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Ultrasonic chemical synthesis of CdS-reduced graphene oxide nanocomposites with an enhanced visible light photoactivity
Accepted Manuscript Full Length Article Ultrasonic chemical synthesis of CdS-reduced graphene oxide nanocomposites with an enhanced visible light photoactivity Yi-Chen Lin, Du-Cheng Tsai, Zue-Chin Chang, Fuh-Sheng Shieu PII: DOI: Reference:
S0169-4332(18)30330-1 https://doi.org/10.1016/j.apsusc.2018.01.305 APSUSC 38444
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
6 November 2017 23 January 2018 31 January 2018
Please cite this article as: Y-C. Lin, D-C. Tsai, Z-C. Chang, F-S. Shieu, Ultrasonic chemical synthesis of CdSreduced graphene oxide nanocomposites with an enhanced visible light photoactivity, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.01.305
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Ultrasonic chemical synthesis of CdS-reduced graphene oxide nanocomposites with an enhanced visible light photoactivity Yi-Chen Lina, Du-Cheng Tsaia, Zue-Chin Changb, Fuh-Sheng Shieua,* a
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan
b
Department of Mechanical Engineering, National Chin-Yi University of Technology, Taichung 41170, Taiwan
Abstract
In this study, we report a facile ultrasonic method to prepare a series of CdS and reduced graphene oxide (CdS/rGO) composites with different weight ratios of graphene at temperature as low as 70 °C for 20 min by employing ammonia as a complexing agent of Cd2+ ions and reducing agent of graphene oxide (GO). Pure CdS particles had a poor crystallinity and aggregated to large particles size. As GO was incorporated into CdS, a uniform dispersion of CdS particles with high crystallinity on rGO sheets was clearly observed. The as-prepared CdS/rGO composites have a wide and strong photo absorption in the visible region and display a substantially improved photocatalytic activity for the degradation of methylene blue under visible light irradiation by forming a heterojunction of rGO and CdS. However, too much rGO will shield the light of the active sites for the CdS nanoparticle surface and thus limit further improvement in the photocatalytic efficiency.
Keywords: Reduce graphene oxide; Cadmium sulfide; Photocatalytic activity.
* Corresponding author at: Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan. Tel.: +886 4 2284 0500; fax: +886 4 2285 7017. E-mail address: [email protected] (F.S. Shieu).
1. Introduction
Photocatalysts have gained much attention for wastewater treatment because of their rapid oxidation, high oxidation effect, and low or no formation of byproducts [1-4]. Until now, TiO2 and ZnO photocatalysts have been widely applied for the photodegradation of dyes because of their high chemical stability and high photocatalytic activity [5,6]. However, one of their main drawbacks is that they only absorb UV light, which is only about 3% of the solar spectrum. Thus, various materials with smaller band gaps have been explored for the photodegradation of dyes under visible light irradiation [7]. Among these materials, CdS has shown a high potential as a visible-light-driven photocatalyst because of its suitable band gap (around 2.4 eV) and high absorption coefficient (>104) [8]. Nevertheless, the small surface area, serious photocorrosion, and high electron-hole recombination of CdS results in a low photocatalytic activity [9]. Many strategies for improving the photocatalytic activities of CdS have been applied over the past several years. Among these strategies, heterogeneous photocatalysis has been extensively considered to lower the electron-hole recombination rate of CdS to improve its photocatalytic activity. Graphene-based materials have a unique sheet-like morphology, high electron conductivity, and high mobility [10-12]. Therefore, combining CdS and graphene can accelerate the charge transportation and separation, and remarkably improve the photocatalytic efficiency under visible light irradiation [13]. Recently, several methods have been reported for synthesizing graphene/CdS composites. For example, Zhiyong et al. [14] prepared graphene-CdS directly from graphene oxide (GO) with ethylene glycol through a simple solvothermal method and showed an excellent degradation for 70% CdS. Nan et al. [15] synthesized graphene/CdS nanocomposites using solvent-exfoliated graphene instead of GO and found that it could decrease the defect density of graphene and efficiently enhance the photocatalytic activity. Fengzhen et al. [16] synthesized graphene/CdS nanorods using a solvothermal method with ethylene diamine and tested the photocatalytic degradation of rhodamine B under visible light illumination. Xinwei et al. [17] synthesized ZnCdS/reduced GO (rGO) using a one-pot hydrothermal reaction in an autoclave (180 °C for 12 h); it provided an excellent degradation (98%) of methylene blue (MB) in 120
min and showed a good stability and reproducibility. Hongwei et al. [18] synthesized a ternary hybrid of meso-TiO2/rGO/CdS using a photo-assisted treatment to reduce GO; they achieved degradation of methylene orange using meso-TiO2 and improved the degradation rate of the loaded-CdS by approximately a factor of 17. In these reports, CdS can be well dispersed on graphene sheets, preventing the aggregation of CdS particles and inhibiting the stacking of graphene. Efficient charge transfer occurs between CdS and graphene because of their uniform heterogenous junction. However, their synthesis methods require an autoclave, high temperature process, and/or complicated manipulation, hindering the progress of the above methods. Therefore, reducing the processing time, temperature, and cost, and developing an environmentally friendly strategy is very important for the synthesis of CdS/graphene composites. In this study, we develop an improved synthetic method for preparing CdS/graphene composites. Cadmium acetate and thiourea were used as cadmium and sulfur sources, respectively. Ammonia water was used as a complexing agent of Cd2+ ions and as a reducing agent for GO. A series of CdS/graphene composites with different graphene weight ratios was synthesized by a facile, one-step ultrasonic chemical method. The process temperature can be as low as 70 °C, and the synthesis time can be as low as 20 min. The raw materials are readily available, and the synthetic procedure is simple and safe. The ultrasonic reaction involves two mechanisms: reduction of GO to rGO and deposition of CdS particles onto the surface of the rGO sheets. The electron-rich Cd atoms and electrophilic carbon atoms of rGO show a strong interaction, which allows the CdS particles to tightly adhere to the rGO sheets. Meanwhile, the ultrasonic condition facilitates rGO to accommodate the maximum amount of CdS particles with an unstacked active surface. Such a composite structure inhibits particle aggregation and offers nucleation and growth platforms for CdS, resulting in a higher product crystallinity and a smaller particle size distribution. As expected, the resultant CdS/rGO composite improves the photocatalytic efficiency for the degradation of MB under visible light irradiation.
