Physical properties of Zn doped TiO2 thin films with

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 105–112

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Physical properties of Zn doped TiO2 thin films with spray pyrolysis technique and its effects in antibacterial activity A. Arunachalam a, S. Dhanapandian a,⇑, C. Manoharan a, G. Sivakumar b a b

Department of Physics, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India Centralised Instrumentation and Service Laboratory (CISL), Department of Physics, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

Zn-doped TiO2 thin films were

deposited on glass substrates by spray pyrolysis technique. XRD study confirms the Zn-doped TiO2 films are polycrystalline nature with tetragonal structure. From the optical study a slight shrinkage of band gap has been observed in the case of the film doped with Zn. The high transmittance of about 85% in the visible and near infra red regions. The doped TiO2 thin film yielded the high antibacterial activity against Bacillus subtilis.

Titanylacetylacetonate + Ethanol

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 28 August 2014 Received in revised form 29 October 2014 Accepted 5 November 2014 Available online 20 November 2014 Keywords: Sprayed nanocrystalline Zn-doped TiO2 thin films Porous nature Antibacterial activity

Spray Pyrolysis Technique

Titanium dioxide thin films plays an important role in various applications such as gas sensors and dye-sensitized solar cells, because of its high efficient photocatalytic activity, high refractive index, resistance to photo corrosion and chemical stability [1]. TiO2

http://dx.doi.org/10.1016/j.saa.2014.11.016 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

Antibacterial Activity

Zinc doped Titanium dioxide (TiO2: Zn) thin films were deposited onto glass substrates by the spray pyrolysis technique with the substrate temperature 450 °C. The structural, optical, photoluminescence (PL) properties and morphological studies were investigated for the films deposited with various doping concentration (0, 2, 4, 6 and 8 at.%) of zinc. The results of X-ray diffraction (XRD) had shown the presence of anatase peak with a strong orientation along (1 0 1) plane at 8 at.% of Zn-doped TiO2 film. Scanning electron microscopy (SEM) study showed the uniform distribution of grains with porous nature. Atomic force microscopy (AFM) observations indicated the tetragonal shape at 8 at.% of Zn-doped TiO2 with the particle size and decrease in surface roughness. The emission at 398 nm was observed at the 8 at.% of Zn-doped TiO2 thin film. The carrier concentration and Hall mobility was increased with doping. The antibacterial activity was highly yielded for the Zn-doped TiO2 thin films. Ó 2014 Elsevier B.V. All rights reserved.

Introduction

⇑ Corresponding author.

Films

exits in three main phases such as anatase (tetragonal), rutile (tetragonal) and brookite (orthorhombic) [2]. TiO2 thin films are also successfully used in photodecomposition of water, purification of environmental pollutants, antireflection coatings, ceramic membrane, and wave guide [3]. Antibacterial agents can be broadly classified into organic and inorganic. Organic antibacterial materials are often less stable particularly at high temperatures and/or pressures compared to inorganic antibacterial agents [4].

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A. Arunachalam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 105–112

Inorganic materials such as metal and metal oxides have attracted more attention over the past decade due to their ability to withstand harsh process conditions. They are generally regarded as safe materials to human beings and animals [5]. The inorganic materials such as silver, gold, and copper, CuO, TiO2, and ZnO have profound antibacterial activity. Antibacterial agents are used extensively in hospitals, healthcare settings, water and air purifications [6]. The most recent application of TiO2 thin film is the fabrication of dye sensitized solar cell (DSSC). Many researchers have used the pristine TiO2 thin film as photoanode in the fabrication dye-sensitized solar cell. To improve the efficiency of the TiO2 as a photoanode many modification methods such as noble metal doping, composite semiconductors and transition metal doping are used [7]. Ta doped TiO2 has enhanced the efficiency of DSSC upto 8.18% [8]. When Zn-doped TiO2 films are synthesized the microspheres are found and this leads to a significant increase of performance of DSSC than the cell based on undoped TiO2 [9]. The efficiency Zn-doped TiO2 thin films based DSSC have been reported scarcely. TiO2 thin films have been prepared by various techniques such as sputtering [2], sol–gel [10], pulse laser deposition [11], chemical bath deposition [12], and spray pyrolysis technique [3]. Among these, spray pyrolysis technique is economic, very simple and suitable for large area thin film preparation. In our present work, the properties of TiO2 thin films are tailored in order to enhance the optical, electrical properties for the fabrication of DSSC onto a microscopic glass substrate and to yield high antibacterial activity of the films with Zn as dopant.

