Surface-protective assistance of ultrasound in synthesis of superparamagnetic magnetite nanoparticles and in preparation of mono-core magnetite-silica nanocomposites

Fig. 8 shows TEM images of as-prepared nanocomposites, which all show a core-shell structure according to the magnetite-silica composite materials. The dark magnetite nanoparticles can b[r]

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Original Article

Surface-protective assistance of ultrasound in synthesis of

superparamagnetic magnetite nanoparticles and in preparation of

mono-core magnetite-silica nanocomposites

Thanh Quang Bui

, Hoa Thi My Ngo, 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 March 2018 Received in revised form 23 June 2018

Accepted July 2018 Available online 11 July 2018 Keywords:

Fe3O4nanoparticles

Fe3O4@SiO2nanocomposites

Shock-wave Ultrasonic assistance Co-precipitation St€ober method

Crystalline structure-magnetic behaviour relationships

a b s t r a c t

Ultrasound was throughout employed to enhance the co-precipitation process for the synthesis of magnetite nanoparticles (Fe3O4) and the St€ober approach for the preparation of magnetite-silica nanocomposites (Fe3O4/SiO2) The synthesised magnetite nanoparticles exhibited single-domain nano-crystallites with a uniform spherical morphology, a narrow size distribution (ca 10 nm), and negligible coercive ﬁeld (~5 Oe) The prepared magnetite-silica nanocomposites possessed a mono core-shell structure with spherical morphology, biologically coherent size (ca 100 nm), and discrete mono-domain behaviour The crystalline structure-magnetic behaviour relationships of the nanomaterials were investigated to imply the presence of a surface protection at nanoscale The speculation indicated that shock waves took place as the surface-protective role rather than the original mechanical interaction of ultrasound with a larger scope of impact

© 2018 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

Magnetite (Fe3O4), well-known as the most strongly

mag-netic mineral in nature, has been attracting diverse re-searches and applications[1,2] Magnetite nanoparticles possess numerous potential applications in magnetic recording technol-ogy, pigments, catalysis, photocatalysis, medical uses, and envi-ronmental processes because of their good biocompatibility, strong superparamagnetic property, low toxicity, and easy preparation[3]

In biomedical applications, magnetite nanoparticles have been applied in targeted drug delivery, hyperthermal treatment, cell separation, magnetic resonance imaging, immunoassay, and separation of biomedical products[4e6] Hence, many prepara-tion methods of magnetite nanoparticles have been developed, such as co-precipitation, hydrothermal synthesis, solvothermal synthesis, sonochemical synthesis, and micro-emulsion[7]

Co-precipitation is the simplest and most efﬁcient synthesis route to obtain the magnetic particles, based on the following reaction: Fe2ỵỵ 2Fe3ỵỵ 8OH/ Fe3O4ỵ 4H2O[8]; in which, a

stoichio-metric mixture of ferrous and ferric precursors in aqueous me-dium is used as an iron source, yielding magnetite nanoparticles after introducing an alkaline solution into it However, the size distribution of as-prepared nanoparticles by co-precipitation is relatively broad because of the presence of both nucleation and particle growth throughout the synthesis process[4] Whilst most of the mentioned biochemical applications require magnetite nanoparticles with chemical stability, biocompatibility, biologi-cally coherent size, and superparamagnetic property [9]a very

recent ﬁnding revealed a negative correlation between the

superparamagnetism of iron oxide nanoparticles and their heat-ing efficiency (specific absorption rate, SAR) [10], which is an imperative factor for magnetic hyperthermia applications In or-der to retain such properties, the prepared magnetite nano-particles also need a further proper modification for better fluidity and chemical stability[11,12]

St€ober et al., in 1968, reported a simple method to synthesise silica submicro-sized particles, based on the hydrolysis of tetrae-thoxysilane (TEOS) in aqueous alcohol solutions with the presence

