Microwave hydrothermal synthesis of Sr2+ doped ZnO crystallites with enhanced photocatalytic properties
Introduction
Zinc oxide (ZnO) is a semiconductor photocatalysis with a wide direct band gap (3.3 eV) and a high free-excition binding energy (60 meV) [1], [2]. It is known as one of the important photocatalysts due to its advantages, including the large initial rates of activities, many active sites with high surface reactivity, low price and environmental safety [3], [4]. However, it also presents some drawbacks like fast recombination rate of the photogenerated electron–hole pair and a low quantum yield in the photocatalytic reaction in aqueous solutions, highly restraining its photocatalytic activity under visible light [5], [6], [7]. Recently, how to enhance the photocatalytic activity of ZnO has drawn much attention from researchers all over the world. It is accepted that the surface area and lattice defects play important roles in photocatalytic activities of metal oxide semiconductors [8], [9], [10]. Researchers also found that doping is an effective and facile method to improve the photocatalytic properties because the variation of the surface area, the incorporation of dopant ions is able to generate lattice defects and variation of band gap energy [11], [12]. Consequently, doping of transition metals, noble metals and non-metals is a very expedient way to improve the photocatalytic activity. Many elements such as Al [9], Ta [13], Cr [14], La [15], Ag [16] and I [17] have been used as dopants and showed better photocatalytic performance.
Alkaline earth metal ions can be taken as the candidate dopants to regulating and controlling the photocatalytic properties. Venkatachalam et al. [18] found that doping of TiO2 nanoparticles with Mg2+ and Ba2+ produces higher photocatalytic activities than those of undoped TiO2 nanoparticles or commercial TiO2. Nevertheless, the entry of alkaline metal ions into the TiO2 lattice also results in the creation of significant lattice defects because of the charge compensation and the ionic radius mismatch between Mg2+ (or Ba2+) and Ti4+, which may put huge uncertainties to the origin of photoactivities. It is well documented that ZnO is superior semiconductor photocatalysis over TiO2 in producing hydrogen peroxide, which allows its uses in efficient photodegradation of organic dye [19], [20]. Hence, alkaline earth metal ions doped ZnO may also improve its photocatalytic properties greatly. Qiu et al. [21] prepared Mg2+ doped ZnO samples by a novel rheological phase reaction route and all Zn1−xMgxO samples exhibit high photoactivities comparable to pure ZnO by degrading methylene blue dye solution under UV light irradiation. Nevertheless, the research of alkaline earth metal ions doped ZnO to study its photocatalytic activity under visible-light irradiation has rarely been reported. As one of the important alkaline-earth metals, strontium is widely used for many applications including the electronics, metallurgy, chemical industry, military industry, optics and some other fields, it is also believed to be a good potential candidate material doped ZnO structures.
It is well known that ZnO crystallites can be doped with different ions through the hydrothermal method. Although the preparation of the doped ZnO has good optical performance via this method, but the conventional hydrothermal method usually costs long time and has more difficulties in technique[22], [23], [24]. Microwave-assisted hydrothermal (M–H) has been considered to be one of significant methods to prepare doped metal oxide crystallites in recent years. Microwaves energy has been demonstrated to enhance organic chemical reaction, increasing the net rate early in the heating process. This method could be attributed to the difficulties in controlling the simultaneous growth of the crystal and recombination of interparticles by the microwave heating [25], [26], [27].
In the present work, we try to use a novel microwave hydrothermal approach to prepare Sr2+ doped ZnO (Zn1−xSrxO) crystallites and try to reveal their photocatalytic activity under visible-light irradiation.
Section snippets
Source materials
The starting materials used in this experiment were analytical without further purification. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (analytical grade, Guangdong Guangghua Sci-Tech Co., Ltd.) and strontium nitrate (Sr(NO3)2) (analytical grade, Sinopharm Chemical Reagent Co., Ltd.) were used as zinc and strontium sources, respectively. Sodium hydroxide (NaOH) (analytical grade) was purchased from Tianjing Hengxin Chemical Reagent Co., Ltd. Ethanol absolute (Tianjing Fuyu Industry of fine
XRD patterns of Zn1−xSrxO
In order to investigate the crystal structure lattice parameters of the Sr2+ doped ZnO crystallites, XRD analysis is used. Fig. 1 shows the XRD patterns of pure ZnO and Sr2+ doped ZnO crystallites. For the Sr2+ doped ZnO crystallites prepared by the microwave hydrothermal method, the XRD analysis showed that a secondary phase occurred in the XRD pattern when the Sr2+ content is 0.3% (Fig.1A(d)). The major phase observed in the XRD pattern is the ZnO hexagonal wurtzite structure according to the
Conclusion
ZnO crystallites were successfully doped with different amounts of Sr2+ via a microwave hydrothermal method. Results show that the increasing the doping concentration of Sr2+ led to the increase of ZnO crystal lattice with morphology change from lamellar-like into a hexagonal columnar structure. UV–vis results further present that these ZnO crystallites exhibit higher absorptions in visible-light region with decreased optical band gaps when increasing the Sr2+ doping concentration. Moreover,
Acknowledgments
This work has been supported by the National Natural Science Foundation of China (50942047), Innovation Team Assistance Foundation of Shaanxi Province (No. TD12-05), International Science and Technology Cooperation Project of Shaanxi Province (2011KW-11) and the Graduate Innovation Foundation of Shaanxi University of Science and Technology.
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2022, Materials Science in Semiconductor ProcessingCitation Excerpt :ZnO is an n-type II-IV semiconductor with a 3.37 eV band gap, a strong excitation binding energy of 60 meV, and a high visible transmission [22]. Low cost, environmental safety, high initial activity rates, and a large number of active sites with strong surface reactivity are some of the advantages of ZnO [47,48]. ZnO absorbs mainly in the UV because of its wide band gap of 3.37 eV (wavelength = 380 nm).