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Visible light responsive Cu2MoS4 nanosheets incorporated reduced graphene oxide for efficient degradation of organic pollutant
Accepted Manuscript Title: Visible light responsive Cu2 MoS4 nanosheets incorporated reduced graphene oxide for efficient degradation of organic pollutant Authors: R. Rameshbabu, R. Vinoth, M. Navaneethan, S. Harish, Y. Hayakawa, B. Neppolian PII: DOI: Reference:
S0169-4332(17)30488-9 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.126 APSUSC 35235
To appear in:
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Received date: Revised date: Accepted date:
22-10-2016 13-2-2017 15-2-2017
Please cite this article as: R.Rameshbabu, R.Vinoth, M.Navaneethan, S.Harish, Y.Hayakawa, B.Neppolian, Visible light responsive Cu2MoS4 nanosheets incorporated reduced graphene oxide for efficient degradation of organic pollutant, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.126 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 proof before it is published in its final 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.
Visible Light Responsive Cu2MoS4 Nanosheets Incorporated Reduced Graphene Oxide for Efficient Degradation of Organic Pollutant
R. Rameshbabua, R. Vinotha, M. Navaneethanb, S. Harishb, Y. Hayakawab B. Neppoliana*
a
SRM Research Institute, SRM University, Kattankulathur, Kanchipuram-603203, Tamil Nadu, India
b
Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, Japan
*Corresponding author: Ph. No: 044-2741-7916, E-mail: [email protected]
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Research Highlights
Cu2MoS4 nanosheets are incorporated on reduced graphene oxide sheets. Electronic interaction between rGO and Cu2MoS4 nanosheets increased the absorption. Insertion of rGO sheets increased the surface area of Cu2MoS4/rGO nanocomposite. 99% methyl orange degradation is achieved for Cu2MoS4/rGO nanocomposite.
ABSTRACT Visible light active copper molybdenum sulfide (Cu2MoS4) nanosheets were successfully anchored on reduced graphene oxide (rGO) using facile hydrothermal method. During the hydrothermal reaction, reduction of graphene oxide into rGO and the formation of Cu2MoS4 nanosheets were successfully obtained. The charge transfer interaction between the rGO sheets and Cu2MoS4 nanosheets extended the absorption to visible region in comparison with bare Cu2MoS4 nanosheets i.e without rGO sheets. Furthermore, the notable photoluminescence quenching observed for Cu2MoS4/rGO nanocomposite revealed the effective role of rGO towards the significant inhibition of electron-hole pair recombination. The photocatalytic efficiencies of bare Cu2MoS4 and Cu2MoS4/rGO nanocomposite was evaluated for the degradation of methyl orange dye under visible irradiation (λ > 420 nm). A maximum photodegradation efficiency of 99% was achieved for Cu 2MoS4/rGO nanocomposite, while only 64% photodegradation was noted for bare Cu2MoS4. The enhanced optical absorption in visible region, high surface area, and low charge carrier recombination in the presence of rGO sheets were the main reasons for the enhancement in photodegardation of MO dye. In addition, the
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resultant Cu2MoS4/rGO nanocomposite was found to be reusable for five successive cycles without significant loss in its photocatalytic performance.
KEYWORDS: Cu2MoS4 Nanosheets Graphene Photocatalysis Charge separation Methyl Orange.
1. INTRODUCTION TiO2 semiconductor photocatalyst has been considered as a promising material for the photocatalytic degradation of organic pollutants [1, 2]. However, it is mostly active in ultraviolet (UV) light due to its wide band-gap energy associated with UV region of the solar spectrum [3,4]. Thus, it further limits the photocatalytic application of these semiconductors in visible region, as the solar spectrum contains only less than 5% of UV light [5]. On the other hand, fast recombination of photoexcited charge carriers leads to low quantum efficiency and thereby affects the photocatalytic performance [6]. Recently, sulphides [7], oxides [8] and nitrides [9] based visible light active photocatalysts have been developed for effective degradation of organic pollutants. Especially, ternary Cu2MoS4 transition metal sulphides (TMSs) based photocatalysts have attracted much attention in recent times owing to their promising photocatalytic activity [10]. Generally, Cu2MoS4 exists in two phases: with space group P42m and I42m. I-Cu2MoS4 has good visible-light absorption capability because of its favorable low bandgap value of 1.71 eV [11]. Tran et al. have successfully prepared Cu2MoS4 and utilized as novel electrocatalyst for hydrogen evolution reaction [12]. In general, photocatalysts with different morphological
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nanostructures greatly influences the photocatalytic activity of the photocatalysts. Recently, K. Zang et al. has synthesized cubic like Cu2MoS4 photocatalyst for photodegradation of methyl orange dye under visible light illumination [13]. Similarly, ultrathin Cu2 MoS4 nanobelts were also synthesized using hydrothermal technique and employed as an efficient visible light photocatalysts for methyl orange degradation [10]. Very recently, Chen et al. has reported an enhanced visible light photocatalytic activity with indented Cu 2MoS4 nanosheets without any supports [14]. In addition to tailoring the morphology, using of conductive solid support like graphene along with photocatalysts can enhance the photocatalytic activity due to its excellent properties, such as high electron mobility, high theoretical surface area and chemical stability [15]. For example, Tran et al. have succeeded in synthesizing Cu2O/rGO composites and demonstrated an improvement in the photocurrent and photo stability of pure Cu 2O with the use of Cu2O/rGO composites [16]. In our previous reports, we have extensively used rGO as a solid support along with TiO2-CuO, TiO2-Cu2O, C3N4-Cu2O and AgI-TiO2 photocatalyst for photocatalytic water splitting and photocatalytic organic pollutants degradation applications [17-20]. The insertion of rGO can act as carrier separation and transportation platform and thereby greatly enhance the photocatalytic activity. In the present work, we designed sheet like Cu2MoS4 photocatalyst along with rGO sheet using hydrothermal technique. The photocatalytic activities of the as prepared samples were evaluated by the degradation of methyl orange (MO) under visible light irradiation (λ > 420 nm). Cu2MoS4/rGO nanocomposite showed an enhanced photocatalytic performance than the previously reported hierarchical Cu2MoS4/rGO hollow sphere [21]. The photocatalytic activity of Cu2MoS4/rGO nanocomposite was higher than the pure Cu2MoS4 nanosheets by almost 1.5 4
times. Furthermore, the photocatalytic activity of the synthesized Cu 2MoS4/rGO nanocomposite was quite stable and showed an excellent reusability. 2. Experimental Section 2.1 Materials and methods Copper nitrate trihydrate (Cu(NO3)2.3H2O), sodium molybdate (Na2MoO4.2H2O), thioacetamide (C2H5NS), ethylene glycol (C2H6 O2), Methyl orange (MO) dye and Graphite powder were purchased from Sigma-Aldrich. Poly (vinyl pyrrolidone) (PVP), Ascorbic acid (C6H8O6), Sodium hydroxide (NaOH) were received from SRL chemicals, India.
