In 2003, Bawendi et al. observed the separation of the electron and the hole between the core and shell materials through (core) shell semiconductor heterostructures, in which the conduction and valence bands of the core and shell material are staggered [
73]. Li and coworkers reported that the flower-like MoS
2/BiVO
4 composite with heterojunction showed outstanding performance for photodegradation of methylene blue due to the staggered band alignment shaped between MoS
2 and BiVO
4 [
74,
75]. Based on these studies, the 2D-layered MoS
2 is found to have immense potential to be used for photocatalysis applications. It is of great importance to indicate that MoS
2 has some disadvantages, such as inadequate charge segregation and underprivileged charge mobility [
76]. Both of them will lead to low photocatalytic behavior. Figure 5(a) is the UV absorption spectra of BiVO
4, MoS
2, and BiVO
4@MoS
2 where the bare MoS
2 nanosheets express noteworthy absorption both in the ultraviolet and visible regions. Meanwhile, the absorption wavelength for pure BiVO
4 lies around 500 nm and attributes to the intrinsic band gap absorption. By depositing MoS
2 nanosheets on the surface of BiVO
4, a better visible-light photocatalytic performance is observed for the composite in comparison with the pure BiVO
4. Moreover, the formula of (
ahn)
n =
A (
hn-Eg) is being used to compute the band gap energy (
Eg) of BiVO
4 and MoS
2, where
h,
n,
Eg, and
A are the absorption coefficient, Planck’s constant, light frequency, band gap, and a constant, respectively. In addition, the index
n would be determined based on the electronic transition of the semiconductor. For instance
n = 2 for direct-gap semiconductor and
n = 0.5 for indirect-gap semiconductor would be considered. Therefore, the value of index
n is equal to 2 for MoS
2 and BiVO
4. The band gap energy can be projected from the intercept of the tangent to the plot of (
ahn)
2 versus the radiation energy (
hn), as can be seen in Figs. 5b–5f. They reported the band gap energy for BiVO
4 and nanosheet MoS
2 as 2.50 eV and 1.62 eV, respectively [
77], while the bulk MoS
2 has shown a band gap energy of 1.23 eV [
78], can be assigned to the strong quantum confinement effect of the thin nanosheets, which also makes the MoS
2 nanosheets an effective visible-light photocatalyst [
79]. The values of band gap energy are reported as 2.19, 2.09 and 1.90 eV for BiVO
4@MoS
2 (2 wt%), BiVO
4@MoS
2 (5 wt%), and BiVO
4@MoS
2 (10 wt%), respectively. Since the photocatalytic property of the photocatalyst is contributed to its band structure, there is a simple approach to determine the band edge positions of both the conduction band (
ECB). Besides, the valence band (
EVB) at the point of zero charge (pH
ZPC), can be calculated by
ECB=
X-Ee- 0.5
Eg [
80]. The
X is the absolute electronegativity of the semiconductor,
Ee is the energy of free electrons on the hydrogen scale (4.5 eV),
Eg is the band gap, and the valence band edge (
EVB) can be determined by
EVB =
ECB +
Eg. Therefore, the
ECB values of BiVO
4 and MoS
2 are determined to be 0.29 and 0.03 eV, respectively, where the
EVB values of BiVO
4 and MoS
2 are estimated to be 2.79 and 1.65 eV, respectively. Figure 6 demonstrates the photocatalysis mechanism of p-n heterojunction photocatalyst and the schematic drawing of the electron-hole separation process.