Effects of synthesis parameters on controllable growth of highly-ordered TiO2 nanotube arrays

Shipu Li1,a, Shiwei Lin1,b, Jianjun Liao1, Danhong Li1, Yang Cao1, Jianbao Li1 1Key Laboratory of Ministry of Education for Application Technology of Chemical Materials in

Hainan Superior Resources, School of Materials and Chemical Engineering, Hainan University,

Haikou 570228,People’s Republic of China

alishipu2008@126.com,b swlin77@gmail.com

Key words: Titanium oxide, Nanotube, Fabrication, Pretreatment, Potential

Abstract. Titania nanotube arrays were fabricated in deionize water and glycerol mixed electrolyte

containing a certain amount of NH4F. Three different polishing methods were used for pretreatment

of Ti substrates: polished by hand with abrasive paper, by polishing machine, or by chemical

polishing fluid (HF:HNO3=1:4, in volumetric ratio). The morphology of three different samples

were imaged by scanning electron microscopy, and their photoelectrical properties were studied as

well. Experimental results showed that Titania nanotube arrays grown on the Ti substrate and

polished by polishing fluid has highly-ordered and well-defined nanotube structure. The effects of

anodization potential and duration on synthesis of highly-ordered TiO2 nanotubes were also studied

in this paper. Both the layer thickness and nanotube diameter linearly increase with the increasing

potential. The layer thickness also increases with prolongation of anodization time. By optimizing

the preparation conditions, we can successfully control the geometrical structure of TiO2 nanotube

arrays with diameters in the range between 50 and 200 nm and the layer thickness between 800 and

2000 nm.

Introduction

TiO2 is one of the most promising oxide semiconductors for photoelectrochemical applications,

particularly due to its low cost, non-toxicity, and stability against photocorrosion. TiO2 nanotubes

are of great interest due to their high surface-to-volume ratios and size-dependent properties. As the

most attractive photocatalyst with excellent photocatalytic properties, it has widespread application

prospect in dye sensitization solar cells [1], sensors [2], hydrogen generation by water

photoelectrolysis [3], photocatalytic degradation of pollutants [4] and biomedicines [5].

To date, TiO2 nanotubes have been produced through a variety of methods which include

templating, hydrothermal treatment and anodic oxidation [6]. The anodic oxidation is shown to be a

powerful method. In 2001, Grimes and co-workers firstly reported the formation of uniform TiO2

nanotube arrays by anodic oxidation of titanium in an hydrofluoric (HF) electrolyte [7]. But the

TiO2 nanotube arrays grown in HF electrolytes are difficult in applications because of its limited

thickness of the layers [8,9]. Then organic electrolytes containing NaF or NH4F instead of HF are

widely used to prepare longer nanotubes[10,11]. The nanotubes produced by anodization in organic

electrolytes can permit a careful control over their nanotube diameter, layer thickness and wall

thickness, obtaining structures vertically oriented from the surface. As it is beneficial to application,

the study for controllable preparation of highly-ordered TiO2 nanotube arrays has profound

significance. The geometrical structures of the TiO2 nanotube arrays by anodization of Ti foils has

been found to strongly depend on the anodization potential, sweep rate, anodization time, and

electrolyte compositions. But there are few reports about the effects of pretreatment conditions on

synthesis of the TiO2 nanotube arrays.

Advanced Materials Research Vols. 284-286 (2011) pp 791-795Online available since 2011/Jul/04 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.284-286.791

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Herein, we applied three different methods to pretreat Ti substrates before anodic oxidation:

polished by hand with abrasive paper, by polishing machine, or by chemical polishing fluid.

Experimental results showed that the last treatment method could lead to a uniform nanotube arrays

by chemical anodization. Furthermore, the effects of anodization potential and duration on the

growth of highly-ordered TiO2 nanotubes are discussed in this paper.

