Metal-free graphitic carbon nitride/black phosphorus quantum dots heterojunction photocatalyst for the removal of ARGs contamination

doi:10.21203/rs.3.rs-2891868/v1

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Metal-free graphitic carbon nitride/black phosphorus quantum dots heterojunction photocatalyst for the removal of ARGs contamination

https://doi.org/10.21203/rs.3.rs-2891868/v1

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published 29 Jul, 2023

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Antibiotic resistant bacteria (ARB) and genes (ARGs) have become hot topics in the field of water purification. In this paper, graphite carbon nitride (g-C₃N₄) and black phosphorus quantum dots (BPQDs) were used as raw materials to fabricate a non-metallic heterojunction composite photocatalyst (H-g-C₃N₄/BPQDs) by hydrothermal impregnation, high-temperature calcination, and ice-assisted ultrasound. The H-g-C₃N₄/BPQDs was used to remove antibiotics and biological pollution from water under visible light irradiation. Based on the porous structure and high specific surface area of H-g-C₃N₄, the the obtained type II heterojunction structure promoted the absorption of visible light, accelerated the interfacial charge transfer, and inhibited the recombination of photogenerated electron-hole pairs. Under visible light irradiation, the degrading efficiency of TC by H-g-C₃N₄ /BPQDs exceeded 91% in 30 min, and E. coli K12 M1655 can be completely inactivated in 4 h. In addition, the maximum inactivation rate of H-g-C₃N₄ /BPQDs for E. coli HB101(RP4) was 99.99% in 4 h, and the degradation efficiency of RP4 was more than 85%. This study provides not only a new idea for the design of green g-C₃N₄-based non-metallic heterojunction photocatalysts but also a broad prospect for the application of g-C₃N₄-based photocatalysts for the removal of ARGs in water treatment.

g-C3N4

BPQDs

Photocatalytic

Heterojunction

ARGs

Since the 1940s, antibiotics have been widely used to treat various diseases and even once became one of the main means to control bacterial growth [1–3]. However, only part of antibiotics are involved in metabolism, and more than 90.0% of antibiotics are released into the water environment as precursor drugs [4]. These residual antibiotics are difficult to biodegrade and completely removed, and in the long term will form persistent toxicity in the water and soil environment [5]. In addition, the selective pressure of antibiotic abuse on microbial communities will eventually lead to the rapid reproduction and spread of antibiotic resistance genes (ARGs) and antibiotic resistant bacteria (ARB) [6]. It is also unfortunate that most current antibiotics do not have a strong DNA-damaging capacity to prevent the spread of ARGs and ARB [7]. Therefore, it is imperative to develop a "one-stop" treatment technology to simultaneously remove antibiotics and biological pollutants in the water while minimizing their damage to the ecological environment and human health.

The current common water purification methods, including UV irradiation, chlorination, and ozone oxidation, still have limitations, such as low oxidation capacity, high energy consumption, and secondary pollution, etc. [8–13]. In recent years, semiconductor photocatalysis technology, one of the most active research fields, has mild working conditions, strong oxidation capacity, and environmental friendly [14–17], which makes it a competitive candidate solution to replace traditional disinfection methods. However, most semiconductor catalysts suffer from high prices, complexity, and low efficiency [18].

Graphitic carbon nitride (g-C₃N₄) is a novel metal-free semiconductor photocatalyst with the attractive advantages of low cost, excellent stability, wide raw material source, environmentally friendly, etc. [19–21]. The relatively narrow band gap (~ 2.7 eV) of g-C₃N₄ can significantly improve its utilization of visible light [22, 23]. Therefore, the photocatalytic treatment by g-C₃N₄ has great advantages for water purification. However, the photocatalytic performance of pristine g-C₃N₄ is severely limited by its low charge separation efficiency and rapid recombination of photogenerated electron-hole pairs [24–26]. At present, great efforts have been made to develop g-C₃N₄ with good visible light responsiveness and redox ability, including element doping [27–29], defect engineering [30, 31], construction of heterojunctions [32], and morphological regulation [33, 34], etc. Among these modification methods, it has unique advantages to enhance the photocatalytic performance of g-C₃N₄ by constructing heterojunction structures with other semiconductors. The adjustment of the band gaps of heterojunction structures can improve the absorption and utilization rate of visible light, promote the separation and transfer of photogenerated charge carriers, and improve redox capacity, etc. [35, 36].

