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La(OH)3:Ln3+ and La2O3:Ln3+ (Ln = Yb/Er, Yb/Tm, Yb/Ho) Microrods: Synthesis and Up-conversion Luminescence Properties Xiao Zhang,† Piaoping Yang,*,† Dong Wang,† Jie Xu,† Chunxia Li,‡ Shili Gai,† and Jun Lin*,‡ †

Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, People's Republic of China ‡ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130021, People's Republic of China S Supporting Information *

ABSTRACT: One-dimensional La(OH)3:Ln3+ (Ln = Yb/Er, Yb/ Tm, Yb/Ho) microrods have been successfully synthesized using molten composite-hydroxide (NaOH/KOH) as a solvent. La2O3:Ln3+ nanostructures with retained striplike shape were achieved by a subsequent annealing process. The phase, structure, morphology, and fluorescent properties have been well investigated by various techniques. It is found that the reaction time plays a key role in confining the growth of the microrods. Both La(OH)3:Ln3+ and La2O3:Ln3+ nanostructures have rodlike shapes with a typical width of 50−400 nm. The up-conversion (UC) photoluminescence (PL) properties of the samples have been studied in detail. Under 980 nm laser excitation, both La(OH)3:Ln3+ and La2O3:Ln3+ microrods exhibit the characteristic emissions of Er3+, Tm3+, and Ho3+ and give green, blue, and blackish green emission colors, respectively. Additionally, the doping concentration of Yb3+ has been optimized by fixing the Er3+ concentration. It should be noted that the up-conversion emission of La2O3:Er3+ microrods can be significantly improved in comparison with that of their bulk counterpart under the same excitation conditions. emissions and narrow bands.14 In particular, RE3+ doped upconversion materials have much potential in photonics, LCD back lighting, and advanced bioanalysis as a result of their low effective density, low phonon energy, and transparence to visual light.20−24 In contrast to the case of the conventional downconversion organic dyes and quantum dots, the UC materials are more attractive because of the minimum photodamage to living organisms, the feasibility of inexpensive use, high detection sensitivity, weak interference background, and high penetration depth in tissues.25,26 Among various RE3+-activated UC materials, yttrium-based lattices, such as Y2O3, NaYF4, and YF3 have been extensively investigated as regards their upconversion luminescent properties.27 However, development of La-based fluorescent materials is still inadequate in spite of their cheaper price and abundant storage. Thus, more research on the La-based materials should be highly fundamental and practical. As we know, lanthanum oxide is a promising host matrix for its good chemical durability, thermal stability, and low phonon energy, which result in high UC efficiency by efficiently hindering nonradiative loss.27−29 RE doped downconversion La(OH)3:Tb3+ and La(OH)3: Eu3+ have been reported as luminescent materials,30,31 while up-conversion

1. INTRODUCTION One-dimensional (1D) nanostructures such as nanobelts, nanowires, nanorods, and nanotubes have attracted considerable interest since the discovery of carbon nanotubes,1 which have a great potential for addressing some basic issues related to dimensionality and space-confined transport phenomena and for practical applications.2−4 These materials should play a significant role in the fabrication of electronic and optical devices, chemical and biological sensors, and field effect transistors due to their unique optical, electrical, mechanical, and thermal properties.5−9 As a typical 1D nanostructure, nanobelts not only extend the understanding of the relationship between structure and property but also can be conducive for building functional devices.10−12 Moreover, 1D fluorescent nanostructures hold both shape-specific and quantum confinement effects; therefore, they have been extensively explored as drug carriers, as photonics, as displays, and for advanced bioanalysis. In particular, their various emission lifetimes and increased luminescence efficiencies are much different from those of their bulk counterparts.10,13 Rare earth ions (RE) doped luminescent materials have been extensively studied due to their fascinating optical characteristics and stabilities.14−19 The emission from the RE3+ dopants is mainly due to electric and magnetic dipole optical transitions based on their unique intra 4f transitions, which are shielded by the outer 5s and 5p orbitals and, consequently, lead to sharp © 2011 American Chemical Society

