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Processing of Iron Oxide Nanoparticles by Supercritical Fluids Un Teng Lam,† Raffaella Mammucari,† Kiyonori Suzuki,‡ and Neil R. Foster*,† School of Chemical Sciences and Engineering, The UniVersity of New South Wales, Sydney, NSW 2052, Australia, and Department of Materials Engineering, Monash UniVersity, Melbourne, VIC 3800, Australia
Supercritical fluids possess unique characteristics that make them effective and environmentally friendly processing media for nanomaterials. Of the nanomaterials of widespread applicative interest, iron oxide nanoparticles have an important role. The importance of iron oxide nanoparticles is due to the multidisciplinary nature of their industrial applications. However, the use of supercritical fluids in the processing of iron oxide nanoparticles is not well-recognized. In this review, special emphasis is placed on applying supercritical fluids technology for iron oxide nanoparticle synthesis, mesoporous iron oxide composite generation, and encapsulation of iron oxide nanoparticles. The role of supercritical fluids in hydrothermal synthesis, nanoscale casting, supercritical drying, and antisolvent techniques is discussed in detail. Finally, potential applications of supercritical fluid technology for the processing of iron oxide nanoparticles are presented. 1. Introduction The term supercritical fluid (SCF) refers to a substance that is above both its critical temperature and pressure.1 When a substance is placed above the critical point, only a single phase exists. Supercritical fluids possess characteristics intermediate to those of liquids and gases. The mass-transfer properties (diffusivity and viscosity) are similar to those of gases, whereas the density and solvating capability are more similar to those of liquids.2 Another attractive feature of SCFs is the ease of tuning various physical properties by varying the operating pressure and temperature. Because of these unique features, SCFs serve as excellent processing media to replace or minimize the use of volatile and toxic organic solvents in many industrial processes and chemical reactions.3 Supercritical fluids have been exploited extensively in nanomaterial processing, ranging from chemical synthesis, polymerization, film generation, nanoparticle generation, nanoencapsulation, and mesoporous material generation to the impregnation or deposition of nanoparticles in porous materials. Thorough reviews on the application of SCFs focusing on different classes of material (such as polymeric materials,4,5 pharmaceutics,2,6 and porous materials7) are available in the literature. In this review, special attention is given to the application of SCFs for the processing of iron oxide, which is a compound with increasing potential, because of its remarkable success as a functional nanomaterial in many industrial applications. The purpose of this review is to outline current and prospective applications that involve SCFs for the synthesis and post-processing of iron oxide nanoparticles. 1.1. Physical and Chemical Properties of Iron Oxide. Iron oxide is a widely used and well-recognized compound, which exists in 16 identified forms.8 In this review, the focus will be placed on three forms of iron oxide, namely, hematite (R-Fe2O3), magnetite (Fe3O4), and maghemite (γ-Fe2O3), because they are the principal forms used in industrial applications. Some of their physical and magnetic properties are summarized in Table 1. * To whom all correspondence should be addressed. Tel.: 00-612-9385 4341. Fax: 00-61-2-9385 5966. E-mail address: n.foster@unsw. edu.au. † School of Chemical Sciences and Engineering, The University of New South Wales. ‡Department of Materials Engineering, Monash University.
