nano - sized pdo loaded sno2 nanoparticles by reverse micelle method

Sensors and Actuators B 136 (2009) 99–104 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Nano-sized PdO loaded SnO 2 nanoparticles by reverse micelle method for highly sensitive CO gas sensor Masayoshi Yuasa a,∗ , Takanori Masaki b , Tetsuya Kida a , Kengo Shimanoe a , Noboru Yamazoe a a Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, 6-1 Kasuga-Koen, Kasuga, 816-8580 Fukuoka, Japan b Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan article info Article history: Received 12 June 2008 Received in revised form 22 October 2008 Accepted 14 November 2008 Available online 30 November 2008 Keywords: CO sensor Nanoparticle Reverse micelle method abstract A reverse micelle method was investigated for preparing nano-sized PdO loaded on SnO 2 nanoparticles. PdO–SnO 2 nano-composite was prepared by precipitating Pd(OH) 2 and Sn(OH) 4 inside a reverse micelle. The microstructure and thegas sensing properties of obtained nanoparticles were investigated. Although the particle size of SnO 2 was as same as ca. 10 nm at each observed sample, the particle size of PdO got larger as increasing with loading amount of PdO because of agglomeration of PdO nanoparticles each other. As a result of the gas sensing measurement, it was found that the particle size of PdO on SnO 2 nanoparticle influences the gas sensing property closely. That is, the sensor response declined gradually with increasing the particle size of PdO although the maximum of the sensor response was obtained in PdO =0.1 mol%. In this method, small amount of PdO loading can be achieved as compared with PdO-loaded SnO 2 sensor prepared by the conventional impregnation method. © 2008 Elsevier B.V. All rights reserved. 1. Introduction For semiconductor gas sensors, tin oxide (SnO 2 ) has been one of the attractive materials because of its high sensitivity and chemical stability. Duringthe past decades,the physicaland chemical proper- ties of SnO 2 have been well studied with the aim of improving the performance of SnO 2 -based gas sensors [1–6]. More importantly, recent extensive studies have found three basic factors concerning the sensing properties of semiconductor gas sensors. In particular, the followings areproposed asthe mostinfluential factors: (1)grain size of particles [7], (2) microstructure of the sensing body [8], and (3) surface modification of particles (noble metal loading) [9–14]. For the grain size effect (first factor), Xu et al. have reported that the sensor response increased drastically as thegrain size decreased to less than 6 nm, which value is twice as large as the thickness of depletion layers in SnO 2 [7]. On the basis of these findings, we pre- pared almost mono-dispersed SnO 2 nanoparticles (mean diameter: 4 nm) suspended in an aqueous solution by hydrothermal treat- ment of tin hydroxide gel, and succeeded in achieving a significant increase in the sensor response to H 2 gas [15]. In addition, we con- tinued to investigate the nature of the grain size effect and reported that small crystals can be depleted of conduction electrons beyond the scheme of convention depletion theories [16]. On the other hand, for the second factor, it has been proved experimentally and theoretically that the sensor performances such as sensitivity and ∗ Corresponding author. Tel.: +81 92 583 7539; fax: +81 92 583 7538. E-mail address: yuasa@mm.kyushu-u.ac.jp (M. Yuasa). response speed depend largely on the rates of diffusion of a tar- get gas and its surface reaction (with oxygen adsorbed on SnO 2 ) [17–22]. This clearly demonstrates the importance of microstruc- ture control of sensing layers, and indicates that sensing layers with porous structures allow detection of larger sized gases by facilitat- ing their diffusion deep inside the sensing layer. Indeed, we have achieved a higher response to CO larger than H 2 by controlling the microstructure of the sensing body [8]. For the surface modifica- tion effect (third factor), it is now well accepted that loading of small amounts of noble metals, such as Pd and Pt on the SnO 2 , promotes gas response as well as the rate of response. In partic- ular, Pd has frequently been loaded on commercial SnO 2 -based gas sensors. In this case, the sensitization originated in electronic inter- action between Pd (actually PdO) and SnO 2 , as