Ultrasonic-assisted Synthesis of Fe Nanoparticles in the Presence of Poly(N-vinyl-2-pyrrolidone)

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* E-mail: [email protected]; Tel.: 0086-021-62233508; Fax: 0086-021-62233508 Received September 27, 2010; revised March 15, 2011; accepted April 13, 2011. Project supported by Shanghai Education Development Foundation (No. 2008CG30)

Chin. J. Chem. 2011, 29, 1829—1836 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1829

Ultrasonic-assisted Synthesis of Fe Nanoparticles in the Presence of Poly(N-vinyl-2-pyrrolidone)

Xu, Yinga(徐颖) Zhang, Xiaoyana(张小燕) Hsing, Yiming*,b(邢怡铭) Fang, Yuzhi*,a(方禹之)

a Department of Chemistry, East China Normal University, Shanghai 200062, China b Department of Chemical Engineering, Bioengineering Graduate Program, the Hong Kong University of Science

and Technology, Hong Kong, China

Zero-valent iron particles were prepared by wet reduction chemistry assisted with ultrasonic treatment. Such prepared particles have uniform size, exhibit crystalline structure and show strong paramagnetic property. Their surface modification by coating poly(N-vinyl-2-pyrrolidone) (PVP) was investigated. The resulting Fe(0)-PVP particles were monodispersed and possessed enhancing magnetization saturation. Those synthesis conditions to control the particle size and distribution were exploited.

Keywords nanoparticles, magnetic properties, ultrasonic-assisted wet reduction, poly(N-vinyl-2-pyrrolidone)

Introduction

Magnetic particles have attracted great attention due to their special properties for many important applica-tions. For example, they are used as multi-terabit mate-rials for magnetic storage, ferrofluids for wastewater treatment, and catalysts for preparing carbon nanotubes and silicon carbide nanowires. Also, they are employed in magnetic separation, drug carrier, magnetic resonance imaging (MRI), hyperthermic therapy, etc.1 Although a number of literatures have been reported on the prepara-tion of magnetic particles, most of them emphasized on iron alloys and iron oxide particles.2-5 The reports on zero-valent iron Fe(0) magnetic particles are relatively limited in spite of their better magnetic properties. Unlike the synthesis of other metal particles, well-dispersed and crystalline Fe(0) particles are diffi-cult to be synthesized due to their easy oxidation and sensitivity to environmental pH. Additionally, without a special surface modification, Fe(0) nanoparticles would suffer further oxidation and aggregation. Till now, most of the reported Fe(0) synthesis routes applied cus-tom-made chemical precursors and yielded amorphous iron particle powder or particles in organic solvent. These routes include chemical vapor condensation,6,7 sonochemical decomposition,8 pyrogenation,9-11 micro-wave plasma and elastic wave pulse methods.12,13 Chemical reduction inside reverse micelles is another method to synthesize nanoparticles,14 where particles grow inside micelles. Herein, we demonstrated a simple wet chemistry method by using ultrasonic treatment to synthesize Fe(0) nanoparticles. Briefly, in the ultra-

sonic-assisted condition, Fe2＋ ions were reduced to zero valent state by using NaBH4 as the reducing agent. The as-prepared nanoparticles have uniform size, ex-hibit crystalline feature and possess strong super-paramagnetic property. To prevent these particles from self-aggregation, coating poly(N-vinyl-2-pyrrolidone) (PVP) to the particle surface was introduced during the reduction process.

During nanoparticle synthesis procedure, protective coating is one important step to prevent particle aggre-gation and facilitate further surface modification. Till now, many coating materials have been reported, in-cluding noble metals,15 polymers,16,17 ceramic materi-als,18 dextran,19 oils20 and silicon dioxide.21 Herein, poly(N-vinyl-2-pyrrolidone), i.e. PVP, was chosen in our study, as polymer is preferred as particle stabilizer due to its strong steric and electrostatic repulsion forces, and PVP itself has the advantages of bio-compatibility, retaining the particle conductivity and catalytic activ-ity.22,23 In our work, when PVP was present in the reac-tion solution, monodispersed magnetic particles were successfully synthesized at sub-micro size. Several techniques were used to characterize the as-prepared Fe(0)-PVP particles. Key synthesis conditions to control the particle size and dispersion, and the effect of PVP on the formation of Fe(0)-PVP nanoparticles were in-vestigated carefully.

