3.2.2. Z-Scheme Heterojunction Photocatalysts

In 2017, Lu et al. [81] reported a Z-scheme photocatalyst that improved the photocatalytic hydrogen production of g-C3N4 nanosheets by loading porous silicon (PSi). The Z-scheme heterostructure improved the photocatalytic H2 evolution performance by loading PSi onto the g-C3N4 photocatalyst. g-C3N4/PSi composites were prepared by the facile polycondensation reaction of PSi with urea at various PSi content ratios and included pure g-C3N4 that was not PSi loaded for comparison. The photocatalytic performance of the g-C3N4/PSi composites and pure g-C3N4 in Figure 8a was evaluated by H2 evolution from water under visible-light irradiation. For composite materials loaded with PSi on g-C3N4 nanosheets, the rate of H2 evolution was better than that of pure g-C3N4 (427.28 μmol g−<sup>1</sup> h<sup>−</sup>1). In particular, the g-C3N4/2.50 wt% composite exhibited the highest photocatalytic activity with a hydrogen evolution rate of 870.58 μmol g−<sup>1</sup> h<sup>−</sup>1, which is around twice as high as that of pure g-C3N4. However, in the case of the Si-based photocatalyst, a passive oxide film was formed on the Si surface, and thus the stability suffered. When the PSi content was larger than 2.50 wt%, the H2 generation activity was reduced. Figure 8b depicts an energy band diagram of g-C3N4/PSi with the redox potential of the photocatalytic reaction. The Z-scheme heterostructure system is recognized as the photocatalytic mechanism for the g-C3N4/PSi composite, and the electrons excited from the CB of PSi in the photocatalyst system can be transferred to the VB of g-C3N4. In addition, the holes generated in g-C3N4 can move to the CB of PSi through the interface formed between g-C3N4 and PSi. The recombination at the interface between the electrons and the holes accumulates a large number of bonds and acts as a recombination center for the electron–hole pairs [82,83]. As a result, the efficiency

of the photogenerated electron–hole pairs is improved, thereby improving photocatalytic hydrogen production under visible-light irradiation.

**Figure 8.** (**a**) Photocatalytic H2 evolution with 100 mg pure g-C3N4 and g-C3N4/PSi composite photocatalysts under visible light (400 nm) and (**b**) a schematic diagram of the g-C3N4/TiO2 heterojunction system. Reproduced with permission from [81]; copyright (2017), Elsevier.
