Co₃O₄/graphene nanocomposite: pre-graphenization synthesis and photocatalytic investigation of various magnetic nanostructures

Abstract

For the first time, a novel technique for preparing cobalt(II) acetyl acetonate [Co(acac)₂] nanostructures has been developed by using the sublimation process. It is found that the sublimation temperature has a main role in creating the special morphology and size of [Co(acac)₂] and Co₃O₄ nanostructures. The nano-sized Co₃O₄ were obtained by thermal decomposition of [Co(acac)₂] nanostructures at 400 °C for 120 min. The produced nanostructures were characterized by powder X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) analysis, Raman spectroscopy, transmission and scanning electron microscopy (TEM, SEM). Magnetic and optical properties of the final product were measured at room temperature by a vibrating sample magnetometer (VSM) and UV-vis spectroscopy, respectively. The Co₃O₄ nanostructures were anchored onto the graphene sheets and the Co₃O₄/graphene (Co₃O₄/GR) nanocomposite was synthesized through pre-graphenization successfully. To investigate the catalytic properties of Co₃O₄/G nanocomposites, photooxidation of methyl orange (MO) was performed. The performed photocatalytic test shows that the methyl orange degradation was about 54.9% after 75 min irradiation of UV light.

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1. Introduction

Recently, very much attention has been focused on nanomaterials because of their unique properties. Inorganic nanomaterials could be used in various fields of application, because of their fascinating optical, magnetical and electrical properties.¹ Furthermore, nanoparticle size and shape are two factors that affect the optical and electronical properties and of course these factors can be changed by the synthesis method employed.

Graphene (GR) is an allotrope of carbon with two-dimensional material properties. Graphene has attracted a great deal of attention due to its many unexpected properties.² Recently, graphene-based metal oxide nanocompoites have been studied extensively, because of their potential application in fuel cells, catalysis, supercapacitors and light energy conversion.³ Furthermore, they create a new perspective for utilizing single-atom-thick graphene as a support to form new nanocomposites and for using their unique properties. The transition metal oxides are used for various applications such as: catalyst, pigment, sensor, magnetic material, in Li-ion rechargeable batteries, and in solar energy storage devices.^4–8 Graphene-based metal oxides nanocompoites represent an enhanced properties compare to their components individually,^9–11 because of their specific structure of 2-dimensional graphene sheets that cause to the charge transfer at the interface of graphene sheets and metal oxides, the metal oxides can be dispersed on the graphene sheets and show a synergistic effect.¹² Separation of electron–hole is a key parameter in photocatalysts because delay their recombination process therefore photocatalytic reactions will be done well. Recombination process in metal oxide/graphene nanocompoites will be decreased because of rapidly electronic transfer from graphene to metal oxide and vice versa, so, photocatalyst performance will be improved. Because of concerns about environmental pollutions, finding a photocatalyst with high performance is been a challenge. So, the use of graphene-based metal oxides nanocompoites as photocatalyst can be a response to this problem.

Recovering and reusing of heterogenous catalysts is a main parameter in catalyst reactions, so, the synthesis of magnetic photocatalysts such as TiO₂ nanolayer coating on cobalt ferrite nanoparticles,¹³ Bi₂O₃/SrFe₁₂O₁₉,¹⁴ ZnFe₂O₄/BiVO₄,¹⁵ and TiO₂/SiO₂@Fe₃O₄ (ref. 16) have developed recently because of the magnetic photocatalysts can be recovered simply through magnetic fields.

One of the most common transition metal oxides is Co₃O₄ that can be used as magnetic photocatalyst. The bulk Co₃O₄ is a p-type semiconductor with stable direct optical band gap at 1.48 and 2.19 eV (ref. 17) and an antiferromagnetic solid¹⁸ but the Co₃O₄ nanostructures has different optical and magnetic properties that are size dependent.^19,20

Among various methods developed for the synthesis of Co₃O₄ like thermal dissociation of complex,⁶ hydrothermal,²¹ co-precipitation,²² microwave,²³ and thermal decomposition of organometallic compounds the last one is more applicable due to its various advantages, including better control of purity, composition, phase, homogeneity and also, this method does not need the expensive and complicated equipment.^19,20

Our group, recently developed an appropriate route to prepare coordination compounds in nano-scale using of sublimation method as a new vapor-phase deposition route that followed by synthesizing metal oxides via thermal decomposition of these precursors.^24,25 Here, we present a synthesis method of Co₃O₄ nanostructures through thermal decomposition of [Co(acac)₂] nanostructures as a precursor and using pre-graphenization technique for preparing of Co₃O₄/GR nanocomposites. It is the first report on the synthesis of cobalt oxide nanostructures by using nano-size precursor that is prepared through sublimation method. Furthermore, methyl orange (MO) degradation was carried out to study the photocatalytic activity of synthesized Co₃O₄/GR nanocomposites through pre-graphenization under UV irradiation.

