二氧化钛纳米阵列的制备及其气敏性能研究

Chinese Science Bulletin

? 2008 SCIENCE IN CHINA PRESS

Springer

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Chinese Science Bulletin | May 2008 | vol. 53 | no. 9 | 1352-1357

Preparation and hydrogen gas sensitive characteristics of highly ordered titania nanotube arrays

JI HuiMing ?, LU HuiXiang, MA DongFang, YU JianJun & MA ShiCai

Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, School of Materials of Tianjin University, Tianjin 300072, China

In this paper, we report the growth and characteristics of titania nanotube arrays prepared by anodic oxidation and then annealed in an oxygen atmosphere at 500℃. The titania nanotube arrays presented high sensitivity to hydrogen gas. The crystalline phase of the samples was checked by X-ray diffraction (XRD). The differences in the nanotubes morphology attributed to the etched samples due to anodiza-tion potential, reaction time and the electrolyte concentration were analyzed by scanning electron mi-croscopy (SEM). The gas sensitive parameters of the samples were obtained from resistance, the re-sponse time and the recovery time at different temperatures.

titanium dioxide, ordered nanotube arrays, preparation, hydrogen gas sensor

Titanium dioxide is a versatile material for different ap-plications. It is used as heterogeneous catalyst, photo-catalyst in solar cells, gas sensors and white pigments (in paints, cosmetics, etc.). Also it has electronic and electrical applications in MOSFET (as a gate insulator) and varistors [1].

Titanium dioxide is also a fascinating material from a surface science point of view. Properly made titania surfaces are very useful for different electronic applica-tions especially as gas sensors and solar cells [2]. The prime requirement for these important applications is high active surface area. Making nanosized material is a efficacious technique to increase the active surface area. The simplest approach to fabricate titania nanotubes is electrochemical anodic oxidation. Gong et al.[3] devel-oped uniformly oriented porous titania nanostructures by anodic oxidation of high purity titanium in hydrofluoric acid medium under potentiostatic bias.

Titania has attracted much attention for its oxygen

sensing capability [4―

6]. Furthermore, with proper ma- nipulation of the microstructure, crystalline phase and/or addition of proper impurities or surface functionalization titania can also be used as a reducing gas sensor [7,8].

Hydrogen has numerous applications in industry,

such as petroleum distillation, chemical production, cryogenic cooling, semiconductor manufacturing proc-esses, fuel cell technology, and rocket engines [9]. Its wide range of applications motivates our work on the development of highly sensitive, specific, robust, and affordable hydrogen sensors, enabling its safe and accu-rate use.

1 Experimental

1.1 Fabrication of the highly ordered titania nano-tube arrays

Industrial pure titanium foil was degreased by sonicating in acetone, ethanol and DI water, and dried in a nitrogen stream. The equipments of the electrochemical anodiza-tion are shown in Figure 1. Anodization experiments are commonly conducted with magnetic agitation of the electrolyte which reduces the thickness of the double layer at the metal/electrolyte interface, and ensures uni-form local current density and temperature over the Ti

Received August 23, 2007; accepted December 18, 2007 doi: 10.1007/s11434-008-0120-0 ?

Corresponding author (email: jihuiming@https://www.360docs.net/doc/7611937387.html, )

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Figure 1 A three-electrode electrochemical cell in which the Ti samples are anodized.

electrode surface.

The anodization was performed in a hydrofluoric acid-based electrolyte. The electrolyte concentration is 0.5%－2%, and electrolyte pH was adjusted by the addi-tion of sodium hydroxide, using a platinum foil cathode. Nanotube length and pore diameter changed depending on the electrolyte concentration (0.5%－2%), the ano-dization potential (5－40 V), and the time of anodization (3－60 min).

After being annealed at 500℃in pure oxygen ambi-ent for 2 h, the titania nanotubes would present a gas sensitivity to hydrogen. The sensitivity was influenced by the testing temperature and the hydrogen concentra-tion.

The crystalline phase and the nanotubes morphologies were clarified by X-ray diffraction (XRD) and scanning electron microscopy (SEM).

1.2 The sensitivity testing of the titania nanotube array

The equipments of sensitivity testing of the titania nanotube arrays are shown in Figure 2. Two spring- loaded parallel platinum foils 10 mm by 2 mm are used

as the electrodes, which are pressed on the nanotubes. The hydrogen gas was mixed in appropriate ratios with nitrogen to create the necessary test gas ambient.

