APPLIED PHYSICS LETTERS 89, 153116 共2006兲

Influence of Ag particle size on ethanol sensing of SnO1.8 : Ag nanoparticle films: A method to develop parts per billion level gas sensors Rakesh K. Joshia兲 and F. Einar Kruisb兲 Fachgebiet Nanostrukturtechnik, Abteilung Elektrotechnik und Informationstechnik, Fakultät Ingenieurwissenschaften, Universität Duisburg-Essen, Campus Duisburg, Bismarckstr. 81, 47057 Duisburg, Germany

共Received 15 March 2006; accepted 22 August 2006; published online 12 October 2006兲 The influence of Ag particle size on ethanol sensing of SnO1.8 : Ag films composed of size-selected nanoparticles with independently controlled size and concentration of Ag is reported in the present study. The study shows that Ag nanoparticles are acting as catalyst for chemical sensitization through a spillover effect. The catalyst particles are observed to be more active on decreasing their size, resulting into an improved sensor response. A response time of 2 s for 1000 ppm ethanol has been achieved. Detection of 100 ppb ethanol in air has been demonstrated using this well-defined technique. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2360245兴 Current trend of research in gas sensor technology is dedicated towards the development of sensors which are applicable for the detection of gases at very low concentrations. Nanobelts,1 nanowires,2 and nanorods3 of several semiconductor oxides have shown their applicability for the parts per billion 共ppb兲 level detection of several gases. Thick film technology has been used to develop the ppb-level film sensors.4 However, a well-defined technique for the tailored fabrication of ppb-level sensors composed of films with monodisperse nanoparticles is yet to be reported. Sensors based on metal doped metal oxides, such as tin oxide 共SnOx兲 doped with noble metals, have drawn great attraction in the recent years.5–7 Many metals such as Pt,5 Pd,8 Au,9,10 and Ag 共Refs. 11 and 12兲 have been added to the SnOx in order to enhance the gas sensing characteristics. Ag, the material of interest for the present study, is one of the members of a class of metals which have been used as a catalyst for the redox reactions at the semiconductor surfaces. Furthermore, Ag-based metal oxide composites have been used for the oxidation of alcohols.13–15 It is thus expected that Ag has the potential to improve the sensing characteristics of the gas sensor due to the catalytic nature of the Ag particles toward the oxidation and reduction of gases. The decomposition of Ag2O, if formed during processing, into metal Ag at the temperatures above ⬃200 ° C 共Refs. 16 and 17兲 ensures the availability of pure metallic Ag for the sensing application at the operating temperatures which are above 200 ° C. Using a well-defined aerosol route we made an attempt to develop ppb-level gas sensors composed of monodisperse SnO1.8 : Ag nanoparticle films by varying the Ag particle size as well as its concentration. The distinct advantage of the applied aerosol route over alternative synthesis methods is a very high level of control over grain size and doping level by generating both SnOx nanoparticles as well as dopant 共Ag兲 nanoparticles separately, selecting the desired particle sizes, and then mixing them homogeneously as aerosols for adjusting the concentration of both aerosols in order to achieve the required Ag content in SnOx films. a兲

Electronic mail: [email protected] Electronic mail: [email protected]

b兲

The method of preparation for the size-selected SnO1.8 : Ag nanoparticles for 关Ag兴 = 0.1%, 1%, and 5% is discussed elsewhere.18–20 Figure 1 shows the schematic of the synthesis procedure. The nanoparticle aerosol composed of agglomerates due to Brownian coagulation of the primary particles is charged using a radioactive source 共Kr85兲 and then allowed to flow into a differential mobility analyzer 共DMA兲 for size fractionation. The DMA selects the particles on the basis of their electrical mobility, which is a function of the particle size, charge level, and shape. The nanoparticle size can be varied by varying the electric field strength in the size-selection zone of the DMA. In our previous reports we have reported transmission electron microscopy 共TEM兲 studies for size-selected SnO1.8 and Ag nanoparticles.20–22 It was shown that the particle size for SnO1.8 and Ag nanoparticles estimated using transmission electron microscopy was identical to the value selected by the DMA. Furthermore, an inflight sintering at high temperature stabilizes the SnOx against grain growth in the film, which results into long term stability.18 The Ag particles which were shown to be approximately spherical even without sintering20 are added to the films at the concentrations of 0.1%, 1.0%, and 5%, based on

