Hollow titanium nitride nanoshells for enhanced plasmon-driven hot electron generation and improved photocatalytic and photovoltaic applications

Abstract

Nowadays, plasmonic titanium nitride (TiN) is widely employed as a potential alternative to noble metals in semiconductor–metal hybrid nanoparticles (S–M HNPs) for improving the utilization efficiency of solar energy in photocatalytic and photovoltaic systems. In semiconductor–TiN nanosystems, TiN NPs convert solar energy into highly energetic (hot) electrons that can be transmitted to the attached semiconductor for enhanced applications. In this paper, we propose TiN nanoshells with a nonabsorbing dielectric core as an improved energy conversion component in S–M HNPs, compared to homogenous TiN nanospheres, with higher geometrical optimization flexibility, wider absorption range tuneability, and effective hot electron generation and utilization due to the reduced plasmonic-shell size. For understanding the impact of the core material on the functionality of the nanoshells, we assume three core materials with different refractive indices (air, silica (SiO₂), and magnesium oxide (MgO)). The exact Mie theory is utilized to calculate the absorption coefficient and the plasmon field of the proposed TiN nanoshells. To quantify the absorbance effectiveness on the solar spectrum, we calculate a relevant figure of merit (FoM) that depends on the spectral features of the absorption coefficient. By optimizing the geometrical parameters of nanoshells, it is found that hollow TiN nanoshells with the lowest core refractive index exhibit the highest FoM of solar energy absorption. Also, the plasmon field intensity of hollow TiN nanoshells is higher and more concentrated in a smaller volume of TiN material in comparison to the field intensity of other nanoshells (SiO₂–TiN and MgO–TiN nanoshells) and TiN nanospheres. Factors affecting the utilization of the generated hot electrons, including the radiative damping of plasmons and the spreading of the plasmon field inside the nanoparticles, have been investigated. In view of the temporal dynamics of hot electrons, it is shown that using the hollow TiN nanoshells with thin shells greatly enhances the effectiveness of the generated hot electrons to reach the attached semiconductor. In fact, the reduced plasmonic-shell thickness results in a trade-off between a longer radiative relaxation time and less solar energy absorption with regard to the selected core material.