阅读说明：本技术 垂直支化石墨烯 (Vertically branched graphene ) 是由 Z·韩 C·楚尼斯 R·阿玛尔 于 2020-02-28 设计创作，主要内容包括：提供了制备垂直支化石墨烯的方法,其包括在缺乏引入的碳源的情况下用惰性等离子体处理原始垂直石墨烯,以产生垂直支化石墨烯。该方法还以可包括用惰性等离子体预处理基底表面；通过使基底表面与包含碳源气体的沉积等离子体接触一段沉积时段,将原始垂直石墨烯沉积到基底表面上。还提供了附着于基底表面的垂直支化石墨烯,该垂直支化石墨烯具有从基底表面延伸的主体部分,所述主体具有随着与基底表面的距离增加而增加的支化度；和具有近端和远端的独立式支化石墨烯,近端包括主体部分,主体部分具有支化度,并且该支化度随着与近端的距离增加和与远端的距离减少而增加。(Methods of preparing vertically branched graphene are provided, which include treating pristine vertical graphene with an inert plasma in the absence of an introduced carbon source to produce vertically branched graphene. The method may further comprise pretreating the substrate surface with an inert plasma; pristine vertical graphene is deposited onto a substrate surface by contacting the substrate surface with a deposition plasma comprising a carbon source gas for a deposition period. Also provided is a vertically-branched graphene attached to a substrate surface, the vertically-branched graphene having a bulk portion extending from the substrate surface, the bulk having a degree of branching that increases with increasing distance from the substrate surface; and free-standing branched graphene having a proximal end and a distal end, the proximal end including a body portion, the body portion having a degree of branching, and the degree of branching increasing with increasing distance from the proximal end and decreasing distance from the distal end.)

1. A method of preparing vertically branched graphene, comprising the steps of:

pristine vertical graphene is treated with an inert plasma in the absence of an introduced carbon source to produce vertically branched graphene.

2. The method of claim 1, wherein the inert plasma used to generate the vertically branched graphene is an Ar plasma.

3. The method of claim 2, wherein the Ar plasma is applied at a pressure <5 Pa.

4. The method of claim 2, wherein the Ar plasma is applied at a pressure of 0.5 to 2 Pa.

5. The method of any one of claims 2 to 4, wherein the Ar plasma is applied at a radio frequency of 10-15 MHz.

6. The method as claimed in any one of claims 2 to 5, wherein the Ar plasma is applied at a power of 500-2000W.

7. The method as claimed in any one of claims 2 to 6, wherein the Ar plasma is applied at a power of 900 and 2000W.

8. The method of any one of claims 2 to 7, wherein the Ar plasma is applied at a radio frequency of 13.56MHz and a power of 1000W at a pressure of 1.5 Pa.

9. The method of any one of the preceding claims, wherein the vertical graphene is treated with an inert plasma for a predetermined period of time to produce a predetermined level of branching.

10. The method of claim 9, wherein the predetermined period of time is at least 1 minute.

11. The method of claim 9, wherein the predetermined period of time is at least 5 minutes.

12. The method of claim 9, wherein the predetermined period of time is at least 10 minutes.

13. The method of claim 9, wherein the predetermined period of time is at most 20 minutes.

14. A method according to any preceding claim, wherein the only heating applied is heating by the plasma.

15. The method of any preceding claim, wherein the substrate temperature is less than 500 ℃.

16. A method of preparing vertically branched graphene, comprising the steps of:

a) pretreating the surface of the substrate with inert plasma;

b) depositing pristine vertical graphene onto the substrate surface by contacting the substrate surface with a deposition plasma comprising a carbon source gas for a deposition period;

c) treating the vertical graphene with an inert plasma in the absence of an introduced carbon source to produce a vertically branched graphene having a vertically branched graphene structure.

17. The method of claim 16, wherein the pristine vertically branched graphene comprises an interconnected network of vertical graphene sheets.

18. The method of claim 16 or claim 17, wherein the inert plasma used to pre-treat the substrate surface and/or generate the vertically branched graphene structure is an Ar plasma.

19. The method as claimed in claim 18, wherein the Ar plasma is applied at a radio frequency of 10-15MHz and a power of 500-.

20. The method of claim 19, wherein the Ar plasma is applied at a radio frequency of 13.56MHz and a power of 1000W at a pressure of 1.5 Pa.

21. The method of any one of claims 16 to 20, wherein the substrate surface is pretreated for a period of 10 minutes.

22. The method of any one of claims 16 to 21, wherein the deposition plasma comprising the carbon source is prepared by introducing a carbon source gas and hydrogen gas to the substrate surface without interrupting the inert plasma.

23. The method of any one of claims 16 to 22, wherein the carbon source is a single source introduced directly into the reaction chamber as a gas stream.

24. A method according to claim 22 or 23, wherein the carbon source gas is methane.

25. The method of any one of claims 22 to 24, wherein the deposition plasma is at a pressure of less than 2 Pa.

26. The method of any one of claims 22 to 25, wherein the deposition plasma is a plasma comprising argon, methane and hydrogen at 1.5-1.8 Pa.

27. The method of any one of claims 16 to 26, wherein depositing vertical graphene occurs without external heating of the substrate, and wherein the only heating applied is heating by the plasma.

