SCIENCE CHINA Technological Sciences • RESEARCH PAPER •
June 2010 Vol.53 No.6: 1576–1582 doi: 10.1007/s11431-010-3102-9
Hydrothermal route to Eu doped LuO(OH) and Lu2O3 nanorods QIU HuaJun1, SHI Ying1*, XIE JianJun1, XIE Jie1, ZHANG LinLin2 & XU FangFang2 1
School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China; 2 Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Received November 13, 2009; accepted January 5, 2010
A facile method was developed to synthesize Eu doped LuO(OH) nanorods through hydrothermal processing and Lu2O3 nanorods by subsequent calcining. The microstructural morphologies of the Lu-based nanostructures could be controlled by simply varying the concentration of NaOH in hydrothermal processing as mineralizer. TEM observation revealed that the obtained LuO(OH) nanorods after hydrothermal processing had a uniform diameter of 10−25 nm and a length around 100 nm. After heat treatment at 600−700°C for 2 h, the high length/diameter ratio was sustained in the obtained Lu2O3 nanorods with different sizes depending on the calcining temperatures. nanorods, hydrothermal processing, LuO(OH), Lu2O3 Citation:
1
Qiu H J, Shi Y, Xie J J, et al. Hydrothermal route to Eu doped LuO(OH) and Lu2O3 nanorods. Sci China Tech Sci, 2010, 53: 1576−1582, doi: 10.1007/s11431-010-3102-9
Introduction
In recent years, rare earth hydroxides and oxides with various nanosized morphologies have been synthesized in order to explore their novel optoelectronic and chemical characteristics. Li’s group [1, 2] has done a systemic research on synthesis of a series of rare-earth hydroxide nanowires/ nanorods (Y(OH)3, La(OH)3, Pr(OH)3, Nd(OH)3, Sm(OH)3, Eu(OH)3, Gd(OH)3, Tb(OH)3, Dy(OH)3, Ho(OH)3, Er(OH)3, Tm(OH)3, YbOOH) with different aspect ratios by hydrothermal method, and the corresponding rare earth oxides by subsequent dehydration. It was foreseeable that these onedimensional nanostructures could be easily functionalized and applied in catalysis, optoelectronics, magnetics and luminescence applications. As one of the important phosphors materials having an extremely high density of 9.42 g/cm3, high Z-number of Lu (71) and analogous structure to Y2O3, more and more attentions have been paid to fabricat-
*Corresponding author (email:
[email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2010
ing Lu2O3 based low dimensional nanostructures, which might exhibit novel optical properties. A conventional method to synthesize Lu2O3 powders was the combustion method using urea ((NH2)2CO) as a fuel [3]. Recently, Chen et al. fabricated nanosized lutetia powders through a modified co-precipitation synthesis using the mixed NH3·H2O and NH4HCO3 as the precipitant [4]. Nano-crystalline Lu2O3:Eu powders from water-naphthalene emulsion were obtained by using polyvinyl alcohol (PVA) as a surfactant and urea as the precipitating agent [5]. However, none of these procedures paid enough attentions to morphology design of Lu-based nanostructures. Thus, it is believed that improvement of synthesis processing would be of great significance to control the morphology of lutetium based nanostructures. Yang et al. have reported three-dimensional flowerlike Lu2O3 and Lu2O3:Ln3+ (Ln=Eu, Tb, Dy, Pr, Sm, Er, Ho, Tm) microarchitectures via ethylene glycol (EG)mediated hydrothermal method followed by a subsequent heat treatment [6]. Wang et al. have successfully prepared Eu doped Lu2O3 nanoflakes, nanoquadrels, and nanorods through hydrothermal method and subsequent calcining. tech.scichina.com
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The key parameter controlling the shape of the product particles was supposed to be the concentration of the starting [Lu3+] [7]. In this paper, we report a facile approach to prepare uniform Eu doped LuO(OH) nanorods by hydrothermal method in the presence of NaOH. It is found that the morphology of the synthesized LuO(OH) products was greatly dependent on the concentration of NaOH in the hydrothermal processing. After subsequent calcinations at 600−700°C, Eu doped Lu2O3 nanorods were successfully obtained.
