NiO–TiO2 composite nanoparticles have been prepared by a modified ammonia-evaporation-induced synthetic method, sintered at 450°C and characterized by powder X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, UV-visible diffuse reflectance spectroscopy, photoluminescence spectroscopy, and electrochemical impedance spectroscopy. Doping shifts the optical absorption edge to the visible region but increases the charge-transfer resistance and decreases the capacitance. Under visible light, the composite nanoparticles effectively catalyze the Escherichia coli inactivation. The prepared oxide is selective in photocatalysis; with UV light, its photocatalytic activity to degrade sunset yellow, rhodamine B, and methylene blue dyes is less than that of the undoped one. However, it degrades phenol faster than TiO2 P25.