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PAPER
www.rsc.org/nanoscale | Nanoscale
Synthesis and characterization of one-dimensional flat ZnO nanotower arrays
as high-efficiency adsorbents for the photocatalytic remediation of water
pollutants†
Da Deng, Scot T. Martin and Shriram Ramanathan*
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Published on http://pubs.rsc.org | doi:10.1039/C0NR00537A
Received 26th July 2010, Accepted 7th September 2010
DOI: 10.1039/c0nr00537a
We report on facile fabrication of 1-D flat ZnO nanotower arrays on various substrates, including
a metal, a semiconductor and an insulator. The nanotowers have a unique flat basal section near the
substrate and taper in stages to wire-like at the tip. Electron microscopy and X-ray photoelectron
spectroscopy are used to characterize these new nanostructures, revealing that their morphologies are
significantly influenced by reaction temperature. A qualitative formation mechanism is proposed based
on the experimental observations. A proof-of-concept demonstration shows that the ZnO nanotower
arrays are highly effective at adsorbing and subsequently photo-remediating a model pollutant (Eosin
B) from water. These observations could promote new applications of photocatalytic adsorbents for
wastewater treatment utilizing oxide semiconductor nanostructures.
1. Introduction
ZnO-based materials have size and morphology dependent physical and chemical properties at the nanoscale which could offer
tremendous opportunities for advanced technologies.1–3 For
example, nanostructured ZnO is being actively explored in lightemitting diodes,4 laser diodes,5 sensors,6 pizeoelectronics,7,8 solar
cells,9 UV-blocking components,10 photocatalysis,11–13 and transparent conductive glass coatings.14 Various kinds of 1-D ZnO
nanostructures have been reported, ranging from uniform nanobelts,15 nanotubes16 to nanowires.17 It is known that the properties
of nanostructured materials are size and shape dependent. Unique
structures could introduce additional tunable properties.
However, it is still challenging to fabricate nanoarrays such as
those of 1-D ZnO with deterministic morphologies on chosen
substrates by simple methods. While most of the reported 1-D
ZnO have a near uniform diameter along the nanostructure length,
nanostructures with non-uniform diameter or stages along the 1-D
nanostructure length and associated asymmetrical physical properties may be of interest toward electronic, thermal and vibration
transport control in future nanodevices. A simple method for
large-scale and low-cost fabrication of 1-D ZnO nanostructures
having non-uniform diameters on various substrates could be of
potential interest for further studies of their functional properties.
Here, we note that non-uniform 1-D ZnO nanotowers with
hexagonal cross-sections have been reported previously.18–21
Preparation methods included high temperature (>950 C)
chemical conversion of ZnS,19 chemical bath despostion,21 thermal
oxidization with controlled oxygen18 and two-step hydrothermal
methods using ZnO seed layers.20
In the context of semiconductor photocatalysts for water
treatment, 1-D ZnO nanostructures could offer various
Harvard School of Engineering and Applied Sciences, Cambridge, MA,
02138, USA. E-mail: shriram@seas.harvard.edu
† Electronic supplementary information (ESI) available: SEM images of
ZnO nanotowers on wafer and zinc substrates; 2nd run adsorption test.
See DOI: 10.1039/c0nr00537a
This journal is ª The Royal Society of Chemistry 2010
advantages. Non-biodegradable dyes widely used in textile,
paper and other industrials pose severe threats to aqueous
environments when released without treatment. The elimination
of harmful components (including dyes) from wastewater is an
important goal for environmental control in industrial settings.
Adsorption and chemical coagulation are traditionally used for
the treatment of such wastewater.22 However, the traditional
methods transfer dyes, and the solid waste containing dyes is
hard to handle. On the other hand, semiconductor photocatalysts such as ZnO utilizing photo-induced oxidation process
could decompose the dyes effectively.12,22
Both powder and thin film forms of ZnO have been explored
to date for environmental applications.22,23 Powder-form ZnO
semiconductor photocatalysts normally show higher photoefficiency due to the increased active surface area. However, the
powder form photocatalysts used in suspension state in water
have certain limitations due to particle aggregation and technical
challenges in photocatalyst separation and recovery.24 In
comparison, thin film ZnO can be easily recovered, yet its surface
area is low leading to poor overall efficiency.11 In comparison, 1D nanostructured ZnO arrays with high surface-to-volume ratio
grown on substrates could offer advantages of both high surface
area and easy recovery.25 Furthermore, due to the large surface
area offered by 1-D ZnO nanoarrays, they could be efficient
adsorbents. The adsorbed pollutants can be subsequently photocatalytically decomposed on the surface and the photocatalyst
can be reused.
