synthesis of pd or pt titanate nanotube and its application

A vailable online at www.sciencedirect.com Sensors and Actuators B 128 (2007) 320–325 Synthesis of Pd or Pt/titanate nanotube and its application to catalytic type hydrogen gas sensor Chi-Hwan Han a , Dae-Woong Hong a , Il-Jin Kim a , Jihye Gwak a , Sang-Do Han a,∗ , Krishan C. Singh b a Photo- & Electro-Materials Research Center, Korea Institute of Energy Research, 71-2, Jangdong, Yuseong, Daejeon 305-343, Korea b Department of Chemistry, Maharshi Dayanand University, Rohtak 124001, India Received 29 March 2007; received in revised form 20 June 2007; accepted 20 June 2007 Available online 27 June 2007 Abstract A catalytic combustible gas sensor has been developed by using Pd and Pt/titanate nanotubes. Pd and Pt/titanate nanotube catalysts were synthesized by a hydrothermal synthesis method. Sensors were fabricated by screen-printing of the catalytic material and a compensating material on an alumina plate with a platinum heater. The sensor with Pd and Pt/titanate nanotubes showed higher response than that with conventional Pd and Pt catalysts. This seems to be due to the evenly dispersed Pd and Pt catalysts on the titanate nanotubes at a nano-scale level, and the better adsorption of hydrogen on the titanate nanotube surface which facilitates the oxidation of hydrogen by the Pd and Pt catalysts. The present flat-type catalytic combustible hydrogen sensor is a good candidate for detection of hydrogen. © 2007 Elsevier B.V. All rights reserved. Keywords: H 2 ; Gas sensor; Catalytic sensor; Pd/titanate nanotube; Pt/titanate nanotubes 1. Introduction Catalytic combustion sensors are used primarily to detect combustible gases. Combustible gas mixtures do not burn till they reach an ignition temperature. However, in the presence of certain chemical media, the gas ignites at lower temperature. This phenomenon is known as a catalytic combustion [1–4]. Both metals and metal oxides have these catalytic properties. Platinum and palladium are excellent catalysts for combustion. The very high rate of reaction on the noble metals makes them ideal for the detection, by calorimetric methods of reducing gases at concentrations around lower explosive limits (LEL). Catalytic sensors for detection of hydrogen up to 100% LEL have been developed by many groups [1,5,6]. In these sensors, reaction of hydrogen and oxygen on the sensing element (Pd and Pt catalysts) causes a rise in its temperature. The tempera- ture of the sensing element is generally compared with that of a compensating element without catalysts. Commercially avail- able catalytic gas sensors consist of a catalytic surface and a ∗ Corresponding author. Tel.: +82 42 860 3449; fax: +82 42 860 3307. E-mail address: hanchi@kier.re.kr (S D. Han). platinum wire as a temperature sensor and heater to maintain the catalyst at the operating temperature. The catalytic surface is generally prepared by sintering noble metal particles (Pt and Pd) on a high surface area material like ␥-Al 2 O 3 , SnO 2 ,TiO 2 and their mixtures. However, there are still certain limitations associated with them. These sensors show low sensitivity due to lack of adsorption sites for hydrogen, can only be operated at high temperature, and are prone to catalytic poisoning [4,7]. Recently titania has attracted much attention for its oxygen sensing capability [8–10]. Furthermore with proper manipula- tion of the microstructure, crystalline phase and/or addition of proper impurities or surface functionalization, titania can also be used as a reducing gas sensor [11–13]. The interaction of a gas with a metal oxide semiconductor is primarily a sur- face phenomenon; therefore nano-porous metal oxides offer the advantage of providing a large sensing surface area. Recently, semiconducting oxide nano-wires which are usually stoichio- metrically better defined and have a greater level of crystallinity than the multi-granular oxides have been used in semiconductor sensors for hydrogen [14–16]. The application of Pd or Pt dispersed on titanate nanotubes (Pd and Pt/titanate nanotubes) in combustion type sensors has not been exploited yet. We have synthesized Pd and Pt/titanate 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.06.025 C H. Han