2. Experimental
2.1 Materials All chemicals were of analytic grade and used without further purification. Graphite powder (Choneye Pure Chemicals, Taipei, Taiwan) was used to prepare GO. Cadmium acetate (Echo Chemical, Miaoli, Taiwan), ammonia water (Choneye Pure Chemicals, Taipei, Taiwan), and thiourea (Shimakyu's Pure Chemicals, Osaka, Japan) were used as precursors to obtain CdS.
2.2 Synthesis of GO GO was prepared according to a modified Hummer’s method [19]. Graphite powder (2 g) was mixed with 0.6 g of NaNO3 in a 20-mL H2SO4 solution in an ice bath and stirred for 30 min. Then, 10 g of KMnO4 was added to the mixture, and the solution was stirred at 35 °C for 3 h. Next, 180 mL of deionized (DI) water was slowly added and maintained at 98 °C for 2 h. Afterward, 20 mL of 30% H2O2 was added to the solution to reduce the residual KMnO4. Finally, the precipitate was centrifuged and washed with 5% HCl and DI water until the pH value reached 7, and then dried at 70 °C for 24 h.
2.3 Synthesis of CdS and the CdS/rGO composite material A mixed solvent containing 50 mL of DI water, various amounts of GO, 0.1 g of cadmium acetate, and 1.1 g of thiourea was prepared. Then, ammonia was added to the solvent to adjust the pH to 11. The synthesis process for the resulting mixture was carried out under a 50/60-Hz ultrasonic shock using an ultrasonic apparatus (E30, Elma Co., Germany) for 20 min at 70 °C. After the reaction was complete, the mixture was centrifuged, washed several times with DI water and absolute ethanol, and then dried at 70 °C for 24 h to obtain the CdS/rGO composites. The as-synthesized CdS/rGO samples with 3, 5, 10, and15 wt% rGO, named as CdS/rGO-3%, CdS/rGO-5%, CdS/rGO-10%, and CdS/rGO-15%, respectively, were isolated by filtration. Typically, CdS/rGO-5% contained 5 mg of GO and 0.1 g of CdS. For reference, the pure CdS
particles were synthesized using a similar method without GO. According to the above details, a possible synthesis route for the CdS/rGO composite is shown in Fig. 1.
2.4 Characterization Microstructural characterization of the synthesized samples was performed using a wide-angle X-ray diffractometer (XRD; MAC MXPIII) operated at 40 kV and 30 mA with Cu Kα radiation. The particle size and morphology of the samples were analyzed by transmission electron microscopy (TEM, JEOL 200CX) and scanning electron microscopy (SEM, JEOL JSM-6700F). The Brunauer-Emmett-Teller (BET) specific surface area and porosity were analyzed based on nitrogen adsorption-desorption isotherms using a Micromeritics ASAP 2020 nitrogen adsorption apparatus. Fourier transform infrared (FTIR) spectroscopy measurements were carried out using a Nicolet 380 infrared spectrometer. Raman spectra were recorded on an Andor DU401-BV multichannel confocal microspectrometer with 632-nm laser excitation. X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe) was performed using monochromated Al Kα radiation (1486.8 eV). The optical properties were measured using a Shimadzu UV3600 spectrophotometer.
2.5 Measurement of the photocatalytic degradation of MB Photocatalytic activity experiments were performed for the photodegradation of MB in a 300-mL double-walled glass beaker with cooling water at 20 °C. A 500-W xenon arc lamp (Prosper OptoElectronic Co., Ltd.) with a heat filter, UV-cutoff filter (420 nm), and IR-cutoff filter (750 nm) was used as a visible light source to trigger the photocatalytic reaction and was positioned 20 cm from the reactor. The focused intensity on the flask was about 700 W/m2, which was measured using an SPM-1116SD visible light radiometer (Lutron Electronics, Inc.) in the range of 400-1100 nm. In a typical experiment, 10 mg of the photocatalyst was suspended in 150 mL of an MB (0.01 mg/mL) solution under magnetic stirring. The temperature of the solution was maintained at room temperature by a flow of cooling water. The degradation of the MB solution with irradiation time was determined by recording UV-VIS absorption spectra using a Shimadzu UV3600 spectrophotometer.
Fig. 2 shows the process of the visible photocatalytic measurement in a typical photocatalytic experiment.
3. Results and discussion
3.1 Structure, morphology, and composition of CdS/rGO Fig. 3 shows the XRD patterns of pristine graphite, GO, CdS, CdS/rGO-3%, CdS/rGO-5%, CdS/rGO-10%, and CdS/rGO-15%. The graphite powder presents a strong diffraction peak at 2θ = 26.5° with an interlayer spacing of 0.33 nm, which can be indexed to the (002) plane of the hexagonal structure of graphite. After oxidation through the modified Hummers process, the GO powder is obtained and a (001) diffraction peak is observed at 2θ = 10.29°, which is typical for GO [20]. The increased interlayer distance is due to the introduction of many oxygen-containing groups on the basal plane [21]. The diffraction peaks of the bare CdS powder are well indexed to the crystal planes of a cubic CdS phase (JCPDS NO, 80-0019) [22]. The weak intensity of the peak indicates the poor crystallinity of the pure CdS powder. The diffraction peaks of the CdS/rGO composite are also well matched to that of cubic CdS. However, the peak intensities are much stronger than those for pure CdS. The patterns do not contain characteristic diffraction peaks of the carbon species because of the low diffraction intensity of graphene. The XRD patterns also indicate that the enhanced graphene content increases the crystallinity of the CdS/rGO particles [23]. After introducing 5% graphene, the XRD peaks of CdS/rGO-5% become stronger and narrower due to the improved crystallinity of CdS particles. Thus, rGO constitutes a superior platform for nucleation and growth of CdS, implying a strong interaction between CdS and rGO. Fig. 4 shows the SEM images of the CdS and CdS/rGO-5% powders. The morphologies of CdS/rGO-3%, CdS/rGO-10%, and CdS/rGO-15% (not shown) are similar to that of CdS/rGO-5%. The CdS powder exhibits a sphere-like morphology and agglomerated particles with diameters 150 nm (Fig. 4a). Form the observation of the CdS/rGO-5% composite (Fig. 4b), GO has a sheet-like morphology with a highly wrinkled surface and many folded edges with a high degree of curvature.
