Investigating Unexpected Magnetism of Mesoporous Silica-Supported

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Investigating Unexpected Magnetism of Mesoporous SilicaSupported Pd and PdO Nanoparticles Hyon-Min Song,†,‡,§ Jeffrey I. Zink,*,‡ and Niveen M. Khashab*,† †

Division of Physical Sciences and Engineering, and Center for Advanced Membranes and Porous Materials (AMPM), 4700 King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia ‡ Department of Chemistry, University of California, Los Angeles, California 90095-1569, United States § Department of Chemistry, Dong-A University, Busan 604-714, South Korea S Supporting Information *

ABSTRACT: The synthesis and magnetic behavior of matrix-supported Pd and PdO nanoparticles (NPs) are described. Mesoporous silica with hexagonal columnal packing is selected as a template, and the impregnation method with thermal annealing is used to obtain supported Pd and PdO NPs. The heating rate and the annealing conditions determine the particle size and the phase of the NPs, with a fast heating rate of 30 °C/min producing the largest supported Pd NPs. Unusual magnetic behaviors are observed. (1) Contrary to the general belief that smaller Pd NPs or cluster size particles have higher magnetization, matrix-supported Pd NPs in this study maintain the highest magnetization with room temperature ferromagnetism when the size is the largest. (2) Twin boundaries along with stacking faults are more pronounced in these large Pd NPs and are believed to be the reason for this high magnetization. Similarly, supported PdO NPs were prepared under air conditions with different heating rates. Their phase is tetragonal (P42/mmc) with cell parameters of a = 3.050 Å and c = 5.344 Å, which are slightly larger than in the bulk phase (a = 3.03 Å, c = 5.33 Å). Faster heating rate of 30 °C/min also produces larger particles and larger magnetic hysteresis loop, although magnetization is smaller and few twin boundaries are observed compared to the supported metallic Pd NPs.

1. INTRODUCTION Ferromagnetism observed in Pd nanoparticles (NPs) and similar noble metal NPs is believed to result from the confined geometry of those nanosize materials.1 This postulated confined geometry is described in many different ways, such as a thin layer film,2,3 a thin wire,4 a few nanometer size clusters,5,6 grain boundaries and twin boundaries,6,7 reduced or unsaturated coordination of surface atoms,8 and the expansion of lattice spacings.9 Of these, twin boundaries are of special interest because they are ubiquitous in noble metal NPs and also contain some of the characteristics of confined geometry, such as lattice spacing expansion and coordinately unsaturated surface atoms around the twin boundaries. Although the occurrence of twin boundaries is dependent on the size of the NPs (with larger NPs containing more twin boundaries), it is also known that even 2 nm size NPs, such as palladium6 and gold,10−12 contain twin boundaries. In the case of unsupported Pd NPs, it is known that smaller size NPs maintain higher magnetization. In Pd NPs with a diameter between 10 and 30 nm, the 10 nm size NPs are reported to maintain higher magnetization.13 In the size regime between 2 and 10 nm, the 2 nm NPs are reported to preserve the highest magnetization.14 One exception is that, in the diameter range between 0.9 and 3.5 nm, the larger NPs showed higher magnetization.5 This deviation from the trend is likely due to the magnetic instability of cluster-size small particles. © XXXX American Chemical Society

The occurrence of magnetism at the nanoscale in otherwise nonmagnetic materials is thought to stem from the unstable surface atoms in those nanosize particles. Especially, surface atoms around twin boundaries are more vulnerable to the defects and symmetry change. Their lattice spacings tend to be larger than those in the crystalline structure, and the coordination of surface atoms around twin boundaries will be affected to be more unsaturated. All these structural features are thought to contribute to the magnetism of Pd NPs. In this study, impregnation of the template followed by thermal annealing was used to synthesize mesoporous silicasupported Pd and PdO NPs. Two shapes of matrices for the imbedded Pd or PdO NPs are chosen: mesoporous silica nanospheres with 2-dimensional hexagonal columnal porosity templated by cetyltrimethylammonium bromide (CTAB),15 and rod-shaped mesoporous silica synthesized using CTAB and perfluorooctanoic acid as the shape-directing agent.16 The silica support was impregnated with H2PdCl4 and annealed at 400 or 500 °C for 4 h under vacuum (to produce metallic Pd nanocrystals) or in air (to produce PdO nanocrystals). The heating rate is the most important parameter to control the particle sizes of the imbedded nanocrystals in the matrix. The Received: July 10, 2014 Revised: September 29, 2014

