The influence of cobalt doping in SnO2 crystal lattice tailored the optical, structural, and surface properties of SnO2. Co2+ was successfully doped in SnO2 (Co-SnO2) via green synthesis using Tradescantia spathacea aqueous leaf extract. Powder X-ray diffraction patterns of the synthesized nanoparticles showed a rutile structure with no impurities. As Co-doping was increased, the average crystallite size increased from 13.25 nm to 32.32 nm and BET results showed reduced surface area. The presence of organic compounds of the aqueous leaf extract was confirmed by Fourier-transform infrared spectroscopy. UV-visible diffuse reflectance spectroscopy showed a red shift suggesting a band gap reduction with Co-doping. The photoluminescence study showed a peak quenching with the increase in Co-doping. Spherical and smaller particles were observed by scanning electron microscopy. The density of states was proposed using X-ray photoelectron spectroscopy and UV-visible diffuse reflectance spectroscopy data. A novel antioxidant study of SnO2 and Co-SnO2 nanoparticles was done under visible light irradiation using 2,2-diphenyl-1-picrylhydrazyl free radicals and was compared to conventional antioxidant method in the dark. Photocatalytic 4-nitrophenol conversion was also conducted in the dark and under visible light irradiation. The enhancement in the photoantioxidant activities and photocatalytic conversion of 4-nitrophenol to 4-nitrophenolate using SnO2 and Co-SnO2 was observed under visible light irradiation.