Controlling the Morphology of Erbium-Doped Yttrium Fluoride Using

DOI: 10.1021/jp1032564. Publication Date (Web): November 17, 2010 ... Lining Sun , Ruoyan Wei , Jing Feng , Hongjie Zhang. Coordination Chemistry Revi...
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Controlling the Morphology of Erbium-Doped Yttrium Fluoride Using Acids as Surface Modifiers: Employing Adsorbed Chlorine Ions to Inhibit the Quenching of Upconversion Fluorescence Da Li,† Cairong Ding,† Gang Song,† Sizhou Lu,† Zhen Zhang,† Yanuo Shi,† Hui Shen,*,† Yueli Zhang,†,‡ Hongqun Ouyang,† and Hai Wang† State Key Laboratory of Optoelectronic Materials and Technologies and School of Physics and Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, and State Key Laboratory of Crystal Material, Shandong UniVersity, Jinan, 250100, China ReceiVed: April 12, 2010; ReVised Manuscript ReceiVed: September 20, 2010

Well-shaped YF3-Er3+ particles with different sizes are prepared by a novel hydrothermal method employing acids as surface modifiers; a simple but effective way is adopted to control the particle size in the present synthesis system. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy results demonstrate that the obtained particles with uniform shape and size are well crystallized. Octahedral particles are prepared employing hydrochloric acid and nitric acid as additives, and short prisms with rhombus bottoms are obtained using acetic acid as modifier. However, there is a large difference in upconversion quantum efficiency between them. The ratio is about 1:195:287 using acetic acid/nitric acid/hydrochloric acid, respectively, for the 537 nm emission. We ascribe the difference mainly to the different vibrational energy values of cation-anion couples on the surfaces of particles. 1. Introduction Rare earth ion-doped fluorides based on LnF3 and NaLnF4 (Ln ) rare earth element) have been intensively investigated due to their attractive features such as a high refractive index, low phonon energy, low toxicity, large effective Stokes shifts, high resistance to photobleaching, and wide applications in light display systems, optical telecommunication, biochemical probes, medical diagnostics, solid state lasers, and solar cells.1-13 Compared with an oxygen-based system, rare earth fluorides normally have lower energy phonons, thus leading to a weaker quenching of the excited states of the rare earth ions. Traditionally, the precise manipulations of lanthanide-doped materials with well-defined morphologies and accurately tunable sizes are realized by using templates and organic additives,14-25 which are complicated and not environmentally responsible. In these works, related regulating reagents include oleic acid (OA), ethylenediaminetetraacetic acid (EDTA), trisodium citrate (Cit3-), ethylene glycol, cetyl trimethyl ammonium bromide (CTAB), and so forth. Employing these additives can effectively control the morphologies of the ultimate products and realize the preferential growth; however, many problems occur when the obtained particles are used for further applications such as biological labeling and imaging. First, the capping organic ligands are not inherently compatible with in vivo systems, and some of them have poor dispersibility in water. Second, superfine particles coated with organic ligands may suffer from terrible energy quenching due to high-energy vibrational models of capping ligands.21 Low-cost inorganic acids such as hydrochloric acid and nitric acid, which are usually used to dissolve metal-related compositions and adjust the pH value of precursor solutions, have not * To whom correspondence should be addressed. E-mail: shenhui1956@ hotmail.com. † Sun Yat-Sen University. ‡ Shandong University.

