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reduced graphene oxide nanocomposites for visible-light-driven photocatalytic degradation of organic contaminant
Author’s Accepted Manuscript Radiation Synthesis of CdS/Reduced Graphene Oxide Nanocomposites for Visible-Light-Driven Photocatalytic Degradation of Organic Contaminant Xiaoyang Fu, Youwei Zhang, Pengfei Cao, Huiling Ma, Pinggui Liu, Lihua He, Jing Peng, Jiuqiang Li, Maolin Zhai www.elsevier.com/locate/radphyschem
PII: DOI: Reference:
S0969-806X(16)30060-3 http://dx.doi.org/10.1016/j.radphyschem.2016.02.016 RPC7073
To appear in: Radiation Physics and Chemistry Received date: 21 November 2015 Revised date: 6 February 2016 Accepted date: 8 February 2016 Cite this article as: Xiaoyang Fu, Youwei Zhang, Pengfei Cao, Huiling Ma, Pinggui Liu, Lihua He, Jing Peng, Jiuqiang Li and Maolin Zhai, Radiation Synthesis of CdS/Reduced Graphene Oxide Nanocomposites for Visible-LightDriven Photocatalytic Degradation of Organic Contaminant, Radiation Physics and Chemistry, http://dx.doi.org/10.1016/j.radphyschem.2016.02.016 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 galley proof before it is published in its final citable 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.
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Radiation Synthesis of CdS/Reduced Graphene Oxide Nanocomposites for Visible-Light-Driven Photocatalytic Degradation of Organic Contaminant Xiaoyang Fua, Youwei Zhangb, Pengfei Caoa, Huiling Mac, Pinggui Liub, Lihua Heb, Jing Penga*, Jiuqiang Lia, Maolin Zhaia*
a
Beijing National Laboratory for Molecular Sciences, Radiochemistry and Radiation
Chemistry Key Laboratory of Fundamental Science, the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b
Aviation Key Laboratory of Science and Technology on Stealth Materials, Beijing
Institute of Aeronautical Materials, Beijing 100095, China c
Beijing Key Laboratory of Radiation Advanced Materials, Beijing Research Center for
Radiation Application, Beijing 100015, China
*Corresponding author. Tel/Fax: +86-10-62757193; Email: [email protected] *Corresponding author. Tel/Fax: +86-10-62753794; Email: [email protected]
ABSTRACT CdS/reduced
graphene
oxide
(CdS/RGO)
nanocomposites
were
successfully
synthesized via a one-step gamma-ray radiation-induced reduction method. The composition and structure of the prepared nanocomposites were characterized by thermal gravimetric analysis, micro FTIR spectroscopy, UV-vis spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. It was found that increasing dose could improve the degree of reduction of graphite oxide
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(GO), and the feed ratio of GO to CdCl2·2.5H2O significantly influenced the size and dispersion of the CdS nanoparticles. The nanocomposites prepared under dose of 300 kGy and the feed ratio of GO to CdCl2·2.5H2O 1.0 wt% exhibited high visible-light photocatalytic performance for the degradation of Rhodamine B with degradation efficiency of 93%. This work provides a novel and facile method to produce the nanocomposites as efficient photocatalysts for the removal of organic contaminants from aqueous solution. Keywords: Gamma-ray radiation synthesis, CdS/reduced graphene oxide (CdS/RGO) nanocomposites, Photo-catalytic performance
1. Introduction Energy crisis and environmental pollution have made people pay more attention to the development of sustainable clean energy (Xu et al., 2015). Solar energy utilization, such as photovoltaic cells (Lee et al., 2015a; Lee et al., 2015b), photo catalysis (Pan and Xu, 2015), and photo electrochemical water splitting (Wang et al., 2014), is extremely attractive because of abundant sunlight resource and less carbon emission. Among them, photo catalytic degradation of organic pollutants using semiconductor photo catalysts has attracted significant amount of research interest because it is considered as the most potential solution to solve the environmental pollution (Pawar et al., 2014; Tang et al., 2015). Thus, the design and development of efficient photocatalysts is the key issue and main challenge for the degradation of organic pollutants. CdS has been extensively studied as one of the most popular semiconductor photocatalysts for various applications, such as degradation of pollutants, hydrogen production from splitting water and organic transformation (Henglein, 1989; Li et al.,
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2011b; Liu et al., 2013; Pathak et al., 2015; Ren et al., 2014). The band gap (2.4 eV) of CdS is well coincident with the spectrum of sunlight, which makes CdS active under sunlight or visible light (Li et al., 2015). The absorption spectrum is dependent on the size of CdS due to the quantum-size effect (Henglein, 1989). However, the inherent disadvantages of CdS photocatalysts such as low separation efficiency of photo generated electron-hole pairs and easy photocorrosion, have limited its cyclic operation in environmental applications (Yan et al., 2013). Therefore, many researchers have focused on further improving the photoactivities and photostabilities of CdS. One of the approaches is to combine CdS with supporting materials to accelerate the charge separation and migration in the photocatalytic procedure (Ren et al., 2014; Yan et al., 2014). Graphene, as a rising star in the carbon family, exhibits outstanding physical, chemical properties and excellent electrocatalytic ability (Cho et al., 2015; Wang et al., 2015). Many previous studies have proven that the integration graphene with other semiconductor could significantly improve the photocatalytic performances of graphene based semiconductor nanocomposites, such as TiO2/graphene (Dembele et al., 2015; Gao et al., 2015) and ZnO/graphene (Dang et al., 2015; Hu et al., 2015). For CdS/graphene nanocomposites, the introduction of graphene can enhance the photocatalytic performances, which has been widely reported in previous literatures (Li et al., 2015; Tang et al., 2015). The reinforcement of photocatalytic performances can be attributed to two reasons. Firstly, graphene can reduce the possibility of electron-hole pair recombination to improve the photocatalytic performance and quantum efficiency of the nanocomposites. Secondly, graphene, serving as the supporter, could improve the degree of dispersion of the CdS nanoparticles. Smaller particle size and larger surface
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area can usually offer more reactive sites and enhance light harvesting, thus enhancing photo catalytic activity. As is reported in the recent literatures, the CdS/graphene nanocomposites are mostly synthesized with the methods of chemical reduction (Nethravathi et al., 2009) and solvent thermal reduction (Li et al., 2011a; Liu et al., 2013). However, these methods usually require high energy consumption and harsh conditions of synthesis. Recently, some groups and our group have reported the preparation of reduced graphene oxide (RGO) and their nanocomposites using gamma ray irradiation (Wang et al., 2013; Zhang et al., 2012). Gamma-ray irradiation exhibits various advantages such as being cost-effective, high-yielding, and environmentally friendly (Kharazmi et al., 2013). However, the radiation synthesis of the CdS/RGO nanocomposites has not been reported in the literature to date. In this work, we implemented a straightforward approach to prepare the CdS/RGO nanocomposites using gamma-ray radiation-induced reduction reaction from precursors of graphite oxide (GO), Na2S2O3 and CdCl2·2.5H2O. The structure of CdS/RGO nanocomposites prepared under different doses and feed ratios of GO to CdCl 2·2.5H2O were studied and the visible light photocatalytic performances of the CdS/RGO nanocomposites were tested by the degradation of Rhodamine B (Rh. B) which has been commonly used as the model of organic dye. 2. Experimental 2.1. Raw materials GO was purchased from the Six Element (Changzhou) Materials Technology Co., Ltd (China). All the other chemical reagents were obtained from Beijing Chemical Works (China). All reagents were analytical-grade and were used as received without further purification.
