Fullerene modification CdSe/TiO2 and modification of photocatalytic activity under visible light
© Meng et al.; licensee Springer. 2013
Received: 16 October 2012
Accepted: 7 March 2013
Published: 23 April 2013
CdSe, CdSe-TiO2, and CdSe-C60/TiO2 composites were prepared using sol–gel method, and their photocatalytic activity was evaluated by measuring the degradation of rhodamine B solutions under visible light. The surface area, surface structure, crystal phase, and elemental identification of these composites were characterized by nitrogen adsorption isotherms, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and UV-visible (vis) absorption spectrophotometry. XRD showed that the CdSe-C60/TiO2 composite contained a typical single and clear anatase phase. SEM of the CdSe-C60/TiO2 composites revealed a homogenous composition in the particles. EDX revealed the presence of C and Ti with strong Cd and Se peaks in the CdSe-C60/TiO2 composite. The degradation of dye was determined by UV–vis spectrophotometry. An increase in photocatalytic activity was observed and attributed to an increase in the photoabsorption effect by fullerene and the cooperative effect of the CdSe. The repeatability of photocatalytic activity was also tested in order to investigate the stability of C60 and CdS-C60/TiO2 composites.
KeywordsFullerene CdSe Visible light UV–vis TEM
TiO2 is the most widely used photocatalyst for effective decomposition of organic compounds in air and water under irradiation of UV light with a shorter wavelength, corresponding to its bandgap energy, due to its relatively high photocatalytic activity, biological and chemical stability, low cost, nontoxic nature, and long-term stability. However, the photocatalytic activity of TiO2 (the bandgap of anatase TiO2 is 3.2 eV which can be excited by photons with wavelengths below 387 nm) is limited to irradiation wavelengths in the UV region [1, 2]. However, only about 3% to 5% of the solar spectrum falls in this UV range. This limits the efficient utilization of solar energy for TiO2. Some problems still remain to be solved in its application, such as the fast recombination of photogenerated electron–hole pairs. Therefore, improving the photocatalytic activity by modification has become a hot topic among researchers in recent years [3, 4].
Photosensitization of stable, large bandgap semiconductors such as SnO2, TiO2, and ZnO in visible light using semiconducting photosensitizers such as CdS, CdSe, and CdTe  has been a long-sought, continuing goal in the area of photoelectrochemical solar energy conversion. Cadmium selenide is a kind of semiconductor with a forbidden zone of 1.7 eV, and its valence electrons can be easily evoked to conduction band when the light wavelength of evoking light is ≤730 nm [6–9]. However, in practical applications, the photoelectrical properties and photocatalytic efficiency of CdSe require improvement.
Conjugated material is proposed to be a good candidate for improving the transportation of photocarriers in the photocatalysis process by forming an electronic interaction with TiO2 due to its unique properties in electron or hole transporting . Among them, fullerene has a variety of special chemical and physical properties due to its delocalized conjugated structure and has been studied quite extensively . Fullerene can efficiently promote a rapid photo-induced charge separation and slow down charge recombination in the dark. Therefore, fullerene has been used to raise the performances of solar cell and medicinal chemistry [12, 13]. Kamat et al. have demonstrated the charge transfer between fullerene clusters and titanium dioxide under visible light; fullerene can be reduced by one-electron function in colloidal TiO2 suspensions and form C60. Besides, many works focused on improving the efficiency of dye sensitization-based photochemical solar cells by adding C60. Those researches were mostly focused on the electron transfer between TiO2 particle and C60 cluster. Photon conversion efficiency can be improved by C60 cluster due to high separation efficiency for the photo-induced electrons and holes. Although the use of fullerene for scavenging photogenerated electrons from titanium dioxide particles has been demonstrated, a few efforts are made to utilize the unique properties of fullerenes to increase the efficiency of photocatalysis; however, the interior mechanism is yet not very clear. A systematical study on a purpose of understanding the interaction between C60 molecules and TiO2 and further effect on the photocatalytic activity is still necessary and important [15–17].
