Visible light photocatalytic degradation of MB using UiO-66/g-C3N4 heterojunction nanocatalyst
Introduction
Semiconductor photocatalysis is an efficient method for environmental detoxification, which is able of converting toxic and non-biodegradable organic compounds into carbon dioxide, water and inorganic salts (Li and Li, 2002; Hu et al., 2018; Zhang et al., 2018b, 2018c). As a good semiconductor photocatalyst, graphitic carbon nitride (g-C3N4) has attracted an increasing attention of scientists recently (Hu et al., 2016; Chen et al., 2017). It is composed of carbon and nitrogen only and has a graphite-like layered structure. In addition the optical band gap of g-C3N4 is approximately 2.7 eV that exhibits a strong response for the visible-light region (Thomas et al., 2008). Owing to its advantages of visible light response, low cost, chemical-stable and non-toxicity (Thomas et al., 2008; Ge and Han, 2012; Niu et al., 2012), g-C3N4 has been used as photocatalyst for water or air purification (Yu et al., 2013, 2014; Tong et al., 2015; Hong et al., 2016; Ye et al., 2016; Giannakopoulou et al., 2017). Even though, the photocatalytic activity of g-C3N4 is somewhat unsatisfactory by the low specific surface area and the rapid recombination of photogenerated electrons and holes because of its narrow band gap. Strategies such as different precursor (e.g. urea (Liu et al., 2011; Zhang et al., 2012; Zhang et al., 2014)) preparation methods, hard template method (e.g. SBA-11 (Zhang et al., 2017), SBA-15 (Chen et al., 2009), SiO2 (Bu et al., 2014; Zhu et al., 2015, 2016)), soft template method (e.g. Pluronic P123 (Zhang et al., 2017)) and exfoliating bulk g-C3N4 to g-C3N4 sheet method (Yang et al., 2013; Wang et al., 2016) were applied to increase the specific surface area. In addition strategies such as coupled with other compounds (e.g. Fe2O3 (Christoforidis et al., 2016), SnO2 (Zang et al., 2014), TiO2 (Chen et al., 2017), Bi2WO6 (Ma et al., 2016), BiPO4 (Pan et al., 2012), Ag3PO4 (Zhang et al., 2013), CdS (Fu et al., 2013)), doping with metal or nonmetal species (e.g. S (Liu et al., 2010), B (Yan et al., 2010), carbon nanotubes (Ge and Han, 2012), Au/Ag nanoparticles (Cheng et al., 2013; Bai et al., 2014)) and coupled with graphene (Ma et al., 2016) were used to ameliorate the rapid recombination of photogenerated electrons and holes to improve the photocatalytic efficiency. However, very few of methods can simultaneously solve the problem of low specific surface area and rapid recombination of electron and holes.
Metal organic frameworks (MOFs), as hybrid organic-inorganic compounds, are high porous materials synthesized through the coordination of metallic ions and organic ligands (Li et al., 2012). Due to their ultrahigh porosity and incredibly high internal surface areas, they have been applied in catalysis (Timofeeva et al., 2014; Arrozi et al., 2015; Kaur et al., 2016; Crake et al., 2017), storage/separation (Huang et al., 2016; Seo et al., 2016; Sarker et al., 2018) and sensing (Hu et al., 2015). In recent years, MOFs are of interest for photocatalysis because of their high surface area, tunable pore size and high light harvesting capacity. Until now, several MOFs including, HKUST-1 (Mosleh et al., 2016), UiO-66 (Xu et al., 2017), MIL-68 (In) (Yang et al., 2017), MOF-235 (Li et al., 2016), MOF-46 (Rad and Dehghanpour, 2016) and MOF-5 (Zhang et al., 2018b), etc. have been studied as photocatalysts. Among many MOFs materials, zirconium-based MOF (UiO-66) attracted widely attention because of its higher chemical stability in water and thermostability compared to other MOFs materials (Shi et al., 2015). It has been reported that due to the synergistic effect arising from the catalytically reactive sites (metal centers/organic linkages) of MOFs, upon its exposure to UV, UiO-66 can act as semiconductors (Shen et al., 2013; He et al., 2014; Sha and Wu, 2015; Ding et al., 2017). Based on the above mentioned superior properties, UiO-66 can be used as a photocatalyst for eliminating organic pollutants from wastewater. Nevertheless, the band gap of UiO-66 