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Structure-switchable mesoporous carbon hollow sphere framework toward sensitive microwave response
Journal Pre-proof Structure-switchable mesoporous carbon hollow sphere framework toward sensitive microwave response Yan Cheng, Huanqin Zhao, Yue Zhao, Jieming Cao, Jing Zheng, Guangbin Ji PII:
S0008-6223(20)30149-4
DOI:
https://doi.org/10.1016/j.carbon.2020.02.011
Reference:
CARBON 15061
To appear in:
Carbon
Received Date: 27 December 2019 Revised Date:
3 February 2020
Accepted Date: 5 February 2020
Please cite this article as: Y. Cheng, H. Zhao, Y. Zhao, J. Cao, J. Zheng, G. Ji, Structure-switchable mesoporous carbon hollow sphere framework toward sensitive microwave response, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2020.02.011. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Graphical abstract
Structure-switchable mesoporous carbon hollow sphere framework toward sensitive microwave response
Yan Chenga, Huanqin Zhaoa, Yue Zhaoa, Jieming Caoa, Jing Zhengb, Guangbin Jia,*
a
College of Materials Science and Technology, Nanjing University of Aeronautics
and Astronautics, Nanjing 211100, P. R. China b
Department of Chemistry and Materials Science, College of Science, Nanjing
Forestry University, Nanjing 210037, P. R. China
*Corresponding Author E-mail: [email protected] (Prof. Guangbin Ji)
Abstract Mesoporous carbon hollow sphere (MCHS) is one of the most significant functional materials in various fields. Unfortunately, systematically regulating microstructure of MHCS has been rarely investigated so far. In this work, using one unique simultaneous-hydrolyzation-polymerization process, we are able to precisely control the morphology configurations of MCHS, including its shell thickness and integrate size, hollow void and mesopore in the shell. The shell thickness and integrate size are associated with the synergistic effect of hydrolyzation and polymerization reactions. The hollow void and mesopore are determined by the SiO2 template morphology. Thus, a series of MCHSs with controlled microstructure were achieved through elaborately tuning the precursor. More importantly, different types of MCHSs demonstrate significantly different dielectric and microwave absorbing properties due to the discrepancy of volume ratio between pores and carbon compositions. The typical sample could achieve a broad effective absorption bandwidth of 5.9 GHz and 6.5 GHz with a thickness of 2.35 mm and 2.65 mm, respectively, at a filling ratio of only 20 wt.%. These encouraging results significantly promote a deeper understanding of the fundamental chemistry mechanism for constructing MCHS as well as using the material as potential candidate for solving microwave interference issue. Keywords: mesoporous carbon; hollow void; porous; tunable; microwave response
1. Introduction Nowadays, modern society is interlinked by ubiquitous electromagnetic wave emitted by electronic devices [1-4]. While bringing convenience to life, however the electromagnetic wave poses a great threat to human health at the same time [5, 6]. Therefore, eliminating external electromagnetic wave interference is a critical prerequisite for ensuring electromagnetic compatibility of precision electrical equipment. Particularly, the attached microwave absorbing coating is powerful to solve this issue. In many filler candidates, carbon materials, such as graphene, carbon nanotubes, carbon sphere, have been widely investigated due to their reliable chemical stability, lightweight superiority and dominated dielectric dissipated ability [7-10]. Among them, carbon spheres derived from organic precursor captures much research interests due to their attractive advantages of low cost, high yield, and adjustable graphitization level. The ordinary carbon spheres are generally confined by limited dielectric loss capability, making it very difficult to become a qualified absorber. Under the premise of appropriate graphitized degree of carbon composition, the most efficient way to boost the dielectric ability is processing microstructure design, especially pores design [11-13]. The introduction of pores can not only improve specific areas and reduce density of carbon spheres, but also perfect the impedance matching feature and increase the possibility of microwave attenuation [14]. Following this thought, we believe that MCHS which inherits the advantages of carbon sphere and contains multilevel pore structure including interior hollow void and exterior mesopores in the
shell makes itself become a potential excellent absorber. Referring the design concept of MCHS, hollow void and mesopores are responsible for boosting impedance matching properties, and the hybridized carbon compositionsorder carbon and disorder carbon - are in charge of the microwave dissipation via conductive loss [15-17]. As such, the variations in pores or carbon compositions both have significant impact on final dielectric and microwave absorption (MA) properties. The microstructure parameters, including hollow void, integrate size, shell thickness and mesopores, positively depend on the volume ratio between pores and carbon compositions of MCHS. Thus, precisely controlling the microstructure and investigating the corresponding effect on dielectric properties are highly required for MCHS. In this work, based on the unique reaction mechanism, the microstructures of MCHS, including hollow void, integrate size, shell thickness, mesopores, are controlled by tuning the hydrolyzation of tetrapropoxysilane (TPOS) and polymerization of phenolic resin for precursors. Thus, we rationally regulated the precursor through changing reaction conditions, then prepared a series of MCHSs with controllable hollow voids (89.3-204.5 nm), mesopores (7.1-11.3 nm), thicknesses (68.1-148.7 nm) and sizes (204.0-268 nm). As-synthesized MCHSs show microstructure-determined dielectric properties and MA performances owing to the difference of volume ratios between pores and carbon compositions. It is observed that only when the impedance matching feature and attenuated ability reach balance in a
specific
microstructure,
the
MCHS
presents
excellent
MA properties.
Representatively, the sample of MCHSV-2 (hollow void: 135.1 nm, shell thickness: 60.4 nm) presents attractive MA properties with an effective absorption bandwidth (EAB) of 5.9 GHz and a reflection loss (RL) of -58.2 dB at 2.35 mm, which may be an ideal filler for solving microwave interference issue. In addition, the as-prepared MCHS can be applied in various research fields, e.g. energy storage, pollutant adsorption, biology cure, electrocatalysis, etc. 2 Experimental 2.1 Materials TPOS, tetraethyl orthosilicate (TEOS), resorcinol and ammonium hydroxide (NH3·H2O, 25%) were purchased from Aladdin; formaldehyde (37%), hydrofluoric acid (HF, 25%) and absolute ethanol were received from Nanjing Chemical Reagent Co., Ltd. All chemicals used without further purification. Deionized (DI) water used in the whole reaction process. 2.2 Synthesis of different types of MCHS The synthesized procedure and as-prepared MCHSs are displayed in Scheme 1. First, 3.46 mL of TPOS (12 mmol) was added to the mixed solution containing 70 mL of ethanol, 10 mL of DI water and 3 mL of NH3·H2O. Then, after 15 min of stirring, 0.4 g of resorcinol and 0.56 mL of formaldehyde were casted into the solution with a further 24 h of stirring. The obtained SiO2@SiO2/phenolic resin (PF) precursor were collected through centrifugal washing with DI water and ethanol for three times, respectively. When the precursor was dried in vacuum oven over one night, 2 g of precursor was put into porcelain boat for high-temperature calcination under argon
atmosphere to convert PF into carbon material. The calcined condition set as 700 ºC for 5 h with a ramping rate of 5 ºC/min. Finally, the MCHS was received after etching SiO2 core via HF solution (25 %). The as-prepared MCHS is denoted as MCHS-1. Then, we tried to systematically control various structure parameters of MCHS, including hollow void size, integrate size, carbon shell thickness, mesopores in carbon shell, through regulating experimental conditions in this work. (I) Hollow void size. The void size is depended on the size of SiO2 core, so prolonging hydrolysis time can enlarge SiO2 core. The stirring time was prolonged from 15 min to 1 h and 2 h, respectively, during the initial TPOS addition stage. The obtained MCHS were denoted as MCHS-1, MCHSV-2 and MCHSV-3, respectively. (II) Integrate size. Enlarging SiO2 core could expand hollow void size, but the integrate size of MCHS increased accordingly, so we employed part of TEOS substituting TPOS as silicon source to stabilize integrate size. If add part of TEOS (total silica source remains unchanged), the SiO2 core would grow larger than pure TPOS during the same hydrolyzation time due to the faster hydrolyzation rate of TEOS. Thus, the hollow void expands. Correspondingly, the carbon shell thickness is reduced owing to the decrease amount of TPOS. In this case, the integrate size of MCHS can be maintained. The added molar ratio between TPOS and TEOS (total 12 mmol) was regulated as 12:0, 9:3, 6:6, 3:9 and 0:12. The obtained MCHS were denoted as MCHS-1, MCHSS-2, MCHSS-3, MHCSS-4, MCHSS-5, respectively. (III) Carbon shell thickness. The shell thickness is determined by phenolic resin polymerization, so we considered whether the carbon shell thickness can be adjusted while remaining the
hollow void size unchanged. The added amount of resorcinol and formaldehyde is 0.5 times, 1.5 times and 2.0 times than that of MCHS-1. The synthesized MCHSs are marked as MCHST-0, MCHST-2, MCHST-3, respectively. (IV) Mesopores in carbon shell. The mesopores in carbon shell are depended by SiO2 nanoparticle size in the shell, so which can be tuned via controlling the volume ratio of water to ethanol due to the faster hydrolysis rate of TPOS in water than in ethanol. The volume ratio of ethanol to DI water (total 80 mL) is controlled as 70 :10, 65: 15, 60: 20, 55: 25, 50: 30. The obtained MCHSs are denoted as MCHS-1, MCHSP-2, MCHSP-3, MCHSP-4, MCHSP-5.
