Synthesis and characterization of Co-Al mixed oxide nanoparticles via thermal decomposition route of layered double hydroxide

Journal of Molecular Structure 1206 (2020) 127679

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Synthesis and characterization of Co-Al mixed oxide nanoparticles via thermal decomposition route of layered double hydroxide M.H. Abdel-Aziz a, b, *, M. Sh. Zoromba a, c, **, M. Bassyouni a, d, M. Zwawi e, A.A. Alshehri f, A.F. Al-Hossainy g, h a

Chemical and Materials Engineering Department, King Abdulaziz University, Rabigh, 21911, Saudi Arabia Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt Chemistry Department, Faculty of Science, Port-Said University, 42521, Port-Said, Egypt d Department of Chemical Engineering, Faculty of Engineering, Port Said University, Port Fouad, Egypt e Mechanical Engineering Department, King Abdulaziz University, Rabigh, 21911, Saudi Arabia f Electrical Engineering Department, King Abdulaziz University, Rabigh, 21911, Saudi Arabia g Chemistry Department, Faculty of Science, Northern Border University, 13211, Arar, Saudi Arabia h Chemistry Department, Faculty of Science, New Valley University, 7251, Al-Wadi Al-Gadid, Al-Kharga, Egypt b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 November 2019 Received in revised form 13 December 2019 Accepted 2 January 2020 Available online 6 January 2020

Cobalt-aluminum layered double hydroxide (Co-Al LDH) is synthesized in the presence of functional amino-organic compounds, including glycine, acetamide and urea. Mixed cobalt-aluminum oxide nanoparticles were prepared from Co-Al LDH by the calcination method. The mixed oxides nanoparticles are characterized by a number of diverse techniques such as FTIR, TGA, XRD, SEM and TEM. The presence of functional amino-organic compounds affects the XRD patterns of Co-Al LDH and the computed lattice parameters. Thin films from Co-Al LDHs with 100 ± 5 nm thickness were fabricated by thermal evaporation method. The average optical energy gaps for Co-Al LDHs films were 2.325 eV with the optical characterization ratio of s2 : s1 and ε1 : ε2 resulted from thin film. Based on the obtained results mixed Co-Al oxides nanoparticles are considered promising alternative materials for producing clean and renewable energy and for enhancing the power conversion efficiency of the prototype of solar cell. © 2020 Elsevier B.V. All rights reserved.

Keywords: Layered double hydroxides Mixed Co-Al oxide nanoparticles Thermal decomposition Optical properties

1. Introduction Advantages of Layered Double Hydroxides (LDHs) such as big surface area, supercapacitors [1], highly positive surface charge, bioactive nanocomposites [2,3] and low coat play important role in the effective removal of negatively charged organic dyes [4,5]. In addition, the (LDHs), transition metal oxides with organic cyanide dyes, are a kind of ionic thin layers materials following the set of the anionic materials [6e9]. To achieve hardly synthesized Mixed Metal Oxides (MMO) of high thermal chemical stability and large specific surface areas, the LDHs go through structural transformation and decomposition after calcination at temperatures ranges from 400 to

* Corresponding author. Chemical and Materials Engineering Department, King Abdulaziz University, Rabigh, 21911, Saudi Arabia. ** Corresponding author. Chemical and Materials Engineering Department, King Abdulaziz University, Rabigh, 21911, Saudi Arabia. E-mail addresses: [email protected], [email protected] (M.H. AbdelAziz), [email protected] (M.Sh. Zoromba). https://doi.org/10.1016/j.molstruc.2020.127679 0022-2860/© 2020 Elsevier B.V. All rights reserved.

