Alcohol Vapor Sensor Based on Quasi-2D Nb2O5
Derived from Oxidized Nb2CTz MXenes

Hanna Pazniak, Ilya A Plugin, Polina M Sheverdyaeva, Laetitia Rapenne, Alexey S Varezhnikov, Antonio Agresti, Sara Pescetelli, Paolo Moras, Konstantin B Kostin, Alexander V Gorokhovsky, et al.

► To cite this version:

Hanna Pazniak, Ilya A Plugin, Polina M Sheverdyaeva, Laetitia Rapenne, Alexey S Varezhnikov, et al.. Alcohol Vapor Sensor Based on Quasi-2D Nb2O5 Derived from Oxidized Nb2CTz MXenes. Sensors, 2023, 24 (1), pp.38. 10.3390/s24010038 . hal-04803666

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Distributed under a Creative Commons Attribution 4.0 International License
1 Article

2 Alcohol Vapor Sensor Based on quasi-2D Nb₂O₅ Derived from
3 Oxidized Nb₂CTz MXenes

4 Hanna Pazniak ¹*, Ilya A. Plugin ², Polina M. Sheverdyaeva ³, Laetitia Rapenne ¹, Alexey S. Varezhnikov ², Antonio
5 Agresti ⁴, Sara Pescetelli ⁴, Paolo Moras³, Konstantin B. Kostin ², Alexander V. Gorokhovsky ², Thierry Ouisse ¹ and
6 Victor V. Sysoev ²,*

7 ¹ Université Grenoble Alpes, CNRS, Grenoble INP, LMGP, Grenoble, CS 50257, 38016 Grenoble Cedex 1, France

8 ² Yuri Gagarin State Technical University of Saratov, ul. Polytechnicheskaya 77, Saratov 410054, Russia

9 ³ Instituto di Struttura della Materia-CNR (ISM-CNR), SS 14, Km 163.5, I-34149 Trieste, Italy

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11 ⁴ Center for Hybrid and Organic Solar Energy, Electronic Engineering Department, University of Rome Tor Vergata, Rome, Italy

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14 * Correspondence: hanna.pazniak@grenoble-inp.fr; vsysoev@sstu.ru

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**Abstract:** MXenes are two-dimensional (2D) materials with a great potential for sensor applications due to their high aspect ratio and fully functionalized surface that can be tuned for specific gas adsorption. Here, we demonstrate that the Nb₂CTz-based sensor exhibits high performance towards alcohol vapors at heating up to 300-350 °C, with the best sensitivity towards ethanol. We attribute the observed remarkable chemiresistive effect of this material to the formation of quasi-2D Nb₂O₅ sheets as the result of the oxidation of Nb-based MXenes. These findings are supported by synchrotron X-ray photoelectron spectroscopy studies together with X-ray diffraction and electron microscopy observations. For analyte selectivity, we employ a multi-sensor approach where the gas recognition is achieved by linear discriminant analysis of the vector response of the on-chip sensor array. The reported protocol demonstrates that MXenes layers are efficient precursors for the derivation of 2D oxide architectures, which are suitable for developing gas sensors arrays.

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**Keywords:** MXene; gas sensor; niobium oxide; X-ray photoelectron spectroscopy, multisensor array

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29 **Citation:** To be added by editorial
30 staff during production.

31 Academic Editor: Firstname
32 Lastname

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34 Received: date

35 Revised: date

36 Accepted: date

37 Published: date

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Copyright: © 2023 by the authors

40 Submitted for possible open access

41 publication under the terms and

42 conditions of the Creative Commons

43 Attribution (CC BY) license

44 (https://creativecommons.org/licenses/by/4.0/)

45 s/by/4.0/).