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2.2. Synthesis of CdS/RGO nanocomposites 1.60g CdCl2·2.5H2O, 1.86g Na2S2O3 and 2.82g ethylene diamine tetra acetic acid disodium salt (EDTA-2Na) was added into a beaker and diluted in 20 mL ethylene glycol (EG). Then pH was adjusted to approximately 11 by adding NaOH and diluted to 40 mL by pure water ([Cd2+]=0.2 mol L-1). Dividing the sample into 4 glass tubes uniformly, one of them does not contain GO for preparing CdS. GO was added into other 3 glass tubes for preparing CdS/RGO nanocomposites. And the mass ratios of GO to CdCl2·2.5H2O were 1.0 wt%, 2.3 wt% and 10.0wt%, respectively. The suspensions were treated by an ultrasonicator for 2 h, bubbled with argon for 30 min and then sealed. After that, the samples were exposed to gamma-ray irradiation using a 60Co source at a dose rate of approximately 100 Gy min-1 at room temperature and the total dose was 300 kGy. After irradiation, the product was separated by filtration through a 0.22 μm nylon membrane filter, washed with pure water and ethanol several times, and finally dried in vacuum at 60 oC to obtain CdS or CdS/RGO nanocomposites. For comparison, CdS/RGO nanocomposites were also prepared under the dose of 150kGy and the feed ratio of 2.3wt% without changing other conditions. RGO was also prepared under the similar conditions with the only addition of GO (1 mg mL-1) at the dose of 150 kGy. The abbreviations of sample and variations of preparation conditions are listed in Table 1. Table 1. The abbreviations of sample and variations of preparation conditions. Name
Sample
Dose/kGy
Feed Ratio/wt%
RGO
RGO
150
CdS
CdS
300
0
NCP-I
CdS/RGO
300
1.0
NCP-II
CdS/RGO
300
2.3
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NCP-III
CdS/RGO
300
10.0
NCP-IV
CdS/RGO
150
2.3
2.3. Characterization TG analysis was carried out by using a TA Q600 thermal analyzer in nitrogen atmosphere at a heating rate of 10 oC min-1 from room temperature to 600 oC. MicroFTIR analysis was performed on a Nicolet (NICOLET iN10 MX) spectrometer. The spectra were measured in absorbance mode in the wavenumber ranging from 4000 to 650 cm-1 with a resolution of 4 cm-1. UV-vis absorption spectroscopy was conducted with a Hitachi UV-3010 spectrophotometer with the wavelength ranging from 200 to 800 nm. XRD patterns were obtained by using a Rigatu Dmax diffractometer with Cu Ka radiation (k = 1.54A°) at a generator voltage of 40 kV and a generator current of 40 mA. XPS analysis was performed with an Axis Ultra instrument from Kratos Analytical using monochromatic Al Ka radiation as the X-ray source for excitation. TEM measurements were performed on a Tecnai F20 transmission electron microscope with an acceleration voltage of 200 kV.
2.4. Visible light catalytic degradation of Rh.B. The visible light photo catalytic performance of the CdS/RGO nanocomposites was estimated by the degradation of Rh.B. In every experiment, 20 mg catalyst was added into 100 mL of Rh. B (12 ppm) solution. The mixtures were treated by a supersonicator for 30 min and stirred by a magnetic stirring apparatus at room temperature for 30 min in the dark to achieve the adsorption/desorption equilibrium of Rh. B. The light source was a PL-X300D Xenon lamp equipped with UV and IR filters which was placed about 20 cm above the sample. Samples were withdrawn regularly at time t, t = 30, 60, 90, 120, 180, 240, 300 min, and immediately centrifuged for 30 minutes to separate any
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suspended solid before analysis. The concentration of Rh. B was calculated by measuring the highest absorbance of the solution samples at around λmax = 554 nm with a UV-Vis spectrophotometer (blue shifts occurred during the degradation of Rh.B). All the experiments above were carried out in duplicate and average values were reported. Standard deviations were found to be within ±3 %. 3. Results and discussion
Scheme 1. Schematic of radiation synthesis of CdS/RGO nanocomposites.
The synthesis route of the CdS/RGO nanocomposites is shown in Scheme 1. The interaction of gamma ray with solvent can produce solvated electrons (Kharazmi et al., 2013), which could reduce S2O32- to S2- (Kharazmi et al., 2013) and GO to RGO (Zhang et al., 2012). It was proposed that Cd(II) would react with S2- to form CdS nanoparticles, and then could be loaded on the nanosheets of RGO to produce the CdS/RGO nanocomposites. EG served as scavenger of the oxidative hydroxyl radicals generated by radiolysis of solvent (Belloni et al., 1998; Kharazmi et al., 2013). pH was adjusted to approximately 11, because under alkaline environment the solvated electrons were more
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stable and GO was more easily to be reduced to RGO (Zhang et al., 2013). In addition, EDTA-2Na prevented the formation of Cd(OH)2 precipitation at high pH. Relevant reactions during the process of synthesis are listed below. →
(1) (2) (3) (4)
( )
(5)
3.1. Characterization of CdS/RGO nanocomposites
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Fig. 1. (a) TGA curves of GO, RGO and CdS/RGO nanocomposites. (b) TGA curves of CdS and CdS/RGO nanocomposites prepared under dose of 300 kGy and different feed ratios.
The TGA curves of CdS/RGO nanocomposites prepared under different doses and feed ratios are shown in Fig. 1 (a) and (b), respectively. For GO in Fig. 1(a), there is a significant weight loss at about 200 oC, which is ascribed to the removal of the unstable oxygen-containing functional groups, while the weight loss of RGO is much smaller due to the reduction of the oxygen-containing functional groups during radiation (Ganguly et al., 2011; Wang et al., 2013). For CdS/RGO nanocomposites in Fig. 1 (a), there is no significant weight loss at 200 oC. Besides, the CdS/RGO nanocomposites prepared at 300 kGy exhibits a better thermostability than the nanocomposites prepared at 150 kGy with the same feed ratio. The results indicate that RGO prepared at a higher dose has a higher degree of reduction and lower content of unstable oxygen containing groups. In Fig. 1 (b), the weight loss of the nanocomposites increases with the increasing of the feed ratio of GO to CdCl2·2.5H2O. It can be attributed to an increasing amount of RGO in the nanocomposites and RGO has a lower thermal stability than CdS.
