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Characterization of Cobalt Sulfide Thin Films Synthesized from Acidic Chemical Baths

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Characterization of Cobalt Sulfide Thin Films Synthesized from Acidic Chemical Baths Hindawi Advances in Materials Science and Engineering Volume 2020, Article ID 2628706, 9 pages https://doi.org/10.1155/2020/2628706 Research Article Characterization of Cobalt Sulfide Thin Films Synthesized from Acidic Chemical Baths Tizazu Abza ,1 Dereje Gelanu Dadi,1 Fekadu Gashaw Hone,1 Tesfaye Chebelew Meharu,1 Gebremeskel Tekle,1 Eyobe Belew Abebe,2 and Kalid Seid Ahmed2 1Hawassa University, Department of Physics, Hawassa, Ethiopia 2Adama Science and Technology University, Department of Materials Science and Engineering, Adama, Ethiopia Correspondence should be addressed to Tizazu Abza; [email protected] Received 13 December 2019; Accepted 26 March 2020; Published 30 April 2020 Academic Editor: Isabel J. Ferrer Copyright © 2020 Tizazu Abza et al. ,is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Cobalt sulfide thin films were synthesized from acidic chemical baths by varying the deposition time. ,e powder X-ray diffraction studies indicated that there are hexagonal CoS, face-centered cubic Co3S4, and cubic Co9S8 phases of cobalt sulfide. ,e crystallite size of the hexagonal CoS phase decreased from 52.8 nm to 22.5 nm and that of the cubic Co9S8 phase increased from 11 nm to 60 nm as the deposition time increased from 2 hrs to 3.5 hrs. ,e scanning electron microscopic images revealed crack and pinhole free thin films with uniform and smooth background and few large polygonal grains on the surface. ,e band gap of the cobalt sulfide thin films decreased from 1.75 eV to 1.3 eV as the deposition time increased from 2 hrs to 3.5 hrs. ,e photoluminescence (PL) spectra of the films confirmed the emission of ultraviolet, violet, and blue lights. ,e intense PL emission of violet light at 384 nm had red shifted with increasing deposition time that could be resulted from the increase in the average crystallite size. ,e FTIR spectra of the films indicated the presence of OH, C-O-H, C-O, double sulfide, and Co-S groups. As the deposition time increased, the electrical resistivity of the cobalt sulfide thin films decreased due to the increase in both the crystallite size and the films’ thickness. 1. Introduction [17–19], optical waveguides, thermal sensors, solar selective coatings, and optical filters [20]. Metal chalcogenide thin films are getting increasing atten- Currently, various deposition technologies, including tion in a variety of electrical, optical, and optoelectronic chemical bath deposition, are used to deposit metal chal- devices due to their physical and optoelectronic properties cogenide thin films [1]. Chemical bath deposition (CBD) has [1, 2]. Cobalt sulfide is one of the magnetic transition metal been employed as a thin film deposition technique for metal chalcogenides that exists in a number of phases such as sulfides, selenides, oxides, and others for a period of nearly a Co4S3, Co1-xS, Co2S3, CoS, CoS2, Co3S4, and Co9S8. ,ese century and half [21]. Despite an edged technique, CBD still multiphase structures are resulted from the existence of has a big unexploited potential to deposit different chal- cobalt in the oxidation state of +2, +3, or +4 [3, 4]. ,erefore, cogenides with intended properties by controlling the de- cobalt sulfide has a complicated chemical composition. position parameters [22]. ,e first formal deposition of ,ese different phases of cobalt sulfide are of particular cobalt sulfide thin films by the CBD method is credited to interest due to their unique applications as supercapacitors Basu and Pramanik, almost 120 years after the CBD tech- [5–7], water splitter to produce hydrogen [8, 9], catalysts nique had been used as a thin film deposition technique [23]. [10–12], diluted magnetic materials [13, 14], counter- Review of literatures indicates that there are very few reports electrodes for dye synthesized solar cells [15, 16], anode on chemical bath-deposited cobalt sulfide thin films materials for advanced sodium and lithium ion batteries [3, 20, 24–26]. Lokhande reported chemical bath-deposited 2 Advances in Materials Science and Engineering cobalt sulfide thin films using sodium thiosulfate as a sulfur 3. Results and Discussion source, cobalt sulfate as cobalt source, and disodium EDTA as complexing agents in an acidic solution [27]. As far as the 3.1. Structural Analysis. Figure 1 shows the XRD patterns of authors are aware, there is no report on cobalt sulfide thin cobalt sulfide thin films deposited at different deposition films deposited under acidic baths except Lokhande’s report. time. ,e XRD patterns of the films deposited at 2 hrs and ,erefore, characterization of cobalt sulfide