One-step hydrothermal synthesis of a ternary heterojunction g-C₃N₄/Bi₂S₃/In₂S₃ photocatalyst and its enhanced photocatalytic performance

Abstract

In recent years, photoelectrocatalysis has been one of the hotspots of research. Graphite-like carbon nitride (g-C₃N₄) is one of the few non-metal semiconductors known and has good potential in the field of photocatalysis due to its simple preparation method and visible light effects. In this study, a method for compounding two semiconductor materials, In₂S₃ and Bi₂S₃, on the surface of g-C₃N₄ via a one-step hydrothermal method is reported, and it was found that this resulting material showed remarkable properties. The advantages of this method are as follows: (1) the formation of a heterojunction, which accelerates the separation efficiency of photogenerated carriers; (2) a large number of holes and defects on the surface of g-C₃N₄ are conducive to the nucleation, crystallisation and growth of In₂S₃ and Bi₂S₃. Compared with its counterpart catalysts, the CN/In₂S₃/Bi₂S₃ composite catalyst has significantly improved performance. Due to its high degree of crystallinity, the adsorption capacity of the catalyst itself is also significantly improved. In addition, the stability of the composite material maintains 90.9% after four cycles of use, and the structure is not damaged. In summary, CN/Bi₂S₃/In₂S₃ composite materials are believed to have broad application potential in the treatment of dye wastewater.

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1 Introduction

With the development of science and technology, the application of photoelectrocatalytic technology^1–5 is of great benefit to the water pollution caused by the of the global industrialization process.^6–9 As a result, different countries and regions worldwide have made great efforts to protect the environment on which people depend.^10–12 Fortunately, at the same time, there have been continuous developments in science and technology toward alleviating pollution in water. Common methods that have been developed for dealing with organic pollution in water can be divided into three categories: physical, chemical and biological methods. Adsorption is the main physical method used,¹³ which mostly involves the use of activated carbon and diatomaceous earth as adsorbents. Other adsorbents have been also reported recently for wastewater treatment, e.g., sepiolite mineral nanofibers.¹⁴ However, the costs associated with this method are high and complete adsorption of the pollutants from water cannot be achieved. Oxidation reactions are the main chemical methods used in the remediation of water pollutants.^15,16 Therefore, it is urgent to develop an environmentally friendly method to solve the current water pollution issues. As photocatalytic technology has the advantages of being green, highly efficient and causing no secondary pollution, it is one of the most promising methods for degrading pollutants in water that could be applied as a future strategy.^17–19

At present, the reports on photocatalysts in the literature are mainly based on semiconductor materials, including titanium dioxide (TiO₂),²⁰ molybdenum sulfide (MoS₂),²¹ Bi₂S₃,²² zinc oxide (ZnO),²³ graphitic carbon nitride (g-C₃N₄).²⁴ The unique forbidden band structures of semiconductors lead to the generation of electrons and holes (e⁻ and h⁺) under the excitation of light of a certain wavelength. These e⁻ and h⁺ then react with water molecules and oxygen in water to form strong oxidising substances, which oxidise and decompose the pollutants present in water into environmentally friendly molecules. Among these semiconductors, in 2009 g-C₃N₄ was first used by Wang et al. in the photocatalytic production of hydrogen. Due to the excellent photoelectric properties of g-C₃N₄, it has attracted the attention of many researchers in the field of photocatalysis.^19,24 In addition to photocatalytic application,²⁵ graphitic carbon nitride based materials are also promising for biomedical applications.²⁶ However, some studies have shown that the rapid recombination of e⁻ and h⁺ severely limits the photocatalytic performance of g-C₃N₄.²⁷ To solve and overcome these difficulties, many researchers have made tremendous efforts,^28,29 such as using different strategies to improve the catalytic performance of g-C₃N₄. For example, a higher number of active sites have been introduced in g-C₃N₄ by increasing its specific surface area,³⁰ however, the coverage of dyes and other small molecules on the catalyst surface and in the pores makes the catalyst less stable. The latest research shows that a strategy of constructing heterojunction binary and multi-component catalysts not only accelerates the separation of e⁻ and h⁺ but also expands the range of light response of these materials has attracted great attention.^31–33 It has been reported in the literature that the construction of heterostructured catalysts leads to systems that have greatly improved photocatalytic performance.^34–37 C. Auttaphon et al.³⁸ synthesised a Z-scheme Bi₂S₃/ZnIn₂S₄ heterostructure for methylene blue degradation, which showed higher photocatalytic activity than the uncombined catalysts alone. Geioushy et al. used a one-pot method to prepare BiPO₄/Bi₂S₃ for the reduction of harmful Cr(VI) driven by visible light.³⁹ ABi₂S₃/Bi₂W₂O₉ composite has also previously been prepared that shows 80% degradation of phenol after 120 min of irradiation, a performance that is substantially higher than those of pure Bi₂S₃ and Bi₂W₂O₉ under visible-light illumination.⁴⁰ An et al. synthesised a new Z-scheme In₂S₃/graphene heterojunction with a core–shell structure, which showed a methyl orange degradation rate of almost five times higher than that of pure In₂S₃ (ref. 41) and reduced Cr(VI) under visible light. For degradation of rhodamine B (RhB), these photocatalysts have been reported recently.^42,43 In summary, from the perspective of development, photocatalytic technology has great application prospects in the degradation of pollution and environmental protection.^44,45

