Fabrication of Highly Sensitive YCeO Chemo-resistive Gas Sensor for Selective Detection of CO2

A room-temperature-operated CO2 gas sensor based on YCeO nanocomposite was effectively prepared by the simple hydrothermal technique to detect low traces of CO2 (50–250 ppm). The YCeO granular morphological features were observed using field-emission scanning electron microscopy, which confirmed successful fabrication of nanocomposite of Y2O3 and CeO2. X-ray diffraction of YCeO showed the Cubic structure of space group Fm3m having density 6.74 gmcm−3. Rietveld refinement was performed for the analysis of complete crystal structural property. Surface porosity and specific surface area were observed by Brunnauer-Emmet Teller analysis. Optical properties were observed using UV-Visible spectroscopy. The band gap, optical conductivity, and refractive index calculated were 3.44 eV, 2.63 × 106, and 0.1164, respectively. Fourier transform infrared spectroscopy was done to analyze the functional and elastic properties of as-prepared nanomaterial. The highest sensor response recorded was 2.14. The response and recovery time at 50 ppm observed were 75.6 and 107.3 s, respectively. The YCeO chemo-resistive sensor confirmed long-term stability and selectivity to CO2 as compared to other gases viz. LPG, NH3, CH4, H2S, NO2 and H2. The relative humidity exposure was also performed at 15, 55 and 95% RH, in which it was confirmed that the sensor would give best response at mid humidity level i.e. 55 %RH. Sensing characteristics curve of YCeO nanocomposite at different temperature (30 °C–90 °C) at 50 ppm confirmed that YCeO sensor performed excellent at room temperature. This report unlocks an innovative opening for the fabrication of sensing devices that are room-temperature-operatable, highly sensitive and selective for quick detection of CO2 gas for its commercialization.

CO 2 emissions in Earth's atmosphere are primarily attributed to the carbon cycle, with man-made or anthropogenic emissions mainly arising from fossil fuel combustion. 1 The increase in indoor CO 2 concentration over the past two centuries has significantly impacted human health, making indoor air quality monitoring crucial.3][4] Today's sensor systems for monitoring CO 2 are primarily electrochemical cells, capacitive sensors, or NDIR.However, these methods have disadvantages such as high costs, complicated operation conditions, and the need for qualified operators.6][7][8] CO 2 is primary a greenhouse gas emitted by human activities, such as fossil fuels and chemical reactions.0][11] Research on CO 2 monitoring and detection has focused on room temperature (RT) and low-power gas sensors for various applications, including air-quality monitoring, air-conditioners, agricultural production, clean energy technologies, engine exhausts, and the chemical industry.Chemo resistive gas sensors have the potential to realize CO 2 smart sensing in various applications, such as monitoring the atmosphere, controlling indoor air quality, and cultivating crops in greenhouses or plant factories.][14][15][16][17][18][19][20] Recent research has focused on improving sensitivity, response time, and working temperature of chemo sensors based on transition metal oxides.Semiconductor materials like CeO 2 , SnO 2 , Sn-Sb, ZnO 2 , CuO, CuO 2 , Ga 2 O 3 , GaN, and WO 3 have proven to be effective gas sensors due to their unique properties.CeO 2 , a member of the rare-earth metal oxides, has been explored for advanced applications in electronics, optics, and heterogeneous catalysis.Metal oxide-based gas sensors have shown their excellence in monitoring and detecting environmental pollutants.2][23][24][25] Nanocomposites formed by n-n metal oxide materials have gained attention due to their electronic and chemical sensitizations, resulting in increased sensing performance even at low concentrations.Noble metal doping in metal oxide nanocomposites has also shown significant improvements in gas sensing performance.Cerium dioxide (CeO 2 ) is a promising gas sensing material due to its abundant reserves, low cost, excellent thermal and structural stability, and abundant oxygen vacancies in its crystal structure.7][28][29] However, there are presently fewer reports on the investigation of novel CeO 2 /Y 2 O 3 composite materials with cubic structure and mesopores, which are exciting circumstances for a variety of applications due to their massive surface area, stability, and low noxiousness.There are not many reports on the studies on the nanocomposite or doping behavior between CeO 2 and Y 2 O 3 for different applications, from different synthesis route as shown in Table I.To the best of our knowledge gas sensing of CO 2 has yet not been performed in CeO 2 and Y 2 O 3 nanocomposite.In this present study we synthesized YCeO from simple and cost-effective hydrothermal route and further characterized by different techniques, and finally selective CO 2 gas sensing was performed successfully.Then NaOH was added drop wise slowly to the solution at room temperature until the pH became 10-11 with continuous magnetic stirring to attain an entirely homogeneous solution.After that, the whole mixture was moved into a 100 ml Teflon-lined autoclave and kept in oven at 140 °C for 6 h.Then the autoclave was taken outside and allowed to cool at room temperature; a little pale-yellow precipitate was formed, which was kept in the oven at 80 °C for drying.The resultant powder of nanocomposite material was annealed at 550 °C and allowed to cool to room temperature.The prepared nanocomposites having 10% doping of yttrium in cerium oxide which are given in the following chemical Eqs.Characterization.-Thefabricated nanocomposite was examined by using various characterization techniques for the validation of different parameters to understanding the nature and performance of as-prepared sample.Firstly, for surface morphological examination, Field Emission Scanning Electron Microscope (FE-SEM, JEOL; JSM 7610f) was used at different magnification.The structural analysis was executed by X-ray Diffractometer (XRD, RIKAGU; Ultima IV) using CuK α radiation (40 kV and 25 mA), with 4°/min scanning speed.For finding the absorption wavelength and band gap, Ultra Voilet Visible Spectroscope (UV-Vis, NexGen's PTS) was used.The Fourier Transform Infrared Spectroscopy (FTIR, SHIMADZU IR; Affinity IS) was used to investigated different functional groups present in the as-prepared nanomaterials.Brunauer-Emmett-Teller (BET, BELSORP-mini Ver 2.5.4) was used for examining the physical structure as the area of prepared material's surface and how that the sample would interact with its environment.

