Thermal Stability of CoreâShell Nanoparticles: A Combined in Situ

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Thermal stability of core-shell nanoparticles: A combined in situ study by XPS and TEM Cecile S. Bonifacio, sophie carenco, Cheng Hao Wu, Stephen D House, Hendrik Bluhm, and Judith C Yang Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 29 Sep 2015 Downloaded from http://pubs.acs.org on October 3, 2015

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Chemistry of Materials

Thermal stability of core-shell nanoparticles: A combined in situ study by XPS and TEM Cecile S. Bonifacio1, Sophie Carenco2,3, Cheng Hao Wu4, Stephen D. House1, Hendrik Bluhm,2 and Judith C. Yang1,5* 1

Department of Chemical and Petroleum Engineering, and 5Physics, University of Pittsburgh, 4200 Fifth Ave, Pittsburgh, PA, USA, 15260

2

Chemical Science Division, Lawrence Berkeley National Lab., 1 Cyclotron Road, Berkeley, CA, USA, 94720 3

Sorbonne Universités, UPMC Univ Paris 06, CNRS, Collège de France, Laboratoire de Chimie de la Matière Condensée de Paris, 11 place Marcelin Berthelot, 75005 Paris, France. 4

Department of Chemistry, University of California, Berkeley, CA, USA, 94720

*Corresponding Author: Prof. Judith C. Yang Department of Chemical and Petroleum Engineering and Physics University of Pittsburgh 208 Benedum Hall 3700 O’Hara St. Pittsburgh, PA 15261 USA Ph: 412-624-8613 Email: [email protected]

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Thermal stability of core-shell nanoparticles: A combined study by in situ XPS and TEM Cecile S. Bonifacio1, Sophie Carenco2,3, Cheng Hao Wu4, Stephen D. House1, Hendrik Bluhm,2 and Judith C. Yang1,5* 1

Department of Chemical and Petroleum Engineering, and 5Physics, University of Pittsburgh, 4200 Fifth Ave, Pittsburgh, PA, USA, 15260

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Chemical Science Division, Lawrence Berkeley National Lab., 1 Cyclotron Road, Berkeley, CA, USA, 94720 3

Sorbonne Universités, UPMC Univ Paris 06, CNRS, Collège de France, Laboratoire de Chimie de la Matière Condensée de Paris, 11 place Marcelin Berthelot, 75005 Paris, France. 4

Department of Chemistry, University of California, Berkeley, CA, USA, 94720

Abstract In situ techniques of transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) were used to investigate the thermal stability of Ni-Co core-shell nanoparticles (NPs). The morphological, structural, and chemical changes involved in the coreshell reconfiguration were studied during in situ annealing through simultaneous imaging and acquisition of elemental maps in the TEM, and acquisition of O 1s, Ni 3p, and Co 3p XP spectra. The core-shell reconfiguration occurred in a stepwise process of surface oxide removal and metal segregation. Reduction of the stabilizing surface oxide occurred from 320 to 440 °C, initiating the core-shell reconfiguration. Above 440 °C, the core-shell structure was disrupted through Ni migration from the core to the shell. This resulted in the formation of a homogeneous Ni-Co mixed alloy at 600 °C. This study provides a mechanistic description of the alteration in the coreshell structures of NPs under vacuum conditions and increasing annealing temperature, which is crucial for understanding these technologically important materials.

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1. Introduction Bimetallic catalyst systems are found in many important technologies – such as in pollution control, catalytic reforming1, 2, electrocatalysis in fuel cells3, and alcohol oxidation4 – as they can exhibit superior performance than their monometallic counterparts. The chemical and physical properties of bimetallic nanoparticles (NPs) are tunable by varying the composition, atomic ordering and size of clusters5. Accordingly, the surface structure, composition, and elemental segregation6,

