Interaction of hydrogen with defects in ZnO nanoparticles - studied by positron annihilation, Raman and photoluminescence spectroscopy

The interaction of hydrogen with defects in ZnO nanoparticles under H-2-Ar mixed gas was investigated by positron annihilation, Raman and photoluminescence spectroscopy. Two series of experiments were performed, including annealing the samples in pure Ar gas and H-2-Ar mixed gas. The thermal stability of hydrogen-related defects was also carried out by subsequent isochronal annealing in air. The positron annihilation results showed that more vacancy clusters were formed in the hydrogenated samples due to the reducing effect of hydrogen when compared to those samples annealed in pure Ar gas, which was consistent with the smaller grain size, and more surface and interfaces observed by X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. Raman, electron paramagnetic resonance, and photoluminescence spectra indicated that there existed two main forms of hydrogen in the ZnO nanoparticles: hydrogen occupying the oxygen vacancy H-O and interstitial hydrogen H-i. H-i diffused out at 150 degrees C, while H-O was completely removed at 700 degrees C. The results are helpful to understand the role of hydrogen in nanoscale ZnO and the applications in UV optoelectronics.