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J. Phys. Chem. C 2008, 112, 10242–10246
Surface Segregation of PdxNi100-x Alloy Nanoparticles Kuan-Wen Wang,*,† Shu-Ru Chung,‡ and Chen-Wei Liu† Institute of Materials Science and Engineering, National Central UniVersity, Taoyuan 320, Taiwan and Material and Chemical Research Laboratories, ITRI, Hsinchu 30011, Taiwan ReceiVed: January 30, 2008; ReVised Manuscript ReceiVed: April 11, 2008
Alloy nanoparticles of PdxNi100-x (x ) 20, 30, 70, and 80) were prepared by the coprecipitation method. The surface segregation of nanoparticles, driven by the surface energy, strain energy, and reduction potential, was studied systematically. The surface composition, surface species, and alloy structure of as-prepared PdxNi100-x and heated Pd30Ni70 alloy nanoparticles was characterized by ESCA, TPR, and XRD, respectively. The structure of as-prepared PdxNi100-x alloy nanoparticles is not homogeneous and changes with the alloy content x. The surface is Pd enriched for as-prepared Pd80Ni20 alloy due to the strain energy effect. In contrast, the surface is Ni enriched for as-prepared PdxNi100-x alloy with x ) 20, 30, and 70 due to the reduction potential effect. In addition, a combinatory process of surface segregation and homogenization during the heat treatment was observed on the surface of Pd30Ni70 alloy nanoparticles after heating at T > 620 K. 1. Introduction Surface segregation is a well-known phenomenon in nanoalloy systems in which one of the constituents preferentially migrates to the surface, resulting in a surface composition different from that of the nominal composition.1,2 Surface segregation may be driven by many thermodynamic factors, such as surface energy and strain energy.3–11 The surface composition due to surface segregation can be described by the following thermodynamic equation
(X sA ⁄ X sB) ) (X bA ⁄ X bB)exp(-∆Ha ⁄ kT)
(1)
Here A and B are the components, Xs and Xb refer to the surface and bulk composition, respectively, and ∆Ha is the enthalpy of the segregated component.3–5 This can estimate the change in enthalpy when one atom migrates to the surface while in another atom migrates into the center. This is the driving force behind segregation and a function of the surface energy of the components. It implies that the surface energies of A and B play an important role in surface segregation. The thermodynamic basis of the driving force for surface segregation is the drive to decrease surface energy.12 Upon heating, the system tends to reach the most stable state by means of atomic migration as the surface atoms rearrange stepwise.5,13 Finally, the atoms with smaller surface energies enrich the surface until they predominate. Thus, both the surface energy of the alloy system is lowered and the composition in each nanoparticle is inhomogeneous across its diameter (concentration gradient) due to surface segregation.7 In the literature, surface compositions and structures of PdxNi100-x alloy have been comprehensively studied using several techniques, such as low-energy ion scattering (LEIS), Auger electron spectroscopy (AES), and electron spectroscopy for chemical analysis (ESCA). All measurements at temperatures near 900 K displayed a remarkable segregation of Pd.14 Miegge and co-workers proposed that the surface composition of Pd in * To whom correspondence should be addressed. Phone: 886-34227151ext 34014. Fax: 886-3-2805034. E-mail:
[email protected]. † National Central University. ‡ Material and Chemical Research Laboratories.
