Multivariate Analysis of Light-Activated SMOX Gas Sensors.

Chemoresistive gas sensors made from SnO2, ZnO, WO3, and In2O3 have been prepared by flame spray pyrolysis. The sensors' response to CO and NO2 in darkness and under illumination at different wavelengths, using commercially available LEDs, was investigated. Operation at room temperature turned out to be impractical due to the condensation of water inside the porous sensing layers and the irreversible changes it caused. Accordingly, for sensors operated at 70 °C, a characterization procedure was developed and proven to deliver consistent data. The resulting data set was so complex that usual univariate data analysis was intricate and, consequently, was investigated by correlation and principal component analysis. The results show that light of different wavelengths affects not only the resistance of each material, both under exposure to the target gases in humidity and in its absence, but also the sensor response to humidity and the target gases. It was found that each of the materials behaves differently under light exposure, and it was possible to identify conditions that need further investigations.

Modeling the Conduction Mechanism in Chemoresistive Gas Sensor Based on Single-Crystalline Sn3O4 Nanobelts: A Phenomenological In Operando Investigation.

Investigating the sensing mechanisms in semiconducting metal oxide (SMOx) gas sensors is essential for optimizing their performance across a wide range of potential applications. Despite significant progress in the field, there are still many gaps in comprehending the phenomenological processes occurring in one-dimensional (1D) nanostructures. This article presents the first insights into the conduction mechanism of chemoresistive gas sensors based on single-crystalline Sn3O4 nanobelts using the operando Kelvin Probe technique. From this approach, direct current (DC) electrical resistance and work function changes were simultaneously measured in different working conditions, and a correlation between the conductance and the surface band bending was established. Appropriate modeling was proposed, and the results revealed that the conduction mechanism in the single-crystalline one-dimensional nanostructures closely aligns with the behavior observed in single-crystalline epitaxial layers rather than in polycrystalline grains. Based on this assumption, relevant parameters were further estimated, including Debye length, concentration of free charge carriers, effective density of states in the conduction band, and position of the Fermi level. Overall, this study provides an effective contribution to understanding the role of surface chemistry in the transduction of the electrical signal generated from gas adsorption in single-crystalline one-dimensional nanostructures.

In Operando Investigation of the Concentration Dependent NO2 Sensing Mechanism of Bi2S3 Nanorods at Low Temperatures and the Interference of O3.

The demand for gas sensors that can detect gases selectively at low temperatures has increased steadily over recent years. Most devices use semiconducting metal oxides as sensing materials which often require high operation temperatures and suffer from a lack of selectivity. Semiconducting metal sulfides were found to be a reasonable alternative for the application in sensing devices at low temperatures. Since metal sulfides are a relatively new class of materials applied in gas sensors, there is little work on sensing mechanisms and overall sensing characteristics of these materials. In this work, the authors investigated the sensing performance of Bi2S3 nanorods operated at 50 °C in the presence of several target gases and found a selective response to oxidizing gases. With the help of DC resistance measurements, diffuse reflectance infrared Fourier transform spectroscopy and work function measurements in a Kelvin Probe setup, the NO2 and O3 sensing mechanisms of Bi2S3 nanorods were revealed. While initially sulfur vacancies were the predominant reaction sites, the formation of nitrates became the key reaction in higher NO2 concentrations. Additionally, it was found that the reaction with O3 healed sulfur vacancies effectively inhibiting the reaction with NO2.

Role of Heterojunctions of Core-Shell Heterostructures in Gas Sensing.

Heterostructures made from metal oxide semiconductors (MOS) are fundamental for the development of high-performance gas sensors. Since their importance in real applications, a thorough understanding of the transduction mechanism is vital, whether it is related to a heterojunction or simply to the shell and core materials. A better understanding of the sensing response of heterostructured nanomaterials requires the engineering of heterojunctions with well-defined core and shell layers. Here, we introduce a series of prototypes CNT-nMOS, CNT-pMOS, CNT-pMOS-nMOS, and CNT-nMOS-pMOS hierarchical core-shell heterostructures (CSHS) permitting us to directly relate the sensing response to the MOS shell or to the p-n heterojunction. The carbon nanotubes are here used as highly conductive substrates permitting operation of the devices at relatively low temperature and are not involved in the sensing response. NiO and SnO2 are selected as representative p- and n-type MOS, respectively, and the response of a set of samples is studied toward hydrogen considered as model analyte. The CNT-n,pMOS CSHS exhibit response related to the n,pMOS-shell layer. On the other hand, the CNT-pMOS-nMOS and CNT-nMOS-pMOS CSHS show sensing responses, which in certain cases are governed by the heterojunctions between nMOS and pMOS and strongly depends on the thickness of the MOS layers. Due to the fundamental nature of this study, these findings are important for the development of next generation gas sensing devices.

