Nanostructured Materials Synthesized in Supercritical Fluid Part 1 (Nanotechnology)

INTRODUCTION

Supercritical fluid (SCF) approach is a novel and emerging technology to generate nanomaterials in small areas, high-aspect-ratio structures, complicated surfaces, and poorly wettable substrates with high uniformity, high homogeneity, and minimum environmental problems.

Through hydrogen reduction of metal-p-diketone complexes in supercritical CO2, a rapid, convenient, and environmentally benign approach has been developed to synthesize a variety of nanostructured materials: 1) metal (Pd, Ni, and Cu) nanowires and nanorods sheathed within multiwalled carbon nanotube (MWCNT) templates; 2) nanoparticles of palladium, rhodium, and ruthenium decorated onto functionalized MWCNTs. These highly dispersed nanoparticles are expected to exhibit promising catalytic properties for a variety of chemical or electrochemical reactions; 3) Cu, Pd, or Cu-Pd nanocrystals deposited onto SiO2 or SiC nanowires (NWs). Different types of nanostructures were achieved, including na-nocrystal-NW, spherical aggregation-NW, shell-NW composites, and ”mesoporous” metals supported by the framework of NWs.

BACKGROUND

Supercritical fluid synthesis and processing of nano-structured materials have attracted an increased attention during the past decade.[1-6] SCFs exhibit a novel hybrid of liquid-like and gas-like properties. They have appreciable densities and can dissolve solid compounds like liquid solvents yet they have low viscosities, low surface tension, 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. Because of these unusual properties, the synthesis and processing of nanostructured materials using SCFs show significant advantages over conventional processes: 1) SCFs facilitate permeation, diffusion, and penetration to small areas, high-aspect-ratio structures, complicated surfaces, and poorly wettable substrates to attain high uniformity and homogeneity, therefore being capable of fabricating nanostructured materials which are difficult to accomplish through traditional methods; 2) SCFs allow higher concentrations of starting materials than chemical vapor deposition (CVD) does and provide diffusivities higher than liquid solvents do, therefore SCF synthesis and processing could be very fast; 3) the solvent strength of a SCF can be varied by manipulation of fluid temperature and pressure, thus allowing a degree of control and rapid separation of products which is not possible using conventional solvents; and 4) some SCFs, such as supercritical CO2 (scCO2), leave no solvent residues and are recyclable, thus being environmentally benign. Furthermore, unreacted materials, by-products, and contaminants in SCFs can be easily removed from the system; therefore products of high purity can be obtained. The synthesis and processing of nanostructured materials such as nanoparticles, nanowires, nanorods, nanotubes, nanocomposites, and thin solid films with nanoscale thickness have been achieved through a number of SCF physical and chemical transformations in which a SCF can act as a medium either for transporting solute species or for chemical reactions, or both. In some cases, the SCF itself can also take part in the reactions. Table 1 summarizes the typical SCF approaches to a variety of nanostructured materials.[1-6] The aim of this article is to show how to use SCFs in the synthesis and processing of nanostructured materials templated by MWCNTs and nanowires (NWs).

Table 1 Typical SCF approaches to nanostructured materials

	Nanoparticles, nanowires, nanorods nanotubes, nanocomposites	Thin films
Physical transformation	Rapid expansion of supercritical solutions (RESS)	Physical RESS deposition
	Supercritical-assisted nebulization and atomization	Physical deposition in SCFs
	Physical impregnation in SCFs without expansion
	Supercritical antisolvent precipitation
	Supercritical drying processes
	Size-selective supercritical dispersion and dissolution
	Physical deposition processes in SCFs
Chemical transformation	RESS into liquid solvents	SCF transport and chemical deposition
	Hydrothermal reactions	SCF transport chemical vapor
		deposition
	Water-in-SCF microemulsion reactions	SCF immersion deposition
	Arrested precipitation in SCFs	SCF chemical deposition
	SCF-liquid-solid approach	Chemical fluid deposition
		SCF Deposition of Self-Assembled
		Monolayers
		Electrodeposition in SCFs
		Hydrothermal leaching

