Microwave-Solvothermal Synthesis of Mesoporous CeO2/CNCs Nanocomposite for Enhanced Room Temperature NO2 Detection

Nitrogen dioxide (NO2) gas sensors are pivotal in upholding environmental integrity and human health, necessitating heightened sensitivity and exceptional selectivity. Despite the prevalent use of metal oxide semiconductors (MOSs) for NO2 detection, extant solutions exhibit shortcomings in meeting practical application criteria, specifically in response, selectivity, and operational temperatures. Here, we successfully employed a facile microwave-solvothermal method to synthesize a mesoporous CeO2/CNCs nanocomposite. This methodology entails the rapid and comprehensive dispersion of CeO2 nanoparticles onto helical carbon nanocoils (CNCs), resulting in augmented electronic conductivity and an abundance of active sites within the composite. Consequently, the gas-sensing sensitivity of the nanocomposite at room temperature experienced a notable enhancement. Moreover, the presence of cerium oxide and the conversion of Ce3+ and Ce4+ ions facilitated the generation of oxygen vacancies in the composites, thereby further amplifying the sensing performance. Experimental outcomes demonstrate that the nanocomposite exhibited an approximate 9-fold increase in response to 50 ppm NO2 in comparison to pure CNCs at room temperature. Additionally, the CeO2/CNCs sensor displayed remarkable selectivity towards NO2 when exposed to gases such as NH3, CO, SO2, CO2, and C2H5OH. This straightforward microwave-solvothermal method presents an appealing strategy for the research and development of intelligent sensors based on CNCs nanomaterials.


Introduction
Over the past few decades, the escalating emission of nitrogen oxides (NO x ), particularly nitrogen dioxide (NO 2 ) stemming from industrial fuel combustion and vehicle exhaust, has posed an increasingly severe threat to the environment and human health.Prolonged exposure to NO 2 not only has detrimental effects on the respiratory system, heightening the risk of mycoplasma respiratory infections, asthma, and pneumonia [1,2], but is also a major contributor to acid rain formation, causing damage to crops and environmental contamination [3].Consequently, there is an urgent need for the development of an efficient nitrogen dioxide gas sensor.Chemoresistive gas sensors, particularly those based on Metal-Oxide-Semiconductor (MOS) technology, have attracted considerable attention in the field of gas sensing.This is primarily due to their simple structure, ease of preparation, and cost effectiveness [4].MOS-based gas sensors demonstrate exceptional selectivity and sensitivity to specific gases, attributable to their distinctive micro-/nanoscale structure and morphology [5].Both p-type MOS (such as Co 3 O 4 [6], CuO [7], NiO [8]) and n-type MOS (such as ZnO [9], TiO 2 [10], Al 2 O 3 [11]) have been extensively investigated for sensing applications.Ceria (CeO 2 ), characterized as a semiconductor material with a wide bandgap (3.2 eV), holds significant promise across various fields, including energy Nanomaterials 2024, 14, 812 2 of 16 storage devices [12], magnetic wave absorbers [13], and photocatalytic applications [14].Furthermore, ceria emerges as a potentially economical nanomaterial for gas sensing due to its chemical stability, mixed valence, and high oxygen absorption capacity [15].Researchers have also explored the use of doped ceria with metal/metal oxide, which has shown fast response times and high selectivity [16,17].However, the intrinsic low conductivity of ceria mandates high-temperature operation, thereby restricting its practical applications.It is crucial to identify a suitable material for modifying the properties of cerium oxide to enable detection at room temperature (RT).
In recent years, extensive research has been conducted on carbon nanomaterials due to their remarkable ability to transport electrons, immense surface area, and excellent electrical conductivity.Carbon nanotubes (CNTs), graphene oxide (GO) and reduced graphene oxide (rGO) have emerged as promising materials for toxic gas detection at RT, benefiting from their myriad functional groups and superior conductive network [18][19][20][21].Nevertheless, their limited selectivity and prolonged reaction times have impeded their widespread utilization in gas sensors.To tackle this challenge, a potential solution involves integrating metal semiconductors with carbon materials to enhance both gas adsorption and electrical conductivity.For instance, Liu et al. [22] synthesized a ZnO/rGO nanocomposite through a thermal reduction and soft solution process, showcasing a heightened response and shorter response-recovery behavior compared to a pure ZnO sensor when employed as a nitrogen dioxide sensor at RT. Nonetheless, the multilayer stacking overlap of graphene oxide nanosheets in this composite failed to provide sufficient active sites, resulting in diminished sensing sensitivity.Therefore, there remains a necessity to identify an alternative nanostructured material possessing both a large specific surface area and an abundance of active sites for effective adsorption of target gases.
The as-grown carbon nanocoils (CNCs), comprised of coiled carbon nanotubes (CNTs) connected by amorphous carbon nanofibers (CNFs), represent a distinctive structural configuration [23,24].Characterized by a unique helical structure amalgamating sp 2 grains and sp 3 amorphous structures [23], CNCs exhibit physical properties that set them apart from both CNTs and CNFs [24].This spiral polycrystalline-amorphous nature renders CNCs highly advantageous for a spectrum of applications, including field-emission devices, electromagnetic wave absorbers, humidity sensors, and acoustic sensors [25][26][27][28].Moreover, the helical architecture of CNCs contributes to an augmented surface area, facilitating gas molecule adsorption by providing an abundance of active sites.CNCs have demonstrated promising potential as active materials in gas sensing applications.For example, a NO 2 sensor based on CNCs decorated with NiO nanosheets has been developed using a hydrothermal method.This sensor has a detection concentration limit of 60.3 ppb and demonstrates selectivity towards NO 2 at RT [29].However, it should be noted that this sensor requires a lengthy hydrothermal reaction time and exhibits a relatively subdued response.
In this study, we propose a rapid and straightforward microwave-solvothermal (MWS) method for synthesizing CeO 2 /CNCs nanocomposites with a mesoporous structure, achieved by embellishing CNCs with CeO 2 nanoparticles.The nanoparticles produced through the MWS method demonstrate characteristics of a narrow particle size distribution, minimal agglomeration, and uniform morphology [30,31].In this way, the crystalline CeO 2 nanoparticles can be evenly anchored onto the chemical vapor deposition (CVD)-grown CNCs in a short period.Furthermore, the as-obtained CeO 2 /CNCs composites exhibit a mesoporous structure and helical morphology, thereby providing a greater number of exposed active sites and larger specific surface areas for the adsorption and diffusion of gas molecules.Additionally, the exceptional conductivity of CNCs facilitates the establishment of a complete electric conduction network, reducing the operational resistance of the original CeO 2 and improving its detection capabilities at room temperature.Moreover, the electron hopping within the helical CNCs contributes to the efficient transport of charge carriers, resulting in an elevated signal level [24].Simultaneously, the composite exhibits a significant presence of oxygen vacancies, further contributing to its exceptional redox capacity.As a result, the CeO 2 /CNCs nanocomposites demonstrate high sensitivity and exceptional selectivity in detecting NO 2 at RT.The facile synthesis of CeO 2 /CNCs composites using the microwave-solvothermal approach represents an innovative and promising avenue for the development of low-temperature nitrogen dioxide-sensing materials.

