The scalable and economical synthesis approach of ZnO has always been demanded, since it is very useful material in the degradation of hazardous dyes. Herein, a scalable synthesis approach for ZnO nanoparticles by precipitating the precursors such as zinc sulphate heptahydrate and ammonium hydroxide has been proposed. Calcinations at suitable temperatures can improve crystalline nature of materials. Therefore as-synthesized ZnO nanoparticles were calcinated at different temperatures 400 °C, 650 °C, 900 °C, 1150 °C, and subsequently their physical and morphological properties were characterized. The TEM analysis revealed the morphology, whereas XRD and SAED confirmed the purity and crystalline nature of ZnO. Moreover, photo catalytic degradation/adsorption results for methylene blue dye of lower (2.5 ppm) and higher concentrations (5 ppm) in the presence and absence of sunlight and ZnO nanoparticles are also reported. Addition of ZnO nanoparticles to methylene blue resulted in enhanced rate of photo degradation in the presence of sunlight. The complete photodegradation of 2.5 and 5 ppm methylene blue in sunlight occurred in 90 min and 165 min, respectively in presence of ZnO nanoparticles. In general many photocatalytic degradation studies had been reported using an artificial light source; however in the present work direct sunlight has been utilized.