Photoactivated Processes on the Surface of Metal Oxides and Gas Sensitivity to Oxygen.

Photoactivation by UV and visible radiation is a promising approach for the development of semiconductor gas sensors with reduced power consumption, high sensitivity, and stability. Although many hopeful results were achieved in this direction, the theoretical basis for the processes responsible for the photoactivated gas sensitivity still needs to be clarified. In this work, we investigated the mechanisms of UV-activated processes on the surface of nanocrystalline ZnO, In2O3, and SnO2 by in situ mass spectrometry and compared the obtained results with the gas sensitivity to oxygen in the dark and at UV irradiation. The results revealed a correlation between the photoactivated oxygen isotopic exchange activity and UV-activated oxygen gas sensitivity of the studied metal oxides. To interpret the data obtained, a model was proposed based on the idea of the generation of additional oxygen vacancies under UV irradiation due to the interaction with photoexcited holes.

Humidity Effect on Low-Temperature NH3 Sensing Behavior of In2O3/rGO Composites under UV Activation.

The nature of the constituent components of composite materials can significantly affect the character of their interaction with the gas phase. In this work, nanocrystalline In2O3 was synthesized by the chemical precipitation method and was modified using reduced graphene oxide (rGO). The obtained composites were characterized by several analysis techniques-XRD, TEM, SEM, FTIR and Raman spectroscopy, XPS, TGA, and DRIFTS. The XPS and FTIR and Raman spectroscopy results suggested the formation of interfacial contact between In2O3 and rGO. The results of the gas sensor's properties showed that additional UV illumination led to a decrease in resistance and an increase in sensor response at room temperature. However, the presence of humidity at room temperature led to the disappearance of the response for pure In2O3, while for the composites, an inversion of the sensor response toward ammonia was observed. The main reason may have been the formation of NH4NO3 intermediates with further hydrolysis and decomposition under light illumination with the formation of nitrite and nitrate species. The presence of these species was verified by in situ DRIFT spectroscopy. Their strong electron-accepting properties lead to an increase in resistance, which possibly affected the sensor signal's inversion.

Synergistic Effect of Surface Acidity and PtOx Catalyst on the Sensitivity of Nanosized Metal-Oxide Semiconductors to Benzene.

Benzene is a potentially carcinogenic volatile organic compound (VOC) and its vapor must be strictly monitored in air. Metal-oxide semiconductors (MOS) functionalized by catalytic noble metals are promising materials for sensing VOC, but basic understanding of the relationships of materials composition and sensors behavior should be improved. In this work, the sensitivity to benzene was comparatively studied for nanocrystalline n-type MOS (ZnO, In2O3, SnO2, TiO2, and WO3) in pristine form and modified by catalytic PtOx nanoparticles. Active sites of materials were analyzed by X-ray photoelectron spectroscopy (XPS) and temperature-programmed techniques using probe molecules. The sensing mechanism was studied by in situ diffuse-reflectance infrared (DRIFT) spectroscopy. Distinct trends were observed in the sensitivity to benzene for pristine MOS and nanocomposites MOS/PtOx. The higher sensitivity of pristine SnO2, TiO2, and WO3 was observed. This was attributed to higher total concentrations of oxidation sites and acid sites favoring target molecules' adsorption and redox conversion at the surface of MOS. The sensitivity of PtOx-modified sensors increased with the surface acidity of MOS and were superior for WO3/PtOx. It was deduced that this was due to stabilization of reduced Pt sites which catalyze deep oxidation of benzene molecules to carbonyl species.

Heterobimetallic Ru(II)/M (M = Ag+, Cu2+, Pb2+) Complexes as Photosensitizers for Room-Temperature Gas Sensing.

