In Situ Pressure Controlled Growth of ZnO Nanoparticles: Tailoring Sizes, Defects, and Optical Properties

ZnO, being an inexpensive, wide band gap semiconductor that possesses high mechanical, thermal, and chemical stabilities and suitable for a wide range of optical and electronic applications, is the preferred semiconductor of this era. In an effort to fully utilize its potential features, ZnO research is receiving increasing attention. This study investigates the influence of pressure on the crystallinity, defect density, size, and morphology of ZnO nanoparticles, synthesized using nonaqueous sol-gel method, and their respective impact on the optical properties. High-crystalline ZnO nanocrystals with a hexagonal wurtzite structure were synthesized at various pressures, including ambient pressure, 25, 37.5, 50, and 100 bars inside a high-pressure reactor. With the increase in pressure, a reduction in particle size was observed, reaching a minimum size (∼10 nm) at 50 bar pressure (ZnO-50). Further increase in pressure causes an enhancement in the particle size. This trend of size variation with pressure is attributed to a tradeoff between esterification and nucleation processes. Contrary to the expectation, smaller ZnO nanocrystals synthesized by the present method possess lesser number of defects, suggesting that high-pressure synthesis is a unique way that offers smaller ZnO nanocrystals of sub-10 nm sizes having high crystallinity and lesser defects in a shorter time span. Also, the optical transmittance of the systems could be greatly enhanced by carefully tuning the particle sizes, with ZnO-50 (∼10 nm particle size) having the highest transmittance (∼95% at 600 nm) among all samples. High crystallinity, uniform morphology, excellent visible transparency, wide band gap, and low defect density make these smaller ZnO nanocrystals a preferred choice for ultraviolet sensors and other optoelectronic devices.