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Nanostructured nickel manganese oxide: aligned nanostructures and theirmagnetic properties

Menaka,a Neha Garg,a Sandeep Kumar,a Deepak Kumar,a Kandalam V. Ramanujachary,b Samuel E. Loflandc

and Ashok K. Ganguli*a

Received 10th May 2012, Accepted 18th July 2012

DOI: 10.1039/c2jm32951d

The present study describes the control of the assembly of NiMn2O4 nanospheres and hexagonal

particles by tuning the precursor (nickel manganese oxalate) morphology. The aspect ratio (2 to 24) of

the nickel manganese oxalate precursors has been achieved by the variation of the surfactant,

co-surfactant and non-polar phase by the reverse micellar route. Slow decomposition of the oxalate

precursor (aspect ratio $ 3) enables us to maintain the anisotropic morphology in the NiMn2O4

nanorods. However, the decomposition of the cuboid-shaped oxalate precursor (aspect ratio < 3) led to

the formation of spherical nanoparticles. The ferrimagnetic transition temperature Tc (126–72 K) and

magnetization decrease with the decrease in the aspect ratio of the nanorods. Hexagonal nanoparticles

of NiMn2O4 show the lowest Tc (�72 K) and magnetization.

1. Introduction

Controlled synthesis of low-dimensional nanostructures

including nanorods, nanowires and nanotubes, has attracted

significant interest because of their size and morphology depen-

dent properties and technological applications compared with

their bulk counterparts.1 The development of viable methodol-

ogies for the assembly of nanospheres into well-defined arrays

has relevance in optoelectronics,2–4 catalysis5 and magnetic

devices.6,7 There have been earlier reports on the control of the

shape and size of binary metal oxalate and metal oxides via

chemical routes.8–12 Among these methods, the microemulsion

method has been efficiently used to control the shape and size of

metal oxalate,10 carbonate11 and borate nanostructures.12 The

alignment of nanospheres into a nanorod has been observed in

CoFe2O4 by the decomposition of nanorods of the iron cobalt

oxalate precursor.13 The assembly of nanospheres into rods is

possible due to the anisotropic morphology of the precursor

which is maintained during the decomposition of the precursor.

The motivation of the present study is to control the aspect ratio

of oxalate precursors and determine the critical aspect ratio

which on decomposition leads to the oxide nanorods comprising

of aligned nanoparticles.

In the Ni–Mn–O system14 several phases (NiMn2O4,

Ni6MnO8, Ni2MnO4, NiMnO3 and many others) are known,

aDepartment of Chemistry, Indian Institute of Technology, Hauz Khas,New Delhi 110016, India. E-mail: ashok@chemistry.iitd.ernet.in; Fax:+91-11-26854715; Tel: +91-11-26591511bDepartment of Chemistry and Biochemistry, Rowan University,Glassboro, NJ-08028, USAcDepartment of Physics and Astronomy, Rowan University, Glassboro, NJ-08028, USA

This journal is ª The Royal Society of Chemistry 2012

among which NiMn2O4 has important applications especially

due to its magnetic and catalytic properties.15–17 NiMn2O4 is also

known for its applications as a solid electrolyte,19 negative

temperature coefficient (NTC) thermistor20 and as a sensor

material.21 NiMn2O4 crystallizes in the spinel structure18 where

Ni2+, Mn2+, Mn3+and Mn4+ (manganese in three different

oxidation states) are distributed among the tetrahedral and

octahedral spinel sites.18 NiMn2O4 has been synthesized by the

solid state,17–21 co-precipitation22 and sol–gel routes.23 Though

there are various reports of nanostructured nickel oxide and

manganese oxide synthesized by a microemulsion-mediated

process, the synthesis of nanostructured nickel manganese oxide

(NiMn2O4) by microemulsion-mediated routes has not been

reported so far. We have earlier reported the synthesis of pure

nickel oxalate, pure manganese oxalate and a mixture of zinc

oxalate and manganese oxalates using the reverse micellar

route.10,11,24,25However, to the best of our knowledge there are no

reports on the synthesis of nanostructures of bimetallic nickel

manganese oxalate using the reverse micellar route.10,11,24,25 It is

known that the possibility of getting binary metal oxalates to

precipitate to form a compound and not a mixture of two

different metal oxalates (manganese oxalate and nickel oxalate)

