J. Phys. Chem. B 2006, 110, 18191-18195
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Novel Route to WOx Nanorods and WS2 Nanotubes from WS2 Inorganic Fullerenes Yan-Hui Li,† Yi Min Zhao,† Ren Zhi Ma,‡ Yan Qiu Zhu,*,† Niles Fisher,§ Yi Zheng Jin,| and Xin Ping Zhang| School of Mechanical, Materials and Manufacturing Engineering, the UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, United Kingdom, AdVanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, NanoMaterials Ltd., Weizmann Science Park, 18 Einstein Street St., Nes Ziona 74140, Israel, and CaVendish Laboratory, UniVersity of Cambridge, JJ Thomson AVenue, Cambridge, CB3 0HE, United Kingdom ReceiVed: April 20, 2006; In Final Form: July 18, 2006
WOx (2 < x < 3) and WS2 nanostructures have been widely praised due to applications as catalysts, solid lubricants, field emitters, and optical components. Many methods have been developed to fabricate these nanomaterials; however, most attention was focused on the same dimensional transformation from WOx nanoparticles or nanorods to WS2 nanoparticles or nanotubes. In a solid-vapor reaction, by simply controlling the quantity of water vapor and reaction temperature, we have realized the transformation from quasi-zerodimensional WS2 nanoparticles to one-dimensional W18O49 nanorods, and subsequent sulfuration reactions have further converted these W18O49 nanorods into WS2 nanotubes. The reaction temperature, quantity of water vapor, and pretreatment of the WS2 nanoparticle precursors are important process parameters for long, thin, and homogeneous W18O49 nanorods growth. The morphologies, crystal structures, and circling transformation mechanisms of sulfide-oxide-sulfide are discussed, and the photoluminescence properties of the resulting nanorods are investigated using a Xe lamp under an excitation of 270 nm.
1. Introduction WOx nanomaterials have recently attracted numerous research interests because they can be valuable catalysts,1 special optical components,2 and good electric field emitters.3 In particular, one-dimensional WOx has been considered as the most efficient precursors for another type of very useful materials, namely WS2 inorganic fullerenes (IFs) or inorganic nanotubes (INs), depending predominantly on their initial spherical or rod shapes. The outstanding property of WS2-IFs (or INs) in applications such as in solid lubricants,4 catalysts,5 scanning tunneling microscopy probes,6 field emissions,7 and shock absorbing8 has initiated many efforts to synthesis them. WS2-IFs (or INs) have been prepared by a series of processes such as the microwave treatment of W(CO)6 reacting with H2S,9 ultrasonic irradiation of W(CO)6 solution mixed with diphenylmethane and sulfur, followed by heating at 800 °C,10 activation processing of a WS2 commercial sample,11 iodine transport method,12 direct pyrolysis of WS42- and CTAB,13 and chemical vapor deposition of WCl6;14 however, the most commonly used technique suitable for industrial-quantity manufacture is the oxide-to-sulfide conversion using H2S at high temperature under a reducing atmosphere, where the original spherical or rod morphology remains almost unchanged after conversion.15,16 Using this technique, NanoMaterials Ltd has been successful in commercializing IFs for products applied to a wide range of industries.17,18 According to this direct conversion mechanism, high-quality WOx nanorods, particularly W18O49 (WO2.72), are required for INs production because W18O49 has been reported to be a good candidate for nanotube conversion. Accordingly, * Corresponding author. E-mail:
[email protected]. † School of Mechanical, Materials and Manufacturing Engineering. ‡ Advanced Materials Laboratory, National Institute for Materials Science. § NanoMaterials Ltd.. | Cavendish Laboratory, University of Cambridge.