Scheme 1. The synthesized procedure of different types of MCHSs; Group I: expanded hollow void and decreased thickness with enlarged integrate size; Group II: expanded hollow void and decreased thickness with controlled integrate size; Group III, expanded hollow void and thickness with enlarged integrate size; Group IV, controlled hollow void and thickness with extended mesopores. 2.3 Characterizations The X-ray diffraction (XRD) pattern was defined by a Bruker D8 ADVANCE X-ray diffractometer using Cu Kα radiation (λ=1.54178 Å) with 40 kV scanning voltage and 40 mA scanning current. The morphology and porous structure were detected by a Hitachi S4800 type scanning electron microscope (SEM) and a Tecnai G2 F30 S-TWIN transmission electron microscope (TEM). The specific surface areas and pore information were obtained on a Brunauer-Emmet-Teller (BET) analyzer equipped with micrometric ASAP 2020 system. The carbon state was identified by a Renishaw Invia Raman spectrometer. The chemical state of carbon atom was tested by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe). The electromagnetic parameters of MCHS in the frequency range of 2-18 GHz were measured via an Agilent PNA N5224A vector network analyzer based on coaxial-line method. The measured toroidal-shaped ring (Φout=7.00 mm, Φin=3.04 mm) was pressed after uniformly mixing MCHS with paraffin wax (mass ratio = 80%: 20%). Then, the electromagnetic parameters between 2-18 GHz are calculated based on the software installed in the vector network analyzer (Agilent PNA N5224A, coaxial-line method). The RL intensity vs frequency at different thickness is simulated according to following equations [18-21]:
Zin = Z 0( µr/εr)1/2 tanh[ j (2πfd / c)( µrεr)1/2 ]
(1)
RL(dB) = 20 log | ( Zin − Z 0) ( Zin + Z 0) |
(2)
where εr (εr=ε′-jε″) and µr (µr=µ′-jµ″) mean complex permittivity and permeability, respectively. f represents the frequency of electromagnetic wave and d is the thickness of microwave absorber. 3 Results and Discussions The morphology and microstructure of the as-prepared different types of MCHS are observed through SEM and TEM identifications. Fig. 1 presents different hollow void-enlarged MCHSs via extending hydrolyzation time of TPOS. As shown in Fig. 1a, the initial MCHS-1 displays clearly typical hollow void feature and strip-like pores in the carbon shell, which obtains hollow void size of 89.3 nm and shell thickness of 68.1 nm. SEM observations demonstrate a good dispersive state of MCHSs and integrate size concentration at 204 nm. While prolonging the hydrolyzation time of TPOS to 1h, the hollow void size obviously increases to 135.1 nm with thickness reducing to 60.4 nm for MCHSV-2, which size is mainly centered at 255.5 nm (Fig. 1b). This is due to the growth of SiO2 core at the extended hydrolyzation time of TPOS, thus the SiO2 nanocores in the shell is declined, giving rise to the decrease of shell thickness. With further prolonging hydrolyzation time to 2h, the hollow void size expands to 196.9 nm and the thickness decreases to 44.6 nm (Fig. 1c). The integrate size of MCHSV-3 improves to 268.0 nm through SEM collection. In this hollow void regulation, the extension of hydrolyzation time of TPOS can not only expand the hollow void size, but also decrease the thickness. The
integrate size is greatly increased as well. For application, the as-synthesized MCHSs are employed for solving microwave issue due to the unique porous structure and hybridized carbon composition. As a typical dielectric material, the magnetic loss of MCHS can be neglected, thus the complex permittivity of ε' and ε" are presented, which represent dielectric storage and loss ability, respectively [22, 23]. The corresponding dielectric parameters and MA properties are presented in Fig. S1 (Supporting Information). It is observed that MCHSV-2 exhibits stronger dielectric loss capability than MCHSV-3, which is ascribed to the expanded hollow void based on the Maxwell-Garnett (MG) theory [23]. For MCHSV-2, the superior dissipated ability enables it to achieve two broad EABs (12.1 to 18 GHz at a thickness of 2.35 mm and 9.7 to 16.2 GHz at a thickness of 2.75 mm).
Fig. 1 The morphology and microstructure observations from SEM and TEM images (a) MCHS-1, (b) MCHSV-2, (c) MCHSV-3; insets are size distribution from SEM images; the scale bars are 200 nm, 100 nm, 50 nm corresponding to TEM images of 1,
2, 3, respectively, 1 µm and 500 nm corresponding to SEM images of 4 and 5, respectively. Then, we explored that if the integrate size of MCHS can be maintained when expanding the hollow void. Since TEOS has one less methyl group than TPOS, it holds a relatively faster hydrolyzation rate in alkaline solution system, so which can produce larger SiO2 core during the same time, leading to expanded hollow void. Hence, we employed TEOS to substitute part of TPOS as silicon source. Meanwhile, fixed total silicon source can retain the integrate size of MCHS. As seen from Fig. 2a, the hollow void size expands to 118.1 nm with thickness reducing to 45.5 nm when adding 3 mmol of TEOS. The total carbon size stabilizes at 212 nm. As increasing TEOS amount to 6 mmol and 9 mmol, observed from Fig. 2b, c, the hollow void sequentially expands to 142.0 nm and 189.1 nm, along with the thickness of 35.9 nm and 24.4 nm. Total carbon size is 214.0 nm and 216.0 nm, respectively. When TPOS is completely replaced by TEOS, the hollow void further enlarges to 204.5 nm with a very thin thickness of 16.9 nm (Fig. 2d). Total carbon sphere size centers at about 217.0 nm. These modified MCHSs have enhanced hollow void space and reduced thickness than MCHS-1 (hollow void of 89.3 nm and thickness of 68.1 nm). As a result, it is revealed that the hollow void can be boosted without altering integrate size of MCHS via part of TEOS substituting TPOS. Correspondingly, the thickness is consecutively decreased because of the lessened TPOS amount. It is noted that when TEOS is 9 mmol, the MCHSs are broken to some extent from SEM observation. For MCHSS-5, more MCHSs are broken than MCHSS-4, which can be resulted from the greatly reduced carbon shell thickness.
