18. Photoredox Reaction

A photoelectrochemical actuator based on poly(acrylic acid) gel loaded with TiO₂ nanoparticles and copper(II) ions swells in a water-ethanol mixed solution under ultraviolet (UV) light due to photocatalytic reduction of copper(II) to copper(0) nanoparticles and oxidation of ethanol. After removal of UV light, the hydrogel gradually shrinks again due to aerobic oxidation of copper nanoparticles to copper(II) ions. A photoelectrochemical actuator based on poly(acrylic acid) gel loaded with TiO₂ nanoparticles and silver(I) ions also swells in water under UV light due to photocatalytic reduction of silver(I) to silver(0) nanoparticles and oxidation of water. The hydrogel shrinks again under visible light due to re-oxidation of silver nanoparticles to silver(I) ions and reduction of oxygen molecules by the plasmon-induced charge separation. Partial swelling and shrinking of the hydrogel are also possible.