Reduced graphene oxide uniformly decorated with Co nanoparticles: facile synthesis, magnetic and catalytic properties

Abstract

Herein, Co nanoparticles with high dispersivity were grown in situ on reduced graphene oxide (RGO) nanosheets by an environmentally friendly and facile one-step strategy. The as-synthesized products were characterized by X-ray powder diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The magnetic and catalytic properties of the RGO/Co nanocomposites were systematically investigated. The results reveal that the RGO/Co nanocomposites have room-temperature ferromagnetic characteristics with Co particle size below single domain size. In addition, these RGO/Co nanocomposites also exhibit excellent catalytic activities toward the reduction of 4-nitrophenol by NaBH₄ and enhanced electrocatalytic properties for the oxidation of glucose. It is believed that this eco-friendly and facile route can be extended to synthesize other metal nanostructures on RGO nanosheets with various functions, and provides a new opportunity for the application of graphene/metal nanocomposites.

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Introduction

Recently, metal nanoparticles have gained increasing attention because of their unique physical and chemical properties and potential applications in the fields of catalysis, environment and clean-energy.^1–3 Noble metals possess high chemical stability and catalytic activity on the nanoscale, and have been a focus of intensive research in practical applications.^4–6 However, the scarcity and high cost of noble metal nanocatalysts limits their commercial application. Therefore, substitute catalysts composed of nonprecious metals have been in high demand. Aside from expensive noble metals, 3d-block transition metals (such as Fe, Ni, and Co) have also attracted intense interest in industry due to their low cost and easy availability.⁷ As is well known, Co represents a class of important transition metal material, which is readily available and has been extensively applied in catalytic reactions.^8,9 In addition, Co nanoparticles, which possess excellent magnetic characteristic, could provide an opportunity to realize controllable on–off reactions using a simple magnetic process. Although Co nanoparticles could present high catalytic performance, aggregation was found for this kind of materials because of the effects of magnetic force and smaller particle, leading to lower catalytic efficiency in long-term application. Thus, the development of active and stable Co-based catalysts is highly desirable.

Generally, to overcome the above disadvantage, Co nanoparticles are desired to disperse finely on a suitable solid support with low cost, high surface area, and superior chemical and physical properties. Lots of materials have been used as supports for the deposition of Co nanoparticles. Recently, carbon nanomaterials as the solid supports to disperse and stabilize the metal nanoparticles in catalytic process have received a great attention.^10–12 As a novel form of carbon, graphene is a promising matrix to support metal nanoparticles to realize new catalysts due to its unique physical, chemical and remarkable tunability.^5,13,14 Compared with the unsupported metal nanoparticles, graphene may enhance the stability and improve the catalytic properties of them.^8,15 Moreover, charge transfer occurs across the graphene–metal interface, which can also enhance the catalytic properties on certain metal nanoparticles.⁹ Although some investigations have been done in the synthesis of graphene/Co nanocomposites, these preparation methods usually suffer from the use of harmful organic reagents or surfactants, and the Co nanoparticles possess uneven sizes and poor dispersion on the surface of graphene. In addition, little work has been done in magnetic graphene/Co nanocomposites for the catalytic reduction of 4-nitrophenol and electrocatalytic oxidation of glucose.

In this work, we demonstrate an environmentally friendly and facile one-step strategy to prepare Co nanoparticles with good controllability and high dispersivity on RGO nanosheets. The structure, morphology, magnetic property and catalytic activities were investigated in detail. The as-synthesized RGO/Co nanocomposites exhibit excellent ferromagnetic property and enhanced catalytic activities toward the reduction of 4-nitrophenol. Especially, the RGO/Co nanocomposites also show prominent electrocatalytic response to glucose with lower detection limit, higher sensitivity, wider linear range and excellent selectivity.

