Synthesis, Characterization, and Magnetic Properties - American

Article pubs.acs.org/Langmuir

Water-Soluble Pd Nanoparticles Capped with Glutathione: Synthesis, Characterization, and Magnetic Properties Sachil Sharma, Bit Kim, and Dongil Lee* Department of Chemistry, Yonsei University, Seoul 120-749, Korea S Supporting Information *

ABSTRACT: The synthesis, characterization, and magnetic properties of water-soluble Pd nanoparticles capped with glutathione are described. The glutathione-capped Pd nanoparticles were synthesized under argon and air atmospheres at room temperature. Whereas the former exhibits a bulklike lattice parameter, the lattice parameter of the latter is found to be considerably greater, indicating anomalous lattice expansion. Comparative structural and compositional studies of these nanoparticles suggest the presence of oxygen in the core lattice when Pd nanoparticles are prepared under an air atmosphere. Both Pd nanoparticles prepared under argon and air show ferromagnetism at 5 K, but the latter exhibits significantly greater coercivity (88 Oe) and magnetization (0.09 emu/g at 50 kOe). The enhanced ferromagnetic properties are explained by the electronic effect of the incorporated oxygen that increases the 4d density of holes at the Pd site and localizes magnetic moments.

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INTRODUCTION In recent years, noble metal nanoparticles with sizes of less than 5 nm have drawn immense research interest because of their unique optical, electrochemical, and catalytic properties and their promising applications ranging from catalysis1,2 to sensors3,4 to nanoelectronics.5,6 These metal nanoparticles are generally synthesized by capping the metal surface in situ with specific ligands such as thiols,7,8 amines,9,10 and phosphines11 to render them stable in solution as well as in dried forms. Glutathione (GSH; γ-Glu-Cys-Gly) is a very special tripeptide having two −COOH groups, one −NH2 group, and one −SH group. Because noble metals have a high affinity for S, the GSH molecule can be an effective capping ligand for their synthesis. The carboxylate and amine functionalities contained in glutathione can be further utilized for anchoring the nanoparticles onto metal oxides such as ZnO and TiO2 to prepare composite catalysts12,13 and clinical drugs for cancer therapy.14 Biomolecular glutathione has been extensively used as the capping ligand for the synthesis of water-soluble nanoparticles of Au,8,15 Ag,14,16,17 Pt,18,19 ZnSe,20 Zn1 − xCdxSe,20 CdS,21 and CdTe.21 In particular, there has been significant progress in the synthesis and applications of monodisperse glutathione-capped Au8,15,18,22−24 and Ag14,16,17 nanoparticles with definite core sizes during the past few years. However, much less progress has been made in the synthesis of thiol-capped Pd nanoparticles,25,26 and to the best of our knowledge, there is no report on the synthesis of water-soluble Pd nanoparticles capped with glutathione. Unlike Au nanoparticles, Pd has less resistance toward oxidation,27,28 and thus the potential oxidation of Pd nanoparticles during their solution-phase synthesis has to be taken into account. However, the oxidation process and its effect on the physical and chemical properties of Pd © 2012 American Chemical Society

nanoparticles are not sufficiently understood yet, although there have been studies that revealed the presence of oxygen as a surface oxide (PdO) shell on Pd nanoparticles.29,30 For example, Coronado et al.29 found 10.7 to 45.8% oxide present in the Pd nanoparticles and assigned it as the surface oxide shell surrounding the metal core. Li et al.30 have also reported the surface oxidation process occurring on n-alkylamine-stabilized Pd nanoparticles by conducting a spectral analysis of Pd 3d Xray photoelectron spectroscopy (XPS). We report here the synthesis of water-soluble Pd nanoparticles capped with glutathione in the size range of 1−4 nm. The glutathione-capped Pd nanoparticles were prepared under air and argon atmospheres at room temperature. Anomalous lattice expansions were observed for Pd nanoparticles prepared under air, whereas there was no lattice expansion observed in the case of Pd nanoparticles prepared under an argon atmosphere. Comparative structural and compositional studies have suggested the presence of oxygen in the core lattice of Pd nanoparticles prepared under an air atmosphere. The presence of oxygen in the core has a significant influence on the structural and magnetic properties of Pd nanoparticles.

