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Photocatalytic Dye Decomposition Effect of Binary Copper (I) Selenide-graphene Nanocomposites Synthesized with Facile Microwave-assisted Technique

Here, we examined the photo-degradation efficiency of Cu2Se-graphene nanocomposites synthesized by a facile and fast microwave-assisted technique. The prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, XPS and UV-Vis spectrophotometry. The photocatalytic performance was studied through the decomposition of Rhodamine (Rh B) as a standard dye under visible light radiation. A 95% of Rh B degradation after visible light irradiation for 180 min indicates that the Cu2Se-graphene composite exhibited significant photodegradation efficiency. Therefore, it can be concluded that the synthesized Cu2Se-graphene can be used as a suitable catalyst for decomposing dye pollutants.


Introduction
Recently, dyes are the most prominent toxin in aquatic environments because of their enormous volume of production from industries, slow biodegradation character, low decoration character and toxicity [1]. During the last decades a chain of experiments have been attempted all over the world to find proper materials to degrade toxic pollutants. Synthetic dyes are largely used in commercial industries. Thousands of different types of commercial dyes and pigments are produced million tons annually. During the dyeing process, approximately 10-15% of the dyes are released into the environment that plays a key role in water pollution [2]. Many techniques used for the removal of dyes form wastewater, such as membrane filtration [3], absorption [4], elerocoagulation [5], ozonation [6], Fenton oxidation [7], ion flotation [8], sonocatalytic degradation [9], and photocatalytic degradation [10,11]. Among those techniques, photocatalytic degradation has been found to be favorable process for dye wastewater treatment due to these technique easily covert harmful organic contaminants into carbonaceous products [12]. Therefore a worldwide researchers have been focuses the degradation techniques of dye pollutants for a long time. Several catalysts have been used for photocatalytic degradation of dyes such as Ag-Au-ZnO [13], polyethylene-TiO2 [14], Bi2WO6 [15], (Yb, N)-TiO2 [16], hollow cobalt (Co) nanoparticles [17], Si/Six core-shell nanowires [18], SnS Nanorods [19], biogenic silver nanoparticles (AgNPs) [20], Ni nanoparticles decorated SiO2/TiO2 magnetic spheres (Ni-SiO2/TiO2) [21] and carbon-modified antimony sulfide (Sb2S3) [21]. But these catalysts have some drawbacks in either limiting by its UV activation requirement or complexity of preparation process, therefore new catalysts have recently been developed. In present time, Cu2Se used is a promising photocatalyst and it has been vastly used for photocatalytic degradation of pollutant, due to its unique properties and wide applications in solar cell, gas sensors, thermoelectric converts etc. direct band gap of the Cu2Se in the range of 1.9-2.3 eV and indirect band gap in the range 1.2-1.7 eV [22]. Among these, Cu2-xSe nanocrystals show efficient photocatalytic activity (50% with irradiation time of 2 h) towards the photodegradation of RhB aqueous solution under visible light due to its suitable band gap [23]. Apart from this, recently graphene plays a vital role in the photocatalysis filed, graphene-based materials have has been widely studied due to excellent properties (e.g. high thermal conductivity ≈ 5000 Wm -1 k -1 and highly surface area ≈ 2600 m 2 g -1 ). These remarkable properties increase the charge transfer separation of the electron and holes [24,25]. These photogenerated holes and electron play an important role in pollutant degradation and photocatalytic redemption.
Furthermore Microwave irradiation techniques are very effective in the fast and facial route for preparation of binary or ternary compound, MW-assisted techniques are low consumption of energy, and a low production cost and successfully decreasing the reaction time up to a few minutes and also suppresses side reactions, with high phase purity and high yield [26,30]. There is no doubt that the microwave technique has the potential to fulfill all areas of synthetic chemistry. Especially, the synthesis of nanoparticles and nanostructures, nanostructured, carbonmaterials, nanoporous nanomaterials, metal oxide nanoparticles supported on carbon nanotubes, and polymer nanocomposites [31,35].
Here, we reported Cu2Se nanoparticles were attached onto the graphene through microwave-assisted techniques.In this procedure, GO is merged with copper (ii) and Se precursor materials under proper condition. During microwave irritation, graphene oxide converts to graphene and attachment of Cu2Se nanoparticles on the graphene sheets was observed in ethylene glycol. The photocatalytic activities were tested with Rh. B as a standard dye under UV/Vis light.