diverse approaches have been investigated for WOx nanorod growth, typical examples including direct heating of W foil with SiO2, O2, and water vapors, as well as hydrothermal reactions starting with W-containing salts. In short, earlier efforts have been focused on using metallic W or W salt to produce appropriate forms of WOx, nanospheres or nanorods/wires, then the sulfuration process being employed to convert them into corresponding WS2-IFs or WS2-INs.15-19 During our thermal analysis study on IFs, we realize that spherical IFs will be oxidized at certain temperature. En route to pursue the detailed morphological transformation of these IFs, we discovered, for the first time, that by using an interesting reverse reaction (sulfide-to-oxide), we have developed a new and simple technique to create one-dimensional WOx nanorods from these IF spheres, which can then be transformed to onedimensional WS2 nanotubes following the oxide-to-sulfide conversion process. In this paper, we describe the details of this novel process for the production of W18O49 nanorods and WS2 nanotubes, in relatively large scale, by simply heating WS2-IFs, realizing transformation from WS2-IFs f WOx nanorods f WS2-INs. The process parameters that affect the chemical and dimensional changes are also discussed. 2. Experimental Section The WS2-IF precursors, supplied by NanoMaterials Ltd., were deposited on a 20 mm × 50 mm quartz plate. Two deposition methods were used to spread the powders. One was to directly spread the dry WS2-IF powder, the other was to scatter the powders in deionized water after ultrasonic treatment. The sample on the quartz plate was then placed into a 1.5 m long quartz tube situated inside a horizontal furnace. Ar was used to flush the quartz tube, at a flow rate of 180 sccm, to thoroughly remove the air residue in the tube. When rising the
10.1021/jp062427j CCC: $33.50 © 2006 American Chemical Society Published on Web 08/26/2006
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Figure 1. SEM images showing (a) raw WS2-IF precursor, (b) and (c) W18O49 nanorods formed at 900 °C for 1 h from WS2-IF precursor without ultrasonic treatment, (d) W18O49 nanorods grown at 1000 °C for 1 h, (e) and (f) W18O49 nanorods prepared at 900 °C for 1 h from WS2-IF well-dispersed precursor treated by ultrasonic.
temperature, the quartz plate supporting the IF samples was initially located outside the heating zone. Upon reaching the reaction temperature, the sample was moved to the central hot zone; meanwhile, water vapor was introduced into the reaction tube by Ar bubbling though a wash bottle containing deionized water. The Ar flow rate was controlled by a mass flow meter, and a typical flow rate of 40 sccm was used. After 1 h, the quartz plate containing the dark-blue W18O49 nanorods was shifted out of the reaction zone and cooled to room temperature rapidly. The conversion of W18O49 nanorods to WS2-INs was carried out in a similar furnace by heating the resulting nanorod sample at 1000 °C for 30 min under a H2S gas flow rate of 100 sccm. Thermal stability of the WS2-IFs precursor in air was investigated using a TA Instruments Q600 thermogravimetric analyzer with a compressed air flow rate of 100 sccm and ramped heating rate of 10 °C/min. X-ray diffraction (XRD) spectra were recorded using a Siemens D500 diffractometer with Cu KR radiation (λ ) 1.5418 Å) at a scanning rate of 0.02°/ min and time step of 2 s, 2θ ranging from 10 to 80°. The products were also characterized by transmission electron microscopy (TEM, Jeol, JEM-2000FX), high-resolution transmission electron microscopy (HRTEM, Jeol, JEM-3000F), and selected area electron diffraction (SAED). Photoluminescence (PL) spectrophotometry of W18O49 nanorods was performed on a Cary Eclipse fluorescence spectrophotometer using a Xe lamp as the excitation light source (excitation at 270 nm) at room temperature. 3. Results and Discussion Figure 1a shows the uniformity of the WS2-IF precursors, with average diameters of 100-120 nm. After 1 h reaction at
