Visible-light-driven green hydrogen and hydrogen peroxide production using a 2D porous organic polymer engineered with 2D SnS₂

Abstract

The transformation of solar radiation into chemical energy or valuable chemical compounds has garnered significant research interest, particularly in light of the global energy crisis. Hydrogen and hydrogen peroxide serve as sustainable energy sources in fuel cells, producing electricity with zero carbon emissions. Recently, the eco-friendly synthesis of H₂ and H₂O₂ from water and oxygen using porous organic polymers (POPs) as photocatalysts has drawn considerable attention. However, their applications have been limited due to low absorption of visible light and the rapid recombination of photoinduced charge carriers, while noble metal co-catalysts remain essential in all POP-based photocatalysts to achieve high rates of hydrogen evolution and hydrogen peroxide production, as well as to enhance charge separation in semiconductor photocatalysts. In this study, we demonstrate a more effective heterojunction photocatalyst—2D–2D SnS₂@TAPA-BPDA—which has a significant effect on photocatalytic H₂ evolution and H₂O₂ production. When exposed to visible light, the SnS₂@TAPA-BPDA composite achieves a hydrogen evolution rate of 1818.8 μmol h⁻¹ g⁻¹, which is approximately 30 times higher than that of the bare TAPA-BPDA POP. Similarly, for hydrogen peroxide production, the same catalyst reaches 3013.3 μmol h⁻¹ g⁻¹, nearly 14 times greater than the bare catalyst. These results highlight the significant enhancement in photocatalytic H₂ evolution and H₂O₂ generation, leading to highly effective solar-to-chemical energy conversion.

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Introduction

The accelerating impacts of climate change and the global depletion of fossil fuel reserves have intensified the search for clean, renewable, and sustainable energy solutions.^1–3 Among the various alternatives, molecular hydrogen (H₂) has emerged as a promising green energy carrier due to its high energy density and versatility in energy conversion and storage systems.^4,5 It plays an essential role in electricity generation via fuel cells and is also extensively utilized in industrial applications such as petroleum refining and ammonia synthesis.^6,7 However, the conventional production of hydrogen, primarily through coal gasification,^8,9 methane reforming,^10,11 and industrial water electrolysis,^12,13 remains energy-intensive,¹⁴ heavily reliant on fossil fuels, and environmentally unsustainable.¹⁵ These challenges have driven growing interest in solar-driven water splitting technologies,^16,17 including photocatalytic^18,19 and photoelectrochemical methods,^20,21 which offer the potential for green hydrogen production under ambient conditions with minimal environmental impact.²²

In parallel, hydrogen peroxide (H₂O₂) is recognized as a vital green oxidant²² with widespread use in water purification,^23–25 medical sterilization,^26–28 bleaching,^29,30 and organic synthesis.^31,32 More recently, H₂O₂ has been investigated as a clean energy carrier in fuel cells and propulsion systems, as it decomposes into only water and oxygen posing minimal ecological risks.^33–36 However, the industrial-scale production of H₂O₂ relies on the anthraquinone oxidation process, which involves palladium-based catalysts, toxic solvents, and high-pressure and high-temperature conditions.^37–39 This method is not only energy-intensive but also results in the generation of hazardous chemical waste, highlighting the need for cleaner and more sustainable alternatives. Photocatalytic synthesis has emerged as a compelling strategy for the simultaneous generation of H₂ and H₂O₂, offering key advantages such as low energy input, eco-friendly raw materials, and continuous operation under mild conditions. Recent studies have shown that H₂O₂ can be sustainably produced via artificial photosynthesis, using only sunlight, dioxygen (O₂), and water (H₂O) through proton-coupled electron transfer (PCET).^40,41 This approach has drawn significant attention due to its safe, valuable, and pollution-free products. However, a major challenge arises from the competing formation of superoxide anions (O₂⁻) via single-electron reduction of oxygen. These reactive species are difficult to activate further and strongly inhibit PCET, leading to extremely low photocatalytic efficiency for H₂O₂ production in aqueous systems.^42,43

Photocatalytic synthesis has emerged as a compelling strategy for the simultaneous generation of H₂ and H₂O₂, offering key advantages such as low energy input, eco-friendly raw materials, and continuous operation under mild conditions. Yet, despite notable progress, traditional photocatalysts like TiO₂,⁴⁴ ZnO,⁴⁵ and CdS⁴⁶ face limitations. Their wide band gaps restrict light absorption to the UV region, which accounts for only ∼5% of the solar spectrum. Moreover, these materials often require noble metal co-catalysts (e.g., Pt, Pd, Au) to enhance activity, further increasing the cost and complexity. These materials, while active under visible light, suffer from toxicity and poor stability.

To overcome these obstacles, researchers have turned toward metal-free photocatalysts, particularly porous organic polymers (POPs). POPs are a class of highly tunable materials with advantages such as adjustable band structures, high photochemical stability, and excellent catalytic selectivity. Crystalline POPs, such as covalent organic frameworks (COFs) and covalent triazine frameworks (CTFs), have shown promise for solar fuel production.⁴⁷ However, their poor structural stability, harsh synthesis conditions, and reliance on sacrificial agents limit their scalability and practical application.

In contrast, non-crystalline POPs offer flexibility and structural versatility but often still require high-temperature/pressure conditions and external additives. Structural engineering particularly through heterojunction formation has been proposed as an effective method to improve photocatalytic activity, facilitating enhanced charge separation and light absorption. In this context, 2D POPs are especially attractive due to their periodic columnar π-arrays, pre-organized transitions of long-lived excited states, and high charge mobility, making them ideal for visible-light photocatalysis.^48–50

Simultaneously, 2D metal chalcogenides (MDCs) like SnS₂, MoS₂, and WS₂ have garnered significant attention for their small band gaps, high surface areas, and excellent electronic properties.^51–54 SnS₂, in particular, is a naturally occurring n-type semiconductor with a hexagonal CdI₂ type layered structure comprising three stable S–Sn–S layers.⁵⁵ Its low band gap (∼2.2 eV) enables absorption across the full solar spectrum, from UV to near-infrared, and the exposed sulfur-rich crystal planes facilitate strong hydrogen bonding with water molecules,^56,57 ideal for photocatalytic applications. Previous studies have demonstrated the potential of SnS₂ in both hydrogen and hydrogen peroxide generation. For instance, Park et al. reported efficient H₂O₂ production using SnS₂/MoO₃ hollow nanotubes,⁵⁸ while Zhen et al. developed monodisperse SnS₂ nanosheets with outstanding H₂ generation performance.^59–62 To enhance photocatalytic efficiency further, integrating SnS₂ with 2D POPs into hybrid heterojunction systems has emerged as a powerful strategy. The intimate interfacial contact in such 2D/2D heterostructures facilitates charge transfer, reduces recombination losses, and improves light harvesting and surface reactivity. Additionally, combining POPs with SnS₂ eliminates the need for expensive noble metal co-catalysts and toxic sacrificial agents.^63,64 Notably, Wang and Domen pioneered the use of Pt-loaded graphitic carbon nitride (g-C₃N₄) with sacrificial donors for hydrogen production,⁶⁵ while researchers like Thomas, Cooper, and Lotsch developed various crystalline POPs for photocatalysis.^66–68

Composite photocatalysts have gained significant attention because they allow different materials to work together, overcoming the drawbacks of single components. Among the possible architecture, two-dimensional (2D) composites stand out over their one-dimensional (1D) and three-dimensional (3D) counterparts.⁶⁹ Their thin, layered structures provide a larger surface area, more exposed active sites, and closer interfacial contact, all of which make it easier for charges to move in a directed way. By carefully matching the band structures of the chosen materials, it is possible to promote efficient separation of photogenerated electrons and holes—an essential step for driving both hydrogen evolution and oxygen reduction into hydrogen peroxide.^70,71

In this work, we combine SnS₂ nanosheets with a porous organic polymer (TAPA-BPDA POP) to build a 2D/2D heterostructure. This design not only improves light absorption and charge transport but also creates open pathways for oxygen diffusion, enabling the simultaneous production of H₂ and H₂O₂. What sets this study apart from earlier POP–semiconductor composites is the deliberate design of the SnS₂@TAPA-BPDA heterostructure through a simple one-step hydrothermal method, resulting in both high performance and excellent stability.

