Sp²/sp³ Framework from Diamond Nanocrystals: A Key Bridge of Carbonaceous Structure to Carbocatalysis

Diamond nanocrystals in robust sp³ hybridization are appealing carbonaceous materials in the material community, whose structure can be transformed into unique sp²/sp³ nanohybrids as bulky nanodiamonds (NDs) and sp² concentric onion-like carbons. Functionalized NDs have been used as carbocatalysts to drive a diversity of heterogeneous reactions, presenting promising catalytic performances, great stability/durability, and low toxicity compared with other carbonaceous and metal materials. More importantly, the tunable configurations of NDs-related materials from sp³ to sp²/sp³ and sp² carbons endow them as ideal chemical probes to elucidate the intrinsic nature toward metal-free catalysis. Herein, a comprehensive overview is presented in the synthesis, properties, functionalization, and characterization of NDs-based materials as well as their recent applications in fuel cell reactions, carbon dioxide reduction, photocatalysis, organic synthesis, oxidative dehydrogenation reactions, and advanced oxidation processes. More importantly, we provide an insightful discussion on unveiling the intrinsic catalytic centers and structure-reactivity chemistry of NDs in redox reactions from an atomic level. Advanced protocols were proposed for regulating the electronic structures of NDs by surface and structural engineering toward better carbocatalysis, which assists to provide valuable guidance for the rational design of ND-based materials toward target catalytic processes. Finally, future research opportunities were proposed to address the current dilemmas in materials synthesis, to facilitate mechanistic studies by theoretical computations, to enable structural/surface functionalization of NDs for advanced catalysis, and to expand the NDs-based materials toward other promising chemical reactions.