Nanocrystalline SnO2 and In2O3 as materials for gas sensors: The relationship between microstructure and oxygen chemisorption

The correlations between microstructure of nanocrystalline TCO SnO₂ and In₂O₃ and parameters of oxygen chemisorption are analyzed. Nanocrystalline SnO₂ and In₂O₃ were prepared by wet chemical method. The sample's microstructure was characterized by TEM, XRD and low-temperature nitrogen adsorption. Electrical properties of TCO were studied at 200-400 °C depending on the oxygen partial pressure. Increase of TCO grain size leads to the increase of the fraction of atomic forms of chemisorbed oxygen at the fixed temperature. It could be due to the decrease of surface barrier resulting in the decrease of activation energy of dissociation of molecular ion O_2(ads)⁻.     

A New Strategy for Humidity Independent Oxide Chemiresistors: Dynamic Self‐Refreshing of In2O3 Sensing Surface Assisted by Layer‐by‐Layer Coated CeO2 Nanoclusters

The humidity dependence of the gas sensing characteristics of metal oxide semiconductors has been one of the greatest obstacles for gas sensor applications during the last five decades because ambient humidity dynamically changes with the environmental conditions. Herein, a new and novel strategy is reported to eliminate the humidity dependence of the gas sensing characteristics of oxide chemiresistors via dynamic self‐refreshing of the sensing surface affected by water vapor chemisorption. The sensor resistance and gas response of pure In₂O₃ hollow spheres significantly change and deteriorate in humid atmospheres. In contrast, the humidity dependence becomes negligible when an optimal concentration of CeO₂ nanoclusters is uniformly loaded onto In₂O₃ hollow spheres via layer‐by‐layer (LBL) assembly. Moreover, In₂O₃ sensors LBL‐coated with CeO₂ nanoclusters show fast response/recovery, low detection limit (500 ppb), and high selectivity to acetone even in highly humid conditions (relative humidity 80%). The mechanism underlying the dynamic refreshing of the In₂O₃ sensing surfaces regardless of humidity variation is investigated in relation to the role of CeO₂ and the chemical interaction among CeO₂, In₂O₃, and water vapor. This strategy can be widely used to design high performance gas sensors including disease diagnosis via breath analysis and pollutant monitoring.     

Surface potentials of magnetron sputtered transparent conducting oxides

Work functions, ionization potentials (electron affinities) and Fermi level positions measured in-situ by photoelectron spectroscopy at surfaces of transparent conducting oxides are presented. Thin films of ZnO, ZnO:Al, SnO₂, SnO₂:Sb, In₂O₃, In₂O₃:Sn, and In₂O₃:(Zn,Sn) are prepared by magnetron sputtering. The Fermi level position is strongly affected by the oxygen content in the sputter gas. The ionization potential and work function of ZnO are strongly affected by surface orientation. In contrast, SnO₂-based and In₂O₃-based materials show pronounced changes of ionization potential and work function induced by surface oxidation and reduction. Unlike SnO₂, the oxidation of the In₂O₃-based TCO surfaces does not occur during deposition but can be induced by post-deposition treatments.     

Preparation and characterization of indium-doped tin dioxide nanocrystalline powders

The precursors of SnO₂ or In₂O₃/SnO₂ nanocrystalline powders have been prepared by the sol-precipitation method. The precursors are calcined at different temperatures to prepare SnO₂ or In₃O₃/SnO₂ nanocrystalline powders with different particle sizes. The reaction conditions, such as the concentration of reactants, the surfactant, pH value, the amount of indium oxide doped, and the reaction temperature and time, have been studied. The nanocrystallites are examined by differential thermal analysis (DTA)/thermogravimetric analysis (TGA), X-ray diffraction (XRD), IR spectroscopy and transmission electron microscopy (TEM).     

