Microstructure and performance of Z-scheme photocatalyst of silver phosphate modified by MWCNTs and Cr-doped SrTiO3 for malachite green degradation
Graphical abstract
Introduction
Over the past decades, the increasingly severe energy crisis and environmental pollution have raised widespread concern, and the utilization of solar energy in environmental remediation has been paid close attention [[1], [2], [3]]. Semiconductor-based photocatalysis technique is such a promising and environmentally friendly technology [[4], [5]]. Despite of its potential advantages, there are still challenges in this field such as maximizing the use of the solar energy, especially the visible-light energy [6]. TiO2, the most frequently used photocatalyst, can only absorb the ultraviolet light energy that accounts for merely 4% of the sunlight energy, thus greatly impeding the use of the solar energy [[7], [8]]. Therefore, investigations on developing efficient visible-light-driven photocatalysts have become a focus [9]. More recently, silver phosphate (Ag3PO4) stands out among numerous photocatalysts due to its excellent photocatalytic performance under visible-light irradiation, which can achieve quantum efficiencies of more than 90% (λ > 420 nm). Moreover, its high photooxidative capabilities for using dissolved oxygen in water and outstanding ability for degrading organic pollutants have been reported [10]. Unfortunately, the photocorrosion, crystal size, and morphology of the photocatalyst hampered its photocatalytic activity and consequent practical applications. For example, Yang et al. found that Ag3PO4 polyhedra with an average diameter of more than 15 μm showed much less photocatalytic activity than sphere-like polyhedral morphology with an average diameter of 450 nm [11].
To overcome these problems, a lot of investigations have been performed, which mainly focus on improving synthesis procedures in controlling morphology and size and adding other materials to prepare silver phosphate-based heterostructured composite photocatalysts [[10], [12]]. For instance, adding graphene to synthesize Ag3PO4-GR composites by hydrothermal approach has been reported as an effective way in control the morphology and size of Ag3PO4 [12]. The formation of heterojunction is considered effective to inhibit the recombination of photogenerated electron-hole and to enhance photocatalytic activity. Several such heterostructured catalysts, including Ag3PO4/TiO2 [13], AgX/Ag3PO4(X = Cl,Br,I) [14], Ag3PO4/BiVO4 [15], Ag3PO4/grapheme [16] and Ag3PO4/Ag/WO3-x [17] have been reported. Compared with above mentioned semiconductors, SrTiO3 is a stable semiconductor with the great potential in water splitting, and its minimum conduction band and maximum valence band are both negative compared to those of Ag3PO4 [18]. Thus, Ag3PO4/Cr-SrTiO3 heterojunction photocatalyst, Cr-doped SrTiO3 with narrower band gap coupling with Ag3PO4 could exhibit higher photodegradation efficiency for isopropyl alcohol [19]. Nevertheless, the photocatalytic activity of this binary system still needs improving. Furthermore, a more efficient Z-scheme heterojunction system has not been proposed and confirmed in this binary system, and low photocatalytic efficiency due to the size and morphology of Ag3PO4 crystals could not be overcome neither.
Z-scheme system is inspired by photosynthesis of green plants which could turn water and CO2 into carbohydrates under visible light irradiation by two photoexcited process (photosystems I and II) [[20], [21]]. Z-scheme heterojunction therefore means combining two different photocatalysts using an appropriate shuttle redox mediator to form two photochemical systems and an electron-transfer system, which could exhibit better performance in separating photogenerated electrons from holes and increasing their activity [[22], [23]]. A Z-scheme photocatalytic system BiVO4/Au@CdS showed better photocatalytic performance through the Z-scheme photocatalytic system than BiVO4/Au and BiVO4/CdS [24]. In this study, Ag3PO4 and Cr-doped SrTiO3 are expected to be the photochemical system I (PS I) and PS II, respectively, and the small amount of Ag produced during the photocatalytic irradiation process as the electron-transfer system. This three-component system has the potential to construct the Z-scheme photocatalytic system. On this basis, considering the combination with other materials to further overcome the problems of photocorrosion, crystal size and morphology of Ag3PO4.
