A facile and general route for the synthesis of semiconductor quantum dots on reduced graphene oxide sheets

Abstract

Owing to its unique graphitized basal plane nanostructure and intriguing physicochemical properties, graphene is considered as an ideal support for developing nanocomposites for various applications. In this study, a facile and general method was developed for the first time to synthesize a variety of semiconductor quantum dots (SQDs) supported on reduced graphene oxide (RGO) sheets, including RGO/metal oxide and RGO/metal sulfide nanocomposites. The as-prepared nanocomposites were investigated by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectra, X-ray photoelectron spectroscopy and transmission electron microscopy. It was found that by using octadecylamine (ODA) as both reductive and dispersing agent, the resulting metal oxide and sulfide SQDs were all homogeneously decorated on the surface of RGO sheets. The optical properties of the as-synthesized RGO/SQDs nanocomposites were studied through ultraviolet-visible and photoluminescence spectroscopy. To demonstrate one potential application, the RGO/NiO nanocomposites were used as electrode materials for electrochemical supercapacitors, which exhibit enhanced capacitive performance and long cycle life. It is expected that our prepared RGO/SQDs nanocomposites could serve as promising candidates for power source, catalysis, optical sensitizer and optoelectronic applications.

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Introduction

In recent years, semiconductor quantum dots (SQDs) have attracted a tremendous amount of attention from both the experimental and theoretical scientific communities due to their peculiar physical and chemical properties and extensive potential applications in nanoelectronics and devices.^1,2 As an important type of semiconductor, metal oxides and sulfides stand out as versatile materials for their numerous technological applications in power sources, catalysts, optical sensitizers, transparent electronics, quantum devices, etc.^3–7 The metal oxides and sulfides SQDs possess unique size- and shape-dependent physical and chemical properties, which are different drastically from their bulk forms. However, due to the large surface area of these metal oxides and sulfides SQDs, they own high surface energy and tend to aggregation and changing in size and shape.⁸ Therefore, it is desirable to explore facile and effective strategies to synthesize well-dispersed metal oxides and sulfides SQDs with good size controllability.

As a new class of carbon-based materials with two-dimensional nanostructure, graphene has triggered considerable attention from both the fundamental scientific and the technological points of view since its discovery by Geim and co-workers in 2004.^9–12 Due to its unique sheet morphology, ultrahigh electron conductivity, extraordinary high surface area and superior chemical stability, graphene has emerged as the unrivaled candidate in many technological fields, such as nanoelectronics, sensors, drug delivery, batteries, supercapacitors and photocatalysis.^13–17 However, graphene sheets have a tendency to agglomerate and restack irreversibly due to the strong van der Waals interaction among the sheets.^18,19 The aggregation of graphene inhibits its practical application in many fields because most of the unique properties of graphene are only associated with single or a few layers of sheets.²⁰ Recently, the development of graphene-based nanocomposites has been fueled up.²¹ The irreversible restacking can be overcome by integrating graphene with other functional nanomaterials to form unique nanocomposites. The functional nanomaterials can enlarge the space between graphene sheets, inhibit their restacking effectively and maintain the intriguing properties of graphene sheets.²² At the same time, as an ideal template, graphene can prevent the aggregation, control the structures and improve the stability and dispersion of the attached nanocrystals. In addition, the synergetic effects between graphene and the guest materials can enhance the properties and improve the functionalities of the graphene-supported nanocomposites.^23–27 With these in mind, a variety of nanomaterials, such as metal, oxide, sulfide and hydroxide have been dispersed on graphene sheets for various applications. Though great progress has been made in the synthesis of graphene/metal oxides and graphene/metal sulfides nanocomposites, these metal oxides and sulfides nanoparticles supported on graphene sheets usually possess bigger size rather than their quantum dots. Therefore, a facile and general method is still highly desirable for the synthesis of graphene/SQDs nanocomposites, which inherit unique structural and electronic properties of graphene synergized with peculiar physical and chemical properties of SQDs.²⁸

