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

Sivakumar Aswathappa ^a, Lidong Dai *^a, Simon A. T. Redfern ^bc, S. Sahaya Jude Dhas ^d, Xiaolei Feng ^c, Eniya Palaniyasan ^e and Raju Suresh Kumar ^f
aKey Laboratory of High-temperature and High-Pressure Study of the Earth's Interior, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, Guizhou 550081, China. E-mail: dailidong@vip.gyig.ac.cn
^bAsian School of the Environment and School of Materials Science and Engineering, Nanyang Technological University, Singapore – 639798, Singapore
^cSchool of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore – 639798, Singapore
^dSaveetha School of Engineering, Saveetha Institute of Medical and Technical Sciences, Saveetha University, Chennai, Tamil Nadu 602105, India
^eDepartment of Physics, Periyar University, Salem, Tamilnadu 636011, India
^fDepartment of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia

First published on 6th August 2024

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Introduction

The nature and properties of sp², sp³ carbon structures and sp²-to-sp³ type phase transitions have long been a topic of fascination for researchers.^1–8 Graphite to diamond conversion is one of the best examples of sp²-to-sp³ type phase transition. Extensive efforts have been made to create sp³ carbon structures from simple carbon materials such as graphene, graphite, and carbon nanotubes, using pressure and thermal engineering approaches.^9–11 Despite years of research into converting low-pressure carbon polymorphs into diamonds, the transitional routes remain elusive and the commercial application of laboratory experiments in industrial processes remains a challenge. Graphite, among the diamond precursors, is particularly notable and it is well-recognized as the initial material for diamond formation under extremely high-pressure and high-temperature conditions based on the sp²-to-sp³ type phase transition. Detailed transformation mechanisms from graphite to diamond have been reported in many studies, from both theoretical¹² and experimental^13–16 perspectives. Techniques that manipulate static temperature and static pressure,¹³ dynamic shock compression (driven by explosives, gas guns, and flyer plate impacts)^7,14,15 and laser shock compression^8,16 have all been adopted in experiments using graphite as the starting material, with most resulting in diamond-like structures at pressures exceeding 50 GPa. In general, in the case of graphite, a martensitic phase transition operating on a nanosecond timescale has been widely accepted to be important as an essential process, in which compression along the z-axis of graphite is key to achieving the high-pressure phases of graphite with non-sp² hybridizations, such as disordered graphite and diamond phase.^13–16 However, slower, medium-fast (of the order of milliseconds) martensitic phase transition mechanisms are thought to be more effective in inducing the sp²-to-sp³ type phase transitions.¹⁷ Additionally, under static compression, the layered graphite remains stable up to 20 GPa and, on further compression, it undergoes the disordered phase and thereby complete amorphous graphite has been observed at 30.1 GPa,¹⁸ but when temperature factors in, diamond particles are produced. For instance, at 12 GPa and 3273 K, graphite is converted into diamond under static pressure compression conditions.¹⁹ On the other hand, with dynamic shock compression, ordered graphite enters a disordered state at 46.8 GPa, while hexagonal diamond formation occurs at 49 GPa and the cubic phase begins at 59.5 GPa.⁷ Kraus et al. reported a nanosecond laser-shock compression experiment on graphite and found the disordered graphite phase at 47 GPa and lonsdaleite phase at 170 GPa.⁸

In recent years, we have seen acoustic shock wave methods yield ever more prominent results for transformations of solid-state materials, allowing for effective recrystallization within a millisecond to microsecond timescale, especially in carbon materials. However, while promising results on phase transitions have been reported using acoustic shock waves at very low transient pressures (Ex: 2 MPa), a comprehensive experimental exploration of millisecond shock wave recovery on carbon allotropes remains largely uncharted. The first reports of dynamic shock compression assisting the synthesis of diamond structures were made in the early 1960s²⁰ and research in this field remains active.^20,21 The exact pressure and temperature required to achieve such phase transitions are still uncertain. This is due to the fact that in static compression techniques, structural relaxation occurs over minutes, while with dynamic compression techniques; structural relaxation takes place over nanosecond timescales. The mechanism of dynamic shock wave-induced structural changes remains unclear.^16–22

