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Kirkendall Effect vs Corrosion of Silver Nanocrystals by Atomic Oxygen: From Solid Metal Silver to Nanoporous Silver Oxide Abdel-Aziz El Mel, Nicolas Stephant, Leopoldo Molina-Luna, Eric Gautron, Yousef Haik, Nouar Tabet, Pierre-Yves Tessier, and Romain Gautier J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06030 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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The Journal of Physical Chemistry

Kirkendall Effect vs Corrosion of Silver Nanocrystals by Atomic Oxygen: From Solid Metal Silver to Nanoporous Silver Oxide Abdel-Aziz El Mel1*, Nicolas Stephant1, Leopoldo Molina-Luna2, Eric Gautron1, Yousef Haik3, Nouar Tabet4, Pierre-Yves Tessier1 and Romain Gautier1 1. Institut des Matériaux Jean Rouxel, IMN, Université de Nantes, CNRS, 2 rue de la Houssinière B.P. 32229, 44322 Nantes cedex 3, France 2. Technische Universität Darmstadt, Department of Material- and Geosciences, AlarichWeiss-Strasse 2, 64287 Darmstadt, Germany 3. College of Science and Engineering, Hamad Ben Khalifa University, P. O. Box 34110, Doha, Qatar 4. Qatar Environment and Energy Research Institute (QEERI), Hamad Ben Khalifa University, Qatar Foundation, Doha, Qatar

ABSTRACT The corrosion of silver upon exposure to atomic oxygen is a unique effect reported in the 80s and was highly studied to overcome the fast degradation of space shuttles in low earth orbit. In this work, we explored the conversion mechanisms of nanostructures from solid silver to nanoporous silver oxide upon exposure to radio frequency air plasma. A broad panel of silver nanostructures with various shapes, sizes and morphologies were considered to carefully examine the different stages of the oxidation process which evolve according to the considered model-system (e.g. nanosphere, nanowire, nanocube or nanotriangle). Through a set of time-lapse studies and very specific experiments, we explained the generation of nanoporosity according to a mechanism based on two effects: i) the high strain in the oxide shell generated as a consequence to the oxidation process and amplified by the bombardment of the material by the energetic species created by the radio-frequency air plasma and ii) the Kirkendall effect occurring at the Ag/Ag2O interface as a consequence to the unbalanced diffusion rates of silver and oxygen ions through the oxide shell.

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INTRODUCTION The oxidation of metal nanocrystals has attracted considerable attention during the last two decades from both fundamental and practical point of view. As demonstrated in the pioneering work of Yin et al., when oxidizing metal nanocrystals, the Kirkendall effect comes into play and transforms the material from solid to hollow.1 In such a case, the Kirkendall effect originates from the unbalanced diffusion rates of metal and oxygen ions through the oxide shell resulting in the injection of vacancies at the interface within the metal phase which condense and form nanovoids.1-2 In case of a nanocrystal, the nanovoids were found to merge together and form a single large void giving rise to a hollow nanostructure.2-13

The most common way to oxidize metals is by using a thermal process. In such a case, the diffusion of the metal and oxygen ions is triggered by the relatively high temperature applied during the oxidation process. An important number of papers have reported on the use of thermal oxidation involving the Kirkendall effect which allows transforming metal nanocrystals from solid to hollow.1-2, 5-9, 11-17 A less known oxidation process, highly explored in the 80s, is the oxidation of spacecraft materials upon exposure to high-density atomic oxygen present in low earth orbit (LEO)18-22 and generated by the photo-dissociation of diatomic oxygen molecules present in the upper atmosphere by absorption of UV solar radiations.23-26 In the case of bulk silver, such a corrosion process was reported to result in the formation of cracked and porous silver oxide instead of a compact pin-holes free oxide layer.24,27-28 The rapid oxidation of silver at room temperature by atomic oxygen can be explained according to a process known as gas flow through micro-pores.23 Contrary to thermal oxidation where a relatively high temperature is required to activate the diffusion of metal and oxygen and trigger the formation of the oxide layer,1, 29 the oxidation by atomic oxygen is able to oxidize silver in a parabolic manner and can take place at room temperature.


