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].