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Immobilization of zinc oxide nanoparticles on graphene sheets for lithium ion storage and electromagnetic microwave absorption
Materials Chemistry and Physics 245 (2020) 122766
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Immobilization of zinc oxide nanoparticles on graphene sheets for lithium ion storage and electromagnetic microwave absorption Shouchun Bao a, 1, Tianqi Hou a, 1, Qingke Tan a, Xiangli Kong a, Haijie Cao a, b, **, Maoxia He c, Guanglei Wu a, b, ***, Binghui Xu a, b, * a b c
Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China State Key Laboratory of Bio-Fibers and Eco-Textiles, Qingdao University, Qingdao, 266071, China Environment Research Institute, Shandong University, Qingdao, 266237, China
H I G H L I G H T S
� A one-pot method to immobilize ZnO nanoparticles (ZnO NPs) with RGO is demonstrated. � ZnO NPs function as an effective catalyzer for GO deoxygenation. � Theoretical calculation and XPS characterization techniques were first used to verify the reaction mechanism. � The ZnO/RGO composite showed good lithium ion storage and electromagnetic microwave absorption capability. A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene oxide Graphene Zinc oxide Lithium ion batteries Electromagnetic microwave absorption
A simple one-step method to immobilize zinc oxide nanoparticles (ZnO NPs) in reduced graphene oxide (RGO) supporting framework is reported in this work. Under mild hydrothermal condition, ZnO NPs function as an effective catalyzer for few-layered graphene oxide (GO) deoxygenation to construct an RGO protecting frame work and in the meantime ZnO NPs are immobilized between the RGO sheets. The synthesized ZnO/RGO composite not only delivered good lithium ion storage capability with a reversible capacity of 560 mA h⋅g 1 for 250 cycles under a current density of 500 mA g 1, but also exhibited good electromagnetic microwave ab sorption performance with the maximum reflection loss value of 67.13 dB when the thickness is 3.2 mm while the effective absorption bandwidth is 7.44 GHz with the thickness of 2.8 mm. The improved performances of the ZnO/RGO composite compared with bare ZnO NPs are ascribed to the synergistic effect from well-dispersed ZnO NPs and RGO framework, particularly, the formation of Zn–O–C bond between the two components brings in good immobilization between the two components, which was verified by both theoretical calculation and XPS characterization techniques. The method to prepared ZnO/RGO composite is facile, eco-friendly and effective, which has great potential to find wider applications for massive production.
1. Introduction ZnO is one of the most extensively studied semiconducting materials with a wide direct band gap of 3.37 eV and large excitation binding
energy of 60 meV [1]. And nano-sized ZnO materials have exhibited attractive electrical, mechanical, chemical and optical properties due to the surface and quantum confinement effects. Nano-sized ZnO materials are being considered to have great potential in the fields of electronics,
* Corresponding author. Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China. ** Corresponding author. Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China. *** Corresponding author. Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China. E-mail addresses: [email protected] (H. Cao), [email protected] (G. Wu), [email protected], [email protected] (B. Xu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.matchemphys.2020.122766 Received 25 October 2019; Received in revised form 15 January 2020; Accepted 4 February 2020 Available online 4 February 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
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for 250 cycles under a current density of 500 mA g 1 and good elec tromagnetic microwave absorption performance with the maximum reflection loss value of 67.13 dB when the thickness is 3.2 mm while the effective absorption bandwidth is 7.44 GHz with the thickness of 2.8 mm, respectively. The starting materials in this work are only GO aqueous suspension and commercially available ZnO NPs, the reaction temperature is 95 � C and the reaction time is 12 h. Therefore, compared with the reported ZnO/RGO composite synthesis methods, such as hydrothermal/solvothermal [12,13], chemical vapor deposition [14] and atomic layer deposition [15], the method in this work can be regarded as facile, eco-friendly and effective, which has great potential to find wider applications for large-scale production.
optics and photonics, antibacterial and photocatalytic etc. [2–4] The inherent agglomeration nature of nano-sized ZnO materials, a common problem for nano-materials due to the increased specific surface area, always requires further stabilization from supporting matrix. Graphene possesses attractive properties, such as large surface area, good elec tronic conductivity and high mechanical strength, which has been regarded as an ideal supporting additive for nano-sized active materials [5,6]. Reduced graphene oxide (RGO), synthesized from graphene oxide (GO) deoxygenation, offers good potential for cost-effective and large-scale production of RGO supported materials. Therefore, improved performances for ZnO/RGO composites compared with bare ZnO NPs (ZnO NPs) in various fields have been reported [7–10]. To explore simplified route to synthesize RGO stabilized ZnO NP composite with unique structure for lithium ion storage, our group has recently reported a two-step preparation method for a hierarchical ZnO/ RGO@RGO composite including a metal Zn reducing GO step and particularly a further ZnO NP catalyzed GO deoxygenation step [11]. However, deeper insight into the reaction mechanism between ZnO NP and GO sheets and further investigation of the ZnO and RGO composite for wider applications are still of significance. In this work, a one-pot synthesis method to immobilize commercially available ZnO NPs with RGO supporting framework is demonstrated. Compared with blank GO sample under mild aqueous condition, ZnO NPs play the critical role of catalyst for the acceleration of GO deoxygenation, during which GO sheets are reduced to RGO with ZnO NPs encapsulated and a Zn–O–C linkage bond forms between ZnO NP and RGO sheet. Theoretical calculation and XPS characterization techniques were employed to verify the above formation mechanism for the first time. Moreover, the lithium ion storage and electromagnetic microwave absorption proper ties of the as-obtained ZnO/RGO composite have been tested. As a result, the ZnO/RGO composite showed a reversible capacity of 560 mA h⋅g 1
2. Materials and methods 2.1. Sample preparation GO was prepared followed our reported method [16]. ZnO NPs (50 nm, 100 mg) were dispersed in de-ionized water (100 ml) under soni cation for 1 h. Then the prepared GO aqueous suspension (1.0 mg mL 1, 100 ml) was slowly added to the above ZnO suspension by a peristaltic pump under magnetic stirring. The reaction temperature was main tained at 95 � C for 12 h. Finally, the mixed suspension became trans parent and the black sediments (ZnO/RGO) was collected with a freeze-drier. The intermediate sample collected after reacting for 1 h was also collected (ZnO@GO) for comparison. 2.2. Characterization X-ray diffraction (XRD) measurement was conducted on a Rigaku Ultima IV X-ray diffractometer with Cu-Kα radiation source (λ ¼
Fig. 1. XRD patterns of sample GO, ZnO NPs, ZnO@GO and ZnO/RGO (A); survey XPS spectra of sample GO and ZnO/RGO (B); high-resolution XPS outline spectrum of C 1s for sample GO (black dash line) and deconvoluted spectrum of sample ZnO/RGO (C); high-resolution spectrum with peak fittings in the O 1s of sample ZnO/ RGO (D). 2
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Fig. 2. FESEM images of sample ZnO/RGO under different magnifications (A, B); TEM image of sample ZnO/RGO with an inset HRTEM image (C); EDS elemental mapping (D) and analysis (E) of sample ZnO/RGO.
0.15418 nm). X-ray photoelectron spectroscopy (XPS) was obtained from a PHI 5000 Versa Probe III spectrometer. The surface structures, morphologies and element mapping were characterized by a JEOL JSM7800F field-emission scanning electron microscope (FESEM) with an energy dispersive X-ray (EDS) accessory and a JEOL JEM-2100 Plus transmission electron microscope (TEM).
Zn12O12 nano-cage. The k-mesh sampling was set as (1 � 1 � 1) with Γ-centered. The structures of referred transition states were located by climbing images nudged elastic band (CI-NEB) [20,21] and dimmer methods [22]. Al the atoms were fully relaxed during the geometrical optimization of stationary points. To reduce the computational expense, the implicit solvation model [23,24] was used with the relative permittivity of 78.4 F/m to representing water solvent.
