Atomic Cu²⁺ Clusters vs Single Cu²⁺ Atoms as Optimal Cocatalysts on NaTaO₃ for Enhanced Noble-Metal-Free H₂O/H₂...

Engineering of single-atom catalysts (SACs) or subnanoclusters (SnCs) as cocatalysts is a powerful strategy for enhancing photocatalytic performance. However, a more concrete understanding of the role of SAC vs SnC remains a challenge. A prerequisite to achieving this task is a systematic selective synthesis of SACs and SnCs as cocatalysts for a given process. Herein, we have employed a modified flame spray pyrolysis (FSP) process for deposition of either single Cu²⁺ atoms or Cu²⁺ subnanoclusters on NaTaO₃ nanoparticles, producing a library of Cu-(SAC)@NaTaO₃ or Cu-(SnC)@NaTaO₃ with controlled amounts of Cu-(SAC) and Cu-(SnC), respectively. In situ Electron Paramagnetic Resonance (EPR) spectroscopy was used as a state-of-the-art tool to map the precise configuration of the Cu²⁺ species on NaTaO₃, as well as the photoinduced electron transfer from NaTaO₃ to surface-anchored Cu²⁺. Photocatalytic H₂ production from H₂O demonstrates that Cu-(SnC)@NaTaO₃ i.e. decorated with Cu²⁺ subnanoclusters, exhibits significantly superior activity vs their single Cu-atom counterpart, achieving enhanced H₂ photogeneration of >10,900 μmol/g/h, corresponding to an apparent quantum yield of 1.5%, with no noble metal added as a cocatalyst. EPR data show that Cu²⁺ subnanoclusters in Cu-(SnC)@NaTaO₃ are ∼300% more efficient electron acceptors compared to Cu²⁺ monomers in Cu-(SAC)@NaTaO₃. Transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) data reveal that FSP-deposited Cu²⁺ SnC forms a tight interface with the NaTaO₃ surface, leading to an improved energy-level configuration. Overall, the present data showcase, for this particular photocatalytic system, that Cu subnanoclusters rather than the currently believed single Cu atoms are the preferable cocatalyst in H₂O photocatalysis by NaTaO₃. This postulate probably can be verified for other pertinent systems. Moreover, we demonstrate that FSP-made Cu-(SnC)@NaTaO₃ is a highly promising, noble-metal-free photocatalyst.