Temperature-controlled fabrication of SnO<Subscript>2</Subscript> nanoparticles via thermal heating of Sn powders

We report a method by which we have produced nano-sized crystalline tin oxide (SnO₂) particles with a rutile structure. We have employed thermal evaporation of solid Sn powders in ambient air. Samples were characterized by scanning electron microscopy, X-ray powder diffraction, transmission electron microscopy, and photoluminescence (PL) spectroscopy. The size of SnO₂ particles in an agglomerated state was found to decrease on decreasing the synthesis temperature in the range of 700–850 °C. The product synthesized at a low temperature of 700 °C was comprised of a trace amount of tetragonal SnO phase. Photoluminescence spectra showed visible light emission, with its overall intensity being increased on increasing the synthesis temperature. PACS 81.07.Wx; 81.05.Hd; 61.10.Nz; 68.37.Hk; 68.37.Lp     

锡薄膜等温氧化研究

采用电子束蒸发制备金属锡薄膜,将其在250—400℃温度范围内进行等温氧化,研究锡薄膜的热氧化动力学机制.采用台阶仪、扫描电子显微镜、俄歇电子能谱仪和X射线衍射仪等方法研究锡薄膜氧化过程中厚度、组分、结构等演变.实验结果表明,在250—400℃温度范围内,锡膜氧化后氧化层按抛物线规律生长;转变活化能为0.34eV;锡膜氧化受到氧扩散机制的控制.研究得到氧化层的生长首先从形成SnO相开始,随着氧化的深入,SnO相分解形成Sn₃O₄相,最后转变为SnO_{2< 关键词：}     

XANES and XPS studies of processes initiated by high-vacuum annealing in SnO x /MWCNT composite layers

The initial and thermally treated composites based on SnO _x /MWCNT have been studied by the XANES and XPS methods using the equipment of the BESSY II Russian-German beamline of synchrotron radiation and by the AES and HRTEM methods. The characteristic mechanisms of chemical and structural transformations in the SnO _x phase have been determined depending on the vacuum annealing temperature. It has been found that the basic process in the metal-oxide component at annealing temperatures not exceeding 500°C is the tin monoxide SnO disproportionation reaction with the formation of the dioxide SnO₂ phase and metallic tin. An increase in the annealing temperature to 800°C results in the activation of carbothermal reduction of metallic tin in contact areas of oxide clusters and MWCNT, as well as in the formation of nanocrystalline structures in the metal-oxide component of composite.     

Synthesis of tin oxides SnO2–x in the entire composition range (x = 0 to 1) by ion‐beam sputter‐deposition

Ion‐beam sputter‐deposition (IBSD) was used to reactively deposit tin oxide crystalline films at oxygen fluxes of 3–15 sccm and at substrate temperatures of 100–600 °C. Analysing the samples by X‐ray diffraction and Raman spectro‐ scopy yields a map of the crystalline structures in dependence on the growth parameters. In addition to SnO₂, pure SnO films of high quality and an intermediate phase such as Sn₂O₃ or Sn₃O₄ can be reproducibly obtained. Thus, IBSD is, to our knowledge, the only thin‐film deposition technique verified yet to reliably produce samples in the entire composition range of tin oxides. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

A novel hydrothermal route to synthesize solid SnO2 nanospheres and their photoluminescence property

Solid tin oxide (SnO₂) nanospheres were fabricated via one-step hydrothermal treatment of tin foil in aqueous alkaline solutions at 180°C. Based on the time-dependent experiments, the evolution behavior of Ostwald ripening is proposed to explain the formation mechanism. Besides a broad emission band at 600 nm, the SnO₂ nanospheres, synthesized for 24 h, show an additional emission band at 490 nm that has a great sensitivity to the content of oxygen. Spectral examinations and analyses reveal that the two emission bands originate from the electronic states determined by the oxygen interstitials, intrinsic surface states, and oxygen vacancies.     

