One-pot synthesis of ternary Ag2CO3/Ag/AgCl photocatalyst in natural geothermal water with enhanced photocatalytic activity under visible light irradiation
Graphical abstract
Introduction
Semiconductor photocatalysis has attracted considerable attention since Fujishima and Honda reported the photoelectrochemcial evolution of H2 on the TiO2 electrode in 1972 [1]. Many researches focused on the photocatalytic degradation of organic pollutants [2], [3], [4], [5] and the photocatalytic H2-generation from water splitting [6], [7], [8], [9]. TiO2, as a typical semiconductor, has been widely used for the application in photocatalysis due to its high chemical stability and prefect photocatalytic activity [10], [11], [12]. Unfortunately, TiO2 can only be activated by UV light which accounts for ca. 5% of the solar energy because of its large band gap (3.2 eV for anatase TiO2), resulting in the low quantum efficiency. Many efforts have been devoted to finding and designing visible-light-responsive photocatalysts according to the band gap and energy levels of the valence band and conduction band. It is worth noting that the coupling of various semiconductors with different band gap levels has been widely investigated to enhance the visible-light absorption capability. Compared with the single-component photocatalysts, semiconductor-semiconductor composites, mainly including sensitization, p–n heterojunction and Z-scheme, usually show excellent photocatalytic activity [13], [14], [15], [16], [17]. In the case of a Z-scheme system, photoexcited electrons transferred from the valence band to conduction band in both semiconductors. The photoexcited electrons of the semiconductor with a higher conduction band acted as the reduction sites and the holes of the semiconductor with a lower valence band are used for the oxidation. Meanwhile, the photoinduced electrons of the semiconductor with a lower conduction band recombined with the photogenerated holes of the semiconductor with a higher valance band. As a result, the Z-scheme photocatalysts have stronger reduction and oxidation potential than the individual photocatalyst [16], [18]. Recently, Ag-based nanomaterials have been considered as promising photocatalysts for the degradation of organic pollutants under visible light irradiation, such as Ag/AgCl [19], [20], [21], Ag3PO4 [22], Ag2O [23], [24], Ag2CO3 [25], [26]. Among them, Ag/AgX (X = Cl or Br) plasmonic photocatalysts stand out owing to the high efficiency and excellent stability under visible light irradiation [3], [21], [27], [28]. The absorption ability in visible light region results from the strong localized surface plasmon resonance (LSPR) of photoexcited Ag nanoparticles. In order to further improve their photocatalytic performance, many hierarchical nanostructures have been reported, including Ag/AgCl/WO3 [29], Ag/AgBr/Bi2MoO6 [30], Ag3VO4/AgBr/Ag [31], AgCl/Ag/γ-TaON [32], and Ag/AgCl/Bi20TiO32 [33]. Compared with the one-component or two-component products, these hierarchical composites indeed enhanced the photocatalytic activity. Therefore, it is critically important to design novel visible-light-driven photocatalysts for the efficient degradation of toxic organic compounds and high energy conversion of solar energy to chemical energy.
Natural geothermal water is a clean, renewable and cheap resource, which is widely distributed and has been used in many fields of industry and life in the world, such as the generation of electric power and physical therapy [34], [35], [36]. Geothermal energy is the natural heat energy contained in the depth of the earth, primarily attributed to the magma and the decay of the radioactive isotopes [37]. Geothermal resource is a kind of renewable energy, which can provide clean and green energy. The development and utilization of renewable energies have been considered as important measures to solve resource shortage and relieve environment pollution pressure. The formation of geothermal water originates from the ground water heated by geothermal energy and hot magma. Then the heated water gushes out of the surface cracks owing to the existence of hydrostatic pressure gradient. It is general that geothermal water contains Cl−, HCO3−, SO42−, Na+, Mg2+ and so on. To the best of our knowledge, there has been no report about the synthesis of visible-light-driven photocatalysts using geothermal water as raw material. In this work, we firstly report a one-pot precipitation method for the direct synthesis of Ag2CO3/Ag/AgCl composite photocatalyst by dropwise addition of silver nitrate (AgNO3) aqueous solution into natural geothermal water which served as chlorine and carbonate sources. Herein, the representative geothermal water was chose from Changbai Mountain, Jilin Province, China. The physical-chemical properties of the as-obtained products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectra (XPS). The Ag2CO3/Ag/AgCl photocatalyst exhibited excellent photocatalytic performance and stability for the visible-light-driven photocatalytic degradation of methylene blue (MB) dye due to the efficient charge carrier separation through a Z-scheme system. This new work might extend the potential application of cheap geothermal water in environment purification.
Section snippets
Synthesis of Ag2CO3/Ag/AgCl composite
All the chemicals were of analytical grade and used as purchased without any further treatment. The geothermal water in this work was taken from Changbai Mountain, Jilin Province, China. The ternary Ag2CO3/Ag/AgCl composite was synthesized by a one-pot precipitation method under ambient conditions. In a typical process, 20 mL of AgNO3 aqueous solution (0.01 M) was dropwise added into 20 mL of natural geothermal water at room temperature with strongly magnetic stirring under sunlight. Then, the
Results and discussion
The Ag2CO3/Ag/AgCl composite was successfully synthesized via a one-pot precipitation method by simply adding an aqueous solution of AgNO3 into natural geothermal water. A white suspension could be observed immediately while AgNO3 solution was dropped into the geothermal water and the color of the product gradually changed to dark gray after being stirred for 0.5 h. The phase structure of the as-synthesized products was investigated by XRD and the typical XRD patterns of Ag2CO3, Ag/AgCl, and Ag2
Conclusion
In this work, a plasmonic Z-scheme photocatalyst of ternary Ag2CO3/Ag/AgCl has been successfully synthesized by a one-pot green method in geothermal water at room temperature, wherein silver nitrate serves as silver source and geothermal water serves as the source of chlorine and carbonate. The as-prepared Ag2CO3/Ag/AgCl shows a higher photocatalytic performance than Ag2CO3, Ag/AgCl, Ag2CO3/AgCl and the mechanical mixture of Ag2CO3 and Ag/AgCl for the degradation of methyl blue (MB) aqueous
Acknowledgements
This project is supported financially by the National Natural Science Foundation of China (No. 51272107), the Natural Science Foundation of Jiangsu Province, China (No. BK2011024), the Specialized Research Fund for the Doctoral Program of Higher Education, China (20133219110015), and A Project Funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.
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