hydrogen sensing using titania nanotubes

This stability of structure, which is one of the essential criteria of a gas sensing material, prompted us to study the gas sensing behavior of these nanotubes to technologically importa

ray diffraction (GAXRD) are used to study the surface morphology and crystal structure of the nanotubes.

# 2003 Elsevier Science B.V All rights reserved

Keywords: Titania; Nanotube; Hydrogen; Nanoporous; Gas sensor

1 Introduction

We recently reported[1]the fabrication of self organized

titania nanotube arrays using an anodization technique

Although the as prepared nanotubes are amorphous, they

crystallize on annealing at elevated temperatures and are

structurally stable to at least 600 8C This stability of

structure, which is one of the essential criteria of a gas

sensing material, prompted us to study the gas sensing

behavior of these nanotubes to technologically important

gases, such as oxygen, carbon monoxide, ammonia, carbon

dioxide and hydrogen

Titania has earned much attention for its oxygen sensing

capabilities[2–7] Furthermore with proper manipulation of

the microstructure, crystalline phase and/or addition of

proper impurities or surface functionalization titania can

also be used as a reducing gas sensor[8–19] The interaction

of a gas with a metal oxide semiconductor is primarily a

surface phenomenon, therefore nanoporous metal oxides

[14,15,20,21]offer the advantage of providing large sensing

surface areas

Hydrogen sensing is needed for industrial process control,

combustion control, and in medical applications with

hydro-gen indicating certain types of bacterial infection In this

work we report on the hydrogen sensing properties of titania

metal oxide hydrogen sensors are generally based on

Pt/TiO2by hydrogen[22–24] Elevated temperature hydro-gen sensors examine the electrical resistance with hydrohydro-gen concentration; for example, Birkefeld et al [25]observed that the resistance of anatase phase of titania varies in presence of carbon monoxide and hydrogen at temperatures above 500 8C, but on doping with 10% alumina it becomes selective to hydrogen

2 Experimental

(99.5% pure from Alfa Aesar, Ward Hill, MA, USA) of thickness 0.25 mm The anodization was performed in an electrolyte medium of 0.5% hydrofluoric acid (J T Baker-Phillipsburg, NJ, USA) in water, using a platinum foil cathode Well-defined nanotube arrays were grown using anodizing potentials ranging from 12 to 20 V Nanotube length increases with anodization time, reaching a length of

400 nm in approximately 20 min, and then remains constant For the present study the samples were anodized for 25 min The samples were then annealed at 500 8C in a pure oxygen ambient for 6 h, with a heating and cooling rate of 1 8C/min

A field emission scanning electron microscope (FESEM) from JEOL (model JSM6300), Peabody, MA, USA was used

to study the surface morphology of the nanotubes A glan-cing angle X-ray diffractometer (GAXRD) from Philips (model X’pert MRD PRO), The Netherlands was used to determine the crystalline phase

The electrode geometry of the titania nanotube sensors is

*

Corresponding author.

E-mail address: cgrimes@engr.psu.edu (C.A Grimes).

0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V All rights reserved.

doi:10.1016/S0925-4005(03)00222-3

metal foil with a nanotube array grown on the top An

insulating barrier layer separates the nanotubes from the

conducting titanium foil Preliminary studies using

evapo-rated gold films as well as silver paste as interdigital

electrodes showed that these materials diffuse into the titania

nanotubes at elevated temperatures resulting in sensor

con-tamination Hence a pressure contact was used to electrically

contact the nanotubes, with two spring-loaded parallel

10 mm by 2 mm platinum contact pads (100 mm thickness)

A schematic diagram of the experimental set up used for

the gas sensing studies is shown inFig 1b The test chamber

consists of a 1.3 l quartz tube, with stainless steel end caps,

placed inside a furnace (Thermolyne, USA model 21100

tubular furnace) The electrical contact was formed by

attaching the platinum pads to the ends of a spring-loaded

‘U’ shaped quartz rod Gas flow through the test chamber

was controlled via a computer-controlled mass flow

con-troller (MKS instruments, Austin, TX, USA) The electrical

resistance of the titania sensors were measured using a

computer-controlled digital multimeter (Keithley, USA

model 2000) Prior to data collection the test chamber

was initially evacuated using a mechanical pump,

where-upon nitrogen (99.999% pure) was passed while the sensor

under test was heated to the desired temperature The test gases examined, oxygen, carbon dioxide, ammonia, carbon monoxide or hydrogen, were mixed in appropriate ratios with nitrogen to create the necessary test gas ambient

