Syntesis by Combustion Method

Introduction

Rare earth (RE) doped nanophosphors are widely used in light emitting diodes (LED’s), field emission displays (FEDs), scintillators, medical imaging and drug delivery. There exceptional electronic and optical properties are derived from f-f and f-d transitions. However, many reported synthesized nanophosphors degrade rapidly when exposed to critical conditions. So, the need of hour is synthesis of a nanophosphor, which is stable in high vacuums with high brightness, thermal and chemical stability and possess adequate mechanical resistance.

Inorganic oxide based nanophosphor have received considerable attention for the generation of color tunable materials due to their characteristic high brightness, long lifetime, low power consumption and eco friendly nature. Earl Danielson in 1998, reported a novel inorganic oxide compound (Strontium-cerium oxide), synthesized by a combinatorial chemical procedure, which crystallize in an orthorhombic crystal system with the space group Pbam (No.55) and cell parameters: a=6.11897(9) Å, b=10.3495(2) Å, and c=3.5970(1).

This Strontium-Cerium oxide (Sr2CeO4) is a blue-white emitting oxide and an excellent phosphor with high thermal and chemical stability, which makes it a promising matrix for various activators. The photoluminescene of Sr2CeO4 is due to energy transfer between O2- and Ce(IV) localized in one-dimensional chains of the edge-sharing CeO6 octahedron. The regularly located luminescent elements govern the luminescent properties of the oxide, and the interaction of these regular optical centers which promotes spatial delocalization of excitation energy is prominent feature.

Sr2CeO4 doped with rare earth (RE) ions results in emergence of astounding materials with a broad excitation band in UV range and a tunable emission. This is due to effective excitation energy transfer from the host to doped centers emitting another spectral range colors. Moreover, luminescence properties depend on the RE dopant being used and the concentration of the dopant. Several methods have been developed for the synthesis of efficient single phase RE doped luminescent Sr2CeO4 nanophosphor. These procedures are still evolving from various drawbacks of stability, control over size and morphology.

In a contribution to overcome these limitations, herein we have first time reported synthesis of rod like luminescent Sr2CeO4:Eu3+(mol% of Eu=0, 0.1, 0.5 ,1, 1.5, 2) by combustion method. A systematic methodology has been followed for the synthesis of this highly stable nanophosphor. Eu3+ was chosen as it acts a good activator ion with red or red orange emission in various hosts. The results show a white light generation at lower concentration, while red luminescence at higher concentrations of the dopant. The report also covers synthesis and characterization aspects of RE doped complex along with for their potential use in lighting and optoelectronic devices.

Experimental

Synthesis of Sr2CeO4:Eu3+ (0.1-2 mol %) nanophosphors The Eu3+ doped nanophosphors were prepared by a modified solution combustion method. The source of Sr, Ce, Eu were Strontium nitrate (Sr(NO3)2).9H2O; 99.99%,Merk Ltd), Cerium nitrate (Ce(NO3)3.6H2O ; 99.99% Sigma Aldrich Ltd) and Europium nitrate(Eu(NO3)3.xH2O; 99.99% Sigma Aldrich Ltd) respectively. Numerous fuels like citric acid, glycine, oxalic acid, Oxalyl dihydrazide(ODH), urea, etc. have been used for the synthesis of nanophosphor materials.

Citric acid produced voluminous fluffy product, whereas urea being eco-friendly, low in cost and easily available was used compared to hydrazine derivative fuels. The stoichiometric amounts of strontium, cerium, europium nitrate were taken in a beaker of 200 ml capacity in order to synthesize Sr2-xCeO4:Eux3+ (x=0.1-2mol %) nanophosphors, taking into the account of Eu3+ ions replacing the Sr2+ ions in the structure.

The required amount of urea was added to the mixture and to dissolve the mixture 20 ml of deionized water was added, mixture was well dispersed using the magnetic stirrer at 80°C for 10-15 min. The stoichiometry of redox mixture used for the combustion was calculated using the total oxidizing and reducing valencies of the compounds. Balancing the total oxidizing and reducing valencies of the compound the stoichiometry of the redox mixture was calculated for the combustion. Into the pre heated muffle furnace maintained at 550+10°C the homogeneous mixture was introduced, first the solution boiled and undergone dehydration and followed by release of large amount of gases CO2, H2O, N2. Then a spontaneous spark occurred and underwent combustion reaction.

