The document summarizes a study that investigated the effect of phase and crystallite size on the formation of titania nanotubes through hydrothermal treatment. Specifically, it examined nanotube formation from commercial and self-prepared anatase and rutile with different crystallite sizes. The results showed that both phase and crystallite size affected nanotube formation, with rutile requiring higher temperatures than anatase to form nanotubes. Increases in crystallite size also appeared to slightly impede nanotube formation by requiring greater driving forces. Well-formed nanotubes were observed by transmission electron microscopy.

1. Advanced Materials Research Vols. 29-30 (2007) pp 211-214
Online available since 2007/Nov/15 at www.scientific.net
© (2007) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMR.29-30.211
Synthesis and Characterisation of Titania Nanotubes:
Effect of Phase and Crystallite Size on Nanotube Formation
D. L. Morgan1,a, E. R. Waclawik1,b and R. L. Frost1,c
1
Inorganic Materials Research Program, School of Physical and Chemical Sciences
Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia
a
dl.morgan@qut.edu.au, be.waclawik@qut.edu.au, cr.frost@qut.edu.au
Keywords: anatase; rutile; titanate; crystallite size; nanotube; XRD; TEM; Raman spectroscopy
Abstract. Nanotubes were produced from commercial and self-prepared anatase and rutile which
were treated with 7.5 M NaOH over a temperature range of 100 – 200°C in 20°C increments. The
formation of nanotubes was examined as a function of starting material type and size. Products
were characterised by X-Ray Diffraction (XRD), Transmission Electron Spectroscopy (TEM), and
Raman Spectroscopy. The results indicated that both phase and crystallite size affected the
nanotube formation. Rutile was observed to require a greater driving force than anatase to form
nanotubes, and increases in crystallite sizes appeared to impede formation slightly.
Introduction
Recently there has been considerable interest in the controlled manipulation of nanostructured low-
dimensional metal oxides. Since the inherent properties of the bulk metal oxides are transferable to
their nanostructured forms incremental tailoring of these properties is possible [1,2]. Titanium
dioxide (TiO2) is one of the many metal oxides currently under investigation as it is readily
converted into various nanostructured forms including nanotubes. Although multiple methods of
nanotube production are available in this study only the soft-chemical hydrothermal treatment
developed by Kasuga et al. is of interest [3]. This simple synthesis involves the conversion of a
titania precursor into well formed nanotubes when treated in caustic solution at raised temperatures.
The advantage of synthesising nanotubes with ease from a common source lends the procedure to
conveniently produce viable technologies from the nanotube product. Such applications include:
gas sensors, ion-storage devices (H+ and Li+); biomedicine (bone remediation and regeneration); and
for environmental purification (photocatalysis, reactive sorbents) [2,4]. Currently, studies of
potential applications are conducted concurrently with investigations into the phase, composition
and mechanism of formation of the nanotubes. Characterisation of the titania nanotubes is a
complex problem, finite size effects and the size and curvature of the nanotubes makes the concise
indexing of phase difficult. For instance, these effects cause broadening and displacement of XRD
results which then do not correlate directly to known titania or titanate species.
Titania nanotubes have been successfully produced throughout the literature from anatase and
rutile (synthetic and from natural rutile sands) dispelling earlier preconceptions that rutile could not
be morphologically altered or that the nature of the rutile surface energy and crystal structure
impedes titania nanotube formation [5]. Works by Lan et al. and Thorne et al. have shown
nanotubes formed in abundance using rutile as the starting material. Both these investigations used
10 M NaOH to treat the rutile precursor over a range of treatment times (2 to 72 hours) [6,7].
Titanate precursors have also been trialed for nanotube formation however; the resultant products
were not similar to titania precursors. When Na2Ti3O7 was treated with NaOH block-like ‘stripes’
were produced rather than nanotubes [8]. However, when deionised water was used as the
hydrothermal liquor large brookite-like nanotubes were obtained [9]. The focus of this study was to
examine the nanotube formation from different titania phase precursors to further understand the
mechanism of formation. The investigations specifically examining the effects of phase and
crystallite size on product nanopowders.
