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IEEE SENSORS JOURNAL, VOL. 5, NO. 1, FEBRUARY 2005
Low-Temperature Catalyst Adding for Tin–Oxide Nanostructure Gas Sensors Sung-Jei Hong and Jeong-In Han
Abstract—Thick-film gas sensors are successfully fabricated using the nanostructure tin–oxide powder. In order to suppress the coarsening of the nanostructure tin–oxide particles during the adding process, the low-temperature catalyst adding (LTCA) method is proposed in this paper. LTCA is an adding method of noble Pd catalyst onto the nanostructure tin–oxide particles under the lower temperature below 300 C by excluding chloride. It turned out that the adding without particle coarsening is successfully carried out by means of LTCA. Applying LTCA to the fabrication of the thick film using nanostructure tin–oxide powder having a size smaller than 5 nm leads to an excellent performance with respect to the methane gas sensing. After aging at 400 C, a good sensitivity ( 3500 1000 ) of 0.66 is obtained for the sensor doped with 5 wt% of Pd catalyst. Also, the sensitivity of the sensor is so stable that the deviation of the electrical resistance is within 3% after 400 h of aging. Index Terms—Gas sensor, low-temperature catalyst adding (LTCA), nanostructure tin–oxide powder.
Fig. 1.
Thermal analyses (TGA and DTA) of Pd acetate.
I. INTRODUCTION
S
EMICONDUCTOR tin–oxide gas sensors are widely used for various applications in gas sensing. The sensing mechanism of these sensors is based on the change of the electrical dc resistance of a gas-sensitive layer due to adsorption reactions of reacting gaseous species [1], [2]. The resistance of the gas-sensitive films strongly depends on the particle size and phase structure of the gas-sensitive material [3]–[5]. Commonly, a smaller size of the particles of the gas-sensitive material gives rise to a better sensitivity because of the enlarged surface area. Accordingly, the nanostructure tin–oxide powder is being spotlighted [6]. In order to cause a reaction of the ultrafine particles with the gas under lower temperature, a noble metal, such as Pd, is applied as a catalytic dopant [7], [8]. The chemical compounds used in a conventional catalyst adding process include a chloride component [9], [10]. In order to remove the chloride, all the nanosize particles as well as the chloride should be heated above 700 C. The grain growth occurs during the heat treatment, and, therefore, a size control of nanodimensional particles through the conventional process is very difficult. Moreover, it is a very dangerous process because the chloride is very harmful. So, in this paper, an adding method is newly developed and applied to the fabrication of the gas-sensitive films using nanostructure tin–oxide powder. Because the adding is done at a lower
Manuscript received August 20, 2002; revised January 8, 2004. The associate editor coordinating the review of this paper and approving it for publication was Prof. Michiel Vellekoop. The authors are with the Information Display Research Center, Korea Electronics Technology Institute, KyungGi-Do, 451-865, Korea (e-mail:
[email protected];
[email protected]). Digital Object Identifier 10.1109/JSEN.2004.838919
Fig. 2. XRD analysis of Pd acetate heat treated at 300 C (2 scan;
1:5405 A).
=
temperature, the present method is referred to as the low-temperature catalyst adding (LTCA). LTCA is useful for sensing materials operated at a temperature lower than 300 C. The nanostructure powder and fabricated thick films were analyzed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Brunauer, Emmett, and Teller (BET) surface area measurements, and field effect scanning electron microscopy (FESEM). The sensitivity of the thick-film gas sensors was tackled by measuring the variation of the electrical dc resistance of the sensors in an atmosphere containing traces of methane. II. FEASIBILITY OF LTCA For LTCA, Pd acetate was adopted instead of Pd chloride which is currently used in the conventional method. In order to
1530-437X/$20.00 © 2005 IEEE
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Fig. 3. HRTEM observation of the synthesized nanostructure tin–oxide powders MDE by IGC. (a) Powder A. (b) Powder B. (c) Powder C.
determine the feasibility of the compound, thermal analyses, such as thermogravimetric analysis (TGA) and differential thermal analysis (DTA), were carried out. Changes in weight and heat flow of the Pd acetate were observed in dependence on temperature. As a result, a change in weight was found at the temperature ranging from 200 C to 260 C, as shown in Fig. 1. Also, a change in heat flow was observed at similar temperature range. It was assumed that the Pd acetate was changed to Pd above 260 C. That is, the acetate component consisting of C, H, and O reacted with O in air above 200 C, and it was oxidized and separated from Pd by reaching 260 C. For certifying these phenomena, the Pd acetate was heat treated at 300 C under air conditioning. After that, the composition of the remained powder was analyzed by means of
Fig. 4. XRD analyses of the synthesized nanostructure tin–oxide powders = 1:5405 A). (a) Powder A. (b) Powder B. made by IGC (2 scan and (c) Powder C.