Black phosphorus (BP), an emerging two-dimensional (2D) semiconductor, has been increasingly used in photocatalysis due to its excellent electrical and optical properties, good charge mobility, and wide light absorption range [37, 38]. Li et al. fabricated a novel layered BP/BiOBr nanoheterojunction by coupling stripped BP nanosheets with BiOBr nanosheets [39]. Compared with pure BiOBr, BP/BiOBr exhibited better tetracycline (TC) degradation performance and faster oxygen and H₂O₂ production rate. Hu et al. designed a 2D/2D Co₂P@BP/g-C₃N₄ heterojunction photocatalyst [40]. The CO₂ reduction rate of Co₂P@BP/g-C₃N₄ is 5.4 times higher than that of the original g-C₃N₄. In most previous studies, lamellar BP is the preferred material for composite, but the loading of BP in composite materials is high, and the cost of BP is high, so researchers have shifted their attention to black phosphorus quantum dots (BPQDs). Compared with the original BP, BPQDs also have other favorable characteristics such as quantum confinement effect, higher specific surface area, prominent edges, higher absorption coefficient, and easy recombination with other materials [41, 42], making it possible to reduce the BP loading without affecting the photocatalytic activity.

Herein, hydrothermally modified g-C₃N₄ (H-g-C₃N₄) was prepared by using urea as a precursor via hydrothermal impregnation and high temperature calcination. Then BPQDs were loaded onto H-g-C₃N₄ by a simple, green, and effective ice-assisted ultrasonic method. The obtained H-g-C₃N₄/BPQDs heterojunction composite photocatalytic material was used for the degradation of TC, inactivation of E. coli, and degradation of ARGs. Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analyzer (XRD), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), photoluminescence (PL), and electrochemical workstation were used to analyze and discuss the physical, chemical, and optical properties of the prepared photocatalyst. In addition, combined with the optical characterization results, the photocatalytic mechanism of H-g-C₃N₄/BPQDs was investigated using free radical trapping experiments and electron paramagnetic resonance (EPR) spectroscopy.

2.1 Materials

Urea was bought from Shanghai Macklin Biochemical Co., Ltd., China. BPQDs dispersion solution was obtained from Nanjing XFNANO Materials Tech Co., Ltd., China. TC solution was purchased from Sangon Biotech (Shanghai) Co., Ltd., China. L-histidine and 1,4-benzoquinone (BQ) were provided by Shanghai Rhawn Chemical Technology Co., Ltd., China. Sodium oxalate was bought from Yuanye Biotechnology Co., Ltd., China. Isopropyl alcohol (IPA) was obtained from Tianjin ZhiYuan Reagent Co., Ltd., China. TIANprep Mini Plasmid Kit was obtained from TianGen Biotech (Beijing) Co., Ltd., China. Ethanol absolute (AR) was purchased from Tianjin jingdongtianzheng Precision Chemical Reagent Factory. Sodium sulfate anhydrous (Na₂SO₄) was provided by Shanghai Aladdin Biochemical Technology Co., Ltd., China. Nafion solution was purchased from Tianjin Incole Union Technology Co., Ltd., China. Ultrapure deionized water (18 MΩ) was produced by a Milli-Q water purification system (Millipore Corporation, Billerica, MA, USA).

2.2 Preparation of photocatalysts

2.2.1 Preparation of g-C₃N₄

g-C₃N₄ was prepared by the hydrothermal-calcination method. Firstly, 10 g urea was added to 20 mL deionized water, stirred and ultrasonic until completely dissolved. Then, the urea solution was transferred to the polytetrafluoroethylene (PTFE) high-pressure reactor and sealed, and hydrothermal treatment in an oven at 150 ℃ for 2 h. After the hydrothermal impregnation, the high-pressure reaction kettle was cooled to room temperature, and the solution was dried in an oven at 60 ℃ until the urea recrystallization. The precursor was moved to the tubular furnace and calcined at 500 ℃ for 2 h (under a nitrogen environment, the heating rate was 5 ℃ min^− 1). Finally, the light-yellow bulk g-C₃N₄ obtained after calcination was ground and stored for later use. As a contrast, g-C₃N₄ was synthesized via direct calcination of recrystallized urea under the same conditions except for the absence of thermal impregnation reaction. The g-C₃N₄ obtained by the above two methods was denoted as H-g-C₃N₄ and D-g-C₃N₄, respectively.

2.2.2 Preparation of H-g-C₃N₄/BPQDs heterojunctions photocatalyst

H-g-C₃N₄/BPQDs heterojunction photocatalyst was prepared by electrostatic self-assembly process (Fig. 1). Simply, 20 mg H-g-C₃N₄ was well dispersed in 20 mL deionized water, and then different volumes (2, 6, 10 mL) of BPQDs dispersions were added. The mixed solution was sonicated in an ice water bath for 4 h and subsequently stirred overnight. After completing the stirring, the precipitation was collected using low temperature high-speed centrifugation, followed by drying under a vacuum at 60℃. The composites with mass ratios of BPQDs and g-C₃N₄ were respectively 1, 3, and 5wt%, and the corresponding H-g-C₃N₄/BPQDs samples were denoted as 1wt% H-g-C₃N₄/BPQDs, 3wt% H-g-C₃N₄/BPQDs, and 5wt% H-g-C₃N₄/BPQDs, respectively.

