V.
Rocher
a,
J.
Manerova
b,
M.
Kinnear
a,
D. J.
Evans
a and
M. G.
Francesconi
*a
aDepartment of Chemistry, University of Hull, Cottingham Road, Hull, HU6 7RX, UK. E-mail: m.g.francesconi@hull.ac.uk; Fax: +44 (0)1482 46 6410; Tel: +44 (0)1482 46 5409
bChemical and Biological Engineering Department, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, UK. E-mail: j.manerova@sheffield.ac.uk; Fax: +44 (0)114 222 7501; Tel: +44 (0)114 222 7500
First published on 26th November 2013
A novel and simple preparation of amine-modified γ-Fe2O3 nanoparticles is described. The presence of amine groups on the surface, instead of hydroxyl groups, will allow conjugation of biologically active molecules to the iron oxide nanoparticles without the need for a size increasing silica shell. Furthermore, the outer amine-layer increases the temperature of the γ-Fe2O3 to α-Fe2O3 structural transition in a similar way to previously reported cationic substitutions. This may suggest the formation of an oxide–nitride outer layer. Re-dispersion of the amine-modified γ-Fe2O3 nanoparticles led to the preparation of stable ferrofluids.
Ferrofluids are most commonly dispersions of either γ-Fe2O3 (maghemite) or Fe3O4 (magnetite) nanoparticles or a mixture of both. The two structures of γ-Fe2O3 and Fe3O4 are both based on a FCC lattice of O2− anions, with Fe3O4 containing both Fe2+ and Fe3+ cations, and γ-Fe2O3 Fe3+ cations and cation vacancies to maintain charge neutrality.8–10 Under thermal treatment cubic, ferromagnetic maghemite transforms irreversibly into the rhombohedral antiferromagnetic hematite (α-Fe2O3).11 This structural transition is being investigated with the aim of increasing the temperature limit of stability of the maghemite phase to maintain its magnetic properties and widen the applicability of γ-Fe2O3. For example, γ-Fe2O3 shows high sensitivity and selectivity in sensors for hydrocarbon gases, without the need for a noble metal.12 Cation doping of γ-Fe2O3 nanoparticles seems to be the most effective way to increase the temperature of the γ-Fe2O3/α-Fe2O3 transition, but very little has been reported on transition temperature variations caused by functionalisation. Here we report that reacting γ-Fe2O3 nanoparticles with NH3 (g) at 200 °C for 2–4 hours leads to direct functionalisation of the nanoparticles with amine groups and increases the temperature of the structural transition from γ-Fe2O3 (maghemite) to α-Fe2O3 (hematite) up to 550 °C. This suggests the possibility of formation of an outer layer of iron oxide–nitride.
Ferrofluids were prepared by dispersion of γ-Fe2O3 nanoparticles and amine-modified γ-Fe2O3 nanoparticles. A sample of approximately 100 mg was added to 2 mL of aqueous solution of HNO3 (pH = 2). This suspension was then subjected to 30 minutes of ultrasonic radiation to break the larger aggregates. The resulting colloidal dispersions were left to rest for 24 hours to test their stability.
Powder X-ray diffraction was carried out using a Siemens D5000 diffractometer using the Cu Kα radiation. Data were recorded from 2θ = 10° to 2θ = 110° over 72 hours, with a step size of 0.02°.
Thermogravimetric analyses were carried out using a Mettler TGA/DSC 1 Starsystem. 10–15 mg of nanoparticles were placed in an alumina pan and heated at a constant rate (30 °C min−1) to 900 °C under air, with the weight pattern and heat flow recorded as functions of the temperature.
The nitrogen content of the N-doped samples was measured using a CE Instruments 1108 CHN analyzer and results were expressed as weight percentages.
Nitrogen adsorption isotherms were recorded on a Micromeritics Tristar 3000. Size distribution was calculated from these data using the BJH model.
To demonstrate qualitatively the presence of surface amine groups, portions of ferrofluids formed with γ-Fe2O3 nanoparticles and γ-Fe2O3 nanoparticles reacted with ammonia were reacted for 1 h with a solution of fluorescamine (4′-phenylspiro[2-benzofuran-3,2′-furan]-1,3′-dione) in acetone (1 mg in 5 mL). The nanoparticles were then removed by filtration and the presence of fluorophors was revealed by examining the solutions under UV light.