Study of the structural phase transformation,and optical behavior of the as synthesized ZnO–SnO2–TiO2 nanocomposite

Bulk nanocomposites ZnO–SnO₂–TiO₂ were synthesized by solid-state reaction method. The X-ray diffraction patterns and Raman spectra of bulk nanocomposite as a function of sintering temperature (700 °C–1300 °C) indicate that the structural phases of SnO₂ and TiO₂ depend on the sintering temperature while the ZnO retains its hexagonal wurtzite phase at all sintering temperatures and SnO₂ started to transform into SnO at 900 °C and completely converted into SnO at 1100 °C, whereas the titanium dioxide (TiO₂) exhibits its most stable phase such as rutile at low sintering temperature (≤900°C) and it transforms partially into brookite phase at high sintering temperature (≥ 900 °C). The optical band gap of nanocomposite ZnO–SnO₂–TiO₂ sintered at 700 °C, 900 °C, 1100 °C and 1300 °C for 16 hours is calculated using the transformed diffuse reflectance ultra violet visible near infra red (UV–VisNIR) spectra and has been found to be 3.28, 3.29, 3.31 and 3.32 eV, respectively.     

Selective fabrication of n‐ and p‐type SnO films without doping

Undoped n‐ and p‐type tin monoxide (SnO) films have been selectively fabricated by pulsed laser deposition with a Sn target and careful control of oxygen partial pressure. The films are epitaxially grown in optimal growth conditions on yttria‐stabilized zirconia (YSZ) substrates with out‐of‐plane and in‐plane orientation relationships of (001)_SnO//(001)_YSZ and [110]_SnO//[100]_YSZ, respectively. Both Seebeck and Hall measurements show consistent results on the carrier types of the films. The electron Hall mobility is approximately 11 cm²/Vs at room temperature and the carrier activation energy is 0.14 eV for the n‐type film. The growth at increased oxygen partial pressure yields p‐type films, demonstrating the selective fabrication of both n‐ and p‐type SnO films without doping. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Temperature dependent structural and optical properties of tin oxide nanoparticles

Nanocrystalline tin oxide (SnO₂) powders were synthesized through wet chemical route using tin metal as precursor. The morphology and optical properties, as well as the effect of sintering on the structural attributes of SnO₂ particles were analyzed using Transmission electron microscopy (TEM), UV–visible spectrophotometry (UV–vis) and X-ray diffraction (XRD), respectively. The data revealed that the lattice strain plays a significant role in determining the structural properties of sintered nanoparticles. The particle size was found to be 5.8 nm, 19.1 nm and 21.7 nm for samples sintered at 300 °C, 500 °C, and 700 °C, respectively. Also, the band gaps were substantially reduced from 4.1 eV to 3.8 eV with increasing sintering temperatures. The results elucidated that the structural and optical properties of the SnO₂ nanoparticles can be easily modulated by altering sintering temperature during de novo synthesis.     

Tin oxide nanocrystals embedded in nanopore arrays on stainless steel surface for photocatalytic applications

The immobilization of SnO₂ nanocrystals on solid substrates for practical photocatalytic applications suffers from poor adhesion that will lead to loss of photocatalytic activity and short service life. An efficient hydrothermal synthesis of SnO₂ nanocrystals embedded in nanopore arrays on stainless steel surface was presented in this paper. The morphology, chemical composition and microstructure of the embedded tin oxide nanocrystals were investigated by X-ray diffraction, field-emission scanning electron microscope, X-ray photoelectron spectroscopy and UV-visible diffuse reflectance spectroscopy. The photocatalytic activity and stability of SnO₂ nanocrystals was evaluated by photodegradation of methylene blue. SnO₂ nanocrystals embedded in nanopore arrays on stainless steel surface existed in a tetragonal rutile structure. The increasing of the hydrothermal temperature will lead to the improvement in photocatalytic activity of SnO₂ nanocrystals. The SnO₂ nanocrystals prepared at 220 °C performed the highest photocatalytic activity and good photocatalytic stability, indicating the effective immobilization of SnO₂ nanocrystals on anodized stainless steel.     

