Table 4 Different types of heterojunctions

1

Type I (Straddling Gap)

In a Type I heterojunction, the VB and CB edges of SC-B are entirely nested within those of semiconductor SC-A. This band alignment promotes the migration of both e⁻ and h⁺ from SC-A to SC-B, where the targeted redox reactions subsequently occur¹⁷¹.

2

Type II (Staggered Gap)

In a Type II heterojunction, the VB and CB edges of SC-B are offset relative to those of SC-A, forming a “staggered” or “staircase” band alignment. In this configuration, the CB minimum resides in SC-B, while the VB maximum lies in SC-A. Upon photoexcitation, e⁻ tend to migrate to the CB of SC-B, whereas holes (h⁺) migrate to the VB of SC-A. This spatial separation of charge carriers promotes efficient charge separation, thereby enhancing photocatalytic performance¹⁷¹.

3

Type III (Broken Gap)

A Type III heterojunction, also known as a broken-gap alignment, resembles the Type II configuration in spatial arrangement but differs fundamentally in electronic structure. In this case, the bandgaps of the two semiconductors do not overlap, resulting in a discontinuity where the CB of SC-B lies below the VB of the SC-B. This pronounced energy offset severely limits charge carrier migration across the interface, thereby rendering the junction inefficient for PEC applications¹⁷¹.

4

p-n Heterojunction

A p–n heterojunction is formed at the interface between an n-type SC and a p-type SC. In the dark, e⁻ diffuse from the n-type to the p-type region, h⁺ diffuse in the opposite direction. This bidirectional carrier movement continues until the E_F of both materials align, establishing equilibrium and forming a built-in electric field at the junction. Under illumination, photogenerated e⁻ in the CB of the p-type SC migrate toward the CB of the n-type SC, while photogenerated h⁺ move toward the VB of the p-type. This charge carrier redistribution induces a shift from the equilibrium condition, altering the E_F alignment and enhancing charge separation across the junction²⁵⁵.

5

Schottky Junction

A Schottky junction is established at the interface between a metal (or metal-like material) and a SC, giving rise to a Schottky barrier as a result of the difference in their E_F. When the metal possesses a higher work function than the SC—such as in the case of BiVO₄—the resulting band alignment facilitates the transfer of photogenerated e⁻ from the CB of the SC to the metal. This directed e⁻ flow promotes efficient charge carrier separation by preventing e⁻/h⁺ recombination, thereby enhancing the performance of PEC systems^{256,257,258,259}.

6

Z-scheme Heterojunction

The Z-scheme heterojunction structurally resembles a Type II configuration but operates via a distinct charge transfer mechanism. In this system, charge carrier migration is driven by the IEF established between the two SCs. Upon photoexcitation, e⁻ from the CB of SC-B recombine with h⁺ in the VB of SC-A at the interface. This selective recombination preserves the photogenerated e⁻ in the CB of SC-A and h⁺ in the VB of SC-B, which possess strong reducing and oxidizing potentials, respectively; thereby enhancing photocatalytic efficiency^170,255,260.

7

S-scheme Heterojunction

The step-scheme (S-scheme) heterojunction is an advanced variation of the traditional Z-scheme, designed to optimize charge separation and redox capability by incorporating distinct OPs and RPs. A critical requirement is that the RP possesses a higher EF than the OP. Upon contact, an IEF is established at the interface, accompanied by energy band bending and Coulombic interactions, which collectively drive charge carrier transfer. This configuration promotes the recombination of low-energy e⁻ and h⁺ while preserving high-energy e⁻ in the RP and high-energy h⁺ in the OP, thus enhancing the redox potential and overall photocatalytic performance^170,255,261.

8

Surface Heterojunction

A surface heterojunction is formed at the interface between different crystal facets of a single SC. Owing to the slight variations in electronic structure and band edge positions among these facets, an internal potential gradient is established, which drives directional charge carrier migration. This behavior resembles that of a Type II heterojunction, facilitating spatial separation of photogenerated e⁻ and h⁺. However, the overall applicability of surface heterojunctions remains limited due to the relatively small band offsets and the dependence on precise facet engineering²⁵⁵.