Size-dependent magnetic responsiveness of magnetite nanoparticles synthesised by co-precipitation and solvothermal methods

The considerable inconsistency in the obtained results from M- 02 size measurements, between TEM images and XRD analysis, could be explained by the formation mechanism of the magnetite n[r]

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Original Article

Thanh Quang Bui*, Suong Nu-Cam Ton, Anh Trong Duong, Hoa Thai Tran

Department of Chemistry, College of Sciences, Hue University, Viet Nam

a r t i c l e i n f o

Article history:

Received 11 October 2017 Received in revised form 31 October 2017

Accepted November 2017 Available online 14 November 2017 Keywords:

Fe3O4

Magnetite nanoparticles Co-precipitation Solvothermal

Magnetic responsiveness-particle size relationship

a b s t r a c t

The dependence of the magnetic responsiveness on magnetite nanoparticle size has been studied Monodisperse magnetite nanoparticles of 10 nm diameter were prepared by an ultrasonically enhanced co-precipitation procedure A carboxyl-functionalised solvothermal approach was applied to synthesise magnetite nanoparticles with an average size of 30 nm The particle sizes and their distribution have been determined by analysing TEM images and considering nanoparticle formation mechanisms The magnetic characterisation revealed an inverse dependence between the magnetite nanoparticle size and its ability to respond to external magneticﬁelds, which was explained by the decrease of magnetic di-poles inside the tailing-away crystal of the magnetite nanoparticles Negligible hysteresis with a small value of Oe was found for the 10 nm nanoparticles, while the larger value of 80 Oe was determined for the 30 nm nanoparticles

© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Nanostructured magnetic particles have been of great scientific and technological interest over the past decades because of their unique properties and highly potential applications Magnetite, well-known as the most strongly magnetic mineral in nature, has attracted diverse researches and applications such as magnetic recording technology, pigments, catalysis, photocatalysis, biomed-ical uses, and environmental processes[1]due to their high bio-logical compatibility, room-temperature superparamagnetism, low toxicity, and easy preparation [2] In biological applications, magnetite nanoparticles have been considered as an ideal candidate for the development of hyperthermal therapies, localising hyper-thermia by converting magnetic energy to heat at micro-scale based on their excellent magneticfield responsiveness[3] However, the ability of their response to an external magneticfield technically depends on the magnetic structure of the material, which originates from crystalline particle size[4,5]

Many preparation methods of magnetite nanoparticles have been developed and reported in varied research works, such as co-precipitation, hydrothermal synthesis, solvothermal synthesis, sonochemical synthesis, and micro-emulsion[5,6]

Co-precipitation is the simplest and most efﬁcient synthesis route to obtain the magnetic particles, based on the implementa-tion of reacimplementa-tion (3) in aqueous soluimplementa-tions[7] In co-precipitation, a stoichiometric mixture of ferrous and ferric precursors in aqueous medium is used as an iron source, which yields superparamagnetic nanoparticles after adding an alkaline solution However, the size distribution of as-prepared nanoparticles by co-precipitation is relatively broad if insufﬁcient assisting controls provided due to the presence of both nucleation and particle growth throughout the synthesis process[8]

Precursor agent/ Reducing agent (1) Reducing agent ỵ Fe3ỵ/ Fe2ỵ ỵ By products (2) Fe2ỵ ỵ Fe3ỵ þ 8OH/ Fe3O4 þ 4H2O (3) Solvothermal method is one of the most popular methods for synthesising high-quality magnetite nanoparticles, based on the implementation of reaction (3) in non-aqueous organic solvents In solvothermal approaches, a mixture of suitable basic source, sta-biliser, viscous solvent, reducing agent, and ferric salt as raw pre-cursors, is heated to a speciﬁc temperature to create the required conditions for reaction (1) to initiate This leads to the next stage of partially reducing ferric as described in reaction (2) to obtain ferrous source for the formation of the magnetite nanoparticles * Corresponding author 77 Nguyen Hue, Hue, Viet Nam

E-mail addresses:thanh.qt.bui@gmail.com,thanh.qt.bui@husc.edu.vn(T.Q Bui) Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2017.11.002

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following the reaction (3) Besides, by the utilisation of appropriate precursor agents, the synthesised magnetite nanoparticle sizes are efﬁciently tunable

To the best of our knowledge, there have been several re-searches in the literature emphasising on the particle size-magnetic properties However, they were commonly neither comparisons between samples synthesised from different routes [3,9], nor attention to differences between samples from the two most popular approaches[10], nor sufficient discussions on specific magnetic behaviours[11,12] Therefore, a distinguishable observa-tion of the size-dependent magnetic responsiveness of magnetite nanoparticles synthesised from the two most popular approaches would be a significant contribution to the application-oriented investigations of nano-sized magnetic materials in general and magnetite nanomaterials in particular

