Synthesis of nanostructured materials using supercritical CO<Subscript>2</Subscript>: Part II. Chemical transformations

This article, the second part of our review series on the use of supercritical carbon dioxide (scCO₂) for synthesis of nanostructured material deals with the production techniques that involve chemical transformations. Taking advantage of both solvent and anti-solvent tunable properties of scCO₂, many nanostructured materials including supported/unsupported nanoparticles, quantum nanodots, nanofilms, nanorods, nanofoams, and nanowires can be prepared. Furthermore, material surfaces can be functionalized using scCO₂. scCO₂ can also be used as a carbon source for the controlled synthesis of carbon nanotubes and fullerenes or as an oxygen source for metal oxide nanostructures. Moreover, materials produced using scCO₂ does not usually need additional purification or drying steps. Depending on surface properties, the morphology of the final material can be adjusted by tuning the process conditions and the reactant concentrations.     

Sorption of U(VI) onto TiO<Subscript>2</Subscript>/ZrO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> Mesoporous Materials from Sulfate Solutions

New nanostructured mesoporous materials of the composition TiO₂/ZrO₂/SiO₂ were prepared by the template sol–gel method using a siloxane–acrylate emulsion as a template. The morphology and structure of these materials and their ability to take up U(VI) were studied. The influence of various factors (ZrO₂ content, pH of solution) on the sorption properties was studied. The suggested materials allow efficient sorption of U(VI) from sulfate solutions with low U(VI) concentrations and can be used in final purification processes.     

Production and characterization of metastable Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> ceramic materials

Producing nanostructured materials through metastable phases is interesting in the field of ceramic materials. Metastable phases can be obtained by the Atmospheric Plasma Spray (APS) technique which, is a well-known technique to produce coatings. The initial powders are melted during the spraying obtaining a homogenized phase due to their solubility in the liquid state. Afterwards, the molten droplets are quenched in a cooled medium, producing the sought metastable phases. Finally, during material consolidation, the metastable structure evolves due to a dual structure. A suppression of the grain growth is produced as a consequence of the immiscibility of both phases in the solid state. Due to their small grain size and uniform structure, these nanostructured materials exhibit very interesting properties such as higher hardness and toughness. The aim of this research has been to produce nanostructured Al₂O₃–TiO₂ ceramic powders through APS + quenching route, starting from commercially available micron-sized powders. A complete characterization of the obtained structures using XRD, SEM, FESEM and EDS has been carried out in the Thermal Spray Center (CPT) of the University of Barcelona.     

Nanostructured Pt-γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CeO<Subscript>2</Subscript> catalytic materials and coatings

A new process is described for the suspension synthesis of polycrystalline Pt-γ-Al₂O₃-CeO₂ catalytic materials and coatings, with maltose, C₁₂H₂₂O₁₁, as a reductant and structure former. The process parameters have been optimized in terms of the dispersion medium composition and the way in which Pt is introduced. The coatings are highly uniform in chemical and phase composition, as evidenced by quantitative analysis, optical microscopy, and x-ray microanalysis results. Characteristically, the coatings have a highly porous structure and good adhesive properties. Catalysts prepared by the proposed process show high activity for the oxygen oxidation of CO. The process can be used to fabricate Pt-Pd-Rh catalysts on block supports for the purification of vehicle exhaust gases and industrial off-gases. It has the advantage that high-porosity multicomponent catalytic coatings can be produced on cordierite block supports in a single step.     

Conductivity of the nanostructured ceramic material Zr<Subscript>0.88</Subscript>Sc<Subscript>0.1</Subscript>Ce<Subscript>0.01</Subscript>Y<Subscript>0.01</Subscript>O<Subscript>1.955</Subscript> prepared from mechanically activated powders

The structure of the Zr_0.88 Sc_0.1Ce_0.01Y_0.01O_1.955 solid solution, a candidate for the use as a solid electrolyte in fuel cells with a low temperature, has been investigated using x-ray powder diffraction and Raman spectroscopy. Single-phase ceramic materials have been produced from powders prepared by the mechanochemical synthesis from ZrO₂ nanoprecursors purified of the impurities introduced during grinding of commercial zirconia. The solid solution has a rhombohedral structure at room temperature owing to the partial ordering of oxygen vacancies. The electrical conductivity of the ceramic materials sintered at temperatures below 1570 K exhibits a hysteresis due to the delay of the martensitic transition from the cubic phase to the rhombohedral phase upon cooling of the sample. The nanostructured ceramic materials are characterized by a high mechanical strength and unusually close values of the activation energies for bulk and grain-boundary electrical conduction.     

