Research ArticleWood-based carbon quantum dots for enhanced photocatalysis of MIL-88B(Fe)
Graphical abstract
Introduction
The dye pollution problem is endangering human water safety. Therefore, how to effectively deal with organic dyes in water has caused widespread concern. Fenton-like degradation can be used for treating pollutants in water, i.e., the use of hydrogen peroxide (H2O2) to generate strong oxidizing free radicals, thereby degrading the target pollutants using advanced oxidation technology [[1], [2], [3]]. In recent years, more and more Fenton-like photocatalysts have been studied [[4], [5], [6], [7]]. MIL-88B (Fe) is an iron-based metal organic framework (MOF) with an appropriate band gap (approximately 2.0–2.3 eV) for photocatalysis [8,9]. Moreover, MIL-88B (Fe) is rich in open channels and has a larger specific surface area than traditional photocatalysts [10,11]. Under visible-light irradiation, an organic ligand can efficiently absorb solar energy and then transfer the energy to the active site. These active sites can contact pollutants in large areas through pores, and at the same time activate H2O2 to generate free radicals to degrade them [12,13]. Therefore, MIL-88B(Fe) is a very promising heterogeneous Fenton-like photocatalyst (see Table 1).
Although MIL-88B(Fe) has many advantages, its photogenerated carrier recombination rate is high, and the solar energy conversion efficiency under visible light is low, restricting the catalytic performance of MIL-88B(Fe) [14]. Mixing with carbon materials is one of the effective methods for improving the separation efficiency of photogenerated carriers. Hang Zhang combined carbon nanotubes with MIL-88B(Fe) to improve the photocatalytic efficiency [15]. Tuan A. Vu prepared a composite of graphene oxide and MIL-88B(Fe) and improved the degradation effect of pollutants using the composite [16]. Carbon quantum dots (CQDs) are an emerging zero-dimensional carbon-based material. Wood powder, a processing waste, is a good carbon source for CQDs. Compared with other carbon materials, wood-based CQDs have diverse raw material sources, low cost, excellent optical properties, and outstanding electron-inducing ability [17,18]. The optical properties of CQDs could be further improved by N-doping, so that CQDs could be used in photocatalysis [20,26]. When combined with other materials, the light absorption capacity of a composite is enhanced. Furthermore, CQDs can be used as electron donors or electron acceptors, so that the composite has good light-induced electron transfer performance [19].
To the best of our knowledge, CQDs have been rarely used to promote Fenton-like reaction of MIL-88B(Fe). Therefore, in this study, CQDs were prepared using hydrothermal method with wood powder as a raw material, and CQDs were added to a precursor solution of MIL-88B(Fe) to successfully prepare CQDs@ MIL-88B(Fe) photocatalyst. The Fenton-like degradation performances of MIL-88B(Fe) and CQDs@MIL-88B(Fe) on methylene blue (MB) under visible light were compared. The results show that the photocatalytic performance of MIL-88B(Fe) loaded with CQDs considerably improved. Various experimental methods were used to study the photochemical properties and photocatalytic principles of these photocatalysts in detail.
Section snippets
Material and reagents
Poplar wood flour was used as the raw material for CQDs. The reagents, including ethylenediamine (en), citric acid (CA), ferric chloride (FeCl3), sodium hydroxide (NaOH), terephthalic acid (PTA), N,N-Dimethylformamide (DMF), methyl alcohol, methylene blue (MB), superoxide dismutase (SQD), isopropyl alcohol (IPA) were purchased from Tianjin Kermel chemical reagent limited company and used as received without further purification.
Sample preparation
CQDs were produced using hydrothermal method. 2 g of poplar wood
Molecular structure and composition
The chemical composition of CQDs was analyzed by XPS. Strong peaks were observed corresponding to C 1s, N 1s, and O 1s (Fig. 1a). The high-resolution C 1s and N 1s spectra indicate the existence of various chemical bonds on the surface of CQDs. The C 1s spectrum can be distinguished into three independent peaks: 287.7 eV (CO), 286.0 eV (C–O/C–N), and 284.5 eV (C–C/CC), while the spectrum of N 1s can be distinguished into two peaks: 400.4 eV (pyrrolic N) and 399.5 eV (C–N–C) (Fig. 1b and c) [[21]
Conclusions
Wood-based CQDs were successfully introduced into MIL-88B(Fe) by hydrothermal method. It was found that CQDs@MIL-88B(Fe) utilize sunlight more efficiently and have a better band structure than pure MIL-88B(Fe) material. A lower recombination rate of photogenerated carriers makes CQDs@MIL-88B(Fe) degrade MB faster, leading to a higher MB removal rate. In addition, the existence of a small number of Fe2+ also plays a role in the increase of optical system.
CRediT authorship contribution statement
Huadong Zhang: Conceptualization, Methodology, Validation, Writing - original draft. Xinchao Gong: Investigation, Formal analysis. Zihui Song: Investigation, Formal analysis. Shuo Zhang: Investigation, Formal analysis. Wenxin Du: Investigation, Formal analysis. Tat Thang Nguyen: Data curation, Visualization. Minghui Guo: Resources, Supervision, Funding acquisition, Writing - review & editing. Xing Gao: Resources, Supervision, Funding acquisition, Writing - review & editing.
Declaration of competing interest
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (31971587).
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