- Email: [email protected]
Electrochemical study of electroless deposition of Fe–P alloys
Electrochimica Acta 51 (2006) 4471–4476
Electrochemical study of electroless deposition of Fe–P alloys Gui-Fang Huang ∗ , Wei-Qing Huang, Ling-Ling Wang, Yan Meng, Zhong Xie, B.S. Zou Department of Applied Physics, Hunan University, Changsha 410082, PR China Received 17 August 2005; received in revised form 30 November 2005; accepted 17 December 2005 Available online 19 January 2006
Abstract Fe–P deposits were prepared by electroless deposition and the inducing effect of coupled aluminum on Fe–P deposition was studied. The partial reactions of reducer oxidation and metal ions reduction in electroless deposition Fe–P bath were investigated by electrochemical investigation. In situ mixed potentials of the process for electrolytes with different pH were measured. The plating rate of iron alloys was measured gravimetrically and calculated by the deposition current density. The results show that coupled aluminum induces the Fe–P deposition by shifting the potential negatively and decreasing the polarization resistance of anodic and cathodic reaction. The test of the mixed potential theory was performed by comparison between direct experimental values of the mixed potential and plating rate with those derived theoretically from the current–potential curves for partial reactions. Due to the hydrogen evolution, the plating rate determined by electrochemical measurement is higher than the average plating rate determined from gravimetrical measurements. © 2006 Elsevier Ltd. All rights reserved. Keywords: Electroless deposition; Inducing effect; Electrochemical; Mixed potential theory; Fe–P alloy
1. Introduction Since Brenner and Riddell first reported electroless deposition of nickel films from aqueous solutions on a catalytic surface, many researchers have made great efforts to follow and demonstrate the application of this method for the deposition of both pure metals, like Ni, Co, Cu, and Ag, and their multi-component alloys [1–7]. Electroless deposition, resulting from the anodic reactions (oxidations of reducing agent) and cathodic reactions (reduction of the metallic species, reducing agent and protons), takes place on the catalytic surface immersion in the electrolyte, without the need for electrical contact. This convenience has led to electroless deposition finding widespread use, such as printed circuit boards, magnetic storage media, microelectronics, radioelectronics, computer engineering, aerospace techniques and metallization of plastic [7–12]. The investigation of basic mechanisms and processes of electroless deposition is of the primary interest for the introduction of new deposits and the improvement of the deposit’s performance. It is widely accepted that electroless deposition proceeds along the electrochemical mechanism as a simultaneous reaction of
∗
Corresponding author. Tel.: +86 731 8643310; fax: +86 731 8822332. E-mail address: [email protected] (G.-F. Huang).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.12.026
cathodic metal deposition and anodic oxidation of a reductant at the same catalytic surface. All these postulations were made on a base of the Wagner–Traud mixed potential theory of corrosion process. Electroless deposition of metals can be regarded as the sum of two partial reactions occurred at the same catalytic surface, the cathodic reduction of the metal combined with the anodic oxidation of the reducing agent. In addition, there might be also some interference effects between the reactions. In order to establish basic mechanism of the electroless deposition, i.e., metal reduction and reducing agent oxidation, calculate instantaneous plating rate, investigate the role of electroless deposition solution components, and confirm mixed potential theory, many studies of independent partial reactions were carried out. Good agreement between the measurement of instantaneous plating rate and that calculated from partial reaction data in the case of Au, Cu, Ni–P, and Co–P was obtained [13–15]. Iron alloys are widely used in magnetic devices, computer, aviation materials and memory alloys due to its excellent soft magnetism, good resistance to corrosion and excellent catalytic performance [16–20]. Ruscior first prepared Fe–P films using electroless deposition by coupling the copper substrates with an aluminum foil and analyzing the composition of deposits [21]. By introducing other elements, Wang et al. followed and prepared multi-component iron alloys [22–25], which were also prepared by coupling aluminum foil with the substrate since
4472
G.-F. Huang et al. / Electrochimica Acta 51 (2006) 4471–4476
the plating rate of iron alloys is too slow to be observed. These studies on electroless deposition iron alloys were focused on the composition, structure, and the magnetism, mechanical and anticorrosion behavior of deposits [21–25]. To date, however, very little attention was paid to the inducing effect of coupled aluminum and the deposition mechanism of electroless iron alloy. This work presents the electrochemical investigation of the electroless Fe–P deposition system. The partial reactions of metal ions reduction and reducer oxidation using electroless Fe–P solution was studied by changing the concentration of metal salt, reducer, and complex, the value of pH and temperature. The inducing effect of aluminum on electroless Fe–P deposition was investigated by analyzing the mixed potential change and the polarization resistance of anodic and cathodic reaction. In situ measurement of mixed potentials of the electroless process was recorded. The plating rate of Fe–P was determined by gravimetrical method and calculated from the deposition current density. The test of the mixed potential theory was performed by comparison between direct experimental values for the mixed potential and plating rate with those derived theoretically from the current–potential I–E curves for partial reactions. 2. Experimental The Fe–P deposits were prepared on brass substrates by electroless plating from a sulfate bath. The bath composition and process parameters are given in Table 1. The pH of the solution was adjusted using 15 wt.% NaOH solution and measured by electronic pH-meter Orion model 720A. The brass substrate and aluminum foil were orderly polished with #600, #1200, and #2000 emery paper, and then rinsed by de-ionized water, diluted HCl, de-ionized water, acetone, de-ionized water, alcohol in sequence prior to deposition. During deposition, the substrate was connected with aluminum foil through a copper wire in accordance with the results of Ruscior et al., showing that it is necessary for the brass substrate to be in contact with aluminum, zinc or magnesium [21]. Other groups followed and used the copper substrate in contact with an aluminum foil to deposit Fe–B or iron alloys by electroless deposition [22–29]. To perform the gravimetrical measurements, electroless Fe–P deposits were prepared on brass foil (25 mm × 15 mm × 2 mm) connected or disconnected with aluminium (25 mm × 18 mm × 1.2 mm). The weight of deposit was calculated by weighing the samples before and after deposition by electronic microbalance model HT-300. The average plating rate was determined by dividing the deposit weight by the surface area of substrate
Fig. 1. Effect of time on the thickness of electroless Fe–P deposits.
