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Growth mechanism and magnetic properties of dendritic nanostructure prepared by pulse electrodeposition
Journal of Alloys and Compounds 694 (2017) 1239e1245
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Growth mechanism and magnetic properties of dendritic nanostructure prepared by pulse electrodeposition Lihu Liu a, b, Liqian Qi a, Rushuai Han a, Huihui Zhang a, Yanlu Wang a, Huiyuan Sun a, b, * a b
College of Physics Science & Information Engineering, Hebei Normal University, Shijiazhuang, 050024, China Key Laboratory of Advanced Films of Hebei Province, Shijiazhuang, 050024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2016 Received in revised form 22 September 2016 Accepted 11 October 2016 Available online 12 October 2016
CoeCu dendrites were synthesized by pulse direct current electrodeposition (DC-PED) on polycrystalline Au substrate from a Co and Cu-sulphamate-based solution in a three-electrode system. Scanning electron microscopy images showed that the CoeCu dendrites consist of a long central backbone with secondary branches, which were composed of many neighbouring cubic-like nanoparticles. Tuning the pulse time was believed to be the key for the formation of a dendritic nanostructure. The X-ray diffraction (XRD) patterns indicated that the dendrite was composed of a solid solution of face-centered cubic CoeCu with preferred orientation of {111} planes and a face-centered cubic Co phase. The energy dispersive spectrometry (EDS) demonstrated that Co and Cu element composition distributed uniformly in the dendrite. Our results demonstrate that the co-deposition of Co and Cu ions is possible during the pulse electrodeposition process and which was dominated by the mechanism of thermodynamics and kinetics perspectives. Magnetic measurements showed that the CoeCu dendritic films exhibit ferromagnetism and easy-axis direction of the magnetization is perpendicular to the film plane. © 2016 Elsevier B.V. All rights reserved.
Keywords: Pulse direct current electrodeposition Dendrite CoeCu alloy Growth models
1. Introduction Dendritic micro/nano structures have received much attention both for fundamental studies and for potential applications in photography, catalysis, optoelectronics, biosensors and surface enhanced Raman scattering due to their special hierarchical structure characteristics [1]. In recent years, there have been many reports on the study of metallic hierarchical dendritic structures and their formation in both theoretical and experimental aspects. Several methods have been used for the preparation of dendritic metal structures, such as galvanic displacement reaction [2], electrodeposition [3,4], hydrothermal approach, and photocatalytic reduction [5]. Of the various methods available to prepare dendritic micro/nano scale materials, electrodeposition is a simple and efficient method that provides versatility and more accurate control by adjusting the growth rate conditions through the applied electric potential, the composition of the plating solution and temperature [6e8]. The nanomaterial shapes and sizes can be readily controlled by adjusting the preparation conditions.
* Corresponding author. College of Physics Science & Information Engineering, Hebei Normal University, Shijiazhuang, 050024, China. E-mail address: [email protected] (H. Sun). http://dx.doi.org/10.1016/j.jallcom.2016.10.097 0925-8388/© 2016 Elsevier B.V. All rights reserved.
At the same time, the interest in cobalt copper system nanomaterials has increased tremendously in recent years for the behavior of resistance under magnetic field [9,10] and a CoeCu alloy film fabricated by electrodeposition provides an economic alternative for potential applications such as magnetic sensors [11]. Despite the unique intrinsic properties and relevant applications, the controlled synthesis of CoeCu dendrites has achieved very limited success. By using pulse current for electrodeposition of metals and alloys, it is possible to exercise greater control over the properties of electrodeposits and to improve them by modifying their microstructures. Herein, we report a novel approach using electrochemical method to synthesize unusual CoeCu dendrites which have never been found in natural occurrence and synthetic materials. On the basis of investigation results, it has been found that the formation of CoeCu dendrites is dominated by both diffusion process controlled by thermodynamics and aggregationbased growth process controlled by kinetics factor. 2. Experimental A classical three electrode electrochemical configuration was used to achieve CoeCu electrodeposition. The counter electrode was platinum plate, reference electrode was a saturated calomel