FIG. 1. Schematic of the procedure used for synthesis of SnO1.8 : Ag mixed nanoparticle films where sections of the setup are described as follows: 1, evaporation furnace; 2, aerosol charger 3, DMA in which the voltage 共V兲 controls the size; 4, sintering furnace; 5, particle counter; and 6, film deposition. Stages of the synthesis are described as follows: A, polydisperse SnO particles; A1, polydisperse Ag particles; B, charged SnO particles, B1, charged Ag particles; C, size-selected SnO particles; C1, size-selected Ag particles; D, size-selected SnO1.8 particles; N, concentration of SnO1.8 particles, N1, concentration of Ag particles; and E, SnO1.8 : Ag particle film.

0003-6951/2006/89共15兲/153116/3/$23.00 89, 153116-1 © 2006 American Institute of Physics Downloaded 12 Oct 2006 to 133.21.215.80. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

153116-2

R. K. Joshi and I. E. Kruis

number concentration. Low-pressure impactor was used for producing SnO1.8 : Ag nanoparticle films with a small area 共⬃1 mm2兲 for gas sensing application. The films were deposited on substrates with gold electrodes in which the electrode structures were bonded to a DIL-16 chip carrier through gold bond wires. Resistance measurements of the nanoparticle films in different gas environments were carried out by using an automated setup including a picoammeter with an internal voltage source. Details of the gas sensing measurement setup with definitions for sensor signal, response time, and recovery time are described in a previous report.20 An increase in the value of sensor signal and decrease in response time with increase of Ag concentration 共up to 5%兲 for the SnO1.8 : Ag mixed films has been observed.20 Further increase of the Ag concentration 共关Ag兴 = 10% 兲 results into a decrease of the sensor signal.20 We study the variation of the sensor signal, and response and recovery times for SnO1.8 : Ag 共关Ag兴 = 10%, 1%, and 5%兲 nanoparticle films with an estimated thickness of ⬃1.5 ␮m, prepared from the size-selected nanoparticles on decreasing the size of the Ag particles in the SnO1.8 : Ag mixed films, keeping the size of SnO1.8 particles at 20 nm. The enhancement in gas sensing behavior on the addition of Ag in SnO1.8 nanoparticle films has been explained on the basis of the phenomenon of chemical sensitization23,24 due to spillover effect at the surface of the mixed films.20 This was explained on the basis of increased catalytic behavior at the surface by the addition of Ag nanoparticles. We have also discussed the improbability of electronic sensitization23,24 in the mixed sensors and concluded that the chemical sensitization via a spillover effect of the Ag nanoparticles is the predominant mechanism responsible for enhancement of gas sensing characteristics on adding Ag nanoparticles. The metal, Ag in this case, enhances the catalytic oxidation of the gas on the semiconductor surface through chemical sensitization. In chemical sensitization, the metal particles act as centers for surface-gas adsorption, which is then followed by spillover onto the oxide surface to react with the negatively charged chemisorbed oxygen. The sensing results of mixing sintered Ag nanoparticles to SnO1.8 support the phenomenon of chemical sensitization. It has been observed that the sensors with sintered Ag particles are less sensitive towards ethanol vapors in comparison with the sensors with nonsintered Ag particles. This phenomenon was understood by the fact that the chemical sensitization by catalytic effect is less in the case of sintered Ag nanoparticles, which are the catalytic centers, due to a loss of active surface area due to sintering. The catalytic effect of the Ag nanoparticles is expected to be enhanced on decreasing their size. Therefore, we study the gas sensing response at 400 ° C for the sensors with 关Ag兴 = 10%, 1%, and 5% with different sizes of Ag nanoparticles keeping the size of SnO1.8 at 20 nm. We observed an enhancement in the gas sensing response on decreasing the size of Ag particles in the sensors for all Ag concentrations. The variations are shown in Fig. 2. The significance of the Ag particle size effect was also reflected in terms of the improvement of response and recovery times, as shown in Figs. 3共a兲 and 3共b兲. The sensors are observed to show decrease in the response and recovery times on decreasing the size of Ag nanoparticles and values as small as 2 s 共response time兲 and 45 s 共recovery time兲 have been obtained for 1000 ppm ethanol at 400 ° C using the SnO1.8 : Ag 共关Ag兴

