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Variation of surface composition and sintering of binary Pd70Ag30 nanoparticles
Journal of Alloys and Compounds 415 (2006) 62–65
Variation of surface composition and sintering of binary Pd70Ag30 nanoparticles Kuan-Wen Wang a , Shu-Ru Chung a , Ling-Yun Jang b , Jyh-Fu Lee b , Tsong-Pyng Perng a,∗ a
Department of Materials Science and Engineering, National Tsing Hua University, 101 Sec. 2 Kuang Fu Road, Hsinchu 30043, Taiwan b National Synchrotron Radiation Research Center, Hsinchu, Taiwan Received 8 June 2005; accepted 29 June 2005 Available online 19 September 2005
Abstract The variation of surface composition and sintering of the binary Pd70 Ag30 nanoparticles prepared by chemical reduction were studied. The composition of the as-prepared Pd70 Ag30 nanoparticles across the diameter is inhomogeneous due to different reduction potentials of Pd and Ag. Upon heating, Ag will migrate to the surface because of its lower surface energy. Surface modification results in variation of surface composition during the sintering process. Stearic acid acts as a grain growth inhibitor. The migration of Ag to the surface is suppressed and the sintering is retarded. Modification by polyethylene glycol leads to more migration of Ag atoms from the core to the surface. This makes the nanoparticles easier to sinter. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Pd–Ag; Sintering; Surfactant; Stearic acid; Polyethylene glycol; ESCA; XANES
1. Introduction Palladium–silver binary alloy nanoparticles can be prepared by chemical reduction in a nitrate solution [1]. Formation of nanoparticles is controlled by the reduction potentials of the constituent ions. Since the reduction potential of Ag is higher than that of Pd (0.34 V versus 0.07 V in alkaline solution) [2], Ag will precipitate first and act as the nucleus for growth of the nanoparticles [3–7]. The particle size can be controlled to be smaller than 10 nm. It has been observed, however, that the composition across the diameter of as-prepared binary Pd–Ag nanoparticles is not homogeneous [7,8]. When subjected to heat treatment, Ag will migrate to the surface since it has a lower surface energy (γ Ag = 930 erg/cm2 and γ Pd = 1500 erg/cm2 ) [9]. Therefore, the surface composition of binary Pd–Ag nanoparticles can be varied and shifted from the nominal stoichiometry by heating. ∗
Corresponding author. Tel.: +886 3 5742634; fax: +886 3 5723857. E-mail address: [email protected] (T.-P. Perng).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.06.081
It is well known that surface modification may affect the surface properties of nanomaterials [10–13]. A common surface modification method is to cover the nanoparticle with a layer of surfactant to prevent agglomeration and to increase dispersity of the particles [14]. If the surface energy of nanoparticles is changed by surface modification, redistribution of atoms within the particle will take place during heating. In this study, the variation of surface composition in Pd70 Ag30 nanoparticles, depending on heating and surface treatment, is examined. 2. Experimental Nanocrystalline Pd70 Ag30 powder was prepared by a chemical precipitation method, as described previously [1]. Briefly, the first step was to prepare Pd black. Fresh Pd black and commercial Ag powder were respectively dissolved in nitric acid. The solutions of Pd(NO3 )2 and AgNO3 were mixed, based on the desired stoichiometry of 70:30, and then reduced by formaldehyde in a basic environment. The average particle size of Pd70 Ag30 was controlled at 8–10 nm.