Preparation of graphene
Graphene oxide was prepared in the laboratory following the Hummer's-Offeman method as highlighted in different articles [36][37][38]. In brief, 20 g natural graphite and H2SO4 (400 mL) were homogeneously mixed at 0 ℃ with constant magnetic stirring. Then, 40 g of KMnO4 slowly mixed to the above solution and the temperature was kept below 14 ℃. The resulting mixture was continuously stirred at 35 ℃ until it became dim brownish. And then diluted to 150 mL using De-ionized (DI) water and kept stirring at below 90 ℃. After adding De-ionized (DI) water, the container was sealed and kept at 100 ℃ with vigorous stirring for 30 min, meanwhile 20% H2O2 added drop wise within 5 min and then the solution was washed with acetone and 10% HCl several time to remove residual metal ions. The solution was then heat-treated in the dry oven at 90 ℃ for 12 h to obtain graphite oxide power, then take 250 mg graphite oxide power were added to 200 mL DI water vigorously stirred for 30 min and then ultrasonicated (using Ultrasonic Processor, VCX 750) for 2 h. Finally the resulting solution were refined and washed several times with hot water and kept in a dry oven for 6 h to obtain graphene oxide power.

Preparation of copper selenide composite
In a typical synthesis process, 1.5 g of anhydrous sodium sulfite (Na2SO3) and 0.3 g crude selenium (Se) power were dispersed in 200 mL of ethylene glycol continuously stirring until homogenous mixture(selenium salt) was obtained. In next step 0.675 g of copper (ii) nitratetrihydrate Cu(NO3)2⋅3H2O was mixed to selenium salt solution and stirred for 30 min obtain a stable solution. Finally solution was transferred to a 500 mL reaction vessel placed in a conventional microwave for 500 s, with 10 sec on and 10 sec off state. The Cu2Se precipitates was then cooled to room temperature and filtered with using 47 mm Whatman filter paper, and heated at a temperature of 350 K for 10 h to obtain a Cu2Se power.

Preparation of Cu2Se-graphene nanocomposite
Cu2Se-graphene nanocomposite was obtained by following above method. A borosilicate glass sealed reaction vessel specially designed for microwave techniques. Graphene oxide (200 mg) was added in 100 mL ethylene glycol and then exfoliated to generate graphene oxide nanosheets (GONS) dispersion solution by ultrasonication for 30 min and then 0.675 g of copper (ii) nitratetrihydrate Cu(NO3)2⋅3H2O were dispersed in 100 mL ethanol and water mixed in equal volumetric ratios of 1 : 1 by ultrasonication for 1 h using a digital sonifer to obtain graphene oxide nanosheets (GONS)/Cu +2 solution. In addition selenium powder and 6 mL NH4OH (28 wt%) were added to the solution, the mixture was vigorously stirred for 30 min, then the solution irradiated by microwave, and solution irradiated by microwave at full power for 10 sec on and 10 sec off (for total 500 s), and cooled at room temperature washed several time with hot water and transferred into a dry oven. The weight ratios of graphene oxides to Cu2Se were taken 1, 2 and 3.5% and then obtained were labeled Cu2Se-graphene 1, Cu2Se-graphene 2 and Cu2Se-graphene 3 respectively.

Photocatalytic degradation experiment of Rhodamine B(RhB)
The adsorption and photocatalytic performance of the as-prepared Cu2Se-graphene was evaluated by the degradation of Rh.B dye under visible light. A xenon (8 W, λ > 420 nm) used as a visible light source. In the experiment, 20 mg of the Cu2Se-graphene catalytic sam-   ple was dispersed 100 mL solution of Rh.B (2.0 × 10 -6 mol/L). In order to reach adsorption-desorption equilibrium, the solution was kept in dark for 2 h. Before switched on the light source, a sample was collected from the solution and kept in a centrifuge at 10,000 rpm for the elimination of solid materials. After that, the light source was tune on and samples were collected in every 30 min and then the samples were centrifuged for 10 min to remove any suspended solid. All the samples were irradiated for 180 min to compare their catalytic efficiencies.

