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The outside-in approach to construct Fe3O4 nanocrystals/mesoporous carbon hollow spheres core-shell hybrids towards microwave absorption Yan Cheng, Jieming Cao, Yong Li, Zhaoyong Li, Huanqin Zhao, Guangbin Ji, and Youwei Du ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03846 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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ACS Sustainable Chemistry & Engineering

The outside-in approach to construct Fe3O4 nanocrystals/mesoporous carbon hollow spheres core-shell hybrids towards microwave absorption

Yan Chenga, Jieming Caoa, Yong Lia, Zhaoyong Lia, Huanqin Zhaoa, Guangbin Ji a, *, Youwei Dub a

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China b

Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

*Corresponding Author: Prof. Dr. Guangbin Ji. Tel: +86-25-52112902; Fax: +86-25-52112626 E-mail: [email protected] Address: 29# Yudao Street, Nanjing 210016, P.R China

Listed authors: Yan Cheng, Jieming Cao, Yong Li, Zhaoyong Li, Huanqin Zhao.

Mailing address: 29# Yudao Street, Nanjing 210016, P.R China

Youwei Du.

Mailing address: 22# Hankou Road, Nanjing 210093, P. R. China ACS Paragon Plus Environment

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ABSTRACT It is widely accepted that building core-shell structure is an effective way to modify impedance matching behavior and induce interfacial polarization relaxtion. In this work, we adopted a novel impregnation process and subsequent calcination treatment to construct Fe3O4 nanocrystals/mesoporous carbon hollow spheres (MCHS) core-shell hybrids. From the perspective of microwave absorption, MCHS possess favorable dielectric loss ability and lightweight feature. Fe3O4 could produce magnetic loss and regulate impedance matching characteristic. Besides, hollow voids and mesoporous facilitate microwave absorption, meanwhile, multiple interfacial polarization induced between Fe3O4 nanocrystals and MCHS contributes to microwave attenuation. Combining these advantages, a maximum reflection loss value of -60.2 dB at 15.5 GHz and a broad effective bandwidth of 5.7 GHz can be achieved for the as-prepared sample at thickness of 2.3 mm. Furthermore, the excellent microwave absorption properties with a ultra-wide bandwidth of 8.0 GHz (10.0-18.0 GHz) at 2.6 mm of thickness was achieved. Therefore, the as-prepared hybrids can act as a lightweight and broadband microwave absorber, moreover, the corresponding method could be a sustainable and low-cost route for designing multicore-shell hybrids. Key words: Fe3O4; mesoporous carbon hollow sphere; multicore-shell; impregnation method; microwave absorption

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INTRODUCTION Due to the fast development of electronic technology, electromagnetic wave interference issues are pervasive in our daily life, which can not only give serious threats to human beings health, but also affect the operation of precise electronic instruments. Moreover, it is of great importance for military equipment to absorb radar waves to achieve the purpose of stealth. Consequently, it is essential to develop advanced microwave absorption (MA) materials with ‘strong, wide, thin, light’ virtues to perfect funtionality. 1-6 Up to now, the feature of lightness has been of the most significant factor while designing MA materials. Under this circumstance, pure carbon and/or carbon-based composites like carbon nanotubes (CNTs), carbon foams, porous carbon spheres and graphene have aroused intensive attention in MA field.7-11 As compared to other carbon materials, porous carbon sphere is a kind of light, easy-prepared and affordable material that has potential to be applied in MA. For instance, Xu et al. prepared hollow carbon spheres with outstanding dielectric loss and MA properties.12 Rong and his co-workers synthesized yolk-shell C@C microspheres with a maximum reflection loss (RL) value of -39.4 dB.13 Moreover, our group designed carbon spheres/graphene composites that showed adjustable effective absorbing bandwidth.14 These researches enlighten us that porous carbon spheres have superior dielectric loss ability and can obtain great RL value. Nevertheless, the lack of permeability is a main barrier wihich restricting its impedance matching characteristic, thus, giving rise to a narrow absorption bandwidth. Accordingly, it is a good choice to combine carbon spheres with magnetic substance. As we realized, Fe3O4 is a typical sort of ferromagetic material with strong magnetism, favorable magnetic loss and low cost, which was taken early in MA field.15-16 Some related works including Fe3O4@C, CNTs/Fe3O4 nanocoating and Fe3O4/graphene capsules demonstrated that Fe3O4 had an obvious role in promoting impedance matching behavior and magnetic loss, resulting in excellent MA performance.17-19 In addition, it is widely known that constructing core-shell structure is an effective way to enhance dielectric loss ability. Because a mass of charge carriers would accumulate in the interface of ACS Paragon Plus Environment


