Bulk-quantity SnO2 nanorods synthesized from simple calcining process based on annealing precursor powders

Tin dioxide (SnO₂) nanorods have been successfully synthesized in bulk quantity by a calcining process based on annealing precursor powders in which sodium chloride, sodium carbonate, and stannic chloride were homogeneously mixed. Transmission electron microscopy shows that the as-prepared nanorods are structurally perfect and uniform, with widths of 10–25 nm, and lengths of several hundreds nanometers to a few micrometers. X-ray diffraction and energy-dispersive X-ray spectroscopy analysis indicate that the as-prepared nanorods have the same crystal structure and chemical composition found in the tetragonal rutile form of SnO₂. Selected area electron diffraction and high-resolution transmission electron microscopy reveal that the as-prepared nanorods grow along the [1 1 0] crystal direction. We found that the calcined temperature has a strong influence on the size and morphology of SnO₂ nanorods. The growth process of SnO₂ nanorods is suggested to follow an Ostwald ripening mechanism. Our findings indicate that other nanorods or nanowires may be manipulated by using this technique, and might provide insight into the new opportunities to control materials fabrication.     

Structural evolution of SnO2 nanostructure from core–shell faceted pyramids to nanorods and its gas-sensing properties

Tin oxide (SnO₂) nanorods were synthesized through an aqueous hexamethylenetetramine (HMTA) assisted synthesis route and their structural evolution from core–shell type faceted pyramidal assembly was investigated. Structural analysis revealed that the as-synthesized faceted SnO₂ structures were made of randomly arranged nanocrystals with diameter of 2–5 nm. The shell thickness (0–80 nm) was dependent on the molar concentration of HMTA (1–10 mM) in aqueous solution. It was revealed that the self-assembly was possible only with tin (II) chloride solution as precursor and not with tin (IV) chloride solution. At longer synthesis hours, the pyramidal nanostructures were gradually disintegrated into single crystalline nanorods with diameter of about 5–10 nm and length of about 100–200 nm. The SnO₂ nanorods showed high sensitivity towards acetone, but they were relatively less sensitive to methane, butane, sulfur dioxide, carbon monoxide and carbon dioxide. Possible mechanisms for the growth and sensing properties of the nanostructures were discussed.     

Phase transformation and crystal growth behaviors of La3+/Ce3+-doped TiO2–20 wt% SnO2 gels

The transformation behaviors of La³⁺/Ce³⁺-doped TiO₂–SnO₂ gels were studied by using differential thermal analysis and X-ray diffraction methods so as to improve the phase transformation and decrease the granularity of crystals. Experimental results show that, anatase, rutile and SnO₂ nanocrystals can exist in the sintering products by varying La³⁺/Ce³⁺ contents and sintering temperatures. 0.8–1.1 wt% of La₂O₃ or CeO₂ doping greatly depresses the growth of anatase and rutile crystals, obtaining nanosized crystals when sintered up to 600 °C for 2 h. With La³⁺/Ce³⁺-doping and increasing their contents, the transformations of gel to anatase and anatase to rutile, as well as the growth of anatase and rutile crystals can be depressed, while the transformation temperature of anatase to rutile receives much less affect. Moreover, the La³⁺-doping has stronger effects on them than Ce³⁺ doping, but has a weaker inhibiting effect on precipitation and growth of SnO₂ crystals.     

Well‐defined Sb2S3 nanostructures: citric acid‐assisted synthesis,electrochemical hydrogen storage properties

Well‐defined (three‐dimensional) 3‐D dandelion‐like Sb₂S₃ nanostructures consisted of numerous nanorods have been achieved via a facile citric acid‐assisted solvothermal process. The as‐prepared products were characterized by X‐ray diffraction (XRD), field‐emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high‐resolution TEM (HRTEM), respectively. The influence factors of the formation of the hierarchical Sb₂S₃ nanostructures are discussed in details based on FESEM characterizations. By simply controlling the quantity of citric acid, the nucleation and growth process can be readily tuned, which brings the different morphologies and nanostructures of the final products. On the basis of a series of contrastive experiments, the aggregation‐based process and anisotropic growth mechanism are reasonably proposed to understand the formation mechanism of Sb₂S₃ hierarchical architectures with distinctive morphologies including nanorods, and dandelion‐like nanostructures. Charge‐discharge curves of the obtained Sb₂S₃ nanostructures were measured to investigate their electrochemical hydrogen storage behaviors. It revealed that the morphology played a key role on the hydrogen storage capacity of Sb₂S₃ nanostructure. The dandelion‐like Sb₂S₃ nanostructures exhibited higher hydrogen storage capacity (108 mAh g⁻¹) than that of Sb₂S₃ nanorods (95 mAh g⁻¹) at room temperature.     

