Photoluminescence Studies of ZnO, ZnO:Eu and ZnO:Eu Nanoparticles Covered with Y2O3 Matrix


Development of advanced display and lighting technology such as field emission displays and plasma display panels requires phosphor which has a high efficiency and low degradation. Particle sizes and the locations of dopants in the hosts take an important role in the luminescence emissions of phosphors. ZnO nanoparticles are widely employed in plasma field emission display devices and well investigated; however, lanthanide (Ln3+) doped ZnO needs more investigations. In ZnO:Eu the lanthanide ions (Eu3+) may occupy either Zn2+ lattice site or on surface of ZnO crystal. The emissions of Eu3+ ion on the surface are the characteristic of Eu2 O3 , which leads to weak luminescence emission. To observe such phenomena, nanoparticles of ZnO, 2 at.% Eu3+ doped ZnO (ZnO:Eu) and ZnO:Eu covered with yattria matrix were prepared by wet chemical method at low temperature. The prepared nanoparticles were characterized by XRD and TEM. XRD data reveal the significant phase segregation of the annealed nanoparticles compared with lower heated samples. This phase segregation of Eu3+ ion establishes responsible for the decrease luminescence intensity of annealed ZnO:Eu nanoparticles compared with the as-prepared ZnO:Eu nanoparticles. Improvement on luminescence emissions could be achieved only for the as-prepared ZnO:Eu nanoparticles while covered with yattria matrix.

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Singh, L. (2015) Photoluminescence Studies of ZnO, ZnO:Eu and ZnO:Eu Nanoparticles Covered with Y2O3 Matrix. Materials Sciences and Applications, 6, 269-278. doi: 10.4236/msa.2015.64032.

1. Introduction

ZnO is a direct band gap semiconductor of band gap 3.2 eV at room temperature and one of the potential materials in various display devices [1] [2] . Luminescent properties of ZnO can be improved by doping ZnO with transition metals or lanthanide ions. Extensive luminescence studies have been carried out some of the systems like ZnO:Eu, ZnO:Mn, ZnO:V, ZnO:W etc. [3] -[7] , since the last three decades. As the luminescent properties of Eu3+ ions in different crystallographic environments are well understood, it can be used as a probe to monitor the structural changes taking place around it, when doped in different inorganic materials. Among them, luminescent properties of ZnO:Eu system have been investigated by a number of authors [3] -[7] keeping in mind its potential applications as materials in field emission display devices. Our previous luminescence studies of ZnO:Eu nanoparticles showed that there were no energy transfer between the host ZnO and Eu3+ ions [8] . As the Eu3+ ions have very poor solubility in ZnO, most of the Eu3+ ions are lying on the surface of the ZnO particles, only few ions may be in the lattice site while annealing at high temperature results migration of Eu3+ ions towards the grain boundaries (distorted environment) and segregation of Eu3+ into separate Eu2O3 phase. Both the phenomena produce significant reduction of their luminescent properties. Hence low temperature synthesis will be suitable Eu3+ doped ZnO nanoparticles. Low temperature synthesis normally leads to smaller particle size. One way to circumvent this problem is to cover the surface of the nanoparticles with suitable inorganic materials or dispersion in organic/inorganic materials wherein the Eu3+ ions can migrate without phase separation. Y2O3 turned out to be a suitable choice as it was a suitable host for the lanthanide ions. Shell formation with Y2O3, it is expected that there will not have any nanoparticle aggregation and phase separation of Eu2O3.

In this article, ZnO:Eu and ZnO:Eu nanocroystals covered with yatria matrix (ZnO:Eu/Y2O3) prepared at low temperature (140˚C) and critical luminescence studies were also carried out for ZnO, ZnO:Eu and ZnO:Eu/ Y2O3 nanoparticles of different particle sizes.

