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

Abstract

The in-situ Raman spectroscopy oxidation of the accordion-like V₂CT_x MXene has been studied. It was found that a nanocomposite of V₂CT_x/V₃O₇ composition was formed as a result. The elemental and phase composition, the microstructure of the synthesized V₂CT_x powder and MXene film as well as the V₂CT_x/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 V₂CT_x and V₂CT_x/V₃O₇ films have an increased sensitivity to ammonia and nitrogen dioxide, respectively, at room temperature and zero humidity. It was shown that the V₂CT_x/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 V₂CT_x MXene.

1. Introduction

The family of two-dimensional (2D) transition d-metal carbides with the general formula M_n+1C_nT_x has attracted the attention of the scientific community in recent years due to its unique properties and high variability of the surface chemistry [1,2]. Currently, the most studied MXenes are the Ti₂CT_x and Ti₃C₂T_x carbide compounds to which the vast majority of the work has been devoted [3,4,5,6]. However, there are more and more studies devoted to other two-dimensional transition metal carbides, including Mo₂CT_x, Nb₂CT_x, V₂CT_x [7,8,9,10] and various nanocomposites based on them [11,12,13,14], are also emerging. At the moment, researchers continue to search and find different ways to synthesize and apply MXenes. The vast majority of works on V₂CT_x are devoted to their use in the composition of lithium-ion batteries and supercapacitors [15,16,17]. V₂CT_x MXene is also used in ferromagnetics [18], antibacterial coatings [19], memristors [20], hydrogen storage devices [21], in catalysis during the oxygen evolution reaction (OER) [22], as well as in chemoresistive gas sensors [23].

The strong interactions of gases with the surface of MXenes corresponding to high negative adsorption energies (as confirmed by DFT calculations) allow this class of compounds to be successfully used as a sensitive materials in chemoresistive gas sensors [24,25]. The mixed and metallic conductivity of individual MXenes allows for obtaining responses at room temperature with a high signal-to-noise ratio (SNR) [26], which distinguishes them from metal oxide semiconductors (MOS), the classical receptor materials for chemoresistive gas sensors [27,28,29,30].

For V₂CT_x MXene, there is sporadic work in the literature on its use in the composition of chemoresistive gas sensors. In [31] the responses to various gases capable of hydrogen bonding (NH₃, triethylamine, ethanol, methanol, acetone, and formaldehyde) were studied depending on the type of etching agent. It has been shown that after the synthesis, the surface functionalization of V₂CT_x MXenes changes and different types of responses (so-called n- and p-type, which correspond to the decrease and increase in electrical resistance when the analyte is injected) to the same gases are observed. In [32], for the obtained V₂CT_x MXene, the authors managed to fix the response to 5–50 ppm NO₂; however, data on the responses to other gases are not given, which makes it difficult to assess the selectivity of the obtained MXene. In [33], Eunji Lee et al. carried out a comprehensive study of the gas-sensitive chemoresistive properties of V₂CT_x MXenes, and found that the greatest response of a large number of analyzed gases was obtained for hydrogen. Xingwei Wang et al. [34] described a synergistic process to fabricate a supercapacitor and simultaneously a gas sensor for ammonia based on a polyaniline/V₂C MXene composite with autonomous power supply from an electromagnetic–triboelectric hybrid generator. The authors were able to obtain responses of 0.3–10 ppm NH₃ at 20 °C with high selectivity. The described approach is extremely promising and state-of-the-art.

The use of MXenes as sensitive materials at room temperatures is due to the fact that they are prone to oxidation at increasing operating temperatures in an air atmosphere. In [35], using TG/DTA and XRD analyses it was shown that V₂CT_x MXene is completely oxidized to V₂O₅ at temperatures above 500 °C. The thermograms presented in the article show an intense exothermic effect with a maximum at 332 °C, accompanied by an increase in mass, when heating the MXenes in an air atmosphere, which is associated with the oxidation process of the MXenes. Unfortunately, the authors did not present data on the phase composition of oxidized MXene in the specified temperature range, which complicates the interpretation of the given data. In [36] the authors studied in situ changes in the phase composition of V₂CT_x MXene in different gas environments using Raman spectroscopy. According to the data obtained, it can be seen that in the air atmosphere the oxidation of MXene begins around 400 °C. Such studies are extremely important for fine control of the phase composition in the context of obtaining V₂CT_x/VO_x nanocomposites for various applications, including chemical gas sensing.