* Corresponding author Building D, 77 Nguyen Hue, Hue, 53000, 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

https://doi.org/10.1016/j.jsamd.2018.07.002

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of ammonium hydroxide as a catalyst[13] The work afterwards initiated the preparation of silica-nanocomposite materials NPs@SiO2 (NPs for nanoparticles) by forming cross-linked silica

shell to protect the core nanoparticles extendedly Beside the hy-drophilic property derived from an enormous number of silanol groups (eSieOH) on the material surface, these groups also possess high potential to be further modiﬁed for acquiring speciﬁc compatibility of the composites through facilitatory hydrolysis re-actions[14] Also, by holding propagation transparency to light and magnetism, the silica-coated nanoparticles still retain their original optical and magnetic properties Finally, the most important feature is that the silica layer provides a chemically and physically inert surface for the core nanomaterials in biological systems[14] Besides, there are three other major methods that have been popularly utilised for preparation of silica-coated magnetic nano-particles, i.e in situ formation of magnetic nanoparticles inside the pre-synthesised silica matrix[15], aerosol pyrolysis[16], and water/oil microemulsion method[16] However, by relying on un-complicated apparatuses and mild reacting conditions, St€ober method is still considered as the most common approach

However, fast hydrolysis of TEOS[17]and aggregate nature of magnetite nano-sized materials [18] could lead to formation of morphologically heterogeneous and considerably large microsized-composites Fe3O4/SiO2, which not only degrade mobility, but also

deteriorate individual magnetite nanoparticle magnetism Ultrasound has been widely utilised in chemical synthesis and

sol dispersion [19] With velocity in water around

1000 m s1e1500 m s1and typical frequencies of laboratory

ul-trasonic irradiations from 20 KHz to 15 MHz, experimental ultra-sonic wavelengths vary from 10 cm down to 100

m

To the best of our knowledge, the powerful energy of shock waves, derived from extremely high velocity, temperature, and pressure, is mainly responsible for well-known treatments and processes such as cleaning, sonochemistry, and erosion[7,24e28] Otherwise, there are only vague explanations for the mechanism of ultrasonic assistance at the nano-scale[29,30] Besides, although magnetite nanoparticles sonochemically coated with silica has been widely reported, the strong agglomeration of the magnetite nanoparticles during the coating process still remains a challenging

task to obtain the nanocomposite with morphological and sized uniformity[31,32]

In this study, coprecipitation-based experiments in synthesis of magnetite nanoparticles and St€ober approach in preparation of magnetite-silica nanocomposites have been proceeded to demon-strate the submicro-effect of shock waves in the reacting medium through the explanation for obtained characteristic results, rather than ultrasonic waves otherwise at much higher scale The intro-duction of basic solution into the iron precursor has also been examined A mild frequency ultrasonic source, 42 KHz and 100 W, was applied to limit high-energy interparticle collisions, which could change particle morphologies and surface compositions Experimental

2.1 Chemicals and reacting conditions

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

hexa-hydrate (FeCl3$6H2O), ammonia solution (NH3, 28% v/v), ethanol

(C2H5OH), TEOS, trisodium citrate dihydrate (Na3C6H5O7$2H2O), and

sodium silicate solution (Na2O(SiO2)x$xH2O, 8% of Na2O and 28% of

SiO2) were analytical grade, and were purchased from Xilong

Chemical Co., Ltd All chemicals were used without further puri ﬁca-tion In all experiments, reacting solutions were deoxygenated by Nitrogen bubbling for 30 before use Ultrasonic condition was deployed with a cleaner-type bath (Cole-Parmer-8892, USA) with 42 KHz, 100 W

2.2 Preparation of magnetite nanoparticles

Magnetite nanoparticles were synthesised by coprecipitation-based method following processes, notated by M-01, M-02, and M-03 In typical synthesis procedure, 1.5 mmol FeCl2$4H2O and

3.0 mmol FeCl3$6H2O were dissolved in 50 mL deoxygenated

distilled water Under the assistance of mechanical stir or ultrasonic irradiation, 10 mL ammonia was differentially introduced to the iron precursor solutions at 40C, bubbling with N2 The assisted

conditions were being kept in the further 30 for ageing pur-pose.Table 1summarises the differences in synthesis procedure of M-01, M-02, and M-03 experiments, respectively

