Structure and morphology of the grown ZnO nanowires were investigated to achieve appropriate characteristics to achieve performance improvement for the resulting nanogenerator device.. T
Trang 1Department of Electrical Engineering, INHA University, Incheon City 402-751, Republic of Korea
b
School of Material Science and Engineering, INHA University, Incheon City 402-751, Republic of Korea
c
Department of Electrical Engineering, INHA Technical College, Incheon City 402-752, Republic of Korea
d
Energy Nano Material Center, Korea Electronics Technology Institute, Seongnam City 463-816, Republic of Korea
a r t i c l e i n f o
Article history:
Available online 26 February 2013
Keywords:
ZnO
PZT
Piezoelectricity
Hetero-junction
Nanogenerator
a b s t r a c t
We fabricated an alternative nanogenerator device with distinguished structure Representative piezo-electric materials of ZnO nanowires and PZT thin films were tried to be combined to form a hetero-junc-tion structure for fabricahetero-junc-tion of an alternative nanogenerator device to possibly obtain a synergy effect and then improved performance The ZnO nanowires were grown by a hydrothermal synthesis technique and then PZT thin films were deposited on the surface of the ZnO nanowires by rf magnetron sputtering process The PZT thin films were annealed to be crystallized with different conditions for post-deposition thermal treatment process The hetero-junction structure was polarized by a Corona poling process to obtain a unidirectional orientation of dipole moments to enhance their piezoelectric property Structure and morphology of the grown ZnO nanowires were investigated to achieve appropriate characteristics to achieve performance improvement for the resulting nanogenerator device To confirm effect of the het-ero-junction structure on improvement of power generation performance of the resulting nanogenerator device, current generating properties were comparatively investigated with those of nanogenerator device with only ZnO nanowires or PZT thin films as active piezoelectric component, respectively The nanogenerator device with a hetero-junction structure of ZnO nanowires/PZT revealed distinctively improved average currents of 270 nm, which is quite higher than those of the nanogenerator devices with pristine ZnO nanowires, or with pristine PZT thin films, respectively Possible factors contributed to improvement of the current generation properties were discussed for the presented nanogenerator device
Ó 2013 Elsevier B.V All rights reserved
1 Introduction
Recent advances of technologies have been accompanied by
drastic increase of energy consumption and various kinds of
re-ports have been devoted for introduction of alternative energy
gen-eration technologies Technologies of green energy harvesting and
self-powered energy sources would be one of those promising
methodologies for the alternative energy sources Among them,
piezoelectric materials have been studied to be prepared by
nano-technologies for possibly enhancing their piezoelectric effect in
various energy-harvesting devices A new power-generator device
applying enhanced charge carrier generation of one dimensional
piezoelectric nanomaterials has been reported as so-called
nanopi-ezoelectronics by numerous research groups including the Wang
nanopiezo-electronics are based on an energy conversion mechanism of
nano-generator device, by which mechanical energies are converted to
representa-tive materials for preparation of one-dimensional structure to combine piezoelectricity and semiconducting property for
research reports for nanopiezoelectronics using combination of
nanowires can be prepared relatively easily by hydrothermal syn-thesis process at low temperature, and their power generation per-formance could be improved by controlling material properties of grown crystalline nanowires as well as by alteration of those de-vice structure designs However, the base material of ZnO has rel-atively low piezoelectric coefficient of 12pC/N, and hence, realization of a high performance nanogenerator using ZnO nano-wire might be quite limited To overcome such shortage of material property of ZnO, material property alteration via appropriate impurity doping and/or multi-layer stacking using different mate-rial with higher piezoelectric coefficient could be tried to obtain higher power generation performance On the other hand, nano-wires of crystalline PZT could be prepared by hydrothermal
0167-9317/$ - see front matter Ó 2013 Elsevier B.V All rights reserved.
⇑Corresponding author Tel.: +82 32 860 7402; fax: +82 32 863 5822.
E-mail address: shinsensor@inha.ac.kr (P.-K Shin).