Synthesis of CNCs and CeO 2 /CNCs Nanocomposite
A large amount of CNCs were prepared via a CVD process with catalyst of Fe/Sn [23,24].Specifically, the mole ratio of Fe to Sn in the Fe (NO 3 ) 3 /SnCl 4 precursor solution was controlled to be 60:1.The CNCs were synthesized by using a thermal CVD technology at 720 • C for 4 h with introducing 30 and 300 sccm of C 2 H 2 and Ar gases, respectively.Then, the obtained CNC clusters were refined for additional processing by being dissolved in ethanol solution using an ultrasonic device for 30 s, thereafter collected the purified CNC samples from supernatant to remove impurity particles.
The synthesis of the CeO 2 /CNCs nanocomposite was carried out using a microwavesolvothermal method, as illustrated in Figure 1a.Initially, 20 mg of CNCs functionalized by acid solution (HNO 3 :H 2 SO 4 in 1:3 (v/v) ratio) were dispersed in 20 mL of N, N-dimethylformamide (DMF) and subjected to sonication for 30 min.Subsequently, 0.7 g of cerous nitrate hexahydrate (Ce (NO 3 ) 3 •6H 2 O) was dissolved in 20 mL of anhydrous ethanol and sonicated for 30 min.Then, the Ce (NO 3 ) 3 solution was added dropwise to the previous solution while continuously stirring magnetically for 1 h.After that, the as-prepared solution was shifted to a 50 mL autoclave, and subsequently treated under a microwave system (microwave 600 W) at 140 • C for 1 h.After naturally cooling to room temperature, the precipitate was collected, washed with deionized water and pure ethanol, and dried at 60 • C for 12 h.Finally, the obtained powders were calcined at 300 • C under vacuum conditions for 2 h, resulting in the formation of CeO 2 /CNCs nanocomposites.
Moreover, the electron hopping within the helical CNCs contributes to the efficient transport of charge carriers, resulting in an elevated signal level [24].Simultaneously, the composite exhibits a significant presence of oxygen vacancies, further contributing to its exceptional redox capacity.As a result, the CeO2/CNCs nanocomposites demonstrate high sensitivity and exceptional selectivity in detecting NO2 at RT.The facile synthesis of CeO2/CNCs composites using the microwave-solvothermal approach represents an innovative and promising avenue for the development of low-temperature nitrogen dioxidesensing materials.

Synthesis of CNCs and CeO2/CNCs Nanocomposite
A large amount of CNCs were prepared via a CVD process with catalyst of Fe/Sn [23,24].Specifically, the mole ratio of Fe to Sn in the Fe (NO3)3/SnCl4 precursor solution was controlled to be 60:1.The CNCs were synthesized by using a thermal CVD technology at 720 °C for 4 h with introducing 30 and 300 sccm of C2H2 and Ar gases, respectively.Then, the obtained CNC clusters were refined for additional processing by being dissolved in ethanol solution using an ultrasonic device for 30 s, thereafter collected the purified CNC samples from supernatant to remove impurity particles.
The synthesis of the CeO2/CNCs nanocomposite was carried out using a microwavesolvothermal method, as illustrated in Figure 1a.Initially, 20 mg of CNCs functionalized by acid solution (HNO3:H2SO4 in 1:3 (v/v) ratio) were dispersed in 20 mL of N, N-dimethylformamide (DMF) and subjected to sonication for 30 min.Subsequently, 0.7 g of cerous nitrate hexahydrate (Ce (NO3)3•6H2O) was dissolved in 20 mL of anhydrous ethanol and sonicated for 30 min.Then, the Ce (NO3)3 solution was added dropwise to the previous solution while continuously stirring magnetically for 1 h.After that, the as-prepared solution was shifted to a 50 mL autoclave, and subsequently treated under a microwave system (microwave 600 W) at 140 °C for 1 h.After naturally cooling to room temperature, the precipitate was collected, washed with deionized water and pure ethanol, and dried at 60 °C for 12 h.Finally, the obtained powders were calcined at 300 °C under vacuum conditions for 2 h, resulting in the formation of CeO2/CNCs nanocomposites.In order to conduct a comparative analysis, we obtained samples containing varying amounts of cerous nitrate hexahydrate (0.3 g, 0.5 g, 0.7 g, and 0.9 g).These samples were designated as CeO 2 /CNCs-0.3,CeO 2 /CNCs-0.5,CeO 2 /CNCs-0.7,and CeO 2 /CNCs-0.9,respectively.Furthermore, employing similar parameters, we synthesized nanocomposites with different compositions by manipulating the temperature of the solvent thermal reaction (100 • C, 120 • C, 140 • C, and 160 • C), and labeled them as sample 1, sample 2, sample 3 and sample 4, respectively.Additionally, for the purpose of further comparison, we prepared bare CNCs under identical experimental conditions.