and deposition time. The deposit thickness was calculated by the mass–density relationship assuming a bulk density of 7.8 g/cm3 for the deposits. The electrochemical measurement was carried out with the electrochemical analyzer CHI-660B. The brass electrode (φ = 20 mm) embedded in an epoxy resin was used as substrate. Mixed potential of electrodes was measured versus saturated calomel electrode during deposition. The potential was recorded after the electrode immersing into the bath. Polarization experiments were carried out with a three-compartment cell. A platinum foil with size 20 mm × 40 mm was used as an auxiliary electrode and a saturated calomel electrode were used as reference electrode. The working electrode was the brass electrode connected with aluminum foil in deposition. In order to simulate partial cathodic or anodic reactions, the investigation of the electrolytes in the absence of either the Na2 H2 PO2 reducer (oxidation solution) or the FeSO4 (reducing solution) was carried out. 3. Results and discussion 3.1. The inducing effect of coupled aluminum on Fe–P deposition Experiments found that there was no deposit on brass substrate without aluminum coupled, while the deposit thickness increased with time linearly when the brass substrate was connected with aluminum foil during deposition, as shown in Fig. 1. The results show that aluminum induces the Fe–P deposition.
Table 1 The bath compositions (g/L) and plating conditions of electroless Fe–P depositions Bath solution
FeSO4 ·7H2 O
NaH2 PO2 ·H2 O
KNaC4 H4 O6 ·4H2 O
H3 BO3
C12 H12 O11
Base Oxidation Reduction Plating
– 9 – 3–12
– – 15 5–25
36 36 36 36
3 3 3 3
1.2 1.2 1.2 1.2
pH: 8–11.5; T: 50–85 ◦ C.
G.-F. Huang et al. / Electrochimica Acta 51 (2006) 4471–4476
4473
Fig. 2. The potential dependence on time with the Al connected or disconnected from the electrode.
To investigate the inducing mechanism of coupled aluminum on Fe–P deposition, we begin with the study of the potential dependence on time with aluminum connected or disconnected. Fig. 2 shows that the potential shifts negative abruptly from the potential of brass in electroless bath of −0.62 V (versus SCE) to about −1.2 V (versus SCE) as the brass electrode is connected with aluminum foil, and changes little during the whole deposition. When the brass electrode is disconnected from the aluminum foil, the potential shifts to about −0.8 V (versus SCE). We also simultaneously measured the deposit thickness and potential dependence on time. In the experiment, the brass substrate was first connected with aluminum in bath for 30 min, then the aluminum was departed from the brass, the potential was recorded and the deposit thickness was calculated with time. It can be seen from Fig. 3(a) and (b) that the thickness of deposits increases with time and the potential keeps at about −1.2 V at the initial 30 min. As the aluminum foil was departed from the brass substrate, however, the deposit thickness begins to decrease with time, accompanying with the potential shift to about −0.8 V sharply. This means that the deposits begin to dissolve with the coupled aluminum disconnection. The potential begins to shift positively again from about 2 h and reaches the brass potential of −0.63 V at about 2.5 h (see Fig. 3(b)). These results indicate that the Fe–P deposition reaction occurs only when the potential is lower than certain negative value (about −0.9 V versus SCE). Thus, the deposition reaction stopped since the potential shifted to the deposit potential (about −0.8 V versus SCE) in bath as the coupled aluminum is disconnected from brass substrate. It is also evidently seen from Fig. 3(a) that the deposit thickness decreases near linearly after the aluminum is disconnected. The linear dependence can be fitted by the equation Y = 1.37 − 0.57X, where X and Y represent time and the deposit thickness, respectively. From the equation, it is easily to obtain the time where the deposits dissolves completely at about 2.4 h, which accords with the time (about 2.5 h) determined from the change of potential for deposit dissolve completely (see Fig. 3(b)). The results indicate that it is necessary for brass substrate to be coupled with aluminum for Fe–P deposited by chemical reduction with hypophosphite as reducing agent.
Fig. 3. (a) The dependence of thickness of deposits on time. (b) The potential dependence on time during deposition or with the electrode disconnected from Al.
To ulteriorly reveal the inducing effect of aluminum on Fe–P deposition, the partial anodic and cathodic polarization curves were measured with or without the coupled aluminum foil. It is well known that the electroless deposition is classified as autocatalytic redox reactions with the metal as a final product in which both the cathodic reduction of the metal and the anodic oxidation of the reducing agent take place on the same catalytic surface. The difference between the redox potential of the reducing agent and that of the metal, E, is the force to drive deposition and related to the reaction rate. When the potential difference E is lower than zero, the deposition is difficult to take place. The I–E curves of brass in oxidation solution or reducing solution in Fig. 4(a) show that there is no intersection between the two curves, and the potential of brass electrode in reducing solution and oxidation solution were about −0.4 and −0.7 V versus SCE, respectively. Thus, the value of potential difference E is negative and there is no force to drive deposition. In comparison, the potentials of brass connected with aluminum foil in reducing and oxidation solutions were about −1.28 and −1.06 V versus SCE, respectively, as displayed in Fig. 4(b). The difference between the potential of the reducing agent and that of the metal, 0.22 V, is the electromotive force to drive deposition.
