Elsevier

Journal of Alloys and Compounds

Volume 636, 5 July 2015, Pages 229-233
Journal of Alloys and Compounds

Letter
Enhanced photocatalytic activity in ZnFe2O4–ZnO–Ag3PO4 hollow nanospheres through the cascadal electron transfer with magnetical separation

https://doi.org/10.1016/j.jallcom.2015.02.176Get rights and content

Abstract

Magnetically separable ZnFe2O4–ZnO–Ag3PO4 hollow nanospheres were successfully synthesized using phenolic formaldehyde microspheres as template via impregnating-calcination process. The photocatalytic activity under visible light irradiation increases in the order of ZnFe2O4–ZnO–Ag3PO4 > ZnFe2O4–Ag3PO4 > Ag3PO4 > ZnFe2O4–ZnO > ZnFe2O4. The enhanced activity could be attributed to the cascade electron transfer from ZnFe2O4 to ZnO then to Ag3PO4 through the interfacial potential gradient in the ternary hybrids conduction bands, which facilitates the charge separation and retarded the charge pair recombination. Furthermore, ZnFe2O4–ZnO–Ag3PO4 hollow nanospheres could be conveniently separated by using an external magnetic field, and the study also provides a general and effective method in the composite hollow nanomaterials with sound heterojunctions that may show a variety of applications.

Introduction

Hollow nanomaterials are effective to enhance photocatalytic activities because of minimizing particle size, so as to achieve high surface area and more active catalytic sites [1]. However it brings another negative effect, when the particle size is decreased to nanoscale, the separation and the recycling of the photocatalysts from the treated water are practical obstacles, which hinder their industrial application, even though they have high photocatalytic activity. Thus, some researchers are working on magnetic photocatalysts that can be separated from the treating system by applying an external magnetic field [2], [3]. Spinel-structured ZnFe2O4 with strong magnetism, outstanding photochemical stability and low cost, as well as a narrow bandgap about 1.9 eV, which gives rise to the visible-light response, has become increasingly attractive in photocatalysis [4]. However, as single phase photocatalysts, their activity is low, because of its fast recombination of charge carriers. Combining two or more semiconductors with appropriate band positions to improve the photocatalytic performance are an established idea, because it can lead to an enhanced charge separation efficiency and enlonged charge carrier life. Recently, many binary ZnFe2O4-based composites have been studied, such as ZnFe2O4–CaFe2O4 [5], ZnFe2O4–TiO2 [6], [7], ZnFe2O4–SrFe12O19 [8], ZnFe2O4–ZnO [9], and their photocatalytic properties all are conspicuous enhanced compared to single ZnFe2O4 photocatalysts. We think that the overall activity can be even more enhanced, if a ternary hybrids system is employed. However, the photocatalytic and electron-transfer processes occurring in the ternary hybrids have not received much attention.

Herein, we choose the ZnFe2O4–ZnO–Ag3PO4 as a ternary hybrids composite photocatalyst. Only with proper band structure, charge separation and transfer can efficiently be obviously improved. For this purpose, we chose ZnFe2O4, ZnO and Ag3PO4 as components in the ternary hybrids. ZnFe2O4 has a wider absorption range (ca.540 nm), which easily produces electron–hole pairs under visible light irradiation. And ZnO has the conduction band (CB) edge (ECB = −3.1 VNHE) that lies below that of ZnFe2O4 (ECB = −0.5 VNHE) and above that of Ag3PO4 (ECB = 0.44 VNHE). Therefore, the CB positions in the ternary hybrids can serve as the form of a cascade structure. When the electron–hole pairs are generated in ZnFe2O4, electrons can be separated from holes by migrating to ZnO and then to Ag3PO4 in a cascade structure along the potential gradient. To test the photocatalytic activity, the results of RhB degradation with the ZnFe2O4–ZnO–Ag3PO4 ternary hybrids are compared with those of bare ZnFe2O4, Ag3PO4 and other binary hybrids under visible light irradiation. And the characterization and the enhanced photochemical behaviors of the ternary hybrids are discussed in detail.

Section snippets

Preparation ZnFe2O4–ZnO hollow nanospheres

Phenolic formaldehyde microspheres (PFS) were prepared following a previously reported method [10]. 0.5 g PFS were added into 20 mL solution with 1.5 M Zn(NO3)2 and 2 M Fe(NO3)3, and ultrasonic dispersion for 15 min. Subsequently, the resulting suspension was aged for 3 h, then filtered, washed, and dried at 50 °C for 24 h. The resultant microspheres were heated in air at 1 °C min−1 up to 650 °C, kept this temperature for 3 h. ZnFe2O4, ZnFe2O4–Fe2O3 and ZnFe2O4–ZnO hollow nanospheres were prepared by

Results and discussion

The phases of ZnFe2O4, ZnFe2O4–ZnO, ZnFe2O4–Ag3PO4 and ZnFe2O4–ZnO–Ag3PO4 were identified by XRD in Fig. 1A, respectively. The peaks located at 18.34°, 30.02°, 35.30°, 37.02°, 42.88°, 53.24°, 56.74° and 62.32° in curve a can be ascribed to the characteristic peaks of ZnFe2O4 (JCPDS No. 77-0011), and the corresponding positions are also detected in curve b, c and d. XRD pattern at 31.58°, 34.30°, 36.08°, 47.58° and 62.94° in curve b and d are corresponded to ZnO (JCPDS No. 89-0510). Moreover,

Conclusions

In this work, the ternary hybrids of ZnFe2O4–ZnO–Ag3PO4 hollow nanospheres were successfully synthesized. The highlight of the article more likely resulted from the following reasons: 1. The average diameter of ZnFe2O4–ZnO–Ag3PO4 is about 230 nm and possesses a shell with a thickness of about 25 nm; 2. The results show that the catalytic activity can be greatly enhanced by coupling ZnFe2O4, ZnO and Ag3PO4, organized as a cascade structure of ZnFe2O4–ZnO–Ag3PO4. 3. ZnFe2O4–ZnO–Ag3PO4 could be

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No. 51202136), Special Fund from Shaanxi Provincial Department of Education (2013JK0939), the Academic Back bone Cultivation Program of SUST (XSGP201202) and the Postgraduate Innovation Fund of SUST.

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