Phase and morphology controlled in the synthesis of iron oxide particles: dimension-based carbonaceous materials as modifiers

Abstract

Different phases and morphologies of iron oxide and its hybrid nanomaterials can be selectively synthesized by a hydrothermal process in the presence of different dimension carbonaceous material modifiers. This opens up a new method for directly controlled synthesis of metal oxides and their hybrids with carbonaceous materials.

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Iron oxide has continued to draw considerable interest because of its wide range of applications in photo/electro catalysts, magnetic fluids, biomedicine, lithium ion battery components, environmental remediation and more.^1–5 Iron oxide has a number of chemical compositions, each of which exhibits various crystal phases. Particularly, magnetite (Fe₃O₄) and hematite (α-Fe₂O₃) are of interest due to their potential applications in nanotechnology such as use as electrode materials, photo/electro catalysts, and probes for sensing and imaging.^6–8 It is commonly accepted that the performance, such as activity, of nanoparticles strongly depends on their sizes, morphologies and crystal phases.^9–12 Moreover, hybrid nanomaterials (rGO/α-Fe₂O₃, rGO/Fe₃O₄, Fe@Fe₃O₄, and so on), which allow us to discover new properties by changing compositions or combinations of the constituent elements, showed higher performance than just a single component.^13–18 However, the synthesis of these materials with controlled phases and morphologies while forming hybrid materials by a single process has long been a challenge.

Carbonaceous materials including small molecules, 0D fullerene (C₆₀), carbon quantum dots (CQDs), 1D carbon nanofibers (CNFs), carbon nanotubes (CNTs), 2D graphene oxide (GO) or graphene are very active materials due to their unique properties and easy-to-engineer surfaces by cutting.¹⁹ The use of organic templates or additives in directing the synthesis of iron oxide nanomaterials has been studied.^20,21 However, modifiers based on other carbonaceous materials are little known. In this study, we describe a novel strategy by which different dimension carbonaceous materials (sodium citrate, CQDs and GO, both molecular structure and surface functional groups being discrepant) were chosen for synthesizing iron oxide. We speculated that use of a modifier may be an ideal way for controlling morphologies and crystal phases, and forming hybrid materials all in a single process.

Hydrothermal treatment of a dispersion solution made of FeCl₃, diethylene glycol, sodium acetate and given carbonaceous materials (2D GO, 0D CQDs and sodium citrate as a small molecule) at 200 °C for 6 h led to the formation of different crystal phases as shown in Fig. 1. The structural information and phase identification can be obtained from the X-ray diffraction (XRD) patterns. All reflection peaks of the different products can be easily indexed as Fe₃O₄ (JCPDS card no. 19-0629), α-Fe₂O₃ (JCPDS card no. 33-0664), and a mixture of two phases α-Fe₂O₃/Fe₃O₄ (JCPDS card no. 19-0629 and 33-0664), respectively. In addition, the diffraction peaks were sharp and intense indicating that the as-prepared products were of high purity and good crystallinity.

Fig. 1 XRD patterns of the particles obtained using different modifiers.

