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Original article
13 (
1
); 346-356
doi:
10.1016/j.arabjc.2017.04.013

A study on solution deposited CuSCN thin films: Structural, electrochemical, optical properties

, , , , , , ,
a Department of Physics & Astronomy, University of Nigeria, Nsukka, Nigeria
b Nanoscience African Network (NANOAFET), iThemba LABS-National Research Foundation, Capetown, South Africa
c UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, College of Graduate Studies, University of South Africa, South Africa
d Department of Physics and Material Science &Engineering, Faculty of Science &Engineering, National University, Singapore 117542, Singapore

⁎Corresponding author at: Department of Physics & 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

A cost-effective successive ionic layer adsorption and reaction (SILAR) method was used to deposit copper (I) thiocyanate (CuSCN) thin films on glass and steel substrates for this study. The deposited thin films were characterized for their structural, morphological, optical and electrochemical properties using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–visible spectroscopy and VersaSTAT potentiostat. A direct band gap of 3.88 eV and 3.6 eV with film thickness of 0.7 μm and 0.9 μm was obtained at 20 and 30 deposition cycles respectively. The band gap, microstrain, dislocation density and crystal size were observed to be thickness dependent. The specific capacitance of the CuSCN thin film electrode at 20 mV/s was 760 F g−1 for deposition 20 cycles and 729 F g−1 for deposition 30 cycles.

Keywords

CuSCN
Successive ionic layer adsorption and reaction (SILAR)
Band gap
Micro strain
Photoluminescence
Electrochemical activities
1

1 Introduction

In recent times the unique combination of the optical and electronic properties of CuSCN has been exploited for optoelectronic devices. Early descriptions showed that the high optical transparency and suitable energy levels of CuSCN along with its solution process-ability and chemical stability made it another promising choice for use as a hole-transporting layer in organic photovoltaic (OPV) and organic light emitting diode (OLED) devices. The high optical transparency of this material is indisputable as it is clearly evident from the transmission spectra (Jaffe et al., 2010; Wu et al., 2005; Pattanasattayavong, 2014). Copper (I) thiocyanate (CuSCN), a p-type semiconductor is usually used as a window layer in thin film solar cells (Sankapal et al., 2004).

CuSCN that has been reported to possess a wide band gap (∼3.6 eV) (O’Regan and Lenzmann, 2004; Fernando et al., 2002; Tennakone et al., 1987) exists in two polymorphic forms: α and β, the β-CuSCN being more stable and readily available (Perera et al., 2005; Pattanasattayavong et al., 2013). The α-phase has an orthorhombic crystal lattice, while the β-phase can be a hexagonal or rhombohedral structure (Jaffe et al., 2010), the β-CuSCN has layers of SCN ions separating the plains of Cu atoms and a strong Cu—S bond that interconnects three dimensionally as shown in Scheme 1 (Ji et al., 2012).

Structure of β-CuSCN.
Scheme 1 Structure of β-CuSCN.

Its polymer-like structure has made it a distinct semiconductor material, because CuSCN thin films free of defects and pinholes can be deposited on large surface areas (Perera et al., 2005; Hehl et al., 2000; Tennakone et al., 1984). CuSCN has received so much attention because of its application as a solid electrolyte in dye-sensitized solar cell (Kumara et al., 2001; O’Regan et al., 2002; Perera et al., 2005). So far, the p-type copper (I) thiocyanate (CuSCN) has been studied mainly for use as a hole-transporting layer for photovoltaic applications (Snaith and Schmidt-Mende, 2007; Yanagida et al., 2009), quantum-dot-sensitized solar cells (QDSCs) (Ruhle et al., 2010), extremely thin absorber (ETA) solar cells (Briscoe and Dunn, 2011), hole-transporting thin film transistors (TFTs) (Pattanasattayavong et al., 2013; Perumal et al., 2014), high efficiency organic bulk hetero-junction (BHJ) cells (Xu et al., 2015), as an interfacial layer in conventional polymer solar cells (Takahashi et al., 2007; Chappaz-Gillot et al., 2014).