Because of their small size, high chemical stability, high-aspect-ratio cavities, and large surface-area-to-volume ratio, CNTs have been considered as templates for confining and directing the growth of metallic nanowires, nanorods or tubular structures, or as supports for metal nanoparticles which can be impregnated in the cavities or attached to the external walls of the CNTs.[26-36] The produced metal/CNT composites can be used as catalysts, sensors, semiconductor devices, data storage, and processing devices, contrast agents in magnetic resonance imaging, new reinforced metal-nanofiber materials, and in xerography.[26,27,35,37-41] Metal impregnation in the hollow interiors of the CNTs can be achieved in situ during CNT growth by incorporating the metals or metal precursors along with the carbon source. Although Fe, Co, Ni, Ti, Cu, and certain lanthanide and transition-metal carbides have been successfully trapped in CNTs using this method, harsh conditions such as high temperature or arc evaporation are usually required, and impurities could be produced as encapsulated carbon clusters and soot.[42-44] Capillary drawing of low-melting metals into cavities of CNTs provides a simple approach for metal loading;[28,45] however, CNTs are not wetted by liquids with surface tensions higher than 100-200 mN m-1, thereby excluding most metals and other elements in the periodic table.[46] The most promising and flexible approach to metal loading is then to deposit metals into the cavities or onto the external walls of CNTs through a chemical reaction such as CVD or wet chemical process.[13,15,26,29-32,35,36,47-55] However, by virtue of the small inner diameter and the extremely high aspect ratio of CNTs, this approach requires high temperatures or extensive reaction times for filling metals into the inner cavities of CNTs, and, consequently, the percentage of filled CNTs often is low. The CVD procedure may suffer from the limited volatility of metal precursors and the resulting low-vapor-phase concentration and mass-transfer-limited reactions, in addition to the high temperature for the decomposition or reduction of metal precursors. In the case of wet chemical procedures, a hindrance could be the slow process for concentrating and impregnating reactants into the cavity of CNTs. As the pristine surface of the CNTs is rather inert and poorly hydrophilic, this approach also results in unsatisfactory adhesion and coverage control of metal nanoparticles coated onto the outer walls of CNTs, and metal agglomeration into fewer larger particles as well.[53,54] For most of the catalytic applications, catalyst particles loaded on the exterior of the CNTs are preferred because they are more accessible to the reactant molecules than those encapsulated inside the internal channels. In order to obtain a specific nucleation of metals on the outer surface with good adhesion and control, functionalization of CNTs before metal deposition is required and can be accomplished by chemical treatments using a myriad of oxidants, such as HNO3, KMnO4, OsO4, HNO3/H2SO4, and RuO4 to generate -COOH, -OH, and other functional groups on the external walls of CNTs.[15,31,32,36,55,56] Also, one-step or two-step sensitiza-tion-activation methods have been used for introducing catalytic nuclei (often Pd-Sn alloys or Pd nuclei) to the otherwise noncatalytic CNT surface to achieve a better metal loading through electroless deposition.[13,30] However, the known wet chemical processes usually involve tedious and time-consuming treatment of CNTs and generate aqueous wastes.

Considerable efforts have also been spent to the modification of NWs to produce hybrid nanocomposites, in which NWs were decorated with nanoparticles, or sheathed by thin films, shells, and molecular layers, and most methods for modifying are solution-based.[17-25]

Based on hydrogen reduction of metal-p-diketone complexes in supercritical CO2, we have developed a rapid, direct, and clean approach for the modification of MWCNTs and NWs to achieve nanocomposites as follows: metal nanowires or nanorods sheathed within MWCNT templates, MWCNTs decorated with catalytic metal nanoparticles, NWs decorated with metal nanocrys-tals, spherical aggregations of metal nanocrystals strung up by NWs, NWs wrapped by metallic shells, and ”mesoporous” metals supported by the framework of NWs.[57-59]

HYDROGEN REDUCTION OF METAL-p-DIKETONE COMPLEXES IN ScCO2

Hydrogen reduction of metal precursors such as metal-p-diketone complexes in scCO2 has proven to be one of the most successful approaches for synthesizing steri-cally stabilized metal nanocrystals.[60,61] ScCO2 reactions and particle nucleation occur in the presence of organic capping ligands, which bind to the surface of the agglomerates to form monolayers and quench further growth, providing size control and nanocrystal stabilization. Moreover, the steric stabilization of nano-crystals in scCO2 varies with the tunable density and solvation power of scCO2, enabling reversible stabilization and destabilization of colloidal dispersion, which could improve many aspects of nanocrystal processing, such as size-selective separation, synthesis, and self-assembly. Robust, highly crystallized, relatively size-monodisperse, and sterically stabilized silver, iridium, and platinum nanocrystals ranging in diameter from 20 to 120 A were synthsized in scCO2 by reducing metal-p-diketone complexes with H2 in the presence of fluori-nated thiol ligands.