Pd1Ni99 and Pd5Ni95 alloys is about 20 and 50 atom % Pd, respectively.15 Ni is alloyed with highly segregated Pd on its surface at very low concentrations. Since Pd surface segregation is predicted based on surface energy, enthalpy, and atomic radii, the surface is occupied by the costly and noble Pd atoms which are electronically modified by Ni. The activity of Pd in 1,3butadiene hydrogenation is improved when associated with Ni, and this is a synergetic effect of Pd and Ni. Such alloys demonstrate excellent performance, and their catalytic and chemical properties can be strongly modified by surface segregation and alloying.16 Although surface segregation and surface composition of alloy nanoparticles affects their properties significantly, there is a distinct lack of quantitative approaches to alloy nanoparticles in the literature. Surface scientists focus on the surface properties of clean and single-crystal systems instead of on those of complex alloy nanoparticles. Therefore, relatively little information exists regarding the surface properties of alloy nanoparticles. If the surfaces of alloy nanoparticles can be controlled through surface modification and surface engineering systematically, the properties and functions of nanoparticles can be manipulated. In this study, PdxNi100-x (x ) 20, 30, 70, and 80) alloy nanoparticles were prepared using the coprecipitation method.17 The surface composition of the as-prepared PdxNi100-x and Pd30Ni70 alloy nanoparticles heated at the given temperatures was elucidated systematically and analyzed by the temperatureprogrammed reduction technique (TPR) and ESCA. The TPR technique has been used to reduce the oxygen chemisorbed on bimetallic crystallites.18–20 It can probe the oxidation states of noble metals and surface composition of PtRu and PdAg alloys successfully. Since the reduction peak temperature varies with the surface composition of the bimetallic samples, TPR can probe the surface composition and surface segregation of the topmost layer ( 620 K. Acknowledgment. This work was supported by the National Science Council of Taiwan under contract no. NSC-96-2218E-008-011. References and Notes (1) Liu, Y.; Wynblatt, P. Surf. Sci. 1993, 290, 335. (2) Kumar, V.; Kumar, D.; Joshi, S. K. Phys. ReV. B 1979, 19, 1954. (3) Wynblatt, P.; Ku, R. C. Surf. Sci. 1977, 65, 511. (4) Mezey, L. Z.; Giber, J. J. Phys. C: Solid State Phys. 1983, 16, 5683. (5) Mervyn, D. A.; Baird, R. J.; Wynblatt, P. Surf. Sci. 1979, 82, 79. (6) Hofer, W.; Mezey, L. Z. Fresenius J. Anal. Chem. 1997, 358, 169. (7) Mezey, L. Z.; Giber, J. Jpn. J. Appl. Phys. 1982, 21, 1569. (8) Mezey, L.Z.; Hofer, W. Surf. Sci. 1995, 331, 799. (9) Cao, Z. X. J. Phys.: Condens. Matter. 2001, 13, 7923. (10) Hofer, W.; Mezey, L. Z. Fresenius J. Anal. Chem. 1995, 353, 631. (11) Vazquez, A.; Pedraza, F. Appl. Surf. Sci. 1996, 99, 213. (12) Gumen, L. N.; Feldman, E. P.; Yurchenko, V. M.; Melnik, T. N.; Krokhin, A. A. Surf. Sci. 2000, 445, 526. (13) Wynblatt, P.; Landa, A. Comput. Mater. Sci. 1999, 15, 250. (14) Helfensteyn, S.; Luyten, J.; Feyaerts, L.; Creemers, C. Appl. Surf. Sci. 2003, 212-213, 844. (15) Miegge, P.; Rousset, J.; Tardy, B.; Massardier, J.; Bertolini, J. C. J. Catal. 1994, 149, 404.
10246 J. Phys. Chem. C, Vol. 112, No. 27, 2008 (16) Bertolini, J. C. Appl. Catal. A: Gen. 2000, 191, 15. (17) Wang, K. W.; Chung, S. R.; Hung, W. H.; Perng, T. P. Appl. Surf. Sci. 2006, 252, 8751. (18) Wang, K. W.; Huang, S. Y.; Yeh, C. T. J. Phys. Chem. C 2007, 111, 5096. (19) Huang, S. Y.; Chang, S. M.; Lin, C. L.; Chen, C. H.; Yeh, C. T. J. Phys. Chem. B 2006, 110, 23300. (20) Chou, C. W.; Chu, S. J.; Chiang, H. J.; Huang, C. Y.; Lee, C. J.; Sheen, S. R.; Perng, T. P.; Yeh, C. T. J. Phys. Chem. B 2001, 105, 9113.
Wang et al. (21) Hansen, K. Constitution of Binary Alloys; McGraw Hill: New York, 1958. (22) Denton, A. R.; Ashcroft, Phys. ReV. A 1991, 43, 3161. (23) Chen, M. I.; Cheng, C. T.; Yeh, C. T. J. Catal. 1985, 95, 346. (24) Khanra, B. C.; Bertolini, J. C.; Rousset, J. L. J. Mol. Catal. A 1998, 129, 233.
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