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electrode (SCE) and the substrate was glass slide covered with a thin polycrystalline gold coating. Before electrodeposition, Au substrates were cleaned in concentrated H2SO4, followed by rinsing with demineralized water. Electrodeposition was carried out by imposing potentiostatic pulses for fixed periods of time without stirring the electrolyte solution and the electrochemical measurements were performed on a model VSP electrochemical potentiostat (Biologic, France) controlled by a personal computer. The temperature of the cell was maintained at 30 C and the solution was deaerated by purified nitrogen for 0.5 h before electrodeposition. All chemicals used were analytical grade and used without further purification. The electrolyte bath containing 0.3 M cobalt sulphate heptahydrate [CoSO4$7H2O] and 0.05 M copper sulphate pentahydrate [CuSO4$5H2O] was prepared using demineralized water. The pH of the electrolyte was adjusted to 3.0 using H3BO3 and which used as complexing agent at the same time. In pulse DC electrodeposition, one pulse consisted of applying U1 for t1 and U2 for t2. U1 value was set at 0.02 V/SCE and U2 was set at 0.5 V/SCE, respectively. The typical potential profile is demonstrated in Fig. 1. 90 cycles were applied, which took deposition time of 9e15 min. The products, which were recorded at a scanning rate of 0.05 /s with the 2q range from 40 to 80 , were characterized by X-ray diffraction (XRD) using an X-ray diffractometer with high-intensity Cu Ka radiation (l ¼ 0.154178 nm) at 40 kV and 40 mA. Field emission scanning electron microscopy (FE-SEM) equipped with an energy dispersive X-ray spectroscopy was performed using JEOL JSM-6700 FESEM. Magnetic measurements were performed using a physical property measurement system (PPMS-6700) with a superconducting quantum interference device (SQUID). 3. Result and discussion As we know, the standard electrode potential of Cu is þ0.34 V (vs. standard hydrogen electrode, SHE) and that of Co is 0.28 V (vs. SHE), which means that the co-deposition of Cu and Co is difficult under the normal condition. In the first step of this investigation, the electrolyte was characterized by cyclic voltammetry (CV) to find appropriate cathode potentials for the deposition of CoeCu precursor. The scan was started in the cathodic direction from þ0.8 to 0.8 V and the potential scanning rate was 25 mV/s. The CV curve of the electrolyte used to deposit CoeCu alloy film is given in Fig. 1a. One cathodic peak can be found at around 0.02 V which corresponds to reduction of Cu2þ ions. This peak is due to the fact that the steady state is not obtained instantaneously. However, before the current density increases again (if the potential is made more negative than about e0.74 V), a current plateau with low current density occurs, indicating the codeposition of both Cu and Co elements may be achieved in this potential range. Furthermore,
two crossovers can be founded at the potential of 0.02 V and 0.06 V from the CV curves in Fig. 1a, known as the nucleation overpotential and the crossover potential. The appearance of these two crossovers indicated 3-D nucleation and subsequent grain growth process [12]. In electrodeposition, as well known, it is rather difficult to control the reduction rate of two elements using fixed deposition potential. The pulse potential can control the flux of each element approaching the electrode in diluted electrolytes, and then PED method is allowed to choose appropriate potentials to control the composition of binary alloy materials films. Based on the CV curve, the potential U1 is chosen to be the starting potential (nucleation potential) at which Cu is reduced and U2 is chosen to achieve the co-deposition of both elements. Fig. 1b demonstrated the typical potential square wave signal as a function of time during PED mode. The duration of U1 was held constant at t1, while the duration of U2 was held constant at t2. The pulse potentials significantly affect the compositional, structural and morphological properties of the binary material films. The morphology and structure of the products were observed by FESEM. Fig. 2 shows the typical SEM images of the CoeCu dendrites prepared through pulse electrodeposition method with same deposition voltage but various pulse time t1 and t2. It can be seen that all the as-fabricated CoeCu nanostructures have a dendritic morphology with length of up to tens of micrometers. So, a sample can be chosen arbitrarily to discuss the formation mechanism and magnetic properties of the dendritic structure. For example, a lowmagnification SEM image of the sample synthesized at t1 ¼ 2s and t2 ¼ 8s is shown in Fig. 3a, revealing that those CoeCu dendrites consist of a long central backbone with secondary branches, which exhibits good symmetry and self-similarity characteristic of fractals. The width of the backbone is around 1.5e2 mm, and its whole length is up to tens of micrometers. The more highly magnified images give more details about the morphology of the CoeCu dendrites. From the local enlarged image (Fig. 3b), we can see that the backbone is virtually a chain of pyramidal-like structures. Those pyramidal-like structures, recomposed of many neighbouring three dimensional cubic-like nanoparticles with a mean size of 500 nm, link together head-to-tail to form backbone structure, as depicted by those green triangles in Fig. 2b. The images taken of the center and the outer parts of the structure imply that the branches have grown in a different manner (Fig. 3c). Interestingly, some ridge structures can be seen clearly perpendicular to the growth direction of the branches, and the distance between the ridges is about 2.5 ± 0.5 mm. On the other hand, the branches are not separated individually but are connected with adjacent ones. The cubic-like nanoparticles sit side by side and appear with smooth surface and sharp edge (Fig. 3d), which may be attributed to the repeated
Fig. 1. (a) Cyclic voltammetry curve of the electrolyte used to deposit CoeCu films. The potential scanning rate is 25 mV/s. (b) Deposition current pulse forms for the prepared CoeCu dendritic structures. Typical pulse potential and time are U1 for t1 ¼ 2 s and U2 for t2 ¼ 8 s, respectively.