CuSCN has been prepared and deposited by various techniques including electrochemical deposition (Chappaz-Gillot et al., 2014; Tennakone et al., 1995; Wu et al., 2005; Ni et al., 2007; Selk et al., 2008; Kamiya et al., 2012), successive ionic layer adsorption and reaction (SILAR) (Sankapal et al., 2004; Gao et al., 2008) and chemical bath deposition (Ahirrao et al., 2011). Previously, the electrodeposition of CuSCN was carried out in organic solvents (Chappaz-Gillot et al., 2014; O’Regan et al., 2000) and recently it is now performed in aqueous electrolytes containing Cu2+ chelating agents so as to improve its stability (Tennakone et al., 1995; Wu et al., 2005; Ni et al., 2007; Kamiya et al., 2012). It is also used as a material for the synthesis of polymer electrolytes (Sindhu et al., 1993) and used to improve the long-term durability and stability of nanocrystalline photovoltaic cells. CuSCN, a molecular metal pseudo-halide (polyatomic groups that incorporate a pseudo-halogen anion such as thiocyanate) of singly-ionized copper behaves like halide ions in chemical reactions (Wijeyasinghe and Anthopoulos, 2015).

In this work, we studied the optical and electrochemical properties of CuSCN thin films deposited at room temperature on glass and stainless steel substrates using SILAR method. Sankapal et al. (2004) and Gao et al. (2008), have done some work on SILAR deposited CuSCN thin films; however, they did not study the electrochemical properties. Extensive work has been done on CuSCN regarding its hole transporting and conducting properties (Jaffe et al., 2010; Ji et al., 2012). Here, we present an in-depth study of the optical and electrochemical properties of CuSCN thin films. Electrochemical capacitors also known as supercapacitors or ultracapacitors are energy storage devices capable of charging and discharging in a few seconds, with life cycle in the order of thousands of cycles (Makino et al., 2015). We were able to show for the first time, the properties of CuSCN thin films for electrochemical energy storage application. This, to the best of the knowledge of the authors has not been studied before now.

2

2 Experimental details

All the chemicals used were of analytical grade and include copper sulphate pentahydrate (CuSO4·5H2O), sodium thiosulphate (Na2S2O3·5H2O), sodium thiocyanate (NaSCN), sodium sulphate (Na2SO4), sodium thiosulphate (Na2S2O3) and sodium thiocyanate (NaSCN). The substrates: glass (microscopic glass slide with the dimension of 75 × 25 × 1 mm) and steel (of dimension of 2.5 × 1.4 cm) were washed with detergent, rinsed with distilled water, soaked in acetone for 30 min, rinsed with distilled water and then ultrasonicated for 20 min and rinsed again, after which the substrates were dried in a sterilizing oven at 60 °C for 20 min, and then kept in an air tight box to prevent contamination.

0.1 M of CuSO4, the cationic precursor was dissolved in distilled water and complexed with 0.1 M of Na2S2O3. The anionic precursor was prepared by dissolving 0.2 M of NaSCN in distilled water. The substrates were immersed in beaker A containing the cationic precursor for 10 s, rinsed in distilled water in beaker B for 3 s and immersed in the anionic precursor in beaker C for 15 s and rinsed again in distilled water in beaker D to remove the weakly held ions from the substrates as shown in Fig. 1(A) & (B). The process of immersing the substrates through all four beakers gives a complete SILAR cycle. The samples were then dried in the oven for 10 min at 60 °C before been characterized. For this study 20 and 30 deposition cycles were employed so as to study the effect of number of cycles on the properties of the thin films.

(A) Schematic diagram of SILAR deposition of CuSCN thin film, (B) schematic formation of the nanorods of CuSCN deposition on glass and steel substrates.
Figure 1 (A) Schematic diagram of SILAR deposition of CuSCN thin film, (B) schematic formation of the nanorods of CuSCN deposition on glass and steel substrates.

