Synthesis of a Ni_0.8Zn_0.2Fe₂O₄–RGO nanocomposite: an excellent magnetically separable catalyst for dye degradation and microwave absorber

Abstract

A Ni_0.8Zn_0.2Fe₂O₄ reduced graphene oxide nanocomposite has been synthesized by a simple ‘in situ co-precipitation’ technique. This composite exhibited an ability to act as an excellent magnetically separable catalyst towards the degradation of various dyes as well as a toxic herbicide (trifluralin). It also demonstrated very good microwave absorption properties.

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In the past few years graphene and graphene based composites have generated immense interest. Graphene possesses superior electrical, thermal, and mechanical properties as well as chemical stability due to its one atom thick planar sheet with a hexagonally arranged sp² bonded carbon atom network.^1–3 Its interesting properties have been exploited for a plethora of applications in myriad fields including electronics, lithium ion batteries, hydrogen storage, microwave absorption, magnetic resonance imaging (MRI) contrast agents, heterogeneous catalyst, etc.^4–10

Graphene based composites are promising microwave absorbing materials because of their light weight, wide absorption frequency, and high thermal stability. Moreover, in comparison with CNTs, graphene based materials are expected to be more attractive, because they offer large surface area, lower price and more stable properties.¹¹ Graphene based nanomaterials also exhibit their potential in heterogeneous catalysts due to their large surface area, remarkable electrical conductivity, excellent absorptivity, etc.^9,12,13

The main objective of our current research is to develop a multifunctional material which can be utilized as microwave absorbing material as well as catalyst. We have chosen these two important applications of graphene based composites because of the following reasons (i) in modern lifestyle extensive usage of mobile phones, personal computers, local area network, radar systems, television image interference of high rising buildings, etc. cause over exposure of electromagnetic waves to human bodies as well as other living beings. Excessive exposure of microwave radiation is harmful to biological systems such as increasing possibility of cancer, weakening immune response, DNA strand damage in brain cells, increasing heart rate, etc.^14–16 Moreover, microwave absorbing materials find applications in stealth technology of military applications. (ii) Water pollution caused by the discharge of colored effluents from various dye base industries (e.g. paper and pulp manufacturing, plastics, clothing, lather treatment, printing, etc.) is a serious environmental concern. Strong color of dyes and pigments pose serious aesthetic and ecological problems to the receiving aquatic ecosystems, such as inhibition of benthic photosynthesis. Moreover, some of the dyes are toxic and carcinogenic in nature.¹⁷

To address the above mentioned issues, we have attempted to synthesize a composite consist of Ni_0.8Zn_0.2Fe₂O₄ (NZF) nanoparticle and reduced graphene oxide (RGO). This NZF–RGO composite is expected to be capable of exhibiting good microwave absorbing property as well as high catalytic efficiency towards degradation of various synthetic dyes.

Though, variety of ferrite based materials have been reported as microwave absorber^18–26 but most of these materials suffer from one or more of the following limitations such as (i) high density, (ii) poor environmental stability, (iii) narrow wave absorbing band, (iv) poor flexibility, etc.^23,24 These factors restrict their wide applications as microwave absorber. Whereas, in case of NZF–RGO composites, RGO component offers light weight, high surface area, superior dielectric property, good thermal and environmental stability. On the other hand, NZF being a soft ferrite exhibits high saturation magnetization value, Snoek's limit, good coercivity, which result in high complex permeability values at wide frequency ranges.¹⁶ Hence, we have expected decent microwave absorbing properties of NZF–RGO nanocomposite.

In case of degradation or decolorization of dyes in aqueous medium, reduction of dyes by using heterogeneous catalysts in presence of reducing agents (such as NaBH₄) is a very popular approach.^9,10,27–29 In this reaction metal nanoparticles (e.g. Pt, Au, Ag, Cu, etc.) are widely used as catalyst.^28–35 Though, these nanoparticles show reasonably good catalytic activity, but they suffer from an inherent limitation, which is their separation problem after the reaction. Due to nanosize of these catalysts, their easy separation from the reaction mixture by filtration is not possible. Recently, development of magnetically separable catalysts has been chosen as an attractive strategy to address this issue.^9,10,27,28 Here, we have designed NZF–RGO composites to act as catalyst for this reaction because of two main reasons. As in this reaction, reduction of dye proceeds via electron transfer from BH₄⁻ to dye molecule, which causes to reduce the functional groups (e.g. –NO₂, –N=N–, etc.) of dye and decolorize the dye, easy electron transfer is an important factor. In NZF–RGO composite, presence of RGO play in dual role: high surface area of RGO is expected to help to absorb BH₄⁻ and dye molecules on catalyst and the high electrical conducting nature of RGO may facilitate the electron transfer from hydride to dye molecule. Moreover, NZF makes this catalyst magnetically separable by providing necessary magnetic character.

