Photocatalytic degradation of Brilliant Green dye ...

Chinese Journal of Catalysis 38 (2017) 2150–2159

催化学报 2017年第38卷第12期 | www.cjcatal.org

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Article (Special Issue on Photocatalysis in China)

Photocatalytic degradation of Brilliant Green dye using CdSe quantum dots hybridized with graphene oxide under sunlight irradiation N. Thirugnanam a, Huaibing Song b, Yan Wu b,* Department of Physics, Annamalai University, Annamalai Nagar 608002, Tamil Nadu, India Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan), Wuhan 430074, Hubei, China

a

b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 3 October 2017 Accepted 1 November 2017 Published 5 December 2017

Keywords: CdSe quantum dots Graphene oxide Nanocomposite Photocatalytic activity Brilliant green dye

CdSe quantum dots (QDs) hybridized with graphene oxide (GO) are synthesized by a facile chemical precipitation method. The absorption of the CdSe/GO nanocomposite is increased with a significant blue shift with respect to CdSe QDs. The specific surface area of the CdSe/GO nanocomposite is 10.4 m2/g, which is higher than that of CdSe QDs (5 m2/g). The PL intensity of the CdSe/GO nanocompo‐ site is lower than that of the CdSe QDs owing to the inhibition of the recombination of electron‐hole pairs in the composite. In Raman analysis, the two bands of the CdSe/GO nanocomposite are shifted to higher wavenumbers with respect to graphene oxide, which is attributed to electron injection that is induced by CdSe QDs into graphene oxide. Using the Brilliant Green dye, the photocatalytic reduction efficiency of CdSe QDs and the CdSe/GO nanocomposite under sunlight irradiation for 90 min are approximately 81.9% and 95.5%, respectively. The calculated photodegradation rate con‐ stants for CdSe QDs and the CdSe/GO nanocomposite are 0.0190 min–1 and 0.0345 min–1, respec‐ tively. The enhanced photocatalytic activity of the CdSe/GO nanocomposite can be attributed to the high specific surface area and the reduction of electron‐hole pair recombination because of the introduction of graphene oxide. © 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

1. Introduction 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, semiconduc‐

tor‐based photocatalysts have attracted a lot of attention be‐ cause of the increasing energy demand and growing environ‐ mental 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 photo‐ catalytic 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 pro‐

* Corresponding author. Tel: +86‐27‐67883731; E‐mail: [email protected] This work was supported by the National Natural Science Foundation of China (51774259) and Engineering Research Center of Nano‐Geo Materials of Ministry of Education (NGM2017KF004 and NGM2017KF012). DOI: 10.1016/S1872‐2067(17)62964‐4 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 12, December 2017

N. Thirugnanam et al. / Chinese Journal of Catalysis 38 (2017) 2150–2159

longed irradiation time that may not be suitable in real situa‐ tions. Hence, the dyes should be degraded in a more rapid, sim‐ ple, 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 elec‐ tronic properties, processing versatility and low cost [17]. Con‐ trolling 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 at‐ tracted tremendous attention owing to its various optoelec‐ tronic 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 ultravio‐ let‐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 semi‐ conductor for the photocatalytic degradation of organic pollu‐ tants [22]. In particular, the photocatalytic activity of CdSe can be enhanced when it is combined with carbon‐based nano‐ materials. 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 devel‐ oping 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 hy‐ brid 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 semi‐ conductor suggests the possibility of fabricating new multi‐ component composite materials that exhibit synergistic physi‐ cochemical properties [32]. Graphene has demonstrated its promising function as an efficient electron acceptor and trans‐ porter 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]. Fur‐ thermore, 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 indus‐ trial dyes [38,39].

