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Copper nanowall array grown on bulk Fe–Co–Ni alloy substrate at room temperature as lithium-ion battery current collector
Thin Solid Films 518 (2010) 6876–6882
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Copper nanowall array grown on bulk Fe–Co–Ni alloy substrate at room temperature as lithium-ion battery current collector Yingying Hu ⁎, Jinping Liu, Ruimin Ding, Kai Wang, Jian Jiang, Xiaoxu Ji, Yuanyuan Li, Xintang Huang ⁎ Center for Nanoscience and Nanotechnology, Central China Normal University, Wuhan 430079, Hubei, PR China
a r t i c l e
i n f o
Article history: Received 11 August 2009 Received in revised form 24 June 2010 Accepted 8 July 2010 Available online 14 July 2010 Keywords: Copper nanostructure Room temperature Li-ion battery
a b s t r a c t Large-scale copper nanowall array on the bulk Fe–Co–Ni alloy substrate has been prepared in aqueous solution at room temperature via an electroless deposition method. The thickness of the nanowalls is about 15 nm. A possible growth mechanism of the nanowalls was proposed. The effects of reaction temperature, reaction time and the amount of critical agent (Fe3+) on the morphology and crystalline phase of the nanowalls were investigated. Furthermore, the electrochemical performance of Sn film supported on the asprepared copper nanowalls current collector is enhanced in comparison with that on the commercial copper foil when used as anode for Li-ion batteries with the operating voltage window of 0.01–2.0 V (vs. Li). After 20 cycles, the discharge capacity of Sn–Cu nanowalls anode still remained 365.9 mAh g− 1, that is, 40% retention of the reversible capacity, while the initial charge capacity of Sn film cast on commercial Cu foil was 590 mAh g− 1, dropping rapidly to 260 mAh g− 1 only after 10 cycles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Owing to their unique structure, electrical, thermal and optical properties, metal nanomaterials are being extensively investigated and used in applications such as catalysis [1,2], batteries [3], electronics [4], optics [5–7], optoelectronics [8], biochemical sensing [1,9], surface-enhanced Raman scattering [10], magnetics [11] and biodiagnostics [12]. Many metal nanostructures (e.g., wires, cubes, walls and dendritic structures) have been synthesized by welldefined methods including electrochemical preparation [13–16], chemical vapor deposition (CVD) [17–19], soft and hard templatedirected synthesis [20,21], photochemical route [22], nanocasting [23,24], and so on [25–27]. Nowadays, copper, as one of the most common metals, is becoming increasingly important for various fields of industries especially for electronic devices due to its low price and stability at high frequencies. Compared with bulk Cu, metallic copper nanostructures grown on substrates with controlled architectures are more attractive for their merits: firstly, they possess larger surface area and thus are more suitable for some applications such as catalyst beds in biomedical reactions [2,28]; secondly, controlled architectures give great opportunities for much more applications than untunable
⁎ Corresponding authors. Tel.: + 86 27 67867004; fax: + 86 27 67861185. E-mail addresses: [email protected] (Y. Hu), [email protected] (X. Huang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.07.035
structures [29]. Although a number of methods have been reported to fabricate copper nanomaterials, attempts to simplify the synthetic conditions are highly desirable. As a typical example, Xue and Chen reported respectively a modified electroless deposition route and a thermal-assisted photoreduction method as attractive routes to fabricate Cu nanostructures [28,30]. Electroless deposition, an important route used to form metal thin films or nanosturctures (e.g., Au, Ag, and Cu) on conductive and nonconductive surfaces, was started in the 1960s. This deposition process based on the galvanic displacement mechanism involves the reduction of metal ions and the oxidation of the substrate surface, the driving force for which depends on half-cell potentials [31]. Some successful examples are palladium at bare and templated liquid/liquid interfaces, Cu on gold, Au nanoparticles on carbon nanotubes, Ag on aluminum alloyed with copper, Ag nanowires on Si (111) surfaces, Cu nano-dot lines on hydrogen-terminated Si (111) surfaces, and so on [32–37]. Recently, a modified electroless deposition route has received increasing attention because it is easy to handle and of very low cost and high efficiency compared to CVD and electrochemical methods [30]. In this paper, we report a facile, room temperature electroless deposition method without the aid of the external reducing agent for rapid synthesis of large-scaled uniform copper nanowall array on bulk Fe–Co–Ni alloy substrate. A possible growth mechanism of copper nanowalls and the effects of reaction conditions are discussed. Moreover, the electrochemical performance of Li-ion batteries (LIBs) using Sn film supported on the as-prepared copper nanowalls as the anode (S-anode) is investigated compared with that on commercial copper foils (C-anode).
Y. Hu et al. / Thin Solid Films 518 (2010) 6876–6882
Fig. 1. Optical image of as-prepared Cu nanowall array on a large-area curly Fe–Co–Ni alloy substrate.
