3.1. XRD pattern and EIS test
The crystal structures and XRD patterns of the nanocomposites are shown in Figure 1a. The black diffraction peak indicated the XRD pattern of In2O3, which well-matched with the JCPDS No. 0.6-1416 card. The sharp peak and FWHM decrease indicated that the as-prepared In2O3 nanocomposite had a decent crystal structure. The broad diffraction peak at 35.51° 2-θ degrees indicated the (111) crystal plane of the β–SiC fiber (JCPDS No. 29-1129). In the XRD pattern of SiC-In2O3, the peak of the In2O3 was significantly changed to low-sharpness, indicating that MO could cover the surface of β–SiC-fibers with successful interactions between each nanocomposite. Figure 1b shows the electrochemical resistance spectra of SiC-In2O3 binary nanocomposites containing different amounts of In2O3.
The resistance and conductivity of the samples are mainly related to their semicircle profile in the EIS test. From the above results, it was found that the 0.1 M In2O3 loaded nanocomposite had a small semicircle with low resistance properties. It is effective for the electrochemical coefficient of the SiC-In2O3 nanocomposite during the test. The SiC-fiber had good electrochemical conductivity. The effectiveness of the In2O3 to β–SiC was lacking, and In2O3 did not reduce or change the fundamental properties of the β–SiC fiber. The EIS profiles of the samples were remarkably changed when the loading amount of In2O3 was increased from 0.1 M to 0.5 M, which might be due to the interconnection between the SiC fiber and MO.
3.2. EDX and SEM analysis
The surface state, morphology, and atomic amount of each element in the nanocomposite were analyzed by EDX and SEM. From the EDX results, the main elements were found in high amounts as shown in Figure 2. The N-element was found in the EDX result of In2O3. This might be derived from the precursor material used for In2O3 synthesis. After combining In2O3 with β–SiC fiber followed by calcination, there was no amount of N-element in the result. Other elements were not found either, confirming that the final sample was successfully synthesized without impurities using a simple hydrothermal method.
The surface morphology and profile of the as-prepared nanomaterials were analyzed by SEM. Pure β–SiC fibers had a smooth and clear surface with all nanoparticles regularly agglomerated as shown in Figure 3 (a, b). The thickness of the nanoparticles was the same, with a width of 11.3 nm and non-similar length. The smooth surface of the SiC fiber supports the location of In2O3 on the surface, which can prevent MO from irregularly spreading on the surface. Pure In2O3 had irregularly shaped primary nanoparticles such as cubes and spheres, with all primary nanoparticles were agglomerated. The size of the cube-shaped nanoparticles was approximately 3.23 µm. The surface was smooth and thicker than sphere-shaped (in Figures 3c, d). The surface images of the different amount of metal oxide loaded SiC-In2O3 nanocomposite were displayed in Figure 3 (e-j). The metal oxide agglomeration on the SiC fibers were non-similar due to the loading amounts. Especially, the agglomeration of the metal oxide was becoming strong in 1:0.5 M SiC-In2O3 (in Figure 3i, j).
In SEM images of SiC-In2O3, sphere-shaped In2O3 was mainly observed on smoother β–SiC fibers. All nanoparticles were agglomerated, resulted in a porous structure profile with different sizes. Moreover, pure In2O3 well-dispersed onto the surface, which can effectively increase the gas sensing performance.
