Research paperInsight into enhanced visible-light-driven Cr(VI) reduction with ZnO/ZnSe hierarchical n-p heterojunction
Graphical abstract
Introduction
At present, with the continuous development of the social economy, the constant expansion of production sale and the rapid development of urbanization, lots of harmful substances have inevitably entered into the water, air and soil systems, causing serious pollution problems. Among them, the water pollution caused by highly toxic heavy metal ions (Cr, Ni, Cu, As, Cd and Pb) has become a serious world-wide problem because of increasing industrial application[1]. Hexavalent chromium Cr(VI) is recognized as a primary pollutant due to its highly mobile in the aqueous solution and toxic to microorganisms, plants, animals and humans[2]. Therefore, efficient removal of Cr(VI) from water system has been an urgent problem to be solved for a better quality of life. Recently, much research has focused on the removal of Cr(VI) from aqueous media, and many remediation technologies have been developed, such as physicochemical adsorption, bioremediation and chemical reduction etc. Although these technologies have been researched for a long time and also have effectively degrade Cr(VI), there are still some problems that must be faced, such as low metal removal rate, complex technological process, and high cost [3].
Recently, the photocatalytic reduction of Cr(VI) to Cr(III) ions has been widely investigated as an attractive treatment method for Cr(VI) removal from wastewaters because Cr(III) is less toxic and immobile, and it is also essential to humans in trace concentration [4]. Therefore, it is very important to design and construct efficient photocatalytic materials for Cr(VI) reduction. In general, if the bottom of the conduction band is more negative than the redox potential of Eθ(Cr2O72−/Cr3+ (1.232 V vs NHE) under acidic condition or Eθ(CrO42−/Cr(OH)3 (−0.13 V vs NHE) under alkaline condition[3], Cr(VI) ions adsorbed on semiconductor surface will be reduced to Cr(III) by electrons under illumination. Therefore, to promote the surface migration of photo-excited electrons, rational construction of photocatalytic materials will be helpful to the Cr(VI) reduction.
Nanostructured metal oxide semiconductors have been widely applied in environmental remediation, such as TiO2[5], CdS[6], C3N4[7], ZnFe2O4[8] and BiWO6 [9]etc. Compared with them, ZnO (Eg~3.37 eV) photocatalyst attracts more attention because of its excellent biocompatibility, non-toxicity, controlled morphologies and chemical stability[10]. Moreover, the photocatalytic activity of ZnO is much better than that of traditional photocatalytic materials such as TiO2 due to its higher hydroxyl ion formation rate and electron mobility[11]. Nonetheless, the wide band gap, the fast recombination of photo-generated carriers, and the strong photocorrosion effect immensely hinder its actual application [12]. Therefore, many methods have been made to improve the photocatalytic efficiency of ZnO, such as morphological control, ion-doping, noble metal loading and so on. Among them, coupling with another well-matched narrow band gap semiconductor is reckoned as an ideal scheme. ZnSe is a promising photocatalytic material due to its low toxicity and natural abundance. Most importantly, ZnSe can form a type-II heterostructure with ZnO to enhance the separation efficiency of photo-excited charge carriers, and thus is in favor of the photogenerated electrons transferring to the ZnO surface upon illumination because of its suitable band-gap (2.67 eV) and conduction band position[13]. Various methods have been reported for the preparing ZnSe/ZnO heterostructures for high efficient photocatalytic activity [14]. However, to the best of our knowledge, most of the studies focus on the photocatalytic decomposition of an organic dye and net photoconversion efficiency, the investigations on the photocatalytic reduction of Cr(IV) and the behavior of charges separation and transfer at the surface or interface for ZnSe/ZnO heterostructures are still lacking.
Herein, we report a novel facile strategy for the fabrication of porous center-hollow ZnO/ZnSe microspheres via a simple surfactant-free water bath and in suit ion-exchange method. The properties of samples were characterized by different advanced techniques, and the photocatalytic activity is also evaluated by the reduction of aqueous Cr(VI) under visible light illumination. Furthermore, the deep mechanism was discussed in detail in this paper.
Section snippets
Materials
All the reagents used in our experiment were analytical grade and without further purification. Deionized water was used in all of the experiments.
Preparation of ZnO multiporous microspheres
ZnO multiporous microsphere was prepared by means of template-free method. In a typical procedure, 1.789 g Zn(NO3)2·6H2O and 0.482 g C6H5Na3O7·2H2O were dissolved in a beaker containing 100 mL of deionized water under vigorous stirring. 1.2654 g C6H12N4 was dissolved in another beaker containing 100 mL of deionized water with stirring. After the
Structure and morphology of ZnO and ZnO/ZnSe composites
The crystal phase structures of as-prepared samples were investigated by powder XRD. As shown in Fig. 1, all the peaks are well indexed to the hexagonal wurtzite ZnO (JCPDS No. 36–1451) and cubic sphalerite ZnSe (JCPDS No. 37–1463), respectively. No other peaks or peaks shifting were observed in XRD pattern of ZnO/ZnSe samples, indicating the high purity of ZnO/ZnSe [15]. In addition, with increasing loading content of ZnSe, the intensities of the characteristic peaks of ZnO gradually weakened.
Conclusion
In summary, three-dimensional ZnO/ZnSe hierachical microspheres were successfully synthesized by a facile template-free method. The results demonstrate that all the ZnO/ZnSe samples present enhanced visible photocatalytic reduction ability toward Cr(VI) compared with pure ZnO. The loading of ZnSe formed by in-situ conversion on the surface of ZnO not only extend the light response to visible region, more importantly, the formation of effective interface electric field between ZnO and ZnSe is
CRediT authorship contribution statement
Jinteng Zhang: Formal analysis, Investigation, Writing - original draft, Methodology. Peng Zhao: Data curation. Yingchen Li: Data curation. Yuehong Cao: Data curation. Tengfeng Xie: Funding acquisition. Yanhong Lin: Funding acquisition. Zhao Mu: Conceptualization, Validation, Resources, Writing - review & editing, Supervision, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the 12th Oil Production Plant of PetroChina Changqing Oilfield Company and the National Natural Science Foundation of China (No. 21872063).
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