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heterojunction and bring about interfacial polarization relaxtion while in alternating electric field. Combined with the above analysis, the Fe3O4 nanocrystals/MCHS core-shell hybrids were synthesized by means of a impregnation process and subsequent calcination. This method may be a novel and sustainable route to bring in multiple cores of metal oxides into porous materials. Furthermore, the RL results confirmed our design ideas are effective while taking into account of attenuation ability and impedance matching. In comparison with Jian’s work, besides the differences in composition and microstructure, we intended to modulate Fe3O4 status to modify complex permittivity of hybrids.19, 20 The excellent MA properties could be obtained for Fe3O4 nanocrystals/MCHS with a maximum RL value of -60.2 dB and a broad bandwidth of 5.7 GHz at 2.3 mm.

EXPERIMENTAL SECTION Materials Absolute ethanol, concentrated ammonia water (25 wt%), formaldehyde (37%) were purchased from Nanjing chemical reagent Co. Ltd. Resocinol, hydrofluoric acid and ferric nitrate were acquired from Sinopharm chemical reagent Co. Ltd. Tetrapropoxysilane (TPOS) was from Energy Chemical. All of reagents were used without further purification. Synthesis of Fe3O4 nanocrystals/MCHS core-shell hybrids The Fe3O4 nanocrystals/MCHS was prepared by two steps. Typically, 3.46 mL of TPOS was added to the mixed solution contains 70 mL of ethanol, 10 mL of deionized water and 3 mL of concentrated ammonia water with magnetic stirring for 15 min. Next, 0.4 g of resorcinol and 0.56 mL of formaldehyde were added to the solution under stirring. Then, the obtained products were calcined at 700 oC for 5 h under nitrogen atmosphere. Finally, MCHS were gained through etching hard template of silica. Then, the impregnation method was employed to fabricate the composite. Specifically, 0.2 mol/L of Fe(NO3)3 solutions was prepared firstly. Afterwards, 0.1 g of MCHS was added to 100 mL of Fe(NO3)3 solution with stirring for 3 h, next, standing 24 h to bring ferric nitrate into MCHS adequately. Finally, the hybrid Fe(NO3)3-MCHS precursor was decomposed under nitrogen atmosphere for 2 h with different temperature of 400, 500 and 600 oC, denoted as S400, S500 and S600, respectively. ACS Paragon Plus Environment

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Characterizations The X-ray diffraction (XRD) patterns of samples were measured by a Bruker D8 ADVANCE X-ray diffractometer (Cu Kα radiation (λ=0.154178 nm with 40 kV scanning voltage and 40 mA scanning current, 10-80 º)). Transmission electron microscopy equipped with an energy dispersive X-ray spectroscopy (TEM, JEOL JEM-2100) was used to observe morphology. The area surface and pore size distributions of samples were investigated through a micromeritics ASAP 2010 system. The carbon status was detected by Raman spectrum (Renishaw Invia). The magnetic performance was tested by vibrating sample magnetometer (VSM, Lakeshore, model 7400 series). In order to obtain the electromagnetic parameters, the tested samples were fabricated by uniformly mixing the sample with paraffin (weight ratio=2:8). Then, a vector network analyzer (Agilent PNA N5244A) was applied to measure the complex permittivity and permeability based on coaxial line method.

RESULTS AND DISCUSION In order to verify the formation of MCHS and the inner structure of S500, TEM images are presented in Fig. 1. From Fig. 1a, we can find that the MCHS have been prepared successfully with ca. 120 nm of diameter at first step. As shown in Fig. 1b, the nanoparticles were produced and located in the interior of MCHS for S500, indicating the nanocrystals have been introduced into MCHS. The selected area electron diffraction (SAED) pattern in Fig. 1c exhibits three obvious diffraction points, which can be assigned to crystal planes of (111), (311), (511) and (622) of Fe3O4. Clearly, the high angle annular dark field-scanning transmission electron microscopy (HADDF-STEM) image of S500 (Fig. 1d) further confirms multi-cores have been introduced to MCHS, along with some are embedded in the shell or out of spheres, which is in agreement with Fig. 1b. Elemental mapping was also done to illustrate that the existence of C, Fe, and O, where Fe and O mainly distributed on the core sites, demonstrating the formation of ferric oxide. Additionally, in order to realize the synthesis process readily, the schematic illustration is displayed in Fig. 1e. Firstly, the precursor of SiO2/SiO2@phenolic resins (Fig. S1a) and SiO2/SiO2@carbon (Fig. S1b) were prepared through hydrolysis and pyrolysis procedure, respectively. ACS Paragon Plus Environment