1. Introduction

Air quality is one of the most important issues in both environmental monitoring and health care industry. Safety rules and standards in these fields are constantly modified to reduce the allowable concentration of toxic gases [1]. Ethanol vapors, which are produced in large amount in distillation, winemaking and chemical industries, are among these harmful gases. Inhalation of ethanol vapors can cause serious health problems, while their storage at high concentration can lead to dangerous accidents [2]. To ensure safe working and environmental conditions, gas sensors based on wide-band-gap metal oxide semiconductors like NiO [3], ZnO [4], SnO₂ [5], WO₃ [6] have been developed and employed to monitor an ethanol concentration. The working principle of such sensors exploits the change of charge carrier concentration and resistance of the sensing material upon adsorption of gas molecules called as a chemiresistive effect. However, the ability of pristine oxides to adsorb gases is moderate and must be improved to enhance sensor performances [7]. Moreover, many sensors based on semiconductor oxides use bulk structures that have rather low sensitivity. For this scope, current research in many laboratories deals with, i) doping strategies, ii) designing heterostructures of two or more materials, and/or iii) enhancing the specific surface area of sensor materials via
nano-structuring by growing nanorods, nanowires or spheres, for instance [8].

In this regard, two-dimensional (2D) materials show a great promise for sensing applications due to their high specific surface area, which offers a large number of active sites for gas adsorption [9]. Among the expanded family of 2D materials, MXenes are of particular interest due to their tunable terminations and related functional properties. MXenes are early transition metal (M) carbides or nitrides (X), which derive by selective chemical etching of the A layer (typically Al) from bulk MAX phase precursor. As the result, MXene surfaces become fully functionalized with abundant groups inherited from the etchants. These groups are usually mixtures of =O, -OH, and -F when MXenes are produced by wet chemistry [10] or mixtures of halogen (-Cl, -Br) and =O when molten salts are used as an etchant [11]. Since their discovery in 2011 [12], MXenes have demonstrated excellent performance in gas-sensing applications. In particular, Ti₃C₂Tₓ, V₂CTₓ and Mo₂CTₓ are reported to be good for the selective detection of ammonia [13, 14], nonpolar gases [15], and water vapors [16] at low concentrations. In addition, due to the hydrophilicity of MXenes, solution processed deposition methods can be easily adapted for the fabrication of gas sensors.

Over the last years, various MXene chemistries have been synthesized and investigated from the fundamental and applicative point of view [17, 18]. Nb₂CTₓ is one of the promising phases whose properties drastically depend on the nature of the surface functional groups [19]. Nb₂CTₓ has already been tested for applications in energy storage [20] and biomedicine [21], but limited information on the gas sensing properties of this phase is available. There are only few studies demonstrating the sensitivity of Nb₂CTₓ-based optics fiber sensor for tyramine [22] and humidity [23] detection or reporting the performances of Nb₂CTₓ-based chemiresistive sensor to NO₂ [24] and NH₃ [25] vapors. The enhanced sensitivity of these sensors was achieved either by functionalization of Nb₂CTₓ with amino groups [24] and Au nanoparticles [22], or by mixing Nb₂CTₓ with nano- [25] or microfibers [23].

Herein, we explore the gas-sensing properties of pristine Nb₂CTₓ MXenes with respect to various volatiles, including alcohols, ammonia, and water vapors. We show that sensors based on pristine Nb₂CTₓ exhibit no response at room temperature. Instead, when the operating temperature of sensor is increased, a reliable sensor performance is obtained at 300-350 °C. We ascribe this behavior to the oxidation of MXenes to the 2D Nb₂O₅ oxide phase as directly evidenced by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). These results are in line with our previous research where a substantial improvement of chemiresistive properties of pristine MXenes is attained via their oxidation to form 2D oxide heterostructures [26].

## 2. Materials and Methods

### 2.1. Synthesis and Characterization of Nb₂CTₓ MXenes

Nb₂CTₓ MXenes were synthesized by a selective chemical etching of Al layers from bulk Nb₂AlC MAX phase precursor in a one-step procedure using a mixture of LiF and HCl according to protocol reported by J. Xiao et al. [27]. In detail, 2.0 g of LiF was slowly added to 20 ml of 12M HCl solution and left under a constant stirring for 5 min. The vial with the solution was placed in an ice bath where Nb₂AlC powder was carefully added in minor portions. After that, the mixture was kept for 90 h at 60 °C under a continuous stirring. Following the reaction, the solution was washed multiple times with a deionized water with further centrifugation at 3500 rpm for 5 min until nearly neutral pH value was obtained. The presence of Li ions in the etching solution allows one a spontaneous Li intercalation between the MXene layers during the etching. This effect favors further delamination of multilayers into single or few-layered sheets using a vortex shaking process. After the vortex shaking, the mixture was centrifuged again until a stable suspension of well delaminated Nb₂CTₓ sheets appears. This step was repeated several times and the supernatant was collected over the PVDF membrane through a vacuum-assisted

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filtration to be dried under vacuum, of residual pressure down to approx. 30 mbar, at room temperature (RT).

