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One-pot synthesis of cuprous oxide-reduced graphene oxide nanocomposite with enhanced photocatalytic and electrocatalytic performance
Author’s Accepted Manuscript One-pot synthesis of cuprous oxide-reduced graphene oxide nanocomposite with enhanced photocatalytic and electrocatalytic performance Fugui Han, Heping Li, Jun Yang, Xiaodong Cai, Lu Fu www.elsevier.com/locate/physe
PII: DOI: Reference:
S1386-9477(15)30285-X http://dx.doi.org/10.1016/j.physe.2015.11.020 PHYSE12203
To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 12 October 2015 Accepted date: 16 November 2015 Cite this article as: Fugui Han, Heping Li, Jun Yang, Xiaodong Cai and Lu Fu, One-pot synthesis of cuprous oxide-reduced graphene oxide nanocomposite with enhanced photocatalytic and electrocatalytic performance, Physica E: Lowdimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2015.11.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-pot synthesis of cuprous oxide-reduced graphene oxide nanocomposite with enhanced photocatalytic and electrocatalytic performance Fugui Han1, Heping Li1, Jun Yang1, Xiaodong Cai2 and Lu Fu1,2* 1
Institute of Botany, Jiangsu Province and Chinese Academy of Sciences, Nanjing Botanical Garden, Mem. Sun Yat-Sen, Nanjing 210014, China 2
Modouo Science and Technology Communication Center, Maowei Science and Technology Ltd. China
Corresponding author email: [email protected]
Abstract We report on the facile one-step synthesis of porous cuprous oxide nanoparticles on reduced graphene oxide (Cu2O–RGO) by synchronously reducing Cu2+ ions and GO with ethylene glycol. The basic chemical components, crystal structure and surface morphology of prepared nanocomposite was carefully characterized. The photocatalytic activities of the as-prepared nanocomposite was investigated by photodegrading methylene blue (MB) under visible light. The electrocatalytic property of the nanocomposite was investigated by electrocatalytic determination of paracetamol. The results indicate that the corporation of RGO with Cu2O nanoparticles could high enhance the both photocatalytic and electrocatalytic properties. Moreover, we found that the content of RGO introduced into nanocomposite could highly affect the product properties. Keywords: cuprous oxide; reduced graphene oxide; photocatalytic; electrocatalytic; MB; paracetamol
Introduction Recent research has witnessed rapid advances in syntheses of nanostructured semiconductors of different sizes, shapes and compositions for use in photocatalysis, adsorption, solar cells, supercapacitor and chemical sensors. Cuprous oxide is a p-type semiconductor with unique optical and electrical properties, which presents itself as a promising material in the fields of solar energy conversion, photocatalytic degradation, catalysis and sensors (1-4). Also it was reported that the copper/copper oxide nanostructures can inhibit the growth of bacteria in visible light irradiation (5). Moreover, Cu2O is abundant and nontoxic, and has low production cost. However, there
are intrinsic drawbacks of Cu2O, including the instability in wet air, photo-corrosion under light irradiation, particularly the fast recombination of the photo-generated electron–hole pairs, which lead to the deactivation of Cu2O and thus limit its practical applications (6, 7). In order to better use Cu2O, many work has been concentrated coupling Cu2O nanoparticles with other materials, such as noble metals, semiconductor nanoparticles and carbon materials (8-10). Graphene, a newly developed form of carbon, has attracted increasing attention recently due to its unique physical and electrochemical properties. In electrochemistry field, using graphene as modifier showed potential advantages of high surface area, ease of processing and safety (11). Besides, graphene has a large theoretical surface area (2630 m2/g) and superior electrical conductance (64 mS/cm) (12, 13). Moreover, graphene also exhibits a large potential window, low charge-transfer resistance and fast electron transfer rate. Many reports also pointed out that graphene could enhance the electrocatalytic and photocatalytic performance of semiconductor nanoparticles (14, 15). So far, materials produced by combining RGO with metal oxide semiconductors have emerged as promising products fora wide range of potential applications in electronic devices, drug delivery, photocatalysis, energy conversion and storage (16-19). Herein, we report a facile and efficient route to load Cu2O nanoparticles on RGO via a simple one-pot solvothermal method. The Cu2O nanoparticles are uniform in diameter and are well-dispersed on the 2D graphene sheets. The as-prepared Cu2O/RGO nanocomposites were characterized using a series techniques. The photocatalytic activities of the as-prepared nanocomposite was investigated by photodegrading MB under visible light. The electrocatalytic property of the nanocomposite was investigated by electrocatalytic determination of paracetamol. We found the Cu 2O/RGO nanocomposites owing excellent photocatalytic and electrocatalytic properties compared to the pure Cu2O nanoparticles and RGO. Moreover, the effect of content of RGO in the nanocomposite was also studied.
