Bright Red Luminescence and Structural Properties of Eu³⁺ Ion Doped ZnO by Solution Combustion Technique

Sebastián López-Romero, María Jesús Quiroz-Jiménez, Manuel Hipólito García, Alfredo Aguilar-Castillo

Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México Circuito Interior S/N Ciudad Universitaria México, México, D.F., México.

DOI: 10.4236/wjcmp.2014.44024                

Abstract

Pure, and Europium ion doped Zinc oxide nanocrystals (ZnO:Eu³⁺) were synthesized by a solution combustion technique. The X-ray diffraction patterns (XRD) reveals the existence of the Eu₂O₃ phase. From the results of both, X-ray diffraction and photoluminescence spectra (PL) reveal that Eu³⁺ ions successfully substitute for Zn²⁺ ions in the ZnO lattice, moreover, when the amount of doped Europium was varied, this changes are showed in changes in the luminescence intensity. The PL is broad and a set of colors was emitted which originates from ZnO and the intra 4f transitions of Eu³⁺ ions. The existence of the Zn-O, Eu³⁺-O and O1s bonding energies were confirmed by X-ray photoelectron spectroscopy (XPS) technique. The samples morphology was registered by a scanning electron microscopy (SEM) technique, and reveals that Europium ions are present on the surface of the ZnO nanocrystals.

Keywords

ZnO, Europium Ion, Solution Combustion, Photoluminescence

1. Introduction

In the last few decades, the semiconductor zinc oxide (ZnO) with a wide band gap energy (3.37 eV) at room temperature and high exciton binding energy (60 meV) [1] , has been used as host material for the doping of rare-eart (RE) and transition metals (TM) ions, which exhibit optical and magnetic activity [2] - [6] . The RE- doped ZnO nanocrystals have an high potential to be used in integrated optoelectronic devices such as infrared and visible (blue, green, red) luminescent devices because they present a highly efficient luminescence even at room temperature [7] -[11] ; the emission process is determined by the internal dynamics of the RE³⁺ electronics transitions governed by the relative energy of the 4f emitting level including the direct 4f-4f and indirect process ⁵D₀ → ⁷f_i with i = 0 - 4. From all the RE³⁺ ions, Eu³⁺ ion is the most representative and most widely studied and actually continue being used as dopant in many host compounds. Its color emission characteristic is the red color which is used in the fabrication of light emitting devises. Actually, it is possible to obtain and dope by several methods of the ZnO semiconductor in all your existent nanostructures: powders, nanowires, nanorods, nanobelts, nanonedles, nanorings, and nanoflowers [12] - [17] . Such methods are: radio frequency magnetron sputtering [4] , spray pyrolysis deposition [18] , hydrothermal synthesis [19] , sol gel technique [20] , thermal evaporation reac- tive [21] , chemical vapor deposition [22] , reactive magnetron sputtering [23] , and solution combustion method [24] . The last method is considered as fast, simple, easy controlabilited, low cost and great scale production. In this work we synthesized ZnO intrinsic and Eu³⁺ doped ZnO (ZnO/Eu³⁺) by a solution combustion method as a function of the Eu³⁺ ion concentration in wt% and after annealing at 900˚C by 24 h, also, the photolumines- cence emission intensity of the ZnO:Eu³⁺ as a function of the Eu³⁺ ion concentration is fited empirically by a exponential function and a phenomenological description and interpretation of the observed data is given.

