Process for preparing graphene

Abstract

A process for preparing a product comprising one or more graphene layers, the process comprising: producing hydrodynamic cavitation in a liquid medium comprising a diaromatic component to synthesise the one or more graphene layers from the diaromatic component.

Claims

1. A process for preparing a product comprising one or more graphene layers, the process comprising: producing hydrodynamic cavitation in a liquid medium comprising a diaromatic component comprising one or more optionally substituted fused or linked diaromatic hydrocarbons to synthesize the one or more graphene layers from the diaromatic component.

2. The process of claim 1, wherein the hydrodynamic cavitation is produced by flowing a feed of the liquid medium through a constriction.

3. The process of claim 2, wherein the constriction has a maximum width of less than 1 mm.

4. The process of claim 2, comprising flowing a feed of liquid medium into a conduit having a principal axis, wherein the conduit is arranged to direct the liquid medium against an impact head having a face perpendicular or predominantly perpendicular to said principal axis; the impact head and the conduit being arranged so that said constriction results between an end of the conduit proximate to the impact head and the impact head.

5. The process of claim 2, wherein the feed of liquid medium is pressurised to a pressure of at least 300 bar or a pressure drop from the feed to the end of the constriction is at least 300 bar.

6. The process of claim 1, wherein the liquid medium is kept at a temperature within a range of plus/minus 5° C.

7. The process of claim 1, wherein the liquid medium is recycled and cavitation is repeatedly produced therein.

8. The process of claim 1, wherein the one or more graphene layers comprise one or more heteroatom impurities.

9. The process of claim 1, wherein the one or more graphene layers are oxidized.

10. The process of claim 1, wherein the one or more graphene layers are functionalized.

11. The process of claim 1, wherein the product comprises a substrate-borne graphene material formed by synthesizing the one or more graphene layers on substrate particles.

12. The process of claim 1, wherein the diaromatic component is a diaromatic hydrocarbon component consisting of one or more optionally substituted fused or linked diaromatic hydrocarbons.

13. The process of claim 1, wherein the diaromatic component comprises one or more diaromatic hydrocarbon compounds of Formula A or Formula B, optionally substituted with one or more moieties at one or more of the numbered positions: ##STR00005##

14. The process of claim 13, wherein the diaromatic component comprises one or more diaromatic hydrocarbon compounds of Formula A or Formula B substituted with one or more moieties selected from methyl, ethyl, and halides.

15. The process of claim 1, wherein the diaromatic component comprises 1-methylnaphthalene.

16. The process of claim 1, wherein the liquid medium comprises a stabilizing component for stabilizing a dispersion of graphene nanomaterial.

17. The process of claim 16, wherein the stabilizing component comprises N-Methyl-2-pyrrolidone (NMP).

18. A process for preparing a product comprising one or more graphene layers, the process comprising: producing hydrodynamic cavitation in a liquid medium comprising a diaromatic component comprising methylnaphthalene, and optionally naphthalene to synthesize the one or more graphene layers from the diaromatic component, wherein the hydrodynamic cavitation is produced by flowing a feed of the liquid medium pressurized to a pressure of at least 300 bar through a constriction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) One or more non-limiting examples of the invention will now be described with reference to the accompanying drawings, in which:

(2) FIG. 1 is a TEM of sample A.sub.1-MN obtained in Example 1;

(3) FIG. 2 is a TEM of sample B.sub.NMP obtained in Example 1;

(4) FIG. 3 is a C1s XPS of sample A.sub.1-MN obtained in Example 1;

(5) FIG. 4 is a C1s XPS of sample B.sub.NMP obtained in Example 1;

(6) FIG. 5 is a Raman spectrum of sample B.sub.NMP obtained in Example 1;

(7) FIG. 6 shows SEM images obtained from the first run in Example 2;

(8) FIG. 7 shows SEM images obtained from the second run in Example 2;

(9) FIG. 8 shows a Raman spectrum obtained from Example 3;

(10) FIG. 9a is an SEM image of silicon starting material in Example 4;

(11) FIGS. 9b to 9e are SEM images of graphene product in Example 4;

(12) FIG. 10 shows UV-vis spectra of dispersions of samples obtained in Example 5;

(13) FIG. 11 shows the particle number density of dispersions of samples obtained in Example 5;

