We report the synthesis of different Cu(OH) 2 and CuO nanostructures (nanowires, rectangles, seedlike, beltlike, and sheetlike) in a solution phase with high yield at low cost by simple reduction of aqueous solution of copper nitrate (Cu(NO 3 ) 2 ) 0.2 M) with different alkaline solutions of sodium hydroxide (NaOH ) 0.1, 0.25, 0.50, 0.75, and 1.0 M). The morphology of the synthesized nanostructures is significantly influenced by the feeding concentration of alkaline NaOH solution. Cu(OH) 2 rectangles and nanowires can be readily obtained by the reduction of Cu(NO 3 ) 2 solution with different molar concentrations of NaOH solution and the synthesized nanomaterials get transformed into different nanostructures of CuO by subsequent heat treatment at 80 °C for half an hour. Well-defined rectangle-like structures of hydrated copper hydroxide Cu(OH) 2 • H 2 O and different CuO nanostructures, such as seedlike, beltlike, and sheetlike, were synthesized by thermal dehydration of corresponding different shaped and sized Cu(OH) 2 nanomaterials. The Raman spectra of different CuO nanomaterials obtained at different molar concentrations of NaOH (0.25, 0.50, 0.75, and 1 M) were recorded in the region 1050-1300 cm -1 . A tentative mechanism has been given for the formation and transformation of different nanostructures.

Cuprous oxide (Cu 2 O) nanostructures have been synthesized by anodic oxidation of copper through a simple electrolysis process employing plain water (with ionic conductivity ∼6 µS/m) as an electrolyte. No special electrolytes, chemicals, and surfactants are needed. The method is based on anodization pursuant to the simple electrolysis of water at different voltages. Platinum was taken as cathode and copper as anode. The applied voltage was varied from 2 to10 V. The optimum anodization time of about 1 h was employed for each case. Two different types of Cu 2 O nanostructures have been found. One type was delaminated from copper anode and collected from the bottom of the electrochemical cell and the other was located on the copper anode itself. The nanostructures collected from the bottom of the cell are either nanothreads embodying beads of different lengths and diameter ∼10-40 nm or nanowires (length ∼600-1000 nm and diameter ∼10-25 nm). Those present on the copper anode were nanoblocks with a preponderance of nanocubes (nanocube edge ∼400 nm). The copper electrode served as a sacrificial anode for the synthesis of different nanostructures. A tentative mechanism for the formation of Cu 2 O nanostructures has been suggested. The present work represents the first such attempt where Cu 2 O nanostructures were formed under the oxidation induced by as simple a process as electrolysis of plain water. Both anodization potential and time influence the morphology of nanostructures of Cu 2 O. Thus, nanothreads are formed at 6 V during 15-30 min, whereas nanowires result when anodization time is extended to 45-60 min. Also two different types of Cu 2 O nanostructures, one which is present in the solution (nanothreads and nanowires) and the other which is located on the copper anode (nanocubes), are synthesized in the same electrolysis run. The optical band gap as calculated from the UV-visible absorption spectra of the nanothreads and nanowires corresponds to 2.61 and 2.69 eV, respectively, which is larger than the known band gap (2.17 eV) of bulk Cu 2 O.

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