Photoinduced synthesis of green photocatalyst Fe3O4/BiOBr/CQDs derived from corncob biomass for carbamazepine degradation: The role of selectively more CQDs decoration and Z-scheme structure
Graphical abstract
Introduction
The environmental problems caused by emerging pollutants such as Pharmaceuticals and Personal Care Products (PPCPs) have been paid much attention in recent years [1]. Thereinto, carbamazepine (CBZ) as the representative central pain analgesic, has played positive role in treating trigeminal neuralgia, epilepsy or other mental diseases, yet has also brought negative effects to the environment [2], [3], [4]. CBZ residuals existed in the surface water, underground water and drinking water would not bring acute toxicity due to low exposure levels (ng/L-µg/L), however, chronic toxicity to creatures was confirmed in the reported literature [3], [5]. For example, the mRNA level of mussel gill tissue of invertebrates in water could be significantly increased after 1 day of CBZ exposure, while the mRNA levels of superoxide dismutase and metallothionein of digestive gland gills were decreased under CBZ exposure for a long time, showing the obvious inhibition effect [6]. Hard biodegradation property makes CBZ persistent and bioaccumulative due to the unique chemical structure of symmetrical aromatic heterocycle [7]. Thus, how to effectively eliminate CBZ residuals in aqueous is a hot topic to be discussed and solved by researchers.
Semiconductor photocatalysis technology as one of the Advanced Oxidation Processes (AOPs) can effectively degrade refractory organic pollutants because of its strong oxidation capacity, fast reaction rate, easy operation and less secondary pollution [8], [9], [10]. Bismuth oxybromide (BiOBr) as the novel visible-light-response bismuth-based photocatalyst is consisted of [Bi2O2]2+ layers and double slabs of bromide atoms, which has attracted much attention and interest due to its moderate band gap energy (∼2.6 eV), outstanding optical property and excellent stability [11], [12], [13]. Besides, a few reported researches pointed out (1 1 0) facet of BiOBr exhibited higher photocatalytic activity because of its greater surface energy and stronger internal electric field [11], [14]. However, photogenerated charge carriers recombination and low photocatalytic efficiency are the vital challenges to restrict the practical application of BiOBr [15], [16]. Recently, interfacial regulation and constructing heterojunction have been considered as efficient strategies for the activity optimization of semiconductor photocatalysts [17], [18], [19].
Notably, carbon quantum dots (CQDs), novel “zero-dimensional” carbon-based nanomaterials, are considered to be outstanding candidates of interfacial regulation due to their intrinsic natures of up-conversion photoluminescence, excellent electron-transfer ability and good biocompatibility [20], [21]. There are many sources for preparing CQDs, while hydrothermal preparation of CQDs derived from waste biomass is a more environmental-friendly method than that from chemical reagents such as ethylenediamine and citric acid [22], [23]. Moreover, how to perform excellent synergetic role between the electron transfer ability of biomass CQDs and the crystal surface characteristics of BiOBr is a challenge worth further consideration. Using common impregnation or adsorption methods can result in random distribution and unstable binding of CQDs, while some researches demonstrated that photogenerated electrons and holes of photocatalyst with strong redox ability could be distributed spatially to different crystal planes under light irradiation [24], [25], [26]. Therefore, we suppose that light-driven synthesis method such as photodeposition method is an innovative choice for biomass CQDs as electron acceptors to be more completely and firmly distributed on the crystal planes of BiOBr, and the experimental results in this research confirmed our prediction.
Furthermore, constructing magnetic bismuth-based heterojunction photocatalyst can not only settle with the problem of photogenerated charge recombination, but also promote the separation and regeneration abilities with the help of external magnetic field for practical application [27], [28]. Common magnetic materials mainly include ferrites (MnFe2O4, CoFe2O4, ZnFe2O4), metal oxides (Fe3O4, Co3O4, Mn3O4, γ-Fe2O3), and pure metals (Fe0, Co) [29], [30], [31]. Thereinto, Fe3O4 as the representative iron oxide is widely used to form bismuth-based/magnetic heterojunction photocatalysts due to its intrinsic benefits of good stability, strong magnetism and low cost [32], [33]. It owns inverse spinel structure Fe3+[Fe2+Fe3+]O42-, and its interaction between Fe2+ in octahedral sites and Fe3+ in tetrahedral sites is beneficial for the strong magnetism [34]. Meanwhile, the intrinsic n-type semiconductor property of Fe3O4 is good for photogenerated charge transfer [35]. Therefore, combining it with BiOBr can solve the difficult problems of nanoparticles recovery and photogenerated charge recombination.
Herein, inspired by photodeposition method and environmental-friendly strategy, constructing novel magnetic biomass CQDs-based photocatalyst Fe3O4/BiOBr/CQDs via photoinduced method to photodegrade CBZ is the object of the research. Firstly, corncob biomass CQDs and novel heterojunction Fe3O4/BiOBr/CQDs were fabricated successfully by hydrothermal and photoinduced methods. Secondly, the differences between photoinduced synthesis and common hydrolysis synthesis on crystal type, crystal morphology, light absorption ability and catalytic activity of photocatalyst Fe3O4/BiOBr/CQDs were emphatically studied. Thirdly, considering the practical application, the properties of Fe3O4/BiOBr/CQDs were investigated from the aspects of material proportion optimization, common influencing factors of natural water, regeneration and stability of photocatalyst. Moreover, the degradation activity on CBZ using the photocatalyst Fe3O4/BiOBr/HC produced by corncob charcoal residue during the synthesis of CQDs was also discussed. Lastly, according to characterization and experimental results, detailed and plausible CBZ degradation mechanism and pathways by Fe3O4/BiOBr/CQDs were proposed.
Section snippets
Chemicals and reagents
Waste corncob biomass from Yuzhong county, Lanzhou City of China was applied as biomass carbon quantum dots precursor. Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), sodium bromide (NaBr) and ferroferric oxide (Fe3O4) were all analytical grades. Benzoquinone (BQ), isopropyl alcohol (IPA), and ethylenediamine tetraacetic acid (EDTA) with analytical grades were used as scavengers of superoxide radicals, hydroxyl radicals, and photogenerated holes, respectively. Target pollutant CBZ was produced by
Morphologies of photocatalysts
Morphology and element distribution of as-prepared photocatalysts were analyzed by SEM and EDS-mapping images as presented in Fig. 1. Photocatalyst BiOBr presented thin sheet shape and magnetic Fe3O4 nanoparticles mainly exhibited nanosphere morphology (Fig. 1(a and b)). Fe3O4 nanoparticles were dispersed on the interface of BiOBr when the above two materials were composed together to form Fe3O4/BiOBr (Fig. 1(c)). Besides, similar hierarchy morphologies were also observed on the ternary
Conclusion
In this work, magnetic, green and environmental-friendly photocatalyst Fe3O4/BiOBr/CQDs decorated by corncob biomass CQDs was fabricated successfully via photoinduced method. More CQDs with diameter of 5–7 nm were deposited on the (1 1 0) facet of BiOBr for FeBrCQDs-4 than FeBrCQDs-4 (NL) through EDS and TEM analyses. FeBrCQDs-4 exhibited higher photocatalytic performance on CBZ degradation, where the removal efficiencies of CBZ and TOC within 120 min visible LED light irradiation could reach
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 was supported by the National Natural Science Foundation of China (Grant 42077330) and National Key R&D Program of China (grant number: 2018YFC1903700).
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Xiaoyun Xie and Shan Li equally contributed to this work as co-first authors.