CN113772742B - Core-shell heterogeneous graphene aerogel microsphere, preparation method and application - Google Patents
Core-shell heterogeneous graphene aerogel microsphere, preparation method and application Download PDFInfo
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Abstract
The invention discloses a core-shell heterogeneous graphene aerogel microsphere, a preparation method and application thereof, and the preparation method comprises the following steps: step 1: FeCl is added3·6H2Adding O into the aqueous dispersion of the graphene oxide, and uniformly dispersing to obtain an external spinning solution; step 2: adding chitosan into a solvent, and uniformly dispersing to obtain an inner layer precursor spinning solution; and 3, step 3: adopting liquid nitrogen as a collector, and obtaining aerogel microspheres through coaxial electrostatic spinning-freezing casting; and 4, step 4: thermally reducing the aerogel microspheres, and cooling to obtain the required core-shell heterogeneous graphene aerogel microspheres; the core-shell structure of the core-shell heterogeneous graphene aerogel microsphere obtained by the invention can enable electromagnetic waves to enter and attenuate in sequence, and enhance the impedance matching performance and the reflection and scattering of the electromagnetic waves; the biomass derived carbon is introduced into the core, so that a new heterogeneous interface and a multi-reflection channel are provided, and the conductivity of the microsphere is reduced; the resonant cavity in the aerogel microsphere is increased, so that the impedance matching performance of the aerogel microsphere is improved.
Description
Technical Field
The invention relates to the technical field of graphene-based aerogel, in particular to a core-shell heterogeneous graphene aerogel microsphere, a preparation method and application.
Background
The graphene-based aerogel is a three-dimensional porous material with an interconnected network structure, which is formed by assembling two-dimensional graphene-based materials. Because of its excellent characteristics of high specific surface area, high elasticity, low density, porous structure and the like. Graphene-based aerogels are considered to be a new class of materials with great application potential and research value. Can be applied to the fields of water treatment adsorbing materials, electromagnetic wave absorbing materials, energy storage materials, electro-catalytic materials and the like. In the foreseeable future, research on the preparation method and application development of graphene-based aerogel will be one of the research hotspots.
In recent years, the research on the preparation and performance of graphene-based aerogel is becoming mature, but there is still a challenge in how to economically and quantitatively prepare and adjust the shape and structure of graphene-based aerogel to meet different application research and practical requirements. At present, the main methods for preparing graphene-based aerogel include a template method, a deposition method, a carbonization method, 3D printing and the like. However, the preparation method has strong dependence on equipment, and the obtained aerogel is often difficult to match with a cavity with a complex structure. In addition, the disadvantages of high cost, low yield, difficult operation and the like make the scale production impossible. And most of the traditional graphene-based aerogel materials are block materials, and the block aerogel materials have some major defects in production and preparation. For example, the preparation of the graphene aerogel with a larger size is complex in process, the aerogel material with a special shape is difficult to prepare, and the block aerogel is difficult to compound and blend with other materials.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a core-shell heterogeneous graphene aerogel microsphere with excellent effective absorption bandwidth and adjustable high-performance electromagnetic wave absorption, a preparation method and application.
The technical scheme adopted by the invention is as follows: a preparation method of core-shell heterogeneous graphene aerogel microspheres comprises the following steps:
step 1: FeCl is added3·6H2Adding O into the aqueous dispersion of the graphene oxide, and uniformly dispersing to obtain an external spinning solution;
step 2: adding chitosan into a solvent, and uniformly dispersing to obtain an inner layer precursor spinning solution;
and step 3: adopting liquid nitrogen as a collector, and obtaining aerogel microspheres through coaxial electrostatic spinning-freezing casting;
and 4, step 4: and thermally reducing the aerogel microspheres, and cooling to obtain the required core-shell heterogeneous graphene aerogel microspheres.
Further, the stepsGraphene oxide and FeCl in step 13·6H2The mass ratio of O is 1: 5-5: 1.
Further, the solvent in the step 2 is acetic acid, the concentration of the acetic acid is 2% v/v, the mass ratio of the chitosan to the acetic acid is 15:1, and the mass ratio of the chitosan to the graphene oxide is 3: 1-8: 1. .
Further, in the coaxial electrostatic spinning process in the step 3, the speed of the external spinning solution is 0.011 mL/min-0.019 mL/min; the speed of the precursor spinning solution of the inner layer is 0.008 mL/min.
Further, the coaxial electrostatic spinning in the step 3 is carried out under the voltage of 5kV, and the distance from the needle tip to the collector is 10 cm.
Further, after the step 3 of freeze casting is finished, freeze drying is carried out for 48 hours, and then the step 4 is carried out.
Further, the thermal reduction process in step 4 is as follows: thermally reducing at 400 ℃ for 2h at the heating rate of 5 ℃/min under the argon atmosphere, and cooling to room temperature.
Further, in the graphene oxide aqueous dispersion in the step 1, the concentration of graphene oxide is 4 mg/mL; and carrying out ultrasonic treatment for 20min for dispersion.
A shell layer of the core-shell heterogeneous graphene aerogel microsphere is provided with a regularly distributed macroporous structure and a 3D network structure, and the aperture range is 10-30 mu m; the inner core is in an irregular three-dimensional porous network structure, the pore diameter is smaller than that of the outer layer, and the pore diameter range is 10 mu m; the shell and the core have an integral interface therebetween.
The application of the core-shell heterogeneous graphene aerogel microspheres is characterized in that the core-shell heterogeneous graphene is used for a microwave absorbent.
