Augmenting the Photocatalytic Performance of Direct Z-Scheme Bi2O3/g-C3N4 Nanocomposite

Abstract

Huge demands for photocatalytically efficient visible-light-induced catalysts have spurred widespread interest in building adaptable heterojunctions. Here, we used in situ thermal polymerization to synthesise the Z-scheme Bi₂O₃/g-C₃N₄ heterojunction. The optical, structural, chemical, compositional and photocatalytic behaviours of the samples were analysed through various analytical techniques and photocatalytic methylene blue (MB) dye degradation reaction. Among the various ratios of Bi₂O₃/g-C₃N₄ heterojunction composites, the 1:1 ratio showed improved visible-light-induced catalytic activity, which attained 91.2% degradation efficiency after 120 min of visible-light exposure. The dye degradation efficiency was calculated under various environmental conditions by varying the dye concentration, solution pH and catalyst dosage. A improved Z-scheme photocatalytic mechanism was proposed in light of the results. A potential mechanism was suggested to explain the photocatalytic activity, and trapping experiments supported it. Last but not least, this strategy might be helpful to prepare the heterojunction photocatalyst for the degradation of organic pigments.

1. Introduction

Overpopulation and the depletion of natural resources have had a serious impact on ecosystems and human life. Conventional physicochemical approaches have not adequately addressed the issue of ecosystem quality and health [1]. Rapid urbanization and industrialization have created significant global environmental issues with the release of large amounts of untreated wastewater into the ecosystem. Generally, wastewater contains pollutants, including pesticides, organic dyes, heavy metals and pharmaceutical waste. In particular, several synthetic dyes, such as methylene blue (MB), methyl orange (MO), rhodamine B (RhB), congo-red (CR), malachite green (MG) and methyl red (MR), are widely used in the paint, printing and textile industries. These organic dyes are highly stable due to their complex structure; they are not bio-degradable and have a carcinogenic effect on the water environment and human health [2,3,4]. Researchers have developed several treatment methods to reduce the impact of wastewater. However, most waste treatment methods have failed to remove these toxic pollutants from wastewater. Finding a potential alternative has become crucial for this purpose. Various eco-friendly advanced oxidation processes have been widely used in multiple areas of toxic agent removal. Specifically, semiconductor-based photocatalysts have gained much attention due to their high degradation efficiency cost-effectiveness and wide utilisation range of the solar spectrum [5]. In particular, metal oxides and their composites have been used as catalysts for the photocatalytic pollutant removal process, using solar energy utilization for the conversion of organic dyes into harmless products [2]. Several nanostructured metal oxides, such as TiO₂, ZnO, CdO, CeO₂, CuO, Co₃O₄, Fe₂O₃, WO₃, V₂O₅, and NiO, have been developed for photocatalytic degradation applications. Among these materials, TiO₂ and ZnO have received much attention due to their high redox ability. However, traditional TiO₂ and ZnO photocatalysts are excited by only 2 to 4% of the solar spectrum. In addition, faster recombination of photo-generated excitons in single-phase components forbids the extension of activity [6]. So, great interest has been paid to the development of photocatalysts comprising larger exciton separation, wider absorption of visible spectra and better efficiency retention over multiple cycles.

The adjustable electronic structure of Bi₂O₃, from around 2 to 3.96 eV, has stimulated researchers’ attention. It offers easy synthesis procedures, is earth-abundant, has a favourable electronic arrangement, and demonstrates a high harvesting efficiency of the solar spectrum. Its efficiency in the removal of organic dyes has been analysed [7]. Unfortunately, the material fails to meet requirements due to the rapid recombination of charge carriers, which greatly affects its widespread application at the industrial level. To overcome the stated drawback, several strategies have been used. Some of them are (i) structural modification, (ii) doping, (iii) surface modification and (iv) developing a heterostructure [8]. Developing a heterostructure was found to be the most successful way of reducing the rate of recombination. In addition, it also enhanced photocatalytic degradation efficiency. As of now, a number of semiconductors, such as CeO₂, MoS₂ and BiOCl, have successfully been linked with Bi₂O₃ to create a stable structure that is conducive to the degradation of organic dyes such as methylene blue, methyl orange and malachite green [9].