Characterization
X-ray diffraction radiation (XRD, MiniFlex600, Rigaku) was used to determine the crystallographic phase of the as-prepared samples, with a CuKa radiation in the range from 10 • to 85 • .The Raman spectra patterns were acquired on a Raman spectrometer in the energy range of 200-2000 cm −1 , the laser utilized to record with an excitation source of 638 nm.Field-emission scanning electron microscope (FE-SEM, FEI Scios, Germany) was used to investigate the morphological characteristics.The TEM images were taken using a JEOL 2200FS electronic microscope operating at 200 kV.The surface area and the pore size were estimated by N 2 adsorption-desorption isotherms using an ASAP 2460 (Micromeritics, USA).The specific surface area of the samples was calculated based on the multi-point adsorption data of the N 2 adsorption isotherm, employing the Brunauer-Emmett-Teller (BET) theory.The pore size distribution was analyzed using the Barrett-Joyner-Halenda (BJH) technique.The chemical states of the materials were determined by X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB Xi+), which generated X-ray with an energy of 1486.7 eV to identify the valence states of the elements.

Fabrication and Measurement of the Gas Sensor
15 mg of the as-synthesized CeO 2 /CNCs composite were mixed with 1 mL of ethanol to prepare dispersion that were finally deposited on the alumina substrate, and following an ultrasonically dispersing for 30 min.Then, 20 µL of the solution was dropped onto the Au interdigital electrode and dried at 60 • C overnight, the schematic of the process is illustrated in Figure 1b.
The gas sensing properties were investigated on a dual-channel gas system (BONA TECH BN2914), which obtained the resistance change curve by introducing the mixture gases of test gas and dry air.In particular, the target gas (NO 2 ) was used to test the sensing performance, and dry air was selected as the carrier gas.Moreover, the effect of humidity on the sensing material can be analysed by adjusting the relative humidity in controlled environment chamber.
The study assessed the performance of the gas sensor under examination through measuring changes in electrical resistance at various concentrations of NO 2 under standard temperature (24 • C).The NO 2 gas concentration test sequence was 10, 20, 30, 50 and 80 ppm, respectively.The percent sensor response (S%) can be calculated by S = (R o − R g ) /R o × 100 where, R o and R g represent the resistance of the sensor when exposed to dry air and target gas, respectively.The resistance of materials can not completely recover to the initial value in a short time, because of the robust adsorption effect of composites on the target gas.Hence, solely the 5-min response to the initiation of the test gas and the subsequent 5-min recuperation following the discontinuation of the test gas introduction were documented.To guarantee the validity of the followed experiment, the samples were preserved at 60 • C for 12 h before the next test.

