Chemoresistive Gas Sensors Research Articles

Dataset of the Optimization of a Low Power Chemoresistive Gas Sensor: Predictive Thermal Modelling and Mechanical Failure Analysis

Over the last few years, employment of the standard silicon microfabrication techniques for the gas sensor technology has allowed for the development of ever-small, low-cost, and low-power consumption devices. Specifically, the development of silicon microheaters (MHs) has become well established to produce MOS gas sensors. Therefore, the development of predictive models that help to define a priori the optimal design and layout of the device have become crucial, in order to achieve both low power consumption and high mechanical stability. In this research dataset, we present the experimental data collected to develop a specific and useful predictive thermal-mechanical model for high performing silicon MHs. To this aim, three MH layouts over three different membrane sizes were developed by using the standard silicon microfabrication process. Thermal and mechanical performances of the produced devices were experimentally evaluated, by using probe stations and mechanical failure analysis, respectively. The measured thermal curves were used to develop the predictive thermal model towards low power consumption. Moreover, a statistical analysis was finally introduced to cross-correlate the mechanical failure results and the thermal predictive model, aiming at MH design optimization for gas sensing applications. All the data collected in this investigation are shown.

Chemoresistive gas-sensing properties of highly dispersed Nb2O5 obtained by programmable precipitation

Abstract Nb2O5 powder obtained by programmable precipitation was used to form a thick gas-sensing film as part of a chemoresistive gas sensor, by screen-printing. The coating of orthorhombic Nb2O5 consisted of nanoparticles with a size of 41.0 ± 2.5 nm. XPS revealed Nb5+, Nb4+ and Nb2+ as well as oxygen vacancies in the crystal structure of niobium oxide. As a result of studying the chemoresistive gas-sensing properties of Nb2O5, it has been shown that among the analysed gases (H2, CO, NH3, H2S and О2), the greatest sensitivity was observed for oxygen and hydrogen sulphide. Nanocrystalline niobium oxide showed a high and reproducible response to 0.02–20% О2 (S1 = 1.1–19.0) at a very low detection temperature of 200 °C for oxygen sensors. At an operating detection temperature of 250 °C, a high and reproducible response to low concentrations of hydrogen sulphide of 4–100 ppm (S2 = 1.2–6.6) was detected for Nb2O5. The influence of humidity on the received signals when detecting oxygen and hydrogen sulphide was studied in detail: there was a decrease in the resistance and the response value at 95% humidity in the medium of both gases. However, unlike the process of detecting H2S (when the response of S2 was almost lost), when determining oxygen, the response of S1 was reduced by a factor of two only, which suggests the possibility of determining the content of O2 in high humidity conditions.

Metal oxide-based gas sensors for the detection of exhaled breath markers.

Exhaled breath test is a typical disease monitoring method for replacing blood and urine samples that may create discomfort for patients. To monitor exhaled breath markers, gas biomedical sensors have undergone rapid progress for non‐invasive and point‐of‐care diagnostic devices. Among gas sensors, metal oxide‐based biomedical gas sensors have received remarkable attention owing to their unique properties, such as high sensitivity, simple fabrication, miniaturization, portability and real‐time monitoring. Herein, we reviewed the recent advances in chemoresistive metal oxide‐based gas sensors with ZnO, SnO2 and In2O3 as sensing materials for monitoring a range of exhaled breath markers (i.e., NO, H2, H2S, acetone, isoprene and formaldehyde). We focused on the strategies that improve the sensitivity and selectivity of metal oxide‐based gas sensors. The challenges to fabricate a functional gas sensor with high sensing performance along with suggestions are outlined.

Optimization of a Low-Power Chemoresistive Gas Sensor: Predictive Thermal Modelling and Mechanical Failure Analysis.

The substrate plays a key role in chemoresistive gas sensors. It acts as mechanical support for the sensing material, hosts the heating element and, also, aids the sensing material in signal transduction. In recent years, a significant improvement in the substrate production process has been achieved, thanks to the advances in micro- and nanofabrication for micro-electro-mechanical system (MEMS) technologies. In addition, the use of innovative materials and smaller low-power consumption silicon microheaters led to the development of high-performance gas sensors. Various heater layouts were investigated to optimize the temperature distribution on the membrane, and a suspended membrane configuration was exploited to avoid heat loss by conduction through the silicon bulk. However, there is a lack of comprehensive studies focused on predictive models for the optimization of the thermal and mechanical properties of a microheater. In this work, three microheater layouts in three membrane sizes were developed using the microfabrication process. The performance of these devices was evaluated to predict their thermal and mechanical behaviors by using both experimental and theoretical approaches. Finally, a statistical method was employed to cross-correlate the thermal predictive model and the mechanical failure analysis, aiming at microheater design optimization for gas-sensing applications.

