Accepted Manuscript

Title: Improvement of Gas Sensing Performance of TiO2

towards NO2 by Nano-Tubular Structuring

Authors: Yakup Gonullu, Guillermo Cesar MondragonRodrıguez, Bilge Saruhan, Mustafa Urgen

PII: S0925-4005(12)00410-8DOI: doi:10.1016/j.snb.2012.04.050Reference: SNB 14088

To appear in: Sensors and Actuators B

Received date: 12-7-2011Revised date: 12-4-2012Accepted date: 18-4-2012

Please cite this article as: Y. Gonullu, G.C.M. Rodriguez, B. Saruhan, M. Urgen,Improvement of Gas Sensing Performance of TiO2 towards NO2 by Nano-TubularStructuring, Sensors and Actuators B: Chemical (2010), doi:10.1016/j.snb.2012.04.050

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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Improvement of Gas Sensing Performance of TiO2 towards NO2 by Nano-Tubular Structuring

Yakup Gönüllüa, Guillermo Cesar Mondragón Rodrígueza, Bilge Saruhana*, Mustafa Ürgenb

a German Aerospace Center, Institute of Materials Research, 51147 Cologne, Germanyb Istanbul Technical University, Dept. of Metallurgical and Materials Engineering, Maslak, Turkey

*Corresponding author: Tel: +49 2203 601 3228 Fax: +492203 696480 E-mail: Bilge.Saruhan@dlr.de

Abstract:

Aligned nano-tubular TiO2 layers were synthesized and their gas sensing abilities were compared

to those of magnetron sputtered titania films towards NO2 and CO. TiO2 nano-tubes were grown

on the titanium foils, via anodic oxidation method (hereafter anodization) using of fluoride

containing ethylene glycol (EG)-based electrolytes. Both amorphous, nano-tubular titania

structures and magnetron sputtered titania layers were annealed at 450°C and 700°C. Magnetron-

sputtered titania films were anatase on deposition and converted to rutile on heat-treatment in air

for 3h at 800°C. Nano-tubular titania layers were converted to anatase at about 400°C and above

600°C to rutile. The sensor performance of the nano-tubular TiO2 layers annealed at 450°C

(anatase) and at 700°C (anatase and rutile) were investigated at 300° to 500°C towards NO2

concentrations of 10, 25, 50 and 100 ppm and CO concentrations of 25, 50 and 75 ppm and

compared to those of the magnetron sputtered TiO2-layers annealed at 450°C (anatase) and

800°C (rutile and anatase). The nano-tubular TiO2-sensor layers yield very promising results for

sensing NO2 with some cross-sensitivity towards CO in relatively small concentrations and in a

wide concentration range up to 500°C.

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1. Introduction

In the last decades, atmospheric pollution and the resulting global warming has reached critical

levels. High-temperature processes involving combustion in airplanes, energy, power production

plants, vehicles and industrial settlements are the main sources of this pollution. NOx-emission is

one of the greenhouse pollutants which are produced during combustion processes, whatever the

quality of the fuel is. Gas sensors are extensively used and continuously developed for the

precise determination of the quantity and chemistry of gas emissions, mainly for the purpose of

controlling the combustion processes [1, 2]. Thus, in situ detection of NOx at high temperatures

coming from the exhaust and combustion gases is becoming necessary and is a challenging issue

[3].

For decades, the oxides of the transition metals such as SnO2, WO3, MoO3, Ga2O3 and Nb2O5

have been investigated and employed as electrochemical gas sensing materials. The lack of

precise operation at high temperatures is a limiting factor for the use of transition metal-oxides.

Table 1 shows some of these metal-oxides and their operation temperatures for detecting NO2.

Un-doped metal-oxides such as SnO2 or WO3 are good candidates for NO2-detection at

moderately low temperatures (below 300°C), but above 300°C they start losing their sensing

ability. TiO2 is another candidate for gas sensing applications at higher temperatures (>500°C)

relying on its excellent chemical stability, semi-conductor property, non-toxicity and relatively

low-cost supply [3, 4]. Considering the challenges, certainly TiO2-based systems may still

require some improvements. Further required improvements are the stability of sensing at

intermediate high-temperatures, increase of selectivity and decrease of response times at these

high temperatures. One of the suggested methods for improvement of the performance of TiO2-

based sensing electrodes is to introduce some doping elements, such as Pt which is a known

catalyst in Lambda devices for better oxygen response [5, 6], or Y and Cu to improve the

selectivity towards CO [7, 8]. Recently, it has been reported that doping TiO2 with Cr can change

its semi-conductivity related properties [9, 10] and improve the NO2-gas sensing ability of TiO2-

based gas sensors [3, 11].

