Preparation and characterization of tio2 nanotube arrays via anodization of titanium films deposited on FTO conducting glass at room temperature

Chinese edition available online at www.whxb.pku.edu.cn ARTICLE Preparation and Characterization of TiO2 Nanotube Arrays via Anodization of Titanium Films Deposited on FTO Conductin

Volume 24, Issue 12, December 2008

Online English edition of the Chinese language journal

Cite this article as: Acta Phys -Chim Sin., 2008, 24(12): 2191−2197

Received: July 14, 2008; Revised: September 17, 2008

*Corresponding author Email: taojie@nuaa.edu.cn; Tel: +8625-52112900

The project was supported by the Natural Science Foundation of Jiangsu Province (BK2004129) and the Aeronautical Science Foundation of China (04H52059) Copyright © 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University Published by Elsevier BV All rights reserved Chinese edition available online at www.whxb.pku.edu.cn

ARTICLE

Preparation and Characterization of TiO2 Nanotube Arrays

via Anodization of Titanium Films Deposited on FTO

Conducting Glass at Room Temperature

Yuxin Tang, Jie Tao*, Yanyan Zhang, Tao Wu, Haijun Tao, Zuguo Bao

College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P R China

conducting glass in NH 4 F/glycerol electrolyte by electrochemical anodization of pure titanium films deposited by radio frequency magnetron sputtering (RFMS) at room temperature The samples were characterized by means of field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and photoelectrochemistry methods The results showed that Ti films prepared at the condition of Ar pressure 0.5 Pa, power 150 W, and 0.5 h at room temperature possessed the zone T model structure with good homogeneity and high denseness When the anodization time was prolonged from 1 to 3 h at the voltage of 30 V, the pore diameter

of TiO 2 nanotubes increased from 50 to 75 nm, and the length increased from 750 to 1100 nm and then gradually decreased to 800

nm, while their wall morphology changed from smooth to rough Also with increasing the anodization voltage, the pore diameter

became larger, and the remaining oxide layer reduced, which could be easily removed by ultrasonic- chemical cleaning in 0.05% (w,

mass fraction) diluted HF solution Moreover, the photocurrent response curves and electrochemical impedance spectroscopy (EIS) results indicated that UV-illumination clearly enhanced the effective separation of the electron-hole pairs and the crystallized electrodes from the annealing treatment of as-anodized electrodes at 450 °C exhibited a better photoelectrochemical performance.

NH 4 F/glycerol

The growth of metal film in vacuum is a deposition process

of massive atoms or atomic groups arriving on the substrate,

which is affected by the deposition parameters, such as

sub-strate temperature, sputtering pressure, and power The

micro-structure of the films is dominated by the relative substrate

there are evaporation method and sputtering method to

pre-pare metal films by vacuum deposition The former easily

conducted in simple equipment can manufacture the films

with poor compactness and several bubbles The latter can

produce firm and dense films with good homogeneity and

re-producibility in a large area owing to high-energy bombarding

particles Therefore sputtering method has become an

impor-tant way that has been used very widely

Recently, titanium film has drawn extraordinary attention

for its applications in microelectronics, machinery, aerospace, and medical industry owing to its remarkable photoelectric

ar-rays from titanium thin films deposited on glass substrates at

500 °C by anodization The well-orderly and perpendicularly-

surface area and strong absorption capacity, but also superior electron lifetimes owing to providing excellent pathways for electron percolation Thus they exhibited high photoelectric

deposit Ti films at high temperature on a variety of substrates (e.g silicon) However, the length of nanotube arrays prepared

in the electrolytes was less than 700 nm since the increasing of

tubes and make these devices microminiaturized and flexible

titania films on the transparent substrate, the distribution of

the pores is not ordered and highly oriented According to our

film and the anodization parameters are critical to the

forma-tion process of titania nanotubes Therefore, on the basis of

the fabrication of high-qualify Ti films, it is necessary to

se-lect an ese-lectrolyte with low chemical dissolution rate to

in-crease the thickness-conversion ratio of Ti film In this study,

we attempt to deposit compact and uniform Ti films on

con-ducting glass at room temperature, and then fabricate large

electrolyte Additionally, the growth process and

investi-gated, and a method for eliminating the cover layer on the

nanotube surface is also discussed

1

1 Experimental

1.1 Deposition of the titanium film at room temperature

on conducting glass

Ti films (ca 1.47 µm) were deposited on fluorine-doped tin

oxide (FTO) conducting glass at room temperature by RFMS

(radio frequency magnetron sputtering) (JPG500, China) A Ti

disk (99.9% purity, TianYuan Co., Ltd., Shenzhen, China)

