Synthesis of silica nanotubes from kaolin clay
Wenjun Dong,ac Wenjiang Li,b Kaifeng Yu,c K. Krishna,b Lizhu Song,c Xiaofeng Wang,a Zichen
Wang,*c Marc-Olivier Coppensb and Shouhua Feng*a
a State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, College of Chemistry, Jilin
University, Changchun 130023, China. E-mail: shfeng@mail.jlu.edu.cn; Fax: +86-431-5671974;
Tel: +86-431-8499952
b DelftChemTech, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, 2628 BL
Delft, The Netherlands. E-mail: m.o.coppens@tnw.tudelft.nl
c
College of Chemistry, Jilin University, Changchun, 130023, China. E-mail: wangzc@mail.jlu.edu.cn
Received (in Cambridge, UK) 10th January 2003, Accepted 14th April 2003
First published as an Advance Article on the web 6th May 2003
Silica nanotubes were synthesized from kaolin clay using
surfactant intercalation, sulfuric acid and hydrothermal
treatments.
significantly destroyed by the intercalation of the surfactant
followed by the sulfuric acid and hydrothermal treatments.
After treatment with CTAB, H2SO4 and hydrothermal aging,
the well-defined structure of silica nanotubes is clearly seen in
TEM images (Fig. 2), which show irregular rod-shaped hollow
tubes (Fig. 2a). High-resolution TEM images (Fig. 2b, c) show
tubular structures with open ends, due to peeling off of the outer
layers, an inner diameter of about 50 nm and an outer diameter
of 80 nm, and a length of less than 1 µm on average.
Since the discovery of carbon nanotubes in 1991,1 many other
nanosized tubular materials have been synthesized.2,3 Due to
the outstanding structural versatility of nanotubes, significant
attention has been focused on these materials with respect to
possible applications in advanced catalysis, sensor/actuator
arrays, energy storage/conversion and opto-electronic devices.4
Recently, various preparation methods like sol–gel, template
assisted, and replica process have been used to synthesize silica
nanotubes.5 However, compared to silica porous materials,
silica nanotubes are not easy to synthesise and the final
nanotube products usually are unstable. In this communication,
we present a relatively easy and inexpensive synthesis proce-
dure for silica nanotubes by converting layered kaolin clay, as a
silica source, into stable nanotubes. Kaolin is a naturally
occurring clay and is extensively used as a catalyst, in paints and
as a filler for reinforcing polymers. Kaolin has a layered silicate
structure, which can be easily intercalated and exfoliated by
organic cations.6 Using the above properties of kaolin, we have
developed a new process for the synthesis of silica nanotubes.
The synthesis of silica nanotubes involved the following
steps. The white colored kaolin powder was obtained by
calcining the original kaolin (Jiangsu province, China) at 750 °C
for 4 h in air. The soluble salts were removed by washing the
above material with distilled water and dried at 100 °C. 5 g of
the dried kaolin was mixed with 20 mL of 0.1 M CTAB aqueous
solutions with vigorous stirring. This mixture was aged for 10 h
while stirring. 2 mL of concentrated sulfuric acid was then
slowly added to the mixture, and stirred for 20 h. The thus
obtained suspension was heated to 120 °C in an autoclave for 24
h under autogenous pressure. After hydrothermal treatment, the
resulting material was filtered, washed, dried and calcined at
550 °C for 6 h in air, which resulted in a white powder of silica
nanotubes. The starting materials and silica nanotubes were
characterised by XRD, HRTEM, MAS-NMR and TG-DTA
techniques.
The solid-state 29Si MAS-NMR spectra of the kaolin, and of
the products at different steps in the synthesis, provide
additional evidence of the conversion of the layered silicate
structure of kaolin to a more condensed structure (Fig. 3). The
observed MAS-NMR signals between dp = 280 to 2120 ppm
are due to different environments of Si. Q2 (297 to 298 ppm),
Q3 (2101 to 2102 ppm) and Q4 (2110 to dp 2111 ppm)
species, which represent the Si structure as depicted in Fig. 3,
are formed during various treatments.8 Kaolin shows a sharp
signal typical of Q2 species, indicating that all the Si elements
are present in a similar environment (Fig. 3a). During CTAB
exchange, the intensity of Q2 species decreases, while that of
Q3 and Q4 species appears and increases with treatment time
(Fig. 3b, c). All the Q2 species disappear on sulfuric acid
treatment, and a dominating signal corresponding to Q4 species
is observed (Fig. 3d), indicating the change of the layered to a
condensed amorphous structure.9 Moreover, our data are in
agreement with those reported in the literature for silica
nanotubes prepared from amorphous SiO2, after hydrothermal
treatment, where a similar spectrum was observed.5 The XRD,
MAS-NMR and HRTEM observations complement each other.
The elemental analysis by ICP-AES showed that the final
products only contain pure SiO2.
The formation process of silica nanotubes from kaolin could
be explained as follows. CTAB in the first step helps in
exfoliating the layered structure. Sulfuric acid removes the
framework Al ions as Al2(SO4)3, which results in many
irregular exfoliated layered-silica/surfactant composites. This
can be confirmed from TG-DTA (Fig. 4), which shows a weight
Fig. 1. Shows the XRD patterns of the kaolin, and of the
materials at different steps in the synthesis, as indicated in the
legend. The calcined material displays the typical layered
silicate structure of kaolin.7 After treatment with CTAB for 5 h,
the material clearly shows a shift in d100 peak towards lower 2q
values, due to expansion of the silicate layers by intercalated
CTAB. At the same time the material starts losing its ordered/
crystalline structure. With further increase in CTAB treatment
to 10 h, the characteristic sharp peaks of kaolin disappear and a
broad peak is seen between 2q = 20 to 40°, along with a
significant decrease in intensity of the d100 peak (Fig. 1c).
Notably, the sample treated with CTAB shows a broad peak
typical of an amorphous material along with a very weak d100
peak after sulfuric acid and hydrothermal treatment (Fig. 1d).
The above observations indicate that the layered structure was
Fig. 1 XRD of (a) original kaolin, after CTAB exchanging for (b) 5 h, (c) 10
h, and (d) after sulfuric acid/hydrothermal treatment.
1302
CHEM. COMMUN., 2003, 1302–1303
This journal is © The Royal Society of Chemistry 2003
Dong, Wenjun
