Tin dioxide nanoparticles: Reverse micellar synthesis and gas sensing properties

Tin dioxide (SnO₂) nanoparticles have been synthesized by reverse micellar route using cetyltrimethyl ammoniumbromide (CTAB) as the surfactant. Monophasic tin dioxide (SnO₂) was obtained using NaOH as the precipitation agent at 60 °C, however, when liquor NH₃ was used as precipitating agent then crystalline SnO₂ nanoparticles are obtained at 500 °C. SnO₂ prepared using NaOH show crystallite size of 4 and 12 nm after heating at 60 and 500 °C respectively using X-ray line broadening studies. Transmission electron microscopy (TEM) studies show agglomerated particles of sizes 70 and 150 nm, respectively. The grain size was found to be 6-8 nm after heating the precursor obtained (using liquor NH₃) at 500 °C by X-ray line broadening and the TEM studies. Dynamic light-scattering (DLS) studies show the aggregates of SnO₂ nanoparticles with uniform size distribution. Mössbauer studies show an increase of s-electron density at the Sn sites compared to bulk SnO₂ and a finite quadrupole splitting indicative of lowering of symmetry around tin atoms. The gas sensing characteristics have also been investigated using n-butane which show high sensitivity and fast recovery time.     

Effect of calcination on the microstructures of titania nanoparticles prepared in W/O microemulsions

Monodispersed titania nanoparticles were prepared from reacting TiOCl₂ with NH₄OH in water/Triton X-100/n-hexanol/cyclohexane microemulsions. The effect of calcination on the microstructures of the particles was investigated. The particles synthesized were amorphous, transformed into the anatase phase at 300°C, and further into the rutile phase at 850°C. The crystallite size of the particles was 9.7 to 35.6 nm in the temperature range between 300 and 900°C. Secondary particles, agglomerates of finer primary particles, were about 20 nm in size at 200°C and increased markedly by a factor of 10 to 20 at 900°C due to a significant interagglomerate densification. With increasing calcination temperature from 300 to 900°C, the specific surface area of the particles decreased rapidly from 317.5 to 8.4 m²/g, whereas the average pore radius increased considerably from 2.9 to 31.8 nm as the result of shrinkage of the agglomerates, destruction of the minute intercrystallite pores, and interagglomerate densification.     

Hydrothermal Synthesis of Nanocrystalline SnO2 for Gas Sensors

Nanocrystalline SnO₂ is prepared by hydrothermal synthesis (130–250°C, 2–5 h) using three different precursors and is characterized by x-ray diffraction, transmission electron microscopy, and nitrogen BET measurements. The crystallite size of SnO₂ powders (d = 4–5 nm) prepared from amorphous stannic acid gels is found to vary very little with process temperature and duration. Air anneals at 500°C for 1–20 h demonstrate that the highest stability toward crystallite growth is offered by the samples prepared by oxidizing SnSO₄ with H₂O₂ (the crystallite size increases only slightly, from 4–5 to 5–7 nm), whereas the crystallite size of the samples prepared by high-temperature hydrolysis of SnCl₄ increases markedly, from 4–5 to 16–17 nm. Nanocrystalline NiO-doped SnO₂ is prepared by hydrothermal treatment, and its physicochemical properties are investigated. Both SnO₂ and SnO₂NiO exhibit gas sensitivity, as demonstrated by consecutively exposing the samples to different gaseous atmospheres: O₂ N₂ O₂ and O₂ N₂ + C₂H₅OH O₂.     

Mössbauer studies and magnetic properties of SnO2 doped with Fe

Transparent conducting SnO₂ powders doped with 10% Fe content were prepared by a polymerized complex method under acidic solutions, and annealed finally at 550 °C, and at 600 °C. These samples were characterized by X-ray diffraction, magnetization, and Mössbauer spectrometry at room temperature. Rutile SnO₂ phase was obtained for both samples, and the crystallite sizes were in the range of 13-14 nm. Both samples exhibit magnetization and the saturation magnetization was smaller for the sample annealed at 600 °C than for sample annealed at 550 °C. Room temperature Mössbauer spectra for both samples showed the presence of two different paramagnetic iron sites but no magnetic sextets. These results suggest that ferromagnetism originates from magnetic defects and not directly from iron ions.     

