Metal-organic frameworks-driven ZnO-functionalized carbon nanotube fiber for NO₂ sensor

1. INTRODUCTION

Excessive inhalation of nitrogen dioxide (NO₂) at low concentrations (–1 ppm) can cause pulmonary disease and severe damage to the respiratory system, making NO₂ one of the most harmful gas species to human health [1]. The National Aeronautics and Space Administration (NASA) reported air pollution trends in major cities worldwide using high-resolution global satellite maps [2], revealing a significant increase in NO₂ concentration in 2014, particularly in East Asia, due to growing economies and the demand for fossil fuels. In view of this, new types of sensing platforms with potential flexible and wearable functions should be developed to realize real-time and rapid NO₂ detection.

Carbon nanotubes (CNTs) are attractive electrical transducer layers for chemical sensors [3-5]. However, the major limitations of pristine CNTs are their low response and selectivity owing to the stable surface chemical states for gas sensor applications [6]. Therefore, functionalization of surface additives and elevating the operating temperature are general approaches for improving the gas-sensing response and selectivity [7,8].

Metal-organic frameworks (MOFs) have been employed as functional materials in gas-sensing layers mainly because of their high porosity and large surface area [9-12]. For example, various carbon nanocomposites have been developed by the functionalization of MOFs on templates such as carbon fibers (CFs) [13], macro-porous carbon (MPC) [14,15], graphene oxide (GO) [16], CNTs [17-19], and fullerene [20]. In addition, MOF-driven nanostructures to form porous metal oxides have been demonstrated to facilitate the p-n junction effect for improved gas-sensing performance [21].

In this study, we propose porous polyhedral ZnO-functionalized carbon nanotube fibers (ZnO/CNTFs) for improved NO₂ sensors. To this end, carbonaceous fibers were first prepared via a wet-spinning process assisted by the liquid crystal behavior of CNTs [5,22]. Liquid phase growth of ZIF-8 was performed by immersing CNTF in a precursor-containing solution, resulting in the homogeneous precipitation of ZIF-8 on the surface of CNTF (ZIF-8/CNTF). Subsequent heat treatment in an inert atmosphere was performed at elevated temperatures to form porous ZnO nanocubes on the CNTFs (ZnO/CNTF). The ZnO/CNTF exhibited an increased response toward NO₂ gas as compared with pristine CNTF, enabling the self-heating property of CNTFs to control the operating temperature. The MOF-driven ZnO/CNTF composite sensor can be integrated with textile materials for applications in wearable NO₂ sensors.

2. EXPERIMENTAL

3. RESULTS AND DISCUSSIONS

The synthesis processes for ZnO/CNTF are schematically illustrated in Fig. 1. First, a one-pot synthesis method was conducted to form ZIF-8 on the pristine CNTF. Separate solutions of Zn(NO₃)₂·6H₂O and 2-MeIM in methanol were mixed together, and CNTF was immersed into the mixed solution to simultaneously precipitate ZIF-8 on the surface of the CNTF (Fig. 1(a), (b)). The CNTF possesses numerous functional groups on its surface, which provide nucleation sites for Zn²⁺ and 2-MeIM clusters, allowing homogeneous growth of ZIF-8 on the CNTF [23]. The one-pot synthesis is advantageous considering that ZIF-8 polyhedrons are tightly bonded to the CNTF chemically, leading to efficient transduction properties during the gas-sensing process compared to physical attachments. Subsequent heat treatment was performed at elevated temperatures to form porous ZnO nanocubes on CNTF (ZnO/CNTF) (Fig. 1(c)).

Fig. 1.

Schematic illustrations of the synthesis of ZnO/CNTF: (a) Before and (b) after one-pot synthesis of ZIF-8 on CNTF. (c) ZnO/CNTF after calcination in N2 atmosphere at elevated temperatures.