In the last few years, preparations of ferrite–RGO composites have been reported by several researchers. Generally, solvothermal/hydrothermal (e.g. ZnFe₂O₄/RGO,³⁶ CoFe₂O₄/RGO,³⁷ NiFe₂O₄/RGO,³⁸ Fe₃O₄/RGO³⁹), vapor diffusion technique (CoFe₂O₄/RGO),⁴⁰ co-precipitation (Fe₃O₄/RGO),¹¹ etc. methods are employed to prepare various ferrite–RGO composites. However, in most of these methods usage of reducing agents (like NaBH₄, N₂H₄, etc.) is very common to convert GO to RGO.

In this paper, we are reporting an in situ co-precipitation technique to synthesize Ni_0.8Zn_0.2Fe₂O₄–RGO nanocomposites where no extra reducing agent was used. In this synthetic methodology, a mixture of GO and aqueous solution of metal nitrates were refluxed in presence of NaOH. NaOH reduced the oxygen containing functional groups of GO to convert it to RGO^41,42 and simultaneously precipitated metal hydroxides, which eventually converted to NZF nanoparticles during reflux.⁴³ To the best of our knowledge, preparation of NZF–RGO nanocomposites and their applications as microwave absorber as well as catalyst for dye and herbicide degradation reactions have not yet been reported.

The synthesized NZF–RGO nanocomposites were characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), Raman spectroscopy, thermogravimetric analysis (TGA), Transmission Electron Microscope (TEM), Energy Dispersive X-ray Analysis (EDAX) and Vibrating Sample Magnetometer (VSM) (details of the techniques used for structural characterization are mentioned in ESI†). Catalytic activity of NZF–RGO nanocomposites were determined towards degradation of various dyes (4-nitro phenol (4-NP), methyl orange (MO), Rhodamine B (RhB)) as well as herbicide (trifluralin) in presence of excess NaBH₄ and the reactions were monitored using an UV-vis spectrophotometer (detail experimental procedure for catalysis reactions and analysis of results are mentioned in ESI†). Microwave absorption property in X-band region of these composites were evaluated using Vector Network Analyzer (VNA) and reflection loss was calculated using the measured values of complex permittivity and permeability. The electromagnetic parameters (relative complex permittivity ε_r = ε′ − jε′′ and relative complex permeability μ_r = μ′ − jμ′′) of the synthesized NZF and NZF–RGO were measured at room temperature to determine their microwave absorption properties. It is well known that, the real permittivity (ε′) and real permeability (μ′) represent the storage ability of dielectric and magnetic energy, while the imaginary permittivity (ε′′) and imaginary permeability (μ′′) symbolize the electrical energy dissipation and magnetic loss respectively.^18,19,44,45 The reflection loss (RL) was estimated from the complex relative permeability and permittivity at a given frequency and absorber thickness employing single layered plane wave absorber model, proposed by Naito and Suetake⁴⁶ (details of microwave absorption measurement and data analysis are mentioned in ESI and Fig. S1 (ESI†)).