600
C. LDHs material before calcination such as Co (Ni)eFeeLDH [10], MneFeeLDH [11], MgeAleLDH [12], ZneAleLDH [13] and their resulting Layered Double Oxides (LDOs) [14] have been studied. The calcination of synthetic LDHs at different temperatures to produce mixed metal oxides (MMOs) which make it suitable for their use as catalysts can achieve good basic characteristics and great surface area. The preparation of numerous organic moieties is probable via the precise calcination of LDHs such as nitrous oxide (N2O) decomposition [15,16] and condensation methanation [17,18]. Commonly, LDHs are weak bases and attain enriched basic characteristics especially when it is activated by thermal calcination route [19,20]. In contrast to the originating LDH, calcination of LDHs for mixed metal oxide results in an increasing surface area and reactivity. In many fields, LDHs and calcined LDHs are applicable, including; catalysts [21], ecological cleaning by adsorption or ion exchange process [22,23], selective nano-reactors [24], membrane separation technology [25], and photoactive ingredients [26]. In the last decades the developments of alternative renewable

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energy sources have a great concern due to the dangerous ecological influences correlated to the consumption of fossil fuels [27,28]. As known, the breakdown of the layered configuration leads to creation of (MMOs) with high specific surface area by thermal calcination process. By the well-validated production procedures of LDHs, fine dispersible bimetal oxides are attained from the LDHs by calcination process due to the even spreading of the cations in the brucite layers of LDHs. MMOs such as Zno and TiO2 are considered good choice to fabricate electrodes in solar cells of the type DSSC (Dye Sensitized Solar Cell) due to their related characteristics such as, injection efficiency, band gap energy, and photo response [29,30]. Also because of the big specific surface area of MMOs it can find promising requests in energy storage applications as for Li batteries [31]. This study aims to synthesize cobalt-aluminum layered double hydroxide (Co-Al LDH) in the presence of functional amino-organic compounds, including glycine, acetamide and urea, and to study the structural, optical, and electric properties of the fabricated film from these LDHs. Also the work aims to prepare mixed cobaltaluminum oxide nanoparticles from Co-Al LDH by the calcination method. The thermal stability of the as-prepared (Co-Al LDHs) are thoroughly examined using thermogravimetric analysis (TGA) and FT-IR techniques. Also, the configuration and morphology description of the Co-Al LDH films are studied.

labeled by Co-Al LDH, Co-Al/A LDH, Co-Al/G LDH and Co-Al/U LDH, respectively.

2.3. Mixed Co(II)-Al(III) nanoparticle synthesis The Co(II)-Al(III) LDHs are calcined during 190 min at 600
C to produce mixed aluminum and cobalt oxide nanoparticles. 2.4. Fabrication of (Co-Al)/A LDH thin film Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films on washed quartz substrates (chromic acid/deionized water) are produced with a high vacuum coating system (Edwards type E 306 A, England) by standard thermal evaporation at a vapor pressure of about 1  104 Pa. These films were vacuum-heated to 200
C and then gradually cooled down to 25
C. The stoichiometry of the uploading film was supported by a crystal-thickness screen for quartz (Model FTM4). The platinum metal electrode was produced with a 100 ± 2 nm thin layer of platinum film using magnetron sputters. Depending on the dipping operations, the average thickness of all Co-Al LDH, Co-Al/G LDH, Co Al/A LDH and Co-Al/U LDH films has been edited to 100 ± 5 nm.

2. Experimental details

2.5. Characterization

2.1. Materials

Characterization methods and typical conditions are listed in Table 2.

The list of chemicals used in conducting the current experiments are presented in Table 1. All chemicals were utilized as received without supplementary refinement.

3. Results and discussion 3.1. Fourier transform infrared (FT-IR)

2.2. The LDHs synthesis The LDH synthesis has been achieved at a room temperature via co-precipitation method with a molar ratio of AlCl3: CoCl2$6H2O: NaCl ¼ 1:2:3. Typically, 2.5 g of AlCl3 has been dissolved in an appropriate volume of distilled water, 8.92 g of CoCl2$6H2O was added to the AlCl3 solution while being agitated with magnetic stirrer at 850 rpm, and 3.28 g NaCl added to the mixture as a host material. Distilled water was added to the solution mixture until the volume reached 200 ml in order to precipitate Co-Al LDH NaOH solution (25% wt) was added at ambient temperature drop by drop until pH reaches 10.5. The resulting slurry was left in the water bath for 24 h at temperature up to 75
C, then it was filtered followed by washing with distilled water for several times to remove excess soluble ions, and the washing process was continued till filtrate pH reaches 7. The separated solid was dried in an oven at 80
C. In the presence of acetamide, glycine and urea the same procedure was repeated. Ratio between AlCl3: CoCl2. 6H2O: NaCl: acetamide, glycine or urea was 1:2:3:6. The elaborated Co-Al LDH in the absence and in the presence of acetamide, glycine and urea were