with 60 mm diameter and 5 mm thickness was used as the

the distance between the target and sample was 60 mm To

remove pinhole defects in the titanium film, the specimen was

cleaned by ultrasonic in acetone, ethanol, and deionized water

for 15 min, respectively Before deposition, the target was

prior sputtered for 10 min to remove the oxide layer or other

impurities on the titanium target surface During the

for 0.5 h, the samples were kept in vacuum for 6 h

anodization method

Anodization was carried out using a two-electrode

configu-ration Titanium film/FTO with an exposed surface area of 1.0

cm×1.5 cm) with a copper wire was used as the

counter-elec-quently dried in air Some samples were heat treated for 3 h at different temperatures ranging from 300 to 550 °C in air at a

in the furnace

2 Results and discussion

2.1 Characterization of titanium films deposited at room temperature

The atom deposition process can be divided into three steps, namely the gas phase atom′s deposition or adsorption, the sur-face diffusion, and the bulk self-diffusion The morphological features of metal films are given on the basis of the relative

melting point of the deposited material) and the energy of deposition atoms The morphological features were named as zone 1, zone T, zone 2, and zone 3 for high melting point

sputtering pressure and power will be formed between zone 1 and zone T Compared with zone 1, the critical nucleus size of

Ti grains is still small and the structure is also consisted of an array of fibrous grains in the zone T, however, the surface dif-fusion of the atom is significant, causing the relatively com-pact structure at grain boundaries without holes and tapered crystals Apparently, it is necessary to choose the appropriate sputtering parameters to promote the appearance of zone T structure At suitable low sputtering pressure and high sput-tering power, the Ti atom will obtain higher energy Therefore, the surface diffusion is improved and the substrate tempera-ture increases, yielding a movement of the structempera-ture to zone T region

FESEM images, XRD pattern, and EDX spectrum of Ti

and t=0.5 h at room temperature are shown in Fig.1 In

Fig.1(a−c), it is found that a close-packed hexagonal structure

of α-Ti film with (002) preferred orientation appears when

150 W power is applied The microstructure of the film is uniform with the grain size of 100 nm, and a banding distribu-tion of columnar structure belonged to the typical zone T structure with high density is seen from the cross-section of the film The Ti atom has high energy to migrate and occupy the equilibrium sites of titanium crystal lattice when the Ti

film is deposited at appropriate high sputtering power and low

sputtering pressure, which results in the growth of the

colum-nar structure (zone T) Moreover, the result of the chemical

analysis by energy dispersive X-ray (EDX) spectroscopy

(Fig.1(d)) indicates that an almost pure Ti film is formed with

a low oxygen contamination level (below 1%) Fig.2 depicts

ti-tanium films deposited at different conditions in 0.5% (w) HF

solution at 10 V When Ti film is deposited at 500 °C

zone 2 form In this condition, the surface and bulk diffusion

of the film are significant and the deposited atoms migrate

adequately, so that the shadowing effect is weakened, leading

to a columnar structure of the film with high denseness

Therefore, the substrate is frequently heated at high

tempera-ture to create a dense, uniform, and crystal titanium film

morpho-logical feature of uniform and organized titania nanotubes

(Fig.2(a)) anodized from Ti films deposited at room

tempera-ture is similar to Fig.2(b), which indicates that the dense and

uniform Ti film obtained directly at room temperature is

fa-vorable for the formation of ordered nanotube arrays

2

arrays on FTO glass substrates

Fig.3 depicts the current–time behaviors recorded at 10 V

F/glyc-erol electrolytes It can be seen that the behaviors are not

electrolyte owing to its high dielectric constant and coefficient

of viscosity Also, a low current density is found in the viscous

indicating that the anodization process is controlled by diffu-sion Hence a dependence of the diffusion constant on the

