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Biomolecule-Assisted, Environmentally Friendly, One-Pot Synthesis of CuS/Reduced Graphene Oxide Nanocomposites with Enhanced Photocatalytic Performance Yingwei Zhang,†,⊥ Jingqi Tian,†,‡,⊥ Haiyan Li,† Lei Wang,† Xiaoyun Qin,† Abdullah M. Asiri,§,∥ Abdulrahman O. Al-Youbi,§,∥ and Xuping Sun*,†,§,∥ †

State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100039, China § Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ∥ Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia S Supporting Information *

ABSTRACT: In this work, we develop a novel environmentally friendly strategy toward one-pot synthesis of CuS nanoparticle-decorated reduced graphene oxide (CuS/rGO) nanocomposites with the use of L-cysteine, an amino acid, as a reducing agent, sulfur donor, and linker to anchor CuS nanoparticles onto the surface of rGO sheets. Upon visible light illumination (λ > 400 nm), the CuS/rGO nanocomposites show pronounced enhanced photocurrent response and improved photocatalytic activity in the degradation of methylene blue (MB) compared to pure CuS. This could be attributed to the efficient charge transport of rGO sheets and hence reduced recombination rate of excited carriers.

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Currently, the copper sulfides (CuxS), which are p-type semiconducting materials, caused by copper vacancies within the lattice,35,36 have drawn intense attention owing to their special properties and application in solar cells, solar controllers, nonlinear materials, lithium-ion batteries, gas sensors, and catalysts.37−40 There are five stable CuxS phases, and the stoichiometric factor x in CuxS varies in a wide range between 1 and 2, accompanied by varying band gap and structure.41,42 Among them, the covellite copper sulfide (CuS) is very attractive since it has an additional absorption band in the NIR.39,42,43 Moreover, CuS maintains transmittance in the infrared and exhibits low reflectance in the visible and relatively high reflectance in the near-infrared region, which makes it a prime candidate for solar energy adsorption.44 Recently, the quantum dots/rGO composites such as CdS/ rGO and ZnS/rGO have aroused extensive concern due to their applications in light harvesting and energy conversion.45,46 Up to now, many methods for the fabrication of such composites have been developed using Na2S, thiourea, and dimethyl sulfoxide as the sulfur source, posing environmental and health risks.45,47−49 Until recently, biomolecule-assisted synthetic methods have become the new and environmentally-

ybrid nanocomposites by combining different nanomaterials have aroused extensive interest over past decades due to their new optical, electronic, thermal, mechanical, and catalytic properties.1−9 Graphene, a flat monolayer of sp2bonded carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, has been an attractive choice as the support for anchoring nanoparticles, due to its high surface area (∼2600 m2/g), high chemical stability, and unique electronic and mechanical properties.10,11 To date, numerous studies on nanocomposites that integrate graphene or reduced graphene oxide (rGO) with different nanoscaled materials have been conducted in which their catalytic, electrochemical, and photoelectrochemical performance was considerably enhanced, due to the increased active surface areas and efficient electron transfer provided by graphene.12−15 However, most of these reports are mainly concentrated in metal and metal oxides supported on graphene,16−30 and less attention has been paid to the nanocomposites combining graphene and chalcogenide nanomaterials,31−34 which also possess excellent chemical and physical properties. In addition, the chemical reduction process of GO to rGO usually results in environmental and health risks due to the use of toxic or hazardous chemical reductants.1,2 Therefore, it is very important to develop an environmentally friendly pathway to synthesize new chalcogenide−graphene nanocomposites with high performance. © XXXX American Chemical Society

