Accepted Manuscript Original article Optical and Electrochemical Capacitive Properties of Copper (I) Iodide Thin Film Deposited by SILAR Method Blessing N. Ezealigo, Assumpta C. Nwanya, Aline Simo, Rose U. Osuji, R. Bucher, Malik Maaza, Fabian I. Ezema PII: DOI: Reference:
S1878-5352(17)30018-7 http://dx.doi.org/10.1016/j.arabjc.2017.01.008 ARABJC 2046
To appear in:
Arabian Journal of Chemistry
Received Date: Revised Date: Accepted Date:
17 October 2016 30 December 2016 12 January 2017
Please cite this article as: B.N. Ezealigo, A.C. Nwanya, A. Simo, R.U. Osuji, R. Bucher, M. Maaza, F.I. Ezema, Optical and Electrochemical Capacitive Properties of Copper (I) Iodide Thin Film Deposited by SILAR Method, Arabian Journal of Chemistry (2017), doi: http://dx.doi.org/10.1016/j.arabjc.2017.01.008
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Optical and Electrochemical Capacitive Properties of Copper (I) Iodide Thin Film Deposited by SILAR Method Blessing N. Ezealigo a, Assumpta C Nwanyaa, Aline Simob,c, Rose U. Osujia,c, R. Bucherc, Malik Maazab,c, Fabian I. Ezemaa,b,c*, a
Department of Physics and Astronomy, University of Nigeria Nsukka
b
Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation,
1 Old Faure road, Somerset West 7129, P.O. Box 722, Somerset West, Western Cape Province, South Africa. c
UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, College of Graduate Studies,
University of South Africa (UNISA), Muckleneuk ridge, P.O. Box 392, Pretoria-South Africa,
ABSTRACT Economical Successive Ionic Layer Adsorption and Reaction (SILAR) method was used to deposit copper iodide (CuI) thin films on amorphous glass and stainless steel (SS) substrates at room temperature. The resulting thin films were characterized for their structural, morphological and optical properties using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and UV-Vis Spectroscopy respectively. The energy band gap observed for the material was 2.98 and 2.78 eV at 20 and 30 cycles respectively. The electrochemical properties of this p-type semiconductor were characterized by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) in Na2SO4 electrolyte. The CuI film on SS gave a specific capacitance of 93 Fg-1 at a scan rate of 2 mV/s with an excellent long-term cycle and reversible stability.
KEY WORDS: CuI, Photoluminescence, Successive Ionic Layer Adsorption and Reaction (SILAR), Specific Capacitance, electrochemical impedance spectroscopy (EIS), Supercapacitor
Author to whom corresponding should be addressed (F.I. Ezema):
E-mail address:
[email protected], +2348036239214
1
1.0 INTRODUCTION Copper iodide (CuI) is an inorganic compound, which occurs in nature as a mineral called marshite, and can also be synthesized by redox reactions of copper and iodine. CuI appears in two colors such as white when it is pure and brown or tan when it is impure. It is odorless and has a molar mass of 190.45g/mol, a density of 5.67g/cm3, a melting point of 606 o C and a boiling point of 1290 o C. CuI is soluble in ammonia and potassium solutions, but insoluble in water and dilute acid [1]. CuI has attracted steadily growing interest because of its spectacular features such as large band gap, negative spin-orbit splitting, large temperature dependence, anomalous diamagnetism behavior, large ionicity and high pressure phase [2]. It also have an ultrafast scintillation property with a decay time of 90 ps at room temperature [3]. CuI is used in dye sensitized solar cells as a solid transparent hole transporting electrolyte [4,5] and it also shows improved hole conductivity in perovskite solar cell where it acts as a hole transport layer [6-8]. Its stability makes it readily vacuum deposited [9,10]. CuI has been used for heterojunction diode [11]. Recently, CuI has also emerged as an effective reusable catalyst for various organic transformations [12]. CuI belongs to the I–VII semiconductors with Zinc-blende (cubic) structure. Its electrical and optical properties made it useful in various applications due to its coordination chemistry which readily couple with other inorganic and organic ligands [13]. CuI thin films have been prepared and deposited by various physical and chemical techniques which include reactive sputtering [14], pulsed laser deposition and electrodeposition [15-17], laser assisted molecular beam deposition (LAMBD) [18], polymer assisted reaction [19], iodination of thin copper films [20], wet chemical synthesis using CuO suspensions [21]. Other methods that have been used include vacuum evaporation [22], ethanol thermal method [23], hybrid electrochemical/chemical method [24, 25], hydrothermal method [26], as a composite [27], colloidal synthesis [28], green synthesis [29, 30], chemical bath method [31], successive ionic layer adsorption and reaction (SILAR) [13, 31] etc. Studies have shown that SILAR method produces a better structure compared to other methods of deposition [13, 31]. SILAR has other advantages such as low cost, low temperature and ease of deposition [32]. Electrochemical capacitors, also known as supercapacitor [33], are of great importance in electronic devices and power systems. Its high-power characteristics and long life cycle make them desirable as energy storage and delivery device. Supercapacitors, do not have a 2