Koteeswara Reddy et al. performed millisecond shock wave recovery experiments (acoustic shock waves) using conventional high enthalpy free piston shock tube methods and found that density increases in graphitic carbon nanoparticles at 50 MPa.²³ The graphitization process has been found in amorphous carbon nanoparticles, and nanotubes under acoustic shocked conditions.^24–26 In addition to that, using acoustical shock waves, a number of other interesting phase transitions in other materials have been reported at very low shock transient pressures (2 MPa) than that of the conventional pressure-induced phase transitions.^27,28 From these reports, it seems clear that a medium-fast martensitic phase transition process is more favorable to induce the structural phase transitions. However, the experimental results of acoustic shock wave recovery for the carbon allotropies have not yet been clearly understood.

Here, we demonstrate the sp²-to-sp³ type phase transition using millisecond acoustic shock waves at a transient pressure of 2 MPa, which leads to the conversion of layered crystalline graphite to non-layered amorphous graphite. We find that this technique, which uses very low-pressure acoustic shock waves, could potentially facilitate the sp²-to-sp³ type phase transition in graphite.

Experimental methods

Commercially available graphite crystals were purchased from TECHNINSTRO Company and the chemical purity of the sample was 99.98%. We prepared bulk-size single crystals with dimensions of 5 × 5 × 1 mm³ along with the (002) plane and they were used for the shock wave recovery experiment. Before the shock wave loading, the title crystals were checked with Raman spectroscopic data and all the test crystals had the same I_D/I_G ratio. For the present experiment, we have chosen 7 identical crystals to be used for the experimental analysis. Among the seven crystals, one crystal has been kept as the control crystal (initial sample) and the remaining six crystals have been utilized for the experimentation so that different numbers of shock wave loadings such as 1, 2, 3, 4, 5, and 500 pulses could be exposed. Detailed information on the shock wave loading procedure and analytical instrument details is presented in the ESI.†

Results and discussion

Acoustic shock wave-induced transformation from ordered to disordered graphite

The control graphite crystal exhibits the first-order Raman scattering lines (Fig. 1(a)) at the characteristic peaks of 1336 cm⁻¹ and 1563 cm⁻¹, which is in good accordance with the previously reported A_1g symmetric mode (also nominated as D-band) and E_2g symmetric mode (also nominated it as G band), respectively.²⁹ It is very obvious that the initial sample shows a relatively stronger intensity in the G-band relative to the D-band in Raman spectroscopy, revealing a highly-ordered graphene layer within the hexagonal lattice. Significantly, the intensity of the Raman 2D band directly correlates with the number of graphene layers in the graphite crystal. The structural stability of graphite is largely dictated by the density of sp² carbon bonds. Over the first five shock pulses, no significant changes are noticeable beyond minor peak shifts and intensity variations of the D and G bands. This suggests that the applied shock waves do not cause substantial alterations to the graphite lattice. We note that for these first few shock pulses (1–5), the D-band intensity slightly increases while the G-band intensity marginally decreases, and the second-order Raman band features remain constant. However, upon further increasing the count of applied shock pulses to 500, the Raman spectrum (Fig. 1(b)–(e)) reveals considerable changes in both first-order and second-order Raman bands. Indeed, after 500 shock pulses, the G-band intensity is nearly nullified, and most importantly, the second-order Raman band features completely disappear. This suggests that the crystalline hexagonal graphite has transitioned to a disordered graphite state with non-sp² hybridizations (Fig. 1(d) and (e)).