2.3. Theoretical calculation
2.4. Lithium ion storage
Spin-polarized calculations were carried out using density functional theory (DFT) on the Vienna Ab initio Simulation Package (VASP) [17, 18]. The electron exchange-correlation energy is performed using generalized gradient approximation functional with the Perdew-Burke-Ernzerh correction [19]. The energy-cutoffs for all sys tems were set to be 400 eV, the criteria for convergence of forces were set to be 0.02 eV Å 1. The thickness of vacuum was set to be 15 Å along the c axis and 6 Å along the a and b directions. The lattice parameter for the simulated cell was a ¼ b ¼ 26 Å, c ¼ 20 Å. The modelled GO mol ecules contain 90 carbon atoms, 27 oxygen atoms and 25 hydrogen atoms, including nine epoxy groups, two hydroxyl groups and nine carboxyl groups. The ZnO nanostructure was modelled by constructing a
The electrochemical testing of the as-prepared ZnO/RGO composite for lithium ion batteries were measured in the form of CR2016 coin-type half cells. The cells were assembled in an argon-filled glovebox (H2O, O2 < 0.01 ppm). The working electrode was constituted by ZnO/RGO composite, carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 8:1:1 on a copper disk. On the other end, a metallic Li disk was used as both counter and reference electrode. Between the two elec trodes, a microporous polyethylene membrane was placed as a sepa rator. The above components were immersed in an electrolyte with 1 M LiPF6 in a mixed solvent of ethylene carbonate (EC) and dimethyl car bonate (DMC) and ethyl methyl carbonate (EMC) with a volume ratio of 3
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1:1:1. Galvanostatic testing for the ZnO/RGO composite was conducted on a NEWARE Battery Testing System at room temperature with a fixed voltage range from 0.01 to 3.00 V (vs Li/Liþ). Cyclic voltammogram (CV) was performed on a CHI660E electrochemistry workstation with a scanning rate of 0.2 mV s 1 between 0.01 and 3.00 V at room temperature. 2.5. Electromagnetic microwave absorption The relative complex permittivity (εr ¼ ε0 -jε") and the relatively complex permeability (μr ¼ μ0 -jμ") of the as-prepared ZnO/RGO com posite with GO and ZnO NPs as comparison were investigated by the vector network analyzer (N5234A; Agilent, USA) with the frequency range between 2.0 and 18.0 GHz. Paraffin was selected as the substrate material, which accounted for 95% of the total mass. The mixture was compressed into a ring shape (Φout ¼ 7.0 mm, Φin ¼ 3.04 mm). 3. Results and discussion 3.1. Structure and morphology From the XRD patterns of the samples in Fig. 1A, it can be seen GO shows an evident characteristic diffraction peak at 11.4� two theta and the characteristic peaks for ZnO NPs match quite well with the standard PDF card. After reacting at 95 � C for 1 h by mixing GO aqueous sus pension with ZnO NPs, from the XRD pattern of sample ZnO@GO, it is observed the ZnO NPs still maintain in the system while the diffraction peak of GO slightly shifts to 12.1� two theta. The broad peak at about 23.9� two theta belonging to RGO (002) facet becomes more obvious, implying the oxygen-containing groups on the GO sheets are partially removed along with the recovery of corresponding sp2 carbon network. Previous work has demonstrated the partial deoxygenation of GO in water at the reaction temperature of 100 � C with a long reaction time up to 5 days [25]. In this work, after reacting for 12 h at 95 � C, only the characteristic diffraction peaks for ZnO and RGO can be found in the ZnO/RGO sample. The XRD testing result illustrates the ZnO/RGO sample is constituted by ZnO NPs and RGO supporting framework, and ZnO functions as an effective catalyst for GO deoxygenation. To further obtain the information of the surface composition changes of GO and investigate the function of ZnO NPs in the above reaction, Xray photoelectron spectroscopy (XPS) characterizing technique was employed. From the XPS spectrum of the GO sample in Fig. 1B, only O 1s and C 1s peaks can be clearly identified with the atomic percentages of 66.11 and 33.89, respectively. By contrast, from the XPS spectrum of the ZnO/RGO sample, strong Zn 2p, O 1s and C 1s peaks are detected with the corresponding atomic percentages of 6.00, 30.52 and 63.48. Therefore, it is estimated that the atomic ratios between C and O orig inating from the oxygen-containing functional groups increases from 2:1 (GO) to 2.6:1 (ZnO/RGO). Moreover, from the comparison of the highresolution C1s spectrum of GO sample (black dash line) and deconvo luted C1s spectrum of ZnO/RGO sample in Fig. 1C, it can be obvious to observe the significantly reduced intensity of the peak constituted by the oxygen-containing functional groups of GO at the higher binding energy after the reaction with the presence of ZnO NPs. These results confirm the catalytic role of the ZnO NPs in the deoxygenation process of GO sheets. In particular, from the O 1s spectrum with peak fittings of sample ZnO/RGO in Fig. 1D, the fitting peaks located at the binding energies of 531.3 and 533.0 eV can be attributed from the crystal structure of ZnO NPs [26]. Apart from the O 1s of ZnO NPs, the fitting peak at 533.0 eV – O group in RGO sheets. The third may also be attributed from the C– fitting peak at 532.2 eV between the two fitting peaks belonging to crystal ZnO NPs in sample ZnO/RGO can the explained by the linkage of Zn–O–C bond between RGO sheets and ZnO NPs, which matches with the fitting peak of C–O/C–O–Zn at 286.9 eV in the C 1s spectrum in Fig. 1C. The existence of Zn–O–C bond in the ZnO/RGO sample
Fig. 3. DFT results for the reduction of GO with the presence of ZnO NPs: top view and side view of GO used during DFT calculations (A); potential energy profile for the reaction of GO and water molecule (B); potential energy profile for the self-decomposition of GO (C); potential energy profile for the reduction of GO by ZnO quantum dots (D). The atoms are colored: Zn in blue, O in red, C in gray, and H in white.
contributes to the enhancement of the immobilization of ZnO NPs in the RGO supporting architecture. The morphology information of the ZnO/RGO sample was obtained by SEM and TEM characterization protocols. From the low-magnified SEM image in Fig. 2A, it can be clearly seen that nano-sized white ZnO NPs without heavy aggregation are coated by large-sized thin RGO sheets. And in the high-magnified SEM image of sample ZnO/RGO in Fig. 2B, it can be notified that the nano-sized ZnO is small cluster constituted by ZnO NPs around 50 nm, particularly, there is space be tween the RGO coating layers and inside ZnO crystals. The TEM image in Fig. 2C matches well with the SEM images, indicating the good immo bilization effect of RGO framework towards ZnO NPs. From the inset HRTEM image of the ZnO/RGO sample in Fig. 3C, the ZnO NPs are anchored on the few-layered RGO sheets and the inside ZnO crystal shows an interplanar distance about 0.26 nm corresponding to the (002) direction. From the EDS mapping result of the ZnO/RGO sample in Fig. 2D, three elements are detected and they are Zn, C and O, and this result is in good agreement with the former XRD and XPS results in Fig. 1A and B. It is well worthy to point out that most of the O element overlaps with that of Zn element while C element distributes on the rest location from the EDS mapping. Fig. 2E shows the EDS analysis result of the ZnO/RGO 4
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sample and the weight percentages of the C, O and Zn elements are 41.37, 16.00 and 42.63, respectively, corresponding to the atomic per centages of 67.55, 19.60 and 12.85. Therefore, the atomic ratios be tween C and O for RGO by excluding the influence from ZnO is estimated to be 10:1. The increased ratio between C and O based on EDS analysis (10:1) than that based on XPS analysis result (2.6:1) in Fig. 1B indicates more thorough reduction of the inside RGO sheets than the surface ones due to the fact that most ZnO NPs are immobilized inside the RGO supporting architecture.
The geometry of stable point Z-IM4 suggests that exposed Zn atoms in ZnO NPs tend to join with epoxy oxygen atoms to fulfill the saturation. Compared to the decomposition route, the further combination reaction has thermodynamic priority. This theoretical calculation result matches quite well the XPS characterizing result in Fig. 1C and D. By analyzing the potential energy profiles of above the deoxygen ation routes of GO, it can be concluded that bimolecular reduction be tween ZnO NPs and GO is of high priority in terms of thermodynamics and kinetics due to the lowest activation energy and slightly exother micity. The results indicate that the presence of ZnO NPs has a catalytic ability towards the removal of the oxygen-containing functional groups on GO sheets. The exposed Zn atoms from ZnO NPs prefer to associate with epoxy oxygen atoms from GO sheets rather than directly abstracting epoxy oxygen atoms. Along with the formation of new Zn–O bonds, an interaction system of Zn–O–C is formed to ensure a good structure stability of the ZnO/RGO composite.
3.2. Reaction mechanisms DFT simulation protocol is used to further reveal the function of ZnO NPs on the deoxygenation of the oxygen-containing functional groups from GO sheets in the reaction. As shown in Fig. 3A, a GO monolayer decorated by epoxy and carboxyl groups are selected for simulation. Results of the charge density distribution implies the epoxy group, the hydroxyl group and the carboxyl group are electron-withdrawing, and the oxygen atoms exhibit electronegative properties by abstracting 0.80–1.15 electrons from surrounded carbon atoms. In the ZnO NPs, the surface Zn2þ ions are unsaturated and exhibit positive electricity by 1.14. Therefore, the dangling bonds of Zn can be saturated by interac tion with the active oxygen especially the epoxy of GO. Here, this paper focused on the interface interaction between ZnO NPs and GO, In the following simulation, three possible pathways to convert the sp3 hy bridized carbon atoms to sp2 hybridized ones in the reaction are simu lated and discussed. The effect of H2O molecules for the deoxygenation of GO is first investigated in the reaction. As shown in Fig. 3B, H2O can stabilize GO sheets in the aqueous system via forming hydrogen bond with the oxy gen atoms from GO. However, the potential energy profile clearly shows that direct the reduction of epoxy group (W-IM1) to hydroxyl group (WIM2) by water is difficult to proceed due to the high endergonic prop erties in this process. Therefore, H2O molecules in this system play the role of stabilizer for GO rather than reducing agent. The possible route for the deoxygenation of GO is that GO may un dergo self-decomposition via removal of oxygen molecules. As shown in Fig. 3C, epoxy functional groups on GO sheets may undergo rear rangement with adjacent ones to form a four-member ring intermediate IM1. In the transition state TS1, the outer C–O bonds are elongated by ~0.60 Å, and the bond length of adjacent O atoms becomes 1.979 Å. The formation of O–O bond is quite endothermic, which has to overcome a high energy barrier of 1.74 eV. The C–O–O–C four-member ring struc ture is unstable and inclines to relax via decomposition of oxygen molecule with an energy barrier of 0.72 eV. Finally, the impact of ZnO NPs on the deoxygenation of GO is investigated by simulating the association process between GO and ZnO nanostructure. As shown in Fig. 3D, ZnO NPs have both Zn and O atoms exposed, where the unsaturated exposed Zn atoms are more likely to react with the active oxygen in GO. Therefore, Z-IM1 is generated with the formation of a new Zn–O bond, and the ZnO nano-cage is anchored on the surface of GO sheets with an exothermic energy of 0.29 eV. Since the bond length of the new formed Zn–O bond is 2.174 Å, 0.20 Å longer than that of original ZnO nanostructures (1.970 Å), the Z-IM1 may decompose to Z-IM2 by the cleavage of epoxy O and GO. This process encounters an energy barrier of 1.55 eV, which is much lower than that in the second GO self-decomposition speculation in Fig. 3C (GO→TS1→IM1). After the removal of ZnO species, the hybridization of epoxy-linked carbon atoms changes from sp3 to sp2. The results provide the evidence that ZnO NPs function as a catalyst to accelerate the GO deoxygenation process by significantly reducing the energy barrier. More importantly, further simulation result shows that instead of detaching from GO sheets, the adsorbed ZnO species are prone to continuously interact with GO by forming a second Zn–O–C bond. This process carries out by the exposed Zn atom associating with neighboring epoxy oxygen atom and has to absorb an energy of 0.23 eV.
