Amorphous TiO2 nanotube arrays for low-temperature oxygen sensors

IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 19 (2008) 405504 (7pp)

doi:10.1088/0957-4484/19/40/405504

Amorphous TiO2 nanotube arrays for low-temperature oxygen sensors Hao Feng Lu1,2 , Feng Li2 , Gang Liu2 , Zhi-Gang Chen2, Da-Wei Wang2 , Hai-Tao Fang3 , Gao Qing Lu4 , Zhou Hua Jiang1 and Hui-Ming Cheng2 1 School of Material and Metallurgy, Northeastern University, No. 11, Lane 3, WenHua Road, Shenyang 110004, People’s Republic of China 2 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 WenHua Road, Shenyang 110016, People’s Republic of China 3 School of Materials Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, People’s Republic of China 4 ARC Centre of Excellence for Functional Nanomaterials, School of Engineering and Australian Institute of Bioengineering and Nanotechnology, University of Queensland, Brisbane, QLD 4072, Australia

E-mail: [email protected] and [email protected]

Received 5 June 2008, in final form 10 July 2008 Published 20 August 2008 Online at stacks.iop.org/Nano/19/405504 Abstract Titania nanotube arrays (TNTA) were synthesized on a titanium substrate using anodic oxidation in an electrolyte containing ammonium fluoride and evaluated for low-temperature oxygen sensing. Their sensing properties were tested at different temperatures (50, 100, 150, 200, 250 and 300 ◦ C) when exposed to various oxygen concentrations. The as-prepared TNTA are amorphous and exhibit much higher carrier concentration than that of annealed TNTA. Such amorphous TNTA show much higher sensitivity than that of annealed TNTA, SrTiO3 and Ga2 O3 sensors. This sample demonstrates the lowest detectable oxygen concentration of 200 ppm, excellent recovery and good linear correlation at 100 ◦ C. These results indicate that TNTA are indeed very attractive oxygen-sensing materials. (Some figures in this article are in colour only in the electronic version)

gas in iron and steel smelters. However, this type of sensor has some remarkable drawbacks: (1) the working temperature of the solid electrolyte is above 1100 K; (2) a reference electrode is needed during testing, which is not suitable to detect O2 concentration close to the oxygen partial pressure of the reference electrode; (3) the sensor is complex and expensive to use and maintain. Therefore, the development of a highly sensitive, portable, low working temperature and convenient sensor is highly desirable and still a great challenge. To develop better oxygen sensors, TiO2 , SrTiO3 and Ga2 O3 with various morphologies have been explored [12–15]. Titania as an important functional material has been widely investigated in broad areas including photo-water splitting, dye-sensitized solar cells, electron field emission electrochemical lithium storage and gas sensing [16–24]. However, the main drawbacks of a TiO2 -based oxygen

1. Introduction The electrical conductivity of semiconducting metal oxides changes when exposed to active gas atmospheres [1–3], which is why semiconducting metal oxides have been widely investigated as candidates for gas sensing. Several metal oxides, such as SnO2 , ZnO, TiO2 , Ga2 O3 , WO3 , MoO3 and In2 O3 are found to have a pronounced sensitivity to gases, such as H2 , NO, NO2 , CO, alcohol and other species [4–8]. Recently, the demands for oxygen sensors are rapidly increasing. Oxygen concentration is one of the most widely used parameters in many fields, e.g. to control the air/fuel mixture in automobile engines [9, 10]. Up till now, commercialized oxygen sensors employed the solid electrolyte of ZrO2 [11] based on the Nernst principle. Such sensors have also been used to detect the oxygen content in exhausted 0957-4484/08/405504+07$30.00

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H F Lu et al

Figure 1. SEM and TEM images of TNTA: (a) and (c) the as-prepared sample, (b) and (d) the sample after annealing at 450 ◦ C in air for 2 h.