An alternative solution for achievement of higher performance in gas sensors is to increase the

surface area of the sensing electrodes by micro- or nano-scaled structuring. As early as 1991,

Yamazoe et al. have shown that the reduction of crystallite size strongly increases the gas

sensing performance and selectivity of SnO2 based sensors [24]. On contact of the oxidizing or

reducing gases with the surface electrons and/or pre-adsorbed lattice oxygen at the semi-

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conductive metal oxide surfaces, electrochemical reactions occur. Thus, superficially

concentrated charge carriers cause changes in resistance or conductivity of the sensing material.

Since, all these interactions occur on free accessible surfaces of the sensor material, the larger the

surface area of a sensing material, the higher the sensitivity increase will be. The changes in

resistance or conductivity are caused by the increasing or decreasing amount of the charge

carriers. Thus, the challenges in the area of chemical gas sensors are shifted on to the fabrication

of sensing electrodes as small-scaled and highly crystallized materials. One-dimensional metal-

oxides are advantageous due to high “surface area to volume” ratio as well as easy and simple

production methods [25, 26]. It is already demonstrated that TiO2 nano-tubular layers exhibit

excellent gas sensing properties towards H2 [25, 27]. Investigation of hydrogen sensing

properties of highly ordered TiO2 nano-tubular layers produced by anodization show that sensors

having such layers are sensitive to broader H2-concentrations varying from 20 ppm to 1000 ppm

even at room temperature. This increase in sensitivity is attributed to the chemisorptions of H2

molecules on the large and highly active surfaces of TiO2 nano-tubes [28].

Although, magnetron sputtered layers of Al, Cr, Y-doped titania were tested as CO and NO2-

sensors [3, 11]. However to our knowledge there exists up-to-date no study which uses nano-

tubular titania electrodes for sensing CO and NO2.

This study describes the use of TiO2-nano-tubular layers for detection of NO2 at temperatures

>300°C. In this study, we demonstrate that this type of sensor electrodes have great potential to

act as highly sensitive NO2 gas sensor operating at intermediate high temperatures (300°C to

500°C) and thus, to fill an application gap. TiO2 nano-tubular layers for the preparation of sensor

devices, a simple sensor design and circuitry electrodes of platinum is employed. The sensing

properties towards NO2 (10-100 ppm) and CO (25-75 ppm) in argon carrier gas were

investigated at the temperature range of 300°C to 500°C. These results were compared with

sensor devices prepared by magnetron-sputtered TiO2 sensing layers.

2. Experimental Methods

Sensor devices of this study consist of nano-tubular TiO2-sensing layer and two parallel running

platinum circuits which were sputtered by masking as 1 mm wide bars. The distance between the

bars was 2 mm. For the devices having magnetron sputtered TiO2-layers, inter-digited platinum

circuits which were previously screen-printed on alumina substrates were used. The sensing

layer was deposited by reactive sputtering of Ti.

2.1 Synthesis of the Nano-tubular TiO2 Layers

TiO2 nano-tubes were grown on the commercially available pure titanium foil (99.6 %) via the

anodisation method. Mirror polished Ti foil substrates with a thickness of 0.25mm were

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subjected to the anodisation process in an Ethylene Glycol (C2H6O2) (EG)-based electrolyte with

98 vol. % EG, 2 vol. % H2O, 0.3 wt. % NH4F where an anodization voltage of 60V was used. A

DC power supply station from STATRON capable of supplying 150V and 8A was used during

anodisation process. An austenitic stainless steel sheet was employed as the counter electrode.

The distance between the anode and cathode was kept constant (20 mm). The samples were

anodised at room temperature under continuous magnetic stirring. After the anodisation step was

completed, the samples were rinsed with ethanol and dried in static air prior to characterisation.

Considering the high-temperature instability encountered in previous studies, the nano-tubular

TiO2 layers were subjected to annealing steps varying from 400° to 800°C. An annealing

temperature of 700°C at a holding time of 3h in static air yielded morphologically stable and

well-crystallised nano-tubular-structures. Annealing temperatures exceeding 700°C may destroy

the nano-structures.

2.2 Synthesis of Magnetron Sputtered TiO2 layers

Al2O3 sensor platforms were coated with an approximately 1 µm thick TiO2 by reactive

magnetron sputtering equipment (LA 250 S from von ARDENNE) using Ti-target as the metal

source. Sputtering was carried out with a pure argon and oxygen mixture in which oxygen acted

as the reactive gas. Argon and oxygen were mixed prior to admission in the sputtering chamber

in proportions of 70 % argon and 30 % oxygen. The chamber pressure was maintained at 7.7 x

10-3 mbar during the deposition process. A sputtering power of 250W was applied by using DC-

Power supply. As-deposited layers were annealed in a furnace under static air atmosphere at

800°C for 3 hours, prior to sensor measurements. In order to avoid the effect from the coarse

grains on sensor properties, the annealing temperature was limited with 800°C.

