Research Article | Open Access
Preparation of Chloro Penta Amine Cobalt(III) Chloride and Study of Its Influence on the Structural and Some Optical Properties of Polyvinyl Acetate
Chloro penta amine cobalt(III) cloride [Co(NH3)5Cl]Cl2 was prepared and then characterized by Fourier transform infrared spectroscopy and X-ray diffraction. The obtained results indicated the formation of orthorhombic [Co(NH3)5Cl]Cl2 nanoparticles of ≈28.75 nm size. Polymeric films based on polyvinyl acetate (PVAc) doped with chloro penta amine cobalt(III) cloride [Co(NH3)5Cl]Cl2 in different weight percent ratios were prepared using the solvent cast technique. The complexation of the additive with the polymer was confirmed by FTIR and SEM studies. The XRD pattern revealed that the amorphousicity of PVAc polymer matrix increased with raising the [Co(NH3)5Cl]Cl2 content. Parameters such as extinction coefficient, refractive index, real and imaginary parts, and optical conductivity were studied by using the absorbance and measurements from computerized UV-visible spectrophotometer in the spectral range 190–800 nm. This study showed that the optical properties of PVAc were affected by the doping of [Co(NH3)5Cl]Cl2 where the absorption increased by leveling up [Co(NH3)5Cl]Cl2 concentration. The nature of electronic transition from valence band to conduction band was determined and the energy band gaps of the composite films samples were estimated by UV-visible spectrum. It was observed that the optical conductivity increased with photon energy and with the increase of [Co(NH3)5Cl]Cl2 concentration.
Polymers can exhibit various mechanical, electrical, and optical properties depending on the synthesis conditions and chemical properties of the backbone . If a polymer is exposed to ultraviolet light, its chemical properties, such as solubility, of the polymer in the exposed area are changed. Photolithography, which is a well-known process in electronics, uses this principle .
Polymers are used in an amazing number of applications. More recently, significant developments have occurred in the area of flexible electronic devices based on the useful piezoelectric, semiconducting, optical, and electrooptical properties seen in some polymers .
Polymeric materials have special interest because, in combination with suitable metal salts, they give complexes which are useful for the development of advanced high energy electrochemical devices, for example, batteries, fuel cells, electrochemical display devices, and photo electrochemical cells with ease of fabrication into desirable sizes . Also polymers have unique properties such as light weight, high flexibility, and ability to be fabricated at low temperature and low cost . Optical communications, including polymer optical fibers, optical waveguides, and optical connectors due to their ease of process, relatively low cost, and mass production, are compared to silica-based optical materials. They also have potential advantages for applications in optical storage systems, such as high thermal stability, low absorption loss, and the ability of refractive index changing upon exposure to light . The electrical and optical properties of polymers have attracted a great attention in view of their applications in optical devices with remarkable reflection, antireflection, interference, and polarization properties .
Commercial vinyl polymers such PVAc (C4H6O2)n are intensively studied because of their broad applications in industry. Polyvinyl acetate is thermoplastic polymer. PVAc-based composites materials were significantly manufactured by resin emulsifier, adhesive, paper, paint, and textile industries owing to high-bond reinforced, film-like, odorless, and nonflammable characteristic and substrate for PVA production . The incorporation of various metallic additives into polymer matrices can produce polymer-matrix composites and improves its properties for specific applications .
Coordination compounds or metal complexes are metal ions surrounded by ligands. Ligands are either anions or molecules that can donate electrons into the d-orbitals of the metal ion and form a bond. Examples of common ligands are chloride ion, cyanide ion, ammonia, ethylenediamine, and ethylenediaminetetraacetateion (EDTA). The metal ions that form coordination compounds are from a group of metals known as transition metals. These metals have more than one oxidation state. This property allows the transition metals to act as Lewis acids . The metal complex used in this paper is chloro pentammine cobalt(III) chloride which is a paramagnetic compound . It decomposes upon heating above 150°C. Its solubility is 0.4 g per 100 mL at 25°C .
In this paper an effort has been made to study the effect of addition of [Co(NH3)5Cl]Cl2 on structural and optical properties of polyvinyl acetate by FTIR, XRD, SEM, and UV-visible spectrometer techniques. The results obtained from these measurements have been analyzed and discussed.
2.1. Preparation of Chloro Penta Amine Cobalt(III) Chloride [Co(NH3)5Cl]Cl2
Chloro penta amine cobalt(III) chloride [Co(NH3)5Cl]Cl2 was prepared by the procedure reported in the literature .