The reactive adsorption of CCl4 on GaAs(100) surfaces at 300 K

Auger electron spectroscopy has been used to monitor the adsorption of CCl₄ on an As-rich GaAs(100) surface at 300 K. Intensities of the Ga (55 eV), As (34 eV), C (270 eV) and Cl (181 eV) transitions have been used to estimate surface number densities at saturation and relative C : Cl stoichiometry of the surface species. Number densities of (4.3 ± 0.2) × 10¹⁴ and (2.0 ± 0.2) × 10¹⁴ cm ⁻² are obtained for carbon and chlorine respectively, suggesting that coverage saturates near one theoretical monolayer and that the C : Cl stoichiometry is approximately 2:1. These data are discussed in terms of a reactive adsorption mechanism.     

Vacancy-like defects in nanocrystalline SnO2: influence of the annealing treatment under different atmospheres

The study of electronic and chemical properties of semiconductor oxides is motivated by their several applications. In particular, tin oxide is widely used as a solid state gas sensor material. In this regard, the defect structure has been proposed to be crucial in determining the resulting film conductivity and then its sensitivity. Here, the characteristics of vacancy-like defects in nanocrystalline commercial high-purity tin oxide powders and the influence of the annealing treatment under different atmospheres are presented. Specifically, SnO₂ nanopowders were annealed at 330 °C under three different types of atmospheres: inert (vacuum), oxidative (oxygen) and reductive (hydrogen). The obtained experimental results are discussed in terms of the vacancy-like defects detected, shedding light to the basic conduction mechanisms, which are responsible for gas detection.     

Structural,photoconductive, thermoelectric and activation energy measurements of V-doped transparent conductive SnO<Subscript>2</Subscript> films fabricated by spray pyrolysis technique

This report investigated the structural, optical and electrical properties of V-doped SnO₂ thin films deposited using spray pyrolysis technique. The SnO₂:V films, with different V-content, were deposited on glass substrates at a substrate temperature of 550°C using an aqueous ethanol solution consisting of tin and vanadium chloride. X-ray diffraction studies showed that the SnO₂:V films were polycrystalline only with tin oxide phases and the preferred orientations are along (1 1 0), (1 0 1), (2 1 1) and (3 0 1) planes. Using Scherrer formula, the grain sizes were estimated to be within the range of 25–36 nm. The variation in sheet resistance and optical direct band gap are functions of vanadium doping concentration. Field emission scanning electron microscopy (FESEM) revealed the surface morphology to be very smooth, yet grainy in nature. Optical transmittance spectra of the films showed high transparency of about ～69–90% in the visible region, decreasing with increase in V-doping. The direct band gap for undoped SnO₂ films was found to be 3.53 eV, while for higher V-doped films it shifted toward lower energies in the range of 3.27–3.53 eV and then increased again to 3.5 eV. The Hall effect and Seebeck studies revealed that the films exhibit n-type conductivity. The thermal activation energy, Seebeck coefficient and maximum of photosensitivity in the films were found to be in the range of 0.02–0.82 eV (in the low-temperature range), 0.15–0.18 mV K^?1 (at T = 350 K) and 0.96–2.84, respectively.     

Cems characterization of SnO2 films obtained by sputtering and sol-gel route

Conversion electron Mössbauer spectroscopy (CEMS) has been used to study tin oxide films prepared by sol-gel dipping and sputtering. The spectra of films prepared by sol-gel route result close to that of crystalline SnO₂ after heat treatment at a temperature as low as 150°C. The Mössbauer parameters of as sputter deposited films indicate that the structure of the deposited stannic oxide has an amorphous character more pronounced for thinner samples. The structure becomes predominantly that of crystalline SnO₂ by heating at 550°C for 30 min provided the film thickness is higher than 10 nm.     

