Post on 27-Jan-2017
Effects of RF Power on The Optical Properties of ZnO/TiO2 Nanocomposites Prepared by RF Magnetron Sputtering and Solution-
Immersion Method
N.A.M. Asib1 2, a, A.N. Afaah1 2, b, A. Aadila1 2, c, M. Rusop1 3, d, Z. Khusaimi1, e 1NANO-SciTech Centre, Institute of Science, Universiti Teknologi MARA, 40450 Shah Alam,
Selangor, Malaysia
2Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia
3NANO-ElecTronic Centre, Faculty of Electrical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia
aamierahasib@yahoo.com, bafaahabdullah@yahoo.com, caadilaazizali@gmail.com, dnanouitm@gmail.com, ezurai142@salam.uitm.edu.my
Keywords: RF power, Optical properties, ZnO/TiO2 nanocomposites, RF magnetron sputtering, Solution-immersion method
Abstract. Nanocomposites of ZnO/TiO2 were fabricated by two methods. Firstly, deposition of
TiO2 nanoparticles by Radio Frequency (RF) magnetron sputtering. Secondly, growths of ZnO
nanostructures on the TiO2 nanoparticles by solution-immersion method with aqueous solution of
Zinc nitrate hexahydrate as precursor solution and stabilizer hexamethylenetetramine (HMTA) in
water as solvent. The optical properties of ZnO/TiO2 nanocomposites were examined by
Ultraviolet-Visible (UV-Vis) spectroscopy, Raman spectroscopy and Photoluminescence (PL)
spectroscopy. UV-vis spectra of ZnO/TiO2 nanocomposites display high absorption in the UV
region and high transparency in the visible region. There is improvement in UV absorption for
ZnO/TiO2 nanocomposites compared to pure TiO2 due to imperfect alignment of ZnO
nanostructures. Raman analysis shows the presence of wurtzite hexagonal ZnO in all the films and
presence of anatase structure of TiO2 in the film deposited at 200 W. PL spectra of the films show
the emissions in the UV and visible regions. Intensity of PL emission in UV region (λ< 400 nm) is
maximum for film deposited at 200 W and minimum for film deposited at 300 W resulting from the
change in the surface state density. A broad peak from ~ 600-700 nm also was found for all the
films.
Introduction
Recently, worldwide attention have been attracted to metal oxide nanoparticles for their superior
photocatalytic properties that is important in environmental applications such as self-
decontamination of large areas including indoors and outdoors areas by decomposing organic
compound, viral, bacterial and fungal species that adsorbed on the surface, also by reducing air
pollution by means of decreasing the amounts of NOx and COx in air [1].
In this work, there are two different metal oxide materials used which are TiO2 and ZnO to
deposit ZnO/TiO2 nanocomposites. Both materials has many common properties that can make
ZnO as a suitable alternative to TiO2 especially when band-gap energy is concerned [2]. ZnO is an
n-type semiconductor with a direct band gap of 3.37 eV that corresponds to the emission in UV
region and has higher electron mobility (60 meV) at room temperature. TiO2 also has almost similar
band gap energy approximately 3.2 eV, which lies in the near UV region (388 nm) when compared
to ZnO. TiO2 is known to be a good candidate for the degradation of environmental contaminants
due to its high photocatalytic activity, low cost, non-toxic and excellent chemical stability [3].
However, TiO2 has lower photocatalytic efficiency compared to ZnO [2].
From previous study, many efforts have been made to modify electronic structure of metal oxide,
in order to enhance the activity of the catalyst [4] in the near-UV and visible spectral ranges,
because of the individual intrinsic photocatalytic activity of pure nano-TiO2 and nano-ZnO films or
Advanced Materials Research Vol. 832 (2014) pp 607-611Online available since 2013/Nov/21 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.832.607
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nanoparticles is considerably weak [1, 5]. For instance, selective doping such as nitrogen, boron and
carbon is used to control properties of metal oxides by increasing the absorption of visible light and
allowing for bandgap narrowing. Another method is by mixing two or more different kinds of
metals into oxide systems such as Co3O4–BiVO4, Cu2O–TiO2, RuO2–WO3 and TiO2–ZnO [1].