Experimental
Device and fabrication.-Thesensing film of YCeO was fabricated on Indium tin oxide coated (ITO) glass substrate by using drop cast method.In this procedure, the ITO glass substrate was taken with 1 × 1 cm 2 dimension and cleaned by distilled water and, acetone alternatively, using ultrasonication methodically.The substrate was then dried out on hot plate at 60 °C for 5-7 min to remove the lasting organic residues.On the other hand, the prepared nanocomposite of YCeO was dispersed in Dimethyl sulfoxide (DMSO) to form a homogeneous solution.After that glass substrate was placed on spin coater and dropped with the obtained solution of YCeO keeping spin-coating at 2500 rpm for 60 s and dried 10 min on a hot plate at 60 °C.This procedure was repeated three times to achieved required thickness, then the sensing film was annealed at 550 °C for 1 h in a furnace.Additional, for sensing, Ag electrodes was dropped on the fabricated sensing film with silver paste.The process of fabrication device is shown in Fig. 1.
The prepared sensing device was positioned inside the sensing chamber and connected to Keithley electrometer.The sensing device was exposed to CO 2 gas inside the chamber and the then electrometer detect the electrical signal and displays on connected computer.Figure 2 shown the schematic diagram of gas sensing measurement setup.
The concentration of gas inside the chamber was calculated using the following Eq. 3.  ECS Sensors Plus, 2024 3 014401 Here C is concentration (ppm), V x is injected volume (ml), V is volume of chamber (ml), T r is the room temperature and T c is operating temperature.