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EXPERIMENTAL SECTION

Chemicals. Palladium(II) chloride (PdCl2, 99.999%, Aldrich), reduced L-glutathione (GSH, ≥98%, Aldrich), sodium borohydride (99%, Aldrich), hydrochloric acid (2.0 M solution, Aldrich), isopropyl alcohol (99.9%, Duksan), and hexane (95%, Duksan) were used as received. Water was purified using a Millipore Milli-Q system (18.2 MΩ·cm). Received: August 16, 2012 Revised: October 22, 2012 Published: October 23, 2012 15958

dx.doi.org/10.1021/la303326u | Langmuir 2012, 28, 15958−15965

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Characterization. Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2100F high-resolution transmission electron microscope (HRTEM) with an acceleration voltage of 200 kV or a JEOL JEM-2010 high-contrast electron microscope operating at 200 kV. TEM or HRTEM data analysis was carried out using digital micrograph software (version 3.11.0). TEM samples of water-soluble glutathione-capped Pd nanoparticles were prepared by phase transferring them to toluene using tetraoctylammonium bromide as a phase-transfer reagent and then drop casting the toluene solution on a 400 mesh Formvar/carbon-coated copper grid (01814-F, Ted Pella). UV−vis absorption spectra were collected using a Shimadzu (UV-3600) UV−vis−NIR spectrophotometer. XPS measurements were carried out on an XPS system (VG Escalab 220i-XL) using a monochromatic Al Kα X-ray source (1486.6 eV). Binding energies (BE) were referenced to the C 1s BE at 284.8 eV. The peak position and integrated intensities were obtained by curve fitting using surface chemical analysis software (Thermo Avantage version 5.35). The field-dependent magnetization data (M−H curves) were recorded using a superconducting quantum interference device (SQUID, Quantum Design) magnetometer at 5 K in the range of −5 to +5 T. The diamagnetic contribution from the sample holder was subtracted from the magnetization data. Transition-metal impurities in Pd nanoparticles were quantified using an inductively coupled plasma mass spectrometer (ICP-MS, DRC II, Perkin-Elmer). Synthesis of Pd Nanoparticles under an Air Atmosphere. Water-soluble glutathione-capped Pd nanoparticles were prepared under an air or argon atmosphere for their comparative study. Both types of Pd nanoparticles were synthesized at room temperature by reducing H2PdCl4 with NaBH4 in the presence of glutathione as a capping ligand. In these syntheses, the molar ratios of [GSH]/ [H2PdCl4] and [NaBH4]/[H2PdCl4] were fixed at 0.35 and 10, respectively. In a typical synthesis under an air atmosphere, a homogeneous aqueous solution of H2PdCl4 (10 mM) was prepared first by adding 0.177 g of PdCl2 in 1 mL of 2 M HCl solution, which was subsequently diluted to 100 mL and stirred overnight. To 39.2 mL of a H2PdCl4 solution (10 mM) placed in a 100 mL beaker, glutathione solution prepared by dissolving 0.0420 g of glutathione powder in 10.8 mL of water was swiftly added. Upon the addition of glutathione, the yellow color of the H2PdCl4 solution was changed to a turbid orange color within 2 min as a result of the formation of the Pd−glutathione complex (Figure 1 inset). The reaction mixture was further stirred at 360 rpm for 1 h. Subsequently, a fresh aqueous solution of NaBH4 was prepared by dissolving 0.148 g of NaBH4 powder in 6 mL of water and added dropwise to the reaction mixture. Upon addition of NaBH4 solution, the turbid orange color of the reaction solution was changed to black, indicating the formation of Pd nanoparticles. The black reaction solution was then stirred at 220 rpm