4474
G.-F. Huang et al. / Electrochimica Acta 51 (2006) 4471–4476
Fig. 5. Dependence of the mixed potential and current density on the pH for electroless Fe–P deposition.
The instantaneous plating rate, v2 (mg/cm2 h), can be calculated from the deposition current density, idep (A/dm−2 ), using Faraday’s law: v2 =
Fig. 4. I–E curves for reduction of Fe2+ ions and for oxidation of hypophosphite: (a) brass electrode, (b) brass electrode coupled with aluminum.
The cathodic and anodic curves intersect at one point where the potential and the current being −1.19 V and 0.013 A, respectively. According to the mixed potential theory, the deposition reaction was carried out with a current of 0.013 A. The resistance for cathodic and anodic reaction (Rpa and Rpc ) derived from the polarization curves are 9.39 and 8.67 , respectively, which are much less than those (136.6 and 38.3 , obtained from the polarization curves in Fig. 4(a)) without coupled aluminum. From the above discussion, the coupled aluminum shifts the potential negatively, and decreases the anodic and cathodic reaction resistance, thus provides an active surface and induces the Fe–P deposition.
idep W = 0.373Nidep = 10.44idep (mg cm−2 h−1 ) F n
where F is the Faraday constant, W the atomic weight of deposits (supposing W = 56), n the number of electrons obtained by iron ion (n = 2), N = W/2 = 28. The pH has the most significant influence on the deposition current density and mixed potential of the process. The dependence of the mixed potential and deposition current density on the pH, as derived from the polarization curves, is presented in Fig. 5. It can be seen that the mixed potential shifts cathodically and the deposition current density increases with the pH increasing. The mixed potential shifts by approximately 300 mV and the deposition current density changes by more than a factor of 2 when the pH varies from 9 to 11.5. This trend was also observed for electroless Co–W–P and Co–P deposition [30]. Fig. 6 depicts the dependence of the mixed potential and deposition current density on the temperature, as derived from electrochemical measurements. When the temperature varies from 50 to 80 ◦ C, the mixed potential shifts cathodically slightly
3.2. The effects on the polarization measurement We now investigate the effects of the pH, temperature and the constitution concentration of electroless bath on the polarization measurement. The data presented in this section were obtained from polarization measurements, which were carried out in the absence of either FeSO4 (partial anodic curve) or NaH2 PO2 (partial cathodic curve). The intersection of these two polarization curves represents mixed potential and deposition current density according to the mixed potential theory from Evans diagrams.
Fig. 6. Dependence of the mixed potential and current density on the bath temperature.
G.-F. Huang et al. / Electrochimica Acta 51 (2006) 4471–4476
Fig. 7. The dependence of the mixed potential and deposition current density on FeSO4 concentration.
Fig. 8. The dependence of the mixed potential and deposition current density on NaH2 PO2 concentration.
and the current density increases with increasing temperature. The dependence of both mixed potential and current density on the temperature in the present investigations is similar to that observed in Co–P deposition [30]. At low temperatures, the process is very slow and may not start below 50 ◦ C. Fig. 6 also shows that the mixed potential and deposition current density change little as the temperature higher than 80 ◦ C. The effect of FeSO4 concentration on the mixed potential and deposition current density is shown in Fig. 7. It can be seen that the effect of concentration of the metal on the mixed potential and deposition current density is not pronounced. The mixed potential shifts anodically and the current density increase slightly with increasing FeSO4 concentration. Fig. 8 displays the effect of NaH2 PO2 concentration on the mixed potential and deposition current density. It can be seen that the mixed potential shifts cathodically slightly while the current density increases with increasing NaH2 PO2 concentration. 3.3. Test of the mixed potential theory As discussed above, the coordinates of intersection of anodic and cathodical polarization curves represent the deposition current density and the mixed potential (E1 ). In order to examine the mixed potential theory, measured values of the mixed potential during deposition and the rate of deposition were compared with those derived theoretically from I–E curves for partial reactions. The average mixed potential (E2 ) is determined for 1 h (from Fig. 9) and the average plating rate is obtained by gravimetrical
4475
Fig. 9. The time dependence of mixed potential at different pH.
method. The mixed potential and the plating rate at different pH are listed in Table 2. It can be seen from Table 2 that the mixed potentials determined during the plating process are in good agreement with those obtained from electrochemical measurements. The plating rates determined from two methods show the similar trend, i.e., the plating rates increase with pH and reach a maximum value at pH equal to about 11. These results indicate the electroless Fe–P deposition obeys the mixed potential theory. The slight decrease of plating rate may be ascribed to the formation of iron hydrate as the pH is higher than 11. Table 2 also
Table 2 The plating rate and mixed potential of electroless Fe–P alloys obtained by gravimetrical and electrochemical methods pH
Mixed potential, E1 (V vs. SCE)
Deposition current density, i (A/dm2 )
Calculated plating rate, v2 (mg/cm2 h)
Mixed potential, E2 (V vs. SCE)
Average plating rate, v1 (mg/cm2 h)
9.0 9.5 10.0 10.5 11.0 11.5
−1.05 −1.11 −1.17 −1.2 −1.23 −1.26
0.240 0.295 0.381 0.416 0.415 0.369
2.509 3.079 3.976 4.343 4.333 3.853
−1.05 −1.08 −1.10 −1.16 −1.21 −1.21
0.972 1.255 1.518 1.634 1.814 1.714
4476
G.-F. Huang et al. / Electrochimica Acta 51 (2006) 4471–4476
to the hydrogen evolution, the plating rate determined by electrochemical measurement is higher than the average plating rate determined from gravimetrical measurements. It is suggested that electrochemical investigation of electroless deposition systems is a suitable tool in defining the favorable technological parameters to produce deposits with controlled characteristics. Acknowledgment This work was supported by Hunan Provincial Natural Science Foundation of China (Grant No. 05JJ30089). References
Fig. 10. The polarization curves at different solution, in which a, b, c and d are the polarization curves in base, oxidation, reduction, plating solution, respectively. And the curve e is the summation of the curves b and c.