1.2. Applications. Iron oxide particles are widely used as ores for the iron and steel industry, as pigments for paints, as catalysts in many industrial processes, and as magnetic pigments in the recording industry. Other uses of iron oxide include ferrofluid, jewelry, and in the production of photochemicals and fertilizers.8 Different forms of iron oxide have been traditionally used as coloring agents for thousands of years, and the three forms of iron oxide discussed previously are still commonly used as synthetic pigment in modern paint, ceramics, and porcelain. As pigments, different forms of iron oxide exhibit a different color; for instance, pigments based on hematite are red, maghemite-based pigments are brown, and magnetite-based pigments are black. As catalysts, hematite and magnetite are used in the Haber process, the Fischer-Tropsch synthesis, water-gas-shift reactions, the dehydrogenation of ethylbenzene to styrene, and the vapor-phase oxidation of alcohols to aldehydes and ketones.8 Magnetite and maghemite are used heavily in the media recording industry and biomedical fields, mainly because of their magnetic properties. On the other hand, hematite is a very weak ferromagnetic material at room temperature and, thus, is rarely used in these areas. Maghemite is the principal magnetic pigment used in many electronic recording devices. Information is stored on the recording medium when a strong magnetic field is generated at the gap of the magnetic head. The maghemite particles embedded in the medium are magnetized by the magnetic field and the magnetic polarity of the particle changes. The magnetically polarized particles remain polarized at the remanent point of the demagnetization process, even if the magnetic field is removed, and, hence, the local field direction of the magnetic head is recorded in the medium. The maghemite particles used for the recording media have a high coercivity, which means that a high intensity of demagnetizing field is required to reduce the magnetic polarization to zero from the saturation state. Therefore, the maghemite particles exhibit a high resistance to the self-demagnetization effect or external magnetic fields, which leads to long-term stability of the information recorded.8 Magnetite is sometimes used in toner and ink for printers, photocopiers, and facsimile machines, and in certain security inks. Apart from being magnetically responsive, magnetite and maghemite are widely used in biomedical applications, because of their biocompatibility and low toxicity in the human body.9,10
10.1021/ie070494+ CCC: $40.75 © 2008 American Chemical Society Published on Web 01/16/2008
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Table 1. Physical and Chemical Properties of Hematite, Magnetite, and Maghemitea Value/Remark property molecular formula crystal structure density (g/cm3) melting point (°C) boiling point (°C) color hardness type of magnetism Curie temperature (K) saturation magnetization, σs , at 300 K (Am2/kg) standard free energy of formation, ∆G0f (kJ/mol) heat of decomposition (kJ/mol) a
hematite
magnetite
maghemite
R-Fe2O3 rhombohedral, hexagonal 5.26 1350
γ-Fe2O3 cubic or tetrahedral 4.87
red 6.5 weakly ferromagnetic or antiferromagnetic 956 0.3
Fe3O4 cubic 5.18 1583-1597 2623 black 5.5 ferrimagnetic 850 92-100
820-986 60-80
-742.7
-1012.6
-711.1
461.4
605
457.6
reddish-brown 5 ferrimagnetic
Adapted from Cornell and Schwertmann.8
Magnetite and maghemite nanoparticles have been extensively studied as drug carriers in magnetic drug targeting,11-13 as contrast agents in magnetic resonance imaging (MRI),14-16 and as heating mediators in magnetic hyperthermia.17,18 Several magnetite/maghemite-based contrast agents for MRI have been commercialized and applied clinically over the last two decades.19 A large number of in vivo tests11,12 and clinical trials20,21 of magnetic drug targeting and magnetic hyperthermia have been conducted using magnetite or maghemite since the mid-1990s. 1.3. Size-Dependent Properties. The particle size of iron oxide is a critical factor for almost all of the aforementioned applications. For instance, surface area, which is a dominating factor for catalysis, is a strong function of the particle size. Smaller particle sizes are generally more valuable in catalysis, because of the greater surface area. The magnetic behavior of iron oxide is also size-dependent.22 It is well-known that ferrimagnetic materials, such as magnetite and maghemite, exhibit superparamagnetism when the particle size is smaller than ∼10 nm.10,23 The superparamagnetic state is induced when the thermal vibration is greater than the magnetic anisotropy energy of the nanoparticle.24 Anisotropy energy is the potential energy that governs the direction of the spontaneous magnetization. The direction of the spontaneous magnetization in each superparamagnetic particle flips randomly with time, because of the large effect of thermal vibration, resulting in the disappearance of the coercivity.24 Hence, superparamagnetic iron oxide nanoparticles exhibit zero magnetic remanence, while ferrimagnetic iron oxide microparticles remain highly magnetized, even in the absence of an applied field.25 Because magnetically polarized particles are attracted to each other, the superparamagnetic behavior is highly desirable for biomedical applications, because it minimizes the agglomeration of the magnetic particles after the removal of an applied magnetic field. Also, the high magnetization response, which is due to the large magnetic moment in the superparamagnetic particles, ensures that movement of the particles can be easily controlled by an external magnetic field. 