Experimental procedure Zn-doped TiO2 nano thin films were deposited onto 75 25 mm2 microscopic glass substrates at the substrate temperature of 450 °C (optimized substrate temperature) by the spray pyrolysis technique. The spray solution was prepared from a mixture of 0.1 M titanyl acetylacetonate (Ti C10H14O5) (AR grade, 99.99% pure MERCK made) was dissolved in pure ethanol, and zinc acetylacetonate was added to starting solution as a dopant. The apparatus used for deposition of TiO2: Zn thin films were obtained by spray pyrolysis in air atmosphere from aqueous solutions in which the atomic ratio of Zn in the spray solution was varied from 0, 2, 4, 6 and 8 at.% of Zn (C10H14O5) (AR grade, 99.99% pure MERCK made) which were dissolved into the solution. It was stirred at 80 °C for 10 min. The transparency of the solution indicated the homogeneity. These prepared solutions were sprayed onto the glass substrates at the substrate temperature of 450 °C. Before preparation of films, the substrates were well cleaned by soap solution followed by HCl, acetone and distilled water. Finally, the cleaned substrates were dried in oven. The experimental set-up used for the spraying process consists of a spray head and heater which was kept inside a chamber having an exhaust fan which can remove the gaseous by products and solvent vapor. The substrate temperature was achieved with the help of heat which is controlled by an automatic temperature controller with an accuracy of ±5 °C. The uniform growth of the film was obtained by moving the spray head in the X–Y direction which was able to scan an area of 200 200 mm at a speed of 20 mm s1 and in steps of 5 mm s1 in the X–Y plane direction respectively. In this unit, the flow rate of the solution was controlled by the stepper motor attached to the solution container. The carrier gas used in this experiment was air. The pressure of the carrier gas was maintained with the help of mechanical gauge. The entire unit is connected to computer with a help of a serial port. The spray parameters were stored in the computer. After deposition, the films were allowed to cool slowly to room temperature and washed with distilled water and then dried. The films were annealed for an hour at 500 °C.