2. Experimental

In this work, bis(acetylacetonato)cobalt(II) powder (100 mesh, 99.998%) was purchased from Sigma-Aldrich and used as a precursor. The magnetic properties of the samples were detected at room temperature using a vibrating sample magnetometer (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran).

2.1. Synthesis of [Co(acac)₂] and Co₃O₄ nanostructures

Preparation of [Co(acac)₂] nanostructures from its bulk was done in a cold finger set under vacuum condition. Each experiment was begun with 0.1 g of bulk powder, which was transferred in the bottom of external pipe. The evaporated material flow is from the hot section (where the initial material is placed) to the cold area that is connected to a vacuum pump. The needle-like depositions which collected at different sublimation temperatures were transferred to crucible and placed in a furnace for synthesis of Co₃O₄ at 400 °C for 120 min. The obtained products were characterized by SEM, TEM, EDS, XRD, VSM and UV-vis.

2.2. Synthesis of graphene (GR) from graphene oxide (GO)

In this work, the used GO was prepared by a modified Hummers method from natural graphite²⁶ and reduced through the described method in previous work.²⁷

2.3. Preparation of Co₃O₄/GR nanocomposite

The Co₃O₄/GR nanocomposite was synthesized via a pre-graphenization technique. Briefly, 0.1 g from obtaining nano-sized Co₃O₄ (0.4 mmol) was then added into the black suspension of graphene (0.5 mg mL⁻¹) and magnetically stirred for 20 min at 90 °C. Finally, the above mixture was put in a household microwave oven (Haier, 2450 MHz, 750 W) for 10 min and then cooled to room temperature naturally. Subsequently, the black precipitation was filtered, washed with distilled water and absolute alcohol, and dried at 100 °C for 12 h in a vacuum oven. The resulting black powder was collected for the following characterization. The obtained product was characterized by Raman, XRD, SEM and UV-vis spectroscopy.

2.4. Characterization

The crystalline phase of the obtained precursor and products were analyzed using a diffractometer of Philips Company with X'PertPro monochromatized Cu Kα radiation (λ = 1.54 Å). Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets. Microscopic morphology of the products was studied by FESEM (Mira3 tescan) and TEM (HT-7700). The energy dispersive spectrometry (EDS) analysis was studied by XL30, Philips microscope. Magnetic properties were measured using a vibrating sample magnetometer (VSM) (BHV-55, Riken, Japan). The Raman analysis system SENTERRA is equipped with high-energy laser diodes that emit both visible and invisible laser radiation in the near infrared region.

3. Results and discussion

The different experiments were carried out for investigation of the sublimation temperature effect on morphology and size of [Co(acac)₂] and Co₃O₄ nanostructures. The preparation conditions are demonstrated in Table 1.

Table 1 Preparation conditions for samples 1–6

Sample (sublimated)	Sublimation temperature (°C)	Production of sublimation (decomposed in 400 °C, 120 min)	Temperature of precursor preparation (°C)
1	160	4	160
2	170	5	170
3	180	6	180

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Fig. 1(a–c) show SEM images of the [Co(acac)₂] nanostructures sublimated at 160 °C (sample no. 1), 170 °C (sample no. 2), and 180 °C (sample no. 3), respectively. As shown in Fig. 1a, when the precursor was sublimated in 160 °C, one-dimensional (1-D) structure obtained that its length to diameter ratio is 25. At 170 °C, the (1-D) structures with length and diameter about 600 and 140 nm along with unknown structures are shown (Fig. 3b). The large and strange structures obtained at 180 °C (Fig. 3c). According to Fig. 1(a–c), length to diameter ratio of products decreased with increasing sublimation temperature. SEM images of Co₃O₄ synthesized by using sublimated precursor at different temperature are illustrated in Fig. 1(d–f). In Fig. 1d, one-dimensional structures together with 20 nm nanoparticles are shown (sample no. 4). Fig. 1e represents nanoparticles with size about 70–80 nm (sample no. 5). SEM image of sample no. 6 is shown in Fig. 1f, the agglomerated particles with various sizes are formed. By considering of SEM images, sample no. 4 was chosen as desired product and so characterized by further analysis.

Fig. 1 SEM images of sample no. (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6.

Fig. 2 (a) and (b) TEM images, (c) XRD pattern and (d) EDS spectrum of samples no. 4.