Fig. 2. Micro FTIR spectra of GO, RGO, CdS and CdS/RGO nanocomposites.
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Fig. 2 is micro FTIR spectra of GO, RGO, CdS and CdS/RGO nanocomposites prepared under 300 kGy. In the spectrum of GO, the peaks at around 3400 cm-1, 2927 cm-1, 1740 cm-1, 1224 cm-1 and 1045 cm-1 are due to the O-H stretching vibrations, CH3 stretching vibrations, C=O stretching vibrations, (C-O-C) stretching vibrations and (C=C-O) stretching vibrations, respectively (Liu et al., 2013). After irradiation, in the spectra of RGO and CdS/RGO nanocomposites, the peaks representing the groups containing oxygen are weaker and a peak appears at 1562 cm-1, corresponding to the skeletal vibration of graphene sheets, which indicates GO is reduced through the removal of the oxygen containing groups and the restoration of the structure of graphene nanosheets (Zhang et al., 2013). The absorption peaks at 628, 1004, 1155 cm-1 are assigned to the Cd–S bond vibrations (Liji Sobhana et al., 2010; Pawar et al., 2014), and these peaks are only observed in the infrared spectra of CdS and CdS/RGO nanocomposites while not found in the infrared spectra of GO or RGO.
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Fig. 3. (a) UV-vis spectra of GO, RGO and CdS/RGO nanocomposites solutions prepared under different doses. (b) UV-vis spectra of CdS and CdS/RGO nanocomposites solutions prepared under different feed ratios at the dose of 300 kGy.
Fig. 3 (a) and (b) show the UV-Vis spectra of these nanocomposites solutions prepared under different doses and feed ratios. For GO, the absorption peak at 232 nm is assigned to the π-π* transition of aromatic C–C bonds. The other shoulder peak at 300 nm corresponds to the n-π* transition of C=O bonds. For RGO, after gamma ray induced reduction, the band at 300 nm becomes less prevalent, and the band at 232 nm is redshifted to 259 nm, indicating the restoration of the electronic π-conjunction structure and the removal of oxygen-containing groups (Zhang et al., 2012). The absorption peak
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in the spectra of CdS/RGO nanocomposites corresponding to the 1sh-1se transition is related to the band-gap energy (Nandakumar et al., 2002). In the spectra of CdS/RGO nanocomposites solutions, the blue-shifts of the absorption peaks are observed due to the quantum size effect of CdS nanoparticles. CdS/RGO nanocomposites, which prepared under the dose of 300 kGy and 150 kGy (Fig. 3(a)) respectively, have almost the same position of the absorption peak, indicating similar band-gap energy and size of the CdS nanoparticles. The result shows that the dose does not have a significant effect on the size of the CdS nanoparticles. In Fig. 3(b), with the feed ratio of GO increasing, the spectrum and the position of the absorption peaks are blue-shifted, indicating the increasing of band-gap energy and the decreasing of the size of CdS nanoparticles (Henglein et al., 1986; Koch et al., 1985). Therefore, it can be concluded that RGO serves as the supporter and helps the CdS nanoparticles to disperse and prevent them from gathering and agglomeration. The absorbance of the peak also decreases along with an increased feed ratio of GO and a decreased amount of CdS load, and the absorbance in the region of visible light also decreases.
Fig. 4. XRD spectra of CdS and CdS/RGO nanocomposites prepared under dose of 300 kGy and different feed ratios.
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The XRD spectra of CdS/RGO nanocomposites prepared under different doses and feed ratios are shown in Fig. 4, respectively. The peaks at 26.7°, 44.0°, 52.1° are assigned to the (002), (110) and (112) crystal planes of hexagonal CdS according to JCPDS card No.41-1049 (a = 4.136A˚, c = 6.713A˚,α=β=90o, =120o). The peak of graphene at 26.0° in the nanocomposites is not observed because the diffraction peak of graphene at 26.0° is overlapped by the peak of CdS at 26.7°. Another possible explanation is that CdS nanoparticles may prevent the restacking of graphene sheets, and the characteristic diffraction peaks disappear (Pham et al., 2010). These diffraction peaks of CdS/RGO nanocomposites are widened to some extent, indicating that the crystallite size of the CdS nanoparticles in the nanocomposites is relatively small according to the Scherrer formula. Furthermore, the sharpness and broadening of the diffraction peaks of CdS/RGO nanocomposites prepared under different doses are similar, which also indicates that the dose does not have an obvious effect on the crystallite size and degree of crystallinity of the CdS nanoparticles. As shown in Fig.4, the sharpness of the diffraction peaks increases along with the increasing of the feed ratio of GO, indicating that the addition of RGO can increase the degree of crystallinity of the CdS nanoparticles in the nanocomposites. A similar phenomenon has been reported and discussed in the previous literature as well (Li et al., 2011b). Therefore, in the presence of RGO, the growth of CdS nanoparticles could be promoted to give a high crystallization degree.
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Fig. 5. (a) XPS spectrum of CdS/RGO nanocomposites (NCP-I). (b), (c) XPS C 1s spectrum of CdS/RGO nanocomposites (NCP-IV, NCP-II).