been seriously considered as inorganic additives to modify the morphology of particles. However, anions such as chlorine ions and nitrate ions are negatively charged; thus, they may be preferentially adsorbed by cations on the surface with high energy and work as surface modifiers to control the shape of particles under certain pH value. On the other hand, serious quenching of the excited states of the rare earth ions may be avoided by replacing organic modifiers with inorganic anions such as chlorine ions that can create an atmosphere of low vibrational energy for the outer layer of obtained particles.26,27 Herein, we report a new method to synthesize size-controllable octahedral YF3-Er3+ particles by using chlorine ions or nitrate ions as surface modifiers. We also use acetic acid to prepare short YF3-Er3+ prisms with bottom rhombuses. The products resulting from using hydrochloric acid as an additive possess the highest upconversion (UC) quantum efficiency compared to those using nitric and acetic acids. 2. Experimental Section 2.1. Synthesis. Uniform YF3-Er3+ micro-octahedra were prepared in the following process: 28.5 mL of aqueous solution containing 0.002 mol of Ln(NO3)3 (Ln ) Y, Er(5%)) was heated to 100 °C; then, this solution was poured into a Teflon container with 0.024 mol of NH4F and 0.006 mol of NaF; the turbid mixture (100 °C mixture) was stirred for 3 min. Subsequently, 15 mL of concentrated hydrochloric acid (HCl, F ) 1.18 g/mL) was added to the resulting mixture. The mixture was stirred for another 3 min and allowed to stand still for 30 min. Finally, the Teflon bottle, which was held in a stainless steel autoclave, was sealed and maintained at 220 °C for 48 h. Diluted hydrochloric acid (15 mL, 0.1 mol/L) was employed to replace 15 mL of concentrated hydrochloric acid in order to investigate the morphological evolution of resulting products. The submicrosized octahedra were prepared in a similar way except that the aqueous solution containing rare earth ions was kept at room

10.1021/jp1032564  2010 American Chemical Society Published on Web 11/17/2010

Controlling the Morphology of Er-Doped YF3 temperature and mixed with fluorides to obtain a room temperature mixture. We also used 15 mL of concentrated nitric acid to replace hydrochloric acid to further study the effects of an inorganic acid on the morphology of the products. For comparison, we employed 15 mL of acetic acid as a type of organic acid instead of an inorganic acid to synthesize YF3-Er3+. In this process, 0.006 mol of NH4F used as a F- source was added with other conditions not changed. Finally, we studied the situation at which 15 mL of water was employed as a replacement for the 15 mL of acid. The temperatures of the ultimate mixtures of the above cases in this paragraph were kept at room temperature, and all the ultimate precursors were heated at 220 °C for 48 h to obtain the ultimate products. 2.2. Characterization. The products were characterized by a powder X-ray diffractometer (RIGAKU D/MAX 2200 VPC), using Cu KR radiation (λ ) 0.154 nm) and a graphite monochromator operating at 40 kV and 30 mA. A scanning electron microscope (JSM-6330F, Japan) was used to characterize the sizes and the shapes of obtained products. Dried samples for the scanning electron microscopy (SEM) were placed on a copper plate and then sputtercoated with platinum. Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) analyses were performed on a JEOL 2010 highresolution transmission electron microscope (HRTEM) equipped with an Oxford instrument EDS system. XPS patterns for the well-washed and dried products were obtained via X-ray photoelectron spectroscopy (XPS, ESCALAB 250). Infrared (IR) spectra of the ultimate products obtained by using different acids as modifiers were recorded with wavenumbers ranging from 4000 to 500 cm-1 with a Nicolet model 759 Fourier transform infrared (FT-IR) spectrometer using a KBr wafer. Raman spectra of products after 1 and 48 h hydrothermal treatment at 220 °C via a laser micro-Raman spectrometer (Renishaw inVia) were also investigated. The upconversion spectra of erbium-doped YF3 were obtained on a TRIAX320 spectrofluorimeter (Jobin-Yvon Inc., Longjumeau, France) with a Hamamatsu R928 photomultiplier tube (PMT) (Hamamatsu Co., Japan) under the excitation of a 980 nm laser diode (Coherent Co., USA). During the detection, a Scog HWB780 nm cutoff filter (Ygyes Co., China) was used in combination with the 980 nm laser diode. To compare the quantum efficiencies of the products prepared by employing different acids as modifiers, welllaminated sheets were made by laminating mixtures containing YF3-Er (5%) powder in dissolved ethylene-vinyl acetate (EVA) with a concentration of 12% (wt %). The upconversion spectra of these sheets were acquired under the excitation of 980 nm with identical testing conditions. 3. Results and Discussion 3.1. Crystal Structure and Morphology. The phase composition and purity of obtained products using hydrochloric acid as a modifier after hydrothermal treatment at 220 °C for different times were examined by XRD technique. As shown in Figure 1, products with no (0 min) and short time heat treatment (30 min) from the 100 °C mixture are crystallized while products obtained from the room temperature mixture are amorphous. It seems that high blending temperature enhances crystallization during the initial stage in our synthesis system. Pure and wellcrystallized products have been obtained by heating the ultimate mixtures at 220 °C for 48 h, and all the corresponding XRD peaks can be perfectly indexed as the orthorhombic YF3 [space group: Pnma (62)]. The calculated lattice constants are as

