Research article
Highly efficient visible photocatalytic disinfection and degradation performances of microtubular nanoporous g-C3N4 via hierarchical construction and defects engineering

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Abstract

Herein, microtubular nanoporous g-C3N4 (TPCN) with hierarchical structure and nitrogen defects was prepared via a facile self-templating approach. On one hand, the hexagonal tubular structure can facilitate the light reflection/scattering, provide internal/external active sites, and endow the electron with oriented transfer channels. The well-developed nanoporosity can result in large specific surface area and abundant accessible channels for charge migration. On the other hand, the existence of nitrogen vacancies can improve the light harvesting (λ > 450 nm) and prompt charge separation by acting as the shallow charge traps. More NHx groups in g-C3N4 framework can promote the interlayer charge transport by generating hydrogen-bonding interaction between C3N4 layers. Therefore, TPCN possessed highly efficient visible photocatalytic performances to effectively inactivate Escherichia coli (E. coli) cells and thoroughly mineralize organic pollutants. TPCN with the optimum bactericidal efficiency can completely inactivated 5 × 106 cfu mL−1 of E. coli cells after 4 h of irradiation treatment, while about 74.4 % of E. coli cells were killed by bulk g-C3N4 (BCN). Meanwhile, the photodegradation rate of TPCN towards methylene blue, amaranth, and bisphenol A were almost 3.1, 2.5 and 1.6 times as fast as those of BCN. Furthermore, h+ and •O2 were the reactive species in the photocatalytic process of TPCN system.

Introduction

In the past few decades, water pollution has become a global problem, which seriously threatened the ecological environment and human health. Among numerous pollutants existed in wastewater, pathogenic bacteria [1], benzene based organic dyes [2], and endocrine-disrupting chemicals (EDCs) [3] are three kinds of major contaminants that arouse wide public concerns. Pathogenic bacteria could exert an influence on aquatic ecosystem and pose an epidemical risk to human health [4]. Benzene based organic dyes, one kind of potential carcinogens, could lead to high chromaticity and toxicity of water body [5]. EDCs could mimic the bioactivity and disturb the metabolic process of natural hormones, which would damage the endocrine function of humans and even cause the fetal malformation [6]. Consequently, it is a significant task to seek an efficient technology to remove pathogenic bacteria, organic dyes, and EDCs from wastewater. Compared with traditional treatments, photocatalysis technology shows a great potential in environmental purification because of its mild reaction conditions, strong oxidation ability, and no harmful by-products [7]. Graphitic carbon nitride (g-C3N4), an organic semiconductor, shows an outstanding potential in the fields of environment remediation due to its low cost, high stability, visible-light response, and appropriate electronic band structure [[8], [9], [10], [11], [12]]. However, poor light utilization, low specific surface area, and rapid charge recombination have severely limited the further photocatalytic application of g-C3N4 [13,14]. Since the polymeric framework of g-C3N4 make it feasible to tune its textural, chemical, and electronic structures, varieties of strategies have been adopted to overcome the above problems and improve the photocatalytic activity of g-C3N4, including hierarchical structure construction [15,16], defect engineering [17,18], morphology manipulation [[19], [20], [21]], elemental doping [22,23], heterojunction designing [24,25], and noble metals decoration [26,27].

Among these approaches, hierarchical structure construction is regarded as a promising way to optimize g-C3N4 as they can strengthen the light harvesting, increase the exposed surface, promote the reactant diffusion, and accelerate the charge transfer [28]. Tubular and porous structures are two kinds of popular morphology for g-C3N4 that have attracted extensive attention. Tubular g-C3N4 possesses the superiorities of both 1D and hollow structures, which bring about several unique advantages, such as facilitating the light reflection/scattering, providing internal and external active sites, and endowing the electron with oriented transfer channels [29,30]. Meanwhile, the well-developed porosity of porous g-C3N4 can lead to large specific surface area, abundant exposed active sites, and low mass transfer resistance [31,32]. Inspired by these respective structural merits, the fabrication of microtubular nanoporous g-C3N4 with hierarchical structure should be an efficient pathway to enhance the photocatalytic performance by taking advantage of both tubular and porous structures. The hierarchical g-C3N4 with microtubular exposed edges and easily accessible nanoporous networks can not only gain higher adsorption capacity for pollutants and offer more active sites for charge transfer, but also avoid the aggregation of catalyst particles and facilitate the separation of catalysts from the reaction system [33]. At the same time, the introduction of special point defects into g-C3N4 framework, such as carbon or nitrogen vacancies, can modify the surface property, tune the electronic structure, and serve as shallow charge trap sites to promote charge separation, and finally enhance the photocatalytic quantum efficiency of g-C3N4 [34,35]. Therefore, it is highly desirable and challenging to achieve g-C3N4 photocatalyst with novel hierarchical structure and point defects simultaneously.

