ARTICLE pubs.acs.org/JPCC
3D Hierarchical Architectures of Sodium Lanthanide Sulfates: Hydrothermal Synthesis, Formation Mechanisms, and Luminescence Properties Yanhua Song,† Haifeng Zou,*,† Ye Sheng,† Keyan Zheng,† and Hongpeng You*,‡ † ‡
College of Chemistry, Jilin University, 130026, People's Republic of China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 130022, People's Republic of China
bS Supporting Information ABSTRACT: Sodium lanthanide sulfate NaLn(SO4)2 3 H2O (Ln = Y, La to Yb) nano/microcrystals with multiform morphologies were successfully synthesized by a facile and effective hydrothermal method. The photoluminescence properties of the Ce3+ and Tb3+ single-doped as well as the Ce3+Tb3+ codoped NaY(SO4)2 3 H2O microsheaves were characterized. The results reveal that the existence of the Ce3+ ions can dramatically enhance the emission intensity of the Tb3+ ions, due to an efficient energy transfer from the Ce3+ to the Tb3+ ions. The critical energy-transfer distance between the Ce3+ and the Tb3+ ions was calculated by methods of concentration quenching and spectral overlapping. Analysis and calculation reveal that the dipolequadrupole interaction should be mainly responsible for the energy transfer.
1. INTRODUCTION The chemical and physical properties of inorganic materials usually have a very close correlation with the morphology, size distribution, and dimensionality. Therefore, the organization of the low-dimensional primary building blocks into controllable threedimensional (3D) superstructures in the solution phase is of particularly great significance and becomes an intensive and hot research topic.1 Synthesis of novel hierarchical structured microand/or nanomaterials is an interesting topic, because they can provide much promise for the fabrication of materials with advanced functionalities.2 A number of materials with 3D hierarchical structures have widespread potential applications in various fields, such as electronics,3 catalysis,4 optics,5 photodegradation,6 water treatment,7 and so on. Although some 3D hierarchical structures can be produced via surfactant- and template-free methods,8 most synthesis methods for hierarchical structures employ templates, which assist in obtaining a uniform morphology and the desired hierarchy. Most commonly used templates are surfactants, such as cetyl trimethylammonium bromide (CTAB)9 and polyvinylpyrrolidone (PVP).10 Recently, much attention has been focused on the synthesis and photoluminescence properties of lanthanide compounds, such as oxides,11 phosphates,12 fluorides,13 oxysulfides,14 and vanadates,15 because of their potential applications in color displays,16 optoelectronics,17 medical and biological detection, and so forth.18 Recently, extensive work has been devoted to the investigation of lanthanide compounds with 3D hierarchical architectures assembled by nanostructured building blocks, such as nanoplates, nanoparticles, nanoribbons, and nanorods.19 Although many investigations on the lanthanide compounds have been reported, the r 2011 American Chemical Society
investigations on the sodium lanthanide sulfates are still limited up to now. Sodium lanthanide sulfates have trigonal structures characterized by space group P3121 with three formulas per cell.20 Each lanthanide ion, located on a 2-fold axis, is coordinated to one oxygen of a water molecule and eight oxygens in sulfate. All lanthanide sites are equivalent. In this study, we present a facile hydrothermal process to synthesize sodium lanthanide sulfates with hierarchical architectures. We also investigate the luminescence properties of NaY(SO4)2 3 H2O:Ce3+,Tb3+ and the energy-transfer mechanism between Ce3+ and Tb3+ ions.