The invention has the beneficial effects that:
(1) the core-shell structure of the core-shell heterogeneous graphene aerogel microsphere obtained by the invention can enable electromagnetic waves to enter and attenuate in sequence, and enhance the impedance matching performance and the reflection and scattering of the electromagnetic waves;
(2) according to the invention, the biomass-derived carbon is introduced into the core, so that a new heterogeneous interface and a multi-reflection channel are provided, and the conductivity of the microsphere is reduced; resonant cavities in the aerogel microspheres are added, so that the impedance matching performance of the aerogel microspheres is improved;
(3) the minimum reflection loss value of the core-shell heterogeneous graphene aerogel microspheres obtained by the invention reaches-61 dB when the thickness is 2.5mm, and the maximum effective absorption bandwidth is 6.88GHz when the thickness is 2.6 mm.
Drawings
Fig. 1 is a schematic structural diagram of an aerogel microsphere obtained in example 3 of the present invention, where a is an optical microscope image, b is a cross-sectional SEM image, c is an SEM image of a partially enlarged cross-section, D is an SEM image of outer layer 3D graphene, e is an SEM image of inner core biomass-derived carbon, and f is an SEM image of an interface.
FIG. 2 is TEM and EDS images of the aerogel microspheres obtained in example 3, wherein a is an SEM image, b 1-b 3 are corresponding element mappings, and c-f are TEM images.
Fig. 3 is a schematic structural diagram of the aerogel microspheres obtained in embodiments 1 to 5, where a is an SEM image of the aerogel microspheres obtained in embodiment 1, b is an SEM image of the aerogel microspheres obtained in embodiment 2, c is an SEM image of the aerogel microspheres obtained in embodiment 3, d is an SEM image of the aerogel microspheres obtained in embodiment 4, e is an SEM image of the aerogel microspheres obtained in embodiment 5, and f is a shell thickness and error statistics of the aerogel microspheres obtained in different embodiments.
Fig. 4 is a schematic raman spectrum of the aerogel microspheres obtained in example 3, where a is a raman spectrum of the aerogel microspheres in the outer shell layer, the inner core layer, and the interface region, b is an XPS total spectrum of the aerogel microspheres, c is a 1s peak, d is an N peak, e is an Fe 2p peak, and f is a stress-strain curve of the aerogel microspheres.
FIG. 5 shows the electromagnetic parameters of the aerogel microspheres obtained in the examples, wherein a is RGO/Fe, example 33O4The real part and the imaginary part (b) of the dielectric constant of the chitosan-derived carbon, c is the real part value, d is the imaginary part value, e is the dielectric loss tangent tan delta epsilon of the aerogel microspheres obtained in the embodiments 1 to 5, and f is the Cole-Cole curve and the fitted average polarization relaxation epsilon of the aerogel microspheres obtained in the embodiment 3c"and conduction loss εp"plot against frequency.
Fig. 6 shows electromagnetic loss values of the aerogel microspheres obtained in the example as an absorbent, where a is the RL value of the aerogel microspheres obtained in examples 1 to 5 as an absorbent when the thickness is 2.5mm, b is the RL value of the aerogel microspheres obtained in example 1 as an absorbent under different thickness conditions, c is the RL value of the aerogel microspheres obtained in example 2 as an absorbent under different thickness conditions, d is the RL value of the aerogel microspheres obtained in example 3 as an absorbent under different thickness conditions, e is the RL value of the aerogel microspheres obtained in example 4 as an absorbent under different thickness conditions, and f is the RL value of the aerogel microspheres obtained in example 5 as an absorbent under different thickness conditions.
FIG. 7 is a magnetic permeability parameter of the aerogel microspheres obtained in the examples, wherein a is a tan δ ε value, b is a real magnetic permeability part, C is a corresponding imaginary part, d is a tan μ value, e is a tan μ value of the microspheres of comparative example RGO/Fe3O4, and f is a vortex coefficient C of the aerogel microspheres obtained in the examples0The value is obtained.
FIG. 8 is a Cole-Cole plot and fitted mean ε c' and ε p "versus frequency for aerogel microspheres obtained in the examples, a for example 1, b for example 2, c for example 4, d for example 5, and e for comparative example RGO/Fe3O4。
FIG. 9 shows the damping constants (. alpha.) of the aerogel microspheres obtained in examples 1 to 5.
Detailed Description
The invention is further described with reference to the following figures and specific embodiments.
A preparation method of core-shell heterogeneous graphene aerogel microspheres comprises the following steps:
step 1: FeCl is added3·6H2Adding O into the aqueous dispersion of graphene oxide, and performing ultrasonic treatment for 20min to obtain a uniform external spinning solution; the concentration of the aqueous dispersion of graphene oxide GO was 4mg/mL, 10 mL. Wherein GO and FeCl3·6H2The mass ratio of O is 1: 5-5: 1.
And 2, step: a certain amount of chitosan powder (Acmec, degree of deacetylation: 85%) was added to an acetic acid solvent (15mL, 2% v/v) to obtain an inner layer precursor spinning solution. The mass ratio of the chitosan to the acetic acid is 15:1, and the mass ratio of the chitosan to the graphene oxide is 3: 1-8: 1.
And step 3: adopting liquid nitrogen (-196 ℃) as a collector, and obtaining aerogel microspheres through coaxial electrostatic spinning-freezing casting; coaxial electrospinning was carried out at 5kV with a tip to collector distance of about 10 cm. The speed of the external spinning solution is 0.011 mL/min-0.019 mL/min, and the speed of the internal precursor is set to 0.008 mL/min.
