The textile, plastic, paper, and pulp industries discharge waste into water bodies without any prior treatment. The waste contains substantial amounts of organic dyes that are usually non-biodegradable and can have severe environmental consequences [1]. To meet the increasing demands for the protection of the environment, highly effective, inexpensive and stable photocatalysts for the degradation of organic chemicals are strongly desirable [2]. In recent years, semiconductor-based photocatalysts have attracted a lot of attention because of the increasing energy demand and growing environmental issues [3, 4]. So far, many semiconductor materials, such as TiO₂, ZnO, ZnS, and CdS, have been widely studied for their photocatalytic activity [5-8]. However, the bandgap energy of TiO₂ is 3.0 eV, ZnO is 3.4 eV and ZnS is 3.6 eV [9, 10]. These wide bandgap semiconductor materials are unfavorable for photocatalytic activity because they require some external source of irradiation, such as a UV source [11]. Using such materials as catalysts for the degradation of dye solutions requires a prolonged irradiation time that may not be suitable in real situations. Hence, the dyes should be degraded in a more rapid, simple, sensitive and reproducible way. This can be achieved by using narrower bandgap semiconductor materials.

During the past few decades, semiconductor quantum dots (QDs) have attracted the attention of researchers and been applied in the degradation of environmental contaminants under visible-light irradiation owing to their unique properties such as quantum confinement effect [12], high surface area [13, 14], advantageous optical properties [15], multiple exciton generation effect [16], and size-dependent optical and electronic properties, processing versatility and low cost [17]. Controlling the size and shape of the QDs allows tuning of their bandgap over a wide range and thus, QDs can be prepared to absorb light over the entire wavelength range of the solar spectrum, which leads to an excellent photocatalytic property. Among the Ⅱ-Ⅵ group of semiconductor QDs, CdSe has attracted tremendous attention owing to its various optoelectronic applications such as light-emitting diodes, laser diodes, photocatalysis, solar cells, and biological labeling. CdSe could be used as a potential material for the photocatalytic degradation of organic dyes, since it exhibits a suitable bandgap of 1.74 eV and rapid generation of electron-hole pairs (charge carriers) [18]. CdSe is an efficient photocatalyst for the oxidation process because of its ability to absorb a decent portion of the ultraviolet-visible region of the solar spectrum along with a positive valance band that is suitable to drive the oxidation reaction [19-21]. Therefore, CdSe is considered as an important semiconductor for the photocatalytic degradation of organic pollutants [22]. In particular, the photocatalytic activity of CdSe can be enhanced when it is combined with carbon-based nanomaterials. Among the promising carbon materials, graphene, a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice structure, is expected to have great potential as a nanoscale building block for developing hybrid materials. This expectation is based on its unique sheet morphology, ultrahigh electron conductivity and mobility [23]. Since the discovery of graphene in 2004, a great number of studies have been performed on graphene/semiconductor composites [24-30]. Typically, graphene decorated with metals or metallic compounds may lead to a variety of advanced hybrid materials with unusual properties [31], which significantly expands the applications of graphene materials owing to their improved performance in fields such as photocatalysis and supercapacitors [25]. The coupling of graphene and a semiconductor suggests the possibility of fabricating new multicomponent composite materials that exhibit synergistic physicochemical properties [32]. Graphene has demonstrated its promising function as an efficient electron acceptor and transporter that enhances the transfer of photogenerated electrons and prolongs the lifetime of photogenerated charge carriers. As a result, graphene-based semiconductor photocatalysts have been widely used in photocatalytic reactions [33-37]. Furthermore, the reports of graphene oxide loaded with CdSe QDs show an excellent photocatalytic activity on the degradation of organic dyes such as malachite green, rhodamine B and industrial dyes [38, 39].

In this work, CdSe/graphene oxide (GO) nanocomposites were synthesized by a chemical precipitation method and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), UV-vis absorption spectroscopy, photoluminescence spectroscopy (PL), Brunauer-Emmett-Teller (BET) analysis and Raman spectroscopy. The photocatalytic activity of the synthesized samples was evaluated by the degradation of a Brilliant Green (BG) dye solution under sunlight irradiation.

