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Article

Ba-Modified ZnO Nanorods Loaded with Palladium for Highly Sensitive and Rapid Detection of Methane at Low Temperatures

by
Yijing Cai
1,†,
Shirui Luo
2,†,
Renjie Chen
2,
Junxia Yu
1,* and
Lan Xiang
2,*
1
Hubei Novel Reactor & Green Chemical Technology Key Laboratory, Key Laboratory for Green Chemical Process of Ministry of Education, School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430205, China
2
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Chemosensors 2022, 10(9), 346; https://doi.org/10.3390/chemosensors10090346
Submission received: 23 July 2022 / Revised: 14 August 2022 / Accepted: 20 August 2022 / Published: 23 August 2022
(This article belongs to the Special Issue Advanced Nanomaterials for Flexible and Wearable Chemo- and Bio- Sensors)
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Abstract

:
Exploring novel sensing materials to rapidly identify CH4 at low temperatures is crucial for various practical applications. Herein, a novel ZnO-xBa/Pd with Ba of cocatalyst loading from 0 to 2.0 wt% was facilely prepared using a two-step impregnation method to improve the sensitivity of the CH4 gas sensor. The microstructure, chemical states of the elements, and surface properties of ZnO-Ba/Pd were characterized, and the gas-sensitive performance of ZnO-Ba/Pd sensors was investigated. Compared to methane sensors based on other inorganic and organic material sensors, the sensor based on ZnO-1.0Ba/Pd exhibited a faster response/recovery time (1.4 s/8.3 s) and higher response (368.2%) for 5000 ppm CH4 at a lower temperature (170 °C). Moreover, the ZnO-1.0Ba/Pd sensor exhibited full reversibility and long-term stability, as well as excellent selectivity at 170 °C. The excellent performance of the ZnO-Ba/Pd sensor was attributed to the electron donation by Ba, which increases the electron density around Pd, thus enhancing the catalytic activity of Pd and promoting oxygen adsorption on the ZnO surface. The present work provides a method for the rational design and synthesis of sensitive materials in practical CH4 detection.

1. Introduction

Human activities are closely related to methane (CH4), which is the main component of coal-bed methane and natural gas, and is widely used in industrial production, households, and other fields [1]. However, CH4 is flammable and highly volatile, with an explosion limit of 5–15% [2]. Therefore, it is critical to design sensors to detect CH4 gas. In the past few decades, many studies have reported on the development of CH4 gas sensors based on inorganic and organic materials [3,4]. Jiao et al. employed a rigid tungsten-capped calix [4] arene-based p-doped conducting polymer with PF6− or ClO4− counter-anions as a transducer in methane detection to achieve experimental and theoretical limits of methane detection (LoD) of <50 and 2 ppm at room temperature, respectively [5]. Zhang et al. integrated a coaxial yarn-shaped G@PVA@GST photoelectrochemical methane sensor in the design and construction, which displayed a rapid response time (0.3 s) and a low detection limit (0.02%) with a linear range of 0.05–0.47% while maintaining favorable selectivity, exceptional bending stability, and sensing reproducibility [6]. Xia et al. presented a novel visible-light photocatalysis-enhanced sensor based on Pd nanoparticle-decorated ZnO/rGO hybrids, enabling high-performance detection of CH4 at room temperature under visible-light illumination [7]. Among them, metal oxide semiconductor sensors (MOSs) are an excellent choice due to their low cost, high response performance, and simple operation [1,8,9,10,11]. Among various MOSs, zinc oxide (ZnO) is widely used, owing to its simplicity of preparation, high electron mobility, and chemical stability [12,13,14]. However, pure ZnO suffers from the disadvantages of high operating temperature (>200 °C), low sensitivity, and slow response/recovery, which hinder its further development in practical applications [15].
To further enhance the gas-sensitive performance of ZnO, various strategies, including noble metal doping, defect engineering, and p-n heterostructure construction, have been employed, where noble metal (Au, Ag, Pd, Pt, etc.) doping is the most effective [16,17,18,19,20,21,22]. Noble metal doping not only promotes the adsorption and desorption processes of oxygen molecules, but also changes the electronic structure of the metal oxide surface, thus enhancing the gas-sensitive response [8,23]. Among them, ZnO/Pd is an important sensing-system combination. In ZnO-based sensors, Pd is a good catalyst for gas sensing and can effectively improve the sensing performance due to the “spillover effect” [24,25]. Wang et al. reported a response of 19.20 for 5000 ppm CH4 at 200 °C using Pd as a sensitizer on ZnO nanosheets, which is 2.13 times higher than that of pure ZnO [12]. Zhang et al. reported that 0.5 %Pd/s-ZnO had the highest sensitivity (6.44) to 500 ppm CH4 at 200 °C, which is about 3.2 times higher than that of pure s-ZnO [26]. Despite this progress, it is still a challenge to develop novel CH4 sensors to meet the growing demands (e.g., lower operating temperatures and faster response/recovery rates). Since the principle of CH4 sensing is the catalytic oxidation of CH4, which is promoted by Pd, it is facile to consider the structural modification of Pd to further improve the CH4 sensing performance [27]. In recent years, researchers have proposed an important strategy: the introduction of a second metal component as a promoter to enhance the catalytic oxidation of CH4 at reasonably low temperatures by increasing the activity of the catalyst, which significantly improves the gas sensing performance [28,29,30]. Among them, using the alkaline earth metal Ba as an electron donor has the advantages of regulating the adsorption/desorption performance of the reaction gas; lowering the gas ignition temperature; stabilizing the active phase; and improving the thermal stability, which could prospectively be introduced as a promoter in Pd-based nanomaterials to achieve fast response/recovery times, high sensitivity, and low operating temperature in CH4 sensing [31,32,33,34].
In this paper, ZnO-xBa/Pd nanorods with Ba content of 0~2.0 wt% were prepared, and the effect of Ba introduction on the CH4 sensing performance of Pd/ZnO was investigated. Compared to ZnO/Pd sensors, the operating temperature of ZnO-1.0Ba/Pd sensors significantly reduced from 230 °C to 170 °C. The response of ZnO-1.0Ba/Pd to 5000 ppm CH4 reached up to 368.2%, which is approximately 1.8 times that of ZnO/Pd (205.0%), and the response/recovery times of ZnO-Ba/Pd (1.4/8.3 s) greatly shortened compared to ZnO/Pd (2.7/13.1 s). The introduction of Ba not only enhances the catalytic activity of Pd, but also the increase in adsorbed oxygen promotes the surface oxidation of CH4, thus enhancing the sensitivity of CH4. This work provides a novel method for the rapid detection of CH4 at low temperatures.