2.3 Characterization

The microscopic morphology of the samples was characterized by field emission scanning electron microscopy (FE-SEM, Sigma 300, Zeiss, German) and transmission electron microscopy (TEM, Talos F200X G2, USA, operating at 100 kV). Fourier transform infrared spectrometer (Nicolet™ iS™ 5, Thermo Scientific, USA) was used to analyze the characteristic peaks of functional groups. X-ray diffraction spectra (XRD) were recorded in the 2θ range from 5° to 80° (scanning speed 5°/min) with Cu Kα (λ = 1.5405 Å) irradiated by an X-ray diffraction analyzer (D8 Advance, Bruker, Germany, operating voltage at 40 kV and current at 150 mA). X-ray photoelectron spectroscopy (XPS) and valence band X-ray photoelectron spectroscopy (VB-XPS) were detected by Al Kα radiation Thermo Fisher Scientific Escalab 250Xi Photoelectron spectrometer (hν = 1486.6 eV) and hemispherical electron spectrometer. UV–vis diffuse reflectance spectroscopy (UV–vis DRS) and photoluminescence (PL) spectra were conducted by a UV–vis-NIR spectrophotometer (UV-3600 Plus, Shimadzu, Japan) and a fluorescence spectrophotometer (360 nm emission spectrum), respectively. The electron paramagnetic resonance (EPR) signals were detected by a Bruker EMXplus spectrometer.

2.4 Evaluation of photocatalytic performance

2.4.1 Photocatalytic degradation of TC

XE 500W (NBeT, China) xenon lamp was used as the light source (500 W), and the degradation efficiency of TC under visible light was taken as the index to evaluate the photocatalytic activity of the prepared samples. The specific experimental process is as follows:

20 mg catalyst was dispersed into 100 mL TC solution (10 mg L^− 1), and then the mixed suspension was stirred in a dark environment for 30 min so that the TC reached the absorption-desorption equilibrium on the catalyst surface. The xenon lamp was subsequently turned on for the photocatalytic reaction. During the reaction, 5.0 mL of the mixture was taken out every 10 min and passed through a 0.22 µm filter membrane to remove the catalyst powder. Finally, the absorbance of the TC solution was measured by UV-vis spectrophotometer (UV-2600i, Shimadzu, Japan), and the photocatalytic degradation efficiency and reaction rate of TC were calculated by Lambert-Beer equation and pseudo-first-order kinetic equation.

Before the reaction started, 1 mM isopropanol (IPA), 1 mM sodium oxalate, 1 mM L-histidine, and 1 mM benzoquinone (BQ) were added to the photocatalytic system as trapping agents for ·OH, holes (h⁺), ¹O₂ and ·O₂⁻ radicals, respectively. After the same time of light, the efficiency of photocatalytic degradation of TC was detected to judge the influence of different free radicals on the photocatalytic effect.

2.4.2 Photocatalytic inactivation of E. coli K12 M1655

The detailed methods of purification, culture, and preservation of strains are presented in Text S1. All glassware before microbiological laboratory, through high-pressure sterilization. E. coli K12 M1655 was cultured in Luria-Bertani (LB) liquid medium at 37 ℃ and 150 rpm shaking table for 12 h, and then centrifuged at 6000 rpm for 5 min to obtain the bacteria. E. coli K12 M1655 was then centrifuged and washed three times with phosphate buffer (PBS), and E. coli K12 M1655 was again resuspended in PBS solution to obtain an appropriate concentration of E. coli K12 M1655 (about 10⁷ CFU mL^− 1). In the experiment of photocatalytic inactivation of E. coli K12M1655, the temperature of the reaction solution was kept below 25°C and stirred with a magnetic stirrer throughout the experiment. 10 mg of the prepared photocatalyst was added to a 50 mL solution containing 10⁷ CFU mL^− 1 bacterial suspension. Then the adsorption was carried out in a dark environment for 30 min to establish the adsorption-desorption equilibrium. Then xenon lamp (500 W with λ > 420 nm UV filter) was turned on, 0.5 mL suspension was taken every hour, diluted with PBS, and 1.0 mL bacterial solution was absorbed and spread on LB solid medium. After incubation at 37°C for 24 h, colonies were counted to determine the viability of the bacteria.

2.4.3 Photocatalytic inactivation of ARB

This paper focuses on the photocatalytic inactivation of E. coli HB101 (RP4). The screening and culture methods of E. coli HB101 (RP4) were shown in Experiment S1. The photocatalytic inactivation experiment of E. coli HB101 (RP4) was basically the same as that of E. coli K12 M1655. During the photocatalytic inactivation experiment, 0.5 mL bacterial suspension was taken every 1 h, diluted appropriately with PBS, and coated on LB solid medium. Bacterial counts were performed after constant culture at 37 ° C.

2.4.4 Extraction and analysis of ARGs

DNA extraction and quantitative polymerase chain reaction (qPCR) were used to investigate the changes in ARGs. In the process of photocatalytic inactivation of E. coli HB101 (RP4), 2 mL of bacterial solution was taken 1 h apart and centrifuged at 4℃ for 3 min (8000 rpm). RP4 was extracted according to the instructions of the TIANprep Mini Plasmid Kit. Finally, the RP4 solution was detected and analyzed by qPCR. The qPCR system is shown in Table S1. The qPCR amplification procedure was described in Table S2.