Detecting methanol in hand sanitizers

SummaryThe coronavirus disease 2019 (COVID-19) pandemic has increased dramatically the demand for hand sanitizers. A major concern is methanol adulteration that caused more than 700 fatalities in Iran and U.S.A. (since February 2020). In response, the U.S. Food and Drug Administration has restricted the methanol content in sanitizers to 0.063 vol% and blacklisted 212 products (as of November 20, 2020). Here, we present a low-cost, handheld, and smartphone-assisted device that detects methanol selectively in sanitizers between 0.01 and 100 vol% within two minutes. It features a nanoporous polymer column that separates methanol selectively from confounders by adsorption. A chemoresistive gas sensor detects the methanol. When tested on commercial sanitizers (total 76 samples), methanol was quantified in excellent (R2 = 0.99) agreement to “gold standard” gas chromatography. Importantly, methanol quantification was hardly interfered by sanitizer composition and viscosity. This device meets an urgent need for on-site methanol screening by authorities, health professionals, and even laymen.

Screening Methanol Poisoning with a Portable Breath Detector.

Methanol poisoning outbreaks after consumption of adulterated alcohol frequently overwhelm health care facilities in developing countries. Here, we present how a recently developed low-cost and handheld breath detector can serve as a noninvasive and rapid diagnostic tool for methanol poisoning. The detector combines a separation column and a micromachined chemoresistive gas sensor fully integrated into a device that communicates wirelessly with a smartphone. The performance of the detector is validated with methanol-spiked breath of 20 volunteers (105 breath samples) after consumption of alcoholic beverages. Breath methanol concentrations were quantified accurately within 2 min in the full breath-relevant range (10-1000 ppm) in excellent agreement (R2 = 0.966) with benchtop mass spectrometry. Bland-Altman analysis revealed sufficient limits of agreement (95% confidence intervals), promising to indicate reliably the clinical need for antidote and hemodialysis treatment. This simple-in-use detector features high diagnostic capability for accurate measurement of methanol in spiked breath, promising for rapid screening of methanol poisoning and assessment of severity. It can be applied readily by first responders to distinguish methanol from ethanol poisoning and monitor in real time the subsequent hospital treatment.

Hydrogen Sensing of Graphene-based Chemoresistive Gas Sensor Enabled by Surface Decoration

Hydrogen (H2) is considered as a new clean energy resource for replacing petroleum because it produces only H2O after the combustion process. However, owing to its explosive nature, it is extremely important to detect H2 gas in the ambient atmosphere. This has triggered the development of H2 gas sensors. 2-dimensional (2D) graphene has emerged as one of the most promising candidates for chemical sensors in various industries. In particular, graphene exhibits outstanding potential in chemoresistive gas sensors for the detection of diverse harmful gases and the control of indoor air quality. Graphene-based chemoresistive gas sensors have attracted tremendous attention owing to their promising properties such as room temperature operation, effective gas adsorption, and high flexibility and transparency. Pristine graphene exhibits good sensitivity to NO2 gas at room temperature and relatively low sensitivity to H2 gas. Thus, research to control the selectivity of graphene gas sensors and improve the sensitivity to H2 gas has been performed. Noble metal decoration and metal oxide decoration on the surface of graphene are the most favored approaches for effectively controlling the selectivity of graphene gas sensors. Herein, we introduce several strategies that enhance the sensitivity of graphene gas sensors to H2 gas.

Nanostructured SmFeO3 Gas Sensors: Investigation of the Gas Sensing Performance Reproducibility for Colorectal Cancer Screening.

Among the various chemoresistive gas sensing properties studied so far, the sensing response reproducibility, i.e., the capability to reproduce a device with the same sensing performance, has been poorly investigated. However, the reproducibility of the gas sensing performance is of fundamental importance for the employment of these devices in on-field applications, and to demonstrate the reliability of the process development. This sensor property became crucial for the preparation of medical diagnostic tools, in which the use of specific chemoresistive gas sensors along with a dedicated algorithm can be used for screening diseases. In this work, the reproducibility of SmFeO3 perovskite-based gas sensors has been investigated. A set of four SmFeO3 devices, obtained from the same screen-printing deposition, have been tested in laboratory with both controlled concentrations of CO and biological fecal samples. The fecal samples tested were employed in the clinical validation protocol of a prototype for non-invasive colorectal cancer prescreening. Sensors showed a high reproducibility degree, with an error lower than 2% of the response value for the test with CO and lower than 6% for fecal samples. Finally, the reproducibility of the SmFeO3 sensor response and recovery times for fecal samples was also evaluated.