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It is the particular porous structure of the formed silver oxide that allows such a process to take place.23 An important number of studies covering the fundamental aspects of the oxidation of silver by atomic oxygen have been published so far since the first reports in the 80s,18-22, 24, 30 but they were limited to bulk silver or thin films. Li et al. have investigated this phenomenon in details by considering bulk silver single crystal at 220 °C as a model system exposed to a beam containing hyperthermal atomic oxygen with a nominal translational energy of 5.3 eV;27 the formation of microporous and defective polycrystalline material out of single crystal silver as an outcome to such an oxidation process was explained according to a mechanism based on the rapid oxidation of Ag by atomic oxygen followed by the reduction of the thermally unstable Ag2O phase at 220 °C. Few reports have focused on the oxidation of silver nanostructures by atomic oxygen but they were limited on the investigation of the chemical state of the nanostructures and no data concerning the morphological transformation were reported.31-35 In 2016, we have published a paper on the generation of nanoporosity in silver nanocolumns by direct exposure to atomic oxygen.28 In the same year, Yu et al. showed that by oxidizing silver nanowires using atomic oxygen generated by air radiolysis using Xrays allows obtaining Ag2O nanotubes.36 They explained the mechanism responsible for such a hollowing process according to the Kirkendall effect.36 Li et al. have also explored the oxidation of silver nanowires using inductively coupled plasma generated by a coil carried out at a relatively low pressure (6.5 Pa);37 they demonstrated that such a process allows obtaining oxide nanotubes with a uniform wall thickness quite similar to the one encountered by Yu et al.36 The results obtained by both research groups36-37 are contrary to what we have demonstrated in our recent work on the oxidation of silver nanocolumns by air plasma.28 This disagreement may originate from the difference between the processes employed to generate the incident flux of atomic oxygen used for oxidation. One has to mention that in our former study, we did not explore the inner structure of the nanoporous columns to investigate if



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whether the Kirkendall effect was present or not.28 Since such a conversion process is expected to be highly influenced by the dimensions, the structure and the shape of the material, in this work we aim to investigate the impact of these parameters on the morphological and structural transformation of silver nanostructures (e.g. nanospheres, nanowires, nanocubes and nanotriangles) occurring upon exposure to atomic oxygen generated by radio frequency air plasma. The inner structure of the nanomaterials is probed by transmission electron microscopy to explore if whether the Kirkendall effect is involved or not when using radio frequency plasma to oxidize silver nanostructures.

EXPERIMENTAL SECTION Synthesis of silver nanostructures. The silver nanostructures were synthesized using the HCl-mediated polyol process following standard protocols already defined in the literature.38 5 ml of ethylene glycol (Sigma-Aldrich, > 99%) was heated in a capped vial with stirring placed in an oil bath at 140 °C for 1 h. HCl (1 mL of a 3 mM solution in ethylene glycol) was added to the solution and the vial was then quickly recapped. After 10 min, a 3 ml solution of silver nitrate (94 mM, Alfa Aesar, > 99.9%) in ethylene glycol was added simultaneously with a 3 ml solution of PVP (147 mM, Sigma-Aldrich, Mr=55 000) in ethylene glycol at the rate of 45 ml per hour. The vial was then recapped and heated at 140 °C. After 45 min, 1 ml of this solution containing the nanospheres was recovered and washed with acetone to remove the ethylene glycol and PVP. After 18 hours, the nanocubes and nanotriangles were formed. To prepare the nanowires, we followed the same protocol but used silver nanospheres as seeds for the one-dimensional growth.

Oxidation by air plasma. The oxidation of silver nanostructures was carried out using a radio-frequency plasma source Evactron® Model 25 De-Contaminator by XEI Scientific, Inc.


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The source was installed on the load-lock of the scanning electron microscope (SEM). In general, such sort of sources uses remote rf plasma to produce the desired radicals to eliminates contaminants from the surface of the specimens before SEM observation. In our case, we used room air as a feeding gas to generate the plasma and oxidize the silver nanostructures. For all the experiments, the distance between the source and the substrate was fixed to 20 mm. The electrical power applied to the source was fixed to 20 W whereas the discharge pressure was 50 Pa. The room humidity was 45% at 300 K which corresponds to a water partial pressure of about 1%.

Electron microscopy. The SEM images of the modified silver nanostructures were recorded using a JEOL JSM 7600 F microscope operating at 5 kV. For scanning transmission electron microscopy (STEM), a Cs-probe corrected JEOL ARM-F operated at 200 kV and provided with a Schottky-FEG was used. Imaging was done using both bright field STEM and high angle annular dark field (HAADF) STEM modes. The data acquisition was done using Digital Micrograph (DM) software, and spatial drift correction was applied. Data analysis was performed with digital micrograph.

