Regular ArticleUltra-thin Bi2WO6 porous nanosheets with high lattice coherence for enhanced performance for photocatalytic reduction of Cr(VI)
Graphical abstract
Introduction
Hexavalent chromium (Cr(VI)) which arises from various industrial activities such as electroplating, leather tanning, paint and pigments represents a serious environmental pollution [1]. Cr(VI) is highly toxic to human beings and has been classified as mutagenic and carcinogenic by the International Agency for Research on Cancer with LD50 values between 50 and 150 mg/kg [2], [3]. Because of the non-biodegradable nature, Cr(VI) can contaminate groundwater and shallow wells with detrimental effects to environment [4]. Consequently, the effective and sustainable detoxification of Cr(VI)-bearing aqueous solution is one of the high-priority research directions. Up to now, adsorption, ion-exchange, chemical precipitation, biological reduction etc. have been applied for detoxification of Cr(VI) aqueous [5], [6], [7], [8], [9]. However, the high cost, complicated process and lengthy processing time limit the application of these techniques. Fortunately, photocatalytic reduction of Cr(VI) to Cr(III) over photocatalysts has been considered to be a promising route for effective and sustainable detoxification of Cr(VI)–bearing solutions. The Cr(III) is about 300 times less toxic than Cr(VI) and can be easily removed from the solution as Cr2O3 or Cr(OH)3 precipitates in alkaline conditions [10]. Therefore, a lot of researchers have striven to neutralize the deteriorating effect of Cr(VI) to less Cr(III) form by photocatalytic reduction reaction [11], [12]. So far, though many materials have been investigated as potential photocatalysts for Cr(VI) reduction, such as Bi2WO6, TiO2, Fe2O3, CoO, InSnS2, ThO2, CeO2 [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. These photocatalysts generally suffer from one or more drawbacks such as poor utilization of solar energy, low electrical conductivity, short lifetime of photogenerated carriers, photocorrosion of the crystal lattice. Therefore, the development of stable and efficient photocatalysts for Cr(VI) reduction in aqueous solutions is of particular interest because of the simplicity and low cost of operation.
Bi2WO6, with a narrow band gap of 2.7 eV, is one of the most attractive materials because of its excellent intrinsic physical and chemical properties, such as nontoxicity, high stability, wide solar response, ferroelectric piezoelectricity, catalytic behavior [26]. Bi2WO6 applied as photocatalyst has been widely used for CO2 photoreduction, organic pollutants degradation, photocatalytic water splitting etc. [27], [28], [29]. However, Bi2WO6 as photocatalyst, the photogenerated charge carriers in excited states are not stable and tend to recombine. Therefore, a lot of approaches, such as morphology modulation, cation ion doping, p-n junction and quantum dot modification have been developed to enhance the photocatalytic performance [30], [31], [32], [33], [34], [35], [36], [37].
Especially, it is widely acceptable that two-dimensional (2D) nanosheets applied for photocatalytic application can facilitate the photogenerated carrier transfer from inside to the surface, and the 2D nanosheets with large fraction of uncoordinated surface atoms could harvest more ultraviolet–visible light. All the characters are beneficial to enhance the photocatalytic activity of photocatalysts. Bi2WO6 has layered structure with octahedral [WO4]2− sandwiched by [Bi2O2]2+. Such layered structure may be stacked with different monolayer oxides by chemical bonds, and may possess oxygen depleted surfaces that expose a huge number of active sites. Therefore, it is desirable to fabricate 2D Bi2WO6 nanosheets and applied in photocatalysis [38]. Z.G. Zou group synthesized ultrathin and uniform Bi2WO6 nanoplates and applied to photocatalytic reduction of CO2 into renewable hydrocarbon fuel under visible light [39]. Y.F Zhu group synthesized square Bi2WO6 nanoplates and applied to photodegradation of organic pollutants [40]. Y. Xie group synthesized {0 0 1}-oriented Bi2WO6 nanosheets with thickness of ∼4 nm and applied as photocatalysts with high efficiency [41]. In addition, the composite, such as carbon quantum dots modified Bi2WO6 nanosheet [42], Bi2WO6/MoS2/RGO heterojunction [43] have been developed to enhance the photocatalytic activity. Ion doping, such as boron doped Bi2WO6 nanosheets as visible photocatalysts has also been reported [44]. However, achieving Bi2WO6 nanosheets with controllable thickness is still a challenge.
Herein, we successfully synthesized thickness controllable Bi2WO6 porous nanosheets (PNS) with high lattice coherence by facile hydrothermal method. The thickness of nanosheets could be controlled from ∼27 nm to ∼16 nm by adjusting the precursor concentration. The efficiency of photocatalytic reduction of Cr(VI) was associated with the thickness of photocatalysts and Bi2WO6 nanosheets with thickness of ∼18 nm (BWO-3) exhibited the highest photocatalytic activity and excellent stability under visible light irradiation. According to Mott-Schottky and VB XPS measurements, BWO-3 had upshifted conduction band, which can enhance the photoreduction capability. Further, the optical and electrochemical measurements exhibited that BWO-3 had increased charge transfer, decreased recombination efficiency and prolonged carriers’ lifetime due to the appropriate thickness, deficiency and porous structure.
Section snippets
Materials
Na2WO4·2H2O, Bi(NO3)3·5H2O, cetyltrimethyl ammonium bromide (CTAB), NaOH, K2Cr2O7, H2SO4, H3PO4, oxalic acid, acetone, and diphenylsemicarbazi were bought from Sinopharm Chemical Reagent Co. Ltd. All chemicals were of analytical grade and used without further purification.
Synthesis of the photocatalysts
The Bi2WO6 PNS were synthesized via hydrothermal method. Typically, 0.165 g (0.5 mmol) Na2WO4·2H2O and 0.025 g CTAB were dissolved in 40 mL of deionized water under magnetic stirring forming clear solution. Then, 0.485 g
Characterization of Bi2WO6
Firstly, the morphology of BWO-3 was investigated by SEM and TEM measurements. The low-resolution SEM image of BWO-3 shown in Fig. 1a reveals the sheet structure and the nanosheets aggregate together. While, the sheet-like structure can be seen more clearly from the magnified SEM image (Fig. 1b). The TEM image of BWO-3 shown in Fig. 1c further verifies the sheet-like structure and the nanosheet is very thin and nearly transparent. Further, apparent pores can be observed from Fig. 1c, which
Conclusions
In summary, we have successfully fabricated thickness controllable Bi2WO6 PNS via facile hydrothermal method. According to the characterization of the structure, morphology, optical and electronic properties, the thickness of Bi2WO6 nanosheets decreased as the decrement of precursor concentration. When the Bi2WO6 PNS were used for photoreduction of Cr(VI), Bi2WO6 PNS with thickness of ∼18 nm (BWO-3) exhibited the best photocatalytic activities resulted from enhanced carriers’ transfer
Acknowledgements
This work is supported by National Natural Science Foundation of China (No. 61204078, 21671059), Program for Innovative Research Team (in Science and Technology) in University of Henan Province (No. 13IRTSTHN026), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (No. 154200510009), Key Project of Science and Technology of Henan Province (No. 17A150033), Henan Science and Technology Program (No. 162300410174, 162300410204), Science and technology key project of Xinxiang (
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