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Atomistic Insight into the Redox Reactions in Fe/Oxide Core-Shell Nanoparticles Shuang Meng, Jiangbing Wu, Ligong Zhao, He Zheng, Shuangfeng Jia, Shuaishuai Hu, Weiwei Meng, Shizhou Pu, Dongshan Zhao, and Jianbo Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03679 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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Chemistry of Materials

Atomistic Insight into the Redox Reactions in Fe/Oxide Core-Shell Nanoparticles Shuang Meng,† Jiangbing Wu,† Ligong Zhao,† He Zheng,*,† Shuangfeng Jia,† Shuaishuai Hu,† Weiwei Meng,† Shizhou Pu,† Dongshan Zhao,† and Jianbo Wang*,†,‡ †

School of Physics and Technology, Center for Electron Microscopy, MOE Key Laboratory of Artificial Micro- and Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan 430072, China ‡ Science and Technology on High Strength Structural Materials Laboratory, Central South University, Changsha 410083, China ABSTRACT: Modern technological pressure for property-directing structural design of Fe/oxide core-shell nanoparticles (NPs) requires the basic understanding of the formation mechanisms and the ability to effectively tune the structures. In this work, in-depth transmission electron microscopy characterization reveals the formation of Fe/Fe3O4 core-shell structure when iron NPs were oxidized at room temperature. More importantly, we present the first atomically resolved dynamical images showing the redox reactions in Fe/oxide NPs at high temperature (400-600 °C). The real-time videos show the unambiguous evidence of the reduction and incorporation of oxygen species along the specific interfaces, leading to the reduction of Fe3O4 to FeO and oxidation of Fe to Fe3O4, which are further investigated based on the theoretical calculations. Meanwhile, it is found that the passive Fe3O4 shell may provide the oxygen ions for the further oxidation of Fe core at high temperature. These findings contribute to a comprehensive scenario for the structural evolution in metal/oxide nanostructures for improved device design and modelling.

1.

INTRODUCTION

hensive scenario for controlling the physical and chemical

As the structural backbone of modern infrastructure

properties. Nonetheless, efforts continue in the direction

and the indispensable element in human body, iron has

to understand and control the complex reaction mecha-

been the focus of research for decades.1-3 In general, Fe is

nisms between Fe and O, possibly opening up new design

chemically reactive due to the multi-valence states and

strategies for advancing a range of nanotechnologies.

can be quickly oxidized when exposed to air or oxygen

Dozens of research groups have responded to the fasci-

including atmosphere, leading to the formation of core-

nating intellectual opportunities by focusing on studying

shell structure. The Fe/oxide core-shell structured nano-

the oxidation mechanisms of pure Fe, which were found to

particle (NP), composed of a ferromagnetic core wrapped

be closely related with temperature,16,17 oxygen concentra-

by a weakly magnetic oxide shell, has been the forefront of

tion, 18 and particle sizes, 19 etc. For example, high-

the research and nanotechnology applications such as

temperature thickening of the oxide layer has been found

4

magnetic recording technology, environmental remedia5,6

7,8

9,10

to follow a parabolic law kinetically, contrasted with the

mag-

barely noticeable growth at room temperature.20 Addi-

netic devices,11,12 drug delivery13 and hyperthermia treat-

tionally, iron oxide will nucleate and begins to grow later-

tion,

14

ment,

energy storage devices,

catalysis systems,

etc. The multiple potential phases of the oxide 15

ally over the surface until the oxygen concentration reach-

shell including FeO, Fe3O4, α-Fe2O3, and γ-Fe2O3, which

es saturation. Despite a rich understanding of the oxida-

serve as the effective medium for the interaction and con-

tion products, the descriptions of their formation

duction of electrons and ions, can contribute to a compre-

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Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 10

Figure 1. The Fe/oxide core-shell structure. (a) The

Figure 2. Atomic-scale structural characterization of the

HAADF-STEM image of an iron NP and the corresponding

Fe/oxide core-shell NPs. (a) A bright-field TEM image. (b) A

EDS elemental mapping of (b) Fe K and (c) O K. (d) The

typical HRTEM image of the squared-area in (a). (c-d) The

overlay of (b) and (c).

