Pd Nanoparticles with

A three-stage formation mechanism of Cu/Pd nanoparticles was proposed in which a Pd reduction ... had good stability and were well dispersed with part...
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12873

2007, 111, 12873-12876 Published on Web 08/14/2007

A Mechanism Study on the Synthesis of Cu/Pd Nanoparticles with Citric Complexing Agent Sylvia H. Y. Lo,*,† Tsan-Yao Chen,‡ Yung-Yun Wang,† Chi-Chao Wan,† Jyh-Fu Lee,§ and Tsang-Lang Lin‡ Department of Chemical Engineering, National Tsing Hua UniVersity, Hsinchu 300, Taiwan, ROC, Department of Engineering and System Science, National Tsing Hua UniVersity, Hsinchu 300, Taiwan, ROC, and National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, ROC ReceiVed: June 28, 2007; In Final Form: August 3, 2007

We have previously synthesized Cu/Pd nanoparticles with a citric complexing agent, demonstrating well the suspension and high catalytic ability of electroless copper deposition. Herein, we report the in situ investigation of the synthesis of Cu/Pd nanoparticles with a citric complexing agent by X-ray absorption near-edge structure (XANES). By characterizing the XANES spectra of Cu and Pd upon the stepwise addition of an alkaline solution, the reaction mechanism of Cu as well as Pd complexing ions was elucidated. Slow reduction of Pd ions and fast reduction of Cu ions induced by zerovalent Pd are found in XANES spectra. A three-stage formation mechanism of Cu/Pd nanoparticles was proposed in which a Pd reduction initial stage, a Cudominated reduction middle stage, and a Pd-dominated reduction final stage were indicated. As a result, a Pd-rich outer shell formed on the surface of synthesized Cu/Pd particles in the final stage. In summary, the formation mechanism and the Pd-rich outer shell structure of synthesized Cu/Pd nanoparticles were found in this citric complexing agent synthesis method.

Bimetallic nanoparticles have long been of interest in the perspectives on their unique catalytic properties involving enhanced activity, selectivity, and stability. The applications of bimetallic nanoparticles include Pt-based1-3 and Pd-based4,5 catalysts for fuel cells, Pd catalyst for electroless copper deposition,6-11 and Pd/Fe,12-14 Pd on Au,15 and Pd16,17 catalyst for groundwater treatment. The structure of bimetallic nanoparticles was found to affect the catalytic ability;18 thus, the method to synthesize bimetallic nanoparticles of desired structure is crucial. We have developed a method with a complexing agent19 to synthesize Cu/Pd nanoparticles in which sodium citrate was added as the complexing agent to the metal precursors. The obtained Cu/Pd nanoparticles had good stability and were well dispersed with particle sizes between 3 and 4 nm. The Cu/Pd nanoparticles were also found to possess excellent catalytic ability for electroless copper deposition in contrast to the noncomplexing agent system which had no catalytic ability. Therefore, the function of the complexing agent in the synthesis of Cu/Pd nanoparticles seems to be more than stability improvement as initially designed and seemingly also affects the reduction of metal ions. An in situ characterization during the formation of nanoparticles is essential to elucidate the mechanism of this synthesis method and the actual role of the added complexing agent. * To whom correspondence should be addressed. Address: Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang Fu Road, Hsinchu 300, Taiwan, Republic of China. E-mail: [email protected]. Tel: 886-03-5715131-33659. Fax: 886-035715408. † Department of Chemical Engineering, National Tsing Hua University. ‡ Department of Engineering and System Science, National Tsing Hua University. § National Synchrotron Radiation Research Center.

