CO2 Sensing Properties of Zr-Added Porous CaFe2O4 Powder

Department of Creative Engineering, National Institute of Technology (NIT), Kitakyushu College, 5-20-1 Shii, Kokuraminami-ku, Kitakyushu, Fukuoka 802-0985, Japan 1Advanced Engineering School of Creative Engineering, National Institute of Technology (NIT), Kitakyushu College, 5-20-1 Shii, Kokuraminami-ku, Kitakyushu, Fukuoka 802-0985, Japan 2Interdisciplinary Graduate School of Agriculture and Engineering, University of Miyazaki, 1-1Gakuenkibanadai-nishi, Miyazaki 889-2192, Japan 3Department of Environmental Robotics, Faculty of Engineering, University of Miyazaki, 1-1 Gakuenkibanadai-nishi, Miyazaki 889-2192, Japan 4International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 Japan 5Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 Japan


Introduction
Recently, there has been increasing demands for the monitoring and/or control of CO 2 concentrations in the office, home, agriculture field, and bio related processes, for example. To date, solid-state electrolyte-based (1)(2)(3)(4) and oxide semiconductor-based (5-7) CO 2 sensors have been intensively investigated. Among these types of CO 2 sensors, an electric-resistance-type sensor using oxide semiconductor has attracted much attention because their electric signal output is directly related to the CO 2 concentration. However, this type of sensor has a problem in that the CO 2 response is fundamentally low owing to its sensing mechanism, i.e., electric resistance change caused by the gas adsorption on the surface of the semiconductor oxide. In order to improve the gas response of this type of sensor, the synthesis of a mesopore structured oxide semiconductor with high specific surface area has been attempted. (8) Oxide-semiconductor-type sensors containing La 2 O 3 or alkaline earth metal oxides (BaO, SrO, CaO) have been investigated because these La 2 O 3 or alkaline earth metal oxides display strong interaction with CO 2 , leading to the enhancement of the electric resistance change, which results in a large output of the sensing signal. (9)(10)(11)(12)(13)(14)(15) The semiconductor-type CO 2 sensor using BaO-containing complex oxides has attracted attention owing to its strong interaction with CO 2 among the alkaline earth metal oxides. (16) However, a material which is abundant, inexpensive, and nontoxic is preferable from the viewpoint of reducing environmental load. As a material which satisfies these conditions, we have focused on calcium ferrite (CaFe 2 O 4 ). We have recently reported that the addition of zirconium (Zr) into CaFe 2 O 4 forms a characteristic porous structure with smaller grains connected into a three-dimensional network, resulting in a higher specific surface area. (17) The porous structure of Zr-added CaFe 2 O 4 is preferable as a semiconductor-type gas sensor material. Therefore, we examined the CO 2 sensing properties of the Zr-added CaFe 2 O 4 materials in the present study.