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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 mi‐ croscopy (HRTEM), UV‐vis absorption spectroscopy, photolu‐ minescence 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. 2. Experimental 2.1. Materials 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 pur‐ chased 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 chemi‐ cals were of analytical reagent grade and used without any further purification. Deionized water was used for all the sam‐ ple preparations. 2.2. Preparation of sodium selenosulfate (Na2SeSO3) solution 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 °C and de‐aerated with nitrogen gas to create an inert environment. After 5 h, the colorless so‐ dium selenosulfate solution was obtained. 2.3. Synthesis of CdSe QDs and CdSe/GO nanocomposites In a typical synthesis of CdSe QDs, cadmium acetate dihy‐ drate (1.06 g) was added to deionized water (50 mL) and stirred magnetically. While stirring, the prepared sodium sele‐ nosulfate solution was added drop by drop. The solution mix‐ ture was stirred continuously for 3 h at 80 °C. Then, the solu‐ tion was cooled to room temperature and the precipitate was obtained. The residue was washed several times with deion‐ ized water and ethanol to remove the impurities. The purified sample was dried at 100 °C 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 permanga‐ nate (6 g) was added gradually and stirred for 3 h while keep‐ ing the temperature of the mixture below 20 °C. After that, the solution mixture was removed from the ice bath and stirred magnetically at 35 °C. 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

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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 solu‐ tion (50 mL), then the same procedure as was done in the CdSe QDs synthesis was followed. Fig. 1 shows the schematic syn‐ thetic procedure of the CdSe/GO nanocomposites. 2.4. Photocatalytic degradation of BG dye The photocatalytic activity of the catalyst was studied by measuring the degradation of a BG dye solution (10 mg/L) un‐ der 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 differ‐ ent time intervals (15, 30, 45, 60, 75 and 90 min). All the ex‐ perimental 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. 2.5. Characterization The XRD spectra of the prepared samples were recorded using an X’Pert‐PRO diffractometer with Cu Kα1 (1.54060 Å) at room temperature. The optical absorption and photolumines‐ cence spectra of the prepared samples were recorded using a Shimadzu‐UV 1800 spectrophotometer and a Perkin Elmer LS55 fluorescence spectrometer, respectively. The morpholo‐ gies 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. Brunau‐ er‐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. 3. Results and discussion 3.1. Structural analysis 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 function‐ al 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 ap‐ peared that corresponded to the (111), (220) and (311) reflec‐ tion 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 na‐ nomaterials. 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 calcu‐ lated using Scherrer’s formula: D 

k (1)  cos

Fig. 1. The schematic synthetic procedure of CdSe/GO nanocomposites.


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where k is a constant (about 0.9), λ = 1.54060 Å (Cu Kα radia‐ tion 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. 3.2. Morphological analysis

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Fig. 3(a) shows the SEM micrograph of the GO sheets. Fig. 3(b) shows the spherically shaped CdSe QDs that were ag‐ glomerated together. The micrograph of the CdSe/GO nano‐ composite (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, cad‐ mium 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 sur‐ faces 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 ob‐ served 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 nanocom‐ posite possessed the crystalline feature of CdSe QDs, which was in agreement with the XRD results. 3.3. UV‐vis absorption spectral analysis Fig. 5 shows that the UV‐vis absorption spectra of GO, CdSe

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Fig. 3. SEM pictures of GO (a), CdSe QDs (b) and CdSe/GO nanocomposites (c) with their respective EDAX spectra (d–f).

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QDs and the CdSe/GO nanocomposite. The typical UV‐vis ab‐ sorption 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* transi‐ tions 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 indi‐ cated that the CdSe/GO nanocomposite would exhibit a better photocatalytic activity than the CdSe QDs. 3.4. Photoluminescence studies 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

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Fig. 6. PL spectra of CdSe QDs and CdSe/GO nanocomposites.


states were formed as a result of the vacancies of Cd2+ or Se2– 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 elec‐ trostatic force of interaction between the CdSe QDs and GO sheets. These interactions caused a variation of the energy lev‐ els, 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 nano‐ composite 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 photo‐ generated 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. 3.5. BET analysis Fig. 7(a) shows the nitrogen adsorption‐desorption iso‐ therms 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 de‐ termined 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 mole‐ cules on the surface of catalysts. Thus, a large specific surface area is beneficial for absorbing more light, increasing the num‐ ber of unsaturated surface coordination sites and the absorp‐ tivity for organic molecules to improve the photocatalytic per‐ formance [48,49]. The excellent photocatalytic activity of the CdSe/GO nanocomposite could be attributed to the high specif‐ ic surface area and the prevention of electron‐hole pair recom‐ bination 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 (Brunau‐ er‐Deming‐Deming‐Teller) classification, which indicated the presence of mesopores. The mesopores could be the transport

