WSN 123 (2019) 87-101 EISSN 2392-2192
Synthesis and investigation of optical, structural, and morphological characteristics of nanostructured
TiO
2-ZnO thin films
Nagham Abdulameer Yasir*, Alzubaidy Muneer Hlail and Ali Kamel Mohsin
Department of Physics, College of Science,Wasit University, Wasit, Iraq *E-mail address: [email protected]
ABSTRACT
TiO2-ZnO thin films have been prepared by using drop casting technique. The optical, structural, and morphological properties of nanostructured TiO2-ZnO thin films deposited by drop casting technique was studied by using several experimental techniques were used to characterize optical, structural, and morphological properties; X-Ray Diffraction (XRD) was used to study the crystal structures, the crystallite size, the dislocation density, and the number of crystallites per unit area of TiO2-ZnO thin films. The surface topography, surface roughness parameters, and average grain size for all samples were done by Atomic Force Microscopy (AFM) micrograph which confirm the existence of Nano-structured thin films. The optical energy gap of the TiO2-ZnO thin films was determined by using UV spectroscopy. It was found that one type of basic electrons transitions, through which the values of the optical energy gap for all the prepared thin films which are allowed direct transition.
Keywords: Crystallite size, drop casting, energy gap, thin films, Titanium dioxide, Zinc oxide, Thin films, Photocatalysis, Sol–gel process, X-Ray Diffraction
1. INTRODUCTION
Titanium dioxide thin films are widely studied because of their interesting chemical, electrical and optical characteristics (high band gap, transparent in the visible range, high refractive index, high dielectric constant, and ability to be easily doped with active ions) which are considered for different utilization such as gas sensors [1], planar waveguides [2], electrochromic systems [3] and photocatalysts [4]. The physical, chemical and photochemical characteristics of the formed TiO2-ZnO composites were largely dependent on the synthesizing method. For instance, Aal et al. have synthesized TiO2-ZnO nanoparticles by hydrothermal method [5]. Zhang et al. synthesized Zn-doped TiO2 thin film by means of pulsed DC reactive magnetron sputtering method utilizing Ti and Zn mixed target [6]. Kim et al. Produced ZnO coated TiO2 nanoparticles for use in flexible dye-sensitized solar cells [7]. Miki-Yoshida et al.
for use in photo-catalytic systems [8]; however, little attention has been paid to their applications in optical coatings. In this paper, we report the study of optical, structural, and morphological properties of nanostructured TiO2-ZnO thin films deposited by using drop casting technique.
Several experimental techniques were used to characterize optical, structural, and morphological properties: X-Ray diffraction, Atomic Force Microscopy (AFM) and UV spectroscopy.
2. EXPERIMENTAL 2. 1. Materials and methods
TiO2-ZnO thin films were prepared using drop casting method, in three steps. The first step: Pure Titanium dioxide were prepared by using the chemical reaction method. (10 ml) of Titanium tetrachloride (TiCl3) (99.9%, Sigma–Aldrich) was diluted in 100 ml of distilled water using a magnetic stirrer for 20 minutes. In the second step, Pure Zinc oxide were prepared by using the chemical reaction method. (10 g) of Zinc chloride (ZnCl2) (Sigma–Aldrich) was grinding for (5 min) by electric grinder device (GOSONIC 500W) in order to obtain a fine powder. ZnCl2 was dissolved in 100 ml of distilled water using a magnetic stirrer for 20 minutes. In the third step, the solutions of TiO2 were doped with (2, 4, 6, and 8 wt %) of ZnO solution. Then, glass substrates with refractive index equal to 1.52 and thickness of 2 mm were carefully cleaned before use. Five drops of this solution were taken and drop casted on to a quartz substrate in order to obtain thin films and kept under ambient conditions for drying.
These thin films were annealed for one hour, and kept at a constant temperature of 300 °C.
2. 2. Characterization
The crystalline structure of TiO2-ZnO thin films examined by X-Ray Diffraction device model (Shimadzu-6000-XRD), with the following Specifications: Target is copper (Cu) with wavelength (λ = 1.5418 Å), voltage (40 kV) and current (30 mA). The scanning angle 2theta is changed in the range of (10-80) degree with a speed of 8.0000 (deg/min). UV-VIS spectra were measured with (Shimadzu UV-1650 PC) in the wavelength range (200-900 nm). Reflectance was recorded by (A SR300 Spectroscopic Reflectometer) in the wavelength range of (200-900) nm. The topography surface study was conducted out using an atomic force microscope (SPM Ntegra NT – MDT).
3. RESULTS AND DISCUSSION
X-Ray Diffraction (XRD) analysis has been done to study the crystal structures, the crystallite size, the dislocation density, and the number of crystallites per unit area of TiO2-ZnO thin films. The results of XRD showed that the samples prepared by the drop casting technique have single crystalline structures for the sample (Pure TiO2 and 2% ZnO) and these samples are a titanium dioxide in the anatase phase and the trend of its growth at the crystalline level (101).