28. The method of any one of claims 16 to 27, wherein depositing vertical graphene is performed at <600 ℃.

29. The method of any one of claims 16 to 28, wherein depositing vertical graphene is performed at 400 ℃.

30. The method of any one of claims 16 to 29, wherein depositing vertical graphene is performed for 10 minutes.

31. The method of any one of claims 16 to 30, wherein after step b) but before step c), the pressure is reduced to<2x10^-2Pa。

32. The method of any one of claims 16 to 31, wherein the pristine vertically branched graphene comprises an interconnected network of vertical graphene sheets.

33. The method of any one of claims 16 to 32, wherein the inert plasma used to generate the vertically branched graphene is an Ar plasma.

34. The method of any one of claims 16 to 33, wherein the Ar plasma is applied at a pressure of 0.5-2 Pa.

35. The method of any one of claims 16 to 34, wherein the Ar plasma is applied at a radio frequency of 10-15 MHz.

36. The method as set forth in any one of claims 16 to 35 wherein the Ar plasma is applied at a power of 500-.

37. The method of any one of claims 16 to 36, wherein the Ar plasma is applied at a radio frequency of 13.56MHz and a power of 1000W at a pressure of 1.5 Pa.

38. The method of any one of claims 16 to 37, wherein the vertical graphene is treated with an inert plasma for a predetermined period of time to produce a predetermined level of branching.

39. The method of claim 38, wherein the predetermined period of time is at least 1 minute.

40. The method of claim 38, wherein the predetermined period of time is at least 5 minutes.

41. The method of claim 38, wherein the predetermined period of time is at least 10 minutes.

42. The method of claim 38, wherein the predetermined period of time is at most 20 minutes.

43. A method according to any one of claims 16 to 42, wherein the only heating applied is heating by the plasma.

44. A vertically branched graphene attached to a substrate surface, the vertically branched graphene having a bulk portion extending from the substrate surface, the bulk having a degree of branching that increases with increasing distance from the substrate surface.

45. The vertically branched graphene of claim 44, prepared by the method of any one of claims 1 to 43.

46. The vertically branched graphene of claim 44 or 45, wherein Raman spectrum I_D/I_GThe relative ratio of peak intensities was 1.1 or more.

47. The vertically branched graphene of claim 46, wherein Raman spectrum I_D/I_GThe relative ratio of peak intensities was 1.4 or more.

48. The vertically branched graphene of claim 47, wherein Raman spectrum I_D/I_GThe relative ratio of peak intensities was 1.7 or more.

49. The vertically branched graphene of claim 48, wherein Raman spectrum I_D/I_GThe relative ratio of peak intensities was 1.9 or more.

50. A free-standing branched graphene having a proximal end and a distal end, the proximal end comprising a body portion, the body portion having a degree of branching, and the degree of branching increasing with increasing distance from the proximal end and decreasing distance from the distal end.

51. The free-standing branched graphene of claim 50, prepared by removing the vertically branched graphene of any one of claims 44 to 49 from a substrate.

52. The free-standing branched graphene of claim 50 or 51, wherein Raman Spectroscopy I_D/I_GThe relative ratio of peak intensities was 1.1 or more.

53. The free-standing branched graphene of claim 52, wherein Raman Spectroscopy I_D/I_GThe relative ratio of peak intensities was 1.4 or more.

54. The free-standing branched graphene of claim 53, wherein Raman Spectrum I_D/I_GThe relative ratio of peak intensities was 1.7 or more.

55. The free-standing branched graphene of claim 54, wherein Raman Spectroscopy I_D/I_GThe relative ratio of peak intensities was 1.9 or more.

56. A catalyst support comprising the vertically branched graphene of any one of claims 44 to 49 or the freestanding branched graphene of any one of claims 50 to 55.

57. Use of the catalyst support of claim 56 in an electrocatalytic process.

58. The use of claim 57 for hydrogen production or oxygen production.

59. The use of claim 57 for the production of a liquid carbon product.

60. RightsThe use of claim 57 for forming from CO₂Production of one or more of reduced n-propanol, ethanol or formate.

61. A catalyst support comprising the vertically branched graphene of any one of claims 44 to 49 or the freestanding branched graphene of any one of claims 50 to 55, in an energy storage device such as a battery or supercapacitor.

Technical Field

The present invention relates to novel graphene having a layered or branched structure, and to a method for preparing the same. The layered or branched graphene can be used in applications where high surface area graphene is desired, such as energy storage devices and catalyst supports.

Background

Graphene exhibits unique electronic, optical, chemical and mechanical properties. Due to their extremely high electron mobility (electrons move approximately 99 times faster through graphene than through silicon), extremely low absorption in the visible spectrum, and relative flexibility and elasticity (compared to inorganics such as indium tin oxide), the use of loaded horizontal graphene sheets as active functional materials has revolutionized the field of flexible, transparent, and ultra-light nanodevices, from optoelectronic elements to sensors. Graphene sheets, in which graphene is exfoliated from any supporting surface, are also a very useful material, which has high thermal and electrical conductivity, and is extremely lightweight.

Although graphene is generally a flat, sheet-like substance, it can also be deposited onto a substrate such that the graphene structure can be produced in a manner that allows for a certain degree of vertical orientation (i.e., the major direction of the graphene plane is orthogonal to the deposition surface). This vertical growth allows for the creation of controlled graphene microstructures that are potentially useful for electron emission, biological recognition, and drug/gene/protein delivery applications, among others. Vertically oriented graphene (often referred to as vertical graphene or graphene nanowalls) provides significantly enhanced functionality compared to horizontally oriented graphene. This is due to the ability to achieve ultra-fast charge transport in the vertical plane through accessible basal planes that also provide a relatively high density of low contact resistance sites for, for example, adsorption and/or immobilization of a range of quantum dot, chemical and biological specific molecules.

