RESEARCH PAPER

Synthesis of nickel nanoparticles in silica by alcogelelectrolysis

Muhammad Z. Rana • Mazhar Mehmood •

Jamil Ahmad • Muhammad Aslam •

Syed K. Hasanain • Sohail Hameed

Received: 26 February 2010 / Accepted: 16 July 2010 / Published online: 11 August 2010

� Springer Science+Business Media B.V. 2010

Abstract We report a novel technique for the

formation of metal nanoparticles, based on electrolysis

of the alcogels containing metal chlorides. The alcogel

was formed from TEOS, water, ethanol, and nickel

chloride, and subjected to galvanostatic electrolysis.

This resulted in successful formation of Ni nanopar-

ticles inside the silica gel. Average particle size of FCC

Ni lies between 18 and 20 nm. The formation of

tetragonal nickel (a sub-oxide of nickel) as well as NiO

were also detected by XRD and SAED. The resistivity

measurements showed that the nickel nanoparticles

were separated from each other by Ni(O) present

between them. Magnetic studies based on ZFC and FC

measurements below room temperature (up to 5 K)

and above room temperature (up to 700 K) were

conducted using SQUID and Magnetic TGA, respec-

tively, which showed strong magnetic irreversibility as

attributable to exchange interaction between metallic

and oxide phases and mutual interactions among

metallic particles in the network structure. The block-

ing temperature (*600 K) of the samples was above

room temperature. M–H studies based on VSM

showed an increase in magnetic coercivity with the

formation of NiO. A magnetic transition associated

with tetragonal nickel was seen at 10 K.

Keywords Sol–gel � Alcogel electrolysis �Nanoparticles � Tetragonal nickel � Exchange

interaction � Synthesis at room temperature �Magnetism

Introduction

Nanomaterials exhibit interesting electrical (Moriarty

2001), optical (Moriarty 2001; Cai and Zhang 1997)

and magnetic properties (Moriarty 2001; Dorman and

Fiorani 1992; Ennas et al. 2004; Cintora-Gonzalez

et al. 2001; Skumryev et al. 2003; Tang et al. 2006),

in addition to high catalytic activity (Polshettiwar and

Molnar 2007).

Nanoparticles of catalytic metals and alloys dis-

persed in mesoporous (oxide) matrices provide with

extra-ordinary catalytic activity in comparison with

their other forms (Polshettiwar and Molnar 2007).

This is due to large surface area of the nanoparticles

(Brinker and Scherer 1989) available for reaction

and/or synergistic effects offered by the nanoparticles

M. Z. Rana � M. Mehmood (&) � J. Ahmad � M. Aslam

Department of Chemical and Materials Engineering

(DCME), National Centre for Nanotechnology (NCN),

Pakistan Institute of Engineering and Applied Sciences

(PIEAS), Islamabad 45650, Pakistan

e-mail: mazhar@pieas.edu.pk

S. K. Hasanain

Department of Physics, Quaid-i-Azam University,

Islamabad, Pakistan

S. Hameed

National Institute for Biotechnology and Genetic

Engineering, Jhang Road, Faisalabad, Pakistan

123

J Nanopart Res (2011) 13:375–384

DOI 10.1007/s11051-010-0040-1

and the matrix phases at the interphase boundaries.

The latter emerge from the fact that the two phases

may adsorb mutually different species facilitating the

reactions at their interfaces. These nanocomposites

may also be extremely useful for electrochemical

catalysts and sensors provided that the nanoparticles

are interconnected among each other.

