Article pubs.acs.org/cm
Europium Chalcogenide Nanowires by Vapor Phase Conversions William L. Boncher,§ Nicholas Rosa, Srotoswini Kar,∥ and Sarah L. Stoll* Department of Chemistry, Georgetown University, Washington, DC 20057, United States S Supporting Information *
ABSTRACT: We have converted Eu(OH)3 single crystal nanowires to Eu2O3 single crystal nanowires and studied the transformation using temperature and atmosphere controlled X-ray powder diffraction. The single crystal Eu2O3 nanowires were investigated as a starting material for the preparation of controlled morphology synthesis of the magnetic semiconductors EuO, EuS, and EuSe using gas phase reagents. In addition, we also explored the synthesis of EuOCl nanowires from Eu2O3 as an alternative route to EuO and EuS nanowires.
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One of the more recently developed concepts to synthesize morphology controlled materials is commonly referred to as ‘chemical transformations of nanostructures’.16,17 The approach is to start with a shape-controlled starting material as a template and then chemically modify under controlled conditions so that the morphology is conserved in the products. Frequently, particular materials that are difficult to make with specific morphologies may be prepared from conversion of materials that easily form a particular nanostructure.18 Conversion chemistry, the exchange or diffusion of atoms into materials can take many forms. These include an alloy approach (new atoms form a solid solution), galvanic replacement (taking advantage of redox chemistry), and cation or anion exchange.16 The high surface area of nanostructures facilitates diffusion and depending on the conversion chemistry can result in novel structures such as hollow nanostructures.19 Cation exchange is perhaps the most successful approach to chemical transformations.20 For example, the exchange of Cd in CdSe with Ag to form Ag2Se is reversible, even at room temperature.20 Cadmium sulfide nanoparticles can also undergo cation-exchange with copper and lead21 as well as platinum and palladium.22 These novel reactions result in high quality, crystalline nanoparticles. The product typically favors the cation salt with the lowest Ksp, but the equilibrium can be shifted using ligand complexation to control the cation solubility. In practice, these exchanges work best for a subset of “soft” metals and are difficult to extend toward more Lewis acidic metals. Also, ion exchange reactions are sensitive to volume changes. The conversion chemistry of nanowires is constrained by the additional mechanical stress along the length of the wire. One of the most impressive examples of nanowire conversion is the reaction of single-crystal tellurium (or selenium) nanowires,
agnetic nanowires have been studied for potential applications in high-density data storage,1 memory logic,2 microwave, and optical devices.3 More recently, efforts to produce storage devices with increased access times and reduced power demands4 have led to Domain Wall Memory5 (or “racetrack” memory) devices.6 Spintronic devices, such as magnetic domain-wall logic, take advantage of the unique magnetic properties of planar nanowires.2 In addition to domain wall movement, fundamental properties of ferromagnets such as magnetic anisotropy7 and coercive field8 are influenced by the shape and dimension of the nanostructure.9 For example, the preferred magnetization direction is found along the axis of the wire independent of crystallographic orientation.10 Magnetic nanowires exhibit enhanced coercivities and remnant magnetizations.3 The importance of nanowires has attracted much synthetic effort, but shape controlled growth remains challenging. Nanowire morphology can be accomplished through a variety of synthetic techniques and has been the subject of several recent reviews.11,12 In some cases, nanowires can be obtained as the natural result of anisotropic crystal growth along one crystallographic axis.12 Solution based approaches include thermolysis where the solvent or capping ligands limit crystal growth along certain hkl directions.13 Alternatively, “oriented attachment”, when nanoparticles aggregate and coalesce through a common crystallographic axis, can also result in nanowire growth.14 The most common methods for synthesizing arrays of nanowires are either through electrodeposition in template porous alumina or by CVD from nanoparticle catalyst seeds.10 One advantage to forming arrays of oriented magnetic nanowires (rather than free-standing wires) is that it can facilitate measurements of bulk properties parallel and perpendicular to the wire axis. However, there have been advances made in assembly of nanowires for a variety of applications.15 © 2014 American Chemical Society
Received: February 12, 2014 Revised: April 30, 2014 Published: May 1, 2014 3144
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sought to determine the conditions for converting the hydroxide to the sesquioxide, maintaining morphology and crystallinity. Nanostructured lanthanide sesquioxide materials are of interest for phosphor applications.48 Using single crystal Eu2O3 nanowires, an ideal starting material, the next goal was to develop conversion chemistry to access nanowires of the series of magnetic semiconductors. Here, we describe the reduction and coupled cation insertion chemistry to form EuO and coupled anion replacement to form EuS and EuSe template formed nanowires. Each reaction involves a gas phase reagent (Eu, H2S, H2Se) heated with preformed Eu2O3 nanowires. In addition, we have also explored the conversion of Eu2O3 to EuOCl, as an alternate route to EuO and EuS nanowires.