K.-W. Wang et al. / Journal of Alloys and Compounds 415 (2006) 62–65
Two chemicals were used to modify the nanoparticles, stearic acid (SA) with a chemical formula CH3 (CH2 )16 COOH (Merck Co.) and polyethylene glycol (PEG) with a formula H(OCH2 CH2 )n OH (Showa Chemical Co.) and a molecular weight near 200 g/mol. About 1 wt.% SA or PEG was added to the nanoparticles to form a thin layer on the surface. The modified samples are named Pd70 Ag30 -s and Pd70 Ag30 -p, respectively. The nanoparticle powder was pressed as a pellet and sealed in an evacuated glass tube and heated to various temperatures up to 450 ◦ C. The phase structure and morphology of the nanoparticles were examined by X-ray diffraction (XRD, Rigaku) with a Cu K␣ radiation and transmission electron microscopy (TEM), respectively. The weight variation of the surfactants and nanoparticle powder during heating was analyzed by thermogravimetric analyzer (TGA, Seiko SSC 5000). The surface morphology of the pellets was examined by field emission scanning electron microscopy (FESEM, JEOL JSM-6500F). The size and weight of the pellets before and after heating in vacuum were measured to calculate the sintering parameters. Electron spectroscopy for chemical analysis (ESCA, Physical Electronics PHI 1600) equipped with a spherical capacitor analyzer was used to study the surface compositions of the binary nanoparticles. The penetration depth and surface resolution was approximately 5 nm. X-ray absorption near edge spectrum (XANES) was measured on the BL15B Tender X-ray beamline at the NSRRC (National Synchrotron Radiation Research Center, Taiwan). The L3 edges of Pd and Ag were obtained in a fluorescence mode to determine the relative surface compositions in the inhomogeneous alloy.
3. Results and discussion The TEM micrograph of the as-prepared Pd70 Ag30 shows that the spherical particles are aggregated together, with the size being approximately 8–10 nm. The thermal stability of the surfactants used in this experiment, SA and PEG, was analyzed by TGA. The decomposition temperature of PEG is lower than that of SA. PEG starts to decompose at 100 ◦ C, and is almost completely decomposed at 270 ◦ C with less than 1.1% of residue. SA has a higher decomposition temperature, being at about 150 ◦ C. There is still 10% of residue at above 300 ◦ C. The surface compositions of Ag by ESCA for different pellet samples heated to various temperatures in a vacuum tube are presented in (Fig. 1). For the as-prepared particles, the surface composition of Ag is only 20 at.%, lower than the nominal composition. This is justified because Ag precipitates first and the core is enriched with Ag. When heated to higher temperatures, Ag moves from the core to the surface, driven by the lower surface energy of Ag. There is 45 at.% Ag on the particle surface after heating to 450 ◦ C in vacuum. On the other hand, the surface composition of Ag for Pd70 Ag30 -s at 450 ◦ C is only about 35 at.%, but it is much higher (67 at.%) for Pd70 Ag30 -p. It seems that PEG may facil-
63
Fig. 1. Surface compositions by ESCA analysis for various Pd70 Ag30 samples heated to different temperatures for 1 h in a vacuum tube.
itate Ag to migrate to the surface. On the contrary, SA has an opposite effect. Similar change of surface composition can also be verified by the XANES technique. As shown in (Fig. 2), for the pellets heated in a vacuum tube to 450 ◦ C, the L3 edge of Ag at 3351 eV in Pd70 Ag30 -p is dramatically increased and therefore the surface is more enriched with Ag compared to the other two samples [15,16]. It is noted that the melting point of Ag (962 ◦ C) is lower than that of Pd (1552 ◦ C), therefore the particles enriched with Ag on the surface will sinter more easily. Based on the above result, it is expected that Pd70 Ag30 -p would sinter better in a vacuum tube than the other two samples, and SA on the surface would retard sintering of the nanoparticles. Indeed this was observed for the sintered pellets. The sintering parameters of three samples sintered in vacuum are shown in (Fig. 3). The parameter ϕ is defined as (ρs − ρg )/(ρt − ρg ), where ρs , ρg , and ρt are sinter density, green density, and theoretical density, respectively. It is seen that Pd70 Ag30 -p sinters better than the other two samples. The retardation of sintering by SA becomes more significant at 400–450 ◦ C. SEM micrographs of the nanoparticle pellet sintered in vacuum at 450 ◦ C are shown in Fig. 4. For unmodified Pd70 Ag30 and Pd70 Ag30 -p, significant sintering is observed. For Pd70 Ag30 -s, although there is some grain growth, SA is effective in retarding the sintering of Pd70 Ag30 nanoparticles and keeping the particles in the nanoscale. The particle size of Pd70 Ag30 -s is only about 30 nm, based on the analysis by XRD patterns. On the contrary, the particles of Pd70 Ag30 have grown to about 120 nm. A schematic illustration of the composition variation and particle growth for the three sam-
64
K.-W. Wang et al. / Journal of Alloys and Compounds 415 (2006) 62–65
Fig. 2. XANES for the L3 edge of Ag in various Pd70 Ag30 samples heated to 450 ◦ C in a vacuum tube.
