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Synthesis and Characterization of Crystalline and Amorphous Palladium Nanoparticles Wei Lu, Bing Wang,* Kedong Wang, Xiaoping Wang, and J. G. Hou* Structure Research Lab, University of Science and Technology of China, Heifei 230026, Anhui, China Received January 29, 2003. In Final Form: May 14, 2003 Ligand-stabilized palladium amorphous nanoparticles with various monodispersed sizes ranging from 2 to 6 nm were synthesized and compared with palladium crystalline nanoparticles. The nanoparticles were characterized by high-resolution electron microscopy (HREM), X-ray photoelectron spectroscopy, and scanning tunneling microscopy/spectroscopy (STM/STS). Both HREM images and electron diffraction patterns show quite clear differences in structures between crystalline and amorphous palladium nanoparticles. STS spectra exhibit fine structures in addition to the Coulomb blockade and the Coulomb staircases in the current-voltage (I-V) curves for the crystalline Pd nanoparticles when the diameters are smaller than 4 nm but only pure effect of Coulomb blockade and Coulomb staircases for amorphous Pd nanoparticles, which further indicates the differences in electronic structures.
I. Introduction Nanoparticles show many novel properties different from their bulk materials. The electronic structure of a nanoparticle determines its fundamental electronic, optical, and magnetic properties, which are usually attributed to the large ratio of surface atoms to core atoms and the finite number of core atoms.1,2 Though many efforts have been made for controlling the electronic structure of nanoparticles via changing their size, shape, and composition,3-5 it is still a challenge to find ways to freely control the properties of nanoparticles. Amorphization is an effective way to modify the electronic and other properties of solid materials.6 It is interesting to know if this method can be extended to nanometer scale and thus to provide an additional way for controlling the electronic structure of a nanoparticle. Recently, some theoretical works predicted the energy structure and stability of amorphous nanoparticles7-10 and a few works reported the formation of composite11-13 and metal14-16 amorphous nanoparticles * Address correspondence to these authors. Email: bwang@ ustc.edu.cn &
[email protected]. (1) Halperin, W. P. Rev. Mod. Phys. 1986, 58, 533. (2) Edelstein, A. S.; Cammarata, R. C. Nanomaterials: Synthesis, Properties and Applications; Institute of Physics: Bristol, PA, 1996; p 541. (3) Petit, C.; Pileni, M. P. J. Magn. Magn. Mater. 1997, 166, 82. (4) Pinna, N.; Weiss, K.; Stack-Kongehl, H.; Vogel, W.; Urban, J.; Pileni, M. P. Langmuir 2001, 17, 7982. (5) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (6) Zallen, R. The Physics of Amorphous Solids; Wiley-Interscience Publication: New York, 1983. (7) Rose, J. P.; Berry, R. S. J. Chem. Phys. 1993, 98, 3262. (8) Garzo´n, I. L.; Michaelian, K.; Beltran, M. R. Phys. Rev. Lett. 1998, 81, 1600. (9) Garzo´n, I. L.; Posada-Amarillas, A. Phys. Rev. B 1996, 54, 11796. (10) de la Fuente, O. R.; Soler, J. M. Phys. Rev. Lett. 1998, 81, 3159. (11) Suzuki, A.; Endo, A.; Tanaka, K. J. Phys. Soc. Jpn. 1999, 68, 3623. (12) Mørup, S.; Sethi, S. A.; Linderoth, S.; Koch, C. B.; Bentzon, M. D. J. Mater. Sci. 1992, 27, 3010. (13) Meldrum, A.; Boatner, L. A.; Ewing, R. C. Phys. Rev. Lett. 2002, 88, 025503. (14) Tehuacanero, S.; Herrera, R.; Avalos, M.; Yacaman, M. J. Acta Metall. Mater. 1992, 40, 1663. (15) Krakow, W.; Jo´se-Yacama´n, M.; Arago´n, J. L. Phys. Rev. B 1994, 49, 10591. (16) Park, S.-J.; Kim, S.; Lee, S.; Khim, Z. G.; Char, K.; Hyeon, T. J. Am. Chem. Soc. 2000, 122, 8581.
