Charge-Transfer-Induced Reactivity in sp Carbon Atomic Wires: Toward 0‑D sp-sp² Nanostructures

Carbon atomic wires (CAWs) are finite linear chains of sp-hybridized carbon atoms. Here, the electrochemical reduction of CAWs in the form of polyynes (i.e., with alternated single-triple bonds) is reported. Upon application of a reducing potential to a solution containing polydispersed hydrogen-capped polyynes, the formation of a black precipitate was observed. Electronic absorption spectroscopy confirmed the irreversible reaction of the carbon chains while excluding the degradation or side reactions. Subsequent analyses revealed that the precipitate consisted of amorphous carbon nanoparticles with a tunable diameter. This control over particle size is attributed to the modulation of growth kinetics through restricted mass transport toward the solid-liquid interface. Raman spectroscopy showed that the resulting material exhibits an amorphous sp-sp² character with a retained sp fraction exceeding 60%. Smaller nanoparticles displayed reduced disorder within the sp² domains, and a broader distribution of sp-chain lengths was preserved in the amorphous matrix. Additional experiments on size-selected polyynes suggest that this synthesis method allows us to better preserve the starting chain length in the final structure. Unlike previously reported amorphous sp-sp² carbon networks, the nanoparticles produced in this study show remarkable stability under ambient conditions, retaining their sp character for times in excess of 6 months. These findings pave the way for future applications, particularly as further diameter tuning may enable access to the quantum dot regime.