et al. / Sensors and Actuators B 128 (2007) 320–325 321 nanotubes by a hydrothermal method and prepared a flat-type catalytic combustion hydrogen sensor on an alumina substrate using both Pd and Pt/titanate nanotubes as catalysts. The work- ing of the sensors fabricated with Pd and Pt/titanate nanotubes and TiO 2 nanoparticles has been compared and explained in the present paper. The choice of H 2 as a test gas is driven by its potential applications as a fuel in internal combustion engines, fuel cell vehicles, and in manufacturing of many industrial chem- icals like ammonia, methanol, gasoline, heating oil, rocket fuel, etc. [17,18]. 2. Experimental The method employed for the synthesis of Pd and Pt/titanate nanotubes was essentially the same as described by Ma et al. [19]. Commercial anatase-type titania powder and PdCl 2 or H 2 PtCl 6 in an equal amount (2 g each) were dispersed in an aqueous solution of NaOH (10 M, 40 ml) and charged into a Teflon-line autoclave. The autoclave was heated at 150 ◦ C for 12 h for hydrothermal treatment. The precipitates were sepa- rated by filtration and washed with dilute HCl and de-ionized water. The synthesized Pd and Pt/titanate nanotubes were dried at 120 ◦ Cinanoven. Synthesized Pd and Pt/titanate nanotubes were examined by powder X-ray diffraction (XRD; Rigaku, Ultima plus diffrac- tometer D/Max 2000). Particle morphology and size were investigated by a field emission scanning electron microscope (FE-SEM; Hitachi, S-4300) and a transmission electron micro- scope (TEM; JEOL, JEM-3000F). Thermal analysis was carried out using a simultaneous thermal analyzer (STA; Scinco, STA S-1500) with a heating rate of 5 ◦ C/min. Fig. 1. Schematic diagram of (a) the sensor structure and (b) the fabricated sensor. Dimensions are in micrometer. Fig. 1 schematically shows the structure and size of the present sensor device. The sensor device was fabricated in the following procedure. A platinum micro-heater was formed on an alumina plate by a screen-printing method with platinum paste (METECH, Platinum conductor PCC 11417) and heat treat- ment at 1000 ◦ C for 10 min. The sensing element was formed by screen-printing of a catalytic layer on the platinum heater, followed by firing at 700 ◦ C for 1 h in a muffle furnace. The compensating element with an inert layer was also formed by the same method. The sensing and compensating elements were linked to signal pins of the sensor body by spot welding (WITH Corporation, WMH-V1) with platinum wire (thickness: 30 ␮m). The compensating element formed one arm of the Wheatstone bridge. The sensor element was connected in series with the bridge, such that nearly the same current flowed through the compensating element and the sensor element. The surface tem- Fig. 2. Schematic view of the measuring system. 322 C H. Han et al. / Sensors and Actuators B 128 (2007) 320–325 perature of the sensor at each applied voltage was measured by an IR radiation thermometer (Minolta IR 0506C). The sensing material as a combustion catalyst for hydro- gen was (a) Pd and Pt/titanate nanotubes (15 wt%) supported on ␥-Al 2 O 3 (70 wt%), and the reference material to com- pensate the heat capacity of it in a bridge circuit was an inactive ␥-Al 2 O 3 film. For comparison, three type of sensors coated with the following compositions were also pre- pared; (b) titanate nanotubes (15 wt%) + Pd and Pt (PdCl 2 and H 2 PtCl 6 15 wt%) + ␥-Al 2 O 3 (70 wt%), (c) TiO 2 nanoparticles (15 wt%) + Pd and Pt (15 wt%) + ␥-Al 2 O 3 (70 wt%), and (d) Pd and Pt (30 wt%) + ␥-Al 2 O 3 (70 wt%). TiO 2 nanoparticles were purchased from Nanostructured & Amorphous Materials Inc. The catalyst layers were screen-printed with a viscous paste, which was a mixture of oxide powder and an organic vehicle. The metal oxide powder material was mixed with an organic vehicle at a concentration of 20 wt%, followed by ball-milling for 24 h, to prepare the pastes suitable for screen-printing. The organic vehicle was prepared by dissolving 10 g polyvinyl alco- hol resin in a mixed solution of 13 mmol n-butyl alcohol and 350 mmol ␣-terpineol, followed by vigorous stirring at 80 ◦ C. All sensing experiments were carried out using a thermostatic environmental test chamber connected with a signal