Making Nanomaterials in Supercritical Fluids: A Review - Journal of

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Making Nanomaterials in Supercritical Fluids: A Review Xiangrong Ye and C. M. Wai* Department of Chemistry, University of Idaho, Moscow, ID 83844-2343; *[email protected]

Supercritical fluids, particularly supercritical CO 2 (scCO2), have been used in areas ranging from materials cleaning, natural products extraction, chemical reactions, sample preparation, and environmental remediation (1–12). Generally speaking, a supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point and a density close to or higher than its critical density. A fluid is considered near-supercritical when it is still a liquid but has begun to show some of the properties normally associated with SCFs. The most distinguishing feature of SCFs is that their physicochemical properties can be altered continuously between gaslike and liquidlike limits by adjusting the system pressure or temperature. SCFs, like liquid solvents, can dissolve solid compounds yet they have low viscosities and high diffusivities like gases. As a result of their high compressibility, SCFs offer a convenient means of accessing a wide range of solvent properties without physically changing the solvent. Physical or chemical transformations in supercritical or near-supercritical fluids have been a subject of considerable interest to material scientists particularly in nanoparticle– nanowire generation and thin solid-film deposition (13–18). Much of the impetus comes from the advantages of supercritical fluid processes over conventional solution processes, such as: (a) SCFs provide high diffusivity and low viscosity and are therefore capable of attaining high uniformity and penetration into small areas, (b) the solvent strength of a SCF can be varied, thus allowing rapid separation of solute by manipulation of fluid pressure (density), and (c) some SCFs, such as scCO2, are environmentally benign. Because of these properties, the byproducts and the contaminants in SCFs can be easily removed from the system; therefore, nanoparticles, nanowires, and thin films of high purity can be obtained. In

materials processing, a SCF can act as a medium either for transporting solute species or for chemical reactions, or both. In some cases, the supercritical fluid itself can also take part in the reactions. This review summarizes recent developments in physical or chemical processes for synthesizing nanoparticles, nanowires, and thin solid films in supercritical or near-supercritical fluids. Physical SCF Processes for Preparing Nanoparticles

Rapid Expansion of Supercritical Solutions Rapid expansion of supercritical solutions (RESS) is a physical process for synthesizing fine powders including nanoparticles (19–28). Figure 1 shows a typical apparatus used in RESS. In this process, solutes such as metal compounds or polymers are dissolved in a SCF. When the SCF solution is allowed to expand rapidly into a region of much lower pressure, a significant drop in the solubility of the solutes occurs, resulting in their precipitation. The precipitated solutes form very small molecular clusters, ion pairs, or dispersed individual molecules. Usually aerosols rather than vapors are formed and the aerosol particles are much smaller than those formed by the nebulization of ordinary liquid solutions followed by desolvation, such as in spray pyrolysis. Various morphologies and particle sizes can be thus produced. Formation of composite materials, such as pharmaceutical or polymer microcomposite materials for controlled drug releasing, is also possible by precipitation of two different substances dissolved in the same SCF solution. The distinguishing features of RESS are the fast attainment of uniform conditions and high supersaturations in SCF, which favor the formation of small particles with narrow size distribution. However, the disadvantage of this physical RESS

FR: fluid reservoir R: pressure regulator P: pump OA: optional autoclave V: valve HR: heated region EN: expansion nozzle CS: collection surface C: collection chamber AR: adiabatic region IsR: isentropic region RS: RESS spray BER: background expansion region

Figure 1. Physical RESS apparatus applied for supercritical water. Inset: the RESS expansion.

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method is that the nanoparticle to be formed must be soluble in the SCF because the particle is chemically identical to the starting material. Many desirable materials, for example, semiconductors, metals, metal oxides, and mixed-metal oxides, are insoluble in common SCFs such as scCO2 and scN2O. SiO2 and GeO2 have been shown to be soluble in scH2O, but the experimental conditions were rather extreme (445 ⬚C, 800 psi) (29–30).

