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Multifunctional Co–Mo films fabricated by electrochemical deposition
Electrochimica Acta 106 (2013) 258–263
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Multifunctional Co–Mo films fabricated by electrochemical deposition Q.F. Zhou a , L.Y. Lu b , L.N. Yu a , X.G. Xu a,∗ , Y. Jiang a,∗ a State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China b School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 17 January 2013 Received in revised form 22 May 2013 Accepted 23 May 2013 Available online 31 May 2013 Keywords: Co–Mo film Electrochemical deposition Magnetic properties Hydrogen evolution
a b s t r a c t Homogenous and crack-free Co–Mo films have been prepared by electrochemical deposition. The composition of the films was adjusted by varying MoO4 2− concentration, pH values and applied potentials. The magnetic properties of the Co–Mo films have been studied. The Co–Mo film with 18 at.% Mo has the highest saturation magnetization of 205 emu/g. The Co3 Mo film with a hexagonal close-packed structure shows the largest coercivity. The electro-catalytic activity of the Co–Mo films has also been studied via hydrogen evolution reaction. Electrochemical tests in 1 mol L−1 NaOH solution demonstrate that the Co–Mo films have both good electro-catalytic properties and corrosion-resistance. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Iron group metals have been widely used as magnetic materials in magnetic storage devices and micro-electromechanical systems. The addition of molybdenum in iron group metals can significantly modify their magnetic properties and result in versatile properties for their synergistic effects. Cobalt–molybdenum alloys are characterized by high hardness, high corrosion and thermal resistance, and good catalytic electrodes for hydrogen evolution reaction (HER). [1,2] Co–Mo films can be obtained by both magnetic sputtering [3] and electrochemical deposition in solutions with additives such as ammonium citrate. [4–6] Electrochemical deposition of Co–Mo films is an induced co-deposition since molybdenum cannot be obtained in aqueous solutions by itself. The stoichiometry of electrochemically deposited homogenous Co–Mo films is also sensitive to the addition of organic compound such as sodium citrate in the electrolyte solution. [7,8] The mechanism of the electrochemical deposition process still remains unclear even though several hypotheses have been discussed. [9–12] Co–Mo thin films have been predicted to be one of the promising candidates for highdensity magnetic recording media because of their high magnetic anisotropy energy and tunable magnetic isolation. [13] Meanwhile, Co–Mo films with low Mo contents have potential applications in magnetic sensors due to their high saturation magnetization
∗ Corresponding authors. Tel.: +86 10 62333209; fax: +86 10 62333209. E-mail addresses: [email protected] (X.G. Xu), [email protected] (Y. Jiang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.094
(Ms ) and low coercivity (Hc ). Co–Mo alloy is also an important electro-catalyst for HER because of its significant synergistic effect achieved by the combination of hyper d-electronic phase Co and hypo d-electronic phase Mo. [14] Co is the main source of catalytic activity, while Mo enhances mechanical strength, corrosion resistance and catalytic activity, originating from the high free energy on the metastable surface. [15] The overall electro-catalytic activity depends on the activation energy, hydrogen coverage on the surface and metal-hydrogen bond strength. [16] Therefore, it is important to study the microstructure, magnetic behavior and HER activity of electrochemically deposited Co–Mo films, especially for the specific hexagonal close-packed (hcp) Co3 Mo phase. In this paper, we report our study on the homogenous Co–Mo films with wide range of Mo contents. The properties involving both magnetic behavior and HER activity are studied. The mechanism of synergistic effects is further explained.
2. Experimental A series of Co–Mo alloy films were prepared in a conventional three-electrode cell by electrochemical deposition at room temperature. All reagents are of analytical grade. Electro-deposition was carried out on copper substrates serving as cathodes. Platinum foil and Ag/AgCl electrode were used as counter and reference electrodes, respectively. Two different electrolyte solutions with 0.005 mol L−1 and 0.6 mol L−1 Na2 MoO4 , respectively, were used. Both of electrolyte solutions also contain 0.1 mol L−1 CoSO4 and 0.2 mol L−1 sodium citrate. The pH value was adjusted to near neutral by adding citric acid. The substrates were degreased in acetone
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and Mo promoted by both the complexation of MoO4 2− and CoCit− with H+ and hydrogen evolution. However, at a higher pH value of about 8.0, two reduction peaks, peak 1 and peak 2, emerge in the cyclic voltammogram in Fig. 1. Peak 1, the one at more negative potential, represents the formation of MoO2 and Mo induced by small quantity of CoCit− . Peak 2 corresponds to the reduction of Co. It is reinforced by the increment of Co content in the Co–Mo films deposited at pH 8.0. Therefore, the pH value of 7.0 is ideal for the preparation of homogenous Co-Mo films in electrolyte solutions with 0.6 mol L−1 of Na2 MoO4 . More negative potentials of −1.75 to −2.25 V were applied to minimize the formation of molybdenum oxides and reduce hydrogen evolution. The corresponding atomic ratio of Mo in the Co–Mo films ranges from 37% to 45%. 3.2. Morphological and structural analysis of Co–Mo films
Fig. 1. Cyclic voltammograms for the solutions with different pH values: pH = 6.5, pH = 7.0, and pH = 8.0.