Here we report on the facile synthesis of 1-D ZnO flat nanotower arrays on various substrates by thermal oxidation. The
substrates explored here include glass (insulator), a silicon wafer
(semiconductor) and zinc metal foil (conductor). The 1-D ZnO
has a unique tower like structure with flat basal section near the
substrate and tapers in stages to wire-like at the tip. To test the
absorptive and photocatalytic capacities of the ZnO nanotower
arrays, we use the model dye wastewater pollutant Eosin B. This
red dye, commonly used as a biological stain and stable in
normal conditions, is harmful if swallowed, inhaled, or absorbed
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Fig. 1 FESEM images of the 1-D flat ZnO nanotower arrays formed on glass slide substrate at 550 C: (a) low magnification top view and (b) side view;
(c) high magnification side view of the base of the ZnO nanotower arrays; the inset of (c) is the corresponding low magnification side view; (d) high
magnification top view of a few nanotowers indicating flat structure; and (e) a typical ZnO nanotower with stages clearly observed.
through skin, and has been used as a model dye in the evaluation
of photocatalysis.12,26 The application of the these unique 1-D
flat ZnO nanotower arrays as adsorbent and subsequently used
for photocatalytic decomposition in air has not been previously
reported to the best of our knowledge. For our proof-of-concept
demonstration, the typical Eosin B absorbance peak at 518 nm
can be readily monitored.
2. Results and discussion
FESEM images of ZnO nanotowers fabricated on glass slides at
550 C are shown in Fig. 1. The low magnification image
(Fig. 1a) viewed from the top shows that a dense array of ZnO
nanotowers forms on the glass substrate. Each nanotower has
a broad and flat basal section near the substrate which then
narrows to a wire at the tip (Fig. 1b). In contrast to other
reported 1-D ZnO nanoarrays,16 the ZnO nanotower arrays
reported here are not perpendicular to the substrate but instead
are tilted at 60 to 70 . Fig. 1c shows a high-magnification side
view of the base region indicating flat morphology at the basal
sections. The inset of Fig. 1c shows the corresponding low
magnification view, revealing that a layer of ZnO thin film is
formed on the glass slide. The flat tower morphology is evident in
the high-magnification top view shown in Fig. 1d. The line
highlighted by a white arrow emphasizes the contact between the
nanotower and substrate. A typical ZnO nanotower characterized by stages is apparent in Fig. 1e. The nanotower arrays could
also be fabricated with similar morphology on a semiconducting
silicon wafer surface (see ESI, Fig. S1†). In addition to glass
substrate and silicon wafer, similar ZnO nanotower arrays could
be fabricated directly on Zn foil by thermal oxidation in air, as
revealed by FESEM images at different magnification shown in
Fig. S2, ESI†. Hence, such ZnO arrays can be fabricated on
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various substrates. Without Pd/Pt coating on glass slide or silicon
wafer, flat ZnO nanotowers were not observed. This indicates
that Pd/Pt coating could provide the wetting surface for deposition and nucleation of ZnO and 1-D growth.
The XRD patterns (Fig. 2a) confirm that ZnO phase with
wurtzite (hexagonal) structure are formed on both glass slide
substrate and on the Zn foil upon thermal oxidation. The XRD
peaks could be assigned according to JCPDS card no. 65-3411
for wurtzite. Fig. 2b shows a broad scan survey XPS spectrum of
the ZnO arrays. The survey spectrum is typical of ZnO and the
Fig. 2 (a) XRD spectrum of the ZnO nanotower arrays fabricated on
glass slide and Zn foil surface; (b) XPS spectrum of the ZnO nanotower
arrays; and (c) high resolution of XPS spectrum of O 1s and (d) Zn 3s.
