Accepted Manuscript Investigations of the Structural, Morphological and Electrical Properties of multilayer ZnO/TiO2 thin films, deposited by Sol-Gel Technique M.I. Khan, K.A. Bhatti, Rabia Qindeel, Leda G. Bousiakou, Norah Alonizan PII: DOI: Reference:
S2211-3797(16)00016-4 http://dx.doi.org/10.1016/j.rinp.2016.01.015 RINP 253
To appear in:
Results in Physics
Received Date: Accepted Date:
19 November 2015 4 January 2016
Please cite this article as: Khan, M.I., Bhatti, K.A., Qindeel, R., Bousiakou, L.G., Alonizan, N., Investigations of the Structural, Morphological and Electrical Properties of multilayer ZnO/TiO2 thin films, deposited by Sol-Gel Technique, Results in Physics (2016), doi: http://dx.doi.org/10.1016/j.rinp.2016.01.015
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Investigations of the Structural, Morphological and Electrical Properties of multilayer ZnO/TiO2 thin films, deposited by Sol-Gel Technique. M.I. Khan1, K.A. Bhatti2, Rabia Qindeel3, Leda G. Bousiakou4, Norah Alonizan3 1. Department of Physics, The University of Lahore, Lahore, Pakistan. 2. Department of Physics, University of Engineering and Technology, Lahore, Pakistan. 3. Department of Physics and Astronomy, College of Science, King Saud University, Riyadh, 11451, Saudi Arabia. 4. Department of Automation Engineering, Piraeus University of Applied Sciences, Egaleo Athens, 12244, Greece. Corresponding Author: Muhammad Iftikhar Khan
Email:
[email protected]
Abstract Investigations of the structural, morphological and electrical properties of multilayer ZnO/TiO2 thin films deposited by sol-gel technique on glass substrate. Sol-gel is a technique in which compound is dissolved in a liquid in order to bring it back as a solid in a controlled manner. TiO2 solution was obtained by dissolving 0.4g of TiO2 nano powder in 5ml ethanol and 5ml diethylene glycol. ZnO solution was obtained by dissolving 0.88g zinc acetate in 20ml of 2methoxyethanol.
X-Ray Diffraction (XRD)
(PW 3050/60
PANalytical X'Pert
PRO
diffractometer) results showed that the crystallinity is improved when the number of ZnO/TiO2 layers increased. Also it shows the three phases (rutile, anatase and brookite) of TiO2. Surface morphology measured by Scanning Electron Microscopy (SEM) (Quanta 250 fei) revealed that Crakes are present on the surface of ZnO/TiO2 thin films which are decreased when number of ZnO/TiO2 layers increased. Four Point Probe (KIETHLEY Instrument) technique used to investigate the electrical properties of ZnO/TiO2 showed the average resistivity decreased by
increasing the number of ZnO/TiO2 layers. These results indicated that the multilayer thin films improved the quality of film crystallinity and electrical properties as compared to single layer. Key Words: Semiconductors, Sol-gel, XRD, SEM, Four Point Probe.
Introduction The electrical and optical properties of semiconductors can be increased by coupling of two semiconductors than a single semiconductor [1-11]. Titanium dioxide (TiO2) is an important semiconductor having vast applications in photocatalysis, solar cells and photonic devices [1, 12–14]. It has rutile, anatase and brookite phases [15–20]. The presence of any one or more than one phases in the material can effect on the micro-structural, optical and electrical properties of the material. Among these phases ruttlie is stable phase while the other two phases are metastable which are difficult to synthesise and is continue to study [21]. In many publications the stability of these phases have discussed [19, 22–25]. Zinc oxide (ZnO) is abundant, non-toxic and low cost attractive material having vast applications because of its high optical and electrical properties [26-35]. Both zinc oxide (ZnO) and titanium oxide (TiO2) materials are abundant, nontoxic, wide band gap ceramic materials and have been widely applied in the electronic and optoelectronic products because of their unique optical and electronic properties [36]. ZnO has high electron mobility than TiO2 and both ZnO and TiO2 have similar band gap energy [37]. A negative shift is present in the valence and conduction band of ZnO as compared to those of TiO2. Also it is claimed that the photodegradation mechanism of organic dyes in ZnO has been similar to that of TiO2. As a consequence, the photocatalytic activity of the ZnO-TiO2 composite system has proven to be superior to that of other mixed oxides [38]. It is suggested that the mixing of these two semiconductors can enhance the performance of optoelectronic devices [39].
