High-sensitivity humidity sensor based on SnO2 nanoparticles synthesized by microwave irradiation method

Tin oxide hexagonal-shaped nanodiscs (SnO) and spherical nanoparticles (SnO₂) have been prepared by using a simple household microwave irradiation method with an operating frequency of 2.45 GHz. This technique permits us to produce gram quantity of homogeneous nanoparticles in just 10 min. The crystallite size was evaluated from powder X-ray diffraction (XRD) studies and was in the 20 to 25 nm range. Transmission electron microscopy (TEM) analysis showed that the as prepared SnO form as hexagonal-shaped nanodiscs and upon subsequent annealing at 500 °C for 5 h in air, the SnO gets converted to spherical-shaped nanoparticles of SnO₂. The SnO₂ sample shows good sensitivity towards the relative humidity. The calculated response and recovery time were found to be 32 s and 25 s respectively. These results indicate promising applications of SnO₂ nanoparticles in a highly sensitive environmental monitoring and humidity controlled electronic devices. The samples were further subjected to thermal analyses (TG–DTA) and UV–VIS diffusion reflectance spectroscopy (DRS) studies.     

Microwave-assisted synthesis and investigation of SnO2 nanoparticles

Microwave technique was adopted for preparation of tin dioxide nanoparticles with particles size ranging from 10 to 11 nm within 10 min. The formation of monocrystalline SnO₂ nanoparticles was confirmed by the XRD (X-Ray Diffraction) and TEM (Transmission Electron Microscopy) as well as with SAED (Selected Area Electron Diffraction) analysis. The structure of the SnO₂ crystal was found to be Cassiterite type tetragonal structure. The FT-IR results further supported the formation of tin dioxide from tin hydroxyl group without any post annealing. The samples were further characterized by thermo gravimetric analysis (TGA), electrical resistance measurements and photoluminescence spectrum.     

Growth and characterization of tin oxide thin films and fabrication of transparent p-SnO/n-ZnO p–n hetero junction

p-Type and n-type tin oxide thin films were deposited by rf-magnetron sputtering of metal tin target by varying the oxygen pressure. Chemical composition of SnO thin film according to the intensity of the XPS peak is about 48.85% and 51.15% for tin and oxygen respectively. Nearest neighbor distance of the atoms calculated from SAED patterns is 2.9 Åand 2.7 Åfor SnO and SnO₂ respectively. The Raman scattering spectrum obtained from SnO thin films showed two peaks, one at 113 cm⁻¹ and the other at 211 cm⁻¹. Band gap of as-deposited SnO_x thin films vary from 1.6 eV to 3.2 eV on varying the oxygen partial pressure from 3% to 30% which indicates the oxidization of metallic phase Sn to SnO and SnO₂. p-Type conductivity of SnO thin films and n-type conductivity of SnO₂ thin films were confirmed through Hall coefficient measurement. Transparent p–n hetero junction fabricated in the structure glass/ITO/n-ZnO/p-SnO shows rectification with forward to reverse current ratio as 12 at 4.5 V.     

Fabrication of resistive CO gas sensor based on SnO2 nanopowders via low frequency AC electrophoretic deposition

A resistive CO gas sensor has been fabricated using AC electrophoretic deposition (ACEPD) technique. SnO₂ thick films are deposited by applying low frequency (0.01–1,000 Hz) AC electric field to a stable suspension of SnO₂ nanoparticles in acetyl acetone. A carbon film base electrode is used as deposit substrate. Effect of CO gas exposure on conductivity of the SnO₂ film at 300 °C is investigated. Results show that the sensor is sensitive and its response is repeatable. This work shows that ACEPD can be used as an easy and cheap technique for fabrication of electronic devices such as ceramic gas sensors.     