and polished in dilute sulfuric acid. The electrolyte solution was stirred to maintain homogeneous composition near the electrodes during deposition. Surface morphology and elemental composition of the films were examined by using a LEO-1450 scanning electron microscope with an energy-dispersive X-ray spectroscopy (SEM-EDX). The structure was analyzed by X-ray diffraction (XRD) on a Rigaku ˚ D/MAX-RB diffractometer using Cu K␣ radiation ( = 1.54056 A). The electro-catalytic tests of the Co-Mo films as electrodes for HER were performed at 25 ◦ C in 1 mol L−1 NaOH aqueous solution at a scan rate of 1 mV/s on an LK98BII microcomputer-based electrochemical analyzer. Magnetic properties of the films were studied by a physical property measurement system (PPMS) at room temperature. 3. Results and discussion 3.1. Electrochemical deposition and compositional analysis The Co–Mo films were deposited by using DC potentiostatic method. The solution containing 0.005 mol L−1 Na2 MoO4 , 1 mol L−1 CoSO4 and 0.2 mol L−1 sodium citrate was used to study the influence of the applied potential on the composition of the Co–Mo films. The pH value of the solution was adjusted to 6.5 (near neutral) to avoid the simultaneous evolution of hydrogen. The EDX results reveal that the atomic ratio of Mo in the deposits increases from 18% to 30% when the applied potential becomes more negative, from −0.8 V to −1.8 V. However, for the solutions with high MoO4 2− concentrations, the deposition of Co–Mo films is hindered by the formation of molybdenum oxides. The influence of pH value was studied to minimize the formation of molybdenum oxides by using electrolyte solutions containing 0.6 mol L−1 Na2 MoO4 , 0.1 mol L−1 CoSO4 and 0.2 mol L−1 sodium citrate. It is found that the percentage content of molybdenum in the Co–Mo films decreases slightly with the increase of pH values. The reduction process of Co and Mo in electrolyte solutions differs at various pH values. As shown in Fig. 1, the reduction peak in the cyclic voltammogram shifts to more negative potential with pH increasing from 6.5 to 8.0, indicating that more negative potential are needed to deposit Co–Mo films using electrolyte with a higher pH value. Meanwhile, a distinct reduction peak with a greater current density (j) can be observed at pH 6.5, which is resulted from the simultaneous deposition of Co
The morphology of the Co-Mo films were studied by SEM and the images are presented in Fig. 2(a)–(c). It can be seen that the Co–Mo films obtained from solutions with 0.005 mol L−1 MoO4 2− are all homogenous and crack-free. When the applied potential is −1.0 V, the atomic ratio of Co and Mo is 3:1. The morphology of the Co3 Mo film is fine-grained as shown in Fig. 2(a). The corresponding EDX pattern of the film is shown in Fig. 2(d). Comparing Fig. 2(a) with (b), the grain size increases gradually with a more negative potential. Fig. 2(c) shows the morphology of the Co3 Mo film with a larger thickness of about 6 m. The morphology transforms from orbicular to polyhedral with the increase of the thickness. Even thicker films of about 32 m have been reported to be acicular. [17] According to Fig. 2(c), the acicular morphology can also be observed. For the solutions with high MoO4 2− concentration (e.g. 0.6 mol L−1 ), pH values significantly affect the uniformity of the Co–Mo films. At pH 7.0, the films are smooth, bright and crack-free at an applied potential of −1.75 V. While there are a lot of bubbles on the surface of the films due to hydrogen evolution at pH 6.5. Fig. 3 shows XRD patterns of the Co–Mo films with the Mo atomic ratios ranging from 18% to 45%. The strong diffraction peaks at 2 = 43.22◦ , 50.47◦ and 73.99◦ belong to copper substrates. The diffraction peaks at 2 = 40.54◦ , 46.30◦ and 50.24◦ correspond to the (2 0 0), (0 0 2) and (2 0 1) planes of the hexagonal Co3 Mo, respectively. The small full width at half maximum (FWHM) values of the diffraction peaks reveal that all the films have a fine crystalline structure. A pure hcp structure is obtained in the Co–Mo films with low Mo atomic ratios from 18% to 25%. However, MoO2 peaks at 2 = 25.95◦ , 36.99◦ , 53.55◦ can be observed in the films with Mo atomic ratios more than 28%. According to the equilibrium-phase diagram of binary Co–Mo alloy, hcp -Co3 Mo phase and ε-Co7 Mo6 phase are stable at low temperature (<1000 ◦ C). However, the XRD patterns shown in Fig. 3 demonstrate that only -Co3 Mo phase was obtained, which indicates that Mo is hard to be obtained by electrochemical deposition in electrolyte solutions with relatively high concentration of MoO4 2− . Meanwhile, the positions of all the diffraction peaks gradually shift toward lower diffraction angles with the increase of Mo concentration. It is due to the lattice expansion induced by Mo atoms, since the atomic radius of Mo is larger than that of Co. 3.3. Magnetic properties of Co–Mo films Magnetic properties of Co–Mo films were studied by measuring the in-plane and out-of-plane magnetic hysteresis (M-H) loops. The films were annealed to refine the crystalline structure before measurement. According to the M-H loops, the Mo content affects the magnetic properties significantly. The pure Co film shows clearly in-plane magnetic anisotropy, having a good square shape as shown in Fig. 4(a). For the Co–Mo films with low concentrations of Mo
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Fig. 2. SEM images of the Co-Mo films deposited in different potentials and depositing time duration: (a) −1.0 V, t = 15 min; (b) −1.5 V, t = 15 min; (c) −1.0 V, t = 90 min; (d) EDX result of Co3 Mo film.
