Magnetic nanofilms of nickel prepared at the liquid-liquid interface

Article abstract of DOI:10.1016/j.materresbull.2011.04.021

Thin films of metallic nickel with a thickness of the order of 20 nm have been prepared at the organic-aqueous interface at room temperature by the reaction of nickel cupferronate [Ni(C₆H₅N ₂O₂)₂] in toluene medium and sodium borohydride (NaBH₄) in aqueous medium. The films were characterized with transmission electron microscopy, scanning electron microscopy and atomic force microscopy. Thicker Ni films could be prepared by carrying out the reaction at the interface at 60 °C. The Ni nanofilms exhibit superparamagnetic behavior.

Full text of DOI:10.1016/j.materresbull.2011.04.021

Materials Research Bulletin 46 (2011) 1500–1503
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Materials Research Bulletin
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Magnetic nanofilms of nickel prepared at the liquid–liquid interface
*
Neenu Varghese, C.N.R. Rao
Chemistry and Physics of Materials Unit, New Chemistry Unit and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research,
Jakkur P.O., Bangalore 560064, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Thin films of metallic nickel with a thickness of the order of 20 nm have been prepared at the organic–
aqueous interface at room temperature by the reaction of nickel cupferronate [Ni(C6H5N2O2)2] in toluene
medium and sodium borohydride (NaBH4) in aqueous medium. The films were characterized with
transmission electron microscopy, scanning electron microscopy and atomic force microscopy. Thicker
Ni films could be prepared by carrying out the reaction at the interface at 60 8C. The Ni nanofilms exhibit
superparamagnetic behavior.
Received 11 April 2011
Received in revised form 20 April 2011
Accepted 21 April 2011
Available online 4 May 2011
Keywords:
ß 2011 Elsevier Ltd. All rights reserved.
A. Interfaces
A. Magnetic materials
A. Thin films
C. Atomic force microscopy
D. Magnetic properties
1. Introduction
Most of the synthetic methods for Ni nanostructures reported in
the literature use high temperature, surfactants, or long chain
Magnetic nanomaterials are of value due to their potential
applications in magnetic fluids, magnetic storage, biomedical
applications and catalysis [1–4]. In this context, nickel nanos-
tructures have been prepared by various methods employing
sputtering, pyrolysis, aqueous and nonaqueous chemical reduc-
tion, sonochemical deposition and water–oil microemulsions [5–
9]. Huo and Gao [10] prepared Ni nanoparticles by the reduction of
nickel acetylacetonate with sodium tetrahydridoborate in the
presence of hexadecyl amine and studied their magnetic proper-
ties. The synthesis of hcp Ni nanoparticles in polyethylene glycol
under refluxing conditions in the presence of oleic acid and oleyl
amine has been reported by Tzitzios et al. [11]. Reduction of nickel
acetylacetonate by hydrazine in oleyl amine also yields hcp or a
mixture of hcp and fcc Ni nanoparticles depending on the reaction
temperatures [12]. Synthesis and magnetic properties of single-
crystalline nanorods and ultra-thin sheets of nickel prepared by
the decomposition of nickel acetate in the presence of long-chain
amines under solvothermal conditions have been reported by
Ghosh et al. [13]. Han et al. synthesized Ni nanoparticles with fcc
and hcp structures by the thermal decomposition of Ni oleate in
dodecylamine and octadecene system [14]. Ni nanobelts have been
obtained by the reduction of a Ni tartrate complex under
surfactant-assisted hydrothermal conditions [15].
amines. It was our interest to prepare Ni nanostructures at room
temperature in the absence of any surfactants or amines. For this
purpose, we have employed the organic–aqueous interface to
prepare thin films of nickel. The organic–aqueous interface has
been used to generate various nanostructures, specially single
crystalline films of inorganic materials [16]. In this article, we
report the synthesis and characterization of nickel nanosheets
prepared by the interface reaction between nickel cupferronate in
toluene medium and sodium borohydride in the aqueous medium.
2. Experimental
Nickel nanofilms were prepared by the reaction of nickel
cupferronate (Ni(Cup)2, Ni(C6H5N2O2)2) as the nickel source and
sodium borohydride (NaBH4) as the reducing agent. Ni(Cup)2 was
prepared by the reaction of nickel acetate and cupferron at 0 8C.
The product formed was filtered, washed with water and dried at
40 8C in an oven. In a typical reaction, 25 mL of 0.1 mM of Ni(Cup)2
in toluene was added slowly to 25 mL of 5 mM of NaBH4 in water.
The interface that formed between the two layers turned silverish
white after a few hours. The interface was kept undisturbed for
30 h for the growth of the nickel nanofilm to occur. The film formed
at the interface was lifted onto a silicon substrate for characteri-
zation. The film was taken on a holey carbon grid for transmission
electron microscopic (TEM) measurements and on a quartz plate
for UV–vis spectroscopic measurements. To examine the effect of
temperature on the formation of the film, the reaction was carried
out at 60 8C keeping the other conditions same.
* Corresponding author. Tel.: +91 80 22082760; fax: +91 80 22082760.
E-mail address: cnrrao@jncasr.ac.in (C.N.R. Rao).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.04.021

