Effect of Li concentration on the structural and optical properties of Co 1-x Li x Cr 2 O 4 chromate nanoparticles prepared by sol-gel method

: CoCr 2 O 4 and doped lithium Co 1-x Li x Cr 2 O 4 chromate powder and nanoparticles were prepared by modified by sol-gel method. The morphological and structural properties of nano chromates were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and Photoluminescence (PL). Nanoparticles of doped lithium were synthesized by adding appropriate amount of cobalt nitrate, chromium nitrate, lithium nitrate and 1,2 Ethanediol as a complexing agent. The sample were heated at 105 ˚C for ten hours in oven to obtain dry gel. Calcination temperature for these samples was 700˚C for 3 hours in a furnace. The particle size of parent compound ranges from 4.4 nm to 11 nm, determined by SEM. The tendency of particles to form the aggregates with the increased annealing temperature was observed. The SEM and optical characterization of this compound has shown the sol gel derived material may be successfully used as an effective doped lithium cobalt ceramic pigment with controlled variation in structural and optical properties. SEM images showed that spherical like doped particle have diameter 33nm. From PL spectra Nano structure shows band gap 2.5ev and when we doped Lithium in it band gap decreases and become 1.19ev, which is associated to band gap transition.


Introduction
A tiny particle as small as 1 to 100 nanometers in size is called a nanoparticle. Due to its very low size a human eye cannot see or detect a nanoparticle. The unique aspect of nanoparticles is that they can exhibit significantly different physical and chemical properties to their bulk material counterparts [1,2].
Metallic and non-metal nanoparticles have been gaining attraction due to their unique applications in environmental, biomedical, optical and electronic industries [3,4]. Metal oxides nanoparticles are among the widest used manufactured nanomaterials because of their unique properties [5]. The properties that make the nanophase structures indispensable tools in modern nanotechnology are their various nonlinear optical properties, higher ductility at elevated temperatures than the coarse-grained ceramics, cold welding properties, superparamagnetic behaviour, Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 6 May 2021 doi:10.20944/preprints202105.0085.v1 unique catalytic, sensitivity, and selective activity. For example, the melting point of the nanosized material is lower than that of a bulk material with the same composition [6]. At the same time, nanoparticles exhibit unusual adsorptive properties and fast diffusivities and they are not stable in critical conditions [7].
Cobalt chromite nanoparticles are usually synthesized at various temperatures by hydrothermal treatment of chromium and cobalt nitrates in a mixture of water and ethylene glycol. CoCr2O4 nanoparticles have different properties like high temperature, high resistance to light and chemicals they are widely used in industrial and commercial scale [8,9].
Nanoparticles Metal oxides and their mixtures have been used as ceramic pigments. Pigments have been in demand for several applications like chemical and thermal stability, hiding and tinting power, particle size. Mixed metal oxides with general spinel formula AB2O4 are characteristics for high thermal and chemical stability and for mechanical resistance [10,11]. Spinel structure has two types, one is known as normal spinel, which is characterized by A +2 cations occupying tetrahedral position and the other characterized by cations B +3 occupying octahedral structure. The subordinate possible structure is inverse spinel, that is also characterized by A +2 occupying one half of the tetrahedral site and B +3 another half of the octahedral position and all tetrahedral coordination positions [12]. Inverse spinel structure can be affected by temperature and pressure during its synthesize [13].
CoCr2O4 are normal ferromagnetic spinel with ions Co +2 occupying the tetrahedral A sites and the octahedral B sites occupied by Cr +3 ions [14]. The compound system undergoes a long-term order of ferromagnetism below ferromagnetic curie temperature of Tc=94 K [15]. Magnetic tangled phase diagram is obtained by the interaction of Co +2 and Cr +3 [16]. Under the spiral ordering at Ts=26K temperature the system is multiferroic [17]. The phase transition present occurs at the Lock-in transition temperature Tl=15K [18]. Nanoparticles of CoCr2O4 possess magnetic properties [14][15][16]. Strong cluster glass like performance have been described for nanoparticles size of 3.1 nm [19]. Since above 310 K, CoLiCr2O4 crystallizes and attains cubic normal spinel structure, therefore, the crystal structure becomes tetragonal due to Jahn-Teller manipulation in that temperature range [20]. Lowering temperature below 65 K, the crystal structure farther transforms into orthorhombic phase [21]. A deformation within the same orthorhombic phase has been distinguished at T=30 K [13]. The magnetic properties of CoCr2O4 and CoLiCr2O4 are different so for Ni doped CoCr2O4, co-precipitation method used to synthesize particles which contain standard size 80-100 nm, are delineated [22]. The property of magnetization of applied magnetic field displays a thin characteristic measured for Co0.5Ni0.5Cr2O4 nanoparticles developed through sol-gel process at T=10 K, which is missing in CoLiCr2O4 and CoCr2O4 [23].
In our present work, we have synthesized cobalt doped lithium chromates nanoparticles by sol gel method, which is easy to use, and environment friendly. Sol gel method requires low temperature and shorter calcination to obtain the desired product [24]. Comparison between two compounds base and doped cobalt chromates with different transition metal concentration with a step of 0.2 is made. The phase composition crystalize size and new spinel composition were investigated in this study.