Experimental

Synthetic procedures

Preparation of the 2D–2D SnS₂@TAPA-BPDA-POP heterostructure. The synthesis of the 2D–2D SnS₂@TAPA-BPDA-POP involved dissolving 140 mg of TAPA-BPDA-POP and 115 mg of SnCl₄·5H2O in 20 mL of distilled water while stirring for 30 minutes. After an additional 30 minutes of stirring, 300 mg of thioacetamide was added to the solution, creating a homogeneous suspension that was promptly transferred to a 50 mL Teflon container. The samples were then placed in an oven at 100 °C for seven hours. Once cooled to room temperature, the sample was centrifuged for three minutes and thoroughly washed three times with acetone, distilled water, and absolute alcohol. Finally, the cleaned product was dried in a vacuum oven at 60 °C for eight hours (Scheme 1).⁷²

Scheme 1 Schematic illustration of the synthetic procedure for the preparation of SnS2@TAPA-BPDA composite.

Results and discussion

The careful selection of linker units and linkages is critical in designing materials for photocatalytic applications, as these components serve as bridges that facilitate exciton generation and charge transport along the polymer backbone. The structure and formation of linkages in the POP backbone were verified using Fourier Transform Infrared (FTIR) spectroscopy. The FTIR spectra of the two monomeric precursors, an aldehyde and an amine, display characteristic peaks (Fig. S1). The bands at 3409 cm⁻¹ and 3333 cm⁻¹ correspond to N–H stretching vibrations, while the strong peak at 1691 cm⁻¹ is attributed to C=O stretching, confirming the presence of amino and aldehyde functional groups in the monomers. In the FTIR spectrum of BPDA, additional peaks observed at 2743 cm⁻¹ and 2838 cm⁻¹ are assigned to C–H stretching vibrations. After polymerization, the FTIR spectrum of TAPA-BPDA-POP shows a prominent absorption band at 1603 cm⁻¹ (Fig. 1a), which is characteristic of C=N stretching vibrations. This confirms the successful condensation reaction between the –NH₂ groups of TAPA and the –CHO groups of BPDA, resulting in the formation of the imine linkage, a hallmark of the final polymer structure. Following the grafting of SnS₂ nanosheets onto the surface of TAPA-BPDA-POP,⁷³ the C=N stretching peak at 1603 cm⁻¹ remains unaltered, indicating that the structural integrity of the POP framework is retained. Additionally, a new peak emerges at 531 cm⁻¹, which corresponds to the Sn–S bond,⁷⁴ providing clear evidence for the successful integration of SnS₂ onto the POP surface.

Fig. 1 (a) FT-IR spectra of SnS₂, TAPA-BPDA POP and SnS₂@TAPA-BPDA, (b) wide angle PXRD pattern of SnS₂@TAPA-BPDA, (c) small angle PXRD pattern of SnS₂@TAPA-BPDA, (d) N₂ adsorption–desorption isotherm of SnS₂@TAPA-BPDA, (e) pore size distribution of SnS₂@TAPA-BPDA, (f) UV-DRS of TAPA-BPDA POP and SnS₂@TAPA-BPDA, (g) PL spectra of TAPA-BPDA POP and SnS₂@TAPA-BPDA, (h) Tauc plot of TAPA-BPDA POP and SnS₂@TAPA-BPDA, (i) Kubelka–Munk diagram for the SnS₂@TAPA-BPDA composite catalyst showing direct and indirect band gap.

The long-range structural periodicity of the TAPA-BPDA-POP was investigated using powder X-ray diffraction (PXRD) analysis (Cu Kα radiation; λ = 1.5418 Å). The small-angle PXRD pattern of the SnS₂@TAPA-BPDA composite (Fig. 1c) displays distinct diffraction peaks at 2θ = 2.82°, 4.94°, and 9.9°, corresponding to the (110), (100), and (210) crystal planes, respectively. These reflections confirm the presence of ordered, periodic structures within the composite material. Further structural information was obtained from the wide-angle XRD pattern (Fig. 1b), which reveals sharp diffraction peaks at 2θ = 15.3°, 28.7°, 32.7°, 50.4°, and 52.3°. These peaks are well-matched with the characteristic reflections of hexagonal SnS₂, corresponding to the (001), (100), (011), (110), and (111) planes, as indexed by JCPDS no. 23-0677, thereby confirming the crystalline nature of the SnS₂ component in the hybrid system. Additionally, peaks observed at 2θ = 18.4° and 43.2° are assigned to the (201) and (213) planes, respectively, and are attributed to the TAPA-BPDA POP. A separate peak at 26.6°, corresponding to the (001) plane, arises due to π–π stacking interactions between the two-dimensional layers of the POP framework, further supporting the layered structure of the composite.

X-ray photoelectron spectroscopy (XPS) was employed to analyze the chemical states and structures of the samples. It was confirmed that the composite sample of SnS₂@TAPA-BPDA POP contained the elements C, N, Sn, and S (Fig. 3). The high-resolution XPS spectra for SnS₂@TAPA-BPDA POP include C 1s, N 1s, S 2p, and Sn 3d. In the C 1s core-level spectra (Fig. 3a) of the SnS₂@TAPA-BPDA POP composite, the two distinct peaks at binding energies of 284.6 eV and 285.3 eV correspond to C–C, C=C, and C=N bonds. The peak at 400.4 eV in the N 1s spectra (Fig. 3b) is typically attributed to sp² hybridized nitrogen (C=N), while the peak at 399.3 eV is associated with the amino group containing a hydrogen atom (C–N–H).⁷⁵ The S 2p spectrum (Fig. 3c) reveals two distinct peaks: the S 2p_3/2 peak is observed at 163.5 eV, while the S 2p_1/2 peak is found at 164.8 eV. Additionally, the Sn⁴⁺ in SnS₂ is indicated by two peaks in the Sn 3d spectrum (Fig. 3d), specifically the 3d_5/2 and 3d_3/2 peaks, which appear at 487.3 eV and 495.8 eV, respectively.⁷⁶

The XPS results clearly highlight strong interfacial interactions between SnS₂ (Fig. S7) and the porous organic polymer (POP) (Fig. S8). For pristine TAPA-BPDA POP, the C 1s spectrum mainly reflects C–C/C=C and C–N bonds, while the N 1s signal indicates pyridinic and graphitic nitrogen species. After incorporating SnS₂, noticeable chemical shifts appear, the C 1s peak moves slightly to higher binding energy, whereas the N 1s peak shifts to lower values. This opposite behavior suggests a redistribution of charge within the POP, with carbon sites losing electrons and nitrogen sites becoming more electron-rich, consistent with electron transfer toward SnS₂.

In the composite, the Sn 3d spectrum retains the characteristic Sn 3d_5/2 and Sn 3d_3/2 doublet of SnS₂ but appears at lower binding energies, confirming that Sn atoms gain electrons through interfacial charge transfer. Meanwhile, the S 2p doublet shifts upward, pointing to electron depletion at sulfur sites. Together, these complementary changes in the POP (C, N) and SnS₂ (Sn, S) regions verify the formation of a built-in electric field at the TAPA-BPDA POP–SnS₂ junction.⁷⁷

Overall, the XPS analysis demonstrates that integrating SnS₂ into the TAPA-BPDA POP matrix drives charge redistribution, promotes defect formation, and establishes an interfacial electric field—factors that work in synergy to boost the electronic and catalytic performance of the SnS₂@TAPA-BPDA composite.

