Applications of supercritical carbon dioxide in materials processing and synthesis

Abstract

Supercritical carbon dioxide (scCO₂) is carbon dioxide that is held beyond supercritical conditions of 31.1 °C and 7.4 MPa. As a non-toxic and environmentally benign green solvent, it has been widely used in the food and pharmaceutical industries for extraction. However, scCO₂ also has many unique properties and thus has great potential for advanced, green materials processing. This concise review focuses on its use as a solvent and an anti-solvent in materials processing and synthesis. Different experimental routes are described that are used to synthesize bulk materials, thin films, coatings, particle suspensions and powders. Examples from the literature are highlighted to illustrate the different experimental set-ups and applications of the resulting materials. This review endeavours to reveal the potential and versatility of scCO₂ in materials processing and synthesis, aiming to encourage a wider application of scCO₂ to open more opportunities in innovative green processing of both traditional and functional materials.

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Xiaoxue Zhang

Dr Xiaoxue Zhang received her BSc from Beijing University of Technology, China, and her MSc and PhD (with distinction) in materials science from Tampere University of Technology, Finland. She has over ten years' research experience both in academia and in industry. She has broad research interests and expertise in materials science, materials chemistry, ceramics, functional materials and materials characterization. She is specialized in sol–gel synthesis, supercritical carbon dioxide processing, superhydrophobic multifunctional materials, engineering ceramics and materials structural characterization. She is author of 25 peer-reviewed journal articles, including another review. She has an h-index of 11.

Saara Heinonen

MSc Saara Heinonen is a doctoral student in the Department of Materials Science at Tampere University of Technology (TUT), Finland. She received her BSc and MSc in ceramic materials from TUT. Her research focuses on functional materials, especially superhydrophobic, antibacterial and photocatalytic surfaces, and supercritical carbon dioxide assisted synthesis.

Erkki Levänen

Dr Erkki Levänen is Professor of ceramics materials and head of the Department of Materials Science at Tampere University of Technology, Finland. Professor Levänen's research interests are in functional ceramics especially in energy and environmental applications. His work ranges from materials synthesis to novel processing techniques and advanced characterization methods as well as application oriented research. The nanoparticle and thin film synthesis include sol–gel, supercritical carbon dioxide assisted synthesis and pyrolysis methods aimed at multifunctional materials with enhanced durability and self-recovery. Professor Levänen is currently author of 180 publications of which 48 are peer-reviewed.

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1. Introduction

Nanostructured materials have been adopted widely in many applications and top-down & bottom-up are two typical strategies to process them. The top-down approach starts with a bulk material and then breaks it into small pieces, while the bottom-up approach builds up nanoscale objects via chemical reactions between nanometric units such as molecular-scale precursors.¹ Despite of the many existing processing methods, it is still challenging to control the composition, structure and morphology of the obtained material. Besides, the processing often involves excessive use of solvents, high energy consumption and costly purification steps.² Therefore new processing method is needed in green processing of traditional materials or advanced processing of new materials. Compared to traditional processing, green processing requires non-use of toxic reagents and solvents, minimized ecological impact, reduced emission of potentially hazardous and toxic organic chemicals as well as non-production of toxic by-products. Meanwhile advanced processing refers to the development of new functional or multifunctional materials in different forms of 3D bulks, coatings, and powders for different applications.

To contribute to the realization of advanced green processing, this short review explains the basics and unique advantages of supercritical CO₂ in materials processing, and further reveals its versatility in synthesis of 3D bulks, coatings and powders for different applications. This review in overall provides a condensed view of the technique and aims to attract more scientists to work on this technique to process and synthesize their targeted structures.

1.1 What is supercritical CO₂ (scCO₂)?

A supercritical fluid is defined as a single phase which occurs when held above its critical temperature and pressure. Of the supercritical fluids, supercritical CO₂ has become a popular one due to its moderate critical temperature of 31.1 °C and pressure of 7.4 MPa (73.8 bar), as shown in Fig. 1.³ As comparison, supercritical water exists at temperature of above 374 °C and pressure of over 22.1 MPa, which consumes much more energy to realize.⁴ Supercritical CO₂ can act different roles of solvent, anti-solvent, solute and reaction medium in materials processing. This review mainly focuses on its use as a solvent and an anti-solvent to process nanostructured materials in forms of 3D bulk, coating, and powder. Excellent and comprehensive reviews to cover other roles of supercritical CO₂ in materials processing can be found elsewhere.^2,5–10

Fig. 1 Phase diagram of CO₂. Reprinted by permission from Macmillan Publishers Ltd: [Nature] (W. Leitner, Green chemistry: designed to dissolve, Nature, 2000, 405, 129–130), copyright (2000).

1.2 Why use of supercritical CO₂ in materials processing and synthesis?

In the supercritical region carbon dioxide becomes a supercritical fluid, a state that is neither a gas nor a liquid, having properties (e.g. density, viscosity, diffusion coefficient) intermediate between those of liquid and gas (see Table 1).¹¹ Supercritical CO₂ is superior in advanced materials processing. Firstly, the solubility of a solute in supercritical CO₂ is a function of the density of supercritical CO₂, which can be tuned by changing the pressure and temperature in the supercritical region. Secondly, the gas like viscosities accelerate the chemical reaction kinetics.¹² Thirdly, supercritical CO₂ has a zero surface tension (as shown in Fig. 2), which provides a good wetting of the surface to allow chemical reaction happen on the surface. It also facilitates a better penetration of the reactants into a porous structure. Besides, CO₂ is easy to separate from the synthesized materials by releasing pressure and the final products are dry without residuals. Therefore supercritical CO₂ processing often results in materials with superior properties such as higher surface area, better distribution of secondary material in matrix, less agglomeration and better-defined nanostructures than those obtained using conventional solvents at ambient pressure.