Physical Composite Processes without Expansion A SCF can also be used as a solvent for dissolving and dispersing desired materials within a matrix that is also soluble in the SCF. After desolvation, nanoparticles of the desired materials form in the matrix. For instance, metal-complex nanoparticle–polymer composites were prepared by dissolving a metal complex in scCO2 and infusing it within an scCO2 “swollen” low-density polymer matrix (31). These metal complex–polymer composites can function as precursors to surface-selective catalyst systems. Supercritical Antisolvent Precipitation Supercritical antisolvent (SAS) precipitation has been applied successfully to make microparticles and nanoparticles (32). Its application has been explored in a variety of different fields including: explosives, polymers, pharmaceutical compounds, coloring matter, superconductors, catalysts, and inorganic compounds. SAS precipitation is based on the fast dissolution of a liquid solution in a SCF. The addition of a SCF leads to a lower solubility of the solute, thus forcing the solute to precipitate into microparticles or nanoparticles. SAS has unique characteristics that include very large diffusivities when compared with those of liquids and one-step complete elimination of the solvent from the precipitates. The most commonly used SAS is CO 2 . By using scCO 2 as the antisolvent, microparticles and sub-microparticles of amoxicillin have been produced (33). Nanoparticles of yttrium, samarium, and neodymium acetates were precipitated as precursors of catalysts or high-temperature superconductors with morphologies dependent on the different expansion levels of the liquid solution (34–35). More recently, scCO2 antisolvent process-based production of fullerene nanoparticles has been reported (36). Supercritical Drying Processes Supercritical drying process is another important physical approach to nanoparticles (37). Nanoparticle powders for

Figure 2. Experimental setup for the chemical RESS method in the preparation of nanoparticles.

a wide variety of applications, for example, pharmaceutical compositions or high-performance ceramics, are produced from sols by contacting the sol with the SCF. The sol is sprayed into the SCF. All or a portion of the original solvent system is separated by dissolution in the SCF, and the resulting SCF is filtered for recovery of the particles. The solids are washed using fresh SCF and dried. Solvent-free inorganic and organic particles can be thus produced with particle sizes ranging from 20–500 nm. Chemical SCF Processes for Preparing Nanoparticles and Nanowires

Chemical Rapid Expansion of Supercritical Solutions In the original RESS process, a supercritical solution is expanded rapidly into a vacuum or air (Figure 1). A supercritical solution can also be rapidly expanded into a liquid solution to initiate a chemical reaction leading to the formation of a new product. Based on this concept, Sun et al. developed a facile and flexible method for preparing stable suspensions of polymer-protected metal or semiconductor nanoparticles, such as nickel, cobalt, iron, silver, Ag2S, CdS, and PbS (38–41). Figure 2 is a schematic of the experimental setup. The nanoscopic metal ion “solute droplet” produced in the RESS process is captured by a reducing agent or sulfide anions in the receiving liquid solution to form metal or metal sulfide nanoparticles. The nanoparticles thus produced have a reasonably narrow-size distribution and form solutionlike stable suspensions in the presence of a stabilization polymer. Some of these nanocrystallines or nanoparticles exhibit excellent nonlinear optical-limiting properties while others are excellent magnetic materials. Direct Chemical Processes without Expansion Chemical reactions occurring within the SCF system can also lead directly to nanoparticles. This direct method for preparing polymer-borne metal clusters was reported by Kryszewski (42). Depositing metallic nanoparticles in polymer systems was achieved via dissolution of a metal precursor in a SCF and its subsequent reduction or decomposition during the polymerization step. Reactions using water in subcritical and supercritical states have been suggested as prospective ways to synthesize nanocrystal oxide catalysts, Ce 0.5 Zr 0.5 O 2 , Ce 0.1 Y xZr 0.9-x O 2 , Zr 1−x Y x O 2 , Zr 1−x In x O 2 , La 2 CuO 4 ; supported catalysts, Pd兾Rh兾ZrO 2 and Pd兾Rh兾TiO2; and supports, CeO2, ZrO2, TiO2 (43). The proposed technique is characterized by high productivity. It is ecologically friendly and multicomponent oxide catalysts are obtained with chemical and phase compositions and properties that can be changed within a large range. More recently, Holmes et al. have developed a SCF solution phase approach to grow nanowires and nanocrystals of silicon. Bulk quantities of defect-free silicon nanowires with nearly uniform diameters ranging from 40 to 50 Å were grown to a length of several micrometers using sterically stabilized gold nanocrystals as uniform seeds to direct one-dimensional nanowire formation (44). Figure 3 is a schematic illustration of the nanocrystal-directed, nanowire-selfassembly process. Dodecanethiol-capped Au nanocrystals were dispersed in supercritical hexane with diphenylsilane at 500 ⬚C and 270 bar (or 200 bar in some cases). At this temperature

