RU2738818C1 - Dry synthesis of graphene from liquid reagents - Google Patents

Abstract

FIELD: nanotechnology.

SUBSTANCE: plasma-generating flow containing initial carbon-containing material and buffer gas is supplied to plasmatron. Carbon-containing material used is carbon-containing liquid, which is introduced under nitrogen pressure into a stream of buffer gas to form a plasma-forming stream at a ratio of carbon-containing liquid: buffer gas of 1:(3÷5) determined by spectral analysis. For conversion in a plasma jet, the formed stream in the form of aerosol is fed into a discharge gap of the plasmatron at low pressure. Products of high-temperature dissociation are subject to sharp cooling.

EFFECT: invention enables synthesis of graphene without using a substrate, use of systems for converting liquid into vapor, transfer of synthesized graphene and purification thereof of undesirable impurities and thereby simplifying the process; besides, requirements to initial raw materials are reduced due to use of wide spectrum of carbon-containing liquids, including wastes.

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Description

The invention relates to the field of nanotechnology and is intended for the production of graphene materials. The obtained nanostructured material can be effectively used in electronics, automotive industry, aerospace vehicles, in the field of small-scale power engineering, for cleaning systems, and fire retardant coatings.

Graphene is theoretically one pure layer of graphite, even though the term graphene is used to refer to a material with multiple stacked atomic layers of graphite, or a graphite layer with minor defects that retains material properties similar to pure graphene.

Due to the successful development of methods for obtaining graphene materials, interest in research and their practical use has sharply increased.

