Efficient hot carrier injection in plasmonic semiconductor heterojunction for artificial photosynthesis of ammonia

We developed a plasmonic semiconductor p-n junction by<i>in situ</i>growing p-type Cu₃BiS₃in n-type Bi₂S₃nanorods by an ion exchange method. The formation of plasmonic semiconductor heterojunctions was verified through high-resolution transmission electron microscopy, Mott-Schottky tests, x-ray photoelectron spectroscopy-based valence band spectra, and powder x-ray diffraction. Additionally, the rapid transfer of hot carriers between the heterojunctions was investigated using ultrafast transient absorption spectroscopy (TAS). The plasmonic p-n junction shows strong localized surface plasmon resonance (LSPR) absorption in the near-infrared (IR) range and delivers a 61-fold enhancement of the ammonia production rate under full spectrum irradiation in pure water. It can achieve an apparent quantum efficiency of 0.45% at 400 nm and 0.16% at 1000 nm.<i>In situ</i>Fourier-transform IR reveals that the plasmonic semiconductor heterojunction promotes the nitrogen chemisorption and activation. Based on TAS measurements, we found that LSPR induced hot carriers can be efficiently injected from plasmonic Cu₃BiS₃to non-plasmonic Bi₂S₃, with sufficient energy to drive water oxidation reaction. We further confirmed that photothermal effects have negligible contribution to the photocatalytic performance in the water-particle suspension system. The present study shows a potential strategy utilizing plasmonic semiconductors made of earth-abundant elements for green ammonia synthesis.