Crystal growth control of rod-shaped ε-Fe2O3 nanocrystals

Herein we report crystal growth control of rod-shaped ε-Fe2O3 nanocrystals by developing a synthesis based on the sol–gel technique using β-FeO(OH) as a seed in the presence of a barium cation. ε-Fe2O3 nanocrystals are obtained over a wide calcination temperature range between 800 °C and 1000 °C. A low calcination temperature (800 °C) provides an almost cubic rectangular-shaped ε-Fe2O3 nanocrystal with an aspect ratio of 1.4, whereas a high calcination temperature (1000 °C) provides an elongated rod-shaped ε-Fe2O3 nanocrystal with an aspect ratio of 3.3. Such systematic anisotropic growth of ε-Fe2O3 is achieved due to the wide calcination temperature in the presence of barium cations. The surface energy and the anisotropic adsorption of barium on the surface of ε-Fe2O3 can explain the anisotropic crystal growth of rod-shaped ε-Fe2O3 along the crystallographic a-axis. The present work may provide important knowledge about how to control the anisotropic crystal shape of nanomaterials.


Introduction
Ferrite magnets are ubiquitous functional materials for industry due to their ferromagnetic properties. [1][2][3][4][5][6] Among ferrite magnets, gamma-diiron trioxide (g-Fe 2 O 3 ) has been extensively used due to its so magnetic functionalities and chemical stability. [7][8][9][10] On the other hand, epsilon-diiron trioxide (3-Fe 2 O 3 ) has drawn increasing attention in recent years. [11][12][13][14][15][16] In 2004, a single phase of 3-Fe 2 O 3 , which was articially synthesized via a nanoscale synthesis, displayed a huge magnetic coercive eld over 20 kOe at room temperature. 11 Since then, various fundamental studies have been reported. [17][18][19][20][21][22][23] Furthermore, practical applications such as high-density magnetic recordings and high-frequency electromagnetic (EM) wave absorption have been considered. [24][25][26][27] From a synthetic viewpoint, different methods for 3-Fe 2 O 3 have been reported, including the reversemicelle and sol-gel combination, mesoporous SiO 2 template, pulsed laser deposition (PLD), and chemical vapor deposition (CVD). 17,[28][29][30][31][32][33][34] The 3-Fe 2 O 3 phase is only formed in the nanosize region. Typically, 3-Fe 2 O 3 nanoparticles have a spherical shape, but rod-shaped crystals can appear under specic conditions. 12 Rod-shaped 3-Fe 2 O 3 has received attention from the viewpoint of applications as oriented bers and probes for magnetic force microscopy. 12,35 For example, small rod-shaped 3-Fe 2 O 3 crystals less than a hundred nanometers are attractive for an oriented optical material because light scattering is eliminated. By contrast, elongated rod-shaped 3-Fe 2 O 3 crystals are desirable as a probe in magnetic force microscopy. Hence, controlling the particle size of rod-shaped 3-Fe 2 O 3 nanocrystals is an important issue. To date, rod-shaped 3-Fe 2 O 3 nanocrystals have only been obtained using a combination of reverse-micelle and sol-gel techniques. 12 However, this method produces 3-Fe 2 O 3 in a very limited calcination temperature region (i.e., 960-1040 C), highlighting the difficulty in controlling the particle size. A method to control the particle size of rod-shaped 3-Fe 2 O 3 has yet to be developed. In this work, we report a synthetic method to prepare rod-shaped 3-Fe 2 O 3 nanocrystals with a wide range of calcination temperatures to control the particle size and investigate a long-standing problem: the mechanism of anisotropic crystal growth of rod-shaped 3-Fe 2 O 3 .
Here, we report the synthesis of rod-shaped 3-Fe 2 O 3 nanocrystals based on the sol-gel technique using b-FeO(OH) as a seed with a SiO 2 matrix. X-ray powder diffraction (XRPD) and Rietveld analyses indicate that 3-Fe 2 O 3 can be obtained by calcination over a wide temperature range (i.e., 800-1000 C). Transmission Electron Microscopy (TEM) shows that the size of the rod-shaped crystals is well controlled over a wide range of calcination temperatures. The anisotropic crystal growth of rod-shaped 3-Fe 2 O 3 can be explained using the surface energy and anisotropic adsorption of barium on the crystal surface. Fig. 1 schematically illustrates the synthesis of rod-shaped 3-Fe 2 O 3 nanocrystals based on the sol-gel technique using b-FeO(OH) as a seed with a SiO 2 matrix. Barium nitrate (Ba(NO 3 ) 2 ) (0.5 g) was added to water-dispersible b-FeO(OH) (4 g) solution (420 mL) (Taki Chemical; Fe-C10). An aqueous solution of 25% ammonia (19.2 mL) was added to a b-FeO(OH) dispersed solution. The mixture was stirred at 50 C for 30 min. Then tetraethoxysilane (TEOS, Si(C 2 H 5 O) 4 ) (24 mL) was added, and the solution was stirred at 50 C for 20 h. The resultant orange gel was collected by centrifugation, washed with water, and dried at 60 C for 1 day. The obtained orange powder was calcinated at 800-1000 C for 4 h in air. The calcinated powder was etched with a NaOH aqueous solution at 70 C to remove the SiO 2 matrix that covered the iron oxide nanocrystals. The etched powder was washed with hydrochloric acid. In this work, we prepared 7 samples calcinated at different temperatures: 800 C (1), 850 C (2), 875 C (3), 900 C (4), 950 C (5), 975 C (6), and 1000 C (7).