Received: August 19, 2011 Revised: October 7, 2011 Published: November 23, 2011 306

dx.doi.org/10.1021/cg201091u | Cryst. Growth Des. 2012, 12, 306−312

Crystal Growth & Design

Article

luminescence of RE3+ doped La(OH)3 has rarely been reported. Furthermore, the dehydration of RE3+ doped lanthanum hydroxide also provides a straightforward and facile process to obtain lanthanum oxide phosphors.27,32,33 To date, 1D lanthanum hydroxide and lanthanum oxide have been synthesized by various routes, including the solvothermal method,34 hydrothermal approach,35,36 electrochemical process,37 coprecipitation procedure,38 and template route.39,40 Herein, uniform Yb3+/Er3+, Yb3+/Ho3+, and Yb3+/Tm3+ codoped La(OH)3 microrods have been synthesized in molten composite-hydroxide (NaOH/KOH) medium. The La2O3:Ln3+ microrods were obtained by a subsequent heating process. The UC properties of La2O3:Ln3+ microrods have been explored in detail. The UC luminescent properties of La2O3:Yb3+/Er3+ microrods are investigated by varying Yb3+ doping concentrations. Notably, La2O3:Yb3+/Er3+ microrods exhibit brighter luminescence than the corresponding bulk phosphors, suggesting potential applications in the fields of optoelectronic devices and various color displays.

Figure 1. XRD patterns of the as-synthesized (A) pure La(OH)3, (B) La(OH)3:1%Er3+, (C) La(OH)3:3%Yb3+/1%Er3+, and (D) La(OH)3:7%Yb3+/1%Er3+ microrods.

Er3+, and La(OH)3:7%Yb3+/1%Er3+ and the standard data for pure La(OH)3 (JCPDS No. 36-1481), respectively. It is obvious that all four samples are of high crystallinity, and the diffractions can be readily indexed to pure hexagonal La(OH)3 (JCPDS No. 36-1481) in the P63/m (176) space group. No peaks from other phases are detected, indicating that the samples are of high purity and Yb3+ and Er3+ ions have been effectively incorporated into the La(OH)3 host by substituting La3+ sites. The calculated d spacing corresponding to the (hkl) values and the lattice constants for the four samples are summarized in Table 1. It is found that these values are well

2. EXPERIMENTAL SECTION 2.1. Materials. All materials were used as received without further purification. La(NO3)3, Yb(NO3)3, Er(NO3)3, Tm(NO3)3, and Ho(NO3)3 were prepared by dissolving the corresponding La2O3, Yb2O3, Er2O3, Tm2O3, and Ho2O3 (99.99%, Sinopharm Chemical Reagent Co., Ltd., China) in HNO3 solution at elevated temperature followed by evaporating superfluous HNO3 under vacuum. In a typical synthesis procedure for the synthesis of 0.5 mmol of La(OH)3:Yb3+/Er3+ microrods, stoichiometric amounts of La(NO3)3, Yb(NO3)3, and Er(NO3)3 and 2 mL of deionized water were added into a 30 mL Teflon-lined autoclave, followed by adding 18 g of mixed hydroxides (the mole ratio of NaOH/KOH is 0.515/0.485). The autoclave was preheated to 200 °C for 30 min, and then the molten hydroxide solution was thoroughly shaken to ensure uniformly mixed reactants. After the reaction progressed for 24 h, the vessel was taken out and cooled to room temperature. The product was filtered and washed with deionized water and hot water several times until the pH value was about 7. Finally, the obtained powders were dried at 60 °C for 12 h. La(OH)3:Yb3+/Tm3+ and La(OH)3:Yb3+/Ho3+ were prepared by the same procedure. La2O3:Ln3+ microrods were obtained by heating the corresponding lanthanum hydroxide at 700 °C for 6 h. For comparison, bulk La2O3:3%Yb3+/1%Er3+ phosphors were prepared by solid state reaction at 700 °C for 6 h. 2.2. Characterization. X-ray powder diffraction (XRD) measurements were performed on a Rigaku TTR III diffractometer at a scanning rate of 10 min in the 2θ range from 10 to 80°, with graphite monochromatic Cu Ka radiation (λ = 0.15405 nm). Thermogravimetric and differential scanning calorimetry (TG-DSC) data were recorded with a thermal analysis instrument (SDT 2960, TA Instruments, New Castle, DE) with the heating rate of 10 °C/min in an air flow of 100 mL/min. SEM micrographs and energy-dispersive X-ray (EDX) spectra were obtained using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi). Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) patterns were recorded using a FEI Tecnai G2 S-Twin with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiple CCD camera. Up-conversion emission spectra were obtained using 980 nm LD Module (K98D08M-30W, China) as the excitation source and detected by R955 (HAMAMATSU) from 400 to 900 nm. All the measurements were performed at room temperature (RT).

