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Assembly and Fine Analysis of Ni/MgO Core/Shell Nanoparticles Sergio D’Addato,*,†,‡ Vincenzo Grillo,†,§ Salvatore Altieri,‡ Stefano Frabboni,†,‡ Francesca Rossi,§ and Sergio Valeri†,‡ †
Centro S3, Istituto Nanoscienze-CNR, Via G. Campi 213/a, 41125 Modena, Italy Dipartimento di Fisica, Universita di Modena e Reggio Emilia, Via G. Campi 213/a, 41125 Modena, Italy § IMEM-CNR, Parco Area delle Scienze 37/A - 43100 Parma, Italy ‡
bS Supporting Information ABSTRACT: We obtained assemblies of Ni/MgO core/shell nanoparticles (NPs) by using a special experimental setup, in which a beam of preformed Ni nanoclusters, obtained by sputtering and aggregation, is co-deposited with evaporated Mg on a substrate, in a controlled O2 atmosphere. In situ X-ray photoelectron spectroscopy (XPS) and ex situ energy-filtered transmission electron microscopy (EF-TEM) show that the Ni core remains metallic, whereas Mg, located in the shell, is completely oxidized. Detailed high-resolution transmission electron microscopy and geometric phase analyses reveal the multitwinned icosahedral structure of the Ni core and details the arrangement of the MgO shell. This combined effort of controlled assembly of NPs and fine analysis offers intriguing possibilities in the design of nanoscale materials.
’ INTRODUCTION Metal nanoparticles (NPs) are the focus of extensive study because of their unique properties compared to those of bulk materials and their resultant technological applications in various areas. For instance, magnetic NPs show very interesting potential in the development of high-density memory1,2 and in selected medical applications, such as biodetection, targeted drug delivery, and magnetic fluid hyperthermia.3 Moreover, the realization of metal/metal oxide core/shell NPs presents different and fascinating possibilities, such as shell-driven magnetization stability4,5 and improved contrast to enhance magnetic resonance imaging (MRI).3 The possibility of creating M/MO core/shell NPs can, in fact, be exploited to overcome the superparamagnetic limit in magnetic NPs, as the magnetic exchange coupling induced at the interface between ferromagnetic and antiferromagnetic systems (such as Ni and NiO) can lead to magnetization stability.4,5 These results have opened the way to the investigation of magnetotransport properties in granular films with the presence of exchange bias. A great number of methods are available for creating assemblies of NPs, the most important being chemical synthesis,2,3,6 self-assembly,7 9 and deposition of preformed gas-phase metal NPs onto surfaces using cluster sources.10 12 The last technique was used to study the properties of Fe and Co granular films,13,14 and more recently, it has been extended to the realization and study of Ni NP films,15 providing insight into the NP shape, film morphology, and collective magnetic phenomena due to interparticle interactions. Co-deposition of NPs obtained from a gasaggregation source and matrix material produced by thermal evaporators has been used to investigate systematically the properties of r 2011 American Chemical Society
magnetic clusters embedded in nonmagnetic materials.11 These studies could shed light on the effects of interparticle interactions on collective magnetic properties, by varying the particle volume fraction (i.e., the interparticle distance in the system).13 Regarding M/MO core/shell NPs, formation of a native oxide at the surface of the NPs is the simplest possibility for creating an oxide shell around an NP core,3 5,16 19 but creation of an artificial shell can also be exploited,20 22 allowing for a greater variety in the choice of the proper material for the required function.23 A detailed study on the morphology, atom geometry, and chemistry of M/MO NPs is a strict necessity for improvements in this field. In this work, we report the results of a study of Ni NPs with two types of oxide shells, made of native nickel oxide and of MgO. We obtained preformed Ni NPs with a gas-aggregation source, and we deposited them in an oxygen atmosphere, codepositing Mg evaporated by an effusion cell. The techniques used in our investigations included X-ray photoelectron spectroscopy, to obtain information about the chemical states of Ni and Mg in the system; energy-filtered transmission electron microscopy (EF-TEM), to obtain a chemical mapping of single NPs; and high-resolution transmission electron microscopy (HR-TEM), to determine the NP crystal structure. Our results show that, by properly choosing the O2 partial pressure and the relative fluxes of preformed NPs and co-deposited metals, it is possible to fabricate M/MO core/shell NPs such as Ni/MgO, Received: March 9, 2011 Revised: June 16, 2011 Published: June 20, 2011 14044