Cermet compact made from semiconducting InSb with constant electrical resistance in the 4–400 K range

A study of the electrophysical properties of samples prepared by phase transformation of stoichiometric InSb into lnSb-Sb-In₂O₃ cermet compact has been performed (InSb, In₂O₃-semiconductors, antimony-metallic conductivity). Samples were prepared by isothermal partial oxidation at 200–500°C for 1–50 h. Bulk and thin-film samples annealed at 400°C for 1–50 h possess relatively constant electrical resistance over the wide temperature interval measured: 4–400 K. The conversion degree, β, and molar ratio, f = In₂O₃/2Sb were calculated from the isothermal thermogravimetry data according to the reaction equation 2InSb+3/2O₂ = In₂O₃+2Sb at temperatures T < 400°C, when no ascertainable amount of antimony is escaping from the system. The β-value increases with temperature, T, and time of oxidation annealing, t. However, instead of being constant, i.e. f = 0.5, f increases for T > 400°C and t > 1 h. The X-ray powder diffraction, thermogravimetry, differential thermogravimetry and differential thermal analysis measurements and studies revealed that metallic antimony escapes partially from the InSb-Sb-In₂O₃ system obtained at T ≥ 400°C. As a result, the mutual volume ratio of individual InSb, Sb and In₂O₃ components is changed, and so also is the overall character of the electrical resistivity of the samples. Due to the partial escape of Sb↑ from the system at T ≥ 400°C, the following reaction is appropriate: 2InSb + 3/2O₂ = In₂O₃ + (2 − z) Sb↑ = In₂O₃+Sb/f+ zSb, where z is the volatilized portion of Sb and f is the molar ratio of the reaction products, i.e. f = In₂O₃/(2 − z)Sb = 1/(2 − z). The SEM observations revealed a growing grain size with temperature and time of annealing, lowering the grain-boundary density and thus also the resistivity of the samples. The properties of the obtained ternary compact may be influenced significantly, if instead of stoichiometric InSb, the initial In-Sb with a variable In/Sb ratio is used.     

H2S sensing properties of La-doped nanocrystalline In2O3

The nanocrystalline powders of pure and La³⁺-doped In₂O₃ with cubic structure were prepared by a simple hydrothermal decomposition route. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and microstructure by transmission electron microscopy (TEM). All the compositions exhibited a single phase, suggesting a formation of solid solution in the concentration of doping investigated. Gas-sensing properties of the sensor elements were tested by mixing a gas in air at static state, as a function of concentration of dopant, operating temperature and concentrations of the test gases. The pure In₂O₃ exhibited high response towards H₂S gas at an operating temperature 150 °C. Doping of In₂O₃ with La³⁺ increases its response towards H₂S and La³⁺ (5.0 wt.% La₂O₃)-doped In₂O₃ showed the maximum response at 125 °C. The selectivity of the sensor elements for H₂S against different reducing gases was studied. The results on response and recovery time were also discussed.     

Tuning the Surface Electron Accumulation Layer of In2O3 by Adsorption of Molecular Electron Donors and Acceptors

In₂O₃, an n-type semiconducting transparent transition metal oxide, possesses a surface electron accumulation layer (SEAL) resulting from downward surface band bending due to the presence of ubiquitous oxygen vacancies. Upon annealing In₂O₃ in ultrahigh vacuum or in the presence of oxygen, the SEAL can be enhanced or depleted, as governed by the resulting density of oxygen vacancies at the surface. In this work, an alternative route to tune the SEAL by adsorption of strong molecular electron donors (specifically here ruthenium pentamethylcyclopentadienyl mesitylene dimer, [RuCp*mes]₂) and acceptors (here 2,2′-(1,3,4,5,7,8-hexafluoro-2,6-naphthalene-diylidene)bis-propanedinitrile, F₆TCNNQ) is demonstrated. Starting from an electron-depleted In₂O₃ surface after annealing in oxygen, the deposition of [RuCp*mes]₂ restores the accumulation layer as a result of electron transfer from the donor molecules to In₂O₃, as evidenced by the observation of (partially) filled conduction sub-bands near the Fermi level via angle-resolved photoemission spectroscopy, indicating the formation of a 2D electron gas due to the SEAL. In contrast, when F₆TCNNQ is deposited on a surface annealed without oxygen, the electron accumulation layer vanishes and an upward band bending is generated at the In₂O₃ surface due to electron depletion by the acceptor molecules. Hence, further opportunities to expand the application of In₂O₃ in electronic devices are revealed.     

