It is often said that “necessity is the mother of invention”; when there is a need, people will invent a way to fulfill it. In the case of the present study, serendipity played a major role in finding a way to reverse the redox disproportionation of Cu₂O that has plagued its usefulness for CO₂ reduction to synthetic fuels. This chance discovery, described below, begins with copper foam.

    A surface oxidized copper foam was immersed into dilute HCl by accident, and the surprise observation of the resultant nanocubes on the foam by SEM inspired us to probe what those nanocubes were and what happened on the foam. A series of experiments revealed the many faces of Cu₂O nanocubes with redox active surface frustrated Lewis pairs (SFLP) comprised of mixed oxidation state copper Cu (0, I, II) sites, oxygen vacancies [O] and hydroxyl OH groups. The nucleation and growth of the aforementioned Cu₂O nanocubes, denoted as CF-Cu₂O, was found to occur via a complex sequence of dissolution, complexation, precipitation, hydrolysis, hydroxylation processes as illustrated in Figure 1. 

    CuCl nanocubes were firstly formed on the intricate pore surface of the copper foam, followed by the hydrolysis and formation of Cu₂O nanocubes. Then the oxidation and hydroxylation of Cu₂O nanocubes triggered by water with residual Cl and dissolved O₂ developed a surface layer of Cu₂(OH)₃Cl, which together with Cu₂O created the redox active SFLP on CF-Cu₂O nanocubes.

    Compared with the Cu₂O nanocubes sample devoid of the redox active SFLP, the CF-Cu₂O nanocubes promoted the photocatalytic conversion of CO₂ to CO, superior in dissociating H₂, adsorbing CO₂ and converting CO₂ even at RT without light. Based on combined in-situ DRIFTS results and physical and chemical characterization data, a reaction pathway was proposed for the photocatalytic hydrogenation of CO₂ by CF-Cu₂O nanocubes (Figure 2). In this mixed valence SFLP scheme, H₂ dissociates heterolytically on the surface of CF-Cu₂O as shown in step 3 to step 4 to step 5. Water is eliminated in step 5 to step 1 creating an [O] vacancy and reducing Cu(I) to Cu(0). The adsorption of CO₂ at an O and [O] site as a surface carbonate like species ensues in step 1 to step 2 facilitating the conversion to CO in step 2 to step 3, thereby completing the photocatalytic cycle. Both in-situ DRIFTS results at RT and in the light for CF-Cu2O indicate the quick appearance of Cu⁰, so the surface of CF-Cu₂O will rapidly turn into mixed oxidation state copper Cu(0, I, II), oxygen vacancies [O] and hydroxyl groups OH and the photocatalytic cycle will start from step 1 and complete the reaction in the sequence of step 1 to step 2 to step 3 to step 4 to step 5.

    The newfound ability to stabilize and activate Cu₂O nanocubes with a redox active SFLP, provides a key step towards the practical utilization of this earth abundant, low cost, non-toxic material in myriad photocatalysis applications.

    To read the full article, please click https://www.nature.com/articles/s41929-019-0338-z.