# A Facile Synthesis and Optical Properties of Bundle-Shaped TbPO4·H2O Nanorods

Authors: Dan Yue; Wen Luo; Wei Lu; Ruiyong Wang; Chunyang Li; Jiazhong Chang; Zhenling Wang
Journal: Advances in Condensed Matter Physics
 (2013)

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## Abstract

Bundle-shaped TbPO4·H2O nanorods have been prepared by a facile hydrothermal technique and characterized by XRD, SEM, TEM, UV-Vis diffuse reflectance spectrum (DRS), photoluminescence (PL) spectrum, and lifetime. The results indicate that the obtained sample has hexagonal structure of TbPO4·H2O and is composed of nanorods bundles which is assembled from many single crystalline nanorods with the diameter of around 45 nm and the length of 2.3 μm. The growth of the single crystalline nanorod is along the (001) plane direction. Under the UV light irradiation, TbPO4·H2O nanorods bundles exhibit bright green emission corresponding to the D54→F7J (J=6,5,4,3) transitions of the Tb3+ ions, and the lifetime is determined to be about 0.24 ms.


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## Body

## 1. Introduction

In recent years, inorganic nanostructures with well-defined shapes and sizes have attracted growing attention because of their unique size- and shape-dependent properties [1–3]. Among many kinds of nanostructured materials, lanthanide orthophosphates (LnPO4) with uniform size and various morphologies have been prepared by some mild and controllable methods [4, 5] and attracted great interest because of their unique properties including very low solubility in water (The solubility product constant, pKsol=25–27) [6], high thermal stability, high index of refraction, and high luminescent efficiency [7, 8]. These materials have been used as active components in a wide range of applications such as phosphors, laser hosts, and biolabeling [9–12]. The chemical and optical properties of one-dimensional (1D) LnPO4 nanostructures (e.g., CePO4 : Tb3+ nanowires, LaPO4 nanorods, CePO4 peanut-liked nanostructures, etc.) can be successfully tailored, which makes these materials have significant potential applications in fabricating the next generation of information storage, optoelectronic, sensing devices, and nanoscale devices [13–15].As an important sort of lanthanide phosphates, TbPO4 has been investigated mainly focusing on its physical low-temperature properties (magnetic properties, birefringence measurements, and mean-field calculations) in previous literatures [16, 17]. Recently, much attention has been focused on the synthesis and properties of the TbPO4 with various morphologies. It is known that the hydrothermal technique is a common method in the field of material science. Using this technique, many materials with uniform morphology and satisfying crystallinity can be obtained at relatively low reaction temperature, usually without any further calcinations at high temperature [18, 19]. For example, TbPO4 : Eu3+ square-like particles were prepared by hydrothermal method with citric acid as the organic additive, and the reaction temperature is as low as 160°C [20]. As a contrast, TbPO4 hollow spheres can be obtained through solid state method when the annealing temperature is increased to 1150°C [21, 22]. In this work, uniform TbPO4·H2O bundle-shaped nanostructures composed of single crystalline nanorods were synthesized at 180°C through a facile hydrothermal technique and characterized by XRD, SEM, TEM DRS, PL spectra, and so forth. The possible mechanism leading to bundle-shaped structures, phase structure, morphology, and optical properties were discussed in detail.


## 2. Experimental Section

### 2.1. Synthesis of Bundle-Shaped TbPO4·H2O Nanorods

Tb4O7 (99.99%) and (NH4)2HPO4 (≥98.5%) were used as starting materials without any further purification. Tb(NO3)3 was prepared by dissolving Tb4O7 in diluted nitric acid, and the water in the solutions was distilled off by heating. Bundle-shaped TbPO4·H2O nanorods were prepared by hydrothermal technique. Typically, 2 mmol of (NH4)2HPO4 was added to 20 mL of 0.1 mol/L Tb(NO3)3 aqueous solution and the mixture was continuously stirred for 2 h. The obtained suspension was then transferred into a Teflon bottle held in a stainless steel autoclave, which was sealed and hydrothermally treated at 180°C for 24 h. After the autoclave was cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with ethanol and distilled water twice, respectively, and dried at 70°C for 24 h to obtain the sample.