In this study, 10 nm magnetite nanoparticles were prepared by an enhanced procedure of co-precipitation based on ultrasonic assistance [13,14] Whilst, 30 nm magnetite nanoparticles were synthesised by a carboxyl-functionalised solvothermal approach [15] The magnetic responsiveness of the materials was derived from a room-temperature magnetic analysis The particle sizes and their distribution were evaluated by a data integrating investigation from various characterisations However, the represented dedica-tion also disregarded the inﬂuence of agglomeration and aggrega-tion states since these are natural trends of magnetite nanomaterials[16]and there are very few studies from the litera-ture supporting the respective inﬂuence on the obtained coercivity. Experimental

2.1 Chemicals and reacting conditions

Ferrous chloride tetrahydrate (FeCl2$4H2O), ferric chloride

hexahydrate (FeCl3$6H2O), hydrochloric acid (HCl, 37%), ammonia

solution (NH3, 28%), ethylene glycol (C2H6O2), trisodium citrate

dihydrate (Na3C6H5O7$2H2O), urea (NH2CONH2) were of analytical

grade, purchased from Xilong Chemical Co., Ltd All chemicals were used without further puriﬁcation In all experiments, reactant so-lutions were deoxygenated by Nitrogen bubbling for 30 before use Ultrasonic condition (Cole-Parmer-8892, USA) was deployed with 42 KHz, 100 W

2.2 Preparation of magnetite nanoparticles

Magnetite nanoparticles were synthesised by co-precipitation and solvothermal methods following two procedures, notated as M-01 and M-02, respectively

In the co-precipitation procedure (M-01 experiment), 1.5 mmol FeCl2$4H2O and 3.0 mmol FeCl3$6H2O were dissolved in 50 mL

deoxygenated distilled water Under ultrasonic irradiation, 10 mL ammonia was rapidly introduced into the as-prepared iron pre-cursor solutions at 40C, bubbling with N2 The black precipitates

were isolated from the solutions by magnetic decantation and washed by deoxygenated distilled water several times, then dried in an oven at 70C

In the solvothermal procedure (M-02 experiment), 7.2 mmol FeCl3$6H2O, 0.6 mmol Na3C6H5O7$2H2O, and 60 mmol NH2CONH2

were completely dissolved in 60 mL ethylene glycol with the aid of ultrasound and vigorous mechanical stirring The solution was sealed in a Teﬂon lined stainless-steel autoclave and then heated at 200C for h After being cooled down to room temperature, the black sediments were separated by magnetic decantation and washed by deoxygenated ethanol and distilled water several times, then dried in an oven at 70C

Table 1summarizes the differences in synthesis procedure of M-01 and M-02 samples, respectively

In addition, 3.5 g of as-prepared M-01 precipitates and 1.5 g trisodium citrate dihydrate were mixed with 100 mL deoxygenated distilled water for h with the aid of ultrasonic irradiation to create citrate-capped magnetite suspensions for band-gap energy ana-lysing purpose

2.3 Characterisation

The crystal structure of theﬁnal products was characterised by X-ray diffraction (XRD, D8-Advance-Bruker, Germany) equipped with Cu Karadiation (l¼ 1.5406 Å), and the mean crystalline size of the nanoparticles was calculated by Scherrer's equation using XRD data[17,18] The morphology of the synthesised products were observed by a transmission electron microscopy (TEM, Jeol-1010, Japan) The magnetic properties were investigated through collected magnetisation-hysteresis (M-H) curves using a vibrating sample magnetometer (VSM, PPMS-6000, USA) at room tempera-ture The molecular structure of the citrate-capped magnetites were characterised with an infrared spectrometer (IR, Shimadzu IR-Prestige-21, Japan) The ultraviolet-visible absorption spectra (UV-Vis spectrometer, Jasco V-660, Japan) were recorded for deriving the band gap analysis of the nanoparticles from as-prepared suspensions

3 Results and discussion

3.1 Surface modiﬁcation characterisation

Both samples of magnetite nanoparticles and citrate-capped magnetite nanoparticles from M-01 experiment wereﬁrstly char-acterised by IR spectroscopy (Fig 1) to demonstrate the capping role of citrate on the surface of magnetite to form as-prepared suspended solutions

Two main absorption bands FeeO, at 565.14 cm1, 397.34 cm1

in magnetite spectra (seeFig 1a) and at 569.00 cm1, 385.76 cm1 in the one of citrate-capped magnetite (seeFig 1b), are observed corresponding to the intrinsic stretching vibrations of metal-oxygen at tetrahedral site (Fetetra-O) and at octahedral site (Feocta

