in Figure 1d. Based on the inhomogeneous sputter behavior of Zn and Cu, the standard deviations of the chemical composition are symbolized by hachured regions. We observed that the composition in the top layers are around 95 at.% and 5 at.% for Zn and Cu, respectively. Within the first 30 nm of the film depth, the Zn content slightly decreases to around 89 ± 5 at.%, while the Cu content increases to 11 ± 5 at.%. Afterwards, the Zn content is almost unchanged within the depth of 30 – 300 nm. Very interestingly, the XPS depth profile clearly reveals that a mixture process between Cu and Zn atoms at the metal-metal interface occurred during the mild annealing process at 150 °C for 2 hours. The obtained alloyed phase is extended up to the surface of the film, where we initially electrodeposited around 2 µm Zn on the Cu substrate. In addition, the atomic Zn:Cu ratio of around 85±6:15±6 in the deeper layers is in good agreement with the resulting crystal phase of CuZn4 via GI-XRD technique. 3.2

Chemical Dealloying of Zn-Cu Alloy Films. The dealloying time (3.5, 8, 24,

48, and 72 h) was varied in 0.1 M HCl and 1.3 M NaOH at room temperature to study the dynamic evolution of nanoporous copper (np-Cu) films. Generally, electrochemical dissolution



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process of Zn depends on the pH value, kind of electrolyte and electrolyte concentration as visualized below: Zn  Zn2+ + 2 e-

pH 1

∆E0 = - 0.76 V vs NHE equation [1]

Zn + 4 OH-  [Zn(OH)4]2- + 2 e-

pH 14

∆E0 ≈ - 1.22 V vs NHE equation [2]

The pH-depending potential for Zn dissolution versus the normal hydrogen electrode (NHE) has been taken from the Pourbaix diagram.25 Considering the respective potential shift for hydrogen evolution at pH 1 and pH 14, the electrochemical Zn oxidation is thermodynamically favored in acidic environment than that in alkaline. For the dealloying experiments, electrolytes of 0.1 M for HCl and 1.3 M for NaOH were chosen to compare our results with those in the literature.13, 26-27 Apart from the comparability, the ionic strength of the solutions is strong enough to entirely dissolve the Zn from the alloyed film in a proper dealloying time. Series of SEM images for the np-Cu films after dealloying time of 3.5, 8, 24, 48, and 72 hours in 0.1 M HCl are displayed in Figure 2. Throughout the paper we will denote the various np-Cu samples treated at different dealloying times and electrolyte with the following label: np-Cu (time/electrolyte).


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Figure 2. Series of SEM images for the np-Cu films as a function of the dealloying time (3.5, 8, 24, 48, and 72 hours) in 0.1 M HCl. Insert: high-resolution SEM image of np-Cu (8 h/HCl) displays the formation of uniform nanometer-sized ligaments.



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In Figure 2b, we observed that the grains are less pronounced after a dealloying time of 3.5 h compared to the initial non-dealloyed film (see Figure 2a). It is obvious that with increasing dealloying time the dynamic evolution of the ligaments to form a three-dimensional structure is enhanced. At the beginning, the dealloying process (3.5 h) generates large cracks at the surface. These cracks, yet, disappear with increasing dealloying time, while the ligament structure appears more and more uniform. On the other hand, we observed that the ligament size increases with additional dealloying time, signifying its dynamic growth. This observation will be discussed later. Finally, after 72 h of dealloying time the surface of the dealloyed film appears dense and solid without any porous structure. The loss of the sponge-like structure is associated with the entire dissolution of Zn. The observed dealloying features in acidic environment strongly differ from those in alkaline. Figure 3 displays the time-resolved SEM images for the treated Zn-Cu alloy films after dealloying time of 3.5, 8, 24, 48, and 72 h in 1.3 M NaOH at room temperature. In the initial dealloying processes, the surface of the bimetallic film (3.5 h/NaOH) shows numerous cracks. We observed that these cracks mainly formed along the grain boundaries of the Zn-Cu alloy films. Very interestingly, the large grains exhibit the formation of nanometer-sized pits. These initial pits seem to be the initiator for the evolution and gradual growth of pores within the bimetallic films. More precisely, for the np-Cu (8 h/NaOH) the observed pits clearly grow and form a sponge-like structure. Apart from the ligament evolution, the np-Cu (24 h/NaOH) also shows few faceted crystals, symbolized with white arrows in the SEM image (Figure 3d). Both features, few nanometer-sized ligaments and very large faceted crystals, appear more often at the surface of the np-Cu films with increasing dealloying time. It is noted that the crystals exhibit an octahedral shape and were identified as Cu(I) oxide by using GI-XRD technique (see supporting information). Finally, for the dealloyed samples (72 h/NaOH) the ligament structure entirely disappears, whereas only larger octahedral Cu(I) oxide crystal