FFTs of the two squared-areas in (b), respectively. (e) The HAADF-STEM image of an individual Fe NP. (f) The EELS

mechanisms as well as reduction of iron oxide remain largely phenomenological, which poses a serious threat to the reliability of related applications in nano-devices. In

spectra collected from the three regions marked by different colors shown in the insert, respectively. The insert is the enlarged view of the boxed area in (e).

this context, the atomic-scale redox processes in nanosized iron/iron oxides are desperately desired.

inside the JEOL JEM-2010 FEF (UHR) electron micro-

In what follows, applying the in situ high resolution

scopes equipped with a heating holder purchased from

transmission electron microscopy (HRTEM),21-32 we pre-

DENS solutions (Lightning D6+). The degree of vacuum in

sent the real-time atomic-scale redox processes of the

the microscope is 1.2×10-5 Pa, which gives an O2 partial

Fe/oxide core-shell NPs at elevated temperatures. The

pressure of 2.4×10-6 Pa. Selected area electron diffraction

atomically resolved dynamical images show the formation

(SAED) was conducted with JEOL JEM-2010 (HT) electron

of Fe3O4 during the oxidation of Fe NPs. Moreover, by uti-

microscopes. The high angle annular dark field scanning

lizing the electron beam (e-beam), the reduction of Fe3O4

transmission electron microscopy (HAADF-STEM) images,

to FeO is directly observed at high temperature (400-

EDS mapping, and electron energy loss spectroscopy

600 °C). The detailed redox mechanisms are discussed

(EELS) were acquired using the JEOL ARM-200F with

based on both experimental observations and theoretical

cold-emission gun.

calculations.

3.

2.

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Firstly, the iron NPs can be oxidized instantly while ex-

The iron NPs with sizes ranging from tens to hundreds

posed to air or oxygen-including atmosphere, leading to

nanometers (Figure S1a) were purchased from Aladdin

the formation of 3-9 nm thick oxide shell on the surface

Company. The in situ heating experiments were performed

(Figure 1a). The corresponding EDS elemental mappings


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Figure 3. The hierarchical-structure evolution at high temperature. (a) The Fe3O4 layer nucleates on the Fe NP surface. (b) The hierarchical structure: Fe: Fe3O4: FeO. (c) A thin FeO layer on the Fe NP surface. (d-f) Schematic illustrations of the structural evolutions in the oxide shell corresponding to (a-c), respectively. The yellow and blue dashed lines indicate the Fe/oxide and Fe3O4/FeO interfaces, correspondingly.

of iron and oxygen (Figure 1b-d) clearly show the core-

In order to further analyze the compositions of the oxide

shell structure of the Fe/oxide NPs. Further structural

shell, the EELS spectra of the NP (Figure 2e) were collected

investigation indicates that the oxide shell is mainly con-

in three regions from the Fe/oxide interface to free surface

sisted of magnetite Fe3O4 (space group: Fd 3m ). Specifi-

(marked by “A”, “B”, and “C” in Figure 2f). It is found that

cally, the SAED pattern (Figure S1b) of an individual NP

the half peak width of Fe L3 and Fe L2 in all three spectra

can be indexed based on the co-existence of α-Fe (space

are 3.5 eV and 3.6 eV, respectively, corresponding to the

group: Im3m ) core and Fe3O4 shell (Figure S1c-d), as evi-

electronic structure of Fe3O4 rather than γ-Fe2O3.34 This is

denced in the high resolution TEM images (Figure 2a-b).

consistent with the size dependent formation of oxide

Based on the SAED pattern (Figure S1b) as well as the fast

shell discovered on iron NPs, whereas Fe3O4 shell prefers

Fourier transforms (FFTs) images (Figure 2c-d), there

to grow on the Fe core with diameter larger than 11 nm,

exist two major equivalent orientation relationships (ORs)

while the γ-Fe2O3 phase is dominant when the diameters

between Fe and Fe3O4: [001]Fe // [001]Fe3O4 , (010) Fe //

of Fe range from 7 to 11 nm.19

(220) Fe3 O4 and [001]Fe // [110]Fe3 O4 , (010) Fe // (002) Fe3 O4

Afterwards, the NPs were directly heated inside the

(see the schematic illustration in Figure S2), in well

TEM. At elevated temperatures (< 400 °C), the thickness

33

agreement with the previous results.