using silver to form single crystal Ag2Te (or Ag2Se) nanowires.23 These can be further transformed through cation-exchange to form CdTe, ZnTe, and PbTe.23 Anion exchange can be more challenging due to the larger volume of anions compared with cations and the role of anions in forming the framework of the crystal structure. Much of the chemistry has targeted the conversion of hydroxides and oxides to chalcogenides. In some cases conversion chemistry is in solution, for example converting Cd(OH)2 nanowires to CdSe nanotubes using NaHSe.24 However, gas phase reactions have also successfully converted nanostructured materials into metal sulfides. For example, nanowires of CuCl have been converted to CuS nanotubes using gas phase reactions with H2S.25 Unusual morphologies have been observed such as the hollow fullerene-like MS2 formed from MO3‑x (M = Mo, W) and H2S.26 Hollow nanostructures (nanoparticles and nanotubes) are common in anion replacement due to the Kirkendall effect.27 The void is thought to form because the diffusion of cations is faster than the diffusion of anions.16 For example, arrays of single crystal, faceted, ZnO nanowires grown through electrodeposition are easily converted to ZnS nanotubes, using sulfur vapor or H2S.28 Although the products are crystalline with shape control, it is rare for anion exchange to produce single crystal nanostructures.29 Our goal was to investigate the conversion chemistry of Eu(OH)3 and Eu2O3 nanowires to form nanowires of a family of magnetic semiconductors. The europium chalcogenides include the ferromagnetic semiconductors, EuO (TC 69K) and EuS (TC 16 K), and the metamagnet EuSe (antiferromagnetic at low fields; ferromagnetic at high fields).30 The europium chalcogenides have ordering temperatures that are too low for practical applications but are ideal systems for understanding and manipulating spin polarized currents.31 The ferromagnetic coupling is communicated through the conduction electrons and can be enhanced by electron doping.32 Spin-filter tunneling has been demonstrated for EuO and EuS,33 but questions remain about how the dimensions might alter this effect.34 We initially turned to conversion chemistry for morphology control and to expand our studies of nanostructured europium chalcogenide materials beyond EuS. We have previously developed a molecular precursor route to the synthesis of nanoparticles of the europium sulfides,35 but this approach cannot be extended to EuO. Air-stable, trivalent europium dithiocarbamate complexes thermally decompose to divalent EuS. The formation of the monosulfide is aided by the fact that Eu2S3 does not appear in the binary phase diagram. By contrast, although EuO is attractive for its higher TC (69 K vs 16 K), highly reducing synthetic conditions are required due to the thermodynamic stability of Eu2O3. The synthesis of EuO nanoparticles has been reported from solution routes such as liquid ammonia36 and photochemical reduction in solution,37 but both examples are poorly crystalline with little size or shape control. Polycrystalline nanorods of EuO have been reported previously,38 but there have been no reports of nanowires of EuS, despite the many groups that have investigated EuS nanoparticles.35−42 Europium selenide has attracted even less attention,43−46 and in our studies we have found this material to be quite air-sensitive. The report of high aspect ratio, single crystal, nanowires of lanthanide hydroxides 47 inspired us to investigate the conversion chemistry of europium hydroxide. We found that both the concentration and ratio of hydroxide to europium ions control the aspect ratio of the hydroxide nanowires. We first
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EXPERIMENTAL SECTION
General Information. Europium nitrate hexahydrate and europium oxide were obtained from Strem. Sodium hydroxide and ammonium chloride were obtained from Sigma-Aldrich. Europium metal was obtained from Alfa Aesar. Chemicals were used as received. Thermal analysis was performed on an SDT Q600 TA Instruments. Simultaneous TGA-DTA data were studied from samples in an alumina pan from 25 to 1000 °C under a N2 flow of 20 mL/min, with a heating rate of 10 °C/min. Eu(OH)3 Nanowires. In a typical experiment, Eu(NO3)3·6H2O (0.25 g, 0.56 mmol) was dissolved in 5 mL of H2O. 0.5 g of NaOH was added to make a 2.5 M solution, which was stirred for 20 min; a white, flocculent precipitate formed