Hydrogen peroxide is considered a "green" reagent: when it decomposes, it leaves behind water and oxygen. But its industrial production is energy-intensive and leaves a noticeable carbon footprint. The alternative is photocatalysis: a material absorbs light and uses its energy for the reaction without fossil fuels.
Researchers from the Max Planck Institute of Colloids and Interfaces in Potsdam, together with international colleagues, proposed controlling not the composition of the photocatalyst but the arrangement of its "building blocks." The work was published in the journal Advanced Materials.
To do this, they took organic-inorganic ionic cocrystals, compounds in which the ions are already ordered. When heated, the crystals turn into the final material, and the preset order is preserved. The resulting photocatalyst was named CALiK: it is a crystalline carbon nitride of the polyheptazine imide class containing lithium and potassium ions in a controlled environment.
"These crystals were known, but we were the first to use them as precursors for carbon nitrides," says the head of the study, Christian M. Pelicano. According to him, the crystals contain the same components as conventional precursors, but the ions in them are already arranged in a specific way.
In conventional methods, ions are introduced randomly, for example through a salt melt, which gives only limited control. Comparative experiments with materials obtained by other methods helped assess the influence of the synthesis strategy itself.
Under the tested conditions, CALiK produced about three times more hydrogen peroxide than conventionally prepared KPHI, and about 40% more than the best material with a single type of cation, CALi. Under light with a wavelength of 410 nanometers, the quantum yield was 58%, the share of photons converted into electrons participating in the reaction. This is the best figure among the compared catalysts.
"Not only the type and amount of ions matter, but also their position in the lattice," notes first author Anna Lo Presti. Spectroscopy and electrochemical measurements showed that in CALiK the charges generated under light separate more efficiently and remain available for reaction with oxygen longer.
Calculations indicate that the distribution of lithium ions changes the electronic environment and facilitates oxygen activation. However, they are based on simplified models and do not fully reflect the real material. Nevertheless, the work shows how to optimize polymeric photocatalysts already at the synthesis stage.