The deposited CuSCN films were characterized for their structural properties using X-ray diffraction (XRD), with a scanning range angle of 10–100° and radiation of wavelength (λ = 1.5406 Å). The optical properties of CuSCN thin films were determined using UV-1800 Schimadzu spectrophotometer which gave a measurement of the absorbance of the films in the range of wavelengths 300–1000 nm, and the photoluminescence (PL) emission and excitation spectra were recorded at room temperature using a Shimadzu RF-5301-PC spectrofluorometer. Zeiss Scanning Electron Microscope (SEM) was used at various magnifications to study the morphological properties of the thin films. Quantitative composition of the deposited thin films on glass substrate was determined using Energy Dispersive X-ray (EDX) spectroscope with counts up to 1000 in the energy range of (0–20) eV. Finally, the electrochemical properties were investigated using Princeton Applied Research VersaSTAT (potentiostat) in a three-electrode configuration which consists of the working electrode (CuSCN films on stainless steel substrate), graphite counter electrode and saturated Ag/AgCl reference electrode, using 0.1 M sodium sulphate (Na2SO4) solution used as electrolyte.

3

3 Results and discussion

3.1

3.1 XRD analysis

The XRD pattern of the deposited CuSCN thin films at 20 and 30 deposition cycles is shown in Fig. 2 for samples deposited on glass substrate. The prominent peaks were observed at 2θ angles of 16.16°, 27.22°, 32.64°, 34.57°, 47.07°, 49.87° and 50.10° which correspond to a rhombohedral phase orientations of (0 0 3), (1 0 1), (0 0 6), (1 0 4), (1 1 0), (0 0 9) and (1 1 3), respectively. The deposited CuSCN thin films match the reference β-CuSCN diffraction pattern (JCPDS 29-0581).

XRD pattern of CuSCN thin film 20 & 30 deposition cycles.
Figure 2 XRD pattern of CuSCN thin film 20 & 30 deposition cycles.

The crystallite sizes were obtained using Debye Scherrer’s formula (Vinila et al., 2014):

(1)
D = k λ β cos θ where D is the crystallite size, β is the full width of the observed diffraction line at half its intensity maximum, k = 0.9 and λ = 1.5406 Å is the wavelength of X-ray source, θ is the Bragg’s diffraction angle at the peak position. The crystallite sizes were estimated to be 11.67 ± 0.05 nm and 11.88 ± 0.05 nm at 20 and 30 deposition cycles, obtained at (0 0 6) and (1 0 1) plane respectively, and this proved that the deposited CuSCN thin film is nanocrystalline. The result obtained is in range with values reported by Hatch et al. (2013) and Tennakone et al. (1987).

Stress resulting from micro strain ( ε ) in the films can negatively affect the structural properties of the films. The micro strain was calculated using the formula (Moreh et al., 2013):

(2)
ε = β cos θ 4 Dislocation density (δ) was obtained using the formula (Henry et al., 2016):
(3)
δ = 1 D 2

The films possessed a micro strain of 0.0296, 0.0292 ± 0.0001 at 20 and 30 deposition cycles respectively. It was observed that the value of the micro strain decreases with an increase in film thickness which may be related to reduction in inter-planar spacing between the atoms, thereby reducing the stacking fault in the films (Henry et al., 2016; Chen et al., 2005). The dislocation density that is a measure of imperfection of the crystal associated with misalliance of a part of the lattice of the crystal with respect to another part was also seen to decrease with an increase in film thickness. At 20 cycles the thin film has a dislocation density of 0.74 m−2 and 0.709 ± 0.001 m−2 at 30 cycles.

3.2

3.2 Morphological studies

The morphology of the deposited CuSCN thin films on glass substrate showed a well-defined rod-like shaped crystal at 20 cycles, which is consistent with the report by Chappaz-Gillot et al. (2013) for CuSCN electrodeposited at room-temperature from an aqueous copper sulphate solution stabilized by EDTA. As the deposition cycles increased to 30 cycles the rods became broader and closely packed. The rod-liked shaped surface morphology shown in Fig. 3(A-D) is electrochemically favourable, because it provides high surface area for electrochemical reaction.

(A) SEM micrograph of CuSCN at 20 cycles at a magnification of 10 KX, (B) SEM micrograph of CuSCN at 20 cycles at a magnification of 50 KX, (C) SEM micrograph of CuSCN at 30 cycles at a magnification of 10 KX, (D) SEM micrograph of CuSCN at 30 cycles at a magnification of 50 KX, (E) Energy dispersive X-ray (EDX) at 20 cycles and (F) EDX at 30 cycles deposited on glass substrates.
Figure 3 (A) SEM micrograph of CuSCN at 20 cycles at a magnification of 10 KX, (B) SEM micrograph of CuSCN at 20 cycles at a magnification of 50 KX, (C) SEM micrograph of CuSCN at 30 cycles at a magnification of 10 KX, (D) SEM micrograph of CuSCN at 30 cycles at a magnification of 50 KX, (E) Energy dispersive X-ray (EDX) at 20 cycles and (F) EDX at 30 cycles deposited on glass substrates.