2. EXPERIMENTAL SECTION 2.1. Preparation. Ln(NO3)3 (Ln = Y, La, Ce, Nd, Sm, Eu, Gd, Dy, Ho, Tm, and Yb), Pr(NO3)3, and Tb(NO3)3 aqueous solutions were obtained by dissolving Ln2O3 (99.99%), Pr6O11 (99.99%), and Tb4O7 (99.99%) in dilute HNO3 solution under heating with agitation. In a typical procedure, 2.0 mL of Y(NO3)3 (1 M) was dissolved into 25 mL of ethylene glycol (EG), then 2.0 g of poly(vinyl pyrrolidone) (PVP K30, M = 40 000) was added into the above solution. After vigorous stirring for 10 min, the PVP was dissolved thoroughly. An ethanol solution (10 mL) containing 4 mmol of H2SO4 was then added dropwise into the above solution. NaOH (1 mL, 2 M) was then introduced to the solution. After additional agitation for 30 min, the as-obtained Received: April 27, 2011 Revised: August 30, 2011 Published: September 01, 2011 19463
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white colloidal suspension was transferred to a 50 mL autoclave, which was heated at 200 °C for 4 h. The as-obtained white precipitates were washed with ethanol three times, centrifuged, and dried at 60 °C in air. A similar process was employed to prepare Ce3+,Tb3+ doped NaY(SO4)2 3 H2O and NaLn(SO4)2 3 H2O samples. 2.2. 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, XFlash-Detector 4010, Bruker). The transmission electron microscopy (TEM) was obtained using an FEI Tecnai G2 S-Twin transmission electron microscope at the accelerating voltage of 200 kV. Excitation and emission spectra were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. All measurements were performed at room temperature.
3. RESULTS AND DISCUSSION
Figure 1. XRD patterns of NaY(SO4)2 3 H2O:Ce3+(a), NaY(SO4)2 3 H2O: Tb3+(b), and NaY(SO4)2 3 H2O:Ce3+,Tb3+(c). The standard data for NaY(SO4)2 3 H2O (JCPDS No. 40-1484) is presented for comparison.
Figure 2. SEM and TEM images of the NaY(SO4)2 3 H2O samples synthesized at 200 °C for 4 h. (a) An overall SEM image of the sample. (b, c) High-magnification SEM images of an individual sheaflike architecture. (d) TEM image of the tail of the sheaflike architecture.
3.1. Structures. Figure 1 shows the X-ray diffraction patterns of the as-prepared NaY(SO4)2 3 H2O:Ce3+, NaY(SO4)2 3 H2O: Tb3+, and NaY(SO4)2 3 H2O:Ce3+,Tb3+. One can observe that all peaks are in good agreement with those of the hexagonal NaY(SO4)2 3 H2O in the JCPDS file 40-1484. No additional peaks of other phases have been found, indicating that Ce3+ and Tb3+ ions have been successfully doped into the host lattice. 3.2. Morphology. The general morphologies and the structures of the as-prepared samples are independent of the doped Ln3+ ions in NaY(SO4)2 3 H2O. The typical SEM images for NaY(SO4)2 3 H2O samples are shown in Figure 2ac. The product looks like straw-sheaves with two fantails consisting of a bundle of outspread nanorods, which are closely bonded to each other in the middle (Figure 2a). The individual straw-sheaf has a length in the range of 7.511 μm. The individual nanorods composing the two fantails of these superstructures have diameters of 110250 nm. TEM analysis provides further insight into the morphology and microstructural details of the NaY(SO4)2 3 H2O sheaf. Figure 2d shows a typical TEM image of asprepared products, clearly showing that the sheaflike sample is further composed of nanorods, which is consistent with the results shown in the SEM image (Figure 2ac). Lanthanide atoms have close and gradually changed ionic radii, and as a result, some other rare-earth sodium lanthanide sulfates, NaLn(SO4)2 3 H2O (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb), have also been synthesized under similar reaction conditions. Table 1 summarizes crystal structures, morphologies, and sizes of the NaLn(SO4)2 3 H2O. The XRD patterns of asobtained NaLn(SO4)2 3 H2O samples all exhibit the peaks of the
Table 1. Optimal Experimental Conditions and Morphologies of Rare-Earth Sodium Sulfates NaLn(SO4)2 3 H2O
temperature
time
[°C]
[h]
NaLa(SO4)2 3 H2O NaCe(SO4)2 3 H2O
200 200
24 24
NaPr(SO4)2 3 H2O
200
NaNd(SO4)2 3 H2O
200
morphology
standard deviations
diameter
length
average diameter
average length
[μm]
[μm]
[μm]
[μm]
diameter
length
bundle bundle
0.260.49 0.0600.094
1.221.32 0.630.92
0.31 0.067
1.27 0.78
0.066 0.014
0.032 0.074
24
nanorod
0.0420.074
0.430.63
0.058
0.53
0.010
0.065
24
sheaf
0.0680.18
0.700.78
0.12
0.74
0.036
0.021 0.13
NaSm(SO4)2 3 H2O
200
24
nanorod
0.0270.048
0.751.16
0.038
0.96
0.007
NaEu(SO4)2 3 H2O
200
24
nanorod
0.0280.068
0.440.97
0.057
0.79
0.012
0.15
NaGd(SO4)2 3 H2O
200
24
nanorod
0.0200.054
0.501.22
0.045
1.11
0.010
0.18
NaTb(SO4)2 3 H2O
200
24
