Application of Titanium Carbide MXenes in Chemiresistive Gas Sensors

The titanium carbide MXenes currently attract an extreme amount of interest from the material science community due to their promising functional properties arising from the two-dimensionality of these layered structures. In particular, the interaction between MXene and gaseous molecules, even at the physisorption level, yields a substantial shift in electrical parameters, which makes it possible to design gas sensors working at RT as a prerequisite to low-powered detection units. Herein, we consider to review such sensors, primarily based on Ti3C2Tx and Ti2CTx crystals as the most studied ones to date, delivering a chemiresistive type of signal. We analyze the ways reported in the literature to modify these 2D nanomaterials for (i) detecting various analyte gases, (ii) improving stability and sensitivity, (iii) reducing response/recovery times, and (iv) advancing a sensitivity to atmospheric humidity. The most powerful approach based on designing hetero-layers of MXenes with other crystals is discussed with regard to employing semiconductor metal oxides and chalcogenides, noble metal nanoparticles, carbon materials (graphene and nanotubes), and polymeric components. The current concepts on the detection mechanisms of MXenes and their hetero-composites are considered, and the background reasons for improving gas-sensing functionality in the hetero-composite when compared with pristine MXenes are classified. We formulate state-of-the-art advances and challenges in the field while proposing some possible solutions, in particular via employing a multisensor array paradigm

Microextrusion Printing of Multilayer Hierarchically Organized Planar Nanostructures Based on NiO, (CeO₂)_0.8(Sm₂O₃)_0.2 and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ

In this paper, NiO, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and (CeO2)0.8(Sm2O3)0.2 (SDC) nanopowders with different microstructures were obtained using hydrothermal and glycol–citrate methods. The microstructural features of the powders were examined using scanning electron microscopy (SEM). The obtained oxide powders were used to form functional inks for the sequential microextrusion printing of NiO-SDC, SDC and LSCF-SDC coatings with resulting three-layer structures of (NiO-SDC)/SDC/(LSCF-SDC) composition. The crystal structures of these layers were studied using an X-ray diffraction analysis, and the microstructures were studied using atomic force microscopy. Scanning capacitance microscopy was employed to build maps of capacitance gradient distribution over the surface of the oxide layers, and Kelvin probe force microscopy was utilized to map surface potential distribution and to estimate the work function values of the studied oxide layers. Using SEM and an energy-dispersive X-ray microanalysis, the cross-sectional area of the formed three-layer structure was analyzed—the interfacial boundary and the chemical element distribution over the surface of the cross-section were investigated. Using impedance spectroscopy, the temperature dependence of the electrical conductivity was also determined for the printed three-layer nanostructure

Microplotter Printing of a Miniature Flexible Supercapacitor Electrode Based on Hierarchically Organized NiCo₂O₄ Nanostructures

The hydrothermal synthesis of a nanosized NiCo2O4 oxide with several levels of hierarchical self-organization was studied. Using X-ray diffraction analysis (XRD) and Fourier-transform infrared (FTIR) spectroscopy, it was determined that under the selected synthesis conditions, a nickel-cobalt carbonate hydroxide hydrate of the composition M(CO3)0.5(OH)·0.11H2O (where M–Ni2+ and Co2+) is formed as a semi-product. The conditions of semi-product transformation into the target oxide were determined by simultaneous thermal analysis. It was found by means of scanning electron microscopy (SEM) that the main powder fraction consists of hierarchically organized microspheres of 3–10 μm in diameter, and individual nanorods are observed as the second fraction of the powder. Nanorod microstructure was further studied by transmission electron microscopy (TEM). A hierarchically organized NiCo2O4 film was printed on the surface of a flexible carbon paper (CP) using an optimized microplotter printing technique and functional inks based on the obtained oxide powder. It was shown by XRD, TEM, and atomic force microscopy (AFM) that the crystalline structure and microstructural features of the oxide particles are preserved when deposited on the surface of the flexible substrate. It was found that the obtained electrode sample is characterized by a specific capacitance value of 420 F/g at a current density of 1 A/g, and the capacitance loss during 2000 charge–discharge cycles at 10 A/g is 10%, which indicates a high material stability. It was established that the proposed synthesis and printing technology enables the efficient automated formation of corresponding miniature electrode nanostructures as promising components for flexible planar supercapacitors

Paper-Based Humidity Sensors as Promising Flexible Devices, State of the Art, Part 2: Humidity-Sensor Performances

This review article covers all types of paper-based humidity sensor, such as capacitive, resistive, impedance, fiber-optic, mass-sensitive, microwave, and RFID (radio-frequency identification) humidity sensors. The parameters of these sensors and the materials involved in their research and development, such as carbon nanotubes, graphene, semiconductors, and polymers, are comprehensively detailed, with a special focus on the advantages/disadvantages from an application perspective. Numerous technological/design approaches to the optimization of the performances of the sensors are considered, along with some non-conventional approaches. The review ends with a detailed analysis of the current problems encountered in the development of paper-based humidity sensors, supported by some solutions

Synthesis of ((CeO₂)_0.8(Sm₂O₃)_0.2)@NiO Core-Shell Type Nanostructures and Microextrusion Printing of a Composite Anode Based on Them