Vapor phase growth and optical properties of single-crystalline SnO2 nanobelts

Mass production of single-crystalline SnO₂ nanobelts was successfully achieved through a thermal evaporation of metallic Sn powders at 900 °C. The as-prepared SnO₂ nanobelts were typically 30-200 nm in width, 10-50 nm in thickness, and about tens of micrometers in length. In addition to the classical Raman models, two new Raman bands at 498 and 698 cm⁻¹ are observed for rutile-phased SnO₂ nanobelts, which can be attributed to the IR-active A_2u TO and A_2u LO modes, respectively. Photoluminescence (PL) spectrum of SnO₂ nanobelts featured an emission band at 615 nm (with a small shoulder at 585 nm), which might correspond to the existence of oxygen deficiencies in the produced belts. The formation of SnO₂ nanobelts followed a vapor-solid (VS) growth mechanism.     

Fine-tuning of temperature coefficients of capacitance (TCC) and dielectric constant (TCK) of Mg2SnO4 via second phase addition

Fine-tuning of the temperature coefficients of capacitance and dielectric constant of magnesium orthostannate (Mg₂SnO₄) has been attempted by means of Sn (IV) and Zr (IV) oxide incorporation as a second phase. The additives were also employed to enhance the density and minimize or eliminate porosity at lower sintering temperatures. Phase-pure magnesium stannate powder was synthesized via conventional solid-state reaction. It was mixed with ZrO₂ and/or SnO₂ and sintered in the temperature range 1500°–1600°C for up to 6 h. Electrical measurements using an AC immittance spectroscopic technique over the temperature range 25°–300°C, on Mg₂SnO₄ compacts containing 5 wt.% of additives and sintered at 1500 °C/ 6 h, were carried out. Data analyses revealed that the capacitance and the derived dielectric constant remained invariant over more than 3 decades of frequency in the kilo to megahertz regime. It was also found that addition of ZrO₂ and SnO₂ has a benign effect on both temperature coefficient of capacitance (TCC) and temperature coefficient of dielectric constant (TCK) as it resulted in smaller dependence of capacitance and dielectric constant compared to pure Mg₂SnO₄. Typically, the TCC values were 5 and 30 ppm/°C and TCK values were 20 and 30 ppm/°C for 5 wt.% ZrO₂- and 5 wt.% SnO₂- added Mg₂SnO₄, respectively, in the temperature range 25°–300°C.     

Selective detection of hydrogen sulfide using copper oxide-doped tin oxide based thick film sensor array

In this work, copper oxide-doped (1, 3 and 5 wt%) tin oxide powders have been synthesised by sol–gel method and thick film sensor array has been developed by screen printing technique for the detection of H₂S gas. Powder X-ray diffraction pattern shows that the tin oxide (SnO₂) doped with 3 wt% copper oxide (CuO) has smaller crystallite size in comparison to 0, 1 and 5 wt% CuO-doped SnO₂. Furthermore, field emission scanning electron microscopy manifests the formation of porous film consisting of loosely interconnected small crystallites. The effect of various amounts of CuO dopant has been studied on the sensing properties of sensor array with respect to hydrogen sulfide (H₂S) gas. It is found that the SnO₂ doped with 3 wt% CuO is extremely sensitive (82%) to H₂S gas at 150 °C, while it is almost insensitive to many other gases, i.e., hydrogen (H₂), carbon monoxide (CO), sulphur dioxide (SO₂) and liquefied petroleum gas (LPG). Moreover, at low concentration of gas, it shows fast recovery as compared to response time. Such high performance of 3 wt% CuO-doped SnO₂ thick film sensor is probably due to the diminishing of the p–n junction and the smallest crystallite size (11 nm) along with porous structure.     

Synthesis of novel hexagon SnO2 nanosheets in ethanol/water solution by hydrothermal process

The novel hexagon SnO₂ nanosheets are successfully synthesized in ethanol/water solution by hydrothermal process. The samples are characterized by X-ray diffraction (XRD), infrared ray (IR) and transmission electron microscopy (TEM). By changing the reaction conditions, the size and the morphology can be controlled. Comparison experiments show that when the temperature increased from 140 °C to 180 °C, the edge length of the hexagon nanoparticles increases from 300-450 nm to 700-900 nm. On the other hand, by adjusting the ratios of water to ethanol from 2 to 0.5, SnO₂ nanoparticles with different morphologies of triangle and sphere are obtained. When the concentration of NaOH is increased from 0.15 M to 0.30 M, a hollow ring structure can be obtained.     

