Directed co-catalyst deposition on organic semiconductor heterojunctions: A new strategy for efficient photocatalytic hydrogen production

Yuwu Zhong's team at the Institute of Chemistry, Chinese Academy of Sciences, focused on the deposition of metal co-catalysts on the surface of organic semiconductor heterojunctions, and they recently proposed a scheme to construct heterojunctions using a small organic molecule containing multi-terminal pyridine (TAPyr) and graphitic carbon nitride (CN). They effectively regulated the dispersion state and deposition amount of the Pt co-catalyst on the heterojunction surface, and combined comparative experiments and theoretical calculations to clarify the important role of the pyridine group. This method of constructing heterojunctions also effectively promotes charge separation. For photocatalytic hydrogen production, a maximum hydrogen production rate of 6.6 mmol·h^-1· g_cat^-1 can be achieved, which is more than 30 times higher than that of the single-component CN system and maintains excellent stability, providing new research ideas for the conversion and storage of solar energy. These results were published as an open access article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background:
Photocatalytic water splitting to produce hydrogen is a highly promising technology for efficient solar energy conversion and storage. Polymer-based organic semiconductors, with their advantages of high visible light absorption efficiency, easily tunable band structures, low cost, and excellent photothermal stability, show great potential for photocatalytic hydrogen production. However, inherent limitations of organic semiconductors, such as short exciton diffusion distances and high Frenkel exciton binding energies, restrict the separation of electron-hole pairs, limiting photocatalytic efficiency. Currently, constructing heterojunctions using multicomponent organic semiconductors is an effective means of promoting photogenerated electron-hole separation and is widely used to construct efficient photocatalytic systems. Furthermore, the dispersion and deposition of metal cocatalysts (such as Pt) on the organic support surface are crucial to catalytic performance. Regulating the dispersion and loading of metal cocatalysts by modifying the organic support surface environment is of great significance. Therefore, developing organic semiconductor heterojunctions that combine efficient charge separation with precise control of cocatalyst deposition is an important approach to improving photocatalytic hydrogen production performance.

Highlights of this article:
The organic small molecule TAPyr forms a stable heterojunction with CN through π-π interactions and hydrogen bonding, effectively improving charge separation efficiency. The pyridine groups also serve as anchoring sites, promoting the extensive deposition and uniform dispersion of Pt on the support surface. Under identical conditions, heterojunctions constructed with the pyridine-free small molecule PhPyr exhibit significantly reduced Pt deposition and exhibit significant aggregation, demonstrating the crucial role of the pyridine groups in the deposition and dispersion of metal atoms. At 1 wt% TAPyr, 1 wt% Pt precursor, and pH 9, the TAPyr/CN heterojunction achieved a hydrogen production rate of 6.6 mmol·h⁻¹· g⁻¹ and an AQY of 1.8% under 500 nm monochromatic light, outperforming most similar catalysts and maintaining high activity for nearly 90 h. The role of the pyridine groups in promoting metal photodeposition, as well as the charge transfer and reaction pathways during the photocatalytic process, were investigated in detail using EPR, transient absorption spectroscopy, and density functional theory (DFT) calculations.

Summary and Outlook:
In summary, this study loaded the polypyridine small molecule TAPyr onto the CN surface to construct a stable heterojunction, expand the visible light absorption range, form a built-in electric field to enhance the separation of photogenerated charges, and achieve directional dispersed deposition of Pt co-catalysts through the terminal pyridine groups, significantly improving the utilization of catalytic active sites, increasing the catalytic activity by more than 30 times compared with the CN system, and maintaining the efficient hydrogen production process stable for nearly 90 hours; experiments and theoretical calculations elucidated the important role of organic small molecules containing polypyridine anchor groups in the directional co-catalyst deposition and their catalytic mechanism in the heterojunction system. In the future, more multifunctional small molecules can be designed to adapt to non-precious metal co-catalysts and be applied to different organic semiconductor substrates. Combined with the design of the full water splitting system and in situ characterization and multi-scale theoretical calculations, the existing stability problems can be solved and the mechanism research can be deepened, pushing this technology towards industrial solar hydrogen production.

This work was published as a Research Article in CCS Chemistry. The first author is Qi Zhao, a doctoral student at the Institute of Chemistry, Chinese Academy of Sciences. The corresponding authors are Yuwu Zhong, a researcher at the Institute of Chemistry, and Kun Tang, a postdoctoral fellow. This work was supported by the National Natural Science Foundation of China and the Youth Innovation Promotion Association of the Chinese Academy of Sciences.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.