Experimental Procedure
CaFe 2 O 4 powder was synthesized by a malic acid complex method. (17) Ca(NO 3 ) 2 ·4H 2 O, Fe(NO 3 ) 3 ·9H 2 O, and malic acid in a 1:2:3 molar ratio were dissolved in ethanol to form a mixed solution. The addition of Zr was accomplished by introducing Zr[OC(CH 3 ) 3 ] 4 solution into the abovementioned mixed solution. The amount of Zr was set at 5 mol% with respect to Fe. The mixed solution was heated on a hot plate to prepare the precursor. The precursor was calcined at 700 °C for 12 h in air. The heating ratio was 10 °C min −1 in all cases. Figure 1 shows a schematic drawing of the CO 2 sensor and measuring circuit used in the present study. The CaFe 2 O 4 -based powders were mixed with α-terpineol containing 5 wt% ethyl cellulose, and the resulting paste was applied on an alumina tube attached to a pair of Pt-wire electrodes. The sensor element was fabricated by heating the entire assembly at 600 °C for 2 h in air. The CO 2 sensing properties were measured in a conventional gas flow apparatus equipped with heating facilities in the temperature range of 250-450 °C. The CO 2 concentration was varied in the range of 0-5000 ppm by diluting pure CO 2 gas with dry air. The sample gases were allowed to flow over  the sensor element at a rate of 0.1 dm 3 min −1 . The gas response (S) was defined as R air /R gas , where R air and R gas were the electric resistances of a sensor element in air and in a sample gas, respectively. The electrical resistances were measured on the basis of a conventional circuit in which the element was connected with an external resistor in series. The output voltage across the external resistor at a circuit voltage of 5 V was used to evaluate the electric resistance of the element.
Diffuse reflectance spectra (DRS) were collected for the sensor materials using an IR spectrometer in the wavenumber range of 1100-2500 cm −1 at 350 °C in air, CO 2 , or N 2 . Figure 2 shows the XRD patterns for Zr-added and unadded CaFe 2 O 4 powders prepared from a malic acid complex. XRD peaks of unadded CaFe 2 O 4 powder calcined at 700 °C in air could be ascribed to the CaFe 2 O 4 phase. On the other hand, the XRD peaks of 5 mol% Zr-added product could also be ascribed to the CaFe 2 O 4 phase, and small impurity peaks appeared in the Zr-added CaFe 2 O 4 powders. These diffraction peaks of the impurity phase were identified as the Ca 2 Fe 2 O 5 . Chemical valence states of Zr-added and unadded CaFe 2 O 4 powders were analyzed by an XPS measurement. As a result, it was found that the binding energies of the Ca 2p, Fe 2p, Zr 3p, and O1s spectra are in good agreement with those of the Ca 2+ , Fe 3+ , Zr 4+ , and O 2− valence states, respectively. (17) Figure 3 shows the transient responses of the sensors made from pure CaFe 2 O 4 and 5 mol% Zradded CaFe 2 O 4 powders to 5000 ppm CO 2 in air at 350 °C. When the atmosphere was changed from dry air to 5000 ppm CO 2 in air, the electric resistances of the sensor decreased, suggesting that CO 2 adsorbs on the surface of CaFe 2 O 4 as a negatively charged species. CaFe 2 O 4 is a p-type 10 Figure 4 depicts the dependence of the gas response and 90% response time (t 90 ) to 5000 ppm CO 2 of the sensors made from pure and 5 mol% Zr-added CaFe 2 O 4 at the operation temperature. It is well known that the sensing performance of resistive-type sensors markedly depends on the operation temperature. (18) As for the reducing gases such as H 2 , the temperature dependence of the gas response reaches maximum in accordance with the mixed effect of gas diffusion and reaction at the sensor surface. This is because the resistance change was caused by the reaction between target gases and the negative charged adsorbed oxygen on the surface of the sensor. On the other hand, the present response to CO 2 would be caused by the adsorption of CO 2 to form negatively charged species. It is speculated that the temperature dependence toward less active gases such as CO 2 would be determined by the rate of adsorption and desorption of the target gas. In the present study, the CO 2 gas response reached a maximum at 300 °C. On the other hand, the 90% response time was too slow for practical use at 300 °C. Thereafter, the CO 2 sensing properties of Zr-added CaFe 2 O 4 were mainly examined at 350 °C owing to the quick response, although the CO 2 gas response was slightly lower than that at 300 °C. Figures 5(a) and 5(b) show the response transients to stepwise changes in CO 2 concentration and the relationship between the gas response and the CO 2 concentration for 5 mol% Zr-added CaFe 2 O 4 . When the CO 2 concentration was changed from 0 to 500 ppm in the measuring chamber, the gas response was 1.6. The gas response became higher as the CO 2 concentration was increased to 5000 ppm and finally reached 3.0. Each 90% response time for stepwise changes in the CO 2 concentration was estimated to be within 90 s. The gas response showed a linear correlation with the logarithm of CO 2 concentration in the range of 500-5000 ppm.

