Tailoring of crystal phase, morphology, and optical properties of ZnO nanostructures by starch-assisted co-precipitation synthesis and annealing

Abstract

ZnO nanostructures have been synthesized by simple co-precipitation method in the presence or absence of surfactant (starch) and subsequent annealing at 500 and 800 °C for 5 h. The XRD results show ZnO and Zn5(OH)8Cl2·H2O phases in the absence, but only ZnO phase in the presence, of starch. The secondary phase diminishes upon annealing and only pure ZnO phase with improved crystallinity is realized at 800 °C. FTIR endorses different phases along with the starch surfactant by the representative modes. Diverse morphologies: hexagonal; tubular; spherical; and cluster morphologies are observed for different synthesis and annealing conditions. The direct band gap is found to be ~ 3.2 eV and experienced a red shift on annealing. The PL spectra consists of characteristic near band edge (NBE) emission in UV region and a broad emission in visible region corresponding to direct excitonic transitions and oxygen defect transitions, respectively. Samples having rod-like morphology as predominant exhibit strong UV and weak defect emissions, while those with spherical morphology as dominant display weak UV and broad defect emissions in PL. The quenching of NBE emission takes place on annealing, which is indicative of increase in non-radiative transitions or deformation of crystallinity or change in morphology. The PL emissions are strongly influenced by surfactant and annealing temperature. Such results offer new prospects for tuning and optimizing the properties of ZnO semiconductor for applications where the optical properties are decisive.