CuO/SiO2 modified amine functionalized reduced graphene oxide with enhanced photocatalytic and electrochemical properties
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Graphene, an ideal two dimensional material, has attracted much attention due to its unique structural and physicochemical properties. Herein, we report synthesis of CuO/SiO2 modified amine functionalized reduced graphene oxide (rGO) and its excellent potentialities in environmental remediation and energy storage application. The structure, purity, functional groups and morphology of as-prepared CuO/SiO2:rGO nanocomposites were characterized by XRD, FTIR, FESEM and TEM. The catalytic activity of the CuO/SiO2:rGO in different ratios was evaluated by degradation of methylene blue (MB) under visible light condition. The CuO/SiO2:rGO (mass ratio 1:3) exhibits high catalytic activity with 99% degradation of 20 ppm MB in 7 min. CuO/SiO2:rGO nanocomposites (1:3) also demonstrate good electrochemical performance with specific capacitance of 235 F g−1, which is about fivefold higher than CuO nanoparticles. The nanocomposite (1:3) also reveals excellent cycling stability at 4 A g−1 for 1000 cycles and the capacitance retention was found to be 95% after 1000 cycles. These results validate that the development of a new class of composite necessitates the proper loading of metal oxide and GO to be used for energy and environmental applications.
KeywordsGraphene Degradation Supercapacitor Amine functionalized Methylene Blue
One of the trending two dimensional sp2 hybridized carbon materials is graphene. It is inspired in the research community due to its exceptional characteristics such as high conductivity, high theoretical surface area (2600 m2 g−1), high thermal conductivity [1, 2]. The physical properties of graphene which comprises a single atomic layer of carbon atoms arranged in honeycomb network structure. Graphene, being a zero overlap semi-metal with both electrons and holes as charge carriers thereby having high charge carrier mobility 250,000 cm2 V−1 s−1 at room temperature . The other inherent properties of graphene is its mechanical strength with Young’s modulus of 1 TPa and high thermal conductivity (~ 5000 W m−1 k−1). However, the strong cohesive van der Waals force adhering graphitic sheets to one another necessities strong exfoliation of π-stacked graphene sheets. Hence, it requires suitable solution-chemistry based approaches to produce chemically modified graphene. Synthesis of graphene by chemical methods facilitates functionalization with polymeric material, semiconductor metal oxides (SMO’s), biomaterials etc. enabling the modification of surface properties. Chemical based approaches relies on initial oxidation of graphite to graphite oxide, and subsequent exfoliation to graphene oxide. The oxidation and exfoliation causes physical changes in the graphite layers by intercalation, chemical functionalization and thermal expansion, thereby making graphene oxide hydrophilic . However, the physicochemical properties of the nanocomposites depend on the distribution of SMO’s over graphene layers. The functional groups of GO render them to be hydrophilic and enable the decoration of SMO via Van der Waals interaction [4, 5]. Among the various SMO’s, CuO has been investigated for supercapacitor [6, 7], photocatalyst  and sensor [9, 10] applications owing to its non-toxicity, abundance and low cost.
CuO is a p-type semiconductor material with a narrow band gap of ~ 1.2–1.5 eV [11, 12]. Beside the development of CuO photocatalyst, the study of supporting material, SiO2 combined to them is also carried out . The chemically inert mesoporous siliceous (SiO2) materials are often employed as a support because they possess high specific surface area, large pore volume, excellent thermal stability and are transparent to UV radiation. To diminish the recombination rate, CuO/SiO2 has to be combined with electron accepting materials such as graphene. Therefore, loading of CuO/SiO2 over graphene is advantageous and could be a good candidate for photocatalyst with improved degradation efficiency [14, 15]. Combination of SiO2 with graphene and other inorganic materials exhibits superior property when compared to bare GO and silica nanoparticles. Combining silica and CuO with graphene tunes the band gap energy levels thereby providing hybrid nanocomposites with highly efficient photocatalytic behaviour.
On the other hand, when CuO is used as an electrode material for supercapacitor, it suffers from poor specific capacitance. To further improve the performance of the supercapacitor, CuO/SiO2 is made as composite with grapheme and offers better performance benefitting from both electric double layer capacitance (EDLC) and pseudocapacitance mechanisms. In addition, incorporation of CuO/SiO2 into graphene layers improves the electrode–electrolyte accessibility and also prevents agglomeration/restacking of graphene layers [6, 16, 17].