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of nanoalloys are important since these structural aspects affect chemical

reactivity and activity8-10. The core-shell architecture, which results from the chemical ordering of one element (A) forming a shell that covers a core of another element (B), usually denoted as A-B core-shell, is one of the most studied bimetallic nanoalloys. The high cost of noble metals has renewed interest in utilizing inexpensive first-row transition metals (Fe, Co, Ni, etc.), even though NPs preparation with these metals presents a challenge due to their greater negative redox potential11. Tailoring nanoalloys with the core-shell structure presents a cost-effective way to produce catalysts by using the less expensive metal in the core and the more expensive but active metal catalyst in the shell. Besides, transition metal core-shell NPs have revealed novel physical properties approaching those of noble metals11-13. Because of this, the potential for greater catalytic activity and selectivity from core-shell NPs compared to monometallic NPs is a strong motivation for studying core-shell configurations of bimetallic NPs. Such catalytic behavior from the core-shell structures arises from the complex interplay of strain and ligand effects, through the heterometallic bonding interaction between the core and shell that acts either co-operatively or competitively14. By fine-tuning the chemical composition, surface oxidation, structure, and dimensions of the core and/or shell, the catalytic properties of the NP can be controlled14, 15.

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In some cases, exceptional catalytic properties have been attributed to the resulting core-shell structure after thermal annealing and oxidation-reduction cycles of initially bimetallic mixed nanoalloys

16-20

. Hence, experiments starting with core-shell NPs are necessary to confirm,

understand, and exploit the catalytic properties of these NP structures as viable catalysts in industrial applications. In fact, the material’s structural stability under the high-temperature conditions of normal operations21 is an important factor to investigate since these nanostructures may be unstable at temperatures far below the melting temperature of the bulk22. Existing annealing studies21, 22 have reported on the instabilities of bimetallic NPs, including core-shell structures resulting in nanoparticle reconfiguration. However these studies did not address the elemental distributions of the intermediate reaction states, limiting the understanding of the structural reconfiguration processes. This bimetallic NP reconfiguration leads to formation of homogeneous NP structure which depends on the miscibility and lattice mismatch between elements. In fact, complex structures such as hetero-dimers from core-shell or vice versa, coreshell from hetero-dimer, are formed after thermal annealing due to the thermally induced diffusion as minimization of interfacial strain22-27. Consequently, systematic studies under environmental conditions providing elemental distributions and measurements of the temperature ranges of core-shell structure thermal stability and/or instability are necessary to obtain a complete description of the driving forces and mechanisms of NP reconstruction. We selected the Ni-Co bimetallic catalyst system to study as it exhibits interesting catalytic activity for the production of methane28. Nickel as catalyst produces methane as a major product29. On the other hand, Co catalyzes the formation of C-C bonds, which makes it a promising Fischer-Tropsch (F-T) catalyst30. Recent studies13, 31, 32 have shown improved activity and selectivity by alloying cobalt with Cu, Pd, or Pt. However, few studies exist on Ni-Co coreshell NPs28,

33-35

. Recently, we reported ex situ TEM observations and in situ X-Ray

photoelectron spectroscopy (XPS)of morphological changes of Ni-Co core-shell NPs exposed to C. Bonifacio et al.


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O2 and H2 gases, which revealed the formation of voids as a result of NPs restructuring as a consequence of reaction with the gases36. In this study, we used in situ scanning transmission electron microscopy (STEM) and XPS techniques to investigate the thermal stability of Ni-Co core-shell NPs under vacuum, throughout the whole process of reduction and annealing. The high surface sensitivity and ability to probe large areas, i.e., an ensemble-average method, makes XPS an effective tool in probing chemical and electronic changes in catalysts. TEM, being a local technique, provides atomic-scale elemental distributions, a vital complement for interpreting XPS results. The important role of oxygen in the stability of the core-shell NPs, which is neglected in most existing thermal stability studies of NPs, is discussed.

2. Experimental Details The Ni-Co core-shell nanoparticles with sizes ranging from 25 to 45 nm were synthesized in a two-step process, as described in Ref. 36. Ni-Co core-shell NPs and hexane were mixed using an ultrasonic bath for 2 minutes until the solid NPs were well dispersed in hexane. This NP solution was drop-casted on a Si wafer and on the holey silicon carbide (SiC) membrane of a MEMS device for XPS and TEM in situ experiments, respectively. During in situ TEM experiments the NPs were heated to constant temperatures from 60 to 600 °C at 20 °C increments for 200 sec under vacuum conditions. XPS measurements were conducted at beamline 11.0.2 in the Advanced Light Source in Berkeley, California37. Each XP spectrum was collected within a few minutes with the photon energy tuned to probe the nanoparticle surface at various depths: 0.5 nm, 1.1 nm and 1.3 nm with corresponding photon energies of 250 eV, 700 eV and 875 eV, respectively, for Ni 3p and Co 3p spectra. The mean free paths were evaluated using NIST SRD 7138 using an average binding

