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.