In the current study, graphene encapsulated CuO/SiO2 nanocomposites were prepared using electrostatic self-assembly method. A facile synthesis route was instigated in which SiO2 spheres were first synthesized. Then copper chloride was reduced using sodium hydroxide and dextrose over SiO2, followed by amine functionalization and electrostatically self-assembled over reduced graphene oxide sheets. Though there are lots of reports on CuO and graphene based nanocomposites for photocatalytic and supercapacitor applications, the report on combining siliceous material with CuO/graphene by electrostatic self-assembly method is limited . The resulting hybrid nanostructure is electrochemically studied and is found to exhibit good electrochemical performance with excellent cycling stability.
2 Materials and methods
Graphite powder, sodium nitrate (NaNO3), sulphuric acid (H2SO4, 98%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 50 wt%), thiourea (SC(NH2)2), ferrous sulphate heptahydrate (FeSO4∙7H2O), 3-aminopropyldimethoxysilane ((CH3O)3 Si(CH2)3NH2, APS), copper chloride dihydrate (CuCl2.2H2O) ammonia (NH3), hydrochloric acid (HCl) deionised water (DI water) was used in the whole experiment.
2.1 Preparation of SiO2 nanospheres
To prepare SiO2 nanospheres, 14.5 mL of aqueous ammonia and 76 mL of absolute ethanol were added to 50 mL of distilled water containing 2.5 g of n-hexadecyltrimethylammonium bromide under magnetic stirring for 15 min. It was followed by the addition of 5 mL of triethylorthosilicate (TEOS) and continuously stirred for 2 h. The resulting white precipitate was filtered and washed with distilled water and ethanol (2 × 10 mL). After drying under vacuum overnight, the samples were calcined at 500 °C for 5 h.
2.2 Preparation of CuO/SiO2 composite
To prepare CuO/SiO2 nanospheres, 0.34 g of calcined SiO2 was dispersed in 100 mL of distilled water at 60 °C for 30 min. To the above solution, 0.34 g of CuCl2.2H2O was added and stirred for 15 min. It was followed by the dropwise addition of 0.2 M of NaOH until pH of 11. Simultaneously, 0.108 g of dextrose in 30 mL distilled water was added and continued to be stirred for 30 min at 60 °C. The resulting precipitate was centrifuged and washed with water several times (3 × 10 mL). After drying under vacuum overnight, the samples were calcined at 500 °C for 3 h. The similar procedure was followed to prepare CuO, but without the addition of SiO2.
2.3 Preparation of amine modified CuO/SiO2 composites
Amine modification was carried out by measuring 0.1 g of CuO/SiO2 in 50 mL isopropanol followed by the addition of 3-aminopropyl tri-methoxysilane (2 mL) and heated at 80 °C for 24 h. Finally the samples were centrifuged, dried in vacuum and dried at 60 °C for 1 h.
2.4 Synthesis of CuO/SiO2:rGO nanocomposites
GO was prepared by modified Hummers method [18, 19, 20, 21]. In the preparation process, 50 mg of amine modified CuO/SiO2 was sonicated for few minutes in 50 mL of distilled water. To the above solution, 50 mg mL−1 of GO was added and heated at 80 °C. Then 1.5 g of urea was added and continued to be heated at 80–90 °C for 4 h. The resulting solution was washed with water several times (3 × 10 mL) and allowed to dry at 60 °C overnight, the product being labelled 1:1 (CuO/SiO2:rGO). Similar experimental procedure was adopted to prepare other variable ratios of CuO/SiO2 and GO (CuO/SiO2:rGO in 1:2 (50 mg (CuO/SiO2): 100 mg GO), 1:3 (50 mg (CuO/SiO2): 150 mg GO), 2:1 (100 mg (CuO/SiO2): 50 mg GO), 3:1 (150 mg (CuO/SiO2): 50 mg GO) respectively).
2.5 Materials characterization
Crystalline nature was analyzed using Xpert-Pro diffractometer with Cu-Kα radiation (λ = 1.54060 Å) in the 2θ range between 5° and 90°. UV–visible diffuse reflectance spectra (DRS) analysis was conducted using UV–Vis spectrophotometer and reflectance was converted to absorbance by Kubelka–Munk function. Raman studies were carried at room temperature using Jobin–Yvon spectrometer using 514 nm argon lasers. FT-IR spectra of the samples were collected using ATR-FTIR (Bruker, Germany). The morphology of the samples were analyzed using field emission scanning electron microscopy (FE-SEM) with a FEI Quanta 3D.
