3.1 Surface morphology study
It is very important to study the surface characteristics of CuS film since the charge storage in a supercapacitor is surface oriented. Fig. 2a-c exposes the surface images of the CuS films deposited at different cycles. The film deposited at 100 cycles contains irregular microspheres, pores and rod-like features scattered over the surface of the substrate. This porosity exhibited by the film enables redox interaction to occur . A clear change in the surface formations of the film is seen as the deposition cycle increased to 150. The irregular microspheres became larger in size and were scantily scattered over the surface. The wider pores together with the irregular microspheres are suitable for supercapacitor applications because ions in the electrolyte easily reach the microsphere and increase redox activity on the surface of the CuS film. More so, as the number of cycles increased further to 200, the morphology was transformed to flower-like agglomerated features which clustered at the center of the substrate showing increased redox interaction .
3.2 Structural study
XRD is an indispensible tool in crystal structure analysis because it produces information about the phase structure of the material. The CuxS orthorhombic phase structure corresponds to JCPDS card number: 23-0961. Two crystalline peaks were prominently observed in Fig. 3 at (262) and (116) planes for the CuS films deposited at 150 and 200 cycles. It is interesting to know that the film deposited at 100 cycles did not yield any prominent crystalline peak; probably because the deposition time was not sufficient to transform the phase of the material . As the number of deposition cycle increased, more peaks were produced due to the accumulation of more ions on the surface of the material. This is in proper alignment with previous report . Additionally, the peaks produced by the 150 and 200 cycles deposited films could have resulted from the effect of deposition time since the films were subjected to the same annealing conditions.
3.3 Elemental composition
Fig. 4a-c displays a set of EDX spectra for the deposited CuS films. There exist a very high peak of copper (Cu) and shorter peaks of sulfur (S) along with other elements such as silicon (Si), calcium (Ca), and sodium (Na) which originated from the glass substrate and is noticable in all the deposited materials. Oxygen (O) emanated as a result of the reaction between the CuS electrode and the atmosphere. It can be observed that the films in each of the cycles showed the same pattern of peaks indicating the uniform distribution of the CuS electrode on the glass slides. The presence of peaks of Cu and S confirms the deposition of CuS film on the substrate.
3.4 Optical studies
Fig. 5a shows the absorbance in (%) of CuS plotted against wavelength in (nm) for the films deposited at 100, 150 and 200 cycles at room temperature. The spectra clearly revealed a high absorption in the visible region with absorption edge within 300 and 450 nm. There exists an exponential decay by the absorption as the wavelength increased form 450-850 nm brought about by annealing treatment given to the samples . The broad absorbance in the visible region indicates that the CuS electrode is a desirable candidate for pseudocapacitor. It is important to note here that the film deposited at 200 cycles showed the highest peak followed by 150 and 100 cycles. Hence, we can say that the number of deposition cycle affected the absorbance of the films. Therefore, the thicker the film, the more the ions inside the film absorb UV radiation [16,17]. From Fig. 5b, annealing the films increased the optical transmittance, this increment could be attributed to the decreasing and rearrangement defects of the films. In addition the annealing lead to improvement of crystallinity of the films structure [16,18,19]. Therefore, CuS thin films can be used as transparent conducting material .
Fig. 5c shows the reflectance curves for CuS films deposited at 100, 150 and 200 cycles. There is an exponential growth in the percentage reflectance of the films with the wavelength. The peak reflectance was noticed by the film at 200 cycles while the least was at 100 cycles. It is evident that at higher cycles, the film was very thick, hence could not reflect much UV radiation. Again, all the films had high reflectance in the NIR regions at wave lengths of 800 nm and 1000 nm. The low reflectance possessed by CuS in the visible region makes it suitable as a coating material.
Fig. 6 gives a graph of plotted against hv. The intercept of the graph was traced along the photon energy axis and the results obtained from the graph are 1.95 eV, 1.97 eV and 1.98 eV for 100, 150 and 200 cycles respectively. It is very crucial to note here that, increase in the deposition cycle lead to increase in the band gap. This is because, at higher number of deposition cycles, more ions are available on the film surface resulting in absorption of UV radiation that is just enough to excite the electrons . The band gap result shows that the CuS is a good material for fabrication of solar cells. This kind of report was also obtained by .
