**4.5 Iodine doping**

Iqbal et al. [23] used a straightforward one-step technique to make a nano-mesoporous g-C3N4 doped with iodine and N vacancy at the same time. Synergistic iodinedoped mesoporous g-C3N4 epidermoid shedding and the introduction of N vacancy due to the release of nitrogen-rich gas has been observed when glucose and NH4I are heated simultaneously. Under simulated solar light, the produced iodine-doped mesoporous g-C3N4 has an exceptional photocatalytic hydrogen generation performance of 7819.2 mol g−1 h−1, approximately 6.5 times greater than bulk g-C3N4 and other types of iodine doped g-C3N4 photo-catalysts. This paper demonstrates a straightforward and practical method for synthesizing high-performance metal-free g-C3N4 photo-catalysts by altering the electronic transition via a well-designed band structure (**Figure 5**).