Experimental

Materials

FeSO4•7H2O ( ≥ 99.0%, Sigma-Aldrich, USA),

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1830 www.cjc.wiley-vch.de © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2011, 29, 1829—1836

NaBH4 (GR for analysis, Merck, Germany), and three kinds of poly(N-vinyl-2-pyrrolidone) (PVP, average Mw is ca. 360000, 40000 and 10000 respectively, Sigma-Al-drich, USA) were used in the experiments without fur-ther treatment. Double distilled H2O with conductivity of 18 MΩ•cm was used in the experiments.

Preparation of zero-valent iron nanoparticles and PVP-covered iron nanoparticles

Zero-valent iron nanoparticles were synthesized by NaBH4 reduction at 0 under ultrasonic condition. To prevent the oxidation of Fe(0) nanoparticles to Fe3O4/Fe2O3, dissolved O2 was removed by bubbling N2. 1 mL of 0.6 mol/L NaBH4 solution was added to 4 mL of 0.025 mol/L FeSO4 solution drop by drop. Fe-PVP nanoparticles were synthesized by adding NaBH4-PVP (6.25×10－3 g/mL) solution into 4 mL of FeSO4 (2.5×10－2 mol/L)-PVP (6.25×10－3 g/mL) solution in the conditions of ultrasonic treatment, N2 saturation and 0 , and the NaBH4 molar amount in the mixture solu-tion was controlled at 2.5 times to that of FeSO4 for to-tally reducing Fe2＋ ions to Fe(0) particles. The mixture solution changing color to black suggested the forma-tion of magnetic particles, and then the reaction solution was incubated for 10 min. The magnetic particles were collected using an external magnet, washed by acetone and ethanol for three times respectively, and then stored in 5 mL of ethanol at 4 . If withour special mention, PVP with MW 360000 was used in the experiments.

Apparatus

A commercial ultrasonic cleaner (Model 9310-1, Type T 700/H, KOU HING HONG Scientific Supplies LTD) was used for the magnetic particle preparation. SP200i syringe pump (WPI) was used to add NaBH4 solution into FeSO4 solution at one controlled rate. JEOL 2010F high-resolution transmission electron mi-croscope (HRTEM) operated with LaB6 filament at 200 kV was used to characterize morphology and lattice structures of the magnetic particles. Crystal line distance was analyzed by DigitalMicrograph Demo software. X-ray photoelectron spectroscopy (XPS, Model PHI 5600 Physical Electronics) was used to study the com-position of the magnetic particles. Sputtering treatment was employed for the magnetic particle depth profile analysis. Vibrating sample magnetometer (VSM, Lake-Shore 7037/9509-P) was used to determine the magnetic properties of the iron particles at room temperature. For XPS and VSM experiments, Fe(0) nanoparticle powder and Fe(0)-PVP nanoparticle powder were prepared by drying the particles-ethanol solution in a vacuum con-centrator at 60 for 30 min. Additionally, for com-parison, Fe(0) nanoparticles were oxidized for XPS and VSM experiments. Zeta potential experiments were car-ried out by using Zeta potential analyzer of Beckman Coulter DELSA 440SX.

Results and discussion

Superparamagnetic nano-sized Fe particles have been synthesized by NaBH4 reducing Fe(II) in the con-dition of ultrasonic treatment. PVP was employed as the particle stabilizer, resulting in well-dispersed magnetic particles. The particles have been investigated by TEM/HRTEM for configuration characterization, Zeta potential experiments for particle surface charge meas-urement, XPS for component analysis and VSM for magnetic property calculation.

Characterizations for the magnetic particles without polymer coating

By using ultrasonic-based wet reduction method, the as-prepared Fe particles as shown in the TEM/HRTEM images (Images 1, 2, Figure 1) are in nanoscale (4—5 nm), exhibiting (100) single crystalline structure based on the distance of crystal lines 0.258 nm.24 Unlike sonochemistry method for iron nanoparticles and nano-rods synthesis,25,26 this ultrasonic-assisted method pro-duced nanoscaled and crystalline magnetic particles without using high temperature and pressure (e.g. 5000 K and 1800 atm) and the additional thermal annealing treatment. Herein, the ultrasonic treatment is vital for sitting particle in a nano-scaled size, as the ultrasonic wave keeps the nucleated iron particles from further growth; otherwise the iron particles would quickly ag-gregate on the addition of NaBH4. The particle size was found to be controlled by the Fe2＋ ion concentration, which was enhanced with the increased FeSO4 concen-tration (Table 1), and all the resulting particles have Fe (100) crystal face.