Fig. 2 The morphology and microstructure observations from SEM and TEM images, (a) MCHSS-2, (b) MCHSS-3, (c) MCHSS-4, (d) MCHSS-5. The scale bars are 200 nm, 100 nm, 50 nm corresponding to TEM images of 1, 2, 3, respectively, 1 µm and 500 nm corresponding to SEM images of 4 and 5, respectively. For clarifying the carbon compositions of MCHSs, corresponding characterizations are performed in Fig. 3. As displayed in Fig. 3a, all MCHSs present a broad peak at about 22.7°, which can be attributed to the (002) plane of graphitic carbon, indicating the MCHSs are partly graphitized. Raman spectra is skilled for demonstrating carbon phase state. As shown in Fig. 3b, there are two prominent peaks at about 1330 cm-1 and 1590 cm-1, respectively, corresponding to D- and G-band of carbon materials [25, 26]. Generally, D-band is derived from sp3 hybridization of carbon atoms with meaning of disorder carbon; G-band is resulted from the stretching vibrations of sp2 bond with existence of graphitic carbon. The intensity ratio of D- and G-band
indicates the graphitized degree of carbon materials. One can find that all MCHSs show close values of 1.0, suggesting the approximate and relatively high graphitization at the same calcination temperature. This is advantageous for the conductive loss of MCHS. Then, we further probe the chemical state of carbon compositions though XPS detection. It is observed that the MCHS presents both C and O elements (Fig. 3c). The O element is derived from phenolic resin precursor, which exists as valence bond of C-O, O=C-C, O=C-O (Fig. 3d, e) [27]. These polar functional groups may be very helpful for introducing dielectric polarization loss [28]. BET measurements were performed to analyze porous structure and surface areas. As presented in Fig. 3f-k, all MCHSs exhibit an obvious hysteresis loop when P/P0 > 0.6, which can be assigned to representative IV, H3-type adsorption isotherm according to IUPAC standard, indicating slit-like mesopores in the carbon shell. The comparable sorption isotherms explain that the MCHSs have similar pore structure. They all display high specific areas and pore volumes (Fig. S2), among which MCHSS-2 owns the largest surface areas of 1375.8 m2/g and pore volume of 2.3 cm3/g. Mesopores in these MCHSs are all close to 7 nm from pore size distribution curve in Fig. S2, illustrating TEOS replacement doesn’t affect pore size. Abundant pore structure and high surface areas of MCHS are conductive to boost impedance matching feature and promote microwave dissipation [29, 30].
Fig. 3 The MCHSs from group II, (a) The XRD patterns, (b) Raman spectra, (c-e) survey spectra, C1s and O1s high-resolution profile of MCHSS-3 of XPS detection, (f-j) adsorption isotherms of BET measurements, (k) surface-area comparisons. Since we have well controlled the hollow void and integrate size of MCHS, in order to investigate whether the carbon shell thickness can be regulated without affecting hollow void, we tried different amounts of phenolic resin additions. As exhibited in Fig. 4a, the MCHS is not able to remain spherical morphology and collapses into bowl-like configuration from TEM and SEM observation. The reason is that a thinner carbon shell thickness is produced by using the limited phenolic resin resource. Hence, it is readily understood that the thin carbon skeleton cannot support the regular spherical microstructure in the scale, thus collapsing into bowl-like structure. When the addition amount of phenolic resin is 1.5 times larger than MCHS-1, not only the shell thickness (129.5 nm) is increased, but also hollow void
size (140.9 nm) and the integrate size are boosted (Fig. 4b). With further increasing addition amount of phenolic resin (2.0 times than that of MCHS-1), the shell thickness improves to 148.7 nm with hollow void expanding to 236.3 nm (Fig. 4c). In addition, we notice that some solid spheres appear in the sample, which is resulted from the self-polymerization of excess phenolic resin without silica involving in the reaction. Therefore, tuning the phenolic resin amount cannot individually adjust shell thickness, which also clearly influences hollow void and integrate size, even morphology of MCHS. Fig. S3 displays complex permittivity and MA performance of shell thickness-regulated MCHSs. In this part, it is shown that the morphology has an obvious effect on its dielectric property. The unique bowl-like MCHS of MCHST-0 performs moderate complex permittivity, thus achieving an excellent absorption between 7.9-12.9 GHz with a strong RL intensity of -54.7 dB at a coating thickness of 3.7 mm. For MCHST-2, the clearly increased thickness results in enhanced dielectric response properties (ε', 9.4-4.6; ε", 9.0-2.7). Although the strong attenuation is confirmed, the impaired impedances matching behavior limits the effective absorption bandwidth. Further, the small solid carbon spheres appeared in MCHST-3 give rise to evidently decreased dielectric values, leading to the reduced MA performance.
Fig. 4 The morphology and microstructure observations from SEM and TEM images (a) MCHST-0, (b) MCHST-2, (c) MCHST-3. The scale bars are 200 nm, 100 nm, 50 nm corresponding to TEM images of 1, 2, 3, 1 µm and 500 nm corresponding to SEM images of 4 and 5, respectively. Our previous works have investigated the regulations of hollow void, integrate size and shell thickness of MCHS, but the study of mesopores in the shell is insufficient. Since the mesopores size is determined by SiO2 nanocores in precursor shell, tuning mesopores requires altering the size of SiO2 nanocores. We realized that the TPOS has a faster hydrolysis speed in DI water than in absolute ethanol, thus properly increasing volume ratio of water can enlarge SiO2 nanocores in the shell. As observed in Fig. 5a, the MCHS keeps good microstructure and morphology with hollow void of 135.5 nm and shell thickness of 52.3 nm when adding a little more water (15 mL). The hollow void and shell thickness are without obvious changes compared with MCHS-1, but the mesopores in the shell look more noticeable. Further increasing volume ratio of water (20 mL), the MCHS turns more transparent than previous one, indicating the
extended mesopores in the shell, of which hollow void and thickness maintain as 135.9 nm and 54.1 nm, respectively (Fig. 5b). Then, the mesopores in MCHSP-4 (Fig. 5c) are further enlarged while continue increasing water volume to 25 mL. In this sample, the pores can be clearly observed from the surface of MCHS, but the uniformity of MCHSs is destroyed to some extent. Accordingly, the microstructure of MCHS has been completely broken with excess water volume of 30 mL (Fig. 5d). The rapid hydrolysis rate of TPOS leads SiO2 production to be completed before phenolic resin polymerization, thus the carbon shell cannot grow on SiO2 cores, resulting in the collapse of MCHS. The microstructure observation gives a visible process of pores evolution and good preservation of integrate size (ca. 230 nm). It is acknowledged that the mesopores derived from the SiO2 nanocores in the shell, thus we get the hard template via calcining the precursor in air. Fig. S4 exhibits a visualized illustration for the hard template transformation, which reflects well the pores development. Furthermore, the pore size distributions from BET measurement (Fig. S5) reveal the gradually increase of 7.1 nm, 10.1 nm and 11.3 nm, corresponding to samples of MCHSP-2, MCHSP-3 and MCHSP-4. These results prove that the mesopores in the carbon shell can be well regulated via tuning volume ratio of absolute ethanol to DI water. The dielectric and MA properties of mesopores-regulated MCHSs are exhibited in Fig. S6. We can see that the samples show obvious mesopores-depended dielectric response properties. From MCHS-1 to MCHSP-3, the dielectric values present an increasing trend. Then, with the mesopores further growing and the microstructure
collapsing, the dielectric values gradually decreased. This illustrates that proper mesopore design can boost dielectric loss capability, but too much mesopores will reduce dielectric values. In this case, MCHSP-4 (ε', 9.2-5.7; ε", 6.0-2.2) presents feasible dielectric values satisfying good microwave attenuated ability and impedance matching feature, thus achieving a wide EAB of 5.0 GHz (9.7-14.7 GHz) and strong RL intensity of -52.3 dB at 2.65 mm.
Fig. 5 The morphology and microstructure observations from SEM and TEM images (a) MCHSP-2, (b) MCHSP-3, (c) MCHSP-4, (d) MCHSP-5. The scale bars are 200 nm, 100 nm, 50 nm corresponding to TEM images of 1, 2, 3, respectively, 1 µm and 500 nm corresponding to SEM images of 4 and 5, respectively.