In this study, to verify the heterostructure photocatalyst theory, the ternary heterostructured photocatalyst g-C₃N₄/Bi₂S₃/In₂S₃ was prepared via a one-step hydrothermal method. The modification of microscale porous spherical Bi₂S₃ and dendritic In₂S₃ on the surface of porous g-C₃N₄ with a large specific surface area significantly improves its optical, physical and photoelectric conversion properties. Compared with the performances of the g-C₃N₄, Bi₂S₃ and In₂S₃ catalysts alone, and Bi₂S₃/In₂S₃ binary heterojunction catalysts, the obtained g-C₃N₄/Bi₂S₃/In₂S₃ ternary heterostructured photocatalyst shows significantly improved degradation of rhodamine B (RhB). In addition, by carrying out powder X-ray diffraction (XRD), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) and other characterisation methods, the composition of the catalyst in terms of its elemental content, its chemical valence state, morphology, and charge carrier transfer behaviour were carefully analysed, and the possible catalytic mechanism of the catalyst was explored and is discussed in detail. This work may provide a feasible practical example of a way of improving the photocatalytic performance of g-C₃N₄ through its combination with Bi₂S₃ and In₂S₃ in a heterogeneous composition as a reference for subsequent research.

2 Experimental

2.1 Materials

Thiourea, absolute ethanol, RhB, urea, bismuth nitrate pentahydrate, indium nitrate, and other chemicals used in the experiments were of analytical purity (99%) and used without further purification. Deionised water was used throughout all of the experimental work in this study. All medicines come from Titan.

2.2 Sample preparation

2.2.1 Preparation of the g-C₃N₄ photocatalyst. An appropriate amount of urea was added to a crucible and closed the lid, and heated in a muffle furnace at a heating rate of 2.5 °C min⁻¹ to 550 °C, and held at this temperature for 4 h to obtain a yellow solid of g-C₃N₄, which was ground into a powder for subsequent use.

2.2.2 Pretreatment scheme of g-C₃N₄. An appropriate amount of g-C₃N₄ powder was added to a 50 mL beaker, into which 20 mL of DMF (N,N-dimethylformamide) was poured and the reaction mixture was stirred at room temperature for 12 h to prepare a solution referred to as solution A.

2.2.3 Preparation of g-C₃N₄/Bi₂S₃/In₂S₃. 70 mL of HON₃ solution with a concentration of 1 mol L⁻¹ was added to a 100 mL beaker and stirred for ten min before adding 0.1 mmol of Bi (NO₃) 5H₂O, and stirring the resulting reaction mixture for 30 min. Then, to this mixture 0.1 mmol of In (NO₃) xH₂O was added and the mixture was stirred for 30 min. After this time 0.2 g of cetrimonium bromide (CTAB) was added and the reaction mixture was ultrasonicated for 30 min, to produce a solution referred to as solution B. The above solution A was slowly added dropwise to solution B to mix them together uniformly, then 0.6 g of thiourea was added and the reaction mixture was stirred for 30 min, before being poured into a 100 mL polytetrafluoroethylene liner and heated to 140 °C under hydrothermal reaction for 12 h. The resulting liquid was washed three times with ethanol and deionised water, and then dried in an oven overnight at 60 °C. g-C₃N₄/Bi₂S₃/In₂S₃ catalysts with different mass fractions of g-C₃N₄ were prepared, and the catalysts g-C₃N₄ and Bi₂S₃/In₂S₃ were prepared for comparison.