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dx.doi.org/10.1021/cm502448p | Chem. Mater. XXXX, XXX, XXX−XXX

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ferromagnetism of both compositions of nanocrystals is also affected by the heating rate and the particle size. The ferromagnetic properties of pure, nonalloyed metallic Pd NPs are in and of themselves quite interesting because they are in contrast to the more common and popularly known paramagnetism of bulk Pd. The magnetism of mesoporous silicasupported Pd and PdO NPs is discussed in terms of structural relationships caused by the heating rate, size of NPs, twin boundaries, and stacking faults in the NPs.

2. EXPERIMENTAL METHODS Powder X-ray diffraction (XRD) measurements were performed with a Panalytical X’Pert Pro X-ray powder diffractometer using Cu Kα radiation (45 V, 40 mA, λ = 1.54056 Å) in a θ−θ mode from 15 to 100 degrees (2θ) in wide angle diffraction measurements and from 1 to 10 degrees in small angle diffraction measurements. Transmission electron microscopy (TEM) images were obtained with either a Tecnai G2 Spirit TWIN, 20−120 kV/LaB6 Transmission Electron Microscope or an FEI Titan 80−300 kV S/TEM using a field emission gun operating at 300 kV. Scanning transmission electron microscopy (STEM) images were obtained with a high-angle annular dark field (HAADF) detector by adjusting the camera length to 190 mm. TEM samples were prepared by sonicating 0.5 mg of each sample in 200 μL of methanol for 2 min, followed by dropcasting onto carbon-coated Cu grids and drying them in air for 30 min before the measurement. Spherical and rod-shape mesoporous silica was prepared following a literature method.16 A Superconducting Quantum Interference Device (SQUID) was used to measure magnetic properties. Specifically, a Magnetic Property Measurement System (MPMS) was adopted to measure field-dependent magnetization. Synthesis of mesoporous silica-supported Pd NPs. Pd(II)Cl2 (CAS 7647-10-1, 354.6 mg, 2.0 mmol) was mixed with HCl (37.5%, 389 mg, 4.0 mmol) in a one-neck round bottomed flask (10 mL). The mixture was stirred at 50 °C for 1 h. Deionized water (5 mL) was added, and the mixture was sonicated for 1 min. After 30 min, the deep brown reaction mixture was filtered with a 25 mm GD/X - nonsterile syringe filter (0.2 μm, by Whatman). Filtered H2PdCl4 aqueous solution was used to impregnate mesoporous silica. Before placing on top of the glass filter, mesoporous silica was finely grinded and dried under vacuum for 2 h. With vacuum applied, aqueous H2PdCl4 solution was dropped on mesoporous silica. After repeated infiltration (×10), mesoporous silica was dried under vacuum. Then, the dried brownish mesoporous silica was transferred to the annealing crucible. Thermal annealing was performed under nitrogen atmosphere at the heating rate of 30 °C/min or 2.5 °C/min. The annealing temperature was 400 or 500 °C. After reaching the annealing temperature, the sample stayed for an additional 4 h. Synthesis of mesoporous silica-supported PdO NPs. Similarly to the synthesis of supported Pd NPs, finely grinded mesoporous silica was infiltrated repeatedly (×10) with H2PdCl4 and dried under vacuum. Brownish mesoporous silica was transferred to the annealing crucible. Thermal annealing was performed under air conditions at the heating rate of 30 °C/min or 2.5 °C/min. The annealing temperature was 500 °C. After reaching the annealing temperature, that temperature was maintained for an additional 4 h.