900 °C, W18O49 nanorods were formed (Figure 1b), exhibiting widths of 60-150 nm (Figure 1c) and lengths of several microns (Figure 1b). A portion of nanoparticles were observed (Figure 1b), presumably the unreacted WS2-IF precursors due to the large agglomerates of the WS2-IF precursors, low reaction temperature, and short reaction time. Increased temperatures (950-1050 °C, reaction time 1 h) result in the majority of the nanorods with enlarged width up to a micron (Figure 1d), while prolonged reaction time (2 h, 900 °C) led to the appearance of yellow WO3 nanorods, which is due to the excessive oxygen offered by water vapor. Experimental results suggest that the well-dispersed and ultrasonic-pretreated WS2-IF precursor has a significant impact on the fabrication of homogeneous-size W18O49 nanorods. The WS2-IF precursor usually agglomerates into large particles with a size of 40-200 µm (Figure S1a, Supporting Information). After reaction, only the surface of the large particles was transformed into W18O49 nanorods, and the overall particles appeared to be intact, resembling a “bird nest” like structure (Figure S1b, Supporting Information). The large agglomerates were easily broken into uniform and small particles by ultrasonic treatment, and a homogeneous deposit layer was formed on the quartz plate (Figure S1c, Supporting Information). The uniformly distributed WS2-IF nanoparticles turned to grow into W18O49 nanorods with high purity (free of nanoparticles, Figure 1e) and homogeneous width (Figure 1f), eliminating the “bird nest” like W18O49 nanorod structures (Figure S1d, Supporting Information). Figure 2a shows an XRD spectrum of the original WS2IFs. After oxidation, monoclinic W18O49 was identified (Figure 2c) according to JCPDS card no. 05-0392, exhibiting similar peaks with different intensities, which is due to the onedimensional configuration of the nanorod sample when com-
WOx Nanorods and WS2 Nanotubes from WS2 Fullerenes
Figure 2. XRD profiles: (a) raw WS2-IF precursor, (b) WS2 nanotubes, and (c) W18O49 nanorods prepared at 900 °C for 1 h using ultrasonic treated WS2-IFs precursor. Peaks of W18O49 represented by *.
pared with the standard JCPDS card (from uniaxial particles). The occurrence of WS2 peaks in Figure 2c is due to the WS2 residual, which is also observed in later TEM examination. On further reaction with H2S for 1 h at 1000 °C, the resulting W18O49 nanorods were converted to WS2, as shown in Figure 2b. Thus, a sulfide f oxide f sulfide circling conversion is complete but with a significant geometry change. The d002 spacing (0.6188 nm) of the resulting WS2-INs exhibits a slight contraction when compared with the starting WS2-IFs (0.6201 nm). Although the XRD results here cannot confirm that the resulting WS2 are all nanotubes (potentially accompanied by WS2 platelts), further TEM studies confirm that the majority of them are indeed nanotubes. A previous report showed that, during the transformation from WO3 nanoparticles to WS2-IFs, the discrepancy caused by the increase of particle radius can lead to stresses and lattice expansions along the c-axis of WS2.17 The crystal planes of a WS2-IN body behave more like those in a two-dimensional system rather than as in a quasi-zero-dimensional highly curved spherical WS2-IFs, thus curvature stresses may be relaxed and subsequently induce a upshift of 2θ, from 14.272° to 14.303° (Figure 2, curves a and b). Accordingly, a 0.2% lattice contraction between two adjacent WS2-INs layers along the c-axis occurs. TEM study shows a profound regular and sharp cross sectional feature of the W18O49 nanorods (Figure 3a). Such a typical crystalline feature may originate from their natural morphology or may result from fractures caused by the ultrasonic treatment or rapid cooling due to their intrinsic shear structure.25 Figure 3b shows a nanorod with a width of 86 nm, together with its SAED pattern and the corresponding HRTEM image. We have indexed its {010} and {103} planes according to the measured d spacing of 0.378 and 0.373 nm, respectively. The growth direction of the nanorods is along the [010] axis, which is perpendicular to the close-packed plane (010) of the monoclinic W18O49. The reaction temperature has a sensitive effect on the WS2IFs to W18O49 nanorod conversion. TGA results show only a slight weight loss of 2% before 390 °C, which includes a possible water vapor loss that absorbed from the air (Figure 4a); a sharp weight loss from 390 to 510 °C, associated with a strong exothermic peak in the differential thermal analysis (DTA) curve (Figure 4b), suggests that WS2-IFs start to be oxidized to form tungsten oxides from 390 °C and completely change to WO3 at 510 °C. The endothermic peak appeared in the DTA curve at 1209 °C (Figure 4b) may correspond to the melting point of the fully oxidized WO3, whereas further weight loss from 1251 °C in Figure 4a is ascribed to the evaporation
