Hybrid nanostructures of Au nanocrystals and ZnO nanorods: Layer-by-layer assembly and tunable blue-shift band gap emission

A layer-by-layer assembly technique was developed to synthesize the hybrid nanostructures of Au nanocrystals with diameter of about 5 nm and ZnO nanorods via the electrostatic interaction. In comparison with ZnO nanorods, the Au-ZnO hybrid nanostructures exhibited the broadened and red-shifted surface plasmon band, enhanced band gap emission, and suppressed defect emission due to the strong interfacial coupling between Au and ZnO. Moreover, the band gap emission of the Au-ZnO hybrid nanostructures is controllably blue-shifted with decreasing distance between the Au nanocrystals and ZnO nanorods tuned by the amount of the polyelectrolyte layers due to the exciton and plasmon interactions.     

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.     

Solvothermal synthesis of ceria nanoparticles with large surface areas

Ceria nanoparticles were obtained by the calcination of precursors synthesised via the solvothermal reaction of cerium acetate. The CeO₂ samples obtained by the thermal decomposition of Ce(C₇H₁₅COO)₃·xH₂O synthesised by solvothermal reaction in 1,4-butanediol in the presence of octanoic acid had an extremely large surface area of 180 m²/g. The Ru catalyst supported on this CeO₂ sample showed a high catalytic activity for benzyl alcohol oxidation.     

Self-assembly of ZnO nanosheets into nanoflowers at room temperature

A singularity flower-like ZnO nanostructure was prepared on a large scale through a very simple solution method at room temperature and under ambient pressure in a very short time. The flower-like ZnO nanostructures were self-assembled by thin and uniform nanosheets, with a thickness of around 5 nm. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to characterize the structure and morphology. The possible growth mechanism was discussed based on the reaction process. The blue shift in the UV-vis spectra of the ZnO nanostructures was also observed.     

A novel method to low temperature synthesis of nanocrystalline forsterite

Nanocrystalline forsterite (Mg₂SiO₄) powder was synthesized using sucrose as a chelating agent and template material from an aqueous solution of magnesium nitrate and colloidal silica. The synthesized powders were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), simultaneous thermal analysis (STA), and scanning electron microscopy (SEM). The synthesized nano-powder had particle size smaller than 200 nm and average crystallite size of powders calcined at 800 °C for 3 h was in the range of 10-30 nm. Also the effect of addition 2 and 4 wt.% forsterite seed on nucleation temperature and crystallite size of forsterite was investigated. The presence of small amounts of Mg₂SiO₄ as seed obviously accelerated the crystallization of forsterite. According to DTA results the inceptive formation temperature of Mg₂SiO₄ without any seed was 760 °C, while this temperature for the specimen containing 4 wt.% seed was 700 °C.     

Novel hydroxide precursors for low temperature synthesis of selected ternary oxides

A novel method of using hydroxide precursors to reduce the synthesis temperature for few selected ternary oxides has been presented here. This technique is very useful and advantageous when the ternary oxides contain an alkaline earth element. The selected compositions for this method are BaCeO₃ (BC), BaBiO₃ (BB), La_0.7Sr_0.3CoO₃ (LSCO) and BaBi₂Nb₂O₉ (BBN). Commercially purchased strontium (or barium) hydroxide and freshly prepared lanthanum, cobalt, bismuth and niobium hydroxides were mixed thoroughly in stoichiometric ratio and heated at different temperatures ranging from 100 to 700 °C for 10 h for corresponding compositions. The sequence of the reaction and evolution of the product phase were studied by the X-ray diffraction (XRD) studies. The phase purity and lattice parameters were also determined by XRD investigations. All the product phases in each case were formed at relatively low temperature than when they were prepared by co-precipitation or solid state method. The morphology and average particle size of these powders were investigated by scanning electron microscopy (SEM).     

