Interfacial Quantum States in ZrO2 Quantum Dots as a Strategy to Activate Photocatalytic Efficiency

Ultrasmall quantum dots (QDs) exhibit electronic behavior inaccessible to bulk counterparts, yet the role of structurally disordered interfaces remains unresolved, particularly in wide-band gap oxides. Herein, using the flame spray pyrolysis (FSP) technology, we engineered ZrO2/ZrO2–x QDs comprising tetragonal ZrO2 QDs (2–5 nm) tightly interfaced with a semiamorphous, subnanometric ZrO2–x layer. Rather than forming a conventional heterojunction, electronic coupling between a {surface distorted hydroxylated ZrO2–x}, and the {quantum-confined crystalline core} generates shallow interfacial quantum states (IQS). This single-metal-oxide architecture elucidates the synergistic contributions of quantum confinement and interfacial states to photocatalytic H2 evolution. The ZrO2/ZrO2–x QDs exhibit an unprecedented H2 production rate of 32,943 μmol g–1 h–1 and an apparent quantum yield of 3.49% at 252 nm, outperforming bulk-like ZrO2. Experimental results indicate that IQS activate otherwise photoinactive ZrO2, while density functional theory (DFT) calculations elucidate their origin and electronic character. Photoelectrochemical measurements reveal a negative shift in flat-band potential and enhanced interfacial charge conductivity. X-ray photoelectron spectroscopy identifies a highly reduced, hydroxylated surface environment in the ZrO2 QDs, whereas electron paramagnetic resonance (EPR) reveals that {ZrO2/ZrO2–x QDs} form photoactive Zr3+ centers. Additionally, reversible photoinduced (g < 2) and (g > 2) signals consistent with electron and hole trapping, respectively. These findings establish IQS engineering as a novel interphase concept, beyond the quantum-dot limit, to promote charge separation and H2 evolution even in wide-band gap metal oxides. More broadly, coupling metal-oxide QDs with subnanometric, oxygen-deficient layers of the same metal offer a simple chemical route to activate otherwise inefficient oxide photocatalysts.