Regular ArticleEnhanced visible-light photocatalytic nitrogen fixation over semicrystalline graphitic carbon nitride: Oxygen and sulfur co-doping for crystal and electronic structure modulation
Graphical abstract
O-S co-doped semicrystalline g-C3N4 was synthesised by a simple hydrothermal method. The obtained photocatalyst had high activity and stability in visible-light photocatalytic nitrogen fixation.
Introduction
Nitrogen fixation is one of the most important chemical processes in nature. Nitrogen element is required for the synthesis of biomolecules like proteins and nuclei acids, while most organisms are unable to metabolize natural nitrogen directly. The Haber-Bosch process is an artificial nitrogen fixation technology and ammonia is produced from the reaction of hydrogen and nitrogen. However, high pressure and temperature is required in the Haber-Bosch method, which limits its development in practical application. Therefore, it is considerable significant to investigate a low energy consumption and eco-friendly process to develop nitrogen fixation.
Photocatalysis, as an environmentally benign technology, has a great potential in nitrogen fixation [1], [2]. To date, a variety of photocatalysts has been explored to realize photocatalytic nitrogen fixation. Schrauzer et al. [3] reported that the nitrogen could be reduced to ammonia by electrons generated from the light excitation of Fe doped TiO2 powders. The reaction could be described as 6H2O + 2N2 → 4NH3 + 3O2. Janet et al. [4] synthesized Pt loaded hierarchical ZnO for nitrogen photofixation. The Pt metal played an important role in the activation of NN bonds, improving the efficiency of nitrogen fixation. However, these semiconductor materials mentioned can only be excited by ultraviolet light (5% of the solar light). Therefore, it is highly desired to explore efficient visible-light photocatalysts for nitrogen fixation recently. In order to enhance the nitrogen fixation efficiency under visible-light irradiation, Sun et al. [5] prepared ultrathin MoS2 to accelerate nitrogen reduction reaction by introducing light induced trions. Li et al. [6], [7] reported that oxygen vacancies of BiOCl and BiOBr nanosheets could absorb and active N2, and transfer electrons as well, reducing the electron-hole recombination rate and improving the photocatalytic activity for nitrogen fixation. Meanwhile, Dong et al. and Wu et al. [8], [9] reported that more nitrogen molecules could be absorbed on nitrogen vacancies and be reduced to NH3 under visible-light irradiation. Until now, most of the photocatalysts used in nitrogen fixation were expensive and low abundant. Non-noble metals and even metal-free photocatalysts were expected to be used for the application of visible-light photocatalytic nitrogen fixation.
Graphitic carbon nitride (g-C3N4) has been deemed a promising heterogeneous metal-free semiconductor photocatalyst in recent years. It has attracted considerable attention due to its high physicochemical stability, appealing electronic structure, and appropriate band positions [10], [11], [12], [13]. However, g-C3N4 is also subject to fast electron-hole recombination and low photocatalytic activity. Generally, the activity is greatly influenced by the structure, including of the crystal structure, electronic structure, and chemical structure [14]. Recently, many efforts for developing g-C3N4 have been made, such as crystal structure modulation [15], chemical structure adjustment [16], and electronic structure optimization [17]. Jürgens et al. [18] designed the crystal of g-C3N4 during condensation of melamine rings by using an ionthermal approach. Wirnhier et al. [19] prepared a high crystalline poly(triazine imide) (PTI)/Li+Cl− using dicyandiamide in molten salts. More recently, Wang et al. [20] prepared semicrystalline g-C3N4 by a hydrothermal method using bulk g-C3N4 and ammonium chloride. It was emphasized that the exciton (bounded electron-hole pairs, which are formed by Coulomb interactions) dissociation could occur at the order-disorder interfaces in the semicrystalline g-C3N4. And the dissociated exciton in g-C3N4 could release electrons and holes, which could transfer to the ordered and disordered chains, respectively, developing the photocatalytic activity. In addition, Paquin et al. [21], [22] also determined that excitons between crystalline (ordered) and amorphous (disordered) phases tended to dissociate in semicrystalline polymeric semiconductors.