3.3. Molecular dimensional analyses by Raman spectra and TEM
The symmetric motion, chemical bonding, and interaction of nanomaterials were analyzed by Raman spectroscopy. Vibrational frequencies are specific to the symmetric motion of molecules and chemical bonds in the final nanocomposite [22, 23]. Full Raman spectra are displayed in Figure 4. Raman peaks of pure MO were observed below 1200 cm−1 Raman shift regions. A total of six Raman peaks appeared in the In2O3 nanocomposite. The first four peaks were related to E1g, E2g, and A1g-Raman active-modes. Indium hydroxide Raman peaks appeared in the 720.8 cm−1 and 1053.2 cm−1 frequency regions. These peaks might be due to the conversion of small amounts of In ions to In(OH)3 during the synthesis process . The pure β–SiC had two-sharp Raman peaks at 1332.2 cm−1 and 1596.6 cm−1 regions related to sp2-hybridized carbon and the optical branch of the second-order Raman spectra, respectively. One-broad peak was obtained at 2747.4 cm−1 Raman shift region. It was classified to 2D symmetric mode which can be obtained from the overtone motion of TO-phonons due to activation by double resonance scattering. After combining the SiC-fiber with MO, no MO-peak appeared in Raman result, although the peak intensity was increased. Such increases in the Raman intensity are related to a particular mode of vibration that appears in a specific bond to allow a specific Raman active mode. in otherwise, it is related to the expression of supressing and dominating bonds which are formed at the specific frequency energy.
The size and shape of the nanomaterial were analyzed by TEM. Obtained images are shown in Figure 5. In Figure 5 (a, b),rod-shaped with short length and long-length β–SiC-fibers were obtained; the long-length SiC-fibers were dominant, which is more favourable for MO and allows uniform distribution on the surface. TEM images of In2O3 revealed that particles were agglomerated and stacked and created the spherical and grain-shaped particles. The agglomerated particles had cleavage steps, indicating a nonsmoothed surface (Figures 5c, d). In SiC-In2O3, the most MO wrapped the β–SiC-fiber surface and there was less agglomeration on the surface. MO nanoparticles were obtained as light black-grey coloured images. β–SiC-fibers were obtained as dark black colored rod-shaped images and the widths of the fibers were identical. The lower agglomeration of MO on β–SiC fibers can have favourable electron-transfer action during an electrochemical test. To summarize, the simple hydrothermal method could be used to synthesize well-spread In2O3 on the surface of β–SiC fibers. This provides favourable conditions for efficient electronic conduction and good electrochemical operation.
3.4. Chemical bonding and XPS analysis
The element-composition and surface state of the nanomaterials were examined by the XPS. Full XPS survey spectra (Figure 6a) showed the coetaneousness of Si, C, In, and O in SiC-In2O3. The Si2p XPS spectra indicate that three electronic structures and chemical bonding states, Si-O, Si-O2, and Si-C are on the surface of the SiC fiber (Figure 6b). The binding energies of Si-O2 and Si-O obtained in the 101.28 and 100.14 eV regions, respectively, had higher intensities than the Si-C XPS peak. The appearance of Si-O was higher than Si-C bonds, indicating that Si-O form of Si existed on the surface of the SiC fiber. Additionally, the O-element appearance can be derived from In2O3 which is located on the surface of the SiC-fiber . The binding energy of Si-C bonding is located at 99.05 eV regions. Figure 6c shows the C1s XPS spectrum of the β–SiC fiber as-well-as the possible deconvolution spectra of the C-element. A total of four different bonding appearances were obtained in the C1s spectra: C-O, C-C, C=C, and C-Si in the surface. The C-C and C=C bonding derived from the SiC and the peak intensities of C-C and C=C bonding were higher than that of C-O bonding. The C-Si bonding energy was obtained at 281.72 eV region, indicating the metal appearance of the SiC-fiber furnace . The peak intensity of C-O was quite similar to that of C-Si. Both oxygen and silica were derived from MO, showing successful interaction between MO and SiC-fibers. The XPS spectrum of In3d (in Figure 6d) was deconvoluted into two peaks: In3d 3/2 at 450.81 eV and In3d 5/2 at 443.34 eV . Figure 6e shows O1s XPS spectrum. The spectrum had four different chemical bonding states . In the spectrum, Si-O, O-Me, C-O-C, and O-C bonding at 532.68, 530.79, 528.59 and 528.92 eV binding energy regions were observed. Si-O and O-Me bonds are attributed to the contribution of MO. XPS results confirmed that each element had interconnection and that In2O3 metal oxides successfully junctioned with SiC fibers and chemical bonds on the surface.
