1230
Langmuir 1992,8, 1230-1231
Notes Mechanism of Copper(I1) Reduction by Formaldehyde Studied by On-Line Mass Spectrometry
Cu2++ 2CD,O
Zenonas Jusys' and Algirdas Vagkelis Institute of Chemistry and Chemical Technology of Lithuanian Academy of Sciences, Goitauto 9, 232600, Vilnius, Lithuania Received April 16, 1991. In Final Form: October 22, 1991
Introduction The main reaction of the widely used electroless copper plating process is the autocatalytic reduction of copper(11) ions by formaldehyde. A somewhat unusual feature of this reaction is the formation of molecular hydrogen along with the metallic copper. The reaction stoichiometry as determined in' and confirmed by other researchers in a simplest form (taking into account neither the formation of methylene glycol and the dissociation of it in alkaline medium nor the chelation of Cu2+ in the presence of complexing agent) may be expressed as cu Cu2++ 2CH,O + 40H- Cuo + 2HC00- + 2H,O + H,
-
(1)
The mechanism of this reaction is assumed by most of the researchers to be an electrochemical one: two partial reactions, anodic oxidation of formaldehyde cu 2CH,O + 40H- 2HC00- + 2H,O + H, + 2e- (2) and cathodic reduction of Cu(I1) cu CU'+ + 2eCuo (3) are expected to proceed simultaneously on the copper surface, with electrons being transferred via metal. The electrochemical mechanism was proved experimentally in the 1960~,-~by means of electrochemical modeling of reaction 1. Later studies showed the above electrochemical half-reactions to interact; i.e., the acceleration of cathodic copper deposition by anodic formaldehyde oxidation was found to take place together with the acceleration of the latter by copper cathodic reductiona5t6 The origin of hydrogen evolving in reactions 1 and 2 could be determined using a deuterium tracer. Using the isotope labeling, hydrogen was found to evolve entirely from the C-H bond of formaldehyde during the anodic oxidation of it on ~ i l v e r .Thus, ~ formaldehyde anodic oxidation on Cu may be expected to proceed by
-
-
cu
2CD,O + 40H- 2DC00- + 2H,O + D, + 2e- (2a) The overall reaction of copper reduction by formaldehyde in this case should be as follows:
0743-746319212408-1230$03.00/0
cu
Cuo + 2DC00- + 2H,O
+ D,
(la) However, the gas evolved during electroless copper deposition in CD2O solution was determined in the recent worke to contain more than 99 96 of HD. The mechanism was suggested,8which explained the hydrogen formation in equal parts from formaldehyde and water. Therefore, a further investigation of electroless copper deposition is necessary. It seemed to us of great interest to apply an on-line mass spectrometric analysiss for studying this process under various conditions in the range of real electroless copper plating solutions, including also those of ref 8. The feasibility of differential electrochemical mass spectrometry (DEMS)lO for investigating the formaldehyde anodic oxidation on Au electrode was demonstrated in the recent work." Experimental Section Solutions and Materials. Two series of experimental runs were carried out in electroless copper plating solution, containing (mol L-') CuS04(0.04), EDTA (0.04), and CDzO (or CH20) (0.08) a t pH 12.0 or 12.5 and t o = 20 or 70 "C. One of them contained deuterated ("heavy") formaldehyde (CDzO) (deuterium content 98 mol % ) as a reducing agent, while "light" water (H20) as well as other materials including light hydrogen was used. The other contained CH20 and D20 (D content 99.8 mol 72 ) as a solvent as well as water-free salts of CuSO4 and tetrasodium ethylenediaminetetraacetate; pH was adjusted with NaOD (Dcontent 99 mol 5%) in this solution. In both cases paraformaldehyde was used for the solution preparation. Solutions were deaerated with Ar. Analysis of the Evolved Gas. A mass spectrometer MI1201 (USSR) was used for the conducting isotopic analysis of the gas evolved, which was sucked through a porous Teflon membrane (thickness 5 pm) into the ion source of the mass spectrometer and detected on-line. Electroless copper plating was carried out on the outer side of the membrane covered with a -0.1-pm copper layer sputtered in vacuum (the geometric area of 1 cm2). Electroless copper plating was initiated by a cathodic switch of potential to -0.6 V (vs Ag/AgCl/KCl saturated electrode) for 1-2 S.
The experimental setup used for the on-line mass spectrometric gas analysis was the same as in ref 12. Deuterium content was calculated by the equation Dmol% =
+
I(D2+) I(HD+)/2 I(H:)
+ I(HD+) + I(D:)
100
where I(Hz+), I(HD+), and I(Dz+) are mass intensities of Hz+ (m/z = 2), HD+ (miz = 3), and Dz+ ( m / z = 4) (PA). Other volatiles had been cooled with liquid nitrogen before the gases reached the ion source of the mass spectrometer. Isotopic gas composition had been measured on 3-5 copper electrodes for 20 min, and the mean value was calculated. An accuracy of mass spectrometric analysis depended on mass intensities and was ranging from *0.2 to f0.5 mol % . The values of mass intensities presented were detected immediately after (8) Ogura, T.; Malcolmson, M.; Fernando, Q. Langmuir 1990,6, 1709. (9) Bruckenstein, S.; Gadde, R. R. J . A m . Chem. SOC.1971, 93, 793. (10) Wolter, 0.;Heitbaum, J. Ber. Bunsen-Ges. Phys. Chem. 1984,88,
(1) Lukes, R. M. Plating 1964, 5 1 , 1066. (2) Saito, M. J . Met. Finish. SOC.J p n . 1965, 26, 300.
