Facile Synthesis, Microstructure, and Gas Sensing Properties of NdCoO3 Nanoparticles

1Nanociencias y Nanotecnologı́a, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, 07360 México City, Mexico 2Departamento de Ingenieŕıa de Proyectos, CUCEI, Universidad de Guadalajara, 44410 Guadalajara, JAL, Mexico 3Facultad de Ciencias, Universidad de Colima, 28045 Colima, COL, Mexico 4Departamento de Quı́mica, CUCEI, Universidad de Guadalajara, 44410 Guadalajara, JAL, Mexico 5Departamento de Ingenieŕıa Eléctrica-SEES, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, 07360 México City, Mexico 6Departamento de Ciencias Computacionales e Ingenieŕıas, CUVALLES, Universidad de Guadalajara, 46600 Ameca, JAL, Mexico 7Departamento de Fı́sica, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, 07360 México City, Mexico


Introduction
Perovskite-type oxides with ABO 3 structure and related materials constitute a group of compounds with a significant technological value because of their interesting physical and chemical properties [1][2][3][4][5][6][7][8].Within this family of compounds, cobaltites, of general formula RECoO 3 (RE = rare earth like La, Nd, Gd, etc.), have been widely studied for their important applications as catalysts, thermoelectric compounds, cathodes for solid-oxide fuel-cells, and gas sensors [9][10][11][12].Several investigations have shown that the presence of cobalt ions, with different oxidation states, plays an important role in the transport properties and catalytic activity of perovskites [13,14].Furthermore, the ability to modify their microstructures, such as porosity, distribution, shape, and particle size can induce changes in their physical properties [15,16].Owing to the nanometric size of the perovskite particles, many applications are potentiated.Nowadays, various synthesis methods are investigated in order to achieve control of the size and shape of the nanoparticles.These methods include sol-gel routes, hydrothermal, solid-state reaction, coprecipitation, spray pyrolysis, solution-combustion processes, and physical vapor deposition [17].In particular, "soft" chemistry methods allow a better control on the stoichiometry and a reduction of the synthesis temperature, which permits obtaining homogeneous nanoparticles [18].
The perovskite-type oxides are of particular interest in the field of gas sensors because they exhibit a high melting point, chemical stability under reducing and oxidizing environments, and a dependency of the partial pressure of oxygen on the concentration of point defects [19].Among the mixed valence perovskites, NdCoO 3 stands out as a strong candidate for gas sensors because it has a high sensitivity to gases, mainly carbon monoxide (CO) [20,21].One reason for the high response in gases is that the oxide crystallizes in a perovskite-type structure, where neodymium ions (Nd 3+ ) are dodecahedrally coordinated, while cobalt has an octahedral coordination but not an integer oxidation state [20].This latter occurs because cobalt can fluctuate between two stable oxidation states producing oxygen-deficient perovskites (RECoO 3− ) with a mixed valence state for cobalt: Co 2+ and Co 3+ , where Co 3+ dominate [22].The equilibration of oxygen in the gas phase with oxygen vacancies in the oxide favors the response of the perovskite as gas sensor [19].However, it is necessary to optimize the performance of this material giving emphasis to its microstructure in order to achieve a functional behavior as gas sensor.In particular, the development of sensors for the detection of carbon monoxide (CO) and gaseous hydrocarbons requires special attention because of their potential hazards [23].CO is one of the most abundant toxic gases and is characterized by being odorless and colorless.Prolonged exposure to this gas can cause damage to the human body and even death, while gaseous hydrocarbons are usually flammable gases with no odor or color.In this regard, one of the objectives of this work was to synthesize NdCoO 3 nanoparticles by a simple method using relatively low calcination temperatures.Another objective was to measure the response of the material's sensitivity in CO and propane, several concentrations of them, and operating temperatures.