Double
distilled water was used throughout the experiments. 2.2 Synthesis of Cu2O nanospheres The Cu2O nanospheres were prepared from the previous report with minor modification [22]. Briefly, 0.0985 of Cu (NO3)2.3H2O and 11.666 g of poly (vinyl pyrrolidone) (PVP) were dissolved in 350 mL of deionized water. Subsequently, an aqueous solution of NaOH (2.0 M, 70.0 mL) and aqueous ascorbic acid solution (0.6 M, 70.0 mL) were then successively added drop wise with vigorous magnetic stirring. After 1 hour, a turbid yellow liquid formed. All the synthesized steps were carried out at room temperature. Finally the precipitate was collected after centrifugation and washed with deionized water and ethanol for several times. The final product was dried in hot air oven at 70 oC for few hours. 2.3 Synthesis of Cu2MoS4/rGO nanocomposite Graphene oxide was synthesized using modified Hummer’s method [23]. Hydrothermal method was followed for the synthesis of rGO loaded Cu2MoS4 photocatalyst. In brief, 60 mg of Cu2O powder was dissolved in 38 ml deionized water and the solution was ultra sonicated for 5 5
min. Then, 162 mg of ammonium tetrathiomolybdate was added into the solution and magnetically stirred for 5 min. After wards, the solution was transferred and sealed in a Teflonlined steel autoclave (50 ml) of 80% capacity of the total volume under airtight condition and maintained at 160 °C for 9 h. The precipitate was washed with deionized water and ethanol for several times to purify it and dried in hot air oven at 60 °C for few hours to obtain the final product. For comparison, Cu2MoS4 photocatalyst was prepared as reported earlier [10].
2.4 Characterization Technique Phase and structure of the as-prepared samples were analyzed by X-ray powder diffraction pattern, recorded using PAN analytical X’pert pro X-ray diffractometer with CuKα radiation (λ = 1.5406 A°). The morphological studies were carried out using a Field EmissionScanning electron microscopy (FEI Quanta FEG 200 HR-SEM). Transmission electron microscopy (TEM) images were recorded using a JEOL JEM 2100F microscope at an accelerating voltage of 200 kV. TEM specimens were prepared by dispersing the product powders in ethanol forming a suspension which was then placed onto a carbon-coated Cu grid after ultrasonic treatment. Diffuse reflectance spectra (DRS) were obtained using Analytikjena specord 210 plus, integrating sphere accessories. The photoluminescence spectra of the samples were obtained using a Jobin Yvon Horiba Fluorolog-3-Tau Spectrofluorometer. Raman spectra were obtained using (Horiba Jobin Yvon) using He–Ne Laser. X-ray photoelectron spectroscopy (XPS) measurements were obtained using K-Alpha instruments, USA, Al as a source, size 400 microns and range 0 to 1350 eV). Surface area and porosity measurements were carried out using a Quantachrome Nova-1000 surface analyzer at liquid nitrogen temperature. 2.5 Photocatalytic experiments 6
To investigate the photocatalytic activity of the photocatalysts, the photocatalytic degradation of MO was carried out under visible light. A 250 W Xe lamp with a λ ˃ 420 nm UV cutoff filter was used as the visible light source. The test solution was prepared by mixing photocatalyst (50 mg) in 50 mL of 10 mg/L MO solution (0.03 mM) and stirred for 30 min in dark condition to achieve the adsorption–desorption equilibrium between the photocatalyst and MO. The solution was then kept under the constant illumination and sampled (3 mL aliquot) at every 10 min of intervals. The MO concentration in each aliquot was examined by detecting the absorbance at the characteristic band of λ = 464 nm using Analytikjena specord 210 plus spectrophotometer. 3. Results and Discussion 3.1 Structural analysis The XRD patterns of the pure Cu2MoS4 and Cu2MoS4/rGO nanocomposites are shown in Fig. 1. It is conspicuous that the major diffraction peaks of Cu2MoS4/rGO composites are similar to that of pure Cu2MoS4 and corresponds to tetragonal-phase Cu2MoS4 (Space group: I-42m) [21]. Furthermore, no diffraction peaks for Cu2O, MoS2 and P-Cu2MoS4 phase are observed, suggesting the high purity of the nanocomposite. Moreover, loading of rGO does not induce any new crystal orientations or changes in preferential orientations of Cu 2MoS4. In addition, no characteristic diffraction peak for rGO is observed for Cu2MoS4/rGO nanocomposite, which might be due to low content of rGO or relatively low diffraction intensity of rGO in the nanocomposite [24]. 3.2 Morphological studies
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FE-SEM and TEM analyses were performed to examine the morphology of pure Cu2MoS4 and Cu2MoS4/rGO nanocomposite. The FE-SEM micrographs of the as-synthesized Cu2MoS4 and Cu2MoS4/rGO nanocomposite at different magnifications are shown in Fig. 2. As shown in Fig. 2(a&b), Cu2MoS4 shows sheet like nanostructure. Similarly, it can be also seen that the Cu2MoS4/rGO nanocomposite also possess nanosheet like morphology, which further decorated on rGO sheets [Fig. 2(c&d)]. Fig. 3 shows the TEM and HR-TEM images of Cu2MoS4 and Cu2MoS4/rGO nanocomposite. As displayed in Fig. 3(a), pure Cu2MoS4 exhibits a well defined sheet like morphology. Moreover, the rGO sheets with wrinkled surface are clearly seen in Cu2MoS4-rGO nanocomposite indicates the formation of interfacial contact between Cu2MoS4 nanosheets and rGO. Thus, the electronic interaction between Cu2MoS4 and rGO sheet can improves the charge carrier separation. Fig. 3(c&d) shows the HR-TEM images of Cu2MoS4 and Cu2MoS4-rGO nanocomposite. The calculated lattice spacing value for both Cu 2MoS4 and Cu2MoS4-rGO nanocomposite is measured to 0.51 nm and it corresponds to (002) plane of Cu2MoS4 [21]. 3.3 UV-vis DRS spectra The optical properties of the Cu2MoS4 and Cu2 MoS4/rGO nanocomposites are studied using the UV-vis absorption spectra in DRS mode, as shown in Fig. 4. Pure Cu2MoS4 nanosheets consist of a characteristic absorption peak at around 775 nm, whereas Cu2MoS4/rGO nanocomposite exhibits red shift and the absorption edge observed at 826 nm [Fig. 4(a)]. This account for the rGO content it also and suggests the existence of strong coupling between the rGO sheets and Cu2MoS4 nanosheets, which is desirable for the photocatalytic applications. Fig. 4(b) shows the Tauc plot of Cu2MoS4 and Cu2MoS4/rGO nanocomposite. The bandgap of pure Cu2MoS4 and Cu2MoS4/rGO are calculated to be 1.6 and 1.5 eV, respectively. This reduction in 8