Table 1 List of chemicals used and their molecular formulas. Chemical

Molecular formula

Supplier

Glycine Acetamide Urea Sodium chloride Sodium hydroxide Cobalt chloride hexahydrate Aluminum chloride

C2 H5 NO2 C2 H5 NO CH4 N2 O NaCl NaOH CoCl2 6H2 O AlCl3

Aldrich Aldrich Aldrich Merck Merck Sigma Sigma

FT-IR spectra of the products are given in Fig. 1, confirming the presence of water molecules intercalated in the interlayer. The absorption band at approximately 3437e3533 cm1 corresponds to the stretching vibrations in the interlayer and then in the hydroxyl group of OeH water molecules [32,33]. Metal e oxygen (MeO) stretching and bending modes result in further absorption bands below 836 cm1 [34,35]. The assignments of free Co-Al LDH and CoAl/G LDH film matrix are examined in comparison. Co-Al/G LDH matrix gives four n(C]O), n(NeH), n(CeH) stretching, and (OeH) bending absorptions occurring at y 1645, 1564, 1459 and 1412 cm1, which have been assigned to the symmetric and antisymmetric stretching vibrations of n(C]O), n(NeH), n(CeH) stretching, and (OeH) bending groups, respectively [36]. In contrast, the Co-Al/A LDH matrix gives the symmetric and anti-symmetric vibrations of NH2 groups at 3440 and 3287 cm1, respectively. In the frequency field the vibration of the acetamide (C]O) occupies (1766 cm1) and the acetamide NH2 bending vibration (1478 cm1) is related [37]. Fig. 1 displays the bands in the Co-Al/A LDH matrix region 1119e1237 cm1, these bands are attribute to the n(CeO) and n(CeC) stretching bands of the ester association. The peak of 470e743 cm1 is attributed to the bonds stretching n(CoeO) and n(AleO). The band at 743 cm1 is attributed to the n(AleO) bond. Finally, Co-Al/U LDH matrix provides an extremely sharp NH2 absorption band at 3533 cm1, which has been assigned to NH2 group symmetric and antisymmetric stretching vibration. The spectral area includes the stretching band of acetamide (1829 cm1) and NH2 (1458 cm1) due to the bending vibration of acetamide [38]. In the Co-Al/A LDH matrix, the 589 and 883 cm1 bands are assigned to the CoeO and AleO bonds, respectively.

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Table 2 List of characterization methods. Characterization method

Model used and specifications

FT-IR TGA

Perkin-Elmer FT-IR type 1650 spectrophotometer (wavelength range 4000e200 cm1). Perkin Elmer Thermogravimetric Analyzer TGA 4000 was employed. Temperature range 50e995
C and heating rate is 10
C/min in nitrogen atmosphere. Philips X-ray diffractometer (model X’pert) with mono-chromatic Cu Ka radiation operated at 40 kV and 25 mA. Scanning electron microscopy (SEM; Inspect S, FEI, Holland), operated at an accelerating voltage of 3.0 kV TEM, JEOL ARM-200F, JEOL, Japan. Edwards type E 306 A, England Model FTM4. Edwards, England SHIMADZU UV-3101 UVeviseNIR pc.

XRD SEM TEM Thermal Evaporator Film thickness and rate monitor UV

Fig. 1. FT-IR spectra of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH.

3.2. TGA analysis Fig. 2 shows that the thermal stabilization of LDHs is influenced by the presence of glycine, acetamide and urea molecules. From their analysis, the complete thermal degradation of Co-Al LDH, CoAl/G LDH, Co-Al/A LDH and Co-Al/U LDH occurs in four stages. A first step within 20e165
C shows that the moisture in Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH can be removed. Within the same range of temperature, the thermal degradation process demonstrated one clear plateau at 80
C with the weight decreasing at a constant temperature. From Fig. 2, Co-Al/G LDH, CoAl/A LDH and Co-Al/U LDH thermal stability is more stable than that of Co-Al LDH. The LDHs matrix are transformed into unstructured mixed metal oxides by the calcination over 165
C. The oxides obtained in this way are evenly distributed for non-similar cations in a mixed oxide structure, which is an important motivation for the use of the LDH in the use of catalytical applications. At 100 and 200
C, the Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH demonstrate slightly different behaviors in the same order that is illustrated above. The mass loss up to 200
C comes from the deposition of the inner layer and adsorbed H2O [39]. Third stage of the decomposition (308e655
C), the residual weights of Co-Al LDH, Co-Al/U LDH, Co-Al/A LDH and Co-Al/G LDH from the decomposition of interlayer hydroxyl and chloride anions are 80.02, 81.19, 82.04 and 82.51%, respectively. In the LDH sample the

