Heterostructured Ti3C2Tx/carbon nanohorn-based gas sensor for NH3 detection at room temperature

In this study, a two-dimensional Ti₃C₂T_x MXene compounded with carbon nanohorn (CNH) by an electrostatic self-assembly method was proposed and then fabricated as room temperature ammonia (NH₃) gas sensors. The successful preparation of the Ti₃C₂T_x/CNH nanocomposite has been characterized in detail. The NH₃ sensing performance based on Ti₃C₂T_x/CNH also has been tested at room temperature. The optimal Ti₃C₂T_x/CNH sensor has a response value of 21.6% to 100 ppm NH₃ at room temperature, which is 10 times higher than that of the pure Ti₃C₂T_x sensor. Furthermore, this sensor is endowed with excellent selectivity, reliable long-term stability, and reproducibility. The enhanced sensing performance is associated with the interconnected structure and the synergistic effect of Ti₃C₂T_x and CNH. This work provides an effective way to prepare MXene-based sensitive materials for NH₃ sensors, which shows excellent NH₃ detection potential at room temperature.

1 Introduction

Ammonia (NH₃) detection is important in the industrial environment and for early diagnosis of medical diseases. Excess NH₃ is harmful to human health as it increases the incidence of respiratory and cardiovascular diseases (Li et al., 2016; Achary et al., 2018; Wang et al., 2018; Wu et al., 2019). Long-term exposure may cause blindness or ammonia poisoning in human beings. NH₃ exposure limitations have been set as 35 ppm for 10 min and 25 ppm for 8 h (Fedoruk et al., 2005; Gangopadhyay et al., 2008). In addition, it can be directly life-threatening when the concentrations of NH₃ are up to 300 ppm (Raj et al., 2010). NH₃ is also a natural body metabolite, and high levels of NH₃ in exhaled gas may reflect several liver and lung-related diseases (Narasimhan et al., 2001; Pagonas et al., 2012; Obermeier et al., 2017), which is potentially applied for noninvasive disease diagnostics. The concentration of exhaled NH₃ for healthy people is in the range of 0.4–1.8 ppm, but that in the breath of end-stage renal disease patients is normally beyond 4.9 ppm. Thus, NH₃ sensors can potentially be applied for noninvasive diagnosis of renal disease (Ji et al., 2019).

MXene is a group of transition metal carbides, nitrides, and carbonitrides with a layered structure similar to that of two-dimensional graphene (Su et al., 2022). Due to their unique surficial and electrical properties, multiple MXenes with a rich chemical composition have a wide range of promising applications in energy storage, hydrogen storage, chemical biosensors, and purification (Zhu et al., 2017; Zhang et al., 2018). Ti₃C₂T_x MXenes with high electrical conductivity, numerous active sites, and abundant terminal functional groups have been widely used as an excellent sensitive material for gas detection at room temperature. The type and number of oxygen-containing functional groups can change the conductivity of Ti₃C₂T_x MXenes and also effectively act as functional sites for gas adsorption. Ti₃C₂T_x MXene is a potential NH₃-sensitive material (Anasori et al., 2017), which displays more sensitivity to NH₃ than CO₂, O₂, and H₂ because the etched Ti₃C₂T_x surface is enriched with oxygen functional groups (Wu et al., 2019). However, pristine Ti₃C₂T_x MXenes with relatively lower capability have rarely been used as sensing materials directly.

The methods to improve the sensor performance mainly include modification of second-phase materials to enhance the activity of the sensitive material or the design of new structures to further promote the practical production applications of gas sensors. The construction of heterostructures effectively improves the gas sensing performance of Ti₃C₂T_x MXene. The electronic properties regulated by the interface of heterostructures may result in enhanced charge transport. Researchers have fabricated the gas sensing materials based on the MXene heterostructures with carbon nanostructures of reduced graphene oxide and carbon nanotubes (Lee et al., 2020; Hassan et al., 2021; Seekaew et al., 2023). These heterostructures combine the high conductivity and structural stability of carbon nanostructures with the merit of Ti₃C₂T_x MXene (Keshtkara et al., 2017). Carbon nanohorn (CNH) is a carbon nanomaterial with an angular appearance and aggregated subspherical shape and has abundant active sites and good electrical conductivity (Zhu et al., 2009; Zhu et al., 2019). It has also been widely used for immunosensing, storage, and electrochemical detection of gases such as methane and hydrogen (Ojeda et al., 2014; Chen et al., 2017; Zheng et al., 2020). Zhou et al. (2022) prepared In₂O₃/CNH nanocomposites and fabricated ideal low-power H₂S gas sensors. Therefore, combining Ti₃C₂T_x MXene with CNH may be promising for improving the sensing properties.