Received: June 19, 2012

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as the exciting source. Photoelectrochemical measurements were composed of a CHI 660D electrochemical analyzer (CH Instruments, Inc., Shanghai), a 500 W xenon lamp (CHFXQ500W, Beijing) with cutoff filter ion (λ > 400 nm), and a homemade three-electrode cell using a KCl-saturated Ag/ AgCl electrode, a platinum wire, and CuS/rGO as the reference, counter and working electrodes, respectively. The CuS/rGO modified ITO electrode was prepared by the dipcoating method. Before modification, the ITO substrates were washed with deionized water and then rinsed with isopropyl alcohol to remove any organic residue. Then, 100 μL of CuS/ rGO suspensions was dip-coated onto a 0.5 cm × 4 cm indium−tin oxide (ITO) glass electrode to form a homogeneous film. The electrode was finally exposed to an infrared light to eliminate the solvent and stored in the dark for photoelectrochemical measurements. For rGO and CuS alone modified ITO electrode, the same method was followed. The supporting electrolyte was 1 M Na2SO4, which was purged with high-purity nitrogen for at least 15 min prior to experiments. Photocatalytic tests: photodegradation studies of MB were carried out in a homemade quartz photochemical reactor. Before light irradiation, the photocatalyst (1 mg) was added into an aqueous solution of MB (10 mL, 4 mg L−1). The mixture was first sonicated for 5 min and then kept in the dark for 1 h with stirring to reach the adsorption−desorption equilibrium. The visible-light source was a 500 W xenon lamp with UV cutoff filter (λ > 400 nm). At the given time intervals, the concentration analysis of MB was determined by using a UV5800 spectrophotometer. For the stability measurements, catalytic poisoning after each cycle is removed by ethanol washing and subsequent drying of the catalyst. To demonstrate general morphology of the CuS/rGO nanocomposites thus formed, Figure 1a shows a typical TEM image of the nanocomposites. It is seen that all the rGO sheets have been densely decorated with a large amount of small nanoparticles. A close view of these nanoparticles reveals that they are relatively well distributed, as shown in Figure 1b. To gain further insight into the nanoparticles, a high-resolution TEM (HRTEM) image taken from a typical area of one piece

friendly focus of nanomaterial preparation. Among the numerous biomolecules, L-cysteine, an amino acid that contains multifunctional groups (−SH, −NH2, and −COO−),31,41,50−53 has been of great interest to researchers and widely applied for the synthesis of metal sulfide.50−53 Herein, inspired by those works, we develop an environment-friendly strategy toward one-pot synthesis of CuS/reduced graphene oxide (CuS/rGO) nanocomposites with the use of L-cysteine as a reducing agent, sulfur donor, and linker to anchor CuS nanoparticles onto the surface of rGO. The formation of CuS/rGO nanocomnposites occurs in a single-step process, carried out by hydrothermal treatment of aqueous dispersion of GO and CuCl2 in the presence of L-cysteine. It suggests that the CuS/rGO nanocomposites show significant photocurrent response under visible light irradiation (λ > 400 nm) and good photocatalytic activity in the degradation of methylene blue (MB), which could be ascribed to the efficient charge transport of rGO sheets and hence reduced recombination rate of excited carriers. Graphite powder, H2O2 (30 wt %), CuCl2, NaNO3, H2SO4 (98 wt %), KMnO4, and L-cysteine were purchased from Aladin Ltd. (Shanghai, China). All chemicals were used as received without further purification. The water used throughout all experiments was purified through a Millipore system. GO was prepared from natural graphite powder through a modified Hummers’ method54 using graphite powder, H2SO4, NaNO3, and H2O2 (30%) as the staring materials. As-synthesized GO was dispersed into individual sheets in distilled water at a concentration of 0.2 mg/mL with the aid of ultrasound for further use. The preparation of CuS/rGO nanocomposites was carried out as follows. In a typical experiment, 23 mg of Lcysteine was added into the 6 mL of 0.2 mg/mL GO dispersion under ultrasonic irradiation for 5 min. Next, 20 μL of 1 M CuCl2 aqueous solution was mixed with the above L-cysteine− GO solution, followed by an obvious color change from brown to deep blue-gray. At last, the resulting mixture was aged in an autoclave at 160 °C for 12 h. The products in the brown black dispersion were centrifuged and washed twice with distilled water and redispersed in water for characterization and further use. Moreover, a control experiment with the use of Na2S instead of L-cysteine was also performed. CuS nanoparticles were prepared as follows. In a typical experiment, 12 mg of Lcysteine was added into the 6 mL of water under ultrasonic irradiation for 5 min. Next, 50 μL of 1 M CuCl2 aqueous solution was added into the above L-cysteine solution. Then, the resulting mixture was aged in an autoclave at 160 °C for 12 h. The products were centrifuged and washed twice with distilled water and redispersed in water for characterization and further use. Raman spectra were obtained on J-Y T64000 Raman spectrometer with 514.5 nm wavelength incident laser light. Powder X-ray diffraction (XRD) data were recorded on a Rigaku D/MAX 2550 diffractometer with Cu Ka radiation (k = 1.5418 A). Diffraction patterns were collected under ambient conditions at a scanning rate of 10°/min. Transmission electron microscopy (TEM) measurement was made on a HITACHI H8100 EM (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of sample solution on carbon-coated copper grid and dried at room temperature. UV−vis spectra were obtained on a UV5800 Spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALABMK II X-ray photoelectron spectrometer using Mg

Figure 1. (a) Low and (b) high magnification TEM images, (c) HRTEM images, and (d) EDS of as-prepared CuS/rGO nanocomposites. B