The synthesized CdS nanoparticles are sphere-like and distributed over the rGO sheets, indicating an excellent interfacial interaction between CdS and rGO. Aggregated CdS nanoparticles are not observed on the rGO plane supports. The TEM image of the CdS/rGO-5% composite displayed in Fig. 4c shows that the CdS particles are distributed homogeneously over the rGO sheets; the CdS particles are 25 nm, which further confirms their good combination and thus avoids CdS nanoparticle agglomeration. High-resolution TEM analysis showed a lattice fringe with a d-spacing of 0.25 nm, which was assigned to the (111) lattice planes of cubic CdS (Fig. 4d). The addition of rGO provided small CdS particles with a high crystallinity. To
further
investigate
the
specific
surface
area
and
pore
structure,
nitrogen
adsorption/desorption isotherms of the sample were measured. As shown in Table 1, the BET surface area increased from 6.979 m2/g for pure CdS to 80.35 m2/g for the CdS/rGO-15% composite; this is most likely due to the contribution of rGO to the surface area. The high surface area structure of the CdS/rGO composite can improve the photocatalytic activity. Fig. 5 shows typical nitrogen adsorption/desorption isotherms and the corresponding curves of the pore size distribution. According to the IUPAC classification, adsorption isotherms can be grouped into six types. Typical pure CdS exhibits a type II isotherm, indicating a microporous structure [24]. On the other hand, the CdS/rGO composites exhibited a type IV isotherm with a typical H3 hysteresis loop in the relative pressure range of 0.4-1.0, suggesting the mesoporous structure of the materials [25]. The mesoporous structures can improve the transfer of photogenerated carriers within the hybrid to promote the kinetics of the photocatalytic reactions [26]. The pore size distribution was calculated using the Barrett–Joyner–Halenda model. The insets in Fig. 5 show the pore size distribution of the CdS/rGO composites. The pore size distribution ranges from 2-100 nm and is centered at 3.1 nm. However, the pore size of CdS is mainly in the range of 7-30 nm.
3.2 Chemical bonding of rGO/CdS FTIR spectroscopy was employed to analyze the presence/absence of oxygenated groups in the samples. Fig. 6 shows the FTIR spectra of the pristine graphite, GO, and CdS/rGO-5% powders.
The spectra contain bands associated with －OH at 3421 cm-1, C=O at 1740 cm-1, C=C at 1637 cm-1, O－H at 1400 cm-1, and C－O at 1039 cm-1 [27,28]. These bands, which are related to oxygenated groups, predominantly appear when graphite is converted to GO. These oxygen-related bands in the CdS/rGO-5% composite become weak or even vanish, indicating the removal of oxygenated groups and significant reduction of GO. The Raman spectra also confirm the reduction of GO. Fig. 7 shows the Raman spectra of the GO and CdS/rGO-5% powders. Two prominent peaks appear at around 1349 and 1613 cm-1, which are assigned to the D band and G band, respectively [29,30]. The intensity ratio of the D and G bands (ID/IG) increased from 1.060 for GO to 1.176 for the CdS/rGO-5% composite. This gives a clear indication of the effective reduction of GO. XPS was used to understand the surface chemical composition and the function groups of the samples. Fig. 8 shows the XPS spectra of the GO and CdS/rGO-5% powders, in which the major peaks, corresponding to Cd, S, C, and O elements, suggest the absence of impurities. The smaller O 1s peak for the CdS/rGO-5% composite than that of GO can be attributed to the incomplete reduction of GO and the presence of atmospheric H2O adsorbed on the surface. Fig. 8b and 8c show the C 1s spectra of the GO and rGO/CdS-5% powders, respectively, wherein the major binding energy contributions are assigned to bonding of C－C, C－O, and O－C=O groups. The intensity of the C－C peak increases and that of the oxygen-related peaks decreases, further confirming the reduction of GO [31]. Fig. 8d and 8e show the Cd 3d and S 2p peaks of the GO and CdS/rGO-5% powders, respectively. The binding energies for the Cd 3d and S 2p peaks are slightly higher than the standard values reported in the literature for CdS [32]. Pure CdS easily aggregated, while CdS uniformly deposited on the graphene sheets in the CdS/rGO composites. CdS/rGO with uniform and small CdS nanoparticles shows a high binding energy according to published results [33]. The correlated red shifts reflect electron transfer from the graphene sheets to CdS, leading to a strong interaction between the CdS nanoparticles and graphene sheet. The XPS data are consistent with the FTIR results, and these outcomes further identify the reduction of GO and the formation of composites in the ultrasonic process. Using ammonia as a solvent and reducing agent reduces GO to
rGO.
3.3 Optical properties of rGO/CdS Fig. 9 shows the UV-VIS absorbance spectra of the pristine graphite, GO, and CdS/rGO-5% powders. GO shows two strong absorbance peaks at 230 and 305 nm, corresponding to the π-π* transition of the sp2 C=C bonds and n-π* transition of the C=O bonds, respectively [34]. In comparison, the n-π* transition of the rGO/CdS-5% composite disappears and the π-π* transition is strongly red shifted from 230 to 240 nm; this indicates that many oxygenated groups were removed and the π-conjugation network was restored [35]. In comparison, the visible light absorption of the CdS/rGO-5% composite only shows a very marginal increase because of the very low CdS content. Nevertheless, based on the heterogeneous junction effect, a high photocatalytic efficiency can be expected.
3.4 Photodegradation of MB and the stability of CdS/rGO Fig. 10a shows the photocatalytic degradation efficiency of the MB solution under visible light irradiation for the CdS/rGO powders with various rGO contents. The absorptions at 0 min show the adsorption abilities of the photocatalysts in the dark. The direct decomposition of MB is very low under visible light irradiation, suggesting that MB is very stable under visible light. CdS exhibits a very poor photocatalytic performance upon visible light irradiation. The addition of rGO allows sufficient sensitivity to visible light. All the CdS/rGO composites exhibit good photocatalytic decomposition characteristics. Among them, CdS/rGO-10% has the best photocatalytic performance upon visible light irradiation. The considerable improvement in the photodegradation ability is due to the higher crystallinity, higher surface-to-volume ratio of the CdS particles, and lower carrier recombination rate, which is the result of tightly anchoring the CdS particles onto the rGO sheets. The photoefficiency depends on the charge transfer, electron-hole recombination, and light-absorption properties. During the reduction process, the uniform CdS nanoparticles grow well on the rGO sheets. Upon further increasing the rGO content (CdS/rGO-15%), the photocatalytic degradation efficiency of MB decreases, and too
much rGO can shielding the light of the active sites on the CdS nanoparticle surface. Because rGO can absorb some visible light, the increase of the rGO content, which causes light harvesting competition between CdS and rGO, decreases the photocatalytic performance. The suspension solution of passing by the photogenerated carriers is decreased; this has also been observed by other investigators [36,37]. The reuse and stability of the photocatalyst is a basic condition for practical applications. Fig. 10b shows a three-cycle photodegradation sequence for MB (at the same initial concentration) using the CdS and CdS/rGO-10% composite, respectively. There is no obvious deactivation of RGO–CdS-10% and the photodegradation efficiency after each run was 90.1%, 89.2% and 88.7%, while that for the pure CdS was 7%, 6.4% and 5.8%. This demonstrates that rGO can improve the stability of the nanocomposites for photocatalysis. rGO has a high electron conductivity that can accept and shuttle the photogenerated electrons, effectively prevent photocorrosion, and thus enhance the stability of CdS under visible light irradiation.