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Fig. 2. SEM images of CoeCu dendrites deposited at U1 ¼ 0.02 V/SCE and U2 ¼ 0.5 V/SCE with different pulse time (a) t1 ¼ 2 s and t2 ¼ 8 s, (b) t1 ¼ 2 s and t2 ¼ 6 s, (c) t1 ¼ 2 s and t2 ¼ 4 s.
nucleation of neighbouring grains and Oswald ripening in which the CoeCu dendrites were grown at the expense of small nanoparticles. Lim et al. have previously observed this type of crystal growth in the formation of dendritic platinum nanostructures [13]. As a result, the dendritic structures can be produced through coalescence or attachment of small nanoparticles to the branches. However, the nanoparticles formed are not so big for the low growth speed [3]. It is well known that the electrochemistry growth process of a crystal is determined by the thermodynamics dominating process and/or the kinetics dominating process [14]. We believe that the observations here imply a competition between thermodynamic and kinetic factors. In the initial stage, when we select the low deposit potential U1 at 0.02 V/SCE, Cu nanoparticles preferentially nucleated on the Au substrate and acted as seeds for further growth of freshly produced CoeCu. Low deposit potential is not high enough to lead to massive metal deposition, so the initial stage is a thermodynamics dominating process. However, when the potential turns to a high value of 0.5 V/SCE and keep this value for 8 s, the dominating factor will transform from thermodynamics to kinetics. At the same time, co-deposition of Cu and Co ions will be achieved and cubic structural CoeCu composited nanoparticles will be obtained. In this process, the obtained CoeCu cubic nanoparticles aggregate directionally to pyramidal shape for the backbone and ridge shape for the branches. After that, because the potential turns from the high value U2 to the low one (U1) again, directional aggregation of metallic nanoparticles is interrupted and a buffer area appears between two pyramids for the backbone or two ridges for the branches, respectively. As a result of the continuous growth,
under the assistance of boric acid, which is a structure-directing agent [15], CoeCu dendrites with chain-like backbone and caterpillar branches are finally formed. By measuring the width of the buffer between two pyramids (l1) and the height of the pyramidal-like nanoparticles clusters (l2), which were demarcated by black and white line segments as shown in Fig. 4a, they are about 0.7 ± 0.1 mm and 3 ± 0.1 mm, respectively. The ratio between the two sizes is about l1:l2 ¼ 1:4, which is almost equal to the ratio of the pulse time (i.e. t1:t2). For comparison, as shown in Fig. 4b, a proportional line segment chequered with black and white was drawn side by side with the potential square wave signal. Furthermore, a similar result can be found in other samples synthesized under a same condition with the same pulse potential but different pulse time. So, we can conclude that the formation of the chain-like backbone of the CoeCu dendrites in this condition may follow the rule,
l1 t1 ¼ l2 t2 We think that the pulse potential may play an important role in the formation of this chain-like backbone, which ensures the codeposition of Cu and Co at the same time. Therefore, the buffer areas and particles aggregation is dominated by thermodynamics process and kinetics process, respectively. On the basis of the above results, the growth mechanism of the CoeCu dendrites can be explained in view of thermodynamics and kinetics control theory accompany with particle attachment [16,17]. The forming process of link-like backbone of the CoeCu dendrites is presented in Fig. 4 c. At the very early stage, prior to the co-
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Fig. 3. (a) SEM image of as-deposited CoeCu dendrite; (b) High-magnification SEM image of the central backbone of the CoeCu dendrite; (c) Low-magnification SEM image of the branches of the CoeCu dendrite; (d) High-magnification SEM image of the branch.
deposition reaction of Co and Cu, because of the different reduction potential of Co2þ and Cu2þ, only the Cu2þ concentration near the electrode is consumed and Cu core nucleates on the Au substrate, but the initial equilibrium condition isn't break down easily. So we can consider this reaction as a relaxation process of ions. When the potential turns to 0.5 V/SCE, a potential high enough to bring about the co-deposition reaction of Co and Cu, the reaction system spontaneously engage in a transition from equilibrium to nonequilibrium, CoeCu buds will form and which will grow to bigger nanoparticles further with increasing the reaction time. At the same time, the generated CoeCu nanoparticles aggregate to clusters in columnar shape because the dominator of kinetics. And then, the reaction system will turn to a short quasi-equilibrium or equilibrium condition again with the potential change from 0.5 V/ SCE to 0.02 V/SCE. The nanoparticles on the top of the columnar clusters will spread to the bottom of them because the dominator of thermodynamics. Finally, the backbone structure composed by pyramidal-like nanoparticles clusters is formed when the reaction time was continuously prolonged. Furthermore, we don't need to describe the forming process of the branch parts anymore because it is similar to the one of the backbone zone. From the view of thermodynamics and kinetics, the CoeCu dendrite, by virtue of its extended surface, has a considerably increased surface energy. Therefore, the aggregation process of the nanoparticles may be much faster than in the equilibrium condition controlled by low potential, which may be the reason buffer areas between two triangles for the backbone and ridges for the branches are formed. The crystalline structure of the as prepared CoeCu dendrites is determined by XRD analysis and the obtained result is shown in Fig. 5. Analysis indicates the CoeCu dendritic structure containing