Appl. Phys. Lett. 89, 153116 共2006兲

FIG. 2. Variation of sensor signal with size of Ag particles at different Ag concentrations 共indicated in legend兲 in SnO1.8 : Ag sensors at 400 ° C for 1000 ppm ethanol.

= 5 % 兲 with SnO1.8 of 20 nm and Ag particles of 5 nm. The increase of gas sensor signal and decrease in response and recovery times on reducing the size of Ag particles confirms our hypothesis of chemical sensitization due to the spillover effect. It is well known that on decreasing the size of metal nanoparticles, their catalytic behavior increases. Since decrease of the particle size leads to an increase of active surface area which plays a key role for the surface phenomena involved in gas sensing, it can be said that the larger the area of the catalytic center is, the better the gas sensing response. The specific surface area values for the monodisperse Ag nanoparticles of sizes of 5, 10, and 20 nm are 114.3, 57.1, and 28.5 m2 / g, respectively. The radius of the particles was estimated using TEM analysis and it was found to be in agreement with the value selected by the DMA. Our method of tuning the Ag particle size independently from its concentration is useful for optimizing the sensors for

FIG. 3. 共a兲 Variation of response time at 400 ° C for 1000 ppm ethanol with size of Ag particles in SnO1.8 : Ag sensors and 共b兲 variation of recovery time at 400 ° C in synthetic air with size of Ag particles in SnO1.8 : Ag sensors. Downloaded 12 Oct 2006 to 133.21.215.80. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

153116-3

Appl. Phys. Lett. 89, 153116 共2006兲

R. K. Joshi and I. E. Kruis

by the above mentioned aerosol technique for the detection of ppb-level ethanol have high degree of reproducibility for the sensor signal because the size of the nanoparticles and concentration of Ag have been controlled with high accuracy. In summary, the Ag particle size dependence on the gas sensing behavior of SnO1.8 : Ag mixed films has been studied using tailored nanoparticle films. The sensor’s behavior was found to be strongly dependent on the particle size of the Ag nanoparticles. Sensors with optimized Ag concentration of 5% and Ag particle size of 5 nm have been used to detect ethanol at a concentration as low as 100 ppb in synthetic air. The demonstration of the ppb-level gas detection by the nanoparticle film sensors prepared by this well-defined synthesis technique is expected to be useful for developing advanced sensing materials for lower concentration of gases. 1

FIG. 4. 共a兲 Variation of sensor signal with ethanol concentration and 共b兲 response behavior for 100 ppb ethanol for SnO1.8 : Ag 共关Ag兴 = 5 % 兲 sensors with SnO1.8 particles of 20 nm and Ag particles of 5 nm at operating temperature of 400 ° C.

the detection of ethanol at low concentrations. Variation of the sensor signal with ethanol concentration is shown in Fig. 4 for SnO1.8 : Ag 共关Ag兴 = 5 % 兲 mixed films with SnO1.8 particles of 20 nm and Ag particles of 5 nm, which was found to be optimal in performance from Figs. 2 and 3. The particle size effect was also clearly seen at low concentrations of ethanol. For example, in the case of 10 ppm ethanol in synthetic air, the response time was noticed as 1 min with 5 nm Ag particles and 30 min 共Ref. 20兲 with Ag of 20 nm in SnO1.8 : Ag 共关Ag兴 = 5 % 兲 sensors. The SnO1.8 : Ag 共关Ag兴 = 5 % 兲 sensors with 5 nm Ag particles are observed to show a sensor signal value of ⬃3 with a response time of ⬃5 min for 100 ppb of ethanol in synthetic air. The response behavior for 100 ppb ethanol is shown in Fig. 4共b兲. Only the first cycle is shown in the figure; further cycles showed the same behavior. This study clearly demonstrates the detection of ethanol at very low concentration by means of reducing the size of Ag particle in mixed nanoparticle films. Sensors made