2. Experimental details All of the reagents were purchased from Shanghai Chemical Reagent Co., Ltd. All Fe–Co–Ni alloy foils (Fe, Co, and Ni with the atomic ratio of 52.23:18.07:29.70, 30 mm × 30 mm × 0.25 mm) and copper foils (purity: ca. 99%, 30 mm × 30 mm × 0.15 mm) were cleaned with ethanol and distilled water before use. In a typical experiment, an aqueous solution (200 mL) containing 3.24 g of FeCl3·6H2O was vigorously stirred to be transparent. A alloy foil and a copper foil were suspended simultaneously into the solution at room temperature (~ 25 °C) for 30 min. After the reaction, the alloy foil covered tightly by a film of light red products was taken out of the
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solution, rinsed with distilled water for several times and then dried in the oven at 60 °C. Control experiments were carried out by adjusting the reaction temperature, the concentration of FeCl3·6H2O (CF) and the reaction time, while other reaction parameters were unchanged. The crystal structure of the sample was determined by X-ray diffraction (XRD) using Bruker D-8 Avance (Cu Kα irradiation, λ = 1.54178 Å) at a scanning rate of 0.06°/s in 2θ ranging from 20° to 80°. The morphology of the as-prepared products was characterized by transmission electronmicroscopy (TEM and HRTEM, JEM-2010FEF, 200 kV), selected-area electron diffraction (SAED) and field-emission scanning electron microscopy (SEM JEOLJSM-6700F) operated at an acceleration voltage of 5 kV. Li-ion batteries consisted of several parts in Teflon molds: a tin film coating on as-prepared Cu nanowall array by vacuum deposition as the anode electrode, a Li metal disc as the counter and reference electrode, a ionexchange membrane layer between two electrodes, LiPF6 (1 M) in ethylene carbonate and dimethyl carbonate (1:1 by volume) as the electrolyte. The entire assembly process was completed in an argonfilled glove box (Mbraun, Unilab, Germany). The performance tests of batteries were implemented by BTS Series (Neware Technology Limited Company). 3. Results and discussions 3.1. Synthesis of Cu nanowall array by electroless deposition Fig. 1 illustrates a typical optical image of Cu film uniformly grown on a large-area alloy substrate. As is obvious from the image, the Cucovered substrate can be severely rolled without visible signs of degradation, indicating the flexibility and mechanical robustness of the as-prepared film. The SEM images of Cu nanowalls on the surface
Fig. 2. SEM patterns of the product under the typical reaction conditions: (a) low-magnification SEM image of large scale product; (b) local high-magnification SEM image of panel a; (c) SEM image of the film (view from 45° tilted); (d) XRD pattern of the typical product.
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observations and mixed potential theory. The main reactions are shown in the following equations: 3þ
2þ
3þ
2þ
Cu þ 2Fe →Cu
2þ
þ 2Fe
Fe þ 2Fe →3Fe 2þ
Cu
Fig. 3. (a) TEM images of the typical product; the inset is the corresponding SAED pattern; (b) High-resolution TEM (HRTEM) image of several particles in the circular area in (a).
of the alloy foil prepared with CF = 0.06 M (pH 1.9) for 0.5 h at room temperature are shown in Fig. 2. Fig. 2a indicates that the products are uniform Cu nanowalls. The thickness of the nanowalls is ~15 nm, as shown in Fig. 2b. In addition, the nanowall film is made of many interconnected individuals (Fig. 2c). Fig. 2d shows the XRD pattern of as-prepared Cu nanowalls. The first two peaks centered at 50.926 and 74.916 can be indexed to a face-centered cubic copper material (Joint Committee on Powder Diffraction Standards (JCPDS) file No. 851326), corresponding to (200) and (220) planes of the copper crystals, respectively. The lack of (111) peak can be attributed to the specific growth direction of copper nanowall array. A typical TEM image of Cu nanowalls shown in Fig. 3a reveals a clear laminated structure, which agrees with the SEM images. The corresponding selected-area electron diffraction (SAED) pattern (inset in Fig. 3a) could be indexed as a pure cubic phrase of Cu, revealing its single crystalline nature. A high-resolution transmission electron microscopy (HRTEM) image is illustrated in Fig. 3b. Interplanar distances of 3.5 and 2.65 Å can be determined for two separate single-crystal particles, corresponding to the (200) and (220) facets of copper, respectively. 3.2. Possible growth mechanism of Cu nanowall array Time-dependent evolution of Cu nanowall structures under typical growth conditions is depicted in Fig. 4. A possible formation mechanism of the nanowalls is proposed based on our experimental
ð1Þ ð2Þ
2þ
þ Fe→Cu þ Fe
ð3Þ
In the initial growth stage, the two reactions, Eqs. (1) and (2), took place almost simultaneously to generate a large amount of Cu2+ in the solution. According to the half-cell reactions, Fe metal lost electrons, whereas Cu2+ accepted two electrons on the surface of the alloy foil to become copper (Eq. (3)), which dissolved swiftly while CF was extremely high after 2 min reaction. Thus, it was observed from Fig. 4a that the alloy foil was eroded rather than covered with copper. Subsequently, as the reactions proceed, CF decreased so that a small quantity of copper started to deposit with unobvious orientations (Fig. 4b). With prolonging the reaction time to 10 min, a few copper nanowalls were deposited on the surface with distinct direction, that is, perpendicular to the surface (Fig. 4c). Finally, after 15 min reaction, the amount of Cu2+ and Fe3+ reached a relative proper level that allowed Cu nanowalls to grow