CRYSTAL GROWTH & DESIGN
Crystalline Nanowires of Ln2O2S, Ln2O2S2, LnS2 (Ln ) La, Nd), and La2O2S:Eu3+. Conversions via the Boron-Sulfur Method That Preserve Shape
2008 VOL. 8, NO. 2 739–743
Yi-Zhi Huang,†,‡ Ling Chen,†,§ and Li-Ming Wu*,† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China, Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China, and State Key Lab of Rare Earth Materials Chemistry and Applications, Beijing 100871, People’s Republic of China ReceiVed August 8, 2007; ReVised Manuscript ReceiVed NoVember 6, 2007
ABSTRACT: Three different kinds of pure-phase nanowires of Ln2O2S, Ln2O2S2, and LnS2 with diameters of several tens of nanometers, ∼20 nm for the La-series and ∼30 nm for the Nd-series, and lengths of up to several micrometers have been successfully converted from the same starting Ln(OH)3 (Ln ) La or Nd) nanowires via a boron-sulfur method under mild conditions, 400–500 °C for 10 min or 24 h. The identity of the product is controlled by the reaction stoichiometry. The commercial red phosphor, La2O2S:Eu3+, that emits at 623 nm has also been obtained in the form of nanowires by this method. Such conversions are processes by which the morphology of the starting Ln(OH)3 is well preserved. The phase characterizations and morphologies of the nanowires, key parameters of the synthetic conditions, the postulated conversion mechanism, and the luminescence properties are reported.
1. Introduction Boron sulfides have recently been found to be efficient, safe, and convenient agents for the in situ syntheses of a broad range of metal sulfides from the corresponding oxides at intermediate temperatures, quite different from the conventional sulfidation agents such as H2S, CS2, A2S (A ) alkali metal), elemental sulfur, and organic thio compounds.1 The boron-sulfur method has been established as a unique means by which nanoparticles of Nd2O3 may be successfully converted into NdS2 nanoparticles without a significant shape change.2 This retention of morphology between Nd2O3 and NdS2 was originally hypothesized to come from the similarity of their crystallographic structures, which was considered to favor their lattice matching during the conversion.2 That speculation brings forward a new question of whether a consistency of chemical structures between the starting oxide and the final sulfide is necessary for such shape preservable conversion. Our recent attempts give a negative answer. In the present work, we report the relative results, a series of conversions realized via the boron-sulfur method, in which the morphologies of the starting materials have been well preserved in the final products without any crystallographic resemblance between them. Our attempts at present have been focused on one-dimensional (1D) nanostructures because of their widely established applications in fabrication of optical, electronic, and magnetic nanoscale devices.3–5 The lanthanide hydroxide nanowires were selected as sources because they were found to function similarly to the oxide2 and their syntheses are indeed convenient.6,7 On the other hand, studies on nano lanthanide sulfides of concern in this work have focused mostly on doped or undoped La2O2S and the like.8–12 Regarding their 1D morphology examples, only undoped La2O2S and Eu2O2S are known as nanorods9,11b and Y2O2S as nanotubes.13 The reason for such a deficiency is probably because these materials are extremely difficult to prepare as 1D nanomaterials via other methods. In the present * Corresponding author. E-mail:
[email protected]. † Fujian Institute of Research on the Structure of Matter, CAS. ‡ Graduate School of the Chinese Academy of Sciences. § State Key Lab of Rare Earth Materials Chemistry and Applications.
work, crystalline nanowires of lanthanide hydroxides have been incompletely or completely sulfidized in situ by boron sulfides into pure crystalline nanowires of oxysulfides (Ln2O2S and Ln2O2S2) or disulfides (LnS2) with a remarkable morphological monodispersity. Nanowires of Eu3+-doped La2O2S (La2O2S: Eu3+), an efficient red phosphor,14 have also been obtained by this means, and their photoluminescent properties were characterized at room temperature.
