Fabrication of novel ternary heterojunctions of Pd/g-C3N4/Bi2MoO6 hollow microspheres for enhanced visible-light photocatalytic performance toward organic pollutant degradation

doi:10.1016/j.seppur.2018.09.061

Separation and Purification Technology

Volume 211, 18 March 2019, Pages 1-9

https://doi.org/10.1016/j.seppur.2018.09.061 Get rights and content

Highlights

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The Bi₂MoO₆ was modified by g-C₃N₄ coupling and metallic Pd deposition simultaneously.
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The Pd/CN/BMO were fabricated via solvothermal precipitation-calcination and photoreduction method.
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2Pd/10CN/BMO heterojunctions exhibited the highest photocatalytic efficiency.
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The higher activity is attributed to the heterojunction structures and SPR effect of Pd.
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Three main active species in degradation of RhB are in the order of O₂⁻ > h⁺ > OH.

Abstract

To improve the photocatalytic performance of Bi₂MoO₆, novel ternary heterojunctions of Pd/g-C₃N₄/Bi₂MoO₆ (Pd/CN/BMO) hollow microspheres were fabricated through g-C₃N₄ coupling via solvothermal precipitation-calcination and metallic Pd deposition by photoreduction method. The optimal ternary 2Pd/10CN/BMO photocatalyst exhibited the highest photocatalytic efficiency towards Rhodamine B (RhB) degradation with a degradation rate of 97% under 40 min visible light irradiation, 77%, 61% and 18% higher than that of the single BMO (20%), binary 2Pd/10CN (61%) and 10CN/BMO (79%), respectively. The enhanced photocatalytic performance was mainly ascribed to the synergetic effect of heterojunction structures between g-C₃N₄ and Bi₂MoO₆ and the surface plasmon resonance (SPR) effect of Pd doping, resulting in the better optical absorption ability and lower combination rate of photogenerated charge carriers. Additionally, the 2Pd/10CN/BMO composite presented excellent photo-stability in recycling experiments. The trapping experiments demonstrated that the O₂⁻, h⁺ and OH were separately the dominant, minor and the least important reactive species toward RhB degradation.

Graphical abstract

The figure shows a possible photocatalytic mechanism of the g-C₃N₄ and Pd comodified Bi₂MoO₆ hollow microspheres. Considering that both of the E_VB and E_CB of g-C₃N₄ are more negative than those of Bi₂MoO₆, the heterojunction structures form between Bi₂MoO₆ and g-C₃N₄. Moreover, the Pd nanoparticles distribute on the g-C₃N₄ layer after introducing them. When irradiating under visible-light, the Pd nanoparticles along with g-C₃N₄ and Bi₂MoO₆ are excited to generate the electrons and holes because of the SPR effect. The photogenerated electrons obtained from Pd will gradually transfer to CB of g-C₃N₄ and then to CB of Bi₂MoO₆ because of the most positive CB potential of Bi₂MoO₆. Simultaneously, the electrons produced on the CB of g-C₃N₄ will shift to the higher CB potential of Bi₂MoO₆ and the retained holes on the VB of Bi₂MoO₆ will easily transfer to the lower potential of g-C₃N₄, which effectively facilitate the separation of electron-hole pairs. The photogenerated electrons in the process of transition possess enough ability to react with adsorbed surface or dissolved O₂ to generate O₂⁻ in view of the fact that the CB of g-C₃N₄ and Bi₂MoO₆ are more negative than E₀ (O₂/O₂⁻ = −0.046 eV). The accumulated holes on the VB of Bi₂MoO₆ can react with OH⁻ to produce OH(E₀ (OH⁻/OH = 2.4 eV)). Moreover, owing to the lower VB potentials, the enriched holes on the VB of g-C₃N₄ cannot react with H₂O or OH⁻ to form OH, which can directly degrade RhB by oxidation.

Introduction

As well-known efficient visible light driven (VLD) photocatalysts, bismuth system semiconductors present prominent application prospects in environmental purification and remediation, especially for the organic pollutant degradation [1], [2], [3], [4], [5]. Notably, bismuth molybdate (Bi₂MoO₆) emerges from various VLD photocatalysts owing to its inexpensiveness, non-toxicity, high stability and VLD properties [6], [7]. Nevertheless, the limited light harvesting ability and high combination rate of photo-induced charge carriers restrict the activity of Bi₂MoO₆ [8], [9]. Many efforts have been taken in an attempt to solve these problems, such as morphology optimization [10], [11], metal element doping [12], [13], [14] and heterojunction construction [15], [16].

Among various approaches, heterojunction construction with graphite like carbon nitride (g-C₃N₄) has been proved to be an effective means to modify Bi₂MoO₆ [17], [18], [19], [20]. Considering the unique g-C₃N₄ with two-dimensional structure, both the energy levels of valence band (E_VB) and conduction band (E_CB) are more negative than those of Bi₂MoO₆, which is suitable for the heterojunction construction between them [21]. The photogenerated electrons or holes from one photocatalyst could transfer to another coupled photocatalyst, inhibiting the charge carrier combination efficiently. The accelerated separation of electron-hole pairs in the heterostructures through the internal electric field become the dominant driving force for the improvement of photocatalytic performance.