1.7 g of ammonium chloride NH4Cl was completely dissolved in ~10 mL of concentrated ammonia NH3 in a 400 mL beaker. With continuous stirring, 3.3 g of cobalt(II) chloride CoCl2 was added to the mixture gradually. When brown color slurry was obtained, 2.7 mL of 30% hydrogen peroxide H2O2 was added slowly. After the effervescence had stopped, ~10 mL of concentrated Hydrochloric acid HCl was add slowly. With continued stirring, the mixture is heated on a hot plate and maintains 85°C for 20 minutes, and then the mixture is cooled to room temperature in an ice bath and filter (using a Buchner funnel). The crystals of [Co(NH3)5Cl]Cl2 are washed with 5-6 times, 5 mL portions of ice water (distilled water cooled in ice) and then 5-6 times, 5 mL portions of ethanol C2H6O. All chemicals used in preparation of chloro penta amine cobalt(III) chloride were purchased from Sigma-Aldrich
2.2. Sample Preparation
Poly(vinyl acetate) (PVAc) with molecular weight 100,000 was purchased from Aldrich. PVAc/[Co(NH3)5Cl]Cl2 composites films were fabricated by the solvent casting technique. At first emulsion of PVAc using distilled water was stirred for 10 h. The necessary weight fractions of [Co(NH3)5Cl]Cl2 were first dispersed in distilled water with a magnetic stirrer for 1 h and then added gradually into the polymeric emulsion with continuous stirring and kept under string for 2 h. Finally, the solution was poured onto cleaned Petri dishes and allowed to evaporate slowly at room temperature for a week. After drying, the films were peeled from Petri dishes and kept in vacuum desiccators until use. The thickness of the obtained films was in the range of ≈120–150 μm.
X-ray diffraction scans were obtained using DX-2700 diffractometer using Cu Kα radiation ( = 1.5406 Å) operating at 40 kV and 30 mA, taken for the range of 5–50°. Measurements were carried out at room temperature. The diffracted intensity as a function of the reflection angel was plotted automatically by the X-ray diffractometer. The various peaks obtained in the diffraction pattern gave the information about the size and interplanar spacing of the compound. FTIR was recorded on Fourier transform infrared spectrophotometer, Shimadzu, model IR-Prestige 21, using KBr pellets. FT-IR spectra of the samples were obtained in the spectral range of (4000–400) cm−1. Ultraviolet-visible (UV-VIS) absorption spectra were measured in the wavelength region of 190–800 nm using double-beam spectrophotometer UV-1800 Shimadzu. The morphology of the films was characterized by scanning electron microscope using Bruker Nano GmbH, Germany, operating at 5 kV accelerating voltage.
3. Results and Discussion
3.1. X-Ray Diffraction (XRD)
A typical XRD pattern for [Co(NH3)5Cl]Cl2 is shown in Figure 1. It can be seen that many sharp peaks were observed in the X-ray profile. The crystalline nature of synthesized [Co(NH3)5Cl]Cl2 was observed by the various sharp crystalline peaks in the XRD pattern. It shows diffraction peaks at 15.8313, 25.6011, 32.6249, and 34.8279 corresponding to the (011), (221), (122), and (040) planes [Co(NH3)5Cl]Cl2 which could be indexed to orthorhombic structure which were consistent with the literature data of Materials Data, Inc. . The average particle size can be calculated using the first sphere approximation of Debye-Scherrer formula : where is the average diameter of the crystals, is the wavelength of X-ray radiation, and is the full width at half maximum intensity of the peak (FWHM). The obtained particle size of [Co(NH3)5Cl]Cl2 is 28.75 nm. The structural parameter such as diffraction angle (deg.), interplanar (Å), relative intensity , and full width at half maximum FWHM (deg.) are in Table 1.
PVAc are semicrystalline polymers as indicated from their XRD patterns illustrated in Figure 2(a). The crystalline nature of PVAc is emphasized by the diffraction peaks at = 19.54°, 40.54°, with a hallow shoulder at = 23° representing the amorphous phase in PVAc .