Ultrasensitive and light-activated NO2 gas sensor based on networked MoS2/ZnO nanohybrid with adsorption/desorption kinetics study

In this work, a high-performance room temperature NO2 chemoresistive gas sensor is fabricated using a MoS2/ZnO nanohybrid. The nanohybrid is synthesized by drop-casting liquid exfoliated MoS2 nanosheets on ZnO nanorods. The uniform network-like distribution of MoS2 nanosheets on the nanorods is confirmed by various characterization techniques. Under UV-activation, the nanohybrid sensor exhibits remarkable responses of 91% and 2310% at 5 and 500 ppb NO2, respectively. In addition, full recovery to the base resistance is observed in each sensing cycle. The low concentration sensitivity is 0.135 ppb−1 and the lowest detection limit is estimated to be around 0.2 ppb. These two values are superior when compared with other reports. The adsorption/desorption kinetics has been studied in detail using the Langmuir adsorption model. The saturated response, the adsorption, and the desorption constant are determined to be 2744%, 3.52 × 10–5 ppb−1 s−1, and 3.50 × 10–3 s−1, respectively. The outstanding performance of the sensor can be attributed to the synergetic effects of MoS2 and ZnO including creation of abundant adsorption sites and fast charge carrier migration.

A Skin‐Inspired Substrate with Spaghetti‐Like Multi‐Nanofiber Network of Stiff and Elastic Components for Stretchable Electronics

AbstractMimicking the skin's non‐linear self‐limiting mechanical characteristics is of great interest. Skin is soft at low strain but becomes stiff at high strain and thereby can protect human tissues and organs from high mechanical loads. Herein, the design of a skin‐inspired substrate is reported based on a spaghetti‐like multi‐nanofiber network (SMNN) of elastic polyurethane (PU) nanofibers (NFs) sandwiched between stiff poly(vinyldenefluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)) NFs layers embedded in polydimethylsiloxane elastomer. The elastic moduli of the stretchable skin‐inspired substrate can be tuned in a range that matches well with the mechanical properties of skins by adjusting the loading ratios of the two NFs. Confocal imaging under stretching indicates that PU NFs help maintain the stretchability while adding stiff P(VDF‐TrFE) NFs to control the self‐limiting characteristics. Interestingly, the Au layer on the substrate indicates a negligible change in the resistance under cyclic (up to 7000 cycles at 35% strain) and dynamic stretching (up to 35% strain), which indicates the effective absorption of stress by the SMNN. A stretchable chemoresistive gas sensor on the skin‐inspired substrate also demonstrates a reasonable stability in NO2 sensing response under strain up to 30%. The skin‐inspired substrate with SMNN provides a step toward ultrathin stretchable electronics.

CO2 sensing with gas sensors based on rare-earth compounds: Material exploration

Rare-earth oxycarbonates Ln2O2CO3 (Ln = rare-earth element) have been identified as materials for chemoresistive CO2 gas sensors. Among them, previous studies identified monoclinic La2O2CO3 as the best performing one. However, not all rare-earth elements have been investigated and, moreover, La2O2CO3 monoclinic phase is metastable and this can influence the long term performance. In this work, we have synthesized rare-earth oxycarbonates Ln2O2CO3 (Ln = La, Nd, and Sm) including monoclinic and hexagonal La2O2CO3, rare-earth oxides Ln2O3 (Ln = Nd, Sm, Gd, Dy, Er, and Yb) and LnO2 (Ln = Ce) by calcination of oxalate hydrate or the acetate hydrate precursors in air. All the materials, except for CeO2 and Nd2O3, were sensitive to CO2. All CO2 sensitive materials, except for monoclinic La2O2CO3 and Nd2O2CO3, were stable and their performance is sufficient for practical use. Hexagonal La2O2CO3 shows the best overall performance. The results of operando investigations indicate that the origin of CO2 sensing is the competitive adsorption between carbonates and hydroxyl groups.

Bridging or in-Plane Oxygen Vacancies for a High-Performance Chemoresistive Gas Sensors? a First-Principles Study