Previously, we have obtained titanium carbide MXenes [37,38], and studied the phase transformation of the Ti₂CT_x MXenes film using in situ Raman spectroscopy and studied the effect of the oxidation process on its chemoresistive properties [39]. The chemoresistive properties of the complex vanadium–titanium carbide Ti_0.2V_1.8C have also been previously studied [40]. The present work is devoted to the study of the oxidation process of V₂CT_x MXene film using in situ Raman spectroscopy and to the study of the gas-sensitive chemoresistive properties of the obtained nanomaterial to a wide group of analyte gases.

2. Materials and Methods

2.1. Synthesis and Application

Reagents: vanadium metal powders (99.9%, 0.5–100 µm, Ruskhim, Moscow, Russia), aluminum (99.2%, 30 µm, Ruskhim, Moscow, Russia), graphite (MPG-8 grade, Ruskhim, Moscow, Russia), potassium bromide KBr (99%, Ruskhim, Moscow, Russia), sodium fluoride NaF (>99%, Reahim, Moscow, Russia), hydrofluoric acid (50%, Honeywell International Inc, Charlotte, NC, USA), hydrochloric acid (36%, Sigma Tek, Moscow, Russia).

The synthesis of V₂CT_x MXene was carried out by selective etching of aluminum contained in the MAX-phase of V₂AlC under the influence of hydrofluoric and hydrochloric acid solutions. Methods for the synthesis of V₂AlC and V₂CT_x, which are close to those used in this work, are described in detail in [40]. Briefly, to obtain V₂AlC powders of aluminum, vanadium, graphite and potassium bromide were mixed in the ratios n(V):n(Al):n(C) = 2:1.2:0.8 and m(V + Al + C) = m(KBr), co-milled, compacted into tablets and subjected to heat treatment in a muffle furnace at 1000 °C [39,40].

To obtain an accordion-like V₂CT_x MXene, a 1 g MAX-phase V₂AlC sample was usually typically added to a solution containing 12 mL of HF (50%) and 8 mL of HCl (36%). After stirring at room temperature for 30 min, the system was heated to 40 ± 5 °C and kept under these conditions under stirring for 120 h. The resulting powder was separated by centrifugation and washed repeatedly with distilled water until pH ~5–6. The sample was then washed twice with ethanol and centrifuged again. The resulting precipitate was redispersed in ethanol in an ultrasonic bath for 30 min, most of the impurities (primarily MAX-phase particles) were separated by centrifugation of the dispersion at 1000 rpm for 5 min. The dispersion of MXene in ethanol was evaporated, and the resulting phase was dried in a vacuum at 150 °C.

The MXene receptor layer was deposited on a special Al₂O₃ substrate with platinum interdigital electrodes and a heater (on the reverse side) by microextrusion printing (10 µm resolution) using a three-coordinate positioning system equipped with a pneumatic and capillary dispenser in the form of a hollow needle with an internal diameter of 150 µm. A dispersion of V₂CT_x MXene in butanol with a concentration of ~5 mg/mL was used. The coating was dried in a desiccator and then held in a vacuum at 150 °C for 5 min.

2.2. Instrumentation

Micrographs and the chemical composition of the surface of the samples were obtained by scanning (NVision 40 scanning electron microscope, Carl Zeiss, (Oberkochen, Germany), secondary electron detector, accelerating voltage 1–10 kV) and transmission electron microscopy (JEOL, JEM-1011, Akishima, Japan), and by X-ray spectral elemental microanalysis (INCA X-MAX 80 energy dispersive X-ray (EDX) spectrometer, Oxford Instruments (Oxford, UK), accelerating voltage 20 kV). X-ray diffraction was performed on powders and films using a D8 Advance (Bruker, Billerica, MA, USA, CuKα = 1.5418 Å, Ni filter, E = 40 keV, I = 40 mA; 2θ range: 5–45°; resolution: 0.02°; point accumulation time: 0.3 s).