Cadmium acetate dihydrate (Cd(CH₃COO)₂·2H₂O), selenium (Se), sodium sulfite (Na₂SO₃), Brilliant Green (C₂₇H₃₄N₂O₄S, molecular weight = 482.62 g/mol), sodium nitrate (NaNO₃), potassium permanganate (KMnO₄), hydrogen peroxide (H₂O₂), sulfuric acid (H₂SO₄) and hydrochloric acid (HCl) were purchased from SD Fine Chem. Limited., India. Graphite powder and hydrogen peroxide (30 wt%) were obtained from LOBA Chemie, and Nice Chemicals, India, respectively. All the chemicals were of analytical reagent grade and used without any further purification. Deionized water was used for all the sample preparations.

Sodium sulfite (3.78 g) and selenium (0.39 g) were added in a three-neck flask containing deionized water (100 mL). The reaction was carried out at 80 ℃ and de-aerated with nitrogen gas to create an inert environment. After 5 h, the colorless sodium selenosulfate solution was obtained.

In a typical synthesis of CdSe QDs, cadmium acetate dihydrate (1.06 g) was added to deionized water (50 mL) and stirred magnetically. While stirring, the prepared sodium selenosulfate solution was added drop by drop. The solution mixture was stirred continuously for 3 h at 80 ℃. Then, the solution was cooled to room temperature and the precipitate was obtained. The residue was washed several times with deionized water and ethanol to remove the impurities. The purified sample was dried at 100 ℃ for 5 h in an oven.

GO was prepared by modified Hummer's method [40]. In a typical synthesis, graphite powder (2 g) and NaNO₃ (2 g) were mixed together and added with concentrated sulfuric acid (96 mL, H₂SO₄). This solution mixture was stirred constantly in an ice bath using a magnetic stirrer. Then, potassium permanganate (6 g) was added gradually and stirred for 3 h while keeping the temperature of the mixture below 20 ℃. After that, the solution mixture was removed from the ice bath and stirred magnetically at 35 ℃. Subsequently, deionized water (200 mL) was added slowly to the mixture. Then, H₂O₂ (10 mL) was added to the solution mixture which turned yellow and the mixture was stirred for 12 h.The resulting solution mixture was washed separately with HCl and H₂O. After the filtration and centrifugation, the product was dried in a hot air oven and obtained as GO powder.

For the synthesis of the CdSe/GO nanocomposites, GO powder (0.5 g) was added to cadmium acetate dihydrate solution (50 mL), then the same procedure as was done in the CdSe QDs synthesis was followed. Fig. 1 shows the schematic synthetic procedure of the CdSe/GO nanocomposites.

The photocatalytic activity of the catalyst was studied by measuring the degradation of a BG dye solution (10 mg/L) under sunlight. CdSe QDs (30 mg) was chosen as the optimum amount for photocatalytic studies. The same amount of the CdSe/GO nanocomposite was also studied against the BG dye degradation. The BG dye degradation was monitored at different time intervals (15, 30, 45, 60, 75 and 90 min). All the experimental works were carried out between 11:00 am and 2:00 pm under sunlight so that the solar intensity (1250 × 100 Lu ± 100) was maintained as constant.

The XRD spectra of the prepared samples were recorded using an X'Pert-PRO diffractometer with Cu K_α1 (1.54060 Å ) at room temperature. The optical absorption and photoluminescence spectra of the prepared samples were recorded using a Shimadzu-UV 1800 spectrophotometer and a Perkin Elmer LS55 fluorescence spectrometer, respectively. The morphologies of the prepared samples were observed by a Jeol JSM 6390 scanning electron microscope, with an accelerating voltage of 20 kV. The morphology and size of the samples were studied using a Jeol/JEM 2100 high resolution transmission electron microscope with an operating voltage of 200 kV. Brunauer-Emmett-Teller (BET) analysis was carried out by a Quantachrome Nova-1000. The Raman spectra were recorded for the prepared samples using a Raman microscope with a 514 nm laser, Reni Shaw, UK.