The morphology of the as-synthesized particles was further investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A phase composition model of particles has been obtained for each sample, estimated from XRD analysis. SEM imaging (Fig. 2a) showed that the particles prepared with GO as modifier consist of platelets approximately 70 nm in diameter. This morphology is consistent with that typically observed in the TEM image in Fig. S1,† where the particles have typical particle size of 70 nm and thin rGO shells that were 4 nm thick. HRTEM was used to obtain more detail of the particle structure as shown in Fig. 2d. The interplanar distance is about 0.369 nm, which corresponds to (012) planes of α-Fe₂O₃. A phase composition model for GO modifier particles showed a single α-Fe₂O₃ phase (Fig. 2a, inset). For 0D CQDs as modifier, SEM analysis (Fig. 2b) found that the particles were of uniform sphere-like morphology with an average size of 80 nm. In more detail, the TEM image in Fig. S2† showed that the nanoparticles are made up of many smaller particles. HRTEM studies (Fig. 2e) revealed that the particles possessed a mixed phase of α-Fe₂O₃ and Fe₃O₄, because the lattice fringes were separated by 0.252 nm and 0.296 nm, which agreed well with the (110) and (220) lattice spacing of α-Fe₂O₃ and Fe₃O₄, respectively. Those results were highly consistent with the XRD characterization. A phase composition model for CQDs modifier particles showed a mixed phase of α-Fe₂O₃ and Fe₃O₄ in a single particle. By using the same method except replacing the modifier with a small molecule, i.e., sodium citrate, a Selaginella uncinata morphology at the micron scale could be observed in the SEM image (Fig. 2c). As can be seen from Fig. 2f, the HRTEM image confirmed that the Selaginella uncinata morphology particles were single crystals of Fe₃O₄ phase. The adjacent lattice fringes were about 0.296 nm, which corresponds to (220) planes of the Fe₃O₄ single crystal. The phase composition model of particles could be considered as Fe₃O₄ single phase, shown in inset image of Fig. 2c. Those results confirmed that different phase and morphology could be synthesized by the choice of an appropriate modifier. The main mechanism was that molecular structure and surface functional groups of the modifier determined the crystal growth direction and crystallization degree. Nitrogen adsorption–desorption isotherms were used to measure the specific surface area of different iron oxide particles. Results are shown in Table S1.† The BET surface areas of α-Fe₂O₃@rGO, α-Fe₂O₃/Fe₃O₄ spherical nanoparticles and Fe₃O₄ Selaginella uncinata particles were 14.5, 59.1 and 146.6 m² g⁻¹, respectively. Due to its surface groups and flexibility of its layer structure, GO can serve both as a support for the growth of spherical like particles and as a surface modifier.²² The possible formation mechanism of the “Selaginella uncinata” morphology Fe₃O₄ nanostructure by using a small molecule (i.e., sodium citrate) as a modifier can be described in terms of the particle growth being via diffusion-controlled and oriented attachment. Li et al. have recently found that synthetic parameters such as concentrations of citric acid have an effect on ZnO dendritic structural manipulation.²³ The formation of H/M mischcrystal by CQDs can be described as follows. Previous reports indicated that there are abundant groups such as hydroxyl and carboxyl groups on the surface of CQDs. Firstly, iron ions were coordinated and aggregated onto CQDs, then hydrolyzed and formed CQDs/Fe(OH)₃. Under hydrothermal condition, globally Fe(OH)₃ crystallized into α-Fe₂O₃ but locally Fe(III) was reduced to Fe(II) due to the reductive ability of CQDs, and resulted in the formation of Fe₃O₄ phase.^24,25

Fig. 2 SEM (a), HRTEM (d), and phase composition model (a, inset) images of iron oxide particles using GO as a modifier; SEM (b), HRTEM (e), and phase composition model (b, inset) images of iron oxide particles using CQDs as a modifier; SEM (c), HRTEM (f), and phase composition model (c, inset) images of iron oxide particles using sodium citrate as a modifier.

Materials with mixed phases are very interesting and useful. Ma observed that the magnetic characteristics of Fe₃O₄/α-Fe₂O₃ hybrid cubes were quite different from those of pure Fe₃O₄ or α-Fe₂O₃ phase.²⁶ In addition, a rectifying behavior was observed in current-perpendicular-to-plane transport properties of polycrystalline Fe₃O₄/α-Fe₂O₃ heterostructures.²⁷ The mischcrystal or hybrid material of hematite and magnetite can enhance or even create new performance, making it worth studying. As stated previously, we obtained particles with a mixture of α-Fe₂O₃ and Fe₃O₄ phases when using CQDs as a modifier. The hematite (α-Fe₂O₃)/magnetite (Fe₃O₄) mischcrystal is referred to as H/M mischcrystal in the following. The composition and properties of H/M mischcrystal including controlled phase composition and magnetism were studied.

As illustrated in Fig. 3a, CQDs exhibited two peaks around 220 and 300 nm in the UV-visible spectrum. The obtained H/M mischcrystal showed a very wide adsorption exceeding 800 nm, and two wide and low characteristic peaks of CQDs appeared at about 220 and 300 nm (inset), revealing the successful formation of hybrid materials of CQDs and iron oxide. As shown in Fig. 3b, the peaks in photoluminescence spectra were assigned to CQDs, and the peak shift with different excitation wavelength attributed to the excitation wavelength-dependent properties of CQDs, indicating the photoluminescence properties and potential applications of H/M mischcrystal.

Fig. 3 (a) UV-visible spectra of CQDs and H/M mischcrystal. (b) Detailed photoluminescence spectra, with different excitation wavelengths, of H/M mischcrystal.

