Highly Sensitive H2 Sensors Based on Co3O4/PEI-CNTs at Room Temperature

Gao, Laixu; Li, Songquan; Xu, Xiaoxue; Zou, Changwei; Zhang, Guo

doi:https://doi.org/10.1155/2022/4743040

Journal of Nanomaterials

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Abstract Introduction Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2022 | Article ID 4743040 | https://doi.org/10.1155/2022/4743040

Highly Sensitive H₂ Sensors Based on Co₃O₄/PEI-CNTs at Room Temperature

Laixu Gao,¹Songquan Li,¹Xiaoxue Xu,²Changwei Zou,¹and Guo Zhang³

Academic Editor: Zhi Li Xiao

Received21 Oct 2021

Revised26 Dec 2021

Accepted11 Jan 2022

Published29 Jan 2022

Abstract

The highly dispersed Co₃O₄ on the surface of CNTs modified with polyethylenimine (PEI) was synthesized using the hydrothermal method. In the CNT-Co₃O₄ composite materials, CNTs not only provide the substrate for the Co₃O₄ nanoparticles but also prevent their aggregation. Furthermore, the interaction between Co₃O₄ and CNTs modified with polyethylenimine (PEI) helps to improve the gas sensing performance. In particular, the CNT-Co₃O₄ composite synthesized at 190°C shows the outstanding sensitive characteristics to H₂ with a lower detection limit of 30 ppm at room temperature. The obtained CNT-Co₃O₄ sensor displays excellent selectivity and stability to H₂. The energy band model of the conductive mechanism has been built to explain the resistance change when the gas sensor is exposed to the H₂. Hence, the CNT-Co₃O₄ composite material presents highly promising applications in H₂ gas sensing.

1. Introduction

Hydrogen energy is the most ideal clean and renewable energy with high heating value, so it is widely used in spacecraft, fuel cell, and internal combustion engine fuel [1–4]. Hydrogen can be stored in the form of gas, liquid, or even solid metal hydrides; thus, the hydrogen can be very potentially convenient to be employed. However, hydrogen gas is colorless, odorless, highly volatile, and flammable. Explosion is very easy to occur when hydrogen gas concentration exceeds 4% in dry air; therefore, safety has always been a big issue in hydrogen storage and use [5, 6]. Developing a sensor for hydrogen gas is essential to monitor its amount for their applications in energy and environmental fields.

Metal oxide semiconductors in nanostructure have been widely used as the active materials for gas sensing because of their high catalytic activities and improved selectivity from the distinctive structure [7–10]. Cobalt oxide (Co₃O₄) is a -type semiconductor and could be potentially employed for supercapacitors, electrochemical devices, and gas sensors due to its excellent electrocatalytic properties, high biocompatibility, and low cost [11–16]. A variety of harmful and toxic gases were detected using Co₃O₄ nanoparticles, such as volatile organic compounds (VOCs), H₂S, and NH₃ [17–20]. To improve the catalytic effectiveness and stability of Co₃O₄ nanoparticles, the design of novel nanostructures and hybrid nanomaterials based on Co₃O₄ nanoparticles is highly in demand.

Carbon nanotubes (CNTs) have shown the effects on the enhancement in catalytic activity for metal oxide gas sensing owing to the distinguished physical and chemical properties, such as excellent mechanical strength, thermal stability, hydrophilicity and stability, and electrical conductivity [21–24], because of their unique surface structures. There are many oxygen-containing groups (–COOH), graphite, and defects on the surface structures of CNTs from acidized treatment [25–27]. Zhang and coworkers showed that the CNTs could reduce the reaction temperature and enhance the gas sensitivity to CO gas for a metal oxide gas sensing system [28, 29]. Nguyen synthesized CNT films via CVD and displayed fast response and high sensitivity for NH₃ sensing [30, 31]. CNTs also have been applied in the NO₂ gas sensor, and the NO₂ surface reaction can be explained by NO and NO₃ which are produced on the surface [32–34].

