Enhanced Microwave Absorption Properties by Tuning Cation

Research Article www.acsami.org

Enhanced Microwave Absorption Properties by Tuning Cation Deficiency of Perovskite Oxides of Two-Dimensional LaFeO3/C Composite in X‑Band Xiang Liu, Lai-Sen Wang,* Yating Ma, Hongfei Zheng, Liang Lin, Qinfu Zhang, Yuanzhi Chen, Yulong Qiu, and Dong-Liang Peng* Department of Materials Science and Engineering, Collaborative Innovation Center of Chemistry for Energy Materials, College of Materials, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: Development of microwave absorption materials with tunable thickness and bandwidth is particularly urgent for practical applications but remains a great challenge. Here, two-dimensional nanocomposites consisting of perovskite oxides (LaFeO3) and amorphous carbon were successfully obtained through a one pot with heating treatment using sodium chloride as a hard template. The tunable absorption properties were realized by introducing A-site cation deficiency in LaFeO3 perovskite. Among the A-site cation-deficient perovskites, La0.62FeO3/C (L0.62FOC) has the best microwave absorption properties in which the maximum absorption is −26.6 dB at 9.8 GHz with a thickness of 2.94 mm and the bandwidth range almost covers all X-band. The main reason affecting the microwave absorption performance was derived from the A-site cation deficiency which induced more dipoles polarization loss. This work proposes a promising method to tune the microwave absorption performance via introducing deficiency in a crystal lattice. KEYWORDS: microwave absorption, LaFeO3/C, dipole polarization, deficiency, perovskites

1. INTRODUCTION To satisfy the fast development of electronic devices to be smarter, faster, and more miniaturized, the operating frequency is increased to a higher frequency range to the greatest extent.1−3 When the electronic devices are operating, the unwanted electromagnetic (EM) wave emission to the surrounding environment is detrimental to the device and human health.4−7 To avoid such a situation, microwave absorption materials (MAMs) should be applied to reduce the undesirable EM emissions. An effective EM absorption material should have the following features: lightweight, broad absorption band, strong absorption, good thermal stability, and antioxidant ability. Recently, tremendous effort has been devoted to the research of composite MAMs formed with magnetic and dielectric materials to satisfy the aforementioned features.7−18 To render the proposed materials with features of being lightweight and having broad absorption band, extensive research has been devoted to two-dimensional (2D) materials with special electrical and optical properties.19−23 The huge © 2017 American Chemical Society

superficial area and the network structure of 2D materials provide the internal multiple reflection phenomenon of EM wave and that good impedance matching effect enhances the absorption band. Carbon materials like magnetically decorated graphene have always been regarded as the most promising candidate for MAMs in 2D materials because of their advantages of being thermally stable and durable.2,13,16,24−30 Insufficiently, the synthesis process of the magnetic/carbon composite materials is tedious and has multiple steps. Lanthanum ferrite (LaFeO3), as a typical multiferroic material, has coexistence of antiferromagnetic and ferroelectric properties at room temperature. LaFeO3 (LFO) is one of the most important oxides in the family of perovskites (ABO3). The crystal structure of LFO is the rare-earth element La with larger ionic radius occupying A place located in the cube vertices, variable valence Fe with smaller ionic radius more Received: November 30, 2016 Accepted: February 1, 2017 Published: February 1, 2017 7601

DOI: 10.1021/acsami.6b15379 ACS Appl. Mater. Interfaces 2017, 9, 7601−7610

Research Article

ACS Applied Materials & Interfaces

3. RESULTS AND DISCUSSION The formation procedure of perovskite oxides decorated amorphous carbon nanosheet is schematically divided into three steps illustrated in Scheme 1. The purchased NaCl