In this study, we develop a facile and general route to rapidly synthesize various metal oxides and sulfides SQDs supported on reduced graphene oxide (RGO) sheets for the first time. Specifically, as shown in Fig. 1, the octadecylamine (ODA) has been employed to serve as both reductive and dispersing agent. The metal oxides and sulfides SQDs with a size of several nanometers were decorated on the RGO sheets with good dispersity and high uniformity. The optical properties of these RGO/SQDs nanocomposites have been studied. Moreover, as a demonstration of their potential applications, the RGO/NiO nanocomposites have been used as an electrode material for electrochemical supercapacitors. The RGO/NiO electrode exhibits enhanced capacitive performance and long cycle life, which make it a suitable and promising electrode material for supercapacitors.

Fig. 1 Illustration for the synthesis of RGO/SQDs nanocomposites.

Experimental

Materials

Natural flake graphite (150 μm, 99.9%) was purchased from Qingdao Guyu Graphite Co., Ltd. ODA (90%) and polytetrafluoroethylene (PTFE) in 60 wt% dispersion was obtained from Aladdin Industrial Corporation (Shanghai, China). All of the other chemical reagents employed in this study are commercially available analytical-grade products, and used as received without further purification.

Synthesis of RGO/metal oxide nanocomposites

Graphite oxide was prepared from the natural flake graphite through a modified Hummers method as described previously.^29,30 In a typical synthesis of the RGO/metal oxide nanocomposites, 40 mg of graphite oxide was dispersed in 40 mL of ODA with ultrasonication for about 2 h. The resulting mixture was then transferred into a 100 mL round-bottomed flask and heated to 200 °C under magnetic stirring. Subsequently, 200 mg of Ni(NO₃)₂·6H₂O (or Zn(NO₃)₂·6H₂O) was added. After reaction for 30 min at 200 °C, the mixture was allowed to cool to 80 °C and precipitated using ethanol as a bad solvent. The solid products were collected by centrifugation, washed thoroughly with chloroform and ethanol, and finally dried in a vacuum oven at 45 °C for 24 h. By using the same procedure, bare RGO was also prepared in the absence of metal nitrates.

Synthesis of RGO/metal sulfide nanocomposites

In a typical procedure, 40 mg of graphite oxide was dispersed in 40 mL of ODA with ultrasonication for about 2 h, and then the resulting mixture was heated to 200 °C under magnetic stirring. Subsequently, 150 mg of Cd(NO₃)₂·4H₂O (or Zn(NO₃)₂·6H₂O) and 20 mg of sulfur powder was added in sequence. After reaction for 30 min at 200 °C, the system was allowed to cool to 80 °C and precipitated using ethanol. The as-prepared RGO/metal sulfide nanocomposites were collected by centrifugation, washed with chloroform and ethanol, and finally dried in a vacuum oven at 45 °C.

Instrumentation and measurements

Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 7° min⁻¹. The dimension and morphology of the products were determined by transmission electron microscopy (TEM, JEOL JEM-2100) at an accelerating voltage of 200 kV. Samples for TEM observation were prepared by dropping the products on a carbon-covered-copper grid after ultrasonic dispersion in absolute ethanol. Fourier transform infrared (FT-IR) spectra were acquired using a Nicolet Nexus 470 spectrometer with KBr pellets in the 4000–400 cm⁻¹ region. Raman spectra of the samples were collected using a DXR Raman microscope with a 532 nm Ar⁺ laser as an excitation source at room temperature. The X-ray photoelectron spectroscopy (XPS) measurements were performed by using a PHI 5000 VersaProbe. The compositions of the nanocomposites were determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Vista-MPX). Ultraviolet-visible (UV-vis) absorption spectra were recorded with a UV-1800PC UV-vis spectrophotometer in ethanol dispersion. Photoluminescence (PL) spectra were collected on a Varian Cary Eclipse Fluorescence Spectrophotometer at room temperature.