The vertical graphene need not be supported on a surface in order to be useful. For example, isolated vertically formed graphene sheets can be readily produced with surface areas in excess of 400m²(ii) in terms of/g. This is much higher than the surface area per unit density of commercial graphene powder prepared from conventional chemically produced graphene flakes.

The vertically formed graphene generally has an open and three-dimensional (3D) structure, a high density of reactive edges, about 3-15 graphite layers, and about 400m²Surface area per gram, and good electrical conductivity. It is possible to control the vertical height of the graphene sheet, for example, 0.5-20 μm vertical graphene can be easily produced as desired.

When viewed from above, looking down towards the substrate on which the vertically formed graphene sheets have been deposited, it can be observed that the vertically formed graphene sheets form an interconnected network, wherein the graphene sheets are produced from a plurality of growth lines on the surface of the substrate. This structure has been described in some publications as "maze-like vertical graphene" to reflect the appearance of a somewhat irregular thin-walled maze when viewed from above.

Vertically formed graphene is readily prepared using typical plasma-assisted or plasma-enhanced vapor deposition processes, and can be formed on virtually any surface that is thermally stable at about 400 ℃. This includes various metal substrates such as stainless steel, Ni, Al, and Cu; semiconductor materials such as silicon wafers, quartz and sapphire, and non-conventional deposition substrates such as carbon paper and carbon fiber, nickel foam and copper foam, among others. The process is relatively simple and does not require a catalyst.

In addition, the vertical graphene sheets can be easily peeled off from the graphene growth substrate by mechanical means, such as scraping or vibration (ultrasonic vibration), so as to produce free-standing graphene powder. Even when the vertically formed graphene is removed from the formation surface by relatively simple mechanical means, many of its useful properties are retained.

Vertical graphene can also be easily functionalized. Carbon atoms at the tip of the vertically grown graphene have much higher reactivity than carbon in the basal plane, and less steric hindrance than carbon atoms in the bulk of the vertical graphene, which may be hindered by adjacent vertical graphene particles. For example, vertical graphene may be oxidized to produce a graphene surface with increased hydrophilicity.

The potential of graphene to reach very high surface areas per unit volume has led to various approaches to try to further increase the surface area through various forms of branching. Although vertical graphene can be grown on a wide range of substrates without the use of any catalyst, the methods used to branch graphene generally rely on adjusting the concentration of carbon precursors or using high carbon content precursors (e.g., C) in a one-step plasma-assisted chemical vapor deposition process₂H₂) And typically requires the use of external heating of the substrate in addition to the heating caused by the deposition plasma. This requirement for temperature (usually>700 deg.c) means that the branched form of graphene has been limited to growth on certain substrates.

Vertical graphene is potentially useful in a number of applications, including:

super capacitor

Lithium ion batteries

Catalyst support for hydrogen generation (both hydrogen evolution reaction HER and oxygen evolution reaction OER)

For CO₂Reduced catalyst support

Fuel cell (both of the hydrogen oxidation reaction HOR and the oxygen reduction reaction ORR)

Solid and flexible energy storage devices

Flow cell

Structural and multifunctional devices

Thus, new vertically prepared graphene seems likely to be a potentially useful material in numerous fields.

There is also a need to produce novel vertical graphene exhibiting desirable controllable electronic, optical and mechanical properties.

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

SUMMARY

According to a first aspect, the present invention provides a method of preparing vertically branched graphene, comprising the step of treating pristine vertical graphene with an inert plasma in the absence of an introduced carbon source to produce graphene.

Preferably, the pristine vertically branched graphene comprises an interconnected network of vertical graphene sheets.

Preferably, the inert plasma used to generate the vertically branched graphene is an Ar plasma. Preferably, the Ar plasma is applied at a radio frequency of 10-15MHz and a power of 500-2000W at a pressure of <5Pa (e.g., 0.5-2Pa), or more preferably at a radio frequency of 13.56MHz and a power of 1000W at a pressure of 1.5 Pa.

The vertical graphene may be treated with an inert plasma for a predetermined period of time to produce a predetermined level of branching. For example, the predetermined period of time is at least 1 minute, or at least 5 minutes, or at least 10 minutes, or at most 20 minutes.

Preferably, the only heating applied is heating by plasma. Preferably, the substrate temperature is less than 500 ℃.

According to a second aspect, the present invention provides a method of preparing vertically branched graphene, comprising the steps of:

a) pretreating the substrate surface with an inert plasma, wherein the only heating applied to the substrate is the heating provided by the plasma;

b) depositing pristine vertical graphene onto a substrate surface by contacting the substrate surface with a deposition plasma comprising a carbon source gas for a deposition period;

c) the vertical graphene is treated with an inert plasma in the absence of any introduced carbon source to produce a vertically branched graphene having a vertically branched graphene structure.

According to a third aspect, the present invention provides a method of preparing vertically branched graphene, consisting of the steps of:

a) pretreating the substrate surface with an inert plasma, wherein the only heating applied to the substrate is the heating provided by the plasma;

b) depositing pristine vertical graphene onto a substrate surface by contacting the substrate surface with a deposition plasma comprising a carbon source gas for a deposition period;

c) the vertical graphene is treated with an inert plasma in the absence of any introduced carbon source to produce a vertically branched graphene having a vertically branched graphene structure.