Experimental GO was synthesized by the modified Hummers method using natural graphite as precursor (20). Briefly, 2 g of graphite powder and 1.25 g NaNO3 were added to 60 mL of concentrated H2SO4. 7.5 g of KMnO4 was added gradually with stirring and cooling to maintain the mixture below 20 °C. The mixture then stirred at 35 °C for 30 min. 120 mL of distilled water was slowly added to the mixture and the temperature increased to 98 °C, then the mixture was maintained at this temperature for 15 min. The reaction was terminated by adding 350 mL of distilled water followed by 10 mL of 30% H 2O2 solution. The solid product was separated by centrifugation, washed repeatedly with 5%
HCl solution until sulfate could not be detected with BaCl2, and then washed three times with ethanol and dried in vacuum at 60 °C overnight. Cu2O-RGO nanocomposites were synthesized by chemical reduction of copper(II) acetate in ethylene glycol solution in the presence of GO. In a typical procedure, 50 mg of GO and 100 mg of copper(II) acetate monohydrate were dispersed in 50 mL of EG. The mixture was transferred into 100 mL steel autoclave. The steel autoclave was sealed, maintained at 180 °C for 2 h and then cooled naturally to the room temperature. The mixture was filtrated and washed copiously with ethanol for several times to remove the remaining ethylene glycol and soluble byproducts, and then dried in a vacuum desiccator. The resulting product was labeled as Cu2O-RGO. For comparison, Cu2O and RGO samples were also achieved by the same procedure. To investigate the effect of GO content on the photocatalytic activity of the Cu 2O-RGO nanocomposites, the weight percentages of GO to Cu2O were varied from 0 to 0.5 (0.05, 0.1, 0.2, 0.3 and 0.5 wt.%) and the resulting samples were labeled as Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5, respectively. The photocatalytic activity of the samples were compared by monitoring the decolouration of heterocyclic dye MB under visible light irradiation. In a typical process, 20 mg of smaple were added into a quartz tube containing a MB solution (50 mL, 20 mg/L), which was placed with a 15 cm distance from the lamp. Prior to the illumination, the suspension was magnetically stirred in the dark for 30 min to reach the adsorption– desorption equilibrium. At given time intervals, 2 mL of suspension was sampled and centrifuged, the supernatant was collected for absorption analysis on a UV-vis spectrophotometer. The absorbance of MB at 664 nm was used for measure the residual dye concentration. For electrocatalytic activity test, A glassy carbon electrode (GCE) was polished by 0.3 and 0.05 μm alumina slurry followed by thoroughly rinsing with ethanol and water. For the electrode surface modification, 5 μL of as-prepared Cu2O-RGO nanocomposites dispersion (0.5 mg/mL) was dropped onto the GCE surface and dried at room temperature. All electrochemical measurements were performed on a CHI430a electrochemical workstation (USA) at room temperature. A conventional three electrode system containing a modified GCE as working electrode, a platinum wire as auxiliary electrode and a Ag/AgCl (3M KCl) electrode as reference electrode was used throughout the electrochemical experiments.
Results and discussion SEM was used for observing the morphology of synthesized Cu 2O/RGO nanocomposite. As shown in Figure 1A, it can be clearly seen that the Cu 2O/RGO nanocomposite was successfully synthesized. The RGO sheets show a corrugated structure. The Cu2O
nanoparticles are decorated on the both sides of RGO sheet, which could effectively prevent the stacking of RGO sheets. The average size of Cu2O nanoparticle formed via galvanic displacement is calculated as 40 nm. The crystal information of GO and Cu2O/RGO nanocomposite are shown in Figure 1B. As shown in the figure, the XRD pattern of GO has a characteristic diffraction peak (002) at around 10°, corresponding to a d-spacing of 0.772 nm, which is larger than the interlayer distance of the (002) peak for graphite. This pheromone could be ascribed to the introduction of oxygenated functional groups, such as epoxy, hydroxyl (–OH), carboxyl (–COOH) and carbonyl (–C=O) groups attached on both sides and edges of carbon sheets. The XRD pattern of Cu2O/RGO nanocomposite shows diffraction peaks correspond to (110), (111), (200), (220) and (311) crystal planes of cubic Cu 2O (JCPDS 782076). Moreover, there is no peaks of impurities are detected, indicating that the formed nanocomposites are pure and well crystallized. Figure 2A shows the Raman spectrum of the Cu2O-RGO nanocomposite. The Raman bands at 218, 401 and 624 cm−1 are assigned to 2Γ12─, 4Γ12─ and Γ12─(2) vibration modes of Cu2O, respectively (21). The bands at 1572 and 1335 cm─1, which was assigned to the graphite (G band, first-order scattering of E2g phonons by sp2 carbon atoms) and diamondoid (D band, breathing mode of κ-point photons of A1g symmetry) bands, respectively (22). The 2-D peak of Cu2O-RGO nanocomposites appears at 2707 cm–1, shifts to the higher wave number value of, and becomes broader