Nanostructured In2O3 Research Articles

Gas Sensing with Nanoporous In2O3 under Cyclic Optical Activation: Machine Learning-Aided Classification of H2 and H2O

Clean hydrogen is a key aspect of carbon neutrality, necessitating robust methods for monitoring hydrogen concentration in critical infrastructures like pipelines or power plants. While semiconducting metal oxides such as In2O3 can monitor gas concentrations down to the ppm range, they often exhibit cross-sensitivity to other gases like H2O. In this study, we investigated whether cyclic optical illumination of a gas-sensitive In2O3 layer creates identifiable changes in a gas sensor’s electronic resistance that can be linked to H2 and H2O concentrations via machine learning. We exposed nanostructured In2O3 with a large surface area of 95 m2 g−1 to H2 concentrations (0–800 ppm) and relative humidity (0–70%) under cyclic activation utilizing blue light. The sensors were tested for 20 classes of gas combinations. A support vector machine achieved classification rates up to 92.0%, with reliable reproducibility (88.2 ± 2.7%) across five individual sensors using 10-fold cross-validation. Our findings suggest that cyclic optical activation can be used as a tool to classify H2 and H2O concentrations.

Synthesis of nanostructured In2O3 ceramics via a green and chemical method for the mineralization of crystal violet dye: A comparative study

Herein, the photocatalytic capabilities of indium oxide nanoparticles obtained by Aerva javanica extract synthesis and surfactant-mediated co-precipitation synthesis were studied using crystal violet (CV) as a model dye. Indium oxide nanoparticles synthesized via green (In2O3-G) and chemical route (In2O3-C) were characterized through different spectroscopic, structural, electrical and morphological analyses. The plant extract’s metabolites (flavonoids) effectively reduce the indium metal and induce oxygen vacancies, which develop sub-energy levels below the conduction band and hence increase the current conductivity and reduce the optical band of the photocatalyst. The above-mentioned structural modifications improve the In2O3-G photocatalyst's light-harvesting capacity and charge transport characteristics. Optical and current–voltage measurements show that the In2O3-G photocatalyst has more outstanding electrical conductivity (σ = 1.76 × 10-3 Sm−1) and bandgap (Eg = 2.93 eV) more compatible with the visible light than its counterpart (In2O3-C). Due to their amine, carboxyl, and hydroxyl functionalities, the plant extract's metabolites controlled and facilitated the synthesis of In2O3-G in the nanosized range. The integration of some crucial features such as higher surface area, faster charger transport properties, and a visible light-compatible band gap enable our In2O3-G photocatalyst to mineralize 93.75% CV dye in just 80 min with 0.027 min−1 rate-constant. This research demonstrates that a green approach to producing ceramic photocatalysts is preferable to chemical methods, which are more costly and create additional environmental issues.

Screen printed Zn-doped nanostructured In2O3 thick films, characterizations, and enhanced NO2 gas sensing at low temperature

The pristine and various mole% (1, 3, 5, 7, and 9) Zn-doped In2O3 nanocrystalline materials have been successfully prepared via the facile sol-gel and screen-printing thick film methods. The crystalline and morphological information for the samples were investigated using various techniques such as XRD, FESEM, HR-TEM, EDAX analysis, and AFM. All the fabricated sensor devices were employed to investigate sensing properties. The resistivity and activation energy properties of all samples were investigated. The 7 % mole Zn doped In2O3 sensor showed superior gas sensing properties and showed excellent selectivity towards NO2 gas than other doped and pure In2O3 sensors. The sensors were tested at different operating temperatures for NO2 gas. The highest response of 117 was revealed for 100 ppm NO2 gas at 50 °C temperature. The optimal sensor device was tested for different NO2 gas concentrations (50–500 ppm) and long-term stability (90 days). The plausible sensing mechanism was briefed. The Zn doped In2O3 materials could be a potential candidate for highly selective, low-temperature commercial NO2 sensor device production.

Unraveling the Surface Hydroxyl Network on In2O3 Nanoparticles with High-Field Ultrafast Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy.

