Unveiling hole trapping and surface dynamics of NiO nanoparticles†

Abstract

Mesoporous NiO is used as p-type material in photoelectrochemical energy conversion devices. The presence of two kinds of hole traps can affect device performance. Here, after band-gap excitation, the relaxation of the hole into two different traps was observed and characterized.

Abstract

The research effort in mesoporous p-type semiconductors is increasing due to their potential application in photoelectrochemical energy conversion devices. In this paper an electron–hole pair is created by band-gap excitation of NiO nanoparticles and the dynamics of the electron and the hole is followed until their recombination. By spectroscopic characterization it was found that surface Ni ³⁺ states work as traps for both electrons and holes. The trapped electron was assigned to a Ni ²⁺ state and the trapped hole to a “Ni ⁴⁺” state positioned close to the valence band edge. The recombination kinetics of these traps was studied and related with the concept of hole relaxation suggested before. The time scale of the hole relaxation was found to be in the order of tens of ns. Finally the spectroscopic evidence of this relaxation is presented in a sensitized film.

Notes

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c7sc03442c