for 14 h. The resulting product solution contains various sized Pd nanoparticles in the range of 1−4 nm, and size purification was carried out by solvent fractionation. Precipitates of Pd nanoparticles were formed upon the addition of different amounts of a solvent mixture (hexane + isopropyl alcohol) and were subsequently separated by centrifugation at 4000 rpm for 10 min. The first fraction was obtained by the addition of 21 mL of hexane and 42 mL of isopropyl alcohol to the product solution with constant stirring. The other three fractions were subsequently obtained by the addition of 15 mL of isopropyl alcohol to the supernatant from the previous fraction with constant stirring. The collected Pd nanoparticles were washed three times with methanol. The Pd nanoparticles obtained from the first fraction were highly agglomerated. Reasonably monodisperse Pd nanoparticles with three core sizes (2.8 ± 0.4, 2.4 ± 0.3, and 1.1 ± 0.1 nm) were obtained from the second, third, and fourth fractions, respectively. Synthesis of Pd Nanoparticles under an Argon Atmosphere. Pd nanoparticles were also synthesized similarly under an inert argon atmosphere while keeping the molar concentrations of H2PdCl4, glutathione, and NaBH4 constant. Unlike the synthesis under an air atmosphere, the solution of the complex formed between glutathione and H2PdCl4 was a clear wine-red color under an argon atmosphere (Figure S1 inset, Supporting Information). In this synthesis, monodisperse Pd nanoparticles with four core sizes of 3.7 ± 0.8, 2.8 ± 0.3, 2.4 ± 0.2, and 1.7 ± 0.2 nm were obtained from the second, third, fourth, and fifth fractions, respectively, and the first fraction was highly agglomerated. The detailed experimental procedure is described in the Supporting Information.

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RESULTS AND DISCUSSION Figure 1 shows the UV−vis absorption spectra of aqueous solutions of H2PdCl4, the Pd−glutathione complex, and Pd nanoparticles. The H2PdCl4 solution shows two broad bands at 420 and 302 nm due to metal to ligand charge transfer31 and a broad band and a sharp peak at 237 and 208 nm, respectively, due to ligand to metal charge transfer.32 Upon the addition of glutathione solution to the H2PdCl4 solution, the bands at 420, 302, and 237 nm disappeared and a new broad absorption band appeared at around 270 nm (Figure 1). The new band at around 270 nm can be assigned to the thiolate S to PdII chargetransfer band, suggesting the formation of the PdII−glutathione complex.33 The formation of the PdII−glutathione complex can be understood by the hard and soft Lewis acids and bases concept34 by which the soft acid (Pd2+ ion) has a high affinity for the soft base (sulfur) as reported for Pd complexation33,35,36 with sulfur-containing peptides such as L-glutathione and Lcysteine. It was observed, however, that precipitation occurred at a high molar ratio of [GSH]/[H2PdCl4] during the complexation step and thus the [GSH]/[H2PdCl4] ratio was fixed at 0.35. The absorption spectrum of the Pd nanoparticle solution displays a monotonic decrease in absorbance with increasing wavelength and no surface plasmon resonance band observed as expected for Pd nanoparticles.30 The absorption spectra of all different sized Pd nanoparticles obtained are shown in Figure S2 (Supporting Information). The steeper increase in the absorbance from the visible to the UV region is observed with decreasing Pd nanoparticle size. Figure 2a−c shows bright-field TEM images of 2.8, 2.4, and 1.1 nm Pd particles prepared under air, respectively. The HRTEM images of the Pd nanoparticles and their corresponding fast Fourier transformation (FFT) patterns are also shown in the insets of Figure 2. The FFT diffraction analysis reveals that all of the prepared Pd nanoparticles have face-centered cubic (fcc) structure, and the dhkl spacing calculated for 2.8 nm Pd nanoparticles is d111 = 2.30 ± 0.01 Å, which is substantially larger than the standard JCPDS value for the bulk fcc Pd metal

Figure 1. UV−vis absorption spectra of aqueous solutions of H2PdCl4 (yellow), the Pd−glutathione complex (turbid orange), and Pd nanoparticles (black) formed during the course of particle synthesis under an air atmosphere. 15959


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Figure 2. Bright-field TEM images of glutathione-capped Pd nanoparticles synthesized under an air atmosphere with core sizes of (a) 2.8 ± 0.4, (b) 2.4 ± 0.3, and (c) 1.1 ± 0.1 nm. Insets show the HRTEM images of Pd nanoparticles and their corresponding FFT patterns. The HRTEM image of 1.1 nm Pd nanoparticles shows that the particles indicated by circles are poorly crystalline with multiple grain boundaries.