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2. 212 Advanced Materials and Processing IV
Experimental
Reagents, Synthesis and Characterisation. Commercial anatase and rutile powder (Sigma-
Aldrich, 99.8% and 99.9+% respectively), sodium hydroxide (Chem-Supply, 98% purity) and
hydrochloric acid (Univar, AR reagent, 32 w/w%) were all used without further purification. The
self-prepared anatase and rutile were produced through the reaction of titanium-isopropoxide in a
mixture of ethanol, acetic acid and acetic acetate (ratio: 0.03 TPT : 0.5 EtOH : 0.015 AcOH : 0.015
AcAc) before calcining at 450°C for 5 hours for anatase and 850°C for 36 hours for rutile. No
impurities in the resulting product materials were observed as determined by XRD analysis.
Approximately 0.5g of each precursor was next treated with 7.5 M NaOH solution (30 – 35 mL) and
hydrothermally treated over 100 – 200°C in 20°C increments. The samples were statically heated
within PTFE-lined Parr bombs in a SEM convectional oven for 20 hours. The resultant powder was
then washed with 0.1 M HCl then deionised H2O until the supernatant measured pH 7.
The produced TiO2 powders were characterised through powder X-ray diffraction (XRD), Raman
spectroscopy and transmission electron microscopy (TEM). XRD patterns were obtained via a
Philips PANalytical X’Pert PRO X-ray diffractometer operating at 40 kV and 40 mA using Cu-Kα1
radiation (λ = 1.54 Å). Bragg-Brentano geometry was used for analysis of the sample from 5 –
70° 2θ. A Renishaw In-Via Raman microscope coupled to a Leica microscope was used for Raman
studies using a Renishaw double Nd:YAG laser (532 nm excitation radiation). A 532 nm Laser
RSSF notch filter was used for Rayleigh rejection when the samples were analysed from 50 –
1200 cm-1. TEM examinations were carried out on a Philips CM200 TEM operated at 200 kV.
Results and Discussion
The formation mechanism of titania nanotubes is believed to occur through the delamination and
subsequent ‘rolling up’ of precursor nanosheets into scrolled nanotubes, it is conjectured that both
the titania precursors phase and crystallite size should affect the outcome of this process.
Considering the formation of the nanosheets is believed to occur either through direct shearing off
of lamellar sheets or the dissolution of the titania precursor into basic octahedral Ti-O building
blocks which reassemble through a condensation and polymerisation process [1,10,11], crystallite
size would especially have an effect. In this study formation of nanotubes from anatase and rutile
with differing crystallite size has been examined (Table 1).
Table 1. Approximate Crystallite Sizes of Starting Materials
Crystallite Size (nm)
Crystallite size determined
Commercial Anatase 109 (A), 248 (R) through applying the Scherrer
Commercial Rutile 112 (R), 92 (A) equation to the [101] and [110]
Self-prepared Anatase 23 reflections for anatase (A) and rutile
(R) respectively.
Self-prepared Rutile 224
The impact of phase and crystallite size on identically hydrothermally treated starting materials
was evident in both the Raman and XRD studies (Fig.1 and Fig. 2) where the initial formation of
nanotubes was observed at different hydrothermal treatment temperatures for the four starting
materials. Nanotube presence can be observed in XRD through the general broadening of peaks,
but mainly through the observation of large d-spacing at an angle ~12° 2θ which is considered
indicative of nanotubes. This peak and these d-spacings have been attributed to the interlayer
spacings of the nanotubes previously [4]. The presence of nanotubes can also be observed in
Raman spectra with samples C, D and H (Fig. 1) exhibiting ‘typical’ nanotube spectra. The
presence of nanotubes in Raman spectra can be observed as a peak within the 320 – 220 cm-1 as
observed in sample A (Fig. 1). Finally, nanotube formation was confirmed through TEM
investigations. Well-formed nanotubes in these samples were observed (Fig. 3).

