Preparation and spectroscopic study of nanosized aluminate spinels Al 2 XO 4 ( X = Cd, Ni and Co) synthesized by solution combustion technique

The aluminate nano-powders Al 2 XO 4 (X = Cd, Ni and Co), crystallizing in a cubic spinel, were prepared by combustion method at 900°C. The effects of cations on the structure, optical properties and distribution of the cations as well as the binding energy of the samples were studied by X-ray diffraction (XRD), UV-Vis diffuse re�ectance and FT-IR) spectroscopy. The XRD analysis con�rmed the spinel structure of all samples, and showed that the average crystallite size of Al 2 XO 4 increases from 23.89 to 32.72 nm, while the lattice parameter changes from 8.0490 to 8.1066 Å when going from Ni to Co. As expected, the FT-IR spectroscopy of Al 2 XO 4 showed the presence of the fundamental absorption peaks in the region (400– 745 cm − 1 ), con�rming the formation of the aluminate spinels. The slight shift toward longer wavenumbers is due to the low electronegativity of Cd. The forbidden bands (E g ), determined by diffuse re�ectance, were found to be 3.78, 3.82 and 3.75 eV respectively for Al 2 CdO 4 , Al 2 NiO 4 and Al 2 CoO 4 , assigned to the charge transfer O 2− : 2p → X 2+ : d orbital, making our spinels good candidates for optoelectronic and photocatalytic applications.

The aim of the present research is to synthesize the spinels Al 2 XO 4 with nanosized morphology using the solution combustion method at room temperature with three cations (Cd 2+ , Ni 2+ and Co 2+ ).This technique allows the preparation of a variety of nanoparticles, including mixed oxides, sul des and metals alloys.It combines the chemical combustion and solution process for the elaboration of aluminate nanopowders crystallizing in a normal spinel and is considered as a sol-gel auto-combustion.
The method has many advantages including simple equipment of preparation, high purity, good homogeneity and crystallinity, nano-crystallites and a stoichiometry close to the nominal composition.
Then, the effect of the element X on the structural parameters and optical properties of the prepared oxides is studied using the XRD analysis, FT-IR spectroscopy and UV-Visible spectrophotometry.

Characterizations
The phase composition and crystal structure of the Al 2 XO 4 nanopowders were analyzed by X-ray diffraction (XRD).The analysis was taken with a MiniFlex 600 powder diffractometer with Cu K α radiation (λ = 1.5406Å) in the range (3° − 90°) with a step of 0.01° at room temperature.The optical properties of the powders were determined by absorbance spectra by using the ultraviolet-visible diffuse re ectance spectroscopy (Shimadzu UV-1650-PC) by measuring the absorption.The FTIR spectra of powders were recorded on Alpha Bruker FT-IR spectrometer in the wavenumber range between of 350 and 4000 cm − 1 .

X-ray diffraction (XRD)
The XRD patterns of Al The average crystallite sizes, interplanar spacing, lattice parameters, cell volumes and structure type of the prepared aluminate were evaluated from the XRD data.The average crystallite size (D) of the prepared powders was calculated from the full width at half maximum intensity (FWHM = b) by using the empirical relation [30][31][32]: where k is the crystallite shape factor (equal to 0.9),  is the maximum diffraction peak (radians).
The  V SHE ), evidenced by a thermal decomposition above 900 °C.
The Al-Al length, in the spinel is greater that the inter-ionic distance of orbital overlap for itinerant electrons and the Al 2 CoO 4 exhibits a semiconducting behavior [37].
The three powders present similar IR spectra.The broad absorption bands in the range (3010 -3340 cm -1 ) are vibrations modes of O-H [38].The absorption peaks between 1446 and 1553 cm -1 are related to the metal-oxygen bonds such as Al-O [39].However, the weak peak at 2281 cm -1 is assigned to CO 2 adsorbed on the powder surface when the samples are handled in air [9].Generally, the absorption of inorganic compounds occur below 1000 cm -1 .The FT-IR spectra of all powders have three strong absorption peaks in the range (400 -745 cm -1 ) correspond to sites of the Cd, Ni and Co [39,40], which is the common property of all aluminate spinels.

UV-Vis spectroscopy
In order to study the optical properties of prepared samples, the UV-Vis absorption spectra were recorded between 200 and 900 nm.give colored oxide.However, this transition if observed, has a weak intensity because it is forbidden by the Laporte because the energy of d orbital depends on the metal.The rule states also that, if the complex is centro-symmetric, the transitions within d orbitals are forbidden.
The band gap energy (E g ) were calculated from the intercept of the tangent with the X-axis, according to the Tauc plots [44], as shown in Figs.In summary, the aluminates appear to have a resistivity of ~ 10 3 W cm and therefore exhibit a semiconducting behavior with a wide depletion width and a gap sensitive under UV light.These properties render them promising in photocatalysis under solar radiation, which accounts for 5% of the solar ux, ~ 50 W m -2 of pure UV light.

