Fabrication of Z-scheme ZnO/g-C3N4/ZnS nanocomposites using high power laser for methylene blue degradation

Photocatalysis plays a vital role in addressing environmental challenges by harnessing solar energy for efficient pollutant degradation. In this study, we investigate the photocatalytic activity of a ZnO/g-C3N4/ZnS composite system in the degradation of methylene blue, a widely used dye with detrimental effects on aquatic ecosystems. The composite materials were synthesized using a facile and scalable approach, and their structural properties, morphologies, sizes, and elemental compositions were characterized using different analytical techniques. The ZnO/g-C3N4/ZnS composite exhibited enhanced photocatalytic performance compared to individual components. Remarkably, the degradation efficiency reached 80% for the composite with a 30% ZnO composition, surpassing the efficiencies of ZnS alone (29%) and ZnS/g-C3N4 (65%). The composite’s higher degradation efficiency is due to synergistic semiconductor effects, enhancing charge transfer and reducing electron–hole recombination. ZnO incorporation increases active sites and surface area, improving interaction with methylene blue. The favorable band edge positions of ZnO aligned with ZnS and g-C3N4, facilitating the utilization of a broader spectrum of solar light. The composite’s photocatalytic activity was achieved under UV light irradiation, demonstrating its potential for sustainable and energy-efficient applications. This study highlights the significance of composite design and the Z-scheme concept in photocatalysis, offering insights into the development of advanced materials for environmental remediation. The findings contribute to the understanding of efficient solar-driven pollutant degradation and pave the way for the design and optimization of innovative photocatalytic systems for sustainable environmental solutions.