Table 1 Density, diffusion coefficient and viscosity of gaseous, supercritical and liquid CO₂

	Density (g cm^−3)	Diffusion (cm^2 s^−1)	Viscosity (g cm^−1 s^−1)
Gas	10^−3	10^−1	10^−4
Supercritical	10^−1 to 1	10^−4 to 10^−3	10^−4 to 10^−3
	Liquid like	Liquid like	Gas like
Liquid	1	<10^−5	10^−2

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Fig. 2 Surface tension of CO₂ in different temperature and pressure. Reprinted with permission from {R. Sui and P. Charpentier, Synthesis of metal oxide nanostructures by direct sol–gel chemistry in supercritical fluids, Chem. Rev., 112, 2012, 3057–3082}. Copyright {2012} American Chemical Society.

Besides, supercritical CO₂ has a number of positive impacts on green processing. Carbon dioxide is inert, non-flammable, non-toxic, inexpensive, and readily available from the by-products of many industrial processes. Therefore Supercritical CO₂ has been recognized as a non-toxic and environmental benign green solvent which could replace many solvents such as CCl₄, benzene and chlorofluorocarbons (CFCs) used in traditional materials processing.¹³ These solvents are typically carcinogenic, environmentally hazardous, volatile or legislatively regulated. Although carbon dioxide is a “greenhouse” gas, the CO₂ used in supercritical CO₂ processing can be obtained from other industrial chemical processes, which will not produce any additional carbon dioxide and would not contribute to greenhouse gas emission. Furthermore, the CO₂ used can be recycled and the use of supercritical CO₂ offers a way to recover industrial emissions prior to their release to the environment. Finally, supercritical CO₂ processing is energy efficient thanks to the moderate critical temperature and pressure.

1.3 Use of co-solvent in supercritical CO₂

In order to utilize supercritical CO₂ as a solvent or an anti-solvent in material processing and synthesis, it is important to understand the solubility of a solute in it. Generally supercritical CO₂ is a non-polar solvent, but it possesses a large quadrupole moment and a polar C=O bond which make a variety of materials (e.g. with hydroxide, carbonyl or fluoride groups) soluble.¹⁴ Water is not soluble in supercritical CO₂, however water can be utilized to produce water-in-CO₂ emulsion as a nanoreactor for materials synthesis. The details of water-in-CO₂ emulsion will be explained in Section 5.2. Polar molecules such as sugars and other inorganic salts are not soluble.¹⁵ Chelate complexes and organometallic compounds are soluble in supercritical CO₂.⁵ Complexes with fluorinated ligands have substantially higher solubility compared with their non-fluorinated analogs.¹⁶ In order to increase the solubility of reactants in supercritical CO₂, small amounts of co-solvents are often introduced to dissolve the reactants, thus making the solution soluble in supercritical CO₂. For example, alcohols such as ethanol,¹⁷ methanol,¹⁸ isopropanol,¹⁹ acetone,²⁰ hexane,²¹ formic acid²² and acetic acid²³ are miscible with scCO₂ and also have shown excellent solubility to a large amount of reactants and precursors. With the use of such co-solvents, sol–gel chemistry can be directly utilized in supercritical CO₂ by preparing the precursor solution using such co-solvent. In this way, the amount of organic solvents in materials synthesis can be largely decreased, and the resulted materials are also readily dried after venting the CO₂. Besides, under certain temperature and pressure, these co-solvents may also reach supercritical state. Table 2 gives the critical temperatures and pressures of some common fluids.² Crystalline oxides can also be prepared utilizing sol–gel precursor in supercritical CO₂ under temperature,²³ thus eliminating the post heat-treatment step.

Table 2 Critical temperatures and pressures of some fluids

	Tc (°C)	Pc (MPa)
Carbon dioxide	31.1	7.4
Water	374.1	22.1
Ethane	32.5	4.91
Propane	96.8	4.26
Methanol	240	7.95
Ethanol	243.1	6.39
Isopropanol	235.6	5.37
Acetone	235	4.76

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1.4 Traditional applications of supercritical CO₂ – extraction, drying and cleaning

Supercritical CO₂ has been investigated in academy and industry as a process solvent in extraction since 1950's. Large scale extraction processes are established and there are about 150 commercial process plants worldwide.²⁴ It has been already widely used in the food industry for the decaffeination of coffee or extraction of hops. In extraction, typically the material to be extracted is placed in a high pressure vessel, which is heated and pressurized above the critical point. With CO₂ circulating, it extracts the desired compounds from the solid material, passing to another vessel where the pressure is reduced to separate the gas CO₂ from the extracted compounds. After the separation, the gas CO₂ is circulated back to the high pressure vessel.