Here, ZnO will be deposited on TiO2 as a photocatalyst to enhance their photocatalytic activity at
longer wavelengths for large area applications. Although there were various chemical,
electrochemical, and physical deposition techniques have been used to integrate ZnO/TiO2
composite films, however, template-assisted approach has been proven to be effective for the
growth of ordered nanostructures [5].Therefore, to study the effect of RF power on the optical
properties of ZnO/TiO2 nanocomposites, we synthesized nanostructured zinc oxide on titanium
dioxide nanoparticles using combination method of RF magnetron sputtering and solution-
immersion method.
Experimental Procedure
The TiO2 nanoparticles were deposited on a glass substrate by using RF magnetron sputtering. A
sintered TiO2 (purity 99.99%, diameter 4 inch, thickness 0.25 inch) was used as a target. Glass
substrates were mounted at a distance of 22 cm from the target. Oxygen was used as reactive gas at
flow rate of 50 sccm, and argon was supplied as working gas at 2 sccm. The depositions were
performed at differents RF powers of 100 W, 150 W, 200 W, 250 W, and 300 W under constant
working pressure of 5 mTorr. All depositions were carried out for 1 hour. After deposition, the
samples were annealed for 1 hour at 450 ⁰C using annealing chamber to achieve crystallization.
The annealed samples were then immersed in centrifuged tubes which contain solution to grow
ZnO on TiO2 nanoparticles. The precursor solution is aqueous solution of zinc nitrate hexahydrate
(Zn (NO3)2.6H2O) and stabilizer hexamethylenetetramine, HMTA (C6H12N4) in water acting as the
solvent. To prepare the solution, it was stirred thoroughly with a magnetic stirrer for 1 hour at 60 ⁰C
and then aged for 24 hours. After that, the centrifuged tubes were placed in a water bath at 70-90 °C
for 4 hours. Nanostructured ZnO were grown on the TiO2 nanoparticles as ZnO/TiO2
nanocomposites. The samples were dried in oven and annealed for 1 hour at 500 ⁰C
Characterization techniques will revolve around UV-Vis spectroscopy, Raman spectroscopy and
temperature dependent (PL) emission spectroscopy.
Results and Discussion
Optical Studies. Fig. 1 (a) shows absorbance measurements as a function of wavelegth for TiO2
nanoparticles prepared by RF magnetron sputtering at various RF powers from 100-300 W
increased at 50 W. After deposition of ZnO on the TiO2 thin film by solution-immersion method,
the absorbance spectra were recorded as shown in Fig. 1 (b).
300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
2.5
Abso
rba
nce (
a.u
.)
Wavelength (nm)
100W
150W
200W
250W
300W
300 400 500 600 700 800
0
1
2
3
4
5
6
Ab
sorb
an
ce
(a
.u.)
Wavelength (nm)
100W
150W
200W
250W
300W
Fig. 1, UV-vis absorbance spectra of (a) TiO2 deposited at various RF powers (b) ZnO/TiO2
nanocomposites deposited at various RF power.
a b
608 Nanoscience, Nanotechnology and Nanoengineering
From Fig. 1 (b), the fabricated ZnO/TiO2 nanocomposites displayed good transparency in the
visible region (400-800 nm) and low transparency in UV region (below 400 nm), which means have
high absorption [3, 6]. The UV absorption properties also improved significantly in the ZnO/TiO2
nanocomposites compared to pure TiO2 nanoparticles due to the existence of ZnO that modifies the
optical absorption edge [7], where, there were trapped UV light caused by interface scattering. The
scattering increase the optical path length inside the film, thus improved the UV light absorption
[6]. At 250 W, both of spectra from Fig.1 (a) and (b) show highest absorption in UV region
compare to other samples due to roughness increasing effect by high energy sputtering [8, 9]
The improvement of UV absorption in the ZnO/TiO2 films indicates the reduction in
transmittance of the nanocomposites compared to the pure TiO2 films. The reason might be due to
imperfect alignment of ZnO nanostructures, which were distributed vertically and horizontally
towards the TiO2 nanoparticles surface. The way of this arrangement and less uniform dispersal of
ZnO caused the rough surface of the thin film and lead to the random scattering of the incident light
between the voids, the vertically and horizontally aligned ZnO nanostructures. As a result, the
scattering reduced the optical transmittance in the visible region. C. M. Firdaus et al. and Prabitha
B. Nair et al. reported that the high transmittance in visible could be achieved if the thin films have
smoother surface morphology, less grain boundaries and good homogeneity [10, 11]
Raman Studies. Raman spectra of the ZnO/TiO2 nanocomposites deposited at various RF powers
were displayed in Fig. 2. There was one main peak were observed at 444 cm-1
in all the samples and
another two small peaks were recorded in sample of 200 W at 143 cm-1
and 200 cm-1
.