the bandgap of composite can be ascribed to the electronic interactions between rGO and Cu2MoS4 nanosheet [25]. 3.4 Photoluminescence spectra To characterize the rate of electron-hole pair recombination, PL analysis was performed for Cu2MoS4 and Cu2MoS4/rGO nanocomposite. As displayed in Fig. 5, bare Cu2MoS4 and Cu2MoS4/rGO nanocomposite exhibit a PL emission peak at around 526 nm when excited by 320 nm laser (Fig. 5), which is enough to promote electronic transitions from the VB to the CB of Cu2MoS4, as evident from DRS spectra (Fig. 4). It can be clearly seen that the notable quenching of PL emission intensity for Cu2MoS4/rGO nanocomposite compared to that of bare Cu2MoS4 indicates the effective separation of electron-hole pairs in the presence of rGO sheet. 3.5 Raman spectra In order to confirm the existence of rGO in Cu2MoS4/rGO photocatalyst, Raman spectra were recorded for pure GO, Cu2MoS4 and Cu2MoS4/rGO photocatalyst. Fig. 6 shows the Raman spectra of GO, Cu2MoS4 and Cu2MoS4/rGO photocatalysts. The Cu2 MoS4 and Cu2MoS4/rGO photocatalysts display several characteristic Raman peaks in the frequency range of 100-1100 cm-1correspond to Cu2MoS4 [13, 14]. The pure GO exhibits the characteristic D and G bands at 1348 cm-1and 1582 cm-1, respectively. Similarly, the same D and G band observed in Cu2MoS4/rGO photocatalyst confirm the presence of rGO in the resultant nanocomposite (inset of Fig. 6). In addition, the ID/IG ratio of GO and Cu2MoS4/rGO photocatalyst was calculated to be 0.84 and 1.0, respectively. Thus, increasing I D/IG ratios of Cu2MoS4/rGO nanocomposite than GO not only represent the reduction of GO but also confirm the incorporation of Cu2 MoS4 on the rGO sheets [26, 27].
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3.6 X-ray photoelectron spectra X-ray photoelectron spectroscopy (XPS) measurement was carried out to investigate the chemical states of elements in the Cu2MoS4/rGO nanosheet photocatalyst. The survey scan (Fig 7a) spectrum proves the presence of Cu, Mo, S, C and O elements. As shown in Fig. 7b, the binding energy peaks noted at 932 eV and 952 eV are assigned to Cu 2p3/2 and Cu 2p1/2, respectively [28, 29]. As displayed in Fig. 7(c), the high resolution Mo 3d spectrum provides two peaks with the binding energies located at 226 and 230 eV correspond to Mo 3d 5/2 and Mo 3d3/2, respectively [14]. Similarly, the high resolution S 2p spectrum can be deconvoluted into two peaks (160 and 161 eV) indicate that the S is in sulfide state [Fig. 7(d)]. Thus, the XPS technique implies that the valence state of the elements (Cu, Mo and S) present in the Cu 2MoS4/rGO photocatalyst are Cu2+, Mo6+ and S2−, which are in good agreement with the literature report [14]. A shown in Fig. 7(e), the high resolution C1s spectrum is deconvoluted into three peaks. An intense peak noted at 283 eV corresponds to C=C or sp 2C. In addition, the low intense peaks observed at 284 eV and 286 eV are assigned to C-C and C-O, respectively [25]. Thus, a less intense C-O group observed in C1s spectrum clearly confirmed that the GO was reduced to rGO during the hydrothermal preparation of nanocomposite. Besides, a peak identified at 531 eV of O1s spectrum corresponds to O-C=O. 3.7 BET analysis The Brunauer-Emmett-Teller (BET) and the Barrett-Joyner-Halenda (BJH) analysis were used to obtain specific surface area and pore sizes of the nanocomposites. As depicts in the inset of Fig. 8, the average pore diameters of Cu2 MoS4 and Cu2MoS4/rGO nanocompoistes are observed to be 11.1 nm and 8.3 nm, respectively. Thus, the slight decrease in the pore size of Cu2MoS4/rGO nanocomposites would be attributed that the loading of rGO sheet blocked or 10
masked the pores of Cu2MoS4. Similar results were also reported in the literature for rGO based photocatalysts [30, 31]. The BET surface area of Cu2MoS4 and Cu2MoS4/rGO nanocomposite are measured to be 14.3 and 41.72 m2 g−1, respectively. This result clearly reveals that the incorporation of rGO sheet with Cu2MoS4 can increase the surface area and enhance the photocatalytic performance. Moreover, high surface area with small pores of the prepared Cu2MoS4/rGO nanocomposite can minimize the diffusion path for electrons and ions. Furthermore, this can also suppress the recombination rate of photoinduced electrons and holes. 3.8 Effect of GO loading In order to optimize the loading of rGO, different wt% (1, 2, 3, 4 and 5) of rGO loaded Cu2MoS4 photocatalysts were synthesized and its photocatalytic performance was evaluated for the degradation of the MO dye. Fig. 9, showed that around 67, 70, 79, 99.95 and 68% degradation of MO was observed with various % loading of GO (1, 2, 3,4 and 5 wt%). Thus, a maximum MO degradation of ~99.95% was achieved with 4wt% rGO that was found to be the optimized rGO loading for the degradation of MO. However, the further increase of rGO loading to 5 wt% decreased the photodegradation efficiency of the photocatalyst. The reason could be that the high loading of rGO can shield the incoming photons into the photocatalyst surface and thereby suppressed the photocatalytic activity [32, 33]. 3.9 Photocatalytic activity Fig. 10(a) shows the photocatalytic degradation of MO dye with different amount of catalyst loading. As displayed in Fig. 10a, the photodegradation efficiencies of 59 %, 67 %, 76 %, 99.9 %, 86 % are achieved for 20, 30, 40, 50 and 60 mg/50 mL, respectively. The optimized catalyst loading was found to be 50 mg and it showed the maximum photodegradation efficiency of 99% within 60 min of visible light illumination. The pH of the solution plays a vital role in the photocatalytic degradation process by affecting the surface charge properties of the catalyst, the 11