Figure 1. (a) TEM and (d) STEM images of spherical mesoporous silica-supported Pd NPs prepared at a heating rate of 30 °C/min and at an annealing temperature of 400 °C. (b) TEM and (e) STEM images of supported Pd NPs prepared at a heating rate of 2.5 °C/min and at an annealing temperature of 400 °C. (c) TEM and (f) STEM images of rod-shaped mesoporous silica prepared at a heating rate of 2.5 °C/min and at an annealing temperature of 400 °C. All scale bars indicate 100 nm.

10.8 (±3.02) (Figure 1b, 1e) and 10.4 (±2.56) nm (Figure S10) were obtained, respectively. Supported Pd NPs in rodshape mesoporous silica were obtained in a similar method by a heating rate of 30 °C/min and at an annealing temperature of 400 °C (Figure S4). A heating rate of 2.5 °C/min and annealing temperature at 400 and 500 °C produce supported Pd NPs in rod-shape mesoporous silica with average diameters of 10.0 (±2.42) nm (Figure 1c, 1f) and 9.4 (±2.23) nm (Figure S16), respectively. The slow heating rate produces dispersed Pd NPs on the surface of mesoporous silica with a narrower size distribution than those synthesized with a faster heating rate. Large size supported Pd NPs under fast thermal annealing, in particular 29.3 nm Pd NPs annealed at 400 °C, are believed to be due to the combined effect of fast coalescence between Pd NPs with partial thermal melting of mesoporous silica. This partial melting of mesostructures is observed in HRTEM (Figure 2a), and under this annealing condition, Pd NPs diffuse and coalescence readily to produce large size Pd NPs. Silica mesostructures were examined with TEM ad STEM (Figure 2) after annealing. Mesostructures were well maintained after annealing, although partial melting occurred at the heating rate of 30 °C/min and at the annealing temperature of 400 °C (Figure 2a). When thermal heating is slow (2.5 °C/ min), obvious hexagonal (P6/m) columnal mesoporous silica was observed. The mesoporous structure is viewed along the [110] direction (Figure 2b) or viewed along the [100] direction (Figure 2c) with a hexagonal [HKL] index.17 A porous structure is also observed in the STEM image, in which metallic Pd with high electronic density can be clearly identified compared to Si with lower electronic density (Figure 2d). Distinctive twin boundaries were observed in supported Pd NPs. Especially in large size Pd NPs which were prepared at a heating rate of 30 °C/min, the region of the twin boundary is broad, and it is not a single twin boundary, but there are parallel multiple twin boundaries with stacking faults (Figure 2e). The Fourier-filtered image also indicates twin boundaries and stacking faults (Figure 2f). The average size of Pd NPs obtained in the fast thermal heating is large (29.3 nm), and they are not exactly spherical. Enormous strain follows in maintaining spherical shape NPs, especially in face-centeredcubic (fcc) phase crystals, and twin boundaries and stacking

3. RESULTS AND DISCUSSION To synthesize supported Pd NPs, MCM-41 type mesoporous silica was impregnated with H2PdCl4 and underwent thermal annealing at 400 or 500 °C for 4 h under an inert atmosphere. The heating rate is the main parameter to change the size of matrix-supported Pd NPs. At the heating rate of 30 °C/min and at the annealing temperature of 400 °C, Pd NPs with an average diameter of 29.3 (±6.26) nm were obtained, as seen in TEM (Figure 1a) and STEM (Figure 1d) images. At the heating rate of 2.5 °C/min and annealing temperature at 400 and 500 °C, supported Pd NPs with an average diameter of B