Morphological and Structural Characteristics
The crystal structures of CeO 2 , CNCs, and CeO 2 /CNCs were examined by analyzing XRD patterns, as shown in Figure 2. The primary peaks observed in the XRD patterns of CeO 2 and CeO 2 /CNCs correspond to a cubic CeO 2 structure (JCPDS, ICDD NO. 34-394), specifically the (111), ( 200), ( 220), (311), ( 222), (400), (331), and (420) planes.Furthermore, the presence of the (002) orientation in the pattern of the CeO 2 /CNCs composite confirms the successful synthesis of the complex.Notably, the crystallinity of the CeO 2 /CNCs composite is diminished as a result of the polycrystalline amorphous characteristics of CNCs, leading to a lower peak intensity in comparison to pure CeO 2 nanoparticles.In addition, no other peaks were detected in the XRD patterns, indicating the high purity of the sythesized samples.
CeO2 and CeO2/CNCs correspond to a cubic CeO2 structure (JCPDS, ICDD NO. 34-394), specifically the (111), ( 200), ( 220), ( 311), ( 222), ( 400), (331), and (420) planes.Furthermore, the presence of the (002) orientation in the pattern of the CeO2/CNCs composite confirms the successful synthesis of the complex.Notably, the crystallinity of the CeO2/CNCs composite is diminished as a result of the polycrystalline amorphous characteristics of CNCs, leading to a lower peak intensity in comparison to pure CeO2 nanoparticles.In addition, no other peaks were detected in the XRD patterns, indicating the high purity of the sythesized samples.The morphology of CeO2, CNCs, and CeO2/CNCs was examined through the utilization of SEM images, as depicted in Figure 3. Figure 3a shows the morphology of the CeO2 nanoparticles, with the inset providing a magnified view of the agglomerate consisting of several aggregates of CeO2 nanoparticles. Figure 3b,c display images of the helical structure of CNCs, with the rough surface (as observed in the inset image of Figure 3c) potentially contributing to gas sensing capabilities.The SEM images of CeO2/CNCs nanocomposite at varying magnifications are presented in Figure 3d-f.A closer look at the SEM image of CeO2/CNCs composite (Figure 3f) clearly reveals that the CeO2 nanoparticles are uniformly modified on the surface of the spiral CNCs.The presence of CNCs plays a significant role in preventing agglomeration, particularly when compared to pure CeO2.This effect leads to a notable improvement in the contact between the surface of material and the NO2 gas molecules, ultimately resulting in a noticeable increase in the efficiency of detection.The morphology of CeO 2 , CNCs, and CeO 2 /CNCs was examined through the utilization of SEM images, as depicted in Figure 3. Figure 3a shows the morphology of the CeO 2 nanoparticles, with the inset providing a magnified view of the agglomerate consisting of several aggregates of CeO 2 nanoparticles.Figure 3b,c display images of the helical structure of CNCs, with the rough surface (as observed in the inset image of Figure 3c) potentially contributing to gas sensing capabilities.The SEM images of CeO 2 /CNCs nanocomposite at varying magnifications are presented in Figure 3d-f.A closer look at the SEM image of CeO 2 /CNCs composite (Figure 3f) clearly reveals that the CeO 2 nanoparticles are uniformly modified on the surface of the spiral CNCs.The presence of CNCs plays a significant role in preventing agglomeration, particularly when compared to pure CeO 2 .This effect leads to a notable improvement in the contact between the surface of material and the NO 2 gas molecules, ultimately resulting in a noticeable increase in the efficiency of detection.
In order to gain a better understanding of the internal structures of the CNCs, a highresolution image of their morphology was obtained using TEM, as shown in Figure S1.It is evident from the image that the inner spiral shape of the CNCs consists of a hollow carbon tube structure.This unique structure promotes electron hopping, thereby facilitating the transport of charge carriers.Additionally, the helical arrangement of the CNCs contributes to an increased surface area, which facilitates the adsorption of gas molecules by providing a greater number of adsorption sites.To further investigate the influence of specific surface area on the gas-sensitive properties, porosity analysis was performed on the CNCs and CeO 2 /CNCs material using N 2 adsorption-desorption isotherm experiments (Figure S2).The results reveal that both the CNCs and CeO 2 /CNCs material exhibit type IV isotherms, with the adsorption hysterias loop attributed to the presence of ordered mesoporous channels in the materials.The specific surface areas and pore characteristics of the CNCs and CeO 2 /CNCs were determined and presented in Table 1.The uncoated CNCs have a specific surface area of 38.8186 m 2 g −1 and an average pore size of 8.054 nm, while with a micropore area of 0.000158 cm 3 g −1 , which restricts the diffusion of gas molecules within the material.In contrast, the CeO 2 /CNCs nanocomposite exhibit higher porosity, with a specific surface area of 83.6895 m 2 g −1 , a micropore area of 0.020243 cm 3 g −1 , and an average pore size of 7.2005 nm.These characteristics provide an increasing number of active sites for the interaction with target gases.In order to gain a better understanding of the internal structures of the CNCs, a highresolution image of their morphology was obtained using TEM, as shown in Figure S1.It is evident from the image that the inner spiral shape of the CNCs consists of a hollow carbon tube structure.This unique structure promotes electron hopping, thereby facilitating the transport of charge carriers.Additionally, the helical arrangement of the CNCs contributes to an increased surface area, which facilitates the adsorption of gas molecules by providing a greater number of adsorption sites.To further investigate the influence of specific surface area on the gas-sensitive properties, porosity analysis was performed on the CNCs and CeO2/CNCs material using N2 adsorption-desorption isotherm experiments (Figure S2).The results reveal that both the CNCs and CeO2/CNCs material exhibit type IV isotherms, with the adsorption hysterias loop attributed to the presence of ordered mesoporous channels in the materials.The specific surface areas and pore characteristics of the CNCs and CeO2/CNCs were determined and presented in Table 1.The uncoated CNCs have a specific surface area of 38.8186 m 2 g −1 and an average pore size of 8.054 nm, while with a micropore area of 0.000158 cm 3 g −1 , which restricts the diffusion of gas molecules within the material.In contrast, the CeO2/CNCs nanocomposite exhibit higher porosity, with a specific surface area of 83.6895 m 2 g −1 , a micropore area of 0.020243 cm 3 g −1 , and an average pore size of 7.2005 nm.These characteristics provide an increasing number of active sites for the interaction with target gases.

Gas Sensing Properties
The effects of different amounts of CeO 2 /CNCs nanocomposites on NO 2 sensing performance were studied, as shown in the Figure 4a-c.It can be clearly seen that the sensitivity increases gradually with the increase in content of Ce within the range of 0.3 to 0.7.
However, the response decreases when the CeO 2 content is further increased.The responses of CeO 2 /CNCs-0.3,CeO 2 /CNCs-0.5, and CeO 2 /CNCs-0.9samples to 50 ppm concentration of NO 2 are approximately 14.3%, 27.8%, and 29.3%, respectively (see in Figure 4c).When comparing to above samples, the mass ratio of CeO 2 /CNCs-0.7 demonstrates superior sensing performance, which exhibits about two to three times greater sensitivity (65.2%) than that of obtained composites.The diminished responsiveness observed in CeO 2 /CNCs-0.9compared to CeO 2 /CNCs-0.7 can be attributed to the heightened crystallinity of CeO 2 , leading to increased resistance that partially obstructs electron transport within the system, consequently yielding lower response magnitudes.As shown in Figure 4d, the responses of CeO 2 /CNCs-0.7 and bare CNCs to varying concentrations of NO 2 (10-80 ppm) reveal that the sensitivity increases with gas concentration.Furthermore, the CeO 2 /CNCs composite sample delivers a better sensing performance than bare CNCs.In particular, the response of CeO 2 /CNCs to 30 ppm (47.3) is nearly ten times higher than the response of bare CNCs (4.69).In fact, pure CNCs display low efficiency in detecting NO 2 gas due to its low resistance, which is insufficient to produce significant changes in conductivity.Therefore, the sensing performance can be improved by appropriately increasing the content of CeO 2 , and highly sensitive CeO 2 /CNCs-0.7 composites are considered potential sensing materials for the following studies.