Cathodoluminescence and its mapping of flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures

The cathodoluminescence (CL) properties including intensity and distribution of the band to band and defect emission of the flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures have been investigated. It is indicated that the Ultraviolet (UV) emission at 380 nm of the flower-like ZnO nanostructures due to the band to band emission is weaker than their yellow emission at 600 nm induced by interstitial oxygen. Moreover, the UV emission of the ZnO nanorods unevenly distributes from the tip to the end. The UV emission on the tip is stronger than that of others due to the waveguide. On the contrary, the yellow emission at 600 nm is uniform. Furthermore, the UV emission of ZnO has been greatly enhanced and the yellow emission has been inhibited by the formation of ZnO/ZnS core-shell nanostructures in the sulfuration process due to the elimination of interstitial oxygen. However, the polycrystalline tube-like ZnS nanostructures shows the uniform and weak defect emission due to S vacancies.     

CuO nanostructures on copper foil by a simple wet chemical route at room temperature

Uniform CuO nanostructures have been synthesized on copper foil substrates by oxidation of Cu in alkaline condition by a simple wet chemical route at room temperature. By controlling the alkaline condition (pH value) different CuO nanostructures like nanoneedles, self-assembled nanoflowers and staking of flake-like structures were achieved. The phase formation and the composition of the films were characterized by X-ray diffraction and energy dispersive analysis of X-ray studies. X-ray photoelectron spectroscopic studies indicated that the samples were composed of CuO. The morphologies of the films were investigated by scanning electron microscopy. A possible growth mechanism is also proposed here. Band gap energies of the nanostructures were determined from the optical reflectance spectra. The different CuO nanostructures showed good electron field emission properties with turn-on fields in the range 6-11.3 V μm⁻¹. The field emission current was significantly affected by the morphologies of the CuO films.     

Morphology-controlled synthesis of ZnS nanostructures via single-source approaches

ZnS nanoparticles of various morphologies, including hollow or solid spherical, and polyhedral shape, were synthesized from single-source precursor Zn(S₂COC₂H₅)₂ without using a surfactant or template. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy. The results indicate that ZnS hollow and solid spheres assembled by nanoparticles can be easily generated by the solution phase thermalysis of Zn(S₂COC₂H₅)₂ at 80 °C using N, N-dimethylformamide (DMF) and ethylene glycol (EG) or water as solvents, respectively, whereas solvothermal process of the same precursor led to ZnS nanoparticles of polyhedral shape with an average size of 120 nm. The optical properties of these ZnS nanostructures were investigated by room-temperature luminescence and UV-vis diffuse reflectance spectra.     

Surface characterization of LaCoO3 synthesized using citric acid

In preparation for synthesizing LaCoO₃, gel was prepared by adding citric acid to the aqueous solution of La(NO₃)₃·6H₂O and Co(NO₃)₂·6H₂O. The infrared spectrum indicates that the gel prepared with 0.007 mol of citric acid is the mixture of LaCo(C₆H₅O₇)(NO₃)₃, lanthanum nitrate, and cobalt nitrate, while the gel prepared with 0.011 mol of citric acid is LaCo(C₆H₅O₇)₂. Perovskite-type LaCoO₃ was obtained by firing the gel above 600°C. The LaCoO₃ surface was characterized by X-ray photoelectron spectroscopy and with respect to the catalytic activity of CO oxidation.     

An efficient room-temperature route to uniform ZnO nanorods with an ionic liquid

Rod-like ZnO nanocrystals have been synthesized via an ultrasound-assisted way by the reaction between Zn(CH₃COO)₂·2H₂O and NaOH in the ionic liquid 1-butyl-3-methyl imidazole six hexafluorophosphoric acid salts ([BMIM][PF₆]) aqueous solution. The products were characterized by XRD, EDX, FESEM, TEM, HRTEM, UV-Vis and PL techniques. The as-prepared ZnO nanorods have a diameter of about 50 nm and a length of 1-2 μm. A plausible four-step mechanism was proposed to explain the formation of ZnO nanorods. It was found that the lowered ion diffusion velocity in the water-ionic liquid medium could largely contribute to the formation of the ZnO nanorods. The effects of experimental parameters on the formation of the products were also explored.     

