Materials Research Bulletin 48 (2013) 739–746

Nanorods, nanospheres, nanocubes: Synthesis, characterization and catalyticactivity of nanoferrites of Mn, Co, Ni, Part-89

Supriya Singh, Pratibha Srivastava, Gurdip Singh *

Department of Chemistry, D.D.U. Gorakhpur University, Gorakhpur 273009, India

A R T I C L E I N F O

Article history:

Received 30 August 2012

Accepted 6 November 2012

Available online 16 November 2012

Keywords:

A. Nanostructures

A. Oxides

C. X-ray diffraction

C. Thermogravimetric analysis

D. Catalytic properties

A B S T R A C T

In this paper, the nanoferrites of Mn, Co and Ni were synthesized by wet chemical method and

characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy

dispersive, X-ray spectra (EDS), transmission electron microscopy (TEM) and high resolution

transmission electron microscopy (HR-TEM). It is catalytic activity were investigated on the thermal

decomposition of ammonium perchlorate (AP) and composite solid propellants (CSPs) using

thermogravimetry (TG), TG coupled with differential scanning calorimetry (TG–DSC) and ignition

delay measurements. Kinetics of thermal decomposition of AP + nanoferrites have also been investigated

using isoconversional and model fitting approaches which have been applied to data for isothermal TG

decomposition. The burning rate of CSPs was considerably enhanced by these nanoferrites. Addition of

nanoferrites to AP led to shifting of the high temperature decomposition peak toward lower

temperature. All these studies reveal that ferrite nanorods show the best catalytic activity superior to

that of nanospheres and nanocubes.

� 2012 Elsevier Ltd. All rights reserved.

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Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Synthesis of spinel ferrite nanostructures is getting enormousattention in recent years due to their surface effect (large surface-to-volume ratio) and quantum confinement effects (size depen-dent properties). Binary spinel ferrites have the structure AB2O4 inwhich A and B display tetrahedral and octahedral cation sites,respectively, and O indicates the oxygen anion site [1]. Structuralparameter of nanoferrites, such as, pore size, large specific surfacearea and large number of cations, for co-ordination sites, cangreatly enhance the rate of chemical reaction and are increasinglygained popularity as reactive nanocrystallites [2,3].

Nanostructures like nanorods, nanotubes, nanowires, nano-belts, nanoplates, nanorings, nanocubes and nanoparticles possessunique electrical, optical, magnetic and catalytic properties incomparison with bulk materials [4–8]. Morphology-controlledsynthesis of nanostructured materials, such as single-crystallineone-dimensional (1D) nanomaterials (wires, rods, belts, cubes andtubes), may present an opportunity for the synthesis of catalyticmaterials with desirable features because these novel materialsnucleate and grow in epitaxial manner, exposing defined crystalplane [9]. Synthesis of nanoferrites has become an important partof modern ceramic research. The wet-chemical synthesis of highly

* Corresponding author. Tel.: +91 551 2200745/2202856; fax: +91 551 2340459.

E-mail address: gsingh4us@yahoo.com (G. Singh).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.11.015

reactive nanocrystallites has advantages over other conventionalmethods to decrease the sintering temperature of ferrites. Thenovelty of this method is to choose the appropriate pH range andcalcinations temperature, various morphology of nanoparticlescan be fabricated. Some of the typical results obtained by thismethod are presented to illustrate its versatility.

Composite solid propellants (CSPs) are the major source ofchemical energy in space vehicles and missiles. Ammoniumperchlorate (AP) is one of the main oxidizing agents that hasbeen used in various CSPs. The burning behavior of propellants ishighly relevant to thermal decomposition of AP. The ballisticproperties of CSPs can be improved by adding a catalyst such asmetal oxides, metal ferrites etc. which accelerates the rate ofdecomposition of AP. Recent investigations have shown that thetransition metal ferrite nanoparticles are highly effective toimprove the thermal decomposition efficiency of AP [10–13].

Herein, we have synthesized nanoferrites with variousmorphologies such as nanorods, nanospheres and nanocubes bywet-chemical method [13] under different synthesis conditions.These were further characterized and used to improve the thermaldecomposition properties of AP and CSPs. We have reporting here acomparative study on the catalytic activities of nanoferrites,including nanorods, nanospheres and nanocubes. The morpholo-gy-dependent catalytic effect of nanoferrites is closely related tothe nature of the exposed active crystal planes on the surface. Anenhanced catalytic activity of nanorods has been found for thermaldecomposition of AP and CSPs over nanosphere and nanocubes.

Fig. 1. XRD pattern of MnF, CoF, and NiF nanoferrites.