2. Materials and Methods

2.1. Synthesis

Based on previous work, ZnO nanorods were synthesized as starting materials using ε-Zn(OH)2 as precursors [15]. In a typical synthesis process of ZnO/Pd, 0.38 g of ZnO nanorods and 0.01 g of PdCl2 were added to 80 mL of ethylene glycol to form a suspension. Then, the suspension was heated at 100 °C for 3 h. Finally, the precipitate was washed and dried overnight at 55 °C to obtain ZnO/Pd. For the synthesis of ZnO-Ba/Pd, a certain amount of Ba(NO3)2, corresponding to 0, 0.5, 1.0, 1.5, and 2.0 wt% of BaO in the final samples, was firstly dissolved in deionized water to form transparent solutions. Then, 0.38 g of ZnO nanorods was dispersed in the solution, followed by continuous stirring for 1 h. After drying at 60 °C for 12 h and calcining at 250 °C for 1.5 h in air, ZnO-Ba materials were obtained. Subsequently, 0.38 g ZnO-Ba and 0.01 g PdCl2 in 80 mL of ethylene glycol were dispersed to form a suspension. Afterward, the suspension was stirred at 100 °C for 3 h. Finally, the precipitate was washed at 55 °C and dried overnight to obtain ZnO-Ba/Pd. According to the designed BaO loading, the materials were denoted as ZnO-0.5Ba/Pd, ZnO-1.0Ba/Pd, ZnO-1.5Ba/Pd, and ZnO-2.0Ba/Pd, respectively.

2.2. Characterization

The microstructure and morphology of the samples were examined via field emission scanning electron microscopy (FESEM, JSM 7401F, JEOL, Akishima City, Akishima City, Japan), and high-resolution transmission electron microscopy (HRTEM, JEM-2010, JEOL, Japan), respectively. EDX (Energy Dispersive X-ray Spectroscopy) analysis was performed on several areas for elemental analysis. An inductively coupled plasma (ICP) spectrometer (Thermo IRIS Intrepid II type) was used to detect the actual Ba amount in the samples. A Ba standard solution of 100 ppm was gradiently diluted to establish the calibration curve, and the sample was fully dissolved with 1% dilute sulfuric acid and tested to obtain the accurate concentration of Ba according to the calibration curve. The crystal structure of the samples was analyzed via X-ray powder diffraction (XRD, Bruker D8 Advance), with Cu kα (λ = 0.154178 nm) incidence-radiation. The surface composition of the materials was characterized using X-ray photoelectron spectroscopy (XPS, PHI-5300, PHI, USA). The Software package XPSPeak41 was used for spectral deconvolution. The catalytic decomposition rate of the target gases was determined via gas chromatography (GC, 9790IIT-2, 78 FULI, China) equipped with a flame ionization detector (FID).

2.3. Sensor Fabrication and Response Measurement

The prepared samples were ground in an agate mortar and dispersed in appropriate deionized water to form a homogeneous paste. The paste was then applied to the surface of an alumina ceramic tube with four Pt electrodes on it. The prepared ceramic tubes were dried at 60 °C for 1 h and then calcined at 250 °C for 2 h. A Ni-Cr heating wire was inserted into the tubes to adjust the sensor’s operating temperature. The gas sensor was aged at 230 °C for 7 days to improve the stability before sensor testing. Gas sensing tests of the fabricated sensors were performed on the gas sensing analysis system of CGS-8 system (Beijing Elite Tech. Co., Ltd., Beijing, China). The measuring system consists of a data acquisition system, heating system, and test software, and the ambient temperature and relative humidity were 25 °C and 25–30%. The calculated volumes of CH4 (1000~5000 ppm) gas were injected into a hermetic chamber with a volume of 500 mL using a micro syringe. The response ( R a − R g R g × 100 % ) of the sensor is defined as the ratio of the resistance of the sensor in the air (Ra) to the resistance in the target gas (Rg). The time required to reach 90% of the total resistance change is used to estimate the response and recovery time.

2.4. Catalytic Measurements

The catalytic activity of samples was evaluated in a steel microreactor (8.0 mm i.d. × 0.7 m) for the oxidation of CH4 at ambient pressure. The catalyst (0.4 g) was ground to a 200–300 mesh powder and placed at the center of the microreactor. The CH4 (37.5 mL min−1) was charged, then mixed with air (75 mL min−1 (STD)). The mole ratio was CH4:O2:N2 = 1:1:1. The sample was collected online at 120 °C and quantified by a gas chromatograph (GC, 9790IIT-2, FULI, China) equipped with a flame ionization detector (FID) and a packed column (Porpark N, 3 mm × 5 mm, Hichina Zhicheng Technology Ltd., Shenzhen, China). The methane decomposition ratio was determined by measuring the ratio of input methane to output methane.