of Fe3O4 layer increased slowly from 9 nm to about 14 nm



Page 4 of 10

(Figure 3a). The oxidation occurs via the adatom process

rate results in the complete transformation of Fe3O4 to

(Figure S3): the iron diffuses to the oxide/gas interface

FeO, and thus the formation of Fe: FeO core-shell struc-

which combines with the residual oxygen in the TEM

ture (Figure 3c and f). The OR between the antiferromag-

chamber, and results in the formation of voids in the iron

netic FeO and Fe core is consistently detected during the

core, in accordance with both the experimental observa-

current experiments and determined to be: [001]Fe //

tions35,36 and theoretical predictions,37 i.e. Fromhold-Cook

[110]FeO , (010) Fe // (001) FeO . It is noted that such OR is

(FC) theory, whereas iron ions are considered to be the

different with those reported during the bulk Fe oxida-

moving species during the oxide growth. The enhanced

tion,43 which may be induced by the different formation

diffusivity of iron ions at elevated temperatures contrib-

mechanisms, i.e. the FeO nucleated via the reduction of

utes to the thickening of the oxide layer.16 However, the

Fe3O4 in our experiment. During the above process, the

oxide nucleation stopped when the thickness of Fe3O4

outmost FeO of the oxide layer decomposed slowly all the

layer increased to 14 nm, which may act as an effective

time, and the core-shell NP became a single crystal α-Fe

barrier for successive ion transport and thus decrease the

NP finally. As far as the whole process is concerned, the

oxidation rates. Subsequently, an interesting atomic-scale

structure of oxide layer progressively evolves (Figure 3d-f),

solid-solid phase transition from spinel Fe3O4 to rocksalt

suggesting an alternative approach to control the physical

FeO (space group: Fm3m ) was directly observed within

properties (e.g. magnetic performance) of Fe/oxide struc-

400-600 °C (Figure 3, Movies S1 and S2). Similar phase

ture with atomic precision.

transition process has been reported under ion irradia38-40

tion,

To our best knowledge, the real-time atomistic reduc-

but remains largely unexplored. The reduction of

tion and oxidation processes in Fe/oxide were rarely re-

Fe3O4 to FeO on the NP surface, leads to the formation of

ported, which are further depicted in Figure 4. As indicat-

a novel hierarchical structure: Fe: Fe3O4: FeO (Figure 3a-b,

ed by the dashed lines and arrows, the reduction of Fe3O4

d-e), contrasted with the core-shell structure on bulk iron

occurs via the {002}Fe3O4 and {111}Fe3O4 planes, accompa-

during the high temperature oxidation: Fe: FeO: Fe3O4:

nied with the further oxidation along {010}Fe and {110}Fe

41,42

Fe2O3 from the innermost Fe-to-oxide interface.

The

planes (Figure 3e, Figure 4). It should be mentioned that

OR between Fe3O4 and FeO can be unambiguously identi-

the oxidation of Fe core is highly dependent on the reduc-

fied as: [110]Fe3 O4 // [110]FeO , (111)Fe3O4 // (111) FeO (Figure

tion of Fe3O4. First of all, the oxidation only initiated after

S4).

the reduction of Fe3O4 to FeO. This could be rationalized

Accompanied with the reduction of Fe3O4, the iron core

because it was well-documented that Fe3+-containing lay-

was continuously oxidized (Figure 3a-b, d-e). It is surpris-

er (i.e. Fe3O4) may serve as the barrier for the ion diffu-

ing to note that, unlike the adatom process presented

sion, and reducing the Fe3+ to Fe2+ can obviously increase

above (Figure S3), the oxidation proceeds at the Fe/Fe3O4

the oxidation rate.37 Besides, it is interesting to note that

interface (Movies S1 and S2), indicating the diffusion of

the Fe/Fe3O4 and Fe3O4/FeO interfaces always keep the

oxygen species rather than the Fe ions via the oxide layer.

similar shape as delineated in Figure 4. For instance, the

The higher reduction rate compared with the oxidation

appearance of “bump” at Fe3O4/FeO interface would lead


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Figure 4. Time-lapsed images showing the atomic-scale structural evolution process in oxide shell at 600 °C viewed along the [001]Fe zone axis. (a-f) Fe3O4 transforms into FeO along {002}Fe3O4 and {111}Fe3O4 planes accompanying with Fe3O4 extending

along the {010}Fe and {110}Fe planes. The yellow and blue dashed lines indicate the oxidation and reduction interfaces, respectively. The arrows indicate the forward directions of the two interfaces. The movement of interface steps at Fe/Fe3O4 and Fe3O4/FeO interfaces correspond to the incorporation and reduction of oxygen species, respectively.

