Mesoporous Effects on SiC Fiber Matrix Combined with Active Metal Oxides for Atmospheric Gas Sensing under High Temperature Conditions

CO 2 and O 2 gas sensing tests were conducted on mesoporous WO 3 (mWO 3 ) and β −SiC/In 2 O 3 /SnO 2 /mWO 3 (SCISOW) quaternary structured gas sensor materials at room temperature and under various high-temperature conditions. The recorded results indicated the incorporation of mWO 3 effectively improved gas detection responses compared to the results of our previous studies. The electrochemical properties and surface morphology of the nanocomposites were analyzed by XRD, SEM, TEM, EDX, XPS, Raman, EIS, PL, Tafel and LSV methods. All quaternary-structured nanocomposites showed a good response to gas at different temperature variations. Therefore, the excellent electronic conductivity and topography of the nanocomposite increased its gas-sensing ability and changed the usability of the β −SiC/In 2 O 3 /SnO 2 /mWO 3 nanocomposite. Gas sensing tests indicated that SCISOW103 nanocomposites showed high resistivity with excellent gas sensing response to CO 2 , compared to other quaternary nanocomposites and mWO 3 . Therefore, the electrochemical and technical parameters that change its value are directly related to the electrical conductivity of the sensor material, and the conductivity/resistivity of the sensor material plays a critical role in adjusting the sensitivity of a gas sensor. We believe that the quaternary SCISOW nanocomposites we propose offer new advanced materials for gas sensing tests and open up new opportunities in the �eld of research.


Introduction
Detecting hazardous gases is a mandatory eld of study to solve environmental problems and mitigate global warming.Among the studied gas sensing materials, nanostructured sensing materials exhibit an extensive response in gas-purged environments [1,2].In particular, metal oxide semiconductors containing gas sensor materials have attracted the attention of gas sensing manufacturers [3][4][5].In addition, the most promising gas sensor material is formed by combining mesoporous metal oxide with a 2D-structured nanomaterial/other metal oxide semiconductor (MOS).The CO 2 gas detection and search for pure, doped and composite materials have been extensively reported in the literature.One of the best MOS gas sensors is tungsten oxide (WO 3 ), which is characterized by its low price, the wide bandgap energy and different morphological structure with polymorphs such as mesoporous, powdery, monoclinic, tetragonal, and orthorhombic [6].In this study, we prepared a mesoporous WO 3 using a mesoporous silica solid template.This method offers a large surface area and a wellarranged porous structure on the WO 3 .The most common mesoporous silica materials include small-pore hexagonal MCM-41, cubic MCM-48, cubic KIT-6, large-pore hexagonal SBA-15 and cubic SBA-16, as well as HMS, MSU materials, and some other silica templates [7][8][9].Several mesoporous structured gas sensor materials have been reported, such as mesoporous NiO for HCHO gas sensing tests at 300°C [10], mesoporous Ag 2 O-SnO 2 for H2S gas sensing tests at 50°C [11] and mesoporous In 2 O 3 for NH 3 gas sensing tests at room temperature [12], with quite high gas detection limits.In our previous study, we synthesized a SnO 2 -loaded β-SiC-In 2 O 3 ternary nanomaterial for CO 2 using the O 2 gas sensing test [13].Gas sensing tests were performed at room temperature and at various temperatures.All gas sensors showed signi cant responses; among them, ternary structured nanocomposites signi cantly improved gas sensing responses compared to binary [14], pure MOSs and β-SiC bers.High-temperature conditions for the detection of toxic gases are mandatory tests; however, hightemperature sensing above 600°C has not been reported.In this study, we synthesized mWO3 using a solid-state silica template and combination with a β-SiC − In 2 O 3 -SnO 2 ternary nanocomposite.The nal quaternary gas sensing material was used for CO 2 and O 2 gas sensing tests under different temperature variations.The crystalline structure and morphological properties of the quaternary nanocomposites were analyzed and all results showed that mWO 3 was successfully fabricated and anchored to form a quaternary nanocomposite.In addition, sensing responses were signi cantly improved compared to those of ternary nanocomposites.