Experimental

Materials

Natural flake graphite with a particle size of 45 μm and cobalt acetylacetonate (Co(acac)₃) were obtained from Aladdin Industrial Corporation (Shanghai, China). All other chemicals are of analytical grade and used without further purification.

Preparation of RGO/Co nanocomposites

Graphite oxide was prepared from the natural flake graphite according to a modified Hummers method.^16,17 The typical procedure for the synthesis of RGO/Co nanocomposites is as follows: 25 mg of graphite oxide was dispersed in 50 mL of triethylene glycol with ultrasonication for 1 h to form a homogeneous dispersion. Subsequently, a certain amount of Co(acac)₃ was added into the above mixture gradually under vigorous stirring. After stirring for about 15 min, the resulting mixture was then transferred into a 100 mL round-bottomed flask and heated at 270 °C under magnetic stirring for 1 h, and followed by cooling to room temperature naturally. The solid products were collected by centrifugation, washed with deionized water and absolute ethanol for several times, and then dried in a vacuum oven at 45 °C. The obtained products were denoted as RGO/Co-0.15, RGO/Co-0.2 and RGO/Co-0.25 for feeds of 0.15, 0.2 and 0.25 g of Co(acac)₃, respectively.

Instrumentation and measurements

The morphology and size of the products were examined by transmission electron microscopy (TEM, JEOL JEM-2100). The phase structures of the as-synthesized products were characterized using X-ray diffraction (XRD, Shimadzu XRD-6100 Lab) with Cu-K_α radiation (λ = 1.5406 Å) at a scanning rate of 5° min⁻¹. Raman scattering was performed on a DXR Raman spectrometer using a 532 nm laser source. The instrument employed for the X-ray photoelectron spectroscopy (XPS) studies was a Thermo ESCALAB 250XI spectrometer. The magnetic measurements were carried out using a vibrating sample magnetometer (VSM, Nanjing University HH-15) at room temperature (300 K). Ultraviolet-visible (UV-vis) spectroscopy measurements were performed on a UV-1800PC UV-vis spectrophotometer.

Catalytic reduction of 4-nitrophenol

In a typical reaction, 5.0 mg of the nanocomposites as the catalyst was added into 100 mL of distilled water. Then, a freshly prepared aqueous solution of NaBH₄ (2.0 mL, 1.0 M) was added in and stirred for 15 min. Finally, 0.5 mL of 4-nitrophenol aqueous solution (10 mM) was added into the mixing solution to start the reaction, and the reaction system was kept stirring until the solution became colorless. During the reaction process, 2.0 mL of the reaction solution was withdrawn at a given time interval, which was immediately recorded in the UV-vis spectrophotometer in a scanning range of 250–500 nm at ambient temperature.

Electrocatalytic oxidation of glucose

The electrocatalytic activities of the as-prepared nanocomposites towards glucose were carried out on a beaker-type three-electrode setup using a CHI 760D electrochemical analyzer (Chen Hua Instruments, Shanghai, China) at room temperature. Platinum wire and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. Glassy carbon electrode (3 mm in diameter) coated with the as-prepared nanocomposites was used as the working electrode. Prior to the surface modification, the glassy carbon electrode was polished using 1.0, 0.3, and 0.05 μm Al₂O₃ powders sequentially, followed by sonication in absolute ethanol and distilled water successively. Then the washed glassy carbon electrode was dried at room temperature and ready for modification. To fabricate a working electrode, catalyst ink was prepared by dispersing 5 mg of catalyst into a mixed solvent containing 1 mL of absolute ethanol and 20 μL of Nafion solution (5 wt%, Alfa Aesar), and then ultrasonicated for 30 minutes to form a homogeneous ink. Then, 6 μL of the catalyst ink was spread onto the surface of the glassy carbon electrode and dried in ambient air. The catalytic activity of the RGO/Co electrode in oxidation of glucose was determined by cyclic voltammetry in 0.1 M NaOH containing different concentrations of glucose in a potential range of 0–0.6 V versus SCE. Amperometric response to the changes of glucose concentration was performed under constant stirring at room temperature in the 0.1 M NaOH solution.