2. Experimental

2.1. Preparation

Nanoparticle of ZnO, 2 at.% Eu3+ doped ZnO (ZnO:Eu), ZnO:Eu covered with in Y2O3 (ZnO:Eu/Y2O3) were prepared by wet chemical in polyethylene glycol medium at 140˚C. The preparation procedures were similar to our earlier reports [8] -[10] . To prepare ZnO:Eu covered with in Y2O3, 98 at.% Zn2+, 2 at.% Eu3+ and 100 at.% Y3+ were used, the precursors used for production of Zn2+, Eu3+ and Y3+ are Zn(CH3COO)2∙2H2O (99.05%, E-Merck), Eu2O3 (99.99% Aldrich) and Y2(CO3)3∙3H2O (99.99%, Alfa Aesar) respectively. Polyethylene glycol acts as reaction medium as well as the cappant during the preparation process. To prepare ZnO doped with 2 at.% Eu3+, 0.50 g of Zn(CH3COO)2∙2H2O was dissolved in 50 ml of polyethylene glycol mixture (form by 80:20 ration of polyethylene and ethylene glycol), warm the mixture at 60˚C within few minutes a clear solution could be observed. Then, 0.00734 g of Eu2O3 dissolved in dil∙HCl, warmed the solution several times in double distill water to get rid out excess acids from the solution. This solution containing Eu3+ ions were simply mixed to the above solution. Another solution containing 0.2 M of NaOH (Merk-GR) was made and simply added to the above mixtures then heated linearly up to 140˚C. The reaction temperature was maintained for one hour. A cloudy white precipitate could be observed and the reaction continued another two hours. The precipitate so obtain was extracted by centrifugation. The powder so obtained were the ZnO:Eu nanoparticles. In the second step, to cover ZnO:Eu nanoparticles with Y2O3 matrix, the former solution containing ZnO:Eu refluxed again simply by addition of another solution containing Y3+ (to get Y3+, 0.4785 g of Y2(CO3)3∙3H2O dissolve in dil∙HCl, warm the solution several times in excess double distilled water). Further 20 ml solution of 0.2 M strength of NaOH added to the above mixture then heated slowly at the same reaction temperature. A thick milky white precipitate could be obtained, the reaction continues another three hours, the precipitate so obtained was extracted by centrifugation in excess ethanol. This powder sample was the ZnO:Eu/Y2O3 nanoparties. The powder samples so obtained were dried for further characterization and luminescence measurements.

2.2. Characterization

X-ray diffraction studies were carried out using a Philips powder X-ray diffractometer (model PW 1071) with Ni filtered Cu-Ka radiation. The lattice parameters were calculated from the least square fitting of the diffraction peaks. The average crystallite size was calculated from the diffraction line width based on Scherrer relation: d = 0.9l/Bcosq, where l is the wavelength of X-rays and B is the half maximum line width.

All luminescence measurements were carried out at room temperature with a resolution of 3 nm, using a Hitachi Instrument (F-4500) having a 150 W Xe lamp as the excitation source. Powder samples were mixed with methanol, spread over a quartz plate, dried at 100˚C and mounted inside the sample chamber. All Transmission Electron Microscopy (TEM) images are done using CM 200 Fillips.

