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The mechanism of electroless Ni-P deposition studied by electrochemical mass spectrometry
247
J. Electroanal. Chem., 325 (1992) 247-255 Elsevier Sequoia S.A., Lausanne
JEC 1916
The mechanism of electroless Ni-P deposition studied by electrochemical mass spectrometry Z. Jusys, J. Liaukonis and A. VaSkelis Institute of Chemistry, Lithuanian Academy of Sciences, Goitauto 9, 232600 Vilnius (Lithuania) (Received 15 April 1991; in revised form 1 November 1991)
Abstract
An acetate electroless nickel plating solution containing H,PO; as a reducing agent and D,O as a solvent at pH 4.5 and to = 80°C was studied with the aid of electrochemical mass spectrometry. The isotopic composition of the evolved gas and rates of light and heavy hydrogen formation were determined to depend on electrode potential and on both nickel(II) and hypophosphite concentration. The rate of anodic hypophosphite oxidation was found to increase in the cathodic region and to decrease in the anodic region in the presence of nickel(II). Nickel(I1) was found to have a retarding effect on the cathodic deuterium evolution from water. The data obtained are discussed in terms of an electrochemical mechanism of electroless plating.
INTRODUCTION
The electroless deposition of Ni-P alloys is an important process from both a theoretical and practical point of view. However, a study of it is rather complicated owing to the process being autocatalytic and hypophosphite being used in parallel reactions of nickel(H) reduction, hypophosphite oxidation by water and hypophosphite reduction to phosphorus. A mass spectrometry analysis of evolved gas combined with isotope labelling made it possible to understand the process mechanism better. The reaction of nickel(I1) reduction by hypophosphite was found [ll to proceed by Ni* + 2H,PO;
+ 2D,O?ZHDPO;
+ Ni + 2D++ H,
and the reaction of catalytic hypophosphite expressed as [1,2] H,PO;
+ D,oTHDPO;
0022-0728/92/$05.00
(1)
oxidation by water on nickel could be
+ HD
0 1992 - Elsevier Sequoia S.A. All rights reserved
(2)
248
Using mass spectrometry the reaction of phosphorus formation during electroless plating in acidic medium was determined to have a stoichiometry [3]: 4H,PO,
+ D,O + D+ 53HDPO;
+ P + 2SH,
As a consequence of reactions (l)-(3), light hydrogen hypophosphite prevails in gas evolved in the course of Such an isotopic gas composition is explained, as a rule, point of view [2,4,5]; the electrons generated in the hypophosphite oxidation H,PO;+D,O2HDPO;+H+D++e-
(3) from the P-H bond of electroless nickel plating. from the electrochemical anodic step of catalytic
(4)
are involved in the simultaneous cathodic reactions Ni*++ 2e- TNi D++
e-
(5)
?D
(6)
+ 4D++ 3e- ?P + 2D,O + H,
(7)
and H,PO;
A change in the isotopic gas composition while changing the nickel potential can be expected according to the reactions (4)-(7X The electrochemical mass spectrometry (EMS) technique 161is very convenient for monitoring the occurrence of the electrochemical mechanism of electroless nickel plating. EXPERIMENTAL
Gaseous products were sucked through a porous Teflon membrane (5 pm thickness) and determined mass spectrometrically (MI-1201, mass spectrometer, Russia). A Teflon membrane was covered with a nickel layer about 0.1 pm thick sputtered in vacuum (geometric area, 1 cm*). The potential of the nickel was controlled by a PI-50-l potentiostat (Russia). Two platinum sheets of 1 cm* area each served as counterelectrodes and Ag/AgCl/KCl,, was used as a reference (all potentials are given with respect to a standard hydrogen electrode (SHE)). Experimental details have been described elsewhere [7]. The deuterium content was calculated from [ll D; + HD+/2 D/mol%
=
H; + HD++ D;
100
(8)
where Hi, HD+ and Dl are mass intensities of Hl (m/z = 2), HD+ (m/z = 3) and DC (m/z = 4) in mA. Other volatiles had been condensed with liquid nitrogen before the gases reached the ion source of the mass spectrometer. To avoid the influence of permeability changes which result from the cathodic nickel deposition,
249 TABLE
1
Deuterium content in gas and values of open-circuit potential E, of nickel in electroles solution ([CH,COONa],0.15mol 1-l; pH 4.5; to = WC; solvent,D,O)
[H,PO; l/mol I-’ 0.19 0.19 0.19 0.19 0.10 0.05 0.02 0.01
[Ni*‘]/mol
0.02 0.04 0.08 0.08 0.08 0.08 0.08
1-l
D/mol%
-L/V
36 8 6 5 5 5 6