3. Results

3.1. Structural and Morphological Characteristics

The construction process of the ZnO-Ba/Pd is schematically illustrated in Figure 1. The material morphology was characterized by SEM and TEM. As seen in Figure 2a, the ZnO-1.0Ba/Pd material presented an urchin-shaped structure that was assembled by uniform nanorods, benefiting the construction of conduction tunnels [15]. According to Figure 2b, Pd nanoparticles with a diameter of 2–5 nm were highly dispersed on the surface of ZnO nanorods. As shown in Figure 2c, the lattice spacing of 0.224 nm observed in HRTEM images was fitted well with the (111) plane of metallic Pd. The elemental content of the sample was detected by EDX (Figure 2d). Several areas of ZnO-1.0Ba/Pd were examined, and no significant Ba was localized due to the limit of instrumental precision. However, 0.63 wt% of the Ba element was identified by ICP analysis. Therefore, it was concluded that Ba was well dispersed on the ZnO-1.0Ba/Pd, which was further confirmed in the following XPS analysis.
The crystal structure and phase composition of ZnO-1.0Ba/Pd were characterized via X-ray diffraction (XRD). As shown in Figure 3a, the prepared samples exhibit three major diffraction peaks at 31.8°, 34.5°, and 36.3°, corresponding to the (100), (001), and (101) crystal planes of ZnO (PCPDF 36-1451). The peak situated at 40.2° can be assigned to the (111) crystallographic facet of Pd (PCPDF 89-4897), revealing that Pd NPs were successfully loaded on ZnO. No significant Ba diffraction peaks were observed from the X-ray diffraction patterns of ZnO-1.0Ba/Pd, which may be ascribed to the ultrasmall size and low content of Ba. XPS analysis was used to investigate the chemical state of the elements in the samples. The peaks of Zn, O, Pd, and Ba elements can be seen in a full-range XPS spectrum (Figure 3b). Figure 3c shows two peaks at binding energies of 1021.3 and 1044.4 eV in ZnO-1.0Ba/Pd, corresponding to Zn 2p3/2 and Zn 2p1/2, respectively, with a gap of 23.1 eV in the splitting energy levels, which indicates that Zn exists in the form of Zn2+. The high-resolution O 1s XPS spectra of ZnO/Pd and ZnO-1.0Ba/Pd are displayed in Figure 3d. For the ZnO/Pd sample, three peaks located at 529.8, 530.4, and 531.6 eV can be observed, which correspond to the lattice oxygen (Olat), defect oxygen (Odef), and surface-adsorbed oxygen species (Oads), respectively. The Oads component is usually attributed to chemisorbed and dissociated oxygen species (O2−, O2−, or O−) and other oxygen species (H2O, OH−, CO2, etc.) [35]. The percentages of Odef and Oads in pristine ZnO/Pd were 26.4 and 39.7%, respectively, and after the addition of Ba, both increased to 29.5 and 42.23%, respectively. Therefore, ZnO-1.0Ba/Pd provides more adsorbed and defective oxygen than ZnO/Pd, which may facilitate the improvement of gas sensitivity. Figure 3e shows two peaks at binding energies of 780.5 and 795.9 eV, corresponding to Ba2+ 3d5/2 and 3d3/2, respectively, with a gap of 15.4 eV in the splitting energy levels. In Figure 3f, the Pd 3d spectrum of ZnO-1.0Ba/Pd presented dominant peaks located at 334.6 and 339.9 eV corresponding to Pd 3d3/2 and Pd 3d5/2, respectively, which were verified as the Pd0 state [15]. In addition, the deconvolution results show two peaks at 335.5 eV and 341.0 eV, which are attributed to Pd2+ 3d5/2 and Pd2+ 3d3/2, indicating the presence of PdO on the surface, which may be the result of partial oxidation of Pd nanoparticles [25]. The Pd 3d spectrum of the ZnO/Pd sample can be deconvoluted into four peaks: the peaks of Pd0 (334.9 and 340.2 eV) and Pd2+ (335.6 and 341.1 eV). Compared to ZnO/Pd, the Pd° and Pd2+ of ZnO-1.0Ba/Pd were negatively shifted by 0.3 eV and 0.1 eV, respectively, indicating the electron density around Pd in ZnO-1.0Ba/Pd increases as a result of the electron donation from the Ba species.

