Yb/Er codoped Gd(OH)3 was synthesized firstly via a simple hydrothermal treatment of the corresponding nitrate in the presence of alkali by tuning the pH values. The Gd2O3:Yb/Er nanocrystals were obtained via sintering the corresponding hydroxides precursors. The as-prepared samples were characterized by the typical X-ray diffraction, energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, and spectrophotometer. The results revealed that two shapes of the as-prepared Gd2O3:Yb/Er nanocrystals can be readily tuned from lemon-like particle to rod-like structure via tuning pH values from 7 to 14. Moreover, compared with the samples prepared at pH 7, the Gd2O3:Yb/Er nanocrystals prepared at pH 14 exhibit enhanced red upconversion emission and higher upconversion luminescence intensity under the excitation of 980？nm laser. 1. Introduction Rare-earth (RE) doped upconversion (UC) nanocrystals have been comprehensively used in high-quality lighting devices, magnets, and other functional materials due to their optical, electronic, and chemical properties arising from the 4f shell of their ions [1–6]. These capabilities are highly sensitive to the bonding states of RE ions. RE doping induced photoluminescence property in fluorides, and oxides systems have been universally reported [7–11]. For instance, Yb/Tm codoped Y2O3 nanocrystals can achieve remarkable green UC luminescent emission [8]. In Yb/Er codoped Lu2O3 system, UC spectra revealed that simultaneous green and red emissions can be obtained [11]. However, the intense UC emission located in the red region by doping Yb/Er is an enormous challenge owing to the fact that Yb/Er codoped UC materials have a multipeak emissions nature. Recently, Li’s group have developed a simple and controlled synthetic method [3–5], in which the shape transformation, phase evolution, and UC emission tuning of a series of RE oxides nanocrystals can be reached by simply adjusting reaction parameters, such as pH value, reaction time, and temperature. Among various developed RE-based oxides UC nanomaterials, gadolinium oxide (Gd2O3) has its unique UC luminescent and magnetic properties. Therefore, RE (Nd3+, Er3+, and Tm3+) doped Gd2O3 nanocrystals have been selected for optical and magnetic studies, because of their high chemical durability, photothermal and photochemical stabilities, low photon-energy, and unpaired electrons of the 4f shell [12–17]. Of course, there are also many investigations of the UC luminescence nanocrystals mainly focused on the RE doped Gd2O3 with phase/structure
References
[1]
G.-Y. Adachi and N. Imanaka, “The binary rare earth oxides,” Chemical Reviews, vol. 98, no. 4, pp. 1479–1514, 1998.
[2]
J. A. Capobianco, J. C. Boyer, F. Vetrone, A. Speghini, and M. Bettinelli, “Optical spectroscopy and upconversion studies of Ho3+-doped bulk and nanocrystalline Y2O3,” Chemistry of Materials, vol. 14, no. 7, pp. 2915–2921, 2002.
[3]
X. Wang, X.-M. Sun, D. P. Yu, B.-S. Zou, and Y. D. Li, “Rare earth compound nanotubes,” Advanced Materials, vol. 15, no. 17, pp. 1442–1445, 2003.
[4]
Z.-L. Wang, J. H. Hao, H. L. W. Chan, W.-T. Wong, and K.-L. Wong, “A strategy for simultaneously realizing the cubic-to-hexagonal phase transition and controlling the small size of NaYF4:Yb3+, Er3+ nanocrystals for in vitro cell imaging,” Small, vol. 8, no. 12, pp. 1863–1868, 2012.
[5]
X. Wang and Y. D. Li, “Synthesis and characterization of lanthanide hydroxide single-crystal nanowires,” Angewandte Chemie International Edition, vol. 41, no. 24, pp. 4790–4793, 2002.
[6]
J. J. Zhou, G. X. Chen, E. Wu et al., “Ultrasensitive polarized up-conversion of Tm3+-Yb3+ doped β-NaYF4 single nanorod,” Nano Letters, vol. 13, no. 5, pp. 2241–2246, 2013.
[7]
G. Z. Ren, S. J. Zeng, and J. H. Hao, “Tunable multicolor upconversion emissions and paramagnetic property of monodispersed bifunctional lanthanide-doped NaGdF4 nanorods,” Journal of Physical Chemistry C, vol. 115, no. 41, pp. 20141–20147, 2011.
