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Preparation, characterization and photocatalytic behavior of WO3-fullerene/TiO2 catalysts under visible light
© Meng et al; licensee Springer. 2011
Received: 6 April 2011
Accepted: 20 July 2011
Published: 20 July 2011
WO3-treated fullerene/TiO2 composites (WO3-fullerene/TiO2) were prepared using a sol-gel method. The composite obtained was characterized by BET surface area measurements, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, and UV-vis analysis. A methyl orange (MO) solution under visible light irradiation was used to determine the photocatalytic activity. Excellent photocatalytic degradation of a MO solution was observed using the WO3-fullerene, fullerene-TiO2, and WO3-fullerene/TiO2 composites under visible light. An increase in photocatalytic activity was observed, and WO3-fullerene/TiO2 has the best photocatalytic activity; it may attribute to the increase of the photo-absorption effect by the fullerene and the cooperative effect of the WO3.
Textile manufacturing involves several processes which generate large quantities of wastewaters. These effluents are highly variable in composition with relatively low biochemical oxygen demand and high chemical oxygen demand contents and are typically characterized as follow: first: strong color due to residual dyes, second: recalcitrance due to the presence of compounds such as dyes, surfactants, and sizing agents; and third: high salinity, high temperature, and variable pH [1–3]. The textile effluents effective treatment usually requires a combination of various physical, chemical, and biological technologies. Some studies researched the treatment of model solutions containing various commercial dyes with emphasis on azo dyes since these are extensively used in dyeing processes. These azo dye molecules are chemically stable and hardly biodegradable aerobically. Most attention has been paid on the oxidative degradation of MB and MO representative mono-azo dyes by oxidation processes [4, 5]. TiO2 is the most widely used photocatalyst far effective decomposition of organic compounds in air and water under irradiation of UV light with wavelength shorter than corresponding to its band gap energy, due to its relatively high photocatalytic activity, biological and chemical stability, low cost, non-toxic nature, and long-term stability. However, the photocatalytic activity of TiO2 (the band gap of anatase TiO2 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 [6, 7]. 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 photocatalytic activity by modification has become a hot topic among researchers in recent years [8, 9].
For the improvement of the photocatalytic activity of TiO2, TiO2 has been coupled with other semiconductors such as SnO2  which can induce effective charge separation by trapping photogenerated electrons. TiO2 coupled with other semiconductors has been reported to perform both the abovementioned functions. This has been realized by coupling the WO3  semiconductor with TiO2. Because of its band gap (E g = 2.6 eV to approximately 3.0 eV) , WO3 mainly absorbs in the near ultraviolet and blue regions of the solar spectrum. As a basic function, WO3 has a suitable conduction band potential to allow the transfer of photogenerated electrons from TiO2 facilitating effective charge separation. However, in practical applications, the photoelectrical properties and photocatalytic efficiency of WO3 require improvement.
C60 has attracted considerable interest for its interesting properties owing to the delocalized conjugated structures and electron-accepting ability. One of the most remarkable properties of C60 in electron-transfer processes is that it can efficiently arouse rapid photoinduced charge separation and relatively slow charge recombination . Therefore, a combination of photocatalysts and C60 might provide an ideal system to achieve enhanced charge separation by photoinduced electron transfer. Some fullerene-donor linked molecules on an electrode were reported to exhibit excellent photovoltaic effects upon photo-irradiation.
A conjugated two-dimensional π-system is suitable not only for synthetic light-harvesting systems but also for efficient electron transfer because the uptake or release of electrons results in minimal structural and solvation change upon electron transfer. Fullerenes contain an extensively conjugated three-dimensional π-system and are described as having a closed-shell configuration consisting of 30 bonding molecular orbitals with 60 π-electrons. This material is also suitable for efficient electron-transfer reduction because of the minimal changes in structure and salvation associated with electron transfer [14, 15].
Unfortunately, deposited metal particles or coupled with other semiconductors only serve as electron trapping agent, or transfer of photogenerated electrons and are not effective to enhance the adsorption of the pollutants. Fullerene-treated TiO2 coupled with other semiconductors has been reported to perform both the abovementioned functions . In addition, C60 is one of the promising materials because of its band gap energy, about 1.6 to 1.9 eV. It has strong absorption in the ultraviolet region and weak but significant bands in the visible region. In general, the coupled systems exhibit higher degradation rate as well as the increased extent of degradation . The studies for comparing the coupled semiconductors with visible light, however, are scarce.
In this paper, WO3-treated fullerene, fullerene-supported TiO2, and WO3-fullerene/TiO2 were synthesized and exhibited enhanced vis-photocatalytic activities compared to the pure TiO2. This study focused on the fabrication and characterization of WO3-fullerene/TiO2 composite in a preparation procedure. Structure variations, surface state, and elemental compositions were examined for the preparation of WO3-fullerene/TiO2 composites. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), and UV-visible (UV-vis) were used to characterize these new photocatalysts. The catalytic efficiency of the WO3-fullerene/TiO2 composite was evaluated by the photo degradation of methyl orange (MO, C14H14N3NaO3S).
