Preparation of ZnO nanoparticles localized on SiC@SiO2 nanocables by a physical templating method

Impregnation of interlaced SiC@SiO₂ nanocables (SiC NWs sheathed by SiO₂ coatings) into a ZnO sol at 0 °C yielded, after thermal treatment up to 600 °C under argon, SiC@SiO₂@ZnO nanostructures. These novel nanostructures consist in SiC@SiO₂ nanocables covered by numerous agglomerated ZnO nanoparticles. The latter are less than 5 nm in diameter. This result is to our knowledge the first example of a physical templating technique involving SiC-based nanowires. Moreover, we have obtained localized ZnO nanoparticles. This localization can be of interests for a further study of their physical properties. When a similar experiment was conducted with pure SiC nanowires, there was no interaction between the nanowires (NWs) and the solution, resulting in the formation of agglomerated ZnO NPs embedded into the 3D NWs network.     

Coupling CoFe2O4 and SnS2 nanoparticles with reduced graphene oxide as a high-performance electromagnetic wave absorber

The reduced graphene oxide (RGO)/CoFe₂O₄/SnS₂ composites have been successfully synthesized by two-step hydrothermal processes. TEM results show that CoFe₂O₄ and SnS₂ nanoparticles with both diameters about 5–10 nm are well dispersed on the surface of graphene. Compared with RGO/CoFe₂O₄ composites, the as-prepared RGO/CoFe₂O₄/SnS₂ composites exhibit excellent electromagnetic (EM) wave absorption properties in terms of both the maximum reflection loss and the absorption bandwidth. The maximum reflection loss of RGO/CoFe₂O₄/SnS₂ composites is −54.4 dB at 16.5 GHz with thickness of only 1.6 mm and the absorption bandwidth with the reflection loss below −10 dB is up to 12.0 GHz (from 6.0 to 18.0 GHz) with a thickness in the range of 1.5–4.0 mm. And especially, they cover the whole X band (8.0–12.0 GHz), which could be used for military radar and direct broadcast satellite (DBS).     

Controlled synthesis of Fe3O4@SnO2/RGO nanocomposite for microwave absorption enhancement

A ternary nanocomposite of Fe₃O₄@SnO₂/reduced graphene oxide (RGO) with different contents of SnO₂ nanoparticles was synthesized by a simple and efficient three-step method. The transmission electron microscopy and field emission scanning electron microscopy characterization display that plenty of Fe₃O₄@SnO₂ core–shell structure nanoparticles are well distributed on the surface of RGO sheets. The X-ray diffractograms show that the products consist of highly crystallized cubic Fe₃O₄, tetragonal SnO₂ and disorderedly stacked RGO sheets. The magnetic hysteresis measurement reveals the ferromagnetic behavior of the products at room temperature. The microwave absorption properties of paraffin containing 50 wt% products were investigated at room temperature in the frequency range of 2–18 GHz by a vector network analyzer. The electromagnetic data show that the maximum reflection loss is −45.5 dB and −29.5 dB for Fe₃O₄@SnO₂/RGO-1 and Fe₃O₄@SnO₂/RGO-2 nanocomposite, respectively. Meanwhile, the reflection loss less than −10 dB is up to 14.4 GHz and 13.8 GHz for Fe₃O₄@SnO₂/RGO-1 and Fe₃O₄@SnO₂/RGO-2 nanocomposite, respectively. It is believed that such nanocomposite could be used as promising microwave absorbers.     

Electrospun ZnO–SnO2 composite nanofibers with enhanced electrochemical performance as lithium-ion anodes

ZnO–SnO₂ composite nanofibers with different structures were synthesized by a simple electrospinning approach with subsequent calcination at three different temperatures using polyacrylonitrile as the polymer precursor. The electrochemical performance of the composites for use as anode materials in lithium-ion batteries were investigated. It was found that the ZnO–SnO₂ composite nanofibers calcined at 700 °C showed excellent lithium storage properties in terms of cycling stability and rate capability, compared to those calcined at 800 and 900 °C, respectively. ZnO–SnO₂ composite nanofibers calcined at 700 °C not only delivered high initial discharge and charge capacities of 1450 and 1101 mAh g⁻¹, respectively, with a 75.9% coulombic efficiency, but also maintained a high reversible capacity of 560 mAh g⁻¹ at a current density of 0.1 A g⁻¹ after 100 cycles. Additionally, a high reversible capacity of 591 mAh g⁻¹ was obtained when the current density returned to 0.1 A g⁻¹ after 50 cycling at a high current density of 2 A g⁻¹. The superior electrochemical performance of ZnO–SnO₂ composite nanofibers can be attributed to the unique nanofibrous structure, the smaller particle size and smaller fiber diameter as well as the porous structure and synergistic effect between ZnO and SnO₂.     