steadily. Fig. 4d revealed well-defined Cu nanowalls were obtained. In this case, it appears that further optimization of the synthesis parameters will provide selective control over the morphology of Cu nanowall array. We found that the galvanic displacement reaction between iron and Cu2+ plays an important role in the whole reaction process. According to the previous study of galvanic replacement reaction, the replacement reaction herein involves a number of processes, including diffusion of Cu2+ to the surface of the alloy substrate, diffusion of iron atoms from bulk to surface, formation of Fe2+ and Cu species, and deposition of Cu atoms on the surface of the alloy foil [38,39]. Schematic illustration of the formation process for the Cu nanowalls is illustrated in Fig. 5. After the Cu2+ ions have been added to the dispersion of the iron atoms, the replacement reaction will start from the sites with relatively high surface energies, such as steps, point defects, and stacking faults [40]. The reaction is like a corrosion process when Fe starts to dissolve. The interior Fe atoms will diffuse to the reaction sites and react with Cu2+ to generate Cu atoms, which will be deposited on the surface of the alloy foil. In general, the face-centered cubic (fcc) metals such as gold, silver, copper, palladium and nickel naturally prefer to grow with a (111) orientation. However, the crystal growth direction depends on the surface energy which mostly lies on components of electrolyte in the given deposition conditions. For example, the effects of H+ ions absorption on growth orientation of Ni nanowires were reported by Pan and co-workers [41]. In our experiment, cupric salt is not used directly, thereby the requirement of reducing surface energy possibly benefits the growth of (220) and (200) planes, leading to the preferred orientation of Cu nanowall array. 3.3. Effects of CF, reaction temperature and time on the morphology and crystalline phase of Cu nanowall array In order to optimize the experimental parameters, we attempted to synthesize Cu nanowalls under different growth conditions. The achievement of the products with different morphologies was typically accomplished by controlling the CF parameter. Fig. 6 displays SEM images of the products prepared at room temperature for 0.5 h with different CF. The results can be explained as follows. When CF = 0.03 M (pH 2.1), the nanowalls become sparse owing to lack of Cu2+ generated in the solution to compete with Fe3+ (Fig. 6a). The morphology of Cu nanowalls is basically preserved when CF increases
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Fig. 4. SEM images of Cu nanowalls obtained with 0.06 mol/L FeCl3·6H2O at different growth time: (a) 2 min; (b) 5 min; (c) 10 min; (d) 15 min.
to 0.08 M (pH 1.8, Fig. 6b). The structure of the product prepared with CF = 0.3 M (pH 1.5) break up due to excessive Fe3+ restraining copper from deposition (Fig. 6c). Thus, if other parameters are suitable, Cu nanowalls can be obtained only when the pH of the solution controlled by the CF parameter is in the range of 1.7 to 2. Apart from the amount of Fe3+, the reaction time is also crucial for the formation of the Cu nanowalls. According to the previous results, well-defined Cu nanowalls have already been gained within the initial 15 min. When prolonging the reaction time to half an hour, the morphology of the product is maintained (Fig. 7a). However, the nanowalls start to adhere and assemble with each other from the surface of substrate after 1 h (Fig. 7b). Fig. 7c shows that the product obtained for 5.5 h of reaction is the complex consisting of a few nanowalls with blocks under them. It is reasonable that the concentration of Cu2+ increases, whereas that of Fe3+ decreases increasingly along with the proceeding of reaction, and then vastly improved bottom-up fill is carried out. Therefore, it is observed that nanowalls disappear and blocks appear when the reaction time is 8 h (Fig. 7d). It is observed that 0.5 h is the optimal reaction time to obtain the well-defined nanowall morphologies. The controlled morpholo-
gies of Cu architectures from nanometer to micrometer scale have been achieved. Furthermore, when we changed the reaction thermal environment during growth, morphologies of the products were different. Table 1 illuminates the summary of morphologies of Cu nanowalls deposited in the electrolyte of 0.06 M FeCl3·6H2O for 1 h at 5 °C below zero, room temperature and 70 °C, respectively. All the XRD patterns are indexed to pure fcc-Cu for samples A1–A3, but the growth of (111) plane of copper is obvious when the growth temperature reaches 70 °C (see Fig. 8). Only when the reaction took place at freezing temperature for 1 h, the best Cu nanowalls of the three were obtained. In general, a high reaction temperature leads to accelerate the reaction rate, and the opposite applies at low reaction temperature. This means that a longer period of time at low temperature or a shorter period of time at high temperature must be needed to obtain the same result gained by reacting at room temperature for 15– 30 min. Therefore, the optimal copper nanowalls were fabricated by using the concentration of precursor solutions for every deposition within 0.04–0.1 M for 15–30 min at room temperature or for 1 h at freezing temperature.
Fig. 5. Schematic illustration of the formation process for the Cu nanowalls.
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Fig. 6. SEM images of the products on alloy substrates attained at room temperature for 0.5 h with different CF: (a) 0.03 mol/L; (b) 0.08 mol/L; (c) 0.3 mol/L.
