Amorphous copper oxide (Cu(II)) nanoclusters function as efficient electrocatalysts for the reduction of carbon dioxide (CO2) to carbon monoxide (CO). In addition to promoting electrocatalytic activity, Cu(II) nanoclusters act as efficient cocatalyts for CO2 photoreduction when grafted onto the surface of a semiconductor (light harvester), such as niobate (Nb3O8(-)) nanosheets. Here, the photocatalytic activity and reaction pathway of Cu(II)-grafted Nb3O8(-) nanosheets was investigated using electron spin resonance (ESR) analysis and isotope-labeled molecules (H2(18)O and (13)CO2). The results of the labeling experiments demonstrated that under UV irradiation, electrons are extracted from water to produce oxygen ((18)O2) and then reduce CO2 to produce (13)CO. ESR analysis confirmed that excited holes in the valence band of Nb3O8(-) nanosheets react with water, and that excited electrons in the conduction band of Nb3O8(-) nanosheets are injected into the Cu(II) nanoclusters through the interface and are involved in the reduction of CO2 into CO. The Cu(II) nanocluster-grafted Nb3O8(-) nanosheets are composed of nontoxic and abundant elements and can be facilely synthesized by a wet chemical method. The nanocluster grafting technique described here can be applied for the surface activation of various semiconductor light harvesters, such as metal oxide and/or metal chalcogenides, and is expected to aid in the development of efficient CO2 photoreduction systems.
Keywords: CO2 reduction; copper oxide; electrocatalysis; nanocluster; nanosheet; niobate; photocatalysis.