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main photoelectron peaks can be assigned to Zn- and O-derived
bands.27 Compared to Fig. 2b, Fig. 2c and d show high-resolution analysis. The O 1s band has a large asymmetry (Fig. 2c),
corresponding to Gaussian components centered at 529 and
530.6 eV. The lower binding energy spectrum of the O 1s could be
attributed to the O2 ions in the wurtzite structure of stoichiometric ZnO. The shift of O 1s to a higher binding energy indicates
the presence either of point defects or chemisorbed oxygen,
possibly as OH, H2O or –CO3.28 The high-resolution analysis of
Zn 3s band shows high symmetry (Fig. 2d), which could be
assigned to the Zn2+ ions in ZnO. The atomic ratio based on XPS
analysis for O : Zn also agrees with that of commercial ZnO
powder analyzed by XPS.
The novel morphology of the ZnO nanostructures is vividly
revealed by the TEM characterization shown in Fig. 3. A typical
ZnO nanotower is shown in Fig. 3a at low magnification. The
broad basal section narrows stage by stage toward the tip. The
ripple-like contrast along the nanotower is typically observed for
flat ZnO nanostructure.15 The high-resolution TEM (HRTEM)
in Fig. 3c indicates that the nanotower is single crystalline, as
further confirmed by selected-area electron diffraction (SAED)
shown in the inset. The diffraction spots correspond to the
wurtzite structure ZnO. The emphasized region in Fig. 3d shows
a crystallinity having a d-spacing of 0.26 nm that matches well to
the d(002) planes of wurtzite. The SAED collected at both the tip
and basal sections of the nanotower shows similar patterns all
indicating the ZnO nanotower growth in the direction perpendicular to [001]. This growth direction is different from previously reported non-flat ZnO nanotowers which were oriented
along the [001] diection.19,20,29 Non-flat ZnO nanotowers reported in literature have hexagonal crosssection and are grown along
c-axis; surrounded by equivalent facets of (101), (110), (011) to
minimize energy.30 However, in our case, the c-axis is in-plane
and the growth occurs by extension into a unique flat
morphology likely to reduce the exposure of unstable (002)
planes to minimize surface energy.20,31 This could qualitatively
Fig. 3 TEM images of the ZnO nanotowers: (a) the stages in the flat nanotower clearly observed in the low magnification image; (b) high magnification
view of the stages; (c) HRTEM image of the nanotower and the inset showing a selected area diffraction pattern; (d) close-up view of the area highlighted
in (c) clearly shows the single crystal structure; (e) ZnO nanotower with branch at the basal section; (f) a typical ZnO nanotower with ripple like contrast
and branch at the tip section; and (g) illustration to show the structure.
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Fig. 4 FESEM images of the ZnO nanotowers fabricated on Zn foil at 650 C for 2 h: (a) low magnification view of the ZnO nanotowers; (b) ZnO
nanotowers fabricated on glass slide at 650 C; (c) ZnO nanotower with high aspect ratio; (d and e) the zoom-in view of the basal and tip region of
highlighted in c shows a large ‘‘base-tip ratio’’; (f) the initial stage of growth with nanoparticles and (g) ZnO nanowires formed at 400 C.
explain the difference between our flat ZnO nanotower to those
literature reported ZnO nanotowers with hexagonal crosssection.
Fig. 3e shows a typical nanotower having a branch on the
shoulder and growing independently into another flat tower
structure. Fig. 3f shows the ripple like contrast along a typical
ZnO nanotower. A branched structure is formed at the top
section for this nanotower.
The experimental results of Fig. 4 demonstrate that the fabrication temperature strongly influences the morphology of the 1-D
ZnO nanotower arrays. At 650 C, the nanotowers formed on Zn
foil substrate broaden and increase in length compared to those of
lower temperatures. The stages are also less clearly differentiated,
approaching the structure of a needle (Fig. 4a). Fig. 4c shows
a typical ZnO nanotower fabricated at 650 C, with the tip and
basal sections of this ZnO tower enlarged in Fig. 4d and 5e. In
addition to the aspect ratio, the ‘‘base-to-tip’’ ratio, which is the
ratio of the width of the basal section to tip section, is 28 : 1. The
ZnO nanotower structure also can be grown on a glass substrate at
650 C, as expected and shown in Fig. 4b. At 400 C, ZnO
nanowires form primarily (Fig. 4f and g).