10.1021/jp075057n CCC: $37.00

X-ray absorption spectroscopy (XAS) is a very powerful in situ method for investigating nanoparticles. Although a number of studies have been done on the structure of bimetallic nanoparticles by using extended X-ray absorption fine structure (EXAFS),20-23 the mechanism of the related synthesis reaction was rarely discussed. Only very few works have been done with the aid of X-ray absorption near-edge structure (XANES) on related aspects,24,25 but no clear mechanism has been indicated. In fact, by understanding the reduction mechanism of metal ions, the rough structure of bimetallic nanoparticles can also be inferred without complicated EXAFS studies. In this study, XANES was employed to study the behavior of the complexing agent and the change in valence state of both Cu and Pd ions during particle formation. From examining the degree of change in the XANES spectrum, the reaction route of both metal ions and the process for the genesis of Cu/Pd nanoparticles can be unfolded. The Cu K-edge XANES of Cu citric complexing ions (noted as CuCit below) shows a characteristic feature of a divalent Cu tetragonal symmetry (i.e., four-coordinated Cu(II)) in Figure 1a. Besides the weak 1s f 3d absorption feature at 8979 eV, the CuCit exhibits only a broad absorption in the region below 8985 eV.26-28 After addition of the protecting agent PVP into the CuCit solution, the Cu XANES is similar to that of CuCit, indicating that adding PVP into the CuCit solution has no effect on the stability of the CuCit complexing ions and that the Cu ions are well-protected by the complexing agent. In this experiment, the metal precursors palladium nitrate and copper sulfate pentahydrate in a molar ratio of 2/1 as well as the citric complexing agent were mixed with PVP and reduced by formaldehyde in alkaline solution in a beaker with constant © 2007 American Chemical Society

12874 J. Phys. Chem. C, Vol. 111, No. 35, 2007

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Figure 1. (a) Cu K-edge XANES of a Cu foil reference, a CuO reference, a Cu2O reference, Cu citric complexing ions, and Cu citric linked with PVP; (b) Cu K-edge XANES with progressive addition of alkali solution.

stirring. The fast oxidation of formaldehyde took place under alkaline conditions with the consumption of the alkali solution and achieved a “quenched state” at pH ) 6 with no further oxidation of formaldehyde, that is, no reduction of the metal ions. Thus, we could slow down and quench the reduction of metal ions by the sequential addition of insufficient alkali solution (relative to the molar number of metal ions) six times so that six quenched states were obtained for the XANES measurements. To avoid the changing of stoichiometry and acquire data identical to those of our previous work,19 the amount of alkaline solution added was exactly the same as that in our previous study. At each quenched state, a small amount of liquid sample was taken into a transparent zipper bag to do the XANES measurement. The detailed experimental method is given in Supporting Information. The commonly used sampling method for XAS, that is, precipitation of nanoparticles from solution, was not considered here since a much longer needed sample preparation time would make the XANES measurements ineffective and may not reflect the real condition during the reduction of metal ions. Thus, the investigation of XANES in this work mainly focused on the qualitative analysis of changes in the valence state. More detailed particle structural information should be given by the curve fitting from EXAFS data. Unfortunately, our dilute system made it difficult to acquire EXAFS data, and the EXAFS analysis is beyond the scope of this work. The XANES spectra in Figure 1b show six stages with quenched states obtained at the end of each alkali addition along with the initial stage before alkali addition. The Cu K-edge XANES changed at a discordant pace, slowly at the beginning (red line) and more quickly in the middle of reaction (green and blue lines), and then reached the steady state (cyan, magenta, and wine lines). At the initial stage, the absorption above 8987 eV slightly moved to a lower energy, and the appearance of an absorption at 8979 eV suggests the consumption of CuCit tetragonal ions and the formation of zerovalent Cu. With

continuous alkali addition, absorption at 8979 eV increased rapidly, indicating the reduction of a large amount of CuCit ions to zerovalent Cu. There was no difference between the fifth and sixth quenched states, indicating no further change in the Cu state. The final Cu state displayed similar characters to that of zerovalent Cu. The absorption resonances of Cu in Cu/Pd nanoparticles samples were not as clear as those in the Cu foil, implying the formation of the nanostructured zerovalent Cu29 (with possible existence of some residual divalent Cu) in the system. CuO is not considered to exist in this system since no absorption at 8984 eV was observed. Pd K-edge XANES spectra of Pd citric complexing ions (noted as PdCit below) were recorded with a Pd foil as the reference. In Figure 2a, the shape of the Pd K-edge spectrum of PdCit showed no difference with or without the addition of PVP, indicating good protection of the Pd ions by the complexing agent. The Pd K-edge spectra were recorded using the same samples as those prepared by the quenching method for the Cu K-edge measurement. In the detection of the Pd K-edge, we took two more measurements than at the Cu K-edge because PdCit ions showed a slower reaction rate. With the successive addition of alkali solution, the intensity of the broad peak between 24360 and 24380 eV gradually decreased, and the peak shape changed progressively to resemble that of Pd foil. Two small absorption resonances similar to that of Pd foil were observed at the eighth quenched stage (red line in Figure 2b). The observation of the Pd K-edge XANES could illustrate the continuous slow conversion of PdCit ions to zerovalent Pd by the reducing agent, formaldehyde. The mechanism of Cu/Pd nanoparticle formation can also be conjectured from the visible observation of CuCit and PdCit complex ions. From the visible observation, CuCit can only be reduced by formaldehyde under alkaline conditions in the presence of Pd colloid, the CuCit reduction being triggered by the zerovalent Pd. Thus, the formation of a pure Cu nanoparticle

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Figure 2. (a) Pd K-edge XANES of a Pd foil reference, Pd citric complexing ions, and Pd citric linked with PVP; (b) Pd K-edge XANES with progressive addition of alkali solution.