2.5 Photoelectrochemical performance measures

The electrochemical properties of the prepared photocatalysts were analyzed by PARSTAT 4000A (AMETEK, USA) electrochemical workstation with standard three electrodes. 0.5M Na₂SO₄ solution was used as the electrolyte. Pt sheet and Ag/AgCl were used as counter electrodes and reference electrodes, respectively. The working electrode was fabricated on a fluorine-doped tin oxide (FTO) conductive glass. The specific methods are as follows: First, the FTO conductive glass was cleaned: the FTO conductive glass was placed in ethanol, acetone, ethanol, and deionized water for 30 min, and then naturally air-dried at room temperature for later use. Secondly, the working electrode was prepared: 4 mg photocatalyst, 50 µL Nafion solution (5 wt%), 1 mL ethanol, and 1 mL deionized water were mixed with ultrasonic for 30 min, and then 50 µL of the mixed solution was added to FTO conductive glass with an effective area of 1 cm² by drops and then dried naturally under nitrogen. Finally, electrochemical impedance spectroscopy (EIS) detection was carried out.

3.1 Morphological and structural characterization

FE-SEM and TEM were used to observe the microstructure of the prepared photocatalytic samples (Fig. 2). FE-SEM images showed that D-g-C₃N₄, H-g-C₃N₄, and 5wt% H-g-C₃N₄/BPQDs all presented porous structures stacked with each other (Fig. 2a-c). In addition, in order to further analyze the element composition ratio, a DES image was used to analyze the total element content (Fig. 2d). Obviously, the proportion of P elements is extremely small due to the small amount of load. To further observe the successful recombination of H-g-C₃N₄ and BPQDs, TEM observations were performed on the photocatalytic samples. D-g-C₃N₄ presents a stacked morphology with a good pore structure (Fig. 2e), and a more obvious pore appearance can be observed in the image of 5wt% H-g-C₃N₄/BPQDs, along with BPQDs (Fig. 2f). To prove the existence of BPQDs, 5wt% H-g-C₃N₄/BPQDs were analyzed by high-resolution transmission electron microscopy (HRTEM) and EDX elemental imaging. In HRTEM (Fig. 2g-h), the interface between H-g-C₃N₄ and BPQDs could be obviously observed, and there was a lattice stripe (0.22 nm) belonging to the (002) crystal plane of BPQDs. In addition, in the EDX element mapping image of 5wt% H-g-C₃N₄/BPQDs (Fig. 2i-l), C, N, and P all appeared in the image, which proved the successful combination of H-g-C₃N₄ and BPQDs. However, due to the low loading of BPQDs, the P element pattern was not obvious, which was mutually confirmed with the EDS image of FE-SEM.

The samples were characterized by FTIR, XRD and XPS in order to study the structure and crystal phase information of the samples. The chemical functional groups of the photocatalyst were characterized by FTIR (Fig. 3a). The results showed that there were tensile vibration peaks of O-H and N-H bonds in the range of 3000–3700 cm^− 1 for all samples [5], and the characteristic peak of typical aromatic C-N heterocyclic tensile vibration appeared in the range of 1200–1700 cm^− 1 [43]. In addition, the absorption peak at 811cm^− 1 was attributable to the respiratory vibration pattern of the tri-s-triazine ring [44, 43]. FTIR results showed that compared with D-g-C₃N₄, except for the slight difference in peak value, the characteristic peaks of H-g-C₃N₄ and H-g-C₃N₄/BPQDs groups did not change significantly, indicating that the chemical structure of g-C₃N₄ did not change during the hydrothermal treatment and recombination process, and there was a polymerized C-N skeleton in the complex compound. In addition, the infrared signal of BP was not detected in the FTIR spectrum, which may be due to the low content of BPQDs and the weak binding bond energy (van der Waals force) between BPQDs and H-g-C₃N₄ [45].

XRD was used to detect the crystalline phase and phase purity of D-g-C₃N₄, H-g-C₃N₄, 1wt% H-g-C₃N₄/BPQDs, 3wt% H-g-C₃N₄/BPQDs and 5wt% H-g-C₃N₄/BPQDs (Fig. 3b). There are two diffraction peaks of 13.0° (100) and 27.2° (002) in D-g-C₃N₄ and H-g-C₃N₄, corresponding to the stacking of tri-s-triazine repeat units and the interlayer stacking of the conjugated aromatic system of graphite-like materials (JCPDS No. 87-1526), respectively [46]. The XRD characteristic peaks of the composite were similar to those of pure g-C₃N₄, and there were no significant differences in peak intensities. However, the characteristic absorption peak of the (020) crystal plane of BPQDs appeared at 17.3° in the composite photocatalyst. 5wt% H-g-C₃N₄/BPQDs also appeared a weak characteristic peak of the (040) crystal plane at 34.0°, while the same absorption peak was not detected in 1wt% H-g-C₃N₄/BPQDs and 3wt% H-g-C₃N₄/BPQDs (JCPDS No. 73-1358). [47] In addition, the strength of the diffraction peak corresponding to BPQDs was weak in all composites, which could be attributed to the limitation of BPQDs loading capacity. Fortunately, characteristic peaks belonging to two single phases appeared in the composite photocatalyst, which confirmed the successful synthesis of H-g-C₃N₄/BPQDs binary composite.