Besides, supercritical CO₂ is also used in drying and cleaning organic residuals. For example, in the micro-electro-mechanical systems (MEMS) organic solvents are trapped in the narrow gaps in the MEMS device. The conventional drying by heating causes large capillary forces in the narrow gaps from the evaporation of the liquid and thus causes collapse. Thanks to the zero surface tension of supercritical CO₂, it can reach the valley of the narrow gaps, dry and remove the residuals completely by dissolving them without structural deformation.²⁵ This is utilized widely for example in the drying process of aerogels which is described in more details in Section 2. Supercritical drying has been also used in other processes such as preparation of nanosized copper borate particles²⁶ and nanocrystalline titania aerogel powder with higher photocatalytic activity and more effective control of the crystallite size compared to xerogel TiO₂ powder obtained by direct drying. In powder processing, supercritical fluid drying enables the controlling of agglomeration, leading to smaller particle size and increased specific surface area as well as higher homogeneity of the powder.^27,28

1.5 Experimental set-up

Numerous experimental set-ups to realize scCO₂ processing have been observed in literature, with the batch autoclave and continuous flow mode being the most common ones. The autoclave is typically installed with an agitator, pressure meter, heating system and rupture disk for safety protection. In a very simple design, dry ice in different weight is placed in an autoclave to reach different pressure under different temperature.²⁹ Such design does not involve high pressure pump, however it is fairly difficult to control the pressure as dry ice evaporates fast before fully tightening the autoclave. In a more complex system, a high pressure pump and a co-solvent pump is connected to the autoclave. After the autoclave reaches supercritical state, the feed of CO₂ is stopped and the reactor becomes a closed system.³⁰ In continuous flow mode, typically a high pressure pump, co-solvent pump, together with a back pressure regulator are connected with the reactor (or vessel). The whole system can be fully computer controlled, and CO₂ is flowing continuously but maintaining the supercritical state in the vessel thanks to the back pressure regulator. Such system can be based on a commercial supercritical extraction rig.³¹ The reactor (vessel) can also be connected to a nozzle, through which the solutes dissolved in the supercritical CO₂ can expand rapidly into a region of much lower pressure. This process is called RESS (rapid expansion of supercritical solutions), resulting in a substantial drop in the solute's solubility and thus a subsequent precipitation.³² The precipitates either form nanostructured particles or coatings if collected on substrates. Overall, the readers should note that there are many other experimental set-ups (such as semi-batch mode and multistage flow processes) which are designed more specifically and not covered in this review.

2. Supercritical CO₂ processing of 3D aerogels

A typical aerogel is a 3D shape, consisting of both meso-(2–50 nm) and micropores (<2 nm) and therefore exhibiting a large surface area. Such high surface area and pore structure provides better properties for applications such as catalysis, insulation, fire retardance, and supercapacitor. For example, spinel nickel cobaltite aerogels were produced with ultrahigh specific capacitance,³³ and copper– and silver–zirconia aerogels were prepared for better catalytic performance in synthesizing methanol from carbon dioxide.³⁴ Since early 1980s, scCO₂ has been extensively used to remove organic solvents from wet gels to form aerogels.^33–36 Fig. 3 illustrates the formation scheme of aerogel compared with the formation of xerogel.¹⁰ Wet gels are typically produced via sol–gel chemistry, in which sol is a colloidal suspension of nanosized solid particles in a liquid phase. Upon proper circumstances, these particles form a 3D network called a gel, which still contains a large amount of organic solvents. Drying the gel by evaporation under ambient pressure gives rise to capillary pressure which causes shrinkage of the network, leading to the formation of xerogel. In scCO₂ drying, scCO₂ diffuses into the pores and exchanges the organic solvents such as ethanol. As a result, the solvent in the pores is discharged to the continuous flow of scCO₂, and the solid network with its pores retains its structure without any collapse.

Fig. 3 Illustration of the formation of aerogel by scCO₂. Modified with permission from {R. Sui and P. Charpentier, Synthesis of metal oxide nanostructures by direct sol–gel chemistry in supercritical fluids, Chem. Rev., 2012, 112, 3057–3082}. Copyright {2012} American Chemical Society.

In the formation of aerogel, the first important step is to form the wet gel from the desired precursors. Strategies such as slow heating,³⁷ use of acid such as acetic acid³⁴ and epoxide³³ have been used to form wet gel. The second primary step is to age the wet gel either in water or in solvent. In this step, hydrolysis and condensation reactions are continued, leading to the enhancement in mechanical strength and stiffness of the gel. Aging is also critical to prevent formation of cracks during drying and the properties of the final aerogel are strongly impacted by the aging process.³⁸ In the final step of supercritical CO₂ drying the drying time, pressure, and temperature are critical and need to be optimized to avoid the partial shrinkage (see Fig. 4(b) (ref. 39)) of the obtained aerogels. Different temperatures can also be coupled with supercritical drying to avoid the post heat-treatment to convert the precursor to desired material phase.

Fig. 4 A well-shaped silica aerogel in (a). Reprinted with permission from {D. A. Loy, E. M. Russick, S. A. Yamanaka and B. M. Baugher, Direct formation of aerogels by sol–gel polymerizations of alkoxysilanes in supercritical carbon dioxide, Chem. Mater., 1997, 9 2264–2268}. Copyright {1997} American Chemical Society. Aerogels with partial shrinkage in (b). Reprinted from C. A. Garcia-Gonzalez, M. C. Camino-Rey, M. Alnaief, C. Zetzl and I. Smirnova, Supercritical drying of aerogels using CO₂: effect of extraction time on the end material textural properties, J. Supercrit. Fluids, 66, 297–306, Copyright (2012), with permission from Elsevier.

