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Original article
12 (
8
); 5380-5391
doi:
10.1016/j.arabjc.2017.01.008

Optical and electrochemical capacitive properties of copper (I) iodide thin film deposited by SILAR method

, , , , , ,
a Department of Physics and Astronomy, University of Nigeria, Nsukka, Nigeria
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

⁎Corresponding author at: Department of Physics and Astronomy, University of Nigeria, Nsukka, Nigeria. fabian.ezema@unn.edu.ng (Fabian I. Ezema)

Received: , Accepted: ,
© The Author(s) 2017
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

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.

Keywords

CuI
Photoluminescence
Successive ionic layer adsorption and reaction (SILAR)
Specific capacitance
Electrochemical impedance spectroscopy (EIS)
Supercapacitor
1

1 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 190.45 g/mol, a density of 5.67 g/cm3, a melting point of 606 °C and a boiling point of 1290 °C. CuI is soluble in ammonia and potassium solutions, but insoluble in water and dilute acid (Perrin and Tarn, 1990). 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 (Johan et al., 2012). It also have an ultrafast scintillation property with a decay time of 90 ps at room temperature (Xin and Wan, 2006). CuI is used in dye sensitized solar cells as a solid transparent hole transporting electrolyte (Mohamed et al., 2015; Zhao et al., 2015) and it also shows improved hole conductivity in perovskite solar cell where it acts as a hole transport layer (Kumara et al., 2004; Christians et al., 2014; Huangfu et al., 2015). Its stability makes it readily vacuum deposited (Makha et al., 2012; Yan et al., 2011). CuI has been used for hetero-junction diode (Xiong and Yao, 2015). Recently, CuI has also emerged as an effective reusable catalyst for various organic transformations (kalita and Phukan, 2007).

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 (Dhere et al., 2010).

CuI thin films have been prepared and deposited by various physical and chemical techniques which include reactive sputtering (Reichld and Florian, 1978), pulsed laser deposition and electrodeposition (Sirimanne et al., 2003; Tennakone et al., 1995; Kang et al., 2010), laser assisted molecular beam deposition (LAMBD) Wijekoon et al., 1993, polymer assisted reaction (Bokshits et al., 2003), iodination of thin copper films (Amalina and Rusop, 2013), and wet chemical synthesis using CuO suspensions (Yang et al., 2004). Other methods that have been used include vacuum evaporation (Tennakone et al., 1998), ethanol thermal method (Liu et al., 2001), hybrid electrochemical/chemical method (Chakang et al., 2009; Huang et al., 2012), hydrothermal method (Nejand et al., 2014), as a composite (Tavakoli and Salavati-Niasari, 2014), colloidal synthesis (Yongsheng et al., 2013), green synthesis (Tavakoli et al., 2013; Jiang et al., 2011), chemical bath method (Bulakhe et al., 2013), successive ionic layer adsorption and reaction (SILAR) (Dhere et al., 2010; Bulakhe et al., 2013). Studies have shown that SILAR method produces a better structure compared to other methods of deposition (Dhere et al., 2010; Bulakhe et al., 2013). SILAR has other advantages such as low cost, low temperature and ease of deposition (Pathan and Lokhande, 2004).

Electrochemical capacitors, also known as supercapacitor (Wang et al., 2012), 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 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 (Devillers et al., 2014; Zhang et al., 2009; Burke, 2000).

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 are hardly studied. More so, to the best of the knowledge of the authors, the electrochemical capacitive properties of CuI deposited by SILAR method are 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 was 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 reveal 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

2 Experimental details

2.1

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) sulfate (CuSO4·5H2O) was used as the cation precursor, and sodium thiosulfate (Na2S2O3) acts as both the reducing and complexing agent; thereby reducing Cu 2 + to Cu + Bulakhe et al., 2013. Potassium iodide (KI) was the anion precursor. 0.21 M of CuSO4·5H2O and 0.03 M of Na2S2O3 solutions were in beaker A of pH ∼ 5, and 0.03 M of KI solution in beaker B of pH ∼ 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 min. They were further rinsed in distilled water and placed in ultrasonic bath for 20 min 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: Cu 2 + + ( S 2 O 3 ) 2 - ↔ Cu + ( S 2 O 3 ) - Cu + ( S 2 O 3 ) - + I - → CuI ↓ + ( S 2 O 3 ) 2 -

