J. Phys. Chem. B 2001, 105, 1701-1704
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Ab Initio Molecular Orbital Study of the Oxidation Mechanism of Hypophosphite Ion as a Reductant for an Electroless Deposition Process Hiromi Nakai,*,† Takayuki Homma,*,‡ Isao Komatsu,‡ and Tetsuya Osaka‡ Departments of Chemistry and Applied Chemistry, Waseda UniVersity, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan ReceiVed: May 17, 2000; In Final Form: December 6, 2000
The oxidation (electron emission) mechanism of a hypophosphite ion (H2PO2, which is a representative reducing agent for an electroless deposition process, was studied by an ab initio molecular orbital method. Two types of reaction pathways were examined; namely, the pathway via three-coordinate compound obtained by primary dehydrogenation, and the one via five-coordinate compound by primary addition of OH-. The calculated energy profile showed that the oxidation reaction occurs via five-coordinate compounds. The solvation effect is clarified to make the reaction endothermic, indicating that the reaction preferably proceeds at the solid/ liquid interface, i.e., the surface of the deposits, rather than in the solution bulk. The catalytic activity of the metal surface, which is one of the most significant factors for the electroless deposition process, was also investigated using Pdn (n ) 4-7) clusters as a model surface. It was found that one of the most important characters determine the catalytic activity of the deposited metal is the electron-accepting ability from the reductant.
1. Introduction Electroless deposition processes have been used widely in advanced electronic devices as well as various fields of industries featuring their capability to form metal thin films onto nonflat and nonconductive surfaces.1 Unlike the conventional electrodeposition processes, the catalytic oxidation of the reductant on the surface is the source of electrons for the metal deposition. Therefore, it is significant to understand the reaction mechanism in order to achieve further precise control of the total reaction processes. Although numbers of studies have been conducted on the electroless deposition processes, mainly by electrochemical2 and kinetic3 approaches, it has been difficult to study the intermediates and transition species of the oxidation reaction in detail only using the experimental approaches. On the other hand, we have employed the ab initio molecular orbital (MO) method to investigate the oxidation reaction process of dimethylamine borane (DMAB), which is a representative reductant for the electroless deposition;4 one of the most important results in the previous study was to clarify a new oxidation reaction pathway via five-coordinate complexes. In the present study, we theoretically investigate the oxidation mechanism of a hypophosphite ion (H2PO2-), which is also used widely as well as DMAB for the electroless deposition. Furthermore, we examine the effects of the solvation and the metal surface for the oxidation reactions. 2. Computational Method The oxidation of a reductant is generally proceeded by the substitution between H• of the reductant and OH- in the solvent. Based upon our previous result,4 two oxidation reaction pathways for the hypophosphite ion were assumed as shown in Figure 1. * To whom correspondence should be addressed. E-mail: nakai@ mn.waseda.ac.jp,
[email protected]. † Department of Chemistry. ‡ Department of Applied Chemistry.
Figure 1. Reaction pathways via three-coordinate complex (2) and five-coordinate complexes (3).
The first pathway via a three-coordinate complex (HPO2-(2)) corresponds to the widely accepted mechanism proposed by Meerakker.3 The second one via a five-coordinate complex (H2PO2(OH)2- (3′) and H2PO2(OH)- (3)) corresponds to the new mechanism proposed by our previous study for the DMAB reductant.4 Since the electroless deposition reaction takes place at the solid/liquid interface, the reaction is influenced by a number of factors such as solvation effect and catalytic activity of the deposited metal. In this study, we examine the oxidation reaction of the hypophosphite ion not only in the gas phase (or in the nonpolar solvation) but also in the polar solvation, as well as on the metal surface. The solvation effect is taken into account by the self-consistent reaction field method with an isodensity surface polarized continuum model (SCRF-IPCM),5 which uses dielectric constant. In the IPCM calculations, the dielectric constant is changed from 1.0 (in the nonpolar solvent) to 78.3 (in the pure water of the bulk solvent6). As for a model surface for the metal, we use Pdn (n ) 4-7) clusters for investigating the catalytic activity of the metal surface to the oxidation reaction of the reductant. The cluster model is based on the locality of the catalytic activity, which we used for investigating the catalytic activity of the Pd surface to the oxidation reaction of the reductant. Since the electron transfer property and the energetics depend on the size of the cluster, we examine this point by using Pdn (n ) 4-7) cluster. In general, the geometry of the adsorbate is not sensitive for the cluster size, although the effect of the surface geometry such as 2D periodicity and metal film thickness on the catalytic
10.1021/jp001816p CCC: $20.00 © 2001 American Chemical Society Published on Web 02/13/2001
1702 J. Phys. Chem. B, Vol. 105, No. 9, 2001
Figure 2. Energy profile of the oxidation reaction of the hypophosphite ion.