0.400 0.400 0.375 0.355 0.350 0.345 0.340 0.330
10
nickel plating
each measurement was carried out in the cathodic region on a new sample of nickel sputtered onto the Teflon membrane. The accuracy of the mass spectrometry analysis was f0.5 mol%; the reproducibility was t-5 mol%. The solutions used in experiments were prepared from water-free sodium hypophosphite (prepared according to ref. 2) and sodium acetate. D,O (deuterium content, 99.8 mol%) was used as a solvent. The solutions contained the following: CH,COONa, 0.15 mol 1-l; NaH,PO,, 0.00-0.19 mol 1-l; NiCI,, 0.00-0.08 mol I-‘. Experimental runs were carried out at 80°C and pH 4.5 (the pH was adjusted with CD,COOD). The half-time of the homogeneous hydrogen exchange reaction between hypophosphite and water was found to be about 20 h in these conditions, while recording the mass spectra took about 5 min. RESULTS
AND DISCUSSION
System at open-circuit potential E, As can be seen from the data listed in Table 1, the presence of nickel(U) ions in the solution changes the gas isotopic composition a great deal in comparison with nickel(U)-free solution: the deuterium content drops from 36 to 5-10 mol%. These data are in agreement with those of ref. 1. However, the reason for this sharp change in isotopic hydrogen composition while introducing nickel(I1) has been neither explained nor even discussed. According to electrochemical views the open-circuit potential is mixed. The value of E, is determined by the balance of the rate of the anodic and cathodic reactions. The latter may be expressed as j, =j, +jx
(9) where j, is the current density of anodic hypophosphite oxidation (41, j, the current density of cathodic hydrogen evolution from water (6) and j, the current densities of the rest of the cathodic reactions.
250
The deuterium content in gas evolving from the nickel(H)-free solution at pH 4.5 (Table 1) is lower than that expected from either the overall equation (2) or the partial reactions (4) and (6). It could be explained by the influence of such cathodic reactions (j,) as the reduction of oxidized species on the nickel surface or hypophosphite reduction to phosphorus 171.This difference was found to decrease with time (j, + 0); the deuterium content reached the theoretical value after about 1 h [7]. When nickel(H) was introduced into the system, cathodic reactions (5) and (7) occur in parallel with (61, i.e. j, = jS + j, in this case. It is evident that reaction (5) has no direct influence on the isotopic composition of hydrogen evolved. Such a new light hydrogen source as cathodic reaction (7) should be noted to appear alongside anodic reaction (4). However, taking into account that the total amount of hypophosphite used to deposit 1 mol of nickel is about 2.5 mol [31 (j,= 2.5j,) and the phosphorus content in the nickel deposit obtained in a similar solution reaches 20 mol% [81 (j, = 4j,), one can easily find jq = lOj,, i.e. the part of reaction (7) in the total amount of light hydrogen evolved does not exceed 10%. Therefore the influence of reaction (7) on the isotopic composition of gas evolved under open-circuit conditions can be neglected. In the simplest case of independent electrochemical reactions (4)-(71, the introduction of cathodic reactions (5) and (7) should shift E, to more positive values and the deuterium mole percentage should decrease. However, there is no shift of E, at a low nickel(H) concentration (0.02 mol 1-l) as opposed to the nickel(H)-free solution, while the deuterium content of the gas falls from 36 to 8 mol%. EMS data demonstrate that the rate of light hydrogen formation increases in the presence of 0.02 mol Ni(I1) 1-l when compared with the nickel(H)-free solution at E, (Fig. 1). Such an enhancement of anodic hypophosphite oxidation in the presence of nickel(I1) ions is possibly connected with the fact that the catalytic activity of the constantly renewing nickel surface is higher than that in the nickel(H)-free solution. Evidently, the increase in the rate of reaction (4) compensates the influence of reactions (5) and (7) and the value of E, remains constant at 0.02 mol Ni(I1) l- ‘.
‘;I =:
s +i
0.4
0.2 o c 0
0.1
li2F02/ml
lmi
0.04 [iiicrI)]/m011-i
Fig. 1. Dependence of light hydrogen formation rate on [Ni”] (A) and [H,PO; ] (0) in electroless nickel plating D,O solution at open-circuit potential ([CH,COONa], 0.15 mol I-‘; t = 80°C; pH 4.5).
