From Stems (and Stars) to Roses: Shape-Controlled Synthesis of Zinc Oxide Crystals

A novel, scalable, solution-based method suitable for the control of ZnO crystal morphology at moderate temperature from rods/stars to rose-shaped, flowerlike structures is presented here. The synthesized ZnO roselike crystals are composed by wide nanosheets folded several times on themselves. This study represents the first step toward the full understanding of the growth mechanism that led to the formation of such a structure. It is now widely acknowledged that both size and shape of the metal oxide crystals have a profound effect on their properties. The influence that two morphology directing agents, ammonia and citric acid trisodium salt dihydrate, have on the shape-controlled synthesis is illustrated. The analysis of various possible mechanisms and forces that might contribute to the roselike crystals formation, such as the influence of polar surfaces, is discussed. The chemical composition and work function of the ZnO roses is investigated via XPS and UPS. XRD analysis suggests wurtzite as the most likely crystal structure for both the ZnO stars and roses, albeit these latter have a more elusive diffractogram because of their folded configuration. PL data suggest a lower level of interstitial oxygen for both crystals synthesized, starlike and flowerlike. Analysis of the ratio of the emission in the Blue range upon the ultraviolet emission indicates that the ZnO stars are slightly more conductive than the ZnO roses, a factor associated with the lower native defect concentration in these latter structures.