### 2.2. Characterization

Phase structure was characterized by a Bruker D8 Advance X-ray diffractometer (XRD) with Cu-Kα radiation (λ=0.15406 nm). The accelerating voltage and emission current were 40 kV and 40 mA, respectively. Morphology of the samples was observed using a scanning electron microscope (SEM, Quanta 200) with an acceleration voltage of 25 kV. The TEM image and selected area electron diffraction (SAED) pattern were obtained on a JEOL-2010 transmission electron microscope at an accelerating voltage of 200 kV. UV-Vis diffuse reflectance spectrum (DRS) was obtained using a UV/Vis Spectrophotometer (Lambda35, PerkinElmer) equipped with an integrating sphere attachment. Photoluminescence (PL) spectra and lifetime were recorded using an FLS920P Edinburgh Analytical Instrument apparatus equipped with a 450 W xenon lamp and a μF900H high-energy microsecond flash lamp as the excitation sources.


## 2.1. Synthesis of Bundle-Shaped TbPO4·H2O Nanorods

Tb4O7 (99.99%) and (NH4)2HPO4 (≥98.5%) were used as starting materials without any further purification. Tb(NO3)3 was prepared by dissolving Tb4O7 in diluted nitric acid, and the water in the solutions was distilled off by heating. Bundle-shaped TbPO4·H2O nanorods were prepared by hydrothermal technique. Typically, 2 mmol of (NH4)2HPO4 was added to 20 mL of 0.1 mol/L Tb(NO3)3 aqueous solution and the mixture was continuously stirred for 2 h. The obtained suspension was then transferred into a Teflon bottle held in a stainless steel autoclave, which was sealed and hydrothermally treated at 180°C for 24 h. After the autoclave was cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with ethanol and distilled water twice, respectively, and dried at 70°C for 24 h to obtain the sample.


## 2.2. Characterization

Phase structure was characterized by a Bruker D8 Advance X-ray diffractometer (XRD) with Cu-Kα radiation (λ=0.15406 nm). The accelerating voltage and emission current were 40 kV and 40 mA, respectively. Morphology of the samples was observed using a scanning electron microscope (SEM, Quanta 200) with an acceleration voltage of 25 kV. The TEM image and selected area electron diffraction (SAED) pattern were obtained on a JEOL-2010 transmission electron microscope at an accelerating voltage of 200 kV. UV-Vis diffuse reflectance spectrum (DRS) was obtained using a UV/Vis Spectrophotometer (Lambda35, PerkinElmer) equipped with an integrating sphere attachment. Photoluminescence (PL) spectra and lifetime were recorded using an FLS920P Edinburgh Analytical Instrument apparatus equipped with a 450 W xenon lamp and a μF900H high-energy microsecond flash lamp as the excitation sources.


## 3. Results and Discussion

### 3.1. Phase Structure and Morphology

Figure1 shows the XRD patterns of bundle-shaped TbPO4·H2O nanorods (Figure 1(a)) and standard data of TbPO4·H2O powders (Figure 1(b)). It can be seen that all of the diffraction peaks of the bundle-shaped TbPO4·H2O nanorods are in agreement with the standard data of hexagonal structure TbPO4·H2O (JCPDS No. 20-1244), with the space group of P3121 (152). Figures 2(a) and 2(b) show the low- and high-magnification SEM images of TbPO4·H2O nanorods bundles, respectively. It can be seen that these bundle-shaped structures are homogeneous in the large field of vision (Figure 2(a)), the magnified image (Figure 2(b)) indicates that the bundle-shaped structures are actually composed of nanorods, and most of the nanorods are linked together by side-by-side conjunction.XRD patterns of bundle-shaped TbPO4·H2O nanorods (a) and standard data of bulk TbPO4·H2O ((b), JCPDS card 20-1244).
(a)
(b)SEM images (a) and (b) of TbPO4·H2O nanorods bundles.
(a)
(b)To investigate the growth mechanism and microstructure of bundle-shaped structures in detail, the obtained sample was observed by the TEM and high resolution TEM (HRTEM) images equipped with selected area electron diffraction (SAED) pattern (Figure3). It can be clearly seen from Figure 3(a) that the obtained sample is composed of bundle-shaped morphology, which is assembled by many TbPO4·H2O nanorods with the diameter of ~45 nm and the length of ~2.3 μm. A high-magnified image of bundled nanorods (Figure 3(b)) indicates that these single crystalline nanorods as the primary construction unit are relatively uniform. The SAED pattern (Figure 3(b), inset) taken from the upper single nanorod can be indexed to the (100) and (001) planes of TbPO4·H2O single crystalline with the hexagonal phase structure. These findings are consistent with the XRD result mentioned above. The HRTEM image (Figure 3(c)) of the single TbPO4·H2O nanorod marked as an oval in Figure 3(b) displays singlecrystalline nature. The values of interplanar spacing of TbPO4·H2O nanorod are 0.596 and 0.633 nm, which is identical to the (100) and (001) facet distance of bulk TbPO4·H2O powders, respectively. It can be seen that the growth direction of TbPO4·H2O singlecrystalline nanorod is along (001) plane. According to the experimental results and analysis, the growth mechanism of TbPO4·H2O nanorods bundles was proposed. Generally, TbPO4·H2O tends to grow as 1D nanorods, which is possibly due to the 1D characteristics of the infinite linear chains of hexagonal-structured TbPO4 [7]. And then, the surface energy of these nanorods may change under the hydrothermal process [23], so these nanorods aggregates might be assembled and grown along the same direction (oriented attachment) to form bundle-shaped structures. The formation process of TbPO4·H2O nanorods bundles is similar to that of In(OH)3 rod bundles [24].TEM images of bundle-shaped TbPO4·H2O nanorods (a) and (b), SAED pattern (inset in (b)), and HRTEM image (c) of the single nanorod.
(a)
(b)
(c)