Proof of Concept for Operando Infrared Spectroscopy Investigation of Light-Excited Metal Oxide-Based Gas Sensors.

Light-excitation of semiconducting metal-oxide (SMOX)-based gas sensors is a promising means to lower their operation temperature, thereby reducing power consumption, which would allow for their broader application. Despite increased research interest in light-excited gas sensors, progress has been slow because of a lack of mechanistic understanding. Notably, significant differences between light-excitation and, the better understood, thermal-excitation mechanisms have been identified. Insights from operando spectroscopic studies have been key to understanding the surface chemistry that determines the performance of thermally activated SMOX, but they have not yet been performed on illuminated sensors. Here, for the first time, we demonstrate that it is possible to perform operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurements on sensors under illumination. We demonstrate the benefits of the approach and show that under light illumination the splitting of water on the WO3 surface is responsible for the increased resistance of the sensor during exposure to high humidity.

Density Functional Investigation on α-MoO3 (100): Amines Adsorption and Surface Chemistry.

The (100) surface of α-MoO3 should possess overwhelmingly more exposed Mo atoms than the (010), and the exposed Mo has been extensively considered as an active site for amine adsorption. However, α-MoO3 (100) has drawn little attention concerning the amine sensing mechanism. In this research, adsorption of ammonia (NH3), monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) is systematically investigated by density functional theory (DFT). All four of these molecules have high affinity to α-MoO3 (100) through interaction between the N and the exposed Mo, and the affinity is mainly influenced by both the characteristics of the molecules and the geometric environment of the surface active site. Adsorption and dissociation of water and oxygen molecule on stoichiometric and defective α-MoO3 (100) surfaces are then simulated to fully understand the surface chemistry of α-MoO3 (100) in practical conditions. At low temperature, α-MoO3 (100) must be covered with a large number of water molecules; the water can desorb or dissociate into hydroxyl groups at high temperature. Oxygen vacancy (VO) can be generated through the annealing process during sensor device fabrication; VO must be filled with an O2 molecule, which can further interact with adsorbed water nearby to form hydroxyl groups. According to this research, α-MoO3 (100) must be the active surface for amine sensing and its surface chemistry is well understood. In the near future, further reaction and interaction will be simulated at α-MoO3 (100), and much more attention should be paid to α-MoO3 (100) not only theoretically but also experimentally.

The Role of Different Lanthanoid and Transition Metals in Perovskite Gas Sensors.

Beginning with LaFeO3, a prominent perovskite-structured material used in the field of gas sensing, various perovskite-structured materials were prepared using sol-gel technique. The composition was systematically modified by replacing La with Sm and Gd, or Fe with Cr, Mn, Co, and Ni. The materials synthesized are comparable in grain size and morphology. DC resistance measurements performed on gas sensors reveal Fe-based compounds solely demonstrated effective sensing performance of acetylene and ethylene. Operando diffuse reflectance infrared Fourier transform spectroscopy shows the sensing mechanism is dependent on semiconductor properties of such materials, and that surface reactivity plays a key role in the sensing response. The replacement of A-site with various lanthanoid elements conserves surface reactivity of AFeO3, while changes at the B-site of LaBO3 lead to alterations in sensor surface chemistry.

Effects of Gas Adsorption Properties of an Au-Loaded Porous In2O3 Sensor on NO2-Sensing Properties.