Physical measurements
XRPD patterns were measured by a Rigaku Ultima IV using Cu Ka (l ¼ 1.5418 A). Rietveld analyses for the XRPD patterns were performed using Rigaku PDXL soware. TEM images were acquired using a JEOL JEM 2000EX. TEM energy-dispersive Xray spectroscopy (TEM-EDX) was performed using a JEOL JEM 2010F. Raman spectra were measured by NRS-5500 Laser Raman Spectrometer (JASCO Corporation, Japan). Magnetic measurements were performed using a Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer.

Results and discussion
Crystal structure, morphology, and magnetic properties of the nanocrystals Fig. 2 shows the XRPD patterns with Rietveld analyses of the obtained samples. Sample 1 contains 97.2% of 3-Fe 2 O 3 with an orthorhombic crystal structure in the Pna2 1 space group and lattice constants of a ¼ 5.064(8) A, b ¼ 8.729 (14) A, and c ¼ 9.610(9) A ( Fig. 2b and S1, Table S1 †). In addition, a slight amount of g-Fe 2 O 3 with a cubic crystal structure in the Fd 3m  space group and a lattice constant of a ¼ 8.299(4) A) is present. Samples 2-6 consist of 3-Fe 2 O 3 as a single phase of iron oxide ( Fig. 2c-g). In sample 7, 3-Fe 2 O 3 is the dominant phase but a slight amount of a-Fe 2 O 3 with a hexagonal crystal structure in the R 3c space group and lattice constants of a ¼ 5.0353(9) A and c ¼ 13.752(3) A is generated as a nominal phase (Fig. 2h). Fig. S2 † shows the phase fraction versus temperature in a phase diagram for iron oxide. A single-phase of 3-Fe 2 O 3 is obtained in a wide calcination temperature range. In the low calcination temperature region, a slight amount of g-Fe 2 O 3 appears. On the other hand, a-Fe 2 O 3 is generated as a nominal phase in the high calcination temperature region. This phase diagram is consistent with the previous study, which reported that 3-Fe 2 O 3 is generated as a stable intermediate phase between g-Fe 2 O 3 and a-Fe 2 O 3 . 29 Fig. 3 and Table S2 † show the TEM images of the samples and the obtained particle size distributions. In sample 1, the nanoparticles have an almost cubic shape with average sizes of 11.9 AE 3.5 nm for the long axis and 8.4 AE 1.9 nm for the short axis. The particle size distribution follows a log-normal distribution (Fig. S3 †). The aspect ratio (the long axis to the short axis) of 1 is 1.4. In sample 2, the average sizes of the long and short axes are 22.2 AE 8.1 nm and 12.5 AE 3.2 nm, respectively. The aspect ratio is 1.8. As the calcination temperature increases, in samples 3-7, the size of the long axis abruptly increases while the size of the short axis gradually increases (i.e., the aspect ratio increases as shown in Fig. 3h). In sample 7, the nanoparticle has an elongated shape, and the average sizes of the long and short axes are 93.2 AE 105 nm and 28.2 AE 12.2 nm, respectively. This gives an aspect ratio of 3.3. As the calcination temperature increases, the long axis grows, leading to an elongated rod-shape. In the case of the reported spherical-shaped particles, changing the calcination temperature controls the size. For example, the diameter is 5.5 AE 1.6 nm at 902 C, 5.6 AE 1.6 nm at 951 C, and 7.8 AE 2.7 nm at 1002 C. 32 The particle sizes are considerably smaller than the rod-shaped 3-Fe 2 O 3 synthesized by the present method. This difference indicates that barium promotes crystal growth of 3-Fe 2 O 3 . Fig. S4 † shows the magnetic hysteresis loops at room temperature up to 7 Tesla. The magnetic coercive eld (H c ) value increases as the calcination temperature increases (i.e., the particle size increases). This observed trend corresponds to the reported trend where H c increases as the particle size increases because the superparamagnetic effect operates in a small particle region. 32,36,37 The magnetization values for 1-5 become higher as the calcination temperature increases. However, in 6 and 7, the magnetization values are low, even though the calcination temperature is high. One possible reason for the low magnetization is the formation of a slight amount of a-Fe 2 O 3 (Fig. S5 †), which is not observed in the XRD measurement. 38 Another reason, it is speculated that when the particle shape becomes elongated into a rod, the particles are likely to overlap with each other due to the magnetic force. The rod-shaped 3-Fe 2 O 3 particle is conrmed to be a single domain magnet. 35 In such a case, the magnetic moments of 3-Fe 2 O 3 particles may cancel each other. Consequently, the net total magnetization becomes smaller.