Hydroxyl groups are among the major active surface sites over metal oxides. However, their spectroscopic characterizations have been challenging due to limited resolutions, especially on hydroxyl-rich surfaces where strong hydroxyl networks are present. Here, using nanostructured In2O3 as an example, we show significantly enhanced discrimination of the surface hydroxyl groups, owing to the high-resolution 1H NMR spectra performed at a high magnetic field (18.8 T) and a fast magic angle spinning (MAS) of up to 60 kHz. A total of nine kinds of hydroxyl groups were distinguished and their assignments (μ1, μ2, and μ3) were further identified with the assistance of 17O NMR. The spatial distribution of these hydroxyl groups was further explored via two-dimensional (2D) 1H-1H homonuclear correlation experiments with which the complex surface hydroxyl network was unraveled at the atomic level. Moreover, the quantitative analysis of these hydroxyl groups with such high resolution enables further investigations into the physicochemical property and catalytic performance characterizations (in CO2 reduction) of these hydroxyl groups. This work provides insightful understanding on the surface structure/property of the In2O3 nanoparticles and, importantly, may prompt general applications of high-field ultrafast MAS NMR techniques in the study of hydroxyl-rich surfaces on other metal oxide materials.

Bis-Cyclometalated Iridium Complexes Containing 4,4'-Bis(phosphonomethyl)-2,2'-bipyridine Ligands: Photophysics, Electrochemistry, and High-Voltage Dye-Sensitized Solar Cells.

In this report, the synthesis and characterization of two bis-cyclometalated iridium(III) complexes are presented. Single-crystal X-ray diffraction shows that [Ir(ppy)2(4,4'-bis(diethylphosphonomethyl)-2,2'-bipyridine)]PF6 adopts a pseudooctahedral geometry. The complexes have an absorption feature in the near-visible-UV region and emit green light with excited-state lifetimes in hundreds of nanoseconds. The redox properties of these complexes show reversible behavior for both oxidative and reductive events. [Ir(ppy)2(4,4'-bis(phosphonomethyl)-2,2'-bipyridine)]PF6 readily binds to metal oxide supports, like nanostructured SnIV-doped In2O3 and TiO2, while still retaining reversible redox chemistry. When incorporated as the photoanode in dye-sensitized solar cells, the devices exhibit open-circuit voltages of >1 V, which is a testament to their strength of these iridium(III) complexes as photochemical oxidants.

Interaction Behaviour of Nanostructured In2O3 Thin Film Towards Nitric Oxide in Argon

“In-situ changes in the carrier concentrations of In2O3 thin film were measured as a function of temperature in argon and argon containing 25 ppm of NO using high temperature Hall measurement facility. Studies show that the charge carrier concentration of In2O3 in argon at 373 K is 5.4 × 1017 cm−3 which gets reduced to 1.5 × 1015 cm−3 in oxygen due to the strong electron withdrawing character of the adsorbed oxygen. 25 ppm of NO in argon drastically lowers the carrier concentration of In2O3 to 4.8 × 1016 cm−3 at 573 K from 6.1 × 1017 due to its higher electron withdrawing character. “The change in DC conductance during sensing is caused by the adsorption of NO in argon on In2O3 surface which is confirmed by the analysis of N 1s pattern.” Adsorption of NO increases charge depletion length (LD) for NO in argon to 25.9 nm at 573 K from its value of 2.3 nm for pure argon and the temperature dependence of LD for NO in argon is evaluated.”

In2O3-Based Thermoelectric Materials: The State of the Art and the Role of Surface State in the Improvement of the Efficiency of Thermoelectric Conversion

In this paper, the thermoelectric properties of In2O3-based materials in comparison with other thermoelectric materials are considered. It is shown that nanostructured In2O3 Sn-based oxides are promising for thermoelectric applications at moderate temperatures. Due to the nanostructure, specific surface properties of In2O3 and filtering effects, it is possible to significantly reduce the thermal conductivity and achieve an efficiency of thermoelectric conversion inaccessible to bulk materials. It is also shown that a specific surface state at the intergrain boundary, optimal for maximizing the filtering effect, can be achieved through (1) the engineering of grain boundary parameters, (2) controlling the composition of the surrounding atmosphere, and (3) selecting the appropriate operating temperature.