for an increasing number of layers with respect to single-layer graphene (2-D, 2680 cm–1). The peak at 2938 cm–1 (D+D′) is attributed to defects, due to the combination of two phonons with different momentum (23). The data suggest the formation of Cu2O-RGO composites. Figure 2B displays the FTIR spectra of GO and Cu2O/RGO nanocomposite. GO showed a series of oxygen containing groups, the strong band at around 3397 cm─1 is due to the stretching vibration of O―H, the band at 1722 cm─1 reflects the C=O vibration of ―COOH located at edge of GO sheets, the features of O―H, epoxide groups and skeletal bending vibration can be seen at 1618 cm─1, the peak at 1392 cm─1 is attributed to the tertiary C―OH groups stretching vibration, the band at 1225 cm─1 is the C―O stretching vibration, the band at 1050 cm─1 is due to C―O stretching vibrations. On the other hand, the Cu2O/RGO nanocomposite shows the Cu―O vibration of Cu2O at 622 cm─1, suggests the interaction between Cu2O and RGO perturbing the Cu―O bonds. The band at 1587 cm─1 can be attributed to the skeletal vibration of C=C of the RGO sheets, indicating that GO was efficiently reduced. Photoluminescence (PL) was applied to investigate the recombination process involved with the electron–hole pairs. As shown in insert of Figure 3, the RGO shows no photoluminescence effect. The broad peaks from 400 to 550 nm were corresponding to the electron transitions from the different sub-levels of conduction band to the Cu dlevels of the valence band (24). The peaks at 420 nm were ascribed to the top of VB to
the top of conduction levels (3d–4p allowed transitions). In comparison of bare Cu2O, the peak became broad for the Cu2O-RGO-3 nanocomposites, which might be resulted in the surface conjugation (d–π conjugate) of Cu2O and RGO sheets (25). Moreover, it can be clearly seen that Cu2O-RGO-3 nanocomposites had the lower peak intensity than bare Cu2O nanoparticles, implying that the introduction of RGO contributed to decreasing the recombination of electron–hole pairs. From Figure 3, we also could observe that Cu2O-RGO-3 nanocomposite displays the lowest PL intensity, which may lead to the highest photocatalytic activity. The photocurrent densities of RGO, Cu2O, and Cu2O-RGO-3 nanocomposites were measured under visible light illumination. As shown in Figure 3b, the RGO shows no response towards light. On the other hand, the Cu2O, and Cu2O-RGO-3 nanocomposites show clearly current responses when the light was turned on and rapidly decrease the current to zero as soon as the light was turned off. Moreover, the Cu 2O-RGO-3 nanocomposites exhibit much higher photocurrent than that of Cu2O nanoparticles. This result again demonstrates that the Cu2O-RGO nanocomposites composite can effectively suppress photogenerated carriers recombination and probably could result a higher photocatalytic performance. The photocatalytic performance of the prepared nanocomposite was tested by degradation of MB under visible light. Figure 4A compares the relative concentration variations of the MB solution over 2 h using RGO, Cu2O nanoparticles, P25 and Cu2ORGO-3, respectively. It can be clearly seen that the RGO did not show photodegradation towards MB. The pure Cu2O nanoparticles outperformed P25 in the photodegradation of MB, and the Cu2O-RGO-3 nanocomposite further improved the photocatalytic performance over that of the Cu2O nanoparticles. This enhancement can be attributed to the enhanced light absorbance and the extended light absorption range of nanocomposite. Additionally, in comparison with pure Cu2O nanoparticles, the lager surface of RGO sheets can offer more active adsorption sites, resulting in high adsorption of MB molecules. Moreover, the photo-generated electrons can easily transfer to RGO, leading to the enhanced charge separation, which is crucial in enhancing the photocatalytic reactivity. The appropriate amount of RGO in the nanocomposite catalyst is crucial in determining the photocatalytic activity of the material. Therefore, we further tested the photocatalytic performance of Cu2O-RGO nanocomposite with different RGO ratio. As shown in Figure 4B, the Cu 2O-RGO-3 shows the best performance compare with that of other Cu2O-RGO nanocomposite. Therefore, 0.2% GO is the optimum condition for constructing Cu2O-RGO nanocomposite. The electrochemical properties of the RGO, Cu2O and Cu2O-RGO-3 nanocomposite were tested by CV and EIS measurements. Figure 5A shows the CVs of three GCE electrodes modified with RGO, Cu2O and Cu2O-RGO-3 nanocomposite in 0.1 M PBS (pH = 7.4) containing 5 mM K3[Fe(CN)6] at a scan rate of 10 mV/s. The peak to peak separation of