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Figure 1. XRD patterns of products hydrothermally synthesized at 220 °C for (a, e) 0 min, (b, f) 30 min, (c, g) 1 h, and (d, h) 48 h. The corresponding temperatures at which the aqueous solution containing rare earth ions and fluorides are mixed are 100 °C (a, b, c, d) and room temperature (e, f, g, h), respectively.

follows: a ) 6.360 Å; b ) 6.864 Å; and c ) 4.406 Å with a unit cell volume of 192.3 (Å)3 for Figure 1(h). The XRD profile of products that are heat treated for 1 h is similar to that of the particles that received a 48 h heat treatment; positions of some peaks in Figure 1(g) shift some distance relative to positions of similar peaks in Figure 1(h). A 1 h heat treatment is much shorter compared to a 48 h heat treatment, and the crystal structure of 1 h treated products may be not perfect. At this stage, ions may not be well-distributed, and more time is needed to allow the material to be perfectly crystallized. Raman spectra of related products support this viewpoint, which will be discussed later in detail. On the other hand, the stoichiometric ratio of the obtained products after 1 h hydrothemal treatment is different from the 48 h treated products as shown in Figure S3 of the Supporting Information, which may also cause shifts in positions of XRD peaks. All the above reasons may lead to differences in XRD patterns in products heated for a short time compared to those treated for a longer time although the short time treated particles are already well-shaped with an octahedral morphology. Figure 2(a) and (b) shows typical SEM images of submicrostructured and microstructured products; uniform and regular octahedra with an average edge length of about 250 nm and 2.3 µm are presented. Figure 2(c) shows the TEM image of submicrosized octahedra and the HRTEM image of the specified area, which provides further insight into morphological details of octahedral particles. 3.2. Morphology Change of Products. 3.2.1. Reaction Time. Products produced at different stages are investigated. Figure 3 shows the morphology change of products obtained at 220 °C for different times: (a, b) 0 min, (c, d) 10 min, (e, f) 30 min, and (g, h) 1 h. Initial particles are obtained after the aqueous solution containing Y3+ and Er3+ are mixed with fluorides. Higher temperature of aqueous solution leads to larger size of initial particles. After a hydrothermal treatment for 10 min, the initial particles obtained from the 100 °C mixture grow larger; however, no increase is observed for the products from the room temperature mixture. When 30 min has passed, octahedral

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Figure 2. SEM images of products obtained at 220 °C for 48 h: (a) submicrostructured, (b) microstructured YF3-Er octahedra, and TEM image (c) of submicrosized YF3-Er octahedra (inset: HRTEM image from the specified area).

Figure 4. XRD (a) and SEM (b) images of products by hydrothermal treatment of 100 °C mixture with 15 mL added 0.1 mol/L hydrochloric acid at 220 °C for 48 h. The inset in (b) shows the magnified image of a flowerlike NaYF4.