Thin film is superior to bulk for such use even if sufficient surface area can be obtained. The structural, morphological and electrical properties of ZnO/TiO2 multilayer thin films depend on the deposition method. There are many deposition methods such as Pulsed Laser Deposition (PLD) [40], pulsed DC reactive magnetron sputtering [41-43], sol–gel [44], impregnation [45, 46] and chemical vapor deposition [47]. Among them, sol–gel technique is a low-cost [48] and widely used for fabricating films with high specific surface area [1]. From the work done earlier, it can be seen that the study of the properties of multilayer ZnO/TiO2 thin films deposited by sol-gel technique needs to be addressed. In this paper, the structural, morphological and electrical properties of single, three and five layers thin films of Zn/TiO2 deposited by sol-gel technique on glass substrate have been studied. Experimental Details: TiO2/ZnO thin films were prepared using Sol-Gel spin Coating method, rotation speed was 3000 rpm and rotation time was 30 second, we dropped 8 drops of coating solution each time in the rotating substrate. TiO2 solution was obtained by adding (0.4 g) of the dissolved material which was TiO2 nano-powder in the (5 ml) of the solvent which was the ethanol and (5 ml) of the stabilizer which was Diethylene glycol. The mix was stirred using the magnetic stirrer for 3 days at 60°C to get a white homogeneous solution of TiO2. To get (0.2 M) ZnO solution we have add (0.88 g) of the dissolved material zinc acetate dehydrate in the (20 ml) of the solvent 2methoxyethanol, then stirred the solution in the magnetic stirrer at 60°C for 30 minutes, then added Mono-ethanol-amine (MEA), after that continuous the stirring for 90 minutes and the temperature was maintained at 60°C. Then the solution was aged for 24 hours, and finally a clear homogeneous solution of ZnO was obtained. First one layer of the ZnO Solution was coated on the glass substrate then it was heated at 120°C for one minute. After that, coated one layer of the TiO2 solution, and then heated it at 150°C for 10 minutes. Finally, the films were annealed at 450 °C for 1 hour.
The structural properties of multilayer thin films were studied by X-ray diffractrometer (XRD), a PW 3050/60 PANalytical X'Pert PRO diffractometer. This diffractrometer emitting a CuKα radiations working in line mode. The XRD patterns were obtained by continuous scanning in a range of 2 θ of 10 to 65 degrees [49]. The step size was 2 θ of 0.01º with a time of 0.50 s / step [50]. The accelerated voltages and filament current of the tube was 45kV and 40 mA respectively [51]. The film morphology was observed using scanning electron microscopy (Quanta 250 fei). The electrical properties of thin films are calculated by using four point probe technique (KIETHLEY Instrument). The specifications of the Kiethley instrument are given below;
Measure voltage from 1nV to 120 V.
6220DC Current source
Source current from 2nA to 105mA
Results and Discussion: The XRD pattern of single, three and five Layer of ZnO/TiO2 thin films deposited by sol-gel technique have been shown in figure 1. Bragg’s Law is used to calculate d-spacing [52] 2dsinθ = nλ -----------------------------------(1) Here, λ = 1.54Ao is the wavelength of CuKα, θ = angle of diffraction and ‘n’ = order of diffraction [53, 54]. Shearer’s formula is used to calculate the grain size, given by [55] . λ
= θ ----------------------------------(2) Here B = full width at half maximum.
The dislocation density is reciprocal to the square of grain size and it is estimated from the following relation using simple approach of Williamson and Smallman [56]. = 1/D2 --------------------------------------------(3) TiO2 has three different phases which are rutile, anatase and brookite. Rutile and Anatase have tetragonal crystal structure while Brookite has orthorhombic crystal structure. The cell parameters of rutile phase are a = 4.627 Ao, b = 4.627 Ao, c = 2.9757 Ao, α = β = γ = 90°, the anatase phase are a = 3.8101 Ao, b = 3.8101 Ao, c = 9.3632 Ao, α = β = γ = 90° [57]. And the brookite phase are a = 5.456 Ao, b = 9.182 Ao, c = 5.143 Ao, α = β = γ = 90°. In single layer of XRD pattern from figure 1, one peak of TiO2 with anatase tetragonal structure with (113) planes of reflection is obtained at an angle of 44.655o. This is accordance in lines of Rosniza Hussin et. al. [58]. The grain size and dislocation line density of this peak are 1.193 x 10-9 m and 7.021 x 1017 m respectively. Neither ZnO phase nor compounds consist of Ti and Zn could be confirmed in ZnO/TiO2 layer deposited by ZnO-TiO2 alternative layers. It is considered that amorphous ZnO may be formed [59]. In XRD pattern of three layers of ZnO/TiO2, brookite and anatase phases of TiO2 are observed. A very strong brookite peak is observed at an angle of 25.85270, assigned to (111) plane. Other brookite peaks are observed at 2θ of 38.3750 (220), 48.5250 (202), 54.3850 (311), 63.0450 (160). The anatase peaks are observed at 2θ of 27.9150 (003), 44.6950 (113), 55.5450 (211). This fact can be attributed to the presence of particles of Zn. According to Lin et. al. the presence of particulate Zn affects the growth of grain size of the metal matrix preventing the increase of the recrystallization during the process by the action of heat [60]. In five layers XRD pattern of ZnO/TiO2, rutile, anatase and brookite phases of TiO2 are observed, The rutile phase of TiO2 is obtained at 2θ of 27.8750 and 54.3150 with (110) and (121) planes of reflections. The anatase peaks are obtained at 2θ of 44.6850 (113),
