Zn-substituted iron oxide nanoparticles from thermal decomposition and their thermally treated derivatives for magnetic solid-phase extraction

Highlights

• Stoichiometric Zn-doped magnetite particles were prepared by thermal decomposition.

• Cation distribution was obtained from XRF and Mössbauer spectroscopy.

• Redistribution of Zn²⁺ between spinel sites occurred upon thermal treatment.

• Facile preparation of carbon-coated particles as a sorbent was shown.

Abstract

Controlled thermal decomposition of zinc and iron acetylacetonates in the presence of oleic acid and oleylamine provided surfactant-capped magnetic nanoparticles with narrow size distribution and the mean diameter of ≈15 nm. The combined study by XRD, XRF and Mössbauer spectroscopy revealed three important features of the as-prepared nanoparticles. First, the actual ratio of Zn:Fe was considerably lower in the product compared to the initial ratio of metal precursors (0.14 vs. 0.50). Second, a pure stoichiometric Zn-doped magnetite system, specifically of the composition Zn_0.37Fe_2.63O₄, with no signatures of oxidation to maghemite was formed. Third, Zn²⁺ ions were distributed at both tetrahedral and octahedral sites, and the observed preference for the tetrahedral site was only twice as high as for the octahedral site. Furthermore, carbon-coated nanoparticles were achieved by pyrolysis of the surfactants at 500 °C, providing a potential sorbent of organic pollutants with room-temperature magnetization as high as 79.1 emu g⁻¹ and very low carbon content of 5 wt%. The thermal treatment, albeit intended only for the carbonization of surfactants, did alter also the non-equilibrium cation distribution toward the equilibrium one by the relocation of a considerable fraction of the octahedrally coordinated Zn²⁺ to the tetrahedral sites. Preliminary experiments with magnetic solid-phase extraction of β-estradiol from aqueous solutions evidenced applicability and reusability of the carbon-coated product in the separation of steroid pollutants.

Introduction

Magnetic nanoparticles with high magnetization, high surface-to-volume ratio and reasonable chemical stability provide an attractive platform for magnetically assisted separation in analytical chemistry and biochemistry [1], [2], [3], catalysis [4], [5], waste water treatment [6], [7] but also for immunomagnetic separation of cells [8], [9], [10]. The isolation or enrichment of analytes may represent an inevitable step in the chemical analysis of organic compounds and biomolecules in complex and dilute samples, e.g. in the determination of organic pollutants in environmental samples, drinking water and food products. Various strategies based on extraction methods including solid-phase extraction, ultrafiltration, electrophoresis and precipitation have been developed for the treatment of complicated samples to isolate the organic and biochemical analytes, and the combination of a suitable sorbent with magnetic material, denoted as the magnetic solid-phase extraction (MSPE), seems to be especially efficient [11], [12]. The target analyte binds to the adsorbent that is subsequently separated by means of an applied magnetic field, and the bound analyte can be further purified and eventually eluted for the analysis. Importantly, the application of magnetic nanomaterials with large specific area enables one to isolate and determine low-abundance analytes even in complicated samples such as dairy products [13], meat [14] and biological fluids [15], [16].

Although magnetic particles based on magnetite, maghemite and mixtures thereof represent a traditional choice not only for medical applications but also for magnetically assisted separation, doping of these binary compounds with suitable metals can further increase their magnetization and thus performance [17]. Actually, the magnetic force acting on a point-like magnetic dipole can be written as $\mathit{F}=\mathrm{\nabla }\left(\mathit{m}\bullet \mathit{B}\right)$, where B is the magnetic induction and m is the magnetic moment, which is proportional to magnetization and volume of the particle [18]. Therefore, both higher magnetization and larger size of particles facilitate the magnetic separation. The effect of doping can be illustrated on the zinc-doped ferrimagnetic systems of magnetite/maghemite, that exhibit higher magnetization than their undoped counterparts (see, e.g. data in [19], [20]) since the diamagnetic Zn²⁺ cation shows a strong preference for tetrahedral sites in the spinel structure. This preference leads to an increase in the net magnetization, which is given by the difference between the moments of the two ferrimagnetically coupled sublattices, formed by tetrahedral and octahedral sites.

Not only the simple iron oxides but also their doped analogues can be prepared by the thermal decomposition method in the form of fine nanoparticles with a narrow size distribution, high crystallinity and bulk-like magnetic properties, namely non-reduced magnetization [21], [22], [23], [24], [25]. In a typical procedure, suitable metal-organic precursors are decomposed in a high-boiling solvent in the presence of long aliphatic 1,2-diols and surfactants, that chemisorb to the surface of arising crystallites of metal oxides and limit their growth [24], [26]. The resulting nanoparticles are capped by the employed surfactants like oleic acid and oleylamine. Considering the organic nature of these molecules, an additional thermal treatment of the as-prepared nanoparticles under an inert atmosphere at moderate temperatures might lead to the carbonization of the surfactants and thus the formation of carbon-coated particles. The anticipated product, possessing only very thin carbon coating and high magnetization, could exhibit efficient adsorption of various organic molecules, as does the activated carbon, and could be very useful for MSPE.