Consequently, the loss of sp² hybridized carbon bonds facilitates the conversion of the sp²-to-sp³ phase transition. On the other hand, observations of D band and G band shifts (Fig. 2(a) and (b)), the intensity ratio of the D and G band (Fig. 2(c)) and the 2D band intensity (Fig. 2(d)) lead us to speculate on the precise nature and identity of the “500-shock-pulse”-loaded graphite phase. Interestingly, the D and G bands show a higher energy shift with respect to the number of shock pulses. A higher wavenumber shift of the G-band can facilitate the formation of interlayer covalent bonds, leading to the creation of extensive regions of sp³-hybridized atoms within the graphite lattice. As the structure transforms towards a disordered state, the concentration of sp³ carbon bonds significantly increases due to buckling and puckering mechanisms^6,10,11 leading to atomic disordering in the hexagonal lattice. This suggests a potential mechanism for the formation of disordered graphite from ordered hexagonal graphite.

Note that sp³–sp³ bonds typically suppress the G-band intensity, so for the disordered graphite state, the G-band intensity is significantly reduced (Fig. 2(b)). The ratio of the intensity of the D-band to the G-band (I_D/I_G) (Fig. 2(c)) is nearly zero for the control graphite crystal, consistent with highly ordered graphite.¹⁸ However, the ratio of I_D/I_G reveals subtle changes up to 5 shocks, and the value is found to have significantly increased at the 500-shocked condition such that the observed values are 0.03, 0.07, 0.15, 0.08, 0.43, 0.019 and 0.86 for 0, 1, 2, 3, 4, 5 and 500 shocks, respectively. The higher I_D/I_G value at the 500-shocked condition evidently demonstrates the formation of disordered graphite. Thus, the sharp and highly intense 2D Raman band in the control crystal indicates the presence of numerous graphene layers (Fig. 2(d)). However, after 500 shock pulses, the 2D band disappears completely, illustrating the total collapse of the graphite's layered structure. This collapse initiates a new non-layered structural configuration like amorphous graphite.³⁰ Typically, G-band linewidth is considered one of the major key factors in static compression experiments to justify the structural phase transitions in graphitic structures.^18,31,32 The width of the G-band is lower for the crystalline graphite and vice versa for the disordered graphite.¹⁸

To provide further supporting evidence for the formation of the disordered phase, we have calculated the G-bandwidth with respect to the number of shock pulses and the observed profile is presented in Fig. 3(a). As seen in Fig. 3(a), the width of the G-band has significantly increased compared to the control graphite crystal on exposure to 500 shocks and the obtained values are found to be 28, 26, 28, 25, 33, 26 and 100 cm⁻¹ for 0, 1, 2, 3, 4, 5 and 500 shocks, respectively. The higher value of full width at half maximum (FWHM) of the G-band at the 500-shocked condition provides further supporting evidence for the proposed sp²-to-sp³ phase transition. In addition to that, in Fig. 3(b), the G-bandwidth profile of the graphite sample is presented under static compression up to 30 GPa.¹⁸ During the compression, the G-bandwidth value is found to have increased with respect to pressure such that the values are found to be 8, 7, 8, 9, 8, 12, 13, 24 and 29 for 0, 2.3, 6.9, 16.3, 18.5, 20.5, 23.2, 28.2 and 30.1 GPa, respectively. Note that the ΔG-bandwidth is identified to be 21 cm⁻¹ in static compression and 72 cm⁻¹ in acoustic shocked conditions. Based on the obtained results, it is evident that acoustic shock waves can trigger more pronounced sp²-to-sp³-type phase transitions compared to static compression.