Nanocomposites composed of metallic nanoparti-

cles dispersed in oxide phase exhibit interesting

magnetic properties. For instance, magnetic nanopar-

ticles isolated by an oxide (or non-magnetic materi-

als) may exhibit superparamagnetism depending on

the size and distribution of nanoparticles. Although

superparamagnetism is an extremely interesting

physical phenomenon, investigations to explore the

magnetic nanoparticles for memory devices have

compelled the researchers to enhance the magnetic

coercivity of small nanoparticles caused by exchange

interaction between the ferromagnetic core and the

shell of an antiferromagnetic oxide phase (Skumryev

et al. 2003). Both the above phenomena have been

extensively investigated. A new approach has been

suggested in which the interparticle interaction of

magnetic nanoparticles lying at a distance shorter

than exchange length produces high saturation mag-

netization, high initial permeability, and low eddy

current losses (Peng et al. 2008; Ma et al. 2009). This

approach may produce a second generation of core

materials for high frequency applications, replacing

the conventionally employed ferrites. However, these

materials with network structure of the metallic

nanoparticles covered by insulating phase have been

scarcely investigated for their magnetic properties

and synthesis.

The insulating phases which have generally been

employed include ceramic as well as polymeric

materials, such as SiO2 (Polshettiwar and Molnar

2007; Fidalgo and Ilharco 2005), Al2O3 (Liu et al.

2005, 2006), ZrO2 (Tom et al. 2003), TiO2 (Tom

et al. 2003; Boiadjieva et al. 2003), polyurethane

(Jamal et al. 2009), and block copolymers (Chipara

et al. 2004), etc.

In general, it is difficult to prepare metallic

nanoparticles directly through chemical methods

employed for the preparation of nanocomposites

and a subsequent pyrolysis and/or hydrogen reduction

treatment becomes almost essential (Cai and Zhang

1997; Cintora-Gonzalez et al. 2001; Kan et al. 2003;

Wu et al. 2001; Estournes et al. 1997; Ennas et al.

1998). This often results in undue growth of particles

due to high temperature.

In this study, we have suggested a combination of

sol–gel technique and alcogel (wet-gel) electrolysis

to prepare composite of networked metallic (nickel)

nanoparticles in silica matrix at room temperature.

Emphasis has been laid on magnetic properties

including ZFC and FC responses at cryogenic and

elevated temperatures up to Curie temperature, as

well as ACsusceptibility, coercivity, saturation mag-

netization, and conductivity.

Experiment

Synthesis

Typically, a mixture containing TEOS (ACROS 98%),

ethanol (94% by vol.), water (double distilled), and

metallic salt NiCl2�6H2O (Riedel-de-Hauen 97% min.)

was prepared in a conical flask. Then, 0.012 N HCl

(Merck) was added drop-wise with continuous stirring

in about 20 min. Stirring was continued for about 2 h,

keeping a lid in place to avoid drying. The molar ratio

of TEOS:Ethanol:Water:Ni:HCl was maintained

around 1:4:11:0.33:0.005. The resulting sol was then

transferred into an open beaker and heated slowly from

room temperature (25 �C) to reach 50 �C in about

30 min. Within a few minutes afterward, gel-point was

approached resulting in sudden rise in viscosity to

form alcogel. Before its complete solidification, alco-

gel was transferred onto a copper plate, used as a

cathode. Another plate of nickel to be used as anode

was placed over alcogel. A load of 1.5 kg was applied

against electrodes to maintain contact between the

electrodes and the alcogel. The exposed area of one

electrode was of the order of 12 cm2, and separation

between them was about 3–4 mm. The electrolysis

was performed galvanostatically using power supply

G-W INSTEK PSP-405. After several hours, the gel

was removed for characterization. To study the effect

of current density, Ni nanoparticles were synthesized

as above at various current densities ranging from 25 to

416 mA/cm2. To study the effect of concentration,

alcogels with varying concentrations of Ni (i.e., mole

fraction of Ni from xNi = 0.00232 to xNi = 0.021440)

were subjected to electrolysis at a prescribed current

density.

376 J Nanopart Res (2011) 13:375–384

123

Characterization

The crystal structure of nanoparticles was character-

ized by X-ray powder diffraction (XRD) using Bruker

D8 Discover (Cu Ka radiation). The XRD patterns

were analyzed using evaluation softwares EVA 11

and TOPAS-P. Transmission Electron Microscopy

(TEM JEOL-1010) was performed to reveal the

microstructure. Brunauer–Emmet–Teller (BET) sur-

face area measurement was performed on micrometric

adsorption instrument (Quantachrome NOVA 2000)

using N2 as analysis gas. Data analysis was performed

on adsorption isotherm using NovaWin 2.1 software.