XPS was used to further analyze the chemical states and elemental compositions of D-g-C₃N₄ and 5wt% H-g-C₃N₄/BPQDs. XPS survey spectra (Fig. 3c) showed that C, N, and O elements were present in both samples. Among them, the absorption peak at 529.4 eV was attributed to chemisorbed oxygen [48]. In addition, in the survey spectrum of 5wt% H-g-C₃N₄/BPQDs, a weak presence of P element was observed at 128.8 eV, which is related to the low load of BPQDs. In high-resolution C 1s XPS spectra, three absorption peaks were observed for D-g-C₃N₄ at the binding energies of 284.8 eV, 286.5 eV, and 288.3 eV (Fig. 3d). The binding energy at 284.8 eV was attributed to the graphite-carbon bond (C-C) as the standard carbon for the sample. The fitted peaks at 286.5 eV and 288.3 eV belong to the sp³-coordinated carbon bond (C-N) and sp²-coordinated carbon bond (N-C = N) on the aromatic ring of g-C₃N₄, respectively. [49] In addition, in the high-resolution C 1s map of 5wt% H-g-C₃N₄/BPQDs, the absorption peak belonging to the C-N bond shifted about 0.2 eV toward the higher binding energy. This may be due to the strong interaction between H-g-C₃N₄ and BPQDs after the combination [50]. In the high-resolution spectrum of N 1s, the absorption peaks of D-g-C₃N₄ were located at 398.8 eV, 400.4 eV, 401.5 eV, and 404.4 eV, respectively (Fig. 3e). Corresponding to triazine ring sp² hybrid nitrogen (C-N = C), tertiary nitrogen group (N-(C)₃), surface uncondensed amino group (C-N-H), and charge effects generated by π excitation in heterocyclic ring, respectively [51]. Compared with D-g-C₃N₄, the high-resolution N 1s absorption peaks of 5wt% H-g-C₃N₄/BPQDs all moved 0.1 eV in the direction of low binding energy, which again proved that there was a strong interface effect between H-g-C₃N₄ and BPQDs. Five distinct absorption peaks can be observed in the P 2p high-resolution map of 5wt% H-g-C₃N₄/BPQDs (Fig. 3f). Among them, 129.7 eV and 130.5 eV belong to P 2p_3/2 and P 2p_1/2, respectively, and the absorption peaks located at 133.8 eV and 135.1 eV are P_xO_y, P-OH or P-H₂O formed due to the oxidation of phosphorus. [46] In addition, a new absorption peak appeared at 132.6 eV, consistent with the P-N bond of P₃N₅, which proved the formation of the heterojunction between H-g-C₃N₄ and BPQDs.

3.2 Photocatalytic activities

3.2.1 Photocatalytic removal of TC

The photocatalytic activity of the prepared photocatalyst was evaluated by testing the degradation of TC under visible light irradiation. In the blank experimental group (without the addition of photocatalyst), TC concentration did not change significantly within 30 min (Fig. 4a), indicating that TC had good optical stability. D-g-C₃N₄ showed good photocatalytic activity and could degrade 84.8% TC within 30 min. In addition, the photocatalytic rate of H-g-C₃N₄ was further improved after hydrothermal treatment. This may be because g-C₃N₄ after hydrothermal treatment has a higher specific surface area and larger pore volume, generating more active sites and thus promoting the photocatalytic degradation of TC [5]. In addition, the photocatalytic degradation of TC was also almost negligible in the presence of BPQDs alone, indicating that BPQDs had no obvious photocatalytic properties. However, when BPQDs were combined with H-g-C₃N₄, the degradation rate of TC was significantly higher than that of the two single materials. Moreover, no matter how much load of BPQDs, the TC degradation rate can be increased to more than 91% in 30 min, which may be attributed to the rapid separation and transfer of photogenerated carriers promoted by the loading of BPQDs on H-g-C₃N₄ surface [46]. In addition, the physical mixing of BPQDs and H-g-C₃N₄ inhibited the photocatalytic performance of H-g-C₃N₄, indicating that in the H-g-C₃N₄/BPQDs photocatalytic system, the two are not simple physical mixing, and the heterojunction structure may be successfully constructed.

