Effects of Hydrogen Treatment and Air Annealing on Ultrafast Charge Carrier Dynamics in ZnO Nanowires Under in Situ Photoelectrochemical Conditions

Abstract

ZnO nanowires (NWs) grown by the hydrothermal method on fluorine-doped tin oxide glass substrate were characterized by scanning electron microscopy, optical absorption, photoelectrochemical photocurrent density, and ultrafast transient absorption pump probe spectroscopy. The as-grown NWs were annealed in air, pure hydrogen atmosphere, or annealed first with air annealing followed by hydrogen treatment. By TA spectroscopy, the samples exhibited a triple exponential transient bleach recovery with lifetimes on the fast (10–15 ps), medium (100–200 ps), and slow (>1 ns) time scale, attributed to shallow donor mixed with electron hole plasma recombination, donor bound recombination, and donor–acceptor pair recombination (DAP), respectively. The as-grown samples were dominated by donor and DAP recombination. Air annealing improved the crystal structure but had little effect on hole trapping. Significantly, the hydrogen-treated NWs showed a reduction in hole trapping and DAP recombination. Hole trapping was attributed to zinc vacancies (VZn) and hydrogen was proposed to passivate these defects. The photocurrent density of the air annealed and cotreated NWs were measured, the latter of which showed improved performance which was attributed to, in part, decreased hole trap states and improved electrical conductivity. In situ ultrafast TA spectroscopy was used to study the photoanodes under working conditions as a function of applied bias. For both samples, the medium time constant became faster with increasing applied bias. A model was proposed to extract the electron–hole separation time constant.