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Fig. 8. Raman spectra of GO and CdSe/GO nanocomposites.

pathway during the photocatalytic reaction by allowing the rapid diffusion of reactant molecules through the pores [51]. 3.6. Raman spectral analysis Fig. 8 shows Raman spectra of GO and the CdSe/GO nano‐ composite. 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. Subsequent‐ ly, 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 ID/IG ratio for GO was 0.88, which was less than that of the CdSe/GO nanocomposite, which was 0.98. This increase in the ID/IG 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 A1g symmetry, but the G band arose from the E2g phonon mode for the sp2 carbon net‐ work 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 1.5 2

CdSe QDs

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electron rich surface.

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 ac‐ tivity 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.

3.7. Photocatalytic studies 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 wave‐ length and concentration of the BG dye solution were deter‐ mined by UV‐vis absorption spectroscopy. The maximum ab‐ sorption wavelength for the BG dye solution was observed at 624 nm. To determine the response of the photocatalytic activ‐ ity of CdSe QDs and the CdSe/GO nanocomposite, the absorp‐ tion 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 wave‐ length (λmax) 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: 0

(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 degrada‐ tion of the BG dye using CdSe QDs and CdSe/GO nanocompo‐ sites 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

Degradation %

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100

(3) where C0 and C are the BG dye concentration at time 0 and t min, respectively, and k is the constant of the pseu‐ do‐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 fraction‐ al reduction of GO only partially restores the sp2 networks; therefore, the remaining oxygen sites still able to accept elec‐ tron 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 ln

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Fig. 9. UV‐vis absorption spectra of BG dye for CdSe QDs (a) and CdSe/GO nanocomposites (b) under sunlight irradiation at different time intervals with a plot of ln(C0/C) vs. time interval (inset).


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Fig. 10. Schematic diagram of the photocatalytic degradation mechanism of BG by CdSe/GO nanocomposites under sunlight irradiation.

owing to the high charge separation induced by the synergistic effects of GO on CdSe. At the same time, under sunlight irradia‐ tion, 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: CdSe/GO + hν → h+ + e– (4) h++ H2O →OH• + H+ (5) e– + O2 → O2–• (6) O2–• + H+ → HO2• (7) e– + H+ + HO2• → H2O2 (8) e– + H2O2 →OH• + OH– (9) • BG dye + OH → CO2 + H2O + mineralized by products (10)

References [1] L. Zhu, S. Ye, A. Ali, K. Ulla, K. Cho, W. C. Oh, Chin. J. Catal., 2015, 36,

603–611. [2] Y. N. Liu, R. X. Wang, Z. K. Yang, H. Du, Y. F. Jiang, C. C. Shen, K.

Liang, A. W. Xu, Chin. J. Catal., 2015, 36, 2135–2144. [3] Y. Liu, J. Li, B. Zhou, S. Lv, X. Li, H. C. Chen, Q. P. Chen, W. M. Cai,

Appl. Catal. B, 2012, 111–112, 485–491. [4] Y. Miao, G. Pan, Y. Huo, H. Li, Dyes Pigments, 2013, 99, 382–389. [5] Y. Zhou, L. H. Xu, Z. J. Wu, P. Li, J. J. He, Optik, 2017, 130, 673–680. [6] L. X. Yin, D. Wang, J. F. Huang, L. Y. Cao, H. B. Ouyang, X. Yong, J.

Alloys Compd., 2016, 664, 476–480. [7] N. Qutub, B. Pirzada, K. Umar, S. Sabir, J. Environ. Chem. Eng., 2016,

4, 808–817. [8] J. Q. Wen, X. Li, W. Liu, Y.P. Fang, J. Xie, Y. H. Xu, Chin. J. Catal., 2015,

36, 2049–2070. [9] A. Oliva, O. Solis‐Canto, R. Castro‐Rodriguez, P. Quintana, Thin Sol‐

id Films, 2001, 391, 18–35. [10] L. X. Yin, D. D. Zhang, D. Wang, X. G. Kong, J. F. Huang, F. F. Wang, Y.

