Synthesis Characterization of Nanostructured ZnCo 2 O 4 with High Sensitivity to CO Gas

In this work, nanostructured ZnCo 2 O 4 was synthesized via a microwave-assisted colloi- dal method, and its application as gas sensor for the detection of CO was studied. Typical diffraction peaks corresponding to the cubic ZnCo 2 O 4 spinel structure were identified at calcination temperature of 500°C by X-ray powder diffraction. A high degree of poros ity in the surface of the nanostructured powder of ZnCo 2 O 4 was observed by scanning electron microscopy and transmission electron microscopy, faceted nanoparticles with a pockmarked structure were clearly identified. The estimated average particle size was approximately 75 nm. The formation of ZnCo 2 O 4 material was also confirmed by Raman characterization. Pellets fabricated with nanostructured powder of ZnCo 2 O 4 were tested as sensors using CO gas at different concentrations and temperatures. A high sensitiv ity value of 305–300 ppm of CO was measured at 300°C, indicating that nanostructured ZnCo 2 O 4 had a high performance in the detection of CO.


Introduction
Gas sensors technology has numerous applications in the automotive, industrial, domestic, and security sectors. In the automotive and industrial sectors, gas sensors are necessary to detect toxic and harmful gases for environment protection and human health (i.e., carbon monoxide). Sensor materials based on semiconducting metal oxides are one of the several technologies being used in the detection of pollutants [1]. This type of oxide materials is suitable for gas sensor applications due to their interesting structural, functional, physical, and chemical properties. To date, reports indicate that n-type semiconductor materials, such as SnO 2 [2,3], ZnO [4], and TiO 2 [5] are most studied in gas sensing area. By contrast, a limited amount research works on p-type oxide semiconductor gas sensors have been found, the most studied being CuO, Co 3 O 4 , and NiO [6]. However, some sensor parameters such as gas sensitivity and working temperature still need to be improved. Therefore, additional studies are needed to improve the gas sensing characteristics of p-type semiconducting oxides by modifying factors such as synthesis conditions, structure, morphology, and composition.

Synthesis
For the preparation of nanostructured ZnCo 2 O 4 by microwave-assisted colloidal method, first, 0.947 g of Zn(NO 3 ) 2 ⋅xH 2 O (Zinc nitrate hydrate), 2.91 g of Co(NO 3 ) 2 ⋅6H 2 O (cobalt nitrate hexahydrate), and 1 g of C 12 H 27 N (dodecylamine) were dissolved separately in 5 mL of ethanol and kept under stirring for 20 min, at room temperature. Then, the cobalt nitrate solution was added drop wise to the dodecylamine solution under stirring. Then, the zinc nitrate solution was slowly added, producing a wine color solution with pH = 2. This resulting colloidal solution was stirred for approximately 24 h. Then, the solvent evaporation was made by a domestic microwave oven (General Electric JES769WK) operated at low power (~140 W). During the evaporation process, microwave radiation was applied for a period of 1 min, over a period of 3 h. The resulting solid material was dried in air at 200°C for 8 h using a muffle-type furnace (Novatech). Finally, the obtained powders were calcined at 500°C for 5 h. For each thermal treatment, a heating rate of 100°C/h was used. The resulting powders were black. The general synthesis process is illustrated in Figure 1.