Electrical resistance/conductivity of pressed sam-

ples (pellets) were measured using Autoranging Mul-

timeter (Kiethly 175). Samples were pressed in a

SANS electromechanical universal machine (CMT

4304), using a Teflon cylindrical die of 4-mm dia-

meter, provided with removable metallic base and

piston, which also functioned as electrodes for mea-

suring the electrical resistance of the powder sample

under a prescribed load.

M–H plots were obtained on Vibrating Sample

Magnetometer (VSM). Ac-susceptibility was mea-

sured by the mutual inductance method using SR530

lock-in amplifier at a lock-in frequency of 270 Hz,

and AC field of 0.8 Oe. Low temperature M-T (ZFC–

FC) measurements were recorded using a SQUID

device (Quantum Design MPMS XL). Magnetic

Thermogravimetric Analysis (M-TGA/MDTG) was

performed on a Thermo Gravimetric Analyzer (TGA/

SDTA851e Mettler Toledo).

Results and discussion

Figure 1 shows typical XRD patterns of the powder

samples prepared by alcogel electrolysis. FCC nickel

forms predominantly, irrespective of the concentration

of nickel in the gel (Fig. 1a) or applied current density

(Fig. 1b). It shows that the metal–solution interfaces

have moved much deeper inside the porous gel

structure to allow for the formation of a high phase

fraction of nickel. This is also in agreement with visual

inspection, as the color of the gel changes up to a depth

of about 2 mm away from the cathode. The nature of

the process dictates the formation of networked

particulate structures keeping connectivity with the

cathode. At xNi = 0.00232, X-ray reflections of NiO

are also seen in the XRD patterns (Fig. 1a), which

disappear with increasing concentration of nickel in

the precursor solution. However, when the concentra-

tion of nickel in the precursor solution increases, two

other peaks appear at 35� and 59�. Similar peaks were

also observed for samples prepared at different

current densities from gel with xNi = 0.02114 (inset

in Fig. 1b). The intensity of these X-ray reflections

increases with current. Identical X-ray reflections have

been seen by Nayak et al. (2005), who have assigned

them to amorphous oxide (Ni–O) formed by adsorbed

oxygen. Relatively sharper X-ray reflections at the

same positions have been observed by Roy et al.

(2005) who have clearly assigned them to the so-called

tetragonal nickel, proposed as a solid solution of

interstitial oxygen in nickel.

The above groups have formed sub-oxide of nickel

by borohydride reduction of nickel chloride in

aqueous solution. In this study, it seems to be formed

by the surface adsorption of oxygen on small nickel

particles or their partial oxidation.

At lower concentrations of nickel (xNi = 0.00232)

and moderate current densities (50 mA/cm2), solution

phase of gel, in front of the advancing metal–solution

interface, may become depleted in nickel ions due to

limited diffusivity of nickel, facilitating side reaction

of hydrogen evolution. The hydrogen evolution

reaction may cause a localized increase in the OH-

ion concentration or pH, leading to the enhanced

oxidation of small nickel particles, which results in

the formation of NiO. At higher concentrations of

nickel, pH does not increase much, and oxidation of

nickel is diminished. Therefore, as the nickel ion

concentration of the precursor solution is increased,

limited adsorption of oxygen is observed resulting in

the formation of oxygen-stabilized tetragonal nickel.

Increased current density also facilitates the side

reaction of hydrogen evolution resulting in higher

pH. Accordingly, the phase fraction of tetragonal

Ni(O) increases, although no NiO is formed due to

fast kinetics.

The average particle size of FCC Ni, as estimated

from X-ray peak broadening (using Scherrer’s for-

mula), lies in the range of 17–20 nm, irrespective of

the nickel concentration in the gel and applied current

density, as shown in Fig. 1c and d, respectively. The

relatively broader X-ray reflections of tetragonal

Ni(O) suggest an apparent particle size of about

2–3 nm.