And 4, step 4: further freeze-drying for 48h, thermally reducing the aerogel microspheres, and cooling to obtain the required core-shell heterogeneous graphene aerogel microspheres Carbon @ RGO/Fe3O4. Thermal reduction was performed at 400 ℃ for 2h at a temperature rise rate of 5 ℃/min under an argon atmosphere, and cooled to room temperature.
Example 1
A preparation method of core-shell heterogeneous graphene aerogel microspheres comprises the following steps:
step 1: FeCl is added3·6H2Adding O into the aqueous dispersion of the graphene oxide, and carrying out ultrasonic treatment for 20min to obtain a uniform external spinning solution; the concentration of the aqueous dispersion of graphene oxide GO was 4mg/mL, 10 mL. Wherein GO and FeCl3·6H2The mass ratio of O is 2: 1.
Step 2: a certain amount of chitosan powder (Acmec, degree of deacetylation: 85%) was added to an acetic acid solvent (15mL, 2% v/v) to obtain an inner layer precursor spinning solution. Wherein the mass ratio of the chitosan to the acetic acid is 15:1, and the mass ratio of the chitosan to the graphene oxide is 5: 1.
And step 3: adopting liquid nitrogen (-196 ℃) as a collector, and obtaining aerogel microspheres through coaxial electrostatic spinning-freezing casting; coaxial electrospinning was carried out at 5kV with a tip to collector distance of about 10 cm. The external spinning solution rate was 0.011mL/min and the internal precursor rate was set to 0.008 mL/min.
And 4, step 4: further freeze-drying for 48h, thermally reducing the aerogel microspheres, and cooling to obtain the required core-shell heterogeneous graphene aerogel microspheres Carbon @ RGO/Fe3O41, curve a in the figure. The thermal reduction was carried out at 5 ℃ under an argon atmosphereThe temperature rise rate of/min is thermally reduced for 2h at 400 ℃ and cooled to room temperature.
Example 2
A preparation method of core-shell heterogeneous graphene aerogel microspheres comprises the following steps:
step 1: FeCl is added3·6H2Adding O into the aqueous dispersion of the graphene oxide, and carrying out ultrasonic treatment for 20min to obtain a uniform external spinning solution; the concentration of the aqueous dispersion of graphene oxide GO was 4mg/mL, 10 mL. Wherein GO and FeCl3·6H2The mass ratio of O is 2: 1.
Step 2: a certain amount of chitosan powder (Acmec, degree of deacetylation: 85%) was added to an acetic acid solvent (15mL, 2% v/v) to obtain an inner layer precursor spinning solution. Wherein the mass ratio of chitosan to acetic acid is 15:1, and the mass ratio of chitosan to graphene oxide is 5: 1.
And step 3: adopting liquid nitrogen (-196 ℃) as a collector, and obtaining aerogel microspheres through coaxial electrostatic spinning-freezing casting; coaxial electrospinning was carried out at 5kV with a tip to collector distance of about 10 cm. The external spinning solution rate was 0.013mL/min and the internal precursor rate was set to 0.008 mL/min.
And 4, step 4: further freeze-drying for 48h, thermally reducing the aerogel microspheres, and cooling to obtain the required core-shell heterogeneous graphene aerogel microspheres Carbon @ RGO/Fe3O4-2, curve B in the figure. Thermal reduction was performed at 400 ℃ for 2h at a temperature rise rate of 5 ℃/min under an argon atmosphere, and cooled to room temperature.
Example 3
A preparation method of core-shell heterogeneous graphene aerogel microspheres comprises the following steps:
step 1: FeCl3·6H2Adding O into the aqueous dispersion of the graphene oxide, and carrying out ultrasonic treatment for 20min to obtain a uniform external spinning solution; the concentration of the aqueous dispersion of graphene oxide GO is 4mg/mL, 10 mL. Wherein GO and FeCl3·6H2The mass ratio of O is 2: 1.
And 2, step: a certain amount of chitosan powder (Acmec, degree of deacetylation: 85%) was added to an acetic acid solvent (15mL, 2% v/v) to obtain an inner layer precursor spinning solution. Wherein the mass ratio of the chitosan to the acetic acid is 15:1, and the mass ratio of the chitosan to the graphene oxide is 5: 1.
And step 3: adopting liquid nitrogen (-196 ℃) as a collector, and obtaining aerogel microspheres through coaxial electrostatic spinning-freezing casting; coaxial electrospinning was carried out at 5kV with a tip to collector distance of about 10 cm. The external spinning solution rate was 0.015mL/min and the internal precursor rate was set to 0.008 mL/min.
And 4, step 4: further freeze-drying for 48h, thermally reducing the aerogel microspheres, and cooling to obtain the required core-shell heterogeneous graphene aerogel microspheres Carbon @ RGO/Fe3O4-3, i.e. curve C in the figure. Thermal reduction was performed at 400 ℃ for 2h at a temperature rise rate of 5 ℃/min under an argon atmosphere, and cooled to room temperature.
Example 4
A preparation method of core-shell heterogeneous graphene aerogel microspheres comprises the following steps:
step 1: FeCl is added3·6H2Adding O into the aqueous dispersion of the graphene oxide, and carrying out ultrasonic treatment for 20min to obtain a uniform external spinning solution; the concentration of the aqueous dispersion of graphene oxide GO was 4mg/mL, 10 mL. Wherein GO and FeCl3·6H2The mass ratio of O is 2: 1.
