Control of TiO2 crystal facets has attracted enormous interest due to the fascinating shape-dependent photocatalytic activity of this material. In this work, the effect of the ratio of {001} and {101} facets on the photocatalytic CO2-reduction performance of anatase TiO2 is reported. A new "surface heterojunction" concept is proposed on the basis of the density functional theory (DFT) calculations to explain the difference in the photocatalytic activity of TiO2 with coexposed {001} and {101} facets.