size of aggregates formed, the charge of dye molecules and adsorption of dye onto the catalyst surface and the concentration of hydroxyl radicals. The photodegradation efficiencies of 95 %, 96 %, 99.95 %, 99.83 % and 98.4% are achieved at pH 3, 5, 7, 9 and 11, respectively [Fig. 10(b)]. The highest photodegradation efficiency of 99.95% is achieved at neutral pH (pH = 7). From this, it can be inferred that the neutral pH is favourable for efficient degradation of MO dye. Cu2MoS4/rGO becomes unstable when pH of the solution rise to 3 [34], whereas at higher pH, Cu2MoS4/rGO is stable and favors a higher adsorption characteristic of MO dye on Cu2MoS4/rGO nanocomposite. The dye concentration is another crucial factor of the photodegradation process. Fig 7(c) shows the photodegradation efficiency of Cu2 MoS4/rGO nanocomposite with respect to the concentration of MO dye from 0.03 mM to 0.06 mM under visible light irradiation. The photodegradation efficiency of Cu 2MoS4/rGO decreases with the increasing dye concentration. The path length of photons entering the solution decreases at high dye concentration which suppresses the photodegradation efficiency [35, 36]. Also, high concentration of dye may have a challenging effect on the adsorption of oxygen and OH onto the surface of catalyst. This corresponds to the amount of •OH (hydroxyl radical) formation on the catalyst surface and its probability of reaction with dye molecule. In the absence of photocatalyst, the degradation of MO is not identified under visible light irradiation as shown in Fig. 10(d). However, Cu2MoS4 photocatalyst shows 64% of MO degradation in 60 min of visible light irradiation. Fortunately, much high photodegradation efficiency of 99.95% was achieved for Cu2MoS4/rGO nanocomposite under identical experimental conditions. Thus, the enhanced photocatalytic activity of Cu 2MoS4/rGO nanocomposite is mainly attributed to the excellent charge separation and transportation properties of rGO. Moreover, the achieved efficiency in the present study is superior than the
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already reported sphere nanostructured Cu2MoS4/rGO photocatalyst [21] and other rGO based Ag-Cu2O/rGO [37], CuO-TiO2/rGO [17] and AgBr-ZnO/rGO photocatalysts [38]. For testing the reusability, the same photocatalyst was used for five times after collecting it at the end of each experiment. The photocatalytic activity of Cu2MoS4/rGO nanocomposite was observed to be quite stable even after the five successive runs of photodegradation of MO without any significant loss in its photocatalytic activity (Fig. 11). Plausible Mechanism The enhancement in photocatalytic activity mainly due to the ultrafast separation of electron-hole pairs with the help of rGO sheets. Fig. 12 displays the photocatalytic mechanism of Cu2MoS4/rGO nanocomposite for degradation of MO dye. Upon visible light illumination, the electrons (e-) present in the valence band (VB) of Cu2MoS4 excite to the conduction band (CB) and leave the positive holes (h+) in the VB. The formation of Schottky barrier at the interfaces of Cu2MoS4 and rGO sheet induces the easy transfer of electrons from CB of Cu 2MoS4 to CB of rGO [21]. The decreased electron-hole pair recombination of rGO supported Cu2MoS4 nanocomposite was also confirmed from PL spectra (Fig. 5). In addition to this, the excellent electronic properties, such as high electron mobility and electron acceptor properties of rGO sheet can easily withdraw the electrons from the Cu 2MoS4 nanosheet and transport at high speed for the efficient generation of superoxide radicals. On the other hand, holes present in the valence band of Cu2MoS4 oxidize the water molecules to hydroxyl (•OH) radicals. These hydroxyl and superoxide radicals are the major reactive species involving in the decomposition of MO dye.
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4. CONCLUSION Reduced graphene oxide supported Cu2MoS4 nanocomposite was successfully synthesized using the facile hydrothermal method. In Cu2MoS4/rGO nanocomposite, Cu2MoS4 possessed nanosheets like morphology with rGO sheets. The loading of rGO sheets decreased the band-gap of Cu2MoS4 than the bare Cu2MoS4 photocatalyst. The BET surface area of Cu2MoS4/rGO nanocomposite was also found to be high compared to that of Cu2MoS4 alone. The prevention of electron-hole pairs with rGO sheet was strongly confirmed from PL emission spectra. The photocatalytic activities of Cu2 MoS4/rGO nanocomposite and bare Cu2MoS4 were well demonstrated for the degradation of methyl orange (MO) dye under visible light irradiation. The Cu2MoS4/rGO nanocomposite was found to be exhibit almost 1.5 times higher photocatalytic activity than pure Cu2MoS4. Acknowledgement This work was supported by Science and Engineering Research Board-Department of Science and Technology (SERB-DST), New Delhi, India [File No: EMR/2014/000645)]
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Figure captions Fig. 1 XRD patterns of as-prepared (a) Cu2MoS4 nanosheet and (b) Cu2 MoS4/rGO nanocomposite. Fig. 2 FE-SEM images of (a&b) Cu2MoS4 nanosheet and (c&d) Cu2MoS4/rGO nanocomposite with different magnifications. Fig. 3 TEM images of (a) Cu2MoS4 nanosheet, (b) Cu2MoS4/rGO nanocomposite; HR-TEM image of (c) Cu2MoS4 nanosheet and (d) Cu2MoS4/rGO nanocomposite. Fig. 4 (a) UV-vis absorption spectra of Cu2MoS4 and Cu2MoS4/rGO photocatalysts recorded in diffuse reflectance mode (DRS) and
(b) Plots of the (αhν)
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vs. photon energy (hν) for
as-synthesized Cu2MoS4 and Cu2MoS4/rGO photocatalysts. Fig. 5 Photoluminescence (PL) spectra of Cu2MoS4 and Cu2MoS4/rGO nanocomposites Fig. 6 Raman spectra of GO, Cu2MoS4 and Cu2MoS4/rGO composite. Fig. 7 High resolution XPS spectra of (a) Survey, (b) Cu2p, (c) Mo3d, (d) S2p (e) C1s and (f) O1s of Cu2MoS4/rGO photocatalyst. Fig. 8 N2 adsorption–desorption isotherms and pore size distribution curves (inset) of Cu2MoS4 and Cu2MoS4/rGO nanocomposites. Fig. 9 Photocatalytic activity of different 1, 2, 3 4 and 5 wt% rGO loaded Cu2MoS4 photocatalysts for the degradation of MO dye. Fig.10 (a) Effect of (a) catalyst loading, (b) pH, (c) dye concentration and (d) photocatalytic activities of as-prepared photocatalysts on degradation of MO under visible-light irradiation. Fig. 11 Reusability test of Cu2MoS4/rGO nanocomposite on degradation of MO. Fig. 12 Possible photodegradation mechanism of Cu2MoS4/rGO nanocomposite for degradation of MO under visible light irradiation.
