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Original article
03 2022
:15;
103689
doi:
10.1016/j.arabjc.2022.103689

Highly efficient photocatalytic degradation for antibiotics and mechanism insight for Bi2S3/g-C3N4 with fast interfacial charges transfer and excellent stability

Highly efficient photocatalytic degradation for antibiotics and mechanism insight for Bi2S3/g-C3N4
, , , , , , , , , , , , ,
a School of Science, Xi’an Polytechnic University, No.19 of Jinhua South Road, Beilin District, Xi’an 710048, PR China
b Department of Science and Environmental Studies, The Education University of Hong Kong, Tai Po, New Territories, Hong Kong 999077, P. R. China
c State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, PR China
d Department of Physics, School of Pure Science, College of Engineering, Science and Technology, Fiji National University, Lautoka, Fiji
e College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, Zhejiang, China

⁎Corresponding authors at: School of Science, Xi’an Polytechnic University, No.19 of Jinhua South Road, Beilin District, Xi’an 710048, PR China (D.Z. Lu); Department of Science and Environmental Studies, The Education University of Hong Kong, Tai Po, New Territories, Hong Kong 999077, PR China (W.K. Ho); College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, Zhejiang, China (H.M. wang). ludingze@whu.edu.cn (Dingze Lu), keithho@eduhk.hk (Wingkei Ho), hongmei256@163.com (Hongmei Wang)

Received: , Accepted: ,
© The Author(s) 2022
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Bi2S3/g-C3N4 (BSCN) samples with different mass ratios of CN to BS were prepared by a facile and practicable hydrothermal method with 2D g-C3N4 nanosheets (CN). The microscopic morphology and structure of pure CN, BS and BSCN were measured by multiple testing methods. Analysis results show that the BSCN was prepared successfully, and the Bi2S3 nanoparticles closely and uniformly adhered to the surface of CN with sheet-like structure. The introduction of Bi2S3 did not change the structure of the CN. The results of the ultraviolet–visible spectroscopic analysis, photoluminescence spectra and electrochemical performance indicated that BSCN showed superior visible-light response compared with CN, and the separation and transfer efficiency of photogenerated carriers was significantly improved. With the decrease of mass ratio of CN/BS, the photocatalytic activity of BSCN initially increased and then decreased for 20 ppm of Rhodamine B solution (RhB), and the Bi2S3/g-C3N4-B with a mass ratio of 8:1 for CN to BS showed optimal photocatalytic performance (98.98%). Furthermore, the Bi2S3/g-C3N4-B exhibited apparent degradation effects (1.021 x10-2, 0.879 x10-2 and 0.793 x10-2 min−1) to three kinds of antibiotics (tetracycline, ciprofloxacin, and oxytetracycline). The BSCN samples still maintained higher degradation efficiency after five cycles of degradation to tetracycline. The capture experiments and the electron spin resonance (ESR) spectra analysis indicated that the h+ and ·O2− played a major role, and ·OH played secondary role during the photocatalytic reaction.

Keywords

g-C3N4 Nanosheets
Bi2S3/g-C3N4
Photoelectric Performance
Photocatalytic Degradation of Antibiotics
Excellent Stability
1

1 Introduction

At present, environmental problems have become an inevitable worldwide concern with the rapid development of society and industry. Increasing domestic sewage, growth of the printing and dyeing industry, and discharge of wastewater from production enterprises of antibiotics adversely affect the environment (Sun et al., 2020; Yi et al., 2020). As a green way of wastewater treatment, piezoelectric catalysis, photocatalysis and other technologies have been widely studied (Li et al., 2022; Li et al., 2021; Li et al., 2021; Cheng et al., 2021). Photocatalytic technology is a viable approach that could effectively solve problems due to its low cost, high degradation efficiency for pollutants, green and non-toxicity, and secondary pollution-free characteristics (Zheng et al., 2019; Huang et al., 2019; Zhou et al., 2019; Akhundi et al., 2020; Habibi-Yangjeh et al., 2020; Asadzadeh-Khaneghah and Habibi-Yangjeh, 2020; Liu et al., 2021; Wang et al., 2021). As a promising photocatalytic material, the graphite carbon nitride (2D, g-C3N4, CN) foremostly reported by Wang et al. has attracted wide interest among researchers (Wang et al., 2009; Wen et al., 2017; Santosh et al., 2018). CN is considered an optional and suitable candidate material in photocatalysis, such as reduction of CO2 (Luo et al., 2019; Lv et al., 2018; Feng et al., 2016; Li et al., 2020; Hu et al., 2020; She et al., 2021), photocatalytic water splitting (Ji et al., 2019; Xu et al., 2019), and photocatalytic degradation of pollutants because of suitable bandgap (Eg = 2.7 eV), chemical stability, non-toxicity, low price, and easy synthesis method (Moreira et al., 2019; Singh et al., 2019; Li et al., 2019; Yan et al., 2021). g-C3N4 nanosheets (2D) possess high-specific surface area and small thickness compared with other morphologies (0D, 1D and 3D) of C3N4 photocatalysts. A larger specific surface can expose photocatalysts to more light and contact with more dye molecules for photocatalytic reaction, and a small thickness may be beneficial to enhance the separation rate of photo-generated electrons and holes. Thus, g-C3N4 nanosheets becomes a potential photocatalyst.