Introduction:The use of Density Functional Theory (DFT) simulations to predict the chemical-physical properties of nanostructured compounds and heterogeneous interactions (solid-gas) has become almost essential today, both to predict the properties of technologically advanced nanomaterials and to overcome the limits of experimental characterizations.Specifically, the use of DFT calculations can likewise be used in chemoresistive gas sensor applications, to investigate the physical-chemical properties of nanostructured semiconductors and their possible catalytic activity. Among the various gas sensing materials used, the Metal Oxide Semiconductors (MOXS) are the most investigated since the excellent sensitivity, good stability and low cost of production [1,2]. Nevertheless, besides all of these advantages they present also some drawbacks such as signal drift over the time, lack of selectivity and high operating temperature.Investigators have tried to address these problems in several manners, including the synthesis of solid solutions composed of different metal oxides, doping these materials with deferent kinds of elements and also with controlled concentrations of oxygen vacancies. The latter is a field still little explored, but preliminary experimental and theoretical works highlighted interesting results [3,4].When the sensor is exposed to a reducing (oxidizing) gas, the gas molecules interact with the chemisorbed oxygen species from the surrounding atmosphere and then decrease (increase) the resistance of a n-type based gas sensor, and vice versa for a p-type semiconductor. This change in resistance is due to the exchange of electrons between the surface and the conduction band. For a stoichiometric semiconductor, the electrons involved in this reaction that are in the conduction band are coming from the valence band after heating the material, in this case they need a high energy to blow up from the valence band to the conduction band. But when one introduces some defects, new energy level will be created and then they will be a new electrons source, thus the energy that the sensor needs to be thermoactivated decrease; that is energy that electrons need to jump to the conduction band. Scope and Methods: The purpose of this work is to investigate the impact of oxygen vacancies inside MOXS on their physical-chemical properties in general and on the electrons exchange in particular. Indeed, the creation of oxygen vacancies influences the physical-chemical properties of metal oxides. i.e. The bridging oxygen vacancies lead to the formation of energy level in the band gap region (Fig.1), these energy levels will behave as electrons donors that decrease the energy (ε1) needed for the electrons to be in the conduction band. While creation of oxygen vacancies from in-plane positions drives to the formation of impurity levels located slightly below the conduction band minimum (CBM), in this case the energy (ε2) that they need to jump to the conduction band is lower than ε1. Therefore, we can say that in-plane oxygen vacancies can overcome the disadvantage of high working temperatures for MOXS; as much as the electrons resource is close to the conduction band, as much as the ionization of these electron levels will be easy and then enhance the reactions (adsorption process) on the surface of the active element at low temperature.Tin dioxide is a typical n-type semiconducting material with a wide band gap of 3.6 eV. It has attracted the attention of many researchers because of its broad spectrum of physicochemical properties. It has also been used in several fields such as optoelectronic devices, electrocatalysis, ceramics and gas sensors. It is the semiconductor most studied as active element for the production of chemoresistive gas sensors. Then, it represents the best candidate for the proposed innovative work.In this work, DFT was used to compute the physical-chemical properties of the considered material. The Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA), which improve the LDA results taking into account the electron density gradient due to the non-homogeneity of the electron density, are the principal basis of the exchange-correlation functional within the framework of DFT. In this work, the GGA parameterized PBE (Perdew-Burke-Ernzerhof) has been used for structural properties prediction. This approximation has demonstrated, beside the LDA, their ability to predict the structural properties of materials. Nevertheless, their applications underestimate the band gap energy of materials compared to experimental results. To enhance the results concerning the band gap the Tran-Blaha’s modified Becke-Johnson exchange potential model (TB-mBJ) has been applied, this approximation has proved its ability to predict the band gap of materials.A series of first-principles studies has been carried out using the Full Potential Linearized Augmented Plane Wave (FPLAPW) method within the framework of the DFT, as implemented in the Wien2k code. This method is the most effective method to solve the Khon Sham equation.

Manganese Oxide Nanoarchitectures as Chemoresistive Gas Sensors to Monitor Fruit Ripening.

The efficient detection of low-concentration ethylene is a challenging issue of key importance for food quality control end-uses. Herein, we report on the fabrication of MnO₂-based nanoarchitectures by a two-step plasma-assisted process, consisting in the initial chemical vapor deposition of MnO₂ (host) on polycrystalline Al₂O₃ substrates and the subsequent functionalization with Ag and Au-based nanoparticles (guest) by sputtering processes. The resulting composites, characterized by a high Ag/Au dispersion and an effective host-guest contact, were tested for the first time as chemoresistive gas sensors for ethylene recognition at low temperatures. The high sensitivity and promising responses, enhanced by metal particle introduction, candidate the target systems as attractive platforms for the eventual monitoring of vegetables/fruits ripening and ageing.