Electrochemical measurements

The electrochemical properties of the samples were conducted with a CHI 750D electrochemical analyzer (Chen Hua Instruments, Shanghai, China) at room temperature. Beaker-type three-electrode cells were assembled with a working electrode, a counter electrode (platinum foil), and a reference electrode (saturated calomel electrode, SCE) immersed in 3 M KOH aqueous solution as the electrolyte. The fabrication of working electrodes was carried out as follows. Briefly, 80 wt% of electroactive materials was mixed with 10 wt% of acetylene black and 10 wt% of PTFE in the presence of ethanol to form a homogeneous slurry. This slurry was pasted on platinum foil and then heat-treated at 60 °C under vacuum overnight. The electrochemical performance was characterized by means of cyclic voltammetry (CV) measurements. The CV measurements were performed in the potential range between 0 and 0.5 V with scan rates from 5 to 80 mV s⁻¹.

Results and discussion

Structural and morphological characterization

In this work, graphite oxide was exfoliated and reduced into RGO by using ODA as both dispersing and reducing agent. The structural change from graphite oxide to RGO is reflected in their XRD spectra. As shown in Fig. 2a, the peak centered at 2θ of 10.6° corresponds to the (001) reflection of graphite oxide.³¹ For the bare RGO obtained by simply refluxing the suspension of graphene oxide in ODA medium, this peak completely disappeared and a broad (002) diffraction peak located at 2θ of about 21° was observed, which suggests that graphite oxide has been flaked and reduced. The lower 2θ value of the RGO could be attribute to the functionalisation of RGO nanosheets by ODA.³² The TEM image of the bare RGO, displayed in Fig. 2b, shows partially folded and crumpled topology with a lateral dimension up to a few microns in length.

Fig. 2 (a) XRD patterns of graphite oxide and bare RGO; (b) TEM image of bare RGO; (c) Raman spectra of graphite oxide and bare RGO; (d) FT-IR spectra of graphite oxide, ODA and bare RGO.

Raman spectroscopy is strongly dependent upon electronic structure and it can be used as a suitable nondestructive technique to distinguishing disordered and ordered crystal structures of carbon. Fig. 2c presents the Raman spectra of the graphite oxide and the bare RGO. The graphite oxide sample has a D peak at 1349 cm⁻¹, which is related to local defects and disorders that break the symmetry and selection rule,³³ while the G band at about 1590 cm⁻¹ usually arises from the in-plane carbon atom stretching vibrations.³⁴ The G band of the bare RGO displays a shift to lower frequency (1577 cm⁻¹), which is close to the value of pristine graphite, indicating the successful reduction of graphite oxide.³⁵ At the same time, the intensity ratio of D band to G band (I_D/I_G) of the RGO is about 1.90, while the I_D/I_G of graphite oxide is 1.81. This indicates that more disordered graphitic structure was formed during the exfoliation and reduction of graphite oxide.^36–38 Fig. 2d shows the FT-IR spectra of the bare RGO, together with those of graphite oxide and pure ODA. For graphite oxide, the characteristic absorption peaks appear for O–H (3411 cm⁻¹), C=O (1729 cm⁻¹), C–OH (1224 cm⁻¹) and C–O (1045 cm⁻¹) stretching vibrations. The deformation vibration of O–H and skeletal vibrations of unoxidized graphitic domains are observed at 1402 and 1623 cm⁻¹, respectively. In the FT-IR spectrum of bare RGO, the peaks from oxygen-containing functional groups weaken or even vanish, suggesting that the graphite oxide was effectively reduced. However, two peaks at 2917 and 2850 cm⁻¹ appear, which correspond to the asymmetric and symmetric C–H stretching vibrations of the alkyl chain of ODA, revealing that some ODA molecules have been attached onto the RGO sheets. In addition, the peak at 3332 cm⁻¹ corresponding to primary amine group of ODA disappears in the pattern of RGO sample, indicating the reaction between the oxygen-containing functional groups of GO and the amine group of ODA.³⁹