2 Experimental details 2.1
Synthesis of LuO(OH) and Lu2O3 nanorods
All chemical reagents used were of analytical grade. To start with, the commercial Ln2O3 (Lu2O3, Eu2O3) powders were dissolved in diluted nitric acid to prepare 0.927 mol/L Lu(NO3)3 solution and 0.916 mol/L Eu(NO3)3 solution, respectively. A mixed solution was prepared by adding 1.9 mL Eu(NO3)3 solution into 30 mL Lu(NO3)3 solution. Then the mixed solution was gradually added into 50 mL NaOH solution with [OH-]=3.5 mol/L under strongly stirring. After being stirred for about 30 min, the obtained white Lu(OH)3 precipitate was washed repeatedly with deionized water to remove excessive OH- anions until pH reached 7. After it was transferred into a 100-mL poly tetrafluoroethylene (PTFE) container, deionized water was filled into the container up to 70% of its total volume. A proper amount of NaOH was added to make OH- concentration range from 0.01 mol/L to 5 mol/L. The Lu(OH)3 precursor was sealed in PTFE container to carry out a hydrothermal treatment at 180°C for 24 h. The obtained hydrothermal products were collected by filtration, washed with deionized water to remove any possible ionic remnants. After being dried at 60°C in air flow for 10 h, the products were calcined in air in the temperature range of 600−920°C for 2 h. 2.2
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Both excitation and emission spectra were recorded with 0.2 nm resolution at room temperature.
3
Results and discussions
3.1 Effect of NaOH concentration on the morphology of LuO(OH) after hydrothermal processing A series of hydrothermal treatments were carried out under different NaOH concentrations to explore the proper conditions for synthesis of lutetium nanorod compounds. From the corresponding X-ray diffraction (XRD) patterns shown in Figure 1, it is clearly seen that as the NaOH concentration increased from 0.1 mol/L to 5.0 mol/L, similar diffraction patterns were obtained despite the peak intensity varied, of which the strongest diffraction intensity appeared at CNaOH =3.0 mol/L, showing its best crystallinity among the four samples. The main diffraction peaks detected could be ascribed to the monoclinic LuO(OH) phase (JCPDS 72-0928, space group P21/m(11)). The TEM micrographs of the obtained LuO(OH) are shown in Figure 2. From the TEM micrograph of LuO(OH) prepared at CNaOH=0.1 mol/L, T=180°C, t=24 h (Figure 2(a)), it is seen that the powders are in irregular shape and size, distributed randomly in the field. Figure 2(b) shows a representative image of LuO(OH) prepared at CNaOH=1.0 mol/L, T =180°C, t=24 h, displaying the general morphology of the nanowires which is 20−30 nm in diameter and about 1 μm in length. Figure 2(c) displays that products with nanorod morphology were successfully obtained under conditions of CNaOH=3.0 mol/L, T =180°C, t=24 h. It is shown that these nanorods present a uniform diameter of 10−25 nm and varied lengths ranging from 100 nm to several micrometers. As the concentration of NaOH reached 5.0 mol/L, LuO(OH) nanoflakes with a typical length of 400 nm and a width of 200 nm were detected. In order to make a further study on
Characterization
Powder X-ray diffraction (XRD) patterns were measured with D\max-2200 diffractometer using Cu Kα radiation (λ = 1.5418 Ǻ) at 40 kV and 40 mA with a scanning rate of 6°/ min. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were taken with transmission electron microscopy (JEM-2010, JEOL company, Japan) with an energy dispersive spectrometry (EDS) attachment. The thermal characteristics were measured with a thermogravimetry-differential thermal analysis (TG-DTA) thermal analyzer (STA 449/C, Netzsch Company, Germany). The samples were placed in the Al2O3 crucible and heated at a rate of 10.0 K/min from 25 to 1000°C in flowing air. Excitation and emission spectra of the Eu doped Lu2O3 powders were recorded using RF-5300 fluorescence spectroscopy (Shimadzu Corporation, Japan).