Figure 3. SEM images of products hydrothermally synthesized at 220 °C for (a, e) 0 min, (b, f) 10 min, (c, g) 30 min, (d, h) 60 min. The corresponding temperatures at which the aqueous solution containing rare earth ions and fluorides are mixed are room temperature (a, b, c, d) and 100 °C (e, f, g, h), respectively. Insets in (c, g) indicate many primary particles are adsorbed on the surface of second-phase particles.

particles surrounded with primary particles appear. We can also find some incompletely formed octahedra in the obtained products as pointed by black arrows. Well-shaped octahedra are available after 1 h hydrothermal treatment. 3.2.2. Effects of Inorganic AdditiWes. Concentrated hydrochloric acid (15 mL) is added into the room temperature mixture containing rare earth ions and fluorides. Octahedral YF3-Er particles are formed after a 48 h hydrothermal treatment of ultimate precursor (pH ) 1). We also investigate the products obtained by adding 15 mL of diluted hydrochloric acid instead of concentrated acid while other experimental conditions are not changed. The diluted acid has a molar concentration of 0.1 mol/L, and the corresponding pH value of the ultimate precursor is 3-4. Interestingly, NaYF4 of flowerlike microstructure instead of submicrosized YF3 octahedra are obtained after hydrothermal process. The XRD pattern and SEM image of the flowerlike NaYF4: Er are shown in Figure 4; inset in this figure shows

details of a flower composed of several self-assembled long hexagonal rods. If no hydrochloric acid is added instead of adding 15 mL water, then the shape of the ultimate products is irregular, and only fragmentary NaYF4 are formed (Figure 5(c)). It seems that the hexagonal form of NaYF4-Er is preferred at high pH value (pH ) 3-4) while orthorhombic YF3-Er is more stable at low pH value (pH ) 1) in our reaction system. To further study the effects of acids on the morphology of ultimate products, 15 mL of concentrated nitric acid replacing the above hydrochloric acid is added into the room temperature mixture; octahedral YF3-Er with uniform size and shape are also available after 48 h hydrothermal treatment (Figure 6). If we choose acetic acid as the modifier instead of an inorganic acid, the shape of the ultimate products are altered, as shown in Figure 5. The pure short YF3-Er prisms with rhombus bottoms appear. 3.2.3. Growth Mechanism. 3.2.3.1. Size Control. Submicrosized and microsized octahedral YF3-Er particles can be prepared without changing the heating temperature. Adopting a higher temperature at which the aqueous solution containing rare earth ions and F- sources are blended will result in larger size of the ultimate products. Figure 3 shows the morphologies of intermediate products in the formation process of octahedra. Larger primary particles are produced when the mixing temperature is at 100 °C compared to the one of room temperature, as shown in Figure 3(a, e). After 10 min of the hydrothermal process, the primary particles from the high temperature mixture grow even larger; however, there are no remarkable changes in the size of particles from the room temperature mixture. When more time passed, primary particles will dissolve to form thermodynamically more stable particles under high temperature and pressure (Figure 3(c, g)). Microsized and submicrosized octahedra are obtained after 1 h heating at 220 °C for the 100 °C and room temperature mixture, respectively. This process may indicate a dissolution-crystallization process, which is based on dissolution of solid, mass transfer in liquid, nucleation, and growth of solid crystals.21 In this process, a lower mixing temperature leads to smaller primary particles in comparison with a higher mixing temperature, and the smaller primary particles dissolve faster than larger ones due to their higher

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Figure 7. XPS pattern of well-washed intermediate products of hydrothermal reaction in which 15 mL of concentrated hydrochloric acid is employed as modifier.

Figure 5. XRD (a) and SEM (b) images of products obtained by using 15 mL of acetic acid in hydrothermal synthesis. Part (c) denotes the SEM image of products acquired by using 15 mL of water instead of acid as additive.

Figure 6. Representative XRD (a) and SEM (b) images of products obtained by using 15 mL of concentrated nitric acid in hydrothermal synthesis.

