Preparation of γ-Fe2O3/ZnFe2O4nanoparticles by enhancement of surface modification with NaOH
© Chen et al.; licensee Chemistry Central Ltd. 2014
Received: 11 March 2014
Accepted: 16 June 2014
Published: 24 June 2014
During liquid-phase synthesis of γ-Fe2O3 nanoparticles by chemically induced transition in FeCl2 solution, enhancement of surface modification by adding ZnCl2 was attempted by using NaOH. By using transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectrometry, and vibrating sample magnetometry, the dependence of the synthesis on the amount of additional NaOH was studied.
The experimental results show that the surface of the γ-Fe2O3 nanoparticles could be modified by adding ZnCl2 to form composite nanoparticles with γ-Fe2O3/ZnFe2O4 ferrite core coated with Zn(OH)2 and adsorbed FeCl3, and that modification could be enhanced by adding NaOH.
In the experimental conditions, when the concentration of additional NaOH was below 0.70 M, the amounts of ZnFe2O4 and Zn(OH)2 phases increased slightly and that of adsorbed FeCl3 was unchanged. When the concentration of NaOH exceeded 0.70 M, the amount of FeCl3, ZnFe2O4, and Zn(OH)2 increased.
Nanoparticles are typically defined as solids that are less than 100 nm in all three dimensions. Many physical phenomena in both organic and inorganic materials have natural length scales between 1 and 100 nm (102 to 107 atoms) [1, 2]. A nanocomposite is a material composed of two or more phases with at least one phase with nanometer dimensions. Due to combination of different physical or chemical properties, composite nanoparticles may lead to completely novel materials . For example, the type and geometric arrangement of surface coating on a magnetic core determine the overall size of a nanocomposite colloid and play a significant role in its biological fate in biomedical applications .
In another study, surface modification of the particles was undertaken during synthesis by adding ZnCl2 to the FeCl2 solution to prepare γ-Fe2O3/ZnFe2O4 composite nanoparticles . Experimental results show that when the concentration of ZnCl2 in solution did not exceed 2 M (50 mL), γ-Fe2O3/ZnFe2O4 bioxide nanoparticles coated with FeCl3 · 6H2O could be prepared. Generally, alkaline solution could assist the precipitation reaction. In the present work, we attempted to enhance the surface modification by adding NaOH to the processing solution. The morphology, crystal structure, surface and bulk chemical composition, and magnetization of the as-prepared products were characterized. The structure of the particles was proposed and the role of NaOH was revealed.
Preparation of the nanoparticles could be divided into two steps. First, the precursor based on amorphous FeOOH and Mg(OH)2 was synthesized by coprecipitation of FeCl3 and Mg(NO3)2, as described in detail elsewhere . In the second step, the precursor was added to 400 mL of 0.25 M FeCl2 solution, and the resulting mixture was heated to boiling for 20 min. Afterward, a mixture of 50 mL of 1 M ZnCl2 solution and 20 mL NaOH solution at a specific concentration was added to the boiling FeCl2 solution, and the resulting mixture was boiled continuously for 10 min. Subsequently, the mixture was allowed to cool to room temperature, and the as-prepared particles were allowed to settle. The NaOH concentrations used for the preparation were 0.35, 0.70, 1.40, and 2.10 M, corresponding to the as-prepared samples (1), (2), (3), and (4), respectively. For comparison, modified particles were prepared without adding NaOH (sample (0)).
The morphology of the particles in the samples was observed by transmission electron microscopy (TEM, Philips Tecnai 10), and their crystal structure was analyzed by X-ray diffraction (XRD, XD-2). The chemical species were measured using X-ray photoelectron spectroscopy (XPS, XSAM 800), and energy-dispersive X-ray spectroscopy (EDX, Genesis) equipped in scanning electron microscopy (SEM, Quanta-200). The magnetization was measured by using a vibrating sample magnetometer (VSM, HH-15).
Results and analysis
Binding energy data from XPS (eV) for samples prepared without NaOH (0) and with increasing NaOH concentrations: 0.35 (1), 0.70 (2), 1.40 (3) and 2.10 M (4)
Atomic percentages of O, Fe, Cl, and Zn from XPS measurements for samples prepared without NaOH (0) and with increasing NaOH concentrations: 0.35 (1), 0.70 (2), 1.40 (3) and 2.10 M (4)
Atomic percentages of O, Fe, Cl, and Zn from EDX spectrometry measurements for samples prepared without NaOH (0) and with increasing NaOH concentrations: 0.35 (1), 0.70 (2), 1.40 (3) and 2.10 M (4)
The experimental results and analysis above indicate that all of the samples were composed of γ-Fe2O3, ZnFe2O4, Zn(OH)2 and FeCl3, and no ZnCl2. The experimental results show that the ratios of Fe to Cl and Fe to Zn obtained by XPS were less than those obtained by EDX spectrometry, and the ratio of Cl to Zn obtained by XPS agree with that obtained by EDX spectrometry (see Tables 2 and 3). Since the EDX spectrometry measurements are acquired at micrometer depths whereas XPS data are obtained from the surface layer of nanometer thickness [15, 16], the experimental results suggest that the core of the particle is essentially γ-Fe2O3 and the coating layers are Zn and Cl based.