A Renishaw (New Mills Wotton-under-Edge, Gloucestershire, UK, GL12 8JR) inVia Reflex Microscope system equipped with a Peltier-cooled CCD (532-nm Nd:YAG laser line was used) was used to record Raman spectra. The laser light was focused on the sample to ~2 μm through a 50× objective (power on the sample was <0.3 mW). A THMS600 stage (Linkam Scientific Instruments Ltd., Redhill, Unit 9, Perrywood Business Park, Honeycrock Lane, Salfords, Redhill, UK, Surrey RH1 5DZ) was used to record variable temperature Raman scattering. The in-situ film heating rate was 5 °C/min. The sample temperature was pre-calibrated using a high-precision Testo 868 thermal imaging camera. When recording Raman spectra, there is local heating of the imaging area, so the spectra were recorded in different areas of the film, but close to each other. All spectra were normalized with respect to the most intense spectrum.

The chemoresistive responses were performed at room temperature using the laboratory setup described in [41]. A gas-air atmosphere was created in a quartz cell (volume −7 × 10⁻⁵ m³) using two Bronkhorst gas flow controllers with a maximum throughput of 100 and 200 mL/min. The electrical properties of the oxide films obtained were measured using a Fluke 8846A Digit Precision Multimeter with an upper detection limit of 1000 MΩ. The sensor temperature was monitored using a pre-calibrated platinum micro-heater. Prior to gas-sensitive measurements, the film was held in a baseline gas atmosphere until a stable signal was obtained.

Different relative humidities (RH) were generated using a special unit with a bubbler flask. The RH of the gas mixture was measured with a digital flow hygrometer “Excis” (EXIS, Russia, Moscow). The temperature value of the relative humidity was set and then measured at 20 °C.

All gas-sensing measurements were performed at room temperature (RT) and 0% RH. The response to H₂, CO, NH_3, benzene (C₆H₆), acetone (C₃H₆O), methane (CH₄) and ethanol (C₂H₅OH) were calculated using the following ratio:

$${\mathrm{S}}_1=\frac{\left|{\mathrm{R}}_{\mathrm{BL}}-{\mathrm{R}}_{\mathrm{g}}\right|}{{\mathrm{R}}_{\mathrm{BL}}}\times 100\%$$

(1)

where R_BL—baseline resistance (synthetic air was used as the baseline gas), R_g—resistance at a given concentration of analyte gas.

The response to humidity was calculated using the following ratio:

$${\mathrm{S}}_2=\frac{\left|{\mathrm{R}}_{\mathrm{BL}}-{\mathrm{R}}_{\mathrm{RH}}\right|}{{\mathrm{R}}_{\mathrm{BL}}}\times 100\%$$

(2)

where R_BL—resistance at 0% relative humidity, R_RH—at a given relative humidity.

3. Results and Discussion

3.1. The Phase Composition and Microstructure of the Obtained V₂CT_x MXene before Oxidation

Figure 1a shows the X-ray patterns of the V₂AlC MAX-phase powders that were used for the synthesis, as well as an X-ray pattern of the resulting V₂CT_x MXene powder. As can be seen, the set of V₂AlC MAX-phase reflexes correlates well with the data reported in the literature (V₂AlC, ICSD-606283, Space group P6₃/mmc). In addition to the characteristic intense reflex (002) at 2θ = 7.4° on the X-ray pattern of the obtained V₂CT_x MXene sample, there are reflexes of impurity phases: the V₂AlC MAX-phase and the cubic vanadium carbide phase VC (ICSD-159870, Space group Fm-3m), which should not affect the gas sensitive properties of MXene. The position of the observed reflex (002) for V₂CT_x MXene is in good agreement with other experimental data for this compound [42,43,44]. Figure 1b shows an X-ray pattern of the V₂CT_x MXene film before oxidation on an Al₂O₃ substrate. Because of the low thickness of the obtained film, only the characteristic set of α-Al₂O₃ substrate reflexes (PDF 00-005-0712) is clearly visible in the X-ray pattern shown. The inset on the left shows a region of the X-ray pattern in the 2θ = 4.5–12° interval with a long signal accumulation time at the point where a widened reflex (002) of the V₂CT_x MXene phase is observed. Its position is in good agreement with both the literature data and the data obtained for the powder of the corresponding composition in the present study (Figure 1a).