existence of Co(II) and Al(III) in the thermal strength was insignificant. From Fig. 2, the appropriate calcination temperature is 655
C which is essential to the efficient reconstruction of the layered structure. The calcination temperature of the LDHs should be higher than that of the surface decomposition and lower than that of the development of the spinel process in the 4th stage, at remaining 68.18, 69, 34, 70, 25, and 70, 71% for Co-Al LDH, Co-Al/A LDH, and Co-Al/G LDH respectively. Thus, the temperature of calcination for LDHs is usually around area y 308e655
C [40]. 3.3. XRD Fig. 3 shows the XRD patterns of all LDHs thin films with glycine, acetamide and urea before and after calcination [Co-Al LDH] AC. After calcination, all LDHs were converted to mixed metal oxides of cobalt and aluminum as illustrated by [Co-Al LDH] AC pattern. Compared with the as-prepared Co-Al LDH, the peaks of Co-Al/G LDH, Co-Al/U LDH and [Co-Al LDH] AC no shifts in 2q values obviously. After the formation of modified LDHs, the (220), (311), (400), (511) and (440) peaks appears, but lower than original Co-Al LDH corresponding to 2q. And compared with Co-Al/A LDH, the feature diffraction peaks of Co-Al/A LDH appear after adding acetamide as weak peaks. Single nanosheets are obtained by destroying the layer structure, which can be conveniently observed by SEM and TEM images from thin films of Co-Al LDH before and after calcination in

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Fig. 2. Thermal analysis of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH.

Fig. 3. XRD for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films.

Fig. 4a and b, respectively. The d-spacing is calculated by the law of Bragg’s from the maximum position:

nl ¼ 2dsinq

(1)

where l is the wavelength (1.5406 A) of X-rays and n ¼ 1. Table 3 shows the typical values of computed d-spacing, Miller indices hkl, FWHM, D for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films, respectively. The basal spacing (d331) of the peak of (331) was 2.44 nm at 2q ¼ 36.85
for CoeAl LDH. A single step is used for the mediated CoeAl LDH used in the work and an interlay spacing (d331) of the LDH is 2.44 nm [41,42].

Co-Al LDHs calculated XRD data have been checked by means of a special program (Software Crystal Sleuth) and it was found that the peaks are significantly in line with the values mentioned. From Fig. 3 and Table 3, the distribution of the peaks FWHM and D indicate that nanostructure thin films containing Co-Al LDH, Co-Al/ G LDH, Co-Al/A LDH and Co-Al/U LDH are expected to remain stable after crystallization [43]. Symmetry structure (Cubic) refinement is obtainable. The lattice parameters of the Co-Al LDHs nanostructure thin films are space group P23, abc ¼ 8.0910(1); 8.0815(1) & 8.0790(1) Å and V ¼ 529.7(3); 526.0(3) & 529.2(2) Å3 for Co-Al LDH, Co-Al/G LDH and Co-Al/U LDH nanostructure thin films, respectively. In comparing the results from the measurements of SEM and

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Fig. 4. a. SEM and TEM iamges of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH before calcinations. Where the original SEM of Co-Al LDH before calcination is [Co-Al LDH]Sb and TEM befor calcination is [Co-Al LDH]Tb.b. SEM and TEM iamges of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH after calcinations. Where the original SEM of Co-Al LDH before calcination is [Co-Al LDH]Sa and TEM befor calcination is [Co-Al LDH]Ta.

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Fig. 4. (continued).

