Charge separation between wurtzite ZnO polar {0 0 1} surfaces and their enhanced photocatalytic activity

• ZnO nanocrystals with various (0 0 2) orientations were synthesized.
• Their photocatalytic activity depends on the texture coefficient of (0 0 2) plane.
• The exposed {0 0 1} polar surfaces were found to be highly reactive facets.
• The enhanced photoreactivity is attributed to polar structure of the {0 0 1} facets.
• The internal electric field between polar surfaces drives charge separation.

ZnO nanocrystals with various (0 0 2) orientations were synthesized via a solvothermal reaction of zinc acetate with n-butylamine (BA) and tetrahydrofuran at 140 °C for 12 h by varying the molar ratio of BA to Zn(II). The formation of the ZnO nanocrystals with various (0 0 2) orientations and shapes results from the selective adsorption of different amount of BA molecules on ZnO (0 0 1) surface. Photocatalytic activity of the as-prepared ZnO nanocrystals with various (0 0 2) orientations in degradation of methyl orange was studied, it was found that the exposed {0 0 1} facets are reactive facets. The structure and atomic charge distribution of the {0 0 1} facets were studied by periodic density functional theory calculations. Based on polar structure of the exposed {0 0 1} surfaces, a charge separation model between polar ZnO {0 0 1} surfaces was proposed. There is an internal electric field between positive Zn-ZnO (0 0 1) and negative O-ZnO (0 0 1¯) planes due to the spontaneous polarization. The internal electric field provides the driving force for charge separation. The reduction and oxidation reactions take place on the positive (0 0 1) and negative (0 0 1¯) polar planes, respectively. The charge separation model can deepen understanding of charge transfer in the semiconductor nanocrystal with highly photocatalytic activities and offer guidance to design more effective photocatalysts as well as new type solar cells, photoelectrodes or photoelectric devices.

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