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2.3 Microstructure and Sensor Characterization

For the investigation of the morphology, the diameter and the length of the TiO2 nano-tubes, a

scanning electron microscope (SEM) (Ultra 55 from ZEISS) equipped with energy dispersive X-

ray (EDX) analyser was employed.

The phase analysis of the samples was carried out by X-ray diffraction measurements (XRD)

with an equipment model D5000 from SIEMENS employing Cu Kα radiation (λ= 1.54056Å)

and using an acceleration voltage of 40kV.

The gas sensing properties of the nano-tubular TiO2-layers were determined by means of DC

resistance measurement, with a 2635A Source-meter from KEITHLEY, using two parallel

platinum circuits with 2 mm interspace and being deposited on the sensing layers by sputtering.

In order to provide reasonable strength to withstand the sputtering induced mechanical erosion,

the nano-tubular layers were produced as thick as 15 µm, although satisfactory sensing

properties can be achieved with a sensing layer of as thin as 1 µm.

The gas sensing measurements of the nano-tubular TiO2 layers were carried out at 300°C to

500°C in a gas-release reactor placed inside a tubular CARBOLITE furnace. The change in

resistance of the TiO2 sensor devices were measured upon exposure to NO2 with concentrations

of 10, 25, 50 100 ppm CO with concentrations of and 25, 50, 75 ppm in argon used as a carrier

gas.

Raman spectroscopy measurements were carried out in a Raman Shift range of 100-1800 cm-1

using a BRUKER Senterra Raman Spectroscope at the Leibniz University of Hannover.

3. Results

3.1 Microstructure

The top view and cross sectional SEM images of magnetron sputtered TiO2 films are given in

Fig. 1a and b respectively. Cross-sectional images of the TiO2-films (Fig. 1a) reveal a columnar

morphology, which extends through the film thickness (approximately 1 µm). The top-view

image shows typical structure obtained with magnetron sputtered films; namely rough surface

resulting from columnar growth (Fig. 1b). On annealing at 1000°C, a recrystallisation derived

grain growth can be observed. The growth mechanism of the nano-tubular TiO2 layers is

governed by competition between the anodisation of TiO2 and the selective etching of TiO2,

which is driven by fluorite-ions [29]. The titanium oxide nano-tubular structure grows on

oxidation of titanium under the applied potential according to the following equation:

Ti +2H2O TiO2 +4H+ +4e− (1)

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Eventually, TiO2 dissolves by fluoride ions generating the well-defined porous structure:

TiO2 +4H+ +6F−→ [TiF6](aq)2− +2H2O (2)

Fig.2 shows the FE-SEM images of TiO2 nano-tubular layers which were obtained by

anodisation of pure Ti metal in the EG-based electrolyte which contains 0.3 wt. % NH4F and 2

vol. % H2O. The surface and cross-section images of the layer achieved after anodizing for 3

hours are shown in Figs. 2a and 2b.

The thickness of the layer after anodisation was 14-16 µm as measured from cross-sections and

increased with the anodisation time. The increase of anodisation time also increases the pore

diameter and the regularity of the pores. Fig. 1c shows the top view and cross-section images of

the TiO2 nano-tubular layers after annealing at 700°C. Under this annealing condition, no

noticeable change at the morphology of TiO2 nano-tubes has been observed, although a slight

decrease at the nano-tube diameter was observed after annealing.

3.2 Phase Analysis

The phase constituents of the magnetron sputtered titania and nano-tubular titania films produced

in EG based electrolyte were investigated with XRD in as-produced state and after annealing at

the temperatures of 450°C and 800°C for magnetron-sputtered layers and at the temperature

range of 450°C and 700°C for nano-tubular layers (Fig. 3). The magnetron-sputtered TiO2 layers

are of anatase phase in the as-coated state and after annealing at 450°C. After annealing at 800°C

for 3 hours, the sputtered TiO2 layer consist of both the anatase and rutile phases, rutile being in

majority (see Fig 3a). The corundum peaks (designated with C) which are detected in this

spectrum are from the substrate (Fig. 3a).

The XRD results showed that the as-anodized TiO2 nano-tubular layers are amorphous in the

initial state. The XRD spectrum of the as-anodised nano-tubular layers yields only titanium

peaks (designated with T) coming from the substrate. On annealing at 400°C, the nano-tubular

layer converts into the anatase phase. The conversion from anatase to rutile started at 600°C on

annealing for 3 hours at each temperature (Fig. 3b). These observations were obtained after

repeating the annealing experiments of the obtained TiO2-layers.