Influence of gas carrier on morphological and optical properties of nanostructured In2O3 grown by solid-vapour process

Nanostructured indium oxide (In2O3) thin film was prepared by solid-vapour deposition method under NH3 and Ar atmosphere. The influence of gas nature on the growth of In2O3 thin film was investigated in terms of structure, morphology and optical properties. X-ray diffraction, Raman spectroscopy and photoluminescence analyses indicated the formation of pure In2O3 phase with strong preferred orientation along c-axis, from cubic- to needles-like morphologies. The as-fabricated nanostructured In2O3 thin films with tailored morphology, enhanced crystallinity and optical quality can be used for gas sensing, solar cells and other potential applications. In addition, the potential use of NH3 as carrier gas for an efficient control of morphology/size and optical properties can be proposed for the fabrication of other nanostructured oxides.

Structural, optical and XPS study of thermal evaporated In2O3 thin films

The nanostructured In2O3 thin films were deposited on Si n-type (1 0 0) substrates by reactive thermal evaporation. The structural, morphological, and oxidation states of the films were investigated using x-ray diffraction, scanning electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. The optical properties of the films were analyzed by UV–vis spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy. The deposited films showed c-In2O3 crystalline nanostructures with a preferred diffraction peak of (2 2 2). The truncated icosahedron shape’s morphology with a transmittance of 85% was observed in the In2O3 thin films. All the deposited indium oxide films have 3+ oxidation states.

Sensor Effect in Oxide Films with a Large Concentration of Conduction Electrons

This paper presents a joint experimental and theoretical investigation of sensor response of nanostructured In2O3 semiconductor thin films containing a large concentration of conduction electrons. The capture of the conduction electrons by oxygen adsorbates from air causes redistribution of the electrons inside the nanoparticles, resulting in reduction of the subsurface electron density, and the drop of the conductivity of nanoparticle thin films. When CO and H2 reduced gas analytes are introduced to the system, their reaction with previously adsorbed negative atomic oxygen ions O– releases electrons back to the nanoparticles, producing a noticeable increase of thin-film conductivity, which constitutes the sensor effect. This work presents a kinetic model of such processes, which allows us to a quantitative description of the sensor effect including dependence of sensor sensitivity on temperature. Concurrently, experiments are performed to quantify the sensor response by nanostructured In2O3 thin film as ...

Biuret-assisted formation of nanostructured In2O3 architectures and their photoluminescence properties

Nanostructured indium oxide (In2O3) microcrystals were prepared by calcining hydrothermally synthesized indium hydroxide (In(OH)3) samples. A cubic structure and small crystallite sizes of the In2O3 microcrystals are determined from the X-ray diffraction (XRD) analysis. Observation of the scanning electron microscopy (SEM) shows that the crystalline morphology could be changed from micocubes, microrods to microspheres with appropriately control of the mole ratios of In3+ to biuret. Based on the SEM and FTIR analysis, a possible formation mechanism of In2O3 microcrystals with different morphologies is also proposed. The photoluminescence (PL) spectra show that the energy separation of the blue peaks at 2.85eV and 2.78eV corresponds to a phonon in sub band-gap energy levels of oxygen vacancies; and an ultraviolet peak near 3.22eV is observed, possibly originating from the longitudinal optical-phonon replica of the donor–acceptor pair transition.

Template-free synthesis of In2O3 nanoparticles and their acetone sensing properties

Ordered In2O3 nanoparticles (In2O3 NPs) were successfully prepared via a template-free method. The synthesis was conducted in a water-glycerol-ethylene glycol mixed solvent without any templates, and the ultra-fine nanostructured In2O3 has a uniform size of about 20–30nm. The In2O3 nanoparticles exhibited improved sensitivity, fast response, and high selectivity to acetone. The enhanced acetone performance results from a large surface area with enough sensing active sites, and small particle size for effective electron depletion.