In considering the formation mechanisms, with regard to the increase in sp³ carbon bonds in the graphite system, two possibilities arise for the formation of disordered graphite: twisted graphite and tilted graphite.^6,9,11 It should be noted that ordered graphite has perfectly parallel basal planes (stacked along the z-axis). However, graphite disordering begins, and the basal planes become non-parallel due to the rupture of the sp² carbon bonds and the presence of low-bond-energy van der Waals bonds. This leads to non-basal screw dislocations, bond angle disorder, local fragmentation, and the formation of dangling bonds around defect structures.^6,9,11 At this stage, there is a significant enhancement of sp³ hybridized carbons in the hexagonal graphite, which is primarily responsible for the formation of the disordered graphite. As sp² bonds are highly stable, dynamic shock compression experiments (such as flyer plate shock impact experiments) require pressures of ∼45 GPa to break these bonds and form disordered graphite.^6,14,15 Furthermore, due to the high thermal conductivity of graphite (250 W m⁻¹ K⁻¹),³³ in the case of nanosecond shock compression conditions (e.g. dynamic flyer plate or gas gun experiments), the applied shock temperature and pressure diffuse very rapidly (potentially on the order of <ns) across the graphite samples. Because of this fast heat propagation, the material may not receive the required latent heat to induce the phase transition.^34,35 Because this heat propagation is so rapid, very high pressure is required. However, in the case of acoustic shock experiments, the shock pulse duration is considerably longer. Because of this longer period, the local heating of the samples may take place over time scales on the order of milliseconds. This suggests that heat propagation is reduced nearly 10⁶ times, effectively slowing heat propagation. This sustained slow heat propagation can then allow the delivery of the required latent heat, allowing for the necessary structural relaxation time to induce the nucleation of disordered graphite from graphite.^17,36

Shock-induced changes in carbon bonding

The survey XPS spectra of the control and 500 shocked crystals are shown in Fig. 4(a), and the core C 1s spectra are displayed in Fig. 4(b)–(d). The sp² and sp³ bands are assigned in the C 1s spectra according to earlier studies and are characterized by splitting into two peaks. Generally, the sp³ band has a higher binding energy compared to the sp² band.^37,38 In the C 1s spectra, the lower energy band is assigned as sp² while the higher energy band as sp³. The respective band positions are 284.8 and 285.4 eV which are in good agreement with the literature values.³⁰ Additionally, the sp² band has a higher intensity compared to the sp³ band (Fig. 4(c)). This clearly indicates that the control sample has a high number of pi-bonds, which is consistent with the Raman results.

Furthermore, at the 500-shocked condition, from Fig. 4(b) and (d), it is quite clear that the population of pi-bonds is significantly reduced, while the resultant σ-bonds are enhanced. As a result, the perfect hexagonal graphite is converted into amorphous graphite, which must have non-hexagonal configurations as well as non-parallel basal planes across the entire lattice. Note that the C 1s spectrum of the sample with a high concentration of sp² carbon will have a broad, asymmetric tail towards the lower binding energy and one or more satellite features will occur at higher binding energy (Fig. 4(c)). For high concentrations of sp³-bonded carbon, the C 1s peak will have a high symmetric shape and will also be slightly shifted to higher binding energy (Fig. 4(b) and (d)). These two kinds of observations have been witnessed in the present work, which clearly demonstrates the formation of an sp³-based specimen.^39–41 Note that the higher energy peak at 285.2 eV (Fig. 4(e)) belongs to sp³ C–C bonds, which correspond to the disordered graphite phase.³⁹ In Fig. 4(f), we have presented the atomic percentages of the control and 500 shocked graphite. The control graphite has sp² (89.02%) and sp³ (10.98%) carbons, respectively (Fig. 4(f)), whereas the 500-shocked graphite has only sp³ (100%), which is one of the strong pieces of evidence for the proposed sp²-to-sp³-type phase transition.