Following the publication of a study by Wang et al. showing that graphitic carbon nitride demonstrates photocatalytic activity for the generation of H₂ and O₂ through water splitting and the decolorization of organic pollutants using visible spectrum, g-C₃N₄ gained widespread use as a novel metal-free visible-light photocatalyst [10,11]. It potentially withstands chemicals and temperature because of the s-triazine ring and high condensation. The presence of abundant nitrogen provides a greater number of active sites compared to other carbon, which is conductive in the photocatalytic mechanism [12,13].

It is anticipated that the efficient combination of g-C₃N₄ and Bi₂O₃ will answer the demand for an effective photocatalyst. The combination also offers a Z-scheme photocatalytic system, which is more effective in photocatalytic activity than single-phase materials. Compared with the traditional composite systems, the Z-scheme heterojunction systems offer stronger redox capability and higher photocatalytic efficiency [14]. A Z-scheme heterojunction system is formed between two semiconductors due to the short distance between the CB of one semiconductor and VB of the another semiconductor. In this circumstance, the electron at the CB of one semiconductor would be quickly combined with the VB holes of the another semiconductor. Therefore, the excited CB electrons in one semiconductor exhibit strong reduction reaction, while the holes in VB of the another semiconductor produce an excellent oxidation reaction [15]. In 2014, Jinfeng Zhang and co-authors synthesised a Z-scheme Bi₂O₃/g-C₃N₄ nanocomposite by the ball milling assisted calcination method. They reported that the prepared nanocomposites showed better photocatalytic activity compared to those of pure nanoparticles. Moreover, they reported that the optimal composite exhibited 78.1% and 45.6 % of TOC removal efficiency against MB and RhB dye, respectively. Further, they proved that the nanocomposite degraded the dye molecules through the Z-scheme mechanism. He et.al reported the Z-scheme type degradation of phenol pollutants using in situ fabricated Bi₂O₃/g-C₃N₄ nanocomposite at room temperature conditions. They showed that the room temperature in situ process helps to produce quantum-sized Bi₂O₃ on g-C₃N₄ nanosheets. This heterojunction helps to enhance the light absorption ability and phenol degradation efficiency [14].

From the literature survey, Bi₂O₃ has a valence band (VB) and conduction band (CB) at 3.13 and 0.33 eV, respectively. In addition g-C₃N₄ has a valence band and conduction band at 1.57 and −1.13 eV, respectively. When g-C₃N₄ and Bi₂O₃ are combined in this way, the electrons from Bi₂O₃’s conduction band quickly interact with the holes on g-C₃N₄’s valence band. As a result, holes in the valence band of Bi₂O₃ demonstrate stronger oxidation ability than photogenerated electrons on the conduction band of g-C₃N₄ [16,17,18,19].

In this study, we used a straightforward approach to successfully couple Bi₂O₃ with g-C₃N₄, an oxidation type semiconductor with a reduction type semiconductor. Th single-step solid state thermal polymerization method is used to prepare the nanocomposites. This process helps to intercalate the Bi₂O₃ nanoparticles into the g-C₃N₄ interlayers, so that the composite helps to enhance the optical and catalytic behaviour compared with pure nanoparticles. The impact of Bi₂O₃ content on the physical and chemical characteristics as well as on the photocatalytic activity of the composites is thoroughly examined. In light of this, a direct Z-scheme mechanism is discovered for produced composites, which mostly accounts for the augmentation of photoactivity in Bi₂O₃/g-C₃N₄ composites [20]. This research offers a practical method for designing and building g-C₃N₄-based Z-scheme heterojunction photocatalysts for use in solar energy conversion and water purification.

2. Results and Discussion

The synthesised samples’ structure and phase purity was analysed by XRD diffraction pattern and is shown in Figure 1. According to the XRD pattern shown in the figure, pure g-C₃N₄ has two distinctive peaks at 27.73° (002) and 13.1° (100), which correspond to long-range interplanar stacking of the aromatic system and in-plane structural packing, or the sequential distance between the tris triazine units. The existence of new peaks that correspond to Bi₂O₃ nanoparticles, as shown in the Bi₂O₃/g-C₃N₄ XRD pattern, confirms the incorporation of Bi₂O₃. Obviously, the prepared samples are crystalline in nature, and the different peaks are in good arrangement with the Joint Committee on Powder Diffraction Standards (JDPDS) Card No (87-1526) and (78-1793) for g-C₃N₄ and Bi₂O₃, respectively. The absence of peaks related to potential contaminants suggests that the processed samples were pure. We observed a slight decrement in the intensity of the spectra of Bi₂O₃ with the increase in the ratio of g-C₃N₄. The average crystallite size of the prepared nanoparticles was calculated by Scherrers’ formula [2]:

(1)$\mathrm{Crystallite} \mathrm{Size} (\mathrm{D})= \frac{\mathrm{k}\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}$

The calculated crystallite size of the pure Bi₂O₃ nanoparticle is ~34 nm, which is little larger than the 1:1 (23 nm) and 3:1 (30 nm) nanocomposite. This result indicates that the addition of g-C₃N₄ reduces the crystallite size of the Bi₂O₃ nanoparticle in the composite system. This reduction of crystallite size is reflected in the diffraction peak intensity reduction and peak broadening.

Morphology of the as-synthesized samples was examined by SEM analysis and is shown in Figure 2. The SEM image of g-C₃N₄ describes that it has a stacked nanosheet structure and it is well matched with the literature [15]. The pure Bi₂O₃ nanoparticle exhibits sub-micron level particle–like morphology and is shown in Figure 2b. The nanocomposite shows the combination of nanosheets and nanoparticles, which reflects the presence of both g-C₃N₄ nanosheets and Bi₂O₃ nanoparticles. It can be clearly seen from the SEM analysis that the nanoparticles are deposited on the g-C₃N₄ nanosheets. Further, the EDS analysis was performed on 1:1 nanocomposite to determine the elemental distribution and is shown in Figure 2. The EDS spectrum and elemental distribution micrographs show the presence of C, N, Bi and O in the nanocomposite. The EDS spectrum does not show any other peaks other than the above-mentioned elements, which indicates the purity of the prepared sample.

Further, TEM analysis (Figure 3) was performed to illustrate the heterojunction between the nanoparticles (g-C₃N₄ and Bi₂O₃) in the nanocomposite and particle size of the nanoparticle. The TEM image clearly shows that the Bi₂O₃ nanoparticles are deposited on the g-C₃N₄ nanosheet and form a strong heterojunction. The measured particle size of the Bi₂O₃ nanoparticle is about ~20–35 nm in size, which is in good agreement with the crystallitie size of the Bi₂O₃ nanoparticle in the 1:1 g-C₃N₄/Bi₂O₃ nanocomposite. The TEM image clearly shows that the Bi₂O₃ nanoparticles are highly agglomerated, exhibit particle-like morphology, and are anchored on the g-C₃N₄ nanosheet. The HRTEM image of the nanocomposite confirms that a strong interfacial interaction exists between the nanoparticles. The measured d-space value of 0.29 nm is related to the plane (220) of Bi₂O₃ nanoparticle. The polycrystalline nature of the as-prepared nanocomposite is clearly shown by the SAED pattern. The heterojunction between the nanoparticles can be helpful to enhance the optical, electrical and photocatalytic activity.

Figure 4 shows the FT-IR spectra of pure and composite materials to identify the functional groups presented in the nanoparticles. The breathing mode of the s-triazine units and the stretching vibration of the C-N and C=N in heterocycles are attributed to the main absorption band in the range of 805, and 1240–1600 cm⁻¹ in the spectrum of g-C₃N₄ and its composites. The broad band seen in the composite spectrum, which is around 3184 cm⁻¹ and 3400 cm⁻¹, is attributed to the N-H and O-H stretching vibration, respectively. Compared with g-C₃N₄, a small hump was observed at ~1408 cm⁻¹ due to the bending vibrations of C-O and C=O [18]. The peaks at 530 cm⁻¹ correspond to the bending vibration of Bi₂O₃. The broad transmittance peak positioned at 3400 cm⁻¹ is attributed to the surface adsorbed water molecule.

