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The influence of pH and bath composition on the properties of Ni–Co coatings synthesized by electrodeposition
Vacuum 86 (2011) 27e33
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
The influence of pH and bath composition on the properties of NieCo coatings synthesized by electrodeposition Liangliang Tian, Jincheng Xu*, Songtao Xiao Department of Materials Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 November 2010 Received in revised form 30 March 2011 Accepted 30 March 2011
NieCo coatings were produced on Cu substrates by electrodeposition from electrolytes with different pH values and different Co2þ concentration. The current efficiency increases from 52.1% to 81.2% with the pH increasing from 2.0 to 5.4. It is clearly observed that the content of cobalt in the deposited coatings gradually increases from 9.4% to 19.6% as the pH value varies from 2.0 to 5.4. The Co content in the deposited coatings increases from 16.5% to 72.7% as the molar ratio of CoSO4/NiSO4 varying from 1:5 to 1:2 in electrolyte. XRD patterns reveal that the structure of the coatings strongly depends on the Co content in the binary coatings. Both granular and dendritic crystals were investigated by SEM and the different crystallization behaviors were illustrated. The saturation magnetization of the coatings goes up from 96.36 kAm1 to 136.08 kAm1 with the pH value increasing from 2.0 to 5.4. The saturation magnetization (Ms) and coercivity (Hc) move up from 144.84 kAm1 and 15.27 kAm1 to 175.13 kAm1 and 125.20 kAm1 with the increase of Co in the electrolyte, respectively. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: NieCo coating Current efficiency Electrodeposition Crystallization behavior Magnetic property
1. Introduction NieCo alloys have been widely used as important engineering materials in industry because of their high-strength, good wear resistance, heat-conductive, electrocatalytic activity and their utility as a soft magnetic film [1e5]. Cobaltenickel alloys are of practical importance because the alloys have high protective and decorative values, besides a wide range of industrial applications. As an example, the alloy is used in the electroforming of molds for die-casting and plastics [6]. The magnetic properties of the cobaltenickel alloy are of interest in electronic applications such as memory drums, discs, cards and tapes particularly in the computer industry [7]. Among all the methods fabricating NieCo films, electrodeposition is a simple and economic method to produce NieCo films without high temperature and high pressure. Lots of metallic coatings have been synthesized by the electrodeposition method, such as ZneMn [8], ZneCo [9,10], NieCueMo [11], ZneNi [12], composite coatings of NieSiC [13], NieCNTs [14], NieCeO2 [15], FeeCreP [16], bronzeegraphite [17] also can be prepared by the electrodeposition process. NieCo alloys are known as very good magnetic materials. Their alloys possess much better permanent magnetic properties than pure metals [18].
* Corresponding author. Tel.: þ86 931 8913577. E-mail address: [email protected] (J. Xu). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.03.027
Moreover, magnetic composites on the basis of such alloys offer a modification of their magnetic properties (enhanced saturation magnetization, controllable coercivity) and they could have a wide range of applications. As we know, physicochemical properties of NieCo films are seriously affected by their compositions and structures [19]. A reliable control of the composition and structure is an important issue for their wide applications. In our present work, in order to control the composition and magnetic property, the electrochemical method was employed to synthesize NieCo coatings. The influence of pH value and Co2þ concentration in the electrolyte on the surface morphology, composition and magnetic property was investigated. A theory which can be concluded into three factors was introduced to research the influence of pH value. Two growth mechanisms were illustrated to explain the corresponding morphology and the crystallization behavior was discussed. The magnetic properties of the coatings were researched. 2. Experiment The electrodepositions were carried out in a conventional threeelectrode cell employing a Electrochemical workstation with a potential range from 12 V to 12 V, current range from 2 A to 2 A. Cu plates were used as substrates. Before deposition, the Cu plates were polished, degreased and rinsed in cold distilled water, activated in a 15% HCl solution (1 min) and rinsed in distilled water again. The
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L. Tian et al. / Vacuum 86 (2011) 27e33
Table 1 Bath composition and deposition conditions for NieCo binary coatings. Bath composition NiSO4$6H2O CoSO4$6H2O H3BO3 Saccharin
Concentration (mol L1) 0.625, 0.5, 0.375, 0.25 0.125 0.4 0.015
Electrodeposition parameter Current density (Adm2) pH Temperature ( C) Time (s)
Value 2.0 2.0, 3.1, 4.3, 5.4 25 1200
exposed area of these pretreated Cu substrates is equal to 1 cm2 while the other surface areas were insulated with PTFE before the deposition. After pretreatment, the Cu plates were transferred into a 50 cm3 beaker containing 40 cm3 electrolytes. Component of the electrolyte and electrodeposition parameters were listed in Table 1. H3BO3 was used to keep the pH a constant. Saccharin was used to reduce internal stress between the coating and Cu plate. Different electrolytes with mole ratio of Co2þ/Ni2þ 1:5, 1:4, 1:3, 1:2 of pH 3.1 were prepared to do comparison research. pH value of the bath was adjusted by 2 M NaOH and H2SO4 solution. A platinum plate with an area of 2 cm2 was used as the anode, while a saturated calomel electrode (SCE) was used as the reference electrode. All the coatings were deposited without stirring. The chemical composition of the coatings was analyzed by inductively coupled plasma atomic emission spectrometry(ICP-AES). X-ray diffraction (XRD) measurements were performed on a Rigaku D/Max-2400 X-ray diffractometer using Cu Ka radiation (40 kV, 60 mA). The morphology and thickness of the coatings were analyzed on a Hitachi S-4800 field emission scanning
electron microscope (SEM) operating at an acceleration voltage of 5 kV. The magnetic properties of the coatings were measured by a Lake Shore 7304 vibrating sample magnetometer at room temperature. 3. Results and discussion 3.1. Effects of pH value 3.1.1. Influence of pH on the morphologies of the coatings Fig. 1 shows the surface morphologies of the coatings deposited at different pH values. In general, the coatings are not so perfect and have quantities of defects on the surface. As can be seen from Fig. 1, it is clearly indicated that with the increase of pH value, the surface of the coatings becomes less defective [20]. In the system of Ni and Co, ion species of Ni, Co, H, H2, NiH, CoH, NiH2, CoH2, NiOH, CoOH, NiH2O, CoH2O, Ni(OH)2 and Co(OH)2 were observed and the dominated ions changed with pH [21]. At lower pH value (2.0 and 3.1), atomic hydrogen and hydrides dominate in the coatings. Quantities of dark circle holes of 100 nm are investigated on the surface as marked in Fig. 1a, indicating that hydrogen evolution strongly influences the deposition. At higher pH value (4.3 and 5.4), the influence of hydrogen and hydrides becomes less apparent and the dark circle holes nearly disappear. Simultaneously, the deposition ratio becomes larger with the increase of the pH. The higher deposition ratio causes a situation that there is no enough time for internal stress release and cracking of the coatings may occur [21]. Fig. 2 gives the AFM images of the coatings deposited at different pH. It is found that the roughness of the coatings decreases from 266.96 nm to 92.93 nm with the pH value increasing from 2.0 to 5.4. Simultaneously, the coatings tend to be more compact with the increase of the pH value because of the weakening of hydrogen evolution at higher pH.
