Synthesis and photo-degradation application of WO3/TiO2 hollow spheres

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Abstract

A WO3/TiO2 composite, hollow-sphere photocatalyst with average diameter of 320 nm and shell thickness of 50 nm was successfully prepared using a template method. UV–vis diffuse reflectance spectra illustrated that the main absorption edges of the WO3/TiO2 hollow spheres were red-shifted compared to the TiO2 hollow spheres, indicating an extension of light absorption into the visible region of the composite photocatalyst. The WO3 and TiO2 phases were confirmed by X-ray diffraction analysis. BET isotherms revealed that the specific surface area and average pore diameter of the hollow spheres were 40.95 m2/g and 19 nm, respectively. Photocatalytic experiments indicate that 78% MB was degraded by WO3/TiO2 hollow spheres under visible light within 80 min. Under the same conditions, only 24% MB can be photodegraded by TiO2. The photocatalytic mineralization of MB, catalyzed by TiO2 and WO3/TiO2, proceeded at a significantly higher rate under UV irradiation than that under visible light, and more significant was the increase in the apparent rate constant with the WO3/TiO2 composite semiconductor material which was 3.2- and 3.5-fold higher than with the TiO2 material under both UV and visible light irradiation. The increased photocatalytic activity of the coupled nanocomposites was attributed to photoelectron/hole separation efficiency and the extension of the wavelength range of photoexcitation.

Introduction

Semiconductor photocatalysts have attracted enormous attention because of their wide applications in environmental purification, such as air and water purification, and organic pollutant degradation [1], [2], [3], [4]. Among the oxide semiconductor photocatalysts available, titanium dioxide has proven to be the most widely used one for environmental catalysis because it has various merits, such as excellent photocatalytic activity, low cost, nontoxicity, and chemical and biological inertness [5], [6]. However, TiO2 has two major disadvantages that restrict it from being widely used in practical applications: (1) the rapid recombination of photogenerated electron (ecb)/hole (hVB+) pairs, which significantly reduces photocatalytic efficiency [7]; and (2) the location of its light response region in the UV region, which accounts for only about 5% of the energy of sunlight.

Considerable efforts have been made to prevent the recombination of charge carriers in the semiconductor and improve the photocatalytic efficiency of TiO2, such as doping with metal ions, especially transition metal ions into the TiO2 lattice [8], deposition of noble metals [9], dye photosensitization on the TiO2 surface [10], [11], and coupling with other semiconductors, such as ZnO–TiO2 [12], V2O5–TiO2 [13], ZrO2–TiO2 [14], [15], CdS–TiO2 [16], [17] and WO3–TiO2 [18], [19], [20], [21], [22]. Among the oxide semiconductors, coupling TiO2 with WO3 has been the subject of intensive investigations during the last 15 years [18], [22] as an approach that for achieving an efficient charge separation and improving the photocatalytic properties of TiO2.

Morphology and microstructures play an important role in the photocatalytic activity of titania. Many studies concerning the new structures of titania, such as titanium dioxide nanowires, nanorods, nanotubes [23], [24], and so on, have been undertaken. Recently, the preparation of titania hollow microspheres has also attracted much attention because of their large surface area, low density, and highly efficient light-harvesting abilities [7], [25]. Despite the fact that many papers have focused on the fabrication of nano- and micro-scale hollow spheres, most of these studies focus on the synthesis and morphology of pure phase titania.

Several synthetic approaches have been employed for preparing mixed metal–oxide systems with core–shell morphology. In comparison to other particles, hollow spheres can be widely used in several applications, such as drug delivery, protection of light-sensitive biological molecules, controlled release of various substances, nano-reactors, and paints and fillers, among others. Hollow balls have many advantages compared to porous particles because of the relatively low density of the former compared to the latter.

In the present work, hollow spheres composed of mixed WO3/TiO2 metal oxide shells were synthesized using colloidal carbon spheres as the template. The aim of this study was to design and construct WO3/TiO2 hollow spheres that can be effectively applied in air/water purification and organic pollutant degradation.

Section snippets

Materials

All reagents were of analytical grade and were subjected to no further treatment. Tetrabutyl titanate, Ti(OBu)4 (Sinopharm Chemical Reagent Co., Ltd.) was used as the starting material. Glucose, d-(+)-C6H12O6 (Sinopharm Chemical Reagent Co., Ltd.), absolute ethanol, CH3CH2OH (Sinopharm Chemical Reagent Co., Ltd.) and ammonium metatungstate, (NH4)6W7O24·6H2O (Sinopharm Chemical Reagent Co., Ltd.) were used as received. The water used throughout was deionized and doubly distilled.

Preparation of carbon spheres

In a typical

Morphology

Composite photocatalyst WO3/TiO2 hollow spheres were synthesized using a colloidal carbon template. The micrographs of the samples were observed by SEM and TEM. Fig. 1 shows the SEM and TEM images of the carbon sphere template obtained by hydrothermal synthesis. The micrographs of the carbon spheres showed that the carbon balls were nearly mono-dispersed, spherical structures with uniform particle size distributions. In addition, the balls had smooth surfaces, and the diameter of the colloidal

Conclusions

Hollow spheres composed of WO3/TiO2 mixed metal oxide shells were fabricated by the template method. The as-synthetic hollow spheres exhibited high catalytic activity for MB photo-degradation under visible light irradiation. The indirect band gap energies of TiO2 and WO3/TiO2 hollow spheres were 3.0 and 2.6 eV, respectively. WO3-loaded TiO2 can shift the light absorption band from near UV to the visible region. Kinetic studies indicated that the photocatalytic degradation of MB followed

Acknowledgments

This research was supported by a grant from Planned Science and Technology Project of Hunan Province, China (no. 2008SK1001). This study was also partly supported by the State Key Program of National Natural Science Foundation of China (no. 51072232). Here, we are grateful for their financial supports.

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