The Energy Dispersive X-ray (EDX) on glass substrates confirms the presence of CuSCN with peaks of copper (Cu), sulphur (S), carbon (C), and nitrogen (N) which are the constituent elements of CuSCN. Fig. 3(E & F) confirms the formation of CuSCN with peaks of Copper (Cu) 41.53%, sulphur (S) 12.97%, carbon (C) 13.85%, and nitrogen (N) 13.75%. The presence of Si and Ca could be from the glass substrate used for deposition.

3.3

3.3 Optical studies

3.3.1

3.3.1 Absorbance

Fig. 4(A) shows the variation in the absorbance with wavelength. The CuSCN thin films showed a high absorbance within wavelength range of 300–840 nm at 30 cycles and 300–480 nm at 20 cycles. The absorption spectrum is similar to that obtained by Iwamoto et al. (2014). It was observed that an increase in the number of cycles increases the optical absorption capacity of the thin films, and this feature is also seen to affect the band gap of the material.

(A) The variation in the absorbance with respect to wavelength at 20 and 30 cycles; (B) variation of (αhν)2 vs hν of CuSCN for direct band gap at 20 and 30 cycles; (C) variation of (αhν)1/2 vs hν of CuSCN for indirect band gap at 20 and 30 cycles.
Figure 4 (A) The variation in the absorbance with respect to wavelength at 20 and 30 cycles; (B) variation of (αhν)2 vs hν of CuSCN for direct band gap at 20 and 30 cycles; (C) variation of (αhν)1/2 vs hν of CuSCN for indirect band gap at 20 and 30 cycles.

The absorption coefficient α was obtained using Eq. (4) (Camara et al., 2013):

(4)
α = 1 t ln 1 T where t is the thickness of the film, and T is the transmittance. The film thickness was estimated using 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 the density of the film material ( ρ  = 2.85 g cm−3 for CuSCN).

The mass, M of the deposited material was estimated using this relation: M = M final - M initial where M initial M final the mass of the substrate (in grams) before and after deposition respectively.

The measurement was done using Mettler Toledo weighing balance B2404-5.

3.3.2

3.3.2 Band gap

The variation of (αhν)2 vs hν which gave a straight line and a direct band gap of 3.88 and 3.60 eV at 20 and 30 deposition cycles respectively is shown in Fig. 4(B). The shift in band gap may be related to the deposition condition of this study such as molar concentration, number of cycles and dipping time which are basic parameters that affect the thickness of the films and important parameter for the estimation of the band gap of a material (Malliga et al., 2014; Shaban et al., 2015; Gao et al., 2008). The thickness of the films obtained was 0.7 and 0.9 ± 0.05 μm for 20 and 30 cycles respectively. This difference in the thickness implies that the deposition technique used to enable the control of film thickness for a desired value. The values obtained for the band gap are in range with those reported by Gao et al. (2008) and Ahirrao et al. (2011).

In Fig. 4(C), the plot of (αhν)1/2 versus photon energy (hv) for indirect band gap gave an estimated value of 2.85 and 2.6 ± 0.02 eV at 20 and 30 deposition cycles respectively, and these values are in range with the report of Pattanasattayavong et al. (2013).

3.3.3

3.3.3 Extinction coefficient

The graph of extinction coefficient (k) against wavelength is presented in Fig. 5(A). The extinction coefficient that is a measure of absorption of light in a medium, showed a maximum at a wavelength of 300 nm for both 20 and 30 deposition cycles followed by a sharp decrease as the wavelength increases, which indicates light loss due to scattering. The positive value shows that the deposited film absorbs a good amount of the optical energy. The drop in k values could be as a result of absorption of light at the grain boundaries at high wavelength region and also suggests that surface of the thin films is smooth (Bhaskar et al., 2001).