However, some inherent drawbacks for the CN influence its photocatalytic efficiency. The utilization of solar energy for CN is insufficient, especially visible light (Mao and Jiang, 2019; Huang et al., 2020). The quantum efficiency of CN is also relatively low because generated e− and h+ inside the CN cannot easily reach to the surface to participate in the reaction (Hu et al., 2015). To effectively solve these disadvantages and improve the photocatalytic activity, the morphological regulation (Liang et al., 2019; Tang et al., 2019), doping with various elements (Guo et al., 2019; Chen et al., 2019), synthesis of CN-based composites using different semiconductors (Li et al., 2019; Li et al., 2019; Wang et al., 2020) and other methods are usually adopted. The study of CN-based heterojunction semiconductor composites has attracted wide attention (Chen et al., 2019; Zhang et al., 2019; Wang et al., 2020). Many CN-based heterojunction composites composed with metal sulfides have been reported, such as CdS (Qiu et al., 2020; Wang et al., 2018); ZnS (Wang et al., 2019), SnS2 (Deng et al., 2017), MoS2 (Hu et al., 2020), and Bi2S3 (Liu et al., 2019). Similarly, some polymetallic sulfides have been extensively studied, including the binary (CuInS, ZnIn2S4,) (Ye et al., 2019; Deng et al., 2022), ternary (CuInZnS) (Gan et al., 2021) and so on. The metal sulfides exhibit better visible light response than metal oxides due to matched band structure (Gan et al., 2021; Truc et al., 2019; Li et al., 2021; Huo et al., 2021; Ke et al., 2021; Huo et al., 2019; Mei et al., 2020; Hu et al., 2020). The formation of unique heterojunctions such as straddling alignment (type-I) (Qiu et al., 2020; Hu et al., 2020), staggered alignment (type-II) (Yi et al., 2020; Li et al., 2019), p-n heterojunction (Zhang et al., 2020; Lua et al., 2018), Z-scheme (Gan et al., 2021; Wang et al., 2019), and S-scheme systems (Deng et al., 2022; Wen et al., 2021) at the interfaces between metal sulfide and CN not only impedes the recombination of carriers, but also makes the CN higher responsive to visible light. Sun et al. (Sun et al., 2020) prepared the MoS2/g-C3N4 samples successfully adopted freeze-drying method, which can improve effectively the separation efficiency of photo-induced charge carriers as a functional heterojunction. The efficiency of photocatalytic hydrogen evolution of the MoS2/g-C3N4 loaded with 15 wt% of MoS2 was as high as 1124 μmol·h−1·g−1.

The bismuth sulfide (Bi2S3) is a nontoxic grain structured material containing weak Bi − S bonds provided with band gap that ranged from 1.2 eV to 2.5 eV (Liu et al., 2019; Dai et al., 2019). Furthermore, the photoelectric conversion efficiency of Bi2S3 is approximately 5%, which has a greatly broad application prospect in electrochemical hydrogen storage, hydrogen sensing, and visible-light sensitizer (Ye et al., 2019; Deng et al., 2022). Recently, the superior Bi2S3/g-C3N4 (BSCN) heterojunction can be built by various methods due to the matching band structure, which could not only adjust the band gap of CN, but also improve the photo-response ability and the separation rate of the photo-excited charge carriers (Arumugam et al., 2018; Li et al., 2017; Hu et al., 2019). Hao et al. (Hao et al., 2020) prepared the Bi2S3 using a low-temperature method and further synthesized Bi2S3/g-C3N4 composite by ultrasound. CN-BiS-2 (1 g of g-C3N4, 20 mg of Bi2S3) exhibited higher visible light response and stronger photocatalytic degradation than pristine g-C3N4 or Bi2S3. However, among the BSCN photocatalysts reported thus far, some problems still exist, namely, (i) the preparation methods are more complex or demanding; (ii) the separation and transfer principle of e−/h+ pairs is unclear; (iii) enhanced catalytic activity of the BSCN attributed to the heterojunction is not evident. In addition, the Bi2S3 nanoparticles with tiny particle size are extremely beneficial for the rapid migration of photoexcited electrons and holes from the inner part to the surface of the samples (Truc et al., 2019; Li et al., 2021; Yin et al., 2018; Zhou et al., 2015; Rong et al., 2015). Furthermore, the edge of the g-C3N4 nanosheets is so thin that is easy to wrinkle, but the Bi2S3 particles easily agglomerate on the edge (Zhang et al., 2021; Chen et al., 2017; Mousavi et al., 2019). Hence, designing a facile synthesis to construct well-dispersed tiny Bi2S3 nanoparticle loaded g-C3N4 nanosheets with high photocatalysis is essential. The BSCN heterojunction and the principle for the separation and migration path of photo-excited e−/h+ pairs are further studied.