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Figure 2. (a) TEM image of supported Pd NPs prepared at a heating rate of 30 °C/min and at an annealing temperature of 400 °C. (b and c) TEM images and (d) STEM image of supported Pd NPs prepared at a heating rate of 2.5 °C/min and at an annealing temperature of 400 °C. Hexagonal (P6/m) columnal mesoporous silica (b) is viewed along the [110] direction, and (c and d) are viewed along the [100] direction. (e) HRTEM image and (f) Fourier-filtered image of supported Pd NP prepared at a heating rate of 30 °C/min and at an annealing temperature of 400 °C. HRTEM images (g and h) of supported Pd NP prepared at a heating rate of 2.5 °C/min and at an annealing temperature of (g) 400 °C and (h) 500 °C. (i) Small angle and (j) wide angle XRD patterns of mesoporous silica-supported Pd NPs. R denotes rod-shaped and S denotes spherical mesoporous silica.

faults are those to compensate for these large surface energies.17 In supported Pd NPs obtained at a lower heating rate (2.5 °C/ min), the size of Pd NPs is small and a single twin boundary (Figure 2h) or multiple twin boundaries (Figure 2g) in one particle are observed. However, stacking faults around twin boundaries are hardly found. When the heating rate is slow (2.5 °C/min) and the annealing temperature is 500 °C, significant numbers of single crystal Pd NPs without twin boundaries were observed (Figures S11, S17). Surface energy minimization by thermal energy, and not by the formation of twin boundaries, is assumed to be the reason. Small angle XRD patterns of mesoporous silica-supported Pd NPs were measured in a θ−θ mode (Figure 2i), in which the sample is stationary and the X-ray beam and detector move simultaneously. Three major peaks of (100), (110), and (200) were observed, which are from the hexagonal phase packed by ordered columns. Those ordered hexagonal columns are the templates made from CTAB and are characteristic of hexagonal MCM-41 type.15,16 D-spacings vary, as they are around 37 Å in spherical particles and around 38.7 Å in rod-shape particles. In all of the measurements, the pore size of spherical silica is larger

than that in rod-shape mesoporous silica. When the heating rate is fast (30 °C/min), the peaks are not as distinctive and the peak positions also shift toward smaller d-spacings. As is observed in the TEM image in Figure 2a, those particles have mesostructures with locally melted pores with smaller pore sizes. Contrary to the diversity in small angle XRD, metallic Pd is the dominant phase in the wide angle XRD patterns (Figure 2j). In the field-dependent magnetization of supported Pd NPs obtained at a heating rate of 30 °C/min and at the annealing temperature of 400 °C, hysteresis exists with typical ferromagnetic behaviors (Figure 3). Traces of hysteresis expand at high applied magnetic fields, in particular at low temperature measurements. This existence of hysteresis is assumed due to the matrix effect that the spin reorientation according to the change of magnetic field is retarded within the silica matrix. The atomic ratio between Si and Pd is about 94:6 (Figure S3), and this large amount of Si acts as an efficient matrix such that dipolar interaction between neighboring Pd NPs is minimized. The coercivity of Pd NPs in the matrix is 103 Oe at 5 K, 126 Oe at 40 K, and 45 Oe at 300 K. In all of the measurements, C