D is the diffusion constant, kB is Boltzmann′s constant, T is the absolute temperature, η is the dynamic viscosity, and r is the radius of a spherical body According to this formula, D is in-versely proportional to η Therefore, a lower growth and

chemical dissolution rate of the nanotubes is obtained in the viscous electrolytes Also, it takes a long time to reach the stable-state As the anodization proceeds, the steep rise of

electrolyte after going through the stable stage The current increase is because the electrolyte interacts with the FTO sur-face as the last of the Ti film is consumed Meanwhile, the sample becomes translucent and should be quickly removed from the electrolyte; otherwise it would be consumed by the

HF Moreover, compared with smooth current curve in glyc-erol electrolytes, occasional fluctuations of the current curve

film or any exposed FTO substrate at the condition of high

elec-trolyte However, this phenomenon does not appear in the

Fig.1 (a) Top and (b) cross-sectional FESEM images of Ti film and the corresponding (c) XRD pattern and (d) EDX spectrum

Fig.2 FESEM images of TiO 2 nanotube arrays via anodization of

titanium films deposited at different conditions

(a) pAr=0.5 Pa, Ps=150 W, Tsubstrate=room temperature, t=0.5 h;

(b) p =0.5 Pa, P =150 W, T =500 °C, t=1 h

Fig.3 Current density–anodizing time curves of titanium films at

10 V in different electrolytes

(a) 0.5% (w) NHF/glycerol, (b) HF/H PO

with the length of 750 nm, which is longer than that of

ano-dization time is 2 h (Fig.4(d, e)), the morphology of the

nano-tube arrays is getting clear owing to the obvious thinning of

the porous structure The pore diameter and the length of the

nanotubes with roughness and ripples at the bottom of tube

walls increases to 75 nm and 1100 nm, respectively, while the

thickness of Ti film decreases to 200 nm During the

anodiza-tion process, the glycerol electrolyte keeps absorbing moisture

in the air, leading to the drop of the resistance of the

electro-lyte As a result, the rise of effective potential on the Ti/FTO

with the increasing amount of water in the electrolyte

Simul-zation (Fig.4(f, g)) The length of the nanotubes is shortened

to 800 nm with some collapsed nanotubes on the surface ow-ing to the serious corrosion of the electrolyte The whole formed tubes have rough walls with ripples Fig.4(h, i, f) shows FESEM images of samples anodized for 3 h at the voltages of 10, 20, and 30 V, respectively Obviously, the po-tential has a large effect on the tube diameters, ranging from about 35 nm at 10 V to 75 nm at 30 V However, the porous layer existed after 3 h anodization reduces gradually with the increase of anodizing potential This indicates that the higher potential not only increases the electric field intensity of the oxide layer, but also speeds up the diffusion of ions, leading to the higher growth rate of nanotube, and accelerating the field- enhanced dissolution rate of the upper oxide layer Therefore,

Fig.4 Top and cross-sectional FESEM images of TiO 2 nanotubes anodized in 0.5% (w) NH4 F/glycerol electrolyte at

different conditions (a, b) 0.5 h, (c) 1 h, (d, e) 2 h, (f, g) 3 h at 30 V, and top view at 10 V (h), 20 V (i) for 3 h

the increase of the potential can promote dissolution of the

cover oxide layer

In order to remove the porous oxide layer on the surface, a

series of as-prepared samples are cleaned in 0.05% (w) HF

solution by chemical etching with ultrasonic Fig.5 exhibits

the variation on the surface topography of the Fig.4(d)

sam-ples (1100 nm) cleaned at different times Clearly, the as-

prepared sample without cleaning is almost covered by a thin

porous oxide layer which is dissolved gradually as time passes

After 15 s treatment (Fig.5(a)), a small part of the uniform

nanotubes are exposed due to the porous oxide film dissolving

under the combined action of the hydrofluoric acid chemical

dissolution and ultrasonic oscillation, and then clear nanotubes

(Fig.5(b)) appear when the time reaches to 30 s Furthermore,

the porous layer is removed completely after 60 s treatment

(Fig.5(c)), while the topography and the length of nanotubes

are not influenced Therefore, the ultrasonic-chemical

clean-ing is an efficient method for the removal of the covered oxide

layer

before and after heat treatment at different temperatures are

nanotube layer (1100 nm) after anodization The characteristic

peaks of the as-anodized sample (Fig.6(a)) do not arise from

FTO and Ti, contributing to the form of the amorphous sample After heat treatment at 300 °C for 3 h (Fig.6(b)), the amor-phous sample transforms to anatase structures characterized with (101) preferred orientation At higher temperature (450