To investigate the stable porosity of the TAPA-BPDA POP and SnS₂@TAPA-BPDA heterojunction, N₂ desorption/adsorption investigation was carried out at 77 K. Fig. 1d and S3(a) shows a fast N₂ uptake at P/P₀ in the lower range, followed by a continuous increase in N₂ uptake at P/P₀ in the upper range, indicating the existence of diverse pores, from massive micropores to mesopores. N₂ adsorption experiments indicated that both the materials were porous, with moderately high surface area. The isotherm curves for both samples display a characteristic type IV isotherm, suggesting the existence of mesoporous structures. The BET surface area of TAPA-BPDA POP is 266 m² g⁻¹. The total pore volume of TAPA-BPDA POP is 0.34 cm³ g⁻¹. On the other hand, SnS₂@TAPA-BPDA composite had a BET surface area of 171 m² g⁻¹. This result indicated a considerable drop in the BET surface area after adding the SnS₂ nanosheet. Additionally, the overall pore volume of SnS₂@TAPA-BPDA is significantly reduced. Although the SnS₂@TAPA-BPDA composite shows a lower BET surface area than pristine TAPA-BPDA POP, its photocatalytic activity is enhanced. This reduction arises from partial pore blocking or framework densification but is offset by efficient interfacial charge transfer, favorable band alignment, and extended light absorption. The intimate POP–SnS₂ contact promotes charge separation and utilization, while the POP's porosity ensures reactant accessibility. Thus, performance improvement stems from electronic synergy rather than surface area alone.

The peak pore size distribution of TAPA-BPDA POP was measured using nonlocal density functional theory (NLDFT) (Fig. S3b), demonstrating that pore widths are around 5.6 nm. In contrast, the peak pore width for the SnS₂@TAPA-BPDA composite (Fig. 1e) has been reduced to 3.8 nm, indicating the presence of SnS₂ nanosheet at the pore surfaces.⁷⁸ To verify the deposition of SnS₂ on the surface of POP, we conducted SEM (Fig. S4) and TEM imaging along with elemental mapping of the heterocomposite material (Fig. S5). The TEM images reveal a distinct contrast, indicating the presence of multiple phases. Additionally, the EDS elemental mapping reveals an uneven distribution of Sn and S elements throughout the nanosheets. These observations clearly indicate the establishment of a 2D–2D heterocomposite structure between the POP and the ultrathin SnS₂ nanosheets. The large area of contact and intimate interfacial contact enhance the interaction between the components, facilitate charge transfer, potentially enhancing photocatalytic water splitting performance.⁷²

The optical properties of TAPA-BPDA POP and SnS₂@TAPA-BPDA composite were examined using UV-vis diffuse reflectance spectroscopy (DRS). While TAPA-BPDA POP displayed an absorption edge at ∼546 nm, the SnS₂@TAPA-BPDA composite exhibited a pronounced red shift to ∼660 nm, markedly extending visible-light absorption (Fig. 1f). This enhancement is attributed to the strong interfacial interaction between SnS₂ nanosheets and the POP backbone, enabling more efficient light harvesting. The corresponding Tauc plot⁷⁹ (Fig. 1h) analysis revealed optical band gaps of 1.87 eV for the composite, consistent with values from Kubelka–Munk-derived data (Fig. 1i, and S6).

Mott–Schottky⁸⁰ analyses (Fig. 2f) indicated that the E_fb is approximately −1.07 V vs. Ag/AgCl, and has an intercept with the X-axis. Next, the potential versus normal hydrogen electrode (NHE) is represented by E_NHE, the measured potential versus Ag/AgCl is denoted by E, and the Nernst equation is E_(NHE) = E_(Ag/AgCl) + E₀. As a result, −1.07 + 0.197 = −0.87 V is the TAPA-BPDA POP's E_fb value (versus NHE, pH 7). Positive slopes in TAPA-BPDA POP indicate the n-type characteristic of semiconductors.⁸¹ For most n-type semiconductors, the conduction band (CB) minimum lies about 0.1 V above E_fb; thus, the CB edge was estimated at −0.97 V. Considering the band gap, the corresponding valence band (VB) position is determined to be 1.28 V (Fig. 2e).⁸² These positions are suitably aligned with the redox potentials for H₂O₂ generation, the HER, and O₂ reduction, enabling both thermodynamically and kinetically favourable photocatalytic water splitting and reactive oxygen species generation. In particular, the CB lies above the reduction potentials for H⁺/H₂ (−0.41 V) and O₂/˙O₂⁻ (−0.33 V), while the VB exceeds the oxidation potentials for O₂/H₂O (0.82 V) and H₂O/O₂ (1.23 V), confirming that they are ideal candidates for photocatalytic water splitting for the production of H₂ and H₂O₂.^83,84

Fig. 2 (a) CV of TAPA-BPDA POP, (b) CV of SnS₂@TAPA-BPDA, (c) LSV of TAPA-BPDA POP and SnS₂@TAPA-BPDA (d) EIS of SnS₂, TAPA-BPDA POP and SnS₂@TAPA-BPDA, (e) Photocurrent of SnS₂, TAPA-BPDA POP and SnS₂@TAPA-BPDA, (f) VB position of TAPA-BPDA POP, (g) Mott–Schottky plot of TAPA-BPDA POP, (h) rate of photocatalytic hydrogen production of TAPA-BPDA POP, 3 wt% SnS₂@TAPA-BPDA and 6 wt% SnS₂@TAPA-BPDA.

PL spectra (Fig. 1g) showed a pronounced quenching of emission intensity in the SnS₂@TAPA-BPDA composite compared to TAPA-BPDA POP, indicating that the heterojunction significantly suppresses radiative recombination of photogenerated electron–hole pairs. Cyclic voltammetry (CV) measurements in 1 M H₂SO₄ at a scan rate of 10 mV s⁻¹ (Fig. 2a) revealed quasi-reversible redox behavior for both the pristine TAPA-BPDA POP and the SnS₂@TAPA-BPDA composite. Notably, the composite exhibited a lower half-wave potential (E_1/2 = 0.015 V vs. Ag/AgCl) compared to the pristine POP (E_1/2 = 0.045 V vs. Ag/AgCl), indicating that electron transfer is kinetically more favorable in the heterostructure. This shift in redox potential can be attributed to the synergistic interaction between SnS₂ and the POP framework, where intimate interfacial contact facilitates faster charge shuttling and lowers the energetic barrier for redox transitions. Linear sweep voltammetry (LSV) (Fig. 2b) further confirmed that the composite required a lower overpotential for hydrogen evolution, suggesting enhanced catalytic activity due to synergistic effects at the SnS₂–POP interface.⁸⁵

Electrochemical impedance spectroscopy (EIS) further supported these observations (Fig. 2c). The Nyquist plots of TAPA-BPDA POP, SnS₂, and the SnS₂@TAPA-BPDA composite showed that the composite possessed the smallest semicircle diameter, corresponding to the lowest charge transfer resistance among the three samples. This reduction in interfacial resistance highlights the critical role of the SnS₂–POP interface in promoting more efficient electron transport.⁸⁶ Moreover, transient photocurrent response studies (Fig. 2d) under visible-light on/off cycling showed substantially higher photocurrent densities for the composite than for TAPA-BPDA POP, signifying more efficient charge separation and faster carrier migration. These combined photophysical and electrochemical results clearly demonstrate that integration of SnS₂ with TAPA-BPDA POP not only broadens the light absorption range but also accelerates electron transport, suppresses recombination, and ultimately enhances the photocatalytic performance for water splitting and H₂O₂ generation.⁸⁷

Catalytic activity

Considering the high stability, narrowed optical band gap, and well-aligned band positions, the TAPA-BPDA POP and SnS₂@TAPA-BPDA composite were employed for photocatalytic H₂ and H₂O₂ generation utilizing natural air, water, and sunlight under ambient conditions. Control experiments conducted in the absence of either the photocatalyst or light illumination resulted in no detectable H₂ formation, confirming that both are essential for the reaction. The SnS₂@TAPA-BPDA composite proved to be an efficient photocatalytic system for hydrogen evolution, owing to its relatively narrow optical band gap of 1.87 eV, which enables substantial light absorption across the UV-visible spectrum. The photocatalytic performance of this heterojunction catalyst for H₂ production was evaluated (Fig. 2g) using 0.35 M Na₂S/0.25 M Na₂SO₃ as sacrificial agents. The Na₂S/Na₂SO₃ pair is known to accelerate the kinetics of the oxidation reaction, thereby enhancing the overall hydrogen evolution rate.

