Plasmon-Driven Hot Electrons in Titanium Nitride Nanoshells for Enhanced Solar Energy Harvesting: Estimations and Perspectives

In plasmonic titanium nitride (TiN) nanoparticles (NPs), the surface plasmon resonance decays nonradiatively, resulting in the generation of highly energetic hot electrons. In plasmon-enhanced solar energy harvesting applications, such as solar cells and photocatalysts, these hot electrons can effectively be transferred from the NPs to a surrounding active semiconductor─most commonly titanium dioxide (TiO2)─thereby improving the overall harvesting efficiency of the device. The superior performance of TiN-based nanostructures, including nanospheres and nanoshells, in hot carrier generation and electron extraction efficiency surpasses that of noble metals such as gold (Au). This enhanced performance can be attributed to the high thermal stability and excellent optical properties exhibited by TiN. Recently, it has been demonstrated that TiN nanoshells possess a superior Figure of Merit (FoM) for solar energy absorption compared to both Au nanoshells and homogeneous TiN and Au nanospheres. In this study, we present quantitative investigations for describing the hot electron generation in TiN nanoshells. Au nanoshells are also investigated for comparative analysis. Two core dielectric materials are assumed, silica (SiO2) and air, in order to examine the effects of the core material on the optical properties of the nanoshells and consequently on the hot electron dynamics. Furthermore, we consider TiO2 as an adjacent or embedding semiconductor for the analyzed nanoshells. Utilizing Mie theory formalism and optimizing the geometric parameters of the investigated structures, this study demonstrates that TiN nanoshells exhibit superior absorbance effectiveness in the solar spectrum compared to Au nanoshells. Next, we investigate the influence of the geometrical dimensions and material characteristics of both the core and the surrounding medium on the quantum efficiency as well as the hot electron current associated with surface and bulk (volume) photoemission mechanisms. It is shown that volume photoemission is more pronounced than surface photoemission at core and shell sizes corresponding to plasmon resonance near the peak of solar irradiance. Compared to Au nanoshells, TiN nanoshells demonstrate a broader range of plasmon resonance spectra, which in turn facilitates an expanded spectrum of hot electron current. For small TiN nanoshells (<20 nm in size), when embedded in water and TiO2, the efficiency of hot electron generation─quantified by the area under the curve of the hot electron current spectra─surpasses that of Au nanoshells of equivalent size by factors of approximately 1.5 and 2.5, respectively. As the size of the nanoshell increases, TiN nanoshells generate hot electrons with exponentially greater efficiency compared to Au nanoshells. The superior hot electron extraction of TiN nanoshells over other structures has been proven.