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the diphenylsilane decomposes into Si atoms. Silicon and gold form an alloy in equilibrium with pure, solid Si when the Si concentration with respect to Au concentration is greater than 18.6%. Under the specific reaction conditions, the Si atoms most likely dissolve into the sterically stabilized Au nanocrystals until reaching supersaturation, at which point they are expelled from the crystal as a thin, nanometer-scale wire. Likewise, if hexagonal mesoporous silica is used as a template instead of Au nanocrystals, quantum-confined silicon nanowires can be formed within the 5-nm diameter pores of the hexagonal mesoporous silica using this SCF solution phase technique (45). If no template is utilized, robust, highly crystallized, relatively-sized, monodispersed, sterically-stabilized Si nanocrystals ranging from 15 to 40 Å in diameter could be obtained in significant quantities in the presence of octanol at 500 ⬚C and 345 bar (46).

Microemulsion Reactions The poor solubility of many polar compounds in SCFs limits their reactions for the purpose of nanoparticle preparation. One solution to this problem is using water-in-supercritical fluid microemulsions to dissolve the desired polar compounds and ions in the fluid phase (47). Figure 4 shows the structure of a typical water-in-scCO2 microemulsion. The surfactants are amphiphilic molecules containing a hydrophilic head group and a CO2-philic tail where the molecular size and shape of the head and tail are designed to favor aggregation. When such molecules aggregate in a nonpolar SCF, a reverse micelle is formed with the hydrophilic head groups forming a core and the hydrophobic tails interacting with the SCF phase. When water is present, it partitions into the hydrophilic core forming microscopic water pools leading to the formation of water-in-SCF microemulsions. These microemulsions are optically clear, containing spatially-defined, aqueous nanodroplets. They are thermodynamically stable to flocculation when the density of the SCF is above the cloud point of the surfactant tail in the bulk fluid. The mean radii of these water droplets are directly proportional to the water-to-surfactant mole ratio (W). These microemulsions can be used as nanoreactors for synthesizing nanoparticles. Since particle growth occurs within the water droplet, the size of the water core can be used to control the particle dimensions. Moreover, the cloud point depends on density of the fluid phase, which can be manipulated by varying the pressure and temperature, thus providing a tunable medium for reaction and separation of synthesized nanoparticles. Metallic copper nanoparticles (diameter < 20 nm) were synthesized by the reduction of Cu

Figure 3. Nanocrystal-directed, nanowire-selfassembly process in scCO2.

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ions from Cu(AOT) 2 (AOT: sodium bis[2-ethylhexyl] sulfosuccinate) incorporated within AOT reverse micelles dispersed in compressed propane and supercritical ethane solutions (48–49). Water-in-scCO2 microemulsions have been formed by using an ammonium carboxylate perfluoro polyether (PFPE) surfactant, which incorporates a “CO2philic” fluorocarbon chain with a hydrophilic polar carboxylate headgroup (50). Recently, Wai and coworkers reported the synthesis and dispersion of silver nanoparticles in a water-in-scCO 2 microemulsion made of AOT and a PFPE–phosphate cosurfactant (51). The microemulsion system utilizing these two surfactants is stable over a wide W range (up to 34 and 16 for AOT and PFPE, respectively) and with high concentrations of electrolytes in the water core. The procedure for making silver nanoparticles involves the reduction of Ag+ in the water core of the microemulsion with a reducing agent sodium triacetoxyborohydride dissolved in scCO2. This was the first report of the stable suspension of nanometer-sized metallic particles in scCO2 and represented a significant advance for the future application of CO2-based systems for a wide range of particle synthesis. In another report, Wai and coworkers showed that silver and copper nanoparticles could be synthesized in the CO2 microemulsion made of AOT–PFPE–phosphate surfactants using more CO2-soluble reducing agents such as sodium cyanoborohydride and N,N,N´,N´-tetramethyl-p-phenylenediamine (52). In situ spectroscopic measurements indicated the formation of the metal nanoparticles was very rapid (on the order of about 30 seconds) suggesting the microemulsion was dynamic in nature. By injecting an aqueous solution of Na2S into a scCO2 solution containing water-in-scCO2 microemulsions with Cd(NO3)2 in the water core, Holmes et al. synthesized semiconductor nanoparticles of cadmium sulfide (53). The water-in-scCO2 microemulsion in this study was stabilized by an ammonium carboxylate perfluoropolyether (PFPE–NH4) surfactant. Because an aqueous solution was introduced into the scCO2 phase, the W value of the microemulsion was

