In Situ Transmission Electron Microscopic Observation of Double-wall
TiO2 Nanotube Arrays at High Temperature
Chaorui Xue,1 Takashi Narushima,1 Yohei Ishida,1 Tomoharu Tokunaga,2 and Tetsu Yonezawa*1
1
Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University,Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628
2Department of Quantum Engineering, Graduate School of Engineering, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601(E-mail: tetsu@eng.hokudai.ac.jp)

REPRINTED FROM

Vol.43 No.9

2014 p.1514–1516
CMLTAG
September 5, 2014

The Chemical Society of Japan

CL-140486

Received: May 11, 2014 | Accepted: June 17, 2014 | Web Released: September 5, 2014

In Situ Transmission Electron Microscopic Observation of Double-wall
TiO2 Nanotube Arrays at High Temperature
Chaorui Xue,1 Takashi Narushima,1 Yohei Ishida,1 Tomoharu Tokunaga,2 and Tetsu Yonezawa*1
1Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University,Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628
2Department of Quantum Engineering, Graduate School of Engineering, Nagoya University,Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601(E-mail: tetsu@eng.hokudai.ac.jp)
Double-wall TiO2 nanotube arrays were prepared using ananodization method in a glycerol-containing electrolyte. In situ
TEM observation of these samples at high temperature wascarried out with and without oxygen injections. With oxygeninjection, an improved thermal stability of the nanotubes athigh temperature was observed. It may provide more detailed
information about the calcination process of TiO2 nanotubes andassist its morphology and structure modulation.

Titanium dioxide (TiO2) nanotube arrays have eﬀectiveapplication in various ﬁelds such as hydrogen generation,1,2 gassensing,3 dye-sensitized solar cells,4 photocatalysis,5 as well as
biomedical applications.6 To realize the eﬃcient performance ofamorphous TiO2 nanotube arrays, high-temperature annealingis necessary to convert them into crystalline forms.7 Thus, theevolution of the crystal phase and the increase of the surface areaof TiO2 nanotube arrays attract much attention for the improvement of their properties. For example, the anatase phase of TiO2is preferred for photocatalytic applications, whereas the rutile
phase is widely accepted as gas sensing materials.8,9 The
Magnéli phase of substoichiometric titanium oxides, which canbe obtained by thermal treatment under reductive conditions,shows a high conductive property.10 Fang et al. profoundlyinvestigated the growth mechanisms of multilayered TiO2
nanotubes by anodization.11 The stability of the nanotube
structure, which maintains a large surface area even underhigh-temperature conditions, is also essential for eﬀectiveapplications.
In situ TEM (transmission electron microscopic) observation at high temperature is a powerful tool to monitor thecalcination process in real time during heating. It provides
detailed information on the morphological changes and evolution of the crystalline phase.12 Kamino and Saka introduced
a ﬁlament-type heating sample holder in order to observemorphological and crystal changes of nanomaterials at hightemperature.13 So far, we have used in situ TEM observation forunderstanding the sintering process of ﬁne copper particles14,15
and other nanoparticles.16,17 In our system, as in the Kamino­Saka system, oxygen gas can be introduced to a TEM column
during in situ TEM observation, and the gas pressure aroundthe samples can be controlled.12­17 The morphological changesof TiO2 nanotubes at high temperature would vary with theatmosphere, that is, with oxygen gas and under high vacuum
without oxygen. In this study, thermal annealing of TiO2nanotubes prepared in a glycerol-containing electrolyte wasperformed in a TEM column with and without injection ofoxygen, and their morphological changes were observed.

