Fig. 3 XRD patterns of (a) CoP, Co3O4 QDs, and (b) ZCS, ZCS/CoP-x and O/ZCS/P-x samples; XPS spectra of (c) Co 2p and O 1s for Co3O4 QDs, and (d) Co 2p and P 2p for CoP.
In Fig. 4a, the XPS signals of Co 2p in Co3O4 QDs/ZCS and ZCS/CoP is quite weak due to the extremely low content of Co3O4 QDs and CoP in composite samples. It is worth noting that the binding energies of Co 2p in Co3O4QDs/ZCS and ZCS/CoP exhibit a negative shift in comparison with those of Co 2p in alone Co3O4 QDs and CoP, which suggests that it is charges that migrate from ZCS to Co3O4 QDs and CoP. On the contrary, the binding energies of Zn 2p, Cd 3d and S 2p in all composite samples present remarkable positive shift in comparison with those of Zn 2p, Cd 3d and S 2p in original ZCS (Fig. 4b-d), which also indicate that it is ZCS that is the electron donor for highly-efficient photocatalytic hydrogen evolution. In addition, it also can be observed from the XPS spectrum of Zn 2p, Cd 3d and S 2p that their binding energies in O/ZCS/P bi-heterostructure cages increase more compared with those in Co3O4 QDs/ZCS and ZCS/CoP, which strongly reveal that Co3O4 QDs and CoP simultaneously transfer electrons from ZCS. Consequently, the construction of hierarchical O/ZCS/P bi-heterostructure cages can rapidly migrate charges from photo-excited ZCS, so the recombination of electron-hole pairs can be greatly inhibited, thereby the hydrogen evolution performance is enhanced seriously.