Step 2: a certain amount of chitosan powder (Acmec, degree of deacetylation: 85%) was added to an acetic acid solvent (15mL, 2% v/v) to obtain an inner layer precursor spinning solution. Wherein the mass ratio of the chitosan to the acetic acid is 15:1, and the mass ratio of the chitosan to the graphene oxide is 5: 1.
And step 3: adopting liquid nitrogen (-196 ℃) as a collector, and obtaining aerogel microspheres through coaxial electrostatic spinning-freezing casting; coaxial electrospinning was carried out at 5kV with a tip to collector distance of about 10 cm. The external spinning solution rate was 0.017mL/min and the internal precursor rate was set to 0.008 mL/min.
And 4, step 4: further freeze-drying for 48h, thermally reducing the aerogel microspheres, and cooling to obtain the required core-shell heterogeneous graphene aerogel microspheres Carbon @ RGO/Fe3O4-4, i.e.Curve D in the figure. Thermal reduction was performed at 400 ℃ for 2h at a temperature rise rate of 5 ℃/min under an argon atmosphere, and cooled to room temperature.
Example 5
A preparation method of core-shell heterogeneous graphene aerogel microspheres comprises the following steps:
step 1: FeCl is added3·6H2Adding O into the aqueous dispersion of graphene oxide, and performing ultrasonic treatment for 20min to obtain a uniform external spinning solution; the concentration of the aqueous dispersion of graphene oxide GO was 4mg/mL, 10 mL. Wherein GO and FeCl3·6H2The mass ratio of O is 2: 1.
Step 2: a certain amount of chitosan powder (Acmec, degree of deacetylation: 85%) was added to an acetic acid solvent (15mL, 2% v/v) to obtain an inner layer precursor spinning solution. Wherein the mass ratio of the chitosan to the acetic acid is 15:1, and the mass ratio of the chitosan to the graphene oxide is 5: 1.
And step 3: adopting liquid nitrogen (-196 ℃) as a collector, and obtaining aerogel microspheres through coaxial electrostatic spinning-freezing casting; coaxial electrospinning was carried out at 5kV with a tip to collector distance of about 10 cm. The external spinning solution rate was 0.019mL/min and the internal precursor rate was set to 0.008 mL/min.
And 4, step 4: further freeze-drying for 48h, thermally reducing the aerogel microspheres, and cooling to obtain the required core-shell heterogeneous graphene aerogel microspheres Carbon @ RGO/Fe3O4-5, i.e. curve E in the figure. Thermal reduction was performed at 400 ℃ for 2h at a temperature rise rate of 5 ℃/min under an argon atmosphere, and cooled to room temperature.
For comparison with the microspheres obtained by the present invention, solid reduced graphene oxide/Fe was prepared3O4I.e. RGO/Fe3O4And chitosan-derived carbon aerogel microspheres, i.e., the carbon curves in the figure.
Solid reduced graphene oxide/Fe3O4: forming charged microdroplets by a uniaxial electrostatic spinning technology, and obtaining the solid reduced graphene oxide/Fe by freezing casting, freezing drying and thermal reduction technology3O4Aerogel microspheres. Carbon aerogelsThe microsphere is also prepared by a uniaxial electrostatic spinning technology.
In order to characterize the core-shell heterogeneous graphene aerogel microspheres obtained by the method, a field emission scanning electron microscope and an element spectrum are adopted to observe the morphological structure of the aerogel microspheres. The structural difference of the core-shell microspheres was analyzed by laser raman spectroscopy (InVia, Renishaw) using a 532nm argon ion laser. Elemental composition was analyzed using an X-ray photoelectron spectrometer (XPS, Thermo scientific K-alpha +) with a monochromatic Al K α X-ray source (1486.6 eV). A DHR-1 rheometer (TA) is selected to test the cyclic compression performance of the aerogel microspheres. The complex permeability and dielectric constant of the aerogel microspheres were measured with a vector network analyzer (AV3618, CETC) in the frequency range of 2-18 GHz.
The aerogel microspheres obtained in the above example were used as an absorbent and mixed with wax uniformly in an amount of 5.0% by weight to prepare an annular sample having an outer diameter of 7.0mm and an inner diameter of 3.04 mm. For testing, samples of different thicknesses were prepared for testing.
FIG. 1a is a photograph taken by optical microscopy of aerogel microspheres obtained in example 3. Aerogel microspheres can be seen to exhibit a uniform spherical structure with a diameter of about 2.2 mm. Fig. 1b and 1c are SEM images of cross sections of aerogel microspheres obtained in example 3. The aerogel microspheres with a layered core-shell heterostructure can be clearly seen from the figure, and the shell layer presents a unique ordered divergent micro-channel structure from inside to outside. FIG. 1D is an SEM image of the shell layer of the aerogel microsphere obtained in example 3, and it can be seen from the SEM image that the shell layer has a regularly distributed macroporous structure and a 3D network structure, and the pore diameter range is 10-30 μm. FIG. 1e is an SEM image of the core layer of the aerogel microspheres obtained in example 3, from which it can be seen that the inner core has an irregular three-dimensional porous network structure with smaller pore sizes than the outer layer and less than 10 μm. FIG. 1f is an SEM image of the aerogel microsphere interface obtained in example 3, from which it can be seen that a large complete interface is formed between different shell wall heterostructures, increasing the interfacial polarization effect. Focusing on the interface junction can further confirm the difference between the pore structure and the 3D framework of the core-shell heterostructure.