from 18 to 25 at.%, the M-H loops show that the in-plane saturation field (Hs// ) is less than 10 kOe, while the out-of-plane saturation field (Hs⊥ ) is much larger, as shown in Fig. 4(b) and (c). It indicates strong in-plane anisotropy which can be attributed to the pure hcp structure and good texture. However, the magnetic anisotropy gradually transforms to perpendicular anisotropy when the Mo concentration increases higher than 37%, because the incorporation of cubic molybdenum oxides weakens the parallel anisotropy of hcp structure. After annealing at 650 ◦ C, a better crystallization of cubic molybdenum oxides helps the Co-Mo film shift its easy axis to perpendicular orientation, as shown in Fig. 4 (e).
Fig. 4(f) shows the dependence of Hc and Ms on the Mo atomic ratio. Hc and Ms of the Co–Mo films are greatly improved compared to the electrodeposited pure cobalt film, which shows Hc of 110 Oe and Ms of 140 emu/g after annealing at 550 ◦ C in vacuum. When the Co–Mo films have relatively low Mo contents (≤20%), they have very low Hc but high Ms . It implies that low Mo concentration films favor soft magnetic properties. The Co–Mo film with 18 at.% Mo shows the highest Ms (205 emu/g) and the lowest Hc (140 Oe). Further incorporation of Mo in the films results in a clear increase of Hc and decrease of Ms . A maximum Hc value of about 400 Oe is obtained from the Co3 Mo film after annealing at 550 ◦ C for 1 h. Heat treatment plays a significant role in the structure and micromorphology, which greatly affect the magnetic properties of the films. Fig. 5(a) shows the dependence of Hc on annealing temperature for the Co–Mo films with the Mo atomic ratios of 22%, 25% and 28%, respectively. Hc of the as-deposited Co3 Mo film is only 21 Oe. However, it increases from 212 Oe to 650 Oe with the annealing temperature increasing from 500 ◦ C to 650 ◦ C. The enhancement of Hc is due to better crystallization after annealing at higher temperatures. As shown in Fig. 5(b), small crystalline grains emerge when the Co3 Mo film was annealed at 550 ◦ C. With the annealing temperature increasing, more and more crystalline grains can be observed in Fig. 5(c). After annealing at 650 ◦ C, the crystalline grain size becomes uniform. Therefore, Hc of the annealed films are higher than that of the as-deposited film.
3.4. Electrocatalytic properties of Co–Mo films for hydrogen evolution
Fig. 3. XRD patterns of the Co–Mo films with various Mo atomic ratios. All the films have been annealed at 550 ◦ C for 1 h in vacuum.
According to the discussion above, the Co–Mo films with low Mo atomic ratios of around 25% show good magnetic properties due to their better morphology and hcp structure. Therefore, the Co–Mo
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Fig. 4. (a)–(e) M-H loops of Co and the Co–Mo films with 18–37 at.% Mo. (f) Dependence of in-plane Hc and Ms on the Mo concentration in the Co–Mo films. All the films have been annealed in vacuum for 1 h.
films with Mo contents from 18 to 28 at.% have been employed to study the electrocatalytic properties for HER. Fig. 6 shows the polarization curves in 1 mol L−1 NaOH solutions. Corresponding electrochemical parameters are listed in Table 1. According to the experimental results, the Co3 Mo film has the smallest Tafel slope and the largest exchange current density, which indicate that the synergetic effect in Co3 Mo electrode exceeds other Co–Mo films. According to Brewer-Engel valence-bond theory [1], the electrocatalytic activity can be improved by the pure hcp structure of Co3 Mo with high symmetry and overlap of the electron orbitals. This also results in a low overpotential of the Co3 Mo film even at relatively high current density areas (j > 500 mA/cm−2 ), consisting with Fig. 6.