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energy of 65 eV, which lies between the Ni 3p and Co 3p energy regions. Resistive heating of the sample-holder button heater was used to adjust the temperature to the target. The in situ TEM heating experiment was performed using a double-tilt Protochips Aduro sample holder. The MEMS devices specifically for the Aduro sample holder are manufactured by Protochips Inc. (Raleigh, NC). Heating of the sample is achieved by electrical contact between the holder and the embedded heating element in the MEMS device as it is placed on the specimen holder (Figure 1c). TEM experiments were carried-out using a FEI Titan TEM/STEM with ChemiSTEM technology (X-FEG and SuperX with four windowless silicon drift detectors) energy-dispersive X-ray spectrometer (EDS) operated at 200 kV.

3. Results In situ XPS heating Figure 1 shows the XPS results of the Ni 3p and Co 3p region collected with photon energy of 250 eV, 700 eV, and 875 eV. On each spectrum, two main peaks were observed: Ni 3p in the 65–70 eV region and Co 3p in the 57–63 eV region. The position of their maxima as well as their relative area evolved as a function of temperature. Regardless of the photon energy, the first spectra, collected at room temperature (r.t.) barely showed Ni and Co peaks, due to the presence of carbon contamination on the native sample. Annealing to 120 °C resulted in a substantial desorption of contaminants so that a significant signal from the catalyst atoms could be obtained. The Co peaks presented a larger area than Ni peaks at this temperature, consistent with the coreshell structure of the nanoparticles. Moreover, both peaks presented high binding energies of 60.3 eV and 67.8 eV, respectively, indicative of an oxidized state. Their broad shapes suggested that, in addition to the presence of satellites, several oxidation states may be present. Full deconvolution was not attempted here as this was out of the scope of this study. Annealing to

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temperatures higher than 320 °C resulted in the appearance of peaks at lower binding energies of 58.8 eV and 66.0 eV (labeled with + and * in Figure 1) for Co and Ni, respectively. XPS data in the O 1s region, shown in Figure 2, confirm the oxidized initial state of the coreshell NPs. The O 1s region signal contains at least three components at 529.8 eV, 531.0 eV, and 532.7 eV. As for Ni 3p and Co 3p, the spectrum at room temperature was weak due to carbon contamination on the native sample. In all spectra, the large peak at 532.7 eV comes from the surface silica layer of the Si wafer39, while the small peak at 531.0 eV arises from surface hydroxyls40, 41. The signal from the oxide species can be identified as 529.8 eV (marked with a vertical dashed line in Figure 2)42, 43. The oxide species were present at low temperature but they disappeared at temperatures higher than 440 °C (cf. Figure 2).

In situ TEM heating Before annealing, the Ni-Co core-shell NPs were identified to have a FCC structure based on the analyzed selected area electron diffraction (SAED) pattern (cf. inset of Figure 3a) , with the ring patterns corresponding to (111), (200), (220), and (311) metallic planes. Furthermore, additional rings corresponding to the (111) and (220) planes of an FCC oxide (labeled with * in Figure 3a inset) were measured. However, the specific type of FCC metal oxide was not possible to identify due to the close d-spacing values such as in the 111 plane: 2.412 Å for NiO and 2.459Å for CoO (JCPDS files 47-1049 (NiO) and 43-1004 (CoO))11 and the marginal error from the analysis (± 0.042Å). Figure 3b shows an acquired EDS map before annealing of the coreshell NPs displaying the Ni and Co distribution. A 50:50 Ni and Co composition and small amount of oxygen resulted from quantification of the acquired EDS spectra (See Supporting Information, Figure S1). Based on the EDS line profile across the marked NP in Figure 3b, the detected O atoms from XPS and EDS were mostly distributed on the surface of the NPs thus was identified as CoO (cf. Figure 3c). The EDS map (cf. Figure 3b) suggests that the Co and Ni C. Bonifacio et al.


