Gallium(III) oxide is a promising functional wide-gap semiconductor for high temperature gas sensors of the resistive type. Doping of Ga2O3 with tin improves material conductivity and leads to the complicated influence on phase content, microstructure, adsorption sites, donor centers and, as a result, gas sensor properties. In this work, Ga2O3 and Ga2O3(Sn) samples with tin content of 0–13 at.% prepared by aqueous co-precipitation method were investigated by X-ray diffraction, nitrogen adsorption isotherms, X-ray photoelectron spectroscopy, infrared spectroscopy and probe molecule techniques. The introduction of tin leads to a decrease in the average crystallite size, increase in the temperature of β-Ga2O3 formation. The sensor responses of all Ga2O3(Sn) samples to CO and NH3 have non-monotonous character depending on Sn content due to the following factors: the formation of donor centers and the change of free electron concentration, increase in reactive chemisorbed oxygen ions concentration, formation of metastable Ga2O3 phases and segregation of SnO2 on the surface of Ga2O3(Sn) grains.

In this work, p-type oxide semiconductors, Co3O4 and complex oxides NixCo3−xO4 (x = 0.04, 0.07, 0.1), were studied as materials for sub-ppm H2S sensing in the temperature range of 90–300 °C in dry and humid air. Nanocrystalline Co3O4 and NixCo3−xO4 (x = 0.04, 0.07, 0.1) were prepared by coprecipitation of cobalt and nickel oxalates from nitrate solutions and further annealing at 300 °C. The surface reactivity of the obtained materials toward H2S both in dry and humid atmosphere (relative humidity at 25 °C R.H. = 60%) was investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Sensor measurements showed a decrease in sensor signal toward 1 ppm H2S with an increase in Ni content because of a decrease in chemisorbed surface oxygen species. On the other hand, sensor signal increases for all samples with increasing the relative humidity that depends on reactivity of the surface hydroxyl groups, which stimulate the decomposition of surface sulfites and provide better surface regeneration at higher temperature. This assumption was additionally confirmed by the faster saturation of the conductivity curve and a decrease in the sensor response time in humid air.

It is known that adsorptive capacity and surface reactivity of metal oxides depends on chemical composition, which influences the characteristics of metal-oxygen bonds, e.g. degree of covalency, effective atomic charges, bond energy and bond length. Tungsten oxide and bismuth tungstate have perovskite-related structures and close semiconductor properties, but different chemical composition. The presence of bismuth in Bi2WO6 results in distinct W-O bond length and bond energy, respective to WO3, which may be one of the factors controlling the surface reactivity, as well as the occurrence of bismuth-related surface sites. Tungsten oxide is a renowned material for heterogeneous catalysts, photocatalysts, and sensors. Bismuth tungstate has been extensively studied as a photocatalytic material, and an interest in sensor applications of this compound emerged recently. Yet, there is a lack of comparative and systematic studies of the electronic structure and sensing properties of tungsten oxide and bismuth tungstate. Such a study would be promising for the elucidation of the role of W-O bonds in controlling the sensing behavior to different analyte gases. In this work, the comparative study of electronic and sensing properties of Bi2WO6 and WO3 was performed. Band structures, charge distribution and metal-oxygen bonds energies were calculated by first principles quantum chemical approach. Nanocrystalline Bi2WO6, WO3 and composite Bi2WO6 + WO3 were synthesized. Bismuth tungstate showed an improved sensitivity to volatile organic compounds (ethanol, formaldehyde, acetone, benzene) and poor sensitivity to nitrogen dioxide, in contrast to WO3. Based on the experimental and computational data, the effects of W-O bond energy and charge distribution on the sensitivity to target gases of bismuth tungstate and tungsten oxide were rationalized. (C) 2020 Elsevier B.V. All rights reserved.

A new approach to in situ analysis of the composition of photosensitive metal halide perovskite-based materials for gas sensor has been developed. The concentration of Cs, Pb, Br and I in the ZnO layer was determined by in situ micro X-ray fluorescence method using inductively coupled plasma mass spectrometry for calibration. The relationship ‘synthesis conditions–composition–photoresponse’ of the sensor has been established based on the results obtained.

