Engineering traditional materials into the new form of atomic and free-standing two-dimensional structures is of both fundamental interest and practical significance, but it is in general facing challenges especially for metal oxide semiconductors. We herein report an ultragreen method for the cost-effective and fast preparation of atomic metal oxide nanosheets that can be further transformed into nanofilms. The method combines top-down building block synthesis and bottom-up electrophoretic assembly in water under ambient conditions, using only bulk metal and Milli-Q water without involving any additional reagents. The focus is on free-standing polycrystalline ZnO nanosheets that can be produced with a lateral dimension as large as 10 mu m and a thickness of 1 nm (the thinnest free-standing metal oxide nanosheet ever reported). A new electrophoretic assembly mechanism dominated by intrinsic surface polarity was revealed. We also demonstrate potential applications of this approach for wet electronic systems as exemplified by facile and in situ fabrication of dielectric layers and cellular electrets.