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remain, indicating a growth of these crystals during dealloying process in alkaline media. It is noted that the absence of the porous structure signifies the entire dissolution of Zn.

Figure 3. Time-resolved series of SEM images for the np-Cu films after a dealloying time of 3.5, 8, 24, 48, and 72 hours in 1.3 M NaOH. Apart from the ligament formation, the white arrows in Figure 3d denote the presence of octahedral crystals consisting of Cu(I) oxide as a competitive reaction against the Zn dissolution process. 13 ACS Paragon Plus Environment


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Figure 4 shows the time-resolved course of the chemical composition for the np-Cu films dealloyed in 0.1 M HCl as well as in 1.3 M NaOH by using EDX technique. We want to emphasize that the EDX measurements involve chemical information of around 1 – 2 µm depth of these samples.28 First, for the initial alloyed film the atomic Zn:Cu ratio is around 78 ± 3 at.% and 22 ± 3 at.% , respectively, and is very similar to the obtained composition via XPS and crystal structure (Zn4Cu) via GI-XRD. We will start to describe the composition of the np-Cu films as a function of the dealloying time in 0.1 M HCl. In Figure 4a, it is obvious that with increasing dealloying time the content of Zn continuously decreases. The course of the residual Zn content reflects the dissolution rate of Zn from the Zn-Cu alloy film over the dealloying time. Within 3.5 h we observed a dramatic drop of the Zn content from 78 ± 2 at.% to 51 ± 6 at.%, indicating accelerated dissolution rates of Zn from the alloyed surface. Afterwards, the Zn dissolution rates gradually slow down with increasing dealloying time, whereby the Zn content decreases from 51 ± 6 at.% (3.5 h/HCl) to 5 ± 2 at.% (48 h/HCl) in the np-Cu film over a long period. After 72 h of dealloying time no zinc was detected by EDX, signifying that the Zn was entirely dissolved from the alloyed film. Apart from the Zn and Cu, the EDX results show a significant content of oxygen in the np-Cu films after dealloying process in alkaline media (Figure 4b). Therefore, we also plotted the course of the oxygen as a function of the dealloying time. The enhanced presence of O is very likely associated with the formation of Cu and Zn oxide species during the dealloying process. We observed that with increasing dealloying time the Zn content gradually decreases, while the content of oxygen as well as Cu increases. Thus, the measured amount of oxygen shows an opposite behavior compared to the course of the Zn content during dealloying process. More importantly, the time-resolved changes of the Zn:Cu ratios are similar during dealloying processes in both electrolyte solutions.


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Figure 4. Time-resolved courses of the chemical composition for the Zn-Cu alloy films during the dealloying processes in 0.1 M HCl (a) and 1.3 M NaOH (b) obtained from the EDX data. The content of oxygen is only plotted in (b) based on the formation of Cu and Zn oxide species in alkaline media. We also established the mean ligament size of the np-Cu films by evaluating of more than 100 ligaments from the SEM images (see Figure 2 and 3). In Figure 5, the mean ligament sizes are plotted as a function of the dealloying time as well as of the residual content of Zn measured by EDX. This allows us to develop a relationship between the ligament size and residual Zn content. In Figure 5a, we observed that the mean ligament size drastically increases from 33 ± 6 nm (8 h/HCl) to 100 ± 38 nm (48 h/HCl), while the Zn content changes from 44 ± 6 at.% (8 h/HCl) to 5 ± 2 at.% (48 h/HCl), respectively. It is evident that the obtained ligament size is strongly linked with the residual content of the less noble metal. In other words, the ligament size increases with decreasing Zn content during dealloying process in acidic environment. Unlike, we observed that in alkaline media the mean ligament size of the np-Cu is around 33 ± 14 nm (8 h/NaOH) and is nearly unchanged over the entire dealloying time. We sum up that the dealloying process in alkaline media produces np-Cu films with constant



