Poly(vinyl chloride)/Nanocarbon Composites for Advanced Potentiometric Membrane Sensor Design

Polymer nanocomposites filled with carbon nanoparticles (CNPs) are a hot topic in materials science. This article discusses the current research on the use of these materials as interfacial electron transfer films for solid contact potentiometric membrane sensors (SC-PMSs). The results of a comparative study of plasticized poly (vinyl chloride) (pPVC) matrices modified with single-walled carbon nanotubes (SWCNTs), fullerenes-C60, and their hybrid ensemble (SWCNTs-C60) are reported. The morphological characteristics and electrical conductivity of the prepared nanostructured composite films are reported. It was found that the specific electrical conductivity of the pPVC/SWCNTs-C60 polymer film was higher than that of pPVC filled with individual nanocomponents. The effectiveness of this composite material as an electron transfer film in a new potentiometric membrane sensor for detecting phenylpyruvic acid (in anionic form) was demonstrated. Screening for this metabolic product of phenylalanine in body fluids is of significant diagnostic interest in phenylketonuria (dementia), viral hepatitis, and alcoholism. The developed sensor showed a stable and fast Nernstian response for phenylpyruvate ions in aqueous solutions over the wide linear concentration range of 5 × 10-7-1 × 10-3 M, with a detection limit of 10-7.2 M.

Current Trends and Promising Electrode Materials in Micro-Supercapacitor Printing

The development of scientific and technological foundations for the creation of high-performance energy storage devices is becoming increasingly important due to the rapid development of microelectronics, including flexible and wearable microelectronics. Supercapacitors are indispensable devices for the power supply of systems requiring high power, high charging-discharging rates, cyclic stability, and long service life and a wide range of operating temperatures (from -40 to 70 °C). The use of printing technologies gives an opportunity to move the production of such devices to a new level due to the possibility of the automated formation of micro-supercapacitors (including flexible, stretchable, wearable) with the required type of geometric implementation, to reduce time and labour costs for their creation, and to expand the prospects of their commercialization and widespread use. Within the framework of this review, we have focused on the consideration of the key commonly used supercapacitor electrode materials and highlighted examples of their successful printing in the process of assembling miniature energy storage devices.

Hydrothermal Synthesis of a Cellular NiO Film on Carbon Paper as a Promising Way to Obtain a Hierarchically Organized Electrode for a Flexible Supercapacitor

The formation of a cellular hierarchically organized NiO film on a carbon paper substrate under hydrothermal conditions using triethanolamine as a base has been studied. The thermal behavior of the carbon paper substrate with the applied semi-product shell was studied using synchronous thermal analysis (TGA/DSC) and it was demonstrated that such modification of the material surface leads to a noticeable increase in its thermal stability. Using scanning electron microscopy (SEM), it was shown that the NiO film grown on the carbon fiber surface is characterized by a complex cellular morphology, organized by partially layered individual nanosheets of about 4-5 nm thickness and lateral dimensions up to 1-2 μm, some edges and folds of which are located vertically relative to the carbon fiber surface. The surface of the obtained material was also examined using atomic force microscopy (AFM), and the electronic work function of the oxide shell surface was evaluated using the Kelvin probe force microscopy (KPFM) method. The electrochemical parameters of the obtained flexible NiO/CP electrode were analyzed: the dependence of the specific capacitance on the current density was determined and the stability of the material during cycling was studied, which showed that the proposed approach is promising for manufacturing hierarchically organized electrodes for flexible supercapacitors.