Table 1 Tafel slopes and exchange current densities of the Co–Mo films as electrodes for HER. % Mo
18%
22%
25%
28%
Tafel slopes (mV) j0,H (mA/cm−2 )
150 0.07
152 0.12
127 0.13
142 0.11
As shown in Fig. 6(a), the overpotentials of the Co–Mo films are all lower than 200 mV when the current density j = 100 mA/cm−2 . The overpotential increases slightly with the increased Mo atomic ratio from 18 to 28 at.%, except for 22 at.%, the overpotential of which is larger than that of the Co3 Mo film. It is due to the pure hcp structure of Co3 Mo films. The HER process involves adsorption and desorption of hydrogen. At low current densities, the reaction mechanism is a combination of the three steps involving Volmer, Heyrouvsky and Tafel reactions, [18] and the reaction rate is mainly controlled by the adsorption of hydrogen. In this case, the bonding strength and the amount of absorbed hydrogen play a key role in the HER. The relatively low overpotential of Co3 Mo electrodes compared with that of the Co–Mo film with 22 at.% Mo is attributed to the strong bonding between hydrogen atoms and Co3 Mo surface which favors the chemical adsorption process. The density of electrons close to the energy level of metal surface (density of states at the Fermi level) is also an important parameter. [19] There is a stronger intermetallic bonding between Co and Mo in the Co3 Mo films. In this case, the smaller lattice parameter increases the density of states at the Fermi energy and electrochemical reaction rates.
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Fig. 5. (a) Dependence of Hc on the annealing temperature for the Co–Mo films with the Mo atomic ratios of 22%, 25%, 28%. (b)–(d) SEM images for the Co3 Mo films annealed at (b) 550 ◦ C, (c) 600 ◦ C and (d) 650 ◦ C, respectively.
Fig. 6. Polarization curves of the Co–Mo electro-catalysts: (a) - j curves, (b) -lg j curves.
4. Conclusion The homogenous and crack-free Co–Mo films with Mo atomic ratios ranging from 18 to 37% have been prepared by electrochemical deposition. The magnetic properties of the Co–Mo films are unique and tunable by adjusting the Mo concentration and annealing temperature. When the Mo concentration is less than 20 at.%, the samples show typical soft magnetic properties. The highest Ms is 205 emu/g which is 1.4 times as the previously reported value of 145 emu/g, [7] while Hc is 140 Oe. When the Mo concentration is increased to 25 at.%, Ms decreases but Hc is greatly enhanced to 642 Oe after annealing at 650 ◦ C. The magnetic anisotropy turns from parallel to perpendicular to the film plane when Mo concentration increases from 25 to 37 at.%. Due to the pure hcp structure with high symmetry and strong intermetallic bonding that result in improved overlap of the electron orbitals, the Co3 Mo films have both the highest Hc and the better electrocatalytic property among all the Co–Mo films. Therefore, the Co–Mo films have potential
application in magnetic sensors, magnetic recording and electrocatalysis. Acknowledgement This work was partially supported by the National Basic Research Program of China (Grant no. 2012CB932702), National Science Foundation of China (Grant nos. 51071022, 51271020, 11174031), PCSIRT, Beijing Nova program (Grant no. 2011031), the Fundamental Research Funds for the Central Universities, Engineering Research Institute Foundation of USTB (YJ 2011-022), and State Key Lab of Advanced Metals and Materials (2011-Z03). References ´ Advances in electrocatalysis for hydrogen evolution in the light [1] M.M. Jakˇsic, of the Brewer-Engel valence-bond theory, International Journal of Hydrogen Energy 12 (1987) 727–752.
Q.F. Zhou et al. / Electrochimica Acta 106 (2013) 258–263 [2] E. Beltowska-Lehman, P. Indyka, Kinetics of Ni–Mo electrodeposition from Nirich citrate baths, Thin solid films 520 (2012) 2046–2051. [3] K. Oikawa, G.W. Qin, M. Sato, O. Kitakami, Y. Shimada, J. Sato, K. Fukamichi, K. Ishida, Magnetically induced phase separation and magnetic properties of Co–Mo hexagonal-close-packed structure thin films, Applied Physics Letters 83 (2003) 966–968. [4] E. Gómez, E. Pellicer, E. Vallés, Influence of the bath composition and the pH on the induced cobalt–molybdenum electrodeposition, Journal of Electroanalytical Chemistry 556 (2003) 137–145. [5] E. Gómez, E. Pellicer, E. Vallés, Electrodeposition of soft-magnetic cobalt–molybdenum coatings containing low molybdenum percentages, Journal of Electroanalytical Chemistry 568 (2004) 29–36. [6] E. Gómez, E. Pellicer, E. Vallés, Developing plating baths for the production of cobalt–molybdenum films, Surface Coatings Technology 197 (2005) 238–246. [7] E. Gómez, E. Pellicer, E. Vallés, Electrodeposited cobalt–molybdenum magnetic materials, Journal of Electroanalytical Chemistry 517 (2001) 109–116. [8] G. Wei, H. Ge, X. Zhu, Q. Wua, J. Yua, B. Wang, Effect of organic additives on characterization of electrodeposited Co–W thin films, Applied Surface Science 253 (2007) 7461–7466. [9] E. Gómez, E. Pellicer, E. Vallés, Intermediate molybdenum oxides involved in binary and ternary induced electrodeposition, Journal of Electroanalytical Chemistry 580 (2005) 238–244. [10] E.J. Podlaha, D. Landolt, Induced codeposition. I. An experimental investigation of Ni–Mo alloys, Journal of the Electrochemical Society 143 (1996) 885–892.