The UV-Vis-DRS spectrum is used to analyse the optical absorption behaviour of the prepared nanoparticles. The absorption spectrum for pure and 1:1 nanocomposites is shown in Figure 5a. The pure g-C₃N₄ shows an absorption cutoff wavelength around 450 nm, whereas the pure Bi₂O₃ nanoparticle is absorbed at 480 nm. From the UV-Vis absorption spectrum, it is clear that the pure and composite photocatalyst can be activated by visible light irradiation. The visible-light absorption behaviour of g-C₃N₄ reflects the n-π* transition of the nitrogen atom lone pair of electrons in the heptazine unit [21]. It is noticed that the absorption wavelength of the 1:1 nanocomposite exhibits red shift compared to pure g-C₃N₄ and shows blue shift compared with Bi₂O₃. The blue shift of the nanocomposite compared to Bi₂O₃ may reflect the existence of the quantum confinement effect or interaction of low bandgap materials (g-C₃N₄). However, the nanocomposite shows an absorption cutoff wavelength of around 470 nm, which is comparatively higher than bare g-C₃N₄. This optical absorption enhancement reveals that the interfacial interaction between the nanoparticles helps to improve the light utilisation behaviour, and it can be helpful to increase the photocatalytic efficiency. The bandgap of the pure nanoparticles is calculated by Tauc’s plot and is shown in Figure 5. The calculated bandgap of g-C₃N₄ and Bi₂O₃ is 2.91 eV and 2.78 eV, respectively.

Further, the PL analysis was conducted to determine the charge separation efficiency of the nanocomposite for effective photocatalytic activity. The emission spectrum for g-C₃N₄, Bi₂O₃ and 1:1 ratio nanocomposites is shown in Figure 5b. When compared to pure g-C₃N₄ and Bi₂O₃, the nanocomposite’s PL emission intensity was significantly lower; this indicates that the photo-excited electron–hole pairs are separated for the photocatalytic degradation reaction. Thus, it is confirmed that the nanocomposite is an effective visible-light active material for a photocatalytic reaction by optical analyses.

Evaluation of Photocatalytic Activity

The degradation of MB dye under ambient LED light irradiation allowed us to examine the photocatalytic activity of the as-prepared samples. Figure 6 shows the relationship between time and dye concentration at various times (t). Figure 6a makes it clear that the breakdown of MB dyes by the photolysis process exhibits minimal efficiency in the absence of catalysts. The MB dye degradation is increased in nanocomposites compared to pure photocatalysts. Pure g-C₃N₄ and Bi₂O₃ nanoparticles show relatively lower degradation efficiency, which reflects the lower charge separation behaviour and poor visible-light utilization behaviour. The nanocomposites show higher degradation efficiency compared to pure samples; in particular, the 1:1 nanocomposite exhibits maximum degradation efficiency compared to any other photocatalysts. The 1:1 nanocomposite achieved 91.2% efficiency after 120 min of visible-light irradiation. This efficiency is about 4.8- and 2.85-fold higher than g-C₃N₄ and Bi₂O₃ photocatalysts, respectively. The recorded MB degradation efficiency of 1:3 and 3:1 nanocomposite is 46.8% and 64.2%, respectively. The improved MB dye degradation efficiency demonstrates that the interfacial interaction between the nanoparticles helps to improve the light utilization efficiency and helps to separate the photo-induced electron–hole pairs.

The following relation was used to compute the dye degradation process’ rate constant [22,23].

ln (C/C₀) = −kt(2)

where C₀ represents the dye solution concentration at time t = 0, C represents the dye solution concentration at time ‘t’ and k signifies the rate constant.

The calculated rate constant (Figure 6b) of the 1:1 nanocomposite is 0.016 min⁻¹ for MB dye degradation after 120 min of LED light irradiation. The “k” value for MB dye degradation using g-C₃N₄ and Bi₂O₃ is 0.001 min⁻¹ and 0.003 min⁻¹, respectively.

Therefore, it can be inferred from the degradation process that the binary composite performs best in visible light when compared to pure catalysts. The heterojunction between g-C₃N₄ and Bi₂O₃, which functions as an electron acceptor to enhance the passage of electrons and effectively inhibits the photo exciton recombination, is what gives the binary composite its improved photocatalytic performance. As a result, the charge carrier recombination effect was significantly diminished, and the photo-induced electron hole pairs were separated. The heterojunctional g-C₃N₄/Bi₂O₃ photocatalyst would therefore be a promising candidate for the oxidation of organic dyes when exposed to visible light.

