Visible-light-activated nanocomposite photocatalyst of Cr2O3/SnO2
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Visible-light-activated Cr2O3/SnO2 nanocomposite photocatalyst was prepared by coprecipitation method and characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption-desorption measurement, and UV–vis diffuse reflectance spectroscopy. The results show that phase composition, crystallite size, Brunauer-Emmett-Teller surface area, and optical absorption of samples varied significantly with the heat treatment temperatures. The Cr2O3/SnO2 photocatalyst (the molar ratio Cr to Sn is 1:2) calcined at 400°C for 2 h exhibited maximum photocatalytic activity because it has a smaller particle size of 10.05 nm and a higher surface area of 38.75 m2/g. Under visible-light (λ > 400 nm) irradiation, the degradation rate of Rhodamine B reached 98.0% in 60 min, which is about 3.5 times higher than that of the standard P25 photocatalyst.
KeywordsCoprecipitation method Coupled photocatalyst Photocatalytic activity Photoelectron spectroscopy
In recent years, photocatalytic degradation of various kinds of organic–inorganic pollutants using semiconductor powder as a photocatalyst has been extensively studied[1, 2]. Among various oxide semiconductor photocatalysts, TiO2 was intensively investigated because of its biological and chemical inertness, strong oxidizing power, nontoxicity, and long-term stability[3, 4, 5]. However, the photocatalytic activity of TiO2 (the bandgap is 3.2 eV, and it can be excited by photons with wavelengths below 387 nm) is limited to irradiation wavelengths in the UV region so that the effective utilization of solar energy is limited to about 3% to 5% of the total solar spectrum. Furthermore, the fast recombination of photo-generated electron–hole pairs hinders the commercialization of this technology. The decomposition of adsorbed organic compound is closely correlated with the density of space charge-separated electron–hole pair on TiO2. Therefore, it is of great interest to separate the electron–hole pairs effectively to increase the photon efficiencies and develop new visible-light photocatalyst to extend the absorption wavelength range into the visible-light region. In this sense, an interesting approach to deal with the issue is carried out by coupled semiconductor technique. Recently, there are a number of studies related to the photocatalytic activity of coupled semiconductor photocatalyst such as ZnO/SnO2, SnO2-TiO2, Au/TiO2-CeO2, and TiO2-CdS. The results show that nearly all the composite semiconductors have presented higher photocatalytic activity than single ones. Indeed, this effect could be due to the synergistic effect of the coupled semiconductor photocatalyst. It was also reported that the chromium-doped TiO2 has been found to exhibit superior photocatalytic activity under visible-light irradiation because chromium atom can effectively narrow the energy bandgap of TiO2[10, 11]. Chromium-based catalysts have been widely examined for polymerization, partial oxidation, and aromatization reaction because of the peculiar characteristics of Cr oxide species on the surface of the support, including oxidation state and coordination environment. However, there have been few studies clearly elucidating the roles of the surface chromate species and supporting the oxidation reaction, particularly the decomposition of environmental pollutants.
In this study, a series of Cr2O3/SnO2 photocatalysts with different calcination temperatures were prepared by coprecipitation method, and their phase compositions, crystalline structures, and particle sizes were studied. We also explored their photocatalytic degradation performance in treating Rhodamine B solution under visible-light (λ > 400 nm) irradiation.
Results and discussion
Effect of calcination temperature on average particle size and BET surface area of Cr 2 O 3 /SnO 2 samples
Calcination temperature (°C)
BET surface area (m2/g)
Grain size (nm)
UV–vis DR spectral analysis
The bandgap energy of Cr2O3 is 2.5 eV and can be activated by the light below 560 nm, when it couples with SnO2 semiconductor, the conduction band of SnO2 acts as a sink for photo-generated electrons. The photo-generated holes move in the opposite direction and accumulate in the valence band of the Cr2O3 particle, which leads to increasing the charge separation efficiency and extending the photo-responding range to visible light. When the heat treatment temperature reaches 500°C, the crystallite sizes become larger, and the absorption edge shows more red shift.