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atoms concentrated on the surface and in the center of the NP, respectively, i.e., a core-shell structure. Although some particles exhibited a thicker shell than others, all NPs were surrounded by a shell of cobalt. (See Supporting Information, Figure S2, for the EDS line profiles of other NPs from Figure 3b). To avoid carbon build-up during the in situ experiment, EDS maps were recorded after each heating cycle as carbon contamination was observed with simultaneous STEM imaging and EDS acquisition. No electron beam irradiation damage was observed during the in situ experiments. Figure 4 shows the EDS maps from two different areas of the sample annealed from room temperature to 600 °C. A change in the NP morphology to a more spherical shape was observed starting at 280 °C (cf. Figures 4a and b). Based on the line profile across the NPs from the EDS maps, the core-shell structure of NPs was maintained up to annealing of 440 °C. At 550 °C, Ni atoms previously found in the core were detected at the shell (cf. Figure 4e). Further annealing to 600 °C resulted to complete Ni and Co mixing within the NP (Figure 4f). The line profile across the particle at 550 °C and 600 °C (cf. Figure 4e and f) indicated the formation of a mixed alloy of Ni and Co. Further verification of the core-shell structure reconfiguration was obtained through the acquired temperature-dependent SAED patterns (See Supporting Information, Figure S3). The weakening of the SAED rings for the FCC metallic structure – diminishing into individual spots beginning at 550 °C – was consistent with the structural change to a uniform phase detected through EDS that occurred during formation of the mixed alloy, as is discussed in detail in the next section. The reconfiguration of the core-shell structure starting at this temperature was observed in all areas of the annealed TEM sample. The effect of the structural changes – and eventually the mixed alloy formation – on the particle size/volume of the NPs was examined. Table 1 summarizes the measured average NP area and diameter (dave) as a function of temperature from the two areas investigated in this

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study. (See Supporting Information, Figure S4, for EDS maps used and procedure for particle size distribution analysis.) Table 1. Average NP size measured from EDS maps acquired during the in situ TEM heating Temperature Average Projected Average diameter [°C] Area [nm2] (dave) [nm] 1135.7 ± 4.15 36.5 ± 2.3 25 280

1121.7 ± 4.15

36.6 ± 2.3

320

1112.8 ± 4.15

36.5 ± 2.3

440

1101.1 ± 4.15

36.4 ± 2.3

550

1107.1 ± 4.15

36.7 ± 2.3

600

1092.0 ± 4.15

36.4 ± 2.3

The dave for the temperature range 25 to 320 °C was measured to be 36.5–36.6 ± 2.3 nm while the dave was 36.4–36.7 ± 2.3 nm for 440 to 600 °C. The error of 2.3 nm corresponds to an uncertainty in NP edge demarcation of ±1.5 pixels during the measurement of the NP areas. The percent change in the measured NP diameters between the annealing steps at 25 to 320 °C and 440 to 600 °C was 0.3% and 0.2%, respectively. The small change in dave of 0.2–0.3% suggests no significant NP size variation occurred during the observed particle reconfiguration.

4. Discussion With the combined XPS and TEM studies, the ensemble and local elemental composition were obtained, allowing for an understanding of the stability limitations of the Ni-Co core-shell NP structure. The thermal stability of the Ni-Co core-shell NPs was determined by in situ annealing of the Ni-Co core-shell NPs from room temperature up to 650 °C under vacuum conditions. The annealing process produced a slight morphology change of the NPs starting at 280 °C, which became more spherical upon further annealing. However, the chemical structure of Ni-Co core-shell NPs did not change significantly. The in situ TEM results revealed that

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above 320 °C, the chemical distribution and structure within the core-shell structure started to change with increased temperature. The reconfiguration of the core-shell structure of NPs was observed in a stepwise process of surface oxide reduction followed by Ni diffusion. The increasing temperature of the system drove the diffusion, resulting in sintering of the particles at 650 °C (See Supporting Information, Figure S5). The percent change in the measured NP diameters of 0.2–0.3% during the reconfiguration of the core-shell structure is reasonable considering that alloying phenomena can be accompanied by a slight modification of density, hence volume contraction or dilatation36. However, the effect of alloying to the NP volume is judiciously interpreted here because the volume of the individual NP can only be approximated given the non-spherical shape of the as-synthesized NPs and the similar radii of Ni and Co. For a more precise determination of NP volume and its changes, electron tomography is necessary. The observed reconstruction of the core-shell structure of the NPs indicated limited thermal stability which is important for the application of the NPs as catalysts. This chemical rearrangement only occurred within each NP and did not affect the distribution of the NPs. A mechanistic description of the nanoparticle reconfiguration for core-shell structures under vacuum conditions and increasing annealing temperatures is described below.

