Combination of ultrafast dye-sensitized-assisted electron transfer process and novel Z-scheme system: AgBr nanoparticles interspersed MoO3 nanobelts for enhancing photocatalytic performance of RhB

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Highlights

  • AgBr nanoparticles interspersed MoO3 nanobelts photocatalyst was synthesized by an oriented diffusing and charge induced deposition method.

  • Ultra-short photocatalytic time (5 min) and Ultra-high degradation efficiency (95%).

  • Noticeable photocurrent response was detected under visible light irridiation, and radical dotO2 was confirmed to be a dominant reactive specie by experiments.

  • Dye-sensitized-assisted electron transfer process was proposed.

  • Redox ability of electrons was enhanced by photon-generated carriers recombination on the ohmic contact interface.

Abstract

Inspired by the natural photosynthesis in green plants, artificial heterogeneous Z-scheme photocatalytic systems are widely used to settle environmental concerns and energy crises, and their excellent characteristics come from long-term stability, wide absorption range, high charge-separation efficiency, and strong redox ability. However, the contribution of the surface-adsorbed dyes antenna molecule is seldom considered in the process of Z-scheme photocatalysis. In this study, we construct AgBr quantum dots decorated MoO3 nanobelts as a novel Z-scheme photocatalyst by an oriented diffusing and charge induced deposition. For the first time, we find the synergistic effect caused by the suitable energy band match among RhB dyes, AgBr nanoparticles, and MoO3 nanobelts, leading to the ultrafast dye-sensitized-assisted electron transfer process. This is responsible for excellent photocatalytic activities of the achieved AgBr/MoO3 monolithic catalyst for degrading RhB under visible light irradiation. Simultaneously, changing of the band gaps and detailed mechanism for high efficiency degradation is analyzed and explored by theoretical calculations and designing further experiments. It is proposed that ultrafast degradation of the RhB on the AgBr/MoO3 nanocomposites is due to both the photocatalytic process and the dye sensitization; the superoxide radical radical dotO2, which is produced by accumulated dye-sensitization-induced abundant electrons with powerful potential in the CB of AgBr accompanying by quick combination of electrons in the CB of MoO3 with photogenerated holes in the VB of AgBr, is a dominant reactive species for the degradation of RhB under visible light irradiation.

Introduction

As an advanced oxidation process over the surface of a semiconductor-based photocatalyst, visible-light-driven photocatalysis can be used to deal with growing environmental pollution through the conversion of solar energy to decompose organic pollutants into CO2 and other small molecular substances [1], [2], [3]. Obviously, for the convenient and cost-effective single-component photocatalyst, the photoinduced carriers of the narrow band gap single-component photocatalyst can easily recombine while it is difficult to generate photogenerated electrons and holes for wide band gap single-component photocatalyst, which directly caused the poor quantum efficiency and low photocatalytic performance [4]. As one of many methods developed to enhance the photocatalytic performance [5], [6], [7], heterojunction structure created between the connected p- and n-type semiconductors recently attracts the most interst of researchers [8], [9], [10]. Driven by the inner-electric field, the photoinduced carriers are efficiently separated and transferred which inhibits their recombination. Unfortunately, both the oxidizability and reducibility of corresponding h+ and e usually become weakened after charge transfer, consequently decreasing the photocatalytic efficiency. Inspired by the natural photosynthesis in green plants, the construction of artificial heterogeneous Z-scheme photocatalytic system is an ideal and effective means because it possesses the high charge-separation efficiency and strong redox ability at the same time. According to the previous reports, most of the synthesized Z-scheme photocatalytic systems usually contained noble metal (Ag, Au, Pt, Ru) or redox pair (IO3−/I, Fe3+/Fe2+), which will restrict their practical application [11], [12], [13], [14], [15]. Actually, different types of carriers (such as electrons or holes) always do not play the same roles for the photocatalytic process, and it is more important to activate the redox ability of specific carriers by appropriate methods. Therefore, it is significant to construct a new Z-scheme system with two appropriate photocatalysts based on the specific reactive species.

