high h2 sensing performance in hydrogen trititanate-derived tio2

The TiO2sensors fabricated by the H2Ti3O7 powders calcined at 700◦C ion exchanged exhibited an excellent gas response S = 30,000 to 1.0% H2/N2at 500◦C, which was three orders of magnitud

High H 2 sensing performance in hydrogen trititanate-derived TiO 2

School of Materials Science and Engineering and Nano Systems Institute-National Core Research Center,

Seoul National University, Seoul 151-742, Republic of Korea

Received 15 November 2005; received in revised form 26 January 2006; accepted 26 January 2006

Available online 28 February 2006

Abstract

Two types of H2Ti3O7powders were prepared by ion exchange and hydrothermal methods, and the H2sensing properties of the H2Ti3O7-derived TiO2sensors were examined In the ion exchange method, Na2Ti3O7 was first synthesized via a solid-state reaction, and H2Ti3O7was obtained from Na+/H+exchange on Na2Ti3O7 H2Ti3O7was also prepared via a hydrothermal reaction of TiO2powder in a NaOH solution The morphology, size, and phase evolution of H2Ti3O7were found to be strongly dependent on the preparation methods The TiO2sensors fabricated by the H2Ti3O7 powders calcined at 700◦C (ion exchanged) exhibited an excellent gas response (S = 30,000) to 1.0% H2/N2at 500◦C, which was three orders of magnitude higher than that of the hydrothermally synthesized powder and commercial anatase powder even though its specific surface area was smaller The higher H2gas response in the TiO2sensor derived from the ion exchanged H2Ti3O7is discussed in terms of the metastable␤-TiO2

and anatase phases

© 2006 Elsevier B.V All rights reserved

Keywords: H2Ti3O7; TiO2; H2 gas sensor; Phase evolution

1 Introduction

Hydrogen has attracted a great deal of attention as a

clean, efficient, and sustainable energy source[1], which can

be used directly for the combustion or as a fuel in fuel

cells For such applications, a reliable hydrogen sensor is

needed to detect a leakage from the storage and

transporta-tion as well as to monitor the concentratransporta-tion over a wide

range

It has been recently reported that TiO2 thin films with

well-dispersed sub-micron pores fabricated by the anodic

oxi-dation of a Ti plate exhibited a gas response greater than

103 to 1.0% H2 [2,3] Varghese et al [4] obtained the high

gas response (∼104) to 1000 ppm H2 in a well-defined TiO2

nano-tube array formed by anodic oxidation Jun et al [5]

showed an extremely high gas response (1.2× 106) to 1.0%

H2in thermally oxidized TiO2films consisting of short cracks

(non-continuous) and continuous cracks These results

sug-gest that a hydrogen sensor with a high gas response can be

∗Corresponding author Tel.: +82 2 880 6273; fax: +82 2 883 8197.

E-mail address: shhong@plaza.snu.ac.kr (S.-H Hong).

achieved using TiO2with various nano-dimensional architec-tures

Recently, low-dimensional nano-structured TiO2 materials (nano-tube, nano-fiber, and nano-wire) have been prepared by

a hydrothermal reaction of TiO2powders in an alkaline solu-tion [6] Among them, the nano-tube has an extremely high specific surface area (>200 m2/g), and was identified to be a hydrogen trititanate (H2Ti3O7)[7] H2Ti3O7is known to have

a ramsdellite structure[8]and was considered to be a potential solid oxide fuel cell electrolyte due to its appreciable protonic conductivity[9] The nano-tubes were sintered into nano-rods after calcination but the diameter of the nano-tube was nearly unchanged[6], indicating the preservation of the high surface area even after calcination at high temperatures H2Ti3O7was also synthesized by ion exchange from Li2Ti3O7or Na2Ti3O7, and was reported to transform into rutile TiO2through a defec-tive and hydrated form or ␤-TiO2 [8,10] The high surface-to-volume ratio of the H2Ti3O7-derived powders appears to make their electrical response extremely sensitive to the species adsorbed on the surface, but no attempt has been made to confirm this

The present study was aimed to prepare the H2Ti3O7powders

by hydrothermal and ion exchange methods, and investigate the phase evolution of H2Ti3O7powders as well as the H2sensing 0925-4005/$ – see front matter © 2006 Elsevier B.V All rights reserved.