After the wet-spinning process, the pristine CNTF exhibited a continuous fibrous structure with high mechanical stability and flexibility (Fig. 2(a)). The microstructure and surface morphology were investigated via scanning electron microscopy (SEM) before and after the growth of ZIF-8. For the pristine CNTF, a rough surface morphology was observed with the aligned CNT bundles (Fig. 2(b), (c)). The CNTF was immersed in a solution containing Zn²⁺ and 2-MeIM to precipitate ZIF-8 on the surface, wherein the transparent solution turned into a white solution (Fig. 2(d)). The microstructure in which CNTFs homogeneously decorated with polyhedral ZIF-8 by one-pot synthesis was confirmed (Fig. 2(e),(f)).

Fig. 2.

(a) Photograph image of pristine CNT fiber. (b) SEM image of pristine CNT fiber and (c) its magnified SEM image. (d) One-pot synthesis of ZIF-8/CNTF in a mixture solution. (e) SEM image of ZIF-8/CNTF and (f) its magnified SEM image.

Microstructural evolution during the calcination process at different temperatures (i.e., 400, 500, and 600 °C) in an N₂ atmosphere was investigated (Fig. 3). The ZnO/CNTF-400 exhibited a minor change in the shape and structure of ZIF-8 (Fig. 3(a), (b)). In addition, porous ZnO nanocubes were not obtained because of insufficient thermal energy at low temperatures [24]. On the other hand, ZnO/CNTF-500 (Fig. 3(c), (d)) and ZnO CNTF-600 (Fig. 3(e), (f)) exhibited a highly porous structure after calcination of the ZIF-8/CNTF. The polyhedral ZIF-8 was converted into porous ZnO nanocubes after the calcination process, which was mainly achieved through the reaction between Zn from ZIF-8 and the oxygen functional groups on the CNTF. However, calcination at sufficiently high temperatures (i.e., 600 °C) induces agglomeration of ZnO nanocubes, as shown in the case of ZnO/CNTF-600 (yellow circles in Fig. 3(e)).

Fig. 3.

SEM images of ZnO/CNTF calcined at (a)–(b) 400 °C (ZnO/CNTF-400), (c)–(d) 500 °C (ZnO/CNTF-500), and (e) –(f) 600 °C (ZnO/CNTF-600) in an N2 atmosphere for 3 h.

To improve the NO₂-sensing response, we utilized the self-heating effect by applying a voltage to the ZnO/CNTF, facilitating activated surface reaction kinetics. A suspended ZnO/CNTF sensor was fabricated on a glass substrate (2.5 cm × 1.0 cm) and a DC voltage was applied through the two probe tips (Fig. 4(a),(b)). An increased temperature of 100 °C was achieved for the ZnO CNTF by Joule heating when the DC voltage was applied in the range of 2–3 V. For example, the heating temperature of the ZnO CNTF-500 sensor was confirmed by an infrared camera, wherein a temperature of 100 °C was obtained under an applied voltage of 2.4 V (Fig. 4(c)). To investigate the heating property of ZnO CNTF-500, voltage-dependent temperature changes were confirmed by slowly increasing the applied voltage (Fig. 4(d)-(f)). When voltage is applied, the temperature rises owing to the self-heating behavior of the CNTF [5]. In particular, ZnO/CNTF-500 exhibited step-like current-resistance transitions by increasing the voltages in the range of 0–2.4 V with a 0.4 V step. When 2.4 V was applied to the ZnO/CNTF-500, the maximum current and resistance of 64 mA and 38 Ω were obtained, respectively, resulting in an operating temperature of 100 °C. Reversible temperature transitions were observed as the sensor recovered the initial current by decreasing the voltage

Fig. 4.

(a) Camera image of ZnO/CNTF sensor for the detection of NO2. (b) Camera image and (c) infrared camera image of ZnO/CNTF-500 sensor maintain operating temperatures at 100 °C under an applied voltage of 2.4 V. (d) Current, (e) resistance, and (f) temperature transitions of ZnO/CNTF-500 sensor under applied voltages in the range of 0–2.4 V with a 0.4 V step.