Figure 1 XRD patterns of the lutetium hydroxide prepared by hydrothermal processing at 180°C for 24 h with different NaOH concentrations. a, [NaOH]=0.1 mol/L; b, [NaOH]=1.0 mol/L; c, [NaOH]=3.0 mol/L; d, [NaOH]=5.0 mol/L.
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the effect of NaOH, powders were also prepared under conditions with CNaOH=8.0 mol/L while the other parameters were kept the same. From the morphology shown in Figure 2(e), it is obvious that the obtained products are in irregular shape and particle sizes are several times larger than those of Figure 2(a). It is suggested that the obtained LuO(OH) morphologies were greatly dependent on the concentration of NaOH in hydrothermal environment. The detailed microstructure of the LuO(OH) nanorods prepared at CNaOH=3.0 mol/L, T=180°C, t=24 h was further observed by HRTEM. Figure 3(a) shows a single LuO(OH)
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nanorod with a diameter of 20 nm. From the HRTEM image and corresponding selected area electron diffraction (SAED) pattern of the single LuO(OH) nanorod, it is deduced that the obtained LuO(OH) nanorods possessed nano-crystalline structure and their preferential growth direction was along [110]. The EDS result in Figure 3(c) indicates that the elements detected are Lu, Eu, O, Cu, Si and C. Of all the elements detected, Lu, Eu, O originated from the starting materials, whereas Cu, Si and C were derived from the sample holder or the environment when preparing the samples for TEM observation. The atomic proportion of Lu/O =0.55
Figure 2 TEM images of the lutetium hydroxide prepared by hydrothermal processing at 180°C for 24 h with different NaOH concentrations. (a) Irregular powders prepared at CNaOH=0.1 mol/L; (b) nanowires prepared at CNaOH=1.0 mol/L; (c) nanorods prepared at CNaOH=3.0 mol/L; (d) nanoplates prepared at CNaOH=5.0 mol/L; (e) irregular powders prepared at CNaOH=8.0 mol/L.
Figure 3 (a) A single LuO(OH) nanorod prepared at CNaOH=3.0 mol/L, T=180°C, t=24 h; (b) HRTEM image of the single nanorod, inside is the corresponding SADE pattern; (c) EDS pattern of the nanorod.
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(Lu=34.99, O=63.12) is approximately equal to the theoretical value Lu/O =0.5 in LuO(OH) and Eu/Lu=5.4% (Lu= 34.99, Eu=1.89) is also near the originally designed composition value of Eu/Lu=6%. 3.2
Thermal decomposition of the LuO(OH) nanorods
Figure 4 shows the thermogravimetric (TG) and differential thermal analysis (DTA) plots for the LuO(OH) nanorods. In the TG curve, there exist two major temperature intervals coupled with two significant mass losses. The first one is located at the interval between 40°C and 330°C, which mostly corresponds to the elimination of adsorption water. The second is in the range of 330°C−600°C, corresponding to the crystallization of LuO(OH) into Lu2O3 phase. The measured weight loss in this step is 4.88%, which is slightly higher than the theoretical weight loss value (4.32%) according to the reaction: LuO(OH)→1/2 Lu2O3+1/2 H2O. For the DTA curve, two main characteristic exothermic peaks at 277°C and 423°C were detected, corresponding to the mass loss at the first and second steps in the TG curve, respectively. As temperature exceeded 600°C the thermal decomposition was almost finished. 3.3 Characteristics of Lu2O3 nanorods derived from LuO(OH) nanorods Figure 5 shows the XRD patterns of the products derived from LuO(OH) nanorods after being calcined at 600°C −920°C for 2 h, respectively. It is clearly seen that all the diffraction peaks from the four patterns can be ascribed to Lu2O3 (JCPDS file No. 65-3172) and the 2θ values are in good agreement with values of pure Lu2O3 phase, demonstrating that the LuO(OH) phase has been transferred into pure Lu2O3 phase, no peaks of europium compounds have been detected, indicating Eu3+ has entered the Lu2O3 host lattice effectively. Figure 6 presents the TEM micrographs of the Lu2O3 nanorods calcined at different temperatures from LuO(OH)
Figure 5 XRD patterns of the Lu2O3 nanorods calcined at 600°C, 700°C, 800°C and 920°C for 2 h, respectively.