3.5 Photocatalytic mechanisms Fig. 11 shows the mechanism of photodegradation. Based on the reported conduction band (−0.95 eV) and valence band edge potential (1.45 eV) of CdS, and the work function (−0.08 eV) of graphene, the photoinduced electrons of CdS in the conduction band under visible light irradiation are transferred to the rGO sheets, leaving holes on the CdS valence band [38]. rGO can accelerate charge transportation and efficiently hinder the recombination rate of the photoinduced electron–hole pairs and separation because of its high carrier mobility. The electrons located on rGO (−0.08 eV) can reduce O2 to O2-, and the holes at the top of the valence band of CdS (1.45 eV) will simultaneously oxidize OH- (derived from absorbed water) to OH. These productions, such as O2−, OH, and h+, have a strong oxidative ability and can decompose the MB solution to CO2, H2O, or other small molecules. These well-separated photoinduced electrons and holes provide the high photocatalytic performance of the CdS/rGO composites. However, excess rGO content inhibits light from reaching the heterojunction between CdS and rGO and thus reduces the photocatalytic activity of the CdS/rGO composites.
4. Conclusion In summary, we presented an ultrasonic approach for a facile and simple synthesis of CdS/rGO composites by choosing ammonia as both the complexing agent of the Cd2+ ions and a reducing agent of GO. During the process, rGO and CdS were simultaneously reduced and exhibited strong interactions between them, providing a uniform dispersity of CdS particles on the rGO sheets. The pure CdS particles have a much poorer crystallinity and higher aggregation than the CdS/rGO composites. This observation indicates that rGO provides a good platform for nucleation and growth of CdS particles. No free CdS particles were observed outside of the graphene sheets. The photocatalytic efficiency for the degradation of MB under visible light irradiation utilizing CdS/rGO heteronanostructures was significantly higher than that using bare CdS particles because of the improved crystallinity, larger surface-to-volume ratio of CdS particles, and the suppression of electron–hole pair recombination. Based on these data, ultrasonic synthetic CdS/rGO composites may provide a manufacturing option for controllable and efficient visible light photocatalysis.
Acknowledgments The authors gratefully acknowledge the financial support for this research by the Ministry of Science and Technology of Taiwan under Grant No. NSC 106-2221-E-005-072-MY3. The present work was also supported in part by the Center for Micro/Nano Science and Technology of the National Cheng Kung University.
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hydrogels for energy storage, CrystEngComm, 18 (2016) 1090-1095. [32] P. Goa, J. Liu, D.D. Sun, W. Ng, Graphene oxide–CdS composite with high photocatalytic degradation and disinfection activities under visible light irradiation, J. Hazard. Mater, 250-251 (2013) 412-420. [33] J.Li, S.Tang, L.Lu, H.C. Zeng, Preparation of nanocomposites of metals, metal oxides, and carbon nanotubes via self-assembly, J. Am. Chem. Soc, 129 (2007) 9401–9409. [34] Y.N. Singhbabu, K.K. Sahu, D. Dadhich, A.K. Pramanick, T. Mishra, R.K. Sahu, Capsule-embedded reduced graphene oxide: synthesis, mechanism and electrical properties, J. Mater. Chem. C, 1 (2013) 958-966. [35] Y. Yang, X. Yang, W. Yang, S. Li, J. Xu, Y. Jiang, Porous conducting polymer and reduced graphene oxide nanocomposites for room temperature gas detection, RSC. Adv, 4 (2014) 42546-42553. [36] M. Fu, Q. Jiao, Y. Zhao, One-step vapor diffusion synthesis of uniform CdS quantum dots/reduced graphene oxide composites as efficient visible-light photocatalysts, RSC. Adv, 4 (2014) 23242-23250. [37] X. Zhao, S. Zhou, L.P. Jiang, W. Hou, Q. Shen, J.J. Zhu, Graphene–CdS Nanocomposites: Facile One-Step Synthesis and Enhanced Photoelectrochemical Cytosensing, Chem. Eur. J. 18 (2012) 4974-4981. [38] X. Wang, H. Tian, Y. Yang, H. Wang, S. Wang, W. Zheng, Y. Liu, Reduced graphene oxide/CdS for efficiently photocatalystic degradation of methylene blue, J. Alloys. Compd, 524 (2015) 5-12.
Fig. 1 Scheme of the one-step synthesis of CdS/rGO nanocomposites.
Fig. 2 Process of visible photocatalytic measurement.
Fig. 3 XRD patterns of sample graphite, GO, CdS, and CdS/rGO (3%, 5%, 10%, 15%).
Fig.4 SEM images of (a) CdS and (b) CdS/rGO-5%; TEM images of CdS/rGO-5% (c-d).
Fig.5 Nitrogen adsorption-desorption isotherms and corresponding pore size distribution curves (inset) of samples CdS and CdS/rGO (3%, 5% 10%, 15%).
Fig. 6 FTIR spectra of Graphite, GO, and CdS/rGO-5% composite.
Fig. 7 Raman spectra of GO and CdS/rGO-5% composite.
Fig.8 (a) XPS spectra of GO and CdS/rGO-5% ;(b and c) C1s peak of GO and CdS/rGO-5%; (d and e) Cd3d and S2p peak of CdS/rGO-5%.
Fig.9 UV-vis absorption spectra of Graphite, GO and CdS/rGO-5% composite.
