Controlled synthesis and characterization of iron oxide nanostructures with potential applications for gas sensors and the environment

Abstract

In the present research, large iron oxide microparticles with large sizes in the range of 1–5 μm have been facilely synthesized by a modified polyol method with NaBH₄ as a versatile strong reducing agent. We found that the highly homogeneous iron oxide microparticles' novel structure is the best pure crystal phase of α-Fe₂O₃ in terms of polyhedral morphology and shape in existence. There are no diffraction peaks of other crystal phases from impurities in α-Fe₂O₃ microparticle products in the crystal growth. Interestingly, a new method of heat treatment or atomic surface deformation allowed for the discovery of a new large α-Fe₂O₃ structure with controlled specific α-Fe₂O₃ oxide grains in the crystal structure. The severe surface deformation of sharp, polyhedral, large α-Fe₂O₃ microparticles under a sintering treatment was found to give un-sharp, polyhedral large α-Fe₂O₃ microparticles with specific grains and boundaries.

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1. Introduction

Typically, magnetic nanoparticles (MNs) such as nickel (Ni) and cobalt (Co) are used in catalysis, biology and medicine. Iron (Fe) based nanoparticles, and magnetic Fe based nanostructures offer increasingly excellent performance in the aforementioned applications due to their great magnetic properties,^1–4 such as their previously undiscovered ferromagnetic, antiferromagnetic, and ferrimagnetic magnetism. So far, the crystal nanostructures of various Fe oxides and magnetic Fe oxide nanoparticles have the same specific crystal nanostructures as magnetite (Fe₃O₄) and maghemite nanoparticles (γ-Fe₂O₃) and hematite (α-Fe₂O₃).^5–10 Recently, scientists and researchers have discovered important practical applications of α-Fe₂O₃ nanoparticles and nano-structures in lithium ion batteries, energy storage and materials for various gas sensors. In recent work, the addition of metal nanoparticles has led to better sensitivity and better selectivity in various oxide sensor devices.^11–15,46 In addition, a reduced graphene oxide platelet/Fe₂O₃ nanoparticle composite can be used in the anode for Li-ion batteries with high-performance as well as high durability and stability.¹⁶ In most cases, the characteristics such as size, shape, morphology, and particle composition of the Fe based nanoparticles need to be controlled with the addition of various metals: Ni, Co, Zn, Cu, etc. For example, the special nanostructures of modified MFe₂O₄ ferrite nanoparticles (M = Co, Ni, Zn), or magnetic multimetal oxide nanoparticles, can be potentially used in magnetic resonance imaging (MRI) technology. During recent years, super-paramagnetic iron oxide nanoparticles (SPIONs), such as superparamagnetic magnetite Fe₃O₄ nanoparticles and maghemite γ-Fe₂O₃ nanoparticles under size and morphology control, have been used in drug delivery vehicles.^17–20 At present, magnetic iron metal and iron oxide nanoparticles also have important applications in experimental catalysts, high contrast agents for magnetic resonance imaging (MRI), and therapeutic agents for the treatments of dangerous tumors and cancers.^17–24 Beside the interesting magnetic properties of iron alloy and iron oxide based nanostructures and nanomaterials, some of the most important characteristics are that iron alloy and iron oxide based nanostructures and nanomaterials have ultra-high durability and stability. Magnetite Fe₃O₄ nanoparticles can be used for magnetic hyperthermia, high contrast agents and MRI technology, and targeted drug delivery vehicles.^17–24 Due to their high biocompatibility and relatively low toxicity in animals and humans, maghemite γ-Fe₂O₃ nanoparticles can also offer great applications in biomedicine. So far, various synthesis and preparation methods of MNs have been utilized to control the size characteristics, the surface shape and morphology characteristics, and the internal characteristics (large and small crystal structures, large and small crystal surfaces, as well as low and high porosity etc.).^21–24

In this research, we present a novel synthesis process to control the size, shape and morphology of large polyhedral Fe oxide microparticles with α-Fe₂O₃ structure in the range of 1–5 μm. Herein, we have successfully used a modified polyol method with the addition of an extra amount of NaBH₄ in ethylene glycol (EG).