Figure 3. Bright-field TEM images of glutathione-capped Pd nanoparticles synthesized under an argon atmosphere with core sizes of (a) 3.7 ± 0.8, (b) 2.8 ± 0.3, (c) 2.4 ± 0.2, and (d) 1.7 ± 0.2 nm. Insets show the HRTEM images of Pd nanoparticles and their corresponding FFT patterns.

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(d111 = 2.245 Å, lattice parameter a = 3.89 Å, JCPDS card no. 46-1043). Even greater lattice expansion (>6%) is observed for 2.4 nm Pd nanoparticles, the d111 spacing of which is calculated to be 2.38 ± 0.01 Å. The lattice expansion appears to vary with the core size; that is, different lattice expansions are observed for 2.4 and 2.8 nm Pd nanoparticles. This result seems to arise from the different levels of oxygen incorporated into Pd nanoparticles as revealed by the XPS compositional analyses (vide infra). We were unable to calculate the dhkl spacing for 1.1 nm Pd nanoparticles because they were found to be poorly crystalline in nature with multiple grain boundaries (Figure 2c inset and Figure S3 in the Supporting Information). The lattice expansion observed here is rather surprising because bulklike lattice spacing values have been found for Pd nanoparticles synthesized under various conditions.10,25,37 However, the presence of an oxide layer on the surface of Pd nanoparticles has been reported, indicating the tendency toward oxidation. Considering the aerobic atmosphere used in this synthesis and the high tendency of Pd toward oxidation,29,30,38 it is likely that such an anomalous expansion in the d111 spacing observed here for Pd nanoparticles is associated with the presence of oxygen in the lattice. This observation prompted us to prepare Pd nanoparticles under an inert argon atmosphere and to compare the structural characteristics. Figure 3a−d shows bright-field TEM images of 3.7, 2.8, 2.4, and 1.7 nm Pd particles prepared under an argon atmosphere. As can be seen in the figures, the particle size distributions of Pd nanoparticles become narrower as the particle size decreases from 3.7 to 1.7 nm and the 2.4 and 1.7 nm Pd particles were found to be highly monodisperse in nature. Furthermore, it is interesting that the uniformity of the particle size is generally improved when the particles are synthesized under an argon atmosphere. This result appears to be associated with the formation step of the PdII−glutathione complex. As shown in the Figure S1 inset (Supporting Information), unlike the complex formed under an air atmosphere (Figure 1 inset), the color of the complex under argon is clear wine red, suggesting uniform complexation between PdII and glutathione. It is well known that glutathione tends to be oxidized to a disulfide form in an aerated aqueous solution,39 resulting in various forms of Pd−glutathione complexes. In addition, the dissolved oxygen can have a great influence on the stability of the complexes. It is therefore not surprising that more polydisperse particles are generated from the mixed complex precursor under aerobic conditions. HRTEM analysis reveals that the d111 spacing values of the Pd nanoparticles prepared under an argon atmosphere are 2.24, 2.22, and 2.24 Å (uncertainty = ± 0.01 Å) for 3.7, 2.8, and 2.4 nm Pd nanoparticles, respectively. This result shows that there is no lattice expansion because the lattice spacing values of the Pd nanoparticles are consistent with that of the fcc Pd metal (d111 = 2.245 Å). However, a d200 spacing of 2.0 ± 0.01 Å is observed for 1.7 nm Pd particles, which is slightly larger than that expected for the d200 plane of the fcc Pd metal (d200 = 1.945 Å, JCPDS card no. 46-1043). This result may reflect the lattice distortion of the very fine Pd nanoparticles as reported by Sun et al.,40 whereas the effect of oxygen incorporated into the Pd lattice cannot be completely ruled out. Taken together, the structural comparison of the Pd nanoparticles prepared under air and argon atmospheres clearly shows that anomalous lattice expansion is observed for Pd particles prepared under an air atmosphere, suggesting the possibility of oxygen incorporation into the lattice. To gain further insight into the composition and its consequence on the

nanoparticle structure, XPS measurements were performed. Figures 4 and 5 display the XPS spectra of the Pd 3d scan for Pd nanoparticles prepared under air and argon atmospheres, respectively.