Fig. 1. SEM micrographs of NieCo coatings deposited at different pH values from the electrolyte containing 0.625 M NiSO4 þ 0.125 M CoSO4 þ 0.4 M H3BO3 at room temperature (a) pH 2.0, (b) pH 3.1, (c) pH 4.3, (d) pH 5.4.
L. Tian et al. / Vacuum 86 (2011) 27e33
29
Fig. 2. The AFM images of the coatings deposited at different pH values. (a) pH 2.0, (b) pH 3.1, (c) pH 4.3, (d) pH 5.4.
3.1.2. Effects of pH value on Co content in the binary coatings Composition of the deposited coatings is precisely measured by ICP and shown in Fig. 3. It is found that the content of Co increases from 9.4% to 19.6% as the pH value varying from 2.0 to 5.4. The electrodeposition process is mainly dominated by three factors: (1) The dissolution of the freshly deposited metal atoms on the substrate because of the acid circumstance in the electrolyte; (2) The formation and absorption of metals hydroxides on the electrode surface; (3) The normal electrodeposition of the metals. A lower pH
value favors the dissolution of freshly deposited metal and depresses the formation and absorption of metal hydroxides. The process is predominated by the normal deposition of the metal, resulting in lower cobalt content in the coatings due to the lower Co2þ in the electrolyte. Nevertheless, a higher pH value favors the formation and absorption of metal hydroxides and depresses the dissolution of the freshly deposited metal. The process is predominated by the second factor, the preferential absorption of cobalt hydroxides results in áková studied higher cobalt content in the coatings [22]. Renáta Orin the Co content in the coatings deposited at different pH values (2.0, 3.0, 4.0) and achieved the same results [21]. 3.1.3. Approximate calculation of current efficiency at different pH values In order to further study the influence of pH, the current efficiency at different pH values was calculated. According to the definition of the current efficiency, the current efficiency h can be expressed as follows:
mco mNi F Jk St h ¼ z þ Mco MNi
(1)
where Jk is the current density, S is the superficial area of the cathode, t is the deposition time, z is the charge of the metal ions, F is the Faraday constant. mCo and mNi are the weight of Co and Ni in the coatings, respectively. MCo and MNi are the atomic weight of the Co and Ni, respectively. It is well known that MCo and MNi are 58.9 and 58.7, respectively. Under approximate condition, MCo approximately equals to MNi. Therefore, the Eq. (1) can be shown as Eq. (2). Fig. 3. The dependence of Co content in the coatings on pH value. (the mole ratio of CoSO4/NiSO4 ¼ 0.125 M/0.625 M, T ¼ 25 C, t ¼ 20min).
mco þ mNi m F ¼ z F Jk St h ¼ z M M
(2)
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L. Tian et al. / Vacuum 86 (2011) 27e33
where M is the atomic weight of Co or Ni, m is the weight of the deposited coatings. On the basis of above discussion, the current efficiency h can be calculated by Eq. (3).
h¼
zmF MJk St
(3)
The current efficiency was calculated and the results were shown in Table 2. It is clear that the current efficiency increases from 52.1% to 81.2% with the pH increasing from 2.0 to 5.4. At lower pH value, higher Hþ concentration favors the evolution of hydrogen and more current is consumed on hydrogen evolution resulting in the lower current efficiency. Nevertheless, when the pH is high, lower Hþ concentration depresses the evolution of hydrogen and less current is consumed on hydrogen evolution leading to a higher current efficiency. 3.2. Effects of Co2þ concentration on the Co content in the coatings 3.2.1. CV curves analysis Cyclic voltammetry was used to determine the reduction and oxidation potentials for the characterization of nickel and cobalt electrodeposition. In the early work, A. Dolati studied the cathodic electrodeposition behaviors of NieCo alloys by cyclic voltammetry and researched the influence of Co2þ concentration on the cathodic process [23]. However, the anodic process was ignored. In this work, the anodic process was investigated. Cyclic voltammograms for the electroplating of NieCo coatings were measured at 20 mV s1 pH 3.1 and displayed in Fig. 4. We can observe that three reaction regions corresponding to the anodic dissolution (E > 0.3 V), the double-layer responses (0.8V < E < 0.3 V) and the cathodic deposition (E < 0.8) are clearly found on all CV curves in Fig. 4. The straight line positions are the oxidation peak positions corresponding to Co (0 V) and Ni (0.18 V). In the case of cathode, note that at potentials negative to 0.85 V on both positive and negative sweeps of all curves, cathodic currents are clearly found, indicating that nickel and cobalt ions can be deposited onto the substrate in the cathodic deposition region. Furthermore, the deposition currents increase with the Co2þ concentration increasing. In the case of anode, it is clear that the current contributions of Co are always much higher than that of Ni even at low Co2þ concentration, demonstrating the existence of anomalous codeposition in the NieCo system. Moreover, the increase in the Co2þ concentration displaces NieCo alloy oxidation peaks to more negative potential with higher Co current distributions. In addition, the oxidation currents of NieCo alloy increase with the increase of Co2þ concentration.