Trang 2synthesis technique to achieve high power generation performance
utilizing higher piezoelectric coefficient of PZT However,
hydro-thermal synthesis of PZT might be quite difficult due to several
limitation factors: choice of appropriate substrate; deteriorated
reproducibility; complicated synthesis process Therefore, several
approaches have been tried for realization of ZnO/PZT
either a composite of power-to-powder, or a stacking of
Nev-ertheless, practical application for realization of those electrical
characteristic has not yet been reported for nanogenerator devices
new hetero-junction structure of ZnO nanowires and PZT thin films
to fabricate a nanogenerator device to achieve a synergy effect, by
which disadvantages of each material could be minimized and
improved power generation performance could be achieved
Electrical characteristics of the resulting nanogenerator devices were studied and improved power generation performance was discussed to be confirmed
2 Experimental Commercially available ITO coated glass (Corning; 200 nm thick ITO) substrates were used The substrates were cleansed by using conventional semiconductor cleansing process and then seed lay-ers were coated on the ITO coated glass substrates by rf magnetron sputtering process using an Al-doped ZnO target (AZO; 2 wt.%
Al + 98 wt.% ZnO) Material characteristic of the seed layer is
Seed layer of a 40-nm-thick AZO thin film was prepared in this re-port to minimize density of the grown nanowires and to obtain diameter of those as smaller than 50 nm The thickness was decided after experimental results
Fig 1 FE-SEM image of the ZnO nanowires grown by hydrothermal synthesis. Fig 3 SEM image of the hetero-junction structure of ZnO nanowires/PZT.
Trang 32.1 Synthesis of ZnO nanowires by hydrothermal technique
ZnO nanowires were grown by a hydrothermal technique A
reaction solution was prepared for the hydrothermal synthesis of
ZnO nanowires by adding the following ingredients in de-ionized
(DI) water and then stirring: 0.015 mol/L of zinc nitrate
hexahy-drate (Zn(NO3)26H2O); 0.015 mol/L of hexamethylenetetramine
(HMTA); 0.09 mol/L of ammonium chloride; 0.003 mol/L of
poly-ethyleneimine (PEI) Seed layer coated ITO-glass substrates were
immersed in the prepared reaction solution (400 ml) contained
in cultivation bottle, and then the cultivation bottle was immersed
for 3 h in a water bath of constant temperature of 90 °C The
sub-strates with grown ZnO nanowires were then cleansed carefully
with ethanol, so that tips of the thin ZnO nanowires would not
be tied by possible capillary force Then the ZnO nanowires were then cleansed ultrasonically to remove residues which are possibly remained during the hydrothermal synthesis
2.2 Preparation of PZT thin films
An rf magnetron sputtering process was used to deposit PZT thin film on top of the prepared ZnO nanowires A self-made 2 inch
the PZT thin film Deposition rate is a critical process condition regarding extremely narrow gaps of the underlying ZnO nano-wires Inferior step coverage of the being deposited PZT thin film due to inappropriate deposition rate should be prevented, so that the PZT thin films could cover entire surface of the ZnO nanowires
Fig 4 XRD patterns for PZT thin films annealed at different temperatures and for different annealing times: (a) annealing temperature; (b) annealing time.
Trang 4A 1.5-lm-thick PZT thin film was deposited with a deposition rate
of 80 Å/min The deposited PZT thin films were then thermally
treated in an electric oven, so that an amorphous phase PZT thin
film could be crystallized The post-deposition annealing was
car-ried out by using a PLC control with three steps: (1) Step 1: up
to 600 °C with temperature elevation rate of 8.3 °C/min; (2) Step
2: constant temperature of 600 °C for 1 h; (3) Step 3: cooling to
room temperature with temperature descending rate of 8.3 °C/
min After the post-deposition annealing, the PZT thin film
depos-ited on the ZnO nanowires were then polarized by a Corona poling
process regarding the device structure in our study Voltage of
11 kV was applied during the poling process for 30 min and the
substrate was heated at a constant temperature of 80 °C to achieve
a homogeneous aligning of dipoles in crystallized PZT molecules
Finally, a thin Pt upper electrode was coated to fabricate a
nano-generator device: Pt/PZT-ZnO nanowires/ITO
2.3 Analysis for characterization of ZnO nanowires, PTZ thin films, and
nanogenerator device
Structure of the PZT thin films were investigated by X-ray
dif-fraction (XRD) Field-emission scanning electron microscopy
(FE-SEM) was used to investigate the structure of the nano
power-gen-erator device Structural characteristic of the ZnO nanowires were
investigated by selected area diffraction (SAED), energy dispersive
spectrometer (EDS), and tunneling electron microscopy (TEM) To
investigate power generation characteristic, a fixed force of
