Elsevier

Chemical Engineering Journal

Volume 353, 1 December 2018, Pages 636-644
Chemical Engineering Journal

Construction of a Bi2MoO6:Bi2Mo3O12 heterojunction for efficient photocatalytic oxygen evolution

https://doi.org/10.1016/j.cej.2018.07.149Get rights and content

Highlights

  • A Bi2MoO6:Bi2Mo3O12 heterojunction was prepared hydrothermally.

  • Improved photocatalytic activity can be achieved by the heterostructured material.

  • The staggered band energy levels are beneficial for efficient charge separation.

  • Selective photodeposition was applied to illustrate the charge transfer pathway.

Abstract

In this study, two different crystallographic phases of bismuth molybdate, α-Bi2Mo3O12 and γ-Bi2MoO6 were synthesized hydrothermally. A series of Bi2Mo3O12/Bi2MoO6 heterostructures with different phase ratios were readily prepared by tuning the concentration of the molybdenum precursor while retaining the same bismuth precursor concentration. The Bi2Mo3O12/Bi2MoO6 heterojunction was found to substantially enhance the photocatalytic water oxidation capability relative to neat Bi2Mo3O12 and Bi2MoO6. The Bi2MoO6 and Bi2Mo3O12 were found to possess a complementary band alignment between which enabled the charge transfer process across the heterojunction interface. Ag and MnOx photodeposition was conducted on the heterostructures to affirm the charge transfer direction between the two crystal phases. Under visible light irradiation, photogenerated holes transfer from the active Bi2MoO6 phase to the inactive Bi2Mo3O12, suggesting efficient charge separation occurs between the two components, explaining the better photocatalytic performance.

Introduction

Although photocatalysis is a promising technology to directly utilize solar energy for producing solar fuels and degrading pollutants, practical application of the technology remains challenging [1], [2], [3]. Conventional photocatalysts such as TiO2, ZnO, and WO3 or emerging photocatalysts such as BiVO4, SrTiO3, and g-C3N4 exhibit water splitting or pollutant degradation capabilities although their photocatalytic performance continues to be below expectation [1], [4], [5], [6], [7], [8]. Solar energy conversion efficiency, light absorption capability, charge separation efficiency and surface redox reaction kinetics are all key factors influencing performance. For instance, while TiO2 has been extensively studied for water splitting, major issues including it being only excitable by UV light and fast charge recombination still require addressing [1], [9], [10], [11]. As an alternative, semiconductor composites comprising multicomponent heterojunctions have been developed to address some of the drawbacks of single photocatalyst materials. Heterojunction formation has been shown to be able to enhance the photocatalytic performance because of the wide light-absorption range; the suitable band structure to promote electron-hole separation and the improved photocatalyst stability [12], [13], [14], [15].

Recently, bismuth-based semiconductors, such as BiVO4, Bi2MO6 (M = W, Mo), BiOX (X = Cl, Br, I) [16], [17], have attracted great attention due to their comparatively smaller band gaps and capacity to split water under light irradiation. The performance is attributed to Bi 6s and O 2p orbital hybridization in the valence band, resulting in an upshift of the band position [18]. Of the bismuth-based semiconductors, bismuth molybdate is interesting and can occur in one of three different crystallographic phases - α-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6. α-Bi2Mo3O12 exhibits a highly selective oxidative ability [19], [20], [21], [22], [23], although its wide band gap energy (∼3.1 eV) limits its practical application under visible light [24]. β-Bi2Mo2O9 is unstable and can decompose into α- and γ-phases in the temperature range of 400–550 °C. The stable region of β-phase bismuth molybdate is 550–670 °C [25], [26], [27]. γ-Bi2MoO6, with a perovskite layered structure, has been found to possess photocatalytic activity for oxygen evolution from an aqueous AgNO3 solution under visible light illumination [25]. However, the overall solar energy conversion of γ-Bi2MoO6 remains low due to fast charge recombination, poor carrier transport and sluggish water oxidation kinetics. Amongst the various available approaches, constructing a Bi2MoO6-based heterojunction is one effective way to improve the photocatalytic efficiency of Bi2MoO6. For instance, BiOBr-Bi2MoO6 [28], Bi2O3-Bi2MoO6 [29], and RGO-Bi2MoO6 [30] heterojunctions have been found to demonstrate a significant enhancement in photocatalytic performance.