doi:10.1016/j.snb.2006.01.043

to 50 ml of a 10 M NaOH aqueous solution and hydrothermally

treated in an autoclave at 130◦C for 72 h The reaction product

was washed thoroughly with distilled water and a 0.1 M HCl

aqueous solution until the pH of the washing solution became

lower than 7 in order to remove sodium (Na)[6,7] The

pow-ders prepared by ion exchange and hydrothermal methods are

referred to as i- and h-, respectively, and the calcination

tem-perature will follow the hyphen For example, i-700 designates

the powder synthesized by ion exchange method and calcined

at 700◦C.

The phases of the as-dried and calcined powders were

deter-mined by X-ray diffraction (XRD, Cu K␣, λ = 1.5406 ˚A) For

the phase evolution, the as-prepared H2Ti3O7 powders were

calcined at temperatures from 200 to 1000◦C with an interval

of 100◦C for 1 h In addition, thermal analysis was conducted

by differential thermal analysis (DTA) and

thermogravimet-ric analysis (TGA) The specific surface area of the powders

was measured by a BET method (ASAP 2010, Micromeritics)

Scanning electron microscope (SEM) and transmission electron

microscope (TEM) were used to observe the morphology of the

powders

2.2 Fabrication and characterization of sensors

For the electrical measurements, comb-like Pt electrodes

were formed by sputtering Pt on an alumina substrate through a

mask, and Pt lead wires were attached to them using a Pt paste

[11] The sensors were fabricated by printing the slurry, which

was made by mixing the calcined H2Ti3O7powders (>600◦C)

with a 1 wt.% cellulose aqueous solution, on the electroded

substrate and heat-treating at 600◦C for 1 h For comparison,

sensors were also fabricated using commercial anatase powder

and synthesized Na2Ti3O7powder

The H2 and CO sensing properties were determined by

measuring the changes in the electric resistance between

200–10,000 ppm H2 (or CO) balanced with N2 (or air) and

ultra pure N2(or air) at 500–550◦C The electrical resistance

was measured using a multimeter (2000 Multimeter, Keithley,

USA) The magnitude of the gas response (S) is defined as the

ratio (Ro/Rg) of the resistance in N2(or air) (Ro) to that in the

sample gas (Rg) The response time (t90%) is defined as the time

required for the sensor to reach 90% of its final signal

Fig 1 XRD patterns of the H2Ti3 O7 powders prepared by the ion exchange method: (A) as-dried and calcined at (B) 400 ◦C, (C) 600◦C, and (D) 800◦C.

3 Result and discussion

3.1 Characterization of H 2 Ti 3 O 7 powder

The phase evolution of the H2Ti3O7 powders prepared by the ion exchange method is shown inFig 1as a function of the calcination temperature The XRD pattern of the as-dried pow-der showed very sharp diffraction peaks (Fig 1(A)), indicating

a well-crystallized structure, and this pattern was in good agree-ment with that of the monoclinic H2Ti3O7(JCPDS Card #41-1092) except several minor peaks These peaks were identified

to be those of residual, unsubstituted Na-containing compound (Na2Ti6O13) The Na content in the as-dried powder was 1 wt.%,

as determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) When heated, H2Ti3O7 was trans-formed into␤-TiO2, which was accompanied by a 7% weight loss at approximately 250◦C on the TGA curve correspond-ing to the dehydration of H2Ti3O7 As a result, only␤-TiO2 phase was observed at 400◦C (Fig 1(B)), which existed up to

700◦C.␤-TiO2was first prepared by the hydrolysis of K2Ti4O9 and subsequent heat treatment[12] Its structure is less compact than that of the other forms of TiO2and it slowly transformed into anatase between 600 and 700◦C[12] With further heat-ing, peaks for anatase were observed in the specimen calcined

at 600◦C (Fig 1(C)) and kept present up to 900◦C Rutile began

to appear at 800◦C (Fig 1(D)) and all the powders completely transformed into rutile at 1000◦C The phase evolution was fur-ther supported by fur-thermal analysis Two endofur-thermic peaks at