2.6 Photocatalytic activity
The typical photocatalytic activity test was carried out under visible light condition on sunny days of October 2017 in the city of Tiruchirappalli (geographical location 10°39′26″N 78°44′46″E) between 10 am to 4 pm. All the photocatalytic degradation experiments were carried out with 20 mg of catalyst and the concentration of methylene blue was fixed as 20 ppm. Prior to irradiation, above mentioned solution mixture was stirred in dark for 2 min to obtain adsorption/desorption equilibrium after which the reaction solution was kept under visible light. Sample solution of 2 mL was syringed out at regular time intervals and supernatant was filtered. The concentration of the dye in the solution was measured as the function of time. UV–visible spectrophotometer was used to analyse the degradation of dye solution as the function of concentration.
2.7 Electrochemical measurement
The electrochemical performance of the prepared electrode material was investigated by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) techniques on an electrochemical workstation (Metrohm, Netherlands) using a three compartment cell at ambient temperature. The synthesized electrode materials, platinum wire and Ag/AgCl were used as working, counter and reference electrodes, respectively. The working electrodes were prepared by mixing active material (70%), acetylene black (20%) and PVDF (polyvinylidene fluoride) (10%) as a binder in agate mortar. N-methyl-2-pyrrolidene (NMP) was used as a solvent. The homogenous slurry was coated on carbon felt (1 cm × 1 cm). The resulting samples were dried at 80 °C for 12 h. All electrochemical measurements were done in 1 M H2SO4 electrolyte.
3 Results and discussion
3.1 X-ray diffraction analysis
The influence of calcination over the formation of CuO and CuO/SiO2 nanostructures was also examined by X-ray diffraction (Fig. S1). When CuO/SiO2 nanocomposites were surface modified with APS, the diffraction peak intensities were decreased, in which positively charged NH3+ groups anchored in the surface of CuO/SiO2, which consequently resulted in the reduction in the size of nanostructures. There are no additional peaks in the diffraction pattern obtained; rather there is change in the diffraction intensities when CuO/SiO2 nanostructure is made composite with variable ratios of GO. In CuO/SiO2:rGO (mass ratio of 1:3), the major diffraction intensities of CuO is reduced completely, which ensures that CuO/SiO2 nanostructures are anchored on rGO sheets. In other CuO/SiO2:rGO mass ratio composites, there is not much decrease in the diffraction intensities.
3.2 Raman analysis
3.3 Microscopic studies
3.4 UV–visible diffuse reflectance spectra analysis
3.5 Photocatalytic activity
3.6 Electrochemical studies
Nanostructured CuO/SiO2:rGO hybrid composites were synthesized by a simple low temperature wet-chemical method. The X-ray diffraction pattern confirms that CuO/SiO2:rGO nanocomposites are in monoclinic crystal system with C2/C space group. The Raman spectrum of CuO/SiO2:rGO nanocomposite corroborates the sp3 and sp2 carbon vibrations in graphitic lattice. Microscopic analyses results demonstrate the dispersion of CuO/SiO2 over rGO sheets. Investigations on photocatalytic degradation of MB reveal that CuO/SiO2:rGO (1:3) nanocomposites afford complete degradation of MB in 7 min. The electrochemical studies suggest that the synergistic effect enables the CuO/SiO2:rGO (1:3) nanocomposite to exhibit excellent electrochemical performance with high specific capacitance and good cycling stability even after 1000 cycles. In principle, these findings suggests that CuO/SiO2:rGO nanocomposites were found to be an excellent catalyst for the degradation of organic dye pollutant under visible light and a promising candidate for electrochemical capacitor electrode material.
The author PJS acknowledges the receipt of fellowship from TEQIP-II, BIT campus. KN and MS acknowledge the CSIR for partial funding. PB acknowledges CSIR-HRDG for providing support under Scientist Pool Scheme (No: 13(8778-A)/2015-Pool). TDT acknowledges CSIR for financial support (No. 01(2818)/14/EMR-II). The authors acknowledge SNR Sons and Charitable Trust, Coimbatore, for electrochemical work station and XRD characterization facilities.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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