Fig. 3 (a) Low, (b) high magnification FESEM images, and (c) XRD of the Co₃O₄/GR nanocomposite.

The TEM images of the as-prepared Co₃O₄ (sample no. 4) synthesized by sublimated at 160 °C are shown in Fig. 2a and b. Fig. 2a shows that 15 nm nanoparticles are coated on nanorods with diameter about 100 nm. This structure can increase the specific surface area of the sample and utilize for some potential applications such as catalysts and chemical supports. Fig. 2b is the magnification of the Fig. 2a, which shows the results of this figure clearly.

The composition of the as-prepared product (sample no. 4) was determined by the typical X-ray diffraction (XRD) pattern (Fig. 2c). All of the diffraction peaks in this pattern correspond to the reflections of cubic phase Co₃O₄ (space group: Fd3̄m, JCPDS = 73–1701), with lattice constants a = b = c = 8.0835. The particle size of this sample was calculated from the major diffraction peak of the corresponding Co₃O₄ (3 1 1) by using the Scherrer formula and estimated about 10 nm.²⁸

The chemical composition and purity of synthesized Co₃O₄ nanostructures (sample no. 4) was analyzed by the EDS measurement, and the result has been shown in Fig. 2d. In the EDS spectrum of sample no. 4, only the characteristic peaks of Co and O are observed. The EDS results confirm the purity of the final product.

Fig. 3a and b show the low and high magnification FESEM images of the Co₃O₄/GR nanocomposite. From the high magnification image (Fig. 3b), it is clear that products are sheet-like that consists of 20 nm sized nanoparticle.

The purities of the Co₃O₄/GR nanocomposite were confirmed by X-ray diffraction (XRD) pattern. As shown in Fig. 3c, all the identified (1 1 1), (2 2 0), (3 1 1), and (2 2 2) diffraction peaks were observed in the synthesized Co₃O₄/GR nanocomposite. A broad peak located at 24.5 corresponds to layered GR.²⁹ The existence of diffraction peaks in this pattern (Fig. 3c) can be confirmed the presence of Co₃O₄ crystals in the graphene-based composite, which is also supported by the results of SEM.

The magnetic properties of the Co₃O₄ nanostructures (sample no. 4) shown in Fig. 4a. The hysteresis loop of this sample depicts a paramagnetic behavior, although bulk Co₃O₄ is antiferromagnetic,³⁰ this change in magnetic property can be ascribed to finite size effects.³¹

Fig. 4 Magnetization versus applied magnetic field at room temperature for (a) pure Co₃O₄ nanostructures and (b) Co₃O₄/GR nanocomposite.

By decreasing the size of magnetic particles: (1) finite size effect role becomes more significant and (2) the presence of defects, broken nature of exchange bonds, fluctuations in the number of atomic neighbors and lattice expansion cause disorder of surface spin and frustration.³² The magnetic hysteresis loop measurements of the Co₃O₄/GR nanocomposites were carried out at 298 K with the applied magnetic field sweeping from −10 kOe to 10 kOe (Fig. 4b).

The hysteresis loop of Co₃O₄/GR nanocomposite exhibits typical weak ferromagnetic behavior at room temperature, and indicates the values of the coercivity force (H_c) and remnant magnetization (M_r) to be 236 Oe and 0.003 emu g⁻¹, respectively. It is easy to find that the coercivity force and remnant magnetization of Co₃O₄/GR nanocomposite are larger than those of pure Co₃O₄ nanostructures and magnetic properties of Co₃O₄/GR nanocomposite and pure Co₃O₄ is different. It has been demonstrated that magnetization of materials is very sensitive to component, size and morphology of the materials.³³

The band gap is a main parameter in determining characteristics of semiconductor and often is estimated from the UV absorption spectrum.

Fig. 5a shows the UV-vis spectrum of sample no. 4, where the corresponding absorption edge appears at 320, 370, and 720 nm. Optical band gap (E_g) may be evaluated based on the optical absorption spectrum using the [(Ahν)ⁿ = B(hν − E_g)] equation,³⁴ where hν is the photon energy, A is absorbance, B is a material constant and n is 2 or 1/2 for direct and indirect transitions, respectively. The value of hν extrapolated to (Ahν)² = 0 gives the absorption band gap energy. The linear relation was found for n = 2, suggesting that the as-prepared Co₃O₄ nanostructures are semiconductors with direct transition at this energy. Two regions with linear relationships, giving two E_g values of 2.0 and 3.2 eV (Fig. 5b). The band gap of 2.0 eV can be attributed to the O⁻² → Co⁺² charge transfer phenomena (electron excitation from valence to conduction band), while the 3.2 eV band gap relates to the O⁻² → Co⁺³ charge transfer (with the Co⁺³ level located below the conduction band).⁶ By considering previous works, can be found that the age values of synthesized Co₃O₄ nanostructures in here, are larger than those of bulk Co₃O₄ (E_g = 1.77 and 3.17 eV, respectively) and Co₃O₄ nanoparticles that reported, formerly.^6,20,23 The increase in the band gap of the Co₃O₄ nanostructures may attribute to the quantum confinement effects of nanomaterials.