XRD. As a result, seen in Fig. 2, most of the peaks indicate Pd. It means that the whole components except Pd were removed. Accordingly, the Pd acetate was present below 300 C and
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IEEE SENSORS JOURNAL, VOL. 5, NO. 1, FEBRUARY 2005
TABLE I GRAIN SIZE AND SURFACE AREA OF THE NANOSTRUCTURE TIN–OXIDE POWDER OF A, B, AND C
Fig. 6. Cross-sectional view of nanostructure gas sensor.
III. EFFECT OF LTCA ON THE MICROSTRUCTURE OF GAS SENSITIVE TIN–OXIDE POWDERS A. Synthesis of Nanostructure Tin–Oxide Powders
Fig. 5. HRTEM micrographs of Pd-added nanostructure tin–oxide powders by LTCA. (a) Powder A. (b) Powder B. (c) Powder C.
LTCA using this compound (without chloride) was found being more feasible at lower temperature than the current adding method.
Raw material powders were made in order to observe the effect of LTCA on the microstructure of gas-sensing materials. As raw materials, nanostructure tin–oxide powders were synthesized by inert gas condensation (IGC). IGC is used to synthesize the nanostructure tin–oxide powder under vacuum condition. The nanostructure tin–oxide particles were synthesized on the surface of a cold substrate by reacting the evaporated tin with O gas. During synthesization, the vacuum chamber for IGC was filled with 1.0 torr He and O was supplied with a constant flow rate of 100 SCCM. Using IGC, three kinds of raw powders different in particle size or in phase structure were synthesized, respectively. We named these raw powders as A, B, and C in this paper. The powder A was synthesized to have a particle size below 5 nm and a mixture of two phase [SnO T and SnO (T), (T) means tetragonal strtucture]. The powder B was synthesized to have a particle size of about 15 nm and also the two-phase-like powder A. The powder C was synthesized to have a similar particle size like powder B but a single phase [SnO T ]. After synthesis, the particle size, specific surface area, and structure of the three synthesized powders were observed and analyzed by HRTEM, BET, and XRD, respectively. As a result, the grains of powder A show a particle size smaller than 5 nm, as shown in Fig. 3(a). The average particle size of the grains of powder B is about 15 nm, as seen in Fig. 3(b). Also, in case of powder C, the average particle size is found to be similar as that of powder B, as intended. The result of specific surface area certified the particle size of the powders as seen in Table I. gas The specifiec surface area was measured by adsorbing molecules on the surface of the powders and then obtaining the equilibrium adsorption amount. So, in principle, the surface area is increased as the particle size of the powder is smaller. The specific surface area of the powder A, B, and C were 74.68 m g, 44.98 m g, and 40.65 m g, respectively. The measured specific surface area of powder A was larger than those of power B and C. This result corresponds with the HRTEM observation
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Fig. 7.
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Power consumption of the test sensors according to operating temperature using a RuO heater.