The apparent rate constant k of the photocatalytic sample was analyzed using a pseudo-first-order kinetic model (Fig. 4b). It is not difficult to find that the k values of the composite photocatalysts are all higher than those of g-C₃N₄. Among them, 5wt% H-g-C₃N₄/BPQDs had the maximum degradation rate constant (0.082 min^− 1), which was higher than that of D-g-C₃N₄ (0.064 min^− 1) and H-g-C₃N₄ (0.065 min^− 1). In addition, we summarized the final concentration and k value of TC degradation by g-C₃N₄-based photocatalysts studied previously (Fig. 4c). By comparison, the H-g-C₃N₄/BPQDs heterojunction photocatalyst prepared in this experiment can obtain an ideal degradation rate even if the addition amount is small.

3.2.2 Photocatalytic inactivation of E. coli K12 M1655

The viability of E. coli K12 M1655 was used as the evaluation index to study the sterilization activity of photocatalytic samples under visible light irradiation (Fig. 4d). When only light existence without catalyst, the number of bacterial cells did not change significantly within 4 h, indicating no toxic effect of visible light irradiation on E. coli K12 M1655. Different from the photocatalytic degradation of TC, BPQDs have a certain degree of cytotoxicity due to their small enough size, although they have no photocatalytic activity. In addition, both D-g-C₃N₄ and H-g-C₃N₄ showed good photocatalytic sterilization ability, and the inactivation rates of bacterial cells were 92.33% and 94.81%, respectively, after 2 h of visible light irradiation. Obviously, the recombination with BPQDs will produce a vfaster and higher sterilization effect. The loading of BPQDs increases from 1wt% to 5wt%, and the inactivation rate of E. coli K12 M1655 can basically reach 100% in 4 h. To visually demonstrate the bactericidal effect, we recorded photographs of viable bacterial colonies on solid LB agar plates exposed to different photocatalysts for 2 h (Fig. S1). Among them, 5wt% H-g-C₃N₄/BPQDs had the fastest sterilization rate, and the deactivation efficiency could reach 99.97% in 2 h, indicating that the interaction between H-g-C₃N₄ and BPQDs played an important role in improving the photocatalytic sterilization activity. This interaction can be attributed to the construction of heterojunction, which effectively inhibits the recombination of photogenerated electron-hole pairs and improves the separation rate of photogenerated charge, thus promoting photocatalytic sterilization activity. On the other hand, the cytotoxicity of BPQDs should not be overlooked in the bactericidal procedure due to their nanoscale. H-g-C₃N₄/BPQDs were firstly adsorbed on the surface of bacterial cells, and then BPQDs might destroy the membrane integrity to make cells more vulnerable to photocatalytic inactivation.

3.2.3 Photocatalytic inactivation of E. coli HB101(RP4)

E. coli HB101(RP4) was used as the target ARB to further analyze the sterilization activity of the photocatalytic samples (Fig. 4e). In the experiments of photocatalytic inactivation of E. coli HB101(RP4), similar results can be observed as those of E. coli K12 M1655 (Fig. S2). After 4 h of visible light irradiation, 1wt% H-g-C₃N₄/BPQDs, 3wt% H-g-C₃N₄/BPQDs, and 5wt% H-g-C₃N₄/BPQDs still showed better sterilization effect. These results indicated that the compound could produce more reactive oxygen species (ROSs) to kill E. coli HB101(RP4).

During ARB inactivation, cytoplasmic contents are released into the environment when the cell membrane breaks down. At this point, the extracellular DNA carrying ARG is likely to be transferred to other bacteria, leading to the rapid propagation and spread of antibiotic resistance [52]. To address this problem, this paper used qPCR to quantify the abundance of RP4 plasmid (Fig. 4f). The results showed that after 4 h of visible light irradiation, RP4 was degraded in different degrees under the action of the photocatalyst. Among them, 3wt% H-g-C₃N₄/BPQDs and 5wt% H-g-C₃N₄/BPQDs had the most significant degradation effect. These two experimental groups could reach a large enough degradation rate (more than 80%) in 2 h, and there was no significant increase in RP4 concentration in the next two hours. This indicates that H-g-C₃N₄/BPQDs have a good photocatalytic ability to degrade ARG and effectively reduce the diffusion of ARG.

3.3 Photocatalytic mechanism

3.3.1 Optical and electrochemical analysis

The photochemical properties of the photocatalytic samples were analyzed by UV-vis DRS, VB-XPS, PL, and electrochemical workstation. In the UV-vis DRS spectra of 1wt% H-g-C₃N₄/BPQDs, H-g-C₃N₄, and BPQDs (Fig. 5a), BPQDs has a wide absorption range at 250–800 nm, and H-g-C₃N₄ shows an obvious absorption edge at about 440 nm. It could also be observed that 1wt% H-g-C₃N₄/BPQDs and H-g-C₃N₄ had similar absorption spectra, but the absorption edge of 1wt% H-g-C₃N₄/BPQDs showed a slight redshift (about 446 nm). This indicated that the introduction of BPQDs could expand the visible light absorption range of H-g-C₃N₄ or formed heterogeneous structure with reduced band gap energy [53]. The band gap energies (E_g) (Fig. 5b) of 1wt% H-g-C₃N₄/BPQDs, H-g-C₃N₄ and BPQDs were calculated as 2.60 eV, 2.70 eV, and 1.79 eV, respectively, using the Tauc method [46]. The reduction of band gap after recombination could generate more photogenerated charge carriers and improve photocatalytic efficiency. The VB potential of semiconductor materials was estimated by VBXPS spectrogram (Fig. 5c), and the VB values of H-g-C₃N₄ and BPQDs were 2.12 eV and 0.71 eV, respectively. Combined with the E_g values in UV-vis DRS, the conduction band (CB) values of H-g-C₃N₄ and BPQDs were − 0.58 eV and − 1.08 eV, respectively.

