Carrier dynamics in cuprous oxide-based nanoparticles and heterojunctions

Abstract

Nanoparticles of cuprous oxide with and without Pd-coatings are explored for charge carrier dynamics that can influence efficiency in applications of solar photocatalysis. Transient absorption/reflection spectroscopy is used to determine the excited-state dynamics, which depends on size and shape. Nanoparticles of a few tens of nanometers exhibit simple resonances toward the blue end of the visible spectrum, while particles over 100 nm exhibit various Mie resonances that enhance their performance in applications and lead to more complicated dynamics. Global and inversion analysis are used to determine the rate dynamics of the photocarriers. Small spheres exhibit Auger scattering in the high photocarrier injection regime, whereas the low carrier injection regime is characterized by Shockley-Read-Hall dynamics. Adding a near uniform, 2-nm thick Pd coating results in a heterojunction between Cu2O and Pd that modifies the relaxation dynamics. The reduction of slope value from 3, characterizing Auger scattering, to the lower values on Pd coated samples shows the admixture of possibly two or more recombination mechanisms. Also, the intermediate injection regime exhibits radiative recombination/trap-assisted scattering in the Pd-coated Cu2O nanoparticles. Power-dependent analysis, by solving the rate equation at the higher injection regime, validates the suppression of Auger scattering in Pd-coated samples. This result indicates the possibility of charge separation across the heterojunction, either from metal to the semiconductor or vice-versa, lowering the effective photocarrier overlap, reducing Auger scattering, and potentially improving photocatalytic activity.