Facile synthesis of ZnO nanostructures and enhancement of their sinterability via short-time cryo-milling

Zinc oxide (ZnO), a wide band-gap semiconductor, has received a great interest due to its potential applications in various fields both as nanostructures and as sintered compacts. In this study, we report on the synthesis of the ZnO nanostructures and facilitation of their sintering for the production of fine-grained dense compacts. The facile synthesis of gram scale ZnO nanostructures was achieved by thermal decomposition of zinc acetate dihydrate (Zn(Ac)₂·2H₂O) or Zn(Ac)₂·2H₂O/graphite mixtures at 300 °C for 12 h. Thermal decomposition of Zn(Ac)₂ resulted in the formation of mostly ZnO nanoparticles with wurtzite structure along with ZnO nanorods, while the addition of graphite significantly promoted the growth of ZnO nanowires. Microstructural and phase properties of the obtained ZnO nanostructures were determined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) techniques, all of which revealed the successful synthesis of high quality ZnO nanostructures. In addition to synthesis and characterization of the ZnO nanostructures, we report on the enhancement of their sinterability by a subsequent cryogenic milling for a short duration of 5 min. As a result of the applied cryo-milling, fabrication of highly dense (96.2%) sintered compacts with fine grain sizes (572 nm) could be achieved after pressureless sintering at 1000 °C for 2 h.     

Characterization and gas sensing properties of bead-like ZnO using multi-walled carbon nanotube templates

We report bead-like ZnO nanostructures for gas sensing applications, synthesized using multi-walled carbon nanotube (MWCNT) templates. The ZnO nanostructures are grown following a two-step process: in the first, ZnO nanoparticles are synthesized on MWCNTs by thermal evaporation of a Zn powder; and in the second, the hybrid nanostructures are heat-treated at 800 °C. Scanning and transmission electron microscopy images indicate that the bead-like ZnO nanostructures have surface protuberances with nanoparticle sizes ranging from 20 to 60 nm, and a well-crystallized hexagonal structure. Gas sensors based on multiple-networked bead-like ZnO showed considerably enhanced electrical responses and better stability to both oxidizing (NO₂) and reducing (CO) gases compared with previously reported nanostructured gas sensors, even if the response to CO gas was slow to increase. Both the NO₂ and CO gas sensing properties increased dramatically when the working temperature was increased up to 300 °C. The response sensitivities measured were 2953%, 5079%, 9641%, 3568%, and 3777% to 20 ppm NO₂ at 200, 250, 300, 350 and 400 °C, respectively. For CO gas on the other hand, the response sensitivities were 107%, 110%, 114%, 118%, and 122% at 5, 10, 20, 50, and 100 ppm concentrations, respectively. For concentrations between 5 and 20 ppm, the recovery time of the oxidizing gas was much shorter than the response time. The origin of the NO₂/CO gas sensing mechanism of the bead-like ZnO nanostructures is discussed.     

Highly stable field emission properties from well-crystalline 6-Fold symmetrical hierarchical ZnO nanostructures

Herein, we report the growth, characterization and field emission application of well-crystalline 6-fold symmetrical hierarchical ZnO nanostructures grown on silicon substrate by thermal evaporation process. The detailed morphological characterizations revealed that the prepared material possess six-fold structures in which ZnO nanoneedles are symmetrically grown on each facets of core hexagonal ZnO nanorods in such a manner that they made beautiful 6-fold symmetrical hierarchical structure. The detailed structural studies confirmed that the grown hierarchical structures possess well-crystallinity with wurtzite hexagonal phase. The room-temperature photoluminescence (PL) spectrum exhibited a strong UV emission confirming good optical properties. The Raman-scattering revealed the wurtzite hexagonal phase for as-grown hierarchical structures. The field emission properties of the 6-fold symmetrical hierarchical ZnO nanostructures were tested and a turn-on voltage equal to 2.8 kV, corresponds to emission current of 65 nA, was observed. A threshold voltage of 4.6 kV with a maximum emission current of about 9.36 µA was also recorded. A high emission current stability profile over a period of ~7000 s was noted for the fabricated FE device.     

