Microporous sponge structure with copper-cobalt oxide hybrid nanobranches

Accepted Manuscript Microporous sponge structure with copper–cobalt oxide hybrid nanobranches Woo-Sung Choi, Heon-Cheol Shin PII:

S0925-8388(16)32842-0

DOI:

10.1016/j.jallcom.2016.09.090

Reference:

JALCOM 38924

To appear in:

Journal of Alloys and Compounds

Received Date: 17 June 2016 Revised Date:

27 August 2016

Accepted Date: 7 September 2016

Please cite this article as: W.-S. Choi, H.-C. Shin, Microporous sponge structure with copper– cobalt oxide hybrid nanobranches, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2016.09.090. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Microporous Sponge Structure with Copper-Cobalt Oxide Hybrid

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Nanobranches

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Woo-Sung Choi, Heon-Cheol Shin*

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School of Materials Science and Engineering,

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Busandaehak-ro 63 beon-gil, Geumjeong-gu, Busan 46241, South Korea

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*Corresponding author.

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Pusan National University,

E-mail address: [email protected] (H.-C. Shin). Tel.: +82 51 510 3099, Fax: +82 51 512 0528

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Abstract

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A sponge structure featuring a micropore network and nanodendritic wall consisting of

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Cu-Co oxide core-shell like hybrid nanobranches is reported. First, microporous sponge

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structure with Cu-Co hybrid nanobranches was formed by hydrogen-templated

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electrolytic deposition. Next, Cu-Co branches were transformed to Cu-Co(OH)2

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branches by selective anodic oxidation of Co. Cu-CoO branches were then obtained by

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subsequent thermal dehydration. During the selective anodization and subsequent

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dehydration of the as-deposited sample, the overall structures of micropore network and

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core-shell like nanodendritic branches were preserved.

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Keywords: Core-shell, Cobalt oxide, Electrodeposition, Porous structure.

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1. Introduction Additive-free porous thin films have been widely studied for use as electrodes in

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functional microelectronic devices because they possess large surface area and thus

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possibly lead to high rate and efficient operation of the devices [1-3]. However, the

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conducting pathway for an electron becomes longer as the structure becomes more

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porous and tortuous. As a result, bulk resistance for electron transport increases,

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resulting in a large Ohmic polarization and a low rate capability, especially for porous

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films consisting of inorganic compounds that usually have low electrical conductivity.

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One potential means of reducing the bulk resistance is the formation of core-shell

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nanorods, where the metal core is directly connected to the conducting substrate [4-6].

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To prepare the nanorod arrays, porous anodic alumina and track-etched polycarbonate

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membranes are typically used as templates. However, because the application and

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removal of a template complicate the preparation process and the template is typically

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expensive, template-assisted methods may not be a practical way of preparing the

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nanorod structure, especially on a large scale. Cobalt has different oxide forms because its oxidation states range from −3 to 4.

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Among them, Co3O4, CoO, and Co(OH)2 have attracted great attention due to their uses

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in various functional devices. The Co3O4 and CoO can be used as the electrode

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materials for oxygen generation systems [7], lithium batteries [8], supercapacitors [9],

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and sensors [10]. The Co(OH)2 can be used for the applications of oxygen generation

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systems [11], alkaline batteries [12], and supercapacitors [13,14].

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Herein, we prepared microporous sponge structures with Cu-Co oxide core-shell

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like hybrid nanobranches. First, a sponge structure with Cu-Co branches was

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electrochemically deposited under the concurrent H2 gas generation. Next, Cu-Co(OH)2

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branches were obtained by the selective anodic oxidation of Co, and Cu-CoO branches

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were acquired upon subsequent thermal dehydration.

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2. Materials and methods

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A microporous sponge layer featuring Cu-Co core-shell like hybrid nanobranches

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(hereafter, a Cu-Co sponge) was electrodeposited onto a stainless steel foil (Type 304,

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0.025 mm, Alfa Aesar) in an aqueous solution of 0.4M CoSO4, 0.04M CuSO4, 1M

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H2SO4, and 1M NaCl. A platinum wire was used as the counter electrode and a

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constant current of 2 A cm−2 was applied for 20 s. The deposition was performed at 25 o

C in a typical beaker cell without agitation and deaeration.