requires the optimization of conditions where the solubility

product of the desired bimetallic oxalate should be lower than the

individual metal oxalates. Donkova et.al.26 have shown the

formation of a solid solution of nickel manganese oxalate

particles (40 mm) having the composition of NixMn1�x-

C2O4$2H2O (x¼ 0.11 and 0.34). In our study, we have attempted

to tune the aspect ratio of nickel manganese oxalate nanorods

NixMn1�xC2O4$2H2O (x ¼ 0.33), by optimizing the effect of the

surfactant, co-surfactant and non-polar phase. Further, the

anisotropic nickel manganese oxalate nanostructures have been

J. Mater. Chem., 2012, 22, 18447–18453 | 18447

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converted to the corresponding oxide without destroying the

anisotropy of the nanorods by optimizing the kinetics and

temperature of decomposition. The bimetallic oxalates synthe-

sized with varying microemulsions led to the formation of

nanostructured NiMn2O4 (containing mixed-valent manganese)

with different morphologies and size of individual nanoparticles.

Varying with the size and morphology of the nanostructures, the

assembly of nanoparticles gives a higher magnetic susceptibility

and magnetization compared to disordered nanostructures.

Also, nearly five-fold enhancement in the magnetic susceptibility

is observed in anisotropic nanostructures of NiMn2O4 compared

to agglomerated nanostructures (synthesized via the surfactant-

less route) at the same applied field of 0.1 T.

Fig. 1 Powder X-ray diffraction pattern of nickel manganese oxalate

Ni0.33Mn0.67(C2O4)$2H2O obtained by (a) the surfactant-less route, and

using the microemulsion systems (b) CTAB/isooctane/butanol/aq., (c)

CTAB/cyclohexane/butanol/aq., (d) Tergitol/cyclohexane/butanol/aq.,

(e) Tergitol/cyclohexane/hexanol/aq., and (f) Tergitol/cyclohexane/octa-

nol/aq.

2. Results and discussion

In the present study we have focused on three points: (a) control

of the shape and size of the nanostructured oxalate precursor

Ni0.33Mn0.67(C2O4)$2H2O by variation of the surfactant, co-

surfactant and non-polar phase, (b) optimization of the condi-

tions for the thermal decomposition of the oxalate precursor to

obtain nanostructured NiMn2O4 with controlled size, aspect

ratio and (c) investigation of the magnetic behavior of aniso-

tropic nanostructures of NiMn2O4.

