Sensing Properties of Impedancemetric Solid-Electrolyte NO x Sensor Using Perovskite-Type Lanthanum Manganite-Based Receptor

Solid-state impedancemetric NO x sensor devices composed of the perovskite-type oxide (La 0.8 A 0.2 MnO 3 ; A = La, Na, K) as a receptor material and a Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 (LATP) disc as a solid-electrolyte transducer were investigated for the detection of NO x (NO and NO 2 ) in the range of 100–500 ppm at 400°C. The lanthanum manganite/LATP-based sensor devices showed NO x sensing responses that could be divided into two components of resistance and capacitance. While the sensitivities to NO x were largely affected by the type of receptor material used, the La 0.8 K 0.2 MnO 3 /LATP-based impedancemetric sensor element showed excellent NO sensing properties. FT-IR measurements revealed that the interaction between NO and La 0.8 K 0.2 MnO 3 was the strongest among the receptor materials.


Introduction
Nitrogen oxides (NO x ; NO, NO 2 ) of exhausts from automobiles and industrial combustible factories are becoming the cause of atmospheric pollutants, such as photochemical smog, acid rain, and so on. Therefore, monitoring the exact amount of nitrogen oxides has become very important for the protection of the global environment in recent years.
We have proposed a new type of solid-state simple NO x sensor combined with a solid-electrolyte as an impedance transducer and an oxide as a receptor. (15)(16)(17) The impedancemetric sensor offers an advantage in its single sensor structure, in which there is no need for reference and/or counter electrodes at the opposite side of the sensing electrode. The impedancemetric solid-state sensor using a solid-electrolyte transducer attached with an oxide layer as a gas sensing receptor showed relatively good NO x sensing properties; however, the impedance changes as the response intensity were as low as several percent for practical applications. (17) In this study, we have attempted to improve the receptor function using cation-doped lanthanum manganite-based perovskite-type oxide, that is, La 0.8 A 0.2 MnO 3 (A = La, Na, K) nanopowders prepared by a polymer precursor method. LaMnO 3 -based oxides have been chosen as the receptor because of its relatively good NO x sensing properties among the perovskite-type oxides (LaBO 3 ; B = Cr, Mn, Fe, Co, Ni). (17) It was revealed that the use of potassium-doped lanthanum manganite (La 0.8 K 0.2 MnO 3 ) as a receptor resulted in about fivefold higher response intensity than that of the nondoped LaMnO 3 . Here, we report the NO x sensing performance and the effect of the receptor material on the impedancemetric solid-electrolyte NO x sensor.

Synthesis of sensor materials
Perovskite-type oxide La 0.8 A 0.2 MnO 3 (A = La, Na, K) nanopowders were prepared by a polymer precursor method. (19)(20)(21) Metal (La, Na, K, Mn) nitrates were dissolved in 30 mL of ethylene glycol (EG), which was then mixed with 40 mmol of acetyl acetone (AcAc) and 7 wt% polyvinylpyrolidone (PVP). The solution was treated in an ultrasonic bath for 30 min, then dried at 85°C. The obtained metal-complex precursor was precalcined at 350°C for 7 h, and finally calcined at 650°C for 2 h.

Fabrication of sensor devices
The sensor structure consisted of an LATP disc, a perovskite-type oxide layer, and Au electrodes at the opposite surface of the disc as shown in Fig. 1. The Au electrodes were tightly coated with an inorganic adhesive (Aron Ceramic, TOAGOSEI). A paste prepared with receptor powder, turpentine oil, and PVP was painted onto the surface of the LATP disc, and dried and sintered at 500°C for 2 h. NO x sensing experiments were carried out in a conventional flow apparatus equipped with a heating facility at 300-500°C. Sample gases containing NO or NO 2 were prepared from each parent gas, i.e., NO diluted with nitrogen or NO 2 diluted with a dry synthetic air (N 2 + O 2 gas mixture), by mixing it with N 2 (99.999 vol%) or air at a total flow rate of 100 mL/min. The sensor responses (resistance and capacitance) were measured with an impedance analyzer (LCR 3532-50, HIOKI) between 50 Hz and 5 MHz. The value of the relative sensitivity (S R or S C ) was defined as S R = (R gas − R air ) / R air × 100 (%) or S C = (C gas − C air ) / C air × 100 (%), in which R gas (C gas ) or R air (C air ) is resistance (capacitance) in gas or air, respectively.

