Article pubs.acs.org/cm
Spatial and Temporal Confinement of Salt Fluxes for the ShapeControlled Synthesis of Fe2O3 Nanocrystals Amanda K. P. Mann, Jie Fu, Christopher J. DeSantis, and Sara E. Skrabalak* Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *
ABSTRACT: Here, molten salt syntheses (MSS) are coupled with ultrasonic spray pyrolysis to yield single-crystalline Fe2O3 nano- and microparticles with controlled shapes and phases. It was previously demonstrated that aerosol-assisted MSS can produce single-crystalline nanoplates. Now, by selecting different molten salt flux components, various crystalline phases and particle shapes are accessed via the dissolution of Fe2O3 colloids, followed by precipitation of the iron oxide products from molten alkali carbonates that are spatially and temporally confined in the aerosol phase. This confinement limits crystal growth to the nanoscale and provides access to products at different stages of supersaturation. The resulting powders consist of hexagonal nanoplates (α- or γ-Fe2O3), rhombohedra (α-Fe2O3), or octahedra (LiFe5O8) depending on the selected molten salt flux. Significantly, this synthetic approach represents a continuous and potentially general route to the generation of shape- and phase-controlled nano- and microcrystals given the diversity of materials previously prepared by molten salt techniques. KEYWORDS: pyrolysis, nanostructures, shape control, molten salt, iron oxide
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INTRODUCTION Single-crystalline Fe 2O 3 nano- and microparticles with controlled shapes and phases are prepared using aerosolassisted molten salt syntheses (MSS). This new technique was first demonstrated with the synthesis of hexagonal NaInS2 nanoplates for use as photoanode material, and it was found that the quality of the material was significantly improved for this application compared to samples made by traditional methods.1 Although this initial demonstration represented a continuous route to single-crystalline nanoparticles, the utility of the synthetic approach could be enhanced if a diversity of particle shapes and phases could be accessed through the selection of different molten salt flux conditions. Historically, the shape of the large crystals prepared by MSS was observed to change with flux composition;2,3 however, with few notable exceptions, the changes were subtle and not akin to the diversity of shape-controlled crystals achieved recently by colloidal methods to nanomaterials.4−6 The lack of diversity in crystal shape for solids prepared via MSS is surprising given that product formation occurs as it does in colloidal methods (i.e., in solution via homogeneous or heterogeneous nucleation followed by growth) and ions are well-known to influence crystal shape.7−9 We attribute this observation to both the prolonged heating associated with the technique and Ostwald ripening10,11 and anticipated that spatially and temporally confining MSS would provide access to products produced at different stages of supersaturation and growth, and thus shape. Herein, we demonstrate with a model iron oxide system that coupling MSS with ultrasonic spray pyrolysis (USP) can generate a variety of shape- and phase-controlled particles via © 2013 American Chemical Society
the dissolution of precursor Fe2O3 colloids and subsequent precipitation of product from molten alkali carbonates spatially and temporally confined in the aerosol phase. The products that arise from this process include powders composed of hexagonal nanoplates (either α- or γ-Fe2O3), rhombohedra (αFe2O3), or octahedra (LiFe5O8). The resulting shape and phase are correlated to the phase of the precursor colloids (α- or γFe2O3) and the carbonate flux used (Li2CO3, Na2CO3, K2CO3, Rb2CO3, or Cs2CO3). Significantly, this simple synthetic approach represents a potentially general platform for the continuous generation of shape- and phase-controlled nanoscale materials. Iron oxides are finding use in magnetic applications,12 lithium ion batteries,13 water treatment,14 and catalysis15−17 and as sensors.13 The expression of different facets and phases can alter the efficacy of iron oxide for these applications.18 Typically, the synthesis of iron oxide is accomplished by colloidal,19,20 hydrothermal,21,22 molten salt,23,24 or sol−gel25,26 methods, and USP has been used to generate polycrystalline Fe2O3 microspheres27 and films.17,28 USP traditionally yields polycrystalline spheres from sintering of subparticles generated within individual droplets; however, bystander salts incorporated into USP can act as a matrix and inhibit subparticle agglomeration to produce single-crystalline particles.29−31 Also, we demonstrated that these salts can melt at high temperatures to form molten salt droplets that serve as micrometer-sized Received: November 25, 2012 Revised: April 16, 2013 Published: April 30, 2013 1549
dx.doi.org/10.1021/cm3038087 | Chem. Mater. 2013, 25, 1549−1555
Chemistry of Materials
Article
Figure 1. SEM images, TEM images (inset, ED), and XRD of the products obtained using α-Fe2O3 precursor colloids and (A−C) K2CO3/Bi(NO3)3 (product, α-Fe2O3), (D−F) Cs2CO3/Bi(NO3)3 (HR-TEM inset in panel D; product, α-Fe2O3), and (G−I) Na2CO3/Li2CO3/Bi(NO3)3 (product, LiFe5O8) flux component systems. XRD reflections denoted with asterisks are artifacts of the sample holder.
fluid reactors for the synthesis of single-crystalline shapecontrolled nanoplates.1 Here, we illustrate that the selection of salt flux and precursor colloid facilitates the generation of a range of iron oxide-based phases and particle shapes, which highlights the potential versatility of aerosol-assisted MSS.
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0.0015 mol). To ensure full dispersion, the colloidal suspension was briefly sonicated and vortexed (∼2 min). The suspension was then sparged with house air at 210 sccm for 30 min, followed by nebulization. The furnace temperature was 900 °C, except in the case of the α-Fe2O3/Cs2CO3/Bi(NO3)3 mixture where the temperature was 915 °C. The product was collected in three-gas washing bottles containing deionized water. It was then concentrated by centrifugation and removal of the supernatant (which was clear and colorless, indicating complete sedimentation). Insoluble flux components were removed by dispersing the concentrated product in 0.1 M HCl in a 1.5 mL Eppendorf tube that is suspended in an ultrasonic bath for 2 min. The product is then reisolated by centrifugation followed by two additional washes with deionized water. Synthesis of α-Fe2O3 Precursor Colloids. α-Fe2O3 colloids were synthesized as reported by Philipse et al.32 Briefly, 100 mL of a 0.002 M HCl solution was heated to 100 °C. FeCl3·6H2O (0.541g, 0.002 mol) was added to the solution without stirring. After 24 h, the reaction mixture was removed from the heat. The colloids were collected by centrifugation and washed twice with water. They were then dried under vacuum at room temperature and stored until they were used. Product Characterization. Iron oxide samples and intermediates were characterized by a number of techniques. Scanning electron microscopy (SEM) was conducted with an FEI Quanta 600 FEG instrument operating at 30 kV. It was interfaced with an Oxford Inca detector for energy dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) and electron diffraction (ED) were conducted with a JEOL 1010 instrument operating at 80 kV or a JEOL JEM-3200FS TEM instrument operating at 300 kV. Powder Xray diffraction (XRD) was performed on a Scintag diffractometer, a Bruker D8 Advance instrument, or a Panalytical Empyrean instrument
EXPERIMENTAL SECTION
Materials. All chemicals were handled in air and used as received. HCl (36.5%-38.0% HCl) was purchased from Macron. Li2CO3 (99.0%) was purchased from Mallinckrodt. Bi(NO3)3·5H2O (98%) was purchased from Alfa Aesar. γ-Fe2O3 nanopowder (