immediately. For experiments to control the aspect ratio, varying the concentration [OH−] (0.39 M− 5.15 M) and [Eu3+] (0.058 M to 0.112 M). The solution was put into a 20 mL Teflon lined Parr Bomb and heated at 180 °C for 6 h. Once cooled to room temperature, the supernatant was decanted and discarded. The vessel was filled to 10 mL with H2O, and the resulting solution was centrifuged for 10 min. The supernatant was decanted and discarded. This was repeated with 10 mL of ethanol. The precipitate was dried under vacuum for 1 h, and the material was lightly ground with a mortar and pestle to increase surface area. Powder X-ray diffraction patterns matched that of Eu(OH)3, PDF 00017-0781 (ICDD, 1979). Eu2O3 Nanowires. 200 mg Eu(OH)3 nanowires were heated in a quartz tube in a Lindberg/Blue M horizontal tube furnace at 500 °C for 3 h under dynamic vacuum. Powder X-ray diffraction patterns matched that of the cubic phase of Eu2O3, PDF 00-034-0392 (ICDD, 1984), Supporting Information Figure S2. EuO Nanowires. In a N2 atmosphere glovebox, 10 mg Eu2O3 nanowires were placed on top of 100 mg of Eu foil (0.1 mm thick) on a graphite boat in a quartz tube. The tube was taken out of the glovebox and heated in a Lindberg/Blue M horizontal tube furnace at 750 °C for 4 h under dynamic vacuum. The powder X-ray diffraction pattern was matched with that of EuO, PDF 00-015-0886 (ICDD, 1965). EuS Nanowires. Eu2O3 nanowires were heated on a graphite boat in a quartz tube in a Lindberg/Blue M horizontal tube furnace at 200 °C for 1 h under dynamic vacuum, and then the temperature was increased to 800 °C. A mixture of 5% H2S, 95% N2 was then flowed through the quartz tube over the material for 10 min. After heating, the tube was removed from the furnace, and once cooled to room temperature, the stopcocks and valves were closed, and the flow of gas was ceased. Powder X-ray diffraction patterns were indexed to EuS, PDF 00-026-1419 (ICDD, 1974). EuSe Nanowires. Eu2O3 nanowires were heated on a graphite boat in a quartz tube in a Lindberg/Blue M horizontal tube furnace at 200 °C for 1 h under dynamic vacuum, and then the temperature was increased to 800 °C. In a connected vessel, H2O was carefully added to Al2Se3, forming H2Se, which then flowed through the quartz tube over the material for 10 min. After heating, the tube was removed from the furnace, and once cooled to room temperature, the stopcocks and valves were closed, and the flow of gas was ceased. Powder X-ray 3145
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diffraction patterns were indexed to EuSe, PDF 00-10-0279 (ICDD, 1939). EuOCl Nanowires. Eu2O3 nanowires (20 mg, 0.06 mmol) were lightly ground in a mortar and pestle with excess NH4Cl (10 mg, 0.19 mmol) and dried in an oven at 120 °C for 2 h in a graphite cup. In a glovebox, this was loaded into a flame-dried quartz tube, put under vacuum briefly, for approximately 2 s, and then left under static vacuum. The material was heated at 400 °C for 2 h. A powder X-ray diffraction pattern was matched with that of EuOCl, PDF 00-012-0163 (ICDD, 1962). EuS from EuOCl. EuOCl nanowires (23.1 mg, 0.1 mmol) were heated at 200 °C for 45 min under dynamic vacuum, and then the temperature was raised to 500 °C for 2 h under flowing H2S (5% in nitrogen). Powder X-ray diffraction patterns were indexed to EuS, PDF 00-026-1419 (ICDD, 1974). Electron Microscopy. Selected-area electron diffraction images were taken with a JEOL JEM 2100F Field Emission Gun Transmission Electron Microscope, at 200 kV. SEM images were taken with a Zeiss SUPRA 55-VP scanning electron microscope, at an acceleration voltage of 20 kV with an in-lens detector. High resolution TEM and selected-area electron diffraction were performed with a JEOL JEM 2100F Field Emission Gun Transmission Electron Microscope, at 200 kV. Powder X-ray Diffraction. Powder X-ray diffraction patterns were obtained using a Rigaku Ultima IV X-ray powder diffractometer with CuKα radiation at 40 kV and 44 mA with a D/teX silicon strip detector. For high-temperature PXRD studies, material was loaded on a platinum sample plate, which was heated according to the specifications of the experiment. Generally, the sample was heated 10 °C/min to the target temperature and then held at temperature for a 2Θ scan (10−80°). Scans were taken at T = RT, 50°, 100°, 150°, 200°, 250°, 275°, 300°, 325°, 350°, 375°, 400°, 450°, 500°, 600°, 700°, 800°, 900°, 1000°, 1200°.
