JBCS

INTRODUCTION

Carbon monoxide (CO) is a colorless, odorless, and tasteless gas.¹ It is a common chemical compound, typically produced by the incomplete combustion of organic materials such as wood, coal, and natural gas.² The primary uses of CO include industrial production, fuel, and metal smelting.^3-5 However, CO is highly toxic as it can bind with hemoglobin, preventing the transport of oxygen in the bloodstream.^6,7 This can lead to CO poisoning, which can be fatal in severe cases.⁸ Symptoms of CO poisoning include headaches, nausea, vomiting, weakness, and loss of consciousness.⁹ Therefore, it is important to ensure indoor air quality by timely ventilation and using CO detectors.

Gas sensors are devices designed to detect and measure the concentration of gases in the surrounding environment.^10,11 For CO detection, common types of gas sensors include electrochemical, infrared, and semiconductor sensors.^12-15 The quartz crystal microbalance (QCM) has garnered interest in gas sensing due to its exceptional accuracy, high stability, and low power consumption.^16-21 Its sensing mechanism involves changes in load mass after the sensing layer absorbs gases, altering the resonance frequency and causing a frequency shift.²² Consequently, the nanomaterials composing the sensing layer on QCM are crucial for detecting target gases.^23-25 Typically, scientists design the sensing process based on the adsorptive interaction between nanomaterials and target gases, including hydrogen bond adsorption, weak acid-base adsorption, Schiff base interaction, and so forth.^26-28

There are many reports^29-31 on QCM sensing platforms for inorganic gases, such as CO₂, H₂S, and NH₃. However, there is still a lack of CO detection work based on the QCM platform. So far, only the research groups of Gomes et al.³² and Antonaroli et al.³³ have conducted preliminary investigations. These studies only assessed the response to relatively high concentrations of CO and did not conduct systematic exploration and analysis of sensing performance parameters. Ceric dioxide (CeO₂) materials are commonly used for gas detection in semiconductor platform, but have not yet been used in QCM platform.^34,35 Here, we investigated the application of CeO₂ nanorods as sensitive materials coated on the surface of QCM, pioneering a systematic exploration of various performance indicators of this sensor at ppb-level concentrations of CO. We found that parameters such as sensitivity, selectivity, response speed, repeatability, and long-term stability were all satisfactory. Additionally, we explored the variations in CO response under different humidity conditions.

 

EXPERIMENTAL

Chemical raw materials and devices

The raw materials required for synthesizing CeO₂ nanorods and their acquisition methods are detailed in the Supplementary Material. The parameters of the QCM chip and purchasing information are also detailed in the Supplementary Material.

Synthesis of CeO₂ nanorods

The process of synthesizing the CeO₂ nanorods are displayed in the Supplementary Material.

Characterization, fabrication and test methods of the QCM sensor

The characterization instruments and methods for CeO₂ nanorods are detailed in the Supplementary Material.

Fabrication and test procedures for the CeO₂ nanorods based QCM sensor follow previously reported report, also outlined in the Supplementary Material. Refer to Figure 1S for a schematic of the testing system. To evaluate the effect of the deposited amount of CeO₂ nanorods on sensor efficiency, sensors were fabricated with 1, 2, 3, and 4 μL of CeO₂ nanorods dispersion deposited on the electrodes of QCMs. These sensors were respectively labeled as CeO₂-1, CeO₂-2, CeO₂-3, and CeO₂-4.

 

RESULTS AND DISCUSSION

Materials characterization

We synthesized pure-phase CeO₂ nanorods using a solvothermal synthesis method. The crystal structure of CeO₂ nanorods was determined by the X-ray diffraction (XRD) pattern (Figure 1a). Figure 1a shows that the diffraction peaks of the CeO₂ nanorods were observed at 2θ = 28.56, 33.16, 47.55, 56.48, and 59.29º, corresponding to the (111), (200), (220), (311), and (222) crystal faces of face-centered cubic (FCC) CeO₂ crystals (JCPDS 43-1002).³⁶ The morphology of CeO₂ nanorods was examined by transmission electron microscope (TEM). Figure 1b shows the TEM image of CeO₂ nanorods with magnification. The scale bar of Figure 1b is 50 nm. The CeO₂ nanorods synthesized exhibit a nanorod structure, with widths ranging from approximately 20 to 40 nm, and they demonstrate an orderly two-dimensional arrangement.