B. Wu, Mater. Sci. Eng. B, 2016, 208, 15–21.

3. Conclusions

[11] T. Ghosh, J. H. Lee, Z. D. Meng, K. Ullah, C. Y. Park, V. Nikam, W. C.

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, re‐ spectively. 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 environmen‐ tally friendly, which are excellent characteristics for the practi‐ cal applications in photocatalysis.

[12] M. M. D. Kumar, S. Devadason, Superlattices Microstruct., 2013, 58,

Oh, Mater. Res. Bull., 2013, 48, 1268–1274. 154–164. [13] Z. Tachan, I. Hod, M. Shalom, L. Grinis, A. Zaban, Phys. Chem. Chem.

Phys., 2013, 15, 3841–3845. [14] L. Chen, C. Lee, C. Chen, Mater. Lett., 2015, 148, 14–16. [15] J. Zhao, M. Holmes, F. Osterloh, ACS Nano, 2013, 7, 4316–4325. [16] X. Chen, G. Wu, J. M. Chen, X. Chen, Z. X. Xie, X. R. Wang, J. Am. Chem.

Soc., 2011, 133, 3693–3695. [17] N. Zhang, Y. Zhang, X. Pan, X. Fu, S. Liu, Y. Xu, J. Phys. Chem. C, 2011,

115, 23501–23511. [18] A. Khataee, F. Mohamadi, T. Rad, B. Vahid, Ultrason. Sonochem.,

2017, 40, 361–372. [19] X. L. Liu, C. C. Ma, Y. Yan, G. X. Yao, Y. F. Tang, P. W. Huo, W. D. Shi,

Y. S. Yan, Ind. Eng. Chem. Res., 2013, 52, 15015–15023.

2158


Graphical Abstract Chin. J. Catal., 2017, 38: 2150–2159 doi: 10.1016/S1872‐2067(17)62964‐4 Photocatalytic degradation of Brilliant Green dye using CdSe quantum dots hybridized with graphene oxide under sunlight irradiation N. Thirugnanam, Huaibing Song, Yan Wu * Annamalai University, India; China University of Geosciences (Wuhan), China

CdSe/GO nanocomposites show an excellent photocatalytic activity against Brilliant Green dye degradation under sunlight irradiation. This enhanced photocatalytic activity can be attributed to the high surface area and high charge separation efficiency of GO. 2015, 5, 1500010.

[20] C. Harris, P. Kamat, ACS Nano, 2010, 4, 7321–7330. [21] R. Chikate, B. Kadu, M. Damle, RSC Adv., 2014, 4, 35997–36005. [22] M. Chen, W. Oh, Nanoscale Res. Lett., 2011, 6, 398–405. [23] K. Novoselov, A. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Du‐

[38] R. Lotfi, Y. Saboohi, Physica E, 2014, 60, 104–111. [39] S. Q. Liu, N. Zhang, Z. Tang, Y. Xu, ACS Appl. Mater. Interfaces, 2012,