Results and Discussion
Morphological analysis.-Ininspecting the interactions between nanomaterials, the purity and shape of the nanoparticles are incontestably essential rudiments.Here, the morphological analysis was analyzed through FE-SEM which is shown in Figs.3a, 3b.The FE-SEM micrograph was taken under different magnifications and observed the diameter ranging from 4-15 nm of granular shape.From figure it is clearly seen that yttrium and cerium combined and developed into a nanocomposite structure, or it can be said that that both nanoparticles generally showed the same morphology as fused and aggregated nanoparticles.These granular structures show the admirable, interminable surface areas as well as porous morphology.By evaluating these nanocomposites, it can be inferred that this is a precise way of preparing YCeO with a low-cost technique.These small granulated shapes of YCeO were broadening the surface area and porosity of the material which is very precious for the adsorption of analytes to improve the gas sensing performance.
Structural analysis.-Thepowder XRD method was used for the structural information as well as phase identification of the fabricated nanomaterials using CuKα radiations with 2θ range between 20°to 80°and having scanning rate of 4°/min.Figure 4a shows the XRD pattern of pure CeO 2 and YCeO nanocomposite.The detected peaks designate the polycrystalline nature of as-prepared sample.The diffraction peaks of CeO 2 observed at 28.72°, 33.38°, 47.69°, 56.56°, 59.52°, 69.63°, 76.94°, and 79.27°are indexed to (111), ( 200), ( 220), (311), (222), and (400) respectively.All the peak positions and comparative intensities are perfectly matched with JCPDS no.01-078-0694 which indicates the cubic crystal from Fm3m space group.Whereas the diffraction peak of YCeO was observed at same position of CeO 2 but an additional peak was  identified at 31.35°.Here, all the peak positions and relative intensities are perfectly matched with JCPDS no.01-075--0175which also indicates the cubic crystal from Fm3m space group.Additionally, the crystallite size of each diffraction peak was estimated by using Scherrer formula as given - Here, k = shape constant (0.94), λ = wavelength of X-ray source (0.15406 nm), β = full width at half maxima (FWHM), and θ is angle of diffraction.The peak broadening at a lower angle is important factor for the calculation of crystallite size; therefore, the crystallite size of as-synthesized nanoparticles were calculated using high-intensity peak; The crystallite size was 70.21 nm for (111) at 2θ value 28.72°for CeO 2 and for YCeO, the maximum intensity peak (111) occurred at 2θ value 28.72°and the crystallite size was 17.62 nm.This confirms that by adding Y 2 O 3 in CeO 2 the crystallite size was substantially reduced.Notably, less is the crystallite size, higher is the surface area, and higher is the sensing performance. 43Additionally, Fig. 4b shows the Rietveld refinement graph of as-prepared YCeO.Rietveld refinement is done through the Fullprof Suite software.Mechanical parameters such as scale factor, unit cell parameters, structural elements, positional and volume parameters have got refined.The position of the respective peaks and magnitudes accurately indicate a good fitting pattern.From this figure it can be undoubtedly seen that the detected and calculated graphs fits very well.Table II.shows fine-tuned structured calculated data.The goodness of fit for the top shape and position, structure and background were measured by the R and chi2 (χ2) factors of the conformation.Sensibly lower Rp, Rwp, Rexp and χ2 values are well-thought-out a good refinement of the profile.However, the standards of the profile parameter R are only one condition for the preeminence of the Rietveld refinement.Furthermore, Fig. 4c shows the crystal structure of pure CeO 2 containing 182 atoms, 304 bonds, 38 polyhedral and Fig. 4d shows the crystal structure of YCeO analyzed using Vista software containing 182 atoms, 304 bonds and 38 polyhedral but as compared to Fig. 4c doping of yttrium was visible in Fig. 4d So.From the above Rietveld refinement it was clearly seen that yttrium beautifully combined with cerium oxide, or it can be said that yttrium interfused with cerium oxide particle to successfully formed YCeO nanocomposite.
Adsorption desorption isotherm.-Brunauer-Emmett-Teller(BET) method was used to study the surface area of synthesized nanomaterial.Here N 2 (nitrogen) was adsorbed or desorbed on the surface of the nanomaterials to detect the surface area and porosity of the as-prepared samples.Here we use Nitrogen dioxide at saturated pressure 101.54 kPa at a sample weight 21 of 0.05 g with 9.792 standard volume at an adsorption temperature of 77k. Figure 5a shows the N 2 adsorption and desorption isotherm graphs of YCeO and Fig. 5b shows the observed porosity of as-prepared nanocomposite, where the pore graph of the material shows its porosity and from this, the relative pressure or total pore volume (p/ po) was 0.098 cm 3 /g, and the mean pore diameter calculated was 49.983 nm.As it was undoubtedly detected that YCeO has a protracted surface area calculated 41.72 m 2 g −1 which is much greater.This result features the confirmation of yttrium and cerium oxide nanocomposite which covers large surface area.Thus, from sensing point of view this improved specific surface area will provide the dynamic path to the gas molecules which will upsurge the proficiency of gas sensing performance.