Table 1. d-Spacing Values and the Lattice Constants for La(OH)3, La(OH)3:1%Er3+, La(OH)3:3%Yb3+/1%Er3+, La(OH)3:7%Yb3+/1%Er3+ Determined by XRD results d-spacing values (Å)

La(OH)3:3% La(OH)3:7% Yb3+/1%Er3+ Yb3+/1%Er3+

standard pattern (JCPDS 36-1481)

hkl

La(OH)3

La(OH)3:1% Er3+

100 110 101 200 201 210 211

5.659 3.267 3.186 2.829 2.281 2.139 1.870

5.651 3.262 3.173 2.825 2.275 2.136 1.866

5.649 3.262 3.176 2.825 2.276 2.135 1.867

5.643 3.258 3.175 3.821 2.274 2.133 1.864

5.652 3.263 3.188 2.827 2.281 2.137 1.870

a c

6.535 3.856

6.524 3.836

6.523 3.844

6.516 3.841

6.528 3.859

consistent with the standard data (JCPDS No. 36-1481). The slightly lower lattice constants may be caused by the smaller ionic radii of the substituted Yb3+ and Er3+. It should be noted that the markedly strengthened intensities of typical peaks may be caused by the preferred growth orientation of the samples. An assertion is further proved by the following HRTEM and SAED analysis. Typical SEM images with different magnification, TEM, HRTEM images, SAED, and EDS of La(OH)3:3%Yb3+/1%Er3+ microrods are displayed in Figure 2. In the low-magnification SEM (Figure 2A), it is evident that the product mainly consists of numerous well-defined microrods with length ranging from

3. RESULTS AND DISCUSSION 3.1. Phase, Structure, and Morphology of La(OH)3:Ln3+ Microrods. Figure 1 shows the XRD patterns of as-prepared La(OH)3, La(OH)3:1%Er3+, La(OH)3:3%Yb3+/1% 307



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Figure 2. FE-SEM images with (A) low magnification, (B) high magnification, (C) single microrod, (D) TEM image, (E) HRTEM image, (F) SAED, and (G) EDS of La(OH)3:3%Yb3+/1%Er3+ microrods.

linear chains, which are parallel to the c-axis, and La atoms at 2c (1/3, 2/3, 1/4) are chained by nine oxygen atoms (Figure 3B). Each chain is surrounded by three other chains, which are displaced by c/2 with respect to the central one and organize hexagonal tunnels when the atoms are all connected (Figure 3C). It is clear that hexagonal La(OH)3 has a highly isotropic structure along the c-axis. The growth direction of the La(OH)3 microrods is largely confined to the [001] direction, which corresponds well to the HRTEM observation. To further understand the formation mechanism of La(OH)3:3%Yb3+/1%Er3+ microrods, time-dependent experiments on the size and shape of La(OH)3:3%Yb3+/1%Er3+ were explored. It is found that the morphology and size depend greatly on the reaction time. In Figure 4A for the sample prepared at 200 °C for 4 h, the microrods display uniform morphology and have self-assembled microrods along their long axis, and then they grow into large-scale ordered patterns. Parts B−D of Figure 4 exhibit La(OH)3:3%Yb3+/1% Er3+ prepared for 12, 24, and 48 h, respectively. It is obvious that with the increase of reaction time the microrods grow much longer and then interweave together, but remain the same diameter. At the initial reaction stage, once the hydroxides are molten, Ln3+ cations can easily react with hydroxyl groups due to the opposite electric charge to form La(OH)3 nuclear along the width direction. Whereas a large amount of Ln3+ and OH− ions still exist in the molten solution, they will be adsorbed on the preferred surfaces of seeds. Therefore, microrods grow along the [001] direction with the reaction time and form the long microrods ultimately. Because of the