The mechanism of anisotropic crystal growth
Next, the origin of high anisotropic crystal growth of rod-shaped 3-Fe 2 O 3 is examined. The TEM images show that the 3-Fe 2 O 3 particle is a single crystal and the longitudinal direction of rodshaped 3-Fe 2 O 3 is the crystallographic a-axis (i.e., the [100] direction) (Fig. 4a). 39 Hence, rod-shaped 3-Fe 2 O 3 nanocrystal is This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 39611-39616 | 39613 a rectangular-type rod with the long axis in the [100] direction (Fig. S6 †).
To understand the anisotropic crystal growth of the nano-rod along the [100] direction, we calculated the surface energies of the (hkl) planes, a hkl , based on the binding energy of each surface. a hkl is expressed by a hkl ¼ SE hkl /S hkl , where SE hkl is the sum of the binding energies of the broken bonds at the (hkl) plane and S hkl is the area of the lattice plane. 40 Assuming equal binding energies between all Fe and O sites, the magnitude of the surface energies for typical lattice planes are calculated as: 19.2 nm À2 (100), 18.7 nm À2 (010), 13.4 nm À2 (001), 19.0 nm À2 (110), 11.9 nm À2 (011), 19.7 nm À2 (101), and 17.8 nm À2 (111) (Table S3 †). The (100), (101), and (111) planes possess high surface energies (Fig. 5), whereas the (001) and (011) planes show low surface energies (Fig. S7 †). Hence, the (100), (101), and (111) planes are candidates for the growth crystallographic planes.
Next, we focused on the barium adsorption effect. Fig. 4b shows the results of EDX measurement in TEM to investigate the distribution of the barium around the particles. The green cylinders indicate the areas of the EDX measurements. The calculated molar ratios of barium for each area with respect to the Fe 2 O 3 formula indicate larger values at the longitudinal side of the rods compared to the edge of the short axis. These results suggest that a larger amount of barium is adsorbed on the longitudinal side of the rod than on the edge surface of the growth direction. We considered the mechanism of anisotropic crystal growth of 3-Fe 2 O 3 from the viewpoint of anisotropic adsorption of barium on the surface of 3-Fe 2 O 3 particles. In the calcination process, melting SiO 2 matrix (Si 4+ and O 2À ) and Ba + ion exist around 3-Fe 2 O 3 particles. Since Ba + ion takes an octahedral BaO 6 coordination geometry with oxygen, 41,42 barium should be adsorbed on the surface of 3-Fe 2 O 3 in the form of pseudo-octahedral BaO 6 . On the other hand, on the surface of 3-Fe 2 O 3 particles, there are two kinds of broken bonds (i.e., broken octahedral FeO 6 and broken tetrahedral FeO 4 ). Since BaO 6 should be adsorbed at the octahedral FeO 6 site rather than at the tetrahedral FeO 4 site, a surface containing a large amount of FeO 6 -broken bonds should be covered with adsorbed barium, suppressing crystal growth (i.e., the growth surface should be the one with a small amount of FeO 6 -broken bonds). We estimated the number of FeO 6 -broken bonds per area, N hkl . Fig. 5 and

Conclusions
Rod-shaped 3-Fe 2 O 3 nanocrystals are synthesized based on the sol-gel technique using b-FeO(OH) as a seed in the presence of a barium cation. In this method, changing the calcination temperature can control the size of rod-shaped 3-Fe 2 O 3 nanocrystals. A low calcination temperature provides almost cubic rectangular-shaped 3-Fe 2 O 3 nanocrystals, whereas a high calcination temperature provides elongated rod-shaped 3-Fe 2 O 3 nanocrystals. Although the mechanism of anisotropic crystal growth of rod-shaped 3-Fe 2 O 3 has been a long-standing issue, in this work, the surface energy and the anisotropic adsorption of barium on the surface of 3-Fe 2 O 3 can explain the anisotropic crystal growth of the rod-shaped 3-Fe 2 O 3 along the   (Table S3 †). Red, blue, and gray spheres in the crystal structure indicate the Fe at octahedral site, Fe at tetrahedral site, and O, respectively. Large a and small N values indicate the easy-growth plane.
crystallographic a-axis. These ndings should realize hard magnetic ferrite 3-Fe 2 O 3 nanocrystals for practical applications and provide important knowledge about how to control the anisotropic crystal shape of the nanomaterials.

Conflicts of interest
The authors declare no competing nancial interest.