Enhancement mechanism of full-solar-spectrum catalytic activity of g-C3N4-x/Bi/Bi2O2(CO3)1-x(Br, I)x heterojunction: The roles of plasma Bi and oxygen vacancies
Graphical abstract
Introduction
The rapid development of industry, agriculture and animal husbandry has brought great convenience to the production and life of human beings, but also causes serious environmental pollutant. The solar energy, as a typical clean and renewable energy source, can be used for environmental remediation through semiconductor photocatalysis technology [1], [2]. Recently, some novel photocatalysts (Bi2O2CO3, ZnIn2S4, CuBi2O4, SrBi4Ti4O15 and so on) [3], [4], [5], [6] have been designed and explored to overcome the shortcomings of traditional photocatalysts in practical application, such as, low utilization of solar energy, rapid carrier recombination, etc. Among them, Bi2O2CO3 has a unique two-dimensional structure formed by alternating [Bi2O2]2+ and CO32– layers, resulting in a large internal electric field between layers and relatively high separation efficiency of photo-induced carriers [7], [8]. However, the application of Bi2O2CO2 is often limited by low solar energy utilization due to the large bandgap energy (∼3.1–3.5 eV) [9], [10].
Doping and oxygen vacancy (OV) engineering are two relatively simple and effective methods to improve the optical performance of semiconductor materials by forming defect states between the forbidden bands [11], [12], [13], [14], [15]. However, the exposed surface defects are easily inactivated in air and water, which will affect the subsequent photocatalytic reaction and the cycle stability of the catalyst [16], [17]. In addition, the excessive dopants and oxygen vacancies usually exist in the subsurface layer of the catalyst in the form of defect clusters, which may act as the recombination centers of electron-hole pairs and result in a decrease in photocatalytic activity [18], [19], [20]. Studies have shown that building the internal electric field through gradient concentration doping and passivating surface defect states by decorating molecules/ions or coupling with other semiconductors [21], [22], [23], [24], [25] are effective strategies to improve charge separation efficiency and stabilize surface defects. Unfortunately, the above modification methods are usually at the expense of the excellent optical performance of the catalyst, and the weakened light absorption is not beneficial for the further improvement of the photocatalytic performance [26], [27].
It is an urgent problem in the field of photocatalysis that how to stabilize surface defects and improve charge separation efficiency while making full use of solar energy. Wang et al. prepared a ternary Zn2SnO4/BiO2-x/Bi2O2.75 heterojunction via in-situ hydrothermal reaction, which exhibited stable full-spectrum absorption and high charge separation efficiency compared with BiO2-x/Bi2O2.75 due to the increased OVs concentration induced by the interface polarization charge transfer [28]. Chen et al. synthesized Bi@OV-Bi2O2CO3 and found that the deactivation of OVs could be inhibited by decorating with Bi nanoparticles [29]. In addition, similar to the noble metal Au and Ag, Bi also has unique localized surface plasmon resonance (LSPR) effect [30]. When the photon frequency matches the natural frequency of Bi surface electrons, the collective oscillations of free electrons in Bi are driven by coupling with the electromagnetic field of incident light. LSPR excitation can not only produce high-energy hot carriers to participate in photocatalytic reactions, but also improve the optical properties of the material system [31], [32]. Therefore, plasma Bi has been widely used in the modification of semiconductor photocatalysts [33], [34].
Based on the above reports and analysis, in this work, OVs and plasma Bi co-modified g-C3N4-x/Bi/Bi2O2(CO3)1-x(Br, I)x heterojunction was designed and prepared via solvothermal reaction. The formation of OVs was promoted induced by the polarization charge transfer from Bi/Bi2O2(CO3)1-x(Br, I)x to g-C3N4-x, while the deposition of Bi on the surface of g-C3N4-x and Bi2O2(CO3)1-x(Br, I)x was attributed to the esterification between CH3CH2OH and NaBiO3 and the subsequent anoxic thermal reduction. The roles of OVs and plasma Bi in the improvement of the full-spectrum catalytic performance of g-C3N4-x/Bi/Bi2O2(CO3)1-x(Br, I)x heterojunction were studied, and the corresponding enhancement mechanism was proposed.
Section snippets
Fabrication of g-C3N4-x/Bi/Bi2O2(CO3)1-x(Br, I)x heterojunction
g-C3N4-x containing nitrogen vacancies (NVs) was prepared according to the previous report [35]. g-C3N4-x/Bi/Bi2O2(CO3)1-x(Br, I)x heterojunction was prepared via solvothermal method. Firstly, 3 mmol of NaBiO3 was dispersed in 40 mL of absolute ethanol. After magnetic stirring for 30 min, 5 mL of HNO3 was added to the above suspension, and the color of reaction system changed from light brown to reddish brown. Then, 3 mmol of the prepared g-C3N4-x, 0.8 mmol of NaIO3 and a certain amount of KBr
Formation mechanism
The XRD patterns of the samples were shown in Fig. S1(a). The diffraction peaks of g-C3N4-x at 12.8° and 27.8° corresponded to the (1 0 0) and (0 0 2) crystal faces of graphite-like carbon nitride, respectively. The XRD patterns of BOCO, IBOCO, BBOCO and IBBOCO revealed the characteristic peaks of Bi (JCPDS No. 85–1329) and Bi2O2CO3 (JCPDS No. 84–1752). Compared with BOCO, IBOCO and BBOCO, the stronger diffraction peaks of Bi in IBBOCO suggested the increased Bi content, suggesting that the
Conclusion
In summary, due to the synergistic effect of plasma Bi and OVs, g-C3N4-x/Bi/Bi2O2(CO3)1-x/(Br, I)x heterojunction showed significantly enhanced full-spectrum response catalytic activity, while maintaining excellent stability due to the inhibited inactivation of OVs by the deposition of metal Bi. The improved TC adsorption and optical performance were achieved due to the promoted OVs formation induced by polarization charge transfer from Bi/Bi2O2(CO3)1-x(Br, I)x to g-C3N4-x. The separation
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the Project of the National Natural Science Foundation of China (Grant No. 52172215, 51772180), the Shaanxi Province Key Research and Development Plan (2018GY-107) and the Graduate Innovation Fund of Shaanxi University of Science and Technology (SUST-A04).
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