to the occurrence of similar “bump” at the Fe/Fe3O4 inter-

in the Supporting Information. According to the above

face (indicated by arrow heads in Figure 4f). Hence, it is

experimental results, two kinds of Fe/Fe3O4 interfaces,

speculated that the generated oxygen species during the

namely (010) Fe / (002)Fe3O4 and (110)Fe / (111)Fe3O4 , are consid-

reduction of Fe3O4 to FeO may contribute to the oxida-

ered, while the low index (100) Fe / (110)Fe3O4 interface is

tion of Fe core. In general, both the two aspects could

adopted for comparison. Meanwhile, two kinds of

accelerate the O2+ diffusion inward to the Fe/Fe3O4 inter-

Fe 3 O 4 /FeO interfaces, namely (002)Fe3O4 / (001)FeO

face and thus change the oxidation behaviors. Further-

(111)Fe3O4 / (111)FeO , are taken into consideration, and the

more, the oxidation process stopped after the complete

low index (110)Fe3O4 / (110)FeO interface is chosen for com-

transformation of Fe3O4 to FeO (Figure 3c), implying

parison. Based upon the surface energy studies,45-47 the

again that the oxygen ions from the passive Fe3O4 shell

most stable O-terminated (002)Fe3O4 and (110)Fe3O4 surfaces

contribute to the further oxidation of Fe core.

were selected to set up corresponding interfaces; and the

and

To further understand the underlying redox reactions,

stable Fe-terminated and O-terminated (111)Fe3O4 surface

interface energy calculations were performed by using the

were chosen to construct the Fe/Fe3O4 and Fe3O4/FeO

VASP Package.44 Detailed calculation set-up can be found

interfaces, respectively. In this work, interfaces are



(010)Fe

Page 6 of 10 /

(002)Fe3O4

interface.

Apparently,

the

(100)Fe / (110)Fe3O4 interface exhibits higher interface energy within whole Fe chemical potential region, implying that it is less likely to be observed in experiments. Similarly, the (111)Fe3O4 / (111)FeO and (002)Fe3O4 / (001)FeO interfaces are the most stable interfaces under the Fe-deficient (-3.00 eV < μFe < -1.61 eV) and Fe-rich conditions (-1.61 eV < μFe < 0 eV), correspondingly (Figure 5b). Thus, based on the theoretical calculations, the redox reactions should proceed along the interfaces with lower interface energies, i.e. {002}Fe3O4 and {111}Fe3O4 planes during reduction of Fe3O4; {010}Fe and {110}Fe planes during oxidation of Fe. In addition, similar redox reactions have been observed along the [110]Fe // [100]Fe3O4 zone axis during heating (Figure S6a-c). The ORs between Fe, Fe3O4, and FeO and are consistent with those detected along the [001]Fe // Figure 5. Interface energy changes as a function of the iron

[110]Fe3O4 zone axis (Figure S4), illustrating that the reac-

chemical potential (µFe) for (a) Fe/Fe3O4 interfaces and (b)

tions occur regardless of the e-beam irradiation directions.

Fe3O4/FeO interfaces.

It should be mentioned that the {111}Fe3 O4 steps observed

simulated with supercell structures (Figure S5) and the distance of two end surfaces of each substance is more than 14 Å in order to prevent interaction between two interfaces. Additionally, the detailed information of inter-

along the [110]Fe3O4 direction (Figure 3b, Figure 4) correspond to the appearance of (020)Fe3O4 steps at the Fe3O4/ FeO interface along the [100]Fe3O4 direction (Figure S6 and S7). Furthermore, it is consistently found that the oxide layer grown on the {001}Fe planes is thicker than that on

facial atomic models is listed in Table S1.

the {110}Fe planes (Figure S8), indicating that the oxidaFor the crystal structure optimizations, the calculated lattice constants are a = 2.822 Å (α-Fe), 4.310Å (FeO) and

tion rate on {001}Fe planes is faster than that on {110}Fe planes, in accordance with the findings in bulk iron.48

8.391 Å (Fe3O4), closed to the experimental values of 2.860 Å, 4.309 Å, 8.384 Å, respectively. Figure 5a shows the interface energies of three types of Fe/Fe3O4 interface as a function of Fe chemical potential: (i) Under the Fedeficient condition (-3.00 eV < μFe < -0.95 eV), the

(110)Fe / (111)Fe3O4 interface is the most stable interface with the lowest energy. (ii) Under Fe-rich condition (-0.95 eV < μFe < 0 eV), the most stable interface can be

It is noted that there are three major factors leading to the reduction and oxidation processes: (i) Thermal heating. Without heating, e-beam irradiation only leads to the thickening of the Fe3O4 layer (Figure S9), consistent with the previous report.33 The elevated-temperature would contribute to the faster inward diffusion of oxygen in Fe3O4,49 and facilitate the decomposition of oxide layer; (ii)


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E-beam irradiation (the electron beam density is about 102

4.