251
The mutual acceleration of both cathodic (5) and anodic (41 reactions in the process of electroless nickel plating in acetate solutions was established earlier [4] by electrochemical measurements. Nevertheless, the increase in the rate of light hydrogen formation would be insufficient to change the deuterium content in the evolved gas from 36 to 8 mol%, with the reaction (6) rate being constant while nickel(I1) ions were introduced into the solution. Hence, the fall in the rate of deuterium evolution in the presence of nickel(I1) should be expected. The rate of reaction (4) was determined to be independent of nickel(I1) concentration when the latter was over 0.02 mol l-‘, the surface state of the catalyst being similar during electroless plating. The dependence of the light hydrogen formation rate on hypophosphite concentration displayed first-order kinetics up to 0.02 mol I- ‘, and zero-order kinetics over 0.1 mol I-’ (Fig. 1) and E, was found to take more positive values with diminishing hypophosphite concentrations, while the deuterium mole percentage in the gas evolved was low (Table 1). The kinetics of hypophosphite oxidation reaction (4) in electroless plating solution should be expressed according to Fig. 1 as
- d[H,POiI dt
=k[H,PO;]“[Ni’+]‘=
where n = 1 at 0 < [H,PO;] the rate constant.
k[H,PO,]”
d 0.02 or n = 0 at [H,PO,]
(10) > 0.1 mol 1-l and k is
System at controlled potential
The potential dependences of hydrogen isotopic composition (Fig. 2) as well as light and heavy hydrogen formation rates (Fig. 3) give a more detailed picture of the interaction of electrochemical reactions (4)-(7X The deuterium percentage in the gas evolved increases gradually and regularly from about 0.5 mol% at -0.1 V to 96 mol% at - 1.0 V (Fig. 2a) when the nickel potential in the hypophosphite solution is changed; the cathodic deuterium evolutions shows an exponential dependence, and the dependence of anodic oxidation of hypophosphite on potential is more complicated (Fig. 3a) (see also the previous work 171). When nickel(I1) is introduced, a considerable decrease in the deuterium mole percentage in the evolved gas is observed in the cathodic region together with the cathodic current density increase owing to nickel00 reduction both close to an open-circuit potential and in a more negative region; for example, in the presence of a nickel(I1) content of 0.08 mol l-l, the deuterium content in gas drops from 70 to 4 mol% at -0.5 V and decreases from 96 to 55 mol% at - 1.0 V (Figs. 2a-2d). As is obvious from Figs. 3a-3d, the change in the deuterium-to-hydrogen ratio results from the change in the rate of hydrogen formation from both water and hypophosphite; reaction (6) is retarded in the presence of nickel(II1, and hydrogen
1.0
0.5
1.0
0.5 -B/v
Fig. 2. Dependence of current density (0) and deuterium content (0) in evolved hydrogen on nickel potential (to = 80°C; pH 4.5). The D,O solution contained the following: CH,COONa, 0.15 mot 1-l (a-h); NaH,PO, 0.19 mol I-’ (a-d), 0.01 mol I-’ (e), 0.02 mol I-’ (0, 0.05 mol I-’ (g) and 0.10 mol I-’ (h); NiCI,
0.0 mol I-’ (a), 0.02 mol I-’ (b), 0.04 mol I-’ (cl, and 0.08 mol I-’ (d-h).
formation from hypophosphite is accelerated at potentials more negative than -0.35 v. The change in the rate of reaction (6) is particularly great; the presence of 0.08 mol Ni(I1) 1-i shifts the cathodic curve of deuterium formation from water to more negative potential values by about 0.25 V. This change is not related to the changes in solution composition, such as a possible pH decrease at the nickel surfaces [8], because an acceleration of reaction (6) can be predicted in this case. The lowering of the rate of deuterium evolution from water is evident both at open-circuit potential and in the cathodic region with the increase in nickel(I1) concentration (Figs. 3a-3d). The rate of reaction (6) is not affected by the change in hypophosphite concentration (Figs. 3e-3h), although the acceleration of deuterium evolution from water by hypophosphite was found to occur in the nickel(IIl-free solution [71. The above data show the change in the rate of reaction (6) to be related to either the presence of nickel(II) or the reduction of nickel(I1) by reaction (5). It is rather unusual from a common electrochemical point of view and there is no simple explanation in terms of independent parallel cathodic reactions.
253
Fig, 3. Dependence of light (0) and heavy (0) hydrogen formation rate on nickel potential (to = 80°C; pH 4.5). The D,O solution contained the following: NaH,PO, 0.19 mol I-’ (a-d), 0.05 mol 1-l (e), 0.02 mol I-’ (f), 0.01 mol I-’ (g) and 0.0 mol I-’ (h); NiCI, 0.0 mol I-’ (a), 0.02 mol I-’ (b), 0.04 mol I-’ (c) and 0.08 mol I-’ (d-h); CH,COONa, 0.15 mol I-’ (a-h).