Fabrication of KIT-6 silica template
The solid-state KIT-6 silica template was prepared using the following method: Highly puri ed DI water (217 ml) and 37% HCl (6.87 ml) were mixed with continuous stirring.Then, 6 g of Pluronic P123 was dissolved at 35°C for 5 h.7.48 ml of isobutyl alcohol was added to the mixture and stirred for more than 1 h at the same temperature.
Then, 12.9 g of tetraethyl orthosilicate (TEOS) was added and the mixture was kept at 35°C for 1 d to form a geltype mixture.The gel mixture was transferred to a 150 ml Te on autoclave, heated to 100°C, and held for 1 d.The drying process was then performed to remove residual liquid, followed by calcination for 6h at 550°C to construct a solid-state silica template KIT-6.

Fabrication of mWO 3
The mesoporous WO 3 was prepared using the KIT-6 template by a one-casting method.1.20 g of PW12 (12phosphotungstic acid) was dissolved in 5 ml of pure ethanol with 0.4 g of KIT-6 template.The mixture was heated to 40°C to evaporate the ethanol.The mixture was then dried at 80°C and calcined at 500°C for 2 h.The calcined powder was added to the HF solution to remove the solid-state template and then washed with DI water to neutralize the pH.After adjusting the pH to 7, the resulting product was dried at 30°C and renamed mWO 3 .

Synthesis of quaternary
A simple stirring process and a calcination method were used to fabricate the quaternary nanocomposites.The preparation method of β − SiC/In 2 O 3 was described in a previous study.The mole ratio of In 2 O 3 was controlled at 0.1, 0.3, and 0.5 M. 0.4 g of β − SiC/In 2 O 3 was dissolved in 30 ml of ethanol, 0.2 g of SnO 2 was dissolved in 15 ml of ethanol and 0.4 g of mWO 3 was dissolved in 15 ml of ethanol.The mixture was stirred for 1 h and then continuously mixed.The mixture was kept for 3h at normal room temperature.The solvent was evaporated at 40°C.The obtained product was dried at 80°C and then calcined.The temperature was set to 500°C and held for 2 h.The samples were named SCISOW101, SCISOW103 and SCISOW105, respectively.

Fabrication and testing of gas sensors
The gas sensor electrode was made using the Doctor Blade method.The electrode was installed in an alumina tube chamber equipped with gas-in and gas-out channels on both sides.The gas-ow rate was controlled with a mass-ow controller at 6 kg/cm 2 for 60 min.The CV curve was measured when the gas purged to the reactor using a PGP201 potentiostat (A41A009).The units of resistance and resistivity (speci c resistance) of the gas sensor electrode were calculated using Eq.[1]: where R = resistance, ρ = resistivity, L = length (cm), A = cross-sectional area (cm 2 ).
The characterization method and detailed information on the gas sensing reactor are described in the Supplementary le.

XRD and electrochemical analysis
Crystal structures of SCISOW nanocomposites were analyzed by XRD. Figure 1 shows the XRD patterns of SCISOW NCs, pure mWO 3 and reference data.The main characteristic peaks of β-SiC, In 2 O 3 , and SnO 2 were obtained, indicating that mWO 3 anchoring and synthesis did not change the crystallinity.The SCISOW NCs' crystallinity was high, as indicated by their sharp XRD peaks.Crystal facet and crystallinity affected the gas sensing response of the material.The detected gas interacts with the surface of the sensor materials, and the crystallinity refers to the arrangement of atoms and molecules, moreover, the sensitivity is based on the change in conductivity and resistance, which is done through the role of electron mobility, and the reason that the crystallinity and gas sensing has a relationship.Figure 1b shows XRD patterns of the KIT-t silica solid-state template and mWO 3 .The KIT-6 template had a cubic la3d structure and exhibited diffraction peaks at 10-30 2theta degrees.The mWO 3 diffraction peak indicates that MO was successfully fabricated using a silica template with an ordered mesostructured state [6].