Generally, to obtain the especial architecture, hard/soft-templating approach is commonly adopted [36,37]. However, this kind of preparation method is neither cost-efficient nor environmental-friendly, as it usually requires complex procedures of template modification and the involvement of poisonous etchants [38]. Additionally, hard-templating often restrain the subsequent functionalization while soft-templating may lead to the lattice disorder in the matrix, seriously restricting the widespread application [39]. Thus, it is an urgent desire for developing an effective, economical, and green approach to synthesize the microtubular nanoporous g-C3N4. In recent years, molecular self-assembly has emerged as a versatile self-templating method for preparing hierarchical materials with unique morphology under mild conditions [40]. It enables two or more kinds of organic molecules to regularly assemble into large-sized and ordered supramolecular aggregates by noncovalent interactions including hydrogen bonding and π-π stacking due to their strong direction and saturation properties [41]. To date, specially-shaped g-C3N4 has been obtained using hydrogen bonding and π-π stacking induced supramolecular intermediate as the precursor by controlling the synthesis temperature and adjusting the pH value of aqueous solution [42]. Therefore, the design of ordered supramolecular precursor is very crucial to the fabrication of g-C3N4 with micro-nanostructure.

In this work, microtubular nanoporous g-C3N4 (TPCN) with hierarchical structure and nitrogen defects was prepared via a molecular self-assembly approach. As melamine can hydrolyze into cyanuric acid at an appropriate pH value, it was utilized as the raw material to in situ produce hexagonal melamine-cyanuric acid supramolecular precursor under acetic acid-assisted hydrothermal process. The acetic acid could not only adjust the pH value to prompt the hydrolysis process, but also fine tune the skeletal structure of precursor to construct defect engineered TPCN. The formation mechanism of the prism-like supramolecular intermediate and TPCN with micro-nanostructure was discussed in detail. The morphology, structure, and properties of TPCN were investigated by various techniques. The disinfection and degradation activities of TPCN photocatalyst were carefully evaluated under visible light irradiation towards typical pathogenic bacteria, organic dyes, and EDCs, which were remarkably enhanced in comparison with bulk g-C3N4 (BCN). Moreover, the important roles of the hierarchical structure and nitrogen defects played in the photocatalytic process of TPCN system were also systematically elucidated to understand the enhanced activities.

Section snippets

Synthesis of microtubular nanoporous g-C3N4 photocatalysts

Firstly, 2 g of melamine was put into a flask with 90 mL of acetic acid solution in a specific concentration (2%, 5%, 8% (v/v)) and refluxed in a heating mantle at 100 °C for 30 min. Then the solution was transferred into a stainless autoclave with a Teflon-inner-liner. The autoclave was sealed and put into an oven, kept at 180 °C for 10 h. After the hydrothermal process, the suspension was centrifuged, washed with deionized water, and dried at 60 °C. The obtained solids were calcinated at

The formation process of microtubular nanoporous g-C3N4 with nitrogen defects

The synthetic process of TPCN photocatalyst including four steps was illustrated in Scheme 1. Firstly, melamine was dissolved in acetic acid solution after heating, and then partially in situ hydrolyzed into cyanuric acid at a rather slow rate during the acetic acid-assisted hydrothermal process. Secondly, the generated cyanuric acid and the remaining melamine immediately self-assembled into melamine-cyanuric acid supramolecular hexamer in the same plane through multiple hydrogen-bond

Conclusion

In conclusion, TPCN with hierarchical structure and nitrogen defects was successfully prepared through a facile self-templating approach. The 1D hollow structure and well-developed porosity of TPCN photocatalyst could increase the specific surface area and exposed active sites, provide multiple light reflection and scattering channels, and facilitate the fast and long-distance charge migration. Additionally, the introduction of nitrogen defects in g-C3N4 skeleton improved the light harvesting (λ

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21707052), the Jiangsu Agriculture Science and Technology Innovation Fund (No. CX(18)2025), the Fundamental Research Funds for the Central Universities (Nos. JUSRP11905 and JUSRP51714B), and the Key Research and Development Program of Jiangsu Province (No. BE2017623).

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