Results and discussion

Characterization of the RGO/Co nanocomposites

In this paper, the typical Co nanoparticles grown on RGO nanosheets were obtained by simultaneously reducing Co(acac)₃ as well as GO in triethylene glycol medium. The typical XRD patterns of graphite oxide and RGO/Co-0.2 nanocomposites are shown in Fig. 1a. The diffraction pattern of graphite oxide shows a strong peak at 2θ of about 11° corresponds to its (001) reflection.¹⁸ For the RGO/Co-0.2 nanocomposites, this (001) peak completely disappeared and a broad (002) diffraction peak located at 2θ of about 24° was observed, which suggests that graphite oxide has been flaked and reduced. All of the other diffraction peaks can be indexed to hexagonal Co (JCPDS 05-0727), which indicates that RGO/Co nanocomposites have been successfully prepared. Raman spectra of graphite oxide and RGO/Co-0.2 nanocomposites are presented in Fig. 1b. Both display two prominent peaks at about 1348 and 1593 cm⁻¹, corresponding to the well-documented D band and G band, respectively. It is known that the D band is associated with structural defects and disorders, while the G band is usually assigned to the E_2g mode observed for C sp² carbon domains.^19,20 The intensity ratio of D band to G band (I_D/I_G) is usually used as a measure of the defects and the degree of disorder in carbon structure.^21,22 From Fig. 1b, an increment in I_D/I_G (1.55) is obtained for RGO/Co-0.2 nanocomposites as compared with that of graphite oxide (1.32), indicating that more defects and disorders were produced after the introduction of Co nanoparticles on the surface of RGO sheets.

Fig. 1 (a) XRD patterns and (b) Raman spectra of the graphite oxide and RGO/Co-0.2 nanocomposites.

In situ refluxing reactions using Co(acac)₃ and GO in triethylene glycol led to the formation of nanocomposites with the Co nanoparticles uniformly decorated on RGO nanosheets as revealed by TEM observations. From Fig. 2a, partially folded and crumpled RGO nanosheets can be observed, and Co nanoparticles can hardly be observed from this TEM image owing to their small size. Fig. 2b and c present the higher magnification TEM images of RGO/Co-0.2 nanocomposites. It can be seen that Co nanoparticles are exclusively deposited on RGO sheets with a highly uniform distribution. Neither free Co nanoparticles nor bare RGO sheets were observed during the TEM investigation, indicating a highly efficient combination between the RGO support and the Co nanoparticles. After the graphite were oxidized, large amounts of defects and oxygen-containing groups will be uniformly formed on the surface of GO. When Co(acac)₃ was mixed with the GO suspension, some Co³⁺ cations would attach to the surface of GO through electrostatic interaction and serve as nucleation sites. In the presence of a reductant, Co³⁺ ions were in situ reduced to form Co nanocrystallites, while GO nanosheets were reduced to RGO in the meantime. Thus, these defects and oxygen-containing groups on GO would benefit anchoring of Co nanoparticles on GO sheets with high dispersion.^23,24 On the other hand, it is difficult to synthesize stable metal nanoparticles that anchored on graphene supports because the nanoparticles tend to aggregate into larger ones, especially under the harsh reaction conditions such as strong reductant and subsequent deoxidization processes.⁶ Therefore, triethylene glycol, a mild reductant will also aid in the preparation of uniformly distributed Co nanoparticles. High resolution transmission electron microscopy (HRTEM) image (Fig. 2d) of these nanoparticles revealed clear lattice fringes with an interplanar distance of 0.208 nm, which corresponds to the (002) lattice spacing of hexagonal Co. The size of the Co nanoparticles is only about 4 nm. It is reported that highly dispersed metal nanoparticles with smaller particle size on supports have advantages in catalytic activity.²⁵ Therefore, the resultant RGO/Co nanocomposites promise great potential as functional nanomaterials for application in catalysis. In this study, the distribution density of Co on the RGO sheets can be fine tuned by adjusting the feeding amount of Co(acac)₃. As shown in Fig. S1,† the distribution density of the Co nanoparticles on the RGO sheets shows an obvious increase when the feeding amount of Co(acac)₃ is increased to 0.25 g.

