Effect Synthesis Time of CeO2 Nanoparticles by Microwave-Assisted Hydrothermal as a Sensing Device on CO Gas Sensitivity

The study presents CeO2 particles synthesized by the microwave-assisted hydrothermal method using different synthesis times for the analysis of sensitivity to carbon monoxide gas, aiming at its application as a sensor material to prevent poisoning caused by this highly toxic gas. Structural, morphological and spectroscopic and electrical behaviors were analyzed by X-ray diffraction, Rietveld refinement, transmission electronic microscopy, Raman scattering spectroscopy, optoelectronic characterization chamber and FT-IR. The samples presented fluorite-type cubic structures, increase in crystallinity and particle size with the variation synthesis time from 5 to 8 minutes. From the micrographs it was observed that the nanoparticles initially were spherical with a surface domain (200), and the synthesis time made them cubic/polyhedral with a surface (111), showing differences in defects and influencing in its properties. The sample synthesized at 8 minutes showed the best result with response and recovery time of 16 and 2 seconds, respectively, at 390 °C, therefore promising for the fabrication of CO gas selective sensing device.


Introduction
Cerium oxide (CeO 2 ) also called ceria is a promising compound for several technological applications mainly because its physical and chemical properties are easily modified through a control over the composition, type of structure, morphology, and size of the particles during its synthesis and process 1,2 .Some of the essential properties of this material are oxygen storage capacity, redox potential, thermal stability, chemical stability, low toxicity, electronic conductivity, and high oxygen mobility [3][4][5] .
Studies report that the properties of CeO 2 nanocrystals are directly related to the morphological structure, particle size and the synthesis method 6,7 .Furthermore, CeO 2 has a fluorite-type cubic crystal structure, Fm3m space group, and a lattice parameter of 0.5411 nm at room temperature 8 .
CeO 2 particles can be prepared by different routes, such as polymeric precursor method, sol-gel method, flow method, precipitation, organometallic decomposition, among others [9][10][11][12][13][14] .On the other hand, these chemical routes have some disadvantages, such as high temperatures, long processing times and the use of organic precursors/solvents 15 .Currently, two methods stand out in the preparation, growth and morphological control of different metallic oxides, conventional hydrothermal (HT) and microwave-assisted hydrothermal (MAH) methods 15,16 .Studies report that MAH has great advantages, below 200 ºC it is possible to obtain pure oxides, high crystallinity, various morphologies, low processing/synthesis time and energy 8,17 .
Therefore, the combination of CeO 2 physicochemical properties makes it a versatile material with a range of applications, such as optical devices, catalysts, solid oxide fuel cells, hydrogen storage materials, gas sensor, among others [18][19][20][21][22][23][24][25][26][27][28] .Oliveira et al. 7 studied morphological, optical and electronic properties of cerium oxide varying the synthesis time by the microwave-assisted hydrothermal method.It was observed that CeO 2 nanoparticles when exposed to a dry air atmosphere increases the electrical resistance of the material, and when exposed to a vacuum atmosphere the electrical resistance decreases as a temperature function and morphologies found in the samples.These researchers concluded that vacuum promotes oxygen desorption on the CeO 2 nanoparticles surface, and that the electrons loss in the 4f state of cerium causes a reduction in electrical resistance.López-Mena et al. 3 synthesized nanostructured CeO 2 microspheres by co-precipitation, testing their application in carbon monoxide detection.They obtained a response time of 9 s at 275 °C, for the 200 ppm CO detection at a 100 kHz frequency.The result revealed that the CeO 2 material showed a reproducible and stable detection response.
In this work, we discuss the obtainment and characterization of CeO 2 powders synthesized by the MAH method and the thick films preparation of these powders to evaluate their response as a possible sensor material in CO detection; and also, to link these responses to material-induced changes in synthesis time.

Synthesis and characterization of the CeO 2
CeO 2 powders were prepared by microwave assisted hydrothermal method.Ce(NO 3 ) 3 .6H 2 O (99.9% purity, Sigma-Aldrich) was dissolved in deionized water under constant stirring.To this solution, we slowly add, until pH 10, a few 1 M KOH solution milliliters.The system obtained was heat treated with the aid of a domestic microwave (2.45 GHz, maximum power of 800 W) adapted with a Teflon autoclave.For the process, a temperature of 100 °C was used, with a heating rate of 10 °C min -1 in 5 and 8 min times.Subsequently, the autoclave was cooled to room temperature and then the CeO 2 powders were collected and washed with deionized water several times, and dried at 100 °C in an oven.
For the characterization of the crystalline structure of the powders, a diffractometer was used, with a copper tube (λ = 1.5406Å) in a range from 20° to 110° with 1° min -1 .Rietveld refinement was made with the software GSAS (General Structure Analysis System) 29 and the graphical user interface EXPGUI developed by B. H. Toby 30 .Morphological structure analysis of the films was performed by transmission electron microscopy (TEM) on a FEI instrument.For this, the samples were previously dispersed in ethanol, sprayed on Cu grids (300 mesh) and finally dried.
To identify the functional groups, present in the samples which would indicate the presence of possible second phases which in turn could degrade the electrical property.In this work, Fourier transform infrared absorption spectroscopy (FT-IR) was performed using a Perkin Elmer Frontier model spectrometer in transmittance mode.The spectra were measured with 64 scans in the region between 4000 cm -1 and 400 cm -1 using a resolution of 1 cm -1 .All these measurements were made at room temperature.