As can be seen from fig. 2, a three-dimensional structure assembled from many folded graphene sheets and biomass carbon and Fe3O4 magnetic nanoparticles.
As can be seen from fig. 1, a dendritic cross structure can be observed between large-layer graphene sheets with strong connectivity, which is beneficial to improving the mechanical properties of the graphene sheets and meets the requirements of practical applications.
FIG. 3 is a schematic structural diagram of aerogel microspheres obtained in examples 1 to 5. It can be seen from the figure that the flow rate of the internal and external spinning solutions in the coaxial electrostatic spinning process is one of the factors influencing the core-shell structure of the graphene-based aerogel microsphere. The flow rate of the inner core layer is maintained at 0.008mL/min, and a series of aerogel microspheres with different shell thicknesses are obtained by changing the flow rate of the outer spinning solution. As can be seen from fig. 3a to 3e, the thickness of the external RGO aerogel increases with the increase of the outer layer speed, and the adjustment and control of the shell layer parameters mainly affect the balance relationship between the conductive loss capability of the aerogel microspheres and the multiple reflections of the core resonant cavity. The average thickness and error of the aerogel microspheres prepared at different flow rates were counted, as shown in fig. 3 f. It is further demonstrated that the shell thickness is positively correlated to the flow rate.
Fig. 4 is a schematic raman spectrum of the aerogel microspheres obtained in example 3, where a is a raman spectrum of the aerogel microspheres in the outer shell layer, the inner core layer, and the interface region, b is an XPS total spectrum of the aerogel microspheres, c is a 1s peak, d is an N peak, e is an Fe 2p peak, and f is a stress-strain curve of the aerogel microspheres.
From fig. 4a, two strong peaks are clearly observed, corresponding to the D and G peaks, respectively. The D peak is generated by the respiratory vibration of sp2 hybridized carbon atoms, reflecting the defects of the internal structure of graphite. The G peak is caused by the stretching movement of sp2 hybridized atoms in the carbon ring or long chain, corresponding to the symmetry of the graphite structure. Raman ID/IGThe ratio is widely used to evaluate the degree of disorder of the carbon material. The higher the ratio, the greater the degree of disorder and the more defects. Intensity ratio of D peak and G peak from shell structure to core structure (I)D/IG) An increase from 0.95 to 1.07 indicates an increase in disorder and an increase in structural defects. The results indicate that the order of RGO after thermal reduction is superior to internal biomass prepared porous carbon. The increase of the defects from outside to inside is beneficial to the resistanceThe anti-matching degree and the electromagnetic wave attenuation are gradually improved.
From FIG. 4b, it can be seen that C (C1s 285.4eV), O (O1s 530.6eV), N (N1s 399.2eV) and Fe (Fe 2p 710.4eV) are present in all samples. From fig. 4C it can be seen that the C1s spectrum (fig. 3C) is divided into four sub-peaks, including C-C/C ═ C (284.6eV), C-N (285.4eV), C-O (286.5eV) and C ═ O (288.1 eV). As can be seen from FIG. 4d, the N1s spectrum is divided into three sub-peaks, representing the N atom pyridine-N (398.8eV), pyrrole-N (400.3eV) and graphite-N (401.3eV), respectively. The N atom in the carbon skeleton has an important influence on the dielectric properties thereof. In one aspect, the N atom can provide an unpaired electron conjugated to a pi-conjugated ring, which is beneficial for enhancing electron transport. On the other hand, the electronegativity of the C atom and the N atom is different, which means that the nitrogen atom can act as a polarization center under an alternating electromagnetic field, resulting in a dipolar polarization process. It can be seen from FIG. 4e that the Fe 2p spectrum shows two broad peaks at 711eV and 724eV, which are attributable to the ionization by Fe 2p3/2 and Fe 2p 1/2. No sub-peak is found near 719.2eV, which indicates that the synthesized Fe3O4The nanoparticles have high purity.
FIG. 4f is a cyclic compressive stress-strain curve of the aerogel microspheres obtained in the example, obtained at 50% strain. After 100 cycles, the compressed composite aerogel balls recover to the original shape after releasing the load, and the recycling performance is high.
Fig. 6 shows electromagnetic loss values of the aerogel microspheres obtained in the example as an absorbent, where a is the RL value of the aerogel microspheres obtained in examples 1 to 5 as an absorbent when the thickness is 2.5mm, b is the RL value of the aerogel microspheres obtained in example 1 as an absorbent under different thickness conditions, c is the RL value of the aerogel microspheres obtained in example 2 as an absorbent under different thickness conditions, d is the RL value of the aerogel microspheres obtained in example 3 as an absorbent under different thickness conditions, e is the RL value of the aerogel microspheres obtained in example 4 as an absorbent under different thickness conditions, and f is the RL value of the aerogel microspheres obtained in example 5 as an absorbent under different thickness conditions.
Solid RGO/Fe3O4The aerogel microspheres have poor wave-absorbing performance inAt a high frequency of 18GHz, the minimum reflection loss RLmin is-20.4 dB. The main reason is RGO/Fe3O4The high conductivity of aerogel microspheres results in impedance mismatch with incident electromagnetic waves. Meanwhile, the RLmin value of the chitosan derived carbon aerogel microspheres does not exceed-10 dB, which shows that the chitosan derived carbon aerogel microspheres have low dielectric loss capacity.