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paramagnetism in the field-dependent measurements even at 5 K. In addition, twin boundaries are hardly observed in the TEM images of nonsupported metallic Pd NPs. A matrix effect on the enhancement of magnetism of Pd NPs has been demonstrated in the examples of carbon matrix,18 graphite matrix,19 apoferritin matrix,20 normal SiO2 matrix21 and C60 matrix.22 There is an early study about the diffusion of Pd ions into a SiO2 matrix.23 Unlike Ag and Cu, which readily diffuse into SiO2 and exchange with Si cations, Pd and Au are found not to diffuse into SiO2. A low solid solubility of these two metals is thought the reason. The diffusion of a small number of metal cations into the position of tetrahedral Si4+ is in fact well-known. Exchange of cations changes physical properties, such as magnetism and the mechanical strength of the composites. It can be assumed from this early study23 that without exchanging Pd cations with Si4+, Pd NPs are much more affected mechanically by stress and strain than electronically. The enhanced magnetism observed in our study is thought to stem from two factors. One is the intrinsic stress experienced by Pd NPs during the fast thermal annealing, which makes the structure of Pd NPs more deviated from equilibrium status and, hence, causes structural instability. The second factor is the outer factor from the silica matrix, as more mechanical stress is driven toward Pd NPs by the silica matrix during fast thermal annealing. The theoretical results of Reddy, Khanna, and Dunlap suggested the importance of nanocrystal shape in the magnetism of Pd NPs.24 They studied the magnetic properties of 13-atom gas-phase clusters of Pd, Rh, and Ru, and found all of them ferromagnetic in the icosahedral shape. Twin boundaries in icosahedral and decahedral NPs are common features.18 In sufficiently large spherical NPs, especially in noble metal NPs, severe strain sometimes makes it hard to maintain spherical shapes, and with the assistance from twin boundaries, they eventually get the faceted shapes.25 Decahedrons (5 tetrahedons) and icosahedrons (20 tetrahedrons) are formed from tetrahedrons with the most stable and most common {111} planes in fcc noble metals, but twin boundaries accompany the formation due to the misfit regions.17 Ferromagnetism in these decahedrons and icosahedrons was observed in Pd NPs experimentally as well.26 Twins are most

Figure 3. Field-dependent magnetization of spherical mesoporous silica-supported Pd NPs prepared at a heating rate of 30 °C/min and at an annealing temperature of 400 °C.

diamagnetism from the silica matrix was subtracted by measuring control samples of the annealed silica matrix. In the temperature-dependent magnetization measurements, the magnetic transition was observed at around 188 K, below which it changes from paramagnetism to ferromagnetism (Figure 4). This transition appears in measurements with applied magnetic fields of 100 Oe (Figure 4a) and 1000 Oe (Figure 4b). The inset in each figure is T vs 1/M of field-cooled measurements and manifests this magnetic transition by showing the deviation from Curie−Weiss law and the linear pattern of paramagnetism above the transition temperature. In all of these measurements, the zero-field-cooled (zfc) measurement started first; that is, the sample was cooled under no magnetic field before the measurement. For the comparison of this unusual magnetic activity of annealed Pd NPs in the mesoporous silica matrix, nonsupported Pd NPs were prepared and annealed under N2 conditions to 400 °C at a rate of 30 °C/min (Supporting Information, Figure S20). Annealed particles are greatly coalesced and aggregated, and they show more likely

Figure 4. Temperature-dependent magnetization of spherical mesoporous silica-supported Pd NPs prepared at a heating rate of 30 °C/min and at an annealing temperature of 400 °C. Measurements were carried out with an applied magnetic field of (a) 100 Oe and (b) 1000 Oe. The measurements started from zero-field-cooled (zfc, solid symbol) and were followed by field-cooled (fc, open symbol) measurements. D



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Figure 5. (a) TEM and (e) STEM images of spherical mesoporous silica-supported PdO NPs prepared at a heating rate of 30 °C/min, and (b) TEM and (f) STEM images obtained at a heating rate of 2.5 °C/min. (c) TEM and (g) STEM images of rod-shaped mesoporous silica-supported PdO NPs prepared at a heating rate of 30 °C/min, and (d) TEM and (h) STEM images obtained at a heating rate of 2.5 °C/min. All scale bars indicate 50 nm.

