Khakpoor et. al.: Optical Properties Improvement TiO2 Thin Films with Adding the Au, Ag or Cu Nanoparticles  

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International Materials Physics Journal vol. 1 No.2 2013 8-13

Optical Properties Improvement TiO2 Thin Films with Adding the Au, Ag or Cu Nanoparticles A. A. Khakpoor1, R. Borjian1, M. Hoseinzade2 Abstract In this study, we used TiO2 thin films doping with Au, Ag or Cu nanoparticles to increase absorption efficiency in a solar cell system. RF Sputtering method was used to prepare four different samples of thin films (TiO2-Au, TiO2-Ag, TiO2-Cu, and TiO2-Au-Ag-Cu). In fact, nanoparticles with different sizes can absorb different wavelength, in preparing samples we used nanoparticles with different grain size in the range of 1-50nm to increase the light absorption by cells. The UV–vis spectra were obtained by UV–Visible spectrophotometer to determine absorption, bad gap variation and conducting of thin films. X-ray diffractometer (XRD) was used to characterize the crystalline structure of the films. In addition, surface morphologies of the films were observed by scanning electron microscopy (SEM). Results show a high absorption efficiency of our samples to compare to other studies. Moreover, sample D (see Table 1) among all of three different nanoparticles has the highest absorption efficiency. Key words: Thin films, Titanium dioxide, RF Sputtering, Nanoparticles, Absorption efficiency 1 . Islamic Azad University-Central Tehran branch, Tehran, Iran 2

. Payam Noor University, Mashhad ,Iran

1. Introduction Semiconductor has a wide range of phenomena respect to conductor and insulator properties, nowadays is interested in different applications in particular physics and electronics technology. .Interesting characteristics and fascinating applications of 2D structures cause especial attention to thin film layers. Titanium dioxide is a white powder with three different phases of anatase, rutile and brookite .TiO2 band gap is about 3.2 eV (for anatase phase) and is about 3.0 eV (for rutile phase) and so it can absorb light in the ultraviolet range. Also, TiO2 is a n-type semiconductor and so it has some interesting characteristics like strong oxidizing power, photoinduced hydrophilicity, nontoxicity and long-term photostability. Titanium dioxide (TiO2) remains one of the most

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promising materials because of its high efficiency, low cost, chemical inertness, ecofriendly nature and Photostability (Fujishima and Hlonda 1927) In optic applications, thin film layers with thickness less than light wavelength (100nm) are used for manufacturing of reflector and transmissive devices [1]. However, the widespread use of TiO2 is hindered by its low utilization of solar energy and self-cleaning properties in the visible region (about 3.5 %) because of the wide band gap TiO2. Therefore, many efforts were concentrated on narrowing the optical band gap of Titania. In order to improve thin layer properties, they are doped with different metal nanoparticles (Fe, Pb, Au, Ag, Sn etc.) and these doped thin layers are investigated with a view to electrical, photocatalytic, hydrophilicity, optical properties, [2-5]. Doping with metal or non-

International Materials Physics Journal

Int. Mat. Phys. J. Vol. 1 No. 2 December 2013 

 

metal ions were resulted in considerable increase in the photocatalytic activity. In particular, the deposition of silver in Titania has been of considerable interest for both mechanistic [3]. Variety of nanocomposite semiconductor materials has been synthesized in recent years to improve the selectivity and efficiency of photocatalytic processes. As an example, it can be pointed to silver nanoparticles investigations in increasing absorption coefficient of TiO2 thin layers [6-11]. In [10] that Ag–TiO2 nanocomposite thin film was prepared on quartz substrates by Laser Pulse Deposition (LPD) method, a transition of the absorption peak to red shift and increase absorption compare to pure TiO2 thin film was observed. Effects of doping TiO2 thin film with Cu nanoparticles was investigated in [12]. The results have shown a significant increase in absorption coefficient in the range of 550nm to 650nm. Moreover, the effects of Au nanoparticles in different dimensions on TiO2 thin film properties have been investigated [13-14], which is caused increase absorption coefficient in the range of 500nm to 600nm. One major concern in manufacturing of solar cells is the improvement absorption coefficient of them. Various metal nanoparticles with different sizes have different color and each of them absorb especial wavelengths of light. So, we are trying to increase the absorption coefficient of photocatalytic activity by doping TiO2 thin films with various metal nanoparticles with different sizes. In addition to solar cells, these thin films are used for detectors, visible and ultraviolet light emitters, laser internal mirrors, and etc.

given through momentum transfer, being doped or pure target is not important. TiO2 powder that was used in this work was prepared form Kimia Eksir Company. The dopant is added to TiO2 powder in the weight percentage as Table 1. Four different samples (TiO2-Au, TiO2-Ag, TiO2-Cu, and TiO2-AuAg-Cu) were prepared. Table 1: weight percentage in different samples.