Herein, a series of BSCN samples was prepared by a facile and practicable hydrothermal method. The microstructure and chemical structure of the as-prepared samples were characterized by the electron transmission microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectra, BET analysis, and X-ray photoelectron spectroscopy (XPS). The photoelectric properties of the BSCN samples were evaluated by the ultraviolet–visible spectroscopic analysis (DRS), photoluminescence spectra (PL), and electrochemical test. The photocatalytic performance was evaluated by degradation of the Rhodamine B (RhB) and antibiotic (tetracycline, ciprofloxacin, and oxytetracycline) solution with a concentration of 20 mg/L, and the stability of the as-prepared samples was measured by five cycles of degradation for tetracycline. The reactive free radicals for the BSCN samples were investigated by the trapping experiments and electron spin resonance (ESR) spectra. Finally, the logical and possible mechanisms are proposed to enhance the photocatalytic activity of CN.

2

2 Experimental

2.1

2.1 Preparation of the samples

The chemicals used in the experiment are all analytical samples. The pure g-C3N4 nanosheets were prepared by heat treatment of urea at 550 °C in an alumina crucible for 3 h and then cooled down naturally to ambient temperature (Wang et al., 2009; Xu et al., 2019; Moreira et al., 2019). As shown in Fig. 1a, the BSCN samples were obtained by a facile hydrothermal method. A certain amount of Bi(NO3)3·5H2O was blended with 35 mL of acetone solution, and magnetic agitation was conducted for 30 min. Then, the CN of certain quality was added to the solution with a continuous stirring. Subsequently, corresponding amount of Na2S2O3·5H2O was added with continuous stirring for 20 min until dissolved homogeneous mixture was obtained at room temperature. Finally, the as-obtained mixture was stored in an oven at 180 °C for 24 h. The mass ratios of CN to BS are 10:1, 8:1, 5:1, and 3:1, respectively, which are labelled as Bi2S3/g-C3N4-A, Bi2S3/g-C3N4-B, Bi2S3/g-C3N4-C, and Bi2S3/g-C3N4-D.

(a) Schematic of the preparation procedure for the CN and BSCN; (b-e) TEM and HRTEM images of the Bi2S3/g-C3N4-B at different magnifications.
Fig. 1
(a) Schematic of the preparation procedure for the CN and BSCN; (b-e) TEM and HRTEM images of the Bi2S3/g-C3N4-B at different magnifications.

2.2

2.2 Characterization

TEM (JEOL 2100F) was used to observe the morphology and microstructure of the samples. The crystal structure of the samples was tested using Japanese desktop MiniFlex600 X-ray diffractometer. Raman spectra were obtained using a laser Raman spectroscopy analyzer (RENISHAW, UK, INVIA) provided with a 750 nm laser. The functional groups and vibrational modes of the samples were identified by the Fourier infrared spectroscopy (Nicolet 5700 FTIR spectrometer). The physical properties of the sample morphology were explored through the specific surface area analyzer (JW-BK100B, Beijing). The X-ray photoelectron spectroscopy was used to probe the surface chemical composition and oxidation states of the samples (Thermo Fisher Scientific ESCALAB 250xi). The UV–vis diffuse reflectance spectra (Hitachi U-3310, Japan) was measured to assess the optical properties and photo-response range of the as-prepared samples. The photoluminescence spectra (iHORIBA, Fluoro Max) were tested to evaluate the recombination efficiency of the photo-induced e−/h+ pairs. The tests for free radical (·O2− and ·OH) production were carried out using the electron spin radical resonance (BRUKER A300, Germany) under continuous visible light irradiation. The solution for 10 mg of samples mixed with 5 mL of deionized water was ultrasonic treated, mixed with DMPO solution of 100 mM, and finally loaded into a capillary tube for test.

2.3

2.3 Photocatalytic activity measurements

Photocatalytic degradation of Rhodamine B: The 0.05 g of samples was placed into a photocatalytic test tube with 50 mL of 20 mg/L of RhB solution. The tubes were placed in the photocatalytic reaction chamber (CEL-SPH2N, Beijing China Education Au-light Co., Ltd.) for degradation test with the circulating water system to maintain reaction temperature. Initially, the dark reaction was performed for 30 min to achieve adsorption–desorption equilibrium. At this time, the concentration of the solution was considered the initial concentration to start the light reaction, namely, “C0”. Then, the solution was illuminated by visible light (300 W of xenon lamp with 420 nm of UV filter). During the reaction, 1.6 mL of the solutions were obtained every 20 min and centrifuged to separate the samples (12000 r/min, 2 min) from the solution. The entire experiment was conducted for 120 min. The absorption spectra of the as-obtained suspension solution were monitored by the fiber optic spectrometer (NOVA-IDH2000, Shanghai Fuxiang Optics Co. Ltd.). Furthermore, the capture experiments about active radicals were conducted by introducing different trapping agents. The hydroxyl radical (∙OH), hole (h+) and superoxide radicals (∙O2−) could be effectively captured by tert-butyl alcohol (TBA), triethanolamine (TEOA) and 1,4-benzoquinone (BQ) during the degradation of RhB process, respectively.