Deposition of Nanostructured Indium Oxide Thin Films for Ethanol Sensing Applications

We present the preparation of a semiconductor gas sensor based on porous nanostructured In2O3 thin films. The In2O3 thin films have been deposited on preheated glass substrates by a spray pyrolysis technique at three substrate temperatures (i.e., 400°C, 450°C, and 500°C). The structural and morphological properties of the films were investigated by means of x-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and fourier transform infrared spectroscopy. The substrate temperature during the film synthesis is found to be the most important factor and must be controlled with precision. It was observed that grain size of the films increased, and the surface roughness decreased with elevating substrate temperature. The sensitivity of the synthesized films was also measured across a range of operating temperature and ethanol concentration. Gas-sensing properties of ethanol shows that the cubic In2O3 nanostructures deposited at the lowest substrate temperature had the highest response.

Memristive properties of In2O3/LaNiO3 heterostructures grown by pulsed laser deposition

Resistance switching properties of nanostructured In2O3 films grown on LaNiO3 (LNO) bottom electrode (BE) have been investigated for non volatile memory applications. Ag/In2O3/LNO/SiO2 heterostructures were fabricated by pulsed laser deposition. Polycrystalline growth of oxides LNO and In2O3 was confirmed by GIXRD, while AFM show nanostructured growth with smooth surface morphology. Two terminal I–V characteristics showed reproducible two distinct resistance states in the film and unipolar type switching. Typical resistance switching ratio (R on /R off ) of the order of 112 % has been estimated at room temperature. A possible mechanism of the formation and rupture of conducting filament is proposed based on the Joule heating effect with external electron injection at the Ag/In2O3 top interface, whereas the LNO BE acts as oxygen reservoir at the In2O3/LNO interface.

Optical and photoelectrical properties of nanocrystalline indium oxide with small grains

Optical properties, spectral dependence of photoconductivity and photoconductivity decay in nanocrystalline indium oxide In2O3 are studied. A number of nanostructured In2O3 samples with various nanocrystals size are prepared by sol–gel method and characterized using various techniques. The mean nanocrystals size varies from 7 to 8nm to 39–41nm depending on the preparation conditions. Structural characterization of the In2O3 samples is performed by means of transmission electron microscopy and X-ray powder diffraction. The combined analysis of ultraviolet–visible absorption spectroscopy and diffuse reflectance spectroscopy shows that nanostructuring leads to the change in optical band gap: optical band gap of the In2O3 samples (with an average nanocrystal size from 7 to 41nm) is equal to 2.8eV. We find out the correlation between spectral dependence of photoconductivity and optical properties of nanocrystalline In2O3: sharp increase in photoconductivity was observed to begin at 2.8eV that is equal to the optical bandgap in the In2O3 samples, and reached its maximum at 3.2–3.3eV. The combined analysis of the slow photoconductivity decay in air, vacuum and argon, that was accurately fitted by a stretched-exponential function, and electron paramagnetic resonance (EPR) measurements shows that the kinetics of photoconductivity decay is strongly depended on the presence of oxygen molecules in the ambient of In2O3 nanocrystals. There is the quantitative correlation between EPR and photoconductivity data. Based on the obtained data we propose the model clearing up the phenomenon of permanent photoconductivity decay in nanocrystalline In2O3.

NO2 Sensing Properties of Thermally or UV Activated In2O3 Nano-octahedra

In the present work, we have synthesized single crystalline In2O3 octahedra via a vapour phase transport at high temperatures over Si/SiO2 substrates. Gas sensing measurements (NO2) have been performed under temperature variations and UV irradiation. Optimum working conditions for each case have been found. A comparison between nanostructured In2O3 and commercial In2O3 powder has been performed in order to show the advantages of using a nanostructured based sensor. As a result, the detection of low concentrations of NO2 (50 ppb) has found to be possible with sensitivities for higher concentrations comparable to those shown in recent publications.