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The Journal of Physical Chemistry C that is, systems of potential interest not only in nanomagnetism, but also in catalysis, such as for steam reforming of coke oven gas for hydrogen production24 and for production of COx-free H2 through NH3 decomposition.22 Moreover, we performed multislice simulations to match experimental HR-TEM images, finding that the Ni core presents a Mackay-type icosahedral structure25 in Ni/NiO and Ni/MgO NPs, probably because of high quenching rates during Ni NP formation in the source, as was previously found with Au and other metals26 31 and predicted theoretically.32
’ EXPERIMENTAL METHODS AND ANALYSIS The Ni NPs were generated and characterized in an experimental system with three interconnected vacuum chambers. The first chamber was equipped with a NP source (NC200U, Oxford Applied Research) and a quadrupole mass filter (QMF). In the source, Ni was evaporated by magnetron sputtering, and it was condensed into NPs by an inert gas carrier (in our case, Ar). The charged NPs in the produced beam were mass-selected by the QMF and entered the deposition chamber, where they were deposited on a substrate. Deposition can occur in O2 atmosphere (which is allowed into the chamber by a leak valve) and also in a solid matrix, as it is possible to co-deposit different materials by thermal evaporation. The atom beam produced by the evaporators impinged on the substrate at the same position where the NPs were deposited. The deposition rates were monitored with a quartz microbalance. We always checked the size distribution of the deposited particles by scanning electron microscopy (SEM)15 and TEM. After deposition, the sample was transferred to the third chamber, equipped with a rotating anode X-ray source (XR50, Specs), generating Al KR photons (hν = 1486.7 eV), and an electron hemispherical analyzer (Phoibos 150, Specs) for in situ XPS and photoexcited Auger electron spectroscopy (AES) analysis. During the experiments reported in this work, most of the samples were produced with an NP beam generated with a magnetron discharge power of P = 55 W and an Ar flow of f ≈ 80 sccm. Under these conditions, we could obtain Ni NPs with a linear size distribution between 3 and 8 nm, as measured with the QMF and directly verified by examination of TEM images. The typical size distributions had a width of σ = 1.2 nm over an average diameter of Ædæ = 5.5 nm, as obtained by SEM images15 (see also Figure S1 in the Supporting Information). Ni NPs were either deposited in O2 atmosphere or co-deposited with Mg in O2 atmosphere. In particular, during the Ni Mg O2 co-deposition process, the O2 partial pressure and Mg deposition rate were specially adjusted so as to minimize and possibly prevent the oxidation of the Ni NP surface during the formation of the fully oxidized MgO shell. Toward this end, preliminary Ni and, separately, Mg oxidation and XPS/AES experiments were systematically performed. These experiments showed that, under the chosen growth conditions, the minimum O2 partial pressure necessary to oxidize the deposited Mg and Ni NPs was on the order of 10 8 and 10 6 mbar, respectively, consistent with the well-known much higher reactivity toward oxidation of Mg compared to Ni. Based on these experiments, the typical O2 pressure values chosen for the Ni O2 deposition and the Ni Mg O2 co-deposition processes were pO2 = 1 10 6 mbar and pO2 = 8 10 8 mbar, respectively, and the MgO and Ni NP deposition rates were 0.25 and 0.20 nm/min, respectively. The amounts of deposited Ni NPs and the resulting MgO (see the Results and Discussion) are given in terms of the nominal
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thickness, t, of an equivalent continuous film with the same density as bulk fcc Ni (tNi) or rock-salt MgO (tMgO). Therefore, the units for equivalent thickness and deposition rate values are nanometers and nanometers per minute, respectively. The substrates used during deposition were highly oriented pyrolytic graphite (HOPG, for some of the XPS analysis) and carboncoated Cu grids for TEM. An HOPG single crystal was peeled in air and immediately introduced into the chamber. HOPG was chosen for its surface stability and inertness. After the preparation and XPS experiments, the samples were transferred to N2 atmosphere. The TEM experiments were performed using a JEOL JEM-2200FS instrument working at 200 keV and equipped with a Schottky field emission gun (SFEG) and an Ω filter for energy loss analysis. The instrument had an objective lens spherical aberration coefficient of 0.5 mm, providing a point-to-point resolution of 0.19 nm. The images were subsequently elaborated using the geometric phase analysis (GPA) method (implemented in the STEM_CELL package) to evaluate the local strain.33,34 Image were simulated to interpret the HR-TEM image contrast using the Kirkland routines for multislice samples (started from within STEM_CELL).35 Energy-filtered TEM (EF-TEM) mapping was also performed using the Mg L2,3 and Ni M2,3 shells at about 52 and 68 eV, respectively. Standard background subtraction was performed using two pre-edge images.