bonos, I. V. Grigorieva, A. Firsov, Science, 2004, 306, 666–669. Q. Xiang, J. Yu, M. Jaroniec, Chem. Soc. Rev., 2012, 41, 782–796. Q. Li, B. Guo, J. Yu, J. Ran, B. Zhang, H. Yan, J. Gong, J. Am. Chem. Soc, 2011, 133, 10878–10884. Y. Xu, Y. P. Mo, J. Tian, P. Wang, H. G. Yu, J. G. Yu, Appl. Catal. B, 2016, 181, 810–817. H. G. Yu, P. Xiao, J. Tian, F. Z. Wang, J. G. Yu, ACS Appl. Mater. Inter‐ faces, 2016, 8, 29470–29477. X. Li, R. C. Shen, S. Ma, X. B. Chen, J. Xie, Appl. Surf. Sci., 2017, http://dx.doi.org/10.1016/j.apsusc.2017.08.194. L. Putri, W. Ong, W. Chang, S. Chai, Appl. Surf. Sci., 2015, 358, 2–14. L. Sim, K. Leong, P. Saravanan, S. Ibrahim, Appl. Surf. Sci., 2015, 358, 122–129. Q. J. Xiang, J. G. Yu, M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575–6578. J. X. Low, J. G. Yu, W. Ho, J. Phys. Chem. Lett., 2015, 6, 4244–4251. Q. J. Xiang, B. Cheng, J. G. Yu, Angew. Chem. Int. Ed., 2015, 54, 11350–11366. X. Li, J. G. Yu, S. Wageh, A. A. Al‐Ghamdi, J. Xie, Small, 2016, 12, 6640–6696. S. Hsieh, W. Chen, T. Yeh, Appl. Surf. Sci., 2015, 358, 63–69. X. F. Wu, L. L. Wen, K. L. Lv, K. J. Deng, D. G. Tang, H. P. Ye, D. Y. Du, S. N. Liu, M. Li, Appl. Surf. Sci., 2015, 358, 130–136. Q. Li, X. Li, S. Wageh, A. A. Al‐Ghamdi, J. Yu, Adv. Energy Mater.,

[40] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339. [41] C. Hontoria‐Lucas, A. J. Lopez‐Peinado, J. de D. Lo´pez‐Gonza´ lez,

[24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]

4, 6378–6385.

[42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53]

M. L. Rojas‐Cervantes, R. M. Martin‐Aranda, Carbon, 1995, 33, 1585–1592. F. Tavakoli, M. Salavati‐Niasari, A. badiei, F. Mohandes, Mater. Res. Bull., 2015, 63, 51–57. S. Chen, J. W. Zhu, X. D. Wu, Q. F. Han, X. Wang, ACS Nano, 2010, 4, 2822–2830. S. Kumar, A. Ojha, B. Walkenfort, J. Photochem. Photobiol. B, 2016, 159, 111–119. W. E. Mahmoud, A. M. Al‐Amri, S. Yaghmour, Opt. Mater., 2012, 34, 1082–1086. J. H. Bang, P. V. Kamat, ACS Nano, 2011, 5, 9421–9427. R. B. Jin, X. J. Jiang, Y. Y. Zhou, J. F. Zhao, Chin. J. Catal., 2016, 37, 760–768. T. Chen, L. M. Dai, J. Mater. Chem. A, 2014, 2, 10756–10775. M. N. Huang, J. H. Yu, C. S. Deng, Y. H. Huang, M. G. Fan, B. Li, Z. F. Tong, F. Y. Zhang, L. H. Dong, Appl. Surf. Sci., 2016, 365, 227–239. W. D. Zhang, F. Dong, W. Zhang, Appl. Surf. Sci., 2015, 358, 75–83. M. S. Akple, J. X. Low, Z. Y. Qin, S. Wageh, A. A. Al‐Ghamdi, J. G. Yu, S. W. Liu, Chin. J. Catal., 2015, 36, 2127–2134. W. Zou, J. Zhu, Y. Sun, X. Wang, Mater. Chem. Phys., 2011, 125, 617–620. M. J. Zhou, D. L. Han, X. L. Liu, C. C. Ma, H. Q. Wang, Y. F. Tang, P. W.


Huo, W. D. Shi, Y. S. Yan, J. H. Yang, Appl. Catal. B, 2015, 172–173, 174–184. [54] J. Debgupta, V. Pillai, Nanoscale, 2013, 5, 3615–3619. [55] J. Jang, E. Kim, S. Choi, D. H. Kim, H. G. Kim, J. S. Lee, Appl. Catal. A, 2012, 427–428, 106–113. [56] J. H. Wang, S. Liang, L. Ma, S. J. Ding, X. F. Yu, L. Zhou, Q. Q. Wang,

2159

CrystEngComm, 2014, 16, 399–405. [57] P. V. Kamat, Phys. Chem. Lett., 2011, 2, 242–251. [58] B. Yuan, J. X. Wei, T. J. Hu, H. B. Yao, Z. H. Jiang, Z. W. Fang, Z. Y. Chu,

Chin. J. Catal., 2015, 36, 1009–1016. [59] Z. Meng, L. Zhu, S. Ye, Q. Sun, K. Ullah, K. Y. Cho, W. C. Oh, Nanoscale

Res. Lett., 2013, 8, 189–198.

