Synthesis of Ti₃C₂T_x nanosheets / ZnO nanowires composite material for NO₂ gas sensing

2.1 Raw materials and equipment

The chemicals used were Zn(NO₃)·6H₂O, NaOH, C₆H₁₂N₄ (HMTA), NaF, CH₃CH₂OH, LiF (purity ≥ 99 %, Aladdin's reagent), HCl, Ti₃AlC₂ MAX powder, and polydiallyl dimethyl ammonium chloride (PDDA, Aladdin's reagent). Deionized water was used throughout the experiment.

The experimental equipment included JJ1023BC electronic scale, TG16-WS centrifugal machine, SK330BT supersonic cleaner, DHG-9145A drying oven, XinYi-10 freeze drier, OTF-1200X tube furnace, DF-101S magnetic stirrer, and SD101 fourchannel gas sensitive measuring instrument.

2.2 Sample Preparation

• Synthesis of one-dimensional ZnO nanowires

The ZnO nanowires were fabricated by a hydrolysis approach. Firstly, solution A was obtained by dissolving 1.19 g Zn(NO₃)·6H₂O and 0.169 g NaOH in 40 ml distilled water under continuous stirring. Then, 0.28 g C₆H₁₂N₄(HMTA) and 0.84 g NaF were dissolved in 40 ml distilled water to obtain solution B. Next, the solutions A and B were mixed for 0.5 h through magnetic stirring. Subsequently, the mixed solution was transferred to a polytetrafluoroethylene (PTFE)-lined autoclave, which was then heated at 120 ℃ in a constant-temperature furnace for 10 h. The white precipitate was Collected, then. The precipitate was separated by centrifugation and cleaned with deionized water for several times until the solution was neutral. Finally, the samples were dried in a drying oven at 80 °C for 8–10 h to obtain a white powder for further use (Wang et al., 2004). The one-dimensional (1D) ZnO nanowires were obtained by heating the dried sample to 400 ℃ at a rate of 5 ℃/ min in a tube furnace and maintaining it for 2 h.

• Preparation of 2D Ti₃C₂T_x

Firstly, 1 g LiF powder was dissolved in 10 mL HCl with a concentration of 9 mol/L and stirred at room temperature for 30 min. In an ice bath, 1 g Ti₃AlC₂ MAX powder was slowly added to the above solution to avoid initial overheating of the solution due to exothermic reaction, and then the solution was transferred to an oil bath at 35 °C and continuously stirred for 24 h (Li et al., 2018; Kim et al., 2018; He et al., 2021; Pei et al., 2021). Then, the powder was dispersed for 1 h to obtain Ti₃C₂T_x. After centrifugation at 3500 rpm for 5 min, the supernatant was discarded, and the precipitate was washed with deionized water to a pH of 6. Finally, the sample was vacuum dried for 10 h to obtain the Ti₃C₂T_x powder.

• Fabrication of Ti₃C₂T_x/ZnO composites by electrostatic self-assembly

Firstly, 1 g of the synthesized 1D ZnO nanowires was dispersed in 300 mL of 5 % PDDA, followed by ultrasonication for 3 h. After filtration, the PDDA-modified ZnOPDDA was washed with deionized water and dried for 12 h in a drying oven at 70 ℃. Subsequently, 1 g PDDA-modified ZnO powder was dissolved in 10.00 mL deionized water. Then, a certain amount of Ti₃C₂T_x was added, and the powder was vigorously stirred under the protection of argon gas for 10 h. Here, the composite material samples with the Ti₃C₂T_x mass fraction of 14 %, 25 %, and 40 % are denoted as M1, M2, and M3, and the pure Ti₃C₂T_x MXene is called M.

2.3 Characterization of Ti₃C₂T_X/ZnO composites

The prepared Ti₃C₂T_x/ZnO composites were characterized by X-ray diffraction (XRD; A25X, Bruker, Germany), scanning electron microscopy (SEM; NOVA NANOSEM −450, FEI, USA), high-resolution transmission electron microscopy(HRTEM, JEM-2100Plus, JEOL Ltd., Japan), X-ray photoelectron spectroscopy (XPS; Thermo Fisher, Thermo Scientific, USA), and Brunauer-Emmett-Teller (BET) surface area analysis (BELSORP, Microtrac MRB, Japan).