Introduction
Organic dyes are the predominant class of contaminants found in effluent from the textile, paper printing, rubber and plastic manufacturing, wood, and silk industries.It is well known that the dyes used in these industrial processes are toxic and carcinogenic, posing a grievous threat to human and animal health (Litter 1999).According to estimates, 12% of the synthetic textile dyes used each year, such as red indigo, Red 120, Rhodamine B, Methylene Blue, and Eriochrome Black T, are lost during the manufacturing process, and 20% of these colors are found in industrial effluent and thus the environment.Because organic dyes are resistant to light and oxidizing agents, biological, physical, and chemical processes (adsorption or coagulation) are ineffective at removing them (Vaiano et al 2017).
As a component of a class of technologies known as Advanced Oxidation Technologies, the technique use for wastewater treatment and air purification is now the primary focus of research and development in the area (AOTs).In contrast to conventional technologies such as activated carbon, these methods eliminate contaminants (Babuponnusami and Muthukumar 2014) (Vaiano et al 2017).In this manner, organic and inorganic compounds, as well as microorganisms, are degraded or transformed into less hazardous forms.All AOTs share the ability to generate highly reactive free radicals, predominantly hydroxyl radicals.AOTs include thermal H 2 O 2 or ozone reactions in addition to light-induced processes such as direct homogeneous photolysis (including vacuum UV), UV/H 2 O 2 photolysis, UV/ozone photolysis, heterogeneous photolysis, radiolysis, and indirect electrolysis (Vaiano et al 2017).
Photocatalysis refers to a photoinduced process that is accelerated by the presence of a catalyst.Photocatalysis is becoming a viable option for the Photocatalytic Degradation of Organic Dyes because it can utilize sunlight to drive the disinfection process using a solid catalyst such as titanium dioxide (Ramalingam et al 2022) (Gorito et al 2022).The primary oxidant is the highly reactive hydroxyl radical (radical dot OH), which can also deactivate viruses, bacteria, spores, and protozoa (Dalrymple et al 2010).Recent research has focused on the use of photocatalysis to remove pigments from wastewaters, in part because this technique has the potential to completely mineralize the desired contaminants (Silva et al 2006).
When a photon carrying sufficient energy is absorbed by a catalyst, it initiates a reaction by promoting an electron from the valence band to the conduction band of the semiconductor catalyst, leading to charge separation.While most studies focus on TiO 2 as a photocatalyst, it has limitations such as deactivation in the presence of ion scavengers and slow degradation rates with high dye concentrations in water (Dalrymple et al 2010).ZnO emerges as a promising alternative semiconductor for photocatalysis.It exhibits comparable degradation efficiency to TiO 2 (Kondo et al 2013) (Fernández et al 1995) and does not suffer from deactivation in the presence of ion scavengers (Akpan and Hameed 2009).Therefore, ZnO demonstrates potential as a viable photocatalyst for various applications, offering advantages over TiO 2 in terms of performance and stability.
The exploration of heterogeneous photocatalysis for the degradation of various organic pollutants has shown promising potential in breaking down dyes in water using solar or artificial light (Kondo et al 2013).However, when dealing with high concentrations of dyes, the penetration of light radiation throughout the entire solution volume becomes a challenging task (Ramalingam et al 2022).This presents a significant hurdle for the effective photocatalytic treatment of aqueous solutions containing a high concentration of dyes.
Utilizing single-component materials for photocatalysis is not as efficient as doping two or more photocatalysts (Li et al 2022).Poor charge carrier separation and rapid recombination of electron-hole pairs limit the performance of single-component photocatalysts.In contrast, doping enhances photocatalytic activity, charge carrier separation, spectral range, selectivity, and stability (Eshete et al 2023, Li et al 2022).Introducing dopant atoms modifies the electronic band structure, reducing the bandgap and extending the absorption range.This enables more efficient generation of electron-hole pairs and promotes enhanced light utilization .Doping also facilitates charge carrier separation, preventing recombination and extending their lifetime.Specific dopants can be used to customize selectivity or improve the degradation of specific pollutants.Additionally, doping enhances the stability and durability of photocatalysts, making them more resistant to oxidation and photocorrosion (Nair et al 2022).
Carbon nitride (g-C 3 N 4 ) has garnered significant interest in material science due to its layered structure, distinctive electronic band structure, and various desirable properties (Chaves et al 2020, Nair et al 2022).The g-C 3 N 4 allotrope, with its wide stability range, conjugated electronic structure, and bandgap of 2.7 eV, is extensively used in water splitting, selective oxidation, and environmental pollutant degradation (Kumar et al 2016, Wudil et al 2023).Pure g-C 3 N 4 , however, has limited photocatalytic activity due to poor charge carrier separation and quick recombination.Doping or coupling g-C 3 N 4 with other materials enhances its photocatalytic efficacy (Kumar et al 2016, Wudil et al 2023).
Zinc oxide (ZnO) is widely studied for photocatalysis due to its low cost, non-toxicity, and excellent activity.However, pure ZnO has limitations that can be overcome by doping with other elements (Lam et al 2012) Doping modifies the band structure, enabling absorption of visible light and improving solar energy utilization.It enhances charge carrier dynamics, suppresses recombination, and increases photocatalytic activity.Doping also introduces active sites and defects, enhancing catalytic efficiency and allowing for tailored selectivity (Lam et al 2012).
Zinc sulfide (ZnS) is a promising photocatalyst that utilizes visible light, making it suitable for solar energy conversion and pollutant degradation.It exhibits excellent photocatalytic activity, generating reactive species upon light absorption (Chankhanittha et al 2023).ZnS offers chemical stability, a high surface area for increased contact with pollutants, and customization potential through doping.These properties make ZnS attractive for various photocatalytic applications (Chankhanittha et al 2023).
The Z-scheme is crucial in photocatalysis as it efficiently utilizes solar energy for catalytic processes (Murillo-Sierra et al 2022, Schumacher and Marschall 2022).By combining semiconductors with appropriate energy levels, the Z-scheme enables enhanced light absorption, effective charge separation, and improved catalytic performance.It expands the usable solar spectrum, prevents electron-hole recombination, and synergizes different semiconductor properties (Abdul Nasir et al 2021, Dong et al 2018).There are numerous reports based on Z-scheme photocatalytic degredations.For instance, in reference (Soltani et al 2012), a two-step hydrothermal method was employed to design Z-scheme CdS/WS 2 heterojunctions.These heterojunctions exhibit high-efficiency photocatalytic degradation for organic dyes and photoreduction for Cr(VI).Similarly in (Xie et al 2020), a Z-scheme In 2 O 3 /WO 3 heterogeneous photocatalyst system was designed for degradation of methylene blue and rhodamine B with promising degradation efficiency.Lastly, in (Mullakkattuthodi et al 2022), authors fabricated Fe 2 O 3 / g-C 3 N 4 based on Z-scheme for degrading methylene blue.
This study introduces a novel aspect to the field of photocatalysis by focusing on the unique combination of ZnO nanosphere, g-C 3 N 4 , and ZnS in a Z-scheme configuration for the degradation of methylene blue.While previous research has explored various Z-scheme photocatalytic systems for the degradation of organic dyes and reduction of toxic compounds, such as Cr(VI) and others (George et al 2022, Jayaraman et al 2015), the specific utilization of ZnO nanosphere/ g-C 3 N 4 /ZnS in the degradation of methylene blue has not been extensively studied before.
By employing ZnO/g-C 3 N 4 /ZnS photocatalysts, this study aims to accelerate the degradation process of methylene blue under UV light.The successful breakdown of methylene blue into harmless byproducts not only reduces its toxicity but also minimizes its environmental impact.This novel Z-scheme photocatalyst system offers several key advantages, including high efficiency, broad-spectrum activity, and the utilization of renewable solar energy.Additionally, this approach promotes environmentally friendly processes by avoiding the use of harsh chemicals and minimizing the generation of harmful byproducts.By exploring the photocatalytic activity of ZnO /g-C 3 N 4 /ZnS in the degradation of methylene blue, this study presents a novel and innovative contribution to the literature The results obtained from this research can pave the way for further advancements in the design and development of Z-scheme photocatalytic systems for the treatment of wastewater, environmental remediation, and the removal of hazardous pollutants, ultimately contributing to the preservation and protection of aquatic ecosystems.The presented research introduces a pioneering approach in photocatalysis, employing high-power laser synthesis to fabricate Z-scheme ZnO/g-C3N4/ZnS nanocomposites.This method allows significant control over structural properties, morphologies, and compositions, enabling precise tuning for optimal photocatalytic performance.The ZnO/g-C3N4/ZnS composite system demonstrates an 80% degradation efficiency in methylene blue, a significant enhancement due to the synergistic effects of the unique fabrication process.This aspect emphasizes the innovative nature of the synthesis technique.The design and synthesis of the Z-scheme system within the composite material contribute to efficient charge separation and transfer, resulting in high photocatalytic activity.The study also highlights potential environmental applications, including water purification and dye degradation.The scalability and effectiveness of the approach offer promising real-world prospects, aligning the research with broader sustainability goals.Finally, the comprehensive characterization provides insightful analyses of the synthesized materials, contributing to the understanding of their unique properties.It establishes a substantial milestone, opening avenues for further exploration and application.