-O), respectively [19e21] The shifts between the corresponding spectra suggest a partial cleavage of these metal-oxygen bonds on the surface of the magnetite nanoparticles forming (citrate) COOeFe bonds Also, high-intense peaks appearing at 3442.94 cm1 and 1622.13 cm1 in Fig 1b are assigned to the stretching vibrating modes of OeH and C]O, accordingly, in the capping-citrate molecules Also, a medium-intense peak at 1394.53 cm1 in Fig 1b is inferred from the existence of CeH bending vibration in the molecule In addition, two weak peaks observed at 3429.83 cm1and 1627.92 cm1inFig 1a could be referred toeOH vibrations of absorbed moisture on the surface of the magnetite nanoparticles[22e24]

3.2 Morphology and particle size characterisation

The TEM images shown in Fig reveal the observed morphology and sized distribution of the as-prepared

Table

Procedural differences of M-01 and M-02 samples

Sample Synthesis method Assistance Surfactant M-01 Co-precipitation Ultrasonic irradiation None

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nanoparticles M-01 sample (Fig 2aeb) appears to have homoge-nously spherical nanoparticles with an average diameter of 10 nm, in form of nanoparticle agglomerates While M-02 sample (Fig 2ced) possesses a unidentifiable morphology and seemingly compacted clusters of particles with a broad size distribution, ranging from tens to hundreds nanometre in diameter Closer ex-aminations on the TEM images of M-02 sample indicate that it might contain smaller nanocrystals with an average size ca 30 nm The analysis of UV-Vis spectra shown inFig 3could provide a further confirmation of the size-observed differences As seen in Fig 3a, there is no pronounced peak in the spectrum of the sodium citrate solution Whilst, a steady increase of absorbance in M-02 suspension spectrum (seeFig 3b), for wavelengths drifting from 900 nm to 250 nm, could be related to the negligibly narrow band gap of original semi-metallic materials (< 0.1 eV)[25] Hence, M-02 nanoparticles absorbed continuously the increasing optical-energy, leading to a smooth change appearing on the plot However, a significantly noticeable peak, centred at the wavelength of 470 nm in the spectrum of M-01 suspension (Fig 3c), infers to the presence of a determined band gap which causes valence electrons to only absorbed a specific energy package in overcoming the respective insulated barrier This could be the effect of the band-gap stretching due to the decrease of the nanoparticle size In addition, the band gap of M-02 nanoparticles is 2.64 eV, which was calculated by Planck's formula[26]

Fig 4illustrates and explains the band-gap stretching phenom-enon by particle size tailing-off Bulk-sized particles of originally conductive materials, with an enormous number of available states for occupation, will absorb photon packages according to a wide continuous spectrum of wavelength However, by decreasing the number of atoms inside a particle as the consequence of reducing the particle size, the density of states inside the particle tends to be discrete Hence, the valence electrons could majorly be excited by absorbing a speciﬁc wavelength than otherwise, whose energies seem either too small that likely appear in far-infrared region or too high that likely fall into X-rays region This leads to a speciﬁc peak appearing in the UV-Vis spectrum (250 nme900 nm) of M-01 sus-pension, and a steady increase of absorption in the UV-Vis spectrum of M-02 considerably bulk-particle suspension

The XRD patterns shown inFig 5reﬂect the crystal structure of the synthesised nanoparticle samples, M-01 (Fig 5a) and M-02 (Fig 5b) The recorded XRD patterns show a good consistency with the standard data of magnetite crystal (Fe3O4, JCPDS

00-001-1111) without any extra noticeable peaks Five pronounced char-acteristic peaks could be clearly identiﬁed correspondingly to (200), (311), (400), (511), and (440) crystalline planes of magne-tite crystalline structrure The interplanar spacings, calculated and referenced as shown in Table 2, indicate that all samples possess the inverse cubic spinel structure of magnetite with over 99 % being consistent with the referenced data The average grain sizes of the magnetite nanoparticles in M-01 and M-02 samples were calculated from Scherrer's equation with full-width at half-maximum values of the corresponding strongest peaks (311) as nm and 28 nm, respectively In addition, the XRD patterns of M-01 sample appearing with lower intensity and more broaden reﬂections could be referred to the decrease of grain size of the Fig IR spectra of (a) magnetite nanoparticles, and (b) citrate-capped magnetite

nanoparticles

Fig TEM images of (aeb) M-01 sample and (ced) M-02 sample

Fig UV-Vis spectra of (a) sodium citrate solution, (b) M-02 suspension, and (c) M-01 suspension

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nanoparticles, leading to X-rays being more strongly and widely diffracted than the one obtained from M-02 sample[27]