Reduction reactions The surface oxide detected by XPS and SAED can be identified as FCC CoO based on the Co2(CO)8 used here to form the Co shell. CoO forms on the surfaces of Co nanocrystals from the decomposition of Co2(CO)8 to metallic cobalt during the synthesis, and subsequent oxidation upon exposure of the sample to air as in previous studies36, 44-46. The initial oxidation on the surface of the Ni-Co core-shell NPs, i.e., CoO formation, is a rapid process11, 46, 47 which can occur during exposure of NPs to air at room temperature after synthesis. It does not affect the NPs morphology or the Ni:Co ratio. C. Bonifacio et al.


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The reduction reactions in the NPs were driven by annealing in vacuum as shown by the XPS and EDS results. The increasing shift of the Ni and Co peaks (labeled with plus and stars in Figure 1) and decreasing area under the oxide peak (labeled with dotted lines in Figure 2) from the XPS data indicated reduction of the surface oxide. The identified peaks in Figure 1 at lower binding energies of 58.8 eV and 66.0 eV for Co and Ni, respectively, suggested that the nanoparticles surface was partially reduced under annealing. Before annealing, the NPs were found to be covered by carbon species possibly leftover ligands from the synthesis process. In fact, the initial undistinguishable XPS signal (cf. r.t. of Figure 1) can be attributed to carbon contamination at the surface. Annealing from room temperature to 120 °C resulted to partial reduction of the surface oxide through carbo-reduction process where the surface oxide and carbon species react and desorb from the surface. At 320 °C, reduction of the NPs was more pronounced at the very surface (photon energy of 250 eV) than deeper within the nanoparticles (photon energy of 875 eV), although peaks from the reduction process eventually appeared at higher temperatures. Altogether, as suggested by the dotted lines, the surface underwent reduction under annealing. It should be noted that no carbide formation was observed by XPS during this process. The area under the peak at 529.8 eV of Figure 2 was measured and is plotted as a function of temperature in Figure 5a. For a comparative analysis on the surface of the particles, quantification of the oxygen atomic % composition from the EDS spectra around the shell of the NP was performed (cf. Figure 5b). Both XPS and EDS showed a decreasing oxygen concentration as the NPs were annealed from room temperature to 320 °C. From the XPS data, the oxide layer at 1.1-1.3 nm, 700 eV and 875 eV, respectively, below the surface of the NP, was completely removed by 440 °C to 550 °C. Further below the surface, approximately 8 nm across the shell, the oxygen atomic % composition from the EDS decreased from 52.4 ± 3.3 atomic % to less than 25.0 ± 1.4 atomic % starting at 360 °C to 550 °C (cf. Figure 5b). Such results of CoO C. Bonifacio et al.


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layer reduction can be explained by the thermodynamic relationships of the Ni, Co and its oxides in terms of the standard Gibb’s free energy of change. In this case, an equilibrium temperature of Teq = 400 °C can be identified for Ni and Co. Below Teq, Ni metal served as a catalyst for the reduction of CoO to Co

48

. Furthermore, the results suggest that annealing at vacuum pressure

created a reducing environment of the Ni-Co core-shell NPs. These results correlate to reduction of surface oxides from previous in situ annealing of NPs 16, 49, 50.