3. Supercritical CO₂ processing of coatings

As aforementioned, RESS (rapid expansion of supercritical solutions) can be applied for coating processing, in which coating material dissolves in scCO₂ and then the solution is rapidly decompressed via a nozzle or an orifice. During depressurization, the density of CO₂ changes significantly, leading to a high supersaturation followed by precipitation on a substrate to form a coating. For example, superhydrophobic alkyl ketene dimer (AKD) coatings were formed on untreated paper surfaces by RESS and Fig. 5 illustrates its experimental set-up.⁴⁰ In their experiments, AKD granule of 300 mg was loaded in the vessel in which temperature was controlled to be 40 °C or 60 °C and pressure was 100 to 300 bar. The AKD granule was then dissolved in scCO₂ and then AKD–scCO₂ solution was sprayed through a nozzle into the substrate in an expansion chamber at atmospheric pressure. Spraying distances of 10 and 50 mm was studied. Fig. 6 shows the typical morphology of the formed coating, which consists of randomly aligned flakes. Such flaky morphology together with the low surface energy of AKD resulted in superhydrophobicity, i.e. contact angle for water of above 150°. Superhydrophobic surface has been regarded as a new type of multifunctional material, with easy-to-clean property,⁴¹ bacterial adhesion reduction,⁴² water repellence, broadband anti-reflection,⁴³ anti-icing,⁴⁴ drag-reduction in fluid flow,⁴⁵ non-adhesive property⁴⁶ and wetting control.^47,48 Based on this route, ceramic and composite coatings are also possible to be processed by tailoring precursors and experimental parameters. In order to have good solubility, co-solvents will be likely needed. Depressurization speed, temperature and pressure also need to be carefully optimized to avoid blocking the nozzle during spraying.

Fig. 5 A typical set-up for coating processing using RESS. Reprinted from C. Quan, O. Werner, L. Wagber and C. Turner, Generation of superhydrophobic paper surfaces by a rapidly expanding superhydrophobic carbon dioxide–alky ketene dimer solution, J. Supercrit. Fluids, 49, 117–124. Copyright (2009), with permission from Elsevier.

Fig. 6 SEM of the AKD superhydrophobic coating by RESS at 300 bar, 40 °C with a spraying distance of 10 mm. Bar = 2 μm. Modified from C. Quan, O. Werner, L. Wagber and C. Turner, Generation of superhydrophobic paper surfaces by a rapidly expanding superhydrophobic carbon dioxide–alky ketene dimer solution, J. Supercrit. Fluids, 49, 117–124, Copyright (2009), with permission from Elsevier.

In another route, metallic films such as Cu, Ag, Co, Pt and Ni have been deposited using scCO₂.⁴⁹ Typically, the metal precursor is firstly dissolved in scCO₂, and then adsorbed on the substrate in the vessel. The metal coating is formed from the adsorbed metal precursor either thermally, or by a conversion agent such as H₂ in scCO₂. For example, dimethyl(acetylacetonate)gold(III) was dissolved in scCO₂ at 60 °C and 138 bar, and the substrate was placed in the same vessel with scCO₂.⁴⁹ After 30 min, a 20–100 molar excess of H₂ was slowly added to the reaction vessel. After H₂ addition, the reactions were allowed to proceed for 4 to 24 h. Because of the zero surface tension of scCO₂, it can lead to exceptional step coverage and thus the fabrication of the high aspect ratio nanostructures by using SiO₂ substrate with nanopillars, illustrated in Fig. 7.^5,50 In this route, it is important to have precursors with high solubility in scCO₂ by using for example the chelate complexes, organometallic compounds, and alcohols as co-solvents to enhance the solubility of precursors.

Fig. 7 (a) SEM of gold arrays obtained by H₂ assisted reduction of gold precursor solution in scCO₂ at 125 °C and 209 bar, and (b) illustration of its formation scheme. Modified with permission from {A. Cabanas, D. P. Long and J. J. Watkins, Deposition of gold films and nanostructures from supercritical carbon dioxide, Chem. Mater., 2004, 16, 2028–2033}. Copyright {2004} American Chemical Society.

4. Supercritical CO₂ exfoliation and intercalation of layered materials

Supercritical CO₂ has also shown great potential in exfoliating and intercalating layered materials. Its high diffusivity, low viscosity, and zero surface tension facilitates its penetration into the layered structure.^{31,51,56–59,62,63} Upon rapid depressurization, the CO₂ expands to gaseous state and further breaks the bonds to separate the layers. In this process, different precursors dissolved in scCO₂ are also able to diffuse into the layered structures. By a rapid depressurization, the solubility of the precursor decreases, and thus the materials precipitate on the surface of the layers.

Graphite is a common layered material, consisting of graphene layers held together by van der Waals force. Graphene has been widely recognized as a wonder material, and thus exfoliation of graphite into graphenes has attracted significant interests in academy and industry. For example, graphene was first exfoliated from graphite by a simple scotch tape,⁵² though this is not a practical production process for graphene. Exfoliation of graphite is typically done by introducing additional external forces in mechanical or thermal ways to overcome the van der Waals force between the layers. In mechanical exfoliation, stirring, shaking and sonication are the main approaches with the assistance of intercalating agents, surfactants to diminish the van der Waals force.⁵³ In thermal exfoliation, the functional groups attached to the graphitic layers decompose upon heating and produce gases to build up pressure to exceed the van der Waals force.⁵³ In general, the yield in the mechanical and thermal exfoliation is rather low for large scale use. Therefore, graphite is often oxidized first, followed by mechanical or thermal exfoliation and reduction, which commonly refers to as chemical exfoliation.⁵⁴ The production yield is high for large scale use, but the oxidation and reduction processes require strong acids and need careful handling. These steps also make manufacturing more complicated, time-consuming and costly. Significant disadvantages of chemical exfoliation are that the oxidation and reduction processes produce many irreversible defects, reduce surface active sites and lead to a weak general quality.⁵⁵