Development of sensor materials based on metal oxide semiconductors (MOS) for selective gas sensors is challenging for the tasks of air quality monitoring, early fire detection, gas leaks search, breath analysis, etc. An extensive range of sensor materials has been elaborated, but no consistent guidelines can be found for choosing a material composition targeting the selective detection of specific gases. Fundamental relations between material composition and sensing behavior have not been unambiguously established. In the present review, we summarize our recent works on the research of active sites and gas sensing behavior of n-type semiconductor metal oxides with different composition (simple oxides ZnO, In2O3, SnO2, WO3; mixed-metal oxides BaSnO3, Bi2WO6), and functionalized by catalytic noble metals (Ru, Pd, Au). The materials were variously characterized. The composition, metal-oxygen bonding, microstructure, active sites, sensing behavior, and interaction routes with gases (CO, NH3, SO2, VOC, NO2) were examined. The key role of active sites in determining the selectivity of sensor materials is substantiated. It was shown that the metal-oxygen bond energy of the MOS correlates with the surface acidity and the concentration of surface oxygen species and oxygen vacancies, which control the adsorption and redox conversion of analyte gas molecules. The effects of cations in mixed-metal oxides on the sensitivity and selectivity of BaSnO3 and Bi2WO6 to SO2 and VOCs, respectively, are rationalized. The determining role of catalytic noble metals in oxidation of reducing analyte gases and the impact of acid sites of MOS to gas adsorption are demonstrated.

Development of new signal processing approaches is essential for improvement of the reliability of metal oxide gas sensor performance in real atmospheric conditions. Advantages statistical shape analysis (SSA) method are presented in comparison to previously reported signal pre-processing techniques - principal component analysis (PCA), discrete wavelet transform (DWT), polynomial curve fitting (PCF) - used in combination with machine learning (ML) algorithm for improvement of detection selectivity. An enhanced identification of chemically related gases (methane and propane) at a concentration range of 40-200 ppm under variable real atmospheric conditions has been demonstrated using working temperature modulated metal oxide gas sensors. Laboratory samples of sensors based on nanocrystalline SnO2 modified with Au and Pd were used. The proposed data preprocessing algorithm is less sensitive to sensor response and baseline drift and fluctuations compared to other methods during two months of continuous operation and work with periods of inactivity. The collected dataset and signal processing code are made public. The advantages of SSA signal pre-processing method are also demonstrated with the use of independent publicly available dataset for the task of CO selective quantitative detection in the air with variable humidity in the 2.2-20 ppm concentrations range.

The review deals with issues related to the principle of operation of resistive semiconductor gas sensors and the use of light activation instead of thermal heating when detecting gases. Information on the photoelectric and optical properties of nanocrystalline oxides SnO2, ZnO, In2O3, and WO3, which are the most widely used sensitive materials for semiconductor gas sensors, is presented. The activation of the gas sensitivity of semiconductor materials by both UV and visible light is considered. When activated by UV light, the typical approaches for creating materials are (i) the use of individual metal oxides, (ii) chemical modification with nanoparticles of noble metals and their oxides, (iii) and the creation of nanocomposite materials based on metal oxides. In the case of visible light activation, the approaches used to enhance the photo- and gas sensitivity of wide-gap metal oxides are (i) doping; (ii) spectral sensitization using dyes, narrow-gap semiconductor particles, and quantum dots; and (iii) addition of plasmon nanoparticles. Next, approaches to the description of the mechanism of the sensor response of semiconductor sensors under the action of light are considered.

In the present work the possibility is considered of a chemical sensor synthesis for quantitative glutathione (GSH) determination. Sensor is based on a composite working electrode containing an array of micron-sized Ag particles immobilized on a conductive substrate (Ti) coated by dielectric TiO2 film. To determine GSH in biological fluids, particularly, in saliva, electrochemical silver-based sensors can be used, since such sensors contain -SH group. With the use of cyclic voltammetry (CV) with a composite working electrode containing an Ag microparticles array, the threshold of quantitative GSH determination is reduced to nM level. Since other modern analogues are inferior at least one order of magnitude in the limit of quantitative GSH detection, we assume that the proposed sensor may be of great interest for clinical diagnosis.