’ RESULTS AND DISCUSSION Figure 1 shows Ni 2p XPS spectra taken from Ni NPs codeposited with MgO on a TEM grid (spectrum a), clean Ni NPs deposited on HOPG (spectrum b), and Ni NPs deposited in the presence of O2 (pO2 = 1 10 6 mbar) (spectrum c). In spectrum a, the amounts of deposited Ni NPs and Mg, as measured with the quartz microbalance in terms of equivalent thicknesses t, were tNi = 1.0 nm and tMgO = 2.0 nm. Regarding the Ni NPs, the overall amount of deposited Ni with tNi = 1.0 nm corresponds to a film of 0.28 layers of Ni particles, modeled as spheres with a diameter of d = 5 nm. Spectra a and b are very similar; they both exhibit the typical line shape of metallic Ni, with the presence of two main peaks, corresponding to emission from the spin orbit split 2p3/2 (binding energy BE = 852.7 eV) and 2p1/2 (BE = 870.0 eV) levels, and of satellite structures, the most relevant one positioned at a relative binding energy of 6 eV from the 2p3/2 peak. This suggests that the Ni NPs are not oxidized during the formation of the MgO shell, as indeed expected considering that the O2 partial pressure used in the Mg co-deposition process is more than 1 order of magnitude smaller than the minimum oxygen pressure necessary to oxidize Ni (see Experimental Methods and Analysis). We remark, however, that, because the Ni NP size is significantly larger than the Ni 2p photoelectron mean free path, it would be not easy to recognize the precence of one or two atomic layers of oxidized Ni atoms at the surface of the Ni NPs by simply comparing with the XPS spectra of fully metallic Ni NPs (spectrum b). Nevertheless, the strong similarity between spectra a and b clearly indicates that the possible presence of oxidized nickel at the surface of the Ni NPs, if any, is limited to at most a few atomic layers. We can therefore deduce that the Ni NP chemical state is not significantly affected by the co-deposition procedure. Our XPS and Auger data also show evidence of full oxidation of Mg and formation of MgO. Indeed, in the Mg 1s XPS data and AES Mg KL23L23 data (shown in the inset of Figure 1), the characteristic satellites associated with plasmon excitation, which are very intense in the case of metallic Mg but are totally absent 14045
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The Journal of Physical Chemistry C in MgO, cannot be observed.36 Therefore, we can conclude that, in our co-deposition experiments, Ni in the NPs remained essentially metallic, whereas Mg was completely oxidized. Instead, when only Ni NPs were deposited in the presence of O2, the Ni 2p spectrum changed significantly, as shown in Figure 1c. In particular, an intense shoulder appeared on the main 2p3/2 peak, at higher binding energy. Comparison of spectrum c with the spectra from the metallic Ni NPs (spectra a and b) indicates that
Figure 1. Ni 2p XPS spectra of (a) Ni NPs co-deposited with MgO on a TEM grid; (b) Ni NPs deposited on HOPG; (c) Ni NPs deposited on HOPG in O2; (d) Ni NPs deposited on HOPG in O2 after background subtraction, along with relative components. Inset: Mg KLL Auger spectrum of Ni NPs co-deposited with MgO on a TEM grid.