thin films 2.5 hrs exhibited two clearly visible peaks at 2θ values of synthesized by changing various deposition conditions 14.8° and 21.2° with preferred orientation along 21.2° due to under acidic medium could have paramount importance for (102) plane of the hexagonal CoS structure (Pdf# 011-279). technologies that use cobalt sulfide thin films. In this work, Five additional faint peaks were also observed for the film we report the synthesis and characterization of cobalt sulfide deposited at 2 hrs as shown in Figure 1 and Table 1. ,e thin films from acidic baths using thioacetamide, cobalt peak at 16.4° is the reflection from the (111) plane of the ° acetate, and EDTA as starting materials. face-centered cubic Co3S4, and the peak at 24.4 is the reflection from the (220) plane of the cubic Co9S8. ,e peak ° 2. Experiment Procedure at 45.64 could not be assigned for a particular phase as both the cubic Co9S8(Pdf#19-0364) and the face-centered cubic Before the actual deposition of the cobalt sulfide thin films, Co3S4(Pdf#02-1338) structures have a common (400) plane the preparation conditions were optimized. Aqueous solu- about this 2θ value. ,e peak at 50.4° is also common for tions of analytical grade cobalt acetate (Loba Chemie), Co3S4, Co9S8, and Co4S3 as can be observed from the listed thioacetamide (Titan), and disodium EDTA (Fine Chem- Pdf card numbers on the same table. ,e peaks at 45.64° icals) were used as starting reagents. In a typical experiment, and 50.4° disappeared and six new peaks appeared at 17.26°, 9 ml (1 M) of cobalt acetate was mixed with 3 ml (0.2 M) of 18.36°, 22.32°, 23.52°, 29.54°, and 30.76° for the cobalt sulfide disodium EDTA in a 150 ml beaker under continuous thin film deposited at 2.5 hrs. ,e new observed peaks are stirring. ,en, 63 ml of deionized water was added to make related to the hexagonal CoS and cubic Co9S8 with no new up the volume to 75 ml. ,e pH of the solution was adjusted peaks related Co3S4 and Co4S3 phases. A new peak related to 4.6 before the addition of thioacetamide. After the ad- to the hexagonal CoS structure was observed at 2θ value of dition of 15 ml (1 M) thioacetamide, the pH was readjusted 20.4° when the deposition period increased to 3 hrs. As the to 4.6 by a drop wise addition of HCl. ,e solution color at deposition period increased to 3 and 3.5 hrs, the intensity of this instant was pink. ,e stock solution was transferred to the prominent peaks decreased and the faint peaks van- the water bath of temperature adjusted to 85°C. ,e pre- ished gradually. However, the intensity of the peaks at cleaned soda lime glass substrates were suspended vertically 18.36° and 22.32° which started to appear from 2.5 hrs in the stock solution. Four samples were prepared by taking deposition time increased gradually. For the deposition out a deposit in 30 minutes interval starting from 2 hrs after period of 2 hrs, 2.5 hrs, and 3 hrs, the preferred orientation the deposition began. ,e color of the films changed from was along the (102) plane of the hexagonal CoS structure. brown to black as the deposition time increased. ,e films ,e preferred orientation changed to the (200) plane of the were partially reflecting like a mirror. ,e reflectivity of the cubic Co9S8 structure for 3.5 hrs deposition period. In films increased with increasing the deposition time, and addition, the preferred orientation of the hexagonal CoS films deposited at 3 hrs and 3.5 hrs reflect as a plane mirror. structure changed from about 21.26° to 20.34° at 3.5 hrs ,e gravimetry film thickness measurement method was deposition time. ,e peak at 12.4° observed in the cobalt used to estimate the thickness of the cobalt sulfide thin films. sulfide thin films deposited at 2 hrs, 2.5 hrs, and 3 hrs does ,e obtained film thickness for cobalt sulfide thin films not match to any of the observed cobalt sulfide phases. ,e deposited at 2 hrs, 2.5 hrs, 3 hrs, and 3.5 hrs was 356 nm, structural analysis showed that as the deposition period 361 nm, 385 nm, and 434 nm, respectively. As the deposition increased, the presence of the face-centered Co3S4 [28] and time increases, ions get sufficient time for deposition on the the hexagonal CoS structures decreased and the Co9S8 substrate surface, thus increasing the layer thickness [20]. phase increased [12]. Crystallographic study of the thin films was carried out ,e average crystallite size of the films was calculated by a Shimadzu X-ray diffractometer with Cu K-α mono- using Scherer’s equation as follows [29]: chromatic radiation (λ � 0.15406 nm) operating at 40 kV and kλ 30 mA. ,e optical absorption of the samples was investi- D � ; (1) gated by a Shimadzu UV-3600 Plus UV-VIS-NIR spectro- β cos θ photometer, and the FTIR spectrum was recorded using a Perkin Elmer Frontier MIR/FIR infrared spectrometer.
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