Fig. 2 (a–c) TEM and (d) HRTEM images of the RGO/Co-0.2 nanocomposites.

For detecting the element chemical composition of the RGO/Co product, XPS analysis was performed. Fig. 3a shows the XPS spectra of graphite oxide and as-prepared RGO/Co-0.2 nanocomposites. Compared to that of graphite oxide, the XPS spectrum of RGO/Co-0.2 nanocomposites exhibits additional Co 2p and 2s peaks. XPS spectrum of the Co 2p in RGO/Co-0.2 nanocomposites can be fitted with four components corresponding to Co(0) and Co(II) peaks. The peaks at 781.2 and 797.1 eV are consistent with the Co(0) 2p_3/2 and Co(0) 2p_1/2 binding energies of Co in a zero valent state.²⁶ This indicates that a metallic product is formed during the refluxing process. The higher energy bands with relatively low intensities observed at 785.9 and 802.9 eV can be attributed to a higher oxidation state of cobalt, formed by inevitable oxidation upon the exposure to air.²⁷ The C 1s XPS spectra of the graphite oxide as well as RGO/Co-0.2 nanocomposites are shown in Fig. 3c and d. The deconvolution spectra show four different peaks centered at 284.8, 286.6, 287.1, and 288.8 eV, corresponding to C–C in aromatic rings, C–O (epoxy and hydroxy), C=O (carbonyl), and O=C–O (carboxyl) groups, respectively.^28,29 In comparison with the C 1s spectra of graphite oxide, the intensities of these oxygen-containing peaks decrease significantly in RGO/Co-0.2 nanocomposites, indicating that most of the oxygen-containing functional groups have been removed during the reaction process.

Fig. 3 (a) XPS survey scan of graphite oxide and RGO/Co-0.2 nanocomposites; (b) high-resolution Co 2p spectrum of RGO/Co-0.2 nanocomposites; high-resolution C 1s spectra of (c) graphite oxide and (d) RGO/Co-0.2 nanocomposites.

Magnetic properties of the RGO/Co nanocomposites

Magnetic property of the as-synthesized RGO nanocomposites was measured with a vibrating sample magnetometer. The magnetic hysteresis loop of RGO/Co-0.2 at room temperature is shown in Fig. 4. As can be seen, the RGO/Co-0.2 nanocomposites exhibit typical ferromagnetic behavior with saturation magnetization, remanent magnetization, and coercivity of 30.3 emu g⁻¹, 6.3 emu g⁻¹, and 560.5 Oe, respectively. The saturation magnetization of RGO/Co-0.2 nanocomposites is lower than the reported value of bulk Co (168 emu g⁻¹),³⁰ mainly owing to the presence of weak magnetic RGO nanosheets, their smaller size of building blocks in the nanocomposites and the unavoidable surface oxidation evidenced by XPS spectrum. Compared to bulk cobalt (the coercivity of which is a few tens of oersteds),³¹ the obtained RGO/Co-0.2 nanocomposites exhibit enhanced coercivity. This may be originated from the smaller size of Co, because the small particle size may change the magnetization reversal mechanism and result in a higher coercivity value.³² It is known that Co nanoparticles with a size of about 4 nm are single magnetic domain often and usually display a superparamagnetic behavior under ambient conditions.³³ In this case, the ferromagnetic-like behavior of our prepared RGO/Co-0.2 nanocomposites inherently correlates with the existence of interaction between RGO and Co, through which electronic structure modifications and symmetry breaking could influence the magnetic anisotropy term.³⁴ This situation has been observed in our previous study.^35,36 Such ferromagnetic RGO/Co nanocomposites would have potential application in catalysis, because it might facilitate the separation of catalyst species from the reaction media by an external magnetic field.