Expanding on the idea of a heterojunction, one meaningful option is to construct a heterojunction between different phases of a semiconductor with the same chemical composition [31], [32]. A heterojunction comprising different crystal phases of the one semiconductor has the benefit of simple preparation by a one-pot synthesis route and suitable band alignment between the phases favors effective migration of the generated charge between them. For instance, Aeroxide P25, which possesses a mixture between anatase and rutile TiO2, has been widely used in various photocatalytic systems [33], [34]. The difference in conduction band (CB) and valence band (VB) levels between the anatase and rutile promotes the efficient charge carrier separation, which increases the photoactivity. α-β phase Bi2O3 [35], and β-γ phase TaON junctions have also been reported [36], [37].

In the work presented here, neat Bi2Mo3O12, neat Bi2MoO6 and Bi2Mo3O12/Bi2MoO6 heterojunctions were prepared by a facile one-pot hydrothermal synthesis process and assessed for photocatalytic water oxidation. To the best of our knowledge, constructing a heterojunction between different crystallographic phases of bismuth molybdate has not been previously considered. The photocatalytic water oxidation results show that the heterostructure possesses an enhanced activity relative to the neat materials whereby there exists an optimal Bi2Mo3O12 and Bi2MoO6 heterojunction ratio. The selective photodeposition of Ag and MnOx was used to examine the charge separation process between the segregated phases and define the origin of the beneficial heterojunction property.

Section snippets

Sample preparation

Bismuth molybdates (Bi2MoO6 and/or Bi2Mo3O12) were synthesized using a modified one-pot hydrothermal method [25], [26]. By varying the mole ratio between the Bi and Mo precursors, neat Bi2MoO6, neat Bi2Mo3O12, and Bi2MoO6/Bi2Mo3O12 composites were successfully prepared. As depicted in Fig. S1, the synthesis involved using a constant (5 mmol) amount of Bi(NO3)3·5H2O as the bismuth precursor, while the Mo precursor (NH4)Mo7O24·4H2O amount was varied from 0.36 to 1.08 mmol. Typically, Bi(NO3)3·5H2

Results and discussion

The crystal structure of final product and possible phase changes during hydrothermal synthesis was examined by powder XRD with the patterns provided in Fig. 1. For the sample comprising 0.36 mmol of Mo precursor, the XRD patterns clearly show diffraction peaks at 2θ = 10.9°, 28.3°, 33.1° etc. which can be assigned to orthorhombic γ-Bi2MoO6 (JCPDS: 00-021-0102). On gradually increasing the Mo precursor concentration, peaks at 2θ = 28.0°, 29.2°, 31.0° etc. representing monoclinic α-Bi2Mo3O12

Conclusions

Bismuth molybdate particles comprising a segregated mixture of α-Bi2Mo3O12 and γ-Bi2MoO6 phases were prepared and examined for their water oxidation capabilities. The extent of each phase was adjusted by simply changing the initial molybdenum precursor concentration during synthesis. Analysis of the band structure indicated a type II heterojunction was formed between the Bi2Mo3O12 and Bi2MoO6, which is beneficial for spatial charge separation. The findings demonstrated that, at a suitable

Acknowledgements

This work was supported by the Australian Research Council (ARC) Discovery Project (DP140102781 and DP170101467) programme. The authors acknowledge the use of facilities within the UNSW Mark Wainwright Analytical Centre.

References (45)

  • L.D. Krenzke et al.

    J. Catal.

    (1980)
  • E. Luévano-Hipólito et al.

    Appl. Catal. A.

    (2013)
  • M. Bettahar et al.

    Appl. Catal. A.

    (1996)
  • P.A. Batist et al.

    J. Catal.

    (1972)
  • A. Martinez-de La Cruz et al.

    Mater. Chem. Phys.

    (2008)
  • S. Wang et al.

    Appl. Surf. Sci.

    (2017)
  • R.I. Bickley et al.

    J. Solid State Chem.

    (1991)
  • T. Ohno et al.

    J. Catal.

    (2001)
  • J. Hou et al.

    Appl. Catal. B.

    (2013)
  • H. Li et al.

    Mater. Chem. Phys.

    (2009)
  • A.-W. Xu et al.

    J. Catal.

    (2002)
  • A. Fujishima et al.

    Nature

    (1972)
  • F. Wang et al.

    ACS Catal.

    (2014)
  • T. Kawai et al.

    Nature

    (1980)
  • Ü. Özgür et al.

    J. Appl. Phys.

    (2005)
  • S.H. Baeck et al.

    Adv. Mater.

    (2003)
  • A. Kudo et al.

    Catal. Lett.

    (1998)
  • J. Ng et al.

    Adv. Funct. Mater.

    (2010)
  • X. Wang et al.

    Nat. Mater.

    (2009)
  • A.L. Linsebigler et al.

    Chem. Rev.

    (1995)
  • I. Izumi et al.

    J. Phys. Chem.

    (1980)
  • U. Bach et al.

    Nature

    (1998)
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