Structural and morphological characterization of thin films based on zinc oxide
F.Touri, A. Sahari, A.Zouaoui
Laboratory growth and characterization of new semiconductor (LCCNS) Setif, 19000, Algeria.
E-mail: faridatouri@gmail.com Abstract-Zinc oxide (ZnO) is a semiconductor wide direct gap
(3.37 eV), which has many interesting properties (chemical, piezoelectric, optical, catalytic ...) .A wide range of applications makes it one of the most studied materials in the last decade, especially in nanostructured form. In this work, we study the electrochemical synthesis of thin films by anodizing a wafer of zinc in aqueous solution. It was found that the morphology and the structure of final deposits are sensitive to conditions preparation (temperature; concentration and pH of the solution).
The structure and morphology are studied by means techniques of X-ray diffractometry (XRD and scanning electron microscopy (SEM)
Keywords— Zinc oxidation, thin layer; Anodic polarization;
Zinc oxide)
I-INTRODUCTION
In the last years, zinc oxide (ZnO) has attracted much attention owing to its various applications in the fields of optoelectronics, sensing, piezoelectric effect, magnetism or photovoltaic [1–3]. One important aspect of ZnO is the possibility to prepare a large range of nanostructures such as nanorods, nanotubes or nanorings [4,5].
Hierarchical ZnO nanostructures can be synthesized at low-temperatures by various method including chemical bath deposition, effect of the sweeping rates [6] hydrothermal synthesis,[7] and eletrodeposition. [8] Among these solution based growth methods, electrochemical deposition (ECD) is a rapid and cost-effective approach for the fabrication of hierarchical ZnO nanostructures.[9] Peulon et al.[10] and Izaki et al.[11, 12] had performed the pioneering work in the field of electrodeposition of ZnO thin films and nanorods on ITO substrates. There have been a variety of reports on the electrochemical synthesis of ZnO nanostructures on various substrates, including GaN,[13] FTO, [14] Au/Si, [15] Zn foils[16]. However, there are only a few reports on the electrodeposition of ZnO nanostructures on the functional materials modified electrodes. Recently, Seong et al. prepared ZnO nanosheets and nanorods structures by electrodeposition on single-walled carbon nanotubes modified electrodes.[17] In particular, Ryan et al. had successfully electrodeposited ZnO nanostructures onto organic-semiconducting substrates comprising copper phthalocyanine and pentacene molecular thin films using a two-step electrochemical process. [18]
Anodizing is an electrolytic passivation process used to increase the thickness and density of the natural oxide layer on the surface of metal parts. Besides increasing corrosion and wear resistance, anodizing is a very cost-effective method to produce uniform and adhesive oxide films on metals. [19]
Anodic depositions is a process that combines simplicity, cost-
effectiveness, and ease in morphology control. [20],[ 21]
Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. Recently, much interest has been drawn toward the anodization method as it enables the preparation of nanostructure metal oxides with unique physical and chemical properties. [22], [23], [24, 25]
II. EXPERIMENTAL DETAILS
The chemicals used in this study are analytical grades.
NaCl were supplied from Sigma Aldrich, the electrolyte consisted of on 0.1M NaCl solution aquese with an initial pH adjusted to 8. High purity (99.0%) zinc foils with a thickness of 0.20 mm and area 1.5 cm2. were used as a starting material in this study. The zinc foils were first degreased with acetone in an ultrasonic bath for 15 min, rinsed with ultrapure water, the bath temperature was held at 25 °C.A conventional three electrode cell was used with zinc foils as the working electrode. A platinum sheet was used as the counter electrode and a saturated calomel reference electrode (SCE,+0.241 V vs.
SHE). saturated electrode as the reference electrode to electrodeposit ZnO films. Electrochemical studies were carried out with a Voltalab PGZ 301 made up of a potentiostat-galvanostat equipped with Volta Master 4 software. The structure and morphology are studied by means techniques of X-ray diffractometry (XRD and scanning electron microscopy (SEM).