3.3. Lithium ion storage performances Compared with the current commercialized graphite anode for lithium ion batteries, ZnO has the advantage of higher theoretical ca pacity while suffers from heavy volume change and low electron con ductivity [27,28]. Constructing RGO supporting architecture immobilized ZnO NP composite anode have been designed and proven to be effective due to the shortened diffusion pathway of Liþ, buffered volume changes and improved overall electron conductivity for the composite electrode [29]. The lithium ion storage performance of the ZnO/RGO composite is investigated. Fig. 4A shows that the lithium ion storage behavior of the ZnO/RGO composite electrode characterized by cyclic voltammetry technique between 0.01 and 3.00 V (vs Li/Liþ) under the scanning rate of 0.2 mV s 1. From the cathodic scanning curves, two reduction peaks at about 0.70 and 0.22 V can be seen, which are ascribed to the reduction of ZnO to Zn and the following formation of LixZn alloy, respectively. It is well worthy to point out that a strong peak at about 0.22 V was clearly observed in the first cathodic process, the intensity of which is signifi cantly deceased. The reductive peak can be ascribed to the formation and growth of the solid electrolyte interphase (SEI) apart from the alloying reaction [30]. This result also indicates most SEI layer forma tion process takes place in the first discharge step. In the remaining cathodic scanning curves, the two reduction peaks located at around 0.70 and 0.22 V becomes more stable and overlapped. By contrast, the anodic scanning curves almost maintains the shape and a series of peaks located at 0.29, 0.37, 0.56, 0.69, and 1.32 V can be detected, corre sponding to the dealloying reactions of the LixZn alloy [31]. It is worthy mentioned that the broad last peak at about 1.32 V can be interpreted by the conversion type reaction between Zn and Li2O to regenerate ZnO crystals [30,32]. Therefore, the lithium storage properties of ZnO based electrode are constituted by both alloy and conversion reactions, which is consisted with the literatures [33,34]. The overall lithium ion storage behavior of the ZnO/RGO electrode follows the below equations: ZnO þ 2Liþ þ 2e ↔ Zn þ Li2O
(1)
Zn þ xLiþ þ xe ↔ LixZn
(2)
Fig. 4B shows charge and discharge curves of the initial two and the 100th cycles for the ZnO/RGO composite electrode at a current density of 500 mA g 1 between 0.01 and 3.00 V. from the first discharge curve, two apparent voltage plateaus can be seen at around 0.25 V and 0.70 V, which are corresponding to the reduction of ZnO to Zn and formation of LixZn alloy, respectively. And from the first charge curve, minor voltage plateaus between 0.20 and 0.70 V and an obvious voltage at about 1.3 V can be observed. These results are consisted with the lithium ion storage behavior from the CV curves in Fig. 3A. The curves for 2nd and 100th are similar in terms of shape except for the difference of the specific ca pacities. The ZnO/RGO composite electrode delivers an initial discharge 5
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Fig. 4. CV curves for the initial 5 cycles (A) and charge-discharge voltage profile (B) of the ZnO/RGO composite electrode.
and charge specific capacities (calculated based on the mass of the ZnO/ RGO composite) are about 1205 and 692 mA h⋅g 1, respectively, with a coulombic efficiency of 57.4%. The low coulombic efficiency for the initial cycle is mainly ascribed to the formation of SEI layer due to electrolyte decomposition and side reactions [35,36]. For the second cycle, specific capacities for the discharge and charge processes are about 718 and 596 mA h⋅g 1, respectively. And the corresponding coulombic efficiency is calculated to be 83%. After 100 cycles, the ZnO/RGO electrode delivers the discharge and charge specific capacities of about 564 and 556 mA h⋅g 1, respectively, with a coulombic effi ciency of 98.6%. The gradually increased coulombic efficiency indicates the successful activation process taking place.
Fig. 5A compares the cycling performances of the ZnO/RGO com posite, RGO prepared by thermal reduction of GO and bare ZnO elec trodes at current densities 500 mA g 1. For the ZnO/RGO composite electrode, it exhibits initial discharge and charge capacities of about 1021 and 625 mA h⋅g 1, respectively. Since then, the reversible capacity gradually decreases to 460 mA h⋅g 1 in the following 40 cycles, origi nating from the gradual activation of the ZnO/RGO composite electrode [26]. Then the reversible capacity of the ZnO/RGO composite electrode displays a stable increasing trend, which is ascribed to more ZnO NPs taking part in the electrochemical reaction due to the excellent sup porting effect from the RGO architecture for the composite. The reversible capacity reaches approximately 560 mA h⋅g 1 after 250
Fig. 5. Cycling performance of the ZnO/RGO composite with RGO and ZnO electrodes as comparison (A) and rate capability of the ZnO/RGO composite elec trode (B). 6
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Fig. 6. 3D representation of RL values versus frequency and thickness for sample GO (A), ZnO (B) and ZnO/RGO (C), respectively; theoretical RL curres versus frequency and thickness of sample ZnO/RGO (D).
continuous cycles. By contrast, for the RGO electrode, it shows lower initial discharge and charge capacity of about 819 and 576 mA h⋅g 1, respectively. The lithium ion storage for RGO electrode follows a double layer adsorption mechanism (C6 þ 2Liþ þ 2e ↔ Li2C6), corresponding to a theoretical capacity of about 740 mA h⋅g 1 [6,37–39]. However, the RGO electrode could not provide stable voltage output and no obvious peak could be identified for RGO in the CV curves of the ZnO/RGO electrode in Fig. 4A. And more rapid reversible capacity fading for this electrode in the following 40 cycles have been observed. After 250 cy cles, the reversible capacity declines to 365 mA h⋅g 1. For the ZnO electrode, although it delivers a high initial discharge capacity of about 1122 mA h⋅g 1, yet the initial reversible capacity for this electrode is only 399 mA h⋅g 1. Even worse, the reversible capacity of the ZnO electrode maintains a sharp declining trend in the remaining cycles, and reaches approximately 100 mA h⋅g 1 after 10 cycles. After 250 cycles, the reversible capacity maintains only 44 mA h⋅g 1. This is probably ascribed to the heavy aggregation of nano-sized ZnO and subsequently losing electrochemical contact with the current collector. The improved cycling performance for the ZnO/RGO composite electrode compared with RGO electrode and bare ZnO electrode is the result of the good synergistic effect from both the inside ZnO NPs and outside the RGO supporting framework, the good immobilization from RGO ensures the stable electrochemical performance of the active ZnO NPs in the ZnO/RGO composite electrode for a long cycle life. The rate performance of the ZnO/RGO composite electrode was also investigated by cycling at stepped current densities between 0.2 and 20.0 A g 1. As shown in Fig. 5B, the ZnO/RGO composite electrode delivers average reversible discharge capacities of 640, 510, 400, 320, 260, 170 and 110 mA h⋅g 1 at 0.2, 0.5, 1.0, 2.0, 5.0, 10.0 and 20.0 A g 1, respectively. When the current density is returned to the initial value of 200 mA g 1, the reversible capacity of the ZnO/RGO composite elec trode reverts to 540 mA h⋅g 1. The rate testing result indicates that the ZnO/RGO composite electrode has good battery performance under a wide range of current densities.
3.4. Electromagnetic microwave absorption performances Besides the application in lithium ion batteries, ZnO/graphene composites have been studied for the improved electromagnetic wave absorbing properties than single component due to the polarization at the graphene/ZnO interface [40,41]. However, graphene supported composites are generally facing the problem of poor dispersion resulting from the poor structure stability of the corresponding composite mate rials. The electromagnetic microwave absorption capability of the ZnO/RGO composite is investigated. To investigate the microwave absorption properties of the ZnO/RGO composite sample, the reflection loss (RL) values of all samples were obtained according to the following equations [42,43]: Where Zin and Z0 represent the input impedance of the metalsupported electromagnetic wave absorbing layer and the impedance of the free space, respectively; f stands for the microwave frequency; d is the thickness corresponding to the sample; c is the velocity of electro magnetic waves in free space; εr is the relative dielectric constant, and μr corresponds to magnetic permeability. The calculated RL values of bare ZnO, GO and ZnO/RGO composite in the frequency range from 2 to 18 GHz are shown in Fig. 6. The maximum RL value for GO is 11.09 dB at 5.68 GHz with the thickness is 7.8 mm in Fig. 6A and the maximum RL value for ZnO is 5.76 dB when the thickness is 8.4 mm in Fig. 6B. By contrast, the ZnO/RGO composite delivers the best microwave absorption performance. As can be seen from Fig. 6C and D, the maximum RL values can reach 67.13 dB when the thickness is 3.2 mm while the effective absorption bandwidth is 7.44 GHz with the thickness is 2.8 mm for the ZnO/RGO composite. To further understand mechanism of the outstanding absorption performance the ZnO/RGO composite absorber, the complex permit tivity (εr¼ ε0 -jε") and complex permeability (μr¼ μ0 -jμ") were tested with the vector network analyzer in the range of 2–18 GHz. In general, the real part of the complex permittivity and complex permeability are used to represent the ability to store microwave energy while the imaginary 7
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Fig. 7. Electromagnetic parameters of the ZnO/RGO composite with ZnO NPs and GO for comparison: the real/imaginary of permittivity (A and B); the real/ imaginary of permeability (C and D); dielectric loss tangent (E); magnetic loss tangent (F).