X-ray photoelectron spectroscopy (XPS) analysis (Fig. 3) was further performed to elucidate the changes of the oxygen-containing functional groups and the bonding chemistry between ODA and RGO. C1s, O1s and N1s peaks can be observed in the XPS survey scan of bare RGO. The intensity of the O1s peak dramatically decreases as compared with that of graphite oxide after the reduction. Further analysis of this XPS spectrum indicates that the C/O molar ratio increases from 2.3 of graphite oxide to 21.2 of RGO, indicating the effective removal of oxygen-containing functional groups. In addition, as proved by the FT-IR spectrum analysis, the reduction of GO was accompanied by nitrogen incorporation from the reducing agent ODA. The appearance of N1s peak in the XPS survey scan spectra of RGO further confirms that the surface of RGO has been functionalized with ODA. Fig. 3b shows the high-resolution C1s spectra of the graphite oxide and RGO. The deconvolution spectra of graphite oxide show four different peaks centered at 284.5, 286.7, 287.7 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.^40,41 In comparison with the C1s spectra of graphite oxide, the intensities of these peaks decrease significantly in RGO and a new peak centered at 285.6 eV can be corresponded to a C–N bond.

Fig. 3 (a) XPS survey scan and (b) high-resolution C1s spectra of graphite oxide and bare RGO.

As the RGO was successfully prepared, we then evaluated its ability to anchor different metal oxide SQDs by simply adding metal nitrates into the reaction system. The crystal structure and phase purity of the as-synthesized RGO/metal oxide nanocomposites were firstly examined by powder XRD. As shown in Fig. 4, in addition to the peak at 2θ of about 21° corresponding to the (002) reflection of RGO, all of the other diffraction peaks in RGO/NiO and RGO/ZnO nanocomposites can be easily indexed to cubic phase of NiO (JCPDS 47-1049) and hexagonal phase of ZnO (JCPDS 36-1451), respectively. No peaks from other compositions are detected in these samples. The XRD analyses demonstrate the formation of the respective metal oxide on RGO sheets. Therefore, in this synthesis, ODA not only works as reducing agent for the reduction of GO, but also works as solvent for the thermal decomposition of nitrate salts forming oxides.

Fig. 4 XRD patterns of RGO/NiO and RGO/ZnO nanocomposites.

In order to reveal the detailed morphology, dispersivity and size of the metal oxide particles on RGO, TEM and high resolution TEM (HRTEM) measurements were performed. The typical (HR)TEM images of RGO/NiO and RGO/ZnO nanocomposites are displayed in Fig. 5, from which it can be clearly seen that in both the cases the metal oxide nanoparticles are exclusively attached on RGO sheets with highly uniform distribution. This indicates that our approach is efficient and can be applied to synthesize various RGO/metal oxide nanocomposites. In the case of RGO/NiO nanocomposites (Fig. 5a), individual NiO SQDs are well separated from each other and homogeneously dispersed on the RGO sheets. Almost no free NiO SQDs were observed outside of the RGO sheets, indicating the perfect combination between the NiO SQDs and the RGO support. The observed lattice spacing of 0.239 nm in Fig. 5b matches well with the interplanar distance of the (111) planes of cubic NiO. The average diameter of NiO nanoparticles is about 5.1 nm. For RGO/ZnO nanocomposites, ZnO SQDs are hardly observed from the TEM image of Fig. 5d owing to their small size. From the HRTEM image (Fig. 5e), the lattice fringes with interplanar distance of approximately 0.286 nm can be indexed to the (100) crystal plane of hexagonal ZnO. The size of the ZnO SQDs is determined to be about 2.7 nm (Fig. 5f). Such small-sized ZnO SQDs on RGO support with large surface area promise great potential as functional nanomaterials for catalysis, sensing and power storage applications.⁴²

Fig. 5 TEM images of the as-prepared (a and b) RGO/NiO and (d and e) RGO/ZnO nanocomposites. The size distribution of (c) NiO and (f) ZnO nanoparticles in the nanocomposites.

