Microwave-Assisted Synthesis of SnO2@ZnIn2S4 Composites for Highly Efficient Photocatalytic Hydrogen Evolution

This research successfully synthesized SnO2@ZnIn2S4 composites for photocatalytic tap water splitting using a rapid two-step microwave-assisted synthesis method. This study investigated the impact of incorporating a fixed quantity of SnO2 nanoparticles and combining them with various materials to form composites, aiming to enhance photocatalytic hydrogen production. Additionally, different weights of SnO2 nanoparticles were added to the ZnIn2S4 reaction precursor to prepare SnO2@ZnIn2S4 composites for photocatalytic hydrogen production. Notably, the photocatalytic efficiency of SnO2@ZnIn2S4 composites is substantially higher than that of pure SnO2 nanoparticles and ZnIn2S4 nanosheets: 17.9-fold and 6.3-fold, respectively. The enhancement is credited to the successful use of visible light and the facilitation of electron transfer across the heterojunction, leading to the efficient dissociation of electron–hole pairs. Additionally, evaluations of recyclability demonstrated the remarkable longevity of SnO2@ZnIn2S4 composites, maintaining high levels of photocatalytic hydrogen production over eight cycles without significant efficiency loss, indicating their impressive durability. This investigation presents a promising strategy for crafting and producing environmentally sustainable SnO2@ZnIn2S4 composites with prospective implementations in photocatalytic hydrogen generation.


Introduction
Recently, excessive environmental pollution and severe energy shortages have adversely affected human life and health, prompting research into renewable energy sources [1].Hydrogen, characterized by its zero-carbon emission properties, is cost-effective, sustainable, and environmentally friendly, making it poised to become a crucial energy source shortly [2].As an efficient and green method, photocatalysis has been extensively studied for energy conversion, utilizing inexhaustible solar energy to address the issues [3].Through photocatalytic reactions, solar energy is converted into chemical energy using photocatalysts to decompose water and produce hydrogen [4].Various semiconductor metal oxide photocatalysts have recently emerged, showcasing elevated photocatalytic efficiency, non-hazardous properties, economic feasibility, and robust chemical resilience [5,6].Among them, semiconductor metal oxide materials like tin dioxide (SnO 2 ) have attracted considerable attention because of their outstanding physical and chemical characteristics [7][8][9].While predominantly employed for the photocatalytic degradation of organic pollutants, their use in the photocatalytic water splitting process for hydrogen generation remains somewhat limited [10].
Tin dioxide (SnO 2 ) is classified as an n-type semiconductor material, possessing a wide band gap of approximately 3.6 eV at room temperature [8,11,12].It is currently regarded as one of the most promising metal oxides due to its non-toxicity, reliable stability, and excellent optoelectronic properties [13].SnO 2 has been widely utilized in gas sensors, electrode materials, lithium-ion batteries, solar cells, and photocatalytic degradation of organic and inorganic pollutants [14,15].It possesses good electron mobility and a large specific surface area, facilitating its use in photocatalysis and electron transfer [16].However, its relatively wide bandgap exhibits optimal photocatalytic activity only in ultraviolet, with reduced performance under visible light [17].Therefore, incorporating other materials or doping other metals is necessary to enhance SnO 2 's absorption of visible light and consequently improve its visible light photocatalytic activity [18][19][20].
Zinc indium sulfide (ZnIn 2 S 4 ) is a ternary metal sulfide known for its layered structure, exhibiting three different crystal phases: cubic, hexagonal, and rhombohedral [21][22][23][24].This photocatalyst is considered a conventional material for visible light absorption and is categorized as a direct band gap semiconductor with adjustable band gaps ranging from 2.06 to 2.85 eV [23,25,26].ZnIn 2 S 4 exhibits minimal toxicity, straightforward synthesis, and exceptional chemical durability, rendering it extensively investigated in areas such as photocatalysis, thermoelectricity, and electrochemical energy storage [27,28].However, the material's ability to catalyze through light exposure is limited by quick recombination rates of electron-hole pairs, low mobility of charge carriers, and structural flaws [29,30].To address these limitations, elemental doping and heterojunction construction increase catalytic active sites and enhance photocatalytic reaction rates [31][32][33].In previous studies, the combination of SnO 2 and ZnIn 2 S 4 has not been explored for the photocatalytic decomposition of tap water to produce hydrogen.This study addresses the respective limitations of SnO 2 and ZnIn 2 S 4 by combining them to form SnO 2 @ZnIn 2 S 4 composites for enhancing their photocatalytic tap water splitting.The composites showed significantly higher efficiency than pure SnO 2 nanoparticles or ZnIn 2 S 4 nanosheets, credited to visible light utilization and improved electron transfer across the heterojunction, leading to efficient separation of electron-hole pairs.Additionally, the composites demonstrated remarkable recyclability, maintaining high levels of photocatalytic hydrogen production over eight cycles without significant efficiency loss, suggesting their impressive durability and potential for environmentally sustainable applications in photocatalytic hydrogen generation.