Microstructural and optical properties of SnO2–ZnSnO3 ceramics

This work deals with some physical investigation on SnO₂–ZnSnO₃ ceramics grown on glass substrates at different temperatures (450 °C and 500 °C). Structural and optical properties were investigated using X-Ray diffraction (XRD), Raman, infrared (IR) absorption (FTIR), UV–visible spectroscopy and Photoluminescence (PL) techniques. XRD results revealed the existence of a mixture of SnO₂/ZnSnO₃ phases at different annealing temperatures. Structural analysis showed that both phases are polycrystalline. On the other hand, the optical constants (refractive index, extinction coefficient and the dielectric constants) have been obtained by the transmittance and the reflectance data. The optical band gap energy changed from 3.85 eV to 3.68 eV as substrate temperature increased from 450 °C to 500 °C. Raman, FTIR modes and PL reinforced this finding regarding the existence of biphasic (SnO₂ and ZnSnO₃) which is detected also by X-Ray diffraction analysis. Finally, the Lattice Compatibility Theory was evoked for explaining the unexpected incorporation of zinc ions in a rhombohedral structure within SnO₃ trigonal lattice, rather than the occupation of SnO₂ available free loci. All the results have been discussed in terms of annealing temperature.     

The O2-dependent growth of ZnO nanowires and their photoluminescence properties

ZnO nanowires were massively synthesized on a Ni(NO₃)₂-coated silicon substrate under oxygen-containing argon atmosphere by a simple chemical vapor deposition method. The average diameter of the ZnO nanowires was about 50 nm and the average length was about 20 μm. The morphologies of the ZnO nanowires strongly depended on oxygen content in the growth atmosphere. At low oxygen concentration (about 5–10 ppm), ZnO nanocones and nanoneedles were obtained, while at high oxygen concentration (about ∼250 ppm), ZnO nanoparticles deposited on the substrate. The room temperature photoluminescence (PL) spectrum of the ZnO nanowires revealed that a strong UV band at 384 nm dominated the whole spectrum. These results indicate that the ZnO nanowires grown under oxygen-containing atmosphere possess better crystalline quality and UV luminescence properties than those grown in reducing hydrogen atmosphere. Based on the analysis of oxygen effect on the ZnO nanostructures, a vapor–liquid–solid mechanism assisted by the redox growth mode was proposed to understand the growth of the ZnO nanowires.     

Feasibility of using a rotating packed bed in preparing coupled ZnO/SnO2 photocatalysts

In this work, coupled ZnO/SnO₂ photocatalysts were prepared in a rotating packed bed (RPB) via co-precipitation. The precursors of coupled ZnO/SnO₂ photocatalysts were formed from solutions of zinc sulfate, tin tetrachloride and sodium hydroxide. The calcinations of these precursors yielded coupled ZnO/SnO₂ photocatalysts. The effect of calcination temperature on the characteristics and photocatalytic activity of coupled ZnO/SnO₂ photocatalysts was studied. The photocatalytic activity of coupled ZnO/SnO₂ photocatalysts was evaluated using the photocatalytic decolorization of methylene blue. The experimental results reveal that coupled ZnO/SnO₂ photocatalysts that were obtained by calcination at 600 °C for 10 h were the most efficient in decolorizing methylene blue.     

TiO2–SnO2 nanomaterials for gas sensing and photocatalysis

Nanopowders of TiO₂–SnO₂ over a full composition range extending from 100 mol% TiO₂ to 100 mol% SnO₂ are obtained by the sol–gel method from TTIP and SnCl₂·5H₂O precursors of Ti and Sn, respectively followed by calcination at 400 °C. The samples are characterized by means of BET, XRD and TEM. Optical properties of the prepared nanomaterials are studied as well. TEM images indicate that the nanoparticles are regular in shape. The specific surface area, SSA of TiO₂ is 95 m²/g while that of SnO₂ amounts to 129 m²/g. The highest SSA of 156 m²/g is achieved at 20 mol% of TiO₂. Occurrence of rutile, anatase and brookite polymorphic forms depends on the chemical composition of nanopowders. Formation of rutile-type solid solution of TiO₂–SnO₂ over the range of 0–80 mol% TiO₂ is confirmed by Vegard rule applied to lattice constants. Electronic band gap decreases with Ti content from 3.84 eV (100 mol% SnO₂) to 3.18 eV (100 mol% TiO₂).     

