A donor-chromophore-catalyst assembly for solar CO2 reduction

摘要

We describe here the preparation and characterization of a photocathode assembly for CO2 reduction to CO in 0.1 M LiClO4 acetonitrile. The assembly was formed on 1.0 μm thick mesoporous films of NiO using a layer-by-layer procedure based on Zr(iv)–phosphonate bridging units. The structure of the Zr(iv) bridged assembly, abbreviated as NiO|-DA-RuCP2²⁺-Re(i), where DA is the dianiline-based electron donor (N,N,N′,N′-((CH2)3PO3H2)4-4,4′-dianiline), RuCP²⁺ is the light absorber [Ru((4,4′-(PO3H2CH2)2-2,2′-bipyridine)(2,2′-bipyridine))2]²⁺, and Re(i) is the CO2 reduction catalyst, Re^I((4,4′-PO3H2CH₂)₂-2,2′-bipyridine)(CO)₃Cl. Visible light excitation of the assembly in CO₂ saturated solution resulted in CO₂ reduction to CO. A steady-state photocurrent density of 65 μA cm^–2 was achieved under one sun illumination and an IPCE value of 1.9% was obtained with 450 nm illumination. The importance of the DA aniline donor in the assembly as an initial site for reduction of the RuCP²⁺ excited state was demonstrated by an 8 times higher photocurrent generated with DA present in the surface film compared to a control without DA. Nanosecond transient absorption measurements showed that the expected reduced one-electron intermediate, RuCP⁺, was formed on a sub-nanosecond time scale with back electron transfer to the electrode on the microsecond timescale which competes with forward electron transfer to the Re(i) catalyst at t_1/2 = 2.6 μs (k_ET = 2.7 × 10⁵ s^–1).