sensor is its low sensitivity and poor recovery rate [12, 15]. 1D TiO2 nanotube arrays (TNTA) are of considerable interest as they exhibit unique architecture, and remarkable and better properties compared to their bulk counterparts. Considering the nature of gas sensing via the interaction of a semiconducting surface with adsorbed gas molecules [25–27], it is thought that TNTA, grown on titanium substrates by a simple and straightforward anodization process with the characteristics of highly ordered open tubes and large specific surface area, can provide abundant sites and channels for gas adsorption, diffusion and chemical reaction. According to a recent report, titania nanotube arrays exposed to 1000 ppm hydrogen exhibit an unprecedented variation in resistance at room temperature [28]. Because the surface resistancecontrolled mechanism suits most gas sensors, it is reasonable to believe that TNTA have the potential for highly sensitive oxygen detection. It is well known that oxygen vacancies exist in n-type semiconducting titania acting as donors. The structure of amorphous titania nanotubes is more disordered than that of crystalline nanotubes, thereby having many more defects (oxygen vacancies) that can provide higher carrier concentration and more active sites. This may enhance the sensitivity when exposed to oxygen. In this work, we explored the oxygen sensing property of amorphous and anatase TNTA at low temperatures over a wide range of oxygen concentrations. The oxygen sensing properties and sensing mechanism of TNTA-based sensors were discussed in detail.

2. Experimental details TiO2 nanotube arrays were prepared from a titanium foil of approximately 250 μm in thickness (99.5% pure from Alfa Aesar). Potentiostatic anodization was performed in a twoelectrode electrochemical cell, connected to a DC power source under a constant potential of 20 V using a platinum foil as counter electrode. The electrolyte consisted of 0.5 wt% NH4 F and 1.0 M (NH4 )2 SO4 in deionized water. The anodic oxidation process was conducted at room temperature for 2 h. The as-prepared TNTA were amorphous, and anatase TNTA were obtained by annealing the as-prepared sample at 450 ◦ C in air for 2 h with a heating rate of 5 ◦ C min−1 . A field emission scanning electron microscope (FESEM, LEO SUPRA 35) was used to observe the morphology of the TNTA. An x-ray diffractometer (XRD, RINT 2200, Cu Kα ) was used to determine the crystalline phase. Transmission electron microscope (TEM) images of TNTA were obtained by a JEOL 2010 at 200 kV. Mott–Schottky curves were measured by a Princeton Applied Research PARSTAT 2273, using a three-electrode system. Two platinum strips (10 mm × 2 mm) at a distance of 3 mm were fixed onto the TNTA surface [15] by two quartz glasses and four screws. Two platinum wires were soldered on platinum sheets with the ohmic connection to a computercontrolled current–time measurement instrument (CHI630I) to measure the current passing through the nanotube arrays under constant potential. There is an oxide barrier layer (insulating 2

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Figure 2. XRD patterns of the as-prepared (curve a) and annealed (curve b) TNTA.

layer) between metallic Ti and TNTA, and this barrier layer can prevent current getting across metallic Ti [15]. The varied concentrations of test gas (oxygen) were controlled by two mass flow controllers, and the carrier gas employed was nitrogen. The sensitivity term S is defined as

S=

Rgas − R0 R0

(1) Figure 3. (a) The resistance response and (b) sensitivity variation of the amorphous TNTA under different oxygen concentrations at 50 ◦ C.

where Rgas and R0 represent the resistance of the sensors in mixed gases including oxygen and nitrogen, respectively.

3. Results and discussion rutile TNTA occurs during the annealing process. Crystalline TiO2 is highly advantageous for H2 detection, but for oxygen crystalline TiO2 exhibits a very poor recovery property [15]. Therefore, we mainly focused on the investigation of the gas sensing responses of amorphous TNTA for oxygen at low temperatures. Figure 3(a) shows the isothermal response of the electrical resistance of the amorphous TNTA sensor at a working temperature of 50 ◦ C when the oxygen concentration is varied from 1.2% to 4.0%. The sample holder was flushed with nitrogen after each exposure to oxygen. The resistance of TNTA increased in the presence of oxygen and recovered in nitrogen. The sensitivity of this sensor in the concentration range studied is plotted in figure 3(b) and there is a good linear relationship with oxygen concentrations at 50 ◦ C. To check the behaviour of the amorphous TNTA sensor in a wider concentration range at higher temperatures, experiments were carried out at 100, 150, 200, 250 and 300 ◦ C, while the concentration of oxygen varied from 200 ppm to 20%. The resistance response at the temperature of 100 ◦ C of the amorphous TNTA sensor for oxygen is shown in figures 4(a)–(c). It can be seen that there are remarkable changes in the resistance of TNTA without obvious hysteresis, and after nitrogen flushing in every cycle the resistance regains its original value. The sensitivity of amorphous TNTA was studied at different temperatures as shown in figure 5(a). In this figure we can see that the sensitivity of amorphous TNTA roughly increases when temperature increases, but does