Therefore it is required to explore eco-friendly methods to exfoliate graphite into high quality graphenes in large scale, and scCO₂ exfoliation has shown its great potential.^51,56 Fig. 8 shows an illustration of graphite exfoliation. Besides, co-solvents can be introduced to enhance exfoliation and also to construct functional groups on the exfoliated graphenes. For example, pyrene and three derivatives (1-pyrenecarboxylic acid, 1-pyrenebutyric acid and 1-pyrenamine) have been reported to assist the graphite exfoliation.⁵⁶ Graphite and pyrene or its derivatives were dispersed in dimethylformamide, and then charged with scCO₂. The pyrene and its derivatives diffused into the graphitic layers. Upon a rapid depressurization, the graphite was exfoliated into graphenes and the pyrene and its derivatives were remained and firmly adsorbed on the surface of the graphenes. The authors also utilized the adsorbed pyrene and its derivatives as interlink to form gold nanoparticles on the graphene surface.

Fig. 8 Illustration of supercritical CO₂ exfoliation of graphite.

Other layered materials such as BN, MoS₂ and WS₂ have also been successfully exfoliated using scCO₂ assisted with ultrasound.^57,58 The experiment set-up is shown in Fig. 9(a). 100 mg of BN, MoS₂ or WS₂ powder material was placed in the vessel, and heated to 45 °C and charged with CO₂ until 10 MPa. Then the ultrasonic probe was started to run for 40 min while the pressure and temperature were maintained at 10 MPa and 45 °C. Ultrasonication enhanced the diffusion of scCO₂ into the layered structure. After the ultrasonication, the products were sprayed along with CO₂ either into the absolute ethanol or onto a substrate. During spraying, the pressure was rapidly decreased and the layered materials were exfoliated. The exfoliated BN, MoS₂ and WS₂ suspended in ethanol are shown in Fig. 9(b).

Fig. 9 Illustration of the scCO₂ exfoliation device with ultrasonic probe in (a), and the digital photographs of the ethanol solutions of the exfoliated BN, MoS₂ and WS₂ in (b). Reprinted with permission from {Y. Wang, C. Zhou, W. Wang and Y. Zhao, Preparation of two dimensional atomic crystals BN, WS₂ and MoS₂ by supercritical CO₂ assisted with ultrasound, Ind. Eng. Chem. Res., 2013, 52, 4379–4382}. Copyright {2013} American Chemical Society.

Besides, supercritical CO₂ has also been applied to improve the exfoliation of layered silicate in polymer matrix to enhance mechanical property,⁵⁹ as the mechanical property improvement is highly dependent on the silicate morphology in the final composite⁶⁰ and it is a common challenge to achieve a fully exfoliated silicate state in such composite.⁶¹ For example, scCO₂ was successfully used to improve the exfoliation of layered silicate in a poly(ethylene terephthalate) (PET) matrix by treating the nanosilicate loaded PET with scCO₂.⁵⁹ Similarly, scCO₂ can also be applied to intercalate layered materials or expand porous materials. For example, natural clay was intercalated with sugar acetate between the layers of the clay,⁶² and the pores within mesoporous silicas were expanded using scCO₂ as the processing medium.⁶³

5. Supercritical CO₂ processing of powder materials

Besides the use of scCO₂ in preparing 3D aerogels, coatings, exfoliating and intercalating layered materials, a wider use of scCO₂ in materials processing in literature is the processing of powder materials for different applications. Indeed the different routes described in this section can also be utilized in processing aerogels, coatings and intercalating layered materials.

5.1 Sol–gel chemistry in scCO₂

Sol–gel method has been adopted as a modern technique to prepare inorganic materials with use of metal alkoxides or inorganic salts precursors. The sol–gel chemistry involves hydrolysis, condensation and polycondensation reactions when the precursors are mixed with water and solvents, as shown in Fig. 10,⁶⁴ where M could be Al, Si, Ti etc. The metal alkoxide precursor is hydrolysed first to form (M–OH) group. These groups will further react to form (M–O–M) structure in the condensation and polycondensation reactions, which leads to the formation of a sol suspension. Sol–gel chemistry corresponds to a molecular way to design materials (particle size, morphology, surface area, pore diameter, etc.) by controlling the conditions of the process (temperature, precursor, concentration of reagents, amount of water, drying methodologies, etc.).⁶⁵

Fig. 10 Reactions involved in sol–gel chemistry. Reprinted from J. Wang, Sol–gel materials for electrochemical biosensors, Anal. Chim. Acta, 399, 21–27, Copyright (1999), with permission from Elsevier.

Sol–gel chemistry has been applied widely to design precursors to form metal oxides, which is either obtained by heat-treatment,^66,67 or hydrothermal,⁶⁸ solvothermal⁶⁹ treatment. In hydrothermal and solvothermal method, the sol precursor is placed in a Teflon liner in autoclave which is heated in an oven. In the autoclave, the water or solvents experience a pressure increase upon heating, however normally the system does not reach supercritical state yet, though it already has many benefits in materials processing such as lowered synthesis temperature, new phase structure⁶⁸ and increased surface area. Therefore, it is expected that use of sol–gel precursor in supercritical CO₂ can bring even more benefits in material processing, as the chemical reaction happens under supercritical state and it is much easier to separate CO₂ from the formed product.¹⁰ For example, a comparative study was carried out using titanium alkoxides as precursor to prepare TiO₂ in liquid and supercritical ethanol.⁷⁰ The results showed that the TiO₂ prepared in supercritical ethanol consisted of anatase in size of 20–60 nm agglomerated into 2 μm spherical spheres while that prepared in liquid ethanol was amorphous in size of 300–700 nm growing into irregular larger particles. Besides sol–gel synthesis has also been carried out in water-in-CO₂ emulsion, on supports materials and templates in supercritical CO₂, which are covered in more details in later sections.