Fig. 2 shows the XRD spectra of GO, CdSe QDs and CdSe/GO nanocomposite. GO showed a characteristic reflection peak centered at 2θ = 10.7° for the (001) reflection plane, which corresponded to an interplanar spacing of 0.84 nm [41], and a very low intensity peak near 2θ = 42.9° [42], which indicated the complete oxidation of graphite and hence the formation of GO. The inter-layer spacing of GO (0.84 nm) was found to be larger than that of graphite (0.336 nm). This was proposed to arise from the attachment of oxygen-containing functional groups on the graphite sheets [43]. The oxygen-based functional groups attached at both sides of the graphite sheets create atomic defects in the graphite structure and have the tendency to exfoliate the structure to a few layers of GO in an aqueous medium [44]. For CdSe QDs, three main diffraction peaks appeared that corresponded to the (111), (220) and (311) reflection planes, which belong to the zinc blende cubic structure of CdSe (JCPDS: 19-0191) [45]. In CdSe/GO nanocomposites, the diffraction peaks corresponding to CdSe alone were detected, which indicated the chemical precipitation method used in this study showed the good formation of a CdSe/GO nanocomposite and graphene oxide was properly hybridized with CdSe. This result agrees with a previous work [22], which used graphene oxide as the support material to prepare graphene-based nanomaterials. The disappearance of the (001) reflection peak in the CdSe/GO nanocomposite may result from the formation of a few layers of reduced graphene oxide (RGO). The crystallite size (D) of CdSe and CdSe decorated on GO sheets was calculated using Scherrer's formula:

(1)

where k is a constant (about 0.9), λ = 1.54060 Å (Cu K_α radiation wavelength), β is the full width at half maximum and θ is the Bragg's diffraction angle. The particle sizes of CdSe and CdSe decorated GO samples were 4.4 and 4.0 nm, respectively. This obviously showed that the size of the synthesized CdSe particles were smaller than Bohr exciton radius of CdSe (5.6 nm), which indicated that the synthesized particles could be termed as quantum dots.

Fig. 3(a) shows the SEM micrograph of the GO sheets. Fig. 3(b) shows the spherically shaped CdSe QDs that were agglomerated together. The micrograph of the CdSe/GO nanocomposite (Fig. 3(c)) shows the CdSe QDs grown on GO sheets. Moreover, the EDAX spectra of GO, CdSe QDs, and the CdSe/GO nanocomposite confirm the presence of carbon, oxygen, cadmium and selenium elements in the prepared samples (Fig. 3(d)-(f)). The morphology and crystalline nature of GO, CdSe QDs and the CdSe/GO nanocomposite were studied by HRTEM and selected area electron diffraction (SAED), respectively. Fig. 4(a) shows that the prepared GO is in the form of thin layers. The two-dimensional structure of the GO sheets and their surfaces were very smooth. In Fig. 4(b), the number of GO layers was found to be 3 or 4. The prepared QDs were spherical. The CdSe QDs were agglomerated and decorated on the surface of the GO sheets. The sizes of the CdSe QDs and the CdSe QDs decorated on GO sheets were 4.4 and 4.1 nm, respectively. In Fig. 4(d) and (f), three consecutive concentric rings were observed in the SAED patterns for the CdSe QDs and the CdSe/GO nanocomposite, which were assigned to the diffraction planes of the zinc blende cubic phase of CdSe, that is, (111), (220) and (311). The SAED pattern revealed that the CdSe/GO nanocomposite possessed the crystalline feature of CdSe QDs, which was in agreement with the XRD results.

Fig. 5 shows that the UV-vis absorption spectra of GO, CdSe QDs and the CdSe/GO nanocomposite. The typical UV-vis absorption spectrum of an aqueous solution of GO showed a plasmon peak near 236 nm owing to the π-π^* transition and a hump around 300 nm that is often attributed to n-π^* transitions of C=O. As shown in the figure, the absorption edges for CdSe QDs and the CdSe/GO nanocomposite appeared at 583 and 556 nm, respectively. The absorption edge of CdSe QDs was blue-shifted with respect to its bulk CdSe (716 nm) owing to the quantum confinement effect. In the case of the CdSe/GO nanocomposite, the absorption was increased with a significant blue-shift. The size of CdSe QDs on the GO layers was reduced when compared with pure CdSe QDs, which resulted in the blue-shift of the optical absorption wavelength for the CdSe/GO nanocomposite with respect to the CdSe QDs owing to the quantum confinement effect. The increased absorption indicated that the CdSe/GO nanocomposite would exhibit a better photocatalytic activity than the CdSe QDs.