Preparation and characterization of the sensor device
Initially, a paste containing cerium oxide powders and binder (glycerol) was prepared.Powder/binder ratio was 1.6 g mL -1 .Then, this paste was deposited on an alumina substrate with interdigitated platinum electrodes.After deposition, the films were dried at 100 °C for 24 h in a dry air atmosphere to evaporate the binder.Subsequently, the heat was treated in a dry air atmosphere with steps of 1 °C min -1 to 380 °C and maintained at this temperature for 2 h to evaporate the residual binder or any impurities presented.
Raman scattering spectroscopy was performed in a Synapse spectrometer (HORIBA Jobin Yvon, France), 32 scans in the region from 200 to 800 cm -1 and 4 cm -1 spectral resolution, coupled with an optical microscope model BX41 (Olympus, Japan) and equipped with an Nd-YAG laser (λ = 514.5 nm), operated at 100 mW.
To perform the electrical measurements, an optoelectronic characterization chamber formed in a closed chamber was used 31,32 .In it, three measurements of electrical response of heating and cooling as a function of temperature were performed, in which vacuum, air and CO environments were chosen under 50 mmHg constant pressure.

Results and Discussion
XRD patterns and structural refinement for CeO 2 nanoparticles obtained at 100 °C for 5 to 8 min are shown in Figure 1. Figure 1a indicates samples monophasic with a fluorite-type cubic structure and Fm3m space group 33 .All diffraction peaks conform to the Inorganic Crystal Structure Database (ICSD) card nº 72155 34 .The sample obtained in 5 min has sharp but broad diffraction peaks indicating to be composed of small poor crystallinity primary particles.Meanwhile, the sample obtained in 8 min presents narrow peaks characteristic with the presence of larger particles and good crystallinity.Table 1 reports the experimental data of Rietveld refinement.Data show that the lattice parameters and unit cell volumes are close to the values published in the literature [35][36][37] and a Joint Committee on Powder Diffraction Standards (JCPDS) card nº 34-0394 for CeO 2 powder where the theoretical lattice parameter was 5.411 Å 38 .An increase in the lattice parameter and unit cell volume is observed, which is normally associated with the synthesis time that influences the particle growth mechanism and, consequently, changes in morphology, crystallinity and deformation of the compound 7 .There is an association between the observed experimental XRD patterns and the theoretical, which can be observed by the same parameter GoF.To explain the growth of CeO 2 particles for hydrothermal conditions two mechanisms are proposed by the literature: Ostwald ripening (OR) and oriented fixation (OA).The OA mechanism took place through the effective collision of cores, using the same crystallographic orientation responsible for obtaining the primary particles of low crystallinity.The OR process required more time to dissolve and recrystallize the material into a larger crystal, increasing crystallinity and reducing defects and surface energy.OA mechanism was the dominant process in this synthesis method, because of the microwave radiation, leading to anisotropic growth of nanoparticles, for the electrostatic interactions in the colloidal system.More time used, there was clearer concision to control the recrystallization process, having the OR mechanism 7,39 .Therefore, the increase in synthesis time increases the crystallinity of the nanoparticles (Figure 1b, c), and this caused a variation in the structural defects, that is, in the oxygen vacancies, distortion in the bonds, tensions and deformations in the crystal lattice, in addition, it affected the surface defects that influence the material's sensing properties.
Morphology and particle size of the samples were examined by TEM. Figure 2a, c shows CeO 2 nanoparticles   using 5 and 8 min with average particle size of 5 and 10 nm, respectively.In addition, agglomerated particles were observed using water as a solvent 36 .These agglomerates were formed by the small size of the particles, and high-density defects and energy.As time was added, particle size enlarged, and reduced surface free energy 7 .Figure 2a shows typical characteristics of the OA process.In this figure, we can observe a mesocrystal-like structure with clear boundaries compared to the oriented primary particles.Using more time (Figure 2c), the presence of the grain boundary became less evident, presenting a more regular surface, indicating the occurrence of self-recrystallization 39 .
The recorded SAEDs patterns (Figure 2b, d) corroborate the XRD result, that the samples are crystalline belonging to the fluorite-type cubic structure.The spaces of the lattice fringes were 0.280 and 0.310 nm (Figure 2b, d) for the CeO 2 synthesized in 5 and 8 min, respectively, calculated from the TEM images correspond to the (200) and ( 111) lattice planes of a fluorite-type cubic structure 35 .Adding time to the synthesis, nanoparticles agglomerate tends to organize themselves to form polyhedral/cube. Considering that, crystal morphologies grow from the ideal crystal (111), we can conclude that the facets (111) of the samples were exposed to minimize the particles surface energy.So, a polyhedron morphology was obtained by adding time, and the particles form ideal cubes or crystals 7,39 .Defects of different types can be found on each exposed surface, for example, surface (111) needs more energy to form facets (100).Therefore, we can observe that the synthesis time changes the concentration of surface defects, one of which is the oxygen vacancies, contributing to the detection properties, as reported by Maekawa et al. 40 .
Figure 3 shows the FT-Raman spectrum of the CeO 2 film.Active Raman vibrational modes are observed in the range between 200-1000 cm -1 .Main Raman (F 2g ) mode referred to an intense peak located at approximately 449 and 462 cm -1 , for 5 min and 8 min, respectively, and was attributed to the symmetrical vibration mode of elongation of the oxygen atoms around each cerium atom 41 .According to the literature 42 , the fluorite-like cubic structure of cerium oxide presents a triply degenerate F 2g mode at ~465 cm -1 displacements, being shifted to smaller particles.Ceria nanoparticles prepared by the microwave-assisted solvothermal method 43 showed bands shifted to a lower frequency, this effect being explained by the expansion of the lattice with the decrease in its size.The lower intensity bands around 250 cm -1 and 600 cm -1 were more pronounced in the sample 5 min and were attributed in the literature to short-range structural defects, the most common being oxygen vacancies.As the synthesis time was added, the modes got thin and the additional peaks practically disappeared, indicating more short-range structural ordering 7,44,45 .