sheaf
1.101.68
2.834.40
1.39
3.62
0.18
0.11
NaDy(SO4)2 3 H2O NaHo(SO4)2 3 H2O
200 200
24 24
half-sheaf half-sheaf
3.245.06 9.3812.74
1.871.96 13.3013.61
4.15 11.06
1.92 13.45
0.054 0.062
0.012 0.035
NaTm(SO4)2 3 H2O NaYb(SO4)2 3 H2O
200
24
nanorod
0.350.82
12.9023.34
0.59
18.12
0.13
0.15
200
24
nanorod
0.540.69
2.745.20
0.62
3.97
0.074
0.16
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The Journal of Physical Chemistry C pure crystalline hexagonal phase (Supporting Information, Figure S1). The products take the shape of either nanorods or hierarchical architectures constructed by nanorods (Supporting Information, Figure S2). For the 1D LnNa(SO4)2 3 H2O (Ln = Pr, Sm, Eu, Gd, Tm, Yb), the growth process is an anisotropic growth process that is a commonly used mechanism for discussing the growth of 1D structures. The growth process of the nanorods contains two steps: an initial nucleating stage and a crystal growth stage. After the nucleating stage, PVP acts as a structure-directing reagent binding to the surface of the crystals, kinetically controlling the growth rate of different crystal faces through selective adsorption and desorption.21 For the 3D LnNa(SO4)2 3 H2O (Ln = Y, La, Ce, Nd, Tb, Dy, Ho), the possible growth process will be proposed on the basis of the time-dependent experiment of YNa(SO4)2 3 H2O. 3.3. Formation Mechanism. To understand the possible formation mechanism, the influence of reaction time was further investigated. Figure 3 shows the SEM images of the products obtained with different reaction times and the same other reaction conditions. When the reaction time is 5 min, lots of peanut-like products can be obtained. With prolonging the reaction time to 15 min, the products grew larger and sheaflike structures appeared. In the subsequent reaction process, the macroscopic shape did not change much. Careful observation of the microscopic shape shows that the bundles composing the sheaf split into nanorods with a diameter of about 50 nm and a length of about 160 nm. On the basis of the above results and analysis, the products may grow according to the crystal-splitting theory proposed by the literature to form the final sheaflike structure.22 At the beginning of the experiment, NaY(SO4)2 3 H2O nuclei were obtained in the solution, which grew the building blocks for the final structure. In the following growth process, the nuclei grew into nanorods under the effects of the crystal structure and external powers. At the same time, branches were developed on the nanorod stems, leading to the splitting. With the increase of the reaction time, the branches further split into the next generation of branches. Crystal splitting is favored in a situation where the organic surfactant is a very potent surface stabilizer. The addition of PVP facilitates the growth of large sheaflike rods and the splitting (Figure 4). The samples obtained without PVP are composed of sheaflike rods with little splitting, which only take place on the two ends of the rods, and their splitting degree is small (Figure 4a). When 2.0 g of PVP was added into the solution, the fanning out tails appeared and the size was increased (Figure 4b). This result should be attributed to the special structure and properties of PVP. We assume that, in the formation process of hierarchical NaY(SO4)2 3 H2O, PVP may have double functions in controlling the superstructure morphology. First, PVP may act as potential crystal face inhibitors in the system, which benefited the formation of oriented nucleation, leading to the construction of anisotropic growth of the nanorods. Second, the PVP stabilizer may be adsorbed onto the surfaces of NaY(SO4)2 3 H2O nanorods by coordinating with both nitrogen and oxygen atoms in the polar pyrrolidone groups. This may exhibit steric hindrance and prompt the formation of hierarchical sheaflike architectures from individual nanorods due to its cross-linking ability since PVP has a linear structure and multiple coordinating sites.23 However, the exact role of PVP in the growth of NaY(SO4)2 3 H2O microarchitectures needs further investigation. The changes in morphology and size for the as-prepared NaY(SO4)2 3 H2O with different molar ratios of Y3+/SO42 have been investigated (Supporting Information, Figure S3). As the SO42 content increases, the water content in the system also increases and the sheaflike structure has been broken down and transferred
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Figure 3. SEM images of NaY(SO4)2 3 H2O microsheaves obtained at different reaction times: (A) 5 min, (B) 15 min, (C) 45 min, and (D) 2 h.
