Current Applied Physics 9 (2009) S79–S81

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Current Applied Physics

journal homepage: www.elsevier .com/locate /cap

Preparation and gas sensitivity of SnO2 nanopowder homogenouslydoped with Pt nanoparticles

Young-In Lee a, Kun-Jae Lee a, Don-Hee Lee a,b, Young-Keun Jeong c, Hee Soo Lee c, Yong-Ho Choa a,*

a Department of Fine Chemical Engineering, Hanyang University, Ansan 426-791, Republic of Koreab LG Electronics Institute of Technology, 16 Woomyoen-dong, Seocho-gu, Seoul 137-724, Republic of Koreac National Core Research Center for Hybrid Materials Solution, Pusan National University, Busan 609-735, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Available online 31 August 2008

PACS:82.45.-h81.16.Hc

Keywords:Tin oxidePlatinumGas sensorZeta potentialCatalyst

1567-1739/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.cap.2008.08.024

* Corresponding author.E-mail addresses: dalmajunior@naver.com (Y.-I. Le

Lee), choa15@hanyang.ac.kr (Y.-H. Choa).

Platinum (Pt)-doped SnO2 nanopowders were prepared by a new doping method which controls the sur-face charge by controlling the pH. Individual Pt particles were homogenously doped on the surface of theSnO2 nanoparticles at pH 6. Subsequently, a heat treatment was conducted in a hydrogen atmosphere toremove contaminants and to increase the number of oxygen vacancies of the SnO2 nanopowders. To rec-ognize the sensitivity of the powders, Pt-doped SnO2 gas sensors were fabricated using a dispensing tech-nology on a silicon substrate and tested at 400 �C in ethanol and formaldehyde gas, and then comparedwith sensors fabricated with commercial powders. The test results showed that the SnO2 gas sensorshomogenously doped with Pt have sufficient sensitivity for detecting reducing gas.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction using surface charge as affected by pH values was applied to hom-

Tin dioxide (SnO2), which is a typical n-type oxide semiconduc-tor, has been widely studied and used in reducing gas sensors be-cause of its high sensitivity, low cost, and quick response [1]. SnO2

gas sensors can detect reducing gases through conductancechanges in surface phenomena such as adsorption in air anddesorption in a reducing gas. The surface reactions of SnO2 are af-fected by various factors such as the microstructure, the concentra-tion of oxygen vacancies, and the elemental composition includingany doping or impurity constituents [2,3]. In general, gas sensorsprepared with only SnO2 do not possess adequate sensing proper-ties. Therefore, to improve the gas sensing properties of the SnO2

gas sensors, the addition of noble metals such as Pt and Pd to theSnO2 gas sensor has been widely studied [4,5]. It is important thatthe catalysts are homogenously doped on the surfaces of the sensormaterials to improve the gas sensing properties [6]. Several dopingmethods such as sputtering [5], evaporation [7], sol–gel [8], spraypyrolysis [9], and chemical–vapor deposition [10] have been re-ported. In these methods, however, it is difficult to homogenouslydisperse the catalyst particles on surface of sensor materials with-out a complicated process. In this study, a new doping method

ll rights reserved.

e), paulo9@hanmail.net (K.-J.

ogenously dope Pt on the surface of SnO2. In addition, the Pt-dopedSnO2 sensor was fabricated through a dispensing technology and itwas analyzed in a reducing gas.

2. Experimental procedure

SnO2 nanopowders were obtained by the attrition milling ofSnO2 powders (20 lm) prepared by the low-temperature phasetransformation method [11]. Platinum (Pt) nanopowders were syn-thesized by the modified electrochemical method using a Pt elec-trode (99.99%) and a reductant in a high applied voltage (380 V).The SnO2 and Pt (0.3 wt%) nanopowders were mixed in the aque-ous solution and the pH was adjusted to 6 and 8 by additions ofHCl and NaOH solutions, respectively. After drying the powders,a heat treatment was conducted at 160 �C for 1 h in a hydrogenatmosphere.

The surface charge of SnO2 and Pt at each pH values was mea-sured by electrophoretic light scattering (ELS, Otsuka ELS-8000)and the microstructure, the particle size, and crystal structure ofthe powders were characterized by field emission scanning elec-tron microscopy (FE-SEM, Hitachi S-4800), the Brunauer, Emmett,and Teller method (BET, Quantachrome AUTOSORB-1-C), X-ray dif-fraction (XRD, Rigaku D/MAX-2500/PC), and high resolution trans-mission electron microscopy (HR-TEM, JEOL JEM-2100F). The solfor sensor fabrication was prepared by mixing 1 g of 0.3 wt%

Fig. 1. (A) FE-SEM image of a SnO2 nanopowder and (B) TEM image of a Pt nanopowder.

S80 Y.-I. Lee et al. / Current Applied Physics 9 (2009) S79–S81

Pt-doped SnO2 nanopowder, silica sol (0.15 g), and propylene gly-col (0.75 g). The sensor, which included heating and sensing elec-trodes, was fabricated by dispensing the sol onto siliconsubstrates. The sensor was sintered for 30 min at 500 �C in air.We analyzed the sensitivity of the sensors to ethanol and formal-dehyde gas at 400 �C.

3. Results and discussion

The FE-SEM image shows the microstructure of the SnO2 nano-powder (Fig. 1A). The morphology was angular due to the use ofthe mechanical milling method, and the size was relatively homo-

Fig. 2. (A) Zeta potential values of SnO2 and Pt nanopowders with respect to pH and (Bnanopowders heated in hydrogen.