Samples preparation
ZnS/g-C 3 N 4 /ZnO nanocomposite samples were synthesized using the pulsed laser ablation technique (Scheme 1) (Gondal et al 2009(Gondal et al 2012)).To synthesize ZnS nanoparticles, we added 50mg of ZnS salt to a glass vial filled with 10 mL of deionized water.Nd:YAG laser beam that operates at 130 mJ, 355 nm, 10Hz to focus underneath the surface of the ZnS colloidal.After 30 min of laser irradiation under vigorous stirring, the colloidal solution of ZnS was obtained.To synthesize the ZnS/g-C 3 N 4 and ZnS/g-C 3 N 4 /ZnO nanocomposites, the powder of ZnS, g-C 3 N 4 , and ZnO, were mixed, maintaining a weight ratio of ZnS to g-C 3 N 4 of 1:1 in all mixtures.The weight ratio of ZnO to 0%, 5%, and 30% respectively was varied.We then added the mixture to a glass vial filled with 10 mL and synthesized the nanocomposites using the same pulsed laser ablation technique as before.

Characterization
The surface morphology and structure of the individually prepared nanomaterials (ZnO, g-C 3 N 4 , and ZnS) as well as the ZnO/g-C 3 N 4 /ZnS nanocomposite, synthesized via pulsed laser ablation (PLAL), were comprehensively analyzed using scanning electron microscopy (SEM) (TESCAN VEGA 3, Czech Republic) for macroscopic imaging and transmission electron microscopy (TEM) (Morgagni 268, FEI, Czech Republic) for detailed characterization.For SEM and energy dispersive X-ray spectroscopy (EDX) analysis, the sample suspensions were securely affixed onto metallic stubs using double-sided carbon tape, while for TEM analysis, the sample suspensions were deposited onto carbon film-coated copper grids and dried prior to transfer into the microscope.