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Figure 2. (a) XRD pattern of as-prepared CuS/rGO nanocomposites. (b) Corresponding particle size distribution histogram of the CuS nanoparticles on the surface of the rGO nanosheet.

presence of a doublet peak at 162.0 eV, which is a typical value for metal sulfides.50−53 All the above results are well matched with the reported values. Figure 3d,e shows the C1s XPS spectra of GO and CuS/rGO nanocomposites, respectively. It is known that most of the oxygen-containing functional groups in GO exist in the form of either hydroxyl or epoxide groups, and the successful removal of epoxy and hydroxyl means the formation of rGO. The C1s spectra of GO and CuS/rGO nanocomposites could be deconvoluted three peaks at 284.5, 286.6, and 288.2 eV, which are associated with C−C, C−O (epoxyl and hydroxyl), and CO (carbonyl), respectively.23 It is seen that the peak intensity of C−O is strong in GO (Figure 3d); in contrast, after hydrothermal treatment process, the peak intensity of C−O in CuS/rGO nanocomposites (Figure 3e) tremendously decreases, suggesting most of the oxygencontaining functional groups are successfully removed.59 All the observations support the conclusion that CuS/rGO nanocomposites have been successfully synthesized. In addition, the successive reduction of GO and synthesis of CuS/rGO nanocomposites by L-cysteine was further verified by Raman spectroscopy, as shown in Figure 4. It is well-known that graphene obtained by chemical reduction of GO exhibits two characteristic main peaks: the D band at ∼1350 cm−1, arising from a breathing mode of κ-point photons of A1g symmetry; the G band at ∼1575 cm−1, arising from the first order scattering of the E2g phonon of sp2 C atoms. In our present study (Figure 4a), it is seen that GO exhibits a D band at 1357 cm−1 and a G band at 1604 cm−1, while the corresponding bands of CuS/rGO nanocomposites are 1357 and 1588 cm−1, respectively. The G band of CuS/rGO nanocomposites (Figure 4b) red-shifted from 1604 to 1588 cm−1 is attributed to the high ability for recovery of the hexagonal network of carbon atom. It is also found that CuS/ rGO nanocomposites show relatively higher intensity of D to G band (1.17) than that of GO (0.84). These observations further confirm the formation of new graphitic domains after the heat treatment process.23,60 A strong and sharp band at 475 cm−1 in CuS/rGO nanocomposites, revealing that the lattice atoms of CuS are aligned in the periodic array, agrees well with the observation for the covellite structure of CuS with a hexagonal crystal structure by previous references.61 L-Cysteine is an amino acid that contains three functional groups (−SH, −NH2, and −COO−)31,41,50−53 and has a high affinity for the Cu2+ ions due to the presence of −SH. It should be mentioned that L-cysteine plays a triple role of reducing agent, sulfur donor, and the linker in our system. L-Cysteine can

of the nanosheet shown in Figure 1b was exhibited in Figure 1c. It can be seen that the nanoparticles decorating on the surface of the sheets with interplanar spacing of 0.303 nm, 0.271 nm, and 0.188 nm, corresponding to the d spacing for the (102), (006), and (110) planes of hexagonal structured CuS, respectively.50−53,55 Moreover, the HRTEM result indicates these particles are CuS nanoparticles with good crystallinity. The chemical composition of the nanocomposites was further determined by the energy dispersed spectrum (EDS), and it clearly confirms the presence of Cu, S, and C elements (other peaks originate from the substrate used), as shown in Figure 1d. Figure 2a shows the corresponding XRD pattern of as-prepared CuS/rGO nanocomposites. All the diffraction peaks are readily indexed to a hexagonal phase of CuS (JCPDS No. 06-0464) and no other characteristic peaks are observed for impurities, indicating the formation of pure hexagonal phase CuS with high crystallinity. Additionally, the XRD pattern of the CuS/rGO nanocomposites shows the absence of the (002) diffraction peak of the rGO nanosheets, indicating that the rGO nanosheets do not stack during the hydrothermal process. The reason can be ascribed to that the CuS nanoparticles anchored on the surfaces of rGO prevent the exfoliated rGO sheets from direct stacking after the reduction of the GO.31 Figure 2b gives the size distribution histogram of the CuS nanoparticles. It reveals that the mean particle size of CuS is about 18 nm with size distribution of 8−28 nm, which is in good agreement with the mean crystal size value of 18.9 nm calculated by the Scherrer equation based on the (102) peak from the XRD pattern. All the above structural and morphological characterizations confirm the successful synthesis of CuS/rGO nanocomposites. Important information on the surface electronic state and the composition of the final CuS/rGO products can be further provided by X-ray photoelectron spectroscopy (XPS), as shown in Figure 3. First, a typical survey XPS spectrum in Figure 3a clearly shows the peaks of Cu 2p, S2p, O 1s, and C 1s. The peaks of O and C come from rGO. The Cu 2p and S 2p peaks are further examined by high-resolution XPS spectra, respectively (Figure 3b,c). Figure 3b shows the binding energies of Cu 2p3/2 and Cu 2p1/2 peaks at 931.7 and 951.7 eV, respectively, which are typical values for Cu2+ in CuS.50−53,56 Moreover, two shakeup satellite lines at 943.0 and 963.6 eV are observed, which are the characteristic of materials having d(9) configuration in the ground state, that is, Cu(II), indicating the paramagnetic chemical state of Cu2+.57,58 Another high-resolution spectrum in the S 2p region shows the C