Gas Sensing Properties
The effects of different amounts of CeO2/CNCs nanocomposites on NO2 sensing performance were studied, as shown in the Figure 4a-c.It can be clearly seen that the sensitivity increases gradually with the increase in content of Ce within the range of 0.3 to 0.7.However, the response decreases when the CeO2 content is further increased.The responses of CeO2/CNCs-0.3,CeO2/CNCs-0.5, and CeO2/CNCs-0.9samples to 50 ppm concentration of NO2 are approximately 14.3%, 27.8%, and 29.3%, respectively (see in Fig- ure 4c).When comparing to above samples, the mass ratio of CeO2/CNCs-0.7 demonstrates superior sensing performance, which exhibits about two to three times greater sensitivity (65.2%) than that of obtained composites.The diminished responsiveness observed in CeO2/CNCs-0.9compared to CeO2/CNCs-0.7 can be attributed to the heightened crystallinity of CeO2, leading to increased resistance that partially obstructs electron transport within the system, consequently yielding lower response magnitudes.As shown in Figure 4d, the responses of CeO2/CNCs-0.7 and bare CNCs to varying concentrations of NO2 (10-80 ppm) reveal that the sensitivity increases with gas concentration.The experimental results present the tests of CeO 2 /CNCs nanocomposites with the same mass ratio, synthesized at various MWS reaction temperatures, as depicted in Figure 5. Specifically, in Figure 5a-c, it can be observed that sample 3 exhibited more pronounced sensitivity and faster response times in multiple concentration ranges, when compared to the tests conducted on sample 1 and sample 2. Furthermore, the response values and response times for sample 3 were found to be 64.26% and 180 s in the presence of 50 ppm NO 2 , which were similar to sample 4 (56.48% and 192 s).Conversely, the response of samples 1 and 2, synthesized at lower temperatures, was only 31.2% and 45.6%, respectively, with response times of 223 s and 235 s.Overall, the sensitivity of the as-prepared samples increased gradually with an increase in reaction temperature, reaching its maximum at approximately 140 • C. It should be noted that excessive temperature can promote the crystallization of additional CeO 2 and, to some extent, limit the efficiency of electron transfer in the material.Additionally, the resistance decreases when an oxidizing gas is passed, indicating that the composites exhibit p-type semiconducting properties.However, the resistance of complexes face difficulty returning to their initial state within a short period of time, because the rapid desorption of NO 2 molecules is hindered as the presence of van der Waals forces.Typically, the recovery of sample is accelerated by drying in an oven, and eliminates the effects of moisture present in the air.
ciency in detecting NO2 gas due to its low resistance, which is insufficient to produce significant changes in conductivity.Therefore, the sensing performance can be improved by appropriately increasing the content of CeO2, and highly sensitive CeO2/CNCs-0.7 composites are considered potential sensing materials for the following studies.
The experimental results present the tests of CeO2/CNCs nanocomposites with the same mass ratio, synthesized at various MWS reaction temperatures, as depicted in Figure 5. Specifically, in Figure 5a-c, it can be observed that sample 3 exhibited more pronounced sensitivity and faster response times in multiple concentration ranges, when compared to the tests conducted on sample 1 and sample 2. Furthermore, the response values and response times for sample 3 were found to be 64.26% and 180 s in the presence of 50 ppm NO2, which were similar to sample 4 (56.48% and 192 s).Conversely, the response of samples 1 and 2, synthesized at lower temperatures, was only 31.2% and 45.6%, respectively, with response times of 223 s and 235 s.Overall, the sensitivity of the as-prepared samples increased gradually with an increase in reaction temperature, reaching its maximum at approximately 140 °C.It should be noted that excessive temperature can promote the crystallization of additional CeO2 and, to some extent, limit the efficiency of electron transfer in the material.Additionally, the resistance decreases when an oxidizing gas is passed, indicating that the composites exhibit p-type semiconducting properties.However, the resistance of complexes face difficulty returning to their initial state within a short period of time, because the rapid desorption of NO2 molecules is hindered as the presence of van der Waals forces.Typically, the recovery of sample is accelerated by drying in an oven, and eliminates the effects of moisture present in the air.The limit of detection (LOD) is defined as 3RMS niose /slope.From the fitting curve (Figure S3), the calculated LOD of CeO 2 /CNCs (sample 3) to NO 2 is determined to be 710.3ppb.A comparison with other NO 2 sensors at room temperature also be presented in Table 2. Furthermore, the multi-cycling properties and gas selectivity are the important parameter for gas sensors.The response degree of composites to 50 ppm NO 2 was tested in six cycling curves, which remained reliable stabilisation in the range of 63.5% to 69.2% (Figure 5d).Moreover, the response degrees of composites to NH 3 , CO, SO 2 , CO 2 and C 2 H 5 OH are shown in Figure 5f.It is evident that the sensitivity is much higher when detecting the NO 2 , indicating the perfect selectivity of the sensing material at RT.In fact, this selectivity is attributed to the ability of the sensing materials to react with anion oxygen [38].The defect sites (such as oxygen vacancies and antisite defects) in the composites, as well as the adsorption capacity and catalytic activity for the target gas, are the main factors contributing to the formation of selectivity [39].Furthermore, the impact of environmental humidity on the gas sensing characteristics of the CeO 2 /CNCs nanocomposite are also examined.Figure 5e displays the responses of composites for 20 ppm NO 2 at different relative humidity (RH), and it is observed that the sensitivity decreases rapidly with the increasing of RH.This is because in humid environments, water molecules are adsorbed on the surface of the sensing material, hindering further contact between the active site and the gas molecules [40].In addition, water molecules can react with NO 2 molecules to form HNO 3 , which impairs the gas sensing response.Therefore, humidity plays a significant role in the gas sensing performance.

