Size-Controlled and Localised Nanoparticles on Photoactive Covalent Organic Frameworks for Artificial Photosynthesis

Concerning levels of CO₂ in the atmosphere have urged researchers to develop technologies that can not only reduce its atmospheric concentration, but also use CO₂ as a feedstock for producing carbon-based fuels and value-added chemicals. Solar irradiation, a renewable and abundant source of energy, can be used to drive these chemical transformations in a process known as artificial photosynthesis¹. Recently, porous materials, such as covalent organic frameworks (COFs), have been explored as photo-responsive supports for catalysts due to their remarkable physical and chemical stability, structural diversity and large surface areas². Furthermore, through careful selection of building blocks, a wider photo-absorption window can be targeted, while also tuning the bandgap to extend the lifetime of electron-hole pair separation, thus establishing a thermodynamically favourable process³.

The incorporation of metal catalysts, such as metal nanoparticles (MNPs), into these types of organic, photo-active supports creates a hybrid material which can facilitate redox reactions via an electron “donor-acceptor” type mechanism, i.e., electrons excited within the framework can be accepted by the MNP and subsequently used to carry out CO₂ reduction³. Furthermore the use of another MNP co-catalyst allows for effective separation of charges within the framework, retaining the photogenerated holes and facilitating the oxidation of a sacrificial agent, such as water in this case⁴. MNPs are widely used for catalysis due to their high surface energy and quantum size effects; in particular, gold nanoparticles (Au NPs) are highly selective towards CO₂ reduction to CO⁵. However, their aggregation can result in gradual loss of catalytic activity, therefore uniformly immobilising them on light-harvesting, porous supports is effective in extending their photocatalytic performance⁶. In addition, RuO₂ nanoparticles, which are widely used in oxidation reactions, possess excellent affinity toward O₂ gas with a favourable O₂ binding energy, low overpotential, and high OER activity⁷.

In this work, a photo-absorbing porphyrin-perylene COF has been decorated with Au NP and RuO₂ NPs, thus creating a novel, robust material for the purpose of artificial photosynthesis. The COF was functionalised with thiol groups to assist in localisation and stabilisation of the Au NPs, resulting in the formation of well-distributed and locally separated NPs anchored into the COF network. The size of the Au NPs has been modulated by adjusting the concentration of gold salt precursor vs. the number of thiol ligands. Previously synthesised RuO₂ nanoparticles were incorporated into the COF unoccupied pores to produce the final MNP-COF hybrid material. NPs size effect and metal loading concentration have been evaluated to enhance the material performance. Results of their photocatalytic efficiency for the simultaneous photocatalytic CO₂ reduction and water oxidation under visible light irradiation will be presented.


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