activity was not investigated in the present work using relatively small clusters. The geometry optimizations at the Hartree-Fock (HF) level are used for all intermediates along the two reaction pathways, i.e., via three-coordinate and via five-coordinate in the gas phase, in the solvation, and on the metal surface. The electron correlation effects are taken into account by the second-order Møller-Plesset perturbation (MP2) method. The 6-31G** Gaussian basis sets7 were used for hydrogen, oxygen, and phosphorus atoms: diffuse s- and p-type functions of ξ ) 0.08458 are augmented for oxygen. For palladium, Kr core is replaced by an effective core potential by Hay and Wadt9 and a valence electron is represented by a (5s5p4d)/[3s3p2d] basis set. All molecular orbital method calculations were performed using the Gaussian9810 software package. 3. Results and Discussion We first examine the oxidation reaction of the hypophosphite ion in a gas phase. Figure 2 shows the optimized geometries of the intermediates and the energy profiles for the oxidation process in which the energy is shown in kcal/mol with respect to that of H2PO2- (1). The reaction proceeds from left to right. Two energy levels correspond to the two reaction pathways via either three- or five-coordinate intermediate shown in Figure 1. The total reaction heat is slightly exothermic from H2PO2-(1) to H2PO3- (4), namely, 2.9 kcal/mol. Recombination of produced hydrogen radicals increases the exothermically of the reaction. Formations of the intermediates, HPO2-(2) and H2PO2(OH)- (3), along the two reaction pathways occur endothermically. However, the energy level of the threecoordinate HPO2-(2) is about 50 kcal/mol higher than that of the five-coordinate H2PO2(OH)- (3). Therefore, the present calculations show that the oxidation reaction of H2PO2-(1) occurs via the five-coordinate intermediate rather than via the three-coordinate one. This result is similar to the oxidation mechanism of another reductant, DMAB, which has been theoretically studied by us.4 Figure 2 also shows the energy levels of the dianion intermediates, H2PO2(OH)2- (3′) and H2PO32-(4′), which are led by the addition of OH- to H2PO2- (1) and HPO2- (2), respectively. Since these dianion species are considerably unstable, it is expected that one electron is emitted immediately, producing the corresponding monoanion species, H2PO2(OH)(3) and H2PO3- (4). Next, we examine the solvation effect with the use of the SCRF-IPCM method, focusing upon the reaction heat (Q) defined by
H2PO2-+ OH- f H2PO3- + 1/2H2 + e- + Q (kcal/mol)
Nakai et al.
Figure 3. Reaction heat of the oxidation reaction of the hypophosphate ion in the solution.
Figure 4. Four kinds of adsorbed geometries of H2PO2- (1) on the Pd4 cluster “b in (b) and (d) represents that the center of gravity of the regular triangle. (a) On-top, H-side, (b) hollow, H-side, (c) on-top, O-side, and (d) hollow, O-side.
which is calculated to be 53.5 kcal/mol in the gas phase. The value of Q decreases exponentially as the dielectric constant () becomes higher; namely, from +53.5 kcal/mol at ) 1.0 to -30.0 kcal/mol at ) 78.3. Figure 3 shows the change of Q by taking the solvation effect into account. In this figure, we show two levels corresponding to the dielectric constants of the inner and outer Helmholtz layers, which are estimated by the Bockris-Devanathan-Mu¨ller model;11 that is, ∼ 6.0 and 32.0, respectively. The results in this figure show that only reaction in the inner layer is exothermic and those in the outer layer and in the bulk solution are endothermic. Thus, the oxidation of the reductant is expected to occur near or on the metal surface. Finally, we investigate the catalytic activities of the metal surfaces. Palladium clusters, Pdn (n ) 4-7), are adopted for the model, since palladium is a representative catalyst for the electroless deposition processes. The cluster geometry is fixed in the Pd (111) crystal lattice position with a Pd-Pd distance of 2.751 Å.6 The first step of the catalytic reaction is an adsorption of the reductant. Four kinds of adsorbed geometries are investigated as illustrated in Figure 4. The adsorption energies on the oxygen side are 16.5 kcal/mol smaller than that on the hydrogen side. The difference between the on-top site and the hollow site is only 3.0 kcal/mol. As a result, the most stable geometry is the adsorption of the oxygen side at the hollow site (Figure 4d). Figure 5 shows the optimized geometries of the intermediates and the energy profile for the oxidation reaction on the Pd4 cluster. Following the results described above, all intermediates are assumed to be adsorbed at the hollow site by two oxygen atoms. Two energy levels correspond to the two reaction pathways via three- and five- coordinate intermediates. As is
Hypophosphite as Reductant for Electroless Deposition
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Figure 5. Energy profile of the oxidation reaction of hypophosphite ion on the Pd4 cluster.