2. Experimental Section All the chemicals were used as purchased, without further purification.
2.1. Preparation of the Convertible Sources. Nanowires of Ln(OH)3 (Ln ) La, Nd) and La(OH)3:Eu3+. The Ln(OH)3 nanowires were synthesized according to the hydrothermal method reported by Li’s research group.6,7 Suitable amounts of aqueous KOH were added to Ln(NO3)3 stock solution to make the KOH concentration 5 M, and the subsequent hydrothermal process was carried out at 180 °C for 12 h. The desired white hydroxide nanowires were filtered, washed with water and ethanol several times, and then dried in air. The doped La(OH)3:Eu3+ was obtained similarly from a mixture of La(NO3)3/ Eu(NO3)3 in a 95:5 molar ratio. 2.2. Preparation of the Converted Products. Nanowires of Ln2O2S, Ln2O2S2, LnS2 (Ln ) La, Nd), and La2O2S:Eu3+. The lanthanide oxysulfide or disulfide nanowires were generated from the corresponding hydroxides by the boron-sulfur method.1,2 Two 2-cmlong silica tubes each closed on one end were situated inside a slightly larger silica jacket with one on the top of another; see Figure 1 in ref 1. The top one was loaded with white convertible Ln(OH)3 nanowires, and the bottom one contained suitable amounts of boron and sulfur powders based on the following three chemical equations: 6Ln(OH)3 + 2B + 3S f 3Ln2O2S + B2O3 + 9H2O
(1)
6Ln(OH)3 + 2B + 6S f 3Ln2O2S2 + B2O3 + 9H2O
(2)
2Ln(OH)3 + 2B + 4S f 2LnS2 + B2O3 + 3H2O
(3)
3+
The doped La2O2S:Eu nanowires were obtained according to eq 1 with La(OH)3:Eu3+ nanowires as starting reactant instead. In general, the overall loadings totalled about 15 mg, the component weighings having a precision of 0.1 mg. After the larger silica jacket (1 cm i.d.) was evacuated and flame-sealed (at length of about 12 cm), the assembly was heated in a program-controlled tube furnace according to the different needs. For a 24 h anneal, the temperature was gradually raised
10.1021/cg700751j CCC: $40.75 2008 American Chemical Society Published on Web 01/11/2008
740 Crystal Growth & Design, Vol. 8, No. 2, 2008
Figure 1. (a) XRD pattern of reactant La(OH)3 nanowires. The XRD patterns of converted products under different conditions (b) La2O2S (500 °C for 10 min); (c) La2O2S2 (500 °C for 10 min); (d) LaS2 (400 °C for 24 h). The standard patterns are (e) La(OH)3, (f) La2O2S, (g) La2O2S2, (h) LaS2. All of the observed patterns (a-d) match well with their corresponding standard patterns. to that desired at 50 °C/h, held for 24 h, and subsequently cooled radiatively to room temperature. For a 10 min anneal, the oven was brought to the desired temperature in advance, and the sample assembly was quickly inserted into the hot furnace, held for 10 min, and then quenched in water. The products were usually washed with CS2 and ethanol several times to wash off unreacted sulfur, boron sulfides, and the byproduct B2O3.1 The yields of the products are usually about 100% on the basis of the amount of the starting hydroxides. The experiment conditions and results are summarized in Table 1. 2.3. Sample Characterization. The structure and morphology of the as-synthesized products were characterized by powder X-ray diffraction (XRD; BDX3300) and transmission electron microscopy (TEM; JEM-2010), respectively. Photoluminescence spectrum was recorded on a Varian Cray Eclipse fluorescence spectrophotometer with 1 nm resolution at room temperature.