In this work, Co₃O₄ nanoparticles were successfully loaded onto the polyethylenimine (PEI) modifed CNTs via a hydrothermal method [28, 35]. The functional groups on the sidewalls of CNTs benefited from the binding of Co²⁺ ions or Co₃O₄ nanoparticles onto the CNTs. The Co₃O₄ nanoparticles were highly dispersed with the PEI-CNTs displaying high conductivity and uniform contact with hydrogen. The prepared hybrid material was integrated on the surface of a planar interdigitated gold electrode for hydrogen detection, and the results showed that PEI-CNTs did improve the gas sensing performance. The composite gas sensor exhibits perfect sensitivity, selectively, and response speed upon exposure to H₂ at room temperature.

2. Experiment

2.1. Syntheses of CNT-Co₃O₄ Nanomaterials

The CNT-Co₃O₄ composite materials were synthesized via a hydrothermal method. CNTs (Shenzhen Nanotech Port Co. Ltd., diameter: 30–80 nm, length: 5–30 μm) were functionalized firstly by acid treatment in the solution (HNO₃ : H₂SO₄ in a 1 : 3 () ratio). 80 mL of 0.03 mg/mL CNTs was added in 20 mL of 2 mg/mL PEI (branched PEI, , Cheng Du Micxy Chemical Co., Ltd.) under magnetic stirring, and the pH value of the solution was kept at 9.0. Subsequently, 20 mL of 2 mg/mL Co(NO₃)₂·6H₂O (Tianjin Regent Chemical, Tianjin, China) was added to the above mixed solution during stirring. After that, saturated sodium hydroxide solution was slowly added into the mixed solution to adjust the pH to 13. The air flow was set to the solution with 50 mL/min for 24 h. Then, the mixed solution was exfoliated by ultrasonication at a power of 100 W for 1 h, and then, the mixed solution was placed still for 24 h at room temperature. A black precipitate was filtered and washed with distilled water to neutral. Lastly, the precipitate was dispersed in distilled water and poured into the Teflon-lined stainless steel autoclave. Hydrothermal synthesis was conducted at 180, 190, and 200°C for 3 h, respectively. The obtained CNT-Co₃O₄ composites were named as Co180, Co190, and Co200, respectively.

2.2. Characterization

The crystalline structures of the products was characterized by X-ray powder diffraction (XRD, D/max-III B-40 kV, Japan, Cu-Kα radiation, ). The Fourier transform infrared (FT-IR) spectra were acquired with the FT-IR spectrometer (PerkinElmer Spectrometer, KBr pellet technique). The morphology of the synthesized samples were investigated using TEM (JEOL-JEM-2100, 200 kV).

2.3. Sensor Fabrication and Electrochemical Performance Measurements

The CNT-Co₃O₄ materials were dispersed in ethanol to form a suspension. The suspension was dropcasted to cover the surface of a planar interdigitated gold electrode on a ceramic substrate. Then, the electrode was dried in a vacuum oven at 80°C for 3 h. The electrochemical performances of the sensor were measured in a chamber with a gas flow apparatus. The electrical resistance of the sensor was measured using a computer-controlled multimeter for filtering signals (, Shanghai, China). During the gas sensing test, a certain volume of H₂ (99.9%, Dalian Great Special Gas Co., Ltd.) was injected with air at room temperature (25°C) in a relative humidity (RH) of around 45%. The response of the sensor was defined as the change of resistance in the two different gas circumstances:

where is the resistance in the tested gas and is the resistance in air.

3. Results and Discussion

3.1. Structure Characterizations

The XRD diffraction and the FT-IR spectra of CNT-Co₃O₄ composites are shown in Figures 1(a) and 1(b), respectively. In the XRD patterns shown in Figure 1(a), two types of peaks can be indexed. Diffraction peaks at 23.9°, 34.3°, and 44.7° can be indexed as the (009), (123), and (018) planes of the functional CNTs (JCPDS no. 74–2328, JCPDS no. 50–1086), and the d-spacings were calculated as 3.72, 2.61, and 2.03 Å, respectively. The XRD spectra showed a face-centered cubic Co₃O₄ phase with the corresponding peaks at 31.2°, 36.8°, 59.3°, and 65.2° (JCPDS no. 42–1467) to the planes of (220), (311), (511), and (440). Furthermore, there are no obvious (220) and (440) peaks existing in the Co180. From the XRD of CNT-Co₃O₄, we can find that the width at the half peak height of the same crystal plane is also different, indicating that the crystallinity and grain size of Co₃O₄ were influenced by the hydrothermal temperature. In addition, the high purity of the CNT-Co₃O₄ composites can be proven because of the two types of peaks.