Characterization of catalysts
For measurements of crystallinity of the prepared samples, XRD patterns were taken using monochromatic high intensity Cu Kα radiation (λ = 1.5406 Å) in XRD (Shimadzu XD-D1). The surface area and morphology of the prepared simple were observed using SEM (JS M-5600 JEOL, Japan). For further detailed study of the composite particles was determined by TEM (JE OL, JEM-2010, and Japan). The change in absorbance was measured using UV-Vis spectrophotometer (Optizen POP, Korea). Raman spectra of the prepared samples were observed using a spectrometer (Jasco Model Name NRS-3100) with an excitation laser wavelength of 532.06 nm. The photocatalytic performance of the Cu2Se-graphene was investigated by absorbance spectrometry whit a UV/Vis spectrophotometer (Optizen POP, Mecasys, Korea).

Results and Discussion
The crystal structure and composition of Cu2Se-graphene via microwave-assisted techniques were analyzed by power XRD measurement. The Figure 1 illustrates that these three kinds of nanocomposites have a similar diffraction patterns that correspond to a monoclinic crystal phase (JCPDS PDF#00-06-0680). The XRD diffraction peaks of the Cu2Se-graphene compound present around 2θ of 26.7, 44.5, 52.6 and 65.7°, which can be indexed to the characteristic peak (111), (220), (311), and (400) plane reflections [39,40]. From Figure 1, it can be seen that the diffractogram of graphene exhibits the typical peaks at 26.7°, which corresponds to the (002) hexagonal planes of crystalline graphite. The intensity of the peaks is very small compared to the Cu2Se peaks. The suppressing of the different peaks verifies the minimal development of crystalline phases on the graphene sheets. The intensity fluctu-ation of the diffraction peaks for the nanocomposites also justifies the increased amount of graphene and suppression of the crystalline phases. The peak intensity decreasing also confirms that the lattice structure of Cu2Se is distorted by the interaction with graphene oxides [41,42]. Morphologies of Cu2Se, graphene and Cu2Se-graphene nanocomposites were examined by SEM. Figure 2 (a-c) predicted the overall structure of the nanocomposites. And, it can be clearly indicated that graphene is plate like structure broken off in different direction and Cu2Se nanoparticles are corporate into group of clusters (Figure 2 (a)). The Figure 2 (b) illustrates an irregular structure of graphene and bright spots indicated a pure Cu2Se particles appear as spherical particles and unevenly distributed. Moreover Cu2Se-graphene nanocrystals were partially agglomerated and formed the bunch of clusters around the graphene sheet formed a graphene-based nanostructure composite (Figure 2 (c)). Furthermore, the plate-like structures presented the presence of oxygen containing functional groups on the surface of the graphene sheets [43]. The information of the microscopic structure for the Cu2Se-graphene composite was examined by TEM technique with different magnification as shown in Figure 3.    From Figure 3 (a-b), it revealed that the two dimensional structure of graphene sheet with few layers was retained after microwave assisted hydrothermal treatment, and also observed that average size of the nanoparticles are approximately 0.3 to 0.5 µm (using image J software). Figure 3 (c) suggests that the graphene sheets are well distributed and providing a large microsheet type plate structure to Cu2Se nanoparticles. Furthermore, the Figure 3 (c) also subscribes the interactions between graphene sheet and Cu2Se nanospheres, which may give increase electrocatalytic performance [44]. The Figure 4 shows Raman spectra which provide further information about the nature and quality of the graphene (e.g. number of layers and the crystal structure). The Raman spectra of Cu2Se-graphene nanocomposites displayed D band at 1353 cm -1 and a sharp G band at 1588 cm -1 . These two peaks labeled G-band and D-band gives information on the nature of carbon-carbon bonds and defects [45,46]. The D band corresponds to the vibration of the carbon atom in disordered or defect sites, which is an ordinary feature of sp 3 defects. While the G band peak can be attributed to sp 2 -bonded carbon atoms [47]. The intensity ratio of D band and G band (ID/IG) generally indicates the degree of reduction of GO to graphene. The ID/IG ≈ 1.2 indicated that the increase