Fig. 4 shows the Raman spectra of the TiO2 nano-tubular layers. Two broad peaks near 612 and

467 cm-1 corresponding to amorphous TiO2 were observed. The peak near 1598 cm-1 is

characteristic of sp2 -bonding which may be related to carbonates generated from the ethylene

glycol. It can be attributed that this is not an amorphous carbon black, relying on the absence of

the accompanying band near to 1340 cm-1 which normally relates to the sp3-component. After

annealing in air at 450°C for 3h, all these carbon related peaks disappear and the Raman bands at

635cm-1, 514 cm-1, 396 cm-1 and 197 cm-1 appear, due to the formation of anatase phase (Fig. 4).

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Anatase is well known with a very sharp and intense peak at 144 cm-1[30]. This Raman

measurement demonstrates that the nanotubular titania annealed at 450°C contains no carbonates

or hydroxides.

3.3 Sensor Performance

The resistance changes of the sensor devices were recorded under a constant current of 1x10-6

Amp. Sensors were exposed to NO2-concentrations of 10 to 200 ppm and CO-concentrations of

25 to 75 ppm in argon carrier gas with a flow rate of 400ml/min. Prior to sensor measurements,

the resistance of the sensor devices were recorded under pure argon gas in order to determine the

base-line. The resistance changes obtained upon release of the test gas NO2 are given with the

sensor response curves shown in Figs. 5a and b for sensors having the magnetron sputtered TiO2-

layers, and in Figs. 5c and d for sensors with the nano-tubular TiO2-layers.

The sensors with the magnetron sputtered TiO2-layers were tested towards 50, 100 and 200 ppm

of NO2 at 400°C, while the sensors with the nano-tubular TiO2-layers yielded response even

towards smaller NO2-concentrations of 10, 25, 50 and 100 ppm at 400°C.

The sensors having magnetron sputtered TiO2-layers annealed at 450°C yielded no detectable

response at all NO2-concentrations. Although rather unstable, nevertheless some sensor response

were obtained with the sensors having the magnetron sputtered TiO2-layers annealed at 800°C.

With this sensor, below 50ppm NO2, an irregular response was observed. On exposure to 50ppm

NO2, the resistance value firstly increased and then decreased although NO2-flow was not

ceased. In general, under higher NO2-concentrations, the resistance values of this sensor did not

sink to the base-line level when ceasing the NO2 flow. This behaviour may be attributed to the

change in semiconductor behaviour from n-type to p-type due to the presence of high pO2.

Exposures to 100ppm and 200ppm NO2 in turn resulted in resistance decreases, although, no

stable response was detected with these variations of the NO2-concentrations (Fig.5b).

These results indicate that the sensors produced with magnetron sputtered TiO2-layer are

unstable and show none or very little sensitivity to NO2. On the contrary, the sensors having the

nano-tubular TiO2-layer annealed at 450°C and 700°C yielded stable response signals and

shorter recovery and the response times on exposure to various NO2 concentrations in argon

carrier gas (Fig. 5c and d). The change in the resistance values of these sensors on exposure to

the test gas NO2 was regular. Resistance increase was steady as the NO2-concentration was

increased. The sensitivity was higher with the sensors having nano-tubular TiO2-layers annealed

at 450°C than those annealed at 700°C.

CO is a typical exhaust gas and almost always present with NO2, therefore, we tested all our

sensors also towards CO at 400°C, in order to determine their cross-sensitivity. The sensor

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response towards 25 and concentrations up to 75ppm CO is shown in Fig. 6. The sensor with

magnetron-sputtered TiO2-layer annealed at 450°C yielded no CO-response, while, after

annealing at 800°C, some response towards CO was observed. As the CO-concentration

increased to 50ppm, a slightly better response was detected. The recovery of this sensor was

exteremely unsteady after ceasing the CO-flow and the signal yielded a strong drift (Fig. 6b). On

the other hand, the response of the sensors with the nano-tubular TiO2-layer annealed at 450°C

and 700°C was fast and stable even at CO-concentration of 25 ppm (Figs. 6c and d). The

resistivity change of this sensor was low but this signal showed very low drift. The base-line

resistivity of this sensor was lower than that of the magnetron-sputtered TiO2 sensor.

Previous studies report that TiO2-based sensors are better in sensing CO than NO2 [8, 31]. The

results of this study give proofs also for good NO2-response if TiO2 sensors are produced as

nano-tubular layers. Nevertheless, it is to note that the base resistance as well as the resistance

change with the nano-tubular TiO2-sensors was higher towards NO2.

4. Discussion

This study reports the sensor characteristics of the sensor devices having nano-tubular TiO2-

layers annealed at the temperatures of 450°C and 700°C towards different NO2 and CO

concentrations in argon as carrier gas. In order to demonstrate the capability of nano-tubular

TiO2 as NO2-sensor, sensors having magnetron sputtered TiO2-layers are also prepared and

annealed at 450°C and 800°C. Fig. 7 gives the relative response of all tested sensors at the

temperature range of 300°C to 500°C.

