Elsevier

Applied Surface Science

Volume 469, 1 March 2019, Pages 125-134
Applied Surface Science

Full Length Article
Exploring the effects of crystal facet in Bi2WO6/BiOCl heterostructures on photocatalytic properties: A first-principles theoretical study

https://doi.org/10.1016/j.apsusc.2018.11.006Get rights and content

Highlights

  • The crystal facet has a critical effect on the properties of Bi2WO6/BiOCl composites.

  • The Bi2WO6(0 1 0)/BiOCl(0 0 1) interface has a marginal lattice mismatch.

  • The Bi2WO6(0 1 0)/BiOCl(0 0 1) heterostructure has a favorable valence band offset.

  • A built-in electric field can be formed at the Bi2WO6(0 1 0)/BiOCl(0 0 1) interface.

  • The Bi2WO6(0 1 0)/BiOCl(0 0 1) heterostructure has higher photocatalytic activities.

Abstract

We explore the effects of crystal facet on the interfacial structures and properties of the Bi2WO6/BiOCl heterostructures using first-principles calculations. The Bi2WO6 (0 1 0) facet is used to match the BiOCl (0 0 1) and (0 1 0) facets to construct the Bi2WO6(0 1 0)/BiOCl(0 0 1) and Bi2WO6(0 1 0)/BiOCl(0 1 0) heterostructures. We find that the Bi2WO6(0 1 0)/BiOCl(0 0 1) interface has a marginal lattice mismatch compared with the Bi2WO6(0 1 0)/BiOCl(0 1 0) interface. The density of states analyses reveal that the Bi2WO6(0 1 0)/BiOCl(0 0 1) heterostructure has a favorable valence band offset. Moreover, a built-in electric field can be formed at the Bi2WO6(0 1 0)/BiOCl(0 0 1) interface. The existence of the favorable band offsets and the electric fields in the Bi2WO6(0 1 0)/BiOCl(0 0 1) heterostructure can facilitate the separation of the photo-generated electron-hole pairs, resulting in the higher photocatalytic performance than the Bi2WO6(0 1 0)/BiOCl(0 1 0) heterostructure. Our results show that the crystal facet has an important effect on the photocatalytic properties of the Bi2WO6/BiOCl heterostructures, and also provide a reference for the design of highly efficient heterostructure photocatalysts combined with crystal facet engineering.

Introduction

Photocatalysts have attracted worldwide attention for their potential applications in solving energy and environmental issue [1], [2], [3], [4]. However, the practical applications of the photocatalysts are still restricted by limited visible-light response and the high photo-generated electron-hole pairs recombination rates. Therefore, considerable attention has been paid to develop new highly efficient visible light photocatalysts and modify the existing ones to improve their photocatalytic activities [5], [6], [7].

Recently, bismuth oxychloride (BiOCl) has attracted increasing research interest due to its superior photocatalytic activity and chemical stability [8], [9], [10]. BiOCl has a unique layered structure consisting of [Bi2O2]2+ layers sandwiched between (Cl)n layers, which could induce internal electric fields along the perpendicular direction to the layers [11], [12]. Zhang et al. reported the facet-dependent photoreactivity of BiOCl single-crystalline nanosheets [13]. They found that BiOCl nanosheets with the exposed {0 0 1} facets had superior activity under UV radiation compared to BiOCl nanosheets with the exposed {0 1 0} facets, owing to the internal electric field in the [0 0 1] direction. However, due to the large band gap of about 3.40 eV, BiOCl has limited photocatalytic performance under visible light.

The construction of heterostructures between BiOCl and other semiconductors with narrow band gap has been proved to be an effective approach to enhance the visible light photocatalytic activity of BiOCl [14], [15], [16], [17], [18], [19]. Several BiOCl-based heterostructures have been prepared and turned out to exhibit improved visible light photocatalytic activities than BiOCl, such as g-C3N4/BiOCl [14], BiOI/BiOCl [15], [16], [17], Bi2S3/BiOCl [18], BiOCl/Bi2O3 [19], etc. Among these heterostructures, Bi2WO6 is expected to be a good candidate to couple with BiOCl to form Bi2WO6/BiOCl heterostructures. Bi2WO6 can absorb visible light with a small band gap of 2.75 eV [20]. As a typical n-type semiconductor, Bi2WO6 can couple with BiOCl to form p-n heterostructures with better built-in electric field at the interface than the p-p ones [21], [22]. Besides, Bi2WO6 possesses a layered structure similar to BiOCl, with [Bi2O2]2+ and [WO4]2− layers packed alternately which indicates the existence of self-internal electric field [23]. The synthesized Bi2WO6/BiOCl heterojunctions exhibited excellent photocatalytic activities under visible light irradiation [24], [25], [26].