is about 3.6 eV which limit its optical adsorption in the visible light region. To achieve this, a narrow band gap semiconductors coupling formed a heterojunction method is an ideal choice. The heterojunction can form an inner electric field to facilitate the transfer/separation of photogenerated electron/hole and inhibit the recombination of electrons and hole so as to enhance the photocatalytic activity (Lin et al., 2014; Shen et al., 2015). Based on the above two types of excellent semiconductors, UiO-66/g-C3N4 heterojunction photocatalyst received great attention. Wang et al. prepared UiO-66/g-C3N4 heterojunction through annealing the mixture of UiO-66 octahedrons and g-C3N4 with various mass ratio in Ar atmosphere for enhanced photocatalytic hydrogen evolution under visible light irradiation (Wang et al., 2015a). Zhang et al. synthesized g-C3N4/UiO-66 nanohybrids via solvothermal method for the oxidation of dye under visible light irradiation (Zhang et al., 2018a). However, the preparation methods mentioned above are complicated which need to be annealing in Ar atmosphere or twice solvothermal reaction. Therefore, based on the excellent properties of UiO-66/g-C3N4 heterojunction (UC) hybrids, it is worthy to further study on the synthesis of UC hybrids to find a more simple and effective method.
Herein, we report the synthesis of UiO-66/g-C3N4 heterojunction photocatalyst through simply annealing the mixture of UiO-66 and g-C3N4 nanosheets in air atmosphere and photocatalytic degradation of the MB in water under visible light irradiation. The hybrids were presented as excellent adsorbents and catalysts because the accessible surface area of g-C3N4 is enlarged and a heterojunction interaction is formed between the two components. The ultrahigh porosity and incredibly high internal surface areas structure of MOFs were retained in the UiO-66/g-C3N4 which could solve the low specific surface area problem of g-C3N4, at the same time, the heterojunction between two semiconductors can facilitate the electron transfer and separation, thus preventing the recombination of electrons and holes. Furthermore, UiO-66/g-C3N4 can utilize the visible light owe to the narrow band gap of g-C3N4. To our knowledge, this kind of synthesis method is rarely reported for UiO-66/g-C3N4 heterojunction photocatalyst to organic photodegradation.
Section snippets
Chemicals
Urea, terephthalic acid (H2BDC), N, N dimethyl-formamide (DMF), zirconium chloride (ZrCl4), ethanol (EtOH), acetic acid, methanol, isopropanol (IPA), and methylene blue dye (MB) were purchased from Sinopharm Co. Ltd. All chemicals and reagents used were in analytical grade.
Synthesis of UiO-66 octahedrons
UiO-66 Octahedrons were synthesized by a solvothermal method. 0.2 g ZrCl4, and 0.14 g terephthalic acid (H2BDC) were dissolved in 40 mL N, N-dimethylformamide (DMF) with a continuous stirring for 1 h and then added 8 mL of
Structure characterization
The X-ray diffraction patterns of pristine g-C3N4, UiO-66 octahedrons and UC x:y hybrids are included in Fig. 1 X-ray diffraction (XRD) analysis shows that the diffraction of UiO-66 fits well with the diffraction pattern in references (Cavka et al., 2008; Valenzano et al., 2011). For the pure g-C3N4, the peaks at 27.5° and 13.5°are found corresponding to the typical interplanar stacked graphitic layered structure and the (002) plane (JCPDS No. 87–1526). In addition, all diffraction patterns in
Conclusion
UiO-66/g-C3N4 hybrids have been synthesized from annealing the mixture of UiO-66 and g-C3N4 nanosheets. The results showed that the UC 10:10 hybrids exhibited best photocatalytic performance on MB degradation under visible light irradiation. The enhanced photocatalytic performance was ascribed to the large specific surface area and unique pore structure from MOFs, a narrow band gap and an excellent heterojunction. The former can facilitate more dye molecules absorbed on the active sites of
Acknowledgements
This work was partially supported by the National Natural Science Foundation of China (21277108).
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