The results confirm that the sensors with the nano-tubular TiO2-layer show better response

towards NO2 than the ones with the sputtered TiO2-layer. The annealing temperature of the

sensing layers has an influence on the sensor behaviour. This may come from the phase

constituents of TiO2, but, also may be due to the differences in surface area and grain sizes.

It appears from the results that the preparation method of the sensing layers substantially

influences the sensitivity of the sensors due to the resulting morphology and surface chemistry.

The sensors with nano-tubular TiO2-layer annealed at 450°C detect NO2 at a wide concentration

range, varying from 10 to 100 ppm at all test temperatures. All sensor layers annealed at 450°C

consist of only the anatase phase. Thus, better sensor performance which is achieved with the

nano-tubular TiO2-layers may strongly rely on the nano-structuring derived high surface area.

High-surface area introduced by nano-tubular layers allows more effective adsorption of gas

species (Figs. 5 and 6). The adsorption kinetic of the gas species on the magnetron sputtered

TiO2-layer is very slow, especially at lower concentrations. Under the given flow rate and at

lower test temperatures, the adsorption of gas species cannot occur that easily on larger sized

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grains of magnetron-sputtered TiO2 as it does on finer nano-tubular TiO2-surfaces (Fig. 8). As

previously reported, there exists a linear relationship between the gas sensitivity and accessible

surface area of semi-conducting sensing materials [32]. Nano-carving of TiO2, based on simple

gas-to-solid reactions, yields nano-sized TiO2-fibers in 15 to 50 nm lengths and enhances H2-

sensitivity of TiO2 [33]. Moreover, the characteristics of the catalytic surface reactions and/or

adsorption/desorption kinetics of gas species on these surfaces play a role in the sensitivity of gas

sensors [32]. Investigations related to the semiconducting properties of TiO2-nanotubes

demonstrate that the electronic structures at the pore walls and at the underlying oxide are

different. The walls of oxide tubes contain a high concentration of electronic defects. This gives

a rise in the modification of catalytic redox reactions.[33-34] The underlying oxide layer is

continuous and has a grainy appearance as shown in Fig. 8b. Our nano-tubular sensors were

prepared by depositing the Pt-circuits on top of the anodised films. Generally, the diffusion of

test gas occurs within a few micron depth of sensing layer [34]. Furthermore, the effects which

may come from the electronic structure difference of the underlying oxide will be eliminated due

to relatively high film thickness of the nano-tubular layer. Thus, no great influence of the layer

thicknesses on the sensor properties is expected.

The phase constituents of titania may also influence the sensor behaviour. Depending on the

annealing temperature and preparation route, the TiO2-layers contain different phase

modifications. After annealing of the magnetron sputtered TiO2 at 800°C, a grain growth and

also a substantial phase transformation to rutile was observed. As a result, the corresponding

sensor response was improved towards NO2 and CO. Our observations demonstrate that

anodized nano-tubular TiO2 reduces the temperature for the anatase to rutile phase

transformation, as well as introducing a higher surface area. The nano-tubular titania layers

annealed at 450°C consists of pure anatase phase and yields relatively higher base-line resistance

values, yielding better sensitivity toward NO2 at test temperatures up to 400°C. In order to

provide a stable sensor condition at test temperatures higher than 400°C, the sensors having

nano-tubular TiO2-layers were annealed at 700°C. This brought up the phase transformation to

rutile, although according to the relative intensity of the XRD peaks (Fig. 3b) the anatase phase

was in majority. This mixed phase condition resulted in a reduction of the resistance change and

thus of the sensor sensitivity.

It was reported by Knauth and Tuller [35] that under different oxygen partial pressures, anatase

and rutile exhibit different electrical behaviors. Under high pO2, rutile behaves as a p-type semi-

conductor, which is probably due to the hole-conductivity. Accordingly, it can be stated that

there will be a competition between the electron and hole determined electrical behavior of the

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sensing layers which contain the anatase and rutile mixed phases. Nano-crystalline anatase under

high pO2 yields then a plateau in conductivity because of the high density grain boundaries [35].

This may explain the lack of sensitivity with the grain boundary rich anatase at the magnetron

sputtered and low temperature annealed TiO2-layers.

Anatase and rutile were reported to exhibit n- and p-type semiconducting properties,

respectively. During the transformation to rutile phase, the excess oxygen reacts with titanium

interstitial defects, resulting in this change of semi-conductivity [8]. Moreover, Savage et al. [8]

report that anatase and rutile mixed phases behave as percolating system, yielding n-type

behavior for samples containing less than 75% rutile and p-type only with pure rutile. The

presence of small amounts of rutile may lower the grade of resistance change through

compensation between the n- and p-type responses, however the overall response will be n-type.