Preferably, the pristine vertically branched graphene comprises an interconnected network of vertical graphene sheets. Preferably, the distance between graphene sheets at the surface furthest from the substrate is less than 2 μm.

Preferably, the inert plasma used to pre-treat the substrate surface and/or generate the vertically branched graphene structure is an Ar plasma. The inert plasma used to pre-treat the surface may be the same as the inert plasma used to generate the vertically branched graphene, or may be different.

Preferably, the Ar plasma is applied at a pressure of 0.5-2 Pa.

Preferably, the Ar plasma is applied at a radio frequency of 10-15 MHz.

Preferably, the Ar plasma is applied at a power of 500-.

Preferably, the Ar plasma is applied at a radio frequency of 10-15MHz and a power of 500-2000W at a pressure of 0.5-2Pa, or more preferably, at a radio frequency of 13.56MHz and a power of 1000W at a pressure of 1.5 Pa.

Preferably, the substrate surface is pretreated for a period of 10 minutes.

Preferably, the deposition plasma comprising the carbon source is prepared by introducing a single carbon source gas and hydrogen gas to the surface of the substrate without interrupting the inert plasma. The carbon source gas may be methane. Preferably, the deposition plasma is at a pressure of less than 2 Pa.

In a preferred embodiment, the deposition plasma is a plasma comprising argon, methane and hydrogen at 1.5-1.8 Pa.

Preferably, the depositing of the vertical graphene is performed without external heating of the substrate, and wherein the only heating applied is heating by plasma. Preferably, the deposition plasma is introduced directly as a dry feed gas or as a feed gas supplied by the manufacturer, without bubbling through water. The deposition of the vertical graphene is preferably performed at <600 ℃.

Preferably, the deposition of the vertical graphene is performed at 400 ℃. Preferably, the deposition of the vertical graphene is performed for at least 10 minutes, more preferably 10-20 minutes.

Preferably, after step b) but before step c), the pressure is reduced to<2x10^-2Pa。

In a fourth aspect, the present invention provides a vertically branched graphene attached to a substrate surface, the vertically branched graphene having a bulk (trunk) portion extending from the substrate surface, the bulk having a degree of branching that increases with increasing distance from the substrate surface.

The vertically branched graphene may be prepared by the method of any one of the first, second or third aspects.

The vertically branched graphene preferably has the following raman spectrum I_D/I_GRelative ratio of peak intensities:1.1 or higher, more preferably 1.4 or higher, even more preferably 1.7 or higher and most preferably 1.9 or higher.

In a fifth aspect, the present invention provides a free-standing branched graphene having a proximal end and a distal end, the proximal end comprising a body portion, the body portion having a degree of branching, and the degree of branching increasing with increasing distance from the proximal end and decreasing distance from the distal end.

The free-standing branched graphene may be prepared by removing the vertically branched graphene of any of the foregoing aspects from the substrate.

The free-standing branched graphene preferably has the following raman spectrum I_D/I_GRelative ratio of peak intensities: 1.1 or higher, more preferably 1.4 or higher, even more preferably 1.7 and most preferably 1.9 or higher.

According to other aspects of the invention, there is provided a catalyst support comprising a vertically-branched graphene or a freestanding-branched graphene of the invention. The catalyst support is also provided in an electrocatalytic process (e.g. hydrogen production or oxygen production) or in the production of a liquid carbon product (e.g. formed from CO)₂Production of one or more of reduced n-propanol, ethanol or formate). Catalyst supports comprising the branched graphene of the present invention may also be used in energy storage devices such as batteries or supercapacitors.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is, it is to be interpreted in the sense of "including, but not limited to".

The term "vertical graphene" as used herein refers to a planar graphene sheet or array of sheets fixed to a substrate and substantially orthogonal to the substrate. Such arrays may also be referred to as "pristine vertical graphene. The term "vertical graphene" or "pristine vertical graphene" as referred to in this specification refers to a planar graphene sheet or array of sheets fixed to and substantially orthogonal to a substrate, having the morphology of an interconnected network of vertical graphene sheets.

The term "vertically branched graphene" as used herein refers to a branched graphene of the present invention, wherein the vertical graphene or pristine vertical graphene has been modified to exhibit branching.

The term "free-standing branched graphene" as used herein refers to a vertically branched graphene that is not immobilized on a substrate.

Brief description of the drawings:

fig. 1 shows a schematic of the two-stage process of the present invention, which includes the formation of original vertical slices and subsequent branching of the original vertical slices.

Fig. 2 shows SEM images of pristine vertical graphene deposited without post-treatment of step c) above, looking down towards the deposition substrate. In the SEM image, the edges of the vertical graphene can be seen as lighter lines on the surface.

Fig. 3 shows an SEM image of vertical graphene after being subjected to an inert plasma for about 5 minutes. The overall vertical graphene appearance is retained, but it can be seen that the edges are no longer purely linear sheet edges with smooth texture, but exhibit some degree of branching.

Fig. 4 shows an SEM image of vertical graphene after being subjected to an inert plasma for about 10 minutes. The overall perpendicular graphene appearance is retained, but the degree of branching is more pronounced compared to fig. 2.

Fig. 5 shows an SEM image of vertical graphene after being subjected to an inert plasma for about 20 minutes. The overall vertical graphene appearance is retained, but the degree of branching is more pronounced than in fig. 3 and 4, and it can be seen that at some locations the branches begin to cross, indicating that a branched layered structure is approaching.

Fig. 6 shows raman spectra of vertical graphene after inert plasma post-treatment for about 1 minute, 5 minutes, 10 minutes and 20 minutes. When the post-treatment period was extended, I was observed_D/I_GIncrease in the ratio of peak intensities.

