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ZnO nanoparticles were successfully synthesized using cassava starch (tapioca) as a chelating agent. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analyses revealed that the synthesized nanoparticles were in the form of ZnO with no impurities present. The estimated average crystallite size, determined from Rietveld refined XRD patterns, increased from 20 to 41 nm as calcination temperature increased from 500 to 700 °C. Transmission electron microscopy (TEM) images showed ZnO nanoparticles with an average size ranging from 30 nm (500 °C) to 100 nm (700 °C). The optical band gap was affected by heat treatment, and the lowest value was found at 500 °C (3.16 eV). Photoluminescence spectroscopy (PL) results show that all synthesized ZnO nanoparticles presented emission in the blue region, which was attributed to oxygen vacancies and optimized by calcination at 700 °C. Image 1 • ZnO nanoparticles were synthesized by a tapioca-assisted sol–gel route. • A heat treatment of 500–800 °C was applied to the synthesized ZnO. • TEM confirmed the formation of ZnO nanoparticles. • The calcination affected particle size and optical properties. • The smallest band gap was obtained in the calcined ZnO at 500 °C. [ABSTRACT FROM AUTHOR]