Facile synthesis of 3D Fe 2 O 3 nanostructures: sponge-like cube shape and bird nest-like architecture

. The hierarchical nanostructures (3D) with their large specific surface area and abundant pores usually possess unique physical and chemical properties for various important applications. In this report, we have introduced simple and scalable routes to successfully synthesize 3D iron oxide nanostructures, including porous cubes and bird nest-like architecture. The 3D sponge-like Fe 2 O 3 nanocubes were formed by an annealing process of perfect Prussian Blue (PB) microcubes, which were built from small nanoparticles linked together. Whereas, the 3D bird nest-like Fe 2 O 3 nanostructures were formed by the transformation of C@FeOOH nanoflower precursors, which were constructed by primary nanorods. The results indicated that the obtained materials show monodispersity, uniform morphology, ultra-porosity and extremely high specific surface area. With unique characteristics, the 3D Fe 2 O 3 nanostructures could be potential candidates for various important fields such as catalysts, absorption and gas sensors.


Introduction
The 3D porous nanostructures are built by an interconnected network of pores, which are fantastic materials for many important areas [1].
These unique architectures usually possess high specific surface area, abundant porosity and enhancement of adsorption/reaction sites, which may originate from the combination of macroscopic properties of the total system with nanoscopic scale [2].Semiconducting metal oxide is one type of 3D porous material that has attracted a lot of interest because it combines the unique intrinsic properties of metal oxide semiconductors with the benefits of porous system derived from the high surface area and large pore volume [3], which are useful for catalysts, absorption and sensors [4].However, it is very difficult to synthesize 3D porous metal oxide nanostructures by facile hydrothermal methods.
Due to its appealing chemical and physical properties, nontoxicity, and low cost [5], iron oxide nanostructure is a significant class of materials that have been investigated for use in several fields, including catalysis [6], magnetic materials [7], lithium-ion batteries [8], gas sensors [9], and bio-sensing and medical applications.
Among various iron oxide nanostructures, the porous nanostructures frequently improve their properties in a variety of applications cavities by utilizing a thermal decomposition process [7].By heating uniform PB microcubes in air at different temperatures, Zhang et al. [14] synthesized hierarchical Fe2O3 microboxes with various shell structures.The porous Fe2O3 nanocubes composed of fine nanoparticles were fabricated by the decomposition of PB at high temperatures [15].Besides, several attempts have also applied carbonaceous sphere templates to synthesize 3D highly porous iron oxide nanostructures such as α-Fe2O3 double-layer hollow spheres [16], porous α -Fe2O3 hollow microsphere [17], multi-shelled α-Fe2O3 microspheres [18] and so on.Therefore, using Fe-MOF and carbonaceous spheres as templates is a very important strategy to prepare ultra-porous 3D Fe2O3 structures.
In this report, uniform PB microcubes and carbon microspheres were used as potential templates for the fabrication of 3D porous Fe2O3 nanostructures.The 3D sponge-like γ-Fe2O3 nanocubes were formed by the assembly of numerous fine nanoparticles through the simultaneous oxidative decomposition of PB.The 3D bird nest-like Fe2O3 nanoarchitectures were synthesized by the hydrothermal method using carbon microspherical templates, which were built by the assembly of primary nanorods.

Synthesis of 3D sponge-like iron oxide microcubes
The following chemicals were purchased from Sigma-Aldrich and utilized without additional

