Journal of Science & Technology 100 (2014) 093-097
Synthesis and Characterization of Magnetic Nanoparticles of NiCo Alloy by Polyol Process
Nguyen Ngoc Minh
Hanoi University ofScience and Technology, No.l, Dai Co Viel Str.. Hai Ba Trung, Ha Noi, Viet Nam
Abstract
Magnetic nanoparticles are considered as an interesting material owing to their potential applications In biotechnology, medical technology and other related fields. Magnetic nanopariicles can be alloys, metals and its oxides such as ion nanoparticles. ion oxide nanopariicles, iron-cobalt alloy nanoparticles. In this research, magnetic nanoparticles of NiCo alloy were prepared by polyol process with using polyvinyl alcohol (PVA) and ethylene glycol as solvent while sodium borohydride was the reducing agent to reduce W;2+ and Co2+ ions together in solution. Nanopariicles were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD) while Vibrating Sample Magnetometer (VSM) was used to characterize the magnetic behaviour of the particles.
Keywords: Magnetic nanoparticles, Nanopartide, NiCo alloy nanoparticles.
1. Introduction
Nowadays, magnetic nanoparticles receive considerable attentions because of their wide range of applications in the immobitization of proteins and enzymes, bioseparation, iimnunoassays, drug delivery and biosensors. One of the applications of magnetic nanoparticles is in medical as shown in Fig. 1. when the size of particles are small enough, the magnetic nanoparticles become superparamagnetic. With using these particles as cartiers in drug delivery under magnetic field, the effects on medical treatments are improved significantly.
Fig. 1. Schematic illusttation of the therapeutic sttategy using magnetic nanoparticles [1].
Fig. I shows that magnetic nanoparticles can be used as a tool for cancer diagnosis by magnetic resonance imaging (MRl) or for magneto impedance
* Corresponding author: Tel.: (+84) 972 231 280 Email: [email protected],vn
(MI) sensor. Hyperthermia can then be induced by alternating magnetic field (AMF) exposure and subsequently desttoy the cancer cells. Thus, magnetic nanoparticles can be used for cancer therapy and at the same time for diagnosis [1], In addition, magnetic nanoparticles, which approach the size of a single magnetic domain, have several special applications in magnetic data storage and permanent magnets because of their interesting properties such as high coercivity and remanence [2] In order to get higher storage densities, future developments in the area of magnetic storage media will rely on the ability to develop stable materials within a few nanometers size [3].
For synthesis NiCo alloy particles, some articles had reported on the synthesis of NiCo alloy by reducing from their metal salt [4, 5, 6, 7], The salt of both metals nickel and cobalt were reduced by some reducing agents such as NaBH:i [8, 9], KBH4, LiAlRi [10]. The stmcture of particles can be either in FCC or HCP structure, depending on the composition and condition of the alloy. However, the synthesized particle sizes were not small enough to reach the superparamagnetic property. In this research, nanopartide of NiCo alloy were synthesized using PVA and ethylene glycol as solvent which can keep particle within few nanosize.
2. Experiments
In this study, synthesis of NiCo alloy magnetic nanoparticles was carried out at the various ratio of Ni^^ ions and Co-"^ ions in order to choose the sample with the narrowest particle size distribution. Nickel chloride (NiCl2.6H20) and cobalt chloride (C0CI2.6H2O) were reduced by sodium borohydride (NaBHa) in ethylene glycol (as a solvent) and
Journal ofScience & Technology 100 (2014) 093-097 polyvinyl alcohol (PVA) (as a surfactant). In the
reaction, polyvmyl alcohol forms a stable layer to protect the particles. In this research, molecular weight ofPVA was used at value Mw = 31,000,
Synthesis of NiCo alloy nanoparticles was carried out according to the Flow chat as shown in Fig. 2.
Preparation ofreactanl solutions:
Solution of metal salts: O.IM nickel salt and cobah salt solutions were prepared by addmg I.I9 g of both Ni and Co salts mto 50 ml ethylene glycol in cone flasks. The ratio of Ni^'^ ions and Co^' ions changed in Table I.
Solution of surfactant: A stock solution of Polyvinyl alcohol (PVA) was prepared by dissolving with the ratio of 0,75g PVA in 3 ml H2O at temperature surrounding SO^C. The stock solution was diluted with ethylene glycol to obtain the required concenttation at 4%wt.