2. Synthesis of Iron Oxide Nanoparticles Different forms of iron oxide have been synthesized by methods such as coprecipitation,26,27 microemulsion techniques,28,29 and sol-gel technology.30 In the coprecipitation method, Fe2+ and Fe3+ salts are dissolved in an aqueous solution in specific ratios; subsequently, the precipitation of iron oxide is initiated by the addition of a strong base. The precipitate must be digested in the mother liquor to form crystals. Particle size,
morphology, and crystal structure are strongly dependent on pH and temperature and involve the control of many variables during the synthesis and the purification steps. Because of the fine pH adjustment involved, the coprecipitation method faces difficulties when utilized for producing large amounts of product.31 The microemulsion method for the synthesis of iron oxide nanoparticles involves the formation of a water-in-oil (w/o) emulsion system.31 In the w/o system, microdroplets of an aqueous phase are stabilized within monolayers of a surfactant and are dispersed in a continuous oil phase. Reactants are dissolved in the entrapped aqueous phase. Precipitation occurs during the collision and mixing of the droplets. The major drawback of this process is the intensive use of organic solvents (such as heptane) as the oil phase.29 Product purification is timeconsuming, with a possible risk of residual solvents and surfactants. Sol-gel processes involve the dissolution of precursors in an appropriate solvent. The gelation step usually takes days to complete. Post-annealing at high temperature is required after solvent removal from the gel and has a strong effect on the product crystallinity.32 It is a constant challenge to overcome the limitations of conventional processes for the synthesis of iron oxide nanoparticles, and especially in large-scale production; for this reason, SCF technology can be an attractive option. 2.1. Hydrothermal Synthesis. Hydrothermal synthesis refers to the reaction that is initiated by bringing aqueous solutions to high temperature and pressure. The definition of high temperature and pressure is quite broad. Generally, conditions ranging from the boiling point of water (100 °C, 0.10 MPa) up to near the critical point (374.3 °C, 22.11 MPa) can be considered as hydrothermal. The hydrothermal formation of metal oxide nanoparticles can be performed either by low-temperature hydrothermal synthesis or by hydrothermal synthesis under supercritical conditions. For the purpose of this review, only studies of near-critical or supercritical hydrothermal synthesis are included. Hematite and magnetite have been synthesized successfully in supercritical water (scH2O) via a two-step synthetic route: hydrolysis of Fe2+ ions, followed by dehydration:
Hydrolysis: Mn+ + nH2O f M(OH)n + nH+ n Dehydration: M(OH)n f MOn/2 + H2O 2
Ind. Eng. Chem. Res., Vol. 47, No. 3, 2008 601 Table 2. Summary of Parameters and Product Characterization for Hydrothermal Synthesis of Iron Oxide Nanoparticles precursor
initial concentration (M)
Fe(NO3)3 Fe(NO3)3
0.05 0.05-0.5
Fe(NO3)3 Fe(NO3)3 Fe(NO3)3 Fe(NH4)(SO4)2 Fe(NH4)(SO4)2 Fe(NO3)3 Fe(NO3)3/FeSO4 (1:1) FeSO4
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Fe(NO3)3 Fe(NO3)3 Fe(NO3)3 Fe(NO3)3 Fe2(SO4)3 FeCl2 Fe(NH4)2H(C6H5O7)2
0.01-0.5 0.03 0.001-0.1 0.16 0.16 0.16 0.0066
additional component
temperature (°C)
pressure (MPa)
Batch Hydrothermal Synthesis 400 unspecified 383-394 unspecified
product form
XRD
Particle Size (nm) TEM SEM
R-Fe2O3 R-Fe2O3
30-40
Rapid Thermal Decomposition of Precursors in Solution 225 48-55 R-Fe2O3 300 40-45 R-Fe2O3 400 40-45 R-Fe2O3 200 48-55 R-Fe2O3 400 48-55 R-Fe2O3 300-400 34.5-55.2 R-Fe2O3 400 34.5-55.2 R-Fe2O3 urea 300 34.5-55.2 Fe3O4
,10 11 23 44 42 11-25 32 15
Continuous Supercritical Hydrothermal Synthesis 247-300 23.5-24.4 R-Fe2O3 196-385 24.5-25.3 R-Fe2O3 400 25.0-37.5 R-Fe2O3 400 35 R-Fe2O3 400 35 R-Fe2O3 400 35 R-Fe2O3 400 35 Fe3O4