ples with different surface conditions is displayed in (Fig. 5). Note that the samples were heated in an evacuated glass tube and the above phenomena are observed only for heating in vacuum. Based on the thermal analysis, SA and PEG have
Fig. 4. SEM morphologies of various Pd70 Ag30 pellets sintered at 450 ◦ C for 1 h in a vacuum tube: (A) Pd70 Ag30 , (B) Pd70 Ag30 -p, and (C) Pd70 Ag30 -s.
Fig. 3. Sintering parameters for various Pd70 Ag30 samples sintered in a vacuum tube.
been decomposed at 450 ◦ C, there may be gas molecules like CO2 and H2 O present inside the vacuum tube and even some residual carbon may be precipitated on the particle surface. The particle pellet is exposed to the vapor molecules and
K.-W. Wang et al. / Journal of Alloys and Compounds 415 (2006) 62–65
Fig. 5. Schematic illustration of the composition variation and particle growth for three samples of Pd70 Ag30 subjected to heating in a vacuum tube. Enrichment of Ag is illustrated by gray color, and enrichment of Pd is illustrated by dark color.
some carbon may be attached to the surface. The surfactant SA is more stable and it may be specifically bound to the particle surface, but PEG may not be. Therefore, stearic acid gives a better protective shell of carbon around the particles after heating. As a consequence, SA acts as a grain growth inhibitor during heating [17]. The migration of Ag atoms to the surface is suppressed and sintering of the nanoparticles is retarded in vacuum. In the case of PEG-modified nanoparticle pellet, the migration of Ag atoms from the interior to the surface is facilitated, resulting in more sintering (i.e., more grain growth) of the nanoparticles. Although these two surfactants may decompose or desorb from the nanoparticle surface during heat treatment under static vacuum in the glass tube, we still wonder why the surface segregation is caused by the surfactants. It is common for ceramic materials whose sintering process can be easily suppressed or accelerated by inhibitors or additives. Some surfactants may melt to help sintering and some residual carbon may pin the particles to retard sintering. In this study, however, the sintering behavior of the binary nanoparticles is closely related to the variation of surface composition. When the nanoparticles are modified by surfactants, the surface energy is changed accordingly. The accompanied change of surface energy is the driving force for atomic migration upon heating. The key issue is why SA acts differently from PEG on this binary alloy. Is this related to the chemical properties of the surfactants or the constituent metals? This needs more study, and quantitative measurement of the surface energy change due to modification by these two surfactants with an inverse gas chromatography technique is in progress.
4. Conclusion To summarize, we report an interesting surface segregation phenomenon resulting from surface modification.
65
Nanoparticles of Pd70 Ag30 in a size of 8–10 nm were prepared by wet chemical reduction in an alkaline solution. Silver has a higher reduction potential, it nucleates first and is enriched in the core. It migrates preferentially to the surface upon heating because of lower surface energy than Pd. Surface modification by surfactants results in variation of surface composition and thus affects the sintering behavior of the nanoparticles. The two surfactants used here have opposite effects on sintering of the nanoparticles. Stearic acid acts as a grain growth inhibitor. The migration of Ag to the surface is suppressed and the sintering is retarded. For polyethylene glycol-modified nanoparticles, the migration of Ag to the surface is facilitated and the sintering becomes easier. Therefore, the surface composition of binary Pd–Ag nanoparticles can be varied and shifted from the nominal stoichiometry by heating and surface modification.
Acknowledgements This work was supported by the Ministry of Education of Taiwan under Contract No. A-91-E-FA04-1-4 and National Science Council of Taiwan under Contract No. NSC-912120-E-007-003.