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Table 3 The computed d-spacing, Miller indices hkl, FWHM, D for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films. 2q

System/Cell Parameters

a

b

Cubic [44e46]: abc ¼ 8.0910(1) ; 8.0815(1) & 8.0790(1) a

b

c

V ¼ 529.7 (3) ; 526.0 (3) & 529.2 (2)

abg ¼ 90


c

31.24 36.85 44.74 59.31 65.18

d (nm)

2.86 2.44 2.02 1.56 1.43

hkl

Co-Al LDH

220 311 400 511 440

Average a b c

Co-Al/G LDH

Co-Al/U LDH

FWHM

D

FWHM

D

FWHM

D

0.7488 0.9447 0.8244 1.0514 1.2207 0.9580

10.62 8.420 9.640 7.560 6.510 8.550

0.9017 1.0619 1.5893 1.5678 1.1384 1.2518

8.82 7.49 5.00 5.07 6.98 6.67

1.1640 1.1706 0.9982 0.9860 1.0447 1.0727

6.83 6.79 7.97 8.06 7.61 7.452

Co-Al LDH. Co-Al/U LDH. Co-Al/G LDH.

XRD of the Crystallite (D) of Co-Al LDHs in nano-structure thin film, it was observed that the SEM (D) is greater than the XRD because of its composition nature, but the results of XRD calculations depend on refraction and reflection of XRD rays. 3.4. SEM and TEM Fig. 4a and b demonstrate the SEM and TEM images of Co-AL LDH. As shown, the sample consists usually of spherical platelets. The modified samples were identical to the original Co-Al LDHs. The images by SEM in Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH are tested after drying before calcination (Fig. 4a), then these twisted nanosheets stack together loosely. The ultrathin and wrinkling morphology of Co-Al LDHs nanostructures thin images (Fig. 4a) is also well demonstrated. Most thin films in the nanostructure often disperse on a quartz substrate which creates a crystal matrix. Thin films are suitable for compositing with other substances for calcined Co-Al LDHs. The SEM and TEM images in Fig. 4b are shown in the surface and internal morphology of the calcined Co-Al LDHs self-assemblage in thin films. As illustrated, CoO and Al2O3 are deposited between layers and shape a stacking structure organized (blue color of TEM after calcination (blue color in TEM after calcination [Co-Al LDH]TA Fig. 4b). A lot of CoO and Al2O3 are seen dispersal inside the material in the crevice of nanostructure of the Co-Al LDH thin films, which can be well observed in the Fig. 4b. Metal oxides between surfaces not only increase the contact area of electrolyte ions, but also enhance conductivity. Therefore, the Co-Al LDH electrochemical property is much better than that of Co-Al LDH thin films due to calcination [47,48]. 3.5. Optical properties The basic absorption edge of the UV region is valuable for the elucidation of the crystalline/non-crystalline optical transformations and electronic band structures. Co-Al LDH thin films is utilized as a semiconductor in numerous applications. The effect of 50% from poly-glycine, poly-acetamide and poly-urea on the absorption spectrum and optical band gap of Co-Al LDH thin films was investigated in this study. Fig. 5 depicts the absorption spectra of Co-Al LDH thin films, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH on the absorption spectra optical properties. As noticed, the maximum absorption occurs at the wavelength l y 448 nm for Co-Al LDH thin films. This peak is related to the electronic transitions from the bonding molecular orbits (p) to the anti-bonding (p*) of LDH’s intercalated acetamide from the highest occupied molecular orbital (HOMO) to the lowest occupied molecular orbital (LUMO) [49e51]. The absorption edge has been shifted to lower wavelength, indicating a growing energy band gap and consequently a decrease in photocatalytic activity in the visible