In addition, an elemental trapping experiment using scavengers helped to identify the active radical responsible for the MB dye degradation process. We employed benzoquinone (BQ), triethanolamine (TEOA) and isopropanol (IPA) as scavengers for superoxide, holes and hydroxyl radicals. Figure 6c, showing the trapping experimental plot, demonstrates that the addition of IPA considerably reduced the dye degradation process and decreased the degradation efficiency of binary composites. Efficiency can be reduced by up to 29% when using a hydroxyl radical scavenger. The degradation reaction’s efficiency was slightly altered by the addition of BQ. Therefore, it was determined from the scavenger trapping experiment that the *OH radicals are the main active ingredients in the breakdown of methylene blue. The obtained scavenging data indicate that reactive species played an order of OH* > h⁺ > O₂* function in the photocatalytic degradation of MB.

Further, the recycling test was conducted using the 1:1 Bi₂O₃/g-C₃N₄ nanocomposite for MB degradation under LED illumination. The sample was recovered by the centrifugal process and washed with ethanol/water several times after each cycle. Then, the sample was dried in a hot air oven for further recycling. To estimate the stability of the as-prepared nanocomposite, four successive recycling tests were conducted, and the results are shown in Figure 6d. From the graph, it is clear that the MB dye degradation efficiency is slightly decreased with increasing the recycle count. However the as-prepared nanocomposite exhibited 83.4% of MB dye degradation efficiency under visible-light irradiation. The decreased degradation efficiency is attributed to the loss of photocatalysts during the recycling process or active site reduction in every recycling process. However, the 1:1 nanocomposite exhibits enough stability for the recycling process of MB dye degradation. To highlight the presented work, a comparison table was plotted, as shown in Table 1.

Furthermore, the degradation experiment was conducted under different environmental conditions to elucidate the behaviour of the prepared nanocomposite, and the results are shown in Figure 7. Initially, MB dye degradation experiments were conducted under different solution pH and are shown in Figure 7a. The solution pH was adjusted with the help of diluted HCl solution and diluted NH₃ solution. The prepared nanocomposite degraded the MB dye up to 93.8% after 120 min of LED irradiation under alkaline conditions. This is slightly higher than the efficiency obtained under neutral pH conditions. This enhancement is attributed to formation of more superoxide radicals under alkaline conditions compared to neutral or acidic conditions.

Industrial wastewater contains different concentrations of organic pollutants; therefore, it is necessary to study the influence of dye concentration in photocatalytic degradation reaction. The photocatalytic experiment was conducted using different concentrations of dye solution with neutral pH conditions and 50 mg of photocatalyst. The result clearly shows that the MB degradation decreases with increasing dye concentration. This result reveals that a higher concentration of dye molecules may be adsorbed on the photocatalyst surface, so that it can reduce the active sites of the photocatalyst or limit the visible-light utilization.

Furthermore, the dye degradation study was extended for different concentrations of photocatalyst, and the results are shown in Figure 7c. The MB dye degradation efficiency is increased with increasing the catalyst dosage, and it attains 98.9% efficiency at 100 mg of photocatalyst concentration. The increasing MB dye degradation efficiency is attributed to the availability of more active sites for the degradation reaction.

Based on the experimental results, it is necessary to elucidate the possible MB dye degradation mechanism using the Bi₂O₃/g-C₃N₄ nanocomposite. The possible mechanism is illustrated in Figure 8. The band potential of the g-C₃N₄ and Bi₂O₃ nanoparticle was calculated using the following relation [3]:

E_CB = χ − E_e − 0.5E_g(3)

E_VB = E_CB + E_g(4)

where χ signifies the electronegativity, E_e represents the energy of the free electron in the hydrogen scale, and E_g denotes the bandgap energy. E_CB and E_VB reflect the energy level of the conduction band and valance band, respectively. The calculated valance band potential of g-C₃N₄ and Bi₂O₃ is 1.75 eV and 3.11 eV, respectively. The conduction band of the nanoparticle is −1.16 eV for g-C₃N₄ and 0.33 eV for Bi₂O₃.