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Fig.5 (a) is the XPS spectra of CdS/RGO nanocomposites prepared at the dose of 300kGy and feed ratio 1.0wt%. The peaks corresponding to the C 1s (binding energy at 284.8ev), O1s (binding energy at 531.8eV), Cd 3d5/2 (binding energy at 404.9eV), Cd 3d3/2 (binding energy at 411.6eV) and S 2p (binding energy at 161.3 eV) are observed. From the peak area, the ratio of Cd:S and C:O is estimated to be about 1.1:1, 2.7:1, respectively. The binding energies of cadmium and sulfur in the nanocomposites are slightly different compared with the pure CdS reported from previous literatures, which indicates the interactions of electron transfer between the CdS and RGO nanosheets (Gao et al., 2013). As shown in Fig.5 (b) and (c), the peak at about 286.4 eV is due to the epoxy and hydroxyl carbon atoms, and the peak at about 288.6 eV is assigned to the carbonyl carbon atoms. These peaks indicate that some oxygen-containing groups still exist in the nanocomposites. The peak at 284.8 eV represents the C-C and C=C in the RGO nanosheets. With increasing dose, the content of C-C and C=C groups in the CdS/RGO nanocomposites increases from 71.0% to 78.4%, indicating the higher dose promotes the degree of reduction of GO in the nanocomposites (Zhang et al., 2012). The elemental compositions of CdS/RGO nanocomposites prepared under different conditions are listed in Table 2 as results of XPS. In Table 2, the ratio of C:O in NCPIV and NCP-II decreases contrarily as the dose increases. The ratio of C:O may not be an appropriate factor to measure the degree of reduction of RGO in CdS/RGO nanocomposites since oxygen is not only from RGO. As reported in the literature (Min and Lü 2011), some CdS are oxidized to CdSO4, which is in accordance with the small peak at 168.5eV, indicating the existence of hexavalent sulfur (SO42-) in Fig 5 (a). Additionally, small molecules containing oxygen can also be adsorbed by the CdS/RGO
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nanocomposites, and the oxygen will also be counted into the ratio of C:O. It can be concluded that dose does not greatly influence the elemental composition of the nanocomposites from Table 2. Table 2. Elemental composition of CdS/RGO nanocomposites prepared under different conditions. Sample
xc/%
xo/%
xCd/%
xs/%
wcd/wt%
CdS
26.7
10.4
32.6
30.3
90.5
NCP-I
38.5
14.4
24.2
22.8
83.4
NCP-II
44.6
15.9
20.3
19.2
78.6
NCP-III
65.3
21.1
6.9
6.7
46.9
NCP-IV
46.3
15.6
20.9
17.2
78.3
The elemental compositions are greatly influenced by the feed ratio. As shown in Table 2, when the feed ratio of GO to CdCl2·2.5H2O increases from 0wt% to 10.0wt%, the mass percentage of CdS is gradually reduced from 90.5% to 46.9%. The ratio of C:O seems to increase while the amount of CdS decreases, possibly indicating that some of the oxygen is from CdSO4 as described beforehand.
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Fig. 6. TEM images of CdS and CdS/RGO nanocomposites prepared under different conditions. (a) CdS, (b) NCP-I, (c) NCP-II, (d) NCP-III, (e) NCP-IV.
The morphology of CdS and CdS/RGO nanocomposites are characterized by TEM. As shown in the image of CdS, the size of CdS nanoparticles without RGO as a supporter is larger than 200 nm. In the presence of RGO, CdS nanoparticles could be well dispersed in the RGO, and the size of CdS nanoparticles decreases with the increasing feed ratio of GO to CdCl2·2.5H2O. This indicates that the introduction of RGO is beneficial to the dispersion and the size control of CdS nanoparticles (Li et al., 2011b). However, when the feed ratio of GO to CdCl2·2.5H2O is more than 10%, the load of CdS is comparatively low. The CdS nanoparticles are loaded on the RGO nanosheets due to electrostatic interaction. Comparing the nanocomposites prepared under different doses, the size of the CdS nanoparticles is similar, and these nanoparticles have a similar
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degree of dispersion as well. These results show that the dose has no significant effect on the size of CdS nanoparticles. These results are in accordance with the results of TGA, FTIR, UV-Vis, XRD, and XPS. 3.2. The visible-light photocatalytic performance of CdS/RGO nanocomposites for the degradation of Rh.B
Fig. 7. (a) Degradation of Rh.B under different conditions. (b) Degradation of Rh.B catalyzed by CdS and CdS/RGO nanocomposites prepared at the dose of 300kGy and different feed ratios.
The photocatalytic performances of CdS/RGO nanocomposites are measured by the degradation of Rh.B under visible light (Fig. 7 (a)). The RGO can’t degrade the Rh. B in the absence of CdS. When loaded with the CdS nanoparticles, the nanocomposites
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exhibit good photocatalytic performance. When the dose increases from 150kGy to 300kGy, the catalytic performances of the prepared nanocomposites slightly improve. It is possible that RGO in the nanocomposites prepared under a higher dose has a better degree of reduction and thus has a more restored π-bonding structure. Thus it is more capable to accept the photo electron excited to the conduction band of CdS nanoparticles and thus separate the photo-generated electron-hole pairs efficiently, lengthening the lifetime of the charge carriers and enhancing the photo catalytic performance of the nanocomposites. As show in Fig.7(b), the feed ratio of GO greatly influences the photo catalytic performance of the nanocomposites. RGO in the nanocomposites not only serves as the photoelectron capturer as mentioned above, but also promotes the dispersion of the CdS nanoparticles and leads to smaller particle size and larger specific surface area, which leads to more reaction sites. Smaller particle size could also increase the band gap energy and reduce the recombination of the photogenerated electron–hole pairs, leading to higher quantum efficiency and better photocatalytic performance (Wang et al., 2008; Zhang et al., 2011). Rh.B could also be adsorbed by RGO due to the π-π stacking interactions and may be more easily concentrated to the catalyst surface so that the photo catalytic performance may be improved (Liu et al., 2013; Zhang et al., 2015). It is also reported in the literature that graphene or GO sheets could help inhibit photo corrosions of the semiconductor nanoparticles in the nanocomposites (Gao et al., 2013; Zhang et al., 2015). However, too much graphene can in turn negatively affect the visible light photo catalytic performance of the nanocomposites because too much graphene could also prevent the nanocomposites from absorbing visible light and this negative effect could be quite overwhelming according to previous research (Li et al., 2011b). In the UV-Vis spectra
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above the absorbance of the nanocomposites in the region of visible light gradually decreases with an increasing feed ratio of GO to CdCl2·2.5H2O, which is also in accordance with the explanation above. Besides, the reduced amount of the CdS nanoparticles loaded in the nanocomposites may be a possible explanation as well. The optimized feed ratio of GO to CdCl2·2.5H2O in this experiment is approximately 1.0 wt% (CdS load amount is 83.4wt%). The photo catalytic performance of the nanocomposites is similar to the nanocomposites synthesized with the method of solvent thermal reduction reported in the previous literature (Liu et al., 2013). 4. Conclusions In conclusion, CdS/RGO nanocomposites with good visible-light photocatalytic performance have been prepared successfully in one step using gamma ray irradiation. The degree of reduction of the graphene in the nanocomposites was mainly influenced by the absorbed dose, while the load, size and degree of dispersion of the CdS nanoparticles in the nanocomposites could be adjusted by the feed ratio of GO to CdCl2·2.5H2O. The prepared CdS/RGO nanocomposites containing 83.4wt% CdS demonstrated good photocatalytic performance for Rh. B with a degradation efficiency of 93% under visible light. The one-step and environmentally friendly method could be also applied for the preparation of nanocomposites materials of other semiconductors and RGO. Acknowledgements Financial support from the National Natural Science Foundation of China (11375019， 11405168, 11505011) are gratefully acknowledged.