Recently, methods for the synthesis of graphene have appeared, in which ethanol and other alcohols were used as a reagent. This direction is associated with the possibility of using alcohols formed during the fermentation of agricultural waste. In [LiZ., ZhuH., WangK., WeiJ., GuiX., LiX., LiC, FanL., SunP., WuD. Ethanolflamesynthesisofhighlytransparentcarbonthinfilms // Carbon. - 2010. - V. 49. - N. 1. - P. 237-241.] Thin carbon films were obtained on a copper substrate in the internal flame of an alcohol burner. Also in the flame of the burner, but in the upper (hot) part, received graphene layers of nickel foil doped with nitrogen [Zhang Y., Cao B., Zhang B., Qi X., Pan C. The production of hydrogen-dopedgraphenefrommixedamineplusethanolflames // ThinSolidFilms. - V. 520. -N. 23. - P. 6850-6855]. Ethanol and a mixture of ethanol and ammonia were used as reagents. Authors [Matsushima M, Noda M, Yoshida T., Kato N., Kalita G., Kizuki T., Uchida N., Umeno M, Wakita K. Formation of graphenenano-particle by means of pulsed discharge to ethanol // J. Appl. Phys. -V. 113. - N. 11. - P. 114304.] proposed a method for obtaining a suspension of graphene flakes in ethanol by passing a pulsating discharge through alcohol in an argon atmosphere. It is shown that by varying the frequency of the transmitted current, one can control the size of graphene nanoparticles, as well as the number of layers in them. In the work [Hagino T., Kondo N., Ishikawa K., Kano N., Sekine M, Hori M. Ultrahigh-speedsynthesisofhanographeneusing alcoholin-liquidplasma // Appl. phys. expr. -2012. - V. 5. - N. 3. - P. 035101.] liquid alcohols were exposed to a nonequilibrium plasma, and a suspension of graphene nanoparticles was also obtained. It is also known to use the CVD method for the synthesis of graphene materials from alcohol. The authors [DerrickHam W., AlfredTok I. Theextendedgrowthofgrapheneoxideflakes usingethanol CVD // Nanoscale. - 2013. - V. 5. - N. 7. - P. 2945-2951.] It is proposed to synthesize films of highly aligned graphite (FOG) directly on the surface of SiO2, previously coated with graphene oxide (GO) flakes. As a result of this work, it was shown that during the high-temperature decomposition of ethanol, the initial GO islands are reduced and act as nucleation centers, initiating further film growth. It has been demonstrated that FOG synthesized by the proposed method has a lower defect density compared to samples obtained by other methods (thermal reduction or hydrazine reduction). Authors [John R., Ashokreddy A., Vijayan C, Pradeep T. Single-and-new-layergraphenegrowthonstainlesssteelsubstratesbydirectthermalchemicalvapordeposition // Nanotechnology. - V. 22. - N. 16. - P. 165701.] a graphene film, consisting of one, two or three layers, was deposited from alcohol vapor on the surface of stainless steel. It is shown that the inhomogeneity of the obtained material is due to the presence of manganese and chromium oxides in the substrate material. The authors [Paul R.K., Badhulika S., Niyogi S., Haddon R.C., Boddu V.M., Costales-Nieves C, Bozhilov K.N., Mulchandani A. The productionofoxygenatedpolycrystallinegraphenebyone-stepethanol-chemicalvapordeposition // Carbon. - 2011. - V. 49. - N. 12. - P. 3789-3795.] Found that a carbon film synthesized from ethanol vapor in a stream of hydrogen at 1000 ° C has a polycrystalline structure with an average crystallite size of 2-5 nm ... Such a film contains a small amount of carboxyl groups (3%), which facilitates the subsequent functionalization and attachment of polymer matrices to graphene layers for their further use in various applications, and also determines the p-type conductivity of this material. In [ChenX., ZhaoP., HouB., EinarssonE., ChiashiS., MaruyamaS. Growth of Single-LayerGrapheneUsingAlcoholCatalyticChemicalVaporDepositiononCuSubstrate // APS MarchMeetingAbstracts. - P. 6001.] noted that in a high vacuum (100 Pa), the process of deposition of a graphene film from ethanol vapors at 1000 ° C on the surface of a copper substrate from ethanol vapors is self-limited, and even with prolonged synthesis, the thickness of the resulting film is consistent with a carbon monolayer. The authors [Miyata Y., Kamon K., Ohashi K., Kitaura R., Yoshimura M., Shinohara H. A simple alcohol-chemical vapor deposition synthesis of single-layer graphenes using flash cooling // Appl. Phys. Lett. - V. 96. - N. 26. - P. 263105.] it was shown that high-quality graphene monolayers can be obtained only with extremely fast cooling of the substrate after synthesis. The deposition of a carbon film from ethanol vapors was carried out on a nickel substrate in an argon atmosphere. Under these conditions, nickel foil inevitably contains a natural oxide film on the surface, which hinders the penetration of carbon atoms through it, and then, instead of the process of dissolution of carbon in nickel, diffusion over the surface takes place. However, upon slow cooling after synthesis, carbon can still gradually dissolve in nickel, while instant cooling makes it possible to obtain a continuous monolayer of graphene on the surface. In [Nakamura A., Miyasaka Y., Temmyo J. Direct Growth Properties of Graphene Layers on Sapphire Substrate by Alcohol-Chemical Vapor Deposition // Japanese J. Appl. Phys. - V. 51. - N. 4S. - P. 04DN03.] On an atomically smooth sapphire surface, films with a thickness of several nm with an optical transmission of 80-90% and an electrical resistance of 2-7 kOhm / sq. Were obtained.