Scanning electron microscopy (SEM) of the Nb₂AlC MAX phase and Nb₂CTz MXenes were performed using a FEG ZEISS GeminiSEM 300 instrument (Carl Zeiss AG, Oberkochen, Germany). SEM images of the sensor chip were recorded using an Aspex Explorer microscope (FEI, Delmont, USA) equipped with an energy dispersion X-ray (EDX) detector. X-ray diffraction (XRD) patterns were recorded using a D8 Bruker diffractometer (Bruker, Massachusetts, USA) operating under the Bragg-Brentano geometry and equipped with Cu anode providing X-rays with 1.5418 Å wavelength. Transmission electron microscopy (TEM) images were acquired with an JEOL 2010 microscope (JEOL, Akishima, Japan) at an acceleration voltage of 200 kV. High-resolution X-ray photo-electron spectroscopy (HR-XPS) measurements were performed in the VUV-Photoemission beamline of the Elettra synchrotron facility (Trieste, Italy).

In order to evaluate the thickness of the Nb₂CTz layer coated over the chip, we have employed a stylus profilometry (stylus profiler Dektak 150, Veeco, USA) with a stylus of 2 µm radius under applying a force equal to 1 mg. The resolution of measuring points has been approx. 0.05 µm.

## 2.2 Gas-sensing experiment

To perform gas sensor measurements, we have employed the multielectrode chip platform [28]. In brief, the chip is a Si/SiO₂ substrate patterned with Pt strip electrodes of 50 µm gaps [29]. To deposit the MXene layer on the chip, a dispersion of delaminated Nb₂CTz in isopropanol (0.1 mg/ml), was sprayed onto a working area of the chip, of 0.215 mm², using a spray brush system. The distance between the nozzle tip of the airbrush and the multielectrode chip was adjusted to 7 cm. The flow rate of the MXene suspension was optimized to 5 s/drop by changing the position of the airbrush needle. The deposition pressure was adjusted to 1 bar and deposition temperature to 100 °C. The thickness of Nb₂CTz layer was controlled by the number of deposition cycles.

After the preparation, the chip has been contacted with Al wires, of 38 µm diameter, into a window at the ceramic holder equipped with multiple metallic thin-film roads to contact pads and 50-pin interface (Erni SMC, 1.27 mm, Erni Electronics GmbH, Adelberg, Germany) for connecting the external reading electronic circuits. For analyte gas supplying, the Nb₂CTz-based chip-in-holder has been introduced to an alloy steel-fabricated chamber supplied with two sealing heat-tolerant rubber rings. The chamber has been blown by a gas flow via in-mounted tubes, of 1/8' diameter, to feed the probes primarily to the chip frontside. Figure 1 shows the measuring setup to conduct the gas-sensing measurements with the chip under study. The whole chamber and all the wires were put in a grounded metal box in order to avoid any influence of external electromagnetic field.

All the electrode pairs in the on-chip sensor array were measured upon applying d.c. electric potential, up to 5 V, in sequence by multiplexing relays (Songle Relay, China) to record the current and derive the resistance of each layer area between electrodes. These resistances, up to 10 GOhm, were processed as sensors of chemiresistive type. The current has been recorded by a multimeter (Keithley DAQ6510, Keithley Instruments, Solon, USA) to be fed via analog I/O unit (USB-6259, National Instruments Co., Austin, USA) with a rate up to 300 ms per sensor. The two modes of electrical readings were employed in course of studies, (i) measuring I(V) dependencies while applying the electric bias in the range of [-5,+5] V mainly in background air conditions to evaluate the contact between the layer under study and electrodes, and (ii) measuring I(t) transients converted to R(t) ones while applying a constant bias of 5 V to see the resistance changes upon exposing to various gas probes. The setup has been driven by PC operated with LabView-based (National Instruments Co., Austin, USA) software.