part stands for the dissipation ability of microwave energy [44]. Fig. 7A compares the real part (ε0 ) of the dielectric constant of three samples. It can be clearly seen the epsilon of the ZnO/RGO composite ranging from 7.48 to 3.95 in the 2–18 GHz range, which is remarkably higher than that of the ZnO and GO sample. Similar result can be observed in Fig. 7B concerning the imaginary part (ε") of the dielectric constant of the three samples. The reason for this phenomenon is that there is a synergistic effect between the RGO supporting framework and the ZnO NPs. Furthermore, the polarization phenomenon between different interfaces
is also conducive to the improvement of microwave dissipation ability [45]. Therefore, ZnO/RGO composite has the highest ε" value. Fig. 7C and D shows the real part (μ0 ) and imaginary part (μ") of the permeability of three samples in the frequency range of 2–18 GHz, respectively. Compared with the ε0 and ε" values at the same frequency, the μ0 and μ" values of ZnO/RGO composite materials are relatively small, which in dicates that the magnetic loss capacity of the composite materials is significantly limited. The fluctuation of samples in the vicinity of 6 GHz and 12–18 GHz might be attributed to natural resonance and eddy
Fig. 8. (A) Impedance matching and (B) attenuation constants (α) of ZnO, GO and ZnO/RGO composite. 8
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Fig. 9. Typical Cole-Cole models ZnO/RGO composite.
current loss. Fig. 7E and F shows the relationship between the dielectric loss angle tangent (tanδε ¼ ε00 /ε0 ) and the magnetic loss angle tangent (tanδμ ¼ μ00 /μ0 ) and frequency of the three samples. In Fig. 7E, ZnO/RGO composite shows the highest dielectric loss capability over the entire frequency range compared to the other two samples. This indicates that more interfacial polarization at the interface can be expected in the ZnO/RGO composite, thus enhancing the dielectric loss. However, in Fig. 7F, there is no obvious change at other frequencies except for the resonance peak at 6 GHz and 14–18 GHz. The impedance matching is another important criterion for measuring microwave absorbing materials. An impedance matching image of three samples can be drawn according to formula (1), and the result is shown in Fig. 8A. In the frequency range of 2–18 GHz, the value of |Zin/Z0| of ZnO/RGO composite is closer to 1. This indicates that ZnO/ RGO composite has better impedance matching characteristics [46]. It is easy to understand that the impedance ratio of the impedance of the absorber to the free space is 1, indicating that the incident electro magnetic wave is absorbed by the absorber, and that there is almost no reflected electromagnetic wave on the surface of the absorber. Following the transmission line theory, the electromagnetic attenu ation constant α of the microwave absorbing material can be calculated by the following formula [47]: The attenuation constant α is one of the parameters used to indicate the ability of the absorber to attenuate electromagnetic waves. The larger dielectric losses and magnetic losses can provide a larger α value. It can be clearly seen from Fig. 8B that as the frequency increases, the value of the decay constant α of the ZnO/RGO composite increases from 29.1 to 272.28, which is much larger than the other two samples. And due to the ZnO/RGO composites have suitable impedance matching characteristics. Therefore, ZnO/RGO composites have excellent micro wave absorption properties. The complex permittivity of microwave absorbing materials is related to debye relaxation. According to debye dipole relaxation, the complex dielectric constant is calculated by the following formula [48]: τ and f represent the time and frequency of relaxation, respectively, and εs and ε∞ are respectively used to indicate the dielectric constant and optical permittivity that are static at high frequency limits. There fore, further calculations can be obtained from the following formula [49]: The curves ε0 and ε’’ obtained from the above formula are a semi circular shape, collectively referred to as a cole-cole semicircle. The cole-cole semicircle model of ZnO/RGO composite material is shown in Fig. 9. Many cole-cole semicircles can be clearly shown, suggesting that there may be many processes of dielectric relaxation due to the lamellar structure of the composite material. In general, due to the layered structure of ZnO/RGO composites, many ZnO/RGO and ZnO–ZnO
Fig. 10. The dependence of the RL curve and the matching thickness (tm) on the matching frequency (fm) of the ZnO/RGO composite.
interfaces are introduced, plus the dielectric relaxation phenomenon caused by induced charge delay. These two conditions lead to the accumulation of a large amount of charge on the interface, which leads to the polarization of the interface. At the same time, the lamellar structure of the composite material can provide unsaturated bonds, which bring about dipole polarization. It can be seen from the model that the absorption summit shows a low-frequency movement trend with the increase of layer thickness. This tendency can be explained by a quarter wavelength attenuation, which is shown as below [50–52]: Where tm is the wavelength of the electromagnetic wave, c is the speed of light in free space, fm is the corresponding peak frequency, n is an integer (n ¼ 1, 3, 5 …). Based on the quarter-wave principle, Fig. 10 established the model of RL curve and matching thickness and matching frequency of ZnO/RGO composite material. It can be clearly seen that the yellow pentacle represents the correspondence between the match ing thickness and the absorption peak frequency. Obviously, all yellow marks can be accurately placed around the 1/4 λ curve, which repre sents the ZnO/RGO composite material conforms to the principle of 1/4 wavelength. In general, the structure, shape and properties of the absorber are related to its impedance matching characteristics and dielectric losses, which act together on the microwave absorption performance of the absorber. Fig. 11 shows the microwave absorption mechanism of ZnO/ RGO composite material. Firstly, when ZnO/RGO composite surface receives the incident microwave, only a small amount of microwave is reflected due to the good impedance matching characteristics of ZnO/ RGO composite and most microwave enters the composite. Secondly, the abundant folds and gaps in the ZnO/RGO composite due to the synergetic functions between RGO and ZnO NPs during the synthesis 9
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Fig. 11. Schematic diagram of microwave absorption mechanism for ZnO/RGO composite.
based on effective absorption bandwidth and has a high reflection loss value. It also shows that the absorber is an ideal candidate material for microwave absorption under the conditions of large absorption band width, strong absorption capacity and thin thickness.
Table 1 Microwave absorption properties of ZnO/RGO composites and other related composites. Sample
ZnO/RGO RGO/Fe3O4/ ZnO RGO/ Co@Fe@Cu N-RGA/Ni (600) ZnO–NiC/RGO TiO2/Ti3C2Tx/ RGO RGO/ MWCNTs/ ZnFe2O4
Mass ratio (wt%) 5 33.3 5 10 40 10 50
RLmin (dB)
d (mm)
Frequency range (RL < 10dB, GHz)
Refs
This work [53] [54] [55] [56] [57] [58]
67.13 57
3.2 2.0
7.44 5.0
49.71 60.8 59.3 65.3 22.2
2.1 2.1 2.05 2.5 1.0
6.0 5.1 5.6 4.3 2.3
4. Conclusions In conclusion, a ZnO/RGO composite with ZnO NPs immobilized by RGO supporting architecture is successfully prepared by a ZnO cata lyzing GO reduction process under mild aqueous condition. XPS char acterization result illustrates the oxygen-containing functionally groups are significantly removed for GO and a Zn–O–C linkage bond forms between ZnO NP and RGO sheet in the ZnO/RGO composite. Theoretical calculation further verifies the above process for the first time. The ZnO/ RGO composite showed a reversible capacity of 560 mA h⋅g 1 for 250 cycles under a current density of 500 mA g 1 as anode for lithium ion batteries and good electromagnetic microwave absorption performance with the maximum reflection loss value of 67.13 dB when the thick ness is 3.2 mm while the effective absorption bandwidth is 7.44 GHz with the thickness is 2.8 mm, respectively. The starting materials in this work are only GO aqueous suspension and commercially available ZnO NPs, the reaction temperature is 95 � C and the reaction time is 12 h. Therefore, this work not only opens a new horizon for GO deoxygen ation, but also provides a facile, eco-friendly and scalable approach to preparing ZnO/RGO composite materials for a wide application.
result in multiple reflections and scattering for the microwave. Thirdly, uneven distribution of electrons probably takes place due to the differ ence of the conductivity between ZnO NPs and RGO supporting frame work, which further leads to the accumulation of bound charges between the interfaces and the related interface polarization. Finally, the defects on the surface of ZnO/RGO composite will also break the balance of charge distribution, causing uneven charge distribution, leading to dipole polarization and further enhancing the microwave absorption performance of the composite material. Table 1 compares the microwave absorption performances of ZnO/ RGO composite materials with the reported related composites. The ZnO/RGO composite prepared in this study have strong competitiveness no matter in terms of mass load, minimum reflection loss and effective absorption bandwidth. It can be clearly seen from the table that the composite material synthesized in this study far exceeds other materials
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements [27]
This work was financially supported by China Postdoctoral Science Foundation (No. 2017M612194 and 2017M610409), Natural Science Foundation of Shandong Province (No.ZR2019YQ24), the Qingchuang Talents Induction Program of Shandong Higher Education Institution (Research and Innovation Team of Structural-Functional Polymer Composites), the Thousand Talents Plan, the World-Class University and Discipline, the Taishan Scholar’s Advantageous and Distinctive Disci pline Program and the world-Class Discipline Program of Shandong Province.