Table 1. The microstructure parameters for different types of MCHSs. Samples
Void diameter/nm
Shell thickness/nm
Average pore width/nm
Average microsphere size/nm
MCHS-1
89.3
68.1
7.2
204.0
MCHSV-2
135.1
60.4
~
255.5
MCHSV-3
196.9
44.6
~
268.0
MCHSS-2
118.1
45.5
6.4
212.0
MCHSS-3
142.0
35.9
6.3
214.0
MCHSS-4
189.1
24.4
7.0
216.0
MCHSS-5
204.5
16.9
6.7
217.0
MCHST-0
~
~
~
~
MCHST-2
140.9
129.5
~
~
MCHST-3
236.3
148.7
~
~
MCHSP-2
135.5
52.3
7.1
225.5
MCHSP-3
135.9
54.1
10.1
228.0
MCHSP-4
126.5
60.2
11.3
243.5
MCHSP-5
~
~
~
~
We have tried to systematically adjust the microstructure of MCHSs (Table 1) and analyze the microstructure evolution effect (hollow voids, shell thickness and mesopores) on dielectric properties and MA performance. To this end, we take size-controlled MCHSs as particular illustration. The complex permittivity and MA performance of MCHS-1 to MCHSS-5 are displayed in Fig. 6a-j. As seen in Fig. 6a, b, MCHS-1 exhibits very low ε' and ε", giving rise to poor MA properties due to insufficient dielectric loss. For MCHS-2, the complex permittivity is obviously enhanced. ε' decreases from 6.0 to 3.4 with ε" decreasing from 3.7 to 1.2. owing to
frequency dispersion behavior, which presents acceptable MA performance (Fig. 6c, d). With hollow void expanding, ε" (5.7-1.6) is more approaching to ε' (6.8-3.7) (Fig. 6e). The boosted dielectric loss (Fig. S7) enables attractive performance for MCHS, which performs a wide EAB of 4.9 GHz (7.5-12.4 GHz) with RL of -52.9 dB at thickness of 4.0 mm, covering whole X-band (Fig. 6f). When hollow void dominates MCHS (MHCSS-4 and MCHSS-5), ε" even exceeds ε', meaning inferior impedance matching feature (Fig. S8) and most of microwave reflected from the absorber surface. This cause these two samples with very weak MA performance (Fig. 6g-J). The gradually climbing dielectric parameters should be significantly associated with microstructure evolution. The expanded hollow void would clearly reduce the density of MCHS, thus increasing filled molar quantity at the same weight, which results in monotonically increasing of complex permittivity. When the filling ratio is 20 wt%, MCHSS-3 presents the best absorption capability due to the compatible of impedance matching and attenuated capacity. Through
comprehensively analyzing complex
permittivity variation with
microstructure evolution of all samples, it is found that this attribution can be ascribed to the difference of pores and carbon compositions. Hollow voids and mesopores can promote impedance matching. Carbon compositions are responsible for microwave attenuation. Due to microstructure difference, the molar quantity is different at the same filling ratio. As a result, if pores dominate MCHS, the sample exhibits a lower complex permittivity, such as MCHSV-3. If carbon compositions dominate MCHS, the sample exhibits a higher complex permittivity, such as MCHST-2. Therefore, the
regulation of microstructure of MHCS can effectively tune their dielectric properties and MA performance. Only when the impedance matching and attenuated capacity are compatible, the sample displays excellent MA performance. For positively understanding the MA performance superiority, their 2D graph (Fig. S9) and Table S1 are further provided. As we know, RL intensity and EAB are two important indicators for estimating a microwave absorber, thus which are performed in Fig. 6g, h from this work and recent related works about pure carbon materials and carbon composites. It is observed that MCHSP-4 and MCHSV-2 both show better MA performance (wider EAB and stronger RL) than the related works, indicating the potential application as a qualified microwave absorber [31-50]. We realized that the as-prepared MCHSs perform great MA properties, so the possible dissipated mechanism is illustrated in Fig. 6m. First, the unique multilevel porous structure including hollow void and mesopores may induce multiple scattering or reflection in interior void or adjacent MCHSs, which can prolong the transmission path of microwave and increase the possibility being attenuated by carbon composition. Second, so much mesopores and voids meaning that there is abundant interface (carbon/air) existing in MCHS. This is contributed to inducing space charge polarization. Then, the hybrid carbon compositions are effective medium dissipating microwave. Sp2 graphited carbon is in favor of electron transportation and form microcurrent among carbon network under applied alternating electric field [51, 52]. Sp3 disorder carbon acts as resistance converting electric energy to heat energy (Fig. S10). Further, the dangling groups of C-OH and -COOH can introduce dipole
polarization relaxation, boosting dielectric loss capability [53-55]. Hence, these qualities ensure MCHS with attractive dielectric and MA properties.
Fig. 6 The complex permittivity variation vs frequency and 3D plot of RL properties, (a, b) MCHS-1, (c, d) MCHSS-2, (e, f) MCHSS-3, (g, h) MCHSS-4, (i, g) MCHSS-5; (k, l) the comparison of EAB and RL with similar works; (m) the possible dissipated mechanism for MCHS. 4 Conclusions In this work, we have systematically regulated the microstructures of MCHSs, including hollow void, integrate size, shell thickness and mesopores in the shell, based on the reaction mechanism. Thus, several types of MCHSs were successfully synthesized, i.e., I, expanded hollow void and decreased thickness with enlarged
integrate size; II, expanded hollow void and decreased thickness with controlled integrate size; III, expanded hollow void and thickness with enlarged integrate size; IV controlled hollow void and thickness with extended mesopores. Among them, the hollow voids were ranged from 89.3 nm to 204.5 nm; integrate sizes were between 204.0 and 268.0 nm; shell thicknesses were in the range of 68.1-148.7 nm; mesopores were adjusted from 7.1 to 11.3 nm. Then, the potential application in MA field was performed. After evaluating the dielectric loss ability and MA performance, MCHSV-2 displayed most promising absorption capability with 6.5 (9.7-16.2) GHz of EAB and -50.6 dB of RL value when thickness is 2.75 mm. The relationship between microstructure and dielectric properties can be attributed to the variable of voids and carbon compositions. Voids benefit impedance matching; hybrid carbon compositions charge microwave attenuation; the favorable compatibility between them can lead to attractive MA properties. The systematic as-prepared MCHSs can be used not only to solve microwave interference issue, but also to deal with other problems, like drug delivery, gas storage, etc. Acknowledgments Financial support from the National Nature Science Foundation of China (No.: 51971111), the Funding for Outstanding Doctoral Dissertation in NUAA (No.: BCXJ17-07), Postgraduate Research & Practice Innovation of Jiangsu Province (No.: KYCX17_0252), and the foundation of Jiangsu Provincial Key Laboratory of Bionic Functional Materials are gratefully acknowledged.
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Credit Author Statement
Structure-switchable mesoporous carbon hollow sphere framework toward sensitive microwave response
Yan Chenga, Huanqin Zhaoa, Yue Zhaoa, Jieming Caoa, Jing Zhengb, Guangbin Jia,*
a
College of Materials Science and Technology, Nanjing University of Aeronautics
and Astronautics, Nanjing 211100, P. R. China b
Department of Chemistry and Materials Science, College of Science, Nanjing
Forestry University, Nanjing 210037, P. R. China
The authors state that this manuscript is an original work to be published on Carbon, and has not been published or submitted to other journals.
Declaration of Interest Statement
Structure-switchable mesoporous carbon hollow sphere framework toward sensitive microwave response
Yan Chenga, Huanqin Zhaoa, Yue Zhaoa, Jieming Caoa, Jing Zhengb, Guangbin Jia,*
a
College of Materials Science and Technology, Nanjing University of Aeronautics
and Astronautics, Nanjing 211100, P. R. China b
Department of Chemistry and Materials Science, College of Science, Nanjing
Forestry University, Nanjing 210037, P. R. China
There are no conflicts to declare.