A.R., Algara-Siller G., Hackens В., Pardoen Т., Kaiser U., Dresselhaus M.S., Charlier J.-C, Raskin J.-P. CVD synthesis of mono-and few-layer graphene using alcohols at low hydrogen concentration and atmospheric pressure // Chem. Phys. Lett. - 2013. - V. 584. - P. 142-146] синтезировали моно-, двух- и трехслойный графен. Причем было показано, что для успешного получения качественного материала необходимо использовать предельно низкую (5%) объемную концентрацию водорода в газовой смеси. Отмечено также, что графеновые слои в получаемых образцах довольно слабо взаимодействуют между собой, что позволяет использовать их для получения монослоя графена путем расслоения. Тонкие графитовые пленки, состоящие из нанозерен многослойного графена были получены авторами [Miyasaka Y., Nakamura А., Temmyo J. Graphite thinfilms consisting of nanograins of multilayergrapheneonsapphiresubstratesdirectlygrownbyalcoholchemicalvapordep osition // Jap.J. Appl. Phys - V. 50- P. 04DH12.] на поверхности непланарной сапфировой подложки методом термического разложения этанола в присутствии водорода.When comparing various alcohols as precursors (methanol, ethanol, propanol), it was shown that films obtained from ethanol vapors have the best degree of crystallinity [Guermoune A., Chari T., Popescu F., Sabri SS, Guillemette J., Skulason HS , Szkopek T., Siaj M. Chemicalvapordepositionsynthesisofgrapheneoncopperwithmethanol, ethanol, andpropanolprecursors // Carbon. - V. 49. - N. 13. - P. 4204-4210.]. A comparison is also made of carbon films synthesized from pentane and ethanol vapors. Thin films of several graphene layers were obtained at atmospheric pressure on the surface of a copper substrate by bubbling a mixture of argon and hydrogen through a liquid organic precursor. It has been shown that ethanol is a preferred reagent, since it allows one to obtain GLP with a lower electrical resistance and a higher electron mobility (due to a smaller number of defects and a higher degree of structural perfection). By a similar method the authors [Campos-Delgado J.,

AR, Algara-Siller G., Hackens B., Pardoen T., Kaiser U., Dresselhaus MS, Charlier J.-C, Raskin J.-P. CVD synthesis of mono-and few-layer graphene using alcohols at low hydrogen concentration and atmospheric pressure // Chem. Phys. Lett. - 2013. - V. 584. - P. 142-146] synthesized mono-, two- and three-layer graphene. Moreover, it was shown that to successfully obtain a high-quality material, it is necessary to use an extremely low (5%) volume concentration of hydrogen in the gas mixture. It was also noted that the graphene layers in the obtained samples interact rather weakly with each other, which makes it possible to use them to obtain a graphene monolayer by exfoliation. Thin graphite films consisting of multilayer graphene nanograins were obtained by the authors [Miyasaka Y., Nakamura A., Temmyo J. Graphite thinfilms consisting of nanograins of multilayergrapheneonsapphiresubstratesdirectlygrownbyalcoholchemicalvapordep osition // Jap.J. Appl. Phys - V. 50- P. 04DH12.] On the surface of a nonplanar sapphire substrate by the thermal decomposition of ethanol in the presence of hydrogen.

Thus, the methods for obtaining graphene from alcohol can be divided into two groups. The first group includes: the synthesis of graphene by chemical vapor deposition, epitaxial growth on a metal surface, etc. They allow the formation of high-quality graphene, but are rather long and expensive, since they require the use of complex specific equipment and the fulfillment of strict technological conditions. To obtain graphene in free form, special procedures are required for separation from the substrate and cleaning. The second group combines such methods as micromechanical exfoliation of graphite, liquid-phase exfoliation of graphite, oxidation of graphite, etc. They are simpler to implement, but have significant drawbacks. This is, first of all, a small fraction of the yield of graphene of the required quality and the need for its purification from the accompanying material and used technological media [Sedlovets DM, Redkin AN, Korepanov V.I. Influence of synthesis conditions on the properties of thin carbon films obtained by pyrolysis of ethanol vapors // XIX Mendeleev Congress on General and Applied Chemistry. Collection of conference materials. - Volgograd, 2011. - S. 565.]. In addition, the micromechanical splitting of the lead, carried out for the first time, has become the foundation for a whole series of new techniques based on the possibility of graphite delamination. However, this method does not allow scaling the synthesis process, and at the same time it is not controllable, since the particles are small, irregular in shape and irregular structure. Thus, delamination methods cannot be used to fully exploit the extraordinary properties of graphene and its further commercial application.

A known method for the production of graphene in cryogenic liquids (see Mortazavi SZ, P. Parvin, Reyhani A. Fabrication of graphene based on Q-switched Nd: YAG laser ablation of graphite target in liquid nitrogen. LaserPhysicsReviewLetters, 2012, Vol. 9, No. 7 , P. 547-552 (prototype)), in which graphene is obtained by laser ablation of a graphite target placed in liquid nitrogen using a pulsed nanosecond Q-switched Nd: YAG laser.