(14) FIGS. 12a to e are C 1s XPS spectra of thin films of samples obtained in Example 5. FIG. 12a: sample L7716; FIG. 12b: sample L7826; FIG. 12c: sample L7831; FIG. 12d: sample L7829; FIG. 12e: sample L7832;

(15) FIGS. 13a to e are TEM images of samples obtained in Example 5. FIG. 13a: sample L7716; FIG. 13b: sample L7826; FIG. 13c: sample L7831; FIG. 13d: sample L7829; FIG. 13e: sample L7832;

(16) FIGS. 14a and b are high magnification TEM images of small particles seen in samples L7826 (FIG. 14a) and L7832 (FIG. 14b) of Example 5; and

(17) FIGS. 15a to e are SEM images of samples obtained in Example 5. FIG. 15a: sample L7716; FIG. 15b: sample L7826; FIG. 15c: sample L7831; FIG. 15d: sample L7829; FIG. 15e: sample L7832.

DETAILED DESCRIPTION

(18) The following non-limiting examples illustrate embodiments of the invention and its working.

Example 1 (Preliminary Study)

(19) A mixture of 1-methyl naphthalene (1-MN) and naphthalene (NAP) was cavitated using ultrasound. Specifically, samples were prepared from the cavitation of 1-MN/NAP (ϕ.sub.1-MN=0.8, ϕ.sub.NAP=0.2) using ultrasound produced by a VCX 750 (750 W) ultra-sonic processor (ex Sonics Materials Inc.) and a 13 mm extender horn that delivered 20 kHz ultrasound to 50 mL of sample contained within a jacketed glass beaker. Cold water (10° C.) was passed through the jacket to keep the liquid hydrocarbon below its flash point.

(20) A dispersion was produced under air. This was not stable and after standing for seven days was centrifuged at 3500 r min.sup.−1 for 20 minutes. The brown supernatant was removed by pipette (A.sub.1-MN) and the remaining black sediment added to N-Methyl-2-pyrrolidone (NMP) (50 mL) followed by a short period (10 kJ) of sonication to produce a black dispersion (B.sub.NMP).

(21) FIG. 1 shows TEM of the brown supernatant A.sub.1-MN which is a low concentration suspension of material that is finely dispersed but agglomerated into >1 μm sized structures which are electron opaque, and therefore relatively thick. FIG. 2 shows the TEM of the black colloid B.sub.NMP that is comprised of thin sheets of variable dimension (10-200 nm) that are seen scattered across the surface of a holey carbon grid.

(22) Thin films were prepared by vacuum filtration of dispersions into alumina membranes (0.2 μm Whatman Anodisc inorganic unsupported filter) mounted on a fritted glass filter. Films were washed with iso-propanol (15 mL) and dried in an oven (60° C.) for two days. XPS survey scans showed that the films are primarily comprised of carbon and oxygen. Aluminium was also present in significant levels (4-6 atom %) as a result of fractures in the thin films producing a paving of ˜50 μm fragments with 10-20 μm gaps. Data correction based on an assumption of clean Al.sub.2O.sub.3 in areas not covered by carbonaceous material allows a C/O atom ratio of 12 and 10 to be determined for sample A.sub.1-MN and B.sub.NMP respectively.

(23) FIGS. 3 and 4 show the C1s XPS spectrum of A.sub.1-MN and B.sub.NMP. The main peak was charge-referenced to an approximate binding energy of 285 eV and this feature was then fitted with five peaks: C—C (285.0 eV), C—O (286.5 eV), C═O (287.8 eV), O—C═O (289.5 eV) and a π-π* shake up, as seen in aromatics (291.5 eV). These features are also reported for graphene, although the C/O atom ratio is closer to levels found in reduced graphene oxide (rGO).

(24) FIG. 5 shows the Raman spectra of B.sub.NMP. The G band (˜1590 cm.sup.−1) is shifted to a higher wavenumber than in graphite or graphene (˜1580 cm.sup.−1), reflecting the degree of oxidation and the presence of spa carbon atoms in the sample. The D band (˜1360 cm.sup.−1) also has a higher intensity than in pure forms of graphite and graphene, which can be attributed to defects and disorder at both the edge and on the basal plane of nanosheet structures. The intensity of the 2D band is low and seen in combination with other overtone bands (D+G and 2G). These features, along with the intensity ratio of these bands (I.sub.D/I.sub.G˜0.83) and their full width at half maximum (FWHMG ˜87 cm.sup.−1) are consistent with values reported for nanosheets of graphene with a degree of oxidation.