3.4. The use of as-prepared copper nanowall array as nano-architectured current collector for Li-ion batteries It is well-known that, as a promising anode material for LIBs, tin (Sn) has high theoretical capacity (994 mAh g− 1), but its practical performance is usually hampered by the poor cycling stability inherently resulting from the large volume change and the unstable solid electrolyte interphase on the surface of Sn metal during the lithium insertion/extraction processes. To achieve relatively good capacity retention, many reports have focused recently on nanoarchitectured current collector acting as both an electron conductor and a buffer layer for the uniform volume changes. Honeycomb carbon anode and nanorod copper anode were fabricated by Martin and Tarascone's group respectively [42,43]. The common methods used presently to fabricate nanostructured current collectors are complicated and costly, therefore the Cu nanowall array prepared by a room temperature template-free low-cost method could be a better choice. Here, as-prepared copper nanowalls were used as nano-
Fig. 7. SEM images of the products prepared at room temperature with 0.06 mol/L FeCl3·6H2O at different reaction time: (a) 0.5 h; (b) 1 h; (c) 5.5 h; (d) 8 h.
architectured current collectors and tin was chosen to be the active insertion electrode material of LIBs, compared with commercial copper foils as the current collectors. The thickness of Sn film illustrated in Fig. 9 is about 1.3 μm, from which it is also obvious that
Y. Hu et al. / Thin Solid Films 518 (2010) 6876–6882 Table 1 Summarized of morphologies of cu nanowalls deposited in the electrolyte of 0.06 mol/L FeCl3·6H2O for 1 h at different temperature. Sample no.
Reaction thermal environment
Morphology
A1 A2
5 °C below zero 25 °C
A3
70 °C
Perfect nanowall Partly nanowall, partly gathered into block Mostly gathered into block
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capacity ratio of 34.8% may be due to the side reaction with the electrolytes and the active materials not inserted into the nanowalls. The discharge and charge capacities of S-anode and C-anode versus cycle number plots up to 30 cycles in the voltage range 0.01–2.0 V at 0.5 °C rate (full use of the capacity in 2 h) at ambient temperature are displayed in Fig. 10c. It can be seen that the battery using Cu nanowalls as nano-architectured electrode exhibits better capacity retention than that using planar Cu foil. 33.04% of the initial discharge and 40.00% of the initial charge capacity are retained after 30 cycles. This means that more Sn particles covered on the commercial Cu foil were unusable caused by the pulverized active materials than that covered on the Cu nanowall current collector when the reversible capacity of the Li cell went steady. The reason for the better cycling of S-anode is that copper nanowalls with higher surface area (Fig. 2a,b) limit the very large volume change of Sn nanoparticles during Li+ ions insertion/extraction process, providing an electrons and Li+ ions mini-conductor simultaneously. Unfortunately, the Cu nanowall current collector still shows decreasing discharge capacity because the Sn film is thicker than the copper nanowall film, and part of active materials suffer from the huge volume change. However, this work still represents another example for the development of promising nano-architectured current collector for LIBs in comparison with the commericial copper foil.
4. Conclusions Fig. 8. XRD patterns of the products prepared with 0.06 mol/L FeCl3⋅6H2O at different reaction temperature for 1 h.
the morphology of Cu nanostructures can remain after Sn deposition. The peaks centered at 30.743, 32.157, 43.9 and 45.025 in Fig. 10a can be indexed to a body-centered cubic tin (JCPDS file No. 04-0673). Other observed peaks originate from Cu nanowalls. The average size of the Sn nanocrystals calculated by the Scherrer equation with the XRD data of tin is 71.95 nm. Fig. 10b shows voltage profiles of the Sanode after 1, 2, 5, and 10 cycles, and the first discharge and charge capacities were 935 and 609.8 mAh g− 1, respectively. An irreversible
In summary, uniform copper nanowall array on the surface of the alloy foil has been synthesized in aqueous solution at room temperature via an electroless deposition method based on galvanic displacement reaction without templates. The thickness of the nanowalls is ~15 nm. A growth mechanism of the nanowalls was proposed. The reaction temperature, reaction time and the amount of critical agent play important roles in the morphology and size of the nanowalls. The optimal experimental condition for preparing copper nanowalls is found. Moreover, Sn film supported on the as-prepared copper nanowall electrode shows better electrochemical performance than that on commercial copper foil when used as anode for Li-ion batteries. In addition to the Li-ion battery application discussed above,
Fig. 9. SEM images of Cu nanowall after Sn deposition. The thickness of Sn film is about 1.3 μm, indicating that the function of Cu nanostrctures is restricted. The mophology of Cu nanowall array can be remain after Sn deposition.
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the as-prepared Cu nanowall array may have potential applications in optics, gas sensors, catalysts, and other related fields. Acknowledgment The authors appreciate the financial support from the National Natural Science Foundation of China (No. 50872039 and No. 50802032). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] Fig. 10. (a) XRD pattern of Sn film deposited on as-prepared Cu nanowall array; (b) Voltage profiles of Sn film supported on Cu nanowalls after 1, 2, 5, and 10 cycles in the voltage range 0.01–2.0 V at 0.5C rate; (c) The discharge and charge capacity versus cycle number plots up to 30 cycles for Sn supported on the two kinds of current collectors of LIBs in the voltage range 0.01–2.0 V at 0.5 °C rate at ambient temperature.