To synthesize Ni_0.8Zn_0.2Fe₂O₄–RGO nanocomposites, in the first step Graphene Oxide (GO) was prepared by using modified Hummers and Offeman method⁴⁷ using graphite powder. Then an in situ co-precipitation technique was employed where formation of Ni_0.8Zn_0.2Fe₂O₄ (NZF) nanoparticles and Reduced Graphene Oxide (RGO) flakes occurred simultaneously (details of GO preparations and starting materials used for composite preparation are provided in ESI†). In a typical synthesis, an aqueous solution mixture containing desired amount of metal nitrates salts (e.g. Fe(NO₃)₃·9H₂O, Ni(NO₃)₂·6H₂O, Zn(NO₃)₂·6H₂O) was prepared. In this mixture, an aqueous mixture containing well dispersed GO powder was added. To this reaction mixture, aqueous solution of NaOH (2 M) was added dropwise till the pH reached at ∼8.5–9. This mixture was then refluxed at 120 °C for 12 h, followed by cooling at room temperature. The black precipitate thus obtained was collected by ultracentrifugation and washed several times by distilled water. After separation this precipitate was dried at 60 °C for 12 h. Employing this preparation protocol NZF–RGO nanocomposites were prepared having different weight ratio of RGO and NZF such as 85 wt% NZF–15 wt% RGO (85NZF–15RGO), 75 wt% NZF–25 wt% RGO (75NZF–25RGO), 50 wt% NZF–50 wt% RGO (50NZF–50RGO). We have also prepared pure Ni_0.8Zn_0.2Fe₂O₄ nanoparticles using the same co-precipitation method without adding GO.

XRD pattern of GO showed an intense diffraction peak at 2θ = 9.76° and a small peak at 42.14° which corresponded to (001) and (101) planes of GO.^48,49 NZF–RGO nanocomposite samples exhibited diffraction at 2θ = 30.35°, 35.72°, 37.83°, 43.32°, 53.94°, 57.20° and 62.89° correspond to (220), (311), (222), (400), (422), (511) and (440) planes of Ni_0.8Zn_0.2Fe₂O₄ respectively (JCPDS card no. 52-0278) (Fig. 1). Average crystallite size of NZF nanoparticles was found to be 14 nm, calculated by using Scherer's equation. In the XRD patterns of NZF–RGO composites no peaks at 2θ = 9.76° and 42.14° were observed, which clearly indicated that during preparation of NZF–RGO composites, GO flakes were converted to RGO and RGO sheets were exfoliated.^{4,7,11,36,50–52} In this in situ co-precipitation method NaOH acted in dual role. It not only precipitated hydroxides of metal ions, which eventually converted to NZF nanoparticles,⁴³ but also converted GO to RGO during reflux.^41,42 In this method, as the formation of NZF occurred in presence of RGO flakes, nanoparticles were expected to be well deposited on the surface of flakes. TEM micrographs of pure NZF and NZF–RGO nanocomposite (Fig. 2(A) and (B)) also revealed that, nanometer thin RGO sheets were well separated and NZF nanoparticles (∼7–10 nm) were homogenously deposited on RGO sheets. EDAX analysis of these nanocomposites also confirmed the composition of these composites (Fig. S2 (ESI†)).

Fig. 1 Room temperature wide angle powder XRD pattern of (a) pure NZF, (b) 85NZF–15RGO, (c) 75NZF–25RGO, (d) 50NZF–50RGO and (e) GO.

Fig. 2 (A) TEM images of pure NZF (B) TEM micrograph of 50NZF–50RGO.

Thermogravimetric analysis (TGA) of these composites (Fig. S3 (ESI†)) indicated that, RGO component of the RGO–NZF composites, thermally decomposed to CO₂ in the temperature range of 275–375 °C and NZF remained as residue.¹¹ TGA also showed that, the wt% of RGO and NZF present in NZF–RGO composites are as per the amount expected during synthesis. These results indicated that, this in situ co-precipitation method is capable of producing NZF–RGO nanocomposites having desired compositions (details of TGA are mentioned in ESI†).

Fig. 3 shows FT-IR spectra of pure GO, pure RGO (prepared by refluxing GO at 120 °C for 12 h) and 50NZF–50RGO. It was observed that, in case of GO peaks appeared at (i) 1384 cm⁻¹ corresponding to the stretching vibration of C–O of carboxylic group, (ii) 1720 cm⁻¹ for carbonyl group, (iii) 1226 cm⁻¹ for C–O stretching vibration of epoxy group (iv) 1054 cm⁻¹ for C–O stretching vibration.^11,53 This fact indicated the presence of oxygen containing functional groups (such as epoxy, carbonyl, carboxyl, and hydroxyl) on the surface of GO.

Fig. 3 FT-IR spectra of (A) GO, (B) RGO and (C) 50NZF–50RGO.