Structural stability of In<Subscript>2</Subscript>O<Subscript>3</Subscript> films as sensor materials

A structural stability of In₂O₃ films in gas sensors was studied in conditions of intensive exploitation of sensor device at elevated temperatures. Structural changes of In₂O₃ films as well as a surface electromigration of In atoms were observed. The degradation effects are caused by simultaneous influence on In₂O₃ films of elevated temperatures and conditions of the working device. It was found that a structural degradation of In₂O₃ films in a sensor device could be suppressed using thin substrates. Fabrication of sensors with uniform In₂O₃ films led to improvement of their operational parameters.     

Structural characterization of sputtered indium oxide films deposited at room temperature

Structural evolution of indium oxide thin films deposited at room temperature by reactive magnetron sputtering and annealing in a reducing atmosphere were investigated. The as deposited indium oxide (In₂O₃) films showed a dominating randomly oriented nanocrystalline structure of cubic In₂O₃. The grain size decreased with increasing oxygen concentration in the plasma. Annealing in reducing atmospheres (vacuum, nitrogen and argon), besides improving the crystallinity, led to a partial cubic to rhombohedral phase transition in the indium oxide films. Annealing improved the optical properties of the indium oxide film and shifted the absorption edge to higher energies.     

Crystal and electronic band structures of homologous compounds ZnkIn2Ok+3 by Rietveld analysis and first-principle calculation

Crystal structure of homologous compounds, Zn₃In₂O_6, Zn₄In₂O₇, Zn₅In₂O₈, Zn₇In₂O₁₀, and In₂O₃ were refined by X-ray Rietveld analysis. Band structures of the homologous compounds were evaluated by first-principle calculation (Cambridge Serial Total Energy Package, CASTEP), using the structural data obtained from the Rietveld analysis. According to the results of CASTEP calculations, a sharp cut-off at the Fermi level could be observed when In³⁺ preferentially occupies the tetrahedral site (Zn₃In₂O₆(4)) or the trigonal bipyramid site (Zn₃In₂O₆(5)) in the (InZn_k)O_k+1⁺ layers. The cut-off at the Fermi level could not be observed when In³⁺ and Zn²⁺ are totally disordered at these sites. Electronic structure calculation suggested that Zn₃In₂O₆(4) is a good conductor and that Zn₃In₂O₆(5) is a poor conductor. Results of geometry optimization indicate that the formation enthalpy of Zn₃In₂O₆(4) was lower than that of Zn₃In₂O₆(5). Considering the electronic structure and the formation enthalpy, Zn₃In₂O₆(4) in which In³⁺ in the (InZn_k)O_k+1⁺ layer occupies the tetrahedral site preferentially, is likely to be the favored structure.     

Preparation and Characterization of Nanoceramics for Solid Oxide Fuel Cells

Precursor powders in the ZrO₂–HfO₂–Y₂O₃–CeO₂, In₂O₃–ZrO₂, and NiO–Nd₂O₃ systems for components of solid oxide fuel cells have been prepared by liquid-phase synthesis. We have determined formation conditions and the particle size of ZrO_2- and In₂O_3-based solid solutions and neodymium nickelate (Nd₂NiO₄), demonstrated the feasibility of producing nanocrystalline powders (10–30 nm) of tailored chemical composition in the temperature range 500–900°C, and optimized powder consolidation conditions. Nanoceramics with a crystallite size from 60 to 90 nm have been obtained and their microstructure and phase composition have been investigated. We have studied the electrical properties of the ZrO_2- and In₂O_3-based solid solutions and the Nd₂NiO₄ compound and established the range of their electrical conductivity at temperatures from 300 to 1000°C: 2.27 × 10^–3 to 2.51 S/cm for the ZrO₂-based solid solution, 8.91 × 10¹ to 6.59 × 10³ S/cm for the In₂O₃-based solid solution, and 3.98 × 10² to 5.02 × 10² S/cm for Nd₂NiO₄.     