Fig. 6 shows the PL spectra of CdSe QDs and the CdSe/GO nanocomposite that were recorded at room temperature. The strong broad luminescence peak that appeared at 603 nm in the visible region arose from the defect states. These defect states were formed as a result of the vacancies of Cd²⁺ or Se^2- ions (such as unstoichiometric defects and dangling bonds) on the surface of the CdSe QDs. However, the emission peak for the CdSe/GO nanocomposite was observed at 576 nm, which was blue-shifted in the peak position and was also quenched. The decoration of CdSe QDs on the GO surface changed the van der Waals force of interaction between the GO layers and the electrostatic force of interaction between the CdSe QDs and GO sheets. These interactions caused a variation of the energy levels, and resulted in a blue-shift of the emission peak when compared with that of CdSe QDs. Fluorescence quenching has been widely used to probe photo-induced electron transfer in nanocomposites [46]. The PL intensity of the CdSe/GO nanocomposite was lower than that of the CdSe QDs owing to the recombination of electron-hole pairs being inhibited in the composites. This could be understood in terms of the interfacial charge transfer from CdSe QDs to GO sheets. GO can serve as an acceptor of the generated electrons of CdSe, which effectively decreases the charge recombination and leaves more photogenerated charges to participate in the chemical reaction [47]. The PL study clearly suggested that the decoration of CdSe on GO can improve the photocatalytic performance.

Fig. 7(a) shows the nitrogen adsorption-desorption isotherms and Fig. 7(b) and (c) present the pore size distribution plots of CdSe QDs and the CdSe/GO nanocomposites. The BET specific surface area of the CdSe/GO nanocomposites was determined to be 10.4 m²/g, which was higher than that of CdSe QDs (5 m²/g). It is generally known that the photocatalytic process is related to the adsorption and desorption of molecules on the surface of catalysts. Thus, a large specific surface area is beneficial for absorbing more light, increasing the number of unsaturated surface coordination sites and the absorptivity for organic molecules to improve the photocatalytic performance [48, 49]. The excellent photocatalytic activity of the CdSe/GO nanocomposite could be attributed to the high specific surface area and the prevention of electron-hole pair recombination because of the presence of GO [50]. Furthermore, the isotherms of the synthesized CdSe QDs and the CdSe/GO nanocomposite were type IV according to the BDDT (Brunauer-Deming-Deming-Teller) classification, which indicated the presence of mesopores. The mesopores could be the transport pathway during the photocatalytic reaction by allowing the rapid diffusion of reactant molecules through the pores [51].

Fig. 8 shows Raman spectra of GO and the CdSe/GO nanocomposite. Raman spectroscopy is a powerful tool to analyze graphene, particularly to understand the interaction between graphene and semiconducting nanomaterials [52]. The GO and CdSe/GO nanocomposite were excited at 514 nm. Subsequently, two characteristic peaks of the D and G bands were observed for GO at 1351 and 1604 cm^-1, and at 1361 and 1607 cm^-1 for the CdSe/GO nanocomposite. The I_D/I_G ratio for GO was 0.88, which was less than that of the CdSe/GO nanocomposite, which was 0.98. This increase in the I_D/I_G ratio for CdSe/GO confirmed the decoration of CdSe QDs on GO. The presence of the D band arose from the in-plane longitudinal phonon vibration or breathing mode of k-point phonons of A_1g symmetry, but the G band arose from the E_2g phonon mode for the sp² carbon network of the graphene plane [53]. The two bands of the CdSe/GO nanocomposite were shifted to higher wavenumbers with respect to GO. This red-shift was attributed to CdSe QDs induced electron injection into GO. The electrons of the valence band (VB) in CdSe QDs were excited to the conduction band (CB) owing to the excitation source of 514 nm (2.41 eV). These excited electrons were transferred to GO, which made it an electron rich surface.

The photocatalytic activities of CdSe QDs and the CdSe/GO nanocomposite were evaluated by monitoring the degradation of BG dye under sunlight irradiation. The absorption wavelength and concentration of the BG dye solution were determined by UV-vis absorption spectroscopy. The maximum absorption wavelength for the BG dye solution was observed at 624 nm. To determine the response of the photocatalytic activity of CdSe QDs and the CdSe/GO nanocomposite, the absorption spectra of exposed samples at different time intervals were recorded and the rate of decolorization was observed in terms of the change in intensity of the maximum absorption wavelength (λ_max) at 624 nm of the BG dye.

