Preparation of Honeycomb SnO2 Foams and Configuration

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Preparation of Honeycomb SnO Foams and Configuration-Dependent Microwave Absorption Features Biao Zhao, Bingbing Fan, Yawei Xu, Gang Shao, Xiaodong Wang, Wanyu Zhao, and Rui Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08383 • Publication Date (Web): 09 Nov 2015 Downloaded from http://pubs.acs.org on November 15, 2015

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ACS Applied Materials & Interfaces

TOC GRAPHIC The honeycomb-like SnO2 foams exhibit the superior properties with the features of strong absorption, thin thickness, broad bandwidth and light weight. 117x192mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Preparation of Honeycomb SnO2 Foams and Configuration-Dependent Microwave Absorption Features Biao Zhao †, Bingbing Fan †, Yawei Xu #, Gang Shao † *, Xiaodong Wang #, Wanyu Zhao †, Rui Zhang †,‡ * †

School of Materials Science and Engineering, Zhengzhou University, Zhengzhou

450001, China ‡

Laboratory of Aeronautical Composites, Zhengzhou Institute of Aeronautical

Industry Management, Zhengzhou 450046, China #

School of Materials Science and Engineering, Henan Polytechnic University,

Jiaozuo 45400, China * Corresponding Author: Dr. Gang Shao E-mail address: [email protected] Prof. Rui Zhang E-mail address: [email protected] Tel: +86-371-60632007 Fax: +86-371-60632600 Abstract: Ordered honeycomb-like SnO2 foams were successfully synthesized by means of a template method. The honeycomb SnO2 foams were analyzed by X-ray diffraction (XRD),

thermogravimetric and differential scanning calorimetry

(TG-DSC), laser Raman spectra, scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR). It can be found that the SnO2 foam configurations were determined by the size of polystyrene templates. The electromagnetic properties of ordered SnO2 foams were also investigated by a network analyzer. The results reveal 1


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that the microwave absorption properties of SnO2 foams were dependent on their configuration. The microwave absorption capabilities of SnO2 foams were increase by increasing the size of pores in the foam configuration. Furthermore, the electromagnetic wave absorption is also correlated with the pore contents in SnO2 foams. The large and high amounts pores can bring about more interfacial polarization and corresponding relaxation. Thus, the perfect ordered honeycomb-like SnO2 foams obtained in the existence of large amounts of 322 nm polystyrene spheres showed the outstanding electromagnetic wave absorption properties. The minimal reflection loss (RL) is -37.6 dB at 17.1 GHz and RL less than -10 dB reaches 5.6 GHz (12.4-18.0 GHz) with thin thickness of 2.0 mm. The bandwidth (< -10 dB, 90% microwave dissipation) can be monitored in the frequency regime of 4.0-18.0 GHz with absorber thickness of 2.0−5.0 mm. The results indicate that these ordered honeycomb SnO2 foams show the superiorities of wide-band, high efficiency absorption, multiple reflection and scatting, high anti-oxidation, light weight and thin thickness. KEYWORDS: SnO2 foams, honeycomb-like structure, electromagnetic properties, interfacial polarization, multiple reflection. 1. Introduction Due to their unique structural features and intriguing properties, porous or hollow microstructures with adjustable size, shape and composition are attracted by intense attention for the varieties of utilizations, such as chemical sensors, catalysis, energy storage, and biomedicine.

1, 2

In past decades, electromagnetic waves are being more

and more used in local area networks, wireless communication tools and personal digital assistants. However, the growing usage of microwave devices would lead to serious electromagnetic interference (EMI) issues in both military and civil applications.3, 4 An available method to settle these issues is to explore a new kind of 2



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absorber with high efficiency microwave absorption properties. This absorber can absorb microwaves and convert electromagnetic energy to heat or other types. Moreover, it could be applied to reduce the microwave reflection from target surfaces such as tanks, aircrafts, ships and the walls of anechoic chambers.5, 6 The current ideal electromagnetic wave absorption properties are expected to possess the features of small thickness, light-weight, wide-band, strong absorption and high anti-oxidation.

7, 8

It is well known that the electromagnetic properties were

closely related with their microstructures.