conventional solid dielectric. The capacitance value of an electrochemical capacitor is determined by two storage principles: electric double layer capacitance (EDLC) and pseudocapacitance, both of which contribute to the total capacitance of capacitor [34-36]. Electric double layer capacitance (EDLC) is exhibited mostly by carbonaceous materials whereby charges are accumulated at the interface between the electrode and the electrolyte while pseudocapacitance is as a result of redox processes and is exhibited by metal oxides and conducting polymers. As stated earlier, CuI have found various applications such as in solar cells and as a catalyst, but the electrochemical capacitive properties of CuI is hardly studied. More so, to the best of the knowledge of the authors, the electrochemical capacitive properties of CuI deposited by SILAR method is yet to be studied. In this work, we study for the first time; the electrochemical behavior of CuI deposited by SILAR. The aim of the study is to probe the electrochemical response of the CuI films using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). The electrochemical studies reveals that the CuI electrode is a potential material for supercapacitor application in addition to its other numerous applications. In addition to this, the SILAR method used in the deposition is relatively cheap and can be used for large area deposition. 2.0 EXPERIMENTAL DETAILS 2.1 Material synthesis CuI thin films were deposited by SILAR method (which was modified by eliminating the first rinsing beaker, thereby rinsing the substrate once in a cycle) so as to allow for easy growth of the thin film. Copper (II) sulphate (CuSO4 ·5H2O) was used as the cation precursor, sodium thiosulphate (Na2S2O3) acts as both the reducing and complexing agent; thereby reducing Cu 2 to
Cu [31]. Potassium iodide (KI) was the anion precursor. 0.21M of CuSO4 ·5H2O and 0.03 M of Na2S2O3 solutions were in beaker A of pH 5, 0.03 M of KI solution
6 and
beaker C contained distilled water for rinsing the loosely held ions from the substrate. The glass and steel substrates were washed in detergent, followed by rinsing and soaking in acetone for 30 minutes. They were further rinsed in distilled water and placed in ultrasonic bath for 20 minutes after which they were rinsed again in distilled water, dried, cleaned with soft paper and stored in air tight box. The chemical process can be described as follows: 3
Cu2+ + (S2O3)2-
Cu+ (S2O3)-
Cu+(S2O3)- + I-
CuI + (S2O3)2-
The substrate was immersed in beaker A for 20s, immersed in beaker B for 20s and finally rinsed in distilled water in beaker C for 20s at 60 °C. This was done for 20 and 30 cycles. Optimized conditions were used to keep the thin films adherent on the substrates. The substrates were then dried in an oven for 10 minutes at 60 °C, and then annealed at 300 °C for 1hr before been characterized. The substrates were characterized in batches; after 14 days and 60 days respectively so as to monitor the stability of the films and similar results were obtained for both batches. 2.2 Characterization The films were structurally characterized by X-ray diffraction (XRD), with a scanning range angle of 10 °- 100 ° with Cu-Kα1
d t
w v l gth
λ= 1.5406Å. Optical
properties of the copper iodide thin films were determined by UV- 1800 Schimadzu spectrophotometer which gave a measurement for the absorbance of the films with wavelength range of 300 – 1100 nm. The morphological properties of the thin film was characterized using Zeiss Scanning Electron Microscope (SEM) at various magnifications and at scale bar length of 100 μm. F
lly, E
gy D s
s v X-ray (EDAX) was used to determine the quantitative
composition of the deposited thin films on the glass substrate. The count used was about 1000, with electron volts range of (0 – 20) eV. The electrochemical characterization was carried out using Princeton Applied Research VersaSTAT potentiostat, in a three-electrode configuration, which consist of the working electrode (CuI films on SS), graphite counter electrode and satd. Ag/AgCl reference electrode in 0.1 M solution of sodium sulphate (Na2SO4) solution as electrolyte.