The light hydrogen formation is enhanced most markedly in the presence of nickel(H) at potentials close to E,. The “bump” of the light hydrogen formation rate is observed at potentials ranging from -0.5 to -0.35 V provided that the hypophosphite concentration is high enough (Fig. 3). It can be related to the increase in nickel catalytic activity on account of the continual renewal of the catalyst surface in the course of nickel deposition. This probably results in the formation of a Ni-P coating with a favourable structure. The role that phosphorus plays is not quite clear in this case, because there is no direct evidence that Ni-P electrocatalytic activity in reaction (4) is higher in comparison with that of pure nickel. The acceleration of light hydrogen formation at more negative potentials can also be related to some extent to the increase in surface catalytic activity as a result of cathodic nickel deposition. Even so, reaction (7) is probably the most significant source of additional light hydrogen in this case; its rate increases with a shift of the potential to more negative values. The dependence of the reaction (7) rate on the
254
potential is similar to that of nickel deposition; the phosphorus content in Ni-P coatings deposited from acetate solution has been found to be unaffected by the potential in the cathodic region [4]. Somewhat unexpected results were obtained in the presence of nickel(B) in the anodic region. The increase in deuterium mole percentage was observed in the gas evolved in the potential interval from -0.3 to -0.15 V (Figs. 2b-2hl. Both the simultaneous decrease in anodic current density (Fig. 2) and the data on the rate of light and heavy hydrogen formation (Fig. 3) clearly indicate the deceleration of anodic hypophosphite oxidation to be the only reason for the relative increase in the deuterium content in the gas. A sharp decrease in the rate of reaction (4) at more positive potentials in the presence of nickel(H) should be related to the changes in the nickel surface state. A high rate of hypophosphite oxidation in the nickel(B)-free solution (Fig. 3a) can be connected with both the surface renewal and the increase in the area of the surface as a result of anodic nickel dissolution, which proceeds probably with the formation of intermediate oxides and hydroxides of nickel [9] and should depend on the solution pH. Therefore the nickel dissolution is of even greater importance when the solution pH is lower than that investigated in ref. 7. The presence of nickel(B) shifts the anodic nickel dissolution to more positive potentials. The retardation of hypophosphite oxidation by nickel(B) in the potential range from - 0.3 to - 0.15 V can be attributed to the formation of passivating compounds on the nickel surface. The probable reasons for the passivation of Ni-P alloys during anodic dissolution were discussed in ref. 10. They include both the surface enrichment by phosphorus and the oxidation of the surface. The passive layer can be destroyed by a further anodic shift of the potential up to - 0.1 V; this leads to a rise in the anodic current density and the increase in the rate of light hydrogen formation (Figs. 2b-2h and 3b-3e). However, with low (less than 0.05 mol 1-l) hypophosphite concentrations, transition to a non-passive state is impossible (Figs. 3e-3g).
CONCLUSIONS
The overall data obtained by EMS suggest the mechanism of electroless Ni-P deposition to be electrochemical. Simultaneous electrochemical reactions of anodic hypophosphite oxidation and cathodic reduction of nickel(H), hypophosphite and water are expected to proceed with electrons being transferred via the nickel surface. The dependence of the rate of light hydrogen evolution from the P-H bond of hypophosphite on the nickel potential suggests that the rate of anodic hypophosphite oxidation depends on the electrocatalytic properties of the nickel surface. The latter are determined by electrochemical processes proceeding on the nickel surface at an open-circuit potential as well as at both cathodic and anodic potential shifts. This results in the mutual influence of either cathodic and anodic processes or the mutual influence of cathodic processes.
255 REFERENCES 1 2 3 4
A.A. Sutyagina, K.M. Gorbunova and M.P. Glazunov, Zh. Fiz. Khim., 37 (1963) 2022. J.H. Marshall, J. Electrochem. Sot., 130 (1983) 369. Z. Jusys and J. Liaukonis, Zh. Neorg. Khim., 34 (1989) 337. A. VaSkelis and G. KlimantaviEiDtC, Lietuvos MA Darbai (Trans. Litkuanian Academy of Sciences), 2(81) (1974) 33. 5 J.E.A.M. van den Meerakker, J. Appl. Electrochem., 11 (1981) 395. 6 0. Wolter and J. Heitbaum, Ber. Bunsenges. Phys. Chem., 88 (1984) 2. 7 Z. Jusys, J. Liaukonis and A. VaSkelis, J. Electroanai. Chem., 307 (1991) 87. 8 A. VaSkelis, A. Jagminiene and A. Prokoptchik, Surf. Coat. Technol., 27 (1986) 301. 9 M.R. Barbosa, SC. Real, J.R. Vilche and A.J. Arvia, J. Electrochem. Sot., 135 (1988) 1077. 10 R.B. Diegle, N.R. Sorensen, C.R. Clayton, M.A. Helfand and Y.C. Yu, J. Electrochem. Sot., 135 (1988) 1085.