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Research highlights CdS/RGO nanocomposites were synthesized via a -ray radiation reduction method. CdS/RGO nanocomposites prepared under different conditions were characterized. Good photo catalytic performances were exhibited with the degradation of Rh. B.
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PII: DOI: Reference:
S0969-806X(16)30060-3 http://dx.doi.org/10.1016/j.radphyschem.2016.02.016 RPC7073
To appear in: Radiation Physics and Chemistry Received date: 21 November 2015 Revised date: 6 February 2016 Accepted date: 8 February 2016 Cite this article as: Xiaoyang Fu, Youwei Zhang, Pengfei Cao, Huiling Ma, Pinggui Liu, Lihua He, Jing Peng, Jiuqiang Li and Maolin Zhai, Radiation Synthesis of CdS/Reduced Graphene Oxide Nanocomposites for Visible-LightDriven Photocatalytic Degradation of Organic Contaminant, Radiation Physics and Chemistry, http://dx.doi.org/10.1016/j.radphyschem.2016.02.016 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 galley proof before it is published in its final citable 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.
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Radiation Synthesis of CdS/Reduced Graphene Oxide Nanocomposites for Visible-Light-Driven Photocatalytic Degradation of Organic Contaminant Xiaoyang Fua, Youwei Zhangb, Pengfei Caoa, Huiling Mac, Pinggui Liub, Lihua Heb, Jing Penga*, Jiuqiang Lia, Maolin Zhaia*
a
Beijing National Laboratory for Molecular Sciences, Radiochemistry and Radiation
Chemistry Key Laboratory of Fundamental Science, the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b
Aviation Key Laboratory of Science and Technology on Stealth Materials, Beijing
Institute of Aeronautical Materials, Beijing 100095, China c
Beijing Key Laboratory of Radiation Advanced Materials, Beijing Research Center for
Radiation Application, Beijing 100015, China
*Corresponding author. Tel/Fax: +86-10-62757193; Email: [email protected] *Corresponding author. Tel/Fax: +86-10-62753794; Email: [email protected]
ABSTRACT CdS/reduced
graphene
oxide
(CdS/RGO)
nanocomposites
were
successfully
synthesized via a one-step gamma-ray radiation-induced reduction method. The composition and structure of the prepared nanocomposites were characterized by thermal gravimetric analysis, micro FTIR spectroscopy, UV-vis spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. It was found that increasing dose could improve the degree of reduction of graphite oxide
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(GO), and the feed ratio of GO to CdCl2·2.5H2O significantly influenced the size and dispersion of the CdS nanoparticles. The nanocomposites prepared under dose of 300 kGy and the feed ratio of GO to CdCl2·2.5H2O 1.0 wt% exhibited high visible-light photocatalytic performance for the degradation of Rhodamine B with degradation efficiency of 93%. This work provides a novel and facile method to produce the nanocomposites as efficient photocatalysts for the removal of organic contaminants from aqueous solution. Keywords: Gamma-ray radiation synthesis, CdS/reduced graphene oxide (CdS/RGO) nanocomposites, Photo-catalytic performance
1. Introduction Energy crisis and environmental pollution have made people pay more attention to the development of sustainable clean energy (Xu et al., 2015). Solar energy utilization, such as photovoltaic cells (Lee et al., 2015a; Lee et al., 2015b), photo catalysis (Pan and Xu, 2015), and photo electrochemical water splitting (Wang et al., 2014), is extremely attractive because of abundant sunlight resource and less carbon emission. Among them, photo catalytic degradation of organic pollutants using semiconductor photo catalysts has attracted significant amount of research interest because it is considered as the most potential solution to solve the environmental pollution (Pawar et al., 2014; Tang et al., 2015). Thus, the design and development of efficient photocatalysts is the key issue and main challenge for the degradation of organic pollutants. CdS has been extensively studied as one of the most popular semiconductor photocatalysts for various applications, such as degradation of pollutants, hydrogen production from splitting water and organic transformation (Henglein, 1989; Li et al.,
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2011b; Liu et al., 2013; Pathak et al., 2015; Ren et al., 2014). The band gap (2.4 eV) of CdS is well coincident with the spectrum of sunlight, which makes CdS active under sunlight or visible light (Li et al., 2015). The absorption spectrum is dependent on the size of CdS due to the quantum-size effect (Henglein, 1989). However, the inherent disadvantages of CdS photocatalysts such as low separation efficiency of photo generated electron-hole pairs and easy photocorrosion, have limited its cyclic operation in environmental applications (Yan et al., 2013). Therefore, many researchers have focused on further improving the photoactivities and photostabilities of CdS. One of the approaches is to combine CdS with supporting materials to accelerate the charge separation and migration in the photocatalytic procedure (Ren et al., 2014; Yan et al., 2014). Graphene, as a rising star in the carbon family, exhibits outstanding physical, chemical properties and excellent electrocatalytic ability (Cho et al., 2015; Wang et al., 2015). Many previous studies have proven that the integration graphene with other semiconductor could significantly improve the photocatalytic performances of graphene based semiconductor nanocomposites, such as TiO2/graphene (Dembele et al., 2015; Gao et al., 2015) and ZnO/graphene (Dang et al., 2015; Hu et al., 2015). For CdS/graphene nanocomposites, the introduction of graphene can enhance the photocatalytic performances, which has been widely reported in previous literatures (Li et al., 2015; Tang et al., 2015). The reinforcement of photocatalytic performances can be attributed to two reasons. Firstly, graphene can reduce the possibility of electron-hole pair recombination to improve the photocatalytic performance and quantum efficiency of the nanocomposites. Secondly, graphene, serving as the supporter, could improve the degree of dispersion of the CdS nanoparticles. Smaller particle size and larger surface
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area can usually offer more reactive sites and enhance light harvesting, thus enhancing photo catalytic activity. As is reported in the recent literatures, the CdS/graphene nanocomposites are mostly synthesized with the methods of chemical reduction (Nethravathi et al., 2009) and solvent thermal reduction (Li et al., 2011a; Liu et al., 2013). However, these methods usually require high energy consumption and harsh conditions of synthesis. Recently, some groups and our group have reported the preparation of reduced graphene oxide (RGO) and their nanocomposites using gamma ray irradiation (Wang et al., 2013; Zhang et al., 2012). Gamma-ray irradiation exhibits various advantages such as being cost-effective, high-yielding, and environmentally friendly (Kharazmi et al., 2013). However, the radiation synthesis of the CdS/RGO nanocomposites has not been reported in the literature to date. In this work, we implemented a straightforward approach to prepare the CdS/RGO nanocomposites using gamma-ray radiation-induced reduction reaction from precursors of graphite oxide (GO), Na2S2O3 and CdCl2·2.5H2O. The structure of CdS/RGO nanocomposites prepared under different doses and feed ratios of GO to CdCl 2·2.5H2O were studied and the visible light photocatalytic performances of the CdS/RGO nanocomposites were tested by the degradation of Rhodamine B (Rh. B) which has been commonly used as the model of organic dye. 2. Experimental 2.1. Raw materials GO was purchased from the Six Element (Changzhou) Materials Technology Co., Ltd (China). All the other chemical reagents were obtained from Beijing Chemical Works (China). All reagents were analytical-grade and were used as received without further purification.
