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 EY-˙fluorene
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.