region. Fig. 6 indicates the spectral transmission, T(l) at a normal occurrence of light within the range 250e1000 nm of wavelength of thin films Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH. At wavelength l  408 nm, the transmittance is about 94.77%, 82%, 99.98 and 99.82 for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films respectively. The maximum transmittance recorded was for Co-Al/A LDH thin films. The transmittance edge of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films is centered at 448 nm, 418 nm, 412 nm and 408 nm respectively. Thus, a wavelength shift of D l y 40 nm was observed to lower l (blue shift). Finally, the structural behavior shows that the LDHs film are flat and homogenous. The optical absorption edges contribute to a smaller shift in wavelength, which is attributed to glycine, acetamide and urea in the LDH’s matrix. The spectrum of absorption near the fundamental edge is also useful for study in the type of optic transitions (i.e., direct or indirect band transitions) occurring in any semiconductor, as well as its optical energy gap. The absorption coefficient can distinguish from the valence to the conduction bands the main absorption region, which is equal to electron excitation. The absorption coefficient can be examined at the fundamental edge in many semiconductor materials in the band-to-band transition theory [52,53]. The optical band gap was computed utilizing the Tauc’s equation as [54,55]:




ahn ¼ R hn  Eg

m

(2)

where a is the absorption coefficient (cm1), Eg is the optical energy gap (eV), R is independent photon energy, but depends on the transfer probability and (m) shows the transition process. In the current study, a plot (ahn)2 vs. (hn) defines the allowed direct optical band gap. The band-gap energy can be calculated from the linear part of the lower photon energy region (through zero y-axis extraction). The (ahn)2 dependence on the photon energy incident of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films. According to Fig. 7., Eg direct was 2.103 eV for the Co-Al LDH, it increased to 2.251 eV for Co-Al/G LDH, then it increased to 2.449 eV for Co-Al/A LDH and increased once again for Co-Al/U LDH thin films to 2.498 eV. For Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films with a thickness of 200 ± 5 nm, the measured values of nðlÞ, and kðlÞ are obtained. The Figs. 8 and 9 show the dependency of both the refractive index, n(l), as well as the index of absorption, k(l). In photon Energy, hn direct, from 1.00 eV to 4.70 eV, were found calculated optical constants nðlÞ, and kðlÞ, Fig. 8 shows the distribution behavior of absorption index, k(l) of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films, in, hn, range of 1.00 eVe4.70 eV. All samples present the same photon energy distribution varying from 2.00 eV to 4.70 eV, and revealing an only absorption peaks in the UVevisible regions and a

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Fig. 5. Absorbance spectra as a function of wavelength for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films.

Fig. 6. Transmittance spectra as a function of wavenumbers for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films.

maximum value of kðlÞ ¼ 2.78, 3.20, 3.01 and 3.03 for of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films, respectively. The maximum value corresponding to the broad signal related to the index kðlÞ corresponds to the p / p* benzenoid rings transition. But this signal showed a sudden fall at 2.80 eV. A decrease of

the absorption index, k(l) to approximatively hn ¼ 3.70 eV was observed, then it increases again with raising the wavelength. A red shift is also observed for Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films [56]. In addition, Fig. 9 displays a similar variation of the, nðlÞ, in the

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Fig. 7. Allowed direct energy band gaps of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films.

Fig. 8. kðlÞvisðhnÞ of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films.

incident hn focused on expanding from 2.25 to 4.00 eV, with UVevisible region. The refractive index nðlÞ behavior of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films exhibited a single peak in the range of 1.00e4.70 eV of photon energy with the

maximum value of 2.76, 3.00, 3.04 and 3.03 for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films respectively. A decrease of the refractive index nðlÞ to approximatively hn ¼ 3.70 eV was observed, then it increases again with raising the

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Fig. 9. nðlÞvisðhnÞ for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films.

wavelength and slightly shift towards higher wavelength (red shift) for Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films [57,58]. Any material is specified by its dielectric constant. The real part ε1(u) and the imaginary part ε2(u) are distinguished for the dielectric constant. ε1(u) is connected to the dispersive behavior,

while, the imaginary part ε2 (u) matches to absorptive behavior of the material. The loss factor expressed by the ratio ε2 ðuÞ=ε1 ðuÞ is determined by evaluating the values of ε1(u) and ε2(u). It also demonstrates energy loss in a dielectric substance that slow polarization currents and other dissipative effects. ε1 ðlÞ and ε2 ðlÞ are

Fig. 10. ε1 visðhnÞ for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films.