Herein, Ti₃C₂T_x/CNH nanocomposites have been fabricated by an electrostatic self-assembly method. The CNH surface is positively charged by adding the surfactant and then combined with the negatively charged Ti₃C₂T_x via an electrostatic self-assembly method. CNH has a larger specific surface area, providing more reaction sites and gas adsorption capacity, and the gas sensor made of Ti₃C₂T_x/CNH nanocomposite displays good response, excellent selectivity, and reliable stability for NH₃ detection at room temperature.

2 Experimental section

2.1 Preparation of sensing materials

2.1.1 Preparation of Ti₃C₂T_x MXene

For selective etching of Al from Ti₃AlC₂ MAX, we used a mixed etching solution containing hydrochloric acid and sodium fluoride as follows: 2 g Ti₃AlC₂ powder was slowly added to an etching solution composed of 40 mL 6 M hydrochloric acid and 2 g sodium fluoride (Wu et al., 2019). The mixed dispersion was continuously stirred at 60°C for 48 h in a water bath. The dispersion was then washed with ethanol and deionized water several times to remove the acidic residue until the pH value was 6. The precipitate was dried in a vacuum oven at 60°C for approximately 8 h to obtain the multilayer Ti₃C₂T_x powder. Then, an appropriate amount of Ti₃C₂T_x powder was added to deionized water and dispersed by ultrasonication to obtain 2 mg/mL of Ti₃C₂T_x dispersion.

2.1.2 Synthesis of Ti₃C₂T_x/CNH nanocomposites

Figure 1 shows the synthesis process of Ti₃C₂T_x/CNH nanocomposites, where Ti₃C₂T_x/CNH nanocomposites were successfully synthesized by an electrostatic self-organization method. CNH was obtained from Henan Yingneng Novel Material Technology Co., Ltd. CNH (50 mg) was dispersed in 10 mL of polydimethyldiallylammonium chloride (PDDA) and stirred for 6 h at 25°C to obtain positively charged CNH. Then, different amounts of Ti₃C₂T_x were added (CNH accounted for 0.1, 0.5, 1.0, and 5.0 wt%, respectively), and the dispersions were continued to be stirred for 24 h at 25°C. The obtained dispersion was centrifuged and washed with deionized water and ethanol in turn and dried in a vacuum oven to obtain Ti₃C₂T_x/CNH nanocomposites.

Figure 1

Figure 1. Schematic diagram of the preparation process of Ti₃C₂T_x/CNH nanocomposites.

2.2 Fabrication and characterization of the gas-sensing devices

The component of the sensor mainly includes a ceramic tube with a gold electrode coated with a sensitive material and a sensor base. The process of fabrication is as follows: the sensitive material is mixed with ethanol and sonicated for 15 min to form a stable dispersion. Then, 5 μL of the prepared dispersion was uniformly coated on the ceramic tube by using a pipette, and the gold electrode was completely covered to form a sensitive layer. The gas-sensing performance was analyzed at room temperature and 40%–45% relative humidity (RH) by using a CGS-8 analysis system from Elite Technology Co., Ltd. A static gas-blending method was used to obtain the target gases with different concentrations, and the detailed processes have been listed in the previous work (Han et al., 2023). The gas-sensing response is defined as S = |R_g-R_a|/R_a × 100%, where R_g and R_a represent the sensors’ resistance in the target gas and air, respectively (Han et al., 2023; Yao et al., 2023).