Conclusion
The homogeneous Al 2 CdO 4 , Al 2 NiO 4 and Al 2 CoO 4 nanopowdres crystallizing in the spinel structure were successfully prepared by the solution combustion method.The substitution of X element on the structural and optical properties features of the nanoparticles have been studied.The products obtained were analyzed by different techniques.The crystallographic analysis con rmed the spinel structure for three samples.In addition, the cell volume and average crystallite size depends on the nature of X element and increases from changes Nickel to Cadmium passing by Cobalt.The appearance of peaks in the range (450 -and 650 cm -1 ) in the IR spectra con rms the formation of spinel structures.The UV-Visible diffuse re ectance spectroscopy gives the band gap energies for the three samples, which absorb in the UV region (3.775-3.818eV) in conformity with their white color.All characterizations (Wide range gap and nanometer crystal size) suggested that the prepared aluminates are candidates for diverse applications such as hydrogen production, nuclear waste, photocatalysts, refractory materials, energy storage, photovoltaics and photodetectors.Page 16/20   Plot of (αhν) 2 versus hν for Al 2 CdO 4 lattice constants (a) and inter-planar spacing (d hkl ) Al 2 XO 4 are evaluated using the formula cited below, respectively[33][34][35][36]:where h, k, l are the Miller indices of the reticular plane, and θ is the diffraction angle corresponding to the most intense re ection plane, the cell volumes (V = a 3 ) of prepared nano-powders are also calculated (Table2).The lattice constant (a) increases from 8.0700 to 8.1066 Å, when going from Ni 2+ to Cd 2+ passing by Co 2+ .This is due simply to the replacement of IV Ni 2+ ion (0.70 Å) by IV Co 2+ (0.735 Å) in high spin con guration and larger IV Cd 2+ (0.84 Å), the roman number indicates the coordination site (tetrahedral).

Fig. 6
Fig.6represents the spectra obtained by the solution combustion method.It is clear from these gures that the strong absorption peak (λ max = 310 nm) for all samples in the UV region is in conformity with the white color.The compounds not absorbing in the visible range are those with full or empty d orbitals of elements in the absorption and are transparent to UV light.

Table 1 .
The chemical formulas and quantities used for the synthesis of Al 2 XO 4 are given in Table 1 The chemical formula and quantities of raw materials used to preparation of the spinels Al 2 XO 4 .NO 3 ) 2 +6 Al(NO 3 ) 3 + 20 CH 4 N 2 O → 3 Al 2 XO 4 + 32 N 2 + 20 CO 2 + 40 H 2 O Figure 1 illustrates the different steps of the solution combustion synthesis of Al 2 XO 4 nanopowders.
[29]XRD patterns of Ni-and Co-aluminates nanoparticles showed several common peaks due to the proximity of Ni and Co in the periodic Table and similar ionic radii of Ni 2+ (0.70 Å) and Co 2+ (0.65 Å) in tetrahedral coordination.One can anticipate a high spin con guration with a superparamagnetic behavior[29].On the contrary, while the Al 2 CdO 4 has different peaks owing to the larger radius of Cd 2+ (0.95 Å) which shows a shift toward smaller 2q angles according to the Bragg law (2 d sinq = l).The major phases in these structures are Al 2 CdO 4 , Al 2 NiO 4 and Al 2 CoO 4 .CdO, NiO, CoO, and Al 2 O 3 are not observed on the patterns.Moreover, we did not observe extra peaks in the pattern which indicate the presence of extra phase like the spinel or large scale distortion.The peaks assigned to the spinel Co 3 O 4 of the Jahn Teller effect are also absent.The XRD analysis of Al 2 CdO 4 , Al 2 NiO 4 and Al 2 CoO 4 nanocomposites con rm the formation of a normal spinel crystallizing in a cubic symmetry with the space group Fd-3m.

Table 2 .
Structural parameters extracted from XRD analysis for Al 2 CdO 4 , Al 2 NiO 4 and Al 2 CoO 4 samples.The evolution of the lattice parameters, cell volume, interplanar spacing, and crystallite size as a function of the cation element are gathered in Table 2.The parameters D, a, V and d of aluminates prepared by solution combustion method follows the sequence: Al 2 NiO 4 < Al 2 CdO 4 < Al 2 CoO 4 .In the spinel structure Al 2 XO 4 , the ions O 2-are close packed along the [111] direction with 1/8 of tetrahedral cavities are occupied by X 2+ and one half lled by Al 3+ .The X 2+ cations form a diamond cubic structure.The reticular energy of Al 2 CoO 4 is due partially to high potential of the couple of Co 3+/2+ (1.84 [47].They are equal to 3.778, 3.818 and 3.750 eV for Al 2 CdO 4 , Al 2 NiO 4 and Al 2 CoO 4 , respectively.The E g values of Al 2 CdO 4 , Al 2 NiO 4 and Al 2 CoO 4 determined in this work are higher than those obtained by other methods, due to the quantum effect, a consequence of three-dimensional space quantization of excitons i.e. the bound electron-hole (e --h + ) pairs.The average length at which the (e --h + ) pairs is attracted by the electrostatic forces (exciton Bohr radius) similarly to the hydrogen atom.Quantum con nement occurs when the clusters of the semiconductor are smaller than the exciton Bohr radius.In such a case, the excitons are quantized giving rise to unusual optical and transport properties.Shows ligand to metal charge transfer (LMCT) electronic bands, whose energies and intensities give useful information.The electrons transfer from O 2-: 2p to the central ions X 2+ : d orbital requires a high energy, due to the electronegativity of oxxygenransfer electronegativity.The Eg value of Al 2 CdO 4 prepared hydrothermally found by Kumar et al.[45]exhibits a value of 2.40 eV.The E g values Al 2 NiO 4 synthesized by co-precipitation and sol-gel method are respectively 3.14 and 3.00 eV, found byGayathri et al. [46].Gholami et al.[47]while that found for Al 2 CoO 4 (2.38 eV) synthesized by thermal decomposition.Manikandan et al. [48] reported the Al 2 CoO 4 synthesized by microwave-assisted combustion shows E g value of 3.32 eV.Therefore, it can be concluded the gap energy (E g ) of the spinels is associated with the preparation technique.