Up-conversion luminescence and energy transfer mechanism in ZnTiO3: Er3+,Yb3+ phosphor
Graphical abstract
Introduction
Rare earth (RE) ion doped semiconductors are operative materials for a diverse range of applications in various fields of research, photocatalytic treatment, environmental purification and development of technologies [[1], [2], [3]]. Importantly, up-converting (UC) RE doped semiconductor materials have attracted enormous attention as photocatalysts for numerous applications [3,4]. Fluoride and oxides-based materials are reported as the most effective hosts for UC RE ions to achieve up-conversion process. It is important to note that both fluoride and oxide-based materials exhibit moderate phonon energy which results into high up-conversion luminescence (UCL) intensity [[5], [6], [7], [8], [9]]. However, high thermal, mechanical and chemical stabilities of oxide hosts with moderate phonon energy makes them a suitable alternative of fluorides hosts for the practical up-conversion application [10,11]. Among oxides materials, ZnO and TiO2 are considered most suitable because they exhibit low phonon energies and better chemical and thermal stability [[12], [13], [14], [15]]. Recent studies have demonstrated an interest in the optical properties of the UC RE ion doped and co-doped ZnO–TiO2 system [16,17]. In the present work the effectiveness of the ZnTiO3 ternary oxide host for RE dopant ions is evaluated for UC photoluminescence.
There are several different phases that can be prepared from the ZnO–TiO2 system, that also consist of several different polymorphs. These phases include ZnTiO3, Zn2TiO2 and Zn2Ti3O8, just to mention a few [18]. ZnTiO3 resembles an ABO3 perovskite structure and has high thermal and chemical stability. It has a wide energy band gap, high electron mobility as well as being cost-effective and environmentally friendly [19,20]. The compound is mostly used as a photocatalyst, microwave dielectric resonator and gas sensors for the detection of carbon monoxide and nitric oxide. When doped with a suitable activator ion, its catalytic and electronic properties can be tuned to expand its possible applications [21,22]. Incorporation of RE ions introduces metastates which display luminescence [23]. ZnTiO3 materials have been synthesized using several approaches, which include the solid-state reaction method, the hydrothermal method, the sol-gel method and solution combustion synthesis, just to mention a few [24,25]. In this study the solid-state reaction was used to prepare ZnTiO3 due to its advantages such as controllable crystal growth, energy efficiency, short firing time [26] and ease of incorporation rare-earths dopant ions.
Currently, electrification of remote areas in many developing countries is still a challenge [27]. To address this, alternative forms of energy such as renewable energy are being explored, which do not need to be integrated into the electric grids. Solar and wind energy are common alternatives, which are effective and efficient renewable energy sources [28]. The solar energy is the most explored of the two, because of cost-effectiveness. Even though power conversion efficiency of conventional crystalline silicon solar cell devices is still relatively low to compete with nuclear and fossil fuel sources of energy, they are currently in the commercial market. Their utilization of the solar radiation has limitations because they can only absorb within the narrow range of the electromagnetic wave spectrum of the sun. They also have limited photo-carrier for trapping and recombination and limited electron injection from the excited state into the semiconductor surface [[29], [30], [31]]. The limited absorption can be improved by incorporating materials with a wider absorption range that will absorb the solar energy and convert it to radiation that is absorbable by the photovoltaic cells. Such materials include down-converting (DC) and up-converting (UC) layers of phosphors [29].
The RE doped phosphors have been studied for use in various applications, which include the up-conversion of near-infrared (NIR) photons into ultraviolet or visible photons. This is due to their rich energy level structures, tuneable wavelengths, high photostability, sharp band widths, long emission lifetimes and relatively low toxicity [32]. Among the UC RE ions, erbium (Er3+) ions are commonly used due to their rich energy level structures with several metastable energy levels and long lifetimes. Ytterbium (Yb3+) ions are suitable UC sensitizers because they enhance the luminescence of UC activator ions, due to their large absorption cross section in the NIR region and their high efficiency in transferring energy to activators [[32], [33], [34], [35]]. Co-doping of Er3+ ions with Yb3+ ions has been a hot topic of research in the past several decades to achieve the most efficient up-converted emission. The UC emission properties for Er3+/Yb3+ group can be achieved through effective energy transfer (ET), excitation modulation and cross relaxation energy transfer [[36], [37], [38]].
In the present research, we prepared ZnTiO3:Er3+,Yb3+ for the first time via a simple high temperature solid-state reaction technique. The effects of lanthanides molar concentration on the UC luminescence properties of this phosphor material were investigated and discussed in detail. Most importantly, the prepared phosphor displays an efficient UC luminescence, which makes it potentially useful for photovoltaic solar cells.
Section snippets
Synthesis
The undoped ZnTiO3 powders were synthesized by the conversional solid-state reaction method. The starting materials were commercial ZnO (99.9%), TiO2 (99.7%), Erbium (III) acetate hydrate (99.9%) and Ytterbium (III) acetate hydrate (99.9%). In a typical preparation, stoichiometric amounts of ZnO and TiO2 were first mixed and ground together in a ball mill for 1 h at room temperature. Erbium acetate was incorporated into the ball milled powder and mixed using a pestle and mortar. The same
Structure and morphology analysis
The XRD patterns in Fig. 1 show the phase formation of the zinc titanate samples. These are the patterns of undoped ZnTiO3, together with luminescence intensity optimized ZnTiO3:0.8mol%Er3+ and ZnTiO3:0.8mol%Er3+,0.7mol%Yb3+ phosphors. The patterns are consistent with the ecandrewsite structure of ZnTiO3 with space group R-3 (148) referenced in the ICSD file number 01-085-0547. These results show that the undoped ZnTiO3 sample is a single phase and does not contain any impurities or other
Conclusion
ZnTiO3:Er3+,Yb3+ up-converting phosphors have been successfully synthesized through a solid-state technique. XRD patterns confirmed that the phosphor crystallized in the ecandrewsite structure with space group R-3 (148). The morphology of the phosphors showed faceted particle shapes. The UC properties have been comprehensively investigated and the significant enhancement for UC emission intensity of Er3+ due to energy transfer from Yb3+ in co-doped samples has been observed. The observation of
Authors contributions
S.J. Mofokeng designed, prepared and performed the optical measurements with help from R.E. Kroon. S.J. Mofokeng characterized the material, analyzed the data and prepared the manuscript including inputs from all the authors. L.L. Noto co-wrote the manuscript and participated in discussion to interpret results primarily with S.J. Mofokeng, M.S. Dhlamini, R.E. Kroon and O.M. Ntwaeaborwa. All authors commented and have given approval to the final version of the manuscript.
Declaration of competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was sponsored by the department of physics and the Grow Your Own Timber (GYOT), from the College of Science, Engineering and Technology at the University of South Africa (UNISA) (Grant no. 88028) and NRF Freestanding, Innovation and Scarce Skills Masters and Doctoral Scholarships (Grant no. 102590).
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