Mössbauer spectra were recorded in zero magnetic field at 80 K on an ES-Technology MS-105 Mössbauer spectrometer with a 900 MBq 57Co source in a rhodium matrix at ambient temperature. Spectra were referenced against a 25 μm iron foil at 298 K and spectrum parameters were obtained by fitting with Lorentzian curves. Samples were ground with boron nitride before mounting in the sample holder.
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Fig. 1 TEM images of γ-Fe2O3 nanoparticles (top) and γ-Fe2O3 nanoparticles reacted with ammonia at 200 °C for 2 hours (bottom). |
Stable colloidal suspensions (ferrofluids) were obtained by sonication in de-ionised water using both γ-Fe2O3 nanoparticles and γ-Fe2O3 nanoparticles reacted with ammonia.
Powder X-ray diffraction (PXRD) of γ-Fe2O3 nanoparticles and γ-Fe2O3 nanoparticles reacted with NH3 (g) for 1 and 2 hours at 200 °C are shown in Fig. 2. The PXRD pattern of the γ-Fe2O3 nanoparticles shows the γ-Fe2O3 single phase. The percentage of γ-Fe2O3versus Fe3O4 was calculated via a peak deconvolution method to be 98.66%.14 The γ-Fe2O3 structure is maintained after ammoniation at 200 °C up to 2 hours and no formation of impurities can be detected. The unit cell parameters of all three compounds were refined from the general model for spinel-type structures, i.e. a face-centred cubic unit cell (space group Fdm, number 227). No substantial difference was observed between the unit cell parameters of the three samples analysed (a = 8.349(1) Å for γ-Fe2O3; a = 8.337(2) Å for γ-Fe2O3 + NH3 (g) for 1 hour; a = 8.335(1) Å for γ-Fe2O3 + NH3 (g) for 2 hours).17,18 The findings of the refinements are in agreement with those reported by Petkov et al.19 The nitrogen content (weight%) was determined by elemental analysis to be 0.05% and 0.24% for nanoparticles reacted with ammonia for 1 and 2 hours, respectively.
Reactions of both γ-Fe2O3 nanoparticles and ammoniated γ-Fe2O3 nanoparticles with fluorescamine were carried out to investigate qualitatively the presence of amine groups on the surface of the ammoniated γ-Fe2O3 nanoparticles.15 Fluorescamine does not fluoresce itself but, after reaction with amine functional groups, forms a highly fluorescent fluorophor. Furthermore, fluorescamine is specific for primary amines as it does not react with secondary amines and forms a non-fluorescent adduct with ammonia.16 The formation of fluorophors was observed only for the nanoparticles that had been reacted with ammonia (Fig. 3), indicating that primary amines have replaced a portion of the hydroxide groups on the surface of the γ-Fe2O3 nanoparticles. Further evidence of amine-functionalisation is the pH of the ferrofluid suspensions, which was consistently between 11 and 12.
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Fig. 3 Fluorescamine test showing fluorescence only on reaction with amine-modified γ-Fe2O3 nanoparticles. |
Reactions between Fe3O4, α-Fe2O3 and Fe with NH3 and/or NH3/H2 at temperatures between 350 and 900 °C have resulted in a variety of iron nitrides.20–23 Tessier et al. and Kikkawa et al. reported the formation of Fe16N2via a ‘soft chemistry’ route, i.e. by using low temperatures and long times, reacting α-Fe2O3 with ammonia for 10 days at 110 °C24 or for 100 hours at 130 °C, after reduction of α-Fe2O3 to α-Fe.25 In our work, low temperature and short reaction times have led to breakage of bonds between the surface of the nanoparticles and –OH groups and to their replacement with –NH2 groups, as well as the probable formation of a thin oxide–nitride layer. The fact that PXRD showed no structural changes and/or secondary phases, the small nitrogen content detected via elemental analyses, and the results from the fluorescamine experiment eliminate the formation of an iron nitride and support direct functionalisation of γ-Fe2O3 nanoparticles with amine groups.
Zero-field Mössbauer spectra for γ-Fe2O3 and γ-Fe2O3 nanoparticles after ammoniation were recorded (Fig. 4). The spectrum for each sample is indistinguishable from the other and confirms that the γ-Fe2O3 structure is maintained on ammoniation. There is no evidence for magnetite or significant amounts of impurity being present. Only one hyperfine pattern is observed as expected in zero field and the spectra correspond to those previously reported for γ-Fe2O3.26,27
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Fig. 4 Mössbauer spectrum of γ-Fe2O3 (top) and γ-Fe2O3 after 2 hours treatment with ammonia (bottom) recorded in the solid state at 80 K in zero magnetic field. |
The TGA and heat flow diagram for γ-Fe2O3 nanoparticles heated in air (Fig. 5, top) shows an initial sharp weight loss due to physisorbed and chemisorbed moisture and hydroxyl groups. The heat flow shows a large exothermic peak at 495 °C corresponding to transformation of γ-Fe2O3 into α-Fe2O3, in agreement with the transition temperature reported by E. Darezereshki, who focussed on γ-Fe2O3 nanoparticles of similar size (13 and 19 nm).28 The residue samples from TGA analyses were analysed by PXRD and the patterns showed only diffraction peaks belonging to the α-Fe2O3 phase.