J. Phys. Chem. B, Vol. 110, No. 37, 2006 18193 of molten WO3. In our heating experiments, low temperatures (950 °C) generally lead to rod formation, but with largely increased width (Figure 1d); 900 °C is found to be an ideal temperature for realizing both shape change from quasi-zerodimensional WS2-IFs to one-dimensional W18O49, with relatively uniform and fine width, 100 nm) or complete conversion of WO3 nanorods, which are inappropriate for subsequent WS2 growth. According to a theoretical calculation, a spherical WS2 nanoparticle with a diameter of 100 nm can transform into a 10 µm long tube with inner and outer diameters of 12 and 15 nm, respectively.21 Our W18O49 nanorods have widths of 60100 nm and lengths of 10-100 µm, suggesting that one W18O49 nanorod is unlikely converted from only a single WS2-IF with a diameter of 100 nm. We deduced that once a W18O49 nucleates somewhere on the surface of a WS2 nanoparticle, it is most likely to grow along the [010] axis, leading to selective growth of the W18O49. During the growth, WS2 nanoparticles that agglomerated together may continue to react with surrounding water vapor and W18O49 molecules may diffuse along the needles, which results in very long W18O49 nanorods. In our experiments, only water vapor rather than reducing H2 was used, therefore, the known transformation mechanism of Wf WO3 f W18O49 is unlikely.16 Although no obvious WO2 peaks appear in the XRD pattern (Figure 2c), which is indicative of a direct transformation from WS2 nanoparticles into W18O49 nanorods, we cannot rule out a possible conversion process involving WO2
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Figure 3. TEM images: (a) W18O49 nanorods and (b) a W18O49 nanorod exhibiting the [010] growth direction. Top inset: SAED pattern of the W18O49 nanorod; bottom inset: a HRTEM image of the W18O49 nanorod.
Figure 4. Thermal analyses of raw WS2-IF precursors in air: (a) TGA curve and (b) DTA curve.
f W18O4922 and maybe such an intermediate phase (WO2) exists in an undetectable amount. Furthermore, previous studies of the thermal stability of WS2 nanoparticles also found the presence of W18O49 in a nitrogen atmosphere,23 which was attributed to the oxidation of W metal instead of direct transformation from WS2 to W18O49.23 In fact, a similar topological change from the all-carbon spherical cages (fullerenes and carbon nano-onions) to onedimensional carbon nanotubes has previously been reported.24 In a direct pyrolysis, with the assistance of a metal catalyst (Ni), fullerenes (C60) have been successfully transformed to nanotubes. Comparing these results, we emphasize the similarity between the all-carbon fullerenes and the inorganic fullerenes, both of which exhibit hollow analogous cages. However, the differences between the two types of fullerene are obvious due to different compositions. The conversion from C60 to carbon nanotubes was realized with the assistance of a Ni catalyst.24 In this context, we have not only achieved the topological change from spheres to rods/tubes but also modified their chemical compositions completely in the course of the conversions, significantly leading to two types of one-dimensional nanomaterials, WOx nanorods, and WS2 nanotubes via simple processes in the absence of any catalyst. However, the growth mechanism from the smaller WS2-IFs to the much bigger oxide nanorods (in terms of their volume) remains unclear, and we believe that the agglomeration and diffusion at high temperatures may have played an important part in it. TEM images of WS2-INs converted from the W18O49 nanorods are shown in Figure 5. It is noteworthy that an apparent feature of our WS2-INs is that many of our WS2-INs are opentipped, compared with previous reports.16 It is believed that such an open tip arises from the typical morphology of our W18O49
Figure 5. (a) Schematic presentation of a W18O49 nanorod (top) may break into small pieces of As and Bs (bottom), which further lead to WS2-INs with varying tip features after subsequent oxide-to-sulfide conversion, (b) TEM image showing a WS2-INs with one open tip only (arrows), and (c) a WS2-IN with both tips open (arrow, cut from black rectangle in Figure S2b, Supporting Information).