two phases and the face centered cubic (fcc) phase dominant for all samples. It is known that the CueCo alloy, which is supposed to be a metastable phase at room temperature, is a copper rich CueCo fcc phase with a preferred orientation of (111) planes [18]. In the XRD patterns, the distinct peaks are fcc-Cu (111), accompanied by (200), (220) and Au peaks. Furthermore, the peak has been found at the angular position of 2q ¼ 44.22 , which should belong to the (111) reflection for fcc Co. While a weak shoulder found at 51.08 should belong to the other reflection peak (200) of fcc Co. This means that the (111) and (200) planes of Co and Cu develop independently during the growth of the samples. It is worth noting that the (111) plane is the thermodynamics crystallographic plane and the (200) and (220) are the kinetics crystallographic planes for the fcc lattice, respectively. Therefore, from this point of view, the growth of crystalline planes of the sample is dominated by the thermodynamics and the kinetics factors, which is also supported by the previous growth model. We can obtained the crystallite size of Co is about 30 nm from Scherrer's formula. The stable fcc Co phase is probably attributable to the small grain size and the supersaturated solution of Cu in the electrodeposited CoeCu alloy [19,20]. So the main reason for the formation of dendritic structure with fcc structure is probably the high copper content in the products. The result indicates that the CoeCu system is immiscible as is well known [21,22]. Compositional analysis of the prepared samples by EDS was also made by detecting different zones (the backbone, the side branches and the single particle) of the samples. Fig. 6 shows the EDS spectra of the CoeCu dendrites. From the EDS spectrum, it is clear that all the dendritic samples consist of both Co and Cu segments and the atomic metal content ratio of Cu:Co is 98:2, 98:2, and 99:1 in the
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Fig. 4. (a) The width of the buffer between two pyramids (l1) and the height of the pyramidal-like nanoparticles clusters (l2); (b) Comparison of l1 and l2 with the pulse time; (c) Possible growth process of CoeCu dendrites under the thermodynamics & kinetics dominance model.
Fig. 5. X-ray diffraction patterns of CoeCu dendrites deposited for 0.5 h at U1 ¼ 0.02 V/SCE and U2 ¼ 0.5 V/SCE in 0.3 M Co2þ and 0.05 M Cu2þ; the stars correspond to the Au substrate.
backbone, the side branches and the single particle, respectively. The almost equal ratio of Co to Cu element in different zones indicated that they distributed uniformly in the dendrite. This suggests that the growth of Co and Cu must be simultaneous, which also demonstrates that the co-deposition of Cu and Co ions is possible during the growth process.
As seen in the EDS studies, the content of copper is higher than that of cobalt for all the samples. As we know, the polarization of copper is higher compared to cobalt, because copper is much nobler than cobalt, so the deposition rate of copper is higher at the first stage of the electrodeposition process. At the beginning, copper grains are formed on Au substrate. While as the deposition process progresses, copper deposition process becomes more diffusion limited than that of cobalt due to the higher concentration of cobalt ions in the electrolyte, the product changes from pure copper to the CueCo mixture, either as a metastable solid solution and/or as nanoclusters of cobalt inside a matrix of copper. On the other hand, when cobalt hydride/hydroxide forms, some of the reduction current may be consumed by the reduction of these species, and hence the deposition rate of cobalt is reduced. These may be the reasons why content of cobalt is low detected in the EDS spectrum. Magnetic measurements were performed in magnetic fields applied in the film plane and out-of the film plane at room temperature. As an example, in-plane and perpendicular hysteresis loops of CoeCu dendritic film through pulse electrodeposition with t1 ¼ 2s and t2 ¼ 8s were shown in Fig. 7. It can be clearly seen that the perpendicular hysteresis loop has higher remanent magnetization (4.5 105 emu) and lower coercivity (56 Oe) than the inplane loops for the sample. The remanent magnetization and coercivity are 2 105 emu and 67 Oe, respectively, with the field parallel to the film plane. This indicates that easy-axis direction of the magnetization is perpendicular and hard-axis is parallel to the film plane although the existence of diamagnetic effect. However, a contrary result had ever been reported in the prior literature
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Fig. 6. EDS spectra of pulse electrodeposited CoeCu dendrites (a) at the backbone, (b) at the branches, and (c) at single particle.
structures. Acknowledgement This work is supported by the Natural Science Foundation of Hebei Province (Grant No. A2016205237). References
Fig. 7. Magnetization hysteresis loop measured at room temperature for CoeCu dendritic structural film with pulse potential and time are U1 for t1 ¼ 2 s and U2 for t2 ¼ 8 s.
although a same dendritic structure was found [23]. We can attribute this effect to the difference of microstructure caused by the pulse deposition method.