Photocatalytic degradation experiments
To study the catalytic performance of the obtained nanocomposites, MB dye degradation experiments were conducted under UV light at room temperature 25 °C.In details, 5 mg of the nanocomposites was dispersed in 50 mL of 10 ppm MB dye and stirred for 30 min in the dark at ambient conditions to establish the adsorptiondesorption equilibrium of the MB dye on the catalyst's surfaces.After 30 min, the mixture was irradiated with a UV lamp (9 W) and the degradation of MB dye was investigated by UV-vis spectrophotometer (SolidSpec-3700) with a 1 cm quartz cell every 10 min.

Structural analysis
The structures of the prepared samples were examined using X-ray diffraction (XRD), and the corresponding results, presented in figure 1, showed characteristic diffraction patterns consistent with ZnO, ZnS, and g-C 3 N 4 .The diffraction peaks confirmed the presence of ZnO in the samples, matching the wurtzite phase (JCPDS: 36-1451) (Yergaliuly et al 2022).The XRD patterns of ZnS exhibited perfect matches with the face-centered cubic (FCC) structure of ZnS (ICDD PDF 65-1691) (Soltani et al 2012).The XRD analysis also revealed a broad and strong peak at 27.3°for g-C 3 N 4 , corresponding to the interlayer stacking of graphite-like conjugated triazine aromatic sheets (JCPDS 87-1526) (Kumar et al 2016).The presence of these diffraction peaks in the XRD patterns of the ZnO/g-C 3 N 4 /ZnS nanocomposite confirmed the successful fabrication of the nanocomposite, validating its composition.

Morphological analysis
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are powerful techniques used for morphological analysis in materials science.SEM provides high-resolution images of the surface topography, allowing for the characterization of particle size, shape, and surface features, while TEM offers even higher resolution, enabling the examination of internal structures and finer details at the nanoscale.The individual components, ZnO, g-C 3 N 4 , and ZnS, as well as the ZnO/g-C 3 N 4 /ZnS nanocomposite, were characterized using SEM and TEM techniques.The purpose of the characterization was to determine the size, shape, and structure of the individual components and to confirm the formation of the ZnO/ g-C 3 N 4 /ZnS nanocomposite.The SEM and TEM results, presented in figures 2 and 3, provided valuable insights into the morphology of the materials.The SEM and TEM analyses revealed that ZnO exhibited solid and hollow spherical structures, while g-C 3 N 4 appeared as flakes or sheets.The size and shape of the g-C 3 N 4 flakes/sheets varied between a few micrometers and several micrometers in length.The dimensions of the ZnO spheres ranged from 1 to 5 μm.ZnS, on the other hand, appeared as very small aggregated nanoparticles arranged in a nanoparticle matrix structure.The ZnO/g-C 3 N 4 /ZnS nanocomposite displayed a compact morphology, consisting of a combination of ZnO spheres, g-C 3 N 4 sheets, and ZnO nanoparticles.The SEM and TEM results confirmed the successful preparation of the ZnO/g-C 3 N 4 /ZnS nanocomposite using the pulsed laser ablation technique in liquid.

Chemical analysis
The composite material was subjected to chemical analysis to assess the presence of its individual components, which was accomplished using energy dispersive x-ray spectroscopy (EDX) spectroscopy (figure 4).The obtained EDX results unequivocally confirmed the presence of carbon (C), nitrogen (N), oxygen (O), zinc (Zn), and sulfur (S), which collectively constitute all five components of the composite.Additionally, EDX mapping images visually demonstrated the uniform distribution of each individual element within the composite.This finding serves as robust evidence supporting the successful formation of a homogeneous ZnO/g-C 3 N 4 /ZnS nanocomposite.In order to further confirm the chemical states and surface components of ZnO/g-C 3 N 4 /ZnS nanocomposite, the composite was investigated by XPS and the results are displayed in figure 5. From the survey spectrum, it could be seen that the composite material contains Zn, S, O, and C elements.Zn 2p spectrum shows the presence of both ZnO and ZnS.According to values reported in the literature, the ZnO for the Zn 2p spectra exhibits peak-shit toward greater binding energy than pristine ZnO reveals the charge transfer between the nanocomposite compounds (Postica et al 2019).Due to its asymmetry of XPS spectrum of S 2p, three evident peaks located at 163.0 eV, 165.9 eV, 167 are deconvoluted in figure 5, which are attributed to S 2−