Handheld Sensor for Point-of-Care Methanol Detection in Breath and Liquor

Handheld Sensor for Point-of-Care Methanol Detection in Breath and Liquor

Surface Leaching of Pd-Containing SnO2 Gas Sensors Enhances Sensitivity

Introduction Palladium is commonly used to enhance the performance of chemoresistive metal-oxide gas sensors [1]. Typically, this enhancement is attributed to the presence of Pd clusters on the surface of their metal oxide support (e.g. SnO2). More specific, small Pd clusters on SnO2 were associated to enhanced sensitivity to H2 due to fermi-level control [2] or spillover effects [1]. However, Pd may not be present in metallic but oxidic form, that is catalytically active as well [3]. E.g. PdO can form during fabrication or upon the typical annealing [4] at 400 - 700 °C [3] for a few hours prior to gas sensing. Furthermore, possible Pd incorporation or embedding into the support rarely has been considered [5].Here, SnO2 particles with different Pd contents were prepared by flame spray pyrolysis (FSP). Their surface Pd was removed through leaching with HNO3 to guaranty the surface and embedded Pd content was measured. Lastly, the influence of surface and embedded Pd on SnO2 gas sensors was evaluated with acetone, ethanol and CO at 350 °C and 50% relative humidity. Method Pristine and Pd-containing (0 - 3 mol%) SnO2 nanoparticles were produced by FSP and annealed for 5 h at 500 °C. To remove all Pd from the surface, the particles were reduced in 5% H2 in Ar at 150 °C for 30 min and then leached in 10% HNO3 in water at 60 °C for 4 h (Figure 1a). Inductively coupled plasma-optical emission spectrometry was used to determine the Pd content in the leached solution, that corresponds to the surface amount. Sensing films were prepared by doctor-blading the particles onto 15 × 13 × 0.8 mm Al2O3 sensor substrates followed by annealing at 300 °C for 30 min. These sensors were then heated to 350 °C and tested with different concentrations (5 - 1000 ppb) of acetone, ethanol and CO in a gas sensing setup described elsewhere [6]. Results and Conclusions Figure 1b displays the fraction of surface (or leachable) Pd over the nominal content of flame-made and annealed particles. Only a fraction of Pd content is on the surface whereas the rest is embedded in the bulk SnO2. For instance, at nominal 0.5 mol% Pd, only 30% of it is leached while at 3 mol% Pd it increased to 45%. Apparently, most Pd is embedded and thereby not exposed to the analytes during sensing.Figure 1c shows the sensor responses at 350 °C to 1 ppm of acetone at 50% RH of annealed Pd-containing SnO2 as a function of the nominal Pd content. Before leaching (triangles), the addition of small amounts of Pd (up to 0.2 mol%) slightly increases the responses to acetone (within the error bar of reproducibility). At higher Pd contents, the responses continuously deteriorate until they are hardly detectable anymore at 3 mol% Pd. The circles in Figure 1c show the analyte responses after leaching the surface Pd as a function of the nominal Pd content. While the responses of pristine SnO2 were hardly affected by leaching, surprisingly, adding only 0.2 mol% Pd almost doubled the acetone response from 4 to 7. Overall, SnO2 containing both surface & embedded Pd (i.e. prior to leaching, triangles) results in lower responses than after leaching (circles).We reveal that flame-made Pd-containing SnO2 contains a significant fraction of Pd embedded in the bulk of (and/or strongly surface-bonded to) SnO2. Furthermore, small amounts of embedded Pd nearly double the responses of SnO2 to acetone at 350 °C. In contrast, the presence of surface Pd deteriorates the sensor performance below that of pure SnO2. This might point out, that small amounts of noble metals embedded in metal oxides can be more effective than on their surface.

Selective Ammonia Detection at Ppb-Level with Si-Doped MoO3 Sensors for Food and Environmental Applications