4. Conclusions In this study, CoeCu dendrites consist of a long central backbone with secondary branches were obtained by pulse electrodeposition method. Cyclic voltammetry of CoeCu deposition reveals that there was a current plateau with low current can be found between the reduction peak of Cu2þ and Co2þ ions, and the middle value, i.e. 0.5 V, was select as the deposition potential. Thermodynamics and kinetics control theory accompany with particle attachment are used to explain the growth process of dendritic nanostructures. We expect that the pulse electrodeposition route can be extended to prepare other composite metal nanomaterials with dendritic
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Growth mechanism and magnetic properties of dendritic nanostructure prepared by pulse electrodeposition Lihu Liu a, b, Liqian Qi a, Rushuai Han a, Huihui Zhang a, Yanlu Wang a, Huiyuan Sun a, b, * a b
College of Physics Science & Information Engineering, Hebei Normal University, Shijiazhuang, 050024, China Key Laboratory of Advanced Films of Hebei Province, Shijiazhuang, 050024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2016 Received in revised form 22 September 2016 Accepted 11 October 2016 Available online 12 October 2016
CoeCu dendrites were synthesized by pulse direct current electrodeposition (DC-PED) on polycrystalline Au substrate from a Co and Cu-sulphamate-based solution in a three-electrode system. Scanning electron microscopy images showed that the CoeCu dendrites consist of a long central backbone with secondary branches, which were composed of many neighbouring cubic-like nanoparticles. Tuning the pulse time was believed to be the key for the formation of a dendritic nanostructure. The X-ray diffraction (XRD) patterns indicated that the dendrite was composed of a solid solution of face-centered cubic CoeCu with preferred orientation of {111} planes and a face-centered cubic Co phase. The energy dispersive spectrometry (EDS) demonstrated that Co and Cu element composition distributed uniformly in the dendrite. Our results demonstrate that the co-deposition of Co and Cu ions is possible during the pulse electrodeposition process and which was dominated by the mechanism of thermodynamics and kinetics perspectives. Magnetic measurements showed that the CoeCu dendritic films exhibit ferromagnetism and easy-axis direction of the magnetization is perpendicular to the film plane. © 2016 Elsevier B.V. All rights reserved.
Keywords: Pulse direct current electrodeposition Dendrite CoeCu alloy Growth models
1. Introduction Dendritic micro/nano structures have received much attention both for fundamental studies and for potential applications in photography, catalysis, optoelectronics, biosensors and surface enhanced Raman scattering due to their special hierarchical structure characteristics [1]. In recent years, there have been many reports on the study of metallic hierarchical dendritic structures and their formation in both theoretical and experimental aspects. Several methods have been used for the preparation of dendritic metal structures, such as galvanic displacement reaction [2], electrodeposition [3,4], hydrothermal approach, and photocatalytic reduction [5]. Of the various methods available to prepare dendritic micro/nano scale materials, electrodeposition is a simple and efficient method that provides versatility and more accurate control by adjusting the growth rate conditions through the applied electric potential, the composition of the plating solution and temperature [6e8]. The nanomaterial shapes and sizes can be readily controlled by adjusting the preparation conditions.
* Corresponding author. College of Physics Science & Information Engineering, Hebei Normal University, Shijiazhuang, 050024, China. E-mail address: [email protected] (H. Sun). http://dx.doi.org/10.1016/j.jallcom.2016.10.097 0925-8388/© 2016 Elsevier B.V. All rights reserved.
At the same time, the interest in cobalt copper system nanomaterials has increased tremendously in recent years for the behavior of resistance under magnetic field [9,10] and a CoeCu alloy film fabricated by electrodeposition provides an economic alternative for potential applications such as magnetic sensors [11]. Despite the unique intrinsic properties and relevant applications, the controlled synthesis of CoeCu dendrites has achieved very limited success. By using pulse current for electrodeposition of metals and alloys, it is possible to exercise greater control over the properties of electrodeposits and to improve them by modifying their microstructures. Herein, we report a novel approach using electrochemical method to synthesize unusual CoeCu dendrites which have never been found in natural occurrence and synthetic materials. On the basis of investigation results, it has been found that the formation of CoeCu dendrites is dominated by both diffusion process controlled by thermodynamics and aggregationbased growth process controlled by kinetics factor. 2. Experimental A classical three electrode electrochemical configuration was used to achieve CoeCu electrodeposition. The counter electrode was platinum plate, reference electrode was a saturated calomel
