Solution combustion synthesis of nanomaterials

Solution combustion (SC) is an effective method for synthesis of nano-size materials and it has been used for the production of a variety (currently more than 1000) of fine complex oxide powders for different advanced applications, including catalysts, fuel cells, and biotechnology. However, it is surprising that while essentially all of the studies on SC emphasize the characterization of the synthesized materials, little information is available on controlling combustion parameters and the reaction mechanisms. This paper is devoted to the analysis of the combustion parameters for different SC reaction modes. First, the conventional volume combustion synthesis mode, which involves uniform reaction solution preheating prior to self-ignition, is briefly discussed. Second, for the first time, results of detailed experimental studies on steady-state self-propagating mode of SC synthesis of nano-powders are presented. Finally, the so-called solution + impregnation combustion mode is considered. The relationship between combustion parameters and product microstructures are emphasized. These results are crucial not only from the application stand-point, but more importantly lead to methodological benefits, allowing application of the developed approaches to investigate steady state heterogeneous combustion waves in new classes of reaction systems.     

Deposition of hybrid structures of reduced graphene oxide and tin dioxide thin films,and persistent photoconductivity observation

Reduced graphene oxide (rGO) is deposited on glass substrate by dripping and sol-gel-coating methods giving rise to nanostructures. When in combination with thin films of SnO₂, they form a heterostructure SnO₂:2 at% Eu/rGO, which alters the surface electrical conductivity. SnO₂ and rGO were also combined as a composite, with conductivity strongly affected by ultraviolet excitation, and shows persistent photoconductivity (PPC) phenomenon even very close to room temperature. Both sort o hybrid structures can be applied in electronic devices. The SnO₂ films are deposited via chemical route by sol-gel or by a mixed technique that combines powders generated by drying the sol-gel solution with resistive evaporation of this powder. Resistivity measured as a function of temperature show that the SnO₂:2 at%Eu sample behaves very similarly to the SnO₂:2 at%Eu/rGO heterostructure sample, with the same energy level for the dominant defect, 172 meV, coincident with ionization of oxygen vacancies. Despite not changing the position of this level, the presence of rGO on the surface of the SnO₂ film induces a decrease in conductivity in vacuum, demonstrating the surface interaction.     

Gold-magnetite nanocomposite materials formed via sonochemical methods

Treatment of preformed magnetite nanoparticles with ultrasound in aqueous media with dissolved tetrachloroauric acid resulted in the formation of gold–magnetite nanocomposite materials. These materials maintained the morphology of the original magnetite particles. The loading of gold particles could be controlled by adjusting experimental parameters, including the addition of small amounts of solvent modifiers such as methanol, diethylene glycol, and oleic acid. The nanocomposite materials were magnetic and exhibited optical properties similar to pure gold nanoparticles.     

Surface and catalytic properties of doped tin oxide nanoparticles

Mixed oxides composed of Zn-Sn, Ti-Sn and V-Sn were prepared by a co-precipitation method and evaluated as catalysts for methanol oxidation in an ambient fixed-bed reactor. Surface analysis by X-ray photoelectron spectroscopy (XPS) revealed an electronic interaction between dopant and Sn atoms in the oxide structure and showed the formation of surface states associated with the dopants. Oxygen vacancies were present on the Zn-doped oxide, and the oxidation of methanol to carbon oxides was favored. The Ti-doped oxide exhibited a favorable selectivity to dimethyl ether, related to the oxygen anions near Ti centers. Vanadium dopants not only dramatically increased the catalytic activity but also promoted the partial oxidation of methanol to formaldehyde. Results demonstrate that the bridging dopant-O-Sn bond acts as active sites and influences product distribution.     

Effect of nitrogen pressure on the combustion synthesis of titanium nitride

Abstract

Titanium nitride, TiN, has attractive physical and chemical properties such as hardness, chemical stability and electrical conductivity. It is a typical material with a wide range of stoichiometry. It can be synthesised by high pressure combustion synthesis. The composition and microstructures can vary with the experimental conditions especially with thermal treatment and nitrogen pressure.     

Luminescence in LaCaAl3O7 prepared by combustion synthesis

LaCaAl₃O₇ phosphors activated by several ns² impurities like Pb²⁺ and Bi³⁺, and rare-earth dopants were prepared by combustion synthesis. X-ray diffraction (XRD) results confirmed the formation of single-phase compounds. Variety of activators exhibited interesting photoluminescence in LaCaAl₃O₇ host. Combustion synthesis furnishes a quick method for preparing these phosphors. It is suggested that LaCaAl₃O₇-based materials can be developed as low-cost phosphors.     