This work is devoted to the investigation of heterobimetallic Ru(II) complexes as photosensitizers for room-temperature photoactivated In2O3-based gas sensors. Nanocrystalline In2O3 was synthesized by the chemical precipitation method. The obtained In2O3 matrix has a single-phase bixbyite structure with an average grain size of 13-14 nm and a specific surface area of 72 ± 3 m2/g. The synthesis of new ditope ligands with different coordination centers, their ruthenium complexes, and the preparation of heterobimetallic complexes with various cations of heavy and transition metals (Ag+, Pb2+, or Cu2+) is reported. The heterobimetallic Ru(II) complexes were deposited onto the surface of the In2O3 matrix by impregnation. The obtained hybrid materials were characterized by X-ray fluorescent analysis, FTIR spectroscopy, and optical absorption spectroscopy. The elemental distribution on the hybrids was characterized by energy-dispersive X-ray spectroscopy (EDS) mapping. The gas sensor properties were investigated toward NO2, NO, and NH3 at room temperature under periodic blue LED irradiation. It was identified that the nature of the second binding cation in Ru(II) heterobimetallic complexes can influence the selectivity toward different gases. Thus, the maximum sensor signal for oxidizing gases (NO2, NO) was obtained for hybrids containing Ag+ or Pb2+ cations while the presence of Cu2+ cation results in the highest and reversible sensor response toward ammonia. This may be due to the specific adsorption of NH3 molecules on Cu2+ cations. On the other hand, Cu2+ ions are proposed to be active sites for the reduction of nitrogen oxides to N2. This fact leads to a significant decrease in the sensor response toward NO2 and NO gases.

The Key Role of Active Sites in the Development of Selective Metal Oxide Sensor Materials.

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.

Flame-Made La2O3-Based Nanocomposite CO2 Sensors as Perspective Part of GHG Monitoring System.

Continuous monitoring of greenhouse gases with high spatio-temporal resolution has lately become an urgent task because of tightening environmental restrictions. It may be addressed with an economically efficient solution, based on semiconductor metal oxide gas sensors. In the present work, CO2 detection in the relevant concentration range and ambient conditions was successfully effectuated by fine-particulate La2O3-based materials. Flame spray pyrolysis technique was used for the synthesis of sensitive materials, which were studied with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) and low temperature nitrogen adsorption coupled with Brunauer-Emmett-Teller (BET) effective surface area calculation methodology. The obtained materials represent a composite of lanthanum oxide, hydroxide and carbonate phases. The positive correlation has been established between the carbonate content in the as prepared materials and their sensor response towards CO2. Small dimensional planar MEMS micro-hotplates with low energy consumption were used for gas sensor fabrication through inkjet printing. The sensors showed highly selective CO2 detection in the range of 200-6667 ppm in humid air compared with pollutant gases (H2 50 ppm, CH4 100 ppm, NO2 1 ppm, NO 1 ppm, NH3 20 ppm, H2S 1 ppm, SO2 1 ppm), typical for the atmospheric air of urbanized and industrial area.

Electron injection effect in In2O3 and SnO2 nanocrystals modified by ruthenium heteroleptic complexes.

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.

Electrospun ZnO/Pd Nanofibers: CO Sensing and Humidity Effect.

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.

Enhancement of Lewis Acidity of Cr-Doped Nanocrystalline SnO2 : Effect on Surface NH3 Oxidation and Sensory Detection Pattern.

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.

Quasi Similar Routes of NO2 and NO Sensing by Nanocrystalline WO3: Evidence by In Situ DRIFT Spectroscopy.

Tungsten oxide is a renowned material for resistive type gas sensors with high sensitivity to nitrogen oxides. Most studies have been focused on sensing applications of WO3 for the detection of NO2 and a sensing mechanism has been established. However, less is known about NO sensing routes. There is disagreement on whether NO is detected as an oxidizing or reducing gas, due to the ambivalent redox behavior of nitric oxide. In this work, nanocrystalline WO3 with different particle size was synthesized by aqueous deposition of tungstic acid and heat treatment. A high sensitivity to NO2 and NO and low cross-sensitivities to interfering gases were established by DC-resistance measurements of WO3 sensors. Both nitrogen oxides were detected as the oxidizing gases. Sensor signals increased with the decrease of WO3 particle size and had similar dependence on temperature and humidity. By means of in situ infrared (DRIFT) spectroscopy similar interaction routes of NO2 and NO with the surface of tungsten oxide were unveiled. Analysis of the effect of reaction conditions on sensor signals and infrared spectra led to the conclusion that the interaction of WO3 surface with NO was independent of gas-phase oxidation to NO2.

Sub-ppm Formaldehyde Detection by n-n TiO2@SnO2 Nanocomposites.