several tens to several hundreds of micrometers. The highmagnification SEM images (Figure 2B and C) reveal that La(OH)3:3%Yb3+/1%Er3+ microrods are clearly faceted and have a rectangular cross section with width 50−400 nm. Each microrod is uniform in width and thickness and has smooth surfaces. The TEM image with a rectangle-shaped end is presented in Figure 2D. Smooth microrods with width of about 250 nm are observed, which is consistent with the SEM result. In the HRTEM image (Figure 2E), the apparent lattice fringes indicate the high crystallinity. The calculated distance (0.27 nm) between the adjacent lattice fringes corresponds well to the d200 spacing (0.28 nm) of pure hexagonal La(OH)3 (JCPDS No. 36-1481). The (001) planes are oriented parallel to the growth axis, suggesting that the growth direction of the microrods occurs preferentially along the [001] direction (caxis). SAED (Figure 2F) reveals the single crystalline nature with a typical hexagonal structure of the product. EDS (Figure 2G) confirms the La, Si, Er, and O elements (Si signal arises from the quartz substrate) in the product. In general, the growth of a microrod is closely related to two factors: the surface energy determines the growth preferential, whereas the growth kinetics determine the final structure.41 As no catalyst, template, or surfactant exists in the reaction system to reduce the surface energy, it is reasonable to infer that the inherent crystal structure and the chemical activity govern the growth of La(OH)3. As shown in Figure 3A, the structure of La(OH)3 belongs to hexagonal P63/m, where the unit cell possessing two molecules of La(OH)3 and a La atom is located in the tricapped trigonal prism. The La atoms array as infinite 308



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Figure 4. SEM images of La(OH)3:3%Yb3+/1%Er3+ synthesized at 200 °C for (A) 4 h, (B) 12 h, (C) 24 h, and (D) 48 h. Insets are their corresponding high magnification images.

Figure 3. (A) Unit cell of the hexagonal La(OH)3 nanocrystal; (B) structural model of La(OH)3 showing La chains along the c-axis and that the La atom chains nine oxygen atoms; (C) crystal structure of the hexagonal La(OH)3 with the OH groups inserted. Blue and red balls are La and O atoms. The hydrogen atoms are omitted for clarify.

Figure 5. TG and DSC curves of as-synthesized La(OH)3:3%Yb3+/1% Er3+ microrods.

can be assigned to the decomposition of La(OH)3 to LaOOH, and the third one should be caused by the further decomposition of LaOOH to La2O3 at 430−560 °C. The theoretical weight loss for the two processes are 9.5% and 5.2%, respectively, which is much closer to those obtained from the TGA curve. The last step from 560 to 800 °C may be attributed to the presence of little carbonates which may originate from air. The results are in agreement with previous literature reports.31,37,38 Typical SEM, TEM, HRTEM, and SAED images of La2O3:3%Yb3+/1%Er3+ are given in Figure 6. The lowmagnification SEM image (Figure 6A) reveals that these microrods still keep the precursor’s morphology although experiencing the thermal decomposition process. But close observation from a high-magnification SEM image (Figure 6B) indicates that their size is slightly shrunk, which may be caused by the gradual elimination of H and O during the annealing process. Such a transformation is common for decomposition

large viscosity of the hydroxide, the formation of La(OH)3:Ln3+ nanostructures is slow, and it is not easy for the nanostructures to agglomerate. This is the key to obtain disperse singlecrystalline microstructures from the reaction without using a surface-capping material. 3.2. Phase Identification and Morphology of La2O3:Ln3+ Microrods. Uniform La(OH)3:3%Yb3+/1%Er3+ microrods were used as precursor to fabricate La2O3:3%Yb3+/ 1%Er3+ microrods. The thermal behavior of La(OH)3:3%Yb3+/ 1%Er3+ microrods was investigated by TG-DSC, as shown in Figure 5. From the TGA curve, a slight decrease below 250 °C, a large weight loss stage from the beginning of about 300 °C, and two smaller steps from 410 and 550 °C are observed. The weight loss for the four stages is 1.8%, 9.7%, 4.3%, and 1.3%, respectively. The first loss corresponds to the elimination of physically adsorbed H2O. The second loss from 280 to 430 °C 309



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Figure 6. FE-SEM images with (A) low magnification, (B) high magnification, (C) single microrod, (D) TEM image, (E) HRTEM image, (F) SAED, and (G) EDS of La2O3:3%Yb3+/1%Er3+ microrods.