The whole process was complete within 10 minutes. The foamy product was slowly cooled to room temperature, the product was crushed in pestle mortar and then was calcined at 1000°C for 4 h. The complete combustion equation of the redox mixture used for the synthesis may be written as: 12 Sr(NO3)2).9H2O + 6 Ce(NO3)3.6H2O + 34CH4N2O → 6 Sr2CeO4 + 34 CO2 + 68H2O + 55N2 12Sr1-x (NO3)2 + 6Ce(NO3)3.6H2O + 34CH4N2O + xEu(NO3)3.XH2O → 6Sr2CeO4:xEu + 55 N2 + 104 H2O + 34CO2 2.2

Characterization and instrumentation The phase purity and crystal structure of obtained products were characterized by powder X ray diffraction, using X-ray difractometer (Panalytical’s X’Pert Pro, Germany) with Cu-Kα radiation (λ= 1.5406Å) with a nickel filter is used to obtain the diffraction data. The XRD patterns were collected in the range of 10°<2θ<70° .The UV-Visible absorption spectrum was recorded on Shimadzu 2600 UV-visible double beam spectrophotometer. The photoluminescence (PL) measurements were performed on a Cary Eclipse Fluorescence Spectrophotometer (Agilent Technologies) equipped with a 450-W xenon lamp as the excitation source.

The morphology of the product was examined by FE-SEM (Hitachi SU 70). All measurements were done at room temperature. Color coordinates were calculated by CIE Chromaticity color calculator.

Results and discussion

Pure Sr2CeO4 and doped phases of Sr2-xEuxCeO4 (x=0.1-2 mol%) were synthesized by low temperature combustion method, in steps in order to modify the structural properties and as a consequence, luminescence characteristics through replacing Sr2+ ions by Eu3+ ions in the structure. The compounds are highly crystalline, very pale yellow in color, stable in air, insoluble in water.

XRD analysis

The diffraction patterns of the synthesized Sr2CeO4 doped with 0.5 mol% Eu3+ ions calcined at 1000°C are illustrated in figure 1. On the basis of the measured X-ray diffraction analysis and in comparison of the reported data, it indicates the formation of single orthorhombic phase with space group Pbam. Introduction of the dopant Eu3+ ions into the oxide crystal structure is confirmed with no notable distortion in the diffraction pattern. It confirms that the Eu3+ ions successfully exist in the lattice site rather than interstitial site.

The site substitution of the dopant is can be deduced from the percentage difference in ionic radii of the host and the dopant ions. In our system the ionic radius of Eu3+ is 0.947Å, while that of Se2+, Ce4+ are 1.18Å, 0.87Å respectively under the co-ordination number(CN=6). The percentage difference in in ionic radii (Dr) between host and doped ions is calculated using equation. Dr = Rh(CN) -- Rd(CN) Rh(CN) Where CN is the co-ordination number, Rh(CN)is the radius of host cations, Rd(CN) is the radius of doped ion. The calculate values of Dr between Eu3+ and Sr2+ on six coordinated site is 19.74%, while that between Eu3+ and Ce4+ is -8.85%. This proves that the dopant Eu3+ would clearly substitute the strontium sites.

Therefore Eu3+ most preferably substitutes Sr2+ rather than Ce4+. Similar results have been reported for various rare earth ions doped Sr2CeO4 phosphor. The atomic mobility causes grain growth which results in better crystallanity as all the samples were calcined at higher temperature (1000°C). The average crystallite size was estimated by Debye-Scherrer method. D= [ 0.9λ ] Βcosθ Where D is an average crystallite (grain) size, the factor 0.9 is the Scherrer constant, λ denotes the X-ray radiation wavelength, β is the Full Width half Maxima (FWHM) and θ the Bragg angle of an observed diffraction peak. The particles were found to be smaller in nanometer range and some large grains were also present. This resulted in slight deviation of the Scherer’s calculations. The obtained nanophosphor was composed of nanocrystallites with an average size of 64.16nm .

The increase of dopant concentration introduces the strain and alteration of the lattice periodicity which results in the decrease of crystal symmetry. The crystallite size was also calculated from the powder X-ray diffraction line broadening (β) using the analysis described by Williamson and Hall (W-H) plot. βcosθ= ɛ(4 sinθ) + 1 /D λ λ Where β is the FWHM in radian, ɛ is the strain developed and D is the crsyatallite size. The equation represents a straight line between 4Sinθ/ λ (X-axis) and β cosθ/ λ (Y-axis). The slope of line gives the inhomogeneous strain (ɛ) and intercept (1/D) of this line on Y-axis gives crystallite size (D).