3.5. Gas sensing performances
The gas sensing performance of xIn2O3 loaded with β–SiC-fibers was tested without gas and with (O2 or CO2) gas purging. The CV profile was examined using a PGP201 potentiostat (A41A009). The test was recorded under a (-500 V) to (1000 V) potential range with a (1 A) to (-1 A) current. Cyclic voltammetry is one of the most commonly used electrochemical analysis techniques. The gas sensor material was used with three-different current collectors: Cu foil, FTO glass, and Ni foil. Figure 7 (a, b, c) shows the results of the CV test of as-prepared electrodes without gas purging. Among them, the Ni-foil current collector drastically supported the electron transfer and electrochemical performance of the SiC-In2O3 nanocomposite coated sensing material. In Figure 7a, the high current density was 2.66×10−2 mA/cm2 on 1:0.5 M SiC-In2O3 coated FTO sensor. The current density value was significantly reduced on 1:0.3 M SiC-In2O3 and 1:0.1 M SiC-In2O3 nanocomposite was coated with sensor material. The current value of pure β–SiC fibers was approximately 3×10−6 mA/cm2, which might be due to the fact that pure β–SiC fiber material had low reaction activity and low electrochemical performance at room temperature. The results of CV tests confirmed that the combination of MO and β–SiC-fibers drastically improved the electron transfer and electrochemical performance of the final nanocomposite material. In Figure 7b, the CV graph of the Ni-foil coated sensor shows a high current density value. The Ni foil current collector has more favourable compatibility with the SiC-In2O3 nanocomposite. It drastically supported the electrochemical performance of the sensor material. A higher current value of 6×10−2 mA/cm2 was found for the 1:0.5 M SiC-In2O3 sensor material (in Figure 7b). The current value changeability between each electrode was not high, indicating that Ni-foil current collector had more stable properties on our synthesized SiC-In2O3 binary nanocomposite. The current density value of sensor material coated Cu foil was quite higher than of coated FTO glass as shown in Figure 7c. However, the CV graph had a zig-zag profile. This indicated that the sensor material was not properly coated on the Cu-foil surface, leading to an irregular interconnection. These CV results indicated that sensor materials based on different electrical collectors coated with 1:0.5 M SiC-In2O3 had good electroconductivity and good electrochemical ability, suggesting that these materials might have high gas sensitivity.
Figure 8 shows the results of the cyclic voltammetry test of the as-prepared gas sensor material under CO2 and O2 purging conditions at room temperature (25°C). The gas sensor material was coated on FTO glass and Ni foil current collectors. Figures 8a and 8b display the gas sensing performance of the coated FTO glass and the Ni foil current collector under CO2 gas purging conditions. Current density values of 1:0.5 M SiC-In2O3 were 8.34×10−3 mA/cm2 and 1.79×10−2 mA/cm2, respectively. The current change variation of the gas sensor was different due to conductivity. In the case of Ni foil, it had a porous structure to support the location/coating ability of the material, thus contributing to the electron transport potential. The sensing ability of the sensor material under O2-gas is displayed in Figures 8c and 8d. Current density values of 1:0.5 M SiC-In2O3 were 9.82×10−3 mA/cm2 and 1.23×10−2 mA/cm2, respectively. Under O2 gas purging conditions, the variation of current density was higher than for CO2 gas detection. This indicated that the sensor material might have more excellent and effective sensing ability for O2 gas sensing at standard room temperature (25°C) when the sensor material uses a Ni-foil current collector.
