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Fast recombination of charges in bulk or on the surface of semiconductors seriously hinders the conversion efficiency of solar energy due to the inherent challenge of transferring two opposite charges to redox sites separately and simultaneously. Intrinsic spontaneous polarization electric fields (ISPEFs) are considered a promising strategy to solve these challenges, thereby significantly boosting the solar photocatalytic conversion efficiency. In this article, developments for solar energy conversion efficiency under the action of an ISPEF are concluded and reviewed. The key points are the recent research progress on spatial separation and directional transfer under an ISPEF induced by macroscopic, piezoelectric, ferroelectric, and surface or interfacial polarization. Particularly, basic principles of ISPEFs over different types of polarization materials are systematically discussed. Ultimately, future research directions and opportunities for polarization photocatalyst materials with an ISPEF are proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Novel heterojunction with internal electric field synergizing polarized field. • Stoichiometric H 2 and O 2 evolution with STH of 1.22 % has been achieved. • e- and h+ are boosted to pyridine N and Cu atom as reductive and oxidative sites. • Breaking long-term thought to regulate charge transfer by internal electric field. Flying broom-like heterojunction photocatalysts with internal electric field synergizing polarized field has been constructed by spontaneously assembling CuO nanoparticles on polarized g -C 3 N 4 microfibers (p -C 3 N 4 Fs) with using a facile hydrothermal method. Theoretical prediction and characterizations confirm that, photo-generated electrons and holes are dynamically transferred to reductive sites of pyridine-like N in p -C 3 N 4 Fs and oxidative sites of Cu atom in CuO for H 2 O splitting within the dual electric fields, respectively. Stoichiometric H 2 and O 2 evolution with solar energy conversion into H 2 energy of 1.22 % has been achieved under AM 1.5G irradiation in photocatalytic H 2 O overall splitting by heterojunction photocatalytic mechanism of translation, rotation, transformation of protons and intermediates. The rational photocatalyst design on heterojunction with dual electric fields breaks the long-term thought of regulating charge carriers only by internal electric field, which will provide a unique perspective to deepen the kinetic study on the spatial separation and directional migration of photogenerated charges to the surface redox active sites for solar energy conversion. [ABSTRACT FROM AUTHOR]

Serious recombination of photo-irradiated electrons (e −) and holes (h +) over photocatalysts restricts solar energy conversion, and directional transfer of e −/ h + to reduction/oxidation locations respectively and simultaneously, is enormously difficult under the shackle of Coulomb field. Herein, internal spontaneous polarization electric field (ISPEF) has been constructed by coordination between 4 d 10 orbit of Cd2+ and sp 2-hybridized N to induce a non-overlapping state of two opposite charge centers in g -C 3 N 4 , which could thus boost charge spatial division and migration. The significantly improved efficiency for H 2 evolution, accompanying by consecutive emission of visual H 2 bubbles was realized. Photo-irradiated charge spatial separation and photocatalytic mechanism within ISPEF of CdC 3 N 4 was firstly confirmed by theory prediction and Hall effect characterization. The study strategy supplies a novel perspective to design polarized semiconductor materials for significantly effective solar energy transformation, which will bring about another research highlights for constructing efficiently advanced semiconductor materials. [Display omitted] • Internal spontaneous polarization electric field (ISPEF) was constructed in g -C 3 N 4. • Strong ISPEF boosts charge spatial separation and directional transfer effectively. • g -C 3 N 4 was polarized by coupling Cd2+ 4 d 10orbit with its sp 2-hybridized N. • Visual H 2 bubbles were released over photocatalysts within ISPEF. • Theoretical calculation and Hall effect test were adopted to confirm ISPEF action. [ABSTRACT FROM AUTHOR]