follows. The loading of PdO on SnO 2 increases the electric resistance often by about one order of magnitude, because PdO acts as a strong acceptor of elec- trons and remove electrons from the oxide. On the other hand, the resistance, when PdO is reduced to Pd on contact with the reducing gases, decreases by back electron transfer from Pd to SnO 2 . The dif- ference in the electric resistance of SnO 2 induced by a change in the oxidized and reduced states of Pd is often large, giving rise to a large increase in response to the reducing gases. Matsushima et al. suc- ceeded in loading fine PdO particles of 3–20 nm on SnO 2 particles (mean diameter: 40nm) by several routes including impregnation, colloid adsorption, and chemical fixation methods, and observed more than ten times higher H 2 gas response by the loading [23,24]. Hence, according to the above three factors concerning the sen- sor performance, it can be proposed that the loading of Pd onto porous sensing layers composed of SnO 2 nanoparticles would offer 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.11.022 100 M. Yuasa et al. / Sensors and Actuators B 136 (2009) 99–104 a way to further improve the sensor response. However, the loading of fine Pd nanoparticles onto SnO 2 nanoparticles is not still chal- lenged in contrast to the case where larger SnO 2 particles were used as the matrices [23,24]. Thus, development of a new loading method of PdO onto SnO 2 nanoparticles is required to selectively bind PdO nanoparticles onto SnO 2 nanoparticles without coagu- lation. In this study, we focused on a reverse micelle method as a new route for loading PdO nanoparticles onto SnO 2 nanoparti- cles. It is well known that when small amounts of aqueous solution and surfactant are mixed together with organic solvent in a des- ignated ratio, nano-sized water droplets called as reverse micelles stabilized by the surfactant were formed in the organic solvent. This method utilizes the reverse micelles (abbreviated as “RM”) as nano-spaces for chemical reaction. Preparation of nano-sized par- ticles by RM methods has been reported for various materials, e.g. metals [25–27], metal oxides [28–32], sulfides [33], supported cat- alysts [34–36] and nanocomposites [37]. Recently, we adopted a RM method for preparing carbon-supported LaMnO 3 nanoparti- cles as an electrocatalyst for an oxygen reduction electrode [38,39]. The promising feature of the RM methods is that nanoparticles can be prepared in nano-sized water droplets as a reaction place. Furthermore, other nanoparticles can be deposited subsequently on the nanoparticles formed in nano-sized water droplets, would result in homogeneous deposition of Pd nanoparticles onto SnO 2 nanoparticles. In this study, PdO-loaded SnO 2 nanoparticles were prepared by precipitating Pd(OH) 2 on Sn(OH) 4 of nano-sized pre- cursor with hydroxides inside RMs, followed by calcination. The obtained nanocomposites (PdO-loaded SnO 2 ) were used to form thick film type sensor devices, and the relationship between thegas sensing properties and nanostructure of PdO-loaded SnO 2 particles are discussed. 2. Experimental 2.1. Material preparation The preparation procedure of PdO-loaded SnO 2 nanoparticles by using RM method is schematically shown in Fig. 1. Three dif- ferent RM solutions were prepared in this preparation process. The molar ratio of water to surfactant was in the range of 3–12. The mix- ing ratio of surfactant to organic solution was 4:6 in weight ratio for all RM solutions. The amount of aqueous solutions used was constant at 10 mL. At first, a RM solution containing [Sn(OH) 4 ] 2− (RM-A) was prepared by mixing cyclohexane (C 6 H 12 ), non-ionic surfactant (NP-6, polyoxyethylene (6) nonylphenyl ether), and an aqueous solution containing [Sn(OH) 4 ] 2− (0.1 M). Mixing was per- formed at 10 ◦ C until the solution became colorless. The solution containing [Sn(OH) 6 ] 2− was prepared by dissolving Sn(CH 3 COO) 4 in a tetramethylammonium hydroxide solution (10%). The pH of the aqueous solution was around 13, and as such [Sn(OH) 6 ] 2− was likely formed at this high pH. To precipitate precursor Sn(OH) 2 , the solu- tion RM-A was mixed together with a RM solution containing an aqueous HNO 3 solution (6%, pH 2) (RM-B). The mixing results in a decrease in the pH of the aqueous phase by collision between dif- ferent reverse micelles, precipitating