3. Results and Discussion 3.1. A Series of Nanowire Products Converted from La(OH)3 Nanowires via the Boron-Sulfur Method. We found that La(OH)3 nanowires (about 20 nm in diameter) from a reported hydrothermal method6,7 could be partially or fully sulfidized to give the corresponding compounds with remarkable maintenance of their linear morphology via a boron-sulfur method. The partially sulfidation could be finished within a very short time, that is, 10 min at 500 °C to generate either La2O2S or La2O2S2 nanowires depending only on the loading ratios of La(OH)3/S, 2:1 for the former and 1:1 for the latter. Complete sulfidation to LaS2 nanowires was realized after a much longer period, 24 h, at a lower temperature, 400 °C, with more loaded sulfur (La(OH)3/S ) 1:2). Such a condition can also lead to partial sulfidation when sulfur is loaded strictly according to the stoichiometries, La(OH)3/S ) 2:1 for La2O2S, or 1:1 for La2O2S2 on the basis of equations 1 and 2 (see details in Table 1). The XRD patterns of the reactant La(OH)3, converted oxysulfides, and disulfides are shown in Figure 1 along with the corresponding calculated patterns. The results indicate that each of the products is the single-phase target as no other phases are detected. As shown in Figure 2, for both oxysulfide (b, c) and disulfide (d) products, their 1D linear morphologies and diameters are nearly identical to those of the initial La(OH)3
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nanowires (a). Remarkably, such conversions are observed preserving morphologies without any noticeable fusion of the products. Such reactant-to-product morphology maintenance is comparable to those found under different conditions, for example, the thermal treatment of a homogeneous mixture of Y(OH)3 and sulfur at 700 °C that generates tubular-preserved Y2O2S nanotubes.13 But the flexibility of the boron-sulfur method to produce diverse compounds from the same reactant simply by introducing different loading ratios and the efficiency of the conversion with such a short reaction time, 10 min, and relatively lower temperature, 500 vs 700 °C, are remarkable. 3.2. An Extension of the Boron-Sulfur Method in Lanthanide Series. Lanthanide elements as a special series in the periodic table have always been regarded to be similar to each other in their physical and chemical properties. We had no intension of trying the boron-sulfur method on each of the lanthanide metal hydroxides, but only carried out some selective experiments to show the general application of such method. Nd(OH)3 was chosen because conversions of Nd(OH)3 in present work and Nd2O32 in previous report might present a direct comparison to better understand the nature of the process. Starting with Nd(OH)3 nanowires (about 30 nm in diameter), as expected, three neodymium products, that is, Nd2O2S, Nd2O2S2,15 and NdS2 nanowires, were obtained under reaction conditions similar to those for corresponding lanthanum reactions. A slightly higher reaction temperature, 450 °C, had to be applied in the NdS2-conversion so as to get good crystallization, whereas a parallel reaction at 400 °C resulted in only amorphous nanowires. The XRD patterns Figure 3a-d of this series strongly resemble those of their La counterparts Figure 1a-d but with a consistent shift toward higher 2θ angles because the Ndcompounds have smaller unit cells. The TEM observations again substantiate the morphology and size dependence of the product Figure 4b-d on those of the starting material Figure 4a. Although the three Nd-products are different in component, crystal structure, and stoichiometry, their morphology and size are nearly identical. 3.3. A Red-Emitting Phosphor in the Form of Nanowire. Eu3+-doped lanthanide oxysulfide is an efficient red phosphor in commercial lighting engineering and high-resolution display devices.14 The preparations of these bulk materials are conventionally a solid-state reaction with a refluxing or a solid–gas reaction involving H2S or CS2 at a temperature higher than 1000 °C (1050–1300 °C).16–21 On the other hand, several methods have been developed recently for their nanosyntheses, such as a solvothermal method with or without pressure relief,8–10 direct or multistep polymer thermolysis method,11,22 combustion synthesis,12 or sulfuration of nanosized hydroxide by elemental sulfur.13 To our knowledge, nanocrystalline La2O2S:Eu3+ has only been made in the form of small discrete particles with a mean size of 18 nm,11a and there has been no report of a 1D morphology such as the uniform wires shown in this work except for undoped La2O2S nanorods.9 So, we took on the challenge following the success above-described on LaS2, La2O2S2, and La2O2S. The strategy was to convert the Eu3+doped hydroxide nanowires at the optimal conditions established for the undoped hydroxide, 500 °C for 10 min or 400 °C for 24 h via the boron-sulfur method. The uniform nanowires of target La2O2S:Eu3+ were successfully obtained as shown in Figure 6b, with its phase purity and crystallinity checked by the XRD pattern shown in Figure 5. Comparisons between the La2O2S:Eu3+ and its undoped counterpart indicate that the small proportion of Eu3+ dopant (Eu3+/ La3+ ) 5:95 molar ratio) does not change either the crystal