J Nanopart Res (2011) 13:375–384 377

123

Figure 2a shows a typical TEM image of the

sample prepared at a current density of 50 mA/cm2.

Nanoparticles dispersed in the gel are visible. The

selected area electron diffraction (SAED) pattern, as

shown on Fig. 2b, confirms the formation of FCC

nanoparticles in amorphous silica matrix. Additional

reflections of NiO are also seen in SAED, although

XRD pattern of the sample (Fig. 1b) does not reveal

the formation of NiO. The absence of tetragonal

nickel in the SAED pattern suggests that the small

particles of tetragonal nickel oxidize to form NiO

during sample preparation.

The particle size (Fig. 2a), lying mostly smaller

than 20 nm, is in agreement with the estimates made

30 40 50 60 70 80

50 mA/cm2

xNi

=0.02114

xNi

=0.00468

xNi

=0.00232

Ni-FCCTetragonal Ni(Ni-O)NiO

(220

)

(200

)

(111

)

(310

)

(002

)

(220

)

(200

)

(111

)

Inte

nsi

ty (

a.u

.)

2θ (degrees)

(a)

(220

)

(200

)(111

)

(002

)

(310

)

30 40 50 60 70 80

30 40 50 60 70 80

75 mA/cm2

50 mA/cm2

Inte

nsi

ty (

a.u

.)2θ degrees

416 mA/cm2

Tetragonal Ni (Ni-O)

FCC Ni

75 mA/cm2

200 mA/cm2

50 mA/cm2

Inte

nsi

ty (

a.u

.)

2θ degrees

416 mA/cm2

(b)

0

10

20

30

40

Par

ticl

e S

ize

(nm

)

Current Density (mA/cm2)

0 100 200 300 4000.00 0.01 0.020

10

20

30

40

Par

ticl

e S

ize

(nm

)

Ni Concentration (moles)

(c) (d)

Fig. 1 Typical XRD

patterns of samples, after

electrolysis, prepared at

various nickel

concentrations in the gel

(a) and applied current

densities (b) along with plots

of estimated particle size vs.

nickel concentration (c) and

vs. current density (d)

378 J Nanopart Res (2011) 13:375–384

123

by X-ray peak broadening. Small size of nanocrys-

tallites is also confirmed by a typical dark field

image, as shown in Fig. 2c. Figures 2d–f present,

respectively, the TEM images of the samples

prepared at 25, 75, and 100 mA/cm2, for comparison.

In agreement with Fig. 1d, the particle size remains

almost unchanged with the applied current density.

Figure 2g shows TEM image of a sample prepared

by reduction in hydrogen at 600 �C. Comparing with

Fig. 2a–f, it can be noticed that the particle size

obtained by alcogel electrolysis at room temperature

is smaller than that obtained by elevated temperature

hydro-reduction.

It appears that the alcogel primarily constitutes of

silica-rich solid network, encompassing nickel chlo-

ride-rich liquid. Therefore, limited diffusivity of the

electro-active species and constrictive action of the

solid part of gel seems responsible for the formation

of small nanoparticles of nickel dispersed in silica

matrix. It also explains why the particle size is

independent of nickel ion concentration and current

density.

Silica(Gel)

(111)

(220)

(200)

(311)

NiO

(111)

(220)

(200)

(311)

FCC Ni

(b) (c)

50 nm75 mA/cm2

30 nm

(d)

25 mA/cm2

(e)

Silica Gel only

gel

nanoparticlesin gel

dense sample

100 mA/cm2

40 nm

(f)

(g)

(a)

50 nm

Fig. 2 Bright field TEM image of sample prepared at 50 mA/

cm2 along with corresponding SAED pattern (a and b,

respectively), a dark field image from the same sample (c),

and bright field images of samples prepared at 25, 75, and

100 mA/cm2 (d–f, respectively) along with a bright field image

of sample synthesized by hydrogen reduction of alcogel (g)

J Nanopart Res (2011) 13:375–384 379

123

For applications at high frequency, resistivity of the

magnetic materials should be high. This aspect was

focused upon by measuring the electrical resistance of

the samples. About 30 mg of sample was acquired in a

cylindrical die (4-mm ID) of Teflon, and uni-axial load

was applied using metallic pistons, which were also

used as electrodes to measure the electrical resistance

of the samples as a function of the applied load.

