Figure 5. (a) XPS spectra of Co-P, Cu-P and Cu-Co-2P-2; (b) Co 2p spectra of Co-P and Cu-Co-2P-2; (c) Cu 2p spectra of Cu-P and Cu-Co-2P-2; (d) P 2p spectra of Co-P, Cu-P and Cu-Co-2P-2.
In addition, the results of Brunauer-Emmett-Teller (BET) tests can also prove the effect of incompletely phosphated Cu-MoF on composites. The nitrogen adsorption-desorption isotherms tested at 77 K obtain pore volume, pore size, and porosity and the results are shown in Table S1 and Figure S3. The isotherms of Co-P, Cu-P and partially phosphatized Cu-Co-2P-2 materials produced by fully phosphating belong to the IV isotherm [36]. It has a H3 type hysteresis loop under relatively high pressure, which means that the mesoporous material has a multi-layer adsorption process due to capillary condensation. It is worth noting that Cu-Co-2P-2 material has the highest specific surface area, largest pore volume and average pore diameter.
The excellent light absorption performance of the photocatalyst is a sufficient condition to ensure efficient photocatalytic activity. The photocatalysts were tested in the UV-Vis-NIR DRS (200-800nm) before and after phosphating. The diffuse reflection spectra of ZIF-9(Co), Cu-MOFs and Cu-MOFs@ZIF-9(Co) samples before phosphating are shown in Figure S4(a). Unphosphorylated pure ZIF-9 (Co) has strong absorption in 400-650nm, while pure Cu-MoF is a blue powder and therefore also has strong absorption in the range of 500-800 nm. The Cu-MOFs@ZIF-9(Co) sample has strong absorption in the visible near-infrared region. The successful assembly of Cu-MOFs and ZIF-9 is explained from the side. Figure S4(b) shows the diffuse reflectance spectra of the Co-P, Cu-P and Cu-Co-2P-2 samples obtained from phosphating. The samples all show strong optical absorption density. Absorption in the ultraviolet and near infrared regions means that the catalyst can achieve electronic transitions at higher or lower energies. The stronger light absorption capacity of photocatalyst means the stronger probability of electron transfer under light, which is more favorable for H2 evolution [37, 38]. Co-P, Cu-P and Cu-Co-2P-2 samples continue to maintain high levels of absorbance with increasing wavelength, which means that light energy is more utilized [39]. This is the important reason why both Co-P and Cu-P show a certain photocatalytic activity. The Cu-Co-2P-2 sample has the highest optical absorption density, which indicates that the strong light absorption performance of the black sample may be the main reason for its enhanced hydrogen evolution activity.