Figure 2 shows SEM and TEM micrographs of the synthesized V₂CT_x MXene powder. The microstructure of V₂CT_x MXene has an accordion-like structure typical of this fabrication technique. The average interlayer distance was calculated from the Figure 2a micrograph, and was found to be 22 ± 5 nm. The value obtained is quite high, indicating a large specific surface area of the obtained multilayer V₂CT_x MXene, which is important for gas sensing. The TEM data (Figure 2c) confirm the formation of the multilayer harmonic, which is in good agreement with the SEM data. Figure 3a–c shows SEM micrographs of the V₂CT_x MXene film on an Al₂O₃ substrate. As can be observed from the presented data, the microstructure of the films is in complete agreement with the microstructure of the powder: the accordion-like structure of the MXene is maintained, the interlayer spacing values are in the range of the calculated values for the powder. Using EDX analysis, it was found that besides C, V, forming elements V₂CT_x MXene and aluminum (typical for an impurity of the initial MAX-phase V₂AlC or aluminum fluorides), the MXene surface T-functional groups contain oxygen, fluorine and chlorine (Figure 4a). Their ratio n(F):n(Cl) = 19:1 is determined by peculiarities of the selective etching process by the chosen system (HF + HCl).

3.2. Thermal Analysis of V₂CT_x Powder

The study of the thermal behavior of the obtained V₂CT_x multilayer powder sample during its heating in an air flow (Figure 4b) allows for a more correct planning of the experiment in the in-situ oxidation of the MXene receptor layer during heating during the recording of Raman spectra. Thus, in the initial stages of heating at temperatures <120–230 °C there is a mass loss of ~1.6–1.7%, probably due to the removal of adsorbed water molecules. On heating >240 °C there is a further decrease in mass, which may be due to the removal of MXene functional groups, primarily OH-groups. This process at temperatures above 240–260 °C is overlaid by exothermic processes of the beginning of MXene oxidation: with a maximum at 300 °C for more dispersed MXene aggregates (a stack diameter of 100–300 nm), surface areas of larger V₂CT_x aggregates (~500–1000 nm stack diameter) and internal areas of large accordion-like aggregates, to which diffusion is difficult (with a maximum at 352 °C). In this case, the oxidation process becomes dominant at 350 °C, as the decrease in mass loss due to functional groups removal is replaced by a tendency to increase in mass. A low intensity exothermic effect in the interval 490–570 °C can be attributed to the oxidation of more oxidation-resistant phases VC and V₂AlC. Thus, the onset of the oxidation process of V₂CT_x MXene with simultaneous heating in air and recording of Raman spectra should be expected at temperatures ~240–250 °C.

3.3. In Situ Raman Spectroscopy during Heating of V₂CT_x MXene Film

The main purpose of using in situ Raman spectroscopy in the present study was to study the oxidation process of the V₂CT_x MXene film as well as the formation of the V₂CT_x/VO_x composite for further study of the gas-sensitive properties.

Figure 5 shows the in-situ Raman spectra of the V₂CT_x MXene film when heated in the temperature range RT–250 °C. At room temperature, four MXene characteristic modes ω₁–ω₄ are observed at 269, 431, 689 and 912 cm⁻¹, respectively. These modes can be related to the individual V₂CT_x MXene [36,45,46,47]. In addition, D-(ω_D) and G-modes (ω_G) are present in the spectra at 1354 and 1582 cm⁻¹, which are common to many carbon systems with sp²-hybridization of carbon atoms [48]. The presence of D- and G-modes is characteristic of the entire MXene family; it illustrates the formation of layered carbon in their composition or the formation of a graphene-like carbon impurity during excessive etching of the MAX-phase. The described set of characteristic modes ω₁–ω₄ and ω_D, ω_G is maintained up to 200 °C. It is worth noting that all modes except for the D- and G-bands are of low-intensity, which is a consequence of two factors: the nature of the MXenes themselves, which are not characterized by intense Raman bands, and the conditions of the Raman experiment, since the study was performed at low laser power to avoid local overheating of the coating by the laser itself and MXene oxidation. In addition, as we have recently shown for Ti₂CT_x [40], the MXene modes on the Al₂O₃ substrate are much weaker than on the platinum substrate.