Gas Sensing Mechanism
The utilization of CNCs is significant importance in enhancing the gas sensing performance.To begin with, CNCs have the ability to prevent the aggregation and reformation of CeO 2 nanoparticles.This is because the functionalized CNCs have an abundance of negatively charged oxygen-containing groups, which allows for the adsorption of Ce 3+ ions through electrostatic interaction.As a result, CeO 2 nanoparticles grow uniformly on the surface of the CNCs during the oxidation reaction [38,41].This uniform distribution of CeO 2 nanoparticles on helical CNCs creates a large specific surface area, facilitating the permeation and diffusion of target gases within the composites.Additionally, while pure CeO 2 nanoparticles typically exhibit excellent gas sensing performance at high operating temperatures (300 • C) [42], the composite structure benefits from the excellent electrical conductivity and rapid electronic transfer capability of the CNCs.This improvement in electrical properties enhances the ability of gas detection at room temperature.Furthermore, the hybrid structure contains numerous defects, which serve as additional active sites for the adsorption of gases, ultimately enhancing the sensing performance.

The Effects of Defects on the Gas Sensing Performance
The presence of defects in sensing materials (CeO 2 /CNCs) is crucial in enhancing the performance of response by creating numerous vacancies and active sites [43].The extent of defects can typically be determined through Raman spectroscopy, which quantifies the ratio of the intensity of the D band to the G band, i.e., I D /I G . Figure 6a exhibits the results of the Raman measurements conducted on the samples that were prepared.The CNCs, functionalized CNCs, and CeO 2 /CNCs composites reveals two graphite-like characteristic peak of D and G band, approximately located at wavelengths of 1356 cm −1 and 1582 cm −1 , respectively.It is acknowledged that the D-band is caused by structural defects and disordered atomic arrangements in sp 3 carbon atoms, while the G-band is associated with the E 2g vibrational mode in the sp 2 carbons [23,44].Moreover, a distinct peak at 458 cm −1 is evident in the spectrum of the CeO 2 /CNCs composites, corresponding to the characteristic peak of CeO 2 , thereby confirming the successful synthesis of the complex.In addition, the I D /I G values of the as-prepared materials were calculated in Figure 6a,b.The characteristic peaks are observed in both CNCs and functionalized CNCs, indicating that the structure remains intact despite oxidation caused by acid treatment.However, the I D /I G value for the CNCs increases after functionalization due to the oxidation process, which leads to the destruction of some original bonds and the introduction of new oxygencontaining bonds [45,46].This increase in the I D /I G value can be interpreted as the presence of defects in the structure.Figure 6b illustrates that the I D /I G ratio for the CeO 2 /CNCs nanocomposites maintains in the range of 1.04 to 1.12, which is close to the values reported for the materials of CeO 2 /graphene [32,47,48].In fact, the gas sensing performance depends heavily on the effect of such defects, and is closely associated with the content of the defect.
to the destruction of some original bonds and the introduction of new oxygen-containing bonds [45,46].This increase in the ID/IG value can be interpreted as the presence of defects in the structure.Figure 6b illustrates that the ID/IG ratio for the CeO2/CNCs nanocomposites maintains in the range of 1.04 to 1.12, which is close to the values reported for the materials of CeO2/graphene [32,47,48].In fact, the gas sensing performance depends heavily on the effect of such defects, and is closely associated with the content of the defect.The crucial role of oxygen vacancy defects in sensing material in promoting the adsorption of gas molecules and facilitating electron transfer by providing more active sites has long been recognized [49].In this study, the surface compositions and elemental chemical states of the CeO2/CNCs samples were analyzed using X-ray photoelectron spectroscopy (XPS).By employing X-ray radiation at an energy of 1486.7 eV, one can readily acquire the signals corresponding to the three atoms, namely Carbon (C), Oxygen (O), and The crucial role of oxygen vacancy defects in sensing material in promoting the adsorption of gas molecules and facilitating electron transfer by providing more active sites has long been recognized [49].In this study, the surface compositions and elemental chemical states of the CeO 2 /CNCs samples were analyzed using X-ray photoelectron spectroscopy (XPS).By employing X-ray radiation at an energy of 1486.7 eV, one can readily acquire the signals corresponding to the three atoms, namely Carbon (C), Oxygen (O), and Cerium (Ce).The findings present the detailed high-resolution survey spectrum (Figure 7a) alongside the X-ray photoelectron spectra of the sample depicted in Figures 7b-f and 8.The survey spectrum of the synthesized CeO2/CNCs samples displays a complex  The gas sensing mechanism portrayed in Figure 9 can be described as follows during a dynamic process.Upon exposure to air, oxygen molecules will be adsorbed onto the oxygen vacancies inherent in the sensing material.These oxygen molecules then capture unbound electrons from the conduction band or donor energy level of the composites, resulting in the formation of O2 − .This notably augments the adsorption of oxygen in the materials, thereby facilitating the movement of electrons across the interface.Consequently, an additional electron depletion layer is formed and a Schottky contact is established.In simpler terms, the consumption of electrons is tantamount to the generation of an equal number of holes, which culminates in the development of a hole accumulation layer (HAL) on the surface of material (as shown in Figure 9b).When exposed to NO2, the interaction between NO2 molecules and the surface oxygen species leads to further electron capture on the conduction band of the composites, resulting in a thicker HAL (Figure 9c).Since holes are serve as the conducting carriers in p-type semiconductors, this precipitates a sudden and significant decline in resistance.The