Atomic Cu2+ Clusters vs Single Cu2+ Atoms as Optimal Cocatalysts on NaTaO3 for Enhanced Noble-Metal-Free H2O/H2 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 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 H-2 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 H-2 photogeneration of >10,900 mu mol/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 similar to 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.