J. Electroanal. Chem., 325 (1992) 247-255 Elsevier Sequoia S.A., Lausanne
JEC 1916
The mechanism of electroless Ni-P deposition studied by electrochemical mass spectrometry Z. Jusys, J. Liaukonis and A. VaSkelis Institute of Chemistry, Lithuanian Academy of Sciences, Goitauto 9, 232600 Vilnius (Lithuania) (Received 15 April 1991; in revised form 1 November 1991)
Abstract
An acetate electroless nickel plating solution containing H,PO; as a reducing agent and D,O as a solvent at pH 4.5 and to = 80°C was studied with the aid of electrochemical mass spectrometry. The isotopic composition of the evolved gas and rates of light and heavy hydrogen formation were determined to depend on electrode potential and on both nickel(II) and hypophosphite concentration. The rate of anodic hypophosphite oxidation was found to increase in the cathodic region and to decrease in the anodic region in the presence of nickel(II). Nickel(I1) was found to have a retarding effect on the cathodic deuterium evolution from water. The data obtained are discussed in terms of an electrochemical mechanism of electroless plating.
INTRODUCTION
The electroless deposition of Ni-P alloys is an important process from both a theoretical and practical point of view. However, a study of it is rather complicated owing to the process being autocatalytic and hypophosphite being used in parallel reactions of nickel(H) reduction, hypophosphite oxidation by water and hypophosphite reduction to phosphorus. A mass spectrometry analysis of evolved gas combined with isotope labelling made it possible to understand the process mechanism better. The reaction of nickel(I1) reduction by hypophosphite was found [ll to proceed by Ni* + 2H,PO;
+ 2D,O?ZHDPO;
+ Ni + 2D++ H,
and the reaction of catalytic hypophosphite expressed as [1,2] H,PO;
+ D,oTHDPO;
0022-0728/92/$05.00
(1)
oxidation by water on nickel could be
+ HD
0 1992 - Elsevier Sequoia S.A. All rights reserved
(2)
248
Using mass spectrometry the reaction of phosphorus formation during electroless plating in acidic medium was determined to have a stoichiometry [3]: 4H,PO,
+ D,O + D+ 53HDPO;
+ P + 2SH,
As a consequence of reactions (l)-(3), light hydrogen hypophosphite prevails in gas evolved in the course of Such an isotopic gas composition is explained, as a rule, point of view [2,4,5]; the electrons generated in the hypophosphite oxidation H,PO;+D,O2HDPO;+H+D++e-
(3) from the P-H bond of electroless nickel plating. from the electrochemical anodic step of catalytic
(4)
are involved in the simultaneous cathodic reactions Ni*++ 2e- TNi D++
e-
(5)
?D
(6)
+ 4D++ 3e- ?P + 2D,O + H,
(7)
and H,PO;
A change in the isotopic gas composition while changing the nickel potential can be expected according to the reactions (4)-(7X The electrochemical mass spectrometry (EMS) technique 161is very convenient for monitoring the occurrence of the electrochemical mechanism of electroless nickel plating. EXPERIMENTAL
Gaseous products were sucked through a porous Teflon membrane (5 pm thickness) and determined mass spectrometrically (MI-1201, mass spectrometer, Russia). A Teflon membrane was covered with a nickel layer about 0.1 pm thick sputtered in vacuum (geometric area, 1 cm*). The potential of the nickel was controlled by a PI-50-l potentiostat (Russia). Two platinum sheets of 1 cm* area each served as counterelectrodes and Ag/AgCl/KCl,, was used as a reference (all potentials are given with respect to a standard hydrogen electrode (SHE)). Experimental details have been described elsewhere [7]. The deuterium content was calculated from [ll D; + HD+/2 D/mol%
=
H; + HD++ D;
100
(8)
where Hi, HD+ and Dl are mass intensities of Hl (m/z = 2), HD+ (m/z = 3) and DC (m/z = 4) in mA. Other volatiles had been condensed with liquid nitrogen before the gases reached the ion source of the mass spectrometer. To avoid the influence of permeability changes which result from the cathodic nickel deposition,
249 TABLE
1
Deuterium content in gas and values of open-circuit potential E, of nickel in electroles solution ([CH,COONa],0.15mol 1-l; pH 4.5; to = WC; solvent,D,O)
[H,PO; l/mol I-’ 0.19 0.19 0.19 0.19 0.10 0.05 0.02 0.01
[Ni*‘]/mol
0.02 0.04 0.08 0.08 0.08 0.08 0.08
1-l
D/mol%
-L/V
36 8 6 5 5 5 6
0.400 0.400 0.375 0.355 0.350 0.345 0.340 0.330
10
nickel plating