Fig. 5 (a) UV-vis spectrum, (b) curve (Ahν) n versus hν of pure Co₃O₄ nanostructures, and (c) UV-vis spectrum of Co₃O₄/GR nanocomposite.

The optical properties of Co₃O₄/GR nanocomposite is shown in Fig. 5c. Three absorption peaks at about 275, 360, and 680 nm are shown related to the optical absorption of graphene, and Co₃O₄. The peak at about 230 nm is attributed to the π → π* transitions of the aromatic C=C of GO that is red-shifted for GNS and shows at 275 nm and indicating that the electronic conjugation within the reduced graphene sheets was reconstructed upon reduction of graphene oxide.^35,36 Two peaks at 360 and 680 nm were attributed to the optical absorption of Co₃O₄. The observed blue-shift in Co₃O₄/GR nanocomposite compare to pure Co₃O₄ is due to the chemical bond between Co₃O₄ nanostructures and graphene layers. In fact graphene layers can be caused absorption edge wavelength-shifting of graphene-based nanocomposites through Burstein–Moss effect (Scheme 1).^37,38 The shifting to higher (red-shift)³⁹ or lower (blue-shift)⁴⁰ wavelengths is determined by the work function value of metal oxides compare to pristine graphene layers.

Scheme 1 Illustration of Burstein–Moss effect in Co₃O₄/GR nanocomposite.

The observed blue-shift in this work is due to this issue: when, Co₃O₄/graphene nanocomposite is prepared and chemical bonds are formed between graphene and Co₃O₄, electrons are flowing from graphene to Co₃O₄ levels and compressing at interface graphene and Co₃O₄. Finally, potential barriers are formed that limiting electron transfers between levels of Co₃O₄. So, the required energy for exciting and transferring of Co₃O₄ electrons is increasing.

The photocatalytic activity of Co₃O₄/GR nanocomposite and pure Co₃O₄ were investigated by monitoring the degradation of methyl orange solution (MO) dye under UV light irradiation (Fig. 6).

Fig. 6 Degradation of MO under UV irradiation by Co₃O₄/GR nanocomposite and pure Co₃O₄.

The photocatalytic degradation reaction was performed out in a quartz photocatalytic reactor and carried out with a 10 ppm MO solution containing 0.05 g of photocatalyst (Co₃O₄/GR nanocomposite or pure Co₃O₄). This mixture was aerated for 30 min to reach adsorption equilibrium. Then, the mixture was placed inside the photoreactor in which the vessel was 40 cm away from the UV. The quartz vessel and light sources were placed inside a black box equipped with a fan to prevent UV leakage.

The experiments were performed at room temperature and pH of the MO solution was adjusted 2–3. The MO degradation percentage was calculated by eqn (1) as follows:

where A₀ and A_t are the absorbance value of MO solution at 0 and t min, respectively.⁴¹

According to photocatalytic calculations by eqn (1), the MO degradation percentage was about 54.9 and 34.3 after 75 min irradiation of UV light in presence Co₃O₄/GR nanocomposite and pure Co₃O₄ as photocatalyst, respectively.

By considering the written photocatalytic reaction mechanism and given schematic for Co₃O₄/GR nanocomposite can be said that probability of recombination electron–hole in Co₃O₄/GR nanocomposite is less than pure Co₃O₄ so its photocatalyst performance is better. The photocatalytic degradation of MO with Co₃O₄/GR nanocomposite was explained by the photogeneration of electron–hole pairs between the conduction (CB) and valence bands (VB) due to excitation of Co₃O₄ under UV light illumination and flowing electrons between Co₃O₄ to graphene layers.⁴² In this work, the superior photocatalytic activity may be related to high surface area, the high separation rate of charge carriers, excellent structure, and great electrical and optical properties.⁴³

The role of graphene layers in improving catalyst performance is shown in Scheme 2.

Scheme 2 The role of graphene layers in improving catalyst performance.