Figure 7 shows XPS of vertical graphene after inert plasma post-treatment for about 1min, 5 min, 10 min and 20 min. The oxygen content in all vertical graphene after about 1, 5, 10 and 20 minutes of treatment (which is generally considered as an impurity in the vertical graphene material) is kept at a low level of about 1%.

FIG. 8 shows the scan rates at 2mV/s, 5mV/s, 10mV/s, 20mV/s, 50mV/s, 100mV/s, and 200mV/s when assembled into a vertical graphene electrode with symmetrical unbranched or pristine graphene electrodes and 1MLi₂SO₄Electrochemical Cyclic Voltammetry (CV) measurements of unbranched or pristine vertical graphene in a 2032 type coin cell of electrolyte.

FIG. 9 shows the scan rates at 2mV/s, 5mV/s, 10mV/s, 20mV/s, 50mV/s, 100mV/s, and 200mV/s when assembled into a vertical graphene electrode with symmetrical unbranched or pristine graphene electrodes and 1MLi₂SO₄Electrochemical Cyclic Voltammetry (CV) measurements of vertically branched graphene after approximately 1 minute, 5 minutes, 10 minutes, and 20 minutes of post-treatment in a 2032 type coin cell of electrolyte.

Fig. 10 shows SEM images of Ni-Fe nanoparticle catalysts electrodeposited on vertically branched graphene after about 1 minute post-treatment looking down towards the deposition substrate.

Figure 11 shows OER catalytic performance of pristine vertical graphene, Ni nanoparticle catalyst electrodeposited on carbon fiber paper, Ni nanoparticle catalyst electrodeposited on pristine vertical graphene, Ni-Fe nanoparticle catalyst electrodeposited on pristine vertical graphene, and Ni-Fe nanoparticle catalyst electrodeposited on vertically branched graphene. Among the catalysts tested, Ni-Fe nanoparticle catalysts electrodeposited on vertically branched graphene showed the best OER performance by producing the highest current density at fixed applied potential (vs. Reversible Hydrogen Electrode (RHE)) or the lowest applied potential (vs. Reversible Hydrogen Electrode (RHE)) at fixed current density.

Fig. 12 shows copper (Cu) versus CO electrodeposited on graphite paper substrate, raw VG substrate and branched VG substrate after 1 minute post-treatment₂Faradaic efficiency of the product. After 1.5 hours, in the catalyst tested at an applied potential (vs. rhe) of 1.2, copper electrodeposited on vertically branched graphene treated after 1 minute showed for CO₂Reduction ofThe highest faradaic efficiency of the product, and the highest proportion of C2+ products (e.g., ethylene, ethanol, and n-propanol). About 31% of all carbon products produced consisted of C2+ species.

Figure 13 shows a spherical Aberration Corrected Scanning Transmission Electron Microscope (ACSTEM) image of platinum (Pt) single atoms immobilized on vertically branched graphene post-treated for 5 minutes. Using H₂PtCl₆As a precursor, Pt monoatomic atoms were fixed by a dip-annealing method.

FIG. 14 shows pristine vertical graphene at 1 μ gcm^-2Pt monoatomic on pristine vertical graphene, and at 1 μ gcm^-2Hydrogen Evolution Reaction (HER) performance of Pt monoatomic ions immobilized on vertically branched graphene after 5 minutes post-treatment at 0.5MH₂SO₄The test was performed in a three electrode configuration of the electrolyte. Among the catalysts tested, the Pt monoatomic atom immobilized on vertically branched graphene post-treated for 5 minutes showed the best HER performance by producing the highest current density at a fixed applied potential (vs. rhe), or the lowest applied potential (vs. rhe) at a fixed current density.

Detailed description of the invention

The vertically branched graphene of the present invention may be produced in any suitable Plasma Enhanced Chemical Vapor Deposition (PECVD) apparatus.

In a general procedure of the present invention, a substrate is first loaded into a deposition chamber of a standard PECVD apparatus. Similar to the previously described methods for making pristine vertical graphene, a wide variety of substrates may be used herein-for example, metal substrates such as stainless steel, copper, aluminum, or nickel may be used. Alternatively, the substrate may be a semiconductor, such as a silicon wafer, or an insulator, such as quartz or sapphire, may be used. Even carbon may be used as a substrate. In general, any suitable substrate capable of withstanding temperatures of about 400 ℃ may be used.

The substrate may also be present in any desired form. In general, forms with a relatively high surface area/volume, such as foil, wafer, paper, foam or fiber, may be used. Typically, a conventional metal foil may be used, but other substrates may also be used, such as silicon wafers, quartz or sapphire wafers, carbon paper or carbon fibers, or nickel or copper foams. The branched graphene thin films of the present invention have been successfully prepared on carbon fiber paper (and nickel foam) as a common substrate for electrochemical water splitting.

Importantly, the process does not require the presence of a catalyst at all.

Once the substrate is loaded into the chamber and the chamber is sealed, the pressure is pumped down to less than about<2x10^-2Medium-high vacuum of Pa.

It is important at this stage that the substrate be pretreated with plasma to clean the substrate. Any suitable type of cleaning plasma may be used, typically an inert plasma, such as an Ar plasma or a He plasma. However, Ar plasma is generally most practical for the pre-treatment stage, as it is used later in both the vertical graphene synthesis stage and the post-treatment (branching) stage.