interface and power controller, as shown in Fig. 2. Freshairwas introduced and then the gas inlet and outlet of the chamber were closed. The device was exposed to a hydrogen gas sample for ∼5 s for gas response test, and the device was recovered by exposing to purified air again. Dry hydrogen and air from commercial gas cylinders were mixed in a desired ratio. Mass flow controllers were used to set the hydrogen and air flow rates. The output voltages of the sensor were measured when the chamber reached Fig. 3. The measured and calculated temperature vs. heater voltage. to desirable conditions. The sensor response V was defined as the difference between the output voltage in a sample gas (V g ) and that in air (V a ): V = V g − V a . 3. Results and discussion 3.1. Calibration of platinum heater Changes in the platinum heater resistance were monitored when a linearly increasing current was applied to the heater. The resistance was converted to sensor temperature according to the well-known equation [20]: R T 2 = R T 1 [1 + α(T 2 − T 1 )] Fig. 4. SEM images of (a) Pd/titanate nanotubes, (b) Pt/titanate nanotubes, (c) titanate nanotubes, and (d) TiO 2 nanoparticles. C H. Han et al. / Sensors and Actuators B 128 (2007) 320–325 323 Fig. 5. EDAX mapping results of Pd/titanate nanotubes. Red dots are of Ti and green dots are of Pd. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.) where R T1 is the resistanceat the initial temperature T 1 , R T2 is the resistance at the final temperature T 2 , and ␣ is the temperature coefficient (+0.00377/ ◦ C). The calculated temperature of the heater was compared with the measured temperature by the IR thermometer and presented in Fig. 3. It can be seen that the measured and calculated tem- peratures at different applied heater voltages well agreed with each other. Thus, it could be concluded that the platinum heater fabricated by screen-printing from platinum paste had the same characteristics of the conventional platinum heater. 3.2. Characterization of Pd or Pt/titanate nanotube Fig. 4 showed typical SEM images showing uniform nature of nanotubes having approximately 100 nm diameters. Nanotubes were found to be entangled with each other. Fig. 5 showed the EDAX mapping result of Pd/titanate nanotube. Thedispersion of Pd on the nanotubes is uniform as depicted in Fig. 5. The EDAX spectra also identified that the nanotubes are composed of Ti, Pd and O. There is almost no Na recorded in nanotubes. Taking into consideration of H • , the synthesized titanate nanotubes are attributed to protonic titanate consistent with the previous stud- Fig. 7. XRD results of (a) Pd/titanate nanotubes, (b) Pt/titanate nanotubes, (c) titanate nanotubes, (d) TiO 2 nanoparticles. PdO (*), Pt metal (**). ies [19].InFig. 6, typical TEM images demonstrate uniformly sized nanotubes over which Pt or Pd nanoparticles are randomly distributed. The outer diameter of the nanotubes in TEM image is again approximately 100 nm, and it is consistent with the SEM images. In Fig. 7, XRD patterns of Pd and Pt/titanate nanotubes are presented and compared with that of titanate nanotubes synthesized in the absence of Pd or Pt. All the three samples are basically having the same identical structures. The peaks 2θ = 9.5, 24.5, 28, 48 and 62 can readily be indexed to lepidocrocite-type titanate phase (e.g., orthorhom- bic H 2 Ti 2−x/4  x/4 O 4 , a = 0.3643, b = 1.8735 and C = 0.2978 nm) and correspond well with 0 2 0, 1 1 0, 1 3 0, 2 0 0 and 0 0 2 reflec- tions. For Pd and Pt/titanate nanotubes, additional peaks from PdO and Pt metal were identified, as shown in Fig. 7a and b, respectively. Thus, Pd and Pt exist over the titanate nanotube surface as an oxide form and a metal form, respectively. Fig. 8 showed TG/DTA curves of Pt and Pd/titanate nan- otubes. The nanotubes showed ∼13% weight loss after heated to 1000 ◦ C and nearly 6% weight loss was found up to 300 ◦ C. The curves of Pt and Pd/titanate nanotubes were almost identical. Fig. 6. TEM images of (a) Pd/titanate and (b) Pt/titanate nanotubes. 