(a)

(b)

The FT-IR spectra of CNT-Co₃O₄ composite materials are shown in Figure 1(b). The broad peak around 3506 and 3384 cm^-1 would be attributed to the N–H stretch of secondary and primary amines of PEI in samples [36]. The peak at 1557 cm^-1 is attributed to the C–C stretching vibration in CNTs [37], suggesting the CNTs presenting in the nanocomposites. CH₂ wagging from 1100 to 1500 cm^-1 is a typical mode of PEI [28]. The peak stretching from 990 to 910 cm^-1 and wagging from 840 to 800 cm^-1 can be attributed to the unsaturated hydrocarbon (=C–H) of CNTs. The sharp peaks at 575 and 660 cm^-1 are attributed to the Co(III)–O and Co(II)–O stretching vibrations of Co₃O₄ [38]. Hence, through the FT-IR characterization, it presents that PEI-CNTs and Co₃O₄ nanoparticles have been successfully combined.

Figure 2 shows the TEM and HRTEM images of CNT-Co₃O₄ composite materials fabricated via the hydrothermal method at 180°C, 190°C, and 200°C. Figures 2(a)–2(d) show that Co₃O₄ nanoparticles were integrated onto the surface of CNTs, forming the CNT-Co₃O₄ composite materials with different densities of Co₃O₄ nanoparticles on CNTs. Figure 2(a) displays relative loose distribution of the aggregated Co₃O₄ nanoparticles on CNTs of the Co180 nanocomposite. Figure 2(b) shows that the aggregated Co₃O₄ nanoparticles were densely distributed on the CNTs. The HRTEM image in Figure 2(c) further shows the dense Co₃O₄ nanoparticles in different predominated crystal facets of (111), (220), and (311) with corresponding lattice d-spacing distance of 4.75, 2.91, and 2.46, respectively. The Co₃O₄ nanoparticles obtained at 190°C presented relatively large size (10–12 nm), while at the hydrothermal temperature of 200°C, it can be found from Figure 2(d) that Co₃O₄ nanoparticles almost fully covered the surface of CNTs. The SAED pattern of the Co200 nanocomposite displays the polycrystalline of Co₃O₄ with crystal planes of (111), (220), (311), (400), (511), and (440). The size distribution of Co₃O₄ nanoparticles in the Co200 nanocomposite was calculated, as shown in the inset of Figure 2(c). Relative large size distribution was found, and most of the Co₃O₄ nanoparticles were within the range of 15–20 nm. We found that the hydrothermal synthesis temperature would be able to control the size of Co₃O₄ nanoparticles and the coverage density of Co₃O₄ nanoparticles on CNT surfaces. Of the three synthetic temperatures, it shows the uniform size and morphology for the Co₃O₄ nanoparticles formed at 190°C.

(a)

(b)

(c)

(d)

The hypothesis of the formation mechanism for the nanocomposites is proposed in Figure 3. The CNTs after acid treatment can be endowed a large amount of moieties of –COOH and –OH, which improves the dispersion of CNTs and enables the modification with PEI. In our work, the PEI would bond to the –COO^- groups of CNTs after both are mixed in the aqueous solution under pH 9; thus, PEI would be uniformly modified on the surface of the CNTs (step 1). The Co²⁺ from the precursor is easily dissociated and positively charged in aqueous solution. Then, we monitor the pH of the solution to 13 to unprotonate the PEI, so the Co²⁺ would be bonded to unprotonated PEI on the surface of CNTs and the Co²⁺ is functionalized onto CNT surfaces uniformly via PEI-CNTs (step 2). When air is introduced into the solution, Co²⁺ is oxidized to Co³⁺ gradually (step 3). The subsequent hydrothermal treatment at high temperature allows the in situ formation of Co₃O₄ nanoparticles on the surface of CNTs (step 4).