the RGO, Cu2O and Cu2O-RGO-3 nanocomposite modified GCEs were 0.32 V, 0.29 V and 0.23 V, respectively. In comparison with RGO and Cu2O modified electrodes, the Cu2ORGO-4 nanocomposite modified GCE shows a pair of redox peaks corresponding to the redox of [Fe(CN)6]3─/4─ with superior electrochemical performances, indicating the significant enhancement of the electron transfer rate. Figure 5B shows the EIS measurements using there materials modified electrodes. The simulated values of R ct are 0.38, 0.29 and 0.21 kΩ for the RGO, Cu2O, and the Cu2O-RGO4 nanocomposite modified GCE, respectively. The results further indicate the better electrochemical performance of Cu2O-RGO nanocomposite. The electrochemical responses of paracetamol at various electrodes were investigated using cyclic voltammetry. As shown in Figure 6A, paracetamol only shows an irreversible oxidation peak at a bare GCE and Cu2O modified GCE. In contract, RGO modified GCE exhibits a pair of well-defined redox peak corresponding to the oxidation and reduction of paracetamol. This enhancement is probably due to the large surface area and outstanding conductivity which is provided by the RGO sheets. Moreover, when the Cu2O-RGO-4 nanocomposite was modified onto the GCE surface, the current responses of this pair of well-defined redox peaks from paracetamol increased greatly. These results indicate that Cu2O-RGO modified electrode exhibits high electro-catalytic capability. The influence of the content of RGO on the electrocatalytic activity of the electrode was also investigated. Figure 6B displays the CV profiles of paracetamol detection at Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5 modified GCEs. It can be seen that the anodic peak current increases with increasing RGO content till 0.3%. After that, the current response decreases upon further increasing of the RGO content. Therefore, Cu2O-RGO-4 modified GCE was chosen as optimum combination. Figure 7 depicts the DPV curves of the Cu2O-RGO-4 nanocomposite modified GCE in various concentrations of paracetamol from 0.1 to 20 μM. As shown in Figure 7 the DPV curves exhibit well-defined peaks corresponding to the oxidation of paracetamol. The peak current increases linearly with the concentration of nitrite in the range of 0.1 to 20 μM. The corresponding regression equation is: I (μA)= 1.8652 C(μM) + 2.1564 (R2 = 0.9912). The detection limit was calculated to be 0.02 μM (S/N = 3). The reproducibility of the proposed modified electrode towards the detection of paracetamol was investigated. The results show that the RSD of the measurement is 3.05%. For the same paracetamol solution, the RSD of measurement using ten Cu2O-RGO-4 nanocomposite modified GCE is determined to be 2.98%. The results suggest that the proposed nanocomposite modified electrode is very consistent for reproducible results.
Conclusion
In summary, this work demonstrated that Cu2O-RGO nanocomposite can be prepared via a facial one-pot hydrothermal approach. In this way, Cu2O nanoparticles and RGO sheets were obtained synchronously. The photocatalytic and electrocatalytic properties of prepared Cu2O-RGO nanocomposite were tested by the photodegradation of MB under visible light and electrochemical determination of paracetamol, respectively. The results suggest that the combination of Cu2O nanoparticles with RGO sheets could lead a superior performance in catalytic activity.
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Figure Captions Figure 1: (A) SEM image and (B) XRD pattern of Cu2O/RGO nanocomposite. Figure 2: (A) Raman spectra of Cu2O/RGO nanocomposite. (B) FTIR spectra of GO and Cu2O/RGO nanocomposite. Figure 3: (A) PL spectra of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5. Inset: PL spectra of RGO, Cu2O and Cu2O-RGO-3. (B) Photocurrent densities vs. time for the RGO, Cu2O and Cu2O-RGO-3. Figure 4: (A) Photodegradation profiles of RGO, Cu2O, P25 and Cu2O-RGO-3. (B) Photodegradation profiles of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5. Figure 5: (A) CV curves and (B) Nyquist plots of the RGO, Cu2O and Cu2O-RGO-4 nanocomposite in PBS containing 5 mM K3[Fe(CN)6] at the scan rate of 10 mV/s. Figure 6: (A) Cyclic voltammograms of bare, RGO, Cu2O and Cu2O-RGO-4 nanocomposite modified GCE in 0.2 M pH 7.0 PBS with 0.05 mM paracetamol. (B) Cyclic voltammograms of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5 modified GCE in 0.2 M pH 7.0 PBS with 0.05 mM paracetamol. Figure 7: DPV curves containing different concentrations of the paracetamol in the range from 0.1 to 20 μM.
Figure 1: (A) SEM image and (B) XRD pattern of Cu2O/RGO nanocomposite.
Figure 2: (A) Raman spectra and (B) FTIR spectra of GO and Cu2O/RGO nanocomposite.
Figure 3: (A) PL spectra of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5. Inset: PL spectra of RGO, Cu2O and Cu2O-RGO-3. (B) Photocurrent densities vs. time for the RGO, Cu2O and Cu2O-RGO-3.
Figure 4: (A) Photodegradation profiles of RGO, Cu2O, P25 and Cu2O-RGO-3. (B) Photodegradation profiles of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5.
Figure 5: (A) CV curves and (B) Nyquist plots of the RGO, Cu2O and Cu2O-RGO-4 nanocomposite in PBS containing 5 mM K3[Fe(CN)6] at the scan rate of 10 mV/s.
Figure 6: (A) Cyclic voltammograms of bare, RGO, Cu2O and Cu2O-RGO-4 nanocomposite modified GCE in 0.2 M pH 7.0 PBS with 0.05 mM paracetamol. (B) Cyclic voltammograms of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5 modified GCE in 0.2 M pH 7.0 PBS with 0.05 mM paracetamol.