Figure 6. (a) HRTEM image and (b) Fourier-filtered image of supported PdO NPs prepared at a heating rate of 2.5 °C/min. (c) HRTEM image and (d) SAED pattern of supported PdO NPs prepared at a heating rate of 30 °C/min. The annealing temperature is 500 °C. (e) Small angle and (f) wide angle XRD patterns of mesoporous silica-supported PdO NPs. R denotes rod-shaped and S denotes spherical mesoporous silica.

composites under 1 bar O2 at 500 °C for 24 h,27 and similarly by the oxidative treatment of Pd/SiO2 under 1 bar O2 at 400 °C for 1 h.28 The growth of PdO, however, is affected by the metallic fcc Pd phase such that epitaxial growth was observed with the Pd phase as the core.28 Pure PdO NPs without a metallic Pd core are rare. In HRTEM images, PdO NPs are single crystalline without twin boundaries or stacking defaults. The major plane of (101) with a lattice spacing of 2.64 Å was identified (Figure 6a, 6c). The Fourier-filtered image also proves single crystallinity

common in these shapes, and it is assumed that there is a possible correlation between NP shape and the induction of ferromagnetism. Supported PdO NPs in the mesoporous silica were prepared by the impregnation method similar to that used in the synthesis of supported Pd NPs. Heating rates of 30 °C/min (Figure 5a,5c,5e,5g) or 2.5 °C/min (Figure 5b,5d,5f,5h) in air were adopted. The annealing temperature was 500 °C for 4 h in air. Previously reported examples of matrix-supported PdO NPs were prepared by the oxidation of metallic Pd/Al 2 O 3 E



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refinement (inset, Figure 7), with white balls indicating palladium atoms, and red indicating oxygen atoms. Each Pd has a square planar coordination with the adjacent four O atoms, and each O makes a tetrahedral coordination with the four adjacent Pd atoms. The cell parameters are slightly larger than in bulk PdO (a = 3.02 Å and c = 5.31 Å).32 This lattice expansion is common in nanosize materials, especially in metal oxides.36 Explanations for the lattice expansion are various, such as the size effect37 or the ionic valence change of cations in nanosize.38 Tetragonal and hexagonal cells are more susceptible to this lattice parameter change because elongation or contraction into the a or c axis in the anisotropic cell is easier than in the cubic cell.36,39 Here, elongation into both a and c axes is observed. In the field-dependent magnetization measurement of supported PdO NPs, magnetization is smaller (0.156 emu/g at 5 K), but coercivity is larger than in supported metallic Pd NPs. The coercivity of supported PdO NPs prepared at a heating rate of 30 °C/min is 177 Oe at 5 K and 145 Oe at 300 K (Figure 8a). The higher coercivity of supported PdO NPs is thought to derive from the anisotropy of the tetragonal unit cell of PdO (inset, Figure 7). High magnetocrystalline anisopropy, and hence high coercivity, is in some materials, such as tetragonal FePt and FePd,40 related with the anisotropic unit cell, In the tetragonal unit cell, the layer-by-layer arrangement of Pd and O is also reminiscent of those layered arrangements of Fe and Pd (or Pt) in FePd (or FePt). Moreover, similar coercivity at room temperature (Figure 8a) implies that magnetic moments of supported PdO NPs are blocked even at room temperature. The other noteworthy behavior of supported PdO NPs is that saturation was not reached at high applied magnetic fields when measured at 5 K (inset, Figure 8a). In fact, the hysteresis measured at 5 K is very typical of antiferromagnetic NPs. With coercivity and the trace of hysteresis at high applied field, supported PdO NPs are thought to follow antiferromagnetic behavior at low temperature. This unusual magnetism of PdO NPs is also reflected in the temperature-dependent magnetization, which was measured with an applied field of 1000 Oe (Figure 8b). The magnetic transition is observed at a temperature of 119 K, and the transition is similar to antiferromagnetism. Many metal oxides are antiferromagnetic even at nanosize, and due to this nanosize, surface spins tend to be uncompensated and thus contribute to the coercivity. With relatively large coercivity and the difficulty to saturate, as seen in the field-dependent measurement (Figure 8a), and with the existence of hysteresis at the high applied magnetic fields, these supported PdO NPs are presumed to follow some of the features of hard magnetism, or more likely follow the magnetic behaviors of antiferromagnetic metal oxide NPs.41