Gas adsorption properties of semiconductor-type gas sensors using porous (pr-) In2O3 powders loaded with and without 0.5 wt % Au (Au/pr-In2O3 and pr-In2O3 sensors, respectively) at 100 °C were examined by using diffuse reflectance infrared Fourier transform spectroscopy, and the effect of the Au loading onto pr-In2O3 on the NO2-sensing properties were discussed in this study. We found the following: the resistance of the Au/pr-In2O3 sensor in dry air is lower than that of the pr-In2O3 sensor; the DRIFT spectra of both the sensors show a broad positive band between 1600 and 1000 cm-1 in dry air (reference: in dry N2 at 100 °C), which mainly originates from oxygen adsorbates and/or lattice oxygen, and that this band is much larger for the Au/pr-In2O3 sensor than for the pr-In2O3 sensor; the Au loading also increases the adsorption amount of H2O and the reactivity of NO2 on the pr-In2O3 surface; and the NO2 response of the Au/pr-In2O3 sensor in dry air is marginally higher than that of the pr-In2O3 sensor in the examined concentration range of NO2 (0.6-5 ppm) in dry air. The obtained results strongly support the enhancement of the NO2 adsorption onto the pr-In2O3 surface by Au loading, which contributed to the improvement of the NO2-sensing properties.

Investigations on the Temperature-Dependent Interaction of Water Vapor with Tin Dioxide and Its Implications on Gas Sensing.

This work presents an operando infrared spectroscopic study of the temperature-dependent water adsorption on pristine SnO2 surfaces and discusses the possible implications on the oxygen ionosorption and gas-sensing mechanism. The impact of water on the sensor resistance, CO-sensing performance, and CO conversion was studied, and the obtained phenomenological results provide the basis for discussing the operando spectroscopic investigation findings. The provided information allows identification of three different water adsorption regimes ranging from physisorption and dominant associative adsorption to mainly dissociative water adsorption. In these regions, water has different impacts on the surface composition, sensor resistance, and gas-sensing performance.

Chemoresistive CO2 Gas Sensors Based On La2O2CO3: Sensing Mechanism Insights Provided by Operando Characterization.

Our previous studies demonstrated that rare-earth oxycarbonates Ln2O2CO3 (Ln = La, Nd, and Sm) and rare-earth oxides Ln2O3 (Ln = Nd, Sm, Gd, Dy, Er, and Yb) are sensitive to CO2 and that hexagonal La2O2CO3 is the best among them in terms of sensitivity, stability, and selectivity. In this study, we have conducted a comprehensive operando characterization on a hexagonal La2O2CO3 based sensor for the basic understanding of the sensing mechanism. This was done by performing under actual operating conditions simultaneous DC resistance and work function changes measurements, AC impedance spectroscopy measurements, and simultaneous DC resistance and DRIFT spectroscopy measurements. The results demonstrate that the double Schottky barriers at grain-grain boundaries are dominant contribution to sensor resistance; there is a competitive adsorption between carbonate species and hydroxyl groups, which depends on both CO2 concentration and humidity and leads to the change in height of the Schottky barriers. Finally, we propose a reaction model stating that there are three types of adsorbates, -CO32-, -OH-, and -O2-, and the relative concentration of which is controlled by a reaction with ambient humidity and CO2. This model is able to consistently explain all our experimental findings.

Dominant Role of Heterojunctions in Gas Sensing with Composite Materials.

It is well known that composite materials, consisting of at least two metal oxides, show qualities and sensing behavior very different from the single components. Recently, the preparation of core-shell nanomaterials for gas sensors has become extremely popular. Specifically, these materials have been found to show desirable sensor responses. The preparation of core-shell nanomaterials is, however, complex, limiting the commercial applicability. Composite materials can be more easily attained simply through the mechanical mixing of the various components. Although some studies exist that attempt to compare mechanically mixed composites to those prepared via a synthetic route, these examinations are often flawed, as due to varying preparation methods, the basic characteristics of the materials are not the same. Here, it was possible to separate the role of the contacts between the materials from that of the secondary core-shell structure, by using a soft method to mechanically break apart the structure. This ensures that the difference in morphology is the only change in the material characteristics. It was verified that the composite materials show a different sensing behavior from that of the pure materials. It was also found that regardless of the secondary structure, the composite materials showed very similar sensor responses. By examining materials containing different ratios of Cr2O3 to SnO2, it was possible to attribute the sensor behavior changes to the contacts between the different metal oxides. It was shown that by varying the concentration of each oxide it is possible to attain either an n- or p-type response and at a certain concentration even no response. This work is significant because it identifies that the contact between the materials plays the dominant role in the sensor response and it shows the viability of mechanical mixing for composite sample preparation.