Fig. 4 Room temperature magnetic hysteresis loops of the RGO/Co-0.2 nanocomposites.

Catalytic reduction of 4-nitrophenol

It is well-known that metal nanoparticles are excellent catalysts for hydrogenation reactions. Herein, the catalytic performance of the RGO/Co nanocomposites was firstly examined by catalyzing the reduction of 4-nitrophenol into 4-aminophenol by NaBH₄ in aqueous solution. Although the reaction is a thermodynamically feasible process involving standard reduction potential for 4-nitrophenol/4-aminophenol = −0.76 and H₃BO₃/BH₄⁻ = −1.33 V versus a normal hydrogen electrode, it is kinetically restricted in the absence of a catalyst.³⁷ However, the addition of a certain amount of RGO/Co catalysts into the reaction system, the solution become fading and the reaction mixture turns into colorless at last. As shown in Fig. 5a, the absorption associated with 4-nitrophenol at 400 nm in alkaline conditions gradually declines, indicating that our prepared RGO/Co-0.2 nanocomposites can successfully catalyze the reduction of 4-nitrophenol. To gain more insight into the reduction of 4-nitrophenol, the kinetic process of this reaction was investigated. It is proposed that the reaction follows the pseudo-first-order kinetics with respect to the concentration of 4-nitrophenol when the amount of NaBH₄ is in excess compared to 4-nitrophenol.³⁸ Since the absorbance intensity of 4-nitrophenol is proportional to its concentrations in the medium, the ratio of absorbance at time t to that at time 0 (A_t/A₀) at 400 nm could be equal to the ratio of concentration of 4-nitrophenol at time t to that at time 0 (C_t/C₀). As can be seen from Fig. 5b, good linear correlations of ln(A_t/A₀) versus time t are obtained, suggesting that the reaction follows first-order kinetics. The rate constants are determined from the slops of the linear plots. The rate constants are calculated to be 0.035, 0.082 and 0.054 min⁻¹ for RGO-0.15, RGO/Co-0.2 and RGO/Co-0.25, respectively. Since bare RGO do not show obvious catalytic activity, it is deduced that the catalytic activity of RGO/Co nanocomposites originates from Co, while RGO serves as synergist. Therefore, the increase of the Co content within a certain range is helpful to enhance the catalytic efficiency. However, further increasing the Co content over its optimum value in the nanocomposites, the catalytic activity of RGO/Co nanocomposites deteriorates for the following reason: in the RGO/Co nanocomposites, the RGO component can act as good adsorbent for the enriching of 4-nitrophenol and keep them at the vicinity of Co nanoparticles. Excessive Co nanoparticles would hinder the adsorption of RGO towards 4-nitrophenol, and thus decrease the catalytic activity. The recyclability is the top priorities for practical applications of a catalyst. Herein, the ferromagnetic property of the RGO/Co nanocomposites makes them be easily separated and recollected from the reaction system by an external magnet. The catalytic stability of the RGO/Co-0.2 catalysts was also studied by monitoring catalytic activity during successive cycles of the reduction reaction. After each measurement, the catalysts were magnetically separated from the solution and washed by distilled water. As shown in Fig. S2a,† the catalytic efficiency only displays a slight decrease without significant loss of activity after 20 cycles, indicating the good stability of the catalyst. The slight decrease of the activity could be attributed to the loss of the catalysts during the catalytic cycles since the dosage of the catalysts is very small. In addition, the structural stability of the catalysts after the catalytic reactions was also checked by TEM observation. As shown in Fig. S2b,† the morphology of the catalyst does not display noticeable changes after catalysis, which further confirms the high stability of our prepared RGO/Co nanocomposites.

