In the present work, nitrogen-doped and nitrogen-boron-double-doped manganese oxide (Mn₂O₃) and tungsten oxide (WO₃) nanoparticles were synthesized using precipitation-hydrothermal method for methylene blue degradation under visible light. Materials were characterized using X-ray diffraction (XRD) analysis, Scanning electron microscopy, Energy dispersive X-ray spectroscopy and UV-vis spectroscopy. Results showed that N and B were successfully incorporated into the crystal lattices of Mn₂O₃ and WO₃. XRD showed that WO₃ was crystallized in the form of a monoclinic lattice, while cubic Mn₂O₃ was produced in the cubic form. The crystallite size was found to be decreased due to the substitution of N and B elements which reveals their roles to accelerate the crystal nucleation rate resulting in the decreased size. On the other hand, single and double doping has successfully narrowed the band gaps of the as-synthesised metal oxide photocatalysts resulting in better absorption in the visible light. Band gaps obtained were as follows: 3.02, 2.50, 1.73 and 1.77 eV for N-WO₃ N/B-WO₃, N-Mn₂O₃ and N/B-Mn₂O₃ respectively. Photocatalytic experiments showed that all as-synthesised materials exhibited a photocatalytic efficiency under visible light ≥420 nm. The degradation efficiency of methylene blue (MB) was in the following order: N-B-co-doped metal oxides > N-doped metal oxides > metal oxides. The presence of scavenger molecules such as isopropanol, EDTA-2Na and benzoquinone inhibited MB degradation. Finally, the results showed that these materials can be reused several times without a notable decrease in efficiency.

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