Experimental evaluation of interactions in supercritical CO<Subscript>2</Subscript>/water/rock minerals system under geologic CO<Subscript>2</Subscript> sequestration conditions

The hydrothermal autoclave experiments were conducted to simulate the interactions in the scCO₂/water/rock minerals (quartz, biotite and granite) reaction systems using a Hastelloy C reaction cell at 100 °C. The dissolution characteristics of rock minerals and their surface texture alternation after hydrothermal treatment were examined by ICP-AES and SEM/EDX investigation, respectively. The results suggested that the hydrolysis of plagioclase phase should be mainly responsible for the elements dissolved from the Iidate granite samples. The dissolution was encouraged by the introduction of CO₂ in the water/granite system, and generated an unknown aluminosilicate. No distinct chemical alternations occurred in the water-free scCO₂/granite system, which indicated that rock minerals should be chemically stable in the water-free scCO₂ fluids under the current mild experimental conditions. Both the highest concentration of Ca existing in the scCO₂/vapor/granite system and the SEM observation results of calcite deposit, suggested that a meaningful CO₂ minerals trapping process should be potential in the CO₂-rich field during a short physicochemical interaction period.     

Evolution of silver in a eutectic-based Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–Ag metamaterial

The development of novel manufacturing techniques of nano-/micromaterials, especially metallodielectric materials, has enabled dynamic development of such fields as nanoplasmonics. However, the fabrication methods are still mostly based on time-consuming and costly top-down techniques limited to two-dimensional materials. Recently, directional solidification has been proposed and utilized for manufacturing of volumetric nanoplasmonic materials using the example of a Bi₂O₃–Ag eutectic-based nanocomposite. Here, we explain the evolution of silver in this composite, from the crystal growth through the post-growth annealing processes. Investigation with tunneling electron microscopy shows that silver initially enters the composite as an amorphous AgBiO₃ phase, which is formed as a wetting layer between the grains of Bi₂O₃ primary phase. The post-growth annealing leads to decomposition of the amorphous phase into Bi₂O₃ nanocrystals and intergranular Ag nanoparticles, providing the tunable localized surface plasmon resonance at yellow light wavelengths.     

Physical properties of lead-free BaFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ceramics obtained from mechanochemically synthesized powders

Modern electronics expect functional materials that are eco-friendly and are obtained with lower energy consumption technological processes. The multiferroic lead-free BaFe_1/2Nb_1/2O₃ (BFN) ceramic powder has been prepared by mechanochemical synthesis from simple oxides at room temperature. The development of the synthesis has been monitored by XRD and SEM investigations, after different milling periods. The obtained powders contain large agglomerates built by crystals with an estimated size about 12–20 nm depending on the period of milling. From this powder, the multiferroic BFN ceramic samples have been prepared by uniaxial pressing and subsequent sintering pressureless method. The morphology of the BFN ceramic samples strongly depends on high-energy milling duration. The properties of the ceramic samples have been investigated by dielectric spectroscopy, in broad temperature and frequency ranges. The high-energy milling of the powders has strongly affected the dielectric permittivity and dielectric loss of the BaFe_1/2Nb_1/2O₃ ceramic samples. The usage of the mechanochemical synthesis to obtain the multiferroic lead-free BFN materials reduces the required thermal treatment and simultaneously improves the parameters of the BFN ceramics.     

Thermoelectric properties of Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript> processed by sintering of cold pressed compacts and hot pressing

Sintering of cold die-pressed compacts and hot pressing have been used in order to produce high efficiency, thermoelectric ε-Zn₄Sb₃ bulk specimens. Single-phase Zn₄Sb₃ was successfully produced by sintering of cold die-pressed compacts using mixed elemental powders, and subsequent hot pressing produced single-phase, bulk specimens without microcracks. Phase transformations in this alloy system during synthesis were investigated using DSC, XRD and SEM. Thermoelectric properties as a function of temperature were investigated from room temperature to 600 K and compared with results of other studies. Transport properties at room temperature were evaluated. Thermoelectric properties of single phase Zn₄Sb₃ materials produced by the sintering of cold die-pressed compacts and hot pressing are comparable to the published data for alternate processing routes. Synthesis by sintering of cold die-pressed compacts and hot pressing offers a potential processing route to produce bulk Zn₄Sb₃.     

In Situ X-ray Diffraction Study of the Phase Transition of Nanocrystalline In(OH)<Subscript>3</Subscript> to In<Subscript>2</Subscript>O<Subscript>3</Subscript>

Single-crystalline indium oxide (In₂O₃) nanocubes were successfully prepared via dehydration of indium hydroxide (In(OH)₃) nanocubes synthesized by a hydrothermal method. In situ x-ray diffraction patterns of the phase transition were obtained. Transmission electron microscopy indicated that the morphology of the In₂O₃ nanocrystallites was the same as that of their precursors, In(OH)₃ nanocrystallites. The reported method, the dehydration of In(OH)₃ nanocrystallites prepared by a hydrothermal method, shows a simple and effective synthesis route for In₂O₃ nanostructured materials.__________From Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 702–705.Original English Text Copyright © 2005 by Hongliang Zhu, Naiyan Wang, Lina Wang, Kuihong Yao, Xiaofei Shen.This article was submitted by the authors in English.     