The function group present in the structure of PVAc has a role in increasing the carbon backbones disturbance, thus resulting in appears a crystalline phases in PVAc as shown in XRD pattern Figure 2(a) . Figures 2(b), 2(c), and 2(d) explained the XRD pattern of PVAc/3, PVAc/6, and PVAc/9 wt.% [Co(NH3)5Cl]Cl2, respectively. It can be seen that the intensity of essential peak of PVAc decreased and the band width increased with increasing the concentrations of [Co(NH3)5Cl]Cl2. The essential peak of PVAc represents the crystalline region in PVAc, so, the reduction of the intensity and broadening of this peak refers to decreases in crystallinity and increases in amorphousicity. This behavior demonstrates complexation between the filler and the polymers in the amorphous region . The behavior of PVAc/[Co(NH3)5Cl]Cl2 composite agrees with PVAc/Pb3O4  and PVAc/TiO2 . With 9 wt.% concentration the peaks belong to [Co(NH3)5Cl]Cl2 observed with lower intensity because [Co(NH3)5Cl]Cl2 structure is being capped with PVAc after formation of composites, which agrees with (Roy et al. 2013) . Polymers with 3-dimensional structure, such poly (vinyl acetate) (PVAc)  have rigid pores, that set an upper limit for additive growth inside such polymeric matrix .
Particle size of [Co(NH3)5Cl]Cl2 particles was found according to the preferred direction plane (011) for PVAc/6 wt.% and 9 wt.% of [Co(NH3)5Cl]Cl2 composites film which are around 22.06 nm and 23.50 nm, respectively.
3.2. Fourier Transform Infrared Spectroscopy (FTIR)
FTIR spectra of [Co(NH3)5Cl]Cl2 show peaks at 3278, 1620, 1307, 840, and 486 cm−1 which correspond to the NH3 stretching vibration, degeneration deformation vibration of NH3 ligand, symmetric deformation vibration of NH3, rocking vibration of NH3, and Co–NH3 stretching vibrations, respectively; also Co–Cl peak appeared around 840 cm−1. The FTIR characterization agreed with Najar and Majid (2013)  who investigated [Co(NH3)5Cl]Cl2. The only functional group of [Co(NH3)5Cl]Cl2 is N–H which must be around 3100–3500 cm−1. Figure 3 represents the FTIR spectrum of [Co(NH3)5Cl]Cl2; the N–H is between 3161.34 and 3279.1 cm−1.
The only functional group of PVAc is C=O. Figure 4(a) represents the FTIR spectrum for PVAc, C=O appeared around 1728.22 cm−1 , also C–O–C appeared around 1246 cm−1, while C–H appeared around 2935.66 cm−1 . It is worth noting that the absorption band near 3400 cm−1 is due to the O–H groups . Figures 4(b), 4(c), and 4(d) show that the PVAc absorption peaks are found to be shifted with adding [Co(NH3)5Cl]Cl2. The shifting gives an insight to an interaction of the [Co(NH3)5Cl]Cl2 in the polymer matrix . With increasing the concentration of [Co(NH3)5Cl]Cl2, the IR absorption peaks are increased due to stretching vibration shifted toward higher wave number , the absorption bands which belong to [Co(NH3)5Cl]Cl2 become more sharper, while the intensity of PVAc absorption bands is decreased indicating an obvious presence of [Co(NH3)5Cl]Cl2. The appearance of the absorption band around 1728 cm−1 for samples 3, 6, and 9 wt.% [Co(NH3)5Cl]Cl2 confirms the presence of PVAc in the samples . With 3 wt.% of [Co(NH3)5Cl]Cl2, the N–H is hidden behind the O–H rounded tip while at higher concentrations the N–H appeared as a sharp tip.
3.3. Scanning Electron Microscope (SEM)
Figures 5(a), 5(b), 5(c), and 5(d) show the SEM photographs of PVAc, PVAc/3 wt.% of [Co(NH3)5Cl]Cl2, PVAc/6 wt.% of [Co(NH3)5Cl]Cl2, and PVAc/9 wt.% of [Co(NH3)5Cl]Cl2 composite films, respectively. In Figure 5(a), some bright undissolved PVAc grains appeared. Other spots with different degree of roughness observed on the backscattered images shown in Figures 5(b), 5(c), and 5(d) seem to be agglomerates of [Co(NH3)5Cl]Cl2 particles, which increase with increasing the concentration of [Co(NH3)5Cl]Cl2. The average diameters of these agglomerated particles (grains) are around 0.885, 1.83, and 2.114 μm for PVAc/3, PVAc/6, and PVAc/9 wt.% of [Co(NH3)5Cl]Cl2 composite films, respectively. The change suggests that PVAc molecules may be dispersed in soft-segment phase with little influence on the microphase separation and mixing of the hard and soft segments. The degree of roughness of the film surface increases with increase of the content of [Co(NH3)5Cl]Cl2. This indicates segregation of the filler in the host matrix and this may confirm the interaction and complexation between the additive and the polymer  and also it may refer to growth of [Co(NH3)5Cl]Cl2 particles in PVAc matrix .
