Few-layer graphitic carbon nitride nanosheet with controllable functionalization as an effective metal-free activator for peroxymonosulfate photocatalytic activation: Role of the energy band bending

Highlights

• Electron-withdrawing groups functionalized few layer g-C₃N₄ was prepared.

• FCN-12 exhibits excellent performance in PMS photocatalytic activation.

• Few layer structure and functionalized groups co-boost charge separation.

• This work provides comprehensive insights into the role of energy band bending.

Abstract

In this work, electron-withdrawing groups functionalized few layer graphitic carbon nitride (FCN-12) nanosheet was prepared and utilized for photocatalytic activation of peroxymonosulfate (PMS). Experimental characterization and density functional theory (DFT) calculation reveals that the electronic structure of FCN-12 is efficiently adjusted by forming an upward band bending, as the functionalized carbonyl (C=O) and carboxyl (-COOH) groups can withdraw the electron from the C-N=C skeleton. With the addition of PMS, FCN-12 displays superb photocatalytic activity for chlortetracycline hydrochloride (CTC) degradation, where 83.4% CTC can be degraded within 120 min under visible light irradiation, much higher than that of bulk g-C₃N₄ (34.3%). Besides, even under the different pH condition and co-existed anions environment, FCN-12/PMS/vis system still exhibits favorable applicability. The boosted catalytic performance is resulted from the collective effect of the few-layer feature and the energy band bending, which leads to the effective migration of photogenerated electron from FCN-12 to PMS via C=O and -COOH groups. This work not only provides an exhaustive insight into the role of energy band bending in electron-withdrawing group functionalized g-C₃N₄, but also paves the avenue for the development of metal-free photocatalyst-mediated environmental remediation based on PMS activation.

Introduction

In recent years, environmental pollution caused by recalcitrant organics has become a severe issue in front of the human being. Due to the strong toxicity and non-biodegradability, traditional methods can hardly eliminate these organics from the water environment [1], [2], [3], [4]. For this reason, the exploitation of advanced technology with desirable treatment effect is urgently needed and highly significant. Among the developed techniques, the sulfate radical (SO₄^·−)-based advanced oxidation process has received interdisciplinary attention on account of its high oxidizability and favorable applicability [5], [6], [7], [8], [9]. Commonly, SO₄^·− is yielded from the activation of persulfate (S₂O₈²⁻, PS) and peroxymonosulfate (HSO₅⁻, PMS). To achieve an efficient activation process, multiple external energies such as irradiation, ultrasound, heat and electricity have been utilized [10]. In view of the energy harvesting and cost saving, visible-light-driven PS/PMS activation reaction is regarded as a promising route. Hence, searching an appropriate photocatalyst is the most important prerequisite. Despite notable success has been realized by transition metal-based photocatalysts (e.g., Co [11] and Fe [10]) in PS/PMS activation, the poor stability and metal ion leaching intrinsically hindered their application in realistic wastewater decontamination. Therefore, pursuing a green photocatalyst (metal-free) is more preferable.

So far, owing to its well-known metal-free property, appropriate band structure and high stability, graphitic carbon nitride (g-C₃N₄) has proven to be a rising star photocatalyst. And in previous studies, it is found that g-C₃N₄ is feasible to activate PS/PMS under visible light irradiation [12], [13]. However, the performance of pristine g-C₃N₄ for PS/PMS photocatalytic activation is largely limited by some inherent drawbacks, including sluggish charge carrier migration and severe electron-hole recombination. Because the basic principle of PS/PMS photocatalytic activation is their interaction with the photogenerated charge carriers, thus, how to achieve an efficient charge carrier separation in g-C₃N₄ is essential. Considering that the charge carrier migration is anisotropic in the two-dimensional g-C₃N₄, the electron density is unevenly distributed in the g-C₃N₄ plane. Accordingly, diverse approaches including defect creation [14], morphology regulation [15], element doping [16], heterojunction construction [17] and functional group modification [13] have been developed to engineer g-C₃N₄ to accelerate the separation of photogenerated charge carriers. Among them, introducing electron-withdrawing groups may be an intriguing choice as the negative charges will be efficiently accumulated by these functional groups, and then an anisotropic built-in electronic field is established, which leads to the generation of energy band bending in the space charge region of g-C₃N₄ plane [18]. Subsequently, the formed energy band bending can be used as a driving force to facilitate the migration of photogenerated electron and holes, thereby significantly inhibiting their recombination and inducing a promoted photocatalytic activity [19], [20], [21]. Nevertheless, few reports have focused on the PS/PMS photocatalytic activation with electron-withdrawing group functionalized g-C₃N₄, and a holistic understanding of the energy band bending in boosted activation process remains unclear.

Besides, since the g-C₃N₄ is generally prepared from the thermal-treatment of carbon/nitride-containing chemicals, the derived product is in the form of large aggregates, which limits their contact with PS/PMS and thus suppresses the activation efficiency [13], [22]. To enlarge the exposure of active sites, exfoliating the bulk g-C₃N₄ into ultrathin or even few-layer nanosheet has proven to be a feasible strategy as the dimension along the z-axis is remarkably reduced [23]. Unfortunately, due to the weak bonding between polymeric melem units, the planar atomic structure may be severely damaged during this top-down process, which is adverse for the in-plane migration of photogenerated charge carriers [24]. In contrast, the bottom-up strategy can efficiently avoid this weakness and has been shown to hold great potential for the preparation of well-defined 2D materials [24], but is rarely adopted for the synthesis of few-layer g-C₃N₄ to date.

Herein, in this work, a facile bottom-up strategy was employed for the preparation of few-layer g-C₃N₄ at first, and then the electron-withdrawing groups (mainly carbonyl and carboxyl groups) were functionalized by acid oxidation treatment. By virtue of the merits of few-layer structure and the electron-withdrawing groups, the obtained FCN-12 exhibits outstanding catalytic performance for PMS activation, as probed by the degradation of chlortetracycline hydrochloride (CTC) under visible light irradiation. DFT calculation reveals that an energy band bending is formed in the space charge region of g-C₃N₄, which induces a rapid charge carrier separation during the photocatalytic activation of PMS. Therefore, even under the different operation condition (e.g., pH and co-existed anions), the FCN-12/PMS/vis system still shows appreciable CTC degradation rate. Then, by combining these experimental results and the above theoretical analysis, the corresponding PMS activation mechanism was discussed for this FCN-12 photocatalytic system.