Functionalization of Indium Oxide for Empowered Detection of CO2 over an Extra-Wide Range of Concentrations

Carbon capture, storage, and utilization have become familiar terms when discussing climate change mitigation actions. Such endeavors demand the availability of smart and inexpensive devices for CO₂ monitoring. To date, CO₂ detection relies on optical properties and there is a lack of devices based on solid-state gas sensors, which can be miniaturized and easily made compatible with Internet of Things platforms. With this purpose, we present an innovative semiconductor as a functional material for CO₂ detection. A nanostructured In₂O₃ film, functionalized by Na, proves to enhance the surface reactivity of pristine oxide and promote the chemisorption of even rather an inert molecule as CO₂. An advanced operando equipment based on surface-sensitive diffuse infrared Fourier transform is used to investigate its improved surface reactivity. The role of sodium is to increase the concentration of active sites such as oxygen vacancies and, in turn, to strengthen CO₂ adsorption and reaction at the surface. It results in a change in film conductivity, i.e., in transduction of a concentration of CO₂. The films exhibit excellent sensitivity and selectivity to CO₂ over an extra-wide range of concentrations (250-5000 ppm), which covers most indoor and outdoor applications due to the marginal influence by environmental humidity.

Elucidating the Ambient Stability and Gas Sensing Mechanism of Nickel-Decorated Phosphorene for NO2 Detection: A First-Principles Study

In the field of layered two-dimensional functional materials, black phosphorus has attracted considerable attention in many applications due to its outstanding electrical properties. It has experimentally shown superior chemical sensing performance for the room temperature detection of NO₂, highlighting high sensitivity at a ppb level. Unfortunately, pristine black phosphorus demonstrated an unstable functionality due to the fast degradation of the material when exposed to the ambient atmosphere. In the present work, a deepened investigation by density functional theory was carried out to study how nickel decoration of phosphorene can improve the stability of the material. Further, an insight into the sensing mechanism of nickel-loaded phosphorene toward NO₂ was given and compared to pristine phosphorene. This first-principles study proved that, by introducing nickel adatoms, the band gap of the material decreases and the positions of the conduction band minimum and the valence band maximum move toward each other, resulting in a drop in the conduction band minimum under the redox potential of O₂/O₂ ^-, which may result in a more stable material. Studying the adsorption of O₂ molecules on pristine phosphorene, we also proved that all oxygen molecules coming from the surrounding atmosphere react with phosphorus atoms in the layer, resulting in the oxidation of the material forming oxidized phosphorus species (PO _x ). Instead, by introducing nickel adatoms, part of the oxygen from the surrounding atmosphere reacts with nickel atoms, resulting in a decrease of the oxidation rate of the material and in subsequent long-term stability of the device. Finally, possible reaction paths for the detection of NO₂ are given by charge transfer analyses, occurring at the surface during the adsorption of oxygen molecules and the interaction with the target gas.

Design of a Metal-Oxide Solid Solution for Sub-ppm H2 Detection

Hydrogen is largely adopted in industrial processes and is one of the leading options for storing renewable energy. Due to its high explosivity, detection of H₂ has become essential for safety in industries, storage, and transportation. This work aims to design a sensing film for high-sensitivity H₂ detection. Chemoresistive gas sensors have extensively been studied for H₂ monitoring due to their good sensitivity and low cost. However, further research and development are still needed for a reliable H₂ detection at sub-ppm concentrations. Metal-oxide solid solutions represent a valuable approach for tuning the sensing properties by modifying their composition, morphology, and structure. The work started from a solid solution of Sn and Ti oxides, which is known to exhibit high sensitivity toward H₂. Such a solid solution was empowered by the addition of Nb, which─according to earlier studies on titania films─was expected to inhibit grain growth at high temperatures, to reduce the film resistance and to impact the sensor selectivity and sensitivity. Powders were synthesized through the sol-gel technique by keeping the Sn-Ti ratio constant at the optimal value for H₂ detection with different Nb concentrations (1.5-5 atom %). Such solid solutions were thermally treated at 650 and 850 °C. The sensor based on the solid solution calcined at 650 °C and with the lowest content of Nb exhibited an extremely high sensitivity toward H₂, paving the way for H₂ ppb detection. For comparison, the response to 50 ppm of H₂ was increased 6 times vs SnO₂ and twice that of (Sn,Ti)_xO₂.