Low Temperature Chemoresistive Oxygen Sensors Based on Titanium-Containing Ti2CTx and Ti3C2Tx MXenes

The chemoresistive properties of multilayer titanium-containing Ti₂CT_x and Ti₃C₂T_x MXenes, synthesized by etching the corresponding MAX phases with NaF solution in hydrochloric acid, and the composites based on them, obtained by partial oxidation directly in a sensor cell in an air flow at 150 °C, were studied. Significant differences were observed for the initial MXenes, both in microstructure and in the composition of surface functional groups, as well as in gas sensitivity. For single Ti₂CT_x and Ti₃C₂T_x MXenes, significant responses to oxygen and ammonia were observed. For their partial oxidation at a moderate temperature of 150 °C, a high humidity sensitivity (T, RH = 55%) is observed for Ti₂CT_x and a high and selective response to oxygen for Ti₃C₂T_x at 125 °C (RH = 0%). Overall, these titanium-containing MXenes and composites based on them are considered promising as receptor materials for low temperature oxygen sensors.

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.

Gas-Sensitive Properties of ZnO/Ti2CTx Nanocomposites

At present, a new class of 2D nanomaterials, MXenes, is of great scientific and applied interest, and their application prospects are very broad, including as effective doping components for receptor materials of MOS sensors. In this work we have studied the influence on the gas-sensitive properties of nanocrystalline zinc oxide synthesized by atmospheric pressure solvothermal synthesis, with the addition of 1-5% of multilayer two-dimensional titanium carbide Ti₂CT_x, obtained by etching Ti₂AlC with NaF solution in hydrochloric acid. It was found that all the obtained materials have high sensitivity and selectivity with respect to 4-20 ppm NO₂ at a detection temperature of 200 °C. It is shown that the selectivity towards this compound is best for the sample containing the highest amount of Ti₂CT_x dopant. It has been found that as the MXene content increases, there is an increase in nitrogen dioxide (4 ppm) from 1.6 (ZnO) to 20.5 (ZnO-5 mol% Ti₂CT_x). reactions which the responses to nitrogen dioxide increase. This may be due to the increase in the specific surface area of the receptor layers, the presence of MXene surface functional groups, as well as the formation of the Schottky barrier at the interface between the phases of the components.

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 Ti₃C₂T_x and Ti₂CT_x 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 La0.6Sr0.4Co0.2Fe0.8O3-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.

Atmospheric Pressure Solvothermal Synthesis of Nanoscale SnO2 and Its Application in Microextrusion Printing of a Thick-Film Chemosensor Material for Effective Ethanol Detection

The atmospheric pressure solvothermal (APS) synthesis of nanocrystalline SnO₂ (average size of coherent scattering regions (CSR)-7.5 ± 0.6 nm) using tin acetylacetonate as a precursor was studied. The resulting nanopowder was used as a functional ink component in microextrusion printing of a tin dioxide thick film on the surface of a Pt/Al₂O₃/Pt chip. Synchronous thermal analysis shows that the resulting semiproduct is transformed completely into tin dioxide nanopowder at 400 °C within 1 h. The SnO₂ powder and the resulting film were shown to have a cassiterite-type structure according to X-ray diffraction analysis, and IR spectroscopy was used to establish the set of functional groups in the material composition. The microstructural features of the tin dioxide powder were analyzed using scanning (SEM) and transmission (TEM) electron microscopy: the average size of the oxide powder particles was 8.2 ± 0.7 nm. Various atomic force microscopy (AFM) techniques were employed to investigate the topography of the oxide film and to build maps of surface capacitance and potential distribution. The temperature dependence of the electrical conductivity of the printed SnO₂ film was studied using impedance spectroscopy. The chemosensory properties of the formed material when detecting H₂, CO, NH₃, C₆H₆, C₃H₆O and C₂H₅OH, including at varying humidity, were also examined. It was demonstrated that the obtained SnO₂ film has an increased sensitivity (the sensory response value was 1.4-63.5) and selectivity for detection of 4-100 ppm C₂H₅OH at an operating temperature of 200 °C.

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.