2.3 Characterizations

The crystal phases and chemical bonding of the as-prepared materials were analyzed by X-ray powder diffraction (XRD, Rigaku Ultima IV) and X-ray photoelectron spectroscopy (XPS, Shimadzu/Kratos AXIS SUPRA+), respectively. Field-emission scanning electron microscopy (FESEM, JSM-7001F) and transmission electron microscopy (TEM, JEM-F200) were performed to characterize the morphology and microstructure of the sensitive materials, respectively. The Fourier-transform infrared (FT-IR) spectrum was performed on PerkinElmer Spectrum 3.

3 Results and discussion

3.1 Characterizations of sensitive materials

Figure 2A displays the XRD patterns of Ti₃AlC₂ and Ti₃C₂T_x MXene. The typical diffraction peaks of Ti₃AlC₂-MAX at 9.5, 19.2, 34.1, 36.9, 38.9, 41.8, 48.4, 42.4, 56.5, 60.2, 65.4, 70.2, and 74.0° correspond to (002), (004), (101), (103), (104), (105), (107), (108), (109), (110), (1011), (119), and (1013) crystal planes, respectively (Tian et al., 2022). Compared with Ti₃AlC₂, the characteristic peaks of the (002) and (004) facets of Ti₃C₂T_x were reduced from 2θ = 9.5° to 5.7° and 2θ = 19.2° to 17.2°, respectively. Furthermore, the appearance of new characteristic peaks at 28.8° for (006) and 34.7° for (0010) also implies successful formation of Ti₃C₂T_x. Figure 2B presents the XRD patterns of CNH, Ti₃C₂T_x, and Ti₃C₂T_x/CNH composites. For the XRD patterns of Ti₃C₂T_x/CNH, the characteristic peaks of Ti₃C₂T_x can be observed in all Ti₃C₂T_x/CNH composites with different amounts of CNH, but no pronounced CNH peaks can be found for the low CNH content (Zhou et al., 2022).

Figure 2

Figure 2. XRD patterns of (A) Ti₃AlC₂ and Ti₃C₂T_x and (B) CNH, Ti₃C₂T_x, and Ti₃C₂T_x/CNH composites.

The morphologies of Ti₃AlC₂, Ti₃C₂T_x, CNH, and Ti₃C₂T_x/CNH nanocomposites were characterized by FESEM. As shown in Figure 3A, Ti₃AlC₂ has a dense multilayer structure. SEM images of multilayer Ti₃C₂T_x are displayed in Figure 3B, and there is an obvious spacing between the lamellae, a typical accordion-like structure, and a smooth surface. Figure 3C presents the SEM image of CNH nanospheres with an average diameter of 70–110 nm. The morphology of Ti₃C₂T_x/CNH (0.5 wt%) composite nanomaterials is exhibited in Figure 3D, indicating the successful combination between CNH and Ti₃C₂T_x.

Figure 3

Figure 3. FESEM images of the (A) Ti₃AlC₂, (B) Ti₃C₂T_x, (C) CNH, and (D) Ti₃C₂T_x/CNH (0.5 wt%) nanocomposites.

To further investigate the microstructure of the sensitive material, the Ti₃C₂T_x/CNH sample was characterized by TEM and high-resolution transmission electron microscopy (HRTEM). The TEM images of Ti₃C₂T_x/CNH (0.5 wt%) in Figure 4A also confirm the successful combination between Ti₃C₂T_x and CNH nanospheres. For Ti₃C₂T_x/CNH, it can be seen that Ti₃C₂T_x is composed of thin sheets with a two-dimensional lamellar structure, and the presence of CNH nanospheres is confirmed. The HRTEM images of the Ti₃C₂T_x/CNH composites are shown in Figure 4B. The spacing of the lattice surfaces in Ti₃C₂T_x is 0.237 nm, which can be attributed to the (110) crystal plane of Ti₃C₂T_x (Zhang et al., 2018). The lattice spacing stripes are not obvious due to the poor crystallinity of CNH. The CNH microspheres and Ti₃C₂T_x nanosheets are displayed in Figure 4, which are involved in the successful preparation of Ti₃C₂T_x/CNH nanocomposites.

Figure 4

Figure 4. (A) TEM image and (B) HRTEM image of the Ti₃C₂T_x/CNH (0.5 wt%) nanocomposite.