The pre-treatment stage is important because without Ar plasma pre-treatment, it is observed that the vertical graphene adheres relatively weakly to the substrate, which can be problematic for applying the vertical graphene in device fabrication. When the surface is pretreated with Ar plasma, adhesion to the substrate is generally stronger than untreated or with H₂Those of plasma pretreatment. By H₂Plasma pretreatment results in unacceptably poor adhesion of graphene to the substrate surface.

For the growth step, Ar is typically introduced into the chamber at a flow rate of 10 standard cubic centimeters (sccm) and the pressure is adjusted to 1.5 Pa. An Ar plasma with a certain radio frequency (13.56MHz) at a power of 1000W was ignited to pretreat the substrate surface for 10 minutes. The radio frequency and power are chosen because these are commonly used in commercial plasma devices. The flow, pressure, frequency and power, as well as the treatment time, can all be varied in accordance with standard practice in the art to generate a cleaning plasma for pretreatment.

A preferred gas for depositing graphene in the present invention is methane, as compared to alternative carbon source gases such as acetylene. Methane is low carbon and high thermal stabilityIn an amount to form a relatively clean plasma. In this case, non-carbonaceous species (e.g. halogen compounds, such as C) are contained that can react or be incorporated into the graphene material₂F₆) Is completely unsuitable as a deposition reagent and is of course not provided during the branching step, which does not contain any added carbon source gas.

In some deposition processes, it is desirable to bubble a feed gas (e.g., methane, acetylene, argon, etc.) through water prior to introducing it into the deposition chamber. The purpose of this step is to generate a humid gas, where ionization of water molecules helps to clean unwanted species in the growing vertical graphene. In the present case, this additional step of generating a moist feed gas is not necessary, and therefore, preferably only a dry feed gas (or feed gas in the supplied state) is used in the pretreatment, deposition and branching steps.

After the pretreatment stage, the argon stream is desirably supplemented with both the methane stream and the hydrogen stream in an uninterrupted manner. Argon, methane and hydrogen were each introduced into the chamber at a flow rate of 10sccm while being subjected to 13.56MHz at 1000W. The role of this carbon plasma (deposition plasma) is to form pristine or unbranched vertical graphene. Furthermore, the flow, pressure, frequency and power, as well as the processing time, can all be varied according to standard practice in the art to produce a carbon plasma for generating vertical graphene. Pristine vertical graphene is substantially unbranched. This method of forming vertical unbranched graphene has been reported in the literature, for example, Han et al, j.mater.chem.a5,17293 (2017); bo, Mao, Han et al, chem.soc.rev.44,2108 (2015).

During the phase of forming the pristine vertical graphene, the pressure in the chamber increases slightly to about 1.5 to 1.8 Pa. The growth temperature of the vertical graphene was measured using a remote infrared thermometer and was about 400 ℃.

The growth time of pristine vertical graphene is about 10-20 minutes. In a plasma environment, approximately 10-20 minutes produces the best vertical graphene density due to the interaction of vertical graphene growth and etching. After that time, turn offThe rf power is turned off and the gas flow is turned off. The pressure in the plasma chamber is again pumped down to as low a pressure as practical, usually<2x10^{- 2}Pa. The purpose of this evacuation step is to remove any residual carbon source in the vacuum chamber.

Then, in the absence of any incorporated carbon source, the pristine vertical graphene is subjected to a post-carbon treatment step, which results in branching of the pristine vertical graphene.

After evacuating the chamber after depositing the pristine vertical graphene, argon gas was again introduced into the chamber at a flow rate of 10 standard cubic centimeters (sccm) and the pressure was again adjusted to 1.5 Pa. Approximately 0.5-2Pa may be used as the pressure in the branching step. 1000W of radio frequency (13.56MHz) power was applied to generate an Ar plasma. In the growth step for preparing the pristine vertical graphene, a plasma is generally generated using Ar as a carrier gas. In the post-carbon treatment step of the method of the invention, the ion bombardment effect (sputtering and redeposition) of the Ar plasma is used to form the branches. H₂The method is generally used in the growth of original vertical graphene to obtain an etching effect, and the method is helpful for growing a graphene structure with better quality. H₂The exact effect of (a) is difficult to investigate and depends on the particular growth conditions.

While any suitable inert gas containing plasma may be used for the pre-treatment step, an Ar plasma is preferred for the post-treatment step. It has been found that Ar plasmas are effective for branching because Ar ions can bombard and knock out carbon atoms in the vertical graphene, which results in a vertically branched graphene structure.

The nitrogen gas that can be used to generate a relatively inert plasma is relatively undesirable as a plasma in the branching step because it can dope the graphene, leading to different series of experimental results. Other reactive plasmas (e.g., oxygen plasmas) are also apparently unsuitable because they cause chemical changes in graphene.

Depending on the desired degree of branching, the post-treatment or branching step is carried out for a predetermined time, typically from 1 to 20 minutes. It is noted that less than 5 minutes will produce some branching, but the branching process is far from complete.

Post-treatment for longer than 30 minutes typically results in branched graphene with poor electrical properties due to the introduction of high density disorder in the graphene structure.

Typically, about 5-30 minutes may be used to produce branched graphene. Preferably, about 10-20 minutes under the described conditions is typically employed to produce high quality, highly branched graphene.