Nanowires of Ln2O2S, Ln2O2S2, LnS2, and La2O2S:Eu3+
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Table 1. Optimal Experimental Conditions for the Shape Preservable Conversions by a Boron-Sulfur Methoda starting nanowires loaded ratio reaction reaction product nanowires color of observed lattice lattice parameters (space group) (hydroxide/B/S) temp (°C) time (h) (space group) the product parameters from JCPDS 6:2:3 500 1/6 La2O2S (P3jm1) white a ) b ) 4.047(2) Å a ) b ) 4.051 Å La(OH)3 c ) 6.96(1) Å c ) 6.944 Å (P63/m) 6:2:3 400 24 La2O2S (P3jm1) white a ) b ) 4.044(2) Å a ) b ) 4.051 Å c ) 6.946(6) Å c ) 6.944 Å 3:1:3b light a ) 13.17(5) Å a ) 13.215 Å 500 1/6 La2O2S2 (Cmca) yellow b ) 5.95(1) Å b ) 5.943 Å c ) 5.95(2) Å c ) 5.938 Å 3:1:3 400 24 La2O2S2 (Cmca) light a ) 13.278(4) Å a ) 13.215 Å yellow b ) 5.945(1) Å b ) 5.943 Å c ) 5.937(1) Å c ) 5.938 Å 1:1:2b grayish a ) b ) c ) 8.195(1) Å a ) b ) c ) 8.200 Å 400 24 LaS2 (cubic) yellow Nd(OH)3 6:2:3 500 1/6 Nd2O2S (P3jm1) olivine a ) b ) 3.958(5) Å a ) b ) 3.946 Å c ) 6.796(4) Å c ) 6.790 Å (P63/m) 3:1:3b cyan a ) b ) 4.106(0) Å a ) b ) 4.077 Å 500 1/6 Nd2O2S2 (I4j2m) c ) 13.083(3) Å c ) 12.83 Å 1:1:2b grayish a ) b ) 4.006(1) Å a ) b ) 4.022 Å 450 24 NdS2 (P4/nmm) yellow c ) 8.019(1) Å c ) 8.031 Å La(OH)3:Eu3+ 6:2:3 500 1/6 La2O2S:Eu3+ white a ) b ) 4.044(4) Å a )b ) 4.051 Å (P63/m)c (P3jm1) c ) 6.929(18) Å c ) 6.944 Å 6:2:3 400 24 La2O2S:Eu3+ white a ) b ) 4.049(2) Å a ) b ) 4.051 Å (P3jm1) c ) 6.949(5) Å c ) 6.944 Å
JCPDS No. 27-0263 27-0263 80-0694 80-0694 25-1042 27-0321 ref 15 85-0511 27-0263 27-0263
a X-ray powder diffraction phase analyses and unit cell refinements were performed by the JADE program. b Excess S was usually used to ensure the completeness of the desired conversion. c The crystallographic structures of the doped compounds are identical to their undoped counterparts.
Figure 2. (a) Typical TEM image of reactant La(OH)3 nanowires. The TEM images of converted products under different conditions: (b) La2O2S obtained at 500 °C for 10 min; (c) La2O2S2 obtained at 500 °C for 10 min; and (d) LaS2 obtained at 400 °C for 24 h. Their identities and phase purities are shown in Figure 1.
structure of La2O2S or its nanowire morphology. The solidstate photoluminescence properties of the as-converted La2O2S: Eu3+ nanowires were characterized at room temperature. Figure 7 shows the emission spectrum excited at 330 nm. The fluorescence bands were assigned as the transitions originating from the 5DJ to 7FJ levels of Eu3+. For example, the four emissions at lower energies (593–701 nm) are attributed to the 5 D0 f 7FJ (J ) 1, 2, 4) transitions, whereas the other three (537–585 nm) are identified as the 5D1 f 7FJ (J ) 1, 2, 3) transitions. The positions of all these emissions are less than 3 nm shift from the reported values,10–12 and also agree well with those of the bulk (Table 2).18 The most intense 623 nm emission along with the adjacent weak one at 614 nm, the most prominent transition (5D0 f 7F2) of Eu3+, indicates the red luminescence. 3.4. The Productive and Shape Preservable Conversions via a Possible Template Self-Sacrificing Process. The effort of this work is to realize a shape retentive conversion from a facile nanomaterial to an otherwise inaccessible nanomaterial. The experimental results show that the boron-sulfur method is an effective process with a quite high yield and purity. It is even better that the method can easily produce three different materials from the same starting nanowires, simply by introducing different loading proportions. Of course, such diversity is essential because of structural variations among the lanthanide sulfides. However, such flexibility also accompanies an extraordinary stoichiometric controllability of the method
Figure 3. (a) XRD pattern of reactant Nd(OH)3 nanowires. The XRD patterns of converted products under different conditions: (b) Nd2O2S (500 °C for 10 min); (c) Nd2O2S2 (500 °C for 10 min); (d) NdS2 (450 °C for 24 h). The standard patterns are (e) Nd(OH)3, (f) Nd2O2S, (g) Nd2O2S2, (h) NdS2. All product patterns (a-d) match well with their corresponding standard patterns.