Figure 4. Water-in-scCO2 microemulsion.



changing. However, CdS nanoparticles with relatively narrow size distributions could be prepared by controlling the water to surfactant mole ratio. Wai and coworkers also demonstrated another method of synthesizing nanoparticles by mixing two microemulsions containing different ions in the water core. The contents in the microemulsions can exchange rapidly, resulting in nanometer-sized new compounds. One example of this microemulsion-plus-microemulsion approach is the synthesis of silver halide nanoparticles by mixing two microemulsions containing silver nitrate and sodium halide separately in the water core (54). Similarly, composite nanoparticles can be synthesized using this method.

supercritical N2O solutions using radio-frequency plasma in which radicals and ions from the N2O served as oxidizing agents. YBa2Cu3O7 was prepared by post-deposition oxidization of the metals co-deposited from supercritical pentane. High-quality InP layers were obtained by rapid expansion of a PPh3 and tris(o-dimethylaminomethylphenyl) indium(III) mixture in supercritical CO2, C2F6, and Xe to the heated InP substrate in a vacuum (61).

Various types of chemical deposition processes in SCFs have been reported. Chemical deposition methods in SCFs are distinctly different from the previously described physical processes because the deposited materials are chemically different from precursor compounds.

Chemical Deposition and Chemical Fluid Deposition Chemical deposition processes can also be carried out directly in a SCF medium. For non-line-of-sight application, such as developing ultra-large-scale-integration (ULSI) interconnects or coating porous materials, this method of deposition is preferred. The gaslike properties and the high pressure of a SCF facilitate the delivery of precursors to small holes and narrow tubing, producing thin films in small features that are difficult to accomplish by CVD, SFT–CD or SFT– CVD. Supercritical fluid chemical deposition (SFCD) and chemical fluid deposition (CFD) techniques have been thus developed and a typical deposition system is represented in Figure 6. Louchev et al. described a SFCD process in which a precursor was transported to a heated substrate (700 K) in a SCF where it underwent thermolysis to yield a thin copper film (62). The film produced by this method had an atomic copper concentration of approximately 80% (i.e., 20% impurities). Bocquet et al. proposed a method in which a metal alkoxide dissolved in scCO2 was thermalized at the surface of a substrate to form a thin homogeneous oxide layer (63– 64). Very thin (< 1mm), very adherent films of anatase TiO2 were formed on alumina substrates by decomposition of titanium tetraisopropoxide in a supercritical isopropanol–CO2 mixture. The films consisted of particles with diameter of approximate 100 nm. The reaction time is very short (a few minutes) and the method is adaptable to thicker films. Brasseur-Tilmant et al. prepared a TiO2 particle membrane on macroporous alumina supports by hydrolytic decomposition of titanium tetraisopropoxide in sc2-propanol

Transport and Chemical Deposition Sievers et al. described a supercritical fluid transport and chemical deposition (SFT–CD) process, also called supercritical fluid transport and chemical vapor deposition (SFT– CVD), for thin film production by using the system shown in Figure 5 (58–60). A precursor dissolved in a SCF was discharged through a restrictor, rapidly expanded into an evacuated CVD (chemical vapor deposition) chamber, and formed an aerosol or a vapor of the film precursor. The vaporized precursor was then induced to a chemical reaction at or near a heated substrate surface to form a thin film. Metal films were formed on substrates heated to a temperature range between 500 and 800 ⬚C. Metal oxide films were deposited from a plasma source with the substrate not significantly above 100 ⬚C. Through SFT–CVD, metal films of Al, Ag, Cr, Cu, In, Ni, Pd, Y, and Zr were pyrolytically deposited from metal coordination compounds dissolved in SCFs in which organic ligands or the solvent served as reducing agents for the metal ions. Films of CuO, Al2O3, Cr2O3, SiO2, and SiO2 doped with B and P were deposited on unheated substrates from

Figure 5. SCF–CD apparatus.

