Advances in Materials Physics and Chemistry, 2012, 2, 56-59
doi:10.4236/ampc.2012.24B016 Published Online December 2012 (http://www.SciRP.org/journal/ampc)
Synthesis of Aluminum-doped Zinc Oxide Nanowires
Hydrothermally Grown on Plastic Substrate
Concepción Mejía García1, Elvia Díaz Valdés1, Ana Ma. Paniagua Mercado1, Arturo F. Méndez Sánchez1,
José A . Andr aca Adame2, Velumani Subr amaniam3, Josué Romero Ibarra3
1Departamento de Física, Escuela Superior de Física y Matemáticas, Edif. UPALM Col. San Pedro Zacatenco, Deleg. GAM, México
2Centro de Nanociencias y Micro y Nanotecnologías del IPN, UPALM, Luis Enrique Erro S/N, México
3Depto. de Ing. Eléctrica, Sección de Estado Sólido, Av. IPN No. Col. San Pedro Zacatenco, México
Received 2012
ABSTRACT
We report the synthesis of undoped ZnO and Al-doped ZnO (ZnO:Al) nanowires grown using a two-step process: (a) preparation of
the seed layer, and (b) growth of the nanostructures. In the first step, 10 mM solutions of zinc acetate dihydrate and 1-propanol were
spin coated on polyethylene terephthalate (PET) substrate at 2000 rpm by 30 s. Vertical nanowires were then grown by dipping the
substrates in an equimolar solution of zinc nitrate hexahydrate and hexamethylenetetramine. As doping source, aluminum nitrate
nonahydrate powders were added in the solution. In the solutions, Al doping concentrations were established as 0.5 At %, 1.0 At %,
2.0 At % and 3.0 At %, respectively. The hydrothermal process were carried out with a commercially microwave at 140 W power
setting. The nanowires were characterized optically and morphologically. XRD patterns show the presence of ZnO, Zn(OH)2 and
Zn6Al2O9. Scanning electron microscopy analysis showed that the size of ZnO nanowires was 50 nm in diameter. The undoped ZnO
and ZnO:Al nanowires bandgap energy was obtained from optical transmission spectra.
Keywords: Nanowires; ZnO; ZnO:Al Nanowires; Spin Coating; Hydrothermal Process
1. Introduction
ZnO is a promising material in the field of short wavelength
optoelectronics due to its wide and direct bandgap of 3.37 eV at
room temperature [1]. Nanostructures based on this semicon-
ductor offer the benefit of material quality leading to improved
device efficiency such as UV nanolasers, field effect transistors
and solar cell electrodes [2,3]. Doping ZnO is an effective
method in order to modify the physical properties. The ZnO:Al
nanowire arrays are capable of reaching the highest conductiv-
ity without deterioration in optical transmission and cristallinity,
and thus are promising candidates for field emision displays.
ZnO:Al nanostructures have been prepared by different meth-
ods (e. g. pulse laser deposition and magnetron sputtering) and
investigated the effect of the Al doping concentrations on the
structure. However these methods require complex procedures
like high temperatures or vacuum.
The hydrothermal ZnO nanowire synthesis has advantages
over the methods mentioned previously. Nevertheless, the time
required for the synthesis takes several hours or days. An alter-
native to reduce process time is the use of the rapid microwave
heating process. In this work, the hydrothermal method is per-
formed with a commercially microwave oven for fast heating of
the material. With this procedure the processing time, for ZnO
and ZnO:Al nanowires grown on PET substrates, is decreased.
The undoped ZnO and ZnO:Al nanowires were characterized
morphologically by scanning electron microscopy (SEM),
structurally by X-ray diffraction and optically by transmission
measurements.
2. Experimental Procedure
Undoped ZnO and ZnO:Al nanowires were grown hydrother-
mally based on the method developed by Husnu et al. [4]. This
method consists of two-step process: (a) preparation of the seed
layer, and (b) growth of the nanostructures. First, a 10 mM
solution of zinc acetate dihydrate and 1-propanol was prepared.
The solution was then spin coated on PET substrate at 2000
rpm by 30 s. After three cycles of spin coated deposition, the
deposit on the substrate was annealed at 100°C during 5 min,
and then a seed layer was obtained on the PET substrate. Sec-
ondly, the substrate was dipped in a mixture of equimolar 25
mM zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethyl-
enetetramine (HMTA) solution in deionized water. As the dop-
ing source, aluminum nitrate nonahydrate (Al(NO3)3·9H2O
powders were added in the solution. In the solutions, Al doping
concentrations were established as 0.5 At %, 1.0 At %, 2.0 At
% and 3.0 At %, respectively. Table 1 shows Al concentrations
for the ZnO:Al nanowire samples. The hydrothermal process
was carried out heating with a commercially microwave oven at
140 W power setting at atmospheric pressure during 10 min.
The X-ray patterns were measured in Grazing-incidence small-
angle X-ray scattering (GISAXS) with XPert PRO Diffracto-
meter from Panalytical using Cu radiation (Kα = 1.54 Å), it is a
technique used to study nanostructured surfaces and thin films.
The sample was placed in one degree (θ = 1 °). X-Ray mirror
and soller slit (0.04 Rad) in incident optic and PIXcel ultrafast
X ray detector for diffracted optic were used. The step size was
0.05 and 300 s for time. 45 kV and 40 mA were used for tube

C. M. GARCÍA ET AL. 57
power. Optical transmission measurements were performed in a
UV/VIS spectrometer Lambda Perkin Elmer in the region of
300 nm to 1100 nm. SEM micrographs were obtained with
secondary electrons in a Zeiss MOD Auriga 39-16 microscopy.
