Optical and radioluminescence properties of ZnO:Zn as a function of reduction degree and treatment temperature

The aim of this work is to highlight the correlation between treatment conditions, composition, and structure of reduced zinc oxide (ZnO:Zn) with optical and radioluminescence properties for the production of high yield scintillating powders. Purchased zinc oxide (ZnO) powders (≈0.35 ± 0.15 μm) were thermally reduced in Ar/H2 (3%) atmosphere at temperatures ranging between 600 (≈0.40 ± 0.17 μm) and 1000 °C (≈6.4 ± 1.6 μm), obtaining ZnO:Zn powders. The reduction degree, evaluated through energy dispersive spectroscopy, showed a linear increase from 0% up to 20% (800 °C sample), reaching a plateau condition at about 25% for the highest treatment temperatures (1000 °C sample). Structural analysis highlighted the existence of a threshold temperature, previously reported in the literature, around 800 °C, beyond which (i) the grain size remarkably increases, (ii) the shape changes from hexagonal to round-shaped, and (iii) the intensity of electron spin resonance (ESR)-active defects at g = 1.9596 decreases for a new resonance at g = 2.0105. Additionally, a substantial variation, from 5.63 ± 0.02 down to 5.2 ± 0.1 g/cm3, of the skeletal density and a slight contraction of the optical bandgap are observed. Unlike the structural properties, PL and x-ray radioluminescence showed a continuous increase of the green emission intensity, associated with ESR-active ionized oxygen vacancies and ESR non-active recombination centers, and of the PL defective lifetimes (33 ns < τfast < 88 ns, 395 ns < τslow < 930 ns).