Nanocomposites based on Au- and SiO2-modified SnO2 were studied as sensitive materials for ethanol and benzene detection in dry (RH = 1%) and humid (RH = 20%) air. Modification of SnO2 by amorphous SiO2 (13 mol.%) was effectuated by hydrothermal synthesis; modification by Au nanoparticles (1 wt.%) was carried out via impregnation by citrate-stabilized Au sol. The composition of the samples was determined by X-ray fluorescent spectroscopy and energy-dispersive X-ray spectroscopy. The microstructure was characterized by XRD, HRTEM, and low-temperature nitrogen adsorption. The surface groups were investigated by XPS, TPR-H2, and FTIR spectroscopy. DRIFT spectroscopy was performed to investigate the interaction between ethanol and the surface of the synthesized materials. Studies of the sensor properties have shown that in all cases the most sensitive is the SnO2/SiO2-Au nanocomposite. This material retains high sensitivity even in a humid atmosphere. The obtained results are discussed in terms of the synergistic effect of two modifiers (Au and SiO2) in the formation of sensor properties of SnO2/SiO2–Au nanocomposites.

X-ray photoelectron spectroscopy and electric conduction measurements performed for nanocrystalline WO3 revealed that the ratio of surface-to-bulk oxygen decreased, and the predominant type of chemisorbed oxygen changed from molecular O2 – to atomic O– and O2– species with the increment of WO3 particle size.

Air humidity is one of the main factors affecting the characteristics of semiconductor gas sensors, especially at low measurement temperatures. In this work we analyzed the influence of relative humidity on sensor properties of the hybrid materials based on the nanocrystalline SnO2 and In2O3 and Ru (II) heterocyclic complex and verified the possibility of using such materials for NO (0.25–4.0 ppm) and NO2 (0.05–1.0 ppm) detection in high humidity conditions (relative humidity (RH) = 20%, 40%, 65%, 90%) at room temperature during periodic blue (λmax = 470 nm) illumination. To reveal the reasons for the different influence of humidity on the sensors’ sensitivity when detecting NO and NO2, electron paramagnetic resonance (EPR) spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) investigations were undertaken. It was established that the substitution of adsorbed oxygen by water molecules causes the decrease in sensor response to NO in humid air. The influence of humidity on the interaction of sensitive materials with NO2 is determined by the following factors: the increase in charge carrier’s concentration, the decrease in the number of active sites capable of interacting with gases, and possible substitution of chemisorbed oxygen with NO2− groups.

In this work, the optical characteristics and conductivity under photoactivation with visible light of hybrids based on nanocrystalline SnO2 or In2O3 semiconductor matrixes and heteroleptic Ru(ii) complexes were studied. The heteroleptic Ru(ii) complexes were prepared based on 1H-imidazo[4,5-f][1,10]phenanthroline and 2,2'-bipyridine ligands. Nanocrystalline semiconductor oxides were obtained by chemical precipitation with subsequent thermal annealing and characterized by XRD, SEM and single-point BET methods. The heteroleptic Ru(ii) complexes as well as hybrid materials were characterized by time-resolved luminescence and X-ray photoelectron spectroscopy. The results showed that the surface modification of SnO2 nanoparticles with heteroleptic ruthenium complexes led to an increase in conductivity upon irradiation with light appropriate for absorption by organometallic complexes. In the case of In2O3, the deposition of Ru(ii) complexes resulted in a decrease in conductivity, apparently due to the special structure of the surface layer of the semiconductor.

Metal oxide semiconductors (MOS) in pristine form and functionalized by noble metals are extensively researched for gas detection. There is a lack of fundamental understanding of the relations between materials composition and sensitivity to volatile organic compounds (VOC). In this work, we compared the sensitivities of different pristine metal oxides and Au-functionalized metal oxides to methanol and acetone. Nanostructured MOS of p-type (NiO, CuO, Co3O4) and n-type (In2O3, ZnO, SnO2, TiO2, and WO3) were synthesized by common aqueous deposition techniques. The oxides differ in metal-oxygen bond energy (EM-O) which was chosen as a parameter for the comparison of sensitivity to methanol and acetone vapors. Metal oxides were functionalized by Au nanoparticles via colloid adsorption, and the effect of gold on the sensitivity to VOC was investigated. Acid sites and surface oxygen sites at the materials surfaces were determined by temperature-programmed probe molecule techniques. The interaction routes of methanol and acetone with materials surfaces were examined by diffuse-reflectance infrared spectroscopy (DRIFT). It was found that with the increase of metal-oxygen bond energy the surface acidity of MOS strengthened, which favored the adsorption and improved the sensitivity to methanol. Oxidation of methanol and acetone to formate and acetate species on the materials surfaces was revealed, and the roles of active sites in these reactions were rationalized. An enhanced sensitivity to VOC was observed for Au-functionalized metal oxides, with the most prominence for TiO2/Au nanocomposite. It was rationalized as a combination of a proper Ti-O bond energy and the catalytic effect of gold.