Fig.10 (a) Degradation of MB catalyzed by samples CdS and CdS/rGO (3%, 5% 10%, 15%) under visible light irradiation. (b) Cycling runs in the photodegradation of MB with CdS and CdS/rGO-10% composite.
Fig. 11 Schematic diagram of energy levels of CdS/rGO photocatalyst.
Table 1. The specific surface areas of samples CdS and CdS/rGO (3%, 5%, 10%, 15%). Sample 2 -1
SBET(m g )
Pure CdS
CdS/rGO-3%
CdS/rGO-5%
CdS/rGO-10%
CdS/rGO-15%
6.97
13.6
42.78
59.93
80.35
Highlights
1. One-step synthesis of CdS/rGO nanocomposites use ultrasonic chemical method. 2. Ultrasonic chemical method could accelerate reaction rate, reduce processing
time, increase product yields, and strengthen the stability of composite materials, that method is simple, fast, environmentally benign and convenient.
3. CdS and reduced graphene oxide (CdS/rGO) at temperature as low as 70℃ employing ammonia as a complexing agent of Cd graphene oxide (GO).
2+
for 20 min by ions as well as a reducing agent of
S0169-4332(18)30330-1 https://doi.org/10.1016/j.apsusc.2018.01.305 APSUSC 38444
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
6 November 2017 23 January 2018 31 January 2018
Please cite this article as: Y-C. Lin, D-C. Tsai, Z-C. Chang, F-S. Shieu, Ultrasonic chemical synthesis of CdSreduced graphene oxide nanocomposites with an enhanced visible light photoactivity, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.01.305
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrasonic chemical synthesis of CdS-reduced graphene oxide nanocomposites with an enhanced visible light photoactivity Yi-Chen Lina, Du-Cheng Tsaia, Zue-Chin Changb, Fuh-Sheng Shieua,* a
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan
b
Department of Mechanical Engineering, National Chin-Yi University of Technology, Taichung 41170, Taiwan
Abstract
In this study, we report a facile ultrasonic method to prepare a series of CdS and reduced graphene oxide (CdS/rGO) composites with different weight ratios of graphene at temperature as low as 70 °C for 20 min by employing ammonia as a complexing agent of Cd2+ ions and reducing agent of graphene oxide (GO). Pure CdS particles had a poor crystallinity and aggregated to large particles size. As GO was incorporated into CdS, a uniform dispersion of CdS particles with high crystallinity on rGO sheets was clearly observed. The as-prepared CdS/rGO composites have a wide and strong photo absorption in the visible region and display a substantially improved photocatalytic activity for the degradation of methylene blue under visible light irradiation by forming a heterojunction of rGO and CdS. However, too much rGO will shield the light of the active sites for the CdS nanoparticle surface and thus limit further improvement in the photocatalytic efficiency.
Keywords: Reduce graphene oxide; Cadmium sulfide; Photocatalytic activity.
* Corresponding author at: Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan. Tel.: +886 4 2284 0500; fax: +886 4 2285 7017. E-mail address: [email protected] (F.S. Shieu).
1. Introduction
Photocatalysts have gained much attention for wastewater treatment because of their rapid oxidation, high oxidation effect, and low or no formation of byproducts [1-4]. Until now, TiO2 and ZnO photocatalysts have been widely applied for the photodegradation of dyes because of their high chemical stability and high photocatalytic activity [5,6]. However, one of their main drawbacks is that they only absorb UV light, which is only about 3% of the solar spectrum. Thus, various materials with smaller band gaps have been explored for the photodegradation of dyes under visible light irradiation [7]. Among these materials, CdS has shown a high potential as a visible-light-driven photocatalyst because of its suitable band gap (around 2.4 eV) and high absorption coefficient (>104) [8]. Nevertheless, the small surface area, serious photocorrosion, and high electron-hole recombination of CdS results in a low photocatalytic activity [9]. Many strategies for improving the photocatalytic activities of CdS have been applied over the past several years. Among these strategies, heterogeneous photocatalysis has been extensively considered to lower the electron-hole recombination rate of CdS to improve its photocatalytic activity. Graphene-based materials have a unique sheet-like morphology, high electron conductivity, and high mobility [10-12]. Therefore, combining CdS and graphene can accelerate the charge transportation and separation, and remarkably improve the photocatalytic efficiency under visible light irradiation [13]. Recently, several methods have been reported for synthesizing graphene/CdS composites. For example, Zhiyong et al. [14] prepared graphene-CdS directly from graphene oxide (GO) with ethylene glycol through a simple solvothermal method and showed an excellent degradation for 70% CdS. Nan et al. [15] synthesized graphene/CdS nanocomposites using solvent-exfoliated graphene instead of GO and found that it could decrease the defect density of graphene and efficiently enhance the photocatalytic activity. Fengzhen et al. [16] synthesized graphene/CdS nanorods using a solvothermal method with ethylene diamine and tested the photocatalytic degradation of rhodamine B under visible light illumination. Xinwei et al. [17] synthesized ZnCdS/reduced GO (rGO) using a one-pot hydrothermal reaction in an autoclave (180 °C for 12 h); it provided an excellent degradation (98%) of methylene blue (MB) in 120
min and showed a good stability and reproducibility. Hongwei et al. [18] synthesized a ternary hybrid of meso-TiO2/rGO/CdS using a photo-assisted treatment to reduce GO; they achieved degradation of methylene orange using meso-TiO2 and improved the degradation rate of the loaded-CdS by approximately a factor of 17. In these reports, CdS can be well dispersed on graphene sheets, preventing the aggregation of CdS particles and inhibiting the stacking of graphene. Efficient charge transfer occurs between CdS and graphene because of their uniform heterogenous junction. However, their synthesis methods require an autoclave, high temperature process, and/or complicated manipulation, hindering the progress of the above methods. Therefore, reducing the processing time, temperature, and cost, and developing an environmentally friendly strategy is very important for the synthesis of CdS/graphene composites. In this study, we develop an improved synthetic method for preparing CdS/graphene composites. Cadmium acetate and thiourea were used as cadmium and sulfur sources, respectively. Ammonia water was used as a complexing agent of Cd2+ ions and as a reducing agent for GO. A series of CdS/graphene composites with different graphene weight ratios was synthesized by a facile, one-step ultrasonic chemical method. The process temperature can be as low as 70 °C, and the synthesis time can be as low as 20 min. The raw materials are readily available, and the synthetic procedure is simple and safe. The ultrasonic reaction involves two mechanisms: reduction of GO to rGO and deposition of CdS particles onto the surface of the rGO sheets. The electron-rich Cd atoms and electrophilic carbon atoms of rGO show a strong interaction, which allows the CdS particles to tightly adhere to the rGO sheets. Meanwhile, the ultrasonic condition facilitates rGO to accommodate the maximum amount of CdS particles with an unstacked active surface. Such a composite structure inhibits particle aggregation and offers nucleation and growth platforms for CdS, resulting in a higher product crystallinity and a smaller particle size distribution. As expected, the resultant CdS/rGO composite improves the photocatalytic efficiency for the degradation of MB under visible light irradiation.