Figure 4. SEM images of NaY(SO4)2 3 H2O prepared without (a) and with (b) PVP.
to nanorods. The influence of H2O content on the morphology has been investigated (Supporting Information, Figure S4). As the H2O volume increases, the obtained sample cannot maintain the sheaflike structure and exhibits an anomalous shape. Crystal splitting is a fast crystal growth process, which depends strongly on the oversaturation of the solution. If the oversaturation exceeds a certain “critical level”, splitting was possible to occur.22 On the other hand, when the water content is higher, the solubility of the ions grows higher, which will prohibit the occurrence of the splitting growth process, so only anisotropic growth can take place. 3.4. Luminescence Properties. 3.4.1. Luminescence of NaY(SO4)2 3 H2O:Ce3+. Figure 5 shows the excitation and emission spectra of the NaY(SO4)2 3 H2O:Ce3+. The excitation spectrum monitored at 322 nm shows three excitation bands centered at 230, 248, and 270 nm, which correspond to the transitions from the ground state 2F5/2 of Ce3+ to the different components of the excited Ce3+ 5d states split by the crystal field.24 The emission spectrum of the NaY(SO4)2 3 H2O:Ce3+ sample shows a broad asymmetrical band that extends from 300 to 400 nm with a maximum at about 322 nm when excited by 270 nm. Similarly, this emission band of Ce3+ can be fitted into two well-separated Gaussian components with maxima at 31 289 cm1 (320 nm) and 29 303 cm1 (341 nm) on an energy scale with an energy difference of about 1986 cm1 (Figure 5B), which is in agreement with the theoretical difference between the 2F5/2 and 2F7/2 levels (ca. 2000 cm1). 3.4.2. Luminescence of NaY(SO4)2 3 H2O:Tb3+. The excitation spectrum of Tb3+ ions is usually composed of two groups of fd transitions and a number of narrow ff peaks. The two groups of fd transitions consist of a spin-allowed transition with high energy and a spin-forbidden transition with low energy. The energy difference [ΔETb,Ce = E(Tb,A) E(Ce,A)] is defined as 19465
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Figure 5. Excitation and emission spectra for NaY(SO4)2 3 H2O:Ce3+ (A) and the two Gaussian components on an energy scale (B).
Figure 7. Excitation (A) and emission (B) spectra for NaY(SO4)2 3 H2O:0.03Ce3+,xTb3+ and magnified emission spectra (D) between 400 and 700 nm.
Figure 6. Excitation and emission spectra for NaY(SO4)2 3 H2O:Tb3+.