• In situ growth of graphene on sp 2-hybridized N at triangular caves of g-C 3 N 4. • Polarization field and barrier-free interface are constructed. • Charges are efficiently transferred from g-C 3 N 4 to graphene under the field action. • Visible H 2 bubbles are firstly observed over metal-free photocatalyst. • Charge transfer principle within polarization field is revealed in depth. In-situ growth of graphene on g -C 3 N 4 (GC 3 N 4) with sp 2-hybridized N atoms at triangular caves as growing sites has been achieved with a tight barrier-free interface and intrinsic spontaneous polarization electric field (ISPEF) for simultaneously driving charge segregation and transfer from bulk to redox sites of surface, as confirmed by Hall effect and theoretical predictions. Consequently, the polarized GC 3 N 4 photocatalyst without any cocatalyst demonstrates a super high efficiency of 18.9 mmol∙g−1 h−1 with a large amount of visual H 2 bubbles under visible light illumination, which is the best metal-free photocatalysts for H 2 O splitting into H 2 fuel so far. This research supplies a novel and facile strategy to in situ grow graphene on pristine carbon materials with ISPEF to break the shackle of cocatalyst usage such as noble metals for commercial promotion to solar energy conversion. [ABSTRACT FROM AUTHOR]

• Spontaneous polarization electric field (SPEF) boosts charge separation and transfer. • SPEF was constructed by In3+[Kr]4 d10 orbit coupling with sp 2-hybridized N of g -C 3 N 4. • Visible H 2 bubbles have been released over the polarized photocatalysts. • Catalytic principle in SPEF was revealed by theoretical study and Hall effect test. Fast recombination of charges in bulk or on surface of semiconductors seriously hinders the efficient conversion of solar energy because of the inherent challenge in transferring two opposite charges to redox sites separately and simultaneously. Herein, spontaneous polarization electric field (SPEF) was designed by intercalation coupling In3+ [Kr]4 d10 orbit with sp 2-hybridized N of g -C 3 N 4. Coupling In3+ ions results in nonoverlapping state of positive and negative charge centers of g -C 3 N 4 , forming a SPEF to promote photo-generated charge separation and transfer across interlayers. Much high photocatalytic H 2 O reduction performance has been achieved as presented by continuously releasing of H 2 bubbles. Photocatalytic principle in SPEF was revealed by theoretical study and Hall effect test. This research shows an appliable strategy to construct polarization photocatalysts for efficient solar energy conversion to H 2 fuel, which will open up another research hotspot for designing highly efficient photocatalysts. [ABSTRACT FROM AUTHOR]

• In-plane coupling electric field efficiently drives directional electron transfer. • Li+ couples with sp2-hybridized N ligands at triangular cave of g-C 3 N 4 (LiC 3 N 4). • LiC 3 N 4 shows strong efficiency with a large number of visible H 2 bubbles. • Photocatalytic solar energy conversion participates in Li resource competition. Directional charge transfer across in-plane structure of g -C 3 N 4 is still inefficient because of the uniform delocalization charge distribution around triangular caves of the π-conjugated tri-s-triazine polymer, which severely restricts its application in solar energy conversion into H 2 fuel. Herein, an in-plane coupling electric field is introduced to boost charge directional transfer by coupling Li+ with sp2-hybridized N ligand at triangular cave of g -C 3 N 4 (LiC 3 N 4) for highly efficient H 2 bubble evolution. The constructed LiC 3 N 4 demonstrates much high H 2 -evolution efficiency with a large number of visible H 2 bubbles, which presents absolute performance advantage and industrial potential in comparison with g -C 3 N 4 supported numerous noble metals. Density functional theory and characterizations reveal an efficient charge directional transfer from the 2p z orbit of sp2-hybridized N ligand to Li atom across in-plane structure of LiC 3 N 4 is achieved under the coupling electric field, resulting in an efficient H 2 bubble evolution. This research strategy shows a scientific perspective to boost directional charge transfer for much effective solar energy conversion into H 2 fuel by Li+ coupling effect, which will be bound to introduce the competition for lithium resources between photocatalytic solar energy conversion and lithium batteries. [ABSTRACT FROM AUTHOR]