Table 1 Size of Fe nanoparticles synthesized by different con-centrations of FeSO4 and NaBH4 (mol/L)

Size/nm Da/nm hklb

FeSO4: 0.02 NaBH4: 0.12 4—5 0.258 100

FeSO4: 0.2 NaBH4: 0.12 7.5—8.5 0.281 100

FeSO4: 2 NaBH4: 0.12 12 0.284 100

FeSO4: 0.02 NaBH4: 0.6 6—7 0.284 100

FeSO4: 0.02 NaBH4: 1.2 6—7 0.285 100 a The distance of Fe nanoparticle crystal lines. b d (Å) 1.4332 is for Fe crystal face (200)24

It is known when the size of magnetic material de-creases to several nanometers, each particle becomes a single magnetic domain and then the material exhibits superparamagnetic properties. Therefore, our ultrasonic- prepared magnetic nanoparticles can quickly agglomer-ate and resuspend by adjusting external magnetic force, displaying weak magnetic coercivity (Hc), remanence (Mr) and hysteresis. Their magnetization data in M-H curve (Figure 2a) overlay with those demagnerization date and then showed no obvious M-H hysteresis


Chin. J. Chem. 2011, 29, 1829—1836 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 1831

Figure 1 TEM/HRTEM images of Fe nanoparticles (Images 1, 2) and Fe-PVP particles (Images 3, 4).

Figure 2 Hysteresis curves measured at room temperature for Fe nanoparticles (a), oxidized Fe nanoparticles (a) and Fe-PVP nanoparticles (b).

loop. The magnetization saturation (Ms) at room tem-perature is estimated at 156 emu/g over 5000 Oe, which is comparable to other lab-prepared or commercial iron oxide particles.27 For comparison, the oxidized sample displays very weak magnetic properties as shown in Figure 2a. Additionally, it is found that the magnetic force was controlled by the particle size, which was in-creased when the particle size decreased (data not shown).

XPS experiments found zero-valent iron at 707.0 eV

and iron oxide at 711.0 eV for the Fe(0) nanoparticles (Curve 1, Figure 3a). The iron oxide mostly came from the particle oxidation during the sample treatment. Due to that XPS is a surface-sensitive technology, it over-estimates the components on the sample surface, and the XPS peak of oxide iron is consequently higher than the zero-valent one. When sputtering technology was employed to remove the oxide layer from the parti-cle surface, the zero-valent peak was increased as a re-sult (Curve 1'). While the oxidized sample has only one

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Figure 3 XPS analysis for Fe nanoparticles (a) and Fe-PVP nanoparticles (b—f). a: Curves 1 and 1' are the Fe2p region XPS spectra for Fe nanoparticle powder before and after 30 Å sputtering treatment. Curves 2 and 2' are the Fe2p region XPS spectra for the oxidized Fe nanoparticle powder before and after 30 Å sputtering treatment. b—f: odd number cycle data for 30 Å sputtering treatment were shown.

iron oxide peak at 711.3 eV, even after sputtering treatment, which was shown in Curves 2 and 2'.

Zeta potential measurement showed that the Zeta potential for the Fe(0) nanoparticles was only 1—2 mV, depending on the electrolyte concentration (Figure 4). This value was much less than the Zeta potential thresh-old of colloid stability (±30 mV).28 Therefore, the par-ticles could not effectively repel each other either by electrostatic or by steric force, and consequently they aggregated as shown in the TEM images (Images 1, 2, Figure 1).