WO3-Based Gas Sensors: Identifying Inherent Qualities and Understanding the Sensing Mechanism.

Semiconducting metal oxide-based gas sensors are an attractive option for a wide array of applications. In particular, sensors based on WO3 are promising for applications varying from indoor air quality to breath analysis. There is a great breadth of literature which examines how the sensing characteristics of WO3 can be tuned via changes in, for example, morphology or surface additives. Because of variations in measurement conditions, however, it is difficult to identify inherent qualities of WO3 from these reports. Here, the sensing behavior of five different WO3 samples is examined. The samples show good complementarity to SnO2 (the most commonly used material)-based sensors. A surprising homogeneity, despite variation in morphology and preparation method, is found. Using operando diffuse reflectance infrared Fourier transform spectroscopy, it is found that the oxygen vacancies are the dominant reaction partner of WO3 with the analyte gas. This surface chemistry is offered as an explanation for the homogeneity of WO3-based sensors.

Direct Microscopic Proof of the Fermi Level Pinning Gas-Sensing Mechanism: The Case of Platinum-Loaded WO3.

It is widely known that the sensing characteristics of metal oxides are drastically changed through noble metal oxide surface additives. Using operando infrared spectroscopy it was identified that the Fermi level pinning mechanism dominates the sensor response of platinum-loaded WO3. Spectroscopy, however, provides information about the sample only on average. Traditional microscopy offers structural information but is typically done in vacuum and on unheated sensors, very different than the operation conditions of metal oxide gas sensors. Here, state-of-the-art in situ scanning transmission electron microscopy offers spatially resolved information on heated samples at atmospheric pressure in varying gas atmospheres. As a result it was possible to directly couple microscopically observed structural changes in the surface noble metal nanoclusters with IR spectra and sensor responses. On the basis of the findings, the dominant Fermi level pinning mechanism could be validated. The presented work demonstrates the benefits of coupling in situ microscopy with operando spectroscopy in order to elucidate the sensing mechanism of metal oxides.

Current Understanding of the Fundamental Mechanisms of Doped and Loaded Semiconducting Metal-Oxide-Based Gas Sensing Materials.

Introducing additives in semiconducting metal oxides includes, besides the use of filters, dynamic operation procedures and chemometric approaches, the most common way of tuning the sensitivity, selectivity, and stability of chemoresitsive gas sensors. For the vast majority of commercially used gas sensing materials, the introduction of additives is essential and is one of the longest lasting topics in gas sensor research. This Review discusses the different chemical and electrical sensitization mechanisms of additives as well as the role of different structures. Based on state-of-the-art experimental findings, this Review revises and updates the concepts that are used to explain the mechanisms through which the additives influence the performance of typical gas sensing materials, i.e., oxide nanoparticles arranged in a porous layer. The first sections classify the different additive structures, namely, doped or loaded oxides as well as mixtures of oxides, and describe the basic working principle of pristine semiconducting metal oxide gas sensors. The subsequent sections discuss different chemical and/or electrical contributions to the sensitization by additive structures, their mutual influence on each other, and the way they impact the sensing properties. The presented concepts and models are essential for understanding the complex role of additives and provide the basis for a knowledge-based design of gas sensors based on semiconducting metal oxide nanoparticles, which is outlined in a separate section.

Conductance Model for Single-Crystalline/Compact Metal Oxide Gas-Sensing Layers in the Nondegenerate Limit: Example of Epitaxial SnO2(101).

Semiconducting metal oxide (SMOX)-based gas sensors are indispensable for safety and health applications, for example, explosive, toxic gas alarms, controls for intake into car cabins, and monitor for industrial processes. In the past, the sensor community has been studying polycrystalline materials as sensors where the porous and random microstructure of the SMOX does not allow a separation of the phenomena involved in the sensing process. This led to conduction models that can model and predict the behavior of the overall response, but they were not capable of giving fundamental information regarding the basic mechanisms taking place. The study of epitaxial layers is a definite improvement, allowing clarifying the different aspects and contributions of the sensing mechanisms. A detailed analytical model of the transduction function for n- and p-type single-crystalline/compact metal oxide gas sensors was developed that directly relates the conductance of the sample with changes in the surface electrostatic potential. Combined dc resistance and work function measurements were used in a compact SnO2(101) layer in operando conditions that allowed us to check the validity of our model in the region where Boltzmann approximation holds to determine the surface and bulk properties of the material.