It is important to note that in the present invention, the formation and branching process of the pristine graphene is performed as separate steps, with the branching step being performed in the absence of an introduced carbon source (i.e., the branching step is performed in the absence of any introduced graphene-forming materials (e.g., methane, ethane, ethylene, acetylene, etc.). Ideally, the deposition and branching of pristine vertical graphene is performed in the same apparatus, but this is not required. According to the present invention, raw vertical graphene can be prepared at one site, stored at a separate site and then processed to a branching step, if necessary.

The prior art methods involving the "one-step" formation of branched graphene, in which the formation and branching of the perpendicular graphene is carried out in a single step in the presence of a carbon source, suffer from the requirement that any branching step be carried out in the presence of a carbon source. This means that the branching and deposition processes are mixed, leading to poor results. In such one-step processes, high quality pristine vertical graphene is not formed as a discrete intermediate, rather the formation of vertical graphene sheets is in competitive equilibrium with branching from the outset. In these one-step processes, the graphene does not necessarily undergo branching at the uppermost edge of the sheet, but rather branching occurs at the bottom just as at the top. As a result, the one-step process tends to result in "dirtier" branched materials.

Furthermore, the inventive method very clearly separates the formation of "coarse" structures (of pristine vertical graphene sheets from the formation of "fine" structures (of branches), so that a relatively wide range of deposition and branching conditions can produce consistent and controllable branched graphene products. In a one-step process, the coarse structure formation step is combined with the fine structure formation step, and a change in one parameter can result in unpredictable changes in the vertical slices or branched structures. Thus, the two-step process of the present invention provides more control than the one-step process.

In fact, the inventors have observed that if during the second step (branching step) a carbon source is present, it is difficult to obtain branching. Without being bound by theory, this observation suggests that the mechanism of branching works through ion bombardment (etching) and redeposition of the existing pristine vertical graphene.

Without wishing to be bound by theory, it is believed that the plasma ion bombardment effect results in the removal of carbon from the original graphene and the simultaneous redeposition of carbon atoms under the influence of a plasma-induced electric field. As a result, smaller graphene nanoplatelets emanate from each original graphene sheet, forming a vertically branched graphene structure.

Branched graphene exhibits a layered morphology, consisting of a skeleton of an interconnected network of perpendicular graphene sheets and smaller graphene nanoplatelets emanating from the skeletal graphene sheets, making it a branched structure. Each branch may in turn support a smaller graphene sheet, similar to a feather-like configuration with a central axial skeleton, patterned like an interconnected network of perpendicular graphene sheets (formed during the growth step), with vanes or barbs at the edges. Although the appearance of the structure is feathery, the framework remains similar to that of the original perpendicular graphene. As can be observed from the provided figures, the morphology of the perpendicular graphene backbone is observed to remain unchanged during the post-carbon treatment, branching step.

The degree of branching of the perpendicular graphene is observed by an electron microscope and can be quantified by analyzing the raman spectrum of the graphene layer. With increasing degree of branching, I_D/I_GThe ratio increases. In pristine vertical graphene thin films deposited on nickel foil without branching, I_D/I_GThe ratio is typically about 1: 1.

As the work-up continues, as can be seen by microscopy, the degree of branching increases, while I can be observed_D/I_GThe ratio increases. See fig. 6. As can be seen in fig. 6, the relative ratios change over time, indicating a greater degree of disorder.The results are shown in Table 1 below.

TABLE 1I_D/I_Gv. post-treatment time

Post-treatment time (min)	ID/IG
		1	1.1
5	1.4
		10	1.7
20	1.9

Once formed, the vertically branched graphene may be removed by any suitable means, for example, by means of a blade or other scraping tool (removal) if a powder form is desired. Alternatively, if a suspension of perpendicular graphene is desired, it can be removed by: the vertically branched graphene is removed to produce free standing vertical graphene is very convenient compared to removing flat graphene sheets from a forming substrate.

The free-standing branched graphene appeared as a black powder. XPS spectrum of independent branched graphene shows that the purity is high, the average carbon content is over 99 percent, and oxygen is hardly contained.

The density of the edge planes of the perpendicular graphene can increase by more than a factor of 10 during the post-carbon treatment step, thus allowing a range of catalysts to be deposited at high mass loading using a range of reaction conditions. This configuration also has the added advantage of allowing the reagent to mix with the deposited catalyst. A particular advantage is that the monatomic catalyst is deposited on a substrate that is preferably fixed on the edge plane of the perpendicular graphene.

Examples

Synthesis of Vertically branched graphene (growth step)

The vertically branched graphene is synthesized in a Plasma Enhanced Chemical Vapor Deposition (PECVD) method without using any catalyst. Specifically, the substrate is loaded into the chamber and the pressure is pumped down to<2x10^-2Pa. The substrate may be a metal foil, such as stainless steel, copper, aluminum, or nickel; semiconductors such as silicon wafers; an insulator such as quartz or sapphire; carbon paper; carbon fibers; nickel foam; or copper foam. Then, Ar was introduced into the chamber at a flow rate of 10 standard cubic centimeters (sccm) and the pressure was adjusted to 1.5 Pa. An Ar plasma with a certain radio frequency (13.56MHz) at a power of 1000W was ignited to pretreat the substrate surface for 10 minutes.

After that, both methane and hydrogen were added to the chamber at a flow rate of 10sccm to synthesize pristine vertical graphene without interrupting the plasma. The pressure during the synthesis increased slightly to 1.5-1.8 Pa. The growth temperature was measured using an external infrared thermometer and found to be about 400 ℃. After another 10 minutes, the plasma was turned off and all gases were turned off.

