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

Abstract

This study investigates the influenceof pressure on thecrystallinity, defect density, size, and morphology of ZnO nanoparticles,synthesized using nonaqueous sol-gel method, and their respectiveimpact on the optical properties. ZnO,being an inexpensive, wide band gap semiconductor that possesseshigh mechanical, thermal, and chemical stabilities and suitable fora wide range of optical and electronic applications, is the preferredsemiconductor of this era. In an effort to fully utilize its potentialfeatures, ZnO research is receiving increasing attention. This studyinvestigates the influence of pressure on the crystallinity, defectdensity, size, and morphology of ZnO nanoparticles, synthesized usingnonaqueous sol-gel method, and their respective impact on theoptical properties. High-crystalline ZnO nanocrystals with a hexagonalwurtzite structure were synthesized at various pressures, includingambient pressure, 25, 37.5, 50, and 100 bars inside a high-pressurereactor. With the increase in pressure, a reduction in particle sizewas observed, reaching a minimum size (& SIM;10 nm) at 50 bar pressure(ZnO-50). Further increase in pressure causes an enhancement in theparticle size. This trend of size variation with pressure is attributedto a tradeoff between esterification and nucleation processes. Contraryto the expectation, smaller ZnO nanocrystals synthesized by the presentmethod possess lesser number of defects, suggesting that high-pressuresynthesis is a unique way that offers smaller ZnO nanocrystals ofsub-10 nm sizes having high crystallinity and lesser defects in ashorter time span. Also, the optical transmittance of the systemscould be greatly enhanced by carefully tuning the particle sizes,with ZnO-50 (& SIM;10 nm particle size) having the highest transmittance(& SIM;95% at 600 nm) among all samples. High crystallinity, uniformmorphology, excellent visible transparency, wide band gap, and lowdefect density make these smaller ZnO nanocrystals a preferred choicefor ultraviolet sensors and other optoelectronic devices.