Introduction Ammonia (NH3) is an important marker for food spoilage[1] as well as a toxic environmental pollutant emitted from concrete walls, fertilizers or refrigeration systems[2]. Therefore, low-cost compact sensors for the detection of hazardous NH3 are urgently needed. Metal-oxide (chemo-resistive) gas sensors are attractive for NH3 detection as they offer simple operation, low power consumption and can be integrated into portable devices.[3] In specific, the α-phase of MoO3 is promising for detection of NH3 down to 50 parts per billion (ppb) at dry conditions. However, due to the high operating temperatures of MoO3 gas sensors (250 to 500 °C), the material must be stabilized by doping or the addition of foreign oxides. Here, we show how tailoring material morphology, phase composition and particle size enables high NH3 sensing performance[4]. Method Pure and Si-doped MoO3 (0 – 20 wt%) nanoparticles were made by flame spray pyrolysis (FSP) and directly deposited onto Al2O3 sensor substrates with interdigitated Pt electrodes[4]. Subsequently, the sensor films were annealed at 450 °C for 5 h in an oven for thermal stabilization. The nanoparticles were analyzed using X-ray diffraction (XRD) patterns, nitrogen adsorption (Brunauer-Emmett-Teller, BET) as well as transmission electron microscopy (TEM) and electron diffraction (ED). The sensing performance was characterized as a function of SiO2 content with synthetic gas mixtures at a sensor temperature of 400 °C on a dynamic mixing setup. Furthermore, the selectivity towards possible interfering molecules of different chemical classes (e.g. acetone, CO and NO) was evaluated. Results and Conclusions The as-prepared, pure MoO3 powder shows fine crystallinity confirmed by transmission electron microscopy (TEM) (Figure 1a) and ED patterns (Figure 1b). By annealing the sensor substrates at 450 °C for 5 h, pure MoO x particles grow to ribbon-like structures (Figure 1d) with large crystallites, as indicated by XRD (triangles) and BET (open circles) and confirmed by bright spots in ED patterns (Figure 1c). Doping with Si reduces particle and crystal growth significantly, decreasing the crystal size from 147 to 65 nm and the particle size from 272 to 83 nm resulting in superior thermal stability. Excess Si (above 3 wt%) is not incorporated into the MoO3 lattice, but forms amorphous SiO2 domains, inhibiting further crystal growth, as subjected by the constant dXRD (Figure 1) and visible by TEM. Due to those structural changes, the sensor response and selectivity depend on the SiO2 doping content.Figure 2 shows the response of pure and Si-doped MoO3 sensors to 1000 ppb NH3 (triangles), acetone (circles), NO (squares) and CO (diamonds) at 400 °C. Increasing the SiO2 content from 0 to 3 wt% almost triples the NH3 response from 0.22 to 0.53 with only minor changes for the other analytes. This results from the reduced sinter neck size of MoO3 due to segregated SiO2 domains which locally narrow the conduction channel leading to an increased electron depletion and change the morphology resulting in a higher surface-to-volume ratio[4]. Therefore, charge carrier mobility and thus film resistance are dominated by surface phenomena increasing the sensor’s sensitivity. Moreover, Si-doping (3 wt%) improved the NH3 selectivity toward other interfering gases achieving a response ratio to acetone (S NH3 / SA ) of 3.6 and to NO of 7.9 and no sensitivity for CO (Figure 2). Above 3 wt% SiO2 content, the film resistance continuously increases (inset in Figure 2) due to the formation of large and inert SiO2 domains. In addition to the enhanced response and selectivity towards NH3, the sensor could clearly detect NH3 down to 400 ppb even in the presence realistic relative humidity (up to 90%)[4]. As a result, this sensor is promising for integration into a portable device for food spoilage detection and environmental monitoring.

Noble Metal/Mn3O4 Nanocomposite Systems:from Design to Applications As Ultra-Sensitive Gas Sensors for Nitrogen Mustard Simulant

The efficient detection of chemical warfare agents (CWAs) is of paramount importance in the development of reliable sensing devices for safety applications. In particular, the increased threats of chemical attacks by terrorist organizations have stimulated a significant interest in the early detection of CWAs. Due to the high toxicity and necessity of special infrastructures for storage/manipulation of CWAs, the activities aimed at their detection are carried out on mimicking compounds compatible with lab test security level. In this context, the recognition of di(propylene glycol) monomethyl ether (DPGME), a simulant of the vesicant nitrogen mustard, is of key importance. Among the various devices adopted for CWA sensing, chemoresistive gas sensors based on nanostructured metal oxides offer various advantages, encompassing low cost, limited power consumption, and good stability/sensitivity. To date, DPGME detection has been performed only in a few cases by the use of SnO2-based systems, and the development of highly efficient sensors for its selective monitoring is of primary importance for next-generation devices.Among metal oxides, p-type Mn3O4, a mixed-valence and environmentally friendly multifunctional system, is an attractive candidate for CWAs detection thanks to the tunable Mn redox chemistry and the inherent catalytic properties. On the other hand, gas sensors based on pure p-type materials like Mn3O4 typically feature responses lower than n-type ones, as well as modest sensitivity/selectivity. To circumvent these drawbacks, a viable alternative is the functionalization with noble metals particles, like Ag and Au, that can act as catalytic promoters of the involved chemical reactions at the nanoscale and/or favor the formation of metal/oxide Schottky junctions, improving charge carrier separation.Based on preliminary results on the development of pure Mn3O4 nanomaterials as gas sensors [1,2], we have reported for the first time on the gas-sensing performances of nanocomposite systems based on Mn3O4-M (M = Ag, Au) in DPGME detection [3]. The target materials were obtained by the initial chemical vapor deposition (CVD) of Mn3O4 on polycrystalline Al2O3 substrates, followed by silver or gold radio frequency (RF) sputtering, with the aim of obtaining a high dispersion of metal nanoparticles.The results yielded by a multi-technique characterization highlighted the formation of phase-pure Mn3O4 nanostructures eventually functionalized with Ag or Au and an intimate contact between metal oxide matrix and nanoparticles. These features enabled us to enhance the system sensitivity in the detection not only of standard volatile organic compounds (acetone and ethanol), but also of DPGME, down to the ppb scale. Moreover, the possibility of discriminating between DPGME, on one side, and acetonitrile (CH3CN) or dimethyl methyl phosphonate (DMMP), simulants of HCN (blood agent gas) and Sarin nerve gas, respectively, demonstrates the system selectivity toward the target warfare gas simulant. To shed light on the origin of enhanced selectivity due to functionalization, density functional theory (DFT) based calculations were performed. The results, providing a molecular insight into the interactions between the target systems and DPGME, enabled us to elucidate a dual-site activation mechanism based on the binding of DPGME to both manganese oxide and gold nanoparticles, accounting for the higher functional performances registered for Au-Mn3O4 sensors.As a whole, this study represents an important step forward in the fabrication of highly efficient sensors based on dual metal oxide-metal NP composites. The obtained results pave the way to mastering next-generation sensing devices for real world applications, which will be pursued by additional investigation of the system long-term stability and humidity influence on the resulting functional performances.