4
3.0 RESULTS AND DISCUSSIONS 3.1 Structural study
(511)
( 422 )
(331)
(400)
(311)
(220)
(b) (200)
Intensity (a.u)
(111)
20 cycles 30 cycles
(a)
20
30
40
50
60
70
80
90
2 (Degrees)
Figure 1: The XRD patterns of CuI thin films (a) 20 cycles (b) 30 cycles The XRD study carried out showed that the thin films deposited on glass substrate and annealed at 300 °C possessed a cubic crystal structure. Th XRD
tt
sh ws
s t 2θ
values of 25.5 °, 29.5 ° , 42.2 ° , 49.9 °, 52.3 ° , 61.2 °, 67.4 ° , 69.4 °,77.1 ° and 82.8 °, which corresponds to cubic phase orientations (111), (200), (220), (311), (222), (400), (422) and (511) planes respectively of CuI which is in agreement with JCPDS card number 01-082-2111. The unidentified peaks present are most likely from the substrates. Us g D y Sch
’s
mul :
D
k cos
(1)
where, and D is the crystallite size β is the full width of the observed diffraction line at its halfintensity maximum, k = 0.9 and λ =1.5406 Å is the wavelength of X-ray source, θ is the Braggs diffraction angle at the peak position. The X-ray Diffraction of the two polymorphs has their highest intensity peak at (111) w th th sm ll st 2θ
gul
st
t 25.5 °. Th d
c s
th s
lym
hs
sh w
figure 1. The CuI at 30 cycles, in comparison to that at 20 cycles, presents several additional 5
peaks well identified regarding their intensity, while CuI at 20 cycles has missing or lesser small peaks such as (400), (331), (422), and (511). This may be due to the increased deposition cycle and thickness resulting in a difference of crystal habit. A study made by Mikiyasu Inouie et al. [37], confirm that rapid growth of nanocrystals result from high concentration of the precursor solution. This is in accord to our work where greater number of cycles favor the growth of the CuI grains. The crystallite size of 53 nm and 68.5 nm at 20 and 30 cycles respectively obtained for (111) plane indicates that the CuI material is nanocrystalline. 3.2 Morphological study The morphological studies of the films obtained using a scanning electron microscope (SEM) at various magnifications is shown in figure 2. A
B
C
D
Figure 2: A & B SEM images at 30 cycles, C & D SEM images at 20 cycles at magnification of 100 & 25 KX respectively.
6
30 cycles Cu
SILAR
Cu2+ + I- +
Na2S2O3
I
+
Cu+
-
Cu
Cu+
Cu+
60 ᵒC
Growth
+
I-
Cu+ Cu+
I
-
I-
20 cycles
Figure 3: Schematic of the formation of CuI The schematic steps for the formation of CuI in figure 3 showed the initial stage of absorption of Cu+ and I- ions on the surface of the substrate which are deposited layer by layer to form the CuI nuclei. Continuous growth of the CuI crystals forms a triangular nanostructure. This implies that the surface energy between the substrates and grain aggregation leading to film formation, favors the formation of triangular nanostructure. This type of structure has been successively reported for solution deposited CuI thin films [38-39]. The nanostructures of the films at 20 and 30 cycles are similar. The morphology shows a well-defined triangular shaped crystal. Similar morphology have been reported by Kostra et al. [40], for CuI films obtained by electrochemical deposition. The close packed structure of the CuI thin film allows electrons and holes to move freely without having to travel long distances.