The phase transformation behaviors of Sn2+-doped Titania gels

The transformation behaviors of Sn²⁺-doped titania gels, such as transformation temperature, conversion rate and the crystal granularity, were studied by using thermo-gravimetric/differential thermal analysis and X-ray diffraction methods. Experimental results show that, anatase, rutile and SnO₂ phases can exist in the sintering products by varying Sn²⁺ content and sintering temperature. Sn²⁺-doping greatly depresses the growth of anatase and rutile crystals and so obtaining nanosized crystals The transformation temperatures of gel to anatase and anatase to rutile first decrease and then increase with an increase of Sn²⁺ content, while both of the transformation rates are associated with Sn²⁺ content, change in its valence and the sintering temperature. It is suggested that by adjusting Ti⁴⁺/Sn⁴⁺ ratio and sintering temperature, the crystal granularity and the ratio of anatase to rutile can be tailored to optimize properties of TiO₂–SnO₂ humidity sensing ceramics.     

Effect of growth parameters on the properties of GaN : Zn epilayers

The vapor-phase HCl/Ga/NH₃ method for deposition of GaN : Zn epilayers on sapphire substrates has been investigated to determine the dependence of epilayer resistivity, cathodoluminescence, and surface quality on the growth parameters. Both nucleation and control of the epilayer properties have been significantly improved by introducing HCl directly into the deposition zone in addition to the HCl that passes over the Ga source to produce GaCl. The effect of annealing on the stability of undoped layers and on the cathololuminescence of Zn-doped layers has also been investigated. Electroluminescent devices with reproducible properties have been obtained by growing structures consisting of an undoped n⁺ layer, a Zn-doped n-type layer, and a very thin, Zn-doped, high-resistivity layer whose growth parameters determine the emission wavelength and electroluminescence efficiency.     

Structural study of undoped and (Mn, In)-doped SnO2 thin films grown by RF sputtering

Undoped and 5%(Mn, In)-doped SnO₂ thin films were deposited on Si(1 0 0) and Al₂O₃ (R-cut) by RF magnetron sputtering at different deposition power, sputtering gas mixture and substrate temperature. X-ray reflectivity was used to determine the films thickness (10–130 nm) and roughness (～1 nm). The combination of X-ray diffraction and Mössbauer techniques evidenced the presence of Sn⁴⁺ in an amorphous environment, for as-grown films obtained at low power and temperature, and the formation of crystalline SnO₂ for annealed films. As the deposition power, substrate temperature or O₂ proportion are increased, SnO₂ nanocrystals are formed. Epitaxial SnO₂ films are obtained on Al₂O₃ at 550 °C. The amorphous films are quite uniform but a more columnar growth is detected for increasing deposition power. No secondary phases or segregation of dopants were detected.     

Effect of Electric Field on Dielectric Properties of SnO2 Sb2O3 and their Mixed Thin Films

Thin films of various thicknesses in the MIM structure have been prepared from the the powders of SnO₂, Sb₂O₃ and (SnO₂ + Sb₂O₃) of high purity by the thermal evaporation technique in a vacuum of 10⁻⁵ Torr. Dielectric properties of SnO₂, Sb₂O₃, and their mixed thin films have been studied with ac and dc electric fields and frequency. Capacitance and loss tangent are almost independent on dc voltage upto 1.0 V for SnO₂, 10.0 V for Sb₂O₃ and 2.5 V for mixed films. These capacitors become unstable at 1.0 V for SnO₂ films and 2.5 V for mixed films. For higher film thicknesses the decay in these films starts at higher voltages. Capacitance and loss tangent increases with applied ac voltage in SnO₂, Sb₂O₃, and their mixed films. A comparison of the capacitance values of SnO₂, Sb₂O₃, and their mixed films showed that the capacitance values are less in Sb₂O₃ as compared to SnO₂ films. In mixed films the capacitance is greater than the constituent films. These studies have shown that Sb₂O₃ films are found to be more stable compared to SnO₂ and their mixed films for ac and dc voltages. The results thus obtained on SnO₂, Sb₂O₃, and their films are presented and discussed.     