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2.2. Synthesis of CdS/RGO nanocomposites 1.60g CdCl2·2.5H2O, 1.86g Na2S2O3 and 2.82g ethylene diamine tetra acetic acid disodium salt (EDTA-2Na) was added into a beaker and diluted in 20 mL ethylene glycol (EG). Then pH was adjusted to approximately 11 by adding NaOH and diluted to 40 mL by pure water ([Cd2+]=0.2 mol L-1). Dividing the sample into 4 glass tubes uniformly, one of them does not contain GO for preparing CdS. GO was added into other 3 glass tubes for preparing CdS/RGO nanocomposites. And the mass ratios of GO to CdCl2·2.5H2O were 1.0 wt%, 2.3 wt% and 10.0wt%, respectively. The suspensions were treated by an ultrasonicator for 2 h, bubbled with argon for 30 min and then sealed. After that, the samples were exposed to gamma-ray irradiation using a 60Co source at a dose rate of approximately 100 Gy min-1 at room temperature and the total dose was 300 kGy. After irradiation, the product was separated by filtration through a 0.22 μm nylon membrane filter, washed with pure water and ethanol several times, and finally dried in vacuum at 60 oC to obtain CdS or CdS/RGO nanocomposites. For comparison, CdS/RGO nanocomposites were also prepared under the dose of 150kGy and the feed ratio of 2.3wt% without changing other conditions. RGO was also prepared under the similar conditions with the only addition of GO (1 mg mL-1) at the dose of 150 kGy. The abbreviations of sample and variations of preparation conditions are listed in Table 1. Table 1. The abbreviations of sample and variations of preparation conditions. Name
Sample
Dose/kGy
Feed Ratio/wt%
RGO
RGO
150
CdS
CdS
300
0
NCP-I
CdS/RGO
300
1.0
NCP-II
CdS/RGO
300
2.3
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NCP-III
CdS/RGO
300
10.0
NCP-IV
CdS/RGO
150
2.3
2.3. Characterization TG analysis was carried out by using a TA Q600 thermal analyzer in nitrogen atmosphere at a heating rate of 10 oC min-1 from room temperature to 600 oC. MicroFTIR analysis was performed on a Nicolet (NICOLET iN10 MX) spectrometer. The spectra were measured in absorbance mode in the wavenumber ranging from 4000 to 650 cm-1 with a resolution of 4 cm-1. UV-vis absorption spectroscopy was conducted with a Hitachi UV-3010 spectrophotometer with the wavelength ranging from 200 to 800 nm. XRD patterns were obtained by using a Rigatu Dmax diffractometer with Cu Ka radiation (k = 1.54A°) at a generator voltage of 40 kV and a generator current of 40 mA. XPS analysis was performed with an Axis Ultra instrument from Kratos Analytical using monochromatic Al Ka radiation as the X-ray source for excitation. TEM measurements were performed on a Tecnai F20 transmission electron microscope with an acceleration voltage of 200 kV.
2.4. Visible light catalytic degradation of Rh.B. The visible light photo catalytic performance of the CdS/RGO nanocomposites was estimated by the degradation of Rh.B. In every experiment, 20 mg catalyst was added into 100 mL of Rh. B (12 ppm) solution. The mixtures were treated by a supersonicator for 30 min and stirred by a magnetic stirring apparatus at room temperature for 30 min in the dark to achieve the adsorption/desorption equilibrium of Rh. B. The light source was a PL-X300D Xenon lamp equipped with UV and IR filters which was placed about 20 cm above the sample. Samples were withdrawn regularly at time t, t = 30, 60, 90, 120, 180, 240, 300 min, and immediately centrifuged for 30 minutes to separate any
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suspended solid before analysis. The concentration of Rh. B was calculated by measuring the highest absorbance of the solution samples at around λmax = 554 nm with a UV-Vis spectrophotometer (blue shifts occurred during the degradation of Rh.B). All the experiments above were carried out in duplicate and average values were reported. Standard deviations were found to be within ±3 %. 3. Results and discussion
Scheme 1. Schematic of radiation synthesis of CdS/RGO nanocomposites.
The synthesis route of the CdS/RGO nanocomposites is shown in Scheme 1. The interaction of gamma ray with solvent can produce solvated electrons (Kharazmi et al., 2013), which could reduce S2O32- to S2- (Kharazmi et al., 2013) and GO to RGO (Zhang et al., 2012). It was proposed that Cd(II) would react with S2- to form CdS nanoparticles, and then could be loaded on the nanosheets of RGO to produce the CdS/RGO nanocomposites. EG served as scavenger of the oxidative hydroxyl radicals generated by radiolysis of solvent (Belloni et al., 1998; Kharazmi et al., 2013). pH was adjusted to approximately 11, because under alkaline environment the solvated electrons were more
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stable and GO was more easily to be reduced to RGO (Zhang et al., 2013). In addition, EDTA-2Na prevented the formation of Cd(OH)2 precipitation at high pH. Relevant reactions during the process of synthesis are listed below. →
(1) (2) (3) (4)
( )
(5)
3.1. Characterization of CdS/RGO nanocomposites
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Fig. 1. (a) TGA curves of GO, RGO and CdS/RGO nanocomposites. (b) TGA curves of CdS and CdS/RGO nanocomposites prepared under dose of 300 kGy and different feed ratios.
The TGA curves of CdS/RGO nanocomposites prepared under different doses and feed ratios are shown in Fig. 1 (a) and (b), respectively. For GO in Fig. 1(a), there is a significant weight loss at about 200 oC, which is ascribed to the removal of the unstable oxygen-containing functional groups, while the weight loss of RGO is much smaller due to the reduction of the oxygen-containing functional groups during radiation (Ganguly et al., 2011; Wang et al., 2013). For CdS/RGO nanocomposites in Fig. 1 (a), there is no significant weight loss at 200 oC. Besides, the CdS/RGO nanocomposites prepared at 300 kGy exhibits a better thermostability than the nanocomposites prepared at 150 kGy with the same feed ratio. The results indicate that RGO prepared at a higher dose has a higher degree of reduction and lower content of unstable oxygen containing groups. In Fig. 1 (b), the weight loss of the nanocomposites increases with the increasing of the feed ratio of GO to CdCl2·2.5H2O. It can be attributed to an increasing amount of RGO in the nanocomposites and RGO has a lower thermal stability than CdS.