Figure 4. Pd 3d XPS spectra of Pd nanoparticles synthesized under air with core sizes of (a) 2.8, (b) 2.4, and (c) 1.1 nm. The binding energy values are summarized in Table 1.

As can be seen in Figure 4, the Pd 3d XPS curves for Pd nanoparticles prepared under air were found to be highly asymmetric toward high BE and thus were deconvoluted into two sets of doublets for the Pd 3d3/2 and 3d5/2 peaks. The binding-energy values for the Pd 3d5/2 peak are collated in Table 1. Whereas the BE values in the range of 335.2 to 335.7 eV are consistent with that of Pd0 capped with thiol,41 the higher BE values in the range of 336.8 to 337.4 eV indicate the presence of the oxidized form of Pd (i.e., PdO).42,43 For 2.8 nm Pd nanoparticles, about 48% of the Pd atoms are present in an oxidized form. The fraction of oxidized Pd increases significantly to 64 and to 84% as the particle size decreases to 2.4 and 1.1 nm, respectively. The Pd 3d XPS curves for Pd nanoparticles prepared under an argon atmosphere are also asymmetric toward higher BE, which can be deconvoluted into two curves corresponding to PdO (red lines) and Pd0 (blue lines) as shown in Figure 5. In this case, however, Pd0 is the dominant form and considerably larger in fraction than PdO. As collated in Table 1, the fraction of oxidized Pd is found to be 24% for the 3.7 nm Pd particles and gradually increases to 37% as the particle size decreases to 1.7 nm. The oxidized form of Pd may also arise from the presence of PdII−glutathione. One may expect, however, that the ligand contribution is similar for the same-sized Pd nanoparticles prepared under air and argon atmospheres, and thus the higher fraction of the oxidized Pd observed in Pd nanoparticles prepared under air is most likely due to the 15961