Luminescence in LaBaB9O16 prepared by combustion synthesis

LaBaB₉O₁₆ phosphors activated by various ions belonging to ns², 3dⁿ and 4fⁿ configurations were prepared by combustion synthesis. Phosphors’ synthesis and luminescence spectra are reported. Most of the activators displayed intense characteristic emission. Pr³⁺→Gd³⁺, Ce³⁺→Tb³⁺, Ce³⁺→Dy³⁺, Ce³⁺→Mn²⁺ and Bi³⁺→Mn²⁺ energy transfers were also observed. In particular, Ce³⁺→Tb³⁺ energy transfer leads to an efficient green emitting phosphor.     

Investigation of the influence of silver and tin on the luminescence of trivalent europium ions in glass

Europium-doped aluminophosphate glasses prepared by the melt-quenching technique have been studied by photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). The effects of silver and tin doping, and of further thermal processing on Eu³⁺ ions luminescence have been assessed. For the glass system containing only europium, Eu³⁺ PL observed under UV excitation is suggested to occur through energy transfer from the excited glass host. After silver and tin doping, an enhanced UV excited Eu³⁺ PL has been indicated to occur essentially due to radiative energy transfer from isolated Ag⁺ ions and/or two fold-coordinated Sn centers. Since thermal processing of the material leads to a quenching effect on Eu³⁺ PL and Ag nanoparticles (NPs) formation due to reduction of silver ions by tin, XPS was employed in order to investigate the possibility for Eu³⁺→Eu²⁺ reduction during HT as a potential source of the PL decrease. The data points towards Ag NPs as main responsible for the observed weakening of Eu³⁺ PL.     

溶胶凝胶法制备参氟二氧化锡薄膜

利用溶胶凝胶方法，在硅碱玻璃底板上制备的透明低电阻SnO2:F薄膜，是一种低辐射导电薄膜。将SnCl4·5H2O 和 NH4F 溶解在50%乙醇和50%水的溶液中。制备条件为底板温度450℃，喷嘴与底板之间的距离60mm,载气流速8 L/min，制备时间5分钟。制成的SnO2:F薄膜面电阻为2Ω/□，可重复性好。并且文中还定性给出了SnO2:F薄膜其红外反射率与面电阻之间的关系。     

Fabrication of titanium dioxide nanofibers containing hydroxyapatite nanoparticles

In the present study, we introduce titanium dioxide (TiO₂) nanofibers that contain hydroxyapatite (HAp) nanoparticles (NPs) as a result of an electrospinning process. A simple method that does not depend on additional foreign chemicals has been employed to synthesize HAp NPs through calcination of bovine bones. Typically, a colloidal gel consisting of titanium isopropoxide/HAp was prepared to produce nanofibers embedded with solid NPs by electrospinning process. The SEM results confirmed well oriented nanofibers and good dispersion of HAp NPs over the nanofibers. XRD results demonstrated well crystalline feature of both TiO₂ and HAp. Physiochemical aspects of prepared nanofibers were characterized for TEM and TEM-EDS which confirmed nanofibers were well oriented and had good dispersion of HAp NPs. Accordingly, these results strongly recommend the use of obtained nanofiber mats as a future candidate for hard tissue engineering applications.     

Synthesis of hydrophilic copper nanoparticles: effect of reaction temperature

Synthesis of hydrophilic copper nanoparticles with an additional coating of an hydrophilic polymer has been carried out by use of hydrazine hydrate (HH) and sodium formaldehyde sulfoxylate (SFS) in aqueous medium. The effect of temperature on nanoparticles when synthesized in aqueous medium has been studied. It is observed that an ideal temperature ranges some where between 70 and 80 °C. Nearly phase-pure nanocopper can be obtained when both sodium succinate and polyvinyl alcohol (PVA) are used together to provide double capping in aqueous medium. It is observed that the surface plasmon resonance (SPR) phenomena is sensitive to experimental conditions and handling of the nanoparticles. X-ray diffraction measurements (XRD) revealed a broad pattern for the fcc crystal structure of copper metal. The particle diameter by use of Scherrer’s equation was calculated to be about 43 nm. Thermal analysis (TGA) revealed ~10–60% weight loss due to the presence of surfactants. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that there is clustering of spherical particles in dry state. Dedicated to Prof. David Cole-Hamilton, University of St. Andrews, UK on his 60th birthday.     