48.4850 (020), 55.4550 (211). Similarly, the brookite peaks are observed at 2θ of 25.7550 (111), 38.4450 (220), 63.1950 (160). From XRD results, it is noted that maximum intensity is obtained in five layers of ZnO/TiO2. So the maximum crystallinity is at the five layers of ZnO/TiO2. Grain size and intensity as a function of Numbers of ZnO/TiO2 layers is shown in figure 2. This graph shows that when the number of layers of ZnO/TiO2 increased then grain size and intensity both are increased. Therefore, the crystalinity of films have been improved by increasing the number of layers. Also, when grain size large then there is less stress applied to move dislocation, so the ductility high. This means that the ductility of films increased and its hardness decreased by increasing the number of ZnO/TiO2 layers [51]. This affect is also seen in four point probe results in which when the number of layers of ZnO/TiO2 increased then its resistivity has decreased and conductivity has increased. SEM results Apart from the crystal structure and crystallite size analysis, yet another important structural characteristic of materials relates to the gross structural/morphological properties. The morphology of the investigated single layer, three layers and five layers of ZnO/TiO2 films is demonstrated by the SEM images presented in figure 3 (a-c). In this study, it is seen that the cracks formation take place on the surface of films by increasing the number of layers. Maximum cracks are seen in three layers of ZnO/TiO2 shown in figure 4(b). The number of cracks which are produced, depended upon several factors, including the nature of the substrate material, the substrate temperature during deposition, the rate of heating and cooling of the substrate, the thickness of TiO2 coats, the number of TiO2 coats applied. This is accordance to the literature [61]. In five layers of ZnO/TiO2, the crackes are reduced. This shows that the electrical properties of these films are change due to the interaction of ZnO and TiO2 particles. In
Sol-gel technique, the inner layer can be interacts with upper layer when the films are heated. This fact is clearly seen in SEM micrographs. The film morphology and its electrical properties are depends on grain size, orientation and grain boundaries [26]. A grain boundary is to identify as the interface at which grains move towards and contact with another different crystal orientation. Several kinds of defect are present at grain boundaries such as dangling and impurities. Therefore, the particle shapes may be changed due to multilayer thin films that may change of crystallinity and electrical properties. The crystallinity properties of the films are clearly dependent on the kinetics of arriving species at the surface [26]. From SEM micrograph, it is concluded that when the number of layers of ZnO/TiO2 thin film increased then the surface morphologies of thin film are improved. These results matched with the existing literature [62]. Electrical Properties: The four probe technique is used to find the resistance of the sample by using the formula [63].
( A) …………………(4)
R = ρ( L
Where ‘L’ and ‘A’ are length and area of cross-section of probe. This formula is used for those materials which have uniform current density throughout the material. The resistivity of semiconductor thin film is calculated by using the following formula [64] π
= () . ----------------(5) A graph is plotted between average resistivity and number of ZnO/TiO2 layers. The values of average resistivity of single layer, three layers and five layers of ZnO/TiO2 are 20.8 x 106 ohmm, 9.12 x 106 ohm-m and 7.71 x 106 ohm-m respectively. This graph shows that the value of average resistivity decreased when the number of ZnO/TiO2 increased. Therefore, the conductivity increased by increasing the number of Layers and hence the electrical properties are increased [65]. Also, when the partical size increases then it may improve the contact of surface between particles. Therefore the mobility of electron improved which reduce the resistivity [6668].
Conclusion Multilayer thin films of ZnO/TiO2 have been successfully deposited by Sol-gel technique on glass substrate. The effect of different layers of ZnO/TiO2 thin films on the structural, Morphological and electrical properties were investigated. It is seen that the crystallinity and electrical properties of multilayer thin film have been improved than single layer because of the combination and interaction effect between different metal oxides. These multilayer ZnO/TiO2 thin films can be used in optoelectronics devices.
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(160)
(121)
(211)
(020)
(111)
800
(113)
(220)
(220)
(113)
(111)
(110)
1000
200
(160)
(311)
(211)
(202)
400
(113)
600
(113)
Count Intensity (arb. units)
Single layer Bi Layer Tri Layer
(003)
1200
0 20
30
40
50
60
2 Theta (Degree)
Figure 1: Xrd pattern of Single layer, Bi Layer and Tri Layer of ZnO/TiO2 thin films.
Grain size (nm) Intensity (a.u.)
2.6 90 2.4
2.0 80
1.8 1.6
Intensity (a.u.)
Grain size (nm)
2.2
1.4 1.2
70
1.0 1
2
3
Number of ZnO/TiO2 Layers
Figure 2: Grain size and Intensity as a function of number of ZnO/TiO2 layers.
a
b
c
Figure 3: The SEM micro graph of ZnO/TiO2 thin films of (a) Single layer (b) Three layers (c) Five layers
22
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
14
dem o
dem o
dem o
dem o
12
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
dem o
Average Resistivity (ohm-m)
20 18 16
10 8
dem o 6
1
dem o 2
dem o 3
dem o 4
5
Number of Layers
Figure 4: Graph between average resistivity and number of ZnO/TiO2 layers.
Research highlights 1. Multilayer thin films of ZnO/TiO2 have been deposited on glass substrate by Sol – gel. 2. The crystallinity and electrical properties of the films have been improved when number of layers increases. 3. The roughness of the films decreases by increasing the number of layers.