Optical analysis.-Theoptical analysis for the absorption of UV light of as-prepared YCeO was performed to find out the absorption wavelength as well as the energy band gap.UV-visible absorption spectrum of YCeO nanocomposite is shown in Fig. 6a.The absorption peak of YCeO is found at 304.377 nm which illustrates that the prepared nanomaterial has its highest absorption of UV light at 304.377 nm.Moreover, the absorbance of the material was affected by doping of different material.According to Beer-Lambert's relation, absorption coefficient (α) can be calculated as following Eq. 5.
Here, K is characteristic parameter, h is Planck's constant, and n is power factor (n).The value n = ½ used for direct allowed transition and to estimate the direct optical bandgap of YCeO nanocomposite.The band gap value of YCeO was found to be 3.44 eV which is shown in Fig. 6b.This band gap proves that YCeO nanocomposite was one of the best semiconducting materials for the prominent sensing devices.Additionally, optical conductivity was calculated for YCeO nanocomposite as shown in Fig. 6c using Eq.7 where α is absorption coefficient, n is refractive index and c is speed of light in vacuum.As optical conduction is a condition of tapping electrons in the conduction band, one additional way to attain this phenomenon is to give an enough energy to the electron bound to atoms so as to disrupt the bond and allow it free to move.This can simply be achieved by exposing the UV light on the prepared nanomaterial by which photons do have an energy permitting the breaking of the bonds.In a normal language, photons can encourage electrons from the valence to the conduction band and leave-taking a hole in the valence band. 44These free electrons and holes can then contribute to electrical conduction of the material.Here, we found the optical conductivity of YCeO was 2.63 × 10 6 .And lastly, refractive index was calculated using absorption wavelength of YCeO nanocomposite as the index of refraction is theoretically only well-defined for one wavelength.Higher wavelengths refract to a higher degree and would give higher index of refraction.Refractive index may vary because the density of the prepared sample changes.The higher the density, the larger the refractive index. 45Liquids transform density with temperature more simply than solids so temperature is typically not measured when measuring the index of a solid unless exact precision is required.But in terms of solid, as compression increases, the density becomes greater because you are compressing the material more which makes the index higher.Here the refractive index of YCeO was calculated  Chemical formula Y 0.20 Ce 0.80 O 1.90 3.
Volume (10 at highest absorbed wavelength at 304.377 nm and found to be 0.1164 as shown in Fig. 6d.By this we can say that higher refractive index will denote that the prepared nanomaterial contain high density and high density provides more the adsorption sites for adsorbing the gases molecules which will increase the gas sensing phenomenon.Refractive index was calculated using formula shown in Eqs. 8 and 9.
Here, T s is Transmittance percent and A is absorption wavelength.
For the functional group examination FTIR spectrum were noted using KBr pellet as standard in range of 400-4000 cm −1 .The FTIR spectrum of YCeO nanocomposite is depicted in Fig. 6e  1000 cm −1 (Ce, Y). 47 The bands seen at about 789.87 cm −1 and 8525 cm −1 are attributed to the Y-O as well as O-Ce-O bond stretching vibration close with the wave number stated in the earlier investigation by Schwartz and Schwartz. 48Functional group band of C-O confirms that this material is manifests prominent features for adsorption and desorption of CO 2 .
We resolute the coefficients of elasticity and viscosity of prepared YCeO.This study focuses on assessing the elastic properties of YCeO like density, bulk density, poison's ratio, Bulk modulus, Young's modulus, Shear modulus, Debye temperature and many more which was tabulated in Table III, by using different formulas to know the behaviors of as prepared nanocomposite by FTIR data's wave number.Moreover, due to mechanism of the gas sensors, chemisorptions happen on the surface of the sensing material.Thus, the activated surface area will rise with the temperature rise due to the growth in activation energy.So, it means that temperature would also play essential role in gas sensing properties that is why the Debye temperature was also calculated.The Debye temperature T D is the temperature of the crystal's upper most normal mode of vibration, and it relates the elastic properties with the thermodynamic properties such as thermal conductivity, phonons, specific heat, thermal expansion, and lattice enthalpy.