specific formulae describing this phenomenon are outlined as follows:  The survey spectrum of the synthesized CeO 2 /CNCs samples displays a complex structure.The three peak groups correspond to the energy intervals of C, Ce, and O elements, with no peaks indicating the presence of other elements, suggesting a high level of purity in the synthesized material, see in Figure 7a.The high-resolution C 1s spectrum of CeO 2 /CNCs, as illustrated in Figure 7b, exhibits notable characteristics that are effectively delineated through a fitting utilizing six components.Predominantly featured in the spectrum is a central component positioned at a binding energy of 284.63 eV (BE = 284.63eV) displaying an asymmetrical profile.This constitution is indicative of the sp 2 graphite heterogeneous phase involving carbon-carbon double bonds (C=C) [25].Another constituent is situated at higher binding energies relative to the primary peak (BE = 285.1 eV) and displays a symmetrical profile.This particular component is distinctly associated with the C-C phase, characterized by sp 3 hybridization of the valence electron states of carbon atoms, which corresponds to the diamond heterostructure under the unique helical structure [50].The spectral analysis encompasses various chemical bonding configurations involving oxygen, hydrogen, and carbon resulting from the introduction of numerous functional groups through acidification.These constituents primarily consist of C-O (epoxy and hydroxyl), C=O (carbonyl), and O-C=O (carboxyl) bond categories [51][52][53], each exhibiting binding energies of 286.42 eV, 287.44 eV, and 288.32 eV, respectively.The energy binding peak located at 290.5 eV is assigned to π-π * shakeup satellite.The XPS spectra of Ce 3d in the CeO 2 /CNCs composites revealed six binding energy peaks (U 1 , U 2 , U 3 , U 1 * , U 2 * and U 3 * ) attributed to Ce 3d 5/2 and 3d 3/2 of Ce 4+ ions, as well as V and V * peaks corresponding to Ce 3d 5/2 and 3d 3/2 of Ce 3+ ions, seem in Figure 7c-f.To further investigate the surface oxygen states, the O 1s spectra of the samples were analyzed and divided into three peaks representing O lat (latticeoxygen), O vac (oxygenvacancies) and O ads (chemisorbed oxygen species), as shown in Figure 8a-d.O lat with the binding energy in the range of 529.2-530.4eV represents oxygen ions in the crystal lattice, which does not contribute to the gas response.O vac and O ads , with the binding energies of approximately 531.5 eV and 533.7 eV, play an important role in improving gas sensitivity [38,54,55].According to the relative area of the fitted peaks, the ratios of cerium and oxygen are listed in Table S1.It is observed that at lower reaction temperatures, the dominant species are Ce 3+ ions and lattice oxygen.However, as the synthesis temperature is increased from 100 to 140 • C, the concentration of Ce 3+ decreases from 47.98 to 19.38%, while the concentration of O lat decreases from 84.6 to 53.02%.This decrease in Ce 3+ ions is attributed to their transformation into Ce 4+ under high heat, while the lattice oxygen evolves into oxygen vacancy defects and chemisorbed oxygen.Among these samples, sample 3 synthesized at 140 • C exhibits the highest concentration of O vac at 35.6%.The concentration of O vac slightly decreases to 34.69% when the temperature is further increased to 160 • C (sample 4).While tested at 50 ppm, shows a decrease in response degree of 56.4%, which is smaller compared to sample 3 (65.3%)due to electron transfer barriers during the recrystallization of CeO 2 .These results highlight the significant role of oxygen vacancies and chemisorbed oxygen in gas sensing.Moreover, the content of these components can be adjusted by manipulating the reaction temperature, which in turn enhances the sensing performance.
The gas sensing mechanism portrayed in Figure 9 can be described as follows during a dynamic process.Upon exposure to air, oxygen molecules will be adsorbed onto the oxygen vacancies inherent in the sensing material.These oxygen molecules then capture unbound electrons from the conduction band or donor energy level of the composites, resulting in the formation of O 2 − .This notably augments the adsorption of oxygen in the materials, thereby facilitating the movement of electrons across the interface.Consequently, an additional electron depletion layer is formed and a Schottky contact is established.In simpler terms, the consumption of electrons is tantamount to the generation of an equal number of holes, which culminates in the development of a hole accumulation layer (HAL) on the surface of material (as shown in Figure 9b).When exposed to NO 2 , the interaction between NO 2 molecules and the surface oxygen species leads to further electron capture on the conduction band of the composites, resulting in a thicker HAL (Figure 9c).Since holes are serve as the conducting carriers in p-type semiconductors, this precipitates a sudden and significant decline in resistance.The specific formulae describing this phenomenon are outlined as follows: O 2(gas) → O 2(ads) (1) NO 2(gas) + e − → NO − In addition, cerium oxide exhibits the ability to release lattice oxygen under hypoxic conditions.This process leads to the transformation of Ce 4+ ions into Ce 3+ and the formation of extra oxygen vacancy defects [56,57].Consequently, the concentration of oxygen vacancy defects in the composites is increased, thereby promoting the adsorption and ionization of oxygen on the material.Hence, it is evident that the combined influence of Ce 4+ ions and oxygen vacancy defects greatly enhance the capacity of material to adsorb NO 2 gas, thereby improving its gas-sensitive performance.In addition, cerium oxide exhibits the ability to release lattice oxygen under hypoxic conditions.This process leads to the transformation of Ce 4+ ions into Ce 3+ and the formation of extra oxygen vacancy defects [56,57].Consequently, the concentration of oxygen vacancy defects in the composites is increased, thereby promoting the adsorption and ionization of oxygen on the material.Hence, it is evident that the combined influence of Ce 4+ ions and oxygen vacancy defects greatly enhance the capacity of material to adsorb NO2 gas, thereby improving its gas-sensitive performance.

