Si doped iron-iron oxide core-shell magnetic nanoparticles are presented in this paper for photocatalysis applications. The nanoparticles were fabricated by a gas phase method with control of small size. Enhanced optical absorption was observed in them compared to the ones without Si, particularly in the 310 nm-450 nm wavelength range. This active optical property is beneficial for photocatalysis in terms of the efficient hole transfer from the catalysts to the reacting chemistry groups. The nanoparticles also possess a strong and active magnetic property which provides an easy way for recyclable usage. This work offers a route towards the improvement of a photocatalytic system which integrates a nanoparticulate structure of a large surface area with effective doping and recoverability by a magnetic field.