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In this work we study the influence of grain and particle size on the structural and optical properties of BiFeO 3 (BFO) nanoparticles and the resulting photocatalytic activity. Unexpectedly, the photocatalytic activity is found to decrease while the expected surface reaction area increases by decreasing the particles size. We show that while the global structure, polarization, particle morphology, and band gap are only weakly altered, if at all, some optical features, namely, the Urbach energy and low-energy bands, in the absorption spectra are substantially changed. We argue that these optical modifications related to defects and local distortions are mainly affected at the skin layer that is inherent to oxides like BFO. By reducing the particle size of BFO nanoparticles, the skin layer is thus altered, which in turn changes the photocatalytic properties.