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Construction of heterojunctions by coupling two semiconductors together is one of the most efficient ways to realize the electron-hole pair separation. In this short paper, I systematically discuss the properties of heterojunction photocatalysts in all cases. The formation of space charge regions, built-in electric field and potential barriers at the interface regions of thermally equilibrated heterojunctions are analyzed in details. When the heterojunctions are used for photocatalysis, the transfer behavior and mechanism of photo-excited non-equilibrium carriers between the constituent semiconductors are discussed. It is demonstrated that the heterojunction properties, carrier transfer behavior and photocatalytic mechanism depend highly on the semiconductivity (N-type or P-type), work function (or Fermi level), and CB/VB potentials of the constituent semiconductors. All types of the heterojunction photocatalysts are expected …