Received 27 May 2016; accepted 28 August 2016; published 31 August 2016
Zinc oxide (ZnO) is actually considered to be a II-VI semiconductor material of great importance in basic science as in technologic applications due to its important properties physical and chemicals: the ZnO has a band gap energy of 3.7 eV  -  , an large exciton bonding energy of 60 meV, also has defects as O and Zn vacancies; however it is chemically and thermally stable and friendly with the environment. Due to these properties, the ZnO can be utilized in the fabrication of devices such as: electro-optical devices  , gas sensors  , catalyst  , piezoelectric device  , electro-optical  , photovoltaic  , paramagnetic  , etc. Several methods for the production of ZnO undoped and doped with various types of dopants (rare heart, metals, lanthanides, etc.)  -  have been described, such as, electrodeposition  , evaporation  , vapor-liquid-solid (VLS) growth  , metal organic catalyst assisted vapor-phase epitaxy  , aqueous thermal decomposition  , microwave activated chemical bath deposition (MW-CBD)  , chemical bath deposition (CBD)  , surfactant- assisted hydrothermal method  , solution combustion  etc. This last method is more convenient than other because it is very fast, and less expensive; it has an easier composition control, and coating can be deposited on large area etc. The introduction of Cu2+ ions into ZnO lattice sums or produces changes that improve the ZnO properties such as: band gap tailored, magnetic and electrical properties, passivation of defects, as p-type dopant in the original n-type ZnO semiconductor. However, actually it is well known that the co-doping with Cu2+ ion in Eu3+ doped glasses photoluminescent compounds the Cu2+ ion can quench the Eu3+ photoluminescence  -  with the increase of the Cu2+ ion concentration in glasses matrix; this quenching effect can be used to tuning between ultra-violet and visible emission in devise photoluminescent. In this study co-doped ZnO/Cu2+ + Eu3+solid solution powders have been synthesized by solution combustion method as a function of the Cu2+ ion concentration in %Wt. maintaining the Eu3+ ion concentration constant at a value of 3% Wt. Finally the sample was annealed at 900˚C by 20 h. Also it is demonstrated that the quenching Eu3+ PL by the copper ion action also occurs in ZnO matrix.
The synthesis method solution combustion is simple and fast caused by the chemical reaction oxidation-reduc- tion (REDOX) highly exothermic (1200˚C)  between an oxidizer agent and an fuel, in this experiment Zinc nitrate hexahydrate (Zn(NO3)2∙6H2) was used as oxidizer agent and urea (NH2CONH2) as fuel, copper chloride (CuCl2) and Europium chloride (EuCl3) were used as dopants agents. The next chemical reaction (REDOX) between the oxidizer agent and the fuel is an reaction highly exothermic and is the Stoichiometric reaction obtained with a equivalence ratio value of unity (i.e. f = O/F = 1) where O and F are the total oxidizing and total reduction valences of the components and the energy released by the combustion is at maximum  . With this reaction are obtained the follow products: ZnO, H2O (vapor), CO2, and N2.
Using the atomic weight concept, the Equation (1) was translated to grams/mol and used to obtain 3 gr of ZnO powder. The Cu2+ ion dopant concentration values were 0, 1, 2, 3, 10, and 20 wt%. The Eu3+ ion dopant concentration was maintained constant at 3 Wt% 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 Cu2+ ion concentration plus 3 Wt% Eu3+ ion were mixing with 20 ml of H2O and stirring vigorously in a flask glass and after put on a hot plate at 500˚C, after of few minutes the reactive solution boils, foams, ignites, and burns with an incandescent flame at an approximate temperature of 1200˚C  producing 3 gr of ZnO powders approximately. After all the samples are annealing at 900˚C by 24 h. The ZnO/Cu2++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), the photoluminescence (PL) of the ZnO/Cu2++Eu3+ samples was studied by means of a spectrofluorometer Fluoro Max-P that uses a xenon lampas excitation source, The wavelength excitation was of 270 nm, and finally, the morphology of the ZnO/Cu2++Eu3+ powders was recorded using a scanning electron microscopy (SEM) JEOL JSM 840 A.