without twin defects (Figure 6b). Particle diameters are 12.4 nm (Figure 6a) and 18.5 nm (Figure 6c), and both are larger than the majority of twin boundary-containing Pd NPs prepared in this study. Twin boundaries are commonly found in metals or metal oxides with a cubic phase. Twin boundaries in metals are broadly found, while a few examples are known in metal oxides, such as cubic spinel MgAl2O429 or Fe3O4.30 Nanomaterials with a tetragonal phase are hardly known for their twin boundaries. Tetragonal zirconia (ZrO2) NPs are reported to possess twin boundaries, but they reside on the boundary between two crystal systems of tetragonal and orthorhombic, and not within the tetragonal structure.31 In general, the lowest surface energy in fcc is (111) planes, but close packing produces strains, particularly in small size spherical nanoparticles. In order to reduce surface energy and due to the presence of a misfit region in the spherical NPs, twin boundaries happen universally in fcc metals. The selected area diffraction pattern (SAED, Figure 6d) indicates high crystallinity of supported PdO NPs. The pattern is in accordance with powder XRD patterns (Figure 6f) and is the tetragonal phase. Small angle XRD patterns in Figure 6e show that mesoporous structures are maintained after thermal heating. Three tetragonal phases of PdO have been reported. The most stable form is primitive tetragonal with a space group of P42/mmc with a cell parameter of a = 3.02 Å and c = 5.31 Å (PDF number 41-1107).32,33 Under ambient conditions, PdO was made as this tetragonal phase, as in our supported PdO NPs. A different unit cell was found under high pressure (12 GPa) with a space group of I4/mmm and with a cell parameter of a = 2.982 Å and c = 5.383 Å (PDF number 43-1024). Contraction along the a axis was noticeable.34 An unusual NaCl-type cubic phase (Fm3̅m, Z = 4) with a cell parameter of a = 5.65 Å (PDF number 46-1211) was reported for thin film PdO prepared by a vacuum deposition within an alumina support.35 In all of the matrix-supported PdO NPs in this study, the most stable tetragonal phase of PdO was formed by annealing at 500 °C in air. Based on the Rietveld refinement, cell parameters of a = 3.050 Å and c = 5.344 Å were obtained with a space group of P42/mmc (Figure 7). The unit cell is depicted based on this

4. CONCLUSIONS Magnetism of mesoporous silica-supported Pd and PdO NPs was studied from structural characteristics. Unexpected magnetism was found such that larger Pd NPs with distinct twin boundaries and with stacking defaults maintain higher magnetization than smaller Pd NPs. We also studied the synthesis and magnetism of supported PdO NPs, and observed hard magnetic behavior of these tetragonal phase PdO NPs. Pd metal in the bulk form has exceptionally large susceptibility, and the 4d electronic configuration (4d95s1) is regarded as another factor to differentiate its properties from 5d metals such as Pt

Figure 7. Rietveld refinement of supported PdO NPs prepared at a heating rate of 2.5 °C/min and at an annealing temperature of 500 °C. The inset image is a ball and stick model of a PdO unit cell based on the refinement. White indicates palladium atoms, and red indicates oxygen atoms. F