Optical analysis
The optical properties of ZnS, ZnS/g-C 3 N 4 and ZnO/g-C 3 N 4 /ZnS nanocomposites were evaluated by UV-vis diffuse reflectance spectroscopy as shown in figure 6. ZnS/g-C 3 N 4 and ZnO/g-C 3 N 4 /ZnS nanocomposites show strong UV-vis light absorption in the range of 200 nm-350 nm, which is in agreement with their high photocatalytic activities compared with ZnS nanoparticles.The bandgap of ZnS nanoparticles, ZnS/g-C 3 N 4 and ZnO/g-C 3 N 4 /ZnS nanocomposites was estimated by equation (1) where α is the coefficient of absorption, hν is the photon energy and A is a constant correlated to specific material (Al-Kuhaili et al 2014).The computation of the bandgaps of the material was carried out to observed the impact of bandgap modulation on the photocatalytic degradation efficiency.The photocatalytic efficiency of a material is strongly correlated with its bandgap, which governs the ability to absorb photons and generate electron-hole pairs.Figure 6 shows the Tauc's plot ((αhν) 2 versus hν) and the obtained bandgap values for ZnS nanoparticles, Zns/g-C 3 N 4 and ZnO/g-C 3 N 4 /ZnS nanocomposites were 3.6 eV, 3.4 eV and 2.76 eV, respectively (George et al 2022).Based on the obtained results, the addition of g-C 3 N 4 nanoparticles reduced the bandgap of ZnS/g-C 3 N 4 to 3.4 eV.The reduction in the bandgap of ZnS/g-C 3 N 4 nanocomposites is attributed to their increase in the