N. Varghese, C.N.R. Rao / Materials Research Bulletin 46 (2011) 1500–1503
1501
Fig. 1. (a) FESEM and (b) TEM images of a nickel film formed at the liquid–liquid interface at room temperature. Inset in (b) shows a SAED pattern of the film. The scale bar in
the inset is 20 m.
m
Field emission scanning electron microscope (FESEM) images
were recorded with a FEI NOVA NANOSEM600. Transmission
electron microscope (TEM) images were obtained with a JEOL
(JEM3010) transmission electron microscope operating with an
accelerating voltage of 300 kV. UV–vis absorption spectrum was
recorded with a Perkin-Elmer Lamda 900 UV/VIS/NIR spectrometer.
Atomic force microscope (AFM) measurements were performed
withInnovaatomicforcemicroscope.Magneticmeasurementswere
carried out with a vibrating sample magnetometer in Physical
Properties Measurement System (PPMS, Quantum Design, USA).
a TEM micrograph of a Ni film. The film is crystalline as can be seen
in the selected area electron diffraction (SAED) pattern in the inset
in Fig. 1b. The electron diffraction spots could be indexed on the
˚
˚
hexagonal structure (P63/mmc, a = 2.651 A, c = 4.343 A, JCPDS No.
45-1027). The TEM image also reveals the presence of small
nanoparticles with diameters in the 2–3 nm range. Slightly bigger
particles (8–10 nm diameter) were also seen in some parts of the
film (indicated by black arrows).
Typical AFM images of Ni films are shown in Fig. 2. The images
reveal that the films are smooth and continuous over wide areas.
The three-dimensional image of the film in Fig. 2a and the height
profile in Fig. 2b show that the average thickness of the film is of
the order of 20 nm.
3. Results and discussion
Magnetic properties of the nickel film formed at the interface
were measured. In Fig. 3a, we show the temperature dependence of
In Fig. 1a, we show a FESEM image of a typical nickel film
formed at room temperature (30 8C) at the interface. Fig. 1b shows
Fig. 2. (a) 3D AFM image of a nickel film formed at the liquid–liquid interface and (b) AFM image showing height profile of a film.
Fig. 3. (a) Temperature dependence of magnetization for a nickel film formed at the liquid–liquid interface (recorded at 100 Oe). (b) Magnetization as a function of the applied
field measured at 5 K. The inset shows the enlarged magnetization curve.

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N. Varghese, C.N.R. Rao / Materials Research Bulletin 46 (2011) 1500–1503
Fig. 4. (a) UV–vis spectrum of a Ni film formed at the liquid–liquid interface and (b) I–V characteristics of a Ni film.
Fig. 5. (a) FESEM and (b) TEM images of a nickel film formed at the liquid–liquid interface at 60 8C.
magnetization under zero-field cooled (ZFC) and field-cooled (FC)
conditions at 100 Oe. The ZFC curve shows a maximum at the
blocking temperature (TB) of 13.5 K characteristic of a super-
paramagnetic material. A TB in the range of 12–40 K has been
reported earlier in the literature for nickel nanomaterials [10,17].
The ZFC and FC curves diverge below TB, characteristic of
superparamagnetic materials [18]. The FC curve shows that
magnetization continues to increase with decrease in temperature
below TB. Fig. 3b shows the field-dependent magnetic behavior of a
film at 5 K. The M–H curve shows hysteresis with a saturation
magnetization (MS) of 45 emu/g, which is lower than that of bulk
nickel (55 emu/g). Such a reduced value of MS is expected in a
nanofilm. The coercivity at 5 K is 770 Oe whereas the coercivity
above TB is negligible. The magnetic hysteresis disappears above TB,
as expected of a superparamagnet [10,18,19]. The present results
clearly establish that the nickel film generated at the liquid–liquid
interface is superparamagnetic.
shows the presence of small nanoparticles (2–3 nm) as shown in
Fig. 5b. An AFM image of the film formed at the interface at 60 8C is
shown in Fig. 6. From the z-scale image of the film shown as the
inset in Fig. 6, the thickness is estimated to be of the order of 30 nm.
This film also exhibits a superparamagnetic behavior.
In Fig. 4a, we show the UV–vis spectrum of a nickel film formed
at the interface. The spectrum shows a broad, weak feature around
320 nm due to surface plasmon resonance (SPR). There are
literature reports of SPR bands of Ni nanoparticles, Ni/SiO2 glass
nanocomposites and Ni implanted alumina in this wavelength
region [19–21]. Fig. 4b shows typical I–V characteristics of a nickel
film. The I–V curve is linear indicating the metallic character of the
nickel film.
Nickel films were also prepared at the liquid–liquid interface at
a higher temperature to check the effect of temperature on the film
characteristics. In Fig. 5a, we show a FESEM image of a film formed
at the interface at 60 8C. The film appears to be rough compared to
that formed at the room temperature. The TEM image of the film
Fig. 6. AFM image of the film showing the height profile of a Ni film formed at the
liquid–liquid interface at 60 8C.

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In conclusion, the present study demonstrates that the organic–
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interface reaction between nickel cupferronate and sodium
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temperature are ꢀ20 nm thick and exhibit superparamagnetism.
Ni films prepared at a higher temperature (60 8C) are thicker and
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The authors thank Mr. Nitesh Kumar for magnetic measure-
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