via physical and chemical methods. However, to our best knowledge, there is no a comparative study between crystalline and amorphous nanoparticles with the size of the same order of magnitude on synthesis and property characterization. The possible effects of atomic order or disorder on the physical properties of nanoparticles are still an open question. In this paper, we report the synthesis and characterization of amorphous nanoparticles, in comparison with crystalline nanoparticles. We prepared various crystalline and amorphous palladium nanoparticles with narrow size distributions by modifying the reaction conditions of a two-phase chemical method.17-22 The atomic structures and size distributions of crystalline and amorphous Pd nanoparticles were characterized by high-resolution electron microscopy (HREM). The valent states of Pd particles were examined by X-ray photoelectron spectroscopy (XPS). Their electronic properties were probed using scanning tunneling microscopy/spectroscopy (STM/STS) for individual nanoparticles. The crystalline and amorphous nanoparticles with similar sizes were compared both in atomic structures and in electronic properties. The STS spectra show that the electronic properties between crystalline and amorphous Pd nanoparticles are quite different, which may be correlated to their atomic ordered or disordered structures. II. Experimental Section A. Reagent. Palladium(II) chloride (PdCl2) (>99%) and poly(N-vinyl-2-pyrrolidone) (PVP, K-30, Mw ) 40 000) were purchased from Shanghai Reagent Co. of Chinese Medical. NaBH4 (∼98%) was purchased from Merick. Dodecanethiol (DT) (∼98%) was (17) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (18) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428. (19) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (20) Toshima, N.; Harada, M.; Yonezawa, T.; Kushihashi, K.; Asakura, K. J. Phys. Chem. 1991, 95, 7448. (21) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc., Faraday Trans. 1993, 89, 2537. (22) Pileni, M. P. Langmuir 1997, 13, 3266.
10.1021/la034160a CCC: $25.00 © 2003 American Chemical Society Published on Web 06/11/2003
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Figure 1. Typical FT-IR spectra of sample C3 and A3 in comparison with dodecanethiol (DT) and PVP. purchased from Acros. All the solvents in the experiment were used as received, and the deionized water used in the experiment had been bubbled with nitrogen for 30 min to expel oxygen. B. Synthesis. Preparation of Palladium Crystalline Nanoparticles. A 460 mg portion of PVP was added to 50 mL of ethanol solution containing 12.0 mg of PdCl2. The solution was subsequently mixed with 50 mL of deionized water and then refluxed at 95 °C for 1 h under nitrogen atmosphere. The color of the solution changed from clear yellow to dark brown during refluxing due to the formation of PVP-separated Pd particles. After evaporating all the water-ethanol solvent under vacuum at room temperature (RT), an aqueous solution of PVP-separated Pd particles was obtained by redissolving the residual into 100 mL of water. A 20 mL portion of the solution was taken out and mixed with 20 mL of toluene containing 5 µL of DT. The mixture was stirred sturdily at RT for 30 min, then the Pd particles were capped by DT and then extracted into a toluene layer. It could be observed that the toluene layer became dark brown, while the aqueous layer became discolored. The toluene layer was taken out and evaporated to about 0.5 mL. The Pd particles were deposited by adding 2 mL of ethanol. The precipitate was washed several times with ethanol to remove the excess DT, and the residua were redissolved in 2 mL of toluene forming an optically transparent solution (sample labeled as C1, dark brown). With a similar procedure, we got other samples, C2 (light brown) and C3 (bright yellow), only by changing the PdCl2 dosage to 6.0 mg and 2.0 mg, respectively. Preparation of Palladium Amorphous Nanoparticles. A 12.0 mg portion of PdCl2 was dissolved in 50 mL of 10 mM hydrochloric acid solution, and 460 mg of PVP was then added. Under nitrogen atmosphere, 50 mL of water containing 3 mg of NaBH4 was subsequently injected in the PdCl2 solution with sturdy stirring at RT. Just after the reducing