S0008-6223(20)30149-4
DOI:
https://doi.org/10.1016/j.carbon.2020.02.011
Reference:
CARBON 15061
To appear in:
Carbon
Received Date: 27 December 2019 Revised Date:
3 February 2020
Accepted Date: 5 February 2020
Please cite this article as: Y. Cheng, H. Zhao, Y. Zhao, J. Cao, J. Zheng, G. Ji, Structure-switchable mesoporous carbon hollow sphere framework toward sensitive microwave response, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2020.02.011. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Graphical abstract
Structure-switchable mesoporous carbon hollow sphere framework toward sensitive microwave response
Yan Chenga, Huanqin Zhaoa, Yue Zhaoa, Jieming Caoa, Jing Zhengb, Guangbin Jia,*
a
College of Materials Science and Technology, Nanjing University of Aeronautics
and Astronautics, Nanjing 211100, P. R. China b
Department of Chemistry and Materials Science, College of Science, Nanjing
Forestry University, Nanjing 210037, P. R. China
*Corresponding Author E-mail: [email protected] (Prof. Guangbin Ji)
Abstract Mesoporous carbon hollow sphere (MCHS) is one of the most significant functional materials in various fields. Unfortunately, systematically regulating microstructure of MHCS has been rarely investigated so far. In this work, using one unique simultaneous-hydrolyzation-polymerization process, we are able to precisely control the morphology configurations of MCHS, including its shell thickness and integrate size, hollow void and mesopore in the shell. The shell thickness and integrate size are associated with the synergistic effect of hydrolyzation and polymerization reactions. The hollow void and mesopore are determined by the SiO2 template morphology. Thus, a series of MCHSs with controlled microstructure were achieved through elaborately tuning the precursor. More importantly, different types of MCHSs demonstrate significantly different dielectric and microwave absorbing properties due to the discrepancy of volume ratio between pores and carbon compositions. The typical sample could achieve a broad effective absorption bandwidth of 5.9 GHz and 6.5 GHz with a thickness of 2.35 mm and 2.65 mm, respectively, at a filling ratio of only 20 wt.%. These encouraging results significantly promote a deeper understanding of the fundamental chemistry mechanism for constructing MCHS as well as using the material as potential candidate for solving microwave interference issue. Keywords: mesoporous carbon; hollow void; porous; tunable; microwave response
1. Introduction Nowadays, modern society is interlinked by ubiquitous electromagnetic wave emitted by electronic devices [1-4]. While bringing convenience to life, however the electromagnetic wave poses a great threat to human health at the same time [5, 6]. Therefore, eliminating external electromagnetic wave interference is a critical prerequisite for ensuring electromagnetic compatibility of precision electrical equipment. Particularly, the attached microwave absorbing coating is powerful to solve this issue. In many filler candidates, carbon materials, such as graphene, carbon nanotubes, carbon sphere, have been widely investigated due to their reliable chemical stability, lightweight superiority and dominated dielectric dissipated ability [7-10]. Among them, carbon spheres derived from organic precursor captures much research interests due to their attractive advantages of low cost, high yield, and adjustable graphitization level. The ordinary carbon spheres are generally confined by limited dielectric loss capability, making it very difficult to become a qualified absorber. Under the premise of appropriate graphitized degree of carbon composition, the most efficient way to boost the dielectric ability is processing microstructure design, especially pores design [11-13]. The introduction of pores can not only improve specific areas and reduce density of carbon spheres, but also perfect the impedance matching feature and increase the possibility of microwave attenuation [14]. Following this thought, we believe that MCHS which inherits the advantages of carbon sphere and contains multilevel pore structure including interior hollow void and exterior mesopores in the
shell makes itself become a potential excellent absorber. Referring the design concept of MCHS, hollow void and mesopores are responsible for boosting impedance matching properties, and the hybridized carbon compositionsorder carbon and disorder carbon - are in charge of the microwave dissipation via conductive loss [15-17]. As such, the variations in pores or carbon compositions both have significant impact on final dielectric and microwave absorption (MA) properties. The microstructure parameters, including hollow void, integrate size, shell thickness and mesopores, positively depend on the volume ratio between pores and carbon compositions of MCHS. Thus, precisely controlling the microstructure and investigating the corresponding effect on dielectric properties are highly required for MCHS. In this work, based on the unique reaction mechanism, the microstructures of MCHS, including hollow void, integrate size, shell thickness, mesopores, are controlled by tuning the hydrolyzation of tetrapropoxysilane (TPOS) and polymerization of phenolic resin for precursors. Thus, we rationally regulated the precursor through changing reaction conditions, then prepared a series of MCHSs with controllable hollow voids (89.3-204.5 nm), mesopores (7.1-11.3 nm), thicknesses (68.1-148.7 nm) and sizes (204.0-268 nm). As-synthesized MCHSs show microstructure-determined dielectric properties and MA performances owing to the difference of volume ratios between pores and carbon compositions. It is observed that only when the impedance matching feature and attenuated ability reach balance in a
specific
microstructure,
the
MCHS
presents
excellent
MA properties.
Representatively, the sample of MCHSV-2 (hollow void: 135.1 nm, shell thickness: 60.4 nm) presents attractive MA properties with an effective absorption bandwidth (EAB) of 5.9 GHz and a reflection loss (RL) of -58.2 dB at 2.35 mm, which may be an ideal filler for solving microwave interference issue. In addition, the as-prepared MCHS can be applied in various research fields, e.g. energy storage, pollutant adsorption, biology cure, electrocatalysis, etc. 2 Experimental 2.1 Materials TPOS, tetraethyl orthosilicate (TEOS), resorcinol and ammonium hydroxide (NH3·H2O, 25%) were purchased from Aladdin; formaldehyde (37%), hydrofluoric acid (HF, 25%) and absolute ethanol were received from Nanjing Chemical Reagent Co., Ltd. All chemicals used without further purification. Deionized (DI) water used in the whole reaction process. 2.2 Synthesis of different types of MCHS The synthesized procedure and as-prepared MCHSs are displayed in Scheme 1. First, 3.46 mL of TPOS (12 mmol) was added to the mixed solution containing 70 mL of ethanol, 10 mL of DI water and 3 mL of NH3·H2O. Then, after 15 min of stirring, 0.4 g of resorcinol and 0.56 mL of formaldehyde were casted into the solution with a further 24 h of stirring. The obtained SiO2@SiO2/phenolic resin (PF) precursor were collected through centrifugal washing with DI water and ethanol for three times, respectively. When the precursor was dried in vacuum oven over one night, 2 g of precursor was put into porcelain boat for high-temperature calcination under argon
atmosphere to convert PF into carbon material. The calcined condition set as 700 ºC for 5 h with a ramping rate of 5 ºC/min. Finally, the MCHS was received after etching SiO2 core via HF solution (25 %). The as-prepared MCHS is denoted as MCHS-1. Then, we tried to systematically control various structure parameters of MCHS, including hollow void size, integrate size, carbon shell thickness, mesopores in carbon shell, through regulating experimental conditions in this work. (I) Hollow void size. The void size is depended on the size of SiO2 core, so prolonging hydrolysis time can enlarge SiO2 core. The stirring time was prolonged from 15 min to 1 h and 2 h, respectively, during the initial TPOS addition stage. The obtained MCHS were denoted as MCHS-1, MCHSV-2 and MCHSV-3, respectively. (II) Integrate size. Enlarging SiO2 core could expand hollow void size, but the integrate size of MCHS increased accordingly, so we employed part of TEOS substituting TPOS as silicon source to stabilize integrate size. If add part of TEOS (total silica source remains unchanged), the SiO2 core would grow larger than pure TPOS during the same hydrolyzation time due to the faster hydrolyzation rate of TEOS. Thus, the hollow void expands. Correspondingly, the carbon shell thickness is reduced owing to the decrease amount of TPOS. In this case, the integrate size of MCHS can be maintained. The added molar ratio between TPOS and TEOS (total 12 mmol) was regulated as 12:0, 9:3, 6:6, 3:9 and 0:12. The obtained MCHS were denoted as MCHS-1, MCHSS-2, MCHSS-3, MHCSS-4, MCHSS-5, respectively. (III) Carbon shell thickness. The shell thickness is determined by phenolic resin polymerization, so we considered whether the carbon shell thickness can be adjusted while remaining the
hollow void size unchanged. The added amount of resorcinol and formaldehyde is 0.5 times, 1.5 times and 2.0 times than that of MCHS-1. The synthesized MCHSs are marked as MCHST-0, MCHST-2, MCHST-3, respectively. (IV) Mesopores in carbon shell. The mesopores in carbon shell are depended by SiO2 nanoparticle size in the shell, so which can be tuned via controlling the volume ratio of water to ethanol due to the faster hydrolysis rate of TPOS in water than in ethanol. The volume ratio of ethanol to DI water (total 80 mL) is controlled as 70 :10, 65: 15, 60: 20, 55: 25, 50: 30. The obtained MCHSs are denoted as MCHS-1, MCHSP-2, MCHSP-3, MCHSP-4, MCHSP-5.