Figure 3 is a plot of the electrical resistance as a

function of the applied load. It may be noticed that

about 100-kg load is required for complete electric

contact (short circuit). It appears that particles are

mostly separated from each other by non-conducting

oxide—silica or Ni–O. The oxidation of small nano-

particles and inter-connecting necks may partly be

responsible for the separation of the metallic nanopar-

ticles in addition to breaking of the necks caused by

stresses developed in the gel due to shrinkage during

subsequent aging. Complete short circuit is possible

when nanoparticles are pushed together or extruded

through the oxide particles at a load of 100 kg. When

the sample prepared by alcogel electrolysis is subse-

quently reduced in hydrogen atmosphere at 600 �C,

electric contact is easily established through the

powder at a small load of only about 10 kg, which

supports our presumption that the nickel oxide (or sub-

oxide) isolates the nanoparticles of FCC nickel.

Porosity of samples is often desirable for catalytic

activity. The specific surface area of a typical

(50 mA/cm2) as-synthesized Ni-SiO2 nanocomposite

was determined from N2 adsorption experiments. The

BET area was found to be about 250 m2/g suggesting

that the present technique can be used for preparing

metal–ceramic catalysts.

In order to determine the magnetic properties of the

samples, hysteresis loops were obtained using vibrat-

ing sample magnetometer. Figure 4a shows typical

M–H curves of the composite samples as a function of

solution chemistry. The saturation magnetization

values as obtained from these curves have been

plotted in Fig. 4c. It may be noticed that the saturation

magnetization increases with an increase in the nickel

ion concentration in the precursor solution up to

xNi = 0.01, which seems to be due to an increase in

the amount of metallic nickel in the sample. A further

increase in concentration of nickel in precursor

solution does not increase saturation magnetization

and, consequently, the amount of metallic nickel.

Applying higher current densities (at xNi = 0.02) does

not enhance the nickel yield as saturation magnetiza-

tion does not vary with current density (Fig. 4b, c, d).

The maximum saturation magnetization, which is

obtained for composite samples prepared at

xNi = 0.02, is about 30 emu/g. It is known that

saturation magnetization of pure nickel is about 55

emu/g, suggesting that the maximum metallic nickel

content of our samples may be about 55 wt%, which

decreases with nickel in the precursor solution below

xNi = 0.01.

Figure 4e shows magnetic coercivity of the sam-

ples with nickel concentration in the precursor

solution. Coercivity of all the samples is much higher

as compared to bulk nickel (about 0.7 Oe.). It

decreases substantially with increase in the nickel

concentration in the precursor solution. This change

in coercivity may be related with the reduction in the

amount of NiO (Fig. 1a) and exchange bias existing

at the interface between the ferromagnetic nickel and

antiferromagnetic Ni(O) (Ohldag et al. 2001; Sun and

Dong 2002). The coercivity as measured from M–H

curves does not vary with current density (Fig. 4f).

AC susceptibility of the samples was measured in

the range of 298–725 K, typically as shown in Fig. 5.

The magnetic susceptibility diminishes above the

Curie temperature of FCC nickel (about 623 K). A

broad peak appears at about 523 K, which is close to

Neel temperature of NiO. A relatively sharp transition

around this temperature has been observed by van

Lierop et al. (2002) as related with the existence of

1E-3

0.01

0.1

1

10

100

1000

20 40 60 80 100 120 140

30 40 50 60 70 80

Tetragonal Ni(Ni-O)

FCC Ni

(200

)(111

)

(220

)

(002

)

(310

)

As-synthesized

H2-Reduced

Inte

nsi

ty (

a.u

.)

