Green preparation of tertiary-phase titanium oxide nanopowder using Chlorella sp. aqueous extract

[Display omitted] •Anatase-brookite-rutile TiO2 NPs were synthesized by a simple precipitation method.•An aqueous algae extract of Chlorella sp. was used as a capping agent.•The effects of the volume concentration of the algae extract were studied.•The algae extract improved thermal stability and size reduction of TiO2 NPs.•The rutile transition and the growth of TiO2 NPs were dependent on the concentration of the algae extract. Herein, a novel green synthesis of tertiary-phase titanium dioxide nanoparticles (TiO2 NPs) using an aqueous extract of Chlorella sp. was reported for the first time. The TiO2 NPs were prepared using various extract volume concentrations (0, 5, 10, 15 and 20 % v/v) and subsequently calcinated at 600 °C. The X-ray diffraction (XRD) results revealed that the concentration ranging from 5 to 15 % v/v algae extract resulted in mixed phases of anatase, brookite, and rutile, while the extract concentration of 20 % v/v yielded mixed phases of anatase and brookite. The phase transformation from anatase to rutile decreased with increasing extract concentration. Transmission electron microscopy (TEM) images showed that the average grain size of the control TiO2 was 90 nm, whereas that of algae mediated TiO2 NPs was 10–20 nm. The concentration of algae extract was a very significant parameter for the phase transformation and particle size reduction of TiO2 NPs. The tertiary phase 10 % and 15 % algae mediated TiO2 powder also exhibited reaction rate constants (k) similar to the binary phase 20 % TiO2, despite containing lower anatase content. The present work demonstrated the successful synthesis of tertiary phases TiO2 nanopowder using microalgae Chlorella extract as a biogenic capping agent.