each measurement was carried out in the cathodic region on a new sample of nickel sputtered onto the Teflon membrane. The accuracy of the mass spectrometry analysis was f0.5 mol%; the reproducibility was t-5 mol%. The solutions used in experiments were prepared from water-free sodium hypophosphite (prepared according to ref. 2) and sodium acetate. D,O (deuterium content, 99.8 mol%) was used as a solvent. The solutions contained the following: CH,COONa, 0.15 mol 1-l; NaH,PO,, 0.00-0.19 mol 1-l; NiCI,, 0.00-0.08 mol I-‘. Experimental runs were carried out at 80°C and pH 4.5 (the pH was adjusted with CD,COOD). The half-time of the homogeneous hydrogen exchange reaction between hypophosphite and water was found to be about 20 h in these conditions, while recording the mass spectra took about 5 min. RESULTS
AND DISCUSSION
System at open-circuit potential E, As can be seen from the data listed in Table 1, the presence of nickel(U) ions in the solution changes the gas isotopic composition a great deal in comparison with nickel(U)-free solution: the deuterium content drops from 36 to 5-10 mol%. These data are in agreement with those of ref. 1. However, the reason for this sharp change in isotopic hydrogen composition while introducing nickel(I1) has been neither explained nor even discussed. According to electrochemical views the open-circuit potential is mixed. The value of E, is determined by the balance of the rate of the anodic and cathodic reactions. The latter may be expressed as j, =j, +jx
(9) where j, is the current density of anodic hypophosphite oxidation (41, j, the current density of cathodic hydrogen evolution from water (6) and j, the current densities of the rest of the cathodic reactions.
250
The deuterium content in gas evolving from the nickel(H)-free solution at pH 4.5 (Table 1) is lower than that expected from either the overall equation (2) or the partial reactions (4) and (6). It could be explained by the influence of such cathodic reactions (j,) as the reduction of oxidized species on the nickel surface or hypophosphite reduction to phosphorus 171.This difference was found to decrease with time (j, + 0); the deuterium content reached the theoretical value after about 1 h [7]. When nickel(H) was introduced into the system, cathodic reactions (5) and (7) occur in parallel with (61, i.e. j, = jS + j, in this case. It is evident that reaction (5) has no direct influence on the isotopic composition of hydrogen evolved. Such a new light hydrogen source as cathodic reaction (7) should be noted to appear alongside anodic reaction (4). However, taking into account that the total amount of hypophosphite used to deposit 1 mol of nickel is about 2.5 mol [31 (j,= 2.5j,) and the phosphorus content in the nickel deposit obtained in a similar solution reaches 20 mol% [81 (j, = 4j,), one can easily find jq = lOj,, i.e. the part of reaction (7) in the total amount of light hydrogen evolved does not exceed 10%. Therefore the influence of reaction (7) on the isotopic composition of gas evolved under open-circuit conditions can be neglected. In the simplest case of independent electrochemical reactions (4)-(71, the introduction of cathodic reactions (5) and (7) should shift E, to more positive values and the deuterium mole percentage should decrease. However, there is no shift of E, at a low nickel(H) concentration (0.02 mol 1-l) as opposed to the nickel(H)-free solution, while the deuterium content of the gas falls from 36 to 8 mol%. EMS data demonstrate that the rate of light hydrogen formation increases in the presence of 0.02 mol Ni(I1) 1-l when compared with the nickel(H)-free solution at E, (Fig. 1). Such an enhancement of anodic hypophosphite oxidation in the presence of nickel(I1) ions is possibly connected with the fact that the catalytic activity of the constantly renewing nickel surface is higher than that in the nickel(H)-free solution. Evidently, the increase in the rate of reaction (4) compensates the influence of reactions (5) and (7) and the value of E, remains constant at 0.02 mol Ni(I1) l- ‘.
‘;I =:
s +i
0.4
0.2 o c 0
0.1
li2F02/ml
lmi
0.04 [iiicrI)]/m011-i
Fig. 1. Dependence of light hydrogen formation rate on [Ni”] (A) and [H,PO; ] (0) in electroless nickel plating D,O solution at open-circuit potential ([CH,COONa], 0.15 mol I-‘; t = 80°C; pH 4.5).
