In a photoelectrochemical cell for water splitting,the critical issue is charge separation and transport,which is usually completed by designing semiconductor heterojunctions.TiO2 anatase-rutile phase junctions are found to largely improve photocatalytic water splitting,but usually exhibit negative effects on photoelectrochemical cells.Here,we design and prepare TiO2 phase junctions with perfect structures: two phases are separated in different spaces to form FTO/anatase/rutile and FTO/rutile/anatase junctions.Anatase films with various thicknesses are deposited by atomic layer deposition technology and rutile nanorod arrays are synthesized by hydrothermal method.Two types of phase junctions as photoanodes have completely opposite effects: FTO/anatase/rutile junctions can not only increase the photocurrent density but also decrease the onset potential,while FTO/rutile/anatase junctions result in significantly negative performances.The working mechanism is discussed by investigating the photoresponse,light absorption and reflectance,and electrochemical impedance spectra.This work firstly presents a clear answer that anatase-rutile phase junctions can improve performances of photoelectrochemical cells,mainly depending on the right structure and sequence of phases in space.