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S0169-4332(17)30488-9 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.126 APSUSC 35235
To appear in:
APSUSC
Received date: Revised date: Accepted date:
22-10-2016 13-2-2017 15-2-2017
Please cite this article as: R.Rameshbabu, R.Vinoth, M.Navaneethan, S.Harish, Y.Hayakawa, B.Neppolian, Visible light responsive Cu2MoS4 nanosheets incorporated reduced graphene oxide for efficient degradation of organic pollutant, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.126 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 proof before it is published in its final 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.
Visible Light Responsive Cu2MoS4 Nanosheets Incorporated Reduced Graphene Oxide for Efficient Degradation of Organic Pollutant
R. Rameshbabua, R. Vinotha, M. Navaneethanb, S. Harishb, Y. Hayakawab B. Neppoliana*
a
SRM Research Institute, SRM University, Kattankulathur, Kanchipuram-603203, Tamil Nadu, India
b
Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, Japan
*Corresponding author: Ph. No: 044-2741-7916, E-mail: [email protected]
1
Research Highlights
Cu2MoS4 nanosheets are incorporated on reduced graphene oxide sheets. Electronic interaction between rGO and Cu2MoS4 nanosheets increased the absorption. Insertion of rGO sheets increased the surface area of Cu2MoS4/rGO nanocomposite. 99% methyl orange degradation is achieved for Cu2MoS4/rGO nanocomposite.
ABSTRACT Visible light active copper molybdenum sulfide (Cu2MoS4) nanosheets were successfully anchored on reduced graphene oxide (rGO) using facile hydrothermal method. During the hydrothermal reaction, reduction of graphene oxide into rGO and the formation of Cu2MoS4 nanosheets were successfully obtained. The charge transfer interaction between the rGO sheets and Cu2MoS4 nanosheets extended the absorption to visible region in comparison with bare Cu2MoS4 nanosheets i.e without rGO sheets. Furthermore, the notable photoluminescence quenching observed for Cu2MoS4/rGO nanocomposite revealed the effective role of rGO towards the significant inhibition of electron-hole pair recombination. The photocatalytic efficiencies of bare Cu2MoS4 and Cu2MoS4/rGO nanocomposite was evaluated for the degradation of methyl orange dye under visible irradiation (λ > 420 nm). A maximum photodegradation efficiency of 99% was achieved for Cu 2MoS4/rGO nanocomposite, while only 64% photodegradation was noted for bare Cu2MoS4. The enhanced optical absorption in visible region, high surface area, and low charge carrier recombination in the presence of rGO sheets were the main reasons for the enhancement in photodegardation of MO dye. In addition, the
2
resultant Cu2MoS4/rGO nanocomposite was found to be reusable for five successive cycles without significant loss in its photocatalytic performance.
KEYWORDS: Cu2MoS4 Nanosheets Graphene Photocatalysis Charge separation Methyl Orange.
1. INTRODUCTION TiO2 semiconductor photocatalyst has been considered as a promising material for the photocatalytic degradation of organic pollutants [1, 2]. However, it is mostly active in ultraviolet (UV) light due to its wide band-gap energy associated with UV region of the solar spectrum [3,4]. Thus, it further limits the photocatalytic application of these semiconductors in visible region, as the solar spectrum contains only less than 5% of UV light [5]. On the other hand, fast recombination of photoexcited charge carriers leads to low quantum efficiency and thereby affects the photocatalytic performance [6]. Recently, sulphides [7], oxides [8] and nitrides [9] based visible light active photocatalysts have been developed for effective degradation of organic pollutants. Especially, ternary Cu2MoS4 transition metal sulphides (TMSs) based photocatalysts have attracted much attention in recent times owing to their promising photocatalytic activity [10]. Generally, Cu2MoS4 exists in two phases: with space group P42m and I42m. I-Cu2MoS4 has good visible-light absorption capability because of its favorable low bandgap value of 1.71 eV [11]. Tran et al. have successfully prepared Cu2MoS4 and utilized as novel electrocatalyst for hydrogen evolution reaction [12]. In general, photocatalysts with different morphological
3
nanostructures greatly influences the photocatalytic activity of the photocatalysts. Recently, K. Zang et al. has synthesized cubic like Cu2MoS4 photocatalyst for photodegradation of methyl orange dye under visible light illumination [13]. Similarly, ultrathin Cu2 MoS4 nanobelts were also synthesized using hydrothermal technique and employed as an efficient visible light photocatalysts for methyl orange degradation [10]. Very recently, Chen et al. has reported an enhanced visible light photocatalytic activity with indented Cu 2MoS4 nanosheets without any supports [14]. In addition to tailoring the morphology, using of conductive solid support like graphene along with photocatalysts can enhance the photocatalytic activity due to its excellent properties, such as high electron mobility, high theoretical surface area and chemical stability [15]. For example, Tran et al. have succeeded in synthesizing Cu2O/rGO composites and demonstrated an improvement in the photocurrent and photo stability of pure Cu 2O with the use of Cu2O/rGO composites [16]. In our previous reports, we have extensively used rGO as a solid support along with TiO2-CuO, TiO2-Cu2O, C3N4-Cu2O and AgI-TiO2 photocatalyst for photocatalytic water splitting and photocatalytic organic pollutants degradation applications [17-20]. The insertion of rGO can act as carrier separation and transportation platform and thereby greatly enhance the photocatalytic activity. In the present work, we designed sheet like Cu2MoS4 photocatalyst along with rGO sheet using hydrothermal technique. The photocatalytic activities of the as prepared samples were evaluated by the degradation of methyl orange (MO) under visible light irradiation (λ > 420 nm). Cu2MoS4/rGO nanocomposite showed an enhanced photocatalytic performance than the previously reported hierarchical Cu2MoS4/rGO hollow sphere [21]. The photocatalytic activity of Cu2MoS4/rGO nanocomposite was higher than the pure Cu2MoS4 nanosheets by almost 1.5 4
times. Furthermore, the photocatalytic activity of the synthesized Cu 2MoS4/rGO nanocomposite was quite stable and showed an excellent reusability. 2. Experimental Section 2.1 Materials and methods Copper nitrate trihydrate (Cu(NO3)2.3H2O), sodium molybdate (Na2MoO4.2H2O), thioacetamide (C2H5NS), ethylene glycol (C2H6 O2), Methyl orange (MO) dye and Graphite powder were purchased from Sigma-Aldrich. Poly (vinyl pyrrolidone) (PVP), Ascorbic acid (C6H8O6), Sodium hydroxide (NaOH) were received from SRL chemicals, India.