The disadvantages of this method include the fact that it takes a long time (20 minutes) to obtain graphene, and the target surface is processed with a stationary laser beam, which limits the synthesis area by the boundaries of the radiation focusing spot. The combination of these disadvantages reduces the performance of the method under consideration.

Many of the current plasma technologies targeting graphene synthesis include plasma-assisted chemical vapor deposition (PECVD). These methods require substrates and low pressure media (below 10 Torr). The synthesis and growth of PECVD materials occurs via surface reactions and, therefore, depends on the conditions created by the substrate. In general, the disadvantage of this direction is the complexity of the preparation of the substrate surface, the use of expensive vacuum equipment, the presence of an additional stage of separating the structure from the substrate, which, in combination, increases the cost of the final product.

From the point of view of the simplicity of the method and the synthesis of graphene without substrates, of interest is the approach using the Aerosol Though-Plasma Method (Engineering Particles Using the Aerosol Though-Plasma Method), the so-called ATP method, which is usually implemented using an argon microwave or HF plasma at atmospheric and slightly reduced pressure.

A known method for the synthesis of graphene without using a substrate in the gas phase as a result of passing liquid drops of ethanol in argon plasma in a microwave plasma reactor with atmospheric pressure [Dato, A .; Radmilovic, V .; Lee, Z .; Phillips, J .; Frenklach, M. Substrate-freegas-phasesynthesis of graphenesheets. NanoLett. 2008, 8, 2012-2016]. The experiments were carried out in a microwave (2.45 GHz) plasma reactor at atmospheric pressure while passing an argon flow at a rate of 1.71 L / min. An aerosol consisting of argon gas (2 L / min) and ethanol drops (4 × 10 ^-4 L / min) was fed directly into the argon plasma. Within 10 ^-1 s, the ethanol droplets quickly evaporate and dissociate in the plasma, forming a solid product, which is collected on nylon membrane filters. The rate of obtaining the carbon material on the filters was 2 mg / min with the mass of introduced carbon in ethanol 164 mg / min. The disadvantage of this approach is the low rate of product formation, and the impossibility of scaling up the method, since the product must be washed out of the filter.

, J

, G Zhang, S Sun, A Perez, О Royuela,

-GagoandMDCalzadaPlasmaPhys. Control. Fusion 60 (2018) 014009] и подача уже паров в аргоновую плазму. Всей системе преобразования этанола через плазму аргона авторы дали название TIAGO. Скорость расхода Ar составляла 1 л/ мин. Расход этанола-2 г ч^-1, что не превышало 2% от объема смеси Ar-EtOH. Этанол испарялся с помощью управляемого смесителя испарителя (СЕМ, Bronkhorst) перед тем, как вводится в плазму. После зажигания плазмы Ar газовую смесь Ar-EtOH добавляли к плазме TIAGO через стальную трубку, нагретую до 110°С, для предотвращения конденсации этанола. Система состоит из регулятора потока жидкости Кориолиса (LIQUID-Flow®) для этанола, регулятор массового расхода (EL-FLOW®) для Ar, который действует как газ-носитель и устройство контролируемого смешивания / испарения. Этанол был помещен в небольшой резервуар под давлением гелия для обеспечения правильной работы регулятора расхода жидкости. Достоинство метода - отсутствие нейлоновых фильтров. Существенный недостаток - аппаратурно сложный метод, при котором помимо перевода жидкости в пар, необходимо поддержание 110°С в системе, чтобы предотвратить обратный процесс.A close technical solution for obtaining high-quality graphene based on the A-T-P method is the transfer of liquid ethanol to a vapor state [C Melero, R