(3) Vaikelis, A.; Salkauskas, M. Liet. T S R Mokslu Akad. Darb. 1967, 8 4 (51), 3. (4) Paunovic, M. Plating 1968, 5 5 , 1161. ( 5 ) VaHkelis, A.; JaEiauskiene, J. Elektrokhimiya 1981, 17, 1816. (6) Wiese,H.; Wei1,K. G . B e r . Bunsen-Ges.Phys. Chem. 1989,91,619. (7) Hoyer, H. 2. Naturforsch. 1949, 4a, 335.
+ 40H-
2. (11) Baltruschat, H.; Anastasijevic, N. A.; Beltowska-Brzezinska, M.; Hambitzer, G.; Heitbaum, J. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 996. (12) Jusys, 2.; Liaukonis, J.; Vaikelis, A. J . Electroanal. Chem. 1991, 307. 87.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 4, 1992 1231
Notes Table I. Mass Intensities of I&+, HD+,and Dz+and Isotopic Composition of Gas Evolved during Electroless Copper Plating. expt no. ~
1
2 3 4 5 6 7 8
DH 12.0 12.0 12.5 12.5 12.0 12.0 12.5 12.5
t.OC 20 70 20 70 20 70 20 70
I(Hz+), I(HD+), I(Dz+), uA MA LLA 9.30 0.04 0.34 1.00 24.50 0.15 17.25 0.10 0.60 46.00 0.30 2.00 0.04 0.00 9.30 0.01 24.75 0.06 0.00 17.80 0.05 0.02 46.10 0.20
D, mol% 97.8 f 0.5 97.6f 0.3 97.7 f 0.4 97.7 0.2 0.5 f 0.5 0.4 f 0.3 0.4f 0.4 0.2 f 0.2
*
0 Solution contained (mol L-l) CuSO, (0.04), EDTA (0.04),and formaldehyde (0.08). Experiments 1-4 were carried out in H2O solution containing CDzO (D content 98 mol 5%) and 5-8 in D20 (D content 99.8 mol % ) solution containing CHz0.
initiating the electroless plating because of membrane permeability changes during copper deposition.
Results and Discussion Our results showed all the amount of gas evolved during electroless copper deposition to originate from formaldehyde (Table I). These results were obtained in the solution containing heavy formaldehyde and light water as well as in the "mirror" system of light formaldehyde and heavy water. Isotopic gas composition was determined to be the same as that of formaldehyde used for copper(I1) reduction; i.e., no excess of water hydrogen isotope was detected in the gas as compared to formaldehyde within the measurement error. These data are consistent with the electrochemicalmechanism of electrolesscopper plating. In the case when heavy formaldehyde and light water are used, reaction l a is the total reaction of anodic and cathodic half-reactions 2a and 3. In the other case, when light formaldehyde and heavy water are used, reaction 1 may be expressed as Cu2++ 2CH,O
+ 40D-
cu
Cuo + 2HC00-
+ 2D,O + H,
(1b) and reaction of anodic formaldehyde oxidation may be
written as
2
+
2CH,O + 40D- 2HC00- + 2D,O + H, 2e- (2b) According to our recent datal3 obtained by DEMS combined with isotope labeling, anodic formaldehyde oxidation on copper in Cu(I1)-free solution also results in hydrogen evolution from formaldehyde alone as could be expected from reactions 2a and 2b. It should be noted that anodic formaldehyde oxidation reaction 2a,b is expected to be not elementary but a complex one. The most popular reaction scheme14J5 includes a primary step of catalytic formaldehyde dehydrogenation as a result of its dissociative adsorption on Cu and disruption of C-D (or C-H) bond with the formation of labile formyl: cu DCDO,, Dad + DCO,, (4) Recombination of hydrogen adsorbed causes the molecular hydrogen to evolve. The metastable formyl is anodically oxidized: cu DCO,, 20H- DCOO- H,O e(5) The data obtained are consistent with this scheme of reactions. The data may be concluded to confirm the electrochemical mechanism of electroless copper(I1) reduction by formaldehyde. However, they are conflictingwith those of ref 8. Such a disagreement may be explained by the preparation for analyzing the samples of gas evolved,8 including the gas elution through an iron-coated alumina column and the conversion of it into water over CuO at high temperature. Possibly, deuterium exchange with adsorbed light water took place on the iron-coated alumina prepared apparently in a light water solution. Therefore, the data obtained may be distorted, and the mechanism proposed8 on the basis of these data is a doubtful one. Registry No. Cu, 7440-50-8;CH20, 50-00-0;Hz, 1333-74-0; CD20,1664-98-8;DzO,7789-20-0.
-
+
-
+
+
(13)Jusys, 2.;Vaikelis, A. Submitted for publication in J . Electroanal. Chem.
(14)Meerakker van den, J. E. A. M. J. Appl. Electrochem. 1981,11, 395. (15)Buck, R. P.; Griffith, L. R. J.Electrochem. SOC.1962,109,1005.