3. Advanced Materials Research Vols. 29-30 213
Figure 1. Raman Spectra of Selected Samples Figure 2. XRD Patterns of Selected Samples
Self-prepared Anatase 100°C (A) and 200°C (C); Commercial Anatase 100°C (B) and 200°C (D); Self-prepared
Rutile 120°C (E) and 200°C (G); and Commercial Rutile 120°C (F) and 200°C (H). Symbols: rutile, anatase
The onset of formation of nanotubes was observed at 100°C for the self-prepared anatase
(Fig. 2); with a crystallite size of 23 nm this titania precursor possessed the smallest crystallite used
in this study. The commercial anatase sample required higher temperature at 120°C to initiate
nanotube formation indicating that a greater driving force was required from larger precursor
crystallites. The ~4× increase in crystallite size did not therefore appear to severely impede
nanotube formation once the hydrothermal temperature was raised. Surprisingly the commercial
anatase appears to completely convert to nanotubes at 160°C with negligible traces observed in the
XRD, where the self-prepared anatase still contained traces of the reactant at 200°C. This suggests
that the smaller crystallite size helps initiate but does not maximise the conversion of anatase into
nanotubes.
Crystallite size effects were also observed in
the rutile samples where a 2-fold increase in
crystallite size was present between the
commercial and self-prepared rutile. The
hydrothermal temperature for the detected onset
of nanotube formation was observed at 120°C
and 160°C respectively suggesting that the
smaller crystallite size enabled for nanotube
formation at lower temperatures, as also
observed for anatase. However, we must note
that trace amounts of anatase present in the
commercial rutile as evidenced through XRD.
Conversion of anatase into nanotubes could
attribute to the nanotube presence at 120°C.
Qualitative examinations of relative peak
intensities in the XRD were therefore carried
out. These indicated that both anatase and rutile Figure 3. TEM images of nanotubes formed
convert into nanotubes at 120°C, albeit with with 7.5 M NaOH @ 140°C from Self-prepared
rutile converting at a slower rate. All traces of Anatase (A) and Commercial Rutile (B). Both
anatase were absent by 160°C indicating samples display well formed nanotubes.
complete conversion into nanotubes confirming

4. 214 Advanced Materials and Processing IV
the results from the commercial anatase.
Thus, the presence of nanotubes from 160°C to 200°C was due solely to the conversion of rutile
with negligible traces of rutile remaining at 200°C. This suggests that the driving force required for
considerable nanotube formation from rutile is achieved at 160°C. This was evidenced in the self-
prepared rutile and from the trace amounts of rutile in the commercial anatase rutile traces no longer
remained at 160°C. When comparing the results obtained, a further notable observation was that
the lengths of the product nanotubes were much larger than the crystallite size in the case of the
precursor titania nanoparticles prepared in-house. The mechanism of nanotube formation is
therefore unlikely to be a simple delamination of individual crystallite sheets. The higher surface
area precursor materials were more reactive and therefore yielded nanotubes under “softer”
hydrothermal treatment conditions. Smaller crystals are likely to possess more dangling bonds and
edge defects than large crystals and possess greater reactivity. The titania octahedral rearrangement
mechanism described by Wang et al. is thus considered to be more consistent with the observed
results [11]. The results obtained were compared to other studies of nanotube formation from rutile.
Although these studies used differing NaOH concentrations and synthesis times (eg. 10 M NaOH
and 72 hours), the results are instructive. Lan et al. observed initial nanotube formation at 100°C
when reacted for 48 hours in 10 M NaOH [6]. The increase in NaOH concentration and time would
both affect the driving force which in this study was observed as a 20°C increase in temperature.
Similarly, Thorne et al. observed that the optimal condition for the maximum production of
nanotubes was 10 M NaOH at 150°C for 72 hours [7]. This compares to the significant conversion
of rutile to nanotubes at 160°C in this study. However, the effect of crystallite size cannot be
confirmed between these studies.
Conclusions
Nanotube formation is affected by both the crystallite size and phase of the starting material, as
observed in this study. This was observed through examination of the products of the reaction of
both anatase and rutile precursors. Rutile appeared to require harsher hydrothermal conditions to
form nanotubes compared to anatase. Considering the commercial anatase and rutile had
comparable crystallite sizes, anatase was observed to form more readily between 120 – 160°C. A
significant crystallite size effect was observed with both the anatase and rutile, with large crystallite
size appearing to require a greater driving force to produce nanotubes.
References
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5. Advanced Materials and Processing IV
10.4028/www.scientific.net/AMR.29-30
Synthesis and Characterisation of Titania Nanotubes: Effect of Phase and Crystallite
Size on Nanotube Formation
10.4028/www.scientific.net/AMR.29-30.211
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