Characterizations of PVP-coated magnetic particles

In order to increase the iron magnetic particle dis-persion, poly(N-vinyl-2-pyrrolidone) (PVP) was em-ployed as the particle stabilizer, which has been used for stabilizing Au, Pt, Pd and their alloy nanoparti-cles.22,29-31 It was found that monodispersed magnetic particles have been synthesized in the presence of PVP as shown in TEM image (Images 3, 4, Figure 1). As PVP is a neutral polymer, it stabilized the magnetic par-ticles by steric repulsion force, and therefore decreased the particles’ Zeta potential as shown in Figure 4. M-H analysis (Figure 2b) shows that the PVP modification



Figure 4 Zeta potential measurements for Fe nanoparticles () and Fe-PVP nanoparticles ().

did not influence the particles’ superparamagnetic prop-erties, which increases the particles’ Ms to 194 emu/g. It can be explained by the fact that one sub-micro sized PVP-modified particle consists of a great number of mini Fe(0) nanoparticles inside with PVP polymer film outside.

XPS depth profile analysis for the PVP-modified particles shows that with sputtering treatment, the atomic concentration (Figure 3b) of C1s and N1s were continuously decreased, and Fe2p became the major component in the particles. It can be explained that sputtering treatment removed the PVP layer outside and exposed the inner Fe. And the atomic concentration of O1s was firstly increased and then continuously de-creased, which can be attributed to the existence of iron oxide between the PVP shell and the Fe(0) particle core due to the particle oxidization during the dryness pro-cedure. XPS peak intensity studies (Figure 3c—3f) showed that C1s and N1s XPS intensities were con-tinuously decreased until the 9th sputtering treatment, thus it can be concluded that the PVP layer was around 24—27 nm. HRTEM imaging showed one clear layer of PVP shell outside the particle (Image 4, Figure 1). The PVP polymer film coats outside the Fe particles as a protective shell mostly through the chelating interaction between the N and O atoms of PVP chains with the iron atoms, whose formation mechanism was primarily in-troduced for PVP-protected noble metal nanoparticles.31 The O1s and Fe2p XPS intensities were stabilized after the 15th sputtering treatment, thus we can conclude that Fe(0) core mostly existed away from surface around 45 nm, during which region Fe2O3 existed.

It can be calculated that one Fe-PVP particle in the diameter of 336 nm is made up of about 2.315e＋9n1 of Fe atoms (ø＝0.254 nm) by using n1＝VFe-PVP particle/VFe

atom, and there are around 1.750e＋6n2 of Fe atoms on one Fe-PVP particle surface by using n2＝SFe-PVP parti-

cle/SFe atom. Herein, V and S were referred to the volume and the surface area of Fe-PVP particle or Fe atom. Considering that the Fe2＋ ions in the reaction solution has been totally reduced into Fe(0) particles, it can be

estimated that (1) 4 mL of 2.5×10－2 mol/L FeSO4 produced about 2.600e＋10(n3) of Fe-PVP particles by using n3＝0.025×4×10 － 3

×NA/n1. Herein, NA is Avogadro's number, 6.02205×l023/mol; (2) after the reaction, the increased weight of Fe was found to be 0.0007 g, which came from PVP shell formation. Therefore, the amount of PVP chains (n4) on one Fe-PVP particle was about 4.592e＋4 by using n4＝

(0.0007/360000)×NA/n3. Consequently, by using n4/n2

we can obtain that on the Fe-PVP particle surface, one PVP chain interacts with thirty-eight Fe atoms through multi-plot adsorption model.

Key parameters for well dispersed PVP-coating magnetic particle synthesis

During magnetic particle synthesis by adding NaBH4-PVP solution into FeSO4-PVP solution, follow-ing results were found. (1) the particle size was adjust-able by the Fe2＋ concentration, which decreased from around 1 µm to several hundred nanometers (Table 2). (2) the concentration and adding rate of NaBH4 solution, on the other hand, was found to control the particles’ dispersion and configuration. Firstly, as displayed in Table 2 at a fixed adding rate, higher NaBH4 concentra-tion resulted in particle aggregation, attributing to the fast reaction rate. Secondly, when the adding rate was slow enough, even very high NaBH4 concentration produced well-dispersed sphere particles with the di-ameter of about 330 nm as shown in Figure 5. (3) The added volume of NaBH4 solution, however, has no ob-vious influence on the particle size and dispersion (Ta-ble 3).