Sn(OH) 4 nanoparticles inside the reverse micelles collided. The final pH of the aqueous phase in the mixed solution was around 9. Then, the mixed solution was fur- ther mixed with the RM solution containing an aqueous Pd(NO 3 ) 2 solution (0.1–5.0mM, pH 4)(RM-C) to precipitate Pd(OH) 2 particles onto Sn(OH) 4 particles inside RMs. The loading amount of Pd was controlled between 0.05 and 5.0 mol% by changing the concentra- tion of Pd ions in the solution RM-C. By adding ethanol to break RMs containing Pd(OH) 2 –Sn(OH) 4 , the resulting precipitates were col- lected by centrifugation and they were washed with ethanol. After drying at 120 ◦ C, the obtained powder was calcined at 600 ◦ Cfor3h to form PdO–SnO 2 nanocomposites. Fig. 1. Schematic diagram of the preparation method of PdO-loaded SnO 2 nanopar- ticles by a reverse micelle method. 2.2. Material characterization The diameter of RMs in solutions containingSn(OH) 4 or Pd(OH) 2 particles was measured bya dynamiclight scattering analyzer (DLS) (ELS6000/8000, Otsuka electronics Co., Ltd.). The morphology of the composites was observed by TEM (JEM-2000EX, JEOL Co., Ltd., Japan). Qualitative and quantitative analyses of PdO in the obtained samples were performed by a wavelength dispersion-type X-ray fluorescence spectrometer with LiF analyzing crystals and Pd K␣ X- ray source (ZSX-mini, Denki Co. Ltd., Japan). The crystalline size of the samples was calculated by Scherrer’s formula from their XRD patterns measured by an X-ray diffractmeter with nickel-filtered Cu K␣ (1.5418Å) source (RINT2100, Rigaku Denki Co., Ltd., Japan). 2.3. Sensor fabrication and measurement Sensor devices werefabricated by ascreen-printing method. The obtained PdO (0.05–5 mol%)–SnO 2 powders were mixed mechani- cally with diethanolamine asa binderto formpasts forprinting. The PdO–SnO 2 powders were pasted on alumina substrates attached with a pair of comb-type Au electrodes (at a space of 90 ␮m between the electrodes) through patterned-screens to fabricate sensor devices. Then, the devices were heat-treated at 600 ◦ Cfor 3 h in air to burn the organic binder. The sensor device thus fabri- cated was settled in a quartz tube and heated by an electric furnace for sensing property measurements. The sensor device was con- nected with a standard resistor in series, and the voltage across M. Yuasa et al. / Sensors and Actuators B 136 (2009) 99–104 101 the standard resistor was measured under an applied voltage of dc 4 V to evaluate the electrical resistance of the device. The electri- cal signal of the sensor devices was acquired with an electrometer. The electric resistances of the devices in air and in air containing target gas (200 ppm CO) were measured at 300 ◦ C which was the most suitable operation temperature. As a target gas, we choose CO which don’t generate a by-product in order to evaluate only loading effect. Sensor response (S =R air /R gas ) was defined as the ratio of the electric resistance in air (R air )tointargetgas(R gas ). 3. Results and discussion 3.1. Characterization of PdO–SnO 2 nanocomposites For reverse micelle formation, the molar ratio of water to surfac- tant called as R w value (R w =[H 2 O]/[surfactant]) is a critical factor; the size of water droplets substantially depends on this value. The stability of reverse micelles is also affected by the R w value. Hence, to prepare precursor hydroxide particles with desired sizes, the effects of the R w on the diameter of reverse micelles were first examined. Fig. 2 shows the dependences of the mean diame- ters of reverse micelles containing precursor Pd(OH) 2 and Sn(OH) 4 nanoparticles. The reverse micelles containing Pd(OH) 2 were pre- pared by mixing the solution RM-C with a reverse micelle solution containing a tetramethylammonium hydroxide solution (10%) as the precipitating agent. The diameters of the two different reverse micelles increased monotonically with increasing their R w values. This tendency can be explained as follows: for reverse micelles to form, the head of the hydrophilic group of surfactant molecules (here, –(CH 2 CH 2 O)–) has to adsorb on the surface of nano-sized water droplets in an organic solvent [40,41]. When the amount of surfactant molecules is decreased, the small water droplets seem to cohere for reducing theinterfacefree energy between water droplet and organic solvent. Accordingly, the