The substrate was immersed in beaker A for 20 s, immersed in beaker B for 20 s and finally rinsed in distilled water in beaker C for 20 s 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 min 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

2.2 Characterization

The films were structurally characterized by X-ray diffraction (XRD), with a scanning range angle of 10–100° with Cu Kα1 radiation of wavelength of λ = 1.5406 Å. 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 were characterized using Zeiss Scanning Electron Microscope (SEM) at various magnifications and at scale bar length of 100 μm. Finally, Energy Dispersive 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 consists of the working electrode (CuI films on SS), graphite counter electrode and satd. Ag/AgCl reference electrode in 0.1 M solution of sodium sulfate (Na2SO4) solution was electrolyte.

3

3 Results and discussions

3.1

3.1 Structural study

The XRD study carried out showed that the thin films deposited on glass substrate and annealed at 300 °C possessed a cubic crystal structure. The XRD pattern shows peaks at 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 (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2) and (5 1 1) 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.

Using Debye Scherer’s formula:

(1)
D = k λ β cos θ where D is the crystallite size, β is the full width of the observed diffraction line at its half-intensity maximum, k = 0.9 and λ = 1.5406 Å is the wavelength of X-ray source, and θ is the Braggs diffraction angle at the peak position.

The X-ray Diffraction of the two polymorphs has their highest intensity peak at (1 1 1) with the smallest 2θ angular position at 25.5°. The differences in those polymorphs are shown in Fig. 1. The CuI at 30 cycles, in comparison with that at 20 cycles, presents several additional peaks well identified regarding their intensity, while CuI at 20 cycles has missing or lesser small peaks such as (4 0 0), (3 3 1), (4 2 2), and (5 1 1). This may be due to the increased deposition cycle and thickness resulting in a difference of crystal habit. A study made by Inoue and Hirasawa (2013), confirms that rapid growth of nanocrystals results from high concentration of the precursor solution. This is in accord with our work where greater number of cycles favors the growth of the CuI grains.

The XRD patterns of CuI thin films: (a) 20 cycles and (b) 30 cycles.
Fig. 1 The XRD patterns of CuI thin films: (a) 20 cycles and (b) 30 cycles.

The crystallite size of 53 nm and 68.5 nm at 20 and 30 cycles respectively obtained for (1 1 1) plane indicates that the CuI material is nanocrystalline.

3.2

3.2 Morphological study

The morphological studies of the films obtained using a scanning electron microscope (SEM) at various magnifications are shown in Fig. 2.

(A and B) SEM images at 30 cycles, (C and D) SEM images at 20 cycles at magnification of 100 & 25 KX respectively.
Fig. 2 (A and B) SEM images at 30 cycles, (C and D) SEM images at 20 cycles at magnification of 100 & 25 KX respectively.

The schematic steps for the formation of CuI in Fig. 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 (Yang et al., 2016; Mahdi and Ali, 2014). The nanostructures of the films at 20 and 30 cycles are similar. The morphology shows a well-defined triangular shaped crystal. Similar morphology has been reported by Kosta et al. (2015), 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.

Schematic of the formation of CuI.
Fig. 3 Schematic of the formation of CuI.

Fig. 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.

EDAX spectrum of CuI thin Film at (a) 20 and (b) 30 cycles.
Fig. 4 EDAX spectrum of CuI thin Film at (a) 20 and (b) 30 cycles.

3.3

3.3 Optical studies

Fig. 5.1(a), shows the variation in the absorbance with respect to wavelength. The CuI thin films present a high absorbance in the visible region (300–580 nm) for the 30-cycle film and (300–420 nm) 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 Fig. 5.1(b).

(a) Variation of optical absorbance with wavelength and (b) variation of (α h ν)2 with h ν for CuI thin films.
Fig. 5.1 (a) Variation of optical absorbance with wavelength and (b) variation of (α h ν)2 with h ν for CuI thin films.

The optical band gap is obtained by the formula (Garadkar et al., 2009):

(2a)
( α h ν ) = A ( h ν - E g ) n where α is the absorption coefficient, Eg is the energy band gap, A is a constant, n =  1 2 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 (α h ν)2 vs h ν (photon energy), gave a straight line in the domain of higher energies, indicating a direct optical transition. The extrapolation of these curves to zero absorption coefficient (α = 0) gave band gaps of 2.98 eV and 2.78 eV at 20 and 30 deposition cycles respectively. The slight decrease in the band gap 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. (2010) and Sankapal et al. (2004). The thickness of the films was estimated using the formula (Bulakhe et al., 2013):

(2b)
T = M ρ A where T is film thickness, M is mass of the film material in grams, A is area of the film in cm2 and ρ is density of the film material (ρ = 5.67 g/cm3). The thickness of CuI was 556 nm and 870 nm.