Fabrication of SnO 2 Nanoparticles
An adapted protocol from the existing literature was employed to synthesize SnO 2 nanoparticles utilizing a microwave-assisted synthesis approach [16].A total of 0.45 g of Tin(IV) chloride, 0.1 g of citric acid, and 0.4 g of sodium hydroxide were dissolved in 50 mL of deionized water and subjected to ultrasonic vibration for approximately 20 min to ensure homogeneous mixing.Subsequently, the solution was transferred to a 100 mL Teflon reaction vessel and subjected to microwave-assisted hydrothermal heating at 180 • C for 60 min.Upon cooling the reaction to ambient temperature, the solution underwent multiple washes with deionized water and ethanol, followed by centrifugation for 3 min, and ultimately dried in a 70 • C oven to produce the white powder.

Fabrication of SnO 2 @ZnIn 2 S 4 Composites
The SnO 2 @ZnIn 2 S 4 composites were synthesized using a microwave-assisted synthesis method.Different weights of SnO 2 nanoparticles were mixed uniformly with zinc chloride (1.25 mM), indium chloride (2.5 mM), and TAA (5 mM) in a ratio of 1:3 with ethanol and deionized water, treated with ultrasonication, transferred into 100 mL Teflon reaction bottle, and underwent a microwave-assisted hydrothermal method to heat at 180 • C for 60 min.Once the reaction concluded and cooled to ambient temperature, the solution underwent rinsing with deionized water and ethanol, followed by centrifugation for 3 min, and subsequently dried in a 70 • C oven to yield the yellow powder of the composite material.

Photocatalytic Hydrogen Production Experiment
A multi-port reaction system produced hydrogen through photocatalysis with specially made photocatalysts.This setup involved stirring with a magnetic stirrer and a 5 W blue LED light source with a peak wavelength of 420 nm powered via PCX50 B Discover with Perfect Light Technology from Beijing, China.In accordance with the standard procedure, 25 mg of the photocatalysts were introduced into a solution comprising 0.1 M of different sacrificial agents (including Na 2 S, Na 2 SO 3 , CH 2 O 2 , and CH 3 OH) and 50 mL of deionized water.When using 0.1 M Na 2 S as the sacrificial reagent in deionized or tap water, the pH values were 13.0 and 12.9, respectively.Before the commencement of the experiment, a degassing process was conducted for a duration of 10 min to eliminate air from the system.Subsequently, the generated hydrogen was measured utilizing gas chromatography (GC, Shimadzu GC-2014, Kyoto, Japan) equipped with a thermal conductivity detector (TCD).

Results and Discussion
Characterization of SnO 2 @ZnIn 2 S 4 Composites Figure 1 illustrates the reaction process of SnO 2 @ZnIn 2 S 4 composites using a two-step microwave-assisted hydrothermal method.Firstly, SnO 2 nanoparticles were prepared by microwave-assisted hydrothermal treatment of SnO 2 precursor (SnCl 4 , NaOH, and citric acid) at 180 • C for 60 min.Subsequently, varying amounts of SnO 2 nanoparticles were dispersed in the precursor solution of ZnIn 2 S 4 (ZnCl 2 , InCl 3 , and TAA), followed by another round of microwave-assisted hydrothermal treatment at 180 • C for 60 min to obtain the SnO 2 @ZnIn 2 S 4 composites.