Vibration mode at 830 cm -1 indicates that there is a reduction from Ce 4+ to Ce 3+ in the samples, as this band is related to the change in short-range symmetry.As this oxide reduction reaction is associated with the defect's generation such as oxygen vacancies, it can be said that this defect type is present in the material structure 28 .Therefore, a higher rate of oxygen diffusion rate in the network is favored by the presence of oxygen vacancies, to assist in the process reduction from Ce 4+ to Ce 3+ , and occurs for the shortest synthesis time 7 .However, the presence of these defects influences the electrical properties of the material, as can be seen in Figure 4 regarding the electrical resistance analyses.
Figure 4 presents the electrical transport of CeO 2 nanoparticles in relation to the n-type semiconductor behavior.Ionized and adsorbed O 2 -, O -and O 2-species formation occurs due to the great availability of unpaired electrons from the oxygen vacancies existing on the sensor device surface 46,47 .According to Equations 1-3, oxygen species adsorbed on the material surface react with CO molecules, oxidizing them to CO 2 , releasing electrons in the conduction band, which reduces the electrical resistance of the sensor device 48,49 .
( ) ( ) ( ) Figure 4a illustrates the relative resistance evaluated from the expression R VAC /R CO in temperature function for thick films, where R VAC is resistance in the vacuum atmosphere and R CO is resistance in the CO atmosphere 50 .Sample synthesized in 8 min showed exponentially higher relative resistance for temperatures >320 °C, having a greater response when compared to sample synthesized in 5 min, with lower R CO values.Relative resistance was obtained at a maximum value at 390 °C for a relative resistance of ~25, this sensor device working temperature value is close to those reported in the literature 28 .Therefore, CO gas causes oxygen desorption on the sample surface, as well as the vacuum atmosphere pointed out by Oliveira et al. 7 , generating less electrons in the 4f state of Ce as well as electrical resistance of the sample.The authors emphasized that this effect depends on the different surfaces, such as the exposed faces, obtained in the nanoparticles according to the synthesis time used.They observed a reduction in the samples obtained with times longer than 8 min of synthesis using a temperature of 180 °C in the MAH method.In addition, López-Mena et al. 3 concluded that less agglomerated particles presented better electrical response.That´s may be the reason why the sample obtained in shorter synthesis time did not show a good result.
Arrhenius Law associated with linear regression (Figure 4b) was used to define the activation energy (Ea) using a vacuum atmosphere to ensure that gas adsorption/ desorption and oxygen diffusion do not occur during heating and cooling processes 51 .Ea values for the samples synthesized for 5 and 8 min were 0.39 eV and 0.30 eV, respectively.Results that corroborate with Figure 4a, where the sample synthesized in 8 min showed better electrical response to gas.Electrical response is due to the contribution of different levels of intermediate energy within the band gap, related to the electron in the 4f orbital of cerium, being the probable cause of the formation of surface defects, such as oxygen vacancies, resulting from changes in morphology of the film nanoparticles.For the sample synthesized in 8 min, surface dominance (111) was observed with the presence of only one oxygen vacancy, which forms more stable holes that reduce the electrical resistance in the CO atmosphere.Whereas the 5 min sample had a surface domain (200) with two oxygen vacancies stabilizing the electron/hole pair 7 .
Once the working temperature (Figure 4c) was determined, we studied the resistance behavior as a time function at 390 °C, for the thick film of the sample synthesized at 8 min.This analysis was performed by alternating the atmosphere from synthetic air (∼20% oxygen) to vacuum and 50 mmHg of CO (99.99%).The response/recovery time was investigated when exposed to CO gas, and the values were 16 s and 2 s, respectively, lower than that reported by Ortega et al. 49 , using a different CeO 2 precursor because of the presence of more oxygen vacancies 52 .Response time is defined as the period of time the material takes to use its resistance by 90% of the initial value when exposed to the target gas.Recovery time, on the other hand, is the time required to recover 90% of initial strength after exposure to the gas.For this material, the electrical conduction mechanism is dominated by the tunneling current, in which defects are created with the presence of oxygen vacancies, which allows, in the 4f states of Ce, an increase in the number of electrons.When an electron goes to the 4f 0 state of Ce due to the process of vacancy formation, we observe a reduction in electrical resistance 7,53 .Therefore, the effect attributed to the morphology of this sample, which formed polyhedral/cubes with exposed faces that had more stable defects, improved the material sensitivity.Figure 5 illustrates the vibrational spectra in the infrared region of CeO 2 powders, in the region from 400 cm -1 to 4000 cm -1 , both samples show bands at the same wavelengths, the differences in spectral configuration can be attributed to higher crystallinity of the sample formed after 8 min.According to group theory, CeO 2 has 2 active modes in the infrared region that are triply degenerate with two cluster-related absorption bands [CeO 8 ].The two bands located at 1500 cm -1 and 1300 cm -1 are asymmetric stretching vibrations of Metal -O bonds 54,55 .On the other hand, the symmetric bending modes corresponding to these connections were verified around 500 cm -1 54,56,57 .The other bands attributed to the metal-oxygen bond were not detected due to the spectral limit of the equipment, since they are located in wavenumbers lower than 400 cm -1 56 .Other absorption bands were found in the spectra of these powders.The bands around 3400 cm -1 are attributed to the stretching vibration of the O -H bonds due to moisture adsorbed on the surface of the sample wafer in KBr and are accompanied by a band at and 1600 cm -1 due to angular deformation of H -O -H [55][56][57][58] .The band around 2300 cm -1 comes from carbon dioxide (CO 2 ) adsorbed on metallic cations, due to the C = O stretching vibration of CO 2 present in the air, as well as those observed in the region at about 1000 cm -1 54,58-60 .The weak bands around 1600, 1500 and 1000 cm -1 are similar to the FT-IR spectra of commercial powders of CeO 2 nanoparticles 54,55 .Thus, it is confirmed that cerium oxide nanocrystals were indeed formed during MAH synthesis.