Double
distilled water was used throughout the experiments. 2.2 Synthesis of Cu2O nanospheres The Cu2O nanospheres were prepared from the previous report with minor modification [22]. Briefly, 0.0985 of Cu (NO3)2.3H2O and 11.666 g of poly (vinyl pyrrolidone) (PVP) were dissolved in 350 mL of deionized water. Subsequently, an aqueous solution of NaOH (2.0 M, 70.0 mL) and aqueous ascorbic acid solution (0.6 M, 70.0 mL) were then successively added drop wise with vigorous magnetic stirring. After 1 hour, a turbid yellow liquid formed. All the synthesized steps were carried out at room temperature. Finally the precipitate was collected after centrifugation and washed with deionized water and ethanol for several times. The final product was dried in hot air oven at 70 oC for few hours. 2.3 Synthesis of Cu2MoS4/rGO nanocomposite Graphene oxide was synthesized using modified Hummer’s method [23]. Hydrothermal method was followed for the synthesis of rGO loaded Cu2MoS4 photocatalyst. In brief, 60 mg of Cu2O powder was dissolved in 38 ml deionized water and the solution was ultra sonicated for 5 5
min. Then, 162 mg of ammonium tetrathiomolybdate was added into the solution and magnetically stirred for 5 min. After wards, the solution was transferred and sealed in a Teflonlined steel autoclave (50 ml) of 80% capacity of the total volume under airtight condition and maintained at 160 °C for 9 h. The precipitate was washed with deionized water and ethanol for several times to purify it and dried in hot air oven at 60 °C for few hours to obtain the final product. For comparison, Cu2MoS4 photocatalyst was prepared as reported earlier [10].
2.4 Characterization Technique Phase and structure of the as-prepared samples were analyzed by X-ray powder diffraction pattern, recorded using PAN analytical X’pert pro X-ray diffractometer with CuKα radiation (λ = 1.5406 A°). The morphological studies were carried out using a Field EmissionScanning electron microscopy (FEI Quanta FEG 200 HR-SEM). Transmission electron microscopy (TEM) images were recorded using a JEOL JEM 2100F microscope at an accelerating voltage of 200 kV. TEM specimens were prepared by dispersing the product powders in ethanol forming a suspension which was then placed onto a carbon-coated Cu grid after ultrasonic treatment. Diffuse reflectance spectra (DRS) were obtained using Analytikjena specord 210 plus, integrating sphere accessories. The photoluminescence spectra of the samples were obtained using a Jobin Yvon Horiba Fluorolog-3-Tau Spectrofluorometer. Raman spectra were obtained using (Horiba Jobin Yvon) using He–Ne Laser. X-ray photoelectron spectroscopy (XPS) measurements were obtained using K-Alpha instruments, USA, Al as a source, size 400 microns and range 0 to 1350 eV). Surface area and porosity measurements were carried out using a Quantachrome Nova-1000 surface analyzer at liquid nitrogen temperature. 2.5 Photocatalytic experiments 6
To investigate the photocatalytic activity of the photocatalysts, the photocatalytic degradation of MO was carried out under visible light. A 250 W Xe lamp with a λ ˃ 420 nm UV cutoff filter was used as the visible light source. The test solution was prepared by mixing photocatalyst (50 mg) in 50 mL of 10 mg/L MO solution (0.03 mM) and stirred for 30 min in dark condition to achieve the adsorption–desorption equilibrium between the photocatalyst and MO. The solution was then kept under the constant illumination and sampled (3 mL aliquot) at every 10 min of intervals. The MO concentration in each aliquot was examined by detecting the absorbance at the characteristic band of λ = 464 nm using Analytikjena specord 210 plus spectrophotometer. 3. Results and Discussion 3.1 Structural analysis The XRD patterns of the pure Cu2MoS4 and Cu2MoS4/rGO nanocomposites are shown in Fig. 1. It is conspicuous that the major diffraction peaks of Cu2MoS4/rGO composites are similar to that of pure Cu2MoS4 and corresponds to tetragonal-phase Cu2MoS4 (Space group: I-42m) [21]. Furthermore, no diffraction peaks for Cu2O, MoS2 and P-Cu2MoS4 phase are observed, suggesting the high purity of the nanocomposite. Moreover, loading of rGO does not induce any new crystal orientations or changes in preferential orientations of Cu 2MoS4. In addition, no characteristic diffraction peak for rGO is observed for Cu2MoS4/rGO nanocomposite, which might be due to low content of rGO or relatively low diffraction intensity of rGO in the nanocomposite [24]. 3.2 Morphological studies
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FE-SEM and TEM analyses were performed to examine the morphology of pure Cu2MoS4 and Cu2MoS4/rGO nanocomposite. The FE-SEM micrographs of the as-synthesized Cu2MoS4 and Cu2MoS4/rGO nanocomposite at different magnifications are shown in Fig. 2. As shown in Fig. 2(a&b), Cu2MoS4 shows sheet like nanostructure. Similarly, it can be also seen that the Cu2MoS4/rGO nanocomposite also possess nanosheet like morphology, which further decorated on rGO sheets [Fig. 2(c&d)]. Fig. 3 shows the TEM and HR-TEM images of Cu2MoS4 and Cu2MoS4/rGO nanocomposite. As displayed in Fig. 3(a), pure Cu2MoS4 exhibits a well defined sheet like morphology. Moreover, the rGO sheets with wrinkled surface are clearly seen in Cu2MoS4-rGO nanocomposite indicates the formation of interfacial contact between Cu2MoS4 nanosheets and rGO. Thus, the electronic interaction between Cu2MoS4 and rGO sheet can improves the charge carrier separation. Fig. 3(c&d) shows the HR-TEM images of Cu2MoS4 and Cu2MoS4-rGO nanocomposite. The calculated lattice spacing value for both Cu 2MoS4 and Cu2MoS4-rGO nanocomposite is measured to 0.51 nm and it corresponds to (002) plane of Cu2MoS4 [21]. 3.3 UV-vis DRS spectra The optical properties of the Cu2MoS4 and Cu2 MoS4/rGO nanocomposites are studied using the UV-vis absorption spectra in DRS mode, as shown in Fig. 4. Pure Cu2MoS4 nanosheets consist of a characteristic absorption peak at around 775 nm, whereas Cu2MoS4/rGO nanocomposite exhibits red shift and the absorption edge observed at 826 nm [Fig. 4(a)]. This account for the rGO content it also and suggests the existence of strong coupling between the rGO sheets and Cu2MoS4 nanosheets, which is desirable for the photocatalytic applications. Fig. 4(b) shows the Tauc plot of Cu2MoS4 and Cu2MoS4/rGO nanocomposite. The bandgap of pure Cu2MoS4 and Cu2MoS4/rGO are calculated to be 1.6 and 1.5 eV, respectively. This reduction in 8