Moreover, the peak at 1621 cm⁻¹ can be assigned to the contribution from the skeletal vibration of the graphitic domains.^11,40,54,55 In case of pure RGO it was observed that the peak at 1720 cm⁻¹ (for carbonyl group) has disappeared and intensities of the peaks at 1226 and 1054 cm⁻¹ (corresponding to C–O) have decreased. Appearance of a peak at 1544 cm⁻¹ was also observed. The band at 1621 cm⁻¹ (in GO sample), which can be assigned to the C=C skeletal vibration of graphitic domains of GO, has been red shifted to 1544 cm⁻¹ for RGO and indicated the partial restoration of π–π conjugation of graphene sheet in RGO.⁵⁵ Absorption bands at the same positions were observed in FT-IR spectra of NZF–RGO nanocomposite samples. Here also carboxylic group vibration band (ν_C=O at 1720 cm⁻¹) was found to be disappeared and the absorption intensity corresponding to C–O at 1226 and 1054 cm⁻¹ were decreased. These results implied that, most of the oxygen containing groups of GO, particularly carboxyl groups, had been removed and some of the hydroxyl and epoxy groups were remained on the surface of RGO in NZF–RGO nanocomposites. Moreover, appearance of peak at 592 cm⁻¹ indicated the presence of ferrite in NZF–RGO nanocomposite.^11,40

Raman spectra of NZF–RGO (Fig. S4 (ESI†)) composites also confirmed the presence of RGO in the composites. In these spectra peaks at 1349 cm⁻¹ and 1586 cm⁻¹, which are characteristic peaks at D band and G band of RGO,⁵⁶ were observed. In case of pure GO the value of I_D/I_G was reported as 0.9.⁴⁰ However, this ratio was increased to 1.03 for NZF–RGO composites, which can be attributed to the decrease in the average size of sp² domains upon reduction of GO during formation of NZF–RGO composites.^40,56

To investigate the catalytic efficiency of the synthesized NZF–RGO nanocomposites, reduction of synthetic dyes (4-nitro phenol (4-NP), methyl orange (MO), Rhodamine B (RhB)) in presence of excess NaBH₄ in aqueous medium was investigated. To understand the effect of presence of RGO in NZF–RGO composites same reactions were performed with pure NZF nanocatalyst. Reduction of 4-NP was first performed in presence of pure NZF and it was observed that, the reduction reaction was completed after 15 min. Similarly, in cases MO and RhB, the times required for completion of reduction were 17 min and 19 min respectively. When NZF–RGO nanocomposites were used as catalysts progressive decrease of time, required for completion of reaction, with increasing RGO content in the catalyst was observed. For example, when 50NZF–50RGO was used as catalyst, time required to complete the reduction of 4-NP, MO and RhB were 6, 5 and 7 min respectively (Fig. 4(A)–(C)) (Table S1 (ESI†)). The time dependent UV-vis spectra of reduction of dyes when catalyzed by different NZF–RGO composites (i.e. NZF, 85NZF–15RGO, 75NZF–25RGO, 50NZF–50RGO and 25NZF–75RGO) are provided in ESI (Fig. S5 and Table S2 (ESI†)). This enhancement of catalytic efficiency of NZF–RGO nanocomposites can be explained by considering following points: (i) these reduction reactions proceeds via relaying of electrons from the BH₄⁻ donor to the acceptor dye molecules.^{28,29,57–59} In aqueous medium BH₄⁻ was first absorbed on the surface of the catalyst. The hydrogen atoms, which were formed from the hydride, transfer electrons to the dye molecule to reduce its functional groups (e.g..–NO₂ group for 4-NP, –N=N– group for MO, –C=N for RhB). This electron transfer (ET) induced hydrogenation of dye molecules occurred spontaneously and NZF nanoparticles play a role of storing electrons after ET from hydride.^9,28 In case of NZF–RGO this nanocomposites, the synergistic effect between NZF and RGO sheets affects positively in the reduction of reaction time. Presence of RGO not only enhances the absorption of dye molecules onto the catalyst through π–π stacking but also facilitates the electron transfer to the dye molecule via electrostatic interaction.^9,60,61 However, the catalytic efficiency of NZF–RGO catalysts was found to be decreased when RGO component in the catalyst was more than 50 wt% (Fig. S5 and Table S2 (ESI†)). This might be due to the decrease of NZF component in the catalyst.⁹ As the concentration of NaBH₄ remains almost constant (because of its excess concentration) throughout the reaction, we have considered that, this catalytic reaction follows pseudo first order kinetics²⁸ and values of apparent rate constant (k_app) were found to be 0.73 m⁻¹, 0.95 m⁻¹ and 0.71 m⁻¹ for 4-NP, MO and RhB respectively when 50NZF–50RGO was used as catalyst (Fig. 4(E)). As it has been observed that, 50NZF–50RGO can act as an efficient catalyst towards reduction of –NO₂ group of 4-NP, we have utilized this catalyst for reduction of trifluralin, which is a herbicide⁶² and toxic in nature and present in water as residue. Trifluralin molecule contains two –NO₂ groups. It has been observed that, 50NZF–50RGO is capable to decolorize the aqueous solution of trifluralin by reducing its –NO₂ groups to –NH₂ group within 20 min in presence of excess NaBH₄ (Fig. 4(D)). The reusability of the catalyst was tested after recover the catalyst from reaction mixture and it was observed that, 50NZF–50RGO retained its activity almost same till five cycle and then slight decrease was observed (Fig. S6 (ESI†)). XRD and TEM analysis of the recycled catalysts showed that, after catalysis reactions any noticeable change in the crystal structure and microstructure of the catalyst did not occur (Fig. S7 (ESI†)).