2 Results and discussion

2.1 The effect of the voltage on the surface mor-phology of titania nanotube arrays

Figure 3 is the SEM images of the surface morphology

of the titania nanotubes at 5, 10, 15, 20, 30, 40 V. The anodization was performed in an electrolyte medium of

13 vol% acetic acid and 0.5 vol% hydrofluoric acid for

30 min.

The surface morphology of the titania nanotube ar-rays (Figure 3(a)) was similar to the spongy Al2O3 ob-tained by anodizing at 5 V, and the nanotubes were connected to each other via tube wall, presenting an in-

ner diameter of~20 nm and wall thickness of ~8 nm. Each the nanotube did not have an intact pore wall, but one wall connecting to two pores. The pore diameter would increase with the increase of voltage. When the voltage was 10 V (Figure 3(b)), the shapes of nanotubes appeared in some areas and their pore diameter and wall thickness arrived at 30 nm and 12 nm, and the wall be-tween pore and pore began to separate. When the volt-age was 15 V, the shapes of nanotubes appeared in the whole area, and their pore diameter and wall thickness reached to 55 nm and 15 nm (Figure 3(c)). When the voltage was 20 V, the pore diameter became bigger and the structure of nanotubes would become more inde-pendent (Figure3 (d)). However, when the voltage got to

30 V, the shapes of nanotubes would be destroyed, and the surface would become unsmooth (Figure 3(e)).

Figure 2 The schematic representation of the experimental apparatus.

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Figure 3 SEM top-view images of titanium oxide films anodized at different voltages: (a) 5 V; (b) 10 V; (c) 15 V; (d) 20 V; (e) 30 V; (f) 40 V .

When at 40 V, the whole surface would become unor-dered, we could hardly see the pore (Figure 3(f)).

The curves of pore diameter and wall thickness of ti-tania nanotubes against anodized voltage can be seen in Figure 4. We can find that the wall thickness increases linearly with the increase of voltage, and the pore di-ameter increases slowly when voltage ranges from 5 to 10 V, and then increases quickly.

Thus it can be seen that the voltage is important to the surface morphology of the titania nanotubes, and we can prepare titania nanotubes at proper voltage.

2.2 The effect of the reaction time on the surface morphology of the titania nanotubes

Figure 5 shows SEM images of surface structures ano-dized for 3 min, 30 min and 1 h, in an electrolyte me-dium of 1 wt% hydrofluoric acid at 20 V. After reacting for 3 min (Figure 5(a)), some parts of the titania nano-

tubes appeared to be illegibility, and some parts of the

Figure 4 Pore diameter and wall thickness of titania nanotubes against anodized voltages.

surface were still covered with oxidation layers. After 30 min, ordered shapes appeared independently in the whole area (Figure 5(b)). The side elevation of the

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Figure 5 SEM images of surface structures obtained at different time: (a) 3 min; (b) 30 min; (c) 1 h. (d) Side elevation of nanotube obtained at 30 min.

nanotubes is shown in (Figure 5(d)), and the pore di-ameter had a little change, indicating that anodizing time had a little influence on pore diameter. When reaching to 1 h, we can see that the surface became complexity and ordered tube shapes had been destroyed (Figure 5(c)). Comparing Figure 5(b) with Figure 5(c), we can evidently find that the pore diameter decreased to some degree.

2.3 The effect of the electrolyte concentration on the surface morphology of the titania nanotubes

Figure 6 shows SEM images of surface structures in electrolyte medium of 0.5, 1, 1.5, 2 wt% hydrofluoric acid anodized at 20 V for 3 min, respectively. We can find that the wall thickness and pore diameter changed little, so the electrolyte concentration had little influence on the surface morphology of the nanotubes, but some samples appeared hollow, due to the connection of de-stroyed tubes, when the concentration was higher. 2.4 The sensitivity of the titania nanotube arrays to hydrogen gas

The titania nanotube arrays used in sensitivity testing to hydrogen gas were prepared under an anodization po-tential of 20 V in an electrolyte medium of 13 vol% ace-tic acid and 0.5 vol% hydrofluoric acid for 30 min. The nanotubes were approximately 500―600 nm in length and presented a barrier layer with the wall thickness of 24 nm and the pore diameter of 82 nm.