Room temperature interactions in Ni/metal thin film couples

Interactions in Ni/metal thin film couples at room temperature were studied. It was found that nickel reacts with gallium, indium and tin to form NiGa₄, Ni₂In₃, Ni₁₀In₂₇ and NiSn, but no reaction was observed with bismuth, cadmium, germanium, magnesium, manganese, antimony, samarium and zinc under the conditions employed.     

Solid-state spreading and crystallization of In2O3: Structure and morphology of films on single-crystal A12O3 and yttria-stabilized zirconia substrates

Spontaneous solid-state spreading of In₂O₃ over the surface of single-crystal A1₂O₃ and Y₂O₃-stabilized ZrO₂ (YSZ) substrates was studied by a variety of techniques. In both systems, the resultant films consisted of cubic In₂O₃. The films grown on YSZ were found to dissolve significant amounts of Zr(IV). No new phases were detected at the In₂O₃/Al₂O₃ interface. The surface morphology of the films was examined by scanning electron and atomic force microscopy techniques. The films grown on the YSZ and A1₂O₃ substrates were found to be essentially identical in surface morphology: both types of films have a dense microstructure and a smooth surface. The effect of the substrate material on the qualitative and quantitative aspects of solid-state spreading is examined in relation to the thermodynamics and kinetics of the process     

The reaction mechanism of the spray Ion Layer Gas Reaction process to deposit In2S3 thin films

The spray Ion Layer Gas Reaction (ILGAR) is a well-established, patented and commercial process used primarily to deposit In₂S₃ as buffer layers in thin film solar cells. In this paper we investigate the growth mechanism of the spray In₂S₃ ILGAR process by characterising the intermediate growth stages of films, following the growth mechanism with a quartz crystal microbalance and tracking the gaseous side-and-intermediate products during film growth, using a mass spectrometer. A basic growth mechanism model is then proposed based on an aerosol assisted chemical vapour deposition of an In(O_x,Cl_y,(OH)_z) film, as the first stage process, followed by the conversion of the intermediate film using H₂S gas to In₂S₃.     

Chemical and structural properties of the system Fe2O3-In2O3

The system Fe₂O₃-In₂O₃ was studied using X-ray diffraction,⁵⁷Fe Mössbauer spectroscopy and infrared spectroscopy. The samples were prepared by chemical coprecipitation and thermal treatment of the hydroxide coprecipitates. For samples heated at 600 °C, a phase, α- (Fe_1?x In _x )₂O₃, isostructural with α-Fe₂O₃, exists for 0?x?0.8, and a phase C-(Fe_1?x In _x )₂O₃, isostructural with cubic In₂O₃, exists for 0.3?x?/1. In the two-phase region these two phases are poorly crystallized. An amorphous phase is also observed for 0.3?x?0.7. For samples heated at 900 °C the two-phase region is wider and exists for 0.1?x?0.8 with the two phases well crystallized. In these samples an amorphous phase is not observed.⁵⁷Fe Mössbauer spectroscopy of samples prepared at 600 °C indicated a general tendency of the broadening of spectral lines and the decrease of numerical values of the hyperfine magnetic field (HMF) with increasing molar fraction In₂O₃ in the system Fe₂O₃-In₂O₃. The samples prepared at 900 °C, in the two-phase region, are characterized by a constant HMF value of 510 kOe at room temperature. Infrared spectroscopy was also used to follow the changes in the infrared spectra of the system Fe₂O₃-In₂O₃ with gradual increase of molar fraction of In₂O₃. A correlation between X-ray diffraction, Mössbauer spectroscopic and infrared spectroscopic results was obtained.     