Photocatalytic activity studies
Factors influencing the photocatalytic activity
Effect of catalyst loading
Effect of pH
The novel visible-light-activated Cr2O3/SnO2 nanocomposite photocatalyst was prepared by coprecipitation method. The characteristic patterns of XRD, BET, TEM, and UV–vis DRS displayed that the sample calcined at 400°C for 2 h (the molar ratio of Cr to Sn is 1:2) has better crystallization, smaller crystal size, and stronger response to visible light. The Cr2O3/SnO2 photocatalyst showed remarkable photocatalytic activity compared with the standard P25 photocatalyst. Rhodamine B (98.0%) can be degraded in 60 min under illumination of the visible light (λ > 400 nm). At high catalyst loadings, penetration of the light inside the reaction medium was reduced because of the light scattering and shielding effect by catalyst particles. In addition, basic pH level of suspension was found to be beneficial for photocatalytic degradation.
Preparation of photocatalysts
The reagent grade chemicals used in preparing the samples, CrCl3·6H2O and SnCl4·5H2O were used as the starting materials. CrCl3·6H2O and SnCl4·5H2O were mixed (the molar ratio of Cr to Sn is 1:2) and dissolved in minimum amount of ethanol. The cationic surfactant cetyltrimethylammonium bromide (5%; 20 mL) in ethanol was dropped into the solution. The system was kept under constant stirring and sustaining the pH of 7 by simultaneous addition of ammonium hydroxide to form the precipitate. The precipitate was filtered and washed with deionized water until no Cl¯ was found in the filtrates. Then, the wet powder was dried at about 100°C in air to form the precursor of the Cr2O3/SnO2 photocatalyst. Finally, the precursors were calcined for 2 h at different temperatures in air to prepare the photocatalyst powders.
Characterization of photocatalysts
To determine the crystallite sizes and identities of the Cr2O3/SnO2 nanocomposite photocatalysts, XRD analysis was carried out at room temperature using a model D8 Bruker AXS (Madison, WI, USA)with monochromatic Cu radiation (40 kV and 30 mA), over the 2θ collection range of 20° to 80°. The shapes of the samples were tested using transmission electron microscopy; FEI, Tecnai F30 HRTEM, FEG (FEI, Hillsboro, OR, USA) operated at 300 kV. The BET surface areas were determined using a Micromeritics ASAP 2010 N2 adsorption apparatus (Norcross, GA, USA). XPS was used to study the chemical composition of the sample. The monochromatic X-ray beams of Al Kα (hν = 1486.6 eV) and Mg Kα (hν = 1253.6 eV) radiations were used as the excitation source. A hemispherical sector analyzer and multi channel detectors were used to detect the ejected photoelectrons as a function of their kinetic energies. XPS spectra were recorded at pass energy of 50 eV, 5-mm slit width, and a take-off angle of 55°. The spectrometer was calibrated by determining the binding energy values of Au 4f7/4 (84.0 eV), Ag 3d5/2 (368.4 eV), and Cu 2P3/2 (932.6 eV) levels using spectrograde materials. The instrumental resolution under these conditions was 1.6 eV full-width at half-maximum for Au 4f7/4 level. The Cls (285 eV) and Au 4f7/4 (84.0 eV) were used as internal standards whenever needed.
UV–vis DRS were recorded in air at room temperature in the wavelength range of 200 to 800 nm using a PE LAMBDA35 spectrophotometer (PerkinElmer, Waltham, MA, USA).
Rhodamine B adsorption experiment
where n(ads) was the number of moles adsorbed; ΔC was the difference between the initial concentration, C0 and equilibrium concentration, Cc; and V was the volume (50 mL).
Photocatalytic activity measurements
Photocatalytic degradation of Rhodamine B in aqueous solution (0.5 g/L) was carried out using a Quartz reactor under visible-light irradiation (tungsten lamp, 500 W). Air was bubbled into the solution throughout the entire experiment. A cutoff filter was placed outside the Quartz jacket to completely remove all wavelengths less than 400 nm to ensure irradiation with visible light (λ > 400 nm). About 0.5 g of photocatalyst was immersed into a 50-mL aqueous Rhodamine B. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to ensure establishment of an adsorption-desorption equilibrium among the photocatalyst, Rhodamine B, and atmospheric oxygen. At given irradiation time intervals, 10 mL of the suspensions are collected, and then filtered through a Millipore filter to separate the photocatalyst particles. The changes in Rhodamine B concentration were analyzed by UV-visible spectroscopy.
This work is financed by the University Grants Commission, New Delhi, India (Grant no. 47–2028/11).
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