200 400 600 800 1000
200 c
m-1
444 c
m-1
143 c
m-1
Inte
nsi
ty (
a.u
.)
Raman shift (cm-1)
100 W
150 W
200 W
250 W
300 W
Fig. 2, Raman spectra of ZnO/TiO2 nanocomposites deposited at various RF powers.
Raman active modes of vibration were observed at 143 cm-1
and 200 cm-1
which corresponding
to anatase phase [11] for the sample prepared at 200 W . There were additional peaks located at 444
cm-1
were observed in all the samples. For ZnO, the peak at 444 cm-1
represented the characteristic
peak of the Raman active mode of the wurtzite hexagonal ZnO or the oxygen atom in the ZnO
hexagonal structure [6, 12, 13]. Meanwhile, for TiO2 the peak is corresponding to rutile phase [11].
The highest peak was recorded in sample of 200 W, which related to the smallest size of
nanostructures in the film.
Photoluminescence Studies. Fig. 3 shows PL spectra of the ZnO/TiO2 nanocomposites, deposited
at different RF powers. The nature of PL spectrum is known depends on the method of fabrication
used [14]. Usually, PL spectra are featured by emissions in both UV and visible region. Defects
levels deep inside the band gap are trap levels for both electrons and holes which contribute to
emission in visible region.
Advanced Materials Research Vol. 832 609
300 400 500 600 700 800 900 1000
0
200
400
600
800
Inte
nsity (
a.u
.)
Wavelength ( nm)
100W
150w
200w
250w
300w
Fig. 3, PL spectra of ZnO/TiO2 nanocomposites deposited at various RF powers.
Intensity of PL emission in UV region (λ< 400 nm) is maximum for film deposited at 200 W and
minimum for film deposited at 300 W as shown in Fig. 3 above. Prabitha, Morta and Adachi have
reported that the decrease in PL intensity with RF powers is corresponds to a change in the surface
state density. Thus, non-radiative surface recombination is enhanced and leads to the
photoluminescence [11]. The result is consistent with the FESEM images that reported more
nanoparticles of TiO2 were deposited on a glass substrate at 200 W compared to 300 W [8]. A broad
peak from ~ 600-700 nm was found for all the films. Peak at 525 nm (green region) is hardly seen
which indicated decrease of oxygen defect in the films, as the peak is related to the oxygen defect
[14].
Conclusion
Nanocomposites of ZnO/TiO2 were fabricated by RF magnetron sputtering and solution-
immersion method in a glass substrate. The deposition of TiO2 nanoparticles were varied at different
RF powers from 100-300 W, with increment rate of 50 W followed by growth of ZnO on the films.
UV-vis spectra of ZnO/TiO2 nanocomposites displayed good transparency in the visible region
(400-800 nm) and low transparency in UV region (below 400 nm), which means high absorption.
The UV absorption properties improved significantly in the ZnO/TiO2 nanocomposites compared to
pure TiO2 nanoparticles due to the existance of ZnO that modifies the optical absorption edge. In
contrast, the improvement of UV absorption in the ZnO/TiO2 films indicated the reduction in
transmittance of the nanocomposites. The reason might be due to the imperfect alignment of ZnO
nanostructures on TiO2 nanoparticles. Then Raman analysis showed the presence of 444 cm-1
peak
which indicated the existence of wurtzite hexagonal ZnO and rutile TiO2 in all of the films. In
addition, the peak at 143 cm-1
and 200 cm-1
represented the anatase structure of TiO2 in the film
deposited at 200 W. Lastly, PL spectra of the films showed the emissions in both UV and visible
regions. Intensity of PL emission in UV region (λ< 400 nm) is maximum for film deposited at 200
W and minimum for film deposited at 300 W due to the change in the surface state density. A broad
peak from ~ 600-700 nm also was found for all the films.
Acknowledgement
We would like to express our gratitude to Research Management Institute, Universiti Teknologi
MARA (UiTM), Shah Alam, Selangor, Malaysia for financial support.
