Vanadium dioxide nanobelts: Hydrothermal synthesis and magnetic properties

VO₂ (B) nanobelts were prepared by a hydrothermal method at 180 °C using V₂O₅·nH₂O sol and H₂C₂O₄·2H₂O as starting agents. The obtained nanobelts have diameters ranging from 50 to 100 nm in width, 20-30 nm in thickness with lengths up to 1.5 μm. Measurements of the static magnetic susceptibility provide evidence for two phase transitions at T₁ = 225 K and T₂ = 290 K, respectively. Below T₁, the data suggest the presence quasi-free as well as of strongly antiferromagnetic correlated spins associated to V⁴⁺-ions.     

Silver nanoparticles: Large scale solvothermal synthesis and optical properties

Silver nanoparticles have been successfully synthesized by a simple and modified solvothermal method at large scale using ethanol as the refluxing solvent and NaBH₄ as reducing agent. The nanopowder was investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), UV-visible and BET surface area studies. XRD studies reveal the monophasic nature of these highly crystalline silver nanoparticles. Transmission electron microscopic studies show the monodisperse and highly uniform nanoparticles of silver of the particle size of 5 nm, however, the size is found to be 7 nm using dynamic light scattering which is in good agreement with the TEM and X-ray line broadening studies. The surface area was found to be 34.5 m²/g. UV-visible studies show the absorption band at ∼425 nm due to surface plasmon resonance. The percentage yield of silver nanoparticles was found to be as high as 98.5%.     

Sonochemical synthesis of ZnO nanoparticles: The effect of temperature and sonication power

ZnO nanoparticles were synthesized by drop-wise addition of ZnCl₂ solution to KOH, in different temperatures and sonication output powers. The morphologies and structure of resulting materials were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The morphology of the products was determined to be a mixture of nano rods as well as nanoparticles depending to sonication output power and temperature. A reduction in nanoparticles and aggregates sizes as well as amount of byproducts was observed by increasing the sonication output power. A combination of sonication output power and temperature was also studied on the properties of the product. At 70 °C and 45 W sonication output power, the smallest nanoparticles as well as narrower range of sizes in addition to minimum amount of byproducts was obtained in this work.     

Nickel titanate microtubes constructed by nearly spherical nanoparticles: Preparation, characterization and properties

In this paper, we report the successful synthesis of NiTiO₃ microtubes constructed by nearly spherical nanoparticles via a simple solution-combusting method employing a mixture of ethanol and ethyleneglycol (V/V = 60/40) as the solvent, nickel acetate as the nickel source, tetra-n-butyl titanate as the titanium source and oxygen gas in the atmosphere as the oxygen source. The as-obtained product was characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray spectrometry (EDS). The UV-vis absorption spectrum of the product showed two absorption peaks centered at 258.6 and 350.1 nm, respectively. The Brunauer-Emmett-Teller (BET) surface area of the product was 14.06 m²/g and the pore size distribution mainly located from 20 to 30 nm. The photocatalytic degradation property of the product for organic dyes showed that the as-obtained porous NiTiO₃ microtubes could strongly promote the degradation of organic dyes including Pyronine B, Safranine T and Fluorescein.     

Zinc oxide micro- and nanoparticles: Synthesis, structure and optical properties

Zinc oxide (ZnO) spherical nanoparticles (SNPs) and bitter-melon-like (BML) microparticles were synthesized by a hydrothermal route using a zinc (Zn) plate as a source and substrate at various synthesis conditions. The structural analysis confirmed the formation of ZnO with hexagonal wurtzite phase on the hexagonal Zn substrate with growth of the ZnO microparticles along the [1 0 1] direction. The UV-vis absorption spectra of the ZnO microparticles indicated absorption peaks in the UV region which can be attributed to the band gap of ZnO. The room temperature photoluminescence (PL) of the ZnO microparticles exhibited a broad emission band, which is fitted with four Gaussian peaks and were assigned to transitions involving free excitons and various defect centers. The growth model for the formation of ZnO micro- and nanoparticles is presented.     