Photocatalytic degradation of methylene blue (MB)
The photocatalytic performance of ZnS nanoparticles and ZnS/g-C 3 N 4 , 10% ZnO/g-C 3 N 4 /ZnS and 30% ZnO/g-C 3 N 4 /ZnS nanocomposites was evaluated by measuring the MB degradation under ultraviolet light irradiation (UV).Figure 7 illustrates process of photocatalytic degradation of MB dye using different catalysts.Figure 7(a) demonstrates the degradation using ZnS nanoparticles, while figures 7(b)-(d) exhibit the degradation using ZnS/g-C 3 N 4 , 10% ZnO/g-C 3 N 4 /ZnS, and 30% ZnO/g-C 3 N 4 /ZnS nanocomposites, respectively.These figures show the removal of MB by different photocatalyst as a function of time.For each of the photocatalyst, the removal process progresses with time and the best removal efficiency was obtained for 30% ZnO/g-C 3 N 4 /ZnS as exhibited in figure 7(d).
The degradation efficiency (D) of MB samples was calculated using the degradation rate calculation formula as shown in equation (2).
where C o is the concentration of the MB solution at time t = 0, and C t is the concentration of the MB solution at the time = t (George et al 2022).Bare ZnS nanoparticles shows only 29% degradation of MB dye under ultraviolet light irradiation in 90 min.However, the ZnS/g-C 3 N 4 nanocomposites show 65% degradation efficiency of MB dye in 90 min under similar conditions.This means that the use of ZnS nanoparticles alone is not sufficient to cause significant catalytic degradation of MB dye, and this is because ZnS nanoparticles themselves are unable to absorb the photon beyond 300 nm (Raza et al 2023).On the other hand, 10% ZnO/g-C 3 N 4 /ZnS and 30% ZnO/g-C 3 N 4 /ZnS nanocomposites display significant degradation efficiency of about 73% and 80% under similar conditions, as shown in figure 7(b).The higher degradation efficiency of 10% ZnO/g-C 3 N 4 /ZnS and 30% ZnO/g-C 3 N 4 /ZnS nanocomposites can be attributed to the heterogeneous structure of ZnO/ZnS and the good dispersion of ZnO/ZnS on the surface of g-C 3 N 4 nanoparticles which enhanced the photocatalytic adsorption and reduced the recombination rate of electron hole (Shirazi et al 2023) (Dong et al 2018).In the present study, we meticulously examined the bandgaps of ZnS, ZnS/g-C 3 N 4 , and ZnO/g-C 3 N 4 /ZnS composites by varying the percentages of ZnO, with the goal of discerning their influence on   the photocatalytic degradation efficiency.The findings in terms of computed bandgaps and observed degradation efficiencies have furnished essential insights, elucidated as follows: 1.For ZnS (Bandgap of 3.6 eV): A degradation efficiency of 29% was achieved over 90 min., indicating moderate photocatalytic activity.The relatively elevated bandgap constrains the absorption of light, thereby diminishing its photocatalytic capability.
2. For ZnS/g-C 3 N 4 Composite (Bandgap of 3.4 eV): By forming a composite with g-C 3 N 4 , the bandgap was reduced, leading to a significant enhancement in degradation efficiency to 65%.This finding exemplifies how  bandgap tuning augments photon absorption and expedites charge transfer, culminating in superior photocatalytic performance.
3. For 10% ZnO/g-C 3 N 4 /ZnS Composite (Bandgap of 2.76 eV): The integration of 10% ZnO resulted in a degradation efficiency of 73%.This optimized bandgap enables more extensive utilization of the solar spectrum, fostering electron-hole pair creation and amplifying the photocatalytic reaction rate.
4. For 30% ZnO/g-C 3 N 4 /ZnS Composite: By elevating the ZnO percentage to 30%, we ascertained the peak degradation efficiency of 80%.The additional decrease in the bandgap assures more proficient absorption and employment of UV light, culminating in a more vigorous and resilient photocatalytic response.
The manifest correlation between the reduction in bandgap and the improvement in degradation efficiency emphasizes the critical role of tailoring the electronic bandgap of the materials to realize the desired photocatalytic effects (Xiong et al 2018).This study, therefore, contributes not only to the scientific understanding of these materials but also to the practical application in environmental remediation and energy conversion technologies.
The kinetics associated with the UV light degradation of the MB dye using ZnS nanoparticles and ZnS/g-C 3 N 4 , 10%ZnO/ZnS/g-C 3 N 4 and 30%ZnO/ZnS/g-C 3 N 4 nanocomposites were utilized using the Langmuir-Hinshelwood kinetic model as shown in equation (3).To demonstrate that the composite material facilitates effective separation of charge carriers, photoluminescence (PL) measurements were conducted for the ZnO and the composite, specifically the 30% ZnO/g-C 3 N 4 /ZnS (Chawla et al 2008, Zhang andMu 2007).These measurements provided insights into the electron transitions and energy band alignments, underscoring the role of the composite structure in enhancing charge separation efficiency, a critical factor in the photocatalytic degradation of substances such as MB. Figure 9(b) provides the PL measurements of two photocatalysts, ZnO and 30% ZnO/g-C 3 N 4 /ZnS, which were analyzed over a wavelength range of 360 nm to 400 nm.The PL spectra displayed a normal distribution, with a prominent peak observed for ZnO, while the composite catalyst, 30% ZnO/g-C 3 N 4 /ZnS, exhibited a reduced intensity, approximately half of that of ZnO.The high PL intensity of ZnO, although indicative of a higher number of electron-hole pairs due to strong radiative recombination, is not favorable for photocatalytic reactions (Etacheri et al 2015).The presence of surface defects or traps might lead to this higher luminescence, hindering the efficient separation of electron-hole pairs required for effective photocatalysis (Etacheri et al 2015, Wang et al 2021).On the other hand, the reduction in PL intensity for the composite catalyst reveals a more promising scenario for photocatalytic applications.The lower intensity suggests better separation of electronhole pairs, with fewer recombining radiatively.The synergy between ZnO, g-C 3 N 4 , and ZnS in the composite facilitates a favorable band alignment, thus enhancing charge separation and transfer.This observation aligns well with the efficiency values and rate constants presented in table 1.The composite catalyst demonstrated a remarkable degradation efficiency of 80%, along with a higher rate constant of 0.02272 min −1 , as compared to pure ZnS with a degradation efficiency of 29%.The reduced PL intensity signifies more effective utilization of photon energy, which translates into increased photocatalytic performance.In conclusion, the PL measurements provide valuable insights into the underlying photocatalytic behavior of the materials.The trends observed in PL intensity are consistent with the photocatalytic efficiency and reaction rates, underscoring the importance of charge separation in achieving superior performance.