2.1 Nickel manganese oxalate Ni0.33Mn0.67(C2O4)$2H2O

precursor

The powder X-ray diffraction pattern of the oxalate precursor

obtained from Tergitol (non-ionic surfactant, ME: Tergitol/

cyclohexane/A/aq., A ¼ 1-butanol, 1-hexanol and 1-octanol) as

well as the CTAB surfactant (cationic surfactant, ME: CTAB/A/

1-butanol/aq., A ¼ cyclohexane or isooctane) at room temper-

ature by the microemulsion method led to the formation of nickel

manganese oxalate dihydrate which is isostructural with nickel

oxalate Ni(C2O4)$2H2O (Fig. 1a–f) which crystallizes in the

monoclinic system with the space group C2/c and refined lattice

parameters of a ¼ 11.771(2) �A, b ¼ 5.334(5) �A, c ¼ 9.833(4) �A

and b ¼ 127.21(5)�. No additional reflection due to manganese

oxalate dihydrate Mn(C2O4)$2H2O was detected which confirms

the formation of a solid solution of nickel manganese oxalate

dihydrate (Ni0.33Mn0.67C2O4)$2H2O instead of a mixture of

nickel oxalate and manganese oxalate. Studies on bulk nickel

manganese oxalates NixMn3�x(C2O4)3$6H2O have shown that a

solid solution is formed in the composition range of 0.5 < x < 2.26

Further, to ascertain the Ni : Mn ratio in the oxalate precursor

atomic absorption spectroscopic analysis (AAS) and quantitative

TEM-EDX analysis of the products showed that the ratio of

Ni : Mn is close to 1 : 2. The details of compositions evaluated by

atomic absorption studies (AAS) have been given in Table 1

which shows that there is no marked change from the loaded

composition. Thus, we have successfully obtained solid solutions

of nanostructured nickel manganese oxalates having composi-

tions Ni0.33Mn0.67(C2O4)$2H2O using the reverse micelle route.

It may be noted that the earlier26 nickel manganese oxalate solid

solution was obtained only by the co-precipitation route which

gives micron-sized particles. The benefit of using the oxalate

ligand as a chelating agent is to maintain the stoichiometry of

nickel and manganese in the starting precursor. These precursors

18448 | J. Mater. Chem., 2012, 22, 18447–18453

are very useful in obtaining the oxides of the desired composi-

tion. Thermogravimetric studies of the oxalate precursor

obtained from CTAB microemulsions show that loss of water

starts at 140 �C and the subsequent decomposition of oxalate

takes place at 310 �C (Fig. 2). We have also studied the surfac-

tant-less process which leads to the formation of micron-sized

irregular particles (Fig. 3a). To control the shape and size of the

oxalate precursor, the effect of the non-polar phase, surfactant

and co-surfactant on the morphology of the oxalate precursor

has been studied.

The synthesis of the oxalate precursor (Ni0.33Mn0.67-

C2O4$2H2O) has also been carried out by varying the nonpolar

phase (isooctane and cyclohexane) with CTAB as the surfactant.

The precursor obtained using isooctane shows the formation of

highly uniform rods (150 nm� 2.5–3.5 mm, aspect ratio¼ 16–23)

(Fig. 3b), while using cyclohexane as the non-polar phase leads to

the formation of uniform nanorods with much lower aspect ratio

(0.55 mm � 2 mm, aspect ratio ¼ 3.6) (Fig. 3c). The marked

decrease in the aspect ratio (nearly fivefold) from 16 to 3.6 is

attributed to the viscosity of the non-polar phase (cyclohexane:

0.89 cP and isooctane: 0.50 cP). During the mixing of the reverse

micelles exchange of reactants takes place leading to precipita-

tion within the surfactant aggregates (reverse micelles in this

case). This intermicellar exchange has an important role in

controlling the size as well as morphology of the particles. In the

present situation, it may be possible that the more viscous

cyclohexane hinders the process of intermicellar exchange of

reactants and hence the growth of the rods slows down compared

to the reactions using the less viscous isooctane. So, in the

presence of isooctane, the exchange rate would be high and hence

the observed aspect ratio of the rods (AR ¼ 16–23) is much

higher than the rods obtained using cyclohexane (AR ¼ 3.6). In

the present study, one of our main objectives is to control the

This journal is ª The Royal Society of Chemistry 2012

Table 1 Compositional analysis by AAS and TEM-EDX studies of the nickel manganese oxalate precursor and nickel manganese oxides

S. no. SystemLoaded composition(Ni : Mn molar ratio)

Atomic absorptionanalysis of the oxalateprecursor, (Ni : Mnmolar ratio), numbers in bracketsindicate the standarddeviation calculated from repetitions ofthe sameexperiment for six sets

Atomic absorption analysis ofnickel manganese oxideobtained at 500 �C(Ni : Mn ratio)

1. Co-precipitation 1 : 2 AAS: 1 : 1.998 (5), TEM-EDX:1 : 1.991 (2)

AAS: 1 : 1.992(5), TEM-EDX:1 : 1.899(4)

2. CTAB + 1-butanol + isooctane+ aq.

1 : 2 AAS: 1 : 1.995(2), TEM-EDX:1 : 1.989

AAS: 1 : 1.997(5), TEM-EDX:1 : 1.192 (5)