Experimental techniques
The structural characterization was performed by XRD using a PANalytical Empyrean system (CuKα, λ = 1.546 Å). The XRD patterns were recorded, at room temperature, in the 2θ range of 10-70° using a step size of 0.02°. The morphological characterization was made by SEM, TEM, high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray (EDS), and high-angle annular dark-field/scanning transmission electron microscopy (HAADF-STEM). For SEM studies, a FEI-Helios Nanolab 600 system operated at 20 kV was used, while a FEI Tecnai-F30 system operated at 300 kV was used for TEM, HRTEM, EDS, and HAADF-STEM analysis. The optical characterization was done by Raman spectroscopy using a Thermo Scientific DXR confocal Raman with a 633 nm excitation source. The Raman spectrum was recorded from 150 to 800 cm −1 , at room temperature, using a Laser power of 5 mW. The gas response (sensitivity) was acquired on pellets of ZnCo 2 O 4 in the presence of several concentrations (1, 5, 50, 100, 200, and 300 ppm) of CO. The sensor devices were fabricated with 0.350 g of the nanostructured powder of ZnCo 2 O 4 , forming pellets with a thickness of 0.5 mm and a diameter of 12 mm. A TM20 Leybold detector was used to control the gas concentration and the partial pressure, and a digital-multimeter (Keithley) was put into use for the measurement of the electric resistance. A general schematic diagram of the gas sensing measurement system is shown in Ref. [33]. The sensitivity was defined as S = R a /R g , where R a is the resistance measured in air and R g is the resistances in gas [21,24].   [38], using the XRD (311) plane, was ~24.7 nm. Since no secondary phase was detected in the XRD pattern of the ZnCo 2 O 4, the synthesis procedure developed in this work allowed to obtain the crystalline phase of ZnCo 2 O 4 without additional diffraction peaks. Thus, this method of synthesis might also be useful for the preparation of other oxide materials. Figure 3 shows the surface morphology of the ZnCo 2 O 4 powder calcined at 500°C with different magnifications. Figure 3a exhibits a SEM image at low magnification, which revealed a surface with a high degree of porosity with pores of irregular shape. The average pore size was calculated around 724 nm. This extensive porosity has been associated with the emission of gas during the removal of organic matter in the calcination process [28]. At high magnification (Figure 3b), a large number of nanoparticles with irregular shape and size in the range of 50-110 nm were clearly observed. In colloidal chemistry, it is known Figure 2. XRD pattern of the ZnCo 2 O 4 powder after a thermal treatment at 500°C in air [37].

Morphological investigations
that the formation of nanoparticles follows a nucleation and a growth mechanism [39]. In this mechanism, first, the nucleation is produced when the concentration of reagents reach the supersaturation limit for a short period of time. Consequently, the formation of large number of crystal nuclei occurs. Later, the process of growth of the particles is developed by diffusion. In our synthesis, the final solution does not present the supersaturation, although the nucleation process could occur when the zinc nitrate solution was added to the cobalt and dodecylamine solution, and the process of growth carried out is during the stirring of the final colloidal solution [31,40]. On the other hand, the dodecylamine plays a key role in the microstructure of ZnCo 2 O 4 particles, since the dodecylamine in the colloidal solution affects the particle growth via the saturating of nanocrystal surfaces and hence, results in the formation of ZnCo 2 O 4 nanoparticles with a peculiar morphology (faceted nanoparticles were obtained) [28]. With thermal treatment at 500°C, the dodecylamine was finally removed from the ZnCo 2 O 4 sample. Figure 4 shows the typical TEM images of the ZnCo 2 O 4 powder calcined at 500°C. Figure 4a exhibits a high concentration of nanoparticles, which was also observed by SEM. As can be seen Figure 4b, faceted nanoparticles with a pockmarked structure were clearly identified. The average particle size was ~75 nm, with a standard deviation of ±12.6 nm. A typical HRTEM image is displayed in Figure 4c. This image confirmed the presence of faceted nanoparticles and a value of 0.286 nm corresponded to the inter-planar d-spacing of the (200) plane of ZnCo 2 O 4 spinel structure.
In order to investigate the nanoparticle's composition, EDS line scan was performed on ZnCo 2 O 4 powder. Figure 5 shows the corresponding analysis. Figure 5a shows a HAADF-STEM image of the ZnCo 2 O 4 nanoparticles. The image confirms the presence of faceted nanoparticles with a pockmarked structure, which is consistent with the TEM images. In the EDS line scan, zinc, cobalt, and oxygen are observed across the linear mapping, confirming the presence of the expected elements, as seen in Figure 5b. In the central region X 2 , a  decrease of the element composition is observed in comparison to point X 1 , which can be due to the irregular surface of the nanoparticle ( pockmarked zone). It is also evident that cobalt exists in larger amount than zinc, corresponding to the target ratio of 1:2. However, the EDS line scan shows carbon (C) and copper (Cu) compounds, which are due to the sample support.