Conclusion
This study focuses on the photocatalytic activity of ZnO/g-C 3 N 4 /ZnS in the degradation of methylene blue.Methylene blue degradation is important for environmental remediation and wastewater treatment due to its detrimental effects on aquatic ecosystems.Using ZnO /g-C 3 N 4 /ZnS photocatalysts, the degradation process can be accelerated.It was observed that when using ZnS alone as the catalyst, a degradation efficiency of 29% was achieved while the combination of ZnS with g-C 3 N 4 resulted in a significant improvement, with a degradation efficiency of 65%.Further enhancement was observed with the incorporation of ZnO in the ZnO/ g-C 3 N 4 /ZnS composites, reaching degradation efficiencies of 73% and 80% for the 10% and 30% ZnO compositions, respectively.Further research and optimization of the composite composition and synthesis methods can pave the way for the development of even more advanced and effective photocatalytic systems for a wide range of environmental applications.

Scheme 1
Scheme 1 Schematic representation of the fabrication of the ZnO/g-C 3 N 4 /ZnS nanocomposite.
o and C t are the concentration of MB at C = 0 and t, respectively(Shirazi et al 2023).The plots of Ln(C t /C o ) versus time shows that the photocatalytic degradation of MB obeys pseudo-first-order kinetics as shown in figure 8(a).Based on the obtained results, 30% ZnO/g-C 3 N 4 /ZnS nanocomposite shows higher value of pseudo-first-order rate constant (k = 0.02272 min −1 ) compared to bare ZnS (k = 0.0049 min −1 ) and ZnS/g-C 3 N 4 nanocomposite (k = 0.01316 min −1 ).Additionally, the obtained results show that the degradation efficiency and the rate constant increased as the content of ZnO nanoparticles increased from 10% to 30% as shown in figures 8(a) and (b).The degredation efficiency of the photocatalysts are summarized in table 1.
3.6.Double Z-scheme mechanism for ZnO/g-C 3 N 4 /ZnS nanocomposite.Figure 9 illustrates the Z-scheme mechanism for the ZnO/g-C 3 N 4 /ZnS composite.Upon exposure to simulated solar light, the ZnO/ZnS/g-C 3 N 4 ternary structure undergoes a series of electron transitions essential for the photocatalytic degradation of MB (Murillo-Sierra et al 2022, Schumacher and Marschall 2022).Electrons in the valence bands (VB) of ZnO, ZnS, and g-C 3 N 4 are excited to their respective conduction bands (CB) (Alansi et al 2018).This initiation triggers a Z-shaped flow of electrons.Electrons in the CB of ZnS transfer to the VB of g-C 3 N 4 .Concurrently, electrons in the CB of ZnO transition to the VB of ZnS (Etacheri et al 2015, Wang et al 2021).This process aligns with the energy band alignments (Wang et al 2019).The transitions lead to the accumulation of electrons in the CB of g-C 3 N 4 and holes in the VB of ZnO, minimizing recombination and enhancing efficiency (Dong et al 2018).Subsequently, the positive potential in ZnO's VB generates hydroxyl radicals, while the negative potential in g-C 3 N 4 's CB drives reduction reactions (Mullakkattuthodi et al 2022, Wang et al 2022).These reactive species interact with MB, leading to its decomposition (figure 9(a)).Key factors include the double Z-scheme, which enhances charge separation efficiency, and the role of g-C 3 N 4 , expanding light absorption to the visible region.In summary, the ZnO/ZnS/g-C 3 N 4 structure's alignment of energy bands, Z-shaped electron flow, and material composition provides an effective solution for MB degradation.This mechanism offers a practical approach to environmental protection and sustainability, with broad applications.

Figure 9 .
Figure 9. (a) A plausible mechanism showing double Z-scheme e/h pair transfer occurring onto the surface of ternary ZnO/g-C 3 N 4 / ZnS under UV-vis radiation, and (b) PL spectra of ZnO and 30% ZnO/g-C 3 N 4 /ZnS nanocomposite.

Table 1 .
The efficiency values and k app for each photocatalyst.