Photocatalytic Decolorization of Methylene Blue Dye in Water Using B-SnO2 Nanoparticles

This research investigates the preparation of boron-doped tin dioxide (B-SnO2) nanoparticles via a simple sol–gel method at various B loadings. X-ray diffraction (XRD) showed that B preserved the SnO2 crystal structure while reducing the crystallite size from 5.05 to 3.14 nm. X-ray photoelectron spectroscopy (XPS) confirmed the presence of B in 3% B-SnO2. Field-emission scanning electron microscopy (FESEM) showed more rounded domains in 3% B-SnO2, and transmission electron microscopy (TEM) showed the average particle size decreasing from 12.61 nm for SnO2 to 10.61 nm. Ultraviolet–visible diffuse reflectance spectroscopy (UV-DRS) showed a decrease in the band gap from 3.60 eV (SnO2) to 3.24 eV (3% B-SnO2). Under ultraviolet-C (UV-C, 200–280 nm) irradiation, 3% B-SnO2 decolorized 94.80% of methylene blue (MB) in 90 min, compared with 76.00% for SnO2, 9.40% for photolysis, and 11.77% for dark adsorption. Because UV-C photons exceed the band gap of all samples, the improvement is more likely related to smaller crystallites and B-induced defects than to band-gap narrowing alone. Scavenger experiments implicated superoxide radicals (•O2−), hydroxyl radicals (•OH), and photogenerated holes (h+). Efficiency fell moderately to 81.25% after four cycles. Thus, 3 mol% B is an effective level for UV-C decolorization of MB by SnO2.