Orthorhombic Pbcn SnO2 nanowires for gas sensing applications

Pure monocrystalline orthorhombic SnO₂ nanowires decorated and non-decorated with cassiterite SnO₂ nanoclusters are analyzed and compared with pure monocrystalline cassiterite SnO₂. We corroborate the coexistence of both, cassiterite and orthorhombic phases, having a higher growth speed for the cassiterite one, in the obtained nanowires by the evaporation/condensation technique. For both phases, the building blocks are the [SnO₆]^8? octahedron which are forming chains of edge-sharing octahedral along the [0 0 1] direction for the cassiterite phase, while in the orthorhombic phase, chains run in a zigzag fashion and contains four octahedra on each unit of chain instead of two previously reported for orthorhombic material obtained at high pressure conditions as Pbcn SnO₂ orthorhombic structure. Results obtained reveal singular structural characteristics of these synthesized orthorhombic nanowires.     

Self-assembled tin-doped ZnO nanowire and nanoplate structures grown by thermal treatment of ZnS powder

Sintering of a ZnS–SnO₂ mixture under argon flow leads to the growth of columnar nanoplate arrays as well as arrays of nanowires, nanorods and nanoplates with six-fold symmetry. The six-fold nanoplate structures correspond to a more advanced stage of growth than the nanowire structures. Cathodoluminescence (CL) in the scanning electron microscope (SEM) shows that the structures contain Sn, but the amount of this element is normally under the detection limit of X-ray energy-dispersive spectroscopy (EDS). The formation of branches in the hierarchical structures depends on the presence of Sn and on defects in the mixture powder.     

Two routes of forming nanorods on the base of nanoparticles

With ZnS nanoparticles as raw materials, two kinds of nanorods were fabricated. When ZnS nanoparticles were dispersed in Zn(NO₃)₂ solution and were treated in a sealed autoclave at 150 °C for 24 h, ZnO nanorods formed. ZnS nanoparticles work as a source of monomers for the growth of ZnO nanorods. With increase of concentration of Zn(NO₃)₂ solution, the products present long nanorods, short nanorods, star structure and bulk particle sequentially. When ZnS nanoparticles were kept in a solution at room temperature, ZnS nanorods formed after 21 days. ZnS nanoparticles work as assembly parts. They fit together spontaneously and present ZnS nanorods with many parallel stripes along the lengthways surface. To check the generality of self-assembling, PdS and Y₂S₃ nanorods were also fabricated.     

Rare earth centers properties and electron trapping in SnO2 thin films produced by sol–gel route

Some very relevant optical, electrical, and structural properties of SnO₂ doped with rare-earth ions Er³⁺ and Eu³⁺ are presented. Films are produced by the sol–gel-dip coating process, and may be described as a combination of nanoscopic dimension crystallites (about 3–10 nm) with their respective intergrain potential barriers. The Er³⁺ and Eu³⁺ ions are expected to act as acceptors in SnO_2, which is a natural n-type conductor, inducing a high degree of charge compensation. Electron trapping and emission spectra data are presented and are rather distinct, depending on the location of the rare-earth impurity. This behavior allows the identification of two distinct centers: located either in the SnO₂ lattice or segregated at the particles surface. Based on a model for thermally activated cross-section defects, the difference between the capture energy of the photo-excited electron and the intergrain potential barrier is evaluated, leading to distinct values for high and low symmetry sites. A higher distortion in the lattice of undoped SnO₂ and SnO₂:Eu (1 at.%) was evaluated from Rietveld refinements of X-ray diffraction data. This was confirmed by Raman spectra, which are associated with the particles size and disorder. By comparing the samples with the same doping concentration, it was found that this disorder is higher in Eu-doped SnO₂ than in Er-doped SnO₂, which is in agreement with a higher energy for the lattice relaxation in the trapping process by Eu³⁺ centers.     

Broad blue-green-red emissions of SnO2/Pr3+ co-doped phosphate glasses

The photoluminescence properties of SnO₂/Pr^3 + co-doped strontium phosphate glasses (75P₂O₅-25SrO) are studied. An ultrabroad emission band covering blue, green and red is observed in co-doped glasses. In co-doped samples, three downward peaks appear in blue emission region, these coincide with Pr^3 + excitation peaks, indicating the energy transfer through cross-relaxation between SnO₂ and Pr^3 +. The mechanism has been detailed based on the energy level diagrams of SnO₂ and Pr^3 +. The chromaticity coordinates of the co-doped samples with varying doping ratio of SnO₂ to Pr^3 + are calculated. The result demonstrates the possibility of generating white light in the SnO₂/Pr^3 + co-doped phosphate glasses.     

