Reduction reactions and densification during <i>in situ</i> TEM heating of iron oxide nanochains

Bonifacio, Cecile S.; Das, Gautom Kumar; Kennedy, Ian M.; Benthem, Klaus van

doi:10.1063/1.5004092

Cited by 6 publications

(9 citation statements)

References 44 publications

(56 reference statements)

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“…As a result, variation of the absolute L 3 /L 2 intensity ratios from the reference sample is observed. However, the relative changes observed in this study are consistent and impart reliable evidence for the variation in 3d transition metal oxidation states . In fact, the convexity of the Co L 3 /L 2 ratio correlated with valence state of Co in the XPS results from II‐H2 to IV‐H2 steps in Figure (left).…”

Section: Resultsmentioning

confidence: 99%

Ensemble versus Local Restructuring of Core‐shell Nickel–Cobalt Nanoparticles upon Oxidation and Reduction Cycles

Carenco

Bonifacio

Yang

2018

Chemistry A European J

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Bimetallic nanoparticles are widely studied, for example in catalysis. However, possible restructuring in the environment of use, such as segregation or alloying, may occur. Taken individually, state-of-the-art analytical tools fail to give an overall picture of these transformations. This study combines an ensemble analysis (near-ambient-pressure X-ray photoelectron spectroscopy) with a local analysis (environmental transmission electron microscopy) to provide an in situ description of the restructuring of core-shell nickel-cobalt nanoparticles exposed to cycles of reduction and oxidation. It reveals a partial surface alloying accompanied by fragmentation of the shell into smaller clusters, which is not reversible. Beyond this case study, the methodology proposed here should be applicable in a broad range of studies dealing with the reactivity of mono- or bi-metallic metal nanoparticles.

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Section: Resultsmentioning

confidence: 99%

Ensemble versus Local Restructuring of Core‐shell Nickel–Cobalt Nanoparticles upon Oxidation and Reduction Cycles

Carenco

Bonifacio

Yang

2018

Chemistry A European J

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“…Above 800°C sublimation of isolated particles is observed, while adjacent nanoparticles within nanochains began coarsening around 900°C (ref. [16] and Figure 1S). The different evolution of nanoparticle and nanochain morphologies with increasing temperature indicates a change in their redox behavior.…”

Section: Resultsmentioning

confidence: 93%

“…Temperatures as low as 400°C, however, reveal the stabilization of the FeO phase for nanochains with lengths equivalent to 6-8 particle diameters. The assembly of individual nanoparticles into chain-like geometries and partial sintering between particles ( Figure S1 and reference [16]) leads to the formation of particle/particle contacts with grain boundary energies lower than the specific surface energies. This effect alongside potential particle rotation and faceting decreases the total surface energy of the particle ensemble and stabilizes the FeO phase.…”

Section: For Individual Nanoparticles and Nanochains The Phase Transfmentioning

confidence: 99%

Stabilization of metal(II)oxides on the nanoscale

Bonifacio

Majidi

et al. 2019

Materials Research Letters

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Increasing surface-to-volume ratios for nanoscale materials may cause metal(II)oxides phases to be thermodynamically unstable compared to their bulk counterparts. For instance, previous studies have found FeO to be unstable for nanoparticles with dimensions below 100 nm. In this study insitu TEM was used to gradually reduce nanoparticles and nanochains of Fe 2 O 3. Electron energy-loss spectroscopy and selected area diffraction at different temperatures not only confirm earlier predictions, but also reveal the unexpected stabilization of the FeO phase for nanochains with a minimal critical length. Hence, dimensionally constrained phases were stabilized on length-scales that were previously considered unattainable. IMPACT STATEMENT This study provides direct experimental evidence for the previously unanticipated stabilization of metal(II)oxides with dimensions well below 100 nm, which has exciting potential for catalyst technologies and next generation memory devices.

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“…Bonifacio and colleagues have recently carried out in-situ heating experiments to study the sintering behavior of iron oxide nanochains [2]. g-Fe2O3 nanoparticles with diameters around 40-50 nm were assembled into 1-dimensional chains using a H2/air diffusion flame inside a magnetic field [3].…”

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confidence: 99%

“…The oxidation states of Fe during in-situ heating were probed by electron energyloss spectroscopy (EELS) measurement. The L3/L2 intensity ratios of the Fe L2,3 ionization edges were determined from spectra acquired at different temperatures [2]. It was found that consolidation of iron oxide nanochains is accommodated by the stepwise reduction of g-Fe2O3 at room temperature to metallic iron above 900 °C.…”

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confidence: 99%

Phase Stability of Iron Oxide Evaluated Through Selected Area Electron Diffraction During In-Situ Heating Experiments

Thron

Benthem

2019

Microsc Microanal

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The phase stability of transition metal oxides and the thermodynamic equilibrium of their reductionoxidation reactions were previously reported to be size-dependent [1]. As the metal oxide's dimension decreases to the nanoscale, the free energies for redox reactions and phase stability temperatures alter drastically for most compounds, including iron oxides [1]. Navrotsky and colleagues [1] have calculated phase diagrams for nanoscale Fe-O system using calorimetric surface energy data and thermodynamic property values of the bulk systems. It was predicted that g-Fe2O3 nanoparticles smaller than 100 nm undergo reduction to Fe3O4 with increasing temperature, before transforming directly to metallic Fe, hence bypassing the formation of the FeO phase.Bonifacio and colleagues have recently carried out in-situ heating experiments to study the sintering behavior of iron oxide nanochains [2]. g-Fe2O3 nanoparticles with diameters around 40-50 nm were assembled into 1-dimensional chains using a H2/air diffusion flame inside a magnetic field [3]. The assembled chains were subsequently utilized for in-situ TEM heating experiments using the Protochips Aduro sample holder. The oxidation states of Fe during in-situ heating were probed by electron energyloss spectroscopy (EELS) measurement. The L3/L2 intensity ratios of the Fe L2,3 ionization edges were determined from spectra acquired at different temperatures [2]. It was found that consolidation of iron oxide nanochains is accommodated by the stepwise reduction of g-Fe2O3 at room temperature to metallic iron above 900 °C.In this study, in-situ selected area electron diffraction (SAED) heating experiments were carried out for g-Fe2O3 nanoparticles with an aberration corrected JEOL JEM 2100F/Cs scanning transmission electron microscope. Images and diffraction patterns were acquired in TEM mode under parallel illumination using a Gatan Rio16 camera. Figure 1(a) shows a bright field TEM image recorded at room temperature that displays nanoparticle chains with lengths around 600 to 800 nm. Figure 1(b) shows the same nanochains at 800°C. SAED patterns and bright field TEM images were collected at various temperatures. After holding times of 10 min, no more changes of the diffraction patterns were detected, which suggests completion of any phase transformations. Diffraction patterns recorded at each temperature were subsequently indexed for phase identification. The SAED patterns of the chain-like g-Fe2O3 nanoparticles under room temperature and 800 °C are shown in Figure 2(a) and 2(b), respectively.

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Reduction reactions and densification during in situ TEM heating of iron oxide nanochains

Cited by 6 publications

References 44 publications

Ensemble versus Local Restructuring of Core‐shell Nickel–Cobalt Nanoparticles upon Oxidation and Reduction Cycles

Ensemble versus Local Restructuring of Core‐shell Nickel–Cobalt Nanoparticles upon Oxidation and Reduction Cycles

Stabilization of metal(II)oxides on the nanoscale

Phase Stability of Iron Oxide Evaluated Through Selected Area Electron Diffraction During In-Situ Heating Experiments

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