The different metal oxides, e.g., Co 3O 4, ZnO, TiO 2 metal chalcogenides, e.g., ZnSe, ZnTe metal sulfides, e.g., CuS, CdS, ZnS and their nanocomposites, are extensively employed as photocatalysts owing to their ease of synthesis, low cost and excellent photocatalytic properties.

Many researchers have reported various photocatalysts with an excellent dye degradation efficiency. The other techniques, such as membrane filtration, electro-catalysis, precipitation, adsorption and biological treatment, are expensive and require advanced setups, but photocatalytic degradation needs an effective catalyst and sunlight.

Among the different techniques, photocatalytic dye degradation is the most effective due to its environmentally friendly, reusable and cost-effective nature. Various strategies have been employed and various are under way to overcome water pollution. This enhanced photocatalytic activity was due to the synergic effect of ZnSe and Co 3O 4 that reduced the electron/hole recombination and caused suitable bandgap alignment. The prepared samples were employed for the photocatalytic degradation of Congo red dye and the nano-heterostructure (ZnSe/Co 3O 4 3:1) shows an exceptional photocatalytic degradation efficiency of 96%. The UV/visible absorption spectrum of ZnSe depicts a sharp absorption at around 480 nm, while Co 3O 4 demonstrates two prominent peaks, 510 nm and 684 nm. The EDX results show that all the elements are present in accordance with their anticipated amounts in the products. The scanning electron micro-images of ZnSe show a mixed morphology of nanoparticles (≈16 nm), including spherical and distorted cubes, while Co 3O 4 has a worm-like morphology (≈20 × 50 nm). The X-ray diffraction results corroborate the good crystallinity and purity of all synthesized products, i.e., ZnSe, Co 3O 4 and their nanocomposites. In the present paper, simple hydrothermal and solid-state methods are reported for the synthesis of metal chalcogenide (ZnSe), metal oxide (Co 3O 4) and their nano-heterostructure (ZnSe/Co 3O 4 3:1, 1:1 and 1:3 ratios by weight), while their photocatalytic efficiencies are also investigated.

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