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more oxygen tends to be incorporated into the Pd nanoparticles when synthesized under an air atmosphere and be present not only on the surface but also in the core lattice of the particles. The FFT diffraction in the Figure 2 inset shows that the core structure of Pd particles prepared under air is still the fcc structure, not PdO. Thus, it is likely that oxygen is incorporated presumably in the interstitial sites of the fcc lattice of Pd nanoparticles when prepared under air, leading to significant lattice expansion as observed in the Figure 2 inset. The presence of oxygen in the core may have a significant influence on the physical and chemical properties of Pd nanoparticles, and we examine the magnetic properties of 2.4 and 2.8 nm Pd nanoparticles prepared under air and argon atmospheres here. Ferromagnetic behavior has been observed at 5 K for Pd nanoparticles with various sizes and structural features.44,45 Two different mechanisms have been proposed to explain the permanent magnetism observed for Pd nanoparticles. For pure metallic Pd nanoparticles without a depletion shell structure on the surface, the structural and electronic factors, for example, surface anisotropy and twin boundaries, can contribute to the increase in the density of states near the Fermi level resulting in ferromagnetic order.44 When the surface is capped with covalently linked protecting molecules or an oxide passivation layer, the electronic interaction with the capping molecules on the surface can give rise to high local magnetic anisotropy.45 Figure 6a (blue line) displays the field-dependent magnetization (M−H) curves recorded at 5 K for 2.4 nm Pd nanoparticles prepared under argon. As can be seen in the figure, the presence of hysteresis (inset of Figure 6a) clearly indicates their ferromagnetic behavior at low temperature. The value of the coercive field (Hc) at 5 K was found to be 37 Oe with a magnetization value (M) of 0.07 emu/g at 50 kOe. The lack of saturation in the magnetization curves suggests that the paramagnetic fractions are still dominant in the sample even at 5 K. These values are comparable to those reported for 2.3 nm Pd nanoparticles capped with dodecanethiol prepared under an N2 atmosphere45 (Hc ≈ 40 Oe and M = 0.02 emu/g). The observed ferromagnetic behavior of dodecanethiol-capped Pd nanoparticles was ascribed to the thiol bond that increases the density of holes at the 4d band of Pd nanoparticles, leading to an increase in the localized magnetic moments. To examine the thiol effect on the magnetic behavior, we have prepared similarly sized gold nanoparticles (2.4 nm) capped with glutathione according to the procedure described elsewhere12 and compared the magnetic behavior. Ferromagnetic behavior with a high value of the coercive field (860 Oe at 5 K) was previously reported for dodecanethiol-capped Au nanoparticles.46 By contrast, Jin and co-workers8 found that 2 and 4 nm glutathione-capped Au nanoparticles exhibit paramagnetism even at 5 K. The origin of this discrepancy is unclear at present,47 but the magnetic behavior of the glutathione-capped Au nanoparticles in Figure 6a is consistent with the paramagnetic character; that is, they exhibit a linear increase in magnetization with the field without any hysteresis at 5 K. Additional XPS analysis in Figure S4 (Supporting Information) shows that the gold atoms in the particles are all metallic and no indication of gold oxide is present, unlike for the Pd nanoparticles. These results suggest that the presence of oxygen in the lattice and the nature of the metal are also responsible for the appearance of ferromagnetism for which Pd is presumably closer to satisfying the Stoner criterion than Au.48,49 The coercivity value for 2.4 nm Pd nanoparticles

Figure 5. Pd 3d XPS spectra of Pd nanoparticles synthesized under argon with core sizes of (a) 3.7, (b) 2.8, (c) 2.4, and (d) 1.7 nm. The binding-energy values are summarized in Table 1.

Table 1. Summary of Core Sizes, Lattice Parameter (a) Values, and Binding Energy Values of fcc Pd Nanoparticles Prepared under Air and Argon Atmospheres Pd 3d5/2 synthesis conditions air

argon

size (nm) 2.8 2.4 1.1 3.7 2.8 2.4 1.7

± ± ± ± ± ± ±

0.4 0.3 0.1 0.8 0.3 0.2 0.2

lattice parameter (a) (±0.01 Å)

Pd0 (eV)

PdO (eV)

Pd/PdO (±2%)

3.98 4.12

335.7 335.6 335.2 335.6 335.6 335.4 335.4

337.4 336.9 336.8 337.2 336.9 336.8 336.8

52/48 36/64 16/84 76/24 71/29 70/30 63/37

3.88 3.85 3.88 4.0

presence of oxygen in the nanoparticles. Considering the bulklike lattice parameter (a) found for the fcc Pd nanoparticles prepared under argon as discussed earlier, it is likely that PdO is predominantly present on the surface, not in the core, of Pd nanoparticles. In these particles, the surface oxygen appears to be generated during their storage under air as it increases with increasing storage time, typically over weeks. Accordingly, it is expected that the fraction of PdO increases with decreasing particle size (i.e., with increasing surface area) as observed in Figure 5. However, the fact that significantly higher fractions of PdO were found for particles prepared under air strongly suggests that oxidized Pd exists not only on the surface but also in the core of Pd nanoparticles when prepared under an air atmosphere. These XPS results clearly explain the lattice expansion found for Pd nanoparticles prepared under an air atmosphere. That is, 15962


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Figure 6. M−H curves of (a) 2.4 nm glutathione-capped Pd nanoparticles prepared under air and argon atmospheres and 2.4 nm glutathione-capped Au nanoparticles and (b) 2.8 nm glutathione-capped Pd nanoparticles prepared under air and argon atmospheres at 5 K. Insets show an enlarged view of the M−H curves around 0 Oe.