The extinction coefficient (k) refers to the quantity of absorbed energy in the film. The decrease in k was due to the variation in the absorbance. The extinction coefficient in Fig. 7a increases with increase in the photon energy and decreases with increase in wavelength. The hump signifies that the deposited CuS films absorbed reasonable amount of optical energy. Since k decreases with increasing wavelength, it shows that the transparent films seem to behave like transparent insulators, preventing high energy UV and high energy temperature NIR radiation into a building while allowing only visible light . The graph of optical absorption coefficient, α vs photon energy, hν is shown in Fig. 7b for the CuS films. It is clear that the films have a high absorption coefficient that means all films have direct band gap. The minimum is value is 0.01 which occurred at 1.2 eV and the maximum value lies between 2.2 at 3.7 eV.
As depicted in Fig. 8a, the optical conductivity increased linearly and almost equally with photon energy up to about 3.52 eV. The slope of the curve changed with each cycles until a peak was reached at 3.76, 3.8 and 3.83 eV for 200, 150 and 100 cycles. After this peak, there was a sharp decay in the conductivity graph. These two different slopes indicate that the conductivity is governed by two mechanisms. In the linear part, the conductivity is proportional to photon energy. After 3.52 eV (exponential part), electrons produced probably have enough energy to ionize other atoms leading to production of electron whole pairs which eventually leads to an exponential increase in conductivity. It is pertinent to stress here that the decrease after the peaks was probably due to recombination.
3.5 Electrochemical studies
Electrochemical analysis is an important way of obtaining the chemical properties of the material with respect to the potentiometer and the electrolyte. In order to check the electrochemical performance of CuS as a powerful electrode material for supercapercapacitor, the cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) techniques were employed. Usually, in a typical electrochemical analysis, the material synthesized is the working electrode, the Ag/AgCl is known as the reference electrode while the graphite is the counter electrode in a 3-electrode system.
3.5.1. Cyclic Voltammetry (CV) Analysis.
Cyclic voltametry among other analytical techniques is the most popular technique that discloses the information about the electrochemical processes occurring at the interface of the electrochemical cell and the material. The cyclic voltammogram displays the plot of the current density (s) in function of the potential (V). Fig. 9a-c shows the cyclic voltametry (CV) plots carried out between -1 to +1 V vs Ag/AgCl at scan rate of 50, 100, 150, and 200 mV/s in the presence of 1 M of potassium hydroxide (KOH) solution at room temperature for the CuS electrodes produced at 100, 150, and 200 cycles.
The shape of the curve produced for all the materials are rectangular, indicating that CuS is good pseudocapacitive material . The materials produced at 150 cycles possess wider rectangle curves indicating higher capacitance . Additionally, there was no noticeable redox peaks in all the deposited electrode suggesting high stability and good reversibility of CuS . Specific capacitance was obtained from the CV by applying equation (1) [23–25] given as
Here, m1 and m2 are masses before and after deposition of the films, (Vf and Vi ) is the potential range from which the analysis was done that is, (-1.0 to +1.0 V vs Ag/AgCl), I represents the current density (mAcm-2), V is the scan rate and represents the area under the CV curve. The graph of the specific capacitance against the scan rate for the CuS electrode deposited at different cycles is well represented in Fig. 10. From the graph, it was noticed that the specific capacitance experienced decay for all the electrodes when the scan rate was constantly increasing. This is a behavior often associated with supercapacitors owing to their dependence on charge and discharge current and frequency . The dependency of the frequency could be attributed to the departure between electrode and the ions. The highest specific capacitance 15.497 Fg-1 was obtained at the lowest scan rate 50 mVs-1 with the material at 200 cycles. This is as a result of the cooperation between more ions present in the electrode compared to other cycles. Similar result was reported by  for a good supercapacitor material.
The Nyquist plot usually represents the plot of the Zim (Ohms) against the Zreal (Ohms). It is basically employed in the EIS analysis because it provides a very useful means of checking the qualities of the charge carrier in a material . Fig. 11 represents the Nyquist diagrams of the CuS films deposited at different cycles at room temperature in the presence of 1 M of KOH solution at a potential range of -1.0 to +1.0 V vs Ag/AgCl. The films clearly exhibit interesting curves having two sections. The first part resembles a semicircle which is located at the high frequency region. This behavior is common with the double layer capacitor. While the second was found at the vertical part at the low frequency area, there was a reasonable difference in the behavior of the films. This difference in behavior could have emerge from the KOH electrolyte used . This behavior is often associated with pseudocapacitance material.