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Then, the hydrofluoric acid was used to etch silica so as to obtain MCHS (Fig. S1c). Next, ferric nitrate was employed as iron source, which would be adsorbed in MCHS at the effect of capillary force in aqueous solution acccording to previous work.21 During calcination process, ferric nitrate would be decomposed into Fe2O3 firstly, then, it could be reduced to Fe3O4 by carbon component at high temperature. The chemical reaction can be interpreted as follows22: 4Fe(NO3)3→2Fe2O3+12NO2+3O2 C+3Fe2O3→2Fe3O4+CO

(1) (2)

This shows that the nanoparticles embedded in the shell or out of spheres are due to the rapid decomposition of ferric nitrate adsorbed in the pores or on the surfaces of MCHS. The morphologies of all samples are shown in Fig S2. It can be observed that the three composites look alike as compared to single MCHS. To clearly observe Fe3O4 nanocrystals variation, the HADDF-STEM images of three composites are presented in Fig S4. We can see intuitively that the Fe3O4 nanocrystals in the composite grow up with annealed temperature increasing.


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Fig. 1 TEM images of (a) MCHS, (b) S500, (c) the SAED parttern of S500, (d) the HADDF-STEM image and elemental mapping of S500, and (e) schematic illustration of the synthesis process of S500. The XRD patterns of all samples are shown in Fig 2a. It is discovered that all samples except for S600 show the broad diffraction peak at 24.1º, which associates with the characteristic peak of (002) for graphitic carbons. The (002) peak of S600 moves left slightly due to the increased interlayer distance, which is resulted from the higher temperature.23 For S400, the diffraction peaks at 35.5º, 43.1º, 57.0º, 62.7º can be attributed to (122), (220), (232), (040) crystal planes of Fe3O4 (PDF#76-0956), respectively. With respect to S500, the peaks of 35.4º, 56.9º, 62.5º corresponding to the lattice planes of (311), (511), and (440), respectively, can be assigned to Fe3O4 (PDF#19-0629). In terms of S600, the peaks’ positions are obviously different with S400 and S500, suggesting diverse crystalline phase. Further, the enhanced peaks demonstrate the increase of crystallinity, which match well with the PDF card of Fe3O4 (PDF-75#1609). In addition, in order to identify the struture of Fe3O4 nanocrystals in three composites, the Rietveld refinements of composites are displayed in Fig. S4 and Table S1. Clearly, ACS Paragon Plus Environment


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the structure of Fe3O4 decomposed from Fe(NO3)3 are highly influenced by annealing temperature, where crystal system, space group, lattice parameters, volume are different respectively. The grain sizes of Fe3O4 in composites are also calculated based on the Debye-Scherrer equation24: D=

0.89λ βCOSθ

(3)

Where λ is the wavelength of X-ray (λ=0.15406 nm), β is the half-width of diffraction peaks, θ represents diffraction angle. The calculated results are listed in Table 1. The X-ray Photoelectron Spectroscopy (XPS) measurement was also done to analyze the composition on the samples’ surface. As observed in Fig. S5, the survey scan of MCHS and S500 are highly similar both including C and O elements. The Raman spectra are presented in Fig. 2b to explore the bonding state of carbon atoms. One can find that all samples exhibit two distinguished peaks situated at 1330 and 1590 cm-1 which are ascribed to D- and G- band. As we known, D-band is a breathing mode of A1g and represents disorder carbon, and G-band corresponds to E2g mode due to sp2 vibrations of carbon atom meaning graphitic carbon.25, 26 In general, the intensity ratio of D- and G-band (ID/IG) is a common standard to evaluate graphitization degree of carbon materials. Correlating the TEM image in Fig. 1a and XRD analysis with specific ID/IG ratio, it can be concluded that the state of MCHS in composite was mainly amorphous and had relatively low graphitization. It is acknowledged that ID/IG ratio of carbon materials has significant influence on microwave absorbing properties of carbon materials, reflecting in the variation of complex permittivity25. It is well known that the high graphitization degree (e.g. graphene) and good conductivity of carbon materials will produce the ideal attenuated microwave ability through conduction loss.