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electrode (SCE) and the substrate was glass slide covered with a thin polycrystalline gold coating. Before electrodeposition, Au substrates were cleaned in concentrated H2SO4, followed by rinsing with demineralized water. Electrodeposition was carried out by imposing potentiostatic pulses for fixed periods of time without stirring the electrolyte solution and the electrochemical measurements were performed on a model VSP electrochemical potentiostat (Biologic, France) controlled by a personal computer. The temperature of the cell was maintained at 30 C and the solution was deaerated by purified nitrogen for 0.5 h before electrodeposition. All chemicals used were analytical grade and used without further purification. The electrolyte bath containing 0.3 M cobalt sulphate heptahydrate [CoSO4$7H2O] and 0.05 M copper sulphate pentahydrate [CuSO4$5H2O] was prepared using demineralized water. The pH of the electrolyte was adjusted to 3.0 using H3BO3 and which used as complexing agent at the same time. In pulse DC electrodeposition, one pulse consisted of applying U1 for t1 and U2 for t2. U1 value was set at 0.02 V/SCE and U2 was set at 0.5 V/SCE, respectively. The typical potential profile is demonstrated in Fig. 1. 90 cycles were applied, which took deposition time of 9e15 min. The products, which were recorded at a scanning rate of 0.05 /s with the 2q range from 40 to 80 , were characterized by X-ray diffraction (XRD) using an X-ray diffractometer with high-intensity Cu Ka radiation (l ¼ 0.154178 nm) at 40 kV and 40 mA. Field emission scanning electron microscopy (FE-SEM) equipped with an energy dispersive X-ray spectroscopy was performed using JEOL JSM-6700 FESEM. Magnetic measurements were performed using a physical property measurement system (PPMS-6700) with a superconducting quantum interference device (SQUID). 3. Result and discussion As we know, the standard electrode potential of Cu is þ0.34 V (vs. standard hydrogen electrode, SHE) and that of Co is 0.28 V (vs. SHE), which means that the co-deposition of Cu and Co is difficult under the normal condition. In the first step of this investigation, the electrolyte was characterized by cyclic voltammetry (CV) to find appropriate cathode potentials for the deposition of CoeCu precursor. The scan was started in the cathodic direction from þ0.8 to 0.8 V and the potential scanning rate was 25 mV/s. The CV curve of the electrolyte used to deposit CoeCu alloy film is given in Fig. 1a. One cathodic peak can be found at around 0.02 V which corresponds to reduction of Cu2þ ions. This peak is due to the fact that the steady state is not obtained instantaneously. However, before the current density increases again (if the potential is made more negative than about e0.74 V), a current plateau with low current density occurs, indicating the codeposition of both Cu and Co elements may be achieved in this potential range. Furthermore,
two crossovers can be founded at the potential of 0.02 V and 0.06 V from the CV curves in Fig. 1a, known as the nucleation overpotential and the crossover potential. The appearance of these two crossovers indicated 3-D nucleation and subsequent grain growth process [12]. In electrodeposition, as well known, it is rather difficult to control the reduction rate of two elements using fixed deposition potential. The pulse potential can control the flux of each element approaching the electrode in diluted electrolytes, and then PED method is allowed to choose appropriate potentials to control the composition of binary alloy materials films. Based on the CV curve, the potential U1 is chosen to be the starting potential (nucleation potential) at which Cu is reduced and U2 is chosen to achieve the co-deposition of both elements. Fig. 1b demonstrated the typical potential square wave signal as a function of time during PED mode. The duration of U1 was held constant at t1, while the duration of U2 was held constant at t2. The pulse potentials significantly affect the compositional, structural and morphological properties of the binary material films. The morphology and structure of the products were observed by FESEM. Fig. 2 shows the typical SEM images of the CoeCu dendrites prepared through pulse electrodeposition method with same deposition voltage but various pulse time t1 and t2. It can be seen that all the as-fabricated CoeCu nanostructures have a dendritic morphology with length of up to tens of micrometers. So, a sample can be chosen arbitrarily to discuss the formation mechanism and magnetic properties of the dendritic structure. For example, a lowmagnification SEM image of the sample synthesized at t1 ¼ 2s and t2 ¼ 8s is shown in Fig. 3a, revealing that those CoeCu dendrites consist of a long central backbone with secondary branches, which exhibits good symmetry and self-similarity characteristic of fractals. The width of the backbone is around 1.5e2 mm, and its whole length is up to tens of micrometers. The more highly magnified images give more details about the morphology of the CoeCu dendrites. From the local enlarged image (Fig. 3b), we can see that the backbone is virtually a chain of pyramidal-like structures. Those pyramidal-like structures, recomposed of many neighbouring three dimensional cubic-like nanoparticles with a mean size of 500 nm, link together head-to-tail to form backbone structure, as depicted by those green triangles in Fig. 2b. The images taken of the center and the outer parts of the structure imply that the branches have grown in a different manner (Fig. 3c). Interestingly, some ridge structures can be seen clearly perpendicular to the growth direction of the branches, and the distance between the ridges is about 2.5 ± 0.5 mm. On the other hand, the branches are not separated individually but are connected with adjacent ones. The cubic-like nanoparticles sit side by side and appear with smooth surface and sharp edge (Fig. 3d), which may be attributed to the repeated
Fig. 1. (a) Cyclic voltammetry curve of the electrolyte used to deposit CoeCu films. The potential scanning rate is 25 mV/s. (b) Deposition current pulse forms for the prepared CoeCu dendritic structures. Typical pulse potential and time are U1 for t1 ¼ 2 s and U2 for t2 ¼ 8 s, respectively.