Synthesis of 3D bird nest-like Fe2O3 nanostructures
The hydrothermal approach was used to design the 3D nest-like α-Fe2O3 nanoarchitectures utilizing carbonaceous spherical templates.We started by getting the carbonaceous sphere ready.
In a typical procedure, 4 g of glucose (Sigma- were added to this suspension.At room temperature, the mixture was vigorously stirred for 24 h.The precipitation was separated and purified multiple times with water and ethanol.
The obtained material was dried for 10 h at 80 °C.
The 3D bird nest-like Fe2O3 nanoarchitectures were produced by calcining the resulting composites at 600 °C for 2 h.
CN)6•3H2O), Polyvinyl Pyrrolidone (PVP).The 3D ultra-porous α-Fe2O3 nanocubes were easily prepared by directly annealing the Prussian blue (PB).For the manufacture of the PB nanocube precursor, 0.11 g of K4Fe(CN)6•3H2O and 3.8 g of polyvineypirrolydone (PVP, K30, MW ~ 40000) were dissolved in 50 mL of HCl 0.1 M aqueous solution by magnetic stirring for 30 pISSN 1859-1388 eISSN 2615-9678 DOI: 10.26459/hueunijns.v132i1D.705557 min to form a homogeneous yellow solution, which was transferred into a 100 mL Teflon-lined autoclave, and then aged for 24 h at 90 o C. The obtained PB blue crystals (Prussian blue) were collected, repeatedly washed with water and ethanol, and then dried at 80 o C for 24 h.The PB nanocubes were annealed for 3 h at 500 °C to form 3D sponge-like γ-Fe2O3 nanocubes.
Aldrich) was added into 40 mL of deionized water to obtain a homogeneous solution.This solution was then transferred into a Teflon-lined autoclave (100 mL).The mixture was aged for 4 h at 140 o C and then for 4 h at 180 o C. The carbonaceous sphere was collected, repeatedly washed with deionized water and ethanol, and then dried for 24 h at 60 o C. To synthesize C@FeOOH microspheres, 0.08 g of as-prepared carbonaceous microspheres were ultrasonically dispersed in 50 mL of deionized water.After that, 2 mmol of FeSO4•7H2O and 0.3 g of CH3COONa diffraction patterns (XRD) were measured on a Bruker D8 Advance x-ray diffractometer with a Cu-Kα line source (λ ~ 1.5406 Å).Scanning Electron Microscopy (SEM) was performed on JSM-5300LV instrument.Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) were conducted on JEOL JEM 1230.N2 adsorption/desorption isotherm (Micromeritics Tristar 3030) was used to determine the specific surface area and pore size distribution of samples The morphology of the PB precursor and the sponge-like Fe2O3 nanocube is analyzed by SEM and TEM techniques, as shown in figure 1.For the PB precursor, the SEM image (1a) indicates that the as-synthesized PB has a typical cubic shape with smooth surfaces that is uniform and well dispersed.The edge length of the PB cubes is in the range of 200-300 nm.TEM image (figure 1b) provides additional details about the perfect cubical structure of PBs.Similar to the SEM result, the TEM image likewise shows cubes with sharp edges and corners.After being annealed at 500 o C for 3 h in air, the calcined material still retains the cubic shape of PB precursor.However, the morphology reveals a sponge-like cube shape with an extremely rough surface (figure 1c).The sponge cubes have a unique 3D porous network, as seen in the TEM image (figure 1d), which is made up of interconnected pores and nanoparticles that arise from the oxidative decomposition of Fe4[Fe(CN)6]3 [19].The phase of sponge-like Fe2O3 nanocubes was determined by SAED and HRTEM, as illustrated in figure 2. All concentric diffraction rings that may relate to the diffraction (220), (311), (400), (422), (511) and (440) planes of γ-Fe2O3 are assigned by the SAED result (figure 2a).In addition, the HRTEM in figure 2b clearly presents lattice fringes with a spacing of 0.25 nm, indicating the (311) planar spaces of γ-Fe2O3 nanostructure.XRD patterns were also used to determine the crystal structures of sponge-like γ-Fe2O3 nanocubes.The XRD pattern of the sample in figure 3a closely matches cubic spinal γ-Fe2O3 (JCPDS card no.39-1346).The XRD curve shows no peak for any other phases, which demonstrates that the PB precursor is completely transformed into maghemite after the annealing process.The XRD of the sponge-like γ-Fe2O3 nanocubes shows a weak intensity and broad reflection, which is consistent with their assembly of many small nanoparticles [20].