Results and Discussion
Characterization of SnO2@ZnIn2S4 Composites Figure 1 illustrates the reaction process of SnO2@ZnIn2S4 composites using a two-step microwave-assisted hydrothermal method.Firstly, SnO2 nanoparticles were prepared by microwave-assisted hydrothermal treatment of SnO2 precursor (SnCl4, NaOH, and citric acid) at 180 °C for 60 min.Subsequently, varying amounts of SnO2 nanoparticles were dispersed in the precursor solution of ZnIn2S4 (ZnCl2, InCl3, and TAA), followed by another round of microwave-assisted hydrothermal treatment at 180 °C for 60 min to obtain the SnO2@ZnIn2S4 composites.The morphology of the SnO2 nanoparticles and SnO2@ZnIn2S4 composites synthesized using the two-step microwave-assisted hydrothermal method was examined using an FESEM, as shown in Figure 2. The SnO2 nanostructures are depicted, comprising numerous nanoparticles with a diameter of approximately 20 nm, uniformly dispersed and aggregated, as shown in Figure 2a. Figure 2b displays the FESEM image of the SnO2@ZnIn2S4 composites following the incorporation of ZnIn2S4.Notably, a multitude of nanoparticles is observed covering the nanosheet structure.Furthermore, flower-like structures with a similar laminar stacking arrangement can be observed in the low-magnification FESEM image (Figure 2c).Further analysis of the distribution of individual elements through FESEM-EDS mapping is illustrated in Figure 2d.It can be observed from the figures that the SnO2@ZnIn2S4 composites contain Sn, O, Zn, In, and S, and they are evenly distributed.Therefore, it can be demonstrated that SnO2 and ZnIn2S4 successfully combine to form SnO2@ZnIn2S4 composites.The morphology of the SnO 2 nanoparticles and SnO 2 @ZnIn 2 S 4 composites synthesized using the two-step microwave-assisted hydrothermal method was examined using an FESEM, as shown in Figure 2. The SnO 2 nanostructures are depicted, comprising numerous nanoparticles with a diameter of approximately 20 nm, uniformly dispersed and aggregated, as shown in Figure 2a. Figure 2b displays the FESEM image of the SnO 2 @ZnIn 2 S 4 composites following the incorporation of ZnIn 2 S 4 .Notably, a multitude of nanoparticles is observed covering the nanosheet structure.Furthermore, flower-like structures with a similar laminar stacking arrangement can be observed in the low-magnification FESEM image (Figure 2c).Further analysis of the distribution of individual elements through FESEM-EDS mapping is illustrated in Figure 2d.It can be observed from the figures that the SnO 2 @ZnIn 2 S 4 composites contain Sn, O, Zn, In, and S, and they are evenly distributed.Therefore, it can be demonstrated that SnO 2 and ZnIn 2 S 4 successfully combine to form SnO 2 @ZnIn 2 S 4 composites.
cles and SnO2@ZnIn2S4 composites at 486.7 eV and 495.1 eV, respectively, as depicted in Figure 4b.These peaks are identified as the Sn 3d5/2 and Sn 3d3/2 peaks of SnO2 [35].The O 1s binding energy of the SnO2 nanoparticles and SnO2@ZnIn2S4 composites is analyzed in Figure 4c.It is separated into two peaks with binding energies at 530.6 eV and 531.8 eV, representing lattice oxygen (O lattice, OL) and oxygen-deficient region (O defect, OD) in SnO2, respectively [36,37].This outcome shows many oxygen defects in the SnO2 nanoparticles and SnO2@ZnIn2S4 composites.The Zn 2p spectrum displays two distinct peaks at 1021.3 eV and 1044.3 eV, corresponding to the Zn 2p3/2 and Zn 2p1/2 states, respectively, as illustrated in Figure 4d [32].The In 3d spectrum (Figure 4e) reveals peaks at 445.0 eV and 452.5 eV, which are associated with the In 3d5/2 and In 3d3/2 states of ZnIn2S4.These findings align with existing literature [38].The S 2p spectrum (Figure 4f) exhibits two distinct peaks located at 161.6 eV and 162.8 eV, which have been identified as corresponding to the S 2p3/2 and S 2p1/2 states, respectively [39].These results collectively confirm the presence of Zn 2+ , In 3+ , and S 2− chemical states, with no indications of impurity peaks in the spectra.TEM analysis was performed to examine the morphology and crystal structure of the SnO2@ZnIn2S4 composites more efficiently.Figure 5a displays the TEM image, revealing the complete integration of SnO2 nanoparticles with ZnIn2S4 nanosheets.Figure 5b exhibits the SAED pattern of SnO2@ZnIn2S4 composites, showing a polycrystalline ring.The crystal structures consist of tetragonal SnO2 (PDF# 01-075-2893) and hexagonal ZnIn2S4 (PDF# 04-009-4783).Moreover, upon examination using high-resolution transmission electron microscopy (HRTEM) imaging (Figure 5c), lattice spacings of 0.264 nm indicative of the (011) plane of SnO2 (PDF# 01-075-2893) and 0.322 nm corresponding to the (101) TEM analysis was performed to examine the morphology and crystal structure of the SnO 2 @ZnIn 2 S 4 composites more efficiently.Figure 5a displays the TEM image, revealing the complete integration of SnO 2 nanoparticles with ZnIn 2 S 4 nanosheets.Figure 5b exhibits the SAED pattern of SnO 2 @ZnIn 2 S 4 composites, showing a polycrystalline ring.The crystal structures consist of tetragonal SnO 2 (PDF# 01-075-2893) and hexagonal ZnIn 2 S 4 (PDF# 04-009-4783).Moreover, upon examination using high-resolution transmission electron microscopy (HRTEM) imaging (Figure 5c), lattice spacings of 0.264 nm indicative of the (011) plane of SnO 2 (PDF# 01-075-2893) and 0.322 nm corresponding to the (101) plane of ZnIn 2 S 4 (PDF# 04-009-4783) were identified.This observation serves to validate the crystal structures of SnO 2 and ZnIn 2 S 4 .TEM-EDS mapping image (Figure 5d) was performed for qualitative and semi-quantitative compositional analysis of SnO 2 @ZnIn 2 S 4 composites, revealing an even distribution of the individual elements Sn, O, Zn, In, and S within the composites.This outcome confirms the effective production of composite materials consisting of SnO 2 and ZnIn 2 S 4 .