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this shoulder results from the superposition of two components, one associated with metallic Ni and the other associated with oxidized Ni. In fact, the main peak, labeled A in spectrum d of Figure 1, has the same BE as the Ni 2p3/2 line of spectra a and b, whereas shoulder B lies at BE = 855.7 eV. Moreover, a third peak, labeled S, appears after oxidation at BE = 861.5 eV. To investigate the oxide-related component, we removed a Shirley background from spectrum c, normalized it to its area, and subtracted spectrum b, which gives the metallic contribution. According to ref 37, the criterion of choice for the relative weight of the metallic contribution was to subtract the largest amount of this component, provided that the intensity in the energy region between 845 and 890 eV remained non-negative. The result of this procedure is shown on Figure 1 (spectrum d and relative components). The component due to oxidized Ni has a relative weight (with respect to the total area) of 0.55. The line shape of the oxidized component resembles the spectrum from stoichiometric NiO, with the presence of two main peaks in the Ni 2p3/2 structure, due to different final-state configurations after the photoelectric process, with different numbers of holes in the oxide ligand band.38 40 The main peak at lower binding energy (B) has a 3d9L character, whereas the second one (S) is due to a final state with 3d8 character.37 40 Our results indicate that exposure of NPs to O2 during deposition leads to the formation of an oxide shell with defects and irregularities around a metal core, which remains stable even after exposure to the atmosphere for 15 h. Furthermore, the presence Ni in the oxidized phase in the external shell (i.e., close to the NP surface) is a further cause for lack of coordination. This partially defective oxide phase can coexist with nickel hydroxyl species. To visualize the effect of co-deposition of Mg and Ni NPs in O2, we performed an analysis based on energy-filtered TEM (EF-TEM) and high-resolution TEM (HR-TEM). In fact, EF-TEM is more specific in distinguishing between different elements, whereas HR-TEM can give a dual type of information: on one hand, the amplitude contrast produces a qualitative difference in intensity in materials with different scattering power, and on the other, the superimposed phase contrast allows for the resolution of different lattice parameters, and therefore different crystal structures. Figure 2 shows the EF-TEM images from NP Ni/MgO samples grown under different conditions: In the first case (Figure 2a, sample A), the amount of co-deposited MgO corresponds to
Figure 2. EF-TEM maps of Ni (red) and Mg (green), for Ni NPs co-deposited with MgO: (a) tMgO = 5 nm, scale bar length l = 20 nm; (b) tMgO = 2 nm, l = 20 nm. 14046
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Figure 3. HR-TEM image of a single particle for Ni NPs co-deposited with MgO: (a) tMgO = 5 nm, scale bar length, l = 5 nm. Indicated fringe spacing values are d = 0.24 nm (blue) and d = 0.12 nm (yellow). (b) tMgO = 2 nm, l = 2 nm. The circles indicate the MgO region outside the first shell. (c) Sample without Mg deposition and reduced air oxidation, l = 2 nm. (d) Atomistic model of the Mackay icosahedron oriented as in the experiment in panel b. (e) Multislice simulation using a nine-shell icosahedron with orientation as in panel b. Other relevant parameters are as follows: defocus = 43 nm, semiconvergence = 0.2 mrad, focal spread = 5 nm. (f) GPA of the lattice parameter expansion relative to panel a in the direction indicated by arrows.