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During the electrodeposition, H2SO4 and NaCl act as a source of an H2 bubble

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template for the micropore network formation and a CuCl- catalytic intermediate

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compound for Cu-Co dendritic growth, respectively [15]. To fabricate the Cu-Co(OH)2 sponge, the Co in the Cu-Co sponge was subjected to

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selective oxidation in 0.1M NaOH solution with a Pt wire as the counter electrode and a

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Hg/HgO electrode as the reference electrode. A constant potential of −0.6 V vs. Hg/HgO

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was applied until the current dropped below 1 mAcm-2. The resulting sample was then

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dehydrated at 200 °C in an Ar atmosphere for 5 min to prepare the Cu-CoO sponge.

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The morphologies and compositions of the samples were analysed using a field-

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emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan) equipped

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with an energy-dispersive X-ray spectrometer (EDS, 7593-H EMAX, Horiba, Japan).

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Moreover, a single nanobranch was cut parallel to its central axis using a focused ion

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beam (FIB, Versa3D LoVac, FEI, USA) and subjected to transmission electron

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microscopy (TEM) / EDS analysis (Titan G2 ChemiSTEM Cs Probe, FEI, USA) to gain

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an in-depth understanding of the composition. For structural characterization, X-ray

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diffraction (XRD) patterns of the samples were recorded using an X-ray diffractometer

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with Cu Kα radiation (XRD, D8 Advance, Bruker, Germany).

3. Results and discussion The preparation procedure of the Cu-Co, Cu-Co(OH)2, and Cu-CoO sponges is

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depicted in Scheme 1. The first step, which entails the electrochemical formation of the

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Cu-Co sponge, was inspired by previous studies on the electrolytic formation of

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nanostructured Ni-Cu thin films [15-17]. It has been reported that co-electroplated Ni-

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Cu consists of two distinguishable phases: a Cu-rich core and a Ni-rich shell. In

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particular, Ni-Cu sponge structures with micropore network could be obtained by

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carefully controlling the plating conditions in our previous work [15]. Similar to the

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case of Ni-Cu thin films, the co-electroplating of Cu and Co has been found to provide

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Co-rich nanodendrites in a Cu-rich matrix [18]. Therefore, there is a great possibility for

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a Cu-Co co-deposit to have a core-shell structure under specific deposition conditions.

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The second step is the selective oxidation of Co to form a Co(OH)2 shell. One

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common means of obtaining metal hydroxides is anodic oxidation in alkaline solution.

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When the oxidation potentials of Cu and Co are clearly distinguishable, with the

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oxidation potential of Co being smaller than that of Cu, it is quite likely that Co is

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oxidized to Co(OH)2 without Cu oxidation under the careful control of anodizing

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potential. The third step is the thermal dehydration of Co(OH)2 to form a CoO shell [19]. To prevent the oxidation of the Cu core, dehydration must be performed in an inert gas

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atmosphere. It should be mentioned that direct formation of CoO from Co by thermal

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oxidation is not suitable for the purposes of this study because partial oxidation of the

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Cu core would be unavoidable. Fig. 1 shows the microporous sponge structure created by the co-deposition of Cu

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and Co. The structure featured micron-sized pores, and branches with widths less than a

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hundred nanometers on its wall. EDS analysis of the deposit cross-section revealed that

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the content ratio of Co to Cu decreased with increasing deposit thickness (Fig. 1b).

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To obtain an in-depth understanding of the deposit composition, a single nanobranch

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was cut parallel to its central axis (please see the branch in the dotted circle of Fig. 2a),

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and the composition of its cross-sectional area was analysed. Notably, the Cu-rich inner

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region was clearly separated from the Cu-Co outer region (Fig. 2b), which was also

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supported by EDS mapping of the dotted-square area in Fig. 2a (Figs. 2c and d). This

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strongly implies that Cu dendrite formation was followed by Co deposition. That is,

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during the electrolytic coating, the Cu, being less active than Co, formed first in a

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dendritic shape. Next, the Cu dendrite is thickened and at the same time the Co, which

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tends to grow in layers, grew on the surface of the Cu dendrite to form Cu-Co outer

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region. The selected area electron diffraction (SAED) pattern revealed that there were no other phases than pure Cu and Co, which were polycrystalline, having hexagonal close-packed and face-centred cubic structures, respectively (inset of Fig. 2a). It is known that the Cu and Co have almost no mutual solid solubility and have no

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alloy forms at equilibrium. Thus, it is likely that two elements might coexist as a form

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of composite. It should be mentioned, however, there might be non-thermodynamic

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phases in the deposits as in the cases of some binary co-deposits [20,21] although such

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metastable phases of Cu-Co deposits have not been reported yet as far as we know.

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Further works need to be done to clearly understand in which way Cu and Co coexist in

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the electro-deposits.