2.4 Fabrication of gas sensing device

An appropriate amount of Ti₃C₂T_x/ZnO was placed in a beaker, and a small amount of pre-prepared organic solvent was added to form a thick liquid slurry. The slurry sample was printed on an alumina matrix by using screen printing to form a material film with a thickness of 5–10 μm. Then, these alumina matrixes were dried at 70℃ for 40 min and calcined at 350 ℃ for 2 h, followed by heat treatment at 550℃ for 2 h to enhance the mechanical bond of nanowires in the films (Hou et al., 2021). Finally, a thick porous film was formed for gas-sensing performance test.

2.5 Gas-sensing performance test

The gas-sensing performance of the samples was evaluated using a four-channel testing device (SD101, Wuhan Huachuang Ruike Tech. Co. Ltd., Wuhan, China). The test system included a computer, a gas flow controller, and a four-channel test chamber. During the test, the test chip was placed in a closed chamber filled with air. When the resistance showed a stable baseline, different working temperatures and different concentrations of target gas were set. At least three samples of each type were tested, and the average values were calculated. The gas response can be evaluated as follows: $$S=R_g/R_a$$ where R_a is the sample resistance in air, and R_g is the sample resistance in the presence of NO₂ gas.

3.1 Morphology and composition

The crystal structure of the prepared samples was examined by XRD, and the results are shown in Fig. 1. It can be seen that the ZnO sample has narrow and sharp characteristic peaks without any impurity peak. The diffraction peaks are located at 31.8°, 34.3°, 36.35°, 47.54°, 56.53°, 62.92°, 67.9°, and 69.1°, which correspond to the (100), (002), (101), (102), (103), (112), and (201) crystal planes, respectively (Keerthana et al., 2022; Gong et al., 2014; Srinivasan et al., 2022) in accordance with the Standard JCPDS Card No. 36–1451. The XRD results suggest that the prepared ZnO sample is a typical feldspathic zincite structure.. The diffraction peaks of Ti₃AlC₂ precursor are in good agreement with the Standard JCPDS Card No. 52–0875 (Li et al., 2019), and there are no impurity peaks, which proves that Ti₃AlC₂ is of high purity. The typical characteristic peaks at 9.65°and 39.13°correspond to the (002) and (104) crystal planes, respectively. However, these diffraction peaks of Ti₃AlC₂ precursor are not observed in the spectra of pure Ti₃C₂T_x and composite samples M1, M2 and M3, while the strong characteristic peak corresponding to (002) plane of Ti₃C₂T_x appears at 6.9°. This indicates that through selective etching of Ti₃AlC₂ precursor by LiF + HCl solution, Ti₃C₂T_x with high purity has been successfully prepared. Comparing the XRD patterns of composite materials M1, M2 and M3 with those of pristine ZnO and Ti₃C₂T_x, the presence of the independent characteristic diffraction peak of ZnO can be verified. Meanwhile, the characteristic diffraction peak of Ti₃C₂T_x appears at 6.9°, indicating that ZnO and Ti₃C₂T_x are successfully combined. Besides, the relative diffraction peak intensity of Ti₃C₂T_x decreases as the relative content of Ti₃C₂T_x decreases from 40 % (M3) to 25 % (M2) and then to 14 % (M1).

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Fig. 1

XRD patterns of Ti₃AlC₂, Ti₃C₂T_x MXene sheets, ZnO nanowires, and M1, M2 and M3 composite materials.

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The micromorphology of the samples was investigated by SEM. Fig. 2(a-d) show the SEM images of Ti₃AlC₂ MAX phase, pure Ti₃C₂T_x, ZnO nanowires, and Ti₃C₂T_x/ZnO composite material, respectively. Fig. 2(e-f) show the SEM image of Ti₃C₂T_x/ZnO composite material at high magnification. It is clear from Fig. 2(a-b) that the Ti₃AlC₂ layers are tightly adhered together without stratification. After etching, the spacing between the layers becomes significantly larger, resembling a typical open stacked sheet structure (Han et al., 2020). This indicates that the Al atom layer in the Ti₃AlC₂ precursor structure can be selectively etched by LiF + HCl system, resulting in a typical 2D accordion morphology, with obvious microscopic pores between the layers. Besides, it can be seen in. Fig. 2(c) that the surface of ZnO nanowires is smooth and compact, with excellent crystallinity, and the dispersion is uniform. Fig. 2(e-g) show that the 1D ZnO nanowires are uniformly distributed on the surface and between the layers of 2D Ti₃C₂T_x. The composite samples have a 2D layered structure, where the ZnO nanowires are inserted in the 2D layered structure of Ti₃C₂T_x, indicating the successful synthesis of Ti₃C₂T_x/ZnO. EDS elemental mapping was employed to reveal the spatial distribution of elements in the as-formed Ti₃C₂T_x/ZnO, as shown in Fig. 2(h-l). It is shown that the C, O, Ti, and Zn elements are uniformly distributed throughout the heterostructures. It is showed that the surface of Ti₃C₂T_x sample formed through etching by LiF + HCl has −O and −F functional groups, while the nanowires on the surface of sheet structure are ZnO, which is consistent with the results of SEM and TEM.

