High-quality Cu₂O nanowire (NW) photocathode was successfully fabricated upon annealing Cu(OH)₂ NW precursor. The precursor was prepared via a wet chemical oxidation method [Cu foil + (NH₄)₂S₂O₈ + NaOH] which is always accompanied with unfavourable nanostructured CuO. However, the present study has synthesized the precursor without the emergence of CuO with modified growth solution at room temperature. Under proper growth condition, the height and width of pure, vertically erected Cu(OH)₂ NW has increased proportionally to the ageing time for growing NW. The photoelectrochemical (PEC) measurement of Cu₂O NW under 100 mW cm⁻² light irradiation and 0.5 M Na₂SO₄ electrolyte solution, has generated − 5.45 mA cm⁻² at − 0.6 V vs. Ag/AgCl. The high photocurrent density was attributed to the high carrier density (2.69 × 10²¹ cm⁻³) accompanied with a low charge transfer resistant under illumination. The inexpensive and highly scalable Cu₂O NW is a promising photocathode for PEC solar to energy application.