Facile synthesis and characterization of ZnO, ZnO:Co, and ZnO/ZnO:Co nano rod-like homojunction thin films: Role of crystallite/grain size and microstrain in photocatalytic performance

•The first comparative study about photocatalytic action ZnO, ZnO:Co 5%, and ZnO/ZnO:Co 5% heterojunction photocatalysts.•Chemically derived nano rod-like ZnO/ZnO:Co 5% heterojunction photocatalyst.•Overall, the best photocatalytic action observed for the nano rod-like ZnO photocatalyst.•Photocatalytic Reduced photocatalytic efficiency in the ZnO/ZnO:Co 5% heterojunction photocatalyst. [Display omitted] Herein, to investigate the photodegradation of methylene blue (MB) under UV light irradiation and the photocatalytic activity of chemically derived ZnO, ZnO: Co 5% nano rod-like thin films and ZnO/ZnO: Co 5% homojunction, the role of grain size and microstrain on their structural, optical, electrical and photocatalytic properties were studied. Hexagonal wurtzite structure, the doping of Co2+ into ZnO host lattice, reduced crystallite/grain size, and enhanced microstrain have been confirmed by structural and morphological analysis. The presence and the atomic content of Zn/Zn2+, Co/Co2+, O/O2- elements, and ions have been confirmed by EDS and XPS measurements. The n/p-type conductivity in nano rod-like film and p-n bilayers have been verified by Hall and I-V characterizations. The PL analysis showed that the decreased luminescence intensity and the formation of defect/trapping centers by Co2+ doping and formation of p-n homojunction as crystallite/grain size decreased while microstrain increased. With 5% Co-doping and the emergence of p-n homojunction, the photocatalytic efficiency (ղ %) was reduced compared with parent ZnO photocatalyst, while the crystallite/grain size decreases and microstrain enhances. The decreased ղ % was attributed to the reduced crystallite/grain size, increment of microstrain, and the generation of defects/trapping centers via 5% Co-doing and formation of ZnO/ZnO:Co 5% nano rod-like homojunction. These results demonstrate that the fabrication of p-n homojunction will not always result in an enhanced ղ % even if it puts down the recombination of light-induced charge transporters.