The structural characterization of the deposited films were carried out by X-ray diffraction technique on SHIMADZU-6000 0 (monochromatic Cu Ka radiation, k = 1.5406 Å A). The XRD patterns were recorded in 2h interval from 10° to 90° with the steps of 0.05° at room temperature. The surface morphology was studied by using SEM (JEOL-JES-1600) with a magnification of 15 kv 10,000 1 lm. Optical absorption spectrum was recorded in the range of 300–1200 nm using JASCO V-670 spectrophotometer. The photoluminescence spectrum (PL) was studied at room temperature using prolog 3-HORIBAJOBINYVON with an excitation source wavelength of 375 nm. The surface topological studies were carried out using Atomic force Microscope (Nano surf Easy scan2) AGILENT-N9410A-5500. The electrical resistivity, carrier concentration and mobility were measured by automated Hall Effect measurement (ECOPIA HMS – 2000 version 2.0) at room temperature in a van der Pauw (VDP) four-point probe configuration. Antibacterial activity Preparation of test solution and disc The test solution was prepared with known weight of fractions in 10 mg/mL, dissolved in 5% dimethyl sulphoxide (DMSO). Sterile discs Himedia Ltd., Mumbai. (6 mm) were impregnated with 20 ll of the TiO2 and Zn-doped TiO2 (corresponding to 100, 200 and 300 mg/mL) allowed to dry at room temperature. Disc diffusion method The agar diffusion method (Bauer et al., 1966) reported [13] was followed for antibacterial susceptibility test. Petri plates were prepared by pouring 20 mL of Mueller Hinton Agar allowed to solidify for the use in susceptibility test against bacteria respectively. Plates were dried and 0.1 mL of standardized inoculums suspension was poured and uniformly spread. The excess inoculums were drained and the plates were allowed to dry for 5 min. After drying, the discs with TiO2 and Zn-doped TiO2 were placed on the surface of the plate with sterile forceps and gently pressed to ensure the contact with the agar surface. Gentamycin (30 mg/disc) were used as the positive control and 5% DMSO was used as blind control in these assays. Finally, the inoculated plates were incubated at 37 °C for 24 h. The zone of inhibition was observed and measured in millimeters. Results and discussions Structural properties The X-ray diffractograms of the pure TiO2 and Zinc doped TiO2 thin films deposited at substrate 450 °C are shown in Fig. 1. The result indicates that all of the films are polycrystalline with a tetragonal structure of anatase TiO2. All the diffraction peaks agreed with the JCPDS card no 21-1272. Pure TiO2 thin film depicts a sharp single phase of anatase peak of (1 0 1) plane. The spectra of the Zn–TiO2 film at 2 at.%, the intensity of (1 0 1) decreased which may be due to the partially crystallization of TiO2 [14]. Further increase of zinc (4 at.%) as a dopant, the peaks corresponding to the anatase phase TiO2 increases without the appearance of the secondary phase such as ZnTiO3. Therefore, it is noticed that at 4 at.% of Zn tends to stabilizes the orientation along (1 0 1) plane and enhances the intensity of the (1 0 1) plane. This stabilizing effect on the crystal structure of anatase phase indicates the improvement of crystallinity of anatase phase [15]. The peak of (1 0 1) plane increases with increase of 6 at.% zinc which can be attributed to influence of Zn2+ into TiO2 lattice structure [16]. A narrower peak of (1 0 1) plane at 8 at.% of Zn demonstrates that the films are preferentially oriented mainly along (1 0 1) direction and is due to strong influence of Zn i.e., for

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It is observed that as the doping concentration increases, the dislocation density also increases and it is attributed to the difference in ionic radii of Ti4+ and Zn2+. It is an indication for the better crystallinity of the film and which clearly show an improvement of the nanoscopic crystallinity of the film at 8 at.% of Zn- doped TiO2 film [20]. The micro stress of the prepared films are calculated using

e rstress ¼ E

ð4Þ

2

where E is the Young’s modulus of the material (282.76 GPa), e is the strain of the film [21]. From Table 1, it is noticed that negative values of stress are estimated in doped films. The negative sign indicates that the film is in compressive stress. The formation of the compressive stress in these film is attributed to the difference in size of Ti4+and Zn2+ [22]. The texture coefficient (TC) represents the texture of (1 0 1) plane and is defined by

IðhklÞ =I0ðhklÞ TCðhklÞ ¼ 1 P IðhklÞ =I0ðhklÞ n Fig. 1. XRD pattern of pure TiO2 and Zn-doped TiO2 thin films. 2+

4+

every 4 unit cells of anatase Zn replaces Ti atoms and other peaks of (1 1 1), (1 1 2), (2 1 1), (2 1 3) and (1 1 6) planes are also observed. The anatase TiO2 peak (1 0 1) become broad and weaker with increase of Zn concentration which is not shown in the XRD pattern. This is due to lack of arrangements caused by higher doping concentration and also the difference between the ionic radius of Zn2+ and Ti4+ [17]. The crystallite size D is calculated using Scherrer’s formula [1].

D¼

kk b cos h

ð1Þ

where k = 0.9 is the shape factor, k is the wavelength used (1.5405 Å), b is the full width at half maximum (FWHM) in radians and h is the Braggs angle. The crystallite size decreases which contributes to the kinetic segregation of dopants [7] whereas the crystallite size of 4 and 6 at.% of Zn thin films slightly vary. This can be attributed to the increasing of produced nucleation centers due to higher concentration of Zn atoms on the surface [18]. The strain (e) and dislocation density (d) are determined by using following relations [1]

e¼

b cos h 4

ð2Þ

where b is FWHM in radians and h is the Bragg angle. The strain of these films was found to be increased which may be due to the crystallization process in polycrystalline thin film. The dislocation density is defined as the length of dislocation lines per unit volume of the crystal, and it is estimated from the following relation using simple approach of Williamson and Smallman [19].

d¼

1

ð3Þ

D2

ð5Þ

where TC(hkl) is the texture coefficient, I(hkl) is the XRD intensity and n is number of diffraction peaks considered. I0(hkl) is the standard intensity of the plane (h k l) taken from JCPDS data. It is seen that TC(hkl) of higher concentration of zinc, shows the preferred orientation along (1 0 1) plane with increased number of grains along the plane as well as good crystallinity and well adherent [23].

