Oxygen vacancies – Cu doping junction control of δ-Bi2O3 nanosheets for enhanced photocatalytic nitrogen fixation
Graphical abstract
2D ultrathin Cu-δ-Bi2O3 nanosheets possess excellent photocatalytic nitrogen fixation without any sacrificial agent and remarkable ability of the photocatalytic degradation of RhB. The surface oxygen vacancies, doping Cu induced defect and ultrathin nanosheet structure can effectively separate the photoinduced electron-hole pairs, improve the molecular chemisorption, and heighten the photocurrent and photocatalytic performance.
Introduction
The photocatalytic nitrogen fixation technology has an unparalleled advantage [1], [2], [3], [4], such as, high efficiency. For example, the technical nitrogen fixation through the Haber-Bosch process requires not only high temperature and high pressure but also large consumption of hydrogen [5], [6]. The photocatalytic nitrogen fixation technology has an unparalleled advantage, such as high efficiency, low energy and environmental protection, which make up for the shortage of industrial nitrogen fixation [7], [8]. Despite great efforts for the photocatalytic nitrogen fixation, to achieve an efficient N2 conversion efficiency, there is still some aspects to be clarified, such as, providing rapid migrate of photogenerated carrier from the interior to the surface, suppressing of recombination of photogenerated carrier, and constructing sufficient active sites for facilitating N2 activation.
Recently, the development of ultrathin 2D nanosheet material is considered an effective strategy to increase surface active sites, improve rapid migration of photogenerated carrier, and reduce bulk charge recombination [9], [10]. Therefore, a series of ultrathin materials have been developed for photocatalytic N2 fixation [11], [12], [13], [14], [15]. However, as far as migration of photogenerated carrier of ultrathin 2D nanosheet material, the ultrafast transformation can easily lead to rapid recombination [16]. To address this bottleneck, fabricating surface oxygen vacancies assist to forming capture trap, which can serve as active sites to promote the adsorption and activation of target molecules. In this way, the surface-reached electrons / holes are used for photocatalytic reaction in replace of recombination. Therefore, designing ultrathin 2D nanosheet material with surface oxygen vacancies may be feasible approaches to improve charge transformation, boost separation efficiency and supply active sites for high-efficiency photocatalytic N2 fixation.
The bismuth based compounds, such as BiOX(X = Cl, Br, I) [17], [18], Bi2MoO6 [19], Bi2WO6 [20], BiVO4 [21], Bi2O3 [22], BiFeO3 [23], Bi4Ti3O12 [24], Bi3NbO7 [25], possess good photocatalytic properties because of small effective electron mass and suitable Fermi wavelength. Especially, Bi2O3 has been widely used in the fields of optical thin film, fuel cells and catalysis because of its adjustable band gap, high performance oxygen ion and polycrystalline property [26], [27], [28], [29], [30]. However, rapid recombination of photogenerated electron-hole and weak photoabsorption limit the potential application of δ-Bi2O3 as an effective photocatalyst [31], [32], [33]. Recently, doping modification is an effective way to improve the photocatalytic activity of semiconductor materials [34]. When the elements are doped into the semiconductor photocatalyst, the doping level can be formed. These energy levels can become a trap of the photogenerated carrier, and thereby effectively inhibit the recombination of the electron-hole [35]. Meanwhile, doping elements into δ-Bi2O3 inevitably lead to coordinative unsaturation and structural distortion of surface Bi-O bonds, which facilitates the escape of partial surface oxygen atoms to form oxygen vacancies.
Therefore, addressing the aforementioned issue, to construct 2D δ-Bi2O3 nanosheet is great promise to alleviate the resistance of charge transfer, shorten the migration distance of charge, and prompt carrier migration from bulk to surface. At the same time, to artificial control and rational design of bi-defects of surface oxygen vacancies and impurity defect is a practical approach for tuning the spatial charge separation. Herein, we reported an effective method for tuning of transfer and spatial separation of carrier charge by design 2D δ-Bi2O3 nanosheet with bi-defects of surface oxygen vacancies and impurity defect. The influence of prepared conditions on the structural, crystallinity, surface areas and optical properties of photocatalyst was systematically studied by several means. Photocatalytic nitrogen fixation was evaluated without any sacrificial agent under light irradiation. The charge carrier transfer was evidenced by the X-ray photoelectron spectroscopy, photoluminescence, and electrochemical impedance spectra.
Section snippets
Preparation
1 mmol Bi(NO3)3·5H2O was dissolved in a mixed solution contained 8 mL ethylene glycol and 32 mL tertbutyl alcohol, followed by vigorous stirring for 3 h and ultrasonic for 30 min. Subsequently, the mixture was put into a Teflon-lined stainless steel autoclave with a capacity of 50 mL. The autoclave was heated and maintained at 140 °C (160 °C, 180 °C) for 8 h and then cooled to room temperature and dried in vacuum at 60 °C for 48 h. The samples were designated as BiO-1, BiO-2, BiO-3.
The Cu
Phase structure analysis
The XRD patterns of the as-prepared samples are shown in Fig. 1a. The diffraction peaks of the as-prepared samples index to well cubic phase of δ-Bi2O3 (JCPDS No. 27–0052). The peaks at 2θ of 27.8°, 32.3°, 46.3°and 54.6° can be indexed to (111), (200), (220), (311) diffraction planes of cubic structured δ-Bi2O3, respectively. Especially, the main diffraction peaks the as-prepared BiO-1, BiO-2 and BiO-3 samples is located in 2θ of 27.8°, 32.3°, 46.3°and 54.6°, which indicates that treatments
Conclusions
Cu doping δ-Bi2O3 nanosheets with uniform thickness of approximately 2.7 nm was prepared by hydrothermal method. The ultrathin sheets structure is benefit to the formation of the surface oxygen vacancies, meanwhile the impurity defect is formed due to the substitutive Cu doping of δ-Bi2O3. Under visible light irradiation, the highest rate of the photocatalytic nitrogen fixation was 142.8 µmol h−1 g−1 for 5% Cu-δ-Bi2O3 without any sacrificial agent. The remarkable ability of photocatalytic
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 has received the finical support from the National Natural Science Foundation of China (21766039), and the project of department of science and technology of Shaanxi province (2021GY-124).
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