, J

, G Zhang, S Sun, A Perez, O Royuela,

-GagoandMDCalzadaPlasmaPhys. Control. Fusion 60 (2018) 014009] and the supply of vapors to argon plasma. The authors gave the name TIAGO to the entire system for converting ethanol through argon plasma. The Ar consumption rate was 1 L / min. Ethanol consumption - 2 g h ^-1 , which did not exceed 2% of the volume of the Ar-EtOH mixture. Ethanol was evaporated using a controlled evaporator mixer (CEM, Bronkhorst) before being injected into the plasma. After ignition of the Ar plasma, an Ar-EtOH gas mixture was added to the TIAGO plasma through a steel tube heated to 110 ° C to prevent ethanol condensation. The system consists of a Coriolis fluid flow controller (LIQUID-Flow®) for ethanol, a mass flow controller (EL-FLOW®) for Ar which acts as a carrier gas and a controlled mixing / vaporization device. Ethanol was placed in a small pressurized helium tank to ensure proper operation of the fluid flow regulator. The advantage of the method is the absence of nylon filters. A significant drawback is a complex hardware method, in which, in addition to transferring liquid into steam, it is necessary to maintain 110 ° C in the system in order to prevent the reverse process.

Analysis of the literature showed that the use of ethanol as a carbon precursor seems to be quite promising, since ethanol can be obtained by fermentation of agricultural waste. The disadvantage of the known methods is their low productivity, and the impossibility of scaling them due to the peculiarities of the microwave discharge; increased requirement for the purity of alcohol; the complexity of using other liquid reagents, since it is necessary to take into account their physicochemical properties and adjust the equipment accordingly: a multistage system (for example, a mixture of argon with alcohol is fed into argon) does not allow complete control of the process.

Technically, the problem of efficient synthesis of graphene without the use of a substrate, including the supply of a plasma-forming flow containing an initial carbon-containing material and a buffer gas, to a plasmatron, conversion in a plasma jet, and subsequent sharp cooling of the products of high-temperature dissociation, consists in the fact that a carbon-containing liquid is used as a carbon-containing material, which is introduced under nitrogen pressure into a stream of buffer gas to form a plasma-forming flow at a ratio of carbon-containing liquid: buffer gas of 1: (3 ÷ 5), determined using spectral analysis, and for conversion, the formed stream in the form of an aerosol is fed into the discharge gap of the plasmatron at reduced pressure.

At the same time, unlike the prototype, to create a plasma-forming flow, additional mixing with a plasma-forming gas (argon) is not required, no additional systems are required to convert the liquid into a vapor state. The formed cold flow is fed into the discharge gap of the plasma generator at a rate of 3-3.5 g⋅s ^-1 , where it undergoes high-temperature dissociation. The resulting plasma flow is rapidly cooled in the volume of the reactor. With the experimentally found ratio of components, a significant amount of carbon can condense in the core of the vapor-gas flow, which leads to the appearance of a region oversaturated with vapor of the condensing component - the region of formation and growth of graphene, which ensures the scalability of the process. Moreover, temperatures of 8000-10000 K are created in the system, and this makes it possible to reduce the requirements for the quality of liquid reagents. The main way in which the research was carried out was a sharp increase in the efficiency of the scheme of the technological line "raw material - finished product bin". As a plasma-forming gas, you can use cheap nitrogen, pure helium, and, as in prototypes, argon.

The technical result of the invention is that in a cold aerosol-containing plasma-forming flow, the ratio of the carbon-containing liquid and the buffer gas is 1: 3 ÷ 5.

The claimed ratio of plasma gas and aerosol 1: 3 ÷ 5 provides a high rate of solid phase formation. Exceeding the found ratio will lead to discharge quenching. Reduction above 1: 5.0 in soot formation.

The proposed method has the following advantages:

1. Simultaneous injection of a carbon-containing source and a buffer gas into the plasmatron;

2. To simplify the technological scheme for obtaining graphene, to increase its efficiency;

3. Due to the direct supply to the discharge gap, the requirements for the feedstock are reduced, which makes it possible to use fermented agricultural waste;

4. The synthesized graphene is low-defect, low-layer, and provides the possibility of expanding its areas of application by reducing its cost.