Formaldehyde (HCHO) is an important indicator of indoor air quality and one of the markers for detecting lung cancer. Both medical and air quality applications require the detection of formaldehyde in the sub-ppm range. Nanocomposites SnO2/TiO2 are promising candidates for HCHO detection, both in dark conditions and under UV illumination. Nanocomposites TiO2@SnO2 were synthesized by ALD method using nanocrystalline SnO2 powder as a substrate for TiO2 layer growth. The microstructure and composition of the samples were characterized by ICP-MS, TEM, XRD and Raman spectroscopy methods. The active surface sites were investigated using FTIR and TPR-H2 methods. The mechanism of formaldehyde oxidation on the surface of semiconductor oxides was studied by in situ DRIFTS method. The sensor properties of nanocrystalline SnO2 and TiO2@SnO2 nanocomposites toward formaldehyde (0.06-0.6 ppm) were studied by in situ electrical conductivity measurements in dark conditions and under periodic UV illumination at 50-300 °C. Nanocomposites TiO2@SnO2 exhibit a higher sensor signal than SnO2 and a decrease in the optimal measurement temperature by 50 °C. This result is explained based on the model considering the formation of n-n heterocontact at the SnO2/TiO2 interface. UV illumination leads to a decrease in sensor response compared with that obtained in dark conditions because of the photodesorption of oxygen involved in the oxidation of formaldehyde.

Gold Decoration and Photoresistive Response to Nitrogen Dioxide of WS2 Nanotubes.

Composites of WS2 nanotubes (NT-WS2 ) and gold nanoparticles (AuNPs) were prepared using aqueous HAuCl4 solutions and subjected to surface analysis. The obtained materials were jointly characterized by X-ray photoelectron (XPS), Raman scattering (RSS), and ultraviolet photoelectron (UPS) spectroscopies. Optical extinction spectroscopy and electron energy loss spectroscopy in the scanning transmission electron microscopy regime (STEM-EELS) were also employed to study plasmon features of the nanocomposite. It was found that AuNPs deposition is accompanied by a partial oxidative dissolution of WS2 , whereas Au-S interfacial species could be responsible for the tight contact of metal nanoparticles and the disulfide. A remarkable sensitivity of n-type resistance of NT-WS2 and Au-NT-WS2 to the adsorption of NO2 gas was also demonstrated at room temperature using periodical illumination by a 530 nm light-emitting diode. Au-NT-WS2 nanocomposites are found to possess a higher photoresponse and enhanced sensitivity in the 0.25-2.0 ppm range of NO2 concentration, as compared to the pristine NT-WS2 . This behaviour is discussed within the physisorption-charge transfer model to explore sensing properties of the nanocomposites.

Co3O4 as p-Type Material for CO Sensing in Humid Air.

Nanocrystalline cobalt oxide Co3O4 has been prepared by precipitation and subsequent thermal decomposition of a carbonate precursor, and has been characterized in detail using XRD, transmission electron microscopy, and FTIR spectroscopy. The sensory characteristics of the material towards carbon monoxide in the concentration range 6.7-20 ppm have been examined in both dry and humid air. A sensor signal is achieved in dry air at sufficiently low temperatures T = 80-120 °C, but the increase in relative humidity results in the disappearance of sensor signal in this temperature range. At temperatures above 200 °C the inversion of the sensor signal in dry air was observed. In the temperature interval 180-200 °C the sensor signal toward CO is nearly the same at 0, 20 and 60% r.h. The obtained results are discussed in relation with the specific features of the adsorption of CO, oxygen, and water molecules on the surface of Co3O4. The independence of the sensor signal from the air humidity combined with a sufficiently short response time at a moderate operating temperature makes Co3O4 a very promising material for CO detection in conditions of variable humidity.

Effect of Donor Nb(V) Doping on the Surface Reactivity, Electrical, Optical and Photocatalytic Properties of Nanocrystalline TiO2.

In this work, we primarily aimed to study the Nb(V) doping effect on the surface activity and optical and electrical properties of nanocrystalline TiO2 obtained through flame-spray pyrolysis. Materials were characterized using X-ray diffraction, Raman spectroscopy and IR, UV and visible spectroscopy. The mechanism of surface reaction with acetone was studied using in situ DRIFTs. It was found that the TiO2-Nb-4 material demonstrated a higher conversion of acetone at a temperature of 300 °C than pure TiO2, which was due to the presence of more active forms of chemisorbed oxygen, as well as higher Lewis acidity of the surface. Conduction activation energies (Eact) were calculated for thin films based on TiO2-Nb materials. The results of the MB photobleaching experiment showed a non-monotonic change in the photocatalytic properties of materials with an increase in Nb(V) content, which was caused by a combination of factors, such as specific surface area, phase composition, concentration of charge carriers as well as their recombination due to lattice point defects.