CO 2 Sensing Analysis
The YCeO sensor confirmed the greater performance even at low CO 2 concentrations.The response of the fabricated sensor based on YCeO nanocomposite were tested in the range of 50-250 ppm CO 2 gas is shown in Fig. 7a.Here, we detected an even graph with less variations which is suggesting that the response enhancing was not due to CeO 2 but rather promoted to the effective fabrication between Y 2 O 3 nanoparticles and the surface of CeO 2 spheres.The response to CO 2 at 30 °C reached its maximum peak with 55% RH (Relative Humidity) thus, this was suggesting that the finest operating temperature 30 °C should be chosen.The sensor response time toward CO 2 is an important parameter for approximating material performance, as it determines the delay to reach the exact gas concentration value and the time required to prepare the sensor for the subsequent measurement. 49he response time (t res ) is defined as the time taken for the relative resistance change to reach 90% of the steady 50,51 state value after CO 2 injection, while the recovery time is for the delay required to reduce the difference between highest and lowest resistance.The YCeO sensor manifested decent response at 30 °C and exhibited almost a linear increase with CO 2 concentrations as shown in Fig. 7b.The sensor based on YCeO for 50-250 ppm CO 2 at 30 °C was further studied, showing a current decrease upon injection into 50 ppm CO 2 and recurring to its original value after CO 2 vapor release.The lowest response time and recovery time were determined to be 75.6 s and 107.3 s, respectively.As the concentration enhanced the electrical current of sensing film also enhanced.Lower concentrations of CO 2 go through weaker interaction among adsorbed oxygen species and CO 2 molecules, higher CO 2 concentrations show significant increase in the current of sensing film.For different concentration, the sensor response increases very slow by increasing ppm in presence of CO 2 and air.Furtherly, sensitivity is calculated by slope of linear fitting curve of sensor response vs concentrations as following Eqs. 10 and 11.The linear fitting curve of sensor response vs concentration revealed the sensor response approximately linearly increased when concentration was increased as shown in Fig. 7c.The linearity coefficient (R 2 ) was found to be 0.99 which suggested the goodness of linear fitting curve.The lowest and highest sensor response of YCeO was calculated as 1.73 and 2.14 respectively.Additionally, the slope of consistent sensor response vs concentration curve resolute the sensitivity of YCeO nanocomposite sensor.Based on the findings of the experimentations, the Limit of Detection (LOD) can be calculated from the linear calibration curve by calculating standard deviation (SD) of the sensor sensitivity, by the following Eq.12.
Here, m is slope of linear fit curve and σ is standard deviation of intercept.The LOD of YCeO nanocomposite sensor was found to be 1.8.The response and recovery times of YCeO nanocomposite sensor for every concentration were estimated by following Eqs.13 and 14.
This recommends that the gas sensor created on the YCeO nanocomposite can comprehend faster detection of CO 2 vapor at low concentration.The high sensor response and rapid response-recovery capabilities of a sensor must be repeated by effective use of the sensor due to large surface accessibility.In comparison to previously reported work based on CO 2 sensor operated at room temperature, YCeO nanocomposite sensor showed high sensing performance as mentioned in Table IV.
Repeatability is an essential factor for testing sensors.The present study examined the repeatability and stability of YCeO. Figure 7d shows 6 cycles of dynamic sensor response curves of YCeO nanocomposite to 50 ppm for reducing CO 2 gas at room temp (30 °C with 55% RH).The outlines of response and recovery curves is nearly indistinguishable, since it sustained the same performance regarding response time and reversibility.Also, even after numerous repetitions, the response value variation was small, which demonstrate a good repeatability and steadiness of the sensor.Debye Temp.T D (K) a 478 13.
Universal Anisotropy 0.91 ECS Sensors Plus, 2024 3 014401 The YCeO nanocomposite sensor's long-term dependability was examined by measure its responses to 50 ppm CO 2 at room temperature (30 °C) for 40 days shown in Fig. 8a.The sensor exhibited very little fluctuations in current and response values which designated the good long-term dependability.The sensor shows the quick sensor response after 20 and 40 days was 1.732 and 1.723 respectively shown in Fig. 8b.This consequence specifies that the interaction between CO 2 gas and the surface of the as-prepared is subjugated by physisorption, whereas chemisorption plays a negligeable role.This long-term stability delivers further evidence for its probable applications in the industry.
Sensor selectivity can be well-defined as the capability of the sensor to respond to a certain gas in the presence of other gases. 56,57n simple words, the selectivity can be appraised by comparing the effects of different gases (oxidizing or reducing) towards the sensor response.Here, Fig. 8c shows the sensor response towards various gases at room temperature (30 °C) such as LPG, NH 3 , CH 4 , H 2 S, NO 2 , H 2 and CO 2 and calculate sensor response 0.7, 0.87, 0.96, 1.06, 1.09, 1.12 and 1.74 respectively.Sensor response for CO 2 as compare to H 2 and LPG are 55% and 149%, respectively which are quite high.Thus, in the presence of all these gases the response to CO 2 will be maximum i.e. for CO 2 the sensing element has the maximum selectivity toward CO 2 .This confirms the highest absorptivity of CO 2 gas molecules than other gasses.The electronic properties of sensing materials for carbon gas sensors are affected by gaseous composition and concentration fluctuations.The sensing mechanism of carbon gas sensors is mainly explained by gas-solid interactions, such as ion oxygen adsorption and direct charge transfer, which is the receptor function of sensitive materials.However, the majority of carbon gas sensors undergo electron exchange between their surfaces and gas molecules, leading to weak selectivity.To improve sensor selectivity, the target response and/or interfering signal can be enhanced.The receptor function of carbon gas sensors is determined by the material surface's ability to interact with the target gas, which is related to adsorption affinity, catalytic ability, surface acidity/alkalinity, and gas reactivity. 58nhancing receptor function depends on the type and microstructure of the sensing material, which can be achieved through chemical methods like doping, composite construction, surface modification, filter membranes, gas sensor arrays, and temperature/humidity  regulation.Thus, by adding yttrium in cerium as a composite can increase the porosity, surface adsorption and roughness of the sensing material by which the sensitivity will improve and makes YCeO more selective towards CO 2 gas.Additionally, the amenity life of a gas sensor is decisive for its economic development.