The gas-sensing measurements have been performed when supplying analyte gases via tubes, 1/4' diameter, joint by Swagelok fittings (Swagelok Co., Solon, USA) at the
flowrate of 400 sccm to be managed by high-precision mass-flow controllers (Bronkhorst, Ruurlo, Netherlands). As the background air, we have employed a lab one to be filtered and dried by a corresponding generator (Purge PG14L, Peak Scientific, Glasgow, UK) purged with a compressor (Precision Compressed Air 230v, Peak Scientific Instruments Ltd., Glasgow, UK) yielding 40 psi pressure to feed the total gas delivery system.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** The experimental setup to measure the response of Nb₂CTₓ MXene-based chip to various analyte gases emitted via bubbling.</figcaption></figure>

To enrich the air with analyte probes, we have used bubblers (Gebr. Rettberg GmbH, Göttingen, Germany) containing liquids of distilled water, various alcohols (methanol, ethanol, isopropanol, butanol), and 25 % NH₃ solution in water. The saturated vapors of each analyte emitted primarily following the bubbling were diluted by air at necessary ratios to yield analyte probes in concentrations of 50–8000 ppm range where the limits were managed by the employed flow controllers and physical properties of the analyte liquids. In case of measurements in a humid air as a background, the dry air was first passed through water-based bubbler containing 25 % of relative humidity or ca. 10400 ppm of H₂O concentration. The analyte gas exposures to the chamber with installed chip were monitored independently by a commercial semiconductor sensor (MQ3, Winsen Co., Zhengzhou, China). The analytes of each type/concentration were supplied to the chip for 90 min to be further cleared in by a pure background air, dry or humid one depending on the targeted experimental conditions, for 180 min at least. The measurements of the chemiresistive response of the Nb₂CTₓ film as a layer in the chip have been conducted at the temperature range of 25–300 °C to be maintained by chip in-built meander heaters. The power dissipated over the heaters was managed by proportional-integral-derivative (PID) controller accounting for data from local thermoresistors which were initially calibrated with high-resolution IR camera (InfRec R500EX-P-D, Nippon Avionics Co., Yokohama, Japan) equipped with 21 µm close-up lens.

### 3. Results and Discussion

#### 3.1. Synthesis and Characterization of Nb₂CTₓ MXenes

To produce Nb₂CTₓ MXenes, a selective chemical etching of Al layers from bulk Nb₂AlC was carried out in a one-step procedure using a mixture of LiF and HCl as detailed in Materials and Methods. These etchants are widely used for the synthesis of Ti₃C₂Tₓ MXenes and allow one to produce large flakes under a mild delamination condition. We have adapted this procedure to obtain Nb₂CTₓ to be schematically illustrated in Figure 2. Nb₂AlC MAX phase consists of alternating Nb₂C and Al layers (Figure 2a) and exhibits a densely packed structure (Figure 2e) as revealed by SEM. After the selective removal of Al, a characteristic accordion-like multilayer structure appears
(Figure 2f) due to the release of H₂ gas arisen during the reaction of Al and HF. This structure indicates the conversion of MAX phase to MXenes, which was additionally confirmed by XRD. The synthesis of Nb₂CTz under optimized conditions is reproducible to yield MXenes with the same structural characteristics.

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<figure><img src="image_4.png" /><figcaption>**Figure 2.** Schematic depiction of Nb₂CTz MXene synthesis and Nb₂CTz-based multielectrode chip: a) structure of Nb₂AlC MAX phase precursor; b) structure of Nb₂CTz sheets produced via a selective chemical etching of Al layer in the mixture of LiF and HCl; c) photo of delaminated suspension of Nb₂CTz MXenes; d) cartoon of the chip fabrication, e-g) SEM images of MAX precursor (e), multilayered MXenes (f) and delaminated Nb₂CTz sheets in the layer (g); i-h) optical and SEM images of the chip covered by Nb₂CTz layer via a spray casting and EDX spectrum taken in the inter-electrode gap (h).</figcaption></figure>

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The XRD plots given in Figure 3a show that (002) peak of the parent MAX precursor is significantly shifted after the MXene synthesis to the lower 2θ position, which corresponds to a change in the c-parameter from 13.88 Å to 17.36 Å. This is due to increasing the interplanar spacing as a result of Al etching and subsequent intercalation of water and Li ions.