energy. The sudden increase of the growth species concentration in aqueous solution from the quick dissolution of small primary

particles will produce a large amount of crystal nuclei, and then the growth species can be easily used up in the subsequent growth process leading to a small size of ultimate particles. The case is changed for the situation of the 100 °C mixture from which large primary particles are formed; less crystal nuclei relative to the former case appear as a result of less abrupt increase of growth species concentration caused by slower dissolution of larger primary particles and continued dissolution of large primary particles provides a continuous supply of component source for further growth of crystal nuclei, which is responsible for the formation of microsized octahedra. 3.2.3.2. Morphology Modifying. It is well-known that planes appear because the crystal faces have the lowest growth rate relative to other faces. During the crystal growing process, facets with high surface energies are easily blocked by foreign chelating ligands such as EDTA21 and CTAB;22 thus, the surface energies can be effectively reduced, and the growth mode can be controlled. The extent of energy reduction is related to the quantity of certain ion-chelating agent couples, which depends on the metal atom density of each face. At present, the (111) facet of the orthorhombic structure of YF3 has the highest metal cation density, which means the cations on the (111) face are most likely to be blocked by chelating ligands. Therefore, adsorption of anions such as Cl-, NO3-, and CH3COO- via electrostatic interaction with metal cations on a high energy surface such as the (111) face to reduce its high surface energy may happen in our synthesis system when a large amount of acid is added. Adsorption of anions onto different planes leads to different morphologies of the ultimate products. Octahedral particles can be prepared when anions are preferentially linked with metal cations on (111) faces. The XPS image of the wellwashed intermediate product when hydrochloric acid is used indicates that chloric ions are available on the surface of product, as indicated in Figure 7. X-ray photoelectron spectroscopy is commonly used to analyze sample surface elements and can penetrate only to a depth of 1-5 nm. Replacing the added acid with water with other conditions not changed will make the shape of products irregular (Figure 5(c)). Using diluted hydrochloric acid instead of concentrated acid induces the formation of hexagonal phase NaYF4-Er rather than orthorhombic phase YF3-Er. Although the diluted acid has a small molar concentration of 0.1 mol/L, the anions from the acid are still likely to be adsorbed onto certain faces with high surface energy, which leads to preferred growth along the [0001] direction and the production of microsized NaYF4-Er rods selfassembled to form flowerlike structures (Figure 4). YF3-Er prisms with rhombus bottoms can also be obtained when acetic acid is added to replace inorganic acid. Different

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Figure 8. Schematic illustration of the growth process when concentrated hydrochloric acid, diluted hydrochloric acid, concentrated nitric acid, acetic acid, and water are used as additives in the hydrothermal preparation.

morphology compared to the cases of hydrochloric acid and nitric acid may be due to a different pH value. The acetic acid cannot tune the pH value to 1 as the above inorganic acids can. However, this case does support our conclusion of employing anions from acids to control the morphology of products under certain pH values. Based on above discussion, a schematic illustration of possible growth processes is shown in Figure 8. 3.3. Infrared-to-Visible UC of Hydrothermally Synthesized Products. 3.3.1. UpconWersion of Powder Obtained by Using HCl as an AdditiWe. Under the pumping of a 980 nm diode laser at 310 mW, both submicrosized and microsized octahedral YF3-Er3+ that are obtained after a 48 h hydrothermal treatment exhibit considerably strong visible emission even though without any ytterbium ions to sensitize, as shown in Figure 9(d) and (h). The main emissions around 547 and 657 nm are pumped by two photos, which are demonstrated by a double-logarithmic plot of luminescent emission intensity (at a given energy) versus incident excitation power (P (mW)) in Figure S1 of the Supporting Information. The mechanism for frequency upconversion of YF3-Er is simply established as shown in Figure S2 of the Supporting Information: excited by the pump light with the wavelength of 980 nm, transition from the 4I15/2 to 4I11/2 of erbium ion occurs, energy transfer from another erbium ion or excited state absorption will further excite the erbium ion from its 4I11/2 to 4F7/2; nonradiative decays from 4 7 F /2 to the 2H11/2 and 4S3/2 are then believed to follow. Subsequent transition from 2H11/2 and 4S3/2 to the ground state 4I15/2 yields green emissions at 547 and 526 nm, respectively. The red emission around 657 nm is from the electronic transition 4F9/2 f 4I15/2 of erbium ions. The appearance of satellite peaks in Figure 9 indicates strong stark splitting of the energetic levels for active ions because of the strong electric field created by the surrounding host material. Noticeable blue emission is also observable under the excitation of a 980 nm diode laser with the power varying between 430 and 680 mW. This upconversion process is expected to be a 3 photons process (Figure S2 of the Supporting Information); however, the double-logarithmic plot in Figure 10 indicates a 6 photons process. Probably other processes such as the feedback of excited-state absorption and cross-relaxation