When the core-shell structure is introduced into the aerogel microspheres, the attenuation performance of electromagnetic waves can be improved through sequence loss, multi-level heterostructure cooperation and multi-resonance coupling; from FIG. 6, it can be seen that Carbon @ RGO/Fe at a frequency of 15.92GHz3O4RLmin value of-1 is-35 dB, effective absorption bandwidth fEReaching 6.72 GHz. At a frequency of 9.28GHz, the minimum reflection loss value RLmin can reach-51 dB (figure 6c), and the effective absorption bandwidth fE6.40GHz at a thickness of 3.6 mm. Particularly, when the shell layer reaches about 205 μm, the minimum reflection loss value RLmin of the aerogel microspheres reaches the lowest value, the best wave absorbing performance is shown, the wave absorbing performance is reduced to-61 dB (shown in figures 6a and C) at 13.84GHz, and the effective absorption bandwidth f of 6.88GHz is realizedE. With the further increase of the large-sheet three-dimensional graphene network, the RLmin value at the frequency of 17.84GHz is reduced to-25 dB, and f isEThe value can reach 7.52GHz (FIG. 6a, D). After further increasing the thickness of the outer layer, Carbon @ RGO/Fe3O4-5 minimum reflection loss value of-18 dB of aerogel microspheres, but effective absorption bandwidth fEStill maintaining 6.32GHz (fig. 6a, E). An effective balance between high dielectric loss and impedance matching is the core shell Carbon @ RGO/Fe3O4The wave-absorbing performance of the aerogel microspheres tends to be improved and then reduced, and finally an optimal value is caused.
To reveal the influence of the core-shell double-layer structure on the corresponding electromagnetic wave absorption characteristics, RGO/Fe was respectively tested3O4Chitosan-derived Carbon, Carbon @ RGO/Fe3O4-3 relative complex dielectric constant of the aerogel microspheres, as shown in figure 5. As can be seen in fig. 5, the real and imaginary parts of the dielectric constant of the aerogel microspheres both decrease with increasing frequency. The main reason is that according to typical dispersion behavior, polarization hysteresis occurs due to the aggravation of the high-frequency electric field variation. Solid RGO/Fe3O4The real part epsilon 'of the dielectric constant of the aerogel microspheres is far higher than the epsilon' value of the chitosan-derived carbon microspheres, and the epsilon 'value of the chitosan-derived carbon microspheres is slightly higher than the epsilon' value of the core-shell double-layer aerogel microspheres. FIG. 5b clearly shows RGO/Fe3O4The values of epsilon "for the aerogel microspheres are highest, while the values of epsilon" for the inner core carbon aerogel microspheres are lowest. When a core-shell structure is introduced, Carbon @ RGO/Fe3O4The imaginary part epsilon 'value of the dielectric constant of the aerogel microspheres is between the two values, which shows that the influence of the double-layer core-shell structure on the epsilon' value is large. The reduction of the imaginary part epsilon' of the dielectric constant is beneficial to the entering of electromagnetic waves into Carbon @ RGO/Fe3O4Inside the aerogel microspheres.
As can be seen from FIGS. 5c and 5d, the core shells Carbon @ RGO/Fe increased with increasing RGO aerogel shell thickness3O4The values of epsilon' and epsilon "of the aerogel microspheres continued to rise, primarily due to the increase in the 3D graphene conductive network. From FIG. 5e, Carbon @ RGO/Fe3O4Due to the existence of the chitosan-derived carbon in the aerogel microspheres, the tan delta epsilon value is reduced, and the electromagnetic waves can enter the aerogel microspheres and the overall impedance matching performance of the core-shell aerogel microspheres can be optimized.
FIG. 7 is the permeability of the aerogel microspheres obtained in the example, and FIG. 7a shows the Carbon @ RGO/Fe prepared at different flow rates3O4Tan delta epsilon and dielectric constant epsilon of aerogel microspheresrWith the same tendency to increase progressively with increasing outer layer flow rate. As can be seen from FIGS. 7 b-d, it is shown that the compounds with RGO/Fe3O4Aerogel microsphere comparison, Carbon @ RGO/Fe3O4The μ', μ "and tan μ of the microspheres decrease with the intervention of the inner core layer due to the presence of the non-magnetic chitosan-derived carbon. From FIG. 7e, a clear resonance peak can be seen, illustrating Fe3O4Magnetic nanoparticles cause the material to generate magnetic losses to electromagnetic waves, mainly due to natural resonance, exchange resonance and eddy current losses. The losses at low frequencies (2-6GHz) are mainly caused by natural resonances. From FIG. 7f, it can be seen that the eddy current coefficient C0(C0＝μ″(μ′)-2f-1) The frequency change (6-18GHz) remains substantially unchanged, indicating that the aerogel microspheres areThe magnetic losses are mainly caused by eddy current losses. In addition, the dielectric loss tangent tan δ ∈ is much larger than the magnetic permeability tangent tan μ, indicating that the magnetic nanoparticles Fe3O4The resulting magnetic losses are weak. Thus Carbon @ RGO/Fe3O4Aerogel microspheres achieve attenuation of incident electromagnetic waves primarily through dielectric loss.