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<figure><img src="image_5.png" /><figcaption>**Figure 3.** Structural characterization of Nb₂CTz MXenes. a) XRD patterns of Nb₂AlC MAX phase precursor and derived Nb₂CTz MXenes by the chemical etching of Nb₂AlC; b) TEM image of Nb₂CTz flakes with corresponding SAED pattern (inset).</figcaption></figure>

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XRD pattern of the MXenes reveals also the presence of low-intensity peaks of the initial precursor. However, unreacted MAX phase and non-delaminated MXene multilayers are easily separated from the delaminated suspension.

The presence of Li ions in the etching solution favors further delamination of multilayers into single or few-layer sheets. After delamination, Nb<sub>2</sub>CT<sub>z</sub> suspension exhibits a characteristic light blue color when diluted (Figure 2c) [30]. We drop-casted the delaminated suspension over Si/SiO<sub>2</sub> substrate and observed the poly-flake morphology of the deposited layers by SEM (Figure 2g). It is clearly seen that the film consists of translucent flakes, irregular in shape and of rather large sizes (1-2 µm). From the observation of single MXene flakes in TEM, we notice that Nb<sub>2</sub>CT<sub>z</sub> exhibits typical flake features, which are illustrated in Figure 3b, in analogy to delaminated Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub> [26]. The selected area electron diffraction (SAED) pattern of the flakes shows (inset, Figure 3b) the hexagonal arrangement of atoms which retains the symmetry of the parent precursor and confirms the high crystallinity of the produced 2D sheets. It is important to note here that as-prepared pristine Nb<sub>2</sub>CT<sub>z</sub> MXenes do not exhibit any oxidation, as evidenced by the absence of oxide peaks in XRD and SAED patterns.

To clarify the physical processes occurring in the Nb₂CTz MXene layer under heating, we performed high-resolution X-ray photo-electron spectroscopy (HR-XPS) measurements. The data are summarized in Figure 4: top row spectra refer to as-synthesized Nb₂CTz while bottom row are spectra to Nb₂CTz exposed to 350 °C. Figure 4a reveals the presence of two Nb 3d doublets, which correspond to Nb-C in the MXene (Nb 3d<sub>5/2</sub> at 203.95 eV, metallic lineshape) and to Nb<sub>2</sub>O<sub>5</sub> (Nb 3d<sub>5/2</sub> at 207.00 eV) that forms spontaneously during the process of exfoliation. In the C1s spectrum, there are four components (Figure 4b). The peak located at 282.00 eV is the counterpart of the Nb-C doublet and has an asymmetric shape, thus confirming the metallic nature of Nb₂CTz. The other peaks are assigned to adventitious carbon (284.50 eV), C-O (286.00 eV) and O=C-OH (287.95 eV). The O1s spectrum can be described with three peaks (Figure 4c) corresponding to Nb<sub>2</sub>O<sub>5</sub> (529.9 eV), C-O (530.75 eV) and the convolution of adsorbed water, O-containing termination of MXenes and O=C-OH (532.55 eV).

<figure><img src="image_6.png" /><figcaption>**Figure 4.** HR-XPS data of as-prepared Nb<sub>2</sub>CT<sub>z</sub> MXenes (a-c) and after oxidation at 350 °C (d-f). XRD pattern and SEM image (inset) for the oxidized Nb<sub>2</sub>CT<sub>z</sub> MXenes (g).</figcaption></figure>