III-RESULTS AND DISCUSSION
The polarization of zinc foils obtained in aqueous solution with 0.1M NaCl at 25 °C, scan rates of Vb = 1 mV/s and pH=8 relative to ZnO films deposition to are shown in Fig. 1 through the dissolution or oxidation of Zn in the electrolyte solution, leading to the formation of a zinc ion complex deposited onto the surface of the anode and finally the formation of ZnO. As reported in most studies, the possible mechanism for the process can be expressed as follows: [20], [25]
Zn → Zn2+ + 2e− (1)
Zn2+ + 4OH− → Zn (OH) 42− (aq) (2)
Zn (OH)42− (aq) → Zn(OH)2 (s) + 2OH− (3)
2Zn (OH)2 (s) → 2ZnO (s) + 2H2O (aq) (4)
The initial stage of the reaction is the active dissolution of Zn at (-0.21V/ECS) in Eq. (1), which is attributed to the formation of Zn (OH) 42− in Eq. (2). When the concentration of Zn(OH) 42− exceeds the solubility product of Zn(OH)2 at ( - 0.02 V/ECS) , precipitation of a compact layer of Zn(OH)2 will occur on the anode surface, at ( 0.47 V/ECS) as in Eq.
(3) at Lastly, ZnO will be formed, in the passivation region as described in the Eq. (4).
-1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0
0 5 10 15 20 25
i(mA/cm2)
E(V/ECS)
vb=1mv/s
Fig. 1: Electrochemical polarization of zinc foils obtained in aqueous solution with 0.1M NaCl at 25
°C scan rates of vb. = 1 mV/s and pH=8
1. -Effect of the scan rates
To determine the nature of the limiting stage and the reaction mechanism on the electrode, we plot the polarization curves of pure zinc as a function of the scan rates fig 2(a,b,c) realized at different scan rates 1,2and3 mV/s respectively, Note when the scan rates increases and obviously there peak intensity increases and the light shifting potential anodic peaks toward more positive values, thus indicating that the system is not reversible.
-1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0
-5 0 5 10 15 20 25 30 35
i(mA/cm2)
E(V/ECS) a
b c
Fig. 2: Electrochemical polarization of zinc foils for three different scan rates of: (a) Vb = 1 mV/s, (b) Vb = 2 mV/s, (c) Vb = 3 mV/s, in aqueous solution with 0.1M NaCl at 25 °C,
and pH=8
.
Figure 3 show the XRD patterns of the samples anodized at different scan rates. The diffraction peaks at 2θ values of 36.25◦, 39.070◦, 43.34 ◦ corresponded to the (101), (210), (204) planes of hexagonal ZnO, respectively (JCPDS 36-1451).Other peaks corresponded to pure zinc (JCPDS 65-3358), which clearly originated from the foil.
20 30 40 50 60 70 80
0 100 200 300 400 500 600 700 800 900 1000 1100 1200
V1 ("V1")
Sample identification (Comment - 1) a
b c
(101)
(210) (204)
Fig. 3: XRD pattern of zinc foil anodized in aqueous solution with 0.1M NaCl at 25 °C, pH=8 and different scan rates vb
=1mV/s, b) vb=2mV/s and vb=3mV/s
fig 4 shows the SEM images of the ZnO nanoflake
arrays prepared by anodization of zinc foil in
aqueous solution with 0.1M NaCl at 25 °C, pH=8
and different scan rates. At vb =3mV/s (Fig 4c),
incomplete formation of tiny nanoflake structure was observed, while vb =2mV/s (Fig 4b) and vb
=1mV/s (Figure 4a) of anodization showed a more complete formation of nanoflake arrays. The size of nanoflakes increased as the anodization of scan rates increased. The nanoflake arrays formed after vb =1mV/s was more packed and uniform compared to the one prepared at vb =2mV/s. This indicates that anodization scan rates play a role in the determination of nanoflake size and distribution.
Fig. 4: SEM images of zinc foil anodized in aqueous solution with 0.1M NaCl at 25 °C, pH=8 and different scan rates vb
=1mV/s ,b) vb=2mV/s an vb=3mV/s
VI-CONCLUSION
ZnO of various nanostructures and sizes was successfully prepared via anodization of Zn foil under different anodic conditions.
The morphology of ZnO was found to be affected by anodization scan rates. The size of the nanoflakes increased as the anodization scan rates increased. XRD analyses confirmed the formation of hexagonal ZnO wurtzite
Avec une méthode moins chère on a réalisé des couches minces de ZnO wurtzite pour des futures applications en optique
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