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Based on the above characterization and analysis, we qualitatively propose a formation mechanism for the ZnO nanotower,
as illustrated in Fig. 5. At the initial stage of thermal oxidation,
Fig. 5 Schematic diagram showing the proposed growth steps for ZnO
nanotowers.
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implying a high concentration of Zn, a thin layer of ZnO is present
on the substrates (i.e., glass slide or silicon wafer). For the case of
Zn foil, ZnO grows by oxidation at the surface. For the glass slide
and the silicon wafer, ZnO is formed by the condensation of Zn
onto the Pd/Pt wetted surface, followed by oxidation. Zn/ZnOx
clusters then form on the surface (step 1), functioning as nucleation sites for further deposition. The initial stage formation of
nanoparticles was experimentally observed as shown in Fig. 4f.
With the continued supply of evaporated Zn from metal source
and O2 from air at the high temperature, ZnO starts to adsorb on
the surface of the clusters and further deposits at the interface of
the Zn/ZnOx, eventually growing as a nanowire (step 2). The
formation of nanowires was experimentally observed as shown in
Fig. 4g. The ZnO adsorbed on the tip of the nanowire promotes
further vertical growth, while the ZnO adsorbed on the side of the
nanowire causes lateral growth. Due to higher concentrations at
the base, additional nucleation and growth occur (step 3) and
continue in subsequent stages (step 4). This is experimentally
observed in Fig. 4b. Since the Zn/ZnOx structures are exposed to
air at high temperature, they can be oxidized to ZnO, forming
a nanotower (step 5). A fully grown nanotower is illustrated in
Fig. 5a. For the branched 1-D flat ZnO nanotower, the excess
supply of Zn at the basal section could promote the formation of
additional structures. Subsequently, it forms a branch, as illustrated in 6b and as observed experimentally. Detailed studies at
the initial stages of growth are required to fully understand the
nanowire growth mechanism.
A proof-of-concept demonstration was carried out for the
potential use of the ZnO nanotower arrays as photocatalytic
adsorbents. Fig. 6a shows the ultraviolet-visible (UV-Vis) spectra
of an Eosin dye solution for progressive contact times with ZnO
nanotower arrays (without UV exposure). The absorbance
decreases continuously over time, indicating that the ZnO nanotower arrays effectively adsorb the dye from the solution. There
are no other absorbance peaks observed, indicating that the
process is physical sorption with no new chemical components
generated. The mechanism of dye adsorption could be similar to
that of activated carbon with large surface area for surface
adsorption.22,32 Fig. 6b shows the normalized concentration of the
dye in solution decreases with time, with 80% loss of dye from
solution over 350 min for the first run of adsorption. For
comparison, we tested a ZnO thin film that did not have a nanotower morphology: it was fabricated on Zn foil by heating a zinc
foil substrate on hot plate for several hours. In the case of this
lower surface area thin film, only 10% of the dye was adsorbed
over a similar 350 min period. Fig. 6c shows the gradual decrease
in color of the solution (upper) and gradual increase of the color of
the ZnO nanotower arrays (lower).
Upon exposure to ultra-violet radiation, the adsorbed red dye
on the surface was decomposed. The white photo-bleached
sample is shown in the lower-right of Fig. 6c. As a control
experiment, exposure to UV radiation alone without any ZnO
present did not cause dye decomposition, in agreement with
earlier reports.26 The mechanism of dye decomposition on the
ZnO nanotower arrays is based on the redox reactions that occur
on the semiconductor surface, as promoted by the photo-induced
electron–hole separation.22,25,33 ZnO has a band gap of 3.2 eV,
corresponding to an onset excitation of 380 nm. A photon
promotes electrons from the valence band to the conduction
This journal is ª The Royal Society of Chemistry 2010
band, creating a hole in the valence band. The electron–hole pairs
can recombine in the volume or at the surface to unproductively
release heat or alternatively can interact with surface-adsorbed
molecules to induce chemistry. With the presence of oxygen as an
electron donor, the dye adsorbed can be oxidized and possibly
mineralized on the semiconductor surface via common photocatalysis process (although we do not have any direct evidence
for this in the present work).33,34 The restore of the surface color
of the ZnO nanoarrays covered substrate observed experimentally indirectly evidenced the mineralization of the adsorbed dye.