The effect of SnS₂ loading on photocatalytic activity was investigated under identical conditions. Bare TAPA-BPDA POP showed a low H₂ production rate of 363 μmol g⁻¹ in 6 h. Incorporation of 3 wt% SnS₂ significantly enhanced the activity, yielding 5183 μmol g⁻¹ in 6 h. The optimum performance was obtained for the 6 wt% SnS₂@TAPA-BPDA composite, which achieved an impressive H₂ evolution rate of 10.913 mmol g⁻¹ in 6 h (Fig. 2g). This represents a ∼9-fold increase for the 3 wt% composite and a ∼30-fold increase for the 6 wt% composite compared to the pristine POP. At λ = 480 nm, the apparent quantum efficiency (AQE) of the 6 wt% SnS₂@TAPA-BPDA catalyst was measured to be 1.996%. The enhanced activity is attributed to efficient heterojunction formation, where photoexcited electrons generated in the light-responsive TAPA-BPDA POP are transferred to the conduction band of SnS₂ (E_g = 2.0 eV), driving the reduction of water to H₂. This interfacial charge transfer promotes spatial separation of photogenerated electron–hole pairs and suppresses recombination, thereby boosting hydrogen production.

Given its favourable stability, appropriate band gap, and well-aligned band positions, the SnS₂@TAPA-BPDA composite was also applied for photocatalytic H₂O₂ generation using only oxygen, water, and visible light under ambient conditions. The process was carried out in ultrapure water under visible-light irradiation without any organic solvents, representing a simple, cost-effective, and sustainable approach. The reaction progress was tracked at specific time intervals of 5, 10, 20, 30, 35, 40, 50, 55, 60, 70, 80 and 90 minutes, with each time point representing a distinct experiment to accurately assess the photocatalytic activity of H₂O₂ generation (Fig. 2h). The presence of H₂O₂ was initially confirmed using a peroxide test kit and quantified by UV-vis spectroscopy with titanium potassium oxalate. The results revealed a linear increase in H₂O₂ concentration over the 90 min reaction period. Experiments performed under visible-light irradiation (420 < λ < 700 nm) showed that even without an external electron sacrificial agent, the composite achieved a remarkable H₂O₂ yield of 4520 μmol g⁻¹ after 1.5 h, corresponding to an apparent quantum yield of 2.27% at 420 nm surpassing most reported porous organic polymer photocatalysts. The hollow and porous architecture of the composite facilitates mass transfer and enhances access of O₂ and H₂O molecules to the active sites, further improving reaction efficiency.

The optimal performance at 6 wt% SnS₂@TAPA-BPDA for the production of H₂ as well as H₂O₂ arises from the balance between possessing adequate active sites and possessing suitable SnS₂ dispersion in the TAPA-BPDA POP matrix. With increased loading levels (e.g., 9 wt%), a reduced photolysis for both the H₂ and H₂O₂ production has been observed (Fig. S9 and S10) due to excess SnS₂ agglomeration with consequent pore blocking, decrease in accessible surface area and prevention of efficiency of charge transfer.

A series of control experiments (Table 1) confirmed the crucial role of the photocatalyst and oxygen. In the absence of the SnS₂@TAPA-BPDA composite (entry 2), no H₂O₂ was formed. Similarly, no product was detected without light irradiation (entry 3) or under 1 atm O₂ without illumination (entry 4). When air was used in place of pure oxygen (entry 5), only a minor yield was obtained, while sunlight irradiation under ambient conditions (entry 6) produced a satisfactory yield. Under a nitrogen atmosphere (entry 7), H₂O₂ generation was significantly suppressed, indicating that the primary pathway is a two-electron oxygen reduction rather than water oxidation. A light on/off experiment (Fig. S9) further demonstrated that H₂O₂ production occurred exclusively during light irradiation, with no activity in the dark. These results collectively confirm that the SnS₂@TAPA-BPDA composite operates via a visible-light-driven photocatalytic mechanism, with heterojunction formation playing a pivotal role in efficient charge separation and transfer for both hydrogen and hydrogen peroxide generation.

Table 1 Control experiments for optimizing the reaction conditions

Entry	Catalyst 30 mg	Reactant	Light	H2O2 (μmol g^−1)
1	SnS2@TAPA-BPDA	O2	300 W Xe lamp	769.9
2	—	O2	300 W Xe lamp	0
3	SnS2@TAPA-BPDA	O2	—	0
4	SnS2@TAPA-BPDA	—	300 W Xe lamp	0
5	SnS2@TAPA-BPDA	Air	300 W Xe lamp	48.11
6	SnS2@TAPA-BPDA	O2	Sunlight	194.8
7	SnS2@TAPA-BPDA	N2	300 W Xe lamp	0

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Proposed mechanism

Understanding the mechanism of H₂O₂ generation in covalent organic frameworks (COFs) and porous organic polymers (POPs) is essential for improving photocatalytic efficiency and stability. Identifying the reaction pathways and active sites allows for targeted modifications, which is crucial for sustainable H₂O₂ production and its applications in green chemistry and environmental remediation.

Upon visible-light irradiation, the TAPA-BPDA POP absorbs photons and generates electron–hole (e⁻/h⁺) pairs. The photoexcited electrons are promoted from the valence band (VB) to the conduction band (CB), while the holes remain in the VB. In the SnS₂@TAPA-BPDA heterostructure, the CB potential of TAPA-BPDA POP (−0.97 V vs. NHE, pH 7) is more negative than that of SnS₂ (−0.47 V vs. NHE), enabling facile electron transfer from the POP's CB to that of SnS₂. Concurrently, holes migrate from SnS₂'s VB to the VB of TAPA-BPDA POP. This bidirectional charge migration leads to effective electron–hole separation, suppressing recombination and enhancing photocatalytic performance, as confirmed by photoluminescence and electrochemical analyses.

There are two probable mechanisms for H₂O₂ generation, water oxidation and/or oxygen reduction. The generation of H₂O₂ is dramatically reduced when pure O₂ is replaced with air, demonstrating that O₂ has a considerable influence on photocatalytic H₂O₂ production. The oxidation potential of the valence band is inadequate for the direct conversion of H₂O to H₂O₂ (2H₂O → H₂O₂ + 2H⁺ + 2e⁻ E⁰ = +1.76 V vs. NHE).

The simultaneous production of H₂ and H₂O₂ involves two distinct reduction pathways: (i) O₂ reduction through successive electron transfer steps that generate *O₂⁻ and intermediates, ultimately leading to H₂O₂, and (ii) proton reduction that yields H₂. Our ESR and scavenger experiments confirm the formation of radical intermediates associated with the O₂ reduction route, thereby establishing H₂O₂ generation as a dominant pathway. In parallel, the observation of H₂ evolution indicates that protons are also reduced under the same reaction conditions.⁸⁸

Although these pathways appear competitive, their coexistence can be rationalized by considering the electronic and structural properties of the catalyst surface. The applied potential window is thermodynamically sufficient to drive both O₂ and H⁺ reduction reactions. Kinetically, surface-active sites with different adsorption affinities and electronic configurations may preferentially channel electron transfer toward either O₂ activation or proton reduction. Such site-specific behaviour could allow both processes to occur simultaneously without completely suppressing one another. Moreover, spatial heterogeneity across the catalyst surface may facilitate localized separation of reaction zones, where O₂-rich regions promote H₂O₂ generation, while proton-accessible sites enable H₂ evolution.⁸⁹

