According to Bravais–Friedel–Donnay–Harker analysis, the growth speed of CuO crystal is proportional to l/dhkl, therefore, the rectangular crystals elongate along the [0 1 0] direction [33,34]. The slowest growing phase determines morphology of crystals [35,36]. Thus, under the experimental conditions and the kinetic difference in the growth planes, the slowest growing (0 1 0) planes dominate the crystal into the typical rectangular shaped structure (e). The formation of nanobat-like structure can be attributed to oriented attachment mechanism. In the process of oriented aggregation, the single crystalline nanobat-like structure forms through oriented arrangement by rotating adjacent divergent sized rectangular structure (e) to allocate identical crystallographic orientation [37,38]. It can be realized that controlled aggregation likely takes place along [0 1 0] direction which has highest reactivity [39]. In general, during the anisotropic crystal growth, tiny primary crystals at their high-energy surfaces are energetically favored since the formation of large crystals can greatly reduce interfacial energy. Thus, we can conclude that the ultimate crystal with well-defined shape and morphology can be attained by adjusting crystal along the three preferred directions pursued by reorganization and crystallization process. To substantiate the formation of rectangular shaped nanobat-like structure via orientation attachment, TEM study was carried out
( f ).