References [1] C.Y. Huang, H.J. Chiang, J.C. Huang, S.R. Sheen, Nanostruct. Mater. 10 (1998) 1393. [2] Handbook of Chemistry and Physics, 54th ed., CRC Press, Boca Raton, FL, 1973, p. D120. [3] M.L. Wu, D.H. Chen, T.C. Huang, Langmuir 17 (2001) 3877. [4] D.H. Chen, C.J. Chen, J. Mater. Chem. 12 (2002) 1557. [5] C. Damle, A. Kumar, M. Sastry, J. Phys. Chem. B 106 (2002) 297. [6] N. Toshima, T. Yonezawa, N. J. Chem. 11 (1998) 1179. [7] H.C. Chu, S.R. Sheen, C.T. Yeh, T.P. Perng, J. Alloys Compd. 322 (2001) 198. [8] C.W. Chou, S.J. Chu, H.J. Chiang, C.Y. Huang, C.J. Lee, S.R. Sheen, T.P. Perng, C.T. Yeh, J. Phys. Chem. B 105 (2001) 9113. [9] H.E. Boyer, T.L. Gell (Eds.), Metals Handbook, Desk edition, ASM, 1985, pp. 2–19. [10] A. Meier, I. Uhlendorf, D. Meissner, Electrochim. Acta 40 (1995) 1523. [11] L. Yang, P. Alexandridis, Curr. Opin. Colloid Interface Sci. 5 (2000) 132. [12] A.B. Bourlinos, A. Bakandritsos, V. Georgakilas, D. Petridis, Chem. Mater. 14 (2002) 3226. [13] J. Ji, Y. Chen, R.A. Senter, J.L. Coffer, Chem. Mater. 13 (2001) 4783. [14] S.F. Lomayeva, E.P. Yelsukov, G.N. Konygin, G.A. Dorofeev, V.I. Povstugar, S.S. Mikhailova, A.V. Zagainov, A.N. Maratkanova, Colloid Surf. A 162 (1999) 279. [15] G. Chen, T. Gua, J. Phys. Chem. B 106 (2002) 5833. [16] H.Y. Lee, T.B. Wu, J.F. Lee, J. Appl. Phys. 80 (1996) 2175. [17] X. Teng, H. Yang, J. Mater. Chem. 14 (2004) 774.
Variation of surface composition and sintering of binary Pd70Ag30 nanoparticles Kuan-Wen Wang a , Shu-Ru Chung a , Ling-Yun Jang b , Jyh-Fu Lee b , Tsong-Pyng Perng a,∗ a
Department of Materials Science and Engineering, National Tsing Hua University, 101 Sec. 2 Kuang Fu Road, Hsinchu 30043, Taiwan b National Synchrotron Radiation Research Center, Hsinchu, Taiwan Received 8 June 2005; accepted 29 June 2005 Available online 19 September 2005
Abstract The variation of surface composition and sintering of the binary Pd70 Ag30 nanoparticles prepared by chemical reduction were studied. The composition of the as-prepared Pd70 Ag30 nanoparticles across the diameter is inhomogeneous due to different reduction potentials of Pd and Ag. Upon heating, Ag will migrate to the surface because of its lower surface energy. Surface modification results in variation of surface composition during the sintering process. Stearic acid acts as a grain growth inhibitor. The migration of Ag to the surface is suppressed and the sintering is retarded. Modification by polyethylene glycol leads to more migration of Ag atoms from the core to the surface. This makes the nanoparticles easier to sinter. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Pd–Ag; Sintering; Surfactant; Stearic acid; Polyethylene glycol; ESCA; XANES
1. Introduction Palladium–silver binary alloy nanoparticles can be prepared by chemical reduction in a nitrate solution [1]. Formation of nanoparticles is controlled by the reduction potentials of the constituent ions. Since the reduction potential of Ag is higher than that of Pd (0.34 V versus 0.07 V in alkaline solution) [2], Ag will precipitate first and act as the nucleus for growth of the nanoparticles [3–7]. The particle size can be controlled to be smaller than 10 nm. It has been observed, however, that the composition across the diameter of as-prepared binary Pd–Ag nanoparticles is not homogeneous [7,8]. When subjected to heat treatment, Ag will migrate to the surface since it has a lower surface energy (γ Ag = 930 erg/cm2 and γ Pd = 1500 erg/cm2 ) [9]. Therefore, the surface composition of binary Pd–Ag nanoparticles can be varied and shifted from the nominal stoichiometry by heating. ∗
Corresponding author. Tel.: +886 3 5742634; fax: +886 3 5723857. E-mail address: [email protected] (T.-P. Perng).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.06.081