The observed balance between H₂ and H₂O₂ yields suggests that the catalyst tunes electron partitioning between the two competing reduction reactions. While the precise mechanistic details of this coordination are not yet fully understood, our findings indicate that surface electronic structure and reaction kinetics play a critical role in enabling the dual production pathway. Nevertheless, the present work provides a foundation for understanding how a single catalytic system can couple H₂O₂ and H₂ generation within the same reaction environment. On the other hand, for the simultaneous production of H₂ and H₂O₂ the conduction band (CB) potential of the POP must be sufficiently negative to reduce both dissolved O₂ (via successive single-electron transfers to *O₂⁻ and intermediates, leading to H₂O₂) and protons (H⁺) to H₂. Simultaneously, the valence band (VB) potential should be positive enough to oxidize water or sacrificial donors, thereby sustaining continuous charge flow. The electronic band structure of the SnS₂@TAPA-BPDA POP (CB: −0.62 V vs. NHE; VB: +1.25 V vs. NHE) (Fig. S11) suggests that it is thermodynamically capable of driving both H₂ and H₂O₂ production under visible-light irradiation. The conduction band potential (−0.62 V) is significantly more negative than the reduction potentials of both H⁺/H₂ (0 V) and O₂/H₂O₂ (+0.68 V), enabling photogenerated electrons to simultaneously reduce protons to H₂ and molecular oxygen to H₂O₂ through stepwise single-electron transfer pathways. Meanwhile, the valence band potential (+1.25 V) is more positive than the H₂O/O₂ oxidation potential (+1.23 V), indicating that photogenerated holes possess sufficient oxidative power to oxidize water or sacrificial donors, thereby maintaining charge balance during the overall photocatalytic process. Therefore, the band alignment of this POP fulfills the thermodynamic requirements for the concurrent photocatalytic production of H₂ and H₂O₂, with electron transfer partitioned between proton and O₂ reduction pathways.^90,91

The trapping experiments studied the intermediates involved in oxygen reduction. The incorporation of p-benzoquinone (p-BQ, a scavenger of O₂˙⁻) into the reaction system greatly reduces the creation of H₂O₂ with insignificant yield. This suggests that BQ quenched the excited electron and inhibited the generation of O₂˙⁻, resulting in decreased H₂O₂ formation. The progressive decline in the BQ peak at 246 nm (Fig. 3f) supports the generation of H₂O₂via the superoxide route, in which conduction band electrons play a larger part in catalysis than valence band holes. POPs produce electrons (e⁻) that reduce O₂, resulting in O₂˙⁻.⁹²

Fig. 3 High-resolution XPS of (a) C 1s, (b) N 1s, (c) S 2p, (d) Sn 3d and (e) XPS survey scan of SnS₂@TAPA-BPDA, (f) oxidation of benzoquinone (BQ) in the presence of superoxide radical (O₂˙⁻), (g) oxidation of 1,3-diphenylisobenzofuran (DPBF) in the presence of singlet oxygen (¹O₂), (h) EPR spectra of DMPO–O₂˙⁻, (i) EPR spectra of .

On the other hand, ¹O₂ is produced by an energy transfer mechanism in which singlet excitons are converted to triplet excitons and then they react with ground-state oxygen (³O₂). To further understand the mechanism, radical scavenger tests were performed to detect the presence of singlet oxygen using 1,3-diphenylisobenzofuran (DPBF). DPBF is a quencher for singlet oxygen (¹O₂) and can be detected by oxidizing it to 1,2-dibenzoylbenzene. The progressive decline in the DPBF signal at 410 nm (Fig. 3g) verifies the formation of H₂O₂via the singlet oxygen route.^93,94 In order to generate O₂˙⁻ from molecular oxygen, it is clear that the POP composite is essential for speeding up conduction band electrons and promoting the charge transfer process. Furthermore, through an energy transfer mechanism, the POP composite accelerates the conduction band electrons, producing ¹O₂ from molecular oxygen (Fig. 4).

Fig. 4 Plausible reaction mechanism for the photocatalytic hydrogen and hydrogen peroxide generation.

To further elucidate the mechanism of H₂O₂ production by SnS₂@TAPA-BPDA, electron spin resonance (ESR) with DMPO spin trapping was employed to confirm the presence of superoxide radicals (O₂˙⁻). As depicted in Fig. 3h, a characteristic 1 : 1 : 1 : 1 quartet signal, indicative of DMPO–O₂˙⁻ adducts,⁹⁵ was observed after 10 minutes of irradiation. Complementary ESR spin trapping using TEMP was also performed to detect the generation of singlet oxygen during H₂O₂ production. Singlet oxygen oxidizes TEMP to form TEMPO (Fig. 3i), which exhibits a distinctive three-line ESR spectrum with almost equal intensity.⁹⁶ These ESR findings provide further evidence that the formation of H₂O₂ is mediated by both superoxide radicals and singlet oxygen as key intermediates, arising from the reduction of O₂ on the SnS₂@TAPA-BPDA composite. This suggests that oxygen reduction is the primary pathway for H₂O₂ production and that, rather than occurring in a single step, the overall process over SnS₂@TAPA-BPDA involves multiple sequential reactions.

Scale up challenges

Scaling-up of photocatalytic hydrogen and hydrogen peroxide production in SnS₂@TAPA-BPDA is constrained by a number of inherent limitations. Catalyst loading must be precisely controlled since over-deposition of SnS₂ would clog the pore network and hinder photon use, whereas under-loading restricts access to active sites.⁹⁷ Manipulation of light becomes more complicated at higher scales since penetration depth is smaller, scattering influences occur, and irradiation is inhomogeneous, all contributing to reduced photocatalytic activity.⁹⁸ In addition, effective product separation of H₂ from H₂O₂ must be guaranteed to avoid back reactions and safe harvesting.⁹⁹ Photocatalyst stability in practical water matrices is another factor because soluble salts, organics, and microbial impurities might cause deactivation, fouling, or structure damage.¹⁰⁰ All these complex barriers need to be overcome in scaling SnS₂@TAPA-BPDA systems from laboratory to practical large-scale use.

Recyclability of the photocatalyst

To assess the reusability of the photocatalyst, photocatalysis was performed under ideal reaction conditions with a catalytic quantity of POP composite. The catalyst was separated from the reaction mixture through a centrifugal method after the hydrogen peroxide production and associated procedures were completed. Subsequently, impurities were removed using ethanol, and the photocatalytic system was restored. The recovered POP composite was then utilized for the next experiment. Under optimal conditions, the POP composite can be recycled up to six times (Fig. 5) before its performance begins to decline significantly.

Fig. 5 Recycling diagrams of the recovered catalyst for SnS₂@TAPA-BPDA catalyzed H₂O₂ production reaction.

Conclusion

In conclusion, we have successfully developed a heterogeneous catalytic system for effective photocatalytic hydrogen evolution and hydrogen peroxide production. This organic polymer (SnS₂@TAPA-BPDA) is thoroughly characterized by various analytical techniques, which confirms the successful decoration of metallic SnS₂ over the POP surface. Intriguingly, the composite catalyst can promote photocatalytic hydrogen evolution and hydrogen peroxide production under mild conditions (room temperature, atmospheric pressure, co-catalyst-free). Overall, this work describes an intriguing heterogeneous platform for efficient hydrogen and hydrogen peroxide production.

Conflicts of interest

The authors declare no competing financial interests.

Data availability

Data will be provided after receiving request from the authors.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5se01138h.

Acknowledgements

SMI acknowledges the BRNS, Govt of India, (project reference no. 58/14/15/2022-BRNS/37077) Govt of India, for providing financial support. We are grateful to Prof. Asim Bhaumik, School of Materials Science, IACS, Kolkata, India, and Dr Rajaram Bal, IIP Dehradun for providing various support in our research. This work was also funded by the Ongoing Research Funding program (ORF-2025-672), King Saud University, Riyadh, Saudi Arabia. We are thankful to Dr Gautam Pramanik, UGC-DAE CSR Kolkata for providing various support in our research. SMI is also thankful to UGC-DAE for providing financial support (Proj. Ref. CRS/2022-23/02/830). We acknowledge the Department of Science and Technology, DST, Govt of India for providing funds to the Department of Chemistry, University of Kalyani under the DST FIST program.

