Metastability Triggered Reactivity in Clusters at Realistic Conditions: A Case Study of N-doped (TiO₂)_n for water splitting

Catalytic activity studies of nanoclusters have usually focused on the global minimum structure. However, by taking a prototypical model system (N-doped (TiO₂)_n clusters) for water splitting, we have found that metastable clusters can dominate the observed activity due to their unique fundamental properties. Hence, consideration of the global minimum structure alone can severely underestimate the activity. Therefore, we have presented a suite of massively parallel cascade genetic algorithm to design N-doped (TiO₂)_n clusters with desired properties: favorable formation energy, high vertical electron affinity (VEA), and low vertical ionization potential (VIP). We find that N-substitution (N_O) prefers to reside at highly coordinated oxygen site, whereas N-interstitial ((NO)_O) and split-interstitial ((N₂)_O) favor the dangling oxygen site. Interestingly, each types of defect reduce the fundamental gap and excitation energy substantially. Next, we understand the thermodynamic and electrochemical (Pourbaix) stability of the designed clusters in the reactive atmosphere using ab initio atomistic thermodynamics. The probability profiles confirm the existence of metastable phases near the phase boundary that contribute significantly to the overall activity of the catalyst.