SEM pictures of the hydrothermally grown ZnO nanowire array on the inkjet-printed Zn acetate patterns. (a) ZnO nanowire array size variation at increased substrate heating; room temperature, 30°C, 40°C, 50°C, 60°C, and 70°C heating from left to right. Inset schematics show the cross sectional view of the ZnO nanowire array. (b) Magnified SEM pictures of 50°C, 60°C, and 70°C from left to right. Blue dotted lines indicate the elevated ZnO array at the center of the droplet due to substrate heating.