Physical SCF Deposition Processes Physical RESS Deposition Processes The physical RESS processes for preparing nanoparticles can be modified for deposition of thin solid films (19–22, 55). When RESS occurs, the solutes can be sprayed onto an exposed substrate surface forming uniform films. Coating thin films onto solid particles can also be achieved by in situ simultaneous nucleation and deposition of dissolved materials out of a SCF solution (56). A powder modification process was developed by RESS of a binder into a fluidized bed to coat the surface of core particles with fine particles (57). An scCO2 solution containing the binder was expanded into the fluidized bed of the mixtures of core particles and fine particles through a nozzle mounted at the center of the distributor. The deposition of the binder incorporated the fine particles into the layers around the core particles. This leads to uniform coating granulation without the agglomeration of particles. Chemical SCF Deposition Processes


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(65). TiO2 particles were deposited on the substrate surface and inside the pores. In a recent patent, a high electric-constant material, BaSr–Ti–O film was formed on a platinum thin film by reactions of organometallic precursors in scCO2. An oxide or a nitride film may also be formed by performing oxidation or nitridation at a low temperature using supercritical or subsupercritical H2O (66). A series of studies on CFD was conducted by Watkins et al. (67–72). A precursor of the material is dissolved into a supercritical or near-supercritical solvent and a substrate (or porous solid) is exposed to the solution. A reaction agent is then mixed into the solution and initiates a chemical reaction involving the precursor, thereby yielding high purity films onto the substrate surface. Metal, metal oxide, and metal sulfide films can be deposited using this method. This method can also be applied to deposit material particles into porous solids. For making metal films, CFD is essentially a hybrid technique that uniquely blends the advantages of CVD and electroless plating. High-purity films of Cu, Pt, Pd, Au, and Rh have been fabricated onto inorganic and polymer substrates by the H2 reduction of organometallic compounds in scCO2. Recently, they succeeded in depositing continuous palladium films at controlled depths within porous alumina disk (72). In SFCD and CFD techniques, the precursors react in the supercritical solution itself (without depressurization), and the apparatus is essentially the same as for SFT–CD or SFT– CVD but without the RESS stage. Subsequent cleaning of metal surfaces can be achieved by washing with neat supercritical fluid.

Immersion Deposition Ye et al. recently developed a supercritical fluid immersion deposition (SFID) method for producing thin films of metals and alloys on substrates or into porous solids of silicon, germanium, and other “fluorine-philic” elemental semiconductors from supercritical or near-supercritical CO2 solutions at modest temperature (73). The deposition setup is shown in Figure 7. The organometallic precursor was dissolved in a reservoir by supercritical or liquid CO2. The solution was then pumped into a vessel where the loaded polymer supported fluorinating agent released HF into CO2

Figure 6. Schematic view of a typical SCFD and CFD apparatus.

