Studying Structural and Optical Properties of Thin Films LiNixMn2-xO4 (x = 0, 0.4, 0.5, 0.6) Prepared by Sol-Gel Method
Adnan Hafez Mini1, Mohammad Bashir Karaman2, Ahmed Khaled Kbetri1, *
1Department of Physics, College of Science, Tishreen University, Latakia, Syria
2Department of Physics, College of Science, Aleppo University, Aleppo, Syria
To cite this article:
Adnan Hafez Mini, Mohammad Bashir Karaman, Ahmed Khaled Kbetri. Studying Structural and Optical Properties of Thin Films LiNixMn2-xO4 (x = 0, 0.4, 0.5, 0.6) Prepared by Sol-Gel Method. American Journal of Nanosciences. Vol. 2, No. 4, 2016, pp. 46-50. doi: 10.11648/j.ajn.20160204.12
Received: October 14, 2016; Accepted: November 18, 2016; Published: December 12, 2016
Abstract: Thin films LiNixMn2-xO4 (x=0, 0.4, 0.5, 0.6) were prepared by Sol-Gel method employing spin-coated technique. The films were annealed at 600°C for 2 hours. The structural properties of the films were studied by XRD technique. The films were found in cubic spinel structure with decreasing in the lattice constant by increasing the substitution ratio until x=0.5. After this ratio additional peaks appearing in XRD pattern, this indicates a phase transition of LiNi0.6Mn1.4O4 thin film. Optical properties of the LiNixMn2-xO4 thin films were investigated by UV–VIS spectroscopy in [400-900] nm rang. The study indicated the presence of a high absorbency values in some part of visible range spectrum of the LiNixMn2-xO4 thin films. Direct optical band gap for thin films LiNixMn2-xO4 were estimated.
Keywords: Thin Films, LiNixMn2-xO4, Sol-Gel, XRD Properties, Optical Properties
In recent years, electronic devices have been considerably improved in integration and power consumption but improvements in size of the power source have been relatively slow compared with others. Therefore, establishing thin-film technologies for lithium ion batteries is an important step towards all-solid-state thin films batteries that are attractive for portable devices [1-3]. Thin films battery consist of three fundamental components: two electrodes referred to as cathode and anode, and an electrolyte . There are efforts in the development of improved electrolyte and anode materials, but the focus of this paper is on the cathode materials.
Cathode materials are typically oxides of transition metals, and one of the most promising cathode materials for rechargeable lithium batteries is spinel LiMn2O4, due to low toxicity, low cost and much safer in comparing with layered structure, like LiCoO2 and LiNiO2 .
Thin-film electrodes are often prepared by physical vapor deposition methods such as pulsed laser deposition , sputtering deposition [7,8]. These methods ensure control over uniformity and thickness of the films, but are limited with respect to substrate sizes and pricing. Wet chemical Sole-Gel techniques are low cost, flexible with regard to substrate size and geometry , and can overcome the disadvantages of physical vapor deposition methods growth, such as slow deposition rate.
LiMn2O4 exhibits a normal spinel structure with the Li+ ions occupying the tetrahedral sites and a 1:1 mixture of Mn3+ and Mn4+ ions being randomly distributed over the octahedral sites [10,11]. Fig. 1 is shown crystal structure of spinel LiMn2O4 .
The existence of Mn3+ ions in the octahedral sites of LiMn2O4 tends to induce a Jahn-Teller (JT) effect that is likely to cause structural instability during charge discharge cycling . An efficient way to improve this spinel is to substitute Mn by transition metal (M) such as Ti, Co, Mg, Ni and Al . In the present study, thin films LiNixMn2-xO4 (x= 0, 0.4, 0.5, 0.6) were synthesized using a Sol-Gel method. The effects of Ni substitution on the structural and the optical properties were investigated by using X-ray diffraction (XRD) and UV–VIS spectroscopy technique.
LiNixMn2-xO4 thin films were deposited on sodalime glass substrates using a sol–gel method. For LiNixMn2-xO4 thin films, the precursors solutions was prepared by dissolving LiCl, (CH3COO)2.Mn.4H2O and Ni(NO3)2.6.H2O powders together in ethanol with adding dimethylamine. All chemicals that used were supplied by Sigma – Aldrich. The molar ratio Li:Ni:Mn in the precursor solution was as it shown in table 1.
The substrate was spin-coated by the precursor solution at 4000 rpm for 30 s and then heated at 170°C for 2 min followed by 300°C for 5 min after each deposition in order to remove the organic substance. This process was repeated until the desired film thickness was attained. Then the precursor films were annealed in air at 600°C for 2 h. Prior to the film deposition, the substrates were cleaned by alkali solution method. The thickness of the obtained films by the above process was about 100 nm, determined by spectra manager software program linked with spectroscopy device JASCO v650.
The crystalline structure of the samples was monitored by XRD using Cu Kα radiation, using Leybold X-RAY Apparatus device (λ = 1.5405 Å), The diffraction data were recorded for 2θ between 10° and 70°. The optical properties of the films were investigated by using JASCO v650 spectroscopy device in range [400-900] nm.