Photocatalytic degradation of antibiotics: The Bi2S3/g-C3N4-B (0.05 g) was placed into 20 mg/L of antibiotic solution (50 mL of tetracycline, ciprofloxacin and oxytetracycline). The experimental process was similar to the degradation of the RhB solution. The dark reaction lasted 30 min, and then the Xenon lamp with the 420 nm of ultraviolet filter (300 W) was turned on. Subsequently, the suspension in the reaction was sampled every 20 min, and the entire reaction process lasted for 120 min. Finally, the concentration of the supernatant was detected by UV–vis diffuse reflectance spectroscopy. The characteristic absorption peaks of tetracycline, ciprofloxacin, and oxytetracycline were located at 357, 275, and 277 nm, respectively. Finally, the degradation of tetracycline was carried out five times to evaluate the stability of the samples. Besides, a comparative test of Bi2S3, g-C3N4 and catalyst-free was carried out to degrade tetracycline through the same experimental procedure as above.

2.4

2.4 Electrochemical test

Initially, 0.01 g of samples was dispersed ultrasonically into 1 mL of ethanol for 60 min to obtain uniform solution. The obtained suspension was spin-coated onto the conductive surface of the ITO conductive glass, and then dried at 50 °C. Then, the electrochemical test was carried out by the three-electrode method on the electrochemical workstation (CHI-660E, Chenhua) (Guo et al., 2019). The as-prepared samples were used as the working electrodes, and the calomel electrodes act as the reference electrodes. Meanwhile, the well-known platinum wire was selected as the counter electrode. The 0.1 mol/L of Na2SO4 solution was used as electrolyte. Finally, the photocurrent was tested under alternately varied dark and illumination environments with the 300 W of xenon lamp. The time interval of data acquisition was 0.1 s, and each sample was continuously tested for 400 s.

3

3 Results and discussion

3.1

3.1 Analysis of microscopic morphology, structure, surface composition, and chemical valence

As shown in Fig. 1a, the BSCN samples were obtained by a facile hydrothermal method. The TEM images of the different BSCN composites (Fig. 1b ∼ 1e) depict the 2D sheet-like structure of the CN at various magnifications from small to large, and the Bi2S3 nanoparticles with diameters of 20 ∼ 30 nm in the dense dark region are compactly and uniformly adhered to the surface of transparent CN nanosheets. The BSCN composite possesses a lamellar structure. The HRTEM image shown in Fig. 1e describes that the interplanar crystal spacing of crystal lattice is ∼ 0.2 nm, which is derived from the plane set of (0 0 2) crystal of the Bi2S3 nanoparticles and consistent with previous reports (Liu et al., 2015). The construction of BSCN composite structures can be described as follows: initially, the Bi3+ ions are effectively adsorbed to the appearance of the CN, and then the S2- ions from the hydrothermal treatment react with the Bi3+ ions to form Bi2S3 nanoparticles ultimately. The ultrasonication for 30 min may help adhere the attachment of Bi2S3 nanoparticles more evenly to the surface of the CN.

Fig. 2 shows the characterization results of XRD regarding the crystal face of the samples. The XRD patterns of pure CN, BS and BSCN are shown in Fig. 2a. Two well-defined characteristic peaks are observed for pure CN (PDF#87–1526) at 27.6° and 12.7°, corresponding to the (0 0 2) and (1 0 0) planes of the CN, respectively (Yu et al., 2017; Liu et al., 2016). The strongest peak at approximately 27.6° originated from stacking of conjugated aromatic structure of CN. However, the less intense peak at 12.7° attributed to the motifs repeatedly occur at the in-plane of the continuous tris-triazine network. The pure BS nanoparticles prepared corresponds very well with the standard PDF card (PDF#17–0320) and has a good crystallinity. The faint peak at approximately 39.8° is observed in the Bi2S3/g-C3N4-B and Bi2S3/g-C3N4-C samples, corresponding to the characteristic peaks of the (1 4 1) planes of the orthogonal structure of Bi2S3 (Mousavi et al., 2019); which confirm the successful synthesis of the BSCN complex constructed. Nevertheless, other characteristic diffraction peaks of Bi2S3 nanoparticles are not clearly observable, probably resulting from the low concentrations of loaded Bi2S3 nanoparticles.

XRD spectra (a), FTIR spectra (b), and Raman spectra (c–d) of pure g-C3N4, Bi2S3 and different Bi2S3/g-C3N4.
Fig. 2
XRD spectra (a), FTIR spectra (b), and Raman spectra (c–d) of pure g-C3N4, Bi2S3 and different Bi2S3/g-C3N4.

The vibration of chemical bonds for pure CN and BSCN was experimentally investigated by FTIR patterns (Fig. 2b). The basic molecular structure and functional groups of the samples are consistent with those of typical CN (Fig. S1). Fig. 2b shows that the absorption peaks at 1234 ∼ 1641 cm−1 of pure CN arise from the stretching vibration of C = N and C − N formed by sp2 hybridization (Lin et al., 2016). The other absorption peak of the CN appears at 808 cm−1, which is caused by the respiration vibration of the triazine ring (Bojdys et al., 2008). No functional groups of Bi2S3 are found in the pattern of the BSCN due to Bi2S3 itself has no characteristic peak in FTIR pattern (Hao et al., 2020). The absorption bands found at around 3073 ∼ 3278 cm−1 are assigned to the O − H groups of H2O and the − NH group in spectrum of all samples. Besides, the position of the absorption peak for the BSCN has no evident displacement compared with the pure CN, indicating that the contact between Bi2S3 and CN does not affect the internal structure of CN. Furthermore, the Raman spectroscopy was used to further testify the existence of CN in BSCN (Fig. 2c–2d). The Raman characteristic modes corresponding to the aromatic triazine ring (N–C = N) containing sp2 hybridized carbon and nitrogen of the CN are at 707.8 and 1233.3 cm−1, which is consistent with previous reports (Hu et al., 2015; Liu et al., 2016). The sharp peak at 1233.3 cm−1 is manifested to the bending mode of N = C of the CN.

