Anisotropic Photoelectrochemical (PEC) Performances of ZnO Single-Crystalline Photoanode: Effect of Internal Electrostatic Fields on the Separation of Photogenerated Charge Carriers during PEC Water Splitting

This work investigates the anisotropic PEC performances of ZnO single-crystalline (SC) photoanodes and the effect of internal electrostatic fields on the separation of photogenerated charge carriers during PEC water splitting. It was found that the internal electrostatic field can greatly influence the bulk charge separation efficiencies during PEC water splitting depending on its orientations, which can only be promoted as the internal electrostatic field in accordance with the direction of the holes’ transportation. Due to the surface stabilization of ZnO polar surfaces, the internal electrostatic field would be gradually decreased to zero near the surface of ZnO SC photoanodes. Therefore, the interfacial charge separation would be mainly determined by the interfacial electric fields in the space charge region formed by the equilibration of the Fermi levels between ZnO and the electrolyte solution. However, the differences on the bulk charge separation efficiencies of ZnO SC photoanodes are much larger than that at the interface, which indicated the bulk charge separation could play a more important role on determining the overall charge separation during PEC water splitting. Therefore, the anisotropic PEC performances of ZnO SC photoanodes during PEC water splitting could be mainly attributed to the internal electrostatic fields. With the assistance of the internal electrostatic field, O-SCs yield a record high solar to hydrogen conversion efficiency of 0.78% at 0.7 V vs RHE and a maximum photocurrent density of 1.84 mA cm –2 at 1.23 V vs RHE with η b and η i of 91.6 and 99.5%, respectively. The results demonstrate the effectiveness of internal electrostatic fields in polar single crystals on promoting the bulk charge separation during PEC water splitting, and indicate that polar single crystals could be good candidates to fabricate high efficient PEC photoanodes with high conversion efficiencies.