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Figure 3. Typical XPS spectra of the CuS/rGO composite: (a) survey spectra, (b) Cu 2p region XPS spectrum, and (c) S 2p region XPS spectrum. C1s spectra of (d) GO and (e) CuS/rGO nanocomposites.

schematic diagram (not to scale) to illustrate the formation process of the CuS/rGO nanocomposites. We further explored the application of the composites for photocurrent generation in the visible range of light. Figure 5 displays the UV−vis absorption spectra of aqueous dispersions of CuS/rGO nanocomposites and CuS nanoparticles. The absorption beyond 800 nm is ascribed to free carrier interband transitions from valence states to the unoccupied states.55 The strong absorption in the visible range implicates the potential application of this nanocomposites in photocurrent generation in visible range of light.

release H2S when heated, which acts a sulfide source as well as a reducing agent, resulting in the formation of metal sulfide nanoparticles and the reduction of GO to rGO.31,41 We also performed a control experiment to confirm the role of Lcysteine as a linker to anchor Cu2+ for the well-distributed growth of the CuS nanoparticles on the rGO sheets. As shown in Figure S1, Supporting Information, when L-cysteine was replaced by Na2S, it is clearly seen that the dispersion of CuS nanoparticles on the rGO sheets was very poor (Figure S1a) and that there are some free particles in solution not on the surface of the rGO sheets (Figure S1b). Scheme 1 presents a D

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Figure 4. Room-temperature Raman spectra of (a) GO and (b) the synthesized CuS/rGO nanocomposites. Figure 6. Photocurrent responses of rGO, CuS, and CuS/rGO nanocomposites modified ITO electrodes in a standard threecompartment cell along with a Pt wire counter electrode and a reference electrode (Ag/AgCl) under white light illumination (λ > 400 nm; input power, 100 mW/cm2; electrolyte,: 1 M Na2SO4 aqueous solution; electrode potential of 0 V vs Ag/AgCl).

Scheme 1. Schematic Diagram (Not to Scale) to Illustrate the Preparation Process of the CuS/rGO Nanocomposites with the Use of L-Cysteine

modified electrode shows almost no photoelectrochemical effect under illumination (green curve), and the CuS modified ITO electrode produces a small cathodic photocurrent ca. 10 nA in intensity (blue curve). In contrast, the CuS/rGO nanocomposites show a 34-fold enhancement in photocurrent (red curve), the photocurrent intensity of which is about 355 nA, indicating that the rGO has a positive role on the photocurrent generation of the CuS/rGO nanocomposites. Scheme 2 shows a schematic illustration for the photocurrent Scheme 2. Schematic Diagram (Not to Scale) to Illustrate the Energy Level Alignment and Photocharge Carrier Dissociation at the CuS−rGO Interface

enhancement mechanism involved. The separated electrons will inject into rGO (∼0 V vs NHE) from the conducting band of CuS, and the collected carriers in the rGO are rapidly transferred to electrolyte and then to the counter electrode due to the excellent carrier mobility of the rGO sheet.62 The holes in the valence band of CuS are transferred to the ITO electrode and external circuit at the same time. Thus, we come to the conclusion that rGO has a positive role on the photocurrent generation of the modified electrode. On one hand, rGO can accept the electrons from the CuS; on the other hand, rGO can facilitate electrons transfer within the composite film.63,64 Next, to evaluate the photocatalytic ability of the nanocomposites, we examined the decomposition of MB dye in solution over the samples under visible-light irradiation (λ > 400 nm) as a function of time. As shown in Figure S2a,b,

Figure 5. UV−vis absorption of the CuS/rGO nanocomposites and CuS nanoparticles.

