Synthesis and Characterization of SILAR-Deposited Leaf-Like Copper Sulphide Films


 Successful synthesis of copper sulphide films deposited via successive ionic layer adsorption and reaction (SILAR) method at 100, 150, and 200 cycles. The deposited films were characterized for their morphological, structural, elemental, optical, and electrochemical features. A leaf-like morphology was obtained for the CuS films. Orthorhombic crystal structure with distinct peaks at (262) and (116) planes were observed. Good optical features, high absorbance with band gap energies ranging from 1.95 eV to 1.98 eV. The synthesized films possessed good electrochemical features with supercapacitive abilities. The synthesized materials find potential applications in optical devices and supercapacitors.


Introduction
A major concern for researchers and scientists in the world today is obtaining a sustainable energy that will address the issue of global warming, pollution, and help prevent the production of CO 2 which pose a threat to human life [1]. Renewable energy sources on the other hand, are alternative and sustainable sources that are natural, abundant, green, and renewable. Researchers and scientists delved into an extensive study on supercapacitors owing to their usefulness and wide range of applications, not only in storage devices but also in making energy readily available especially in remote areas [1]. Their business is to produce high power density but they are de cient in production of energy density in respect to batteries [2]. Supercapacitors or ultracapacitors keep electrical energy by dual means: double layer capacitance (electrostatic storage process) or pseudocapacitance (Faradaic redox storage process) [3].
Many years of research has shown that acidic and alkalinic electrolyte solutions produce high speci c capacitive materials while organic electrolyte yields high operating voltage [4]. Several compounds such as CoS, CuS, MnS, and NiS have been proven to be good electrode materials for supercapacitors [5] because of the sulfur content that is rich in energy density, and different oxidation states which support reversible redox reaction in energy storage devices [6].
Copper sul de is a p-type semiconductor known for its high extinction coe cient, high refractive index, speci c capacitance, band gap, and high conductivity which makes it of great interest to the optoelectronic [7], catalytic [8], and gas sensing [9] industries. Diverse processes have been adopted in fabricating CuS such as successive ionic layer adsorption and reaction (SILAR), chemical bath deposition (CBD), sol-gel, atomic layer deposition, spray pyrolysis [9] etc. In SILAR process, lm is prepared by dipping the glass slide into cationic and anionic precursor solutions with rinsing in the middle to remove the unbounded ions. SILAR method was employed because it is affordable, easy synthesis process, for depositing material at low temperatures, for fabricating exible photovoltaic devices.
Copper sul de lms have been deposited on glass substrates using cupric sulphate and sodium sul de aqueous solutions as precursors on glass substrate by SILAR method at room temperature with an obtained band gap energy of 2.14 eV [10]. CBD method was also utilized in growing CuS thin lms on glass substrates at 300 K with band gap energies ranging from 1.60 to 2.40 eV [11]. SILAR method was also used to prepare CuS lms from solutions of CuC and N S at two different molarities on glass substrates at room temperature with the band gap energy found to vary from 2.2 to 2.8 eV.
This work focuses on studying the effects of varying deposition cycles on SILAR-deposited CuS lms using thioacetamide as anionic precursor. The morphological, structural, elemental, optical, and electrochemical features of the deposited lms have been discussed.

Synthesis of CuS lm
Prior to the deposition, the substrates were washed with clean water, put in acetone for 25 minutes, rinsed in distilled water, placed in ultrasonicator for 25 minutes, and dried in an oven set at 60 o C for 15 minutes.
The substrates were weighed and kept in a neat container as the precursor solutions were prepared. For the cationic precursor solution, 0.21 M of cupric sulphate (CuSO 4 ) was mixed with 50 ml of distilled water and stirred for 10 minutes. 0.03 M of sodium thiosulphate (Na 2 S 2 O 3 ) was dissolved in 50 ml of distilled water and stirred for 5 minutes. The solutions were mixed and stirred continuously, with 1 ml of NH 3 added drop wise to maintain the solution at a pH of 6. For the anionic precursor solution, 0.025 M of thioacetamide (C 2 H 5 NS) was dissolved in 50 ml of distilled water and stirred for 5 minutes. A weighed and cleaned glass slide was placed in the cationic precursor for 10 seconds to allow the adsorption of Cu 2+ , rinsed in the rst beaker of distilled water for 5 seconds to remove the unbounded ions, placed in the anionic precursor for 10 seconds to allow reaction with S 2− , and nally rinsed in distilled water for 5 seconds to remove the in rmly bonded ions. The same procedures were taken to deposit the CuS on stainless steel substrates. The process was repeated for 100, 150, and 200 cycles at room temperature and annealed at 200 o C for 1 hour. The lms were weighed before and after deposition to provide for thickness values. The processes involved in the deposition of the CuS lms are described by Fig. 1 with the arrow head giving the sequence of deposition.

Characterization Techniques
The synthesized lms were characterized for their morphological, structural, elemental, optical, and electrochemical features using Zeiss scanning electron microscope, Siemens X-ray diffractometer, energy l 2 a 2 dispersive X-ray spectroscope (EDX), Shimadzu double beam spectrophotometer of model: UV-1800 in the wavelength range (200-1000 nm), and a Princeton applied research VersaSTAT potentiostat in a 3electrode system respectively. The lm thickness was measured using gravimetric weight difference method.

