Ag2S-doped core-shell nanostructures of Fe3O4@Ag3PO4 ultrathin film: Major role of hole in rapid degradation of pollutants under visible light irradiation
Graphical abstract
Introduction
Organic micropollutants (OMPs), such as neonicotinoid insecticides and antibiotics, adversely affect water quality and aquatic food webs [1], [2], thus cost-effective options for OMPs treatments have attracted much attention over the past years [3], [4], [5]. A variety of advanced treatment processes have been developed to remove OMPs from water efficiently, including membrane filtration, advanced oxidation/reduction processes, and adsorption [6], [7], [8], [9], [10]. Compared to membrane filtration and adsorption, photocatalytic degradation provided a potential for efficient and rapid removing OMPs from water [11], [12], [13], [14]. More importantly, membrane filtration and adsorption would likely re-release OMPs when regeneration of the used membrane and adsorbents, whereas photocatalytic degradation would destroy the OMPs molecularly. Silver phosphate (Ag3PO4) with a direct transition of 2.4 eV, being superior compared to a traditional ultraviolet photocatalyst like TiO2 (e.g., 3.2 eV for anatase and 3.0 eV for rutile) [15], [16], showed a quantum efficiency up to 90% under visible light [17], [18], exhibiting high efficiency for degradation of organic pollutants [19], [20], [21]. However, Ag3PO4 would undergo severe photocorrosion during its photocatalytic process because the separation efficiency of photogenerated electron-hole pairs was poor and thus pair recombination was rapid due to the narrow band gap. Accordingly, this would drastically decrease the structural stability of Ag3PO4 and thus reduce its photocatalytic activity [22], [23]. Photocorrosion would readily exhaust Ag3PO4 and leach dissolved silver from Ag3PO4, which indicated that additional Ag3PO4 would be consumed, resulting in high cost of Ag3PO4 for practical uses. Moreover, background constituents in polluted water like natural organic matter (NOM) would generally inhibit photocatalytic activity of catalysts through scavenging of produced reactive oxygen species (ROS) at the surface and in solution, competitive adsorption of inhibitors, and the inner filtering of the excitation illumination [24]. These issues together with catalysts separation/collection seriously limited an extensive application of Ag3PO4 to remediate pollutants in environment.
To improve the stability and thus photocatalytic activity of Ag3PO4, researchers fabricated Ag3PO4-based nanocomposites by adjusting the band gap of Ag3PO4 [25], [26]. Silver sulfide (Ag2S), one of metal sulfides with fast charge-exchange feature, has a narrow band gap and shows very high stability [27]. Epitaxial growth of Ag2S may potentially protect Ag3PO4 from photocorrosion and further improving photocatalytic activity of Ag3PO4. Micron sized Ag3PO4 core with particulate Ag2S shell showed high stability, yet the photocatalytic activity toward organic dye was not ideal [28], being likely related to the limited interface between dodecahedron Ag3PO4 core and particulate Ag2S coating. We hypothesized that the epitaxial growth of Ag2S crystals into an ultrathin Ag3PO4 film may provide well interfacial connection, which was very crucial in promoting the photogenerated charge carrier transfer/separation and thus the photocatalytic performance. Also, an ultrathin Ag3PO4 film on magnetic particles has potentials for efficient separation/collection of catalysts under an external magnetic field. In the present paper, we reported for the first time the visible-light photocatalytic performance and stability of Ag2S-doped core-shell nanostructures of Fe3O4@Ag3PO4 ultrathin film.
Herein, we designed and prepared Ag2S-doped core-shell nanostructures of Fe3O4@Ag3PO4 ultrathin film (denoted as Ag2S/Fe3O4@Ag3PO4) via solvothermal deposition of Ag3PO4 on Fe3O4 nanoparticles, followed by an in-situ anion-exchange reaction between Ag3PO4 and Na2S at room temperature (Scheme 1). We further adopted neonicotinoid insecticides (e.g., imidacloprid and thiacloprid) and antibiotics (e.g., difloxacin hydrochloride and sulfadizine) as model OMPs to examine photocatalytic activity and stability of the novel catalyst in visible light. The effect of environmental background constituents in particular NOM and inorganic salts on the photocatalytic activity was evaluated. Furthermore, potential mechanisms for the photocatalytic activity and anti-photocorrosion of the novel catalyst were proposed on the basis of scavenging experiments and catalyst characterization analysis.
Section snippets
Materials
Silver nitrate (AgNO3), dibasic sodium phosphate (Na2HPO4), iron(II) chloride tetrahydrate (FeCl2·4H2O), iron(III) chloride hexahydrate (FeCl3·6H2O), ammonium hydroxide, sodium sulfide (Na2S), sodium chloride (NaCl), sodium dichromate (Na2Cr2O7), ethylenediaminetetraacetic acid disodium salt (Na2-EDTA), sodium nitrate (NaNO3), sodium oxalate (Na2C2O4), calcium chloride (CaCl2), p-benzoquinone (PBQ), ethanol and tert-butyl alcohol (TBA) were purchased from Sinopharm Chemical Reagent Co. Ltd
Characterization of photocatalysts
The XRD patterns were performed to examine crystal phase of the prepared nanocomposites. The diffraction peaks of pure Ag3PO4 were at 21.00, 29.80, 33.40, 36.65, 47.85, 55.10, 57.35 and 61.75°, being well indexed to the {1 1 0}, {2 0 0}, {2 1 0}, {2 1 1}, {3 1 0}, {3 2 0}, {3 2 1} and {4 0 0} planes of the cubic structure of Ag3PO4 (JCPDS No. 06–0505) (Fig. S1). Compared to Fe3O4, new diffraction peaks appearing in the XRD of Fe3O4@Ag3PO4 nanostructures could be well assigned to Ag3PO4 (Fig. S1
Conclusion
In summary, the b-Ag2S/Fe3O4@Ag3PO4-100 photocatalyst exhibited highly efficient photocatalytic activity toward OMPs including imidacloprid, thiacloprid, difloxacin hydrochloride and sulfadizine under visible light irradiation. Compared to Ag3PO4, the b-Ag2S/Fe3O4@Ag3PO4-100 showed enhanced photocatalytic activity and anti-photocorrosion. Moreover, high photocatalytic degradation efficiencies of OMPs could be observed even in environmental water, showing possible potentials for practical
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
We thank the Natural Science Foundation of Zhejiang Province (LY18B070011) and the National Natural Science Foundation of China (21806141, 21806143) for financial support. The authors thank the anonymous reviewers for their valuable comments and suggestions on this work.
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