The chemical bonding of Ti₃C₂T_x/CNH samples was further analyzed by XPS, and the corresponding results are exhibited in Figure 5. Figure 5A shows the C 1s spectrum of Ti₃C₂T_x/CNH with peaks at 293.3, 286.4, 284.8, and 281.4 eV, which are attributed to C=O, C–O, C–C, and C–Ti functional groups, respectively (Zhou et al., 2022). For the O spectrum in Figure 5B, the peak at 531.1 eV is typical of surface lattice oxygen, while the peak located at 532.9 eV is characteristic of surface-adsorbed oxygen species (O^2– and O⁻) (Andreeva et al., 1998; Hu et al., 2018; Lee et al., 2018). The surface-adsorbed oxygen reacts with the target gas molecules through redox reactions, thus effectively enhancing the gas-sensitive properties (Wang et al., 2015). The high-resolution Ti 2p spectrum is displayed in Figure 5C, which corresponds the peaks at 460.7, 459.3, 456.6, and 453.6 eV to Ti–X (Ti⁺), T–C (Ti²⁺), Ti_xO_y (Ti³⁺), and Ti–O (Ti²⁺), respectively, where Ti–X corresponds to the sub-stoichiometric ratios of titanium carbide or titanium oxide nitride (Halim et al., 2014). The F 1s spectrum is presented in Figure 5D, and the peaks appearing at 688.9 and 687.5 eV can correspond to F–Ti and F–C, respectively (He et al., 2021). Therefore, the XPS results indicate the successful preparation of Ti₃C₂T_x/CNH composites, which is consistent with the XRD, SEM, and TEM results. Supplementary Figure S1 shows the FT-IR spectra of Ti₃C₂T_x, CNH, and Ti₃C₂T_x/CNH. The bands lower than 1,000 cm⁻¹ correspond to the vibrations of Ti–O and C–O. The bands at 1,350 and 1,630 cm⁻¹ may be related to the vibrations of O–H stretching and C=O bonding, respectively (Zhou et al., 2022).

Figure 5

Figure 5. (A) C 1s, (B) F 1s, (C) O 1s, and (D) Ti 2p fine spectrum of Ti₃C₂T_x/CNH (0.5 wt%) composites, respectively.

3.2 Gas-sensing performance

To investigate the effect of CNH content on the NH₃-sensing performance of Ti₃C₂T_x/CNH sensors, the sensing characteristics of Ti₃C₂T_x/CNH composite sensors with different CNH contents have been tested at room temperature. Figure 6A shows I–V curves of the Ti₃C₂T_x, CNH, and Ti₃C₂T_x/CNH (0.5 wt%) sensors, which display the typical Ohmic contacts, and thus, the resistances of the gas sensors are decided by the sensitive materials rather than the contact characteristics (Yuan et al., 2018; Han et al., 2021). As shown in Figure 6B, Supplementary Figure S2, the responses of pure Ti₃C₂T_x, CNH, Ti₃C₂T_x/CNH (0.1 wt%), Ti₃C₂T_x/CNH (0.5 wt%), Ti₃C₂T_x/CNH (1.0 wt%), and Ti₃C₂T_x/CNH (5.0 wt%) sensors to 100 ppm NH₃ are 2.0, 8.7, 21.6, 10.8, and 8.7%, respectively. The Ti₃C₂T_x/CNH (0.5 wt%) sensor is endowed with the highest response, which is approximately 10 times the response value of previously reported Ti₃C₂T_x-based sensors, indicating that CNH nanospheres significantly enhance the response to NH₃ (Wu et al., 2019). As the CNH content increases, the response to NH₃ increases accordingly, and the best peak-response (∼21.6%) is obtained at a CNH content of 0.5 wt%, indicating that the cooperation between Ti₃C₂T_x and CNH significantly enhances the NH₃-response. However, as the CNH content in the composite continues to increase, the response of the composite to NH₃ decreases instead. It may be because CNH has a large specific surface area, which can provide more reactive sites and help in improving the gas-sensitive response, while excess CNH covered with Ti₃C₂T_x decreases the number of reactive sites on the Ti₃C₂T_x surface, which affects the adsorption of NH₃ by Ti₃C₂T_x. Selectivity is considered to be an important key metric for evaluating gas sensors. The response and recovery time of the gas sensor are important parameters to be examined when the sensor is used in practice. As shown in Figure 6C, when the gas sensor is exposed to 100 ppm NH₃, the resistance value of the gas-sensing element reaches 90% of the equilibrium value in about 48 s, and the sensor recovery time is calculated as 196 s (Chen et al., 2022). The response of the Ti₃C₂T_x/CNH (0.5 wt%) sensor at room temperature to various interfering gases at 100 ppm is displayed in Figure 6D. The sensor has responses of 2, 1, 2, 2, 1, 21.6, and 3% to 100 ppm hydrogen sulfide, isopropyl alcohol, ethylene glycol, acetone, ethanol, ammonia, and formaldehyde, respectively. The Ti₃C₂T_x/CNH-based sensor shows excellent selectivity for NH₃ gas. As listed in Table 1, the response of the Ti₃C₂T_x/CNH-based sensor is competitive with that of the reported Ti₃C₂T_x-based NH₃ sensors.