According to the band potential of the nanoparticles, bandgap energy and scavengers’ test, a possible MB dye degradation mechanism was proposed, and the schematic of the heterojunction charge transfer is shown in Figure 8. From the band potential values, it can be clearly noticed that the VB potential of Bi₂O₃ and CB potential of g-C₃N₄ is adequate to execute the oxidation and reduction reaction, respectively. The VB of Bi₂O₃ has a higher value, oxidizing the water molecule into •OH radicals, compared to the standard oxidation (1.98 V vs. NHE) potential. The CB potential of g-C₃N₄ has higher reduction potential to reduce the oxygen molecule into superoxide radicals in comparison with standard reduction potential (−0.33 V vs. NHE) [24]. However, the CB of Bi₂O₃ and VB of g-C₃N₄ are not suitable for reduction and oxidation reaction due to lesser band potential compared to the standard redox potential values. Therefore, it not possible to generate the superoxide radicals and hydroxyl radicals at CB of Bi₂O₃ and VB of Bi₂O₃, respectively. The conventional type-Ⅱ heterojunction is not suitable for demonstrating the MB dye degradation mechanism of the Bi₂O₃/g-C₃N₄ nanocomposite due to inconsistency with the scavengers’ result. Therefore the reaction mechanism is described by the Z-scheme process with the help of the trapping experiment and literature reports. Both of the semiconductors are visible-light active materials and can produce electron–hole pairs during the light irradiation. According to the Z-scheme mechanism, the excited conduction band electrons from the Bi₂O₃ are transferred to and combined with the valance band holes of the g-C₃N₄ through the interfacial contact. Thereafter, the combined electrons are again excited to the CB of g-C₃N₄ during the light incident. In this way, the electrons and holes are separated for photocatalytic reaction. Therefore, the accumulated CB electrons at g-C₃N₄ and holes at VB of Bi₂O₃ generate the superoxide radicals and hydroxyl radicals for the degradation reaction. This element of the Z-scheme mechanism is well matched with the scavengers’ experiment and the literature reports [23,24,25,26]. The possible charge transport and degradation mechanism of the Bi₂O₃/g-C₃N₄ nanocomposite is given below [1,2,3,4]:

Bi₂O₃ + hυ → e⁻ (Bi₂O₃) + h⁺(Bi₂O₃)(5)

g-C₃N₄ + hυ → e⁻ (g-C₃N₄) + h⁺(g-C₃N₄)(6)

e⁻ (g-C₃N₄) + O₂ → •O₂⁻(7)

•O₂⁻ + e⁻ + 2H⁺ → H₂O₂(8)

•O₂⁻ + H₂O₂ → •OH + OH⁻ + O₂(9)

2H₂O₂ → 2•OH(10)

h⁺ (Bi₂O₃) + OH^-/H₂O → Bi₂O₃ + •OH(11)

•O₂⁻/•OH + dye → Intermediate product → CO₂ + H₂O(12)

3. Materials and Methods

Melamine was directly polymerized at 500 °C for 4 h and 520 °C for 2 h in order to prepare g-C₃N₄. A stoichiometric quantity of Bi(NO₃) and melamine was first dissolved in 10 mL CH₃OH to synthesis the Bi₂O₃/g-C₃N₄ nanocomposite. After stirring for two hours, the homogenous suspension was dried at 100 °C. The dry mixture was then ground and annealed for 4 h at a rate of 5 °C/min at 500 °C and 520 °C for 2 h. The sample procedure was followed to prepare pure Bi₂O₃. The sample was then designated as Bi₂O₃/g-C₃N₄ x:x, where x is the mass of Bi₂O₃ added to g-C₃N₄. By changing the mass of Bi₂O₃ under the same reaction conditions, samples of 1:3, 1:1 and 3:1 Bi₂O₃ to g-C₃N₄ were created [27,28].