7
(a)
(b)
Figure 4: EDAX spectrum of CuI thin Film at (a) 20 (b) 30 cycles Figure 4 shows that the chemical composition of the film analyzed by EDAX technique confirms the elemental composition of CuI with the unique presence of Cu: 35%, 38%, and I: 33%, 37.3% in CuI thin films for both 20 and 30 cycles. 3.3 Optical studies Figure 5(a), show the variation in the absorbance with respect to wavelength. The CuI thin films present a high absorbance in the visible region (300 – 580nm) for the 30-cycle film and (300 - 420nm) for the 20-cycle film. We can infer that an increase in the number of cycles increases the optical absorption capacity of the thin film because the thickness of the film retards the easy passage of the light rays. This feature is seen to affect the band gap of the material as shown in figure 5(b). The optical band gap is obtained by the formula [41]: (h ) A(h E g ) n
(2a)
Where α is the absorption coefficient, Eg is the energy band gap, A is a constant, n= for a direct band gap and n=2 for indirect band gap. Therefore CuI thin films have unique properties for application for various purposes such as window material in solar cells The plot of variation (αhν)2 vs hν (photon energy) , gave a straight line in the domain of higher energies, indicating a direct optical transition. The extrapolation of these curves to zero s
t
c
c
t (α= 0) g v
dg s
2.98 eV and 2.78 eV at 20 and 30 deposition
cycles respectively. The slight decrease in the bandgap of the 30-cycle film with respect to the 20-cycle film could be due to the larger crystal size of the 30-cycle film. The values of the band gap are in the same range with that reported by Dhere et al. [13] and Sankapal et al. [42]. The thickness of the films was estimated using the formula [42]:
8
T
M A
(2b)
where T is film thickness, M is mass of the film material in grams, A is area of the film in cm2 and is density of the film material ( = 5.67 g/cm3 ). The thickness of CuI was 556 nm, 870 nm
Absorbance (a.u)
2
(A)
20 cycles 30 cycles
(B)
1
0
400
600
800
Wavelength (nm)
1000
Figure 5.1 (a): Variation of Optical absorbance with wavelength, (b): V for CuI thin films
t
(αhν)2 with hν
. 3.3.1 Refractive Index of the thin films Refractive index (n) is one of the basic properties of an optical material. The refractive index of a material is the ratio of the sine of angle of incidence to the sine of angle of refraction. The refractive index can be calculated using formula from reflectance spectra. If the absorption is high with no or minimal interferences the sample refractive index, n can be calculated using the equations [44]:
R
(n 1) 2 (n 1) 2
(3a)
Re-writing equation (3a) by making n subject of formula,
n
1 R 1 R
(3b)
9
(1 R) 4R k2 (1 R) (1 R)2
n
(3c)
The refractive index can also be related to the extinction coefficient using equation (3c) [45]. It was observed that the value of refractive index, n increases with increase in photon energy to a maximum value of the 2.64 at 2.2 eV, 2.65 at 3.25 eV for 20 and 30 cycles respectively, then decreased as photon energy increased further for the CuI thin films as seen in figure 5.2. 2.6
(A)
Refractive index (n)
2.4 2.2 2.0 1.8 1.6 1.4
20 cycles 30 cycles
1.2 1.0 1.5
2.0
2.5
3.0
3.5
4.0
Photon energy (eV)
Figure 5.2: Refractive index spectra CuI thin films 3.3.2 Extinction coefficient (k) The extinction coefficient (k) is a measure of light lost due to scattering and absorption per unit volume. The value of k was calculated using the following relation [46]: k
4
(4)
where α is the absorption coefficient, and λ is the wavelength. The positive value shows the absorbing nature of the material [47] which can be ascribed to the smoothness and uniformity of the material [48]. The graph of extinction coefficient against wavelength is shown in figure 5.3. The extinction coefficient which is a measure of absorption of light in a medium, showed a maximum at a wavelength of 4.1 eV for both 20 and 30 cycles followed by a sharp decrease for CuI films.