reagent was added, the color of the solution changed from clear yellow to dark brown, denoting the formation of the Pd particles. A 20 mL portion of the dark brown solution was taken out and mixed with 20 mL of toluene containing 5 µL of DT. Following a similar procedure as that used to prepare sample C1, we got the sample A1 (dark brown). By changing the dosage of PdCl2 to 6.0 mg and 2.0 mg while keeping the same ratio of NaBH4 to PdCl2, we obtained two additional samples: A2 (light brown) and A3 (bright yellow). C. Infrared Spectrum. FT-IR spectra were obtained at RT using a Magna-IR 750 spectrometer. The specimens were prepared by spreading several drops of toluene solution containing the Pd nanoparticles on KBr crystal plates and evaporating the solvent slowly. The spectra of samples Ci and Ai (i ) 1, 2, and 3) show nearly the same γC-H and δC-H absorption bands as that of DT, while the γCdO band of PVP is absent. The typical spectra of samples C3 and A3 are plotted in Figure 1, compared with those of DT and PVP. The results indicate that PVP have been removed from the samples and the Pd particles are capped with DT. D. Microscopy and Spectroscopy. The specimens for HREM and STM/STS measurements were prepared by spreading drops
Figure 2. HREM images and electron diffraction patterns of samples C1, C2, and C3 and A1, A2, and A3. Insets: distributions of particle sizes. of the toluene solutions containing the Pd nanoparticles on several pieces of carbon-coated copper mesh grids and Au films, respectively. The Au films used were freshly grown on mica with thicknesses of about 160 nm and typical Au(111) terraces of 100200 nm. The HREM images of the Pd particles were obtained with a JEOL 2100 high-resolution electron microscope operated at an accelerating voltage of 200 kV. In the HREM images, only the Pd cores give a contrast, which allows us to determine the size distributions of Pd particles for each sample. The particles spread on copper substrates were examined by the XPS with a VG ESCALAB MKII system. The source employed was monchromated Mg KR radiation. We typically first recorded an energy distribution curve for each sample over the available energy range 0-1400 eV, followed by high-resolution scans over the C 1s, S 2p, and Pd 3d photoelectron binding energy region. In STM/STS measurements, an ultrahigh vacuum lowtemperature scanning tunneling microscope (Omicron SCALA) operated at 4.2 K was used to characterize the Pd particles on the Au substrates. The current-voltage (I-V) curves of individual Pd particles were measured by positioning the tip of the microscope over a selected Pd nanoparticle while turning off the feedback of the microscope. Here, the tip of the microscope, the DT-capped Pd nanoparticle, and the Au substrate formed a double barrier tunneling junction (DBTJ).23,24 The electrochemically etched Pt/Ir tip was carefully treated and cleaned in a vacuum before it was used.
III. Results and Discussion A. Structure of the Crystalline Particles. The upper part of Figure 2 shows the HREM images and electron diffraction patterns of the Pd nanoparticle samples labeled Ci (i ) 1, 2, and 3). The insets in Figure 2 also give the size distributions of the nanoparticles. The size distributions are relatively narrow, with average diameters of about 5.8, 4.5, and 1.9 nm for samples C1, C2, and C3, respectively. The electron diffraction patterns (upper) show relatively sharp rings, indicating that the Pd particles in samples C1, C2, and C3 are in crystalline form. The HREM images show that more than 80 of the particles are lattice resolvable, which further confirms that most (23) Hou, J. G.; Wang, B.; Yang, J. L.; Wang, X. R.; Wang, H. Q.; Zhu, Q. S.; Xiao, X. D. Phys. Rev. Lett. 2001, 86, 5321. (24) Wang, B.; Wang, H. Q.; Li, H. X.; Zeng, C. G.; Hou, J. G.; Xiao, X. D. Phys. Rev. B 2001, 63, 035403.
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Figure 3. (a) HREM image of crystalline Pd particles with resolvable atomic lattice. (b) Image of a crystalline Pd particle with atomic resolution and nearly hexagonal shape.
Figure 4. (a) HREM image of a amorphous Pd particles showing a branchlike structure. (b) HREM image of a amorphous Pd particle showing a shell-like structure.