[8]
S. F. Zhou, N. J. Jiang, K. Miura et al., “Simultaneous tailoring of phase evolution and dopant distribution in the glassy phase for controllable luminescence,” Journal of the American Chemical Society, vol. 132, no. 50, pp. 17945–17952, 2010.
[9]
S. J. Zeng, J. J. Xiao, Q. B. Yang, and J. H. Hao, “Bi-functional NaLuF4:Gd3+/Yb3+/Tm3+ nanocrystals: structure controlled synthesis, near-infrared upconversion emission and tunable magnetic properties,” Journal of Materials Chemistry, vol. 22, no. 19, pp. 9870–9874, 2012.
[10]
S. J. Zeng, M.-K. Tsang, C.-F. Chan, K.-L. Wong, and J. H. Hao, “PEG modified BaGdF5:Yb/Er nanoprobes for multi-modal upconversion fluorescent, in vivo X-ray computed tomography and biomagnetic imaging,” Biomaterials, vol. 33, no. 36, pp. 9232–9238, 2012.
[11]
E. W. Barrera, M. C. Pujol, F. Díaz et al., “Emission properties of hydrothermal Yb3+, Er3+ and Yb3+, Tm3+-codoped Lu2O3 nanorods: upconversion, cathodoluminescence and assessment of waveguide behavior,” Nanotechnology, vol. 22, no. 7, Article ID 075205, 2011.
[12]
H. Guo, N. Dong, M. Yin, W. P. Zhang, L. R. Lou, and S. D. Xia, “Visible upconversion in rare earth ion-doped Gd2O3 nanocrystals,” Journal of Physical Chemistry B, vol. 108, no. 50, pp. 19205–19209, 2004.
[13]
H. Guo, Y. F. Li, D. Y. Wang et al., “Blue upconversion of cubic Gd2O3:Er produced by green laser,” Journal of Alloys and Compounds, vol. 376, no. 1-2, pp. 23–27, 2004.
[14]
T. Hirai and T. Orikoshi, “Preparation of Gd2O3:Yb, Er and Gd2O2S:Yb, Er infrared-to-visible conversion phosphor ultrafine particles using an emulsion liquid membrane system,” Journal of Colloid and Interface Science, vol. 269, no. 1, pp. 103–108, 2004.
[15]
D. Dosev, I. M. Kennedy, M. Godlewski, I. Gryczynski, K. Tomsia, and E. M. Goldys, “Fluorescence upconversion in Sm-doped Gd2O3,” Applied Physics Letters, vol. 88, no. 1, Article ID 011906, 2006.
[16]
L. D. Sun, J. Yao, C. H. Liu, C. S. Liao, and C. H. Yan, “Rare earth activated nanosized oxide phosphors: synthesis and optical properties,” Journal of Luminescence, vol. 87–89, pp. 447–450, 2000.
[17]
A. Brenier and G. Boulon, “Laser heated pedestal growth and spectroscopic investigations of Nd3+-doped Gd2O3 single crystal fibres,” Journal of Luminescence, vol. 82, no. 4, pp. 285–289, 1999.
[18]
A. Brenier, “Laser-heated pedestal growth of Er3+-doped Gd2O3 single crystal fibres and up-conversion processes,” Chemical Physics Letters, vol. 290, no. 4–6, pp. 329–334, 1998.
[19]
X. Bai, H. W. Song, G. H. Pan et al., “Improved upconversion luminescence properties of Gd2O3:Er3+/Gd2O3:Yb3+ core-shell nanorods,” Journal of Nanoscience and Nanotechnology, vol. 9, no. 4, pp. 2677–2681, 2009.
[20]
M.-A. Fortin, R. M. P. Petoral Jr., F. Soderlind, et al., “Polyethylene glycol-covered ultra-small Gd2O3 nanoparticles for positive contrast at 1.5 T magnetic resonance clinical scanning,” Nanotechnology, vol. 18, no. 39, Article ID 395501, 2007.
[21]
Y. P. Li, J. H. Zhang, Y. S. Luo, X. Zhang, Z. D. Hao, and X. J. Wang, “Color control and white light generation of upconversion luminescence by operating dopant concentrations and pump densities in Yb3+, Er 3+ and Tm3+ tri-doped Lu2O3 nanocrystals,” Journal of Materials Chemistry, vol. 21, no. 9, pp. 2895–2900, 2011.