### 3.2. Optical Properties

Figure4(a) shows the UV-Vis DRS spectrum of TbPO4·H2O nanorods bundles. The absorption peaks at 216 and 310 nm are due to the spin-allowed 4f-5d transition and the spin-forbidden transition of the Tb3+ ions, respectively. The absorption peak at 487 nm is assigned to the transitions from the ground level 7F6 to the excited level 5D4 of Tb3+ ion [25]. The excitation spectrum of TbPO4·H2O nanorods bundles was obtained by monitoring the emission of Tb3+ due to 5D4→7F5 transition at 543 nm, as shown in Figure 4(b). It can be seen that the excitation peak at 255 nm is assigned to intra 4f8 transitions between the 4f75d1, and most of the excitation peaks can be clearly assigned (352 nm: 7F6→5D2; 368 nm: 7F6→5G6; and 378 nm: 7F6→5D3). Under the UV light irradiation (378 nm), the emission spectrum is composed of four well-resolved peaks at 490, 543, 588, and 612 nm, which is corresponding to the D54→F7J (J=6,5,4,3) transitions of Tb3+ ions as labeled in Figure 4(c). Figure 4(d) shows the PL decay curves of TbPO4·H2O nanorods bundles with the excitation wavelength (378 nm) and emission wavelength at 543 nm. The PL decay curve for Tb3+ in the TbPO4·H2O nanorods bundles can be well fitted into a single exponential function as I(t)=I0exp(-t/τ) (τ is 1/e lifetime of Tb3+ ion) [26]. The lifetime for Tb3+ in TbPO4·H2O nanorods bundles is determined to be 0.24 ms.Figure 4
UV-Vis DRS (a), PL excitation (b), emission (c) spectra, and decay curve (d) of bundle-shaped TbPO4·H2O nanorods.


