6050
J. Phys. Chem. C 2009, 113, 6050–6055
Highly Uniform Gd(OH)3 and Gd2O3:Eu3+ Nanotubes: Facile Synthesis and Luminescence Properties Guang Jia, Kai Liu, Yuhua Zheng, Yanhua Song, Mei Yang, and Hongpeng You* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed: January 9, 2009; ReVised Manuscript ReceiVed: February 13, 2009
Uniform Gd(OH)3 nanotubes have been prepared via a simple wet-chemical route at ambient pressure and low temperature, without any catalysts, templates, or substrates, in which Gd(NO3)3 was used as the gallium source and ammonia as the alkali. SEM and TEM images indicate that the as-obtained Gd(OH)3 entirely consists of uniform nanotubes in high yield with diameters of about 40 nm and lengths of 200-300 nm. The temperature-dependent morphological evolution and the formation mechanism of the Gd(OH)3 nanotubes were investigated in detail. Furthermore, the Gd2O3 and Eu3+-doped Gd2O3 nanotubes, which inherit their parents’ morphology, were obtained during a direct annealing process in air. The corresponding Gd2O3:Eu3+ nanotubes exhibit the strong red emission corresponding to the 5D0-7F2 transition of the Eu3+ ions under UV light or low-voltage electron beam excitation, which might find potential applications in the fields such as light-emitting phosphors, advanced flat panel displays, or biological labeling. 1. Introduction One-dimensional (1D) nanostructures, such as nanorods, nanowires, nanotubes, and nanobelts, have attracted extensive interest over the past several years due to their potential applications in a wide range of fields.1-3 Among the various 1D nanostructures, nanotubular materials are expected to have unusual characteristics amplified by their marked shape-specific and quantum size effects. The possibility of modifying the outer and inner surfaces and edges also enhances the advantageous characteristics of nanotubes. The interest in nanotubes thus stimulates researchers to enlarge the range of inorganic nanomaterials, from carbon-based substances to sulfides,4,5 nitrides,6 and oxides.7-9 As we know, rare earth compounds have been widely used as high-performance luminescent devices, catalysts, and other functional materials based on the electronic, optical, and chemical characteristics arising from their 4f electrons. Therefore, much attention has been paid to the rare earth materials with nanotubular morphology due to their shape-specific properties. Several strategies have been developed for the growth of nanotubes, such as template and catalyst-based methods.10-12 Among the methods used in the synthesis of nanotubes, hydrothermal or solvothermal techniques have emerged as powerful methods due to their great chemical flexibility and synthetic tenability. Recently, a variety of rare earth hydroxide and oxide compound nanotubes, such as Y(OH)3, Dy(OH)3, Eu2O3, CeO2, and Tb(OH)3, have been synthesized through hydrothermal process.13-18 Compared with other rare earth compounds, there have been few reports on the synthesis of nanotubular gadolinium hydroxide, which can be used as catalyst, sorbent, and precursor of Gd2O3.19-22 As we know, Eu3+-doped Gd2O3 phosphor is one of the important red-emitting phosphors. Although the investigations on Gd2O3:Eu3+ phosphors are extensive,23-25 few studies have been reported on the * To whom correspondence
[email protected].
should
be
addressed.
E-mail:
synthesis of dispersed, uniform, and well-defined Gd2O3:Eu3+ nanotubes and their corresponding luminescent properties. Recently, Li et al. have prepared the Gd(OH)3 nanowires and nanotubes through a hydrothermal method.15,26 However, for application at ambient pressure, the hydrothermal reaction is not suitable for large-scale and industrial preparation due to the pressure limitation. So it is significant to develop more facile, efficient, and low-cost techniques to fabricate large-scale and well-crystallized Gd(OH)3 nanotubes. In this work, we present a simple wet-chemical approach to the fabrication of Gd(OH)3 nanotubes at ambient pressure and low temperature without any catalysts, templates, or substrates. The Gd2O3 nanotubes, which inherit their parents’ morphology, can be obtained during the calcination process. This novel approach is of significant importance in industrial applications as a consequence of its low costs, benignancy to environment, and synthetic convenience. 2. Experimental Section Gd(NO3)3 and Eu(NO3)3 aqueous solutions (pH ) 4) were obtained by dissolving Gd2O3 (99.99%) and Eu2O3 (99.99%) in dilute HNO3 solution under heating with agitation. 2.1. Preparation of Gd(OH)3 Nanotubes. In a typical synthesis, 2 mmol of Gd(NO3)3 aqueous solution was added to 25 mL of deionized water to form a clear solution in a bottle under magnetic stirring. Then 25 wt % of ammonia solution (A. R., Beijing Chemical Company) was introduced rapidly to the vigorously stirred solution until pH ) 10. After additional agitation for 10 min, the mixture was heated to 75 °C for 18 h with vigorous stirring. The resulting white precipitate was collected, centrifuged, and washed with deionized water and ethanol for several times respectively and finally dried at 60 °C in air. Different reaction temperatures (18, 40, and 60 °C) and alkaline sources (NaOH) were selected to investigate the effects of these factors on the morphological and structural properties of the samples.
10.1021/jp9002164 CCC: $40.75 2009 American Chemical Society Published on Web 03/19/2009
Gd(OH)3 and Gd2O3:Eu3+ Nanotubes
Figure 1. XRD patterns of the samples prepared at (a) room temperature (18 °C), (b) 40 °C, (c) 60 °C, and (d) 75 °C. The standard data for hexagonal phase Gd(OH)3 (JCPDS No. 83-2037) is also presented in the figure for comparison.
2.2. Thermal Conversion of Gd(OH)3 to Gd2O3. The asprepared Gd(OH)3 sample was annealed at 600 °C for 2 h in air with a heating rate of 2 °C min-1 to obtain the Gd2O3 product. In order to investigate the luminescence of Eu3+ in the host Gd2O3 nanotubes, the Eu3+-doped Gd2O3 sample was prepared by introducing a proper amount (5%, molar ratio) of Eu(NO3)3 solution instead of Gd(NO3)3 to the solution as described above. 2.3. Characterization. The samples were characterized by powder X-ray diffraction (XRD) performed on a D8 Focus diffractometer (Bruker). The morphology and composition of the samples were inspected using a scanning electron microscope (SEM; S-4800, Hitachi) equipped with an energy-dispersive X-ray spectrum (EDX; JEOL JXA-840). Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained using a JEOL 2010 transmission electron microscope operating at 200 kV. Photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The cathodoluminescence (CL) measurements were carried out in an ultrahigh-vacuum chamber (
