Insight into the role of Ti3+ in photocatalytic performance of shuriken-shaped BiVO4/TiO2−x heterojunction
Graphical abstract
Introduction
Photocatalysis has received considerable attention for its potential application in many fields, such as environmental remediation using solar energy [1], [2], [3]. Various metal oxides have been widely studied as candidate materials for photocatalysis because of their stability and relative abundance. BiVO4 is recognized as one of the promising photocatalysts, owing to its excellent stability against photocorrosion and chemical corrosion, narrow bandgap (∼2.4 eV) in the monoclinic phase, and low cost [4], [5], [6]. The direct narrow bandgap makes BiVO4 as a good light absorber, but its carrier diffusion length (Ld) around 70 nm3 is relatively short due to a high recombination rate of charge carriers [7], which becomes a main factor that restricts its practical applications. Additionally, low surface area and weak surface adsorption ability of micron-sized BiVO4 are also important issues that strongly limit its application.
To overcome the above stated shortcomings of BiVO4, various strategies including nanostructure fabrication, heterojuction and surface modification have been explored [8], [9]. Among these strategies, heterojunction construction is proposed as one of the most effective approaches to overcome the barrier of charge transfer [10], [11]. The built-in electric field formed in the heterojunctions makes the photo generated electrons and holes move into opposite directions, thus prolonging the lifetime of the carriers. The noble metals (such as Ag, Au, or Pt), the carbon nano-materials (graphene) and the semiconductors (such as TiO2, WO3, CeO2, Bi2WO6, and CdS) are widely adopted for combining with BiVO4 to achieve a high efficiency in photocatalysis performance [12], [13], [14], [15], [16], [17]. Among these matierials, TiO2 [18], [19] is one of the most representative photocatalysts. TiO2 is proved to be a promising photocatalyst due to its practicality and strong photocatalytic oxidation capacity [20]. But its wide band gap (∼3.2 eV) [21] limits its light absorption to UV range. Therefore, combining of TiO2 with BiVO4 could be a potential pathway not only to extend the light absorption of TiO2 to visible range, but also to enhance the transfer of charge carriers by forming heterojunction at the interface. However, the energy band matching is a key factor for achieving a highly effective BiVO4/TiO2 heterojunctions. It is known that the conduction band of anatase TiO2 is −0.290 eV vs NHE, [22] while for monoclinic BiVO4, it is 0.074 eV vs NHE [23]. As a result, when the BiVO4/TiO2 heterojunction is formed, an interfacial energy barrier is present in the interface. Therefore, under visible light irradiation, it is impossible for the generated electrons of BiVO4 to climb over the energy barrier migrating to the conduction band of TiO2. The unmatched energy band alignment critically affects the charge carrier migration in the formed BiVO4/TiO2 heterojunctions, which is one of the reasons for very limited success achieved in this regard.
In our previous work, various strategies were developed to introduce a defect electronic band in TiO2 by self-doping of Ti3+ in the TiO2 lattice [24], [25], [26]. The location of Ti3+ induced electronic band is below the conduction band of TiO2 [27], [28] as illustrated in Scheme 1, which reduces the interfacial energy barrier between BiVO4 and TiO2, and makes it possible for the migration of electrons from BiVO4 to the conduction band of TiO2 (Scheme 1). The Ti3+ in TiO2 matrix can also trigger the visible-light activity of TiO2 [29], [30], [31], [32]. On the other hand, the generated holes of TiO2 can also transfer to the valance band of BiVO4. In addition, the TiO2−x prepared according to our previous work exhibits extremely high surface area (263.95 m2 g−1) [24], which could provide abundant reactive sites for photocatalytic reaction. As supported by the test of photo degrading phenol, the construction of heterojunctions between BiVO4 and TiO2−x is an effective approach towards high photocatalytic performance.
Section snippets
Material preparation
The BiVO4 and TiO2−x heterojunction was prepared via a two-step hydrothermal process and denoted as BiVO4/TiO2−x. Typically, the shuriken-shaped BiVO4 samples were synthesized using an aqueous solution of NH4VO3 (6 mM) and Bi (NO3)3·5H2O (6 mM) in 2 M HNO3 (30 mL) at room temperature, with addition of 100 μL TiCl3 solution (20%) as a structure directing agent [33]. The pH of the solution was adjusted to 5 with ammonia (28 wt.%) under vigorous stirring. The obtained mixture was transferred to a Teflon
Result and discussion
Fig. 1 depicts the morphology of BiVO4 and the BiVO4/TiO2-x(1.8 mmol) heterojunction. As shown in Fig. 1a, the shuriken-shaped BiVO4 is successfully developed with TiCl3 as a structure directing agent, which is composed of four main branches (about 10 μm) just like the shuriken stars. But the synthesized BiVO4 sample also has tens of secondary (4–8 μm) and tertiary (1–2 μm) branches around the main branches to form a unique architecture, which, to some extent, increases the growing sites for the
Conclusions
Shuriken-shaped BiVO4/TiO2−x heterojunctions are successfully prepared via a two step hydrothermal process. The obtained BiVO4/TiO2−x shows higher photocatalytic activity compared to its counterparts, pure BiVO4 and the physic mixture of BiVO4 and TiO2−x. The improved photocatalytic performance is attributed to the formed heterojunction built between BiVO4 and TiO2−x, which enhances the separation of photogeneration of electron and hole pairs. The Ti3+ defect energy level diminishes the
Acknowledgements
Financial support by the National Nature Science Foundation of China (Grant Nos. 21507157, 21473248), the International Science &Technology Cooperation Program of Xinjiang Uygur Autonomous Region (20166021), the “Western Light” Program of Chinese Academy of Sciences (XBBS201410), the CAS/SAFEA International Partnership Program for Creative Research Teams, the STS project of Chinese Academy of Sciences (KFJ-SW-STS-179), and the High-Technology Research & Development Project of Xinjiang Uyghur
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These authors contributed equally to this work.