Synthesis of Vertically branched graphene (post-treatment step)

Then the vacuum chamber is pumped out to<2x10^-2Pa. Ar was introduced into the vacuum chamber at a flow rate of 10 standard cubic centimeters (sccm) and the pressure was again adjusted to 1.5 Pa. A 1000W Ar plasma at radio frequency (13.56MHz) power was activated to post-treat the vertical graphene for a specified time (e.g., 1 minute, 5 minutes, 10 minutes, or 20 minutes). Smaller due to plasma ion bombardment effect and redeposition of carbon atomsGraphene nanoplatelets can emanate from pristine graphene sheets to form a vertically branched graphene structure. The height of the vertically branched graphene can be controlled by the height of the pristine graphene, which can be varied by a number of plasma processing parameters (e.g., deposition time, flow rate, plasma power, and pressure).

Applications using vertically branched graphene

Carbon powder is a well-known material in electrochemical devices, and the electrochemical properties of the vertically branched graphene of the present invention were investigated.

Using symmetric pristine vertical graphene electrodes and 1MLi as electrolyte₂SO₄And constructing the button cell. The vertically branched graphene of the present invention was used to construct otherwise identical cells.

Cyclic Voltammetry (CV) for each cell was plotted at scan rates of 2mV/s, 5mV/s, 10mV/s, 20mV/s, 50mV/s, 100mV/s, and 200mV/s (curves). The vertically branched graphene shows similar impedance and stability to the original vertically branched graphene, but exhibits a specific capacitance (Cs)1.5 times higher than that of the original unbranched graphene.

Preliminary data also show that the free-standing branched graphene powder performs better in energy storage devices, as well as hydrogen generation and CO₂A reduced catalyst support.

Non-noble metal catalysts of Ni and Ni-Fe alloys were electrodeposited on both pristine vertical graphene (on carbon fiber paper) and vertically branched graphene (on carbon fiber paper). Their catalytic performance for Oxygen Evolution Reaction (OER) was tested in alkaline 1m koh electrolytes.

As can be seen in fig. 11, Linear Sweep Voltammetry (LSV) shows that among the original vertical graphene (on carbon fiber paper), the Ni catalyst electrodeposited on carbon fiber paper, the nickel catalyst electrodeposited on the original vertical graphene (on carbon fiber paper), and the Ni-Fe catalyst electrodeposited on the original vertical graphene (on carbon fiber paper), the Ni-Fe catalyst electrodeposited on branched graphene has the best OER performance.

Various branched stones are investigated and researchedGraphene as catalytic carrier in CO₂In electrochemical reduction. The results are presented in fig. 12, fig. 12 shows copper (Cu) versus CO electrodeposited on a graphite paper substrate, a pristine vertical graphene (on graphite paper) substrate, and a vertically branched graphene (on graphite paper) substrate after 1min post-treatment₂Faradaic efficiency of the product. After 1.5 hours, copper electrodeposited on vertically branched graphene (on graphite paper) after 1min in the catalyst tested at an applied potential (vs. rhe) of 1.2 was shown to be for CO₂The highest faradaic efficiency of the reduction products, and the highest proportion of C2+ products (e.g., ethylene, ethanol, and n-propanol). About 31% of all carbon products produced consisted of C2+ species.

Table 2 lists the CO of copper (Cu) electrodeposited on a graphite paper substrate, a vertical graphene (on graphite paper) substrate, and a vertically branched graphene (on graphite paper) substrate post-treated for 1min₂And (5) reducing the result. After 1.5 hours, of the catalysts tested at an applied potential (vs. rhe) of 1.2, the copper electrodeposited on the vertically branched graphene (on graphite paper) after 1min of treatment showed the highest total average current density for CO₂Highest average current density of reduction products and highest amount of CO produced₂Reduction products, including both gaseous and liquid products, include C2+ products.

Thus, non-noble metal catalysts of Cu nanoparticles electrodeposited on vertically branched graphene in CO compared to Cu catalysts electrodeposited on pristine vertical graphene and carbon fiber paper₂Higher yields of liquid carbon products (e.g., n-propanol, ethanol, and formate) are obtained in the reduction of (a).

In further embodiments, the present invention may be used to prepare hydrogen-producing catalysts.

Pristine vertical graphene was prepared as described above, followed by 5 minutes post-treatment (branching). Subsequently, using H₂PtCl₆The subsequent vertically branched graphene is subjected to a dip-annealing process as a precursor to produce fixed Pt monoatomic atoms on the graphene surface. FIG. 13 shows a spherical aberration correction sweep of platinum (Pt) monoatomic atoms immobilized on vertically branched grapheneTransmission Electron Microscope (ACSTEM) images were drawn.

FIG. 14 shows pristine vertical graphene at 1 μ gcm^-2Pt monoatomic on pristine vertical graphene, and at 1 μ gcm^-2Hydrogen Evolution Reaction (HER) performance of Pt monoatomic immobilization on vertically branched graphene after 5 minutes post-treatment at 0.5MH₂SO₄The test was performed in a three electrode configuration of the electrolyte. HER is a well-known process in the art involving H₂Conversion of O to H₂And O₂. As described herein, catalytic performance (i.e., H generated) can be observed by current density at a fixed applied potential (vs. rhe) or applied potential (vs. rhe) at a fixed current density₂Amount of). Among the catalysts tested, the Pt monoatomic atom immobilized on vertically branched graphene post-treated for 5 minutes showed the best HER performance by producing the highest current density at a fixed applied potential (vs. rhe), or the lowest applied potential (vs. rhe) at a fixed current density.