1514 | Chem. Lett. 2014, 43, 1514–1516 | doi:10.1246/cl.140486(a)(b)(c)(d)

Figure 1. (a) SEM image of the surface, (b) SEM image of thebottom, (c) cross-sectional TEM image of the root segments, and(d) schematic illustration of the cross section of the obtained
double wall TiO2 nanotube arrays by anodization.
The anodization of a Ti foil was carried out in a twoelectrode system using a DC power supply (Kikusui KX-100H,Japan). A Pt foil was used as the counter electrode, and a Tifoil (10 mm © 15 mm, thickness 250 ¯m, 99.6% purity, Nilaco,Japan) was used as the anode. Anodization voltage and time
were 60 V and 24 h, respectively. An electrolyte containing
96.5 wt % glycerol (Junsei), 3 wt % H2O (puriﬁed by Organo/
ELGA Purelabo system, >18 M³), and 0.5 wt % NH4F (Wako)
was used. The surface of the prepared samples was washedcarefully with puriﬁed water for several times and dried underair. A JEOL-JSM 6701F ﬁeld emission scanning electron
microscope (FE-SEM) was used to observe the surface morphology of the obtained samples. TEM imaging including in situ
observation was conducted using a Hitachi H-9500 (300 kV)
microscope. The behavior of the TiO2 nanotube samples uponheating was observed by using a ﬁlament-type specimen heating
holder (see TOC Figure).12­17 Oxygen gas was introduced froman oxygen gas cylinder under regulated pressure.
Figure 1 shows the representative SEM and TEM images ofthe double-wall TiO2 nanotube arrays prepared by anodization.
From the top surface (Figure 1a), the nanotubes demonstrate
only single-wall structures with an inner diameter of 200­250 nm. The tube walls are very thin (<30 nm). However, from© 2014 The Chemical Society of Japan(b)(a)(a)(c)(d)

Figure 2. (a) Temperature proﬁle of in situ TEM observation
with introduction of oxygen (Pressure of the TEM column:
10¹4 Pa) and (b), (c), and (d) in situ TEM images of TiO2
nanotube under oxygen atmosphere at various temperatures.
(b) At room temperature, (c) at 600 °C, and (d) at 820 °C (Images
collected at the points shown in (a)).
the bottom (Figure 1b), the tube walls become thicker(>100 nm), and the inner diameter decreases to 90­100 nm.
Judging from these SEM and TEM images, we can conclude thatthe inner side of the tubes has a cone-like structure as illustrated
in Figure 1d. This unique structure is probably derived from theexcessive etching of the top of the tubes. In addition, the tube
wall also splits to a double-layered structure as can be observed
from the inset circle area in Figure 1b. The cross-sectional
TEM image of the root segments of the nanotubes (Figure 1c)
conﬁrms their double-wall structure. A similar situation wasreported in the case of anodization using a polyol electrolyte.18,19
These nanotubes are considered as amorphous at room temperature, as previously reported in the case of single-wall titaniumoxide nanotubes with a thicker side wall.7 Selected area electron
diﬀraction (SAED) measurement of our sample unfortunately
destroyed their structures during the observation owing to beamfocusing.
In situ TEM observation at high temperatures was carriedout with the obtained nanotube array samples. Figure 2 shows
the morphological changes of the nanotube arrays at hightemperature with introduction of oxygen gas. The pressure of theTEM column was kept at 10¹4 Pa. The temperature proﬁle up to820 °C during this experiment is shown in Figure 2a. At roomtemperature, a nanotube structure was clearly identiﬁed. Whenthe temperature was increased stepwise to 600 °C and subsequently the sample was heated at this temperature for 12 min,the contrast of the image (Figure 2c) became sharper than that atroom temperature (Figure 2b). Many protruding ends were stillobserved, and no structural collapse was detected (Figure 2c).
According to another in situ TEM annealing of single-wall TiO2
nanotubes,7 this change in contrast was due to crystallization athigh temperature. On the other hand, structural changes occurred
where ﬂower-like structures with small titanium oxide nanoparticles in the upper right area (Figures 2b and 2c) changed
their arrangements. These changes can also be attributed to thecrystallization of titanium oxide. Next, after heating the sample