The specific surface area (m2·g−1), average pore diameter (nm), and total pore volume (cm3·g−1) were characterized by BET measurements. The N2 adsorption–desorption isotherms are displayed in Fig. S2. The isothermal adsorption curves of pure CN and BSCN samples conform to the trend of type-Ⅳ, and the hysteresis loop is in the form of type-H3 (Xia et al., 2017). The specific values of the physical properties of the samples are illustrated in Table 1. The specific surface areas of g-C3N4, Bi2S3/g-C3N4-B, and Bi2S3/g-C3N4-A are 73.60, 78.52, and 80.82 m2/g, respectively. Compared with pure CN, the specific surface area of BSCN increases slightly due to the formation of the loaded Bi2S3 nanoparticles over the edges of lamellar CN, which has a certain inhibitory effect on the crimping phenomenon of CN.

Table 1 Physical properties of pure g-C3N4 and Bi2S3/g-C3N4 composites with different g-C3N4 contents.
Samples BET surface area (m2·g−1) Average pore diameter (nm) Total pore volume (cm3·g−1)
g-C3N4 73.60 28.63 0.57
Bi2S3/g-C3N4-B 78.52 31.84 0.66
Bi2S3/g-C3N4-A 80.82 17.14 0.36

The surface valence states of the Bi2S3, g-C3N4, and Bi2S3/g-C3N4-B samples were tested by the XPS. As illustrated in Fig. 3a, the survey spectra of the as-prepared Bi2S3/g-C3N4-B depict the presence of elements C, N, Bi, S, and O. The existence of O elements may be due to oxygen in the air. The high-resolution XPS spectra of various elements (C 1 s, N 1 s, Bi 4f and S 2p) are presented in Fig. 3(b–d) after Gaussian–Lorentzian deconvolution. Fig. 3b shows the high-resolution XPS spectra of C 1 s. The peak situated at 288.23 (CN) and 288.10 (BSCN) eV assigned to the N = C − N bond formed by sp2 hybridization of C and N elements in the aromatic ring of CN (Hao et al., 2016). Another binding energy peak at approximately 284.8 eV originated from the extrinsic carbon dioxide in the air (C − C) (Shao et al., 2019). Fig. 3c illustrates the high-resolution XPS spectra of N 1 s, which is deconvoluted into three significant binding energy peaks, as follows: (i) the peak at 398.77 (CN) and 398.63 (BSCN) eV is caused by the sp2 hybridized N atoms of the tri-s-triazine ring (C − N = C) (Wang et al., 2017); (ii) the binding energy peak of the tertiary N atoms connected to the triazine ring N−(C)3 is located at 400.66 and 400.50 (BSCN) eV (Fu et al., 2019); (iii) the small peak at 404.38 (CN) and 404.21 (BSCN) eV is related to charging effect of π-excitation (Zhang et al., 2021). Fig. 3d shows the high-resolution spectra of the Bi 4f and S 2p. For the pure Bi2S3, the two binding energy peaks at 158.35 and 163.77 eV were fitted, among which the first one can be ascribed to Bi 4f7/2, and the second one is caused by Bi 4f5/2. The doublet separation of 5.42 eV confirms that the Bi element exists in the form of Bi3+. Meanwhile, the XPS spectrum also confirms the existence of sulfur species from the appearance of an S 2p peak at 161.15 and 162.91 eV, which is consistent with the literature (Ke et al., 2017; Kim et al., 2018; Wang et al., 2017). Compared with pure Bi2S3, the binding energy of Bi 4f and S 2p of Bi2S3/g-C3N4-B shift towards higher direction, which due to the change in the electron cloud density attributed to the van der Waals force between g-C3N4 and Bi2S3 indicating the formation of heterojunction between Bi2S3 and g-C3N4. The formation of the heterojunction composites may favor the electron transfer between g-C3N4 and Bi2S3. In addition, the weight percentages of the elements for the optimal photocatalyst (Bi2S3/g-C3N4-B) were measured using XPS analyses (Thermo Fisher Scientific ESCALAB 250xi). The analysis results are shown in Table 2, and the XPS results are consistent with the theoretical value.

