Regular ArticlePhotocatalytic nitrogen fixation: Oxygen vacancy modified novel micro-nanosheet structure Bi2O2CO3 with band gap engineering
Graphical abstract
Introduction
Easily accessible in nature, N2 accounts for about 78% of the earth’s atmosphere volume. However, the “fixed” nitrogen, necessary form for living organisms [1], cannot be obtained easily due to negative electron affinity, high ionization energy, and strong nonpolar triple bond of N2 molecule [2], [3], [4], [5]. Haber − Bosch process, the industrial nitrogen fixation method, has to be performed under drastic conditions (15–25 MPa, 300–550 °C) to overcome high activation energy, thus it has the disadvantages of high energy consumption and large CO2 emissions [6], [7], [8]. Therefore, it is desirable to develop more economical and environmentally friendly methods for ammonia production. Compared to the traditional industrial method, the greatest advantage of photocatalytic nitrogen fixation is that molecular N2 can be activated under ambient conditions and the hydrogen source can be replaced from H2 to H2O. However, the main restricting factors of this process are the limited absorption range of light, large recombination of photo-generated charges and difficulty to activate extremely stable N2 molecules [9], [10], [11], [12], [13]. Thus, the more subtle and artificial design of semiconductor photocatalysts to meet the high energy gap of N2 as well as maintain a small band gap to permit maximum visible light irradiation seems to be the main challenge for nitrogen fixation photocatalysts development [14].
As a member of the austenite phase oxide, Bi2O2CO3 possesses a unique layered structure and intrinsic distortion, which is advantageous to the separation of photo-generated charges [15]. The large band gap provides appropriate redox potential for various photocatalytic reactions. Nonetheless, to achieve better efficiency on photocatalytic nitrogen fixation, it is vital to construct an electron-donating structure for N2 activation and a more feasible kinetic path for further reaction. Defects formation on semiconductor photocatalysts is a very promising way of modification to provide abundant active sites for reactants adsorption and activation, effectively lowering activation energy barriers for thermodynamically challenging reactions. Among all forms of defects, surface oxygen vacancies (OVs) have attracted much attention [16], [17], [18]. Firstly, surface OVs can regulate the band gap of semiconductors in a certain range, expanding the light-responding area from ultraviolet to visible light [19]. Secondly, defect states can trap abundant localized electrons and inhibit the recombination of photo-generated charges [20]. Photo-generated electrons injection from surface OVs can provide abundant specific sites for molecular N2 chemisorption and activation, significantly lowering the activation energy and facilitating surface chemical reactions [21], [22], [23]. Moreover, the properties of nanomaterials vary greatly with different morphology and particle size. Therefore, size regulation is another effective mean of improving the performance and efficiency of photocatalysts. It has been proved that the specific surface area and light absorption efficiency are prone to increase with the reduction on the size of photocatalysts [24], [25], which is beneficial for generation of more active sites toward photocatalytic reaction and accelerating surface adsorption and reactions.
Normally, defects are more inclined to occur during a nanomaterials down-sizing process since the atomic-escape energy can be largely reduced [26]. Surface atoms are more prone to escape from the 2D lattice, resulting in surface defects generation. In this work, surface OVs are introduced via a nanomaterial size-reducing process under room temperature. Surface OVs modified Bi2O2CO3 (namely BOC/OV) are successfully fabricated with a uniform micro-nanosheet structure (down-sizing to 10 × 10 nm). The synergetic efforts of surface OVs modification and small size effect of BOC/OV are beneficial for adsorption and activation of N2 molecule while facilitating photo-generated charges separation and effectively improving the photo-fixation ability of N2 under visible light irradiation. With the accumulation of surface OVs, the energy band is consecutively tuned and the conduction band all shifted to a much lower position. The whole photocatalysts preparing and N2 photo-fixation processes are conducted under ambient conditions without extra heat energy required. Comparing with the traditional hydrothermal method and severe conditions for OVs formation (such as hydrogen reduction and calcination in reducing atmosphere), it is simpler operated and fully meets the demand for energy conservation and sustainable development.
Section snippets
Materials synthesis
In this study, all the chemical reagents used were purchased from Macklin (Shanghai, China) and were all analytical grade used without further purification. Ultrapure water was used in the whole study.
Typically, 2 mmol Bi(NO3)3·5H2O and 6 mmol Na2CO3 were dispersed in 20 mL deionized water respectively. After sonicating and stirring for a while to fully dissolve the reagents, Na2CO3 solution was added dropwise to Bi(NO3)3·5H2O solution under magnetic stirring. After stirring at room temperature
Crystal phase and chemical structure characterization
XRD was performed to confirm the basic crystal structure. As shown in Fig. 1a, all samples applied well to JCPDS no. 84–1752. The main diffraction peaks at 2θ = 12.9, 23.9, 30.3, 32.7, 42.3, 47.0, 52.2 and 56.9° could be assigned to (0 0 2), (0 1 1), (0 1 3), (1 1 0), (1 1 4), (0 2 0), (1 1 6) and (1 2 3) planes of the tetragonal structure of Bi2O2CO3, respectively, indicating that basic crystalline structure and phase composition of BOC/OV were not altered. Furthermore, the intensity and width of the peaks
Conclusions
We report an effective method to constructing surface OVs modified Bi2O2CO3 (BOC/OV) micro-nanosheets for photocatalytic nitrogen fixation. Surface OVs were successfully introduced via nanomaterial size-reducing process with the presence of glyoxal. The concentration of OVs and the band gap values can be deliberately and consecutively tuned by the adding amount of glyoxal. Herein, with surface OVs acting as active sites, all BOC/OV samples delivered higher activity on N2 photo-fixation owing to
CRediT authorship contribution statement
Yalan Feng: Conceptualization, Investigation, Methodology, Formal analysis, Writing - original draft. Zisheng Zhang: Resources, Funding acquisition. Kai Zhao: Validation, Investigation, Writing - review & editing. Shuanglong Lin: Formal analysis, Writing - review & editing. Hong Li: Writing - review & editing, Project administration. Xin Gao: Resources, Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgment
This work was supported by the National Key R&D Program of China (2018YFB1501600), Doctoral Research Foundation of Shijiazhuang University, China (20BS003) and the National Natural Science Foundation of China (No. 21476161).
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