The possible mechanism of degradation of dye by photocatalysts can be summarized as follow:

Reaction photocatalytic of Co₃O₄/GR nanocomposite:

Co₃O₄ + hν → (e_CB⁻) + (h_VB⁺) → graphene (e⁻ + h⁺)

Co₃O₄ (h_VB⁺) + OH_ads⁻ → ˙OH + Co₃O₄

dye + ˙OH → degradation of dye

Reaction photocatalytic of Co₃O₄/GR nanocomposite:

UV + Co₃O₄ → Co₃O₄˙ (h⁺ + e⁻)

h⁺ + H₂O → H⁺ + ˙OH

2h⁺ + 2H₂O → 2H⁺ + H₂O₂

H₂O₂ → ˙OH + ˙OH

e⁻ + O₂ → ˙O₂⁻

˙O₂⁻ + 2OH˙ + H⁺ → H₂O₂ + O₂

H₂O₂ → ˙OH + ˙OH

˙OH + dye → degradation of dye

The used Co₃O₄/GR nanocomposite as photocatalyst was recovered and used for two other times. Degradation percentage of MO was reduced (Fig. 7) from 54.9 (step 1) to 28.3 (step 2) and to 10.6 (step 3) because of the active sites in catalyst were decreased at each stage.

Fig. 7 Degradation of MO under UV irradiation by Co₃O₄/GR nanocomposite at three step.

The Raman spectrum of the prepared Co₃O₄/GR nanocomposite in this work is shown in Fig. 8. A broad D band (1341.87 cm⁻¹), a sharp G band (1581.20 cm⁻¹), and a small 2D band (2683.6 cm⁻¹) are observed in this spectrum. Usually, D band is known as defect band or disorder band that is very weak in graphite and pure graphene and its intensity is directly proportional to the amounts of defects or disorders in the prepared products.

Fig. 8 Raman spectrum of Co₃O₄/GR nanocomposite.

Herein, can be said that the composite formation and the presence of anchored Co₃O₄ on graphene layers are caused to create disorder and defect in graphene lattice, so a broad D band is observed. The G band is related to in-plane bond-stretching motion of the pairs of C sp² atoms and its position is a main factor for determination of the number of graphene layers. 2D band is used to determine thickness of graphene layers and its position is not important for characterizing but its shape is a key parameter for the thickness determination of graphene layers.⁴⁴

Graphene layers have high specific area and unique electronic properties, so synthesis of graphene-based nanocomposites cause to improve photocatalytic activity due to increasing of its surface and it cause to delay recombination process. Synthesis of graphene layers and its existence in Co₃O₄/GR nanocomposite was approved by Raman spectroscopy. In this spectrum, G′ peak of graphene is shown at 2650 cm⁻¹ that is shifted toward higher wavelength compared to graphite (Fig. 9). Therefore, improvement of as-prepared nanocomposite photocatalytic activity is proved by the given Raman spectrum (Co₃O₄/graphene nanocomposite) in this work.

Fig. 9 The shifting of G′ peak of graphene and graphite.

4. Conclusion

In summary, an eco-friendly, economical and simple method to synthesize Co₃O₄ nanostructures by solid-state decomposition of [Co(acac)₂] precursor was represented. The precursors were prepared via a sublimation process as a novel vapor-phase deposition method at different temperatures. By increasing the sublimation temperature the obtained structures become irregular and therefore the size of the final product was increased. The one-dimensional structures [Co(acac)₂] were achieved in sublimation temperature 160 °C, these rods are known as desired precursor that have a length to diameter ratio about 30. The obtained Co₃O₄ represents a paramagnetic that is different from magnetic property of bulk Co₃O₄, this is because of the size finiteness effect. The calculated optical absorption band gaps of the Co₃O₄ nanostructures were estimated about 2.3 and 3.6 eV, which compared to the values of the bulk sample show blue shift. The Co₃O₄ nanostructures anchored onto graphene sheets and Co₃O₄/GR nanocomposite synthesized through pre-graphenization, successfully. The absorption peaks of Co₃O₄/GR nanocomposite in compared to Co₃O₄ nanostructures were blue-shifted that related to the interaction between the Co₃O₄ nanostructures and the graphene sheets. To investigate the catalytic properties of Co₃O₄/GR nanocomposite, photooxidation of methyl orange (MO) was performed. The performed photocatalytic test shows that the methyl orange degradation was about 54.9% after 75 min irradiation of UV light. This approach provides a simple route for the preparation of Co₃O₄ nanostructures and Co₃O₄/GR nanocomposite with high purity.

Acknowledgements

The authors are grateful to Council of University of Kashan for providing financial support to undertake this work by Grant no. (159271/125).

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