Figure 7: DPV curves containing different concentrations of the paracetamol in the range from 0.1 to 20 μM.
PII: DOI: Reference:
S1386-9477(15)30285-X http://dx.doi.org/10.1016/j.physe.2015.11.020 PHYSE12203
To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 12 October 2015 Accepted date: 16 November 2015 Cite this article as: Fugui Han, Heping Li, Jun Yang, Xiaodong Cai and Lu Fu, One-pot synthesis of cuprous oxide-reduced graphene oxide nanocomposite with enhanced photocatalytic and electrocatalytic performance, Physica E: Lowdimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2015.11.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-pot synthesis of cuprous oxide-reduced graphene oxide nanocomposite with enhanced photocatalytic and electrocatalytic performance Fugui Han1, Heping Li1, Jun Yang1, Xiaodong Cai2 and Lu Fu1,2* 1
Institute of Botany, Jiangsu Province and Chinese Academy of Sciences, Nanjing Botanical Garden, Mem. Sun Yat-Sen, Nanjing 210014, China 2
Modouo Science and Technology Communication Center, Maowei Science and Technology Ltd. China
Corresponding author email: [email protected]
Abstract We report on the facile one-step synthesis of porous cuprous oxide nanoparticles on reduced graphene oxide (Cu2O–RGO) by synchronously reducing Cu2+ ions and GO with ethylene glycol. The basic chemical components, crystal structure and surface morphology of prepared nanocomposite was carefully characterized. The photocatalytic activities of the as-prepared nanocomposite was investigated by photodegrading methylene blue (MB) under visible light. The electrocatalytic property of the nanocomposite was investigated by electrocatalytic determination of paracetamol. The results indicate that the corporation of RGO with Cu2O nanoparticles could high enhance the both photocatalytic and electrocatalytic properties. Moreover, we found that the content of RGO introduced into nanocomposite could highly affect the product properties. Keywords: cuprous oxide; reduced graphene oxide; photocatalytic; electrocatalytic; MB; paracetamol
Introduction Recent research has witnessed rapid advances in syntheses of nanostructured semiconductors of different sizes, shapes and compositions for use in photocatalysis, adsorption, solar cells, supercapacitor and chemical sensors. Cuprous oxide is a p-type semiconductor with unique optical and electrical properties, which presents itself as a promising material in the fields of solar energy conversion, photocatalytic degradation, catalysis and sensors (1-4). Also it was reported that the copper/copper oxide nanostructures can inhibit the growth of bacteria in visible light irradiation (5). Moreover, Cu2O is abundant and nontoxic, and has low production cost. However, there
are intrinsic drawbacks of Cu2O, including the instability in wet air, photo-corrosion under light irradiation, particularly the fast recombination of the photo-generated electron–hole pairs, which lead to the deactivation of Cu2O and thus limit its practical applications (6, 7). In order to better use Cu2O, many work has been concentrated coupling Cu2O nanoparticles with other materials, such as noble metals, semiconductor nanoparticles and carbon materials (8-10). Graphene, a newly developed form of carbon, has attracted increasing attention recently due to its unique physical and electrochemical properties. In electrochemistry field, using graphene as modifier showed potential advantages of high surface area, ease of processing and safety (11). Besides, graphene has a large theoretical surface area (2630 m2/g) and superior electrical conductance (64 mS/cm) (12, 13). Moreover, graphene also exhibits a large potential window, low charge-transfer resistance and fast electron transfer rate. Many reports also pointed out that graphene could enhance the electrocatalytic and photocatalytic performance of semiconductor nanoparticles (14, 15). So far, materials produced by combining RGO with metal oxide semiconductors have emerged as promising products fora wide range of potential applications in electronic devices, drug delivery, photocatalysis, energy conversion and storage (16-19). Herein, we report a facile and efficient route to load Cu2O nanoparticles on RGO via a simple one-pot solvothermal method. The Cu2O nanoparticles are uniform in diameter and are well-dispersed on the 2D graphene sheets. The as-prepared Cu2O/RGO nanocomposites were characterized using a series techniques. The photocatalytic activities of the as-prepared nanocomposite was investigated by photodegrading MB under visible light. The electrocatalytic property of the nanocomposite was investigated by electrocatalytic determination of paracetamol. We found the Cu 2O/RGO nanocomposites owing excellent photocatalytic and electrocatalytic properties compared to the pure Cu2O nanoparticles and RGO. Moreover, the effect of content of RGO in the nanocomposite was also studied.