Solution of reducing agent: I.425g of sodium borohydride was dissolved m 50 ml of ethanol for making the solution with 0.75M. The stirrer was used to enhance the dissolving rate for reducing Ni^'^ and Co^"* tons in solution.
Precipitation pf nanoparticles in colloidal condition:
In this stage of preparation, about 3ml of hydrazine (80%) was added into 38 ml o f t h e black colloidal solution. After 2 hours at constant stirring, a lot of black cluster powder was suspended at the bottom of beaker while the liquid portion became clearer. Centtifiige was used to separate the nanopartide from the liquid portion. The powder was washed with ethanol several times.
Hydrothermal treatment
The nanoparticles of NiCo alloy were synthesized from the flow chart in Fig. 2 resulting in amorphous form, so the XRD pattern is not so high enough to discriminate with other forms. Thus, these nanoparticles were used with hydrothermal tteatment to increase the crystal structure before testing with XRD. As a result, the suspension of NiCo alloy nanoparticles was ttansferred into a borosilicate glass vessel-lined autoclave (Parr) with a stainless steel shell and it was hydrothermally tteated at 200''C for 6 hours. The obtained powders were characterized by using XRD.
Table 1. Samples with different ratio ofNi^^ ions and Co ions
X o o f
samples Cl C2 C3 C4 C5
•kN?*):'kCii**) 9:1 7:3
3:33:7 1:9
VfljUNiCU (ml)
5.4 4.2 3.0 1.8 0.6
VojMCocn (ml)
o.a
1 8 3-0 4.2 5-4
(moi) 6x10^
6x10^
6x10^
6x10-"
6x10^
Witli n(N?*) and n(Co^*j are number of mole of ion Ni^^and Co^* in solution VO.IMNICH is volume of 0 1M NiChwas used
PVA in Ethylene glycol Add (CoCb solution + NiCb soluUon) Add (NaBH4 solution)
Centrifiigation and wash with ethanol
Add [N2H5OH (80%)] <
1 Stir Colloidal solution
NiCo alloy nanopartide powders
Fig. 2. Flow chart for the synthesis of NiCo alloy nanoparticles
Journal ofScience & Technology 100 (2014) 093-097
"^ ^i|4iiWJlW ^ * I W I M » » * A I I M . A A ^
Fig. 3. XRD pattern of Ni-Co alloy powder, (a) samples without heating and (b) after using hydrothermal treatment at 200 "C for 6 hours.
25 Mean size ~ 5.0 nm 20-
15- 10- 5-
L Frequency (%)
4 0 4 3 4,5 4.7 4 9 5.2 5,5 5,7 5.8 6,2 Particle size (nm)
25 Mean size ~ 4,2 nm 20
15 10 5 0 (c)
Frequency (%
3.2 3 4 3,6 3-8 4.0 4,3 4.5 4 7 4 9 Particle size (nm) 25
20 15 10 5 0 (e)
Mean size -- 3.5 nm
tfi 1111 IVr^
2.8 3,0 3.2 3.4 3 6 3,8 4.0 4.3 4,5 4.7 Particle size (nm)
Mean size ~ 4.8 nm 20-
15- 10- 5-
k Frequency (%)
P
3.8 4 0 4 3 4,5 4 7 4,9 5 2 5,5 5.7 5 8 (o) Particle size (nm)
25 ^ Mean size ~ 4.1 nm 20
15 10 5 0 (d)
Frequency (%) i
^
\k
3.0 3,2 3 4 3 6 3 8 4 0 4.3 4.5 4.7 4 9 Particle size (nm)
f^'C_ x "Nanoparticles ' I H P ^ ^ ^ ! ' ^ ? •" ' - ^
Fig. 4. Particle size distributions of sample with different ratio of nickel ion and cobah ion: (a) sample Cl; (b)
sample C2; (c) sample C3; (d) sample C4, (e) sample C5 and (f) TEM image of colloidal solution containing
nanopartide.
Journal of Science & Technology 100 (2014) 093-097 3. Results a n d Discussions
XRD results: structural analysis by XRD was carried out for two samples as shown in Fig. 3. Fig.