Hydrothermal Synthesis of Ag Thin Films and Their SERS Application

In this article, a facile, one-step method for the formation of silver thin-film nanostructures on the surface of Al₂O₃ substrates using the hydrothermal method is proposed. The dependence of the SERS effect intensity of the formed films during the detection of methylene blue (MB) low concentrations on the synthesis conditions, additional temperature treatment, and laser radiation wavelength (532 and 780 nm) in comparison with similar dye films on commercial SERS substrates is shown. The detection limit of the analyte used for the indicated lasers is estimated. The effect of the synthesis temperature on the particle size, crystal structure, and microstructure features of the obtained thin films based on silver nanoparticles is demonstrated. Using spreading resistance microscopy, the interface between the substrate and Ag particles is studied, and the dependence of the size of the corresponding gap between them and the nature of microstructural defects on the parameters of hydrothermal treatment of reaction systems in the presence of Al₂O₃ substrates is shown. As a result of the study, the factors associated with the properties of the obtained SERS substrates and the parameters of recording the spectra, which affect the amplification factor of the spectral lines intensity of the analyte, are revealed.

Microplotter printing of planar solid electrolytes in the CeO2-Y2O3 system

The formation process for planar solid electrolytes in the CeO₂-Y₂O₃ system has been studied using efficient, high-performance, high-resolution microplotter printing technology, using functional ink based on nanopowders (the average size of crystallites was 12-15 nm) of a similar composition obtained by programmed coprecipitation of metal hydroxides. The dependence of the microstructure of the oxide nanoparticles obtained and their crystal structure on yttrium concentration has been studied using a wide range of methods. According to X-ray diffraction (XRD), the nanopowders and coatings produced are single-phase, with a cubic crystal structure of the fluorite type, and the electronic state and content of cerium and yttrium in the printed coatings have been determined using X-ray photoelectron spectroscopy (XPS). The results of scanning electron (SEM) and atomic force microscopy (AFM) have shown that the coatings produced are homogeneous, they do not contain defects in the form of fractures and the height difference over an area of 1 µm² is 30-45 nm. The local electrophysical characteristics of the oxide coatings produced (the work function of the coating surface, capacitance values, maps of the surface potential and capacitive contrast distribution over the surface) have been studied using Kelvin-probe force microscopy (KPFM) and scanning capacitive microscopy (SCM). Using impedance spectroscopy, the dependence of the electrophysical characteristics of printed planar solid electrolytes in the CeO₂-Y₂O₃ system on yttrium content has been determined and the prospects of the technology developed for the manufacture of modern, intermediate-temperature, solid oxide fuel cells have been demonstrated.

Microplotter-Printed On-Chip Combinatorial Library of Ink-Derived Multiple Metal Oxides as an "Electronic Olfaction" Unit

Information about the surrounding atmosphere at a real timescale significantly relies on available gas sensors to be efficiently combined into multisensor arrays as electronic olfaction units. However, the array's performance is challenged by the ability to provide orthogonal responses from the employed sensors at a reasonable cost. This issue becomes more demanded when the arrays are designed under an on-chip paradigm to meet a number of emerging calls either in the internet-of-things industry or in situ noninvasive diagnostics of human breath, to name a few, for small-sized low-powered detectors. The recent advances in additive manufacturing provide a solid top-down background to develop such chip-based gas-analytical systems under low-cost technology protocols. Here, we employ hydrolytically active heteroligand complexes of metals as ink components for microplotter patterning a multioxide combinatorial library of chemiresistive type at a single chip equipped with multiple electrodes. To primarily test the performance of such a multisensor array, various semiconducting oxides of the p- and n-conductance origins based on pristine and mixed nanocrystalline MnO_x, TiO₂, ZrO₂, CeO₂, ZnO, Cr₂O₃, Co₃O₄, and SnO₂ thin films, of up to 70 nm thick, have been printed over hundred μm areas and their micronanostructure and fabrication conditions are thoroughly assessed. The developed multioxide library is shown to deliver at a range of operating temperatures, up to 400 °C, highly sensitive and highly selective vector signals to different, but chemically akin, alcohol vapors (methanol, ethanol, isopropanol, and n-butanol) as examples at low ppm concentrations when mixed with air. The suggested approach provides us a promising way to achieve cost-effective and well-performed electronic olfaction devices matured from the diverse chemiresistive responses of the printed nanocrystalline oxides.