The changes in the Raman spectra of the V₂CT_x MXene film start to appear at 250 °C. In addition, new modes are added to the V₂CT_x MXene ω₁–ω₄ bands: intense ω₅ at 144 cm⁻¹ as well as ω₆–ω₁₁ at 280, 306, 409, 525, 700 and 995 cm⁻¹, respectively. The Raman bands described are characteristic of α-V₂O₅ (orthorhombic crystal lattice, Space group Pmmn) [49]: B_2g, B_1g, A_1g, A_1g, A_1g and (A_1g and B_2g) modes, respectively [50]. It should be noted that the D- and G-modes are maintained at the heating temperature of 250 °C, which also confirms the fact that the MXene oxidation did not proceed completely and the V₂CT_x/V₂O₅ composite was formed at these temperature conditions. Thus, the temperature of 250 °C was chosen as optimal for in situ heating of the V₂CT_x MXene film using Raman spectroscopy.

After cooling the sample to room temperature, Raman spectra were also recorded for the obtained oxidized V₂CT_x MXene film. It was found that the spectrum obtained is significantly different from that recorded for the sample heated to 250 °C (Figure 5). The spectrum contains a completely different set of Raman modes: ω₁₂–ω₂₀ at 168 (intense), 224, 261, 298, 845, 880 (intense), 936, 990 and 1031 cm⁻¹, respectively, which correspond with high accuracy to the characteristic peaks for another vanadium oxide, V₃O₇ (monoclinic crystal lattice, Space group C2/c) [50]. It is to be noted that the ω₁–ω₃ and ω_D–ω_D modes characteristic of V₂CT_x MXene are maintained in the spectrum, indicating that the V₂CT_x/V₃O₇ composite has formed during cooling. Thus, after cooling from 250 °C to room temperature, the obtained film undergoes structural changes: the vanadium (V) oxide α-V₂O₅ formed during MXene oxidation is transformed into the mixed oxide V₃O₇ containing vanadium atoms in the +IV and +V oxidation states. It is worth noting that the formation of V₃O₇ is not a common case for vanadium oxides, which are characterized by individual V₂O₅ and VO₂ oxides, as well as composites containing their individual phases [49,51].

3.4. The Phase Composition and Microstructure of the Obtained V₂CT_x MXene after Oxidation

Figure 1b shows an X-ray pattern of the V₂CT_x/V₃O₇ film on an Al₂O₃/Pt substrate after heating to 250 °C using in situ Raman spectroscopy. It shows intense reflexes related to the substrate materials α-Al₂O₃ and Pt (PDF 00-005-0712 and 00-004-0802, respectively), as well as low-intensity reflexes of the V₃O₇ phase. The resulting set of V₃O₇ reflexes correlates well with the available data in the database (ICSD 2338), which is particularly evident in the corresponding inset in Figure 1b, right. The XRD data obtained are in full agreement with the Raman data and confirm the V₃O₇ content in the films obtained after heating to 250 °C in air.

Figure 3d–f shows SEM microphotographs of V₂CT_x/V₃O₇ films after heating to 250 °C using in situ Raman spectroscopy; it is shown that the microstructure of the films undergoes significant changes. The microphotographs show a shape similar to the initial multilayer harmonic, which is characteristic of MXene, but this is no longer observed after oxidation. The formed structures become more closed and porous with a pore size of ~25–40 nm, which is close to the value of the interlayer distances that were calculated for the individual multilayer aggregates of V₂CT_x MXene.

3.5. Gas-Sensing Chemoresistive Properties

In the first step, chemoresistive responses were determined for the obtained V₂CT_x and V₂CT_x/V₃O₇ films when detecting 100 ppm CO, NH₃, NO₂, benzene (C₆H₆), acetone (C₃H₆O), ethanol (C₂H₅OH), and 1000 ppm methane (CH₄) and H₂ at room temperature and 0% RH. Experimental responses (S1) for these gases are shown in Figure 6a,b. Selectivity diagrams (Figure 6c,d) were constructed from the series of responses obtained. The obtained responses for V₂CT_x do not show high numerical values; nevertheless, due to the high SNR value (which is typical for the whole MXene family) it was possible to efficiently detect different gases. For the original V₂CT_x MXene film, the highest response (1.35%) was observed in the detection of NH₃. Notable responses were also recorded for NO₂ (0.96%) and CO (0.94%), while the response for all other analyzed gases did not exceed 0.55%. For the oxidized V₂CT_x/V₃O₇ MXene, a significant increase in the response value for all analyzed gases was observed: the response for NH₃ increased more than 9 times to 12.7%, and for NO₂ more than 15 times to 14.9%. The response to all other analyzed gases did not exceed 10.3%. The numerical values of the responses to all the studied gases of the MXene film before and after oxidation are shown in Figure 6c,d.