Nanocrystalline tin dioxide SnO2 is of considerable interest for the creation of solar cells, solid-state chemical sensors, and oxidation catalysts. High adsorption properties and reactivity of nanocrystalline SnO2 are due to high concentration of surface adsorption centers, especially the coordinately unsaturated tin cations Sn4c, Sn5c, chemisorbed oxygen, and oxygen vacancies VO. In the recent years, many works have been devoted to the development of various approaches to the synthesis of nanocrystalline SnO2 and to the investigation of its reactivity. The main direction of research of nanocrystalline SnO2 is aimed at increasing the selectivity of adsorption and promoting chemical reactions on the surface. The selectivity of the chemical properties of nanocrystalline SnO2 can be achieved by tuning the type and concentration of surface active centers by bulk doping immobilization of modifiers: nanoparticles based on noble metals or transition elements oxides. The data on the effect of catalytic modifiers on the composition, microstructure, type and concentration of active sites, and reactivity of nanocrystalline SnO2 are systematized in this section.

Nanocrystalline complex oxides NixCo3-xO4 (0 <= x <= 1) were obtained via co-precipitation of oxalates with subsequent thermal decomposition and characterized in detail by ICP-MS, XRD, TEM, XPS and EPR methods. It was found that complex oxides NixCo3-xO4, where x <= 0.25, can be considered as Co3O4 based solid solutions with a normal spinel structure, while NixCo3-xO4 with a high nickel content (x = 0.70, x = 1) have a partially or completely inverse spinel structure. All complex oxides NixCo3-xO4 (0 <= x <= 1) are characterized by p-type conductivity, which sharply increases with the increase in Ni content. Sensor measurements in the presence of 5-20 ppm CO demonstrated that in the temperature range 80-150 degrees C the sensor response of NixCo3-xO4 decreases with an increase in Ni content that can be explained by the decrease in the concentration of oxygen chemisorbed on the NixCo3-xO4 surface. However, the lower limit of CO detection (measured at 120 degrees C) decreases from 1.08 ppm for Co3O4 down to 0.39 ppm for NixCo3-xO4 that makes NixCo3-xO4 complex oxides promising materials for low temperature detection of carbon monoxide at ultra low concentrations. (C) 2020 Elsevier B.V. All rights reserved.

RGO-SnO2 heterojunction has been extensively studied as an efficient sensitive material, while its sensing mechanism in surface chemical system is still needed to be concerned. The aim of this work is to evaluate the effects of rGO and materials processing on surface sites of nanocrystalline tin dioxide and room-temperature sensitivity to NO2. Gas-sensitive heterojunction of tin dioxide with 2-dimensional rGO has been synthesized by a one-step hydrothermal method. Contrastive researches about composition, microstructure and surface species of hydrothermal-obtained pristine SnO2, rGO-SnO2 hybrids were performed. Further reduction of rGO in hydrothermal composite with a post-treatment at over 100 degrees C has been found to affect the hydrophilicity and electrical conduction of rGO-SnO2 composite. Sensitivity of rGO-SnO2 to 2-8 ppm NO2 with fast response was demonstrated at room temperature, while the abrupt change of conductivity type of hydrothermal-obtained rGO-SnO2 heterojunction from p-type to n-type was observed after post-anneal treatment. The conductive type conversion was discussed to be closely related to the higher reduction degree of rGO and enhanced hydrophobic surface of rGO-SnO2 heterojunction.