The process of the hydrothermal synthesis of hierarchically organized nanomaterials with the core-shell structure with the composition ((CeO2)0.8(Sm2O3)0.2)@NiO was studied, and the prospects for their application in the formation of planar composite structures using microextrusion printing were shown. The hydrothermal synthesis conditions of the (CeO2)0.8(Sm2O3)0.2 nanospheres were determined, and the approach to their surface modification by growing the NiO shell with the formation of core-shell structures equally distributed between the larger nickel(II) oxide nanosheets was developed. The resulting nanopowder was used as a functional ink component in the microextrusion printing of the corresponding composite coating. The microstructure of the powders and the oxide coating was studied by scanning (SEM) and transmission electron microscopy (TEM), the crystal structure was explored by X-ray diffraction analysis (XRD), the set of functional groups in the powders was studied by Fourier-transform infrared spectroscopy (FTIR) spectroscopy, and their thermal behavior in an air flow by synchronous thermal analysis (TGA/DSC). The electronic state of the chemical elements in the resulting coating was studied using X-ray photoelectron spectroscopy (XPS). The surface topography and local electrophysical properties of the composite coating were studied using atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). Using impedance spectroscopy, the temperature dependence of the specific electrical conductivity of the obtained composite coating was estimated

Microextrusion Printing of Hierarchically Structured Thick V2O5 Film with Independent from Humidity Sensing Response to Benzene

The process of V2O5 oxide by the combination of sol-gel technique and hydrothermal treatment using heteroligand [VO(C5H7O2)2–x(C4H9O)x] precursor was studied. Using thermal analysis, X-ray powder diffraction (XRD) and infra-red spectroscopy (IR), it was found that the resulting product was VO2(B), which after calcining at 300 °C (1 h), oxidized to orthorhombic V2O5. Scanning electron microscopy (SEM) results for V2O5 powder showed that it consisted of nanosheets (~50 nm long and ~10 nm thick) assembled in slightly spherical hierarchic structures (diameter ~200 nm). VO2 powder dispersion was used as functional ink for microextrusion printing of oxide film. After calcining the film at 300 °C (30 min), it was found that it oxidized to V2O5, with SEM and atomic force microscopy (AFM) results showing that the film structure retained the hierarchic structure of the powder. Using Kelvin probe force microscopy (KPFM), the work function value for V2O5 film in ambient conditions was calculated (4.81 eV), indicating a high amount of deficiencies in the sample. V2O5 film exhibited selective response upon sensing benzene, with response value invariable under changing humidity. Studies of the electrical conductivity of the film revealed increased resistance due to high film porosity, with conductivity activation energy being 0.26 eV

Chemoresistive Properties of V₂CT_x MXene and the V₂CT_x/V₃O₇ Nanocomposite Based on It

The in-situ Raman spectroscopy oxidation of the accordion-like V2CTx MXene has been studied. It was found that a nanocomposite of V2CTx/V3O7 composition was formed as a result. The elemental and phase composition, the microstructure of the synthesized V2CTx powder and MXene film as well as the V2CTx/V3O7 nanocomposite obtained at a minimum oxidation temperature of 250 °C were studied using a variety of physical and chemical analysis methods. It was found that the obtained V2CTx and V2CTx/V3O7 films have an increased sensitivity to ammonia and nitrogen dioxide, respectively, at room temperature and zero humidity. It was shown that the V2CTx/V3O7 composite material is characterized by an increase in the response value for a number of analytes (including humidity) by more than one order of magnitude, as well as a change in their detection mechanisms compared to the individual V2CTx MXene

Microplotter Printing of Hierarchically Organized NiCo₂O₄ Films for Ethanol Gas Sensing

Using a combination of chemical coprecipitation and hydrothermal treatment of the resulting dispersed system, a hierarchically organized NiCo2O4 nanopowder was obtained, consisting of slightly elongated initial oxide nanoparticles self-organized into nanosheets about 10 nm thick, which in turn are combined into hierarchical cellular agglomerates of about 2 μm. Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HR-TEM) allowed to confirm the formation of NiCo2O4 powder with the desired crystal structure via additional heat treatment of the intermediate product. Energy-dispersive X-ray spectroscopy (EDX) was used to confirm the target metal ratio, and the uniform distribution of the elements (Ni, Co and O) was shown by mapping. The resulting nanopowder was employed to prepare functional inks suitable for microplotter printing of the NiCo2O4 film. It was found that an oxide film morphology is fully inherited from the hierarchically organized oxide nanopowder used. Atomic force microscopy (AFM) revealed the film thickness (15 μm) and determined the maximum height difference of 500 nm over an area of 25 μm2. Kelvin probe force microscopy (KPFM) showed that the surface potential was shifted to the depths of the oxide film, and the work function value of the material surface was 4.54 eV, which is significantly lower compared to those reported in the literature. The electronic state of the elements in the NiCo2O4 film under study was analyzed by X-ray photoelectron spectroscopy (XPS). Chemosensor measurements showed that the printed receptor layer exhibited selectivity and high signal reproducibility for ethanol detection. As the relative humidity increases from 0 to 75%, the response value is reduced; however, the sensor response profile and signal-to-noise ratio remain without significant changes

Effect of Ti₂CT_x MXene Oxidation on Its Gas-Sensitive Properties

The oxidation process was studied for the synthesized low-layer Ti2CTx MXene deposited on a special Al2O3/Pt sensor substrate using in situ Raman spectroscopy. It is noted that on the ceramic parts of the substrate (Al2O3), the beginning of oxidation (appearance of anatase mod phase) is observed already at 316 °C, in comparison with platinum, for which the appearance of anatase is noted only at 372 °C. At the temperature 447 °C, the initial MXene film is completely oxidized to TiO2. Using scanning electron microscopy and atomic force microscopy, the microstructure and dispersity of the obtained MXene film were studied. It was found that the obtained films exhibit chemoresistive responses to the detection of a wide group of gases, H2, CO, NH3, C6H6, C3H6O, CH4, C2H5OH and O2, at room temperature and RH = 50%. The highest sensitivity is observed for NH3. The partial oxidation of the Ti2CTx MXene was shown to favorably affect the gas-sensitive properties