ABSTRACT
Direct splitting of water into H2 fuel and O2 gases by sunlight is one of the most promising and potentially low-cost approaches to solve the grand global energy challenge. Recent progresses in electronic structure theory and quantum simulations allow us to directly explore the atomistic mechanism and ultrafast dynamics of water photosplitting on plasmonic nanoparticles. In this chapter, we present a summary on the progresses and the improved understanding from recent research activities to directly simulate ultrafast electron–nuclear dynamics of water splitting on gold nanoparticles upon light exposure using real-time time-dependent density functional theory.
First, we propose that supported gold nanoparticles on oxide thin film can potentially serve as an efficient photocatalysts for water splitting. We identify that the quantum couplings between water molecules and quantum well states in magic gold nanoparticles are key to promote water adsorption and to significantly reduce the water-splitting energy barrier. Strong oscillations are found in water adsorption energy, the dissociation barrier, and the binding energy of the dissociated H atom, resulting in well-defined patterns that are correlated with the wavefunction symmetry of frontier orbitals of selected quantum well states.
Upon light illumination, we identify a strong correlation between light intensity, hot electron transfer, and water-splitting reaction rates. The rate of water splitting is dependent not only on respective optical absorption strength, but also on the quantum oscillation mode of plasmonic excitation. Odd modes are more efficient than even modes, owing to larger amount of charge transfer and faster decaying into hot electrons whose energy matches well the antibonding orbital of water. This finding suggests that photocatalytic activity can be manipulated by adjusting the energy level of plasmon-induced hot carriers, through altering the nanoparticle size and laser parameters, to better overlap adsorbate unoccupied levels in plasmon-assisted water photolysis.
Hydrogen gas generation from solar water splitting provides a renewable energy cycle to produce carbon-free fuels. We directly explore the ultrafast electron–nuclear quantum dynamics on the timescale of ~100 fs during water photosplitting on plasmonic clusters embedded in liquid water. Water molecule splitting is assisted by a rapid proton transport in liquid water in a Grotthuss-like mechanism. We identify that plasmon-induced field enhancement effect dominates water splitting, while charge transfer from gold to antibonding orbital of water molecule also plays an important role. A “chain reaction” like rapid H2 production is observed via combining two hydrogen atoms from different water molecules. These results provide a route toward a complete understanding of water photosplitting in the ultimate time and spatial limit.
