This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 10085–10087 10085

Cite this: Chem. Commun., 2011, 47, 10085–10087

Highly ordered titania nanotube arrays with square, triangular, and

sunflower structures

Bo Chen, Kathy Lu* and Jeffrey Allen Geldmeier

Received 5th June 2011, Accepted 18th July 2011

DOI: 10.1039/c1cc13342j

Focused ion beam guided anodization enables the fabrication of

TiO2 nanotubes in a square arrangement with square cell shapes

and in a graphite lattice arrangement with triangular cell shapes,

which is impossible through self-organized anodization. TiO2

nanotubes in sunflower patterns are also obtained, which demon-

strates the great potential of guided anodization in fabricating

asymmetrical nanotubes.

Vertically aligned TiO2 nanotube arrays have attracted consider-

able attention because of their application potentials in dye-

sensitized solar cells,1–4 water photoelectrolysis,5,6 sensors,7–9

and lithium-ion batteries.10,11 For dye-sensitized solar cells,

TiO2 nanotube arrays provide direct electron transport pathways

and large internal surface area to improve the light harvesting

efficiency and reduce electron–hole recombination. For lithium-

ion batteries, ordered TiO2 nanotube arrays provide not only

close contact between the electrolyte and the nanotubes but

also enhanced electron and lithium-ion transport.

Traditional one-step electrochemical anodization can only

create disordered TiO2 nanotube arrays. Two-step anodiza-

tion,12–14 using better arranged concaves after the first-step

anodization as guiding seeds, has been explored to achieve

ordered TiO2 nanotubes. Similar to anodic Al2O3 nanopores,

the arrangement is only limited to hexagonal even under

the best anodization conditions; other important nanotube

arrangements cannot be obtained.

Focused ion beam (FIB) lithography15–17 and nanoimprint

lithography18,19 are two widely used patterning techniques to

fabricate highly ordered porous anodic alumina with both

hexagonal and square arrangements. Moreover, alumina

nanopore arrays with variable pore densities,20,21 such as

Moire patterns, have been realized by FIB pattern guided

anodization. It is of great interest if similar guided anodization

strategies can be applied to titanium anodization. However,

titanium has very high Young’s modulus (116 GPa) and no

study has been reported on generation of ordered nanotube

arrays through patterning methods.

In this communication, FIB patterning is used to guide the

anodization of TiO2 nanotubes. TiO2 nanotubes in square and

triangular shapes and arrangements are made possible with

the FIB guided anodization of square and graphite lattice

concave arrays, respectively. Asymmetrical TiO2 nanotube

arrays in a sunflower pattern are realized by the FIB guided

anodization.

A titanium substrate (99.6+%, 0.2 mm thick, Goodfellow,

Oakdale, PA) was first washed with acetone, isopropanol, and

methanol in ultrasound for 10 min each, and then electro-

polished in a freezing electrolyte (B1 1C) of glacial acetic acid/

perchloric acid (9/1 volume ratio) at 55 V with 800 rpm stirring

speed for 2 min. The guiding patterns for anodization were

created on the electropolished Ti surface by a FIB microscope

(FEI Helios 600 NanoLab). The patterning ion beam current

was 28 pA under 30 kV acceleration potential, and the ion

exposure time at each patterned concave was 90 ms. The

anodization was carried out in a two-electrode electrochemical

cell in an electrolyte mixture of 0.1 MNH4F and 10 vol%H2O

in ethylene glycol at room temperature with 15 mA cm�2

constant current density for 5 min. The voltage saturated at

88 V within 30 s. In order to separate the TiO2 nanotube layer

from the metallic Ti substrate for backside examination, the

anodic TiO2 foil was ultrasonically agitated in DI water/

ethanol (1/9 volume ratio) for 1 min, then immersed in

0.1 M HCl for 1 h. After rinsing with DI water and drying

in a nitrogen stream, the anodic nanotube layer was easily

peeled off from the substrate by a tape. For morphological

characterization of the samples, scanning electron microscope

(SEM) and atomic force microscope (AFM) were used.

For FIB patterning, an ordered concave pattern with

uniform diameter in square arrangement is created on the

electropolished Ti surface as shown in Fig. 1a. The diameter of

the concaves is 50 nm, and the depth of the concaves is 40 nm.

