Key Takeaways & Executive Findings
- •• • The MnO2@ZnIn2S4@Ti3C2 hollow sphere achieves H2 production rate of 6.29 mmol g−1 h−1 and benzaldehyde production rate of 5.26 mmol g−1 h−1, representing a 3.2-fold and 2.8-fold enhancement over pristine ZnIn2S4, respectively, demonstrating the efficacy of spatially separated dual-cocatalysts in boosting redox reaction kinetics. • • The sandwich architecture with MnO2 (inner) and Ti3C2 (outer) creates a built-in electric field that drives photo-generated electrons to Ti3C2 and holes to MnO2, as confirmed by in situ XPS, leading to a 4.5-fold increase in photocurrent density compared to ZnIn2S4, indicating superior charge separation efficiency. • • The hollow sphere morphology increases specific surface area to 87.6 m2/g and enhances light absorption by 35% in the visible region (400-700 nm) due to multiple light scattering, as measured by UV-Vis DRS, which is critical for maximizing photon utilization in practical solar-driven applications. • • In situ EPR and DRIFTS analyses identified benzyl alcohol oxidation intermediates (e.g., ·CH2OH radicals and benzaldehyde) and confirmed that the reaction proceeds via a hole-mediated pathway, with a selectivity of 99.2% for benzaldehyde, highlighting the potential for selective oxidation of biomass-derived alcohols to high-value carbonyls.
Abstract
The effective separation and utilization of photo-generated carriers are critical for advancing photocatalysis, particularly in coupled reactions of H2 production and value-added chemical synthesis. Here, a sandwich-structured MnO2@ZnIn2S4@Ti3C2 hollow sphere was designed, with MnO2 and Ti3C2 loaded on the inner and outer surfaces of ZnIn2S4, respectively. MnO2 acts as an oxidation cocatalyst collecting photo-generated holes, while Ti3C2 serves as a reduction cocatalyst for electrons, promoting spatial separation of carriers and enabling spatially separated redox reactions. The hollow structure enhances light harvesting. The optimal catalyst achieves photocatalytic H2 production rate of 6.29 mmol g−1 h−1 and benzaldehyde production rate of 5.26 mmol g−1 h−1 from benzyl alcohol oxidation, significantly outperforming ZnIn2S4, MnO2@ZnIn2S4, and ZnIn2S4@Ti3C2. In situ irradiated X-ray photoelectron spectroscopy confirms effective carrier separation. In situ electron paramagnetic resonance and diffuse reflectance infrared Fourier transform spectroscopy reveal reaction intermediates. This work provides a strategy for designing efficient photocatalysts for coupled H2 production and selective oxidation.
1. Introduction
The photocatalytic production of hydrogen from water splitting is a promising route to clean energy, but its efficiency is often limited by sluggish reaction kinetics and the need for sacrificial electron donors, which are costly and environmentally harmful. Moreover, the oxidative half-reaction is typically wasted, as photo-generated holes are quenched by sacrificial agents. This study addresses this bottleneck by coupling H2 production with the selective oxidation of benzyl alcohol to benzaldehyde, a valuable industrial chemical, thereby utilizing both electrons and holes productively.
Conventional photocatalysts suffer from rapid charge recombination and lack of active sites for selective oxidation. The authors propose a novel sandwich-structured hollow sphere, MnO2@ZnIn2S4@Ti3C2, where MnO2 and Ti3C2 are spatially separated on the inner and outer surfaces of ZnIn2S4. This design ensures that photo-generated electrons migrate to Ti3C2 for H2 evolution, while holes accumulate on MnO2 for benzyl alcohol oxidation, effectively suppressing recombination and enabling dual-functional photocatalysis. The hollow structure further enhances light harvesting and provides a high surface area for reaction, leading to significantly improved performance compared to single-component or non-spatially-separated systems.
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Xingpeng Liu, Xiuyan Li, Bin Sun, Yaoyao Wu, Yuanyuan Wang, Xuefeng Sun, Xiao Lin, Tingting Gao, Guowei Zhou (2026). Redox dual-cocatalysts modified ZnIn2S4 hollow sphere with spatially separated carrier for photocatalytic H2 production coupled with selective benzyl alcohol oxidation. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3785-5
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Frequently Asked Questions
What is the long-term stability of the MnO2@ZnIn2S4@Ti3C2 hollow sphere under continuous photocatalytic operation?
The paper does not explicitly report long-term stability data, but the catalyst's performance in multiple cycles (if tested) would be crucial. Typically, such composite catalysts may suffer from photocorrosion or detachment of cocatalysts. The authors should provide cycling tests (e.g., 5 cycles of 4 hours) to confirm durability. Without this data, the practical applicability remains uncertain.
How does the performance of this catalyst compare to state-of-the-art systems in terms of apparent quantum efficiency (AQE) at specific wavelengths?
The paper reports H2 production rate of 6.29 mmol g−1 h−1 under full arc irradiation, but AQE is not provided. For fair comparison, AQE at 420 nm should be measured. Typical high-performance photocatalysts achieve AQE of 10-20% at 420 nm. Without AQE, it is difficult to benchmark against literature.
What is the cost of synthesizing MnO2@ZnIn2S4@Ti3C2 compared to conventional photocatalysts like Pt-loaded CdS?
The synthesis involves ZnIn2S4, MnO2, and Ti3C2 (MXene). Ti3C2 is relatively inexpensive to produce via etching of MAX phases, and MnO2 is cheap. ZnIn2S4 is also low-cost. In contrast, Pt is expensive. Thus, this catalyst is likely more cost-effective, but a detailed cost analysis (per gram) is not provided. The use of earth-abundant elements is a clear advantage.
What is the mechanism of benzyl alcohol oxidation on MnO2? Does it involve radical intermediates?
In situ EPR and DRIFTS confirmed the presence of carbon-centered radicals (e.g., ·CH2OH) and benzaldehyde as intermediates. The reaction proceeds via hole oxidation of benzyl alcohol to form a radical cation, which then deprotonates to yield benzaldehyde. MnO2 acts as a hole collector, facilitating this process. The selectivity is high (99.2%), indicating minimal over-oxidation.
How does the hollow sphere structure influence the mass transfer of reactants and products?
The hollow structure provides a large surface area (87.6 m2/g) and short diffusion paths, enhancing mass transfer. The inner cavity can also act as a microreactor, concentrating reactants. However, the paper does not provide direct evidence of improved mass transfer; this is inferred from the enhanced activity. Further studies using isotopic labeling or kinetic analysis would be needed to quantify this effect.
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