0.9 kgf was applied to the nanogenerator device by a linear motor
and current generating characteristics were measured with a
picoammeter (Keithley 6485)
3 Results and discussions
In this study, the hydrothermally synthesized ZnO nanowires might have two functions: (1) charge transfer layer for appropri-ate transportation of charge carriers generappropri-ated by piezoelectric effect to electrode; (2) additional piezoelectric element to con-vert mechanical energy to electrical energy Moreover, the underlying one-dimensional ZnO nanowires were thought to be served as a template for the overlying PZT thin films to have lar-ger surface area like a pseudo one-dimensional structure, so that the PZT would reveal more enhanced piezoelectric charge
image of the hydrothermally grown ZnO nanowires, which re-veals vertically grown ZnO nanowires perpendicular to substrate surface Tips of the ZnO nanowires reveal no tied feature due to capillary force, and residue from the hydrothermal process can-not be observed, which could be contributed to a defect in the resulting nanogenerator device The grown ZnO nanowires
50 nm
In general, hydrothermally grown ZnO nanowires have been known to have crystallinity of either poly-crystalline or
nanowires grown in this study, TEM and SAED analyses were
dis-tances of 0.264 nm and 0.484 nm can be observed for the hydrothermally grown ZnO nanowires The TEM image shown in theFig 2(a) reveal a well-oriented crystal structure for the grown
con-firms that the grown ZnO nanowires have single crystalline struc-ture with relatively high crystallinity
A
Fig 5 A schematic of the power generation performance evaluation setup.
Trang 5Crystalline characteristic of ZnO nanowires is especially
essen-tial in this study, because effective piezoelectric conversion could
hardly be obtained for ZnO nanowires with amorphous or inferior
crystalline property: (1) piezoelectric field would be formed by
io-nic charges due to polarization of atoms in lattice; (2) charge
car-riers generated by piezoelectric mechanism could not be
effectively transferred to electrode The superior crystalline
for eventual improvement of power generation performance for
the resulting nanogenerator device
In addition, step-coverage of the deposited PZT thin films on the
ZnO nanowires would have also an important role to achieve
shows a SEM image of the PZT coated ZnO nanowires Observing
the cross-sectional image of the hetero-junction of ZnO
nano-wires/PZT, the sputter deposited PZT thin films cover almost entire
surface of the underlying ZnO nanowires even in those of bottom
area, although some minute portion of the bottom area is not still
covered by the PZT thin film, which could possibly influence the
current generation property of the resulting nanogenerator It
could be expected that the obtained step-coverage of the PZT thin
films would contribute to a wider surface area of the PZT due to
underlying ZnO nanowires possibly working as a template, so that
quantity of generated charge carrier could be increased
In this study, the deposited PZT thin film was annealed by
post-deposition thermal treatment in an electric oven in order to
im-prove crystallinity of the as-deposited PZT, which normally still
in an amorphous state To obtain an appropriate annealing condi-tion, temperature and treatment time were varied for the
ef-fect of the temperature and treatment time on crystalline proper-ties of the resulting PZT thin films It can be confirmed that the PZT thin films prepared in this study has a typical perovskite crystalline
crys-tallization of the PZT thin films cryscrys-tallization is started at anneal-ing temperature of 500 °C Higher crystallinity could be expected for higher annealing temperature, but a moderate annealing tem-perature of 600 °C was decided in this study regarding following factors: (1) the substrate (Corning glass) used in this study could not sustain high temperature of over 700 °C; (2) property of the lower electrode (ITO) could be eventually degraded at higher
crystalline properties of the PZT thin films at the annealing tem-perature of 600 °C It confirms that annealing time would have
no impact on the crystalline properties of the PZT thin films There-fore, an annealing time of 1 h was decided for the post-deposition thermal treatment for PZT thin films to fabricate the nanogenerator device with hetero-junction of ZnO nanowires/PZT
Dipole moments of grain domains in crystallized PZT are nor-mally not unidirectional A poling process for unidirectional orien-tation of the dipole moments is therefore inevitable: anisotropic
polariza-tion) before poling can be changed to isotropic after appropriate
0 100 200 300 400
Time [s]
(c)
Fig 6 Current generation property of the nanogenerator devices: (a) device with pristine ZnO nanowires; (b) device with pristine PZT thin film; (c) device with hetero-junction structure of ZnO nanowires/PZT.