(25) Following literature study, it was thought that cavitation of the diaromatic component led cyclodehydrogenation reactions to proceed as follows:

(26) ##STR00002##

(27) Alpha Position Reaction (Peri-Condensation)

(28) ##STR00003##

(29) Beta Position Reaction allows for Cata-Condensation

(30) ##STR00004##

(31) Subsequent reactions can take place to build up the unit cell at adjacent alpha-alpha positions or beta-beta positions.

Example 2

(32) A trial was conducted to determine whether hydrodynamic cavitation of a diaromatic component could deliver similar results to those seen during the ultrasonic cavitation of Example 1.

(33) A liquid medium consisting of 1-methyl naphthalene was continuously circulated through a high pressure homogenizer at a fixed pressure over a given time.

(34) The homogenizer was a Benchtop Panda Plus 2000 unit (Serial Number 9759), which works on the principle of forcing liquid through an annular constriction formed between an impact head and an impact ring. Feed pressure of the unit was adjusted by varying the size of the constriction. Back-pressure was set to 30 barg using a secondary valve.

(35) In a first run, a volume of 250 mL 1-methyl naphthalene was circulated at a feed pressure of 600 barg for 10 minutes. The starting temperature of the 1-methyl naphthalene was recorded as 18.5° C. and the end temperature was 45° C.

(36) In a second run, a volume of 200 mL 1-methyl naphthalene was circulated at a feed pressure of 1000 barg for 10 minutes. The starting temperature of the 1-methyl naphthalene was recorded as 39° C. and the end temperature was 66° C.

(37) In each run, the 1-methyl naphthalene starting material was straw-coloured. After the 10 min, a black dispersion was visible in the 1-methyl naphthalene. There were some black solid particles left at the bottom of the collection beaker. The material was dried down on hot plates and some phase separation occurred.

(38) Total solids were found to be 0.2 g in 250 mL for the first run and 0.3 g in 200 mL for the second run. With reference to FIGS. 6 and 7, SEM images showed the presence of platelets.

(39) It was concluded that cavitation within the homogeniser led to the formation of particles similar to those obtained in Example 1.

Example 3

(40) A trial was conducted out with the same general method and equipment as used in Example 2, but with 1-methyl naphthalene recovered from a previous run.

(41) A volume of 200 mL 1-methyl naphthalene was circulated at a feed pressure of 1000 barg for 10 minutes. A water jacket was used in an attempt to stabilise temperature.

(42) The following temperatures were noted:

(43) TABLE-US-00001 Time (min) Temperature (° C.)  0 21 10 59 15 64 20 68 25 73 30 76

(44) The batch was then centrifuged in an IEC Centra 8 Centrifuge for 1100 rpm for 20 minutes. Material was collected at the bottom of the tubes but the dispersion still appeared to contain material. Therefore it was centrifuged at 1100 rpm for an additional 20 minutes.

(45) On drying down, 0.7-0.8 g of solids was obtained. This is equivalent to 3.5-4 g/L of solids. This run showed that it is possible to recover and reuse 1-methyl naphthalene and also confirmed that yield increased over time.

(46) Sheet resistance of the dried product was tested and found to be about 24 Ω/sq.

(47) With reference to FIG. 8, a Raman trace was obtained and showed that graphene was present in combination with graphitic carbon.

(48) It was concluded that the obtained product particles comprised graphene layers.

Example 4

(49) A trial was conducted to deposit conductive graphene onto the surface of silicon particles. The same homogenizer as in Examples 1 to 3 was employed.

(50) 150 mL of 1-methyl naphthalene and 1.5 g of silicon powder (99.9% purity, 325 mesh (11.5 micron average particle size)) were circulated at a feed pressure of 1000 bar for 10 min.

(51) The following temperature rises in the system were observed:

(52) TABLE-US-00002 Time (min) Temperature (° C.) 0 23.7 3.5 47 7 61 10 71

(53) The sample discoloured, consistent with 1-methyl naphthalene being converted to graphene platelets.

