Construction of carbon dots modified MoO3/g-C3N4 Z-scheme photocatalyst with enhanced visible-light photocatalytic activity for the degradation of tetracycline
Graphical abstract
Introduction
In the past decades, tetracycline (TC) has been widely used to treat human and animal infections [1]. It has been frequently detected in various environmental matrices. The presence of TC in the environment could cause antibiotic resistance problem, which poses a serious threat to the well-being of human and animals. However, due to the stable chemical structure and recalcitrance to biological degradations, TC cannot be effectively removed through conventional wastewater treatment processes [2,3]. Therefore, new techniques for the treatment of TC in wastewater are required. Recent years, photocatalytic degradation of TC from wastewater has received a lot of attention due to its high efficiency and long-term reliability. Some of traditional photocatalysts, e.g., TiO2 [4], have been proven to have the ability to degrade TC under UV light irradiation. However, the poor utilization of solar light energy of these photocatalysts hindered their practical application. Therefore, the sunlight -driven photocatalysts are needed for the photocatalytic treatment of TC.
Graphitic carbon nitride (g-C3N4) as a new visible light-driven photocatalyst has attracted enormous attention due to its low toxicity, high stability, and low cost [5]. As a metal-free polymeric semiconductor, g-C3N4 has a narrow band gap of ca. 2.7 eV and thus has a strong visible-light response. This property results in electrons and holes easily produced under visible-light excitation [6]. However, the high recombination rate of the photogenerated electron-hole pairs and the low specific area of pure g-C3N4 greatly limited its photocatalytic activity [7,8]. Therefore, various methods have been developed to improve the photocatalytic performance of the pure g-C3N4, including nonmetal doping [9,10], metal deposition [11], coupling with other materials to form hybrids [12,13], and using nano-sized structures g-C3N4 [14,15]. In particular, combining g-C3N4 with other semiconductors to form the Z-scheme heterostructured photocatalyst would efficiently separate the photogenerated electrons and holes, thus enhance the photocatalytic activity of g-C3N4 under visible light.
MoO3 is a well-known p-type metal oxide semiconductor and has been considered to be a promising candidate to form the Z-scheme heterostructured photocatalyst due to its unique energetic and electrical properties [16]. It has been demonstrated that combining MoO3 with other photocatalyst, such as TiO2 [17], CdS [18], and polyimides [19], enabled the composites with the excellent photocatalytic activity through the hindering of charge recombination and improving of charge transfer processes. Recent studies also found that the combination of MoO3 with g-C3N4 could form the Z-scheme photocatalyst with enhanced photocatalytic performance [16,20]. Because of the suitable band gaps between the two semiconductors, the photogenerated charge carrier can be effectively separated and hence generated more reactive species. Nevertheless, it is still needed to improve the visible light absorption of the composites to achieve their high photocatalytic activity in their practical applications.
Carbon quantum dots (CDs), as a new discovered carbon-based nanomaterial, displayed the excellent up-conversion photoluminescence feature, as well as the outstanding photoinduced electron transfer and reservoir properties, all of which enabled the photocatalysts to utilize the sunlight efficiently [21]. Due to these remarkable properties, CDs have been successfully integrated with many semiconductors to improve their photocatalytic activities by decreasing electron-hole recombination and broadening the photo-absorption region. Our former studies also found that CDs could effectively improve the photocatalytic performances of TiO2 [22], BiPO4 [23], and g-C3N4 [24] for the degradation of PPCPs in water. Therefore, we are motivated to rationally design the carbon dots modified MoO3/g-C3N4 Z-scheme photocatalyst with enhanced visible-light photocatalytic activity for potential practical application.
In this study, we reported the facile fabrication of CDs/g-C3N4/MoO3 composites, which showed excellent visible-light photocatalytic performance compared to that of the pristine g-C3N4, MoO3, and g-C3N4/MoO3 hybrids. The microstructures, morphologies, phases, and optical properties of the samples were investigated via a series of characterization. Tetracycline (TC), as one of the most widely used antibiotics, was used to evaluate the photocatalytic activities of the prepared composites. The influences of MoO3 and CDs contents on the photocatalytic performance of the CDs/g-C3N4/MoO3 composite were systematically evaluated. Electron spin resonance (ESR) and the reactive species trapping experiments were conducted to qualitatively detect the roles of the different active species during TC degradation. Subsequently, possible photocatalytic transformation pathway and mechanisms for TC degradation were proposed. Finally, the reduction of antibacterial activity of TC during the photocatalytic process was evaluated.
Section snippets
Catalyst preparation
All reagents (Taitan Company, China) employed for catalyst preparation were analytical grade and used without further purification. Deionized (DI) water from a Milli-Q apparatus (>18 mΩ cm, Germany) was used. MoO3 powder was synthesized through the solid-state decomposition reaction of (NH4)6Mo7O24·4H2O at 500 °C for 4 h in ambient air condition. The obtained product was then washed with DI water three times and dried at 60 °C for 5 h.
Graphitic carbon nitride (g-C3N4) was synthesized by
Structure and morphology characterizations
The XRD pattern of pure g-C3N4, MoO3, CM3 and 0.5CCM3 are displayed in Fig. 1. Two diffraction peaks of pure g-C3N4 are observed at 13.0° and 27.4°, respectively corresponding to the in-plane structural packing motif and interlayer stacking structure of aromatic segments, which can be perfectly indexed to (001) and (002) crystal planes of graphitic materials. Pure MoO3 has the main peaks at 12.8°, 23.4°, 25.7°, 25.8°, 27.3°, and 39.0°, respectively relating to the (020), (110), (040), (120),
Conclusions
In this study, we constructed a high-efficient Z-scheme CDs/g-C3N4/MoO3 photocatalyst through a facile calcination method. The prepared 0.5CCM3 photocatalyst showed excellent photocatalytic properties on tetracycline degradation under visible light comparing with g-C3N4/MoO3 and pristine g-C3N4. The analysis of reactive species confirmed that h+ played dominant role in the photocatalytic degradation of TC. CDs as electron reservoir enhanced the separation and optical converter, and hence
Acknowledgments
This work was supported by the National Natural Science Foundation of China (21707019 and 21677040), the Innovative Team Program of High Education of Guangdong Province (2015KCXTD007), and the Science and Technology Planning Project of Guangdong Province (2017A050506052, 2017A020216010, and 2017B020216003).
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