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Figure 8. (a) Field-dependent and (b) temperature-dependent magnetization of spherical mesoporous silica-supported PdO NPs prepared at a heating rate of 30 °C/min and at an annealing temperature of 500 °C. (5) Coronado, E.; Ribera, A.; Garcia-Martinez, J.; Linares, N.; LizMarzan, L. M. J. Mater. Chem. 2008, 18, 5682−5688. (6) Sampedro, B.; Crespo, P.; Hernando, A.; Litrán, R.; Sánchez López, J. C.; López Cartes, C.; Fernandez, A.; Ramírez, J.; González Calbet, J.; Vallet, M. Phys. Rev. Lett. 2003, 91, 237203. (7) Alexandre, S. S.; Anglada, E.; Soler, J. M.; Yndurain, F. Phys. Rev. B 2006, 74, 054405. (8) Shinohara, T.; Sato, T.; Taniyama, T. Phys. Rev. Lett. 2003, 91, 197201. (9) Jeon, Y. T.; Lee, G. H. J. Appl. Phys. 2008, 103, 094313−5. (10) Iijima, S.; Ichihashi, T. Phys. Rev. Lett. 1986, 56, 616−619. (11) Gao, P.-Y.; Kunath, W.; Gleiter, H.; Weiss, K. Z. Phys. DAt. Mol. Cl. 1989, 12, 119−121. (12) Gao, P.; Gleiter, H. Acta Metall. 1987, 35, 1571−1575. (13) Taniyama, T.; Ohta, E.; Sato, T. Europhys. Lett. 1997, 38, 195− 200. (14) Angappane, S.; Jeongmi, P.; Youngjin, J.; Hyeon, T.; Park, J. G. J. Phys.: Condens. Matter 2008, 20, 295209. (15) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W. J. Am. Chem. Soc. 1992, 114, 10834−10843. (16) Li, Z.; Nyalosaso, J. L.; Hwang, A. A.; Ferris, D. P.; Yang, S.; Derrien, G.; Charnay, C.; Durand, J.-O.; Zink, J. I. J. Phys. Chem. C 2011, 115, 19496−19506. (17) Hofmeister, H. Z. Kristallogr. 2009, 224, 528−538. (18) Kulriya, P. K.; Mehta, B. R.; Avasthi, D. K.; Agarwal, D. C.; Thakur, P.; Brookes, N. B.; Chawla, A. K.; Chandra, R. Appl. Phys. Lett. 2010, 96, 053103. (19) Mendoza, D.; Morales, F.; Escudero, R.; Walter, J. J. Phys.: Condens. Matter 1999, 11, L317−L322. (20) Gálvez, N.; Valero, E.; Domínguez-Vera, J. M.; Masciocchi, N.; Guagliardi, A.; Clemente-León, M.; Coronado, E. Nanotechnology 2010, 21, 274017. (21) Kulriya, P. K.; Mehta, B. R.; Agarwal, D. C.; Kumar, P.; Shivaprasad, S. M.; Pivin, J. C.; Avasthi, D. K. J. Appl. Phys. 2012, 112, 014318. (22) Ghosh, S.; Tongay, S.; Hebard, A. F.; Sahin, H.; Peeters, F. M. J. Magn. Magn. Mater. 2012, 349, 128−134. (23) McBrayer, J. D.; Swanson, R. M.; Sigmon, T. W. J. Electrochem. Soc. 1986, 133, 1242−1246. (24) Reddy, B. V.; Khanna, S. N.; Dunlap, B. I. Phys. Rev. Lett. 1993, 70, 3323−3326. (25) Hofmeister, H. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: 2004; Vol. 3, pp 431−452.

and Au. Along with its inertness to air and moisture and its different magnetism from other noble metals, the study of magnetism of Pd NPs has received more attention. In this regard, this work presents another aspect of mesoporous silicasupported Pd NPs obtained by unconventional thermal annealing conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Additional TEM, STEM, HRTEM images, EDS analysis and size distribution analysis, and additional magnetic properties of mesoporous silica-supported Pd and PdO nanoparticles, and TEM, XRD, magnetic data of unsupported Pd and PdO nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected]. * E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from Dong-A University, King Abdullah University of Science and Technology (KAUST), and NSF Grant DBI-1266377. We also acknowledge Bei Zhang in the Nanofabrication core lab at KAUST for his help with the magnetic measurements. The work at UCLA also leveraged the support provided by the National Science Foundation and the Environmental Protection Agency under Cooperative Agreement Number, DBI 0830117.

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REFERENCES

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