It is well known that surface modification may affect the surface properties of nanomaterials [10–13]. A common surface modification method is to cover the nanoparticle with a layer of surfactant to prevent agglomeration and to increase dispersity of the particles [14]. If the surface energy of nanoparticles is changed by surface modification, redistribution of atoms within the particle will take place during heating. In this study, the variation of surface composition in Pd70 Ag30 nanoparticles, depending on heating and surface treatment, is examined. 2. Experimental Nanocrystalline Pd70 Ag30 powder was prepared by a chemical precipitation method, as described previously [1]. Briefly, the first step was to prepare Pd black. Fresh Pd black and commercial Ag powder were respectively dissolved in nitric acid. The solutions of Pd(NO3 )2 and AgNO3 were mixed, based on the desired stoichiometry of 70:30, and then reduced by formaldehyde in a basic environment. The average particle size of Pd70 Ag30 was controlled at 8–10 nm.
K.-W. Wang et al. / Journal of Alloys and Compounds 415 (2006) 62–65
Two chemicals were used to modify the nanoparticles, stearic acid (SA) with a chemical formula CH3 (CH2 )16 COOH (Merck Co.) and polyethylene glycol (PEG) with a formula H(OCH2 CH2 )n OH (Showa Chemical Co.) and a molecular weight near 200 g/mol. About 1 wt.% SA or PEG was added to the nanoparticles to form a thin layer on the surface. The modified samples are named Pd70 Ag30 -s and Pd70 Ag30 -p, respectively. The nanoparticle powder was pressed as a pellet and sealed in an evacuated glass tube and heated to various temperatures up to 450 ◦ C. The phase structure and morphology of the nanoparticles were examined by X-ray diffraction (XRD, Rigaku) with a Cu K␣ radiation and transmission electron microscopy (TEM), respectively. The weight variation of the surfactants and nanoparticle powder during heating was analyzed by thermogravimetric analyzer (TGA, Seiko SSC 5000). The surface morphology of the pellets was examined by field emission scanning electron microscopy (FESEM, JEOL JSM-6500F). The size and weight of the pellets before and after heating in vacuum were measured to calculate the sintering parameters. Electron spectroscopy for chemical analysis (ESCA, Physical Electronics PHI 1600) equipped with a spherical capacitor analyzer was used to study the surface compositions of the binary nanoparticles. The penetration depth and surface resolution was approximately 5 nm. X-ray absorption near edge spectrum (XANES) was measured on the BL15B Tender X-ray beamline at the NSRRC (National Synchrotron Radiation Research Center, Taiwan). The L3 edges of Pd and Ag were obtained in a fluorescence mode to determine the relative surface compositions in the inhomogeneous alloy.
3. Results and discussion The TEM micrograph of the as-prepared Pd70 Ag30 shows that the spherical particles are aggregated together, with the size being approximately 8–10 nm. The thermal stability of the surfactants used in this experiment, SA and PEG, was analyzed by TGA. The decomposition temperature of PEG is lower than that of SA. PEG starts to decompose at 100 ◦ C, and is almost completely decomposed at 270 ◦ C with less than 1.1% of residue. SA has a higher decomposition temperature, being at about 150 ◦ C. There is still 10% of residue at above 300 ◦ C. The surface compositions of Ag by ESCA for different pellet samples heated to various temperatures in a vacuum tube are presented in (Fig. 1). For the as-prepared particles, the surface composition of Ag is only 20 at.%, lower than the nominal composition. This is justified because Ag precipitates first and the core is enriched with Ag. When heated to higher temperatures, Ag moves from the core to the surface, driven by the lower surface energy of Ag. There is 45 at.% Ag on the particle surface after heating to 450 ◦ C in vacuum. On the other hand, the surface composition of Ag for Pd70 Ag30 -s at 450 ◦ C is only about 35 at.%, but it is much higher (67 at.%) for Pd70 Ag30 -p. It seems that PEG may facil-
63
Fig. 1. Surface compositions by ESCA analysis for various Pd70 Ag30 samples heated to different temperatures for 1 h in a vacuum tube.