Preparation of sensing material.
XRD patterns of the La 0.8 A 0.2 MnO 3 (A = La, Na, K) perovskite-type oxides prepared by the polymer precursor method are shown in Fig. 2(a). They showed well-crystallized and almost single-phase orthorhombic-type perovskite-type oxides. Figure 2(b) shows  Table  1 summarizes the lattice parameters and BET surface areas of these La 0.8 A 0.2 MnO 3 (A = La, Na, K) oxides. The distance at the (111) plane (d 111 ) of La 0.8 Na 0.2 MnO 3 and La 0.8 K 0.2 MnO 3 decreased and increased, respectively, compared with that of LaMnO 3 , which corresponds to an increase and a decrease in the value of 2θ angles. The lattice volume of La 0.8 Na 0.2 MnO 3 was smaller than that of LaMnO 3 , which indicates that the cation exchange in the crystal was caused by the difference in the ion radii of Na + (0.97 Å) and La 3+ (1.14 Å). On the other hand, the lattice parameter of b in La 0.8 K 0.2 MnO 3 was expanded by introducing K + (1.33 Å) ion, which has a larger ion radius than La 3+ (1.14 Å). On the other hand, the lattice parameters of a and c in La 0.8 K 0.2 MnO 3 were shrunk and the lattice volume of La 0.8 K 0.2 MnO 3 was not changed much from that of LaMnO 3 . It was also found that the orthorhombic distortion of LaMnO 3 was eased to the cubic phase by K + doping in LaMnO 3 , as shown in Table 1. The specific surface areas of the La 0.8 A 0.2 MnO 3 (A = La, Na, K) oxides prepared by the polymer precursor method were 16.2, 14.1, and 10.8 m 2 /g, respectively, as also shown in Table 1.

Sensing behavior of impedance-metric NO x sensors
The NO and NO 2 response properties of the impedancemetric sensor using nondoped LaMnO 3 receptor and LATP transducer were firstly examined. Figure 3 shows Nyquist's plots of the LaMnO 3 /LATP sensor at a frequency range from 50 Hz to 5 MHz in air, 100 ppm NO, and 100 ppm NO 2 at 400°C. This sensor showed the impedance change by changing the NO gas from air to NO (or NO 2 ) especially at the lower frequency region of the interface impedance area. However, the noise in the sensor response was increased at lower frequency area, so we investigated the response characteristic of the sensor at the fixed frequency of 100 Hz as a sensing signal. The impedance response of the LaMnO 3 / LATP sensor could be divided into two components: resistance (R) and capacitance (C).   The response sensitivities of resistance (S R ) and capacitance (S C ) of various sensor elements for both NO and NO 2 are summarized in Fig. 7. The La 0.8 A 0.2 MnO 3 (A = Na, K)/LATP sensors showed higher response sensitivity to both NO and NO 2 at 400°C than that of using LaMnO 3 oxide, in spite of the low specific surface areas of La 0.8 A 0.2 MnO 3 (A = Na, K) oxides. Figure 8 shows response sensitivities of the La 0.8 K 0.2 MnO 3 /LATP    sensor to NO, NO 2 , N 2 O, and CO 2 at 400°C, 100 Hz. The La 0.8 K 0.2 MnO 3 /LATP sensor showed the highest sensitivity of the resistance component among all the sensors examined for NO, although no remarkable difference in capacitance of the sensors was observed. Furthermore, the sensor showed no response or a small response to N 2 O and CO 2 , respectively.
A large difference was found in the sensor responses to NO at 400°C, as stated above. NO absorption properties on La 0.8 A 0.2 MnO 3 (A = La, Na, K) surfaces were characterized by FT-IR spectrometry. Figure 9 shows FT-IR spectra of the oxides before and after treatment with 100 ppm NO at 400°C for 30 min. The starting adsorption peaks at around 600 cm −1 of all the oxides could be assigned as ν (Mn-O) in the perovskitetype oxides. After the NO treatment, adsorption peaks at 1385 cm −1 , which could be assigned as ν (NO 3 − ), were observed. Adsorption peaks of NO 3 − could be produced by the reaction between NO and adsorbed oxygen. The strongest ν (NO 3 − ) peak at 1385 cm -1 was detected on La 0.8 K 0.2 MnO 3 . (24)(25)(26) This should be one of the reasons for the high NO sensitivity of the La 0.8 K 0.2 MnO 3 /LATP sensor. Adsorption peaks at 1600 and 2400 cm −1 could be noticed in the region of ν (C=O) and ν (CO 2 ), which could come from atmospheric carbon dioxide. (26,27) However, the sensing mechanism of the present electrochemical device still needs further investigation.
The sensor mechanism of this new type of solid-electrolyte impedancemetric is shown in Fig. 10. As the p-type oxides were used as the receptor, electrons trapped by adsorbed oxygen were flowed into the receptor when the adsorbed oxygen was consumed with NO or NO 2 . Consequently, the amount of carrier holes in the receptor was decreased. Then, Li + in the solid electrolyte moved more freely because of the electrical force from the holes in the receptor. The resistance of the solid electrolyte should be increased by the expansion of the distance of mobile Li + , and the capacitance of the solid electrolyte was also increased with increasing amount of mobile Li + . As the interface between the receptor and transducer seems to play an important role in the sensing mechanism, the design of the optimum sensor structure in the receptor and transducer layers should improve the sensor performance. Further investigations on the sensing mechanism and sensor structure are now in progress.

Conclusions
An impedancemetric sensor has been designed using perovskite-type (La 0.8 A 0.2 MnO 3 ; A = La, Na, K) oxide receptors and Li + -ionic conductor (Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 ; LATP) transducer. The impedance responses of the NO x sensors could be obtained as components of resistance and capacitance. Moreover, they showed dependence on each NO x concentration and selectivity of NO and NO 2 . The sensitivities of these sensors were affected by the ability of receptor materials to detect the gas at different NO x concentrations and the La 0.8 K 0.2 MnO 3 receptor gave the highest sensitivity among the sensors.