Elsevier

Journal of Alloys and Compounds

Volume 509, Issue 41, 13 October 2011, Pages 10115-10120
Journal of Alloys and Compounds

Luminescent properties and X-ray photoelectron spectroscopy study of ZnAl2O4:Ce3+,Tb3+ phosphor

https://doi.org/10.1016/j.jallcom.2011.08.054Get rights and content

Abstract

Cerium (Ce3+)- and terbium (Tb3+) co-doped zinc aluminate (ZnAl2O4) nanocrystals were successfully prepared by a combustion method using urea as fuel. The as-prepared samples were annealed in hydrogen atmosphere to improve their optical properties and crystallinity. The structure and morphology of the samples analyzed using X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) showed that ZnAl2O4 crystallized in a well known cubic spinel structure. As deduced from X-ray photoelectron spectroscopy (XPS) data, there was partial inversion of cations in as-prepared (unannealed) samples implying that the ZnAl2O4 did not crystallize in a cubic normal spinel. Instead, the XPS data demonstrated possible structural readjustment from inverse to normal spinel after annealing. The excitation and emission data collected when ZnAl2O4:Ce3+,Tb3+ samples, with different concentrations of Ce3+ and Tb3+ ions, were excited at different wavelengths showed that green emission of Tb3+ was sensitized by Ce3+, i.e. there was energy transfer from Ce3+ to Tb3+ resulting in improvement of green emission from Tb3+. This study therefore, sets out to discuss the sensitizing effect of Ce3+ and the effect of annealing on the structure of ZnAl2O4:Ce3+,Tb3+.

Highlights

► Highly crystalline ZnAl2O4:Ce3+,Tb3+ powder phosphors were prepared using solution combustion method at low temperature. ► Structural readjustment from normal to inverse spinel confirmed from XPS data. ► Energy transfer from Ce3+ to Tb3+ demonstrated.

Introduction

Zinc aluminate (ZnAl2O4) is a well known semiconductor with a wide bulk band gap of 3.5–3.9 eV [1], [2], [3], [4], [5], [6]. It belongs to a class of mixed-metal oxides called the spinels which are commonly represented by a general chemical formula AB2O4 where A and B are divalent (2+) and trivalent (3+) cations, respectively. In AB2O4 spinels, 8 of the 64 tetrahedral interstices are occupied by A2+ cations while 16 of the 32 octahedral interstices are occupied by B3+ cations [7]. It is well known that ZnAl2O4 can crystallize in a cubic normal or inverse spinel structure depending on the preparation procedure. In the normal spinel structure, the 3+ ions occupy the octahedral site while the 2+ ions occupy the tetrahedral site. In an inverse spinel the divalent and trivalent ions are not just exchanged but there is a mixed occupation by different amounts of A2+ and B3+ on the octahedral site while the tetrahedral site is only occupied by the B3+ cations. In most cases, intermediate structures between normal and inverse spinels are crystallized [8]. Traditionally, ZnAl2O4 with normal, intermediate or inverse spinel structure is widely used as a catalyst or ceramic [9]. Today, it is used in many applications such as optoelectronics, sensor technology and information display technology [1], [2], [5], [6] because of its excellent optical and hydrophobic properties and high chemical and thermal stability [10]. For application in display technologies, ZnAl2O4 is used as host matrix for trivalent rare-earth ions (e.g. Tb3+, Eu3+ and Dy3+) [11], [12], [13] or transition metals (e.g. Mn2+ and Cr3+) [14], [15] to prepare phosphors emitting mostly in the visible range of the electromagnetic spectrum. Researchers in this study are particularly interested in the performance of a nanocrystalline ZnAl2O4 because of the speculation that nanocrystalline materials may have better optical properties than their bulk counterparts [11]. Different synthesis methods such as sol–gel [16], [17], hydrothermal [18], [19], combustion [6], [15], [20] and solid state reaction [21] are commonly used to prepare rare-earth/transition metal doped nanocrystalline ZnAl2O4 phosphors. In this study, the solution combustion method was used to prepare Ce3+–Tb3+ co-activated nanocrystalline ZnAl2O4 phosphors. Compared to other methods, the combustion method has advantages such as cost-effectiveness, low processing temperature, extremely shorter reaction time, high purity and homogeneity of the final product. The flame temperature during urea assisted combustion was sufficient to enable some Al ions in the zinc aluminate spinel structure to occupy tetrahedral sites (spinel inversion) [22]. The objective of this study was to prepare an efficient green emitting phosphor through sensitization of Tb3+ by Ce3+. It is well known that Ce3+ can absorb UV photons and sensitize emission of other rare-earth by a down-conversion process [23], [24]. This study was also intended to investigate the effects of different parameters such as relatively low activator concentrations, annealing temperature and excitation wavelengths on emission efficiency of nanocrystalline ZnAl2O4:Ce,Tb phosphor. In addition, the X-ray photoelectron spectroscopy (XPS) was used to determine the chemical and electronic states of the elements present in as prepared and post-preparation annealed samples. This phosphor was evaluated for application in display technologies and also as a UV down-converting layer for improved efficiency of photovoltaic cells.

Section snippets

Powder preparation

The raw materials used were analytical reagent (AR) grade zinc nitrate hexahydrate, aluminum nitrate, rare-earth nitrates (cerium nitrate and terbium nitrate), and urea (ACS reagent). All these were of AR grade from Merck South Africa with ∼99% purity, while the rare-earth (Ce3+ and Tb3+) nitrates with ∼99.99% purity were from Aldrich–Sigma. Stochiometric amounts of zinc nitrate, aluminum nitrate and urea were dissolved in triple de-ionized (DI) water. A homogeneous transparent solution was

XRD analysis

The XRD patterns of pure/undoped and Ce3+–Tb2+ co-activated ZnAl2O4 are, respectively, shown in Fig. 1, Fig. 2. Undoped ZnAl2O4 samples in Fig. 1 were (a) as prepared and (b) annealed in air at 600 °C and (c) 700 °C for 4 h, respectively. Fig. 2 shows the room temperature XRD patterns of the ZnAl2O4:Ce3+,Tb3+ samples annealed at 700 °C in a hydrogen atmosphere for 4 h. The concentrations of Ce3+ and Tb3+ were (a) 1.33 mol%, 0.66 mol%; (b) 1.14 mol%, 0.86 mol%; (c) 1 mol%, 1 mol%; (d) 0.66 mol%, 1.33 mol%;

Conclusions

In conclusion, the ZnAl2O4:Ce3+,Tb3+ powder phosphors were successfully synthesized using a one-step combustion technique. As confirmed from the X-ray diffraction data, the ZnAl2O4 was highly crystalline with or without post-synthesis annealing. The X-ray photoelectron spectroscopy data confirmed that there was structural readjustment from inverse to normal spinel as a result of annealing. The TEM data showed that the particles were spherical in shape, with some degree of faceting, and their

Acknowledgements

The authors would like to thank the South African National Research Foundation (NRF), National Research Foundation of Korea and Nanomaterials Cluster fund of the University of the Free State for the financial support. Authors also give special thanks to Dr. Weon-Sik Chae, Korea Basic Science Institute, for the fluorescence lifetime measurement.

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