In this work, nanostructured ZnCo2O4 was synthesized via a microwave-assisted colloidal method, and its application as gas sensor for the detection of CO was studied. Typical diffraction peaks corresponding to the cubic ZnCo2O4 spinel structure were identified at calcination temperature of 500°C by X-ray powder diffraction. A high degree of porosity in the surface of the nanostructured powder of ZnCo2O4 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 ZnCo2O4 material was also confirmed by Raman characterization. Pellets fabricated with nanostructured powder of ZnCo2O4 were tested as sensors using CO gas at different concentrations and temperatures. A high sensitivity value of 305–300 ppm of CO was measured at 300°C, indicating that nanostructured ZnCo2O4 had a high performance in the detection of CO.
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 . 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 SnO2 [2, 3], ZnO , and TiO2  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, Co3O4, and NiO . 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.
Among the p-type semiconductors, zinc cobaltite (ZnCo2O4) is a material with spinel-type structure, which has been mainly used as electrode for Li-ion batteries [7–11] and supercapacitors [12–15], due to its higher electrochemical performances and higher conductivities. To date, sensor devices based on nanostructured spinel ZnCo2O4 have particularly exhibited an excellent sensitivity toward liquefied petroleum gas [16–18], ethanol [19, 20], acetone , Cl2 , formaldehyde , and Xylene . Additionally, several references [18–21, 25] reported its poor sensitivity to carbon monoxide (CO). On the other hand, a variety of synthesis methods have been developed on the preparation of ZnCo2O4 such as combustion , thermal decomposition , co-precipitation/digestion , W/O microemulsion , hydrothermal [9, 14], sol-gel , and surfactant-mediated method . Recently, the colloidal route assisted by microwave radiation has provided an efficient and low cost synthesis method to obtain different types of nanostructured materials [28–31]. In this simple synthetic process, the addition of a surfactant agent plays a key role in the material's microstructure because the surfactant's ligands adsorb on the particles' surface inhibiting the particle growth and modifying the particles' microstructure [31–34]. Also, microwave radiation provides a rapid evaporation of the precursor solvent and a short reaction time in comparison with conventional heating [35, 36]. With this in mind, the synthesis of nanostructured ZnCo2O4 was done via a microwave-assisted colloidal method using zinc nitrate, cobalt nitrate, dodecylamine (as surfactant), and ethanol. Consequently, nanostructured ZnCo2O4 powder was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. The potential application of nanostructured ZnCo2O4 as gas sensor was studied by measuring its sensitivity toward different CO concentrations and working temperatures.
2. Synthesis, characterization, and gas sensing application of ZnCo2O4
For the preparation of nanostructured ZnCo2O4 by microwave-assisted colloidal method, first, 0.947 g of Zn(NO3)2⋅xH2O (Zinc nitrate hydrate), 2.91 g of Co(NO3)2⋅6H2O (cobalt nitrate hexahydrate), and 1 g of C12H27N (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.
2.2. 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 ZnCo2O4 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 ZnCo2O4, 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. . The sensitivity was defined as S = Ra/Rg, where Ra is the resistance measured in air and Rg is the resistances in gas [21, 24].
2.3. Structural analysis
Figure 2 shows a typical XRD pattern of the ZnCo2O4 powder obtained at a calcination temperature of 500°C. In this pattern, the main phase detected was the ZnCo2O4 crystal structure, which was identified by the JCPDF #23-1390 file. The sharp and strong peaks indicated a good crystallinity of the ZnCo2O4 sample. The diffraction peaks corresponded well to the (111), (220), (311), (222), (400), (422), (511), and (440) planes of the ZnCo2O4 spinel phase situated at 2θ = 18.95°, 31.21°, 36.80°, 38.48°, 44.73°, 55.57°, 59.28°, and 65.14°, respectively. The average crystal size, which was calculated by Scherrer's formula , using the XRD (311) plane, was ~24.7 nm. Since no secondary phase was detected in the XRD pattern of the ZnCo2O4, the synthesis procedure developed in this work allowed to obtain the crystalline phase of ZnCo2O4 without additional diffraction peaks. Thus, this method of synthesis might also be useful for the preparation of other oxide materials.