Experimental

The TiO2 nanotube arrays were prepared by electrochemical anodization of titanium foils (0.5 mm,

99.6% purity). Firstly, the titanium foils were cut into small pieces (1×5 cm). Three samples

(namely, sample 1, 2, and 3) were polished by chemical polishing fluid (HF:HNO3=1:4, in

volumetric ratio), by hand with abrasive paper, and by polishing machine, respectively. Then all the

samples were degreased by sonicating in acetone, methanol, rinsed with deionized water (DI) and

dried in air in steps. The anodization was performed in a two-electrode configuration with titanium

foil as the working electrode and stainless steel foil as the counter electrode. The samples were

anodized at 30 V and in solutions containing 0.27 M NH4F consisting of mixtures of DI water and

glycerol (1,2,3-propanetriol) prepared in volumetric ratio (50:50%) for 3h. After the electrochemical

treatment the samples were rinsed with deionized water and dried in air. Thermal annealing was

performed in ambient air at 450 ˚C for 3 h. Furthermore, after Ti foils were pretreated by chemical

poilshing fluid, different potentials (10~35 V), and time (1~10 h) were applied in the systhesis of

TiO2 nanotube arrays. Microstructures of the samples were characterized by scanning electron

microscope (SEM, Hitachi S-3000N, Japan). Electrochemical characterization of the samples was

carried out via electrochemical workstation (Zanher zennium, Germany) and performed using a

three-electrode configuration with samples as the working electrode, saturated Ag/AgCl as a

reference, and platinum foil as a counter electrode. Linear sweep voltammograms for the titania

nanotubes arrays were measured in 1 M KCl solutions at sweep rates of 100 mV/s.

Results and discussion

Figure 1(a) shows the digital photo of three samples after anodization at 30 V and in electrolyte

containing 0.27 M NH4F consisting of mixtures of DI water and glycerol for 3 h. The three samples

were polished by chemical polishing fluid (1), by hand with abrasive paper (2), and by polishing

machine (3), respectively. The samples have two different regions; light and dark parts. Sample 1

has only a small light region, but samples 2 and 3 have a large light region. In the dark region seen

in Fig.1(b) uniform TiO2 nanotube layers with approximately 1800 nm thickness and 160 nm

diameter were grown. In this region the tubes have complete structures and ordered arrangements.

However, in the light region, as shown in Fig. 1(c) oxide layer with a thickness of 1200 nm was

produced. In this region the structures are irregular and the surface looks grassy with few

nanotubes.

The TiO2 nanotube arrays on Ti substrates after different pretreatment were crystallized by

thermal treatment. And their photoelectrical properties were investigated with/without UV

irradiation. Figure 2(a) shows the linear sweep voltammograms for the sample polished by hand,

which was measured in 1 M KCl solutions at a sweep rate of 100 mV/s. The current density

decreases with increasing potential when measured without UV irradiation. But when the sample

was exposed to UV irradiation, the current density increased significantly. It shows that the TiO2

nanotube arrays are active under UV irradiation. For comparison, three samples with different

polishing methods were also measured under UV irradiation. Figure 2(b) shows the current density

792 Materials and Design

of three samples at the same potential of 1 V. It can be seen that the current density of the sample

polished by chemical polishing fluid is much higher than those of the other two samples. From Fig.

1, there are a large area of irregular structure on samples 2 and 3, which has lower photoresponse.

Experimental results above show that the sample polished by polishing fluid is the feasible

polishing method which can obtain high quality TiO2 nanotube arrays with good

photoelectrochemical property.