3. Results and Disscusion
3.1. X-Ray Diffraction Patterns
Figure 1 shows the X-ray diffraction patterns of the PET sub-
strate, the undoped ZnO nanowires (sample Z1) and the ZnO:Al
(sample Z3). There were identified several crystalline phases as
(C10H8O4)n, ZnO, Zn(OH)2 and Zn6Al2O9. The PET phase
(C10H8O4)n has a triclinic structure with crystal lattice dimen-
sion of 4.53Åx5.92Åx10.77Å (99.92ºx118.62ºx111.37º). Its
reflections of highest intensities are located at 22.72º and 26.11º
2degrees and its corresponding hkl are ( -110) and (100).
The ZnO phase has a hexagonal structure with crystal lattice
dimension of 3.25Åx3.25Åx5.20Å (90ºx90ºx120º). The reflec-
tions of highest intensities are located at 31.76º, 34.32º and
36.25º 2degrees and its hkl are (100), (002) and (010)
respectively. The Zn(OH)2 phase has a tetragonal structure with
crystal lattice dimension of 8.22Åx8.22Åx14.34Å
(90ox90ox90o). The reflections of highest intensities are located
at 19.58o, 24.90o and 30.57o 2 degrees and its corresponding hkl
are (112), (004) and (213). The reflections of highest intensities, of
the Zn6Al2O9 phase, are at 32.17o, 34.60o and 36.49o 2 degrees.
Figure 2, in the region from 30º to 60º 2 degrees, shows in
detail the presence of the crystalline phases identified in the
samples. From sample Z1 we appreciate reflections associated
to PET, as well as, reflections of ZnO. Sample Z3 shows the
presence of the ZnO, Zn6Al2O9 and Zn(OH)2 crystalline phases.
It is clearly observed that when the ZnO is doped with Al the
Zn6Al2O9 phase is formed. The reflections of Zn6Al2O9 phase
are observed as shoulders in the bands of ZnO, in addition to
the reflections present at 47.56º and 56.78º 2 degrees. On the
other hand, additional reflections from Zn(OH)2 can be ob-
served at 42.84º, 45.76º and 50.91º 2 degrees and its hkl are
(314), (107) and (008) respectively.
3.2. Scanning Electron Microscopy
Figure 3 shows the typical SEM images of the undoped ZnO
(sample Z1) and ZnO:Al (sample Z4) arrays grown at 140 W
by 10 min. Figure 3(a) shows the SEM image of a high density
of vertically undoped ZnO nanowire arrays It can be seen that
the nanowires grown vertically aligned on the PET substrate.
The diameters of the ZnO nanowires are less than 50 nm. Fig-
ure 3(b) shows the image of ZnO doped with 2.0 At % Al.
Table 1. Conditions of growth parameters.
Sample Al At %
Z1 undoped
Z2 0.5
Z3 1.0
Z4 2.0
Z5 3.0
Figure 1. X-Ray diffraction patterns of the PET substrate, undoped
ZnO (sample Z1) and ZnO:Al at 1.0 At % (sample Z3) processed in
a microwave oven at 140 W during 10 min.
Figure 2. X-Ray diffraction patterns, in the 30o to 60o 2 region of
the PET substrate, undoped ZnO (sample Z1) and ZnO:Al at 1.0 At
% (sample Z3) processed in a microwave oven at 140 W during 10
min.
Besides the high density of ZnO nanowire arrays, it can be
found that there are other nanostructures like nanosheets. Due
to the doping with Al ions, the coexistence of the ZnO nanowires
and nanosheets was observed. We also observed that the num-
ber of nanosheets was increasing with doping concentration.

C. M. GARCÍA ET AL.
58
3.3. Optic al Tr ansmission
Figure 4 shows the optical transmission spectra in the range of
300 and 1100 nm for all samples. It can be observed a good
transparency from the samples doped with Al in comparison
with the undoped ZnO sample Z1. In our previous work, an
undoped ZnO sample grown on glass substrate with a 140 W
and 15 min, an optical transmission around 60% was reported
[5]. In this work it was possible to obtain ZnO wires with the
same power setting but less time (10 min) grown on PET sub-
strate. With Al doping concentration, the optical transmission
was increased from 5 % to 65%.
From these spectra it can be seen that the absorption edge is
around 360 nm - 390 nm. From the transmission spectra the
energy band gap Eg was obtained for each sample. Table 2
shows these results. It was no possible to determine the energy
band gap of the undoped sample through this method. The en-
ergy band gap decreases when the Al concentration increases.
(a)
(b)
Figure 3. Scanning electron microscopy images of the undoped
ZnO nanowires (sample Z1) and ZnO:Al (sample Z4) grown on
PET substrate at 140 W for 10 min.
Figure 4. Optical transmission spectra of undoped ZnO and
ZnO:Al nanowires on PET substrate.
Table 2. Band gap energy of the undoped and ZnO:Al nanowires.
Sample Eg(eV)
Z1 -
Z2 3.32
Z3 3.32
Z4 3.33
Z5 3.27
4. Conclusions
Undoped ZnO nanowire arrays and ZnO:Al nanostructures
grown on PET substrate were successfully synthesized by the
hydrothermal method using a commercial microwave oven.
With just 10 min of heat treatment, ZnO nanowires with less
than 50 nm of diameter were obtained. The coexistence of the
nanowires and nanosheets was observed by introducing Al ions.
Optical transmission spectra showed that ZnO nanowire arrays
doped with Al increases the optical transmission in comparison
with the arrays of undoped ZnO. The ZnO:Al nanostructures
treated with the hydrothermal method showed a great promise
for applications as flexible devices, processed with low power,
short growth time, and low cost.
5. Acknowledgements
The authors thank Esteban Basurto Cazares for the technical
support.
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