References

1. S. Singh, Energy: Crises, Challenges and Solutions, 2021, vol. 1, pp. 1–17.
2. F. Johnsson, J. Kjärstad and J. Rootzén, Clim. Policy, 2019, 19, 258–274.
3. I. Dincer, Renewable Sustainable Energy Rev., 2000, 4, 157–175.
4. (a) S. O. Akpasi, I. M. S. Anekwe, E. K. Tetteh, U. O. Amune, S. I. Mustapha and S. L. Kiambi, Clean Energy, 2025, 9, 52–88; (b) V. Krishnan, L. Gonzalez-Marciaga and J. A. McCalley, Energy, 2014, 73, 943–957.
5. H. Sun, Z. Wang, Q. Meng and S. White, Int. J. Hydrogen Energy, 2025, 105, 10–22.
6. A. Yapicioglu and I. Dincer, Renewable Sustainable Energy Rev., 2019, 103, 96–108.
7. A. Valera-Medina, F. Amer-Hatem, A. K. Azad, I. C. Dedoussi, M. De Joannon, R. X. Fernandes and M. Costa, Energy Fuels, 2021, 35, 6964–7029.
8. G. J. Stiegel and M. Ramezan, Int. J. Coal Geol., 2006, 65, 173–190.
9. J. Li and W. Cheng, Int. J. Hydrogen Energy, 2020, 45, 27979–27993.
10. L. Chen, Z. Qi, S. Zhang, J. Su and G. A. Somorjai, Catalysts, 2020, 10, 858.
11. R. Kumar, A. Kumar and A. Pal, Mater. Today: Proc., 2021, 46, 5353–5359.
12. K. Zeng and D. Zhang, Prog. Energy Combust. Sci., 2010, 36, 307–326.
13. K. G. Dos Santos, C. T. Eckert, E. De Rossi, R. A. Bariccatti, E. P. Frigo, C. A. Lindino and H. J. Alves, Renewable Sustainable Energy Rev., 2017, 68, 563–571.
14. A. Franco and C. Giovannini, Sustainability, 2023, 15, 16917.
15. L. Samylingam, N. Aslfattahi, C. K. Kok, K. Kadirgama, M. Schmirler, T. Yusaf, D. Ramasamy and M. F. Ghazali, Korean J. Chem. Eng., 2024, 41, 2961–2984.
16. (a) X. Tao, Y. Zhao, S. Wang, C. Li and R. Li, Chem. Soc. Rev., 2022, 51, 3561–3608; (b) S. Senapati, R. Bal, M. Mohapatra, N. Dalai and B. Jena, Energy Technol., 2024, 12, 2300979.
17. C. Xiang, A. Z. Weber, S. Ardo, A. Berger, Y. Chen, R. Coridan, K. T. Fountaine, S. Haussener, S. Hu, R. Liu and N. S. Lewis, Angew. Chem., Int. Ed., 2016, 55, 12974–12988.
18. (a) Y. Y. Wang, Y. X. Chen, T. Barakat, Y. J. Zeng, J. Liu, S. Siffert and B. L. Su, J. Energy Chem., 2022, 66, 529–559; (b) S. Kumar, A. Kumar, A. Kumar and V. Krishnan, Catal. Rev., 2020, 62, 346–405.
19. J. Liu, Y. Zhang, L. Lu, G. Wu and W. Chen, Chem. Commun., 2012, 48, 8826–8828.
20. J. W. Ager, M. R. Shaner, K. A. Walczak, I. D. Sharp and S. Ardo, Energy Environ. Sci., 2015, 8, 2811–2824.
21. (a) J. Seo, T. Takata, M. Nakabayashi, T. Hisatomi, N. Shibata, T. Minegishi and K. Domen, J. Am. Chem. Soc., 2015, 137, 12780–12783; (b) S. Senapati, L. Pradhan, B. Mohanty, R. Bal, M. Mohapatra and B. Jena, J. Phys. Chem. Solids, 2025, 192, 112924.
22. (a) M. Bowker, Green Chem., 2011, 13, 2235–2246; (b) S. S. Acharyya, S. Ghosh, N. Siddiqui, L. S. Konathala and R. Bal, RSC Adv., 2015, 5, 4838–4843.
23. S. P. Teong, X. Li and Y. Zhang, Green Chem., 2019, 21, 5753–5780.
24. W. R. Sanderson, Pure Appl. Chem., 2000, 72, 1289–1304.
25. M. F. Kabir, E. Vaisman, C. H. Langford and A. Kantzas, Chem. Eng. J., 2006, 118, 207–212.
26. (a) B. McEvoy and N. J. Rowan, J. Appl. Microbiol., 2019, 127, 1403–1420; (b) S. A. Ali, I. Sadiq and T. Ahmad, Langmuir, 2024, 40, 10414–10432; (c) I. Sadiq, S. A. Ali and T. Ahmad, ACS Appl. Energy Mater., 2025, 8, 5544–5563.
27. M. S. Noh, S. H. Jung, O. Kwon, S. I. Lee, S. J. Yang, E. Hahm and B. H. Jun, ACS Omega, 2020, 5, 29382–29387.
28. M. Rothe, E. Rohm, E. Mitchell, N. Bedrosian, C. Kelly, G. String and D. Lantagne, medRxiv, 2020, 2020–11.
29. (a) M. Rezende, L. Ferri, S. Kossatz, A. D. Loguercio and A. Reis, Oper. Dent., 2016, 41, 388–396; (b) Z. Liu, Y. Bian, G. Dawson, J. Zhu and K. Dai, Chin. Chem. Lett., 2025, 36, 111272.
30. (a) L. Ding, Z. Pan and Q. Wang, Chin. Chem. Lett., 2024, 35, 110125; (b) F. C. Marson, R. S. Gonçalves, C. O. Silva, L. T. Â. Cintra, R. C. Pascotto, P. D. Santos and A. L. F. Briso, Oper. Dent., 2015, 40, 72–79.
31. G. G. O. Oor, Catalytic Oxidations with Hydrogen Peroxide as Oxidant, 1992, vol. 13.
32. B. C. Nyamunda, F. Chigondo, M. Moyo, U. Guyo, M. Shumba and T. Nharingo, J. At. Mol., 2013, 3, 23.
33. O. C. Esan, X. Shi, Z. Pan, Y. Liu, X. Huo, L. An and T. S. Zhao, J. Power Sources, 2022, 548, 232114.
34. S. Luo, W. Pan, Y. Wang, X. Zhao, K. W. Leong and D. Y. Leung, Appl. Energy, 2022, 323, 119610.
35. H. Jabri, A. Sahibeddine and R. Amrousse, Innovative Materials for Environmental and Aerospace Applications, 2025, 247–278.
36. M. Ventura and P. Mullens, The 35th Joint Propulsion Conference and Exhibit, 1999, p. 2880.
37. J. M. Campos-Martin, G. Blanco-Brieva and J. L. Fierro, Angew. Chem., Int. Ed., 2006, 45, 6962–6984.
38. M. Besson, P. Gallezot and C. Pinel, Chem. Rev., 2014, 114, 1827–1870.
39. D. Kovačič, Direct Synthesis of Hydrogen Peroxide Using Monometallic and Bimetallic Pd Based Catalysts Supported on TiO₂, PhD thesis, Cardiff University, 2022.
40. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80.
41. Y. Shiraishi, T. Takii, T. Hagi, S. Mori, Y. Kofuji, Y. Kitagawa, S. Tanaka, S. Ichikawa and T. Hirai, Nat. Mater., 2019, 18, 985–993.
42. A. Chakraborty, A. Alam, U. Pal, A. Sinha, S. Das, T. Saha-Dasgupta and P. Pachfule, Nat. Commun., 2025, 16, 503.
43. H. Hou, X. Zeng and X. Zhang, Angew. Chem., Int. Ed., 2020, 59, 17356–17376.
