Visible-light photocatalytic degradation of methylene blue with laser-induced Ag/ZnO nanoparticles
Highlights
► The catalytic behavior of Ag/ZnO nanoparticles in the visible-light range is notably improved by the method of laser-induction. ► The maximum efficiency of 92% photodegradation of MB is reached with 2.0 wt% Ag/ZnO at pH 11. ► Increasing the content of AgNO3 in the preparation process will increase the size of Ag particles on ZnO matrix.
Introduction
Industrial effluents have long been a serious problem of the environmental protection issues [1]. Most of the dyestuffs that the textile and paper industries produced are difficult to decompose, due to the relatively stable chemical structures of aromatic dyes [2], and hence cause severe contaminations to the drinking water and irrigation systems in some areas. Methylene blue (MB), as illustrated in Scheme 1, is one of the frequently used dyes for the agriculture, textile, paper-making, cosmetic, and pharmaceutical purposes [3]. General treatments of such wastewater include adsorption [4], [5], reverse osmosis [6], or chemical coagulations [7]. However, those methods are somewhat inefficient because they require further treatments and the dealing costs are relatively expensive.
Semiconductor photocatalysts have been a potential field for treating various water pollutants [8], [9], [10]. The efficient oxidation reaction of contaminants by photocatalysts under irradiation processes provides an alternative way of managing these pollutants properly. Over 30 years of widely studies, semiconductor photocatalysts which have frequently been investigated include oxide compounds of TiO2, ZnO, WO3, SnO2, ZrO2, and sulfide compounds of CdS, ZnS, and so on [11], [12], [13], [14], [15]. Among these compounds, zinc oxide has been widely investigated for its photocatalytic activities [16], [17]. It has drawn great attention in research and industries in recent years owing to its powerful oxidation capability, non-toxicity, chemical stability, and low cost.
With a direct band gap of 3.2 eV, ZnO absorbs light less than 388 nm in wavelength and promotes the valence band electrons up to the conduction band, leaving holes in valence band as in Eq. (1). The holes produced could subsequently react with hydroxide ions (OH−) or water molecules in aqueous solution to form powerful oxidants such as hydroxyl radicals (OH) in Eq. (2). The electrons on conduction band could transform the dissolved oxygen to superoxide in Eq. (3) and further to hydrogen peroxide in Eq. (4). These strong oxidants could be used to mineralize organic pollutants from wastewater.ZnO + hv → ZnO(e− + h+)h+ + OH− → OHe− + O2 → O2−O2− + 2H+ + e− → H2O2
To expand the usage of this photocatalyst, many studies employed modifications onto ZnO nanoparticles in order to improve its catalytic efficiency through enhancements of absorbance in the visible-light region. The advantages of photocatalysts modifications are to ensure the separation of electron–hole pairs, broadening the absorption spectrum, and to facilitate some specific reactions on the surface of catalysts [18]. Those methods consist of doping ZnO with manganese [19], cobalt [20], silver [21], [22], polymer-modification [23], and so forth.
In this article, the authors present a novel preparation method of Ag/ZnO nanoparticles by using laser-induction of ZnO and AgNO3 in 2-propanol medium. The applied laser source induced evenly distributed nanoparticles of ZnO and the silver ions were reduced to silver particles. The structures, surface morphologies and optical properties of Ag/ZnO were examined by UV–vis spectroscopy (UV–vis), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and selected area electron diffraction (SAED). The photocatalytic activities of Ag/ZnO nanoparticles were evaluated through the degradations of methylene blue solutions initiated by visible-light irradiation with a high pressure sodium lamp.
Section snippets
Materials
Zinc oxide (ZnO; 99.7%) was purchased from Panto Co. Laser-induction medium 2-propanol (C3H8O; HPLC grade) was purchased from EDTEA Co. Silver nitrate (AgNO3; 99.5%) and methylene blue (C16H18ClN3S; reagent grade) were purchased from Shin-An-Shin Co. These chemicals were used without further purification. Deionized water was used throughout the entire experiments.
The preparation of laser-ablated ZnO nanoparticles
Each 10 mL 2-propanol with 0.2 g ZnO was first put in a 20-mL vial under ultrasonic vibration for 10 min. A Q-switch Continuum Powerlite
Characterization of laser-ablated ZnO and Ag/ZnO nanoparticles
Very little light can be absorbed in the visible-light spectrum with wavelength greater than 450 nm by semiconductor ZnO. Nevertheless, after various amounts of Ag were doped onto ZnO nanoparticles, with 0.5, 1.0, 2.0, 5.0, 10.0, and 30.0 wt% of AgNO3 relative to ZnO added in preparation process, the absorbance of Ag/ZnO nanoparticles in the visible-light region was notably observed from the UV–vis spectrum, with the absorbance ranging from 0.08 for 2.0 wt% to 0.2 for 30.0 wt% Ag/ZnO
Conclusions
Laser-induced Ag/ZnO nanoparticles exhibit enhanced absorption in the visible-light range by the UV–vis measurements. With EDX, TEM, and XRD measurements, corresponding amounts of Ag particles were found deposited onto the surface of ZnO matrix with relative contents of Ag during the preparation processes. The heterogeneous degradation efficiencies of ZnO semiconductor under visible-light irradiation are greatly enhanced by the modification of ZnO through laser-induction and the doping of
Acknowledgement
This work was supported by National Science Council of Taiwan and National Cheng Kung University.
References (27)
- et al.
J. Hazard. Mater.
(2006) - et al.
Bioresour. Technol.
(2001) - et al.
J. Hazard. Mater.
(2008) - et al.
J. Hazard. Mater.
(2007) - et al.
Appl. Catal. B
(1998) - et al.
J. Hazard. Mater.
(2006) Catal. Today
(1999)Spectrochim. Acta A
(2011)- et al.
Chem. Eng. J.
(2010) - et al.
Appl. Catal., A
(2009)
J. Hazard. Mater.
Mater. Sci. Eng. B
Chemosphere
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