Facile preparation of SnC2O4 nanowires for anode materials of a Li ion battery

A simple method of creating densely-packed nanostructures of functional metal oxides is attractive, but it has always been a challenge. Here, we synthesize well-distributed nanostructures of Sn complexes (SnC₂O₄ and SnO₂) via a simple chemical anodization technique followed by annealing. Chemical anodization of Sn surface in oxalic acid, using various organic solvents, provides one-dimensional nanostructures of SnC₂O₄. Length and packing density were precisely controlled by several parameters: solubility of oxalic acid, dielectric constant of organic solvents, and the ion transfer of proton and oxalate anion. Further thermal decomposition converts the SnC₂O₄ nanowires into SnO₂ nanowires, maintaining the nanostructure form in the process. In addition, we expect that the mixture of SnC₂O₄ and SnO₂ nanowires synthesized by this approach might be potential alternative anode materials for prompt charging and discharging Li ion batteries.     

Synthesis of SnO2−x nanoparticles tuned between 0 x 1 in a premixed low pressure H2/O2/Ar flame

The transition from tin monoxide to tin dioxide is investigated by doping a low pressure premixed H₂/O₂/Ar flame with dilute concentrations of tetramethyl tin (TMT) Sn(CH₃)₄. The H₂/O₂ ratio was varied between 1.97 and 0.97; the reactor pressure was set to 30 mbar. The inlet gas velocity was kept constant and the precursor concentration was varied between 200 and 700 ppm. Mean particle diameters between 3 nm d_p 7 nm were measured using a particle mass spectrometer (PMS), while X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis revealed information on crystal structure as well as morphology of the synthesized oxides. A gray SnO film was observed in the reactor at low oxygen concentrations, while for higher oxygen concentrations light yellow sub-stoichiometric tin oxides and white SnO₂ were obtained. Further analysis of the sub-stoichiometric oxides, using atomic electron spectroscopy AES revealed that SnO_2−x (0.2 x 0.6) was obtained. Furthermore, electrical characterization of all materials, deposited on interdigital capacitors, showed a decrease in the conductivity of the nanoparticles with increasing oxidation (decreasing x).     

Effect of electromagnetic field strength on Ni0.35Zn0.65Fe2O4 as performed by combustion synthesis

Ni_0.35Zn_0.65Fe₂O₄ ferrite is prepared through combustion synthesis in the external electromagnetic field. The highest magnetic field strength for the experiment is 1.1 T. Reactions temperatures were monitored by infrared radiation thermometer, the synthesized ferrite prepared in different magnetic fields is analyzed by XRD, SEM, and VSM. The results indicate that the coercivity of ferrite gradually decrease with the increase of magnetization. When the magnetic field strength is 0.54 T, the saturation magnetization is improved up to 56.05 emu/g (42%) as compared to that of ferrite in zero magnetic field. Through SEM analysis of Ni_0.35Zn_0.65Fe₂O₄ ferrite, homogeneous grains of the crystal are observed. With the increase of external magnetic field, the ferrite grain improved. This paper also systematically explores the effect of the electromagnetic field on ferrite by combustion synthesis.     

Rapid reduction of titanium dioxide nano-particles by reduction with a calcium reductant

Micro-, submicron-, and nano-scale titanium dioxide particles were reduced by reduction with a metallic calcium reductant in calcium chloride molten salt at 1173 K, and the reduction mechanism of the oxides by the calcium reductant was explored. These oxide particles, metallic calcium as a reducing agent, and calcium chloride as a molten salt were placed in a titanium crucible and heated under an argon atmosphere. Titanium dioxide was reduced to metallic titanium through a calcium titanate and lower titanium oxide, and the materials were sintered together to form a micro-porous titanium structure in molten salt at high temperature. The reduction rate of titanium dioxide was observed to increase with decreasing particle size; accordingly, the residual oxygen content in the reduced titanium decreases. The obtained micro-porous titanium appeared dark gray in color because of its low surface reflection. Micro-porous metallic titanium with a low oxygen content (0.42 wt%) and a large surface area (1.794 m² g⁻¹) can be successfully obtained by reduction under optimal conditions.     