(A) Plot of extinction coefficient versus wavelength, (B) plot of refractive index of CuSCN thin films versus photon energy, (C) plot of real part of dielectric constant versus photon energy, (D) plot of imaginary part of dielectric constant versus photon energy, (E) plot of optical conductivity versus photon energy deposited on glass substrates, (F) photoluminescence plot.
Figure 5 (A) Plot of extinction coefficient versus wavelength, (B) plot of refractive index of CuSCN thin films versus photon energy, (C) plot of real part of dielectric constant versus photon energy, (D) plot of imaginary part of dielectric constant versus photon energy, (E) plot of optical conductivity versus photon energy deposited on glass substrates, (F) photoluminescence plot.

3.3.4

3.3.4 Refractive index

The values of the refractive index shown in Fig. 5(B) were obtained using Eq. (5) for semiconductors, where the extinction coefficient (k) and refractive index (n) are related by the given equation (Guneri et al., 2015):

(5)
n = ( 1 + R ) ( 1 - R ) + 4 R ( 1 - R ) 2 - k 2 where R is the reflectance.

In the visible range of the electromagnetic spectrum, the refractive index (n) at 20 cycles was 2.65 and 2.68 ± 0.04 at 30 cycles at photon energy of 2.5 eV and 1.75 eV respectively. These values are in range with the report by Pattanasattayavong et al. (2013). The magnitude of the refractive index decreases as it approaches the higher energy region for both samples. The sharp decrease could be said as a result of surface imperfections as well as internal reflections or photon energy trapped within the grain boundary (Ong et al., 2000). According to Ravindra et al. (2007), the refractive index originates from narrow bands of absorption; this proves the similarity between the absorbance and refractive index curve.

3.3.5

3.3.5 Dielectric constant

The real and imaginary dielectric constants were calculated using the formulas respectively (Wiktorezyk, 2002):

(6)
ε r = n 2 - k 2
(7)
ε i = 2 nk The real part of the dielectric constant ( ε r ) in Fig. 5(C) shows the dispersion that takes place when a beam of light falls on the film, while the imaginary part of the dielectric constant ( ε i ) shown in Fig. 5(D) is a measure of the dissipative rate of the wave medium. The dielectric properties of the films vary with refractive index. The imaginary dielectric constant represents the energy absorption from the dipole motion of an electric field (Taskin and Podder, 2014; Kamruzzaman et al., 2014). The film at 30 cycles showed a distinctive hump which can be attributed to deceleration of the light beam as it passes through the material.

3.3.6

3.3.6 Optical conductivity

The photoconductivity increased with photon energy as a result of the generation and transport of holes and electrons, as a result of electrons excited by photon energy. The values of the optical conductivity obtained were 8.31 × 1013 ± 0.05 S/cm and 10.5 × 1013 ± 0.05 S/cm at 20 and 30 deposition cycles respectively. In Fig. 5(E) at higher energy level, the samples have a high conduction rate, which is significant at 30 cycles compared to the 20 cycle sample. This may be as a result of large number of carriers donors present as the concentration increases and the energy level shifts towards the conduction band (Henry et al., 2016).

3.3.7

3.3.7 Photoluminescence

The photoluminescence (PL) property showed an emission peak at 400 nm for both 20 and 30 deposition cycles, excited at 350 nm. The 30 cycle film showed a higher emission peak which indicates that the PL response is affected by the thickness of the film and it is consistent with the report by Verma and Joshi (2009). The PL spectra in Fig. 5(F) showed a peak in blue region about 407 nm and a broad peak in the blue region. The peak at 407 nm is as a result of the recombination of free excitons through an exciton – exciton collision process (Parganiha et al., 2015) while a broad blue region with slight emission peak was observed at 510 nm.

3.4

3.4 Electrochemical studies

3.4.1

3.4.1 Cyclic voltammetry (CV)

The cyclic voltammetry (CV) measurements of CuSCN thin films were performed at a scan rate of 5, 20, 30, 40, 50, 75 and 100 mV s−1 and the potential window was between −1.0 and +1.0 V Ag/AgCl.

The specific capacitance, C s obtained by CV measurement was calculated by using the relation:

(8)
C s = ∫ I ( V ) dV m ∗ v ∗ Δ V where I is the response current, m is the mass of the active CuSCN material of the electrode, v is the scan rate in V s−1 and Δ V is the range of the potential window.

