Using a pure ionic liquid-assisted solid-state-like synthesis method under 80 degrees C, high quality zinc oxide (ZnO) nanorods in 1-alkyl-3-methylimidazolium and bromide anions ionic liquids C(n)mimBr-+(-) (n = 2, 4, 6, 8) were realized. The size effect of the N-alkyl chains of ionic liquids hindered the further growth of ZnO nanorods and the aspect ratio (length/diameter, L/D) of ZnO nanorods (from 40 : 1, 20 : 1, 10 : 1, to 5 : 1) were realized by rationally tuning the length of the organic cation groups. Long octyl chains (9.93 angstrom) resulted in short ZnO nanorods (with L/D = 5 : 1), otherwise, short ethyl chains (2.53 angstrom) resulted in long ZnO nanorods (with L/D = 40 : 1). The growth mechanism of the above ZnO nanorods was mainly due to electrostatic interactions between cations of the ionic liquids and the ZnO crystal nucleus, causing the preferential growth on the 0001 directions of the hexagonal ZnO. The as-obtained short ZnO nanorods (with L/D = 5 : 1) exhibited defect-related green-yellow emission under UV excitation, and acted as effective recyclable photocatalysts during six photodegradation cycles of the organic dye molecule Rhodamine B, suggesting promising potential for environmental applications. This systematic study highlighted that the structures and morphology of ZnO nanomaterials could be controllable via specifically varying the molecular structure of 1-alkyl-3-methylimidazolium cation-based ionic liquids, and then with the tuned photocatalytic properties and optical and electronic functions of them.