X-ray photoelectron spectroscopy of the Bi2S3, g-C3N4 and Bi2S3/g-C3N4-B: (a) Survey spectrum; high-resolution XPS spectra of C 1 s (b), N 1 s (c), and Bi 4f and S 2p (d).
Fig. 3
X-ray photoelectron spectroscopy of the Bi2S3, g-C3N4 and Bi2S3/g-C3N4-B: (a) Survey spectrum; high-resolution XPS spectra of C 1 s (b), N 1 s (c), and Bi 4f and S 2p (d).
Table 2 Weight percentages of the elements for the Bi2S3/g-C3N4-B.
Elements Theoretical value
(wt.%)		 XPS value
(wt.%)
Bi 9.11 8.93
S 2.02 1.96
C 34.71 34.92
N 54.16 54.19

3.2

3.2 Analysis of the photoresponse ability and photoelectric performance

The photo-response ability of acquired samples was tested and assessed by DRS in Fig. 4a. The Bi2S3 had a high photo-response ability in the wavelengths range of 200 ∼ 900 nm attributed to the black powders, and the light absorption capacity of CN is enhanced evidently due to the introduction of Bi2S3, especially in the visible light range. Furthermore, a redshift occurred at the photo-absorption edge of the BSCN sample compared with the pure CN, indicating a broadening of the range of visible light absorption possibly because the combination of BS and CN forms a heterojunction at the interface to adjust the bandwidth of CN, which is beneficial to the photocatalytic performance.

UV–vis DRS patterns (a), PL patterns (b), surface photocurrent with time (c), and schematic of electrochemical test (d) for pure g-C3N4 and Bi2S3/g-C3N4 samples.
Fig. 4
UV–vis DRS patterns (a), PL patterns (b), surface photocurrent with time (c), and schematic of electrochemical test (d) for pure g-C3N4 and Bi2S3/g-C3N4 samples.

The recombination efficiency of photogenerated carriers inside the catalyst was estimated by photoluminescence spectra with the excitation wavelength of 277 nm. The relative intensity of fluorescence can reflect the recombination rate of photogenerated e−/h+ pairs (Li et al., 2016). In Fig. 4b, the fluorescence peak of the pure CN can be clearly observed at approximately 450 nm, which originates from the energy-level transitions of band gap energy of pristine CN. The photoluminescence intensity decreases significantly with the loading of Bi2S3 nanoparticles, indicating a suppressed recombination rate of photogenerated charge carriers in the samples.

The surface transient photocurrent response of the samples was measured under illumination and dark conditions subsequently and presented in Fig. 4c. The effective separation of photogenerated e−/h+ pairs is directly related to the enhancement of photocatalytic capacity. Only a weak photocurrent of the CN and BS is produced under the illumination. However, the PC response of the BSCN is higher than that of CN, and the intensity of the Bi2S3/g-C3N4-B is approximately 20 times as much as the pure CN after four on–off cycles. Besides, the pure CN and BS has a large interface transfer resistance (Fig. 4d), but the interface resistance of the composites is significantly reduced, indicating that the transfer rate of photo-excited carries are the improved attributed to the interface effect between BS and CN. The conclusion is in agreement with the results of PL. Under the illumination condition, the built-in electric field at the interface of the samples can promote effectively the separation and transfer efficiency of the photo-induced e− and h+ pairs.

3.3

3.3 Evaluation of photocatalysis performance

The photocatalytic activity of the pure CN, BS and BSCN samples was monitored by the degradation test of RhB solution (20 ppm). The UV–vis absorption spectra during photocatalytic degradation of Bi2S3/g-C3N4-B were shown in Fig. 5a. The Bi2S3/g-C3N4-B samples can achieve almost complete degradation for the RhB with a concentration of 20 ppm within 120 min. Fig. 5b shows the catalytic degradation effect curve of BSCN composites with different mass ratio of CN to BS across 120 min of illumination. The photocatalytic activity initially enhances and then decreases with the increase in the content of BS. The best photocatalytic degradation efficiency of the RhB solution is obtained for the Bi2S3/g-C3N4-B (with a mass ratio of 8:1 for CN to BS), and the degradation efficiency is approximately 98.98%. Data fitting indicates that the results satisfy the first-order reaction kinetic equation − ln(C/C0) = kt as shown in Fig. 5c, and the corresponding histogram prepared based on the slope of the kinetic curve is presented in Fig. 5d, which further validate the above rule (Table S1). The analysis results are consistent with that of the PL and electrochemical performance test. Additionally, two blank control experiments showed that only under light conditions, samples can be excited to produce large numbers of electron hole pairs and a series of free radicals, which could adsorb organic contaminants to participate in the reaction. Further, the capture experiments for active radicals were performed and the results as shown in Fig. S3. The photodegradation of RhB was almost not changed with the TBA, suggesting that ∙OH hardly play a role in degradation reactions. However, while the BQ and TEOA were added the RhB solution, Bi2S3/g-C3N4-B almost lost degradability, revealing that the ∙O2− and h+ were the main active species in the photocatalytic reaction. Additionally, the total carbon analysis (TOC; vario TOC select, Elementar, Germany) technology was used to study total organic carbon changes from photocatalytic degradation process of RhB molecules to ensure complete degradation of the selected pollutant. Fig. S4 shows the ability of mineralization of Bi2S3/g-C3N4-B samples for the RhB solution in terms of removal of TOC, which is slightly slower than the photodegradation of the RhB solution. However, the TOC removal has reached ∼ 97.88% by 180 min of irradiation time. The results show that almost no by-products are produced after complete photocatalytic reaction, which is consistent with that of LC-MS (Supporting Information). The Bi2S3/g-C3N4-B samples before and after photocatalytic degradation of RhB solution were characterized by XRD spectra for crystal structure. As shown in Fig. S5, the XRD patterns the prior to photocatalytic (Bi2S3/g-C3N4-B-Raw) and after reaction (Bi2S3/g-C3N4-B-Rea) for Bi2S3/g-C3N4-B was exhibited. A comparative study indicates that the position and the intensity of diffraction peaks for the Bi2S3/g-C3N4-B is unaffected after photocatalytic reaction, indicating that as-prepared samples have good stability during the photocatalytic reaction process.