Effect of composition and temperature on conductive and sensing properties of CeO2 + In2O3 nanocomposite films

The sensory response of nanostructured In2O3+CeO2 composite films to hydrogen and carbon monoxide in air ambience is investigated for varying film composition and temperature ranging from 280°C to 520°C. The temperature dependence of the sensor response S (S=R0/R, where R0 and R are respectively the film resistance in pure air and air containing the sample gas), exhibits the trend typical of such sensors, specifically, curves with maximum Smax at a certain temperature Tmax. The values of Smax, characterizing the sensor response of the films significantly increase when a small amount of CeO2 is added to In2O3. Addition of CeO2 to In2O3 also results in a decrease in Tmax. The measured XPS spectra show that at low CeO2 composition, the composite film structure is characterized by clusters with a high concentration of oxygen vacancies, which increase the chemisorption of reagents and oxygen. The maximum sensor response is observed in In2O3+CeO2 composite films containing 3–10wt.% CeO2. Further enrichment of the composite with CeO2 produces a sharp decrease in sensor response, which at 40wt.% CeO2 is less than the response of pure In2O3. The response mechanism in the In2O3+CeO2 composite is also investigated, considering the promotion of sensory reactions by small CeO2 nanoclusters located on the surface of the In2O3 nanocrystals.

Effect of the tin impurity on the energy spectrum and photoelectric properties of nanostructured In2O3 films

It is shown that the doping of colloidal indium-oxide nanocrystals with tin shifts the absorption edge to shorter wavelengths of the visible spectral region and induces considerable absorption in the infrared region. The shape of the absorption and transmittance spectra in the infrared region is characteristic of the local surface plasmon resonance. According to estimations, the concentration of free charge carriers in In2O3(Sn) reaches 1019 cm−3. The temperature dependences of the photoconductivity in the range 77–300 K are indicative of hopping conduction in nanostructured films based on In2O3(Sn) nanocrystals. The mechanisms responsible for the effect are discussed.

Charge carrier transport mechanisms in nanocrystalline indium oxide

The charge transport properties of nanocrystalline indium oxide (In2O3) are studied. A number of nanostructured In2O3 samples with various nanocrystal sizes are prepared by sol–gel method and characterized using various techniques. The mean nanocrystals size varies from 7–8nm to 18–20nm depending on the conditions of their preparation. Structural characterizations of the In2O3 samples are performed by means of transmission electron microscopy and X-ray diffraction. The analysis of dc and ac conductivity in a wide temperature range (T=50–300K) shows that at high temperatures charge carrier transport takes place over conduction band and at low temperatures a variable range hopping transport mechanism can be observed. We find out that the temperature of transition from one mechanism to another depends on nanocrystal size: the transition temperature rises when nanocrystals are bigger in size. The average hopping distance between two sites and the activation energy are calculated basing on the analysis of dc conductivity at low temperature. Using random barrier model we show a uniform hopping mechanism taking place in our samples and conclude that nanocrystalline In2O3 can be regarded as a disordered system.

Thermoelectrical properties of spray pyrolyzed indium oxide thin films doped by tin

The search for materials with thermoelectric parameters capable of operating at high temperatures continues to be of great interest; n-type metal oxides are promising candidates. Here, two series of thin (~100nm) indium oxide films doped by tin (from 0 to 50at.%) were deposited by spray pyrolysis at 350°C and 450°C. Characterization of the films was performed using X-ray diffraction, scanning electron microscopy and atomic force microscopy. Thermoelectric properties, i.e., the conductivity and the Seebeck coefficient, were then studied over a temperature range of 20–450°C. It was shown that these parameters as well as their nanostructure were strongly dependent on the Sn content and deposition temperature. Specifically, the conductivity had maxima near 5% and 20% for films deposited at 350°C and 450°C, respectively. The power factor (PF) as a function of Sn content also demonstrated non-monotonous behavior with two maxima; for films deposited at 350°C these maxima were again observed near 5% and 20% of Sn content. The maximal PF value equaled to 4.7mW/(m·K2) at a temperature of 450°C was observed at 5at.% Sn. This result is one of the best ever obtained for metal oxides in a given temperature range. The optimal films were characterized by a cubic-like crystallite nanostructure with {400} surface faceting. A model explaining such high parameters was subsequently proposed. We also determined the effect of ambient humidity on the thermoelectric properties of nanostructured In2O3:Sn films at an operating temperature range below 400°C, which is caused by the change of surface conductivity under the influence of water vapor.