Fig. 4. CV curves for the electroplating of NieCo coatings on platinum filament (with diameter of 1 mm) from the baths with different mole ratio of Co2þ/Ni2þ. (the concentration of Co2þ was kept at 0.125 M).
1:4, no other diffraction peaks are found except the NieCo fcc peaks located at 44.5 , 51.8 , 76.4 and 93.1. It is mainly because the Co atoms are incorporated into the Ni lattice since the microstructure of Ni and Co are resemble. At Co2þ/Ni2þ 1:3 and 1:2, besides the diffraction peak of (200), the peaks of hcp Co ((100) and (101)) appear revealing the formation of different phases of Co in the coatings. Moreover, the diffraction peak of NieCo located at 51.8 splits into two peaks corresponding to Ni (200) and Co (200). On the basis of above discussion, the structure of the deposited NieCo coatings strongly depends on the Co content in the deposited coatings [24]. 3.2.3. ICP analysis Fig. 6 reveals the relationship between Co2þ concentration in the electrolyte and the composition of deposited coatings. It is
3.2.2. XRD analysis Fig. 5 demonstrates the XRD patterns of the NieCo coatings deposited from the electrolyte containing different concentration of Co2þ. From Fig. 5, no diffraction peaks of Cu are found in all the patterns indicating that the Cu plates are completely covered by the coatings. The diffraction peaks corresponding to NieCo alloys are found on all the curves, indicating the microcrystalline structure of the coatings [19]. As noted in Fig. 5, when the Co2þ/Ni2þ are 1:5 and
Table 2 Current efficiency at different pH values. pH value
Current efficiency
2.0 3.1 4.3 5.4
52.1% 62.3% 74.7% 81.2%
Fig. 5. XRD patterns of NieCo binary coatings deposited from the electrolyte with different mole ratio of Co2þ/Ni2þ at room temperature, t ¼ 20min, pH ¼ 3.1. (the concentration of Co2þ was kept at 0.125 M).
L. Tian et al. / Vacuum 86 (2011) 27e33
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Fig. 6. Effect of molar ratio of CoSO4/NiSO4 on the coating composition. (the concentration of Co2þ was kept at 0.125 M, pH ¼ 3.1).
clearly revealed that the content of Co increases from 16.5% to 72.7% as the molar ratio of CoSO4/NiSO4 varying from 1:5 to 1:2. The results are consistent with the research of electroless deposition of NieCoeB [18]. Anomalous codeposition was investigated during the electrodeposition process. This phenomenon has been reported mainly in codeposition of iron-group metals (iron, cobalt and nickel) and zinc with iron-group metals. The hydroxide suppression mechanism was introduced as an explanation [25,26]. On the basis of above discussion, the anomalous codeposition can also be attributed to the three factors mentioned above. The effect of the second factor is more important because the normal codeposition always exists and the dissolution of fleshly deposited metal atoms is less apparent at pH 3.1. In the system of NieCo, when pH value exceeds the critical value for forming metal hydroxide, both nickel and cobalt hydroxides may form. The anomalous codeposition occurs because of the preferential absorption of cobalt hydroxides. When pH value remains below the critical value, Ni2þ and Co2þ exist as the form of NiOHþ and CoOHþ [21]. It is believed that both the monohydroxides and hydroxides of Co are favored in aqueous solution [19]. Generally speaking, it is the competition absorption of hydroxide or monohydroxide results in the anomalous codeposition.
Fig. 7. The micrographs of the deposited coating at pH 5.4 from the electrolyte containing 0.625 M NiSO4 þ 0.125 M CoSO4 þ 0.4 M H3BO3 at room temperature.
formation of two-dimensional crystal nuclei is driven by the overpotential in the solution. The nucleation rate J can be expressed as follows:
lgJ ¼ A
B jD4j
where A and B are constant, jD4j is modulus of the overpotential. With the increase of overpotential, the size of the crystal nuclei
3.3. The crystallization behavior of the coatings The surface SEM micrographs for the deposited coating are shown in Fig. 7. From Fig. 7, we can see that the deposited coating is composed of equiaxed granules of about 15e20 nm in size (Fig. 7a). The surface morphology of the coating shows coexistence of two features: dendritic crystals (Fig. 7b) embedded in a fine-grained matrix (Fig. 7a). The two kinds of morphologies are corresponding to different crystallization behaviors. The SEM image of Fig. 7a shows granular morphology related to nucleation and growth mechanism (Fig. 8). Large quantities of tiny steps exist on the surface of Cu substrates because of the polish before electrodeposition. Ni and Co atoms are adsorbed onto the steps after being reduced from the electrolyte due to the lower nucleation energy on the surface of steps. The tiny steps disappear after being filled up with the adsorbed atoms. Then the twodimensional crystal nuclei form by the aggregation of adsorbed atoms to provide fresh steps for the electrocrystallization. The
(4)
Fig. 8. The schematic illustration of nucleation and growth mechanism.