3. CTAB + 1-butanol + cyclohexane+ aq.

1 : 2 AAS: 1 : 1.997 (4), TEM-EDX:1 : 1.192 (6)

AAS: 1 : 1.995(4), TEM-EDX:1 : 1.995 (3)

4. Tergitol + 1-butanol + cyclohexane+ aq.

1 : 2 AAS: 1 : 1.991 (3), TEM-EDX:1 : 1.195 (2)

AAS: 1 : 1.997 (3), TEM-EDX:1 : 1.891 (2)

5. Tergitol + 1-hexanol + cyclohexane+ aq.

1 : 2 AAS: 1 : 1.993 (5), TEM-EDX:1.188 (3)

AAS: 1 : 1.998 (5), TEM-EDX:1.189 (5)

6. Tergitol + 1-octanol + cyclohexane+ aq.

1 : 2 AAS: 1 : 1.994 (5), TEM-EDX:1 : 1.191(3)

AAS: 1 : 1.992 (4), TEM-EDX:1 : 1.195 (4)

Fig. 2 Thermogravimetric analysis of the nickel manganese oxalate

precursor obtained using the Tergitol/cyclohexane/butanol/aq.,

microemulsion.

Fig. 3 TEM micrograph of Ni0.33Mn0.67(C2O4)$2H2O obtained using

(a) the surfactant-less route, and the microemulsion systems (b) CTAB/

isooctane/butanol/aq., (c) CTAB/cyclohexane/butanol/aq., (d) Tergitol/

cyclohexane/butanol/aq., (e) Tergitol/cyclohexane/hexanol/aq., and (f)

Tergitol/cyclohexane/octanol/aq.

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critical aspect ratio beyond which the oxalate precursor leads to

the formation of oxide nanorods (self aligned particles). Also,

our earlier studies on binary metal oxalate nanorods10,11 suggest

that the cationic (CTAB) surfactant leads to the formation of

nanorods of higher aspect ratio compared to the non-ionic

surfactant (Tergitol and TX-100). So, to obtain rods of small

aspect ratio, the charge on the surfactant was varied and Tergitol

(a non-ionic surfactant) instead of the cationic surfactant CTAB

was used as a surfactant in the presence of 1-butanol (co-

surfactant) and cyclohexane (non-polar phase). The oxalate

precursor obtained using Tergitol (surfactant) and 1-butanol (co-

surfactant) leads to the formation of cuboidal particles

(0.49 mm� 0.56 mm, aspect ratio¼ 1.12) (Fig. 3d) while using the

cationic surfactant (CTAB) we obtained a rod-shaped

morphology as shown in Fig. 3b and c. TEMmicrographs of the

nickel manganese oxalate precursor obtained using 1-hexanol

This journal is ª The Royal Society of Chemistry 2012

and 1-octanol as co-surfactants in the presence of Tergitol and

cyclohexane show the formation of rods of length ¼ 0.70 mm,

dia¼ 0.35 mm and aspect ratio¼ 2 (Fig. 3e) and length¼ 1.5 mm,

dia ¼ 0.5 mm, aspect ratio ¼ 3 (Fig. 3f) respectively. It may be

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noted that with the increase of the chain length of the co-

surfactant (from C-4 to C-8), the aspect ratio of the nanorods

increases. This is due to the increase in surfactant film flexibility

which leads to increase in the intermicellar exchange rate. This is

in close agreement with our earlier study12 in which we have

found that the aspect ratio of nanorods of nickel borate increases

by increasing the carbon chain length of the co-surfactant.