Additionally, the humidity effect of sensing measurement is compulsory for low temperature operated sensor.The CO 2 gas sensing measurement at 50 ppm of YCeO nanocomposite sensor was executed at different humidity levels as shown in Fig. 9a.The humidity inside the chamber was amplified by humidifier (K 2 SO 4 saturated solution) for 15% to 95% and relative humidity measured by hygrometer.The difference in electrical current of YCeO nanocomposite sensor at changed humidity levels as 15, 55, and 95 %RH, respectively are shown in Fig. 9a.At the lower humidity levels (15-30 %RH), the distinctions in current are high with exposure to CO 2 whereas as the humidity levels increase the distinction in current decreases.For the purpose that at lower humidity area only the chemisorption layer was formed so the variation in current is create very high whereas in mid humidity (30-60 %RH) region, few physisorption layer was formed which concentrated the interaction of oxygen ions to gas molecules so, the variation in current was reduced. 59At higher humidity levels (60-95 %RH) the development of physisorption increases which weakens the interface of oxygen ions to CO 2 gas molecules.As shown in Fig. 9b the sensor response of YCeO nanocomposite sensing film decreases as the humidity increases.The humidity effect predicts the YCeO sensor can be affected by humidity but in mid humidity region, tiny changes have happened.So, the YCeO sensor can be effectively used for the detection of low concentrations of CO 2 in mid-humidity region.
Operating temperature plays a significant role in the sensing parameters of semiconducting materials, as it requires surface activation energy for the generation of oxygenated vacancies.1][62][63][64][65] The sensor response and recovery times are useful parameters, calculated by exponentially rising and decaying curves fitted between 30 °C, 60 °C and 90 °C respectively as shown in Fig. 9c.Most sensing requirements have a set time limit in which test gases are exposed to certain concentrations.The response and recovery times of our samples are significantly decreased with increasing temperature.These times do not follow a regular pattern as with the optimum response.The current changes become nonlinear after the optimum temperature.At the optimum temperature, the chemisorption of a gas molecule is high, indicating that the ratio of O − and O 2− species to the O 2− species is greatly increased.These two species are dominant at higher temperatures and frozen at the surface due to fast temperature change.The highest response is observed in the YCeO sensors at temperatures nearly 90 °C.YCeO sensors with the largest number of adsorption-desorption sites provides its maximum response at the lowest temperature.Figure 9d shows the sensor response calculated by YCeO at different temp have been shown.At high temperature the sensor response was high but it consumes a lot of power consumption.So, we can say that our device will better operate at room temperature which consumes the power as well as makes it cost effective.
Gas sensing mechanism.-Numerousstudies have been steered to enhance the gas sensing properties of metal oxide semiconductors, such as doping, multi-layer integration, surface activation and nanocomposites.Besides this, the surface conditions play a crucial role in gas sensing properties. 65The gas sensing bend is divided into three stages: steadiness, adsorption/response, and recovery.Moreover, controlling electrical contacts based on Ohmic and Schottky contacts suggestively changes the chemical sensor response of metal oxide.][68] When exposed to air, YCeO sensor adsorbed oxygen molecules, capturing free electrons from the conduction band to form chemisorbed oxygen species resulting in an increase in effective potential barrier height as shown Fig. 10a.Y 2 O 3 transfers electrons into the lower-energy conduction band, forming an accumulation layer on the surface of CeO 2 .The higher chemisorbed and dissociated oxygen species component of YCeO sensor attracts more free electrons, resulting in an increase in electron concentration and decrease current.The adsorption of atmospherics oxygen can be explained as by following Eqs.15 When exposed to CO 2 which is a reducing gas, the surface region of CeO 2 can be improved by the electrons from oxygen anions.This process releases surrounded electrons back to the conduction band, reducing the potential barrier at their interfaces resulting in an increase in current.These molecules react with adsorbed oxygen species, releasing trapped electrons back to the conduction band and increasing electron concentration which increases the current and decreasing the potential barrier at their interfaces and resulting in an increase in current as shown in Fig. 10b.Gas sensors based on YCeO have ultra-sensitive and low operating temperature properties due to heterojunctions designed between Y 2 O 3 and CeO 2 .Thus, the heterojunctions designed between dissimilar semiconductor oxides have an important effect on gas sensing performance.Since the conduction band advantage of YCeO is situated at higher potential that is why electrons present in the conduction band of YCeO will wander until their Fermi energy level is up to equilibrium.This process will deplete the extra electrons of YCeO composite interface, which will enhance the gas sensing properties.The adsorbed CO 2 reduces the number of carriers on the YCeO surface, resulting in enhanced current change.They also provide more surface-active sites for the reaction of adsorbed oxygen and CO 2 gases, making them ultra-sensitive and a prominent sensor. 69,70The hybrid composite, combination of CeO 2 and Y 2 O 3 , boosts better electron transportation and gas adsorption.The nanocomposite has high conductivity, plenty of active sites, and composites cause substantial changes in electrical properties before and after revelation to the target gas at room temperature.It is proposed that the environmental factors such as oxygen content and humidity will be studied in future research.