Heavily Doped Zinc Oxide Thin Films for Nitrogen Dioxide Optical Gas Sensing

Introduction In this work heavily n-doped ZnO doped with gallium, germanium, aluminum and silicon are studied for NO2 detection, using the sensibility of localized surface plasmonic resonance in the near infrared to the change of environmental conditions. Trivalent cations (Al3+ and Ga3+) or tetravalent cations (Si4+ and Ge4+) are used as dopant agents with nominal concentration of 20%, following a non-aqueous heat up colloidal synthesis [1]. No presence of other phases like metal, dopant oxides, and binary dopant-ZnO oxides is detected by XRD analysis. It is proved by spectrophotometric analysis that introduction of doping agent causes the formation of a surface plasmon resonance in the near infrared, tunable changing dopant nature. We have demonstrated that strong sensitivity of the plasmon resonance to chemical variations at the surface of the nanocrystal make this category of materials very suitable for detection of 1000 ppm NO2 balance in air, finding the best compromise between sensitivity, signal to noise ratio and stability of the sensor in 20% germanium doped ZnO. Materials and Methods Synthesis: Doped ZnO NPs were synthetized through a previously reported non-aqueous heat up colloidal method [2]. Film preparation: Doped zinc oxide thin films were prepared by spin coating technique on fused silica substrates. The films were thermal treated in a tube furnace filled with a 5% H2−95% Ar gas mixture at 450°C for 2h [3]. The final thickness of the films were of 500-600 nm. Film characterization: Nanoparticles morphology and dimension were observed by transmission electron microscope (TEM). Morphology of the films and an estimation of doping efficiency were investigated using a scanning electron microscope (SEM). Crystalline structure of thin films was determined by grazing incidence X-ray diffraction (XRD). Gas sensing measurement were performed in a Harrick gas flow cell connected to a gas handling system via flow meters and coupled with a Jasco V-570 spectrophotometer. Optical absorption was collected using the Jasco V-570 spectrophotometer in the 350-2500 nm wavelength range at operative temperature (OT) set at 150°C in 0.4 L/min flow of synthetic air or of target gas. In this work, 1000 ppm of NO2 in dry air was used as target gas. Optical Absorption Change (OAC = Absgas - Absair) was evaluated to identify the suitable wavelength to perform the dynamic sensing. Results and Conclusions In general, at equilibrium in dry air condition, atmospheric molecular oxygen is adsorbed onto sensor’s surface, due to the interaction with oxygen vacancies, and ionizing. In this work sensing test are performed at 150°C, at this temperature O2¯ is considered stable. If nitrogen dioxide is introduced, equilibrium changes due to the gas’ high oxidizing attitude, which increase the concentration of adsorbed molecules and cause an increment of electrons’ extraction. Therefore, the thickness of Electron Depletion Layer as well as the resistance of material increase. Due to this phenomenon, sensor response can be defined in several different ways as function of differences in measurement criteria. In particular, changing in resistance is most commonly monitoring parameter used for chemoresistive gas sensors, but besides the obvious electrical effects, it can be observed from formula (1) that decreasing in carrier density the angular frequency of LSPR maximum peak’s position decrease and, as consequence, its wavelength increase, obtaining a red-shift of the plasmon position.In Figure 1 are presented the OAC of the various thin films tested and two cycles of Air/NO2 exposition. We found that all the prepared samples are able to detect 1000 ppm of NO2 in dry air but with different dynamic characteristic. In particular, a trade-off between sensibility and signal to noise ratio versus stability was highlighted, which depend from the amount of free charge generated by the dopant agent. In fact, GZO thin film exhibit higher sensitivity and signal to noise ratio, but lower recovering of the baseline due to progressive irreversible degradation of the sensor. On the contrary, SZO has approximately total recovering of baseline but the signal difference between air and gas exposition is lower. GeZO exhibit intermediate proprieties with both good sensitivity and baseline recovering, while AZO present the worst sensing behaviors. The use of colloidal approach to the synthesis overcome some limitations of the sol-gel method, making this process competitive by an industrial point of view compared to other common investigated deposition technique, like magnetron sputtering. In the same time allow to obtain nanoparticles easily dispersible and depositable at room temperature and in ambient condition.Future works will investigate cross-sensitivity and detection limit of so obtained NO2 sensors. Analyzed sensing proprieties coupled with optical transparence in visible light, paving the way to the fabrication of new invisible and highly efficient sensors for detection of hazardous gases.

