Separation of photogenerated electron-hole pairs is a key step to enhance the photocatalytic activity of semiconductor photocatalysts. Internal electric filed,as a potential driving force to separation of the carriers,has become one of the research hotspots in photocatalytic field recently.In this paper, the literatures about the enhancement of photocatalytic performance based on the internal electric field are reviewed. To align their potentials (E_f), charge transfer occurs between two different component semiconductor materials. This charge redistribution region is known as the space charge region. After the charge transfer, the accumulation of electrons on the semiconductor surface leads to upward band bending. The internal electric field can be formed due to the redistribution of charges, which may in turn facilitate the separation of electrons and holes for reactions. The upward or downward bending can drive the holes/electrons to run up for an oxidation reaction/a reduction reaction, respectively. Internal electric fields within photocatalysts can arise from ferroelectric phenomena, p-n/polymorph junctions,polar surface terminations,and nonlinear optical material. The internal electric fields within photocatalysts mitigate the effects of recombination and back-reaction, then to increase photochemical reactivity. The strategies for manipulation of internal fields are also discussed for the design of efficient photocatalysts. Finally,we highlight some crucial issues in engineering internal electric fields and provide tentative suggestions for future research on increasing their photocatalytic performance. Especially, the importance of using advanced physical technology and theoretical calculation method to characterize the distribution of internal electric field is emphasized.

Contents
1 Introduction
2 Internal electric fields in ferroelectrics
2.1 Internal electric fields in ferroelectrics and photocatalysis reactions
2.2 Factors impacted on the internal electric fields in ferroelectrics
2.3 The application of organic-inorganic perovskite structures in photocatalysts
3 Internal electric fields from p-n junction
3.1 Polaring process within p-n junction
3.2 Photocatalysts with p-n junction
3.3 Factors impacted on p-n junction
4 Internal electric fields from polymorph junctions
5 Polar surface terminations
5.1 Polar surface, internal electric fields and photocatalysis
5.2 Internal electric fields between crystal faces
6 Internal electric fields within noncentronsymmetric compounds
6.1 Nonlinear optical photocatalysts
6.2 Polaring in the photocatalysts with sillenite structure
7 Conclusion and outlook