5.2 Water-in-CO₂ microemulsion as a nanoreactor for nanoparticle formation

Water-in-CO₂ microemulsion results from the insolubility of water in scCO₂ and the different solubility of metal precursors in water and in scCO₂. That is, metal precursors can be dissolved in water, but it is not soluble in scCO₂. Therefore with use of proper surfactants, discrete water droplets are dispersed and stabilized in the continuous scCO₂ phase, thus forming water-in-CO₂ microemulsion. An illustration of such water-in-CO₂ microemulsion is shown in Fig. 11, where perfluoropolyether (PEFE) and sodium bisethylhexyl sulfoccinate (AOT) act as surfactants.⁶ Such surfactants are amphiphilic molecules containing both a CO₂-phobic and a CO₂-philic portion. The CO₂-phobic head displays poor solubility in scCO₂ and therefore prefers to reside away from the continuous scCO₂ phase, while the CO₂-philic tail has good solubility in scCO₂ and extends out into the scCO₂ phase. As water is not soluble in CO₂, it stays in the core of the surfactant micelles and thus water-in-CO₂ microemulsion forms. The choice of surfactants can be challenging, as it is generally difficult to overcome the strong attractive van der Waals interactions between water droplets to obtain a stable water-in-CO₂ microemulsion.⁷¹ Therefore incorporation of fluorinated or silicone units in the CO₂-philic portion of the surfactant encourages its good solubility in scCO₂ and a better stability of the water-in-CO₂ micelles. In addition to the choice of surfactants, the water to surfactant ratio is also an important parameter in the microemulsion stability.

Fig. 11 Water-in-CO₂ micelles stabilized by a perfluoropolyether (PEFE) surfactant in (a) and stabilized by sodium bisethylhexyl sulfoccinate (AOT) with PFPE as a co-surfactant in (b). Reproduced from ref. 6 with permission from The Royal Society of Chemistry.

The water-in-CO₂ microemulsion provides a nanoreactor for synthesizing a variety of nanoparticles in scCO₂ by using water soluble precursors and restricting the chemical reactions in the water-in-CO₂ micelles. The particle size is restricted by the micelle diameter as the surfactant surrounding water droplet act as a phase separator.⁷² Therefore the maximum nanoparticle size is controllable by adjusting the core size which can be tuned by adjusting processing parameters such as scCO₂ pressure, temperature, and water to surfactant ratio.⁷³ However, during depressurization, agglomeration of nanoparticles may still occur.

Water-in-CO₂ microemulsion has also been utilized to form metal nanoparticles. For example, silver and copper nanoparticles have been prepared using the water-in-CO₂ microemulsion technique.⁷⁴ In a typical process, AOT was used as surfactant, and the metal precursors were dissolved in the water core. After 30 min of stirring at 38 °C and 200 atm in a high pressure vessel, formation of water-in-CO₂ microemulsion was visually observed. After that, stirring was stopped and a reducing agent of NaBH₃CN in an ethanol solution was injected into the vessel to reduce Ag⁺ and Cu²⁺ into the elemental metal particles. More examples of synthesizing Au,⁷⁵ Pd,⁷⁶ Pt,⁷⁷ CdS,⁷⁸ ZnS⁷⁹ and AgS⁸⁰ can be found in the literature.

5.3 Composites: nanoparticles on support materials

5.3.1 Impregnating nanoparticles to support materials. Heterogeneous catalysis is a very popular application of structures of nanoparticles on supports. The activity of such catalysts depends on the metal composition, particle size, morphology and particle distribution. Therefore high surface area porous materials are often applied to support the specific metal nanoparticles in order to confine their particle growth, lead to homogeneous distribution, and thus enhance the catalytic performance. Different methods using liquid solvents have been applied to produce such structures; however, two main challenges are the impregnation of the nanocrystals into the nanoporous matrix, and the adsorption of the nanocrystals from the solution phase to the support. For example, due to the large viscosity and surface tension, most liquid solvents cannot wet the support surface sufficiently to diffuse into the pores, therefore leading to formation of poorly dispersed nanoparticles with a non-homogeneous distribution. Besides, the subsequent drying process may cause structural changes with a significant reduction of the support material's surface area and thus its catalytic activity.⁸¹

As a comparison, supercritical CO₂ is ideal to produce such catalyst structures based on its low viscosity, high diffusivity, zero surface tension and easy separation. A number of papers have reported the formation of such metal particles on supports structures.^81–84 For example, Pd was impregnated into mesoporous silica SBA-15 by dissolving palladium hexafluoroacetylacetonate in scCO₂ and impregnating it into the SBA-15 support. Then the Pd precursor was reduced either with a H₂–CO₂ mixture or in pure H₂ after CO₂ depressurization. As a result, Pd nanoparticles were evenly distributed into the support and the cluster size was limited by the pore size of the support silica and the amount of the precursors (see Fig. 12). The other example is the impregnation of SnO₂ nanoparticles into CNTs, as shown in Fig. 13,⁸⁵ based on a simple process. The authors used SnCl₂ as precursor and dissolved it in ethanol. The mixture was loaded into a high pressure vessel and filled up with scCO₂ for 5 h. After slow depressurization, the product was washed with absolute ethanol and vacuum dried at 60 °C for 6 h. The TEM images show successful impregnation of SnO₂ into the core of carbon nanotubes, which was confirmed by the lattice fringes in Fig. 13(b).