Dynamic light scattering studies show that the droplet sizes of

the Ni2+ and Mn2+ microemulsions containing CTAB and Ter-

gitol surfactants are 10–20 nm and 8–10 nm respectively. Though

there is no marked variation in the droplet size with respect to the

surfactant/co-surfactant, however, a drastic change in the surface

charges has been observed (Table 2). The surface charge of nickel

manganese oxalate obtained via the co-precipitation route was

found to be negative, �23.55 mV (Table 2), while nickel

manganese oxalate obtained using CTAB and Tergitol shows a

surface charge of +3.74 mV to +7.57 mV and �11.7 mV

to �12.8 mV respectively (Table 2). So it appears that the

cationic surfactants (CTAB) having a positive charge on their

head group facilitate the assembly of surfactant molecules on the

surface of the nanorods due to the electrostatic interaction

between negatively charged nickel manganese oxalate and CTAB

which subsequently prevents the growth along the diameter (with

resultant positive zeta potential) and results in the formation of

nanorods with high aspect ratio. However, the nonionic surfac-

tants (Tergitol) will not have such an assembly of surfactants due

to the absence of electrostatic interactions between the surfactant

and nickel manganese oxalate surfaces and hence growth will be

more uniform which explains the formation of cuboids and rods

of low aspect ratio. These studies are in close agreement with

earlier reports on other metal oxalate nanorods and nano-

cubes.10,11 So, apart from the surfactant film flexibility control by

the co-surfactant chain length (as discussed earlier), the surface

charge of the surfactant also plays a crucial role in stabilizing

anisotropic nanostructures. In the oxalate precursor, the cationic

surfactant (CTAB) leads to the formation of nanorods with high

aspect ratio (16–24) while the non-ionic surfactant (Tergitol)

leads to the formation of cuboids. However, by the variation of

the co-surfactant chain length (even while using the non-ionic

Tergitol surfactant), it is possible to increase the aspect ratio of

the rods upto 3.

2.2 Nickel manganese oxide

The major challenge in the oxalate precursor route is to maintain

the anisotropy during the decomposition of the oxalate precursor

to the corresponding oxide. In most of the earlier reports, the

synthesis of metal oxides from anisotropic metal oxalates leads to

spherical nanoparticles of oxides in which anisotropy of the

Table 2 Zeta potential studies of nickel manganese oxalate

System Zeta (in mV)

Co-precipitation �23.55CTAB + 1-butanol + isooctane + aq. +3.74CTAB + 1-butanol + cyclohexane + aq. +7.57Tergitol + 1-butanol + cyclohexane + aq. �12.8Tergitol + 1-hexanol + cyclohexane + aq. �11.8Tergitol + 1-octanol + cyclohexane + aq. �11.7