Besides of heterojunction construction, photoreduction of noble metal nanoparticles is another outstanding method to elevate photocatalytic properties of Bi₂MoO₆, such as Ag [22], Pt [23] and Pd [24]. It is reported that the deposition of metallic Palladium on the surface of Bi₂MoO₆ not only accelerate the separation of photo-induced e⁻/h⁺ pairs, but enhance the optical harvesting ability because of surface plasmon resonance effect (SPR), leading to a significantly enhancement of photocatalytic performance toward organic pollutant degradation [25], [26], [27].

In present work, to improve the light harvest ability and accelerate the separation rate of photogenerated charge carriers of Bi₂MoO₆, novel ternary heterojunctions of Pd/g-C₃N₄/Bi₂MoO₆ (Pd/CN/BMO) hollow microspheres were fabricated through g-C₃N₄ coupling via solvothermal precipitation-calcination and metallic Pd deposition by photoreduction method. The novel ternary heterojunctions of Pd/g-C₃N₄/Bi₂MoO₆ photocatalysts displayed brilliant photocatalytic performance in the process of Rhodamine B (RhB) degradation under visible light irradiation. There has been few previous researches using both two approaches to modify Bi₂MoO₆. The chemical compositions, morphology structures, optical properties, photocatalytic activities and photostabilities were fully characterized and discussed. Additionally, a possible mechanism of Pd/g-C₃N₄/Bi₂MoO₆ sample for the enhanced photocatalytic performance was also proposed on the base of the reactive species quenching experiments.

Section snippets

Preparation of g-C₃N₄

To obtain g-C₃N₄ nanosheets, 30 g of melamine was added into a covered crucible and then was transferred into a muffle furnace. Heated to 550 °C for 2 h at the rate of 10 °C/min. The obtain yellow g-C₃N₄ were then grounded into powder and named as CN.

Synthesis of Bi₂MoO₆

The pure Bi₂MoO₆ was synthesized by a solvothermal method. 3.3732 g of Bi(NO₃)₃·5H₂O was dissolved in 30 mL of ethylene glycol (EG) under magnetic stirring to prepare solution A and 0.842 g of Na₂MoO₄·2H₂O was dissolved in the same volume of EG to

XRD analysis

To identify the crystal phases of CN, BMO, xCN/BMO and mPd/10CN/BMO samples, XRD patterns of them are illustrated in Fig. 1. Two distinct peaks of g-C₃N₄ at 13.1° and 27.4° could be well consistent with the standard g-C₃N₄ (JCPDS 87-1526) [28]. All diffraction peaks of Bi₂MoO₆ are readily indexed to the orthorhombic bismuth molybdate phase (JCPDS 72-1524) [29]. Four strong peaks located at 28.38°, 32.68°, 47.02° and 55.58° match well with the (1 3 1), (0 0 2), (2 0 2), and (1 3 3) planes

Conclusions

In summary, the novel ternary heterojunctions of Pd/g-C₃N₄/Bi₂MoO₆ hollow microspheres were successfully obtained via g-C₃N₄ coupling and metallic Pd photoreduction deposition on Bi₂MoO₆ surface. The introduced g-C₃N₄ formed heterojunctions with Bi₂MoO₆, which greatly facilitated the separation of electron-hole pairs. The deposited Pd nanostructures distributing on the g-C₃N₄ layer lead to an enhanced optical absorption ability due to the SPR effect. Because of the synergetic of g-C₃N₄ coupling

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China for Youth (21207093), Liaoning Excellent Talents in University (LJQ2014023) and the Natural Science Foundation of Shenyang Science and Technology Bureau (No. 18-013-0-02).

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Fabrication of novel ternary heterojunctions of Pd/g-C3N4/Bi2MoO6 hollow microspheres for enhanced visible-light photocatalytic performance toward organic pollutant degradation

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Preparation of g-C3N4

Synthesis of Bi2MoO6

XRD analysis

Conclusions

Acknowledgements

Trans. Nonferrous Met. Soc. China

J. Coll. Interf. Sci.

Chem. Eng. J.

J. Phys. Chem. Solids

Mater. Res. Bull.

J. Alloy. Compd.

Appl. Catal. B: Environ.

Mater. Lett.

Chem. Eng. J.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Catal. B: Environ.

Nano Energy

J. Hazard. Mater.

Appl. Catal. B: Environ.

Sep. Purif. Technol.

Mat. Sci. Semicon. Proc.

J. Catal.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Catal. B: Environ.

J. Mol. Catal. A-Chem.

Sep. Purif. Technol.

Appl. Catal. B: Environ.

Fabrication of novel ternary heterojunctions of Pd/g-C₃N₄/Bi₂MoO₆ hollow microspheres for enhanced visible-light photocatalytic performance toward organic pollutant degradation

Preparation of g-C₃N₄

Synthesis of Bi₂MoO₆