3. Results and Discussion

3.1. XRD and TEM Study

Figure 1(a) shows the XRD patterns of as-prepared ZnO and Figures 1(b)-(d) gives the XRD pattern of as- prepared 2 at.% Eu3+ doped ZnO (ZnO:Eu) nanoparticles along with their 500˚C and 900˚C heated samples. The particles are crystalline with hexagonal structure with S. G. P63mc, all the diffraction peaks corresponds to the JCPDS 36-1451. No other characteristic peaks of europium impurity such as Eu2O3 could be detected at least within the resolution limit of the diffractometer. The broadening of the diffraction peaks are the characteristic of nanosized ZnO materials. The lattice parameters of as-prepared sample are found to be a = 3.249(1) Å and c = 5.205(1) Å. Bulk value of ZnO has lattice parameters a = 3.249 Å, c = 5.206 Å (JCPDS 36-1451). Lattice parameters are close to bulk value. The crystallite sizes are calculated from the peak position of ZnO:(101) plane. The crystallite size of as-prepared sample is found to be 12 nm. With Eu3+ incorporation, there is slight shift in the diffraction peak maxima towards lower 2q values compared to as-prepared ZnO (Figure 1(b)), indicating the lattice expansion of the host lattice with Eu3+ incorporation. The lattice parameter values are found to be a = 3.254(1), c = 5.208(1) Å. It is understandable as the ionic radius of Eu3+ (0.95 Å) is higher than that of Zn2+ (0.70 Å) and hence the Eu3+ incorporation in the ZnO lattice is associated with lattice expansion. It may be quite possible that many of the Eu3+ ions must be occupying the Zn2+ lattice site of ZnO nanoparticles. Annealing the samples at 500˚C and 900˚C result in the slight shifting of the diffraction peaks to higher 2q values, revealing that the lattice parameter decrease with increase in heat treatment temperatures. The lattice parameters of 500˚C heated sample are a = 3.252(1) Å and c = 5.208(1) Å, and for 900˚C heated sample the corresponding values are a = 3.249(1) Å and c = 5.207(1) Å respectively. From these results it can be confirmed that there are partial removal of Eu3+ ions from the ZnO lattice with heat treatment. The crystallite sizes are calculated from the peak position of ZnO:(101) plane by Gaussian fitting. The crystallite sizes of 2 at.% Eu3+ doped ZnO (ZnO:Eu) nanoparticles:as-prepared, 500˚C and 900˚C heated samples are 8, 16 and 36 nm respectively suggesting the agglomeration of particles on higher heat-treatment.

Figure 1. XRD patterns of (a) as-prepared ZnO, (b) 2 at.% Eu doped ZnO and heat-treated ZnO (c) at 500˚C and (d) at 900˚C.

Figures 2(a)-(c) show XRD patterns of as-prepared 2 at.% Eu3+ doped ZnO (ZnO:Eu) nanoparticles covered with in Y2O3 (ZnO:Eu/Y2O3) along with their 500˚C and 900˚C heated samples. In as-prepared sample only the characteristic peaks of ZnO could be observed indicating that Y2O3 phases are amorphous. However on heating the samples at 500˚C and 900˚C, along with diffraction peaks characteristic of ZnO the characteristic peaks of cubic Y2O3 are also appeared. The * indicates the characteristics peaks of Y2O3 phases. The crystallite sizes are calculated from the peak position of ZnO:(101) plane. The crystallite sizes of 2 at.% Eu3+ doped ZnO (ZnO:Eu) nanoparticles covered with Y2O3 are 8, 13 and 32 nm respectively for the as-prepared, 500˚C and 900˚C heated samples, indicating the increase of particle size with heat-treatment. The lattice parameters of ZnO present in ZnO:Eu/Y2O3 are a = 3.256(1) Å, c = 5.208(1) Å for as-prepared sample and for the 500˚C and 900˚C heat- treated samples are a = 3.251(1) Å, c = 5.208(1) Å and a = 3.249(1) Å, c = 5.208(1) Å for 900˚C respectively. Bulk value of ZnO has lattice parameters a = 3.249 Å, c = 5.206 Å, and volume, V = 47.62 Å3. It suggests that lattice parameters decreases with the heat-treatment indicating Eu3+ coming out from ZnO lattice with heat- treatment. The lattice parameter and unit cell volume of Y2O3 present in ZnO:Eu/Y2O3 are a = 10.621(1) Å for 500˚C heat-treated sample; and a = 10.649(1) Å for 900˚C heat-treated sample. The lattice parameters for the bulk value of Y2O3 is a = 10.604 Å, and volume, V = 1192.40 Å3 [11] . Unit cell parameter of Y2O3 in ZnO:Eu/ Y2O3 samples increase with heat-treatment. It indicates that Eu3+ions which are coming out from the ZnO lattice diffuse into Y2O3 lattices occupying Y3+ sites.