2.3 Characterisation of the photocatalysts

XRD patterns were recorded using a Bruker D8 Advance powder X-ray diffractometer equipped with a Cu-Kα radiation source. A Nicolet iS10 Fourier-transform infrared (FTIR) spectrometer was employed to record FTIR spectra of the samples in the wavenumber range of 500–4000 cm⁻¹. XPS measurements of the prepared samples were recorded using a PHI 5000 Versa Probe spectrometer, in which the binding energy positions were calibrated against the C–C bond peak that has a binding energy of 284.8 eV. The morphology characteristics of the samples were investigated using transmission electron microscopy (TEM, Tecnai G F20, Hitachi, HT7700) and scanning electron microscopy (SEM, Hitachi S-4800). UV-visible diffuse reflection spectroscopy (DRS) measurements were carried out using a UV-visible spectrophotometer (Shimadzu UV-3600) across a wavelength range of 200–800 nm, employing BaSO₄ as a benchmark. Fluorescence emission and time-resolved fluorescence spectra were measured using a fluorescence spectrometer (Hitachi F-7000) over a wavelength range of 340–800 nm at an excitation wavelength of 320 nm, employing a 300 W xenon lamp (CEL-HXF300) as a light source.

2.4 Photocatalytic activity

To determine the photocatalytic activity of the synthesised photocatalyst, the catalytic degradation of RhB was evaluated under simulated sunlight using a 500 W xenon lamp as a light source in a photoreactor (CEL-HXF300, Beijing, China). In a typical experiment, before being exposed to light the reactants and catalyst were placed in a quartz tube and magnetically stirred for 30 min in the dark to achieve adsorption–desorption equilibrium. At a given time interval after beginning the visible light irradiation, samples were taken from the reaction suspension (each with a volume of 5 mL), centrifuged at 8000 rpm for 10 minutes, and then the supernatant was removed. Then, the concentration of RhB in the sample solutions was analysed using a (UV-31/32/3300) UV-visible spectrometer (MAPADA) at its maximum absorption wavelength. To ensure the repeatability of the results, each run was repeated to obtain an average for each set of conditions. A blank test was also carried out by irradiating the reactant solution in the absence of the catalyst to evaluate its photo-induced self-sensitised photodegradation.
where η is the photocatalytic efficiency; C₀ is the concentration of the reactant before illumination and C_t is the concentration of the reactant after illumination for t hour.

3 Results and discussion

3.1 Morphology and structure

3.1.1 XRD analysis. As shown in Fig. 1, XRD was conducted to identify the crystallographic and phase structures of Bi₂S₃, CN, In₂S₃, Bi₂S₃/In₂S₃ and CN-xBi₂S₃/In₂S₃. The XRD pattern of pure CN shows two distinct diffraction peaks at 2θ = 12.43° and 27.54°, which can be indexed to the (002) and (100) diffraction planes, respectively, of g-C₃N₄ (JCPDS (87-1526)), attributed to its typical stacked aromatic rings and interlayer π–π stacking.⁴⁶ Fig. 1 shows distinct diffraction peaks at 2θ = 11.14°, 15.80°, 15.90°, 17.58°, 22.39°, 23.72°, 24.92°, 25.20°, 28.60°, 31.65°, 31.79°, 32.93°, corresponding to the (110), (020), (200), (120), (220), (101), (130), (310), (211), (040), (221), (301) diffraction planes respectively, of Bi₂S₃ (JCPDS No. 17-0320).²² The profile of In₂S₃ exhibits the characteristic diffraction peaks of β-In₂S₃ (JPCDS No. 65-0459).⁴⁷ However, the XRD pattern of In₂S₃/Bi₂S₃ only shows the diffraction peaks of Bi₂S₃, showing that it contains little In₂S₃ content. In addition, one new weak characteristic peaks at 27.54° is present in the patterns of the CN/Bi₂S₃/In₂S₃ composites, which can be assigned to the (0 0 2) peak of g-C₃N₄. With an increase in the amount of g-C₃N₄ present in the catalyst samples, the diffraction peak of Bi₂S₃/In₂S₃ at 2θ = 24.92° went from being of high intensity to low intensity, reflecting the influence that it has on the phase crystallinity of Bi₂S₃/In₂S₃.