1 2

d

2

¼

2

2

h þk l þ 2 a2 c

ð6Þ

The lattice constants calculated for TiO2 thin films are shown in Table 1. The ‘a’ and ‘c’ values are in concordance with the standard values of TiO2 single crystals (a = 3.785 nm and c = 9.513 nm) which indicate that the quality of TiO2 films is good crystalline in nature. The lattice parameters a and c are less than the bulk which is strong indication of stress in the films. The similar trend is observed for Al doped ZnO thin films [14,24]. Morphological studies Surface morphology The SEM micrographs of pure TiO2 and Zn-doped TiO2 thin films are shown in the Fig. 2(a)–(e). It is observed that the films are uniform without cracks and with dense morphology that covers entire substrate surface area. The SEM image of pure TiO2 film Fig. 2(a) shows the homogeneous with spherical shaped micro crystals. As the doping concentration increased (2, 4 and 6 at.%), the grains become compact and uniformly arranged. The grains size of the higher doping concentration of Zn-doped TiO2 (8 at.%) is much lesser than that pure TiO2 (from the XRD analysis), which is due to the agglomeration of the grains in Zn-doped TiO2 and also exhibits porous nature of the film, which is suitable for the fabrication of DSSC [25].

Table 1 Structural parameters of pure TiO2 and Zn-doped TiO2 thin films. At.% of Zn

0 2 4 6 8

D (nm)

17.092 29.9902 12.7856 12.4909 4.9553

e 103 2.027 0.2889 2.711 2.775 6.9963

d 1014 lines/m2

3.423 11.118 6.1172 6.4102 4.0429

r (GPa)

4.396 1.753 1.498 1.429 1.597

TC

0.794 2.1428 2.4193 2.3622 2.5252

Lattice parameters (Å) a

c

3.782 3.734 3.741 3.743 3.738

9.512 9.534 9.534 9.532 9.534

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Fig. 2. SEM images of (a) pure TiO2, (b) 2 at.%, (c) 4 at.%, (d) 6 at.% and (e) 8 at.% Zn-doped TiO2 films.

Fig. 3. AFM images of (a) pure TiO2 and (b) 8 at.% of Zn-doped TiO2 films.


Surface topography Fig. 3(a) and (b) shows the AFM images of pure TiO2 and 8 at.% of Zn-doped TiO2 thin films. From the images, it is observed that the films do not have any voids or pin holes and are porous in nature. The image shows that the film is uniform, consisting of homogeneous and densely packed with small grains over the entire surface of the scanned area. The pure TiO2 thin film exhibits the spherical granular structure and the surface roughness of 9 nm (Fig. 3a). With higher doping of Zn-doped TiO2 (8 at.%), the film exhibits the clear tetragonal structure with a columnar arrangement of the grains. The 3D image of the doped film exhibits the mushroom like formation with the surface roughness of 5 nm (Fig. 3b). Optical properties Transmittance Fig. 4 depicts the transmission spectra of pure TiO2 and Zn-doped TiO2 in the wavelength range 300–1200 nm. The transmittance in the visible region for the pure TiO2 is over 75%. But as the doping concentration increase the optical transmission enhances upto 85%. The increase in the transmittance is attributed to the well-crystallization of films [26].