The black Fe3O4precipitates were isolated from the solutions by

magnetic decantation and washed by deoxygenated distilled water several times; then dried in an oven at 50C for h

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2.3 Preparation of magnetite-silica nanocomposites

Magnetite-silica nanocomposites were synthesised by St€ober approach following processes, notated by MS-01, and MS-02 In typical preparation procedure, 35 mg M-03 magnetite nano-particles powder and 200

m

summarises the differences in the synthesis procedure of MS-01, and MS-02 experiments, respectively

The Fe3O4/SiO2colloids were isolated from the liquid by

mag-netic decantation and washed with distilled water and ethanol several times; then dried in an oven at 100C for 12 h

2.4 Characterisation

The crystal phase of products were characterised by X-ray diffraction (XRD, D8-BRUKER, Germany) equipped with Cu K

a

l

[33,34] The morphology of the synthesised products was observed by transmission electron microscopy (TEM, JEOL-1010, Japan) The magnetic properties were investigated through collected magnetisation-hysteresis (M-H) curves using a vibrating sample magnetometer equipment (VSM, PPMS-6000, USA) at room temperature The molecular structure of silica-coated magnetite materials was examined by infrared spectrometer (IR spectrometer, SHIMADZU IR-PRESTIGE-21, Japan)

3 Results and discussion 3.1 Magnetite nanoparticles

XRD patterns shown inFig 2are utilised to demonstrate the crystal structure of the synthesised nanoparticle samples, M-01 (Fig 2a), M-02 (Fig 2b), and M-03 (Fig 2c) The measured diffrac-tion angles of all samples show a good consistency with those from standard XRD pattern of magnetite (Fe3O4, PDF card No 11-614)

[35] with ﬁve pronounced peaks, appearing at 30.09, 35.42,

43.05, 56.94, and 62.51correspondingly to (200), (311), (400), (511), and (440) lattices of magnetite crystalline structure The interplanar spacings, calculated by Bragg's Law[36]and referenced as shown inTable 3, indicate that all samples possess the inverse cubic spinel structure of magnetite with over 95% being consistent with the referenced date The average crystalline size of magnetite nanoparticles in M-01, M-02, and M-03 samples, estimated from

Scherrer's equation with full-width at half-maximum values ob-tained from the corresponding strongest peaks (311), are 27 nm, nm, and nm, respectively In addition, XRD patterns of sample M-02 and sample M-03 appearing with lower intensities and broader reﬂections could be referred to the decrease of crystallite size of the nanoparticles, leading to the X-rays being more strongly and widely diffused than the one observed from sample M-01[37] TEM images shown in Fig 3reveal the morphology and size distribution of as-prepared nanoparticles Sample M-01 (Fig 3a and b) appears with an unidentiﬁed morphology, widely distributing from tens to hundreds nanometres Whilst, sample M-02 (Fig 3c and d) and sample M-03 (Fig 3e and f) likely comprise homoge-nously spherical nanoparticles with the average diameter ca 10 nm, in form of tightly nanoparticle aggregates The upper bound on particle sizes, in comparing with the calculations from XRD anal-ysis, could be explained by inhomogeneous strain and crystal lat-tice imperfections on the surface of the nanoparticles, leading to smaller calculated values in Scherrer's equation[38](illustrated in

Fig 5) Besides, in aqueous suspensions, a combination of Lifschitz-van der Waals and magnetic forces would result magnetite particles in tending to aggregate into considerably large nano-particle clusters (>1

m

The inhomogeneous morphology of sample M-01 could be explained by simultaneous presences[40]of crystalline nucleation and growth throughout the synthesis process, induced from slowly dropping ammonia into the precursor solution This led to het-erogeneous nucleation and condensational growth (Ostwald ripening process)[41], to form heterogeneous nano-sized particles as the product (illustrated inFig 4a) In contrast, by pouring rapidly ammonia into reactant solution, the medium state promptly reached to solubility threshold and swiftly overcame the conden-sational growing process, leading to homogeneous mass nucleation