Comparative studies between synthetic routes of SiO2@Au composite nanoparticles

We studied two different methods for the deposition of Au nanoparticles (Au NPs) onto the functionalized silica microspheres. One method was to mix the two kinds of particles together and react at room temperature overnight. The other one was to reduce hydrochloroauric acid to Au NPs with sodium citrate in the presence of the functionalized silica microspheres (one for the naked silica microspheres, the other for the Au-attached silica microspheres). We investigated the morphologies of SiO₂@Au composite nanoparticles synthesized by these two methods, and found that the latter achieved a denser Au coverage on silica microspheres. Furthermore, we studied the effect of the pH values in a wide range, respectively, for the two methods. A possible mechanism was put forward to interpret the formation of SiO₂@Au composite nanoparticles and the effects of the synthetic routes and pH values on the morphologies and optical properties.     

Microwave-assisted hydrothermal synthesis of coralloid nanostructured nickel hydroxide hydrate and thermal conversion to nickel oxide

Coralloid nanostructured nickel hydroxide hydrate has been successfully synthesized by a simple microwave-assisted hydrothermal process using nickel sulfate hexahydrate as precursor and urea as hydrolysis-controlling agent. A pure coralloid nanostructured nickel oxide can be obtained from the nickel hydroxide hydrate after calcination at 400 °C. The thermal property, structure and morphology of samples were characterized by thermogravimetry (TG), temperature-programmed reduction (TPR), X-ray (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).     

Low temperature synthesis of bismuth ferrite nanoparticles by a ferrioxalate precursor method

The synthesis of bismuth ferrite by solid-state reaction of Bi₂O₃ and Fe₂O₃ results in the formation of multiphase products. Even coprecipitation followed by calcination leads to the formation of impurity phases. Here, we report the synthesis of magnetoelectric bismuth ferrite by a ferrioxalate precursor method. In this process, bismuth ferrite, synthesized through solutions of some specific salts led to the formation of phase pure (perovskite) nanocrystalline powder (11–22 nm as evident from X-ray diffraction analysis) at a temperature of 600 °C. The synthesized powders were characterized by X-ray diffractometry, thermogravimetry and differential thermal analysis, Fourier transformation infrared spectroscopy and scanning electron microscopy. The synthesis route is simple, energy saving and cost-effective. Such nanosized bismuth ferrite powder may have a potential application in making lead free piezoelectric materials for actuators as well as magnetoelectric sensors.     

Electrical and gas sensing properties of self-aligned copper-doped zinc oxide nanoparticles

Electrical and gas sensing properties of nanocrystalline ZnO:Cu, having Cu X wt% (X = 0.0, 0.5, 1.0, and 1.5) in ZnO, in the form of pellet were investigated. Copper chloride and zinc acetate were used as precursors along with oxalic acid as a precipitating reagent in methanol. Material characterization was done by X-ray diffraction (XRD), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and inductive coupled plasma with optical emission spectrometry (ICP-OES). FE-SEM showed the self-aligned Cu-doped ZnO nano-clusters with particles in the range of 40-45 nm. The doping of 0.5% of copper changes the electrical conductivity by an order of magnitude whereas the temperature coefficient of resistance (TCR) reduces with increase in copper wt% in ZnO. The material has shown an excellent sensitivity for the H₂, LPG and CO gases with limited temperature selectivity through the optimized operating temperature of 130, 190 and 220 °C for H₂, LPG and CO gases, respectively at 625 ppm gas concentration. The %SF was observed to be 1460 for H₂ at 1% Cu doping whereas the 0.5% Cu doping offered %SF of 950 and 520 for CO and LPG, respectively. The response and recovery time was found to be 6 to 8 s and 16 s, respectively.     

Microwave radiation as heating method in the synthesis of titanium dioxide nanoparticles from hexafluorotitanate-organic salts

Nanocrystalline anatase was obtained from ionic liquid-like precursors containing hexafluorotitanate-organic salts and less than 25 wt.% of water, and using boric acid as fluoride scavenger. Two alternative heating methods were explored using either a conventional oven or a domestic microwave apparatus. A significant reduction in the reaction time from 24 h to only few minutes was obtained using the microwave route. The as-prepared materials were characterized by X-ray diffraction, transmission electron microscopy, nitrogen sorption analysis, and attenuated total reflectance Fourier transformed infrared, X-ray photoelectronic and Raman spectroscopes. The convenience of using the microwave heating option was a function of the organic cation present in the precursor. Thus, organic ammonium cations containing only hydrocarbon substituents, such as diethylammonium, phenylammonium and benzyltrimethylammonium led to the precipitation of nanocrystalline anatase powder with high specific surface area (up to 120 m² g⁻¹) in a short processing time (1-3 min). Otherwise, alcohol and carboxylate functionalized cations decomposed under microwave treatment. Moreover, the choice of the organic cation allowed tuning several properties of the end material, such as particle size and pore morphology.     