Ni diffusion from the core to the shell The acquisition of the EDS maps and SAED patterns was crucial in resolving the structural changes in the core-shell NPs. However the quantitative analysis of the XPS and EDS results was the key in identifying the onset of the NP reconstruction. The temperature for the core-shell structural reconfiguration at rates in the minute range was identified at 440 °C through analysis of the SAED pattern, Ni and Co XPS signals and EDS atomic percent compositions. Up to 440 °C, the EDS map showed no indication of Ni species outside the core of the NP (cf. Figure 4d) due to very slow diffusion below this temperature. However, the SAED patterns (Supporting Information, Figure S3) changed from thick to a single diffused ring from 320 °C to 440 °C, respectively, indicating an onset of structural change in the core-shell structure. The thick ring from the SAED pattern comes from the core and shell structures of the NP. The change to a single ring indicated the formation of a uniform phase as in the case for the reconstruction of AuPd core-shell NPs during in situ TEM annealing in Ref. 21. Figure 6 shows a plot of Ni/Co ratio from the XPS data in Figure 1 and from quantified atomic % compositions from the EDS maps acquired at the NP’s shell (8 nm across the shell). From room temperature to 360 °C, the Ni/Co was constant indicating that Ni is mostly concentrated at the core (c.f Figure 7a and b). At 440 °C, an increase in the Ni/Co peaks with temperature was observed indicating an increase in Ni species on the surface. Regardless of the photon energy for the XPS results, Ni/Co ratio was C. Bonifacio et al.


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about two times higher at 550 °C than at 120 °C. This effect was even more pronounced at the very surface (photon energy of 250 eV). Similarly, the Ni/Co from the EDS of the NP’s shell at 550 °C (cf. Figure 7b) increased 4 times (0.70 versus 0.17) compared to Ni/Co at room temperature. These indicate nickel has migrated through the cobalt shell which is supported by the EDS maps (Figure 4e and f) at 550 °C and 600 °C. The observed Ni diffusion starting at 440 °C and above was also related to the reduction reactions based on the thermodynamic relationships of Ni, Co and its oxides. Above the Teq = 400 °C from the Ellingham diagram48 identified from the previous section, the reducing agent for the reaction is reversed, Co instead of Ni. In this case, the leftover oxide as suggested by the quantified O atomic % above 440 °C (cf. Figure 5b) was reduced by Co. Due to the increased temperature during annealing, diffusion of Ni to the surface is favored with the lower surface energy of Ni compared to Co51. This leads to a reduction of the overall surface energy of the NPs, correlating to the observed morphological change of increasing spherical shape of the NPs and structural change with annealing.

Mechanism of the core-shell reconstruction The reconstruction of the core-shell NPs during annealing under vacuum can be described in a stepwise process of surface oxide removal and metal segregation to the surface, I and II, respectively, in Figure 7. Part I shows the depletion of the surface oxide (blue in Figure 7). Synthesized NPs usually have inherent native oxide due to exposure to air. Based on the XPS and EDS results, this native oxide is reduced when annealed in vacuum conditions (cf. Part I in Figure 7) correlating to previous works (cf. Refs.16, 49, 50). At low annealing temperatures, the presence of the oxide layer stabilized the structure of the NP and prevented any transport process to occur. An example of a transport process is sintering, which is a diffusion driven process. It has been shown that sintering was hindered due to the presence of oxide layer as in the study by C. Bonifacio et al.


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Munir in Ref52. Similarly, an increase in thermal stability of the NPs preventing sintering was observed for the O2 pre-treated Pt NPs on γ-Al2O3 substrate in Ref.53. In this case, the complete surface oxide layer removal by 550 °C initiated the diffusion process with increasing annealing temperature (cf. part II of Figure 7). Part II of Figure 7 represents the segregation of Ni atoms (green in Figure 7) from the core to the shell, along with the migration of Co atoms (red) from the shell to the core, which consequently resulted to assimilation of Ni and Co atoms forming a mixed alloy of Ni and Co (yellow in Figure 7). With further annealing, the core-shell structure is disrupted forming a NP with a homogeneous mixed alloy of Ni and Co. Core-shell NPs are synthesized based on the atomic size, relative strengths of homonuclear bonds and surface energies of elements5. In most cases, the smaller atomic size element resides in the core while the larger comprised the shell due to steric constraints5,

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. However once exposed to high annealing temperatures, the surface

energy dictates the diffusion of the element either from the core or shell5, 20 such as for the Ni and Co in this study. The segregation of Ni from the core to the shell (Part II in Figure 7) is due to the lower surface energy of Ni than Co51. This segregation of the element to the surface disrupts the core-shell structure with annealing at high temperatures under vacuum conditions. Subsequently, a homogeneous mixed alloy structure is formed. This resulting homogeneous structure is expected due to the complete solid solubility of Ni and Co11. Homogeneous structures are formed after thermal annealing of core-shell NPs from completely soluble solid systems such as Au-Pd21 and Co-Pt55 as similar to the resulting structure from this study. On the other hand, hetero-dimers or nanohybrid structures are to be expected in bimetallic systems with immiscible and lattice mismatched elements due to interfacial strain minimization during thermally induced 22-26 .

