Molecular-polaron-coupling-enhanced photocatalytic CO2 reduction on copper phthalocyanine/NiMgFe layered double hydroxide nanocomposites

Abstract

The quenching of photogenerated carriers in photocatalysis is an intrinsic adverse effect that cannot be coupled with the surface reaction step, which has a longer lifetime. To this end, we constructed organic/inorganic composites, copper phthalocyanine tetrasulfonate/NiMgFe-layered double hydroxides (CuPcS/NMF-LDHs), using an electrostatic assembly method. The unique photogenerated-carrier transfer mechanism was probed: strong coupling of charge transfer (CT) states to molecular vibration results in the formation of molecular polarons, which further accelerates carrier separation due to the confinement or shielding effect of polarons. Based mostly on transient absorption spectroscopy, in situ irradiated X-ray photoelectron spectroscopy, and electron paramagnetic resonance techniques, the carrier transfer mechanism could be clarified in more detail. Specifically, when photogenerated carriers are transferred from the NMF-LDHs to CuPcS, the electrons (e⁻) in CuPcS and the holes (h⁺) of the NMF-LDHs form a charge transfer singlet-state (¹CT). Then, heavy-atom-induced spin–orbit coupling promotes an electron spin-state flip to generate a CuPcS triplet excited state (T₁) with a 58.24 ns lifetime and forms a charge transfer triplet-state (³CT). Moreover, the photogenerated electrons in the NMF-LDHs lead to continuous injection of electrons into the CuPcS anions, inducing polarization. When the transfer of photoelectrons from the ligand to the metal (LMCT) occurs, the ³CT state will couple with the CuPcS molecular vibration to generate a [Cu(I)PcS]⁻ polaron, resulting in photogenerated-carrier localization. A series of long-range Förster energy transfers prolong the lifetime of photogenerated carriers to match the timescale of surface reaction. Meanwhile, the high exciton separation efficiency and active [Cu(I)PcS]⁻ polaron can efficiently convert CO₂ to CO with a yield 8.7 times that of unmodified NMF-LDHs and 5.2 times that of CuPcS. This work experimentally demonstrated the generation of [Cu(I)PcS]⁻ polarons induced by NMF-LDHs and their potential to activate CO₂. As far as possible, the barriers between photocatalysis, of interest in chemistry, and polarons, of interest in solid-state physics, are bridged.