Figure 6

Figure 6. (A) I–V curves of comparison of Ti₃C₂T_x, CNH, and Ti₃C₂T_x/CNH (0.5 wt%). (B) Response of different ratios of Ti₃C₂T_x/CNH samples to 100 ppm NH₃ at room temperature. (C) Dynamic response/recovery characteristic of the Ti₃C₂T_x/CNH (0.5 wt%) sensor to 100 ppm NH₃ at room temperature. (D) Response of the Ti₃C₂T_x/CNH (0.5 wt%) sensor to different gases at 100 ppm.

Table 1

Table 1. Comparison of the Ti₃C₂T_x/CNH sensor with the reported Ti₃C₂T_x-based NH₃ sensors.

The dynamic response of the optimal Ti₃C₂T_x/CNH (0.5 wt%) sensor to different concentrations (1–50 ppm) of NH₃ gas is displayed in Figure 7A. Figure 7B shows that the corrected curve is almost linear, with a correlation coefficient R² of 0.9877. The detection limit of this Ti₃C₂T_x/CNH sensor is calculated to be about 250 ppb (Han et al., 2023). The response of the Ti₃C₂T_x/CNH (0.5 wt%) gas sensor increases linearly with increasing NH₃ concentration, showing a good linearity. Reproducibility and long-term stability are also important indexes in the practical application of the sensor. Exposure of the sensor to 50 ppm NH₃ has been performed for six consecutive cycles at room temperature (Figure 7C). The results show that the composite sensor has good cycling reproducibility with no significant change in NH₃ detection during the continuous adsorption–desorption process. Moreover, as shown in Figure 7D, the stable response of the sensor to 100 ppm NH₃ over 30 days indicates that the device also exhibits relatively good long-term stability. Furthermore, Supplementary Figure S3 shows the response/recovery characteristics of the Ti₃C₂T_x/CNH sensor to 50 ppm NH₃ at 20, 40, and 55% RH conditions. The results show that the response values of the Ti₃C₂T_x/CNH sensor to 50 ppm NH₃ remain stable at about 10% within the RH range of 20%–55%.

Figure 7

Figure 7. (A) Dynamic response/recovery curve of the Ti₃C₂T_x/CNH (0.5 wt%) sensor at room temperature for a concentration range of 1–50 ppm NH₃. (B) Fitting curves between gas concentrations and response values of the Ti₃C₂T_x/CNH (0.5 wt%) sensor. (C) Repeatability of the Ti₃C₂T_x/CNH (0.5 wt%) sensor to 50 ppm NH₃ for six continuous cycles. (D) Long-term stability of the Ti₃C₂T_x/CNH (0.5 wt%) sensor to 100 ppm NH₃.