the bandgap of composite can be ascribed to the electronic interactions between rGO and Cu2MoS4 nanosheet [25]. 3.4 Photoluminescence spectra To characterize the rate of electron-hole pair recombination, PL analysis was performed for Cu2MoS4 and Cu2MoS4/rGO nanocomposite. As displayed in Fig. 5, bare Cu2MoS4 and Cu2MoS4/rGO nanocomposite exhibit a PL emission peak at around 526 nm when excited by 320 nm laser (Fig. 5), which is enough to promote electronic transitions from the VB to the CB of Cu2MoS4, as evident from DRS spectra (Fig. 4). It can be clearly seen that the notable quenching of PL emission intensity for Cu2MoS4/rGO nanocomposite compared to that of bare Cu2MoS4 indicates the effective separation of electron-hole pairs in the presence of rGO sheet. 3.5 Raman spectra In order to confirm the existence of rGO in Cu2MoS4/rGO photocatalyst, Raman spectra were recorded for pure GO, Cu2MoS4 and Cu2MoS4/rGO photocatalyst. Fig. 6 shows the Raman spectra of GO, Cu2MoS4 and Cu2MoS4/rGO photocatalysts. The Cu2 MoS4 and Cu2MoS4/rGO photocatalysts display several characteristic Raman peaks in the frequency range of 100-1100 cm-1correspond to Cu2MoS4 [13, 14]. The pure GO exhibits the characteristic D and G bands at 1348 cm-1and 1582 cm-1, respectively. Similarly, the same D and G band observed in Cu2MoS4/rGO photocatalyst confirm the presence of rGO in the resultant nanocomposite (inset of Fig. 6). In addition, the ID/IG ratio of GO and Cu2MoS4/rGO photocatalyst was calculated to be 0.84 and 1.0, respectively. Thus, increasing I D/IG ratios of Cu2MoS4/rGO nanocomposite than GO not only represent the reduction of GO but also confirm the incorporation of Cu2 MoS4 on the rGO sheets [26, 27].
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3.6 X-ray photoelectron spectra X-ray photoelectron spectroscopy (XPS) measurement was carried out to investigate the chemical states of elements in the Cu2MoS4/rGO nanosheet photocatalyst. The survey scan (Fig 7a) spectrum proves the presence of Cu, Mo, S, C and O elements. As shown in Fig. 7b, the binding energy peaks noted at 932 eV and 952 eV are assigned to Cu 2p3/2 and Cu 2p1/2, respectively [28, 29]. As displayed in Fig. 7(c), the high resolution Mo 3d spectrum provides two peaks with the binding energies located at 226 and 230 eV correspond to Mo 3d 5/2 and Mo 3d3/2, respectively [14]. Similarly, the high resolution S 2p spectrum can be deconvoluted into two peaks (160 and 161 eV) indicate that the S is in sulfide state [Fig. 7(d)]. Thus, the XPS technique implies that the valence state of the elements (Cu, Mo and S) present in the Cu 2MoS4/rGO photocatalyst are Cu2+, Mo6+ and S2−, which are in good agreement with the literature report [14]. A shown in Fig. 7(e), the high resolution C1s spectrum is deconvoluted into three peaks. An intense peak noted at 283 eV corresponds to C=C or sp 2C. In addition, the low intense peaks observed at 284 eV and 286 eV are assigned to C-C and C-O, respectively [25]. Thus, a less intense C-O group observed in C1s spectrum clearly confirmed that the GO was reduced to rGO during the hydrothermal preparation of nanocomposite. Besides, a peak identified at 531 eV of O1s spectrum corresponds to O-C=O. 3.7 BET analysis The Brunauer-Emmett-Teller (BET) and the Barrett-Joyner-Halenda (BJH) analysis were used to obtain specific surface area and pore sizes of the nanocomposites. As depicts in the inset of Fig. 8, the average pore diameters of Cu2 MoS4 and Cu2MoS4/rGO nanocompoistes are observed to be 11.1 nm and 8.3 nm, respectively. Thus, the slight decrease in the pore size of Cu2MoS4/rGO nanocomposites would be attributed that the loading of rGO sheet blocked or 10
masked the pores of Cu2MoS4. Similar results were also reported in the literature for rGO based photocatalysts [30, 31]. The BET surface area of Cu2MoS4 and Cu2MoS4/rGO nanocomposite are measured to be 14.3 and 41.72 m2 g−1, respectively. This result clearly reveals that the incorporation of rGO sheet with Cu2MoS4 can increase the surface area and enhance the photocatalytic performance. Moreover, high surface area with small pores of the prepared Cu2MoS4/rGO nanocomposite can minimize the diffusion path for electrons and ions. Furthermore, this can also suppress the recombination rate of photoinduced electrons and holes. 3.8 Effect of GO loading In order to optimize the loading of rGO, different wt% (1, 2, 3, 4 and 5) of rGO loaded Cu2MoS4 photocatalysts were synthesized and its photocatalytic performance was evaluated for the degradation of the MO dye. Fig. 9, showed that around 67, 70, 79, 99.95 and 68% degradation of MO was observed with various % loading of GO (1, 2, 3,4 and 5 wt%). Thus, a maximum MO degradation of ~99.95% was achieved with 4wt% rGO that was found to be the optimized rGO loading for the degradation of MO. However, the further increase of rGO loading to 5 wt% decreased the photodegradation efficiency of the photocatalyst. The reason could be that the high loading of rGO can shield the incoming photons into the photocatalyst surface and thereby suppressed the photocatalytic activity [32, 33]. 3.9 Photocatalytic activity Fig. 10(a) shows the photocatalytic degradation of MO dye with different amount of catalyst loading. As displayed in Fig. 10a, the photodegradation efficiencies of 59 %, 67 %, 76 %, 99.9 %, 86 % are achieved for 20, 30, 40, 50 and 60 mg/50 mL, respectively. The optimized catalyst loading was found to be 50 mg and it showed the maximum photodegradation efficiency of 99% within 60 min of visible light illumination. The pH of the solution plays a vital role in the photocatalytic degradation process by affecting the surface charge properties of the catalyst, the 11