Conclusions
The outcome of YCeO based chemo resistive gas sensor for the detection of CO 2 at room temperature (30 °C) were deliberate.YCeO was synthesized from simple and cost-effective hydrothermal route and further characterized by different techniques, such as XRD, FESEM, BET, UV-Visible and FTIR spectroscopy.Rietveld refinement analysis was performed for the structural information of YCeO nanocomposite.The average crystallite size of YCeO nanocomposite calculated by Scherrer formula was 17.62 nm.The exposure of CO 2 was performed at room temperature in the range of 50-250 ppm.The lowest and highest sensor responses of CO 2 at 50 or 250 ppm were 1.73 and 2.14, respectively.The fast response and recovery times at 50 ppm were 75.6 s and 107.3 s, respectively.The sensor response enhancement was due to the successful fabrication of nanocomposite between the Y 2 O 3 and CeO 2 .The YCeO based chemo resistive gas sensor authenticated high long-term stability, outstanding sensor response, fast response/recovery time and selectivity to CO 2 as compared to other gases.The relative humidity exposure was also performed at 15, 55 and 95% RH, in which it confirmed that sensor would give best response at mid humidity level i.e. 55 %RH and at last Sensing characteristics curve of YCeO nanocomposite at different temperature (30 °C-90 °C) at 50 ppm shown which confirms that YCeO sensor performs excellent at low temperature or room temperature.This report demonstrates that YCeO chemo resistive sensor has huge potential for detection of low traces of CO 2 gas.