The crystal facet effects on the photocatalytic activities of the heterostructures have been studied in the g-C3N4/BiOCl [14] and BiOI/BiOCl [15] composites. Their studies showed that the BiOCl(0 1 0)-based composites displayed higher photocatalytic performance than that of BiOCl(0 0 1)-based composites. It can be seen that the crystal facet has a critical effect on the photocatalytic performance of the heterostructures. However, the crystal facet effects on the photocatalytic activities of the Bi2WO6/BiOCl heterostructures are still unclear. In addition, the synergistic effects of the self-internal electric field and the built-in electric field at the interface on the charge transfer behavior have been rarely reported. Therefore, exploring the atomic and electronic structures of the Bi2WO6/BiOCl heterostructures and the effects of electric fields on the migration of charge carriers may provide a reference for designing higher efficiency heterostructures photocatalysts.

In the present work, first-principles calculations are used to investigate the interface geometric, electronic structures and photocatalytic mechanism of the Bi2WO6(0 1 0)/BiOCl(0 0 1) and Bi2WO6(0 1 0)/BiOCl(0 1 0) heterostructures. We find that the crystal facet has an obvious effect on the photocatalytic properties of the heterostructures. Due to the favorable valence band offset and the built-in electric field at the interface, the Bi2WO6(0 1 0)/BiOCl(0 0 1) heterostructure has higher photocatalytic performance than the Bi2WO6(0 1 0)/BiOCl(0 1 0) heterostructure.

Section snippets

Computational details

The calculations are performed using the plane-wave pseudopotential DFT method as implemented in the CASTEP code [27]. The generalized-gradient approximation with the PBEsol [28] forms is used to describe the exchange and correlation potential. Ultrasoft pseudopotentials [29] are used to deal with core electrons, and the valence atomic configurations are 6s26p3 for Bi, 3s23p5 for Cl, 2s22p4 for O, and 5s25p65d46s2 for W. The calculated lattice constants for Bi2WO6 and BiOCl agree well with the

Interface lattice match

The interface lattice match is one primary factor for building the interface models. It can be investigated by examining the surface geometry structures of the Bi2WO6 and BiOCl phases. The lattice parameters of BiOCl (0 0 1) surface are aBiOCl(0 0 1) = bBiOCl(0 0 1) = 3.87 Å, and the lattice parameters of BiOCl (0 1 0) surface are aBiOCl(0 1 0) = 7.40 Å and bBiOCl (0 1 0) = 3.87 Å, respectively. The lattice parameters of Bi2WO6 (0 1 0) surface are aBi2WO6 (0 1 0) = 5.49 Å and bBi2WO6 (0 1 0) =

Conclusions

In summary, the interfacial structures and the effects of crystal facet on the photocatalytic properties of the Bi2WO6/BiOCl heterostructures are studied by first-principles calculations. We find that the Bi2WO6(0 1 0)/BiOCl(0 0 1) heterostructure has a suitable band alignment and a built-in interface electric field to separate the photo-gennerated carriers, while the Bi2WO6(0 1 0)/BiOCl(0 1 0) heterostructure could not satisfied. As a result, the Bi2WO6(0 1 0)/BiOCl(0 0 1) heterostructure

Acknowledgment

This work is supported by the National Natural Science Foundation of China (Grant Nos. 51502076, 11447183, 11404089, and 11347188), the Natural Science Foundation of Hebei Province (Grant Nos. A2015205141 and A2015205142), the Science and Technology Research Project of Higher Education in Hebei province (Grant No. QN2014046 and ZD2017041). The calculations were performed on the Quantum Materials Simulator of Hebtu.

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