tMgO = 5 nm, whereas in the second case (shown in Figure 2b), tMgO = 2 nm (sample B). The EF-TEM maps of Mg (green) and Ni (red) show, particularly in Figure 2a, that the main part of the particle is made of Ni with a diameter of d ≈ 5 nm, whereas Mg is localized around the Ni. The Mg extends to a shell of thickness of t ≈ 2 nm (composed of MgO) about the Ni and, in the case of sample A, to a broader and irregularly speckled 5 8 nm outside the first shell. This external coverage is practically absent in the case of sample B. Our EF-TEM data show clearly the effect of codeposition: We have evidence of the formation of a MgO shell around the metallic Ni NPs, and the amount of co-deposited MgO is reduced, as in the case of sample B, the MgO is concentrated only around the Ni NP core. In this case, the Mg and Ni distributions are practically superimposed, so that the Mg shell is certainly very thin. We argue that such a peculiar distribution of MgO around the Ni NPs might be due to the higher affinity that MgO has toward Ni than toward the substrate, so that the Ni NPs effectively behave as centers of preferential nucleation for the MgO. The crystallinity of the Ni particle (in contrast to the amorphous substrate) might also play a role in favoring MgO preferential nucleation. In Figure 3a,b, we report the HR-TEM images from samples A and B, respectively. The image from sample A shows different series of fringes corresponding to regions having different scattering contrasts. The outermost set of fringes, visible, for example, in the top right corner of Figure 3a and highlighted with a circle, is irregular and occasionally extends a large distance from the particle center corresponding to regions with lower scattering power. According to the discussion related to the EF-TEM image, these
regions are rich in Mg: The lattice spacings at 0.24 and 0.14 nm are compatible with MgO but are not compatible with any metallic Ni periodicity. A detailed analysis of the fringe distribution demonstrates that the MgO fringes are visible in almost all of the speckles outside the first shell. Similar fringes appear as very faint traces (indicated by circles) in Figure 3b (see also Figure S2 in the Supporting Information). For the interior part of the particle in Figure 3a, the image is characterized by a complicated superposition of fringes. Such contrast appears similarly but more clearly in Figure 3b, where the amount of MgO is reduced. To rule out spurious effect of the MgO fringes, we performed experiments on standalone Ni nanoparticles with different degrees of oxidation. In Figure 3c, we show HR-TEM images of Ni particles with a very low level of oxidation. It is worth noting that the main fringes here are very similar to those in Figure 3a,b, with particular reference to the fringes at 0.21 nm that dominate the patterns with different orientations. In all cases, the pattern can be correctly explained by considering a multitwinned icosahedral particle.25 A model for such a particle, generated with Mackay icosahedron structure generator software,41 is shown in Figure 3d. The particle here is oriented consistently with the experimental image. The multislice simulation for this structure is shown in Figure 3e, and it is in good agreement with the experimental contrast in Figure 3b,c. The same pattern also resembles the one in Figure 3a in the central area, but the image appears more confused because of the superposition with MgO fringes. Figure 4 shows an HR-TEM image of sample A and the relative map of occurrence of two relevant periodicities in the fringes. Figure 4a, obtained with a slight defocus, provides evidence 14047
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Figure 4. Left: High-resolution TEM image of sample A. Right: Map of the amplitude of the 0.21-nm (red) and 0.14-nm (green) fringes. Scale bar length l = 10 nm.
of the Ni particle and MgO shells with different scattering contrasts. The scattering contrast permits confirmation of the core/ shell nature of the particles (see also Figure S2, Supporting Information) in practically all analyzed cases. Figure 4b can be used to map crystallographic information. The method used to obtain this map is described in the Supporting Information. The image of Figure 4b in particular refers to the periodicities at 0.14 nm (green) and 0.21 nm (red). To interpret these maps, it is important to notice that, because of their multitwinned nature, the Ni particles expose mainly the {111} fringes at 0.21 nm, but these fringes are also sometimes visible in the MgO. In contrast, the 0.14-nm fringes are a characteristics of only the MgO. This means that the red areas are dominantly due to Ni, whereas the green areas are due only to MgO. The maps qualitatively confirm the results of EF-TEM in terms of the distribution of the two phases, but some extra information can be inferred. First, the outer speckle outside the first shell is also largely crystalline, as inferred from Figure 3a,b. Second, the MgO shell immediately around Ni is probably largely crystalline but not a single crystal, as the visibility of the fringes is not uniform. In particular, analysis of the image in Figure 3a (and in Figure S1 in the Supporting Information) shows MgO crystal fringes aligned with the Ni ones with an epitaxial relationship at the interface. To study the interface relationship between Ni and MgO in greater detail, we performed GPA of the lattice deformation. Figure 3e refers to the GPA of Figure 3a. This image shows the deformation in the direction indicated by arrows, using (111) fringes in the orthogonal direction. (For a discussion of the reliability of the method, see ref 19.) The fringes are visible in a large part of the Ni particle, roughly corresponding to the bright blue region in the map. The map shows, on the right side of the Ni core, a deformed shell (in red and orange) with a thickness of about 1 nm. A 15% deformation is compatible with the difference in the (111) periodicity between Ni (0.21 nm) and MgO (0.24 nm) as already observed. As discussed in a preceding article,19 this variation would also be compatible with the presence of NiO, but comparison with the average shell size deduced by EF-TEM excludes the possibility that NiO might extend more than one monolayer. It is further worth mentioning that, in this projection, the (111) faces and, therefore, the interfaces are oriented parallel to the beam direction, so they are visible without superposition.
