Nickel (Ni) doped Mn₃O₄ nanoparticles (NPs) were synthesized by a quick and facile chemical precipitation technique to investigate their performance in the degradation of methylene blue (MB) in the absence of light. XRD, FESEM, TEM, AAS, XPS, and FT-IR were used for the investigation of the structural, surface morphological, and elemental composition of Ni doped Mn₃O₄ NPs. XRD confirms the formation of a tetragonal phase structure of pure Mn₃O₄ and 1% and 3% Ni doped Mn₃O₄ NPs. However, mixed phases were found in the case of 5 to 10% Ni doped Mn₃O₄ NPs. Well-defined spherical-shaped morphology was presented through FESEM. Particle sizes decreased linearly (58.50 to 23.68 nm) upon increasing the doping concentration from 0% (pure Mn₃O₄) to 7% respectively, and then increased (48.62 nm) in the case of 10% doping concentration. TEM further confirmed spherical shaped 32 nm nanoparticles for 7% Ni doped Mn₃O₄. The elemental composition and oxidation state of the prepared NPs were confirmed by using XPS spectra. Mixed valence Mn²⁺ and Mn⁴⁺ states were found in pure Mn₃O₄ and 1% and 3% Ni doped Mn₃O₄ NPs in the ratio of 2MnO-MnO₂. In addition, three different oxidation states Mn²⁺, Mn³⁺, and Mn⁴⁺ were found in 5 to 10% Ni doped Mn₃O₄ NPs. Moreover, as a dopant Ni exists as Ni²⁺ and Ni³⁺ states in all Ni doped Mn₃O₄ NPs. The synthesized NPs were then applied as potent oxidants for the degradation of MB at pH 3. With the increase of doping concentration to 7%, the degree of degradation was increased to 79% in the first 10 min and finally, it became about 98%. The degradation of MB follows the pseudo-first-order linear kinetics with a degradation rate of 0.0342 min^-1.

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