The WH calculated particle size was 54.64nm . The dislocation density was estimated by using the relation δ=1/D2. There is a slight difference in crystallite size determined from the W-H plots and from those calculated using Scherrer’s formula. The small variation is due to the fact that in Scherrer’s formula strain component is assumed to be zero and observed broadening of diffraction peak was considered as a result of reducing grain size only.

FE-SEM analysis 

It shows agglomerated fluffy and porous kind of morphology due to combustion synthesis. However, the FE-SEM pictures obtained in this study shows rod like structures with diameters in nanometer. During the combustion synthesis, the temperature immediately increases to a higher level and sustains there for few seconds and quickly comes down to a lower temperature. In a very short span of time crystals undergo a fast shape evolution process for formation of various morphologies.

UV-Vis absorption spectroscopy 

The optical absorption spectra were recorded in the wavelength region of 200-450 nm. A flagrant absorption peaks around 330nm and 260 nm is observed in all the samples whereas 220 nm peaks is present in concentration above 0.1mol% of dopant. With changing the dopant concentration the absorption wavelength varies slightly as seen in increasing the doping concentration the absorption edge shifts towards higher wavelength. The optical energy gap, Eg of doped samples were calculated using the Tauc relation. αhν ~ (hν - Eg)n Where hν is the photon energy and α is the optical absorption coefficient near the fundamental absorption edge.

The absorption coefficient α is calculated from the optical absorption spectra. The values of the optical band gap energy is obtained by plotting (αhν)n Vs hν in the high absorption range followed by extrapolating the linear region of the plots to (αhν)n=0. Eg is the optical band gap and n is the constant associated to the different types of electronic transition n = ½, 2,3/2, 3 for direct allowed, indirect allowed, forbidden and indirect forbidden transition respectively. The optical energy band gap of the nanophosphors varies 3.08 and 3.21 eV.

The variation in Eg values with different concentration of Eu3+ in Sr2CeO4 is mainly attributed to structural defects such as vacancies, degree of structural disorder in the lattice, which is able to change the intermediary energy level distribution within the band gap.

Photoluminescence studies

Luminescence properties of single phase Sr2CeO4 Blue light generation. Fig shows the excitation spectra of the strontium-cerium oxide. It consists of two peaks, a broad band at 260 nm and a shoulder at 340 nm. Due to different types of Ce4+ and O2- lattice bonds, the spectra shows two excitation peaks which are because of different charge transfer transitions. Characteristic emission of the blue phosphor Sr2CeO4 is connected with the CT phenomena from orbitals of O2- ions to the empty 4f shell of Ce4+ ions.

The higher energy band (245 nm) originates from O1àCe4+ transition, where O1 is the terminal oxygen ion in the structure of Sr2CeO4 , and the peak at 330 nm results from the CT transition between the equatorial oxygen ion and the Ce4+ ion (O2-àCe4+). Fig shows the emission spectra of the strontium-cerium oxide. It is a simple broad band with its centre located at 475 nm attributed by the Ce4+ charge transfer emission, in Ce4+ the 4f shell is vacant, therefore only possible transition is one in which an electron is excited from the oxygen ligand to the Ce4+ion: a charge transfer transition. Under UV radiation, the excitation of the ground state to one of the excited states, t1u-f or t1g-f, associated with two groups of O2- ions (equatorial and terminal) occurs.

Due to spin forbidden transitions t1u-f, the related absorption or excitation band is less intense as compared to the band connected with the t1g-f transition. Emission in the range of 400-600 is connected with the radiative relaxation process from the excited CT state of the CeO6 complex. It is associated with a reduction of the crystallite size. It directly influenced the energy band gap of Sr2CeO4, enlarging the distance between the ground and excited CT states.

Luminescence properties of doped Sr2CeO4:Eu3+(0-2 mol%) White light emission at low concentration and red light at higher concentration is observed. The Excitation spectra were recorded by fixing the highest emission intensity of the Eu3+(615 nm) as the observing wavelength for all the dopant concentration. The recorded spectra shows same features for all the samples measured with a broad band in the range of 200-320 nm and a shoulder at 340 nm, which are allocated to the Ce4+-O2- CT transition of undoped Sr2CeO4.

The Eu3+ ions in oxide exhibit their own CT states, they can also take part in ET. The broad band peak around 260 nm is the because of the overlap of the charge transfer bands from the oxygen to europium (O-Eu) and the host band (Ce-O). The intra-configrurational 4f-4f transitions of Eu3+ corresponds to weaker peaks in the range 320-450 nm. The transitions 7F0à5H3, 5L9, 5L7, 5L6 and 5D4 are corresponds to 320,360,380,395 and 420 nm peaks respectively.