The interesting observation from GPA is that, in the symmetrical surface on the left, no MgO fringe is visible. This indicates that the coverage is, in this case, not always uniform or crystalline although most facets are completely covered, but most important, it shows that lattice continuity between Ni and MgO is not maintained. This is certainly due to the complicated crystallography of the multitwinned particle that cannot be fitted to any single-crystal structure for MgO. Similar evidence is shown in Figure S1 of the Supporting Information. Evidently, the MgO coverage nucleates separately for each {111} surface.
’ CONCLUSIONS We have presented a method for assembling core/shell Ni/ MgO NPs by co-deposition of preformed Ni nanoclusters and Mg in a controlled O2 atmosphere. In situ XPS/AES and ex situ EF-TEM analyses demonstrate that the Ni core remains metallic, whereas the shell is composed of completely oxidized Mg. A detailed HR-TEM investigation combined with an advanced multislice simulation revealed that the metal Ni core is composed of multitwinned crystals with an icosahedral shape. GPA shows deformation on the external region of the Ni core compatible with the difference in the (111) periodicity between Ni and MgO, but lattice continuity between Ni and MgO is not maintained, because of the complex structure of the core. We suggest that significant improvement of the crystalline quality and uniformity of the oxide shell could be achieved by exploiting, for both the substrate and the preformed metal nanoclusters, temperature ranges higher than room temperature as used in the present work. The recognition of the importance of grain internal structure and weak intergrain coupling in granular materials exhibiting spectacular magneto-transport and chemical phenomena is attracting great attention to M/MO nanoparticles, and the possibility of atomic-scale control of chemical and structural parameters in these nanoscale systems might be a central issue for emerging technologies. For this reason, the search for new nanoscale fabrication and analysis methods is necessary. Compared to other nanoparticle fabrication methods, our approach offers the possibility of taking advantage of a full set of spectroscopic and microscopic techniques with both chemical and spatial sensitivity that can be applied in situ and that often are not accessible to other types of nanoparticle fabrication approaches. Moreover, the combination 14048
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The Journal of Physical Chemistry C of preformed nanocluster production and out-of-equilibrium growth methods based on ultra-high-vacuum reactive deposition, as proposed in our work, might open up new possibilities in the field of nanocluster and granular film materials science. In this respect, the main challenge remains the ability to control, at the subnanometer scale, atomic structure and lattice matching between different materials with inhomogeneous surfaces and highly curved interfaces. We believe that our study has a two-fold implication: a controlled method of assembling core/shell NPs that can be extended to different materials and a complementary and thorough system of analysis for this class of systems.
’ ASSOCIATED CONTENT
bS
Supporting Information. (1) SEM images and analysis results of Ni NPs deposited on Si/SiOx. (2) Methodology of fringe mapping used to obtain Figure 4 of the article. (3) HRTEM images of samples A and B in a different projection from that shown in Figure 4. (4) Additional low-magnification TEM images showing clear evidence for the existence of a complete but not uniform shell. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: +390592055254. Fax: +390592055235.
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