of the hydroxide nanowires under flowing nitrogen (see Supporting Information S1 for the TGA). The measured weight loss as a function of temperature indicates a two-step decomposition. The first step, which occurs at 324 °C, corresponds to a loss of mass consistent with the formation of EuO(OH).49 A second mass loss at 440 °C corresponds to the formation of Eu2O3 and is confirmed by powder X-ray diffraction (PXRD) of the final product. Supporting Information S2 has the X-ray powder diffraction pattern of Eu2O3, and the same pattern is plotted as logI-vs-2Θ in Supporting Information S3. This is consistent with prior studies of the dehydration of bulk Eu(OH)3.50,51 Studies of the morphology of the Eu(OH)3 nanowire material converted under two atmospheric conditions (ambient air and vacuum) had striking differences. The europium sesquioxide formed in air was quite agglomerated with total loss of the nanowire morphology (see Figure 2a). However, the
RESULTS/DISCUSSION Single-crystal nanowires of the series of lanthanide hydroxides, Ln(OH)3, have been reported previously.47 The nanowire morphology of hexagonal Ln(OH)3 materials is thought to be due to anisotropic growth under hydrothermal conditions. Depending on the lanthanide, the concentration of hydroxide appears to control different morphologies observed, for example, nanosheets, nanobelts, and nanotubes.47 Focusing on the europium hydroxide, we explored the effect of hydroxide concentration and the ratio of [OH−]:[Eu3+], on the morphology. We found nanoplates forming under dilute hydroxide, with nanowires forming at moderate concentrations and nanorods at the highest hydroxide concentrations and highest [OH−]:[Eu3+] ratio. The range of morphologies we have observed is shown in Figure 1 (a-c). We also studied various conditions for the formation of Eu2O3 from single crystal Eu(OH)3 nanowires. In initial studies we monitored the conversion using thermogravimetric analysis
europium sesquioxide that formed under vacuum maintained the nanowire morphology (Figure 2b). We optimized the conversion by heating under vacuum between 300 and 700 °C and found the highest aspect ratio nanowires were for 400 °C for 2 h. To further investigate the transformation mechanism of Eu(OH)3 to Eu2O3, the material was monitored by X-ray powder diffraction during heating from room temperature to 1200 °C. Initially, dehydration was monitored in an ambient atmosphere. The progress was tracked by a series of PXRD scans, sequentially with increasing temperatures (listed in experimental) as shown in Figure 3. The first scans at low temperatures (black in Figure 3) index to europium hydroxide. At 350 °C, the peak intensities for the hydroxide diminish, and the pattern appears amorphous until ∼600 °C (illustrated in blue). At this point, the material appears to crystallize and can be indexed to Eu2O3 (in green). These are the conditions that
Figure 2. Scanning Electron Microscopy images of Eu2O3 formed under air (left) and under dynamic vacuum (right). Scale bar for (a) is 300 nm; scale bar for (b) is 100 nm.
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Figure 1. Scanning Electron Microscopy images of different Eu(OH)3 morphologies. In (a), dilute base ([OH−] = 0.728 M, [Eu3+] = 0.115 M), scale bar is 1000 nm, (b) moderate base ([OH−] = 2.4 M, [Eu3+] = 0.113 M) scale bar is 800 nm, and (c) concentrated base ([OH−] = 5.2 M, [Eu3+] = 0.056 M), scale bar is 600 nm.