18450 | J. Mater. Chem., 2012, 22, 18447–18453

oxalate (carboxylate) precursor is lost during thermal decom-

position.24,27 However, the key idea of the present work is to

make oxalate precursor nanorods of various aspect ratios and

further decomposition of these oxalate precursors to get aligned

nanoparticles (or nanorods) of NiMn2O4. Alignment of

CoFe2O4 nanoparticles13 by thermal decomposition of

CoFe2(C2O4) oxalate rods (aspect ratio of 200) leads to the

formation of aligned nanoparticles (size �10 nm). As discussed

later (in the Experimental section) we have slowly heated the

bimetallic oxalate precursor at 800 �C (heating rate of 70 �C per h

for 12 h). The powder X-ray diffraction (PXRD) pattern of the

oxide obtained from the decomposition of Ni0.33Mn0.67(C2O4)$

2H2O (using CTAB/Tergitol surfactant with a nonpolar phase

cyclohexane/isooctane and cosurfactant (1-butanol/1-hexanol/1-

octanol)) was satisfactorily indexed on the basis of the cubic cell

(space group Fd3m) with a refined cell parameter of 8.403 (2)�A as

reported for nickel manganese oxide (NiMn2O4) (JCPDS # 71-

0852) (Fig. 4). The TEM micrograph of the nickel manganese

oxide NiMn2O4 obtained at 800 �C via the surfactant-less route

shows the formation of agglomerates comprising of 10–15 nm

particles (Fig. 5a). However, the same type of methodology with

a surfactant (CTAB/isooctane/butanol/aq.), leads to the forma-

tion of rods (0.15 mm � 3.5 mm, AR � 24) (Fig. 5b) consisting of

nanoparticles (10–15 nm) (inset of Fig. 5b). On variation of the

solvent (non-polar phase) to cyclohexane, the above micro-

emulsion method results in the formation of rods (L ¼ several

micrometers, dia ¼ 1 mm) (Fig. 5c) made up of hexagonal

particles �100 nm. Further, quantitative analysis using TEM-

EDX studies of NiMn2O4 confirms the presence of Ni and Mn

with a ratio close to 1 : 2 (Table 1). It may be noted that

NiMn2O4 rods obtained using isooctane as the nonpolar phase

assembled from small spherical nanoparticles (10–15 nm) while

hexagonal particles of NiMn2O4 were observed to align into rods

when cyclohexane was used (instead of isooctane). Earlier,

NiMn2O4 nanoparticles (�20 to 40 nm) were synthesized by the

sol–gel route.28 However, to the best of our knowledge there are

Fig. 4 Powder X-ray diffraction pattern of nickel manganese oxide

NiMn2O4 obtained at 800 �C from the oxalate precursor using (a) the

surfactant-less route, (b) CTAB/isooctane/butanol/aq., (c) CTAB/cyclo-

hexane/butanol/aq., (d) Tergitol/cyclohexane/butanol/aq., (e) Tergitol/

cyclohexane/hexanol/aq., and (f) Tergitol/cyclohexane/octanol/aq.

This journal is ª The Royal Society of Chemistry 2012

Fig. 5 TEM micrograph of pure NiMn2O4 obtained at 800 �C from the

oxalate precursor using (a) the surfactant-less route, (b) CTAB/isooctane/

butanol/aq., (c) CTAB/cyclohexane/butanol/aq., (d) Tergitol/cyclo-

hexane/butanol/aq., (e) Tergitol/cyclohexane/hexanol/aq., and (f) Tergi-

tol/cyclohexane/octanol/aq.Fig. 6 HRTEM micrograph of nickel manganese oxide obtained at

800 �C from the oxalate precursor using (a) the surfactant-less route, (b)

CTAB/isooctane/butanol/aq., (c) CTAB/cyclohexane/butanol/aq., (d)

Tergitol/cyclohexane/butanol/aq., (e) Tergitol/cyclohexane/hexanol/aq.,

and (f) Tergitol/cyclohexane/octanol/aq.Dow

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no reports so far on the assembly of the nanoparticles or nanorod

formation of NiMn2O4. Also, the above method leads to the

smallest individual nanoparticles (�10 to 15 nm). So far we have

discussed the variation of the size and aspect ratio of oxide

nanoparticles due to the variation in the surfactant and solvent.

Using the same condition (Tergitol: surfactant, cyclohexane:

nanopolar phase) if the co-surfactant is varied with 1-butanol,

1-hexanol, 1-octanol, we obtain hexagonal particles of diameter

154 nm, 206 nm, and 280 nm respectively (Fig. 5d–f). It may be

noted that NiMn2O4 obtained from the oxalate precursor (aspect

ratio 3) leads to the formation of an assembly limited to few

particles. However, NiMn2O4 obtained by the decomposition of

rods of lower aspect ratio (�2) show agglomerated hexagonal

particles (no assembly). Thus, the anisotropy of the nano-

structured oxalate precursor was maintained in the oxide after

decomposition only when the aspect ratio of the oxalate

precursor is higher than 2. The high resolution TEMmicrograph

of NiMn2O4 obtained using reverse micellar as well as co-

precipitation routes shows the presence of (311) lattice fringes

which confirms the single crystalline nature (Fig. 6a–f).

Fig. 7 Xm vs. T plot of pure NiMn2O4 obtained at 800 �C from the

oxalate precursor using (a) CTAB/isooctane/butanol/aq., (b) CTAB/

cyclohexane/butanol/aq., (c) Tergitol/cyclohexane/octanol/aq., (d) Ter-

gitol/cyclohexane/butanol/aq., (e) Tergitol/cyclohexane/hexanol/aq., and

(f) the surfactant-less route.

2.3 Magnetic properties of nickel manganese oxide (NiMn2O4)

The magnetization of nanostructured NiMn2O4 obtained under

different conditions suggests ferrimagnetic behavior with the

ordering temperature ranging between 72 and 145 K (Fig. 7a–f).