2θ (degrees)

measurement limit

Res

ista

nce

(Ω

)

Load Applied (kg)

As-SynthesizedH

2 Reduced

Fig. 3 A plot of electrical resistance as a function of applied

load for a typical sample after electrolysis and after subsequent

hydrogen reduction

380 J Nanopart Res (2011) 13:375–384

123

exchange bias. In our case, the broad transition may be

due to non-stoichiometric nature of oxide. In addition,

the magnetic coupling between nickel nanoparticles

due to their close proximity to each other may also be

operative. An edge is observed in the susceptibility

curve around 575 K (inset in Fig. 5). It may be related

with the termination of Ni–Ni(O) coupling, after

which only interparticle interaction exists.

A SQUID device was used for studying magneti-

zation, M, as a function of temperature, T, under

applied field strength of 100 Oe, as shown in Fig. 6.

The sample was first zero-field cooled to about 4 K.

ZFC measurements were then performed during

heating up to a prescribed temperature of 300 or

400 K that was followed by cooling to 4 K for FC

measurements. It can be noticed that ZFC and FC

curves are widely separated from each other. The

irreversibility in the magnetic response may be

arising from exchange bias between ferromagnetic

nickel and its sub-oxide along with magnetic inter-

actions among the nickel nanoparticles lying at close

proximity to each other.

It may be noticed that a peak appears in the ZFC

curve at about 10 K. Such a transition has been found

by Roy et al. (2006) at 15–20 K and by Nayak et al.

(2005) at 10–25 K. As proposed by Roy et al., the

presence of interstitial oxygen does not allow parallel

alignment of magnetic spins of nickel atoms, which

is responsible for very weak magnetization of this

phase at room temperature. However, a transition to

0-40

-20

0

20

40

xNi

=0.02114

M (

emu

/g)

H (kOe)

50 mA/cm2

75 mA/cm2

100 mA/cm2

200 mA/cm2

-8 -4 0 4 8-40

-20

0

20

40

50 mA/cm2

M (

emu

/g)

H (kOe)

xNi

=0.00232x

Ni=0.00468

xNi

=0.01068x

Ni=0.02114

(a) (b)

0

20

40

60x

Ni=0.02114

Ms (

emu

/g)

Current density (mA/cm 2)

(d)

60

80

100

120

140

160

180

200x

Ni=0.02114

Hc (

Oe)

Current density (mA/cm2)

(f)

0

20

40

60

Ms (

emu

/g)

Mol fraction Ni in sol (xNi

)

(c) 50 mA/cm2

50 75 100 125 150 175 200

50 75 100 125 150 175 200

0.000 0.005 0.010 0.015 0.020

0.000 0.005 0.010 0.015 0.02060

80

100

120

140

160

180

200

Hc (

Oe)

Mol fraction Ni in sol (xNi

)

50 mA/cm2

(e)

Fig. 4 M–H curves as a

function of nickel

concentration and current

density (a and b,

respectively), magnetization

as a function of nickel

concentration and current

density (c and d,

respectively), and coercivity

as a function of nickel

concentration and current

density (e and f,respectively)

J Nanopart Res (2011) 13:375–384 381

123

ferromagnetic nature possibly occurs at about

10–25 K causing enhanced magnetic susceptibility.

The peak intensity in the ZFC and FC curves at 10 K

increases with the increase in current density along

with an associated decrease in susceptibility at higher

temperatures. This suggests that higher amounts of

phase fractions of tetragonal nickel being formed at

the increased current density leads to enhanced

coercivity (lower susceptibility) of FCC nickel at

low applied field (100 Oe).

In order to compare FC and ZFC magnetic response

at elevated temperatures, Magnetic Thermogravimet-

ric Analysis (M-TGA) was performed as shown in

Fig. 7a. Temperature was first raised above the Curie

temperature of nickel (curve 1) followed by cooling to

room temperature (curve 2) without magnetic field.

Weight changes during heating (curve 1) are due to

drying of the sample. After cooling, a magnet was

placed below the sample, which caused a deflection in

300 400 500 600 700

300 400 500 600

TC=Ni

TN=NiO

Su

scep

tab

ility

(a.

u)

Temperature ( oK)

TC Ni

TN NiO

Applied field= 0.8 Oe

AC

su

scep

tab

ility

(a.

u.)

