Synthesis and highly visible-induced photocatalytic activity of CNT-CdSe composite for methylene blue solution
© Chen and Oh; licensee Springer. 2011
Received: 24 April 2011
Accepted: 26 May 2011
Published: 26 May 2011
Carbon nanotube-cadmium selenide (CNT-CdSe) composite was synthesized by a facile hydrothermal method derived from multi-walled carbon nanotubes as a stating material. The as-prepared products were characterized by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectrophotometer. The as-synthesized CNT-CdSe composite efficiently catalyzed the photodegradation of methylene blue in aqueous solutions under visible-light irradiation, exhibiting higher photocatalytic activity.
Environmental problems such as toxic organic pollutants provide the impetus for fundamental and applied research into environmental areas. Semiconductor photocatalysts have attracted considerable attention for a long time in the fields of photochemistry [1–5] because of their usefulness with regard to solving environmental problems. Over the last few years, considerable efforts have been made in the controlled synthesis of various nanoscaled materials to improve their properties for photocatalysis. Cadmium selenide (CdSe) is an n-type semiconductor. Its bandgap energy was reported to be in the range from 1.65 to 1.8 eV [6–9]. CdSe was found to be suitable for various optoelectronic applications such as light-emitting diodes, laser diodes [10–13], catalysis , solar cells , and biological labeling .
More recently, many groups have synthesized CdSe nanomaterials with high photocatalytic activity in the degradation of organic pollutants under UV light irradiation, such as CdSe-Pt nanorods and nanonets , hybrid CdSe-Au nanodumbbells , CdSe/ZnS-photosensitized nano-TiO2 film . Therefore, as an important semiconductor, CdSe is an effective catalyst for photocatalytic degradation of organic pollutants. However, a few recent papers have discussed the preparation and properties of CdSe combining with carbon nanotubes (CNTs) composite. Since the discovery of the CNTs [19, 20], they have attracted much attention because their unique mechanical, optical, and electrical properties that may impact many fields of science and technology [21–24]. However, the functionalization of CNTs requires chemical modification of their surface, in order to form the functional groups on the surface.
In this paper, the multi-walled carbon nanotubes (MWCNTs) were used as start material and functionalized by m-chlorperbenzoic acid (MCPBA). Then the CNT-CdSe composite were prepared directly via a conventional hydrothermal method. The intrinsic characteristics of resulting composite were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX), transmission electron microscopy (TEM) analysis and UV-vis diffuse reflectance spectrophotometer. The photocatalytic activity of the as-synthesized samples was evaluated by degrading methylene blue (MB) under irradiation of visible light.
Crystalline MWCNTs powder (diameter, 5~20 nm; length, ~10 μm) of 95.9 wt.% purity from Carbon Nano-material Technology Co., Ltd., Pohang-si, Gyungbuk-do, Korea was used as a starting material. For the oxidization of MWCNTs, MCPBA was chosen as the oxidizing agent which purchased from Acros Organics, New Jersey, USA. Benzene (99.5%) was used as the organic solvent which purchased from Samchun Pure Chemical Co., Ltd, Seoul, Korea. Cadmium acetate dihydrate (Cd(CH3COO)2, 98%), selenium metal powder, and ammonium hydroxide (NH4OH, 28%) were purchased from Dae Jung Chemicals & Metal Co., Ltd, Siheung-si, Gyonggi-do, Korea. Anhydrous purified sodium sulfite (Na2SO3, 95%) was purchased from Duksan Pharmaceutical Co., Ltd, Ansan-si, Gyeonggi-do, Korea. The MB (C16H18N3S·Cl, 99.99+%) was used as model pollutant which purchased from Duksan Pure Chemical Co., Ltd, Ansan-si, Gyeonggi-do, Korea. All chemicals used without further purification and all experiments were carried out using distilled water.
Synthesis of CdSe and CNT-CdSe composite
Synthesis of CdSe
For the synthesis of CdSe compound, the sodium seleno sulfite (Na2SeSO3) solution and Cd(NH3)4 2+ solution was prepared at first. Na2SO3 (5 g) and selenium metal powder (0.5 g) were dissolved in 30-mL distilled water and refluxed for 1 h to form Na2SeSO3 solution. Meanwhile, Cd(CH3COO)2 (0.5 g) was dissolved in 2-mL distilled water. NH4OH (6 mL) was added to it and the mixture was stirred till it dissolved completely to form Cd(NH3)4 2+ solution. Finally, Cd(NH3)4 2+ and Na2SeSO3 solutions were mixed together and the mixture was stirred and refluxed for at least 5 h. After the temperature of the mixture was brought down to room temperature, the mixture was filtered through Whatman filter paper. The solid obtained was collected and washed with distilled water for five times. After being dried in vacuum at 353 K for 8 h, the CdSe compound was obtained.