CONCLUSION

A/cm2). To illustrate the influence of e-beam irradiation,

In conclusion, by a combination of in situ observations

the e-beam was focused on a small area in a single Fe NP

as well as theoretical investigations, we have been able to

during heating (Figure S10a-c). Obviously, the thickness

study the atomic-scale redox reactions in Fe/oxide NPs at

of the oxide layer subjected to e-beam irradiation contin-

high temperature (400-600 °C). Firstly, both the SAED

uously decreases (marked by the circles in Figure S10)

patterns and EELS spectra reveal the formation of

accompanied with the reduction of Fe3O4 to FeO, while

Fe/Fe3O4 core-shell structure when iron NPs were oxi-

the oxide layer in the non-irradiated region slowly subli-

dized at room temperature. Increasing the temperature

mated (indicated by arrow heads in Figure S10). (iii) Con-

may increase the saturation oxide thickness, as well-

centration of oxygen. It was reported that under the oxy-

predicted by the FC theory. More importantly, based on

gen-rich environment (e.g. air ambient), the previously-

the time-dependent HRTEM imaging, the reduction of

formed Fe3O4 will be oxidized to the more stable Fe2O3 at

Fe3O4 to FeO and oxidation of Fe to Fe3O4 were captured,

high temperature.50 In contrary, our experiments were per-

which occurred along the specific interfaces which were

formed inside TEM with pretty low oxygen-concentration,

theorized to exhibit lower interface energies. Both ther-

the Fe3O4 can be reduced to FeO induced by e-beam.

mal heating and e-beam irradiation effects were discussed.

There are two main irradiation effects of e-beam: knock-

Our results provide new atomistic insights for the redox

on displacement and ionization effect. We mainly consid-

reactions in Fe/oxide core-shell NPs with an eye toward

er the knock-on displacement for Fe3O4, while the ioniza-

precisely controlling the NPs properties for real applica-

tion effect could be negligible due to its high electrical

tions.

conductivity (2 × 102 S/cm) induced by the disorder be-

ASSOCIATED CONTENT

tween Fe2+ and Fe3+ on the octahedral sites.51-53 For the 200

Supporting Information: Structural characteristics of

keV electron, the maximum energy transferred to the

iron/oxide core-shell NPs; orientation relationships (ORs)

oxygen atom can be calculated to be 32.8 eV based on the

between Fe and Fe3O4; the existence of void; the hierarchical

following equation:

54,55

Emax =

structure in a single Fe NP; the atomic models of interfacial supercell structures; in situ heating of a Fe/oxide core-shell

2 E0 ( E0 + 2 m0 c 2 ) Mc 2

in which M is oxygen mass, E0 is the e-beam energy, m0 is the electron mass, and c is light velocity. Since the roomtemperature threshold displacement energy for oxygen in most oxides is more than 40 eV, which decreases at higher temperature, the oxygen atoms close to the surface of Fe3O4 could be easily displaced: 51,56 (1) the oxygen atoms escaping from the sample results in the reduction of Fe3O4; (2) the oxygen atoms diffusing within the sample leads to the further oxidation of Fe core.50,57

NP as viewed along the [110 ] F e // [100]Fe3 O 4 zone axis; the interfacial shape change between Fe3O4 and FeO; the oxide layers on {001}Fe and {110}Fe planes with different thicknesses; the thickening of the Fe3O4 layer under e-beam irradiation at room temperature; the reduction of Fe3O4 to FeO which only occurs in the e-beam irradiated area at high temperature; real-time videos for the reduction of Fe3O4 to FeO and the further oxidation of Fe core; parameters of interfacial atomic models; calculation methods of interface energy.



AUTHOR INFORMATION Corresponding Author *

Email: [email protected] Email: [email protected]

*

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial internets.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51871169、51671148, 51271134, J1210061, 11674251, 51501132, 51601132), the Hubei Provincial Natural Science Foundation of China (2016CFB446, 2016CFB155), the CERS-1-26 (CERS-China Equipment and Education Resources System), the China Postdoctoral Science Foundation (2014T70734), the Open Research Fund of Science and Technology on High Strength Structural Materials Laboratory (Central South University), and the Suzhou Science and Technology project (No. SYG201619).

ABBREVIATIONS NP, nanoparticle; HRTEM, high resolution transmission electron microscopy; e-beam, electron beam; HAADF-STEM, high angle annular dark field scanning transmission electron microscopy; TEM, transmission electron microscopy; EELS, electron energy loss spectroscopy; SAED, selected area electron diffraction; ORs, orientation relationships; FFT, fast Fourier transform; FC, Fromhold-Cook.

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