The degree of self-heating according to the applied voltage varied with calcination temperatures of the ZnO/CNTFs. For this reason, different DC voltages were applied to the individual ZnO CNTF and pristine CNTF samples, as summarized in Table 1, to equalize the operating temperature for NO₂ sensing as 100 °C. In the case of the ZnO/CNTF, slightly higher voltages (–3 V) should be applied to maintain 100 °C as compared to the pristine CNTF (–2 V), which was mainly attributed to the increased resistance of ZnO/CNTFs after the calcination process.

Table 1.

Applied DC voltages to maintain the operating temperature at –100 °C for individual samples.

A comparative gas-sensing characterization was performed using pristine CNTF and ZnO/CNTF calcined at different temperatures toward NO₂ at an operating temperature of 100 °C (Fig. 5). All the sensors were stabilized under exposure to baseline air for 12 h to maintain a stable baseline resistance followed by the injection of NO₂ gas in the concentration range of 1–20 ppm. Both the pristine CNTF and ZnO/CNTF exhibited decreased resistance upon exposure to NO₂, indicating p-type-sensing properties. Dynamic response (R_air/R_gas) transitions were evaluated by comparing the CNTF and ZnO/CNTF at each calcination temperature. For the sensors with a calcination temperature of 400 °C, a decreased response of ZnO/CNTF-400 (R_air/R_gas=1.051) was obtained as compared to the response of the pristine CNTF-400 (R_air/R_gas=1.029) at 20 ppm NO₂ (Fig. 5(a)). The negligible improvement in the NO₂-sensing response of the ZnO/CNTF-400 sensor is mainly attributed to the insufficient oxidation of ZIF-8 at 400 °C, leading to a less porous structure of ZnO on the surface of the CNTF (Fig. 3(b)). On the other hand, a noticeable improvement in the responses was observed after calcination at 500 and 600 °C. For the ZnO/CNTF-500 sensor, the response was 1.086 at 20 ppm NO₂, which was slightly higher than that of the pristine CNTF-500 (R_air/R_gas=1.06) (Fig. 5(b)). Similarly, we obtained an enhanced response (R_air/R_gas=1.076) for ZnO/CNTF-600 at 20 ppm NO₂ as compared to the response (R_air/R_gas=1.047) of the pristine CNTF-600 (Fig. 5(c)). The increased NO₂-sensing responses after calcination at 500 and 600 °C were mainly attributed to the complete oxidation of ZIF-8 to form ZnO with high porosity (Fig. 3(d), (f)). However, the slightly reduced NO₂ response of ZnO/CNTF-600 compared to that of ZnO/CNTF-500 was attributed to the agglomeration of ZnO nanocubes after excessive heat treatment at an elevated temperature, resulting in a decreased active surface area for the NO₂ reaction.

Fig. 5.

Dynamic response transitions of pristine CNTF and ZnO/CNTF after calcination at (a) 400 °C, (b) 500 °C, and (c) 600 °C toward NO2 gas in the concentration range of 1–20 ppm at the operating temperature of 100 °C.

The improved NO₂ sensing mechanism of ZnO/CNTFs is discussed based on the resistance transitions and charge transfer behavior. Both CNTF and ZnO/CNTF exhibited decreased resistance upon exposure to NO₂. Because NO₂ is an oxidizing gas and has an electron-accepting nature, an increased hole concentration can be achieved in CNTF, resulting in p-type sensing. Considering that the pristine CNTF exhibited a NO₂-sensing response, there was a direct chemical interaction between NO₂ and CNTF. Specifically, surface-adsorbed nitrate (NO₃^–) can be formed on the surface of graphitic carbon based on the following chemical reaction [5]:

this chemical equation explains that the reason for decreasing resistance upon exposure to NO₂ is trapping of electrons, which increases the hole concentrations in the CNTF.