Micro-structural nature of disordered graphite

In Fig. 5, the microstructural features of the control and 500-shocked graphite samples are presented. As seen in Fig. 5(a)–(c), the control graphite has well-defined rectangular morphology, whereas, at the-500 shocked condition, the surface morphology undergoes a high degree of morphological deformation (Fig. 5(d)–(f)) because of the collapse of the layered structure by the impact of shock waves. In Fig. 6(a)–(c), we can observe Morié fringes with highly-ordered patterns along the z-axis (basal planes) and a uniform mosaic spread (orientation distribution) with a d-spacing of 0.35 nm. In addition, Fig. 6(g) shows the SAED pattern of the control graphite with a perfect sextet pattern with an interval of 60°. The brightness of the sextets is proportional to the number of ordered layers in the graphite lattice.⁴² The highly visible six spots (inside the yellow rings in Fig. 6(g)) indicate that the control graphite has a high symmetry of ABAB stacking layers. This configuration is characteristic of a high fraction of sp² bonding in the crystal system and is mirrored in the 2D Raman bands, as shown in Fig. 1(c). The transformation of graphite to disordered graphite under high-pressure and high-temperature conditions involves substantial compression of the basal planes along the z-axis.

This results in bond-angle distortion, puckering-induced ring disorder (rotational/translational ring disorder), wave-like buckling, and slipping of basal planes. These kinds of defects significantly aid in the formation of disordered graphite.^6,7,11 The 500-shocked graphite sample's Morié fringe patterns are presented in Fig. 6(d)–(f) and it has a short-range order with the d-spacing of 0.28 nm. The SAED pattern of the 500-shocked sample is portrayed in Fig. 6(g) which obviously demonstrates the loss of the sextet pattern with the interval of 60° such that a diffused fringe pattern is observed for the (002) plane, which may be due to the disordered graphite. Based on the observed lattice fringe patterns, the graphite undergoes significant lattice compression and thereby loss of the ordered long-range layered structure is witnessed, which is microstructural proof for the proposed phase transition.

It is evident that the applied transient shock pressure disrupts the long-range order and high symmetry of ABAB stacking (Fig. 6(e) and (f)) leading to the formation of disordered graphite. Consequently, the graphite may experience in-plane and out-of-plane vibrations of carbon atoms in the non-basal planes, implying that carbon atoms stochastically vibrate out of a graphite basal layer (up or up/down), which is highly favorable for the formation of a non-layered type structural configuration.

Conclusion

In summary, we have proposed and explored the sp²-to-sp³ phase transition in graphite by employing a direct experiment, which is an alternative technology to induce the phase transition using low-pressure acoustic shock waves such that the results are systematically evaluated by implementing Raman, XPS and HR-TEM analyses. According to the Raman spectral results, the G-band intensity undergoes significant reduction whereas the FWHM value of the G-band is enhanced at the 500-shocked condition. Moreover, the complete disappearance of the 2D band is also witnessed at the 500-shocked condition. The results of the C 1s core XPS band provide superior evidence for the conversion of sp²-to-sp³ carbon bonds. HRTEM images clearly demonstrate the loss of long-range order and ABAB staking at the 500-shocked condition because of the formation of ring disorder. The effects of acoustic shock waves on graphite are most pronounced, especially when compared to the impacts of static pressure, flyer plate shock compression, and laser shock compression processes. This is primarily due to the simultaneous impact of pressure and temperature occurring over the timescales of milliseconds, which is confirmed by Raman, XPS, and TEM results. Moreover, the combined simultaneous effect of comparatively slower temperature and pressure exposure in solids may provide sufficient nucleation time to generate a new phase, in contrast to nanosecond compression experiments. Further optimization of these processes is required, such as optimizing the shock loading plane, the shock pressure, the number of shock pulses, and the sample thickness in order to find the critical point of diamond nucleation, which is currently in progress.

Ethics approval

Not applicable.

Consent to participate

All the authors agree to participate.

Informed consent

All people involved in or responsible for the research were informed and gave consent.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare no conflict of financial interest.

Acknowledgements

The authors thank NSF of China (42072055). This project was supported by the Researchers Supporting Project number (RSP2024R142), King Saud University, Riyadh, Saudi Arabia.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc03216k

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