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Fig. 2. SEM images of CoeCu dendrites deposited at U1 ¼ 0.02 V/SCE and U2 ¼ 0.5 V/SCE with different pulse time (a) t1 ¼ 2 s and t2 ¼ 8 s, (b) t1 ¼ 2 s and t2 ¼ 6 s, (c) t1 ¼ 2 s and t2 ¼ 4 s.
nucleation of neighbouring grains and Oswald ripening in which the CoeCu dendrites were grown at the expense of small nanoparticles. Lim et al. have previously observed this type of crystal growth in the formation of dendritic platinum nanostructures [13]. As a result, the dendritic structures can be produced through coalescence or attachment of small nanoparticles to the branches. However, the nanoparticles formed are not so big for the low growth speed [3]. It is well known that the electrochemistry growth process of a crystal is determined by the thermodynamics dominating process and/or the kinetics dominating process [14]. We believe that the observations here imply a competition between thermodynamic and kinetic factors. In the initial stage, when we select the low deposit potential U1 at 0.02 V/SCE, Cu nanoparticles preferentially nucleated on the Au substrate and acted as seeds for further growth of freshly produced CoeCu. Low deposit potential is not high enough to lead to massive metal deposition, so the initial stage is a thermodynamics dominating process. However, when the potential turns to a high value of 0.5 V/SCE and keep this value for 8 s, the dominating factor will transform from thermodynamics to kinetics. At the same time, co-deposition of Cu and Co ions will be achieved and cubic structural CoeCu composited nanoparticles will be obtained. In this process, the obtained CoeCu cubic nanoparticles aggregate directionally to pyramidal shape for the backbone and ridge shape for the branches. After that, because the potential turns from the high value U2 to the low one (U1) again, directional aggregation of metallic nanoparticles is interrupted and a buffer area appears between two pyramids for the backbone or two ridges for the branches, respectively. As a result of the continuous growth,
under the assistance of boric acid, which is a structure-directing agent [15], CoeCu dendrites with chain-like backbone and caterpillar branches are finally formed. By measuring the width of the buffer between two pyramids (l1) and the height of the pyramidal-like nanoparticles clusters (l2), which were demarcated by black and white line segments as shown in Fig. 4a, they are about 0.7 ± 0.1 mm and 3 ± 0.1 mm, respectively. The ratio between the two sizes is about l1:l2 ¼ 1:4, which is almost equal to the ratio of the pulse time (i.e. t1:t2). For comparison, as shown in Fig. 4b, a proportional line segment chequered with black and white was drawn side by side with the potential square wave signal. Furthermore, a similar result can be found in other samples synthesized under a same condition with the same pulse potential but different pulse time. So, we can conclude that the formation of the chain-like backbone of the CoeCu dendrites in this condition may follow the rule,
l1 t1 ¼ l2 t2 We think that the pulse potential may play an important role in the formation of this chain-like backbone, which ensures the codeposition of Cu and Co at the same time. Therefore, the buffer areas and particles aggregation is dominated by thermodynamics process and kinetics process, respectively. On the basis of the above results, the growth mechanism of the CoeCu dendrites can be explained in view of thermodynamics and kinetics control theory accompany with particle attachment [16,17]. The forming process of link-like backbone of the CoeCu dendrites is presented in Fig. 4 c. At the very early stage, prior to the co-
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Fig. 3. (a) SEM image of as-deposited CoeCu dendrite; (b) High-magnification SEM image of the central backbone of the CoeCu dendrite; (c) Low-magnification SEM image of the branches of the CoeCu dendrite; (d) High-magnification SEM image of the branch.
deposition reaction of Co and Cu, because of the different reduction potential of Co2þ and Cu2þ, only the Cu2þ concentration near the electrode is consumed and Cu core nucleates on the Au substrate, but the initial equilibrium condition isn't break down easily. So we can consider this reaction as a relaxation process of ions. When the potential turns to 0.5 V/SCE, a potential high enough to bring about the co-deposition reaction of Co and Cu, the reaction system spontaneously engage in a transition from equilibrium to nonequilibrium, CoeCu buds will form and which will grow to bigger nanoparticles further with increasing the reaction time. At the same time, the generated CoeCu nanoparticles aggregate to clusters in columnar shape because the dominator of kinetics. And then, the reaction system will turn to a short quasi-equilibrium or equilibrium condition again with the potential change from 0.5 V/ SCE to 0.02 V/SCE. The nanoparticles on the top of the columnar clusters will spread to the bottom of them because the dominator of thermodynamics. Finally, the backbone structure composed by pyramidal-like nanoparticles clusters is formed when the reaction time was continuously prolonged. Furthermore, we don't need to describe the forming process of the branch parts anymore because it is similar to the one of the backbone zone. From the view of thermodynamics and kinetics, the CoeCu dendrite, by virtue of its extended surface, has a considerably increased surface energy. Therefore, the aggregation process of the nanoparticles may be much faster than in the equilibrium condition controlled by low potential, which may be the reason buffer areas between two triangles for the backbone and ridges for the branches are formed. The crystalline structure of the as prepared CoeCu dendrites is determined by XRD analysis and the obtained result is shown in Fig. 5. Analysis indicates the CoeCu dendritic structure containing