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Fig. 2

SEM images of (a) Ti₃AlC₂ MAX phase; (b) pure Ti₃C₂T_x nanosheets; (c) ZnO nanowires; (d-g) Ti₃C₂T_x/ZnO composite materials. (h-l) EDS mapping analysis of C, O, F, Ti, and Zn.

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The microstructure and internal crystal structure of the Ti₃C₂T_x/ZnO composite were examined by HRTEM. Fig. 3(a) shows that the ZnO nanowires are uniformly distributed on the surface and between the layers of Ti₃C₂T_x, which is consistent with the SEM results. Fig. 3(c) presents the HRTEM image of the composite, where the lattice fringe spacings of 0.26 and 0.307 nm are attributed to the (0 0 2) plane of ZnO and the (0 0 2) plane of Ti₃C₂T_x, respectively. These results are consistent with the previous characterizations.

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Fig. 3

(a-b) TEM image of the synthesized Ti₃C₂T_x/ZnO composites from different perspectives; (c) HRTEM image showing the interlayer spacing of the synthesized Ti₃C₂T_x/ZnO composites.

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The specific surface area and pore structure of Ti₃C₂T_x/ZnO and Ti₃C₂T_x samples were analyzed by N₂ adsorption–desorption isotherm measurement. The results are shown in Fig. 4. The isotherm reveals a H3-type hysteresis loop at a relative pressure (P/P₀) of 0.7–1.0, indicating that the samples have an obvious mesoporous structure. The BET specific surface area of M1, M2 and M3 are calculated to be 23.92,24.22 and 24.75 m²/g, respectively.The difference among M1, M2 and M3 is very small. This indicates that the effect of the difference in titanium carbide content in the composites on the specific surface area is minimal.The BET specific surface area of M is calculated to be 7.2923 m²/g. This implies that the specific surface area of the Ti₃C₂T_x/ZnO composite material is nearly three times higher than that of Ti₃C₂T_x, indicating the potential of the proposed structure in gas sensing application.

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Fig. 4

N₂ adsorption–desorption isotherms of Ti₃C₂T_x (M)and Ti₃C₂T_x/ZnO composite.

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XPS was used to examine the elemental composition and chemical state of different materials. Fig. 5(a) shows the full survey spectrum of ZnO, where only Zn, O, and C elements can be observed. This confirms a high purity of the ZnO nanowires. The highresolution XPS spectrum of Zn 2p in Fig. 5(b) shows two strong peaks at 1021.8 and 1044.9 eV, which are attributed Zn 2p_3/2 and Zn 2p_1/2, respectively, indicating the normal oxidation state of Zn²⁺ in ZnO nanowires (Zhao et al., 2021). Besides, the spacing between the two Zn 2p peaks is 23.1 eV, which is consistent with the energy splitting of ZnO. Fig. 5(c) shows the high-resolution XPS spectrum of O1s, where the peak at 529.53 eV can be assigned to O^2– in ZnO nanowires, while the peak at 530.28 eV is ascribed to the surface hydroxyl group formed due to the reaction of chemisorbed oxygen with water in the atmosphere (Zhao et al., 2019).Fig. 5(d) shows the full survey spectrum of Ti₃C₂T_x. It is evident that the Ti₃C₂T_x sheets are composed of Ti, C, O, and F elements. The spectra of individual elements was verified by Gaussian fitting. In the fitted C 1 s spectrum, there are four peaks at the binding energies of 280.63, 283.58, 284.33, and 288.28 eV, which can be indexed to C-Ti, C-C, C-O, and CH_x-C-O bonds (Halim et al., 2016; Wu et al., 2017), respectively, proving a typical carbon bonding in Ti₃C₂T_xMXene. As shown in Fig. 5(e), the Ti 2p XPS spectra of Ti₃C₂T_x can be divided into six peaks at the binding energies of 453.9, 454.98, 458.23, 460.18, and 464.8 eV, which correspond to Ti-C(Ti²⁺), Ti-X(Ti³⁺), TiO₂-X-F_x, C-Ti-F_x, and TiO₂(Ti⁴⁺), respectively (Halim et al., 2014).Fig. 5(g) shows the full survey spectrum of Ti₃C₂T_x/ZnO composite. The characteristic peaks of Ti, O, C, Z, and F can be observed in the full spectrum, which is consistent with the EDS mapping results. The C 1 s spectra shows three peaks corresponding to different carbon bonds: C-O, C-C, and C-Ti-T_x. The fitted Ti 2p spectra shows four different peaks, which represent C-Ti-O_xF_x, Ti-C, C-Ti-O_x, and CTi-F_x, TiO₂. However, compared to the Ti 2p spectrum of Ti₃C₂T_x, the area of C-Ti-F_x peak is significantly decreased because the attached ZnO has added the oxygen terminal to Ti₃C₂T_x.