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[11] E.J. Podlaha, D. Landolt, Induced codeposition. II. Mathematical model describing the electrodeposition of Ni–Mo alloys, Journal of the Electrochemical Society 143 (1996) 893–899. [12] A. Brenner, Electrodeposition of Alloys, Principles and Practice, Academic Press, New York, London, 1963. [13] B. Yang, G. Qin, W. Pei, Y. Ren, N. Xiao, X. Zhao, Abnormal saturation magnetization dependency on W content for Co–W thin films, Acta Metallurgica Sinica (English Letters) 23 (2010) 8–12. ´ O. Popovski, A. Dimitrov, D. Slavkov, E. Lefterova, S. Hadˇzi Jor[14] P. Paunovic, danov, Study of structural and electrochemical characteristics of Co-based hypo-hyper d-electrocatalysts for hydrogen evolution, Electrochimica Acta 52 (2007) 4640–4648. [15] Q. Han, S. Cui, N. Pu, J. Chen, K. Liu, X. Wei, A study on pulse plating amorphous NiMo alloy coating used as HER cathode in alkaline medium, International Journal of Hydrogen Energy 35 (2010) 5194–5201. [16] D. Lupu, P. Mˇarginean, A.R. Biris¸, Hydrogen in some synergetic electrocatalysts, Journal of Alloys and Compounds 245 (1996) 146–152. [17] E. Gómez, E. Pellicer, X. Alcobé, E. Vallés, Properties of Co–Mo coatings obtained by electrodeposition at pH 6.6, Journal of Solid State Electrochemistry 8 (2004) 497–504. ´ M.V. Vojnovic, N.V. Krstajic, Kinetic analysis of hydrogen evolution [18] J.M. Jakˇsic, at Ni–Mo alloy electrodes, Electrochimica Acta 45 (2000) 4151–4158. [19] A. Subramaniaa, A.R. Sathiya Priyaa, V.S. Muralidharan, Electrocatalytic cobalt–molybdenum alloy deposits, International Journal of Hydrogen Energy 32 (2007) 2843–2847.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Multifunctional Co–Mo films fabricated by electrochemical deposition Q.F. Zhou a , L.Y. Lu b , L.N. Yu a , X.G. Xu a,∗ , Y. Jiang a,∗ a State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China b School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 17 January 2013 Received in revised form 22 May 2013 Accepted 23 May 2013 Available online 31 May 2013 Keywords: Co–Mo film Electrochemical deposition Magnetic properties Hydrogen evolution
a b s t r a c t Homogenous and crack-free Co–Mo films have been prepared by electrochemical deposition. The composition of the films was adjusted by varying MoO4 2− concentration, pH values and applied potentials. The magnetic properties of the Co–Mo films have been studied. The Co–Mo film with 18 at.% Mo has the highest saturation magnetization of 205 emu/g. The Co3 Mo film with a hexagonal close-packed structure shows the largest coercivity. The electro-catalytic activity of the Co–Mo films has also been studied via hydrogen evolution reaction. Electrochemical tests in 1 mol L−1 NaOH solution demonstrate that the Co–Mo films have both good electro-catalytic properties and corrosion-resistance. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Iron group metals have been widely used as magnetic materials in magnetic storage devices and micro-electromechanical systems. The addition of molybdenum in iron group metals can significantly modify their magnetic properties and result in versatile properties for their synergistic effects. Cobalt–molybdenum alloys are characterized by high hardness, high corrosion and thermal resistance, and good catalytic electrodes for hydrogen evolution reaction (HER). [1,2] Co–Mo films can be obtained by both magnetic sputtering [3] and electrochemical deposition in solutions with additives such as ammonium citrate. [4–6] Electrochemical deposition of Co–Mo films is an induced co-deposition since molybdenum cannot be obtained in aqueous solutions by itself. The stoichiometry of electrochemically deposited homogenous Co–Mo films is also sensitive to the addition of organic compound such as sodium citrate in the electrolyte solution. [7,8] The mechanism of the electrochemical deposition process still remains unclear even though several hypotheses have been discussed. [9–12] Co–Mo thin films have been predicted to be one of the promising candidates for highdensity magnetic recording media because of their high magnetic anisotropy energy and tunable magnetic isolation. [13] Meanwhile, Co–Mo films with low Mo contents have potential applications in magnetic sensors due to their high saturation magnetization
∗ Corresponding authors. Tel.: +86 10 62333209; fax: +86 10 62333209. E-mail addresses: [email protected] (X.G. Xu), [email protected] (Y. Jiang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.094
(Ms ) and low coercivity (Hc ). Co–Mo alloy is also an important electro-catalyst for HER because of its significant synergistic effect achieved by the combination of hyper d-electronic phase Co and hypo d-electronic phase Mo. [14] Co is the main source of catalytic activity, while Mo enhances mechanical strength, corrosion resistance and catalytic activity, originating from the high free energy on the metastable surface. [15] The overall electro-catalytic activity depends on the activation energy, hydrogen coverage on the surface and metal-hydrogen bond strength. [16] Therefore, it is important to study the microstructure, magnetic behavior and HER activity of electrochemically deposited Co–Mo films, especially for the specific hexagonal close-packed (hcp) Co3 Mo phase. In this paper, we report our study on the homogenous Co–Mo films with wide range of Mo contents. The properties involving both magnetic behavior and HER activity are studied. The mechanism of synergistic effects is further explained.