 

 

Gas sensing properties

The ability of QCM sensors utilizing CeO₂ nanorods to detect CO was assessed by varying gas concentrations and recording frequency shifts. Figure 2a depicts the frequency shifts of the fabricated sensors (CeO₂-1, CeO₂-2, CeO₂-3, and CeO₂-4) in response to CO concentration variations. It is evident that all four sensor types exhibit a decrease in frequency as CO concentration rises. Specifically, as the concentration levels increase from 500 to 4000 ppb, the frequency shifts of CeO₂-1, CeO₂-2, CeO₂-3, and CeO₂-4 sensors become more pronounced. This indicates a significant relationship between frequency shifts in CeO₂ nanorods based QCM sensors and CO concentration. Notably, CeO₂-3, and CeO₂-4 display the most substantial responses to CO concentrations ranging from 500 to 4000 ppb. Thus, considering the importance of the response and the need for cost reduction, we choose to further investigate sensing performance using the CeO₂-3 sensor.

 

 

The frequency shift curves of the CeO₂-3 sensor in response to varied concentrations of CO are depicted in Figure 2b. It is evident that the CeO₂-3 sensor exhibits reversible frequency shifts from the initial value and can return to baseline. Introduction of CO at concentrations of 500, 1000, 2000, and 4000 ppb resulted in similar-shaped frequency shifts for the CeO₂-3 sensor, with the amplitudes of the frequency shifts increasing proportionally with the CO concentrations. Figure 2c illustrates the linear correlation between the frequency shift and the respective CO concentrations. The derived equation is as follows: y = -0.1152x - 0.2565, with an R-squared value of 0.9987, where y represents the frequency shift due to CO (Hz), x denotes the CO concentration (ppb), and R is the regression coefficient. This indicates a linear proportionality between the frequency shift and CO concentration.

Figure 2d illustrates the frequency shift curve of the CeO₂-3 sensor when exposed to 500 ppb CO. Upon injection of 500 ppb CO, the frequency initially decreased. Subsequently, the frequency shift rapidly declined, surpassing 90% of the total value, and reaching -46.8 Hz within 22 s. Following this rapid decrease, the frequency shift exhibited a slower decline, reaching -51.2 Hz. Upon flushing the test chamber with N₂ for 12 s, the frequency essentially recovered by 90%. In summary, the response time and recovery time of the CeO₂-3 sensor to CO are 22 and 12 s, respectively.

Ensuring repeatability and long-term stability are crucial aspects in evaluating gas sensors.^37,38 To assess the repeatability of the CeO₂-3 sensor, it was subjected to three cycles of exposure to 500 ppb CO followed by flushing with N₂. As depicted in Figure 3a, the frequency reduction point marks the introduction of 500 ppb CO, while the frequency rise point indicates N₂ injection. Across the three consecutive cycles, the frequency shift curves exhibited similar shapes with minimal alterations in frequency shift, response time, and recovery time, underscoring the excellent repeatability of the CeO₂-3 sensor to CO.

 

 

To evaluate its long-term stability, the CeO₂-3 sensor was exposed to CO for durations of 1, 2, 3, and 4 weeks, respectively. Subsequently, the frequency shift of the sensor when exposed to 500, 1000, 2000, and 4000 ppb CO was measured, as depicted in Figure 3b. The results demonstrate a mere approximate 5% decrease in frequency shift within 3 months, indicating highly consistent sensing characteristics and long-term stability of the CeO₂-3 sensor.

Selectivity is also important for gas sensors.³⁹ It was investigated by exposing the CeO₂-3 sensor to a variety of common gases along with CO. Figure 4 shows the frequency shifts of the CeO₂-3 sensor upon exposure to CO, CO₂, NO₂, NO, NH₃, O₂ and H₂ in 500 ppb, demonstrating good selectivity for CO compared with other interfering gases. The CeO₂-3 sensor shows the highest frequency shift to 500 ppb CO with a value of up to -51.2 Hz, whereas the frequency shifts of other interfering gases are less than -19.6 Hz. The high selectivity of CO should be attributed to the specific adsorption of CO molecules and Ce atoms of CeO₂.⁴⁰ Yang et al.⁴⁰ reported that on the (111) surface of CeO₂, only weak adsorption of CO is observed, and this weak interaction involves electrostatic interactions. In the weak interaction mode, a nodal plane is present in the electron localization function (ELF) between the carbon atom of CO and the surface Ce atom. This suggests an absence of charge localization between the adsorbate and the surface, indicating the absence of covalent bonding in these modes. Yang et al.⁴⁰ also explores the charge density difference (CDD) for the weak interaction mode. In line with the ELF analysis, there is no charge accumulation between the carbon atom of CO and the Ce atom; instead, there is only a slight charge polarization within the CO molecule and the adjacent Ce atom. Hence, this weak interaction can be attributed, at least partially, to electrostatic interaction between CO and the CeO₂ substrate.