TEM picture shows the as-prepared ZnO:Eu nanoparticles are in spherical shape covered with Y2O3 particles (ZnO:Eu/Y2O3), the particle size is found to be 13 nm as shown in Figure 3(a). The bright portion shows the ZnO:Eu nanoparticles surrounded by dark Y2O3 particles. The inset figure shows the expansion of it, showing ZnO particles are covered with Y2O3 particles. Figure 3(b) shows the TEM picture of 900˚C treated ZnO:Eu/ Y2O3 nanoparticles. It shows that both the ZnO and the Y2O3 particles are crystalline, though Y2O3 fairy covered to ZnO:Eu particles. The particle size is 46 nm. The particle sizes determined by XRD and TEM are in agreement.

3.2. Luminescence Study

Figure 4 shows the emission spectra of pure (a) as-prepared, (b) 500˚C and (b) 900˚C heat-treated ZnO nanoparticles excited at 320 nm. In as-prepared sample the pattern consists of a sharp peak around 390 nm superimposed over a broad peak centered around 500 nm. Weak luminescence emission at 465 nm could be detected in this sample. Based on the previous photoluminescence studies of ZnO nanoparticles [12] -[14] , the peak around

Figure 2. XRD pattern of 2 at.% Eu doped ZnO nanoparticles dispersed in Y2O3 (a) as-prepared, (b) 500˚C and (c) 900˚C heat-treated nanoparticles, the * shows the Y2O3 phase in the pattern.

Figure 3. TEM images of (a) as-prepared 2 at.% Eu doped ZnO nanoparticles covered with Y2O3 along with its expansion in the inset and (b) shows the 900˚C heat- treated ZnO:Eu/Y2O3 nanoparticles.

Figure 4. Photoluminescence emission spectra of (a) as-prepared and heat-treated at (b) 500˚C and (c) 900˚C ZnO nanoparticles excited at 320 nm.

390 nm has been attributed to the near band edge emission due to the exciton emission [12] , that of 465 nm emission is due to excitons on the surface of ZnO nanoparticles and that around 500 nm has been attributed to the defects present in the ZnO lattice or the deep level electrons [11] -[18] . In 500˚C heat-treated sample the emission at 466 nm is dominant over the 390 and 500 nm emissions, this emission is due to the native defect of ZnO nanoparticle or the emission due to surface trap excitons. For the sample heat-treated at 900˚C, the green emission around 500 nm is dominated over the band edge emission 390 nm and that of surface trap emission 466 nm. The green emission of ZnO is still on debate [17] -[23] , but in our observation this emission is due to oxygen vacancy [10] . In order to confirm the exact peak positions of luminescence emissions, the photoluminescence emission spectrum of 900˚C heat-treated ZnO nanoparticle is deconvoluted with Gaussian fitting (chi square = 0.9988) as shown in Figure 5. It is observed two peaks centered at 390 and 504 nm. The emission of native defect state could not be detected. It is observed that the UV-emission and the visible light emissions of ZnO nanoparticles depend on the particle size. With incorporation of Eu3+ in the ZnO lattice, no defect or exciton emissions could be observed. However strong Eu3+ emissions due to the transition 5D0®7F1 (orange emission at 590 nm) and 5D0®7F2 (yellow emission at 615 nm) could be observed on direct excitation of Eu3+ ion at 394 nm (7F0®5L6) as shown in Figure 6(a). Heating the sample at 500˚C there is significant reduction in the Eu3+ luminescent intensity as seen from Figure 6(b) and on further heating at 900˚C no emission could be observed from the sample (Figure 6(c)). Partial phase segregation of Eu3+ ions as well as Eu3+ incorporation in the grain boundaries (distorted regions) are responsible for the significant reduction in the luminescence intensity of heated samples. These results are supported by XRD data, decreased in the lattice parameter values of the ZnO:Eu nanoparticles for higher heat-treatments at 500˚C and 900˚C. It is worth to mention, even though the luminescence emission intensities decrease with the raise heat-treatment temperatures, the peak positions of the emission lines

Figure 5. Deconvolution of 900˚C heat-treated ZnO nanoparticle, ooo signifies the experimental data, ―― indicates the fitted curve and the ...... indicates the deconvoluted curves.

Figure 6. Photoluminescence emission spectra of ZnO:Eu nano- particles (a) as prepared and its (b) 500˚C and (c) 900˚C heat treated samples excited at f-f transition of Eu3+ ion (394 nm).

do not change i.e. the emission lines are still at 615 nm.