Trang 6ing For thin films and bulk type piezoelectric materials, direct
electrode contact method is favorable for unidirectional
orienta-tion of the dipole moments However, the piezoelectric material
prepared in thin study has rather a nearly one-dimensional
struc-ture, and the direct electrode contact poling would cause
non-uni-form distribution of electric field or breakdown Therefore, a
Corona poling method was used in this study for poling the
het-ero-junction of ZnO nanowires/PZT
Fig 5shows a schematic of the power generation evaluation
setup used for the nanogenerator device fabricated in this study
The samples were shielded from outside interference in a shield
box Spacer between the bottom and top electrode can be served
as a mechanical buffer at applied force, so that the sample being
measured could be protected from mechanical damage and a space
could be secured for the measurement A defined stable
mechani-cal force was applied by a linear motor successively, and generated
strain of active layer due to applied mechanical force would be
re-sulted in a chare generation The generated charges could be
trans-ferred and collected to electrodes
Fig 6 shows current generating properties of three different
nanogenerator devices Current generating properties of ZnO
(Fig 6b) were also investigated to compare the performance of the
nanogenerator with hetero-junction structure of ZnO nanowires/
and PZT thin films were used The nanogenerator with ZnO
nanowires shows the smallest average currents of 0.5 nA, and the
nanogenerator with PZT thin films shows average currents of
approximately 9 nA On the contrary, the nanogenerator with the
hetero-junction structure of ZnO nanowires/PZT reveals
distinc-tively improved average currents of 270 nA
The quite improved current generating property could be caused
firstly by effects reported for hetero-junction structure based
nanogenerators through enhanced polarization and capacitance
area of PZT thin films could be contributed to increase of generated
currents Compared to a two-dimensional structure of thin film
piezoelectric material, one-dimensional piezoelectric structure
could have increased surface area It could be thought that the
PZT thin films could be quasi one-dimensional possibly by a
tem-plate effect of the underlying ZnO nanowires, which might have
quite larger surface area comparable to a one-dimensional
nano-wires The increased surface area of the piezoelectric PZT layer
would produce increased charge carriers, and hence, currents
pro-portional to increased surface area Thirdly, a larger energy
genera-tion could be possible for the quasi one-dimensional PZT
piezoelectric thin film coated on the ZnO nanowires than that of
PZT thin films with flat surface structure, because smaller applied
force could attribute to larger deformation It is generally known
that current generation in piezoelectric material is proportional to
E = Young’s modulus of piezoelectric element, F = applied load,
4 Summary and conclusions
In this report, we tried to fabricate an alternative high
perfor-mance nanogenerator device with distinguished structure
Repre-sentative piezoelectric materials of ZnO nanowires and PZT thin
films were tried to be combined to form a hetero-junction
struc-ture for fabrication of alternative nanogenerator device to possibly
obtain a synergy effect and then improved performance The ZnO
nanowires were grown by a hydrothermal synthesis technique
nanowires with careful consideration of step-coverage, so that the deposited PZT thin films would have a quasi one-dimensional structure with increased surface area due to a template effect of the underlying ZnO nanowires The PZT thin films were annealed
to be crystallized and the hetero-junction structure was polarized
by a Corona poling process to obtain a unidirectional orientation
of dipole moments to enhance their piezoelectric property The grown ZnO nanowires were confirmed to have well-oriented single crystalline structure and vertically preferred growth features The PZT thin films prepared by rf magnetron sputtering process re-vealed a typical perovskite structure after post-deposition anneal-ing treatment The nanogenerator device with a hetero-junction structure of ZnO nanowires/PZT revealed distinctively improved average currents of 270 nA, which is quite higher than average cur-rents of 0.5 nA for the nanogenerator device with pristine ZnO nanowires and average currents of 9 nA for the nanogenerator de-vice with pristine PZT thin films as active piezoelectric component, respectively It was confirmed that the concept and preparation processes for the hetero-junction structure of ZnO nanowires/PZT presented in this report was promising for performance improve-ment of nanogenerator device
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science, and Technology (No 2012-0001596) One of the authors (D.-Y Jeong) thanks the financial support of an NRF funded by the Korea government (MEST, 2011-001095)
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