Carbon monoxide (CO) is colorless, odorless and yet highly toxic gas; it can attack hemoglobin from the blood, preventing the supply of needed oxygen to different parts of human body . Sensors for detecting CO gas have been widely used in many areas, for example, the control of industrial wastes and vehicle emissions, the monitoring of indoor atmosphere and coal mine explosion .
Several substituted rare-earth-transition-metal oxides of the perovskite structure (ABO3) have been used as CO sensors. Among these oxides, LaCoO3 and LaFeO3 exhibit excellent CO sensing performance. In our previous work, the characteristics and sensing properties of the LaCo1-xFexO3 system for CO gas sensors have been studied and the LaCo0.3Fe0.7O3 one demonstrated maximal response.
In addition, NdMO3 (M = Co, Fe, etc.) exhibits good ability in CO sensing or CO catalytic oxidation. Among them, NdCoO3 is an excellent sensing material with high sensitivity towards the CO [3-4]. It has been studied for CO sensing and exhibits excellent performances; for example, NdCoO3 thin film showed a good response (about 15%) for CO concentration until 0.1% and the optimal working temperature was found to be around 300 ∘C . On the other hand, NdFeO3 was reported to have good gas-sensing properties for CO. For examples, the NdFeO3 sensor showed a response of 1215% to 0.03% CO gas at 170∘C . In addition, some mixed solution compounds, such as NdFe1−x CoxO3  and La1−x NdxFeO3 , have been prepared and their structural, electrical, and gas sensing properties have been investigated.
Based on the above analyses, La1-xNdxCo0.3Fe0.7O3 could be very promising for the CO sensing; there must be an optimal x value for obtaining the best CO sensing performance. In this work, we prepared La1-xNdxCo0.3Fe0.7O3 nanoparticles by co-precipitation and investigated the CO sensing properties of the La1-xNdxCo0.3Fe0.7O3 system, with an aim to exploit the optimal chemical composition for the best CO sensing performances. It is discovered that the La0.7Nd0.3Co0.3Fe0.7O3 sensor shows the best CO sensing performances.
A chemical co-precipitation method was used to prepare La1-xNdxCo0.3Fe0.7O3 nanoparticles (x = 0÷1.0 with increment 0.1). The stoichiometric amounts of corresponding metal nitrates were dissolved in deionized water. H2O2 was then added to oxidize Co(II) to Co(III). Afterwards, NaOH solution was added to adjust the pH value to 11∼12. The resulted precipitate was rinsed with deionized water until pH=7, and dried at 80∘C for 2 h. Finally, La1-xNdxCo0.3Fe0.7O3 nanoparticles were obtained by calcination at 650∘C for 6 h in air.
The gas sensors were fabricated by dipping. The La1-xNdxCo0.3Fe0.7O3 nanoparticles were dispersed in ethanol ultrasonically. Then, the suspension was dripped on the FTO substrate, on which a gap of about 60 ∘m was cut by laser. Afterwards, the samples were dried naturally to remove ethanol. Platinum wire was connected to the FTO using Ag paste.
The crystal structure of the La1-xNdxCo0.3Fe0.7O3 powder was checked by X-ray diffraction (XRD) (SHIMADZU XRD-7000S) in the 2θ range of 10-90∘. The microstructure of La1-xNdxCo0.3Fe0.7O3 sensors was analyzed using scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectroscope (EDS). The sensor resistance was collected automatically every second using a Keithley 2450 Source Measurement Unit (SMU). A flow system comprising two mass flow controllers was used to introduce gases with specified concentrations of CO in N2 into the sample chamber at a flow rate of 500 standard centi-cubic per minute (SCCM).
3.1. Phase determination and microstructure
Fig. 1 shows the XRD patterns of the La1-xNdxCo0.3Fe0.7O3 nanoparticles prepared by co-precipitation; all of them are single phase. The corresponding spectra of rhombohedral LaCoO3 (orange line), orthorhombic LaFeO3 (violet line), NdCoO3 (red line), and NdFeO3 (blue line) are reported for comparison. When x = 0, the LaCo0.3Fe0.7O3 nanoparticles have the transient phases between LaCoO3 and LaFeO3. When x = 1, the NdCo0.3Fe0.7O3 nanoparticles have the transient phases between NdCoO3 and NdFeO3. In addition, the angle of the Bragg reflections of the La1-xNdxCo0.3Fe0.7O3 nanoparticles increases with an increase of Nd content x. This is because the radius of the Nd3 + (98 pm) ion is smaller than that of La3+ (103 pm), when the La3+ is replaced by Nd3 + at A site, the lattice parameters should become smaller with increase of the Nd concentration. According to the Bragg formalum, the diffraction angle becomes larger.
Typical SEM micrographs of the La0.7Nd0.3Co0.3Fe0.7O3 sensor are shown in Fig. 2. From the Fig. 2a, we can see that the sensing layer is highly porous and the original morphology of particles is well maintained, which is preferred for gas-sensing applications. A clear boundary exists between the La0.7Nd0.3Co0.3Fe0.7O3 layer and the FTO substrate, as indicated in Fig. 2g. Good adhesion between the sensing layer and the substrate is also obvious. The representative EDS elemental mappings of the La0.7Nd0.3Co0.3Fe0.7O3 sensor are illustrated in Figs. 2c-g suggesting that the elements La, Nd, Co, and Fe show the homogeneous distribution throughout the sensor.
3.2. Sensing Properties to CO
The resistance of La1-xNdxCo0.3Fe0.7O3 sensors (x = 0÷1.0 with increment 0.1) were tested under alternating cycles of 100 ppm CO and N2 at 250∘C; the resistance curve of the La0.7Nd0.3Co0.3Fe0.7O3 sensor under alternating cycles of 100 ppm CO and N2 at 250∘C is shown in Fig. 3. When 100 ppm CO was introduced, the sensor resistance increased, and a resistance decrease was observed when CO was cut off. A good repeatability was achieved among individual cycles. The sensor response is defined as: S = R C O ">R C O R N 2 R N 2 , here R C O is the sensor resistance in the presence of CO, and R N 2 is the resistance in N2. It can be calculated from the resistance curve.
Fig. 4 shows the response of the La1-xNdxCo0.3Fe0.7O3 sensors as a function of x at 250∘C. As one can see, the responses of the La1-xNdxCo0.3Fe0.7O3 sensors fluctuate with the change of x. Among them, when x = 0.3, the La0.7Nd0.3Co0.3Fe0.7O3 sensor displays an excellent response (S = 52.8).
The CO sensing properties of La1-xNdxCo0.3Fe0.7O3 were systematically investigated as a function of Nd concentration. Among them, the La0.7Nd0.3Co0.3Fe0.7O3 sensor showed excellent CO sensing performance. Therefore, La0.7Nd0.3Co0.3Fe0.7O3 is a very promising material for the CO sensors.