Facile synthesis of iron and cerium co-doped g-C3N4 with synergistic effect to enhance visible-light photocatalytic performance

https://doi.org/10.1016/j.materresbull.2020.110812Get rights and content

Highlights

  • Fe and Ce bimetallic co-doped g-C3N4 photocatalyst (Fe-Ce/CN) is successfully synthesized by simple calcination and hydrothermal method.

  • Comparing with pristine g-C3N4, Fe-Ce/CN shows more effective separation of electron-hole and narrowed band gap.

  • It exhibits high removal rate of Cr(Ⅵ) in the mixed pollutants solution without pH adjustment under visible-light irradiation.

  • The possible reaction mechanism of simultaneous photoreduction Cr(Ⅵ) and photooxidation organic dye by Fe-Ce/CN is proposed.

Abstract

A novel visible-light-driven Fe and Ce bimetallic co-doped g-C3N4 (Fe-Ce/CN) photocatalyst was successfully fabricated by a straightforward method. Experimental results indicated that no matter in the single pollutant system or mixed pollutants system containing Cr(Ⅵ) and an organic dye, Fe-Ce/CN exhibits super photocatalytic activity. It is mainly caused by the synergistic effect of Fe and Ce ions. Fe inserted into the lattice of g-C3N4 could narrow the band gap and inhibit combination of photogenerated electron-hole pairs. More importantly, the existence of Fe and Ce would improve the redox cycle with each other which is benefit for further increasing charge separation, light absorption and quantum efficiency. Furthermore, the controlled experiments and electron spin resonance (ESR) were performed to identify the relevant reactive species. Possible photocatalytic process of simultaneously removing mixed pollutants and mechanism were proposed. In general, this work provides a facile method to improve the photocatalytic activity of photocatalyst.

Introduction

Owing to the shortage of fossil energy, demands for wastewater treatment and carbon dioxide conversion, Semiconductor-based photocatalytic technology has attracted considerable attention worldwide [[1], [2], [3], [4]]. Therefore, numerous experiments have been directed toward photocatalytic material. Among various semiconductor photocatalyst, nonmetallic graphitic carbon nitride (g-C3N4) with structure of tri-s-triazine is generally considered as a promising photocatalyst. It plays an important role in the development of visible-light-driven photocatalyst on account of suitable narrow band gap (2.7 eV), nontoxic property and low cost with its earth-abundance raw material [[5], [6], [7], [8], [9]]. Besides, g-C3N4 has favorable chemical stability owing to the strong covalent bonds between molecules which make it stable in aqueous (pH = 7), even in acidic (pH = 0) or basic (pH = 14) solutions [10]. However, pristine g-C3N4 can only display poor photocatalytic performance under visible light irradiation due to its low surface area, poor light absorption and high recombination of photoinduced electron(e)-hole(h+) pairs [11,12]. From the aspect of practical application, it is meaningful to enhance the photocatalytic performance of the material to get cheap, high-efficiency, stable and environmental benign catalyst under visible light irradiation.

Up to now considerable efforts have been made to extend the photocatalytic activity of g-C3N4, including but not limited to synthesis method [[13], [14], [15]], impurities doping [[16], [17], [18], [19]], nanostructure design [[20], [21], [22], [23]], construction of heterojunctions [[24], [25], [26], [27]] and surface functionalization [[28], [29], [30]]. In these multifarious modification strategies, it is an effective way to dope g-C3N4 with heteroatoms. As a process of incorporating, it has been attested that doping foreign impurities can modify chemical nature and physical properties of g-C3N4 [31,32]. For elevating the photocatalytic efficiency of g-C3N4, plenty of foreign impurities doping have been developed, including doping with C [33], N [34], P [35], S [36],Fe [17], V [37], et al.

g-C3N4, which has unique structure of six lone-pair electrons of nitrogen, has the quite feasible and appropriate sites for metal inclusion [38]. Lots studies have been carried out for metal doping of g-C3N4. Iron (Fe), as a naturally abundant metallic element, has been regarded as a promising candidate for modification. On the one hand, Fe3+ /Fe2+ can facilitate the separation of photogenerated e-h+ pairs. On the other hand, complexing reaction of g-C3N4 with Fe3+ and Fe2+ can happen. Against this background, many modified methods about Fe have been reported. Most recently, Tian et al. succeed in preparing Fe-doped g-C3N4 and demonstrated that their catalytic performance is great improved to tetramethyl benzidine [39]. Based on interfacial charge transfer effect, Liu et al. adopted Fe(Ⅲ) grafted g-C3N4 and the sample exhibited enhanced photocatalytic activity owing to increasement of visible light sensitive and effective e-h+ separation [40]. Meanwhile, Hu et al. reported that Fe3+ doped g-C3N4 catalysts have strong reducing nitrogen fixation [41].