Figure 3. X-ray powder diffraction patterns as a function of temperature. The black pattern indexes to Eu(OH)3, the blue pattern of an amorphous material, and the green of crystalline Eu2O3. 3146
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led to highly agglomerated material, where the fine nanowire morphology of the starting hydroxide is completely lost. A second diffraction experiment under vacuum provided an interesting comparison, and the data is presented in Figure 4. In
It is interesting that the latter not only maintained morphology but also formed a single crystalline phase. It could be that the lower temperatures for the transformation were important or that the rate of reaction was key. Because of the importance of volume changes for single crystal transformation, nanowire dimensions is one metric investigated for nanowire conversions. For example, previous work on the conversion of Te nanowires to Ag2Te found the increase in dimensions of the Ag2Te wires relative to the Te nanowires could be related to the increased volume of the new unit cell. Based on crystallographic data, the density increases from the hydroxide to the sesquioxide. However, the Eu(OH)3 nanowires synthesized in this report had a wide range of aspect ratios, with most variability seen in the lengths with an average of 971 nm (±567). The dimensions were so polydisperse that it rendered any effort to compare the volume with the converted material as meaningless. Single crystal Eu2O3 nanowires proved to be a useful starting material for magnetic semiconductors. We also find this example quite interesting because most studies of single crystal-to-single crystal or “topotactic” transformations focus on thermodynamic arguments between the structure of the starting material and the final material. It is generally agreed that such transformations only take place in systems where there is little atomic movement from the starting structure to the final structure. The activation barrier to topotactic conversions are thought to be dominated by structural differences and volume changes. Here both temperature and the rate were clearly altered by reaction under vacuum, highlighting the role kinetics can play. Our first conversion target was EuO, a ferromagnet with the highest TC in the series. The reaction with europium vapor is a long established synthesis of bulk EuO,52,53 and this chemistry has previously adapted to produce polycrystalline EuO nanorods.38 The europium vapor reduces the Eu(III) of the starting material to Eu(II), and an additional equivalent of divalent europium has to intercalate into the structure according to the reaction: Eu3+2O3 + Eu0 → 3Eu2+O. The result is conversion of the sesquioxide BCC material into the monoxide FCC lattice (the cell constant is almost exactly half). Using europium metal under dynamic vacuum the Eu2O3 nanowires visibly changed from white to dark red-black. However, if the oxide nanowires were placed too far downstream from Eu powder, incomplete conversion was frequently observed. Consistent results were obtained when Eu2O3 nanowires were placed on top of Eu foil. The foil was approximately 10 times the stoichiometric weight required, so after the reaction the wires physically slid off the thinned foil. After reacting for 4 h at 750 °C, the powder X-ray diffraction pattern was indexed to the sodium chloride lattice of EuO. (See Figure 6b as well as larger images in Supporting Information S4. Supporting Information S5 has the EuO pattern graphed as logI-vs-2Θ as well as Supporting Information Table 1 with 2Θ, Intensity, and hkl, for both the observed and literature patterns of EuO for comparison.) Initially, we anticipated that temperature would be critical for the nanowire conversion. Our premise was that the temperature needed to be high enough for the activation energy required, but not so high a temperature as to cause interdiffusion between wires destroying the morphology. We sought the lowest temperature that still produced EuO, hoping to increase time to allow full conversion. However, studies of the morphology of the EuO under different reaction conditions
Figure 4. X-ray powder diffraction patterns as a function of temperature. The black pattern indexes to Eu(OH)3, the blue pattern indexes to EuO(OH), and the green pattern indexes to crystalline Eu2O3.
contrast to heating in air, under vacuum the transformation of the hydroxide pattern was clearly visible at temperatures as low as 250 °C (almost 100 °C lower than in the first study). Rather than forming an amorphous material, as observed under air, a new crystalline material that formed could be indexed to EuO(OH) (blue in Figure 4). A second transformation was also observed, again at a much lower temperature than observed under air. Under dynamic vacuum, crystalline Eu2O3 (in green) can be identified as low as 325 °C (compared with 600 °C in air). The TGA data supports the phase transformations although the temperatures do not directly correlate with hot stage X-ray diffraction experiments. The atmosphere in TGA is different (flowing nitrogen); however, we believe the shifts in temperature are due to the heating rates. Thermal analysis does support the formation of the europium oxy-hydroxide intermediate, based on the mass lost. We used Transmission Electron Microscopy to investigate the Eu2O3 formed under vacuum. Like the Eu(OH)3 nanowires, the Eu2O3 nanowires appeared to be single crystalline based on the TEM, as shown in Figure 5. The High Resolution
Figure 5. High Resolution Transmission Electron Microscopy and Diffraction (a) is the end of a nanowires − scale bar is 10 nm − and (b) is the center of the nanowire and Selected Area Electron Diffraction pattern (c) of Eu2O3 nanowires.