The photocatalytic efficiency of the fabricated SnO 2 nanoparticles was assessed by quantifying the hydrogen evolution rate (HER) in 50 mL deionized water under blue LED light irradiation.In aqueous settings, sacrificial agents are commonly employed to boost the effectiveness of oxidation reactions in the photocatalytic water splitting process for hydrogen production [40].Figure 6a illustrates the impact of four sacrificial agents (Na 2 S, Na 2 SO 3 , CH 2 O 2 , and CH 3 OH) on the photocatalytic performance of SnO 2 nanoparticles without pH adjustment.All sacrificial agents were utilized at a uniform concentration of 0.1 M. The average hydrogen evolution rate (HER) with SnO 2 nanoparticles follows the order: Na 2 S (22.8 µmol , and CH 3 OH (9.8 µmol•h −1 •g −1 •L −1 ).A possible ratio- nale behind this phenomenon could be the adsorption of sulfide ions (S 2− , originating from dissociated Na 2 S) onto the photocatalyst.These ions could interact with photogenerated positive charges, thereby impeding the recombination of electron-hole pairs [41].Therefore, Na 2 S was used as a sacrificial reagent in the subsequent reactions.
Materials 2024, 17, x FOR PEER REVIEW 7 of 14 plane of ZnIn2S4 (PDF# 04-009-4783) were identified.This observation serves to validate the crystal structures of SnO2 and ZnIn2S4.TEM-EDS mapping image (Figure 5d) was performed for qualitative and semi-quantitative compositional analysis of SnO2@ZnIn2S4 composites, revealing an even distribution of the individual elements Sn, O, Zn, In, and S within the composites.This outcome confirms the effective production of composite materials consisting of SnO2 and ZnIn2S4.The photocatalytic efficiency of the fabricated SnO2 nanoparticles was assessed by quantifying the hydrogen evolution rate (HER) in 50 mL deionized water under blue LED light irradiation.In aqueous settings, sacrificial agents are commonly employed to boost the effectiveness of oxidation reactions in the photocatalytic water splitting process for hydrogen production [40].Figure 6a illustrates the impact of four sacrificial agents (Na2S, Na2SO3, CH2O2, and CH3OH) on the photocatalytic performance of SnO2 nanoparticles without pH adjustment.All sacrificial agents were utilized at a uniform concentration of 0.1 M. The average hydrogen evolution rate (HER) with SnO2 nanoparticles follows the order: Na2S (22. , and CH3OH (9.8 µmol•h −1 •g −1 •L −1 ).A possible rationale behind this phenomenon could be the adsorption of sulfide ions (S 2− , originating from dissociated Na2S) onto the photocatalyst.These ions could interact with photogenerated positive charges, thereby impeding the recombination of electron-hole pairs [41].Therefore, Na2S was used as a sacrificial reagent in the subsequent reactions.
To further boost the effectiveness of photocatalytic water splitting for hydrogen production using SnO2 nanoparticles, this investigation also integrated 0.02 g of SnO2 nanoparticles into different precursor materials before the reaction.Subsequently, the efficacy of photocatalytic water splitting for hydrogen generation using SnO2 nanoparticles in conjunction with various materials was examined under blue LED light exposure, as depicted in Figure 6b.The optimal efficiency for photocatalytic water splitting for hydrogen production (408.9µmol•h −1 •g −1 •L −1 ) is achieved when SnO2 nanoparticles react with ZnIn2S4 precursor materials to form SnO2@ZnIn2S4 composites.This result indicates a 17.9-fold increase in efficiency compared to SnO2 nanoparticles alone, demonstrating that the composite with ZnIn2S4 effectively enhances the photocatalytic efficiency of the material.
Figure 6c illustrates the performance variation in photocatalytic water splitting to produce hydrogen for pure SnO2 nanoparticles, ZnIn2S4 nanosheets, and SnO2@ZnIn2S4 composites formed by reacting different weights of SnO2 nanoparticles with ZnIn2S4 reaction precursors under blue LED light irradiation.Adding 0.02 g of SnO2 nanoparticles exhibits the optimal photocatalytic water splitting efficiency for hydrogen production (408.9 . This result represents a significant enhancement compared to pure SnO2 nanoparticles (22. ) by approximately 17.9-fold and 6.3-fold, respectively.This finding verifies that incorporating an appropriate weight of SnO2 nanoparticles assists in preparing SnO2@ZnIn2S4 composites with optimal hydrogen production efficiency.Figure 7a displays the UV-visible absorption spectra of SnO2 nanoparticles and SnO2@ZnIn2S4 composites.In contrast to SnO2 nanoparticles, the absorption edge of SnO2@ZnIn2S4 composites demonstrates a significant redshift.Moreover, the absorption intensity of SnO2@ZnIn2S4 composites is notably enhanced in the wavelength range of λ > 450 nm, which augments the generation of photogenerated carriers and enhances photocatalytic performance under visible light.Photoluminescence (PL) spectroscopy is a tech- To further boost the effectiveness of photocatalytic water splitting for hydrogen production using SnO 2 nanoparticles, this investigation also integrated 0.02 g of SnO 2 nanoparticles into different precursor materials before the reaction.Subsequently, the efficacy of photocatalytic water splitting for hydrogen generation using SnO 2 nanoparticles in conjunction with various materials was examined under blue LED light exposure, as depicted in Figure 6b.The optimal efficiency for photocatalytic water splitting for hydrogen production (408.9µmol•h −1 •g −1 •L −1 ) is achieved when SnO 2 nanoparticles react with ZnIn 2 S 4 precur- sor materials to form SnO 2 @ZnIn 2 S 4 composites.This result indicates a 17.9-fold increase in efficiency compared to SnO 2 nanoparticles alone, demonstrating that the composite with ZnIn 2 S 4 effectively enhances the photocatalytic efficiency of the material.