(54) The graph below shows the corrected sheet resistance as measured with a 4 point probe, indicating a drop after 10 minutes of production—although it was difficult to form a film to make measurements.

(55) TABLE-US-00003 Corrected Sheet Weight of Sheet Sample Resistance Material/g Resistance Start 20.6M Ω/□ 0.001 68K Ω/□ Extremely thin film 10 mins 60K Ω/□ 0.009 18K Ω/□ Very thin film

(56) With reference to FIG. 9, SEM images suggest that flake-like material derived from 1-methyl naphthalene was grown and deposited onto the surfaces of the silicon particles. After deposition, the silicon particles looked less angular and appeared to have material on the surface.

Example 5

(57) Sample Preparation

(58) The hydrodynamic cavitation of 1-methyl naphthalene was carried out at various upstream pressures (P.sub.u), using a Benchtop Panda Plus 2000 homogeniser as set out above in Example 2. Fluid was circulated at a fixed pressure for a set processing time.

(59) The conditions used and samples prepared are set out in the table below. 1-MN was sourced from Sigma Aldrich at 95% and ≥95% purity. L7832 was produced by combining 1-MN (≥95% purity) with 10% v/v octylamine. A sample of graphene produced by delamination of graphite (L7716) was used as a reference sample.

(60) TABLE-US-00004 Conditions P.sub.u v Time Sample bar CN ms.sup.−1 min Fluid L7826  600 20 337 60 200 mL 95% 1-MN L7831 1000 33 440 60 200 mL 95% 1-MN L7832 1000 33 440 30 270 mL ≥95% 1-MN 30 mL octylamine L7829 1500 50 542 60 200 mL ≥95% 1-MN

(61) CN is sufficiently high in all cases to give high levels of cavitation in fast moving flows of 1-MN. This resulted in the rapid formation of black dispersions in all cases. Solid samples were obtained by drying down the dispersions on hot plates within a fume hood. 1-MN has a boiling point of 240° C. and so the samples were dried on hotplates at 260° C. until there was no more loss in weight.

(62) Dispersions of L7716, L7826, L7831, L7829 and L7832 (5 mg) in 50 mL of N-methyl-2-pyrrolidone (NMP) were formed. Adding 5 mg of L7716 to 50 mL NMP and shaking produces a black dispersion. Establishing the dispersion of L7832 required some shaking and also exposure (5 minutes) to ultrasound in a Branson 200 ultrasonic cleaner bath (30 W). When L7826, L7831 and L7829 were added to NMP only a small fraction of the sample dispersed, even after treatment in the ultrasonic cleaner bath. A 13 mm extender horn powered by a VCX 750 ultrasonic processor (ex Sonics Materials Inc.) was used to apply ultrasound at 45 W (50% amplitude setting) directly to the NMP. This was done until dispersion formation took place with no further darkening in colour (typically after 10 minutes).

(63) All samples ex hydrodynamic cavitation of 1-MN (L7826, L7829, L7831, L7832) had a greyish/black appearance consistent with dispersed graphene flakes (L7716) and contrastingly different from the brown dispersions produced when 1-MN was treated with ultrasound to promote the formation of nanosheets by cavitation. This reflects the smaller size of the flakes and/or the partially oxidised nature of the nanosheets produced under the milder ultrasound conditions. The term partially oxidised graphene (poG) is used hereinbelow to describe this material produced with ultrasound.

(64) The samples were analysed using UV-vis, particle number density, XPS, TEM and SEM analysis.

(65) UV-Vis Analysis

(66) UV-vis spectra were recorded (FIG. 10) using a UV-1699PC VWR Spectrophotometer (VWR International, Radnor, Pa.). The even transmission of light across the 400-1000 nm range for all the samples produced by hydrodynamic cavitation (L7832, L7826, L7831, L7829) is similar to the behaviour of the sample of graphene produced by exfoliation of graphite (L7716). The distinctive spectrum of poG (ex ultrasound cavitation) shows much lower transmission (stronger absorbance) at shorter wavelengths of light.