Transmission Electron Microscopy exhibited clear lattice fringes, and the Selected Area Diffraction Patterns exhibit sharp spots, indicative of single crystals, along the length of the wire. These spots could be indexed to the BCC structure of Eu2O3, looking down the 022 zone. It is perhaps not surprising that the material that transitioned through an amorphous phase might lose morphology, while the material that went through a crystalline intermediate phase might maintain the morphology. 3147
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Eu 2O3(s) + 2NH4Cl(s) → 2EuOCl(s) + H 2O(g) + 2NH3(g)
(2)
This gave successful synthesis of EuOCl, based on powder Xray diffraction (see Supporting Information S8 for diffraction pattern of EuOCl as well as the logI-vs-2Θ for this material in Supporting Information S9), and the nanowire morphology appears to be conserved based on SEM (Supporting Information S10). However, the materials do not appear to be single crystalline based on electron diffraction and generally seem to have more surface roughness than the sesquioxide. Nonetheless we also explored the reaction of the europium oxychloride with H2S, which to our knowledge has not been reported and found this reaction successfully forms phase pure EuS (see Supporting Information S11 for X-ray diffraction pattern of EuS and Supporting Information S12 the graph of logI-vs-2Θ). We presume the reaction involves the formation of water vapor and HCl(g), as products. We continue to explore these reactions further but include this example because it provides a broader view of the scope of chemical conversions of the europium oxides. Our second target for converting Eu2O3 nanowires was europium sulfide, EuS. There are a variety of solid state sulfurizing agents that have been used to convert lanthanide oxides to sulfides, including elemental sulfur, carbon disulfide, and hydrogen sulfide. In previous studies we have found carbon disulfide leaves graphitic impurities, and the pressure of elemental sulfur is difficult to control compared with commercially available H2S (5% in N2). Starting with Eu2O3 nanowires, heated under flowing dilute H2S, we were able to form phase pure, EuS nanowires (see X-ray powder diffraction pattern of EuS in Supporting Information S6 as well as the logIvs-2Θ in Supporting Information S7). A table of 2Θ and Intensity for experimental and literature EuS is also included in Supporting Information Table 2. We found that annealing the Eu2O3 at low temperatures to remove surface water or carbonates was helpful for avoiding the formation of EuO2S2 as an impurity. We have also previously reported that in studies of alkali metal doping (Eu2‑xNaxS2 from 0 to 1), we were able to form cubic EuNaS2, which has no divalent europium, at temperatures as low as 350 °C.56 Thus, the reduction chemistry requires a higher activation energy than the anion exchange. As we observed in the formation of EuO, the nanowire conversion is optimally done at high temperatures and for short reaction times. Longer reaction times cause the material to agglomerate and lose the wire morphology. The EuS shown in Figure 7 reacted at 800 °C for 10 min and has a clean X-ray powder diffraction pattern indexing to EuS. The material appears to be of a wirelike morphology; however, there does appear to be some minor agglomeration, and high resolution TEM studies suggest the material is polycrystalline. Finally, we were also interested in the formation of EuSe nanowires. We were able to prepare H2Se in situ by the reaction of H2O(l) with Al2Se3(s) under highly controlled conditions (warning: the reaction is exothermic and rapidly produces a toxic gas). The white nanowires of Eu2O3 quickly darkened. Using an air-free sample holder with Be windows, we obtained a powder diffraction pattern of the product which indexed to EuSe as well as the presence of unreacted Eu2O3, as shown in Figure 8. We investigated the air oxidation of EuSe and found that as the material greys when exposed to air, the X-ray diffraction pattern develops a pattern which can be indexed to
Figure 6. Transmission Electron Microscopy image of EuO nanowires (a) and X-ray powder diffraction pattern, starred peaks index to EuO (b).
