The employment of spin polarization under an external magnetic field holds great potential for the improvements of photocatalytic performance. However, owing to the huge difference in dielectric properties between ferromagnetic oxide and polymers, the photogenerated excitons with spin states are often limited to the ferromagnetic oxide wells, which leads to unsatisfactory activity. In this paper, a single-atom Co-doped C₃N₄ photocatalyst is successfully synthesized for photocatalytic water splitting and simultaneous oxidation of benzylamine. Under a tiny external magnetic field (24.5 mT), the hydrogen production rate could reach at 3979.0 μmol·g⁻¹·h⁻¹, which is about 340 times that of C₃N₄. Experimental results and theoretical calculations indicate that the interaction of Co d and N p orbital changes the symmetry center of C₃N₄, resulting in an increase in dielectric constant and spin polarization. Moreover, magnetic fields further promote parallel electron spin, and the increased number of charges with the parallel spin-down state is likely to dissociate under the action of an external magnetic field. On the other hand, the Co–N bond provides a huge built-in electric field and active site for strengthening the charge transfer and surface reaction. This work not only deepens the understanding of spin polarization, but also enriches methods to accelerate electron–hole separation.

Polymers are usually restricted on the high exciton binding energies and sluggish electron transfer because of the low dielectric constants. Regulating spin-polarized electrons is regarded as an attractive strategy, but often confined to the d-orbital elements. Here, the nonmetal P and N elements co-mediated the spin polarization of carbon nitrides (PCN) have been elaborately designed. The optimized PCN-3 shows an outstanding hydrogen production (22.2 mmol·g^–1·h^–1) coupled with selective benzylamine oxidation without using any solvent and cocatalysts, which is 200 times of original C₃N₄ and superior to the photocatalysts has been reported to date. Experimental and theoretical results verified that the spin-orbital coupling of N 2p and P 2p remarkably increased the parallel spin states of charge and reduced the formation of singlet excitons to accelerate exciton dissociation in carbon nitride. In addition, charge separation and surface catalysis can be significantly enhanced by the electron spin polarization of carbon nitride with the parallel arrangement, huge built-in electric field and disturbed electronic structure. Our finding deepens the insight into the charge separation and exciton dissociation in spin polarization, and offers new tactics to develop high-efficiency catalysts.