Among the family of semiconductor materials, MoO3 is one of the most noticeable star materials due to its chemical stability, nontoxicity and abundant in source, and has been widely applied in many fields including wastewater treatment [16], capacitors [17], heterojunction photovoltaics [18], cathode material for lithium [19] and sodium-ion batteries and electrocatalytic materials [20]. Despite low quantum yield and large band gap (2.7–3.2 eV) of MoO3 layered material were not conducive to the photocatalytic performance, but its excellent dye adsorption capacity (as showed in SI) is significant for photocatalysis process [21]. To availably degrade the adsorbed pollutants, MoO3 must be collaborated with other applicable semiconductor or element to form heterojunctions or compounds. Belt-like α-MoO3, which possesses rectangular cross section and uniform width or thickness, has attracted particular interest due to its anisotropic structure, fast charge transfer dynamics, larger surface to volume ratio with specifically exposed crystal facets [6]. Up to now, a series of products have been studied, such as α-MoO3@MoS2 [22], TiO2/MoO3 [23], g-C3N4/MoO3 [24], Bi2Mo3O12/MoO3 heterostructures [16] and so on. Even so, to further shorten degradation period and improve degradation efficience is still a challenge. Moreover, there is no report on MoO3-based Z-scheme heterojunctions so far according to our knowledge. Therefore, it is necessary to construct the MoO3-based direct solid-state Z-scheme system with a visible-light-driven semiconductor photocatalyst for enhancing the photocatalytic efficiency.

In this study, we constructed AgBr quantum dots decorated MoO3 nanobelts to combine superior azo-dyes adsorptive capacity of MoO3 nanobelts with the high visible light absorption of AgBr, and the composite prospectively displayed excellent photocatalytic activities for degrading RhB under visible light irradiation. Subsequently, the changing of the band gaps was analyzed by theoretical calculations, and the reason for high efficiency degradation was investigated by designing further experiments including trapping reactive species and degrading visible light-insensitive thiophene. Finally, the probable mechanism for the photocatalysis process was proposed and discussed in detailed. This work provides a novel way for designing photocatalytic system consists of a substrate material with high pollutant adsorption ability and an antenna material with high visible light visible-light response.

Section snippets

Materials

All reagents were analytical grade and used without further purification. Molybdenum powder (Mo, AR), Rhodamine B (C28H31ClN2O3, AR) and barium sulfate (BaSO4, AR) were used as reagents and supplied by Aladdin. Silver nitrate (AgNO3, AR), Absolute ethyl alcohol (C2H5OH, AR), hydrogen peroxide (H2O2, 30%wt), sodium bromide (NaBr, AR), isooctane ((CH3)2CHCH2C(CH3)3, AR), thiophene (C4H4S, AR) were purchased from Sinopharm Chemical Reagent (Shanghai, China). Deionized water was produced using a

Formation and characterization of AgBr/MoO3 nanobelts composite

The proposed formation of AgBr/MoO3 nanocomposite was associated with the oriented diffusion of Br and the subsequent reaction with Ag+ on the MoO3 nanobelts surface, as schematically described in Fig. 1a. When the MoO3 nanobelts were homogeneously dispersed in ethanol solution at room temperature, introducing Br ions into the solutions contributed to surface adsorption ascribed to the positive charges of the MoO3 nanobelts surfaces. Once Ag+ ions were injected, it would easily react with Br

Conclusions

In summary, a composite photocatalytic system of AgBr quantum dots interspersed MoO3 nanobelts has been successfully constructed through a simple and handy way, and the composite displayed high degradation efficiency of 95% for Rhodamine B solution under visible-light irradiation within 5 min. Theoretical calculations indicated that the changing trend of the band gaps is consistent with the experimental results. Further experiments suggested that radical dotO2 played a much more important role than radical dotOH and

Acknowledgements

The work is supported by Jiangsu postdoctoral scientific research fund (1202016C), the National Nature Science Foundation of China (NSFC No. 51372114), Natural Science Foundation (NSF) of Jiangsu Province (BK20151198), the Open Project of Jiangsu Key Laboratory for Environment Functional Materials (SJHG1310) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD).

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