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Based on the suggested formation mechanism of the Cu-Co structure, the variation

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of the Cu:Co ratio with deposit thickness (Fig. 1b) can be explained as follows: a Cu

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core closer to the substrate would have a thicker Co shell because it forms earlier in the

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deposition and thus experiences a longer period of Co growth as the deposition proceeds.

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The microporous sponge with these Cu-Co core-shell like branches, i.e., the Cu-Co

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sponge, could be used as a starting material for the fabrication of Cu-Co(OH)2 and Cu-

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CoO sponges, as depicted in Scheme 1.

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To fabricate the Cu-Co(OH)2 sponge, we attempted to selectively oxidize the Co

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shell of the Cu-Co sponge. To ensure that the oxidation potential of Co is smaller than, and clearly different from, that of Cu, the oxidation potentials of the Cu-Co sponge were

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examined by anodic linear sweep voltammogram (LSV) (Fig. 3a). The LSV of the Cu-

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Co sponge included several distinct anodic peaks. It is generally agreed that the first

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peak at −0.6 V vs. Hg/HgO corresponds to the Co oxidation to form Co(OH)2 [22],

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consistent with the result for pure Co foil (A in the figure). The attribution of the

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following two peaks at −0.35 and −0.10 V vs. Hg/HgO to the Cu oxidation was

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confirmed from the result for pure Cu foil (B and C, respectively, in the figure). This

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result suggests that the Co in the Cu-Co sponge can be selectively oxidized by limiting

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the anodizing potential to less than the Cu oxidation potential.

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To obtain Cu-Co(OH)2 sponge, a constant potential of −0.6 V vs. Hg/HgO was

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applied to the Cu-Co sponge. The XRD pattern of the resulting sample clearly showed

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the formation of Co(OH)2 phase with a hexagonal crystal structure, whereas no peaks of

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Cu oxide phases were found (middle pattern in Fig. 3b). This result indicates that only

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the Co shell was anodically oxidized under the conditions used herein. It is noted that

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the XRD pattern of the pre-oxidized sample (i.e., the Cu-Co sponge) did not show any

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peaks of the crystalline Co phase (bottom pattern in Fig. 3b), contradicting the

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aforementioned SAED analysis. This discrepancy is probably because the grains of the

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Co phase were too small for its crystallinity to be detected by XRD [23]. The microscopic images of the Cu-Co(OH)2 sponge showed that the overall

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microporous structure remained almost unchanged, but the nanobranches expanded

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slightly (Figs. 4a-1 and 4a-2). Notably, the nanobranches at the bottom of the sponge

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(inset in Fig. 4a-2) were much thicker than those at the surface (inset in Fig. 4a-1). This

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difference in branch thickness is due to difference in the thickness of the Co shell on the

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Cu core, as explained earlier in Fig. 1b.

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After the dehydration of the Cu-Co(OH)2 sponge, the XRD pattern confirmed that

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the Co(OH)2 phase completely disappeared and a crystalline cubic CoO phase formed

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(top pattern in Fig. 3b). Almost no morphological changes were found after the

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dehydration (Figs. 4b-1 and 4b-2). It maintained its unique core-shell like structure

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despite a series of oxidation processes as shown in the compositional analysis on the

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cross-section of single nanobranch (Fig. 5). In particular, the mapping of O element

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distribution revealed that O atoms are located at the place where the Co atoms exist.

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These results clearly show that a Cu-CoO sponge was successfully created.

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There have been several reports on superior electrochemical properties of Co-based

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core-shell structure as electrodes for functional electrochemical devices [24-26]. Since

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the structure in this paper features a similar core-shell like hybrid nanobranches and

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additionally highly-open microporous sponge framework, it is expected that our structure will have a great possibility for the electrochemical applications. The

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continuing work is in progress to characterize the electrochemical properties of Cu-CoO

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sponge as an anode in lithium battery, which will be reported soon in a separate paper.

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4. Conclusions

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In this work, microporous sponge structures with Cu-Co oxide core-shell like hybrid

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nanobranches were presented. The controlled anodization of the Cu-Co hybrid

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nanobranches and its subsequent thermal dehydration were found to be effective means

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of fabricating Cu-Co(OH)2 and Cu-CoO hybrid nanobranches, respectively.

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Acknowledgements

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This research was supported by the National Research Foundation of Korea Grants

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funded by the Korea Government (MSIP) (NRF-2011-C1AAA001-0030538 and

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2012M2A8A5025923).