Conclusions
In this word, CeO2/CNCs nanocomposites were successfully synthesized using a microwave-solvothermal method, in which uniformly assembled CeO2 nanoparticles on the surface of CNCs.The microstructures and compositions of the resulting composites were analysed using various characterization techniques and gas-sensing tests.The nanocomposite doped with the mass of Ce precursors (0.7) demonstrated the highest sensitivity,

Conclusions
In this word, CeO 2 /CNCs nanocomposites were successfully synthesized using a microwave-solvothermal method, in which uniformly assembled CeO 2 nanoparticles on the surface of CNCs.The microstructures and compositions of the resulting composites were analysed using various characterization techniques and gas-sensing tests.The nanocomposite doped with the mass of Ce precursors (0.7) demonstrated the highest sensitivity, with a response of 65.4 to 60 ppm NO 2 at room temperature.This sensitivity was nearly 9 times greater than that observed with bare CNCs.Furthermore, the gas sensing properties of the composites could be indirectly modulated by adjusting the appropriate reaction temperature to increase the oxygen vacancy defects.Overall, the microwave-solvothermal method offers a simple approach for designing and synthesizing nanocomposites with unique physicochemical properties, making them suitable for smart applications.

Figure 1 .
Figure 1.Schematic representation of (a) the synthesis process of CeO 2 /CNCs nanocomposites; (b) fabrication of gas sensor.

17 Figure 3 .
Figure 3. SEM images of (a) CeO2 nanoparticles (b,c) CNCs and (d-f) CeO2/CNCs nanocomposite.Insets in a, c and d show the views of the CeO2 nanoparticles, CNCs and CeO2/CNCs nanocomposite at higher magnification.

Figure 3 .
Figure 3. SEM images of (a) CeO 2 nanoparticles (b,c) CNCs and (d-f) CeO 2 /CNCs nanocomposite.Insets in a, c and d show the views of the CeO 2 nanoparticles, CNCs and CeO 2 /CNCs nanocomposite at higher magnification.

Figure 4 .
Figure 4. (a,b) The responses of CeO2/CNC nanocomposite to different concentrations of NO2.(c) Dynamic response-recovery curves of CeO2/CNC nanocomposite with different mass ratios of CeO2 in the presence of 50 ppm NO2.Insets in c show the dynamic response-recovery curves of pure CNCs.(d) Dynamic response-recovery curves of CNCs and CeO2/CNCs upon exposure to NO2 with a concentration range of 10-80 ppm.

Figure 4 .
Figure 4. (a,b) The responses of CeO 2 /CNC nanocomposite to different concentrations of NO 2 .(c) Dynamic response-recovery curves of CeO 2 /CNC nanocomposite with different mass ratios of CeO 2 in the presence of 50 ppm NO 2 .Insets in c show the dynamic response-recovery curves of pure CNCs.(d) Dynamic response-recovery curves of CNCs and CeO 2 /CNCs upon exposure to NO 2 with a concentration range of 10-80 ppm.

Figure 5 .
Figure 5.The responses of CeO2/CNC composites synthesized at various reaction temperatures for (a,b) 10-80 ppm and (c) 50 ppm NO2.(d) Repeatability of CeO2/CNCs on cycling exposure to NO2 in the presence of 50 ppm NO2.(e) Responses of CeO2/CNCs to 30 ppm NO2 with various humidities.(f) Responses of CeO2/CNCs to different target gases at room temperature.

Figure 5 .
Figure 5.The responses of CeO 2 /CNC composites synthesized at various reaction temperatures for (a,b) 10-80 ppm and (c) 50 ppm NO 2 .(d) Repeatability of CeO 2 /CNCs on cycling exposure to NO 2 in the presence of 50 ppm NO 2 .(e) Responses of CeO 2 /CNCs to 30 ppm NO 2 with various humidities.(f) Responses of CeO 2 /CNCs to different target gases at room temperature.

Nanomaterials 2024 ,
14, x FOR PEER REVIEW 11 of 17 spectroscopy (XPS).By employing X-ray radiation at an energy of 1486.7 eV, one can readily acquire the signals corresponding to the three atoms, namely Carbon (C), Oxygen (O), and Cerium (Ce).The findings present the detailed high-resolution survey spectrum (Figure 7a) alongside the X-ray photoelectron spectra of the sample depicted in Figure 7b-f, and Figure 8.

Figure 9 .
Figure 9. (a) Schematic for the gas-sensing mechanism; The adsorption sensing mechanism on the surface of CeO2/CNCs composites for (b) O2 and (c) NO2.

Figure 9 .
Figure 9. (a) Schematic for the gas-sensing mechanism; The adsorption sensing mechanism on the surface of CeO 2 /CNCs composites for (b) O 2 and (c) NO 2 .

Table 1 .
The physical properties of as-prepared samples.

Table 1 .
The physical properties of as -prepared samples.

Table 2 .
Comparison of our prepared NO 2 sensor with previously reported other NO 2 sensor.