3.3 Phase structure and surface chemical state
The Co3O4 QDs, CoP, ZCS and O/ZCS/P-x samples were further performed by X-ray diffraction (XRD) to determine the phase composition. As displayed in Fig. 3a, the XRD pattern of Co3O4 QDs matches well with cubic phase Co3O4 (PDF#42-1467) [30], and the characteristic peaks at 36.6°, 44.8°, 59.4° and 65.1° can be severally indexed to the (311), (400), (511) and (440) crystal planes of cubic phase Co3O4. Meanwhile, the diffraction peaks at 31.6°, 36.2°, 48.1° and 56.7° in the XRD pattern of the bare CoP were in accordance with the (011), (111), (211) and (212) crystal planes of orthorhombic CoP (PDF#29-497) [36,37]. Fig. 3b displays the XRD patterns of ZCS and ZCS-based hybrids. The representative diffraction peaks of at 26.1°, 27.8°, 29.5°, 38.4°, 45.9°, 50.2° and 54.5° can be severally indexed to the (100), (002), (101), (102), (110), (103) and (112) crystal planes of hexagonal Zn0.5Cd0.5S (PDF #89-2943) [38]. However, no visible diffraction peaks of Co3O4and CoP were presented in all ZCS-based samples, which is because of the extremely low content and crystallinity of Co3O4 and CoP. Nevertheless, the presence of Co3O4 and CoP in the O/ZCS/P-x composites has been proved by EDS and element mapping techniques as displayed in Fig. 2.
The X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface composition and chemical states of the as-obtained samples. The Co 2p and O 1s high-resolution XPS profiles of original Co3O4 QDs are illustrated in Fig. 3c and d, respectively. The two peaks at 780.2 and 795.5 eV are attributed to Co 2p3/2 and Co 2p1/2 of Co2+, and other two peaks at 782.1 and 797.9 eV are assigned to Co 2p3/2 and Co 2p1/2 of Co3+ [39], respectively. The peaks centered at 530.0 and 531.6 eV in the O 1 s XPS profiles of Co3O4 QDs belong to Co-OH and Co-O groups, respectively. And the signals at 787.8 and 804.1 eV correspond to the satellite peaks of Co 2p3/2 and Co 2p1/2, respectively. The Co 2p and P 2p high-resolution spectrum of pristine CoP are displayed in Fig. 3e. The XPS spectrum of Co 2p in CoP is similar to that in Co3O4 QDs, but it is very clearly that the binding energies of both Co2+ and Co3+ in Co 2p are bigger than those in Co3O4 QDs, which is due to the stronger electronegativity P elements in CoP. The weak signals at 129.5 and 130.3 eV in the high-resolution spectrum of P 2p (Fig. 3f) belong to the index of Co-P bonds in CoP [40]. While the peak at 134.1 eV is attributed to the oxidized P owing to the inevitable surface oxidation.