Rhodium Oxide Surface-Loaded Gas Sensors.

In order to increase their stability and tune-sensing characteristics, metal oxides are often surface-loaded with noble metals. Although a great deal of empirical work shows that surface-loading with noble metals drastically changes sensing characteristics, little information exists on the mechanism. Here, a systematic study of sensors based on rhodium-loaded WO3, SnO2, and In2O3-examined using X-ray diffraction, high-resolution scanning transmission electron microscopy, direct current (DC) resistance measurements, operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and operando X-ray absorption spectroscopy-is presented. Under normal sensing conditions, the rhodium clusters were oxidized. Significant evidence is provided that, in this case, the sensing is dominated by a Fermi-level pinning mechanism, i.e., the reaction with the target gas takes place on the noble-metal cluster, changing its oxidation state. As a result, the heterojunction between the oxidized rhodium clusters and the base metal oxide was altered and a change in the resistance was detected. Through measurements done in low-oxygen background, it was possible to induce a mechanism switch by reducing the clusters to their metallic state. At this point, there was a significant drop in the overall resistance, and the reaction between the target gas and the base material was again visible. For decades, noble metal loading was used to change the characteristics of metal-oxide-based sensors. The study presented here is an attempt to clarify the mechanism responsible for the change. Generalities are shown between the sensing mechanisms of different supporting materials loaded with rhodium, and sample-specific aspects that must be considered are identified.

Temperature-Dependent NO2 Sensing Mechanisms over Indium Oxide.

The surface species responsible for NO2 gas sensing over indium oxide was studied by operando DRIFTS coupled to a multivariate spectral analysis. It revealed the important roles of surface nitrites on the temperature-dependent gas sensing mechanism and the interaction of such nitrites with surface hydroxyls. A highly hydroxylated surface with high concentration of surface adsorbed H2O is beneficial to enhance the concentration of adsorbed NO2, present as nitrites, thus explaining superior sensing response at lower operating temperatures.

Response of Gallium Nitride Chemiresistors to Carbon Monoxide is Due to Oxygen Contamination.

We report on the influence of oxygen impurities on the gas sensing properties of gallium nitride (GaN) chemiresistors. As shown by XRD, elemental analysis, and TEM characterization, surface oxidation of GaN-for example, upon contact to ambient air atmosphere-creates an oxidative amorphous layer which provides the sites for the sensing toward CO. Treating this powder under dry ammonia at 800 °C converts the oxide layer in nitride, and consequently the sensing performance toward CO is dramatically reduced for ammonia treated GaN gas sensors. Hence the response of GaN sensors to CO is caused by oxygen in the form of amorphous surface oxide or oxynitride.

Understanding the Potential of WO3 Based Sensors for Breath Analysis.

Tungsten trioxide is the second most commonly used semiconducting metal oxide in gas sensors. Semiconducting metal oxide (SMOX)-based sensors are small, robust, inexpensive and sensitive, making them highly attractive for handheld portable medical diagnostic detectors. WO3 is reported to show high sensor responses to several biomarkers found in breath, e.g., acetone, ammonia, carbon monoxide, hydrogen sulfide, toluene, and nitric oxide. Modern material science allows WO3 samples to be tailored to address certain sensing needs. Utilizing recent advances in breath sampling it will be possible in the future to test WO3-based sensors in application conditions and to compare the sensing results to those obtained using more expensive analytical methods.

Selectivity Enhancement by Using Double-Layer MOX-Based Gas Sensors Prepared by Flame Spray Pyrolysis (FSP).

Here we present a novel concept for the selective recognition of different target gases with a multilayer semiconducting metal oxide (SMOX)-based sensor device. Direct current (DC) electrical resistance measurements were performed during exposure to CO and ethanol as single gases and mixtures of highly porous metal oxide double- and single-layer sensors obtained by flame spray pyrolysis. The results show that the calculated resistance ratios of the single- and double-layer sensors are a good indicator for the presence of specific gases in the atmosphere, and can constitute some building blocks for the development of chemical logic devices. Due to the inherent lack of selectivity of SMOX-based gas sensors, such devices could be especially relevant for domestic applications.