Morphologies and electrochemical properties of 0.6Li2MnO3·0.4LiCoO2 composite cathode powders prepared by spray pyrolysis

Nanosized 0.6Li₂MnO₃·0.4LiCoO₂ composite cathode powders are prepared by spray pyrolysis. The micron-sized composite powders are converted into nanosized powders by a simple milling process. The mean sizes of the composite powders measured from the TEM images increase from 20 to 170 nm when the post-treatment temperatures increase from 650 to 900 °C. The Brunauer–Emmett–Teller surface areas of the composite powders post-treated at 650 and 900 °C are 24 and 3 m² g⁻¹, respectively. The XRD patterns indicate that the layered composite powders post-treated at 800 and 900 °C have high crystallinity and low cation mixing. The mean crystallite sizes of the powders, measured from the (003) peak widths of the XRD patterns using Scherrer's equation, are 35 and 56 nm at post-treatment temperatures of 800 and 900 °C, respectively. The initial discharge capacities of the 0.6Li₂MnO₃·0.4LiCoO₂ composite are 262, 267, 264, and 263 mAh g⁻¹ when the post-treat temperatures of the powders are 650, 700, 800, and 900 °C, respectively. The discharge capacity of the composite powders post-treated at 900 °C abruptly decreases from 263 to 214 mAh g⁻¹ by the seventh cycle and then slowly decreases to 198 mAh g⁻¹ with increasing cycle number, up to 30.     

Influence of thickness on H2 gas sensor properties in polycrystalline SnO x films prepared by ion-beam sputtering

Amorphous SnO _x films were deposited on sintered alumina substrates by ion-beam sputtering. They were annealed at 500° C for 2 h in air and polycrystalline films with thickness varying from about 1 to 700 nm were prepared. Film-sensor properties against 0.47% H₂ gas were measured as a function of thickness and the operating temperature for 150 to 350° C. The film thickness exhibiting a sensitivity maximum increased gradually with temperature. The optimum thickness shifted from 7 nm at 150° C to 175 nm at 350° C. Highly sensitive films lay in a narrow thickness range of 60 to 180 nm and films thinner or thicker than this were relatively insensitive at 300 and 350°C. A model was proposed to interpret the sensitivity behaviour in terms of thickness and grain-boundary effect.     

Borohydride synthesis and stabilization of flake-like tetragonal zirconia nanocrystallites

Flake-like particles of tetragonal zirconia (t-ZrO₂) were synthesized using ZrOCl₂·8H₂O as zirconium precursor, and sodium borohydride (NaBH₄) and cetyltrimethylammonium bromide (CTAB) as precipitating agent and surfactant, respectively. Small-sized nuclei, which formed during the borohydride synthesis, play an important role in the formation of small sized crystallites (5 nm) even at 600 °C and stabilization of t-ZrO₂ in both the samples synthesized with and without surfactant. In the sample synthesized without CTAB, a minor m-ZrO₂ phase (5 vol.%) along with the major t-ZrO₂ phase having a crystallite size of 20 nm, was observed at 700 °C. However, the use of surfactant leads to the formation of stabilized t-ZrO₂ nanoparticles with a smaller crystallite size of 15 nm. SEM micrographs of t-ZrO₂ show elliptical as well as elongated shaped flake-like morphology. The formation of small sized crystallites and slow growth of nucleation embryo play an important role in stabilizing the t-ZrO₂ up to as high temperature as 700 °C.     

Synthesis of novel pompon-like porous SnO2 and its application in lithium-ion battery

Novel pompon-like porous SnO₂ with an average diameter of 900 nm has been successfully synthesized via a simple hydrothermal process with subsequent calcination treatment at 600 °C for 2 h in air. The crystalline structure and morphology of the resulting product were characterized by X-ray diffraction, micro-Raman spectrometer, field-emission scanning electron microscopy and transmission electron microscopy. The results indicate that the product is composed of self-assembly SnO₂ pompon with a high purity tetragonal rutile-like structure. The lithium storage property of the obtained pompon-like porous SnO₂ was evaluated by conventional discharge/charge test, showing a high initial discharge capacity of 1895 mAh g^-1 at a current density of 100 mA g^-1.     