9, 10

Among the candidates for the

microwave absorption, the hollow/porous structured materials which possess the features of lightweight and multi-reflection are supposed to be promising electromagnetic wave absorption materials.

11-17

Lv et al. fabricated porous

three-dimensional flower-like Co/CoO by means of a facile method containing hydrothermal and subsequent annealing procedure, and the minimal reflection is −50 dB.

12

Peng et al. designed hollow microspheres with multiple-shell conductive

polymer by utilizing hollow Fe3O4 microspheres as sacrificial templates and the optimal reflection loss of the triple-shelled PEDOT is -39.7 dB with a thickness of 2.0 mm.

13

He and coworkers prepared hollow porous cobalt spheres by combining a

solvent-thermal route and hollow porous Co spheres exhibit enhanced microwave absorption. 14 Wang et al. synthesized hollow Fe3O4 spheres with the diameter of about 500 nm via a solvothermal process with the assistance of polyvinyl-pyrrolidone and the minimum reflection loss of -42.7 dB was located at 2.0 GHz with a thickness of 6.9 mm.16 In our previous paper, CuS hollow flowers self-assembled by nanoflakes were successfully synthesized through a facile solvothermal method. The optimal reflection loss of ‒31.5 dB can be obtained at 16.7 GHz with the thickness of 1.8 mm.17 From these results, it can be concluded that hollow/porous microstructures are in favorable 3


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for the improvement of electromagnetic wave absorption properties. However, the limitation of above materials is easily oxidated when placed in the harsh conditions (high temperature). In this work, for the first, three-dimensionally ordered honeycomb SnO2 foams which can endure high temperature were prepared by means of using polystyrene spheres as sacrificial templates. There are plentiful ways to prepare hollow/porous materials, including template method,

18

self-assemble,

Kirkendall effect,21 and Ostwald ripening.

22

19

Galvanic replacement，

20

Among these mentioned methods, the

template method was proved to be versatile and easily controlled way. Thus, we use polystyrene spheres as templates to design ordered honeycomb SnO2 foams and the size of pore in the foams were easily monitored. This unique ordered honeycomb SnO2 foams are favorable for the enhancement of electromagnetic wave absorption properties. The microwave absorption mechanism was also investigated in detail. 2. Experimental Section 2.1 Materials SnCl4•5H2O was supplied by Tianjin Kemiou Chemical Reagent Co., Ltd. NaOH and ammonia were provided by Xilong Chemical Industry Incorporated

Co., Ltd.

Styrene and Potassium peroxydisulfate (K2S2O8) were purchased from Beijing Chemical Works. All chemical reagents were commercial available and used without further purification. The distilled water used in the experiments was homemade. 2.2 Fabrication of Three-Dimensionally Ordered Polystyrene/SnO2 Hybrids Monodispersed polystyrene (PS) spheres with different sizes (148 nm, 213 nm, 322 nm) were synthesized via an emulsion polymerization process, which is similar with previous literatures.

23, 24

The detail procedure was described in supporting

information. After washing with ethanol and distilled water several times, the purified 4



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PS spheres were re-dispersed in distilled water and allowed to form colloidal crystals for subsequent usage. The SnO2 nanocrystals were prepared through a hydrothermal method. In a typical process, SnCl4•5H2O (0.001 mol) and ammonia (3 mL) were dissolved in the mixture of distilled water (30 mL) and absolute ethanol (30 mL). Then the above solution was transferred into a Teflon-lined stainless-steel autoclave and heated to 150 °C for 24 h. And when the autoclave naturally cooled down to room temperature at last, the products were collected after washing and drying. For the synthesis of polystyrene/SnO2 hybrids, a certain amount of the obtained 148 nm-PS solution was blended with SnO2 solution (0.1 g SnO2 dispersed in 50 mL distilled water) by the means of ultrasonication. The final products was collected, washed, and dried at 50 °C for 12h. 2.3. Fabrication of Three-Dimensionally Honeycomb SnO2 Foam To obtain foam structure, the polystyrene/SnO2 hybrids were pyrolysed in air at 500 °C for 2 h to remove PS templates. Thus the honeycomb 148 nm- SnO2 foam was designed (Sample A). The other two honeycomb SnO2 foams with different diameters of PS spheres (213 nm, 322 nm) were denoted as Sample B and Sample C, respectively. 2.4. Characterization Field emission scanning electron microscopy (FESEM) was taken on a JEOL JSM-7001F microscope operated at 15 kV. Fourier transform infrared (FT-IR) spectra were performed with a Nicolet iS10 FTIR spectrometer in the range of 3500–500 cm-1. The crystallite structures of the prepared samples were investigated by the means of X-ray powder diffraction (XRD; Rigaku Ultima IV). The thermogravimetric (TG) and differential scanning calorimetry (DSC) were recorded on a STA 409/PC thermal analyzer (Netzsch, Germany) with a heating rate of 10 °C/min in flowing air. Laser 5