2. Experimental section

2.1. Chemical

For the chemical synthesis processes to make the pure α-Fe₂O₃ oxide nanoparticles, we used chemicals from Aldrich, Sigma-Aldrich and Wako. These include poly(vinylpyrrolidone) (PVP) (FW = 55 000) as a good protective agent (Aldrich) and FeCl₃·4H₂O (Aldrich, no. 451649 and 236489). In particular, sodium borohydride (NaBH₄) was used as a strong reducing agent for the synthesis of α-Fe₂O₃ microparticles, ethylene glycol (EG) from Aldrich was used as both a solvent and a weak reducing agent, and ethanol, acetone, and hexane were procured from Aldrich or Japanese companies. Here, all chemicals were of standard analytical grade and were used without any further purification. Deionized and distilled water with high purity prepared by a Milli-Pore purification system available in our laboratory was used for the washing and cleaning of containers during experimental synthesis processes.

2.2. Synthesis of α-Fe₂O₃ oxide microparticles

Briefly, 3 mL of EG, 1.5 mL of 0.0625 M FeCl₃, 3 mL of 0.375 M PVP, and 0.028 g NaBH₄ were used for making Sample 1 in a typical process of the controlled synthesis of the large polyhedral α-Fe₂O₃ oxide microparticles. The details and steps of the known process procedures were previously presented.^19,25 In general, FeCl₃ was completely reduced with the extra amount of NaBH₄ in EG at 200–230 °C for 30 min. As a result, black solutions containing polyhedral α-Fe₂O₃ oxide microparticles with large sizes, shapes and morphologies were obtained as the final product. They have a particle size of 1–5 μm with a polyhedral shape and morphology. Similarly to Sample 1, we used the same processes for Sample 2 and Sample 3 for XRD and SEM measurements. Sample 1 was also used for XRD and SEM measurement and analysis. Sample 2 was heated at 500 °C for 1 h for SEM measurement and analysis. Sample 3 was heated at 900 °C for 1 h for SEM analysis. All of the experimental conditions for making Samples 1–3 corresponding to the XRD results are presented in Table 1 and section 3 (Results and discussion).

Table 1 Experimental conditions for the preparation of the samples, and their α-Fe₂O₃ crystal structure

Sample	Chemicals and precursor solution	Experimental	Heat treatment	Crystal structure by XRD
Sample 1	• 3 mL of EG	• 200–230 °C for 30 min	• Drying in air or mixture of oxygen–air	α-Fe2O3 (PDF-89-0597)
	• 1.5 mL of 0.0625 M FeCl3
	• 3 mL of 0.375 M PVP	• Pumping method of stock solutions
	• 0.028 g NaBH4
Sample 2	• 3 mL of EG	• 200–230 °C for 30 min	• Heated at 500 °C in air or mixture of oxygen–air	α-Fe2O3 (PDF-89-0597)
	• 1.5 mL of 0.0625 M FeCl3
	• 3 mL of 0.375 M PVP
	• 0.028 g NaBH4	• Pumping method of stock solutions
Sample 3	• 3 mL of EG	• 200–230 °C for 30 min	• Heated at 900 °C in air or mixture of oxygen–air	α-Fe2O3 (PDF-89-0597)
	• 1.5 mL of 0.0625 M FeCl3
	• 3 mL of 0.375 M PVP
	• 0.028 g NaBH4	• Pumping method of stock solutions

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2.3. Material characterization

2.3.1. X-ray diffraction method. In the XRD method for crystal analysis, we used the as-prepared products of the black solution containing the PVP-αFe₂O₃ oxide microparticles (Sample 1). The PVP-αFe₂O₃ oxide nanoparticles were washed many times in order to obtain clean α-Fe₂O₃ oxide nanoparticles by our standard procedures with the use of a centrifuge. The black solution of Fe based microparticles was dried in order to leave an Fe based nano-powder on the glass substrate for XRD analysis. The high heat treatment of our samples was carried out in a gas/air flow (20 mL min⁻¹) or a mixture of 10 mL min⁻¹ for O₂, and 10 mL min⁻¹ for air at 500 °C and 900 °C for 1 h in ovens. The X-ray diffraction patterns were recorded by an X-ray diffractometer (Rigaku D/Max 2550V) at 40 kV/200 mA using Cu Kα radiation (1.54056 Å). Finally, only the crystal phase of α-Fe₂O₃ was found in the pure as-prepared α-Fe₂O₃ microparticles.