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L. Tian et al. / Vacuum 86 (2011) 27e33
Fig. 9. The schematic illustration of growth of dendritic crystals.
which can steadily exist decreases and the nucleation rate increases. Eventually, the morphology of Fig. 7a forms by the layer and layer growth pattern. Fig. 7b represents the morphology of dendritic crystals related to the resistance of liquid mass transfer (Fig. 9). If the atoms crystallize on the tiny projection of the Cu substrates, the growth speed of the crystals is much faster than the other parts because of the much larger diffusion flux. The diffusion flux Jd can be expressed as follows:
Jd ¼
zFDi c0i r0
(5)
where z is the number of electrons involved into the reaction, F is the Faraday constant, Di is the diffusion coefficient, c0i is the solution concentration, r0 is the radius of curvature of the tip of the dendritic crystals. From the SEM image (Fig. 7b) the r0 of the dendritic crystals is about 10 nm and that value of the other crystals is about 10 mm corresponding to the thickness of the diffusion layer. From Eq. (5), we can know that Jd of the dendritic crystals is much larger than the other crystals and the larger Jd favors the formation of dendritic crystals.
Fig. 11. The dependence of saturation magnetization (Ms) and coercivity (Hc) on the mole ratio of Co2þ and. Ni2þ in the electrolyte.
3.4. Magnetic property analysis The magnetic properties of the NieCo coatings electrodeposited at different pH values were measured at room temperature and shown in Fig. 10. The saturation magnetization (Ms) moves up from 96.36 kAm1 to 136.08 kAm1 with the pH value increasing from 2.0 to 5.4. The coercivity (Hc) of the coatings slightly reduces from 20.26 kAm1 of pH 2.0 to 14.21 kAm1 of pH 5.4. The increase of saturation magnetization (Ms) is caused by the increase of Co which has higher saturation magnetization (Ms). The coercivity (Hc) is sensitive to the phase, defect, stress, thickness of the coatings [18]. At lower pH value (especially pH 2.0), the deposited coatings contain more defect of dark circle holes (Fig. 1a) than that of the higher pH value due to fierce reduction of Hþ. The higher defect concentration results in a higher coercivity (Hc) even though the coating contains less Co. Fig. 11 displays the dependence of saturation magnetization (Ms) and coercivity (Hc) on the mole ratio of Co2þ and Ni2þ. As can be seen from Fig. 11, the saturation magnetization (Ms) and coercivity (Hc) move up from 144.84 kAm1 and 15.27 kAm1 to 175.13 kAm1 and 125.20 kAm1 with the increase of Co in the deposited coatings. The increase of the saturation magnetization (Ms) can be attributed to the increase of Co content in the deposited coatings. The coercivity (Hc) may be effected by two factors: (1) Defects in the coatings influence the coercivity. (2) Co content in the coatings influences the coercivity. In this group of experiment, the effect of defects can be neglected compared with the sharp increase of Co in the coatings. Therefore, the coercivity increases with the increase of Co content in the coatings. 4. Conclusions
Fig. 10. The dependence of saturation magnetization (Ms) and coercivity (Hc) of the NieCo coatings on the pH value.
NieCo coatings were successfully electrodeposited on Cu substrates from the electrolyte with different Co2þ concentration and pH values. The content of cobalt gradually increases from 9.4% to 19.6% as the pH value varying from 2.0 to 5.4. The current efficiency increases from 52.1% to 81.2% with the pH increasing from 2.0 to 5.4. The content of Co increases from 16.5% to 72.7% as the molar ratio of Co2þ and Ni2þ varying from 1:5 to 1:2 in the electrolyte. The coatings with lower Co content are fcc structure. However, the structure of hcp Co appears at higher Co content in the coatings. The crystallization behaviors of the different features are explained by
L. Tian et al. / Vacuum 86 (2011) 27e33
nucleation and growth mechanism and the growth of dendritic crystals. The magnetic performance of the coatings shows that the saturation magnetization (Ms) moves up from 96.36 kAm1 to 136.08 kAm1 with the pH value increasing from 2.0 to 5.4. The saturation magnetization (Ms) and coercivity (Hc) move up from 144.84 kAm1 and 15.27 kAm1 to 175.13 kAm1 and 125.20 kAm1 with the increase of Co in the coatings, respectively. Acknowledgments This work was supported by the National Talent training found of Lanzhou University in basic physics (513-041102). References [1] [2] [3] [4] [5]
Shi L, Sun CF, Gao P, Zhou F, Liu WM. Appl Surf Sci 2006;252:3591e9. Chow GM, Ding J, Zhang J, Lee KY, Surani D. Appl Phys Lett 1999;74:1889e91. Yu ZB, Qiao MH, Li HX, Deng JF. Appl Cata A 1997;163:1e13. Chi B, Li JB, Yang XZ, Gong YL, Wang N. Int J Hydrogen Energ 2005;30:29e34. Narayanan TN, Shaijumon MM, Ajayan PM, Anantharaman MR. Nanoscale Res Lett 2010;5:164e8. [6] Yang H, Pan CT, Chou MC. J Micromech Microeng 2001;11:94e7.