324 C H. Han et al. / Sensors and Actuators B 128 (2007) 320–325 Fig. 8. TG/DTA results of (a) Pd/titanate and (b) Pt/titanate nanotubes. This demonstrates that the presence of Pd nanoparticles over titanate nanotubes does not change the crystalline nature of the tubes. Our TG curves are slightly different from that of Ma et al. [19]. Weight losses of 10% up to 140 ◦ C and 13% after 1000 ◦ C heating have been found in their curves. 3.3. Evaluation of sensor performance Fig. 9 shows the responses of typical sensors pre- pared with the compositions of (a) Pd and Pt/titanate nanotubes (30 wt%) + ␥-Al 2 O 3 (70 wt%), (b) titanate nan- otubes (15 wt%) + Pd and Pt (15 wt%) + ␥-Al 2 O 3 (70 wt%), (c) Fig. 9. Response of sensors using various catalysts. TiO 2 nanoparticles (15 wt%) + Pd and Pt (15 wt%) + ␥-Al 2 O 3 (70 wt%) and (d) Pd and Pt (30 wt%) + ␥-Al 2 O 3 (70 wt%) to 1% hydrogen concentration in air at different heater temperatures. The maximum sensor response (V) can be achieved at nearly 250 ◦ C. The sensor response of composition (a) is almost twice larger than those of the sensors of compositions (b), (c) and (d). The sensor response of composition (a) at 118 ◦ C is almost equal to that of composition (b) at 250 ◦ C. Generally catalytic gas sensors (Pd and Pt dispersed on ␥- Al 2 O 3 ) become sensitive only at high temperature. Various parameters such as crystalline size, film thickness, porosity, amount and nature of dopants, surface oxides and catalysts are known to be important in enhancing the gas sensitivity of the sensors [2]. A thick film of polytetrafluoroethylene (PTFE) on the Pd and Pt dispersed Al 2 O 3 surface is found to be resistant to catalytic poisoning and reduce the sensor’s maximum response temperature to 120 ◦ C [4]. We have also reported reduced maxi- mum temperature of a flat-type catalytic hydrogen sensor using TiO 2 and UV LED [21]. It is well-known that noble metals like Pt and Pd are particu- larly active for oxidation reactions because the heat of adsorption of oxygen on noble metals is sufficiently low to allow relatively low activation energy of oxidation and consequently a rapid rate of reaction. In Fig. 9, the operational temperature for maximum response for all the sensors was around 250 ◦ C. This is the tem- perature at which the rate of reaction on the catalytic surface is fastest, resulting in a large change of temperature and conse- quently a large change of voltage in the circuit. For the sensors (b), (c) and (d), the magnitude of maximum response at 250 ◦ C is almost the same. This indicates that the catalytic activity of Pt and Pd is not affected by the presence or absence of TiO 2 nanoparticles or nanotubes. However, the response of sensor (a) is almost twice as large as those of the sensors (b), (c) and (d). This indicates that the rate of oxidation reaction on Pd and Pt/titanate nanotube surfaces is almost twice as large as those on the other catalysts. This also suggests that the number of adsorp- tion sites or the catalytic surface area has increased considerably. This is only possible if the size of Pd or Pt particles on titanate nanotubes is in nano-scale, which can be observed from TEM Fig. 10. The relation between the sensor response and hydrogen concentration at an applied voltage of 4 V. C H. Han et al. / Sensors and Actuators B 128 (2007) 320–325 325 images of Pd and Pt/titanate nanotubes. Another reason for the enhanced response of sensor (a) may be due to better adsorption of hydrogen on the titanate nanotube surface which facilitates the oxidation of hydrogen reaction by the Pd and Pt catalysts on the titanate nanotube surface. For sensor (b) which is also fabricated with titanate nanotubes and (Pd, Pt) catalysts, no enhancement of response was observed. This may be due to the absence of (Pd, Pt) catalysts on the titanate nanotube surface. In this case, hydro- gen adsorbed on titanate nanotubes could not be easily oxidized because the (Pd, Pt) catalysts existed apart from the nanotubes. Fig. 10 depicts the variation of sensor voltage with the vari- ation of hydrogen concentration at an operating temperature of 250 ◦ C. A linear relationship between the voltage and the hydrogen concentration up to 3% of hydrogen is found. 