Figure 4. (a) TEM image; (b) HRTEM image; (c) EDX image; (d)
SAED pattern and (e) element mapping of Cu-Co-2P-2.
Figure 5 shows the X-ray photoelectron spectroscopy (XPS) of Co-P, Cu-P
and Cu-Co-2P-2. The XPS measurement spectra of Co-P, Cu-P and Cu-Co-2P-2
and the corresponding high-resolution XPS spectra of various elements
are shown in Figure 5(a). It can be easily seen from Figure 5(a) that
MOFs derived Co-P and Cu-P photocatalysts contain Na, C, N and O. And
Cu-Co-2P-2 composite photocatalysts have both Cu and Co signals. Figures
5(b, c and d ) are high-resolution XPS spectra of Co 2p, Cu 2p, and P 2p
respectively. Figure 5(b) shows the Co 2p peaks of Co-P and Cu-Co-2P-2,
respectively. The peak at 793.1eV is assigned to Co
2p1/2, and the binding energy values of 778.1 eV and
777.9 eV correspond to Co 2p3/2. In addition, the
diffraction peaks at 779.9 eV and 800.1 eV/799.9 eV belong to the
vibration satellites of Co 2p3/2 and Co
2p1/2 [31]. From pure CoP toCu3P@CoP
composite, Co 2p3/2 binding energy has a negative shift
to lower binding energy. These observations indicate that there is a
strong electronic interaction between CoP and Cu3P in
Cu-Co-2P-2, which is of great significance for promoting photocatalytic
hydrogen production activity. The test results of the Cu-P photocatalyst
obtained by fully phosphating Cu-MOFs are shown in Figure 5(c). The
high-resolution XPS of Cu-P shows four peaks, two peaks corresponding to
the 2p3/2 and Cu 2p1/2 spin states at
932.6 and 952.5 eV, respectively. The two peaks at 933.5 and 955.3 eV
are satellite peaks [32]. In the Cu-Co-2P-2 sample, since the
Cu-MOFs inside are only partially phosphorylated, the binding energy of
the Cu 2p orbital is different from that of the Cu 2p orbital in the
Cu-P sample. In the same Cu-Co-2P-2 sample, the binding energies of Cu
2p3/2 and Cu 2p1/2 are divided into
932.5 and 952.7 eV. The peak at 936.2 eV is a satellite peak in 2P
orbit, and the broad peak with binding energy value of 944.8 eV is
related to incompletely phosphated Cu-MOFs [33]. The XPS spectrum of
the P 2p energy level is shown in Figure 5(d). The high-resolution XPS
spectrum of the element P of Cu-P, Co-P and Cu-Co-2P-2 samples can be
divided into two peaks. The three signals with a binding energy of
133.7eV (Cu-Co-2P-2), 133.5eV (Cu-P) and 133.4 eV (Co-P) are resulted
from the surface oxidation of the sample when it contact with air, which
is an oxidized P species [32, 34]. The three peaks at 129.5 eV
(Cu-Co-2P-2), 129.1 eV (Cu-P) and 129.4 eV (Co-P) correspond to the
binding energy of the M-P (M = Cu, Co) bond, indicating the formation of
metal phosphides. It is impressive that the peaks of Cu-Co-2P-2 both
shift to a high binding energy compared with Co-P and Cu-P. These
results further indicate that there is a strong electronic interaction
between Cu3P and CoP in Cu-Co-2P-2, which is beneficial
for improving the catalytic performance of the composite photocatalyst
[35].