2.3.2. Scanning electron microscopy. In order to study the size and shape of the as-prepared α-Fe₂O₃ microparticles (Samples 1–3), we used a field emission scanning electron microscope (SEM) (JEOL-JSM-634OF) operated at 5, 10, and 15 kV (5–15 kV), with a probe current around 12 μA. The SEM images of the as-prepared Fe microparticles were focused by using a suitable fine focus level adjustment. To characterize the α-Fe₂O₃ oxide microparticles with very large sizes of 1–5 μm, copper or copper brass grids containing the α-Fe₂O₃ microparticles were maintained under vacuum by using a vacuum cabinet.

3. Results and discussion

Fig. 1 and 2 show the SEM images of the as-prepared large α-Fe₂O₃ oxide microparticles with polyhedral morphologies and shapes of a certain size of about 1–5 μm.^19,20 It should be noted that the as-prepared nanoparticles were observed to have polyhedral morphologies, such as cubes, octahedra, and tetrahedra etc. This is possibly because sodium borohydride (NaBH₄) is a very strong reducing agent, leading to the fast crystal growth of large polyhedral Fe₂O₃ microparticles. Here, the polyhedral α-Fe₂O₃ microparticles have three large crystal surfaces with crystal planes of (100), (011), and (111). They have large homogeneous sizes and sharp, smooth, polyhedral surfaces. In particular, our discovery of a new nano-structure is confirmed from our heat treatment process of the as-prepared Fe oxide microparticles at 900 °C (Fig. 3(b) and (c), S1 and S2 (ESI†)) but no clear and significant structural changes at the surfaces of the large α-Fe₂O₃ microparticles at 500 °C (Fig. 3(a)).

Fig. 1 SEM image of the uniform Fe oxide nanoparticles and large (α)-Fe₂O₃ microparticles synthesized by a modified polyol method (Sample 1).

Fig. 2 (a) SEM image of the large uniform and polyhedral Fe oxide nanoparticles and (α)-Fe₂O₃ microparticles synthesized by a modified polyol method (Sample 1). (b) SEM image of an orthorhombic crystal, and (c) its model.

Fig. 3 (a) SEM images of α-Fe₂O₃ nanoparticles with a sharp polyhedral shape and morphology in a sensor device calcined at 500 °C with no changes in the α-Fe₂O₃ nanostructures (Sample 2). (b) and (c) SEM images of α-Fe₂O₃ nanoparticles with a sharp polyhedral shape and morphology in a sensor device calcined at 900 °C (Sample 3).

Very interestingly, most large polyhedral α-Fe₂O₃ oxide microparticles contained smaller α-Fe₂O₃ nanoparticles in their very large nano-textures. All the large α-Fe₂O₃ crystal surfaces were clearly deformed in the nanoparticle heat treatment at 900 °C. C1 in Fig. 4(c) shows the new micro-nano structure of one as-prepared large polyhedral Fe based microparticle after heat treatment at about 900 °C. Although the particle size of this large microparticle was not significantly changed, all the large crystal surfaces were significantly changed into the new micro-nano surface structures. Fig. 4(d) and (e) present two models for possibility of very slow crack propagation along the grain boundaries for intergranular fractures (red and blue lines). 35 ± 1 grains or 35 ± 1 small or intermediate nanoparticles were observed in the large crystal surfaces or the crystal planes. The various sizes of the nanoparticles were about 100–300 nm for the small α-Fe₂O₃ nanoparticles, and about 300–500 nm for the intermediate α-Fe₂O₃ nanoparticles, compared to the very large α-Fe₂O₃ microparticle of about 3 μm. The arrangements in order show that there are six nanoparticles or grains (6 ± 1) in one row, and six nanoparticles or grains (6 ± 1) in one column. Therefore, there are 35 ± 2 nanoparticles as a raw estimation. Thus, we estimate there are about 266 to 280 small nanoparticles (or grains) in the large microparticle. The oxide grains are strongly connected and linked in one large particle as a three dimensional (3D) microparticle. In addition, all the Fe₂O₃ grains clearly exhibited curvature on their surfaces. Moreover, each α-Fe₂O₃ grain is classified as a single α-Fe₂O₃ crystal by the XRD method. However, the boundaries between the grains are also clearly distinguished.