°C, 550 °C), the peaks of anatase in Fig.6(c, d) become stronger and the (110) characteristic diffraction peak of rutile

nanotubes is a mixed structure of anatase and rutile Also, the rutile phase grows at the interface between the barrier layer of the nanotubes and titanium film where the thermal oxidation

2 2.3 Photoelectrochemical characterization of nanotubular

Figs.7 and 8 show the photocurrent response curves and electrochemical impedance spectra (EIS) of nanotubular

UV illumination, respectively All electrochemical

illumina-tion using three-electrode system at room temperature with the

electrode (SCE) served as the working, counter, and reference electrodes, respectively The distance between the working electrode and the UV lamp is 3 cm The EIS measurement is

Fig.5 Top view of the surface of TiO 2 nanotubes in Fig.4(d) after ultrasonic cleaning by immersing in

0.05% (w) HF solution for different times

(a) 15 s, (b) 30 s, (c) 60 s

Fig.6 XRD patterns of nanotublar TiO 2 /FTO electrodes annealed

at different temperatures (a) as-anodized, (b) 300 °C, (c) 450 °C, (d) 550 °C

Fig.7 Photocurrent response curves of nanotubular TiO 2 /FTO electrodes annealed at different temperatures in 0.1 mol·L −1

Na 2 SO 4 solution under UV pulsed-illumination

carried out by applying 100 kHz to 0.01 Hz frequency range

with oscillation amplitude of 5 mV on a CHI660

electro-chemical workstation As seen in Fig.7, the photoelectric

cur-rent transiently increases and then tends to be stable when UV

light is turned on, which indicates that the nanotube electrode

has a good photoelectric current stability However, once UV

light is turned off, the photocurrent intensity quickly decreases

to initial value as dark current This phenomenon illustrates

that the composite electrodes have the n-type semiconductor

characteristic It is clear that the photocurrent increases along

with the increasing of heat treatment temperature and the

re-sponsive currents are constant at 0.03, 0.38, 1.55, and 4.88 µA

under dark condition and then drastically increase to 1.74,

6.48, 9.78, and 7.95 µA under UV illumination, respectively

illumination is owing to the efficient photo-generated

elec-trons conducting through interfacial region of the nanotubes

under an electric field, and the photocurrent level is affected

by the crystallinity and isomorph type of the titania When the

nanotubes is enhanced obviously, leading to the improvement

of photoelectrochemical performance It is found that the

sample shows better performance at 450 °C However, when

the temperature is higher than 550 °C, the performance of the

electrode degrades due to the decreasing of the specific

damage and collapse of the nanotubes (not shown here) This

found in the amorphous electrode corresponding to higher

radius of semicircle, the smaller will be the capacitance

con-stant, which results in higher value of the impedance of

Fara-day current Therefore, it is more difficult for the chemical

reaction to take place on the electrode owing to the higher

en-ergy barrier However, it is found that the radius of the

semi-circle decreases after heat treatment Moreover, the smallest

radius corresponding to the optimal performance appears at

450 °C This result is consistent with that of the photocurrent

the voltage of 30 V for 2 h With the increasing of the anodi-zation voltage (from 10 to 30 V for 3 h), the pore diameter became larger (from 35 to 75 nm), and the remaining oxide layer reduced, which could be easily removed by ultrasonic-

chemical cleaning in 0.05% (w) diluted HF solution

Further-more, the photoelectrochemistry measurements indicated that the crystallized electrodes from the annealing treatment of as-anodized electrodes at 450 °C exhibited better photoelec-trochemical performance Additionally, the UV-illumination clearly enhanced the effective separation of the electron-hole pairs, so that the photo-induced electrons transferred quickly

to the conducting glass via external circuit and formed

photo-current

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