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Immobilization of zinc oxide nanoparticles on graphene sheets for lithium ion storage and electromagnetic microwave absorption Shouchun Bao a, 1, Tianqi Hou a, 1, Qingke Tan a, Xiangli Kong a, Haijie Cao a, b, **, Maoxia He c, Guanglei Wu a, b, ***, Binghui Xu a, b, * a b c
Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China State Key Laboratory of Bio-Fibers and Eco-Textiles, Qingdao University, Qingdao, 266071, China Environment Research Institute, Shandong University, Qingdao, 266237, China
H I G H L I G H T S
� A one-pot method to immobilize ZnO nanoparticles (ZnO NPs) with RGO is demonstrated. � ZnO NPs function as an effective catalyzer for GO deoxygenation. � Theoretical calculation and XPS characterization techniques were first used to verify the reaction mechanism. � The ZnO/RGO composite showed good lithium ion storage and electromagnetic microwave absorption capability. A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene oxide Graphene Zinc oxide Lithium ion batteries Electromagnetic microwave absorption
A simple one-step method to immobilize zinc oxide nanoparticles (ZnO NPs) in reduced graphene oxide (RGO) supporting framework is reported in this work. Under mild hydrothermal condition, ZnO NPs function as an effective catalyzer for few-layered graphene oxide (GO) deoxygenation to construct an RGO protecting frame work and in the meantime ZnO NPs are immobilized between the RGO sheets. The synthesized ZnO/RGO composite not only delivered good lithium ion storage capability with a reversible capacity of 560 mA h⋅g 1 for 250 cycles under a current density of 500 mA g 1, but also exhibited good electromagnetic microwave ab sorption performance with the maximum reflection loss value of 67.13 dB when the thickness is 3.2 mm while the effective absorption bandwidth is 7.44 GHz with the thickness of 2.8 mm. The improved performances of the ZnO/RGO composite compared with bare ZnO NPs are ascribed to the synergistic effect from well-dispersed ZnO NPs and RGO framework, particularly, the formation of Zn–O–C bond between the two components brings in good immobilization between the two components, which was verified by both theoretical calculation and XPS characterization techniques. The method to prepared ZnO/RGO composite is facile, eco-friendly and effective, which has great potential to find wider applications for massive production.
1. Introduction ZnO is one of the most extensively studied semiconducting materials with a wide direct band gap of 3.37 eV and large excitation binding
energy of 60 meV [1]. And nano-sized ZnO materials have exhibited attractive electrical, mechanical, chemical and optical properties due to the surface and quantum confinement effects. Nano-sized ZnO materials are being considered to have great potential in the fields of electronics,
* Corresponding author. Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China. ** Corresponding author. Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China. *** Corresponding author. Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China. E-mail addresses: [email protected] (H. Cao), [email protected] (G. Wu), [email protected], [email protected] (B. Xu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.matchemphys.2020.122766 Received 25 October 2019; Received in revised form 15 January 2020; Accepted 4 February 2020 Available online 4 February 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
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for 250 cycles under a current density of 500 mA g 1 and good elec tromagnetic microwave absorption performance with the maximum reflection loss value of 67.13 dB when the thickness is 3.2 mm while the effective absorption bandwidth is 7.44 GHz with the thickness of 2.8 mm, respectively. The starting materials in this work are only GO aqueous suspension and commercially available ZnO NPs, the reaction temperature is 95 � C and the reaction time is 12 h. Therefore, compared with the reported ZnO/RGO composite synthesis methods, such as hydrothermal/solvothermal [12,13], chemical vapor deposition [14] and atomic layer deposition [15], the method in this work can be regarded as facile, eco-friendly and effective, which has great potential to find wider applications for large-scale production.
optics and photonics, antibacterial and photocatalytic etc. [2–4] The inherent agglomeration nature of nano-sized ZnO materials, a common problem for nano-materials due to the increased specific surface area, always requires further stabilization from supporting matrix. Graphene possesses attractive properties, such as large surface area, good elec tronic conductivity and high mechanical strength, which has been regarded as an ideal supporting additive for nano-sized active materials [5,6]. Reduced graphene oxide (RGO), synthesized from graphene oxide (GO) deoxygenation, offers good potential for cost-effective and large-scale production of RGO supported materials. Therefore, improved performances for ZnO/RGO composites compared with bare ZnO NPs (ZnO NPs) in various fields have been reported [7–10]. To explore simplified route to synthesize RGO stabilized ZnO NP composite with unique structure for lithium ion storage, our group has recently reported a two-step preparation method for a hierarchical ZnO/ RGO@RGO composite including a metal Zn reducing GO step and particularly a further ZnO NP catalyzed GO deoxygenation step [11]. However, deeper insight into the reaction mechanism between ZnO NP and GO sheets and further investigation of the ZnO and RGO composite for wider applications are still of significance. In this work, a one-pot synthesis method to immobilize commercially available ZnO NPs with RGO supporting framework is demonstrated. Compared with blank GO sample under mild aqueous condition, ZnO NPs play the critical role of catalyst for the acceleration of GO deoxygenation, during which GO sheets are reduced to RGO with ZnO NPs encapsulated and a Zn–O–C linkage bond forms between ZnO NP and RGO sheet. Theoretical calculation and XPS characterization techniques were employed to verify the above formation mechanism for the first time. Moreover, the lithium ion storage and electromagnetic microwave absorption proper ties of the as-obtained ZnO/RGO composite have been tested. As a result, the ZnO/RGO composite showed a reversible capacity of 560 mA h⋅g 1
2. Materials and methods 2.1. Sample preparation GO was prepared followed our reported method [16]. ZnO NPs (50 nm, 100 mg) were dispersed in de-ionized water (100 ml) under soni cation for 1 h. Then the prepared GO aqueous suspension (1.0 mg mL 1, 100 ml) was slowly added to the above ZnO suspension by a peristaltic pump under magnetic stirring. The reaction temperature was main tained at 95 � C for 12 h. Finally, the mixed suspension became trans parent and the black sediments (ZnO/RGO) was collected with a freeze-drier. The intermediate sample collected after reacting for 1 h was also collected (ZnO@GO) for comparison. 2.2. Characterization X-ray diffraction (XRD) measurement was conducted on a Rigaku Ultima IV X-ray diffractometer with Cu-Kα radiation source (λ ¼
Fig. 1. XRD patterns of sample GO, ZnO NPs, ZnO@GO and ZnO/RGO (A); survey XPS spectra of sample GO and ZnO/RGO (B); high-resolution XPS outline spectrum of C 1s for sample GO (black dash line) and deconvoluted spectrum of sample ZnO/RGO (C); high-resolution spectrum with peak fittings in the O 1s of sample ZnO/ RGO (D). 2
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Fig. 2. FESEM images of sample ZnO/RGO under different magnifications (A, B); TEM image of sample ZnO/RGO with an inset HRTEM image (C); EDS elemental mapping (D) and analysis (E) of sample ZnO/RGO.
0.15418 nm). X-ray photoelectron spectroscopy (XPS) was obtained from a PHI 5000 Versa Probe III spectrometer. The surface structures, morphologies and element mapping were characterized by a JEOL JSM7800F field-emission scanning electron microscope (FESEM) with an energy dispersive X-ray (EDS) accessory and a JEOL JEM-2100 Plus transmission electron microscope (TEM).
Zn12O12 nano-cage. The k-mesh sampling was set as (1 � 1 � 1) with Γ-centered. The structures of referred transition states were located by climbing images nudged elastic band (CI-NEB) [20,21] and dimmer methods [22]. Al the atoms were fully relaxed during the geometrical optimization of stationary points. To reduce the computational expense, the implicit solvation model [23,24] was used with the relative permittivity of 78.4 F/m to representing water solvent.