that average grain size formed for the powder A is smaller than that of B and C. Also, the phases in the nanostructure powders were also controlled well. As seen in Fig. 4(a) and (b), XRD analysis revealed that powder A and B consist of a mixed phase of SnO and SnO. In contrast, the powder C has a single phase SnO , as seen in Fig. 4(c). B. Application of LTCA to Nanostructure Tin–Oxide Powders The gas-sensitive materials were made by adding 5 wt% Pd catalyst of the three kinds of synthesized tin–oxide nanopowders. In order to suppress coarsening of the particle size, LTCA was applied for the adding process. The Pd acetate is dissolved in acetone solvent, the nanostructure tin–oxide powder is mixed with the Pd acetatic solution, the acetone solvent is evaporated during the mixing. Then, the remaining mixture is heat treated at 300 C. The phase, particle size and surface area were analyzed by HRTEM and BET, respectively. As a result, it was found that the particle size and surface area of the powders remain almost unchanged. Fig. 5 shows the HRTEM micrographs of Pd added nanostructure tin–oxide powders fabricated by means of LTCA. Compared with the particle size of the raw material powders seen in Fig. 3, it is clearly seen that the particle size is nearly unchanged during the adding process. Also, the change in surface area is small as seen in Table I. It is assumed that LTCA at temperatures below 300 C suppresses the interdiffusion between the adjacent particles, and, therefore, the coarsening of the nanoparticles cannot occur. Therefore, it is certified that the LTCA method is effective to ensure the suppression of the grain growth during adding and useful for control of the size of the nanodimensional particles. IV. EFFECT OF MICROSTRUCTURE OF THE PD-ADDED NANOSTRUCTURE TIN–OXIDE POWDERS ON THE GAS SENSITIVITY OF THEIR SCREEN PRINTED FILMS A. Fabrication of Nanostructure Tin–Oxide Gas Sensor In order to observe the effect of the microstructure of the gassensitive Pd-added tin–oxide on its gas sensitivity, nanostructure
gas sensors were fabricated using the three kinds of Pd-added tin–oxide powders. Fig. 6 depicts the cross-sectional view of the nanostructure tin–oxide gas sensor. The substrate is alumina of which size and thickness are 2 2 mm and 0.25 mm, respectively. A gas-sensing layer is formed on one side and a heating layer is formed on the opposite side. The thickness of the sensing layer is approximately 40 m. The heating layer is needed for the stable-sensing function. In this work, RuO was selected as a heating material. The fabrication procedure is as follows. First, a contact electrode for the heating layer is printed with Au paste on one side of alumina substrate, and dried at 160 C. Next, Au paste as an electode for the sensing layer is printed and dried at the same condition on opposite side of the alumina substrate. After that, the two Au electrodes on both sides are heat treated at at 850 C. The RuO paste is printed as a heating layer, heat treated at 850 C, and trimmed with laser. The heat treatment does not affect the Au electrodes since it has been already heat treated at the same temperature. Then, the gas-sensitive paste, which is composed of Pd-added tin–oxide powder, binder, and solvent, was printed. The printed gas-sensitive layer is heat treated at 600 C for 1 h. Finally, the fabricated nanostructure gas sensors are packaged in TO type headers. Fig. 7 shows the heater temperature according to power consumption. The heater temperature is approximately linearly proportional to the power consumption. It is reported that an appropriate temperature for a stable operation of the gas sensor is higher than 400 C [11]. In this case, the power required to establish that temperature was found to be 450 mW. So, all the sensors in this work were operated at that power. Using the fabrication technology above, five gas sensors of each condition were fabricated for measurement of the gas-sensitive properties. After fabrication, the surfaces of the gas-sensitive layers were observed with FESEM. The thick film fabricated with Pd added nanopowder A is seen in Fig. 8(a). The average particle size is observed as 5 nm. In the case of thick films with Pd-added nanopowder B and C, shown in Fig. 8(b) and (c), the average particle sizes are 15 and 15 nm, respectively. As mentioned, the average particle sizes of the A, B, and C raw material powders are 5, 15, and 15 nm, respectively. The
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IEEE SENSORS JOURNAL, VOL. 5, NO. 1, FEBRUARY 2005
Fig. 9. Analysis of bet surface areas of a raw material, Pd-added at 300 C and thick film at 600 C of the nanostructure tin–oxide powders A, B, and C.
Fig. 10. Resistances of nanostructure gas sensors with different particle sizes (phase: SnO (T)).
sensitive thick-film layers can be fabricated with the ultrafine Pd-added tin–oxide powders by using the presented process including LTCA.
Fig. 8. FESEM microstructures of surfaces of gas sensistive layers. (a) Nanostructure gas sensor A. (b) Nanostructure gas sensor B. (c) Nanostructure gas sensor C.