Experimental GO was synthesized by the modified Hummers method using natural graphite as precursor (20). Briefly, 2 g of graphite powder and 1.25 g NaNO3 were added to 60 mL of concentrated H2SO4. 7.5 g of KMnO4 was added gradually with stirring and cooling to maintain the mixture below 20 °C. The mixture then stirred at 35 °C for 30 min. 120 mL of distilled water was slowly added to the mixture and the temperature increased to 98 °C, then the mixture was maintained at this temperature for 15 min. The reaction was terminated by adding 350 mL of distilled water followed by 10 mL of 30% H 2O2 solution. The solid product was separated by centrifugation, washed repeatedly with 5%
HCl solution until sulfate could not be detected with BaCl2, and then washed three times with ethanol and dried in vacuum at 60 °C overnight. Cu2O-RGO nanocomposites were synthesized by chemical reduction of copper(II) acetate in ethylene glycol solution in the presence of GO. In a typical procedure, 50 mg of GO and 100 mg of copper(II) acetate monohydrate were dispersed in 50 mL of EG. The mixture was transferred into 100 mL steel autoclave. The steel autoclave was sealed, maintained at 180 °C for 2 h and then cooled naturally to the room temperature. The mixture was filtrated and washed copiously with ethanol for several times to remove the remaining ethylene glycol and soluble byproducts, and then dried in a vacuum desiccator. The resulting product was labeled as Cu2O-RGO. For comparison, Cu2O and RGO samples were also achieved by the same procedure. To investigate the effect of GO content on the photocatalytic activity of the Cu 2O-RGO nanocomposites, the weight percentages of GO to Cu2O were varied from 0 to 0.5 (0.05, 0.1, 0.2, 0.3 and 0.5 wt.%) and the resulting samples were labeled as Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5, respectively. The photocatalytic activity of the samples were compared by monitoring the decolouration of heterocyclic dye MB under visible light irradiation. In a typical process, 20 mg of smaple were added into a quartz tube containing a MB solution (50 mL, 20 mg/L), which was placed with a 15 cm distance from the lamp. Prior to the illumination, the suspension was magnetically stirred in the dark for 30 min to reach the adsorption– desorption equilibrium. At given time intervals, 2 mL of suspension was sampled and centrifuged, the supernatant was collected for absorption analysis on a UV-vis spectrophotometer. The absorbance of MB at 664 nm was used for measure the residual dye concentration. For electrocatalytic activity test, A glassy carbon electrode (GCE) was polished by 0.3 and 0.05 μm alumina slurry followed by thoroughly rinsing with ethanol and water. For the electrode surface modification, 5 μL of as-prepared Cu2O-RGO nanocomposites dispersion (0.5 mg/mL) was dropped onto the GCE surface and dried at room temperature. All electrochemical measurements were performed on a CHI430a electrochemical workstation (USA) at room temperature. A conventional three electrode system containing a modified GCE as working electrode, a platinum wire as auxiliary electrode and a Ag/AgCl (3M KCl) electrode as reference electrode was used throughout the electrochemical experiments.
Results and discussion SEM was used for observing the morphology of synthesized Cu 2O/RGO nanocomposite. As shown in Figure 1A, it can be clearly seen that the Cu 2O/RGO nanocomposite was successfully synthesized. The RGO sheets show a corrugated structure. The Cu2O
nanoparticles are decorated on the both sides of RGO sheet, which could effectively prevent the stacking of RGO sheets. The average size of Cu2O nanoparticle formed via galvanic displacement is calculated as 40 nm. The crystal information of GO and Cu2O/RGO nanocomposite are shown in Figure 1B. As shown in the figure, the XRD pattern of GO has a characteristic diffraction peak (002) at around 10°, corresponding to a d-spacing of 0.772 nm, which is larger than the interlayer distance of the (002) peak for graphite. This pheromone could be ascribed to the introduction of oxygenated functional groups, such as epoxy, hydroxyl (–OH), carboxyl (–COOH) and carbonyl (–C=O) groups attached on both sides and edges of carbon sheets. The XRD pattern of Cu2O/RGO nanocomposite shows diffraction peaks correspond to (110), (111), (200), (220) and (311) crystal planes of cubic Cu 2O (JCPDS 782076). Moreover, there is no peaks of impurities are detected, indicating that the formed nanocomposites are pure and well crystallized. Figure 2A shows the Raman spectrum of the Cu2O-RGO nanocomposite. The Raman bands at 218, 401 and 624 cm−1 are assigned to 2Γ12─, 4Γ12─ and Γ12─(2) vibration modes of Cu2O, respectively (21). The bands at 1572 and 1335 cm─1, which was assigned to the graphite (G band, first-order scattering of E2g phonons by sp2 carbon atoms) and diamondoid (D band, breathing mode of κ-point photons of A1g symmetry) bands, respectively (22). The 2-D peak of Cu2O-RGO nanocomposites appears at 2707 cm–1, shifts to the higher wave number value of, and becomes broader for an increasing number of layers with