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Optical bandgap The optical band gap (Eg) for the pure TiO2 and Zn–TiO2 films was calculated on the basis of optical absorption by using

ahm ¼ Aðhm Eg Þn

ð7Þ

where a is the optical absorption co efficient, hm is the photon energy, A is a constant and n is the number that depends on the nature of transition and has values of ½ and 2 for direct and indirect transitions respectively. The typical plot of (ahm)2 vs. photon energy is depicted in Fig. 5. The band gap doped films are appreciably greater than bulk anatase band gap (3.2 eV) and this indicates the formation of nanosize particles. But the band gap decreases from the range of 3.69 to 3.50 eV and becomes sharp with higher doping concentration of zinc. The band gap becomes narrow than the TiO2 which is tailored by zinc element such as ZnO. This induces the occurrence of a red shift in transmittance as seen in Fig. 4 [7,27]. Extinction coefficient and refractive index The extinction coefficient (k) can be obtained from the relation,

k¼

ak 4p

ð8Þ

Fig. 6 represents the extinction coefficient vs. wavelength. From Fig. 6 it is noticed that the extinction coefficient of doped TiO2 decreases with increase of doping concentration. At 380 nm, the k value decreases which correspond to the optical band gap of anatase phase. The film exhibits homogeneity and better crystallinity of the films [28,29]. The refractive index is calculated at different wavelength using the relation [30]

n¼

1 þ R1=2 1 R1=2

ð9Þ

Fig. 7 represents the refractive index for the films deposited at 450 °C with different doping concentration. The refractive index is slightly higher than bulk anatase TiO2 (2.5) which can be attributed to the presence of higher porosity of thin films [26]. Photoluminescence (PL)

Fig. 4. Optical transmittance of pure TiO2 and Zn-doped TiO2 thin films.

The PL spectra of anatase TiO2 materials attributed the three kinds of physical origins such as self trapped excitons, oxygen vacancies and surface states. Most of the surface states are oxygen

Fig. 5. Variation of (ahm)2 vs. hm of the pristine TiO2 and Zn-doped TiO2 thin films.

Fig. 6. The variation of extinction coefficient of theTiO2 and Zn-doped TiO2 thin films with wavelength.

110


and 437 nm. As the doping concentration increases, the PL emission shifts towards lower range at 398 nm which corresponds to the excitonic PL peaks trapped by surface state. The peak intensity is comparatively higher than undoped film because when nucleation and crystallization of TiO2 nanocrystalline occurs, more electrons and holes excited form TiO2 nanocrystallites can be trapped by the defect levels and these lead to the increase in the emission peak intensity. The PL intensity of 8 at.% Zn doped TiO2 is lowest among the doped samples indicate the recombination of electrons and holes which result in higher photocatalytic activity of the film and in solar cell fabrication [27]. A drastic decrease in intensity of the PL peaks is observed for higher doping concentration shows that the crystallization of the film is deteriorated and it is not shown in Fig. 9. The similar observation is observed by Chen et al. for the Sn doped ZnO thin film [31]. Electrical properties

Fig. 7. The variation of refractive index of the pristine TiO2 and Zn-doped TiO2 thin films with wavelength.

Hall effect measurement are carried out at room temperature to measure the electrical properties such as resistivity, mobility, carrier concentration and conductivity of the undoped and doped TiO2 films. The pristine film exhibits a resistivity of 3.42 102 X cm and carrier concentration of 4.99 1021 cm3. When the film is doped with 8 at.% of Zn, the resistivity is decreased upto 2.03 102 X cm, which is due to improved crystallinity of the film, as observed from XRD results. The Hall mobility and conductivity is 2.14 1021 cm3 and 2.91 103 X1 cm1 for TiO2 and Hall mobility, conductivity and carrier concentration is 7.32 1021 cm3, 3.95 103 X1 cm1 and 12.11 cm2/Vs respectively for 8 at.% of Zn-doped TiO2. In addition Hall mobility, carrier concentration and conductivity also undergoes a sharp increase which is due to the weakened carrier scattering process and the improvement of crystallinity. According to the morphological studies, the film exhibits the homogeneous, porous nature, and the particles are densely packed which result in increase in electron mobility from grain to grain. These properties improve the mobility and conductivity of the doped film [27,32,33]. Antibacterial activity

Fig. 8. Photoluminescence emission spectra of TiO2 thin films with various Zn concentrations.

vacancies or the Ti4+ ions adjacent to oxygen vacancies. The photoluminescence spectra of undoped and Zn-doped TiO2 are shown in Fig. 8. The spectra of pristine TiO2 film exhibits emission at 417 nm

Generally, TiO2 is known for its chemical stability and optical competency. It has been used extensively for killing different groups of micro organisms including bacteria, fungi and viruses, because it has high photo reactivity, broad-spectrum antibiosis and chemical stability [34]. Therefore, pure TiO2 and 8 at.% Of Zn-doped TiO2 are taken for antibacterial activity and tested with five bacteria namely Staphylococcus aureus, Klebsiella pneumonia, Pseudomonas aeruginosa, Proteus mirabilis and Bacillus subtilis.