Experimental Procedures
2.1.NdCoO 3 Synthesis.NdCoO 3 nanoparticles with perovskite structure were synthesized by a solution method assisted by microwave radiation.0.004 mole of Nd(NO 3 ) 3 ⋅6H 2 O (Aldrich, 99.9%), 0.004 mole of Co(NO 3 ) 2 ⋅6H 2 O (Jalmek, 98.6%), and 0.030 mole of ethylenediamine (Sigma, ≥99.5%) were separately dissolved in distilled water.The cobalt nitrate and ethylenediamine solutions were mixed slowly and the mixture was later stirred for 20 min at 375 rpm.The neodymium nitrate solution was slowly added to the mixture and the stirring continued for 24 h at room temperature.Evaporation of the solvent was carried out using noncontinuous microwave radiation at 290 W using a domestic microwave oven (LG, model MS1147).The microwave radiation was applied in steps of 60 to 90 s to maintain the suspension's temperature below 90 ∘ C. Control of temperature and exposure times was needed to prevent loss of material.The paste produced from evaporation was dried at 200 ∘ C for 8 h in a normal atmosphere, and the obtained powders were divided into three parts and they were calcined at 500, 600, and 700 ∘ C, respectively.For this, a heating rate of 100 ∘ C/h was employed with a residence time of 5 h.The heat treatment was performed in a programmable oven (Vulcan6) temperature control.NdCoO 3 Powders.All the calcined powders were analyzed by X-ray powder diffraction (XRD) at room temperature using a Rigaku SmartLab diffractometer, with CuK radiation and nickel filter.The 2 scanning range was from 20 to 70 ∘ , with steps of 0.02 ∘ and a duration of 1 s per step.The material's microstructure was analyzed by scanning electron microscopy (SEM, JEOL JSM-6390LV) in high vacuum mode using the secondary electron (SE) emission.The surface chemical composition was analyzed by energy dispersive X-ray spectroscopy (EDS-SEM) with a Bruker 5010 XFlash detector.To observe the nanoparticles, a transmission electron microscope (TEM, JEOL JEM-2010) with an accelerating voltage of 200 kV was employed.For this, a representative sample of the powder was used, which was dispersed in isopropyl alcohol for 5 min and placed on a copper grid.The surface area of the powders was assessed by nitrogen physisorption at 77 K using a Minisorp II BEL Japan equipment.Prior to the nitrogen adsorption process, the chamber with the sample was degassed and maintained under vacuum for 24 h at room temperature.The powder topology was analyzed by atomic force microscopy (AFM) using a JSPM-5200 microscope from JEOL under intermittent contact operations under normal pressure conditions.

Gas Sensitivity Tests.
The sensitivity tests of the NdCoO 3 were performed in atmospheres of carbon monoxide (CO) and propane (C 3 H 8 ).The electrical resistance of NdCoO 3 pellets with a diameter of 12 mm and a thickness of 500 m was measured.The pellets were made with 0.3 g of NdCoO 3 powders at a pressure of 10 tons during 30 min using a Simplex Ital Equip-25 tons pressing machine.Two ohmic contacts were placed on the surface of the pellets using colloidal silver paint (Alfa Aesar, >99%).The pellets were placed into a measuring chamber with a vacuum capacity of 10 −3 torr.The gas concentration inside the chamber was electronically monitored by a Leybold (model TM20) vacuum gauge.The fed gas concentrations were 0, 5, 50, 100, 200, and 300 ppm.The operating temperature was electronically controlled using a type J thermocouple with four fixed values: 25, 100, 200, and 300 ∘ C. The electrical resistance was recorded using a Keithley 2001 digital multimeter.The sensitivity (S) was calculated by the relative difference in electrical conductances using the expression  = (  −  0 )/ 0 , where   and  0 denote electrical conductances in the sampled gas (CO or C 3 H 8 ) and air, respectively.Figure 1 shows a diagram of the system used for the material's characterization in the gases.