3. Results and Discussion
3.1. Structural Characterization
The Figure 1 shows the X-ray diffractograms of the ZnO/Cu2++Eu3+ powders as a function of the Cu2+ ion concentration and annealing at 900˚C by 20 h. From the XRD patterns, can be observed that for the Cu2+ doping of 1, 2, 3, 10 and 20%Wt all the diffraction peaks can be indexed to the majority phase hexagonal wurtzite type ZnO structure for all samples (JCPDS card #89-(102)), also, for the Cu2+ ion concentrations of 1, 2, 3 and 10%Wt a peak at 2θ = 28.5˚ is present and is assigned to the minority Eu2O3 phase (JPDAS card #86-2476). The diffractograms also reveal the high crystallinity of the product for the five Cu2+ ion concentrations, for the higher Cu2+ concentrations of 10 and 20%Wt. A peak corresponding to the Eu2CuO4 molecules is present; effectively: in Figure 2 can be see the peak at the 2theta scale at 24˚ correspond to the Eu2CuO4 phase, however also in Figure 2 appear all the peaks characteristic of the zinc oxide majority phase reported in this text.
Comparatively, contrary to the observation of Iribarren et al.  obtained from your experiments on Cu2+ doped ZnO, in our diffractograms can be not observed a little shift initially toward higher angles of the principal peak (101) until 3%Wt of Cu2+ where an maximum is reached, after at higher Cu2+ concentrations the angular displacement is toward lower angles, in the first case this is due to the substitution of the Zn ion with 0.06 nm size by the smaller Cu2+ ion with 0.057 nm size that cause an lattice parameter shrinkage, in the second case this is attributed to an relative saturation of the Cu2+ ions on the samples surface, in our case the not presence of angular changes can be due to the Eu3+ ions present in the ZnO lattice. No diffraction peaks were detected from other impurities. Using the Scherrer formula, the average crystallite size calculated from characteristic peak (101) was 160 nm for the ZnO intrinsic and decreased to 150 nm for the ZnO/Cu2++Eu3+ samples doped with 10 Wt% of Cu2+ ion concentration.
Figure 1. XRD spectra of the Cu2++Eu3+ doped ZnO as a function of the Cu2+ ion concentration in %Wt.
Figure 2. XRD diffractogram of the ZnO and Eu2CuO4 compound. Cu2+ ion concentration for this figure was of 20 wt%.
3.2. Morphological Study
The Figure 3(a) shows the SEM image of the form of the Eu3+ doped ZnO whit out Cu2+ ions, it is observed that the Eu3+/ZnO crystallites are agglomerated forming amorphous particles of 1.5 µm of large in an approximation. The Figure 3(b) shows the SEM image of a particle of Cu2++Eu3+ doped ZnO polycrystalline particles, it can be see that the individual crystallites are agglomerated forming amorphous particles whit 2 µm side and 3 µm of large approximated.
Figure 3. SEM images of (a) undoped ZnO and (b) Cu2++Eu3+doped ZnO with 10%Wt Cu ions.