The TGA and heat flow diagrams for γ-Fe2O3 nanoparticles reacted with NH3 (g) for 2 hours at 200 °C (Fig. 5, bottom) show a weight loss between room temperature and 400 °C that is probably due to loss of amino groups from the surface of the nanoparticles. The heat flow shows that the exothermic peak corresponding to transformation of γ-Fe2O3 into α-Fe2O3 has shifted from 495 °C to 550 °C.
A comparison of the exothermic heat flow peaks indicating the γ-Fe2O3 to α-Fe2O3 transition temperature, for all three samples, is shown in Fig. 6. The transition temperature increases from 495 °C for the γ-Fe2O3 nanoparticles to 526 °C for γ-Fe2O3 nanoparticles reacted with NH3 (g) for 1 hour, and to 550 °C for the γ-Fe2O3 nanoparticles reacted with NH3 (g) for 2 hours.
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Fig. 6 Comparison of the heat flow diagrams for (a) γ-Fe2O3 nanoparticles (dotted line) and (b) γ-Fe2O3 nanoparticles reacted for 1 hour and (c) for 2 hours. |
The maghemite–hematite structural transition has been discussed in a recent review on polymorphous transformation of nanometric iron(III) oxides.29 The maghemite to hematite phase transition was studied using differential thermal analysis30 and in situ real time X-ray diffraction.31,32 Different transition temperatures have been reported for nanoparticles of maghemite (200–500 °C) and for microsized particles (500 and 600 °C)33 and cation doping has been found to be a useful tool to enhance the transition temperature. Doping of amorphous Fe2O3 with 8.5% Mn3+ led to γ-Fe2O3 nanoparticles after heating for 3 hours at 500 °C34 and Zn2+ doping enhances the phase transformation temperature by circa 100 °C.35 DSC studies under air of ZnxFe3−xO4 (x = 0, 0.2, 0.4, and 0.6) solid solution showed that the γ-Fe2O3 to α-Fe2O3 phase transition temperature increases with increase in zinc content.36 A study on Fe3−xCoxO4 solid solutions showed that for x = 0.1 the temperature of the γ-Fe2O3 to α-Fe2O3 transition was increased by about 100 °C.37 γ-Fe2O3 nanoparticles doped with La3+ showed no sign of phase transition to α-Fe2O3 after 8 hours at 400 °C.38
Here, we show that the γ-Fe2O3 to α-Fe2O3 transition temperature is enhanced up to 550 °C as a consequence of the reaction of γ-Fe2O3 nanoparticles with ammonia. PXRD data show no changes in the patterns indicating no formation of nitrides, but it is likely that a thin layer of oxide-nitride is formed on the surface of the γ-Fe2O3 nanoparticles. It has been reported that the α/γ structural transition occurs as the size of the particles increases; hence an O/N layer is likely to decompose in air as the temperature increases (TGA carried out in air), hence delaying the α/γ structural transition. Whether the presence of primary amines on the surface of γ-Fe2O3 nanoparticles has any influence on the γ-Fe2O3 to α-Fe2O3 transition temperature is difficult to assess. Functionalisation of γ-Fe2O3 nanoparticles with palmitic acid was reported to shift the γ-Fe2O3 to α-Fe2O3 transition temperature up to 400 °C.39 Two sets of γ-Fe2O3 nanoparticles were functionalised with PMMA (polymethyl methacrylate) and with caprylate respectively and their γ versus α phase stability compared.40 The γ-Fe2O3 to α-Fe2O3 transition occurred at 400 °C for the caprylate-capped γ-Fe2O3 nanoparticles and at 500 °C for the γ-Fe2O3/PMMA composite γ-Fe2O3. It was argued that the outer molecular layer creates a barrier, which slows down the aggregation of the nanoparticles and consequent structural transition. However, in our case the coating of the nanoparticles alone may not be very effective in hindering particle aggregation as –NH2 groups provide an outer layer of comparable thickness to the –OH layers.
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