2.3. Theoretical calculation
2.4. Lithium ion storage
Spin-polarized calculations were carried out using density functional theory (DFT) on the Vienna Ab initio Simulation Package (VASP) [17, 18]. The electron exchange-correlation energy is performed using generalized gradient approximation functional with the Perdew-Burke-Ernzerh correction [19]. The energy-cutoffs for all sys tems were set to be 400 eV, the criteria for convergence of forces were set to be 0.02 eV Å 1. The thickness of vacuum was set to be 15 Å along the c axis and 6 Å along the a and b directions. The lattice parameter for the simulated cell was a ¼ b ¼ 26 Å, c ¼ 20 Å. The modelled GO mol ecules contain 90 carbon atoms, 27 oxygen atoms and 25 hydrogen atoms, including nine epoxy groups, two hydroxyl groups and nine carboxyl groups. The ZnO nanostructure was modelled by constructing a
The electrochemical testing of the as-prepared ZnO/RGO composite for lithium ion batteries were measured in the form of CR2016 coin-type half cells. The cells were assembled in an argon-filled glovebox (H2O, O2 < 0.01 ppm). The working electrode was constituted by ZnO/RGO composite, carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 8:1:1 on a copper disk. On the other end, a metallic Li disk was used as both counter and reference electrode. Between the two elec trodes, a microporous polyethylene membrane was placed as a sepa rator. The above components were immersed in an electrolyte with 1 M LiPF6 in a mixed solvent of ethylene carbonate (EC) and dimethyl car bonate (DMC) and ethyl methyl carbonate (EMC) with a volume ratio of 3
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1:1:1. Galvanostatic testing for the ZnO/RGO composite was conducted on a NEWARE Battery Testing System at room temperature with a fixed voltage range from 0.01 to 3.00 V (vs Li/Liþ). Cyclic voltammogram (CV) was performed on a CHI660E electrochemistry workstation with a scanning rate of 0.2 mV s 1 between 0.01 and 3.00 V at room temperature. 2.5. Electromagnetic microwave absorption The relative complex permittivity (εr ¼ ε0 -jε") and the relatively complex permeability (μr ¼ μ0 -jμ") of the as-prepared ZnO/RGO com posite with GO and ZnO NPs as comparison were investigated by the vector network analyzer (N5234A; Agilent, USA) with the frequency range between 2.0 and 18.0 GHz. Paraffin was selected as the substrate material, which accounted for 95% of the total mass. The mixture was compressed into a ring shape (Φout ¼ 7.0 mm, Φin ¼ 3.04 mm). 3. Results and discussion 3.1. Structure and morphology From the XRD patterns of the samples in Fig. 1A, it can be seen GO shows an evident characteristic diffraction peak at 11.4� two theta and the characteristic peaks for ZnO NPs match quite well with the standard PDF card. After reacting at 95 � C for 1 h by mixing GO aqueous sus pension with ZnO NPs, from the XRD pattern of sample ZnO@GO, it is observed the ZnO NPs still maintain in the system while the diffraction peak of GO slightly shifts to 12.1� two theta. The broad peak at about 23.9� two theta belonging to RGO (002) facet becomes more obvious, implying the oxygen-containing groups on the GO sheets are partially removed along with the recovery of corresponding sp2 carbon network. Previous work has demonstrated the partial deoxygenation of GO in water at the reaction temperature of 100 � C with a long reaction time up to 5 days [25]. In this work, after reacting for 12 h at 95 � C, only the characteristic diffraction peaks for ZnO and RGO can be found in the ZnO/RGO sample. The XRD testing result illustrates the ZnO/RGO sample is constituted by ZnO NPs and RGO supporting framework, and ZnO functions as an effective catalyst for GO deoxygenation. To further obtain the information of the surface composition changes of GO and investigate the function of ZnO NPs in the above reaction, Xray photoelectron spectroscopy (XPS) characterizing technique was employed. From the XPS spectrum of the GO sample in Fig. 1B, only O 1s and C 1s peaks can be clearly identified with the atomic percentages of 66.11 and 33.89, respectively. By contrast, from the XPS spectrum of the ZnO/RGO sample, strong Zn 2p, O 1s and C 1s peaks are detected with the corresponding atomic percentages of 6.00, 30.52 and 63.48. Therefore, it is estimated that the atomic ratios between C and O orig inating from the oxygen-containing functional groups increases from 2:1 (GO) to 2.6:1 (ZnO/RGO). Moreover, from the comparison of the highresolution C1s spectrum of GO sample (black dash line) and deconvo luted C1s spectrum of ZnO/RGO sample in Fig. 1C, it can be obvious to observe the significantly reduced intensity of the peak constituted by the oxygen-containing functional groups of GO at the higher binding energy after the reaction with the presence of ZnO NPs. These results confirm the catalytic role of the ZnO NPs in the deoxygenation process of GO sheets. In particular, from the O 1s spectrum with peak fittings of sample ZnO/RGO in Fig. 1D, the fitting peaks located at the binding energies of 531.3 and 533.0 eV can be attributed from the crystal structure of ZnO NPs [26]. Apart from the O 1s of ZnO NPs, the fitting peak at 533.0 eV – O group in RGO sheets. The third may also be attributed from the C– fitting peak at 532.2 eV between the two fitting peaks belonging to crystal ZnO NPs in sample ZnO/RGO can the explained by the linkage of Zn–O–C bond between RGO sheets and ZnO NPs, which matches with the fitting peak of C–O/C–O–Zn at 286.9 eV in the C 1s spectrum in Fig. 1C. The existence of Zn–O–C bond in the ZnO/RGO sample
Fig. 3. DFT results for the reduction of GO with the presence of ZnO NPs: top view and side view of GO used during DFT calculations (A); potential energy profile for the reaction of GO and water molecule (B); potential energy profile for the self-decomposition of GO (C); potential energy profile for the reduction of GO by ZnO quantum dots (D). The atoms are colored: Zn in blue, O in red, C in gray, and H in white.
contributes to the enhancement of the immobilization of ZnO NPs in the RGO supporting architecture. The morphology information of the ZnO/RGO sample was obtained by SEM and TEM characterization protocols. From the low-magnified SEM image in Fig. 2A, it can be clearly seen that nano-sized white ZnO NPs without heavy aggregation are coated by large-sized thin RGO sheets. And in the high-magnified SEM image of sample ZnO/RGO in Fig. 2B, it can be notified that the nano-sized ZnO is small cluster constituted by ZnO NPs around 50 nm, particularly, there is space be tween the RGO coating layers and inside ZnO crystals. The TEM image in Fig. 2C matches well with the SEM images, indicating the good immo bilization effect of RGO framework towards ZnO NPs. From the inset HRTEM image of the ZnO/RGO sample in Fig. 3C, the ZnO NPs are anchored on the few-layered RGO sheets and the inside ZnO crystal shows an interplanar distance about 0.26 nm corresponding to the (002) direction. From the EDS mapping result of the ZnO/RGO sample in Fig. 2D, three elements are detected and they are Zn, C and O, and this result is in good agreement with the former XRD and XPS results in Fig. 1A and B. It is well worthy to point out that most of the O element overlaps with that of Zn element while C element distributes on the rest location from the EDS mapping. Fig. 2E shows the EDS analysis result of the ZnO/RGO 4
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sample and the weight percentages of the C, O and Zn elements are 41.37, 16.00 and 42.63, respectively, corresponding to the atomic per centages of 67.55, 19.60 and 12.85. Therefore, the atomic ratios be tween C and O for RGO by excluding the influence from ZnO is estimated to be 10:1. The increased ratio between C and O based on EDS analysis (10:1) than that based on XPS analysis result (2.6:1) in Fig. 1B indicates more thorough reduction of the inside RGO sheets than the surface ones due to the fact that most ZnO NPs are immobilized inside the RGO supporting architecture.
The geometry of stable point Z-IM4 suggests that exposed Zn atoms in ZnO NPs tend to join with epoxy oxygen atoms to fulfill the saturation. Compared to the decomposition route, the further combination reaction has thermodynamic priority. This theoretical calculation result matches quite well the XPS characterizing result in Fig. 1C and D. By analyzing the potential energy profiles of above the deoxygen ation routes of GO, it can be concluded that bimolecular reduction be tween ZnO NPs and GO is of high priority in terms of thermodynamics and kinetics due to the lowest activation energy and slightly exother micity. The results indicate that the presence of ZnO NPs has a catalytic ability towards the removal of the oxygen-containing functional groups on GO sheets. The exposed Zn atoms from ZnO NPs prefer to associate with epoxy oxygen atoms from GO sheets rather than directly abstracting epoxy oxygen atoms. Along with the formation of new Zn–O bonds, an interaction system of Zn–O–C is formed to ensure a good structure stability of the ZnO/RGO composite.
3.2. Reaction mechanisms DFT simulation protocol is used to further reveal the function of ZnO NPs on the deoxygenation of the oxygen-containing functional groups from GO sheets in the reaction. As shown in Fig. 3A, a GO monolayer decorated by epoxy and carboxyl groups are selected for simulation. Results of the charge density distribution implies the epoxy group, the hydroxyl group and the carboxyl group are electron-withdrawing, and the oxygen atoms exhibit electronegative properties by abstracting 0.80–1.15 electrons from surrounded carbon atoms. In the ZnO NPs, the surface Zn2þ ions are unsaturated and exhibit positive electricity by 1.14. Therefore, the dangling bonds of Zn can be saturated by interac tion with the active oxygen especially the epoxy of GO. Here, this paper focused on the interface interaction between ZnO NPs and GO, In the following simulation, three possible pathways to convert the sp3 hy bridized carbon atoms to sp2 hybridized ones in the reaction are simu lated and discussed. The effect of H2O molecules for the deoxygenation of GO is first investigated in the reaction. As shown in Fig. 3B, H2O can stabilize GO sheets in the aqueous system via forming hydrogen bond with the oxy gen atoms from GO. However, the potential energy profile clearly shows that direct the reduction of epoxy group (W-IM1) to hydroxyl group (WIM2) by water is difficult to proceed due to the high endergonic prop erties in this process. Therefore, H2O molecules in this system play the role of stabilizer for GO rather than reducing agent. The possible route for the deoxygenation of GO is that GO may un dergo self-decomposition via removal of oxygen molecules. As shown in Fig. 3C, epoxy functional groups on GO sheets may undergo rear rangement with adjacent ones to form a four-member ring intermediate IM1. In the transition state TS1, the outer C–O bonds are elongated by ~0.60 Å, and the bond length of adjacent O atoms becomes 1.979 Å. The formation of O–O bond is quite endothermic, which has to overcome a high energy barrier of 1.74 eV. The C–O–O–C four-member ring struc ture is unstable and inclines to relax via decomposition of oxygen molecule with an energy barrier of 0.72 eV. Finally, the impact of ZnO NPs on the deoxygenation of GO is investigated by simulating the association process between GO and ZnO nanostructure. As shown in Fig. 3D, ZnO NPs have both Zn and O atoms exposed, where the unsaturated exposed Zn atoms are more likely to react with the active oxygen in GO. Therefore, Z-IM1 is generated with the formation of a new Zn–O bond, and the ZnO nano-cage is anchored on the surface of GO sheets with an exothermic energy of 0.29 eV. Since the bond length of the new formed Zn–O bond is 2.174 Å, 0.20 Å longer than that of original ZnO nanostructures (1.970 Å), the Z-IM1 may decompose to Z-IM2 by the cleavage of epoxy O and GO. This process encounters an energy barrier of 1.55 eV, which is much lower than that in the second GO self-decomposition speculation in Fig. 3C (GO→TS1→IM1). After the removal of ZnO species, the hybridization of epoxy-linked carbon atoms changes from sp3 to sp2. The results provide the evidence that ZnO NPs function as a catalyst to accelerate the GO deoxygenation process by significantly reducing the energy barrier. More importantly, further simulation result shows that instead of detaching from GO sheets, the adsorbed ZnO species are prone to continuously interact with GO by forming a second Zn–O–C bond. This process carries out by the exposed Zn atom associating with neighboring epoxy oxygen atom and has to absorb an energy of 0.23 eV.