Figure 7 shows the emission spectra corresponding to as-prepared ZnO:Eu/Y2O3 nanoparticles along with their heat-treatments at 500˚C and 900˚C. There are two strong emission lines due to f-f transitions of Eu3+ ions at 5D0®7F1 and 5D0®7F2. Here opposite phenomena could be observed compared to heat-treated ZnO:Eu nanoparticles, strong emission peaks of Eu3+ ions could be detected for the heat-treated ZnO:Eu/Y2O3 samples and luminescence intensity also increased with the increase heat-treatment temperature. It also observes in the emission lines that the peak positions of the heat-treated samples are shifting towards lower wavelength side (at 610 nm) compared to the as-prepared sample (remain at 615 nm). Why such phenomena could be observed in the emission spectra of ZnO:Eu/Y2O3 samples? It is clearly observed the luminescence emission intensity of as- prepared ZnO:Eu/Y2O3 is at 615 nm and strong. In the as-prepared ZnO:Eu/Y2O3 sample, Y2O3 exists as amorphous as revealed by the XRD data in Figure 2. The emission lines of ZnO:Eu/Y2O3 nanoparticles are from the ZnO:Eu particles, not from Y2O3. In such samples the Y2O3 is amorphous that is Y(OH)3, the Eu3+ ions occupy the Zn2+ site in ZnO and results the emission lines. If the emissions are from Y2O3, it will be from surface

Figure 7. Photoluminescence emission spectra of ZnO:Eu nanoparticles covered with Y2O3 (a) as-prepared, (b) 500˚C and (c) 900˚C heat-treated samples excited at f-f transition of Eu3+ (394 nm).

ions i.e. Eu3+ ions by forming weakly bonded Eu2O3 and will be leading weak luminescence emission from such particles [9] . To confirm these emissions are either from ZnO:Eu or Y(OH)3 particles, 2 at.% of Eu3+ ion doped Y2O3 was prepared at the same reaction temperature (the preparation procedure was similar to LR Singh et al. [9] except using NaOH in place of urea). Comparison of luminescence emissions of the as-prepared ZnO:Eu and Y(OH)3:Eu are shown in the supplementary Figure 1. It is observed the luminescence emission of ZnO:Eu is strong whereas that of Y(OH)3:Eu is weak as Y2O3 is amorphous at this temperature and the emissions are from the surface ions i.e. from weakly bonded Eu2O3. Crystallization of Y2O3 starts only when annealed at higher temperatures and substitution of Y3+ by Eu3+ starts at this condition only. When Eu3+ ions occupy Y3+ site strong emissions could be obtained in such systems that is why the emission of heat-treated ZnO:Eu/Y2O3 samples are intense. These results are similar to earlier reports [9] [24] -[26] . It is worth to mention that the peak positions of the heat-treated samples are found at 610 nm, these spectra are identical with heat-treated Y2O3:Eu [9] . The substitution of Y3+ by Eu3+ are supported by XRD data that the unit cell volume of Y2O3 increased with the increase of heat-treatment temperatures. As the heat treatment temperatures increased more Eu3+ ions are leaving ZnO particles then migrate towards the Y2O3 lattice, where it feels an environment different from that of ZnO. That is why narrow and sharp Eu3+ emission lines at 610 nm could be observed in the luminescence emission spectra of annealed ZnO:Eu/Y2O3 samples. Figure 8 shows the comparisons of luminescence emission spectra of as-prepared ZnO:Eu and the ZnO:Eu nanoparticles covered with in yattria matrix (ZnO:Eu/Y2O3), it is obvious that the luminescence emission of ZnO:Eu nanoparticles covered with in yattra matrix can be significantly improved its luminescence intensity compared to ZnO:Eu particles. The luminescence emission intensity of ZnO:Eu nanoparticles in yattra matrix is10 times than the ZnO:Eu nanoparticles. This result will be useful for the further understanding of these systems of particles. It is expected the solubility of Eu3+ ions in ZnO are poor and most of the lanthanide ions may be on the surface of the ZnO particles or on the grain boundaries. These lanthanide ions migrates towards the amorphous environment of Y2O3 consequently reduces surface ions in ZnO. The surface lanthanide ions enhanced non-radiative transition leading weak luminescence emission, due to this ZnO:Eu covered with in Y2O3 have more intense compared to ZnO:Eu nanoparticles. For further confirmation of location of Eu3+ ions in ZnO:Eu/Y2O3 nanoparticles the excitation spectra are taken by monitoring at 615 nm for as-prepared and heat-treated samples as shown in Figure 9. The pattern consists of a broad peak around 250 nm for the heat-treated samples which are the characteristics of Eu-O charge transfer band in Y2O3 lattice [1] [27] . Such Eu-O charge transfer band could not be observed in as-prepared ZnO:Eu/Y2O3nanocrystals but only the f-f absorption band at longer wavelength could be observed. This simply confirms that in as-prepared sample the