Figure 1 .
Figure 1.Spin coating process for the fabrication of YCeO sensing device.

Figure 2 .
Figure 2. Depicts the schematic diagram of gas sensing measurement setup.

Figure 4 .
Figure 4. (a) shows the XRD pattern of synthesized CeO 2 and YCeO, (b) shows the graph of Rietveld refinement, (c) shows the structural model CeO 2 and (d) shows the structural model of YCeO obtained by refinement.

o
Where x = cuvette's thickness, Io = initial photon intensity, I = instantaneous photon intensity and A = absorbance.To excite the electrons from valance band to conduction band energy required is the band gap energy.The energy band gap is calculated by important relation between photon energy and absorption coefficient energy as follows Eq. 6:

Figure 5 .
Figure 5. (a) shows the adsorption and desorption curve and (b) shows the porosity graph of YCeO nanocomposite.
. The O-H stretching vibration in OH groups caused by large bandwidth is seen at 3435.77 cm −1 And 2309.81 cm −1 .According to Hadeer et al., 46 the bands detected between 1250 and 1750 cm −1 are produced by the asymmetric stretching of the C-O band, which might result from the absorption of CO 2 from the atmosphere.It can be shown from the FTIR analysis that the sample is a mixed composite of cerium and yttrium oxide.The lattice vibration modes of Y-O and O-Ce-O correspond to the bands in the low-frequency range of 400 cm −1 to

Figure 6 .
Figure 6.(a) shows the UV-Visible spectrum of YCeO, (b) shows the band gap, (c) shows the optical conductivity, (d) shows the refractive index of YCeO and (e) shows the FTIR spectrum of YCeO nanocomposite.

n
Where [C] stands for analyte gas concentration and A & B are both constants.

Figure 8 .
Figure 8.(a) Durability curve for 40 days towards at 50 ppm exposure of CO 2 , (b) Sensor response curve for 40 days and (c) shows the selectivity curve of YCeO nanocomposite sensor towards exposure of numerous gases.

Table I .
Literature survey on YCeO nanocomposite.

Table II .
Calculated structural parameters from Rietveld refinement.

Table IV .
Comparative analysis on metal oxide based on ammonia gas sensor at 28 °C.