synthesis of pd or pt titanate nanotube and its application

Sensors and Actuators B 128 2007 320–325Synthesis of Pd or Pt/titanate nanotube and its application to catalytic type hydrogen gas sensor Chi-Hwan Hana, Dae-Woong Honga, Il-Jin Kima, Jih

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 Hana, Dae-Woong Honga, Il-Jin Kima, Jihye Gwaka,

Sang-Do Hana,∗, Krishan C Singhb

aPhoto- & Electro-Materials Research Center, Korea Institute of Energy Research, 71-2, Jangdong, Yuseong, Daejeon 305-343, Korea

bDepartment 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: H2; 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␥-Al2O3, SnO2, TiO2 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 addimanipula-tion 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

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 TiO2nanoparticles has been compared and explained in the

present paper The choice of H2 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 PdCl2 or

H2PtCl6 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◦C in an oven.

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.

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 ␥-Al2O3 (70 wt%), and the reference material to

com-pensate the heat capacity of it in a bridge circuit was

an inactive ␥-Al2O3 film For comparison, three type of

sensors coated with the following compositions were also

pre-pared; (b) titanate nanotubes (15 wt%) + Pd and Pt (PdCl2and

H2PtCl6 15 wt%) +␥-Al2O3 (70 wt%), (c) TiO2 nanoparticles

(15 wt%) + Pd and Pt (15 wt%) +␥-Al2O3(70 wt%), and (d) Pd

and Pt (30 wt%) +␥-Al2O3(70 wt%) TiO2nanoparticles 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 alcoalco-hol 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 inFig 2 Fresh air was 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 responseV was defined as the difference between the output voltage in a sample gas (Vg)

and that in air (Va):V = Vg− Va

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(T2− T1)]

Fig 4 SEM images of (a) Pd/titanate nanotubes, (b) Pt/titanate nanotubes, (c) titanate nanotubes, and (d) TiO nanoparticles.

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 resistance at the initial temperature T1, R T2is the

resistance at the final temperature T2, 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

inFig 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 4showed typical SEM images showing uniform nature of

nanotubes having approximately 100 nm diameters Nanotubes

were found to be entangled with each other.Fig 5showed the

EDAX mapping result of Pd/titanate nanotube The dispersion of

Pd on the nanotubes is uniform as depicted inFig 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 H2Ti2−x/4x/4O4, 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 inFig 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.

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%) +␥-Al2O3 (70 wt%), (b) titanate

nan-otubes (15 wt%) + Pd and Pt (15 wt%) +␥-Al2O3(70 wt%), (c)

Fig 9 Response of sensors using various catalysts.

TiO2 nanoparticles (15 wt%) + Pd and Pt (15 wt%) +␥-Al2O3 (70 wt%) and (d) Pd and Pt (30 wt%) +␥-Al2O3(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

␥-Al2O3) 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 Al2O3surface 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 TiO2and 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 InFig 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 TiO2 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.

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 10depicts the variation of sensor voltage with the

vari-ation of hydrogen concentrvari-ation 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 TiO2nanoparticles or titanate nanotubes

The enhanced sensitivity of the Pd and Pt/titanate nanotube

sen-sor may be due to faster reaction between adsen-sorbed O2and H2

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

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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 Research Center, Korea Institute of 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 the National Institute for Materials 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.