(67) Particle Number Density

(68) The particle number density (N) of the NMP dispersions was measured using a Spectrex LPC-2200 laser particle counter (Spectrex Corporation, Redwood City, Calif.), which makes measurements based on the principle of near-angle light scattering. Particles are reliably counted in the 1-100 μm size range. This technique has a detection limit of 1 μm and therefore only counts agglomerated nanosheets. Samples were gently swirled before being left to stand to allow any air bubbles to settle. Number density counts were based on an average of ten consecutive measurements. The results are set out in the table below and shown in FIG. 11. The data show low particle number densities consistent with stable dispersions. Samples with agglomerated particles would typically have particle number densities >×10 larger than the values in the table.

(69) TABLE-US-00005 N ± 2σ (0 days) Sample ×10.sup.6 cm.sup.−3 L7716 0.18 ± 0.01 L7826 0.25 ± 0.03 L7831 0.21 ± 0.01 L7829 0.10 ± 0.01 L7832 0.24 ± 0.03

(70) X-Ray Photoelectron Spectroscopy (XPS)

(71) Thin films were prepared as described for example 1, except that the films were dried under vacuum. The thin films from dispersions produced by hydrodynamic cavitation were grey in colour, compared to the brown films from dispersions produced by ultrasound.

(72) The XPS data were acquired using a bespoke ultra-high vacuum system fitted with a Specs GmbH Focus 500 monochromated Al α X-ray source, Specs GmbH Phoibos 150 mm mean radius hemispherical analyser with 9-channeltron detection, and a Specs GmbH FG20 charge neutralising electron gun. Survey spectra were acquired over the binding energy range 1100-0 eV using a pass energy of 50 eV and high resolution scans were made over the C 1s, O 1s and N 1s photoelectron lines (where detected) using a pass energy of 20 eV. Under these conditions the full width at half maximum (FWHM) of the Ag 3d.sub.5/2 reference line is <0.8 eV. In each case, the analysis was an area average over a region approximately 2 mm in diameter on the sample surface.

(73) The energy scale of the instrument is calibrated according to ISO standard 15472, and the intensity scale is calibrated using an in-house method traceable to the UK National Physical Laboratory. Data were quantified using Scofield cross sections corrected for the energy dependencies of the electron attenuation lengths and the instrument transmission. Data interpretation was carried out using CasaXPS software v2.3.16.

(74) Prior to detailed analysis, the spectra were charge-corrected so that the principal component of the C 1s peak appeared at a binding energy of 285.0 eV, as is standard practice in XPS analysis of insulating samples. However the samples were difficult to charge-neutralise satisfactorily, and the spectra collected represent best-efforts after spending significant time optimising the charge neutralising electron flood gun conditions. In some cases, differential charging between the substrate and the sample was evident.

(75) The C 1s spectra of the samples are shown in FIG. 12.

(76) The spectrum of sample L7716 (ex exfoliated graphite; FIG. 12a) shows a C 1s peak that is consistent with graphene—a narrow asymmetric main peak of sp.sup.2-hybridised carbon at 285 eV (FWHM of 0.63 eV) and weak plasmon loss features around 290-292 eV. The small increase in intensity around 286-288 eV is typical of a very low level of non-specific carbon-oxygen bonding.

(77) L7826 (FIG. 12b), L7831 (FIG. 12c) and L7829 (FIG. 12d) all show a peak due to C—C bonding at approximately 285 eV with a series of features at higher binding energies that would typically be interpreted as consistent with varying degrees of carbon-oxygen bonding.

(78) L7832 (FIG. 12e) shows a narrow asymmetric C 1s peak that is consistent with sp.sup.2-hybridised carbon (FWHM of 0.5 eV)—similar to that seen for the exfoliated graphite sample (L7716). The low level of oxygen observed in the survey scan is reflected in the small proportion of carbon-oxygen bonding. Unambiguous assignment of deconvoluted peaks fitted on the high energy side of this feature to amine (C—N) or amide (C(O)—N) bonds is not possible.

(79) A strong C 1s peak dominated all the spectra, with some signal from the underlying Anodisc substrate (Al, P and O) being seen in all cases. It was possible to assume the area of exposed Anodisc was proportional to the sum of the Al and P signals and to correct the C, O and N concentrations for the contribution from the substrate and re-normalise to give the approximate composition of the thin film overlayer. The elemental compositions (atom %) determined in this way are set out in the table below. RP065 and RP032 are samples prepared by treating 1-MN (RP065) and 1-MN with 10% octylamine (RP032) with ultrasound, for comparison. The detection of Si is assumed to be related to previous experiments carried out using the homogeniser. It is not clear why the Cl signal for sample L7831 is atypically high.