Fig. 4 Plastic and surface deformation: (a) the various grains and boundaries in one two dimensional (2D) large crystal surface of the grains with round boundaries of large α-Fe₂O₃ nanoparticles, as in SEM image C1; (b) the configuration of the boundaries in the 2D system observed in SEM image C3; (c)–(e) proposed models for two cases: (c) no heat treatment; (d) and (e) with heat treatment. (d) Large 3D particle made up of grains with un-sharp curvature boundaries (our results); (e) or with sharp boundaries (our proposal) as estimated in C3.

The large oxide grains have two categories in normal and abnormal grain-growth regimes.^26–32 Certain grains in our new micro-nano structures have both small and large sizes with respect to the abnormal grain-growth regimes. They show un-sharp curvature boundaries (our results) or sharp boundaries (our proposed model). Thus, there are important mechanisms and processes of recovery, re-crystallization, and development of grain growth in every large α-Fe₂O₃ crystal. Surprisingly, the new 3D structures can be considered to be excellent evidence of 3D grain growth without particle collapse or cracking. This is of high importance, it is considered to be the “ideal” grain growth in materials optimization,^29–31 as well as the recrystallization and annealing phenomena of the nanoparticles being of both technological importance and scientific interest at present.³² At present, 3D metal and oxide structures have been simulated under different grain growth regimes, but experimental evidence has not been shown in a micro- or nano-system. Thus, our results are an interesting discovery regarding the structure and synthesis design of new micro-nano sized structures. Here, each large α-Fe₂O₃ microparticle became an oxide nanograin system. Clear and complete models of micro-nano surfaces and boundaries of nano-structures were previously proposed for heat treatments at high temperature.^26–28,30 The important properties of nano-materials and nano-structures involved in the grain and boundary structure were predicted by modeling and simulation.²⁹

At present, the creation of homogeneous and fine oxide grains in very large oxide microparticles is a big challenge for scientists. From our proposed models in Fig. S2(e) and (f) (ESI†) and Fig. 4, the oxide grains have created large crystal surfaces with various different degrees of concave or convex curvature and roughness. In fact, the oxide grains can be split in two categories, the coarse-grain forms and the fine-grain forms. These are very crucial to predict the properties of engineered nanostructures in both theory and practice.^26–35 Here, we have appropriately selected two temperature points for our method of nanoparticle heat treatment to produce nanoparticles with an α-Fe₂O₃ structure the same as the well-known α-FeC equilibrium diagram from 500 to 910 °C.^33,34 This is an interesting finding for making α-Fe₂O₃ microparticles with deformed surface states. For a given method of nanoparticle heat treatment, it is possibly true that the various pure structures of metal, alloy, and oxide nanoparticles are the same as their equilibrium diagrams in metallurgy. After annealing, there is the appearance of small and large α-Fe₂O₃ grains in the large crystal surfaces because of renucleation and recrystallization processes. The severe deformation of the large, flat, and smooth crystal surfaces into concave, convex, rough, and distorted crystal surfaces of α-Fe₂O₃ microparticles is very crucial to achieve new nano-textures (Fig. 3(b) and (c), S1 and S2 (ESI†), and Fig. 4 with models). These Fe₂O₃ structures may be very stiff and permanent after nanoparticle heat treatment in the 500–900 °C range investigated. We suggest that there was plastic deformation in the α-Fe₂O₃ crystal surfaces of every Fe₂O₃ microparticle, but also elastic deformation in the α-Fe₂O₃ microparticles with no significant changes in the particle size distribution. Thus, an appropriate annealing process can control the grain sizes and boundaries among the interfaces of the grains.