RESULTS AND DISCUSSION Oxidation of silver nanospheres. The first oxidation experiments were performed on silver nanospheres since they can be considered as the simplest system in terms of symmetry. The morphological transformation of silver nanospheres with three different diameters (375, 230 and 95 nm) occurring upon exposure to atomic oxygen is presented in Figure 1. When exposing a nanosphere with 375 nm in diameter to atomic oxygen for 10 s, one can clearly remark that the surface roughness becomes more important with the occurrence of nanograins covering randomly the nanosphere surface (Figure 1a). Further increasing the oxidation time



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results in the full coverage of the nanosphere with a continuous oxide layer. When reaching 30 s, the surface adopts an urchin-like structure. At 60 s, the nanosphere surface becomes compact again before it transforms to nanoporous when reaching 140 s. The nanoporous skeleton formed by the end of the oxidation process consists of interconnected nanoligaments with a mean thickness of 17 ± 5 nm; the pore size, defined as the inter-distance between two adjacent nanoligaments, is about 10 ± 4 nm. Throughout the course of the reaction, the global diameter of the nanosphere increases by 63% (from 375 to 600 nm). In overall, these results are very similar to the ones obtained in our previous study of nanocolumns.28 While a similar evolution was observed for a nanosphere with 230 nm in initial diameter (10 ± 4 nm in pore size by the end of the oxidation process) (Fig. 1b), the one with a diameter of 95 nm revealed a different behavior (Fig. 1c). As it can be seen, during the first 20 s, the morphology of the nanosphere has been completely changed. For 30 s of exposure, some nanopores are already formed. When reaching 60 s, the nanosphere becomes fully nanoporous. Increasing the oxidation time to 140 s results in the complete deterioration of the nanosphere. It is important to point out here that for nanospheres with a diameter lower or equal to 50 nm, a single nanopore was found to form in the center region of the nanospheres (Fig. S1). Such behavior, quite different compared to the one encountered in the case of large particles, results from the low amount of silver constituting the small nanocrystal.


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Figure 1. Chronological transformation of silver nanospheres from solid to nanoporous upon exposure to air plasma. Scanning electron microscope images of the oxidized nanospheres with three different initial diameters: (a) 375 nm, (b) 230 nm and (c) 95 nm. For all experiments, the power applied to the plasma source was fixed to 20 W and the pressure to 50 Pa. Scale bar: 100 nm.

To better understand the conversion process from solid to nanoporous, the inner structure of the nanospheres was probed at different stages using transmission electron microscopy (TEM) (Fig. 2). The TEM analysis showed that the Ag2O nanoclusters formed over the surface of the silver nanocrystals during the early stage of the transformation process as a consequence of the reaction between silver and atomic oxygen generated by the plasma (

2 Ag + O → Ag 2 O ) (Fig. 2a) exhibit a single crystal structure (Fig. 2b and 2c). They were also found to be extremely sensitive to the e-beam which reflects their unstable nature (Supplementary movie 1 – SM1). As discussed earlier by Waterhouse et al.,39 in addition to atomic oxygen, other species such as OH radicals and ozone, may also contribute to the oxidation of silver. It is believe that ozone has a low contribution as its the density in such a low pressure discharge is much lower than the one of atomic oxygen.33 On the other hand, the contribution of the OH radicals is expected to be more important; a more detailed study must be carried out at different humidity levels to explore the role of OH radicals on the generation of nanoporosity in silver. In the second stage,

the oxidation process becomes size dependent. For example, a silver nanosphere with a large 7 ACS Paragon Plus Environment


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size (Fig. 2d) or a mid-size (Fig. 2e) tends to form a core/shell structure with a relatively nanoporous shell whereas a small nanosphere becomes fully nanoporous (Fig. 2f). The formation of the compact layer at this stage in case of the large and mid-size particles is a consequence to the coalescence of the silver oxide nanoclusters formed during the early stage of oxidation. The generation of nanoporosity in the material is a direct proof to the existence of a corrosion process occurring during oxidation.

