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5 figures.-- Supporting information available.-- This ACS article is provided under the terms of this ACS AuthorChoice/Editors' Choice via Creative Commons CC-BY agreement: https://pubs.acs.org/page/policy/authorchoice_ccby_termsofuse.html ; Developing highly efficient and stable photoelectrochemical (PEC) water-splitting electrodes via inexpensive, liquid phase processing is one of the key challenges for the conversion of solar energy into hydrogen for sustainable energy production. ZnO represents one the most suitable semiconductor metal oxide alternatives because of its high electron mobility, abundance, and low cost, although its performance is limited by its lack of absorption in the visible spectrum and reduced charge separation and charge transfer efficiency. Here, we present a solution-processed water-splitting photoanode based on Co-doped ZnO nanorods (NRs) coated with a transparent functionalizing metal–organic framework (MOF). The light absorption of the ZnO NRs is engineered toward the visible region by Co-doping, while the MOF significantly improves the stability and charge separation and transfer properties of the NRs. This synergetic combination of doping and nanoscale surface functionalization boosts the current density and functional lifetime of the photoanodes while achieving an unprecedented incident photon to current efficiency (IPCE) of 75% at 350 nm, which is over 2 times that of pristine ZnO. A theoretical model and band structure for the core–shell nanostructure is provided, highlighting how this nanomaterial combination provides an attractive pathway for the design of robust and highly efficient semiconductor-based photoanodes that can be translated to other semiconducting oxide systems. ; A.G.-G., A.K.S., F.M., A.G., D.A.Z., and D.A. acknowledge funding received from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No 722176. W.K.M., A.M.B., and J.H.F. acknowledge financial support from Spanish Ministry MINEICO under Project ENE2016-79282-C5-1-R (AEI/UE/FEDER) and the Gobierno de Aragón (Grupo Reconocido DGA T03_17R, FEDER/UE y DGA T03_20R). ; Peer reviewed

8 figures.-- Supplementary information available. ; The controlled modification of the electronic properties of ZnO nanorods via transition metal doping is reported. A series of ZnO nanorods were synthesized by chemical bath growth with varying Co content from 0 to 20 atomic % in the growth solution. Optoelectronic behavior was probed using cathodoluminescence, time-resolved luminescence, transient absorbance spectroscopy, and the incident photon-to-current conversion efficiency (IPCE). Analysis indicates the crucial role of surface defects in determining the electronic behavior. Significantly, Co-doping extends the light absorption of the nanorods into the visible region, increases the surface defects, shortens the non-radiative lifetimes, while leaving the radiative lifetime constant. Furthermore, for 1 atomic % Co-doping the IPCE of the ZnO nanorods is enhanced. These results demonstrate that doping can controllably tune the functional electronic properties of ZnO nanorods for applications. ; This work has received funding from the Horizon 2020 program of the European Union for research and innovation, under grant agreement no. 722176 (INDEED) and SNSF via project nr-40B2-0_176680 and BSCGI0-157705. WKM, AMB acknowledge Spanish MINEICO under project ENE2016-79282-C5-1-R (AEI/FEDER, UE) and Gobierno de Aragón (Grupo Reconocido DGAT03_17R (FEDER, UE and T03_20R). ; Peer reviewed