The phenomenon of grain growth has been explained even for metals, alloys and oxides.^26–33 The plastic and surface deformation mechanisms and stress–strain behaviors have generally led to various methods to strengthen and regulate the mechanical characteristics of steel (FeC) or ferrite, which are important in metallurgy.^29,33 These allow new methods of nanoparticle heat treatment for engineered nanoparticles. So far, most annealing and heat treatments have generally led to new structures of metals, alloys, glasses, ceramics and oxides with interesting new (and some already known) discoveries such as higher strength, better toughness, higher durability and stability through small grains, creep resistance and reduction through large grains.³² These are due to the special characteristics of both small grain and large grain systems. Plastic and super-plastic deformations were characterized in FeC, Fe oxides, and Fe alloys during steel heat treatment.^26,33 However, we also suggest that the curvature boundaries between the grains were possibly due to both plastic and elastic deformation, permanent plastic deformation of the external surface and internal structure, and elastic deformation where the particle size was retained in the elastic recovery of particle shape. Furthermore, α-Fe₂O₃ microparticles with sharp and straight edges (Fig. 1, 2, and 3a) were observed in both plastic and elastic deformation processes, but higher localized plastic deformation on all surfaces was also observed (Fig. 3(b) and (c) and models (a), (b), (d), and (e) in Fig. 4). Our interesting evidence regarding plastic and elastic atomic surface deformation and grain growth is of importance in simulation and modeling at present.

Although the grain boundaries of the α-Fe₂O₃ grains are clearly distinguishable on the surface of one large microparticle, it is not difficult to observe possible cracking and propagation cracking in the prepared microparticles. The good recovery characteristics of the large shape and morphology of α-Fe₂O₃ microparticles was shown after strong surface deformation and plastic deformation in the formed α-Fe₂O₃ grains. Thus, we can expect that the boundaries of the fine grains in one such large microparticle can be controlled by the nanoparticle sintering process. This also illustrates the well ordered arrangement of the small and intermediate α-Fe₂O₃ oxide nanoparticles with specific fine grain boundaries, and also within the oxide grains on the surfaces annealed at 900 °C for 1 h that were evidenced by one as-prepared large α-Fe₂O₃ oxide microparticle (or large α-Fe₂O₃ oxide crystal). The boundaries of α-Fe₂O₃ oxide grains and α-Fe₂O₃ oxide domains were also observed on the large crystal surfaces and we predict their existence inside the internal structure of the pure large polyhedral α-Fe₂O₃ oxide microparticles. There are two small holes on the crystal surface, which are observed because there is a certain degree of the porosity in the large microparticle. Fig. S2(b) (ESI†) shows the micro-nano structure of the as-prepared large α-Fe₂O₃ oxide microparticles of about 3 μm with an orthorhombic shape and morphology. The crystal α-Fe₂O₃ oxide grains and grain boundaries of finite sizes can be clearly distinguished. The small crystal grains are normally located at the corners of the intermediate α-Fe₂O₃ oxide grains. In the as-prepared large α-Fe₂O₃ oxide microparticles, their shapes and morphologies have flat and smooth crystal surfaces. After an appropriate heat treatment, the Fe based grains (or Fe based nanocrystals) appeared on the surfaces. Each large crystal surface will become more coarse because of the concave and convex local regions of α-Fe₂O₃ oxide grains that are caused. So far, our samples are considered to be the best examples of very large α-Fe₂O₃ microparticles with oxide grains, oxide grain domains, and sharp and un-sharp boundaries for future studies in this field.

In addition, Fig. S2(c) and (d) (ESI†) show interesting α-Fe₂O₃ oxide grains in the forms of micro and nano oxide crystals. Crystallization and re-crystallization transformations were observed in the deformation of sharp, flat, and smooth large oxide crystals into un-sharp, distorted, rough, convex and concave large oxide crystals with specific oxide grains. Our latest results regarding the pure α-Fe₂O₃ oxide microstructures and nanostructures with respect to the structural phase transitions and mechanisms are the most important examples at present. Thus, the high roughness of the small and large α-Fe₂O₃ oxide crystals was caused during heat treatment at 900 °C. The α-Fe₂O₃ oxide grains also appeared at all of the six large crystal surfaces of the as-prepared large polyhedral α-Fe₂O₃ oxide microparticles. The most important thing is that there are no collapses of the nano- and micro-structures of the large as-prepared α-Fe₂O₃ oxide microparticles in the range between 500 °C and 900 °C. Therefore, the important issues of achieving high stability and durability are possibly dealt with by using higher heat treatments for microsystems and nanosystems. Because the α-Fe₂O₃ nanostructures have very large sizes, we did not characterize them by TEM measurements in our subsequent further investigation. As a facile method for the controlled synthesis of Fe oxide based microparticles, we suggest that NaBH₄ can be successfully used for the very strong reduction of Fe precursors in various common solvents, such as EG, alcohols and water. A moderate addition of control agents such as NaOH, NH₄OH, NaI, HCl etc. can be carried out during the controlled synthesis. By this method, NaBH₄ can usually be used in excess amounts for the full reduction of metal precursors.^17–24 Here, we suggest that the fast formation of very small Fe metal nanoparticles during the synthetic process occurred at 200–230 °C.¹⁹ We also suggest that, first, Fe nanoparticles were formed by the full reduction of Fe precursors such as FeCl₃·xH₂O (or FeCl₂·xH₂O) with the addition of NaBH₄. Then, the surfaces of the Fe nanoparticles are oxidized in the initial formation of the Fe oxide shells. According to the synthesis time, the formation of metal oxide shell can be understood to be a gradual and slow oxidation. In general, this can lead to the complete internal structure of the prepared microparticles being completely oxidised. As crucial evidence for this, structural transformations among FeO (Wustite), ε-Fe₂O₃, Fe₃O₄, γ-Fe₂O₃, and α-Fe₂O₃ can be carried out through suitable heat treatments.^9,36 However, there have been many considerable difficulties encountered when scientists have tried to make large nano-textures from smaller nanoparticles by self-assembly methods. It is clear that our as-prepared products, the Fe based nanoparticles with the controlled homogeneous features of size, shape, and morphology, can be used in order to meet the very high demands of sensor materials.