diameter of the water droplets inside reverse micelles tends to increase with increasing the R w value. For the revere micelles containing Sn(OH) 4 , they were sta- ble when the R w value was 6–12. In this case, the diameter of the reverse micelles was in the range of 4–13 nm. On the other hand, for Pd(OH) 2 , stable reverse micelle solutions were obtained when the R w value is around 3–9. This difference is likely due to the difference in the pH of the aqueous phase in the two RMs. Con- sidering the size and the stability of reverse micelles, the R w value of 9 was selected as appropriate for the preparation of precursor Sn(OH) 4 –Pd(OH) 2 composites. Fig. 3 shows the particle size dis- tribution of reverse micelles (R w = 9) in the RM solution containing Pd(OH) 2 (1.0 mol%)–Sn(OH) 4 . Thesize distribution was very narrow Fig. 2. Dependence of the diameter of reverse micelles containing Pd(OH) 2 (open circle) or Sn(OH) 4 particles (closed circle) on the molar ratio of water to surfactant. Fig. 3. Theparticle size distribution of reversemicellescontainingPd(OH) 2 –Sn(OH) 4 (1.0 mol%). and no agglomeration was observed. The above results suggest that Pd(OH) 2 –Sn(OH) 4 nanocomposites of 7–12 nm in diameter was successfully obtained in nanosized water droplets inside reverse micelles. The PdO-loaded SnO 2 nanoparticles obtained by calcination of the above composite powder were characterized. Fig. 4 shows the XRD pattern of PdO (1.0 mol%)–SnO 2 calcined at 600 ◦ C. In the pattern, only peaks ascribable to SnO 2 (tetragonal structure, a =b =4.7382 Å, c = 3.1871 Å, JCPDS 41-1445) were seen, suggest- ing the successful conversion of precursor Sn(OH) 4 to SnO 2 .No peaks of PdO were observed because of its small loading amount (1.0 mol%). The crystalline size of SnO 2 calculated with Scherer’s formula using the XRD peaks was 13.5 nm. This is in nearly good agreement of the size of the precursor composite as shown in Fig. 2. The results suggest that no significant crystal growth occurred in the composite. The qualitative and quantitative analyses of PdO in the composite were performed by X-ray fluorescence (XRF) analy- sis. Fig. 5 shows a representative XRF spectrum (Pd K␣) of the PdO (1 mol%)–SnO 2 nanocomposite, indicating the presence of Pd in the sample. The ratio of Pd to Sn was also determined by the calibration curve obtained with reference samples. For the 1.0 mol% Pd-loaded sample, it was confirmed that the determined loading amount was within 1.0% deviation from the nominal amount. Thus, it is sug- gested that Sn and Pd ions were almost completely precipitated from the precursor solutions in the present method, although the small amount of Pd below 1 mol% loading could not be precisely quantified because of difficulty in separating noise from signal of XRF. Fig. 4. XRD pattern of PdO (1.0mol%)-loaded SnO 2 nanoparticles prepared by the reverse micelle method. 102 M. Yuasa et al. / Sensors and Actuators B 136 (2009) 99–104 Fig. 5. Representative XRF spectrum (Pd K␣) of PdO (1.0mol%)-loaded SnO 2 nanoparticles. The morphology and the particle size of SnO 2 and PdO were observed by TEM. Fig. 6 shows TEM and high-resolution (HR)-TEM images of PdO–SnO 2 nanocomposites with different PdO loading amounts. The obtained TEM images show that the particle size of SnO 2 in all samples was as same as ca 10 nm, in good agreement with the XRDresults.Thus, the observed particles are judged to be of single crystalline without significant sintering even after high tem- perature calcination. This suggests the effectiveness of the present method for preparing thermally stable SnO 2 nanoparticles. On the other hand, the particle size of PdO was different, depending on its loading amount. To differentiate between SnO 2 and PdO particles, lattice images were taken by HR-TEM. For 0.5mol% PdO loading, the particle sizeof PdOwas observed to be lessthan 5nm. Withincreas- ing the loading amount, PdO particles tended to be agglomerated each other and grew up larger. For smaller 0.1 mol% PdO loading, no PdO particles with clear lattice images were observed. However, this issupposed to be owing to smallerparticle sizeof PdO,probably Fig. 7. The dependence of the electric resistance in air at 300 ◦ C on the loading amount of PdO for the devices prepared by the reverse micelle (closed circle) and impregnation methods (closed square). less than 1 or 2 nm. In addition, from the results of electric resis- tance in air, as shown later, it is understood that smaller particles of PdO are loaded on nano-sized SnO 2 . This is in marked contrast to the reported case where PdO particles with a wider size distribu- tion (3–20 nm) by an impregnation method were loaded on larger SnO 2 particles (ca. 50–100 nm) [42]. 