3.3.1

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 (Ndukwe, 1996):

(3a)
R = ( n - 1 ) 2 ( n + 1 ) 2

Re-writing Eq. (3a) by making n subject of formula,

(3b)
n = 1 + R 1 - R
(3c)
n = ( 1 + R ) ( 1 - R ) + 4 R ( 1 - R ) 2 - k 2

The refractive index can also be related to the extinction coefficient using Eq. (3c) Guneri et al., 2015. 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, and 2.65 at 3.25 eV for 20 and 30 cycles respectively, and then decreased as photon energy increased further for the CuI thin films as seen in Fig. 5.2.

Refractive index spectra CuI thin films.
Fig. 5.2 Refractive index spectra CuI thin films.

3.3.2

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 (Ling-min et al., 2006):

(4)
k = α λ 4 π where α is the absorption coefficient, and λ is the wavelength. The positive value shows the absorbing nature of the material (Vettumprumal et al., 2015) which can be ascribed to the smoothness and uniformity of the material (Alwan, 2010). The graph of extinction coefficient against wavelength is shown in Fig. 5.3. The extinction coefficient that 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.
Extinction coefficient pattern of CuI.
Fig. 5.3 Extinction coefficient pattern of CuI.

3.3.3

3.3.3 Optical conductivity (σopt)

The optical conductivity (σopt) is the measure of the electrical conductivity in an alternating field. The optical conductivity of the thin films was calculated using the following relation (Hotra et al., 2012):

(5)
σ = α nc 4 π 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 Fig. 5.4 was as a result of electrons excited by photon energy with a maximum value of 10.8 × 1013 S/cm and 8.0 × 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 photon energy level shifts toward the conduction band (Henry et al., 2016).
Optical conductivity of CuI thin films.
Fig. 5.4 Optical conductivity of CuI thin films.

3.3.4

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 electrons finally return to their equilibrium states, excess energy is released and may include the emission of light.

The photoluminescence (PL) spectra in Fig. 5.5 showed strong emissions at room temperature. The CuI PL spectra have only one peak at 420 nm for both cycles. The peaks originate from the recombination of free excitons and electrons from the conduction bands to the trapped holes (Parganiha et al., 2015; Huang et al., 2008). Evaluating the peak intensity, we observed that the film of 30 deposition cycles has a higher luminescence peak intensity, which shows that the CuI thin films have different recombination efficiency of the optical transition.

Photoluminescence spectra for CuI thin films.
Fig. 5.5 Photoluminescence spectra for CuI thin films.

3.4

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), galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques.

Cyclic voltammetry (CV) measurements of CuI thin films were 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 and +1.0 V/AgCl. The CV curve of the thin films showed that the CuI materials possess a capacitive feature.

Fig. 6a and b shows the cyclic voltammetry curves for CuI thin films electrodes at various scan rates 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 (Dubal et al., 2010). There is a decrease in the specific capacitance at higher scan rates because electrolyte ions interact 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 (Rusi, 2014; Saranya et al., 2012). At slow scan rate there is a reduction in the charging current, when the magnitude of the faradic current raises above noise level.

(a) CV curves for CuI thin films at 20 cycles. (b) CV curves for CuI thin films at 30 cycles.
Fig. 6a and b (a) CV curves for CuI thin films at 20 cycles. (b) CV curves for CuI thin films at 30 cycles.

In Fig. 6c we see the dependence of specific capacitance on scan rate, we observed a decrease in the specific capacitance as the scan rate is increases. This is a common feature in supercapacitors, because their capacitance is frequency and charge/discharge current dependent. The frequency dependence is as a result of the difference in distance of the ions from the electrode. The stability that determines the suitability of the material for long-term usage is shown in Fig. 6d. Cyclic stability was done at 50 mV s−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.

Plot of specific capacitance against scan rate.
Fig. 6c Plot of specific capacitance against scan rate.
Cyclic voltammetry curve showing the 1st and 100th cycle.
Fig. 6d Cyclic voltammetry curve showing the 1st and 100th cycle.