particle sizes of the three nanopowders are rarely changed after fabrication of thick films including heat treatment at 600 C. For more detailed investigation, the surface areas of the heat-treated thick films were measured and compared with those of raw material powders. As presented in Fig. 9, the result indicates that the grain growth of the Pd added nanopowders made by LTCA are suppressed in spite of the high-temperature heat treatment. That is, most particles of the three thick films with LTCA rarely grow less than 2.5% despite of the high-temperature heat treatment. It is not clear which factors give rise to the stability of particle sizes of the Pd added nanopowders yet. We are going to check which factors stabilize the particles size. So, the gas-
B. Effect of the Particle Size on the Sensitivity of the Pd-Added Tin–Oxide Nanostructure Tin–Oxide Gas Sensor The sensitivity was determined by measuring the electrical dc resistance of the thick films in an atmosphere containing 500–10 000 ppm of methane. The sensitivity is defined by the ratio of the resistances measured at a methane concentration of . Thus, the sensitivity is en1000 and 3500 ppm . During the meahanced with decreasing ratio of surement, the sensors were heated to 400 C in an environment with a constant humidity of 60%. Under this environment, the effect of the particle size on the sensitivity of the Pd-added nanostructure tin–oxide gas sensor was investigated. So, the gas sensors using powder A and B are compared. As mentioned, the A and B powder are different in particle size. The particle size of powder A is smaller than that of powder B. The resistance of the two nanostructure gas sensors is shown in Fig. 10. The sensor A exhibits higher resistance than
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Fig. 11. Sensitivities of nanostructure gas sensors with different particle sizes [phase: SnO (T)].
that of sensor B. Higher resistance of the sensor A is attributed to the smaller size of the particles [12]. The difference in resistance of the thick film by the particle size is predicted to affect the sensing properties of the gas sensor. Fig. 11 shows variations of the sensitivity of the sensors A and B as a function of aging time. Whereas the resistance is roughly unchanged by aging in air, the response of the sensors on methane gas varies with the aging treatment. After aging reached the value longer than 1 h, the sensitivity of 0.66 and 0.76 for the sensor A and B, respectively. Consequently, it turned out that the sensor fabricated by using smaller nanostructure particles exhibited a better sensitivity. As menmeans a better sensitioned, the lower value of tivity. This fact is attributed to the increase of the specific surface area by the smaller particle size. In most of conventional Pd adding process, a heat treatment higher than 700 C is required, that inevitably leads to the coarsening of the nanostructure particles. In contrast, LTCA provides a low-temperature adding, that suppresses the coarsening of the particles during adding and thus ensures the application of the nanostructure tin–oxide powder in thick-film gas sensors. C. Effect of the Phase on the Sensitivity of the Pd-Added Nanostructure Tin–Oxide Gas Sensor The sensitivity of nanostructure gas sensors with different phase structure is compared. The particle size of the sensor C is similar to that of sensor B, but the phase is different. The powder B consists of a mixture of SnO T and SnO (T), but the powder C consists of only single phase SnO T . The gas-sensitive layers were analyzed by means of XRD as shown in Fig. 12. Compared to Fig. 4, the ratio of SnO (T)/ SnO T of both the sensor of A and B is reduced. It means that the phase of SnO (T) particles was partially changed. This effect is assumed to be due to the thermal oxidation of SnO (T) above 300 C. The oxidation of SnO (T) is reported to occur at a temperature exceeding 300 C and metastable tin–oxide particle [13]. So, the partial change in the film structure is caused by the heat treatment. In the case of sensor C, however, no such change in structure is
Fig. 12. XRD analysis of phase sturetures of gas-sensitive layers (2 scan; = 1:5405 A). (a) Gas-sensitive layer A. (b) Gas-sensitive layer B. (c) Gas-sensitive layer C.
observed since the structure consists of the stable SnO phase only. Also, as shown in Fig. 13, the resistance of the gas sensor B is lower than that of sensor C. It is assumed that the higher
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Fig. 13. Resistances of nanostructure gas sensors with different phases (particle size: 15 nm).
IEEE SENSORS JOURNAL, VOL. 5, NO. 1, FEBRUARY 2005
Fig. 15.
Long-term stability of electrical resistance of gas sensor C.
V. CONCLUSION In this paper, the low-temperature catalyst adding (LTCA) method is proposed and applied to the fabrication of thick-film gas sensors using nanostructure tin–oxide powder. Because the adding is carried out at the lower temperature than 300 C, the coarsening of nanostructure particles is successfully suppressed. Also, the grain growth of the Pd-added nanopowder is suppressed after fabrication a thick-film gas-sensitive layer heat treated at 600 C. The gas-sensitive layer with ultrafine tin–oxide particles smaller than 5 nm or single phase of of SnO T exhibited the excellent sensitivity 0.66 after aging at 400 C. Also, electrical resistance of the gas-sensitive layer after aged for 400 h was stable of which deviation is within 3%. So, the high-performance nanostructure tin–oxide gas sensor could be fabricated applying the LTCA. Fig. 14. Sensitivities of nanostructure gas sensors with different phases (particle size: 15 nm).