Figure 7a displays the UV-visible absorption spectra of SnO 2 nanoparticles and SnO 2 @ZnIn 2 S 4 composites.In contrast to SnO 2 nanoparticles, the absorption edge of SnO 2 @ZnIn 2 S 4 composites demonstrates a significant redshift.Moreover, the absorption intensity of SnO 2 @ZnIn 2 S 4 composites is notably enhanced in the wavelength range of λ > 450 nm, which augments the generation of photogenerated carriers and enhances photocatalytic performance under visible light.Photoluminescence (PL) spectroscopy is a technique that utilizes the fluorescence produced when unbound charge carriers recombine, providing valuable information on the movement, transfer, and separation of electron-hole pairs generated by light in semiconductor materials [6,42].Figure 7b reveals the measured PL emission spectra of SnO 2 nanoparticles and SnO 2 @ZnIn 2 S 4 composites.SnO 2 @ZnIn 2 S 4 composites exhibit significantly lower emission intensity than SnO 2 nanoparticles, indicating suppression of photogenerated hole-electron recombination in the composites.Photocurrent measurements are conducted to evaluate the efficiency of photogenerated electron-hole pair separation for photocatalytic performance.As depicted in Figure 7c, the photocurrent intensity of SnO 2 @ZnIn 2 S 4 composites exceeds that of SnO 2 nanoparticles, demonstrating the synergistic effect between SnO 2 and ZnIn 2 S 4 in suppressing photogenerated electron-hole recombination.
Figure 8 illustrates the potential mechanism underlying the photocatalytic watersplitting process on the SnO 2 @ZnIn 2 S 4 composites when subjected to blue LED light irradiation.An ion exchange resin is first applied to the indium tin oxide (ITO) glass, creating a coating of SnO 2 and ZnIn 2 S 4 .Subsequently, the flat band potential is determined using cyclic voltammetry [43].The determined valence band (VB) and conduction band (CB) positions of SnO 2 and ZnIn 2 S 4 align with established findings.In particular, the conduction band positions for SnO 2 and ZnIn 2 S 4 are recorded at −0.14 eV and −0.58 eV, respectively, while their valence band positions are noted at 3.35 eV and 1.78 eV, respectively [15,[44][45][46].In addition, previous XPS O1s (Figure 4c) can verify that SnO 2 nanoparticles and SnO 2 @ZnIn 2 S 4 composites exhibited oxygen defects.Notably, the presence of oxygen defects leads to the emergence of new electronic state bands near the lower boundary of the CB in SnO 2 .This phenomenon reduces the band gap, enhancing visible light absorption [47].Upon exposure to blue LED light irradiation, photogenerated electrons within the VB of SnO 2 and ZnIn 2 S 4 are excited to the CB.Subsequently, the photogenerated electrons in the CB of ZnIn 2 S 4 can migrate to the CB of SnO 2 .SnO 2 acts as an electron sink, effectively capturing and reducing hydrogen ions to generate hydrogen.Concurrently, the photogenerated holes within the VB of SnO 2 can transfer to ZnIn 2 S 4 , where they participate in water oxidation to produce oxygen or hydrogen ions.This photocatalytic process facilitates the efficient separation of photogenerated charge carriers, thereby promoting the photocatalytic production of hydrogen.The collaborative influence notably mitigates carrier recombination, consequently amplifying the photocatalytic efficacy of SnO 2 @ZnIn 2 S 4 and facilitating effective hydrogen generation.This synergy of photocatalysis helps to separate charge carriers, promoting the efficient production of hydrogen efficiently.Additionally, the combination of SnO 2 @ZnIn 2 S 4 composites dramatically enhances the ability to absorb light, thereby increasing the effectiveness of generating hydrogen through photocatalysis.Figure 8 illustrates the potential mechanism underlying the photocatalytic watersplitting process on the SnO2@ZnIn2S4 composites when subjected to blue LED light irradiation.An ion exchange resin is first applied to the indium tin oxide (ITO) glass, creating a coating of SnO2 and ZnIn2S4.Subsequently, the flat band potential is determined using cyclic voltammetry [43].The determined valence band (VB) and conduction band (CB) positions of SnO2 and ZnIn2S4 align with established findings.In particular, the conduction band positions for SnO2 and ZnIn2S4 are recorded at −0.14 eV and −0.58 eV, respectively, while their valence band positions are noted at 3.35 eV and 1.78 eV, respectively.[15,[44][45][46].In addition, previous XPS O1s (Figure 4c) can verify that SnO2 nanoparticles and SnO2@ZnIn2S4 composites exhibited oxygen defects.Notably, the presence of oxygen defects leads to the emergence of new electronic state bands near the lower boundary of the CB in SnO2.This phenomenon reduces the band gap, enhancing visible light absorption [47].Upon exposure to blue LED light irradiation, photogenerated electrons within the VB of SnO2 and ZnIn2S4 are excited to the CB.Subsequently, the photogenerated electrons in the CB of ZnIn2S4 can migrate to the CB of SnO2.SnO2 acts as an electron sink, effectively capturing and reducing hydrogen ions to generate hydrogen.Concurrently, the photogenerated holes within the VB of SnO2 can transfer to ZnIn2S4, where they participate in water oxidation to produce oxygen or hydrogen ions.This photocatalytic process facilitates the efficient separation of photogenerated charge carriers, thereby promoting the photocatalytic production of hydrogen.The collaborative influence notably