{TiO₂/TiO₂(B)} Quantum Dot Hybrids: A Comprehensible Route toward High-Performance [>0.1 mol gr^–1 h^–1] Photocatalytic H₂ Production from H₂O

Industrial-scale photocatalytic H2 production from H2O is a forward-looking aim in research and technology. To this end, understanding the key properties of TiO2 as a reference H2 production photocatalyst paves the way. Herein, we explore the TiO2 nanosize limits, in conjunction with the TiO2(B) nanophase, as a strategy to enhance the photocatalytic H2 production at >150 mmol/g/h. We present a targeted engineering realm on the synthesis of quantum dots (QDs) of TiO2 consisting of an anatase core (3 nm) interfaced with a nanometric shell of the TiO2(B) phase, synthesized through a modified flame spray pyrolysis (FSP) process. The {TiO2-anatase/TiO2(B)} core–shell QDs, with high specific surface area SSA = 360 m2/gr, achieve a milestone H2 production yield of 156 mmol/g/h and solar-to-H2 efficiency nSTH = 24.2%. We demonstrate that diligent control of the TiO2-anatase/TiO2(B) heterojunction, in tandem with lattice microstrain, are key factors that contribute to the superior H2 production, i.e., not only the high SSA of the QDs. At these quantum-size limits, the formation of lattice dislocations and interstitial Ti centers enhances photon absorption at ∼2.3 eV (540 nm), resulting in the generation of midgap states around the Fermi energy. EPR spectroscopy provides direct evidence that the photoinduced holes are preferentially localized on the TiO2(B) shell, while the photoinduced electrons accumulate on the anatase nanophase. Combined electrochemical and photocatalytic analyses demonstrate that the presence of an optimal TiO2(B) phase is significant for the photoactivity of TiO2 in all QD materials. High SSA does contribute to enhanced photocatalytic H2 production; however, its role is not the key-determinant. TiO2 lattice-dislocations in QDs provide extra DOS that can additionally assist in the photon utilization efficiency. Overall, the present work reveals a general concept, that is, at the quantum-size scale, lattice microstrain engineering and interstitial-states' formation are spontaneously facilitated by nanolattice physics. Diligent optimization of these properties offers a pathway toward high-end photocatalytic efficacy.