The increasing concentration of Eu3+ ions tunes the Sr2CeO4 for red emission than blue luminescence which is observed due to f-f transitions of the dopant. The emission spectra consist of characteristic lines originating from transitions 5D2, 5D1 and 5D0 levels into the components belonging to the ground state 7Fj. The high level transitions 5D2 and 5D1 are most intense in the low concentrations of Eu3+ which is attributed low vibration energy of CeO44- group.

However, with the increase in concentration the intensities of these transition bands 5D2à7F0(467 nm), 5D2à7F2(491 nm), 5D2à7F3(511 nm),5D1à7F1 (535 nm)and 5D1 à7F2 (555 nm) significantly reduces. The transition peaks 5D0à7F0(580 nm),5D0à7F1(593 nm),5D0à7F2(616 nm),5D0à7F3(655 nm),5D0à7F4(705 nm) show meager change in their intensities. 5D123à7Fj transition lines appear in compounds with low phonon energy which leads to multiphonon relaxation. Weak emissions are due to the mutliphonon relaxation by CeO44- that is not able to bridge the gaps between the higher levels and the 5D0 levels of Eu3+. The orange emission belongs to magnetic dipole transition 5D0à7F1 (589 nn) of Eu3+.

The red emission is due to electric dipole transition 5D0à7F2 (615 nm). The growing amount of defects caused by exchange of Sr2+ ions by higher charged Eu3+ ions results in asymmetry of the Eu3+ environment which also cause the ratio of the 5D0à7F1 and 5D0à7F2 bands change. No compensation for the lacking charge has been used which results in distortion of the matrix structure. The emissions due to the host and the dopant are present in the spectra.

The high ratio between the 5D0à7F2 (615 nm) and 5D0à7F1 (594 nm) bands provides the solution that Eu3+ ions occupy sites with low symmetry and without an inversion centre. Gradual change in the emission from blue to red is the confirmation of ET from matrix to the Eu3+ ions. With change in Eu3+ ions concentration the PL spectra exhibit perceptible changes in the intensity. Firstly with increase in dopant concentration the PL emission intensity increases, reaches a maximum value and then decreases with further increase in Eu3+ due to concentration quenching.

The observed optimized concentration after which concentration quenching occurs is found to be at x=0.5 mol%. To compare the theoretical band gap values, reflectance diffuse spectrum was obtained and the band gap values based on the Kubelka-Munk, equation was determined, and the value found was 3.25 eV. The result s of the theoretical and experimental band gap values are in good agreement when compared with the experimental one (3.17 eV) estimated from diffuse reflectance and Kubelka-Munk equations. To calculate the material performance in color luminescent emission, CIE chromaticity diagram is a evaluation toll for specifying the color of an LED which is a universally standardized way used across the industry. The CIE 1931 is a standard to LED industry relating to color, such as color rendering and color mixing.

The values of a and b show CIE chromaticity diagram of Sr2CeO4:Eu3+ excited under respectively. Color tune from blue to white and reddish orange to dark red at 226 nm excitation wavelengths. The CIE co-ordinates shift from (X=0.1763, Y=0.2363) to (X=0.5566, Y=0.3803) resulting in the potential usage of Sr2CeO4:Eu3+ nanophosphor for the generation of blue, white and reddish orange in lighting industries.

Conclusion

Strontium cerium oxide (Sr2CeO4) doped with Eu3+(0.1-2 mol%) nanophosphors rods were successfully synthesized by simple and economic modified solution combustion. The nanophosphors were characterized with XRD for optimizing the orthorhombic structure, FE-SEM micrograph shows nanorods, UV-Visible and Tauc Plot for the estimation of the absorption band gap energy of 3.17 eV and PL spectra for the color tuning of the matrix. The emission spectra vary with Eu3+ concentration showed characteristic peaks corresponding to the 5D0→7Fj(j=1,2) transitions because of the energy transfer from Ce4+ -O2- CTS to Eu3+ ions. The emission color can be easily tuned by changing the dopant concentration. As a result color tuning from blue to whitish blue to reddish orange to dark red can be achieved, which was further confirmed by CIE chromaticity coordinates. This demonstrated that change of luminesce color depends on the concentration of the dopant. This study suggests that the Sr2CeO4:Eu3+ nanophosphor might be a promising phosphor for wide range of applications.