251
The mutual acceleration of both cathodic (5) and anodic (41 reactions in the process of electroless nickel plating in acetate solutions was established earlier [4] by electrochemical measurements. Nevertheless, the increase in the rate of light hydrogen formation would be insufficient to change the deuterium content in the evolved gas from 36 to 8 mol%, with the reaction (6) rate being constant while nickel(I1) ions were introduced into the solution. Hence, the fall in the rate of deuterium evolution in the presence of nickel(I1) should be expected. The rate of reaction (4) was determined to be independent of nickel(I1) concentration when the latter was over 0.02 mol l-‘, the surface state of the catalyst being similar during electroless plating. The dependence of the light hydrogen formation rate on hypophosphite concentration displayed first-order kinetics up to 0.02 mol I- ‘, and zero-order kinetics over 0.1 mol I-’ (Fig. 1) and E, was found to take more positive values with diminishing hypophosphite concentrations, while the deuterium mole percentage in the gas evolved was low (Table 1). The kinetics of hypophosphite oxidation reaction (4) in electroless plating solution should be expressed according to Fig. 1 as
- d[H,POiI dt
=k[H,PO;]“[Ni’+]‘=
where n = 1 at 0 < [H,PO;] the rate constant.
k[H,PO,]”
d 0.02 or n = 0 at [H,PO,]
(10) > 0.1 mol 1-l and k is
System at controlled potential
The potential dependences of hydrogen isotopic composition (Fig. 2) as well as light and heavy hydrogen formation rates (Fig. 3) give a more detailed picture of the interaction of electrochemical reactions (4)-(7X The deuterium percentage in the gas evolved increases gradually and regularly from about 0.5 mol% at -0.1 V to 96 mol% at - 1.0 V (Fig. 2a) when the nickel potential in the hypophosphite solution is changed; the cathodic deuterium evolutions shows an exponential dependence, and the dependence of anodic oxidation of hypophosphite on potential is more complicated (Fig. 3a) (see also the previous work 171). When nickel(I1) is introduced, a considerable decrease in the deuterium mole percentage in the evolved gas is observed in the cathodic region together with the cathodic current density increase owing to nickel00 reduction both close to an open-circuit potential and in a more negative region; for example, in the presence of a nickel(I1) content of 0.08 mol l-l, the deuterium content in gas drops from 70 to 4 mol% at -0.5 V and decreases from 96 to 55 mol% at - 1.0 V (Figs. 2a-2d). As is obvious from Figs. 3a-3d, the change in the deuterium-to-hydrogen ratio results from the change in the rate of hydrogen formation from both water and hypophosphite; reaction (6) is retarded in the presence of nickel(II1, and hydrogen
1.0
0.5
1.0
0.5 -B/v
Fig. 2. Dependence of current density (0) and deuterium content (0) in evolved hydrogen on nickel potential (to = 80°C; pH 4.5). The D,O solution contained the following: CH,COONa, 0.15 mot 1-l (a-h); NaH,PO, 0.19 mol I-’ (a-d), 0.01 mol I-’ (e), 0.02 mol I-’ (0, 0.05 mol I-’ (g) and 0.10 mol I-’ (h); NiCI,
0.0 mol I-’ (a), 0.02 mol I-’ (b), 0.04 mol I-’ (cl, and 0.08 mol I-’ (d-h).
formation from hypophosphite is accelerated at potentials more negative than -0.35 v. The change in the rate of reaction (6) is particularly great; the presence of 0.08 mol Ni(I1) 1-i shifts the cathodic curve of deuterium formation from water to more negative potential values by about 0.25 V. This change is not related to the changes in solution composition, such as a possible pH decrease at the nickel surfaces [8], because an acceleration of reaction (6) can be predicted in this case. The lowering of the rate of deuterium evolution from water is evident both at open-circuit potential and in the cathodic region with the increase in nickel(I1) concentration (Figs. 3a-3d). The rate of reaction (6) is not affected by the change in hypophosphite concentration (Figs. 3e-3h), although the acceleration of deuterium evolution from water by hypophosphite was found to occur in the nickel(IIl-free solution [71. The above data show the change in the rate of reaction (6) to be related to either the presence of nickel(II) or the reduction of nickel(I1) by reaction (5). It is rather unusual from a common electrochemical point of view and there is no simple explanation in terms of independent parallel cathodic reactions.
253
Fig, 3. Dependence of light (0) and heavy (0) hydrogen formation rate on nickel potential (to = 80°C; pH 4.5). The D,O solution contained the following: NaH,PO, 0.19 mol I-’ (a-d), 0.05 mol 1-l (e), 0.02 mol I-’ (f), 0.01 mol I-’ (g) and 0.0 mol I-’ (h); NiCI, 0.0 mol I-’ (a), 0.02 mol I-’ (b), 0.04 mol I-’ (c) and 0.08 mol I-’ (d-h); CH,COONa, 0.15 mol I-’ (a-h).