Thus, it is plausible that all the tested sensors display n-type behavior, despite containing rutile in

different proportions. Rutile content does not exceed over 75%. The loss of sensitivity towards

NO2 after annealing the nano-tubular TiO2-layer at 700°C may be due to the formation of some

rutile.

Generally, all the investigated sensors show a sensitivity decrease as the sensor test temperature

increases from 300°C to 500°C (see Fig. 9). This may be related to the temperature sensitivity of

TiO2 having a relatively high negative Temperature Coefficient of Resistance (TCR) which can

be calculated by the following formula:

(3)

Morphological and crystallographic changes at nanotubes increase the base line resistance as the

annealing temperature increase.

In the case of the reducing gas, CO reacts with the surface adsorbed oxygen of the anatase phase,

releasing the trapped electron back and increasing its conductivity. With rutile where the

majority of the charge carriers are holes, the injected electron recombines with hole, resulting in

a conductivity decrease [8].

The phase sequences and morphological properties of the sensor layers influence not only the

sensors sensitivity but also its response and recovery behavior. The variation response and

recovery times of both TiO2 sensors with the NO2-concentration and temperature are shown in

Figs. 9a and 9b. The nano-tubular TiO2 sensor layers exhibits much faster response and recovery

times being around 3 to 4 minutes for each tested concentration and temperature than that with

the sputtered TiO2 layers (20 to 30 minutes). Moreover, the recovery and response times of the

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sensors with the nano-tubular TiO2 layer are almost independent from the test temperature.

Previous studies with TiO2 powder based sensors report that an improved NO2 response and

shorter response and recovery times are only achievable by doping of TiO2 with elements such

as: Al, Cr and W [3, 11, 22 and 36]. All investigated TiO2 sensing layers of the present study

contain no dopants. The achieved high NO2-sensitivity is solely a result of the nano-tubular

morphology of TiO2.

The response times get shorter with the magnetron-sputtered layer as the temperature increases.

The surface adsorbed reactants must be rather inactive at lower temperatures (e.g. <300°C). At

higher temperatures, the catalytic activity of the adsorbed reactant species on the surface active

sites increases leading to a faster desorption of the reaction products. Thus, this yield a plausible

explanation for higher recovery times (>30 min) observed with larger grained magnetron

sputtered TiO2-layers. Sensor layers possessing higher surface area, as in the case of the nano-

tubular TiO2-layers, will have higher density of active sites. Thus, it is likely that in this case, the

temperature effect will have an under ordered influence on the response and recovery times of

the sensors.

The charge carrier concentration on the surface of a semiconductor is sensitive to the

composition of the surrounding atmosphere [37]. The thermodynamic equilibrium is in a gas

mixture of NO/NO2 above about 500°C on the side of NO. Thus, during the high temperature

measurements, the concentration of test gas may not contain only NO2 but also NO. This fact

may explain the decrease observed in the sensor sensitivity at temperatures of about 500°C.

Another issue which is important in terms of a gas sensor is its selectivity towards the target gas,

in other words the cross-sensitivity of the sensor. Fig. 10 represents the relative responses of the

nano-tubular TiO2-layers towards a mixed gas containing 50 ppm NO2 and 50 ppm CO in a

temperature range of 300°C to 500°C. The sensor with nano-tubular TiO2 layer shows a higher

relative response toward NO2 than CO at all test temperatures (i.e. 300°C, 400°C and 500°C).

Relying on these results, it can be assumed that the sensor with nano-tubular TiO2-layer will be

more selective towards NO2. In order to test this, the sensor response is measured at 400°C under

NO2 and CO mixed gas flows in argon carrier gas at a concentration range of 50 to 100 ppm

(Fig. 11). Prior to exposure to the mixed gas, the sensors base line is defined under the single gas

exposure. Exposure to NO2 results in a resistance increase, as exposure to CO causes a resistance

decrease. Under low concentration mixed gas exposure (e.g. 50 ppm CO and 50 ppm NO2), the

nano-tubular sensor displays no resistance change compared to the level of that with only 50

ppm CO. This behaviour is repeated with higher single gas concentrations (e.g. 100 ppm).

Increased concentration of the mixed gas (e.g. 100 ppm CO and 100 ppm NO2), results in slight

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resistance change compared to that measured during single CO-exposure. This may indicate a

negligibly small NO2 cross-sensitivity. For the sensor with the nano-tubular TiO2-layer, the CO

selectivity can be anticipated when exposed to CO and NO2 mixed-gas atmospheres, despite the

fact that, under single gas measurements, higher NO2-sensitivity was recorded.