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solution. The solution was transported into a heated reactor (lower than 100 ⬚C) and merged with the “fluorine-philic” substrate mounted there, depositing a metal film onto the substrate surface. High-purity Pd, Ag, and Cu metal films have been thus produced on silicon or germanium substrates. SFID involves redox reactions between an organometallic precursor and an elemental semiconductor in scCO2 using HF as a reagent. The reaction is probably initiated by oxidation of an elemental semiconductor to a fluoride compound with the release of electrons that cause the reduction of an oxidized metal in a precursor to its metallic form. For SFID deposition of copper film onto silicon substrate from scCO2 solution of Cu(hfa)2 (hfa: hexafluoroacetylacetonate), the possible deposition mechanism can be proposed as following: Si + 4HF + 2Cu(hfa)2 → 2Cu + SiF4 + 4Hhfa SFID is potentially useful for depositing thin films in small features and narrow tubing owing to the gaslike properties and the high pressure of the SCFs, combined with the modest reaction temperature. Compared to the conventional CVD approach, chemical deposition processes in SCFs have several advantages. Supercritical media allow one to use relatively nonvolatile, less toxic, thermally unstable, and less expensive precursors. This extends the range of possible precursors and their combinations for production of thin films. Almost any precursor that is soluble in a SCF can be used in the SCF deposition process. Virtually all of the volatile CVD and metal–organic chemical vapor deposition (MOCVD) reagents and many semivolatile or nonvolatile reagents can be used (13–14,59). Furthermore, the deposition of mixed-metal compositions by CVD usually requires more than one precursor chamber and the delivery rates of each of the reagents must be simultaneously controlled to maintain the correct stoichiometric ratio. The preparation of YBa2Cu3O7−x films by MOCVD, for example, has been shown to require separate, individual temperature reservoirs and carrier-flow rate-controlled reservoirs for the Y, Ba, and Cu precursor compounds (74–77). In a SCF deposition process, films that require more than one reagent in the CVD process can be deposited using a single supercritical fluid solution containing all of the precursor reagent(s). The precursor mixture can be prepared by weighing the necessary species, followed by dissolution in a SCF. This allows precise control of stoichiometry and homogeneity of the reagents at the substrate surface because, unlike traditional MOCVD processes, they are from a single source

Figure 7. Setup for immersion deposition process in scCO2



that is well mixed before it enters the deposition chamber. The films obtained were usually very smooth, uniform, and highly reflective. In addition, the deposition rate of CVD process is low, but by using supercritical fluid deposition, the deposition rate can be increased. Unlike sputtering or evaporation deposition methods, supercritical fluid chemical deposition process is not limited to line-of-sight deposition, and complex shaped objects can be coated. SFCD, CFD, or SFID is a promising method to coat nanotubes. SCF Electrodeposition Processes Electrodeposition technique can also be used in a supercritical solution (78). Aluminum has been electrodeposited from supercritical electrolytes consisting of toluene, ethane, AlBr3, and Bu4NBr (or KBr) at temperatures of 75–120 ⬚C and pressures of 1200–1600 psi. Ceramic and carbon films have been deposited from supercritical water solution (79). By forming microemulsions in a SCF, many SCFs can be electrically conductive, thus applicable for electrodeposition. Other Nanomaterials Processing Using SCFs Besides the materials processing techniques discussed above, some other methods have also been developed to make nanoparticles or thin solid films using SCFs. For example, carbon coating can be obtained on silicon carbide fibers by selective etching with scH2O, which removes silicon from the surface layer of the fiber (80). This hydrothermal-leaching method provides a simple and inexpensive route to carbon coatings on the surface of polymer-derived SiC fibers. Nanoporous silica materials have been prepared using an activated carbon as a mold and scCO2 as a solvent (81). Tetraethyl orthosilicate dissolved in scCO2 was in contact with the activated templates of various macroscopic shapes. After the removal of activated carbon templates by calcination in an air or oxygen plasma treatment, microporous and mesoporous SiO2 samples, replicating not only meso-structures, but also macroscopic shapes, of activated carbon molds were obtained. Hazards Silane, HF, and other hazardous materials as well as high pressure and temperatures could be involved in the referenced experiments. Suitable safety precautions should be taken into consideration including the use of hoods and blast screens to prevent possible leakage or explosion. Conclusions Processing materials in a SCF, especially in scCO2, is a novel and emerging technology for preparing nanoparticles, nanowires, and thin solid films. Supercritical CO2 is inert and recyclable; therefore, exposure of personnel to hazardous solvents and disposal of organic liquid wastes can be minimized. Removing unreacted reagents, byproducts, and contaminants from the SCF system can be easily achieved, thus leading to production of nanoparticles, nanowires, and thin solid films with high purity. Because of the gaslike diffusivity of SCFs, diffusion-limited reactions occur more uniformly than in conventional solvents. Reactions involving gases for preparing nanomaterials are more efficient in SCFs

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