Photocatalysis Experiments

In a quartz reactor, the samples’ photocatalytic activity was tested on the degradation of MB dye. First, the as-prepared photocatalyst (50 mg) was added to 50 mL of MB (20 mg/L) dye solution. A water circulation system was used to maintain the solution’s constant stirring in a magnetic stirrer at a reaction temperature of 27 to 30 °C in order to achieve homogenous dispersion. Prior to irradiation, the dye and photocatalyst suspension was constantly agitated for 30 min in the absence of radiation to achieve adsorption and desorption equilibrium. Due to its high-intensity emission in the visible region, the commercial 40 W LED bulb was employed as a source of visible light. The average LED light intensity that was observed was around 37,032 lux. Samples were removed and evaluated using UV-Spectroscopy at 664 nm for MB, which corresponds to the maximal absorption wavelength of dyes, at regular intervals [29]. Under LED illumination, a trapping experiment using various scavengers was conducted to assess the involvement of reactive spices in the degrading process of dye. As scavengers for superoxide anion, hole (h⁺), hydroxyl (•OH) radicals, 0.1 mmol of benzoquinone, 0.1 mmol of ethylenediaminetetraacetic acid, and 5 mL of isopropanol were utilised [30].

4. Conclusions

In conclusion, the binary heterojunction was synthesised using a straightforward technique. The XRD result clearly shows that the crystallite size of the Bi₂O₃ nanoparticles was reduced, while the heterojunction formation enables strong optical utilization behaviour compared to the pure samples. Further, the interaction between the nanoparticles helps to increase the electron–hole separation, which promotes higher MB dye degradation efficiency. Among all the as-synthesised samples, the 1:1 ratio of the Bi₂O₃/g-C₃N₄ nanocomposite exhibits the highest degradation efficiency after 120 min of LED light illumination. The synergistic effect between g-C₃N₄ and Bi₂O₃, which actively slows down the rate of charge carrier recombination and has a broad absorption spectrum for visible light, was responsible for the improved photocatalytic performance. As a result, the heterojunction photocatalyst degraded the dye molecule through the Z-scheme charge transfer process, which was supported by the scavengers’ experiment and band potential calculation. Hence, this simple method can be helpful to for the large-scare preparation of photocatalyst with higher efficiency.

Author Contributions

Conceptualization, B.D.; software, M.P. and M.A.M.; validation, K.M.; formal analysis, P.T.; investigation, R.R. and B.P.; resources, P.T.; data curation, K.M.; writing—original draft, K.M., M.P., B.P. and B.D.; writing—review and editing, B.P. and B.D.; visualization, M.A.M.; supervision, B.D.; project administration, M.S. and B.D.; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through a Research Groups Program under Grant No. R.G.P. 2/215/43.

Conflicts of Interest

The authors declare that there is no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Table

Figure 1. XRD pattern of (a,c) pristine g-C₃N₄ and Bi₂O₃; (b,d,e) different weight percentages of Bi₂O₃/g-C₃N₄ nanocomposite.

Figure 2. SEM image of (a) g-C3N4, (b) Bi2O3, (c) 1:1 Bi2O3/g-C3N4; (d) EDS spectrum/elemental mapping of 1:1 Bi2O3/g-C3N4 nanocomposite.

Figure 3. TEM (a), HRTEM (b) and SAED (c) images of 1:1 Bi2O3/g-C3N4 nanocomposite.

Figure 4. FTIR spectra of prepared samples.

Figure 5. (a) UV-Vis-DRS spectra and (b) PL emission spectra.

Figure 6. (a) Degradation plot, (b) pseudo 1st order kinetics, (c) scavenger’s test and (d) recycling test.

Figure 7. MB dye degradation under different conditions: (a) solution pH, (b) dye concentration and (c) catalyst dosage.

Figure 8. Possible reaction mechanism.

Table 1

Comparison table.

No.	Catalyst	Pollutant (ppm)	Irradiation Time (min)	Efficiency (%)	Reference
1	ZnO-V2O5-WO3	MB (10 ppm)	80 min	99.8%	[2]
2	PANI-CeO2-Fe2O3-NiO	MB (5 ppm)	50 min	99%	[3]
3	Bi2O3/g-C3N4	Phenol (10 ppm)	150 min	>70%	[14]
4	Bi2O3/g-C3N4	MB (10 ppm)	120 min	>90%	[15]
5	Bi2O3/g-C3N4	Rh B (10 ppm)	80 min	98%	[18]
6	Bi2O3/g-C3N4	Amido Black (10 ppm)	120 min	88.71%	[19]
7	NiFe2O4/CD/g-C3N4	Rh B (10 ppm)	120 min	97%	[21]
8	Bi2O3/g-C3N4	MB (10 ppm)	120 min	91.2%	This work

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