When exposing the titania nanotube arrays to hydro-gen gas at different testing temperatures, the resistance of the sample decreases obviously, and the resistance recovers completely after switching back to air from hydrogen ambient. From Figure 7, we can find that the initial resistance of the sample is different at different temperatures. When exposed to 1000 ppm H 2, the resis-tance of samples changed obviously.

Table 1 shows the response time and the recovery time at different testing temperatures. We can find that the response time and recovery time of the titania nano-tubes range from 86 s and 268 s at 240℃ to 46 s and 93s at 300℃, respectively. The environment humidity and temperature were about 30% RH and 15℃, respec-tively.

The titania nanotubes would present gas sensitivity only by heat treatment [10]. Figure 8 shows the XRD pat-terns of a typical sample obtained by anodizing and after annealing at 500℃ for 2 h in an oxygen atmosphere with heating and cooling rates of 3℃/min. We can find that anatase phase appears (A-anatase; T-Ti substrate) after heat treatment. Anatase, the polymorph of titania, has been reported to have high sensitivity towards reduc-ing gases like hydrogen and carbon monoxide [11―

13]. As the diffusing hydrogen atoms go to the interstitial sites and as the c /a ratio of anatase is almost four times that of the normal rutile, it appears that anatase lattice accommodates hydrogen easily and hence has a greater

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Figure 6 SEM images of surface structures obtained at different HF concentrations: (a) 0.5%; (b) 1%; (c) 1.5%; (d) 2%.

Figure 7 Variation of the resistance of the sample with time when ex-posed to hydrogen gas at different temperatures.

Table 1 Response and recovery time variation to hydrogen gas concen-trations

Testing temperature

Response time

Recovery time

240℃ 86 s 268 s 270℃ 52 s 157 s 300℃

49 s

93 s

contribution to hydrogen sensitivity.

When the TiO 2 is exposed to the air ambient, O 2 will be absorbed onto the surface of the material, forming physically absorbed oxygen (O 2 gas) and chemical oxy-gen(O 2ads ?, O ads ?, and O ads 2?)[14]. When the temperature is

low, absorbed oxygen is mainly physical absorbed oxy-

Figure 8 XRD patterns of the titania nanotubes after heat treatment.

gen (O 2 gas) and will capture free electron from the-nanotube arrays, forming oxygen ions (O ads ? and O ads 2?) with the temperature increasing [15]. When the temperature is above 450 K, the absorbed oxygen is mainly O ads ?, the chemical reaction equation is as follows:

O 2 + 2e ?→2O ads ?.

When O 2 became O ads ?, the surface of the sensitive ma-terial presented acceptor state, made the energy band bend up and formed Schottky barrier, creating an elec-tron depletion region and forming surface high resis-tance state, as a result, the resistance of the material in-creased [16]. When the nanotube arrays were exposed to the hydrogen gas at high temperature, the major process

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behind the interaction between the nanotubes and hy-drogen gas was the chemisorption of the dissociated hy-drogen on the titania surface. During chemisorption hy-drogen acted as a surface state and a partial charge transfer took place from hydrogen to the conduction band of titania. This created an electron accumulation layer on the nanotube surface, enhancing its electrical conductance. By removing the hydrogen ambient, elec-tron transfer got back to hydrogen and desorbed, thus restoring the original resistance of the nanotubes [17]. In addition, the special architecture of the nanotubes may enable dramatically improving the gas sensitivity. The titania tubes have nanosize and inside and outside surfaces. As a result, they can create two electron deple-tion regions when absorbing oxygen, thus the most volume of the inter-tubular region may experience high resistance, and the resistance will decrease obviously as the hydrogen gas is injected. 3 Conclusions

Highly ordered titania nanotube arrays were prepared by optimum technology. Titanium foil was anodic oxidation at 20 V in an electrolyte medium of 13 vol% acetic acid and 0.5 vol% hydrofluoric acid in 30 min, and then was annealed in an oxygen atmosphere at 500℃. Hydrogen gas sensitive characteristic were studied. The sensitivity of the titania nanotube arrays was influenced by the test-ing temperature. The response time and recovery time were also influenced by the testing temperature, and they decreased with increasing temperature, and at 300℃ the response time and the recovery time could get to 49 s and 93 s, respectively. We believe that the hydrogen gas chemisorption onto the titania surface where they act as electron donors was the main sensitive mechanism. In summary, titania nanotubes prepared using anodization can successfully be used as hydrogen gas sensors.

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