Fe(III)-C3N4 hybrids photocatalyst for efficient visible-light driven nitrogen fixation

https://doi.org/10.1016/j.matchemphys.2020.123830Get rights and content

Highlights

  • A new strategy of preparing enriched N-vacancy C3N4 by introducing Fe(III) is developed.

  • The high separation rate of electron-hole and broad bandgap of C3N4 are achieved.

  • The Fe(III)-C3N4 effectively enhances the visible-light nitrogen photofixation activity.

Abstract

Solar-driven reduction of nitrogen to ammonia is a promising strategy for energy storage and conversion. Herein, a new strategy of modifying Fe(III) onto carbon nitride is reported showing enhanced photocatalytic nitrogen fixation. The Fe(III) and nitrogen vacancies narrow down the carbon nitride intrinsic bandgap and suppress the radiative electron-hole recombination. Fe(III)-C3N4 therefore shows potential as a new high-efficiency artificial photosynthesis ammonia.

Introduction

Ammonia (NH3) is an essential clean energy carrier and traditional chemical raw material of fertilizers [[1], [2], [3], [4]]. Haber–Bosch reaction consumes more than 1% of the global energy supply, resulting in various harmful environmental consequences [5]. Developing green and clean methods of replacing the Haber–Bosch process is highly demanded [6,7]. Schrauzer and Guth and co-workers reported the first photo-fixation nitrogen by Fe-doped TiO2 in 1977 [8]. Although efforts have been devoted, as a rate-determining step for the photocatalytic nitrogen fixation, the adsorption and activation of N2 are still a significant challenge [9,10]. Two attractive aspects have been focused on improving the adsorption and activation of N2: (1) mimicking the natural nitrogen fixation by plants [[11], [12], [13]], (2) engineering more vacancies at the surface interface of the catalyst [14]. Recent results showed that introducing defects on semiconductor surfaces enhanced the N2 adsorption and activation [15,16]. Carbon nitride (C3N4) is a promising photocatalytic nitrogen fixation candidate due to its suitable electronic band structure, excellent stability, low cost, and easy preparation [17]. Dong et al. found that nitrogen vacancies in C3N4 nanosheets improved nitrogen fixation ability [18]. Xie and co-workers reported the introduction of a single Cu atom to isolate the valence electron remarkably increased the vacancies and accelerated ammonia yields [19]. Wang et al. explored KOH-treated g-C3N4 with high activity because that K doping introduced more surface nitrogen vacancies [20]. There is still plenty of room for improving the photocatalytic efficiency of g-C3N4 for artificial light synthesis nitrogen fixation.

An efficient catalyst of Fe(III)-C3N4 was developed for efficient photocatalytic reduction of N2. This catalyst exhibited an ammonia production rate of 146.25 μmol g−1 in water without any sacrificial agents, which is 3.4 times higher than that of pristine C3N4. The cation Fe incorporation provided a high level of nitrogen vacancies. These high concentration nitrogen vacancies are beneficial to the selectively adsorbing of N2 as an electron trap center. Fe(III)-C3N4 also shows a broader absorption range of light and faster carrier separation efficiency.

Section snippets

Materials

Melamine (≥99.0%), Ferric chloride hexahydrate (FeCl3·6H2O, AR), ammonium hydrogen carbonate (NH4HCO3, AR), potassium hydroxide (KOH, AR), sodium hydroxide (NaOH, 96.0%), ammonium chloride (NH4Cl, 99.5%), sodium citrate dehydrates (C6H5Na3O7·2H2O, 99.0%), silver nitrate (AgNO3, AR), 2-PrOH (AR) were obtained from Sinopharm Chemical Reagent (China). Ethanol was obtained from Beijing Reagent Co. (China). Nessler's reagent (K2[HgI4], 98.0%), potassium sodium tartrate tetrahydrate (C4H4O6KNa·4H2O,

Results and discussion

The TEM image showed that Fe(III)-C3N4 was thin and irregular nanosheets (Fig. 1a). The HRTEM image revealed that there was no characteristic related to Fe2O3, FeOOH quantum dots, or other iron-based compounds in Fe(III)-C3N4 (Fig. 1b). Scanning transmission electron microscopy (STEM) image and corresponding energy disperse spectrometer (EDS) element mapping exhibit a homogeneous distribution of C, N, and Fe element on Fe(III)-C3N4 (Fig. 1c–f). XRD patterns in Fig. 1g showed one distinct

Conclusions

The incorporation of Fe(III) and the resulted abundant nitrogen vacancies in Fe(III)-C3N4 promoted the adsorption and activation of N2 for the photocatalytic N2 fixation. The as-synthesized Fe(III)-C3N4 possessed favorable bandgap and extended visible light absorbance with improved charge separation and transfer efficiency. This work may open new opportunities for developing high-efficiency solar-energy-conversion catalysts.

CRediT authorship contribution statement

Hui Zeng: Conceptualization, Methodology, Resources, Investigation, Writing - original draft. Lulu Liu: Investigation. Dantong Zhang: Investigation. Ying Wang: Software, Formal analysis. Zhenhua Li: Data curation, Validation, Formal analysis. Chang Liu: Software, Validation. Lei Zhang: Supervision, Project administration. Xiaoqiang Cui: Writing - review & editing.

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.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (51872116 and 51571100), the National Key R&D Program of China (Grants 2016YFA0200400), the Program for JLU. Science and Technology Innovative Research Team (JLUSTIRT, 2017TD-09), and Jilin province science and the Technology development program (20190201233JC).

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