of rare earth hydroxide compounds.42 The maintenance of the morphologies is perhaps due to the higher activation energies needed for the collapse of these structures, and this morphology-inheriting method is a facile and general strategy for designing some morphology-dependent functional compounds.43 In Figure 6C for the single microrod, the TEM image shows that the microrods are approximately 300 nm in diameter and their surface is rough, which is in agreement with the SEM result. In the HRTEM image (Figure 6D), the lattice fringes exhibit the imaging characteristics of La2O3, where the d spacing of 0.193 nm corresponds to the distance of the (110) plane and the preferential growth direction is also [001]. The SAED image (Figure 6E) indicates the hexagonal structure of La2O3 microrods. The EDS taken from an individual microrod (Figure 6F) confirms that the microrods are composed of La, O, Yb, and Er elements. No other impurity peaks can be detected, which gives further support to the XRD results. 3.3. PL Properties of La2O3:Ln3+ and La(OH)3:Ln3+ (Ln = Yb/Er, Yb/Tm, Yb/Ho). Figure 7 shows the UC emission spectra of La2O3:Ln3+ microrods and their corresponding luminescent photographs upon 980 nm laser excitation. In the luminescent photographs (insets in Figure 7), La2O3:3%Yb3+/ 1%Er3+, La2O3:3%Yb3+/1%Tm3+, and La2O3:3%Yb3+/1%Ho3+ exhibit bright green, blue, and blackish green emissions, respectively. In the emission spectrum of La2O3:3%Yb3+/1% Er3+ (Figure 7A), a primary strong band in the green emission region maximized at 546 nm can be assigned to the 4S3/2 →

Figure 7. UC emission spectra of (A) La2O3:3%Yb3+/1%Er3+, (B) La2O3:3%Yb3+/ 1%Tm3+, and (C) La2O3:3%Yb3+/1%Ho3+ microrods upon 980 nm excitation. 4

I15/2 of Er3+, and the weak band in the red region at 653 nm is ascribed to the Er3+4F9/2 → 4I15/2 transition. The much stronger green emission than the red light results in the bright green emission, which is confirmed by its CIE chromaticity coordinate (x = 0.2717, y = 0.7133). In Figure 7B for La2O3:3%Yb3+/1%Tm3+, an intense blue emission at 475 nm and a much weaker red emission at 652 nm originate from the 1 G4 → 3H6 and 1G4 → 3F4 transitions of Tm3+, respectively. 310



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Obviously, the emission spectrum of La2O3:3%Yb3+/1%Tm3+ is dominated by the 1G4 → 3H6 transition, which is based on a three-photon process and thus gives a strong blue luminescence with the CIE chromaticity coordinate of x = 0.1357, y = 0.1411. In Figure 7C for La2O3:3%Yb3+/1%Ho3+ microrods, the dominant green emission at 548 nm can be associated with transition of the 5F4, 5S2 levels to the 5I8 ground state and the weak red emission at 661 nm originating from the 5F5 → 5I8 transition can be observed. Clearly, the dominant 5F4,5S2 → 5I8 transition gives a strong green luminescence confirmed by the CIE chromaticity coordinate (x = 0.2746, y = 0.6977). It is well-known that an appropriate codoping sensitizer can increase UC luminescence on the basis of the energy transfer between the donor and acceptor ions in many host materials, but high content may result in concentration-dependent quenching.44,45 Considering the strong effect of dopant concentration on the UC emission intensity, we studied the UC emissions of the samples with various Yb3+ concentrations by fixing the Er3+ concentration (1 mol %), as shown in Figure

shown in Figure 9. It can be seen that the emission intensity of the 4S3/2 → 4I15/2 transition of La2O3:3%Yb3+/1%Er3+ micro-

Figure 9. Comparison of UC emission spectra of (A) La2O3:3%Yb3+/ 1%Er3+ microrods and (B) its bulk counterpart annealed at 700 °C.