250 and 560◦C and one exothermic peak at 720◦C were found

in the DTA curve (not shown here) Based on the XRD results, these peaks were attributed to the transformation from H2Ti3O7

to␤-TiO2, from␤-TiO2to anatase, and from anatase to rutile, respectively

On the other hand, very broad diffraction peaks were observed for the hydrothermally synthesized powder (Fig 2(A))

Fig 2 XRD patterns of the H2Ti3O7 powders prepared by the hydrothermal

method: (A) as-dried and calcined at (B) 400 ◦C and (C) 800◦C.

and the high-resolution transmission electron microscopy

(HRTEM) image revealed the nano-tubular morphology (not

shown here) In accordance with a previous report [7], the

H2Ti3O7nano-tubes were formed during the hydrothermal

treat-ment With heating, H2Ti3O7 was directly transformed into

anatase without formation of␤-TiO2above 400◦C (Fig 2(B)),

which was much lower than the former case In the DTA

curve (not shown here), only one big endothermic peak was

observed around 120◦C, which corresponded to the

dehydra-tion of H2Ti3O7 No evidence of␤-TiO2formation was found

in this study and the previous report[6] It is speculated that

a scroll structure of a single sheet of titanium oxide inhibits

the transformation into␤-TiO2 in the hydrothermally

synthe-sized H2Ti3O7 The phase transition from anatase to rutile

occurred at a comparable temperature to the ion exchanged case

(Fig 2(C))

The morphologies of the calcined powders at 700◦C are

shown in Fig 3 In the ion exchange method, a micron-sized

H2Ti3O7powder was initially produced and particle coarsening

(or growth) was not significant with heat treatment, resulting in

the plate-like particles of 1–2␮m size after calcination at 700◦C

(Fig 3(A)) In contrast, nano-tubular particles were originally

obtained in the hydrothermal method and calcination resulted in

the elongated particles of 20–30 nm wide and 100–200 nm long

(Fig 3(B)) Indeed, HRTEM images indicated that the

nano-tubular structure changed to a nano-rod structure with a circular

cross-section as in the earlier report[6] The specific surface

areas of the calcined powders (700◦C) determined by BET were

3.6 and 34.5 m2/g for the ion exchange and hydrothermal cases,

respectively

3.2 Characterization of sensors

Fig 4(A) shows a response transient to 1.0% H2balanced

with N2of the i-700 sensor measured at 500◦C Upon injecting

a 1.0% H2/N2sample gas, the resistance rapidly decreased by

more than four orders of magnitude The recovery was slightly

Fig 3 SEM micrographs of the calcined powders (700 ◦C) prepared by (A) ion exchange and (B) hydrothermal method.

Fig 4 (A) Response transient of the i-700 sensor to 1.0% H2/N2 at 500 ◦C

and (B) magnitude of gas response (S) and response time (t90%) in the sensors

prepared by i-700, h-700, anatase, and Na2Ti3O7.

ricated from Na2Ti3O7powder alone As shown inFig 4(B),

the magnitude of the gas response for Na2Ti3O7sensor was 6,

which suggests that Na inclusion had a negligible effect on the

H2sensing properties, considering the gas response of 30,000 for

the i-700 sensor Another highly possible cause was considered

to arise from the crystal structure of the calcined powders The

XRD results indicated that the i-700 powder consisted of␤-TiO2

and anatase while the h-700 powder was composed of anatase

Based on this comparison, the presence of␤-TiO2appears to

be responsible for the high H2sensing performance in the i-700

sensor

In order to determine the role of␤-TiO2, the powders obtained

by the ion exchange method were calcined at different

temper-atures (600–800◦C) and the H

2sensing properties were deter-mined More␤-TiO2phase was present in the powder calcined

at 600◦C, and␤-TiO2phase completely disappeared at 800◦C,

which resulted in a mixture of anatase and rutile The electrical

resistances and the magnitude of gas response to 1.0% H2/N2

are shown inFig 5as a function of the calcination temperature

Contrary to our expectation, the i-600 sensor had much lower gas

response than the i-700 even though it contained more␤-TiO2

phase It appears that the high H2sensing performance is not

directly related to the amount of␤-TiO2present As expected,

the i-800 sensor exhibited the lowest gas response possibly due

to the low gas response of the rutile phase

Fig 5 Electrical resistance (Ro in N2 gas and Rg in sample gas (1% H2)),

magnitude of gas response, and response time at 500 ◦C in the i-600, i-700, and

i-800 sensors.