610 Nanoscience, Nanotechnology and Nanoengineering
References
[1] Nihan Kosku Perkgoz, Refik Sina Toru, Emre Unal, Mustafa Akin Sefunc, Sumeyra Tek,
Evren Mutlugun, Ibrahim Murat Soganci, Huseyin Celiker, Gulsen Celiker, and Hilmi Volkan
Demir, Photocatalytic hybrid nanocomposites of metal oxide nanoparticles enhanced towards
the visible spectral range. Applied Catalysis B: Environmental. 105 (2011) 77-85.
[2] Vendula Houskova, Vaclav Stengl, Snejana Bakardjieva, and Nataliya Murafa, Photoactive
materials prepared by homogeneous hydrolysis with thioacetamide: Part 2—TiO2/ZnO
nanocomposites. Journal of Physics and Chemistry of Solids. 69 (2008) 1623–1631.
[3] Mohammad Hossein Habibi and Maryam Mikhak, Titania/zinc oxide nanocomposite coatings
on glass or quartz substrate for photocatalytic degradation of direct blue 71. Applied Surface
Science. 258 (2012) 6745– 6752.
[4] Muzafar A. Kanjwal, Nasser A. M. Barakat, Faheem A. Sheikh, Soo Jin Park, and Hak Yong
Kim, Photocatalytic Activity of ZnO-TiO2 Hierarchical Nanostructure Prepared by Combined
Electrospinning and Hydrothermal Techniques. Macromolecular Research. 18 (2010) 233-
240.
[5] S. Benkara and S. Zerkout, Preparation and characterization of ZnO nanorods grown into
aligned TiO2 nanotube array. J. Mater. Environ. Sci. 3 (2010) 173-188.
[6] Mamat, M.H., et al., Fabrication of ultraviolet photoconductive sensor using a novel
aluminium-doped zinc oxide nanorod–nanoflake network thin film prepared via ultrasonic-
assisted sol–gel and immersion methods. Sensors and Actuators A: Physical. 171 (2011) 241-
247.
[7] Li ShiWang, MingWei Xiao, Xin Jian Huang, and Yan DanWu, Synthesis, characterization,
and photocatalytic activities of titanate nanotubes surface-decorated by zinc oxide
nanoparticles. Journal of Hazardous Materials. 161 (2009) 49-54.
[8] N. A. M. Asib, M.Z.M., S. Abdullah, and M.Rusop, Effect of RF Power on Optimization of
Titanium Dioxide Nanostructures by RF Magnetron Sputtering. Advanced Materials Research
Vol. 667 (2013) 104-109.
[9] Cheol Ho Heo, Soon-Bo Lee, and Jin-Hyo Boo, Deposition of TiO2 thin films using RF
magnetron sputtering method and study of their surface characteristics. Thin Solid Films. 475
(2005) 183– 188.
[10] Firdaus, C.M., et al., Characterization of ZnO and ZnO: TiO2 Thin Films Prepared by Sol-Gel
Spray-Spin Coating Technique. Procedia Engineering. 41 (2012) 1367-1373.
[11] Prabitha B. Nair, V.B.Justinvictor, Georgi P.Daniel, K.Joy, V.Ramakrishnan, and
P.V.Thomas, Effect of RF power and sputtering pressure on the structural and optical
properties of TiO2 thin films prepared by RF magnetron sputtering. Applied Surface Science.
257 (2011) 10869-10875.
[12] W.D. Yu, X.M.L. and X.D. Gao, Self-catalytic synthesis and photoluminescence of ZnO
nanostructures on ZnO nanocrystal substrates. Applied Physics Letters. 84 (2004) 2658–2660.
[13] S.K. Sharma and G.J. Exarhos, Raman spectroscopic investigation of ZnO and doped ZnO
films, nanoparticles and bulk material at ambient and high pressures. Solid State Phenomena.
55 (1997) 32-37.
[14] Z. Khusaimi, S.A., H. A. Rafaie, M. H. Mamat, N. Abdullah, S. Abdullah, and M. Rusop,
Photoluminescence and Structural Properties of Gold Assisted Zinc Oxide Nanorods.
Malaysian Journal of Science 28 (Special Edition). 28 (2009) 197-201
Advanced Materials Research Vol. 832 611
Nanoscience, Nanotechnology and Nanoengineering 10.4028/www.scientific.net/AMR.832 Effects of RF Power on the Optical Properties of ZnO/TiO2 Nanocomposites Prepared by RF
Magnetron Sputtering and Solution-Immersion Method 10.4028/www.scientific.net/AMR.832.607