Dependence of nanocrystalline SnO2 particle size on synthesis route

Very fine SnO₂ powders were produced by (a) slow and (b) forced hydrolysis of aqueous SnCl₄ solutions and (c) hydrolysis of tin(IV)-isopropoxide dissolved in isopropanol (sol-gel route) and then characterized by X-ray powder diffraction, Fourier transform infrared and laser Raman spectroscopies, TEM and BET. The XRD patterns showed the presence of the cassiterite structure. As found from XRD line broadening the crystallite sizes of all powders were in the nanometric range. TEM results also showed that the sizes of SnO₂ particles in all powders are in nanometric range. Very fine SnO₂ powders showed different features in the FT-IR spectra, depending on the route of their synthesis. The reference Raman spectrum of SnO₂ showed four bands at 773, 630, 472 and 86 (shoulder) cm⁻¹, as predicted by group theory. Very fine SnO₂ powders showed additional Raman bands, in dependence on their synthesis. The broad Raman band at 571 cm⁻¹ was ascribed to amorphous tin(IV)-hydrous oxide. The additional Raman bands at 500, 435 and 327 cm⁻¹ were recorded for nanosized SnO₂ particles produced by forced hydrolysis of SnCl₄ solutions. However, these additional Raman bands were not observed for nanosized SnO₂ particles produced by slow hydrolysis of SnCl₄ solution or the sol-gel route. The aggregation effects of nanosized particles were considered in the interpretation of the Raman band at 327 cm⁻¹. The method of low frequency Raman scattering was applied for SnO₂ particle size determination. On the basis of these measurements it was concluded that the size of SnO₂ particles was also in the nanometric range and that, the sol-gel particles heated to 400 °C consisted of several SnO₂ crystallites.     

Growth and photoluminescence properties of vertically aligned SnO2 nanowires

Vertically aligned SnO₂ nanowires (NWs) were grown for the first time by a vapor–liquid–solid method on c-sapphire with gold as a catalyst under Ar gas flow. Electron backscatter diffraction analysis indicated the NWs are single crystalline having the rutile structure, grow vertically along the [1 0 0] direction, and exhibit a consistent epitaxial relationship where lattice mismatch is estimated to be 0.3% along the SnO₂ [0 1 0] direction. The growth of these NWs is sensitive to many parameters, including growth duration, substrate type, source vapor concentration, and the thickness of the catalyst layer. Photoluminescence measurements at room temperature showed that the vertically aligned NWs exhibit an intense transition at 3.64 eV, a near band-edge transition which is rarely observed in SnO₂.     

Crystal growth of tin oxide nano-sheets in aqueous solutions and time variation of N2 adsorption characteristics

Nucleation and crystal growth of tin oxide were realized in aqueous solutions. Nano-sheet structures grew in the solutions containing SnF₂ at 90 °C to develop in-plane size and thickness gradually. Their color, size, morphology, crystal structure and N₂ adsorption property varied with progress of crystal growth. Color of powder changed from white to light yellow, light blown to light green particles with growth. XRD analyses clarified that single phase of SnO₂ was synthesized in the aqueous solutions without high temperature annealing. It is an advantage to conventional ceramic firing process. Growth curves of crystallite size were consistent with morphological observations using Field Emission Scanning Electron Microscopy FE-SEM. Development of sheet structure was caused by crystal growth of SnO₂. Large crystallite size perpendicular to (200) planes indicated that growth along c-axis was suppressed compared to a-axis. Anisotropic crystal growth resulted anisotropic shape of nano-sheets. Chemical composition of fluorine-doped SnO₂ nano-sheets was estimated to Sn : F = 1 : 0.06–0.12. BET surface area increased with crystal growth and reached to 85 m²/g. It gradually decreased with synthesis period. Growth of sheet structure along in-plane direction related to increase of surface area. Further growth in thickness increased volume of the sheets to cause decrease of surface area per weight. N₂ adsorption property strongly related to crystal growth and shape change of the nano-structures. The system has the advantage of morphology controllability of tin oxide nano-structure and low environmental load.     