respect to single-layer graphene (2-D, 2680 cm–1). The peak at 2938 cm–1 (D+D′) is attributed to defects, due to the combination of two phonons with different momentum (23). The data suggest the formation of Cu2O-RGO composites. Figure 2B displays the FTIR spectra of GO and Cu2O/RGO nanocomposite. GO showed a series of oxygen containing groups, the strong band at around 3397 cm─1 is due to the stretching vibration of O―H, the band at 1722 cm─1 reflects the C=O vibration of ―COOH located at edge of GO sheets, the features of O―H, epoxide groups and skeletal bending vibration can be seen at 1618 cm─1, the peak at 1392 cm─1 is attributed to the tertiary C―OH groups stretching vibration, the band at 1225 cm─1 is the C―O stretching vibration, the band at 1050 cm─1 is due to C―O stretching vibrations. On the other hand, the Cu2O/RGO nanocomposite shows the Cu―O vibration of Cu2O at 622 cm─1, suggests the interaction between Cu2O and RGO perturbing the Cu―O bonds. The band at 1587 cm─1 can be attributed to the skeletal vibration of C=C of the RGO sheets, indicating that GO was efficiently reduced. Photoluminescence (PL) was applied to investigate the recombination process involved with the electron–hole pairs. As shown in insert of Figure 3, the RGO shows no photoluminescence effect. The broad peaks from 400 to 550 nm were corresponding to the electron transitions from the different sub-levels of conduction band to the Cu dlevels of the valence band (24). The peaks at 420 nm were ascribed to the top of VB to
the top of conduction levels (3d–4p allowed transitions). In comparison of bare Cu2O, the peak became broad for the Cu2O-RGO-3 nanocomposites, which might be resulted in the surface conjugation (d–π conjugate) of Cu2O and RGO sheets (25). Moreover, it can be clearly seen that Cu2O-RGO-3 nanocomposites had the lower peak intensity than bare Cu2O nanoparticles, implying that the introduction of RGO contributed to decreasing the recombination of electron–hole pairs. From Figure 3, we also could observe that Cu2O-RGO-3 nanocomposite displays the lowest PL intensity, which may lead to the highest photocatalytic activity. The photocurrent densities of RGO, Cu2O, and Cu2O-RGO-3 nanocomposites were measured under visible light illumination. As shown in Figure 3b, the RGO shows no response towards light. On the other hand, the Cu2O, and Cu2O-RGO-3 nanocomposites show clearly current responses when the light was turned on and rapidly decrease the current to zero as soon as the light was turned off. Moreover, the Cu 2O-RGO-3 nanocomposites exhibit much higher photocurrent than that of Cu2O nanoparticles. This result again demonstrates that the Cu2O-RGO nanocomposites composite can effectively suppress photogenerated carriers recombination and probably could result a higher photocatalytic performance. The photocatalytic performance of the prepared nanocomposite was tested by degradation of MB under visible light. Figure 4A compares the relative concentration variations of the MB solution over 2 h using RGO, Cu2O nanoparticles, P25 and Cu2ORGO-3, respectively. It can be clearly seen that the RGO did not show photodegradation towards MB. The pure Cu2O nanoparticles outperformed P25 in the photodegradation of MB, and the Cu2O-RGO-3 nanocomposite further improved the photocatalytic performance over that of the Cu2O nanoparticles. This enhancement can be attributed to the enhanced light absorbance and the extended light absorption range of nanocomposite. Additionally, in comparison with pure Cu2O nanoparticles, the lager surface of RGO sheets can offer more active adsorption sites, resulting in high adsorption of MB molecules. Moreover, the photo-generated electrons can easily transfer to RGO, leading to the enhanced charge separation, which is crucial in enhancing the photocatalytic reactivity. The appropriate amount of RGO in the nanocomposite catalyst is crucial in determining the photocatalytic activity of the material. Therefore, we further tested the photocatalytic performance of Cu2O-RGO nanocomposite with different RGO ratio. As shown in Figure 4B, the Cu 2O-RGO-3 shows the best performance compare with that of other Cu2O-RGO nanocomposite. Therefore, 0.2% GO is the optimum condition for constructing Cu2O-RGO nanocomposite. The electrochemical properties of the RGO, Cu2O and Cu2O-RGO-3 nanocomposite were tested by CV and EIS measurements. Figure 5A shows the CVs of three GCE electrodes modified with RGO, Cu2O and Cu2O-RGO-3 nanocomposite in 0.1 M PBS (pH = 7.4) containing 5 mM K3[Fe(CN)6] at a scan rate of 10 mV/s. The peak to peak separation of