3 Results and discussion The surface morphology of nanotube arrays prepared using an anodization potential of 20 V and annealed at

500 8C for 6 h in a pure oxygen ambient is shown in Fig 2a and b It can be seen from these images that the nanotubes are uniform over the surface The nanotubes are approximately 400 nm in length and have a barrier layer[1]

20 V anodization the average pore diameter, as determined from FESEM images, is 76 nm (standard deviation 15 nm), with a wall thickness of 27 nm (standard deviation 6 nm) The sample anodized at 12 V was found to have an average pore diameter of 46 nm (standard deviation 8 nm) with a wall thickness 17 nm (standard deviation 2 nm) The por-osities of the 20 and 12 V samples were calculated as 45 and 61%, respectively

Fig 1 Schematic representation of (a) the electrode geometry, and (b) the experimental apparatus.

Fig 4shows the response of the 20 V sample as a function

of ambient temperature, as it is switched from a nitrogen environment to one containing 1000 ppm hydrogen, and then back to nitrogen The plot was made using (Rg/R0)1 versus time where R0is the base resistance of the sensor, i.e the sensor resistance before introducing the test gas, and Rg the measured resistance in the presence of test gas The sensor shows increasing hydrogen sensitivity with tempera-ture, with a three order of magnitude change in resistance at

original resistance it recovered without hysteresis

The sensitivity S is defined by the formula

S¼R0 Rgs

Rgs where R0is the resistance of the sensor before passing the gas and Rgsthat after passing gas and reaching the saturation value The sensitivity of a 20 V sample with temperature, to

1000 ppm hydrogen, is shown inFig 5 Sensitivity is seen to increase with temperature to approximately 380 8C where the increase in sensitivity with temperature is beginning to saturate

The response time, defined as the time needed for the sensor to reach 90% of the final signal for a given concen-tration of gas, is plotted against temperature inFig 6 (the time includes that required for the gas to equilibrate inside

Fig 2 The surface morphology of the titania nanotubes after annealing at

500 8C: (a) high, and (b) low magnification images of a 20 V sample, and

(c) a high magnification image of a 12 V sample.

Fig 3 Glancing angle X-ray diffraction pattern of a 20 V sample (glancing angle ¼ 28) annealed at 500 8C in oxygen ambient A, R and T represent the reflections from anatase crystallites, rutile crystallites, and the titanium substrate, respectively.

the measurement chamber, estimated to be
30 s) The

response time reduces exponentially with temperature

To check the behavior of the sensor on repeated hydrogen

exposure, the hydrogen concentration was varied in discrete

steps of 100 ppm from 0 to 500 ppm while keeping the

temperature constant at 290 8C; the chamber was flushed

with nitrogen after each exposure to hydrogen The response

of the 20 V prepared nanotube sensors, kept at 290 8C, is shown in Fig 7 The behavior of the sensor is consistent, recovering its original resistance after repeated exposure to varying hydrogen gas concentrations The sensitivity of the sensor in this concentration range is plotted inFig 8a; there

is a linear increase in sensitivity at low concentrations The sensitivity of the hydrogen sensor over 100 ppm to 4% (the explosive limit in the presence of oxygen) is shown in Fig 8b

Fig 9 shows the hydrogen sensitivities, at 290 8C, of nanotube sensors having a pore diameter of 76 nm, and a pore diameter of 46 nm While smaller diameter nanotubes had greater sensitivity to hydrogen the samples made at lower anodizing voltages tended to be more brittle, and harder to mechanically handle without breaking

The 20 V sample was exposed to oxygen, carbon mon-oxide, ammonia and carbon dioxide at 290 8C The sensor was found to have no detectable variation in resistance on exposure to carbon dioxide The sensitivities of the titania

Fig 4 Variation of resistance R g , normalized with respect to baseline resistance R 0 , of a 20 V sample with time on exposure to 1000 ppm hydrogen at different temperatures It may be noted that the inverse of R g /R 0 was used in the plot for representing data in positive y-direction.

Fig 5 The sensitivity temperature dependence of a 20 V sample to

1000 ppm hydrogen.

Fig 6 Response time variation of a 20 V sample to temperature The dots

represent measured data.