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Copper nanowall array grown on bulk Fe–Co–Ni alloy substrate at room temperature as lithium-ion battery current collector Yingying Hu ⁎, Jinping Liu, Ruimin Ding, Kai Wang, Jian Jiang, Xiaoxu Ji, Yuanyuan Li, Xintang Huang ⁎ Center for Nanoscience and Nanotechnology, Central China Normal University, Wuhan 430079, Hubei, PR China
a r t i c l e
i n f o
Article history: Received 11 August 2009 Received in revised form 24 June 2010 Accepted 8 July 2010 Available online 14 July 2010 Keywords: Copper nanostructure Room temperature Li-ion battery
a b s t r a c t Large-scale copper nanowall array on the bulk Fe–Co–Ni alloy substrate has been prepared in aqueous solution at room temperature via an electroless deposition method. The thickness of the nanowalls is about 15 nm. A possible growth mechanism of the nanowalls was proposed. The effects of reaction temperature, reaction time and the amount of critical agent (Fe3+) on the morphology and crystalline phase of the nanowalls were investigated. Furthermore, the electrochemical performance of Sn film supported on the asprepared copper nanowalls current collector is enhanced in comparison with that on the commercial copper foil when used as anode for Li-ion batteries with the operating voltage window of 0.01–2.0 V (vs. Li). After 20 cycles, the discharge capacity of Sn–Cu nanowalls anode still remained 365.9 mAh g− 1, that is, 40% retention of the reversible capacity, while the initial charge capacity of Sn film cast on commercial Cu foil was 590 mAh g− 1, dropping rapidly to 260 mAh g− 1 only after 10 cycles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Owing to their unique structure, electrical, thermal and optical properties, metal nanomaterials are being extensively investigated and used in applications such as catalysis [1,2], batteries [3], electronics [4], optics [5–7], optoelectronics [8], biochemical sensing [1,9], surface-enhanced Raman scattering [10], magnetics [11] and biodiagnostics [12]. Many metal nanostructures (e.g., wires, cubes, walls and dendritic structures) have been synthesized by welldefined methods including electrochemical preparation [13–16], chemical vapor deposition (CVD) [17–19], soft and hard templatedirected synthesis [20,21], photochemical route [22], nanocasting [23,24], and so on [25–27]. Nowadays, copper, as one of the most common metals, is becoming increasingly important for various fields of industries especially for electronic devices due to its low price and stability at high frequencies. Compared with bulk Cu, metallic copper nanostructures grown on substrates with controlled architectures are more attractive for their merits: firstly, they possess larger surface area and thus are more suitable for some applications such as catalyst beds in biomedical reactions [2,28]; secondly, controlled architectures give great opportunities for much more applications than untunable
⁎ Corresponding authors. Tel.: + 86 27 67867004; fax: + 86 27 67861185. E-mail addresses: [email protected] (Y. Hu), [email protected] (X. Huang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.07.035
structures [29]. Although a number of methods have been reported to fabricate copper nanomaterials, attempts to simplify the synthetic conditions are highly desirable. As a typical example, Xue and Chen reported respectively a modified electroless deposition route and a thermal-assisted photoreduction method as attractive routes to fabricate Cu nanostructures [28,30]. Electroless deposition, an important route used to form metal thin films or nanosturctures (e.g., Au, Ag, and Cu) on conductive and nonconductive surfaces, was started in the 1960s. This deposition process based on the galvanic displacement mechanism involves the reduction of metal ions and the oxidation of the substrate surface, the driving force for which depends on half-cell potentials [31]. Some successful examples are palladium at bare and templated liquid/liquid interfaces, Cu on gold, Au nanoparticles on carbon nanotubes, Ag on aluminum alloyed with copper, Ag nanowires on Si (111) surfaces, Cu nano-dot lines on hydrogen-terminated Si (111) surfaces, and so on [32–37]. Recently, a modified electroless deposition route has received increasing attention because it is easy to handle and of very low cost and high efficiency compared to CVD and electrochemical methods [30]. In this paper, we report a facile, room temperature electroless deposition method without the aid of the external reducing agent for rapid synthesis of large-scaled uniform copper nanowall array on bulk Fe–Co–Ni alloy substrate. A possible growth mechanism of copper nanowalls and the effects of reaction conditions are discussed. Moreover, the electrochemical performance of Li-ion batteries (LIBs) using Sn film supported on the as-prepared copper nanowalls as the anode (S-anode) is investigated compared with that on commercial copper foils (C-anode).
Y. Hu et al. / Thin Solid Films 518 (2010) 6876–6882
Fig. 1. Optical image of as-prepared Cu nanowall array on a large-area curly Fe–Co–Ni alloy substrate.