shows that the instantaneous plating rates calculated from electrochemical measurement are significantly higher than the average plating rates determined from gravimetrical measurements. The possible reason for the magnitude difference between the instantaneous and average plating rates is that the reduction of iron ions and hypophosphite is suppressed to some extent due to the formation of the iron phosphides, hydrogen evolution and/or poisoning of the catalytic surface. The polarization curves at different solution are presented in Fig. 10, where the curves of a, b, c, d represent the polarization curves in base, oxidation, reducing and plating solution, respectively. The curve e represents the compound polarization by adding the curves of b and c, where the dependence between the cathodic and anodic processes is neglected. It is clear that there is difference between the compound curve e and the polarization curve d, indicating that the cathodic and anodic processes occur simultaneously and are interdependent. There is no obvious reducing peak at b curves, the current increase rapidly with the potential shift negatively only at the potential more negative than −1.1 V, which also appear in curve c, suggesting the iron reducing accompanying with the hydrogen evolution. 4. Conclusions It has been shown that a coupled aluminum is necessary for Fe–P electroless deposition. The inducing mechanism of coupled aluminum on electroless Fe–P deposition was investigated by electrochemical measurement. The coupled aluminum induces the Fe–P deposition by shifting the potential negatively and reducing the reaction resistance. The mixed potential theory was verified for the Fe–P electroless deposition. It was carried out by comparison of the average plating rate and the mixed potential determined during deposition process with those that were calculated from the electrochemical measurements. Due
[1] V.M. Dubin, Y. Shacham-Diamand, B. Zhao, P.K. Vasudev, J. Electrochem. Soc. 144 (1997) 898. [2] M. Paunovic, P.J. Bailey, R.G. Schad, D.A. Smith, J. Electrochem. Soc. 141 (1994) 1843. [3] N.M. Hasan, J.J. Mallett, S.G. dos Santos Filho, et al., Phys. Rev. B 67 (2003) 081401(R). [4] J.N. Balaraju, K.S. Rajam, Surf. Coat. Technol. 195 (2005) 154. [5] L. Magagnin, V. Sirtori, S. Seregni, et al., Electrochim. Acta 50 (2005) 4621. [6] H. Ashassi-Sorkhabi, H. Dolati, N. Parvini-Ahmadi, et al., Appl. Surf. Sci. 185 (2002) 155. [7] H. Ashassi-Sorkhabi, S.H. Rafizadeh, Surf. Coat. Technol. 176 (2004) 318. [8] H. Honma, Electrochim. Acta 47 (2001) 75. [9] T. Osaka, T. Asahi, J. Kawaji, et al., Electrochim. Acta 50 (2005) 4576. [10] Y. Shacham-Diamand, A. Inberg, Y. Sverdlov, et al., Electrochim. Acta 48 (2003) 2987. [11] Y. Shacham-Diamond, S. Lopatin, Electrochim. Acta 44 (1999) 3639. [12] T.N. Khoperia, T.J. Tabatadze, T.I. Zedgenidze, Electrochim. Acta 42 (1997) 3049. [13] M. Paunovic, Plat. Surf. Finish. 70 (1983) 62. [14] M. Paunovic, Plating 55 (1968) 1161. [15] I. Ohno, S. Haruyama, Surf. Technol. 13 (1981) 1. [16] S.J. Pang, T. Zhang, K. Asami, Corros. Sci. 44 (2002) 1847. [17] W. Fernelgel, W. Rodewald, R. Blank, et al., J. Magn. Magn. Mater. 196–197 (1999) 288. [18] P. Tartaj, T. Gonza lez-Carren, O. Bomatı-Miguel, et al., Phys. Rev. B 69 (2004) 094401. [19] C. Brosseau, S. Mall´egol, P. Qu´effelec, et al., Phys. Rev. B 70 (2004) 092401. [20] R. Mathieu, P.E. Jonsson, P. Nordblad, Phys. Rev. B 65 (2001) 012411. [21] C. Ruscior, E. Croiala, J. Electrochem. Soc. 18 (1971) 696. [22] L.L. Wang, L.H. Zhao, G.F. Huang, et al., Plat. Surf. Finish. 88 (2001) 92. [23] L.L. Wang, L.H. Zhao, G.F. Huang, et al., Z. Metallkd. 92 (2001) 391. [24] L.L. Wang, W.Q. Huang, G.F. Huang, et al., Z. Metallkd. 93 (2002) 298. [25] L.L. Wang, L.H. Zhao, G.F. Huang, et al., Surf. Coat. Technol. 126 (2000) 272. [26] B.W. Zhang, W.Y. Hu, D.Q. Zhu, Physica B 183 (1993) 205. [27] W.Y. Hu, B.W. Zhang, Trans. Inst. Met. Finish. 71 (1993) 30. [28] W.Y. Hu, B.W. Zhang, Physica B 175 (1991) 396. [29] N. Fujita, A. Tanaka, E. Makino, et al., Appl. Surf. Sci. 113/114 (1997) 61. [30] N. Petrov, Y. Sverdlov, Y. Shacham-Diamand, J. Electrochem. Soc. 149 (2002) C187.