Fig. 1 The typical XRD patterns of CN, Bi₂S₃/In₂S₃ and CN/Bi₂S₃/In₂S₃.

3.1.2 SEM/TEM analysis. In Fig. 2a the layered porous structure of g-C₃N₄ can be seen in CN-xBi₂S₃/In₂S₃, which is beneficial for the adsorption of dyes by the catalyst. Fig. 2b shows an image of the structure of In₂S₃, which is made up of a large number of hexagons with side lengths ranging from 0.3–1 μm. Fig. 2c shows an image of Bi₂S₃, which has a rod-shaped structure, where the rods have diameters in the range of 50–100 nm and a length in the range of 0.5–2.0 μm in size.

Fig. 2 SEM images of (a) g-C₃N₄; (b)In₂S₃; (c) Bi₂S₃; (d) Bi₂S₃/In₂S₃; (e) 0.05CN/Bi₂S₃/In₂S₃; TEM images of (f) g-C₃N₄; (g) Bi₂S₃/In₂S₃ and HRTEM images of (h) 0.05CN/Bi₂S₃/In₂S₃.

Compared with pure In₂S₃ and Bi₂S₃, the morphology of the composite of both materials in Fig. 2d shows a spherical structure of In₂S₃ surrounded by rod-shaped Bi₂S₃. This structure is more conducive to the transfer of photogenerated e⁻ and h⁺ than the single catalysts alone. Fig. 2e shows an SEM image of 0.05CN/Bi₂S₃/In₂S₃, which shows the combination of flower-like In₂S₃ and a spherical Bi₂S₃ structure. Porous layered g-C₃N₄ has a large specific surface area and pores, so it provides more defects with which to facilitate the crystallisation, nucleation and growth of In₂S₃ and Bi₂S₃. In addition, the structures of In₂S₃ and Bi₂S₃ are not damaged. To analyse its structure in more depth, TEM and high-resolution TEM (HRTEM) were used to characterise 0.05CN/Bi₂S₃/In₂S₃. Fig. 2f shows a TEM image of g-C₃N₄, from which it can be seen that the layered g-C₃N₄ of the film structure is consistent with the results presented in the literature. Fig. 2g shows a TEM image of Bi₂S₃/In₂S₃, which is consistent with the image shown in Fig. 2d. Fig. 2h shows a HRTEM image of 0.05CN, in which it can be clearly seen that there are two lattice fringes with different widths of 0.352 and 0.278 nm, corresponding to the (1 3 0) crystal plane of Bi₂S₃ and the (4 0 0) crystal plane of In₂S₃. This result is consistent with the PXRD results.

3.1.3 FTIR analysis. To further demonstrate the surface properties and functional groups present in the samples, FTIR spectra of In₂S₃, Bi₂S₃, g-C₃N₄ and CN-xBi₂S₃/In₂S₃ were recorded and the results are presented in Fig. 3. In the FTIR spectrum of CN-xBi₂S₃/In₂S₃ there is no obvious peak that is characteristic of Bi₂S₃.⁴⁸ The peaks at 1327, 1417, 1559, and 1631 cm⁻¹ in the same spectrum can be assigned as the characteristic absorption bands of g-C₃N₄, attributed to the vibration of the C₆N₇ skeleton and the typical stretching mode of the CN heterocycles.⁴⁹

Fig. 3 The FTIR spectra of CN, Bi₂S₃/In₂S₃ and CN-xBi₂S₃/In₂S₃.

The peak at around 809 cm⁻¹ can be assigned to the breathing mode of the triazine units. In addition, the peaks at 772 and 1196 cm⁻¹ are typical of In₂S₃ and the characteristic peak of Bi₂S₃/In₂S₃ is roughly consistent with In₂S₃.^50,51 The characteristic absorption peak of g-C₃N₄/In₂S₃/Bi₂S₃ is consistent with that of g-C₃N₄, while the peak related to Bi₂S₃/In₂S₃ gradually decreases in intensity, which may be a result of the g-C₃N₄ peak being too intense.