Preparation and characterization of SnO2 nanoparticles incorporated into talc porous materials (TPM)

Tin oxide (SnO₂) particles incorporated into porous material with different Sn/Si molar ratios were prepared. Talc was mechanically milled and subsequently leached to prepare talc porous material (TPM) which was used as host for incorporating SnO₂ nanoparticles. The SnO₂ incorporated TPM (SnO₂/TPM) samples were characterized by X-ray diffraction (XRD), high-resolution TEM (HRTEM), Fourier transformation infrared spectroscopy (FT-IR) and N₂ adsorption techniques. The specific surface area and pore volume decrease significantly from 260 m²/g and 0.51 ml/g for TPM sample to 178 m²/g and 0.32 ml/g for SnO₂/TPM sample with Sn/Si molar ratio of 0.4, respectively. Both of them can be attributed to the presence of SnO₂ particles within the pores of the SnO₂/TPM samples.     

Molten-salt decomposition synthesis of SnO2 nanoparticles as anode materials for lithium ion batteries

SnO₂ nanoparticles were synthesized by a simple, easily scaled-up molten-salt decomposition method with SnSO₄ as the molten salt and the reactive phase. During the synthesis process, the undecomposed molten SnSO₄ makes it possible to obtain SnO₂ nanoparticles by serving as the dispersion medium and keeping the particles from aggregation. The as-prepared SnO₂ had a tetragonal rutile structure with an average particle size of 50 nm. When used as anode materials for lithium ion battery, SnO₂ nanoparticles retained the charge capacity still as high as 402 mAh g^? 1 at a current density of 156 mA g^? 1 after 40 cycles. Moreover, cyclic voltammograms tests showed the formation/deformation of Li₂O was partially reversible.     

Microstructural investigation and SnO nanodefects in spray-pyrolyzed SnO2 thin films

Spray pyrolysis is one of the most cost-effective methods to prepare SnO₂ films due to its ability to deposit large uniform area, low fabrication cost, simplicity and low deposition temperature. Conventionally, scanning electron microscopy (SEM) and X-Ray Diffraction (XRD) are routinely used to investigate microstructure and crystal structure of the SnO₂ films. In the present study, the SnO₂ films were deposited by spray pyrolysis at 300, 400 and 500 °C and the microstructure of the 500 °C film was further examined by using transmission electron microscopy (TEM) and convergent beam electron diffraction (CBED). It was found that large grain-size vertically-aligned columnar SnO₂ grains were formed after a few layers of small grain-size randomly oriented SnO₂ grains. Moreover, CBED showed the presence of SnO nanodefects that had not been reported before and could not be detected by SEM or XRD.     

Rod- and wire-like morphologies of tin oxide developed with plasma oxidation after electrodeposition

A new manufacturing method has been developed to prepare films composed of Sn-rich SnO₂ wires and rods using electrodeposition and subsequent plasma oxidation of pure Sn. The morphology of Sn-rich tin oxide grains varied significantly depending on the deposition current density. After a DC plasma oxidation process, the Sn, SnO, and SnO₂ phases were obtained as spherical grains when the previous electrodeposition was carried out at 6 A/dm² current density. The wire morphology was obtained only when the electrodeposition current was below 3 A/dm². The film produced at 1.5 A/dm² and then plasma oxidized showed wire morphology with single crystals of SnO₂ that formed in the (110) direction.     

Hydrothermal synthesis of SnO2/SnS2 nanocomposite with high visible light-driven photocatalytic activity

SnO₂/SnS₂ nanocomposite with a heterojunction structure (that is, SnO₂ nanoparticles-decorated SnS₂ nanoplates) was synthesized via the hydrothermal reaction between SnO₂ nanoparticles and thioacetamide in 5 vol.% acetic acid aqueous solution at 150 °C for 3 h, and characterized by X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy and UV–vis diffuse reflectance spectra. The photocatalytic activity of the hydrothermally synthesized SnO₂/SnS₂ nanocomposite was tested by degrading methyl orange in distilled water under visible light (λ > 420 nm) irradiation. It was found that the hydrothermally synthesized SnO₂/SnS₂ nanocomposite exhibited superior photocatalytic activity to SnO₂ nanoparticles, SnS₂ nanoplates and physically mixed SnO₂/SnS₂ nanocomposite. The heterojunction structure of the hydrothermally synthesized SnO₂/SnS₂ nanocomposite, which can facilitate interfacial electron transfer and reduce the self-agglomeration of two components, was considered to play an important role in achieving its higher photocatalytic activity.     