Novel ZnSe/NiO heterostructure nanocomposites were successfully prepared by one-step hydrothermal method. The ZnSe/NiO-based sensor exhibits a response of ~ 96.47% to 8 × 10−6 NO2 at 140 °C, which is significantly higher than those of intrinsic ZnSe-based (no response) and NiO-based (~ 19.65%) sensors. The theoretical detection limit (LOD) of the sensor is calculated to be 8.91 × 10−9, indicating that the sensor can be applied to detect the ultralow concentrations of NO2. The effect of NiO content on the gas-sensing performance of the nanocomposites was investigated in detail. The optimal NiO content in the nanocomposite is determined to be 15.16% to achieve the highest response. The as-fabricated sensor also presents an excellent selectivity to several possible interferents such as methanol, ethanol, acetone, benzene, ammonia and formaldehyde. The enhanced sensing performance can be attributed to the formation of p–p heterostructures between ZnSe and NiO, which induces the charge transfer across the interfaces and yields more active sites.

Variable air humidity affects the characteristics of semiconductor metal oxides, which complicates the reliable and reproducible determination of CO content in ambient air by resistive gas sensors. In this work, we determined the sensor properties of electrospun ZnO and ZnO/Pd nanofibers in the detection of CO in dry and humid air, and investigated the sensing mechanism. The microstructure of the samples, palladium content, and oxidation state, type, and concentration of surface groups were characterized using complementary techniques: X-ray fluorescent spectroscopy, XRD, high-resolution transmission electron microscopy (HRTEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray (EDX) mapping, XPS, and FTIR spectroscopy. The sensor properties of ZnO and ZnO/Pd nanofibers were studied at 100–450 °C in the concentration range of 5–15 ppm CO in dry (RH25 = 0%) and humid (RH25 = 60%) air. It was found that under humid conditions, ZnO completely loses its sensitivity to CO, while ZnO/Pd retains a high sensor response. On the basis of in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) results, it was concluded that high sensor response of ZnO/Pd nanofibers in dry and humid air was due to the electronic sensitization effect, which was not influenced by humidity change.

The development of sensor materials of which gas sensitivity activates under light illumination is of great importance for the design of portable gas analyzers with low power consumption. In the present work a ZnO/CsPbBr3 nanocomposite based on nanocrystalline ZnO and colloidal cubic-shaped perovskite CsPbBr3 nanocrystals (NCs) capped by oleic acide and oleylamine was synthesized. The individual materials and obtained nanocomposite are characterized by x-ray diffraction, low-temperature nitrogen adsorption, x-ray photoelectron spectroscopy, high angle annular dark field scanning transmission electron microscopy with energy-dispersive Xray spectroscopy mapping and UV-vis absorption spectroscopy. The spectral dependence of the photoconductivity of the ZnO/CsPbBr3 nanocomposite reveals a well-defined peak that strongly correlates with the its optical absorption spectrum. The nanocomposite ZnO/CsPbBr3 shows enhanced photoresponse under visible light illumination (lambda(max) = 470 nm, 8 mW/cm(2)) in air, oxygen and argone, compared with pure nanocrystalline ZnO. Under periodic illumination in the temperature range of 25-100 degrees C, the ZnO/CsPbBr3 nanocomposite shows a sensor response to 0.5-3.0 ppm NO2, unlike pure nanocrystalline ZnO matrix, which demonstrates sensor sensitivity to NO2 under the same conditions above 100 degrees C. The effects of humidity on the sensor signal and photoresponse are also discussed.

Understanding ammonia oxidation over metal oxide surfaces is crucial for improving its detection with resistive type gas sensors. Formation of NOx during this process makes sensor response and calibration unstable. Cr-doping of nanocrystalline metal oxides has been reported to suppress NO2 sensitivity and improve response towards NH3, however the exact mechanism of such chromium action remained unknown. Herein, by using EPR spectroscopy we demonstrate formation of Cr(VI) lattice defects on the surface of nanocrystalline Cr-doped SnO2. Enhancement of Cr-doped SnO2 surface acidity and ammonia adsorption as a result has been revealed by using in situ IR spectroscopy. Moreover, a decrease in concentration of free electrons in the conduction band has been shown as a result of substitutional Cr(III) defects formation. Weaker NOx chemisorption during ammonia oxidation over SnO2 surface after Cr doping has been found with the use of mass-spectrometry assisted NH3 thermo-programmed desorption. The given example of surface acidity adjustment and electronic configuration by means of doping may find use in the design of new gas-sensing metal oxide materials.