For the electrochemical redox reaction, 0.1 M solution of sodium sulfate (Na2SO4) was used as the electrolyte. The specific capacitance obtained by CV measurement was calculated by using the relation (Dubal et al., 2013):

(6)
C s = ∫ I ( V ) dV m ∗ v ∗ Δ V 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 V s−1 and ΔV is the range of the potential window.

The specific capacitance at scan rate of 2 mV/s was found to be 93 Fg−1 and 73 Fg−1 for 20 and 30 cycles respectively. The ( SO 4 ) 2 - ions take time for intercalation and de-intercalation at slow scan rate and more charges are transferred than at higher scan rates (Gund et al., 2013; Conway, 1991; Suleiman et al., 2010). 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 (Gujar et al., 2006).

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. Fig. 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 for 20 and 30 cycles respectively in the potential range −1.0 to +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 Fig. 8b. The capacity retention of the CuI thin film electrode was found to have 2% depreciation after 1000 cycles. This indicates that the electrode has a good stability.

Galvanostatic charge-discharge curves of CuI electrode at various current densities: (a) 20 cycles and (b) 30 cycles.
Fig. 7 Galvanostatic charge-discharge curves of CuI electrode at various current densities: (a) 20 cycles and (b) 30 cycles.
(a) Variation of specific capacitance with current density and (b) variation of specific capacitance with number of cycles.
Fig. 8 (a) Variation of specific capacitance with current density and (b) variation of specific capacitance with number of cycles.

The shape of the discharge in Fig. 7 shows two sections: a linear part due to voltage drop across the equivalent series resistance (ESR) and a nonlinear part due to faradaic reaction at the electrode surface contributing to the charge storage. This behavior suggests the pseudo-capacitive behavior of the electrodes (Ezema et al., 2015; Yadav et al., 2016; Nwanya et al., 2014). The nature of the discharge curve is assigned to the asymmetric pseudo-capacitive behavior (Dubal et al., 2012; Nwanya et al., 2015; Amaechi et al., 2016; Ghodbane et al., 2009), 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 Eq. (7) Gund et al., 2013:

(7)
Cs = I D ∗ T D m ∗ V ( Fg - 1 )

The energy density was calculated using the formula (Ghodbane et al., 2009):

(8)
E = V ∗ ID ∗ TD m

And the power density was obtained from the formula (Ghodbane et al., 2009):

(9)
P = V ∗ I D m
(10)
Columbic efficiency, η = T D T c ∗ 100 % 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 varies from 5 mA to 6.5 mA.

The Ragone plot in Fig. 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 W h kg−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. (2016), for copper sulfide electrode.

Ragone of CuI electrode.
Fig. 9 Ragone of CuI electrode.

The Nyquist plot obtained at a frequency range of 1–1 MHz at a constant voltage amplitude of 10 mV is shown in Fig. 10. In the high frequency region, the curve intercepts the Zre axis at 8.15 Ω and 28.4 Ω for the 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 Fig. 10.

Nyquist plot of the electrochemical impedance spectrum of the CuI thin film electrode.
Fig. 10 Nyquist plot of the electrochemical impedance spectrum of the CuI thin film electrode.

The plot of frequency against the phase angle is shown in the Bode plot (Fig. 11). The CuI electrode showed a maximum phase angle of 48° and 62° 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.

Bode plot of the phase angle versus the frequency of the CuI thin film electrode.
Fig. 11 Bode plot of the phase angle versus the frequency of the CuI thin film electrode.

Fig. 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 part and imaginary part of the capacitance are 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 purely resistive (Patil et al., 2015).

Frequency dependence of the real and imaginary part of the capacitance values for the CuI thin film electrode.
Fig. 12 Frequency dependence of the real and imaginary part of the capacitance values for the CuI thin film electrode.

4

4 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 Fg−1 at scan rate 2 mV s−1 and shows appreciable stability with only about 2% depreciation after 1000 cycles. The capacitance value was obtained though not competitive compared to the metal oxide supercapacitors, and this research has evidently proven the CuI thin films possess good electrochemical capacitive properties which is the novelty of this study. Therefore, our SILAR deposited CuI thin film electrode is a promising material with potential for supercapacitor application.

Acknowledgments

We graciously 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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