Gas Sensing Properties of Epitaxial LaBaCo2O5.5+δ Thin Films

Chemical reactivity and stability of highly epitaxial mixed-conductive LaBaCo2O5.5+δ (LBCO) thin films on (001) LaAlO3 (LAO) single-crystalline substrates, fabricated by using pulsed laser deposition system, were systematically investigated. Microstructure studies from x-ray diffraction indicate that the films are c-axis oriented with the interface relationship of [100]LBCO//[100]LAO and (001)LBCO//(001)LAO. LBCO thin films can detect the ethanol vapor concentration as low as 10ppm and the response of LBCO thin film to various ethanol vapor concentrations is very reliable and reproducible with the switch between air and ethanol vapor. Moreover, the fast response of the LBCO thin film, as the p-type gas sensor, is better than some n-type oxide semiconductor thin films and comparable with some nanorods and nanowires. These findings indicate that the LBCO thin films have great potential for the development of gas sensors in reducing/oxidizing environments.

Scientific RepoRts | 5:10784 | DOi: 10.1038/srep10784 should be insulate. And when the valence states are mixed with Co +3 and Co +4 or Co +2 and Co +3 , the LBCO should be semiconductor or semimetallic, respectively. Since the LBCO films were grown in the pure oxygen of 250 mTorr, the films are good conductive. Moreover, the Co +4 is not as stable as Co +3 . Therefore, the valence states should be a mixed state of Co +3 and Co +4 . When LBCO thin film is exposed to the reducing gas, parts of Co +4 changed into Co +3 and the resistance increases. In the contrast, the gas change to oxidizing gas, parts of Co +3 changed into Co +4 and the resistance decreases 51,52 . Ethanol vapor is safer and more active than other gas, such as H 2 , ammonia, hydrogen sulfide, which is used as the representative of reducing gases. In this paper, we explore here the transient responses of the LBCO thin films to various ethanol vapor concentrations at different operating temperatures. It was found that even though ethanol vapor concentration is as low as 10 ppm, the LBCO thin film still can detect the transient response. Moreover, the response time can be reduced to ~24 s under the ethanol vapor concentration of 400 ppm with the recovery time of only ~10 s at 375 o C. These results indicate that the LBCO thin films have great potential for the development of the gas sensor applications in reducing/oxidizing environments, expect the applications on giant magnetoresistance and cathode for solid oxide fuel cell.