We investigated the elemental composition of the H/M mischcrystal prepared at 200 °C for 6 h by X-ray photoelectron spectroscopy (XPS). Apart from Cl element, signals from C, O, and Fe were detected (Fig. 4a). Higher-resolution spectra were recorded to further understand the electronic states of the elements. In the Fe 2p spectrum (Fig. 4b), the peaks for Fe 2p^3/2, Fe 2p^1/2, and satellite Fe³⁺ were observed at 710.7, 724.4, and 720.4 eV, which were indicative of the formation of Fe₃O₄ and α-Fe₂O₃ phase in H/M mischcrystal. The binding energy assigned to S 2p at 168.4 eV is characteristic of oxidized sulphur species in the CQDs (Fig. 4c).²⁸ Five different peaks centered at 284.4, 284.8, 285.3, 286.3, and 288.3 eV were observed in the C 1s spectrum, which were respectively attributed to the sulphur C–C, C=C, C–O, C=O, and C–S groups. The XPS results further indicated that the hybrid of CQDs and iron oxide was of mixed phase. Moreover, abundant functional groups gave the materials a good water solubility.

Fig. 4 (a) Survey XPS spectrum of the obtained H/M mischcrystal prepared at 200 °C for 6 h. (b) High-resolution XPS Fe 2p spectrum. (c) High-resolution XPS S 2p spectrum. (d) High-resolution XPS C 1s spectrum.

The crystal structure and phase composition of H/M mischcrystal were investigated by XRD analysis. Fig. 5 shows the XRD patterns of H/M mischcrystal prepared at 200 °C for different reaction times of 3, 6 and 12 h. All three samples of H/M mischcrystal showed a powder XRD pattern originating from a mixture of two phases: α-Fe₂O₃ and Fe₃O₄ (JCPDS card no. 19-0629 and 33-0664, respectively). The phase ratio of α-Fe₂O₃ : Fe₃O₄ in H/M mischcrystal was 0.78, 1.05 and 1.13 for the different reaction times of 3, 6 and 12 h, respectively, which was calculated based on XRD results. The results from XRD analysis indicated that iron oxide particles can be facilely synthesized with precise phase proportion controlled simply by adjusting the reaction time.

Fig. 5 XRD patterns of the obtained H/M mischcrystal prepared at 200 °C for different reaction times of 3, 6 and 12 h.

It is well known that α-Fe₂O₃ nanoparticles are weakly ferromagnetic or antiferromagnetic, while Fe₃O₄ is often ferrimagnetic but superparamagnetic when the size is less than 15 nm. So the magnetic properties of H/M mischcrystal are strongly dependent on phase composition. The determination of the effect of particle morphology on magnetic behavior of the samples prepared using different modifiers is impossible because their crystal phase compositions are quite different. Herein, CQDs as modifier were chosen as an example to synthesize particles of the same morphology (SEM images of the sample are shown in Fig. S3†) but different composition. We mainly discuss the effect of particle composition on magnetic behavior. Field-dependent magnetization (M–H) curves of the 3, 6, and 12 h H/M mischcrystal showed magnetization saturation values of 32.5, 37.5, and 45.8 emu g⁻¹, respectively (Fig. 6). Also, those samples showed a certain coercivity in the M–H curves (Fig. 6, inset). The increase in saturation magnetization values could be attributed to the change in phase ratio of α-Fe₂O₃ : Fe₃O₄ in the H/M mischcrystal samples. This suggested that the H/M mischcrystal possesses tunable phase composition and magnetic responsivity, which is an advantage for studying structure–activity relationships as a model and for related applications.

Fig. 6 Magnetization measurements as a function of applied magnetic field for the obtained H/M mischcrystal prepared at 200 °C for different reaction times of 3, 6 and 12 h. The inset is an enlarged view.

In conclusion, nanoscale synthesis, characterization and correlation of nanostructures to their phase and magnetic properties controlled during one-step hydrothermal reaction were successfully examined. This synthetic strategy, which involves a suitable modifier with different dimensions, might be useful for controlling synthesis and screening for desired important nanomaterials.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This project was supported by the science and technology foundation of the national general administration of quality supervision in China (no. 2012QK053), the education bureau of Fujian province of China (no. JAT160875), natural science foundation of Zhangzhou (no. ZZ2016J31) and college students' innovative experiment project of Fujian.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15315a

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