The photocatalytic reduction efficiency of CdSe QDs and the CdSe/GO nanocomposite was calculated using the following relation:

(2)

where C₀ is the initial concentration of BG dye solution (mg/L) and C is the concentration of the BG dye solution (mg/L) after different time intervals under sunlight using CdSe QDs and the CdSe/GO nanocomposite. Fig. 9(a) and (b) display the degradation of the BG dye using CdSe QDs and CdSe/GO nanocomposites under sunlight irradiation. The concentration of the BG dye gradually decreased with increasing time. In other words, the color of the dye solution increasingly lost its intensity as the dye concentration continued to decrease during photocatalysis at time intervals of 15 min. At the end of 90 min, the intensity of the absorption peak of the BG dye decreased to approximately 81.9% for CdSe QDs, while for the same irradiation time, the intensity of the absorption peak of the BG dye decreased to approximately 95.5% for the CdSe/GO nanocomposite. Hence, the CdSe/GO nanocomposite decolorized the BG dye faster than CdSe QDs. The excellent photocatalytic activity of the CdSe/GO nanocomposite could be attributed to the high specific surface area and the reduction of electron-hole pair recombination because of the introduction of graphene. According to BET analysis, the determined specific surface area of the CdSe/GO nanocomposite was 10.4 m²/g, which was higher than that of CdSe QDs (5 m²/g). However, the excellent photocatalytic activity of the CdSe/GO nanocomposite could also be ascribed to the introduction of graphene oxide, which could effectively reduce the recombination rate of the photogenerated electrons and holes. Similar observations have also been demonstrated in other reports on TiO₂/GR and AgCl/GO [54, 55]. In general, graphene/semiconductor nanocomposites are recognized as active photocatalysts because the addition of semiconductor nanomaterials on a graphene surface prevents the aggregation of graphene layers, which in turn increases the surface area for the removal of organic pollutants from the aqueous solution [56].

The graphs of ln(C₀/C) versus time interval over 0-90 min were plotted, and could be approximated as straight lines, as shown in Fig. 9(a) and (b) (inset). The kinetic studies were performed on the basis of the rate of disappearance of the BG dye. The photocatalytic activities of the prepared samples could be expressed by the Langmuir-Hinshelwood model.

(3)

where C₀ and C are the BG dye concentration at time 0 and t min, respectively, and k is the constant of the pseudo-first-order rate. The calculated rate constant (k) for CdSe QDs and the CdSe/GO nanocomposite were 0.0190 and 0.0345 min^-1, respectively.

The mechanism of the charge transfer process between CdSe QDs and graphene oxide sheet is shown in Fig. 10. When the CdSe/GO nanocomposite was irradiated under sunlight, GO acted as a good electron acceptor. It is known that the fractional reduction of GO only partially restores the sp² networks; therefore, the remaining oxygen sites still able to accept electron and undergo reduction [57]. Graphene oxide attached to CdSe transferred the electrons (e^-) to the CB of CdSe, resulting in an increase in the number of electrons and also the rate of the electron-induced redox reactions [11]. The photocatalytic activity of the CdSe/GO nanocomposite was enhanced, mainly owing to the high charge separation induced by the synergistic effects of GO on CdSe. At the same time, under sunlight irradiation, CdSe generated electrons (e^-) and holes (h⁺), which took part in the oxidation and reduction reactions. The generated electrons (e^-) reacted with dissolved oxygen molecules and produced oxygen peroxide radicals O₂^•-. The positive charge hole (h⁺) reacted with OH^- derived from H₂O to form hydroxyl radicals OH^•. Moreover, some of the reactive oxygen species O₂^•- continued to form into OH^• radicals [58]. The BG molecules could then be photocatalytically degraded by oxygen peroxide radicals O₂^•- and hydroxyl radicals OH^• to CO₂, H₂O, and other mineralization products [59]. The reactions involved in the charge mobility and mineralization of the dyes are as follows:

(4)

(5)

(6)

(7)

(8)

(9)

(10)

CdSe QDs hybridized with GO were synthesized through a chemical precipitation method. The XRD pattern of the CdSe/GO nanocomposite possesses a similar structural pattern to zinc blende cubic CdSe. The particle sizes of CdSe QDs and CdSe QDs decorated on GO are found to be 4.4 and 4.0 nm, respectively. The direct evidence of the formation of GO layers and CdSe QDs hybridized with GO is confirmed by HRTEM analysis. The effect of the CdSe/GO nanocomposite on BG dye degradation is higher than that of pure CdSe QDs. The CdSe/GO nanocomposite synthesized by a chemical precipitation method exhibits simplicity, low cost, and high yield and is environmentally friendly, which are excellent characteristics for the practical applications in photocatalysis.