Results and Discussion
3.1.XRD Analysis.Figure 2 shows the X-ray diffraction patterns of the calcined powders at 200, 500, 600, and 700 ∘ C. From the spectra array it can be observed that paste calcined at 200 ∘ C is amorphous, since the diffractogram does not show defined peaks.The spectra of powders calcined at 500, 600, and 700 ∘ C present the diffraction peaks corresponding to the NdCoO 3 phase with perovskite structure.These peaks were observed at the 2 angular positions 23.6 ∘ , 26.5 ∘ , 33.6 ∘ , 35.8 ∘ , 41.5 ∘ , 48.3 ∘ , 49.8 ∘ , 54.3 ∘ , 55.8 ∘ , and 60.0 ∘ , which are indexed according to the reference from The International Centre for Diffraction Data, ICDD, 00-052-1279-PDF as the following crystal planes: (020), ( 111), ( 121), ( 210), ( 220), ( 202), ( 212), ( 103), (311), and (123), respectively.The NdCoO 3 crystallized in an orthorhombic system with space group Pnma (62) and cell parameters  = 5.33 Å,  = 7.52 Å, and  = 5.34 Å.At higher calcination temperatures, the peaks in the spectra are of higher amplitude, better defined, and with a relative low level of noise, all of which are associated with a better crystallinity, especially the sample obtained at 700 ∘ C. We found in this work that, using the solution method, NdCoO 3 powders can be synthesized at a relative low temperature of 700 ∘ C, which is lower than those used in others methods to produce these compounds.For example, using the sol-gel method (in the presence of urea), NdCoO 3 was obtained using a sintering temperature of 1100 ∘ C [20]; the compound series ACoO 3 has been synthesized by calcining at 1100 ∘ C per 3 h (for A = Pr, Nd, Sm, and Gd) [24], and at 700 ∘ C per 5 h followed by a sintering step at 1000 ∘ C per 8 h (for A = Pr, Nd, Tb, and Dy) [25]; through the conventional solid-state reaction method, NdCoO 3 , Nd 0.8 Ca 0.2 CoO 3 , and Nd 0.8 Sr 0.2 CoO 3 have been synthesized at the temperatures 900-1050 ∘ C with calcination times of 24 h [26]; NdCoO 3 has been also synthesized through calcination at 900 ∘ C from a precursor reactive paste [27].Table 1 shows a summary of the methods commonly employed to synthesize this perovskite.After reviewing literature, the method used in this work can be considered as a simple and efficient route to synthesize NdCoO 3 .

SEM Analysis and N 2 Adsorption. The following results
were obtained for the NdCoO 3 powders obtained at 700 ∘ C. Figures 3(a)-3(d) show typical SEM micrographs of the NdCoO 3 powders.The images were taken at these magnifications: (a) 500x, (b) 2000x, (c) 5000x, and (d) 10000x (inset: 30000x).It can be seen in Figures 3(a) and 3(b) that the oxide's surface has a lot of different sized pores (in a range 0.5 to 25 m), which confers it a sponge-like morphology.In Figures 3(c) and 3(d), at higher magnification, it can be clearly observed an agglomeration of particles with high connectivity between them, forming a network-like structure.In addition, smaller pores can be observed (with a size of about 0.2 m).This porosity is also attributable to the gases released during thermal decomposition of organic matter, mainly water vapor, NO  , and CO 2 [28].Figure 4 shows a typical EDS-SEM spectrum obtained from the NdCoO 3 powders.In this spectrum, the characteristic Nd, Co, and O peaks, according to the material's chemical composition, can be identified.For neodymium (Nd), the peak of greatest intensity in the spectrum corresponds to the characteristic line L, whose energy is 5.22 keV.For cobalt (Co), three peaks located at 0.77, 6.92, and 7.65 keV correspond to the characteristic L, K, and K lines, respectively.For oxygen (O), the corresponding line is K, whose energy is 0.53 keV.The peak located at 0.28 KeV denotes the presence of carbon (C), which is a residue of the thermal decomposition of organic matter, this being one of the disadvantages of using organic reagents in the preparation of inorganic materials [29].In addition, the EDS-SEM elemental composition (Nd : Co : O at. % 20.72 : 21.13 : 58.15) corresponds nearly to the theoretical atomic composition of the NdCoO 3 (Nd : Co : O at. % 20 : 20: 60).The slight deviation can be attributed to traces of metal oxides remaining in the sample.Figure 5 shows the N 2 adsorption isotherm of the NdCoO 3 powders at 77 K and the corresponding BET plot (inset).The shape of the isotherm was characterized as type II according to IUPAC's classification; this isotherm, which shows a weak hysteresis, is typically obtained in the case of macroporous solids.The specific surface area was of 7.52 m 2 /g.In general, these oxides exhibit relatively low surface areas: in the range of 3 to 14 m 2 /g [30][31][32].Our result agrees well in this range.