After exposure of the spray-coated Nb₂CTz film to 350 °C, there is no signature of metallic Nb-C bonds neither in the Nb 3d spectrum (Figure 4d) nor in the C 1s spectrum (Figure 4e). This indicates that Nb₂CTz MXenes tend apparently to oxidize faster than Ti₃C₂Tₓ due to the presence of two transition metal layers instead of three ones that are in a direct contact with air. The Nb 3d spectrum of Figure 4d consists of three doublets (Nb
3d<sub>5/2</sub> at 207.75, 212.05 and 213.85 eV) assigned to Nb<sub>2</sub>O<sub>5</sub> oxide.These shifted doublets originate from charging effects of different magnitude (0.75, 5.05 and 7.85 eV towards higher binding energy with respect to 3d<sub>5/2</sub> in Figure 4a) characterizing different parts of the sample. This interpretation is substantiated by the observation of similar shifted components in the C1s and O 1s spectra (Figure 4e,f). The peak of adventitious carbon in Figure 4e is located 0.60 eV deeper than the corresponding peak in Figure 4b. The two features at 289.90 and 291.25 eV are mainly ascribed to adventitious carbon peaks shifted by stronger charging effects. Similarly, the Nb<sub>2</sub>O<sub>5</sub> peak in the O 1s spectrum of Figure 4c is located 0.55 eV deeper than the corresponding peak in Figure 4f. Also, in this case two additional shifted components are observed (534.40 eV and 536.10 eV) [31]. The XRD pattern confirms the absence of Nb<sub>2</sub>CT<sub>z</sub> and the coexistence of orthorhombic and monoclinic crystallographic phases of Nb<sub>2</sub>O<sub>5</sub> oxide. The large and phase-dependent bandgap of Nb<sub>2</sub>O<sub>5</sub> (3.1- 5.3 eV) can explain the different energy shifts of the photoemission peaks observed in Figure 4d-f. Notably, the SEM image in the inset of Figure 4g shows that the surface of the oxidized sample has a thin flake-like structure that differs from the typical morphology of bulk Nb<sub>2</sub>O<sub>5</sub> particles.

3.2. *Gas-sensing measurements*

To perform gas sensor measurements, we have employed the multielectrode chip platform as detailed in Materials and Methods [28, 29]. While several samples were prepared at different stages of research, here we show the data related to exemplary chip to be fully characterized in order to highlight its major features. The deposited Nb<sub>2</sub>CT<sub>z</sub> sheets (Figure 2i,h) cover the working area of the chip in the form of percolating layer. The profiles of the layer have been taken across the electrodes of the chip. The height of the electrodes is approximately 1.5 µm and is shown by the black dashed line in Figure 5. Nb<sub>2</sub>CT<sub>z</sub> layer is lying between and over a top of the electrodes; its height profile is marked in red, orange and green, corresponding to the different scans. The layer thickness is estimated as the difference between cross-electrode profiles of the pristine and the coated chip and does not exceed 700 nm in the inter-electrode gaps.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** The stylus profile scans taken over multielectrode chip coated with oxidized Nb<sub>2</sub>CT<sub>z</sub> MXene layer following gas-sensor measurements at advanced temperatures.</figcaption></figure>

The EDX plot (bottom part of Figure 2h) indicates clearly the presence of Nb from the MXenes, Si and O from the substrate (due to the small thickness of the spray-coated film), and residual traces of Al.

After the chip preparation and its storage under air conditions, the gas-exposure tests at room temperature revealed no changes in the resistance of the Nb<sub>2</sub>CT<sub>z</sub> film that went beyond the range of 10 GOhm to be possible measured. We consider that the MXenes were spontaneously oxidized upon the deposition, probably in the form of Nb<sub>2</sub>CT<sub>z</sub>/Nb<sub>2</sub>O<sub>5</sub> or Nb<sub>2</sub>O<sub>5</sub> phases, resulting in non-measurable conductivities. Therefore, to
activate the film conductance, we have sequentially heated the chip and could then measure resistance, still in the GOhm range, at temperatures close to 300 °C. Figure 6a shows I-V curves measured when the chip was exposed to dry air. Although the curves look rather linear, these data indicate the existence of slight Schottky barrier between the layer and Pt electrode related to differences in work functions of these materials [32]. When the chip was exposed to test analyte vapors, we could observe a reliable reduction of resistance with a magnitude depending on analyte concentration. The typical R(t) transient is shown in Figure 6b for the case of methanol vapors mixed with dry air, 100-8000 ppm concentration. For comparison, we show the signal of commercial semiconductor sensor given as an electric bias, U, in Volts. As one can see, resistance of Nb₂CTz film fully follows the varying signal of the commercial sensor although at lower amplitude change. Similar behavior was observed when the chip was exposed to other test analytes which is consistent with the chemiresistive effect in n-type wide-band semiconducting oxides [33] that highlights the reducing nature of the test analytes.