(80) TABLE-US-00006 Peak RP065 RP032 L7716 L7826 L7829 L7831 L7832 O 1s 14.64 14.18 2.29 13.16 10.68 6.51 3.83 C 1s 84.07 81.07 97.71 80.81 83.89 84.46 92.21 N 1s 1.30 4.15 — 1.93 1.22 0.49 0.34 Si 2p — — — 2.67 3.98 1.14 1.53 Cl 2p — — — 1.43 0.22 7.39 2.10 Na 1s — 0.25 — — — — — S 2p — 0.35 — — — — — C/O 6 6 43 6 8 13 24

(81) Transmission Electron Microscopy (TEM)

(82) The dispersions were filtered onto holey carbon film 300 mesh copper TEM grids and dried. TEM images were acquired using a Phillips CM20 TEM in bright field transmission mode using a 200 kV beam energy and captured using the side-entry CoolSnap 1k×1k Peltier cooled camera system with Gatan Digital Micrograph software. An exemplary TEM image for each sample is shown in FIG. 13.

(83) The exfoliated graphite sample (L7716; FIG. 13a) shows small thin angular flakes that are typically 1-3 μm in size and with a range of thicknesses. Some of the flake edges indicate that multiple layers are present. The low contrast at the edge of some flakes is indicative a very thin material. L7826 (FIG. 13b) shows irregular particles and flakes ranging from <1 μm to ˜5 μm in size. A range of flake thicknesses is evident and some flakes have smaller flakes (˜100 nm) on their surface. Smaller particles can also be seen in the background of some images. L7831 (FIG. 13c) shows scattered flakes that are typically 1-2 μm in size. Creases and folding is present. Some flakes clearly consist of multiple layers. L7829 (FIG. 13d) again shows flakes of variable size and thickness. Folding/creasing is observable in some images as well as the presence of smaller particles on the surface of some flakes and scattered across the background of the carbon grid. L7832 (FIG. 13e) shows clusters of 2-4 μm flakes. Flake folding is observable and small ˜50 nm particles are scattered across the background of the carbon grid.

(84) High magnification images of the small particles seen in the hydrodynamic samples reveal that they are clusters of 20-50 nm primary particles (FIG. 14a; L7826) or sometimes even smaller ˜10 nm primary particles (FIG. 14b; L7832).

(85) Scanning Electron Microscopy (SEM)

(86) Analysis on powder samples was carried out using a Leo (now Zeiss) 1455VP SEM at 20 kV beam energy with 30 pA beam current. The powder samples were obtained by drying down the dispersions of the samples on hot plates within a fume hood as described above.

(87) Images were acquired of a typical area at ×500 and ×5000 magnification. An exemplary SEM image at ×5000 magnification for each sample is shown in FIG. 15. Flake-like structures are seen in all the high magnification images, with L7716 (FIG. 15a) and L7832 (FIG. 15e) bearing the greatest resemblance.

(88) The elemental composition of the samples was determined using SEM-EDX. EDX analysis was performed using an Oxford Instruments X-Max ultra-thin window EDX detector with Oxford INCA acquisition and processing software. The atom % composition obtained via this method of each of the samples is set out in the table below.

(89) TABLE-US-00007 Element L7716 L7826 L7829 L7831 L7832 C 98.84 76.20 85.44 91.83 97.48 O 1.16 20.64 5.54 4.26 1.34 Si — 2.57 6.48 1.91 0.75 S — 0.17 0.49 0.13 — Cl — 0.12 0.11 1.62 0.30 Ti — 0.10 0.82 0.03 — Fe — 0.06 0.52 0.07 0.05 Zn — 0.13 0.27 0.04 — Al — — 0.20 — — Ca — — 0.12 0.02 — Mo — — — — 0.08 Na — — — 0.08 — C/O 85 4 15 22 73

(90) Discussion

(91) The high pressures employed in the GEA Panda Plus Unit (600-1500 bar) results in thin flakes that are >10 times larger in their two lateral dimensions than the smaller nanosheets produced during ultrasound cavitation. The acoustic pressure developed by the probe used during ultrasound treatment is <10 bar and this would appear to be why TEM images of poG ex ultrasound cavitation show 10-200 nm sized flakes. The higher pressures in the hydrodynamic reactor would be anticipated to result in bubbles growing to a larger size before they collapse. This means that the number of moles of 1-MN vapour in a hydrodynamic cavity will be greater, allowing for the growth of larger sheets. TEM images of the hydrodynamic samples shows flakes that are typically >1 μm and sometimes larger than 5 μm, indicating approximately a tenfold increase in sheet size.