Electrochemical study of electroless deposition of Fe–P alloys Gui-Fang Huang ∗ , Wei-Qing Huang, Ling-Ling Wang, Yan Meng, Zhong Xie, B.S. Zou Department of Applied Physics, Hunan University, Changsha 410082, PR China Received 17 August 2005; received in revised form 30 November 2005; accepted 17 December 2005 Available online 19 January 2006
Abstract Fe–P deposits were prepared by electroless deposition and the inducing effect of coupled aluminum on Fe–P deposition was studied. The partial reactions of reducer oxidation and metal ions reduction in electroless deposition Fe–P bath were investigated by electrochemical investigation. In situ mixed potentials of the process for electrolytes with different pH were measured. The plating rate of iron alloys was measured gravimetrically and calculated by the deposition current density. The results show that coupled aluminum induces the Fe–P deposition by shifting the potential negatively and decreasing the polarization resistance of anodic and cathodic reaction. The test of the mixed potential theory was performed by comparison between direct experimental values of the mixed potential and plating rate with those derived theoretically from the current–potential curves for partial reactions. Due to the hydrogen evolution, the plating rate determined by electrochemical measurement is higher than the average plating rate determined from gravimetrical measurements. © 2006 Elsevier Ltd. All rights reserved. Keywords: Electroless deposition; Inducing effect; Electrochemical; Mixed potential theory; Fe–P alloy
1. Introduction Since Brenner and Riddell first reported electroless deposition of nickel films from aqueous solutions on a catalytic surface, many researchers have made great efforts to follow and demonstrate the application of this method for the deposition of both pure metals, like Ni, Co, Cu, and Ag, and their multi-component alloys [1–7]. Electroless deposition, resulting from the anodic reactions (oxidations of reducing agent) and cathodic reactions (reduction of the metallic species, reducing agent and protons), takes place on the catalytic surface immersion in the electrolyte, without the need for electrical contact. This convenience has led to electroless deposition finding widespread use, such as printed circuit boards, magnetic storage media, microelectronics, radioelectronics, computer engineering, aerospace techniques and metallization of plastic [7–12]. The investigation of basic mechanisms and processes of electroless deposition is of the primary interest for the introduction of new deposits and the improvement of the deposit’s performance. It is widely accepted that electroless deposition proceeds along the electrochemical mechanism as a simultaneous reaction of
∗
Corresponding author. Tel.: +86 731 8643310; fax: +86 731 8822332. E-mail address: [email protected] (G.-F. Huang).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.12.026
cathodic metal deposition and anodic oxidation of a reductant at the same catalytic surface. All these postulations were made on a base of the Wagner–Traud mixed potential theory of corrosion process. Electroless deposition of metals can be regarded as the sum of two partial reactions occurred at the same catalytic surface, the cathodic reduction of the metal combined with the anodic oxidation of the reducing agent. In addition, there might be also some interference effects between the reactions. In order to establish basic mechanism of the electroless deposition, i.e., metal reduction and reducing agent oxidation, calculate instantaneous plating rate, investigate the role of electroless deposition solution components, and confirm mixed potential theory, many studies of independent partial reactions were carried out. Good agreement between the measurement of instantaneous plating rate and that calculated from partial reaction data in the case of Au, Cu, Ni–P, and Co–P was obtained [13–15]. Iron alloys are widely used in magnetic devices, computer, aviation materials and memory alloys due to its excellent soft magnetism, good resistance to corrosion and excellent catalytic performance [16–20]. Ruscior first prepared Fe–P films using electroless deposition by coupling the copper substrates with an aluminum foil and analyzing the composition of deposits [21]. By introducing other elements, Wang et al. followed and prepared multi-component iron alloys [22–25], which were also prepared by coupling aluminum foil with the substrate since
4472
G.-F. Huang et al. / Electrochimica Acta 51 (2006) 4471–4476
the plating rate of iron alloys is too slow to be observed. These studies on electroless deposition iron alloys were focused on the composition, structure, and the magnetism, mechanical and anticorrosion behavior of deposits [21–25]. To date, however, very little attention was paid to the inducing effect of coupled aluminum and the deposition mechanism of electroless iron alloy. This work presents the electrochemical investigation of the electroless Fe–P deposition system. The partial reactions of metal ions reduction and reducer oxidation using electroless Fe–P solution was studied by changing the concentration of metal salt, reducer, and complex, the value of pH and temperature. The inducing effect of aluminum on electroless Fe–P deposition was investigated by analyzing the mixed potential change and the polarization resistance of anodic and cathodic reaction. In situ measurement of mixed potentials of the electroless process was recorded. The plating rate of Fe–P was determined by gravimetrical method and calculated from the deposition current density. The test of the mixed potential theory was performed by comparison between direct experimental values for the mixed potential and plating rate with those derived theoretically from the current–potential I–E curves for partial reactions. 2. Experimental The Fe–P deposits were prepared on brass substrates by electroless plating from a sulfate bath. The bath composition and process parameters are given in Table 1. The pH of the solution was adjusted using 15 wt.% NaOH solution and measured by electronic pH-meter Orion model 720A. The brass substrate and aluminum foil were orderly polished with #600, #1200, and #2000 emery paper, and then rinsed by de-ionized water, diluted HCl, de-ionized water, acetone, de-ionized water, alcohol in sequence prior to deposition. During deposition, the substrate was connected with aluminum foil through a copper wire in accordance with the results of Ruscior et al., showing that it is necessary for the brass substrate to be in contact with aluminum, zinc or magnesium [21]. Other groups followed and used the copper substrate in contact with an aluminum foil to deposit Fe–B or iron alloys by electroless deposition [22–29]. To perform the gravimetrical measurements, electroless Fe–P deposits were prepared on brass foil (25 mm × 15 mm × 2 mm) connected or disconnected with aluminium (25 mm × 18 mm × 1.2 mm). The weight of deposit was calculated by weighing the samples before and after deposition by electronic microbalance model HT-300. The average plating rate was determined by dividing the deposit weight by the surface area of substrate
Fig. 1. Effect of time on the thickness of electroless Fe–P deposits.