Conversely, low graphitization means more amorphous carbon,

benefiting microwave penetration. In comparison with ratios of three composites, S500 is slightly higher than the other two, indicating stronger dissipated ability derived from MCHS. From Fig. 2(c-f) and Table 2, the specific surface area of S500 decreased, but S400 and S600 increased, when they compared with MCHS, which could be associated with Fe3O4 nanocrystals size growth. All samples exhibited high specific surface area. Simutaneously, the pore size distribution of ACS Paragon Plus Environment

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composites still stabilized at 6 nm as well as MCHS. It is noted that a broad peak appeared at the inset of Fig. 2c corresponds to 90 nm, which may be attributed to the diameter of hollow void of MCHS according to the TEM image (Fig. 1a). After the introduction of Fe3O4 nanocrystals, the broad peak in the inset of Fig. 2e shifts to 68 nm. Hence, it may infer that the Fe3O4 nanocrystals were into the hollow void of MCHS, which mean diameter may be 22 nm. It is found that there is no broad peak in the inset of Fig 2d, so the Fe3O4 size in S400 could not be exactly deduced. With regard to S600 (Fig. 2f), the broad peak exhibit at 72 nm and the Fe3O4 size may be 18 nm. But this result is not accurate as S500 and smaller than actual dimension. The role of porosity in the composite, besides could alleviate the samples’ density and weight, is more important to enlarge surface areas, which is a effective way to prolong the microwave transmission path in absorber, especially for a fine pore size distribution.27, 28 Hence, the incident microwave could have more chances to be attenuated. In addition, abundant pores contribute to impedance matching characteristic and make more microwaves into the absorber. Hysteresis loop of S500 is exhibited in Fig. S6. It is observed that S500 has low magnetization value, indicating weak magnetism due to dominated nonmagnetic carbon and little grain size of Fe3O4. Nontheless, the higher coercivity can improve natural resonace frequency for ferrite, thus, which can achieve effective absorption at gigahertz.29



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Fig. 2 (a) XRD patterns and (b) Raman spectra of all samples; (c-f) N2 adsorption-desorption isotherms and pore size distribution of MCHS, S400, S500, S600, respectively. Table 1 The grain size of Fe3O4 calculated based on Debye-Scherrer equation from XRD results. Sample

S400

S500

S600

θ(°)

35.6

35.5

35.3

β(rad)

0.01441

0.014

0.00752

D(nm)

9.99

10.3

19.1


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Table 2 Specific surface areas and total pore volumes of MCHS and S500. Sample

SBET (m2·g-1)

Slangmuir (m2·g-1)

Vpore (cm3·g-1)

MCHS

738

871

2.293

S400

914

1086

2.348

S500

675

801

2.03

S600

1332

1580

3.41

It is known that there are two key factors affecting microwave absorbing properties: one is the whole attenuated abilities of coating, which decides the reflection loss intensity. The other is the impedance matching which determines fundamentally the effective absorption bandwidth. Therefore, a favorable absorber may not only need close impedance with air, but also possess outstanding dissipated abilities, specifically, which is associated tightly with the complex permittivity (εr) and complex permeability (µr). And real parts of the complex permittivity (ε′) and complex permeability (µ′) represents storage ability of electric and magnetic, likewise, the image parts (ε″ and µ″) serves as wastage ability of electric and magnetic.30, 31 As observed in Fig. 3a, the ε′ of all samples exhibits decreased tendency with growing frequency, which can be resulted from the delay of dipoles at high-frequency alternating electric field. Among them, MCHS possess the highest ε′ values between 6.6 and 13.1 and with slight fluctuation at high frequency. After introducing Fe3O4 nanocrystals, the ε′ of three composites obviously gets reduced as compared to pure MCHS due to low dielectric constants of Fe3O4, of which S400, S500 and S600 ranged from 4.5-8.5, 4.2-10 and 5.0-12.7, respectively. Interestingly, the ε′ of the composites enhances as the annealed temperature increased. As we known, ε′ is associated with the total number of free electrons in sample. Thus, this phenomenon coud be ascribed to the expanded contact interface area that led to the increase of charge carriers between multiple interfaces of Fe3O4/carbon under alternating electric field. The increased interface area may be induced by the crystal growth of Fe3O4 with ACS Paragon Plus Environment


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improved temperature. Moreover, for porous materials, Maxwell-Garnett (MG) theory is a symbolic mechanism for exploring variation of the effective permittivity, which can be expressed as32:

ε

MG eff

= ε1

( ε2 + 2ε1 ) + 2 P(ε2 − ε1 ) ( ε2 + 2ε1 ) − P(ε2 − ε1 )