3.a shows the X-ray diffraction pattern of as- synthesized NiCo particles. This pattem showed relatively broadened peaks corresponding to NiCo alloys [3]. The feamre peaks of the NiCo alloy particles were at 20 = 44.5", 51.8", 76,1°, 92.5** and 98.05*' which were the observed positions for FCC crystal structure of NiCo at the planes of ( H I ) , (200), (220), (311) and (222), respectively. Therefore, tiie obtained lines from XRD pattem dearly showed that these lines are characteristic for FCC crystal structure of NiCo. There were no feature peaks at 26 ^ 41.72"
and 47.60", which are the position for HCP a-Co (100) and (lOI), respectively. Therefore, it can be concluded that the NiCo afloy was successfiilly synthesized [11 ]. Besides, there was no phase attributed for nickel/cobalt boride or other nickel/cobalt impurity phases.
In order to have high match of peaks, the black powder of sample was kept in ethylene glycol and heated at 200''C for 6 horns so that significant peaks were obtained because of formation of crystalline structure in nanoparticles (see Fig. 3.b).
Therefore, these results show that sodium borohydride can be used to reduce nickel and cobalt ions in ethylene glycol as indicated in the following reactions [12].
Ni^^- + NaBH4 + 3C2H50H + H2O -> Ni" (metal) + NaOH + B(OC2H5)3 + 4H2
Co^-' + NaBH4 + 3C2H50H + H2O -> Co" (metal) + NaOH + B(OC2H5)3 + 4H2
T E M result: the presence of spherical nanoparticles in colloidal solution was proved by TEM image in Fig, 4.f Fig. 4 also shows particle size log-normal distributions of samples with different ratio of nickel ion and cobalt ion.
With the increasing of nickel content, the mean particle size increased. It is because at higher concentiation of nickel, small Ni metal particle can dissolve in alkaline solution forming [Ni(0H)4]^' ion in presence of air, thus it can redeposit as Ni metal on the surface of small particle by adsorbed hydrogen.
Small particles are efficient catalyst for reduction process; therefore dissolution of metal ion deposits can cause the enlargement ofthe particle. Cobah does not show such characteristics as its particle size does not grow.
mu/g)
C 0
izat Magne
,-..
^ 1
fl 0
tiza
c
y
^ 1 3 2
'
-30000 -20000 -10000
•
"
. . . -•.... - , . « .
^
1 10000 20000 30000
Mean size ~ 3.5 nm
1 0 ™ ^ « ™ 30000
^^^ Mean Size ~
^.."'"'^'A 5.0 nm
^J
Magnetic Field (Oe)
S -|
3 2
'
^
-30000 -20000 -10000 B 10000 20000 30000
(b)
— ^ ~ 3.5 nm s II - 4 . 1 n m 4-1
~' - 5 . 0 nm 3 _ 2 J
'
Mean size ~ 4.1 nm
. „ ; S = =
^^^
\ ^
-30000 -20Q0O -10000 1 lOODO 20000 30000
——^^^^
^ ^ ^ -A
(d) -5 J Magnetic Field (Oe) Fig. 5. The
temperature (;
temperature
hystensis loops of magnetic Ni-Co alloy nanoparticles with different particle size at room b, c) and (d) magnetization comparing between samples with different particle si7,e at room
Journal ofScience & Technology 100 (2014) 093-097 VSM results: the magnetic properties for
NiCo alloy nanoparticles were characterized by usmg vibrating sample magnetometer (VSM) at room temperature (~300"K) to observe the variation in magnetization with particle size. Results are shown in Fig. 5. The magnetic properties were investigated witii an applied field of-20 kOe <H< 20 kOe.
The plots of magnetization versus applied magnetic field for the NiCo alloy nanoparticles show that the saturation magnetization (Ms) increases with decreasing in particle size. Probably different distributions of alloy atoms on the surface of nanoparticles might cause this.
4. Conclusions
- The magnetic nanoparticles of NiCo alloy can be synthesized by using polyol process.
- Mean size of nanopartide depends on the ratio of nikel and cobalt ions, the mean size increases when this ratio increases.
- Polyvinyl alcohol (PVA) can be used as surfactant in synthesis nanopartide.
- Magnetic properties of nanoparticles depend on particle size, saturation magnetization (Ms) increases with decreasing in particle size.
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