and deposition time. The deposit thickness was calculated by the mass–density relationship assuming a bulk density of 7.8 g/cm3 for the deposits. The electrochemical measurement was carried out with the electrochemical analyzer CHI-660B. The brass electrode (φ = 20 mm) embedded in an epoxy resin was used as substrate. Mixed potential of electrodes was measured versus saturated calomel electrode during deposition. The potential was recorded after the electrode immersing into the bath. Polarization experiments were carried out with a three-compartment cell. A platinum foil with size 20 mm × 40 mm was used as an auxiliary electrode and a saturated calomel electrode were used as reference electrode. The working electrode was the brass electrode connected with aluminum foil in deposition. In order to simulate partial cathodic or anodic reactions, the investigation of the electrolytes in the absence of either the Na2 H2 PO2 reducer (oxidation solution) or the FeSO4 (reducing solution) was carried out. 3. Results and discussion 3.1. The inducing effect of coupled aluminum on Fe–P deposition Experiments found that there was no deposit on brass substrate without aluminum coupled, while the deposit thickness increased with time linearly when the brass substrate was connected with aluminum foil during deposition, as shown in Fig. 1. The results show that aluminum induces the Fe–P deposition.
Table 1 The bath compositions (g/L) and plating conditions of electroless Fe–P depositions Bath solution
FeSO4 ·7H2 O
NaH2 PO2 ·H2 O
KNaC4 H4 O6 ·4H2 O
H3 BO3
C12 H12 O11
Base Oxidation Reduction Plating
– 9 – 3–12
– – 15 5–25
36 36 36 36
3 3 3 3
1.2 1.2 1.2 1.2
pH: 8–11.5; T: 50–85 ◦ C.
G.-F. Huang et al. / Electrochimica Acta 51 (2006) 4471–4476
4473
Fig. 2. The potential dependence on time with the Al connected or disconnected from the electrode.
To investigate the inducing mechanism of coupled aluminum on Fe–P deposition, we begin with the study of the potential dependence on time with aluminum connected or disconnected. Fig. 2 shows that the potential shifts negative abruptly from the potential of brass in electroless bath of −0.62 V (versus SCE) to about −1.2 V (versus SCE) as the brass electrode is connected with aluminum foil, and changes little during the whole deposition. When the brass electrode is disconnected from the aluminum foil, the potential shifts to about −0.8 V (versus SCE). We also simultaneously measured the deposit thickness and potential dependence on time. In the experiment, the brass substrate was first connected with aluminum in bath for 30 min, then the aluminum was departed from the brass, the potential was recorded and the deposit thickness was calculated with time. It can be seen from Fig. 3(a) and (b) that the thickness of deposits increases with time and the potential keeps at about −1.2 V at the initial 30 min. As the aluminum foil was departed from the brass substrate, however, the deposit thickness begins to decrease with time, accompanying with the potential shift to about −0.8 V sharply. This means that the deposits begin to dissolve with the coupled aluminum disconnection. The potential begins to shift positively again from about 2 h and reaches the brass potential of −0.63 V at about 2.5 h (see Fig. 3(b)). These results indicate that the Fe–P deposition reaction occurs only when the potential is lower than certain negative value (about −0.9 V versus SCE). Thus, the deposition reaction stopped since the potential shifted to the deposit potential (about −0.8 V versus SCE) in bath as the coupled aluminum is disconnected from brass substrate. It is also evidently seen from Fig. 3(a) that the deposit thickness decreases near linearly after the aluminum is disconnected. The linear dependence can be fitted by the equation Y = 1.37 − 0.57X, where X and Y represent time and the deposit thickness, respectively. From the equation, it is easily to obtain the time where the deposits dissolves completely at about 2.4 h, which accords with the time (about 2.5 h) determined from the change of potential for deposit dissolve completely (see Fig. 3(b)). The results indicate that it is necessary for brass substrate to be coupled with aluminum for Fe–P deposited by chemical reduction with hypophosphite as reducing agent.
Fig. 3. (a) The dependence of thickness of deposits on time. (b) The potential dependence on time during deposition or with the electrode disconnected from Al.
To ulteriorly reveal the inducing effect of aluminum on Fe–P deposition, the partial anodic and cathodic polarization curves were measured with or without the coupled aluminum foil. It is well known that the electroless deposition is classified as autocatalytic redox reactions with the metal as a final product in which both the cathodic reduction of the metal and the anodic oxidation of the reducing agent take place on the same catalytic surface. The difference between the redox potential of the reducing agent and that of the metal, E, is the force to drive deposition and related to the reaction rate. When the potential difference E is lower than zero, the deposition is difficult to take place. The I–E curves of brass in oxidation solution or reducing solution in Fig. 4(a) show that there is no intersection between the two curves, and the potential of brass electrode in reducing solution and oxidation solution were about −0.4 and −0.7 V versus SCE, respectively. Thus, the value of potential difference E is negative and there is no force to drive deposition. In comparison, the potentials of brass connected with aluminum foil in reducing and oxidation solutions were about −1.28 and −1.06 V versus SCE, respectively, as displayed in Fig. 4(b). The difference between the potential of the reducing agent and that of the metal, 0.22 V, is the electromotive force to drive deposition.