itate Ag to migrate to the surface. On the contrary, SA has an opposite effect. Similar change of surface composition can also be verified by the XANES technique. As shown in (Fig. 2), for the pellets heated in a vacuum tube to 450 ◦ C, the L3 edge of Ag at 3351 eV in Pd70 Ag30 -p is dramatically increased and therefore the surface is more enriched with Ag compared to the other two samples [15,16]. It is noted that the melting point of Ag (962 ◦ C) is lower than that of Pd (1552 ◦ C), therefore the particles enriched with Ag on the surface will sinter more easily. Based on the above result, it is expected that Pd70 Ag30 -p would sinter better in a vacuum tube than the other two samples, and SA on the surface would retard sintering of the nanoparticles. Indeed this was observed for the sintered pellets. The sintering parameters of three samples sintered in vacuum are shown in (Fig. 3). The parameter ϕ is defined as (ρs − ρg )/(ρt − ρg ), where ρs , ρg , and ρt are sinter density, green density, and theoretical density, respectively. It is seen that Pd70 Ag30 -p sinters better than the other two samples. The retardation of sintering by SA becomes more significant at 400–450 ◦ C. SEM micrographs of the nanoparticle pellet sintered in vacuum at 450 ◦ C are shown in Fig. 4. For unmodified Pd70 Ag30 and Pd70 Ag30 -p, significant sintering is observed. For Pd70 Ag30 -s, although there is some grain growth, SA is effective in retarding the sintering of Pd70 Ag30 nanoparticles and keeping the particles in the nanoscale. The particle size of Pd70 Ag30 -s is only about 30 nm, based on the analysis by XRD patterns. On the contrary, the particles of Pd70 Ag30 have grown to about 120 nm. A schematic illustration of the composition variation and particle growth for the three sam-
64
K.-W. Wang et al. / Journal of Alloys and Compounds 415 (2006) 62–65
Fig. 2. XANES for the L3 edge of Ag in various Pd70 Ag30 samples heated to 450 ◦ C in a vacuum tube.
ples with different surface conditions is displayed in (Fig. 5). Note that the samples were heated in an evacuated glass tube and the above phenomena are observed only for heating in vacuum. Based on the thermal analysis, SA and PEG have
Fig. 4. SEM morphologies of various Pd70 Ag30 pellets sintered at 450 ◦ C for 1 h in a vacuum tube: (A) Pd70 Ag30 , (B) Pd70 Ag30 -p, and (C) Pd70 Ag30 -s.
Fig. 3. Sintering parameters for various Pd70 Ag30 samples sintered in a vacuum tube.
been decomposed at 450 ◦ C, there may be gas molecules like CO2 and H2 O present inside the vacuum tube and even some residual carbon may be precipitated on the particle surface. The particle pellet is exposed to the vapor molecules and
K.-W. Wang et al. / Journal of Alloys and Compounds 415 (2006) 62–65
Fig. 5. Schematic illustration of the composition variation and particle growth for three samples of Pd70 Ag30 subjected to heating in a vacuum tube. Enrichment of Ag is illustrated by gray color, and enrichment of Pd is illustrated by dark color.
