Low-temperature growth and optical properties of ZnO nanorods on platinum
substrate
M. A. Bakar 1, M. A. A. Hamid1*, A. Jalar2, R. Shamsudin1
School of Applied Physics, Faculty of Sciences and Technology
2
Institute of Microengineering and Nanoelectronics
Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor D. E., Malaysia.
*Corresponding author email: azmi@ukm.my
1
Abstract
A simple solution route employing the reaction of zinc nitrate hexahydrate
(Zn(NO3)2·6H20) and hexamethylenetetramine (C6H12N4) has been demonstrated to
successfully grown aligned hexagonal rod of ZnO on platinum substrate. The general
morphological observations revealed that the distributed uniform hexagonal rods have
obtained on the platinum substrate surface from 1.5 h to 3 h growth time. Sharper and
higher intensity of XRD peaks was observed for ZnO thin film deposit at 3 hour compared
to 1.5 hour growth time. EDX has confirmed the pure ZnO has obtained due to only Zn and
O present in the sample. The optical band gap for ZnO thin film deposited for 3 h is 3.30
eV which is slightly near to the bulk value of ZnO, 3.37 eV. This result present that the
platinum substrate has influence of ordering an array hexagonal rods of ZnO.
Keywords: Thin film; Platinum substrate; Zinc oxide; Aqueous chemical growth; Uniform
hexagonal rods
1.
Introduction
With a direct bandgap of 3.37 eV at room temperature and a large exciting binding energy
of 60 meV which is higher than thermal energy at room temperature, 27 meV [1], Zinc
oxide (ZnO) has been recognized as a promising semiconductor material for electronic and
optoelectronic devices. Various synthesis methods have been used to fabricate ZnO
nanostructures such as thermal evaporation [2], molecular beam epitaxy (MBE) [3],
chemical spray pyrolysis [4], chemical vapor deposition (CVD) [5], etc. These growth
techniques are complicated and require high temperature growth conditions. However, the
chemical solution route has become a promising approach for the large scale production of
nano/microscale materials because of it is less expensive, simple, fast, and requires a low
growth temperature [6]. In addition, the solution growth method can produce quality
nanostructures without using a metal catalyst with higher crystal quality [7]. In order to
avoids oxidation and corrosion of metallic substrates, low temperature deposition is used.
Up to now, aqueous chemical growth (ACG) approaches have gained in importance
because of cost efficient and low temperature growth technique. Postels et al. 2007 reported
that ACG process is based on a nucleation step followed by growth of ZnO nanorods in
aqueous solution at temperatures below 95ºC [8].
A lot of factors playing role for the ZnO nanorods growth such as concentration of
the aqueous solution [9-12], pH [13, 14], temperature [15] etc., that affect the final
morphology of the grown ZnO. Herein, we focus on the controllable fabrication of ZnO
nanorods aligned on platinum substrate without seed layer from 1.5 h to 3 h growth time
process by aqueous chemical growth.
It is well known that types of substrates also have a great influence on the
morphology of the ZnO structures. Moreover, growth of ZnO on platinum substrate has the
great advantage of the integration of the devices. In past decades, platinum has gained
special attention due to its high electrical conductivity, chemically inert [16], and stable and
hard to oxidize [17]. There are reports on ZnO nanostructures grown directly onto
conducting metal substrates such as Zn foil [18], Al [19] and copper [20]. One of the
advantages of growing ZnO directly onto a metal substrate is the formation of robust
electrical contact during growth.
The effect of using platinum substrate on surface morphology and optical properties
of ZnO thin films were investigated. The ZnO films were characterized by X-ray diffraction
(XRD) and field emission scanning electron microscope (FESEM) equipped with Energy
Dispersive X-rays (EDX) for their structural and surface morphology while the optical
measurements was carried out using UV-vis spectrometer. Our results provide clues on the
role of the formation of the array ZnO hexagonal rods.
2.