likely occupying B place located on the center of symmetry of the cube, and oxygen ions in the cube edge length midpoint forming oxygen octahedron.31,32 The special crystal structure gives the LFO outstanding magnetic and electrical properties that make LFO widely applied in catalytic oxidation,33,34 sensors,35−37 visible-light photocatalysis,38,39 and solid oxide fuel cells.40 The special physical properties make it possible for LFO to generate a positive response to EM wave to improve the microwave absorption properties. However, LFO perovskite is rarely reported for the application as MAMs. In particular, La cation site-deficient perovskite oxides can introduce more dipoles to improve the dielectric properties and tune the microwave absorption performance. Here, we demonstrate a one-step procedure to synthesize 2D LaxFeO3/C (LxFOC) nanocomposites and investigate the EM parameters and microwave absorption performance in detail. To the best of our knowledge, 2D LFOC has never been reported as effective MAMs before. The cation deficiency in LFO crystal structure was easily controlled by tuning the molar ratio of La/Fe elements. The related complex permittivity was effectively increased by introducing the cation deficiency in LFO crystal lattice. The experimental results showed that the L0.62FOC composite had prominent microwave absorption properties with small thickness (2.94 mm). The maximum reflection loss (RL) was −26.6 dB at 9.8 GHz and the effective absorption bandwidth (below −10 dB) covered all X-band (8− 12 GHz). Furthermore, we thoroughly discussed the effect of cation deficiency in LFO crystal structure on dielectric properties. This would be a new way to tune frequency in microwave absorption performance.

Scheme 1. Schematic Illustration of the Preparation of the LxFOC 2D Nanocomposite

powders have a cubic morphology with the edge of about 400 μm (Figure S1a of the Supporting Information). First, Fe(NO)3, La(NO)3, and glucose were coated onto the surface of thermally stable NaCl particles which act as a template after evaporation of all water in the mixture (Figure S1b). Second, in the processing of heating to 600 °C in a tube furnace, the glucose granules were melted at about 150 °C and formed thin liquid film that embedded Fe(NO)3 and La(NO)3 on the surface of NaCl. As the temperature increased, Fe(NO)3 and La(NO)3 reacted with glucose to form LaFeO3. In the meantime, glucose thin liquid film was in situ carbonized to 2D amorphous carbon matrix on the surface of NaCl template. Finally, naturally cooled to room temperature (Figure S1c,d), the NaCl templates were peeled off by dissolving in deionized water to obtain the end product which had the 2D structure of LFO nanoparticles embedded in amorphous carbon matrix. Figure 1a shows the XRD patterns of the prepared LxFOC samples. LFOC powders exhibite a pure orthorhombic perovskite structure with a space group of Pnma (PDF#: 371493). The major diffraction peaks at 22.61°, 32.17°, 39.67°, 46.19°, and 57.61° are determined for (101), (121), (220), (202), and (240) lattice planes. When introducing the La deficiency, we can observe that the position of the diffraction peak corresponding to the (121) lattice plane of L0.62FOC shifted slightly to higher angles (32.29°) compared to the pristine LFO shown in Figure 1b, and the positions of the diffraction peaks of other samples are presented in Table S1. The structures of the prepared LxFOC perovskites were further confirmed by Rietveld refinement (Figure S2 and Table S2). Because of the lack of larger ionic La sited at the A position, the crystal lattice is partly shrunk (Figure 1c).40 Furthermore, the peak of the (311) lattice plane belonging to cubic structure maghemite-c becomes more and more obvious as the iron content increased, indicating that an impurity phase appeared. The surface composition and chemical states of LxFOC 2D nanocomposites were examined by XPS. The samples are LFOC, L0.62FOC, and L0.25FOC, respectively. As shown in Figure 2a, the peaks of La 3d3/2 and La 3d5/2 both have two split peaks. The main peaks of La 3d3/2 and La 3d5/2 at binding energies of 851.6 and 834.7 eV are in agreement with the literature.32,41 The satellites peaks at 855.7 and 838.9 eV

