We present an approach to fabricate an efficient noble metal-free photocatalytic platform for H2 evolution and provide evidences that the photocatalyst active state forms via a photo-induced redox process in organic-water mixtures.To fabricate the photocatalytic platform, NiCu bilayers (thickness in the range of a few nm) are deposited by Ar-plasma sputtering on anodic TiO2 nanocavity arrays.[1] A subsequent thermal treatment triggers solid-state dewetting[2,3] of the metal bilayer, i.e. owing to surface diffusion, the Ni and Cu films agglomerate and inter-mix forming NiCu bimetallic nanoparticles at the TiO2 surface.[4] This approach allows for a full control over key features of the NiCu nanoparticles, e.g. size, loading, composition and co-catalytic H2 generation ability. We found that dewetted-alloyed NiCu nanoparticles not only are significantly more reactive than their pure Ni or Cu counterparts, but also lead to H2 generation rates that approach those of noble metal (Pt) modified TiO2 nanocavities.Characterization results (EDS-TEM, XPS and XRD) of the as-formed photocatalyst suggest the co-catalyst nanoparticles to feature a NiCu bimetallic core and an oxide (or hydroxide) shell – the latter presumably forms by surface oxidation under ambient conditions.To identify the chemical state of the NiCu nanoparticles during photocatalysis, we carried out XAS operando experiments[5,6] (beam line P65 at DESY – Petra III, Hamburg, Germany) at the Cu K and Ni K edges, in fluorescence mode, under UV light illumination in degassed ethanol-water solutions.Our results demonstrate that under operando conditions the co-catalyst is subjected to changes of the Ni and Cu chemical state:[7–9] we observe that surface Ni and Cu oxide species are reduced (by TiO2 conduction band electrons) in the early stage of illumination – this converts the co-catalyst nanoparticles into the active metallic NiCu phase.[1] J. E. Yoo, K. Lee, M. Altomare, E. Selli, P. Schmuki, Angew. Chemie Int. Ed. 2013, 52, 7514–7517.[2] C. V. Thompson, Annu. Rev. Mater. Res. 2012, 42, 399–434.[3] M. Altomare, N. T. Nguyen, P. Schmuki, Chem. Sci. 2016, 7, 6865–6886.[4] D. Spanu, S. Recchia, S. Mohajernia, O. Tomanec, Š. Kment, R. Zboril, P. Schmuki, M. Altomare, ACS Catal. 2018, 8, 5298–5305.[5] M. Fracchia, P. Ghigna, A. Vertova, S. Rondinini, A. Minguzzi, Surfaces 2018, 1, 138–150.[6] A. Minguzzi, O. Lugaresi, C. Locatelli, S. Rondinini, F. D’Acapito, E. Achilli, P. Ghigna, Anal. Chem. 2013, 85, 7009–7013.[7] J. S. Schubert, J. Popovic, G. M. Haselmann, S. P. Nandan, J. Wang, A. Giesriegl, A. S. Cherevan, D. Eder, J. Mater. Chem. A 2019, 7, 18568–18579.[8] B. Mei, K. Han, G. Mul, ACS Catal. 2018, 8, 9154–9164.[9] M. J. Muñoz-Batista, D. Motta Meira, G. Colón, A. Kubacka, M. Fernández-García, Angew. Chemie Int. Ed. 2018, 57, 1199–1203.