4. Conclusion It may be concluded that the catalytic gas sensor fabricated with Pd and Pt/titanate nanotubes shows better response than the sensors fabricated with TiO 2 nanoparticles or titanate nanotubes. The enhanced sensitivity of the Pd and Pt/titanate nanotube sen- sor may be due to faster reaction between adsorbed O 2 and H 2 . The direct adsorption of hydrogen on the titanate nanotube sur- face facilitates the oxidation reaction of hydrogen by Pd and Pt catalysts evenly dispersed at a nano-scale level on the titanate nanotubes. Acknowledgements This research was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Programs, funded by the Ministry of Science and Technology of Korea. References [1] D.W. Dabill, S.J. Gentry, P.T. Walsh, A fast response catalytic sensor for flammable gases, Sens. Actuators 11 (1987) 135–143. [2] P.T. Moseley, B.C. Tofield, Solid State Gas Sensor, IOP Publishing Ltds., Bristal, UK, 1987. [3] S.J. Gentry, T.A. Jones, The role of catalysts in solid-state gas sensors, Sens. Actuators 10 (1986) 141–163. [4] V.R. Katti, A.K. Dehnath, S.C. Gadkari, S.K. Gupta, V.C. Sahni, Passivated thick film catalytic type H 2 sensor operating at low temperature, Sens. Actuators B 84 (2002) 219–225. [5] M.G. Jones, T.G. 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Hydrogen Energy 27 (2004) 719–723. [19] R. Ma, Y. Bando, T. Sasaki, Nanotubes of lepidocrocite titanates, Chem. Phys. Lett. 380 (2003) 577–582. [20] J.J. Carr, Sensors & Curcuit, PTR Prentice Hall, Englewood Cliffs, New Jersey, 1993, p. 75. [21] C H. Han, D W. Hong, S D. Han, J. Gwak, K.C. Singh, Catalytic com- bustion type hydrogen gas sensor using TiO2 and UV-LED, Sens. Actuators B 125 (2007) 224–228. Biographies Chi-Hwan Han received his PhD degree in physical chemistry from Korea University, Seoul Korea in 2001. He worked at Bordeaux 1 University as a post- doctoral fellow in 2002–2003. At present he is working in the Photo & Electro Materials ResearchCenter, Korea Instituteof Energy Research (KIER),Daejeon, Korea. His areas of interest are (i) nano sensing materials, (ii) micro-electro mechanical system (MEMS), and (iii) electro luminescent phosphors. Dae-Woong Hong received his BSc degree in electronics engineering from Chungnam National University, Daejeon, Korea in 2006.He is currently a master course student at the Yonsei University, Seoul, Korea. His areas of interest are (i) catalytic reaction, and (ii) catalytic sensor modules. Il-Jin Kim received his PhD degree in the department of electronics engineering from Chung-Nam National University, Daejeon, Korea in 2006. He worked at ASM Genitech Co. Ltd., Korea in 2000–2003. At present he is working in the Photo & Electro Materials Research Center, KIER, Daejeon, Korea. His areas of interest are (i) man machine interface systems (MMI) and programming, (ii) MEMS and (iii) fabrication of micro chemical sensors. Jihye Gwak received her PhD degree in materials from Universit ´ e Montpellier II, Montpellier, France in 2003. She worked at theNational Institute forMaterials Science (NIMS), Japan in 2003–2005. She has been working at KIER, Daejeon, Korea, since September 2005. Her areas of interest are (i) nano-ceramic mate- rials, (ii) luminescent materials & devices, (iii) porous inorganic materials & membranes, (iv) sol–gel science, and (v) chemical sensors. Sang-Do Han received his PhD degree in solid state chemistry from Bordeaux 1 University, France in 1994. He worked at LG Semiconductors Co. Ltd., in 1978–1980. He joined KIER, Daejeon, in 1980. His areas of interest are (i) electronic and electrolyte materials, (ii) chemical sensors and (iii) hydrogen production. Krishan C. Singh received his PhD degree in chemistry from M.D. University Rohtak, Haryana, India in 1980. He has been working as a lecturer and profes- sor since 1980 in the same university. His major research field is the solution thermodynamics, electrochemistry, phosphor materials and chemical sensors. At present he is visiting scientist at KIER, under an agreement between KIER and M.D. University. His research team has collaboration with KIER for 7 years for synthesizing advanced materials. . www.sciencedirect.com Sensors and Actuators B 128 (2007) 320–325 Synthesis of Pd or Pt/ titanate nanotube and its application to catalytic type hydrogen gas sensor Chi-Hwan. substrate using both Pd and Pt/ titanate nanotubes as catalysts. The work- ing of the sensors fabricated with Pd and Pt/ titanate nanotubes and TiO 2 nanoparticles