Decoration of semiconductor photocatalysts with cocatalysts is generally done by a step-by-step assembly process. Here, we describe the self-assembling and self-activating nature of a photocatalytic system that forms under illumination of reduced anatase TiO2 nanoparticles in an aqueous Ni2+ solution. UV illumination creates in situ a Ni+/TiO2/Ti3+ photocatalyst that self-activates and, over time, produces H2 at a higher rate. In situ X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy show that key to self-assembly and self-activation is the light-induced formation of defects in the semiconductor, which enables the formation of monovalent nickel (Ni+) surface states. Metallic nickel states, i.e., Ni0, do not form under the dark (resting state) or 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, in the absence of sacrificial species or noble metal cocatalysts.

Department Materials Science WW 4 LKO Friedrich Alexander University of Erlangen Nuremberg Erlangen 91058 Germany

Dipartimento di Chimica Università degli Studi di Milano Via Golgi 19 Milan 20133 Italy

Dipartimento di Chimica Università degli Studi di Pavia Viale Taramelli 13 Pavia 27100 Italy

ESRF The European Synchrotron 71 Avenue des Martyrs CS40220 Grenoble Cedex 9 38043 France

Nanotechnology Centre VŠB Technical University of Ostrava 17 listopadu 2172 15 Ostrava Poruba 708 00 Czech Republic

PhotoCatalytic Synthesis PCS Group MESA Institute for Nanotechnology University of Twente P O Box 217 Enschede 7500 AE The Netherlands

Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute Palacký University Olomouc Křížkovského 511 8 Olomouc 779 00 Czech Republic

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