 

 

Impact of environmental humidity

Exploring the impact of environmental humidity is crucial for gas sensing applications using QCM platforms.⁴¹ Since these sensors typically operate at room temperature, it is not possible to eliminate interference from water vapor through methods like high-temperature heating. Additionally, some sensing materials may interact with water molecules, and adsorption of water molecules from the environment can affect their response to target gases. Therefore, we tested the response of CeO₂-3 sensor to 500 ppb CO and 4000 ppb CO in different relative humidity (RH) environments, specifically manifested as frequency drift values. The relative humidity levels tested were 0% RH, 20% RH, 40% RH, 60% RH, 80% RH, and 90% RH. The results from Figure 5 indicate that increasing environmental humidity decreases CeO₂-3 sensor's response to 500 ppb CO and 4000 ppb CO. This should be attributed to water molecules in the air occupying adsorption sites on the CeO₂ surface intended for CO, as water molecules can also adsorb onto the CeO₂ surface.⁴² Fronzi et al.⁴² systematically explored the adsorption modes of water molecules on the surface of CeO₂. On the CeO₂ (111) orientation, water molecules can be co-adsorbed by Ce or O atoms in four different ways, undoubtedly greatly reducing the effective adsorption sites for CO. This will consequently diminish the CO adsorption capacity of CeO₂, leading to a weakened response of CeO₂-3 sensor. Therefore, some methods can be adopted to eliminate the influence of environmental humidity on CeO₂-3 sensor's detection of CO in the future, such as post-processing methods like error compensation.

 

 

In order to explore the effect of crystal surface effect on sensing performance, we purchased commercial CeO₂ nanoparticles with the crystal surface of (110) and compared their sensing performance with the CeO₂ nanorods we synthesized with the crystal surface of (111). The load mass of CeO₂ nanoparticles and CeO₂ nanorods is the same as that of CeO₂-3 sensor. As shown in Figure 6a, the frequency shift values of CeO₂ nanoparticles with the crystal surface of (110) towards CO concentrations of 500-4000 ppb are slightly larger than those of CeO₂ nanorods with the crystal surface of (111). However, as shown in Figure 6b, the response and recovery time of CeO₂ nanoparticles with the crystal surface of (110) to 500 ppb concentration of CO are much larger than those of CeO₂ nanorods with the crystal surface of (111). It should be attributed to that the adsorption strength of CO by CeO₂ exposed on crystal plane (110) is larger than that of CeO₂ nanorods exposed on crystal plane (111), and it is also more difficult to desorb.⁴⁰ Therefore, considering the frequency shift and response/recovery time, CeO₂ nanorods with the crystal surface of (111) should be a better CO sensing material.

 

 

CONCLUSIONS

In summary, our research team proposes a novel and systematic method for CO detection using a QCM gas sensing platform. By coating the QCM surface with CeO₂ nanorods, we achieved selective and rapid detection of CO gas with high sensitivity. Additionally, the sensor demonstrates suitable repeatability and long-term stability. We also investigated the sensing mechanism, explored the influence of environmental humidity on sensor response, and referenced potential theoretical explanations. This work offers a new approach for utilizing QCM platforms in CO detection.

 

SUPPLEMENTARY MATERIAL

Synthesis of CeO₂ nanorods; characterization; fabrication and test methods of the QCM sensor; schematic of the gas testing system are available at http://quimicanova.sbq.org.br, in PDF format, with free access.

 

ACKNOWLEDGMENTS

This research was supported by National Natural Science Foundation of China under Grant No. 62001420; Zhejiang Provincial Natural Science Foundation of China under Grant No. LQ21F010017; University-Industry Collaborative Education Program under Grant No. 220400576262052; and Science Foundation of Zhejiang Gongshang University Hangzhou College of Commerce, Zhejiang Gongshang University, China (ZJHZCC) under Grant No. 2222111.

 

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Associate Editor handled this article: Cassiana C. Montagner