mitigates carrier recombination, consequently amplifying the photocatalytic efficacy of SnO2@ZnIn2S4 and facilitating effective hydrogen generation.This synergy of photocatalysis helps to separate charge carriers, promoting the efficient production of hydrogen ef- Deionized water undergoes purification by eliminating diverse ions and contaminants from tap water [46,48].Hence, if photocatalysts could directly generate hydrogen via the decomposition of tap water, it could significantly decrease the expenses and duration associated with purifying tap water into deionized water, thereby facilitating the practical utilization of photocatalysts [42].Additionally, this approach could lead to more sustainable and cost-effective water treatment methods, contributing to environmental conservation efforts [49].To demonstrate the practicality of utilizing SnO 2 @ZnIn 2 S 4 composites for photocatalytic tap water splitting, 50 mg of the composites was dispersed in 50 mL of tap water solution containing 0.1 M Na 2 S without any pH modification.This experimental setup was subjected to blue or white LED light irradiation, as depicted in Figure 9a.The average HER of SnO 2 @ZnIn 2 S 4 composites is 408.9 µmolh −1 g −1 L −1 in deionized water and 670.8 µmolh −1 g −1 L −1 in tap water under blue LED light irradiation.The average HER of SnO 2 @ZnIn 2 S 4 composites is 160.1 µmolh −1 g −1 L −1 in deionized water and 395.8 µmolh −1 g −1 L −1 in tap water under white LED light irradiation.Notably, the average HER of SnO 2 @ZnIn 2 S 4 composites in tap water is approximately 1.64 (blue LED light) and 2.47 (white LED light) times higher than in deionized water, respectively.It is speculated that the possible reason is that more ions or chlorine ions in tap water can facilitate the transfer of photogenerated electron and hole pairs, thereby enhancing photocatalytic hydrogen production [50].These findings highlight the exceptional photocatalytic efficacy of SnO 2 @ZnIn 2 S 4 composites in hydrogen generation, even when dealing with intricate water compositions, without needing pH adjustments.In addition, this result also confirms that blue LED light is more effective than white LED light when employing SnO 2 @ZnIn 2 S 4 composites for the photocatalytic decomposition of tap water to produce hydrogen.The lower photocatalytic efficiency of white LED light compared to blue LED light may be due to the dispersion of light source intensity [41].Deionized water undergoes purification by eliminating diverse ions and contaminants from tap water [46,48].Hence, if photocatalysts could directly generate hydrogen via the decomposition of tap water, it could significantly decrease the expenses and duration associated with purifying tap water into deionized water, thereby facilitating the practical utilization of photocatalysts [42].Additionally, this approach could lead to more sustainable and cost-effective water treatment methods, contributing to environmental conservation efforts [49].To demonstrate the practicality of utilizing SnO2@ZnIn2S4 composites for photocatalytic tap water splitting, 50 mg of the composites was dispersed in 50 mL of tap water solution containing 0.1 M Na2S without any pH modification.This experimental setup was subjected to blue or white LED light irradiation, as depicted in Figure 9a.The average HER of SnO2@ZnIn2S4 composites is 408.9 µmolh −1 g −1 L −1 in deionized water and 670.8 µmolh −1 g −1 L −1 in tap water under blue LED light irradiation.The average HER of SnO2@ZnIn2S4 composites is 160.1 µmolh −1 g −1 L −1 in deionized water and 395.8 µmolh −1 g −1 L −1 in tap water under white LED light irradiation.Notably, the average HER of SnO2@ZnIn2S4 composites in tap water is approximately 1.64 (blue LED light) and 2.47 (white LED light) times higher than in deionized water, respectively.It is speculated that the possible reason is that more ions or chlorine ions in tap water can facilitate the transfer of photogenerated electron and hole pairs, thereby enhancing photocatalytic hydrogen production [50].These findings highlight the exceptional photocatalytic efficacy of SnO2@ZnIn2S4 composites in hydrogen generation, even when dealing with intricate water compositions, without needing pH adjustments.In addition, this result also confirms that blue LED light is more effective than white LED light when employing SnO2@ZnIn2S4 composites for the photocatalytic decomposition of tap water to produce hydrogen.The lower photocatalytic efficiency of white LED light compared to blue LED light may be due to the dispersion of light source intensity [41].
Efficiently recycling catalysts is paramount in photocatalysis [41].Designing photocatalysts that maintain consistent photoactivity across multiple cycles is crucial for minimizing waste and ensuring sustainable processes [51].In the context of photocatalysts, the gradual decline in efficiency over time can lead to increased waste generation throughout their life cycle.Hence, creating photocatalysts that can be easily separated and recycled is crucial to mitigate the depletion of valuable resources within the waste stream [32].This study conducted eight consecutive cycles of photocatalytic tap water splitting to assess the durability of SnO2@ZnIn2S4 composites.The average HER of the SnO2@ZnIn2S4 composites exhibits a sustained high level throughout eight cycles, as depicted in Figure 9b.Additionally, the XRD pattern of the sample after the eight cycles, shown in Figure 9c, does not exhibit any new peaks, indicating the preservation of the composite's structural integrity.These results prove that the SnO2@ZnIn2S4 composites exhibit better durability and recyclability.