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Raman spectroscopy was attained by a LabRAM HR Evolution Confocal Laser MicroRaman spectrometer from 1800 to 200 cm-1 at room temperature. The 532 nm line of the laser was utilized as the excitation sources, with the capability of supplying 250 mW. The SnO2 foams were uniformly blended with paraffin wax at 30% weight fraction (which is transparent to microwave and used as binder). The samples were pressed into a toroidal shape (φ out = 7.00 mm, φ in = 3.04 mm). The relative complex permittivity (εr = ε′ − jε″) and permeability (µr = µ′ − jµ″) would be calculated by the experimental scattering parameters S11 and S21 by the aid of the standard Nicolson−Ross theoretical calculations. 25 3. Results and Discussion Figure 1 illustrates the synthesis protocol for honeycomb SnO2 foams based on the sacrificial template method. In the first step, SnO2 nancrystals and polystyrene (PS) spheres with certain size were synthesized, respectively. Then the obtained PS spheres were served as templates to blend with as-received SnO2 nanocrystals, and the PS spheres were surrounded by plentiful SnO2 nanocrystals. Finally, the obtained PS/SnO2 composites were further pyrolyzed in air, which cause the decomposition of PS spheres to produce unique honeycomb SnO2 foams. Monodispersed PS spheres with a diameter of 148 nm were synthesized by means of a polymerization method, which is similar with previous reports.

23

The TEM

images of SnO2 nanocrystals were provided in Figure S1. Based on the TEM (Figure S1a) and HRTEM (Figure S1b) images, the size of SnO2 nanocrystals was about 4 nm, which is in accordance with XRD result. Moreover, from the selected area electron diffraction (SAED) rings (inset in the Figure S1a), the SnO2 nanocrystals were polycrystalline. The PS spheres were utilized to construct three-dimensionally ordered colloidal crystals through gravimetric sedimentation (inset of Figure 2a). The obtained 6



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colloidal crystals were used as templates and SnO2 nanocrystals can be filled in the interstitial voids among the closely packed PS spheres. As shown in Figure 2(a,b), the 3-D ordered PS spheres were compactly encircled by a large number of SnO2 nanocrystals. After removing PS spheres by the aid of calcination, 3-D honeycomb-like SnO2 forms with uniform macropores of ~148 nm were received (Figure 2(c,d)). Notably, the size of macropore shows a little smaller than that of PS spheres due to the shrinkage of macropore framework during calcination. In addition, some non-pore parts existed in the SnO2 foam configurations were also clearly observed, which might be due to low amounts of PS spheres in the blended hybrids. From the inset of Figure 2d, it should be noted that the interconnected pores were also found in the SnO2 foam configurations, which is helpful for the increasement of electromagnetic wave absorption. The crystal structures of as-synthesized samples were characterized by X-ray powder diffraction (XRD). From the XRD profile (Figure 3a) of the as-synthesized three-dimensionally ordered polystyrene/SnO2 hybrids, the poorly crystalline phase is in accordance with tetragonal rutile SnO2 (JCPDS card no. 41-1445). After removing PS spheres by means of calcination, all the diffraction peaks can be also well match with tetragonal rutile SnO2 phase (Figure 3b). Moreover, it can be found that the intensity of peaks become stronger than those of polystyrene/SnO2 hybrids, which indicates high crystalline of SnO2 foams. The average crystallite size of SnO2 before calcination is calculated to be about 3.7 nm using Scherrer’s formula based on the (110) peak.

