A donor-chromophore-catalyst assembly for solar CO(2) reduction

We describe here the preparation and characterization of a photocathode assembly for CO(2) reduction to CO in 0.1 M LiClO(4) 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-RuCP(2)(2+)-Re(i), where DA is the dianiline-based electron donor (N,N,N′,N′-((CH(2))(3)PO(3)H(2))(4)-4,4′-dianiline), RuCP(2+) is the light absorber [Ru((4,4′-(PO(3)H(2)CH(2))(2)-2,2′-bipyridine)(2,2′-bipyridine))(2)](2+), and Re(i) is the CO(2) reduction catalyst, Re(I)((4,4′-PO(3)H(2)CH(2))(2)-2,2′-bipyridine)(CO)(3)Cl. Visible light excitation of the assembly in CO(2) saturated solution resulted in CO(2) 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 RuCP(2+) 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 × 10(5) s(–1)).