Figure 2. (a) TEM image of Ag2O nanoclusters formed during the first stage of the oxidation process on a silver nanosphere and (b) high-resolution TEM image showing the lattice fringes of a single crystal Ag2O nucleated on the surface of the silver nanosphere and (c) the associated FFT pattern indexed according to the diffraction database file number: ICSD 247821. To obtain the Ag2O nanoclusters, the silver nanosphere was oxidized with the air plasma for 10 s while fixing the electrical power applied to the source to 20 W and the pressure to 50 Pa. (d) HAADF-STEM image recorded on a large nanosphere and bright-field TEM images recorded on (e) a mid-size and (f) a small nanosphere exposed to air plasma for 60 s; the air plasma treatment was performed with a power of 20 W and at a pressure of 50 Pa.

When carefully examining the large (Fig. 2d) and mid-size particle (Fig. 2e), one can remark that in addition to nanopores in the oxide shell, nanovoids are interestingly present at the 8 ACS Paragon Plus Environment

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Ag/Ag2O interface. The nanovoids formed within the large-size particle are discontinuous and separated from each other whereas the ones formed within the mid-size particle are found to constitute a continuous ring separating the Ag core from the Ag2O shell. The existence of these nanovoids is a direct proof to the presence of an unbalanced mass transport of material analogue to the Kirkendall effect.1, 7, 11 In such a case, the Kirkendall effect originates from the unbalanced diffusion rates of silver and oxygen ions through the oxide shell resulting in the injection of vacancies at the interface within the silver phase which condense and form nanovoids (Fig. 5e).1-2, 11, 40-41 As the Kirkendall effect requires the existence of an oxide shell to trigger the unbalanced diffusion of silver and oxygen ions, during the course of oxidation, the Kirkendall effect enters in competition with the corrosion process; this is related to the fact that the corrosion tends to fragment the oxide shell into small discontinuous pieces which hinder the formation of a continuous oxide layer necessary to ensure the unbalanced mass transport of material. As the Kirkendall effect is size dependent, the “hollowing degree” of the mid-size particle is more important than the one of the large-size particle. Moreover, since the Kirkendall effect is a thermally activated process,36, 42 one can conclude that the temperature of the system rises significantly during the oxidation process. The local rise in temperature is expected to originate from the chemical reaction between atomic oxygen and silver (1) which is in nature exothermic.43 To double-check this issue, we used the same equipment operating in the same conditions to expose copper films and polymers to air plasma. As expected, no material’s degradation originating from the oxidation or thermal heating of the material were noticed. In case of silver, such a local rise in temperature is further enhanced as the nanostructures were positioned on silicon substrates covered with native oxide which slows down the heat dissipation in the substrate. At the nanoscale, such a “local” rise in temperature becomes very significant and triggers the solid-state atomic diffusion of silver in the oxide shell. While the local temperature cannot be measured experimentally, the thermal diffusion



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coefficient of silver in the oxide shell can be estimated using Fick’s first law of diffusion.2 For simplicity, let us consider the case of a nanosphere with a metal core and an oxide shell. By applying the boundary conditions, the outward diffusion flux of silver in the oxide shell, J, can be approximated as follow:2 Ag 2 O J = D Ag

ρ Ag O

(2)

2

d

Ag 2 O Where D Ag stands for the diffusion coefficient of silver in the oxide shell, ρ Ag 2O is the

density of Ag2O (considered constant) and d is the thickness of the oxide shell. From the definition of diffusion flux in a spherical shell with an inner surface equal to 4πr2 (with r being the inner radius of the shell), at an instant t one can write: (4 / 3)πr 3 ρ Ag 2O  r 3 − rc3 J = ⋅  r3 4πr 2 ⋅ t 

   

(3)

Where rc represents the radius of the metal core remaining inside the oxide shell after an oxidation time t. Ag 2 O as follow: From equations (2) and (3) one can express the diffusion coefficient D Ag

Ag 2 O D Ag =

rd 3t

 r 3 − rc3 ⋅  r3 

   

(4)

Considering the mid-size nanosphere presented in Figure 2e and oxidized for 60 s, from the TEM image one can determine the inner radius r of the oxide shell (192 nm), the metal core radius rc (130 nm) and the oxide shell thickness d (60 nm). Injecting these parameters in Ag 2 O equation (4) allows estimating the approximate diffusion coefficient, D Ag , in case of the

considered nanosphere to about 4.4×10-13 cm2/s. This value is very close the one found by Yu et al. (1.2 × 10−13 cm2/s) who used X-ray radiolysis of air to form atomic oxygen and oxidize softly silver nanowires and trigger the Kirkendall-induced hollowing process;36 this indicates that the local temperature of the nanostructures triggering the thermal diffusion is more or less 10 ACS Paragon Plus Environment