TEM Analysis. Figures 6(a)-6(e)
show TEM images of the NdCoO 3 synthesized at 700 ∘ C and the particle size distribution.From Figure 6(a) it is clear that the NdCoO 3 powders are comprised of nanometer sized particles, which possess an irregular shape and are connected to each other, in agreement with the SEM analysis (Figure 3).According to the analysis of multiple TEM images, where the particles were clearly identifiable, it was estimated that their size is in the range of 17 to 151 nm; about 80% of the particles are within the range of 17 to 100 nm; their average size is 66 nm with a standard deviation of 37 nm (Figure 6(b)).Figures 6(c) and 6(d) show images obtained by high-resolution transmission electron microscopy (HRTEM), where fringes can be seen on the material's crystal lattice.These images reveal that the interplanar distances are about 2.6 and 3.7 Å (inset in Figures 6(c  In some studies, chelating and structure directing agents are employed for controlling the size and shape of the particles.In particular, ethylenediamine is a known chelating agent, which reacts with transition metals yielding coordination compounds [29].This has been taken advantage of as a strategy for the anisotropic growth of II-VI compound semiconductors such as ZnS, ZnSe, and CdS [33,34].In those studies, the ethylenediamine forms complexes with transition metals, which connect each other, and 1D structures like nanowires and nanotubes are obtained through a condensation reaction [34].Recently, there have been synthesized mixed oxides with trirutile-type structure by "soft" chemistry methods in the presence of ethylenediamine, for example: MgSb 2 O 6 nanorods [35], nanostructured ZnSb 2 O 6 microbars [36], and interconnected (through neck-like structures) mesoporous CoSb 2 O 6 nanoparticles [37].We found by using our proposed method that NdCoO 3 nanoparticles can be processed with sponge-like morphology to form long chains due to the connectivity between them.The microstructure of these materials is influenced by the amine, which is incorporated and contributes to form inorganic network, acting as a template.Finally subsequently yielding nanocrystals can be tailored in porous adsorption configuration [34].These morphologies follow the crystallization principles described by Lamer and Dinegar [30,38].7(a)-7(c) present the global topography from NdCoO 3 powders obtained at 700 ∘ C by tapping mode AFM.The surface is formed by agglomerates of nanoparticles, which are able to leave spaces in the nanoparticles neighborhood.The scanned area indicates a RMS mean roughness of approximately 1.76 nm.The estimated particle size is about 30 to 150 nm.These results are consistent with those obtained from TEM analysis (Figure 6).