Ceria (Ce), as a kind of friendly rare earth material, has valence variation of Ce4+ and Ce3+ due to incompletely occupied 4f orbital [42]. Interfacial charge transfer and separation rate of photogenerated e-h+ pairs will be improved by the redox cycle of Ce4+/Ce3+ [43]. Chen et al. synthetized Ce-doped TiO2/diatomite granule by sol-gel method and evaluated the degradation of oxytetracycline [44]. Moreover, a theoretical arithmetic based on density functional method demonstrated that Ce have energy barrier like that of Pt and g-C3N4 supported Ce can create more active sites [45].

Clearly, Fe or Ce doping can improve photocatalytic performance of g-C3N4 by changing electronic structure, broadening photo-response range and limiting high charge recombination. More importantly, it has been proved that solid solution containing Fe and Ce can mutually facilitate the cycle of Fe3+/Fe2+ and Ce4+/Ce3+ [46]. So far, few researches have been reported to testify the double metallic co-doped g-C3N4 can accelerate the redox cycle and further improve the photocatalytic performance.

Herein we prepared Fe and Ce bimetallic co-doped g-C3N4 (Fe-Ce/CN) photocatalyst via the simple calcination and hydrothermal method to improve photoreduction activity. For comparing their photocatalytic efficiency, different content Ce and Fe were also mono-doped on pure g-C3N4. The photocatalytic experiments were carried out in single and mixed pollutants system under visible light irradiation. The resultants show that Fe-Ce/CN can exhibit hugely improved photocatalytic performance due to its effective separation of e-h+, narrowed band gap and high quantum efficiency.

Section snippets

Material

Urea (CH4N2O, 99 %), potassium bichromate (K2Cr2O7, 99 %), methyl orange (MO) and rhodamine B (RhB) were provided by Tianjin Damao Chemical reagent Factory. Iron(III) nitrate nonahydrate (Fe(NO3)3, 98 %) was obtained by shanghai Macklin Biochemical Co., Ltd. Cerium(III) nitrate hexahydrate (Ce(NO3)3, 99 %) was bought from Tianjin Kemiou Chemical Reagent Co., Ltd and Ethanol (C2H6O, 99 %) was acquired by Tianjin Zhiyuan Chemical reagent Factory. All of the chemicals applied in this work were

Photocatalyst characterization

After preliminary experiment, 3 %Ce/CN and 2.5 %Fe/CN were choose as best photocatalytic activity catalysts in their single doping system (Fig. S1). The discussion of single doping centered around them. The crystal structures of as-prepared products were characterized by XRD. The XRD patterns are given in Fig. 1. There are two distinct characteristic diffraction peaks resulting from in-plane tri-s-thiazine packing motif and interlayer stacking of aromatic system located at about 13.1° (100) and

Conclusion

In this study, a novel Fe and Ce bimetallic co-doped CN was successfully synthesized through facile calcination and hydrothermal method. The as-prepared sample Fe-Ce/CN exhibits well photocatalytic performance for Cr(Ⅵ) reduction and organic dye oxidation in single or mixed pollutants system under visible light irradiation. The substantially improved photocatalytic activity was mainly attribute to synergy effect between redox cycle of two metallic ions. On the one hand, simultaneous doping of

Author statement

Manuscript title: Facile synthesis of iron and cerium co-doped g-C3N4 with synergistic effect to enhance visible-light photocatalytic performance

I have made substantial contributions to the conception or design of the work; I have revised the work critically for important intellectual content.

I agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

All persons who

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.

Acknowledgement

We are genuinely appreciative of all the support offer by Analytical and Testing Center of SCNU of TEM measurement (No. 16 KJ20).

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