Fig 7 Resistance of a 20 V sample when exposed to different concentrations of hydrogen at 290 8C The nitrogen–hydrogen mixture was passed for 1500 s; the chamber was then flushed with nitrogen for

3000 s before passing the nitrogen–hydrogen mixture again.

nanotubes to the other gases is shown in Fig 10 The

sensitivity of the nanotubes to carbon monoxide and

ammo-nia are negligible compared to that of hydrogen The

resis-tance of the nanotubes increased in presence of oxygen, and

did not regain their original electrical conductivity even after

several hours in a nitrogen environment

Since the sensor measurements were conducted in

atmo-spheres without oxygen, the increase in conductivity cannot

be due to hydrogen removing oxygen from the lattice

[27–29] or the removal of chemisorbed oxygen [30–32]

It is likely that the hydrogen molecules get dissociated at the

defects on the titania surface These molecules can diffuse

into the titania lattice, and act as electron donors[25,33,34]

But this process would lead to very slow response and recovery times and complete recovery would be difficult Since the sensor completely regains its original resistance with hydrogen cycling it appears that this is not the domi-nant mechanism behind high hydrogen sensitivity Hence,

we believe that the major process behind the interaction between the nanotubes and hydrogen is the chemisorption

During chemisorption hydrogen acts as a surface state and a partial charge transfer takes place from hydrogen to the conduction band of titania This creates an electron accumulation layer on the nanotube surface that enhances its electrical conductance On removing the hydrogen ambient, electron transfer takes place back to hydrogen and it desorbs, thus restoring the original resistance of the nanotubes

Another factor that may play a role in the hydrogen sensitivity (and selectivity) is the platinum electrodes It

is possible that platinum is acting as a catalyst for interaction

of hydrogen with titania At elevated temperatures hydrogen dissociation can occur on platinum surfaces These

surface where they diffuse into the nanotube surface From the present study it was not clear how significant a role the platinum electrodes play

Anatase, the polymorph of titania has been reported to be

of high sensitivity towards reducing gases like hydrogen and

anatase phase mainly on the walls and rutile in the barrier layer As the diffusing hydrogen atoms go to the interstitial sites[25,33]and as the c/a ratio of anatase is almost four times compared to that of rutile, it appears that anatase lattice accommodates hydrogen easily and hence has a higher contribution to hydrogen sensitivity

The effect of chemisorption can be neglected in the oxygen sensing experiments As the recovery requires several hours it appears that the nanotubes contain oxygen vacancies or titanium interstitial defects in presence of nitrogen On exposing the sensor to oxygen ambient, the lattice reoxidizes and hence the conductivity of the sensor decreases On removing oxygen, the reduction of the lattice

Fig 8 The sensitivity variation of a 20 V sample at 290 8C for (a)

low hydrogen concentrations, and (b) 0.01 to 4% hydrogen

concentra-tions.

Fig 9 A comparison of the variation in resistance of samples having pore

diameters of 46 and 76 nm, vs time, upon exposure to 1000 ppm of

hydrogen at 290 8C.

will not immediately occur hence the sensor requires several

hours to regain its original conductivity

It should be noted that the conducting titanium foil

beneath the nanotubes ultimately limits the sensitivity by

reducing the baseline resistance of the sensor

4 Conclusions

Titania nanotubes prepared using anodization and

annealed in an oxygen atmosphere at a temperature of

500 8C were found highly sensitive to hydrogen The

nano-tube sensors contain both anatase and rutile phases of titania,

and showed appreciable sensitivity towards hydrogen at

temperatures as low as 180 8C The sensitivity increased

drastically with temperature showing a variation of three

orders in magnitude of resistance to 1000 ppm of hydrogen

at 400 8C The response time decreased with increasing

temperature; at 290 8C full switching of the sensor took

approximately 3 min Results were highly reproducible with

no indication of hysteresis Our results showed these sensors

are capable of monitoring hydrogen levels from 100 ppm to

4% At 290 8C nanotubes with smaller pore diameter

(46 nm) showed higher sensitivity to hydrogen compared

to larger pore diameter samples (76 nm) The sensors

showed high selectivity to hydrogen compared to carbon

monoxide, ammonia and carbon dioxide Although the

sensor was sensitive to high concentrations of oxygen, the

response time was high and the sensor did not completely

regain the original condition We believe that the hydrogen

sensitivity of the nanotubes is due to hydrogen

chemisorp-tion onto the titania surface where they act as electron

donors In summary, it was demonstrated that sensors

com-prised of titania nanotubes prepared using anodization can

successfully be used as hydrogen sensors

Acknowledgements

Partial support of this work by the National Science

Foundation through grant ECS-9875104 is gratefully

acknowledged

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