Conclusions
In this study, we have successfully produced composite materials consisting of SnO2@ZnIn2S4 for photocatalytic splitting of tap water, employing a two-step synthesis method assisted by microwave technology.Our investigation delved into the impact of incorporating fixed quantities of SnO2 nanoparticles into various materials to form com- Efficiently recycling catalysts is paramount in photocatalysis [41].Designing photocatalysts that maintain consistent photoactivity across multiple cycles is crucial for minimizing waste and ensuring sustainable processes [51].In the context of photocatalysts, the gradual decline in efficiency over time can lead to increased waste generation throughout their life cycle.Hence, creating photocatalysts that can be easily separated and recycled is crucial to mitigate the depletion of valuable resources within the waste stream [32].This study conducted eight consecutive cycles of photocatalytic tap water splitting to assess the durability of SnO 2 @ZnIn 2 S 4 composites.The average HER of the SnO 2 @ZnIn 2 S 4 composites exhibits a sustained high level throughout eight cycles, as depicted in Figure 9b.Additionally, the XRD pattern of the sample after the eight cycles, shown in Figure 9c, does not exhibit any new peaks, indicating the preservation of the composite's structural integrity.These results prove that the SnO 2 @ZnIn 2 S 4 composites exhibit better durability and recyclability.

Conclusions
In this study, we have successfully produced composite materials consisting of SnO 2 @ZnIn 2 S 4 for photocatalytic splitting of tap water, employing a two-step synthesis method assisted by microwave technology.Our investigation delved into the impact of incorporating fixed quantities of SnO 2 nanoparticles into various materials to form composites, thereby enhancing hydrogen production through photocatalysis.Furthermore, we investigated the effect of different concentrations of SnO 2 nanoparticles in the ZnIn 2 S 4 reaction precursor to improve the synthesis of SnO 2 @ZnIn 2 S 4 composites for photocatalytic hydrogen generation.Notably, the efficiency of photocatalytic hydrogen production in SnO 2 @ZnIn 2 S 4 composites surpasses that of pure SnO 2 nanoparticles or ZnIn 2 S 4 nanosheets.The improved efficiency can be credited to successfully harnessing visible light and promoting photogenerated electrons across the heterojunction, leading to the effective dissociation of electron-hole pairs.Furthermore, reusability tests have validated the exceptional performance of the SnO 2 @ZnIn 2 S 4 composites.Despite undergoing eight cycles, the composites continue to exhibit elevated levels of photocatalytic hydrogen production without experiencing a notable decline in efficiency.This study introduces a forward-looking methodology for synthesizing and fabricating environmentally sustainable SnO 2 @ZnIn 2 S 4 composites, which hold promise for applications in the field of photocatalytic hydrogen generation.

