Oriented electron tunneling transport in hierarchical Ag/SiO₂/ TiO₂ nanobowl arrays for plasmonic solar water splitting

Hierarchical Ag/SiO₂/TiO₂ nanobowl (NB) arrays were fabricated for use as plasmonic photoanodes for solar-hydrogen conversion. The nanobowls had large pore size and were composed of an upper TiO₂ nanoring and a lower TiO₂ nanohole. A thin SiO₂ inter-layer was introduced as an electron transmission channel to change the mechanism of hot electron transport. Simulations were performed to characterize the variation of electron concentration in Ag/SiO₂/TiO₂ NB arrays, taking into account both the optical transition of photogenerated electrons, and electron tunneling. The multiphysics coupling function of COMSOL software provided the light source for optical transition of photogenerated electrons, and a Wentzel-Kramers-Brillouin model was employed to represent the tunneling. The results demonstrate that the TiO₂ nanoring was a transporter, which transmitted electrons downward to the nanohole. The SiO₂ layer replaces the Schottky barrier to become a bridge for tunneling of hot electrons in high- and low-energy states into TiO₂. Moreover, the coverage of the SiO₂ layer helped increase the light absorption of TiO₂, it also reduced the near electric field coupling between Ag and TiO₂. Accordingly, under AM 1.5 light irradiation, the photocurrent density and average hydrogen evolution rate of Ag/SiO₂/TiO₂ were 1.8 and 2.2 times higher, respectively, than those of pure TiO₂, implying far more efficient migration of carriers.