## 3.1. Phase Structure and Morphology

Figure1 shows the XRD patterns of bundle-shaped TbPO4·H2O nanorods (Figure 1(a)) and standard data of TbPO4·H2O powders (Figure 1(b)). It can be seen that all of the diffraction peaks of the bundle-shaped TbPO4·H2O nanorods are in agreement with the standard data of hexagonal structure TbPO4·H2O (JCPDS No. 20-1244), with the space group of P3121 (152). Figures 2(a) and 2(b) show the low- and high-magnification SEM images of TbPO4·H2O nanorods bundles, respectively. It can be seen that these bundle-shaped structures are homogeneous in the large field of vision (Figure 2(a)), the magnified image (Figure 2(b)) indicates that the bundle-shaped structures are actually composed of nanorods, and most of the nanorods are linked together by side-by-side conjunction.XRD patterns of bundle-shaped TbPO4·H2O nanorods (a) and standard data of bulk TbPO4·H2O ((b), JCPDS card 20-1244).
(a)
(b)SEM images (a) and (b) of TbPO4·H2O nanorods bundles.
(a)
(b)To investigate the growth mechanism and microstructure of bundle-shaped structures in detail, the obtained sample was observed by the TEM and high resolution TEM (HRTEM) images equipped with selected area electron diffraction (SAED) pattern (Figure3). It can be clearly seen from Figure 3(a) that the obtained sample is composed of bundle-shaped morphology, which is assembled by many TbPO4·H2O nanorods with the diameter of ~45 nm and the length of ~2.3 μm. A high-magnified image of bundled nanorods (Figure 3(b)) indicates that these single crystalline nanorods as the primary construction unit are relatively uniform. The SAED pattern (Figure 3(b), inset) taken from the upper single nanorod can be indexed to the (100) and (001) planes of TbPO4·H2O single crystalline with the hexagonal phase structure. These findings are consistent with the XRD result mentioned above. The HRTEM image (Figure 3(c)) of the single TbPO4·H2O nanorod marked as an oval in Figure 3(b) displays singlecrystalline nature. The values of interplanar spacing of TbPO4·H2O nanorod are 0.596 and 0.633 nm, which is identical to the (100) and (001) facet distance of bulk TbPO4·H2O powders, respectively. It can be seen that the growth direction of TbPO4·H2O singlecrystalline nanorod is along (001) plane. According to the experimental results and analysis, the growth mechanism of TbPO4·H2O nanorods bundles was proposed. Generally, TbPO4·H2O tends to grow as 1D nanorods, which is possibly due to the 1D characteristics of the infinite linear chains of hexagonal-structured TbPO4 [7]. And then, the surface energy of these nanorods may change under the hydrothermal process [23], so these nanorods aggregates might be assembled and grown along the same direction (oriented attachment) to form bundle-shaped structures. The formation process of TbPO4·H2O nanorods bundles is similar to that of In(OH)3 rod bundles [24].TEM images of bundle-shaped TbPO4·H2O nanorods (a) and (b), SAED pattern (inset in (b)), and HRTEM image (c) of the single nanorod.
(a)
(b)
(c)


## 3.2. Optical Properties

Figure4(a) shows the UV-Vis DRS spectrum of TbPO4·H2O nanorods bundles. The absorption peaks at 216 and 310 nm are due to the spin-allowed 4f-5d transition and the spin-forbidden transition of the Tb3+ ions, respectively. The absorption peak at 487 nm is assigned to the transitions from the ground level 7F6 to the excited level 5D4 of Tb3+ ion [25]. The excitation spectrum of TbPO4·H2O nanorods bundles was obtained by monitoring the emission of Tb3+ due to 5D4→7F5 transition at 543 nm, as shown in Figure 4(b). It can be seen that the excitation peak at 255 nm is assigned to intra 4f8 transitions between the 4f75d1, and most of the excitation peaks can be clearly assigned (352 nm: 7F6→5D2; 368 nm: 7F6→5G6; and 378 nm: 7F6→5D3). Under the UV light irradiation (378 nm), the emission spectrum is composed of four well-resolved peaks at 490, 543, 588, and 612 nm, which is corresponding to the D54→F7J (J=6,5,4,3) transitions of Tb3+ ions as labeled in Figure 4(c). Figure 4(d) shows the PL decay curves of TbPO4·H2O nanorods bundles with the excitation wavelength (378 nm) and emission wavelength at 543 nm. The PL decay curve for Tb3+ in the TbPO4·H2O nanorods bundles can be well fitted into a single exponential function as I(t)=I0exp(-t/τ) (τ is 1/e lifetime of Tb3+ ion) [26]. The lifetime for Tb3+ in TbPO4·H2O nanorods bundles is determined to be 0.24 ms.Figure 4
UV-Vis DRS (a), PL excitation (b), emission (c) spectra, and decay curve (d) of bundle-shaped TbPO4·H2O nanorods.


## 4. Conclusion

In summary, the bundle-shaped TbPO4·H2O nanorods have been successfully prepared by the hydrothermal route. The reaction media are aqueous solution and free of any surfactants or templates, and the synthesis technique is simple and environmentally friendly. The bundle-shaped TbPO4·H2O nanostructures are assembled by many single crystalline TbPO4·H2O nanorods through side-by-side conjunction. And these nanorods bundles yield green emission attributed to the transitions from the 5D4 to the F7J (J=6,5,4,3) energy levels of Tb3+, which makes these nanorods bundles have potential applications in many fields such as lighting and optoelectronic devices with nanometer dimensions.



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*Source: 673945-2013-12-18.xml*