Cation Mixing Controls Photocatalytic Activity of Ionic Carbon Nitrides by Switching Exciton Character and Electron Localization
Year: 2026
Type: preprint
Abstract: Ionic carbon nitrides of the poly(heptazine imide) (PHI) type count among the most versatile polymeric photocatalysts, whereby their performance depends sensitively on the incorporated alkali metal cations.However, the mechanistic origins of this cation dependence remain largely underexplored.Here, the effects of cation mixing on exciton generation and dynamics are elucidated for a series of water-soluble PHIs comprising binary combinations of Li⁺, Na⁺, K⁺, and Cs⁺.Photocatalytic H2O2 production reveals that all investigated mixtures except KCsPHI outperform the single-cation CsPHI benchmark, whereby LiCsPHI exceeds it almost threefold.Using band-resolved analysis of ultrafast transient absorption data recorded in the absence and presence of ethanol as hole quencher, complemented by hybrid-DFT and time-dependent Hartree-Fock calculations, this activity pattern is traced back to the character of the low-energy excitons: only in LiCsPHI do interlayer charge-transfer excitons conjoin with a strong localization of the conduction-band electron density at the reactive edge carbon atoms, which suppresses geminate recombination and accelerates hole extraction by up to an order of magnitude.Notably, this switch in exciton character leaves a spectroscopic fingerprint in the band-resolved decay kinetics of the shallow-trapped carriers, whereby only in LiCsPHI does the minor excited-state absorption band outlive the main band.Moreover, the calculated oscillator strength of the low-energy exciton correlates inversely with the photocatalytic activity, i.e., brighter is not better: the most active LiCsPHI is the one hosting the darkest (least emissive, most CT-like) low-energy exciton.This work establishes cation mixing as a rational strategy for engineering the excitonic landscape of ionic carbon nitrides and provides concrete design guidelines for multi-cation PHI photocatalysts.
Source: ChemRxiv
Language: English
Field: Energy
Domain: Physical Sciences
Open Access status: green
Awards: G1110023947, SUnlight-driven Next Generation Artificial photosynthesis bio-hybrid TEchnology platform for highly efficient carbon neutral production of solar fuels, TRR 234: Lichtgetriebene molekulare Katalysatoren in hierarchisch strukturierten Materialien – Synthese und mechanistische Studien, G369187184, G402306430, G7121873242
OpenAlex ID: W7221116469