[40] with uniformly sized monodispersed nanoparticles as the crystalline nuclei in M-02 and M-03 experiments As the result, the crystallites experienced a condensational clusterisation process by spontaneously clustering together to form larger clusters, which in order to reduce particle-solute concentration (illustrated inFig 4b) However, these nanoparticles without a proper surface-protection could crystallisedly aggregate on neighbouring surface during the

Table

Procedural differences of experiment 01, experiment 02, and experiment M-03

Experiment Ammonia introduction Assistance solution

M-01 Droplet Mechanical stir

M-02 Pouring Mechanical stir

M-03 Pouring Ultrasonic irradiation

Table

Procedural differences of experiment MS-01, and experiment MS-02

Experiment Assistance solution

MS-01 Mechanical shake

MS-02 Ultrasonic irradiation

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synthesis processes, forming the secondary crystalline-aggregated structure that were composed of primary magnetite nano-crystallites, rather than terminating only at agglomeration Hence, responsiveness of the nanostructure to alternative external mag-netic ﬁeld, which applied to utilise superparamagnetic property

[42], would be degenerated Unfortunately, the coalescence was unrecognisable by only TEM and XRD analyses Fig 5illustrates differences between coalescence and agglomeration, and imagi-nations for the unrecognisability

Fig represents the room-temperature M-H curves of as-prepared magnetite samples, measured by cycling the external magneticﬁeld between 11,000 Oe and 11,000 Oe All samples, M-01 (Fig 6a), M-02 (Fig 6b), and M-03 (Fig 6c) exhibit soft magnetic characteristic with large saturation magnetisation (Ms) values of 72.5 emu$g1, 67.7 emu$g1, and 57.7 emu$g1, respectively, which

equal to the sum of the magnetic moments in each cluster of magnetite nanoparticles [43] Whilst, the obtained coerciveﬁeld (Hc) values are considerably distinguishable, with 75 Oe, 25 Oe, and Oe respectively according to coercivities of sample M-01, sample M-02, and sample M-03 The relatively high Hc value of

sample M-01 is inferred from the over-threshold particle size of superparamagnetism, ca 20 nm, of magnetite material [44], explained by the formation of multi-domain magnetite nano-particles[45] The weakest coercivity (5 Oe) of sample M-03 in-dicates the retained superparamagnetic property after completing the synthesis process This observation could be justified by the assumption that the as-prepared tiny and discrete magnetite nanoparticles, which freely alter following the applied external magneticfields, and instantaneously respond to the magnetic al-ternatives Meanwhile, the modest coercivity of sample M-02 suggested the existence of polycrystallite clusters, which might embed many single-domain nanocrystallites by coalescence These domains might resist mutually from responding properly to the applied magneticfield

The submicro-effect of shock waves could be applied to explain for the obtained differences in magnetic properties between sam-ple M-02 and samsam-ple M-03 By continuously forming the rebounding waves derived from the formation and collapse of acoustic cavitations, the liquid medium accelerated M-03 crystal-lites to maintain their oscillating suspended motion microscopi-cally during the stabilisation stage This eventually resulted in only physical agglomeration as the crystalline discretion was still

Table

Comparison of the interplanar spacings of sample M-01, sample M-02 and sample M-03 to the standard data of magnetite (Fe3O4, JCPDS No 11-614)

hkl 200 311 400 511 440

Standard 2q() 30.09 35.42 43.05 56.94 62.51

d (Å) 2.967 2.532 2.099 1.616 1.484

M-01 2q() 32.22 35.64 43.34 57.26 62.86

d (Å) 2.776 2.517 2.086 1.608 1.477

Completeness (%) 95.36 99.41 99.38 99.48 99.54 M-02 2q() 30.08 35.60 43.44 57.26 63.00

d (Å) 2.968 2.519 2.081 1.608 1.474

Completeness (%) 99.95 99.52 99.17 99.48 99.34 M-03 2q() 30.32 35.60 43.10 57.58 62.80