Temperature (K)

Bulk Ni

100 mA/cm2

200 mA/cm2

416 mA/cm2

Fig. 5 AC susceptibility curves of samples prepared at

different current densities and nickel concentration of alcogel

along with that of bulk nickel for comparison

Fig. 6 ZFC–FC magnetization curves for samples prepared at current densities of 50, 75, and 100 mA/cm2 (a–c, respectively):

applied field for FC being 100 Oe

382 J Nanopart Res (2011) 13:375–384

123

the balance indicating a pseudo-rise in weight propor-

tional to magnetic force between the magnet and the

sample. The temperature was then raised to a pre-

scribed temperature for ZFC measurements (e.g.,

curve 3) followed by cooling to record FC response

(curve 4). It may be noticed from ZFC curve that

susceptibility starts decreasing beyond about 520 K.

However, FC and ZFC curves do not meet each other

up to about 600 K. This aspect has been further

confirmed in typical measurements performed below

Curie temperature (Fig. 7b). After obtaining ZFC

curve up to a prescribed temperature, the temperature

was intermittently decreased and then again increased

to obtain several FC curves. When compared with the

behavior of bulk nickel (Fig. 7c), it can be seen that the

samples prepared by alcogel electrolysis exhibit

irreversibility/magnetic blocking at elevated temper-

atures, beyond the Neel temperature of NiO, which

may be due to non-stoichiometry of the oxide as well

as inter-particle magnetic interaction due to close

proximity of nickel nanoparticles.

The Curie temperature of the nickel nanoparticles

as obtained from Fig. 7a is 622 K, which is compa-

rable to the Curie temperature of bulk nickel (625 K):

the variation lying within the experimental

limitations. A relatively sharp transition in suscepti-

bility (ZFC curve) is observed at about 575 K. A

transition at the same temperature was also observed

in the AC susceptibility curve (inset in Fig. 5) and

appears to be associated with termination of bias

coupling between nickel and its oxide.

Summary

A new technique based on electrolysis of alcogels has

been employed for the synthesis of nickel nanoparti-

cles. Nickel chloride as nickel precursor was intro-

duced in the solution, which after gelation resulted in

uniform distribution of nickel precursor throughout

the pores of alcogel. Electrolysis of as-generated

alcogels (i.e., without any subsequent treatment)

resulted in the formation of nickel nanoparticles.

Estimations based on comparison between the satu-

ration magnetization of samples with bulk nickel

indicated a nickel content of 27–55 wt% in samples

depending on the concentration of nickel chloride

precursor in solution. Average size of nickel nano-

particles lied between 17 and 20 nm. The presence of

both tetragonal nickel (a solid solution of nickel

602 K

575 K518 K

4

3

2

1

FC

ZFC

without field

50 mA/cm2

XNi

=0.02114

Mag

net

izat

ion

(a.

u.)

Drying cycle ZFC-FC

(a)

400 500

Mag

net

izat

ion

(a.u

.)

Temperature (K)

XNi

=0.0211450 mA/cm2

Mag

net

izat

ion

(a.

u.) (b)

300 400 500 600 700

Mag

net

izat

ion

(a.

u.)

Temperature (K)

Bulk Ni

(c)

Fig. 7 M-TGA curves of a

typical sample after

electrolysis (a and b) and

that of bulk nickel for

comparison (c)

J Nanopart Res (2011) 13:375–384 383

123

containing oxygen) and NiO has been confirmed by

diffraction experiments. Resistivity measurements

show that nickel nanoparticles are separated from

each other due to the presence of Ni(O) between them.

A ferromagnetic–antiferromagnetic transition corre-

sponding to tetragonal nickel was observed at about

10 K. The samples remained in the blocked state up to

about 600 K seemingly due to exchange bias coupling

between nickel and its oxide and interparticle inter-

actions between nickel nanoparticles lying in close

proximity to each other. A relatively sharp transition

in the susceptibility behavior is observed around

575 K, which seems to be related to termination of

nickel–nickel oxide exchange bias effects.

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