26

However, after calcination at 150 °C, the average size of SnO2

nanoparticles was increased to 6.4 nm due to the growth in the thermal decomposition process. To select the appropriate temperature of the calcination of precursor, the thermal 7


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behavior of precursor in air has been investigated. The curves of thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) are shown in Figure 4a. From the TG curve, it can be clearly seen that there are three stage of weight loss in the calcination process. A weight loss (∼5.68%) around 100 °C in the TG curve is attributed to the loss of absorbed water molecules. The sharp weight loss (about 19.54%) at about 350 °C is due to the pyrolysis of the precursor. However, in the 300-400°C, the residual carbon is still existed in the system. Thus, another weight loss (about 15.07%) was attributed to the remove of residual carbon at about 450 °C. Meanwhile, there are obvious two exothermic peaks in the DSC curve, which are in accordance with two sharp weight losses in the calcination process. According to the DSC and TG curves, it is reasonable that we choose a temperature of 500 °C for thermal treatment of the precursor to ensure it was decomposed completely. The thermal stability of SnO2 foams was investigated and the result was shown in Figure S2. It can be seen that the SnO2 foams hardly exhibit change of weight loss in the temperature range of 20-800 ºC, which indicates the relative high thermal stability of the foams. To further confirm the existence of SnO2 in the foams, the Raman spectra was carried out and the results were shown in Figure 4b. Three fundamental Raman scattering peaks at 476, 629, and 772 cm-1 can be observed, which correspond to those of a rutile SnO2 crystal.27 The peak at 476 cm-1, 629 cm-1 and 772 cm-1 can be assigned to the Eg mode, A1g mode, and B2g mode, respectively. Thus, three Raman spectra present the typical feature of the rutile phase of obtained SnO2 foams.28 In addition to the fundamental Raman scattering peaks of rutile SnO2, the other three scattering peaks are also observed, which may be attributed to the phonon confinement effect in SnO2 nanocrystals.

29

It can be also seen that there is no carbon

existing in final product. The pure PS and PS/SnO2 hybrids were also characterized by 8



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the FT-IR spectra and the corresponding results were shown in Figure S3. The other two sizes of PS spheres (213 nm, 322 nm) were also used to serve as sacrificial templates to design SnO2 foams. The overall morphologies of 213 nm PS spheres and 322 nm PS spheres were shown in the Figure S4. It can be clearly seen that three-dimensionally ordered monodispersed PS spheres with mean diameter of 213 nm (Figure S4a) and 322 nm (Figure S4b) were obtained in the polymerization protocol. The morphologies of SnO2 foams prepared with various sizes of PS templates were shown in Figure 5. From the overall photograph (Figure 5(a,c,e)), it can be clearly seen that three-dimensionally honeycomb SnO2 foams were obtained in this templating process. Notably, some non-pore parts are also existed in the SnO2 foam configurations. Further inspection from the enlarged SEM images (Figure 5(b,d,f)), the sizes of macropore were smaller than those of PS spheres due to the shrinkage in the calcination process and the interconnected pores were also obviously observed. From the above analysis, it can be reasonable concluded that 3-D honeycomb-like SnO2 were successfully synthesized with the acid of various PS templates. However, the SnO2 foam configurations, such as macropores and connected pores, exhibit a significant different, which may cause different microwave absorption properties. It is well known that the electromagnetic wave absorption abilities of absorbers were closely correlated with their complex relative permittivity and permeability as well as the configuration structures.

30, 31

Owning to the absence of magnetic

components in these SnO2 foams, the real part (µ′) and imaginary part (µ″) of complex permeability are almost 1 and 0, respectively, which are not presented in this work. For the complex permittivity, the imaginary part (ε″) and real part (ε′) represent the storage and loss abilities of electromagnetic energy, respectively.

17, 32

The 9


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frequency dependence of ε′ and ε″ for the SnO2 foam/paraffin composites are exhibited in Figure 6. From Figure 6a, it can be found that Sample C (322 nm pores) shows the largest ε′ values. That is to say that the ε′ values increase with increasing the size of pores for the SnO2 foams. Interestingly, as shown in Figure 6b, the ε″ values show the similar trend in comparison with ε′ values. It is reasonable concluded that the complex permittivity is crucially determined by the configuration of SnO2 foams. The ε′ is accounting for the polarizability of a material, which consist of electric polarization and dipolar polarization in microwave frequency.