Additives in Nanocrystalline Tin Dioxide: Recent Progress in the Characterization of Materials for Gas Sensor Applications.

Tin dioxide has huge potential and is widely studied and used in different fields, including as a sensitive material in semiconductor gas sensors. The specificity of the chemical activity of tin dioxide in its interaction with the gas phase is achieved via the immobilization of various modifiers on the SnO2 surface. The type of additive, its concentration, and the distribution between the surface and the volume of SnO2 crystallites have a significant effect on semiconductor gas sensor characteristics, namely sensitivity and selectivity. This review discusses the recent approaches to analyzing the composition of SnO2-based nanocomposites (the gross quantitative elemental composition, phase composition, surface composition, electronic state of additives, and mutual distribution of the components) and systematizes experimental data obtained using a set of analytical methods for studying the concentration of additives on the surface and in the volume of SnO2 nanocrystals. The benefits and drawbacks of new approaches to the high-accuracy analysis of SnO2-based nanocomposites by ICP MS and TXRF methods are discussed.

Effect of Manganese Distribution on Sensor Properties of SnO2/MnOx Nanocomposites.

Nanocomposites SnO2/MnOx with various manganese content (up to [Mn]/[Sn] = 10 mol. %) and different manganese distribution were prepared by wet chemical technique and characterized by X-ray diffraction, scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis and mapping, IR and Raman spectroscopy, total reflection X-ray fluorescence, mass-spectrometry with inductive-coupled plasma (ICP-MS), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) spectroscopy. A different distribution of manganese between the volume and the surface of the SnO2 crystallites was revealed depending on the total Mn concentration. Furthermore, the identification of surface MnO2 segregation was performed via Raman spectroscopy. There is a strong dependence of the sensor signal toward CO and, especially, NO) on the presence of MnO2 surface segregation. However, manganese ions intruding the SnO2 crystal structure were shown to not almost effect on sensor properties of the material.

Highly Active Nanocrystalline ZnO and Its Photo-Oxidative Properties towards Acetone Vapor.

Zinc oxide is one of the well-known photocatalysts, the potential applications of which are of great importance in photoactivated gas sensing, water and air purification, photocatalytic synthesis, among others. However, the photocatalytic performance of ZnO strongly depends on its morphology, composition of impurities, defect structure, and other parameters. In this paper, we present a route for the synthesis of highly active nanocrystalline ZnO using commercial ZnO micropowder and ammonium bicarbonate as starting precursors in aqueous solutions under mild conditions. As an intermediate product, hydrozincite is formed with a unique morphology of nanoplates with a thickness of about 14-15 nm, the thermal decomposition of which leads to the formation of uniform ZnO nanocrystals with an average size of 10-16 nm. The synthesized highly active ZnO powder has a mesoporous structure with a BET surface area of 79.5 ± 4.0 m2/g, an average pore size of 20 ± 2 nm, and a cumulative pore volume of 0.507 ± 0.051 cm3/g. The defect-related PL of the synthesized ZnO is represented by a broad band with a maximum at 575 nm. The crystal structure, Raman spectra, morphology, atomic charge state, and optical and photoluminescence properties of the synthesized compounds are also discussed. The photo-oxidation of acetone vapor over ZnO is studied by in situ mass spectrometry at room temperature and UV irradiation (λmax = 365 nm). The main products of the acetone photo-oxidation reaction, water and carbon dioxide, are detected by mass spectrometry, and the kinetics of their release under irradiation are studied. The effect of morphology and microstructure on the photo-oxidative activity of ZnO samples is demonstrated.

Sensitization of ZnO Photoconductivity in the Visible Range by Colloidal Cesium Lead Halide Nanocrystals.