3.2. Gas Sensing Testing

The response of CNT-Co₃O₄ materials to H₂ gas was investigated through measuring the resistance versus time over two multifinger electrodes (size: ) at room temperature, as shown in Figure 4.

Figure 5 presents the H₂ gas sensing performance of CNT-Co₃O₄ nanocomposite materials, Co180, Co190, and Co200, respectively. The concentration range of H₂gas was adjusted from 1000 ppm to 30 ppm. The nanocomposite of CNTs-Co₃O₄ can play a role as the -type semiconductor with the hole as the carrier. When a -type semiconductor contacts with the reducing gas, the reduction interaction between them would produce electrons and the -type semiconductor would accept the electron; thus, the resistance of the semiconductor would be increased. In Figure 5, we can see that the resistance of the CNT-Co₃O₄ nanocomposite increased when the air with H₂ passed through while its resistance dropped to the original value when air was introduced without H₂. The sensitivity decreases with the decrease in the concentration of the target gas, respectively, at room temperature. It is shown that the sensing response varies with the change in the concentration of the analyte gas.

By comparing the results shown in Figure 5, it can be seen that the response to H₂ is significantly higher for Co190 than the response to H₂ for Co180 and Co200 at room temperature. The response to the 1000 ppm H₂ for the nanocomposite Co190 presents the highest resistance, 81.9 kΩ, which is 2.01 times higher than the original resistance value. The response of the Co190 to the H₂ concentration of 30 ppm decreased, and the increased resistance is 1.13 times higher than the original value at room temperature. For the nanocomposite of Co200, the factor of the increased resistance decreased to 1.26 with 1000 ppm H₂. Hence, the nanocomposite of Co190 exhibited the highest sensing sensitivity among all three nanocomposite materials.

Furthermore, the CNT-Co₃O₄ nanocomposites fabricated at different hydrothermal temperatures displayed remarkably different effects on the recovery process of gas sensors. The possible reasons can be ascribed to the following aspects: (i) the Co₃O₄ nanoparticles were better dispersed on the surface of CNTs at low temperature and the Co₃O₄ nanoparticles were in relatively uniform size (shown in Figures 2(a) and 2(b)) and (ii) higher synthesis temperature caused large size of the Co₃O₄ nanoparticles and wide size distribution. Moreover, higher hydrothermal temperature induced severe nanoparticles’ aggregation and complete coverage on the surface of CNTs (shown in Figure 2(c)). This resulted in a low surface-area-to-volume ratio, which decreased the surface area of exposure to target gas for response.

The roles of the CNTs and PEI for the sensing performance of the nanocomposite materials were investigated as well. The sensing response to H₂ of the pure Co₃O₄ nanoparticles, nanocomposite of Co₃O₄-PEI without CNTs, and nanocomposite of Co₃O₄-CNTs without PEI is shown in Figure 6. It has been found that without CNTs or PEI in the nanomaterial, the sensor exhibited significantly lower response than that of the nanocomposite of Co₃O₄-CNTs with PEI. It could be related to the coupling effect between the Co and CNTs, which had been proposed to contribute to the enhanced oxygen reduction ability [28]. In the Co₃O₄/PEI-CNT composite, when the CNT architecture was employed as conducting scaffolds in a Co₃O₄ semiconductor-based sensor, it not only prevents Co₃O₄ nanoparticles from aggregation but can also boost the electron transfer efficiency. PEI provided high-density homogeneous functional groups on the CNTs’ sidewalls for binding Co₃O₄ nanoparticles. Meanwhile, PEI is helpful for high-density dispersion of Co₃O₄ grains and enhances the interaction between Co₃O₄ grains and CNTs and improves the transport of the carriers to the surface [39, 40]. Therefore, it could be concluded that the PEI played an important role in the sensing performance of the nanocomposites.

We also evaluated the sensing selectivity of the nanocomposites of CNT-Co₃O₄ to H₂ with the interferences such as ethanol, methanol, hydrogen, benzene, and acetone, as shown in Figure 7(a). The response of the sensor to 1000 ppm H₂ was highest and 3–4 times higher than the responses to the other gases at the same concentration, suggesting an excellent selectivity to H₂ at room temperature.