of the particles in samples Ci are in crystalline form. To show this clearly, the magnified HREM images for sample C3 are shown in Figure 3. Figure 3a shows the lattice-resolvable image of the nanoparticles. The image of one particle in Figure 3a is further magnified in Figure 3b to show its quasi-hexagonal shape and clear hexagonal lattice. In most cases, the small crystalline Pd particles show polygonal shapes in HREM images, which are quite similar to the shape of gold prarticles or Pd particles.18,25 The lattice at the edge of the particles is not clear, and in some particles the lattice show some distortion and domain boundary, indicating that the atoms in crystalline particles are not perfectly stacked, but with some defects. This may also influence their electronic structures to some extent. B. Structure of the Amorphous Particles. The HREM images and electron diffraction patterns of samples Ai (i ) 1, 2, and 3) are shown in the lower part of Figure 2. The size distributions are also relatively narrow, and the measured average diameters are about 6.2, 3.7, and 2.1 nm for samples A1, A2, and A3, respectively. The electron diffraction patterns show blurred rings, indicating that the particles are amorphous. Most of particles in samples A1, A2, and A3 exhibit a round shape in HREM images, in contrast to the polygonal shape of the crystalline Pd particles. This indicates that the Pd particles in sample A1, A2, and A3 are spherelike, which further indicates that the structures of amorphous Pd particles are isotropic. In HREM images, most of the particles in samples Ai show random stacked structures, typically, as shown in Figure 4a and Figure 4b, while only a small part (less than 10%) of the particles show periodic lattice. These results are consistent with the electron diffraction patterns. On the basis of this evidence, it is reasonable to conclude that the most of the particles in samples A1, A2, and A3 are amorphous. (25) Veisz, B.; Kira´ly, Z.; To´th, L.; Pe´cz, B. Chem. Mater. 2002, 14, 2882.
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C. Measurement with XPS. XPS measurements were performed for crystalline and amorphous Pd nanoparticles. Overall, the XPS spectra were quite similar for both kinds of the particles. In Figure 5, the representative highresolution C 1s, S 2p, and Pd 3d XPS signals for samples C3 and A3 are depicted. The line shapes of Pd 3d and S 2p were fitted by assuming a Gaussian:Lorentzian (70%:30%) line shape. The line shapes of the high-resolution Pd 3d core level spectra indicate that there are two types of valent states. As shown in Figure 5a, the main component at binding energy of 335.2 eV corresponds to a zerovalent state of Pd, while the minor component at 337.3 eV is assigned to a nonzerovalent state. Here, the nonzerovalent state of Pd may be assigned to the formation of PdS interphase. This is also consistent with the results of S 2p XPS signals. In S 2p core levels shown in Figure 5b, we assign the 2p3/2 binding energy of 162.3 eV to that of sulfur present in a metal sulfide interphase and the binding energy of 163.2 eV to that of an alkanethiolate species present in a self-assembled monolayer, as a quite similar case of alkanethiol on a Pd(111) surface.26 No oxidation of sulfur is observed in the S 2p XPS signals. This indicates that the oxidations were quite slight in both of the Pd particle samples because the syntheses were performed under a nitrogen atmosphere. So, we believe that the nonzerovalent state observed in the Pd 3d XPS binding energy region is most probably due to formation of a PdS interphase other than oxidation. Considering the large ratio of surface atoms to core atoms in Pd nanoparticles, the observation of a relatively large percentage (∼20%) of Pd nonzerovalent state due to PdS interphase is reasonable. In Figure 5c, the C 1s line shape shows a sharp line at 285 eV, typical for condensed alkanes,27 which is also consistent with FT-IR results that the Pd particles were capped with DT. The XPS results indicate that there is little difference in Pd valent state between two kinds of particles, and the main component of Pd is zerovalent state. D. STS Results and Electronic Structures of Crystalline and Amorphous Nanoparticles. In Figure 6, we plotted typical I-V curves and their numerical differentials, dI/dV, for small crystalline and amorphous Pd particles of about 2 nm (samples C3 and A3). It is obvious that nearly equidistant-spaced steps are exhibited in the I-V curve (or equidistant-spaced peaks in dI/dV) for the amorphous Pd particle, while extra fine features in addition to each step appear in the I-V curve (or multipeaks in dI/dV) for the crystalline one. Such significant differences in STS results between the crystalline and the amorphous Pd particles may be attributed to their different electronic properties. To understand this, we should consult the singleelectron tunneling (SET) effect.28 When a metal particle is placed in a DBTJ as a center island coupled with two electrodes, the I-V characteristics of the DBTJ will display Coulomb blockade and Coulomb staircases due to the SET effect if the charging energy e2/2C exceeds the thermal energy kBT, and the resistances of the two tunneling junctions are larger than the resistance quantum h/e2, where C is capacitance (typically in the order of 10-19 F), e is the electron charge, kB is the Boltzmann constant, T is temperature, and h is Planck’s constant. In a semiclas(26) Love, J. C.; Wolfe, D. B.; Haasch, R.; Chabinyc, M. L.; Paul, K. E.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 2003, 125, 2597. (27) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. P.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (28) For a review, see: Single Charge Tunneling; Grabert, H., Devoret, M. H., Eds.; NATO Advanced Study Institute, Series B: Physics; Plenum: New York, 1991; Vol. 294.