Fig. 5 (a) UV-vis absorption spectra of the reduction reaction systems in the presence of RGO/Co-0.2 nanocomposites; (b) pseudo first order plots of ln(A_t/A₀) of 4-nitrophenol versus reaction time over different catalysts.

Electrocatalytic oxidation of glucose

Detecting glucose is of paramount importance to the diagnosis and management of diabetes. In this study, the electrocatalytic performance of our prepared RGO/Co nanocomposites towards the oxidation of glucose was also investigated. The electrochemical properties of the glass carbon electrode modified with RGO/Co-0.2 nanocomposites were examined in 0.1 M NaOH solution using cyclic voltammetry at different scan rates varied from 10 to 200 mV s⁻¹. As can be seen in Fig. 6a, the cyclic voltammograms show an oxidation peak at 0.2–0.4 V during the anodic scan, corresponding to the oxidation of metallic cobalt.³⁹ In the cathodic scan, the cathodic peaks were associated with the reduction of Co oxide formed in the positive cycles. The cyclic voltammograms for RGO/Co-0.2 modified electrode in NaOH solution agree well with other reports.^40,41 With an increase of the scan rate, the anodic and cathodic peak currents are enhanced with the increasing of the scan rate, and the potentials of the oxidation peak and reduction peak shift to the positive and the negative directions, respectively. In addition, the intensities of the redox peak currents exhibited good linearity against the square root of the scan rate with high correlation co-efficient of 0.9929 and 0.9913 (Fig. 6b), indicating that the RGO/Co-0.2 electrode is dominated by a diffusion controlled kinetics during the electrocatalytic process in alkaline solution. Fig. 6c shows the cyclic voltammograms of the RGO/Co-0.2 electrode in 0.1 M NaOH solution containing various concentration of glucose (scan rate of 50 mV s⁻¹). Obviously, the introduction of glucose (0.1, 0.5, 2.0, and 5.0 mM) causes increase of the anodic current at a potential larger than 0.3 V with a slightly positive shift in peak potential. This phenomenon can be ascribed to the glucose oxidation to gluconolactone, and the mechanism of the electrocatalysis can be expressed according to the following reactions:^42,43

Co + 2OH⁻ → Co(OH)₂ + 2e⁻ (1)

Co(OH)₂ + OH⁻ → CoOOH + H₂O + e⁻ (2)

CoOOH + OH⁻ → CoO₂ + H₂O + e⁻ (3)

2CoO₂ + glucose → 2CoOOH + glucolactone (4)

Fig. 6 (a) Cyclic voltammograms of RGO/Co-0.2 nanocomposites electrode at different scan rates from 10 to 200 mV s⁻¹ in 0.1 M NaOH; (b) the variation of the peak current densities for RGO/Co-0.2 electrode as a function of the square root of scan rate; (c) cyclic voltammograms of RGO/Co-0.2 electrode in 0.1 M NaOH in the presence of various concentrations of glucose at scan rate of 50 mV s⁻¹; (d) amperometric responses of RGO/Co-0.2 electrode prepared to successive additions of 100 μM glucose in 0.1 M NaOH solution at different electrocatalytic oxidation potentials.

It is well known that the detection potential strongly affects the amperometric response of the electrode to glucose. The optimal operating potential for sensing glucose with RGO/Co-0.2 electrode was evaluated by amperometric responses toward successive additions of 100 μM glucose into 0.1 M NaOH solution. As can be seen, the amperometric response currents increase as the electrocatalytic oxidation potential increases. However, the noise level is more serious at 0.6 V. Thus, a suitable potential 0.55 V was selected.