Thus, ZnFe2O4 grew epitaxially on the γ-Fe2O3 crystallites and some Zn(OH)2 outside of the ZnFe2O4 layer was preserved. Clearly, additional NaOH enhanced the reaction so that x and y increased with increasing NaOH content. In addition, Fe3+ and Cl- in the liquid phase were adsorbed and were subsequently converted to composite nanoparticles coated with FeCl3. Experimental results show that with increasing NaOH content, the amount of phases based on Zn increased. When the NaOH concentration was lower than 0.70 M, the FeCl3 phase was nearly unchanged since the Fe: Cl ratio is almost invariant (Table 2); thus, only when the NaOH concentration exceeded 0.70 M did the amount of FeCl3 increase clearly with NaOH concentration due to increasing amount of Cl. This means that the value of x in equation (2) increased with NaOH concentration at low NaOH concentrations, i.e., the amount of ZnFe2O4 phase increased clearly with NaOH concentration when the NaOH concentration did not exceed 0.70 M, but it increased slightly with NaOH concentration when the NaOH concentration was > 0.70 M. As the results in Table 2 show a consistent increase of the Zn: Fe ratio with increasing NaOH concentration, it can be judged that Zn(OH)2 increased slightly under low NaOH concentration (<0.70 M) and did clearly under high NaOH consentration (>0.70 M). According to the relation between the specific magnetization and NaOH content, the action of additional NaOH can be discussed further as follows.
Equation (7) shows that increment of the mass fraction of the ZnFe2O4 and Zn(OH)2 phases lowered the value of σ, whereas a decrement in the mass fraction of the FeCl3 phase increased it. For sample (1), x and y in the precipitation reaction described by equation (2) increased slightly compared with those for sample (0); hence, ΔϕZn-Fe, ΔϕZn, and ΔϕCl were very small compared with their counterparts for sample (0). Therefore, σ of sample (1) was about the same as that of sample (0). For sample (2), x and y in the precipitation reaction increased, but the increment of x could be larger than y. Thus, it can be judged from equation (3) that the increment in molar content x/2 of the ZnFe2O4 phase would be larger than that of the Zn(OH)2 phase (y - x/2), i.e., ΔϕZn-Fe > ΔϕZn. Since σγ-Fe - σZn-Fe < σγ-Fe - σZn (≈ σγ-Fe - σCl) and |ΔϕCl| is proportional to Δϕγ-Fe + ΔϕZn, |ΔϕCl| (σγ-Fe - σCl) > ΔϕZn-Fe(σγ-Fe - σZn-Fe) + ΔϕZn(σγ-Fe - σZn). Consequently, the σ value of sample (2) was greater than those for samples (0) and (1).
Therefore, the σ weakened in the order of samples (2) to (4).
During liquid-phase synthesis of γ-Fe2O3 nanoparticles from precursor composed of amorphous FeOOH and Mg(OH)2 by chemically induced transition in FeCl2 solution, Mg(OH)2 dissolved, FeOOH transformed into γ-Fe2O3 nanocrystallites, and Fe2+ was oxidized partially into Fe3+. The surface of the particles could be modified by adding ZnCl2 to form γ-Fe2O3/ZnFe2O4 composite nanoparticles coated with Zn(OH)2 and adsorbed FeCl3. Such composite nanoparticles exhibited stepwise distribution of magnetization from inner to outer regions. Thus, they could be easily dispersed in carrier liquid to form excellent ferrofluids . Experimental results indicate that when the amount of ZnCl2 solution was constant (1 M, 50 mL), the modification could be enhanced by addition of NaOH. When the concentration of additional NaOH was below 0.70 M, the amount of FeCl3 adsorbed was unchanged, but that of ZnFe2O4 and Zn(OH)2 increased slightly, increasing the magnetization of the products. When the concentration of additional NaOH exceeded 0.70 M, the amount of adsorbed FeCl3 and ZnFe2O4 and Zn(OH)2 phases increased, and the specific magnetization of the as-prepared products weakened with increasing amount of NaOH. These results show that surface modification during synthesis of the composite nanoparticles γ-Fe2O3/ZnFe2O4 coated with Zn(OH)2 and FeCl3 could be enhanced by additional NaOH to obtain various proportions of phases in the composite particles. This route could be an interesting route for preparing magnetic composite nanoparticles with novel properties. It could potentially be used to prepare other composite nanoparticles based on γ-Fe2O3. In this regard, it will be investigated further.