2. Experimental A series of Co–Mo alloy films were prepared in a conventional three-electrode cell by electrochemical deposition at room temperature. All reagents are of analytical grade. Electro-deposition was carried out on copper substrates serving as cathodes. Platinum foil and Ag/AgCl electrode were used as counter and reference electrodes, respectively. Two different electrolyte solutions with 0.005 mol L−1 and 0.6 mol L−1 Na2 MoO4 , respectively, were used. Both of electrolyte solutions also contain 0.1 mol L−1 CoSO4 and 0.2 mol L−1 sodium citrate. The pH value was adjusted to near neutral by adding citric acid. The substrates were degreased in acetone
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and Mo promoted by both the complexation of MoO4 2− and CoCit− with H+ and hydrogen evolution. However, at a higher pH value of about 8.0, two reduction peaks, peak 1 and peak 2, emerge in the cyclic voltammogram in Fig. 1. Peak 1, the one at more negative potential, represents the formation of MoO2 and Mo induced by small quantity of CoCit− . Peak 2 corresponds to the reduction of Co. It is reinforced by the increment of Co content in the Co–Mo films deposited at pH 8.0. Therefore, the pH value of 7.0 is ideal for the preparation of homogenous Co-Mo films in electrolyte solutions with 0.6 mol L−1 of Na2 MoO4 . More negative potentials of −1.75 to −2.25 V were applied to minimize the formation of molybdenum oxides and reduce hydrogen evolution. The corresponding atomic ratio of Mo in the Co–Mo films ranges from 37% to 45%. 3.2. Morphological and structural analysis of Co–Mo films
Fig. 1. Cyclic voltammograms for the solutions with different pH values: pH = 6.5, pH = 7.0, and pH = 8.0.
and polished in dilute sulfuric acid. The electrolyte solution was stirred to maintain homogeneous composition near the electrodes during deposition. Surface morphology and elemental composition of the films were examined by using a LEO-1450 scanning electron microscope with an energy-dispersive X-ray spectroscopy (SEM-EDX). The structure was analyzed by X-ray diffraction (XRD) on a Rigaku ˚ D/MAX-RB diffractometer using Cu K␣ radiation ( = 1.54056 A). The electro-catalytic tests of the Co-Mo films as electrodes for HER were performed at 25 ◦ C in 1 mol L−1 NaOH aqueous solution at a scan rate of 1 mV/s on an LK98BII microcomputer-based electrochemical analyzer. Magnetic properties of the films were studied by a physical property measurement system (PPMS) at room temperature. 3. Results and discussion 3.1. Electrochemical deposition and compositional analysis The Co–Mo films were deposited by using DC potentiostatic method. The solution containing 0.005 mol L−1 Na2 MoO4 , 1 mol L−1 CoSO4 and 0.2 mol L−1 sodium citrate was used to study the influence of the applied potential on the composition of the Co–Mo films. The pH value of the solution was adjusted to 6.5 (near neutral) to avoid the simultaneous evolution of hydrogen. The EDX results reveal that the atomic ratio of Mo in the deposits increases from 18% to 30% when the applied potential becomes more negative, from −0.8 V to −1.8 V. However, for the solutions with high MoO4 2− concentrations, the deposition of Co–Mo films is hindered by the formation of molybdenum oxides. The influence of pH value was studied to minimize the formation of molybdenum oxides by using electrolyte solutions containing 0.6 mol L−1 Na2 MoO4 , 0.1 mol L−1 CoSO4 and 0.2 mol L−1 sodium citrate. It is found that the percentage content of molybdenum in the Co–Mo films decreases slightly with the increase of pH values. The reduction process of Co and Mo in electrolyte solutions differs at various pH values. As shown in Fig. 1, the reduction peak in the cyclic voltammogram shifts to more negative potential with pH increasing from 6.5 to 8.0, indicating that more negative potential are needed to deposit Co–Mo films using electrolyte with a higher pH value. Meanwhile, a distinct reduction peak with a greater current density (j) can be observed at pH 6.5, which is resulted from the simultaneous deposition of Co
The morphology of the Co-Mo films were studied by SEM and the images are presented in Fig. 2(a)–(c). It can be seen that the Co–Mo films obtained from solutions with 0.005 mol L−1 MoO4 2− are all homogenous and crack-free. When the applied potential is −1.0 V, the atomic ratio of Co and Mo is 3:1. The morphology of the Co3 Mo film is fine-grained as shown in Fig. 2(a). The corresponding EDX pattern of the film is shown in Fig. 2(d). Comparing Fig. 2(a) with (b), the grain size increases gradually with a more negative potential. Fig. 2(c) shows the morphology of the Co3 Mo film with a larger thickness of about 6 m. The morphology transforms from orbicular to polyhedral with the increase of the thickness. Even thicker films of about 32 m have