Synthesis of Cu vanadate nanorods for visible‐light photocatalytic degradation of gentian violet

Cu vanadate nanorods have been synthesized via the hydrothermal process using polymer polyvinyl pyrrolidone (PVP) as the surfactant. X‐ray diffraction (XRD) shows that the nanorods are composed of monoclinic Cu₅V₂O₁₀ phase. Scanning electron microscopy (SEM) observation shows that the diameter and length of the nanorods are 50–300 nm and 3 μm, respectively. PVP concentration, hydrothermal temperature and duration time play essential roles in the formation and sizes of the Cu vanadate nanorods. A PVP‐assisted nucleation and crystal‐growth process is proposed to explain the formation of the Cu vanadate nanorods. Gentian violet (GV) is used to evaluate the photocatalytic activities of the Cu vanadate nanorods under solar light. The GV concentration clearly decreases with increasing irradiation time, and content of the Cu vanadate nanorods. GV solution with the concentration of 10 mg L⁻¹ can be totally degraded under solar light irradiation for 4 h using 10 mg Cu vanadate nanorods. The Cu vanadate nanorods have good photocatalytic activities for the degradation of GV under solar light.     

Synthesis by organic molten salt method and characterization of chalcopyrite AgInS2 nanorods

In this paper, chalcopyrite AgInS₂ nanorods were synthesized for the first time by a one‐step, ambient pressure, environment friendly organic molten salt (OMS) method at 200 °C. The as‐synthesized products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. The XRD results reveal that the as‐synthesized products at 120–160 °C under ambient pressure contain AgIn₅S₈ which will decrease with the increase of growth temperature. A sample containing only the chalcopyrite AgInS₂ phase is successfully obtained at 200 °C. Furthermore, the elemental compositions are found to become increasingly stoichiometric with increasing temperature. UV‐Vis and photoluminescence (PL) spectra are utilized to investigate the optical properties of AgInS₂ nanorods. By testing on UV‐Vis spectra, it is concluded that the limiting wavelength of the AgInS₂ nanorods is 661 nm and the band gap is 1.88 eV. A broad red emission band peak centered at about 1.874 eV (662 nm) is clearly observed at room temperature, and the intensity of the emission increases with excitation wavelength. In addition, the photoluminescence quantum yield (PLQY) of the nanocrystals at the excitation wavelength of 250 nm was determined to be 13.2%. A possible growth mechanism of AgInS₂ nanorods was discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Development of porous silicon matrix and characteristics of porous silicon/tin oxide structures

Porous silicon (PSi) was formed at different current densities in the range of 5-60 mA/cm² by electrochemical anodized etching in HF for different durations in the range of 10-30 min. Above this PSi structure, SnO₂ films were deposited by the spin coating technique. The PSi has been characterized by X-ray diffraction studies. Peaks pertaining to PSi along with those corresponding to SnO₂ are observed. Atomic force microscopic studies indicate that very fine needle like silicon nanostructures are observed which is the result of the best PSi structure formed at 30 mA/cm². For the SnO₂ covered PSi structures, larger grains are observed with uniform coverage. The PSi samples prepared at current densities above and below 30 mA/cm² show PL spectra with asymmetric and overlapped peaks. The PL profile of thin SnO₂ film coated on PSi shows a peak at 633 nm and a small hump at about 660 nm.     

Microwave hydrothermal synthesis and photocatalytic properties of ZnWO4 nanorods

Cd²⁺‐doped ZnWO₄ nanorods have been synthesized at 200 °C with microwave hydrothermal method, using Zn(NO₃)₂·6H₂O, Na₂WO₄·2H₂O and CdCl₂ as raw materials. The effects of Cd²⁺ doping contents on the structure and morphology of the product were studied. The results show that Cd²⁺ doping into the crystal lattice of ZnWO₄ nanopowder makes the powder orientationally grow along (010), (110) and (200) crystal planes to form the nanorods. The bigger Cd²⁺ doping contents are, the more obviously ZnWO₄ nanorods grow. Meanwhile, the nanopowder is gradually transformed from monoclinic phase into the orthogonal phase. As the charge transference medium between the interfaces, Cd²⁺ restrains the combination of holes and electrons. After doped, the photocatalytic properties of ZnWO₄ nanorods are increased. When Cd²⁺ doping content is 20%, the Cd²⁺‐doped ZnWO₄ nanorods showed the highest degradation rate up to 98% in 2 h.