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Template-assisted generation of three-dimensionally branched titania
nanotubes on a substrate
Kevin R. Moonoosawmy,a Martha Es-Souni,b Robert Minch,a Matthias Dietzea and Mohammed Es-Souni*a
Received 17th August 2011, Accepted 29th September 2011
DOI: 10.1039/c1ce06064c
Due to their high aspect ratio titania nanotubes have more surface area that enhances their ability
towards harnessing light. Herein, we report on a template-assisted method to synthesize branched
titania nanotubes anchored on a substrate. The branched structure consists of units of six or more tubes
connected at their base; each tube has a dimension of approximately 6 mm long, 600 nm wide and 65 nm
thick wall. Our approach takes advantage of hydrothermally processed branched ZnO nanorod (NR)
arrays that act as sacrificial templates. After chemical dissolution of the ZnO film that inherently forms
during ZnO–NR processing, the titania overlayer is coated atop of the template and annealed at 350 C
to produce the anatase nanostructure. Removal of the template under mild acidic conditions reveals the
anatase tubes. The processes employed are well suited for large-scale application. The highly textured
surface of the nanotubes also exhibits low reflectivity when compared to its thin film counterpart.
1. Introduction
Recently, numerous studies have indicated that titania nanotubes have improved properties compared to any other form of
titania for application in photoelectrolysis,1,2 photocatalysis,3,4
sensing,5,6 and photo-voltaics.7,8 Typically titania nanotubes are
fabricated via three main avenues (i) the template-assisted
method,9,10 (ii) the electrochemical anodic oxidation method11
and (iii) the hydrothermal method.12,13 Each method has its own
advantages and limitations. A reasonable control of the scale of
the nanotubes can be achieved by the templating method,
although the removal of the template can weaken the mechanical
stability of the dense nanotube arrays. Anodic oxidation of
titanium foils produces dense and aligned nanotube arrays with
high aspect ratios. Nevertheless, mass production is limited as the
nanotube formation requires low pH with fluoride containing
electrolytes such as HF(aq) which increases health and environmental hazards and the cost of fabrication equipment.
Furthermore, the TiO2 nanotubes prepared by anodization
require annealing, at elevated temperatures such as 450 C, to
transform them from amorphous to anatase phase.14 Hydrothermal treatment offers a cost effective route towards large scale
implementation15 which is modifiable producing well separated,
low dimensional and crystallized nanotubes.
However, the long duration of its reactions, the requirement
for high concentration of NaOH and the difficulty towards
a
Institute for Materials & Surface Technology (IMST), University of
Applied Science, Grenzstrasse 3, 24149 Kiel, Germany. E-mail: me@
fh-kiel.de; Fax: +49 0431 210 2660; Tel: +49 0431 210 2660
b
Faculty of Dentistry, Clinic of Orthodontics, Christian-AlbrechtsUniversity, Arnold Heller Str. 16, Kiel, Germany
474 | CrystEngComm, 2012, 14, 474–479
maintaining size uniformity are subjects of concern with the
latter method.
A better degree of control of the nanotube–array architecture
will impart tunable and functional properties, for example
controlling the texture and surface area that in turn can enhance
light absorption properties. Much effort has been invested in
developing new anodizing procedures aimed at improving the
nanotube length and structure.16 However, these dense arrays
suffer from bundling17 and formation of precipitates18 that
impede electron transport, thus there is a need for discrete
architectures. Furthermore, branched architectures have been
reported to offer more surface area, for charge separation, than
one dimensional structures thereby improving efficiency of the
photocatalyst.19,20 Only few reports in the literature report the
fabrication of hierarchical arrays21 or Y-branched titania nanotubes22 which are suggested to have better electron transport
properties than their 1D counterpart.