Facile synthesis of α-MnO2 nanostructures for supercapacitors

Various α-MnO₂ nanostructures have been successfully synthesized by a simple hydrothermal method based on the redox reactions between the MnO₄⁻ and H₂O in mixture containing KMnO₄ and HNO₃. The effect of varying the hydrothermal time to synthesize MnO₂ nanostructures and the forming mechanism of α-MnO₂ nanorods were investigated by using XRD, SEM and TEM. The results revealed an evolvement of morphologies ranging from brushy spherical morphology to nanorods depending upon the hydrothermal time. The surface area of the synthesized nanomaterials varied from 89 to 119 m²/g. Electrochemical properties of the products were evaluated using cyclic voltammetry and galvanostatic charge–discharge studies, and the sample obtained by hydrothermal reaction for 6 h at 120 °C showed maximum capacitance with a value of 152 F/g. In addition, long cycle life and excellent stability of the material were also demonstrated.     

Low-temperature hydrothermal synthesis of phase-pure rutile titania nanocrystals: Time temperature tuning of morphology and photocatalytic activity

In this paper, a simple and efficient methodology for the low-temperature synthesis of phase-pure nanocrystalline rutile TiO₂ with tuned morphology is reported. Control on morphology has been achieved by simple variation of the hydrothermal process, starting with titanium-tetrachloride without using mineralizers, additives or templating agents. The X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns showed no other phases of TiO₂ establishing the formation of phase-pure rutile titania in the entire temperature range of synthesis (40-150 °C) and most noticeably even at a considerably low temperature (40 °C). Fourier transform infrared (FT-IR) spectra strongly indicated the presence of hydroxyl group or surface adsorbed water and the thermogravimetry and differential thermo-gravimetry (TG-DTG) showed no phase change up to 1000 °C. A combination of reaction parameters (temperature, time) with a thorough transmission electron microscopy (TEM) study demonstrated the formation of phase-pure rutile titania nanocrystals as nano-rods, bunched nano-spindles or spherical nanoparticles depending on the hydrothermal reaction conditions. The photocatalytic activity of the synthesized nanocrystals has been successfully evaluated on the photodegradation of methyl orange (MO), a well-known pollutant azo-dye, as a model reaction.     

Hydrothermal synthesis of spindle-like ZnS hollow nanostructures

Spindle-like hollow nanostructures of zinc sulfide (ZnS) have been successfully synthesized by hydrothermal process using a simple surfactant emulsion template. The morphologies of ZnS nanostructures were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and field-emission scanning electron microscopy (FE-SEM). It is found that most of the products including twin ellipsoids with connected hollow cores are reminiscent of spindle-like structures. The lengths, widths and the thickness of the shell are in the range of 1-2 μm, 300-450 nm and 20-40 nm, respectively. Selected area electron diffraction (SAED) and X-ray powder diffraction (XRD) patterns show that the shell is composed of sphalerite ZnS polycrystals.     

Effect of photocatalytic activities of nano-sized copper molybdate (CuMoO4)-doped bismuth titanate (Bi2Ti4O11) (CMBT) alloy

Different compositions of [CuMoO₄]_x-doped Bi₂Ti₄O₁₁ nanophotocatalyst (x = 0.05, 0.1, 0.5) have been prepared by chemical precursor decomposition (CPD) method using triethanolamine (TEA) and HNO₃. Cu(II) is one of reactive species on the catalyst surface and Mo(VI) ion helps to generate charge compensation of lattice having poor catalytic properties. The photocatalytic properties based on the prepared samples for photodecolorization of thymol blue (TB) solutions are examined by Hg-lamp. The crystal structures of the prepared nano-powders are characterized by XRD, EDAX, UV-vis spectra, specific surface area (BET), and HRTEM analyses. The average particle size of copper molybdate-doped bismuth titanate ranges 32 ± 5 nm measured from TEM. Results show doping of copper molybdate of 5 mol% with bismuth titanate can significantly increase the photoactivity of bismuth titanate compared all the compositions studied except degussa P25 titania. The observed increased photocatalytic activity of copper molybdate-doped bismuth titanate ((CuMoO4)_x(Bi₂Ti₄O₁₁)_1−x; CM_xBT_1−x) is attributed to the strong absorption of OH groups at the surface of the catalyst.     