Effects of In and Mg doping on properties of ZnO nanoparticles by flame spray synthesis

The Mg- and In-doped zinc oxide (Mg _x Zn_1−x O, In _y Zn_1−y O) nanoparticles were successfully prepared by flame spray synthesis method. According to the results obtained from X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–Vis absorption spectra, it was concluded that the Mg or In doping induced the lattice constants to change to some extent; the band gap of Mg _x Zn_1−x O also increased with respect to the decreasing band gap of In _y Zn_1−y O. Moreover, the strong UV emission and weak visible emission were investigated by photoluminescence spectra, while the mechanisms of Mg or In doping on PL spectra have been discussed in detail.     

Combustion synthesis of nitrides: mechanistic studies

Combustion synthesis (CS) is characterized by extremely high heating rates (up to 10⁶ K/s) and temperatures (up to 4000 K), as well as short times of reaction completion (usually less than 1 s, sometimes even 10⁻³–10⁻² s). The above unique conditions lead to materials microstructure formation mechanisms that cannot be observed in the isothermal or quasi-isothermal cases. In turn, evolution of the reaction media microstructure influences characteristics (temperature, velocity) of the combustion wave. In this work, such microstructural effects are demonstrated on the examples of CS in different gas–solid systems (i.e., Si–N₂, Al–N₂, and Nb–N₂). It is shown that dilution of the reaction media by inert high surface area precursors with phase compositions similar to the CS product leads to a significant increase in combustion velocity in the Si–N system, while the temperature remains constant. Also, it is revealed that dispersion of the metal (Nb) particles in the preheating zone might be responsible for constancy of the combustion velocity as a function of the size of the initial reactants. Finally, a complex two-stage mechanism of AlN formation explains the non-monotonic shape of the temperature–time profile observed during CS in this system.     

Thermal oxidation-grown vanadium dioxide thin films on FTO (Fluorine-doped tin oxide) substrates

By deposition of metallic vanadium on FTO substrate in Argon atmosphere at room temperature, the sample was then annealed in furnace for 2 h at the temperature of 410 °C in air ambient. (1 1 0) -orientated vanadium dioxide films were prepared on the FTO surface. A maximum transmittance of ∼40% happened at 900–1250 nm region at room temperature. The change of optical transmittance at this region was ∼25% between semiconducting and metallic states. In particular, vanadium dioxide thin films on FTO exhibit semiconductor–metal phase transition at ∼51 °C, the width of the hysteresis loop is ∼8 °C.     

The size distributions of nanoparticles of the oxides of Mg, Ba and Al in flames: Their measurement and dependence on the concentrations of free radicals in the flame

Flat, pre mixed, laminar, and very O₂-rich flames of C₂H₂ + O₂ + N₂ with [O₂]/[O₂]_stoich  2.8 and a temperature 2000 K have been burned at atmospheric pressure. Trace amounts (13 ppm) of the metals Mg, Ba or A1 were added to the unburnt gases by nebulising an aqueous solution of a halide of the metal, so that e.g., Mg formed molecules of Mg(OH)₂, MgOH and MgO, as well as free atoms of Mg. The relative abundances of these species were governed by well-characterised equilibria and consequently depended on the temperature and also the concentrations of the flame’s free radicals H, OH and O. Transmission electron microscopy showed that nanoparticles of the oxides of these metals formed from their pool of molecular species in these flames. Particle size distributions were also measured (much less tediously) with a mobility analyser (DMS 500, Cambustion) operating at 0.25 bara. The optimal way of continuously sampling the gases at a point along the flame’s axis was investigated and shown to require expanding the sample (to a pressure of 1/3 bara) supersonically through an orifice with a diameter greater than 0.4 mm. In addition, the sample had to be diluted with N₂, with a volumetric flow rate of 10–20 times that of the sample, all at 1/3 bara. The sizes of oxide nanoparticles, as measured by transmission electron microscopy, agreed with the values of 6–10 nm from the mobility analyser. With Mg all the metal appeared very rapidly as nearly spherical nanoparticles of MgO early in a flame’s reaction zone. This was also true for Ba, which, according to thermodynamic considerations at the final temperature of the flame, should not form any particles of BaO. That particles do actually form is due to the reaction zone having a relatively low temperature and super-equilibrium concentrations of the free radicals H, OH and O. Aluminium was expected to form particles of A1₂O₃. However, only a small fraction of the Al formed particles; this is attributed to the production of gas-phase molecules of Al₂O₃ (i.e., the nuclei) from AlO and AlO₂ being by a relatively slow three-body reaction, as well as Al₂O₃ being a very minor member of the gas-phase pool of molecular species containing Al.