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High-density vertical ZnO nanowires (NWs) were synthesized by vapor phase transport deposition without a metal catalyst or template. The work function φ of ZnO NWs was reduced to 3.7 eV by exposure to UV. At a low vacuum of 10-3 Torr, the UV light improved the field emission performance and reduced the turn-on voltage such that they were better than those of normal field emission in a high vacuum. The depletion layer of NWs disappeared because surface O2- oxygen molecules were removed by exposure to UV light. The enhancement by UV remained even after the UV light had been turned off for 20 min.