Materials and methods
ZnO structures were grown by the aqueous chemical growth method on Pt substrate using
an equimolar (0.05 M) aqueous solution of zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and
hexamethylenetetramine (C6H12N4) dissolved in the deionized water. The mixed solution
was magnetically stirred until complete dissolution for one hour. The glass substrates used
in the experiment were cleaned with acetone and ethanol followed by ultrasonic cleaning in
deionized water for 15 min. The cleaned glass substrates were then coated with platinum,
before being immersed in the prepared solution. The substrates were then immersed in the
prepared solution and heated at 90 ºC for different growth time; 1.5 h, 2 h, 2.5 h and 3 h in
an oven without any stirring. Subsequently, the samples were removed and washed
thoroughly with deionized water to eliminate any residual salts and were then dried in air.
The samples were analyzed using field emission scanning electron microscopy (FESEM) to
determine the morphology of the ZnO nanostructures. The phase composition and crystal
structure were determined by X-ray diffraction (XRD) while the element composition of
the zinc oxide nanostructures was examined using energy dispersive X-ray spectroscopy
(EDX).
3.
Result and discussion
Morphology and microstructure of the prepared ZnO were determined by using FESEM.
Fig.1 shows the typical FESEM images of ZnO nanostructures at different growth time.
The 50 kx-magnification of FESEM image in Fig. 1(a) shows the morphology of ZnO
grown at 90 ºC for 1.5 h. It was observed that uniform distribution of ZnO nanorods on the
substrate surface. The nanorods size increase when increasing the growth time. This is
because of “Ostwald ripening” reported by Krichevsky and Stavans [21]. The diameter of
the nanorods from 1.5 h to 3 h is between 0.1-0.2 µm. The length increases with the growth
time from 1-2 µm for 1.5 h and become 1-4 µm for 3 h growth time.
Fig. 1. 50 kx-magnification FESEM images of the ZnO nanorods grown on Pt coated glass
substrate at different growth time: (a) 1.5 h; (b) 2 h; (c) 2.5 h and (d) 3 h.
Various synthesis methods have been successfully grown array ZnO nanorods
structure on various types of substrates. However, the methods they used to obtain the array
structure of ZnO require special experimental condition such as ZnO coated seed layer
substrate [22] and long reaction time such as 12 hour [10,22]. Uekawa et al [23] reported
that the ZnO rod arrays grow because of the decomposition of some soluble species of zinc
hydroxide on the surface of ZnO seeds. The ZnO is a wurtzite structure and exhibits partial
polar characteristics [24]. Tian et al. [25] reported that these wurtzite structure having a
tendency to grow into rod-like structure. It is well known that the shape and size of
inorganic nanostructures have much influence on their properties.
In our experiment, ZnO nanostructures were formed in the weak acid condition. The
pH value of solution decreased from 6.37 to 5.73 for 1.5 h h growth time and become 5.49
for 2 h and finally 5.25 for 3 h. Li et al. reported that the base concentration decreased
because of evaporation of ammonia in the aqueous solution due to long hour experiment
[26]. The decomposition in the aqueous solution can be expressed as follows [27]:
C6H12N4+10H2O → 6HCHO+4NH3·H2O
Zn2+ +4NH3·H2O → [Zn(NH3)4]2+ + 4H2O → Zn(OH)2+4NH4+ + 2OHZn(OH)2 → ZnO+H2O
(1)
(2)
(3)
In the weak acid condition of the aqueous solution, the hexamethylenetetramine
(HMT) decomposed to aldehyde and ammonia (Eqn.(1)). To produce a large number of
Zn2+ amino complexes in the solution, the ammonia combines with the Zn2+. The Zn2+
amino complexes were then hydrolyzed (Eqn.(2)) and Zn(OH)2 were formed, which
subsequently formed ZnO nuclei on the substrate (Eqn.(3)); thus ZnO film grew from the
nuclei.