Fig. 4 Time dependent UV-vis spectral changes of the reaction mixtures of (A) 4-NP, (B) MO, (C) RhB and (D) trifluralin catalysed by 50NZF–50RGO in presence of excess NaBH₄ and (E) pseudo first order kinetic plot of 4-NP, MO, RhB and trifluralin reduction catalysed by 50NZF–50RGO.

As NZF is magnetic in nature, 50NZF–50RGO nanocatalyst offers an additional advantage along with its high catalytic activity. This catalyst can easily be separable from reaction mixture after completion of the reaction by using a magnet externally. This easy magnetic separation of this catalyst helps to overcome the limitation of separation problem associated with nanoparticle catalysts. The magnetic properties (saturation magnetization (M_s) and coercivity (H_c)) of NZF–RGO nanocomposites were determined by VSM (Fig. 5(A)). It has been observed that, with increasing RGO component M_s value of the composites were decreased (Table S3 ESI†). In these composites presence of nonmagnetic RGO along with several other factors (e.g., attachment of NZF nanoparticles on the surface of RGO sheets which might influence particle surface spin, disordered surface spin structure, dipolar inter particle interactions, etc of NZF nanoparticle^63–65) play important roles in determining their M_s values. 50NZF–50RGO nanocomposites possessed M_s and H_c value of 14 emu g⁻¹ and 0.361 Oe. Fig. 5(B) clearly demonstrates that 50NZF–50RGO can easily be separable from reaction mixture by using a simple bar magnet externally.

Fig. 5 (A) Room temperature magnetic hysteresis loops for (a) pure NZF, (b) 85NZF–15RGO, (c) 75NZF–25RGO and (d) 50NZF–50RGO. (B) Magnetic separation of the catalyst by applying a magnet externally after completion of reaction.

Microwave absorbing property of NZF–RGO nanocomposites was investigated considering the fact that, RGO component of the composite will contribute towards microwave absorption due to the presence of residual defects and groups on the surface of the RGO sheets.^11,56 On the other hand, as the M_s value of NZF–RGO nanocomposites are lower than that of pure NZF, the anisotropy energy of NZF–RGO nanocomposites are higher than that of pure NZF. This higher anisotropy energy is expected to be helpful to enhance the microwave absorption properties of the composites.^11,66–70