2. Experimental
2.1 Materials All chemicals were of analytic grade and used without further purification. Graphite powder (Choneye Pure Chemicals, Taipei, Taiwan) was used to prepare GO. Cadmium acetate (Echo Chemical, Miaoli, Taiwan), ammonia water (Choneye Pure Chemicals, Taipei, Taiwan), and thiourea (Shimakyu's Pure Chemicals, Osaka, Japan) were used as precursors to obtain CdS.
2.2 Synthesis of GO GO was prepared according to a modified Hummer’s method [19]. Graphite powder (2 g) was mixed with 0.6 g of NaNO3 in a 20-mL H2SO4 solution in an ice bath and stirred for 30 min. Then, 10 g of KMnO4 was added to the mixture, and the solution was stirred at 35 °C for 3 h. Next, 180 mL of deionized (DI) water was slowly added and maintained at 98 °C for 2 h. Afterward, 20 mL of 30% H2O2 was added to the solution to reduce the residual KMnO4. Finally, the precipitate was centrifuged and washed with 5% HCl and DI water until the pH value reached 7, and then dried at 70 °C for 24 h.
2.3 Synthesis of CdS and the CdS/rGO composite material A mixed solvent containing 50 mL of DI water, various amounts of GO, 0.1 g of cadmium acetate, and 1.1 g of thiourea was prepared. Then, ammonia was added to the solvent to adjust the pH to 11. The synthesis process for the resulting mixture was carried out under a 50/60-Hz ultrasonic shock using an ultrasonic apparatus (E30, Elma Co., Germany) for 20 min at 70 °C. After the reaction was complete, the mixture was centrifuged, washed several times with DI water and absolute ethanol, and then dried at 70 °C for 24 h to obtain the CdS/rGO composites. The as-synthesized CdS/rGO samples with 3, 5, 10, and15 wt% rGO, named as CdS/rGO-3%, CdS/rGO-5%, CdS/rGO-10%, and CdS/rGO-15%, respectively, were isolated by filtration. Typically, CdS/rGO-5% contained 5 mg of GO and 0.1 g of CdS. For reference, the pure CdS
particles were synthesized using a similar method without GO. According to the above details, a possible synthesis route for the CdS/rGO composite is shown in Fig. 1.
2.4 Characterization Microstructural characterization of the synthesized samples was performed using a wide-angle X-ray diffractometer (XRD; MAC MXPIII) operated at 40 kV and 30 mA with Cu Kα radiation. The particle size and morphology of the samples were analyzed by transmission electron microscopy (TEM, JEOL 200CX) and scanning electron microscopy (SEM, JEOL JSM-6700F). The Brunauer-Emmett-Teller (BET) specific surface area and porosity were analyzed based on nitrogen adsorption-desorption isotherms using a Micromeritics ASAP 2020 nitrogen adsorption apparatus. Fourier transform infrared (FTIR) spectroscopy measurements were carried out using a Nicolet 380 infrared spectrometer. Raman spectra were recorded on an Andor DU401-BV multichannel confocal microspectrometer with 632-nm laser excitation. X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe) was performed using monochromated Al Kα radiation (1486.8 eV). The optical properties were measured using a Shimadzu UV3600 spectrophotometer.
2.5 Measurement of the photocatalytic degradation of MB Photocatalytic activity experiments were performed for the photodegradation of MB in a 300-mL double-walled glass beaker with cooling water at 20 °C. A 500-W xenon arc lamp (Prosper OptoElectronic Co., Ltd.) with a heat filter, UV-cutoff filter (420 nm), and IR-cutoff filter (750 nm) was used as a visible light source to trigger the photocatalytic reaction and was positioned 20 cm from the reactor. The focused intensity on the flask was about 700 W/m2, which was measured using an SPM-1116SD visible light radiometer (Lutron Electronics, Inc.) in the range of 400-1100 nm. In a typical experiment, 10 mg of the photocatalyst was suspended in 150 mL of an MB (0.01 mg/mL) solution under magnetic stirring. The temperature of the solution was maintained at room temperature by a flow of cooling water. The degradation of the MB solution with irradiation time was determined by recording UV-VIS absorption spectra using a Shimadzu UV3600 spectrophotometer.
Fig. 2 shows the process of the visible photocatalytic measurement in a typical photocatalytic experiment.
3. Results and discussion
3.1 Structure, morphology, and composition of CdS/rGO Fig. 3 shows the XRD patterns of pristine graphite, GO, CdS, CdS/rGO-3%, CdS/rGO-5%, CdS/rGO-10%, and CdS/rGO-15%. The graphite powder presents a strong diffraction peak at 2θ = 26.5° with an interlayer spacing of 0.33 nm, which can be indexed to the (002) plane of the hexagonal structure of graphite. After oxidation through the modified Hummers process, the GO powder is obtained and a (001) diffraction peak is observed at 2θ = 10.29°, which is typical for GO [20]. The increased interlayer distance is due to the introduction of many oxygen-containing groups on the basal plane [21]. The diffraction peaks of the bare CdS powder are well indexed to the crystal planes of a cubic CdS phase (JCPDS NO, 80-0019) [22]. The weak intensity of the peak indicates the poor crystallinity of the pure CdS powder. The diffraction peaks of the CdS/rGO composite are also well matched to that of cubic CdS. However, the peak intensities are much stronger than those for pure CdS. The patterns do not contain characteristic diffraction peaks of the carbon species because of the low diffraction intensity of graphene. The XRD patterns also indicate that the enhanced graphene content increases the crystallinity of the CdS/rGO particles [23]. After introducing 5% graphene, the XRD peaks of CdS/rGO-5% become stronger and narrower due to the improved crystallinity of CdS particles. Thus, rGO constitutes a superior platform for nucleation and growth of CdS, implying a strong interaction between CdS and rGO. Fig. 4 shows the SEM images of the CdS and CdS/rGO-5% powders. The morphologies of CdS/rGO-3%, CdS/rGO-10%, and CdS/rGO-15% (not shown) are similar to that of CdS/rGO-5%. The CdS powder exhibits a sphere-like morphology and agglomerated particles with diameters 150 nm (Fig. 4a). Form the observation of the CdS/rGO-5% composite (Fig. 4b), GO has a sheet-like morphology with a highly wrinkled surface and many folded edges with a high degree of curvature.