Benzene (99.5%) and ethyl alcohol were purchased as reagent-grade from Duksan Pure Chemical Co. (Ansan-si, Gyeonggi-do, South Korea) and Daejung Chemical Co. (Gwangju-si, Gyeonggi-do, South Korea) and were used as received. Crystalline fullerene [C60] powder (99.9% purity from Tokyo Kasei Kogyo Co. Ltd., Tokyo, Japan) was used as the carbon matrix. Titanium(IV) n-butoxide (TNB, C16H36O4Ti) as the titanium source for the preparation of the WO3-fullerene/TiO2 composites was purchased as reagent-grade from Acros Organics (Morris Plains, NJ, USA). The ammonium metatungstate hydrate (H26N6O40W12·x H2O) purchased from Sigma-Aldrich™ Chemie GmbH (Steinheim, Germany) was used as a raw material to generate WO3 at high temperatures. Methyl orange (MO, C14H14N3NaO3S, 99.9%, Duksan Pure Chemical Co., Ltd) was of analytical grade.
Preparation of WO3-fullerene composites
Nomenclature of the samples prepared with the photocatalysts
3.8 × 10-5 mol H26N6O40W12·x H2O + H2O + MCPBA + 30 mg fullerene
MCPBA+ benzene + 30 mg fullerene + 3 ml TNB
MCPBA+ benzene + 30 mg fullerene + 3.8 × 10-5 mol H26N6O40W12·x H2O + H2O + benzene + 3 ml TNB
Preparation of WO3-fullerene/TiO2 composites
WO3-fullerene was prepared using pristine concentrations of TNB for the preparation of WO3-fullerene/TiO2 composites. WO3-fullerene 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 WO3-fullerene/TiO2 gels and heat treated at 873 K to produce the WO3-fullerene/TiO2 composites.
Characterization of photocatalysts compounds
To measure the structural variations, XRD patterns were obtained using an X-ray generator (Shimadzu XD-D1, Shimadzu Corporation, Kyoto, Japan) with Cu Kα radiation. Scanning electron microscopy (SEM, JSM-5200, JEOL, Tokyo, Japan) was used to observe the surface state and structure of the photocatalyst composites. Energy dispersive X-ray spectroscopy (EDX) was also used for elemental analysis of the samples. The specific surface area (BET) was determined by N2 adsorption measurements at 77 K (Monosorb, Quantachrome Instruments Ltd, Boynton Beach, FL, USA). Transmission electron microscopy (TEM, JEM-2010, JEOL) was used to observe the surface state and structure of the photocatalyst composites at an acceleration voltage of 200 kV. TEM was also used to examine the size and distribution of the titanium and iron particles deposited on the fullerene surface of various samples. The TEM specimens were prepared by placing a few drops of the sample solution on a carbon grid. UV-vis diffused reflectance spectra were obtained using a UV-vis spectrophotometer (Neosys-2000, Scinco, Seoul, South Korea) by using BaSO4 as a reference and were converted from reflection to absorbance by the Kubelka-Munk method.
Photocatalytic degradation of MO
The photocatalytic activities were evaluated by MO degradation in aqueous media under visible light irradiation. For visible light irradiation, the reaction beaker was located axially and held in a visible lamp (8 W, halogen lamp, KLD-08L/P/N, Fawoo Technology, Bucheon Si, South Korea) box. The luminous efficacy of the lamp is 80 lm/W, and the wavelength is 400 nm to approximately 790 nm. The lamp was used at a distance of 100 mm from the aqueous solution in a dark box. The initial concentration of the MO was set at 1 × 10-5 mol/L in all experiments. The amount of the photocatalysts (WO3-fullerene, fullerene-TiO2, and WO3-fullerene/TiO2) composite was 0.05 g per 50 ml solution. The reactor was placed for 2 h in the darkness box in order to make the photocatalyst composites particles adsorbed the MO molecule maximum. After the adsorption state, the visible light irradiation was restarted to make the degradation reaction proceed. In the process of degradation of methyl orange, 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 done for 10, 30, 60, 90, and 120 min, respectively. Samples were withdrawn regularly from the reactor and dispersed powders were removed by a centrifuge. The clean transparent solution was analyzed by UV/vis spectroscopy. The MO concentration in the solution was determined as a function of the irradiation time.