Carbon-assisted synthesis of nanowires and related nanostructures of MgO

MgO nanowires and related nanostructures have been prepared by carbon-assisted synthesis, starting from polycrystalline MgO or Mg without the use of metal catalysts. The study has been carried out with different sources of carbon, all of them yielding the nanostructures with some differences. It has been possible to obtain nanotrees and other interesting nanostructures by this method. It has also been possible to obtain aligned MgO nanowires by carbon-assisted synthesis over Au-coated Si substrates. A vapor-solid mechanism of one-dimensional growth seems to be operative in the reactions carried out in bulk, but a vapor-liquid-solid mechanism applies when Si substrates are used.     

Thermal-treatment effect on the photoluminescence and gas-sensing properties of tungsten oxide nanowires

Single-crystalline non-stoichiometric tungsten oxide nanowires were initially prepared using a simple solvothermal method. High resolution transmission electron microscopy (HRTEM) investigations indicate that the tungsten oxide nanowires exhibit various crystal defects, including stacking faults, dislocations, and vacancies. A possible defect-induced mechanism was proposed to account for the temperature-dependent morphological evolution of the tungsten oxide nanowires under thermal processing. Due to the high specific surface areas and non-stoichiometric crystal structure, the original tungsten oxide nanowires were highly sensitive to ppm level ethanol at room temperature. Thermal treatment under dry air condition was found to deteriorate the selectivity of room-temperature tungsten oxide sensors, and 400 °C may be considered as the top temperature limit in sensor applications for the solvothermally-prepared nanowires. The photoluminescence (PL) characteristics of tungsten oxide nanowires were also strongly influenced by thermal treatment.     

Preparation of highly concentrated and stable suspensions of silver nanoparticles by an organic base catalyzed reduction reaction

Highly concentrated and stable suspensions of silver nanoparticles have been synthesized by chemical reduction from silver nitrate in a formaldehyde reductant using an organic base, triethylamine, as the reaction promoter. In this reaction, a low molecular weight organic compound, thiosalicylic acid (TSA), was used as the protecting agent. The average size of the silver nanoparticles prepared from this method was less than 10 nm, which allowed low-temperature sintering of the metal. The suspensions were further stabilized by the addition of excessive triethylamine, which forms an amine salt with TSA. A 50 wt%, stable suspension has been prepared. The suspensions of silver nanoparticles prepared by this method are free from any metal ion contamination, and are suitable for use in semiconductor industries.     

Coprecipitation-assisted hydrothermal synthesis of PLZT hollow nanospheres

Lanthanum-modified lead zirconate titanate Pb_1−xLa_x(Zr_1−yTi_y)O₃ (PLZT) hollow nanospheres have been successfully prepared via a template-free hydrothermal method using the well-mixed coprecipitated precursors and the KOH mineralizer. The structure, composition, and morphology of the PLZT hollow nanospheres were characterized by XRD (X-ray diffraction), ICP (inductive coupled plasma emission spectrometer), FTIR (Fourier transform infrared spectra), TG/DTA (thermogravimetric analysis and differential thermal analysis), TEM (transmission electron microscopy) and SEAD (selected area diffraction). The results show that the composition and the morphology control of the PLZT products are determined by the KOH concentration. The PLZT hollow nanospheres with uniform size of about 4 nm were synthesized in the presence of 5 M KOH. The crystalline nanoparticles can be prepared at dilute KOH, in contrast to the amorphous powders prepared at concentrated KOH. Formation mechanisms of the PLZT hollow nanospheres are also discussed.     