Gas Sensitivity.
Figure 8 shows the sensitivity graph of NdCoO 3 depending on the (a) CO concentration and (b) the operation temperature.At room temperature (25 ∘ C), no significant changes were recorded in the electrical resistance of the material, so the sensitivity was zero for all gas concentrations.However, variations at the temperatures 100, 200, and 300 ∘ C were detected.Inset in Figure 8  the changes in the electrical resistance of the material at different temperatures in the presence of CO.In particular, at 100 ∘ C, the calculated sensitivity values were 0, 0.01, 0.04, 0.08, 0.13, and 0.20 in the presence of 0, 5, 50, 100, 200, and 300 ppm of CO, respectively; at 200 ∘ C, the magnitude of the sensitivity rose to 0, 0.02, 0.07, 0.21, 0.33, and 0.52, respectively, under the same gas concentrations; at 300 ∘ C the sensitivity increased to the values of 0, 0.03, 0.10, 0.30, 1.3, and 7.2 with the same CO concentrations.Table 2 summarizes the sensitivity variations in CO at 200 and 300 ∘ C. The most accepted mechanism to explain this phenomenon is based on the band bending caused by oxygen adsorption [39].In this model, the oxygen adsorbed on the surface of the semiconductor captures electrons from the solid, forming the chemisorbed species O − and O 2− , decreasing the number of the charge carriers [40,41].The magnitude of the sensitivity clearly rises with the increase in the gas concentration.This is because the number of CO molecules that react with the chemisorbed oxygen increases, donating electrons to the material's surface [41].At higher temperatures (300 ∘ C in this case), the more reactive oxygen species (O − ) predominate; therefore, their formation increases the gas-solid interactions in the presence of CO, causing a significant increase in the response.The highest measured sensitivity was 7.2 at 300 ppm of CO and at a temperature of 300 ∘ C (Figure 8(a), inset).Figure 9 shows the sensitivity of NdCoO 3 as a function of (a) propane concentration and (b) operation temperature.A similar behavior was previously observed (in CO).Inset in Figure 9(b) shows the changes in the electrical resistance of the material at different temperatures in the presence of propane.The perovskite is sensitive to variations in the concentration of propane, obtaining the values of 0, 0, 0.01, 0.02, 0.03, and 0.04 at concentrations of 0, 5, 50, 100, 200, and 300 ppm at 100 ∘ C; at 200 ∘ C, the sensitivity increased to 0, 0.003, 0.01, 0.03, 0.05, and 0.07, respectively, at the same gas concentrations; at 300 ∘ C the values rose to 0, 0.04, 0.15, 0.63, 5.4, and 94.1, respectively, at the same propane concentrations.Again, the highest sensitivity (94.1) was found at a temperature of 300 ∘ C at 300 ppm of the gas (Figure 9(a), inset).Table 3 summarizes the sensitivity variations in propane at 200 and 300 ∘ C. The propane detection mechanism has not been deeply studied so far.It has been suggested that the propane molecules react with the chemisorbed O − oxygen species producing CO 2 , water vapor, and a release of electrons to the material's surface resulting in variations in its electrical resistance and therefore in the sensitivity [42].Furthermore, the sensitivity of the material decreased when the test gases were removed from the chamber, meaning that the pellets possess a high performance for different gas concentrations at different working temperatures.
The gas sensitivity results were compared with similar metal oxides finding that we have succeeded in obtaining a   [37].A sensitivity of ∼17 (CO) and ∼31 (propane) was reached for LaFeO 3 nanoparticles at 350 ∘ C and a gas concentration of 200 and 300 ppm, respectively [43].On the other hand, undoped SnO 2 and ZnO thin films showed sensitivities of 0.7 and 2.5, respectively, at 300 ∘ C and a propane concentration of 300 ppm [44,45].The good response of the NdCoO 3 toward gases can be attributed to the microstructure obtained during the synthesis process.It should be mentioned that among the different type of gas sensors (e.g., catalytic combustion, electrochemical, infrared absorption, and thermal conductive), conductometric sensors based on metal oxides have many advantages, such as ease of use, rapid response time, low detection limits, and high sensitivity (verified during this work) [46].

Conclusions
In summary, NdCoO 3 perovskite-type nanoparticles were successfully synthesized by the solution method assisted by microwave radiation.This method is a sustainable and alternative route to synthesize NdCoO 3 at a relatively low temperature compared with those used by other synthesis methods, mainly solid-state reactions.The characterization was conducted with X-ray diffraction, scanning and transmission electron microscopy, atomic force microscopy, energy dispersive X-ray spectroscopy, and N 2 adsorption.The nanoparticles, with a high connectivity between them, formed a porous structure sensitive to changes in the concentration of carbon monoxide and propane at different operating temperatures.The sensitivity of the material rose with the increasing of the temperature and the gas concentration.The maximum values of sensitivity were found at 300 ∘ C and 300 ppm for both gases.For carbon monoxide, the maximum sensitivity was found of about 7, while for propane the sensitivity markedly increased to a value of 94.In these terms, the perovskite-type NdCoO 3 is a strong candidate to be used as a gas sensor.

Figure 1 :Figure 2 :
Figure1: Schematic of the system used to measure the electrical response in a controlled atmosphere and temperature environment.
) and 6(d)), corresponding to planes (121) and (020), respectively, similar to the maximum diffraction angle of ∼33.6 and ∼23.6 ∘ (2) obtained by XRD.Figure6(e)shows an electron diffraction pattern made on a number of nanoparticles.This result confirmed the crystallinity of the powders previously analyzed by XRD and verified the local crystallinity of NdCoO 3 sample.

Table 1 :
Comparison of the main synthesis methods for NdCoO 3 .

Table 2 :
Sensitivity (S) values of NdCoO 3 in CO atmospheres at 200 and 300 ∘ C.