10
20 cycles 30 cycles
Extinction coefficient (k)
0.14
0.07
0.00
1.0
1.5
2.0
2.5
3.0
Photon energy (eV)
3.5
4.0
Figure 5.3: Extinction coefficient pattern of CuI 3.3.3 Optical conductivity ( opt ) The optical conductivity ( opt ) is the measure of the electrical conductivity in an alternating field. The optical conductivity of the thin films was calculated using the following relation [49]:
nc 4
(5)
where n is refractive index, c is speed of light and α is the absorption coefficient. The photoconductivity increased with photon energy resulting from the generation and transport of holes and electrons. The increase in optical conductivity in figure 5.4 was as a result of electrons excited by photon energy with maximum value of 10.8 x 1013 S/cm and 8.0 x 1013. At high photon energy the samples possess high optical conductivity, which is higher for 20 cycle film compared to the 30-cycle sample. This may be as a result of large number of carriers present, the
Optical conductivity * 10
13
(s-1)
photon energy level shifts towards the conduction band [50]. 20 cycles 30 cycles
10
8
6
4
2
0 2.0
2.5
3.0
3.5
4.0
Photon energy (eV)
11
Figure 5.4: Optical conductivity of CuI thin films 3.3.4 Photoluminescence (PL) Photoluminescence is light emission from any matter after the absorption of photons (photo excitation). This photo excitation causes electrons within a material to move into permissible excited state. When these electron finally return to their equilibrium states, excess energy is released and may include the emission of light. The photoluminescence (PL) spectra in figure 5.5 showed strong emissions at room temperature. The CuI PL spectra has only one peak at 420 nm for both cycles. The peaks originates from the recombination of free excitons and electrons from the conduction bands to the trapped holes [5152]. Evaluating the peak intensity, we observed that the 30 deposition cycles film has a higher luminescence peak intensity, which show that the CuI thin films have different recombination efficiency of the optical transition. 20 cycles 30 cycles
Photoluminiscence
1200000
800000
400000
0 400
600
800
Wavelength (nm)
Figure 5.5: Photoluminiscence spectra for CuI thin films. 3.4 Electrochemical studies To determine the performance of the electrochemical capacitive properties of CuI thin films as an electrode material, the super capacitor properties are evaluated using cyclic voltammetry (CV), galvonostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques.
12
0.004
(a)
(b)
-2
Current Density (mA cm )
CurrentDensity (mA cm -2)
0.004
0.002
0.000 2 mV/s 5 mV/s 10 mV/s 20 mV/s 50 mV/s 75 mV/s 100 mV/s
-0.002
-0.004 -1
0
0.002
0.000 2 mV/s 5 mV/s 10 mV/s 20 mV/s 50 mV/s 75 mV/s 100 mV/s
-0.002
1
-1
0
Potential (V) vs Ag/AgCl
1
Potential (V) vs Ag/AgCl
Figure 6(a): CV curves for CuI thin films at 20 cycles, (b): CV curves for CuI thin films at 30 cycles 80 20 cycle 30 cycle
70
80 60 50
60
40
40
30 20
20 10
0
Specific capacitance (Fg-1)
Specific capacitance (Fg-1)
100
0
0
20
40
60
80
-1 Scan rate (mVs )
100
Figure 6 (c): Plot of Specific capacitance against scan rate 0.04 30 cycles
20 cycles
Current density (mA cm -2)
Current Density (mA cm -2)
0.004
0.000
th
-0.004
100 cycle st
1 cycle
-1
0
Potential (V) vs Ag/AgCl
1
0.00 100
th
cycle
st
1 cycle
-0.04
-1
0
1
Potential (V) vs Ag/AgCl
Figure 6 (d): Cyclic voltammetry curve showing the 1st and 100th cycle. 13
Cyclic voltammetry (CV) measurements of CuI thin films was performed at a constant scan rate of 2, 5, 10, 20, 50, 75 and 100 mV s−1. The electrodes exhibited stable operation in the potential range −1.0
d +1.0 V/AgCl. The CV curve of the thin films showed that the CuI
materials possess a capacitive feature. Figure 6(a) and (b) shows the cyclic voltammetry curves for CuI thin films electrodes at various scan rate for 20 and 30 cycles. The shape of CV curves of CuI film electrode shows a pseudocapacitive behavior which is a characteristic of the electrochemical capacitors [56]. There was a decrease in the specific capacitance at higher scan rates because electrolyte ions interacts with the outer surface of the electrode; the active material at the inner surface becomes inaccessible during the electrochemical process, leading to a reduction in the specific capacitance [57-58]. At slow scan rate there is a reduction of the charging current, when the magnitude of the faradic current above noise level. The stability which determines the suitability of the material for long term usage is shown in figure 6(d). Cyclic stability was done at 50 mVs-1, and a slight shift in the CV curve was observed after the first cycle for both 20 and 30 cycles of the CuI electrode. For the electrochemical redox reaction, 0.1 M solution of sodium sulphate (Na2SO4) was used as the electrolyte. The specific capacitance obtained by CV measurement was calculated by using the relation [56]:
Cs =
I (V )dV
m * v * V
(6)
Where I is the current in Ampere, m is the mass of the active area of the thin film dipped into the electrolyte, v is the scan rate in Vs-1 and V is the range of the potential window. The specific capacitance at scan rate of 2mV/s was found to be 93 Fg-1 and 73 Fg-1 for 20 and 30 cycles respectively. The SO4
2
ions takes time for intercalation and de-intercalation at
slow scan rate and more charges are transferred than at higher scan rates [57-59]. The specific capacitance (Cs) decreases at higher scan rate due to the inability of active sites to sustain the redox reactions. This shows that the surface area is not accessible at high scan rates [60].