Materials and Methods
To synthesize and fabricate CoCr2O4, and Li doped in cobalt chromate for the formation of lithium chromate we used lithium nitrate, cobalt nitrate, and chromium nitrate solution. 100 ml distilled water and 1,2 Ethendiol (as a complexing agent) was added to solution. In this process 100ml purified water was taken in a beaker and excess amount of lithium nitrates, cobalt and chromium were added. The beaker was then heated and constantly stirred at a temperature of 60-65 0 C. The heating continued for an hour until the solution became homogenous then 1,2 ehanediol (as a complexing agent) was added to the reaction solution to form gel. Intensive green gel was obtained after heating it for about 4 hours. This process is shown in Figure 1. The gel was dried in a DHG-9202 Oven at 105 0 C for ten hours. As a result, green colored powder was obtained. A grinder and agate motor were used to grind the powder well. VULCAN-D550 furnace was used to sinter the powder of CoCr2O4 and Co1-xLi x Cr2O4 at 700 0 C for 3 hours to obtain greenish color nanoparticles. At the end, the powder, which is obtained, greenish in color.
This synthesis and fabrication process is shown in figure 1. The nanoparticles were characterized for their structural and optical properties using XRD, SEM, Raman spectroscopy and PL. The fact that majority powders are not in single phase is not critical, since the main goal of pigmentary field is to achieve the proper parameters of the pigments for application in industry [25]. less than 10nm for all temperature in range.
The average particle size can be calculated using Bragg's equation, nλ= 2dSinѲ, where n is an integer, Ѳ is the angle between incident and scattered ray, d is the inter-spacing distance and λ is the wavelength associated with the particle. Crystal size D of a particle can also be calculated by using Scherrer's equation, D= Kλ/βCosѲ, where K is crystalline shape constant.
XRD results show that the powder form nanoparticles achieved from sol gel process are more homogenous than obtained by other technique. However, phase composition is identical to parent compound, yet, morphology of synthesized pigments differed significantly.

Raman spectroscopy:
Raman   To put light on these compounds furthermore, the compounds prepared by the sol-gel synthesis route were formed with higher pronounced crystallinity. The morphological features of the samples derived by both synthesis methods are quite different. Moreover, the size and shape of chromite particles depend not only on the used synthesis method, but also on the nature of the octahedral cation . The origin of cation in the spinel structure displayed the tendency to produce pigments of numerous hues. The increase of content leads to warm black color.
The enrichment in tetrahedral ligand field gives the variety of green shades form bluish green to yellowish green. Only Cu-doped pigments exhibited less different hues and are dark, nearly black. Nonetheless, there are no main differences of the colors between pigments, produced by the sol-gel synthesis methods. Due to these characteristic properties.

Conclusions
Chromites spinel Co1-xMxCr2O4 (M=Li with various concentration 0.2, 0.4 and 0.6) are synthesized by sol-gel method. Calcination temperature for single-phase cobalt chromites and Li-doped chromites were manufactured at 600-700˚C. Li substituted single-phase chromites were found to be dependent on annealing temperature. These spinels were mixture of spinel-type and spinel oxide at higher substitution ratios of 0.6. Outcomes revealed that cobalt chromites spinel structure was magnificently manufactured by conventional sol-gel method using chromium, Cobalt and copper nitrates as precursors under thermal decomposition. This method shows promising potential in the synthesis