The amperometric response of the RGO/Co-0.2 electrode was further evaluated by successive addition different amount of glucose into stirring NaOH solution at an applied potential of 0.55 V. As shown in Fig. 7a, with the addition of glucose, the current increases rapidly, indicating its excellent glucose detection performance. In addition, the current increases linearly with increment of concentration of glucose up to 4.0 mM with a correlation coefficient of 0.9988 (Fig. 7b). The sensitivity of the RGO/Co-0.2 electrode was calculated to be as high as 437.8 μA mM⁻¹ cm⁻² from the slope of the calibration curve. The detection limit can reach up to 3.1 μM based on a signal-to-noise ratio of 3. In order to evaluate the performance of our prepared RGO/Co electrode, the obtained results have been compared with previously reported non-enzymatic glucose sensors, as shown in Table 1. It can be concluded that the sensitivity, detection limit and linear range of the RGO/Co-0.2 electrode are comparable to or even higher than that of the previously reported glucose sensors.

Fig. 7 (a) Typical amperometric response of RGO/Co-0.2 electrode at 0.55 V to successive addition of glucose in 0.1 M KOH; (b) the linear relationship between the response current and the glucose concentration.

Table 1 Comparison of the properties of RGO/Co-0.2 electrode with various nonenzymatic glucose sensors reported previously

Glucose sensor	Linear range (mM)	Sensitivity (μA mM^−1 cm^−2)	Detection limit (μM)	Ref.
RGO/Co-0.2	0.01–4.0	437.8	3.1	This work
Graphene/Co	0.00167–0.47	4700	0.68	39
Co/ITO	0.005–0.18	1720	0.25	41
Co-MOCP/CPE	0.0025–0.95	—	80	43
Co3O4	Up to 2	36.3	0.97	44
Graphene/Co3O4	0.05–0.3	—	10	45
CoPc–GOD	0.02–18	7.71	5	46
RGO/Ag	0.5–12.5	3.84	0.16	47
Au–BSA–GLA–GO	0.005–6	298.2	0.1	48

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Anti-interference studies are important and necessary for an amperometric biosensor. In this study, the specificity of the RGO/Co-0.2 electrode was investigated by using different interferents which have electroactivities similar to the target analyte. Amperometric responses of the sensor to consecutive injection of 0.1 mM glucose and 0.005 mM interfering species such as uric acid (UA), dopamine (DA) and ascorbic acid (AA) in 0.1 M NaOH at 0.55 V were recorded as shown in Fig. 8. One can see that for all the interfering species, the addition of UA, DA and AA hardly provide notable interference for glucose sensing, which indicates the excellent selectivity of our prepared RGO/Co electrode and its potential for practical use.

Fig. 8 Amperometric response of the RGO/Co-0.2 electrode on addition of glucose, UA, DA and AA at an applied potential of 0.55 V in 0.1 M NaOH solution.

Conclusions

In summary, a facile one-step refluxing method has been adopted to prepare RGO supported Co nanoparticles without using harmful organic reagents or surfactants. Structural measurements reveal that the tiny Co nanoparticles with uniform diameter were homogeneously grown on RGO nanosheets. The magnetic and catalytic properties of the RGO/Co nanocomposites were systematically investigated. It is shown that the RGO/Co nanocomposites show ferromagnetic behavior and excellent catalytic activity toward the reduction of 4-nitrophenol. In addition, when used for nonenzymatic glucose detection, the RGO/Co nanocomposites possess many merits such as high sensitivity, large detection range, low detection limit, and good selectivity. Thus, the simple preparation procedure, low cost, unique magnetic properties, and enhanced catalytic performance of RGO/Co nanocomposites can offer a potential strategy for the development of advanced catalysts.

Acknowledgements

The authors are grateful for financial support from the Startup Fund for Distinguished Scholars of Jiangsu University (No. 15JDG023), the Natural Science Foundation of Jiangsu Province (No. BK20150507), Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1601231C), China Postdoctoral Science Foundation (No. 2015M580392 and 2015M580393), and the National Natural Science Foundation of China (No. 51272094 and 51602129).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21515g

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