ХИМИЯ ВЫСОКИХ ЭНЕРГИЙ, 2013, том 47, № 3, с. 217-222
ФОТОХИМИЯ
УДК 541.14
DECOMPOSITION OF RHODAMINE 6G IN THE PRESENCE OF TITANIA COUPLING WITH CARBON SOURCE AS PHOTOCATALYST UNDER VISIBLE
LIGHT EXCITATION © 2013 г. Dongfang Zhang
College of Science, Huazhong Agricultural University Wuhan 430070, PR China E-mail: zdfbb@yahoo.cn Поступила в редакцию 30.03.2012 г. В окончательном виде 11.12.2012 г.
Heterogeneous photocatalytic degradation of N-containing R6G dye was achieved by visible light-activated carbon doped TiO2 (C—TiO2) nanoparticles, synthesized by a low-temperature wet-chemical technique using glucose as carbon source. The structural and physicochemical properties of C—TiO2 were characterized by X-ray diffraction (XRD), Raman spectroscopy, UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and FT-IR spectroscopy. Compared with the pure TiO2, the carbon modified nanomaterials exhibited enhanced absorption in the broad visible-light region together with an apparent red shift in the optical absorption edge. The resulting carbon-doped TiO2 catalyst was employed as an effective photocatalyst for degradation of Rhodamine 6G (R6G) in aqueous solutions under visible light irradiation (k > 420 nm). In addition, the intrinsic mechanism of visible light-induced photocatalytic oxidation of organic compounds on the carbon-doped titania was proposed and discussed.
DOI: 10.7868/S0023119713030150
The textile dyeing process is an important source of contamination that is responsible for the continuous pollution of the environment. The volume of waste water containing processed textile dyes is steadily increasing [1]. Different conventional techniques have been used for the treatment of this type of waste, but photocatalytic treatment is among the new and promising oxidation technologies for the complete degradation of textile waste [2]. The photodegradation process, in general, occurs with the attack of organic substances by the activated oxygen species, such as the hydroxyl radical and super oxide radical, generated on the TiO2 particulate surface by the reduction of dissolved oxygen in solution and/or oxidation of surface hydroxyl by TiO2 [3]. Nevertheless, TiO2 is only activated under UV irradiation because of its wide band gap of 3.2 eV. Its application is significantly limited by the high cost of the UV irradiation source and the low percentage of UV light in the solar spectrum. Therefore, it is necessary to develop a low-cost photocatalyst that operates not only under UV irradiation but also under the visible radiation range.
Coupling of photocatalyst TiO2 with nonmetal elements is a promising possibility to hybridize photoac-tivity with adsorptivity and has been studied recently. Doping of TiO2 with nonmetal atoms such as sulfur,
carbon, and nitrogen, resulted in high photocatalytic activity under visible light owing to band gap narrowing [4—6]. The results have revealed that the existence of the C—O (carbonate) bonds or the Ti—C bond depends critically on the synthetic method (wet chemical or solid state reactions), while both carbon species may even coexist in TiO2, depending on the preparation conditions. Obviously, the above factors are important for the photocatalytic properties of C—'TiO2 since the performances of TiO2 have been found to be determined by the crystalline phase, the microstructure and the form/density of defects, meanwhile the physicochemical properties [7].
In the present work, we develop a low-temperature method to prepare carbon-doped TiO2 by using glucose and titanium tetra-isopropoxide as the carbon and TiO2 sources, respectively. The structural and physicochemical properties of C—'TiO2 were characterized in detail. N-containing dye Rhodamine 6G (R6G) was selected as a target contaminant to evaluate the enhancement of photocatalytic activity of C—TiO2 nanoparticles under visible light irradiation in water. R6G dye is extensively used for coloring leather, paper, silk and wool, which should be treated before they are extruded into the environment. It was found that the resulting carbon-doped TiO2 by this new approach ex-
hibits much higher photocatalytic activity than the undoped counterpart on the degradation of R6G under visible light irradiation.
EXPERIMENTAL SECTION
All of the materials used in this study were analytical reagents (AR) and distilled water was used throughout the synthesis process. The carbon-containing titania composites were synthesized by a hydrothermal method and glucose and titanium tetra-isopropoxide (TTIP) were used as the carbon and titanium sources, respectively. Firstly, a certain amount (0.05 g) of glucose (C5HnO5CHO) was dissolved in 100 mL distilled water. Then, the mixed titanium source containing 15.2 mL TTIP and 55 mL ethanol was added dropwise to it with magnetic stirring. After stirring for 2 h, the milky solution was transferred into a 200 mL stainless steel autoclave with a Teflon liner and kept at 180°C for 10 h. The white or grey precipitates thus obtained were collected, washed thoroughly with distilled water followed by a rinse in ethanol, and then dried in an oven at 80°C. For comparison, un-doped or blank TiO2 was also prepared following the same procedure as above except that the first addition of glucose was absent.
