Photocatalysis is one of the important methods to solve the current environmental pollution and energy crisis. However, the photocatalytic efficiency of most photocatalysts is still low. Improving the separation of photogenerated charges is an effective way to enhance the photocatalytic efficiency. Firstly, the paper summarizes the photocatalysis mechanism, and then reviews the effects of built⁃in electric field in semiconductor p⁃n junction, heterophase junctions, polarized surfaces and ferroelectric material polarization on separation efficiency of photogenerated charges and photocatalytic performance, based on the recent domestic and international research progress in improving the separation efficiency of photogenerated charges. Finally, the future development of built⁃in electric field in photocatalysis is prospected.

A two⁃dimensional nano flake rutile titanium dioxide (TiO ₂) semiconductor material was synthesized under low temperature by hydrothermal method.The effects of HCl concentration on the crystalline phase,crystallinity,morphology,particle size and photoelectric properties of obtained TiO ₂ particles are studied using XRD,SEM and SPV.The results indicate that the micron spherical,nanosheets and nanorods rutile TiO ₂ particles can be synthesized by controlling the HCl concentration at 3,5,7 mol/L,respectively.Moreover,the photocatalytic performance of obtained rutile TiO ₂ particles is further investigated by degradation of rhodamine B. The results show that the activities of micron spherical,nanosheets and nanorods rutile TiO ₂ samples are higher than that of traditional rutile TiO ₂ sample obtained by calcination method.Compared with micron spherical and nanorods rutile TiO ₂ samples,2D nanosheets rutile TiO ₂ sample has better photocatalytic properties.SPV results reveal that the 2D nanosheets rutile TiO ₂ has a significant efficiency on photogenerated charges separation and migration.

X80 pipeline steel specimens were treated by electrochemical hydrogen charging under various current densities and charging time. The polarization resistance and corrosion rate of X80 steel were analyzed in Yingtan simulated soil solution under the different hydrogen charging conditions. The results show that with the hydrogen charging current density increased from 1 mA/cm ² to 20 mA/cm ², the free-corrosion potential of X80 moved negatively, charge transfer resistance Rt dropped from 1 176 Ω·cm ² to 909 Ω ·cm ², double electric layer capacitors increased from 0.000 16 μF/cm ² to 0.001 7μF/cm ² and corrosion current density increased from 3.322 mA/cm ² to 6.880 mA/cm ². When filling hydrogen time increased from 1 h to 5 h, Rt changed from 1 522 Ω ·cm ² down to 749 Ω ·cm ², double electric layer capacitors increased from 0.000 14 μF/cm ² to 0.007 5 μF/cm2 and corrosion current density increased from 2.91 mA/cm ² to 5.01 mA/cm ². This suggests that electrochemical hydrogen charging result in anodic dissolution rate of the weak spots and concentration of metal ions increased. Hydrogen speeded up anodic dissolution of X80 steel and leaded to higher corrosion sensitivity at the steel surface.

Electrochemical impedance spectrum technology and morphology observing were used to study the corrosion behaviors of X70 pipeline steel in 0.5mol/L NaHCO ₃+0.5mol/L NaCl solution.The results show that X70 steel corrosion resistant declined within the elastic load range.The metal corrosion was promoted by applied stress mainly due to two aspects:one is the atoms are arranged when deformed,lattice defects and atomic activation energy increased in the stress concentration area with the applied stress increased,which resulted in the balance potential decreased sharply;another is the surface passivation membrane is damaged or local thinned,which resulted in corrosion occurred at passivation membrane rupture or weak position.X70 steel surface passivation membrane ruptured and sample local plastic deformation increased which lead to local potential changing under the common function of the applied stress and erosive medium.The corrosion micro cell formed aroundt he matrix and created a condition fo rthe pitting nucleation.

Cyclic voltammetry was used to comprehensively investigate the fundamentals of the electrochemical corrosion reactions occurring at the steel／solution interface in Yingtan soil simulated solution． The interrelation between anode polarization and the cathodic hydrogen evolution was studied by changing the sweep rate， the upper potential limit，ions concentration and gas conditions．The results show that the electronically behaviors of X80 steel are irreversible charge transfer process．There is no stable oxide film formed on the steel surface． The anode polarization and cathodic hydrogen evolution reaction are enhanced by the corrosive ions in the solution ．The hydrogen evolution was catalyzed by the stable deposition film which generated during the anode polarizing．The introduction of oxygen could form a stable oxide film，which weakened the
catalysis to the hydrogen evolution．The stress corrosion cracking sensitivity of the welded metal is increased by CO_２ introduced to the solution．

The passive behavior and electrochemical characterization of the passive films formed on X80 and X70 pipeline steels in 0.5 mol/L NaHCO ₃ solution were discussed by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and capacitance measurements. It is show that X80 and X70 pipeline steels present a wide passive anodic region (-0.16~0.90 V), the passive current density (ip) of X80 and X70 steels are 6.76 μA/cm ² and 18.38 μA/cm ², respectively. EIS results indicate that the surface of the passive film formed on X80 steel is much denser and uniformer than that of X70, and the result is also show by AFM. Mott-Schottky analyses displayed that the passive films on X80 and X70 pipeline steels in 0.5 mol/L NaHCO ₃ exhibit n-type semiconductive characters in the potential region of -0.20 V to 0.80 V. Compared with the passive film of X70 steel, the donor density is much lower and the width of space charge layer is thicker for the passive film of X80 steel, so the corrosion resistance of the latter is better.

The corrosion behavior of X70 steel in Yingtan acidic soil was studied by using polarization curve technique and electrochemical impedance spectroscopy (EIS) in 13%~40% water content. It is found that X70 steel corrosion rate is rised with the increasing of water content and reached a maximum value when the water content of soil is saturated, and samples is corroded selectively expanding roundly. Cathode reaction is oxygen reduction when water content is 13%, hydrogen ion then participated in with the water content increasing. Water content and pH both affect X70 steel electrochemical behaviors. Corrosion resisting property of X70 steel is bad in Yingtan acidic soil.