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Gas-sensing performance of Nb₂CTz/Nb₂O₅-based chip at 300 °C: a) I-V curves measured in dry air for three sensor elements; b) typical R(t) transient of single sensor element (top) and voltage signal of commercial sensor exposed to methanol vapors; c) the chemiresistive response of the sensor elements toward various analyte concentrations; d) selectivity diagram of chemiresistive response at analyte concentration equal to 2000 ppm; e) 3D cross-section of total 6D LDA space; the points relate to chip vector signals to analytes, 2000 ppm concentration, ellipses frame areas for vector data scattered under Gaussian distribution with 0.8 confidence around gravity centers of analyte-related clusters; denotings are Amm – ammonia, Iso- isopropanol, But – butanol, Meth-methanol, Eth- ethanol, Hum – humidity.</figcaption></figure>

The major sensor characteristic, the calibration function of the response, S, versus gas concentration, C, is given in Figure 6c in a double log scale. The response is defined as a relative change of film conductance in percent:

$$S = \left( \frac{G_i(\text{gas})}{G_i(\text{air})} - 1 \right) \cdot 100, \qquad (1)$$

where Gᵢ(gas) and Gᵢ(air) are the local conductances of the film located between each couple of electrodes (sensor element) recorded upon an exposure to the analyte gas probe and background air, respectively.

As one can see, all the S(C) curves describing the analytes follow the S~Cⁿ function to be compatible with Freundlich adsorption isotherm. The power exponent values related to the analyte probes are listed in Table 1. The lowest value of n, of ca. 0.04, is
observed for ammonia vapors, which yields a very limited sensor response, down to ca. 1-2 %, in contrast to alcohol vapors where *n* varies between 0.14 and 0.29; still, the response magnitude to alcohols lies in the range of 8-100 %. It is interesting that humidity also exhibits a reduced chemiresistive effect with a magnitude of sensor response similar to ammonia, although the *n* value of 0.16 for this analyte corresponds to those characterizing alcohols.

<table><caption>**Table 1.** The power index, *n*, of S(C) curve characterizing the gas response of sensor elements of sensor elements of Nb<sub>2</sub>CT<sub>z</sub>/Nb<sub>2</sub>O<sub>5</sub>-based chip operating at 300 °C versus various test analytes.</caption><thead><tr><th>Gas</th><th>Ammonia</th><th>Humidity</th><th>Isopropanol</th><th>Butanol</th><th>Methanol</th><th>Ethanol</th></tr></thead><tbody><tr><td>n</td><td>0,04 ± 0,02</td><td>0,16 ± 0,02</td><td>0,15 ± 0,03</td><td>0,14 ± 0,04</td><td>0,29 ± 0,06</td><td>0,15 ± 0,03</td></tr></tbody></table>

To further elucidate the effect of H<sub>2</sub>O vapors, we have performed studies exposing the chip to the same analytes mixed with a wet air, of 25 % RH. These data are summarized in Figure 6d along with data observed in dry background air. As one can see, the presence of H<sub>2</sub>O in a background atmosphere substantially lowered the sensor response to all the analytes, although the signals are still in the range of 2-11 % at a concentration of 2000 ppm. The observed chemiresistive effect is very similar to one reported earlier with non-stoichiometric Nb<sub>2</sub>O<sub>5</sub> oxide [32, 34], which is maturing from the existing of defects, primary oxygen vacancies, and redox surface reactions. Still, we guess that possible contribution from the layer/electrode barrier is driven by the similar nature, when the contact potential depends on the same processes that occur with the material under study itself. As shown above, Nb<sub>2</sub>CT<sub>z</sub> film transforms into oxide phase at the temperatures of 300-350 °C. The latter is known to have a strong non-stoichiometry in oxygen, preferentially observed in high-surface area nanostructured materials [32]. We assume that the Nb<sub>2</sub>O<sub>5</sub> formed as a result of the oxidation of Nb<sub>2</sub>CT<sub>z</sub> is non-stoichiometric, since a well-crystalline oxide is formed at higher temperature of 800 °C [33]. When a non-stoichiometric Nb<sub>2</sub>O<sub>5</sub> film is exposed to a wet air (or, a humid atmosphere), OH-groups are adsorbed on an oxygen lattice defect and reduce the sensor signal from analytes. This non-stoichiometry of derived quasi-2D Nb<sub>2</sub>O<sub>5</sub> can also explain its higher signal towards alcohols compared to ammonia in accordance to other data reported in literature [34]. Interestingly, while our previous study showed that Ti<sub>3</sub>C<sub>2</sub>T<sub>z</sub> MXenes annealed at 350 °C display the highest response towards acetone [26], Nb<sub>2</sub>CT<sub>z</sub> MXenes annealed at the same temperature showed the highest response towards ethanol. This indicates that the selectivity of MXenes is highly dependent on transition metals and derived oxides.

