A donor-chromophore-catalyst assembly for solar CO₂ reduction

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-RuCP₂ ²⁺-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).