Fig. 2. Micro FTIR spectra of GO, RGO, CdS and CdS/RGO nanocomposites.
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Fig. 2 is micro FTIR spectra of GO, RGO, CdS and CdS/RGO nanocomposites prepared under 300 kGy. In the spectrum of GO, the peaks at around 3400 cm-1, 2927 cm-1, 1740 cm-1, 1224 cm-1 and 1045 cm-1 are due to the O-H stretching vibrations, CH3 stretching vibrations, C=O stretching vibrations, (C-O-C) stretching vibrations and (C=C-O) stretching vibrations, respectively (Liu et al., 2013). After irradiation, in the spectra of RGO and CdS/RGO nanocomposites, the peaks representing the groups containing oxygen are weaker and a peak appears at 1562 cm-1, corresponding to the skeletal vibration of graphene sheets, which indicates GO is reduced through the removal of the oxygen containing groups and the restoration of the structure of graphene nanosheets (Zhang et al., 2013). The absorption peaks at 628, 1004, 1155 cm-1 are assigned to the Cd–S bond vibrations (Liji Sobhana et al., 2010; Pawar et al., 2014), and these peaks are only observed in the infrared spectra of CdS and CdS/RGO nanocomposites while not found in the infrared spectra of GO or RGO.
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Fig. 3. (a) UV-vis spectra of GO, RGO and CdS/RGO nanocomposites solutions prepared under different doses. (b) UV-vis spectra of CdS and CdS/RGO nanocomposites solutions prepared under different feed ratios at the dose of 300 kGy.
Fig. 3 (a) and (b) show the UV-Vis spectra of these nanocomposites solutions prepared under different doses and feed ratios. For GO, the absorption peak at 232 nm is assigned to the π-π* transition of aromatic C–C bonds. The other shoulder peak at 300 nm corresponds to the n-π* transition of C=O bonds. For RGO, after gamma ray induced reduction, the band at 300 nm becomes less prevalent, and the band at 232 nm is redshifted to 259 nm, indicating the restoration of the electronic π-conjunction structure and the removal of oxygen-containing groups (Zhang et al., 2012). The absorption peak
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in the spectra of CdS/RGO nanocomposites corresponding to the 1sh-1se transition is related to the band-gap energy (Nandakumar et al., 2002). In the spectra of CdS/RGO nanocomposites solutions, the blue-shifts of the absorption peaks are observed due to the quantum size effect of CdS nanoparticles. CdS/RGO nanocomposites, which prepared under the dose of 300 kGy and 150 kGy (Fig. 3(a)) respectively, have almost the same position of the absorption peak, indicating similar band-gap energy and size of the CdS nanoparticles. The result shows that the dose does not have a significant effect on the size of the CdS nanoparticles. In Fig. 3(b), with the feed ratio of GO increasing, the spectrum and the position of the absorption peaks are blue-shifted, indicating the increasing of band-gap energy and the decreasing of the size of CdS nanoparticles (Henglein et al., 1986; Koch et al., 1985). Therefore, it can be concluded that RGO serves as the supporter and helps the CdS nanoparticles to disperse and prevent them from gathering and agglomeration. The absorbance of the peak also decreases along with an increased feed ratio of GO and a decreased amount of CdS load, and the absorbance in the region of visible light also decreases.
Fig. 4. XRD spectra of CdS and CdS/RGO nanocomposites prepared under dose of 300 kGy and different feed ratios.
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The XRD spectra of CdS/RGO nanocomposites prepared under different doses and feed ratios are shown in Fig. 4, respectively. The peaks at 26.7°, 44.0°, 52.1° are assigned to the (002), (110) and (112) crystal planes of hexagonal CdS according to JCPDS card No.41-1049 (a = 4.136A˚, c = 6.713A˚,α=β=90o, =120o). The peak of graphene at 26.0° in the nanocomposites is not observed because the diffraction peak of graphene at 26.0° is overlapped by the peak of CdS at 26.7°. Another possible explanation is that CdS nanoparticles may prevent the restacking of graphene sheets, and the characteristic diffraction peaks disappear (Pham et al., 2010). These diffraction peaks of CdS/RGO nanocomposites are widened to some extent, indicating that the crystallite size of the CdS nanoparticles in the nanocomposites is relatively small according to the Scherrer formula. Furthermore, the sharpness and broadening of the diffraction peaks of CdS/RGO nanocomposites prepared under different doses are similar, which also indicates that the dose does not have an obvious effect on the crystallite size and degree of crystallinity of the CdS nanoparticles. As shown in Fig.4, the sharpness of the diffraction peaks increases along with the increasing of the feed ratio of GO, indicating that the addition of RGO can increase the degree of crystallinity of the CdS nanoparticles in the nanocomposites. A similar phenomenon has been reported and discussed in the previous literature as well (Li et al., 2011b). Therefore, in the presence of RGO, the growth of CdS nanoparticles could be promoted to give a high crystallization degree.
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Fig. 5. (a) XPS spectrum of CdS/RGO nanocomposites (NCP-I). (b), (c) XPS C 1s spectrum of CdS/RGO nanocomposites (NCP-IV, NCP-II).