For ammonia, for which high responses were observed during detection, the sensitivity to different concentrations was studied. Figure 7a,b shows the responses to 4–100 ppm NH₃ at RT of the MXene film before and after oxidation. As can be seen, there is a consistent increase in the response (S1) with increasing NH₃ concentrations from 4 to 100 ppm in both cases: from 0.18 to 1.35%, and from 2.7 to 12.7% for V₂CT_x and V₂CT_x/V₃O₇, respectively, with a small baseline drift (which can be explained by the high interaction energy of the ammonia molecule with the receptor material). Thus, both V₂CT_x and V₂CT_x/V₃O₇ films demonstrate the ability to precisely detect different analyte content in the gas atmosphere at room temperature.

Figure 7c,d shows the response of the obtained V₂CT_x and V₂CT_x/V₃O₇ films when the relative humidity RH is changed from 0 to 95%. For the original V₂CT_x film, the response (S2) is 10% when the RH is changed to 95%, which is quite high compared to the response for the other gases for which the response does not exceed 1.35%. The oxidized V₂CT_x/V₃O₇ film also shows a significant increase in sensitivity to RH: the response at 70% RH is 85.6%. At the same time, it should be noted that in the range RH = 50–95% the response to humidity changes much less significantly than at humidity <50%. In general, the high sensitivity of MXenes to humidity is typical and well-described in the literature, but we have not found any reports in the available sources of an increase in their sensitivity to humidity during the formation of the V₂CT_x/V₃O₇ composite.

Figure 8 shows a column diagram of the selectivity, taking into account the sign of the response. Positive values correspond to an increase in electrical resistance (p-response), and negative values correspond to a decrease (n-response). As can be seen from the presented data, for the original V₂CT_x MXene film in the detection of all gases (including an increase in RH) there is a p-response, which is typical for MXene, except for NO₂, for which an n-response is recorded. However, for the oxidized V₂CT_x/V₃O₇ film, significant changes are observed: for all gases, except NH₃ and NO₂, the response is reversed from p- to n-response. When NH₃ is detected, the p-type response is retained, and when NO₂ is detected, the n-type response is retained. The observed processes with the change of direction of resistance change are a consequence of the change of mechanisms of gas detection.

In the detection of various gases, the p-response (with an increase of resistance at the analyte injection) is quite typical for various individual MXenes, including V₂CT_x [33]. The resistance increase observed in this case can be explained by the so-called “swelling mechanism”, according to which gases can intercalate into the MXenes’ interlayer space, which causes an additional increase in the interlayer distance, complicating the charge transfer outside the MXenes plane [33]. In addition, it is believed that the sorption of any gases due to the charge transfer between them and the MXenes metal atoms interferes with the transport of charge carriers and reduces their quantity.

As in the case described in the present study, the investigation by Yajie Zhang et al. [32] also recorded an n-response in the detection of NO₂, which the authors associated with possible reactions between the NO₂ molecule and the functional groups -O and -OH on the surface of the V₂CT_x MXene. As a result, ${\mathrm{NO}}_2^{-}$-groups may be formed on the MXene surface, which leads to a decrease in electrical resistance. This mechanism is supported by the fact that, according to EDX analysis, the V₂CT_x powder obtained contains a large number of oxygen-containing fragments (O, OH and probably H₂O in the interlayer space) in addition to F- and Cl-groups, which may also be involved in the gas-sensing mechanism and influence the response characteristics [31]. There are a few studies in the literature on the use of an individual V₂CT_x as a sensor material; nevertheless, the available data are in good agreement with the data obtained in the present study [31,32,33,34].