Our facile protocol can also be extended to synthesize RGO/metal sulfide nanocomposites. When the metal nitrates and sulfur powder were introduced into the reaction system, composites of metal sulfide SQDs dispersed on RGO sheets were formed. The XRD patterns (Fig. 6) of RGO/CdS and RGO/ZnS nanocomposites show that both of the decorated CdS and ZnS SQDs have cubic zinc blende structure. The characteristic peaks are in good agreement with the standard pattern of CdS (JCPDS 42-1411) and ZnS (JCPDS 05-0566). It is known that the characteristic peak intensity of RGO gets weakened gradually with increasing amount of the loaded nanoparticles. The characteristic peak even cannot be detected if the coverage of nanoparticles is dense enough.⁴³ The loading amounts of nanoparticles in the RGO/SQDs nanocomposites are about 55.3, 49.7, 71.1 and 60.2 wt% for RGO/NiO, RGO/ZnO, RGO/CdS and RGO/ZnS nanocomposites, respectively. Therefore, it is reasonable that the characteristic peak intensity of the RGO in the RGO/metal oxides is much stronger than that in the RGO/metal sulfides nanocomposites due to the lower nanoparticle loading. The characteristic peak of RGO nearly disappears in the XRD pattern of RGO/CdS with high CdS-loading (71.1 wt%). Fig. 7 shows the representative TEM images of the RGO/CdS and RGO/ZnS nanocomposites. The homogeneous and dense coverage of CdS and ZnS SQDs on the RGO surface could be observed. No CdS or ZnS SQDs are scattered out of the RGO sheets, giving evidence of the strong interaction between them. The lattice images are shown in Fig. 7b and e, from which the crystalline features of CdS and ZnS SQDs can be clearly observed. The lattice spacings of 0.333 and 0.312 nm agree well with the interplanar distance of the (111) plane of CdS and (111) plane of ZnS, respectively. The particle sizes of the decorated CdS and ZnS SQDs are about 4.7 and 2.9 nm, respectively (Fig. 7c and f). The attachment of CdS and ZnS SQDs on the surfaces of RGO can act as ‘spacers’ to prevent the RGO sheets from direct stacking.⁴⁴ In addition, it is known that the metal sulfide SQDs show unique size dependent optical absorption. The combination of carbonaceous materials with metal sulfide SQDs is expected to improve the prospects for using hybrid materials in optoelectronic applications.⁴⁵ Thus, our prepared RGO/CdS and RGO/ZnS nanocomposites would serve as promising candidates for future optoelectronic devices.

Fig. 6 XRD patterns of RGO/CdS and RGO/ZnS nanocomposites.

Fig. 7 TEM images of the as-prepared (a and b) RGO/CdS and (d and e) RGO/ZnS nanocomposites. The size distribution of (c) CdS and (f) ZnS nanoparticles in the nanocomposites.

Optical properties

To investigate the optical properties of the as-prepared RGO/SQDs nanocomposites, ultraviolet-visible (UV-vis) and PL spectra measurements were carried out. Fig. 8 shows the UV-vis absorption spectra of the as-prepared bare RGO and RGO/SQDs nanocomposites. It can be seen that the bare RGO shows a strong absorption peak at 260 nm, which is generally regarded as the excitation of π-plasmon of graphitic structure.⁴⁶ The RGO/NiO nanocomposites exhibit broadened band at 260 nm, which is consistent with the reported UV-vis spectra of RGO/NiO.⁴⁷ The absorption peak at 371 nm in RGO/ZnO nanocomposites is ascribed to the contribution from ZnO nanoparticles.⁴⁸ Interestingly, the absorption peak of RGO at 260 nm was shifted to 274 nm in the RGO/ZnO nanocomposites, suggesting a coupling effect between ZnO and RGO.⁴⁹ The typical metal sulfide absorption bands are observed in the UV-vis spectra. The RGO/CdS and RGO/ZnS show absorption bands at about 465 and 300 nm, respectively. The broadening and weakening of the bands can be attributed to the coupling effect.⁵⁰

Fig. 8 UV-vis absorption spectra of the as-prepared (a) bare RGO, (b) RGO/NiO, (c) RGO/ZnO, (d) RGO/CdS and (e) RGO/ZnS nanocomposites.