Effect of mechanical activation on physical properties of relaxor ferroelectric Pb<Subscript>2</Subscript>ScNbO<Subscript>6</Subscript> ceramics

Relaxor ferroelectric lead scandium niobate Pb₂ScNbO₆ (PSN) ceramics was obtained by solidstate synthesis and a modified ceramic technology, whereby the sintering stage was preceded by room-temperature compression and shear straining of the synthesized PSN powder in Bridgman anvils. It is established that this mechanical activation leads to the development of dynamic recrystallization processes in PSN grains, which significantly influence the physical properties of the final ferroelectric ceramic material.     

Controllable synthesis of MoS<Subscript>2</Subscript> nanostructures from monolayer flakes,few-layer pyramids to multilayer blocks by catalyst-assisted thermal evaporation

Molybdenum disulfide (MoS₂), as a representative of two-dimensional layered materials, has been extensively investigated due to their unique structure and interesting electronic and optical properties. However, the controllable synthesis of monolayer and pyramidal MoS₂ nanostructures needs improvement, and their growth mechanism requires more investigations. Here, uniform MoS₂ nanostructures from monolayer flakes, few-layer pyramids to multilayer blocks were successfully fabricated by a catalyst-assisted thermal-evaporation-based chemical vapor deposition method via simply adjusting the carrier Ar gas flow rate. After the comprehensive characterization on the obtained materials, their nucleation and growth mechanisms were proposed with specifically highlighting the influence of the carrier gas flow rate, which might also be of help to understand the synthesis processes of other two-dimensional semiconducting transition metal dichalcogenides by similar method.     

Preparation and electrochemical properties of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> by a rheological-phase-assisted microwave synthesis method

By using LiCO₃ and MnO₂, a rheological-phase-assisted microwave synthesis method has been applied in the fast preparation of spinel LiMn₂O₄ in order to reduce the cost of cathode materials. Comparing with the pristine LiMn₂O₄ obtained by the traditional solid-state reaction method, the structure and surface morphology of the samples synthesized by the rheological-phase-assisted microwave synthesis method have been investigated. The powders were used as positive materials for lithium-ion battery, whose charge/discharge properties and cycle performance have been examined in detail. As a result, the powders prepared by the rheologicalphase-assisted microwave synthesis method at 750°C are pure spinel LiMn₂O₄ with regular shapes and uniform distribution, which exhibit higher capacity and much better reversibility than the sample prepared by the traditional solid-state reaction. The text was submitted by the authors in English.     

Preparation of rutile (TiO<Subscript>2</Subscript>) nanostructured materials at low temperature from TiCl<Subscript>4</Subscript> aqueous solution

The synthesis of rutile (TiO₂) nanostructured materials at low temperature from TiCl₄ aqueous solution was described. TiO₂ coatings on polystyrene (PS) particles were prepared by layer-by-layer assembly technique. The samples were characterized by DTA-TG, SEM, XPS, TEM and XRD techniques. The experimental results showed that pure rutile-TiO₂ coatings with nanocrystal structure were synthesized at 100 °C. On the surface of PS particles, sphere-type TiO₂ coatings exhibited uniform shape and a narrow size distribution. The amount of TO₂ (wt%) and shell thickness of particles increased with the adding of coating layers. Hollow TiO₂ spheres were obtained by calcination at 450 °C. TiO₂/PS with 2 coating layers showed higher degradation rate. The photocatalytic activity of hollow TiO₂ spheres was higher than that of TiO₂/PS.     

Formation of phenyl-C<Subscript>61</Subscript>-butyric acid methyl ester nanoscale aggregates after supercritical carbon dioxide annealing

In this study, we successfully developed a novel method to create [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) nanoscale aggregates using supercritical carbon dioxide (scCO₂) annealing and fabricated bulk heterojunction (BHJ) solar cells with the nanoscale PCBM to improve device performance. PCBM forms nanoscale aggregates with a size of approximately 70 nm after scCO₂ annealing at 11 MPa and 50 °C for 60 min. However, PCBM remains amorphous after thermal annealing (TA) at 150 °C for 5 min. The morphology, structure, and crystallinity of poly(3-hexylthiophene) (P3HT) in the scCO₂-treated P3HT film are nearly the same as those in the TA-treated P3HT film. In the P3HT/PCBM blend, the formation of PCBM nanoscale aggregates by scCO₂ treatment decreases the disturbance for P3HT crystallization and improves diffusion and regular packing of P3HT molecular chains. This increases the crystallinity of P3HT so that it becomes higher than that in the TA-treated blend film. The nanoscale aggregates of PCBM and the higher crystallinity of P3HT give the scCO₂-treated P3HT/PCBM BHJ solar cells a maximum power conversion efficiency (PCE) of 2.74%, which is much higher than that of the as-cast device (PCE is 1.70%) and a little higher than the highest PCE (2.64%) of thermally annealed devices. These results indicate that scCO₂ is an effective, mild, and environmental method to modulate the nanoscale aggregates of PCBM and to improve the PCE of BHJ solar cells. However, the size of the PCBM aggregates is a little larger than the most suitable size of the exciton diffusion length, leading to limited improvement of the PCE.     