Minimum reflection loss (RL) values of pure NZF and NZF–RGO composites were determined from their complex relative permeability and permittivity values. It has been observed that, in X band region minimum RL values of these composites are greater than those of pure GO,⁷¹ RGO^72–75 (Table S4†) and NZF. Pure NZF exhibited minimum reflection loss (RL) of −13.61 dB (∼95% loss) at 10.72 GHz with effective bandwidth (i.e., minimum RL < −10 dB) 9.8–12 GHz when absorber thickness was 1.9 mm (Fig. 6(A)). With increasing RGO content in NZF–RGO nanocomposites minimum RL was increased. Composite containing 50 wt% RGO, i.e. 50NZF–50RGO, showed minimum RL of −19.99 dB (∼99% loss) at 11.58 GHz with absorber thickness of 1.8 mm and effective bandwidth was 10.22–12.4 GHz (Fig. 6(B)). When thickness was increased from 1.8 to 1.95 mm, though minimum RL was decreased from −19.99 dB to −18.70 dB but effective bandwidth was increased from 10.22–12.4 GHz to 9.31–12.4 GHz. Hence, NZF–RGO nanocomposites have demonstrated that, incorporation of RGO caused to enhance minimum RL value of the composites. As NZF–RGO composites possess heterogeneous structure, the accumulation of virtual charges at the interface between NZF–RGO, having different dielectric constant, leads to interfacial polarization (known as Maxwell–Wagner polarization). This might be one of the important factors which played critical role in enhancing microwave absorption property of NZF–RGO compare to pure NZF.^76,77 For comparison RL values of several ferrites and ferrite based composites have been listed in Table S5.† It has been observed that RL value of 50NZF–50RGO is comparable and in some cases greater than most of these composites.^{11,22,23,36,39,40,78–90}

Fig. 6 Frequency dependence of reflection loss of synthesized (A) pure NZF and (B) 50NZF–50RGO nanocomposites by varying the thickness of the absorber.

Conclusions

In summary, we have described an in situ co-precipitation technique for preparation of NZF–RGO nanocomposites. Here, reduction of GO to RGO and formation of NZF nanoparticles from metal nitrates occurred simultaneously, which helped to anchor NZF nanoparticles on the sheets of RGO homogenously. Moreover, in this method no extra reducing agent was used to reduce GO. NaOH acted as precipitating agent of metal hydroxides, which formed final NZF nanoparticle as well reducing agent to reduce GO to RGO. To the best of our knowledge, this is the first time synthesis of NZF–RGO composite has been reported which acted as microwave absorbing material as well as magnetically separable catalyst. NZF–RGO nanocomposites having 50 wt% RGO content (50NZF–50RGO) exhibited excellent catalytic activity towards decolorization of various dyes. This also showed its capability to reduce a toxic herbicide (trifluralin) which is found frequently in water as residue. Reduction of trifluralin is not yet well reported in the literature as of now. Magnetic nature of 50NZF–50RGO makes it a magnetically separable catalyst, which solves the separation related problem associated with the nanosized catalysts. 50NZF–50RGO also exhibited ∼99% minimum RL in the X-band region with effective bandwidth at 10.22–12.4 GHz. This nanocomposite not only shows higher minimum RL in comparison with pure NZF but also its light weight, (due to presence of RGO) offers an added advantage. The heteroarchitectural structure of RGO–NZF composites caused to enhance its catalytic property as well as microwave absorption property. 50NZF–50RGO nanocomposites have demonstrated its capability to act as a multifunctional material in the area of heterogeneous catalyst as well as microwave absorber.

Acknowledgements

Dr N. N. Ghosh gratefully acknowledges financial support from DRDO, New Delhi, India (ERIP/ER/1305004/M/01/1523). We also express our thanks to Prof. Paul A. Millner and Martin Fuller, University of Leeds, U K, for recording TEM micrographs.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of characterization technique used, details of catalysis reactions, measurement of microwave absorption of the sample. Fig. S1 frequency dependent permittivity and permeability of (A) pure NZF and (B) 50NZF–50RGO nanocomposite, Fig. S2 EDAX spectra of synthesized 50NZF–50RGO nanocomposites; Fig. S3 thermal analysis of NZF–RGO composites, magnetic property of NZF–RGO nanocomposite, Fig. S4 Raman spectra of GO and NZF–RGO, Fig. S5 catalytic activity towards degradation of 4-NP, RhB and MO in presence of excess NaBH₄ by varying RGO content Fig. S6 reusability study of 50NZF–50RGO catalyst, Fig. S7 (A) XRD and (B) TEM analysis of the recycled catalysts 50NZF–50RGO. See DOI: 10.1039/c5ra26634c

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