Financial support for this work was provided by the National Science Foundation of P.R. China (No. 51375039 and 11074205).
- Willard MA, Kurihara LK, Carpenter EE, Calvin S, Harris VG: Chemically prepared magnetic nanoparticles. Int Mater Rev. 2004, 49: 125-170.View ArticleGoogle Scholar
- Murray CB, Kagan CR, Bawendi MG: Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies. Annu Rev Mater. 2000, 30: 545-610.View ArticleGoogle Scholar
- Szabó DV, Vollath D: Nanocomposites from coated nanoparticles. Adv Mater. 1999, 11: 1313-1316.View ArticleGoogle Scholar
- Reddy LH, Arias JL, Nicolas J, Couvreur P: Magnetic nanoparticles: design and characterization, toxicity and biocompatibility, pharmaceutical and biomedical applications. Chem Rev. 2012, 112: 5818-5878.View ArticleGoogle Scholar
- Sun SH: Recent advances in chemical synthesis, self-assembly, and applications of FePt nanoparticles. Adv Mater. 2006, 18: 393-403.View ArticleGoogle Scholar
- Nogués J, Sort J, Langlais V, Skumryev V, Suriñach S, Muñoz JS, Baró MD: Exchange bias in nanostructures. Phys Rep. 2005, 422: 65-117.View ArticleGoogle Scholar
- Cushing BL, Kolesnichenko VL, O'Connor CJ: Recent advances in the liquid-phase syntheses of inorganic nanoparticles. Chem Rev. 2004, 104: 3893-3946.View ArticleGoogle Scholar
- Zhang QM, Li J, Lin YQ, Liu XD, Miao H: The preparation and characterization of Ni-Fe bioxide composite nanoparticles. J Alloy Compd. 2010, 508: 396-399.View ArticleGoogle Scholar
- Miao H, Li J, Lin Y, Liu X, Zhang Q, Fu J: Characterization of γ-Fe2O3 nanoparticles prepared by transformation of α-FeOOH. Chin Sci Bull. 2011, 56: 2383-2388.View ArticleGoogle Scholar
- Wen BC, Li J, Lin YQ, Liu XD, Fu J, Miao H, Zhang QM: A novel preparation method for γ-Fe2O3 nanoparticles and their characterization. Mater Chem Phys. 2011, 128: 35-38.View ArticleGoogle Scholar
- Chen LL, Li J, Lin YQ, Liu XD, Lin LH, Li DC: Surface modification and characterization of γ-Fe2O3 nanoparticles synthesized by chemically-induced transition. Mater Chem Phys. 2013, 141: 828-834.View ArticleGoogle Scholar
- Narayanaswamy A, Xu HF, Pradhan N, Peng XG: Crystalline nanoflowers with different chemical compositions and physical properties grown by limited ligand protection. Angew Chem Int Ed. 2006, 45: 5361-5364.View ArticleGoogle Scholar
- Seals R, Alexander R, Taylor LT, Dillard JG: Core electron binding energy study of group IIb-VIIa compounds. Inorg Chem. 1973, 12: 2485-2487.View ArticleGoogle Scholar
- Arulmurugan R, Vaidyanathan G, Sendhilnathan S, Jeyadevan B: Co-Zn ferrite nanoparticles for ferrofluid preparation: Study on magnetic properties. Physica B. 2005, 363: 225-231.View ArticleGoogle Scholar
- Tanuma S, Powell CJ, Penn DR: Calculations of Electron Inelastic Mean Free Paths III. Data for 15 Inorganic Compounds over the 50-2000 eV Range. Surf Interface Anal. 1991, 17: 927-939.View ArticleGoogle Scholar
- Srnová-Šloufová I, Vlčková B, Bastl Z, Hasslett TL: Bimetallic (Ag)Au nanoparticles prepared by the seed growth method: Two-dimensional assembling, characterization by energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and surface enhanced Raman spectroscopy, and proposed mechanism of growth. Langmuir. 2004, 20: 3407-3415.View ArticleGoogle Scholar
- Li J, Wang AR, Lin YQ, Liu XD, Fu J, Lin LH: A study of ZnFe2O4 nanoparticles modified by ferric nitrate. J Magn Magn Mater. 2013, 330: 96-100.View ArticleGoogle Scholar
- Lin LH, Li J, Fu J, Lin YQ, Liu XD: Preparation, magnetization, and microstructure of ionic ferrofluids based on γ-Fe2O3/Ni2O3 composite nanoparticles. Mater Chem Phys. 2012, 134: 407-411.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.