Herein, we report the generation of mechanically stable, onsubstrate branched TiO2–anatase nanotubes that are microns
long with nanometre wall-thickness, and possessing a high degree
of porosity. The method encompasses the advantages of large
scale implementation of the hydrothermal method, templating,
on-substrate growth architecture akin to anodization while
limiting their inherent disadvantages. We make use of a hydrothermally grown template of the ZnO branched nanorod structure which takes less time to grow than current methods. The
titania nanostructure is formed by dip-coating our substrate into
a TiO2 precursor solution. Titania as a photocatalyst relies on its
potential to harness solar radiation, a sustainable and renewable
energy vector, while maintaining a cost-effective procedure
geared towards minimizing our carbon footprint during fabrication is highly sought-after.
This journal is ª The Royal Society of Chemistry 2012
2. Experimental
2.1 Chemicals
Hexamethylenetetramine (HMTA), zinc nitrate hexahydrate (Zn
(NO3)2$6H2O), polyethyleneimine (PEI), titanium(IV)-isopropoxide (Ti-Is), acetylacetone, 99% (AcAc), and nitric acid, 69%,
were purchased from Sigma-Aldrich, Germany. Ethanol, 99.9%
(EtOH), and ammonium hydroxide, 25% (NH4OH), were
obtained from Merck Chemicals, Germany. Hydrochloric acid
(HCl) was supplied by Roth, Germany. PEG 400 was purchased
from ABCR GmbH & Co KG, Germany. All chemicals were of
analytical grade purity. All solutions were prepared with deionized water ($18 MU cm).
2.2 Preparation of sol–gel, substrates and templates
Polymeric TiO2 sol–gel was prepared by firstly dissolving Ti-Is
(3.56 ml) in EtOH (5 ml) for 5 min. This solution was added
drop-wise to a pre-mixed solution of AcAc (0.58 ml) in EtOH
(5 ml) and stirred further for 20 min. A pre-mixed solution
containing EtOH (4.64 ml) and H2O (0.86 ml) was subsequently
added to the above and stirred for 30 min. The light yellow
solution was filtered using a 0.2 mm PTFE syringe filter. To
prevent cracking PEG 400 (0.02 g ml1) was added to the filtrate
(20 ml) and the polymeric sol was aged for 1 day in the fridge.
The titania sol used to coat the template (Tipo) was prepared by
hydrolyzing Ti-Is (1.33 ml) with H2O (3.23 ml) with nitric acid as
catalyst. The reaction volume was adjusted to 30 ml with EtOH.
It was then allowed to react for 45 min after which the sol was
filtered using a 0.2 mm nylon syringe filter. The sol was kept at 8
C and was stable over several months.
The preparation process is shown schematically in Fig. 1.
Oxidized silicon wafer was used as substrates. The substrates
were cleaned by a snow-jet cleaning process prior to use. Prior to
ZnO–NR growth a 250 nm thin layer of TiO2 was processed by
sequential spin-coating and subsequent annealing in a pre-heated
furnace held at 350 C for 20 min. ZnO–NRs were grown
following the method described previously.23 The substrate was
immersed in a flask containing 20 ml of the growth solution
(aqueous solution of 12 mM HMTA, 5 mM PEI, 25 mM Zn
(NO3)2$6H2O and 2.5 mM NH4OH). The covered flask was
placed for 1 hour in an oil bath that was preheated to 82–85 C.
Subsequently the templated substrate was rinsed thoroughly with
deionized water and dried in air at 50 C. A low concentration of
HCl (1 103 M, pH ¼ 3) was used to gently etch the underlying
ZnO thin films away for 25 min with the solution held at room
temperature. The surface was soaked in distilled water held at 40
C for another 25 min to remove the ZnCl2 formed during
etching. Dip coating (DC) was employed to coat the ZnO NR
template with a thin TiO2 layer using Tipo sol at a rate of 4 mm
s1. Four sequential layers were needed to enrobe the ZnO NRs,
the fourth DC layer was done at a speed of 0.5 mm s1. After each
dip-coating sequence, the films were annealed in a pre-heated
furnace held at 350 C for 10 min. The final annealing was done
at the same temperature but for 20 min. The underlying ZnO
structure is removed by etching it away with 0.03 M HCl at room
temperature (RT) for 25 min and then soaked in water held at
40 C for another 25 min.
This journal is ª The Royal Society of Chemistry 2012
Fig. 1 Schematics of the steps towards fabrication of the branched
titania nanotubes. In step (a) three layers of titania are spin coated atop
of the oxidized silicon substrate, (b) branched ZnO nanorods (ZnO NRs)
are hydrothermally grown for 1 h at 80 C, (c) a gentle etching (1 mM
HCl) is used to remove the underlying ZnO thin film also produced
during step (b). In the last step (d) dip coating is employed to generate the
titania overlayer and final removal of the template reveals the branched
titania nanotubes.
