1
OPTICAL, STRUCTURAL AND MORPHOLOGICAL CHARACTERIZATION OF ELECTRODEPOSITED
CUPROUS OXIDE THIN FILMS: EFFECT OF DEPOSITION TIME
Samiha. Laidoudi
a*, Mohamed.Redha. Khelladi
b, c, Charif. Dehchar
a, Samah.
Boudour
a, Leila. Lamiri
a, Ouafia. Belgherbi
a, Rabah. Boufnik
daResearch Center in Industrial Technologies CRTI, P.O. Box 64, Cheraga 16014, Algiers, Algeria
bLCIMN Laboratory, Department of Process Engineering, Faculty of Technology, University Ferhat Abbas Setif- 1, 19000 Sétif, Algeria
cDepartment of Materials Science, Faculty of Sciences and Technology, Mohamed El Bachir El Ibrahimi University, Bordj-Bou-Arreridj 34030, Algeria
dResearch Center For Development Of Advanced Technologies, ex-CDTA, Algiers, Algeria. Research Unit In Optics And Photonics UROP, Setif, Algeria.
* E-mail: s.laidoudi@crti.dz
Abstract: The purpose of this work is the development and characterization of a novel electrode material based on copper oxide (Cu2O) for use as electrode in catalytic application. The samples are prepared on an indium doped tin oxide (ITO) glass substrate using a simple electrochemical deposition process from a solution of copper (II) sulfate and citric acid. The Cu2O films are deposited under chronoamperometric control at a potential of −0.50V versus SCE at different deposition times ranging from 2 to 15 minutes. The solution was maintained at a temperature of 60°C and a pH of 11. The effect of the deposition time is mainly examined in terms of the change in structural, morphological and optical properties of the Cu2O films using various characterization techniques. Atomic force microscopy (AFM) images showed that the prepared thin films are homogeneous with a granular shape. Also, the surface of the deposits becomes roughened with increasing deposition time. Scanning electron microscopy (SEM) images showed that the morphology of the prepared thin films is composed of a mixture of cubic and pyramidal shapes regularly distributed over the surface of the substrate. X-ray diffraction (XRD) measurements demonstrated that Cu2O thin films prepared by electrochemical deposition have a pure cubic structure with higher preferred growth orientation (111) and good crystallinity. Characterization by UV-Visible spectroscopy showed that the samples have high absorption in the visible region. The calculated values of the direct band gap are between 1.9 and 2.15 eV. These results represent a good starting point for the development of low cost anode used in catalytic application.
Keywords: Cu2O thin film; Electrodeposition; Deposition time; Optical properties.
2 1. INTRODUCTION
Synthesis of cuprous oxide (Cu2O) thin films has attracted greater attention due to its intrinsic p-type semiconducting properties with cubic crystallinity structure [1, 2]. Chemical stability, non-toxicity and abundance [3] are relevant qualities that have made this material a candidate of choice in various applications, including catalysis [4, 5], glucose sensors [6, 7]
and energy storage devices [8-10]. The Cu2O films are higher absorption coefficient in visible regions with direct band gap of 1.9–2.2 eV [11]. Hence, they are found to be more attractive materials for hetero-junction solar cells [12]. To synthesize Cu2O thin films both physical and chemical methods have been used, such as chemical vapor deposition [13], thermal oxidation [14], sol-gel [15], sputtering [16] and electrodeposition [17, 18]. Among all these methods, electrodeposition is the most attractive because of its low-cost and environmentally friendly approach, ambient pressure processing, low temperature synthesis. In addition, it allows control of film thickness with the possibility of large scale deposition [19]. In this paper, a serie of Cu2O thin films was prepared using cupric sulfate solution with bath pH 11 on indium-doped tin oxide (ITO) substrate. The effects of the deposition time on the morphological, microstructural and optical properties of the as-deposited Cu2O films were investigated in detail. The aims of this study were to optimize a Cu2O layer for anode materials in catalytic application as subsequent step.
2. EXPERIMENTAL PROCEDURE 2.1 Electrodeposition of Cu2O thin films
Cu2O thin films were directly electrodeposited on the ITO (In: InO2) substrate from an aqueous solution containing 0.05 M copper sulfate and 0.05 M citric acid [18]. The pH of the bath solution was adjusted in the range of 11 using sodium hydroxide (NaOH). The citric acid served as the complexing agent to prevent Cu precipitation when NaOH was added to the solution. Chronoamperometry was used to electrodeposit Cu2O films at different deposition times ranging from 2 to 15 minutes. A three-electrode cell workstation was used with a platinum (Pt) wire and saturated calomel electrode (SCE, +0.241 V vs. SHE) as the counter and reference electrode, respectively. Before the deposition, the glass substrates of ITO were cleaned ultrasonically with ethanol, acetone and distilled water for 10 min. Deposition temperature was kept constant for all the experiments at 60 °C. After deposition all samples were rinsed with distilled water and air-dried.