suggested that the optimum balance of conversion while maintaining morphology in fact required the opposite: high temperatures and short reaction times. This echoes the results for the hydroxide conversion. In contrast to previous work, our hope was that by starting with single crystal Eu2O3 nanowires we might produce single crystal EuO nanowires. The conservation of morphology of polycrystalline Eu2O3 nanorods to polycrystalline EuO nanorods has been reported.38 Our Eu2O3 nanowires also conserve morphology to form EuO nanowires. Unfortunately, as seen in Figure 6a, the TEM studies do not support the single crystalline nature of the EuO nanowires. This may not be surprising, as the reaction with europium vapor is not “cation exchange” but cation reduction/insertion. The cell has to accommodate an additional equivalent of europium with a larger ionic radii. The X-ray powder diffraction appears to be single phased, but occasionally we did find starting material (single crystalline Eu2O3) based on electron diffraction. We believe the presence of unreacted Eu2O3 nanowires must be less than 5%. An alternative reaction to form europium chalcogenides is through the oxyhalide, EuOCl. Europium monoxide can be formed using EuOCl with LiH to form EuO, LiCl, and H2. The LiCl can be distilled from the product at temperatures close to 800 °C. Previous work by Bärnighausen54 demonstrated that a solid-state reaction, under vacuum, between europium oxyhalides for X = F, Cl, Br, and lithium hydride yielded singlecrystals of EuO according to Reaction 1 below: EuOX(s) + LiH(s) → EuO(s) + LiX(g) + 1/2 H 2(g) (1)
To form the europium oxychloride, we adapted the synthesis of Corbett,55 using ammonium chloride. Using ammonium chloride with the single crystal nanowires of Eu2O3 at modest temperatures (∼400 °C) formed the europium oxy-chloride according to Reaction 2: 3148
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system we observe that high temperatures and short reaction times lead to the best morphology and phase control, contrary to our expectations. We believe high temperatures are necessary, for the reduction of europium to form the monochalcogenides. The nanowire morphology appears to be converted, and the products are highly crystalline. However, all the systems we explored exhibited some surface roughness and loss of texture to form polycrystalline wires. We also explored the solid-state reactions to form EuOCl nanowires, which provide a potential novel route to EuO. We also observe for the first time the reaction of EuOCl with hydrogen sulfide to form EuS. We have successfully produced magnetic semiconductors with nanowire morphology; however, single crystal nanowires of the magnetic semiconductors remain an important challenge.
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ASSOCIATED CONTENT
* Supporting Information S
The TGA of Eu(OH)3 nanowires and the X-ray powder diffraction patterns for Eu2O3, EuO, EuS, EuOCl, and Eu2O2Se as well as SEM images of EuOCl. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 7. Scanning Electron Microscopy image of EuS nanowires (a) and X-ray powder diffraction, starred peaks index to EuS (b).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Addresses §
WLB Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States. ∥ SK Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research is supported by the NSF award No. CHE112387, and X-ray powder diffraction was obtained using an instrument purchased from the MRI program at NSF (CHE0959546). We acknowledge the support of the Maryland NanoCenter and its NispLab. The NispLab is supported in part by the NSF as a MRSEC Shared Experimental Facility.
Figure 8. Powder X-ray diffraction pattern of EuSe. (Purple * indicates EuSe. Green ! indicates Eu2O3 starting material. The peaks due to Be and BeO from the air-free sample holder are colored red.)
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Eu2O2Se (see the Supporting Information for X-ray powder diffraction pattern of the oxidized product in Supporting Information S13). Although the single crystal structure of the oxyselenide has not been reported, the cell constants have been reported (a = 3.91, c = 6.89 Å).57 Using the JADE software it was possible to match the structure using the oxy-sulfide crystal structure and refine the cell constants to a = 3.90, c = 6.87 Å, as seen in the Supporting Information.
REFERENCES
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CONCLUSIONS The conversion of single crystal hydroxide nanowires under controlled temperatures and atmospheric conditions can form high aspect ratio, single crystal europium sesquioxide nanowires. The morphology control and crystallinity are notably different under vacuum, where the conversions occur at lower temperatures and pass through a crystalline intermediate before forming single crystal nanowires of the Eu2O3. The Eu2O3 nanowires were used as a novel starting material under a variety of gas phase conversion chemistries to form EuO, EuS, and EuSe. Guidelines for gas phase conversions that maintain nanostructured morphology have yet to be developed, but in this 3149
dx.doi.org/10.1021/cm500530r | Chem. Mater. 2014, 26, 3144−3150
Chemistry of Materials
Article
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