Elemental analysis of the preparation
EDX elemental microanalysis, BET surface area, and k app values of photocatalysts
2.24 × 10-4
2.86 × 10-3
1.52 × 10-3
4.75 × 10-3
Surface characteristics of the samples
Table 2 lists the specific surface area (BET) of the materials examined. The BET surface area of pure TiO2 was 18.95 m2/g, and the surface area of pure fullerene was 85.05 m2/g. Tungsten oxide particles were introduced into the pores of fullerene, which decreased the BET surface area. The surface area of fullerene-TiO2 was 64.62 m2/g. Fullerene contains many pores, which can increase the surface area of the photocatalyst. The BET surface area decreased from 85.05 m2/g for pure fullerene to 57.74 m2/g for WO3-fullerene/TiO2. This suggests that the TiO2 and tungsten oxide were introduced into the pores of the fullerenes, which decreased the BET surface area. The WO3-fullerene sample had the largest surface area, which can affect the adsorption reaction.
UV-vis diffuse reflectance spectroscopy
Photocatalytic activity of samples
In the degradation step, Figure 6 shows the results of TiO2, WO3-fullerene, fullerene-TiO2, and WO3-fullerene/TiO2 composites degradation MO solutions under visible light. The relative yields of the photolysis products formed under different irradiation time conditions are shown for the products. The dye concentration was 1.0 × 10-5 mol/l, and the absorbance decreased with increasing irradiation time. This suggests that the light transparency of the dye concentration was increased greatly by the photocatalytic degradation effect. The effect of the high crystallinity of the anatase phase on the photocatalytic degradation of dye was shown. Under visible light irradiation, TiO2 cannot depredate MO molecules, but WO3-fullerene, fullerene-TiO2, and WO3-fullerene/TiO2 composites have good photocatalytic activity. Comparing these three samples, WO3-fullerene/TiO2 composite has the best degradation effect, which is due to the synergistic reaction of WO3, fullerene, and TiO2.
WO3-fullerene also has a barrier degradation effect than pure TiO2, due to the same reason as fullerene-TiO2 system. From Figure 6 and Table 2, we can find that the k app of WO3-fullerene is 2.86 × 10-3, which is larger than that of fullerene-TiO2 (1.52 × 10-3). This is because, with the band gap of WO3 being relatively small, electrons will obtain energy to jump onto the conduction band and become free electrons named photoelectrons when under visible light irradiation. In this system hole and electron pairs were also generated and separated on the interface of fullerene. Fullerene is acted as photosensitize. These electron-hole pairs can recombine or diffuse to the surface where they can initiate redox reactions with surface species, so the degradation effects of TiO2-fullerene and WO3-fullerene/TiO2 were limited.
At WO3-fullerene/TiO2 system, the photocatalytic activities were enhanced mainly due to the high efficiency of charge separation induced by the synergistic effect of fullerene, WO3, and TiO2. Because of the least band gap of fullerene (1.6 to 1.9 eV), hole and electron pairs were generated and separated on the interface of fullerene easily by visible light irradiation, and the electron can transfer easily from the CB of fullerene to a TiO2 molecule and, simultaneously, the holes in the VB of TiO2 can transfer directly to fullerene because both the conduction band (CB) and the valence band (VB) of WO3 were higher than the CB and VB of TiO2 and fullerene. When the hole and electron pairs were also generated and separated on the interface of WO3, electrons at the CB of WO3 migrated to CB of TiO2 and fullerene, and holes at the VB of WO3 migrated to VB of TiO2 and fullerene . This can allow the transfer of photogenerated electrons facilitating effective charge separation and decreased the rate of recombination about the electron-hole pairs. Fullerene also acts as the adsorb facient and increases the surface area of compounds which can increase the adsorption effect for samples, adsorbed more O2 and dye molecules, and make sure this systems take full advantage of yield oxidizing species. Figure 8 is the schematic diagram of the separation of photogenerated electrons and holes on the WO3-fullerene/TiO2 interface. Electrons and holes were used to produce the hydroxyl radicals (OH·) and superoxide ions (O2 ·-). Oxidative degradation of azo dyes occurs by the attack of hydroxyl radicals and superoxide ions, which are the highly reactive electrophilic oxidants. Due to the efficiency of hydroxyl radicals and superoxide ions, azo dyes were decompounded to CO2, H2O, and inorganic.
This study examined the preparation and characterization of WO3-fullerene, fullerene-TiO2, and WO3-fullerene/TiO2. The BET surface area of pristine fullerene was higher than that of the WO3-fullerene/TiO2 composite. XRD revealed the WO3 structure and anatase. TEM showed that TiO2 particles with some agglomerates were dispersed over the surface of fullerene together with WO3 particles. In UV-vis absorption, spectra samples have shown a great adsorption at visible region. Fullerene-TiO2 has a good photodegradation effect under visible light irradiation, due to the photosensitivity, and enhances the BET surface area effect of fullerene. The WO3-fullerene/TiO2 composite showed the best photocatalytic degradation activity of the MO solution under visible light irradiation. This was attributed to the three different effects between the photocatalytic reactions of the supported TiO2, to the energy transfer effects of fullerene and WO3, such as electrons and light, and to the separation effect in this system.
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