3.2. Gas Sensing Properties

The operating temperature is the primary factor affecting the sensitivity of the material to the target gas. The gas response based on the prepared samples of 5000 ppm CH4 was examined at the different working temperatures of 120~300 °C to determine the relationship between the gas response and the working temperature. As shown in Figure 4a, a volcano-like correlation between temperature and response was observed. It was found that the optimal operating temperature of pristine ZnO for CH4 gas is 280 °C, with a maximum response of 32.6%. Significantly, after loading Pd and Ba/Pd, the optimum operating temperature was remarkably reduced and the response increased. The optimal operating temperatures of ZnO/Pd and ZnO-Ba/Pd decreased from 280 °C to 230 °C and 170 °C, respectively. Notably, the optimal operating temperature of ZnO-Ba/Pd plummeted by 110 °C. In addition, the response is affected by the Ba addition, which tends to increase and then decrease in the range of 0.5–2.0 wt%. Among them, the ZnO-1.0Ba/Pd sensor showed maximum sensitivity (368.2%) at 170 °C, which was nearly 1.8 times that of ZnO/Pd (205.0%). This demonstrates the competitive advantage of Ba addition. When a small amount of Ba was added, it was not sufficient to reach full catalytic enhancement (ZnO-0.5Ba/Pd (278.18%)). However, when the Ba addition exceeded 1 wt%, the response values of ZnO-1.5Ba/Pd and ZnO-2.0Ba/Pd were 288.1 and 219.6%, respectively, and the corresponding response values decreased. Such a phenomenon may be due to the decrease in the active sites of sensitive materials caused by the Ba overloading [28]. These results indicate that only the appropriate amount of Ba loading can effectively improve the sensing performance.
The time-dependent response of the sensors at 170 °C in the concentration range of 1000–5000 ppm is shown in Figure 4b, indicating excellent reversibility and repeatability. The ZnO-1.0Ba/Pd exhibited a superior CH4 response (368.2% to 5000 ppm) compared to ZnO/Pd (204.9%), ZnO-0.5Ba/Pd (278.18%), ZnO-1.5Ba/Pd (288.1%), and ZnO-2.0Ba/Pd (219.6%), demonstrating a remarkably improved response due to functionalization of the Ba as a cocatalyst for ZnO/Pd. As shown in Figure 4c, the response values of ZnO-1.0Ba/Pd showed good linearity with wide testing of CH4 concentrations (1000–5000 ppm), indicating a practical application for quantitative CH4 detection. For instance, the linear relationship Response= 0.06C+ 88.07 can be fitted to ZnO-1.0Ba/Pd (R2 = 0.9963), where C is the concentration of the test gas. The response/recovery time is an important parameter in evaluating the practicability of the CH4 sensor. As shown in Figure 4d, the response/recovery times of the ZnO/Pd, ZnO-0.5Ba/Pd, ZnO-1.0Ba/Pd, ZnO-1.5Ba/Pd, and ZnO-2.0Ba/Pd sensors for 5000 ppm CH4 were 2.7/13.1 s, 2.5/12.3 s, 1.4/8.3 s, 1.5/10.7 s, and 2.4/14.2 s, respectively. The fast response of the ZnO-1.0Ba/Pd sensor stems from the following factors: first, the unique urchin structure of ZnO provides a large specific surface area, which facilitates the adsorption of gas molecules; second, the catalytic activation of Pd NPs, which is effectively facilitated by Ba, promotes the chemisorption or dissociation kinetics of CH4, thus accelerating the response/recovery process [36]. When the concentration of the Ba addition was further increased, both reaction/recovery times became longer (1.5/10.7 s and 2.4/14.2 s, respectively), which may be related to the excess Ba loading covering the active site of the material and reducing the reaction rate [37]. Comparing the samples with different Ba concentrations (Figure 5a), the Pd 3d spectrum in ZnO-1.0Ba/Pd showed the highest shift to low-binding energy, indicating the highest electron transfer from Ba to Pd, thus enhancing the catalytic activity of Pd and promoting the catalytic oxidation of CH4. The fact that these sensors were exposed to 5000 ppm CH4 for five consecutive cycles showed that the response–recovery curves were reproducible, with no significant changes (Figure 5b), indicating excellent repeatability. As shown in Figure 5c, the ZnO-1.0Ba/Pd sensor shows a high response of 368.2% to CH4 (5000 ppm), clearly much higher than other typical gases (54.2% for 24 ppm CO, 42.6% for 24 ppm NH3, 20.6% for 1 ppm C7H8, 25.4% for 1 ppm HCHO, 23.2% for 1 ppm C6H6), illustrating excellent selectivity; the maximum allowable concentration of C7H8, HCOH, and C6H6 is 1 ppm, and the safe concentration of CO, NH3, and CH4 in coal mines are 24 ppm, 40 ppm, and 5000 ppm, respectively. Moreover, the long-term stability test for the ZnO/Pd, ZnO-0.5Ba/Pd, ZnO-1.5Ba/Pd, and ZnO-2.0Ba/Pd sensors operated at 170 °C for 30 days at 5000 ppm CH4 with a 17.5,13.1, and 13.9% decrease, respectively, compared to the original response values (Figure 5d). In contrast, the ZnO-1.0Ba/Pd sensor can maintain its original response with a change of less than 8.5% after 30 days, indicating satisfactory reproducibility and long-term stability. A comparison of the present sensors with other reported CH4 sensors is summarized in Table 1; the ZnO-1.0Ba/Pd sensor sensing performances are superior to previous CH4 sensors, indicating that the as-designed sensors are promising for actual CH4 detection.

3.3. Gas Sensing Mechanism

As an n-type semiconductor material, the ZnO sensors follow a surface resistance-controlled sensing model via the adsorption–reaction–desorption mechanism [8]. When n-type ZnO is exposed to air, oxygen (O2) molecules are chemisorbed on the surface of ZnO in the form of O2− (<100 °C), O− (100–300 °C), and O2− (>300 °C) by attracting electrons in the conduction band of ZnO [48]. The reactions are as follows:
O2(ads) + e−→O2−(ads)
O−(ads) + e−→2O−(ads)
O−(ads) + e−→O2−(ads)
Chemisorbed oxygen species thus induce the formation of electron depletion regions on the ZnO surface. Thereafter, when ZnO is exposed to CH4, the resistivity of ZnO decreases due to the surface reaction between CH4 and chemisorbed oxygen, generating by-products CO2 and H2O [49]. This leads to the back-donation of electrons in the conduction band of ZnO (Rair > Rgas) (Figure 6a). The reaction process is described below:
CH4 +4O−(ads)→CO2(gas) + 2H2O(gas) + 4e−
The improved performance of the ZnO-1.0Ba/Pd sensor could be attributed to two aspects. Firstly, introducing the second metal Ba as a promoter helps to improve the catalytic effect of Pd and enhances the sensor performance. The catalytic oxidation properties of CH4 on ZnO/Pd and ZnO-1.0Ba/Pd samples were examined at 100–300 °C, as shown in Figure 6b. The decomposition efficiency of ZnO-1.0Ba/Pd can reach 55.7% at 170 °C. For ZnO/Pd, only about 25.3% of CH4 was catalytically oxidized into CO2 and H2O, which is much lower than that of ZnO-1.0Ba/Pd. The catalytic activity of the ZnO-1.0Ba/Pd sensor was 2.2 times higher than that of the ZnO/Pd sensor. Therefore, the introduction of Ba promotes the high oxidative catalytic activity of Pd, enabling it to oxidize CH4 gas more easily and further improving the CH4 sensing performance. The introduction of Ba to enhance the catalytic activity of Pd is attributed to the strong electron-donating ability of the alkaline earth metal Ba, which can increase the electron density around Pd, improve the catalytic activity of Pd, and promote the catalytic oxidation of CH4 [28].
In addition, with the introduction of Ba, the surface chemisorbed oxygen component greatly increased (as depicted in Figure 3d), and the increase in the surface chemisorbed oxygen component provided more active sites for the redox reactions occurring on the surface of the sensing material, which induced significant changes in the sensor resistance. As shown in Figure 6c, the baseline resistances of ZnO-1.0Ba/Pd (43.8 MΩ) were significantly higher than those of ZnO/Pd (31.9 MΩ). Once the analyte was introduced, the reaction rate between the chemisorbed oxygen species and CH4 was enhanced due to the introduction of Ba, which resulted in a larger decrease in resistivity. As shown in Figure 6d, the work functions of BaO, Pd, and ZnO were 1.65, 5.15, and 4.45 eV, respectively [12]. The transfer of electrons from BaO to Pd was consistent with the results obtained from XPS (Figure 3f), improved the catalytic activity of Pd, and promoted the catalytic oxidation of CH4. The electrons were transferred from ZnO to Pd until the Fermi-level reached equilibrium. Thus, Schottky junctions formed at the interfaces between Pd and ZnO [19]. This process resulted in the extension of the electron depletion layer in the initial state, contributing considerably to the improved CH4 sensing properties of ZnO-1.0Ba/Pd.