Conclusions
Crystalline and monophasic CeO 2 nanoparticles with a fluorite-type cubic structure were synthesized by MAH method according to the results of XRD, Rietveld refinement, Raman and TEM analysis.Increasing the synthesis time, particles with greater crystallinity were less agglomerated, and the transformation from a polyhedral morphology in an exposed (110) surface to a cubic morphology in an exposed surface (100) surface was observed.The response time of 16 s at 390 °C, indicated that the defective structure of CeO 2 -based materials provided mechanisms for its application, based on the transient 4f 1 electrons of Ce that can migrate through tunneling/jumping mechanisms because of the redox processes that may occur.

Figure 1 .
Figure 1.a) XRD patterns of the CeO 2 nanoparticles synthesized at 100 °C for 5 and 8 minutes by MAH method; Rietveld refinement; b) synthesis for 5 minutes; c) synthesis for 8 minutes.

Figure 3 .
Figure 3. Raman spectra of CeO 2 nanoparticles synthesized at 100 °C for (a) 5 min and (b) 8 min in the MAH method.

Figure 4 .
Figure 4. Electrical measurements; a) relative resistance vs temperature; b) conductance during cooling (vacuum atmosphere); c) electrical resistance as a function of time in different atmospheres for the CeO 2 sensor films synthesized at 100 °C by the MAH method.

Table 1 .
Lattice parameters and quality indices of the Rietveld refinement of CeO 2 nanoparticles.