Syngas, a CO and H₂ mixture mostly generated from non-renewable fossil fuels, is an essential feedstock for production of liquid fuels. Electrochemical reduction of CO₂ and H⁺/H₂O is an alternative renewable route to produce syngas. Here we introduce the concept of coupling a hydrogen evolution reaction (HER) catalyst with a CDots/C₃N₄ composite (a CO₂ reduction catalyst) to achieve a cheap, stable, selective and efficient route for tunable syngas production. Co₃O₄, MoS₂, Au and Pt serve as the HER component. The Co₃O₄-CDots-C₃N₄ electrocatalyst is found to be the most efficient among the combinations studied. The H₂/CO ratio of the produced syngas is tunable from 0.07:1 to 4:1 by controlling the potential. This catalyst is highly stable for syngas generation (over 100 h) with no other products besides CO and H₂. Insight into the mechanisms balancing between CO₂ reduction and H₂ evolution when applying the HER-CDots-C₃N₄ catalyst concept is provided.

Photocatalytic overall water splitting using particulate semiconductors is a potential means of storing solar energy in using the intermittency of sunlight as a primary source of power and zero emission of carbon dioxide. Herein, we constructed a composite material with carbon dots (CDs) anchored on the surface of octahedral CoO as a highly efficient and long-term stable photocatalyst for overall water splitting under visible light irradiation (λ > 400 nm). The structure and morphology of the CDs/CoO composite were investigated by a series of characterization methods. The obtained CDs/CoO composites exhibit more-efficient visible light absorption than pure CoO, leading to higher photocatalytic activity for overall water splitting. The optimized photocatalytic H₂ and O₂ production was achieved for the CDs/CoO composite with a content of 5 wt% CDs (5% CDs/CoO), showing a H₂ (O₂) evolution rate of 1.67 μmol h⁻¹ (0.91 μmol h⁻¹) with an expected 2 : 1 stoichiometry, which is up to 6 times as high as that of pristine CoO. Additionally, the 5% CDs/CoO composite also shows outstanding photocatalytic stability for over 15 cycling experiments. This enhanced photocatalytic activity and outstanding stability in CDs/CoO composites could be ascribed to several merits of CDs that not only improved charge separation efficiency and visible-light absorption but also effectively conducted heat generated by the photo-thermal effect of CoO.