2. Experimental details All of the reagents were purchased from Shanghai Chemical Reagent Co., Ltd. All Fe–Co–Ni alloy foils (Fe, Co, and Ni with the atomic ratio of 52.23:18.07:29.70, 30 mm × 30 mm × 0.25 mm) and copper foils (purity: ca. 99%, 30 mm × 30 mm × 0.15 mm) were cleaned with ethanol and distilled water before use. In a typical experiment, an aqueous solution (200 mL) containing 3.24 g of FeCl3·6H2O was vigorously stirred to be transparent. A alloy foil and a copper foil were suspended simultaneously into the solution at room temperature (~ 25 °C) for 30 min. After the reaction, the alloy foil covered tightly by a film of light red products was taken out of the
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solution, rinsed with distilled water for several times and then dried in the oven at 60 °C. Control experiments were carried out by adjusting the reaction temperature, the concentration of FeCl3·6H2O (CF) and the reaction time, while other reaction parameters were unchanged. The crystal structure of the sample was determined by X-ray diffraction (XRD) using Bruker D-8 Avance (Cu Kα irradiation, λ = 1.54178 Å) at a scanning rate of 0.06°/s in 2θ ranging from 20° to 80°. The morphology of the as-prepared products was characterized by transmission electronmicroscopy (TEM and HRTEM, JEM-2010FEF, 200 kV), selected-area electron diffraction (SAED) and field-emission scanning electron microscopy (SEM JEOLJSM-6700F) operated at an acceleration voltage of 5 kV. Li-ion batteries consisted of several parts in Teflon molds: a tin film coating on as-prepared Cu nanowall array by vacuum deposition as the anode electrode, a Li metal disc as the counter and reference electrode, a ionexchange membrane layer between two electrodes, LiPF6 (1 M) in ethylene carbonate and dimethyl carbonate (1:1 by volume) as the electrolyte. The entire assembly process was completed in an argonfilled glove box (Mbraun, Unilab, Germany). The performance tests of batteries were implemented by BTS Series (Neware Technology Limited Company). 3. Results and discussions 3.1. Synthesis of Cu nanowall array by electroless deposition Fig. 1 illustrates a typical optical image of Cu film uniformly grown on a large-area alloy substrate. As is obvious from the image, the Cucovered substrate can be severely rolled without visible signs of degradation, indicating the flexibility and mechanical robustness of the as-prepared film. The SEM images of Cu nanowalls on the surface
Fig. 2. SEM patterns of the product under the typical reaction conditions: (a) low-magnification SEM image of large scale product; (b) local high-magnification SEM image of panel a; (c) SEM image of the film (view from 45° tilted); (d) XRD pattern of the typical product.
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observations and mixed potential theory. The main reactions are shown in the following equations: 3þ
2þ
3þ
2þ
Cu þ 2Fe →Cu
2þ
þ 2Fe
Fe þ 2Fe →3Fe 2þ
Cu
Fig. 3. (a) TEM images of the typical product; the inset is the corresponding SAED pattern; (b) High-resolution TEM (HRTEM) image of several particles in the circular area in (a).
of the alloy foil prepared with CF = 0.06 M (pH 1.9) for 0.5 h at room temperature are shown in Fig. 2. Fig. 2a indicates that the products are uniform Cu nanowalls. The thickness of the nanowalls is ~15 nm, as shown in Fig. 2b. In addition, the nanowall film is made of many interconnected individuals (Fig. 2c). Fig. 2d shows the XRD pattern of as-prepared Cu nanowalls. The first two peaks centered at 50.926 and 74.916 can be indexed to a face-centered cubic copper material (Joint Committee on Powder Diffraction Standards (JCPDS) file No. 851326), corresponding to (200) and (220) planes of the copper crystals, respectively. The lack of (111) peak can be attributed to the specific growth direction of copper nanowall array. A typical TEM image of Cu nanowalls shown in Fig. 3a reveals a clear laminated structure, which agrees with the SEM images. The corresponding selected-area electron diffraction (SAED) pattern (inset in Fig. 3a) could be indexed as a pure cubic phrase of Cu, revealing its single crystalline nature. A high-resolution transmission electron microscopy (HRTEM) image is illustrated in Fig. 3b. Interplanar distances of 3.5 and 2.65 Å can be determined for two separate single-crystal particles, corresponding to the (200) and (220) facets of copper, respectively. 3.2. Possible growth mechanism of Cu nanowall array Time-dependent evolution of Cu nanowall structures under typical growth conditions is depicted in Fig. 4. A possible formation mechanism of the nanowalls is proposed based on our experimental
ð1Þ ð2Þ
2þ
þ Fe→Cu þ Fe
ð3Þ
In the initial growth stage, the two reactions, Eqs. (1) and (2), took place almost simultaneously to generate a large amount of Cu2+ in the solution. According to the half-cell reactions, Fe metal lost electrons, whereas Cu2+ accepted two electrons on the surface of the alloy foil to become copper (Eq. (3)), which dissolved swiftly while CF was extremely high after 2 min reaction. Thus, it was observed from Fig. 4a that the alloy foil was eroded rather than covered with copper. Subsequently, as the reactions proceed, CF decreased so that a small quantity of copper started to deposit with unobvious orientations (Fig. 4b). With prolonging the reaction time to 10 min, a few copper nanowalls were deposited on the surface with distinct direction, that is, perpendicular to the surface (Fig. 4c). Finally, after 15 min reaction, the amount of Cu2+ and Fe3+ reached a relative proper level that allowed Cu nanowalls to grow