The considerable inconsistency in the obtained results from M-02 size measurements, between TEM images and XRD analysis, could be explained by the formation mechanism of the magnetite nanocluster structures, schematically illustrated inFig 6b[15] Due to slow formation of the magnetite nanonuclei, the citrate mole-cules were adsorbed on the surface of the crystallites covalently resulting in electrostatically repelling forces between the neigh-bouring nanocrystals This not only promoted the hydrophilic ability of the magnetite nanoparticles, but also induced electro-static surfactants from the negatively charged citrate molecules covering the surface of the nanoparticles In contrast, the surface tension nature of nanomaterials together with magnetic forces caused the nanoparticle agglomeration toward the reduction of their surface energy As the consequence of the unbalance between the strong surface tension and the weaker electrostatic repulsion, the magnetite nanoparticles spontaneously agglomerated to form the ultimate subunit-nanostructures Therefore, the discrete nanocrystals, of ca 28 nm in average size as calculated by Scherrer's equation from the XRD data, represent the primary structure, irrespective of the larger clusters, with size ranging from several tens to hundreds nanometres observed in the TEM images, exhibit the secondary structure of the magnetite material

The reasonable correspondence of M-01 particle sizes, deduced from XRD data and recognised from TEM images, could be explained by the procedure of initiating the reaction environment and by the original properties of ultrasound in liquid mediums [28,29], schematically illustrated inFig 6a First, by pouring the ammonia rapidly into the iron precursor solution, the medium state promptly reached to the solubility threshold and swiftly overcame the condensational growing process, also known as Ostwald ripening process[30], leading to a homogeneous mass nucleation [31] with uniformly sized monodispersed nanoparticles as the

resulting products Besides, the mechanical ultrasonic interactions in the liquid medium could physically interfere down to the surface of the nano-scaled crystallites, holding them separatedly sus-pended in the reaction medium [14] As a result, the products appeared as discrete mono-sized-distributed nanoparticles, lead-ing to the correspondence between the particle sizes extracted from the both TEM and XRD characterisations

In addition, the lower bound on XRD-calculated particle size, in comparison with the one from TEM images, could be explained by inhomogeneous strain and crystal lattice imperfections on the surface of the nanoparticles[32]

3.3 Magnetic characterisation

Fig 7represents the room-temperature M-H curves of the as-prepared magnetite samples, measured by cycling the external magneticﬁeld between 11000 Oe and 11000 Oe M-01 (Fig 7a), and M-02 (Fig 7b) exhibit soft magnetic behaviour with relatively high saturation magnetisation (Ms) values of 57.7 emu$g1and 67.7 emu$g1, respectively Whilst, the obtained hysteresis values are

considerably distinguishable, with negligible Oe and 80 Oe, respectively, referring to the coercivity values of M-01 sample and M-02 sample

The relatively high coercivity value of M-02 sample is inferred from the over-threshold particle size of superparamagnetism, ca 25 nm, of magnetite material [33] Such over-sized particles perform as multi-domain crystalline granules[5], whose magnetic moments resisting each other from the proper response to an external magnetic ﬁeld (Fig 8b) Whilst, a negligible coercivity value (weak magnetic reluctance) indicates the retained super-paramagnetic property of M-01 sample after the completion of the synthesis process The observed features could be explained by the Fig XRD patterns and identiﬁed lattice indices of (a) M-01 and (b) M-02 samples

Table

Comparison of the interplanar spacings of M-01 and M-02 samples to the standard XRD data of magnetite (Fe3O4, JCPDS 00-001-1111)

hkl M-01 M-02 Standard

data d (Å) Completeness

(%)

d (Å) Completeness (%)

200 2.955 99.59 2.953 99.53 2.967

311 2.521 99.56 2.519 99.49 2.532

400 2.096 99.86 2.090 99.57 2.099

511 1.603 99.26 1.610 99.69 1.615

440 1.479 100.00 1.475 99.73 1.479

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fact that the as-prepared tiny and discrete magnetite nanoparticles contain quasi-mono magnetons, single magnetic dipoles, which freely alter following the applied external magnetic ﬁelds, and instantaneously respond to the magnetic alternatives (Fig 8a) In brief, the decrease of the particle size leads to the decrease of the number of containing magnetic dipoles; and the latter in turn leads to the reduction of the magnetic reluctance caused by the internal coupling interactions between the magnetic moments

4 Conclusion

This study showed the dependence of the magnetic respon-siveness on particle size of magnetite nanoparticles synthesised by co-precipitation and solvothermal methods, and revealed that the magnetic responsiveness increases with the decrease of the particle size The negligible hysteresis of Oe correlates to the particle size of ca 10 nm; whereas, the magnetite nanoparticles with ca 30 nm in diameter exhibit higher magnetic reluctance, indicated by a larger coercivity value of 80 Oe The determination of nanoparticle size could be performed by an integrated analysis of TEM, XRD, and

UV-Vis spectroscopy, in which, TEM images reveal the morphology, XRD data infers the estimated grain size, and UV-Vis spectra further conﬁrm the difference of nanoparticle sizes by band-gap stretching phenomenon observation

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