The UV–vis absorption spectra of the Bi2S3/g-C3N4-B during photocatalytic degradation (a); The degradation efficiency of different samples (b); First-order reaction kinetic equation − ln(C/C0) = kt (c); reaction rate for different samples (d).
Fig. 5
The UV–vis absorption spectra of the Bi2S3/g-C3N4-B during photocatalytic degradation (a); The degradation efficiency of different samples (b); First-order reaction kinetic equation − ln(C/C0) = kt (c); reaction rate for different samples (d).

Furthermore, three antibiotics (tetracycline, ciprofloxacin, and oxytetracycline) with the concentration of 20 mg/L were used to evaluate the photocatalysis performance of the samples. Fig. 6a and 6b depict the degradation of tetracycline, ciprofloxacin, and oxytetracycline by Bi2S3/g-C3N4-B, and the degradation of g-C3N4 and Bi2S3 for tetracycline. The photocatalytic reaction follows the first-order reaction kinetic equation − ln(C/C0) = kt by fitting the data. Fig. 6c shows the histogram based on the slope of the kinetic curve. Pure Bi2S3 and g-C3N4 have no obvious degradation effect on tetracycline, and tetracycline has no self-degradation ability. The figure shows that the Bi2S3/g-C3N4-B samples possess excellent photocatalysis activity on three antibiotics, and the degradation efficiency of tetracycline is the highest within 120 min. Fig. 6d shows the five-cycle degradation curve of tetracycline for the Bi2S3/g-C3N4-B. After five cycles, the degradation efficiency for tetracycline still remains approximately 95%, and the analysis results demonstrate that the as-prepared BSCN samples have high stability during the photocatalytic reaction process.

(a) The visible-light-driven photocatalysis performance for the antibiotics (tetracycline, ciprofloxacin, and oxytetracycline) with the concentration of 20 mg/L; (b–c) first-order reaction kinetic equation − ln(C/C0) = kt and the reaction rate value of Bi2S3/g-C3N4-B; (d) recycling degradation efficiency of Bi2S3/g-C3N4-B for tetracycline.
Fig. 6
(a) The visible-light-driven photocatalysis performance for the antibiotics (tetracycline, ciprofloxacin, and oxytetracycline) with the concentration of 20 mg/L; (b–c) first-order reaction kinetic equation − ln(C/C0) = kt and the reaction rate value of Bi2S3/g-C3N4-B; (d) recycling degradation efficiency of Bi2S3/g-C3N4-B for tetracycline.

3.4

3.4 Mechanism analysis for enhanced photocatalytic activity

3.4.1

3.4.1 Analysis of the reactive free radicals

The ESR patterns of superoxide radical ·O2− and hydroxyl radical ·OH at 0 min, 1 min, 5 min, and 10 min are shown in Fig. 7. The results show that no signals corresponding to ·O2− and ·OH emerge under the dark environment, indicating that the BSCN has no paramagnetic center without illumination. However, the signals of ·O2− and ·OH appear under continuous illumination after 1 min. The presence of ·O2− indicates that photogenerated carriers are generated under illumination conditions, and the CB position of the samples is more negative than the potential of H2O/O2 and O2/·O2−. Thus, the electrons gathered on the CB can be captured by O2 in water and react to produce ·O2−. The emergence of ·OH signals is due to the holes on the VB of the samples that react rapidly with water to produce after irradiation. The intensity of ESR signals increases as the illumination time increases. In addition, the ·O2− production of BSCN composites is significantly higher than that of ·OH under illumination. The results indicated that ·O2− plays a major role, and ·OH plays a secondary role during the photocatalytic reaction process.

ESR spectra of hydroxyl (a) and superoxide radicals (b) captured by DMPO for Bi2S3/g-C3N4-B.
Fig. 7
ESR spectra of hydroxyl (a) and superoxide radicals (b) captured by DMPO for Bi2S3/g-C3N4-B.