4474
G.-F. Huang et al. / Electrochimica Acta 51 (2006) 4471–4476
Fig. 5. Dependence of the mixed potential and current density on the pH for electroless Fe–P deposition.
The instantaneous plating rate, v2 (mg/cm2 h), can be calculated from the deposition current density, idep (A/dm−2 ), using Faraday’s law: v2 =
Fig. 4. I–E curves for reduction of Fe2+ ions and for oxidation of hypophosphite: (a) brass electrode, (b) brass electrode coupled with aluminum.
The cathodic and anodic curves intersect at one point where the potential and the current being −1.19 V and 0.013 A, respectively. According to the mixed potential theory, the deposition reaction was carried out with a current of 0.013 A. The resistance for cathodic and anodic reaction (Rpa and Rpc ) derived from the polarization curves are 9.39 and 8.67 , respectively, which are much less than those (136.6 and 38.3 , obtained from the polarization curves in Fig. 4(a)) without coupled aluminum. From the above discussion, the coupled aluminum shifts the potential negatively, and decreases the anodic and cathodic reaction resistance, thus provides an active surface and induces the Fe–P deposition.
idep W = 0.373Nidep = 10.44idep (mg cm−2 h−1 ) F n
where F is the Faraday constant, W the atomic weight of deposits (supposing W = 56), n the number of electrons obtained by iron ion (n = 2), N = W/2 = 28. The pH has the most significant influence on the deposition current density and mixed potential of the process. The dependence of the mixed potential and deposition current density on the pH, as derived from the polarization curves, is presented in Fig. 5. It can be seen that the mixed potential shifts cathodically and the deposition current density increases with the pH increasing. The mixed potential shifts by approximately 300 mV and the deposition current density changes by more than a factor of 2 when the pH varies from 9 to 11.5. This trend was also observed for electroless Co–W–P and Co–P deposition [30]. Fig. 6 depicts the dependence of the mixed potential and deposition current density on the temperature, as derived from electrochemical measurements. When the temperature varies from 50 to 80 ◦ C, the mixed potential shifts cathodically slightly
3.2. The effects on the polarization measurement We now investigate the effects of the pH, temperature and the constitution concentration of electroless bath on the polarization measurement. The data presented in this section were obtained from polarization measurements, which were carried out in the absence of either FeSO4 (partial anodic curve) or NaH2 PO2 (partial cathodic curve). The intersection of these two polarization curves represents mixed potential and deposition current density according to the mixed potential theory from Evans diagrams.
Fig. 6. Dependence of the mixed potential and current density on the bath temperature.
G.-F. Huang et al. / Electrochimica Acta 51 (2006) 4471–4476
Fig. 7. The dependence of the mixed potential and deposition current density on FeSO4 concentration.
Fig. 8. The dependence of the mixed potential and deposition current density on NaH2 PO2 concentration.
and the current density increases with increasing temperature. The dependence of both mixed potential and current density on the temperature in the present investigations is similar to that observed in Co–P deposition [30]. At low temperatures, the process is very slow and may not start below 50 ◦ C. Fig. 6 also shows that the mixed potential and deposition current density change little as the temperature higher than 80 ◦ C. The effect of FeSO4 concentration on the mixed potential and deposition current density is shown in Fig. 7. It can be seen that the effect of concentration of the metal on the mixed potential and deposition current density is not pronounced. The mixed potential shifts anodically and the current density increase slightly with increasing FeSO4 concentration. Fig. 8 displays the effect of NaH2 PO2 concentration on the mixed potential and deposition current density. It can be seen that the mixed potential shifts cathodically slightly while the current density increases with increasing NaH2 PO2 concentration. 3.3. Test of the mixed potential theory As discussed above, the coordinates of intersection of anodic and cathodical polarization curves represent the deposition current density and the mixed potential (E1 ). In order to examine the mixed potential theory, measured values of the mixed potential during deposition and the rate of deposition were compared with those derived theoretically from I–E curves for partial reactions. The average mixed potential (E2 ) is determined for 1 h (from Fig. 9) and the average plating rate is obtained by gravimetrical
4475
Fig. 9. The time dependence of mixed potential at different pH.
method. The mixed potential and the plating rate at different pH are listed in Table 2. It can be seen from Table 2 that the mixed potentials determined during the plating process are in good agreement with those obtained from electrochemical measurements. The plating rates determined from two methods show the similar trend, i.e., the plating rates increase with pH and reach a maximum value at pH equal to about 11. These results indicate the electroless Fe–P deposition obeys the mixed potential theory. The slight decrease of plating rate may be ascribed to the formation of iron hydrate as the pH is higher than 11. Table 2 also
Table 2 The plating rate and mixed potential of electroless Fe–P alloys obtained by gravimetrical and electrochemical methods pH
Mixed potential, E1 (V vs. SCE)
Deposition current density, i (A/dm2 )
Calculated plating rate, v2 (mg/cm2 h)
Mixed potential, E2 (V vs. SCE)
Average plating rate, v1 (mg/cm2 h)
9.0 9.5 10.0 10.5 11.0 11.5
−1.05 −1.11 −1.17 −1.2 −1.23 −1.26
0.240 0.295 0.381 0.416 0.415 0.369
2.509 3.079 3.976 4.343 4.333 3.853
−1.05 −1.08 −1.10 −1.16 −1.21 −1.21
0.972 1.255 1.518 1.634 1.814 1.714
4476
G.-F. Huang et al. / Electrochimica Acta 51 (2006) 4471–4476
to the hydrogen evolution, the plating rate determined by electrochemical measurement is higher than the average plating rate determined from gravimetrical measurements. It is suggested that electrochemical investigation of electroless deposition systems is a suitable tool in defining the favorable technological parameters to produce deposits with controlled characteristics. Acknowledgment This work was supported by Hunan Provincial Natural Science Foundation of China (Grant No. 05JJ30089). References
Fig. 10. The polarization curves at different solution, in which a, b, c and d are the polarization curves in base, oxidation, reduction, plating solution, respectively. And the curve e is the summation of the curves b and c.