33

It is no hard

to understand that dipolar polarization takes place in SnO2 foams because of small SnO2 nanoparticles (less than 10 nm), which can be considered as dipoles. Based on the free electron theory,

34, 35

ε ′′ ≈ 1 / πε 0 ρf , where ρ is the resistivity, high ε″ means

high conductivity. The relatively high conductivity of Sample C would cause high electric polarization. Furthermore，due to the presence of oxygen vacancies on the surfaces of SnO2,36 these vacancies would induce migration electrons when irradiated by the incident microwave, which could give rise to electric polarization. Moreover, compared with Sample A (148 nm pores) and Sample B (213 nm pores), Sample C possesses the high surface areas between SnO2 nanoparticles and pores. The interfaces could induce the more interfacial polarization and corresponding relaxations, which is in favor of the enhancement of electromagnetic absorption properties. The interfacial polarization often happens in the heterogeneous system.11, 37 In this work, due to the different dielectric constant and electronegativity of SnO2 and air, the interfacial polarization would be generated when SnO2 foams were irradiated under the alternated electromagnetic field. From the above views, we can conclude that the high values of ε′ and ε″ for Sample C result from the dipolar polarization, high electric polarization and more interfacial polarization. 10



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Commonly, the loss tangent (tan δ) is utilized to stand for the loss properties of the electromagnetic wave in absorbing materials.38 The dielectric loss tangent tan δε (tan δε = ε″/ε′) is calculated on the basis of the measured relative complex permittivity of the SnO2 foams/paraffin composites and he results were shown in Figure S5a. It can be clearly seen that Sample C exhibits the highest tan δε among the three samples, which indicates that SnO2 foams with large size of pores possess the high dielectric loss. The high tan δε of Sample C is in accordance with the ε″ values in Figure 6b, which are derived from dipolar polarization, high electric polarization, more interfacial polarization and their corresponding relaxations. Generally, the relaxation process which can be analyzed by the Cole-Cole semicircles has a vital influence on permittivity behaviors of microwave absorbers.

39

Based on the Debye dipolar

relaxation, the relative complex permittivity can be described by the following equation: 40, 41 εr = ε∞ +

εs −ε∞ = ε ′ − jε ′′ 1 + j 2πfτ

(1)

In which ε s , ε ∞ , f , τ are the static permittivity, relative dielectric permittivity at the high-frequency limit, frequency and polarization relaxation time, respectively. From the eqn (1), it can be deduced that ε′ = ε∞ +

ε ′′ =

εs − ε∞ 1 + (2πf ) 2 τ 2

(2)

2πfτ (ε s − ε ∞ ) 1 + (2πf ) 2 τ 2

(3)

Based on eqn (2) and (3), the relationship between ε′ and ε″ can be expressed as: 2

εs + ε∞  2   εs − ε∞  ε′ −  + ( ε ′′ ) =   2    2 

2

(4)

Thus, the plot of ε′ versus ε″ would be a single semicircle, which is usually defined as 11


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the Cole–Cole semicircle, and each semicircle is related to one Debye relaxation process.42 Figure S5 (b-d) manifests the Cole–Cole semicircles (ε″ versus ε′) for three SnO2 foams/paraffin composites in the frequency regime of 1–18 GHz. It can be found that three Cole–Cole semicircles can be seen in the Sample C (322 nm pores) and two semicircles existed in the Sample A (148 nm pores) and Sample B (213 nm pores). These results mean that there are multiple dielectric relaxation processes in the SnO2 foams, which may be ascribed to the multiple interfaces, ordered 3-D configuration and plentiful macropores. However, the distortions of Cole–Cole semicircles would be found, suggesting that besides the Debye relaxation, other mechanisms such as electron polarization, Maxwell–Wagner relaxation (interfacial polarization), and dipolar polarization are also existed in these foams.