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ligament sizes which is decoupled from the residual Zn content. Tuning Zn content independent of the ligament sizes for np-Cu materials is very critical for systematic studies in heterogeneous and electrochemical catalysis.

Figure 5. Ligament size as a function of the residual Zn amount in the np-Cu films after different dealloying times in 0.1 M HCl (left) and 1.3 M NaOH (right). To establish the chemical composition at the surface and sub-surfaces, XPS depth profile investigations on np-Cu films were performed. Figure 6 displays the XPS depth profiles of the np-Cu films dealloyed for 24 h and 48 h in 0.1 M HCl as well as in 1.3 M NaOH, respectively. The hachured areas symbolize the accuracy of the measurements due to the inhomogeneous sputter behavior of Cu and Zn which results in a stronger remove of Zn in the corresponding matrix. Figure 6a shows a XPS depth profile for the np-Cu (24 h/HCl) with an atomic Cu:Zn ratio of around 57±6 : 43±6 at the surface. Very interestingly, within the depth of 10 nm the atomic Cu:Zn ratio dramatically dropped from 57±6 : 43±6 to 89±4 : 11±4, indicating that the top layers of the np-Cu film are significantly enriched with Zn. The enrichment of Zn at the top layers can be explained by its higher oxophilic character compared to Cu which is very critical for the continuous dealloying processes. Otherwise, a higher Cu content would passivate the


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surface without the evolution of pores. In addition, we observed that the Cu:Zn ratio gradually changes from 89±4 : 11±4 to around 70±7 : 30±7 with increasing sputter depth, signifying a clear concentration gradient in the np-Cu film (24 h/HCl). This observed concentration gradient is likely related to the mass transport limitation (e.g. penetration of the electrolyte) during dealloying processes. A similar behavior was observed for np-Cu (48 h/HCl) (Figure 6b). It is obvious that a lower Zn content was measured at the surface compared to that for the np-Cu (24 h/HCl) based on the advanced dealloying processes. The Cu:Zn ratio alters from 75±8 : 25±8 to 88±4 : 12±4 within the first 10 nm of the depth and finally to 94±3 : 6±3 in the deeper layers (up to 250 nm), respectively.



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Figure 6. XPS depth profiles for the np-Cu films treated in (a) 0.1 M HCl for 24 h, (b) 0.1 M HCl for 48 h, (c) 1.3 M NaOH for 24 h and in (d) 1.3 M NaOH for 48 h. Hachured areas represent the error range due to inhomogeneous sputtering processes. In Figure 6c and 6d, the XPS depth profiles of the np-Cu (24 h/NaOH/) and np-Cu (48 h/NaOH) films are shown. For the np-Cu (24 h/NaOH), the atomic Cu:Zn:O ratio at the surface is around 35±4 : 13±2 : 52±6, revealing a significant presence of metal oxide surface species. A strong change of the chemical distribution was observed in the first 10 nm of depth (atomic Cu:Zn:O ratio from 35±4 : 13±2 : 52±6 to 61±7 : 29±7 : 10±2), signifying a decrease of the oxygen content and an increase of the Zn content in the deeper layers. Interestingly, the composition of Zn, Cu and O are nearly stable in the deeper layers (up to 250 nm). A similar behavior was observed for the np-Cu (48 h/NaOH), where the XPS results show a Cu:Zn:O ratio of 32±3 : 8±2 : 60±6 at the surface, indicating the formation of metal oxide species. Within the first 10 nm of the sputter depth profile, the Cu:Zn:O ratio changes from 32±3 : 8±2 : 60±6 to 73±4 : 13±4 : 14±2, signifying a strong enrichment of Cu and Zn with decreasing O content. Afterward, this ratio is almost constant.