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Figure 5. High-resolution XPS data for (a) Pd 3d, (b) S 2p, and (c) C 1s core levels of amorphous (sample A3) and crystalline (sample C3) Pd particles.
Figure 6. I-V curves and numerical derivative differential, dI/dV - V, for crystalline and amorphous Pd nanoparticles of about 2 nm in diameter. The curves for crystalline Pd particles have been shifted for clarity.
sical theory of the SET effect,29 an electronic structure of quasi-continuous energy levels, or a constant density of states, of a metal particle results in a equidistant-spaced Coulomb staircases in the I-V curves, while an electronic structure of discrete energy levels gives extra fine structures in addition to the Coulomb staircases.30-32 On the basis of the fact that our observations in Figure 6 coincide with the theory, we conclude that the equidistantspaced steps in the I-V curve of an amorphous Pd particle in Figure 6 are due to its constant density of states, while the extra fine features in addition to the steps in the I-V curve of the crystalline Pd particle in Figure 6 are due to the electronic structure of discrete energy levels. The differences in electronic structures between crystalline and amorphous Pd particles are mainly caused by their different atomic structures. As we discussed above, when a crystalline Pd nanoparticle is reduced to as small as about 2 nm or less, it contains about 300 Pd atoms. (29) Amman, M.; Wilkins, R.; Ben-Jacob, E.; Maker, P. D.; Jaklevic, R. C. Phys. Rev. B 1991, 43, 1146. (30) Averin, D. V.; Korotkov, A. N.; Likharev, K. K. Phys. Rev. B 1991, 44, 6199. (31) von Delft, J.; Ralph, D. C. Phys. Rep. 2001, 345, 62. (32) Davidovic´, D.; Tinkham, M. Phys. Rev. Lett. 1999, 83, 1644.
Considered its relatively high symmetry, as shown in Figure 3, its discrete electronic energy level spacings are expected to be in the order of several tens of meV, and even up to hundreds of meV due to the confinement effect and the finite number of atoms (or conduction electrons).33 Thus, the discrete energy level spacings, ∆, enter a regime of kBT , ∆ ∼ e2/C. In this case, the electron transport via the crystalline Pd particle is possible only by resonant tunneling through a corresponding quantized levels of the particle, as in the cases of semiconductors and molecules.34,35 Hence, the additional fine structures in the I-V curve of crystalline Pd particles are related to its large electron discrete energy level spacings.31 However, even with a similar particle size, due to the atomic disordered structure in an amorphous Pd particle, the existence of a static effect and a dynamic effect may cause a quite different electronic structure of the amorphous Pd particle from that of the crystalline one. In the static effect, the reduction of the degeneracy of eigenstates as a result of the structural disorder causes a reduced average energy level spacing by a factor of 3 in the amorphous Pd nanoparticles compared to the one in the crystalline Pd particle if we consider a random stacked model for the amorphous Pd particle and a high-symmetry model of Ih or Oh stacking for the crystalline Pd particle. In the dynamic effect, the level broadening due to the reduced lifetime of the electronic states in a disordered system (amorphous), caused by the enhanced electron-electron scattering and electron-phonon scattering, may be more significant than that in an ordered system (crystalline).36,37 The consequence of the two effects may result in a constant density of states in amorphous Pd particles and, thus, cause the disappearance of the effect of quantized levels in I-V curves. A comprehensive understanding of the differences in electronic structure between two kinds of Pd particles needs a further quantitative study. (33) Wang, B.; Wang, K. D.; Lu, W.; Wang, H. Q.; Li, Z. Y.; Yang, J. L.; Hou, J. G. Appl. Phys. Lett. 2003, 82, 3767. (34) Tarucha, S.; Austing, D. G.; Hongda, T.; van der Hage, R. J.; Kouwenhoven, L. P. Phys. Rev. Lett. 1996, 77, 3613. (35) Xue, Y.; Datta, S.; Hong, S.; Reifenberger, R. Phys. Rev. B 1999, 59, R7852. (36) Alhassid, Y. Rev. Mod. Phys. 2000, 72, 895. (37) Pontius, N.; Lu¨ttgens, G.; Bechthold, P. S.; Neeb, M.; Eberhardt, W. J. Chem. Phys. 2001, 115, 10479.