Structural and chemical characterization of SnO2-based nanoparticles as electrode material in Li-ion batteries

Improvement of long-term stability of electrode materials in Li-ion batteries requires a detailed understanding of influence of synthesis parameters on surface chemistry and on properties. Therefore, bare SnO₂ and core/shell nanoparticles with SnO₂ core and a hydrocarbon shell are synthesized in an Ar/20% O₂ microwave plasma, deposited as porous nanoparticle films in situ on heated Ni-substrates, and finally assembled as anodes in Swagelok cells. In a comprehensive study, we investigate structure, particle size, chemistry, morphology, and water content of the nanoparticles using X-ray diffraction, transmission electron microscopy, specific surface area analysis, and coulometric water titration. The thicknesses of the nanoparticle films and their surface chemistry are investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. SnO₂ nanoparticles are crystalline, with a tetragonal cassiterite structure. Primary particle sizes around 3 nm are reached for the bare SnO₂ particles, 5–8 nm for the cores of the core/shell nanoparticles. A minimum microwave power of 900 W is necessary to synthesize SnO₂ nanoparticles without precursor residuals as pristine SnO₂ particles for the subsequent coating step. In the coating step increasing hydrocarbon content can be correlated with increasing carbon-precursor feeding rate. Water uptake, stemming either from the process, or due to atmospheric contamination, can successfully be reduced by a thermal treatment. The still remaining water is a function of specific surface area. Finally, bare SnO₂ versus core/shell nanoparticles are compared regarding the influence of the shell on the electrochemical properties. The principal improved functionality of the developed anodes in Swagelok cells is demonstrated.     

Influence of antimony doping on structure and conductivity of tin oxide whiskers

Tin dioxide whiskers doped with different concentrations of antimony (0-0.25 at.%) have been grown from SnO and Sb₂O₃ mixture in a tube furnace in a flowing mixture of argon and oxygen at a constant source temperature. The whiskers possess high structural perfection. Influence of Sb on crystal structure, morphology and conductivity of SnO₂ whiskers is investigated. Antimony doping allows a decrease in the resistance of SnO₂ whiskers up to 10⁶ times.     

SnO<Subscript>2</Subscript>-based thin films with excellent photocatalytic performance

SnO₂ semiconductor is a new-typed promising photocatalyst, but wide application of SnO₂-based photocatalytic technology has been restricted by low visible light utilization efficiency and rapid recombination of photogenerated electrons–holes. To overcome these drawbacks, we prepared B/Fe codoped SnO₂–ZnO thin films on glass substrates through a simple sol–gel method. The photocatalytic activities of the films were evaluated by degradation of organic pollutants including acid naphthol red (ANR) and formaldehyde. UV–Vis absorption spectroscopy and photoluminescence (PL) spectra results revealed that the B/Fe codoped SnO₂–ZnO film not only enhanced optical absorption properties but also improved lifetime of the charge carriers. X-ray diffraction (XRD) results indicated that the nanocrystalline SnO₂ was a single crystal type of rutile. Field emission scanning electron microscopy (FE-SEM) results showed that the B/Fe codoped SnO2–ZnO film without cracks was composed of smaller nanoparticles or aggregates compared to pure SnO2 film. Brunauer–Emmett–Teller (BET) surface area results showed that the specific surface area of the B/Fe codoped SnO₂–ZnO was 85.2 m² g^?1, while that of the pure SnO₂ was 20.7 m² g^?1. Experimental results exhibited that the B/Fe codoped SnO₂–ZnO film had the best photocatalytic activity compared to a pure SnO₂ or singly-modified SnO₂ film.     