Morphology pro le, EDX and Raman spectrum, XPS and electrochemical analysis
The surface morphology of the nanocomposites was analyzed by SEM and TEM, and the elemental compositions were well-analyzed by EDX to investigate impurities.Pure mWO 3 was spherical in shape as observed in both SEM and TEM images ( Figs. 2a,c and 3a, b).The state of the β-SiC ber was visualized by SEM analysis.The even and shiny surface of the β-SiC ber offers an abundant area for MO to be located.As shown in Fig. 2 (b-h), the MOS was well coated on the β − SiC ber and the surface of the quaternary nanocomposites was smooth without much agglomeration.The same surface morphology pro le was obtained in the TEM analysis of the quaternary nanocomposites, as shown in Figs. 3 (d-k).The bit plot of the atomic weights of the elements is shown in Fig. 3.
The main characteristic elements were well detected by EDX analysis without impurities.No Si was obtained on pure mWO 3 , con rming that the silica solid-state template was completely removed by HF treatment and that mWO 3 was well formed with no impurity elements that could be derived from the base template.Symmetric motion, bonding states and interaction of the elements were measured by Raman spectroscopy and the full spectrum is displayed in Fig. 4. Pure mWO 3 had Raman peaks in the Raman shift regions of 250-1000 cm − 1 .The peaks were located at 241.67, 771.54 and 942.12 cm − 1 [6].The sharpness and shape of the peak changed after combining the β-SiC ber, SnO 2 and In 2 O 3 .The characteristic peaks of β-SiC bers obtained in the shift regions of 1300-1600 cm − 1 were related to sp 2 -hybridized carbon [14,15].In addition, the peaks decreased as the loading amount of MOS increased.Changes in the peak intensity and shifting are mainly due to the inhibitory and dominant bonds that are formed at a speci c frequency.Moreover, the intensity of the In 2 O 3 peak at 1075.23 cm − 1 increased with increasing In 2 O 3 loading.
The elemental composition and binding energy state of SCISOW103 were analyzed by XPS and the data are summarized in Fig. 5. Figure 5a shows the complete XPS spectra of mWO 3 and SCISOW103 NCs.The S1p spectrum showed ve peaks in certain binding energy regions (as shown in Fig. 5b).The Si-C peak ratio was lower than that of other oxygen-containing Si-bonds, indicating that the MOs were effectively introduced into the β-SiC ber. Figure 5c shows the C1s XPS spectrum which includes ve peaks, the rst two peaks being related to C-Si and C-Sn.The other peaks are related to oxygen-containing functional groups.The XPS spectrum of In3d is displayed in Fig. 5d with two peaks assigned to the In3d 3/2 and In3d 5/2 .Sn2d of SnO 2 is shown in Fig. 5e, which includes Sn 3d 3/2 and Sn 3d 5/2 of the SnO 2 MO.All XPS peaks were in good agreement with our previously reported results [16].Figure 5f shows the XPS spectrum of W4f, which has two characteristic peaks: W4f 7/2 and W4f 5/2 [17,18].The W4f comparative study is shown in Fig. 5g, and this data was used to evaluate how mWO 3 loading or cross-linking with other MOSs could change the binding energy.The peak intensity changed drastically and the peak location shifted slightly.The W4f spectrum of mWO 3 is shown in Figure S1.The last XPS spectrum is O1s of SCISOW103 NC which contains six peaks in certain binding energy regions (as presented in Fig. 5h).In conclusion, the results of the XPS analysis indicated that the various structured MOs were well linked with the β-SiC bers forming a successful interfacial connection, thus establishing a stable charge-transfer channel.
The conductivity and charge-transfer resistance of the nanocomposite are factors that affect the activity of the material.Electrochemical impedance spectroscopy (EIS), photoluminescence (PL), Tafel and Linear sweep voltammetry (LSV) analyses were conducted on all nanocomposites.The lifetime of the photoinduced pairs and mobility were analyzed by PL and the spectrum is shown in Fig. 6a.Pure mWO 3 had the lowest PL intensity, indicating a long lifetime of highly mobile pairs.Among the quaternary nanocomposites, SCISOW101 NC was characterized by high PL intensity, which was the reason for the low mobility morphological pro le.PL intensities of SCISOW103 and SCISOW105 were suppressed compared to those of SCISOW101.EIS test results correlated well with PL intensity (as shown in Fig. 6b).The SCISOW101semicircural pro le was large, with low conductivity and high resistance between the electrode and the electrolyte.A small semicircle was observed in the following sequence: mWO3 < SCISOW103 < SCISOW105 NCs.The electric circuit was inserted into the EIS. Figure 6c shows the Tafel and LSV curves.The charge transfer resistance and corrosion rate were closely related to the alternating current of the working electrodes obtained from the Tafel plot.Pure mWO 3 has an electropositive corrosion potential, which indicates that a lower corrosion rate occurs on the electrode surface in the electrolyte [6].
As previously analyzed, SCISOW101 NCs had a high semicircular curve with high PL intensity, and the SCISOW101 electrode was more electronegative with a high corrosion rate.The other two SCISOW nanocomposites exhibited high corrosion resistance.In addition, the electrode surface was less damaged and oxidized in the corrosive electrolyte.The kinetics of the electron transfer reaction at the electrode was studied using the LSV curve, and the measured LSV curve was inserted in Fig. 6c.The summary of the analysis indicated that the composition of the nanocomposite and the loading of the mesoporous MOS effectively in uenced the electrochemical activity and properties of the formed nanocomposites [6, 14].