Results and Discussion
The crystalline quality of the LBCO thin films was characterized by the XRD θ − 2θ scan, rocking curve, ϕ scan and reciprocal space mappings (RSMs). Figure 1 is a typical θ − 2θ pattern for the LBCO thin films on (001) LAO substrates showing that only the (00l) peaks can be detected for the LBCO thin films. It is revealed that the films are c-axis oriented or the c-axis normal to the substrate surface. The rocking curve measurements from the (002) reflections for the LBCO films show that the Full Width of Half Maximum (FWHM) is ~0.92°, as shown in the inset (a) of Fig. 1, indicating that the LBCO films on LAO substrates have good crystalline quality. To understand the in-plane interface relationships between the LBCO films and the LAO substrates, the ϕ scans measurements have been performed. The inset (b) of Fig. 1 is the ϕ scans taken around from the {101} reflections of the LBCO films and LAO substrates. The four-fold symmetry and sharp peaks in the ϕ scan suggest that the films have good epitaxial nature. Therefore, the orientation relationships between the thin films and the substrates can be determined to be [100] LBCO //[100] LAO and (001) LBCO //(001) LAO . RSMs is a very effective method to study the microstructure information, such as lattice parameters, defect density, domain structure, etc. in order to obtain the lattice parameters and interface strain of LBCO films on LAO substrates, the RSMs have been performed taken around from the symmetric (002) and asymmetric (103) reflections of the LBCO films and LAO substrates, as shown in the Fig. 2. According to the Bragg law and the angles' relationship between these crystalline planes, the in-plane (a & b) abd out-of-plane (c) lattice parameters of LBCO films on LAO substrates can be calculated from the RSMs. The in-plane and out-of-plane lattice parameter is 3.90 Å and 3.88 Å, respectively. It is shown that this LBCO film on LAO is full relaxation state.
In order to identify the optimum working temperature for the LBCO gas sensors, the sensing transient response of the LBCO films to 1000 ppm ethanol vapor exposure were investigated as a function of sensor temperature from 250 o C to 450 o C, as shown in the Fig. 3(a). It is clearly seen that the resistance of LBCO thin film increase when it is exposed to the ethanol vapor (reducing gas), then the resistance decrease when the gas change from 1000 ppm ethanol vapor to air, which is in agreement with our previous studies under the gas switch between H 2 and O 2 16,23 . In fact, the LBCO thin films will generate a high density of oxygen vacancies when it is exposed to the reducing gas 53 , such as ethanol vapor gas. This chemical processing can be represented by: The electrons are released into the LBCO films through the oxidation reactions with ethanol vapor reducing gas. Thus, the increased electronic density will result in the increase of the resistance due to the LBCO films are p-type semiconductors, On the other hand, the resistances of the LBCO thin films will decrease when exposed to oxidizing gas, such as air. The response time from air to 1000 ppm ethanol vapor decrease with the temperature increase. Especially, the response time rapidly drop and become faster when the temperature exceed to 375 o C. Although the LBCO thin films can detect ethanol gas at the temperature of 250 o C and 300 o C, the resistance of LBCO thin film can't recovery completely in short time when it exposed to air. Thus, the working temperature above 300 °C is better for the excellent performance as ethanol/reducing gas sensor. Meanwhile, the stability of the LBCO thin film as ethanol/ reducing gas sensor has been examined, as shown in Fig. 3(b). The response and recovery repeatability of the LBCO thin film is examined under 1000 ppm ethanol vapor at 375 °C. It shows that the resistance change of LBCO thin film can keep its initial value after circle operation, revealing that the LBCO thin film as ethanol/reducing gas sensor have good reproducibility.
To investigate the sensitivity of LBCO thin film as ethanol gas sensor, the transient response or the resistance switching behavior during the change between air and various ethanol vapor concentrations at 375 o C was examined. As shown in the Fig. 4, the LBCO thin films have reliable and reproducible responses during the change between air and various ethanol vapor concentrations. Even at very low concentration of 10 ppm, the LBCO films still can detect transient response. The ratio of resistance switching between air to ethanol vapor is strongly dependent upon the ethanol vapor concentrations, or linearly-like increases with the increase of the ethanol vapor partial pressures from 10 ppm to 5000 ppm, as seen in the inset of Fig. 4. The sensitivity (S) of the LBCO thin film as the ethanol gas sensor increase form 1.04 at 10 ppm to 2.92 at 5000 ppm. The S determined by using the formula S = R g /R a , where R a and R g are resistance of the gas sensor in air and in testing gas atmosphere, respectively. The sensitivity can be enhanced by replacing the air with oxygen gas.
It is known that the response and recovery times are the two important quantity factors for the gas sensors, As shown in the inset of Fig. 5, the response time is the time taken by the resistance change from R air to R air + [R gas -R air ]*90%, and the recovery time is the time taken by the resistance change from R gas to R gas -[R gas -R air ]*90%. The response and recovery factors for the LBCO thin films as ethanol gas sensors are shown in Fig. 5    with the increase of ethanol vapor concentrations. Then, the response time increases with the increase of the ethanol vapor partial pressures from 500 ppm to 2000 ppm. Therefore, the optimum response time is ~24 s at the concentration of 400 ppm. The reason for the phenomena is still unclear and will be explored in the following work. There is no transition for the recovery time, which decreases from ~29 s (10 ppm) to ~6 s (2000 ppm) with the increase of the ethanol vapor concentration. Compared with some ethanol sensors, epitaxial LBCO thin film exhibits better quality factors. Table 2 gives the as-reported response times of various ethanol sensors at intermediate temperature ranges. Obviously, the response (recovery) time of the LBCO epitaxial thin films is less than that from the n-type gas sensors based on the NiO thin films, SnO 2 thin film and TiO 2 thick films. Normally, the response of p-type oxide semiconductor gas sensor is slower than n-type semiconductor gas sensor 54 . Moreover, it is comparable with the ZnO nanorods, CuO microsphere, hollow sea urchin-like structure α-Fe 2 O 3 and various nanostructure Co 3 O 4 listed in the Table 2. Nanorods and nanowires normally have fast response due to the large reactivity area. The comparable fast response in LBCO thin films indicates that epitaxial LBCO thin films will be an good gas sensor for reducing gas, especially for ethanol vapor. The response and recovery time definitely can be improved by adjusting the nanostructure of LBCO, like LBCO nanorods and nanowires. Therefore, LBCO materials have great potential for the development of the application of gas sensors in reducing/oxidizing environments.
In summary, the sensing transient response of the epitaxial LBCO thin films during the changes of air to various ethanol vapor exposure have been investigated in the temperature range of 250 o C to 450 o C. It is shown that the LBCO thin films have reliable and reproducible responses to ethanol vapor,  Structural and electrical characterization. The crystalline quality and epitaxial behavior of the LBCO thin films were characterized by high-resolution x-ray diffraction (HRXRD) using PANalytical X'Per MRD. The LBCO thin films have been used for the gas sensing measurements. The ethanol sensing properties of the LBCO thin films were systematically studied using the GS-1TP intelligent gas sensing analysis system (Beijing Elite Tech. Co. Ltd., China) 55 . This analysis system consists of several parts: a heating system, gas distribution system, probe system, vacuum system, measurement system and related control software. The samples can be heated up to 500 °C with a precision of 1 °C. In order to obtain the accurate data, all the samples will be preheated at the working temperature for ~20 min. Once the LBCO gas sensors were stable, the ethanol gas will be injected into the test chamber. After the resistance of the LBCO gas sensor reaches a new constant value, we will open the test chamber to recover LBCO sensor into the air atmosphere.