4. Conclusions

In summary, Ba-modified ZnO/Pd nanorods were synthesized for efficient CH4 detection at a low working temperature. The unique urchin structure of ZnO and uniformly dispersed Ba and Pd were characterized by SEM and TEM, and the sensing performance was investigated. The ZnO-1.0Ba/Pd-based sensors exhibited high response performance at 170 °C, with a maximum response value of 368.2% (5000 ppm CH4) and short response/recovery time (1.4 s/8.3 s), as well as excellent stability. The excellent performance of the ZnO-Ba/Pd sensor is attributed to the electron donation of Ba, which increases the electron density around Pd, thus enhancing the catalytic activity of Pd, and the increase in adsorbed oxygen which also promotes the surface oxidation of CH4. This study introduces the cocatalytic effect of Ba into the gas sensing mechanism and rationalizes the design of ZnO-Ba/Pd sensors with enhanced CH4 gas sensing performance, which is expected to provide insights into the synthesis and development of practical CH4 sensors in the future.

Author Contributions

Conceptualization, methodology, investigation, formal analysis, data curation, visualization, writing original draft, Y.C.; investigation, S.L.; investigation, R.C.; funding acquisition, supervision, J.Y.; resources, supervision, funding acquisition, project administration, L.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 21978153, the National Key Research and Development Program, grant number 2019YFC1905803, and the Innovative Team program of Natural Science Foundation of Hubei Province, grant number 2021CFA032.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Guo, Y.; Liu, C.; Wang, D.; He, R. Advances in the development of methane sensors with gas-sensing materials. Chin. Sci. Bull. 2019, 64, 1456–1470. [Google Scholar] [CrossRef]
  2. Hong, T.; Culp, J.T.; Kim, K.-J.; Devkota, J.; Sun, C.; Ohodnicki, P.R. State-of-the-art of methane sensing materials: A review and perspectives. TrAC-Trend Anal. Chem. 2020, 125, 115820. [Google Scholar] [CrossRef]
  3. Qin, T.; Niu, Y.; Qiao, X.; Guo, W.; Zhang, C.; Yang, Z.; Zhang, Y. Integrated coaxial graphene-based yarn with multidimensional architecture for self-powered photoelectrochemical methane sensor. Sens. Actuators B 2022, 365, 131965. [Google Scholar] [CrossRef]
  4. Dosi, M.; Lau, I.; Zhuang, Y.; Simakov, D.S.A.; Fowler, M.W.; Pope, M.A. Ultrasensitive Electrochemical Methane Sensors Based on Solid Polymer Electrolyte-Infused Laser-Induced Graphene. ACS Appl. Mater. Interfaces 2019, 11, 6166–6173. [Google Scholar] [CrossRef]
  5. Jiao, M.-Z.; Chen, X.-Y.; Hu, K.-X.; Qian, D.-Y.; Zhao, X.-H.; Ding, E.-J. Recent developments of nanomaterials-based conductive type methane sensors. Rare Met. 2021, 40, 1515–1527. [Google Scholar] [CrossRef]
  6. Zhang, C.; Guo, W.; Qin, T.; Qiao, X.; Xiao, Z.; Yang, Z. Ultrasensitive self-powered photoelectrochemical detection of methane based on a coaxial integrated carbonene fiber. Environ. Sci. Nano 2022, 9, 2086–2093. [Google Scholar] [CrossRef]
  7. Xia, Y.; Wang, J.; Xu, L.; Li, X.; Huang, S. A room-temperature methane sensor based on Pd-decorated ZnO/rGO hybrids enhanced by visible light photocatalysis. Sens. Actuators B 2020, 304, 127334. [Google Scholar] [CrossRef]
  8. Bhati, V.S.; Hojamberdiev, M.; Kumar, M. Enhanced sensing performance of ZnO nanostructures-based gas sensors: A review. Energy Rep. 2020, 6, 46–62. [Google Scholar] [CrossRef]
  9. Dey, A. Semiconductor metal oxide gas sensors: A review. Mater. Sci. Eng. B 2018, 229, 206–217. [Google Scholar] [CrossRef]
  10. Debeda, H.; Massok, P.; Lucat, C.; Ménil, F.; Aucouturier, J.L. Methane sensing: From sensitive thick films to a reliable selective device. Meas. Sci. Technol. 1997, 8, 99. [Google Scholar] [CrossRef]
  11. Lawrence, N.S. Analytical detection methodologies for methane and related hydrocarbons. Talanta 2006, 69, 385–392. [Google Scholar] [CrossRef] [PubMed]
  12. Wang, Y.; Meng, X.; Yao, M.; Sun, G.; Zhang, Z. Enhanced CH4 sensing properties of Pd modified ZnO nanosheets. Ceram. Intc. 2019, 45, 13150–13157. [Google Scholar] [CrossRef]
  13. Yuan, H.; Aljneibi, S.; Yuan, J.; Wang, Y.; Liu, H.; Fang, J.; Tang, C.; Yan, X.; Cai, H.; Gu, Y.; et al. ZnO Nanosheets Abundant in Oxygen Vacancies Derived from Metal-Organic Frameworks for ppb-Level Gas Sensing. Adv. Mater. 2019, 31, e1807161. [Google Scholar] [CrossRef] [PubMed]
