Reagent concentration dependent variations in the stability and photoluminescence of silica-coated ZnO nanorods
Silica (SiO2) coating is finding increasing use as a means of improving the properties of ZnO nanomaterials. However, the current literature contains seemingly contradictory reports of the effect of such coatings on their photoluminescence (PL) properties. Two types of ZnO nanorod (henceforth termed Types A and B) were synthesized using the same hydrothermal method (differing only in the chosen precursor concentrations), then subjected to the exact same SiO2-coating procedure. SiO2 coating is seen to have a strikingly different effect on the Type A and B nanorod morphologies and their PL. In the case of Type A nanorods, (i.e. nanorods grown using high precursor concentrations), SiO2 coating has no discernible morphological effect but causes an obvious increase in the intensity of the visible component within the PL emission. Type B nanorods (grown from 20-times less concentrated reactive solution), in contrast, show a very different response to SiO2 coating: morphologically, they appear as many ZnO nanodots in the silica shell that surrounds the etched ZnO core, and their ultraviolet PL is boosted. Systematic investigation of the effects of various other post-treatments on the PL from Type A and B nanorods leads to the conclusion that the different responses to SiO2 coating can be traced to the different nanorod growth chemistries – i.e. by precipitation from Zn(OH)2 intermediates (Type A nanorods) or by direct reaction of Zn2+ and OH− ions (Type B material). The present study advances our understanding both of the controlled synthesis and of routes to optimising the properties of bare and silica-coated ZnO nanomaterials for nanodevice applications.