3.2. Gas sensing properties of PdO–SnO 2 nanocomposite films Fig. 7 shows the dependence of the electric resistance of sensor films in air at 300 ◦ C on the loading amount of PdO. For compar- ison, the electric resistances of the sensor films prepared by the conventional impregnation method [42] were also shown in this figure. In the conventional impregnation method, stannic acid precipi- tated from an aqueous solutionof SnCl 4 with ammoniasolution was Fig. 6. TEM and HR-TEM images of PdO (1.0 mol%)-loaded SnO 2 nanoparticles: (a) 0.5 mol%, (b) 1.5 mol%, and (c) 5.0 mol% PdO loading. M. Yuasa et al. / Sensors and Actuators B 136 (2009) 99–104 103 Fig. 8. The dependence of the sensor response to 200 ppm CO at 300 ◦ C on the loading amount of PdO for the devices prepared by the reverse micelle method. calcined at 900 ◦ C for 5 h to obtain the SnO 2 powder. Then, PdCl 2 solution was impregnated to the above SnO 2 powder, and then the solution evaporated to dryness and reduced in a flow of H 2 gas for 3 h. The particle size of SnO 2 and Pd in the conventional method was 50–100 nm and 3–10 nm, respectively. The electric resistance of SnO 2 nanoparticles increased with loading PdO, and reached the maximum at 0.1 mol% loading. The increase at 0.1 mol% was more than one order of magnitude. The increase observed in the elec- tric resistance is similar to the trend reported in the literature [42], and can be interpreted in terms of the electric interaction between PdO and SnO 2 , for which PdO attracts electrons from SnO 2 and pro- duces electron depleted layers on the SnO 2 surface. The obtained results thus confirm that PdO nanoparticles were effectively loaded on SnO 2 nanoparticles as observed in TEM images. On the other hand, loading more than 0.1mol% decreased the resistance. The further loading of PdO may lead to the agglomeration of PdO par- ticles and impede the formation of effective contacts between PdO and SnO 2 . Note that the observed dependence of the resistance on amount of PdO loading is somewhat different from the reported dependences for samples prepared through impregnation, colloid adsorption, and chemical fixation methods [42]. This means that the electrical resistance depends on preparation methods, namely, the dispersion state of PdO over the SnO 2 surface. In addition, for the reverse micelle method, the loading amount at the maximum resistance was 15 times lower than those for the above methods. It is considered that finer dispersion of PdO was attained by the present method, reducing the optimum PdO loading amount for maximizing the depletion effects. Fig. 8 shows the dependence of the sensor response to 200 ppm CO at 300 ◦ C on amount of PdO loading. The maximum sensor response was obtained at 0.1 mol% PdO loading, reaching a high value of S =320. On the other hand, the sensor response was decreased with further increasing the loading amount. Such a trend is ingood accordance with the dependence ofthe electricresistance as shown above. This good consistency between the resistance and the sensor response indicates that the electrical interaction between PdO and SnO 2 is dominant for the improvement of the sensor response rather than the catalytic effect of PdO that assists the combustion of CO with adsorbed oxygen. As revealed in this study, the developed method can improve the sensor response even by a smaller loading amount of PdO, as compared with the other reported methods. The reduction of the loading amount is the favorable feature of the present method. Note that the size of PdO was decreased by reducing the loading amount, as deduced by the TEM observations. Moreover, based on the obtained results, it can be suggested that the sensor response is associated with the number of contacts between PdO andSnO 2 par- ticles. Itis speculated that the number ofthe contacts was increased bythe sizereductionof PdO.To examine thepossibilityof thiseffect, the number of PdO loaded on SnO 2 was roughly estimated using the representative sizes