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The controlled synthesis of one‐dimensional, two dimensional, and three‐dimensional CuO nanostructures has been achieved by a hydrothermal process in the presence of sodium citrate (SC) by simply controlling the reaction conditions. When the molar ratio of sodium citrate to CuSO₄·5H₂O (represented by SC/Cu²⁺) is below 1.0, 1D CuO nanorods with 30–40 nm diameters and 100–200 nm lengths are formed. In the absence of citrate, 2D flakelike CuO nanostructures with 150–200 nm widths and 300–400 nm lengths are obtained. When the SC/Cu²⁺ ratio is more than 1.0, 3D branchlike CuO nanostructures with lengths of hundreds of nanometers and diameters of 20–100 nm are attained. On the basis of the morphology and X‐ray diffraction patterns of the samples, a possible growth mechanism for the CuO nanostructures is proposed. Moreover, it is shown that the optical bandgap energy (E_g) of resulting …