Sample

Tio2

Cu

Ag

Au

A

86%

14%

-

-

B

80%

-

20%

-

C

63%

7%

10%

20%

D

84%

-

-

16%

Each sample in the Table prepared as a target and placed in the vacuum chamber. The vacuum system was pumped to final pressure of 2×10-5 Torr. Then the sputtering was carried out, layer was deposited and the thickness was measured during the process. A summary of coating parameters was presented in Table 2. Finally, a TiO2 thin film with an average thickness of 150nm was coated on glass substrate in 2×5 cm slide. The pure TiO2 sample was transparent, but the other samples were slightly dark and opaque. Table 2: coating parameters Substrate

Power

Final

Primary

temperature

(W)

vacuum

vacuum

(Torr)

(Torr)

(0C)

25 2. Experimental For deposition of x–TiO2 composite films, LPD sputtering method in the presence of argon gas was used. Impurities (gold, silver and copper) added in a defined amount of the TiO2 powder to utilize high optical and functional properties. Therefore, using LPD methods causes the materials that evaporate faster, cover the substrate and prevent to obtain homogeneous layers with special percent of materials. Accordingly, sputtering method was chosen as an appropriate method in this work. Because the required energy to surface atoms off the target is

100

2×10

-5

10-6

In order to analysis, surface morphologies of thin films, SEM (Philips XL30) images were prepared. The UV–vis spectra in the range of 200 to 1100nm were obtained by UV–Visible spectrophotometer to determine the absorption and conducting of thin films. Also, XRD (Philps model PW1800) pattern of samples was prepared by an X-ray diffractometer to study the crystalline structure of thin films. 3. Results and discussion

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Khakpoor et. al.: Optical Properties Improvement TiO2 Thin Films with Adding the Au, Ag or Cu Nanoparticles  

3.1. Surface morphology analysis SEM images of samples were presented in Fig.   1. It could be observed that particle sizes in all samples are in nano range. However, there is a different size of particle in each sample. These size differences could be related to the variable pressure of the container during the coating process. As pointed previously, existence of particles with different sizes causes each range of particles absorb better a special wavelength of light and it lead to prepared samples absorb a wider range of wavelength. Sample C shows a layer with rough surface morphology and compact structure. So, it is more effective in optic process. Of course, this rough surface morphology and compact structure could be observed slightly in other samples.

Fig.1-c: SEM Image of sample C

Fig.1-d: SEM Image of sample D

Fig.1-a: SEM Image of sample A

Fig1-b: SEM Image of sample B

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3.2.XRD analysis An X-ray diffractometer (XRD) was used to characterize the crystalline structure of the films. The accelerating voltage and the applied current were 40 kV and 30 mA, respectively. Fig. 2 shows XRD patterns of thin films. XRD patterns for pure TiO2 include just anatase peaks. However, as could be observed in Fig. 2, all TiO2 thin films doped with Au, Ag, and Cu nanoparticles are amorphous. Therefore, the pure TiO2 thin film is anatase while the other samples are amorphous. Fig. 3 shows the UV-vis spectra of TiO2 and xTiO2 thin films. As could be observed, all sample absorption coefficient is in range of ultraviolet and more. However, this enhancement is more in samples C and D. As pointed before,

International Materials Physics Journal

Int. Mat. Phys. J. Vol. 1 No. 2 December 2013 

 

absorption was used to calculate thin films band gap. One of the simplest and most regular methods to calculate optical constants is the use of absorption and transmittance spectra of a film with a certain thickness. The optical absorption coefficient α, is defined as, . A (1) α where (2)

3.3.Optical properties analysis TiO2 and Au particles, especially in the range of nanometer, have intense photocatalytic property. So, this enhancement could be related to presence of Au nanoparticles in these two samples. Also, comparison among all samples has shown sample C has the most absorption coefficient. Sample C is included all three kinds of impurities (Au, Ag and Cu) and this enhancement could be related to properties of different impurities in absorption of different wavelengths. In addition, Fig. 3 shows that the particle size distribution leads to wide adsorption peaks.

where I0 is the intensity of incident light to the thin film with thickness of t and I is the intensity of transmitted light. Optical absorption coefficient and optical energy band gap Eg, is related by theory of optical absorption as, (3) where is the energy of indicated photon and k is a constant. Also, n is a constant that its value for semiconductor with direct and indirect energy band gap is 1 and 3, respectively. In Eq. is zero, the energy band gap (3) when will be equal the energy of indicated photon. So, the energy band gap of film could be calculated by extrapolating of curve, and also determined that the energy band gap is direct or indirect. Fig. 4 shows curves for our samples. Because the curves are liner it could be concluded that these semiconductors have direct energy band gaps and their values could be calculated by extrapolating for 0. The energy band gap for our samples have presented in Table 3. According to the results, doping TiO2 thin films with impurities has reduced energy band gaps and the most reduction is related to sample D. Energy band gap reduction of thin film improves its conductivity and electrons will move more easily.