The energy dispersive X-ray spectrum (EDX) of ZnO grown on Pt substrate for 1.5
h, 2 h, 2.5 h and 3 h are shown in Fig. 2. The values of weight % for Zn increased with the
increasing of growth time from 62.81 % for 1.5 h, 64.56 % for 2 h, and 71.93 % for 2.5 h
and become 72.37 % for 3 h growth. The EDX spectrums show that only Zn and O atoms
peaks were detected. The Pt peak observed is due to Pt substrate used in the experiment. No
characteristic peaks from impurities were detected implying that the ZnO obtained were
pure.
Fig. 2. EDX spectra of ZnO grown on the Pt coated glass substrate for (a) 1.5 h; (b) 2 h; (c)
2.5 h and (b) 3 h.
Futhermore, the crystallinity and crystal structures of the grown ZnO thin film were
examined by X-ray diffraction (XRD) as shown in Fig. 3. The observed indexed peaks in
this XRD pattern are fully matched with the corresponding pure wurtzite structures of ZnO
with lattice parameter of a = 3.24982 Å and c = 5.20661 Å (JCPDS Card No. 36-1451). A
few peaks existed at 2θ = 40° (111) and 2θ = 45° (200) corresponding to the Pt substrate
used in the experiment. The peaks of Pt are clearly seen for 1.5 h and 2 h growth time.
Notably, the relative peaks of the (100), (002) and (101) started to appear from 1.5 h
growth time in Fig. 3(a) and become sharper and higher in intensity with the increasing the
growth time. This implying that the ZnO thin film are well crystalline in Fig. 3 (d)
compared to the peaks in Fig. 3(a). Yamada et al. 2004 has reported that the good
crystallinity of ZnO obtained on Pt surface was due to small lattice mismatch between ZnO
c-plane with Pt (111) plane is about 1.4 %. This is important in bulk acoustic resonator
application which requires good electrical characteristics that depend on the crystallinity of
ZnO thin film [17].
(100)
(101)
Intensity (a.u)
(002)
(d) 3 h
(100)
(c) 2.5 h
Pt (111)
(100) (101) Pt (111)
Pt (200)
(002)
(b) 2 h
15
(101)
(002)
(a) 1.5 h
10
(110)
(102)
Pt (111)
Pt (111)
(100)
20
25
30
Pt (200)
35
40
45
50
55
60
2 Theta (deg)
Fig. 3. X-ray diffraction patterns of ZnO thin film grown on platinum substrate for: (a) 1.5
h; (b) 2 h; (c) 2.5 h and (d) 3 h.
Fig. 4 shows the optical transmission spectra for ZnO thin film deposited on
platinum substrate for 3 h.. The transmission value being around 55 % shows that the ZnO
thin film has moderate transmission in UV region. The value of optical band gap can be
calculated by extrapolating of the linear portion of ahv2 vs photon energy (hv) graph. The
optical band gap for ZnO thin film was found to be 3.30 eV which slightly near to the bulk
value of ZnO 3.37 eV.
Fig.4 Optical transmission spectra and Plot of (αhν)2 vs. hν of the ZnO thin film on
platinum substrate for 3 h.
4.
Conclusions
In summary, an aligned of hexagonal rods od ZnO have been successfully produced by
aqueous chemical growth on platinum substrate without using any seed layer. Overall, array
ZnO rods morphology is observed from 1.5 h to 3 h growth time, the only difference being
the diameter and the length size of the rod structures. All of EDX spectrums show that only
Zn and O atoms peaks were detected. The Pt peak observed is due to Pt substrate used in
the experiment. The optical band gap for ZnO thin film deposited for 3 h is 3.30 eV which
is slightly near to the bulk value of ZnO, 3.37 eV. This result present that the platinum
substrate has influence on the morphology and ordering of array hexagonal rods of ZnO.
Acknowledgement
The authors acknowledge the financial support from the research grant, UKM-RRR1-07FRGS0257-2010.
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