steadily. Fig. 4d revealed well-defined Cu nanowalls were obtained. In this case, it appears that further optimization of the synthesis parameters will provide selective control over the morphology of Cu nanowall array. We found that the galvanic displacement reaction between iron and Cu2+ plays an important role in the whole reaction process. According to the previous study of galvanic replacement reaction, the replacement reaction herein involves a number of processes, including diffusion of Cu2+ to the surface of the alloy substrate, diffusion of iron atoms from bulk to surface, formation of Fe2+ and Cu species, and deposition of Cu atoms on the surface of the alloy foil [38,39]. Schematic illustration of the formation process for the Cu nanowalls is illustrated in Fig. 5. After the Cu2+ ions have been added to the dispersion of the iron atoms, the replacement reaction will start from the sites with relatively high surface energies, such as steps, point defects, and stacking faults [40]. The reaction is like a corrosion process when Fe starts to dissolve. The interior Fe atoms will diffuse to the reaction sites and react with Cu2+ to generate Cu atoms, which will be deposited on the surface of the alloy foil. In general, the face-centered cubic (fcc) metals such as gold, silver, copper, palladium and nickel naturally prefer to grow with a (111) orientation. However, the crystal growth direction depends on the surface energy which mostly lies on components of electrolyte in the given deposition conditions. For example, the effects of H+ ions absorption on growth orientation of Ni nanowires were reported by Pan and co-workers [41]. In our experiment, cupric salt is not used directly, thereby the requirement of reducing surface energy possibly benefits the growth of (220) and (200) planes, leading to the preferred orientation of Cu nanowall array. 3.3. Effects of CF, reaction temperature and time on the morphology and crystalline phase of Cu nanowall array In order to optimize the experimental parameters, we attempted to synthesize Cu nanowalls under different growth conditions. The achievement of the products with different morphologies was typically accomplished by controlling the CF parameter. Fig. 6 displays SEM images of the products prepared at room temperature for 0.5 h with different CF. The results can be explained as follows. When CF = 0.03 M (pH 2.1), the nanowalls become sparse owing to lack of Cu2+ generated in the solution to compete with Fe3+ (Fig. 6a). The morphology of Cu nanowalls is basically preserved when CF increases
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Fig. 4. SEM images of Cu nanowalls obtained with 0.06 mol/L FeCl3·6H2O at different growth time: (a) 2 min; (b) 5 min; (c) 10 min; (d) 15 min.
to 0.08 M (pH 1.8, Fig. 6b). The structure of the product prepared with CF = 0.3 M (pH 1.5) break up due to excessive Fe3+ restraining copper from deposition (Fig. 6c). Thus, if other parameters are suitable, Cu nanowalls can be obtained only when the pH of the solution controlled by the CF parameter is in the range of 1.7 to 2. Apart from the amount of Fe3+, the reaction time is also crucial for the formation of the Cu nanowalls. According to the previous results, well-defined Cu nanowalls have already been gained within the initial 15 min. When prolonging the reaction time to half an hour, the morphology of the product is maintained (Fig. 7a). However, the nanowalls start to adhere and assemble with each other from the surface of substrate after 1 h (Fig. 7b). Fig. 7c shows that the product obtained for 5.5 h of reaction is the complex consisting of a few nanowalls with blocks under them. It is reasonable that the concentration of Cu2+ increases, whereas that of Fe3+ decreases increasingly along with the proceeding of reaction, and then vastly improved bottom-up fill is carried out. Therefore, it is observed that nanowalls disappear and blocks appear when the reaction time is 8 h (Fig. 7d). It is observed that 0.5 h is the optimal reaction time to obtain the well-defined nanowall morphologies. The controlled morpholo-
gies of Cu architectures from nanometer to micrometer scale have been achieved. Furthermore, when we changed the reaction thermal environment during growth, morphologies of the products were different. Table 1 illuminates the summary of morphologies of Cu nanowalls deposited in the electrolyte of 0.06 M FeCl3·6H2O for 1 h at 5 °C below zero, room temperature and 70 °C, respectively. All the XRD patterns are indexed to pure fcc-Cu for samples A1–A3, but the growth of (111) plane of copper is obvious when the growth temperature reaches 70 °C (see Fig. 8). Only when the reaction took place at freezing temperature for 1 h, the best Cu nanowalls of the three were obtained. In general, a high reaction temperature leads to accelerate the reaction rate, and the opposite applies at low reaction temperature. This means that a longer period of time at low temperature or a shorter period of time at high temperature must be needed to obtain the same result gained by reacting at room temperature for 15– 30 min. Therefore, the optimal copper nanowalls were fabricated by using the concentration of precursor solutions for every deposition within 0.04–0.1 M for 15–30 min at room temperature or for 1 h at freezing temperature.
Fig. 5. Schematic illustration of the formation process for the Cu nanowalls.
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Fig. 6. SEM images of the products on alloy substrates attained at room temperature for 0.5 h with different CF: (a) 0.03 mol/L; (b) 0.08 mol/L; (c) 0.3 mol/L.