Figure 9. Upconversion emission spectra of YF3-Er excited by a 980 nm laser: products hydrothermally synthesized at 220 °C for (a, e) 0 min, (b, f) 30 min, (c, g) 1 h, and (d, h) 48 h. The temperatures at which the aqueous solution containing rare earth ions and fluorides are mixed are 100 °C (a, b, c, d) and room temperature (e, f, g, h), respectively.

or looping process may occur in the whole 6 photons process under high excitation power,28 which increase the number of absorbed low energy photons to emit a high energy photon compared to a normal 3 photons process. In Figure 1, the peak of the XRD patterns is high and narrow for samples obtained after the 1 h hydrothermal process at 220 °C; however, Figure 9 (c) and (g) shows nearly no emission for them. By investigating the EDX patterns (Inset in Figure S3 of the Supporting Information) of the products obtained after hydrothermal treatment for different lengths of time, we find that the contents of erbium ions in 1 h products are comparable to the 48 h ones. It is possible that only some particles are

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Figure 12. FTIR spectra of the as-prepared YF3-Er (5%) particles obtained from hydrothermal process by adding HCl, HNO3, and CH3COOH as modifiers. Figure 10. Blue-emission spectra of submicrosized and microsized particles obtained after 48 h hydrothermal process when excited by a diode laser at 980 nm with the power of 680 mW. Inset shows the power dependence of emission intensity, and the imposed power is between and 430 and 680 mW.

Figure 11. Upconversion spectra (980 nm excitation, 0.8 W/cm2) of EVA sheets containing 12% (wt%) YF3-Er (5%) particles prepared by using HCl, HNO3, and CH3COOH as additives. Inset in this figure shows the upconversion fluorescence of the sample by using HCl as additive.

crystallized after the 1 h hydrothermal treatment, which is responsible for the diffraction pattern. This viewpoint is supported by the Raman patterns of the related products in Figure S3 of the Supporting Information, which show wider peaks for 1 h treated products compared with 48 h treated products. Wider peaks indicate richer vibrational models in 1 h treated products due to disordered distribution of the related ions. After the 48 h hydrothermal process, products are well crystallized, and component ions including erbium ions are perfectly dispersed in the crystal structure, which render the obtained products a good upconversion property. 3.3.2. Comparison of UC Efficiencies of EVA Sheets Incorporated with Particles Prepared by Employing Different AdditiWes. Previous studies focus on using organic surfactants to control the morphologies of the ultimate products; however, the addition of these modifiers may decrease the UC emission, though they can make well-shaped forms.20-24 Herein, we compare the upconversion quantum efficiencies of products using inorganic and organic acids as modifiers. Upconversion spectra of EVA sheets with 12% (wt%) YF3-Er (5%) particles incorporated are shown in Figure 11. The 980 nm diode laser (0.8 W/cm2) is used to excite the above samples at the same conditions. We can see that the product obtained from using HCl as additive shows the highest quantum efficiency. The corresponding upconversion emission is intense to the naked eye, even the pump power is low, as shown in the inset in Figure 11. The intensity ratio is about 1:195:287 from acetic acid/nitric acid/hydrochloric acid, respectively, at the emission of 537 nm. The sample with hydrochloric acid as the modifier has the