The gas sensing test was mainly realized by current change upon exposure to the target gas environment under a constant voltage . Under room temperature conditions, activation of the electrode was less. Nevertheless, all sensor materials showed good sensing for CO2 and O2 gases. The current change variation on 1:0.5M SiC-In2O3 was higher than those of the other two gas sensor materials at standard room temperature. This provided evidence that this material had a more active sensing ability and that the reaction of gas on the electrode surface was more dynamic. The current change variation on 1:0.1 M SiC-In2O3 was lower than other high amount of MO loaded β–SiC-binary nanocomposites, indicating that 0.1 M In2O3 irregularly spread on the surface of the SiC fiber and could not activate the sensing ability due to the lack of electron exchange.
The SiC-based gas sensor material had strong activation under high-temperature. In addition, the combination of β–SiC fibers with MO can bring excellent electronic conductivity and boost chemical activity (enhanced chemical activity and interactivity of the gas and electrode surface) . Figure 9 shows the results of the electrochemical test of the coated Ni-foil current collector under high temperatures without or with a gas purged state. Under high temperatures, differences among current density values of three different electrodes were not high, suggesting that these prepared electrodes could show quite similar sensing ability. However, the 1:0.5 M SiC-In2O3 nanocomposite coated electrode had a high current density value (Figure 9a). Figures 9b and 9c show electrochemical responses of CO2 and O2-gas with x-amount of In2O3 loaded β–SiC fiber electrodes. The 1:0.5 M β–SiC-In2O3 electrode had high conductivity for O2 gas but low-conductivity for CO2 gas. Figure 10 (a and b)displays the highest current density value of each gas sensor material without or with a gas purging condition at room temperature (25°C) and high temperature (350°C). The unit of current density value is mA/cm2. The sensor material had quite strong sensitivity for O2 at room temperature as displayed in a bit graph. O2 gas had an electron acceptor behaviour. Oxygen gas strongly interacted with the surface of the sensor material. At a high temperature, a strong sensing ability of SiC-In2O3 sensor was observed for CO2 gas. The possible gas sensing reactions on SiC-In2O3 nanomaterial at room and high temperatures are displayed in Figure 11. The gas sensing mechanism is based on the transfer of charges, in which the sensing material acts as an absorber or donor of charges. Charge transfer between the gas molecule and the sensing material will cause changes in sensing material properties. Gases such as O2 and CO2 tend to receive electrons from the sensor surface. The oxidizing gas (receiver) can increase the resistance of the sensor surface and reduce the resistance of the sensor by reducing the gas (donor). Gases such as O2 tend to receive electrons from the surface of the sensor, which is an oxygen-dominated gas that takes electrons from the surface of the metal oxide and converts them into ions that can be rapidly absorbed on the surface of a metal-oxide sensor (Figure 11a and b). As a result, electrons on the surface become trapped, which increases the height of the potential barrier. On the other hand, it affects the surface conductivity of metal oxides or electron conduction. In the case of CO2, it has a linear bond and a stable structure with no lone pair of electrons (Figure 11c and d). The CO2 gas react with surface electrons of the gas sensors and made a form of (CO2−). During a sensing test, surface electrons of the β–SiC-In2O3 sensor are used to sense CO2, or CO2 gas receives electrons from the gas sensor so that the current density value (mA/cm2) of the gas sensor is significantly lower than that of a normal system or a no gas-purged system.
To conclude, the electron transfer ability between the target gases and sensor material strongly defines the sensing ability of SiC-In2O3. Furthermore, the active parts of the surface affect the reaction between the gas and the surface on the sensor surface. SiC-In2O3 nanocomposites contain varying amounts of In2O3 metal-oxide, which makes it possible to determine how they affect activities of gaseous materials. The high load of metal oxides strongly supports the electrochemical performance of β–SiC fibers, resulting in the formation of a high electron density Si-C-O-In bond sensor layer. The charge transfer process then becomes more active under the influence of the interface structure. In addition, functional β–SiC fibers had an abundant surface area on which MO can be homogeneously distributed. All factors mentioned above adequately explained the surface modification of the β–SiC fiber, the change of electron-transfer activity, and the gas sensing ability.