Synthesis of SnO2 nanoparticles inside mesoporous carbon via a sonochemical method for highly reversible lithium batteries

A sonochemical method was introduced to synthesize SnO₂ nanoparticles in the pores of mesoporous carbon without any other agents. The nitrogen adsorption measurement and transmission electron microscopy results revealed that the SnO₂ nanoparticles with the average particle size of around 10 nm were homogeneous distribution in the matrix. The aggregation of SnO₂ was hindered by the three-dimensioned porous frameworks, resulting in a relatively large surface area of 362 m² g^? 1, which is beneficial for lithium-ion storage in batteries. The resultant composites with 43% SnO₂ exhibited a high reversible capacity of 200 mAh g^? 1 even after 300 cycles, which is 186% higher than that of the initial mesoporous carbon matrix. This strategy is expected to incorporate other functional nanoparticles inside mesoporous carbon for many applications.     

Synthesis and characterisation of SnO2 nano-single crystals as anode materials for lithium-ion batteries

Rutile structure SnO₂ nano-single crystals have been synthesized using tin (IV) chloride as precursor by the modified hydrothermal method. Controllable morphology and size of SnO₂ could be obtained by adjusting the concentration of the hydrochloric acid. The SnO₂ nanoparticles were characterised by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and electrochemical methods. The SnO₂ nanoparticles as anode materials in lithium-ion batteries exhibit high lithium storage capacities. The reversible capacities are more than 630 mA h g^− 1.     

Controllable preparation of SnO2 nanoplates and nanoparticles via hydrothermal oxidation of SnS2 nanoplates

A facile hydrothermal oxidation route has been proposed for the controllable preparation of SnO₂ nanoplates and nanoparticles, using the home-made SnS₂ nanoplates as a precursor. It was found that the temperature played an important role in the microstructures of the obtained products. While nanoplates of tetragonal phase SnO₂ were synthesized via hydrothermal treatment of the SnS₂ nanoplates in 15 vol.% H₂O₂ aqueous solution at 180 °C for 24 h, only nanoparticles of tetragonal phase SnO₂ could be obtained via hydrothermal treatment of the SnS₂ nanoplates in 15 vol.% H₂O₂ aqueous solution at 200 °C for 24 h. The as-prepared products were characterized by means of X-ray diffraction, field emission scanning electron microscope, and Raman spectra, and their possible formation mechanisms were also discussed.     

Polymer-embedded stannic oxide nanoparticles as humidity sensors

Stannic oxide (SnO₂) nanoparticles have been suspended in polyvinyl alcohol (PVA) matrix in different PVA:SnO₂ molar ratios ranging from 1:1 to 1:5 using simple chemical route. This suspension was deposited on ceramic substrate and upon drying was carefully detached from the substrate. SnO₂-embedded self-standing, transparent and flexible thin films were hence synthesized. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques show the rutile tetragonal structure of SnO₂ with particle size ~ 5 nm. UV–Visible spectroscopy demonstrates the band gap of 3.9 eV, which does not alter when embedded in polymer. Fourier transform infrared spectroscopy (FTIR) reveals that the properties of SnO₂ do not modify due to incorporation in the PVA matrix. The structures work as excellent humidity sensors at room temperature. For a critical PVA:SnO₂ molar ratio of 1:3, the resistance changes to five times of magnitude in 92% humidity within fraction of second when compared with resistance at 11% humidity. The sample regains its original resistance almost instantaneously after being removed from humid chamber. Nanodimensions of SnO₂ particles and percolation mechanism related to transport through polymer matrix and water molecule as a carrier has been used to understand the mechanism.     