3 2.2. Characterization techniques
The Cu2O thin films were deposited in a potentiostatic mode, using a computer-controlled potentiostat/galvanostat (Voltalab 40) as a potential source. Cyclic voltammetry (CV) was found to offer some noteworthy information on the electrodeposition synthesis of cuprous oxide thin film. Chronoamperometry was used to electrodeposit Cu2O films at different deposition times ranging from 2 to 15 minutes. All the samples were analyzed using an atomic force microscope (AFM) to determine surface roughness and morphology. In this case a representative area of 5×5 μm2 was used. Scanning electron microscope (SEM) was used to study the surface morphology of the deposited films. The structure of the obtained films was identified by a Rigaku SmartLab diffractometer using Cu Ka1 radiation at 45 kV and 200 mA (Cu Ka1, k = 0.154056 nm). Optical absorption spectra of the samples were obtained from a SHIMADZU 2401PC spectrophotometer in the ultraviolet UV–visible region.
3. RESULTS and DISCUSSION 3.1. Electrochemical characterization
To investigate the electrochemical behavior of alkaline Cu-citrate solution and to determine the potential range giving the Cu2O from the corresponding electrolyte, cyclic voltammetric experiments were performed. Fig 1 shows the CV curves of ITO-coated glass substrate in a solution containing 0.05 M copper sulfate and 0.05 M citric acid, while scan rate, temperature, and pH of the bath were maintained at values of 20 mV/s, 60 °C and 11, respectively.
4 Fig.1. (a) Cyclic voltammogram for Cu2O electrodeposited on ITO glass substrate in an alkaline electrolyte bath (pH= 11) containing 0.05 M CuSO4 and 0.05 M citric acid at 60 °C.
(b) Current-time curve of the Cu2O electrodeposition at a fixed potential -0.5 V vs SCE on ITO substrate.
By scanning towards the negative potential side, two different regions may be observed (Fig.1). First, the reduction of cupric complex Cu (2+) to Cu (1+) on ITO electrode (Eq. 1).
The produced Cu+ ions subsequently react with hydroxide (OH-) to form CuOH (Eq. 2) which then transform by dehydration to Cu2O resulting in thin films (Eq. 3), [20, 21], according to the following reactions:
(1) (2) (3)
When the applied potential was swept more -0.6 V, the metallic copper deposition reaction was observable (Eq. 4).
(4)
Upon the reverse scan, a crossover between the anodic and cathodic current appears in the curve, which is characteristic of the formation of a new phase involving a nucleation and growth process at the ITO surface [22].
-1,00 -0,75 -0,50 -0,25 0,00 0,25 0,50
-4,0 -3,5 -3,0 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0
0 50 100 150 200 250 300
-1,6 -1,4 -1,2 -1,0
-0,8 -0,50 V
i (mA/cm2 )
t (s) (b)
i (mA/cm2)
E(V/ECS) (a)
5 An intense anodic peak and a shoulder were observed at 0 and +0.2 V vs. SEC, respectively, which can be attributed Cu to Cu(I) or Cu(I) to Cu(II) oxidation and the formation of a second and mixed layer of Cu(II) oxide, hydroxide, and Cu2O/Cu(OH)2
21,23.
Chronoamperometry experiment was carried out in order to synthesize Cu2O thin films on ITO substrate at different electrodeposition times. According to the cyclic voltammetric measurement (Fig. 1.a), -0.5 V was chosen as the applied potential for the potentiostatic depositions of Cu2O. Potential hold of -0.5 V was chosen in order to make sure that Cu2+ is reduced to only Cu+ layers at the electrode surface. Typical current-time transients obtained at a cathodic potential of -0.5 V/SCE are shown in Fig. 1.b.
In the beginning, the current density increases until the current maximum, imax, is reached at a time tmax, which is corresponding to the double-layer charging at the interface of ITO/electrolyte [24]. After the value of imax, the current density began to decrease because of Cu2O nanostructures that cover the electrode surface. This process due to the nucleation and growth of copper oxide nuclei [24].
After electrodeposition, the deposits of Cu2O were carefully rinsed with distilled water to remove unreacted products from the surface and dried under air.
3.2. Structural analysis
The X-ray diffraction patterns of the Cu2O films electrodeposited at -0.5 V vs. SCE with different deposition times are shown in Fig. 2. The peaks with 2θ values of 36.39°, 42.30°, 61.41° and 73.41° corresponding to (111), (200), (220) and (311) crystal planes, respectively, are assigned to the polycrystalline cubic structure of Cu2O (JCPDS: 05-0667).
From Fig 2, besides the diffraction peaks corresponding to the ITO substrate, no impurities such as cupric oxide (CuO) or metallic Cu are found in the XRD patterns. In addition, the intensity of (111) peak was higher than that of other peaks which indicates a preferential growth of the Cu2O films along the (111) direction. It has been reported that (111) plane has the lowest surface energy as compared to the other peaks surface [25, 26]. The XRD peaks intensities were increased by increasing the deposition time from 2 to 15 min, indicating a structural improvement of Cu2O where the crystallinity increased as the deposition time increased [19].