2. Experimental

Undoped and Eu³⁺ ion doped ZnO nanocrystals ZnO:Eu³⁺ were prepared by solution-combustion synthesis method as a function of the Eu³⁺ ion concentration in wt% using as source materials Zinc nitrate hexahydrated (Zn(NO₃)₂∙6H₂O) as oxidizer, urea (H₂NCONH₂) as fuel, and Europium chloride (EuCl₃) as dopant. In this process the follow redox chemical reaction stoichiometric oxidizer-fuel producing ZnO, H₂O vapor, CO₂, and N₂ is obtained:

(1)

The equation (1) is obtained by taken in account the oxidizer/fuel molar radio (O/F = 1) required for a stoichiometric reactive solution which is determined by summing the total oxidicing and reducing valencies in the oxidizer compound and dividing it by the sum of the total oxidation and reducing valencies in the fuel compound [25] . Accordingly, for the complete combustion of Zinc nitrate-urea reactive solution, the molar ratio becomes 5/3, the molar balanced equation (1) was obtained with this value. Using the atomic weight concept, the equation (1) was translated to grams/mol and used to obtain 3 gr of ZnO powder for all the 0, 1, 3, 5, 7, and 10 wt% Eu³⁺ ion concentrations with respect to 3 gr of ZnO. From the translated grams/mol equation (1) (no shown), 8.31 gr of Zn(NO₃₎)₂∙6H₂O, 3.68 gr of H₂NCONH₂ plus the numerous of grams corresponding to each Eu³⁺ ion concentration were mixing with 20 ml of H₂O and stirring vigourosly in a flask glas and after put on a hot plate at 500˚C, after a few minutes the reactive solution boils, foams, ignites, and burns with a incandescent flame at a approximate temperature of 1200˚C [26] producing 3 gr of ZnO powders approximately. After all the samples are annealing at 900˚C by 20 h. The ZnO:Eu³⁺ samples thus obtained were structurally characterized by X-ray diffraction (XRD) technique using a Philips PW 1800 diffractometer with Cu kα radiation (1.5406 Å), XPS method was used to verify the Zn-O, Eu-O and O1s bonding energies (BE), the photoluminescence (PL) of the ZnO:Eu³⁺ samples was studied by means of a spectrofluorometer FluoroMax-P that uses a xenon lamp as excitation source, The wavelength excitation was of 270 nm, and finally, the morphology of the ZnO:Eu³⁺ powders was recorded using a scanning electron microscopy SEM) JEOL JSM 840 A.

3. Results and discussion

3.1. Structural characterization

figure 1 shows the X-ray diffractograms of the ZnO:Eu³⁺ powders as a function of the Eu³⁺ ion concentration and annealing at 900˚C by 20 h. From the XRD patterns, can be observed that all the diffraction peaks can be indexed to the majority phase hexagonal wurtzite tipe ZnO structure for all samples (JCPDS card #89-(102), moreover, for all the Eu³⁺ ion concentrations a little peak at 2θ = 28.4˚ is observed, which is attributed to the (210) plane of the Eu₂O₃ minority phase (JPDS card #86-2476), Park et al. [27] reported diffraction peaks due to Eu₂O₃ after annealing the Eu³⁺ doped ZnO at temperatures higher 1000˚C in air and vacun conditions, no dif- fraction peaks were detected from other impurities. However, the intensity of the (101) peaks decrease with the

Figure 1. XRD patterns of the ZnO/Eu³⁺ samples as a function of the Eu³⁺ ion concentration in wt%.

increase in the Eu³⁺ concentration which create some disorder in the ZnO structure. This is further verify by the increase of he FWHM of the (101) peak with the Eu³⁺ concentration indicating a decrease in the crystallite size; effectively, using the Scherrer formula, the average crystallite size calculated from characteristic peak (101) was 41 nm for the ZnO intrinsic and decreased to 18 nm for the ZnO/Eu³⁺ samples doped with 10 wt% of Eu³⁺. In addition, in figure 2, are showed the diffraction patterns of the characteristic peaks (100), (002) and (101) of the doped ZnO/Eu³⁺ samples as a function of the Eu³⁺ ion concentration, it can be observed that the three peaks shifted towards a bigger 2θ vaue for 1, 3, and 5 wt% of Eu³⁺ concentration compared with the ZnO intrinsic, and further this peaks returned to the same position what that of the ZnO intrinsic, this change in the diffraction peaks towards a bigger 2θ value show a decrease of the lattice parameter and cell volume, this result is contrary to the results founded by another researchers [4] [28] [29] since the doping of the bigger size Eu³⁺ ion (effective ionic radio r_i = 9.74 nm) compared to that of the smaller Zn²⁺ ion (r_i = 7.40 nm) and therefore its incorporation in the ZnO lattice must lead to a increase in the cell parameters and volume, it is due to a low solubility of the Eu³⁺ ion in the ZnO lattice even for 1 wt% Eu³⁺ concentration, Yang et al. reported that the solution limit in the Eu³⁺ doped is below 0.2 wt% Eu³⁺ ion [10] , this low solubility increase the yield of Eu₂O₃ species with increase of Eu³⁺ concentration at the ZnO nanocrystals surfaces [30] as is showed in the XRD diffraction patterns and at the SEM images showed in figure 4.