Bulk NiMn2O4 has reported Curie temperature Tc values

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between 122 K and 145 K.23,27,28 NiMn2O4 nanoparticles

obtained via the co-precipitation route show a Tc of 126 K. The

assembled nanostructures of NiMn2O4 obtained by thermal

decomposition of nickel manganese oxalate obtained using

CTAB/isooctane/Bu-OH/aq., CTAB/cyclohexane/Bu-OH/aq.

and Tergitol/cyclohexane/octanol/aq. show slightly lower Tc

values of 118 K, 111 K and 103 K respectively. Further, the

agglomerated hexagonal particles of NiMn2O4 (obtained by

thermal decomposition of the oxalate precursor made with

Tergitol/cyclohexane/butanol/aq., Tergitol/cyclohexane/hex-

anol/aq.) show a markedly low Tc of 72 K. The large variations

in the ordering temperature in both bulk and nanoscale parti-

cles may likely be the result of mixing of Ni and Mn in the

octahedral and tetrahedral sites. However, it is important to

note that the effective moment was calculated in the tempera-

ture range of 200–300 K, where the inverse of the magnetic

susceptibility varies almost linearly with temperature, yielding a

value of 6.88 mB/F.U. which is close to the reported magnetic

moment (6.9 mB/F.U.) for bulk NiMn2O4 (Fig. 8).27,28. A similar

trend (as shown by the plots of magnetic susceptibility in

Fig. 8(b)) has been observed for the saturation magnetization

where the aligned nanostructures show larger magnetization

compared to the disordered nanoparticles (Fig. 9).

3. Experimental

Commercially available Tergitol NP-9 (Sigma Aldrich, 99%) and

CTAB (cetyltrimethyl ammoniumbromide) (Spectrochem, AR,

99%) were used as the surfactants in the microemulsion mediated

synthesis of nanostructured nickel manganese oxalate. Nickel

nitrate (Fisher Scientific, 98%), manganese acetate (CDH,

99.5%), ammonium oxalate (Spectrochem, 98%), 1-butanol

(Qualigens, 99.5%), 1-hexanol (Spectrochem, 98%), 1-octanol

(Spectrochem, 99%), isooctane (Spectrochem, 99%) and cyclo-

hexane (Fisher Scientific, 99%) were used in the synthesis.

Different microemulsions (with varying surfactants and co-

surfactants) were used to control the morphology of the nano-

structured nickel manganese oxalate. The details of the process

are as follows:

Fig. 8 Variation of (a) Tc with size and morphology and

18452 | J. Mater. Chem., 2012, 22, 18447–18453

3.1 Synthesis of the Ni0.33Mn0.67(C2O4)$2H2O precursor using

a non-ionic surfactant (Tergitol)

Tergitol NP-9 was used as the surfactant with a surfactant to

water ratio (W0) of 14 and cyclohexane was used as the oil

phase. To study the effect of the co-surfactant, alcohols

with varying carbon chain length (1-butanol, 1-hexanol and

1-octanol) were used. All the other parameters were kept

constant. Three separate microemulsions were prepared each

containing Ni2+, Mn2+ and oxalate ions. The microemulsions

containing nickel and manganese were mixed and stirred for

2 h. To the resulting solution, the third microemulsion con-

taining oxalate ions was added and stirred overnight. After

reducing the volume to half, acetone was added and the solu-

tion was heated slowly to further reduce the volume (to half)

before centrifugation, and then the product was washed

with acetone. We have also synthesized nickel-manganese

oxalate by the co-precipitation method (no surfactant or oil) to

compare the morphology of the oxalates with and without the

microemulsion. In the co-precipitation (surfactant-less) route,

the Ni2+, Mn2+ and oxalate solution were mixed and

stirred overnight. The precipitate was filtered and then dried at

room temperature to obtain the nickel manganese oxalate

precursor.