Synthesis of CNT-CdSe composite
For preparation of the CNT-CdSe composite, the MWCNTs had to functionalize by MCPBA at first. MCPBA (1 g) was melted in 60 mL benzene, and then 0.5 g MWCNTs was put into the oxidizing agent. The mixture was stirred with a magnet for 6 h at 343 K. Then the MWCNTs was dried at 373 K and spared.
XRD (Shimadz XD-D1, Uki, Kumamoto, Japan) result was used to identify the crystallinity with monochromatic high-intensity CuKα radiation (λ = 1.5406 Å). SEM (JSM-5600, JEOL Ltd., Tokyo, Japan) was used to observe the surface state and structure of prepared composite using an electron microscope. Transmission electron microscopy (TEM, Jeol, JEM- 2010, Japan) was used to determine the state and particle size of prepared composite. TEM at an acceleration voltage of 200 kV was used to investigate the number and the stacking state of graphene layers on various samples. TEM specimens were prepared by placing a few drops of sample solution on a carbon grid. The element mapping over the desired region of prepared composite was detected by an EDX analysis attached to SEM. UV-vis diffuse reflectance spectra were obtained using an UV-vis spectrophotometer (Neosys-2000, Scinco Co. Ltd., Seoul, Korea) by using BaSO4 as a reference at room temperature and were converted from reflection to absorbance by the Kubelka-Munk method.
Photocatalytic activity measurements
The photocatalytic activity under visible lamp (KLD-08L, 220 V, 50-60 Hz, 8 W, pure white, λ > 420 nm, Fawoo Tech Co., Ltd., Tokyo, Japan) irradiation of the CNT-CdSe composite was evaluated by using MB as the model substrate. In an ordinary photocatalytic test performed at room temperature, 0.05 g CNT-CdSe composite was added to 50 mL of 1.0 × 10-5-mol/L MB solution, which was hereafter considered as the initial concentration (c 0). Before turning on the visible lamp, the solution mixed with composite was kept in the dark for at least 2 h, allowing the adsorption/desorption equilibrium to be reached. Then, the solution was irradiated with visible lamp. The first sample was taken out at the end of the dark adsorption period (just before the light was turned on), in order to determine the MB concentration in solution after dark adsorption, which was hereafter considered as the initial concentration (c ads). Samples were then withdrawn regularly from the reactor by an order of 30, 60, 90, 120, 180, and 240 min, and immediately centrifuged to separate any suspended solid. The clean transparent solution was analyzed by using a UV-vis spectrophotometer (Optizen POP, Mecasys Co., Ltd, Seoul, South Korea) at wavelength of 665 nm [25–27].
Results and discussion
where α, v, E g , and A are the absorption coefficient, light frequency, band gap, and a constant, respectively. Therefore, the band gap energy (E g ) of CdSe compound can be estimated from a plot of (αhv)1/2 versus photo energy (hv), as shown in figure inset in Figure 6. The band gap energy of CdSe is 1.74 eV, which is fairly close to literature value of 1.65 to 1.8 eV (CdSe) [6–9].
Moreover, the two composite both exhibit strong absorption in the UV light region with wavelength less than 400 nm and visible-light region with wavelength at 400-800 nm, assigned to the band adsorption of CdSe. And the absorption of CNT-CdSe composite is higher than that of CdSe compound in both of UV light and visible-light region, as the MWCNTs act as good electron acceptors can accept the electrons from light irradiation [31, 32], indicating the CNT-CdSe composite would exhibit more excellent photoactivity than CdSe compound.
Degradation of MB solution
The photocatalytic activities of the CNT-CdSe composite were evaluated by the photodegradation of MB aqueous solution under visible-light irradiation. The decreasing concentration of MB in the photocatalytic reaction was used to evaluate the activity of the composite. The characteristic absorption peak of MB solution at 665 nm was chosen as the monitored parameter to detect the concentration of MB solution.
In this study, CNT-CdSe composite was successfully synthesized by a simple hydrothermal method. From the XRD patterns, the cubic crystal structure of CdSe can be observed. TEM image shows that the surface of MWCNTs has been coated with CdSe layers uniformly with particle size of about 10 nm. The EDX results reveal the presence of C, Cd, and Se with high content in prepared composite. The diffuse reflectance spectra suggest the CNT-CdSe composite shows strong photoabsorption at UV light and visible-light range. The photocatalytic activity of the CNT-CdSe composite is investigated by degradation of MB in aqueous solution under visible-light irradiation. The results reveal that CNT-CdSe composite exhibit excellent photocatalytic activity for degradation of MB solution under visible-light irradiation.
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