2.3 Characterization of the nano-structured surface
The nano-structured surface was characterized using a highresolution scanning electron microscope (Ultra Plus, ZEISS,
Germany). The samples were investigated by X-ray diffraction
(XRD) (X’Pert Pro, PANalytical, Holland) in Bragg–Brentano
geometry using monochromatic Cu Ka radiation with l ¼ 1.5418
and a scanning range of 20–90 2q. A Bruker Raman microA
scope was used to acquire spectra over a range of 70–700 cm1,
with a spectral resolution of 3–5 cm1, using a backscattering
configuration with a 20 objective excited with a 532 nm laser
diode. Data were collected on numerous spots on the sample and
recorded with a fully focused laser power of 20 mW. Each
spectrum was accumulated four times with an integration time of
10 s. The Raman signal was recorded using a CCD camera. The
silicon substrate Raman peak position (520 cm1) was used to
calibrate spectral frequency. UV-vis-NIR Diffuse reflectance
spectra were collected with an Ocean Optics DH 2000 BAL with
Spectralon as reference.
3. Results and discussion
3.1 Morphology and structure of the template
Fig. 2a shows the SEM micrographs of the branched ZnO–NR
structure grown on a thin TiO2 layer supported on oxidized
silicon. The individual rod can be described as a frustum of
a hexagonal pyramid. They are 6 mm in height with a width of
600 nm at the top with an approximately 1 mm wide base, where
the other branches are connected as shown in the inset of Fig. 2a.
CrystEngComm, 2012, 14, 474–479 | 475
Fig. 2 SEM micrographs with close-up view (inset) of (a) the branched ZnO NR template and (b) the branched ZnO NRs after gentle etching to remove
underlying ZnO thin films which also results in pitting on the nanorods.
The branches have approximately the same length, which implies
concurrent growth. The tapered structures, which consist of
numerous stacked nanoplates with hexagonal prismatic faces,
can be ascribed to a fast growth rate along the c-axis direction.
The latter indicates the inherent anisotropic growth of ZnO is
favored along the c-axis and the formation of edge dislocations
during a relatively rapid growth can result in the tapered
morphology.24,25 The branching observed seems to suggest
a radiating growth mode from a common junction. The initially
fast nucleation of ZnO nanocrystals and their coalescence
(mimicking twinning) on the substrate can explain the formation
of branched structures from star-like26 monocrystallite nanorods
with incompletely grown crystal planes.27,28 Thereon, the Ostwald ripening process controls the homocentric growth along the
c-axis direction as stacking becomes more energetically
favorable.25
Fig. 3a shows the Raman spectra obtained on these ZnO NRs.
The Raman peaks observed were at 101 cm1, 144 cm1,
197 cm1, 303 cm1, 397 cm1, 439 cm1, 521 cm1 and 639 cm1.
To clarify our data; we also collected Raman spectra (Fig. 3b–e)
on several other surfaces, containing at least one component, in
order to validate our assignment. Fig. 3b depicts the spectra of
TiO2 layer spin-coated on top of oxidized silicon. Anatase has
a tetragonal structure with two TiO2 chemical units in the
primitive cell. It belongs to the D19
4h; space group. Out of the
15 optical modes (1A1g + 1A2u + 2B1g + 1B2u + 3Eg + 2Eu) only
the A1g, B1g along with Eg modes are Raman active while the A2u
as well as Eu modes are infrared active whereas the B2u mode is
both Raman and infrared inactive. We thus assign the peaks
observed at 144 cm1, 197 cm1, 397 cm1 and 639 cm1 in Fig. 3b
to the 2A1g, B1g, and Eg modes of anatase. ZnO has a wurtzite
structure and belongs to the space group C46v; with two formula
Fig. 3 Raman spectra of (a) ZnO NRs, (b) anatase thin film on oxidized Si (Ox Si), (c) ZnO NRs on ZnO TFs, (d) ZnO TFs on Ox Si and (e and f) gently
etched ZnO showing strong fluorescence (red) which is quenched once washed (orange).