After photo-exposure, the initial white color of the ZnO nanotower arrays returns (Fig. 6c), and the recovered arrays are
reusable for further adsorption and photocatalysis. The second
test run shows that the arrays are effective in adsorbing the dye
(Fig. S3, ESI†).
3. Conclusions
A thermal oxidation method was employed to fabricate ZnO
nanotower arrays on various substrates. The novel structure of the
flat towers was characterized by electron microscopy (SEM/
TEM), XRD and XPS. A qualitative formation mechanism for
the nanotowers was proposed. The ZnO nanotower arrays served
as an effective adsorbent for removing the model dye pollutant
Eosin B from water. Following adsorption, the dye was decomposed on the surface by the redox reactions induced by ultraviolet
illumination. We anticipate that these initial results could stimulate further studies on the exploration of solid state ZnO nanostructures for water treatment.
4. Experimental
ZnO nanotower arrays synthesis
For the fabrication of ZnO nanotowers on glass slide (or silicon
wafer), a piece of microscope cover glass slide (1 1 cm2) coated
with Pt/Pd (80 : 20) target using Cressington 208HR sputter
coater was used as a substrate. Zn foil (0.2 1.2 cm2, Sigma
99.9%) was placed in a crucible boat and partially covered with
the glass slide with the Pt/Pd coated side facing downward to Zn
foil. The crucible boat was transferred to a tube furnace and
heated to desired temperature. For the fabrication of ZnO
nanotowers on Zn substrate, the Zn foil was heated directly to
the desired temperature and collected without any further
treatment. The surface turned to grey in color indication the
successful fabrication of ZnO nanotowers.
Materials characterization
The 1-D ZnO nanostructures were characterized by high resolution field-emission scanning electron microscopy and energy
dispersive X-ray spectroscopy (FESEM/EDS, Supra55VP/
Ultra55, Zeiss, Germany), transmission electron microscopy and
selected-area electron diffraction (TEM/SAED, JEOL JEM2100 operating at 200 kV), powder X-ray diffraction (XRD) on
a Scintag XDS2000 X-ray diffractometer using Cu Ka radiation,
and X-ray photoelectron spectroscopy (XPS) performed on
a surface science SSX-100 spectrometer using monochromated
Al-Ka X-rays and calibrated to a C 1s electron peak at 284 eV.
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Fig. 6 Proof-of-concept demonstration of the 1-D flat ZnO nanotower arrays as a recoverable and reusable photocatalytic adsorbent in an environmental application for the remediation of a dye pollutant in water: (a) UV-Vis spectra of the eosin B measured for increasing adsorption times; (b)
normalized concentration for increasing adsorption time (no UV exposure) for a fresh ZnO nanotower arrays; results are compared to a ZnO thin film
prepared by conventional methods; and (c) optical images of the change in color of the solution (upper) and the corresponding ZnO materials (lower)
over time corresponding to the data of panels (a) and (b); the right most image in the lower panel shows that the ZnO nanotower arrays are photobleached after exposure to UV radiation for 30 min; this recovered sample was subsequently used for the second run.
Photocatalytic adsorbent evaluation
2
A piece (0.5 2.5 cm ) of Zn foil covered with ZnO nanotower
arrays prepared by thermal oxidation was placed in a cuvette
containing the model polluted water with 3 10 5 M Eosin B
dye. The cuvette was then placed in dark. The concentration of
Eosin B was monitored at different time intervals by an Agilent
8453 UV-Visible spectrophotometer using the absorbance peak
of Eosin B at 518 nm. As adsorption occurred, the red color
faded from the solution in the cuvette while the red coloring of
the ZnO nanotower arrays saturated over time. The red stained
samples could be reused after exposure to UV-Vis light from
a 150 W UV-enhanced Xenon lamp to completely decompose the
adsorbed Eosin B, restoring the surface to a white color.
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