Fig. 12 TEM images of Pd/SBA-15 composites prepared by impregnating Pd precursor via scCO₂ into SBA-15 for 8 h in (a and b) and for 16 h in (c and d). Reprinted from J. Morere, M. J. Tenorio, M. J. Torralvo, C. Panda, J. A. R. Renuncio and A. Cabanas, Deposition of Pd into mesoporous silica SBA-15 using supercritical carbon dioxide, J. Supercrit. Fluids, 6, 213–222, Copyright (2011), with permission from Elsevier.

Fig. 13 TEM images of the CNTs impregnated with SnO₂. Reprinted from Z. Sun, Z. Liu, B. Han and G. An, Supercritical carbon dioxide-assisted deposition of tin oxide on carbon nanotubes, Mater. Lett., 61, 4565–4568, Copyright (2007), with permission from Elsevier.

Besides, ceramic-polymer composite can also be obtained by impregnation via scCO₂ to tailor and enhance its properties. For example, ZnO/poly(ethylene terephthalate) (PET) composite was obtained by dissolving zinc precursor in scCO₂ and further impregnating it into PET films, followed by treatment of hydrazine alcoholic solution.⁸⁶ Fig. 14 shows the illustration of the process and the surface images of the resulted ZnO/PET composite. The maximum loading rate of the precursor into the PET film was 28%. The SEM images revealed that the PET film surface was completely covered with ZnO nanoparticles in size of 50–80 nm.

Fig. 14 Scheme representation of the impregnation process and the SEM images of the resulted ZnO/PET composite. Reprinted from M. R. Mauricio, F. C. G. Manso, M. H. Kunita, D. S. Velasco, A. C. Bento, E. C. Muniz, G. M. De Carvalho and A. F. Rubira, Synthesis and characterization of ZnO/PET composite using supercritical carbon dioxide impregnation technology, Composites, Part A, 42, 757–761, Copyright (2011), with permission from Elsevier.

5.3.2 Encapsulating nanoparticles with polymers. Encapsulation is often applied to protect materials from rapid degradation and also in the application of controlled drug release. Supercritical CO₂ has also shown promise to produce encapsulated structures (especially encapsulated inorganic particles with polymers) to meet the application requirements.^87–89 For example silica nanoparticles were encapsulated by Eudragit polymer using scCO₂.⁸⁹ The polymer was dissolved in acetone, and silica nanoparticles were suspended in the polymer solution using ultrasonication. The encapsulation was carried out by spraying the suspension in scCO₂ through a nozzle. Fig. 15 shows a schematic illustration of the encapsulation process together with a resulted structure.⁸⁷ When the suspension droplets were in contact with scCO₂, a very fast mutual diffusion into and out of the droplet happened as acetone was highly miscible with scCO₂. The polymer in the droplet became saturated very quickly due to the extraction of acetone from the droplet, and the polymer started to gelate. Meanwhile the scCO₂ diffused into the droplet and dissolved in the acetone solution, leading to swelling of the droplet and formation of an empty shell or balloon structure,⁹⁰ as shown in Fig. 15(a) in the high solvent expansion route. Then such balloon structure burst into very fine viscous droplets containing nanoparticles and polymers. Further extraction of acetone by scCO₂ from the viscous droplets induced glass transition of the polymer, resulting the nucleation and precipitation of the polymer on the surface of nanoparticles. However, the encapsulated nanoparticles within the polymer film were agglomerated, as shown in Fig. 15(b and c).

Fig. 15 Schematic illustration of encapsulating silica with Eudrgit polymer in (a) and the resulted structure with polymer to silica ratio of 1 : 5 in (b) and of 1 : 6 in (c). Reprinted from H. Krober and U. Teipel, Microencapsulation of particles using supercritical carbon dioxide, Chem. Eng. Process., 44, 215–219, Copyright (2005), with permission from Elsevier.

5.3.3 Preparation of hollow inorganic spheres. Inorganic hollow particles have also gained interest for applications such as catalysts,⁹¹ and capsules for controlled release of drugs.⁹² Hollow silica particles have been synthesized through several different routes for instance sacrificial type of template process with colloidal polymer particles⁹³ and combination of a one-step reverse-phase microemulsion route and a post selective etching method.⁹⁴ Simple synthesis of inorganic hollow spheres using supercritical CO₂ has also been reported. The precursors were carried into cross-linked polystyrene (PS) microsphere templates using scCO₂. After sol–gel procedure and calcinations the templates were removed and hollow microspheres of inorganic material were obtained.⁹⁵ Another method to produce hollow silica and titania-containing silica spheres using scCO₂ was introduced by Zhang et al.⁹⁶ A precursor/scCO₂ solution was mixed with cetyltrimethyl-ammonium bromide (CTAB) aqueous solution. After the hydrolysis reaction in the interface of scCO₂ and water, the synthesis resulted in hollow spheres of silica and titania. By adjusting the pressure and CTAB concentration, the diameter of the spheres could be controlled from nano-scale to micron scale. Yet another method for silica hollow sphere production has used copolymeric spheres as core templates. Dimethylamino groups of the copolymeric spheres absorbed water that catalysed the hydrolysis of tetraethyl orthosilicate (TEOS) and it led to deposition of the silica network on the templates followed by the removal of the template by heat-treatment.⁹⁷