The final formation of the pure α-Fe₂O₃ structure in EG was done with the long-term stabilization of various PVP polymers by a facile method with the use of NaBH₄ as a strong reducing agent for the Fe precursors. The successfully controlled synthesis of magnetite nanoparticles has been shown in some recent works. These works also tried to focus on the facile synthesis of the crystal phases of MNs, such as Fe₃O₄, γ-Fe₂O₃, and α-Fe₂O₃ nanoparticles,³⁷ and on producing α-Fe₂O₃ nanoparticles for promising applications in lithium ion batteries,³⁸ as well as nanostructured Fe oxide materials for much more durable and stable advanced energy conversion and storage devices, such as nano-sized transition-metal oxides as negative-electrode materials for high performance lithium-ion batteries.^39,40 In addition, we have evaluated the crystal structure of the as-prepared samples of large Fe₂O₃ microparticles. Fig. 5 shows the typical XRD patterns of one dried sample, and two samples after calcination at 500 and 900 °C. Here, the pure α-Fe₂O₃ oxide microparticles display a rhombohedral crystal structure. The prepared large microparticles analysed by the XRD method had sharp and narrow diffraction peaks, which is evidence of the high crystallization of the pure α-Fe₂O₃ crystal structure produced by the modified polyol method with NaBH₄. The α-Fe₂O₃ crystal structure (hematite system) belonging to the crystallographic space group R3̄C[167] has lattice constants (a,b,c) equal to 5.039 nm, 5.039 nm, and 13.770 nm, respectively, with a ratio of c/a = 2.733 (ICDD/JCPDS PDF-89-0597) using the JADE software (Materials Data) for XRD pattern processing and MDI materials data. Table 2 lists the powder pattern indexing of Samples 1–3. For Sample 1 (c/a = 2.730815) and Sample 2 (c/a = 2.733914), the c/a ratios are the same as that of a standard sample (ICDD/JCPDS PDF-89-0597). For Sample 3, the c/a ratio of our sample is equal to 2.726895 at 900 °C, which is a little smaller than that of the standard sample.

Fig. 5 XRD powder diffraction patterns of the samples of the Fe based nanoparticles prepared at various temperatures: (a) dried sample (Sample 1), (b) 500 °C (Sample 2), and (b) 900 °C (Sample 3). The stability and durability of the α-Fe₂O₃ nanostructure during nanoparticle heat treatment at high temperatures were identified.