SEM images of the as-prepared TNTA and annealed TNTA from top and cross views are shown in figures 1(a) and (b). Both of the TNTA are vertically aligned on a Ti foil substrate. It is clear that the surface structure and outer diameter of the amorphous and annealed samples are very similar. The nanotubes have a uniform outer and inner diameter distribution around 150 and 110 nm. The TNTA are composed of wellaligned nanotubes of about 2.3 μm in length. TEM images of the as-prepared nanotubes and annealed nanotubes are shown in figures 1 (c) and (d). The inserted SAED pattern (figure 1(c)) proves the amorphous nature of as-prepared sample, Figure 1(d) indicates the polycrystalline structure of the annealed nanotubes. We can see that the morphology of TNTA remained the same as the original after heat treatment. The thickness of the tube wall is approximately 20 nm. XRD patterns of the as-prepared and annealed TNTA are shown in figure 2. The peaks of the latter, which were obtained by annealing the asprepared sample at 450 ◦ C in air for 2 h, can be indexed as a main anatase phase corresponding to 2θ = 25.3◦ seen for the (101) crystal orientation. Compared to the amorphous nature of the as-prepared TNTA, the annealed TNTA show an anatase phase plus a negligible rutile phase. Generally, nanostructured TiO2 used for gas sensing (including H2 , CO, NO2 , CH4 , etc) should be annealed in air or oxygen atmosphere at a certain temperature prior to testing in order to form a crystalline structure [29–32]. A transformation of amorphous TNTA into anatase and (or) 3

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Figure 4. Electrical resistance change of the amorphous TNTA when exposed to three different concentration ranges at 100 ◦ C: (a) 200–1980 ppm, (b) 6.1–9.5, (c) 1.8%–20.0%.

not exhibit any linear correlation with oxygen concentrations and irregularly fluctuates at temperatures above 180 ◦ C. Furthermore, the resistance responses above 180 ◦ C show a very poor recovery property according to present results. However, at 100 ◦ C, high sensitivity, an excellent recovery property and a linear relationship with oxygen concentrations can be observed. At this temperature an amorphous TNTA sensor exhibits sharp variation of electrical resistance up to two orders of magnitude (∼102 ), much higher than that of other metal-oxide sensors such as Ga2 O3 thin film (∼1.5) [11], nanoscale TiO2 thick film (∼1.5) [12] and SrTiO3 thick film (∼6.5) [13]. These performances represent higher sensitivity, better recovery and linear correlation compared to these metaloxide semiconductors for oxygen sensing reported in the literature [12, 13, 15]. Meanwhile, the annealed TNTA sample was also tested at 100 ◦ C. The sensitivity of anatase TNTA has no linear relationship with oxygen concentrations and is much lower than that of the amorphous sensor (figure 5(b)). It is obvious that the sensing property of amorphous TNTA is much better than that of annealed TNTA as an oxygen sensor at 100 ◦ C. Varghese et al [15] reported that the resistance of the TiO2 nanotubes increased when exposed to 1% oxygen at 290 ◦ C, but could not recover to its original electrical conductivity even after several hours’ exposure in nitrogen. So it can be concluded that, at low (≈100 ◦ C) and relatively hightemperature (>150 ◦ C), both amorphous and annealed TNTA exhibit different recovery behaviours after exposure to oxygen. In general, the changes in resistance of metal-oxide semiconductor gas sensors when exposed to different

atmospheres are due to a charge carrier exchange of adsorbed gas with the oxide surface [32]. In order to understand the superiority of an amorphous over anatase TNTA sensor in response to oxygen at low temperature and the dissimilar responses of TNTA at different temperatures, we should consider the unique interactions toward these conditions. As TiO2 is an n-type semiconductor, its electrical conductivity relies on free electrons supplied by the donor energy level, which is below the conduction band minimum in the bandgap of titania caused by oxygen vacancies or interstitial titanium atoms. While the TNTA are exposed to oxygen-containing atmospheres, oxygen molecules are adsorbed on the TiO2 surface, then forming some anions which act as acceptors by trapping electrons from the nanotube conduction band and creating a depletion region on the nanotube surface that enhances its electrical resistance [28]. It has been proposed that the adsorbed oxygen molecules transform to various anion species by transferring electrons from the metal oxide to the chemisorbed oxygen according to the following processes [33]: − O2 (gas) ⇔ O2 (ad) ⇔ O− 2 (ad) ⇔ O (ad)

⇔ O2− (ad) ⇔ O2− (lattice).