3.3 Gas-sensing mechanisms

The improved gas-sensing performance of the Ti₃C₂T_x/CNH (0.5 wt%)-based sensor for NH₃ can be explained by the following aspects:

Ti₃C₂T_x has abundant and active surface termination groups, which is beneficial for the sensing performance. The decorated CNH can provide more adsorption sites for the target gas, and the loading of CNH particles on the surface and interlayers of Ti₃C₂T_x nanosheets slows down the accumulation of layered Ti₃C₂T_x nanosheets and increases the Ti₃C₂T_x nanosheet layer spacing. Compared with pure Ti₃C₂T_x, the loaded CNH increases the specific surface area of Ti₃C₂T_x/CNH composites, improves the adsorption and diffusion of NH₃, and contributes to the improved NH₃-response.

A schematic diagram of the gas-sensing mechanism of the Ti₃C₂T_x/CNH sensor is displayed in Figures 8A, B. Ti₃C₂T_x exhibits p-type sensing behavior, and the relevant literature shows that its bandgap is 1.45 eV. CNH is a p-type semiconductor with a bandgap of 0.4 eV. The work functions of Ti₃C₂T_x and CNH are 3.9 and 5.0 eV, respectively (Lee et al., 2017; Schultz et al., 2019; Zhou et al., 2022). As shown in Figure 8C, in the Ti₃C₂T_x/CNH nanocomposite, nanoscale p–p heterojunctions are formed between Ti₃C₂T_x and CNH. Since the work function of CNH is higher than that of Ti₃C₂T_x, electrons will flow from the latter to the former, resulting in the formation of a hole depletion layer (HDL) in CNH and a hole accumulation layer (HAL) in Ti₃C₂T_x (Yao et al., 2023).

Figure 8

Figure 8. Schematic diagram of (A) the gas-sensing mechanism of the Ti₃C₂T_x/CNH sensor, (B) energy band diagram of Ti₃C₂T_x and CNH before forming a complex, and energy band structure of Ti₃C₂T_x/CNH composite in (C) air and (D) NH₃.

In air, oxygen molecules are adsorbed on the surface of the composite (Eqs 1 and 2). The oxygen molecules adsorb on the CNH surface, and the negative charge formed on the surface will increase its work function, thus increasing the work function difference between CNH and Ti₃C₂T_x. This will require transfer of more electrons to compensate. As a result, the HAL thickness in Ti₃C₂T_x increases, and the resistance of the sensor in air is relatively low. When the composite is exposed to NH₃, the NH₃ molecules react with ${\mathrm{O}}_2^{-}$ species, releasing electrons to neutralize the holes (Figure 8D, Eq. 3), reducing the hole density in the nanocomposite, and disrupting the dynamic carrier balance between Ti₃C₂T_x and CNH. The hole transfer from CNH to Ti₃C₂T_x reduces the thickness of HAL on the CNH surface. The thinner HAL at the heterojunction interface contributes to increased sensor resistance (Suh et al., 2018).

${\mathrm{O}}_2\left(\text{gas}\right)\to {\mathrm{O}}_2\left(\text{ads\hspace{0.17em}}\right)(1)$

${\mathrm{O}}_2\left(\text{gas}\right)+{\mathrm{e}}^{–}\to {\mathrm{O}}_2^{–}\left(\mathrm{T}<100°C\right)(2)$

${4\text{NH}}_3+5{\mathrm{O}}_2^{–}\to 4\text{NO}+6{\mathrm{H}}_2\mathrm{O}+5{\mathrm{e}}^{–}(3)$

4 Conclusion

Ti₃C₂T_x/CNH nanocomposites have been prepared by an electrostatic self–assembly method, and the characterization and gas–sensitive sensing properties of the material are investigated. The experimental results show the Ti₃C₂T_x/CNH (0.5 wt%) nanocomposite sensor has good gas–sensing performance for NH₃ gas at room temperature, indicating that CNH can effectively optimize the sensor and make Ti₃C₂T_x MXene more suitable for detecting NH₃ at room temperature.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.

Author contributions

YH: writing–review and editing, conceptualization, funding acquisition, investigation, and writing–original draft. YD: writing–original draft. YY: writing–review and editing. ZL: writing–review and editing. ZZ: writing–review and editing.

Funding

The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. The authors gratefully acknowledge financial supports by the National Natural Science Foundation of China (62104149) and the Shanghai Sailing Program (21YF1431400).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2024.1383538/full#supplementary-material

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