Figure 1 .
Figure 1.Illustrates a schematic representation of the reaction to forming the SnO2@ZnIn2S4 composites.

Figure 1 .
Figure 1.Illustrates a schematic representation of the reaction to forming the SnO 2 @ZnIn 2 S 4 composites.

Figure 6 .
Figure 6.(a) The average HER of SnO2 nanoparticles with different sacrificial reagents.(b) The average HER of SnO2 nanoparticles is decorated with various materials.(c) The average HER of SnO2@ZnIn2S4 composites with varying weights of SnO2 nanoparticles.

Figure 6 .
Figure 6.(a) The average HER of SnO 2 nanoparticles with different sacrificial reagents.(b) The average HER of SnO 2 nanoparticles is decorated with various materials.(c) The average HER of SnO 2 @ZnIn 2 S 4 composites with varying weights of SnO 2 nanoparticles.

Figure 9 .
Figure 9. (a) The average HER of SnO2@ZnIn2S4 composites under deionized water and tap water under blue or white LED light irradiation.(b) Reusability of SnO2@ZnIn2S4 composites for eight cycles.(c) XRD spectrum of SnO2@ZnIn2S4 composites after the eight cycles.

Figure 9 .
Figure 9. (a) The average HER of SnO 2 @ZnIn 2 S 4 composites under deionized water and tap water under blue or white LED light irradiation.(b) Reusability of SnO 2 @ZnIn 2 S 4 composites for eight cycles.(c) XRD spectrum of SnO 2 @ZnIn 2 S 4 composites after the eight cycles.