3.4. UV-VIS Spectra
The absorbance spectra for PVAc/[Co(NH3)5Cl]Cl2 doped films are shown in Figure 6. As indicated in Figure 6 [Co(NH3)5Cl]Cl2 enhances the absorbance of the PVAc host. The UV-visible absorption spectra of PVAc and PVAc/[Co(NH3)5Cl]Cl2 composites films are carried out at room temperature. The pretend spectral dependencies of optical functions unambiguously show that the principle role in the observed spectra plays electron-phonon broadening. The UV optical absorption pattern of PVAc exhibits an absorption band like shoulder at about = 260 nm. This band is attributed to the carbonyl group . It is observed that the wavelength corresponding to the absorption band like shoulder increases with increasing in [Co(NH3)5Cl]Cl2 content; this increase has been attributed to the minor structural inhomogeneities present in PVAc  which are due to growth of [Co(NH3)5Cl]Cl2 inside the polymeric matrix. As the composite films show a red shift behavior, these shifts indicate the complexation between the [Co(NH3)5Cl]Cl2 and the PVAc and may also be due to change in crystallinity with presence of additive . These results were confirmed by XRD results. From Figure 6, a small absorption band at about 500 nm was observed. Formation of new peaks for the samples and also broadening of those peaks with increasing [Co(NH3)5Cl]Cl2 indicate a considerable interaction between additive and host polymer . Also Figure 6 shows that the absorbance increases by adding different weight percentages of [Co(NH3)5Cl]Cl2; this is related to absorbance of [Co(NH3)5Cl]Cl2 or, in other words, the absorbance increases with percentages of absorbed particles . The absorption at any wavelength depends on the number of particles along the bath of the incident light (i.e., it depends on concentration of [Co(NH3)5Cl]Cl2) and on the length of the optical path passing through . These results have a good agreement with Abdelaziz .
Absorption coefficient is defined as the ability of a material to absorb the light of a given wavelength. The absorption coefficient was calculated from the optical absorbance by the following relation : Figure 7 shows the variation of the absorption coefficient with photon energy for PVAc/0, PVAc/3, PVAc/6, and PVAc/9 wt.% of [Co(NH3)5Cl]Cl2 composites films. It is clear that the absorption coefficient is increasing with concentration of [Co(NH3)5Cl]Cl2; this may be attributed to increase in the absorbance . Figure 7 also shows the dependence of absorption coefficient on incident photon energy, indicated from the low value of absorption coefficient with low value of photon energy and vice versa which means that the possibility of electron transition is increasing with photon energy.
From the absorption coefficient previous results, the electron transition of PVAc/[Co(NH3)5Cl]Cl2 is indirect. A good linear fit is obtained for and versus as shown in Figures 8 and 9, respectively. The respective values of are obtained by extrapolating to and for the allowed indirect transition and forbidden indirect transition, respectively. Content is responsible for the formation of some defects in the films. These defects produce the localized states in the optical band gap and overlap. These overlaps give an evidence for decreasing energy band gap when the [Co(NH3)5Cl]Cl2 content is increased in the polymeric matrix as shown in Figures 8 and 9. In other words the decrease in the optical gap reflects the increase in the degree of disorder in the PVAc films. Abdelaziz and Ghannam  observed similar results, respectively. Or it can be attributed to the additive complexation with the polymer matrix . These results agree with FTIR, SEM, and XRD observations.
Figure 10 shows the values of energy gap for indirect transition (allowed and forbidden) for (PVAc/0, PVAc/3, PVAc/6, and PVAc/9 wt.% [Co(NH3)5Cl]Cl2) composites.
The extinction coefficient was calculated using the following equation : The dependence of the extinction coefficient on the wavelength in the range 190–800 nm of PVAc/[Co(NH3)5Cl]Cl2 composites samples is shown in Figure 11. It is clear that the extinction coefficient for pure PVAc sample shows a decrease in values of the all wavelengths (190–800) nm, while it increases for PVAc/0, PVAc/3, PVAc/6, and PVAc/9 wt.% of [Co(NH3)5Cl]Cl2 in the wavelength from 400 nm to 800 nm. Extinction coefficient was increased for PVAc films with increasing the concentration of [Co(NH3)5Cl]Cl2; this is due to the increase in absorption coefficient .
