The stabilization of single-atom catalysts on semiconductor substrates is pivotal for advancing photocatalysis. TiO2, a widely employed photocatalyst, typically stabilizes single atoms at oxygen vacancies-sites that are accessible but prone to agglomeration under illumination. Here, we demonstrate that cation vacancies in Ti-deficient TiO2 nanosheets provide highly stable anchoring sites for Pt single atoms, enabling persistent photocatalytic hydrogen evolution. Ultrathin TiO2 nanosheets with intrinsic Ti4+ vacancies are synthesized via lepidocrocite-type titanate delamination and Pt single atoms are selectively trapped within these vacancies through a simple immersion process. The resulting Pt-decorated nanosheets exhibit superior photocatalytic hydrogen evolution performance, outperforming both Pt nanoparticle-loaded nanosheets and benchmarked Pt single-atom catalysts on P25. Crucially, Pt atoms anchored at Ti4+ vacancies display remarkable resistance to light-induced agglomeration, a key limitation of conventional single-atom photocatalysts. Density functional theory calculations reveal that Pt incorporation into Ti4+ vacancies is highly thermodynamically favorable and optimizes hydrogen adsorption energetics for enhanced catalytic activity. This work highlights the critical role of cation defect engineering in stabilizing single-atom co-catalysts and advancing the efficiency and durability of photocatalytic hydrogen evolution.

Applied Physics Division of Materials Science Department of Engineering Sciences and Mathematics Luleå University of Technology Luleå 971 87 Sweden

Department for Correlative Microscopy and Materials Data Fraunhofer Institute for Ceramic Technologies and Systems Äußere Nürnberger Straße 62 91301 Forchheim Germany

Department of Chemistry Universität Bayreuth Universitätsstraße 30 95447 Bayreuth Germany

Department of Materials Science and Engineering WW4 LKO Friedrich Alexander Universität Erlangen Nürnberg Martensstraße 7 91058 Erlangen Germany

Institute of Micro and Nanostructure Research and Center for Nanoanalysis and Electron Microscopy IZNF Friedrich Alexander Universität Erlangen Nürnberg Cauerstraße 3 91058 Erlangen Germany

Institute of Particle Technology Friedrich Alexander Universität Erlangen Nürnberg Cauerstraße 4 91058 Erlangen Germany

Laboratoire de Chimie de Coordination CNRS Université de Toulouse INPT UPR 8241 205 route de Narbonne Toulouse Cedex 4 31077 France

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

Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute Palacký University Šlechtitelů 27 Olomouc 78371 Czech Republic

Serbian Academy of Sciences and Art Kneza Mihaila 35 Belgrade 11000 Serbia

University of Belgrade Faculty of Physical Chemistry Studentski trg 12 16 Belgrade 11000 Serbia

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