Key Takeaways & Executive Findings
- •• • The Co SA-TT-COF/15 wt% CdS heterojunction with 50.5 nm TT-COF thickness achieves a CO production rate of 14157 μmol g−1 h−1 and 90.9% selectivity, representing a top-tier performance among COF-based photocatalysts, which is critical for industrial-scale solar fuel synthesis. • • The S-scheme heterojunction design enhances the built-in electric field and promotes charge separation, as evidenced by femtosecond transient absorption spectroscopy, leading to accelerated charge transfer kinetics and reduced recombination losses, essential for practical efficiency. • • The optimized TT-COF thickness (50.5 nm) balances active site density and accessibility, directly influencing the CO2 photoreduction rate; this thickness-dependent behavior provides a design guideline for maximizing catalytic output. • • The integration of Co single atoms into the COF matrix increases CO2 adsorption and activation, lowering energy barriers, as confirmed by theoretical calculations, which is pivotal for achieving high selectivity and conversion rates under mild conditions.
Abstract
Photocatalytic CO2 reduction is an attractive route to address sustainable energy crises and environmental issues, yet its efficiency is limited by poor charge separation, narrow light absorption, sluggish kinetics, and low CO2 adsorption/activation. Here, a series of Co SA-TT-COF/CdS S-scheme heterojunction photocatalysts were synthesized by integrating Co single atoms (Co SA) decorated covalent organic frameworks (COFs) with CdS nanotubes via in situ condensation and post-modification. The TT-COF layer thickness on CdS was regulated to optimize active site density and accessibility. The optimal Co SA-TT-COF/15 wt% CdS heterojunction, with a TT-COF thickness of 50.5 nm, achieved a CO production rate of 14157 μmol g−1 h−1 and a selectivity of 90.9%, among the best COF-based photocatalysts reported. Theoretical calculations, experiments, and femtosecond transient absorption spectroscopy revealed that the S-scheme heterojunction enhances the built-in electric field, optimizes energy levels, narrows bandgaps, extends light harvesting, improves charge separation and transfer kinetics, and lowers energy barriers for CO2 adsorption/activation, directly contributing to superior performance.
1. Introduction
Photocatalytic CO2 reduction offers a promising route to convert solar energy into valuable fuels, yet its commercial viability is hindered by low quantum efficiency, poor product selectivity, and rapid charge recombination. Conventional photocatalysts such as TiO2 or CdS suffer from wide bandgaps and insufficient active sites, limiting their performance under visible light. Moreover, the strong C=O bond (750 kJ mol−1) in CO2 requires substantial energy input, and multi-step proton-coupled electron transfer often leads to mixed products, complicating downstream separation and increasing costs.
This work addresses these bottlenecks by constructing an S-scheme heterojunction between a Co single-atom-decorated covalent organic framework (COF) and CdS nanotubes. The design synergistically combines the high surface area and tunable chemistry of COFs with the visible-light absorption of CdS, while the single-atom Co sites provide uniform and highly active centers for CO2 activation. The thickness of the COF layer is precisely controlled to optimize the interface and active site exposure, resulting in a dramatic enhancement in CO production rate and selectivity. This approach not only overcomes the limitations of individual components but also offers a scalable strategy for designing efficient photocatalysts for solar fuel generation.
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Tongxuan Li, Tingting Sun, Shaolong Zhang, Jianzhuang Jiang (2026). A single-atom COF/CdS S-scheme photocatalyst for COF thickness-dependent CO2 photoreduction. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3859-y
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Frequently Asked Questions
What is the role of the TT-COF layer thickness in the photocatalytic performance, and how was it optimized?
The TT-COF layer thickness directly influences the density and accessibility of active sites. An optimal thickness of 50.5 nm (15 wt% CdS) was found to maximize the CO production rate (14157 μmol g−1 h−1) and selectivity (90.9%). Thinner layers may lack sufficient active sites, while thicker layers could hinder charge transfer and light absorption. The thickness was controlled by adjusting the precursor ratio during synthesis.
How does the S-scheme heterojunction enhance charge separation and transfer kinetics?
The S-scheme heterojunction between Co SA-TT-COF and CdS creates a built-in electric field that drives photogenerated electrons from CdS to the COF and holes in the opposite direction, effectively separating charge carriers. This is confirmed by femtosecond transient absorption spectroscopy, which shows accelerated charge transfer and reduced recombination, leading to improved photocatalytic efficiency.
What is the industrial significance of achieving a CO production rate of 14157 μmol g−1 h−1?
This rate is among the highest reported for COF-based photocatalysts, indicating a potential for practical solar fuel production. For scale-up, such performance could translate to significant CO output per gram of catalyst, but further studies on stability, scalability, and cost are needed to assess commercial viability.
How does the Co single-atom decoration improve CO2 adsorption and activation?
Co single atoms provide isolated active sites that enhance CO2 adsorption and lower the energy barrier for CO2 activation, as shown by theoretical calculations. This leads to higher selectivity for CO production and improved reaction kinetics, as the Co sites facilitate the formation of key intermediates.
What are the main challenges for scaling up this photocatalyst system?
Challenges include the scalability of COF synthesis, the stability of the heterojunction under prolonged operation, and the cost of CdS and Co precursors. Additionally, achieving uniform thickness control on a large scale may be difficult. Future work should focus on optimizing synthesis conditions and exploring alternative materials to enhance durability and reduce costs.
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