size of aggregates formed, the charge of dye molecules and adsorption of dye onto the catalyst surface and the concentration of hydroxyl radicals. The photodegradation efficiencies of 95 %, 96 %, 99.95 %, 99.83 % and 98.4% are achieved at pH 3, 5, 7, 9 and 11, respectively [Fig. 10(b)]. The highest photodegradation efficiency of 99.95% is achieved at neutral pH (pH = 7). From this, it can be inferred that the neutral pH is favourable for efficient degradation of MO dye. Cu2MoS4/rGO becomes unstable when pH of the solution rise to 3 [34], whereas at higher pH, Cu2MoS4/rGO is stable and favors a higher adsorption characteristic of MO dye on Cu2MoS4/rGO nanocomposite. The dye concentration is another crucial factor of the photodegradation process. Fig 7(c) shows the photodegradation efficiency of Cu2 MoS4/rGO nanocomposite with respect to the concentration of MO dye from 0.03 mM to 0.06 mM under visible light irradiation. The photodegradation efficiency of Cu 2MoS4/rGO decreases with the increasing dye concentration. The path length of photons entering the solution decreases at high dye concentration which suppresses the photodegradation efficiency [35, 36]. Also, high concentration of dye may have a challenging effect on the adsorption of oxygen and OH onto the surface of catalyst. This corresponds to the amount of •OH (hydroxyl radical) formation on the catalyst surface and its probability of reaction with dye molecule. In the absence of photocatalyst, the degradation of MO is not identified under visible light irradiation as shown in Fig. 10(d). However, Cu2MoS4 photocatalyst shows 64% of MO degradation in 60 min of visible light irradiation. Fortunately, much high photodegradation efficiency of 99.95% was achieved for Cu2MoS4/rGO nanocomposite under identical experimental conditions. Thus, the enhanced photocatalytic activity of Cu 2MoS4/rGO nanocomposite is mainly attributed to the excellent charge separation and transportation properties of rGO. Moreover, the achieved efficiency in the present study is superior than the
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already reported sphere nanostructured Cu2MoS4/rGO photocatalyst [21] and other rGO based Ag-Cu2O/rGO [37], CuO-TiO2/rGO [17] and AgBr-ZnO/rGO photocatalysts [38]. For testing the reusability, the same photocatalyst was used for five times after collecting it at the end of each experiment. The photocatalytic activity of Cu2MoS4/rGO nanocomposite was observed to be quite stable even after the five successive runs of photodegradation of MO without any significant loss in its photocatalytic activity (Fig. 11). Plausible Mechanism The enhancement in photocatalytic activity mainly due to the ultrafast separation of electron-hole pairs with the help of rGO sheets. Fig. 12 displays the photocatalytic mechanism of Cu2MoS4/rGO nanocomposite for degradation of MO dye. Upon visible light illumination, the electrons (e-) present in the valence band (VB) of Cu2MoS4 excite to the conduction band (CB) and leave the positive holes (h+) in the VB. The formation of Schottky barrier at the interfaces of Cu2MoS4 and rGO sheet induces the easy transfer of electrons from CB of Cu 2MoS4 to CB of rGO [21]. The decreased electron-hole pair recombination of rGO supported Cu2MoS4 nanocomposite was also confirmed from PL spectra (Fig. 5). In addition to this, the excellent electronic properties, such as high electron mobility and electron acceptor properties of rGO sheet can easily withdraw the electrons from the Cu 2MoS4 nanosheet and transport at high speed for the efficient generation of superoxide radicals. On the other hand, holes present in the valence band of Cu2MoS4 oxidize the water molecules to hydroxyl (•OH) radicals. These hydroxyl and superoxide radicals are the major reactive species involving in the decomposition of MO dye.
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4. CONCLUSION Reduced graphene oxide supported Cu2MoS4 nanocomposite was successfully synthesized using the facile hydrothermal method. In Cu2MoS4/rGO nanocomposite, Cu2MoS4 possessed nanosheets like morphology with rGO sheets. The loading of rGO sheets decreased the band-gap of Cu2MoS4 than the bare Cu2MoS4 photocatalyst. The BET surface area of Cu2MoS4/rGO nanocomposite was also found to be high compared to that of Cu2MoS4 alone. The prevention of electron-hole pairs with rGO sheet was strongly confirmed from PL emission spectra. The photocatalytic activities of Cu2 MoS4/rGO nanocomposite and bare Cu2MoS4 were well demonstrated for the degradation of methyl orange (MO) dye under visible light irradiation. The Cu2MoS4/rGO nanocomposite was found to be exhibit almost 1.5 times higher photocatalytic activity than pure Cu2MoS4. Acknowledgement This work was supported by Science and Engineering Research Board-Department of Science and Technology (SERB-DST), New Delhi, India [File No: EMR/2014/000645)]
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Figure captions Fig. 1 XRD patterns of as-prepared (a) Cu2MoS4 nanosheet and (b) Cu2 MoS4/rGO nanocomposite. Fig. 2 FE-SEM images of (a&b) Cu2MoS4 nanosheet and (c&d) Cu2MoS4/rGO nanocomposite with different magnifications. Fig. 3 TEM images of (a) Cu2MoS4 nanosheet, (b) Cu2MoS4/rGO nanocomposite; HR-TEM image of (c) Cu2MoS4 nanosheet and (d) Cu2MoS4/rGO nanocomposite. Fig. 4 (a) UV-vis absorption spectra of Cu2MoS4 and Cu2MoS4/rGO photocatalysts recorded in diffuse reflectance mode (DRS) and
(b) Plots of the (αhν)
2
vs. photon energy (hν) for
as-synthesized Cu2MoS4 and Cu2MoS4/rGO photocatalysts. Fig. 5 Photoluminescence (PL) spectra of Cu2MoS4 and Cu2MoS4/rGO nanocomposites Fig. 6 Raman spectra of GO, Cu2MoS4 and Cu2MoS4/rGO composite. Fig. 7 High resolution XPS spectra of (a) Survey, (b) Cu2p, (c) Mo3d, (d) S2p (e) C1s and (f) O1s of Cu2MoS4/rGO photocatalyst. Fig. 8 N2 adsorption–desorption isotherms and pore size distribution curves (inset) of Cu2MoS4 and Cu2MoS4/rGO nanocomposites. Fig. 9 Photocatalytic activity of different 1, 2, 3 4 and 5 wt% rGO loaded Cu2MoS4 photocatalysts for the degradation of MO dye. Fig.10 (a) Effect of (a) catalyst loading, (b) pH, (c) dye concentration and (d) photocatalytic activities of as-prepared photocatalysts on degradation of MO under visible-light irradiation. Fig. 11 Reusability test of Cu2MoS4/rGO nanocomposite on degradation of MO. Fig. 12 Possible photodegradation mechanism of Cu2MoS4/rGO nanocomposite for degradation of MO under visible light irradiation.
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