Carbon-based adsorbents containing magnetic nanoparticles have been already demonstrated as promising materials for MSPE of organic dyes [27], polycyclic aromatic hydrocarbons [28], veterinary antibiotics sulfonamides [14], organophosphorus pesticides [29], etc. These materials have been achieved by several distinct procedures. A number of efficient sorbents were obtained by direct combination of magnetic nanoparticles with graphene/graphene oxides [30], [31], [32] or multi-wall carbon nanotubes [33], [34], that were either introduced into the synthesis of magnetic particles or combined with the particles already prepared. Excellent adsorption properties were reported also for magnetic nanoparticles dispersed in nanoporous carbon that resulted in a single step from the carbonization of a zeolitic imidazolate framework [35], and similar core-shell structures were prepared by the reducing flame spray pyrolysis [36] or arc-discharge methods [37]. Other authors deposited carbon layer onto magnetic particles by decomposition of an organic precursor like glucose under hydrothermal conditions [38], [39]. However, the pyrolysis of surfactants in the surfactant-capped magnetic nanoparticles obtained by the thermal decomposition has not been, as far as we were able to ascertain, suggested.

In most of the studies utilizing the thermal decomposition method, it has been assumed that magnetite results from iron-containing precursors as iron acetylacetonates or oleates in the presence of alcohols as the 1,2-diols. However, conclusive data on the identity of the prepared iron oxides with respect to possible oxidation to maghemite have been rarely reported. Actually, standard powder X-ray diffraction (XRD) patterns, typically used for the phase analysis, do not allow to distinguish reliably between magnetite and maghemite, in particular, if the small size of crystallites delimits the size of coherent diffraction domain and line broadening occurs. The magnetite, Fe₃O₄, possesses the inverse spinel structure described by the formula (Fe³⁺)[Fe³⁺Fe²⁺]O₄, where the brackets () and [ ] denote the tetrahedral and octahedral sites, respectively, and characterized by the space group $\mathit{Fd}\overline{3}m$. The maghemite, γ-Fe₂O₃, possesses also the spinel structure that can be described as (Fe³⁺)[Fe³⁺_5/3□_1/3]O₄, where □ represents a vacancy, i.e. one-ninth of iron atoms is removed compared to magnetite. The vacancies on the octahedral sites of the spinel-like maghemite structure can show either completely random distribution or some degree of ordering. The former case is characterized by the same space group as magnetite, i.e. $Fd\overline{3}m$, whereas partial ordering of vacancies leads to the cubic phase described by the enantiomorphous pair P4₁32/P4₃32, and full vacancy ordering provides the tetragonal phase with symmetry of the P4₁2₁2/P4₃2₁2 pair [40]. Both the ordered maghemite phases can be, in principle, distinguished from the $Fd\overline{3}m$ phases based on extra superstructural reflections, which are, however, rather weak and may be obscured by disorder, low size of crystallites and presence of admixtures.

Compared to XRD, Mössbauer spectroscopy provides a unique insight into the local structure of iron oxides, which enables one to differentiate between the Fe²⁺ and Fe³⁺ at individual crystallographic sites. Therefore, it can be employed for the accurate determination of magnetite and maghemite content in their mixtures or quantification of vacancies in their solid solutions_, which are, from the viewpoint of chemical composition, identical cases. Moreover, Mössbauer spectroscopy enables one to determine unambiguously the cation distribution among different crystallographic sites in ternary spinel systems, like zinc-doped magnetite/maghemite [41]. Although the tetrahedral preference of Zn²⁺ is well-known in bulk samples, nonequilibrium cation distribution can occur in nanocrystalline phases [42], [43].

However, both the stoichiometry and cation distribution may be altered in consequence of the thermal treatment of once prepared spinel samples, as both the oxidation of Fe²⁺ in magnetite structure and the redistribution of cations can occur. Such processes should be envisaged during any treatment at elevated temperatures, for example during the preparation of carbon-coated nanoparticles by pyrolysis of the surfactants, but such changes have never been studied for doped iron oxides prepared by the thermal decomposition method.

Shortly, the present study is devoted to zinc-doped iron oxide nanoparticles of the spinel structure synthesized by the thermal decomposition method, whose chemical composition, stoichiometry and cation distribution are rigorously analysed by X-ray fluorescence spectroscopy (XRF) and Mössbauer spectroscopy, and whose magnetic behaviour is probed by SQUID magnetometry. The structural properties and magnetism are further analysed after the pyrolysis at 500 °C of surfactants, originally attached to the surface of particles, which is done to achieve carbon-coated particles. The comparative analysis aims to reveal possible changes in the oxidation state of iron and redistribution of cations between the two crystallographic sites in consequence of thermal treatment at elevated temperatures. The high-resolution transmission electron microscopy (HRTEM) and thermogravimetric analysis (TGA) are carried out to study the result of carbonization. Finally, the adsorption properties of the product are studied by using methylene blue (MB) as a model organic compound, and preliminary experiments on MSPE of steroid pollutants, namely β-estradiol, are demonstrated.