The formation of the intermediate hydroxide occurs during the hydrolysis step and is an immediate process.33 The hydroxide forms a gelatinous solid network, which dehydrates slowly at room temperature.33 As with other SCFs, the tunable properties of scH2O allow great flexibility in controlling the reaction conditions by varying the operating pressure and temperature. The use of scH2O overcomes the need to utilize organic solvents and totally overcomes the problem of residual solvent. The high self-dissociation and low dielectric constant of scH2O allow the precipitation of iron oxide without the addition of strong bases, which is required in the conventional methods.34 The dehydration rate is accelerated as the reaction is performed at elevated temperatures. In addition, the high diffusivity of reactants in scH2O favors a high overall reaction rate.32 The low solubility of metal hydroxide and oxide in scH2O induces a high supersaturation and thus induces the formation of very fine crystals in the dehydration step.32,34,35 Hydrothermal synthesis is conventionally performed in a batch-type autoclave. Two configurations of continuous singlestep processes were later developed that mainly use a tubular reactor. They are known as rapid thermal decomposition of precursors in solution (RTDS), where the iron salt solution is directly brought up to supercritical conditions, and continuous supercritical hydrothermal synthesis (CSHS), where the metal salt solution is mixed with another stream of scH2O. Studies on hydrothermal synthesis of iron oxide using batch and continuous processes are summarized in Table 2. 2.1.1. Batch Hydrothermal Synthesis. In the batch process, the precursors are dissolved in water and the solution is
32-64
33-63
40-86
ref 36 37
2-10 4-10 20-40 2-10 40-80
38 38 38 38 38 39 39 39
13-32 30-40 5-8 50 50 50 50
32 33 35 40 40 40 40
50 50 50 50
constantly heated to the desired temperature and pressure at which it is aged for several hours or days.41 A mixture of hematite (R-Fe2O3) and indium oxide (In2O3) nanoparticles 3040 nm in diameter have been synthesized under supercritical conditions in a batch-type autoclave at 400 °C.36 The hydrothermal synthesis of iron oxide can also be performed within porous structures. Hematite nanoparticles have been effectively synthesized within the pores of activated carbon pellets.37 The activated carbon was first immersed in the aqueous precursor solution under ambient conditions for a period of time to allow diffusion of the precursors into the pores. The pellets were then brought to the final supercritical conditions in an autoclave to further enhance the diffusion and initiate the hydrolysis and dehydration. Hematite nanoparticles ∼16-36 nm in size were found evenly distributed throughout the pellets, whereas a larger number of particles accumulated at the near-surface regions (see Figure 1). 2.1.2. Rapid Thermal Decomposition of Precursors in Solution (RTDS). The first continuous process for synthesizing fine iron oxide nanoparticles was developed by Matson and coworkers and is known as RTDS.38,39,42,43 In this process, an iron salt solution is pumped through a heated section at a high flow rate to attain supercritical temperature and pressure. Crystallization occurs in the heated section and is terminated when the particle-fluid mixture expands through the nozzle, followed by a rapid quench in the condenser. The formation of nanoparticles is due to the rapid heating rate, high reaction rate, and short residence time in the reactor. The final temperature (200400 °C) is usually reached in less than a few seconds and the
Figure 1. Distribution of hematite nanoparticles at the near-surface regions (left), half of pellet radius (middle), and center (right) of the activated carbon pellets. Adapted with permission from Xu and Teja.37 Copyright 2006, Elsevier.
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Figure 3. Transmission electron microscopy (TEM) images of iron oxide/ 6-line ferrihydrite nanoparticles produced from a 0.1M Fe(NO3)3 solution at increasing RDTS processing temperatures. Reproduced with permission from Matson et al.39 Copyright 1994, American Chemical Society.
Figure 2. Experimental setup for rapid thermal decomposition of precursors (RTDS).
residence time is usually within the range of 5-30 s. Correction to the batch process, dehydration happens when the hydroxide crystals are still very small.40 Thus, dehydration becomes much easier and faster for small hydroxide particles, which will, in turn, give small final oxide particles. A schematic of the experimental setup is presented in Figure 2. Hematite nanoparticles 350 °C. Images of mixtures of hematite and 6-line ferrihydrite produced at different temperatures in the scale-up setup are presented in Figure 3. 2.1.3. Continuous Supercritical Hydrothermal Synthesis (CSHS). After the development of RDTS, various researchers engaged in the design of alternative ways to bring the precursor solutions to supercritical conditions. A modified process, which is commonly known as CSHS, was developed. The key feature of this process is the rapid and intimate mixing of the preheated scH2O stream and the precursor solution stream at a T-mixing point. Nucleation happens primarily at the mixing point. Further crystal growth and aggregation are believed to occur in the reactor.32 A diagram of the experimental apparatus for CSHS is given in Figure 4. Because metal hydroxides have a very low solubility in scH2O, a high supersaturation can be achieved just after the mixing point.33
Figure 4. Experimental apparatus for continuous supercritical hydrothermal synthesis (CSHS).