Fig. 1 Schematic drawing of the experimental apparatus.

Hydrogen reduction of metal-p-diketone complexes in scCO2 has also been demonstrated as an effective method to deposit metal films into high-aspect-ratio structures of inorganic and polymer substrates as well as into mesoporous solids.[62-67] Herein a metal-p-diketone complex is dissolved into scCO2 and a heated substrate (or mesoporous solid) is exposed to the solution. H2 is then mixed into the solution and initiates a chemical reaction involving the precursor, thereby yielding metal films onto the substrate.

Likewise, hydrogen reduction of metal-p-diketone complexes in scCO2 can be applied to the modification of MWCNTs and NWs using a typical experimental setup shown in Fig. i .[57-59] The MWCNTs have diameters of about 20-30 nm and lengths of about 2-3 mm. Two types of NWs, SiO2, and SiC NWs, 40 to 110 nm in diameter, several tens of micrometers in length, and randomly oriented on silicon substrates, were subjected to modification. MWCNTs, SiO2, or SiC NWs on silicon substrates were loaded in a 3.47-mL high-pressure stainless steel reactor along with a metal-p-diketone complex. Following precursor loading, valve V1 was closed while valves V2, V3, and V4 were opened and H2 at 3 atm was allowed to flow through the reactor for 5 min to expel the air inside. Valves V2, V3, and V4 were then closed, and V1 was opened to charge the H2-CO2 mixer with 80 atm of CO2. After mixing of H2 and CO2, valve V3 was opened forcing the mixture into the reactor. Valves V1 and V3 were then closed for the dissolution of the precursor in the CO2 solution. To ensure complete dissolution, the reactor was left undisturbed for 30 min. After that, the reactor was heated gradually to the desired temperature and kept at this constant temperature for 510 min. After the reaction, the reactor was cooled to 35 °C and vented slowly by opening V4. Neat CO2 flow was used to flush the reactor twice to remove the possible unreacted species and by-products. The reactor was then opened to recover the modified MWCNTs or NWs.

Fig. 2 TEM images of metal nanowires or nanorods impregnated in MWCNT templates. (a,b) Paladium. (c) Nickel. (d) Copper.

METAL NANOWIRES AND NANORODS SHEATHED WITHIN MWCNTS

Unfunctionalized MWCNTs were used as the templates for confining and directing the growth of metal nanowires and nanorods caused by the hydrogen reduction of metal-p-diketone complexes in scCO2. The metal-p-diketone complexes used were M(hfa)2 xH2O (M=Pd, Ni, and Cu; hfa=hexafluoroacetylacetonate), and the temperatures of hydrogen reduction were 80-150°C, 250°C, and 250°C for filling paladium, nickel, and copper into MWCNTs, respectively. Transmission electron microscopy (TEM) observation of many different views of the product revealed several forms of foreign materials inside the MWCNTs. The amount of filled MWCNT out of the total number of nanotubes is estimated at about 10%. Fig. 2a and b shows the TEM images of nanowires or nanorods sheathed within carbon nanotubes. The nanowires are 7-9 nm in diameter and can be more than 200 nm in full length. The diameter of nanowires corresponds to the inner diameter of the MWCNTs and varies along the wires due to the fluctuation in the MWCNT diameter. The nanowires can be straight or curved, depending upon the curvature of the CNT wrapping. Fig. 2b also shows several segments of nanowires or nanorods that have been filled into the MWCNT. A typical energy dispersive X-ray spectrum (EDS) conducted on an individual nanowire or nanorod in the TEM is shown as an inset in Fig. 2b. It confirms that the nanowires and nanorods were made purely of palladium. Similarly, nanowires and nanorods of nickel or copper can also be developed within the channels of MWCNT templates, as shown in Fig. 2c and d.