nanorods. When the calcining temperature was 600°C, Lu2O3 nanorods had an average diameter of 30−40 nm and length of several hundred nanometers, their surfaces were smooth and clean as well, preserving the morphology of LuO(OH) nanorod with high ratio of length to diameter; when the calcining temperature went up to 700°C, the morphology was kept quite well with a slight increase in diameter; when the calcining temperature was further raised, the nanorod morphology could be retained, but these Lu2O3 nanorods had the tendency to transfer into bamboo-like nanorods with rough surface. The HRTEM image (Figure 6(e)) taken at the knot of a single Lu2O3 nanorod showed that the Lu2O3 nanorods obtained at 920°C were composed of Lu2O3 grains with size of about 15−20 nm. It could be proposed that the condition (600−700°C/2 h) is suitable for formation of Lu2O3 nanorods starting from LuO(OH) nanorods. The morphology of LuO(OH) nanorods was maintained perhaps due to the higher activation energies needed for the collapse of these nanostructures, the same phenomena were also observed by Wang and Li [1, 2] in a series of rare earth materials, such as Y(OH)3 to Y2O3, La(OH)3 to La2O3, Sm(OH)3 to Sm2O3, Dy(OH)3 to Dy2O3, and so on. 3.4 Mechanism for LuO(OH) nanorods under hydrothermal conditions
Figure 4 TGA-DTA curve of the LuO(OH) nanorods.
It is seen from Table 1 that as the concentration of NaOH increased from 0.1 to 1.0 mol/L, the morphologies of the products after hydrothermal processing changed from irregular powders to nanowires; when [OH−] further increased from 1.0 to 5.0 mol/L, the morphology of hydrothermal products transferred from nanowires into nanoflakes, demonstrating that the aspect ratio became lower and the morphology of final products alternated from one dimension into two dimensions. When [OH−] was adjusted to 8.0 mol/L, the LuO(OH) morphology was irregular powders. It is demonstrated that [OH−] played the key role of preventing
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Figure 6 TEM micrographs of the Lu2O3 nanorods calcined from LuO(OH) nanorod. (a) 600°C/2 h; (b) 700°C/2 h; (c) 800°C/2 h; (d) 920°C/2 h; (e) HRTEM micrograph taken at the knot of a single Lu2O3 nanorod. Table 1
Dependence of morphologies of LuO(OH) samples on conditions of hydrothermal processing No
CNaOH (mol/L)
T (°C)
t (h)
Shape
1
0.1
180
24
irregular powders
2
1
180
24
wires
3
3
180
24
rods
4
5
180
24
flakes
5
8
180
24
irregular powders
the hydrothermal products from growing into three-dimensional grains, and [OH−]=1.0–3.0 mol/L is proved to be the proper range. It is widely accepted that the hydrothermal processing is a heterogeneous reaction in the presence of aqueous solvents or mineralizers under high pressures to dissolve and recrystallize materials which are relatively insoluble under ordinary conditions [8]. As mineralizer, NaOH generated lots of OH− in the hydrothermal processing which could be absorbed on the surface of the precipitate and formed some ligand, thus the precipitate had a high surface energy which enabled them to form low dimensional materials such as nanoplate and nanorod. Under proper range of [OH−], the precipitate would exhibit a high solubility and a high rate of dissolution/recrystallization, which is favorable to the crystal growth. However, the concentration of the NaOH must be in a proper degree, so high a concentration could badly influence the convection current of the solute, consequently hindered the crystal