the difference in the fd energy of Tb3+ with that of the first electric dipole-allowed transition in Ce3+. Its values are known for many different compounds, and they are in first approximation independent on the type of compound the ions doped in. The energy difference ΔETb,Ce between the first fd transition of the Tb3+ ion and that of the Ce3+ ion (270 nm, 37037 cm1) is 13200 ( 920 cm1.25 According to the excitation spectrum of NaY(SO4)2 3 H2O:Ce3+ and the ΔETb,Ce, the spin-allowed fd transition of the Tb3+ ions in NaY(SO4)2 3 H2O should locate at 195203 nm. Figure 6 shows the excitation and emission spectra of NaY(SO4)2 3 H2O:Tb3+. The excitation bands around 200 nm and a shoulder at 221 nm are assigned to the spin-allowed (lowspin) transition of Tb3+ ions. The energy difference between the bands at 236 and 203 nm is 6888 cm1, which is near the average energy difference between the spin-allowed fd transition with
that of the spin-forbidden transition for the Tb3+ ions (6300 ( 900 cm1).26 Therefore, the band at 236 nm is due to the spin-forbidden (high-spin) transition of the Tb3+ ions that involve the 4f8 and 4f75d electronic configurations in the ground and excited states. Besides this, there are several excitation bands between 280 and 400 nm due to the intra 4f8 transitions from the 7 F6 to the 5I6 (286 nm), 5H6 (304 nm), 5D0 (318 nm), 5G2 (341 nm), 5D2 (353 nm), 5G6 (370 nm), and 5D3 (379 nm) levels, respectively. Upon the excitation at 236 nm, the obtained emission spectrum exhibits four obvious lines centered at 491, 545, 585, and 621 nm, which can be attributed to the transitions from the 5D4 excited state to the 7FJ (J = 6, 5, 4, 3) ground states of the Tb3+ ions, respectively. The 5D4 f 7F5 transition at 545 nm is the most prominent group. The comparison of the emission and excitation spectra for the NaY(SO4)2 3 H2O:Ce3+ and NaY(SO4)2 3 H2O:Tb3+ (Supporting Information, Figure S5) reveals a significant spectra overlap between the emission band of the Ce3+ ions and the excitation band of the Tb3+ ions. Therefore, it is expected that an efficient resonance-type energy transfer can occur from the Ce3+ to Tb3+ ions. 3.4.3. Luminescence of NaY(SO4)2 3 H2O:Ce3+,Tb3+. Figure 7 shows the excitation and emission spectra of NaY(SO4)2 3 H2O:0.03Ce3+, xTb3+. The excitation spectrum monitored at 545 nm (Tb3+ emission) shows several excitation bands of Ce3+ and Tb3+ ions, and the intensity becomes stronger with the increasing of Tb3+ doping contents. Under excitation at 270 nm, the emission spectra of 19466
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NaY(SO4)2 3 H2O:0.03Ce3+,xTb3+ shows not only a band of Ce3+ ions but also a green band of Tb3+ ions. With an increase in Tb3+ content, the emission intensity of the Tb3+ ions increases systematically, whereas the emission intensity of the Ce3+ ions simultaneously decreases monotonically from x = 0 to 0.2. This result means that the Tb3+ ions are essentially excited through the Ce3+ ions, thereby indicating efficient energy transfer from the Ce3+ to the Tb3+ ions. The 5d4f transition of Ce3+ is electric-dipole-allowed and is several orders of magnitude stronger than ff transitions of Tb3+. Therefore, the Ce3 + ions can strongly absorb UV light from the ground states to the excited states and then transfer the energy to the Tb3+ ions. As a result, the excitation into the Ce3+ band at 270 nm yields both the emissions of the Ce3+ and Tb3+ ions. 3.4.4. Calculations. To better understand the energy-transfer process, the decay curves have been measured and the lifetimes as well as energy-transfer efficiencies have been calculated. The decay curve of the singly Ce3+-doped NaY(SO4)2 3 H2O can be well fitted into a single-exponential function. For the Ce3+- and Tb3+-doped samples, the doping of the Tb3+ ions significantly modifies the fluorescent dynamics of the Ce3+ ions. The results reveal that the fluorescence decays deviate slightly from a singleexponential rule, indicating the presence of a nonradiative process. The effective lifetime is defined as Z ∞ IðtÞt dt ð1Þ τ ¼ Z0 ∞ IðtÞ dt 0
