Annotation of the results obtained in 2021
During the first year of the project the method of relatively low-temperature (at temperatures of 1000-1200°C) synthesis of МАХ phases in the system Ti2AlC-V2AlC was developed. The temperature features of the mixed phases of compositions Ti1V1AlC, Ti0.5V1.5AlC and Ti0.2V1.8AlC, which require increasing the synthesis temperature to 1100 and 1200°C (for the phase Ti1V1AlC) for ordering. The obtained samples of MAX phases were characterized using up-to-date physico-chemical methods, which made it possible to demonstrate the structural features of these compounds. It was found that as titanium atoms are replaced by vanadium, there is a systematic decrease in the oxidation resistance. Using the obtained MAX phases the methods of aluminum removal from their structure with the formation of the corresponding MXenes in the Ti2C-V2C system have been developed under the influence of hydrofluoric acid solutions, HF+HCl acid mixtures, as well as lithium, sodium, potassium and ammonium fluoride solutions in hydrochloric acid. The influence of the etching process temperature and the exposure time on the composition and practical yield of the products has been studied. The influence of the chemical nature and transition metal content in the MAX phase on the efficiency of synthesis of MXenes of variable composition has been revealed. It was found that the ratio of halogenide end groups changes by an order of magnitude depending on the used method of aluminum removal that should considerably influence both the stability of aqueous dispersions of MXenes and their electrochemical characteristics. The efficiency of titanium,vanadium-containing MXenes delamination process by ultrasonic treatment and also by combination of ultrasonic treatment with simultaneous introduction of tetramethylammonium and tetrabutylammonium hydroxide solutions has been studied. It was found that the use of N(Me)4OH solution makes it possible to obtain stable aqueous dispersions of single-layer (or low-layer) MXenes. A method has been developed to replace the dispersion medium from aqueous to an organic solvent of alcohol class (ethanol, butanol, ethylene glycol) and to obtain on their basis stable dispersions suitable in their rheological characteristics for use as functional inks for creating the receptor layers of multi-sensor systems using microplotter printing. The high stability of the obtained dispersions based on ethylene glycol was shown. A technique for microplotter printing arrays of receptor layers using MXene-based functional inks in the Ti2C-V2C system has been developed and optimized. Coating was applied to the surface of various microchips, which are oxidized silicon substrates with pre-coated platinum microheaters, strip electrodes, and thermoresistors. Receptor layers of different chemical composition and different thicknesses were deposited on 2 chips - in the first case for the set Ti2C and V2C composition MXenes the thickness of coatings was varied, and in the second - an array of 5 MXenes with a gradient of chemical composition (with a gradual replacement of titanium by vanadium) was formed. We evaluated the chemoresistive properties of the sensor array based on the printed array of Ti2C and V2C composition structures on the surface of microchip #1. Five sensor segments were exposed to different concentrations of acetone and ethanol in the ranges: [0; 43] ppm and [0; 128] ppm, respectively. According to the results of the principal component analysis and linear discriminant analysis, the maximum value of the pairwise correlation coefficient is typical for the analyte/dry air pair and was 1.0, and the recognition coefficient between ethanol and acetone was 0.78, which is probably due to the low material sensitivity and a limited range of detectable contents due to low levels of active centers on the material surface. Thus, the proposed approach of combining an electronic nose based on appropriate MXenes with machine learning protocols for express estimation of the type of the analyte being determined can be provided by varying the conditions of synthesis and material deposition on the chip surface, taking into account comparative control of the change in the chemoresistive properties of the obtained Ti2C and V2C films. We have also studied the chemosensory characteristics of the fabricated multisensor #2 containing an array of 5 MXenes of variable composition in the Ti2C-V2C system when exposed to a number of analytes - alcohol vapor (methanol, ethanol, isopropanol), ammonia and water mixed with dehydrated air. Measurements were performed at room temperature, without additional activation. It was shown that all analytes increase the resistance of MXene layers, but to different degrees. This agrees with the known literature data on other MXenes based on titanium carbide. The magnitude of the response to alcohols correlates with the value of their molecular weight. In this case the greatest change in resistance is observed when exposed to water vapor. The performed measurements of the chemoresistive response as a relative change in the resistance of the layers on the concentration of water vapors in a wide concentration range (500-10000 ppm) showed the following of the power law