476 | CrystEngComm, 2012, 14, 474–479
This journal is ª The Royal Society of Chemistry 2012
units per primitive cell. The predicted Raman active modes are
A1 + E1 + 2E2. The two non-polar E2 modes are Raman active,
while the A1 and E1 modes are both Raman and infrared active.
The high-frequency E2H (439 cm1) mode involves the vibration
of oxygen (O) atoms, while the low-frequency E2L (101 cm1)
mode is associated with the vibration of the zinc (Zn)
sublattice.29,30
The reported values corroborate well the peak observed in
Fig. 3c for ZnO NRs grown on ZnO thin films (ZnO TFs) atop of
oxidized silicon. By comparison to the ZnO NRs, the ZnO TFs,
which is depicted in Fig. 3d, show much broader peaks at
101 cm1 and 439 cm1. The films are made by sequential spincoating of the ZnO precursor onto oxidized silicon followed by
an annealing step at 85 C for 10 min. It has been reported in the
literature31 that smaller particle size results in asymmetric
broadening of the peaks. This implies that a higher intensity
observed for the ZnO NR peaks correlates with a better crystallinity of the nanostructure, as substantiated by the SEM
micrographs in Fig. 2a. The peak at 303 cm1 and 521 cm1 is
ascribed to the silicon substrate.32 The subsequent gentle etching
procedure, using 1 103 M HCl, allows elimination of the
underlying ZnO TF that forms during the growth of the NRs,
without dissolving the latter. This is a necessary step that ensures
that the subsequently grown TiO2 will firmly adhere onto the
substrate, thereby conferring the necessary mechanical stability
to the TiO2 structure towards the final removal of the ZnO–NR
template. Furthermore, it conveniently creates pits, as depicted
in Fig. 2b, on the ZnO NRs that improve adhesion of the titania
layer towards the formation of the branched titania nanotubes.
The resulting Raman spectrum of the etched sample, as presented (in red) in Fig. 3e, shows a strong fluorescence. It is
reasonable to presume that the presence of ZnCl2, formed by the
reaction of HCl with ZnO, gives rise to this fluorescence. Once
the substrate is washed, the ZnCl2 is readily dissolved in water
and we recuperate a spectrum (orange in Fig. 3f) similar to that
observed in Fig. 3a. We have found that the removal of the ZnO
TF was a prerequisite to enhance the mechanical stability of the
ensuing formation of the branched titania nanotubes.
3.2 Formation of the branched titania nanotubes
Dip-coating is employed to cover the ZnO NR template. The
titania overcoat follows the same geometrical shape as its parent
mold having a length of about 6 mm with an internal width of
approximately 600 nm. The sample was pyrolized at 350 C and
a highly textured branched titania shell is formed over the
template as shown in Fig. 4a. The ZnO template is etched away
under mild acidic conditions to reveal the branched titania
nanotubes, as seen in Fig. 4b. In order to achieve a mechanically
stable nanostructure and prevent collapsing of the nanorods
subsequent to ZnO etching a critical TiO2 layer/wall thickness
has to be reached. In our case we empirically determined (see
experimental conditions above) that a minimum wall thickness of
approximately 65 nm was necessary for achieving self-standing,
branched TiO2 nanostructures (see the inset of Fig. 4b). The
nanotubes have a wall thickness of approximately 65 nm with
a rough and porous surface that can be potentially useful
towards improving the efficiency of the photoactive layer irrespective of its destined application. Our nanotubes have
optimum length for certain application, such as dye sensitized
solar cells. It has been suggested that efficient cells can be created
with more than 4 mm thick TiO2 layers where the electron
diffusion length is reported to be within the same magnitude.33
Raman spectra were collected on the substrate before (Fig. 5a)
and after etching away the ZnO-template (Fig. 5b). The relatively
prominent ZnO peaks at 101 cm1 and 439 cm1 still observed in
Fig.5a are absent in Fig. 5b that solely shows the peaks corresponding to anatase nanotubes and Si substrate. To confirm that
we have effectively removed the sacrificial ZnO template we also
collected XRD data on both structures. Fig. 5c shows the XRD
patterns collected on the ZnO (PDF 003-0888) coated with
anatase. Several diffraction peaks are observed, with the (002)
reflex being the most prominent, indicating the presence of
a wurtzite ZnO structure. Upon removal of the ZnO template the
resulting XRD (Fig. 5d) reveals only the anatase (PDF 001-0562)
structure. The intense peak observed at 69 is attributed to the
Si substrate. The absence of rutile structure was observed by the
XRD, which is also corroborated by our Raman data.