The interpore distance is 250 nm. During the anodization, the

FIB patterned concaves serve as the seeds to guide the growth

of TiO2 nanotubes. Highly ordered TiO2 nanotube arrays with

square arrangement are obtained (Fig. 1b). The surface and

backside SEM images show that the nanotubes have square

outer walls with 250 nm diameter. There is a thin layer of

nanopores on the top of TiO2 nanotube arrays, and all the

nanotubes develop from the location of the nanopores. The

cross section illustrates the effective guidance of the FIB

patterned concaves and the vertical growth of the nanotubes.

With the FIB guidance, TiO2 nanotube arrays with the

arrangement fundamentally different from the self-organized

hexagonal pattern are attained.

Department of Materials Science and Engineering, Virginia Tech,Blacksburg, Virginia, 24061, USA. E-mail: klu@vt.edu;Fax: +1 540 231 8919; Tel: +1 540 231 3225

ChemComm Dynamic Article Links

www.rsc.org/chemcomm COMMUNICATION

Publ

ishe

d on

10

Aug

ust 2

011.

Dow

nloa

ded

by L

omon

osov

Mos

cow

Sta

te U

nive

rsity

on

20/0

2/20

14 0

0:21

:10.

View Article Online / Journal Homepage / Table of Contents for this issue

10086 Chem. Commun., 2011, 47, 10085–10087 This journal is c The Royal Society of Chemistry 2011

Besides square outer tube shapes and arrangements, the

fabrication of triangular TiO2 nanotube shapes is also

explored here. The FIB patterned concaves with graphite

lattice arrangement and a 225 nm interpore distance are used

to guide the anodization development (Fig. 2a). After the

anodization, all the TiO2 nanotubes grow at the FIB patterned

locations (Fig. 2b). The backside and cross section views of the

TiO2 nanotube arrays demonstrate that no tubes form at the

centers of the graphite lattice structure. The growth of each

nanotube is confined by the three neighboring nanotubes,

thus the nanotube outer wall can only expand to the three

neighboring centers of the graphite lattice structure (as shown

in the inset of Fig. 2a). As a result, anodized TiO2 nanotube

arrays with triangular outer wall shapes in graphite lattice

arrangement are obtained.

With the attainment of the square and graphite lattice

arrangements of the TiO2 nanotubes, more sophisticated

arrays with asymmetrical structures, such as a sunflower

(Fig. 3a), are further investigated here. Fig. 3b shows the FIB

guiding concave pattern, and Fig. 3c shows the corresponding

anodized TiO2 nanotube arrays with a sunflower pattern. Even

though the TiO2 nanotube arrays have asymmetrical nanotube

arrangement, the backside view of the tube bottoms indicates

that the FIB concave pattern can still effectively guide the

growth of TiO2 nanotubes. In this case, the intertube distances of

the TiO2 nanotube sunflower pattern range from 180 to 260 nm.

Similar to a sunflower (Fig. 3a), the TiO2 nanotube sunflower is

made up of nanotubes that are located at the pattern center,

along with the nanotubes distributed outward in an optimal

filling of the space. Each TiO2 nanotube settles into a position

that has a certain rotation angle relative to the previous

nanotube. The pattern can be mathematically described with

polar coordinates:22

r = dk1/2 (1)

y = ka (2)

where r is the distance from the center of the sunflower to each

nanotube, d is the distance of the nearest nanotube to the

center nanotube (180 nm), k is the seed number, y is the

rotation angle of each nanotube, and a is 0.618 (decimal

fraction of golden ratio) of a complete turn (222.51). Two sets

of spirals can be identified for the TiO2 nanotube sunflower,

one curving counterclockwise and the other clockwise. In

Fig. 3c, the TiO2 nanotube arrays can be either grouped into

55 counterclockwise spirals (inset (1) of Fig. 3d) or 34 clockwise

spirals (insert (2) of Fig. 3d), which are two nearby Fibonacci

numbers (the numbers in the sequence: an = an�1 + an�2, such

as 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, . . .). The fabrication

of the TiO2 nanotube sunflower pattern demonstrates the

significant potential of FIB guided anodization to design

nano-devices in the future.

The mechanism of FIB pattern guided Ti anodization can

be understood as follows. During the anodization, the

FIB patterned concaves serve as the seeds to facilitate the

development of the TiO2 nanotubes at the patterned sites.