Flexible polymer composites with enhanced wave absorption properties based on beta-manganese dioxide nanorods and PVDF

Beta-manganese dioxide (β-MnO₂) nanorods have been fabricated on a large scale by a simple hydrothermal process in a wild condition. Several characterizations such as XRD, SEM, TEM and FESEM have been employed. The wave absorption properties of β-MnO₂/PVDF nanocomposites have been investigated. The results indicated that the β-MnO₂/PVDF nanocomposites exhibit enhanced wave absorption properties. The minimum reflection loss of the β-MnO₂/PVDF nanocomposite reaches − 30.1 dB (> 99.9% attenuation) at 8.16 GHz with a filler loading of 40 wt.%, and the frequency bandwidth less than –10 dB is from 7.12 to 9.20 GHz. The main microwave absorbing mechanism has been also discussed.     

Enhanced NO2 sensing properties of Zn2SnO4-core/ZnO-shell nanorod sensors

Zn₂SnO₄-core/ZnO-shell nanorods were synthesized using a two-step process: synthesis of Zn₂SnO₄ nanorods the thermal evaporation of a mixture of ZnO, SnO₂, and graphite powders, followed by atomic layer deposition (ALD) of ZnO. The nanorods were 50–250 nm in diameter and a few to a few tens of micrometers in length. The cores and shells of the nanorods were face-centered cubic-structured single crystal Zn₂SnO₄ and wurtzite-structured single crystal ZnO, respectively. The multiple networked Zn₂SnO₄-core/ZnO-shell nanorod sensors showed a response of 173–498% to NO₂ concentrations of 1–5 ppm at 300 °C. These response values are 2–5 times higher than those of the Zn₂SnO₄ nanorod sensor over the same NO₂ concentration range. The NO₂ sensing mechanism of the Zn₂SnO₄core/ZnO-shell nanorods is discussed.     

Effects of ZnO and NiO on material properties of microwave absorptive glass-ceramic tile derived from iron ore tailings

Effects of ZnO and NiO on structural, morphological, magnetic properties and microwave absorption behavior of glass-ceramic tiles derived from iron ore tailings (IOTs) were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), magnetization measurements, fourier transform infrared spectroscopy (FTIR) and complex permittivity and permeability measurements. Gradual replacement of NiO by ZnO was found to lower crystallinity of the glass-ceramic and reduce average grain size of the only crystalline phase indexed as spinel Ni–Zn ferrite. Ni²⁺ ion oscillations at different coordination environments inside glass network led to several notable dielectric losses in 3.8–15 GHz. Ferromagnetic resonance frequency is positively correlated with the amount of Ni²⁺. These Ni-related effects enhanced the maximum microwave attenuation of the glass-ceramic to reach −34.35 dB with its frequency being tunable in 6.70–11.20 GHz.     

One-step solution synthesis and formation mechanism of flower-like ZnO and its structural and optical characterization

The regular hierarchical flower-like ZnO nanostructures assembled by nanosheets were successfully synthesized by one-step solution route with citrate assistance at room temperature. It was demonstrated that the concentration of citrate and the molar ratio of Zn²⁺/OH⁻ had strong effect on the formation of nanosheets and self-assembly flower-like nanostructures. A reasonable formation mechanism of the flower-like nanostructures was proposed. According to UV–vis spectrum, the flower-like ZnO nanostructures exhibited strong light absorption, and the value of band gap of the obtained ZnO was estimated to be 3.26 eV. Moreover, the room-temperature photoluminescence (PL) spectrum of the sample presented only a near-band edge emission at 382 nm.     

Far infrared reflection spectroscopy of Zn2SnO4 ceramics obtained by sintering mechanically activated ZnO–SnO2 powder mixtures

A mixture of starting ZnO and SnO₂ powders (molar ratio 2:1) were mechanically activated for 10, 40, 80 and 160 min in a planetary ball mill and then isothermally sintered at 1300 °C for 2 h in order to obtain Zn₂SnO₄ ceramics. X-ray diffraction analysis confirmed single-phase polycrystals. Far infrared reflection spectra were measured (100–1400 cm⁻¹). The same oscillators were observed, but the highest intensity of reflectivity peaks was obtained for the powder activated 10 min and it gradually decreased with longer times of mechanical activation. This is in agreement with microstructure analysis where longer times of mechanical activation lead to increased porosity and defects. Using group theory six ionic oscillators were calculated for single crystal Zn₂SnO₄ spectra, but two more oscillators were observed in the obtained experimental spectra, which could be the result of mechanical activation and sintering. The FIR experimental results were numerically analyzed and oscillator parameters were calculated.     