Raman characterization
The Raman spectrum shown in Figure 6 allowed us to confirm the formation of the ZnCo 2 O 4 when a calcination temperature of 500°C was used. As shown in Figure 6, the Raman spectrum of the ZnCo 2 O 4 powder shows five vibrational bands located at 182, 475, 516, 613, and 693 cm −1 corresponding to the five active Raman bands of ZnCo 2 O 4 spinel structure [41]. However, the band at 204 cm −1 is a vibrational mode that could be generated by Co 3 O 4 [42]. The formation of this oxide is due to the cation disorder (substitution of Zn 2+ by Co 2+ ) in  the ZnCo 2 O 4 spinel structure. As the Co 3 O 4 possess a spinel structure same as the ZnCo 2 O 4 ; therefore, they have similar XRD patterns and a deformation in the XRD pattern of ZnCo 2 O 4 is not expected.

Gas sensing application of ZnCo 2 O 4
The sensing performance of the ZnCo 2 O 4 sensor was investigated on pellets fabricated from the nanostructured ZnCo 2 O 4 powders and tested in different concentrations of CO. Figure 7 shows the variation of sensitivity against temperature at different concentrations of CO (1, 5, 50, 100, 200, and 300 ppm). As shown in Figure 7, only minor variations in sensitivity were measured at operating temperatures between 100 and 200°C in whole CO concentration range (1-300 ppm). For operating temperatures above 200°C, the sensitivity increased markedly from 5 to 300 ppm, with the maximum values of the sensitivity registered at 300°C. ppm, respectively. However, at 300°C and with same concentrations, the sensitivity values were 1, 2, 3.3, 84.5, 157.5, and 305, respectively. The observed increase in sensitivity with the concentration is due to increase in gas concentration and operation temperature. The increase of the sensitivity is also associated with increased oxygen desorption at high temperatures [43,44]. Additionally, the ZnCo 2 O 4 sensor showed a decrease in gas response when CO gas were removed from the vacuum chamber.  It is known that the gas sensing mechanism of semiconducting materials is based on the changes of the electrical resistance produced by interaction between the target gas and chemisorbed oxygen ions [45,46]. When oxygen is adsorbed on the semiconductor's surface, oxygen species are generated at the surface by taking electrons from the conduction band of the semiconductor. In general, molecular (O 2¯) and ionic (O − and O 2− ) species are formed below 150°C and above this temperature, respectively [47]. Consequently, a space charge layer with thickness of ~100 nm is formed at the surface [6]. In our tests at temperatures above 100°C, the ionic species that adsorb chemically on the sensor are more reactive than molecular species that adsorb at temperatures below 100°C [30,33,48]. It means that below 100°C, the thermal energy is not enough to produce the desorption reactions of the oxygen and, therefore, an  electrical response does not occur regardless of the gas concentration, as can see in Figure 8a. By contrast, above 100°C (in this case 200 and 300°C), the formation of ionic species at surface of the ZnCo 2 O 4 occurs causing a chemical reaction with the gas and resulting in changes in the electrical resistance of the material (i.e., a high sensitivity is recorded) [48,49]. Additionally, the conductivity mechanism of ZnCo 2 O 4 sensor is strongly related to the crystallite size (D) and the space charge layer (L): if D >> 2L, the conductivity is limited by the Schottky barrier at the particle border; thus, gas detection does not depend on the size of the particle; if D = 2L, the conductivity and the gas sensing depend on the growing of necks formed by crystallites; and when D < 2L, the conductivity depends on the size of the crystallites [2]. In our case, the latter condition occurs while detecting the gases, since the average particle size is less than 100 nm; that is the reason why the conduction of the charge carriers (holes) takes place on the nanoparticles' surface [6,50].

Conclusions
ZnCo 2 O 4 -faceted nanoparticles (~75 nm) were obtained by the simple and inexpensive microwave-assisted colloidal method, using dodecylamine as surfactant. This synthetic method is allowed to obtain the ZnCo 2 O 4 at a calcination temperature of 500°C. The sensing tests showed that ZnCo 2 O 4 sensor is highly sensitive to concentrations of 1-300 ppm of carbon monoxide at working temperatures above 100°C. Specifically, a maximum sensitivity of 305 was obtained for a CO concentration of 300 ppm at a working temperature of 300°C. The CO sensing response of ZnCo 2 O 4 is better than that reported in previous investigations. Therefore, ZnCo 2 O 4 can be considered as a potential candidate for gas sensing applications.