(2)

In these processes, there exists a transition temperature below which oxygen adsorbed on the surface is mainly in the form of O− 2 , whereas above which, chemisorbed oxygen dominates in the form of O− and O2− [34]. This transition temperature is approximately 450 K (≈180 ◦ C) for metal-oxide surfaces [34]. 4

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relationship between carrier concentration and space charge layer capacitance term CSC can be defined as [35, 36]   1 2 kT E − E = (3) fb − 2 εε0 eNd e CSC where k is Boltzmann’s constant (1.38 × 10−23 J K−1 ), ε0 the vacuum permittivity (8.85 × 10−14 F cm−1 ), e the elementary charge (1.6 × 10−19 C), ε the permittivity of TiO2 (for anatase is 48), Nd the donor density and T the Kelvin temperature. From the slopes of the Mott–Schottky plots in their linear range there can be calculated Nd (donor density or carrier concentration) of amorphous and anatase TNTA, Nd (amorphous) = 1.149 × 1019 cm−3 > Nd (anatase) = 4.022 × 1017 cm−3 . The amorphous nanotube structure is more disordered than that of anatase nanotubes, thereby having many more defects (oxygen vacancies or interstitial positions) which provide abundant local donor energy levels. The donor energy levels enhance conductivity [34], i.e. concentration of charge carriers. The larger the base concentration of charge carriers (electrons), the more it changes after exposure to oxygen. This leads to higher sensitivity for an amorphous than an anatase sensor to the same oxygen-containing atmospheres, and better linear correlation with oxygen change over a wider concentration range. When the TNTA sensors work at 100 ◦ C, oxygen is chemisorbed in the form of O− 2 , leading to an increase of resistance. There is a reasonable model in which O− 2 species are mainly bound in the vicinity of vacancies [3]. On removing the oxygen atmosphere, oxygen desorption takes place and electrons transfer back to nanotubes, recovering to the original resistance of the nanotubes. However, the condition is different at higher temperatures. At higher temperatures, O− and O2− ions exist as prevailing species, causing a re-oxidation of the crystal that leads to restructuring of the whole surface [3]. Because the energy of O2− in a lattice site is estimated to be much lower (about 20 eV) than that in an adsorbed state [37], O2− (ad) is unstable and has to be stabilized by diffusing into the lattice. This change reduces defects (oxygen vacancies) which provide abundant local donor energy levels in amorphous TNTA, i.e. charge carrier concentration is decreased. On removing oxygen, the reduction of the lattice cannot occur quickly, hence the time for a sensor to regain

Figure 5. (a) The sensitivity of amorphous TNTA at 50, 100, 150, 250 and 300 ◦ C for different oxygen concentrations. (b) The sensitivity of the amorphous TNTA and annealed TNTA corresponding to different oxygen concentrations at 100 ◦ C.

As to the dominant mechanism behind the phenomena that amorphous TNTA has higher sensitivity and a better linear relationship with oxygen concentrations than those of anatase TNTA, it is reasonably considered that the key point is the change of charge carrier concentration on the nanotube surface. Based on the Mott–Schottky principle, the carrier concentration can be obtained by electrochemically measuring the space charge layer capacitance. The Mott–Schottky plots of amorphous and anatase TNTA at the AC frequency of 1 kHz in 0.25 M Na2 SO4 aqueous electrolyte are shown in figure 6. According to the Mott–Schottky principle, the

Figure 6. Mott–Schottky plots of (a) amorphous TNTA and (b) anatase TNTA in 0.25 M Na2 SO4 , recorded at a frequency of 1 kHz.

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its original conductivity is very long [15]. This is a plausible reason why the TNTA sensors are recoverable for cycled changes between oxygen and nitrogen at low temperature under the transition temperature, but at higher temperature they become unrecoverable.

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4. Conclusion Titania nanotube arrays were prepared by anodization and investigated for their potential application in oxygen sensors. The as-prepared amorphous TNTA show remarkable recoverable responses to oxygen at a low temperature of 50 ◦ C. At 100 ◦ C the sensing properties (sensitivity, recovery, linear correlation with oxygen concentration and response range) are the best and the lowest detectable concentration is 200 ppm. Interestingly, the oxygen sensitivity of TNTA is much better than that of other metal-oxide sensors such as nanoscale TiO2 thick film, SrTiO3 thick film and Ga2 O3 thin film. These results demonstrate that amorphous TNTA structures can be very promising candidates for oxygen sensors, particularly at low temperatures.

Acknowledgments The authors thank Professor Ying Li and Dr Li Liu for their support of the Mott–Schottky curve measurements. This work was supported by the National Science Foundation of China (50602011).

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