The experimental method of chemical synthesis solution combustion [17] , is quite simple, fast and economical in which an oxidizer and a fuel agents are combined in a highly exothermic red-ox chemical reaction stoichiometric, producing ZnO, H₂O (vapor), molecular N₂, and CO₂. In this work using Zinc Nitrate hexahydrate [Zn(NO₃)₂∙6H₂O] as oxidizer, urea [(H₂NCONH₂)] as fuel, yttrium chloride [YCl₃] and copper chloride [CuCl₂] as dopants, undoped and Cu²⁺ + Y³⁺ doped ZnO were synthesized by a solution combustion technique as a function of Cu²⁺ ion concentration in %wt, maintaining the Y³⁺ concentration constant in 3%wt. The samples were later annealed at 835˚C by 2 h.

Pure and Cu²⁺ + Y³⁺ doped ZnO samples were obtained by a solution combustion technique by means of the following redox chemical reaction stoichiometric:

3Zn ( NO 3 ) 2 × 6H 2 O + 5H 2 NCONH 2 + 2 ( YCl 3 + CuCl 3 ) → 3ZnO + 10H 2 O + 5CO 2 + 8N 2 + 2 ( Cu 2+ + Y 3+ ) + 5Cl 2 (1)

The Equation (1) was obtained by taken into account the oxidizer/fuel molar radio (O/F = 1) required for a stoichiometric mixture which is determined by summing the total oxidizing and reducing valences in the oxidizer compound and dividing it by the sum of the total oxidizing and reducing valences in the fuel compound [17] . Accordingly for the complete combustion of zinc nitrate-urea mixture, the molar ratio becomes 5/3, the equation balanced (1) was obtained with this value ratio. Using the weight atomic concept in Equation (1), it was used to obtain 3 gr. of ZnO for all the Cu²⁺ ion concentrations. The dopants yttrium and copper by means its respective chloride were simultaneously integrated into ZnO host in each combustion reaction using the dopants concentrations correct. The samples produced were characterized by x-ray diffraction technique using a Philips PW 1800 diffractometer using Cu Kα radiation, the morphology of the samples was studied by means of a scanning electron microscopy JEOL JSM 840 A , and the photoluminescence spectra was recorded using a Tektronix 792 AD.

The XRD patterns of Y³⁺ doped ZnO with 3%wt. and Cu²⁺ + Y³⁺ co-doped ZnO with 3%wt. of Y³⁺ ion and 1, 3, 5, 10, and 20%wt. of Cu²⁺ ion concentrations and after annealed at 835˚C by 2 h are showed in Figure 1. The observed XRD characteristic peaks positioned in 2θ scale at 31.64, (100), 34.32, (002), 36.02, (101), 47, 45, (102), 56.49, (110), 62.70, (103), 66.30, (201), 67.85, (112) and 69.93, (201) shows the good crystallinity of the material. All the diffraction peaks can be indexed to the hexagonal wurtzite structure of ZnO (JCPDS card No. 36-1451, a = b = 3.249A, c = 5.206A). However, other characteristic peaks at 29˚, (222) and 27˚, (230), corresponding to the yttrium oxide (Y₂O₃) and Y₂Cu₂O₅ phases respectively also were observed in Cu²⁺ + Y³⁺ co-doped ZnO, which can be attributed to the doping of Y³⁺ ion into Zn²⁺ lattice site. Also, it is observed that the peak intensity of the news phases decrease with Cu²⁺ ion concentration until total quenching at 20%wt. due to Cu²⁺ ion incorporation into Zn-Y-O lattice site and segregation of Y³⁺ ion toward ZnO surface. The Y₂Cu₂O₅ phase also is obtained as secondary phase in the YBaCuO superconductor synthesis [22] [23] [24] [25] . The Figure 2 shows a magnification of the (002) plane between 34.1˚ and 34.7˚ in which are observed the changes in peak intensity and peak position as a function of Cu²⁺ ion concentration. The peak intensity is increased when the Cu²⁺ ion is incorporated into Y³⁺-Zn-O lattice until 10%wt. Cu²⁺ ion concentration. This is attributed to the Cu²⁺ interstitial existence sharing the

oxygen with the Zn²⁺ atoms increasing the peak intensity. After 10%wt. the peak intensity decrease, due to excess of Cu²⁺ atoms that are energetically efficient to coalesce into metallic copper cluster decreasing the peak intensity [20] . The peak position along (002) plane of Cu²⁺ + Y³⁺ co-doped ZnO samples is red shifted in 2θ scale and broader up to 10%wt. Cu²⁺ ion concentration, after this value the peak position is increased newly due to the effect by intercalation of Cu²⁺ ion on the interatomic distance of the Y-Zn-O lattice [20] . The average crystal size of the samples was obtained from diffraction (101) plane using the Debye-Scherrer equation:

D = 0.9 λ β cos θ (2)

where λ is the X-ray wavelength used (1.5406Å), β is the full width at half maximum (FWHM) along (101) plane and theta is the Bragg diffraction angle. The Figure 3(a) and Figure 3(b) shows the change in FWHHM and average crystal

size as a function of Cu²⁺ ion concentration respectively, it is observed from Figure 3(a) that the FWHM increases with Cu²⁺ ion concentration until reach a maximum value at 10 %wt., after this value decrease until reach a value minimum. In contrary, from Figure 3(b) it is observed that the crystal size decrease with the Cu²⁺ ion concentration until reach a minimum value in 10%wt. after this value the crystal size increase. This behavior between the FWHM and the average crystal size is predicted by the Debye-Scherrer equation, since for a maximum value of FWHM correspond a minimum value in crystal size. Also it is observed that the Cu²⁺ + Y³⁺ co-doped ZnO crystals have less size than Y³⁺ doped ZnO; the initial reduction size from 75 nm with 0.0%wt. of Cu²⁺ ion to 40 nm whit 10%wt. of Cu²⁺ ion is due to the distortion produced by Cu²⁺ ions intercalated in the Zn-Y-O lattice [20] . At higher Cu²⁺ concentration than 10%wt. the Cu²⁺ atoms excess is combined forming the Y₂Cu₂O₅ compound.

The Figure 4 shows the photoluminescence (PL) spectra of pure ZnO and Y³⁺ doped ZnO with 3%wt. The spectra were obtained in the UV-Vis range, illuminated with an excitation wavelength of 350 nm at room temperature. The PL spectrum of pure ZnO samples, exhibits the typical main band with characteristics peaks centered about 390 nm and corresponds to the near band edge (NBE) emission [6] . At higher wavelengths than NBE diffraction peak appears with less intensity that NBE peak the green emission band centered about 518 nm and attributed to free exciton recombination. From Figure 4 can be observed and calculated a little blue shift when Y³⁺ dopant was incorporated into ZnO host lattice, changing the ultra-violet NBE emission from 390 to 382 nm increasing the ZnO band gap in 0.3 eV. This slight blue shift of the UV peak is caused by 3%wt. Y³⁺ doping [9] and [16] . However it is observed an increase in UV intensity in Y³⁺ doped ZnO compared with UV intensity of pure ZnO. The Figure 5 exhibit the room temperature PL spectra of Cu²⁺ + Y³⁺ co-doped ZnO samples with var-

ious Cu²⁺ ion concentrations values of 2, 3, 5, 10 and 20%wt., it is clearly observed that for all Cu²⁺ concentration the green PL emission due to free exciton recombination has been quenched by the intercalation of Cu²⁺ ions into Y³⁺ + ZnO host lattice; this quenching effect is attributed to a non-radiative recombination process known as non-radiative recombination Auger phenomena which is associated to degenerate electrons, in which the energy released by an electron is immediately recombined and absorbed by another electron and the energy involved is dissipated by phonons. Auger process is considered as the cause major of non-radiative recombination in semiconductor materials. Auger process depends on the doping atoms concentration and defects in the lattice. [19] . However, in Fig. 5 also it is observed that the UV-NBE emission of the samples is diminished by increasing Cu²⁺ ion concentration, indicating that the optical-gap of ZnO semiconductor can be tailored by means of Cu²⁺ doping. This last result can be technologically applied in UV radiation sensors fabrication [26] .

The scanning electron microscope (SEM) technique has been used to observe the surface morphology (SM) of the particles. The Figures 6(a)-(d) exhibits the SEM images of the SM of Cu²⁺ + Y³⁺ co-doped ZnO. The Figure 6(a) shows the SM of Y³⁺ doped ZnO sample, it presents the existence of like-ovoid particles compact, dense structure and forming layers which have its surface covered with grains of size similar that are unevenly distributed. The average size grain is of 30 nm. The Figure 6(b) shows the SM of Cu²⁺ + Y³⁺ co-doped ZnO with 2%wt. of Cu²⁺ ions concentration, it is observed that the grains are most agglomerated

and evenly distributed in the sample. In this case the grains have an average size of 50 nm. The Figure 6(c) exhibits the SM of the Cu²⁺ + Y³⁺ co-doped ZnO with 3%wt. of Cu²⁺ ions concentration. It is seen that further increase in the intercalation of Cu²⁺ ions into Zn-Y-O host lattice improves the crystallinity of the sample. The Figure 6(c) displays the SM of Cu²⁺ + Y³⁺ co-doped ZnO with 3%wt. of Cu²⁺ ions, the particles presents like-spheres form and higher size than Y³⁺ doped ZnO. The average grain size of these particles is 50 nm. Finally the Figure 6(d) exhibits the SM of Cu²⁺ + Y³⁺ co-doped ZnO with 10%wt. of Cu²⁺ ions concentration. In this Cu²⁺ concentration level is observed the presence of the Y₂Cu₂O₅ as is corroborated by the XRD patterns showed in Figure 1. However, also can be seen Cu²⁺ ions interstitial localized into ZnO matrix. The geometry of these particles is like-spheres with an average size of 75 nm.