Visible light photocatalytic activity enhancement of Ag3PO4 dispersed on exfoliated bentonite for degradation of rhodamine B
Graphical abstract
Introduction
Semiconductor-assisted photodegradation of organic pollutants has been receiving a lot of attention because it is an economic and eco-friendly solution for the remediation of environmental pollution with dyes. Recently, silver orthophosphate (Ag3PO4) has been discovered as a novel and highly active visible-light photocatalyst due to the efficient separation of photoexcited electrons and holes [1]. Intriguingly, Ag3PO4 has extremely high photooxidative capabilities for the evolution of O2 from water splitting and for the decolorization of organic dyes under visible light irradiation [2]. Ag3PO4 has a suitable band gap of 2.45 eV and can achieve a quantum yield of up to 90% at wavelengths longer than 520 nm, which is significantly higher than that of other semiconductors reported in the literature [3]. Therefore, Ag3PO4 has been touted as one of the most promising photocatalysts in harvesting solar energy for environmental purification and clean energy production. Moreover, the photocatalytic activity of Ag3PO4 can be further improved by controlling shape [4], morphology [5] and crystal face [6] of Ag3PO4 crystals, and by compositing with other materials, which include semiconductors such as BiPO4 [7], Fe3O4 [8], AgBr [9], ZnO [10], SnO2 [11],TiO2 [12], etc., and carbon materials including graphene [13], oxidized graphene [14] and carbon quantum dots [15]. Therefore, it is very important to search for cost-effective and efficient materials to further improve Ag3PO4 catalyst.
Due to low cost, abundant availability, easy accessibility and environmentally friendly nature, bentonite clay represents an appealing substrate for depositing all kinds of photocatalysts. Bentonite is an industrial name for montmorillonite clay. Montmorillonite is a phyllosilicate mineral consisting of 2:1 stacked layers with two tetrahedral sheets sandwiching an octahedral sheet and possessing a swelling behavior [16], [17]. The isomorphous substitution of Al3+ for Si4+ in the tetrahedral sheet and/or Mg2+ or Zn2+ for Al3+ in the octahedral sheet results in a net negative charge on the clay surface [18]. Compared with other clays, bentonite has excellent adsorption capacities and possesses sorption/exchange sites available within its interlayer space as well as on the outside surfaces and edges [19]. For the above reasons, bentonite is the most effective adsorbent used in removal of dyes [20]. Besides, many studies have shown that bentonite is a good substrate for synthesis of composite photocatalysts with higher photocatalytic activity, for examples, CdS/bentonite [21], ZnS/bentonite [22], TiO2/bentonite [12], g-C3N4/bentonite [23], BiVO4/bentonite [24] and MoS2/bentonite [25] were previously tested. All of these composite photocatalysts showed stronger ability for degradation of organic pollutants because of uniform dispersion of photocatalyst on the clay substrate generating distinct reaction sites. Furthermore, all synthetic composites of bentonite showed greater specific surface area and improved adsorption property, which resulted in higher photocatalytic performance compared to pure catalysts as single phases. By dispersing on a substrate, the amount of photocatalyst needed for degradation of organic pollutants is also reduced.
In our previous study, the silver salt was intercalated into bentonite interlayers for the synthesis of Ag3PO4–Ben composites and they exhibited high catalytic efficiency for Orange II degradation under visible light irradiation. However, catalyst loaded in the interlayer is not conducive to the absorption of the photons. If the bentonite is exfoliated and the photocatalytic nanoparticles were to be supported on exfoliated bentonite nano-layer external surfaces, they will lead to better photocatalytic effect [26]. Therefore, in this study, we assembled Ag3PO4 nanocrystals on exfoliated bentonite nanolayers in order to produce exfoliated bentonite-Ag3PO4 (EB–Ag3PO4) nanocomposites for improving the photocatalytic activity of this visible light-responsive photocatalyst. After exfoliating of bentonite, the huge internal surface area of bentonite is exposed to the outside. Thus, the external surface area of the bentonite can be greatly increased by exfoliation, which in turn enhances the adsorption performance and promotes the catalyst’s role. The work reported here, demonstrates a facile strategy to synthesize EB–Ag3PO4 composites and its application for photocatalytic degradation of organic dyes in water under visible light irradiation. The composition, morphology, and optical properties of the as-synthesized EB–Ag3PO4 composites were characterized. The photocatalytic performance of EB–Ag3PO4 composites was evaluated by the photo-degradation of rhodamine B (RhB) in water under visible light. The effects of exfoliated bentonite amounts in composites on the visible-light-driven photocatalytic efficiency were investigated. Moreover, a possible mechanism for degradation of RhB over EB–Ag3PO4 composite was proposed based on the present experimental results.
Section snippets
Preparation of photocatalysts
All chemicals were of analytical grade and used without further purification. Natural bentonite, composed primarily of Ca2+-montmorillonite, is obtained from Tianyu Co., Ltd. (Inner Mongolia, China). Its structural formula is Ca0.392Na0.016K0.020(Si7.92Al0.08)(Al2.518Fe0.450Mg1.104Ti0.036Mn0.004)O10(OH)2·nH2O, and the charge density is −0.82 e per unit cell. The total cationic exchange capacity (CEC) is 108 meq/100 g [27]. The natural bentonite was ground and sieved by 200-mesh then washed with
XRD analysis
The samples of natural bentonite, exfoliated bentonite, pure Ag3PO4 and EB–Ag3PO4-3 composite were characterized by XRD and their patterns are presented in Fig. 2. According to the XRD pattern of natural bentonite, the predominant peak at 2θ = 5.64° gives the c-axis dimension of 1.48 nm, which is characteristic to montmorillonite. For exfoliated bentonite, the characteristic peak of bentonite at 2θ is broad and weak as expected, which suggested that the layers of bentonite clay were delaminated by
Conclusions
In summary, an excellent hybrid photocatalyst of EB–Ag3PO4 was synthesized via a convenient in situ ion exchange route. XRD, TEM and FTIR revealed that Ag3PO4 nanospheres were successfully attached on to the surface of exfoliated bentonite. The uniform Ag3PO4 nanospheres were evenly and densely spread on the exfoliated bentonite sheets with the diameter of about 6–8 nm, which resulted in improved photocatalytic activity. The RhB degradation by EB–Ag3PO4-3 composite reached about 95% within 21 min
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 21477009), the Project of Jiangsu Province Industry-University-Research joint innovation fund (BY2013024-16) and Changzhou Science and Technology Development Program (CJ20140026). The TEM characterization was under the support of Dr. Xiazhang Li.
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