Preparation and visible-light photocatalytic activity of α-Fe2O3/γ-Fe2O3 magnetic heterophase photocatalyst
Introduction
The photocatalysts of semiconductive transition metal oxides have received increasing attention to address the problem of chemical utilization of solar energy [1], [2], [3]. A key issue with these photocatalysts is the photogenerated carrier lifetime, which would lead to low photocatalytic efficiency if they are not optimized adequately [4], [5]. In order to make the lifetime long enough, one can separate the photogenerated electron–hole pairs spatially. An available strategy is to form heterojunctions. For example, Wang et al. reported an enhanced separation of photogenerated electrons from holes in a TiO2/carbon nanowall heterojunction [6]. We have developed a one-step self-combustion method to prepare anatase/rutile TiO2@C and found that the material exhibits better photocatalytic activity under UV irradiation [7].
As is known, many factors need to be considered in preparing heterojunctions with excellent photocatalytic activity. Among them, the two are more important. One is that the selected materials should be easy to form well-structured interfaces, and the other is that the band structures of the materials should match well with each other [8]. Generally, semiconductors in polymorphs have similar chemical bonds and band structures, and heterophase junctions between them can be formed more easily. It is thus expected that the heterophase junctions formed with the polymorphic semiconductors may show high photocatalytic activity. Fe2O3 has four polymorphs of α-, β-, ε-, and γ-Fe2O3 [9]. Among them, the band gaps of α-Fe2O3 and γ-Fe2O3 are 2.2 and 2.3 eV, respectively [10], [11]. The valence band (VB) and the conduction band (CB) positions of α-Fe2O3 are close to those of γ-Fe2O3, which implies that the heterophase junctions formed between them should be suitable for photocatalysis. Additionally, γ-Fe2O3 is of magnetism which enables the photocatalyst to be magnetically separated from the photocatalytic system [12]. To the best of our knowledge, such a magnetic heterophase photocatalyst with visible-light photocatalytic activity has not been reported so far.
In this work, heterophase junction material of α-Fe2O3/γ-Fe2O3 nanorods was successfully prepared by a facile thermal decomposition and redox method. Visible-light induced photodegradation of model dye RhB was investigated by using the as-prepared material as a photocatalyst. The formation of the heterophase junctions was found to remarkably enhance the visible-light photocatalytic activity of the nanocrystalline Fe2O3, and the used photocatalyst could be magnetically collected easily.
Section snippets
Experimental
A schematic illustration for the preparation processes of α-Fe2O3, γ-Fe2O3, and α-Fe2O3/γ-Fe2O3 nanorods is shown in Fig. 1. First, a yellow ferrous oxalate precursor was prepared by the reaction of Fe2SO4·7H2O and (NH4)2C2O4·H2O. Second, ferrous oxalate precursor was thermally decomposed to α-Fe2O3. Third, α-Fe2O3 was completely reduced by PEG1000 (weight ratio 1:2) in Ar, then oxidized to γ-Fe2O3. Fourth, α-Fe2O3 was partially reduced by PEG1000 (weight ratio 1:1) and oxidized to α-Fe2O3/γ-Fe2
Results and discussion
The XRD patterns of the samples are shown in Fig. 2a. All the diffraction peaks of the as-prepared α-Fe2O3 and γ-Fe2O3 match well with those of standard XRD patterns of α-Fe2O3 (JCPDF 33-0664) and γ-Fe2O3 (JCPDF 39-1346), respectively. The results indicate that both the samples are single phase. For the as-prepared α-Fe2O3/γ-Fe2O3 heterophase material, the XRD pattern contains diffraction peaks of α-Fe2O3 and γ-Fe2O3, which means that the sample is composed of α-Fe2O3 and γ-Fe2O3. The
Conclusion
In conclusion, α-Fe2O3/γ-Fe2O3 heterophase nanorods were prepared by a facile thermal decomposition and redox method. The material exhibited a higher visible-light photocatalytic activity owing to the well-structured interfaces and suitable band structures of the heterophase junctions. The existence of γ-Fe2O3 enables the composite to be magnetically separated easily. The present study suggests a promising new strategy for engineering practical photocatalysts for wastewater treatment. Moreover,
Acknowledgments
The project was financially supported by the National Natural Science Foundation of China (Grant nos. 11179038 and 10974073), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant no. 20120211130005), and the Fundamental Research Funds for the Central Universities (Grant no. 2022012zr0036).
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