Crystal facet engineering of semiconductors is proven to be an effective strategy to increase photocatalytic performances. However, the mechanism involved in the photocatalysis is not yet known. Here, we found that photocatalytic performances of the Cl- ion capped NiO octahedrons with exposed {1 1 1} facets were activated by treatment of AgNO3 and NH3 center dot H2O solutions. The clean NiO {1 1 1} facets were found to be highly reactivity faces. Based on polar structure of the {1 I 1} surfaces, a charge separation model between polar {1 1 1} surfaces is proposed. The internal electric field created by the spontaneous polarization drives effectively separation of photogenerated electrons and holes. The reduction and oxidation reactions selectively take place on the positive Ni-NiO and negative O-NiO planes. The good charge separation results in high photocatalytic activity. Our model suggests the polar surface-engineering as a strategy to develop more effective photocatalysts and new type of solar cells, electronic and photoelectric devices. Published by Elsevier B.V.