3.2. XPS analysis

To check the Eu³⁺ presence in the ZnO host, and its effect as a doping agent, on the surface chemical composi- tion, XPS whole spectra of the ZnO:Eu³⁺ samples was obtained as a function of the Eu³⁺ ion concentration, figure 3 shows the binding energy (BE) for each ZnO:Eu³⁺ samples doped with the Eu³⁺ ion at 0, 1, 3, 5, 7, and 10 wt%, focusing in particular on the binding energies of the typical lines of O, Zn, and Eu. The O1s photoelec- tron peak showed a BE = 530.1 eV [31] [32] , attributed to the lattice oxygen in a Zn-O-Zn network. With re- spect to the Zn ion presence the core-level photoelectron peaks showed a BE = 1021.5 eV corresponding to the core-level Zn2p_3/2 revealing the presence of Zn²⁺ ions in an oxide environment, the before analysis include to both undoped and Eu³⁺ ZnO doped. For all the Eu³⁺ ion concentrations, in your XPS spectrum respective there are two peaks in the Eu³⁺3d region (1110 - 1170 eV) with BE = 1134.5 eV and 1164.3 eV corresponding to Eu³⁺3d_5/2 and Eu³⁺3d_3/2 respectively; both peaks are due to multiple spin-orbit interactions and are consistent with the reported values for Europium-coordinated ions which indicates that the oxidation states of Europium ions are trivalents for the ZnO:Eu³⁺ samples [29] [33] . Non other satelite peak can be seen on the XPS spectra.

3.3. Morphology of the ZnO/Eu³⁺ samples

figure 4 shows the SEM images of the as prepared ZnO:xEu³⁺ (x = 0, 1, 3, 5, 7, and 10 wt%) samples as a

Figure 2. XRD patterns of the (100), (002) and (101) peaks of the ZnO; Eu³⁺ samples as a Function of the Eu³⁺ ion concentration in wt%.

Figure 3. XPS spectra of the ZnO/Eu³⁺ samples as a fuction of Eu³⁺ concen- tration.

function of the Eu³⁺ ion concentration in wt% and after annealing at 900˚C by 24 h. As can be observed, there are not clear morphological differences between undoped and Eu³⁺ doped samples indicating that the Eu³⁺ dopant in the ZnO host do not affect its morphology of the ZnO:Eu³⁺ nanocrystals. The grains of the ZnO have a mean large about 0.6 µm. Moreover, it can be seen that Eu³⁺ crystallites are adhered to the ZnO surface increas- ing with the Eu³⁺ ion concentration.

3.4. Photoluminescence properties

figure 5 shows the room temperature photoluminescence spectra (PL) of the ZnO:Eu³⁺ nanocrystals as a

(a) (b) (c)(d) (e) (f)

Figure 4. SEM images of the ZnO/Eu³⁺ samples as a function of the Eu³⁺ Concentration. (a) 0%; (b) 1%; (c) 3%; (d) 5%; (e) 7%; (f) 10%.