Fig. 3 (a) HRTEM images of a Pd nanorod sheathed within a MWCNT, revealing that the Pd nanorod is crystalline with a fcc structure, (b) enlarged section of (a) showing that the imaging zone axis is [110].

Fig. 3a shows a high-resolution transmission electron microscopy (HRTEM) image of a Pd nanorod sheathed by a MWCNT, revealing that the Pd nanorod is composed of segments of single crystals. The region marked by a dashed square in Fig. 3a was further enlarged and is shown in Fig. 3b. Fourier transform and Fourier filtered HRTEM image are shown as insets in Fig. 3b. The HRTEM image processing indicates that this segment of Pd nanorod possesses the face centered cubic (fcc) structure with a measured lattice constant of 0.38 nm, which is comparable with the reported lattice constant of 0.3887 nm.

Similar to CVD and some wet chemical processes for MWCNT decoration, nucleation of metals as nanoparticles on the outside of MWCNTs occurred unavoidably in supercritical CO2 along with metal filling. As has been reported, the defects in the MWCNT structure can provide favored sites for nucleation and growth of particles. Therefore besides the pure nanowire (or nanorod)/ MWCNT composites, nanowire(or nanorode)/MWCNT/ nanoparticle composites are also observed. As shown in Fig. 3 a, a couple of palladium nanoparticles were attached to the exterior surface of the MWCNT sheathing the nanowire. Based on our experiment, we can somehow adjust the outside or inside loading preference of a metal by functionalization of MWCNTs. Metal deposition occurs only on the external walls of functionalized MWCNTs.

MWCNTS DECORATED WITH CATALYTIC METAL NANOPARTICLES

Functionalization of MWCNTs

The functionalization of MWCNTs was performed by dispersing and refluxing 0.5 g of MWCNTs in 40 mL of concentrated H2SO4-HNO3 mixture (1:1 v/v ratio) for 6 hr to form a dark-brown suspension. The reaction mixture was then diluted with distilled water to 200 mL and stirred for several hours, cooled down to room temperature, and filtered. The recovered black solid was washed several times with distilled water and finally dried at room temperature in vacuum. Previous X-ray photoelectron spectroscopy (XPS) and diffusion reflectance infrared Fourier transform (DRIFT) studies revealed that the surfaces of functionalized MWCNTs become covered with carboxylic (-COOH), Carbonyl (>C=O), and hydroxyl (-COH) groups[31,32,68] These functional groups have been demonstrated to provide favorite nucleation sites for metal nanoparticle growth and to stabilize the nanoparticles by increasing the nanoparticle-CNT interaction.

Fig. 4 TEM images and EDX spectroscopy of MWCNTs decorated with Pd nanoparticles after hydrogen reduction of (A) 10 mg, (C) 20 mg, (D) 30 mg and (E) 50 mg Pd(hfa)2.

Fig. 5 (A) TEM and (B) HRTEM images of MWCNTs decorated with Rh nanoparticles.

Functionalized MWCNTs Decorated with Catalytic Palladium Nanoparticles

Modification of 10 mg of functionalized MWCNTs was carried out through hydrogen reduction of Pd(hfa)2 • xH2O at 80°C. A bright field TEM micrograph of the MWCNTs after scCO2 deposition using 10 mg of Pd(hfa)2 • xH2O is shown in Fig. 4A. Well-dispersed, spherical particles were anchored onto the external walls of MWCNTs, and the size range of these particles was about 5-10 nm. A selected area electron diffraction (SAED) pattern on a nanoparticle is shown as an inset in Fig. 4A, and the bright rings with occasional bright spots signify the crystalline nature of the nanoparticle. EDS examination confirmed the presence of Pd in the nanoparticles decorating MWCNTs (Fig. 4B). For comparison, a commercial Pd on activated carbon catalyst sample was also examined by TEM, and the results showed numerous very large Pd particles irregularly distributed on carbon surfaces. The MWCNT appears to provide a unique template for decoration of nanometer-sized Pd metal particles on the carbon surfaces. By increasing the amount of Pd(hfa)2 precursor, the loading density of Pd nanoparticles on the outer walls of CNTs can be increased (Fig. 4C to E).