ZnFe2O4 multi-porous microbricks/graphene hybrid photocatalyst: Facile synthesis, improved activity and photocatalytic mechanism
Graphical abstract
Introduction
With the background of the increasing global air and water pollutions, semiconductor mediated photocatalysis has attracted considerable attention because it provides a promising pathway for solving energy supply and environmental pollution problems [1], [2]. Of the semiconductors being developed as photocatalysts, TiO2 is currently the most promising because of its special features, such as its low cost, non-toxicity and photochemical stability [3], [4]. However, the band-gap of TiO2 is too large (3.0–3.2 eV) so that it can only utilize solar energy of less than 5% [5], [6]. Therefore, the development of novel visible-light-responsive photocatalysts with high activity is currently an intensive and hot research topic. In recent years, spinel ferrites are among the most studied materials and have been widely used in electronic devices [7], information storage [8], magnetic resonance imaging [9], drug-delivery technology [10] and semiconductor photocatalysis [11], [12], [13]. Particularly, zinc ferrite (ZnFe2O4), a narrow-band-gap semiconductor (band-gap 1.9 eV), has been reported to be a promising visible-light-responsive photocatalyst [14], [15]. It is well known that the properties of ZnFe2O4 are strongly dependent on its morphology and microstructure. Thus, morphology controlled synthesis becomes an important issue. Many recent efforts have been directed toward the synthesis of ZnFe2O4 micro-/nanostructures with diverse morphologies such as nanotubes [11], [16], nanorods [12], [17], nanofibers [18], [19], micro-/nanospheres [14], microtimbers [20], [21] and hollow spheres [22], [23]. However, until now, a simple synthesis strategy for brick-like ZnFe2O4 with a multi-porous structure is still rarely reported. The ZnFe2O4 multi-porous microbricks (ZnFe2O4-MM) is expected to show improved performances in photocatalysis and energy conversion because of its unique structure.
Graphene, a new class of 2D carbonaceous material with atom-thick layer features, has attracted much attention recently for photoelectrochemical and photocatalytic applications due to its high specific surface area and fast electron transfer ability, which can effectively inhibit the recombination of the electron–hole pairs [24], [25], [26]. The unique electronic structures of both ZnFe2O4 and graphene inspired us to design and synthesize graphene-supported ZnFe2O4 hybrid. However, to date, relatively little attention has been concentrated on the construction of ZnFe2O4/reduced graphene oxide (RGO) nanocomposites [27], [28], [29] and there is no study focused on the utilization of the ZnFe2O4-MM/RGO hybrid as photocatalyst for water treatment. Hence, the development of graphene-supported ZnFe2O4-MM hybrid system and investigation of its photocatalytic activity is of fundamental and practical significance.
In this work, a novel ZnFe2O4-MM/RGO hybrid with high visible-light photocatalytic activity was synthesized via a facile deposition–precipitation reaction without use of any templates or surfactants, followed by a hydrothermal treatment. The promoting effect of the ZnFe2O4-MM/RGO hybrid on the photocatalytic activity was observed and the correlation between the structure and the property was discussed based on the results of a systematic characterization. A possible mechanism for the transfer of photogenerated carriers was also proposed.
Section snippets
Synthesis of ZnFe2O4-MM/RGO hybrid
All reagents were of commercially available analytical grade and were used without further purification. ZnFe2O4-MM was synthesized via a facile one-pot route. In a typical preparation, aqueous solutions of 0.1 M ZnSO4·7H2O and 0.2 M FeSO4·7H2O were prepared, respectively. A 100 mL aliquot of each solution was then mixed together at 80 °C. 300 mL 0.1 M (NH4)2C2O4 solution was introduced into the above mixed solution and stirred for 30 min to produce ZnFe2O4-MM precursor. The precursor was further
Structure and morphology properties of ZnFe2O4-MM/RGO hybrid
The crystal structure of the ZnFe2O4-MM/RGO hybrid is characterized by XRD patterns, as shown in Fig. 1a. All of the diffraction peaks of the hybrid can be readily indexed to the ZnFe2O4 (JCPDS 22-1012) and graphene. The peaks at 2θ values of 30.4, 35.5, 43.0, 53.3, 56.9 and 62.4 can be attributed to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) facets of spinel ZnFe2O4. Also, a broad and weak peak at 2θ value of 24.5 can be indexed to (0 0 2) plane of graphene, suggesting the reduction of GO to
Conclusions
In summary, a hybrid photocatalyst of ZnFe2O4-MM/RGO was successfully prepared by a facile two-step strategy, which exhibited enhanced photocatalytic activity in the degradation of 4-CP under visible light irradiation (λ > 420 nm). The kinetic constant of 4-CP removal with ZnFe2O4-MM/RGO hybrid was about 2.08 and 7.29 times higher than that of pure ZnFe2O4-MM and ZnFe2O4-NP, respectively. We attributed this improvement to the integrative effect of the enhanced light absorption, unique morphology
Acknowledgements
This work was supported financially by the National Nature Science Foundation of China (NSFC-RGC 21061160495), the National High Technology Research and Development Program of China (863 Program) (No. 2010AA064902) and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education.
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