The refractive index is a fundamental optical property of polymers that is directly related to other optical, electrical, and magnetic properties and is also of interest to those studying the physical, chemical, and molecular properties of polymers by optical techniques . The refractive index is calculated by  where is reflectance which is obtained from absorption and transmission spectra in accordance with conservation of energy law . Figure 12 represents the refractive index for PVAc/0, PVAc/3, PVAc/6, and PVAc/9 wt.% of [Co(NH3)5Cl]Cl2 composites films in the investigated range of wavelengths. Inspection of Figure 12 indicates for all compositions that the refractive index decreases with increasing wavelength. The figure shows that the refractive index increases as a result of increasing in the percentage of [Co(NH3)5Cl]Cl2 which is due to the increasing of the density of composites film as a result of [Co(NH3)5Cl]Cl2 content. In the literature, the relationship between refractive index and mass density is described as linear . Increasing in refractive index with concentration of [Co(NH3)5Cl]Cl2 is a result of increasing the number of atomic refractions due to the increase of the linear polarizability which agree with Lorentz-Lorentz formula .
Dielectric constant is defined as the response of the material toward the incident electromagnetic field. The dielectric constant of () is given by the following equation : where () and () are the real and the imaginary parts of dielectric constant, respectively, which can be obtained by the following equations : The dependence of the real part on the wavelength is shown in Figure 13 for PVAc/0, PVAc/3, PVAc/6, and PVAc/9 wt.% of [Co(NH3)5Cl]Cl2. It can be noticed, from this figure, that the real part depends on refractive index because the effect of extinction coefficient is very small so, it could be canceling . The real part of dielectric constant is increases with [Co(NH3)5Cl]Cl2 concentration and the curves vertex shifted to higher wavelengths with increasing [Co(NH3)5Cl]Cl2 percentage which may attributed to the dependency of the real part of dielectric constant on refractive index . The imaginary part of dielectric constant as a function of wavelength is shown in Figure 14. It is clear that the imaginary part depends on extinction coefficient especially in the range of wavelength around (390–800) where the refractive index stays almost constant while extinction coefficient increases with wavelength [34, 43].
The absorption coefficient and the refractive index were used to obtain the optical conductivity (), by the following relation : where is the velocity of light in the space. Figure 15 shows the variation of optical conductivity of PVAc/0, PVAc/3, PVAc/6, and PVAc/9 wt.% of [Co(NH3)5Cl]Cl2 composites films, as a function of photon energy. The conductivity of pure PVA is almost constant up to around 5.2 eV of photon energy, after that it increases with increase in photon energy. The [Co(NH3)5Cl]Cl2 concentration caused the increase in optical conductivity, which is due to high absorbance of the polymer composites films. The increase in optical conductance, and decrease in band gap energy of PVAc/[Co(NH3)5Cl]Cl2 with increase of [Co(NH3)5Cl]Cl2 concentration, can be attributed to the increase in number of mobile charge carriers and also to the increase in amorphous nature of host polymer . These results agree with Al-Taa’y et al. .
Polymer films based on PVAc with different concentrations of [Co(NH3)5Cl]Cl2 were prepared using solvent casting technique. XRD reviled that the synthesized [Co(NH3)5Cl]Cl2 was indexed to orthorhombic structure. The formation of an intermolecular interaction and complexation between PVAc and [Co(NH3)5Cl]Cl2 has been confirmed using XRD, FTIR, SEM, and UV. The UV results indicated that [Co(NH3)5Cl]Cl2 can effectively enhance the optical properties of PVAc. The absorption coefficient increased with increasing of the weight percentage of the additive. The increase in optical conductance and decrease in energy band gap of polymer host matrix with increase of [Co(NH3)5Cl]Cl2 concentration were attributed to the increase in number of mobile charge carriers and also to the increase in amorphous nature of the polymer host matrix. The optical constants such as extinction coefficients, refractive index, real and imaginary dielectric constants, and optical conductance are found to depend on the concentration of [Co(NH3)5Cl]Cl2 in the polymer film. PVAc/9 wt.% [Co(NH3)5Cl]Cl2 composites films show the best optical properties. This type of composites could be a suitable candidate for photovoltaic cells, although further studies and enhancement are desired. Also, this work confirms that the refractive index and energy gap are strongly correlated.
In summary, the measurements of optical properties indicate that the [Co(NH3)5Cl]Cl2 is useful additive to simultaneously increase both absorbance and optical conductivity of PVAc. As a result, the PVAc/[Co(NH3)5Cl]Cl2 composite film shows dramatic changes in optical properties which help it in optical devices fabrication.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge Dr. Nadher Najem for helpful discussions during the development of this work; also they want to express their deep procreation for Dr. Mohammed Hadi for his helpful assistance in XRD measurement.
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