Ultraviolet photodetectors made from SnO2 nanowires

SnO₂ nanowires can be synthesized on alumina substrates and formed into an ultraviolet (UV) photodetector. The photoelectric current of the SnO₂ nanowires exhibited a rapid photo-response as a UV lamp was switched on and off. The ratio of UV-exposed current to dark current has been investigated. The SnO₂ nanowires were synthesized by a vapor-liquid-solid process at a temperature of 900 °C. It was found that the nanowires were around 70-100 nm in diameter and several hundred microns in length. High-resolution transmission electron microscopy (HRTEM) image indicated that the nanowires grew along the [200] axis as a single crystallinity. Cathodoluminescence (CL), thin-film X-ray diffractometry, and X-ray photoelectron spectroscopy (XPS) were used to characterize the as-synthesized nanowires.     

Preparation of nanocrystalline perovskite KNbO3 by peroxo-precursor decomposition method

Nanocrystalline perovskite KNbO₃ is prepared by a peroxo-precursor decomposition method at moderate temperatures of 650-900 °C. Peroxo-heteropoly-niobic acid is prepared by direct reaction between NbC powder and H₂O₂ aq, and mixing the peroxo-heteropoly-niobic acid with KOH aq yields an amorphous precursor salt. Perovskite KNbO₃ is obtained by heating the precursor in air for 1 h at 650-900 °C. The X-ray diffraction patterns were well fitted with the space group Pm3m in the Rietveld analysis. The X-ray diffraction peak profiles and field emission scanning electron microscope images indicate the crystallite size is in the range of 25-35 nm.     

Gas sensing properties of lead doped tin oxide thick films

The present investigation deals with the fabrication of liquid petroleum gas (LPG) sensor materials based on semiconducting oxide SnO₂. The gas sensor materials have been prepared by conventional solid-state route. The effect of Pb incorporation, operating temperature, morphology, and sensitivity is discussed using the results of X-ray diffraction (XRD), along with sensing performance. Out of various sensor compositions, Pb doped SnO₂ sintered at 1000 °C for 2 h has shown high sensitivity towards LPG at an operating temperature of 150 °C. Different characterization techniques have been employed, such as surface area analyzer, X-ray diffraction (XRD), to study the formation of SnO₂, surface area and crystallite size, respectively. The results suggested the possibility of utilizing the sensor element for the detection of LPG.     

Nanocrystalline SnO2 and Au:SnO2 thin films prepared by direct current magnetron reactive sputtering

Nanocrystalline pure and gold doped SnO₂(Au:SnO₂) films were prepared on unheated glass substrates by dc magnetron reactive sputtering and, subsequently, the as deposited films were annealed in air. The films structure, surface morphology, photoluminescence, electrical and optical properties were investigated. After annealing the as deposited SnO₂ films, crystallinity increased and the surface roughness decreased. The intensity of PL peaks increases sharply with the annealing temperature. The optical transmittance of the films was around 89% after annealing the as deposited SnO₂ films at 450 °C. The as deposited Au:SnO₂ films show better crystallinity than the as deposited SnO₂ films, the average grain size was around 4.4 nm. The emission peaks of Au:SnO₂ films are slightly blue shifted as compare to undoped SnO₂ films. The Au:SnO₂ films show the lowest electrical resistivity of 0.001 Ωcm with optical transmittance of 76%, after annealing at 450 °C.     

