Dual roles of ZnS thin layers in significant photocurrent enhancement of ZnO/CdTe nanocable arrays photoanode.

The effect of a ZnS thin layer on the photoelectrochemical property of ZnO/CdTe nanocable arrays-on-indium tin oxide was systematically studied by the successive ion layer absorption and reaction (SILAR) of ZnS. The thickness of CdTe on bare ZnO/CdTe nanocable arrays was optimized to approximately 10 nm to achieve a saturated photocurrent density of 6.5 mA/cm(2). Significant photocurrent enhancement was achieved by gradually increasing the ZnS SILAR cycle number from 0 to 10. A "type I" band alignment with conduction and valence band offsets of 0.58 and 1.52 eV, respectively, was deduced for the CdTe/ZnS interface. The detailed microstructure of the CdTe/ZnS interface and the relationship between the photocurrent and the ZnS thickness indicated that ZnS not only serves as a barrier layer that prevents electron injection from CdTe to the electrolyte but also provides an effective tunneling channel for hole transfers to the electrolyte. A ZnO/CdTe/ZnS nanocable photoanode yielded a saturated photocurrent density of 13.8 mA/cm(2) when the thickness of ZnS was controlled to approximately 2 nm.