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Fig. 5

(a) XPS full survey spectrum of ZnO; XPS spectra of (b); Zn 2p and (c) C 1 s for ZnO; (d) XPS full survey spectrum of Ti₃C₂T_x; high-resolution XPS spectra of (e) Ti 2p and (f) C 1 s for Ti₃C₂T_x;(g) XPS full survey spectrum of Ti₃C₂T_x/ZnO; high-resolution XPS spectra of (h) Ti 2p and (i) C 1 s for Ti₃C₂T_x/ZnO composite.

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3.2 Gas-sensing properties

To determine the optimal operating temperature of composites and Ti₃C₂T_x/ZnO pure Ti₃C₂T_x, the sensing response of the samples M, M1, M2, and M3 to 100 ppm NO₂ at different operating temperatures was examined, and the results are shown in Fig. 6(a). As the operating temperature increases, the response values of M, M1, M2, and M3 gas sensors first increase and then decrease. The response of all the samples becomes maximum at 250 ℃. Specifically, the maximum response values of M, M1, M2, and M3 to 100 ppm NO₂ are 4.3, 17.07, 18.66, and 8.5, respectively. Therefore, 250℃ is selected as the optimal operating temperature of all the sensors. Fig. 6(b) shows the dynamic response-recovery curves of different sensors at the optimal operating working temperature. It is evident that the response value of all the composite materials is higher than that of the pure Ti₃C₂T_x. The response of sample M2 reaches a maximum of 18.66. Specifically, compared to the pureTi₃C₂T_x, the sensitivity of Ti₃C₂T_x/ZnO composite is enhanced by almost 4.3 times. Further, within the range of 100 s, the sensitivity of the four different sensor materials rises rapidly.the M2 is determined to represent the best gas-sensing performances of Ti₃C₂T_x/ZnO composites in the future measurements. Moreover, all of the responses of these sensors based on Ti₃C₂T_x/ZnO composites are much higher than thatof their pristine components, indicating that the gas-sensing performances can be improved by the pure Ti₃C₂T_x. Fig. 6(c) shows that the sensor exhibits extremely high sensitivity for ppm-level NO₂ gas at 250 ℃. In particular, the response of the Ti₃C₂T_x/ ZnO composite shows a good linear relationship with the NO₂ concentration (R² = 0.95344), which indicates that the Ti₃C₂T_x/ZnO composite-based sensor can be used for real-time detection of NO₂ gas. Fig. 6(d) shows that good linear fits are also acquired.The limit of detection (LOD) of the sensor based on the Ti₃C₂T_x/ZnO composite is estimated to be 0.105 ppm based on the equation LOD = 3σ_D/σ (here, σ_D is the standard deviation of noise during the response and σ is the slope of the linear part).

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Fig. 6

(a) Response of materials towards 100 ppm NO₂ at different temperatures. (b)Response-recovery of different materials to 100 ppm NO₂ at 250 ℃. (c) Dynamic response-recovery curves for different NO₂ concentrations (20–100 ppm) at 250 ℃. (d) Linear fitting of the response curve of Ti₃C₂T_x/ZnO composite to 0–100 ppm NO₂ at 250 ℃.