Impact of Pd nanoparticle loading method on SnO2 surface for natural gas detection in humid atmosphere

To improve the sensor response to low concentrations of methane (CH₄) at low operating temperatures in humid atmospheres, we prepared Pd-loaded SnO₂ (Pd-SnO₂) nanoparticles via two different Pd-loading processes: (i) a general impregnation method and (ii) a new loading method using poly(N-vinyl-2-pyrrolidone) (PVP) as a protective agent for Pd receptor particles. According to the measured electric resistances, the Pd particles limited the hydroxyl-poisoning of the SnO₂ particle surface. Because Pd is oxidized to PdO, a p–n junction is formed at the interface between PdO and SnO₂, and such interface gives the enlargement of the electron depletion layer. Therefore, Pd further improved the resistance against hydroxyl poisoning of the SnO₂ surface in humid air. In addition, although the sensor based on neat SnO₂ did not respond to low-concentration CH₄ at 200–400 °C, both the sensors based on the Pd-loaded SnO₂ samples exhibited high sensor response to 200 ppm CH₄ in a humid atmosphere. The Pd-SnO₂ obtained by the new loading method exhibited a higher response to CH₄ at lower concentrations in the lower operating temperature range (200–250 °C). This improvement in the sensor response is probably due to the catalytic activity of the larger Pd nanoparticles. According to high-resolution transmission electron microscopy–energy-dispersive X-ray spectroscopy images, the new loading method successfully provided Pd-loaded SnO₂ nanoparticles with Pd nanoparticles dispersed uniformly on the SnO₂ particle surface. The average particle size of Pd nanoparticles loaded on the surface of SnO₂ by the new loading method was slightly larger than that of the Pd nanoparticles loaded by the impregnation method. As the Pd particle size increases, it is thought that crystalline PdO particles are formed more easily, thereby improving the combustion activity of CH₄ under humid conditions. These results are of great significance for further decreasing the energy consumption of the CH₄ sensor and increasing its sensor response in humid atmospheres.

     

Ferromagnetic nanoparticles in Sn-O system

Tin oxide nanoparticles ranging in average size from 12 to 315 nm have been prepared by levitation-jet aerosol synthesis through condensation of tin vapor in a flow of inert gases and oxygen (air). The nanoparticles have been characterized by transmission electron microscopy, X-ray diffraction, BET measurements, vibrating-sample magnetometry, and Raman scattering spectroscopy. The results indicate that the nanoparticles may exhibit room-temperature ferromagnetism, with their magnetization having a maximum at O: Sn = 1. The ferromagnetic order is tentatively attributed to the presence of localized states on the Sn/SnO and SnO/SnO₂ interfaces.     

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.     

The effect of Pt nanoparticles loading on H2 sensing properties of flame-spray-made SnO2 sensing films

SnO₂ nanoparticles loaded with 0.2–2 wt% Pt have successfully been synthesized in a single step by flame spray pyrolysis (FSP) and investigated for gas sensing towards hydrogen (H₂). According to characterization results by X-ray diffraction, nitrogen adsorption, scanning/high resolution-transmission electron microscopy and analyses based on Hume-Rothery rules using atomic radii, crystal structure, electronegativities, and valency/oxidation states of Pt and Sn, it is conclusive that Pt is not solute in SnO₂ crystal but forms nanoparticles loaded on SnO₂ surface. H₂ gas sensing was studied at 200–10,000 ppm and 150–350 °C in dry air. It was found that H₂ response was enhanced by more than one order of magnitude with a small Pt loading concentration of 0.2 wt% but further increase of Pt loading amount resulted in deteriorated H₂-sensing performance. The optimal SnO₂ sensing film (0.2 wt% Pt-loaded SnO₂, 20 μm in thickness) showed an optimum H₂ response of ∼150.2 at 10,000 ppm and very short response time in a few seconds at a low optimal operating temperature of 200 °C. In addition, the response tended to increase linearly and the response times decreased drastically with increasing H₂ concentration. Moreover, the selectivity against carbon monoxide (CO) and acetylene (C₂H₂) gases was also found to be considerably improved with the small amount of Pt loading. The H₂ response dependence on Pt concentration can be explained based on the spillover mechanism, which is highly effective only when Pt catalyst is well-dispersed at the low Pt loading concentration of 0.2 wt%.