highest intensity ratio I537/654 of 2.6; the lowest value of about 1 comes from the sample with acetic acid as the modifier. Chloride has a low phonon energy of about 260 cm-1, which is much lower than the vibrational energy of typical organic ligand-chelated ions. Lower phonon energies are important for minimizing nonradiative (multiphonon) losses and increasing the overall metastable energy lifetime. On the other hand, a lot of active ions on the surfaces of particles are exposed to surroundings, and the emission efficiencies of these ions are seriously affected by the surroundings. The adsorption of organic molecules will lead to terrible fluorescence quenching. Nonradiative losses are common when the vibrational energy is larger than one-fifth of the gap energy between two energy levels at which radiative transition may occur. To better understand the difference of the upconversion efficiencies of our samples, the products are characterized by FT-IR spectra as shown in Figure 12. Since the nonradiative losses are mainly related to high energy vibrational modes, wavenumbers from 4000 to 500 cm-1 are considered. Apparently, particles produced by adding acetic acid as modifier have the most serious absorption at high energy, and the products obtained from using hydrochloric acid have the lowest. The obvious band around 829 cm-1 of particles obtained with acetic acid used arises from rocking vibration of -CH3 groups. Together with the band around 1700 cm-1 arising from the CdO stretch vibration mode, the two bands indicate the adsorption of CH3COO- on the surfaces of YF3 particles. As for the curve of products prepared by using nitric acid, the remarkable absorption band around 1385 cm-1 indicates the existence of NO31-. There is no obvious absorption band for particles produced by using hydrochloric acid as additive compared to the other two samples. It is interesting that the products obtained from using acetic acid as the modifier have the highest absorption band around 3366 cm-1, which is attributed to OHvibration. The lowest one appears from the particles prepared by employing hydrochloric acid as additive. This may be due to the fact that chlorine ions possess the highest electronegativity and acetate ions have the lowest one. Once the chlorine ions are adsorbed on the surfaces of particles, the hydroxide ions are harder to be adsorbed by metal ions on the surfaces of particles compared to the case of using acetic acid as additive. The nitrate ions also have much larger electronegativity compared with acetate ions, but the addition of nitric acid can also provide added hydroxide ions. Therefore, the surfaces of particles obtained from using hydrochloric acid are occupied by metal-chlorine couples with the lowest vibration energy and the least metal-hydroxy couples with high vibration energy compared with the cases of adding nitric acid and acetic acid as modifiers, bringing in the highest quantum efficiency for the

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ultimate product which is 287 times larger than the product prepared by using acetic acid as the modifier. 4. Conclusions In summary, uniform submicrosized and microsized octahedral YF3-Er particles have been successfully fabricated in this study. Acids such as hydrochloric acid, nitric acid, and acetic acid are employed to control the morphology of products. By changing the temperature at which the mixture was obtained, we can control the size of the ultimate products. Analysis for the crystal growth process indicates that the dissolutionrenucleation process occurs and the anions of acids are vital for morphology control under certain pH values. Octahedra and short prisms with rhombus bottoms for YF3-Er have been prepared by adding different acids as modifiers. The ultimate products obtained by using hydrochloric acid as additive have the highest upconversion quantum efficiency compared to the cases of adding nitric acid and acetic acid. The lowest vibration energy of metal-chlorine couples and the least adsorbed hydroxy ions on the surfaces of particles bring the product of the hydrochloric acid case the highest quantum efficiency and intensity ratio I537/654 compared with the other two cases. This work may make us realize the advantage of using inorganic acids as modifiers compared to organic additives, which are vastly used to control the morphology of particles in previous literatures. Acknowledgment. This work was supported by the Science & Research program of Guangdong Province in China (No. 2008A080800007 and No. 2008A011800004) and the Fundamental Research Funds for the Central Universities (10lgzd13). Supporting Information Available: Figures of the dependence of the upconversion emission intensity on excitation power, upconversion processes in Er3+ ions in YF3 particles, and the normalized Raman spectra of products obtained after 1 and 48 h hydrothermal treatment. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Li, C. X.; Zhuang, C. M.; Hou, Z. Y.; Wang, L. L.; Quan, Z. W.; Lian, H. Z.; Lin, J. J. Phys. Chem. C 2009, 113, 2332–2339. (2) Wang, G. F.; Qin, W. P.; Zhang, J. S.; Zhang, J. S.; Wang, Y.; Cao, C. Y.; Wang, L. L.; Wei, G. D.; Zhu, P. F.; Kim, Y. J. J. Phys. Chem. C 2008, 112, 12161–12167.

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