In addition, PL spectroscopy was used to analyze the separation and migration behavior of photogenerated carriers at an excitation wavelength of 360 nm (Fig. 5d). H-g-C₃N₄ showed a strong emission peak around 460 nm, indicating that the photogenerated electron-hole pairs were highly composite and the mobility of photogenerated charge carriers was low [54]. Fortunately, the absorption peak of 1wt% H-g-C₃N₄/BPQDs was lower than that of H-g-C₃N₄, indicating that the addition of BPQDs and H-g-C₃N₄ formed a heterojunction structure, which could inhibit the recombination of photogenerated electron-hole pairs and improve the photocatalytic performance [55]. EIS is an effective technique to research the interfacial charge transfer dynamics of semiconductors. In the EIS measurements of H-g-C₃N₄ and 1wt% H-g-C₃N₄/BPQDs (Fig. 5e), the arc radius of 1wt% H-g-C₃N₄/BPQDs was smaller, indicating that the composite had a smaller resistance of charge transfer and a higher charge separation and transfer rate [53].

3.3.2 Analysis of photocatalytic reactive free radical

Taking TC as the target pollutant, the possible reactive oxygen species (ROSs) in the photocatalysis process were estimated by free radical capture experiment and EPR spectroscopy, and the photocatalysis mechanism was further studied. In the free radical capture experiment of TC photocatalytic degradation by 1wt% H-g-C₃N₄/BPQDs (Fig. 5f), the addition of IPA, sodium oxalate, L-histidine, and BQ all affected the TC removal rate in varying degrees. Among them, the TC removal rate decreased most obviously when BQ was the quencher, indicating that ·O₂⁻ was the main ROS participating in the reaction. Secondly, the degradation of TC was inhibited to some extent in the presence of L-histidine and sodium oxalate. However, in the presence of IPA, ·OH was quenched largely, but the removal rate of TC was reduced but not obvious, indicating that ·OH played a weak role in the photocatalytic process. In conclusion, ·O₂⁻, ¹O₂, and h⁺ constitute the major active species for the photocatalytic degradation of TC.

The role of ·O₂⁻, ¹O₂ and ·OH in the photocatalytic process was further investigated by EPR technique using DMPO as a radical trapping agent under xenon lamp irradiation (Fig. 5g-i). In the EPR maps of the three kinds of ROSs, no EPR signal was generated in the dark conditions, indicating that the generation of ROSs was related to photogenerated charge carriers in the photocatalysis process [43]. In the EPR maps of the light environment, the signal intensity of 1wt% H-g-C₃N₄/BPQDs was stronger than that of H-g-C₃N₄, indicating that the ability of the composite photocatalyst to generate ROSs was enhanced under visible light irradiation [2]. In particular, ·O₂⁻ is generated by electrons (e⁻) transfer, that is, 1wt% H-g-C₃N₄/BPQDs produced more photogenerated electrons [49]. It shows that the introduction of BPQDs promotes the separation and transfer of photogenerated charge, and further confirms that the heterojunction structure is formed between H-g-C₃N₄ and BPQDs.

Based on the above analysis, we proposed the possible photocatalytic reaction mechanism of H-g-C₃N₄/BPQDs composite photocatalyst (Fig. 6). Firstly, under visible light induction, e⁻ in VB of H-g-C₃N₄ and BPQDs were excited to CB with the same amount of h⁺ to form photogenerated electron-hole pairs. Then, thanks to the interlaced sideband position of H-g-C₃N₄ and BPQDs, e⁻ transferred from CB of BPQDs to CB of H-g-C₃N₄ and reacted with O₂ to generate ·O₂⁻ radical. The h⁺ in VB of H-g-C₃N₄ was transferred to VB of BPQDs, and h⁺ could also oxidize TC to produce oxidation products such as CO₂ and H₂O. In addition, ·O₂⁻ could be oxidized to ¹O₂ by h⁺ in VB of BPQDs. Interestingly, the results of the radical trapping experiments showed that L-histidine inhibited the TC removal rate more than sodium oxalate, which could be explained by the improved separation efficiency of charge carriers for light-induced h⁺ after quenching. It should be noted that because the VB potential of BPQDs is not positive enough, H-g-C₃N₄/BPQDs cannot directly oxidize H₂O or OH⁻ to ·OH (OH⁻/·OH = 2.40 eV vs NHE, H₂O/·OH = 2.72 eV vs NHE). However, the EPR results showed the generation of ·OH, which may be because O₂⁻ could capture photogenerated e⁻ reduction to H₂O₂, and then H₂O₂ was further converted to ·OH [43]. The reduction of the band gap caused by the formation of type II heterojunction inevitably led to the deterioration of the redox ability of the catalyst, but the effective separation of photogenerated electron-hole pairs was a more important factor in determining the amount of ROSs formation. The formation of type II heterojunction effectively inhibited the recombination of photogenerated electron-hole pairs and thus improves the photocatalytic activity.