rods is nearly twice that of bulk phosphors, suggesting the significant improvement of the UC emission. It is well-known that defects have a serious drawback on the emission intensity, as they provide nonradiative recombination routes for electrons and holes. In order to get high-efficiency light, the number of electron/hole recombinations via optically active centers must be maximized. If the surface area is greatly reduced, the phosphor with fewer defects shows great improvement in PL intensity.48−50 In the SEM image (Figure S1 of the Supporting Information), the bulk La2O3:3%Yb3+/1%Er3+ sample is composed of multiple irregular blocks and several aggregations. The rough surface and the serious aggregations cause the increase of surface area, which introduces a large number of defects. Therefore, a large number of electrons and holes in the excited state will return to ground state via the optically radiative recombination route, resulting in the lower emission intensity of the bulk phosphors. Different from the intensive research on RE ions doped oxide, the work on La(OH)3 based fluorescent materials has rarely been reported. Figure S2 of the Supporting Information gives the UC emission spectra of La(OH)3:3%Yb3+/1%Er3+, La(OH)3:3%Yb3+/1%Tm3+, and La(OH)3:3%Yb3+/1%Ho3+ microrods, respectively. It can be seen that all three samples show the characteristic emissions of Er3+, Tm3+, Ho3+ with the CIE chromaticity coordinate (x = 0.2203, y = 0.7403) of La(OH)3:3%Yb3+/1%Tm3+ and (x = 0.2390, y = 0.7312) of La(OH)3:3%Yb3+/1% Ho3+. The positions of the emission peaks and the corresponding transition mechanism are very similar to those of La2O3:Ln3+, except for the decrease of the luminescent intensity, which may be caused by the quenching effect of the OH− group in the La(OH)3 matrix.

Figure 8. UC emission spectra of La2O3:Ln3+ microrods with doping concentrations of (A) 1%Yb3+/1% Er3+, (B) 3%Yb3+/1% Er3+, (C) 5% Yb3+/1% Er3+, and (D) 7% Yb3+/1% Er3+.

8. It can be seen that the green emission intensity increases with Yb3+ concentration from 1% to 5% and then decreases with further increased concentration, implying that the optimum Yb3+ concentration is 5 mol % in the La2O3 host. However, only a slightly improved emission is achieved compared with that of the 3 mol % Yb3+ doped sample. Considering the economic reason, we select 3 mol % as the doped concentration for studying the UC properties. A possible interpretation for the change of the emissions is proposed as follows. The low concentration of Yb3+ from 1% to 5% is available to effectively furnish and transfer the energy to the Er3+, resulting in the higher emission intensity. With the further increased amount of Yb3+ sensitizer ions in the La2O3 host lattice, the interatomic distance of Yb3+−Er3+ decreases, which efficiently facilitates the back-energy-transfer process 4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2F5/2 (Yb3+).46,47 The back-energy-transfer consequently suppresses the population in the excited levels of 4S3/2, resulting in the decrease of greenlight emissions (4S3/2 → 4I15/2). For comparison, the emission spectra of La2O3:3%Yb3+/% 3+ Er microrods and their corresponding bulk phosphors are

4. CONCLUSIONS In summary, Yb3+/Er3+, Yb3+/Tm3+, and Yb3+/Ho3+ codoped La(OH)3 microrods were successfully prepared by a facile hydroxide mediated route. After annealing at 700 °C for 6 h, La(OH)3:Ln3+ can be directly converted to the La2O3:Ln3+ microrods, which maintain the inherent morphology. Under 980 nm excitation, bright UC luminescence of 4S3/2 → 4I15/2 of Er3+ in green, the 1G4 → 3H6 emission of Tm3+ in blue, and 5F4, 311



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S2 → 5I8 of Ho3+ in green are obtained in both La(OH)3:Ln3+ and La2O3:Ln3+ microrods. The emission intensities are strongly governed by the concentration of Yb3+ ions. Furthermore, the La2O3:3%Yb3+/1%Er3+ microrods have nearly twice the emission intensity of that of the bulk counterpart. The unique fluorescent properties facilitate a promising candidate of this La2O3 based material in various color display fields on the basis of the high up-converting efficiency and high resistance against laser radiation damage. 5

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ASSOCIATED CONTENT S Supporting Information * SEM image of bulk La2O3:3%Yb3+/1%Er3+ phosphors, and UV emission spectra of La(OH)3:3%Yb3+/1%Er3+, La(OH)3:3% Yb3+/1%Tm3+, and La(OH)3:3%Yb3+/1%Ho3+ microrods. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected].

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ACKNOWLEDGMENTS Financial support from the National High Technology Research Program of China (2011AA03A407), the National Basic Research Program of China (2010CB327704), and the National Natural Science Foundation of China (NSFC 20871035) is gratefully acknowledged.

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