Gas sensing test
The gas sensing properties of the nanocomposites were tested at room and high-temperature conditions with CO 2 and O 2 gas purging.The gas sensor nanocomposite anchored in Ni foil, which has a porous structure, can support the electron transfer path and provide an abundant area where the gas sensor material can be in close contact.
The gas sensing capability was observed based on the difference in recorded current density units without gas and under gas purging conditions.Figure 7 shows the gas sensing test at room temperature.The CV curve was recorded without a gas-purging environment to assess whether the gas sensing material could detect the gas.
Pure mWO 3 exhibited a signi cant response to CO 2 gas compared to O 2 gas ( Fig. 7a).The SCISOW101 nanocomposite showed a very poor response to gases, preventing the gases from reacting with the surface of the sensor materials, and the electrochemical properties of SCISOW101 were very poor ( Fig. 7b).This result was con rmed by the SCISOW103 and SCISOW105 NCs gas sensing tests.Figures 7c and d show the recorded CV curves of SCISOW103 and SCISOW105 NCs at room temperature (25°C).
Figure 8 shows the CV curves of the pure mWO 3 gas sensor material under different high-temperature conditions with and without gas purging.Changes in current density (mA/cm 2 ) were not large when the temperature was increased from 100°C to 600°C.First, the current density of mWO 3 at 100°C was 4.70×10 − 3 mA/cm 2 , which changed to 4.66×10 − 3 mA/cm 2 at 600°C.Under the conditions of CO 2 and O 2 gas purging, there was a noticeable change occurred in the current density in different temperature ranges; in particular, pure mesoporous WO 3 responded better to O 2 gas.Figures 8-11 show the gas sensing tests of the SCISOW101, SCISOW103, and SCISOW105 NCs.SCISOW101 exhibited a poor response to any gas, similar to room-temperature experiments.
The SCISOW103 NC exhibited the lowest current density in each test, with signi cant differences between the gas purge and no purge conditions.In particular, the SCISOW103 gas sensing material had a better response to CO 2 gas than to O 2 gas, indicating that the surface reactivity between the target CO 2 gas and the catalyst surface was good and tended to receive electrons.The recorded CV curves of SCISOW103 in gas-free and gas-purging environments are shown in Figs. 10 (a-c).The last gas sensor material used was SCISOW105.Based on the aforementioned electrochemical experiments, the properties of SCISOW105 are almost identical to those of SCISOW103, suggesting that the material may have high gas sensing capabilities.The current density of the SCISOW105 gas sensor material under non-gas purge conditions is shown in Fig. 11a.Figures 11b and c show the CV curves of the SCISOW105 sensor material under the CO 2 and O 2 gas purging conditions.The de nition of the gas sensing mechanism is a mandatory part that needs to be explained.Carbon dioxide gas is one of the most stable molecules because it consists of a single carbon atom covalently bonded to two oxygen atoms, and this bond is very strong.O 2 is an oxygen-dominant gas, which has a strong tendency to receive electrons from the electrode surface and, with the increase in resistivity, creates an electron barrier [19,20].During the gas sensing test, a change in current density can be evidence of the surface reaction between the gas molecules and the gas sensor electrode.Furthermore, the electron density of the sensor layer and the effective charge transfer and separation properties of the β − SiC − In 2 O 3 -SnO 2 nanocomposites provided better sensing ability; it also improved the usability of the β-SiC ber by loading of MOs [21].
The resistance (k × Ohm) and resistivity (speci c resistance) (k × Ohm × cm) of the gas sensor electrode were evaluated using Eqs.[1]; the recorded current density units (mA/cm 2 ) were used for the calculation.The calculation results are summarized in Tables 1-5; it was observed that as the current density of the gas sensor electrodes decreased, the resistance and resistivity increased.The resistance (k*Ohm*cm 2 ) of the actual electrode surface in contact with the gas was evaluated and is listed in Table S1-S5.The highest resistivity was observed for the SCISOW103 gas sensor electrode.As previously mentioned, the electrochemical results and gas sensing tests were well matched and strongly suggest that the combination of mesoporous metal oxide with the ternary nanocomposite improved the gas sensing capability of the material.However, a heavy load of In 2 O 3 metal oxides has the opposite effect on the charge carrier and reduces the performance of the gas sensor electrode.Our research group focused on the fabrication of quaternary materials for gas sensors.In our previous studies, we studied gas sensing tests on binary and ternary structured nanomaterials, and it has been shown that changing the structure is effective in improving the gas sensor material performance and offers new materials in research elds [13,14].
The electrochemical properties and bandgap of any material determine the energy required to transfer electrons from the top level of the valence band (which is su ciently immobile to allow free conduction) to the ground level of the conduction band.As the name suggests, electrons are freely transferred).Its electrical properties (or special properties) must be different in the presence of gas than in the absence of gas.Therefore, the electrochemical and technical parameters that change its value are directly related to the electrical conductivity of the sensor material.
The conductivity/resistivity of the sensor material plays a key role in adjusting the sensitivity of the gas sensor.In addition, two-dimensional materials are more selective and sensitive to gas molecules than pure two-dimensional materials.The sensitivity and selectivity of gas sensors based on two-dimensional semiconductor materials depend on changes in their electronic properties.Therefore, the electrochemical and technical parameters affecting its value are directly related to the electrical conductivity of the sensor material.By controlling the electrical conductivity of semiconductor materials, desired electrical properties can be achieved.By changing the composition of the two-dimensional material, the conductivity of the resulting material can be changed, since the connection of atoms of the original sensing material with atoms of a doping material with different properties creates a band gap with different values.In addition, we found that the loading amount of the metal oxides strongly affects the crystallinity, chemical structure, morphology and surface reactions between the gas and the sensor materials.We believe that our results will contribute to the development of new materials for gas sensing tests.

