Synthesis, growth mechanism, optical properties and catalytic activity of ZnO microcrystals obtained via hydrothermal processing

Abstract

In the present study, typical ZnO microcrystals exhibiting the wurtzite hexagonal crystal structure were produced successfully, characterized by a high degree of crystallinity, via hydrothermal processing at 120, 150 and 180 °C, assisted by N-cetyl-N,N,N-trimethylammonium (CTAB). The samples were characterised by XRD, Raman and infrared, FE-SEM, UV-Vis by diffuse reflectance and photoluminescence (PL). The experimental results confirm that all hydrothermally synthesised ZnO samples were crystallised into a wurtzite hexagonal structure. The ZnO crystals exhibit the morphology of hexagonal columns in the absence and double hexagonal columns in the presence of CTAB. The length and average diameter of the microstructures decrease with increasing processing temperature. It is evident that all the synthesised samples present very similar profiles and band positions in the PL emission spectra, with an emission band in the violet range at approximately 400 nm, a small peak in the UV range at approximately 380 nm, and highly superposed and intense emission bands between 440 and 750 nm (blue to red emission), with a maximum at approximately 610 nm. Furthermore, a nucleation and growth model was proposed to explain the formation of ZnO microcrystals, based on the experimental conditions, that were preferably grown in the [001] direction. In addition, the ZnO exhibited excellent performance in the photocatalytic degradation of rhodamine B (RhB) and methyl orange (MO), achieving 97% and 99% photodegradation of RhB and MO, respectively, when ZnO obtained at 120 °C, in the absence of CTAB, was used as catalyst.