4.

DISCUSSION

Based on the present data, we have developed a model about the dealloying processes of the formation of np-Cu in acidic and alkaline electrolytes as a function of the reaction time. Figure 7 displays the different evolved stages of the np-Cu structure during dealloying processes in 0.1 M HCl and 1.3 M NaOH solutions. Generally, the dealloying process is a critical interplay between dissolution rates of less noble metal (Zn) and surface diffusion rates of the remaining noble metal (Cu). In other words, the dealloying process results in a surface fluctuation by continuous Zn dissolution and Cu surface diffusion, where a sponge-like structure evolves.


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Figure 7. Model about the time-resolved dealloying processes for the evolution of npCu films in 0.1 M HCl (left) and 1.3 M NaOH (right) solutions. In the initial stage of dealloying process, the strong drop of the Zn content in both electrolytes can be explained by the extremely high Zn concentration at the surface of the alloyed films. In 0.1 M HCl the dissolution of Zn takes place continuously and is not restricted by the high solubility of ZnCl2 during the entire dealloying process. Therefore, the surface diffusion of Cu atoms occurs steadily during dealloying processes, resulting in a gradual growth of ligaments by increasing dealloying time as shown in Figure 7 (left). This visualizes a strong correlation between the ligament size and the Zn content. Consequently, with



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decreasing Zn content the ligament size increases. The obtained ligament structure is mainly uniform; yet, it cannot be decoupled from the Zn content over a broad length scale. On the other hand, in 1.3 M NaOH the evolution of Cu and Zn oxide species at the surface leads to a strong restriction of the surface mobility. It is known that the surface diffusion coefficient of metal oxides might be three magnitudes slower than that for the respective metals.29-31 We suggest that due to the formation of Cu (hydr)oxide species in strong alkaline media the surface mobility of the remaining Cu atoms is drastically reduced compared to that in acidic environment. Therefore, the dissolution process of Zn to form soluble Zn hydroxide species is completed with the dissolution/re-deposition processes of soluble Cu hydroxide species (shown in Figure 7, right). It needs to consider that the solubility of the Zn and Cu oxides strongly depends on the local pH value at the surface and inside the pores. When a critical concentration of soluble Cu species is reached or exceeded at the interface between metallic surface and electrolyte during dealloying, the soluble Cu species start to re-deposit as octahedral Cu(I) oxide at the surface. Since the Cu(I) oxide crystals continuously grow with increasing dealloying time, we assume that the dissolution/re-deposition of soluble Cu prevails over the surface diffusion of the remaining Cu atoms which is highly critical for the dealloying process. Both processes, reduced surface mobility and dissolution/re-deposition of Cu species, strongly influence the ligament size and Zn content in the np-Cu films. Due to the critical interplay between these processes, the ligament size is largely independent of the Zn content in the np-Cu films.

5.

CONCLUSIONS

In this study, we investigated the dynamic formation and growth of ligaments in np-Cu films by variation of the dealloying conditions like reaction time in two different electrolytes and pH values. The np-Cu films were prepared from Zn-rich Zn-Cu alloy films. Our XPS depth profiles 20 ACS Paragon Plus Environment

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indicate an enrichment of the oxophilic Zn metal within 10 nm of depth, which is highly critical for the continuous dealloying processes over the time. The dealloying process of these alloy films results in the evolution of ligaments at least after 8 hours in 0.1 M HCl and 1.3 M NaOH. In both electrolytes, the Zn content decreases with increasing dealloying time and strongly affects the ligament structure during dealloying. We pointed out that in acidic media the ligament size is strongly coupled with the Zn content, resulting in an increase of the ligament size with decreasing Zn content during dealloying. In contrast, in alkaline media the ligament size is independent of the Zn content which allows us to tailor the concentration of the less noble metal over a broad range with similar ligament sizes. This observation is very likely related to the reduced surface mobility of Cu atoms in alkaline media due to the formation of Cu (hydr)oxide species. Simultaneously, the dealloying process in alkaline media partially leads to the dissolution of Cu species as soluble Cu (hydr)oxide complexes. When a critical concentration of soluble Cu (hydr)oxide species close to the surface has been reached or exceeded, Cu(I) oxide crystals preferentially form by re-deposition. Finally, after 72 hours of dealloying time, the Zn is entirely dissolved from the alloyed films, resulting in a dense and solid Cu surface in 0.1 M HCl or in large Cu(I) oxide crystals in 1.3 M NaOH. Controlling the dealloying conditions allows us to prepare np-Cu-films with tunable ligament size and content of less noble metal for various applications in catalysis, sensors and medicine.