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similar in both cases. Despite the similarity between the two oxidation temperatures, in our case nanoporous structures are obtained instead of hollow ones as reported by Yu et al.36 The formation of nanoporosity can be explained by the presence of a physical bombardment of the oxide shell by the relatively energetic species generated by the radio-frequency plasma (the kinetic energy in an oxygen plasma may reach values as high as 20 eV44) which is absent when generating atomic oxygen by X-ray radiolysis of air.36 The nanoporous morphology that we obtained throughout our experiments is very similar to the one encountered in case of silver used during the 1980s in spacecrafts navigating in low earth orbit which due to their high velocity (~8 km.s-1), collision energies of 4.5-5 eV are produced between the spacecraft surfaces and oxygen atoms present in low earth orbit which promotes the chemical reaction between atomic oxygen and silver.18 When an energetic oxygen species is captured by a potential well at or below the surface of silver where it chemically reacts to form an oxide, a fraction of its impinging energy is transferred to the material in a form of heat resulting in a local rise in temperature.18 In addition, as discussed earlier by Li et al.,27 such high incident energy should easily overcome the migration barrier of oxygen in silver (~1 eV) and thus promote the inner diffusion of oxygen through the lattice of the material via an interstitial mechanism. One has to mention that contrary to the study by Li et al. who performed their experiments on bulk silver specimens maintained at 220 °C using a pulsed laser detonation source to generate the atomic oxygen flux,27 we did not observe any detectable reduction in the Ag2O phase to metallic silver; the difference here is probably related to the fact that in our case all the experiments were carried out at room temperature under a continuous incident flux of atomic oxygen and the specimens were not intentionally heated during the plasma treatment. Moreover, the high internal residual stress generated within the oxide shell during the process due to the difference in density between the silver and silver oxide phases and amplified by the ion bombardment drives the weak points, represented by the parts of the



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oxide shell forming the walls of Kirkendall nanocavities, to pop up and transform into nanopores. As a consequence, instead of obtaining hollow nanostructures with uniform walls as reported by Yu et al.,36 nanoporosity is generated in the oxide shell. Thanks to the presence of nanopores in the shell, oxygen can flow through the nanopores, playing the role of channels, and drives the metal silver core to further oxidize.23 In its turn, the newly formed oxide layer tends to crack for the same reason mentioned above allowing the gas to penetrate deeper and deeper into the metal core. The repeated events of oxidation and cracking taking place subsequently during the oxidation process results, at the final stage, in the full transformation of the material from solid to nanoporous.

Oxidation of silver nanowires. After studying the oxidation of nanospheres, the objective was to explore if whether the shape of nanostructures has an impact on the formation of silver oxide. For this purpose, nanowires with three different diameters were considered: 175 nm, 90 nm and 35 nm (Fig. 3).


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Figure 3. Morphological transformation of silver nanowires from solid to nanoporous upon exposure to air plasma. Scanning electron microscope images recorded on nanowires with three different diameters: (a) 175 nm, (b) 90 nm and (c) 35 nm. For all experiments, the power applied to the plasma source was fixed to 20 W and the pressure to 50 Pa. Scale bar: 100 nm.

Contrary to nanospheres where the distribution of the Ag2O clusters over the surface was found to be random, when the nanowires are oxidized for 10 s, rows of aligned silver oxide nanoclusters were found to form over the surface of the nanowires. The number of rows increases as increasing the diameter of the nanowire. Such preferential growth originates from the presence of step-edges in the {100} side facets45 aligned along the nanowire axis where the silver atoms are less bonded to the crystal and thus can be oxidized more easily. For the



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thick nanowire (i.e. 175 nm) (Fig. 3a), for 20 s of oxidation, the previously formed rows of silver oxide nanograins become almost continuous with the appearance of some holes in the surface. The extremities of the nanowire are found to be highly covered with silver oxide nanograins. Reaching 30 s of oxidation allows significantly increasing the coverage density of the nanowire surface by oxide nanograins. When reaching 60 s, the nanowire becomes coated with a continuous layer of silver oxide (Figs. 3a and 4a). One can clearly note that the oxide shell formed at this stage is full of pinholes and defects (Fig. 4a) contrary to the study by Yu et al.36 who were able to obtain well-defined nanotubes with uniform walls thickness. As discussed in the previous section, the reason for such a difference in morphology is related to the fact that we employed radio-frequency plasma, which creates energetic incident species, to oxidize the nanostructures.

