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

Abstract

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 n_STH = 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.