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.