2. EXPERIMENTAL SECTION 2.1. Synthesis. In the preparation of LFOC sample, Fe(NO3)3· 9H2O (1 mmol), La(NO3)3·6H2O (1 mmol), and glucose (1 g) were used as starting materials. All the precursors were dissolved in deionized water (6 mL) and then stirred with NaCl (15 g). The precursor was placed in a drying oven at 90 °C for 16 h to obtain a dry sample. Subsequently, the dry sample was ground into very fine powders by agate mortar. The powders were sintered at 600 °C for 2 h at the heating rate of 10 °C·min−1 under an Ar atmosphere. After being naturally cooled to room temperature, the black powders were washed with deionized water and ethanol three times and finally dried in an oven at 60 °C for 1 day. Similarly, other samples were prepared by adjusting the molar ratio of La(NO3)3·6H2O and Fe(NO3)3·9H2O using the same steps as the preparation for LFOC. The fabricated samples were denoted as LFOC, L0.75FOC, L0.62FOC, L0.49FOC, L0.38FOC, L0.38FOC, and L0.2FOC, respectively. 2.2. Characterizations. The morphologies of the LxFOC samples were characterized using a scanning electron microscope (SEM, Hitachi SU-70) equipped with an energy dispersive X-ray spectrometer. The transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns of samples were obtained by using a transmission electron microscope (TEM, JEM2100, 200 kV). The structures of the samples were characterized by Xray powder diffraction (XRD) on a Bruker-Axe X-ray diffractometer with Cu Kα radiation source. The elemental chemical state was characterized by X-ray photoelectron spectroscopy (XPS) on a PHI Quantun-2000 spectrometer. The hysteresis loops were obtained at room temperature on a vibrating sample magnetometer (VSM, LakeShore 7404). Meanwhile, the relative complex permittivity and permeability were tested at the frequency range of 8−12.4 GHz on an Agilent N5222A vector network analyzer. The tested samples were prepared by mixing the 40 wt % LxFOC powders and 60 wt % wax paraffin uniformly. 7602


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Figure 1. (a) XRD patterns of LFOC, L0.75FOC, L0.62FOC, L0.49FOC, L0.38FOC, L0.25FOC, and L0.2FOC samples. (b) XRD patterns with an expanded region of 2θ = 29°−39°. (c) The crystal lattice structure illustration of the LFO and A-site-deficient LxFO perovskites.

Figure 2. XPS spectra of (a) La 3d, (b) Fe 2p, and (c) C 1s regions of sample LFOC, L0.62FOC, and L0.38FOC. (d), (e), and (f) are the O 1s fitting curve of LFOC, L0.62FOC, and L0.38FOC, respectively.

phase from sample L0.38FOC is γ-Fe2O3, not magnetite. The peaks at about 284.4 eV are the C 1s electron orbits in Figure 2c. It is noteworthy that the O 1s XPS spectra are wide and asymmertric, indicating that there are several kinds of O chemical states in samples shown in Figure 2d−f. So the O 1s XPS spectra of three samples were fitted in Figure 2d−f. For sample LFOC, the O 1 peak at about 532.6 eV is attributed to

correspond to the shake-up state of La 3d due to an electron transferring from the O 2p valence band to an empty La 4f orbit.42 As shown in Figure 2b, the peaks at about 724.4 and 710.7 eV correspond to the Fe 2p1/2 and Fe 2p3/2 orbits. The presence of the satellite peak at around 719.2 eV is characteristic of Fe3+ cations which are in accord with the XRD patterns in Figure 1a. Consequently, the precipitated 7603


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Figure 3. Field emission scanning electron microscope (FESEM) images of (a) LFOC, (b) L0.62FOC, and (c) L0.38FOC; (d), (e), and (f) are the partially magnified images of the above images.

Figure 4. FESEM images of (a) L0.62FOC and corresponding elemental mapping images of (b) C, (c) O, (d) Fe, and (e) La; (f) is the EDS spectrum of the selected area. The scale bars in the above images are all 10 μm.

the contribution of the adsorbed molecular water (H2O). The O2 peak at about 531.3 eV is mainly from the surface-adsorbed oxygen or hydroxyl groups. The relatively strong peak O3 at about 529.7 eV is ascribed from the contribution of La−O and Fe−O in the crystal lattice. However, when the La defects in LFO crystal lattice (L0.62FOC) are introduced, the O3 peak is weakened and broadened (Figure 2e). The main reason is that the crystal lattice defects of LFO lead to the highly oxidative oxygen species (O22−/O−) to generate a new peak (O4) at 530.1 eV and the peak O3 of La−O and Fe−O weakened. This result clearly suggests that the surface oxygen vacancies in the