14
1
-2
1
-2
5 mA cm
(a)
6.0 mA cm
-2
6.5 mA cm
0
-1
2
4
6
8
10
12
14
16
18
-2
5.5 mA cm -2
Potential (V) Ag/AgCl
Potential (V) Ag/AgCl
5.5 mA cm
-2
5 mA cm
(b)
-2
6 mA cm
-2
6.5 mA cm
0
-1
20
2
4
6
8
Time (s)
10
12
14
16
18
20
Time (s)
Figure 7: Galvanostatic charge- discharge curves of CuI electrode at various current densities (a) 20 cycles (b) 30 cycles 14
12
11.40
(b)
11
11 9 10 8 9 7
8
) 13.10
11.25 11.20
13.05
11.15
13.00
11.10 11.05
12.95
-1 11.00
12.90
6
10.95
5 5
6
7
)
6
7
11.35 11.30
-1
10
13.15
Specific capacitance (Fg
12
20 cycles 30 cycles
Specific capacitance (Fg
(a)
13.20
Specific capacitance (Fg-1)
Specific capacitance (Fg-1)
13
20 cycles 30 cycles
200
400
Current density (mA)
600
800
1000
Number of cycles
Figure 8: (a) Variation of specific capacitance with current density (b) variation of specific capacitance with number of cycles. The galvanostatic charge-discharge (GCD) test was carried out on the CuI thin film electrodes using a three-electrode potentiostat system with 0.1 M solution of Na2SO4. Figure 7 shows the galvanostatic charge–discharge curves of CuI thin film electrodes at current densities of 5, 5.5, 6 and 6.5 mA cm−2
20
d 30 cycl s
s ct v ly
th
t t l
g −1.0 t +
1.0 V/AgCl. The long-term stability of the thin film electrode was investigated by potential cycling at a current density of 5 mA/cm2 and the capacitance retention after 1000 cycles is shown in figure 8b. The capacity retention of the CuI thin film electrode was found to have a 2% depreciation after 1000 cycles. This indicates that the electrode has a good stability.
15
The shape of the discharge as shown in figure 7 show two sections: a linear part due to voltage drop across the equivalent series resistance (ESR) and a non-linear part due to faradaic reaction at the electrode surface contributing to the charge storage. This behavior suggest the pseudo-capacitive behavior of the electrodes [61-63]. The nature of the discharge curve is assigned to the asymmetric pseudo-capacitive behavior [64- 67], which is in agreement with result redox reactions at an interface between electrode and electrolyte shown in the CV curves. The Specific capacitance was calculated from the discharge side of GCD curve using equation 7 [57]: Cs
I D * TD m *V
(Fg-1)
(7)
The energy density was calculated using the formula [67]: E
V * ID * TD m
(8)
And the power density was obtained from the formula [67]:
P Columbic efficiency,
V * ID m
(9)
TD *100 % Tc
(10)
where V is the potential window, ID is the discharge current, TD, Tc is the discharge and charge time respectively and m is the active mass. The columbic efficiency of about 98% for each cycle of charge discharge as the current density is varied from 5mA to 6.5mA. 55
45 40 45
35 30
Energy density (Wh kg -1)
Energy Density (Wh Kg-1)
20 cycles
50
30 cycles
36
27
4.5
25
5.0
5.5
Power density (KW kg-1)
5.5
6.0
6.0
6.5
Power density (KW kg-1)
7.0
16
Figure 9: Ragone of CuI electrode The ragone plot in figure 9 of the CuI electrode showed the plot of the power density against the energy density of which the highest energy density was 52.6 Whkg-1 for 20 cycle. It was observed that the energy density decreases significantly with increase in power density. Similar phenomenon was reported by Bulakhe et al. [68], for copper sulphide electrode. -400
-2000
20cycles
30 cycles
-300
-2
Zim ( cm )
Zim ( cm-2)
-1500
-200
-100
Parameter
value
C (F) R()
1 E-5 8.15
W
0
0
100
200
-1000
-500
Parameter C (F)
1E-5
300
Zre ( cm-2)
400
0
500
0
500
1000
Value 1E-5
R()
28.4
W
1E-5
1500 2000 -2 Zre ( cm )
2500
3000
Figure 10: Nquist plot of the electrochemical impedance spectrum of the CuI thin film electrode The Nyquist plot obtained at a frequency range of 1 MHz to 1 mHz at a constant voltage amplitude of 10 mV is as shown in figure 10. In the high frequency region, the curve intercept the Zre x s t 8.15Ω