Vanadium dioxide (VO₂) and vanadium pentaoxide (V₂O₅) are known to be n-type semiconductors [52,53]. The vanadium oxide (V₃O₇) formed in this case is intermediate between VO₂ and V₂O₅ and is also an n-type semiconductor. The n-type response obtained when the V₂CT_x/V₃O₇ film detected all the gases studied except NH₃ can be explained by the classical detection mechanism for MOS-sensors through the ion adsorbed oxygen on the surface of the V₃O₇ semiconductor nanoparticles [54,55,56]. CO, C₆H₆, C₃H₆O, C₂H₅OH, CH₄ and H₂ are reducing gases, so when they react with ionized oxygen ($O_y^{x-}$), they should oxidize to form CO₂ and H₂O, which can lead to the release of additional electrons while the electrical resistance decreases, corresponding to an n-response [57,58,59,60]. The obtained data on the sensing properties agree well with those reported in the literature for V₂O₅ [30] at elevated sensing temperatures. When NH₃ and NO₂ were detected by the V₂CT_x/V₃O₇ nanocomposite film, p- and n-responses, respectively, were observed (i.e., the detection character of V₂CT_x MXene was maintained), which is not typical for n-type semiconductors. In this regard, it can be assumed that the detection of these gases is mainly due to the characteristic of the MXene mechanism.

The increase in response values for the V₂CT_x/V₃O₇ nanocomposite can be associated with the formation of metal defects in the MXene layers during oxidation and the increase in specific surface area that improved adsorption of gases, as well as with the formation of heterojunctions. This leads to an effective separation of charge carriers at the interface and increases the sensitivity to adsorption of electron-donor and -acceptor gases.

It is to be noted that the oxidation of the V₂CT_x film also changes the direction of the humidity-sensing response from the n- to the p-type. For the individual V₂CT_x film, the p-type response with increasing humidity is also traditionally explained by the swelling mechanism resulting from the adsorption of molecules between the MXene layers. On the other hand, the n-type response of the V₂CT_x/V₃O₇ composite film can be explained by the mechanisms of the interaction of water molecules with MOS-materials [54], according to which hydroxylgroups are formed on the surface of the metal oxide semiconductor. As a result, electrons are emitted and a decrease in electrical resistance is observed, which corresponds to the n-response [27].

Separately, it should be noted that there are no data in the literature on the use of V₃O₇ as a gas-sensitive chemoresistive material. This fact can be explained by the metastability of this oxide, and the operation of MOS-gas sensors usually occurs at elevated temperatures. The data obtained in this study are probably the first experimental confirmation of the high gas sensitivity of this oxide.

4. Conclusions

In summary, the present study is the first to investigate the effect on the gas-sensitive properties of accordion-like V₂CT_x MXene synthesized by selective etching of V₂AlC MAX-phase by HF-HCl acid mixture, and doping with nanosized V₃O₇ as a result of MXene oxidation controlled in situ by Raman spectroscopy.

The elemental and phase composition, the microstructure of the synthesized V₂CT_x MXene, the coatings based on it, and the V₂CT_x/V₃O₇ nanocomposite obtained at a minimum oxidation temperature of 250 °C were studied by using a variety of physical and chemical analytical methods.

It was found that the obtained V₂CT_x and V₂CT_x/V₃O₇ films are sensitive at room temperature and zero humidity to a wide range of investigated gases (H₂, CO, NH_3, C₆H₆, C₃H₆O, CH₄, C₂H₅OH and NO₂), but the highest responses were observed for ammonia and nitrogen dioxide. At the same time, the partial oxidation of V₂CT_x leads not only to an increase in the response of the V₂CT_x/V₃O₇ material, but also to some change in selectivity. Thus, if for the original V₂CT_x multilayer MXene the responses at 100 ppm (RT, RH = 0%) decreased in the series NH₃ (1.35%) > NO₂ (0.96%) > CO (0.94%), then for the V₂CT_x/V₃O₇ composite the series of highest responses changes: NO₂ (14.9%) > NH₃ (12.7%) > acetone (10.3). Despite the close response values for NO₂ and NH₃, this does not negatively affect the selectivity due to the different nature of the resistance change when these gases are injected.

It is shown that the oxidation of the V₂CT_x film and the formation of the V₂CT_x/V₃O₇ nanocomposite also leads to a significant increase in the humidity response value (RH = 0–95%) and a change in its character from n- to p-type. At the same time, for V₂CT_x/V₃O₇ there is a saturation of the response value after reaching the relative humidity of 50%, in contrast to the initial V₂CT_x MXene.