This contribution describes the self-assembling and self-activating nature of a TiO₂ photocatalyst that generates hydrogen from water in the absence of sacrificial species or noble metal co-catalysts [1]. Such system forms when nanoparticles of reduced anatase TiO₂ are illuminated in an aqueous Ni²⁺ solution [2,3]. UV illumination creates in-situ a Ni⁺/TiO₂/Ti³⁺ photocatalyst that over time produces H₂ at a higher rate. Thus, UV light is both the synthesis tool that forms this active metastable photocatalytic entity and the energy provider for this entity to enable H₂ evolution from water. Operando X-ray absorption (XAS) and electron paramagnetic (EPR) spectroscopies show that key to self-assembly and self-activation is the light-induced formation of defects in the semiconductor [4], which enables the formation of monovalent Ni⁺ surface states. Metallic nickel states, i.e., Ni⁰, do not form, neither in the dark (resting state) nor under illumination (active state). Once the catalyst is assembled, the Ni⁺ surface states act as electron relay for electron transfer to form H₂ from water. Self-amplifying reaction schemes of this type entail considerable potential for developing a new generation of photocatalysts via simple one-pot synthesis routes. References [1] M. Altomare, S. Qin, V.A. Saveleva, Z. Badura, O. Tomanec, G. Zoppellaro, A. Vertova, A. Taglietti, A. Minguzzi, P. Ghigna, P. Schmuki, under review. [2] S. Qin, Z. Badura, N. Denisov, O. Tomanec, S. Mohajernia, N. Liu, S. Kment, G. Zoppellaro, P. Schmuki, Electrochem. Commun. 122 (2021) 106909. [3] A. Naldoni, M. Altomare, G. Zoppellaro, N. Liu, Š. Kment, R. Zbořil, P. Schmuki, ACS Catal. 9 (2019) 345–364. [4] E. Wierzbicka, X. Zhou, N. Denisov, J.E. Yoo, D. Fehn, N. Liu, K. Meyer, P. Schmuki, ChemSusChem. 12 (2019) 1900–1905.