Metastability triggered reactivity in clusters at realistic conditions: a case study of N-doped (TiO2) n for photocatalysis

Abstract

We present a strategy, by taking a prototypical model system for photocatalysis (viz. N-doped (TiO2) n clusters), to accurately design photocatalyst with enhanced reactivity at a given environmental conditions (i.e. temperature (T), pressure () and doping (µ e )). Since free energy potential energy surface consists of many competing isomers even in a small energy window, computational design of specific metastable photocatalysts with enhanced activity is extremely challenging. This requires fixing various parameters as follows; (i) favorable formation energy, (ii) low fundamental gap, (iii) low excitation energy, (iv) high vertical electron affinity (VEA) and (v) low vertical ionization potential (VIP) to drive the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for water splitting. Here, we show that by integrating several first principles based methodologies that consideration of the global minimum structure alone can severely underestimate the activity. First, we have used a suite of genetic algorithms (namely, searching clusters with conventional minimum total energy ((GA)); searching clusters with specific property i.e. high VEA ((GA)), and low VIP ((GA))) to model the N-doped (TiO2) n ( n = 4–10, 15, 20) (meta)stable clusters. Next, we have identified its free energy using ab initio atomistic thermodynamics to confirm that the metastable structures are not too far from the free energy global minima. We find that N-substitution ((N)) prefers to reside at highly coordinated oxygen site, whereas N-interstitial ((NO)) and split-interstitial ((N) favor the dangling oxygen site. (NO) and (N doped states are thermodynamically stable at finite temperature (T) and pressure (). Interestingly, each types of defect (viz. substitution, interstitials) reduce the fundamental gap and excitation energy substantially. However, (N doped clusters are found to be less probable in the Pourbaix phase diagram, whereas (N) and (NO) doped metastable clusters show significant occupance probability near the phase boundaries. The latter ensures much higher electrocatalytic activity for water splitting than the most stable configurations.