Multi-walled carbon nanotubes (MWCNTs) with unique electrical, optical and chemical properties show the potential to overcome these problems. MWCNTs was considered as one of the most promising carbon-based nanomaterial because of its outstanding properties, including i) high aspect ratio; ii) excellent electrical conductivity (>105 S/m); iii) high Young's modulus (≈1 TPa); and iv) extraordinary tensile strength (≈60 GPa) [[25], [26], [27]]. Thus, MWCNTs could be an ideal additive to modify the properties of composites [28]. Most recently, a few research teams have combined MWCNTs with Ag3PO4 to improve photocatalytic performance [29]. The photocatalyst of Ag3PO4-WO3/MWCNTs were successfully prepared with high-performance for the simultaneous degradations of Cr(VI) and orange II [30]. The AgBr-Ag3PO4/MWCNTs composite fabricated via a chemical precipitation method shows excellent photocatalytic activity for the treatment of dye wastewater [31]. These successful cases demonstrated that MWCNTs could be the electron acceptor to increase the electron migration rate, improve the separation of carrier, and consequently enhance photocatalytic activity of the photocatalysts.
Unfortunately, data and reports on the investigations of sharp changes in size and morphology of Ag3PO4 crystals caused by MWCNTs have not been available. Additionally, the new photocatalyst which combines the advantages of Z-scheme heterojunction Ag3PO4/Ag/Cr:SrTiO3 and MWCNTs should have the potential of excellent visible-light-driven photocatalytic performance and photostability have not been reported. Therefore, Ag3PO4@MWCNTs@Cr:SrTiO3 was prepared and characterized in the present work, and its performance for MG degradation was evaluated. The mechanism about the notable change in the size and morphology of Ag3PO4 crystals caused by the introduction of MWCNTs were discussed in detail. The photodegradation mechanisms were investigated using GC–MS, ESR, and radical trapping experiments.
Section snippets
Material
Strontium acetate (Sr(Ac)2) was purchased from Shanghai Mackin Biochemical Co. Ltd. Titanium isopropylate ([(CH3)2CHO]4Ti) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Multi-walled carbon nanotubes (MWCNTs) with nominal outer diameters of 10–20 nm were purchased from Nanjing Xianfeng Nano Materials Technology Co., Ltd. Disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O), silver nitrate (AgNO3), malachite green (MG) and other reagents were purchased from Sinopharm
Characterization of the as-prepared samples
The morphology and microstructure of as-prepared samples were characterized by scanning electron microscopy (SEM). As presented in Fig. 1a, obviously, Ag3PO4 polyhedra with an average diameter of about 30 μm was observed, to which some small irregular sphere-like polyhedral particles were adhered, and they were Ag3PO4 particles with an average diameter of about 700 nm. Interestingly, after MWCNTs has been introduced into this system, large Ag3PO4 particles with the diameter of 30 μm no longer
Conclusions
A novel type of Z-scheme heterojunction photocatalyst Ag3PO4@MWCNTs@Cr:SrTiO3 was synthesized successfully and reported for the first time. The addition of MWCNTs to the composite catalyst changed the size and micro-morphology of Ag3PO4 from a polyhedron crystal with a diameter of about 30 μm to a spherical-like one with a diameter of 0.28–0.69 μm. The composite of Ag3PO4@MWCNTs@Cr:SrTiO3 exhibited excellent photocatalytic activity for MG degradation in aqueous solutions under both sunlight and
Acknowledgments
This work was supported by the project of National Natural Science Foundation of China (Grant No.: 51478172, 51278464, and 51521006), the Natural Science Foundation of Zhejiang Province of China (Grant No.: LY17E080002), the Department of Science and Technology of Hunan Province of China (Contract No.: 2017JJ2029), and the Department of Science and Technology of Changsha City of China (Contract No.: kh1601187).
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These authors contributed equally to this paper.