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Fig.5 (a) is the XPS spectra of CdS/RGO nanocomposites prepared at the dose of 300kGy and feed ratio 1.0wt%. The peaks corresponding to the C 1s (binding energy at 284.8ev), O1s (binding energy at 531.8eV), Cd 3d5/2 (binding energy at 404.9eV), Cd 3d3/2 (binding energy at 411.6eV) and S 2p (binding energy at 161.3 eV) are observed. From the peak area, the ratio of Cd:S and C:O is estimated to be about 1.1:1, 2.7:1, respectively. The binding energies of cadmium and sulfur in the nanocomposites are slightly different compared with the pure CdS reported from previous literatures, which indicates the interactions of electron transfer between the CdS and RGO nanosheets (Gao et al., 2013). As shown in Fig.5 (b) and (c), the peak at about 286.4 eV is due to the epoxy and hydroxyl carbon atoms, and the peak at about 288.6 eV is assigned to the carbonyl carbon atoms. These peaks indicate that some oxygen-containing groups still exist in the nanocomposites. The peak at 284.8 eV represents the C-C and C=C in the RGO nanosheets. With increasing dose, the content of C-C and C=C groups in the CdS/RGO nanocomposites increases from 71.0% to 78.4%, indicating the higher dose promotes the degree of reduction of GO in the nanocomposites (Zhang et al., 2012). The elemental compositions of CdS/RGO nanocomposites prepared under different conditions are listed in Table 2 as results of XPS. In Table 2, the ratio of C:O in NCPIV and NCP-II decreases contrarily as the dose increases. The ratio of C:O may not be an appropriate factor to measure the degree of reduction of RGO in CdS/RGO nanocomposites since oxygen is not only from RGO. As reported in the literature (Min and Lü 2011), some CdS are oxidized to CdSO4, which is in accordance with the small peak at 168.5eV, indicating the existence of hexavalent sulfur (SO42-) in Fig 5 (a). Additionally, small molecules containing oxygen can also be adsorbed by the CdS/RGO
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nanocomposites, and the oxygen will also be counted into the ratio of C:O. It can be concluded that dose does not greatly influence the elemental composition of the nanocomposites from Table 2. Table 2. Elemental composition of CdS/RGO nanocomposites prepared under different conditions. Sample
xc/%
xo/%
xCd/%
xs/%
wcd/wt%
CdS
26.7
10.4
32.6
30.3
90.5
NCP-I
38.5
14.4
24.2
22.8
83.4
NCP-II
44.6
15.9
20.3
19.2
78.6
NCP-III
65.3
21.1
6.9
6.7
46.9
NCP-IV
46.3
15.6
20.9
17.2
78.3
The elemental compositions are greatly influenced by the feed ratio. As shown in Table 2, when the feed ratio of GO to CdCl2·2.5H2O increases from 0wt% to 10.0wt%, the mass percentage of CdS is gradually reduced from 90.5% to 46.9%. The ratio of C:O seems to increase while the amount of CdS decreases, possibly indicating that some of the oxygen is from CdSO4 as described beforehand.
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Fig. 6. TEM images of CdS and CdS/RGO nanocomposites prepared under different conditions. (a) CdS, (b) NCP-I, (c) NCP-II, (d) NCP-III, (e) NCP-IV.
The morphology of CdS and CdS/RGO nanocomposites are characterized by TEM. As shown in the image of CdS, the size of CdS nanoparticles without RGO as a supporter is larger than 200 nm. In the presence of RGO, CdS nanoparticles could be well dispersed in the RGO, and the size of CdS nanoparticles decreases with the increasing feed ratio of GO to CdCl2·2.5H2O. This indicates that the introduction of RGO is beneficial to the dispersion and the size control of CdS nanoparticles (Li et al., 2011b). However, when the feed ratio of GO to CdCl2·2.5H2O is more than 10%, the load of CdS is comparatively low. The CdS nanoparticles are loaded on the RGO nanosheets due to electrostatic interaction. Comparing the nanocomposites prepared under different doses, the size of the CdS nanoparticles is similar, and these nanoparticles have a similar
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degree of dispersion as well. These results show that the dose has no significant effect on the size of CdS nanoparticles. These results are in accordance with the results of TGA, FTIR, UV-Vis, XRD, and XPS. 3.2. The visible-light photocatalytic performance of CdS/RGO nanocomposites for the degradation of Rh.B
Fig. 7. (a) Degradation of Rh.B under different conditions. (b) Degradation of Rh.B catalyzed by CdS and CdS/RGO nanocomposites prepared at the dose of 300kGy and different feed ratios.
The photocatalytic performances of CdS/RGO nanocomposites are measured by the degradation of Rh.B under visible light (Fig. 7 (a)). The RGO can’t degrade the Rh. B in the absence of CdS. When loaded with the CdS nanoparticles, the nanocomposites
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exhibit good photocatalytic performance. When the dose increases from 150kGy to 300kGy, the catalytic performances of the prepared nanocomposites slightly improve. It is possible that RGO in the nanocomposites prepared under a higher dose has a better degree of reduction and thus has a more restored π-bonding structure. Thus it is more capable to accept the photo electron excited to the conduction band of CdS nanoparticles and thus separate the photo-generated electron-hole pairs efficiently, lengthening the lifetime of the charge carriers and enhancing the photo catalytic performance of the nanocomposites. As show in Fig.7(b), the feed ratio of GO greatly influences the photo catalytic performance of the nanocomposites. RGO in the nanocomposites not only serves as the photoelectron capturer as mentioned above, but also promotes the dispersion of the CdS nanoparticles and leads to smaller particle size and larger specific surface area, which leads to more reaction sites. Smaller particle size could also increase the band gap energy and reduce the recombination of the photogenerated electron–hole pairs, leading to higher quantum efficiency and better photocatalytic performance (Wang et al., 2008; Zhang et al., 2011). Rh.B could also be adsorbed by RGO due to the π-π stacking interactions and may be more easily concentrated to the catalyst surface so that the photo catalytic performance may be improved (Liu et al., 2013; Zhang et al., 2015). It is also reported in the literature that graphene or GO sheets could help inhibit photo corrosions of the semiconductor nanoparticles in the nanocomposites (Gao et al., 2013; Zhang et al., 2015). However, too much graphene can in turn negatively affect the visible light photo catalytic performance of the nanocomposites because too much graphene could also prevent the nanocomposites from absorbing visible light and this negative effect could be quite overwhelming according to previous research (Li et al., 2011b). In the UV-Vis spectra
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above the absorbance of the nanocomposites in the region of visible light gradually decreases with an increasing feed ratio of GO to CdCl2·2.5H2O, which is also in accordance with the explanation above. Besides, the reduced amount of the CdS nanoparticles loaded in the nanocomposites may be a possible explanation as well. The optimized feed ratio of GO to CdCl2·2.5H2O in this experiment is approximately 1.0 wt% (CdS load amount is 83.4wt%). The photo catalytic performance of the nanocomposites is similar to the nanocomposites synthesized with the method of solvent thermal reduction reported in the previous literature (Liu et al., 2013). 4. Conclusions In conclusion, CdS/RGO nanocomposites with good visible-light photocatalytic performance have been prepared successfully in one step using gamma ray irradiation. The degree of reduction of the graphene in the nanocomposites was mainly influenced by the absorbed dose, while the load, size and degree of dispersion of the CdS nanoparticles in the nanocomposites could be adjusted by the feed ratio of GO to CdCl2·2.5H2O. The prepared CdS/RGO nanocomposites containing 83.4wt% CdS demonstrated good photocatalytic performance for Rh. B with a degradation efficiency of 93% under visible light. The one-step and environmentally friendly method could be also applied for the preparation of nanocomposites materials of other semiconductors and RGO. Acknowledgements Financial support from the National Natural Science Foundation of China (11375019， 11405168, 11505011) are gratefully acknowledged.
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Research highlights CdS/RGO nanocomposites were synthesized via a -ray radiation reduction method. CdS/RGO nanocomposites prepared under different conditions were characterized. Good photo catalytic performances were exhibited with the degradation of Rh. B.
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