Fig. 1 (a) SEM image of the FIB guiding pattern in square arrange-

ment with a 250 nm interpore distance. The inset (1) is an AFM image

of (a), and inset (2) is the surface topology along the line in inset (1).

(b) Anodic TiO2 nanotube array after the FIB guided anodization.

The inset (1) and inset (2) in (b) are the backside and cross section

views of the TiO2 nanotube arrays, respectively. All scale bars in the

insets are 500 nm.

Fig. 2 (a) SEM image of the FIB guiding pattern in graphite lattice

arrangement with a 225 nm interpore distance. The inset in (a) is the

schematic drawing of development of triangular nanotubes from a

graphite lattice structure. (b) Anodic TiO2 nanotube array after the

FIB guided anodization. The inset 1 and inset 2 in (b) are the backside

and cross section views of the TiO2 nanotube arrays, respectively. All

scale bars in the insets are 500 nm.

Fig. 3 (a) Photograph of a sunflower head in nature, (b) SEM image

of the FIB guiding pattern, (c) TiO2 nanotube arrays after the FIB

guided anodization, (d) The backside view of the anodic TiO2

nanotube arrays. The inset (1) and inset (2) in (d) are the schematics

of counterclockwise spirals and clockwise spirals in the TiO2 nanotube

sunflower, respectively. The circle with a dashed line in (d) is the

boundary of the FIB guided TiO2 nanotubes.

Publ

ishe

d on

10

Aug

ust 2

011.

Dow

nloa

ded

by L

omon

osov

Mos

cow

Sta

te U

nive

rsity

on

20/0

2/20

14 0

0:21

:10.

View Article Online

This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 10085–10087 10087

At the beginning of the anodization, the TiO2 layer is thin, and

the electric field is large. As a result, the rate of oxidization is

much faster than the rate of dissolution. The TiO2 layer grows

both at the wall and bottom of the FIB patterned concaves. In

other words, the nanotube continuously increases its outer

diameter. However, this development is confined by the high

tube density.23 During the expansion of the tubes, the

neighboring outer walls of the nanotubes will move towards

each other until two walls merge into one thick combined wall.

The thickness of the TiO2 layer at the wall is much thicker than

that at the FIB patterned pore bottom. Therefore, the local

electric field at the pore bottom is large, which facilitates the

growth of the nanotubes at the FIB patterned locations due to

the electric field enhanced oxidization and dissolution rates.

Moreover, Ga+ implantation and Ti amorphization during

the FIB bombardment also facilitate the development of the

nanotubes at the FIB patterned concaves. Additionally, small

radius F� ions migrate to the metal–oxide interface under the

applied electric field. Because F� ions move faster than

O2� ions (due to the smaller size), a significant amount of

F� ions is present at the cell boundaries, which dissolves the

cell boundaries into water-soluble TiF2�6 complexes.24,25 This

explains why TiO2 nanopores change into TiO2 nanotubes.

However, it takes some time for F� ions to migrate to the cell

boundaries in the beginning of the anodization, which leaves a

thin layer of TiO2 nanopore arrays on the anodized surface

with cell boundaries undissolved. A similar phenomenon

has been reported for the two-step anodization of TiO2

nanotubes.12–14 A thin layer of nanopore arrays with the same

arrangement always covers the top of anodic TiO2 nanotubes.

Self-organized Ti anodization only forms hexagonal nanotube

arrangement in order to minimize the volume expansion stress.

For the FIB guided anodization, the intertube distance can be

accurately varied and the tube arrangement can be designed into

different patterns. Because of this flexible patterning ability, the

volume expansion stress during the Ti anodization can be

utilized in different ways to generate new tube outer shapes

and arrangements. The preferential development of TiO2

nanotubes at the FIB patterned locations enables the creation

of square, graphite lattice, and sunflower structures. This

patterning guided anodization technique provides the opportunity

to design TiO2 nanotube-based devices with controlled tube shape

and arrangement, which can greatly improve the property and

efficiency of the devices. Moreover, other patterning techniques

can be exploited in the future as an alternative to FIB patterning

in order to generate large area TiO2 nanotube arrays with a lower

cost, which will further enlarge the applications of patterning

guided anodization.

With the FIB pattern guidance, square TiO2 nanotubes with

square arrangement and triangular TiO2 nanotubes with

graphite lattice arrangement have been obtained by anodization.