44. (a) J. Zhang, L. Zheng, F. Wang, C. Chen, H. Wu, S. A. K. Leghari and M. Long, Appl. Catal., B, 2020, 269, 11877; (b) M. Fazil, J. Ahmed and T. Ahmad, Catal. Today, 2024, 115103.
45. (a) X. Xu, D. Chen, Z. Yi, M. Jiang, L. Wang, Z. Zhou, X. Fan, Y. Wang and D. Hui, Langmuir, 2013, 29, 5573–5580; (b) S. Kumar, N. L. Reddy, H. S. Kushwaha, A. Kumar, M. V. Shankar, K. Bhattacharyya, A. Halder and V. Krishnan, ChemSusChem, 2017, 10, 3588–3603; (c) S. Kumar, N. L. Reddy, A. Kumar, M. V. Shankar and V. Krishnan, Int. J. Hydrogen Energy, 2018, 43, 3988–4002; (d) S. A. Ali, I. Sadiq and T. Ahmad, Chem. Commun., 2025, 61, 8157–8169.
46. (a) S. Thakur, T. Kshetri, N. H. Kim and J. H. Lee, J. Catal., 2017, 345, 78–86; (b) U. Farooq and T. Ahmad, Int. J. Hydrogen Energy, 2024, 64, 290–300.
47. (a) R. Saha, B. Joarder, A. S. Roy, S. M. Islam and S. Kumar, Chem.–Eur. J., 2013, 19, 16607–16614; (b) G. Kumar, M. Singh, R. Goswami and S. Neogi, ACS Appl. Mater. Interfaces, 2020, 12, 48642–48653; (c) V. Saptal, D. B. Shinde, R. Banerjee and B. M. Bhanage, Catal. Sci. Technol., 2016, 6, 6152–6158; (d) M. Sarkar, P. Chakrabortty, M. Sengupta, A. C. Kothari, M. S. Islam and S. M. Islam, Ind. Eng. Chem. Res., 2024, 63, 5573–5590; (e) P. Sarkar, A. H. Chowdhury, S. Biswas, A. Khan and S. M. Islam, Mater. Today Chem., 2021, 21, 100509; (f) R. Sani, T. K. Dey, M. Sarkar, P. Basu and S. M. Islam, Mater. Adv., 2022, 14, 5575–5597; (g) S. Raza, I. Sadiq, S. Shaheen, M. Saniya and T. Ahmad, Sustainability Sci. Technol., 2025, 2, 012002.
48. (a) R. A. Molla, M. A. Iqubal, K. Ghosh, A. S. Roy and S. M. Islam, RSC Adv., 2014, 4, 48177–48190; (b) A. Roy, P. Sarkar, M. S. Islam, K. Yusuf and S. M. Islam, ChemCatChem, 2024, 16, e202400920; (c) S. Roy, T. Chatterjee, B. Banerjee, N. Salam, A. Bhaumik and S. M. Islam, RSC Adv., 2014, 4, 46075–46083; (d) Y. Meng, Y. Luo, J. L. Shi, H. Ding, X. Lang, W. Chen, A. Zheng, J. Sun and C. Wang, Angew. Chem., Int. Ed., 2020, 59, 3624–3629; (e) D. Chakraborty, A. Modak and A. Bhaumik, ChemCatChem, 2025, 17, e00807; (f) A. Ghosh, M. Mondal, R. N. Manna and A. Bhaumik, J. Colloid Interface Sci., 2024, 658, 415–4249; (g) P. Chakrabortty, S. Kumar, A. Chowdhury, A. Khan, A. Bhaumik and S. M. Islam, ChemCatChem, 2024, 16, e202301539.
49. (a) N. Li, Q. Wang and H. L. Zhang, Chem. Rec., 2020, 20, 413–428; (b) N. Haque, S. Biswas, M. Dolai, D. K. Nandi, M. Sarkar and S. M. Islam, Mol. Catal., 2023, 536, 112900.
50. Y. Byun, L. S. Xie, P. Fritz, T. Ashirov, M. Dincă and A. Coskun, Angew. Chem., 2020, 132, 15278–15282.
51. (a) Y. Hou, Y. Zhu, Y. Xu and X. Wang, Appl. Catal., B, 2014, 156–157, 122–127; (b) S. A. Ali, S. Majumdar, P. K. Chowdhury, N. Alhokbany and T. Ahmad, ACS Appl. Energy Mater., 2024, 7, 2881–2895; (c) S. Kumar, A. Kumar, V. Navakoteswara Rao, A. Kumar, M. V. Shankar and V. Krishnan, ACS Appl. Energy Mater., 2019, 2, 5622–5634.
52. B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff and J. K. Nørskov, J. Am. Chem. Soc., 2005, 127, 5308–5309.
53. F. Wang, Y. Zhang, Y. Gao, P. Luo, J. Su, W. Han, K. Liu, H. Li and T. Zhai, Small, 2019, 15, 1901347.
54. B. Li, T. Xing, M. Zhong, L. Huang, N. Lei, J. Zhang, J. Li and Z. Wei, Nat. Commun., 2017, 8, 1958.
55. (a) Y. Sun, H. Cheng, S. Gao, Z. Sun, Q. Liu, Q. Liu, F. Lei, T. Yao, J. He, S. Wei and Y. Xie, Angew. Chem., Int. Ed., 2012, 51, 8727–8731; (b) X. Zhu, X. Luo, H. Yuan and C. Tian, RSC Adv., 2018, 8, 3304–3311.
56. I. Shown, S. Samireddi, Y.-C. Chang, R. Putikam, P.-H. Chang, A. Sabbah, F.-Y. Fu, W.-F. Chen, C.-I. Wu, T.-Y. Yu, P.-W. Chung, M. C. Lin, L.-C. Chen and K.-H. Chen, Nat. Commun., 2018, 9, 169.
57. (a) X. Jia, C. Tang, R. Pan, Y. Long, C. Gu and J. Li, ACS Appl. Mater. Interfaces, 2018, 10, 18073–18081; (b) J. Yu, C.-Y. Xu, F.-X. Ma, S.-P. Hu, Y.-W. Zhang and L. Zhen, ACS Appl. Mater. Interfaces, 2014, 6, 22370–22377; (c) J. Xia, D. Zhu, L. Wang, B. Huang, X. Huang and X.-M. Meng, Adv. Funct. Mater., 2015, 25, 4255–4261.
58. Y. Zhang and S. J. Park, J. Mater. Chem. A, 2018, 6, 20304–20312.
59. J. Yu, C. Y. Xu, F. X. Ma, S. P. Hu, Y. W. Zhang and L. Zhen, ACS Appl. Mater. Interfaces, 2014, 6, 22370–22377.
60. H. Yu, M. Dai, J. Zhang, W. Chen, Q. Jin, S. Wang and Z. He, Small, 2023, 19, 2205767.
61. L. Liu, Z. Wang, J. Zhang, O. Ruzimuradov, K. Dai and J. Low, Adv. Mater., 2023, 35, 2300643.
62. S. Gbadamasi, M. Mohiuddin, V. Krishnamurthi, R. Verma, M. W. Khan, S. Pathak, K. Kalantar-Zadeh and N. Mahmood, Chem. Soc. Rev., 2021, 50, 4684–4729.
63. (a) T. Banerjee, F. Haase, G. Savasci, K. Gottschling, C. Ochsenfeld and B. V. Lotsch, J. Am. Chem. Soc., 2017, 139, 16228–16234; (b) A. Mukherji, R. Bal and R. Srivastava, ChemElectroChem, 2020, 7, 2740–2751.
64. L. Li, W. Fang, P. Zhang, J. Bi, Y. He, J. Wang and W. Su, J. Mater. Chem. A, 2016, 4, 12402–12406.
65. (a) H. Ou, P. Yang, L. Lin, M. Anpo and X. Wang, Angew. Chem., Int. Ed., 2017, 56, 10905–10910; (b) K. Maeda, X. Wang, Y. Nishihara, D. Lu, M. Antonietti and K. Domen, J. Phys. Chem. C, 2009, 113, 4940–4947.