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[7] Abd El-Rehim SS, Abd El-Halim AM, Osman MM. J Appl Electrochem 1985;15: 107e12. [8] Díaz-Arista P, Ortiz ZI, Ruiz H, Ortega R, Meas Y, Trejo G. Surf Coat Tech 2009; 203:1167e75. [9] Lodhi ZF, Mol JMC, Hamer WJ, Terryn HA, DeWit JHW. Electrochim Acta 2007; 52:5444e52. [10] Chen PY, Sun IW. Electrochim Acta 2001;46:1169e77. [11] Beltowska-Lehman E. Surf Coat Tech 2002;151e152:440e3. [12] Beltowska-Lehman E, Ozga P, Swiatek Z, Lupi C. Surf Coat Tech 2002; 151e152:444e8. [13] Kim Sun Kyu, Yoo Hong Jae. Surf Coat Tech 1998;108e109:564e9. [14] Guo C, Zuo Y, Zhao XH, Zhao JM, Xiong JP. Surf Coat Tech 2008;202: 3246e50. [15] Xue YJ, Jia XZ, Zhou YW, Ma W, Li JS. Surf Coat Tech 2006;200:5677e81. [16] Li BS, Lin A, Wu X, Zhang YM, Gan FX. J Alloy Compd 2008;453:93e101. [17] Nickchi T, Ghorbani M. Surf Coat Tech 2009;203:3037e43. [18] Wang Sl. Thin Solid Films 2007;515:8419e23. [19] Bai A, Hu CC. Electrochim Acta 2002;47:3447e56. [20] Vicenzo A, Cavallotti PL. Electrochim Acta 2004;49:4079e89. áková R, Orin ák A, Vering G, Talian I, Smith RM, Arlinghaus HF. Thin Solid [21] Orin Films 2008;516:3045e50. [22] Gomez E, Ramirez J, Valles E. J Appl Electrochem 1998;28:71e9. [23] Dolati A, Sababi M, Nouri E, Ghorbani M. Mater Chem Phys 2007;102:118e24. [24] Wu G, Li N, Zhou DR, Mitsuo K. Surf Coat Tech 2004;176:157e64. [25] G?mez E, Vallés E. J Electroanal Chem 1997;421:157e63. [26] Fabri Miranda FJ, Barcia OE, Diaz SL, Mattos OR, Wiart R. Electrochim Acta 1996;41:1041e9.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
The influence of pH and bath composition on the properties of NieCo coatings synthesized by electrodeposition Liangliang Tian, Jincheng Xu*, Songtao Xiao Department of Materials Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 November 2010 Received in revised form 30 March 2011 Accepted 30 March 2011
NieCo coatings were produced on Cu substrates by electrodeposition from electrolytes with different pH values and different Co2þ concentration. The current efficiency increases from 52.1% to 81.2% with the pH increasing from 2.0 to 5.4. It is clearly observed that the content of cobalt in the deposited coatings gradually increases from 9.4% to 19.6% as the pH value varies from 2.0 to 5.4. The Co content in the deposited coatings increases from 16.5% to 72.7% as the molar ratio of CoSO4/NiSO4 varying from 1:5 to 1:2 in electrolyte. XRD patterns reveal that the structure of the coatings strongly depends on the Co content in the binary coatings. Both granular and dendritic crystals were investigated by SEM and the different crystallization behaviors were illustrated. The saturation magnetization of the coatings goes up from 96.36 kAm1 to 136.08 kAm1 with the pH value increasing from 2.0 to 5.4. The saturation magnetization (Ms) and coercivity (Hc) move up from 144.84 kAm1 and 15.27 kAm1 to 175.13 kAm1 and 125.20 kAm1 with the increase of Co in the electrolyte, respectively. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: NieCo coating Current efficiency Electrodeposition Crystallization behavior Magnetic property
1. Introduction NieCo alloys have been widely used as important engineering materials in industry because of their high-strength, good wear resistance, heat-conductive, electrocatalytic activity and their utility as a soft magnetic film [1e5]. Cobaltenickel alloys are of practical importance because the alloys have high protective and decorative values, besides a wide range of industrial applications. As an example, the alloy is used in the electroforming of molds for die-casting and plastics [6]. The magnetic properties of the cobaltenickel alloy are of interest in electronic applications such as memory drums, discs, cards and tapes particularly in the computer industry [7]. Among all the methods fabricating NieCo films, electrodeposition is a simple and economic method to produce NieCo films without high temperature and high pressure. Lots of metallic coatings have been synthesized by the electrodeposition method, such as ZneMn [8], ZneCo [9,10], NieCueMo [11], ZneNi [12], composite coatings of NieSiC [13], NieCNTs [14], NieCeO2 [15], FeeCreP [16], bronzeegraphite [17] also can be prepared by the electrodeposition process. NieCo alloys are known as very good magnetic materials. Their alloys possess much better permanent magnetic properties than pure metals [18].