The PL spectra of the RGO/SQDs nanocomposites at room temperature are shown in Fig. 9. The PL spectra of the RGO/NiO nanocomposites was obtained with the excitation wavelength of 260 nm. A strong UV emission peak located at 308 nm corresponding to a near band-edge emission is observed. It should correspond to the wide band gap of the NiO nanoparticles due to the recombination of excitons. The PL spectra of the RGO/ZnO nanocomposites with excitation wavelength of 300 nm is shown in Fig. 9b. The spectra shows the emission peak at around 370 nm which originates from the excitonic recombination between the conduction band and the valence band of ZnO.⁵¹ The RGO/CdS shows an emission peak at about 430 nm under an excitation of 355 nm, which corresponds to the band edge emission of CdS nanoparticles.⁵² The band edge emission of ZnS quantum dots occurs at 360 nm. The peaks at 383 and 448 nm can be attributed to shallow-trap emission and deep-trap emission of the ZnS nanoparticles, respectively.^52,53

Fig. 9 PL spectra of (a) RGO/NiO (λ_ex = 260 nm), (b) RGO/ZnO (λ_ex = 300 nm), (c) RGO/CdS (λ_ex = 355 nm) and (d) RGO/ZnS (λ_ex = 280 nm).

Electrochemical studies

Electrochemical supercapacitors are considered as promising candidate for energy storage and conversion devices due to their intriguing properties such as high power density, low temperature sensitivity, fast recharge capability and long life cycles.^54–56 Nickel oxide, as a pseudocapacitive material, has received considerable amount of attention due to its low cost, high theoretical specific capacitance and well-defined redox behavior.⁵⁷ Herein, the RGO/NiO composite was employed as a model to investigate the potential application of our prepared RGO/SQDs nanocomposites. The electrochemical performance of the RGO/NiO electrode materials for supercapacitors was tested by cyclic voltammetry (CV) using a three-electrode system. Fig. 10a shows the corresponding CV curves of RGO/NiO nanocomposites in 3 M KOH aqueous solution at various scan rates. It can be seen that the CV current response of RGO/NiO increases gradually with the increase of scan rate and the anodic peak potential shifts positively, meanwhile, the cathodic peak potential shifts negatively. However, no significant change in the shape of CV curve was detected, which indicates that the RGO/NiO nanocomposite electrode possesses good rate performance. The anodic peak in the potential range of 0.3–0.5 V is due to the conversion from NiO to NiOOH, and the cathodic peak in the potential range of 0.1–0.2 V is related to the reverse process, as shown by eqn (1):⁵⁸

NiO + OH⁻ ↔ NiOOH + e⁻ (1)

Fig. 10 (a) Cyclic voltammetry curves of RGO/NiO nanocomposites measured at different scan rates; (b) specific capacitances of RGO, NiO and RGO/NiO with different feeding amount of Ni(NO₃)₂·6H₂O (150, 200, 250 mg) at various scan rates; (c) TEM image of the bare NiO nanoparticles; (d) the cycle lifetime of RGO/NiO nanocomposites at scan rate of 80 mV s⁻¹.