6 Fig.2. X-ray diffraction patterns of Cu2O thin films for different deposition times.
According to the preferential peak (111), the average crystallites size can be estimated using the Scherrer equation [27]:
( )
(5)
Where D is the mean crystallites size, is the Bragg angle (rad), β is the full width at half maximum (FWHM), and is the wave length of the incident X-ray (0.15406 nm).
The average grain sizes calculated are: 4.03, 36.98, 37.47, and 42.77 nm for 2, 5, 10 and 15 min deposition time, respectively. These values are detailed in Table 1.
Table 1: Structural and morphological parameters of electrodeposited Cu2O thin films.
Deposition time
(min) 2 (°) FWHM (°) D (nm)
2 36.43948 4.10629 4.07
5 36.44910 0.39111 36.98
10 36.48755 0.44646 37.47
15 36.48234 0.45228 42.77
25 30 35 40 45 50 55 60 65 70 75 80
t=15 min
t=10 min
t=5 min
t=2 min Cu2O (311)
ITO
Cu2O (111) Cu2O (200)
Counts (arb.u)
2 (deg)
CuO (220) 2
7 3.3. Morphological analysis
In order to see the effect of deposition times on the morphological properties of different Cu2O thin layers, surface topographical characterization using an atomic force microscope (AFM: MFP-3D Asylum Research), and the morphology by a scanning electron microscope (FE-MEB US 8000) were performed.
Fig.3 shows the 2D and 3D AFM images of the different Cu2O deposits electrodeposited on ITO substrate at different deposition times at E = -0.5 V / ECS. This figure reveals that all the films are uniform, homogeneous and compact with granular morphology regardless of the value of the applied time.
Fig.3. 2D and 3D AFM images of Cu2O thin films for different deposition times.
8 The increase in deposition time from 2 to 15 min did not really influence on the topography of the thin layers, because all the samples have the same type of particles. Also, the size of the latter increases with increasing time while maintaining its topography due to volume diffusion and coalescence of particles.
The values of the root mean square (RMS) roughness surface of the films obtained at with various deposition times are shown in fig.4. It is found that the Cu2O films roughness increases when the deposition time increases.
Fig.4. Variation of the RMS roughness of the Cu2O thin films electrodeposited at different deposition time.
The morphology evolution of the Cu2O films deposited on ITO substrate at different time obtained by scanning electron microscopy is shown in Fig. 5. From the figure, it can be clearly seen that the grain size and crystalline shape of the films depend with time. The film deposited at 2 min is well-defined cubes with small grain size. Then, this morphology begins to change with increasing deposition time to give big grains in three-face pyramid shaped and they completely cover the surface of substrate. Comparing with other time, it is also noted that the film deposited at 15 min shows the three faces pyramid becomes larger and fully covered all over the surface without any voids.
9
10 Fig.5. SEM images of Cu2O thin films deposited at different deposition time: 2, 5, 10 and 15
min.
3.4. Optical analysis
Fig. 6 illustrates the absorption spectrum of Cu2O thin films grown on ITO substrate at different times in the wavelength range of 300 nm to 900 nm. The bare ITO substrate was also used as reference samples when measuring the optical absorbance spectrum.
Fig.6. Absorbance spectra of Cu2O thin films deposited on ITO glass substrates at different time.
11 Firstly, all the films indicated a better absorption in the spectral region of 350-500 nm in the solar spectrum. The absorbance spectra show a high absorbance for of the Cu2O thin films at wavelengths range below 500 nm with the appearance of two sharp optical absorption transitions at 354 and 457 nm. This absorption is due to the inter-band electron transition between the valence band and the conduction band in the Cu2O thin film [28, 29].
Fig. 6 shows that the sample deposited at 15 min exhibits the highest optical absorbance at most wavelengths especially in the wavelength range below 500 nm, and the light absorbance in Cu2O thin films notably reduces increasing the deposition time.
The optical band gap value Eg is obtained using the Tauc's plot [30], by extrapolating the linear region of (αhv)2 versus (hv) plot (Fig. 6). The optical band gap values for Cu2O thin films are decreased from 2.15, 2.10, 2.02 and 1.9 by increase of deposition time.
4. CONCLUSION
In summary, Cu2O thin films were deposited on ITO glass substrate by electrochemical method at different deposition time. Cyclic voltammetry studies were performed to determine the suitable deposition potential range of Cu2O thin films. The XRD measurements demonstrated that Cu2O thin films prepared by electrochemical deposition have a pure cubic structure with higher preferred growth orientation (111) and good crystallinity. The average grain sizes of the prepared films are 4.03, 36.98, 37.47, and 42.77 nm for 2, 5, 10 and 15 min deposition time, respectively. The AFM images showed that the prepared thin films are uniform, homogeneous with a granular morphology. Also, as the deposition time increases the grain size and the RMS roughness increase. The SEM images showed that the morphology of the prepared thin films is composed of a mixture of cubic and pyramidal shapes regularly distributed over the surface of the substrate, and the grains became larger by the increase of deposition time. Characterization by UV-Visible spectroscopy showed that the film deposited at 15 min exhibits the highest optical absorbance. The calculated values of the direct band gap are between 1.9 and 2.15 eV.
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