3.5 Proposed photocatalytic mechanism
Based on Mott-Schottky’s test results, the band structure of CoP and Cu3P can be further analyzed. The band gap energies of pure CoP and Cu3P can be obtained from the literature [24, 50, 51], which are 1.74 eV and 1.55 eV, respectively. The valence band of CoP and theconduction band of Cu3P can be obtained through the formula of EVB = ECB + Eg, whose values are 1.09 V and -0.14 V, respectively. Therefore, before and after the contact, the band positions of p-Cu3P and n-CoP are shown in Figure 10(a). Because p-type semiconductors have Fermi levels close to VB, n-type semiconductors have Fermi levels close to CB [52]. After the contact between p-Cu3P and n-CoP, due to the existence of the potential difference, an electric field was built in the semiconductor to promote charge transfer. During the formation of the p-n heterojunction, when the Fermi levels of CoP and Cu3P reached equilibrium, the direction of this internal electric field was directed from CoP to Cu3P [53]. At the same time, the conduction band of Cu3P will increase, and the conduction band of CoP will decrease with the action of Fermi leverage until the Fermi energy levels of CoP and Cu3P reach the equilibrium [54], which is also confirmed from two linear regions in the Mott-Schottky of Cu-Co-2P-2 composite catalyst.
Under visible light, CoP and Cu3P semiconductors absorb enough energy to generate electron-hole pairs under the action of EY sensitization. The EY molecule as a sensitizer is adsorbed on the surface of the photocatalyst to form a single excited state EY1* under visible light irradiation, and then undergoes a band gap conversion to form a more stable triple excited state EY3*. With TEOA as the electron donor, EY3* is reduced and quenched to form EY with strong reducing ability. The electrons of EY are transferred to the surface of the composite photocatalyst and participate in the reduction reaction to precipitate H2. At the same time, the dye molecules return to the ground state. The incompletely phosphatized MOFs framework provides a support for CoP and Cu3P produced by Cu-MOFs@ZIF-9(Co) phosphating, making dye molecules more easily adsorbed on the semiconductor surface. An interfacial electric field was formed in a p-n junction composite catalyst constructed by Cu3P and CoP, and the direction was from n-type CoP to p-type Cu3P [54]. CoP and Cu3P are excited by visible light, and electrons and holes are generated. The electrons transition from the valence band (VB) of the photocatalyst to the conduction band (CB). Then, under the action of the interface electric field, the electrons on the CB of Cu3P quickly migrate to the CB of CoP, and then participate in the hydrogen evolution reaction of reduced water. At the same time, the holes migrate from the VB of CoP, to the VB of Cu3P, and are finally consumed by TEOA molecules. It is worth mentioning that the generated EYfluorene molecules with strong reducing ability during EY sensitization can transfer electrons to the conduction band of CoP and Cu3P, providing a rich electron source for the reaction system. In the process of photocatalytic water splitting, the pn heterojunction formed by the combination of CoP and Cu3P effectively promotes the transfer of electrons and greatly inhibits the recombination of electron-hole pairs, thereby significantly improving the photocatalytic hydrogen production activity.