according to the Freundlich isotherm. Quantum mechanical studies of the atomic structure, electronic and conductive properties of MXenes in the Ti2C-V2C system at the sequential replacement of titanium by vanadium have been performed. Equilibrium configurations of the supercells of the corresponding systems were obtained. The dimensions of the supercells were chosen in such a way as to allow further surface placement of the various analytes. The analytes used in the calculations were: water, the alcohols (methanol and ethanol), ammonia, acetone, cyclopentane, and cyclohexane. First of all, the character of Fermi energy and conductivity changes during the sequential replacement of titanium by vanadium was studied. It was determined that the lowest Fermi energy values are observed in two cases - for individual Ti2C and V2C MXenes. In all cases with titanium replacement by vanadium the Fermi energy is much higher and is ~-4.8 eV. The resistance exhibits a different behavior - it grows when the amount of vanadium increases up to 75% and then decreases. In the study of water as an analyte, the variant with one water molecule and when the material is completely covered by water was considered. Analysis of the calculated data shows that even 10% replacement of titanium by vanadium leads to an increase of the Fermi energy by ~0.3 eV, further the Fermi energy behaves differently in the case of different analytes. For all cases the values of electrical resistance are calculated. In contrast to the Fermi energy, the value of electrical resistance behaves the same in all cases: the resistance gradually grows with an increase in the vanadium fraction up to 75% and then decreases. Special attention should be paid to the effect of moisture on the resistance value, which is of greater importance for all materials of mixed composition completely covered by water molecules. Thus, the results of the quantum mechanical calculations carried out agree with the experimental results and confirm that varying the composition of MXenes in the Ti2C-V2C system allows to control their electronic properties (in particular, the Fermi level shift). Thus, when the degree of titanium substitution for vanadium is 10% there is an increase in the Fermi energy by ~0.3 eV. It has been also established that regardless of the composition of the analytes under consideration on the surface of the MXenes the maximum resistance value is achieved for the Ti0.5V1.5C composition, and the minimum - for the individual Ti2C MXenes. The results obtained in the first year of the project create a fundamental basis for conducting research to examine in detail the chemoresistive effect of synthesized MXenes of variable composition in the Ti2C-V2C system (including modified surface) when forming corresponding multisensor systems using microplotter printing, which can significantly approach the problem of creating effective gas sensors functioning at room temperature.

 

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Annotation of the results obtained in 2022
Methods for MXenes synthesis in the Ti2CTx-V2CTx system using etching systems in the form of metal fluoride solutions (LiF, NaF, KF, NH4F) in hydrochloric acid (in situ HF) were developed, the higher efficiency of NaF-HCl system was determined experimentally. The effect of variations in hydrochloric acid concentration, temperature and process duration on the composition of the products was studied. The delamination technique of the formed multilayer MXenes was developed by combining their preliminary interaction with tetramethylammonium hydroxide solution with further ultrasound (US) exposure. Before the formation of receptor components based on (TixV1-x)2C/MOx (where M is titanium and vanadium cations) heterostructures obtained by partial oxidation of the corresponding MXenes , the formation of stable disperse systems suitable in their characteristics for application as functional inks was considered. Thus, in the first case stable such systems were formed at the stage of multilayer MXenes delamination, including additional US exposure. In the second case the multilayer (TixV1-x)2C MXenes were subjected to an additional controlled heat treatment (in the temperature range of 100-300°C) in air for partial oxidation of the material surface. With the aim of deagglomeration the disperse systems based on the above nanocomposites were subjected to US treatment, after which they acquired sufficient sedimentation stability for use as functional inks for microplotter printing of receptor components. The obtained functional inks based on (TixV1-x)2C/MOx heterostructure (where M is titanium and vanadium cations) were used in the formation of receptor components on the surface of individual chemosensor chips and single-crystal multielectrode microchips by microplotter printing. At the same time, printing conditions (geometric parameters of capillary dispenser, its movement speed, number of layers of the miniature receptor components formed, character of the digital trajectory used, etc.) were optimized with respect to the used individual sensors and multielectrode microchips peculiarities, whose distance between the strip coplanar electrodes is about 50 μm. Sensitivity to air humidity, as well as to a number of