(a)

(b)

(c)

To validate the sensing reliability and long-term stability of the CNT-Co₃O₄ composite sensor, we repeated the measurement of the dynamic response-recovery five times and tested the sensibility for 30 days. The repeatability of response and recovery curves shows the excellent repeatability in five consecutive experiments, as shown in Figure 7(b). After 30 days of long-term stability testing, the sensitivity of the sensor dropped from an initial 2.01 to 1.83; it retained 91.04% of its initial response, as shown in Figure 7(c). Therefore, CNTs-Co₃O₄ displays potentials in a sensor and wide range of application prospects for an H₂ sensor.

3.3. Gas Sensing Mechanism

The gas sensing mechanism of the metal oxide nanoparticles is strongly related to surface reactions [41], and chemisorbed oxygen also makes important contribution to the sensing mechanism. In general, the chemisorbed oxygen species of , , and can be formed at <150°C, 150~400°C, and >400°C, respectively. Because of the electrostatic interaction between the oppositely charged species, the adsorption of oxygen anions onto -type oxide semiconductors induces the formation of hole accumulation layers. An energy band conduction model was used for the -type oxide semiconductor as the gas sensors [7]. Figure 8 shows the structure and energy band model of the conductive mechanism in the air and reducing gas. When the -type oxide semiconductor was exposed to the air, ambient oxygen can extract electrons from the valence band and form oxygen ions. It can increase the concentration of the holes, forming the hole accumulation layers. It can be depicted that the energy bands exhibited an upward band bending, of which length is the thickness of the hole accumulation layers. When the -type oxide semiconductor is exposed to the reducing gas, the oxygen ions on the surface would be reacted. The decrease in the concentration of the holes, described in the energy band representation as a downward band bending ( in Figure 8), results in the increase in the resistance of the gas sensor. Hence, in our study, the ambient oxygen is absorbed on the surface of the CNTs-Co₃O₄ and traps the electrons at the surface as the oxygen ions () in the room temperature (2). When the CNT-Co₃O₄ was exposed under the ambient H₂, the reaction between reducing gas and oxygen ions () can result in the increase in the resistance of the CNT-Co₃O₄ gas sensor (3).

4. Conclusion

In summary, the CNT-Co₃O₄ nanocomposites with highly dispersed Co₃O₄ were fabricated via the hydrothermal method. The CNT-Co₃O₄ nanocomposites display excellent sensing sensitivity, selectivity, and stability at room temperature to H₂ gas. CNTs play multiple roles in the sensing performance, not only offering the Co₃O₄ substrates to avoid its aggregation but also providing perfect migration of the electron pathway via the synergetic chemical coupling effect between Co₃O₄ and CNTs. PEI also contributes to improving the performance of H₂ sensors. Hence, the CNT-Co₃O₄ composites represent a potential application prospect in H₂ detection.

Data Availability

The experimental data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was funded by the National Natural Science Foundation of China grant (No. 61905105), Guangdong Universities (advanced PVD coating and metal surface modification technology innovation team), Special Fund for Science and Technology Innovation Strategy of Guangdong Province (2020A0505100059), Natural Science Foundation of Guangdong Province (2021A1515011928), Guangdong Universities Innovation Project (No. 2020KTSCX073), and Zhanjiang Science and Technology Project (No. 2019A01044).

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Copyright © 2022 Laixu Gao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Journal of Nanomaterials

Highly Sensitive H2 Sensors Based on Co3O4/PEI-CNTs at Room Temperature

Abstract

1. Introduction

2. Experiment

2.1. Syntheses of CNT-Co3O4 Nanomaterials

2.2. Characterization

2.3. Sensor Fabrication and Electrochemical Performance Measurements

3. Results and Discussion

3.1. Structure Characterizations

3.2. Gas Sensing Testing

3.3. Gas Sensing Mechanism

4. Conclusion

Data Availability

Conflicts of Interest

Acknowledgments

References

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Highly Sensitive H₂ Sensors Based on Co₃O₄/PEI-CNTs at Room Temperature

2.1. Syntheses of CNT-Co₃O₄ Nanomaterials