Synthesis and characterization of tin oxide (SnO2) nanocrystalline powders by a simple modified sol–gel route

This paper reports the synthesis and characterization of nanocrystalline tin oxide (SnO₂) powders by a simple method using a chitosan–polymer complex solution. To obtain SnO₂ nanocrystalline powders, the precursor was calcined at 500–600 °C in air for 2 h. The phase composition of calcined samples was studied by X-ray diffraction (XRD). The XRD results confirmed the formation of a SnO₂ phase with tetragonal structure. The particle sizes of the powder were found to be 22–23 nm as evaluated by the XRD line broadening method. TEM investigation revealed that the SnO₂ samples consist of crystalline particles of 19–21 nm. The corresponding selected area electron diffraction analysis further confirmed the formation of the tetragonal structure of SnO₂ without any impurity phases. The optical properties of the samples were explored by Fourier transform infrared spectroscopy, optical absorption and Raman studies. The estimated band gaps of the samples were in the range of 3.44–3.73 eV.     

AES and XPS studies of a por-Si/SnOx nanocomposite formed using a powerful ion beam of nanosecond duration

The results of the X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM) investigations of tin-oxide nanolayers on samples of por-Si/SnO _x composites with varying matrix porosity, formed using a powerful ion beam of nanosecond duration, are presented. It is shown that rapid melting and crystallization of the surface leads to the formation of Si nanoparticles with a maximal size of 200 nm. It is established that tin is included in the structure of the nanocomposite in an oxidized state with a small inclusion of metallic β tin. With increasing porosity, the phase composition of the tin nanolayers becomes closer to the state corresponding to the highest tin oxide (SnO₂). It is also shown that, upon an increase in the porosity, the intensity of the tin 4d subvalent line increases, which is, apparently, associated with an enhanced degree of hybridization of tin and oxygen atoms. The changes in the elemental composition of the composite and the depth of tin penetration are estimated from the results of ion etching.     

Studies on pyrolytically sprayed SnO2 and Sb-SnO2 thin films for LPG sensor applications

Undoped and antimony doped tin oxide thin films of different thicknesses were prepared on mineral glass substrate by spray pyrolysis method via sol-gel route. Both the films show good transmittance in the visible region. Band gap energy of both films lies between 3 to 3.5 eV. X-ray diffraction studies of undoped and antimony doped tin oxide thin films for various annealing temperature show polycrystalline tetragonal structure of SnO₂ with preferred orientation of (110) and (101), respectively and from the XRD data, grain size were also evaluated. AFM images of Undoped and antimony doped tin oxide thin films annealed at 375 °C depict the film thickness and indicate uniform surface pattern without dark pits and with strains of some ups exceeding the specified limit. The prepared films were tested in a specially developed test rig for Liquefied Petroleum Gas detection at different operating temperatures. The response characteristics of the films for LPG detection show maximum sensitivity and minimum response time at the operating temperature 400 °C. Studies indicate that antimony doped tin oxide thin film are one among the suitable candidates for LPG detection with a detection sensitivity and response time (t90) of 11 and 140 seconds, respectively. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Solid state effects in the Auger spectrum of zinc and oxidised zinc

Measurements of the zinc L_2,3M_4,5M_4,5 Auger spectra are reported. The line shapes in solid zinc are similar to those in zinc vapour but the Auger energies have increased by about 15 eV and the line breadths have broadened from 0.5 eV to 1.0 eV fwhm. The ratio of the L₂:L₃ groups differ from the vapour suggesting that L₂L₃M_4,5 Coster-Kronig transitions occur in the solid but not in the vapour. Changes in the spectra with oxidation have been observed. The Auger lines broaden on oxidation and a line breadth of 3.2 eV fwhm gives the best fit to the spectrum of almost fully oxidised zinc. The oxide L₃M_4,5M_4,5¹G₄ peak progressively shifts from 2.6 eV to 4.2 eV below the metal peak as the oxide thickness increases, the latter value being close to the measured shift in crystalline zinc oxide. Similar energy variation is reported for solid Argon condensed onto clean silver and the shifts are explained in terms of variation in “extra electron relaxation” with film thickness.     