Investigation on Sensing Performance of Highly Doped Sb/SnO2

Tin dioxide (SnO₂) is the most-used semiconductor for gas sensing applications. However, lack of selectivity and humidity influence limit its potential usage. Antimony (Sb) doped SnO₂ showed unique electrical and chemical properties, since the introduction of Sb ions leads to the creation of a new shallow band level and of oxygen vacancies acting as donors in SnO₂. Although low-doped SnO₂:Sb demonstrated an improvement of the sensing performance compared to pure SnO₂, there is a lack of investigation on this material. To fill this gap, we focused this work on the study of gas sensing properties of highly doped SnO₂:Sb. Morphology, crystal structure and elemental composition were characterized, highlighting that Sb doping hinders SnO₂ grain growth and decreases crystallinity slightly, while lattice parameters expand after the introduction of Sb ions into the SnO₂ crystal. XRF and EDS confirmed the high purity of the SnO₂:Sb powders, and XPS highlighted a higher Sb concentration compared to XRF and EDS results, due to a partial Sb segregation on superficial layers of Sb/SnO₂. Then, the samples were exposed to different gases, highlighting a high selectivity to NO₂ with a good sensitivity and a limited influence of humidity. Lastly, an interpretation of the sensing mechanism vs. NO₂ was proposed.

Air Stable Nickel-Decorated Black Phosphorus and Its Room-Temperature Chemiresistive Gas Sensor Capabilities

In the rapidly emerging field of layered two-dimensional functional materials, black phosphorus, the P-counterpart of graphene, is a potential candidate for various applications, e.g., nanoscale optoelectronics, rechargeable ion batteries, electrocatalysts, thermoelectrics, solar cells, and sensors. Black phosphorus has shown superior chemical sensing performance; in particular, it is selective for the detection of NO2, an environmental toxic gas, for which black phosphorus has highlighted high sensitivity at a ppb level. In this work, by applying a multiscale characterization approach, we demonstrated a stability and functionality improvement of nickel-decorated black phosphorus films for gas sensing prepared by a simple, reproducible, and affordable deposition technique. Furthermore, we studied the electrical behavior of these films once implemented as functional layers in gas sensors by exposing them to different gaseous compounds and under different relative humidity conditions. Finally, the influence on sensing performance of nickel nanoparticle dimensions and concentration correlated to the decoration technique and film thickness was investigated.

First-Principles Study of Electronic Conductivity, Structural and Electronic Properties of Oxygen-Vacancy-Defected SnO₂

The use of computer simulations has become almost essential for prediction and interpretation of device's performance. In gas sensing field, the simulation of specific conditions, which determine the physical-chemical properties of widely used metal oxide semiconductors, can be used to investigate the performance of gas sensors based on these kinds of materials. The aim of this work was to evaluate the physical-chemical properties of tin dioxide employed for environmental and health gas sensing application and to investigate the influence of oxygen vacancies on its properties by means of density functional theory. Two samples, having different concentration of oxygen vacancies, were deeply studied in terms of their structural, electronic and electrical properties. It was proved the influence of oxygen vacancies on lattice parameter. By increasing oxygen vacancies concentration, the increased number of impurity states took these closer to the conduction band minimum, which can lead to an easier adsorption process of oxygen species and their availability to be exchanges with the molecules of the target gases. In this way a reduction of the operating temperature can be observed, thus reducing the power consumption of devices, while keeping the catalytic performance of the material.