Chem. Lett. 2014, 43, 1514–1516 | doi:10.1246/cl.140486(c)(b)(d)Figure 3. (a) Temperature proﬁle of in situ TEM observation
without introduction of oxygen gas (Pressure of the TEM
column: 2 © 10¹5 Pa) and (b), (c), and (d) in situ TEM images ofTiO2 nanotube without oxygen gas introduction at varioustemperatures. (b) At room temperature, (c) at 610 °C, and (d) at
800 °C (Images collected at the points shown in (a); A tip on topof tube was shown in square areas).
at 820 °C for approximately 5 min, a clear structural collapse of
the nanotubes was observed (Figure 2d). The tube transformed
into a coral-like structure. This transformation is presumably
induced by the coalescence of crystal grains and sintering at820 °C.
As shown in Figure 3, without oxygen injection, in situ
TEM observation of TiO2 nanotubes at high temperature wasalso performed. The pressure of the TEM column was 2 ©
10¹5 Pa. Some crystalline fringes, which were not observed inTiO2 nanotubes at room temperature, were observed in the highresolution TEM image of the nanotubes at 320 °C. These suggestthat crystallization of our TiO2 nanotubes, which were amorphous at room temperature, proceeded at high temperatures(Figure 4).
The structure of the nanotubes was not highly stable evenat 610 °C. The structural collapse was conﬁrmed on the top of
the tube. Comparing the square areas in Figures 3b and 3c,
we found that the tip on the top of the tube disappeared afterca. 12-min heating at 610 °C. This result suggests that the tube
already started losing its structure at 610 °C. It is diﬀerent fromthe behavior of TiO2 nanotubes under oxygen, where no obvious
structural collapse was observed at ca. 600 °C (Figure 2c).
Around 800 °C, the nanotube architecture collapsed dramatically(Figure 3d). The nanotubes completely decomposed and becamea dense stone-like structure after approximately 5 min. This
structure is highly diﬀerent from the coral-like structure consisting of intercolumnar pores in the presence oxygen (Figure 2d).
These systematic in situ TEM experiments as shown in
Figures 2 and 3 indicate that morphological changes of the TiO2
nanotubes occur faster under high vacuum without oxygen. In
the presence of oxygen gas, a higher structural stability of theTiO2 nanotubes was achieved. For example, at ca. 600 °C, thetubes showed higher stability with oxygen gas. The XP spectrumof our as-prepared TiO2 nanotubes showed that the atomic ratioof Ti:O is 1:2.07. Our TiO2 nanotubes are almost stoichiometric.
© 2014 The Chemical Society of Japan | 1515

Figure 4. High-resolution TEM image of TiO2 nanotube
heated without oxygen at 320 °C. Lattice fringes can be observedat the center, and they indicate the crystalline structure of TiO2.
Under oxygen atmosphere, in the case of these stoichiometric
TiO2 nanotubes, a slower and smaller structural change of TiO2
with the crystallization of amorphous TiO2 is reasonable asshown in Figure 2. Other researchers also reported the highstability of oxygen-rich titania at high temperature.20 Conversely, under a high vacuum without oxygen, morphologicalchanges were observed. According to the previous reports,
oxygen molecules adsorbed on anatase TiO2 desorbed atca. 140 °C under high vacuum.21 In the previous literature,oxygen ion diﬀusion is considered as the rate-determining stepin the sintering of TiO2.22 During the in situ TEM annealing
process without oxygen, oxygen vacancy favors the diﬀusion
according to the general vacancy diﬀusion mechanism. Thus,
sintering of TiO2 was accelerated under high vacuum as shownin Figure 3, and a slower sintering of TiO2 is reasonable tohappen if the concentration of oxygen vacancy is low as shownin Figure 2.
In summary, we performed in situ TEM annealing of
double-wall TiO2 nanotubes. The double-wall TiO2 nanotube
array was prepared by anodization in a ﬂuorinated glycerol
electrolyte. In situ TEM annealing of the TiO2 nanotubes athigh temperature was successfully performed with and without
oxygen. Under an oxygen partial atmosphere, no obvious
morphological changes were observed up to ca. 600 °C, but
crystallization of nanotubes occurred around 300 °C. Based on
the general vacancy diﬀusion mechanism, we concluded that thediﬀerence in structural changes at high temperature was due tothe presence of oxygen gas molecules that stabilized the TiO2structure.

1516 | Chem. Lett. 2014, 43, 1514–1516 | doi:10.1246/cl.140486

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© 2014 The Chemical Society of Japan