The improved NO₂-sensing response upon the functionalization of ZnO on CNTFs can be explained by electronic sensitization. ZnO is a well-known n-type semiconductor metal oxide, which can react with NO₂ via chemical adsorption, as shown below:

resulting in an electron trap on the ZnO surface. Hence, a decreased Fermi level of ZnO can be obtained while lowering the energy barrier between ZnO and CNTF, which results in facile electron transfer from CNTF to ZnO. This observation is consistent with the decreased resistance upon exposure to NO₂ as a result of the increased hole concentration in ZnO/CNTF [7]. Another reason for the improved NO₂ response can be attributed to the p-n junction formation between n-type ZnO and p-type CNTFs. The p-n junction forms a depletion layer, wherein the thickness of the depletion layer can be effectively modulated upon exposure to NO₂, leading to large resistance changes and a high response. In addition, the porous polyhedral structure of ZnO facilitates NO₂ penetration into the sensing layers, which induces efficient surface chemical reactions.

In terms of optimum sensing characteristics with respect to the calcination temperature, ZnO/CNTF-400 exhibited increased baseline resistance after heat treatment, which indicates the formation of ZnO by forming a p-n junction with CNTF. However, mesopores were not noticeably generated on the ZnO nanocubes because of insufficient calcination temperature, resulting in a decreased NO₂ response. For ZnO/CNTF-500, we observed an increased baseline resistance, which is consistent with the formation of the p-n junction. In addition, highly porous ZnO nanocubes were uniformly decorated on the surface of the CNTF, leading to an improved NO₂ response. After calcination at 600 °C in N₂, the ZnO/CNTF-600 exhibited a decreased baseline resistance compared to the pristine CNTF-600. The decreased resistance is mainly attributed to the disappearance of the p-n junction. Indeed, excess heat treatment in an inert atmosphere can reduce ZnO, limiting the formation of the p-n junction [7]. Although porous ZnO nanocubes were formed on the surface of CNTF, the ZnO/CNTF-600 sensor exhibited a slightly decreased NO₂ response because of the inactive modulation of the depletion region formed by the p-n junction. Overall, the optimized calcination temperature was confirmed to be 500 °C considering the efficient charge transfer to ZnO, p-n junction formation, and decoration of porous ZnO nanocubes by forming a ZnO/CNTF composite.

The selective NO₂-sensing properties of ZnO/CNTF-500 were evaluated by exposure to various interfering gases, such as hydrogen sulfide (H₂S), ammonia (NH₃), carbon monoxide (CO), and toluene (C₇H₈) at 20 ppm (Fig. 6). The results confirmed that ZnO/CNTF-500 exhibited the highest response (R_air/R_gas) toward NO₂ (1.083) as compared to the response toward H₂S (1.0003), NH₃ (1.0007), CO (1.0028), and toluene (1.0004).

Fig. 6.

Selective sensing property of ZnO/CNTF-500 sensor toward various gas species of 20 ppm at the operating temperature of 100 °C.

4. CONCLUSIONS

In this work, ZnO/CNTF composites were developed by homogeneous decoration of ZIF-8 on CNTF through one-pot synthesis and subsequent calcination in N₂ atmosphere. The microstructure of the ZnO/CNTF composites was investigated with respect to the calcination temperature. In particular, porous polyhedral ZnO nanocubes were deposited on the surface of CNTFs after calcination at 500 and 600 °C. In addition, the p-n junction was formed after calcination at 400 and 500 °C between the interfaces of n-type ZnO and p-type CNTF, which induces efficient modulation of the depletion layer, leading to improved NO₂-sensing properties. To activate the surface reaction kinetics, we utilized the self-heating property of CNTFs by applying a voltage to induce Joule heating. The operating temperature was maintained at 100 °C during gas-sensing characterization. As a result, an improved NO₂-sensing response (R_air/R_gas=1.086 at 20 ppm) was achieved with ZnO/CNTF-500. In addition, the ZnO/CNTF-500 exhibited high NO₂ selectivity with minor responses toward interfering gas species such as H₂S, NH₃, CO, and toluene. This study demonstrates a new approach for the synthesis of CNT-based sensing composites functionalized with MOF-driven ZnO nanocubes, which can be integrated with textile materials for wearable chemical sensors.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2020R1C1C1010336). This work was also supported by the U.S. Army Combat Capabilities Development Command Soldier Center (DEVCOM SC) and International Technology Center Pacific (ITC-PAC) Global Research Project under contract FA520920P0130, and conducted at Hanyang University.

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