The sensor with nano-tubular TiO2-layer yields a resistance change in different directions

depending on the type of exposed gas species (i.e. reducing or oxidising). For instance a

resistance decrease is observed towards the reducing gas CO, and a resistance increase towards

the oxidising gas NO2. The following surface equations explain these differences [38]:

4)(2

2)( 2 ogog VeCOOCO (4)

2)()(2 2 adsgg ONOeNO (5)

Considering that TiO2 is a highly resistive n-type semiconductor, it is expected that the majority

of charge carriers (electrons) decreases due to the CO2 formation. When the nano-structured

TiO2-sensors are exposed to a reducing gas such as CO, this reacts with the lattice oxygen

species, producing CO2 molecules and releasing electrons back into the conduction band

according to the Eq. (4). This reaction causes an increase in the conductivity. As seen in Figs. 6

and 7, thus, the resistance of the sensor with the nano-tubular TiO2–layer decreases [3, 39].

According to Eq. (5), the exposure to NO2 results in a conductivity decrease and consequently in

a response to the opposite direction to that compared with CO. When the sensor surfaces are

exposed to an oxidising gas such as NO2 in this case, the mechanism does not involve the release

of electrons but the use of these electrons leading to decomposition of NO2. According to this

reaction, NO forms and releases O2− in the system. This released oxygen covers the electrode

surfaces forming a layer of adsorbed oxygen ions (O2-ads) which eventually stops the electron

transfers. Thus, a decrease in conductivity or in other words an increase in resistance of the

nano-tubular TiO2-layers is observed. (See Figs. 5 and 6). Selectivity of the TiO2 nano-tubular

layers towards NO2 can be improved by doping with trivalent oxides. Effect of dopants on

sensing mechanism towards CO and NO2 are presently under investigation.

4. Conclusions

Vertically oriented TiO2 nano-tube arrays were synthesized in EG-based (including 2 vol. % H2O

and 0.3 wt. % NH4F) solution and annealed at 450°C and 700°C. The sensor devices were

produced by deposition of platinum over a mask. A reference sensor was produced by magnetron

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sputtering of TiO2-layer on interdigited Pt-circuits and annealed at 450°C and 800°C. Both

sensors were tested towards NO2 and CO at the temperature range of 300°C and 500°C. Both

low-temperature annealed sensing layers yielded pure anatase phase. The annealing of both

layers at 700°C (nano-tubular) and 800°C (magnetron-sputtered) resulted in the formation of

rutile, the content of which is higher in the magnetron-sputtered TiO2-layer.

Low temperature annealed magnetron sputtered TiO2-layer showed no sensor response towards

both gases. The sensor having nano-tubular TiO2-layer showed faster and more stable response

towards NO2 than the reference sensor after annealing at both temperatures. Some CO-sensitivity

was detected with the nano-tubular TiO2-layer after annealing at both temperatures, but both

responses were lower than those detected towards NO2. Higher temperature annealed magnetron-

sputtered TiO2-sensing layer displayed better sensor response towards CO than NO2. Moreover,

the sensor with nano-tubular TiO2-layer exhibits shorter response and recovery times than those

of sputtered TiO2-layer.

The present results confirm that the nano-structuring the TiO2 layers improves the sensing ability

of TiO2 towards a wider range of NO2 concentrations. Hereby, the phase constituents play a

subordinated role. The comparison of different titania phases in terms of sensing behaviour of

nano-tubular TiO2-layers indicate that the layers with pure anatase phase yield higher sensitivity

than those having rutile and anatase mixed-phases. In all cases, increased surface area at sensor

layers leads to higher sensitivity, stable response and shorter response and recovery times.

Furthermore, the response and recovery times can be reduced.

The sensors with un-doped nano-tubular TiO2-layers yielded higher calculated relative

sensitivities during single gas measurements towards NO2 compared to that of CO at 300°C,

400°C and 500°C. However, under CO plus NO2 mixed-gas exposures, the nano-tubular TiO2-

sensor was less selective towards NO2. Explanation of this phenomenon requires further

evidence which is part of ongoing research activities. The deficit in the cross-sensitivity may be

overcome by doping of the TiO2 layers with trivalent oxides.

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5. Acknowledgement

The authors thank Mr Ayhan Yüce for providing the magnetron sputtered sensor devices and for

his valuable support during sensor characterisation.

The project is partly financed by the International Office of BMBF under the contract number

TUR 09/002. The scholarship grant for Y. G. is provided by DAAD-DLR-research fellowship

programme.

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Table 1: Operation temperature and NO2-gas concentration range for various metal-oxides

Sensor Materials Operation Temperature(°C)

Gas Concentration(ppm)

Ref.