growth. That is why when the concentration was 8.0 mol/L, the powders were in irregular shape and had much bigger sizes. On the other hand, the inherent anisotropic crystal structure of an inorganic compound was very important for the formation of anisotropic nanocrystals [1, 2]. Materials with anisotropic structures, for example monoclinic YbOOH [1, 2], hexagonal ZnO [9], can easily grow into one-dimensional nanocrystal without any templates. In this experiment, LuO(OH) nanorods were obtained without any templates, this result strongly indicates that some anisotropic structures must exist. Figure 1 shows that the obtained LuO(OH) nanorods have anisotropic monoclinic structure which could grow along a certain direction under hydrothermal conditions. A sketch map showing the growth mechanism is illustrated in Figure 7. It is believed that NaOH and the inherent monoclinic crystal structure are two critical factors determining the final morphology of the hydrothermal
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Figure 7 Schematic illustration for the growth mechanism of LuO(OH) nanorods. RT stands for room temperature.
products. The reason why it grew along [110] direction still remains to be investigated further.
4 Luminescence properties of Lu2O3 nanorods The final Lu2O3 nanorods were subjected to a spectrofluorophotometer measurement at room temperature. The excitation (Figure 8(a)) and emission (Figure 8(b)) spectra of the nanorods specimens were recorded at an emission wavelength of 611 nm and an excitation wavelength of 247–533 nm, respectively. The main emission peak was located at about 611 nm, resulting from the hypersensitive electric dipole transition of 5D0→7F2 [10, 12]. Other several peaks located at 580−600 nm and 650−670 nm are ascribed to 5D0→7F1,2 and 5D0→7F3 transitions, respectively [11]. It is clearly seen from the emission profiles that when the excitation wavelength changed from 247 to 533 nm, a similar emission spectrum was acquired. As for the excitation spectrum, the excitations around 466 and 533 nm correspond to the 7F0→5D2 and 7F0→5D1 transition, respectively [13, 14], while the peak at 247 nm is ascribed to the O2−− Eu3+ charge transfer band (CT band) [13, 14].
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Figure 8 Excitation (a) and emission (b) spectra of the as-prepared Lu2O3 nanorods.
Comparing with spectral properties of Eu:Lu2O3 materials reported previously [10−14], it is demonstrated that the excitation and emission spectra (peak positions) are not dependent on the morphology of the Lu2O3 nanostructures, this is because the excitation and emission of Ln3+ arise from f-f transitions which are strongly shielded by the outside 5s and 5p electrons.
5
Conclusion
In summary, we have synthesized LuO(OH) nanorods by a facile hydrothermal processing without any templates under conditions of CNaOH =3.0 mol/L, 180°C, 24 h, and then Lu2O3 nanorods by a subsequent calcining of the as-prepared LuO(OH) nanorods. Under optimized conditions, the diameter and length of Lu2O3 nanorods reached 30−40 nm and several hundred nanometers, respectively. It was proposed that [OH-] and the inherent crystal structure were two important factors leading to the nanorod morphology of the lutetium based nanostructures. The obtained Lu2O3 nanorods exhibited strong light emission at wavelength of 611 nm. The luminescent properties are not related to morphol-
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ogy of Lu based nanostructures.
6
This work was financially supported by the Shanghai Municipal Basic Research Project (Grant No. 09JC1406500), Shanghai Academic Disciplines (Grant No. S30107). The authors are also grateful to Bo Lu at Instrumental Analysis and Research Center of Shanghai University for X-ray diffraction analysis of the samples.
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