Fig.3: the UV-vis spectra of TiO2 and x-TiO2 thin films

Knowing optical constants of thin films are very important in using these thin films for photoelectric devices and waveguides. There are various methods to calculate optical constants of thin films. In this study, basic theory of

Fig.4-a:

-

curve for sample A

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Khakpoor et. al.: Optical Properties Improvement TiO2 Thin Films with Adding the Au, Ag or Cu Nanoparticles  

 

Fig.4-b:

-

curve for sample B

comparison among all samples has shown sample C, that has included all three kinds of impurities, has the most absorption coefficient. In general, the doping with metal impurities lead to reduction of energy band gap and the most reduction is related to sample D. Energy band gap reduction of thin film improves its conductivity and electrons will move more easily. References

Fig.4-c:

-

curve for sample C

Fig.4-d:

-

curve for sample D

Table3: The energy band gap for our samples Sample

Band Gap (ev)

TiO2

3.8

A

3.2

B

3.6

C

3.3

D

2.6

4. Conclusions In this study, pure TiO2 and doping with Au, Ag and Cu nanoparticles thin films were coated on glass substrates by sputtering method. Absorption coefficients of all samples are in range of ultraviolet and more. However, because of a photocatalytic property of Au nanoparticles that were used in samples C and D, the enhancement is more for them. Also, 12   

[1] Leaverand K.D., Chapman B.N., ''Thin films'', The wykeham science, 1971. [2] Rahman M. M., Krishna K. M., Soga T., Jimbo T. and Umeno M., ''Optical properties and X-ray photoelectron spectroscopic study of pure and Pb-doped TiO2 thin films'', Journal of Physics and Chemistry of Solids, Vol.60, 201210. 1999. [3] Mardare D., Tasca M., Delibas M., and Rusu G. I., ''On the structural properties and optical transmittance of TiO2 r.f. sputtered thin films'', Applied Surface Science, Vol.156, 200-206, 2000. [4] Sonawane R. S., and Dongare M. K., ''Sol– gel synthesis of Au/TiO2 thin films for photocatalytic degradation of phenol in sunlight'', Journal of Molecular Catalysis, Vol.243, 68-76, 2006. [5] Wang S. F., Hsu Y. F., Lee Y. S., ''Microstructural evolution and optical properties of doped TiO2 films prepared by RF magnetron sputtering'', Ceramics International,Vol.32, 121–125, 2006. [6] He J., Ichinose I., Kunitake T., Nakao A., ''In Situ Synthesis of Noble Metal Nanoparticles in Ultrathin TiO2−Gel Films by a Combination of Ion-Exchange and Reduction Processes'', Langmuir, Vol.18(25), 10005–10010, 2002. [7] He C., Xiong Y., Chen J., Zha C., Zhu X., ''Photoelectrochemical performance of Ag– TiO2/ITO film and photoelectrocatalytic activity towards the oxidation of organic pollutants'', Journal of Photochemistry and Photobiology A: Chemistry, Vol.157(1), 71-79, 2003. [8] Arabatzis I.M., Stergiopoulos T., Bernard M.C., Labou D., Neophytides S.G., Falaras P., ''Silver-modified titanium dioxide thin films for efficient photodegradation of methyl orange'',

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Applied Catalysis. B, Vol. 42(2), 187-201, 2003. [9] Szabó-Bárdos E., Czili H. and Horváth A., ''Photocatalytic oxidation of oxalic acid enhanced by silver deposition on a TiO2 surface'', Journal of Photochemistry and Photobiology A, Vol.154(2-3), 195-201, 2003. [10] Yu J., Xiong J., Cheng B., Liu S., ''Fabrication and characterization of Ag–TiO2 multiphase nanocomposite thin films with

enhanced photocatalytic activity'', Applied Catalysis B: Environmental,Vol.60(3-4), 211– 221, 2005. [11] Su W., Wei S.S., Hu S.Q. and Tang J.X., ''Preparation of TiO2/Ag colloids with ultraviolet resistance and antibacterial property using short chain polyethylene glycol'', Journal of Hazardous Materials, Vol.172(2-3), 716-720, 2009.

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