The dielectric loss of the composite material is mainly derived from conduction loss and polarization relaxation. The core-shell double-layer structure mainly provides a heterogeneous interface for the aerogel microspheres so as to increase the polarization relaxation of the interface. FIG. 8 is a Cole-Cole curve of aerogel microspheres obtained in the examples. As can be seen from the figure, the Cole-Cole curves with aerogel microspheres all contain a significant number of debye semi-circles, indicating that there are multiple polarization relaxation processes for the graphene-based aerogel microspheres. For solid RGO/Fe3O4Aerogel microspheres, the main cause of polarization relaxation is dipolar polarization of RGO surface functional groups, as well as graphene interlamellar and RGO/Fe3O4Interface polarization caused by non-uniform interfaces therebetween. After the inner core-shell polysaccharide derived carbon is introduced, the heterogeneous shell layer interface between the graphene and the chitosan derived carbon is increased, so that new interface polarization relaxation is caused. Multiple heterogeneous interfaces can lead to the collection and uneven distribution of charge at the interface, ultimately resulting in interfacial polarization of the material. The Cole-Cole curves all show a long tail strip shape, which indicates that the conductive loss is the main loss mechanism of the aerogel microspheres to electromagnetic waves. Relatively speaking, RGO/Fe3O4The conductive loss of the aerogel microspheres is larger than that of core-shell Carbon @ RGO/Fe3O4The aerogel microspheres have strong conductive loss, and the polarization loss capability of the core-shell double-layer heterostructure is better than that of the solid aerogel microspheres.
Carbon @ RGO/Fe with core-shell double-layer structure3O4The aerogel microspheres break through a complete conductive network in the pure graphene aerogel microspheres, the conductivity of the microspheres is modulated to a lower level, and better impedance matching performance is shown. On the basis, by adjusting Carbon @ RGO/Fe with different thicknesses3O4The aerogel microspheres can be used for effectively adjusting the impedance matching performance of the aerogel microspheres, namely, the aerogel microspheres can be used for adjusting the impedance matching performance of the aerogel microspheresThe relationship between electrical loss and impedance matching.
Another important factor determining the performance of the wave-absorbing material is the damping constant, and fig. 9 is the damping constant of the aerogel microspheres obtained in the example. RGO/Fe of three-dimensional porous network structure3O4The aerogel microspheres have the highest value of the damping constant α, corresponding to the strongest damping capacity. Carbon @ RGO/Fe3O4The aerogel microspheres have a high alpha value and an attenuation constant value of over 100 at medium and high frequencies. With the increase of the thickness of the outer layer, the attenuation capability of the aerogel microspheres is gradually enhanced, and the fact that the three-dimensional network structure of the graphene plays a decisive role in the attenuation of electromagnetic waves is shown. Core-shell Carbon @ RGO/Fe with appropriate impedance matching performance and high attenuation capability3O4-3 the aerogel microspheres have excellent absorption efficiency for electromagnetic waves.
Carbon@RGO/Fe3O4The chitosan-derived carbon of the core portion of-3 provides a larger resonant cavity. This unique structure can play an important role in electromagnetic absorption-multiple reflections and scattering in the resonant cavity will result in more dissipation of the electromagnetic energy. In addition, the superior interface between RGO and chitosan-derived carbon adds additional space charge polarization, thereby synergistically improving the loss capability of the core-shell structure. In short, the design strategy of the layered core-shell graphene aerogel microspheres enables the aerogel microspheres to have better absorption performance.
The aerogel microspheres obtained by the invention. Firstly, the core-shell multilevel structure coupling realizes the sequential loss of electromagnetic waves from inside to outside. The electromagnetic waves pass through the three-dimensional porous network of the shell layer graphene, and the three-dimensional resistance network quickly attenuates long-range induced current and converts the induced current into heat energy to generate conduction loss on the electromagnetic waves. Subsequently, the residual electromagnetic wave is incident on the non-uniform interface of the shell wall and is reflected and scattered, and part of the microwave is reflected to the graphene aerogel of the shell layer to realize reabsorption. In addition, a non-uniform interface between the graphene and the chitosan-derived carbon induces strong interface polarization relaxation, improves impedance matching performance, and promotes further absorption and attenuation of microwaves. Finally, the residual electromagnetic waves enter the biomass porous carbon of the inner core layer and the inner porous carbonThe cavity formed by the structure can be regarded as a resonant cavity, and the electromagnetic wave is forced to undergo multiple reflection and scattering inside the cavity, so that the attenuation loss is further enhanced. Therefore, core-shell heterostructure coupling can lead to multi-order loss and multiple resonance effects of electromagnetic waves. On the basis, Carbon @ RGO/Fe is further adjusted by adjusting shell parameters3O4Microwave absorption efficiency of aerogel microspheres.
Varying the thickness of the outer shell layer is primarily to optimize the loss capability of the electromagnetic waves entering the first layer. Carbon @ RGO/Fe with increasing shell thickness3O4The conductive loss capability of the microspheres to electromagnetic waves is increased. And secondly, the core-shell heterostructure effectively optimizes the matching performance of the aerogel microsphere. The solid graphene-based aerogel microspheres have a complete conductive network, corresponding to high conductivity and strong attenuation capacity, but the high conductivity causes poor impedance matching performance of the material and increased reflection. The three-dimensional conductive network of the graphene aerogel can bring high conductivity, and is combined with low-conductivity biomass carbon to adjust the conductivity, so that the impedance matching performance is further improved, the attenuation is enhanced by the coupling of a multilayer structure, and the respective advantages are fully exerted. After adjusting the shell parameters, the core-shell Carbon @ RGO/Fe is found3O4-3 the aerogel microspheres have optimal microwave absorption properties at a certain outer layer thickness. On the premise of ensuring high conduction loss, the impedance matching performance is adjusted to the optimal level. Combination effect of core-shell aerogel microspheres. Single Carbon @ RGO/Fe3O4The aerogel microspheres have high-efficiency microwave absorption. After the aerogel microspheres are combined, the microspheres are contacted with each other, so that multiple reflection and scattering phenomena of electromagnetic waves among the aerogel microspheres are facilitated. And a small amount of unmatched electromagnetic waves can be further captured and attenuated by order losses in each aerogel microsphere. In general, the combined effect enhances multiple reflection and scattering of electromagnetic waves, prolongs the transmission path of the electromagnetic waves, and brings about multi-level loss. In addition, the large number of defects caused by nitrogen atom doping and residual oxygen-containing functional groups provide abundant dipoles, resulting in polarization loss of incident microwaves. At the same time, Fe3O4Contribute to the magnetic loss of aerogel microspheres to achieve magneto-dielectric propertiesSynergistic effect.
