3.4 Photoelectrochemical characterization test
Generally, the photocurrent response is used to reveal the
photoelectrochemical properties that occur on the photocatalyst surface
[47, 48]. Figure 9(a) shows the photocurrent response curves of
Cu-P, Co-P and Cu-Co-2P-2. Compared with Cu-P and Co-P samples,
Cu-Co-2P-2 showed the highest photocurrent density. This high
photocurrent density further indicates that the photogenerated carriers
in the partially phosphated Cu-MOFs@ZIF-9 composite catalyst with a
special structure have a more effective separation efficiency and a
lower recombination rate, thereby enhancing the photocatalytic
H2 Production activity. The polarization curves were
used to explore the changes in the current density of Cu-P, Co-P, and
Cu-Co-2P-2 as a function of voltage and the level of hydrogen production
overpotential.
The LSV curves of Cu-P, Co-P and Cu-Co-2P-2 electrodes are shown in
Figure 9(b). Unsurprisingly, Cu-Co-2P-2 showed the fastest increase in
current density, indicating that Cu-Co-2P-2 has a lower overpotential.
The decrease of the hydrogen-producing overpotential was mainly due to
the rapid electron transfer at the layered Cu-P@Co-P interface, which
indicates that Cu-Co-2P-2 composite catalyst has strong HER activity.
Electrochemical impedance spectroscopy (EIS) plots were collected at
open circuit potentials in the frequency range of 1 MHz to 0.1 Hz. As
shown in Figure 9(c), the size of the curvature radius of the curve
reflects the size of the charge transfer resistance. The relatively
small circular orphan indicates that the charge transfer is faster
[47]. Cu-Co-2P-2 has the smallest radius of curvature, indicating
its excellent electrical conductivity. The combination of Co-P and Cu-P
constitutes a rapid transfer channel. Therefore, the Nyquist curve of
Cu-Co-2P-2 composites shows a significantly reduced semicircle, which
indicates that the charge transfer rate of Cu-Co-2P-2 composites is
significantly enhanced.
The Mott-Schottky tests of Co-P, Cu-P and Cu-Co-2P-2 are shown in Figure
9(d-f). The positive and negative slope of the curve in the figure can
be used as a basis for judging the type of semiconductor. The slope is a
regular description of an n-type semiconductor, otherwise it is a p-type
semiconductor. Pure Co-P in Figure 9(d) has a positive slope, which is
typical n-type semiconductor behavior. The slope of pure Cu-P is
negative, indicating that Cu3P is a p-type
semiconductor. The flat band potentials (Efb) for Cu-P
and Co-P are estimated at 1.21 V and -0.45 V, respectively, which are
relative to SCE. According to the meaning of the flat band potential,
the Fermi level (Ef) position of pure CoP and pure
Cu3P samples can also be roughly estimated. Generally,
the conduction band potential (ECB) of an n-type
semiconductor is more negative -0.1 or -0.2 V than its flat band
potential [47, 49], while the valence band potential of a p-type
semiconductor is corrected by 0.1 or 0.2 V. Therefore, the Co-P
conduction band is roughly estimated to be -0.65 V, and the Cu-P valence
band is roughly estimated to be 1.41 V relative to SCE. For the
Cu-Co-2P-2 composite catalyst, there are two linear regions in the
Mott-Schottky diagram, which can be attributed to the flat band
potentials of the recombined CoP and Cu3P, the values of
which are -0.07 V and 1.02 V, respectively.
This
is due to the formation of a p-n heterojunction after the recombination
of n-type CoP and p-type Cu3P, which results in a Fermi
level shift, that is a shift in the flat band potential.