Raman spectra,photoluminescence and ferromagnetism of pure,Co and Fe doped SnO2 nanoparticles

The pure and transition metal (Co and Fe = 3 and 5 mol%) doped SnO₂ nanoparticles have been synthesized by a chemical route using polyvinyl alcohol as surfactant. These nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman, Fourier transform infrared (FTIR) spectroscopy, photoluminescence (PL) and magnetic measurements. The XRD patterns show that all the samples have tetragonal rutile structure without any extra phase and the value of average particle size using FWHM lies within 12–29 nm is also confirmed by TEM. FTIR spectrum has been used to confirm the formation of SnO bond. Raman spectroscopy shows the intensity loss of classical cassiterite SnO₂ vibration lines which is an indication of significant structural modifications. From PL, an intense blue luminescence centered at a wavelength ～530 nm is observed in the prepared SnO₂ nanoparticles, which is different from the yellow-red light emission observed in SnO₂ nanostructures prepared by other methods. The strong blue luminescence from the as-grown SnO₂ nanoparticles is attributed to oxygen-related defects that have been introduced during the growth process. These Co and Fe-doped SnO₂ nanoparticles exhibit room temperature ferromagnetism and the value of their magnetic moment and phase transition temperature are sensitive to their size and stoichiometric ratio.     

Controllable synthesis of porous CxNy nanofibers with enhanced electromagnetic wave absorption property

Porous C_xN_y nanofibers are controllably synthesized by a simple two-step method. The prepared samples possess uniform micropores and a chemical composition of C_0.73 N_0.27 with a surface area of 329 m² g⁻¹. The obtained C_xN_y nanofibers exhibit remarkable electromagnetic (EM) wave absorption properties when compared with conventional one-dimensional carbon materials. The minimum reflection loss (RL) reaches −36 dB at 2.7 GHz when the ratio of the C_xN_y absorbent added in paraffin matrix is only 1:3. The bandwidth of the RL below −10 dB covers 7.7 GHz (8.1–15.8 GHz) at the sample thickness of 2.5 mm. A possible EM wave loss mechanism was proposed in detail. The multiple reflection and dielectric loss could govern the excellent EM absorption leading the product to a probable application in stealth materials.     

Nanostructured SnO2 thin films for NO2 gas sensing applications

We report the synthesis of nanostructured SnO₂ by a simple inexpensive sol–gel spin coating method using m-cresol as a solvent. This method facilitates rapid synthesis at comparatively lower temperature enabling formation of nanostructures suitable for gas-sensing applications. Various physicochemical techniques have been used for the characterization of SnO₂ thin films. X-ray diffraction analysis confirmed the single-phase formation of tetragonal SnO₂ having crystallite size 5–10 nm. SnO₂ showed highest response (19%) with 77.90% stability toward 100 ppm nitrogen dioxide (NO₂) at 200 °C. The response time of 7 s and recovery time of 20 min were also observed with the same operating parameters. The probable mechanism is proposed to explain the selective response toward nitrogen dioxide. Impedance spectroscopy studies showed that the response to nitrogen dioxide is mainly contributed by grain boundaries. The reproducibility and stability study of SnO₂ sensor confirmed its candidature for detection of NO₂ gas at low concentration (10–100 ppm) and lower operating temperature.     

Tin–Zinc oxide composite ceramics for selective CO sensing

Composite metal oxide gas sensors were intensely studied over the past years having superior performance over their individual oxide components in detecting hazardous gases. A series of pellets with variable amounts of SnO₂ (0–50 mol%) was prepared using wet homogenization of the component oxides leading to the composite tin-zinc ceramic system formation. The annealing temperature was set to 1100 °C. The samples containing 2.5 mol% SnO₂ and 50 mol% SnO₂ were annealed also at 1300 °C, in order to observe/to investigate the influence of the sintering behaviour on CO detection. The sensor materials were morphologically characterized by scanning electron microscopy (SEM). The increase in the SnO₂ amount in the composite ceramic system leads to higher sample porosity and an improved sensitivity to CO. It was found that SnO₂ (50 mol%) - ZnO (50 mol%) sample exhibits excellent sensing response, at a working temperature of 500 °C, for 5 ppm of CO, with a fast response time of approximately 60 s and an average recovery time of 15 min. Sensor selectivity was tested using cross-response to CO, methane and propane. The results indicated that the SnO₂ (50 mol%)-ZnO (50 mol%) ceramic compound may be used for selective CO sensing applications.