The method is based on the introduction of a cold aerosol plasma flow into the discharge gap of the plasma generator. The carbon-containing liquid is introduced into the buffer gas stream to form a pressurized aerosol (mechanical or auxiliary gas), which compensates for possible pressure fluctuations in the flow and which allows the aerosol to be delivered to the center of the stream. Here, the liquid is sprayed with a flow of buffer gas, mixed with it, falling into the discharge gap at a reduced pressure (350-740 Torr), which then rises to atmospheric, quickly and completely evaporates, and then dissociates. This method eliminates the corrosive effect of the solution on the electrodes, the appearance of uncontrolled displacements of the plasma jet, disturbances in its combustion, and ensures the effective entry of the reagent into the plasma without the use of a carrier gas, a special system for drying aerosol.

The method for dry synthesis of graphene from liquid reagents is carried out as follows.

A carbon-containing liquid under the pressure of technical nitrogen is introduced into an argon stream along the flow and is sprayed with the formation of an aerosol. The flow rate of the buffer gas is 0.5-3.5 g / s. This creates a ratio of 1: 3 ÷ 5 liquid reagent: argon (helium, nitrogen) using spectral analysis, which determines the change in carbon concentration. It was established using electron microscopy that graphene structures are formed under these conditions. With the found ratio, the rate of graphene synthesis was 50 g / h.

Alcohols (ethanol), benzene, toluene, kerosene, etc. can be used as a carbon-containing liquid.

In the proposed method, there is a rapid cooling of cold drops in thermal plasma. As a result of the supersaturation of carbon vapors, graphene crystallization centers arise. Graphene grows in the reactor volume without the use of catalytic substrates, and the morphology of graphene is not inferior to the structures grown on a copper or nickel substrate by the CVD method.

The proposed method makes it possible to simplify the process of graphene synthesis, improve the manufacturability of the process, its productivity and reliability.

In fig. 1 shows the morphology of synthesized graphene using helium as a plasma-forming gas.

Examples of the implementation of the proposed method.

Example 1. Implemented a method for producing graphene using a mixture of liquid reagent: plasma gas = 1: 3. Argon is used as the plasma-forming gas, and the liquid reagent is ethanol. When ethanol is added to argon, a cold aerosol containing flow is formed, which is fed to the plasmatron. The argon consumption was 3.0 g / s. The pressure of the medium before the introduction of the liquid reagent was 350 Torr. When this mixture is converted, graphene is formed with a productivity of 50 g / h.

Example 2. Implemented a method for producing graphene using a mixture of liquid reagent: plasma-forming gas = 1: 5. Nitrogen is used as a plasma-forming gas, ethanol is a liquid reagent. When ethanol is added to argon, a cold aerosol containing flow is formed, which is fed to the plasmatron. The nitrogen consumption was 1.5 g / s. The pressure of the medium before the introduction of the liquid reagent was 350 Torr. When this mixture is converted, graphene is formed with a productivity of 30 g / h.

Example 3. Implemented a method for producing graphene using a mixture of liquid reagent: plasma gas = 1: 4. Argon is used as the plasma-forming gas, and the liquid reagent is ethanol. When ethanol is added to argon, a cold aerosol containing flow is formed, which is fed to the plasmatron. The argon consumption was 3.5 g / s. The pressure of the medium before the introduction of the liquid reagent is 710 Torr. When this mixture is converted, graphene is formed with a productivity of 30 g / h.

The inventive method allows the use of raw materials in the form of liquid carbon-containing waste for the synthesis of graphene.

Claims (1)

A method for synthesizing graphene without using a substrate, including supplying a plasma-forming flow containing an initial carbon-containing material and a buffer gas to a plasmatron, conversion in a plasma jet, and subsequent rapid cooling of high-temperature dissociation products, characterized in that a carbon-containing liquid is used as a carbon-containing material, which is introduced under pressure of nitrogen into the stream of buffer gas to form a plasma-forming flow at a ratio of carbon-containing liquid: buffer gas of 1: (3 ÷ 5), determined by spectral analysis, and for conversion, the formed stream in the form of an aerosol is fed into the discharge gap of the plasmatron at reduced pressure.

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