Fig.1 (a): The photo of three samples after anodization; (b) and (c): SEM top-view images of the Ti

foils after anodization for the dark (b) and light (c) regions in (a), respectively. The insets show the

cross-section images of the corresponding nanotube arrays.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

1.5x10-5

2.0x10-5

2.5x10-5

3.0x10-5

3.5x10-5

4.0x10-5

i(A/cm

2)

Potential/V

A

B

A: under UV

B: dark

(a) 1 2 3

0.0

1.0x10-5

2.0x10-5

3.0x10-5

4.0x10-5

5.0x10-5

6.0x10-5

7.0x10-5

i(A/cm

2)

1: polishing fluid

2: by hand

3: polishing machine

(b)

Fig.2 (a): linear sweep voltammograms for the sample polished by hand measured in 1 M KCl

solutions at a sweep rate of 100 mV/s; (b): current density of three samples under the same potential

of 1 V.

To study the effect of anodization potential for synthesis of highly-ordered TiO2 nanotubes,

further experiments were conducted. Figure 3 shows the SEM images taken from the samples

anodized at the different potentials applied between 10 and 35 V for 3 h. The images are top-views,

while the insets are cross-sectional views of the samples. As it can be seen, there is a strong effect

of the applied potential on the nanotube diameters, ranging from 50 nm at 10 V, 100 nm at 20 V, to

almost 200 nm at 35 V. However, when the anodization potential is lower than 10 V, it cannot get

complete tubular structure. Under this condition, the potential seems to be inadequate for

continuous formation of TiO2 film [12]. When the applied potential was raised to 40 V, no nanotube

structure was obtained, either. Thus the potential window of this organic electrolyte to successfully

grow well-defined nanotube arrays is 10~35 V.

Advanced Materials Research Vols. 284-286 793

Fig.3 SEM top and cross-sectional images of the nanotube arrays after anodizatin for 3 h at different

potentials (10 V, 15 V, 20 V, 25 V, 30 V, 35 V).

10 15 20 25 30 350

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

Potential(V)

Layer thickness (nm)

40

60

80

100

120

140

160

180

200

220

240

Tube diameter(n

m)

Fig.4 Nabotube diameter and thickness of the nanotube arrays as a function of applied potentials.

Figure 4 shows the variation of the nanotube diameters and layer thickness in dependence of the

anodization potential, both of the layer thickness and nanotube diameter increase linearly as

increasing potential. The layer thickness can be adjusted in the ranges form 800 nm at 10 V, 1700

nm at 25 V, to 2000 nm at 35 V. In this work, the tube diameter and anodization potential obey the

relationgship of d (nm) ≈ 5 × U (V).

0 2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

3.5

Layer thickness (µµ µµm)

hour(h)

Fig.5 Nanotube layer thickness as a function of anodization time.

794 Materials and Design

The relationship between the nanotube layer thickness and anodization time is shown in Fig.5.

The tube layer thickness increases linearly form 950 nm at 1h to 2650 nm at 5 h. With anodization

time further increasing, the layer thickness increases slowly. After anodization for 10 h, even

prolongated anodization time does not increase the nanotube layer thickness any more.

Conclusions

In summary, highly-ordered TiO2 nanotubes have been grown by anodization of Ti substrates in

mixed electrolytes consisting of water, glycerol and NH4F. The results show that the sample

pretreated by polishing fluid is the feasible polishing method. Also, we studied the relationship

between tube diameter, layer thickness and anodization potential, time. By changing the preparation

parameters, we can successfully achieve controllable growth ot highly-ordered nanotube arrays with

tube diameter in the range of 50 - 200 nm and the layer thickness in the range of 800 - 3200 nm.

Acknowledgement

This work was supported by Program for New Century Excellent Talents in University

(NCET-09-0110), the Key Project of Chinese Ministry of Education (210171), and National

International Cooperation Program (2009DFA92551). We acknowledges Mr. Guizhen Wang for the

analysis of scanning electron microscopy in the Analytical and Testing Center of Hainan

University.

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Materials and Design 10.4028/www.scientific.net/AMR.284-286 Effects of Synthesis Parameters on Controllable Growth of Highly-Ordered TiO2 Nanotube Arrays 10.4028/www.scientific.net/AMR.284-286.791

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