Optical properties of SnO2 nanoparticles

In recent times, considerable research efforts have been focused on the exploration of novel optical properties of nanocrystalline SnO₂ particles such as photoluminescence (PL). In the present work, spherical SnO₂ nanoparticles are synthesized by solid state reaction at different temperatures using tin chloride dihydrate and sodium hydroxide flakes as precursors. Transmission electron micrograph shows uniform and spherical SnO₂ nanoparticles of diameter 30–40 nm. Micro-Raman spectra show an inverse relationship of line-width with SnO₂ particle size. The band gap of SnO₂ is calculated by plotting (αhν)² vs. hν and extrapolating the linear portion of it to α = 0 and found it to be 3.76–4.24 eV. Photoluminescence intensity increases with the increase of solid state reaction temperature up to 873 K. This is attributed to the generation of the oxygen ion vacancies in SnO₂.     

Characterization of clean and oxidized (100)LaB6

We have studied clean and oxidized (100)LaB₆ grown from aluminum melts by making Auger, LEED, evaporation and work function measurements on well-defined surfaces. The clean surface shows no La enrichment when initially heated as high as 1700°C. Its LEED pattern is 1 × 1, indicating no surface reconstruction. Langmuir evaporation studies up to temperatures of 1700°C show only La and B evaporating non-congruently, and LaO. The activation energy for B evaporation from LaB₆ (and from CeB₆ and EuB₆ also) is abot 5.5 eV, very close to that from elemental B. The rare-earth activation energies, however, vary, being highest for the rare-earth whose pure metal vapor pressure is lowest. Oxidation was carried out at room temperature using O₂ pressures up to 10^?7 Torr and at 1000°C using O₂ pressures up to 10^?4 Torr. At room temperature oxygen adsorption proceeeds to a saturated value indicated from LEED behavior to be about one monolayer. It produces a monotonic work function increase, which also saturates (at 1.40 V), varying linearly with the oxygen uptake. Oxidation at 1000°C is much more extensive than at room temperature, involving at least several monolayers, and results in a work function increase of 2.42 V. Results are discussed in terms of a terminal plane composed of La atoms, and adsorbed oxygen which, when given sufficient mobility, prefers bonding to La atoms at sites atop the B octahedra.     

Auger spectroscopic study of the surface composition of Pt/Sn alloys in ultrahigh vacuum and in the presence of oxygen and hydrogen

The surface composition of two Pt/Sn alloys, viz. PtSn and Pt₃Sn, has been followed by means of AES, as a function of annealing in ultrahigh vacuum, oxygen chemisorption and reduction with hydrogen.The results, which were quantitatively interpreted with the aid of a novel calibration technique, reveal the following features: - The surface of PtSn and Pt₃Sn becomes enriched with tin by annealing in vacuum. Ultimate values of 68±5 at% Sn for PtSn and 41±5 at% Sn for Pt₃Sn were attained after annealing at 500°C. - The adsorption of oxygen on the annealed surface of PtSn and Pt₃Sn causes a further enrichment with tin, while severe oxidation of PtSn at 500°C leads to complete disappearance of Pt from the surface. - Oxygen is more strongly and differently bound on a surface containing about 40 at% Sn than on a surface containing about 70 at% Sn. Activated adsorption of oxygen takes place only on the latter. The results suggest the formation of SnO₂ surface complexes on the exposed surface of Pt₃Sn. - Reduction of the alloys at 500°C carries the excess of tin into the bulk and reduces its surface concentration to 35±5 at% for Pt₃Sn and 64±5 at% for PtSn, which is an enrichment of the surface with platinum relative to the annealed state.