The electronic structure of g-C3N4 could be optimized by doping with metal-free atoms to improve the photocatalytic performance [23], [24], [25], [26]. For example, oxygen doped g-C3N4 could modulate electronic and band structure, and expand light absorbance range, enhancing the visible-light photocatalytic activity [27]. In our previous report, the photocatalytic activity of oxygen doped porous g-C3N4 was remarkably improved as compared to that of bulk g-C3N4 [28]. Liu et al. and Wang et al. [23], [29] reported that sulfur doped g-C3N4 elevated the conduction band minimum and enhanced the conductivity of g-C3N4, thus improving the photocatalytic performance of g-C3N4. Besides of single-element doping, many co-doped g-C3N4, such as PO co-doped g-C3N4 [30], OS co-doped g-C3N4 [31], and PS co-doped g-C3N4 [32] also presented excellent photocatalytic performance attributing to tailoring its electronic and optical properties. Compared to single-element doped g-C3N4, co-doped g-C3N4 photocatalysts combine the advantages of these dopants. Nevertheless, it is still a challenge to modify the crystal and electronic structure of g-C3N4 with bi-elements to make g-C3N4 an excellent photocatalyst for visible-light photocatalytic nitrogen fixation.
Herein, we developed a facile one-step hydrothermal treatment route to prepare OS co-doped semicrystalline g-C3N4 (HGCNOS) for visible-light photocatalytic nitrogen fixation by treating g-C3N4 with l-cysteine. The chemical and electronic structure of HGCNOS significantly changed as compared to g-C3N4. Furthermore, the photocatalytic activity of HGCNOS in visible-light photocatalytic nitrogen fixation was evaluated. The results indicated that the exciton dissociation at the order-disorder interfaces and metal-free atoms doping synergistically led to the improved photocatalytic nitrogen fixation activity of HGCNOS.
Section snippets
The synthesis of GCN
GCN photocatalyst was synthesized with melamine as the precursor in a furnace, heating to 550 °C at a ramp rate of 2 °C/min and keeping for 4 h. The as-prepared yellow product was washed with ultrapure water and ethanol by several times and dried at 60 °C overnight to obtain GCN.
The synthesis of HGCNOS
HGCNOS complex was prepared by treating GCN with an l-cysteine assisted hydrothermal method. In a typical synthesis, l-cysteine (0.5 g) and 1 g GCN were dispersed in 100 mL water. Subsequently, the mixture was transferred into
Chemical structure and morphology of HGCNOS
The crystal structures of the as-prepared photocatalysts were analyzed by XRD. As shown in Fig. 1a, the (1 0 0) and (0 0 2) peaks in GCN located at 13.0° and 27.5° corresponded to the in-plane ordering of tri-s-triazine units (melon) and interlayer stacking, respectively (Fig. 1a). The XRD pattern of GCN was similar to that in previous reports [10], [11], [12], [13]. For HGCNOS, both of the two characteristic peaks became narrower and stronger, suggesting its crystallinity and periodicity were
Conclusions
In this paper, oxygen and sulfur co-doped semicrystalline graphitic carbon nitride (HGCNOS) photocatalyst was successfully synthesized by hydrothermal treatment of GCN and l-cysteine. HGCNOS exhibited a higher photocatalytic activity than GCN and materials in previous reports [5], [6], [7], [9] on nitrogen photofixation. The doping of oxygen made HGCNOS semicrystalline, forming disorder-order interfaces. The exciton dissociated on the disorder-order interfaces of semicrystalline HGCNOS,
Acknowledgements
The financial supports from the National Natural Science Foundation of China (Nos. 51478223 and 51678306), China Postdoctoral Science Foundation (2017T100372, 2016M590458, and 2013M541677), the Jiangsu Planned Projects for Postdoctoral Research Funds (1202007B) and the Fundamental Research Funds for the Central University (30915011308) are gratefully acknowledged.
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