resistance of the sensor C is caused by the lower carrier density depending on the phase structure [4], [14]. That is, the vacancy which was owing to SnO generated a higher carrier density, and it causes a lower resistance. As seen in Fig. 14, the sensor C exhibits the better sensitivity compared to sensor B. Thus, the larger change of the resistance of the gas sensor caused by the phase structure enables a better sensitivity with the lower similar to the case of the particle size effect. Also, long-term stability of the gas-sensitive thick film was evaluated using the gas sensor C. The long-term stability test was conducted by observing its electrical resistance, according to aging time, when reacted with the methane gas of which concentration are 0, 1000, and 3500 ppm, respectively. The gas-sensitive thick film was aged at 400 C, and the electrical resistance was measured every 48 h. As shown in Fig. 15, the nanogas sensor C showed stable phenomena of electrical resistance. During aging for 400 h, the resistance variation of the sensor was only within 3%. So, it is judged that the LTCA is a useful technology to produce highly sensitive and stable gas sensor.
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[9] A. Diequez, A. R-Rodriguez, J. R. Morante, U. Weimar, M. S-Berberich, and W. Gopel, “Morphological analysis of nanocrystalline SnO for gas sensor applications,” Sens. Actuators B, vol. 31, pp. 1–8, 1996. [10] A. Cirera, A. R-Rodriguez, J. R. Morante, U. Weimar, M. S-Berberich, and W. Gopel, “New method to obtain stable small-sized SnO powders for gas sensors,” Sens. Actuators B, vol. 58, pp. 360–364, 1999. [11] N. Yamazoe and T. Seiyama, “Sensing mechanism of oxide semiconductor gas sensors,” in Proc. Int. Conf. Solid-State Sensors and Actuators, 1991, pp. 376–379. [12] Q. Pan, J. Xu, X. Dong, and J. Zhang, “Gas-sensitive properties of nanometer-sized SnO ,” Sens. Actuators B, vol. 66, pp. 237–239, 2000. [13] W. K. Choi, H. Sung, K. H. Kim, J. S. Cho, S. C. Choi, H. J. Jung, and S. K. Koh, “Oxidation process from SnO to SnO ,” J. Mater. Sci. Lett., vol. 16, pp. 1551–1554, 1997. [14] S. Semancik and T. B. Fryberger, “Model studies of SnO -based gas sensors: Vacancy defects and Pd additive effects,” Sens. Actuators B, vol. 1, pp. 97–102, 1990.
Sung-Jei Hong received the B.S. and M.S. degrees in metallurgical engineering from Sungkyunkwan University, Korea, in 1991 and 1993, respectively. After 1993, he joined the Korea Electronics Technology Institute, KyungGi-Do, where he serves as a Managerial Researcher in the Information Display Research Center. His research is in the fields of the semiconductor devices, including gas sensors; the design and fabrication of nanostructure materials, such as tin–oxide, indium tin–oxide, and yttrium–oxide; and flat-panel display, such as metal–insulator–metal liquid-crystal display, chip-on-glass packaging, etc. Mr. Hong is a member of the Materials Research Society, the Society for Information Display, and the Korean Institute of Electrical and Electronic Material Engineers.
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Jeong-In Han received the B.S. degree in metallurgical engineering from Yonsei University, Korea, in 1983 and the M.S. and Ph.D. degrees in materials science and technology from the Korea Advanced Institute of Science and Technology (KAIST), Seoul, in 1985 and 1989, respectively. He was a Senior Researcher with the Semiconductor Institute, Semiconductor Division, Samsung Electronics Company, from 1989 to 1992, where he worked on high dielectric materials for DRAM, ferroelectric materials for FRAM, and poly-Si TFT LCD. After 1992, he joined the Korea Electronics Technology Institute, KyungGi-Do, where he serves as a Director in the Information Display Research Center. He was a Visiting Professor in the Advanced Material Science Division, Department of Advanced Industry Engineering, Kyonggi University, Suwon, Korea, from 1998 to 2000. His research is in the fields of the semiconductor devices, including gas sensors, and the design and fabrication of flat-panel display, such as flexible display chip-size packaging on flexible polymer substrates, e-paper, etc. Dr. Han is a member of the Society for Information Display, the Korean Information Display Society, the Korean Materials Research Society, and the Korean Society for Broadcasting Engineering.