3. Results and Discussion
3.1. Structural Properties
The XRD patterns of the LiNixMn2-xO4 (x = 0, 0.4, 0.5, 0.6) samples are displayed in Fig. 2. These patterns were compared with the JCPDS (JCPDS 35-0782) pattern of LiMn2O4 , and that indicates that crystal structure of thin films was formed according to a cubic spinel structure except for thin film LiNi0.6Mn1.4O4. For ratio x=0.6 additional peaks appearing at the right side of the (311) diffraction peak, and on the left side of the (400) diffraction peak as marked by * in Fig. 2, this indicates a phase transition for this ratio and the structure of this ratio is no longer a cubic spinel structure. The diffraction lines shifted towards higher 2θ angles with Ni substitution, indicating that the lattice parameter decreased with Ni content increase. The lattice constant a0 for thin films LiNixMn2-xO4 with cubic spinel structure (x=0, 0.4, 0.5) were calculated using equations (1) .
Where is miller index, d distance between parallel planes.
The distance between parallel planes is calculated using Bragg's law of X-ray diffraction using equation (2) .
Where n is a positive integer and λ is the wavelength of incident wave.
The value of distance between parallel planes, lattice constant and volume of primary cell for thin films LiNixMn2-xO4 with (x=0, 0.4, 0.5) are listed in table 2.
|Sample||Distance of parallel plans d Å||Lattice constant a0 Å||volume of primary cell VÅ3|
These values are compatible with previous studies where a0 for thin film LiMn2O4 is 8.225 Å in reference , for thin film LiNi0.4Mn1.6O4 is 8.174 Å in reference , and for thin film LiNi0.5Mn1.5O4 is 8.170 Å in reference . To indicate the impact of the substitutions ratio x on the lattice constant we draw variation of the lattice parameters as a function of Ni content x in spinel LiNixMn2-xO4 as shown in Fig. 3.
The decrease of the lattice parameters with Ni substitution in LiNixMn2-xO4 can be explained depending on bond length between Mn(Ni) and O in the compound LiNixMn2-xO4. The Mn(Ni)-O bond increased slightly with Ni substitution because of the larger atomic weight of Ni than Mn. The length bond Mn-O in compound LiMn2O4 is 1.823 Å , the length bond Mn(Ni)-O in compound LiNi0.4Mn1.6O4 is 1.804 Å and in compound LiNi0.5Mn1.5O4 is 1.800 Å . So a decreases in volume of primary cell happened.
3.2. Optical Properties
UV–VIS spectroscopy is a very useful technique for estimating the optical band gap of a semiconductor. The optical band gap is very much related to the electronic conductivity of the materials. Fig. 4 shows the UV–VIS absorption spectra of thin films LiNixMn2-xO4. The spectra are plots of absorbance against wavelength of the photon at normal incidence and at room temperature for the thin films. The UV-VIS spectra shows that the films have high absorbance in some parts of the visible region, which decreases gradually as the wavelength increases.
Band gap of the thin films where calculated for direct allowed transition by equation (3) :
Where, A1 is a constant related to characteristics of valence packs and conductivity packs of the semiconductor, Eg the energy band gap, hν energy of the photon, α absorption coefficient.
By presenting (αhν)² values as a function to hν Graphically, and extension of the linear part of the curve graph to crossing the hν axis, then the energy gap will be determined as it is shown in Fig. 5, Fig. 6, Fig. 7, and Fig. 8 for thin films LiMn2O4, LiNi0.4Mn1.6O4, LiNi0.5Mn1.5O4, and LiNi0.6Mn1.4O4 respectively. The direct band gap for thin films were estimated and listed in table 3.
These values are compatible with previous studies where Eg for thin film LiMn2O4 is 2.25 eV in reference . To indicate the impact of the substitutions ratio x on the energy band gap we draw variation of the energy band gap as a function of Ni content x in spinel LiNixMn2-xO4 as shown in Fig. 9.
As it shown in Fig. 9 the band gap is reduced to a value 1.84 eV for x=0.5. It is well known that the band structure of LiMn2O4 or Ni-substitute LiMn2O4 is strongly influenced by hybridization among transition metal 3d and O 2p orbitals, while Li remains in an ionic form. Thus, the decrease in band gap may be explained in terms of the total theoretical density of states (DOS) of pristine LiMn2O4 and LiNi0.4Mn1.6O4, LiNi0.5Mn1.5O4 thin films [22,23]. For thin film LiNi0.6Mn1.4O4 there are a slight increase in energy band gap, it may be due to some impurity phases in the structure of this film.
In this work, we have prepared thin films LiNixMn2-xO4 (x=0, 0.4, 0.5, 0.6) by Sol-Gel method employing spin-coating technique. The films annealed at 600°C for 2 hours. The structural properties of the films were studied by XRD technique. The films were found in cubic spinel structure with decreasing in the lattice constant by increasing the substitution ratio until x=0.5. The decrease of the lattice parameters with Ni substitution in LiNixMn2-xO4 can be explained depending on bond length between Mn(Ni) and O in the compound LiNixMn2-xO4, this bond increased slightly with Ni substitution because of the larger atomic weight of Ni than Mn, and hence improving structural stability. For x=0.6 a phase transition occur and the structure for this ratio is no longer cubic spinel. The UV-VIS spectra shows that the films have high absorbance in some parts of the visible region, which decreases gradually as the wavelength increases. The direct band gap is reduced from 2.11 eV for LiMn2O4 to a value 1.84 eV for LiNi0.5Mn1.5O4. The decrease in band gap may be explained in terms of the total theoretical density of states (DOS) of pristine LiMn2O4 and LiNi0.4Mn1.6O4, LiNi0.5Mn1.5O4 thin films. For thin film LiNi0.6Mn1.4O4 there are a slight increase in energy band gap, it may be due to some impurity phases in the structure of this film.