(92) The flakes produced during hydrodynamic cavitation are thin and similar in appearance to graphene produced by the exfoliation of graphite (L7716). Flakes with multiple layers are sometimes present and folding/creasing of sheets is also observable. The small circular features that appear in some of the TEM images look to be comprised of ≤50 nm primary particles. These may be soot-like material that forms inside small cavitation bubbles. Some of the dimensions are approaching those of graphene quantum dots.

(93) Compositional analysis of material produced by the ultrasound cavitation of diaromatic components has indicated material that is similar to reduced forms of graphene oxide (rGO). The XPS C 1s spectra of poG materials show that some carbon-oxygen bonding is present in the nanosheets. The partial oxidation of the sheets is believed to be a consequence of the composition of cavitation bubbles, which are comprised of a mixture of hydrocarbon vapour and some gas. The gas is derived from dissolved air in the liquid reaction medium. Some—but not all—of this air is degassed during cavitation.

(94) The higher pressures employed during hydrodynamic cavitation would be expected to result in greater levels of de-gassing and therefore lower levels of oxygen in the flakes that are produced. The C 1s peaks for hydrodynamically generated samples at P.sub.u=600, 1000 and 1500 bar (L7826, L7831 and L7829) do show features that are consistent with carbon-oxygen bonding, indicating that degassing is still only partial during cavitation at these high pressures. The trend in the both the XPS and SEM composition data is that the C/O ratio of the material produced during hydrodynamic cavitation becomes larger when increasing P.sub.u from 600 to 1000 bar and then becomes smaller when the pressure is increased further to 1500 bar.

(95) Combining octylamine with 1-MN leads to the formation of flakes that are similar to graphene from exfoliated graphite in size, thickness and composition. Some sheet functionalisation may have taken place, although evidence for alkyl attachment via amide and/or amine linkages is not clear from the XPS data. The significantly increased C/O ratio and the shape of the C 1s XPS peak when using the alkylamine shows that the flakes produced have a purity approaching that of graphene from exfoliated graphite. It is possible that the alkylamine—maybe through changes in the surface tension (σ) and/or viscosity (η) of the reaction medium—facilitates more effective de-gassing during cavitation.

Example 5

(96) A trial was conducted to demonstrate hydrodynamic cavitation of a diaromatic component using a microfluidizer instead of a homogeniser.

(97) The unit used was a Microfluidics M110P Microfluidizer. Clean 1-methyl naphthalene was run through the unit at a pressure of 1000 Bar. Volume<500 mL, number of passes=20. The starting material changed colour from a straw colour to a black dispersion, which then settled out to yield black particles.

(98) Based on the change in colour, it was inferred that the microfluidizer has been successful in terms of converting 1-methyl naphthalene to graphene nanoplatelets in a similar way to the homogenizer.

CONCLUSION

(99) Hydrodynamic cavitation allows for the preparation of products comprising one or more optionally functionalised graphene layers. Such products may take the form of graphene nanomaterials and/or substrate-borne graphene materials.

(100) The process is readily scalable, may be operated in a continuous manner, and is thus more suited to industrial scale production than ultrasonic cavitation.

(101) The hydrodynamic cavitation of diaromatic hydrocarbons offers an opportunity to:

(102) Reproduce the same chemistry observed during the ultrasound cavitation of diaromatic components

(103) Achieve higher production rates than observed during ultrasound cavitation

(104) Scale-up reactors for the commercial production of 2-dimensional carbon materials

(105) Produce partially oxidised forms of graphene

(106) Produce functionalised forms of graphene by introducing suitable components into the liquid diaromatic reaction medium

(107) Produce more pristine forms of graphene (as observed when using alkylamines as a part of the reaction mixture).