Enhanced dual-wavelength sensitive upconversion luminescence of BiPO4:Yb3+/Er3+ phosphors by Sc3+ doping
Graphical abstract
Dual-wavelength sensitive upconversion luminescence of the as-synthesized BiPO4:Yb3+, Er3+ phosphors were significantly enhanced by Sc3+ ions doping under the excitation of 980 nm or 1550 nm.
Introduction
Lanthanide ions (Ln3+) doped inorganic materials showing upconversion (UC) emission properties have been attracted more and more attention due to their potential applications in some fields, such as laser lighting or displays [1], [2], [3], temperature sensors [4], [5], [6], biomedical imaging [7], [8], [9], photodynamic therapy [9], [10] and solar cells [11], [12], [13] etc. Recently, the rare-earth orthophosphates (REPO4) as an important UC material attracted broad attention due to their low sintering temperature, chemical stability and environment friendly characteristics [14]. However, obtaining large quantities of rare-earth ions (REs), in a highly pure form, which is essential when REPO4 is used as the host material, is much more costly. Considering the main group elements like Sb and Bi are relatively much cheaper, the similar ionic radii of Bi3+ and RE3+, and the fact that BiPO4 is isostructural to REPO4 [15], BiPO4 was chosen as the investigated system in this paper. Also, it should be noted that Ln3+ co-doped BiPO4 with low dimensional structure shows promising temperature transformation. It reported that there would be some changes in the position of the nearest oxygen of the Bi3+ ion towards LaPO4, if Bi3+ ions incorporate in LaPO4 host materials. Therefore, it is reasonable to believe that the oxygen coordination of Ln3+ is disordered in the BiPO4 crystal structure [16]. As a result, it can be expected that the doping or substituting foreign ions into the lattice of parent compounds would introduce local distortion, and these local distortions could pin the structure of parent compounds. For the host of UC materials, it is well-known that BiPO4 mainly exists in two crystalline forms namely the hexagonal and monoclinic. The essential difference in the hexagonal and monoclinic form is the coordination number around Bi3+. In the case of hexagonal BiPO4, Bi3+ is surrounded by eight oxygen atoms, whereas in monoclinic BiPO4, Bi3+ has a coordination number of nine, similar to the monazite structure of lanthanide phosphate [17]. Until now, only a few works about UC luminescence properties of BiPO4:RE3+ materials under excitation at 980 nm were reported. In 2013, Zhang’ group [18] synthesized uniform rice-shaped BiPO4 UC submicron particles and obtained multicolor/bright white UC emissions by means of precisely adjusting the concentration of the dopant RE3+ (RE = Yb, Er, Tm and Ho) ions. In 2015, Zhang’ group [19] studied that the phase of BiPO4:Er3+/Yb3+ phosphors changed from hexagon to monocline and the morphology changed from nanorods through nanorugbies to microoctahedra with the extension of aging time.
Actually, besides being excited under 980 nm, the Er3+ ion can also be excited under 1550 nm, because the metastable 4I13/2 level of Er3+ has a long lifetime [20], [21], [22], [23], [24], [25]. Moreover, it is essential to study the excellent UC luminescence performance of Er3+ ions induced by excitation under 1550 nm in order to enlarge the solar spectrum response. By contrast, there are not any investigation about BiPO4:RE3+ UC luminescence under excitation at 1550 nm. Meanwhile, the application of BiPO4:Yb3+/Er3+ phosphors is still very much constrained due to their low UC intensity.
How to improve the UC luminescence intensity? Electronic transition probabilities and self-specific absorption of UC emission are two determinant factors of the UC luminescence intensity. In 2004, François Auzel [26] elaborated UC luminescence and anti-Stokes processes with f and d ions in solids. According to many previous reports, electronic transition probabilities are significantly affected by the local crystal field symmetry of RE ions. It will break the symmetry of the crystal field around the host lattice and RE ions’ surrounding environment by co-doping with foreign ions [27], [28], [29], [30], [31], [32], in order to enhance UC luminescence intensity. On account of these reasons, we consider that the co-doping one smaller trivalent ion (Sc3+) into BiPO4:Yb3+/Er3+ phosphors would decrease the crystal field symmetry around Yb3+ and Er3+ ions, leading to enhancement of the UC luminescence intensity. In fact, Yu et al. [30] reported that the blue, green, red and UV UC emissions of hexagonal NaYF4:Er3+/Yb3+ were obviously enhanced by doping with Sc3+. Yu’ group [31] presented a report on the synthesis of (Y0.1Yb0.05Er0.005ScxAl)2O3. The asymmetry of Er3+ local surrounding has been improved, with the increasing of Sc3+ co-doping concentration. As a result, the UC luminescence intensity has been enhanced several times, and so on. While these studies are only limited to excitation at 980 nm.
Herein, considering the effects of excitation wavelength and structures on UC performance of BiPO4 by different preparation methods have not been reported so far. In this work, dual-wavelength (980 and 1550 nm) sensitive UC luminescence of BiPO4:Yb3+/Er3+/Sc3+ phosphors were prepared by solid-state route and solvothermal synthesis, respectively. The explanation for the power density dependence of the UC emission intensity is given. Additionally, the mechanism of the enhanced UC emission intensity of the as-synthesized BiPO4:Yb3+,Er3+ with Sc3+ ions doped under the excitation of 980 nm or 1550 nm LD was also investigated, which will be a meaningful research work.
Section snippets
Materials
Bi(NO3)3⋅5H2O (AR grade), Yb2O3 (99.99%), Er2O3 (99.99%), Sc2O3 (99.99%), NH4H2PO4 (AR grade), NaH2PO4 (AR grade) and ethanol were purchased from Aladdin Chemical Reagent Factory (China). and were used as starting materials without further purification. Deionized water used as solvent throughout the experiment.
All the rare earth nitrate were obtained by dissolving the corresponding rare earth oxides in dilute HNO3 under heating with agitation followed by evaporating the solvent.
Samples synthesized by a solid-reaction method
The rare earths
Structure and morphology of powders
The crystal phase of samples prepared by two different methods were investigated by XRD. As shown in Fig. 1a, if the samples were prepared by solid-state method, the XRD patterns were found to be in good agreement with monoclinic phase of pure BiPO4 (JCPDS card No. 43-0637). By contrast, samples prepared by solvothermal method display different phases before and after calcining at 800 °C for 4 h. Before calcination (Fig. 1c), the patterns of the obtained samples match well with the standard
Conclusion
Monoclinic and hexagon phases of BiPO4:15% Yb3+, 2% Er3+, x% Sc3+ (x = 0, 1, 3 and 5) samples were prepared via solid-state route and solvothermal synthesis, respectively. Firstly, it is interesting to find that the BiPO4:15% Yb3+, 2% Er3+, x% Sc3+ (x = 0, 1, 3 and 5) samples with hexagon phase could not exhibit any UC emission, while these samples with monoclinic phase shows the strong dual-wavelength sensitive UC emission, depending a lot on the phase structure. Secondly, the dual-wavelength
Acknowledgements
The authors gratefully acknowledge the China Postdoctoral Science Foundation (2016M592308), Chinese Academy of Sciences (Grants XDA09030203).
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