Figure 6 shows the photocurrent responses of rGO, CuS, and CuS/rGO nanocomposites modified ITO electrodes under white light illumination (λ > 400 nm, 100 mW/cm2). The rGO E

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Figure 7. (a) Variation of MB concentration as a function of irradiation time under visible light (λ > 400 nm) with different photocatalysts. C is the MB concentration at time t, and C0 is the concentration of MB after the adsorption equilibrium. (b) Repeated bleaching of MB over recycled CuS/ rGO nanocomposites under visible light.

could also provide a high concentration of MB near to the CuS nanoparticles on rGO, leading to highly efficient contact between them. The concentrated dye molecule environment over the catalyst surface can speed up the reaction with photogenerated active species, which promotes the photocatalytic degradation process.66 Moreover, according to above photocurrent result, the loading of CuS nanoparticles on the surface of the rGO sheet could improve the photogenerated electron−hole pair separation and facilitate the transfer of photogenerated charge due to the efficient charge transport of rGO sheets and hence reduce recombination rate of excited carriers, which is also beneficial to improve the photocatalytic ability of samples. Hence, CuS/rGO nanocomposties show an improved photocatalytic activity in the degradation of MB. At last, the separated electrons or holes can quickly react with adsorbed O2 or H2O to produce O2• or OH• radicals. Then, the so-formed radicals react with the organic dyes to oxidize it and eventually mineralization takes place.55,67 In summary, hydrothermal treatment of aqueous dispersion of GO and CuCl2 in the presence of L-cysteine has been proven to be an effective strategy for preparing CuS/rGO nanocomposites. Such CuS/rGO nanocomposites show an enhanced photocurrent generation and improve photocatalytic activity in the degradation of MB under visible light, due to fast charge separation and the efficient charge transport after the introduction of rGO sheets. Our present study is important because it provides us a convenient, environmentally friendly approach toward one-pot synthesis of CuS/rGO nanocomposites for applications.

Supporting Information, the characteristic absorption peak of MB at 663 nm is used to monitor the photocatalytic dye degradation reaction. The normalized temporal concentration changes of MB during the photodegradation process are shown in Figure 7a. It can be seen that the decomposition of MB is negligible under visible light irradiation without any catalyst or in the presence of rGO sheets. However, an approximate 81% of the dyes are removed from the solution after 2 h in the presence of CuS/rGO nanocomposites. For comparison, the pure CuS nanoparticles with similar sizes were also checked (the corresponding characterizations are given in Figure S3, Supporting Information), and only about 49% of MB had decomposed under the same testing conditions. It demonstrates that CuS/rGO nanocomposties have much higher photocatalytic activity than bare CuS under visible light. It should be pointed out that, before the irradiation with visible light, CuS/rGO nanocomposites also present higher dye adsorption efficiency than CuS nanoparticles alone under the dark reaction. All the above observations indicate that the introduction of rGO support plays an active part in the adsorption and catalytic degradation process of dyes. In addition to photocatalytic activity, the stability of photocatalysts is another important issue for their practical applications; therefore, the photocatalytic stability of CuS/rGO nanocomposites is investigated by recycling them in the repeated MB degradation experiments. As shown in Figure 7b, 81% of the MB has been bleached after 2 h for the first cycle of degradation, and 60% and 52% of the MB has been degraded in the same period for the second and third run, respectively. It is reported that the metal sulfide semiconductors are unstable since they undergo photoanodic corrosion in the photocatalytic reactions,65 leading to the loss of stability of the CuS/rGO nanocomposites during the repeated experiments. As for the mechanism of photocatalysis, it is believed that the efficient adsorption of dye molecules on the photocatalysts and effective separation of photogenerated electron/hole pairs are the most important factors, which determine the efficiency of the photocatalytic reaction. In our photocatalytic experiments, first of all, the enhanced adsorption capacity of CuS/rGO nanocomposites may be ascribed to the more efficient adsorption of cationic dye molecules MB on rGO via electrostatic force field and π−π stacking interactions between rGO and π-rich MB dye molecule.15,47−49 Such adsorption

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ASSOCIATED CONTENT

* Supporting Information S

TEM images of CuS/rGO prepared with Na2S; Timedependent optical absorption spectra of MB degradation using CuS/rGO and CuS; XRD pattern and TEM image of CuS. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: (+86)431-85262065. E-mail: [email protected]. Author Contributions ⊥

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These authors contributed equally to this work. dx.doi.org/10.1021/la303049w | Langmuir XXXX, XXX, XXX−XXX

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21175129) and the National Basic Research Program of China (No. 2011CB935800).

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