Gas sensing mechanism
As previously reported, 5 mol% Zr-added CaFe 2 O 4 powder exhibited three-dimensional porous structures formed by small grains connecting with each other, resulting in a higher specific surface area compared with that of pure CaFe 2 O 4 . This increment in the surface area upon Zr-addition might be effective for enhancing the response to CO 2 gas beyond that of pure CaFe 2 O 4 . However, upon the addition of 5 mol% Zr, the surface increases to only twice that of pure CaFe 2 O 4 . Thus the enhanced gas response of the Zr-added CaFe 2 O 4 -based sensor might originate not only from the increased surface area but also from the effect of Zr itself. Thus, infrared (IR) measurements were conducted in dry air, N 2 , and pure CO 2 atmospheres at 350 °C for Zr-added and pure CaFe 2 O 4 powders to analyze the CO 2 related species on these samples. In dry air, IR absorption bands were observed at 1350-1550 cm −1 and 2300-2400 cm −1 for Zr-added and pure CaFe 2 O 4 powders, respectively, as shown in Fig. 6. Fukuda and co-workers (19) reported the possible adsorption configurations of CO 2 on the CaO surface to be unidentate and bidentate carbonate complexes.
They pointed out that the IR band derived from the Ca-O-C complex configuration appears in the range of 1350-1550 cm −1 . This IR band might be due to Ca-O-C asymmetric vibration. When CO 2 adsorbs on carbonate in a bidentate complex configuration, on the other hand, the adsorption peak is expected to appear at approximately 1750 cm −1 . (19) The results shown in Fig. 5 revealed significant IR bands at around 1350-1550 cm −1 . The IR spectra were also observed in the range of 2300-2400 cm −1 . Dietzel et al. reported that the IR band derived from the O=C=O configuration is observed from 2300 to 2400 cm −1 for the metal oxide surface. (20) This IR band might be derived from the O=C=O stretching vibration. In the present study, IR absorption bands were observed at 1350-1550 cm −1 and 2300-2400 cm −1 for Zr-added and pure CaFe 2 O 4 powders, respectively, so that the configuration of CO 2 adsorption on the CaFe 2 O 4 powder might be a unidentate complex judging from the slight increment in the intensity in the range of 2300-2400 cm −1 . As shown in Fig. 6, when the atmosphere was changed from dry air to CO 2 , the IR spectra of pure CaFe 2 O 4 powder was almost unchanged. In contrast to the pure CaFe 2 O 4 powder, the IR absorption bands of the Zr-added CaFe 2 O 4 powder at 1350-1550 cm −1 and 2300-2400 cm −1 became stronger when the atmosphere was changed from dry air to CO 2 [ Fig. 6 CaFe 2 O 4 is known as a p-type semiconductor, and its majority carriers are holes. (21) Accordingly, it is expected that the reaction of adsorbed CO 2 with a negatively charged oxide ion would bring about an increase in the hole concentration. The sensing signal would originate from the negatively charged CO 2 adsorption, taking into account the results of the IR measurement, although the detailed mechanism requires further study. The present IR analysis and previously reported slight increment in the surface area indicates that the addition of Zr is effective not only for increasing the surface area but also for enhancing the CO 2 adsorption on CaFe 2 O 4 . In other words, added Zr accelerates the adsorption of the CO 2 that is produced by the reaction of CO 2 with negatively charged oxygen species. This implies that the change in electric resistance caused by CO 2 adsorption on the CaFe 2 O 4 surface is enhanced by the mixed effect of Zr addition, i.e., the increment in the surface area, and the enhancement of oxygen-species-assisted CO 2 adsorption.

Conclusion
It was demonstrated that 5 mol% Zr-added CaFe 2 O 4 powder showed a higher CO 2 gas response compared with that of pure CaFe 2 O 4 powder and reached a maximum value at 300 °C. At 350 °C, the 90% response time of Zr-added CaFe 2 O 4 powder was much faster than that at 300 °C. It is conceivable, on the basis of the IR measurements, that the change in the electric resistance of CaFe 2 O 4 is caused by the reaction of adsorbed CO 2 with a negatively charged oxide ion, because of the increase in the hole concentration. Furthermore, the strong gas response of Zr-added CaFe 2 O 4 to CO 2 can be attributed to its high specific surface area as well as enhanced adsorption on the sensor material.