  14. Xia, Y.; Pan, A.; Gardner, D.W.; Zhao, S.; Davey, A.K.; Li, Z.; Zhao, L.; Carraro, C.; Maboudian, R. Well-connected ZnO nanoparticle network fabricated by in-situ annealing of ZIF-8 for enhanced sensitivity in gas sensing application. Sens. Actuators B 2021, 344, 130180. [Google Scholar] [CrossRef]
  15. Luo, S.; Chen, R.; Wang, J.; Xie, D.; Xiang, L. Designed synthesis of ZnO/Pd@ZIF-8 hybrid structure for highly sensitive and selective detection of methane in the presence of NO2. Sens. Actuators B 2021, 344, 130220. [Google Scholar] [CrossRef]
  16. Chen, R.; Wang, J.; Luo, S.; Xiang, L.; Li, W.; Xie, D. Unraveling photoexcited electron transfer pathway of oxygen vacancy-enriched ZnO/Pd hybrid toward visible light-enhanced methane detection at a relatively low temperature. Appl. Catal. B Environ. 2020, 264, 118554. [Google Scholar] [CrossRef]
  17. Korotcenkov, G.; Cho, B.K. Engineering approaches for the improvement of conductometric gas sensor parameters. Sens. Actuators B Chem. 2013, 188, 709–728. [Google Scholar] [CrossRef]
  18. Li, C.; Song, B.-Y.; Lv, M.-S.; Chen, G.-L.; Zhang, X.-F.; Deng, Z.-P.; Xu, Y.-M.; Huo, L.-H.; Gao, S. Highly sensitive and selective nitric oxide sensor based on biomorphic ZnO microtubes with dual-defects assistance at low temperature. Chem. Eng. J. 2022, 446, 136846. [Google Scholar] [CrossRef]
  19. Qin, C.; Wang, B.; Li, P.; Sun, L.; Han, C.; Wu, N.; Wang, Y. Metal-organic framework-derived highly dispersed Pt nanoparticles-functionalized ZnO polyhedrons for ppb-level CO detection. Sens. Actuators B 2021, 331, 129433. [Google Scholar] [CrossRef]
  20. Hieu, N.M.; Lam, D.V.; Hien, T.T.; Chinh, N.D.; Quang, N.D.; Hung, N.M.; Van Phuoc, C.; Lee, S.-M.; Jeong, J.-R.; Kim, C.; et al. ZnTe-coated ZnO nanorods: Hydrogen sulfide nano-sensor purely controlled by pn junction. Mater. Des. 2020, 191, 108628. [Google Scholar] [CrossRef]
  21. Lupan, O.; Ababii, N.; Santos-Carballal, D.; Terasa, M.-I.; Magariu, N.; Zappa, D.; Comini, E.; Pauporté, T.; Siebert, L.; Faupel, F.; et al. Tailoring the selectivity of ultralow-power heterojunction gas sensors by noble metal nanoparticle functionalization. Nano Energy 2021, 88, 106241. [Google Scholar] [CrossRef]
  22. Miller, D.R.; Akbar, S.A.; Morris, P.A. Nanoscale metal oxide-based heterojunctions for gas sensing: A review. Sens. Actuators B 2014, 204, 250–272. [Google Scholar] [CrossRef]
  23. Wang, Y.; Cui, Y.; Meng, X.; Zhang, Z.; Cao, J. A gas sensor based on Ag-modified ZnO flower-like microspheres: Temperature-modulated dual selectivity to CO and CH4. Surf. Interfaces 2021, 24, 101110. [Google Scholar] [CrossRef]
  24. Koo, W.T.; Choi, S.J.; Kim, S.J.; Jang, J.S.; Tuller, H.L.; Kim, I.D. Heterogeneous Sensitization of Metal-Organic Framework Driven Metal@Metal Oxide Complex Catalysts on an Oxide Nanofiber Scaffold Toward Superior Gas Sensors. J. Am. Chem. Soc. 2016, 138, 13431–13437. [Google Scholar] [CrossRef] [PubMed]
  25. Guo, L.; Wang, Y.; Shang, Y.; Yang, X.; Zhang, S.; Wang, G.; Wang, Y.; Zhang, B.; Zhang, Z. Preparation of Pd/PdO@ZnO-ZnO nanorods by using metal organic framework templated catalysts for selective detection of triethylamine. Sens. Actuators B 2022, 350, 130840. [Google Scholar] [CrossRef]
  26. Zhang, S.; Li, H.; Zhang, N.; Zhao, X.; Zhang, Z.; Wang, Y. Self-sacrificial templated formation of ZnO with decoration of catalysts for regulating CO and CH4 sensitive detection. Sens. Actuators B 2021, 330, 129286. [Google Scholar] [CrossRef]
  27. Chen, Y.; Zhang, W.; Luo, N.; Wang, W.; Xu, J. Defective ZnO Nanoflowers Decorated by Ultra-Fine Pd Clusters for Low-Concentration CH4 Sensing: Controllable Preparation and Sensing Mechanism Analysis. Coatings 2022, 12, 677. [Google Scholar] [CrossRef]
  28. Du, J.; Guo, M.; Zhang, A.; Zhao, H.; Zhao, D.; Wang, C.; Zheng, T.; Zhao, Y.; Luo, Y. Performance, structure and kinetics of Pd catalyst supported in Ba modified γ-Al2O3 for low temperature wet methane oxidation. Chem. Eng. J. 2022, 430, 133113. [Google Scholar] [CrossRef]
  29. Klingstedt, F.; Karhu, H.; Neyestanaki, A.K.; Lindfors, L.E.; Salmi, T.; Väyrynen, J. Barium Promoted Palladium Catalysts for the Emission Control of Natural Gas Driven Vehicles and Biofuel Combustion Systems. J. Catal. 2002, 206, 248–262. [Google Scholar] [CrossRef]
  30. Auvray, X.; Lindholm, A.; Milh, M.; Olsson, L. The addition of alkali and alkaline earth metals to Pd/Al2O3 to promote methane combustion. Effect of Pd and Ca loading. Catal. Today 2018, 299, 212–218. [Google Scholar] [CrossRef]