Growth of Rare-Earth-Doped K<Subscript>2</Subscript>LaCl<Subscript>5</Subscript>, K<Subscript>2</Subscript>BaCl<Subscript>4</Subscript>, and K<Subscript>2</Subscript>SrCl<Subscript>4</Subscript> Single Crystals

Processes are described for the synthesis and melt growth of rare-earth-doped K₂LaCl₅, K₂BaCl₄, and K₂SrCl₄, using rare-earth oxides as starting materials. The solubility of rare-earth activators in Bridgman-grown crystals are determined, and the spectroscopic properties of K₂LaCl₅〈Nd〉 crystals are investigated.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 748–752.Original Russian Text Copyright © 2005 by Vinogradova, Galagan, Dmitruk, Moiseeva, Osiko, Sviridova, Brekhovskikh, Fedorov.     

Effect of Au and NiO catalysts on the NO<Subscript>2</Subscript> sensing properties of nanocrystalline SnO<Subscript>2</Subscript>

Nanocrystalline tin dioxide has been synthesized, and its surface has been modified with Au and NiO. Their distributions in the nanocrystalline tin dioxide have been examined by X-ray diffraction and transmission electron microscopy. The NO₂ sensing properties of the materials have been studied in the range 100–1000 ppb. Both gold and nickel enhance the NO₂ response of SnO₂. Codoping with Au and NiO markedly enhances its sensing response and, in addition, lowers the peak response temperature. The observed effect of NO₂ concentration in dry air on the sensing response of the SnO₂〈Au, NiO〉 nanocomposite can be understood in terms of the sequence of processes that take place on the SnO₂ surface upon nitrogen dioxide adsorption in the presence of chemisorbed oxygen.     

On the synthesis,characterization and photocatalytic applications of nanostructured TiO<Subscript>2</Subscript>

Nanocrystalline semiconducting materials are attracting much attention due to their potential applications in solar energy conversion, nonlinear optics, and heterogeneous photocatalysis. In the present investigation, we have synthesized nanostructured TiO₂ photocatalysts, which have been used in the photocatalytic degradation of phenol (one of the most common water pollutants). These catalysts have been prepared through sol-gel technique using titanium tetra-isopropoxide as a raw material for synthesis. Characterization techniques such as XRD, SEM and TEM have been employed for structural/microstructural investigations. XRD results show that the as synthesized TiO₂ nanopowder exhibit anatase phase, TiO₂. The average sizes of the TiO₂ nanopowders are ∼ 5–10 nm. The optical properties of the samples were investigated through UV-visible and fluorescence techniques. It has been observed that absorption edge corresponds to ∼ 410 nm (bandgap, ∼ 3.02 eV). The emission peak in the fluorescence spectrum at ∼ 418 nm corresponds to the bandgap energy of ∼ 2.97 eV. Concentration of phenol (initial concentration, ∼ 100 ppm) with illumination time was monitored by measuring the absorbance of pure and illuminated phenol through UV-visible spectrophotometer. Salient feature of this study relates to the fact that the present sol-gel synthesized TiO₂ nanopowders have been found to be better photocatalysts for phenol degradation than the presently employed commercial TiO₂ (P-25, Degussa) photocatalyst. Thus, whereas phenol concentration, with the presently synthesized TiO₂ nanopowders, the concentration of phenol decreases up to ∼ 32% but for commercial TiO₂ nanopowder (P-25, Degussa), it decreased only up to ∼ 25%. The improved surface area is considered as an important factor for the aforesaid decrease in phenol concentration.     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba(Co<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>

Polycrystalline nonstoichiometric Ba(Co_1/3Nb_2/3)O₃ (BCN) materials have been synthesized and investigated. Deviations from stoichiometry have been shown to lead to the formation of crystalline Ba₆CoNb₉O₃₀ (barium deficiency) and Ba₈CoNb₆O₂₄ (cobalt deficiency). The effect of phase composition on the microwave dielectric properties of BCN has been studied. The results demonstrate that the dielectric properties of BCN-based materials can be tuned by varying cobalt content. The materials obtained are high-Q microwave dielectrics with temperature-stable properties.     

Proton Conductivity and Thermal Properties of Ba(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.