3.4.2

3.4.2 Pathways for the generation, migration, and separation efficiency of photogenerated charge carriers

Fig. 8 shows the VB-XPS, flat band potential, and conduction band position for the g-C3N4 and Bi2S3. The potentials of the valence band (VB) and the conduction band (CB) of the g-C3N4 are + 1.45 and − 1.339 V, respectively. The potentials of the VB and the CB of Bi2S3 are + 2.01 V and + 0.381 V, respectively. Furthermore, the forbidden band width (Eg) of g-C3N4 was calculated based on the DRS spectra, and the measurement value of the Eg for g-C3N4 is 2.802 V as shown in Fig. S6, which is matching the VB and CB of g-C3N4: +1.45 – (–1.339) = 2.789 V. The measured band edge positions of the Bi2S3 and g-C3N4, indicate that the mechanism of charge carrier transfer for the BSCN composites depends on the staggered Type-II heterojunction, which is presented in Fig. 9a. Therefore, the photogenerated electrons must migrate from the conduction band of g-C3N4 to the conduction band of Bi2S3. On the contrary, the photogenerated holes tend to transfer from the valence band of Bi2S3 to the valence band of g-C3N4. The present results contradict the reduction potential for electrons (only 0.381 V on the CB of Bi2S3) that is insufficient to reduce O2 into O2− radical (−0.33 V) or H+ to produce H2 under irradiation (Li et al., 2018).

(a–b) VB-XPS of the g-C3N4 and Bi2S3; (c–d) flat band potential and position of conduction band for the g-C3N4 and Bi2S3.
Fig. 8
(a–b) VB-XPS of the g-C3N4 and Bi2S3; (c–d) flat band potential and position of conduction band for the g-C3N4 and Bi2S3.
Pathways for the generation, separation, and migration of photogenerated charge carriers for the Bi2S3/g-C3N4.
Fig. 9
Pathways for the generation, separation, and migration of photogenerated charge carriers for the Bi2S3/g-C3N4.

Therefore, a possible and reasonable mechanism for the enhanced photocatalytic activity of BSCN composite heterojunction is proposed. The proposed Z-scheme-based charge transfer is a feasible mechanism in the Bi2S3/g-C3N4 heterojunction to justify the improved photocatalytic activity. We suggest that the g-C3N4 forms a close surface contact with Bi2S3 initially, and the heterogeneous junction occurs at the interface. Subsequently, both products are excited to transition the electrons from VB to CB under visible radiation, resulting in many free electrons and holes. Some electrons of the CB of Bi2S3 quickly recombine with the holes of the VB of g-C3N4 due to the synergistic effect of an internal electric field, band bending, and Coulomb interaction. Meanwhile, the remaining electrons and holes retain at the CB of g-C3N4 and VB of Bi2S3, respectively, to participate the redox reaction (Fig. 9b). The electrons and holes generated and clustered by photo-excited can participate in a series of redox reactions such that producing a certain amount of active free radicals include ·O2–, H2O2 (Fig. S7, the test of photocatalytic production of hydrogen peroxide in the Supporting Information), OH–, ·OH and so on, which can further degrade pollutants such as dyes and antibiotics etc. Thus, efficient separation and utilization of charge carriers can be achieved, leading to distinctly boosted photocatalytic performance. Hence, the composite samples exhibit better photocatalytic properties when compared with pure g-C3N4.

4

4 Conclusions

The BSCN samples are prepared by a facile hydrothermal method. The Bi2S3 nanoparticles are well-dispersed on the surface of the CN, and both products form close interface. The structure of CN was not been changed with the introduction of Bi2S3. The ultraviolet–visible spectroscopic analysis, photoluminescence spectra, and electrochemical test results show that BSCN samples exhibit better photo-response ability than that of pure CN in the visible light region, and the separation and transfer efficiency of photogenerated carriers is significantly improved. The photocatalytic reaction results show that when the mass ratio of CN and BS is 8:1 (Bi2S3/g-C3N4-B), the degradation efficiency is the highest and reaches 98.98% for the degradation of RhB solution (20 ppm) for 120 min. Furthermore, the Bi2S3/g-C3N4-B has an evident degradation effect for antibiotics (tetracycline, ciprofloxacin, and oxytetracycline). After photocatalytic reaction of five cycles, the degradation rate of Bi2S3/g-C3N4-B for tetracycline still remains at 95%, indicating that the as-prepared BSCN samples have excellent stability during the photocatalytic reaction. The trapping experiment and the ESR spectrum analysis shows that in the course of the degradation process, the h+ and ·O2− plays a major role compared with ·OH. This research can provide a good idea for the treatment of printing and dyeing wastewater or antibiotic wastewater.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51802245), Shaanxi Province Innovative Talent Promotion Plan-Young Science and Technology Star (No. 2021KJXX-43), Jiaxing Public Welfare Projects (No. 2020AD10021), Science and Technology Guidance Project Plan of China National Textile and Apparel Council (No. 2020004), the Natural Science Basic Research Plan in the Shaanxi Province of China (No. 2020JQ-828), China Postdoctoral Science Foundation Funding (No. 2018M631188), the Shaanxi Provincial Association of Science and Technology Youth Talents Lifting Plan (No. 20180418), Scientific Research Program Funded by Shaanxi Provincial Education Department (No. 18JK0350), Scientific Research Foundation for Ph.D., Xi’an Polytechnic University (No. BS1741), and the Graduate Innovation Foundation of Xi'an Polytechnic University (No. chx2021031).

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.

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Appendix A

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2022.103689.

Appendix A

Supplementary material

The following are the Supplementary data to this article:

Supplementary data 1

Supplementary data 1


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