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5. Conclusions The thermal stability of Ni-Co core-shell nanoparticles was investigated using combined in situ XPS and TEM heating studies. EDS elemental maps revealed the elemental distributions of Ni and Co within the nanoparticles, complementing the XPS results, providing a complete description of the core-shell reconfiguration as function of annealing temperature. The core-shell structure was stable at temperatures below 440 °C. This is attributed to the presence of an oxide layer, confirmed by TEM and EDS analysis, that stabilized the NPs during low temperature annealing. Above 440 °C, the core-shell structural reconfiguration proceeded in a stepwise process of surface oxide removal and metal segregation. During reconfiguration, Ni segregated at an appreciable rate from the core to the shell, forming a Ni-Co mixed alloy that became homogeneous at 600 °C, followed by aggregation. Such significant increase in temperature induced thermal diffusion and considerable mass transport resulting in sintering of the NPs. By observing the structural rearrangements of the nanoparticles in situ as a function of temperature, we were able to determine the thermal conditions under which the specially synthesized structure is stable and the minimum temperature where significant elemental diffusion and aggregation occurs. Conclusions drawn here are specific to the NiCo system, which is one of the less studied despite its relevance for technological applications, each metal being abundant and relatively inexpensive. This work also impacts other fields besides catalysis –including metallurgy, magnetism, etc. – where nanoparticle stability under operating conditions is essential for their technical viability and long-term durability.

Supporting Information Available: EDS elemental mapping and spectra of the Ni-Co coreshell nanoparticle composition, EDS line profiles through nanoparticles confirming core-shell structure and showing shell width variation, in situ selected area electron diffraction (SAED) patterns of nanoparticles as a function of temperature, particle size distributions and C. Bonifacio et al.


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corresponding EDS maps as a function of temperature, EDS maps showing sintering behavior of particles with temperature during in situ annealing. This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgements C.S.B., S.D.H. and J.C.Y. acknowledge financial support by DOE BES under Contract No. DE-FG02-03ER15476. S.C. acknowledges CNRS, UPMC and Collège de France. The electron microscopy was performed at the Molecular Foundry at Lawrence Berkeley National Lab which is supported by the Office of Basic Energy Sciences of the US Department of Energy under Contract No. DE-AC02-05CH11231. C.H.W. acknowledges the ALS doctoral fellowship in residence. The authors thank Dr. Karen Bustillo for the technical support using the ChemiSTEMTM and Ross Grieshaber for the careful review of this paper.

Funding Sources DOE BES under Contract No. DE-FG02-03ER15476 supported C.S.B, S.D.H. and J.C.Y.

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Figures

Figure 1. Photoelectron spectra showing the Ni and Co 3p spectral lines of Ni-Co core-shell nanoparticles during annealing from room temperature to 550 °C in the XPS at photon energies of 250 eV (a), 700 eV (b), and 875 eV (c). Starting at 350 °C, low binding-energy peaks labeled with star (*) and plus (+) symbols for Ni and Co, respectively, were observed indicating surface reduction. Dash lines are a guide to the eye only.

Figure 2. Photoelectron spectra of O 1s region using two photon energies: 700 eV(a) and 875 eV(b) collected at various temperatures from room temperature to 550 °C. Smooth lines underneath the spectra are fits for three components: the oxide at 529.8 eV (labeled with a dashed line) and other oxygenated species (hydroxyls, etc.) at 531.0 and 532.7 eV.

Figure 3. TEM characterization of the Ni-Co core-shell NPs before annealing. (a) Highresolution TEM image of the Ni-Co core-shell NPs and the corresponding selected area electron diffraction (SAED) pattern (inset) indexed as FCC structure with the (111) and (220) planes of the surface oxide identified and labeled with (*). (b) EDS elemental map showing the shell and core comprised of Co (red) and Ni (green), respectively. (c) EDS composition line profile of the marked particle in (b) identifying the oxide as CoO.