As well as, the results showed that the samples (4% ZnO, 6% ZnO, 8% ZnO) were polycrystalline, and the sample (4% ZnO) was a mixture of crystalline phases of titanium dioxide in the anatase phase and zinc oxide and the samples (6% ZnO, 8% ZnO) have a crystalline form of Cubic zinc oxide, and the trend of growth of all films are at the crystalline level (100). This result is consistent with the resulting of the researcher (Kaviyarasu, K.) [9]. Figures (1, 2, 3, 4, and 5) shows the X-Ray diffraction curves of the thin films (Pure TiO2, 2%
ZnO, 4% ZnO, 6% ZnO, and 8% ZnO) respectively. The crystalline size of the prepared thin films was determined by Debye Scherrer equation )1(.
𝐷 =
k λβ cos 𝛳B (1) where, (D) is the crystallite size, (k) is the Scherrer constant and equal 0.94 assuming that the particles are spherical, (λ) is the X-Ray wavelength, (θβ) is the Bragg diffraction angle, and (β) is the full width at half maximum (FWHM) [10]. The numbers of the dislocations lines which cut the crystal per unit area represent dislocation density given by equation (2).
𝛿 =
1𝐷2
(2) where, (𝛿) is the dislocation density, and (D) is the crystallite size [11].
The number of crystallites per unit area, can be obtained using the following relationship (3(.
𝑁
°=
𝑡𝐷3
(3) where, (No) is the number of crystallites per unit area and (t) is the thickness of the film [12]. The relationship between X-Ray wavelength and angle of incidence is given by Bragg’s law (4) [13].
𝑛 λ = 2𝑑ℎ𝑘𝑙sin θ (4)
In this equation, the variable d is the distance between atomic layers in a crystal, lambda (λ) is the wavelength of the incident x-ray beam, and n is an integer representing the order of the diffraction peak. In simple structures, the peaks in an x-ray diffraction pattern are directly related to the atomic distances through equation 4. Table (1) summarizes the structural properties of TiO2-ZnO thin films prepared by using the drop-casting method. According to the results, it was observed that the dhkl values were obtained are somewhat consistent with the values (00-080-0075 Card Data). Also, the crystallite size and dislocation density of the prepared samples are increased by increasing the ZnO concentration. This result is consistent with the resulting of the researchers Bendavid, A. and Karunagaran, B. [14, 15].
Table 1. Structural properties of TiO2-ZnO thin films prepared using the drop casting method.
Fig.1. X-Ray diffraction curve for pure titanium dioxide (pure TiO2) film prepared by a drop casting method.
Sample 2Ө (Deg.) hkl FWHM
(Deg.) dhkl (Å) Crystalline size (nm)
Ave. Cry.
size (nm)
Dislocation Density δ (nm-2)
Pure TiO2 24.3124 TiO2(101) 0.1600 3.65803 53.07 53.07 3.55×10-4 2% ZnO 24.9511 TiO2(101) 0.5200 3.56582 16.35 16.35 3.74×10-3
4% ZnO
15.8342 ZnO(100) 0.1677 5.59241 49.97
62.12 2.59×10-4 31.7340 ZnO(004) 0.1256 2.81743 68.71
25.7297 TiO2(101) 0.1258 3.45966 67.68
6% ZnO
15.6511 ZnO(100) 0.1388 5.65742 60.36
62.43 2.56×10-4 31.7013 ZnO(004) 0.1600 2.82026 53.93
48.1193 ZnO(234) 0.1245 1.88944 73.02
8% ZnO
15.8109 ZnO(100) 0.1352 5.60060 61.98
63.81 2.45×10-4 48.2563 ZnO(004) 0.1380 1.88439 65.91
31.6975 ZnO(234) 0.1358 2.82059 63.54
Fig.2. X-Ray diffraction curve for titanium dioxide doped with the zinc oxide film (2% ZnO) prepared by the drop-casting method.
Fig. 3. X-Ray diffraction curve for titanium dioxide doped with a zinc oxide film (4% ZnO) prepared by drop casting method.
Fig. 4. X-Ray diffraction curve for titanium dioxide doped with zinc oxide film (6% ZnO) prepared by drop casting method.
Fig. 5. X-Ray diffraction curve for titanium dioxide doped with zinc oxide film (8% ZnO) prepared by drop casting method.
Atomic Force Microscopy (AFM) was used to study surface topography, surface roughness parameters, and an average grain size for all samples. Table (2) shows the surface roughness and the average grain size of TiO2-ZnO thin films. It was observed the surface roughness and the average grain size increased, as zinc oxide concentration increased. Figure (6) shows two-dimensional and three-dimensional AFM images of TiO2-ZnO thin films.
Observed that at higher surface roughness parameters, the grain of these surfaces become more homogeneous and nanostructures. This result is consistent with the resulting of the researcher (Cai, K.) [16]. Figure (7) shows the average grain size distribution of TiO2-ZnO thin films with different concentrations (Pure TiO2, 2% ZnO, 4% ZnO, 6% ZnO, 8% ZnO).
Table 2. Surface roughness parameters and average grain size measurements of TiO2-ZnO thin films.