* Corresponding author. Tel.: þ86 931 8913577. E-mail address: [email protected] (J. Xu). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.03.027
Moreover, magnetic composites on the basis of such alloys offer a modification of their magnetic properties (enhanced saturation magnetization, controllable coercivity) and they could have a wide range of applications. As we know, physicochemical properties of NieCo films are seriously affected by their compositions and structures [19]. A reliable control of the composition and structure is an important issue for their wide applications. In our present work, in order to control the composition and magnetic property, the electrochemical method was employed to synthesize NieCo coatings. The influence of pH value and Co2þ concentration in the electrolyte on the surface morphology, composition and magnetic property was investigated. A theory which can be concluded into three factors was introduced to research the influence of pH value. Two growth mechanisms were illustrated to explain the corresponding morphology and the crystallization behavior was discussed. The magnetic properties of the coatings were researched. 2. Experiment The electrodepositions were carried out in a conventional threeelectrode cell employing a Electrochemical workstation with a potential range from 12 V to 12 V, current range from 2 A to 2 A. Cu plates were used as substrates. Before deposition, the Cu plates were polished, degreased and rinsed in cold distilled water, activated in a 15% HCl solution (1 min) and rinsed in distilled water again. The
28
L. Tian et al. / Vacuum 86 (2011) 27e33
Table 1 Bath composition and deposition conditions for NieCo binary coatings. Bath composition NiSO4$6H2O CoSO4$6H2O H3BO3 Saccharin
Concentration (mol L1) 0.625, 0.5, 0.375, 0.25 0.125 0.4 0.015
Electrodeposition parameter Current density (Adm2) pH Temperature ( C) Time (s)
Value 2.0 2.0, 3.1, 4.3, 5.4 25 1200
exposed area of these pretreated Cu substrates is equal to 1 cm2 while the other surface areas were insulated with PTFE before the deposition. After pretreatment, the Cu plates were transferred into a 50 cm3 beaker containing 40 cm3 electrolytes. Component of the electrolyte and electrodeposition parameters were listed in Table 1. H3BO3 was used to keep the pH a constant. Saccharin was used to reduce internal stress between the coating and Cu plate. Different electrolytes with mole ratio of Co2þ/Ni2þ 1:5, 1:4, 1:3, 1:2 of pH 3.1 were prepared to do comparison research. pH value of the bath was adjusted by 2 M NaOH and H2SO4 solution. A platinum plate with an area of 2 cm2 was used as the anode, while a saturated calomel electrode (SCE) was used as the reference electrode. All the coatings were deposited without stirring. The chemical composition of the coatings was analyzed by inductively coupled plasma atomic emission spectrometry(ICP-AES). X-ray diffraction (XRD) measurements were performed on a Rigaku D/Max-2400 X-ray diffractometer using Cu Ka radiation (40 kV, 60 mA). The morphology and thickness of the coatings were analyzed on a Hitachi S-4800 field emission scanning
electron microscope (SEM) operating at an acceleration voltage of 5 kV. The magnetic properties of the coatings were measured by a Lake Shore 7304 vibrating sample magnetometer at room temperature. 3. Results and discussion 3.1. Effects of pH value 3.1.1. Influence of pH on the morphologies of the coatings Fig. 1 shows the surface morphologies of the coatings deposited at different pH values. In general, the coatings are not so perfect and have quantities of defects on the surface. As can be seen from Fig. 1, it is clearly indicated that with the increase of pH value, the surface of the coatings becomes less defective [20]. In the system of Ni and Co, ion species of Ni, Co, H, H2, NiH, CoH, NiH2, CoH2, NiOH, CoOH, NiH2O, CoH2O, Ni(OH)2 and Co(OH)2 were observed and the dominated ions changed with pH [21]. At lower pH value (2.0 and 3.1), atomic hydrogen and hydrides dominate in the coatings. Quantities of dark circle holes of 100 nm are investigated on the surface as marked in Fig. 1a, indicating that hydrogen evolution strongly influences the deposition. At higher pH value (4.3 and 5.4), the influence of hydrogen and hydrides becomes less apparent and the dark circle holes nearly disappear. Simultaneously, the deposition ratio becomes larger with the increase of the pH. The higher deposition ratio causes a situation that there is no enough time for internal stress release and cracking of the coatings may occur [21]. Fig. 2 gives the AFM images of the coatings deposited at different pH. It is found that the roughness of the coatings decreases from 266.96 nm to 92.93 nm with the pH value increasing from 2.0 to 5.4. Simultaneously, the coatings tend to be more compact with the increase of the pH value because of the weakening of hydrogen evolution at higher pH.
Fig. 1. SEM micrographs of NieCo coatings deposited at different pH values from the electrolyte containing 0.625 M NiSO4 þ 0.125 M CoSO4 þ 0.4 M H3BO3 at room temperature (a) pH 2.0, (b) pH 3.1, (c) pH 4.3, (d) pH 5.4.
L. Tian et al. / Vacuum 86 (2011) 27e33
29
Fig. 2. The AFM images of the coatings deposited at different pH values. (a) pH 2.0, (b) pH 3.1, (c) pH 4.3, (d) pH 5.4.
3.1.2. Effects of pH value on Co content in the binary coatings Composition of the deposited coatings is precisely measured by ICP and shown in Fig. 3. It is found that the content of Co increases from 9.4% to 19.6% as the pH value varying from 2.0 to 5.4. The electrodeposition process is mainly dominated by three factors: (1) The dissolution of the freshly deposited metal atoms on the substrate because of the acid circumstance in the electrolyte; (2) The formation and absorption of metals hydroxides on the electrode surface; (3) The normal electrodeposition of the metals. A lower pH
value favors the dissolution of freshly deposited metal and depresses the formation and absorption of metal hydroxides. The process is predominated by the normal deposition of the metal, resulting in lower cobalt content in the coatings due to the lower Co2þ in the electrolyte. Nevertheless, a higher pH value favors the formation and absorption of metal hydroxides and depresses the dissolution of the freshly deposited metal. The process is predominated by the second factor, the preferential absorption of cobalt hydroxides results in áková studied higher cobalt content in the coatings [22]. Renáta Orin the Co content in the coatings deposited at different pH values (2.0, 3.0, 4.0) and achieved the same results [21]. 3.1.3. Approximate calculation of current efficiency at different pH values In order to further study the influence of pH, the current efficiency at different pH values was calculated. According to the definition of the current efficiency, the current efficiency h can be expressed as follows:
mco mNi F Jk St h ¼ z þ Mco MNi
(1)
where Jk is the current density, S is the superficial area of the cathode, t is the deposition time, z is the charge of the metal ions, F is the Faraday constant. mCo and mNi are the weight of Co and Ni in the coatings, respectively. MCo and MNi are the atomic weight of the Co and Ni, respectively. It is well known that MCo and MNi are 58.9 and 58.7, respectively. Under approximate condition, MCo approximately equals to MNi. Therefore, the Eq. (1) can be shown as Eq. (2). Fig. 3. The dependence of Co content in the coatings on pH value. (the mole ratio of CoSO4/NiSO4 ¼ 0.125 M/0.625 M, T ¼ 25 C, t ¼ 20min).
mco þ mNi m F ¼ z F Jk St h ¼ z M M
(2)
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L. Tian et al. / Vacuum 86 (2011) 27e33
where M is the atomic weight of Co or Ni, m is the weight of the deposited coatings. On the basis of above discussion, the current efficiency h can be calculated by Eq. (3).