Plasma-Assisted Synthesis of MnO2-Based Nanoarchitectures As Gas Sensors for Safety and Food Quality Monitoring

The efficient detection of chemical warfare agents (CWAs) for homeland defense and people safety has received a growing attention, due to their high toxicity and dangerous effects on human health even at low concentration levels. In this regard, sensing of acetonitrile, a simulant of cyanide CWAs known to be itself a poisonous gas at low levels in air, is of utmost importance to prevent health hazards. In recent years, efforts have also been devoted to monitor food quality and, in particular, fruit/vegetable ripening/degradation in horticultural industry, for which ethylene is an important marker. Indeed, this analyte is a colorless, odorless and flammable gas whose production by climacteric fruits/vegetables, even at concentrations as low as some ppm, is a tell-tale of their ripeness stage. As a consequence, its efficient monitoring in warehouses is of key importance to extend fruits/vegetables life cycle and suppress detrimental losses of these agricultural products.In this context, the development of low-cost and reliable gas sensing materials capable of detecting the above gases in their early stages is a key issue to be still properly addressed. Among the various devices for acetonitrile and ethylene detection, chemoresistive gas sensors based on metal oxides with tailored morphology offer manifold advantages, including robustness, stability, low cost, and ease of operation. Among metal oxides, MnO2, an n-type semiconductor, exists in various polymorphs (α-, β-, γ-, δ- and ε-types) among which rutile-type β-MnO2 is the most stable one. This structural flexibility has stimulated efforts toward MnO2 use for various applications, among which volt-amperometric sensors. In this context, an interesting option to improve functional performances is the introduction of F as a dopant into the metal oxide matrix, as previously demonstrated for F-doped Co3O4 and Fe2O3 nanosystems [1,2]. On this basis, in this contribution [3] we report for the first time on acetonitrile and ethylene gas sensing performances of supported β-MnO2-based nanosystems synthesized by plasma enhanced-chemical vapor deposition (PE-CVD). In particular, the attention has been devoted to: (i) the modulation of the system morphology and defectivity, taking advantage of the versatility and inherent benefits offered by the above technique for low-temperature processing of inorganic nanomaterials; (ii) the use of Mn(tfa)2•TMEDA (Htfa = 1,1,1-trifluoro-2,4-pentanedione; TMEDA = N,N,N′,N′-tetramethylethylenediamine) as a single-source molecular precursor for both manganese and fluorine, allowing an in-site material doping during growth process.After a thorough characterization of MnO2 systems grown at different oxygen partial pressures, the system sensing performances were tested in the detection of acetonitrile and ethylene gases, investigating the interplay between the system characteristics and gas sensing performances.The results highlight favorable responses already at low operating temperatures with an enhanced detection efficiency toward the target analytes, that could provide a baseline data set for the development of MnO2 sensors for safety and food industry applications outperforming the actual ones.Afterwards, our attention has been focused on a proof-of-concept on the possibility of enhancing performances in ethylene sensing by MnO2 systems basing on the nanoarchitectonics concept [4]. More specifically, we have exploited the system sensitization with noble metals (M = Ag, Au) to boost the resulting functional behavior thanks to the guest catalytic activity at the nanoscale and to the formation of metal-oxide interfaces, yielding an enhanced charge carrier separation. The target materials have been fabricated by an original two-step plasma-assisted route, consisting in the initial PE-CVD of MnO2 nanoarchitectures, followed by the introduction of silver and gold nanoaggregates by mild sputtering processes. Tailored low-dimensionality nanoarchitectures with controllable features and an even distribution of noble metal particles have been successfully prepared taking advantage of the unique versatility offered by plasma processing, resulting in material properties hardly attainable by means of conventional synthetic routes. Tests aimed at ethylene sensing have revealed enhanced responses and sensitivities at low temperatures, with performances directly dependent on the functionalizing guest agent, as discussed in the framework of a comparative material chemico-physical characterization.

New Chemoresistive Gas Sensor Arrays for Outdoor Air Quality Monitoring: A Combined R&D and Outreach Activities

New Chemoresistive Gas Sensor Arrays for Outdoor Air Quality Monitoring: A Combined R&D and Outreach Activities