(4)

Where ε1 represents the permittivity of host (solid) and ε2 is the permittivity of guest (air), and p means the volume fraction of guest. With the Fe3O4 crystal growth, the voids volume decreased which would lead to the increase of effective permittivity. From Fig. 3b, we find that the ε″ of all samples shows similar trend with corresponding ε′ curves, which can be interpreted by the following formula 33:

ε' =

ε" 2πτf

+ ε∞

(5)

The sort order of ε″ for three composites is in accordance with ε′, where S400, S500 and S600 decrease from 7.5 to 2.5, 8.6 to 2.6, 13.5 to 3.8, respectively. It is inferred that this result is highly induced by the advance of the effective permittivity and the enhacement of the interface polarization relaxtion behavior. It is known that Fe3O4 and carbon component has obviously unlike permittivity characteristic. Thus, there are mass of free charges existing at the interface of them. When applied the electric field, these free charges would follow the variation and hopping between Fe3O4 and carbon repeatly, leading to strong interfacial polarization relaxtion to dissipate incident microwave. On the basis of Debye theory, the relation between ε′ and ε″ can be described as34:

(ε' −

ε s + ε∞ 2 ε −ε ) + (ε" ) 2 = ( s ∞ ) 2 2 2

(6)

Hence, when the plot of ε″ versus ε′ is a semicircle, it corresponds to one Debye dipole polarization process. As observed in Fig. S7, all samples displays several small semicircles, indicating multiple dipole polarization process. However, the Cole-Cole semicircles are distorted obviously, which illustrates other mechanisms like the Maxwell-Wagner relaxtion also existed in the composite.35 Furthermore, abundant mesoporous and voids play a significant role in scattering and reflecting microwave in the composite, which ensure outstanding dielectric dissipated capabilities. Generally, ACS Paragon Plus Environment

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dielectric loss factors (tanδe = ε″/ε′) are of importance for assessing dielectric attenuated properties. As shown in Fig. 3c, the values of three composites are almost higher in 2-18 GHz than pure MCHS except for partial lower for S400, demonstrating the improvement of dielectric loss abilities after compounding. This could be ascribed to the increased of interface polarization relaxtion between Fe3O4 and MCHS. And all composites exhibited high dielectric loss factors, which benefits microwave absorption intensity. As shown in Fig. S8a, 8b, µ′ of all the composites fluctuate around 1.05 and µ″ varies about 0.1, which resulted from low magnetization of the composites. This is similar with MCHS and account for very weak magnetic loss. The fluctuation of permeability can be influenced by dielectric variation.36, 37 Namely, magnetic energy may be transformed to electric energy at microwave frequency, expressing as increased ε″ and negative µ″. Also, some peaks of µ″ for the composites can be associated with multiple nature resonance behavior of the Fe3O4 nanocrystals. Magnetic loss factors (tan µe =µ″/µ′) of the composites displayed at Fig. S8c is also fluctuant and actually low, which are much less than dielctric loss factors (Fig. 3c). Accordingly, the excellent microwave absorbing properties of the composites is mainly benefited from the outstanding dielectric dissipated capabilities.


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(a)

MCHS S400 S500 S600

12

ε′

10 8 6 4 2

4

6

8

10

12

14

16

18

Frequency (GHz)

(b) 14 12

MCHS S400 S500 S600

ε″

10 8 6 4 2 2

4

6

8

10

12

14

16

18

Frequency (GHz)

(c) 1.1

MCHS S400 S500 S600

1.0 0.9

tanδ δε

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

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0.8 0.7 0.6 0.5 2

4

6

8

10

12

14

16

18

Frequency (GHz)

Fig. 3 The dependence of frequency on (a) the real part of permittivity, (b) the imagine part of permittivity (c) dielectric loss factors of all samples. In order to evaluate electromagnetic dissipated properties, the RL values for all samples are calculated on the basis of following formulas38-40:

Zin = Z 0( µr/εr)1/2 tanh[ j ( 2πfd / c)( µrεr)1/2 ] RL(dB) = 20 log | ( Zin − Z 0) ( Zin + Z 0) |


(7) (8)

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Where Zin represents the input impedance, f is the frequency of microwave, d stands for the coating thickness of absorber, c means the velocity of electromagnetic wave, εr (εr=ε′-jε″) and µr (µr=µ′-jµ″) are complex permittivity and permeability. Fig. 4 presents the reflection loss curve and effective bandwidth (

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