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.