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
Graphical abstract
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.
Section snippets
Chemicals and reagents
All chemicals were analytical grade and used without further processing. Melamine (≥99%), glycerol (≥99%), ethanol (EtOH, ≥ 99.7%), nitric acid (HNO3, 65 ~ 68%), sulfuric acid (H2SO4, 95 ~ 98%), polyvinyl alcohol (≥99.5%), methanol (≥99.5%), hydrazine hydrate (≥80%), sodium hydroxide (NaOH, ≥ 96%), potassium hydroxide (KOH, 85%), sodium dihydrogen phosphate (NaH2PO4, 99%), sodium chloride (NaCl, 99.5%), sodium nitrate (NaNO3, 99%), sodium sulfate (Na2SO4, 99%), tert-butyl alcohol (TBA, 99%),
Material characterization
The crystal phase of the obtained photocatalyst was first investigated by XRD. As outlined in Fig. 1a, two distinct diffraction peaks situated at 13.0° and 27.5° are perceived, where the former (1 0 0) is mainly stemmed from the in-planar arrangement of heptazine framework, while the later (0 0 2) can be credited to the interlayer stacking of conjugated aromatic system [27]. Compared with BCN, the (0 0 2) peak of FCN becomes much weaker and broader, which is an indicator of few-layer nature [24].
Conclusion
In summary, carbonyl and carboxyl groups co-functionalized few-layer graphitic carbon nitride was prepared and employed for the photocatalytic activation of PMS. The functionalized groups can efficiently modulate the electronic structure of g-C3N4 by forming an upward band bending in the space charge region, as a result of their strong electron-withdrawing capacity. Under the synergistic effect of this energy band bending and few-layer structure, the photocatalytic charge carriers are
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.
Acknowledgements
This work was financially supported by the Key R & D project in Hunan (2018SK2048), the National Natural Science Foundation of China (51541801, 51521006) and Postgraduate Scientific Research Innovation Project of Hunan Province (CX20190294).
References (69)
- et al.
Photo-removal of 2,2′4,4′-tetrabromodiphenyl ether in liquid medium by reduced graphene oxide bridged artificial Z-scheme system of Ag@Ag3PO4/g-C3N4
Chem. Eng. J.
(2019) - et al.
Visible-light-driven photocatalytic degradation of diclofenac by carbon quantum dots modified porous g-C3N4: Mechanisms, degradation pathway and DFT calculation
Water Res.
(2019) - et al.
Insight into the energy band alignment of magnetically separable Ag2O/ZnFe2O4 p-n heterostructure with rapid charge transfer assisted visible light photocatalysis
J. Catal.
(2019) - et al.
Boosting molecular oxygen activation ability in self-assembled plasmonic p-n semiconductor photocatalytic heterojunction of WO3/Ag@Ag2O
Chem. Eng. J.
(2019) - et al.
Graphene- and CNTs-based carbocatalysts in persulfates activation: Material design and catalytic mechanisms
Chem. Eng. J.
(2018) - et al.
Comparison of the reactivity of ibuprofen with sulfate and hydroxyl radicals: An experimental and theoretical study
Sci. Total Environ.
(2017) - et al.
Photocatalytic activation of peroxymonosulfate by TiO2 anchored on cupper ferrite (TiO2@CuFe2O4) into 2,4-D degradation: Process feasibility, mechanism and pathway
J. Hazard. Mater.
(2018) - et al.
Peroxymonosulfate-enhanced visible light photocatalytic degradation of bisphenol A by perylene imide-modified g-C3N4
Appl. Catal. B: Environ.
(2018) - et al.
Ti3C2 Mxene/porous g-C3N4 interfacial Schottky junction for boosting spatial charge separation in photocatalytic H2O2 production
Appl. Catal. B: Environ.
(2019) - et al.
Edge-Functionalized g-C3N4 Nanosheets as a Highly Efficient Metal-free Photocatalyst for Safe Drinking Water
Chem
(2019)