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Fig. 11. ε2 visðhnÞ for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films.

function of nðlÞ and kðlÞ by the following expressions [59,60]:

ε1 ðuÞ ¼ n ðuÞek ðuÞ 2

and

2

ε2 ðuÞ ¼ 2nðuÞkðuÞ (3)

(4)

Figs. 10 and 11 exhibits the behavior of both ε1 (u) and ε2 (u) vs the photon energy Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/ U LDH thin films. The two parts of the dielectric constant behavior

Fig. 12. s1 visðhnÞ for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films.

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Fig. 13. s2 visðhnÞ for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films.

of display only one peak. The real part ε1 (u) according to Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films are described by the presence of a peak at a photon energy 2.78 eV, 2.97 eV, 3.01 eV and 3.01 eV respectively. The real dielectric constant ε1(u) is more intense with the Co-Al LDH and a slight shift to higher photon energy is observed for Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films. Similarly, ε2 (u) displays a single peak at a photon energy 2.78 eV, 3.3 eV, 3.01 eV and 3.04 eV respectively Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films. However, the signal intensity is much more important with the CoAl LDH [61]. The complex optical conductivity sopt (i.e. the description of the photoconductivity properties of these materials) plays an important role in the understanding of optoelectronic applications of any semi-conducting material. sopt is related by the specified equation to the complex dielectric constant [62]: sg opt ðhnÞ ¼ s1 ðhnÞ þ is2 ðhnÞ, where s1 ðhnÞ ¼ uε2 ε0 and s2 ðhnÞ ¼ uε1 ε0 , where s1 is real conductivity and s2 is imaginary conductivity. Figs. 12 and 13 display the s1 and s2 dependency parts of optical conductivity (hn) for CoAl LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films. In addition, s2, has a higher value than s1 of. The obtained results in Figs. 12 and 13 also show that there is only one peak for both s1 and s2 at, hn, ranged from 1.00 to 4.50 eV. This peak may be ascribed to the optical inter-band transitions. At photon energy 3.95 eV, the values for s1 were 5.24  105 U1 m1, 3.32  105 U1 m1, 3.01  105 U1 m1 and 3.04  105 U1 m1 respectively for CoAl LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH thin films. In addition, s2 were found to be 1340 U1 m1, 1320 U1 m-1, 1220 U1 m-1 and 1080 U1 m1 respectively. 4. Conclusion Via co-precipitation method at 25
C, uniform and large-sized platelets of Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U

LDH were successfully synthesized. The plate dimensions can be controlled carefully by adjusting transition metal salt and organic polymers concentrations (Glycine, acetamide and urea). The matrix of organic molecules is interspersed between LDH layers. In the absence and existence of organic polymer matrix, thermal stability of LDHs is measured. The thermal stability Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH is more stable than Co-Al LDH. Thin films of LDHs with a thin thickness 100 ± 5 nm are prepared in high vacuum coating units with a thermal evaporation process. XRD is not sharp and uniform in the Co-A/A LDH system due to the extreme amorphous nature. The particle size was determined using TEM images, the size was located within the range of 7.24e11.71 nm for as-synthesized film form LDHs. The average direct energy gaps for Co-Al LDH, Co-Al/G LDH, Co-Al/A LDH and Co-Al/U LDH nanostructure films are 2.103 eV, 2.251 eV, 2.449 eV and 2.498 eV, respectively. The results of the optical characteristics, ratio s2 : s1 and ε1 : ε2 confirm that a thin film from LDHs constructed heterojunction can be used as a solar cell and transistor.

Author contributions All authors contributed in all parts of the paper as follows: M. H. Abdel-Aziz (Designed the methodology and analysis Collected the data- Contributed data analysis tools - Performed the analysis - Wrote the paper). M. Sh. Zoromba (Designed the methodology and analysis - Collected the data- Contributed data analysis tools - Performed the analysis - Wrote the paper). M. Bassyouni (Designed the methodology and analysis - Collected the dataContributed data analysis tools - Performed the analysis - Wrote the paper). M. Zwawi, (Designed the methodology and analysis Collected the data- Contributed data analysis tools - Performed the analysis - Wrote the paper). A. A. Alshehri (Designed the methodology and analysis - Collected the data- Contributed data analysis tools - Performed the analysis - Wrote the paper). A. F. Al-Hossainy

M.H. Abdel-Aziz et al. / Journal of Molecular Structure 1206 (2020) 127679

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