nanorods. It is noted that shear planes generally exist in a W18O49 cryatal,25 which often cause fractures during rapid cooling, such as when the sample was immediately shifted out of the reaction zone, forming small pieces (Figure 5a, top and bottom). The oxide nanorods to WS2 conversion is believed to follow the well-established oxide-to-sulfide conversion mechanism,17,26 which involves the outer W18O49 surfaces first reacting with H2S to form the outmost WS2 layers and proceeding inward gradually to form the inner layers, and finally W18O49 disappears and WS2 nanotubes result. The inner cavities of WS2 nanotubes are formed as a result of the structural differences between oxide and sulfide.19 In our studies, some WS2 nanotubes seem to be not empty, which may be due to the insufficient reaction time of W18O49 nanorods with H2S. In the presence of H2S, part A of the nanorod will transform into a WS2 nanotube with one tip open and the other one closed (Figure 5b, arrows). W18O49 nanorods grow along the [010] direction, thus the close-packed plane (010) is roughly perpendicular to the nanorods axis. During conversion, sulfur diffusion along the rod body to form WS2 layers is presumably easier than that through the fractured rod tip, being close-packed planes, thus such preferential reaction results in the final opened tips. For some W18O49 nanorods with one round end formed in the experiment, presumably without cleavage-broken planes,
WOx Nanorods and WS2 Nanotubes from WS2 Fullerenes
J. Phys. Chem. B, Vol. 110, No. 37, 2006 18195 Supporting Information Available: SEM images showing the effects of the pretreatment of the precursor on the products. TEM image showing a WS2-INs with two closed ends and a WS2-IN with two open tips. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 6. Photoluminescence spectrum of the resulting W18O49 nanorods at 270 nm excitation.
they tend to react equally at both the body surface and the end surface, leading to closed WS2 nanotube tips. Accordingly, a W18O49 nanorod with two round and smooth ends (Figure 5a, top) is likely to form a WS2 nanotube with both tips closed (Figure S2a, arrow, Supporting Information), similar to WS2 nanotubes transformed from tungsten oxide whiskers.16 A W18O49 nanorod with two fractured ends (part B in Figure 5a) leads to a WS2 nanotube with two opened tips (arrow in Figure 5c and Figure S2b, Supporting Information). Figure 6 shows the room-temperature PL spectrum of W18O49 nanorods. Two PL emission peaks were observed at ultraviolet (358 nm) and blue (423 nm) regions. Lee et al.27 and Feng et al.28 also reported the similar PL properties of W18O49 nanorods. When the W18O49 nanorods sample is on the border of the quantum confinement regime, size only has a weak effect on the PL property, which leads to the slightly increased wavelength when raising the width.27 The length of our W18O49 nanorods is 10-100 µm, much longer than 130 ( 30 nm, leading to a more increased ultraviolet emission at 358 nm than the 349 nm peak reported by Lee.27 The blue emission of W18O49 nanorods is caused by the presence of oxygen vacancies or defects.27 The strong blue emission peak at 423 nm suggests that our long W18O49 nanorods possess many oxygen vacancies. 4. Conclusions In summary, one-dimensional W18O49 nanorods and WS2INs have successfully been prepared by a direct transformation using quasi-zero-dimensional WS2-IFs as precursors via simple heating in an oxidation atmosphere (for oxide nanorods) and a subsequent sulfidization process (for nanotubes), which is fundamentally different from previous studies in terms of the precursors and the reaction direction of the process. The reaction temperature, water vapor, and precursor preparation are important process parameters for long, thin, and homogeneous nanorod growth. It is suggested that the interesting open-tipped feature of the WS2-INs is controlled by the profound regular and sharp W18O49 nanorod cross sections, which may arise from cleavage planes caused by intrinsic shear planes. The open tips make nanotubes an ideal structure for tracking other metals or molecules inside their cavity for catalysts and other applications. By modifying the reaction chemistry, we have demonstrated a sulfide f oxide f sulfide circling conversion and have realized interesting geometry conversion, resulting in two useful onedimensional nanomaterials. The room-temperature PL spectra of W18O49 nanorods show an ultraviolet emission peak at 358 nm and a blue emission peak at 423 nm. Acknowledgment. We thank the EPSRC for financial support.
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