Elsevier

Molecular Catalysis

Volume 433, May 2017, Pages 363-371
Molecular Catalysis

Optical properties and visible-light driven photocatalytic performance of Bi14MoO24 semiconductor with layered δ-Bi2O3-type structure

https://doi.org/10.1016/j.mcat.2017.02.027Get rights and content

Highlights

  • Bi14MoO24 with layered δ-Bi2O3-type structure was synthesized by co-precipitation.

  • The nanoparticles show high absorption in UV–vis wavelength region (Eg = 2.48 eV).

  • The photocatalytic activity was markedly enhanced for Bi14MoO24 (7Bi2O3·MoO3).

  • The mechanism of this photocatalysis system was proposed.

Abstract

The bismuth molybdate oxide of Bi14MoO24 (7Bi2O3·MoO3) was synthesized by the co-precipitation route. The samples developed into the uniform nanoparticles around 50 nm. Crystalline phase was verified via Rietveld XRD structural refinement. Tetragonal Bi14MoO24 is related to that the face-centered cubic (fcc) δ-phase of Bi2O3-like structure. It has more loose space with a big structure-openness than parent δ-Bi2O3. The surface microstructure and components of the photocatalysts were studied by TEM, SEM, BET, EDS and XPS measurements. The UV–vis absorption spectra showed that the band gap energy of Bi14MoO24 was greatly reduced in comparison with δ-Bi2O3. The result confirms that Bi14MoO24 can response to visible wavelength between 200 and 500 nm (Eg = 6.2  2.48 eV). Bi14MoO24 derived from δ-Bi2O3 by incorporating Mo (7Bi2O3:MoO3) in the lattices possesses broader conduction band. This greatly benefits the mobility of the light-induced charges taking part in the photocatalytic reactions. Bi14MoO24 nanoparticles possess efficient photodegradation on Rhodamine B (RhB) dye and phenol solutions. The photocatalysis activities and mechanisms were discussed on the crystal structure characteristics and the measurements such as photoluminescence, exciton lifetime and XPS results. The layered Bi–O polyhedral structure, good photo-absorption, multivalent elements, quenched luminescence and long exciton lifetime were possibly the origin of its high photocatalytic activity in the photo-degradation.

Introduction

Bismuth oxide (Bi2O3) is one of the most widely reported photocatalysts, which has been intensively investigated for preparation methods, electronic structure, wastewater treatment, organic contaminants decomposition, CO2 reduction, water splitting under visible-light irradiation, etc. Firstly, Bi2O3 has suitable band gap (2.1–2.8 eV) responsive to visible light [1], [2]. Bi3+ ion has a d10 configuration with a peculiar 6s2 lone pair inducing more internal polar electric field, which promotes charge separations because that holes and electrons present opposite movements in the electric field. Meanwhile, there is high hybridization between Bi-6s and O-2p orbitals, which makes the large dispersion in valence band (VB). This is beneficial for the mobility of holes promoting high photocatalytic activity [3], [4], [5].

Secondly, the structure has a high openness because of the oxygen vacancies in Bi2O3 lattices. It has an intrinsic vacancy concentration (25%) on the highly disordered oxygen sublattice. It has been confirmed that the structure-openness in a semiconductor could provide ample space for momentary polarizing charge. This effect further accelerates the charge separation between the photo-created holes and electrons and consequently enhances the photocatalytic reactions [6].

Thirdly, among four polymorphic phases of Bi2O3, α-, β-, γ-formations present a low electrical conductivity, however, δ-phase presents very high ionic conductivity [7]. This is an incomparable advantage of δ-phase Bi2O3, i.e., the highest oxygen ionic conductivity (∼1 S/cm at 1023 K). This value is more than two times higher than the well-known electrolyte of yttrium-stabilized zirconia (YSZ) [8]. And this unusually high ionic conductivity is ascribed to the high structure-openness mentioned above together with high polarizability of Bi-6s2 lone pairs. The good conduction of a semiconductor is beneficial for the easy movement of electrons or holes in the process of photocatalysis. However, δ-Bi2O3 is a meta-stable phase, which is only stable in a narrow temperature 730–825 °C [9]. This is the reason for the limitation of the practical application.

In present work, Bi14MoO24 was developed and evaluated by incorporating Mo(VI) into δ-Bi2O3. Mo implies the significant lowering of the symmetry from the fluorite-type O sublattice. This derives a bismuth-rich Bi14MoO24 with lower tetragonal lattices than the defect cubic fluorite-type structure of δ-Bi2O3. Bi14MoO24 was reported to be a fluorite-type superstructure (δ-Bi2O3) with a layered-pseudo pseudo-fcc subcell [10], [11], [12]. Ling et al. [13] proposed that and Bi14MoO24 has octahedral coordination of Mo by O atoms, with each MoO6 octahedron having one axial and one equatorial O site vacant. There is also plenty of Vo in the lattices and Bi–O polyhedral present high distortions because of the steric lone-pair effects. Consequently, high optical absorption and efficient photocatalysis could be expected in Bi14MoO24.

Bi14MoO24 was synthesized by the facile co-precipitation route. The phase formation and the Rietveld refinement were finished. The surface and microstructure were investigated by SEM, EDS, TEM, and BET measurements. The photocatalytic behaviors were measured on degradation of Rhodamine B (RhB) by visible irradiation. XPS, photoluminescence and decay lifetimes were measured to elucidate the photocatalytic activity.

Section snippets

Experimental

Bi14MoO24 was prepared by co-precipitation route. Bi(NO3)3·5H2O (Junsei Chemical Co., 99.0%) and (NH4)6Mo7O24·4H2O (Junsei Chemical Co., 99.0%) with the molar ratio of 14:1 were used as the starting materials. In a typical synthesis, Bi(NO3)3·5H2O (4.85 g) and (NH4)6Mo7O24·4H2O (0.2 g) were dissolved in 100 mL and 50 mL water, respectively. The two solutions were mixed together under strong stirring. The mixture was further stirred for 60 min at 80 °C with magnetic stirring. Then the solutions were

Crystal formation

The XRD pattern of Bi14MoO24 nanoparticles was measured to identify the phase formation as shown in Fig. 1. The pattern could be well indexed to the standard card PDF#49-0281 for Bi14MoO24. No impurity peak was detected. To investigate detailed structural characteristics, the experimental XRD patterns were conducted by Rietveld refinement on the GSAS program shown in Fig. 1. The refinement gave residual errors of 9.82% and 10.53% for Rp and Rwp. The refined unit cell data and the atomic

Discussions

For a semiconductor, the influence on the photocatalysis is very complicated such as band gap energy, band structures, crystal structure, and surface properties etc. on the Bi14MoO24 photocatalyst, the following discussions could be focused on the crystal structure, multivalent elements on the surface, the luminescence and decay properties.

Firstly, Bi14MoO24 has such a rich bismuth-oxide with the coordination environments of the Bi3+ keeping a good representative of that in δ-phase Bi2O3. The

Conclusions

Bi14MoO24 (7Bi2O3·2MoO3) nanoparticles were prepared by the co-precipitation route. The structural Rietveld refinement was finished on the base of the face-centered cubic (fcc) δ-phase of Bi2O3. There are rich oxygen vacancies in Bi14MoO24 than that in the Bi2O3 lattices, which result in high openness of the structure. The samples developed into fine nanoparticles about 50 nm. Bi14MoO24 has a strong absorption in UV and visible wavelength region with direct allowed band gap energy of 2.48 eV.

Acknowledgements

This research was supported by a key project for Industry-Academia-Research in Jiangsu Province, and the fund from Jiangsu Naton Tech. Co., LTD, and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China.

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