The effective lifetime values are calculated to be 14.63 ( 0.05, 13.40 ( 0.06, 11.17 ( 0.08, and 8.95 ( 0.04 ns for NaY(SO4)2 3 H2O:0.03Ce3+,xTb3+ with x = 0.04, 0.08, 0.12, and 0.16, respectively (Supporting Information, Figure S6). The decay lifetime of the Ce3+ ions decreases monotonically with an increase in the Tb3+ doping concentration, which strongly supports energy transfer from the Ce3+ and Tb3+ ions, and the energy-transfer process may happen via a resonant-type mechanism.27 The energy-transfer efficiency (ηT) from the Ce3+ to the Tb3+ ions can be calculated by eq 228 ηT ¼ 1
τS τS0
ð2Þ
where τS and τS0 are the lifetimes of the sensitizer (Ce3+) with and without the activator (Tb3+), respectively. The ηT value (standard deviation = 0.039) of NaY(SO4)2 3 H2O:0.03Ce3+, xTb3+ could be obtained as a function of x and is presented in Figure 8, in which ηT is found to increase gradually with an increase in the Tb3+ dopant content. In many cases, concentration quenching is due to the energy transfer from one activator to another until the energy sink in the lattice is reached.29 Blasse has pointed out that the critical transfer distance (Rc) can be estimated from the following equation !1=3 3V ð3Þ Rc ≈ 2 4πχc N where V is the volume of the unit cell, N is the number of the host cations in the unit cell, and χc is the critical concentration at which the luminescence intensity of the sensitizer (Ce3+) is half that in the sample in the absence of activator (Tb3+); that is to say, χc occurs when ηT = 0.5. For the NaY(SO4)2 3 H2O host, N is 3,
Figure 8. Dependence of the energy-transfer efficiency ηT in NaY(SO4)2 3 H2O:Ce3+,Tb3+ on the Tb3+ content (x).
V = 515.19 Å3, and χc = 0.15. The critical distance of the energy transfer Rc is estimated to be about 12.98 ( 0.04 Å. The resonant energy-transfer mechanism consists of two types: exchange interaction and multipolar interaction.2931 The critical distance between the sensitizer and activator for an exchange interaction should be shorter than 5 Å. The critical distance for the energy transfer from Ce3+ to Tb3+ ions calculated using the concentration quenching method is longer than 5 Å, indicating little possibility of energy transfer via the exchange interaction mechanism. Thus, the electric multipolar interaction can take place for energy transfer between the Ce3+ and Tb3+ ions. On the basis of Dexter’s energy-transfer formula of multipolar interaction and Reisfeld’s approximation, the following relation can be obtained30 ηS0 n=3 µ CCeþTb ηS
ð4Þ
in which ηS0 is the intrinsic luminescence quantum efficiency of the Ce3+ ions and ηS is the luminescence quantum efficiency of the Ce3+ ions with the activator (Tb3+), the values of ηS0/ηS can be approximately calculated by the ratio of related luminescence intensities (IS0/IS), CCe+Tb is the total doping concentration of Ce3+ and Tb3+, and the n = 6, 8, and 10 represent dipoledipole, dipolequadrupole, and quadrupolequadrupole interactions, respectively. The IS0/IS Cn/3 plots are further illustrated in Figure 9, and only when n = 8 does it exhibit a linear relation, indicating clearly that the energy transfer from the Ce3+ to the Tb3+ ions is the dipolequadrupole mechanism. The energy-transfer probability PSA for each multipolar interaction was considered to further investigate the characteristics of multipolar interations, such as dipoledipole, dipolequadrupole, and higher-order interactions. The dipoledipole energytransfer probability from a donor to an acceptor is given as follows Z fd FS ðEÞFA ðEÞ dE dd ¼ 3:024 1012 6 ð5Þ PCeþTb E4 R τS in which fd is the oscillator strength of the involved absorption transition of the acceptor, τS is the radiative decay time of the sensitizer, R is the sensitizeracceptor average R distance [Å], E is the energy involved in the transfer [eV], and FS(E)FA(E)/E4 dE represents the spectral overlap between the normalized shapes of the Ce3+ emission FS(E) and the Tb3+ excitation FA(E) and it is calculated to be about 0.07154 eV5. The critical distance (Rc) is defined as the distance for which the probability of transfer equals the probability of radiative emission of the donor, the distance for which PCe+TbτS0 = 1. Hence, Rc can be obtained from the following 19467
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Figure 9. Dependence of ISO/IS of Ce3+ on (a) C6/3, (b) C8/3, and (c) C10/3.