Table 2 The indexing of powder diffraction patterns of α-Fe₂O₃ oxide

Sample	a (nm)	b (nm)	c (nm)	c/a	Space group	ICDD/JCPDS-PDF
Sample 1	5.056	5.056	13.807	2.730815	R3̄C(161)	ICDD/JCPDS PDF-89-0597
Sample 2	5.051	5.051	13.809	2.733914	R3̄C(161)	ICDD/JCPDS PDF-89-0597
Sample 3	5.053	5.053	13.779	2.726895	R3̄C(161)	ICDD/JCPDS PDF-89-0597

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The narrow and sharp peaks from the XRD show the very high crystallization of large polyhedral α-Fe₂O₃ microparticles without any mixture of other phases, such as FeO (Wustite), α-FeOOH, ε-Fe₂O₃, Fe₃O₄, γ-Fe₂O₃ etc. As shown in Fig. 5, the calcination of samples at 500 and 900 °C also resulted in the formation of α-Fe₂O₃ (PDF-89-0597) crystal phase structure. Importantly, high crystallization of the pure rhombohedral hematite α-Fe₂O₃ was obtained between 500 °C and 900 °C. As shown in Fig. 2(a)–(c) and S1 and S2 (ESI†), the degree of densification of the prepared oxide microparticles at 500 °C is a little smaller than that of the prepared oxide microparticles at 900 °C. Thus, the as-prepared microparticles are single crystals or monocrystallites because they show a sharp polyhedral shape and morphologies that are continuous and unbroken to the edges and corners of the microparticles in Sample 1, without the existence of any grains or boundaries. All the microparticles have sharp, smooth, and flat surfaces, as well as sharp right-angled edges and corners. In the α-Fe₂O₃ microparticles after heat treatment at 500 °C (Sample 2) and 900 °C (Sample 3), the structures were considerably changed. The microparticles were changed in their deformation at 500 °C but show no significant changes in their size and shape in Fig. 3(a). They gradually became polycrystals under high heat treatment. At an annealing temperature of about 900 °C, the microparticles were changed due to their very significant deformations, with the appearance of grains and boundaries between them. The grains have particle sizes in both the microsize range and the nanosize range. Additionally, the polycrystalline α-Fe₂O₃ microparticles or α-Fe₂O₃ polycrystallites have various crystallites of varying size and orientation. Each microparticle became a polycrystal or a polycrystallite. Here, each microparticle has much smaller α-Fe₂O₃ microparticles and α-Fe₂O₃ nanoparticles, or so-called grains. However, each grain can be considered a single crystal or single crystallite because of their continuous shape and morphology.

At present, the α-Fe₂O₃ oxide based nanostructures have special significance for ecosystems and environmental applications,⁴¹ α-Fe₂O₃ tetradecahedra can be used in gas sensing by a facile hydrothermal method with the use of K₄Fe(CN)₆·3H₂O, sodium carboxymethyl cellulose solution, PVP, and N₂H₄·3H₂O solution at room temperature at 200 °C for 6 h,⁴² and other α-Fe₂O₃ structures also have practical applications in gas sensing.^42–45 Interestingly, the important roles of the metal or alloy or oxide grains were known in the significant reduction of lattice thermal conductivity for an enhanced ZT applied in new thermal nano-structured materials.³⁰ The topic of nanoparticle heat treatment will be an important subject for scientists, and specialists. In addition, various methods of nanoparticle heat treatment need to be significantly considered in the development of new nanomaterials with grain and boundary structures.⁴⁵

4. Conclusion

In this research, we have successfully prepared large α-Fe₂O₃ microparticles by a modified polyol method with NaBH₄. A new α-Fe₂O₃ microparticle with α-Fe₂O₃ grains was discovered by chance following external surface deformation, and did not show structural collapse after nanoparticle heat treatment at 900 °C. This new structure of α-Fe₂O₃ microparticles containing micro-grains, nano-grains and boundaries would potentially exhibit good properties in future gas sensors.

Acknowledgements

In this research, we are very grateful to the precious support from the Structural Ceramics Engineering Center, Shanghai Institute of Ceramics, Chinese Academy of Science, Dingxi Road 1295, Shanghai 200050, China. This study was also supported in part by a fund from the National Natural Science Foundation of China (NSFC, contract nos. 51071167 and 51102266). Lastly, we would like to thank the significant efforts of Mr Michael Ignatowich (PhD student), California Institute of Technology for checking and editing the manuscript.

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Footnote

† Electronic supplementary information (ESI) available: SEM images of the very large α-Fe₂O₃ microparticles produced with a modified polyol method with NaBH₄ at 200–300 C for 30 min. SEM images of the very large α-Fe₂O₃ microparticles with surface deformations and the grains under the same conditions and microparticle heat treatment. See DOI: 10.1039/c3ra45925j

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