Fig. 3. TEM images of Pt-doped SnO2 nanopow

geneous. Based on the BET analysis, the specific surface area of theSnO2 nanopowder was founded to be 35.41 m2/g and the averagesize calculated by the specific surface area was 24 nm. The micro-structure of the Pt nanopowder prepared by the modified electro-chemical method was characterized by TEM (Fig. 1B). It was ofspherical shape and had a narrow particle size distribution. Thespecific surface area as analyzed by BET was 26 m2/g and the cal-culated size was 10 nm.

Zeta potential values were measured to confirm the surfacecharge of the SnO2 and Pt nanopowders with respect to pH. Asshown in Fig. 2A, when the pH was 2 < 3.8 < 4.2 < 6.5, the surfacecharges of SnO2 and Pt were opposite each other. Therefore, in the

) X-ray diffraction patterns of (a) SnO2, (b) Pt-doped SnO2, and (c) Pt-doped SnO2

ders prepared (A) at pH 6 and (B) at pH 8.

Fig. 4. The sensitivity of sensors tested (A) in ethanol (air, 2, 6, 14, 30, 70, 110, and 150 ppm) and (B) in formaldehyde (air, 0.45, 0.9, 1.7, 2.5, and 4.2 ppm) gas; tested sensorswere prepared by (a) Pt-doped SnO2 and (b) commercial nano-sized SnO2 powders.

Y.-I. Lee et al. / Current Applied Physics 9 (2009) S79–S81 S81

range of the pH condition, Pt nanoparticles can be homogenouslydoped on the surface of the SnO2 nanoparticles. However, becausethe SnO2 nanopowder was dissolved into a strong acid solution,the mixing of SnO2 and Pt was carried out in a solution of pH 6.

To remove any contaminants and increase the number of oxy-gen vacancies in the SnO2 nanopowder, the prepared Pt-dopedSnO2 nanopowders were heated for 1 h at 160 �C in a hydrogenatmosphere [12]. Fig. 2B shows the XRD patterns of the SnO2 nano-powders, Pt-doped SnO2 nanopowders, and Pt-doped SnO2 nano-powders heated in hydrogen. The XRD patterns indicated thatthe Pt doping and the heat treatment did not cause the transforma-tion of the lattice structure of SnO2. A diffraction peak for Pt wasnot detected because of the low content (0.3 wt%) of the addedPt nanoparticles. A crystallite size of 20 nm for the SnO2 nanopow-ders was calculated from the line broadening according to theScherrer equation [13]. This value corresponded well to the sizeobtained from the BET analysis.

Fig. 3 shows the TEM images of the Pt-doped SnO2 nanopow-ders at different pH values. To distinctly observe the dispersionand aggregation of Pt nanoparticles, samples for TEM analysis wereprepared by adding Pt of 3 wt%. Fig. 3A and B shows the Pt-dopedSnO2 nanopowders after the mixing of SnO2 and Pt at pH 6 and 8.The SnO2 and Pt have a positive and negative surface charge at pH6, respectively. Therefore, Fig. 3A clearly shows that individuallydispersed Pt nanoparticles, which are seen as darker grains andare of spherical morphology, are homogenously doped on the sur-face of the SnO2 nanoparticles due to attractive forces created bythe opposite surface charge. However, as seen in Fig. 3B, it is hardlypossible to find images similar to that in Fig. 3A because the SnO2

and Pt nanoparticles have a repulsive force interaction that resultsin a similar negative charge between the particles at pH 8.

To measure the sensitivity of the Pt-doped SnO2 nanopowdersprepared by the new doping method, a sensitivity test was con-ducted in ethanol and formaldehyde gas at 400 �C and the sensitiv-ity, defined as Rgas/Rair. Rgas, and Rair is the resistance of the sensorin test gas and in the air, respectively. Fig. 4 shows the sensitivityof the sensors prepared by Pt-doped SnO2 and commercial nano-sized SnO2 powders (Aldrich, 20 nm). As shown in Fig. 4, the sensi-tivity of the SnO2 sensor, which was homogenously added to Pt,was better than the sensor made of the commercial powders. Itwas also observed that the resistance of the sensor rapidly de-creased in comparison with the sensor prepared the commercialpowders because of the active desorption of electrons throughthe catalysis of Pt added to SnO2, despite exposure of small vol-umes of ethanol and formaldehyde gas. The addition of catalyticnoble metals is known to increase the sensitivity of SnO2-basedgas sensors [14,15]. The presence of Pt enhances the adsorptionand dissociation of molecular oxygen on the surface of semicon-

ductor [14]. Thus, it was assumed that the homogenous dopingof Pt, which utilized the surface charge of each material, improvedthe sensitivity of the sensor. Also the sensitivity of the Pt-dopedSnO2 nanopowders was reappeared in the second test after 2 days.It means that the synthesized Pt-doped SnO2 nanopowders wasconfirmed a reusability.

4. Summary

A homogenous doping of Pt nanoparticles on the surface of SnO2

nanoparticles was successfully achieved by utilizing the surfacecharge of each powder. In addition, it was confirmed that the sur-face charge with respect to pH had an effect on the dispersionand the aggregation of SnO2 and Pt nanoparticles. The Sensitivitytest results of the sensors prepared by Pt-doped SnO2 nanopowdersand commercial powders indicated that the homogenous doping ofPt improved the sensitivity and the sensor prepared Pt-doped SnO2

had enough sensitivity for applications as a reducing gas sensor.

Acknowledgements

This work was supported by the grants-in-aid for the NationalCore Research Center Program from MOST/KOSEF (No. R15-2006-022-03001-0), and also partially supported by grants-in-aid forthe National Research Laboratory Program from MOST/KOSEF(R0A-2003-000-10320-0).

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