3.1.4 XPS analysis. To study the surface chemical states and chemical compositions of the CN/Bi₂S₃/In₂S₃ heterojunction samples, Fig. 4 shows XPS measurements of the samples, in which it can be seen that the survey spectrum of CN–In₂S₃/Bi₂S₃ exhibits the typical C 1s, S 2p, S 2s, N 1s, In 3d, and Bi 4f peaks expected for CN–Bi₂S₃/In₂S₃, Bi₂S₃/In₂S₃ and CN. The survey spectrum indicates that C and N exist on the surface of g C₃N₄, and Bi, In, and S on the surface of In₂S₃/Bi₂S₃. Besides this, as shown in Fig. 4a, Bi, In and S were found to be present on the surface of the g-C₃N₄ nanosheets, since the peaks at around 443.61, 157.43, and 225.01 eV can be assigned to the In 3d, Bi 4f and S 2s energy levels, respectively. The high-resolution C 1s, N 1s, In 3d, Bi 4f and S 2s XPS spectra of 0.05CN/Bi₂S₃/In₂S₃ are presented in Fig. 4b–f. Moreover, the binding energy peak of the N–C=N groups in the 0.05CN–Bi₂S₃/In₂S₃ heterostructure shifts lowers by 0.02 eV to 288.14 eV, which indicates that there are weak interactions between the Bi₂S₃/In₂S₃ and g-C₃N₄ nanosheets as a result of surface modification effects.⁵² The N 1s Gaussian curve in Fig. 4b can be deconvolved into three peaks that have binding energies of 398.47, 400.20, and 401.90 eV, corresponding to sp²-hybridised nitrogen (C–N=C), the N–(C)3 groups of the skeleton, and the surface uncondensed bridging N atoms with C–N–H functional groups attached.^53,54 Compared to g-C₃N₄, the peak positions in the N 1s spectrum of 0.05CN/Bi₂S₃/In₂S₃ are shifted to higher binding energies. In the In 3d spectrum (Fig. 4d), the In 3d_3/2 and In 3d_5/2 peaks of 0.05CN/Bi₂S₃/In₂S₃ are located at 452.23 and 444.58 eV, which indicates a slight shift to higher binding energies compared to pristine In₂S₃ (451.39 and 443.87 eV). As shown in Fig. 4e, the Bi 4f XPS spectrum of Bi₂S₃ displays clearly two peaks (162.73 and 157.43 eV), which can be attributed to the 4f_5/2 and 4f_7/2 of Bi³⁺. Moreover, compared to Bi₂S₃, the peak positions (163.50 and 158.18 eV) of Bi 4f in 0.05CN/Bi₂S₃/In₂S₃ are shifted to higher binding energies. Since the peak of Bi 4f covers the characteristic peak of S 2p, S 2s is added to illustrate the existence of S²⁻. As shown in Fig. 4f, the S 2s XPS spectrum of Bi₂S₃/In₂S₃ shows three peaks at 221.0, 224.0, and 224.9 eV. Compared with Bi₂S₃/In₂S₃, 0.05CN/Bi₂S₃/In₂S₃ shows peaks that are slightly shifted to higher binding energies (222.6, 225.01, and 227.17 eV). These results suggest the successful heterogeneous growth of In₂S₃ and Bi₂S₃ on the g-C₃N₄ nanosheets.

Fig. 4 XPS spectra of the CN, In₂S₃/Bi₂S₃, and 0.05CN/In₂S₃/Bi₂S₃; (a) survey, (b) C 1s, (c) N 1s, (d) In 3d, (e) Bi 4f, and (f) S 2s spectra.

3.1.5 UV-vis analysis. To investigate the modification effect that the g-C₃N₄ nanosheets have on the Bi₂S₃/In₂S₃ heterostructure, the optical absorption properties of the as-prepared samples were analysed using UV-vis spectroscopy, the results of which are shown in Fig. 5a. The g-C₃N₄ shows good light absorption at a wavelength of less than 450 nm. The light absorption of In₂S₃ and Bi₂S₃ seem much higher than that of g-C₃N₄, at a wavelength of 1000 nm. Compared with Bi₂S₃/In₂S₃, the absorption edge of the g-C₃N₄/Bi₂S₃/In₂S₃ heterostructure exhibits a distinct red shift, which may originate from the modification effect of Bi₂S₃/In₂S₃ on the surface of the g-C₃N₄ nanosheets. This signifies that a strong interaction is formed between g-C₃N₄ and Bi₂S₃/In₂S₃, which is beneficial to improving the separation efficiency of charge carriers and the stability of the heterostructure. In addition, the band gap energies (E_g) of the samples were calculated using the Tauc equation:

(αhν)² = A(hν − E_g)ⁿ

where α, hν, A and E_g are the absorption coefficient, photoenergy, proportionality constant and band gap, respectively. As shown in Fig. 5b, the band gaps of g-C₃N₄, In₂S₃ and Bi₂S₃ are 2.84, 2.2, and 1.30 eV, respectively. In addition, the band gap of Bi₂S₃/In₂S₃ is estimated to be 1.26 eV, and the band gaps of 0.01CN/Bi₂S₃/In₂S₃ and 0.05CN/Bi₂S₃/In₂S₃ are 1.20 and 1.18 eV, respectively. Compared with Bi₂S₃/In₂S₃, the band gap of 0.05CN/Bi₂S₃/In₂S₃ is narrower, which indicates that the heterostructure of 0.05CN/Bi₂S₃/In₂S₃ is more easily excited by visible light and is beneficial for improving photocatalytic performance due to the modification of large holes and defects on the surface of the g-C₃N₄ nanosheets.

Fig. 5 (a) UV-vis DRS of g-C₃N₄, In₂S₃, Bi₂S₃, Bi₂S₃/In₂S₃, 0.01CN/Bi₂S₃/In₂S₃, and 0.05CN/Bi₂S₃/In₂S₃ and (b) plot of (αhv)^n/2 versus hv.

3.1.6 PL spectra and photocurrent analysis. To further understand the reasons for the improvement in the photocatalytic activity of the composite photocatalyst, the PL spectra were measured to investigate the separation efficiency of photo-induced charge carriers.⁵⁵ The PL spectra of g-C₃N₄, Bi₂S₃/In₂S₃ and 0.05 g-C₃N₄/Bi₂S₃/In₂S₃ are shown in Fig. 6a. The PL emission stems from the radiative recombination between the excited e⁻ and h⁺ and the weaker the PL peak intensity is, the slower the recombination rate of the excited e⁻ and h⁺. Comparing the PL emission spectra of the three materials, the 0.05 g-C₃N₄/Bi₂S₃/In₂S₃ heterostructure shows low fluorescence intensity relative to that shown by g-C₃N₄ and Bi₂S₃/In₂S₃. These results indicate that the efficient interfacial charge transfer between Bi₂S₃/In₂S₃ and g-C₃N₄ hinders the recombination of e⁻ and h⁺ pairs. In addition, Fig. 6b is the photocurrent curve of the catalysts. We can clearly see that the photocurrent value of the composite material is the highest, and the conclusion corresponds to Fig. 6a.

Fig. 6 PL spectra (a) and photocurrent (b) of the g-C₃N₄, Bi₂S₃/In₂S₃ and 0.05 g-C₃N₄/Bi₂S₃/In₂S₃ composites.

3.2 Photocatalytic activity

Fig. 7a shows the degradation RhB by the different catalysts. It can be clearly seen that the 0.05 g-C₃N₄ composite catalyst shows the best catalytic activity, which almost completely degrades the dye after 45 min of exposure to light. Fig. 7b shows the first-order kinetic fitting curve of the degradation, and the results obtained are consistent with the results shown in the degradation curve. Fig. 7c shows a histogram of the catalyst fitting coefficient. The fitting coefficient of the 0.05 g-C₃N₄/Bi₂S₃/In₂S₃ catalyst is 0.02826, which is significantly higher than the values for g-C₃N₄, In₂S₃ and Bi₂S₃. Compared with recently published articles, this study has advantages in degradation efficiency.^56–58

Fig. 7 (a) Influence that the illumination time has on the photocatalytic degradation of the catalysts, (b) the pseudo-first-order dynamics, and (c) the apparent rate constant of the degradation reaction.