3.2 Synthesis of the Ni0.33Mn0.67(C2O4)$XH2O precursor

using a cationic surfactant (CTAB)

Microemulsions with cetyltrimethylammonium bromide

(CTAB) as the surfactant, 1-butanol as the cosurfactant, and

isooctane or cyclohexane as the nonpolar phase and the aqueous

phase (containing metals and oxalate ions) were prepared. Three

separate microemulsions were prepared, each containing Ni2+,

Mn2+ and oxalate ions. The microemulsions containing nickel

and manganese were mixed and stirred for 2 h. To the resulting

solution, the third microemulsion containing oxalate ions was

added and stirred overnight. The precipitate obtained was

separated from the apolar solvent and washed with a 1 : 1

mixture of CHCl3 and CH3OH. The precipitate was dried at

room temperature.

(b) Xm with temperature of different morphologies.

This journal is ª The Royal Society of Chemistry 2012

Fig. 9 M–H plot of pure NiMn2O4 obtained at 800 �C from the oxalate

precursor using (a) the surfactant-less route, and the microemulsion

systems (b) CTAB/isooctane/butanol/aq., (c) CTAB/cyclohexane/

butanol/aq., (d) Tergitol/cyclohexane/butanol/aq., (e) Tergitol/cyclo-

hexane/hexanol/aq., and (f) Tergitol/cyclohexane/octanol/aq.

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3.3 Synthesis of nickel manganese oxide (NiMn2O4) from a

nickel manganese oxalate precursor synthesized using CTAB and

Tergitol as the surfactant

Nickel manganese oxalate precursors obtained from the Tergitol

and CTAB microemulsion system were calcined at 800 �C for

12 h in air (the heating rate and cooling rate were maintained at

70 �C per h), leading to the formation of nickel manganese oxide

NiMn2O4.

3.4 Characterization

Powder X-ray diffraction studies were carried out on a Bruker

D8 Advance diffractometer using Ni-filtered Cu-Ka radiation

with a step size of 0.02� and a step time of 1 s. Raw data were

subjected to background correction and Ka2 lines were removed.

TEM studies were carried out using a Tecnai G2 20 electron

microscope operated at 200 kV. TEM specimens were prepared

by dispersing the oxide powder in ethanol by ultrasonic treat-

ment. A few drops were poured onto a porous carbon film sup-

ported on a copper grid and then dried in air. Temperature and

field-dependent magnetization measurements were carried out

with a Quantum Design Physical Properties Measurement

system. The magnetization was measured at temperatures

ranging from 3 to 300 K, in an applied field of 1 Tesla.

4. Conclusions

We have demonstrated the synthesis of pure phases of

Ni0.33Mn0.67(C2O4)$2H2O anisotropic nanostructures using

microemulsion and co-precipitation routes. The variation of the

co-surfactant and surfactant produced a variety of morphologies

of the oxalate (cuboids and rods). Using a non-ionic surfactant

(Tergitol), an attempt to reduce the surfactant film flexibility (by

increasing the hydrophobic chain length of the co-surfactant)

leads to increase in the aspect ratio of the nickel manganese

oxalate precursor. The use of a cationic surfactant (CTAB) leads

to highly uniform rods (length ¼ 2.5–3.5 mm, dia 150 nm, aspect

This journal is ª The Royal Society of Chemistry 2012

ratio ¼ 16–24). The key idea in this study was to maintain the

anisotropy of the oxalate precursor in the oxide nanostructures.

We observe a ‘critical aspect ratio’ of oxalate precursor rods,

beyond which the anisotropy is maintained in the oxide nano-

structures. Ni0.33Mn0.67(C2O4)$2H2O led to the formation of

NiMn2O4 (spinel) nanorods made up of an assembly of spherical

or hexagonal nanoparticles when the aspect ratio of the oxalate

precursor was greater than 2. The magnetic properties of the

spinel are rather complex and may be related to the occupancy of

the transition metal ions in the tetrahedral and octahedral sites as

well as the anisotropy of the structure.

Acknowledgements

Menaka and NG thank UGC, Govt. of India for a fellowship.

AKG thanks CSIR and DST, Govt. of India for financial

assistance. SEL acknowledges support from NSF grant DMR

0908779.

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