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Repeatability and long-term stability are the two primary performance indicators for the practical application of a gas sensor. The repeatability of a gas sensor is measured by changing the gas concentration. Fig. 7(a) shows the dynamic resistance vs. time curve of the M2 composite sample for five cycles under exposure to 100 ppm NO₂. The M2 composite shows an excellent repeatability without any significant attenuation in the response under alternate exposure to air and 100 ppm NO₂. Furthermore, Fig. 7(b) shows the response of Ti₃C₂T_x/ZnO composite exposed to 100 ppm NO₂ for a continuous 30 days. The test was performed once every 5 days for 6 consecutive times, and the results show a slight fluctuation or negligible reduction, indicating good long-term stability. At the optimal operating temperature, the response and recovery curves of samples M and M2 exposed to 100 ppm NO₂ are shown in Fig. 7(c,d). After NO₂ gas is injected, the response of samples M and M1 increases rapidly and becomes stable at the maximum value. The response time and recovery time of M2 are calculated to be 17 and 102 s, respectively, which are both lower than those of Ti₃C₂T_x gas sensor. The results demonstrate that the Ti₃C₂T_x/ZnO composite-based sensor exhibits a comparatively fast response/recovery to NO₂ gas at 250℃. Besides, it can be seen that the sensor cannot fully recover to its initial resistance. This may be because the sensor measures the concentration of target gas at a high temperature, and a certain amount of residual NO₂ gas molecules are adsorbed on the sample, occupying the effective adsorption sites of oxygen, which inevitably reduces the recovery resistance value. The improvement in the gas sensitivity of Ti₃C₂T_x can be ascribed to the formation of heterojunction between Ti₃C₂T_x and ZnO (Wang et al., 2022). The good conductive network provided by 2DTi₃C₂T_x can greatly enhance the mobility of free charge carriers in the gas–solid reaction process, improve the reaction rate at the gas–solid interface, facilitate the gas adsorption, and shorten the response/ recovery time.

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Fig. 7

(a) Cyclic transient sensing response towards 100 ppm NO₂; (b) long-term stability of Ti₃C₂T_x/ZnO composite; (c-d) response and recovery time of the sensors based on M and M2 composites to 100 ppm NO2 at 250 ℃.

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Selectivity is an important characteristic of gas sensors. To evaluate the ability of the gas sensor to recognize NO₂ and avoid the interference of background gases, the selectivity of Ti₃C₂T_x/ZnO composites was tested. Fig. 8 shows the response of Ti₃C₂T_x/ZnO composite-based sensor to six different gases (ethanol, acetone, acetaldehyde, methanol, NO₂ and SO₂) at the optimal operating temperature of 250 ℃. The gas response values of M2 sensor to 1000 ppm ethanol, acetone, acetaldehyde, and methanol are 0, 0, 1.94, and 0.86, respectively. The gas response values of M1 and M2 sensors to 100 ppm SO₂ are 3.87 and 5.02. Thus, the response of Ti₃C₂T_x/ZnO composite-based sensor to NO₂ is much higher than that to other gases, indicating its excellent selectivity to NO₂. This suggests that the M1 and M2 gas sensitive elements show good response and selectively toward NO₂ in the presence of several background gases.

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Fig. 8

Selectivity of Ti₃C₂T_x/ZnO composite to various target gases.

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3.3 Gas sensing mechanism

Herein, a possible mechanism for the gas sensing property of Ti₃C₂T_x/ZnO composite material is established based on the modulation of depletion layer. After the contact in vacuum, the charge carrier transfer process occurs at the Ti₃C₂T_x interface due to the difference in Fermi energy and then stops until the static charge equilibriumis reached when the Fermi level (E_f) flattens under the action of internal electric field, causing band bending, as shown in Fig. 9(c). When the Ti₃C₂T_x/ZnO gas sensor is exposed to air, O₂ molecules can not only capture free electrons from the conduction band of ZnO and convert them into O^2–, but also obtain electrons from Ti₃C₂T_x through surface functional groups (–OH and −O, −F) to produce O^2–. The adsorbed O^2– ions can further gain electrons to form O^-ions, causing an increase in the Schottky barrier and the formation of electron depletion layer (Su et al., 2019), as shown in Fig. 9(d). When the Ti₃C₂T_x/ZnO gas sensor is exposed to NO₂, which is a typical oxidizing gas with high electron affinity, NO₂ can be adsorbed on surface of Ti₃C₂T_x/ZnO composite and capture electrons form NO^2–. The depletion layer at the interface widens, and the resistance increases, as shown in Fig. 9(e). As NO₂ further reacts with a large amount of O^2-on the material surface to form NO^2–, the ZnO conduction band and Ti₃C₂T_x continue to lose electrons. Besides, the depletion layer further widens and the resistance increases, and the adsorbed O^-ions on the surface of ZnO spheres also play a major part in the chemical adsorption of NO₂ (Yang et al., 2016; Al-hadeethi et al., 2017).

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Fig. 9

(a-b) Schematic illustration of the NO₂ gas sensing mechanism of Ti₃C₂T_x/ZnO nanocomposite. Schottky barrier in (c) vacuum, (d) air, and (e) NO₂.

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