A-site cation-deficient L0.62FOC perovskites will play a vital role in the enhancement of dipoles polarization. The morphologies of the synthesized L0.38FOC, L0.62FOC, and LFOC are investigated by SEM (Figure 3). All the samples possess the 2D hierarchical sheet structure seen in Figure 3. To further study the distribution of different elements, the corresponding elemental maps of sample L0.62FOC are presented in Figure 4. In the whole triangular plate region, the elemental maps of Fe, C, O, and La demonstrate a very homogeneous elemental distribution. The element content is verified by energy-dispersive X-ray spectroscopy (EDS) as 7604


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Figure 5. (a)−(c) are TEM (a) and HRTEM images ((b) and (c)) of sample LFOC. (d)-(f) are TEM (d) and high-resolution TEM (HRTEM) images ((e) and (f)) of sample L0.62FOC. The insets in (a) and (d) are the SAED patterns while those in (c) and (f) are the fast Fourier transform (FFT) patterns.

Figure 6. (a) Magnetization versus magnetic field for the samples. (b) Saturation magnetization of different La contents.

embedded on the amorphous carbon nanosheets. The insets in Figure 5a,d are SAED patterns that agree well with the XRD patterns shown in Figure 1a. The higher magnification of samples presented in Figure 5b,e indicates that the nanoparticles have torispherical nanostructure decorated on amorphous carbon with a diameter of about 7 nm. The embedded structure and excellent dispersibility can contribute to improved microwave absorption properties. The lattice fringes with a lattice spacing of 0.278 and 0.274 nm correspond to the (121) crystal plane of the orthorhombic perovskite

shown in Figure 4f. The contents of La, Fe, O, and C are 34.61, 22.24, 29.98, and 13.17 wt %, respectively. The good dispersion of these nanoparticles in amorphous carbon matrix may contribute to microwave absorption properties. To further study the inner structure of the synthesized nanocomposite, the TEM and HRTEM images, and SAED patterns are shown in Figure 5. The samples were dropped onto a common carbon membrane-coated TEM grid of 200 mesh. As shown in Figure 5a,d, the LFOC and L0.62FOC are flat. Meanwhile, the small nanoparticles are uniformly 7605


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Figure 7. (a) Real part (ε′) and (b) imaginary part (ε″) of permittivity, (c) real part (μ′) and (d) imaginary part (μ″) of permeability, (e) dielectric tangent loss values (ε″/ε′), and (f) magnetic tangent loss values (μ″/μ′) of composites.

Figure 8. Schematic illustrations of the dipoles polarization of sample L0.62FOC.

magnetic order throughout the particles would happen on condition that the particle size is small enough. The reason is that the uncompensated surface moments are generated by enhanced surface area. The superparamagnetic behavior is dominated by the uncompensated antiferromagnetic surface local moment contribution that thermally fluctuates at room temperature. The special magnetic performance would contribute to enhancing the microwave absorption properties via superparamagnetic relaxation.45 Figure 7a,b exhibits the frequency-dependent related complex permittivity (εr = ε′ + jε″). It is seen that all samples have a polarization relaxation response to the EM wave and the real part of permittivity (ε′) increases in the X-band as the molar ratio of La/Fe decreases from 1.00 to 0.62. The values of ε′ are in the range of 4.6−4.3, 4.7−4.4, and 7.7−6.6 for LFOC, L0.75FOC, and L0.62FOC, respectively. The increased ε′ is

structure. The insets in Figure 5c,f are the fast Fourier transform (FFT) patterns. The hysteresis loops of three samples measured at room temperature are depicted in Figure 6a. All the samples with nanoparticles embedded in carbon show superparamagnetic behavior with no magnetic hysteresis that is different from the antiferromagnetic nature of bulk LFO, which is derived from the quite small LFO nanoparticles compared to the literature.32,43,44 The values of saturation magnetization (Ms) of different samples are 18.77, 14.34, 10.5, 7.12, 5.87, 3.16, and 1.59 emu/g, respectively. As shown in Figure 6b, the value of Ms increases as the molar ratio of Fe/La increases. The antiferromagnetic property of LFO is understood in terms of 2sublattice (two interpenetrating pseudocubic face-centered sublattices which consist of FeO6 octahedral units) antiferromagnetic ordering. However, a fundamental change in the 7606


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Figure 9. (a), (b), and (c) are the 3D diagrams of RL depending on frequency and thickness of the marked samples; (d), (e), and (f) are the RL of the samples corresponding to the image above at various thicknesses versus frequency.