3.3 Research on the mechanism of photocatalysis

Fig. 8a–c show the valence band spectra of g-C₃N₄, In₂S₃ and Bi₂S₃ respectively (1.95, 0.32, and −0.35 eV). From the results obtained and shown in Fig. 5b, the valence (VB) and conduction bands (CB), respectively, of g-C₃N₄ (1.95 and −0.89 eV), In₂S₃ (0.32 and −1.88 eV), and Bi₂S₃ (−0.35 and −1.65 eV) can be calculated, which facilitate the subsequent analysis of the mechanism of the photocatalyst degradation. Fig. 8e shows the particle capture experiment performed by the catalyst, using isopropanol (IPA), 1,4-benzoquinone (BQ), and Na₂C₂O₄ as the reactive species scavengers for ˙OH, ˙O₂⁻, and h⁺, respectively.⁵⁹ It can be seen that BQ, IPA and EDTA-2NA all have different degrees of influence on the catalytic performance of the catalyst. Among them, IPA has the greatest degree of influence, with a degradation rate of 28.3%, whereas EDTA-2Na has the lowest degree of influence and a degradation rate of 65.3%. This shows that photogenerated e⁻ have a greater impact on the ternary heterostructured nanocomposite g-C₃N₄/In₂S₃/Bi₂S₃ than h⁺. Fig. 8e shows the possible reaction pathway of the photogenerated carriers of the ternary heterostructured nanocomposite g-C₃N₄/In₂S₃/Bi₂S₃ catalyst. When the catalyst is excited by light, e⁻ transition from the VB to the CB. Due to the difference in electronegativity, the excited e⁻ will be transferred from the CBs of In₂S₃ and Bi₂S₃ to the CB of g-C₃N₄. At the same time, h⁺ will be transferred from the VB of g-C₃N₄ to the VBs of In₂S₃ and Bi₂S₃, and can also oxidise pollutants. The transfer speed of h⁺ and e⁻ is improved, and the recombination of photogenerated carriers is suppressed, which improves the photocatalytic performance of the material. It is speculated that the photocatalysis may proceed in the following way:

CN/Bi₂S₃/In₂S₃ + hν → e⁻ + h⁺ (1)

OH⁻ + h⁺ → ˙OH (2)

H₂O + h⁺ →H⁺ + ˙OH (3)

e⁻ + O₂ → ˙O₂⁻ (4)

˙O₂⁻ + H⁺ → ˙HO₂ (5)

2·HO₂ → O₂ + H₂O₂ (6)

H₂O₂⁺ ˙O₂⁻ → ˙OH + OH⁻ + O₂ (7)

Fig. 8 (a)–(c) show the valence band diagrams of g-C₃N₄, In₂S₃ and Bi₂S₃; (d) the free radical trapping experiments for the degradation of RhB over the 0.05CN/Bi₂S₃/In₂S₃ composite. (e) The proposed photocatalytic mechanism for the degradation of RhB on the surface of the 0.05CN/Bi₂S₃/In₂S₃ composite.

Of these, (2), (4), and (7) are thought to be the main photocatalytic reactions that occur.

3.4 Stability testing

Fig. 9a shows the stability test results of the 0.05 g-C₃N₄/Bi₂S₃/In₂S₃ catalyst. After repeating the degradation test four times, the degradation performance of the catalyst still reaches 90.9. Therefore, it can be seen that the heterostructure of the composite catalyst not only improves its activity, but also its stability. To further assess whether the structure of the catalyst is damaged after use, PXRD measurements were carried out on the catalyst before and after its use, and the results are shown in Fig. 9b. It can be seen that there is basically no change in the PXRD peak patterns of the catalyst before and after use. Therefore, it can therefore be concluded that the catalyst structure is not destroyed after use.

Fig. 9 (a) Cycling runs of 0.05CN/Bi₂S₃/In₂S₃ in the degradation of RhB under visible light. (b) PXRD pattern of 0.05CN/Bi₂S₃/In₂S₃ before and after carrying out the photocatalytic experiments.

4 Conclusions

In this paper, a one-step hydrothermal method was used to synthesise a 3D dendritic and porous spherical ternary heterostructure nanocomposite g-C₃N₄/In₂S₃/Bi₂S₃. The g-C₃N₄/In₂S₃/Bi₂S₃ heterostructure catalyst shows significantly improved degradation of RhB compared with the components from which it is made. The reason for the improved photocatalytic performance can be attributed to the increase in the specific surface area of the catalyst, meaning that it has more active sites; the formation of 3D dendritic and porous spherical materials enhances the adsorption performance of the catalyst and the composition of the heterostructure catalyst accelerates the separation of photogenerated carriers, thereby promoting the production of O₂⁻, OH, and h⁺. The research herein describes the design of a new type of ternary heterostructure catalyst. The disadvantage here is that the decomposition process of the degradants was not able to be analysed in detail, which remains a subject for future work.

Conflicts of interest

The authors declare that they have no competing interests.

Acknowledgements

We are grateful for grants from Natural Science Foundation of China (No. 21766029).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra00729g

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