The core-shell graphene-based aerogel microspheres are prepared by electrostatic spinning combined with freeze drying and calcination. In the preparation process, the key technology is to control the formation of a Taylor cone in electrostatic spinning. Oxygen-containing functional groups on GO sheets with Fe3+The cross-linking forms a mixed colloidal solution as an outer spinning solution, the chitosan precursor is used as an inner spinning solution, and in the high-voltage coaxial electrostatic spinning process, concentric microdroplets can be stretched and broken into a small double-layer heterogeneous charged droplet. The inner layer and the outer layer both have a large number of active groups, and hydrogen bonds are formed at the interface of the heterostructure. In the quick freezing process, the graphene outer layer uses the ice crystal as the template to assemble the GO sheets into a 3D aerogel. Meanwhile, the inner layer chitosan solution generates more ice crystal nuclei at the temperature of-196 ℃ and generates a large number of small holes, thereby forming concentric heterogeneous ice microspheres. Subsequently, through freeze drying and thermal reduction, the oxygen-containing functional groups on the outer layer are gradually reduced to form Reduced Graphene Oxide (RGO), and the chitosan precursor in the inner part is converted into amorphous carbon, so that the preparation of the double-layer heterogeneous aerogel microspheres is realized. The core-shell structure obtained by the invention is beneficial to the polarization of a non-uniform interface and the effect of a resonant cavity, and is beneficial to the coordination of conduction loss and impedance matching performance. Carbon @ RGO/Fe3O4The loading of the aerogel microspheres is only 5 wt%, the minimum reflection loss RLmin value of 2.5mm in thickness is-61 dB, and the effective absorption bandwidth can reach 6.88 GHz. By tuning the core and shell heterostructures, Carbon @ RGO/Fe can be found3O4The effective absorption bandwidth of the aerogel microspheres can reach 7.52 GHz. The aerogel microspheres obtained by the invention have a special structure, so that the aerogel microspheres have higher absorption efficiency and wider absorption bandwidth when being used as an absorbent.
Claims (6)
1. The preparation method of the core-shell heterogeneous graphene aerogel microspheres is characterized by comprising the following steps:
step 1: FeCl is added3·6H2Adding O into the aqueous dispersion of the graphene oxide, and uniformly dispersing to obtain an external spinning solution; graphene oxide and FeCl3·6H2The mass ratio of O is 1: 5-5: 1;
step 2: adding chitosan into a solvent, and uniformly dispersing to obtain an inner layer precursor spinning solution; the solvent is acetic acid, the concentration of the acetic acid is 2% v/v, the mass ratio of the chitosan to the acetic acid is 15:1, and the mass ratio of the chitosan to the graphene oxide is 3: 1-8: 1;
and step 3: adopting liquid nitrogen as a collector, and obtaining aerogel microspheres through coaxial electrostatic spinning-freezing casting; in the coaxial electrostatic spinning process, the speed of the external spinning solution is 0.011 mL/min-0.019 mL/min; the speed of the spinning solution of the precursor of the inner layer is 0.008mL/min
And 4, step 4: thermally reducing the aerogel microspheres, and cooling to obtain the required core-shell heterogeneous graphene aerogel microspheres;
the core-shell heterogeneous graphene aerogel microsphere shell layer is provided with a regularly distributed macroporous structure and a 3D network structure, and the pore diameter ranges from 10 to 30 micrometers; the inner core is in an irregular three-dimensional porous network structure, the pore diameter is smaller than that of the outer layer, and the pore diameter range is 10 mu m; the shell and the core have an integral interface therebetween.
2. The preparation method of core-shell heterogeneous graphene aerogel microspheres according to claim 1, wherein the coaxial electrospinning in the step 3 is performed at a voltage of 5kV, and the distance from the needle point to the collector is 10 cm.
3. The preparation method of core-shell heterogeneous graphene aerogel microspheres according to claim 1, wherein after the step 3 of freeze casting is finished, the core-shell heterogeneous graphene aerogel microspheres are freeze-dried for 48 hours, and then the step 4 is performed.
4. The preparation method of core-shell heterogeneous graphene aerogel microspheres according to claim 1, wherein the thermal reduction process in the step 4 is as follows: thermally reducing at 400 ℃ for 2h at the heating rate of 5 ℃/min under the argon atmosphere, and cooling to room temperature.
5. The preparation method of core-shell heterogeneous graphene aerogel microspheres according to claim 1, wherein in the graphene oxide aqueous dispersion in the step 1, the concentration of graphene oxide is 4 mg/mL; and carrying out ultrasonic treatment for 20min for dispersion.
6. The application of the core-shell heterogeneous graphene aerogel microspheres obtained by the preparation method according to any one of claims 1 to 5, wherein the core-shell heterogeneous graphene is used for a microwave absorbent.
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