Preparation of ordered mesoporous Ag/WO3 and its highly efficient degradation of acetaldehyde under visible-light irradiation

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Abstract

A highly active photocatalyst, silver loaded mesoporous WO3, was successfully synthesized by an ultrasound assisted insertion method. The photodegradation of a common air pollutant acetaldehyde was adopted to evaluate the photocatalytic performance of the as-prepared sample under visible-light irradiation. The photocatalytic activity was about three and six times higher than that of pure mesoporous WO3 and nitrogen-doped TiO2, respectively. The photocatalytic mechanism was investigated to understand the much enhanced photocatalytic activity, which was mainly attributed to the largely improved electron–hole separation in the Ag–WO3 heterojunction.

Introduction

In recent years, numerous studies have been reported on semiconductor photocatalysis which can utilize solar energy to decompose harmful organic pollutants in air and aqueous systems [1], [2], [3], [4], [5]. Among the semiconductors employed, TiO2 is the most extensively studied photocatalyst because of its low cost, high efficiency and stability. However, TiO2 is only active in the ultraviolet (UV) light range due to its wide band gap. To obtain visible-light-driven photocatalysts so as to utilize visible light, doping or ion-implanting has been used to modify TiO2 [6], [7], [8], [9], but dopants usually act as recombination centers between the photogenerated electrons and holes, which greatly reduced the photocatalytic activities [10]. Thus researchers are devoted in searching for other candidates to solve this problem.

Tungsten trioxide (WO3), which possesses a small band gap of 2.4–2.8 eV, has many advantages for visible-light-driven photocatalysis such as strong adsorption within the solar spectrum, stable physicochemical properties, and resilience to photocorrosion effects [11], [12], [13], [14]. In addition to the visible-light absorption, there are many other factors influencing the photocatalytic activity, such as the potential levels of energy bands, the separations of photogenerated electron–hole pairs and the microstructures of photocatalysts, etc. Like other simple binary metal oxides, WO3 has a deep valence band which is mainly composed of O 2p orbitals. The deep valence band combined with the small band gap results in a low conduction band level, which limits the photocatalysts to react with electron acceptors [15], [16], [17] and then increases the recombination of photogenerated electron–hole pairs. This was one of the reasons limited the development of WO3 as a practical photocatalyst. Therefore, one of the principles to improve the photocatalytic performance of WO3 is to increase the efficiency of electron–hole separation. Loading noble metals on the photocatalyst has been proved as an effectual approach recently [18], [19], [20]. Among various noble metals, silver is of considerable interest not only because of the resultant enhanced electron–hole separation but also ascribed to the extension of visible-light absorption and enhanced photocatalytic activity from the surface plasmon resonance (SPR) effect of silver nanoparticles [21], [22].

As mentioned above, the microstructures of the photocatalyst also influence the photocatalytic activity significantly. Mesoporous structures exhibit the obvious advantages for the heterogeneous catalysis [23], [24]. Especially, ordered mesoporous structures have been proved to be excellent structures for photocatalysis due to their larger surface area and multiple scattering, enable more light to be harvested and also possess continuous pore channels that facilitate the transfer of reactant molecules [25]. Inspired by the above analysis, we conceive that mesoporous silver loaded WO3 (m-Ag/WO3) may exhibit high photocatalytic activity. However, the preparation and photocatalytic property of m-Ag/WO3 have not been reported up to the present.

In the present paper, photocatalytic active m-Ag/WO3 was synthesized by an ultrasound assisted insertion method. The photooxidation of a common air pollutant acetaldehyde was adopted to evaluate the photocatalytic performance of the as-prepared sample under visible-light irradiation. Comparative studies indicate that the photocatalytic activity of m-Ag/WO3 is much superior to that of pure mesoporous WO3 (m-WO3), silver loaded commercial WO3 (c-Ag/WO3) and nitrogen-doped TiO2 (N-TiO2) nanoparticles under the same conditions. Besides, the photocatalytic mechanism was investigated to understand the much enhanced photocatalytic activity.

Section snippets

Sample preparation

Mesoporous silica with cubic Ia3d symmetry (KIT-6) was prepared according to the reference using tri-block copolymer Pluronic P123 (EO20PO70EO20, MW = 5800, Aldrich) as template in an acidic aqueous solution [26].

Mesoporous WO3 was prepared by a hard template replicating technique. Typically, 1.2 g of 12-phosphotungstic acid (AR, Sinopharm) was dissolved in 10 mL of ethanol, and this solution was incorporated into 0.4 g of as-prepared KIT-6 template under stirring by the impregnation technique.

Structural characteristics

The crystalline phase and mesostructural ordering of the m-WO3 and m-Ag/WO3 samples were characterized by both wide-angle X-ray diffraction (WXRD) and low-angle X-ray diffraction (LXRD) measurements. As shown in Fig. 1a, WXRD demonstrates the m-WO3 sample was well crystallized in a single phase and all of the diffraction peaks can be indexed to monoclinic WO3 (JCPDS 72-0677). The WXRD of the m-Ag/WO3 sample was similar to that of m-WO3 except for the diffraction peak of Ag (1 1 1) at 2θ of 38.1°

Conclusion

Mesoporous silver loaded WO3 has been successfully synthesized by an ultrasound assisted insertion method. The as-prepared m-Ag/WO3 sample exhibits excellent photocatalytic decomposition of a common air pollutant of acetaldehyde under visible-light irradiation. Comparative studies indicated that the photocatalytic activity of the m-Ag/WO3 sample is much superior to that of m-WO3, c-Ag/WO3 and N-TiO2 under the same conditions. The photocatalytic mechanism was investigated to understand the much

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (Nos. 50732004 and 50672117) National Basic Research Program of China (973 Program, 2007CB613305) and Nanotechnology Programs of Science and Technology Commission of Shanghai Municipality (0852nm00500).

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