This paper describes the preparation and characterization of a photocathode assembly for CO2 reduction to CO in 0.1M 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, ReI((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/cm2 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 t1/2 = 2.6 µs (kET = 2.7 × 105 s–1).
Wang, D.; Wang, Y.; Brady, M. D.; Sheridan, M. V.; Sherman, B. D.; Farnum, B. H.; Liu, Y.; Marquard, S. L.; Meyer, G. J.; Dares, C. J.; Meyer, T. J. A Donor-Chromophore-Catalyst Assembly for Solar CO2 Reduction. Chem. Sci. 2019, 10, 4436-4444. http://dx.doi.org/10.1039/c8sc03316a.