two phases and the face centered cubic (fcc) phase dominant for all samples. It is known that the CueCo alloy, which is supposed to be a metastable phase at room temperature, is a copper rich CueCo fcc phase with a preferred orientation of (111) planes [18]. In the XRD patterns, the distinct peaks are fcc-Cu (111), accompanied by (200), (220) and Au peaks. Furthermore, the peak has been found at the angular position of 2q ¼ 44.22 , which should belong to the (111) reflection for fcc Co. While a weak shoulder found at 51.08 should belong to the other reflection peak (200) of fcc Co. This means that the (111) and (200) planes of Co and Cu develop independently during the growth of the samples. It is worth noting that the (111) plane is the thermodynamics crystallographic plane and the (200) and (220) are the kinetics crystallographic planes for the fcc lattice, respectively. Therefore, from this point of view, the growth of crystalline planes of the sample is dominated by the thermodynamics and the kinetics factors, which is also supported by the previous growth model. We can obtained the crystallite size of Co is about 30 nm from Scherrer's formula. The stable fcc Co phase is probably attributable to the small grain size and the supersaturated solution of Cu in the electrodeposited CoeCu alloy [19,20]. So the main reason for the formation of dendritic structure with fcc structure is probably the high copper content in the products. The result indicates that the CoeCu system is immiscible as is well known [21,22]. Compositional analysis of the prepared samples by EDS was also made by detecting different zones (the backbone, the side branches and the single particle) of the samples. Fig. 6 shows the EDS spectra of the CoeCu dendrites. From the EDS spectrum, it is clear that all the dendritic samples consist of both Co and Cu segments and the atomic metal content ratio of Cu:Co is 98:2, 98:2, and 99:1 in the
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Fig. 4. (a) The width of the buffer between two pyramids (l1) and the height of the pyramidal-like nanoparticles clusters (l2); (b) Comparison of l1 and l2 with the pulse time; (c) Possible growth process of CoeCu dendrites under the thermodynamics & kinetics dominance model.
Fig. 5. X-ray diffraction patterns of CoeCu dendrites deposited for 0.5 h at U1 ¼ 0.02 V/SCE and U2 ¼ 0.5 V/SCE in 0.3 M Co2þ and 0.05 M Cu2þ; the stars correspond to the Au substrate.
backbone, the side branches and the single particle, respectively. The almost equal ratio of Co to Cu element in different zones indicated that they distributed uniformly in the dendrite. This suggests that the growth of Co and Cu must be simultaneous, which also demonstrates that the co-deposition of Cu and Co ions is possible during the growth process.
As seen in the EDS studies, the content of copper is higher than that of cobalt for all the samples. As we know, the polarization of copper is higher compared to cobalt, because copper is much nobler than cobalt, so the deposition rate of copper is higher at the first stage of the electrodeposition process. At the beginning, copper grains are formed on Au substrate. While as the deposition process progresses, copper deposition process becomes more diffusion limited than that of cobalt due to the higher concentration of cobalt ions in the electrolyte, the product changes from pure copper to the CueCo mixture, either as a metastable solid solution and/or as nanoclusters of cobalt inside a matrix of copper. On the other hand, when cobalt hydride/hydroxide forms, some of the reduction current may be consumed by the reduction of these species, and hence the deposition rate of cobalt is reduced. These may be the reasons why content of cobalt is low detected in the EDS spectrum. Magnetic measurements were performed in magnetic fields applied in the film plane and out-of the film plane at room temperature. As an example, in-plane and perpendicular hysteresis loops of CoeCu dendritic film through pulse electrodeposition with t1 ¼ 2s and t2 ¼ 8s were shown in Fig. 7. It can be clearly seen that the perpendicular hysteresis loop has higher remanent magnetization (4.5 105 emu) and lower coercivity (56 Oe) than the inplane loops for the sample. The remanent magnetization and coercivity are 2 105 emu and 67 Oe, respectively, with the field parallel to the film plane. This indicates that easy-axis direction of the magnetization is perpendicular and hard-axis is parallel to the film plane although the existence of diamagnetic effect. However, a contrary result had ever been reported in the prior literature
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Fig. 6. EDS spectra of pulse electrodeposited CoeCu dendrites (a) at the backbone, (b) at the branches, and (c) at single particle.
structures. Acknowledgement This work is supported by the Natural Science Foundation of Hebei Province (Grant No. A2016205237). References
Fig. 7. Magnetization hysteresis loop measured at room temperature for CoeCu dendritic structural film with pulse potential and time are U1 for t1 ¼ 2 s and U2 for t2 ¼ 8 s.
although a same dendritic structure was found [23]. We can attribute this effect to the difference of microstructure caused by the pulse deposition method.
4. Conclusions In this study, CoeCu dendrites consist of a long central backbone with secondary branches were obtained by pulse electrodeposition method. Cyclic voltammetry of CoeCu deposition reveals that there was a current plateau with low current can be found between the reduction peak of Cu2þ and Co2þ ions, and the middle value, i.e. 0.5 V, was select as the deposition potential. Thermodynamics and kinetics control theory accompany with particle attachment are used to explain the growth process of dendritic nanostructures. We expect that the pulse electrodeposition route can be extended to prepare other composite metal nanomaterials with dendritic
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