Still, the performance of the derived sensor toward ethanol vapors is comparable with the performance from the literature reported for the pristine Nb<sub>2</sub>O<sub>5</sub> of various morphologies. We have compared these data in Figure 7.

However, we cannot selectively distinguish the analytes taking the signal of a single sensor element. Therefore, for selectivity issue we could consider the vector signals generated by the whole sensor array located on the chip regarding all the test analytes recorded in stationary parts of R(t) curves [43]. The multisensory vector signal has been processed by linear discriminant analysis (LDA) algorithm using homemade software. This technique reduces the original multidimensional signal of the array, whose dimensionality is equal to number of sensors, into an artificial space of features with dimensionality of (N-1), where N is a number of classes (here, analytes) to recognize [44, 45]. In essence, the LDA processing goes via maximizing the inter-class variation of vector signals versus their in-class scatter. The vector array data given for example in case of analytes present in 2000 ppm concentration are shown in the LDA space of Figure 6e, where they are grouped according to origin. Here, we display 2D cross section of the total 6D LDA coordinate system.
<figure><img src="image_9.png" /><figcaption>**Figure 7.** The comparative plot of sensor performance of pristine Nb<sub>2</sub>O<sub>5</sub> structures *versus* ethanol vapors based on literature data (adapted from Refs. [35-42]) regarding the presented here results. S/C is response (S) to concentration (C) ratio.</figcaption></figure>

As one can see, the gravity centers of analyte-related clusters are well distinguished here with average Mahalonobis distance of 42 µn. While the zero point in this space relates to fully noisy data like a “white noise” having no specific features toward inputs under study, this distance could serve as an indication of selectivity. From the obtained data, it is clear that all the analytes could be distinguished with the on-chip array.

## 4. Conclusions

Thus, the present results show that 2D sheets of Nb<sub>2</sub>O<sub>5</sub> appeared following the oxidation of Nb<sub>2</sub>CT<sub>2</sub> MXenes in the temperature range 300-350 °C could be beneficial for applying in gas sensors to detect alcohols vapors, among which the highest response is observed towards ethanol, ~ 100 % at 2000 ppm. The combination of various thorough structural characterizations of the material reveals that the improvement in sensing performance is likely due to the flake-like morphology of derived Nb<sub>2</sub>O<sub>5</sub> oxide and its non-stoichiometry.

Here we should note that room temperature-operated sensors are highly attractive from power consumption viewpoint. However, such an operation in practically all the materials appeared mostly in 2D architectonics suffers from a long-term impact of adsorbed species. It ordinarily yields a significant shift into the baseline under a high influence of humidity and/or other VOCs and gases present in the normal gas atmosphere. So far, even UV illumination does not give a necessary power to accelerate the rate of adsorption/desorption of gas molecules from the material surface. Therefore, heating is remained a gold standard to operate chemiresistors as these new data further support.

The reported protocol is easily applied for developing multisensor arrays of planar type for Enose fabrication with a high sensitivity towards alcohols mixed with dry and wet air in concentrations corresponding to known detection requirements like regulations by U. S. Occupational Safety and Health Administration (OSHA).

**Author Contributions:** Conceptualization, H.P., V.S.; methodology, A.A., S.P.; formal analysis, I.A.P., P.M.S., A.S.V., P.M., K.B.K., A.V.G.; investigation, H.P., I.A.P., L.R., A.S.V., K.B.K.; writing-original draft preparation, H.P., P.M., T.O., V.V.S.; writing-review and editing, H.P., V.V.S. All authors have read and agreed to the published version of the manuscript

**Funding:** I.A.P., A.S.V., V.V.S. thank Russian Science Foundation, project no. 22-29-00793. Authors acknowledge Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities and for financial support under the IUS internal project (proposal nr. 20220537). P. M. S. and P. M. acknowledge a partial support through the project EUROFEL-ROADMAP ESFRI of the Italian MUR.

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**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of interest:** The authors have no conflicts to disclose

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