Figure 4. Series of EDS maps recorded at different temperatures during the in situ heating experiment. A line profile was extracted from the marked particle on each elemental map. The core-shell structure of the NPs was unchanged from room temperature up to 420 °C (a to d).

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Annealing to 550 °C and above (e to f) resulted in the reconfiguration of the core-shell structure into a homogeneous mixed alloy of Ni and Co.

Figure 5. Oxide composition as a function of temperature obtained from the area of oxide peak, O 1s, at 529.8 eV (a) from the XPS data in Figure 2 and O atomic % composition (b) from the analyzed EDS elemental maps obtained from the outermost layer of the NP (marked area of the inset).

Figure 6. Ni/Co ratio as a function of temperature extracted from the Ni and Co peaks (a) from the XPS data in Figure 1 and Ni/Co atomic % composition (b) from the analyzed EDS elemental maps obtained from the outermost layer of the NP.

Figure 7. Schematic of core-shell NP reconfiguration with increasing annealing temperatures. Stage I is the removal of surface oxide (blue) step at low annealing temperatures resulting in diminished oxide thickness. Stage II involves the surface-energy dependent transport process by metal diffusion at higher temperatures resulting in the reconfiguration and formation of a new NP structure. In this case, the core-shell structure was disrupted by Ni (green) and Co (red) diffusion and the subsequently formation of Ni-Co alloy homogeneous structure.

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Photoelectron spectra showing the Ni and Co 3p spectral lines of Ni-Co core-shell nanoparticles during annealing from room temperature to 550°C in the XPS at photon energies of 250 eV (a), 700 eV (b) and 875 eV (c). Starting at 350°C, low binding-energy peaks labeled with star (*) and plus (+) symbols for Ni and Co, respectively, were observed indicating surface reduction. Dash lines are a guide to the eye only. 159x121mm (300 x 300 DPI)



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Photoelectron spectra of O 1s region using two photon energies: 700 eV(a) and 875 eV(b) collected at various temperatures from room temperature to 550°C. Smooth lines underneath the spectra are fits for three components: the oxide at 529.8 eV (labeled with a dashline) and other oxygenated species (hydroxyls, etc.) at 531.0 and 532.7 eV. 105x116mm (300 x 300 DPI)


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TEM characterization of the Ni-Co core-shell NPs before annealing. (a) High resolution TEM image of the NiCo core-shell NPs and the corresponding selected area electron diffraction (SAED) pattern (inset) indexed as FCC structure with the (111) and (220) planes of the surface oxide identified and labelled with (*). (b) EDS elemental map showing the shell and core comprised of Co (red) and Ni (green), respectively. (c) EDS composition line profile of the marked particle in (b) identifying the oxide as CoO. 252x85mm (300 x 300 DPI)



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Series of EDS maps recorded at different temperatures during the in situ heating experiment. A line profile was extracted from the marked particle on each elemental map. The core-shell structure of the NPs was unchanged from room temperature up to 420°C (a to d). Annealing to 550 and above (e to f) resulted in the reconfiguration of the core-shell structure into a homogeneous mixed alloy of Ni and Co. 223x149mm (300 x 300 DPI)


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Oxide composition as a function of temperature obtained from the area of oxide peak, O 1s, at 529.8 eV (a) from the XPS data in Figure 2 and O atomic % composition (b) from the analyzed EDS elemental maps obtained from the outermost layer of the NP (marked area of the inset). 232x108mm (150 x 150 DPI)



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Ni/Co ratio as a function of temperature extracted from the Ni and Co peaks (a) from the XPS data in Figure 1 and Ni/Co atomic % composition (b) from the analyzed EDS elemental maps obtained from the outermost layer of the NP. 214x95mm (300 x 300 DPI)


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Schematic of core-shell NP reconfiguration with increasing annealing temperatures. Stage I is the removal of surface oxide (blue) step at low annealing temperatures resulting to diminished oxide thickness. Stage II involves the surface-energy dependent transport process by metal diffusion at higher temperatures resulting to reconfiguration and formation of a new NP structure. In this case, the core-shell structure was disrupted by Ni (green) and Co (red) diffusion and the subsequently formation of Ni-Co alloy homogeneous structure. 210x72mm (300 x 300 DPI)



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Table of Contents Graphic 62x44mm (300 x 300 DPI)


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