3.4. The use of as-prepared copper nanowall array as nano-architectured current collector for Li-ion batteries It is well-known that, as a promising anode material for LIBs, tin (Sn) has high theoretical capacity (994 mAh g− 1), but its practical performance is usually hampered by the poor cycling stability inherently resulting from the large volume change and the unstable solid electrolyte interphase on the surface of Sn metal during the lithium insertion/extraction processes. To achieve relatively good capacity retention, many reports have focused recently on nanoarchitectured current collector acting as both an electron conductor and a buffer layer for the uniform volume changes. Honeycomb carbon anode and nanorod copper anode were fabricated by Martin and Tarascone's group respectively [42,43]. The common methods used presently to fabricate nanostructured current collectors are complicated and costly, therefore the Cu nanowall array prepared by a room temperature template-free low-cost method could be a better choice. Here, as-prepared copper nanowalls were used as nano-
Fig. 7. SEM images of the products prepared at room temperature with 0.06 mol/L FeCl3·6H2O at different reaction time: (a) 0.5 h; (b) 1 h; (c) 5.5 h; (d) 8 h.
architectured current collectors and tin was chosen to be the active insertion electrode material of LIBs, compared with commercial copper foils as the current collectors. The thickness of Sn film illustrated in Fig. 9 is about 1.3 μm, from which it is also obvious that
Y. Hu et al. / Thin Solid Films 518 (2010) 6876–6882 Table 1 Summarized of morphologies of cu nanowalls deposited in the electrolyte of 0.06 mol/L FeCl3·6H2O for 1 h at different temperature. Sample no.
Reaction thermal environment
Morphology
A1 A2
5 °C below zero 25 °C
A3
70 °C
Perfect nanowall Partly nanowall, partly gathered into block Mostly gathered into block
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capacity ratio of 34.8% may be due to the side reaction with the electrolytes and the active materials not inserted into the nanowalls. The discharge and charge capacities of S-anode and C-anode versus cycle number plots up to 30 cycles in the voltage range 0.01–2.0 V at 0.5 °C rate (full use of the capacity in 2 h) at ambient temperature are displayed in Fig. 10c. It can be seen that the battery using Cu nanowalls as nano-architectured electrode exhibits better capacity retention than that using planar Cu foil. 33.04% of the initial discharge and 40.00% of the initial charge capacity are retained after 30 cycles. This means that more Sn particles covered on the commercial Cu foil were unusable caused by the pulverized active materials than that covered on the Cu nanowall current collector when the reversible capacity of the Li cell went steady. The reason for the better cycling of S-anode is that copper nanowalls with higher surface area (Fig. 2a,b) limit the very large volume change of Sn nanoparticles during Li+ ions insertion/extraction process, providing an electrons and Li+ ions mini-conductor simultaneously. Unfortunately, the Cu nanowall current collector still shows decreasing discharge capacity because the Sn film is thicker than the copper nanowall film, and part of active materials suffer from the huge volume change. However, this work still represents another example for the development of promising nano-architectured current collector for LIBs in comparison with the commericial copper foil.
4. Conclusions Fig. 8. XRD patterns of the products prepared with 0.06 mol/L FeCl3⋅6H2O at different reaction temperature for 1 h.
the morphology of Cu nanostructures can remain after Sn deposition. The peaks centered at 30.743, 32.157, 43.9 and 45.025 in Fig. 10a can be indexed to a body-centered cubic tin (JCPDS file No. 04-0673). Other observed peaks originate from Cu nanowalls. The average size of the Sn nanocrystals calculated by the Scherrer equation with the XRD data of tin is 71.95 nm. Fig. 10b shows voltage profiles of the Sanode after 1, 2, 5, and 10 cycles, and the first discharge and charge capacities were 935 and 609.8 mAh g− 1, respectively. An irreversible
In summary, uniform copper nanowall array on the surface of the alloy foil has been synthesized in aqueous solution at room temperature via an electroless deposition method based on galvanic displacement reaction without templates. The thickness of the nanowalls is ~15 nm. A growth mechanism of the nanowalls was proposed. The reaction temperature, reaction time and the amount of critical agent play important roles in the morphology and size of the nanowalls. The optimal experimental condition for preparing copper nanowalls is found. Moreover, Sn film supported on the as-prepared copper nanowall electrode shows better electrochemical performance than that on commercial copper foil when used as anode for Li-ion batteries. In addition to the Li-ion battery application discussed above,
Fig. 9. SEM images of Cu nanowall after Sn deposition. The thickness of Sn film is about 1.3 μm, indicating that the function of Cu nanostrctures is restricted. The mophology of Cu nanowall array can be remain after Sn deposition.
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the as-prepared Cu nanowall array may have potential applications in optics, gas sensors, catalysts, and other related fields. Acknowledgment The authors appreciate the financial support from the National Natural Science Foundation of China (No. 50872039 and No. 50802032). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] Fig. 10. (a) XRD pattern of Sn film deposited on as-prepared Cu nanowall array; (b) Voltage profiles of Sn film supported on Cu nanowalls after 1, 2, 5, and 10 cycles in the voltage range 0.01–2.0 V at 0.5C rate; (c) The discharge and charge capacity versus cycle number plots up to 30 cycles for Sn supported on the two kinds of current collectors of LIBs in the voltage range 0.01–2.0 V at 0.5 °C rate at ambient temperature.
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