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References

2

[1] H.-C. Shin, J. Dong, M. Liu, Nanoporous structures prepared by an electrochemical

3

deposition process, Adv. Mater. 15 (2003) 1610–1614.

RI PT

1

[2] B. Varghese, M.V. Reddy, Z. Yanwu, C.S. Lit, T.C. Hoong, G.V.S. Rao, B.V.R.

5

Chowdari, A.T.S. Wee, C.T. Lim, C.-H. Sow, Fabrication of NiO nanowall electrodes

6

for high performance lithium ion battery, Chem. Mater. 20 (2008) 3360–3367.

SC

4

[3] X.-Y. Xue, S. Yuan, L.-L. Xing, Z.-H. Chen, B. He, Y.-J. Chen, Porous Co3O4

8

nanoneedle arrays growing directly on copper foils and their ultrafast

9

charging/discharging as lithium-ion battery anodes, Chem. Commun. 47 (2011)

10

M AN U

7

4718–4720.

[4] J. Qu, H. Li, J.J. Henry Jr., S.K. Martha, N.J. Dudney, H. Xu, M. Chi, M.J. Lance,

12

S.M. Mahurin, T.M. Besmann, S. Dai, Self-aligned Cu–Si core–shell nanowire array

13

as a high-performance anode for Li-ion batteries, J. Power Sources, 198 (2012) 312–

14

317.

16 17 18

EP

[5] J. Wang , N. Du , H. Zhang , J. Yu, D. Yang, Cu–Ge core–shell nanowire arrays as

AC C

15

TE D

11

three-dimensional electrodes for high-rate capability lithium-ion batteries, J. Mater. Chem. 22 (2012) 1511-1515.

[6] G.X. Pan, X.H. Xia, F. Cao, P.S. Tang, H.F. Chen, Fabrication of porous Co/NiO

19

core/shell nanowire arrays for electrochemical capacitor application, Electrochem.

20

Commun. 34 (2013) 146–149.

12

ACCEPTED MANUSCRIPT

[7] J. Liu, L. Jiang, B. Zhang, J. Jin, D. S. Su, S. Wang, G. Sun, Controllable synthesis

2

of cobalt monoxide nanoparticles and the size-dependent activity for oxygen

3

reduction reaction, ACS Catal. 4 (2014) 2998−3001.

RI PT

1

[8] S. Grugeon, S. Laruelle, L. Dupont, J.-M. Tarascon, An update on the reactivity of

5

nanoparticles Co-based compounds towards Li, Solid State Sci. 5 (2003) 895–904.

6

[9] J. Deng, L. Kang, G. Bai, Y. Li, P. Li, X. Liu, Y. Yang, F. Gao, W. Liang, Solution

7

combustion synthesis of cobalt oxides (Co3O4 and Co3O4/CoO) nanoparticles as

8

supercapacitor electrode materials, Electrochim. Acta 132 (2014) 127–135.

12 13 14 15 16 17 18

M AN U

[11] Y. Wang, D. Zhang, H. Liu, A study of the catalysis of cobalt hydroxide towards the

TE D

11

related explosive gas sensing applications, Nanotechnology 26 (2015) 415501.

oxygen reduction in alkaline media, J. Power Sources 195 (2010) 3135–3139. [12] T. N. Ramesh, P. V. Kamath, Bi2O3-modified cobalt hydroxide as an electrode for alkaline batteries, Electrochim. Acta 53 (2008) 4721-4726.

EP

10

[10] L. H. Li, J. Xiao, G. W. Yang, Amorphization of cobalt monoxide nanocrystals and

[13] L. Cao, F. Xu, Y.-Y. Liang, H.-L. Li, Preparation of the novel nanocomposite

AC C

9

SC

4

Co(OH)2/ultra-stable Y zeolite and its application as a supercapacitor with high energy density, Adv. Mater. 16 (2004) 1853-1857.

[14] S. Chen, J. Zhu, X. Wang, One-step synthesis of graphene−cobalt hydroxide

19

nanocomposites and their electrochemical properties, J. Phys. Chem. C 144 (2010)

20

11829–11834.

13

ACCEPTED MANUSCRIPT

[15] W.-S. Choi, H.-R. Jung, S.-H. Kwon, J.-W. Lee, M. Liu, H.-C. Shin,

2

Nanostructured metallic foam electrodeposits on a nonconductive substrate, J. Mater.

3

Chem. 22 (2012) 1028-1032.

RI PT

1

[16] R. Qiu, X.L. Zhang, R. Qiao, Y. Li, Y.I. Kim, Y.S. Kang, CuNi dendritic material: 

5

synthesis, mechanism discussion, and application as glucose sensor, Chem. Mater. 19

6

(2007) 4174–4180.