43, 44

In SnO2

paraffin-composites, the multiple interfaces between the SnO2, paraffin matrix, and air bubble can favor the electromagnetic attenuation owning to the interaction of electromagnetic radiation with charged multi-poles at the interfaces. 12 To evaluate the electromagnetic wave absorption properties of as-prepared SnO2 foams, the reflection loss (RL) values could be simulated based on the relative complex permeability (µr = µ′ - jµ″) and permittivity (εr = ε′ - jε″) with a given frequency ( f ) and absorber thickness (d), by means of the following equations: 45-49 RL = 20 log10 (Z in − Z 0 ) /(Z in + Z 0 ) Z in = Z 0

 2πfd µ r ε r µr tanh j  c εr 

   

(5) (6)

where Z 0 is the impedance of free space, Z in is the input characteristic impedance, and c is the velocity of light. Figure 7a shows the simulated theoretical Sample A, Sample B and Sample C paraffin composites with 50wt% loadings at the thickness of 3.0 mm in the measured frequency regime of 1–18 GHz. Since a paraffin matrix is 12



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transparent to electromagnetic waves, the obtained results are generally considered as the electromagnetic wave absorption abilities of the filler itself. It is noteworthy that the electromagnetic wave absorption properties of SnO2 foams were increased with increasing the size of pores in the honeycomb foams. The optimal reflection loss of -15.9 dB can be obtained at 9.8 GHz. The RL values below -10 dB (90% microwave attenuation) were observed in the 8.4-11.2 GHz rang. From the eqn (5) and (6), it can be found that the thickness of the absorbers is one of important factors, which affects the RL values and location of optimal RL. Hence, the reflection losses of three SnO2 foams/paraffin composites with various thicknesses were also calculated and the results were shown in Figure 7(b-d). In comparison with Sample A (Figure 7b) and Sample B (Figure 7c), the Sample C (Figure 7d) presents the enhanced microwave properties. From Figure 7d, the optimal reflection loss is ‒26.6 dB 5.6 GHz. The effective absorption (less than -10 dB) bandwidth can be tuned between 4.8 GHz and 16.3 GHz for the absorbing materials with the thicknesses in 2.0−5.0 mm. The microwave absorption properties of SnO2 nanocrystals were also investigated and shown in Figure S6. The SnO2 nanocrystals present very weak microwave absorption. The minimal reflection loss was only -5.2 dB at 12.4 GHz with the thickness of 4.0 mm. It is noted that the minimal absorption peaks would move to higher frequencies and with the decreasing thickness. This phenomenon can be well account for the quarter-wavelength cancellation model. 50-52 In the absorber system, thermal energy is generated due to the absorption of EM radiation. Therefore, the materials should have sufficient thermal conductivity for heat dissipation. The thermal conductivity of SnO2 13


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is relatively large.53 Based on these results, it can be found that the microwave absorption properties of SnO2 were largely determined by their configurations. With increasing the size of pores in the foam configuration, the microwave absorption properties were improved. As is well known that large size of pores in SnO2 foams would cause more interfacial polarization, which is in favor of the improvement of electromagnetic absorption. In addition, the high electron polarization and dipolar polarization existed in the Sample C are also beneficial for the enhancement of absorption. Thus, Sample C shows the excellent electromagnetic wave performances. In the above results, the configurations affect their microwave absorption properties. The microwave absorption properties were enhanced with increasing the size of pores in the foam configuration and Sample C exhibits the outstanding microwave absorption properties. What about the effect of pore amounts on microwave absorption? Based on this question, we also investigated the effects of pore contents on the microwave absorption properties. The honeycomb SnO2 foams were prepared with two times PS (322 nm) amounts compared with Sample C and their FESEM photographs were shown in Figure 8. From the Figure 8, it can be clearly seen that the ordered 3-D perfect honeycomb-like foam configuration was obtained and each pore was connected by neighbor pores. From the broken areas, it can be observed that the interconnected pores are also existed in this SnO2 foams. The electromagnetic wave absorption properties of this perfect honeycomb SnO2 foams were exhibited in Figure 9. The optimal RL of ‒37.6 dB can be obtained at 17.1 GHz and bandwidth with RL less than -10 dB can reach 5.6 GHz (12.4-18.0 GHz) with thin 14



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thickness of 2.0 mm. The absorption (

Preparation of Honeycomb SnO2 Foams and Configuration

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