Fig. 9. Antibacterial activity on (1) pure TiO2 and (2) 8 at.% of Zn doped TiO2 films (g) Positive control, (c) negative control, (a) 100 mg/mL, (b) 200 mg/mL, and (d) 300 mg/mL.

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A. Arunachalam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 105–112 Table 2 Antibacterial activity of TiO2. Organisms

Control

Gentamycin 30 mg

Staphylococcus aureus Klebsiella pneumonia Pseudomonas aeruginosa Proteus mirabilis Bacillus subtilis

NZ NZ NZ NZ NZ

29 18 13 21 21

Zone of inhibition (mg/mL) 100

200

300

NZ NZ 8 NZ 11

NZ NZ 9 NZ 12

NZ 11 11 NZ 14

NZ – No Zone of inhibition.

Table 3 Antibacterial activity of Zn-doped TiO2 thin film. Organisms

Staphylococcus aureus Klebsiella pneumonia Pseudomonas aeruginosa Proteus mirabilis Bacillus subtilis

Control

NZ NZ NZ NZ NZ

Gentamycin 30 mg

29 18 13 21 21

Zone of inhibition (mg/mL) 100

200

300

NZ NZ NZ NZ 12

NZ 7 NZ NZ 14

8 9 NZ NZ 15

NZ – No Zone of inhibition

Fig. 9 1, 2 represents the clear image of antibacterial activity on pure TiO2 and 8 at.% of Zn doped TiO2 films. The antibacterial activity of pure TiO2 in different concentrations against bacteria (S. aureus, K. pneumonia, P. aeruginosa, P. mirabilis and B. subtilis) was studied. The mean zone of inhibition ranges between 11 mm and 14 mm. Gentamycin is a positive control, and the zone of inhibition ranges from 13 mm to 29 mm respectively. The highest mean zone of inhibition (14 mm) is recorded B. subtilis with against pure TiO2 is presented in Table 2. The antibacterial activity of Zn-doped TiO2 with 8 at.% in different concentrations against bacteria (S. aureus, K. pneumonia, P. aeruginosa, P. mirabilis and B. subtilis) is observed. The mean zone of inhibition range between 12 mm and 15 mm. Gentamycin is a positive control, the zone of inhibition range from 13 mm to 29 mm respectively. The highest mean zone of inhibition (15 mm) is recorded B. subtilis with against Zn-doped TiO2 with 8 at.% is presented in Table 3. Conclusion Highly oriented Zn-doped TiO2 thin films were deposited onto glass substrates by spray pyrolysis technique. The pristine TiO2 and doped films were polycrystalline in nature with tetragonal structure and exhibited a sharp increase of anatase peak which was preferentially oriented along (1 0 1) plane. The strong then and narrowing of anatase peak was observed for 8 at.% of Zn-doped TiO2 thin film. The surface morphology of the doped film at 8 at.%, reveal the decrease in grain size with porous nature which could absorb more dye molecules in the DSSC. The mushroom like formation with decreased roughness was observed from the AFM study. The optical transmittance was enhanced upto 85% with the shrinkage of the band gap, indicated the well-crystallization of nano sized grains. The Pl emission spectra were observed at 398 nm and it was attributing to the applications like photo degradation. The increased Hall mobility, carrier concentration of the film doped with 8 at.% of Zn in TiO2 can be used as transparent conductive oxide electrode in the fabrication of solar cell, photo degradation. Also the inhibition of bacterial growth and mechanism of antibacterial activity of Zn-doped TiO2 thin film was highly yielded for the doped film.

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