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• Sub-nanometer transition metal clusters are formed using photodeposition method. • Co clusters exhibit a much higher photocatalytic activity than that using Co 3 O 4. • The oxygen bridge vacancy acts as colour centre located on the TiO 2 (0 0 1) surface. • The color centers of TiO 2 significantly impact the cluster nucleation and growth. The transition elements (Fe, Co, Ni) are important substitutes for noble metals as the co-catalysts in hydrogen evolution reaction. Enhancing the atom utilization efficiency is highly desirable, but preparation and preservation of the transition metal catalysts with the size downscaling to sub-nanometer are challenging. In this work, Fe, Co and Ni clusters with subnano size as co-catalysts are synthesized by controllable growth on 2D ultrathin TiO 2 (B) nanosheets for hydrogen evolution using a photo deposition method. The ultra-small Fe, Co, and Ni co-catalysts on the TiO 2 (B) nanosheets exhibit remarkably enhanced photo activities, compared to that with the best Pt loading. It is found that the oxygen bridge vacancies act as the colour centres locating at the position adjacent to the O 1 -Ti 1 sites on the TiO 2 (B) (0 0 1) surface. The photo-colouration and photo-bleaching have significantly impact on the nucleation and growth of the metallic clusters. The high density and uniform dispersion of the sub-nano-sized transition metal clusters benefits for the photo reduction under light irradiation, as well as efficient carrier separation, leading to a great improvement of the photo activity. [ABSTRACT FROM AUTHOR]