Ordered Co₃O₄ hierarchical nanorod arrays: tunable superhydrophilicity without UV irradiation and transition to superhydrophobicity

Ordered Co₃O₄ hierarchical nanorod arrays have been fabricated employing pulsed laser deposition (PLD) on polystyrene colloid spheres followed by an annealing process in air. The surface roughness and chemical composition were controlled successfully by adjusting the oxygen pressure during the PLD process and post-annealing temperature, respectively. The as-prepared Co₃O₄ nanorod arrays had superior superhydrophilicity without UV irradiation, which was attributed to the improved roughness related to the hierarchical structures and the abundant OH⁻ groups induced by the PLD and annealing processes. After chemical modification with fluorosilane, the superhydrophilicity was converted to hydrophobicity or superhydrophobicity with controlled water contact angles. Our results demonstrate the fabrication of a new stable superhydrophilic material without UV irradiation; well-controlled Co₃O₄ hierachical nanorod arrays may also find potential applications in the areas of field-emission, catalysis, and lithium-ion batteries.