SnO2 120-220 5-100 12-15

WO3 200 50-1000 ppb 16

ZnO-SnO2 100-300 200-1000 17

ZnO 150-400 40 18-19

SnO2-WO3 100-250 500 20

Al-doped SnO2 300-400 5-20 21

TiO2-WO3 400-800 20 22

Cr-doped TiO2 500 2-50 3,11

Nb-doped TiO2 600 100-300 23

Al-doped TiO2 600-800 10-100 own unpublished work

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Table 1: Operation temperature and NO2-gas concentration range for various metal-oxides

Figure 1: SEM images of TiO2-layer prepared via magnetron sputtering (a) as-coated cross-section (b) as-coated top view (c) top view after annealing for 1h at 800°C

Figure 2: SEM images of TiO2 nano-tubular layer produced by anodisation in EG based electrolyte for 3 hours at 60V (a) as-prepared cross-section (b) as-prepared top view images and (c) top view after annealing at 700°C

Figure 3: XRD spectra of (a) Sputtered TiO2 and (b) Nano-tubular TiO2 layers after annealing at the temperature range of 400°C and 800°C. C: Corundum and T: Titanium from the corresponding substrate materials as shown on the spectra.

Figure 4: Raman Spectra of the nano-tubular TiO2 Layers

Figure 5: Sensor response towards NO2 at 400°C with (a) magnetron-sputtered TiO2-layer after annealing at 450°C (b) magnetron-sputtered TiO2-layer after annealing at 800°C, (c) nano-tubular TiO2-layer after annealing at 450°C (d) nano-tubular TiO2-layer after annealing at 700°C.

Figure 6: Sensor response towards CO at 400°C (a) with magnetron-sputtered TiO2-layer after annealing at 450°C, (b) magnetron-sputtered TiO2-layer after annealing at 800°C, (c) nano-tubular TiO2-layer after annealing at 450°C, (d) nano-tubular TiO2-layer after annealing at 700°C.

Figure 7: Sensitivity of the sensors at 800°C annealed magnetron sputtered (grey lines) and at

700°C annealed nano-tubular TiO2-layers (black lines) on testing towards various NO2

concentrations (10-100ppm) in argon at different temperatures (300°C-500°C).

Figure 8: TEM-micrographs of nano-tubular TiO2-layer at the top (a) and the bottom (b).

Figure 9: Response (a) and recovery (b) times of the TiO2 based sensors on exposure to NO2

Figure 10: Sensitivity of the TiO2 nano-tubular sensors toward CO and NO2 –mixed gas at 300°C, 400°C and 500°C.

Figure 11: Cross-sensitivity of nano-tubular TiO2-sensor electrode in CO and NO2 gas mixture in the concentration range of 50 and 100 ppm at 400°C.

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Figure 4: Raman Spectra of the nano-tubular TiO2 Layers

Figure(s)

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Figure 5: Sensor response towards NO2 at 400°C with (a) magnetron-sputtered TiO2-layer after annealing at 450°C (b) magnetron-sputtered TiO2-

layer after annealing at 800°C, (c) nano-tubular TiO2-layer after annealing at 450°C (d) nano-tubular TiO2-layer after annealing at 700°C.

Figure(s)

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Figure 8: TEM-micrographs of nano-tubular TiO2-layer at the top (a) and the bottom (b).

1 µm

b a

0.5 µm

Figure(s)

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Figure 9: Response (a) and recovery (b) times of the TiO2 based sensors on exposure to NO2

a b

Figure(s)

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Figure 10: Sensitivity of the TiO2 nano-tubular sensors toward CO and NO2 –mixed gas at

300°C, 400°C and 500°C.

Figure(s)

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Figure 11: Cross-sensitivity of nano-tubular TiO2-sensor electrode in CO and NO2 gas

mixture in the concentration range of 50 and 100 ppm at 400°C.

Figure(s)

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Figure 1: SEM images of TiO2-layer prepared via magnetron sputtering (a) as-coated cross-section (b) as-coated top view (c) top view after

annealing for 1h at 800°C

b

1 µm 500 nm

a c

1 µm

Figure(s)

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Figure 2: SEM images of TiO2 nano-tubular layer produced by anodisation in EG based electrolyte for 3 hours at 60V (a) as-prepared cross-section

(b) as-prepared top-view images and (c) top-view after annealing at 700°C.

Figure(s)

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Figure 3: XRD spectra of (a) Sputtered TiO2 and (b) Nano-tubular TiO2 layers after annealing at the temperature range of 400°C and 800°C.

C: Corundum and T: Titanium from the corresponding substrate materials as shown on the spectra.

a

Figure(s)

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Figure 6: Sensor response towards CO at 400°C with (a) magnetron-sputtered TiO2-layer after annealing at 450°C (b) magnetron-sputtered TiO2-

layer after annealing at 800°C (c) nano-tubular TiO2-layer after annealing at 450°C (d) nano-tubular TiO2-layer after annealing at 700°C. a

Figure(s)

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Figure 7: Sensitivity of the sensors at 800°C annealed magnetron sputtered (grey lines) and at

700°C annealed nano-tubular TiO2-layers (black lines) on testing towards various NO2

concentrations (10-100ppm) in argon at different temperatures (300°C-500°C).

Figure(s)