CdSe量子点/氧化石墨烯复合物对灿烂绿染料的光催化降解性能研究 N. Thirugnanam a, 宋怀兵b, 吴

艳b,*

安纳马莱大学物理学院, 泰米尔纳德邦608002, 印度中国地质大学(武汉)材料与化学学院, 湖北武汉430074, 中国 a

b

摘要: 纺织、塑料、造纸和纸浆等工业排放物中含有大量的有机染料, 这些染料通常不可生物降解, 从而产生了严重的环境污染问题. 为了降解这些有机染料废弃物, 人们迫切需要高效、廉价、稳定的有机物降解光催化剂. 近年来, 半导体光催化剂引起了人们的广泛关注, 尤其是窄禁带半导体材料可以实现染料的高效降解. 在半导体II–VI族中, CdSe具有合适的带隙 (1.74 eV)和快速生成的电子-空穴对, 被认为是光催化降解有机污染物的重要半导体材料. 特别是当它与超高的电子导电性的碳基纳米材料结合时, 光催化活性增强. 本文采用一种简单的化学沉淀法成功合成了CdSe量子点与氧化石墨烯(GO)的复合材料. 紫外-可见吸收光谱显示, CdSe量子点和CdSe/GO纳米复合材料的吸收边分别出现在583和556 nm处. 与纯CdSe量子点相比, GO层上的CdSe量子点的尺寸减小, 由于量子限制效应, CdSe/GO纳米复合材料的光吸收波长在蓝移, 从而拓宽了 CdSe/GO纳米复合物的光吸收范围. PL光谱图显示CdSe量子点的可见光区的强宽发光峰出现在缺陷态的603 nm, 而在576 nm处观察到CdSe/GO纳米复合材料的发射峰, 峰位蓝移, 光猝灭. GO表面上CdSe量子点的修饰改变了GO层间相互作用的范德华力和CdSe量子点与GO片相互作用的静电作用力. 这些相互作用导致能级的变化, 使得CdSe/GO纳米复合的发射峰蓝移. 由于复合物中电子-空穴对的复合被抑制, CdSe/GO纳米复合材料的光致发光强度低于CdSe量子点, 此对应于CdSe量子点到GO板的界面电荷转移. PL研究表明, GO修饰CdSe可促进电子-空穴对的分离. EIS测量方法进一步研究了CdSe量子点和CdSe/GO纳米复合材料的电荷输运行为. 结果显示, 加入GO后, CdSe量子点的阻抗值减小, 表明GO的引入降低了电荷转移电阻, 促进了其界面电荷转移. 因此, CdSe/GO纳米复合材料具有较高的电荷分离效率, 可以提高其光催化活性. 拉曼光谱显示, 由于CdSe量子点的激发, 电子注入到GO中, 使得CdSe/GO纳米复合物材料的拉曼光谱向更高的波数转移. 通过BET 性能测试, CdSe/GO纳米复合物的比表面积为10.4 m2/g, 比CdSe量子点的比表面积(5 m2/g)增加了一倍. 我们发现在太阳光的照射下, CdSe量子点和CdSe/GO纳米复合物对灿烂绿染料的光降解率分别为81.9%和95.5%, 各自对应的光降解速率分别为0.0190和0.0345 min‒1. CdSe/GO纳米复合物增强的光催化性能归因于具有较大的比表面积以及氧化石墨烯的加入促进了电子-空穴对的有效分离. 关键词: CdSe量子点; 氧化石墨烯; 纳米复合材料; 光催化活性; 灿烂绿染料收稿日期: 2017-10-03. 接受日期: 2017-11-01. 出版日期: 2017-12-05. *通讯联系人. 电话/传真: (027)67883731; 电子信箱: [email protected] 基金来源 : 国家自然科学基金 (51774259); 纳米矿物材料及应用教育部工程研究中心开放基金 (NGM2017KF004, NGM2017KF012). 本文的全文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).