Figure 6. Effect of the Pd cluster size on instability of adsorbed H2PO2(OH)2- (3′) with respect to adsorbed H2PO2-(1). Structures of Pd clusters are also shown. Cluster size 0 means the gas phase.
seen in Figure 5, it is expected on the Pd4 cluster that the reaction via five-coordinate intermediate is favorable, as was expected from the result for the case of the gas-phase reaction. The time-consuming step in Figure 5 is the coordination of OH- to the adsorbed hypophosphate ion, as was also shown in Figure 2. Here, the adsorbed species of the intermediates are expressed by adding the subscript “ad”. Note that the energy difference between H2PO2-(1ad) and H2PO2(OH)2-(3′ad) on the Pd cluster is much smaller than that in the gas-phase reaction; that is, 61.0 and 117.8 kcal/mol, respectively. Since 3′ and 3′ad are not the transition states, the energy differences do not correspond to the activation energy. However, the great decrease of the values from 117.8 to 61.0 kcal/mol suggests that the oxidation reaction of the reductant is catalyzed by the Pd4 cluster. This great stability is caused by the electron transfer of the dianion species H2PO2(OH)2- (3′) to the Pd4 cluster. In fact, the negative charge of the adsorbate at 3′ad is reduced from -2.00 (in the gas phase (3′)) to -1.53. On the other hand, the
negative charge of -1.00 at 3 changes to -0.78 at 3ad. This difference is also seen in the geometry change. The geometry change from 3 to 3ad is smaller than that from 3′ to 3′ad. For example, the P-O distance changes 1.53 to 1.48 Å at 3′ to 3′ad, whereas that is 1.56 Å both for 3 and 3ad. Figure 6 shows effect of the Pd cluster size of the instability of adsorbed 3′ad with respect to 1ad as well as the number of electron transferred from H2PO2(OH)2-(3′) to Pdn. With an increase in the cluster size, the transferred electron becomes larger and the instability becomes smaller. This indicates that the “catalytic activity” is of the Pd surface to the oxidation reaction due to its electron-accepting ability. 4. Conclusion We have studied the oxidation mechanism of a hypophosphite ion, which is a representative reductant for the electroless deposition process, by using the ab initio MO method. The pathway of its oxidation process was investigated from the
1704 J. Phys. Chem. B, Vol. 105, No. 9, 2001 elementary reaction level, and it was found that the reaction proceeds via five-coordinate complexes, in which OH- coordinates to P before the elimination of H radical, rather than via three-coordinate complexes. This pathway corresponds to that of the reaction of dimethylamine borane (DMAB),4 which is another representative reductant. These results suggest that the reaction pathway via five-coordinate intermediate, in which the electron is emitted after the coordination of OH- could be a general mechanism to the reductant species for the electroless deposition process. It is also suggested that the reaction preferably proceeds at the solid/liquid interface, that is the surface of the deposited metals, rather than in the “bulk” solution. Furthermore, it is expected that the reaction is stabilized considerably at the surface of metal such as Pd due to its ability to accept electron, which enhances the electron-emitting reaction of the reductant. These results demonstrate that the approach using the MO has capability to elucidate the significant (but still unclear) factors of the electroless deposition reaction such as the “catalytic activity” of the metal surface, in theoretical quantitative basis for the first time. Acknowledgment. This work was financially supported in part by Grant-in-Aid for Scientific Research for Encouragement of Young Scientists, the Ministry of Education, Science, and Culture, and also by the Kawakami Memorial Foundation.
Nakai et al. References and Notes (1) Mallory, G. O.; Hajdu, J. B. Electroless Plating, Fundamentals and Applications; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990. (2) Ohno, I.; Wakabayashi, O.; Haruyama. S. J. Electrochem. Soc. 1985, 132, 2323. (3) Van den Meerakker, J. E. A. M. J. Appl. Electrochem. 1981, 11, 397. (4) Homma, T.; Nakai, H.; Onishi, M.; Osaka, T. J. Phys. Chem. B 1999, 103, 1774. (5) Foresman, J. B.; Keith, T. A.; Wiberg, K. B.; Snoonian, J. M.; Frisch, J. J. Phys. Chem. 1996, 100, 16098. (6) The Chemical Society of Japan. Kagaku-binran 1984. (7) Hehre, W. J.; Random, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (8) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (9) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998. (11) Bockris, J. O’M.; Devanathan, M. A. V.; Mu¨ller, K. Proc. R. Soc. (London) 1963, A274, 55.
