Acoustic shock wave-induced sp²-to-sp³-type phase transition: a case study of a graphite single crystal

Achieving facile and simple temperature- and pressure-induced transformation of sp²-to-sp³ remains an important and fascinating challenge within the realm of carbon science and technology. Here, we introduce a new technique that utilizes repeated exposure of low-pressure (2.0 MPa) millisecond acoustic shock waves on a sample to facilitate the successful transformation of sp²-to-sp³ carbon bonds. This transformation is verified through visible Raman spectroscopic, X-ray photoelectron spectroscopic (XPS), and high-resolution transmission electron microscopic (HRTEM) observations. Typically, in general nanosecond dynamic compression experiments, sp³ carbon bond formation occurs only at pressures of ∼45 GPa or more, and under static compression, this transition takes place at ∼30 GPa. However, with our innovative approach, similar results can be achieved with acoustic shock waves operating at significantly lower pressures of 2.0 MPa. Based on the observed analytical results, the sp²-to-sp³ type phase transition occurred at the 500-shocked condition and this transition leads to the conversion of layered crystalline graphite to non-layered amorphous graphite, which may be the pre-state of sp³ bonded diamond formation. The complete disappearance of the 2D band in the Raman spectrum and the conversion of asymmetric to symmetric shape of the C 1s band in the XPS spectrum are the major proof for the proposed sp²-to-sp³ phase transition. Further optimization is currently underway to find the critical point in achieving the probable phase transition of graphite to diamond. The proposed technique put forward a platform for a new impending way to make the sp³ carbons from sp² carbons in indoor laboratories, which may also offer a new science division to understand the formation of diamonds or diamond-like structures under lower transient pressure conditions. Even though the proposed technique is cost-effective and involves a handy tool, to move it from lab to industrial applications, we still have a lot of ground to cover in fundamental aspects.