In this work, colloidal perovskite nanocrystals (PNCs) are used to sensitize the photoconductivity of nanocrystalline ZnO films in the visible range. Nanocrystalline ZnO with a crystallite size of 12-16 nm was synthesized by precipitation of a zinc basic carbonate from an aqueous solution, followed by annealing at 300 °C. Perovskite oleic acid- and oleylamine-capped CsPbBr3, CsPb(Cl/Br)3 and CsPb(Br/I)3 PNCs with a size of 6-13 nm were synthesized by a hot injection method at 170 °C in 1-octadecene. Photoconductive nanocomposites were prepared by applying a hexane sol of PNCs to a thick (100 µm) polycrystalline conductive ZnO layer. The spectral dependence of the photoconductivity, the dependence of the photoconductivity on irradiation, and the relaxation of the photoconductivity of the obtained nanocomposites have been studied. Sensitization of ZnO by CsPbBr3 and CsPb(Cl/Br)3 PNCs leads to enhanced photoconductivity in the visible range, the maximum of which is observed at 460 and 500 nm, respectively; close to the absorption maximum of PNCs. Nanocomposites ZnO/CsPb(Br/I)3 turned out to be practically not photosensitive when irradiated with light in the visible range. The data obtained are discussed in terms of the position of the energy levels of ZnO and PNCs and the probable PNCs photodegradation. The structure, morphology, composition, and optical properties of the synthesized nanocrystals have also been studied by XRD, TEM, and XPS. The results can be applied to the creation of artificial neuromorphic systems in the visible optical range.

Electrospun ZnO/Pd Nanofibers as Extremely Sensitive Material for Hydrogen Detection in Oxygen Free Gas Phase.

The development of safety sensors is an urgent necessity for the successful use of hydrogen in real conditions, which may differ, in particular, by the oxygen content in the surrounding atmosphere. Palladium-modified zinc oxide shows the high sensitivity when detecting hydrogen in air; however, studies of the sensor properties and the operation mechanism of the ZnO/Pd sensor when reducing gases are detected in an oxygen deficient or inert atmosphere have not been effectuated. In this work, we synthesized the ZnO and ZnO/Pd nanofibers by electrospinning and for the first time determined their sensor properties in the detection of CO, NH3 and H2 in different oxygen backgrounds. The microstructure and composition of nanofibers were characterized by electron microscopy, X-ray diffraction, X-ray fluorescent spectroscopy, and X-ray photoelectron spectroscopy. The interaction with the gas phase was investigated in situ by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). The sensor properties of ZnO and ZnO/Pd nanofibers were studied at 100-450 °C towards CO, NH3 and H2 in the N2/O2 gas mixtures containing 0.0005-20% O2. When detecting CO, a decrease in the oxygen concentration from 20 to 0.0005% in the gas phase does not lead to a significant change in the sensor response. At the same time, when detecting NH3 and especially H2, a decrease in oxygen concentration down to 0.0005% results in the dramatic increase in the sensor response of ZnO/Pd nanofibers. This result is discussed in terms of palladium hydride formation, modulation of the potential barrier at the ZnO/Pd interface, as well as changes in the concentration of donor defects and charge carriers in the ZnO matrix. Synthesized electrospun ZnO/Pd nanofibers are extremely promising materials for sensors for detecting hydrogen in an oxygen free atmosphere.

SnS2 Nanosheets as a Template for 2D SnO2 Sensitive Material: Nanostructure and Surface Composition Effects.

Two-dimensional nanosheets of semiconductor metal oxides are considered as promising for use in gas sensors, because of the combination of a large surface-area, high thermal stability and high sensitivity, due to the chemisorption mechanism of gas detection. In this work, 2D SnO2 nanosheets were synthesized via the oxidation of template SnS2 nanosheets obtained by surfactant-assisted one-pot solution synthesis. The 2D SnO2 was characterized using transmission and scanning electron microscopy (TEM, SEM), X-ray diffraction (XRD), low-temperature nitrogen adsorption, X-ray photoelectron spectroscopy (XPS) and IR spectroscopy. The sensor characteristics were studied when detecting model gases CO and NH3 in dry (RH25 = 0%) and humid (RH25 = 30%) air. The combination of high specific-surface-area and increased surface acidity caused by the presence of residual sulfate anions provides a high 2D SnO2 sensor's signal towards NH3 at a low temperature of 200 °C in dry air, but at the same time causes an inversion of the sensor response when detecting NH3 in a humid atmosphere. To reveal the processes responsible for sensor-response inversion, the interaction of 2D SnO2 with ammonia was investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in dry and humid air at temperatures corresponding to the maximum "positive" and maximum "negative" sensor response.