d (Å) 2.945 2.519 2.097 1.599 1.478

Completeness (%) 99.28 99.52 99.91 98.98 99.63

Fig TEM images of (a, b) sample M-01, (c, d) sample M-02, and (e, f) sample M-03

Fig Schematic illustration of nucleations and subsequent condensations

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preserved; hence, the superparamagnetic behaviour was expected (Fig 7c) Whilst, without an efﬁcient surface-protection against the naturally strong surface tension of magnetite nanoparticles, M-02 crystallites spontaneously aggregated on the surface of the neighbouring crystallites during the stage of stabilisation by condensational clusterisation with high-energy collisions This led to not only physical agglomeration, but also crystalline coalescence at the interfaces (Fig 7b) Besides, by slowly nucleating and growing, M-01 co-precipitated nanoparticles were more likely to contain heterogeneous magnetic domains (Fig 7a) In summary, by analysing the observable characteristic results, ultrasonic irradia-tion seems to reduce the aggregairradia-tion effects, thereby increasing the crystalline discreteness of the synthesised nanoparticles

3.2 Magnetite-silica nanocomposites

The diffusive and surface-protective propose of nanoscopic ef-fect of shock waves would be reinforced through explanation for experimental results of silica coating processes onto the surface of magnetite nanoparticles by St€ober-approach experiments

Fig 8shows TEM images of as-prepared nanocomposites, which all show a core-shell structure according to the magnetite-silica composite materials The dark magnetite nanoparticles can be observed to be embedded in the light-grey silica layer However, the morphologies are different by varying the apparatuses Sample MS-01 (Fig 8a and b) appears to have an unidentified morphology and a micro-scaled size The images also reveal clear structure of embedded magnetite clusters, dense agglomeration of numerous primary magnetite nanoparticles, inside the silica matrix Mean-while, sample MS-02 (Fig 8c and d) comprises spherical nano-composites, observed in form of quasi-mono and discrete core of magnetite nanoparticles inside the silica matrix This indicates a significant contribution of the technique since in nano-synthesis, monocore-shell structures constituted by hydrophobic cores and hydrophilic shells can be only efficiently achieved by utilising highly surface-protective agents, such as Brij-30, Triton X-100, Igepal CO-520, or other high-molecular surfactants [46] In addition, the average diameter of MS-02 nanocomposites is ca 100 nm, coherent size for biological applications; e.g., size of cell: 10e100

m

Fig All the nanocomposite samples possessed a high coercivity, ca 70 Oe, in comparison with the value of Oe that obtained from M-03 magnetite nanoparticles The rises could be explained by the presence of the silica layer as the magnetically dead contribution to the total volume and mass of the magnetic nanocomposite The layer is not contributing to the total magnetic moment, and considered as a high temperature antiferromagnetic order covering the ferromagnetic nanoparticles[48] The interaction between the antiferromagnetic shell and the ferromagnetic core induces the increase in coercive force Besides, a signiﬁcant difference of Ms values between the nanocomposites would demonstrate the mor-phologies of as-observed nanocomposite samples First, the negli-gible magnetic saturation of sample MS-02, ca 3.2 emu g1 (Fig 9b), has been expected since the minimal number of magnetite nanoparticles in the core were observed from TEM images (Fig 8a and b) This led to a deﬁciency of total magnetic moment inside

Fig MeH curves of (a) sample M-01, (b) sample M-02, and (c) sample M-03

Fig Formation illustrations of (a) M-01 domain nanoparticles, (b) M-02

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a single nanocomposite particle In contrast, the existence of numerous magnetite nanoparticles in a composite particle of MS-01, as-observed in Fig 8c and d TEM images, consisted with a higher magnetisation value, 50.2 emu g1inFig 9a

Fig 10 illustrates proposed mechanism model of the silica coating processes resulting in the considerably different observa-tions in structures and morphologies between as-prepared magnetite-silica composite materials By applying mechanical shake as diffusion solution, the unbalance between strong surface

tension and signiﬁcantly weaker diffusive driving force led to ag-glomerations of magnetite nanoparticles throughout the rapid hydrolysis and condensation of TEOS Therefore, micro-scaled

composite structure containing dense magnetite clusters

embedded inside silica matrix has been obtained as the product of experiment MS-01 (Fig 10a) In contrast, micro-massive driving force of shock waves would contribute to oscillating discretion of the nanocrystalltes This would lead to localised hydrolysis condensation of TEOS and growth of silica layer on the surface of single magnetite nanoparticles, forming spherical quasi-monocore nanocomposites (Fig 10b)