figure, is a method to estimate Debye relaxation process. In Figure S5, several semicircles were found in all samples, corresponding to the multirelaxation behaviors of LxFOC. Figure 7c,d shows the frequency-dependent complex permeability (μr = μ′ + μ″). It is seen that the values of complex permeability fluctuate slightly at X-band and μ′ is close to 1 and μ′′ close to 0, meaning that the nanocomposites almost are dielectric materials. Obviously, the complex permeability has little effect on the EM wave absorption compared to the ε″ dielectric effects. The dielectric loss (tan δ E = ε ′ ) and

mainly derived from the dipoles orientation polarizations. As illustrated in Figure 8, to maintain the charge balance after decreasing the cation La3+, the concentration of oxygen vacancy (VO) is enhanced as the La defects increase. This result clearly suggests that the Fe−VO dipole pairs enhance the polarization and play a dominant role in the enhancement of real permittivity in an external electric field. When the molar ratio of La/Fe decreased to 0.49, the crystallinity decreased as seen in the XRD pattern in Figure 1, which led to the reduction of the value of ε′ (4.9−4.3). A new phase (Fe2O3) increased the interface polarization and the sample L0.38FOC reached the maximum value (7.2−6.2). To avoid complications due to the effect of Fe2O3 phase, the EM parameters of L0.38FOC, L0.25FOC, and L0.20FOC (shown in Figure S3) are not considered here. The image part of permittivity (ε″) includes the conductance loss (εc′) and relaxation loss (εr″), ε″ = εr″ + εc″ =

(εs − ε∞)ωτ 2 2

1+ωτ

+

σ ε0ω

magnetic loss (tan δM =

μ″ ) μ′

represent the attenuation of the

incident EM wave energy. Figure 7e,f presents the frequencydependent tan δE and tan δM. As described earlier in Figure 7a,b, the sample L0.62FOC has the maximum value of tan δM (0.55−0.62) in all the samples because of the orientation relaxation of Fe−VO dipole pairs. Compared to the tan δE, the value of tan δM is 1 order of magnitude smaller, which can be ignored to contribute to the attenuation. The theoretical reflection loss (RL) values are calculated as follows,

(1)

where εs and ε∞ are the static permittivity and relative dielectric permittivity at the high-frequency limit, ω is angular frequency, τ is relaxation time, σ is conductivity, and ε0 is dielectric constant in vacuum. Because of the low conductivity of composites (Figure S4a) at GHz mainly derived from the 60 wt % insulated paraffin filling, ε″ is primarily dominated by relaxation loss of defect dipoles polarization and interface polarization, and the conductance loss (∼10−11) can be ignored (Figure S4b). The defect dipoles are mainly from the La defects and amorphous graphite. The interfacial polarization is generated by the interfaces which LxFO nanoparticles uniformly embed in 2D carbon nanosheets. Consequence, the sample L0.62FOC has the maximum ε″ (4.22−4.18) which is enhanced by a maximum of orientation relaxation of Fe−VO dipole pairs. The plot of ε′ versus ε″, namely, Cole−Cole

RL = 20 log|(Z in − 1)/(Z in + 1)|

(2)

Z in =

(3)

μr /εr tanh{j(2πfd /c) με } r r

where RL is reflection loss coefficient, Zin is normalized input impedance, μr and εr are the relative permeability and permittivity at frequency f, c is the velocity of EM waves in free space, and d is the thickness of absorber. As seen in Figure 9a−c, the 3D diagrams of RL depending on frequency and thickness demonstrate that the sample of L0.62FOC has excellent microwave absorption properties in a smaller thickness range. Obviously, sample L0.62FOC presents the maximum absorption of −28.5 dB at 11.0 GHz with a thickness of 2.70 mm, superior to other samples (Figure 9d−f). 7607


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ACS Applied Materials & Interfaces Table 1. Microwave Absorption Properties of Some Typical MAMs microwave absorption materials

minimum RL (dB)

thickness (mm)

frequency range (GHz)