Figure 4. TEM and STEM images of (a) large, (b) mid-size and (c) small nanowires after 60 s of oxidation. (d) Electron diffraction pattern recorded on the thin nanowire presented in panel c; the diffraction rings were indexed according to the diffraction database file number: ICSD 247821. The oxidation was carried out using air plasma generated with an electrical power applied to the source of 20 W and a discharge pressure of 50 Pa.

For an oxidation time of 140 s, the nanowire becomes fully nanoporous with a maze-like structure (Fig. 3a). Although the evolution of the nanowire with a diameter of 90 nm was quite similar to the thicker one, three slight differences can still be noticed (Fig. 3b). The first difference can be seen at 10 s of oxidation where the silver oxide nanograins tend to form a single row of nanograins on the surface located at the center region of the nanowire instead of 14 ACS Paragon Plus Environment

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two rows as encountered in case of the thicker nanowire. Two other rows can be also remarked on the sides-surface. The second difference can be noticed at 60 s of oxidation since the nanowire becomes hollow with a nanoporous and irregular shell (Fig. 4b). The third difference can be remarked at 140 s: the nanowire becomes nanoporous and totally flat. Reducing the nanowire’s diameter to 35 nm impacts remarkably the early stages of the oxidation process (Fig. 3c). This is related to the fact that the nanowire’s diameter becomes comparable to the size of the oxide nanograins. At 20 s, the oxide nanograins grow laterally (i.e. perpendicular to the nanowire’s axis). In addition, the nanowire starts breaking in some regions where an important amount of silver has been consumed during the oxidation reaction. At such a low dimension, corrosion becomes the dominant process and it takes over the Kirkendall effect which cannot occur unless a relatively continuous oxide shell is formed. Further increasing the oxidation time to 30 s results in the complete transformation of the nanowire from solid into a skeleton-like structure. It can be noticed that the oxide ligaments constituting the skeleton are oriented perpendicularly to the nanowire’s axis; such orientation was expected as at 20 s the oxide nanograins were found to mainly expand perpendicularly to the nanowire’s axis. When reaching 60 s, the nanowire loses its three-dimensional aspect. At this stage, only Ag2O phase was detected (Fig. 4c and 4d) indicating that silver has been totally consumed and the process does not induce a reduction of the silver oxide phase into metallic silver. As the plasma treatment proceeds in time and reaches 140 s, the oxide phase was found to spread over the substrate surface while maintaining a nanoporous structure. The evolution of the morphology at this stage is probably related to the ion bombardment generated by the plasma which will be discussed more in details in the next section in case of the small nanotriangles.



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Oxidation of silver nanocubes and nanotriangles. To further explore the shape effect on the oxidation process of silver nanostructures, in addition to nanowires we have considered two other systems: nanocubes (Fig. 5) and nanotriangles (Fig. 6). In case of nanocubes, two different sizes were explored: 240 (Fig. 5a) and 85 nm (Fig. 5b). In case of the large nanocubes (i.e. 240 nm), although they are enclosed by {100} facets due to the preferential binding of PVP to Ag(111),46 when oxidized for 10 s, the oxide grains grow randomly over the surface (Fig. 5a) contrary to the case of nanowires discussed in the previous section. As the oxidation process proceeds in time, the size of the oxide grains increases until they coalesce and form a continuous porous layer at 60 s of oxidation. Further increasing the oxidation time to 140 s results in a shape transformation from cubic to spherical; at this stage, the material becomes highly nanoporous. Compared to the large nanocube, the small one was found to deteriorate and loose its shape of origin much faster (Fig. 5b). It is quite interesting to point out that during the first 10 s of oxidation, the small nanocubes were found to oxidize preferentially on their corners. The deformation and shape-transformation of the nanocube start over at 20 s of oxidation. When reaching 30 s, the cubic shape transforms into a spherical one. Further increasing the oxidation time to 60 s leads to an enhancement in the porosity of the material. At 140 s, the material spreads over a large surface area compared to the one covered by the pristine nanocube.


