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

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