shows that the instantaneous plating rates calculated from electrochemical measurement are significantly higher than the average plating rates determined from gravimetrical measurements. The possible reason for the magnitude difference between the instantaneous and average plating rates is that the reduction of iron ions and hypophosphite is suppressed to some extent due to the formation of the iron phosphides, hydrogen evolution and/or poisoning of the catalytic surface. The polarization curves at different solution are presented in Fig. 10, where the curves of a, b, c, d represent the polarization curves in base, oxidation, reducing and plating solution, respectively. The curve e represents the compound polarization by adding the curves of b and c, where the dependence between the cathodic and anodic processes is neglected. It is clear that there is difference between the compound curve e and the polarization curve d, indicating that the cathodic and anodic processes occur simultaneously and are interdependent. There is no obvious reducing peak at b curves, the current increase rapidly with the potential shift negatively only at the potential more negative than −1.1 V, which also appear in curve c, suggesting the iron reducing accompanying with the hydrogen evolution. 4. Conclusions It has been shown that a coupled aluminum is necessary for Fe–P electroless deposition. The inducing mechanism of coupled aluminum on electroless Fe–P deposition was investigated by electrochemical measurement. The coupled aluminum induces the Fe–P deposition by shifting the potential negatively and reducing the reaction resistance. The mixed potential theory was verified for the Fe–P electroless deposition. It was carried out by comparison of the average plating rate and the mixed potential determined during deposition process with those that were calculated from the electrochemical measurements. Due
[1] V.M. Dubin, Y. Shacham-Diamand, B. Zhao, P.K. Vasudev, J. Electrochem. Soc. 144 (1997) 898. [2] M. Paunovic, P.J. Bailey, R.G. Schad, D.A. Smith, J. Electrochem. Soc. 141 (1994) 1843. [3] N.M. Hasan, J.J. Mallett, S.G. dos Santos Filho, et al., Phys. Rev. B 67 (2003) 081401(R). [4] J.N. Balaraju, K.S. Rajam, Surf. Coat. Technol. 195 (2005) 154. [5] L. Magagnin, V. Sirtori, S. Seregni, et al., Electrochim. Acta 50 (2005) 4621. [6] H. Ashassi-Sorkhabi, H. Dolati, N. Parvini-Ahmadi, et al., Appl. Surf. Sci. 185 (2002) 155. [7] H. Ashassi-Sorkhabi, S.H. Rafizadeh, Surf. Coat. Technol. 176 (2004) 318. [8] H. Honma, Electrochim. Acta 47 (2001) 75. [9] T. Osaka, T. Asahi, J. Kawaji, et al., Electrochim. Acta 50 (2005) 4576. [10] Y. Shacham-Diamand, A. Inberg, Y. Sverdlov, et al., Electrochim. Acta 48 (2003) 2987. [11] Y. Shacham-Diamond, S. Lopatin, Electrochim. Acta 44 (1999) 3639. [12] T.N. Khoperia, T.J. Tabatadze, T.I. Zedgenidze, Electrochim. Acta 42 (1997) 3049. [13] M. Paunovic, Plat. Surf. Finish. 70 (1983) 62. [14] M. Paunovic, Plating 55 (1968) 1161. [15] I. Ohno, S. Haruyama, Surf. Technol. 13 (1981) 1. [16] S.J. Pang, T. Zhang, K. Asami, Corros. Sci. 44 (2002) 1847. [17] W. Fernelgel, W. Rodewald, R. Blank, et al., J. Magn. Magn. Mater. 196–197 (1999) 288. [18] P. Tartaj, T. Gonza lez-Carren, O. Bomatı-Miguel, et al., Phys. Rev. B 69 (2004) 094401. [19] C. Brosseau, S. Mall´egol, P. Qu´effelec, et al., Phys. Rev. B 70 (2004) 092401. [20] R. Mathieu, P.E. Jonsson, P. Nordblad, Phys. Rev. B 65 (2001) 012411. [21] C. Ruscior, E. Croiala, J. Electrochem. Soc. 18 (1971) 696. [22] L.L. Wang, L.H. Zhao, G.F. Huang, et al., Plat. Surf. Finish. 88 (2001) 92. [23] L.L. Wang, L.H. Zhao, G.F. Huang, et al., Z. Metallkd. 92 (2001) 391. [24] L.L. Wang, W.Q. Huang, G.F. Huang, et al., Z. Metallkd. 93 (2002) 298. [25] L.L. Wang, L.H. Zhao, G.F. Huang, et al., Surf. Coat. Technol. 126 (2000) 272. [26] B.W. Zhang, W.Y. Hu, D.Q. Zhu, Physica B 183 (1993) 205. [27] W.Y. Hu, B.W. Zhang, Trans. Inst. Met. Finish. 71 (1993) 30. [28] W.Y. Hu, B.W. Zhang, Physica B 175 (1991) 396. [29] N. Fujita, A. Tanaka, E. Makino, et al., Appl. Surf. Sci. 113/114 (1997) 61. [30] N. Petrov, Y. Sverdlov, Y. Shacham-Diamand, J. Electrochem. Soc. 149 (2002) C187.