Fig. 4 SEM micrographs and close-up view (inset) of the (a) titania coated ZnO NR template and (b) branched titania nanotubes.
This journal is ª The Royal Society of Chemistry 2012
CrystEngComm, 2012, 14, 474–479 | 477
up-conversion efficiencies.34 Decreasing the amount of solar
radiation reflected by a surface has been intensively investigated
via the use of anti-reflective coating(s).35,36 Fig. 6 shows the
diffuse reflectance spectra measured at room temperature. The
pale blue spectrum collected on the branched ZnO NR template
exhibits an oscillatory structure. The oscillatory structure
suggests an interference effect which is attributed to multiple
reflection beams from the surface. A suppression of the interference and the reflectance is observed once the surface is coated
with TiO2, as depicted by the dark green spectrum in Fig. 6. As
can be seen in Fig. 4a, the coated titania layer offers a porous
surface that has been promulgated to lower the reflectivity of
a surface.37 Upon removal of the sacrificial ZnO layer, a lower
reflectance of <30% is observed (light green).
This value is lower than the maximum reflectance of 50% and
35% observed (at l ¼ 400 nm) for the ZnO template and the
titania coated template respectively. We have also compared the
branched titania nanotubes with titania thin film made by spincoating of TiO2 sol atop of oxidized Si. The spin-coated TiO2 film
(red spectrum) shows a higher reflectance (50% at l ¼ 400 nm)
with observable interference fringes due to multiple reflection
beams from the surface. In contrast, the presence of branched
titania nanotubes promotes lower reflectivity than its thin film
counterpart, due to the so-called ‘‘Moth-eye’’ structure effects.38
4. Conclusions
Fig. 5 Raman spectra of (a) titania coated on the ZnO NR template and
(b) titania nanotubes in the anatase phase are observed and no ZnO NR
peaks are observed after its removal. X-Ray diffraction patterns of (c)
titania on the ZnO NR template and (d) branched titania nanotubes in
the anatase phase.
3.3 Optical properties of the nanostructure
The energy loss due to reflection from the optical component
reduces the up-conversion efficiency of the impinging radiation.
ZnO nanostructures have been also sought as a potential photoactive material; however, TiO2 has so far provided better solar
Our results show that we can fabricate branched titania nanotubes. Albeit being an indirect route, the approach described
takes considerably less time and less harsh conditions than direct
hydrothermal routes to produce titania nanostructures.
Following dip-coating a relatively lower annealing temperature
of 350 C is required as opposed to higher annealing temperature
required by closely packed anodized titania nanotubes. To the
best of our knowledge; this is the first report on an on-substrate
branched architecture that could, based on literature data,
promote 1D electron transport along the tubes axis but without
the bundling effect that interferes with charge separation. This
process is not limited to Si-substrates but can be expanded to
other substrates. We are currently investigating the use of the
zinc chloride by-product towards in situ generation of ZnS
quantum dots for dye-sensitized solar cells.
Acknowledgements
Financial support of this work is provided by the European
council and the Land of Schleswig-Holstein Project # TraFo
08139.
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Fig. 6 Diffuse reflectance spectra of ZnO NR template, titania coated
atop of the ZnO NR template, branched titania nanotubes and spincoated titania thin film on oxidized Si.
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