Compounds in the system BaO  In2O3

The occurrence of compounds in the system BaOIn₂O₃ has been investigated. In addition to the already known compounds BaIn₂O₄ and Ba₄In₆O₁₃, three new compounds were found: Ba₂In₂O₅, Ba₃In₂O₆ and Ba₅In₂O₈. Ba₂In₂O₅ has a simple, perovskite-like X-ray diagram, which however, could not be indexed. Ba₃In₂O₆ and Ba₅In₂O₈ have tetragonal body-centered unit cells with $\text{a}\text{= 4.1913}\text{A}\text{?}\text{,}\text{c}\text{= 21.6693}\text{A}\text{?}\text{and a}\text{= 4.1742}\text{A}\text{?}\text{,}\text{c}\text{= 29.431}\text{A}\text{?}$ respectively. Ba₃In₂O₆ reacts with most air to form a hydrogarnet: Ba₃In₂(OH)₁₂.     

Zirconia-based nanocrystals in the ZrO2-In2O3 system

Zirconia-based ZrO₂-In₂O₃ nanocrystals are prepared by hydrothermal treatment of coprecipitated zirconium oxyhydroxide and indium hydroxide. Indium oxide is shown to dissolve predominantly in cubic zirconia nanocrystals. Its solubility in nanocrystalline zirconia notably exceeds the equilibrium In₂O₃ solubility in ZrO₂ single crystals.     

Growth and structural characterization of InN/In2O3 coaxial nanocables

A synthesis route to coaxial nanocables has been developed using low dissociation temperature of InN and high melting point of In₂O₃. InN/In₂O₃ coaxial nanocables were successfully synthesized by a reaction of the mixture of In and In₂O₃ powder with a constant flowing ammonia atmosphere through two-stage. High-resolution transmission electron microscopy observations demonstrated that the nanocable consists of wurtzite InN nanowire core, In₂O₃ outer shell separated in the radial direction. The InN/In₂O₃ coaxial nanocables were 20–80 nm in diameter and up to several tens of micrometers in length. It is proposed that the low dissociation temperature property of InN material acts as an important role to form In₂O₃ protecting layers which coated the surface of InN nanowires and high melting point of In₂O₃ prevent InN to further dissociate. The method results in high purity, and high reproducibility of the InN/In₂O₃ nanocable-structures.     

Synthesis, characterization, and gas-sensing property for HCHO of Ag-doped In2O3 nanocrystalline powders

Pure and Ag-doped In₂O₃ nanocrystalline powders with different doping concentrations have been prepared by a sol–gel method, and characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectrum (XPS). The results indicated that these powders had a good crystalline structure with an average crystallite size of 12 nm. The indirect heating structure sensors were fabricated by loading these powders on ceramic tubes with Au electrodes. Gas-sensing measurements indicated that the sensor fabricated from 8 wt.% Ag-doped In₂O₃ could detect HCHO gas down to 2 ppm with a short response time (10–15 s) and an excellent selectivity at 100 °C.     

Ultra-sensitive room-temperature H<Subscript>2</Subscript>S sensor using Ag–In<Subscript>2</Subscript>O<Subscript>3</Subscript> nanorod composites

Ultra-sensitive H₂S sensors operated at room temperature were fabricated using Ag–In₂O₃ nanorod composites synthesized using sol–hydrothermal method followed by NaBH₄ reduction process. TEM proved that the In₂O₃ was nanorod structures of?~?110 nm in length and?~?35 nm in diameter. Ag nanoparticles with diameters from 10 to 15 nm homogeneously decorated on the surfaces of the In₂O₃ naonorods. XRD and XPS analysis proved that the Ag elements existed as zero-valent metallic silver on the surface of the In₂O₃ nanorods. Ag nanoparticles could enhance the formation of chemisorbed oxygen species and interactions between H₂S molecules and oxygen species due to spillover effect, and the electron transfer between Ag and In₂O₃ nanorods also enhanced the sensing properties. Therefore, the H₂S sensors based on the Ag–In₂O₃ nanorod composites showed significantly improved sensing performance than those based on the pure In₂O₃ nanorods. The optimized content of Ag nanoparticles is 13.6 wt%. Operated at room temperature, the H₂S sensors made of 13.6 wt% Ag–In₂O₃ nanorod composites exhibited an ultra-high response of 93719 to 20 ppm H₂S and a superior detection limit of 0.005 ppm. The sensor also showed good reversibility, good selectivity, excellent reproducibility and stability for detection of H₂S gas.