The CSHS treatment of hematite and magnetite nanoparticles 50 nm in diameter was successfully conducted without the addition of strong bases.40 In addition to the precursors used in RTDS, iron(III) sulfate and iron(II) chloride were also used for the synthesis of hematite, while ferric ammonium citrate was used as a precursor for magnetite. In the formation of magnetite, the reduction of Fe3+ species to Fe2+ species occurred. It is believed that the reduction of the Fe3+ species is due to the presence of carbon monoxide (CO), which was generated during the thermal decomposition of ferric ammonium citrate.40 Therefore, it is believed that the final product form can be controlled by the introduction of an oxidizing or reducing gas during the synthesis. Hao and Teja32 conducted a detailed investigation of the effects of different operating conditions on particle size and morphology. It was observed that particle size increases with the concentration of the initial precursor, as can be inferred by the transmission electron microscopy (TEM) images in Figure 5. Particle morphology also varied as a function of the concentration of the starting precursor solution. In fact, spherical particles were produced at low concentration and more rhombic particles were found at higher concentration. Contrary to the RDTS process, temperature did not impact significantly on the particle size, whereas long residence times produced larger crystals with a broader particle size distribution. 2.2. Rapid Expansion of Supercritical Solutions (RESS). Carbon dioxide (CO2) can also be used as a SCF in the synthesis of iron oxide particles. The supercritical synthesis of iron oxide
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Figure 5. TEM images of hematite nanoparticles obtained at a reactor temperature of 300 °C and a residence time of 12 s, with a precursor concentration of 0.03 M (right) and 0.50 M (left). Reproduced with permission from Hao and Teja.32 Copyright 2003, Materials Research Society.
nanoparticles using CO2 as a SCF has been conducted via a modified rapid expansion of supercritical solutions (RESS) process.44 The RESS process is commonly used for the production of fine particles and polymer films. In the RESS process, a solute is first dissolved in the supercritical medium; the supercritical solution is then rapidly depressurized through a nozzle in an expansion chamber. During the expansion, the solvating power and density of the supercritical medium decrease significantly and, therefore, a high degree of supersaturation can be achieved almost instantaneously. The high level of supersaturation achieved during the RESS process favors the formation of small particles. CO2 is, by far, the most commonly used fluid in the RESS process, because of its low toxicity and moderate critical conditions. However, the application of the RESS process to the formation of metal-based compounds is not common, because of the generally low solubility of such compounds in scCO2. The RESS process produces low throughput for products that exhibit a low solubility in the SCF, unless noticeable amounts of SCF are used, which, in turn, makes the process less economically attractive. To overcome this challenge, a CO2soluble precursor, iron carbonyl, has been selected.44 The precursor was first dissolved in scCO2 and the solution was sprayed onto a silicon collection substrate that was attached to the wall of the expansion chamber. Photolysis of the precursor was obtained by exposing the jet to a 10 kW ultraviolet (UV) beam during the entire expansion process. After deposition was completed, the product was further exposed to the UV beam. Carbonyl ligands were cleaved during photolysis to produce pure metallic iron nanoparticles. After the product was removed from the expansion chamber, the iron nanoparticles were quickly oxidized to maghemite and magnetite. Ultrafine particles ∼510 nm in size were produced and collected on a substrate. 3. Generation of Mesoporous Iron Oxide Composites Porous materials have important roles in adsorption, separation, catalysis, ion exchange, chemical sensing, drug delivery and tissue engineering.45 Porous materials are generally classified by the pore size. According to the International Union of Pure and Applied Chemistry (IUPAC) recommendations, a porous material is classified as microporous when the pore size is 50 nm.46,47 Porous materials associated with iron oxide mostly fall into the mesoporous class, which, thus, is within the scope of this review. Special attention will be given to aerogels, the pore size of which is within the mesoporous definition. Aerogels differ from
other mesoporous materials, because of their extremely low density. Conventional preparations of porous materials involve the extensive use of organic solvents; replacing organic solvents with SCFs is a more environmentally friendly alternative. Several solvent-free approaches that use SCFs48,49 have been developed and are particularly useful for applications such as tissue engineering and drug delivery, because of the elimination of toxic solvents. In fact, regulatory agencies impose strict limits on the amount of residual solvents allowed in the human body. The other main advantage of using SCFs as solvents is the preservation of the porous structure during the drying step, because the collapse that results from the phase changes of the organic solvent during conventional drying is avoided. The generation of mesoporous materials using SCFs can be achieved by either physical or chemical routes. Common physical means include foaming,50,51 SCF antisolvent-induced phase separation,52,53 and crystallization of SCF-swollen crosslinked polymers.54 Chemical processes include nanoscale casting,55,56 chemical gelation of SCF solutions, and the templating of SCF emulsions.57 3.1. Nanoscale Casting. Nanoscale casting is often adopted for the synthesis of mesoporous iron oxide. Nanoscale casting is a template-based method that is conventionally performed using a liquid solvent. Activated carbon is used as the template, which has micropores (