The synthesized CdS nanoparticles are sphere-like and distributed over the rGO sheets, indicating an excellent interfacial interaction between CdS and rGO. Aggregated CdS nanoparticles are not observed on the rGO plane supports. The TEM image of the CdS/rGO-5% composite displayed in Fig. 4c shows that the CdS particles are distributed homogeneously over the rGO sheets; the CdS particles are 25 nm, which further confirms their good combination and thus avoids CdS nanoparticle agglomeration. High-resolution TEM analysis showed a lattice fringe with a d-spacing of 0.25 nm, which was assigned to the (111) lattice planes of cubic CdS (Fig. 4d). The addition of rGO provided small CdS particles with a high crystallinity. To
further
investigate
the
specific
surface
area
and
pore
structure,
nitrogen
adsorption/desorption isotherms of the sample were measured. As shown in Table 1, the BET surface area increased from 6.979 m2/g for pure CdS to 80.35 m2/g for the CdS/rGO-15% composite; this is most likely due to the contribution of rGO to the surface area. The high surface area structure of the CdS/rGO composite can improve the photocatalytic activity. Fig. 5 shows typical nitrogen adsorption/desorption isotherms and the corresponding curves of the pore size distribution. According to the IUPAC classification, adsorption isotherms can be grouped into six types. Typical pure CdS exhibits a type II isotherm, indicating a microporous structure [24]. On the other hand, the CdS/rGO composites exhibited a type IV isotherm with a typical H3 hysteresis loop in the relative pressure range of 0.4-1.0, suggesting the mesoporous structure of the materials [25]. The mesoporous structures can improve the transfer of photogenerated carriers within the hybrid to promote the kinetics of the photocatalytic reactions [26]. The pore size distribution was calculated using the Barrett–Joyner–Halenda model. The insets in Fig. 5 show the pore size distribution of the CdS/rGO composites. The pore size distribution ranges from 2-100 nm and is centered at 3.1 nm. However, the pore size of CdS is mainly in the range of 7-30 nm.
3.2 Chemical bonding of rGO/CdS FTIR spectroscopy was employed to analyze the presence/absence of oxygenated groups in the samples. Fig. 6 shows the FTIR spectra of the pristine graphite, GO, and CdS/rGO-5% powders.
The spectra contain bands associated with －OH at 3421 cm-1, C=O at 1740 cm-1, C=C at 1637 cm-1, O－H at 1400 cm-1, and C－O at 1039 cm-1 [27,28]. These bands, which are related to oxygenated groups, predominantly appear when graphite is converted to GO. These oxygen-related bands in the CdS/rGO-5% composite become weak or even vanish, indicating the removal of oxygenated groups and significant reduction of GO. The Raman spectra also confirm the reduction of GO. Fig. 7 shows the Raman spectra of the GO and CdS/rGO-5% powders. Two prominent peaks appear at around 1349 and 1613 cm-1, which are assigned to the D band and G band, respectively [29,30]. The intensity ratio of the D and G bands (ID/IG) increased from 1.060 for GO to 1.176 for the CdS/rGO-5% composite. This gives a clear indication of the effective reduction of GO. XPS was used to understand the surface chemical composition and the function groups of the samples. Fig. 8 shows the XPS spectra of the GO and CdS/rGO-5% powders, in which the major peaks, corresponding to Cd, S, C, and O elements, suggest the absence of impurities. The smaller O 1s peak for the CdS/rGO-5% composite than that of GO can be attributed to the incomplete reduction of GO and the presence of atmospheric H2O adsorbed on the surface. Fig. 8b and 8c show the C 1s spectra of the GO and rGO/CdS-5% powders, respectively, wherein the major binding energy contributions are assigned to bonding of C－C, C－O, and O－C=O groups. The intensity of the C－C peak increases and that of the oxygen-related peaks decreases, further confirming the reduction of GO [31]. Fig. 8d and 8e show the Cd 3d and S 2p peaks of the GO and CdS/rGO-5% powders, respectively. The binding energies for the Cd 3d and S 2p peaks are slightly higher than the standard values reported in the literature for CdS [32]. Pure CdS easily aggregated, while CdS uniformly deposited on the graphene sheets in the CdS/rGO composites. CdS/rGO with uniform and small CdS nanoparticles shows a high binding energy according to published results [33]. The correlated red shifts reflect electron transfer from the graphene sheets to CdS, leading to a strong interaction between the CdS nanoparticles and graphene sheet. The XPS data are consistent with the FTIR results, and these outcomes further identify the reduction of GO and the formation of composites in the ultrasonic process. Using ammonia as a solvent and reducing agent reduces GO to
rGO.
3.3 Optical properties of rGO/CdS Fig. 9 shows the UV-VIS absorbance spectra of the pristine graphite, GO, and CdS/rGO-5% powders. GO shows two strong absorbance peaks at 230 and 305 nm, corresponding to the π-π* transition of the sp2 C=C bonds and n-π* transition of the C=O bonds, respectively [34]. In comparison, the n-π* transition of the rGO/CdS-5% composite disappears and the π-π* transition is strongly red shifted from 230 to 240 nm; this indicates that many oxygenated groups were removed and the π-conjugation network was restored [35]. In comparison, the visible light absorption of the CdS/rGO-5% composite only shows a very marginal increase because of the very low CdS content. Nevertheless, based on the heterogeneous junction effect, a high photocatalytic efficiency can be expected.