Dopant induced variations in microstructure and optical properties of CeO2 nanoparticles

Nanocrystalline CeO₂ particles doped in the range of 0-20% of Ca²⁺, La³⁺, and Zr⁴⁺ have been prepared from hydrothermal synthesis of nitrate solutions at 200 °C and the influences of the dopants on microstructure and optical properties of the nanoparticles have been investigated. The unit cell parameter is found to be modified by −0.39, +0.83 and +0.16% for doping of 20% Zr⁴⁺, La³⁺, and Ca²⁺, respectively. For each batch prepared, nanoparticles with a narrow size distribution of 5-15 nm have been obtained. A high-resolution transmission electron microscopy investigation reveals that these particles are single crystals mostly having hexagonal, square or circular two-dimensional projections. UV-visible spectra of doped powders exhibit shift of the absorption edge and absorption peak with respect to those of the undoped CeO₂ particles and has been attributed to compensation of Ce³⁺ and decreasing crystallite size as result of doping.     

Iron-stabilized nanocrystalline ZrO2 solid solutions: Synthesis by combustion and thermal stability

The synthesis of Fe³⁺-stabilized zirconia by the nitrate/urea combustion route was investigated. Using several characterization techniques, including X-ray diffraction, field-emission-gun scanning electron microscopy and notably Mössbauer spectroscopy, it was possible to determine the appropriate amount of urea that allows to obtain a totally stabilized Zr_0.9Fe_0.1O_1.95 solid solution. The nanocrystalline zirconia solid solution is mostly tetragonal, but the presence of the cubic phase could not be ruled out. An in-depth study of the thermal stability in air showed that the Fe³⁺ solubility in the stabilized solid solution starts to decrease at about 875 °C which results in the formation of hematite (possibly containing some Zr⁴⁺) at the surface of the zirconia grains and further provokes the progressive transformation into the monoclinic zirconia phase.     

Oxalate based non-aqueous sol-gel synthesis of phase pure sub-micron LiFePO4

We report on the synthesis and preliminary characterisation by X-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer spectroscopy and infrared spectroscopy (IR) of C-LiFePO₄. Homogeneous sub-micron sized particles of surface carbon coated phase pure LiFePO₄ are synthesised by a novel non-aqueous oxalate based sol-gel procedure. Our synthetic route successfully overcomes the incidence of Fe³⁺, effectively controls undesirable particle growth and has the potential for upscaling and application as Li-ion battery cathodes.     

Continuous synthesis of controlled size carbon-encapsulated iron nanoparticles

Carbon-encapsulated iron nanoparticles were continuously and selectively synthesised in a thermal plasma jet from ethanol (carbon source) and Fe powders with different grain sizes. The grain size of the Fe powder influenced the size distribution of the as-produced carbon encapsulates. The products obtained from large Fe particles (50-78 μm) were comprised of small encapsulates with diameters between 5 and 10 nm. Larger carbon encapsulates with a broad diameter distribution (10-100 nm) were synthesised from the finest Fe particles (18 μm). It was also found that Fe particle size was the most crucial parameter for determining the encapsulation yield. The encapsulation yield was also influenced by the carbon to iron ratio and the thermal conductivity of the plasma gas.     

Co-synthesis of single-walled carbon nanotubes and carbon fibers

A new vertical floating catalytic technique is developed and used to prepare both single-walled carbon nanotubes (SWNTs) and carbon fibers (CFs). Scanning electron microscopy (SEM) observation shows a clear separation of these two materials. Thin films of SWNTs can be peeled easily from the CF substrate which just acts as a catalyst support for the SWNT growth. The production process is also semicontinuous, resulting in a yield of ∼1.0 g h⁻¹ of SWNTs film with high purity. Structure and vibrational properties of these materials are investigated by electron microscopy and Raman spectroscopy, respectively.     

Effect of KCl, NaCl and CaCl2 mixture on volume combustion synthesis of TiB2 nanoparticles

Preparation of titanium diboride (TiB₂) nanoparticles was carried out by volume combustion synthesis. TiO₂, B₂O₃ and elemental Mg were mixed with 0-60% salt mixture of KCl, NaCl and CaCl₂ with increment of 15% as a low melting temperature diluent. Compressed samples were synthesized in a tubular furnace at a constant heating rate under argon atmosphere. Thermal analysis of the process showed that the addition of the low melting temperature salts mixture led to a significant decrease in ignition and combustion temperatures. Synthesized samples were then leached by nitric and hydrochloric acids to remove impurities. The samples were examined by XRD, SEM and DLS analysis. The results showed the formation of fine deagglomerated particles with the addition of the salts mixture. The results revealed that 45% salts mixture had the smallest average particle size of about 90 nm.