The specific capacitance of the CuSCN electrode obtained at 20 mV/s was 760 F g−1 for 20 cycles and 729 F g−1 for 30 cycles. Fig. 6(A & B) shows the plot of current density against potential at various scan rates for 20 and 30 deposition cycles respectively. The difference in the specific capacitance value at 20 and 30 deposition cycles could be attributed to the charge on the electrode. The specific capacitance (Cs) decreases at higher scan rate because of the incapability of active sites to withstand the redox reactions (Nwanya et al., 2014, 2015) as shown in Fig. 6(C).

(A) CV curves for CuSCN thin film electrodes at 20 cycles at various scan rates, (B) CV curves for CuSCN thin film electrodes at 30 cycles at various scan rates, (C) galvanostatic charge discharge curve of the CuSCN thin electrode at various current densities at 20 cycles, (D) galvanostatic charge discharge curve of the CuSCN thin electrode at various current densities at 30 cycles, (E) variation of specific capacitance with scan rate, (F) variation of capacitance retention of the electrode films with cycle number for both 20 and 30 cycles, (G) ragone plot of CuSCN film electrode deposited on steel substrates.
Figure 6 (A) CV curves for CuSCN thin film electrodes at 20 cycles at various scan rates, (B) CV curves for CuSCN thin film electrodes at 30 cycles at various scan rates, (C) galvanostatic charge discharge curve of the CuSCN thin electrode at various current densities at 20 cycles, (D) galvanostatic charge discharge curve of the CuSCN thin electrode at various current densities at 30 cycles, (E) variation of specific capacitance with scan rate, (F) variation of capacitance retention of the electrode films with cycle number for both 20 and 30 cycles, (G) ragone plot of CuSCN film electrode deposited on steel substrates.

3.4.2

3.4.2 Galvanostatic charge discharge (GCD)

The GCD curves for CuSCN thin films at 20 and 30 deposition cycles respectively at current densities of 5 mA, 5.5 mA, 6 mA and 6.5 mA are shown in Fig. 6(C-D). The specific capacitance decreases with increase in current density, a maximum value of Cs to be 17 F g−1 at a current density of 5 mA/cm2. Though IR drop is inevitable in a charge-discharge test it should not be large, because huge IR drop results in less charge across the electrodes of the cell than the applied voltage thereby reducing the performance of the device (Balakrishnan and Subramanian, 2014). Increase in IR drop reduces the capacitance of a supercapacitor. Therefore, the capacitance value of the CuSCN material, accompanied with a high IR drop as a result of large energy loss due to resistance, makes the CuSCN thin film electrode not so good for supercapacitor application. The capacity retention of the CuSCN film for long-term stability was investigated at a current density of 5 mA obtained was 94% after 1000 cycles as shown in Fig. 6(F). The ragone plot in Fig. 6(G) compared the power density and energy density of the CuSCN thin film electrode for energy storage device application. The energy density was found to decrease with increasing current density while the power density increases with increasing current density which suggested that the discharge capacity is proportional to the charging capacity (Nwanya et al., 2016).

The specific capacitance is calculated from the formula (Gund et al., 2013):

(9)
C s = I D ∗ T D m ∗ v ( F g - 1 )
(10)
Columbic efficiency, η = T D T c ∗ 100 % where V is the potential window, ID is the discharge current, TD and Tc is the discharge and charge time respectively and m is the active mass. The electrodes possess a columbic efficiency of about 98%.

3.4.3

3.4.3 Electrochemical impedance spectroscopy

The EIS technique helps to understand the various mechanisms and kinetics of electrochemical reactions at the electrodes, electrode/electrolyte interface and within the electrolyte. Many impedance parameters of the device such as electrolyte resistance (Rs), the charge transfer resistance (Rct) over the interface between the electrode and the electrolyte, the constant phase element (CPE or Q) and Warburg (W) element can be determined from EIS analysis. The constant phase element is due to inhomogeneity at the electrode/electrolyte surface and dynamic disorder associated with diffusion. The Warburg (W) element gives an indication of the ion diffusion into the electrode materials and is seen in the intermediate frequency and is the 45 line to the imaginary impedance (Z″) axis.