SC

4

[17] J.-K. Chang, S.-H. Hsu, I-W. Sun, W.-T. Tsai, Formation of nanoporous nickel by

8

selective anodic etching of the nobler copper component from electrodeposited

9

nickel-copper alloys, J. Phys. Chem. C 112 (2008) 1371-1376.

13 14 15 16 17 18

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12

45 (1999) 1077–1087.

[19] C.W.F.T. Pistorius, Thermal decomposition of the hydroxides of cobalt and nickel to 100 kilobars, Z. Phys. Chem. 34 (1962) 287-294. [20] T. Omi, S. Kokunai, H. Yamamoto, Structure of amorphous Ni-P electrodeposits,

EP

11

[18] P.E. Bradley, D. Landolt, Pulse-plating of copper–cobalt alloys, Electrochim. Acta

Trans. Jpn. Inst. Met. 17 (1976) 370-377.

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10

M AN U

7

[21] H.-R. Jung, E.-J. Kim, Y. J. Park, H.-C. Shin, Nickel–tin foam with nanostructured walls for rechargeable lithium battery, J. Power Sources 196 (2011) 5122-5127.

[22] A. Foelske, H.-H. Strehblow, Structure and composition of electrochemically

19

prepared oxide layers on Co in alkaline solutions studied by XPS, Surf. Interface

20

Anal. 34 (2002) 125–129.

14

ACCEPTED MANUSCRIPT

[23] H. Natter, M. Schmelzer, M.-S Lo1ffler, C. E. Krill, A. Fitch, R. Hempelmann,

2

Grain-growth kinetics of nanocrystalline iron studied in situ by synchrotron real-time

3

X-ray diffraction, J. Phys. Chem. B 104 (2000) 2467-2476.

RI PT

1

[24] G. Wang, X. Shen, J. Yao, D. Wexler, J. Ahn, Hydrothermal synthesis of carbon

5

nanotube/cobalt oxide core-shell one-dimensional nanocomposite and application as

6

an anode material for lithium-ion batteries, Electrochem. Commun. 11 (2009) 546-

7

549.

9

M AN U

8

SC

4

[25] B. S. Yeo, A. T. Bell, Enhanced activity of gold-supported cobalt oxide for the electrochemical evolution of oxygen, J. Am. Chem. Soc. 133 (2011) 5587–5593. [26] X. Xia, J. Tu, Y. Zhang, X. Wang, C. Gu, X. Zhao, H. J. Fan, High-quality metal

11

oxide core/shell nanowire arrays on conductive substrates for electrochemical energy

12

storage, ACS Nano, 6 (2012) 5531-5538.

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Scheme 1. Preparation procedure of microporous sponges with Cu-Co, Cu-Co(OH)2,

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and Cu-CoO nanobranches.

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Figure Captions

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Fig. 1. (a) Surface and (b) cross-sectional views of Cu-Co deposit. Contents of Co and Cu in the top, middle, and bottom regions of the deposit were presented in (b).

Fig. 2. (a) Cross-sectional view of nanobranches that were cut using FIB and (b) the

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magnified image of the branch in the dotted circle of (a), together with the Co

the dotted-square area in (a).

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and Cu content profiles across it. EDS mapping of (c) Co and (d) Cu taken from

Fig. 3. (a) LSVs of Cu-Co sponge, pure Cu and Co foils (scan rate: 1 mVs-1). The voltammograms of the foils were magnified 10 times for better comparison. (b)

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XRD patterns of Cu-Co, Cu-Co(OH)2, and Cu-CoO sponges (Cu: JCPDS 892838, Co(OH)2: JCPDS 89-8616, CoO: JCPDS 89-7099).

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Fig. 4. Microscopic images of (a) Cu-Co(OH)2 and (b) Cu-CoO sponges. (a-1),(a-2): surface. (b-1),(b-2): cross-section.

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Fig. 5. (a) Cross-sectional view of nanobaranches that

were cut using FIB. EDS

mapping of (b) Cu (c) Co and (d) O taken from (a).

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Fig. 1.

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Fig. 2.

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Fig. 3.

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Fig. 4.

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Fig. 5.

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< Highlights > • Microporous structure with Cu-Co hybrid nanobranch was electrolytically prepared.

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• Anodic oxidation and dehydration led to Cu-CoO core-shell like hybrid nanobranch.

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• Overall structure of micropore and nanobranch was preserved during the treatments.