Another significant result is the creation of novel TiO2 nanotube

arrays with asymmetrical arrangement, such as the sunflower

structure, which demonstrates the great potentials of FIB guided

anodization in fabricating designed TiO2 nanotube arrays. The

ordered TiO2 nanotube arrays with a controllable structure will

have great potentials in novel devices, such as dye-sensitized

solar cells and lithium-ion batteries.

We acknowledge the financial support from National Science

Foundation under grant No. CMMI-0824741 and the

Institute of Critical Technology and Applied Science of Virginia

Tech. We highly appreciate Mr. Lucapost for providing the

delicate sunflower photograph.

Notes and references

1 J. R. Jennings, A. Ghicov, M. L. Peter, P. Schmuki andA. B. Walker, J. Am. Chem. Soc., 2008, 130, 13364.

2 O. K. Varghese, M. Paulose and C. A. Grimes, Nat. Nanotechnol.,2009, 4, 592.

3 K. Zhu, T. B. Vinzant, N. R. Neale and A. J. Frank, Nano Lett.,2007, 7, 3739.

4 G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese andC. A. Grimes, Nano Lett., 2006, 6, 215.

5 G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese andC. A. Grimes, Nano Lett., 2005, 5, 191.

6 N. K. Allam, K. Shankar and C. A. Grimes, J. Mater. Chem.,2008, 18, 2341.

7 Q. Zheng, B. X. Zhou, J. Bai, L. H. Li, Z. J. Jin, J. L. Zhang,J. H. Li, Y. B. Liu, W. M. Cai and X. Y. Zhu, Adv. Mater., 2008,20, 1044.

8 Y. H. Zhang, P. Xiao, X. Y. Zhou, D. W. Liu, B. B. Garcıa andG. Z. Cao, J. Mater. Chem., 2009, 19, 948.

9 M. Paulose, O. K. Varghese, G. K. Mor, C. A. Grimes andK. G. Ong, Nanotechnology, 2006, 17, 398.

10 D. W. Liu, P. Xiao, Y. H. Zhang, B. B. Garcıa, Q. F. Zhang,Q. Guo, R. Champion and G. Z. Cao, J. Phys. Chem. C, 2008,112, 11175.

11 D. W. Liu, Y. H. Zhang, P. Xiao, B. B. Garcıa, Q. F. Zhang,X. Y. Zhou, Y. H. Jeong and G. Z. Cao, Electrochim. Acta, 2009,54, 6816.

12 Y. Shin and S. Lee, Nano Lett., 2008, 8, 3171.13 D. A. Wang, B. Yu, C. W. Wang, F. Zhou and W. Liu, Adv.

Mater., 2009, 21, 1964.14 S. Q. Li, G. M. Zhang, D. Z. Guo, L. G. Yu and W. Zhang,

J. Phys. Chem. C, 2009, 113, 12759.15 N. W. Liu, A. Datta, C. Y. Liu, C. Y. Peng, H. H. Wang and

Y. L. Wang, Adv. Mater., 2005, 17, 222.16 B. Chen, K. Lu and Z. P. Tian, Langmuir, 2011, 27, 800.17 C. Y. Liu, A. Datta, N. W. Liu, C. Y. Peng and Y. L. Wang, Appl.

Phys. Lett., 2004, 84, 2509.18 W. Lee, R. Ji, C. A. Ross, U. Gosele and K. Nielsch, Small, 2006,

2, 978.19 H. Masuda, A. Abe, M. Nakao, A. Yokoo, T. Tamamura and

K. Nishio, Adv. Mater., 2003, 15, 161.20 J. Choi, R. B. Wehrspohn and U. Gosele, Adv. Mater., 2003,

15, 1531.21 B. Chen and K. Lu, Langmuir, 2011, 27, 4117.22 M. Naylor, Math. Mag., 2002, 75, 163.23 Z. X. Su and W. Z. Zhou, Adv. Mater., 2008, 20, 3663.24 S. Yoriya and C. A. Grimes, J. Mater. Chem., 2011, 21, 102.25 A. Ghicov and P. Schmuki, Chem. Commun., 2009, (20), 2791.

Publ

ishe

d on

10

Aug

ust 2

011.

Dow

nloa

ded

by L

omon

osov

Mos

cow

Sta

te U

nive

rsity

on

20/0

2/20

14 0

0:21

:10.

View Article Online