In this work, CdSe-TiO2 and C60-hybridized CdSe-TiO2 photocatalysts showed significantly enhanced photocatalytic activity for the degradation of salicylic acid and formaldehyde under visible-light irradiation. The enhancement of photoactivity was attributed the photosensitization of CdSe and the enhanced interfacial charge separation between C60 layers and TiO2 particles.
Crystalline fullerene (C60) powder of 99.9% purity from TCI (Tokyo Kasei Kogyo Co. Ltd., Tokyo, Japan) was used as the carbon matrix. For the oxidization of C60, m-chloroperbenzoic acid (MCPBA) was chosen as the oxidizing agent and was purchased from Acros Organics (Fair Lawn, NJ, USA). Benzene (99.5%) was used as the organic solvent and was 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). Titanium(IV) n-butoxide (TNB, C16H36O4Ti) as the titanium source for the preparation of the CdSe-C60/TiO2 composites was purchased as reagent-grade from Acros Organics (USA). Rhodamine B (Rh.B, C28H31ClN2O3) was purchased from Samchun Pure Chemical Co., Ltd. (Korea). All chemicals were used without further purification, and all experiments were carried out using distilled water.
Synthesis of CdSe
For the synthesis of CdSe, sodium selenosulfite (Na2SeSO3) solution and Cd(NH3)42+ solution were first prepared. Na2SO3 (4 g) and selenium metal powder (0.2 g) were dissolved in 20 of mL distilled water and refluxed for 1 h to form Na2SeSO3 solution. Meanwhile, Cd(CH3COO)2 (0.675 g) was dissolved in 7 mL of distilled water. NH4OH (2 mL) was added, and the mixture was stirred until it dissolved completely to form Cd(NH3)42+ solution. Finally, the Cd(NH3)42+ and Na2SeSO3 solutions were mixed together, and the mixture was stirred and refluxed for at least 5 h. After the mixture had been brought down to room temperature, the mixture was filtered through a Whatman filter paper. The solids obtained were collected and washed five times with distilled water. After being dried in vacuum at 353 K for 8 h, the CdSe compound was obtained.
Synthesis of CdSe-C60 composite
For the preparation of the CdSe-C60 composite, C60 had to be functionalized by MCPBA at first. MCPBA (ca. 1 g) was suspended in 50 mL of benzene, followed by the addition of fullerene (ca. 30 mg). The mixture was heated under reflux in air and stirred for 6 h. The solvent was then dried at the boiling point of benzene (353.13 K). After completion, the dark-brown precipitates were washed with ethyl alcohol and dried at 323 K, resulting in the formation of oxidized fullerene.
Synthesis of CdSe-C60/TiO2 composites
CdSe-C60 was prepared using pristine concentrations of TNB for the preparation of CdSe-C60/TiO2 composites. CdSe-C60 powder was mixed with 3 mL TNB. The solutions were homogenized under reflux at 343 K for 5 h while being stirred in a vial. After stirring, the solution transformed to CdSe-C60/TiO2 gels and was heat-treated at 873 K to produce the CdSe-C60/TiO2 composites.
X-ray diffraction (XRD; Shimadzu XD-D1, Uki, Kumamoto, Japan) was used to identify the crystallinity of the composite with monochromatic high-intensity Cu Ka radiation (l = 1.5406 Å). Scanning electron microscopy (SEM; JSM-5600, JEOL Ltd., Tokyo, Japan) was used to observe the surface state and structure of the prepared composite using an electron microscope. Transmission electron microscopy (TEM; JEM-2010, JEOL Ltd.) was used to determine the state and particle size of the prepared composite. TEM at an acceleration voltage of 200 kV was used to investigate the number and the stacking state of graphene layers on the various samples. TEM specimens were prepared by placing a few drops of sample solution on a carbon grid. The elemental mapping over the desired region of the prepared composite was determined by an energy dispersive X-ray spectroscopy (EDX) analyzer attached to the SEM. UV-visible (vis) diffuse reflectance spectra were obtained using a UV–vis spectrophotometer (Neosys-2000, Scinco Co. Ltd., Seoul, Korea) using BaSO4 as a reference at room temperature and were converted from reflection to absorbance spectra by the Kubelka-Munk method.