The light hydrogen formation is enhanced most markedly in the presence of nickel(H) at potentials close to E,. The “bump” of the light hydrogen formation rate is observed at potentials ranging from -0.5 to -0.35 V provided that the hypophosphite concentration is high enough (Fig. 3). It can be related to the increase in nickel catalytic activity on account of the continual renewal of the catalyst surface in the course of nickel deposition. This probably results in the formation of a Ni-P coating with a favourable structure. The role that phosphorus plays is not quite clear in this case, because there is no direct evidence that Ni-P electrocatalytic activity in reaction (4) is higher in comparison with that of pure nickel. The acceleration of light hydrogen formation at more negative potentials can also be related to some extent to the increase in surface catalytic activity as a result of cathodic nickel deposition. Even so, reaction (7) is probably the most significant source of additional light hydrogen in this case; its rate increases with a shift of the potential to more negative values. The dependence of the reaction (7) rate on the
254
potential is similar to that of nickel deposition; the phosphorus content in Ni-P coatings deposited from acetate solution has been found to be unaffected by the potential in the cathodic region [4]. Somewhat unexpected results were obtained in the presence of nickel(B) in the anodic region. The increase in deuterium mole percentage was observed in the gas evolved in the potential interval from -0.3 to -0.15 V (Figs. 2b-2hl. Both the simultaneous decrease in anodic current density (Fig. 2) and the data on the rate of light and heavy hydrogen formation (Fig. 3) clearly indicate the deceleration of anodic hypophosphite oxidation to be the only reason for the relative increase in the deuterium content in the gas. A sharp decrease in the rate of reaction (4) at more positive potentials in the presence of nickel(H) should be related to the changes in the nickel surface state. A high rate of hypophosphite oxidation in the nickel(B)-free solution (Fig. 3a) can be connected with both the surface renewal and the increase in the area of the surface as a result of anodic nickel dissolution, which proceeds probably with the formation of intermediate oxides and hydroxides of nickel [9] and should depend on the solution pH. Therefore the nickel dissolution is of even greater importance when the solution pH is lower than that investigated in ref. 7. The presence of nickel(B) shifts the anodic nickel dissolution to more positive potentials. The retardation of hypophosphite oxidation by nickel(B) in the potential range from - 0.3 to - 0.15 V can be attributed to the formation of passivating compounds on the nickel surface. The probable reasons for the passivation of Ni-P alloys during anodic dissolution were discussed in ref. 10. They include both the surface enrichment by phosphorus and the oxidation of the surface. The passive layer can be destroyed by a further anodic shift of the potential up to - 0.1 V; this leads to a rise in the anodic current density and the increase in the rate of light hydrogen formation (Figs. 2b-2h and 3b-3e). However, with low (less than 0.05 mol 1-l) hypophosphite concentrations, transition to a non-passive state is impossible (Figs. 3e-3g).
CONCLUSIONS
The overall data obtained by EMS suggest the mechanism of electroless Ni-P deposition to be electrochemical. Simultaneous electrochemical reactions of anodic hypophosphite oxidation and cathodic reduction of nickel(H), hypophosphite and water are expected to proceed with electrons being transferred via the nickel surface. The dependence of the rate of light hydrogen evolution from the P-H bond of hypophosphite on the nickel potential suggests that the rate of anodic hypophosphite oxidation depends on the electrocatalytic properties of the nickel surface. The latter are determined by electrochemical processes proceeding on the nickel surface at an open-circuit potential as well as at both cathodic and anodic potential shifts. This results in the mutual influence of either cathodic and anodic processes or the mutual influence of cathodic processes.
255 REFERENCES 1 2 3 4
A.A. Sutyagina, K.M. Gorbunova and M.P. Glazunov, Zh. Fiz. Khim., 37 (1963) 2022. J.H. Marshall, J. Electrochem. Sot., 130 (1983) 369. Z. Jusys and J. Liaukonis, Zh. Neorg. Khim., 34 (1989) 337. A. VaSkelis and G. KlimantaviEiDtC, Lietuvos MA Darbai (Trans. Litkuanian Academy of Sciences), 2(81) (1974) 33. 5 J.E.A.M. van den Meerakker, J. Appl. Electrochem., 11 (1981) 395. 6 0. Wolter and J. Heitbaum, Ber. Bunsenges. Phys. Chem., 88 (1984) 2. 7 Z. Jusys, J. Liaukonis and A. VaSkelis, J. Electroanai. Chem., 307 (1991) 87. 8 A. VaSkelis, A. Jagminiene and A. Prokoptchik, Surf. Coat. Technol., 27 (1986) 301. 9 M.R. Barbosa, SC. Real, J.R. Vilche and A.J. Arvia, J. Electrochem. Sot., 135 (1988) 1077. 10 R.B. Diegle, N.R. Sorensen, C.R. Clayton, M.A. Helfand and Y.C. Yu, J. Electrochem. Sot., 135 (1988) 1085.