Both samples of magnetite nanoparticles (M-03) and magnetite-silica nanocomposites (MS-02) were further characterised by IR spectroscopy (Fig 11) to demonstrate the capping role of silica on the surface of magnetite to form as-prepared nanocomposites Two

main absorption bands of FeeO bonds, at 565.14 cm1 and

397.34 cm1in magnetite spectrum (Fig 11a), are observed con-sisting with intrinsic stretching vibrations of metal-oxygen at tetrahedral site (Fetetra-O) and at octahedral site (Feocta-O),

respectively[49,50] While, the corresponding bands are shifted to 559.36 cm1 with signiﬁcantly lower intensity or completely overlapped by an original high-intense shaped peak at 472.56 cm1, which could be assigned to SieOeSi or OeSieO bending modes, in the nanocomposite spectrum (Fig 11c) The shifting and lessening observations between the spectra suggest a partial cleavage of these metal-oxygen bonds on the surface of magnetite

nano-particles to form SieOeFe bonds Besides, the bands at

802.39 cm1, 958.62 cm1, and 1105.21 cm1, appeared inFig 11c,

Fig 10 Formation illustrations of (a) MS-01 multi-core nanocomposites, and MS-02 mono-core nanocomposites

Fig M-H curves of (a) sample MS-01, and (b) sample MS-02

Fig 11 IR spectra of (a) sample M-03, (b) silica, and (c) sample MS-02

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refer to symmetric stretching mode of SieOeSi, symmetric stretching mode of SieO, and asymmetric stretching mode of SieOeSi, respectively [51,52] Also, a medium-intense peak at 1627.97 cm1and a broadening peak at 3433.29 cm1inFig 11c are assigned to OeH bending vibration of SieOeH and OeH stretching vibration, respectively, on the silica-surface of the nanocomposite All the silica-related assignments are further conﬁrmed by the corresponding characteristic peaks appearing in the reference spectrum of silica (Fig 11b) at 470.63 cm1, 790.81 cm1, 968.27 cm1, 1105.21 cm1, 1643.35 cm1, and 3396.64 cm1, entire-respectively In addition, two weak peaks observed at 3429.43 cm1 and 1627.92 cm1 in Fig 11a correspond to OeH vibrations of absorbed moisture on the surface of magnetite nanoparticles[53e55]

Fig 12represents the XRD pattern of MS-02 magnetite-silica nanocomposites Five clear-observed peaks, appearing at charac-teristic 2-theta positions of magnetite inverse cubic spinel structure lattices i.e 30.09(200), 35.42(311), 43.05(400), 56.94(511), 62.51(440), indicate the well structural maintenance of magnetite nanoparticle cores after the formation of the composite material Besides, the amorphous silica matrix, coated onto the surface of magnetite nanocores, is revealed due to a high-intense broad peak observed between 20and 30[56]

4 Conclusion

The report proposed a facile technique based on the utilisation of ultrasonic assistance, consequently creating shock waves as diffusion technique and surface protection, to enhance co-precipitation method in synthesis of magnetite nanoparticles and St€ober approach in preparation of magnetite-silica nano-composites The synthesised magnetite nanoparticles possess uniform spherical morphology and narrow dispersed distribution (ca 10 nm for average size), and exhibit as single-domain nano-crystallites (5 Oe for coercivity) The as-prepared magnetite-silica

nanocomposites possess uniform distributed spherical

morphology (ca 100 nm for average size) and clearly observed monocore-shell structure, and perform discrete mono-domain

behaviour (3.2 emu$g1 for magnetisation) Diffusive and

surface-protective mechanisms at the nanoscopic scale of shock waves, derived from the implosive collapse of acoustic cavitation in liquid medium under ultrasonic irradiation, were supported by investigating the crystalline structumagnetic behaviour re-lationships of the nanomaterials

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