some carbon may be attached to the surface. The surfactant SA is more stable and it may be specifically bound to the particle surface, but PEG may not be. Therefore, stearic acid gives a better protective shell of carbon around the particles after heating. As a consequence, SA acts as a grain growth inhibitor during heating [17]. The migration of Ag atoms to the surface is suppressed and sintering of the nanoparticles is retarded in vacuum. In the case of PEG-modified nanoparticle pellet, the migration of Ag atoms from the interior to the surface is facilitated, resulting in more sintering (i.e., more grain growth) of the nanoparticles. Although these two surfactants may decompose or desorb from the nanoparticle surface during heat treatment under static vacuum in the glass tube, we still wonder why the surface segregation is caused by the surfactants. It is common for ceramic materials whose sintering process can be easily suppressed or accelerated by inhibitors or additives. Some surfactants may melt to help sintering and some residual carbon may pin the particles to retard sintering. In this study, however, the sintering behavior of the binary nanoparticles is closely related to the variation of surface composition. When the nanoparticles are modified by surfactants, the surface energy is changed accordingly. The accompanied change of surface energy is the driving force for atomic migration upon heating. The key issue is why SA acts differently from PEG on this binary alloy. Is this related to the chemical properties of the surfactants or the constituent metals? This needs more study, and quantitative measurement of the surface energy change due to modification by these two surfactants with an inverse gas chromatography technique is in progress.
4. Conclusion To summarize, we report an interesting surface segregation phenomenon resulting from surface modification.
65
Nanoparticles of Pd70 Ag30 in a size of 8–10 nm were prepared by wet chemical reduction in an alkaline solution. Silver has a higher reduction potential, it nucleates first and is enriched in the core. It migrates preferentially to the surface upon heating because of lower surface energy than Pd. Surface modification by surfactants results in variation of surface composition and thus affects the sintering behavior of the nanoparticles. The two surfactants used here have opposite effects on sintering of the nanoparticles. Stearic acid acts as a grain growth inhibitor. The migration of Ag to the surface is suppressed and the sintering is retarded. For polyethylene glycol-modified nanoparticles, the migration of Ag to the surface is facilitated and the sintering becomes easier. Therefore, the surface composition of binary Pd–Ag nanoparticles can be varied and shifted from the nominal stoichiometry by heating and surface modification.
Acknowledgements This work was supported by the Ministry of Education of Taiwan under Contract No. A-91-E-FA04-1-4 and National Science Council of Taiwan under Contract No. NSC-912120-E-007-003.
References [1] C.Y. Huang, H.J. Chiang, J.C. Huang, S.R. Sheen, Nanostruct. Mater. 10 (1998) 1393. [2] Handbook of Chemistry and Physics, 54th ed., CRC Press, Boca Raton, FL, 1973, p. D120. [3] M.L. Wu, D.H. Chen, T.C. Huang, Langmuir 17 (2001) 3877. [4] D.H. Chen, C.J. Chen, J. Mater. Chem. 12 (2002) 1557. [5] C. Damle, A. Kumar, M. Sastry, J. Phys. Chem. B 106 (2002) 297. [6] N. Toshima, T. Yonezawa, N. J. Chem. 11 (1998) 1179. [7] H.C. Chu, S.R. Sheen, C.T. Yeh, T.P. Perng, J. Alloys Compd. 322 (2001) 198. [8] C.W. Chou, S.J. Chu, H.J. Chiang, C.Y. Huang, C.J. Lee, S.R. Sheen, T.P. Perng, C.T. Yeh, J. Phys. Chem. B 105 (2001) 9113. [9] H.E. Boyer, T.L. Gell (Eds.), Metals Handbook, Desk edition, ASM, 1985, pp. 2–19. [10] A. Meier, I. Uhlendorf, D. Meissner, Electrochim. Acta 40 (1995) 1523. [11] L. Yang, P. Alexandridis, Curr. Opin. Colloid Interface Sci. 5 (2000) 132. [12] A.B. Bourlinos, A. Bakandritsos, V. Georgakilas, D. Petridis, Chem. Mater. 14 (2002) 3226. [13] J. Ji, Y. Chen, R.A. Senter, J.L. Coffer, Chem. Mater. 13 (2001) 4783. [14] S.F. Lomayeva, E.P. Yelsukov, G.N. Konygin, G.A. Dorofeev, V.I. Povstugar, S.S. Mikhailova, A.V. Zagainov, A.N. Maratkanova, Colloid Surf. A 162 (1999) 279. [15] G. Chen, T. Gua, J. Phys. Chem. B 106 (2002) 5833. [16] H.Y. Lee, T.B. Wu, J.F. Lee, J. Appl. Phys. 80 (1996) 2175. [17] X. Teng, H. Yang, J. Mater. Chem. 14 (2004) 774.