The crystallographic information such as structure, composition and defect content of the phases was established with powder X-ray diffraction (XRD, Shi-madzu XRD-6000, CuKa radiation, X = 0.15418 nm) in the region 2^ = 10°—90° with a step size of 0.04° was used. The Raman spectra were recorded on a Raman spectrometer (LabRam HR800) equipped with an optical microscope at room temperature. For excitation source, the 632.8-nm line from a He—Ne laser was focused, with an analyzing spot of about 1 ^m, on the sample under the microscope. The percentage of UV-Vis reflectance was measured by DRS for the powder form of the catalysts using a scanning UV-Vis-NIR spectrophotometer (Varian Cary 500) in the region of 200—800 nm. The spectrophotometer was equipped with an integrating sphere assembly, and poly-tetrafluoroethylene was used as a reflectance material. In situ FT-IR spectra were recorded on a Thermo Nicolet 330 FT-IR spectrometer (AVATAR) at a resolution of4.0 cm-1. For each recording of IR spectra, the quantity of KBr was equal, and the concentration of sample was controlled at 2 wt % of the KBr pellet.
The as-prepared catalysts were employed as photo-catalysts in the degradation the aqueous Rhodamine 6G (R6G, C28H31N2O3Cl) dye. The artificial light photocatalytic activity test is conducted in a quartz photoreactor with a cylindrical configuration. A mixture of 200 ml aqueous R6G (25 mg/l) and 60 mg of catalyst powders were firstly stirred for 30 min in a quartz beaker in the dark to establish an adsorption/desorption equilibrium. Then the suspension is irradiated by a 300 W halogen tungsten lamp equipped
with a 420 nm cutoff filter as a visible light source. The distance between the light and the suspension is kept at 12 cm. During the irradiation experiments, samples of 5 ml are withdrawn from the suspension at a given intervals and are immediately centrifuged at 5000 rpm for 10 min to remove solids. The concentration of R6G after illumination is monitored at X = 524 nm using a UV-VIS spectrophotometer. The decolorization efficiency of the R6G can be determined by the formula: decolorization is (c0-c)/c0, where c0 and c represents the concentration of the primal and remaining R6G and c0-c is the concentration of the decomposed R6G. The absorbance of the R6G solution was measured using a UV-VIS spectrophotometer (Shimadzu UV-3101).
RESULTS AND DISCUSSION
In order to characterize the crystalline structure of the doped and undoped samples, their XRD patterns (Fig. 1) were obtained. The pattern of undoped TiO2 shows the presence of anatase (JCPDS, No. 21-1272, space group: I41/amd(141)) and rutile (JCPDS, No. 21-1276, space group: P42/mnm(136)). However, only anatase is present in the XRD pattern of C-doped TiO2, indicate the doping of carbon prohibit the phase conversion from anatase to rutile as well as the formation of rutile [8]. By comparison, the as-prepared C-TiO2 sample showed a dominant anatase phase compared with the reference TiO2, while no other dopant-related crystal phases could be resolved, independently of the incorporation carbon source. This indicated that the contents of introduced carbon were below the detection limit and the (101) peak of TiO2 slightly shifts to a higher degree after carbon doping, which implies that carbon could enter the bulk of TiO2. It can be also found that the crystal size of anatase decrease with the carbon doping by comparing the (101) peak of anatase in Fig. 1. We think this decrease on crystal size can be attributed to dissimilar boundaries created by carbon doping, which can suppress the hydrolysis of titanium alkoxide and the rapid crystallization of the TiO2 particles. Meanwhile, the significant decrease on the intensity ofXRD pattern of TiO2 after carbon doping may arise from the shield effect of these doped carbon species by adsorbing on the TiO2 particle surface. As expected, the average crystalline size calculated by applying the Scherrer formula on the anatase (101) diffraction peak decreased with the addition of carbon source, since the incorporated carbon species reduces the rate of titanium alkoxide hydrolysis and condensation of TiO2 leading to smaller crystal size (~20 nm).
Raman spectroscopy is an effectivr tool to investigate and characterize carbonaceous materials. Figure 2 depicts the Raman spectrum of the as-prepared C-TiO2 sample. It can be seen that carbon-doped TiO2 exhibits the distinct Raman-active modes of the anatase TiO2 phase verifying the phase composition that deter-
DECOMPOSITION OF RHODAMINE 6G
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Fig. 1. Powder XRD patterns for undoped TiO2 and C-doped TiO2 nanostructures prepared by wet-chemical method.
Raman shift, cm 1
F
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