Synthesis of nanoparticles (NPs) is gaining attention as a cost-effective and environmentally acceptable alternative to remove the pollutant by facile photocatalysis process. Role of hydrothermal treatment on Zinc-oxide (ZnO) nanostructures were investigated using non-ionic surfactant diethanolamine (DEA). Further, in order to investigate the effect of DEA on morphological variation different concentration of DEA was used. The samples were thoroughly characterized by XRD, Rietveld analysis, FESEM and TEM to get insight idea about the ZnO structural and morphological properties. Moreover, XPS spectra reveal the variation of surface oxygen defects as hydrothermal treatment induced more defects to ZnO material. BET measurement reveals the alteration of surface area and pore size of ZnO sample. The surface defect-states (mostly oxygen vacancies) of the catalyst nanoparticles can influence the photocatalytic degradation of MB dye activated by ZnO nanoflowers via a non-radiative energy transfer pathway. A steady-state photoluminescence analysis validated the photoinduced electron transport from ZnO to MB dye. Steady state photoluminesence emission spectra established one to one correlation between the defects and colour emission from ZnO. Spectral overlap between donor (ZnO) to acceptor (MB dye) also enhanced greatly after hydrothermal treatment ascribing more Förster resonance energy transfer (FRET) which accelerates photocatalytic degradation efficiency of methylene blue (MB) dye under UV light irradiation. The defect-engineered ZnO nanoparticles synthesized through facile hydrothermal treatment led to an efficient decolourization of MB dye which was strengthened by FRET based on a correlation of photocatalytic degradation and defect mediated colour emission.