EB (GHz)

graphene-wrapped ZnO47 MWCNT−ZnO/SiO248 CNT/G-PDMS49 (Mn0.2Ni0.4Zn0.4Fe2O4)x− (BaFe12O19)1−x50 Al-doped ZnO/ZrSiO451 SrF composites52 BCFO553 CNW/Si3N454 MWCNT/Fe3O4@ZnO55 Fe3O4/C56 L0.62FOC (this work)

−45.05 −21.6 −55 −25 −32 −16 ca. −19 −50.21 −40.9 −27 −26.6

2.2 2.25 2.75 3.5 3.5 5 1.56 3 3.5 3.5 2.94

8.9−11.4 9.2−12.4 ∼8.6−12.0 8.2−10.6 ∼8−11.6 ∼8.1−9.6 8.7−12.1 8.76−12.4 8−12 8−12.4 8−12.4

2.5 3.2 3.4 2.4 3.6 1.5 3.4 3.64 4 4.4 4.4

Figure 10. Schematic illustration of EM wave absorption for 2D LFOC nanocomposites.

ratio of La(NO3)3·6H2O to Fe(NO3)3·9H2O. The results that diffraction peaks corresponding to the (121) lattice plane of LFO shifted to a high angle, the XRD Rietveld refinement, and the O 1s XPS peak broadened indicated that the La defect existed. The L0.62FOC composite had prominent microwave absorption properties with a small thickness (2.94 mm). The maximum reflection loss (RL) is −26.6 dB at 9.8 GHz and the effective absorption bandwidth (below −10 dB) covered all Xband (8−12 GHz). The excellent microwave absorption performance is attributed to the multiple scatterings in 2D structure, the polarization relaxation loss from the defect of amorphous carbon, the dipoles of Fe−VO generated by the La deficiency, and the interface polarization in the nanocomposites. To tune the La deficiency in the crystal lattice, the microwave absorption properties can be effectively improved. Therefore, it turned out that the LFOC nanocomposites are promising materials as MAMs.

Especially, when the thickness (d) of composites increased to 2.94 mm, the effective bandwidth (EB) which the RL value is under −10 dB enhanced to 4.4 GHz from 8 to 12.4 GHz which covered the whole X-band. By comparison, sample LFOC and L0.75FOC have poor EM wave absorption, lower RL values, and narrower EB with a larger thickness. Compared to higher RL values, a broader EB and a smaller thickness play an equally important role in practical application of MAMs.46 In this paper, we mainly focused our study on the microwave absorption properties in X-band and compared with some representative MAMs in Table 1. The sample L0.62FOC possesses excellent microwave absorption properties in broadly effective bandwidth at a small thickness. The massive EM wave absorption of L0.62FOC/paraffins composites can be understood from several proposed mechanisms illustrated in Figure 10. As EM waves strike the sample, part of them are immediately reflected on the surface and the remaining part pass through the sample. In the meantime, the EM waves can be reflected back and forth between the 2D amorphous carbon sheets to provide multiple reflections which are conducive to the interaction between EM waves and 2D L0.62FOC nanosheets. Therefore, the amorphous carbon defects, La defects, and interfaces between L0.62FOC nanoparticles and carbon matrix can create tremendous dipoles when subjected to an external EM field. The created dipoles produced polarization losses responding to the EM waves which ultimately improved the EM wave absorption properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15379. SEM pictures of purchased NaCl powders and the morphologies of coated powders; Rietveld refinement results of XRD patterns of LxFOC powders; the relatively complex permittivity and permeability of L0.38FOC, L0.25FOC, and L0.2FOC; the conductivity and conductance loss of LxFOC; the Cole−Cole figure of LxFOC; the RL−f plots of LxFOC at the thickness of 2.94 and 3.30 mm; the peak position of the (121) lattice plane of LxFOC in XRD patterns (PDF)

4. CONCLUSION Perovskite oxides (LaFeO3) decorated 2D amorphous carbon nanocomposites have been successfully fabricated on a large scale via a one-pot synthesis. The LFOC nanocomposites with different defect concentrations were controlled by adjusting the 7608


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.-S.W.). Tel.: 86-592-2180155. Fax: 86-592-2183515. *E-mail: [email protected] (D.-L.P.) ORCID

Dong-Liang Peng: 0000-0003-4155-4766 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 51371154, 51571167, and 51171158) and the Fundamental Research Funds for the Central Universities (Grant No. 20720140547).

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