Unique hybrid systems for electroreduction of CO 2 under both conventional and visible-light-induced conditions are proposed and designed here by over-coating copper(I) oxide with tungsten(VI) oxide nanowires. When Cu 2 O and WO 3 nanostructures are sequentially deposited on glassy carbon substrate, the resulting system has exhibited high electrocatalytic activity toward reduction of CO 2 in phosphate buffer of pH = 6.1. By introducing WO 3 , the Cu-based system becomes more selective against the competitive hydrogen evolution and exhibits higher CO 2 -reduction currents relative to the performance of single components. During electroreduction, highly catalytic Cu sites are generated or intercalated within WO 3 nanowires partially reduced to mixed-valence hydrogen-absorbing tungsten(VI,V) oxide bronzes, H x WO 3 , co-existing with sub-stoichiometric tungsten(VI,IV) oxides, WO 3-y . Strong adsorption and activation of CO 2 molecule has been demonstrated at the WO 3 -decorated Cu 2 O interface. Under photoelectrochemical conditions involving illumination with sun-light, the proposed hierarchical system of Cu 2 O semiconductor deposited onto the transparent fluorine-doped conducting glass electrode and decorated with WO 3 nanowires is well-behaved and active toward CO 2 -reduction in the Na 2 SO 4 neutral medium. In addition to the Cu 2 O stabilization effect, the heterojunction formed by p-type Cu 2 O and n-type WO 3 semiconductors seems to facilitate charge distribution and separation at the photoelectrochemical interface.
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