3.4 Photodegradation of MB and the stability of CdS/rGO Fig. 10a shows the photocatalytic degradation efficiency of the MB solution under visible light irradiation for the CdS/rGO powders with various rGO contents. The absorptions at 0 min show the adsorption abilities of the photocatalysts in the dark. The direct decomposition of MB is very low under visible light irradiation, suggesting that MB is very stable under visible light. CdS exhibits a very poor photocatalytic performance upon visible light irradiation. The addition of rGO allows sufficient sensitivity to visible light. All the CdS/rGO composites exhibit good photocatalytic decomposition characteristics. Among them, CdS/rGO-10% has the best photocatalytic performance upon visible light irradiation. The considerable improvement in the photodegradation ability is due to the higher crystallinity, higher surface-to-volume ratio of the CdS particles, and lower carrier recombination rate, which is the result of tightly anchoring the CdS particles onto the rGO sheets. The photoefficiency depends on the charge transfer, electron-hole recombination, and light-absorption properties. During the reduction process, the uniform CdS nanoparticles grow well on the rGO sheets. Upon further increasing the rGO content (CdS/rGO-15%), the photocatalytic degradation efficiency of MB decreases, and too
much rGO can shielding the light of the active sites on the CdS nanoparticle surface. Because rGO can absorb some visible light, the increase of the rGO content, which causes light harvesting competition between CdS and rGO, decreases the photocatalytic performance. The suspension solution of passing by the photogenerated carriers is decreased; this has also been observed by other investigators [36,37]. The reuse and stability of the photocatalyst is a basic condition for practical applications. Fig. 10b shows a three-cycle photodegradation sequence for MB (at the same initial concentration) using the CdS and CdS/rGO-10% composite, respectively. There is no obvious deactivation of RGO–CdS-10% and the photodegradation efficiency after each run was 90.1%, 89.2% and 88.7%, while that for the pure CdS was 7%, 6.4% and 5.8%. This demonstrates that rGO can improve the stability of the nanocomposites for photocatalysis. rGO has a high electron conductivity that can accept and shuttle the photogenerated electrons, effectively prevent photocorrosion, and thus enhance the stability of CdS under visible light irradiation.
3.5 Photocatalytic mechanisms Fig. 11 shows the mechanism of photodegradation. Based on the reported conduction band (−0.95 eV) and valence band edge potential (1.45 eV) of CdS, and the work function (−0.08 eV) of graphene, the photoinduced electrons of CdS in the conduction band under visible light irradiation are transferred to the rGO sheets, leaving holes on the CdS valence band [38]. rGO can accelerate charge transportation and efficiently hinder the recombination rate of the photoinduced electron–hole pairs and separation because of its high carrier mobility. The electrons located on rGO (−0.08 eV) can reduce O2 to O2-, and the holes at the top of the valence band of CdS (1.45 eV) will simultaneously oxidize OH- (derived from absorbed water) to OH. These productions, such as O2−, OH, and h+, have a strong oxidative ability and can decompose the MB solution to CO2, H2O, or other small molecules. These well-separated photoinduced electrons and holes provide the high photocatalytic performance of the CdS/rGO composites. However, excess rGO content inhibits light from reaching the heterojunction between CdS and rGO and thus reduces the photocatalytic activity of the CdS/rGO composites.
4. Conclusion In summary, we presented an ultrasonic approach for a facile and simple synthesis of CdS/rGO composites by choosing ammonia as both the complexing agent of the Cd2+ ions and a reducing agent of GO. During the process, rGO and CdS were simultaneously reduced and exhibited strong interactions between them, providing a uniform dispersity of CdS particles on the rGO sheets. The pure CdS particles have a much poorer crystallinity and higher aggregation than the CdS/rGO composites. This observation indicates that rGO provides a good platform for nucleation and growth of CdS particles. No free CdS particles were observed outside of the graphene sheets. The photocatalytic efficiency for the degradation of MB under visible light irradiation utilizing CdS/rGO heteronanostructures was significantly higher than that using bare CdS particles because of the improved crystallinity, larger surface-to-volume ratio of CdS particles, and the suppression of electron–hole pair recombination. Based on these data, ultrasonic synthetic CdS/rGO composites may provide a manufacturing option for controllable and efficient visible light photocatalysis.
Acknowledgments The authors gratefully acknowledge the financial support for this research by the Ministry of Science and Technology of Taiwan under Grant No. NSC 106-2221-E-005-072-MY3. The present work was also supported in part by the Center for Micro/Nano Science and Technology of the National Cheng Kung University.
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Fig. 1 Scheme of the one-step synthesis of CdS/rGO nanocomposites.
Fig. 2 Process of visible photocatalytic measurement.
Fig. 3 XRD patterns of sample graphite, GO, CdS, and CdS/rGO (3%, 5%, 10%, 15%).
Fig.4 SEM images of (a) CdS and (b) CdS/rGO-5%; TEM images of CdS/rGO-5% (c-d).
Fig.5 Nitrogen adsorption-desorption isotherms and corresponding pore size distribution curves (inset) of samples CdS and CdS/rGO (3%, 5% 10%, 15%).
Fig. 6 FTIR spectra of Graphite, GO, and CdS/rGO-5% composite.
Fig. 7 Raman spectra of GO and CdS/rGO-5% composite.
Fig.8 (a) XPS spectra of GO and CdS/rGO-5% ;(b and c) C1s peak of GO and CdS/rGO-5%; (d and e) Cd3d and S2p peak of CdS/rGO-5%.
Fig.9 UV-vis absorption spectra of Graphite, GO and CdS/rGO-5% composite.
Fig.10 (a) Degradation of MB catalyzed by samples CdS and CdS/rGO (3%, 5% 10%, 15%) under visible light irradiation. (b) Cycling runs in the photodegradation of MB with CdS and CdS/rGO-10% composite.
Fig. 11 Schematic diagram of energy levels of CdS/rGO photocatalyst.
Table 1. The specific surface areas of samples CdS and CdS/rGO (3%, 5%, 10%, 15%). Sample 2 -1
SBET(m g )
Pure CdS
CdS/rGO-3%
CdS/rGO-5%
CdS/rGO-10%
CdS/rGO-15%
6.97
13.6
42.78
59.93
80.35
Highlights
1. One-step synthesis of CdS/rGO nanocomposites use ultrasonic chemical method. 2. Ultrasonic chemical method could accelerate reaction rate, reduce processing
time, increase product yields, and strengthen the stability of composite materials, that method is simple, fast, environmentally benign and convenient.
3. CdS and reduced graphene oxide (CdS/rGO) at temperature as low as 70℃ employing ammonia as a complexing agent of Cd graphene oxide (GO).
2+
for 20 min by ions as well as a reducing agent of