The present work demonstrates the field emission characteristics and photocatalytic behavior of ZnCo2O4 marigold flowers synthesized via a facile hydrothermal method. The effect of annealing of these 3D porous hierarchical nanostructures on field emission and photocatalytic performances is studied. When compared with the as-synthesized sample, annealed ZnCo2O4 exhibits a similar to 2-fold improvement in photocatalytic activity for methylene blue (MB) degradation under visible light irradiation. The turn-on, threshold fields and high emission current densities are also strongly influenced as a result of annealing. The turn on field required to draw an emission current density of similar to 1 mu A/cm(2) is found to be similar to 2.4 and similar to 1.8 V/mu m for as-synthesized and annealed ZnCo2O4 marigold flowers, respectively. Field-emission measurements demonstrate remarkably large field enhancement and better stability for annealed samples. The X-ray diffraction and Raman analysis reveal that annealing improves the crystallinity and also help to remove the structural defects in ZnCo2O4. The enhancement in the field emission and photocatalytic activity of annealed ZnCo2O4 marigold flowers is attributed to the modification of electronic properties as a result of dehydration, crystallite growth and reduced surface defects/impurity phases. (C) 2017 Elsevier B.V. All rights reserved.