Figure 3. Three stages during the formation of Cu/Pd nanoparticles: (a) the initial reduction, (b) the CuCit-ions-dominated reduction, and (c) the final PdCit ions reduction.

is not considered since CuCit can only be reduced on the surface of zerovalent Pd. The citric complex agent not only prevented the formation of copper hydroxide but altered the equilibrium potential of divalent Cu.30 Moreover, the citric complexing agent also altered the equilibrium potential of divalent Pd; thus, the reduction rate of complex Pd ions was slower than the free divalent Pd ions without complexing agent. The reduction of PdCit was slow, and the absorption resonance similar to Pd foil did not appear until the eighth addition of alkali solution. The pure Pd nanoparticle is also beyond our consideration since the reduction of CuCit on the Pd surface is so fast both in XANES and in visible observation. Any existing Pd surface can be a reactive site to catalyze the reduction of Cu, and this is exactly the basic idea of electroless copper deposition. Furthermore, the high activation energy of PdCit reduction can be inferred from the slow reduction rate, and this high activation energy may make Pd ions prefer to reduce on an existing particle.

Therefore, the pure Pd nanoparticle is not considered even after the consumption of CuCit. From visible and XANES observations, there are factors interworked in the synthesis of Cu/Pd nanoparticles with the citric complexing agent, fast reduction of CuCit ions triggered by zerovalent Pd, slow reduction of PdCit ions, and the formation of Cu/Pd nanoparticles in a designed Cu-to-Pd molar ratio (Cu/Pd ) 1/2). As a result, a three-stage formation mechanism was proposed, as schematically shown in Figure 3. In the initial reduction phase, PdCit ions were slowly reduced to zerovalent Pd and served as the reducing center for CuCit ions. After repeated alkali addition, the reduction of CuCit ions was promoted by the presence of zerovalent Pd. A significant amount of CuCit ions were reduced in this stage until it reached the fifth and sixth quenched states shown in Figure 1b. Meanwhile, the PdCit ions always maintained a slow reaction rate, as observed from the overlapping of spectra among each

12876 J. Phys. Chem. C, Vol. 111, No. 35, 2007 addition of alkali in Figure 2b. After a large amount of CuCit ions had been reduced, the reaction entered the final stage, only the reduction of PdCit still proceeded upon alkali addition. Thus, Cu/Pd nanoparticles with a Pd-rich outer shell were obtained. This Pd-rich outer shell structure may explain why partial replacement of the expensive Pd by less-active Cu does not abate the activity of the Pd-based catalyst for electroless copper deposition.19 By this citrate complex synthesis method, Pd is preferentially distributed in the outer shell of the Cu/Pd nanoparticles. Thus, a Pd-based catalyst with a decreased Pd content can be prepared without sacrificing the desired catalytic activity. Acknowledgment. This research was financially supported by the National Science Council NSC 952221E007206 and 952221E007237MY2. The National Synchrotron Radiation Research Center (NSRRC) is gratefully acknowledged. Supporting Information Available: Sampling method and XANES measurements. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Liu, H.; Song, C.; Zhang, L.; Zhang, J.; Wang, H.; Wilkinson, D. P. J. Power Sources 2006, 155, 95. (2) Zhang, X.; Chan, K. Y. Chem. Mater. 2003, 15, 451. (3) Antolini, E. Mater. Chem. Phys. 2003, 78, 563. (4) Rice, C.; Ha, S.; Masel, R. I.; Wieckowski, A. J. Power Sources 2003, 115, 229. (5) Li, X.; Hsing, I. M. Electrochim. Acta 2006, 50, 3477. (6) Shipley, C. R. U. S. Patent 3,001,920, 1961. (7) Florio, S. M.; Burress, J. P.; Colangelo, C. J.; Couble, E. C.; Kapeckas, M. J. U.S. Patent 5,611,905, 1986.

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