The results of the optical measurements showed the occurrence of one type of basic electrons transitions, through which the values of the optical energy gap for all the prepared thin films, which are allowed direct transition. The value of the optical energy gap for the allowed direct transition by the relationship (5) was found where the value (r = 1/2) is plotted graphically between (αhʋ) 2 and the photon energy falling (hʋ) and choose the best tangent line for the straight part of the curve, where the value of the optical energy gap of the allowed direct transition represents the intersection point with the photon energy axis. It was noted that the value of the optical energy gap of the allowed direct transition is decreased by increasing the ratio of zinc oxide doped with TiO2.
αhʋ = 𝐵 (hʋ − 𝐸𝑔𝑜𝑝𝑡)𝑟 (5)
where, 𝐸𝑔𝑜𝑝𝑡 is the energy gap between direct transition, B is a constant depended on the type of material, r is exponential constant and its value dependent on type of transition [17].
The effect of the doping titanium dioxide with zinc oxide on the optical energy gap of TiO2-ZnO was investigated. Figures (8, 9, 10, 11, and 12) represents the optical energy gap of (Pure TiO2, 2% ZnO, 4% ZnO, 6% ZnO, 8% ZnO) respectively.
Sample
Surface roughness parameters
Average Grain
Size (nm) Average Surface
Roughness (nm)
Surface Skewness (dimensionless)
Surface Kurtosis (dimensionless)
Root Mean Square RMS
(nm)
Peak-Peak (nm)
pure TiO2 19.7 -0.267 2.28 23.6 100 74.71
2% ZnO 3.4 2.46×10-3 1.8 3.96 14.7 42.78
4% ZnO 3.88 0.0189 1.93 4.34 17.2 51.63
6% ZnO 7.4 -0.0241 1.99 8.79 36 68.17
8% ZnO 9.34 -0.292 2.26 10.8 37.4 87.19
Fig. 6. Shows two-dimensional and three-dimensional AFM images of TiO2-ZnO thin films with different concentrations: (a) Pure TiO2, (b) 2% ZnO, (c) 4% ZnO, 6% ZnO, (e) 8% ZnO.
(a
(b)
3D 3D
2D 2D
(c)
(d)
(e) 2D
2D
2D
3D
3D
3D
Fig. 7. Shows the average grain size distribution of TiO2-ZnO thin films with different concentrations: (a) Pure TiO2, (b) 2% ZnO, (c) 4% ZnO, 6% ZnO, (e) 8% ZnO.
(a)
(c) (b)
(e) (d)
Fig. 8. Energy gap as a function to the photon energy of pure TiO2 thin film prepared by using the drop casting method.
Fig. 9. Energy gap as a function to the photon energy of titanium dioxide doped with the zinc oxide thin film (2% ZnO) prepared by the drop-casting method.
Fig. 10. Energy gap as a function to the photon energy of titanium dioxide doped with the zinc oxide thin film (4% ZnO) prepared by the drop-casting method.
Fig. 11. Energy gap as a function to the photon energy of titanium dioxide doped with the zinc oxide thin film (6 % ZnO) prepared by the drop-casting method.
Fig. 12. Energy gap as a function to the photon energy of titanium dioxide doped with the zinc oxide thin film (8 % ZnO) prepared by the drop-casting method.
Fig. 13. Energy gap as a function to photon energy of titanium dioxide doped with zinc oxide thin film (6 % ZnO) and catalyst by a monochromatic wavelength light (laser-helium neon).
Fig. 14. Energy gap as a function to the photon energy of titanium dioxide doped with the zinc oxide thin film (6 % ZnO) and catalyst by a multi-wavelength light (tungsten-lamp).
The effect of photocatalytic on the optical energy gap was also investigated for TiO2
doped with 6% zinc oxide using two types of light sources; First, shed a beam of monochromatic wavelength light (laser-helium neon) and find out its effect on the energy gap as shown in Figure (13).
Second, shed a beam of multi-wavelength light (tungsten-lamp) and see its effect on the energy gap as shown in Figure (14). It was observed an increase in the value of the optical energy gap of the allowed direct transition for the sample doped with 6% ZnO. At first and before the stimulus (2.55 eV) and becomes (3 eV) when the source of light is laser-helium neon and becomes (5.34 eV) when the source of light is tungsten-lamp.
4. CONCLUSIONS
TiO2-ZnO thin films were successfully prepared by using the drop casting technique. This method produced a large quantity of TiO2-ZnO nanocrystals at relatively high purity and very low cost. XRD analysis showed that the crystallite size and dislocation density of the prepared samples are increased by increasing the ZnO concentration. The AFM results assure the existence of nanostructure. The optical energy gap values were increased as the concentration increase, which is due to the increase in crystal growth leading to an increase in the average grain size. The optical transitions in TiO2-ZnO are direct and the value of the optical energy
gap decreases with increasing ZnO concentration from 3.85 to 2 eV. As well as, It was observed an increase in the value of the optical energy gap of the allowed direct transition for the sample with 6% ZnO concentration and becomes (3 eV) when the source of light is laser-helium neon and becomes (5.34 eV) when the source of light is tungsten-lamp.
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