Scheme 1. The synthesized procedure of different types of MCHSs; Group I: expanded hollow void and decreased thickness with enlarged integrate size; Group II: expanded hollow void and decreased thickness with controlled integrate size; Group III, expanded hollow void and thickness with enlarged integrate size; Group IV, controlled hollow void and thickness with extended mesopores. 2.3 Characterizations The X-ray diffraction (XRD) pattern was defined by a Bruker D8 ADVANCE X-ray diffractometer using Cu Kα radiation (λ=1.54178 Å) with 40 kV scanning voltage and 40 mA scanning current. The morphology and porous structure were detected by a Hitachi S4800 type scanning electron microscope (SEM) and a Tecnai G2 F30 S-TWIN transmission electron microscope (TEM). The specific surface areas and pore information were obtained on a Brunauer-Emmet-Teller (BET) analyzer equipped with micrometric ASAP 2020 system. The carbon state was identified by a Renishaw Invia Raman spectrometer. The chemical state of carbon atom was tested by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe). The electromagnetic parameters of MCHS in the frequency range of 2-18 GHz were measured via an Agilent PNA N5224A vector network analyzer based on coaxial-line method. The measured toroidal-shaped ring (Φout=7.00 mm, Φin=3.04 mm) was pressed after uniformly mixing MCHS with paraffin wax (mass ratio = 80%: 20%). Then, the electromagnetic parameters between 2-18 GHz are calculated based on the software installed in the vector network analyzer (Agilent PNA N5224A, coaxial-line method). The RL intensity vs frequency at different thickness is simulated according to following equations [18-21]:
Zin = Z 0( µr/εr)1/2 tanh[ j (2πfd / c)( µrεr)1/2 ]
(1)
RL(dB) = 20 log | ( Zin − Z 0) ( Zin + Z 0) |
(2)
where εr (εr=ε′-jε″) and µr (µr=µ′-jµ″) mean complex permittivity and permeability, respectively. f represents the frequency of electromagnetic wave and d is the thickness of microwave absorber. 3 Results and Discussions The morphology and microstructure of the as-prepared different types of MCHS are observed through SEM and TEM identifications. Fig. 1 presents different hollow void-enlarged MCHSs via extending hydrolyzation time of TPOS. As shown in Fig. 1a, the initial MCHS-1 displays clearly typical hollow void feature and strip-like pores in the carbon shell, which obtains hollow void size of 89.3 nm and shell thickness of 68.1 nm. SEM observations demonstrate a good dispersive state of MCHSs and integrate size concentration at 204 nm. While prolonging the hydrolyzation time of TPOS to 1h, the hollow void size obviously increases to 135.1 nm with thickness reducing to 60.4 nm for MCHSV-2, which size is mainly centered at 255.5 nm (Fig. 1b). This is due to the growth of SiO2 core at the extended hydrolyzation time of TPOS, thus the SiO2 nanocores in the shell is declined, giving rise to the decrease of shell thickness. With further prolonging hydrolyzation time to 2h, the hollow void size expands to 196.9 nm and the thickness decreases to 44.6 nm (Fig. 1c). The integrate size of MCHSV-3 improves to 268.0 nm through SEM collection. In this hollow void regulation, the extension of hydrolyzation time of TPOS can not only expand the hollow void size, but also decrease the thickness. The
integrate size is greatly increased as well. For application, the as-synthesized MCHSs are employed for solving microwave issue due to the unique porous structure and hybridized carbon composition. As a typical dielectric material, the magnetic loss of MCHS can be neglected, thus the complex permittivity of ε' and ε" are presented, which represent dielectric storage and loss ability, respectively [22, 23]. The corresponding dielectric parameters and MA properties are presented in Fig. S1 (Supporting Information). It is observed that MCHSV-2 exhibits stronger dielectric loss capability than MCHSV-3, which is ascribed to the expanded hollow void based on the Maxwell-Garnett (MG) theory [23]. For MCHSV-2, the superior dissipated ability enables it to achieve two broad EABs (12.1 to 18 GHz at a thickness of 2.35 mm and 9.7 to 16.2 GHz at a thickness of 2.75 mm).
Fig. 1 The morphology and microstructure observations from SEM and TEM images (a) MCHS-1, (b) MCHSV-2, (c) MCHSV-3; insets are size distribution from SEM images; the scale bars are 200 nm, 100 nm, 50 nm corresponding to TEM images of 1,
2, 3, respectively, 1 µm and 500 nm corresponding to SEM images of 4 and 5, respectively. Then, we explored that if the integrate size of MCHS can be maintained when expanding the hollow void. Since TEOS has one less methyl group than TPOS, it holds a relatively faster hydrolyzation rate in alkaline solution system, so which can produce larger SiO2 core during the same time, leading to expanded hollow void. Hence, we employed TEOS to substitute part of TPOS as silicon source. Meanwhile, fixed total silicon source can retain the integrate size of MCHS. As seen from Fig. 2a, the hollow void size expands to 118.1 nm with thickness reducing to 45.5 nm when adding 3 mmol of TEOS. The total carbon size stabilizes at 212 nm. As increasing TEOS amount to 6 mmol and 9 mmol, observed from Fig. 2b, c, the hollow void sequentially expands to 142.0 nm and 189.1 nm, along with the thickness of 35.9 nm and 24.4 nm. Total carbon size is 214.0 nm and 216.0 nm, respectively. When TPOS is completely replaced by TEOS, the hollow void further enlarges to 204.5 nm with a very thin thickness of 16.9 nm (Fig. 2d). Total carbon sphere size centers at about 217.0 nm. These modified MCHSs have enhanced hollow void space and reduced thickness than MCHS-1 (hollow void of 89.3 nm and thickness of 68.1 nm). As a result, it is revealed that the hollow void can be boosted without altering integrate size of MCHS via part of TEOS substituting TPOS. Correspondingly, the thickness is consecutively decreased because of the lessened TPOS amount. It is noted that when TEOS is 9 mmol, the MCHSs are broken to some extent from SEM observation. For MCHSS-5, more MCHSs are broken than MCHSS-4, which can be resulted from the greatly reduced carbon shell thickness.