equation Z Rc6 ¼ 3:024 1012 fd
FS ðEÞFA ðEÞ dE E4
mechanism of the energy transfer is a resonant type that takes place through a dipolequadrupole interaction. ð6Þ
The oscillator strength of the Tb3+ electric dipole transition (fd) is in the order of 106.32 By the above equation, the critical distance Rdd c for a dipoledipole interaction mechanism is calculated as 7.70 ( 0.03 Å. This value largely deviates from that estimated from the quenching concentration data (12.98 ( 0.04 Å), further indicating that the electric dipoledipole interaction is not primarily responsible for the energy transfer from the Ce3+ to the Tb3+ ions in this system. Therefore, an electric dipole quadrupole interaction was considered as a possible mechanism for the energy transfer between Ce3+ and Tb3+ ions in this system. For the dipolequadrupole mechanism, the critical distance can be expressed by Z FS ðEÞFA ðEÞ dE ð7Þ Rc8 ¼ 3:024 1012 λ2S fq E4 where λS (in Å) is the emission wavelength of Ce3+ ions, fq is the oscillator strength of quadrupole electrical absorption transitions for Tb3+, and fq/fd = 103.33 The critical distance Rc was estimated to be 14.75 ( 0.03 Å, which agrees approximately with that obtained by using the concentration-quenching method (12.98 ( 0.04 Å). This result confirms that the energy-transfer mechanism between Ce3+ and Tb3+ ions in the NaY(SO4)2 3 H2O host is a dipolequadrupole interaction.
4. CONCLUSION A simple and facile hydrothermal method was used to prepare NaLn(SO4)2 3 H2O (Ln = Y, La to Yb) with hierarchical architectures or one-dimensional morphologies. The PVP plays an important role in the formation of sheaflike NaY(SO4)2 3 H2O, and the formation mechanism is supposed to be a rapid splitting process. The spectroscopic data indicate that the energy transfer from the Ce3+ to the Tb3+ ions takes place in this host matrix. The critical distances of energy transfer are calculated by concentration quenching and spectral overlapping methods and the results are 12.98 ( 0.04 and 14.75 ( 0.03 Å, respectively. The
’ ASSOCIATED CONTENT
bS
Supporting Information. XRD patterns and SEM images of NaLn(SO4)2 3 H2O (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb), SEM images of NaY(SO4)2 3 H2O with different molar ratios of Y3+/SO42, SEM images of NaY(SO4)2 3 H2O with different volume ratios of H2SO4:H2O, comparison of the normalized emission spectrum of NaY(SO4)2 3 H2O:Ce3+ with the normalized excitation spectrum of NaY(SO4)2 3 H2O:Tb3+, and emission decay curves of NaY(SO4)2 3 H2O:0.03Ce3+,xTb3+. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (H.Z.),
[email protected] (H.Y.).
’ ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (Grant 20771098), the Fund for Creative Research Groups (Grant 20921002), and the National Basic Research Program of China (973 Program, Grants 2007CB935502 and 2006CB601103). ’ REFERENCES (1) Xu, L.; Lu, C.; Zhang, Z.; Yang, X.; Hou, W. Nanoscale 2010, 2, 995–1005. (2) Zhang, Y.; Or, S. W.; Wang, X.; Cui, T.; Cui, W.; Zhang, Y.; Zhang, Z. Eur. J. Inorg. Chem. 2009, 48, 168–173. (3) (a) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66. (b) Yan, H.; He, R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. J. Am. Chem. Soc. 2003, 125, 4728. (4) (a) Perez-Ramirez, J.; Christensen, C. H.; Egeblad, K.; Christensen, C. H.; Groen, J. C. Chem. Soc. Rev. 2008, 37, 2530. (b) Hu, G.; Ma, D.; Liu, L.; Cheng, M.; Bao, X. Angew. Chem., Int. Ed. 2004, 43, 3452. (5) Yang, M.; You, H.; Zheng, Y.; Liu, K.; Jia, G.; Song, Y.; Huang, Y.; Zhang, L.; Zhang, H. Inorg. Chem. 2009, 48, 11559. 19468
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dx.doi.org/10.1021/jp203903b |J. Phys. Chem. C 2011, 115, 19463–19469