Conclusion
In this study, we prepared a mesoporous WO 3 using a KIT-6 silica template, which was used to fabricate the quaternary nanocomposites.mWO 3 , In 2 O 3 and SnO 2 metal oxides were successfully combined with 2D β-SiC bers.The surface area of the β-SiC ber provided potential conditions for receiving all metal oxides without agglomeration, thus ensuring no surface reactions between the gas and the sensor material and electrons.XRD, SEM, TEM, EDX, XPS and Raman analyses were performed to evaluate the crystallinity and chemical bonding states of quaternary nanocomposites, and electrochemical properties were analyzed using EIS, PL, Tafel and LSV methods.Gas sensing responses of mWO 3 and quaternary SCISOW nanocomposites were measured at room temperature and various high temperatures in CO 2 and O 2 gas purging environments.The gas sensing response of the newly modeled quaternary nanocomposites was found to be improved compared to our previous studies.Environmental conditions such as temperature uctuation and gas ow rate affect gas sensing tests.In addition, the chemical structure, effective charge carrier and electron donor/acceptor phases of the gases were considered as potential factors in the gas sensing tests.Our proposed the quaternary SCISOW nanocomposites we proposed showed a strong gas sensing response to both CO 2 and O 2 gases under all temperature variations, and it was also con rmed that loading of metal oxides into the 2D β-SiC bers improved the surface reaction between the gas and the sensor material with effective charge carriers.

Declarations
Acknowledgement Tables

Figures Figure 1 XRD
Figures

Figure 6 a
Figure 6

Figure 7 Gas
Figure 7

Figure 9 Gas
Figure 9

Figure 10 Gas
Figure 10

Table 1 .
The resistance (Ω) value of each electrode from CV graph without gas-purging conditions with Ni foil current collector.

Table 2 .
The resistance (Ω) value of each electrode from CV graph under CO 2 and O 2 gas-purging conditions at normal room temperature (25°C).

Table 3 .
The resistance (Ω) value of each electrode from CV graph electrode under different high temperatures with without gas purging.

Table 4 .
The resistance (Ω) value of each electrode from CV graph electrode under different high temperatures with CO 2 gas purging.

Table 5 .
The resistance (Ω) value of each electrode from CV graph electrode under different high temperatures with O 2 gas purging.