Acoustic shock wave-induced sp3-to-sp2 type solid-state phase transition: a case study on tetrahedral amorphous carbon†

In high-pressure carbon science, the sp²-to-sp³ type phase transition (e.g., graphite-to-diamond) is considerably known, however, the sp³-to-sp² type phase transition remains inadequately understood. Due to the high bulk modulus, manipulating the crystal structure is challenging. Herein, we report the phase transition of tetrahedral amorphous carbon into graphite under transient pressure-dependent acoustic shocked conditions with the values of 2.0 MPa and 15.6 MPa and exposed to different shock pulses such as 0, 250 and 500 such that a shock-wave-induced superheating mechanism is put forth for the observed graphitization process. For clarity of the discussion, we represent case-1 for 2.0 MPa and case-2 for 16.5 MPa which refers to the two shock transient pressures. In the case-1 experiment, the values of the intensity ratio of the D-band and G-band (I_D/I_G ratio) are found to be 0.70, 0.83, and 0.82 for 0, 250 and 500 shocks, respectively. In the case-2 experiment, the I_D/I_G ratio values are found to be 0.70, 0.32, and 0.16 for 0, 250 and 500 shocks, respectively. In case 2, at the 500-shocked condition, the observed I_D/I_G value is nearly equal to the pure graphite single crystal. According to the XPS results, normalized intensity ratio values of the control and case-2 samples sp²/sp³ bands are found to be 1.0 and 7.14 which provides convincing evidence for the occurrence of sp³-to-sp² phase transition and HR-TEM results support the XPS and Raman results. As a result of a high degree of graphite formation, the proposed technique offers a new platform to manipulate the crystal structure of hard sp³ hybridized components, thereby a large-scale synthesis of graphite can be a reality.