Overturning photoreduction product of CO₂ by defect- and COOH-functionalized multi-wall carbon nanotubes

We functionalize MWCNTs with defects and COOH groups to form MWCNTs30 through a novel discharge process. In the photoreduction of CO₂ with pure H₂O in the absence of sacrificial reagent, with CdS as light absorber, MWCNTs30 triggers a selective CH₃OH production, with a CH₃OH selectivity of 100% and an internal quantum efficiency of 3.9%, but CO is the only C-based product by using pristine MWCNTs without functionalization. The defects on MWCNTs30 are active sites for enhancing CO₂ adsorption, H₂O dissociation to produce H and H transfer to CO₂, thus boosting CO₂ hydrogenation to CH₃OH. The COOH groups on MWCNTs30 provide H for promoting CO₂ hydrogenation to CH₃OH. The multiple roles of defects and COOH groups overturn the photoreduction product from CO to CH₃OH. The results open a new way to achieve more efficient photoreduction of CO₂ in the absence of noble metal and sacrificial reagent.