Published on the web September 23, 2011
1107
Fabrication and Photocatalytic Properties
of TiO2 Nanotube Arrays Modified with Phosphate
Kazuya Nakata,*1,2,3 Baoshun Liu,1 Yosuke Ishikawa,1,4 Munetoshi Sakai,1,2 Hidenori Saito,2 Tsuyoshi Ochiai,1,3
Hideki Sakai,4 Taketoshi Murakami,1 Masahiko Abe,3,4 Katsuhiko Takagi,2 and Akira Fujishima*1,2,3
1Photocatalyst Group, Kanagawa Academy of Science and Technology,
KSP Building East 412, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012
2Organic Solar Cell Assessment Project, Kanagawa Academy of Science and Technology,
KSP Building East 308, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012
3Research Institute for Science and Technology, Energy and Environment Photocatalyst Research Division,
Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601
4Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510
(Received June 29, 2011; CL-110548; E-mail: pg-nakata@newkast.or.jp)
We have prepared TiO2 nanotubes by anodization in HF or
HF/H3PO4 mixed electrolyte. The morphologies and photo-
catalytic properties of the nanotubes were changed by electro-
lytes. The nanotubes prepared in HF/H3PO4 mixed electrolyte
showed higher performance to decompose acetaldehyde by
photocatalysis than those prepared in pure HF.
cell. Titanium foil (0.25 mm thick, 5 © 3 cm) was used as the
working electrode, and platinum foil (5 © 1 cm) served as the
counter electrode. The distance between the working and counter
electrodes was 1 cm. A voltage was applied with a DC power
supply (E3612A, Agilent Technologies). Anodization was
performed in three different electrolytes (30 mL) consisting of
0.5 vol % HF for sample 1, 0.5 vol % HF and 1 M H3PO4 for
sample 2, and 0.5 vol % HF and 4 M H3PO4 for sample 3 at 25 V
for 1 h at room temperature. After anodization, the Ti foils were
removed from solution under continued application of potential
and washed with Milli-Q water. The films were calcined at
500 °C under atmospheric conditions for 3 h.
Cross-sectional and top view images of TiO2 nanotube
arrays prepared in different electrolytes are shown in Figure 1.
To prepare the TiO2 nanotubes in sample 1, titanium foil was
anodized in an electrolyte containing 0.5 vol % HF at 25 V
for 1 h, resulting in the formation of a TiO2 nanotube array
containing nanotubes with a length of about 160 nm, inner
diameter of about 95 nm, and wall thickness of about 6 nm
(Figure 1a). A cross-sectional image of sample 1 revealed that
this TiO2 nanotube array is highly ordered and oriented
perpendicular to the titanium substrate. To produce sample 2
(Figure 1b), titanium foil was anodized in a mixed electrolyte
containing both HF and H3PO4. A regular, well-aligned nano-
tube array containing nanotubes with a length of about 505 nm,
average inner diameter of about 96 nm, and wall thickness of
about 8 nm was obtained. When a mixed electrolyte with a high
concentration of H3PO4 was used (sample 3, Figure 1c), the
Titanium dioxide (TiO2) has been widely studied since the
discovery of photoelectrochemical water splitting by Fujishima
and Honda1 because it shows a strong oxidation ability that can
be used not only for water splitting but also many other practical
applications.2-4 For example, TiO2 coatings applied to glass and
ceramic substrates can provide a self-cleaning surface that
removes organic contaminants. For this purpose, TiO2 surfaces
that exhibit high performance for the photocatalytic degradation
of organic compounds are being investigated by many research-
ers. Anodized TiO2 nanotube arrays5,6 are a potential candidate
for this role because they can photocatalytically degrade organic
compounds efficiently. This is a result of their large effective
surface area, straight channels that enable diffusion of oxidizable
species into the TiO2 nanotubes, and less recombination because
the thickness of half of the nanotube wall is significantly less
than the carrier diffusion length in TiO2.7 The photocatalytic
performance of TiO2 nanotubes depends on morphology,
especially their length.5 When the length of a nanotube
increases, its surface area increases, which leads to high
photocatalytic performance. However, longer TiO2 nanotubes
tend to break easily and separate from the substrate.8 Another
method to increase photocatalytic performance is surface
modification with nonmetal atoms (N, S, C, B, P, and F)9-15 or
anion molecules.16-19 For example, Zhao et al. reported that
surface modification of TiO2 powders with phosphate-improved
photocatalytic performance.16 Such surface modification should
increase electron-hole separation, which in turn influences
photocatalytic performance.
In this work, to obtain photocatalysts that exhibit high
performance, TiO2 nanotubes modified with phosphate are
prepared by anodization in a mixed electrolyte containing
hydrofluoric acid (HF) and phosphoric acid (H3PO4). The
structures, crystal phases, and photocatalytic properties of the
resulting materials are investigated.
Figure 1. Cross-sectional and top view images of TiO2
nanotube arrays prepared in different electrolytes. Sample (a) 1,
(b) 2, and (c) 3. Scale bar is 200 nm.
Highly ordered TiO2 nanotube arrays were prepared by
potentiostatic anodization in a two-electrode electrochemical
Chem. Lett. 2011, 40, 1107-1109
© 2011 The Chemical Society of Japan
Nakata, Kazuya