the RGO, Cu2O and Cu2O-RGO-3 nanocomposite modified GCEs were 0.32 V, 0.29 V and 0.23 V, respectively. In comparison with RGO and Cu2O modified electrodes, the Cu2ORGO-4 nanocomposite modified GCE shows a pair of redox peaks corresponding to the redox of [Fe(CN)6]3─/4─ with superior electrochemical performances, indicating the significant enhancement of the electron transfer rate. Figure 5B shows the EIS measurements using there materials modified electrodes. The simulated values of R ct are 0.38, 0.29 and 0.21 kΩ for the RGO, Cu2O, and the Cu2O-RGO4 nanocomposite modified GCE, respectively. The results further indicate the better electrochemical performance of Cu2O-RGO nanocomposite. The electrochemical responses of paracetamol at various electrodes were investigated using cyclic voltammetry. As shown in Figure 6A, paracetamol only shows an irreversible oxidation peak at a bare GCE and Cu2O modified GCE. In contract, RGO modified GCE exhibits a pair of well-defined redox peak corresponding to the oxidation and reduction of paracetamol. This enhancement is probably due to the large surface area and outstanding conductivity which is provided by the RGO sheets. Moreover, when the Cu2O-RGO-4 nanocomposite was modified onto the GCE surface, the current responses of this pair of well-defined redox peaks from paracetamol increased greatly. These results indicate that Cu2O-RGO modified electrode exhibits high electro-catalytic capability. The influence of the content of RGO on the electrocatalytic activity of the electrode was also investigated. Figure 6B displays the CV profiles of paracetamol detection at Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5 modified GCEs. It can be seen that the anodic peak current increases with increasing RGO content till 0.3%. After that, the current response decreases upon further increasing of the RGO content. Therefore, Cu2O-RGO-4 modified GCE was chosen as optimum combination. Figure 7 depicts the DPV curves of the Cu2O-RGO-4 nanocomposite modified GCE in various concentrations of paracetamol from 0.1 to 20 μM. As shown in Figure 7 the DPV curves exhibit well-defined peaks corresponding to the oxidation of paracetamol. The peak current increases linearly with the concentration of nitrite in the range of 0.1 to 20 μM. The corresponding regression equation is: I (μA)= 1.8652 C(μM) + 2.1564 (R2 = 0.9912). The detection limit was calculated to be 0.02 μM (S/N = 3). The reproducibility of the proposed modified electrode towards the detection of paracetamol was investigated. The results show that the RSD of the measurement is 3.05%. For the same paracetamol solution, the RSD of measurement using ten Cu2O-RGO-4 nanocomposite modified GCE is determined to be 2.98%. The results suggest that the proposed nanocomposite modified electrode is very consistent for reproducible results.
Conclusion
In summary, this work demonstrated that Cu2O-RGO nanocomposite can be prepared via a facial one-pot hydrothermal approach. In this way, Cu2O nanoparticles and RGO sheets were obtained synchronously. The photocatalytic and electrocatalytic properties of prepared Cu2O-RGO nanocomposite were tested by the photodegradation of MB under visible light and electrochemical determination of paracetamol, respectively. The results suggest that the combination of Cu2O nanoparticles with RGO sheets could lead a superior performance in catalytic activity.
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Figure Captions Figure 1: (A) SEM image and (B) XRD pattern of Cu2O/RGO nanocomposite. Figure 2: (A) Raman spectra of Cu2O/RGO nanocomposite. (B) FTIR spectra of GO and Cu2O/RGO nanocomposite. Figure 3: (A) PL spectra of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5. Inset: PL spectra of RGO, Cu2O and Cu2O-RGO-3. (B) Photocurrent densities vs. time for the RGO, Cu2O and Cu2O-RGO-3. Figure 4: (A) Photodegradation profiles of RGO, Cu2O, P25 and Cu2O-RGO-3. (B) Photodegradation profiles of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5. Figure 5: (A) CV curves and (B) Nyquist plots of the RGO, Cu2O and Cu2O-RGO-4 nanocomposite in PBS containing 5 mM K3[Fe(CN)6] at the scan rate of 10 mV/s. Figure 6: (A) Cyclic voltammograms of bare, RGO, Cu2O and Cu2O-RGO-4 nanocomposite modified GCE in 0.2 M pH 7.0 PBS with 0.05 mM paracetamol. (B) Cyclic voltammograms of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5 modified GCE in 0.2 M pH 7.0 PBS with 0.05 mM paracetamol. Figure 7: DPV curves containing different concentrations of the paracetamol in the range from 0.1 to 20 μM.
Figure 1: (A) SEM image and (B) XRD pattern of Cu2O/RGO nanocomposite.
Figure 2: (A) Raman spectra and (B) FTIR spectra of GO and Cu2O/RGO nanocomposite.
Figure 3: (A) PL spectra of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5. Inset: PL spectra of RGO, Cu2O and Cu2O-RGO-3. (B) Photocurrent densities vs. time for the RGO, Cu2O and Cu2O-RGO-3.
Figure 4: (A) Photodegradation profiles of RGO, Cu2O, P25 and Cu2O-RGO-3. (B) Photodegradation profiles of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5.
Figure 5: (A) CV curves and (B) Nyquist plots of the RGO, Cu2O and Cu2O-RGO-4 nanocomposite in PBS containing 5 mM K3[Fe(CN)6] at the scan rate of 10 mV/s.
Figure 6: (A) Cyclic voltammograms of bare, RGO, Cu2O and Cu2O-RGO-4 nanocomposite modified GCE in 0.2 M pH 7.0 PBS with 0.05 mM paracetamol. (B) Cyclic voltammograms of Cu2O-RGO-1, Cu2O-RGO-2, Cu2O-RGO-3, Cu2O-RGO-4 and Cu2O-RGO-5 modified GCE in 0.2 M pH 7.0 PBS with 0.05 mM paracetamol.
Figure 7: DPV curves containing different concentrations of the paracetamol in the range from 0.1 to 20 μM.