  31. Friberg, I.; Sadokhina, N.; Olsson, L. Complete methane oxidation over Ba modified Pd/Al2O3: The effect of water vapor. Appl. Catal. B 2018, 231, 242–250. [Google Scholar] [CrossRef]
  32. Xie, J.; Wang, J.; Wang, H.; Li, H.; Wang, J.; Shen, M. Promoted Hydrothermal Stability of Pd/CeO2 Catalyst by Ba Doping. Catal. Lett. 2018, 148, 2596–2607. [Google Scholar] [CrossRef]
  33. Modwi, A.; Taha, K.K.; Khezami, L.; Al-Ayed, A.S.; Al-Duaij, O.K.; Khairy, M.; Bououdina, M. Structural and Electrical Characterization of Ba/ZnO Nanoparticles Fabricated by Co-precipitation. J. Inorg. Organomet. Polym. Mater. 2019, 30, 2633–2644. [Google Scholar] [CrossRef]
  34. Chen, X.; Lin, J.; Zheng, Y.; Zhan, Y.; Zhang, W.; Xiao, Y.; Zheng, Y.; Jiang, L. Catalytic methane oxidation performance over Pd/γ-Al2O3 catalyst optimized by the synergy of phosphorus and MOx (M = La, Ba and Zr). Fuel 2021, 299, 120933. [Google Scholar] [CrossRef]
  35. Lu, Y.; Zhan, W.; He, Y.; Wang, Y.; Kong, X.; Kuang, Q.; Xie, Z.; Zheng, L. MOF-templated synthesis of porous Co3O4 concave nanocubes with high specific surface area and their gas sensing properties. ACS Appl. Mater. Interfaces 2014, 6, 4186–4195. [Google Scholar] [CrossRef] [PubMed]
  36. Wang, G.; Meng, M.; Zha, Y.; Ding, T. High-temperature close coupled catalysts Pd/Ce–Zr–M/Al2O3 (M = Y, Ca or Ba) used for the total oxidation of propane. Fuel 2010, 89, 2244–2251. [Google Scholar] [CrossRef]
  37. Liotta, L. Thermal stability, structural properties and catalytic activity of Pd catalysts supported on Al2O3–CeO2–BaO mixed oxides prepared by sol–gel method. J. Mol. Catal. A Chem. 2003, 204–205, 763–770. [Google Scholar] [CrossRef]
  38. Aghagoli, Z.; Ardyanian, M. Synthesis and study of the structure, magnetic, optical and methane gas sensing properties of cobalt doped zinc oxide microstructures. J. Mater. Sci. Mater. Electron. 2018, 29, 7130–7141. [Google Scholar] [CrossRef]
  39. Zhang, B.; Wang, Y.; Meng, X.; Zhang, Z.; Mu, S. High response methane sensor based on Au-modified hierarchical porous nanosheets-assembled ZnO microspheres. Mater. Chem. Phys. 2020, 250, 123027. [Google Scholar] [CrossRef]
  40. Basu, P.K.; Bhattacharyya, P.; Saha, N.; Saha, H.; Basu, S. The superior performance of the electrochemically grown ZnO thin films as methane sensor. Sens. Actuators B Chem. 2008, 133, 357–363. [Google Scholar] [CrossRef]
  41. Yang, L.; Wang, Z.; Zhou, X.; Wu, X.; Han, N.; Chen, Y. Synthesis of Pd-loaded mesoporous SnO2 hollow spheres for highly sensitive and stable methane gas sensors. RSC Adv. 2018, 8, 24268–24275. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  42. Zhang, D.; Chang, H.; Li, P.; Liu, R. Characterization of nickel oxide decorated-reduced graphene oxide nanocomposite and its sensing properties toward methane gas detection. J. Mater. Sci. Mater. Electron. 2015, 27, 3723–3730. [Google Scholar] [CrossRef]
  43. Zhang, S.; Li, Y.; Sun, G.; Zhang, B.; Wang, Y.; Cao, J.; Zhang, Z. Enhanced methane sensing properties of porous NiO nanaosheets by decorating with SnO2. Sens. Actuators B 2019, 288, 373–382. [Google Scholar] [CrossRef]
  44. Li, X.; Li, Y.; Sun, G.; Zhang, B.; Wang, Y.; Zhang, Z. Enhanced CH4 sensitivity of porous nanosheets-assembled ZnO microflower by decoration with Zn2SnO4. Sens. Actuators B 2020, 304, 127374. [Google Scholar] [CrossRef]
  45. Lu, W.; Ding, D.; Xue, Q.; Du, Y.; Xiong, Y.; Zhang, J.; Pan, X.; Xing, W. Great enhancement of CH4 sensitivity of SnO2 based nanofibers by heterogeneous sensitization and catalytic effect. Sens. Actuators B 2018, 254, 393–401. [Google Scholar] [CrossRef]
  46. Min, B.-K.; Choi, S.-D. Undoped and 0.1 wt.% Ca-doped Pt-catalyzed SnO2 sensors for CH4 detection. Sens. Actuators B Chem. 2005, 108, 119–124. [Google Scholar] [CrossRef]
  47. Vuong, N.M.; Hieu, N.M.; Hieu, H.N.; Yi, H.; Kim, D.; Han, Y.-S.; Kim, M. Ni2O3-decorated SnO2 particulate films for methane gas sensors. Sens. Actuators B 2014, 192, 327–333. [Google Scholar] [CrossRef]
  48. Ramgir, N.S.; Ghosh, M.; Veerender, P.; Datta, N.; Kaur, M.; Aswal, D.K.; Gupta, S.K. Growth and gas sensing characteristics of p- and n-type ZnO nanostructures. Sens. Actuators B 2011, 156, 875–880. [Google Scholar] [CrossRef]
  49. Yang, B.; Zhang, Z.; Tian, C.; Yuan, W.; Hua, Z.; Fan, S.; Wu, Y.; Tian, X. Selective detection of methane by HZSM-5 zeolite/Pd-SnO2 gas sensors. Sens. Actuators B 2020, 321, 128567. [Google Scholar] [CrossRef]
Chemosensors 10 00346 g001 550
