Graphene-enhanced visible-light photocatalysis of large-sized CdS particles for wastewater treatment
© Lü et al.; licensee Springer. 2014
Received: 4 March 2014
Accepted: 16 March 2014
Published: 26 March 2014
The hybrid composites of graphene decorated by large-sized CdS particles (G/M-CdS) were prepared by a one-pot solvothermal route in which the reduction of graphite oxide into graphene was accompanied by the generation of microsized CdS particles. The structure and composition of the obtained nanocomposites were studied by means of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The CdS particles with the average sizes of approximately 640 nm were formed on graphene sheets. The as-prepared composite was used as adsorbent to remove dye from wastewater using the organic dye Rhodamine B as the adsorbate. The G/M-CdS composite reveals a high photodegradation rate under visible light irradiation. Our results demonstrate that the G/M-CdS is very promising for removing organic dyes from wastewater.
KeywordsCdS Graphene Photocatalysis Wastewater treatment
In recent years, the growing concerns about energy and environmental problems have stimulated extensive research on solar energy utilization. It is well known that dyes are widely used in various fields, but their discharge into water could cause environmental pollutions since most of the dyes are harmful. Therefore, various strategies are explored to photocatalytic degradation of organic dyes using semiconductor photocatalysts. In particular, the carbon nanostructures, acting as outstanding electron acceptors and highly conductive scaffolds, have found their applications in photocatalysis[1–4]. Commonly used adsorbents can suffer from low adsorption capacities and separation inconveniences. Therefore, the exploration of new promising adsorbents is still desirable.
Graphene with atomically thin and two-dimensional conjugated structure, exhibits high conductivity as well as thermal, chemical, mechanical, and optical stability and a high specific surface area[5–8]. These outstanding advantages allowed graphene to be utilized as a promising adsorbent supporting material to remove pollutants from aqueous solution[9–14]. CdS is an important II–VI semiconductor, it can be potentially applied in many fields such as light-emitting diodes, thin film transistors, solar cells, and photocatalysts[15–19]. The narrower bandgap of CdS than that of TiO2 facilitates the utilization of visible light, which makes CdS a competitive candidate as photocatalyst. When CdS is irradiated by visible light, electrons located in the valence band can be excited to the conduction band, forming electron-hole pairs, which are responsible for the photocatalytic activity. Disadvantageously, the rapid recombination of the excited electron-hole pairs is an obstacle limiting the photocatalytic activity of catalysts. The ways to delay the electron-hole pair recombination of CdS include the hybrid of CdS with other semiconductors[20, 21], noble metals, or loaded CdS on support materials with high surface areas or combining CdS with conductive supports. The nanocomposites composed of CdS and graphene showed significantly improved properties in electrocatalysis, supercapacitor, high-performance lithium ion batteries, etc. As for graphene-based composite photocatalysts, the π-π conjugation net and the conductivity made graphene an efficient electron acceptor, when the semiconductors were excited, the electrons at the interface could be transferred to graphene and stabilized by the conjugation net, retarding the charge recombination. The applications of graphene-CdS nanocomposites as the adsorbent for the extraction of organic pollutants have been reported[25–30]. The above methods share one common feature: nanoscaled CdS nanocrystals were attached onto the surface of graphene. Very recently, Wang et al. reported the photocatalysis investigation using nest-like CdS-graphene composite, and the nest-like CdS structure with an average diameter of about 1 μm is composed of many branches with approximately 5-nm diameter. It is well known that the size of inorganic materials have a large influence on their widely changing chemical or physical properties. However, the photocatalysis properties of CdS microparticles-graphene composites (G/M-CdS) have not been really reported previously.
Herein, we synthesized the G/M-CdS composites by one-step hydrothermal method. Its practical application potential in the removal of dyes from aqueous solution was investigated. As indicated previously, organic dyes are widely used in various fields, which are the main organic pollutant source in water. These dyes own the same feature on structure in that benzene rings are included. Therefore, in order to evaluate the adsorption performance and photocatalytic activity of the G/M-CdS, one representative organic dye including benzene rings should be chosen. Rhodamine B (Rh.B) is a chemical compound and a typical dye, which is often used as a tracer dye within water and is used extensively in biotechnology applications. Thus, Rh.B was selected as model organic pollutant in this work. The results exhibit that the G/M-CdS composites possesses very efficient adsorption and photodegradation ability. To the best of our knowledge, this is the first attempt to treat wastewater with large CdS particle/graphene composites.
All the chemicals and reagents were of analytical purity and used without further purifications. CdCl2 · 2.5H2O, Na2S2O3 · 5H2O and Rh.B were purchased from Aladdin. Water used in all experiments was doubly distilled and purified by a Milli-Qsystem (Billerica, MA, USA). Transmission electron microscopy (TEM) images were obtained using a JEOL2010 transmission electron microscope (Akishima-shi, Japan). The powder X-ray diffraction (XRD) measurements were performed using a D-MAXIIA X-ray diffractometer (Rigaku, Shibuya-ku, Japan) with CuKa radiation (λ = 1.5406 Å). The concentrations of dye solutions were measured using a UV-2501 spectrophotometer (Shimadzu, Kyoto, Japan).
Graphite oxide (GO) was synthesized from natural graphite powder (spectral requirement, Shanghai Chemicals, Shanghai, China) according to a modified Hummers method. The G/M-CdS composite was prepared according to previous reports[32, 33]. Typically, 9 mg of GO was dispersed in 30 mL of deionized water by ultrasonication for 1 h. Then 1.5 mmol CdCl2 · 2.5H2O was added followed by 30-min stirring. Subsequently, 1.5 mmol Na2S2O3 · 5H2O was added. After 15-min stirring, the solution was transferred into a Teflon-lined stainless steel autoclave (50 mL) and reacted under 160°C for 10 h. After cooling to room temperature, the obtained solution was then centrifuged and washed by deionized water several times. Finally, the formed G/M-CdS composites were dried in a vacuum drier. For comparison, CdS microparticles (MPs) were also synthesized under the same reaction condition without adding GO.
where C0 (mg/L) represents the initial dye concentration, Ceq (mg/L) is the equilibrium concentration of the dye remaining in the solution every test, V (L) is the volume of the aqueous solution, and m (g) is the weight of the G/M-CdS composite.
Photocatalytic experiments were conducted to photocatalytically degrade Rh.B in water under visible light irradiation. A domestic visible light lamp (11 W) was used as a light source and set about 10 cm from the reactor. Experiments were carried out at ambient temperature. The reaction suspension was prepared in the same fashion as in the adsorption experiments. Before irradiation, the solutions were stirred in the dark in order to reach the adsorption-desorption equilibrium. At different irradiation time intervals, analytical samples were taken from the reaction suspension and centrifuged to remove the photocatalyst particles. The concentrations of the remnant Rh.B were monitored by checking the absorbance of solutions.
Results and discussion
In summary, we have successfully prepared G/M-CdS composites via an effective solvothermal method. Their ability of extraction of dye from aqueous solution was examined using Rh.B as adsorbate. The photocatalytic activity measurements demonstrate that the G/M-CdS photocatalysts show superior photoactivity in degradation of Rh.B under visible light irradiation. The present work opens up a new avenue to preparing G-based composite materials and provides new insights into the photocatalytic degradation of dyes under visible light irradiation.
CdS microparticles-graphene composites
transmission electron microscopy
Financial support from the National Natural Science Foundation of China (authorization numbers: 61376020, 21301167) and the Natural Science Foundation of Jilin Province (20130101009JC), China are acknowledged.
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