Surface morphology study
It is very important to study the surface characteristics of CuS lm since the charge storage in a supercapacitor is surface oriented. Fig. 2a-c exposes the surface images of the CuS lms deposited at different cycles. The lm deposited at 100 cycles contains irregular microspheres, pores and rod-like features scattered over the surface of the substrate. This porosity exhibited by the lm enables redox interaction to occur [12]. A clear change in the surface formations of the lm 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 lm. More so, as the number of cycles increased further to 200, the morphology was transformed to ower-like agglomerated features which clustered at the center of the substrate showing increased redox interaction [13].

Structural study
XRD is an indispensible tool in crystal structure analysis because it produces information about the phase structure of the material. The Cu x S 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 lms deposited at 150 and 200 cycles. It is interesting to know that the lm deposited at 100 cycles did not yield any prominent crystalline peak; probably because the deposition time was not su cient to transform the phase of the material [14]. 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 [15]. Additionally, the peaks produced by the 150 and 200 cycles deposited lms could have resulted from the effect of deposition time since the lms were subjected to the same annealing conditions.

Elemental composition
Fig. 4a-c displays a set of EDX spectra for the deposited CuS lms. 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 lms 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 con rms the deposition of CuS lm on the substrate. Fig. 5a shows the absorbance in (%) of CuS plotted against wavelength in (nm) for the lms 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 [14]. 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 lm 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 lms. Therefore, the thicker the lm, the more the ions inside the lm absorb UV radiation [16,17]. From Fig. 5b, annealing the lms increased the optical transmittance, this increment could be attributed to the decreasing and rearrangement defects of the lms. In addition the annealing lead to improvement of crystallinity of the lms structure [16,18,19]. Therefore, CuS thin lms can be used as transparent conducting material [11]. Fig. 5c shows the re ectance curves for CuS lms deposited at 100, 150 and 200 cycles. There is an exponential growth in the percentage re ectance of the lms with the wavelength. The peak re ectance was noticed by the lm at 200 cycles while the least was at 100 cycles. It is evident that at higher cycles, the lm was very thick, hence could not re ect much UV radiation. Again, all the lms had high re ectance in the NIR regions at wave lengths of 800 nm and 1000 nm. The low re ectance 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 lm 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 [20].

Optical studies
The extinction coe cient (k) refers to the quantity of absorbed energy in the lm. The decrease in k was due to the variation in the absorbance. The extinction coe cient in Fig. 7a increases with increase in the photon energy and decreases with increase in wavelength. The hump signi es that the deposited CuS lms absorbed reasonable amount of optical energy. Since k decreases with increasing wavelength, it shows that the transparent lms seem to behave like transparent insulators, preventing high energy UV and high energy temperature NIR radiation into a building while allowing only visible light [21]. The graph of optical absorption coe cient, α vs photon energy, hν is shown in Fig. 7b for the CuS lms. It is clear that the lms have a high absorption coe cient that means all lms 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.

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. The shape of the curve produced for all the materials are rectangular, indicating that CuS is good pseudocapacitive material [13]. The materials produced at 150 cycles possess wider rectangle curves indicating higher capacitance [14]. Additionally, there was no noticeable redox peaks in all the deposited electrode suggesting high stability and good reversibility of CuS [22]. Speci c capacitance was obtained from the CV by applying equation (1) [23][24][25] given as Here, m 1 and m 2 are masses before and after deposition of the lms, (V f and V i ) 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 speci c 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 speci c 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 [13]. The dependency of the frequency could be attributed to the departure between electrode and the ions. The highest speci c 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 [13] for a good supercapacitor material.
The Nyquist plot usually represents the plot of the Z im (Ohms) against the Z real (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 [26]. Fig. 11 represents the Nyquist diagrams of the CuS lms 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 lms clearly exhibit interesting curves having two sections. The rst 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 lms. This difference in behavior could have emerge from the KOH electrolyte used [27]. This behavior is often associated with pseudocapacitance material.

Conclusion
This work demonstrates successfully the synthesis of copper sulphide lms deposited via successive ionic layer adsorption and reaction (SILAR) method at 100, 150, and 200 cycles. All the lms were annealed at 200 o C for I hour after deposition. The deposited lms were characterized for their morphological, structural, elemental, optical, and electrochemical features using scanning electron microscope, X-ray diffractometer, energy dispersive x-ray diffractometer, UV-vis spectrophotometer, and a 3-electrode potentiostat. A leaf-like morphology was obtained for the CuS lms while the structural studies revealed orthorhombic crystal structure with distinct peaks at (262) and (116) planes observed. The CuS lms exhibited good optical features, high absorbance with band gap energies ranging from 1.95 eV to 1.98 eV. The synthesized lms possessed good electrochemical features with supercapacitive abilities. The synthesized materials nd potential applications in optical devices and supercapacitors.       Nyquist plots of the CuS electrodes