3.3. Photoluminescence Study
The Figure 4(a) and Figure 4(b) shows the room temperature photoluminescence spectra (PL) of the Eu3+ doped ZnO without the Cu2+ ion dopant and of the Cu2++Eu3+ doped ZnO (ZnO:Cu2++Eu3+) nanocrystals as a function of the Cu2+ ion concentration in Wt% and after both samples undoped and doped with the Cu2+ were annealing at 900˚C by 24 h, the samples were irradiated with an excitation wavelength of 270 nm (in the UV range). In Figure 4(a) it is observed that the PL spectra correspond to the Eu3+ ion emission in a ZnO matrix, in particular the most intense peak at the visible color red whit 613 nm in wavelength  . However it is observed from Figure 4(b) that the ZnO:Cu2++Eu3+ samples doped with 1, 2, 3, 10, and 20 Wt% are all alike: because they does not present antypical green broad emission band from 400 nm to 600 nm centered about 516 nm  as is showed in Figure 4(a), this band is due to the intrinsic defects emission of ZnO host. However, in the PL spectra of the ZnO:Cu2++Eu3+ samples two peaks at the UV region in 350 and 380 nm are present, in this the intensity of this last peak I this observer that your intensity it is decreased at an concentration of 20% Cu2+, this decrease is due to the capacity of Quenching of the Cu2+ at hide concentration the first peak is attributed to the 5d®7f europium transition, the second band is attributed to near edge band (NEB) emission  , other peak at the violet-blue region centered about 420 nm also is observed, this peak is due to the 5D®4F Eu3+ transition. From the photoluminescence spectra corresponding to the Eu3+ ion also appear: the peaks at 591, 613 and 630 nm are related to the direct intra-4f transitions in Eu3+ ions 5D0®7Fj (j = 1, 2, 3), the most intense emission is associated to the 5D0®7F2 transition in the red spectral region (613 nm) and is due to a allowed electric-dipole transition with inversion antisymmetric   , which results in a large transition probability in the crystal field  -  . The peak at 591 nm is due to the 5D0®7F1 transition, is an allowed magnetic-dipole transition. 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 interstitial sites  -  . It is very important to note from the photoluminescence spectra that as increase the Cu2+ ion concentration in the ZnO:Cu2++Eu3+ samples the intensity of the most intense transition of the Eu3+ ion with red wavelength of 613 nm decrease until an negligible value while the intensity of the wavelength emission at 380 nm situated in the UV region increase until reach an constant value for high Cu2+ ion concentrations: effectively: the Figure 5 shows the emission intensity variation of the red 5D0®7F2 transition of the Eu3+ ion and of the UV emission peak at 380 nm of the ZnO:Cu2++ Eu3+ samples as a function of the Cu2+ ion concentration, the graphic shows that as the Cu2+ ion concentration increase the red color intensity decrease until an minimum value while the intensity of the UV emission increase until obtain an constant value. It has been reported that divalent copper ion can effectively suppress Eu3+ photoluminescence, in an matrix of glass this is due to the quenching effect of Cu2+impuritieson Eu3+ emission photoluminescence quenching resulting from Eu3+→Cu2+ non-radiative energy transfer in which an spectral overlap occurring between Cu2+ absorption and Eu3+ emission(e.g. 5D0→7F2) transition occur   . Thus, in this paper is postulated that the quenching of the Eu3+ photoluminescence also can occur in ZnO matrix.
Figure 4. (a) Photoluminescence spectra of Eu3+ doped ZnO at 3%Wt; (b) Photoluminescence spectra of Cu2++Eu3+ doped ZnO as a function of the Cu2+ ion concentration in %Wt.
Figure 5. Intensity of the emissions Wavelength 380 and 613 nm as a function of the Cu2+ ion concentration in %Wt.
In this study Cu2++Eu3+ doped ZnO solid solution powders were synthesized by solution combustion method maintaining the Eu3+ ion concentration fixed at 3%Wt and after annealing at 900˚C by 24 h. From the XRD study it is confirmed that the ZnO hexagonal wurtzite structure is conserved after the introduction of the Cu2+ ion in the ZnO structure, and the same XRD study showed the presence of the minority phases Eu2O3 and Eu2CuO4. From photoluminescence study it is demonstrated that the Cu2+ ion effectively suppresses the Eu3+ ion photoluminescence due to the PL quenching effect of the Cu2+ ion on the Eu3+ ion.
The authors wish to thank to Adriana Tejeda (IIM) for the XRD measurements. To Omar Novelo Peralta (IIM) (by you SEM study), to M.A. Canseco Martinez and Lázaro Huerta (IIM) by your chemical analysis.
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