This result suggests that the capacity mainly results from the Faradaic reaction pseudocapacitance, which is based on the surface reversible redox mechanism.⁵⁹ In addition, the RGO/NiO nanocomposites also exhibit peaks located at about 0.25 V, which can, in part, be due to the presence of oxygen-containing RGO.⁶⁰ The specific capacitance (C_s) value of the RGO/NiO electrode can be evaluated from the CV curves according to eqn (2):⁶¹

C_s = (∫IdV)/(νmV) (2)

where I is the response current, V is the potential, ν is the potential scan rate, and m is the mass of the electroactive materials in the electrode. The specific capacitance of bare NiO and the RGO/NiO electrodes at various scan rates is shown in Fig. 10b. It can be clearly observed that the specific capacitance gradually decreases with increasing the scan rates for each sample. This is attributed to the different rate of alkali ion to go in and out of the surface of the electrode materials. At high scan rates, diffusion most likely limits the movement of electrolyte ions due to the time constraint, and only the outer active surface can be utilized for charge storage, and thus resulting in a lower specific capacitance.^62,63 In addition, the RGO/NiO electrodes exhibit much higher capacitance at all scan rates compared to bare NiO. The significantly enhancement of specific capacitance is probably attributed to the synergistic effect between NiO and RGO sheets. On the one hand, as shown in Fig. 10c, since there are no substrates to fix the NiO particles, bare NiO tend to aggregate with each other. RGO can offer an environment to effectively prevent the aggregation of these NiO particles, and provide a higher available electrochemically active surface area for exploiting the full advantages of NiO pseudocapacitance. On the other hand, the RGO sheets in RGO/NiO nanocomposites act as not only the support for the dispersion of NiO nanoparticles but also the electronic conductive channels due to the excellent electrical conductivity of RGO.⁶⁴ The intimate combination of NiO nanoparticles with RGO sheets is favorable to the fast transfer of electrons throughout the whole electrode matrix, leading to the higher capacitance. Thus, the structural feature of RGO/NiO nanocomposites ensures the effective utilization of both NiO and RGO in the composite electrode. Moreover, the specific capacitance of RGO/NiO electrodes can be adjusted by change the loading amount of NiO in the composites. With the increase of NiO content in RGO/NiO nanocomposites, the specific capacitance of the RGO/NiO electrode increased at first and then decreased. The RGO/NiO-200 nanocomposites exhibit the highest specific capacitance. In RGO/NiO electrode, NiO predominates in charge storage, while RGO sheets act as electronic conductive channels. Therefore, it is reasonable that the specific capacitance increases with the increase in the NiO loading amount within a certain range. However, if the content of NiO is too high, a decrease of RGO matrix in the nanocomposites occurs. The RGO/NiO-250 with low content of RGO will result in a decrease in electrical conductivity, and thereby leading to a decreased capacitance.

The cycle stability of supercapacitors is a crucial parameter for their practical applications. To confirm the cycle life of our prepared RGO/NiO electrode, the cycling life test was carried out by repeating the CV measurements at a scan rate of 80 mV s⁻¹ for 1000 cycles. Fig. 10d presents the specific capacitance as a function of cycle number. Interestingly, the specific capacitance of RGO/NiO does not degrade but increase continuously until the 600th cycle. This is likely due to the increased effective interfacial area between electrolyte and electrode materials with the increase of reaction time.⁶⁵ Subsequent cycling caused no obvious specific capacitance drop up to 1000 cycles, demonstrating the excellent electrochemical stability and high degree of reversibility in the cycling of the electrode material. The long-term stability suggests that our prepared RGO/NiO nanocomposite is a promising electrode material for supercapacitors.

Conclusions

In summary, various RGO/metal oxides and RGO/metal sulfides nanocomposites were successfully prepared through a facile, rapid and general strategy for the first time. ODA was used as both dispersing agent and reducer for the uniformly deposition of metal oxides and sulfides nanocrystals on the surface of RGO. The prepared RGO/NiO nanocomposites were used as electrode materials for supercapacitor to investigate the potential applications of our prepared RGO-based nanocomposites. The results reveal that the RGO/NiO electrode exhibits enhanced capacitive performance and excellent long-term stability, which make it a promising electrode material for supercapacitors. It is expected that our facile and general method reported here could be extended to synthesize other RGO/SQDs nanocomposites for various applications.

Acknowledgements

The authors are grateful for financial support from the National Natural Science Foundation of China (no. 51272094), the Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20123227110018), and the Undergraduate Practice Innovation Project of Jiangsu Province (201310299022Z).

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