Figure 5. Room temperature photoluminescence spectra of the ZnO:Eu³⁺ samples as a function of the Eu³⁺ ion concentration irradiated with a excita- tion wavelength of 270 nm.

function of the Eu³⁺ ion concentration in wt% and after annealing at 900˚C by 24 h, the samples were irradiated with an excitation wavelength of 270 nm (in the UV range). It is observed that the ZnO:Eu³⁺ samples doped with 1, 3, 5, 7 and 10 wt% are all alike: they present a green broad emission band from 400 nm to 600 nm centered about 516 nm, this band is due to the intrinsic defects emission of ZnO host. however, in addition to the broad band characteristic of defects in ZnO appear the photoluminescence spectra of the Eu³⁺ doped powders: effec- tively, the sharp peaks in 579, 591, 613, 618, 650 and 770 nm are related to the direct intra-4f transitions in Eu³⁺ ions ⁵D₀ → ⁷F_j (j = 0, 1, 2, 3, 4), the most intense emission is associated to the ⁵D₀ → ⁷F₂ emission in the red spectral region (613 nm) and is due to an allowed electric-dipole transitions with inversion antisymmetry [19] , which results in a large transition probability in the crystal field; in our case this emission is split in two compo- nents of 613 and 618 nm, theoretically, the ⁷F₂ level gives three crystal field levels of A1 and 2E with 3C_v sym- metry, because A1 and one of two E levels have close energy levels, two emission peaks (A1 and E at 613 and 618 nm) can be overlapped in the PL spectra [34] [35] . The peak at 591 nm is due to the ⁵D₀ → ⁷F₁ transition, is an allowed magnetic-dipole transition, if the Eu³⁺ ion is situated in a symmetry center in the ZnO matrix, elec- tric-dipole transitions between the 4f⁶ levels are strictly forbidden by the Laporte selection rule (equal parity) while the magnetic-dipole is allowed. Thus, the intensity ratio of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transition known as symmetry ratio can provide information about the quality structural of the material [35] [36] . The symmetry ra- tio in our material about 8 indicating low inversion symmetry when the Eu³⁺ ion is incorporated into hexagonal ZnO host by substitution on Zn sublattice. The peak at 579 nm is due to the forbidden ⁵D₀ → ⁷F₀ transition due to the same total angular momentum, indicating that some Eu³⁺ ions possibly occupy other sites as interticial sites [37] [38] . Figure 6 shows the integrated emission intensity variation of the ⁵D_o → ⁷F₂ transition as a func- tion of the Eu³⁺ ion concentration, the empiric graphic was fited, to the exponential function I(%) = 1.3E6*e^0.23% where % is the Eu³⁺ ions concentration, the solid curve in figure 5 represents the fit to the experimental data, the integrated intensity increases as an exponential function as the doping concentration increase which indicates the enhanced energy transfer between the ZnO host and activator Eu³⁺ ions. The concentration Quenchin effects do not appear in this concentration range.

4. Conclusion

In this work undoped and Eu³⁺ doped ZnO nanocrystals were synthesized by a solution combustion technique. From XRD studies, the ZnO:Eu³⁺ structure resulted in the hexagonal wurtzite ZnO structure for all the Eu³⁺ ion concentrations; and also showed the existence of the Eu₂O₃ phase mixed with the ZnO phase. The effect of Eu³⁺ doping percentage on the nanocrystals showed that with increasing doping percentage, disorder in the nanocrys- tals increases. From the XPS spectra, the oxidation states of the Eu ions are trivalent for the Eu³⁺ doped ZnO nanocrystals. Solution combustion synthesized Eu³⁺ doped ZnO nanocrystals are found to emit red and green

Figure 6. Fit of the intensity emission integrated of the transition ⁵D₀ → ⁷F₂ as a funtcion of the Eu³⁺ ion concentration in wt%.

light. The intensity of the emission was related to doping concentration by means of an exponential fit.

Acknowledgements

The authors wish to tank Adriana Tejeda (IIM) for his support in carrying out X-ray measurements; Omar Novelo (IIM) for their support in the scanning electron microscopy, to M. Canseco Martínez by his support in chemical reactions.

Conflicts of Interest

The authors declare no conflicts of interest.

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