h¼
zmF MJk St
(3)
The current efficiency was calculated and the results were shown in Table 2. It is clear that the current efficiency increases from 52.1% to 81.2% with the pH increasing from 2.0 to 5.4. At lower pH value, higher Hþ concentration favors the evolution of hydrogen and more current is consumed on hydrogen evolution resulting in the lower current efficiency. Nevertheless, when the pH is high, lower Hþ concentration depresses the evolution of hydrogen and less current is consumed on hydrogen evolution leading to a higher current efficiency. 3.2. Effects of Co2þ concentration on the Co content in the coatings 3.2.1. CV curves analysis Cyclic voltammetry was used to determine the reduction and oxidation potentials for the characterization of nickel and cobalt electrodeposition. In the early work, A. Dolati studied the cathodic electrodeposition behaviors of NieCo alloys by cyclic voltammetry and researched the influence of Co2þ concentration on the cathodic process [23]. However, the anodic process was ignored. In this work, the anodic process was investigated. Cyclic voltammograms for the electroplating of NieCo coatings were measured at 20 mV s1 pH 3.1 and displayed in Fig. 4. We can observe that three reaction regions corresponding to the anodic dissolution (E > 0.3 V), the double-layer responses (0.8V < E < 0.3 V) and the cathodic deposition (E < 0.8) are clearly found on all CV curves in Fig. 4. The straight line positions are the oxidation peak positions corresponding to Co (0 V) and Ni (0.18 V). In the case of cathode, note that at potentials negative to 0.85 V on both positive and negative sweeps of all curves, cathodic currents are clearly found, indicating that nickel and cobalt ions can be deposited onto the substrate in the cathodic deposition region. Furthermore, the deposition currents increase with the Co2þ concentration increasing. In the case of anode, it is clear that the current contributions of Co are always much higher than that of Ni even at low Co2þ concentration, demonstrating the existence of anomalous codeposition in the NieCo system. Moreover, the increase in the Co2þ concentration displaces NieCo alloy oxidation peaks to more negative potential with higher Co current distributions. In addition, the oxidation currents of NieCo alloy increase with the increase of Co2þ concentration.
Fig. 4. CV curves for the electroplating of NieCo coatings on platinum filament (with diameter of 1 mm) from the baths with different mole ratio of Co2þ/Ni2þ. (the concentration of Co2þ was kept at 0.125 M).
1:4, no other diffraction peaks are found except the NieCo fcc peaks located at 44.5 , 51.8 , 76.4 and 93.1. It is mainly because the Co atoms are incorporated into the Ni lattice since the microstructure of Ni and Co are resemble. At Co2þ/Ni2þ 1:3 and 1:2, besides the diffraction peak of (200), the peaks of hcp Co ((100) and (101)) appear revealing the formation of different phases of Co in the coatings. Moreover, the diffraction peak of NieCo located at 51.8 splits into two peaks corresponding to Ni (200) and Co (200). On the basis of above discussion, the structure of the deposited NieCo coatings strongly depends on the Co content in the deposited coatings [24]. 3.2.3. ICP analysis Fig. 6 reveals the relationship between Co2þ concentration in the electrolyte and the composition of deposited coatings. It is
3.2.2. XRD analysis Fig. 5 demonstrates the XRD patterns of the NieCo coatings deposited from the electrolyte containing different concentration of Co2þ. From Fig. 5, no diffraction peaks of Cu are found in all the patterns indicating that the Cu plates are completely covered by the coatings. The diffraction peaks corresponding to NieCo alloys are found on all the curves, indicating the microcrystalline structure of the coatings [19]. As noted in Fig. 5, when the Co2þ/Ni2þ are 1:5 and
Table 2 Current efficiency at different pH values. pH value
Current efficiency
2.0 3.1 4.3 5.4
52.1% 62.3% 74.7% 81.2%
Fig. 5. XRD patterns of NieCo binary coatings deposited from the electrolyte with different mole ratio of Co2þ/Ni2þ at room temperature, t ¼ 20min, pH ¼ 3.1. (the concentration of Co2þ was kept at 0.125 M).
L. Tian et al. / Vacuum 86 (2011) 27e33
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Fig. 6. Effect of molar ratio of CoSO4/NiSO4 on the coating composition. (the concentration of Co2þ was kept at 0.125 M, pH ¼ 3.1).
clearly revealed that the content of Co increases from 16.5% to 72.7% as the molar ratio of CoSO4/NiSO4 varying from 1:5 to 1:2. The results are consistent with the research of electroless deposition of NieCoeB [18]. Anomalous codeposition was investigated during the electrodeposition process. This phenomenon has been reported mainly in codeposition of iron-group metals (iron, cobalt and nickel) and zinc with iron-group metals. The hydroxide suppression mechanism was introduced as an explanation [25,26]. On the basis of above discussion, the anomalous codeposition can also be attributed to the three factors mentioned above. The effect of the second factor is more important because the normal codeposition always exists and the dissolution of fleshly deposited metal atoms is less apparent at pH 3.1. In the system of NieCo, when pH value exceeds the critical value for forming metal hydroxide, both nickel and cobalt hydroxides may form. The anomalous codeposition occurs because of the preferential absorption of cobalt hydroxides. When pH value remains below the critical value, Ni2þ and Co2þ exist as the form of NiOHþ and CoOHþ [21]. It is believed that both the monohydroxides and hydroxides of Co are favored in aqueous solution [19]. Generally speaking, it is the competition absorption of hydroxide or monohydroxide results in the anomalous codeposition.