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The absence of discrete energy level effects in the SET spectra of amorphous nanoparticles may find interesting application in nanodevices. A room-temperature single electron device requires the size of nanoparticle to be less than a few nanometers so that Coulomb charging energy (e2/2C) greatly exceeds the thermal energy (kBT). However, the entanglement of the Coulomb staircases with discrete states can lead to complex I-V characteristics that are not desirable for device applications.38 Such complications may be easily avoided if amorphous particles are used instead. E. The Growth of Pd Particles in Relation to Their Crystalline/Amorphous Structures. In the experiments, the Pd(II) ions in the solution were reduced to zero-valence Pd atoms by different reducing agents. The reduced Pd atoms had the tendency to aggregate together and the small particles tended to aggregate into large particles to decrease the total surface energy, which would finally lead to the growth of large Pd particles. When PVP was dissolved, the abundant organic long chains in the solution intersected each other and formed a great number of interspaces. The aggregation of small Pd particles was restricted by the organic long chains, which prevented further growing of the Pd particles. Moreover, the polymer in the solution was evenly dissolved forming evenly separated interspaces, which resulted in the narrow size distributions of Pd particles. There are two main factors that affect the sizes of Pd particles in the experiments: the concentration of Pd(II) ions and the concentration of PVP in the solution. Via adjusting the concentration of the concentration of Pd(II) ions and the concentration of PVP in the solution, we may change the average sizes of Pd particles. In the experiment, we kept the concentration of PVP but changed the concentration of Pd(II) to obtain various average sizes of Pd particles. The more the Pd(II) ions that were in the solution, the larger Pd particles formed when the concentration of Pd(II) ions was in a certain range. (38) Ferry, D. K.; Goodnick, S. M. Transport In Nanostructures; Cambridge University Press: Cambridge, 1997; Chapter 4, pp 244249.
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The atomic structure of Pd particles is dominated by the reduction speed. For a certain concentration of Pd(II) ions and PVP in the solution, we changed the reduction speed by using a different reducing agent. The mechanism of reducing metal ions into metal atoms at zerovalent state using ethanol as weak reducing agent had been reported by Toshima et al.21 This reductive reaction under the effect of PVP is relatively slow, so the formation of Pd atoms is gentle and the atoms have enough time to aggregate into ordered structures, whereas NaBH4 is a strong reductant, with which the reduction is relatively quite fast when the Pd(II) ions are reduced to Pd atoms.17 Hence, the Pd atoms are produced quickly even at room temperature, so the reduced Pd atoms stick on particles without relaxation to balance sites and to produce a disorder structure, i.e., the amorphous form of Pd particles. The use of different reductants also slightly influences the size distribution of the Pd particles. As insets shown in Figure 2, the size distributions for samples Ci are slightly narrower than those for samples Ai, correspondingly. IV. Conclusion Thiol-stabilized palladium particles with various narrow size distributions and different structures were obtained by the two-phase system and ligand-exchange reaction. NaBH4 reduces Pd(II) ions to Pd atoms quickly resulting in the formation of amorphous Pd particles, while ethanol reduces Pd(II) ions gently resulting in the formation of crystalline Pd particles. The HREM images and electron diffraction patterns clearly show different structure in crystalline and amorphous Pd particles. STS results further stress the differences in electronic structure between the two kinds of Pd particles. Acknowledgment. This work was supported by the National Project for the Development of Key Fundamental Sciences in China (Grant No. G2001CB3095), by the Natural Science Foundation of China (Grant Nos. 50121202, 50132030,10074059, 19904012). LA034160A