3.3. Lithium ion storage performances Compared with the current commercialized graphite anode for lithium ion batteries, ZnO has the advantage of higher theoretical ca pacity while suffers from heavy volume change and low electron con ductivity [27,28]. Constructing RGO supporting architecture immobilized ZnO NP composite anode have been designed and proven to be effective due to the shortened diffusion pathway of Liþ, buffered volume changes and improved overall electron conductivity for the composite electrode [29]. The lithium ion storage performance of the ZnO/RGO composite is investigated. Fig. 4A shows that the lithium ion storage behavior of the ZnO/RGO composite electrode characterized by cyclic voltammetry technique between 0.01 and 3.00 V (vs Li/Liþ) under the scanning rate of 0.2 mV s 1. From the cathodic scanning curves, two reduction peaks at about 0.70 and 0.22 V can be seen, which are ascribed to the reduction of ZnO to Zn and the following formation of LixZn alloy, respectively. It is well worthy to point out that a strong peak at about 0.22 V was clearly observed in the first cathodic process, the intensity of which is signifi cantly deceased. The reductive peak can be ascribed to the formation and growth of the solid electrolyte interphase (SEI) apart from the alloying reaction [30]. This result also indicates most SEI layer forma tion process takes place in the first discharge step. In the remaining cathodic scanning curves, the two reduction peaks located at around 0.70 and 0.22 V becomes more stable and overlapped. By contrast, the anodic scanning curves almost maintains the shape and a series of peaks located at 0.29, 0.37, 0.56, 0.69, and 1.32 V can be detected, corre sponding to the dealloying reactions of the LixZn alloy [31]. It is worthy mentioned that the broad last peak at about 1.32 V can be interpreted by the conversion type reaction between Zn and Li2O to regenerate ZnO crystals [30,32]. Therefore, the lithium storage properties of ZnO based electrode are constituted by both alloy and conversion reactions, which is consisted with the literatures [33,34]. The overall lithium ion storage behavior of the ZnO/RGO electrode follows the below equations: ZnO þ 2Liþ þ 2e ↔ Zn þ Li2O
(1)
Zn þ xLiþ þ xe ↔ LixZn
(2)
Fig. 4B shows charge and discharge curves of the initial two and the 100th cycles for the ZnO/RGO composite electrode at a current density of 500 mA g 1 between 0.01 and 3.00 V. from the first discharge curve, two apparent voltage plateaus can be seen at around 0.25 V and 0.70 V, which are corresponding to the reduction of ZnO to Zn and formation of LixZn alloy, respectively. And from the first charge curve, minor voltage plateaus between 0.20 and 0.70 V and an obvious voltage at about 1.3 V can be observed. These results are consisted with the lithium ion storage behavior from the CV curves in Fig. 3A. The curves for 2nd and 100th are similar in terms of shape except for the difference of the specific ca pacities. The ZnO/RGO composite electrode delivers an initial discharge 5
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Fig. 4. CV curves for the initial 5 cycles (A) and charge-discharge voltage profile (B) of the ZnO/RGO composite electrode.
and charge specific capacities (calculated based on the mass of the ZnO/ RGO composite) are about 1205 and 692 mA h⋅g 1, respectively, with a coulombic efficiency of 57.4%. The low coulombic efficiency for the initial cycle is mainly ascribed to the formation of SEI layer due to electrolyte decomposition and side reactions [35,36]. For the second cycle, specific capacities for the discharge and charge processes are about 718 and 596 mA h⋅g 1, respectively. And the corresponding coulombic efficiency is calculated to be 83%. After 100 cycles, the ZnO/RGO electrode delivers the discharge and charge specific capacities of about 564 and 556 mA h⋅g 1, respectively, with a coulombic effi ciency of 98.6%. The gradually increased coulombic efficiency indicates the successful activation process taking place.
Fig. 5A compares the cycling performances of the ZnO/RGO com posite, RGO prepared by thermal reduction of GO and bare ZnO elec trodes at current densities 500 mA g 1. For the ZnO/RGO composite electrode, it exhibits initial discharge and charge capacities of about 1021 and 625 mA h⋅g 1, respectively. Since then, the reversible capacity gradually decreases to 460 mA h⋅g 1 in the following 40 cycles, origi nating from the gradual activation of the ZnO/RGO composite electrode [26]. Then the reversible capacity of the ZnO/RGO composite electrode displays a stable increasing trend, which is ascribed to more ZnO NPs taking part in the electrochemical reaction due to the excellent sup porting effect from the RGO architecture for the composite. The reversible capacity reaches approximately 560 mA h⋅g 1 after 250
Fig. 5. Cycling performance of the ZnO/RGO composite with RGO and ZnO electrodes as comparison (A) and rate capability of the ZnO/RGO composite elec trode (B). 6
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Fig. 6. 3D representation of RL values versus frequency and thickness for sample GO (A), ZnO (B) and ZnO/RGO (C), respectively; theoretical RL curres versus frequency and thickness of sample ZnO/RGO (D).
continuous cycles. By contrast, for the RGO electrode, it shows lower initial discharge and charge capacity of about 819 and 576 mA h⋅g 1, respectively. The lithium ion storage for RGO electrode follows a double layer adsorption mechanism (C6 þ 2Liþ þ 2e ↔ Li2C6), corresponding to a theoretical capacity of about 740 mA h⋅g 1 [6,37–39]. However, the RGO electrode could not provide stable voltage output and no obvious peak could be identified for RGO in the CV curves of the ZnO/RGO electrode in Fig. 4A. And more rapid reversible capacity fading for this electrode in the following 40 cycles have been observed. After 250 cy cles, the reversible capacity declines to 365 mA h⋅g 1. For the ZnO electrode, although it delivers a high initial discharge capacity of about 1122 mA h⋅g 1, yet the initial reversible capacity for this electrode is only 399 mA h⋅g 1. Even worse, the reversible capacity of the ZnO electrode maintains a sharp declining trend in the remaining cycles, and reaches approximately 100 mA h⋅g 1 after 10 cycles. After 250 cycles, the reversible capacity maintains only 44 mA h⋅g 1. This is probably ascribed to the heavy aggregation of nano-sized ZnO and subsequently losing electrochemical contact with the current collector. The improved cycling performance for the ZnO/RGO composite electrode compared with RGO electrode and bare ZnO electrode is the result of the good synergistic effect from both the inside ZnO NPs and outside the RGO supporting framework, the good immobilization from RGO ensures the stable electrochemical performance of the active ZnO NPs in the ZnO/RGO composite electrode for a long cycle life. The rate performance of the ZnO/RGO composite electrode was also investigated by cycling at stepped current densities between 0.2 and 20.0 A g 1. As shown in Fig. 5B, the ZnO/RGO composite electrode delivers average reversible discharge capacities of 640, 510, 400, 320, 260, 170 and 110 mA h⋅g 1 at 0.2, 0.5, 1.0, 2.0, 5.0, 10.0 and 20.0 A g 1, respectively. When the current density is returned to the initial value of 200 mA g 1, the reversible capacity of the ZnO/RGO composite elec trode reverts to 540 mA h⋅g 1. The rate testing result indicates that the ZnO/RGO composite electrode has good battery performance under a wide range of current densities.
3.4. Electromagnetic microwave absorption performances Besides the application in lithium ion batteries, ZnO/graphene composites have been studied for the improved electromagnetic wave absorbing properties than single component due to the polarization at the graphene/ZnO interface [40,41]. However, graphene supported composites are generally facing the problem of poor dispersion resulting from the poor structure stability of the corresponding composite mate rials. The electromagnetic microwave absorption capability of the ZnO/RGO composite is investigated. To investigate the microwave absorption properties of the ZnO/RGO composite sample, the reflection loss (RL) values of all samples were obtained according to the following equations [42,43]: Where Zin and Z0 represent the input impedance of the metalsupported electromagnetic wave absorbing layer and the impedance of the free space, respectively; f stands for the microwave frequency; d is the thickness corresponding to the sample; c is the velocity of electro magnetic waves in free space; εr is the relative dielectric constant, and μr corresponds to magnetic permeability. The calculated RL values of bare ZnO, GO and ZnO/RGO composite in the frequency range from 2 to 18 GHz are shown in Fig. 6. The maximum RL value for GO is 11.09 dB at 5.68 GHz with the thickness is 7.8 mm in Fig. 6A and the maximum RL value for ZnO is 5.76 dB when the thickness is 8.4 mm in Fig. 6B. By contrast, the ZnO/RGO composite delivers the best microwave absorption performance. As can be seen from Fig. 6C and D, the maximum RL values can reach 67.13 dB when the thickness is 3.2 mm while the effective absorption bandwidth is 7.44 GHz with the thickness is 2.8 mm for the ZnO/RGO composite. To further understand mechanism of the outstanding absorption performance the ZnO/RGO composite absorber, the complex permit tivity (εr¼ ε0 -jε") and complex permeability (μr¼ μ0 -jμ") were tested with the vector network analyzer in the range of 2–18 GHz. In general, the real part of the complex permittivity and complex permeability are used to represent the ability to store microwave energy while the imaginary 7
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Fig. 7. Electromagnetic parameters of the ZnO/RGO composite with ZnO NPs and GO for comparison: the real/imaginary of permittivity (A and B); the real/ imaginary of permeability (C and D); dielectric loss tangent (E); magnetic loss tangent (F).