Fig. 7(A) shows the Nyquist plot of the CuSCN thin film electrode in the frequency range of 100 kHz to 10 mHz, with an amplitude of 10 mV and at a bias voltage of 0.7 V. The equivalent circuit and the fitted values of the circuit parameters were obtained using ZSimpWin software and are shown in the inset of Fig. 7(A) (upper left) and Table 1, respectively. The equivalent circuit model Rs is the resistance offered by the electrolyte solution and the value is the high frequency intercept of the semi circle on the real impedance axis. Rct is the charge transfer resistance offered at the interface between the electrode and the electrolyte and the value is the diameter of the semi-circle.

(A) Nyquist plot for CuSCN thin films at 20 & 30 deposition cycles on steel substrates; (B-C) bode plot of CuSCN thin film electrode at 20 and 30 cycles respectively.
Figure 7 (A) Nyquist plot for CuSCN thin films at 20 & 30 deposition cycles on steel substrates; (B-C) bode plot of CuSCN thin film electrode at 20 and 30 cycles respectively.
Table 1 Values of the fitted electrochemical circuit parameters from EIS analysis of the electrodes.
Circuit element 20 cycles 30 cycles
Rs (Ω) 5.96 6.05
CPE (F) 6.41 × 10−3 6.50 × 10−4
n 0.8 0.8
Rct (kΩ) 1.08 1;36

The Rct value for 20 and 30 deposition cycles was 1.08 and 1.36 kΩ respectively and is a function of the electronic and ionic resistances (Dubal et al., 2011; Pandolfo and Hollenkamp, 2006). Eq. (11), (Raju and Ozoemena, 2015), defines the impedance of the CPE:

(11)
Z CPE = 1 Q ( jw ) n where Q is the frequency-independent constant relating to the surface electroactive properties, and w represents the radial frequency. For n = 0, the CPE is a pure resistor; n = 1, CPE is a pure capacitor, n = −1, CPE is an inductor, while n = 0.5 it is Warburg impedance (Zw). The value for n obtained is greater than 0.5 but less than the ideal value of unity for pure electric double layer capacitors (EDLC). This confirms that the electrode material behaves more like a pseudocapacitor. Pseudocapacitors, achieve energy storage by rapid faradaic redox reaction with the charge transfer at the interface between electrode and electrolyte.

From these values obtained, it is obvious that CuSCN electrodes have high charge-transfer resistance; the 30-cycle electrode has a higher value than the 20-cycle electrode. This could be one of the reasons for the higher specific capacitance value obtained at 20 deposition cycles.

The plot of the logarithm of frequency against the phase angle (the Bode plot) is shown in Fig. 7(B-C). CuSCN electrode gave a maximum phase angle of 62° and 68°. These values are characteristic of faradaic reaction and hence confirm the pseudocapacitive nature of the electrodes. Pseudocapacitors have maximum phase angles that are usually less than 90° while pure electric double layer capacitors possess phase angle of 90° (Nwanya et al., 2014). The slight difference in the phase angle between the two cycles could be related to the thickness of the film.

Since the deposited material has proven to possess a good capacitance value of 760 F g−1 and capacitance retention capability for over 1000 charge-discharge cycles, it can be possibly applied in as an electrochemical energy storage material.

4

4 Conclusion

CuSCN thin films have been successfully synthesized by successive ionic layer adsorption and reaction (SILAR) method for electrochemical and optical studies. The extinction coefficient, refractive index and photoluminescence properties of the film were observed to increase with film thickness. The specific capacitance, Cs of the CuSCN electrode obtained at 20 mV/s was 760 F g−1 for 20 cycles and 729 F g−1 for 30 cycles. While these values are appreciable, the CuSCN electrodes have high IR drop as well as high charge transfer resistance (Rct). This could impede its electrochemical energy storage application. However, the electrode showed good stability maintaining about 94% of the stored charge for over 1000 charge-discharge cycles. Therefore more optimization is needed in order to reduce the high IR drop and the charge transfer resistance. Our future studies on this material will be geared towards this aspect in order to improve its electrochemical energy storage capability.

Acknowledgement

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 and for providing some research facilities.

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