been reported to be acicular. [17] According to Fig. 2(c), the acicular morphology can also be observed. For the solutions with high MoO4 2− concentration (e.g. 0.6 mol L−1 ), pH values significantly affect the uniformity of the Co–Mo films. At pH 7.0, the films are smooth, bright and crack-free at an applied potential of −1.75 V. While there are a lot of bubbles on the surface of the films due to hydrogen evolution at pH 6.5. Fig. 3 shows XRD patterns of the Co–Mo films with the Mo atomic ratios ranging from 18% to 45%. The strong diffraction peaks at 2 = 43.22◦ , 50.47◦ and 73.99◦ belong to copper substrates. The diffraction peaks at 2 = 40.54◦ , 46.30◦ and 50.24◦ correspond to the (2 0 0), (0 0 2) and (2 0 1) planes of the hexagonal Co3 Mo, respectively. The small full width at half maximum (FWHM) values of the diffraction peaks reveal that all the films have a fine crystalline structure. A pure hcp structure is obtained in the Co–Mo films with low Mo atomic ratios from 18% to 25%. However, MoO2 peaks at 2 = 25.95◦ , 36.99◦ , 53.55◦ can be observed in the films with Mo atomic ratios more than 28%. According to the equilibrium-phase diagram of binary Co–Mo alloy, hcp -Co3 Mo phase and ε-Co7 Mo6 phase are stable at low temperature (<1000 ◦ C). However, the XRD patterns shown in Fig. 3 demonstrate that only -Co3 Mo phase was obtained, which indicates that Mo is hard to be obtained by electrochemical deposition in electrolyte solutions with relatively high concentration of MoO4 2− . Meanwhile, the positions of all the diffraction peaks gradually shift toward lower diffraction angles with the increase of Mo concentration. It is due to the lattice expansion induced by Mo atoms, since the atomic radius of Mo is larger than that of Co. 3.3. Magnetic properties of Co–Mo films Magnetic properties of Co–Mo films were studied by measuring the in-plane and out-of-plane magnetic hysteresis (M-H) loops. The films were annealed to refine the crystalline structure before measurement. According to the M-H loops, the Mo content affects the magnetic properties significantly. The pure Co film shows clearly in-plane magnetic anisotropy, having a good square shape as shown in Fig. 4(a). For the Co–Mo films with low concentrations of Mo
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Fig. 2. SEM images of the Co-Mo films deposited in different potentials and depositing time duration: (a) −1.0 V, t = 15 min; (b) −1.5 V, t = 15 min; (c) −1.0 V, t = 90 min; (d) EDX result of Co3 Mo film.
from 18 to 25 at.%, the M-H loops show that the in-plane saturation field (Hs// ) is less than 10 kOe, while the out-of-plane saturation field (Hs⊥ ) is much larger, as shown in Fig. 4(b) and (c). It indicates strong in-plane anisotropy which can be attributed to the pure hcp structure and good texture. However, the magnetic anisotropy gradually transforms to perpendicular anisotropy when the Mo concentration increases higher than 37%, because the incorporation of cubic molybdenum oxides weakens the parallel anisotropy of hcp structure. After annealing at 650 ◦ C, a better crystallization of cubic molybdenum oxides helps the Co-Mo film shift its easy axis to perpendicular orientation, as shown in Fig. 4 (e).
Fig. 4(f) shows the dependence of Hc and Ms on the Mo atomic ratio. Hc and Ms of the Co–Mo films are greatly improved compared to the electrodeposited pure cobalt film, which shows Hc of 110 Oe and Ms of 140 emu/g after annealing at 550 ◦ C in vacuum. When the Co–Mo films have relatively low Mo contents (≤20%), they have very low Hc but high Ms . It implies that low Mo concentration films favor soft magnetic properties. The Co–Mo film with 18 at.% Mo shows the highest Ms (205 emu/g) and the lowest Hc (140 Oe). Further incorporation of Mo in the films results in a clear increase of Hc and decrease of Ms . A maximum Hc value of about 400 Oe is obtained from the Co3 Mo film after annealing at 550 ◦ C for 1 h. Heat treatment plays a significant role in the structure and micromorphology, which greatly affect the magnetic properties of the films. Fig. 5(a) shows the dependence of Hc on annealing temperature for the Co–Mo films with the Mo atomic ratios of 22%, 25% and 28%, respectively. Hc of the as-deposited Co3 Mo film is only 21 Oe. However, it increases from 212 Oe to 650 Oe with the annealing temperature increasing from 500 ◦ C to 650 ◦ C. The enhancement of Hc is due to better crystallization after annealing at higher temperatures. As shown in Fig. 5(b), small crystalline grains emerge when the Co3 Mo film was annealed at 550 ◦ C. With the annealing temperature increasing, more and more crystalline grains can be observed in Fig. 5(c). After annealing at 650 ◦ C, the crystalline grain size becomes uniform. Therefore, Hc of the annealed films are higher than that of the as-deposited film.