2.4. Morphological investigations
Figure 3 shows the surface morphology of the ZnCo2O4 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 . 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 that the formation of nanoparticles follows a nucleation and a growth mechanism . 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 ZnCo2O4 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 ZnCo2O4 nanoparticles with a peculiar morphology (faceted nanoparticles were obtained) . With thermal treatment at 500°C, the dodecylamine was finally removed from the ZnCo2O4 sample.
Figure 4 shows the typical TEM images of the ZnCo2O4 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 ZnCo2O4 spinel structure.
In order to investigate the nanoparticle’s composition, EDS line scan was performed on ZnCo2O4 powder. Figure 5 shows the corresponding analysis. Figure 5a shows a HAADF-STEM image of the ZnCo2O4 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 X2, a decrease of the element composition is observed in comparison to point X1, 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.
2.5. Raman characterization
The Raman spectrum shown in Figure 6 allowed us to confirm the formation of the ZnCo2O4 when a calcination temperature of 500°C was used. As shown in Figure 6, the Raman spectrum of the ZnCo2O4 powder shows five vibrational bands located at 182, 475, 516, 613, and 693 cm−1 corresponding to the five active Raman bands of ZnCo2O4 spinel structure . However, the band at 204 cm−1 is a vibrational mode that could be generated by Co3O4 . The formation of this oxide is due to the cation disorder (substitution of Zn2+ by Co2+) in the ZnCo2O4 spinel structure. As the Co3O4 possess a spinel structure same as the ZnCo2O4; therefore, they have similar XRD patterns and a deformation in the XRD pattern of ZnCo2O4 is not expected.
2.6. Gas sensing application of ZnCo2O4
The sensing performance of the ZnCo2O4 sensor was investigated on pellets fabricated from the nanostructured ZnCo2O4 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.
Figure 8 shows sensitivity of the ZnCo2O4 sensor toward different concentrations of CO at 100, 200, and 300°C. As seen in Figure 8a, a sensitivity value of 1 was maintained across the CO concentration range when the ZnCo2O4 sensor was operated at 100°C. On the contrary, when the ZnCo2O4 sensor was working at a temperature of 200°C (Figure 8a) and 300°C (Figure 8b), the sensitivity increased with an increase of CO concentration. At 200°C, the sensitivity values of the sensor were 1, 1.1, 1.5, 2.2, 2.8, and 3.1 for CO concentrations of 1, 5, 50, 100, 200, and 300 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 ZnCo2O4 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 (O2¯) and ionic (O− and O2−) species are formed below 150°C and above this temperature, respectively . Consequently, a space charge layer with thickness of ~100 nm is formed at the surface . 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 ZnCo2O4 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 ZnCo2O4 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 . 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].
In comparison with previous works, our ZnCo2O4 sensor fabricated on faceted nanoparticles showed a superior sensitivity toward CO (a sensitivity of ~305 in 300 ppm of CO at 300°C) than those sensors based on ZnCo2O4 nanoparticles , hierarchical porous ZnCo2O4 nano/microspheres , ZnCo2O4 nanotubes , porous ZnO/ZnCo2O4 hollow spheres , and nanowires-assembled hierarchical ZnCo2O4 microstructure , with sensitivities of 1 (50 ppm at 350°C), 2 (100 ppm at 175°C), 1.69 (400 ppm at 300°C), 1.1 (100 ppm at 275°C), and 29 (10 ppm at 300°C), respectively.
ZnCo2O4-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 ZnCo2O4 at a calcination temperature of 500°C. The sensing tests showed that ZnCo2O4 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 ZnCo2O4 is better than that reported in previous investigations. Therefore, ZnCo2O4 can be considered as a potential candidate for gas sensing applications.
The authors are grateful to PRODEP for financial support under the project F-PROMEP-39/Rev-04 SEP-23-005. The authors also thank PROFOCIE 2016 for financial support and CONACYT-México grants: the National Laboratory for Nanoscience and Nanotechnology Research (LINAN). The authors are grateful to G. J. Labrada-Delgado, B. A. Rivera-Escoto, K. Gomez, Miguel Ángel Luna-Arias and Sergio Oliva for their technical assistance.