In summary, a simple, green ice-assisted ultrasonic method was reported to fabricate H-g-C₃N₄/BPQDs type II heterojunction photocatalyst with excellent photocatalytic properties. The physicochemical properties of the samples were examined by TEM, XRD, XPS, and other means, which showed that BPQDs were successfully loaded on the surface of layered g-C₃N₄. The BPQDs promoted the separation of photogenerated charge carriers. In addition, based on the porous structure and high specific surface area of H-g-C₃N₄, the formation of type II heterojunction promotes the absorption of visible light, accelerates the interfacial charge transfer, and inhibits the recombination of photogenerated electron-hole pairs, which led an excellent photocatalytic performance of H-g-C₃N₄/BPQDs in the degradation of TC and inactivation of E. coli K12 M1655 and E. coli HB101(RP4) under simulated visible light irradiation. This study provides a new idea for the design of green g-C₃N₄-based non-metallic heterojunction photocatalysts and expands the application of g-C₃N₄ in the removal of ARGs contamination.

Funding This work was supported by the National Natural Science Foundation of China (32171717), Natural Science Foundation of Tianjin (22JCYBJC01560), the Young Elite Scientists Sponsorship Program (YESS20200389), the Special Fund (JK2022B041400-017, JK20191A020181-024).

Conflict of interest The authors declare no competing interests.

Authorship contribution statement

Chenyu Li, Lin Dai, and Jingfeng Wang supervised the project. Xinyuan Zhang, Chenyu Li, and Lin Dai designed the experiments. Xinyuan Zhang performed the experiments. All authors discussed experiments and results. Xinyuan Zhang, Chenyu Li, and Lin Dai wrote the manuscript. All authors have given approval for the final version of the manuscript.

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You are reading this latest preprint version

Metal-free graphitic carbon nitride/black phosphorus quantum dots heterojunction photocatalyst for the removal of ARGs contamination

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental section

2.1 Materials

2.2 Preparation of photocatalysts

2.2.1 Preparation of g-C₃N₄

2.2.2 Preparation of H-g-C₃N₄/BPQDs heterojunctions photocatalyst

2.3 Characterization

2.4 Evaluation of photocatalytic performance

2.4.1 Photocatalytic degradation of TC

2.4.2 Photocatalytic inactivation of E. coli K12 M1655

2.4.3 Photocatalytic inactivation of ARB

2.4.4 Extraction and analysis of ARGs

2.5 Photoelectrochemical performance measures

3. Result and discussion

3.1 Morphological and structural characterization

3.2 Photocatalytic activities

3.2.1 Photocatalytic removal of TC

3.2.2 Photocatalytic inactivation of E. coli K12 M1655

3.2.3 Photocatalytic inactivation of E. coli HB101(RP4)

3.3 Photocatalytic mechanism

3.3.1 Optical and electrochemical analysis

3.3.2 Analysis of photocatalytic reactive free radical

4. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

Metal-free graphitic carbon nitride/black phosphorus quantum dots heterojunction photocatalyst for the removal of ARGs contamination

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental section

2.1 Materials

2.2 Preparation of photocatalysts

2.2.1 Preparation of g-C3N4

2.2.2 Preparation of H-g-C3N4/BPQDs heterojunctions photocatalyst

2.3 Characterization

2.4 Evaluation of photocatalytic performance

2.4.1 Photocatalytic degradation of TC

2.4.2 Photocatalytic inactivation of E. coli K12 M1655

2.4.3 Photocatalytic inactivation of ARB

2.4.4 Extraction and analysis of ARGs

2.5 Photoelectrochemical performance measures

3. Result and discussion

3.1 Morphological and structural characterization

3.2 Photocatalytic activities

3.2.1 Photocatalytic removal of TC

3.2.2 Photocatalytic inactivation of E. coli K12 M1655

3.2.3 Photocatalytic inactivation of E. coli HB101(RP4)

3.3 Photocatalytic mechanism

3.3.1 Optical and electrochemical analysis

3.3.2 Analysis of photocatalytic reactive free radical

4. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

2.2.1 Preparation of g-C₃N₄

2.2.2 Preparation of H-g-C₃N₄/BPQDs heterojunctions photocatalyst