3.4. Electrocatalytic properties of Co–Mo films for hydrogen evolution
Fig. 3. XRD patterns of the Co–Mo films with various Mo atomic ratios. All the films have been annealed at 550 ◦ C for 1 h in vacuum.
According to the discussion above, the Co–Mo films with low Mo atomic ratios of around 25% show good magnetic properties due to their better morphology and hcp structure. Therefore, the Co–Mo
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Fig. 4. (a)–(e) M-H loops of Co and the Co–Mo films with 18–37 at.% Mo. (f) Dependence of in-plane Hc and Ms on the Mo concentration in the Co–Mo films. All the films have been annealed in vacuum for 1 h.
films with Mo contents from 18 to 28 at.% have been employed to study the electrocatalytic properties for HER. Fig. 6 shows the polarization curves in 1 mol L−1 NaOH solutions. Corresponding electrochemical parameters are listed in Table 1. According to the experimental results, the Co3 Mo film has the smallest Tafel slope and the largest exchange current density, which indicate that the synergetic effect in Co3 Mo electrode exceeds other Co–Mo films. According to Brewer-Engel valence-bond theory [1], the electrocatalytic activity can be improved by the pure hcp structure of Co3 Mo with high symmetry and overlap of the electron orbitals. This also results in a low overpotential of the Co3 Mo film even at relatively high current density areas (j > 500 mA/cm−2 ), consisting with Fig. 6.
Table 1 Tafel slopes and exchange current densities of the Co–Mo films as electrodes for HER. % Mo
18%
22%
25%
28%
Tafel slopes (mV) j0,H (mA/cm−2 )
150 0.07
152 0.12
127 0.13
142 0.11
As shown in Fig. 6(a), the overpotentials of the Co–Mo films are all lower than 200 mV when the current density j = 100 mA/cm−2 . The overpotential increases slightly with the increased Mo atomic ratio from 18 to 28 at.%, except for 22 at.%, the overpotential of which is larger than that of the Co3 Mo film. It is due to the pure hcp structure of Co3 Mo films. The HER process involves adsorption and desorption of hydrogen. At low current densities, the reaction mechanism is a combination of the three steps involving Volmer, Heyrouvsky and Tafel reactions, [18] and the reaction rate is mainly controlled by the adsorption of hydrogen. In this case, the bonding strength and the amount of absorbed hydrogen play a key role in the HER. The relatively low overpotential of Co3 Mo electrodes compared with that of the Co–Mo film with 22 at.% Mo is attributed to the strong bonding between hydrogen atoms and Co3 Mo surface which favors the chemical adsorption process. The density of electrons close to the energy level of metal surface (density of states at the Fermi level) is also an important parameter. [19] There is a stronger intermetallic bonding between Co and Mo in the Co3 Mo films. In this case, the smaller lattice parameter increases the density of states at the Fermi energy and electrochemical reaction rates.
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Fig. 5. (a) Dependence of Hc on the annealing temperature for the Co–Mo films with the Mo atomic ratios of 22%, 25%, 28%. (b)–(d) SEM images for the Co3 Mo films annealed at (b) 550 ◦ C, (c) 600 ◦ C and (d) 650 ◦ C, respectively.
Fig. 6. Polarization curves of the Co–Mo electro-catalysts: (a) - j curves, (b) -lg j curves.
4. Conclusion The homogenous and crack-free Co–Mo films with Mo atomic ratios ranging from 18 to 37% have been prepared by electrochemical deposition. The magnetic properties of the Co–Mo films are unique and tunable by adjusting the Mo concentration and annealing temperature. When the Mo concentration is less than 20 at.%, the samples show typical soft magnetic properties. The highest Ms is 205 emu/g which is 1.4 times as the previously reported value of 145 emu/g, [7] while Hc is 140 Oe. When the Mo concentration is increased to 25 at.%, Ms decreases but Hc is greatly enhanced to 642 Oe after annealing at 650 ◦ C. The magnetic anisotropy turns from parallel to perpendicular to the film plane when Mo concentration increases from 25 to 37 at.%. Due to the pure hcp structure with high symmetry and strong intermetallic bonding that result in improved overlap of the electron orbitals, the Co3 Mo films have both the highest Hc and the better electrocatalytic property among all the Co–Mo films. Therefore, the Co–Mo films have potential
application in magnetic sensors, magnetic recording and electrocatalysis. Acknowledgement This work was partially supported by the National Basic Research Program of China (Grant no. 2012CB932702), National Science Foundation of China (Grant nos. 51071022, 51271020, 11174031), PCSIRT, Beijing Nova program (Grant no. 2011031), the Fundamental Research Funds for the Central Universities, Engineering Research Institute Foundation of USTB (YJ 2011-022), and State Key Lab of Advanced Metals and Materials (2011-Z03). References ´ Advances in electrocatalysis for hydrogen evolution in the light [1] M.M. Jakˇsic, of the Brewer-Engel valence-bond theory, International Journal of Hydrogen Energy 12 (1987) 727–752.
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