E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, and Z. L. Wang, Appl. Phys. Lett. 81, 1869 共2002兲. 2 A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S. Z. Deng, N. S. Xu, Y. Ding, and Z. L. Wang, Appl. Phys. Lett. 88, 203101 共2006兲. 3 C. Wang, X. Chu, and M. Wu, Sens. Actuators B 113, 320 共2006兲. 4 P. Ivanov, E. Llobet, X. Vilanova, J. Brezmes, J. Hubalek, and X. Correig, Sens. Actuators B 99, 201 共2004兲. 5 L. Mädler, A. Roessler, S. E. Pratsinis, T. Sahm, A. Gurlo, N. Barsan and U. Weimar, Sens. Actuators B 114, 283 共2006兲. 6 Q. Wan and T. H. Wang, Chem. Commun. 共Cambridge兲 30, 3841 共2005兲. 7 A. Salehi and M. Gholizade, Sens. Actuators B 89, 173 共2003兲. 8 A. N. Shatokhin, F. N. Putilin, O. V. Safonova, M. N. Rumyantseva, and A. M. Gas’kov, Inorg. Mater. 38, 374 共2002兲. 9 U. S. Choi, G. Sakai, K. Shimanoe, and N. Yamazoe, Sens. Actuators B 107, 397 共2005兲. 10 O. Wurzinger and G. Reinhardt, Sens. Actuators B 103, 104 共2004兲. 11 V. Lantto and J. Mizsei, Sens. Actuators B 5, 21 共1991兲. 12 L. Jianping, W. Yue, G. Xiaoguang, M. Qing, W. Li, and H. Jinghong, Sens. Actuators B 65, 111 共2000兲. 13 A. N. Pestryakov, V. V. Lunin, A. N. Devochkin, L. A. Petrov, N. E. Bogdanchikova, and V. P. Petranovskii, Appl. Catal., A 227, 125 共2002兲. 14 W.-L. Dai, Y. Cao, L.-P. Ren, X.-L. Yang, J.-H. Xu, H.-X. Li, H.-Y. He, and K.-N. Fan, J. Catal. 228, 80 共2004兲. 15 L.-P. Ren, W.-L. Dai, X.-L. Yang, Y. Cao, H. Li, and K.-N. Fan, Appl. Catal., A 273, 83 共2004兲. 16 J. E. Pierson and C. Rousselot, Surf. Coat. Technol. 200, 276 共2005兲. 17 T. Shima and J. Tominaga, Thin Solid Films 425, 31 共2003兲. 18 R. Ramamoorthy, M. K. Kennedy, H. Nienhaus, A. Lorke, F. E. Kruis, and H. Fissan, Sens. Actuators B 88, 281 共2003兲. 19 F. E. Kruis and R. K. Joshi, China Particuol. 3, 99 共2005兲. 20 R. K. Joshi, F. E. Kruis, and O. Dmitrieva, J. Nanopart. Res. 共to be published兲. 21 M. K. Kennedy, F. E. Kruis, H. Fissan, B. R. Mehta, S. Stappert, and G. Dumpich, J. Appl. Phys. 93, 551 共2003兲. 22 K. K. Nanda, A. Maisels, F. E. Kruis, H. Fissan, and S. Stappert, Phys. Rev. Lett. 91, 106102 共2003兲. 23 N. Yamazoe, Sens. Actuators B 5, 7 共1991兲. 24 N. Yamazoe, Y. Kurokawa, and T. Seiyama, Sens. Actuators 4, 283 共1983兲.

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Influence of Ag particle size on ethanol sensing of ...

(Received 15 March 2006; accepted 22 August 2006; published online 12 October 2006). The influence of Ag ... technique. © 2006 American Institute of Physics.

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