on the atmosphere used (Fig 6) The phase transformation was reversible, which was demonstrated by the repeated atmospheric changes The peak intensity of␤-TiO2was slightly reduced after annealing for 40 h in air, indicating that the phase transforma-tion from␤-TiO2to anatase at 500◦C is quite slow and that the thermally induced transformation is not significant compared

to the atmospheric change From this observation, the high H2 sensing performance of the i-700 sensor can be attributed to the reversible phase transformation of the␤-TiO2phase with atmo-sphere and the accompanying change in resistance The i-600 sensor exhibited the similar behavior with atmosphere change but the peak intensity difference was very small, which might reflect the lower gas response of i-600 sensor At present, the mechanisms for the phase transformation and resistance change with atmosphere are not well understood and in situ phase anal-ysis under sensor operation conditions is further required The concentration dependence of the H2gas response in the i-700 sensor at 550◦C is shown in Fig 7(A) The magnitude

of the gas response increased almost linearly with increasing

H2concentration from 200 to 10,000 ppm The sensor exhib-ited excellent sensing properties over a wide range of the H2 concentrations As a sensing mechanism in a N2 atmosphere,

Fig 6 XRD patterns of the i-700 powders annealed at 500 ◦C in different atmo-spheres: (A) 1% H2/N2 and (B) pure N2

Fig 7 (A) H2 concentration dependence of the gas response and (B) H2

selec-tivity against CO in the i-700 sensor.

Varghese et al.[4]suggested the chemisorption of the

spilled-over hydrogen atoms and the consequent creation of an electron

accumulation layer on the TiO2 surface, which enhanced the

electrical conductance The suggested mechanism was different

from the operating principle of semiconductor-type sensors in

an oxidizing atmosphere

The selectivity between H2and CO gases and the effect of

the balance gas in the i-700 sensor were further investigated at

500◦C (Fig 7(B)) The magnitude of the gas response to 1.0%

CO balanced with N2 was∼15, which was 2000 times lower

than that for H2 Thus, the i-700 sensor exhibited an excellent

selectivity toward H2gas in a N2atmosphere However, the gas

response was extremely low in air The magnitude of the gas

response to 1.0% H2 balanced with air was only 3 and even

lower with CO gas In the TiO2 sensor with Pd electrode, the

lower gas response in air atmosphere was attributed to the

par-tial oxidation of Pd into PdO and the resultant decrease of H

atom dissolution in the Pd electrode[13] However, the

appli-cation of this mechanism to Pt electrode of this experiment is

hardly conceivable Further studies are needed to determine the

sensing mechanism at different atmospheres and to improve

the gas response in the presence of oxygen for the practical

applications

4 Conclusion

Micron-sized, plate-like H2Ti3O7 was obtained in the ion

exchange method and it transformed into␤-TiO2, anatase, and

rutile in sequence with heat treatment Trititanante nano-tubes

were obtained in the hydrothermal method and directly trans-formed into anatase without formation of␤-TiO2 The i-700 sen-sor, which was composed of␤-TiO2and anatase phases, showed

a quick response (t90%= 1 s) and an excellent gas response

(S = 30,000) to 1.0% H2/N2 at 500◦C However, the H

2 gas response was three orders of magnitude lower in the h-700 sen-sor, which was composed of anatase phase, even though the specific surface area was 9–10 times higher than i-700 The higher H2 sensitivity in the i-700 sensor is speculated to be due to the reversible phase transformation between␤-TiO2and anatase depending on the atmosphere used and the resultant change in resistivity The i-700 sensor exhibited the linear con-centration dependence over a wide range of H2concentrations (20–10,000 ppm) and the high H2selectivity against CO How-ever, the gas response in air was extremely small and needs to

be further improved for practical applications

Acknowledgment

This work was supported by the Nano Systems Institute-National Core Research Center (NSI-NCRC) program of Korea Science and Engineering Foundation (KOSEF), Korea

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