Fig. 2 The morphology and microstructure observations from SEM and TEM images, (a) MCHSS-2, (b) MCHSS-3, (c) MCHSS-4, (d) MCHSS-5. The scale bars are 200 nm, 100 nm, 50 nm corresponding to TEM images of 1, 2, 3, respectively, 1 µm and 500 nm corresponding to SEM images of 4 and 5, respectively. For clarifying the carbon compositions of MCHSs, corresponding characterizations are performed in Fig. 3. As displayed in Fig. 3a, all MCHSs present a broad peak at about 22.7°, which can be attributed to the (002) plane of graphitic carbon, indicating the MCHSs are partly graphitized. Raman spectra is skilled for demonstrating carbon phase state. As shown in Fig. 3b, there are two prominent peaks at about 1330 cm-1 and 1590 cm-1, respectively, corresponding to D- and G-band of carbon materials [25, 26]. Generally, D-band is derived from sp3 hybridization of carbon atoms with meaning of disorder carbon; G-band is resulted from the stretching vibrations of sp2 bond with existence of graphitic carbon. The intensity ratio of D- and G-band
indicates the graphitized degree of carbon materials. One can find that all MCHSs show close values of 1.0, suggesting the approximate and relatively high graphitization at the same calcination temperature. This is advantageous for the conductive loss of MCHS. Then, we further probe the chemical state of carbon compositions though XPS detection. It is observed that the MCHS presents both C and O elements (Fig. 3c). The O element is derived from phenolic resin precursor, which exists as valence bond of C-O, O=C-C, O=C-O (Fig. 3d, e) [27]. These polar functional groups may be very helpful for introducing dielectric polarization loss [28]. BET measurements were performed to analyze porous structure and surface areas. As presented in Fig. 3f-k, all MCHSs exhibit an obvious hysteresis loop when P/P0 > 0.6, which can be assigned to representative IV, H3-type adsorption isotherm according to IUPAC standard, indicating slit-like mesopores in the carbon shell. The comparable sorption isotherms explain that the MCHSs have similar pore structure. They all display high specific areas and pore volumes (Fig. S2), among which MCHSS-2 owns the largest surface areas of 1375.8 m2/g and pore volume of 2.3 cm3/g. Mesopores in these MCHSs are all close to 7 nm from pore size distribution curve in Fig. S2, illustrating TEOS replacement doesn’t affect pore size. Abundant pore structure and high surface areas of MCHS are conductive to boost impedance matching feature and promote microwave dissipation [29, 30].
Fig. 3 The MCHSs from group II, (a) The XRD patterns, (b) Raman spectra, (c-e) survey spectra, C1s and O1s high-resolution profile of MCHSS-3 of XPS detection, (f-j) adsorption isotherms of BET measurements, (k) surface-area comparisons. Since we have well controlled the hollow void and integrate size of MCHS, in order to investigate whether the carbon shell thickness can be regulated without affecting hollow void, we tried different amounts of phenolic resin additions. As exhibited in Fig. 4a, the MCHS is not able to remain spherical morphology and collapses into bowl-like configuration from TEM and SEM observation. The reason is that a thinner carbon shell thickness is produced by using the limited phenolic resin resource. Hence, it is readily understood that the thin carbon skeleton cannot support the regular spherical microstructure in the scale, thus collapsing into bowl-like structure. When the addition amount of phenolic resin is 1.5 times larger than MCHS-1, not only the shell thickness (129.5 nm) is increased, but also hollow void
size (140.9 nm) and the integrate size are boosted (Fig. 4b). With further increasing addition amount of phenolic resin (2.0 times than that of MCHS-1), the shell thickness improves to 148.7 nm with hollow void expanding to 236.3 nm (Fig. 4c). In addition, we notice that some solid spheres appear in the sample, which is resulted from the self-polymerization of excess phenolic resin without silica involving in the reaction. Therefore, tuning the phenolic resin amount cannot individually adjust shell thickness, which also clearly influences hollow void and integrate size, even morphology of MCHS. Fig. S3 displays complex permittivity and MA performance of shell thickness-regulated MCHSs. In this part, it is shown that the morphology has an obvious effect on its dielectric property. The unique bowl-like MCHS of MCHST-0 performs moderate complex permittivity, thus achieving an excellent absorption between 7.9-12.9 GHz with a strong RL intensity of -54.7 dB at a coating thickness of 3.7 mm. For MCHST-2, the clearly increased thickness results in enhanced dielectric response properties (ε', 9.4-4.6; ε", 9.0-2.7). Although the strong attenuation is confirmed, the impaired impedances matching behavior limits the effective absorption bandwidth. Further, the small solid carbon spheres appeared in MCHST-3 give rise to evidently decreased dielectric values, leading to the reduced MA performance.
Fig. 4 The morphology and microstructure observations from SEM and TEM images (a) MCHST-0, (b) MCHST-2, (c) MCHST-3. The scale bars are 200 nm, 100 nm, 50 nm corresponding to TEM images of 1, 2, 3, 1 µm and 500 nm corresponding to SEM images of 4 and 5, respectively. Our previous works have investigated the regulations of hollow void, integrate size and shell thickness of MCHS, but the study of mesopores in the shell is insufficient. Since the mesopores size is determined by SiO2 nanocores in precursor shell, tuning mesopores requires altering the size of SiO2 nanocores. We realized that the TPOS has a faster hydrolysis speed in DI water than in absolute ethanol, thus properly increasing volume ratio of water can enlarge SiO2 nanocores in the shell. As observed in Fig. 5a, the MCHS keeps good microstructure and morphology with hollow void of 135.5 nm and shell thickness of 52.3 nm when adding a little more water (15 mL). The hollow void and shell thickness are without obvious changes compared with MCHS-1, but the mesopores in the shell look more noticeable. Further increasing volume ratio of water (20 mL), the MCHS turns more transparent than previous one, indicating the
extended mesopores in the shell, of which hollow void and thickness maintain as 135.9 nm and 54.1 nm, respectively (Fig. 5b). Then, the mesopores in MCHSP-4 (Fig. 5c) are further enlarged while continue increasing water volume to 25 mL. In this sample, the pores can be clearly observed from the surface of MCHS, but the uniformity of MCHSs is destroyed to some extent. Accordingly, the microstructure of MCHS has been completely broken with excess water volume of 30 mL (Fig. 5d). The rapid hydrolysis rate of TPOS leads SiO2 production to be completed before phenolic resin polymerization, thus the carbon shell cannot grow on SiO2 cores, resulting in the collapse of MCHS. The microstructure observation gives a visible process of pores evolution and good preservation of integrate size (ca. 230 nm). It is acknowledged that the mesopores derived from the SiO2 nanocores in the shell, thus we get the hard template via calcining the precursor in air. Fig. S4 exhibits a visualized illustration for the hard template transformation, which reflects well the pores development. Furthermore, the pore size distributions from BET measurement (Fig. S5) reveal the gradually increase of 7.1 nm, 10.1 nm and 11.3 nm, corresponding to samples of MCHSP-2, MCHSP-3 and MCHSP-4. These results prove that the mesopores in the carbon shell can be well regulated via tuning volume ratio of absolute ethanol to DI water. The dielectric and MA properties of mesopores-regulated MCHSs are exhibited in Fig. S6. We can see that the samples show obvious mesopores-depended dielectric response properties. From MCHS-1 to MCHSP-3, the dielectric values present an increasing trend. Then, with the mesopores further growing and the microstructure
collapsing, the dielectric values gradually decreased. This illustrates that proper mesopore design can boost dielectric loss capability, but too much mesopores will reduce dielectric values. In this case, MCHSP-4 (ε', 9.2-5.7; ε", 6.0-2.2) presents feasible dielectric values satisfying good microwave attenuated ability and impedance matching feature, thus achieving a wide EAB of 5.0 GHz (9.7-14.7 GHz) and strong RL intensity of -52.3 dB at 2.65 mm.
Fig. 5 The morphology and microstructure observations from SEM and TEM images (a) MCHSP-2, (b) MCHSP-3, (c) MCHSP-4, (d) MCHSP-5. The scale bars are 200 nm, 100 nm, 50 nm corresponding to TEM images of 1, 2, 3, respectively, 1 µm and 500 nm corresponding to SEM images of 4 and 5, respectively.