part stands for the dissipation ability of microwave energy [44]. Fig. 7A compares the real part (ε0 ) of the dielectric constant of three samples. It can be clearly seen the epsilon of the ZnO/RGO composite ranging from 7.48 to 3.95 in the 2–18 GHz range, which is remarkably higher than that of the ZnO and GO sample. Similar result can be observed in Fig. 7B concerning the imaginary part (ε") of the dielectric constant of the three samples. The reason for this phenomenon is that there is a synergistic effect between the RGO supporting framework and the ZnO NPs. Furthermore, the polarization phenomenon between different interfaces
is also conducive to the improvement of microwave dissipation ability [45]. Therefore, ZnO/RGO composite has the highest ε" value. Fig. 7C and D shows the real part (μ0 ) and imaginary part (μ") of the permeability of three samples in the frequency range of 2–18 GHz, respectively. Compared with the ε0 and ε" values at the same frequency, the μ0 and μ" values of ZnO/RGO composite materials are relatively small, which in dicates that the magnetic loss capacity of the composite materials is significantly limited. The fluctuation of samples in the vicinity of 6 GHz and 12–18 GHz might be attributed to natural resonance and eddy
Fig. 8. (A) Impedance matching and (B) attenuation constants (α) of ZnO, GO and ZnO/RGO composite. 8
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Fig. 9. Typical Cole-Cole models ZnO/RGO composite.
current loss. Fig. 7E and F shows the relationship between the dielectric loss angle tangent (tanδε ¼ ε00 /ε0 ) and the magnetic loss angle tangent (tanδμ ¼ μ00 /μ0 ) and frequency of the three samples. In Fig. 7E, ZnO/RGO composite shows the highest dielectric loss capability over the entire frequency range compared to the other two samples. This indicates that more interfacial polarization at the interface can be expected in the ZnO/RGO composite, thus enhancing the dielectric loss. However, in Fig. 7F, there is no obvious change at other frequencies except for the resonance peak at 6 GHz and 14–18 GHz. The impedance matching is another important criterion for measuring microwave absorbing materials. An impedance matching image of three samples can be drawn according to formula (1), and the result is shown in Fig. 8A. In the frequency range of 2–18 GHz, the value of |Zin/Z0| of ZnO/RGO composite is closer to 1. This indicates that ZnO/ RGO composite has better impedance matching characteristics [46]. It is easy to understand that the impedance ratio of the impedance of the absorber to the free space is 1, indicating that the incident electro magnetic wave is absorbed by the absorber, and that there is almost no reflected electromagnetic wave on the surface of the absorber. Following the transmission line theory, the electromagnetic attenu ation constant α of the microwave absorbing material can be calculated by the following formula [47]: The attenuation constant α is one of the parameters used to indicate the ability of the absorber to attenuate electromagnetic waves. The larger dielectric losses and magnetic losses can provide a larger α value. It can be clearly seen from Fig. 8B that as the frequency increases, the value of the decay constant α of the ZnO/RGO composite increases from 29.1 to 272.28, which is much larger than the other two samples. And due to the ZnO/RGO composites have suitable impedance matching characteristics. Therefore, ZnO/RGO composites have excellent micro wave absorption properties. The complex permittivity of microwave absorbing materials is related to debye relaxation. According to debye dipole relaxation, the complex dielectric constant is calculated by the following formula [48]: τ and f represent the time and frequency of relaxation, respectively, and εs and ε∞ are respectively used to indicate the dielectric constant and optical permittivity that are static at high frequency limits. There fore, further calculations can be obtained from the following formula [49]: The curves ε0 and ε’’ obtained from the above formula are a semi circular shape, collectively referred to as a cole-cole semicircle. The cole-cole semicircle model of ZnO/RGO composite material is shown in Fig. 9. Many cole-cole semicircles can be clearly shown, suggesting that there may be many processes of dielectric relaxation due to the lamellar structure of the composite material. In general, due to the layered structure of ZnO/RGO composites, many ZnO/RGO and ZnO–ZnO
Fig. 10. The dependence of the RL curve and the matching thickness (tm) on the matching frequency (fm) of the ZnO/RGO composite.
interfaces are introduced, plus the dielectric relaxation phenomenon caused by induced charge delay. These two conditions lead to the accumulation of a large amount of charge on the interface, which leads to the polarization of the interface. At the same time, the lamellar structure of the composite material can provide unsaturated bonds, which bring about dipole polarization. It can be seen from the model that the absorption summit shows a low-frequency movement trend with the increase of layer thickness. This tendency can be explained by a quarter wavelength attenuation, which is shown as below [50–52]: Where tm is the wavelength of the electromagnetic wave, c is the speed of light in free space, fm is the corresponding peak frequency, n is an integer (n ¼ 1, 3, 5 …). Based on the quarter-wave principle, Fig. 10 established the model of RL curve and matching thickness and matching frequency of ZnO/RGO composite material. It can be clearly seen that the yellow pentacle represents the correspondence between the match ing thickness and the absorption peak frequency. Obviously, all yellow marks can be accurately placed around the 1/4 λ curve, which repre sents the ZnO/RGO composite material conforms to the principle of 1/4 wavelength. In general, the structure, shape and properties of the absorber are related to its impedance matching characteristics and dielectric losses, which act together on the microwave absorption performance of the absorber. Fig. 11 shows the microwave absorption mechanism of ZnO/ RGO composite material. Firstly, when ZnO/RGO composite surface receives the incident microwave, only a small amount of microwave is reflected due to the good impedance matching characteristics of ZnO/ RGO composite and most microwave enters the composite. Secondly, the abundant folds and gaps in the ZnO/RGO composite due to the synergetic functions between RGO and ZnO NPs during the synthesis 9
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Fig. 11. Schematic diagram of microwave absorption mechanism for ZnO/RGO composite.
based on effective absorption bandwidth and has a high reflection loss value. It also shows that the absorber is an ideal candidate material for microwave absorption under the conditions of large absorption band width, strong absorption capacity and thin thickness.
Table 1 Microwave absorption properties of ZnO/RGO composites and other related composites. Sample
ZnO/RGO RGO/Fe3O4/ ZnO RGO/ Co@Fe@Cu N-RGA/Ni (600) ZnO–NiC/RGO TiO2/Ti3C2Tx/ RGO RGO/ MWCNTs/ ZnFe2O4
Mass ratio (wt%) 5 33.3 5 10 40 10 50
RLmin (dB)
d (mm)
Frequency range (RL < 10dB, GHz)
Refs
This work [53] [54] [55] [56] [57] [58]
67.13 57
3.2 2.0
7.44 5.0
49.71 60.8 59.3 65.3 22.2
2.1 2.1 2.05 2.5 1.0
6.0 5.1 5.6 4.3 2.3
4. Conclusions In conclusion, a ZnO/RGO composite with ZnO NPs immobilized by RGO supporting architecture is successfully prepared by a ZnO cata lyzing GO reduction process under mild aqueous condition. XPS char acterization result illustrates the oxygen-containing functionally groups are significantly removed for GO and a Zn–O–C linkage bond forms between ZnO NP and RGO sheet in the ZnO/RGO composite. Theoretical calculation further verifies the above process for the first time. The ZnO/ RGO composite showed a reversible capacity of 560 mA h⋅g 1 for 250 cycles under a current density of 500 mA g 1 as anode for lithium ion batteries and good electromagnetic microwave absorption performance with the maximum reflection loss value of 67.13 dB when the thick ness is 3.2 mm while the effective absorption bandwidth is 7.44 GHz with the thickness is 2.8 mm, respectively. The starting materials in this work are only GO aqueous suspension and commercially available ZnO NPs, the reaction temperature is 95 � C and the reaction time is 12 h. Therefore, this work not only opens a new horizon for GO deoxygen ation, but also provides a facile, eco-friendly and scalable approach to preparing ZnO/RGO composite materials for a wide application.
result in multiple reflections and scattering for the microwave. Thirdly, uneven distribution of electrons probably takes place due to the differ ence of the conductivity between ZnO NPs and RGO supporting frame work, which further leads to the accumulation of bound charges between the interfaces and the related interface polarization. Finally, the defects on the surface of ZnO/RGO composite will also break the balance of charge distribution, causing uneven charge distribution, leading to dipole polarization and further enhancing the microwave absorption performance of the composite material. Table 1 compares the microwave absorption performances of ZnO/ RGO composite materials with the reported related composites. The ZnO/RGO composite prepared in this study have strong competitiveness no matter in terms of mass load, minimum reflection loss and effective absorption bandwidth. It can be clearly seen from the table that the composite material synthesized in this study far exceeds other materials
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements [27]
This work was financially supported by China Postdoctoral Science Foundation (No. 2017M612194 and 2017M610409), Natural Science Foundation of Shandong Province (No.ZR2019YQ24), the Qingchuang Talents Induction Program of Shandong Higher Education Institution (Research and Innovation Team of Structural-Functional Polymer Composites), the Thousand Talents Plan, the World-Class University and Discipline, the Taishan Scholar’s Advantageous and Distinctive Disci pline Program and the world-Class Discipline Program of Shandong Province.
[28] [29] [30] [31]
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