d 28.4 Ω
th 20- and 30-cycle films respectively. These values
correspond to the equivalent series resistances due to charge transfer processes between electrode and electrolyte. The equivalent circuit and the values of its parameters as fitted with ZsimpWin software are shown as an inset of figure 10. -80
-80 20 cycles
-70
3.0
Phase of Z (deg) log |z|
2.0
-40 -30
1.5
-20 1.0
-10
Phase angle ()
2.5
-50
0
-70
Phase angle log |Z|
3.5
-60
3.0
-50
2.5
-40
2.0
-30
1.5
-20
1.0
-10 -2
-1
0
1
2
3
Log frequency (Hz)
4
5
0.5
0
Log |Z|
Phase angle ()
-60
4.0 30 cycles
0.5 -2
-1
0
1
2
3
4
5
Log frequency (Hz)
Figure 11: Bode plot of the phase angle versus the frequency of the CuI thin film electrode
17
The plot of frequency against the phase angle is shown in the Bode plot (Figure 11). The CuI electrode showed a maximum phase angle of 48º and 62º for the 20- and 30- cycle films respectively. These values confirm the pseudocapacitive nature of the electrodes. The slight difference in the phase angle between the two cycles could be related to the thickness of the film.
Cre Cim
0.010
0.0010
0.005 0.005
0.000
0.000 -2
-1
0
1
2
3
Log freqency (Hz)
4
5
Cre
30 cycles
0.003
Cim
0.002 Cim (F)
0.0015
C im(F)
Cre (F)
0.010
0.015
Cre (F)
20 cycles
0.0005
0.001
0.0000
0.000 -2
-1
0
1
2
3
4
5
Log frequency (Hz)
Figure 12: Frequency dependence of the real and imaginary part of the capacitance values for the CuI thin film electrode. Figure 12 shows the plot of the real (Cre) and imaginary capacitance (Cim) against the logarithm of the frequency. The figure indicates that both the real and imaginary part of the capacitance is frequency dependent (capacitance decreases with increase in frequency) in the low frequency regions. At higher frequencies, the capacitance becomes frequency independent and the capacitive impedance becomes almost zero and the total impedance is purly resistive [69]. 4.0 CONCLUSION In this study we have successfully deposited and characterized CuI thin films for its optical, electrochemical and structural properties. The XRD pattern confirms the formation of cubic-like microstructure of CuI thin films. The band gap of CuI thin film obtained was 2.98 eV and 2.78 eV at 20 and 30 deposition cycles respectively. The variations in the properties of the synthesized material were observed to be dependent on film thickness. Also, the results of the refractive index, extinction coefficient and photoconductivity showed similar feature. The deposited CuI thin film at 20 cycles has a maximum specific capacitance of 93 F g-1 at scan rate 2mVs-1 and shows appreciable stability with only about 2% depreciation after 1000 cycles. The capacitance value obtained though not competitive compared to the metal oxide supercapacitors, 18
this research have evidently proven the CuI thin films possess good electrochemical capacitive properties which is the novelty of this study. Therefore, our SILAR deposited CuI thin films electrode is a promising material with potential for supercapacitor application.
ACKNOWLEDGEMENTS We gracious acknowledge the grant for this project by TETFUND under contract number TETF/DESS/UNN/NSUKKA/STI/VOL.I/B4.33. We thank the US Army Research Laboratory– Broad Agency Announcement (BAA) for the financial support given to this research (under Contract number W911NF-12-1-0588). Also we thank Engr. Emeka Okwuosa for generous sponsorship of April 2014 and July, 2016 conference/workshops on applications of nanotechnology to energy, health &.Environment conference.
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