Atomic Cu Clusters vs Single Cu Atoms as Optimal Cocatalysts on NaTaO for Enhanced Noble-Metal-Free HO/H Photocatalysis.

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 Cu atoms or Cu subnanoclusters on NaTaO nanoparticles, producing a library of Cu-(SAC)@NaTaO or Cu-(SnC)@NaTaO with controlled amounts of Cu-(SAC) and Cu-(SnC), respectively. Electron Paramagnetic Resonance (EPR) spectroscopy was used as a state-of-the-art tool to map the precise configuration of the Cu species on NaTaO, as well as the photoinduced electron transfer from NaTaO to surface-anchored Cu. Photocatalytic H production from HO demonstrates that Cu-(SnC)@NaTaO i.e. decorated with Cu subnanoclusters, exhibits significantly superior activity vs their single Cu-atom counterpart, achieving enhanced H 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 Cu subnanoclusters in Cu-(SnC)@NaTaO are ∼300% more efficient electron acceptors compared to Cu monomers in Cu-(SAC)@NaTaO. Transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) data reveal that FSP-deposited Cu SnC forms a tight interface with the NaTaO 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 HO photocatalysis by NaTaO. This postulate probably can be verified for other pertinent systems. Moreover, we demonstrate that FSP-made Cu-(SnC)@NaTaO is a highly promising, noble-metal-free photocatalyst.