in the number of graphene layers [48]. X-ray photoelectron spectroscopy (XPS) analysis was used for the chemical composition and bonding configuration of the prepared nanocomposites. Figure 5 depicted the full survey spectrum indicated peaks corresponding to all components exhibited the formation of Cu2Se-graphene nanocomposites. From the figure clearly shown that the selenium 3d core level two peaks placed at 54.60 eV and 58.9 eV, which indicated that the lattice Se-2 and an oxidation state of Se [49], and O1s (530 eV) presented C=O and O-C-OH groups [50,51]. The position C 1s (284.5eV) peak has shown the characteristic sp2 carbon corresponding to C-C groups [52]. The main peak of the Cu2p core level corresponding at 934 was observed. The evolution of the Cu2p XPS spectra clearly shown oxidation from Cu + to a mixture of Cu + and Cu +2 [53]. The photocalytic activities of the Cu2Se-graphene nanocomposite using Rh.B as organic dye under visible irradiation was investigated and shown in Figure 6 (a-f). The decline intensity of electronic absorption spectra demonstrates the degradation of Rh.B in the presence of Cu2Se-graphene catalysts. In the degradation process two essential steps are involved (1) the absorption by the photocatalyst and (2) the fast charge transfer route; these two steps are one kind of reason of the decomposition for the organic pollutants. In this study, the gra-phene is acts as an adsorption support material and dye molecules absorbed on the surface of graphene via π-π interaction. The Figure 6 (a-c) clearly indicates that the concentration of Rh.B with the different interval of time changed, which shows the adsorption and electron charge transfer ability of the Cu2Se-graphene nanocomposites. For achieving adsorption-desorption equilibrium, the prepared sample was kept in dark for 2 h, after obtain the adsorption-desorption equilibrium, the solution was kept in closed box and turn on visible light. From the Figure 6 (a-c), it shows the photocatalytic performance of the Cu2Segraphene nanocomposites in term of photo degradation of Rh.B molecules under visible light irradiation. During the increasing of the irradiation time, the intensity of the characteristic absorption band of Rh.B (554 nm) was significantly decreased. The concentration changes of Rh.B solution were recorded and Figure 6 (c-d) clearly shows that approximately 95% of the organic dye was degraded by Cu2Se-graphen-3 nanocomposite after 180 min as irradiation time. The kinetics of the degradation can be expressed by using the Langmuir-Hinshelwood model, can be expressed as -In(Ct/Co) = Kappt, where, Kappt is apparent rate constant, and CO initial concentration at t = 0 and Ct are the concentration of dye at t = t [54]. The kinetic plot and apparent rate constant (Kaap) of the Cu2Se-graphene-3 were shown in Figure 6 (e). It can be clearly seen that Cu2Se-graphene-3 catalyst exhibited highest visible light activity (Kapp = 5.32 × 10 -3 min -1 ), indicating that the Cu2Se-graphene-3 is highly effective catalytic materials for photodegradation.
Besides the excellent photocatalytic efficiency , the stability of the catalytic is also essential to the application of a photocatalyst [55]. To investigating the stability and reusability of the Cu2Se-graphene composites, the cyclic runs in the photocatalyitc degradation of Rh.B in the presence of Cu2Se-graphene under visible light are also studied. Figure  5(f) shows that Cu2Se-graphene minor decrease first two cycles of the photodegradation of Rh. B. The decomposition rates for 1 st 2 nd 3 rd and 4 th cycles were 18.62, 18.10, 16.23 and 12.33% respectively. And, the results concluded that the Cu2Se-graphene composites have moderate stability. It seems like a promising candidate for environmental sanctification.

Conclusion
Cu2Se-graphene has been prepared via microwave-assisted technique. SEM and TEM images clearly show that Cu2Se was unevenly distributed onto the surface of the graphene sheets. The details revealed that Cu2Se-grapheen is effective catalyst for photodegradation. Moreover, the cyclic reusability explained that Cu2Se-graphene nanocomposites are a favorable candidate for environmental mediation. In this study, it presented that graphene is one kind of candidate as based material to develop remarkably efficient binary system for photocatalytic degradation of organic dyes.