of PdO observed in the HR-TEM images. The number of PdO particles per mass (N) for each sample can be calculated using the following equation under the assumption that their sizes are constant for each sample: N = the total volume of PdO per mass the volume of a PdO particle (1) For the estimation of N, the total volume of PdO per mass in the 0.5 mol%-loaded sample is abbreviated as V. Likewise, those val- ues for 1.5 and 5.0 mol%-loaded samples are expressed as 3V and 10V, respectively. The representative particle sizes of PdO in the 0.5, 1.5 and 5.0 mol% PdO-loaded samples was approximately 4,6 and 10 nm, respectively. Thus, when the particle diameter for 0.5 mol% is abbreviated as D, then thosefor 1.5and 5.0 mol% can be expressed as 1.5D and 2.5D, respectively. By using these values for Eq. (1), the number of PdO particles for each sample can be calculated as follows: 0.5mol%:N = V (4/3)(D/2) 3 = 6 ×  V D 3  (2) 1.5mol%:N = 3V (4/3)(1.5D/2) 3 = 3.96  V D 3  (3) 5.0mol%:N = 10V (4/3)(2.5D/2) 3 = 2.88  V D 3  (4) The above simple calculation results indicate that the number of PdO particlestends to increase with decreasing theloading amount. Such an increase in the number of PdO particles is readily expected to cause an increase in the density of contacts between PdO and SnO 2 particles, provided that the size of SnO 2 is constant. Conse- quently, the surface depletion effect, induced by the formation of PdO–SnO 2 junctions, is enhanced. This significantly increases the electric resistance as well as the sensor response even by the quite low PdO loading. 4. Conclusions PdO-loaded SnO 2 nanoparticles were prepared by the reverse micelle method. Stable and mono-disperse reverse micelles of ca. 10 nm containing both Pd(OH) 2 and Sn(OH) 4 were obtained at R w = 9. The calcination of the collected hydroxide composites at 600 ◦ C produced PdO-loaded SnO 2 nanoparticles; the particle size of SnO 2 was ca. 10 nm irrespective of the PdO loading amount. Nano-sized PdO particles of ca. 4nm were prepared at 0.5 mol% loading. However, with increasing the PdO loading amount, PdO particles agglomerated each other and grew up larger. It was found that both of the electric resistance and the sensor response of PdO-loaded SnO 2 were dependent onthe loading amount.The max- imum electric resistance and sensor response were obtained at 0.1 mol% PdO loading. The optimum amount of PdO for maximiz- ing the sensor response was fairly smaller for the reverse micelle method, as compared with those for conventional methods. 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Biographies Masayoshi Yuasa has been an assistant professor at Kyushu University Since 2005. He received his ME degree in materials science in 2003.Hiscurrentresearchinterests include the development of chemical sensors and active electrocatalysts for oxygen reduction and oxygen evolution. Takanori Masaki received his ME degree in materials science in 2007 from Kyushu University. Tetsuya Kida has been an associate professor at Kyushu University since 2006. He received his ME degree in materials science in 1996 and his Dr. Eng degree in 2001 from Kyushu University. His current research interests include the development of chemical sensors, nanoparticle synthesis, and self-assembles inorganic–organic hybrid materials. Kengo Shimanoe has been a professor at Kyushu University since 2005. He received the BE degree in applied chemistry in 1983 and the ME degree in 1985 from Kagoshima University and Kyushu University, respectively. He joined Nippon Steel Corp. in 1985, and received PhD in engineering in 1993 from Kyushu University. His current research interests include the development of gas sensors and other functional devices. Noboru Yamazoe had been a professor at Kyushu University since 1981 until he retired in 2004. He received his BE degree in applied chemistry in 1963 and PhD in engineering in 1969 from Kyushu University. His research interests were directed mostly to development and application of functional inorganic materials. . sensor Nanoparticle Reverse micelle method abstract A reverse micelle method was investigated for preparing nano-sized PdO loaded on SnO 2 nanoparticles. PdO SnO 2 nano-composite. even by the quite low PdO loading. 4. Conclusions PdO- loaded SnO 2 nanoparticles were prepared by the reverse micelle method. Stable and mono-disperse reverse