Fig. 7. The micrographs of the deposited coating at pH 5.4 from the electrolyte containing 0.625 M NiSO4 þ 0.125 M CoSO4 þ 0.4 M H3BO3 at room temperature.
formation of two-dimensional crystal nuclei is driven by the overpotential in the solution. The nucleation rate J can be expressed as follows:
lgJ ¼ A
B jD4j
where A and B are constant, jD4j is modulus of the overpotential. With the increase of overpotential, the size of the crystal nuclei
3.3. The crystallization behavior of the coatings The surface SEM micrographs for the deposited coating are shown in Fig. 7. From Fig. 7, we can see that the deposited coating is composed of equiaxed granules of about 15e20 nm in size (Fig. 7a). The surface morphology of the coating shows coexistence of two features: dendritic crystals (Fig. 7b) embedded in a fine-grained matrix (Fig. 7a). The two kinds of morphologies are corresponding to different crystallization behaviors. The SEM image of Fig. 7a shows granular morphology related to nucleation and growth mechanism (Fig. 8). Large quantities of tiny steps exist on the surface of Cu substrates because of the polish before electrodeposition. Ni and Co atoms are adsorbed onto the steps after being reduced from the electrolyte due to the lower nucleation energy on the surface of steps. The tiny steps disappear after being filled up with the adsorbed atoms. Then the twodimensional crystal nuclei form by the aggregation of adsorbed atoms to provide fresh steps for the electrocrystallization. The
(4)
Fig. 8. The schematic illustration of nucleation and growth mechanism.
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L. Tian et al. / Vacuum 86 (2011) 27e33
Fig. 9. The schematic illustration of growth of dendritic crystals.
which can steadily exist decreases and the nucleation rate increases. Eventually, the morphology of Fig. 7a forms by the layer and layer growth pattern. Fig. 7b represents the morphology of dendritic crystals related to the resistance of liquid mass transfer (Fig. 9). If the atoms crystallize on the tiny projection of the Cu substrates, the growth speed of the crystals is much faster than the other parts because of the much larger diffusion flux. The diffusion flux Jd can be expressed as follows:
Jd ¼
zFDi c0i r0
(5)
where z is the number of electrons involved into the reaction, F is the Faraday constant, Di is the diffusion coefficient, c0i is the solution concentration, r0 is the radius of curvature of the tip of the dendritic crystals. From the SEM image (Fig. 7b) the r0 of the dendritic crystals is about 10 nm and that value of the other crystals is about 10 mm corresponding to the thickness of the diffusion layer. From Eq. (5), we can know that Jd of the dendritic crystals is much larger than the other crystals and the larger Jd favors the formation of dendritic crystals.
Fig. 11. The dependence of saturation magnetization (Ms) and coercivity (Hc) on the mole ratio of Co2þ and. Ni2þ in the electrolyte.
3.4. Magnetic property analysis The magnetic properties of the NieCo coatings electrodeposited at different pH values were measured at room temperature and shown in Fig. 10. The saturation magnetization (Ms) moves up from 96.36 kAm1 to 136.08 kAm1 with the pH value increasing from 2.0 to 5.4. The coercivity (Hc) of the coatings slightly reduces from 20.26 kAm1 of pH 2.0 to 14.21 kAm1 of pH 5.4. The increase of saturation magnetization (Ms) is caused by the increase of Co which has higher saturation magnetization (Ms). The coercivity (Hc) is sensitive to the phase, defect, stress, thickness of the coatings [18]. At lower pH value (especially pH 2.0), the deposited coatings contain more defect of dark circle holes (Fig. 1a) than that of the higher pH value due to fierce reduction of Hþ. The higher defect concentration results in a higher coercivity (Hc) even though the coating contains less Co. Fig. 11 displays the dependence of saturation magnetization (Ms) and coercivity (Hc) on the mole ratio of Co2þ and Ni2þ. As can be seen from Fig. 11, the saturation magnetization (Ms) and coercivity (Hc) move up from 144.84 kAm1 and 15.27 kAm1 to 175.13 kAm1 and 125.20 kAm1 with the increase of Co in the deposited coatings. The increase of the saturation magnetization (Ms) can be attributed to the increase of Co content in the deposited coatings. The coercivity (Hc) may be effected by two factors: (1) Defects in the coatings influence the coercivity. (2) Co content in the coatings influences the coercivity. In this group of experiment, the effect of defects can be neglected compared with the sharp increase of Co in the coatings. Therefore, the coercivity increases with the increase of Co content in the coatings. 4. Conclusions
Fig. 10. The dependence of saturation magnetization (Ms) and coercivity (Hc) of the NieCo coatings on the pH value.
NieCo coatings were successfully electrodeposited on Cu substrates from the electrolyte with different Co2þ concentration and pH values. The content of cobalt gradually increases from 9.4% to 19.6% as the pH value varying from 2.0 to 5.4. The current efficiency increases from 52.1% to 81.2% with the pH increasing from 2.0 to 5.4. The content of Co increases from 16.5% to 72.7% as the molar ratio of Co2þ and Ni2þ varying from 1:5 to 1:2 in the electrolyte. The coatings with lower Co content are fcc structure. However, the structure of hcp Co appears at higher Co content in the coatings. The crystallization behaviors of the different features are explained by
L. Tian et al. / Vacuum 86 (2011) 27e33
nucleation and growth mechanism and the growth of dendritic crystals. The magnetic performance of the coatings shows that the saturation magnetization (Ms) moves up from 96.36 kAm1 to 136.08 kAm1 with the pH value increasing from 2.0 to 5.4. The saturation magnetization (Ms) and coercivity (Hc) move up from 144.84 kAm1 and 15.27 kAm1 to 175.13 kAm1 and 125.20 kAm1 with the increase of Co in the coatings, respectively. Acknowledgments This work was supported by the National Talent training found of Lanzhou University in basic physics (513-041102). References [1] [2] [3] [4] [5]
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