Photocatalytic degradation of dyes
Photocatalytic activity was evaluated by dye degradation in aqueous media under visible-light irradiation. For visible-light irradiation, the reaction beaker was located axially and held in a visible lamp box (8 W, halogen lamp, KLD-08 L/P/N, Korea). The luminous efficacy of the lamp was 80 lm/W, and the wavelength was 400 to 790 nm. The lamp was located at a distance of 100 mm from the aqueous solution in a dark box. The initial concentration of the dyes was set at 1 × 10−5 mol/L in all experiments. The amount of photocatalytic composite used was 0.05 g/50-mL solution. The reactor was placed for 2 h in the dark box to make the photocatalytic composite particles adsorb as many dye molecules as possible. After the adsorption phase, visible-light irradiation was restarted to make the degradation reaction proceed. To perform dye degradation, a glass reactor (diameter = 4 cm, height = 6 cm) was used, and the reactor was placed on the magnetic churn dasher. The suspension was then irradiated with visible light for a set irradiation time. Visible-light irradiation of the reactor was performed for 120 min. Samples were withdrawn regularly from the reactor, and dispersed powders were removed in a centrifuge. The clean transparent solution was analyzed by a UV–vis spectrophotometer (Optizen POP, Mecasys Co., Ltd., Daejeon, Korea). The dye concentration in the solution was determined as a function of the irradiation time.
Results and discussion
EDX elemental microanalysis and BET surface area values
Table 1 lists Brunauer-Emmett-Teller (BET) surface areas of the raw CdSe, CdSe-TiO2, and CdSe-C60/TiO2 photocatalysts. The BET value decreased from 85.00 m2/g of pure fullerene to 47.27 m2/g of CdSe-C60/TiO2. The TiO2 and CdSe particles were introduced into the pores of fullerene, and the value of CdS-C60/TiO2 decreased . Added C60 can increase the surface area because C60 has a relatively larger surface area. The BET values of CdSe and CdSe-TiO2 compounds were 26.71 and 30.47 m2/g, respectively. The BET surface area of the CdS-TiO2 photocatalyst was increased by 55.13 % when the CdSe-TiO2 particles were modified by C60. The pore size and pore volume increased significantly when the particles were modified by C60 because C60 particles have larger surface area and pores.
where K is the reflectance transformed according to the Kubelka-Munk function, R is the reflectance (%), and F(R) is the so-called remission or Kubelka-Munk function .
CdSe-C60/TiO2 composites have the best discoloration effect, which is due to the following reasons: (1) C60 is an energy sensitizer that improves the quantum efficiency and increases charge transfer, (2) C60 can enhance the adsorption effect during the discoloration processes, and (3) CdSe can provide excited electrons for TiO2 and engender hydroxyl radicals (·OH) and superoxide radical anions (·O2−) with the presence of H2O and oxygen. Figure 9b shows a schematic diagram of the separation of photogenerated electrons and holes on the CdSe-C60/TiO2 interface [31, 32].
Photocatalysts were synthesized successfully using a simple sol–gel method. From the XRD patterns, the cubic crystal structure of CdSe was observed. TEM showed that the surface of TiO2 has been coated uniformly with C60 and CdSe layers with a C60 particle size of approximately 20 nm. The diffuse reflectance spectra indicated that the composites showed strong photoabsorption in the UV–vis range, and the presence of C60 enhanced the level of photoabsorption in the visible range. The nitrogen adsorption isotherms show that the added C60 can enhance the adsorption effect significantly. The photocatalytic activity of the CdSe-C60/TiO2 composite was examined by the degradation of MB in aqueous solutions under visible-light irradiation. The CdSe-C60/TiO2 composites showed good adsorption and degradation effects. Overall, within the limits of the degradation ability, the degradation effect can be enhanced by an increase in adsorption capacity. CdSe-C60/TiO2 composites also have good photocatalytic activity in cycle experiment which emphasizes the excellent stability of C60 and photochemical stability of C60-modified photocatalyst.
Thanks very much for all of the Authors, and the professor Oh. They did the job of analyzed and prepared work, and contribution of materials.
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