Direct patterning of SnO2 composite films prepared with various contents of Pt nanoparticles by photochemical metal-organic deposition

The optical and electrical characteristics of SnO₂ composite films with various contents (0, 0.05, 0.1, 0.2, and 0.3 at.%) of Pt nanoparticles were evaluated. The Pt nanoparticles were synthesized by a methanol reduction method and their average size was controlled to 3 nm using poly(N-vinyl-2-pyrrolidone) as a protecting agent. The lowest resistivity of 2.031 × 10^− 2 Ω cm was obtained in the SnO₂ film containing 0.2 at.% Pt nanoparticles after annealing at 700 °C while its average transmittance in the visible region was 85.24%. The enhanced electrical properties were attributed to the increase of the carrier concentration and crystallinity of the films due to donation from Pt nanoparticles as well as the increased annealing temperature. Meanwhile, the slight degradation of the transmittance was due to scattering from the introduction of Pt nanoparticles and the increased crystallite size due to the increase of the annealing temperature to 700 °C. Well-defined 20-μm wide direct-patterned composite SnO₂ films containing Pt nanoparticles were formed by a simple photochemical metal-organic deposition process involving a photosensitive starting precursor, UV exposure, and removal of the unpatterned area by rinsing with solvent. Based on the results of this study, we suggest that direct-patternable SnO₂ films with Pt nanoparticles can be easily applied to transparent electrodes in electrical devices without requiring an expensive and toxic process such as dry etching.     

Effect of Sn/Sb ratio in determining crystallite size of “SnO2-Sb2O5” semiconductors

Six SnO₂-Sb₂O₅ semiconductors with different Sn/Sb ratios were prepared by the gel method. The samples, heated at different temperatures in the range 250 to 1100° C, were characterized by X-ray analysis, surface-area determination, TG-DTA studies and SEM observations. Results indicate that the crystallite size of SnO₂ doped by antimony oxide is affected by Sb/Sn ratio, smaller crystallites being obtained at lower antimony oxide loadings. The presence of a Sb₂O₄ phase separated from SnO₂ and the effect of antimony doping on SnO₂ were considered to be the main parameters ruling SnO₂ crystallite dimensions.     

Synthesis and structural and optical properties of metastable ZrO2 nanoparticles with intergranular Cr3+/Cr4+ doping and grain surface modification

Nanoparticles of stabilized ZrO₂ in a single cubic (c) phase are obtained with an intergranular doping of 4 to 20 at.% Cr³⁺/Cr⁴⁺ additives by a chemical method using a high energy amorphous precursor with polymer molecules of sucrose and polyvinyl alcohol. In the polymer, the metal cations disperse and rearrange in a specific network structure with local symmetry probably similar to that in c-ZrO₂. On heating at 250 to 800°C in air, the polymer network decomposes and burns out spontaneously (with a strong exothermic peak over 350 to 500°C in thermal analysis) in a refined microstructure in 10 to 20 nm diameter particles of near spherical shape. Those are identified to be of c-ZrO₂ by x-ray diffraction. A modified microstructure of 15 to 30 nm crystallites of dispersed tetragonal (t) and/or monoclinic (m) phases in stabilized c-ZrO₂ develops on a prolong heating at 900 to 1000°C from a polymer precursor for 2 h or longer. Particles in t-ZrO₂ are in acicular shape, as long as 450 nm with aspect ratio 5 to 20, while in the shape of platelets in m-phase in an average 300 nm size. It is found that the Cr³⁺/Cr⁴⁺ additives promote formation of c-ZrO₂ by a controlled decomposition and combustion of precursor in small particles at 250 to 800°C temperature. Part of the additives form a thin amorphous surface layer in individual c-ZrO₂ grains so that it prevents them to grow or transform in the equilibrium m-ZrO₂ bulk structure as long as the temperature lies below 900°C. The x-ray diffraction in light of the optical spectrum reveals that part of the Cr⁴⁺ cations occupy Zr⁴⁺ sites in a distorted c-ZrO₂ lattice.     

Low temperature synthesis of SnO2 nanocrystals from tin, sulfur and ammonium chloride powders

Tetragonal phase SnO₂ nanocrystals were synthesized directly by heating the mixture of tin (Sn), sulfur (S) and ammonium chloride (NH₄Cl) powders in air at 400 °C for 2-5 h. The phase, size and purity of the resultant products were characterized by means of powder X-ray diffraction (XRD), filed emission scanning electronic microscope (FESEM), and energy dispersive X-ray (EDX) spectra. Besides, the effects of heating temperature, duration, and composition of the reactants on the phases of the resultant products were investigated. It was found that both of the S and NH₄Cl additives played important roles in the current low temperature (400 °C) synthesis of pure SnO₂.