Elsevier

Chemosphere

Volume 215, January 2019, Pages 280-293
Chemosphere

A recyclable nanosheet of Mo/N-doped TiO2 nanorods decorated on carbon nanofibers for organic pollutants degradation under simulated sunlight irradiation

https://doi.org/10.1016/j.chemosphere.2018.09.182Get rights and content

Highlights

  • Recyclable MNTC nanosheet photocatalysts were fabricated.

  • The mineralization ratio for MB by the MNTC nanosheet is much higher than that by TiO2.

  • Environmental media such as inorganic salts and NOM were evaluated.

  • The possible degradation pathways and mechanism were proposed.

  • The practical application of the MNTC nanosheet indicates the potential in the future.

Abstract

A novel nanosheet of Mo/N-codoped TiO2 nanorods immobilized on carbon nanofibers (MNTC nanosheet) was self-synthesized through two facile steps. The Mo/N-doped TiO2 nanorods dispersed through in situ growth on the network constructed by long and vertical carbon nanofibers (CNFs). The fabricated MNTC nanosheet displayed superb photocatalytic activity of methylene blue (MB), and the degradation ratio by the MNTC nanosheet was nearly twice than that of pure nanoparticles. The photocatalytic activities during the degradation process in the presence of environmental media such as inorganic salts and natural organic matter (NOM) were also determined. Intermediates were analyzed by ion chromatography and electrospray ionization-mass spectrometry to unravel the potential degradation pathways, and the excellent mineralization ratio for MB over MNTC nanosheet was 79.8%. The trapping active species experiments verified that h+ was the main active species in the degradation process. Notably, the recycling experiment proved that the MNTC nanosheet was more stable, and it was successfully applied in purifying practical wastewater. Lastly, the fabricated MNTC nanosheet also displayed remarkable degradation performance towards sulfamethoxazole and bisphenol A.

Introduction

Wastewater represents an alternative to freshwater if it can be treated successfully for reuse applications. The majority of contaminants in wastewater are organic pollutants such as dyes (Li et al., 2017; Mirbagheri and Sabbaghi, 2018), antibiotics (Cheng et al., 2018; Xing and Jin, 2018; Chen et al., 2018) and pharmaceutical and personal care products (Dodgen et al., 2013; Yang et al., 2018). Promising treating techniques involve advanced oxidation processes (AOPs), adsorption (Huang et al., 2018a), electrochemistry (Rezgui et al., 2018; Zhang et al., 2018; Vidal et al., 2018) and membrane separation (Mahdavi et al., 2018; Makhetha and Moutloali, 2018; Liu et al., 2018a). Among them, AOPs have been considered as an efficient technique for the transformation and degradation of organic pollutants in aqueous media.

Heterogeneous photocatalysis is recognized as one of the most desirable method for the wastewater treatment because it may accelerate pollutant degradation through solar energy without leading to harsh conditions and secondary pollution and toxicity (Zhang et al., 2017; Wan et al., 2018a; Ma et al., 2018). Various photocatalysts such as TiO2 (Zhou et al., 2017), CeO2 (Wen et al., 2018), BiVO4 (Chen et al., 2016, 2017a) and NiO (Liu et al., 2018b), etc. have been synthesized for pollutant degradation with excellent photocatalytic activity. Compared with other photocatalysts, TiO2 is a broadly utilized photocatalyst in decomposing organic pollutants, owing to its unique properties, including nontoxicity, high chemical stability, low cost and strong photoreactivity (Liu et al., 2018c; Qin et al., 2018). However, the applications of the TiO2 photocatalyst are restricted because of the fast recombination rate of electrons–holes, narrow sun energy adsorption range, ease for aggregation and weak recycling ability (Yuan et al., 2018).