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Figure 5. Chronological transformation of the morphology of silver nanocubes upon exposure to air plasma from solid to nanoporous. Scanning electron microscope images recorded for nanocubes with two different side widths: (a) 240 nm and (b) 85 nm. For all experiments, the power applied to the plasma source was fixed to 20 W and the pressure to 50 Pa. Scale bar: 100 nm.

The same oxidation experiments were repeated on silver nanotriangles with three different sizes (Fig. 6). Starting with the largest nanotriangle (Fig. 6a), after 10 s, the oxidation was found to occur preferentially along the step-edges of the upper facet of the triangle. Increasing the oxidation time to 20 s results in a significant coverage of the triangle’s surface with silver oxide clusters.

Figure 6. Morphological transformation of silver nanotriangles from solid to nanoporous upon exposure to air plasma. SEM images recorded for nanotriangles with different width sizes: (a) 230, (b) 160 and (c) 100 nm. For all experiments, the power applied to the plasma source was fixed to 20 W and the pressure to 50 Pa. Scale bar: 100 nm.

When reaching 30 s, the oxide clusters percolate and form a highly porous and rough oxide layer over the surface. At 60 s, the triangular shape disappears and the nanostructure adopts a spherical shape. In addition, the oxide layer covering the surface becomes more compact. This



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shape transformation into a spherical shape also encountered in case of nanocubes support our supposition to the presence of a thermally-activated solid-state diffusion mechanism discussed previously. For 140 s of oxidation, both the size and porosity of the nanostructure significantly increase. The evolution of the nanotriangles with the midsize was a little bit different (Fig. 6b). Three differences can be noticed in comparison with the large nanotriangle: i) The oxidation process takes place preferentially at the extremities of the triangle; ii) When examining the mid-size nanotriangle after 20 s of oxidation, one can conclude that the morphological transformation is more advanced at this stage compared to the case of the large nanotriangle; iii) For 140 s of oxidation, the material becomes highly porous and loses at the same time a part of its 3D aspect. The behavior of the smallest nanotriangle was found to be similar to the one of the mid-side (Fig. 6c). The only difference can be identified at 140 s of oxidation as silver in such a case was found to dewett the substrate surface. In addition, the material becomes flat and constituted of in-plane interconnected nanoligaments. The formation of such in-plane elongated nanoparticles originates from the presence of ion bombardment generated by the plasma which may induce a 2D diffusion process of silver over the substrate surface due to a local rise in temperature. The ion bombardment may also trigger a sputter/deposition process during which silver is sputtered from the main body of the nanostructure and deposited in the surrounding region in a form of nanoclusters; the presence of a sputter/deposition mechanism can be supported by the formation, in some cases, of a race-track of small clusters surrounding the silver nanostructures (Fig. S2).


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CONCLUSION In conclusion, we have explored the oxidation process of silver nanostructures by atomic oxygen generated by radio-frequency air plasma. The different stages of the morphological transformation taking place during the oxidation process were carefully explored for four different silver model systems including nanospheres, nanowires, nanocubes and nanotriangles. Through a set of time-lapse experiments, we demonstrated that the transformation stages can be strongly influenced by the shape and the dimensions of the considered model system. Our data showed that during the course of oxidation, the corrosion effect occurs simultaneously to the Kirkendall effect which takes place at the Ag/Ag2O interface resulting in the generation of nanovoids which weaken the interface between the two phases. The generation of nanoporosity was explained according to a mechanism based on the pop-up phenomenon of the Kirkendall nanocavities due to the high strain present within the silver oxide shell forming their walls. The presence of high strain in the oxide shell was attributed to the bombardment of the material by the energetic species generated by the radiofrequency air plasma.

ASSOCIATED CONTENT Competing interests statement The authors declare that they have no competing financial interests. Correspondence and requests for materials should be addressed to A. A. El Mel ([email protected]).

SUPPORTING INFORMATION SEM images of silver nanospheres with different sizes before and after oxidation; SEM image recorded on large nanosphere and nanowire surrounded by a race-track; movie visualizing the instability of Ag2O

nanoclusters under electron beam formed on a silver nanosphere. 19 ACS Paragon Plus Environment


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Kirkendall Effect vs Corrosion of Silver ... - ACS Publications

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