predicted from the total-energy band calculations.50,51 In summary, the comparison of the magnetic properties of the Pd nanoparticles prepared under air and argon atmospheres clearly indicates that the presence of oxygen in the core and the resulting lattice expansion are associated with the enhanced ferromagnetism found for the Pd nanoparticles prepared under an air atmosphere.

prepared under air is found to be 88 Oe, more than twice the value found for same-sized Pd nanoparticles prepared under argon. The magnetization value (0.09 emu/g at 50 kOe) is also greater than for Pd nanoparticles prepared under argon. The M−H curves (Figure 6b) recorded at 5 K for 2.8 nm Pd nanoparticles prepared under air and argon atmospheres also show a similar trend. That is, 2.8 nm Pd nanoparticles prepared under air exhibit a higher coercivity and magnetization (Hc = 56 Oe and M = 0.06 emu/g at 50 kOe) at 5 K than those of the same-sized Pd nanoparticles prepared under argon (Hc = 23 Oe and M = 0.02 emu/g at 50 kOe). This is interesting because the result shows that the magnetic property is indeed related to the structural and compositional differences between the two Pd nanoparticles found in HRTEM and XPS analyses. That is, the presence of oxygen in the core of Pd nanoparticles prepared under air may give rise to a dramatic increase in the ferromagnetic property. The presence of transition-metal impurities such as Fe, Co, Ni, Cr, and Mn in the Pd nanoparticles may also have an effect on the observed ferromagnetism. The XPS wide-scan spectra of 2.4 and 2.8 nm Pd nanoparticles prepared under air in Figure S5 (Supporting Information) show no sign of their presence. More detailed quantitative analyses of 2.4 nm Pd nanoparticles prepared under air and argon atmospheres using ICP-MS revealed that the major transition-metal impurities are less than 10 ppm in both Pd nanoparticles. More importantly, the impurity concentrations in the Pd nanoparticles prepared under an air atmosphere are similar or smaller than those in the Pd nanoparticles prepared under an argon atmosphere as collated in Table S1 (Supporting Information). These results indicate that the effect of the transition-metal impurities is insignificant and lead us to conclude that the enhanced ferromagnetism observed in the Pd nanoparticles prepared under air is associated with the oxygen incorporated into the core. Considering the small size and compositional heterogeneity of the Pd nanoparticles, the permanent magnetism observed does not seem to be related to the metallic ferromagnetic instability of bulk Pd but rather to the strong magnetic anisotropy. In other words, the chemical bonding to depleting atoms (O and S in this case) leads to an increase in the 4d density of holes at the Pd site and localized magnetic moments. In addition to the electronic effect, the volume expansion induced by the incorporated oxygen may additionally contribute to the enhanced ferromagnetism by a depletion of s and p states and a corresponding increase in d states as

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CONCLUSIONS We have shown that reasonably monodisperse, water-soluble Pd nanoparticles capped with glutathione can be prepared in the size range of 1−4 nm. Comparative HRTEM studies of Pd nanoparticles prepared under argon and air atmospheres have found anomalous lattice expansions when Pd nanoparticles are prepared under an air atmosphere. XPS compositional studies of these nanoparticles suggest the presence of oxygen in the core lattice of Pd nanoparticles prepared under an air atmosphere. These particles exhibit significantly enhanced ferromagnetic properties, reflecting the electronic effect of oxygen incorporated into the core lattice. This work provides the first quantitative results demonstrating that the presence of oxygen in the core lattice has a significant influence on the structure and magnetic properties of Pd nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of Pd nanoparticles under an argon atmosphere. Supplementary UV−vis absorption spectra, HRTEM image, XPS spectra, and ICP-MS analysis of Pd nanoparticles. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (82-2)-2123-5638. Fax: (822)-364-7050. Home page: http://chem.yonsei.ac.kr/ ∼nanomat/. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Mid-Career Researcher Program (20110029735), the Basic Science Research Program (20110004697), the World Class University Program (R3215963


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102170), and the Priority Research Centers Program (20110022975) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. We thank the Korea Basic Science Institute-Seoul Center for help with SQUID and ICP-MS analyses.

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