Sn Doping Enhances Light Absorption of Lanthanum Poly(Heptazine Imide) for Efficient Photocatalytic Hydrogen Evolution

Lanthanum poly(heptazine imide) (La‐PHI) is a novel, highly crystalline PHI material. However, its visible‐light absorption remains weak due to the limited contribution of La 3+ to the band structure of PHI. Considering that the ionic radius of Sn 2+ is comparable to that of La 3+ , Sn 2+ can stably occupy the identical sites within the PHI framework. Meanwhile, the 5p orbitals of Sn 2+ can effectively hybridize with the N 2p orbitals, thereby introducing intermediate energy levels. Experimental results demonstrate that Sn 2+ doping generates an intermediate level at approximately 2.30 eV within the forbidden band, effectively broadening the light absorption range. Through optimizing the Sn 2+ doping amount, the photocatalytic activity of the obtained sample under 420 < λ < 780 nm is 5.3 times that of the original La‐PHI. This study indicates that Sn 2 + doping is an effective strategy for extending the light absorption range, and the introduction of intermediate levels provides new insights into the design of highly efficient PHI‐based photocatalysts.