Plasmonic coupling on TiO₂: TDDFT reveals why silver clusters outperform gold and copper

Plasmonic metal nanoparticles can extend TiO₂ photocatalysts into the visible-light range, but their performance depends sensitively on how collective metal excitations couple to the semiconductor surface. In this theoretical study, DFT and TDDFT were used to resolve that coupling at the atomistic and excited-state level for Ag, Au, and Cu clusters supported on rutile TiO₂(110).

The researchers first optimized realistic metal cluster geometries on TiO₂, then used the complex polarizability TDDFT algorithm in ADF to calculate photoabsorption spectra for large cluster models. This made it possible to move beyond ground-state adsorption trends and directly analyze how the TiO₂ support reshapes plasmonic excitations. The calculations showed that metal adsorption fills the TiO₂ band gap with additional electronic states, fundamentally changing the electronic structure of the interface.

The key result is that silver is the most plasmonic of the three metals studied. Ag₂₀ retains a longitudinal plasmon when supported on TiO₂, while Au₂₀ and Cu₂₀ show little or no plasmonic character at comparable cluster size. Larger Ag₃₇ clusters reveal an even clearer structure-property relationship: the orientation of the cluster relative to the surface controls whether plasmon intensity is enhanced, quenched, red-shifted, or blue-shifted. When the longitudinal plasmon points toward the surface, induced dipoles in the cluster and support couple favorably, lowering the excitation energy and strengthening interfacial coupling.

Specialized excited-state analysis was central to these insights. ICM-OS maps distinguished genuinely collective plasmonic resonances from ordinary electronic transitions, fragment analysis separated metal-to-surface, surface-to-metal, and intrafragment contributions, and transition-density plots showed how charge redistribution develops across the metal-TiO₂ interface. This revealed that different supported silver geometries can switch between charge-transfer-assisted plasmon coupling and more metal-localized plasmon behavior.

For photocatalyst design, the study highlights why simply depositing a noble metal on TiO₂ is not enough. The metal identity, cluster size, adsorption geometry, and dipole orientation all influence visible-light absorption and interfacial charge dynamics. Modeling provides a mechanistic filter for identifying promising plasmonic TiO₂ nanostructures before experimental synthesis, with Ag/TiO₂ emerging as the most favorable system among the clusters studied.