Hydrothermal growth of upright-standing ZnO sheet microcrystals

• Upright-standing ZnO sheet microcrystals were hydrothermally fabricated.
• The ZnO sheets were prepared with sodium oxalate at 70 °C without any surfactant.
• The preferable adsorption of oxalate anions causes the formation of ZnO sheet.
• The continuous growth in six directions leads to the formation of hexagonal sheets.

Large-scale upright-standing ZnO sheet microcrystals were fabricated on Zn substrate using sodium oxalate as structure-directing agent by a hydrothermal method at low temperature (70 °C) without any surfactant. The sheets are about 3–5 μm in dimension and 100–300 nm in thickness. The strong and narrow diffraction peaks of ZnO indicate that the sample has a good crystallinity and size. The morphology of sheet-like ZnO varied with the concentrations of sodium oxalate and reaction time. The sheet-like ZnO would transform into rod-like ones when sodium oxalate was substituted by equivalent sodium acetate. The formation of sheet-like ZnO is attributed to the preferable adsorption of oxalate anions on (0 0 0 1) face of ZnO, which inhibits the intrinsic growth of ZnO. Additionally, the continuous growth in six (0 1 −1 0) directions that have the lowest surface energy leads to the formation of hexagonal sheets.