the most practically significant gas analytes was examined for the obtained materials. On the example of delaminated Ti2CTx (synthesized under the influence of NaF-HCl system) and multilayer V2CTx (obtained by etching of V2AlC with HF-HCl) the effect of their partial oxidation on chemoresistive properties was analyzed. For this purpose the combined thermal analysis data (in an air) and in situ Raman spectroscopy results were used in order to found the temperatures at which the formation of the corresponding oxide phases (TiO2 with anatase structure for Ti2CTx and V2O5 for V2CTx) is observed, but the MXene modes are preserved. For the first time, it was revealed that cooling the V2CTx/V2O5 coating from 250°С to room temperature results in a material composition change to V2CTx/V3O7. For delaminated Ti2CTx high humidity sensitivity (at room temperature), which significantly exceeds that of the Ti2CTx/TiO2 heterostructure, especially at high relative humidity values, is noticed. For multilayer V2CTx the opposite situation is observed: as a result of its partial oxidation the response to humidity increases almost 9-fold, as well as the direction of resistance change when interacting with the analyte (from p- to n-type). For the Ti2CTx/TiO2 few-layered two-dimensional heterostructure, the sensitivity to ammonia remains high when detecting under 50% relative humidity and at room temperature, higher responses to NO2 and ethanol are observed, and the response to oxygen decreases sharply. In the case of multilayered V2CTx, the formation of V2CTx/V3O7 heterostructure also results in a change in sensitivity and selectivity: while for V2CTx the highest response is observed for ammonia, the V2CTx/V3O7 nanocomposite was more sensitive to NO2. This may be due to a change in the composition of the MXenes surface functional groups when they are heated in an air medium. It was also found that both Ti2CTx and V2CTx showed better kinetic characteristics under partial oxidation and a significant (in some cases 15-fold) increase in response values to gas analytes, which may be related to a change in the detection mechanism as a result of the MXene surface groups modification and semiconducting TiO2 particles appearance. In addition, the chemoresistive properties of the multisensor array based on the Ti2C/TiO2 and V2C/VOx heterostructures of various thicknesses formed on the surface of the multielectrode microchip were evaluated with respect to acetone and 2-propanol in the concentration range [0; 140] ppm and [0; 147] ppm. Using the classification protocol of the obtained responses, the algorithm found that the pairwise classification coefficient for all analytes combinations was 100%, which indicates a high potential selectivity of the studied heterostructures with relatively high sensitivity for different VOCs detection. The chemoresistive effect in (TixV1-x)2C/MOx heterostructures (M is titanium and vanadium cations), where x varies from 0.1 to 0.9, printed as miniature components on a multielectrode chip, was also examined when exposed to reducing gas vapors mixed with pure air at various operating temperatures ranging from 25 to 400 °С. Thus it was shown that the formed (TixV1-x)2C/MOx (where M is titanium and vanadium cations) heterostructures are sensitive to reducing gas vapors in a mixture with pure air in concentrations from ppm at various operating temperatures up to 320 °C and are promising for gas sensors development. A multisensor approach was used to analyze the vector signal of receptor components array based on various MXene heterostructures. The resulting data clusters corresponding to different analytes are sufficiently distant in the LDA space, which ensures their selective recognition. At the same time, a decrease in the analytes concentration leads to a shift of all clusters along individual nonoverlapping trajectories. Using quantum-chemical calculations describing the peculiarities of material-analite interactions for the obtained (TixV1-x)2C/MOx heterostructures (where M is titanium and vanadium cations) the density of states, Mulliken charge redistribution, electron transmission function, and chemoresistance response for each case of analytes interaction with partially oxidized MXenes with gradual substitution of titanium for vanadium and analytes landing (methanol molecules, ethanol, acetone, ammonia, and water) were calculated. Of all the partially oxidized MXenes studied, the most humidity-sensitive ones were identified. According to the calculations carried out, by applying oxidized MXenes with different vanadium concentrations on the chip, the maximum chemoresistive response to the following analytes can be achieved: methanol, ethanol, acetone, ammonia and water. Regardless of the analyte nature on the material surface, the maximum resistance value is achieved when 50% of titanium atoms are replaced by vanadium, the minimum resistance value is achieved for the individual V2C MXene. The calculation results show that the electrical resistance of the oxidized MXen can change ~6.5 times (from 1.492 kOhm to 9.672 kOhm) when changing the metal ratio in the studied (TixV1-x)2C/MOx heterostructures (where M is titanium and vanadium cations).

 

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