Figure 8. Photoluminescence emission spectra of (a) as-prepared ZnO:Eu nanoparticles and (b) ZnO:Eu nanoparticles covered with Y2O3 matrix (ZnO:Eu/ Y2O3) excited with f-f transition 394 nm.

Figure 9. Excitation spectra of ZnO:Eu/Y2O3 nanoparticles (a) as-prepared monitored at 615 nm and (c) 900˚C monitored at 610 nm.

Eu3+ ions are adapted in the amorphous environment of Y2O3, not forming the bond with Y3+ site. The above observations have revealed that there are significant numbers of surface Eu3+ ions on the ZnO nanoparticles and phase segregation of Eu3+ ions can be significantly reduced by incorporation by the Y2O3 matrix. The ratio of the integrated intensities of magnetic dipole transition 5D0®7F2 and electric dipole transition 5D0®7F1 transitions can be considered indicative of the asymmetry of coordination of Eu3+ ions [9] .

Asymmetric ratio A21 for as-prepared ZnO:Eu/Y2O3 nanoparicle is found to be 3.4 that of heat-treated at 500˚C and 900˚C are found to be 4.8 and 5.3 respectively. This simply indicting the that co-ordination of Eu-O could be improved for the annealed samples, the Eu3+ ions are leaving ZnO and making co-ordination with Y3+. The co-ordinations of Eu-O are also increase with the increase of annealing temperature. But the result is contrary with ZnO:Eu systems, in these system the asymmetric ratio decreases with the increase of annealing temperatures. From the above observations in the luminescence emissions of both the systems (ZnO:Eu and ZnO:Eu/Y2O3) it is evident that covering of ZnO:Eu nanoparticles in Y2O3 can significantly reduced phase segregation of Eu2O3 in these systems.

4. Conclusion

Nanocrystals of ZnO, Eu3+ doped ZnO (ZnO:Eu) and ZnO:Eu covered with in Y2O3 could be successfully prepared at low temperature by wet chemical method in polyethylene glycol medium. Luminescence emission of pure ZnO nanoparticles depends on their particle sizes. However, there is a significant reduction in the luminescence intensity of annealed ZnO:Eu compared with the as-prepared ZnO:Eu which can be attributed to the partial phase segregation of Eu3+ ions as well as the incorporation of Eu3+ ions in the grain boundaries leading to distorted environment. The phase segregation and the incorporation of Eu3+ ions at the grain boundaries can be significantly reduced by incorporating the ZnO:Eu nanoparticles in Y2O3.


The author awes Dr. R. S. Ningthoujam, Scientific Officer E, Chemistry Division of BARC, Mumbai for providing all the facilities of characterization, luminescence measurements and constant encouragement while preparing this manuscript. The author thanks DST New Delhi for providing financial assistance under DST Fast Tract Young Scientist Scheme (SR/FTP/PS-128/2011) during execution of this work and UGC New Delhi for providing financial assistance under UGC Start Up Grant with sanction No.F.20-1(25)/2012(BSR).

Conflicts of Interest

The authors declare no conflicts of interest.


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