Acknowledgments We thank Jonas Knake for laboratory support, the Georg Lichtenberg scholarship of the RTG Nano- and Energy Research, the German Research Foundation (DFG) for funding the XPS



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setup (INST 184/144-1 FUGG) and the Federal Ministry of Education and Research (BMBF, FKZ 03SF0539).


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TOC IMAGE



Figure 1. Structural characterization of the electrodeposited Zn and Zn Cu alloy films on polycrystalline Cu substrate after thermal annealing at 150 °C for 2 h in N2 atmosphere: (a) SEM image of the obtained Zn film with µm-sized grains, (b) SEM image of the thermally treated Zn Cu alloy film, (c) GI-XRD profiles of the pure Zn and Zn Cu alloy films including the reference patterns from the COD database24, (d) XPS depth profile up to 300 nm showing the chemical distribution of Zn and Cu within the bimetallic film. The hachured areas represent the derivations of the composition due to the inhomogeneous sputter process. 529x427mm (96 x 96 DPI)

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Figure 2. Series of SEM images for the np-Cu films as a function of the dealloying time (3.5, 8, 24, 48, and 72 hours) in 0.1 M HCl. Insert: high-resolution SEM image of np-Cu (8 h/HCl) displays the formation of uniform nanometer-sized ligaments. 186x230mm (96 x 96 DPI)



Figure 3. Time-resolved series of SEM images for the np-Cu films after a dealloying time of 3.5, 8, 24, 48, and 72 hours in 1.3 M NaOH. Apart from the ligament formation, the white arrows in Fig. 3d denote the presence of octahedral crystals consisting of Cu(I) oxide as a competitive reaction against the Zn dissolution process. 183x223mm (96 x 96 DPI)


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Figure 4. Time-resolved courses of the chemical composition for the Zn Cu alloy films during dealloying processes in 0.1 M HCl (a) and 1.3 M NaOH (b) obtained from the EDX data. The content of oxygen is only plotted in (b) based on the formation of Cu and Zn oxide species in alkaline media. 311x119mm (96 x 96 DPI)



Figure 5. Ligament size as a function of the residual Zn amount in the np-Cu films after dealloying in 0.1 M HCl (left) and 1.3 M NaOH (right). 307x117mm (96 x 96 DPI)


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Figure 6. XPS depth profiles for the np-Cu films treated in (a) 0.1 M HCl for 24 h, (b) 0.1 M HCl for 48 h, (c) 1.3 M NaOH for 24 h and in (d) 1.3 M NaOH for 48 h. Hachured areas represent the error range due to inhomogeneous sputtering processes. 478x403mm (96 x 96 DPI)



Figure 7. Model about the time-resolved dealloying processes for the evolution of np-Cu films in 0.1 M HCl (left) and 1.3 M NaOH (right) solutions. 149x136mm (96 x 96 DPI)


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Figure S1: Grazing incident X-ray diffraction(GI-XRD) profiles of the treated ZnCu films after dealloying time of 48 h and 72 h in 1.3 M NaOH solution including the reference patterns from the COD database1 (Cu: pattern ID: 4313211 and Cu2O: pattern ID: 1000064). Based on both GI-XRD profiles, two different crystalline phases of Cu and Cu2O are identified, revealing the formation of Cu2O during the dealloying process in alkaline media. 187x150mm (300 x 300 DPI)


Ligament Evolution in Nanoporous Cu Films Prepared by Dealloying

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