66. (a) J. R. Song, W. G. Duan and D. P. Li, Molecules, 2018, 23, 1732; (b) R. S. Sprick, J.-X. Jiang, B. Bonillo, S. Ren, T. Ratvijitvech, P. Guiglion, M. A. Zwijnenburg, D. J. Adams and A. I. Cooper, J. Am. Chem. Soc., 2015, 137, 3265–3270.
67. P. Kuhn, M. Antonietti and A. Thomas, Angew. Chem., Int. Ed., 2008, 47, 3450–3453.
68. (a) S. Kuecken, A. Acharjya, L. Zhi, M. Schwarze, R. Schomacker and A. Thomas, Chem. Commun., 2017, 53, 5854–5857; (b) M. S. Lokolkar, M. K. Pal, S. Dey and B. M. Bhanage, Catal. Lett., 2023, 153, 2359–2367.
69. (a) I. Ahmad, R. Bousbih, A. Mahal, W. Q. Khan, M. Aljohani, M. A. Amin, N. N. Jafar, M. S. Jabir, H. Majdi, A. S. Alshomrany and M. Shaban, Mater. Sci. Semicond. Process., 2024, 180, 108578; (b) I. Ahmad, S. A. AlFaify, K. M. Alanezi, M. Q. Alfaifi, M. M. Abduljawad and Y. Liu, Dalton Trans., 2025, 54, 1402–1417.
70. (a) I. Ahmad, S. Shukrullah, M. Y. Naz, M. A. Rasheed, M. Ahmad, E. Ahmed, M. S. Akhtar, N. R. Khalid, A. Hussain and S. Khalid, Int. J. Hydrogen Energy, 2021, 46, 26711–26724; (b) I. Ahmad, S. Shukrullah, M. Y. Naz and H. N. Bhatti, Dalton Trans., 2023, 52, 6343–6359.
71. (a) I. Ahmad, S. Shukrullah, M. Y. Naz and H. N. Bhatti, React. Chem. Eng., 2023, 8, 1159–1175; (b) I. Ahmad, M. Q. Alfaifi, S. B. Ahmed, M. M. Abduljawad, Y. A. Alassmy, S. A. Alshuhri and T. L. Tamang, Int. J. Hydrogen Energy, 2024, 96, 1006–1066.
72. D. Shang, D. Li, B. Chen, B. Luo, Y. Huang and W. Shi, ACS Sustainable Chem. Eng., 2021, 9, 14238–14248.
73. (a) W. Liu, J. Wang, S. Song, L. Hao, J. Liu, Y. An, Y. Guo, Q. Wu, C. Wang and Z. Wang, Food Chem., 2022, 367, 130653; (b) S. Xiong, Y. Zhang, W. Zhang, N. Yang, F. Lv, J. Guo, X. Cui, K. Fang, M. Chen, C. Wang and C. Hua, J. Electron. Mater., 2024, 53, 2656–2665.
74. Q. Xie, H. Zhou, Z. Lv, H. Liu and H. Guo, J. Mater. Chem. A, 2017, 5, 6299–6309.
75. H. Chang, I. Khan, A. Yuan, S. Khan, S. Sadiq, A. Khan, S. A. Shah, L. Chen, M. Humayun and M. Usman, ACS Appl. Nano Mater., 2024, 7, 10451–10465.
76. (a) L. Jing, Y. Xu, Z. Chen, M. He, M. Xie, J. Liu, H. Xu, S. Huang and H. Li, ACS Sustainable Chem. Eng., 2018, 6, 5132–5141; (b) T. Di, B. Zhu, B. Cheng, J. Yu and J. Xu, J. Catal., 2017, 352, 532–541.
77. (a) H. El Marouazi, V. Keller and I. Janowska, Surf. Interfaces, 2023, 36, 102510; (b) A. Deng, Y. Sun, Z. Gao, S. Yang, Y. Liu, H. He, J. Zhang, S. Liu, H. Sun and S. Wang, Nano Energy, 2023, 108, 108228; (c) T. R. Gengenbach, G. H. Major, M. R. Linford and C. D. Easton, J. Vac. Sci. Technol., A, 2021, 39, 011201.
78. G. Mukherjee, J. Thote, H. B. Aiyappa, S. Kandambeth, S. Banerjee, K. Vanka and R. Banerjee, Chem. Commun., 2017, 53, 4461–4464.
79. S. Song, Z. Liang, W. Fu and T. Peng, ACS Appl. Mater. Interfaces, 2017, 9, 17013–17023.
80. L. Li, J. Yan, T. Wang, Z.-J. Zhao, J. Zhang, J. Gong and N. Guan, Nat. Commun., 2015, 6, 5881.
81. A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi and K. Domen, J. Am. Chem. Soc., 2002, 124, 13547–13553.
82. Z. Wang, P. K. Nayak, J. A. Caraveo-Frescas and H. N. Alshareef, Adv. Mater., 2016, 28, 3831–3892.
83. P. Chakrabortty, S. Ghosh, R. V. Singh, M. R. Pai, K. Yusuf and S. M. Islam, ACS Sustainable Chem. Eng., 2024, 12, 10978–10992.
84. Z. Teng, Q. Zhang, H. Yang, K. Kato, W. Yang, Y.-R. Lu, S. Liu, C. Wang, A. Yamakata, C. Su, B. Liu and T. Ohno, Nat. Catal., 2021, 4, 374–384.
85. M. Liu, P. Xia, L. Zhang, B. Cheng and J. Yu, ACS Sustainable Chem. Eng., 2018, 6, 10472–10480.
86. B. Chen, B. Ge, S. Fu, Q. Li, X. Chen, L. Li, J. Wang, Z. Yang, J. Ding, W. Fan, B. Mao and W. Shi, Chem. Eng. Sci., 2020, 226, 115843.
87. R. Shen, J. Xie, X. Lu, X. Chen and X. Li, ACS Sustainable Chem. Eng., 2018, 6, 4026–4036.
88. Q. Wu, X. Wang, H. Nie, Y. Zhou, H. Huang, M. Shao, Y. Liu and Z. Kang, Chem. Eng. J., 2021, 409, 128184.
89. M. Qasim, M. Liu and L. Guo, Catal. Today, 2023, 409, 119.
90. T. Wang, L. Zhang, J. Wu, M. Chen, S. Yang, Y. Lu and P. Du, Angew. Chem., Int. Ed., 2023, 62, e202311352.
91. B. Yan, Q. Gu, W. Cao, B. Cai, Y. Li, Z. Zeng, P. Liu, Z. Ke, S. Meng, G. Ouyang and G. Yang, Proc. Natl. Acad. Sci. U. S. A., 2024, 121, e2319286121.
92. D. Shao, L. Zhang, S. Sun and W. Wang, ChemSusChem, 2018, 11, 527–531.
93. Y. You, Org. Biomol. Chem., 2018, 16, 4044–4060.
94. T. Entradas, S. Waldron and M. Volk, J. Photochem. Photobiol., 2020, 204, 111787.
95. H. Che, X. Gao, J. Chen, J. Hou, Y. Ao and P. Wang, Angew. Chem., Int. Ed., 2021, 60, 25546–25550.
96. C. Mendoza, A. Désert, L. Khrouz, C. A. Páez, S. Parola and B. Heinrichs, Environ. Sci. Pollut. Res., 2021, 28, 25124–25129.
97. (a) M. Paiu, D. Lutic, L. Favier and M. Gavrilescu, Appl. Sci., 2025, 15, 5681; (b) P. Li, D. Kowalczyk, J. Liessem, M. M. Elnagar, D. Mitoraj, R. Beranek and D. Ziegenbalg, Beilstein J. Org. Chem., 2024, 20, 74–91.
98. (a) S. Wu and X. Quan, ACS ES&T Eng., 2022, 2, 1068–1079; (b) Q. Ruan, ChemCatChem, 2024, 16, e202301082.
99. S. R. Mishra, X. Gadore and M. Ahmaruzzaman, Mater. Today Sustain., 2024, 27, 100819.
100. S. Wang, Z. Xie, D. Zhu, S. Fu, Y. Wu, H. Yu, C. Lu, P. Zhou, M. Bonn, H. I. Wang and Q. Liao, Nat. Commun., 2023, 14, 6891.

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