If FeSO4-PVP solution was dropped into NaBH4-PVP solution, it was found that firstly all the resulting particles have aggregated by varying NaBH4 concentration and adding rate (Table 4, Figure 5). It can be attributed to the fast reaction rate when FeSO4 was added into NaBH4 solution, consequently the Fe nanoparticles did not have enough time to be coated by polymer layers. Secondly, the higher NaBH4 concentra-tion resulted in the decrease in particle size as shown in the TEM images (Figure 5) and Table 4. Miyake et al.31 has found a similar result that when they used alcohols as reducing agents and PVP as protective polymer, a faster reduction rate of [PdCl4]

2－ produced smaller Pt particles.

The enhancement in the PVP concentration has been found firstly to improve the particle dispersion, and then decrease the well-dispersed particles’ size as displayed in Table 5. Miyake et al. obtained a similar result that increasing the PVP amount could decrease the Pt nanoparticle size.29 However, enhancing PVP amount resulted in the increase in the solution viscosity, thus increasing the experimental operation difficulty. Addi-tionally, it was found that the magnetic particles’ size depends on the length of PVP chain, i.e. shorter PVP chain resulted in smaller sized particles as shown in TEM images (Figure 6).

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Figure 5 TEM images of Fe-PVP particles synthesized by the adding rate at 100 (a), 40 (b), and (c) 20 µL/min when NaBH4 was 6.5×10－3 (1), 3.25×10－2 (2), and (3) 6.5×10－2 mol/L.



Table 2 Effect of redox reagent concentration on the Fe-PVP particle synthesis when NaBH4-PVP solution was added into FeSO4-PVP solution at 100 µL/min

FeSO4 concentration (CO＝2.5×10－2 mol/L)

NaBH4 concentration (CO＝6.5×10－3 mol/L)

Size/nm Size RD RSD/% Configuration and dispersion

CO/10 CO 955 13.9 17.9 Sphere, dispersing well

CO CO 336 12.1 16.1 Sphere, dispersing wella

10CO CO — — — No particles

CO 5CO 334 18.8 23.8 Sphere, aggregatinga

CO 10CO — — — Sphere, aggregating badlya

Table 3 The effect of the added volume of NaBH4-PVP solution on the Fe-PVP particles size

NaBH4 adding volume/mL 1 2 10 40

particle size/nm 205 347 349 336

Table 4 The effect of NaBH4 concentration on the Fe-PVP particle synthesis when 4 mL of FeSO4 (2.5×10－2 mol/L)-PVP (6.25×10－3 g/mL) solution was added into NaBH4-PVP (6.25×10－3 g/mL) solution at 100 µL/min

NaBH4 concentration (CO＝6.5×10－3 mol/L) Size/nm Size RD RSD/% configuration and dispersion

CO 538 13.7 16.3 Un-sphere, aggregating

5CO 378 9.4 12.6 Un-sphere, aggregating

10CO 184 8.5 12.5 Un-sphere, aggregating a TEM images were shown in Figure 5.

Table 5 Effect of PVP concentration on the Fe-PVP particle synthesis

NaBH4 concentration (CO＝6.5×10－3 mol/L)

PVP concentration (CO＝6.25×10－3 g/mL)

Size/nm Size RD RSD/% Configuration and dispersion

5CO CO 334 18.8 23.8 sphere aggregating

5CO 10CO 330 12.7 15.6 sphere dispersing well

CO CO 336 12.1 16.1 sphere dispersing well

CO 5CO 242 16.2 20.9 sphere dispersing well

Figure 6 TEM images of magnetic particles synthesized by PVP of Mw 10000 (left) and 40000 (right).

Conclusion

The zero-valent iron nanoparticles were synthesized by using wet chemical reaction, and ultrasonic treatment was found to be an easy and efficient way to control the

particles at nano-sizes. PVP modification was employed to fulfill the particle monodispersion owing to the polymer steric repulsion force. The particles with PVP modification maintained special magnetic properties with high magnetization saturation. Additionally, their

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size and dispersion were found to be controllable by varying the concentration of PVP and redox agents or by changing the reaction solution adding rate. These particles are expected to become interesting magnetic materials for various employments, such as efficient catalyst.

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Documents

Ultrasonic-assisted Synthesis of Fe Nanoparticles in the Presence of Poly(N-vinyl-2-pyrrolidone)