Low temperature pH dependent synthesis of flower-like ZnO nanostructures with enhanced photocatalytic activity

In the present study we have synthesized flower-like ZnO nanostructures comprising of nanobelts of 20 nm width by template and surfactant free low-temperature (4 °C) aqueous solution route. The ZnO nanostructures exhibit flower-like morphology, having crystalline hexagonal wurtzite structure with (0 0 1) orientation. The flowers with size between 600 and 700 nm consist of ZnO units having crystallite size of ∼40 nm. Chemical and structural characterization reveals a significant role of precursor:ligand molar ratio, pH, and temperature in the formation of single-step flower-like ZnO at low temperature. Plausible growth mechanism for the formation of flower-like structure has been discussed in detail. Photoluminescence studies confirm formation of ZnO with the defects in crystal structure. The flower-like ZnO nanostructures exhibit enhanced photochemical degradation of methylene blue (MB) with the increased concentration of ligand, indicating attribution of structural features in the photocatalytic properties.     

Na substitution for La- and Mn-sites in LaMnO3 from alkali halide fluxes: low temperature synthesis, structure and properties

La_1−xNa_xMn_1−yNa_yO₃ (0.12<x<0.16; 0.04<y<0.07) have been synthesized from NaBr and NaI fluxes at relatively low temperature of 850 and 750 °C, respectively. Final composition of these oxides was obtained from chemical analysis of the elements present. The flux grown oxides crystallize in the rhombohedral structure (space group R-3c, No. 167) and are ferromagnetic metals (FMM). Chemical analysis and Rietveld refinement of X-ray data indicate Na ion is substituted for both La- and Mn-sites in the compound prepared from NaBr and NaI fluxes with 33 and 49% of Mn⁴⁺ concentration, respectively.     

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.     

Low temperature synthesis of nanocrystalline lithium ferrite by a modified citrate gel precursor method

Single phase nanocrystalline lithium ferrite is synthesized by a modified citrate gel precursor technique. Ferrite nanoparticles of average size of 8 nm, obtained after calcination of the citrate gel made by the usual method at 450 °C, show superparamagnetic behavior. However, small amounts of -Fe₂O₃ is formed as an impurity phase due to the initial formation of some -Fe₂O₃ phase. On the other hand, when the pH of the mixed solution is increased to 7 after the addition of ammonia solution, a lower calcination temperature of 200 °C is sufficient for the formation of single phase lithium ferrite nanoparticles of size 30 nm. No impurity phases are detected when the nanocrystalline powders are calcined at higher temperatures. The magnetic properties of the ferrite nanoparticles of different sizes obtained by calcining the powders at different temperatures are studied.     

Effect of oxyfluorinated multi-walled carbon nanotube additives on positive temperature coefficient/negative temperature coefficient behavior in high-density polyethylene polymeric switches

Multi-walled carbon nanotubes (MWCNTs) were embedded into high-density polyethylene (HDPE) to improve the electrical properties of HDPE polymeric switches. The MWCNT surfaces were modified by oxyfluorination to improve their positive temperature coefficient (PTC) and negative temperature coefficient (NTC) behaviors in HDPE polymeric switches. HDPE polymeric switches exhibit poor electron mobility between MWCNT particles when the number of oxygen functional groups is increased by oxyfluorination. Thus, the PTC intensity of HDPE polymeric switches was increased by the destruction of the electrical conductivity network. The oxyfluorination of MWCNTs also leads to weak NTC behavior in the MWCNT-filled HDPE polymeric switches. This result is attributed to the reduction of the mutual attraction between the MWCNT particles at the melting temperature of HDPE, which results from a decrease in the surface free energy of the C-F bond in MWCNT particles.