Figure 1. Schematic illustration of the fabrication of ZnO-Ba/Pd sensitive material.
Figure 1. Schematic illustration of the fabrication of ZnO-Ba/Pd sensitive material.
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Figure 2. SEM images of (a) ZnO-1.0Ba/Pd, (b) TEM of ZnO-1.0Ba/Pd, (c) HRTEM image of ZnO-1.0Ba/Pd, (d) EDX spectrum of ZnO-1.0Ba/Pd.
Figure 2. SEM images of (a) ZnO-1.0Ba/Pd, (b) TEM of ZnO-1.0Ba/Pd, (c) HRTEM image of ZnO-1.0Ba/Pd, (d) EDX spectrum of ZnO-1.0Ba/Pd.
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Figure 3. XRD patterns of (a) ZnO-1.0Ba/Pd; (b) overall XPS spectra of ZnO-1.0Ba/Pd, XPS spectra of ZnO-1.0Ba/Pd; (c) Zn 2p spectrum; (d) O 1s spectrum; (e) Ba 4d spectrum; (f) Pd 3d spectrum.
Figure 3. XRD patterns of (a) ZnO-1.0Ba/Pd; (b) overall XPS spectra of ZnO-1.0Ba/Pd, XPS spectra of ZnO-1.0Ba/Pd; (c) Zn 2p spectrum; (d) O 1s spectrum; (e) Ba 4d spectrum; (f) Pd 3d spectrum.
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Figure 4. Gas sensing properties of ZnO/Pd, ZnO-0.5Ba/Pd, ZnO-1.0Ba/Pd, ZnO-1.5Ba/Pd, and ZnO-2.0Ba/Pd for 5000 ppm CH4: (a) response at different working temperatures (120–300 °C), (b) dynamic response (1000–5000 ppm), (c) relationship of the gas response to CH4 gas concentration (1000–5000 ppm), (d) response and recovery curves of sensitive materials.
Figure 4. Gas sensing properties of ZnO/Pd, ZnO-0.5Ba/Pd, ZnO-1.0Ba/Pd, ZnO-1.5Ba/Pd, and ZnO-2.0Ba/Pd for 5000 ppm CH4: (a) response at different working temperatures (120–300 °C), (b) dynamic response (1000–5000 ppm), (c) relationship of the gas response to CH4 gas concentration (1000–5000 ppm), (d) response and recovery curves of sensitive materials.
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Figure 5. (a) The Pd 3d XPS spectra of the different samples; (b) reproducibility of ZnO/Pd, ZnO-0.5Ba/Pd, ZnO-1.0Ba/Pd, ZnO-1.5Ba/Pd, and ZnO-2.0Ba/Pd; (c) selectivity of ZnO-1.0Ba/Pd at the working temperature of 170 °C; (d) stability tests for five ZnO-based sensors.
Figure 5. (a) The Pd 3d XPS spectra of the different samples; (b) reproducibility of ZnO/Pd, ZnO-0.5Ba/Pd, ZnO-1.0Ba/Pd, ZnO-1.5Ba/Pd, and ZnO-2.0Ba/Pd; (c) selectivity of ZnO-1.0Ba/Pd at the working temperature of 170 °C; (d) stability tests for five ZnO-based sensors.
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Figure 6. (a) Schematic illustration of the CH4 gas sensing mechanism; (b) CH4 decomposition ratios for ZnO/Pd and ZnO-1.0Ba/Pd at 100–300 °C; (c) resistance change curves of sensitive materials before and after target gas injection; (d) energy band diagram of BaO, Pd, and ZnO.
Figure 6. (a) Schematic illustration of the CH4 gas sensing mechanism; (b) CH4 decomposition ratios for ZnO/Pd and ZnO-1.0Ba/Pd at 100–300 °C; (c) resistance change curves of sensitive materials before and after target gas injection; (d) energy band diagram of BaO, Pd, and ZnO.
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Table
Table 1. The sensing performance of the CH4 sensor in this work in comparison to previous reports.
Table 1. The sensing performance of the CH4 sensor in this work in comparison to previous reports.
MaterialsTem. (°C)Con. (ppm)Res.Ref.
Pd–Ag/ZnO/Zn22010,0003.81 a[38]
Au/ZnO2501004.16 a[39]
ZnO/rGO190100012.10% b[40]
Pd–SnO23002504.88 a[41]
Pd/ZnO2001008.56 a[12]
NiO/rGO260100015% b[42]
SnO2/NiO33050015.2% b[43]
Pd/s-ZnO1905006.44 a[26]
Zn2SnO4/ZnO250100027.2 a[44]
Pt–SnO235050021% b[45]
Pt–Ca/SnO240050002.3 a[46]
Ni2O3–SnO2400200127% a[47]
ZnO-1.0Ba/Pd1705000368.2% bThis work
Note: The response is defined as: a Ra/Rg, b (Ra − Rg)/Ra×100%.
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Cai, Y.; Luo, S.; Chen, R.; Yu, J.; Xiang, L. Ba-Modified ZnO Nanorods Loaded with Palladium for Highly Sensitive and Rapid Detection of Methane at Low Temperatures. Chemosensors 2022, 10, 346. https://doi.org/10.3390/chemosensors10090346

AMA Style

Cai Y, Luo S, Chen R, Yu J, Xiang L. Ba-Modified ZnO Nanorods Loaded with Palladium for Highly Sensitive and Rapid Detection of Methane at Low Temperatures. Chemosensors. 2022; 10(9):346. https://doi.org/10.3390/chemosensors10090346

Chicago/Turabian Style

Cai, Yijing, Shirui Luo, Renjie Chen, Junxia Yu, and Lan Xiang. 2022. "Ba-Modified ZnO Nanorods Loaded with Palladium for Highly Sensitive and Rapid Detection of Methane at Low Temperatures" Chemosensors 10, no. 9: 346. https://doi.org/10.3390/chemosensors10090346

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