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Figure 4. (a) Typical TEM image of reactant Nd(OH)3 nanowires. The TEM images of converted products are (b) Nd2O2S obtained at 500 °C for 10 min; (c) Nd2O2S2 obtained at 500 °C for 10 min; and (d) NdS2 obtained at 450 °C for 24 h. Their identities and phase purities are listed in Figure 3. Table 2. Transition Assignments for the Room-Temperature Emissions of La2O2S:Eu3+ Nanowires (λex ) 330 nm) and the Corresponding Values for Bulk La2O2S:Eu3+ emission of the nanowires (nm)
Figure 5. (a) XRD pattern of reactant, La(OH)3:Eu3+ nanowires; (b) XRD pattern of the La2O2S:Eu3+ converted at 500 °C for 10 min.
Figure 6. (a) Typical TEM image of reactant, doped La(OH)3:Eu3+ nanowires; (b) typical TEM image of the doped La2O2S:Eu3+ converted at 500 °C for 10 min. Their corresponding XRD patterns are shown in Figure 5.
Figure 7. The room-temperature PL emission spectrum of the asconverted La2O2S:Eu3+ nanowires excited at 330 nm.
over the sulfidation process. And it is noteworthy that the two oxysulfides can be achieved in a short reaction time, 10 min, and no coproduction of unremovable impurities is found. Although a close morphological retention between the starting Ln(OH)3 and the product oxysulfides or disulfides is observed,
537 555 585 593 614 623 701
transition 5
D1 D1 5 D1 5 D0 5 D0 5 D0 5 D0 5
f f f f f f f
7
F1 F2 F3 7 F1 7 F2 7 F2 7 F4 7 7
emission of the bulk (nm)a 538.1 558.3 587.1 594.3 615.5 624.3 699.1
a The values are the energy level barycenter gaps in ref 18 except for the 5D0 f 7F2 transition, which has two values according to the split 7 F2 levels.
there is no corresponding crystallographic control. Both Ln2O2S and Ln(OH)3 belong to the hexagonal class, but with very different structures, a hexagonal close-packed structure for the former and a tunnel structure for the latter. Note that NdS2 nanodisks were made by the boron-sulfur method from Nd2O3 nanodisks previously,2 and herein, NdS2 nanowires from Nd(OH)3 nanowires. Thus, a morphology-dependence of the nanoproduct on the starting nanomaterial without crystallographic control is again suggested. Such conversions from one nanostructured material to another have also been found in some other reports, for example, the formation of Ag2Se (or Ag2Te) nanowires from Se (or Te) nanowires and Bi2Se3 nanorods from Bi2S3 nanorods.23–25 The self-sacrificing template process proposed for those wet-chemistry routes also seems suitable for the present dry conversions. In the solid–gas reactions under such mild temperatures, the nucleation and the subsequent growth of the products were apparently confined to the boundary of each individual reactant island since there was no mass diffusion among the reactant particles and the reaction occurred merely by diffusion between the gaseous species and the solid. That is to say, the starting nano solid reactant served as a self-sacrificing template to confine the growth and shape of the nano product, which resulted in a product with a close external resemblance even though there was no crystallographic similarity between them. In this way, the key to control the morphology appears to be the physical shape of the starting material.
4. Conclusions The newly established boron-sulfur method allows X-ray pure nanowires of Ln2O2S, Ln2O2S2, or LnS2 (Ln ) La, Nd) to be made from easily accessible nanowires of Ln(OH)3. Significantly, the morphology of the product under the optimal conditions depends only on the starting morphology, and the identity of product is controllable simply by controlling the
Nanowires of Ln2O2S, Ln2O2S2, LnS2, and La2O2S:Eu3+
loading ratios. The same principles apply to the synthesis of the doped La2O2S:Eu3+ nanowires, which have an efficient red emission at room temperature. A self-sacrificing template process is proposed for such shape preservable conversions. Recently, we have successfully extended the same method to selenides and even tellurides, which will be reported soon. We strongly believe that the boron-chalcogen method will find a wide range of applications in the field of nanomaterial synthesis. Acknowledgment. This research was supported by the National Natural Science Foundation of China under Projects (20401014, 20401013, 20521101), the State Key Laboratory Science Foundation (070023 and 050097), the NSF of Fujian Province (2004HZ01-1, 2005HZ01-1), and the “Key Project from CAS” (KJCX2.YW.H01).
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