The photocatalytic activity of TiO2 can be enhanced by constructing heterojunction with other photocatalysts or doping with metallic and nonmetallic ions such as iron and nitrogen (Komatsuda et al., 2018; Rajagopal and Ryu, 2018; Huang et al., 2018b). Besides, loading photocatalysts on appropriate substrates such as nanowires (Patnaik et al., 2018; Han et al., 2018; Hoch et al., 2016), microspheres (Kim et al., 2018) and nonwovens (Wan et al., 2018b) is a desirable method to solve the recycling problem. Electrospinning nanofibers are featured by superb properties, such as interconnected pore structures, large surface area, facilely controlled pore-size distribution from the submicron level to a few micrometers, which are excellent candidates as the supports of photocatalysts (Shi et al., 2018; Zhao et al., 2018). Based on the abovementioned advantages, nanofibers have been applied in broad fields such as solid-phase extraction, environment, medicine, electronics and photonic devices (Si et al., 2017; Jia et al., 2014). Recently, CNFs, prepared by calcining electrospun nanofibers, have been highlighted in the areas of catalysis oxidation/reduction because of its excellent conductivity, huge strength, large surface area and flexibility (Munaweera et al., 2014; Park et al., 2018).

Herein, we designed and synthesized a nanosheet-shaped photocatalyst (MNTC nanosheet) by using a simple hydrolysis–calcination method through in situ growth of Mo/N-doped TiO2 nanorods on CNFs. After codoping with Mo and N, the photocatalytic activity of TiO2 has been obviously enhanced. Moreover, the modified TiO2 nanorods are distributed uniformly on the nanofibers. Photocurrent, photoluminescence (PL), electrochemical impedance spectroscopy (EIS), diffuse reflectance spectra (DRS) and Mott–Schottky (MS) measurements were conducted to confirm the electron–hole separation efficiency and band structures of the obtained catalysts. Compared with other organic dyes, methylene blue (MB) is difficult to be degraded and mineralized. Therefore, it was chosen as one of the model pollutants to estimate the photocatalytic activity of obtained catalysts. The ion chromatography (IC) and electrospray ionization-mass spectrometry (ESI-MS) were used to analyze the intermediates and explore the possible degradation pathways. Meanwhile, environmental factors such as initial solution pH, inorganic salts and organic acids were used to test the anti-interference ability of the MNTC nanosheet. The active species trapping experiment was performed to identify reactive species participated in the degradation process. Combining with the intermediates, reactive species and band structures, the possible degradation pathways and mechanisms were proposed in details. Moreover, the fabricated MNTC nanosheet has also been used in the degradation of sulfamethoxazole (SMZ) and bisphenol A (BPA) to evaluate its widespread application. Finally, the recycling and application of the MNTC nanosheet in wastewater purification were studied.

Section snippets

Chemicals and reagents

Polyacrylonitrile(PAN, MW = 150,000), N, N-dimethylformamide (DMF), tetrabutyl titanate (C16H36O4Ti, TBOT), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, AMT) and methylene blue (C₁₆H₁₈ClN₃S, MB) were purchased from Sigma-Aldrich, whereas other chemicals were purchased from Sinopharm Chemical Co., Ltd. All the chemicals were of analytical grade and used as received from suppliers without further purification.

Synthesis of PAN nanofibers

A 15wt% of homogeneous polymer blend solution for electrospinning was obtained by

Structure and morphology

Typically, excellent crystallinities of TiO2 and MNTC nanosheet were displayed in Fig. 2. Five characteristic peaks of anatase TiO2 appear at 25.3°, 37.8°, 48.1°, 53.8° and 62.2°, corresponding to (101), (004), (200), (105) and (213) crystal faces, respectively, which were well matched with the pure anatase TiO2 (JPDS Card No. 71-1166) (Qiu et al., 2018; Roy et al., 2013). No obvious diffraction peaks of CNFs were observed in the MNTC nanosheet, indicating its amorphous structure. For the MNTC

Conclusion

In summary, a novel nanosheet of Mo/N-codoped TiO2 nanorods immobilized on CNFs was successfully prepared by a facile method. It was demonstrated that the CNFs nanosheet afforded the modified TiO2 nanorods dispersion of a superb morphology. This is attributed to the network structure of CNFs, which facilitates the TiO2 nanorods in situ growth. The fabricated photocatalyst exhibits superior photodegradation activity towards MB under simulated sunlight irradiation. The maximum TOC removal ratio

Acknowledgments

The authors gratefully acknowledge the support from the National Natural Science Foundation of P.R. China (Grants 51522805 and 51708281), Natural Science Foundation of Jiangsu Province, China (Grant BK20170647) and the Nanjing University Innovation and Creative Program for PhD candidate (NO. CXCY17-21).

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