5.3.4 Decorating nanoparticles on CNTs and graphene. CNTs have been adopted as ideal supports for metal catalysts due to their small size, high chemical stability and large surface area to volume ratio. However, it is not easy to attach metal nanoparticles on their surface and so far the most established method is to use harsh oxidative pre-treatment to generate functional groups to promote the deposition of foreign materials on their sidewalls.^98,99 With the use of supercritical CO₂, as explained forehand, metal nanoparticles obtained from hydrogen reduction of the organometallic precursors can be homogeneously deposited on the sidewalls of the CNTs,^100,101 while silver nanoparticles were formed on CNTs using glucose as reducing agent in the precursor.¹⁰² Besides, the CNTs have also been decorated with metal oxides such as Ce₂O₃, Al₂O₃ and La₂O₃.¹⁰³ By controlling the reaction parameters, uniform coatings of the metal particles were also readily formed on CNTs. In a typical process, CNTs were dispersed in a metal nitride ethanol solution, and such dispersion was loaded into a high pressure vessel. The vessel was then charged with CO₂ to reach supercritical state at 120 °C. Fig. 16 shows a comparison of the formation of La₂O₃/CNTs with and without the use of scCO₂. With the use of scCO₂, the CNTs were covered with a layer of La₂O₃. Without the use of scCO₂, La₂O₃ formed agglomerates, which were not attached to the CNTs.

Fig. 16 Illustration of the formation of metal oxide–CNT composite. The inserted TEMs images are from La₂O₃–CNT composites produced. Lattice fringes are from the marked area on the CNTs surface. Modified from Z. Sun, X. Zhang, B. Han, Y. Wu, G. An, Z. Liu, S. Miao and Z. Miao, Coating carbon nanotubes with metal oxide in a supercritical carbon dioxide–ethanol solution, Carbon, 45, 2589–2596, Copyright (2007), with permission from Elsevier.

In addition to the use of CNTs as support, graphene has recently become very popular to act as support to produce composites. Depending on how the graphene was obtained, functionalized graphene sheets or graphene oxide sheets have been prepared and used as a support to metal or metal oxide nanoparticles using scCO₂.^104–107 Fig. 17 shows an example of a Pt/graphene composite, in which graphene sheets were produced by rapid thermal expansion of graphite oxide.¹⁰⁴ In this process, the graphene sheets and Pt precursor were loaded to scCO₂ with the use of methanol as co-solvent to improve the solubility of the Pt precursor in scCO₂. After 1.5 h, H₂/CO₂ mixture gas was introduced to the vessel which was heated to 300 °C to reduce the Pt precursor into Pt nanoparticles. As a result, Pt nanoparticles with a mean size of 3.28 nm were evenly distributed on graphene. The electrocatalytic investigations revealed that the current density of methanol electrooxidation with the resultant Pt/graphene catalyst was 3.5 times higher than that of conventional Pt/carbon black catalyst.

Fig. 17 TEM image of a Pt/graphene composite. The black dots are Pt particles and typical lattice fringes of Pt nanoparticles are inserted in (b). Reprinted with permission from {J. Zhao, L. Zhang, T. Chen, H. Yu, L. Zhang, H. Xue and H. Hu, Supercritical carbon dioxide assisted deposition of Pt nanoparticles on graphene sheets and their application as an electrocatalytic for direct methanol fuel cells, J. Phys. Chem. C, 2012, 116, 21374–21381}. Copyright {2012} American Chemical Society.

5.4 Using template

Materials such as SiO₂,^108,109 TiO₂,^110,111 TiO₂/SiO₂,^112–114 Fe₂O₃,¹¹⁵ SiO₂/Fe₂O₃¹¹⁶ and Al₂O₃/Fe₂O₃¹¹⁷ were also deposited via scCO₂ on templates of active carbon,^{108,114–117} carbon cloth,¹¹⁰ silk fiber,¹⁰⁹ polyethylene glycol (PEG)^112,113 and polystyrene (PS) microspheres¹¹¹ which were then removed to form a replicate structure of the template. For example, a porous titania was obtained by using fibrous active carbon cloth as a template.¹¹⁰ The authors dissolved titanium isopropoxide in scCO₂ with isopropanol and deposited it on active carbon cloth. Then the samples were calcined at 600 °C for 2 h and the active carbon cloth was removed afterwards. The fibrous shape of the activated carbon cloth was retained after the removal of the template, as shown in Fig. 18.

Fig. 18 SEM image of the titania sample after removal of active carbon template. Reproduced from ref. 110 with permission from John Wiley and Sons.

6. Summary

Despite of the unique advantages of supercritical CO₂ in advanced green materials processing, it is much less widely applied and studied than other processing methods such as sol–gel and hydrothermal methods. This review explains the basics of applying scCO₂ in material processing, and systematically summarizes the different experimental routes used in scCO₂ processing and categorized them into processing of 3D aerogels, coatings and powders for a number of different applications. The experimental ideas described in the processing of 3D bulks, coatings and powders are indeed interchangeable and can be tailored and combined together. Relating examples from literature are highlighted to reveal the potential and versatility of this technique in materials processing and synthesis, aiming to encourage readers to explore new opportunities of using this technique.

Acknowledgements

This work was supported by the Academy of Finland (decision no. 25987) and by FIMECC Ltd. (Finnish Metals and Engineering Competence Cluster) HYBRIDS – Hybrid Materials program.

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