Table 1. The microstructure parameters for different types of MCHSs. Samples
Void diameter/nm
Shell thickness/nm
Average pore width/nm
Average microsphere size/nm
MCHS-1
89.3
68.1
7.2
204.0
MCHSV-2
135.1
60.4
~
255.5
MCHSV-3
196.9
44.6
~
268.0
MCHSS-2
118.1
45.5
6.4
212.0
MCHSS-3
142.0
35.9
6.3
214.0
MCHSS-4
189.1
24.4
7.0
216.0
MCHSS-5
204.5
16.9
6.7
217.0
MCHST-0
~
~
~
~
MCHST-2
140.9
129.5
~
~
MCHST-3
236.3
148.7
~
~
MCHSP-2
135.5
52.3
7.1
225.5
MCHSP-3
135.9
54.1
10.1
228.0
MCHSP-4
126.5
60.2
11.3
243.5
MCHSP-5
~
~
~
~
We have tried to systematically adjust the microstructure of MCHSs (Table 1) and analyze the microstructure evolution effect (hollow voids, shell thickness and mesopores) on dielectric properties and MA performance. To this end, we take size-controlled MCHSs as particular illustration. The complex permittivity and MA performance of MCHS-1 to MCHSS-5 are displayed in Fig. 6a-j. As seen in Fig. 6a, b, MCHS-1 exhibits very low ε' and ε", giving rise to poor MA properties due to insufficient dielectric loss. For MCHS-2, the complex permittivity is obviously enhanced. ε' decreases from 6.0 to 3.4 with ε" decreasing from 3.7 to 1.2. owing to
frequency dispersion behavior, which presents acceptable MA performance (Fig. 6c, d). With hollow void expanding, ε" (5.7-1.6) is more approaching to ε' (6.8-3.7) (Fig. 6e). The boosted dielectric loss (Fig. S7) enables attractive performance for MCHS, which performs a wide EAB of 4.9 GHz (7.5-12.4 GHz) with RL of -52.9 dB at thickness of 4.0 mm, covering whole X-band (Fig. 6f). When hollow void dominates MCHS (MHCSS-4 and MCHSS-5), ε" even exceeds ε', meaning inferior impedance matching feature (Fig. S8) and most of microwave reflected from the absorber surface. This cause these two samples with very weak MA performance (Fig. 6g-J). The gradually climbing dielectric parameters should be significantly associated with microstructure evolution. The expanded hollow void would clearly reduce the density of MCHS, thus increasing filled molar quantity at the same weight, which results in monotonically increasing of complex permittivity. When the filling ratio is 20 wt%, MCHSS-3 presents the best absorption capability due to the compatible of impedance matching and attenuated capacity. Through
comprehensively analyzing complex
permittivity variation with
microstructure evolution of all samples, it is found that this attribution can be ascribed to the difference of pores and carbon compositions. Hollow voids and mesopores can promote impedance matching. Carbon compositions are responsible for microwave attenuation. Due to microstructure difference, the molar quantity is different at the same filling ratio. As a result, if pores dominate MCHS, the sample exhibits a lower complex permittivity, such as MCHSV-3. If carbon compositions dominate MCHS, the sample exhibits a higher complex permittivity, such as MCHST-2. Therefore, the
regulation of microstructure of MHCS can effectively tune their dielectric properties and MA performance. Only when the impedance matching and attenuated capacity are compatible, the sample displays excellent MA performance. For positively understanding the MA performance superiority, their 2D graph (Fig. S9) and Table S1 are further provided. As we know, RL intensity and EAB are two important indicators for estimating a microwave absorber, thus which are performed in Fig. 6g, h from this work and recent related works about pure carbon materials and carbon composites. It is observed that MCHSP-4 and MCHSV-2 both show better MA performance (wider EAB and stronger RL) than the related works, indicating the potential application as a qualified microwave absorber [31-50]. We realized that the as-prepared MCHSs perform great MA properties, so the possible dissipated mechanism is illustrated in Fig. 6m. First, the unique multilevel porous structure including hollow void and mesopores may induce multiple scattering or reflection in interior void or adjacent MCHSs, which can prolong the transmission path of microwave and increase the possibility being attenuated by carbon composition. Second, so much mesopores and voids meaning that there is abundant interface (carbon/air) existing in MCHS. This is contributed to inducing space charge polarization. Then, the hybrid carbon compositions are effective medium dissipating microwave. Sp2 graphited carbon is in favor of electron transportation and form microcurrent among carbon network under applied alternating electric field [51, 52]. Sp3 disorder carbon acts as resistance converting electric energy to heat energy (Fig. S10). Further, the dangling groups of C-OH and -COOH can introduce dipole
polarization relaxation, boosting dielectric loss capability [53-55]. Hence, these qualities ensure MCHS with attractive dielectric and MA properties.
Fig. 6 The complex permittivity variation vs frequency and 3D plot of RL properties, (a, b) MCHS-1, (c, d) MCHSS-2, (e, f) MCHSS-3, (g, h) MCHSS-4, (i, g) MCHSS-5; (k, l) the comparison of EAB and RL with similar works; (m) the possible dissipated mechanism for MCHS. 4 Conclusions In this work, we have systematically regulated the microstructures of MCHSs, including hollow void, integrate size, shell thickness and mesopores in the shell, based on the reaction mechanism. Thus, several types of MCHSs were successfully synthesized, i.e., I, expanded hollow void and decreased thickness with enlarged
integrate size; II, expanded hollow void and decreased thickness with controlled integrate size; III, expanded hollow void and thickness with enlarged integrate size; IV controlled hollow void and thickness with extended mesopores. Among them, the hollow voids were ranged from 89.3 nm to 204.5 nm; integrate sizes were between 204.0 and 268.0 nm; shell thicknesses were in the range of 68.1-148.7 nm; mesopores were adjusted from 7.1 to 11.3 nm. Then, the potential application in MA field was performed. After evaluating the dielectric loss ability and MA performance, MCHSV-2 displayed most promising absorption capability with 6.5 (9.7-16.2) GHz of EAB and -50.6 dB of RL value when thickness is 2.75 mm. The relationship between microstructure and dielectric properties can be attributed to the variable of voids and carbon compositions. Voids benefit impedance matching; hybrid carbon compositions charge microwave attenuation; the favorable compatibility between them can lead to attractive MA properties. The systematic as-prepared MCHSs can be used not only to solve microwave interference issue, but also to deal with other problems, like drug delivery, gas storage, etc. Acknowledgments Financial support from the National Nature Science Foundation of China (No.: 51971111), the Funding for Outstanding Doctoral Dissertation in NUAA (No.: BCXJ17-07), Postgraduate Research & Practice Innovation of Jiangsu Province (No.: KYCX17_0252), and the foundation of Jiangsu Provincial Key Laboratory of Bionic Functional Materials are gratefully acknowledged.
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Credit Author Statement
Structure-switchable mesoporous carbon hollow sphere framework toward sensitive microwave response
Yan Chenga, Huanqin Zhaoa, Yue Zhaoa, Jieming Caoa, Jing Zhengb, Guangbin Jia,*
a
College of Materials Science and Technology, Nanjing University of Aeronautics
and Astronautics, Nanjing 211100, P. R. China b
Department of Chemistry and Materials Science, College of Science, Nanjing
Forestry University, Nanjing 210037, P. R. China
The authors state that this manuscript is an original work to be published on Carbon, and has not been published or submitted to other journals.
Declaration of Interest Statement
Structure-switchable mesoporous carbon hollow sphere framework toward sensitive microwave response
Yan Chenga, Huanqin Zhaoa, Yue Zhaoa, Jieming Caoa, Jing Zhengb, Guangbin Jia,*
a
College of Materials Science and Technology, Nanjing University of Aeronautics
and Astronautics, Nanjing 211100, P. R. China b
Department of Chemistry and Materials Science, College of Science, Nanjing
Forestry University, Nanjing 210037, P. R. China
There are no conflicts to declare.