S. Singh et al. / Materials Research Bulletin 48 (2013) 739–746740

2. Experimental

2.1. Materials preparation

MnCl2�4H2O, CoCl2�6H2O, NiCl2�6H2O, FeCl3 (Merck), AP wasobtained from CECRI, Karaikudi; NaOH (Merck 98%), HTPB(hydroxylterminatedpolybutadiene) of VSSC, Thiruvanthapuram,IPDI (isophoron diisocynate) of Merck and DOA (dioctyl adipate) ofMerck were of analytical grade and used without furtherpurification. 0.4 M (25 mL) of FeCl3 and 0.2 M (25 mL) of metalchlorides were dissolved in distilled water. The other precipitant isNaOH solution (concentration is 3 mol/L). NaOH solution wasintroduced under magnetic stirring until the pH value of themixture reached to 10–12. The viscous precipitates were thenbrought to a reaction temperature of 80 8C and stirred for 1 h. Afterthe reaction completed, the resultants were washed with distilledwater 2 times then ethanol 2 times to minimize external ions in theproducts and then dried overnight at 100 8C in air. All the productswere calcined at 300 8C, 400 8C and 500 8C for 3 h, respectively, toafford MnFe2O4, CoFe2O4 and NiFe2O4, marked as MnF, CoF and NiFrespectively.

2.2. Characterizations

The average crystallite size and crystallinity of the powderswere analyzed using powder X-ray diffraction by advanced X-raydiffractometer (Bruker AXD8 advance) with Cu Ka (l = 1.5418 A)radiation. The broadening of the peaks indicates nanometer size ofthe ferrites and no peak of impurity was observed in XRD patterns.Particle size of the ferrites was calculated by applying Scherrer’sformula [14]. The morphologies and structures of the calcinedsamples were characterized by field emission scanning electronmicroscopy (FE-SEM). Elemental analysis and atomic ratio of thecalcined products were determined from the, energy dispersive X-ray line spectra (EDS). The particle size, predominantly exposedcrystal planes and distribution of nanoferrites were determinedusing transmission electron microscopy (TEM), high-resolutiontransmission electron microscopy (HR-TEM) and selected areaelectron diffraction (SAED) patterns, respectively.

2.3. Preparation of CSPs

CSPs samples were prepared by dry mixing [15] of AP (100–200mesh) with and without nanoferrites (1 wt%) followed by additionof HTPB. The solid materials were mixed with HTPB in the ratio of3:1 at 70 8C for 1 h. Then, added as binder part curing agent IPDI inequivalent ratio to HTPB and plasticizer DOA, 30% to HTPB. Theprepared propellants were casted into aluminum plates (havingdimensions 1 cm � 3 cm � 10 cm). The samples were cured in anincubator at 70 8C for 9 days [16].

3. Catalytic activity

3.1. Burning rate measurements

The burning rate of cured (CSPs) samples at an ambientpressure was measured by the same method as reported earlier[16–18]. The propellant strands were inhibited by applying PVCtape to check the side burning. The vertically held strands wereignited electrically with the help of a nichrome wire at the top.The time required for a certain length of burning of the strandwas recorded by a stop watch. The burning rate has beencalculated by using l/t where l is length of propellant strand(mm) and t the time (s) of burning. An average of 3measurements were taken within experimental error andresults are reported in Table 2.

3.2. Thermal decomposition

Thermal decomposition was investigated as follows:

3.2.1. Simultaneous TG–DSC

TG–DSC thermograms on AP with and without nanoferrites (bymixing in ratio of 99:1) were recorded on the samples (�12 mg)using Perkin Elmer (Pyris Diamond) under nitrogen atmosphere(flow rate 200 mL/min) at a heating rate of 10 8C min�1 in an openalumina pan.

3.2.2. Non-isothermal TG in static air

Non-isothermal TG studies on AP and CSPs with and withoutnanoferrites (wt � 20 mg) were undertaken in static air at theheating rate of 10 8C min�1 using an indigenously fabricated TG

S. Singh et al. / Materials Research Bulletin 48 (2013) 739–746 741

apparatus [19]. A round bottom gold crucible was used as thesample holder.

3.2.3. Isothermal TG

Isothermal TG of AP with and without nanoferrites(wt � 20 mg) was undertaken in static air using the above saidTG apparatus in temperature range (300–340 8C). The isothermalTG of AP by incorporating varied amount of nanoferrites (0.5, 1, 1.5and 2% by mass) was also done at 300 8C.

Fig. 2. FE-SEM image of (a) MnF nanorods, (b) CoF nanospheres, and (c) NiF

nanocubes.

3.2.4. Kinetic analysis of isothermal TG data

The kinetic analysis using isothermal TG data have been done asreported earlier [20–23].

3.3. Ignition delay (Di) measurements

To understand the thermal stability of AP and CSPs with andwithout nanoferrites, the ignition delay (Di) measurement wereundertaken on a tube furnace technique [24] using 20 mg (100–200mesh) of samples for AP and CSP with and without nanoferrites(temperature range 345–420 8C). The accuracy of the temperature oftube furnace was �1 8C. The sample was taken in an ignition tube(5 cm length � 0.4 cm diameter) and the time interval between theinsertion of the ignition tube into the TF and the moment of appearanceof a flame noted with the help of a stop watch with accuracy of 0.1 sgave the value of ignition delay (Di). The ignition tube clamped in a bentwire was inserted manually into the furnace up to a fix depth (14 cm)just above the probe of the temperature indicator cum controller(Century, Chandigarh). The time for insertion of the ignition tube wasalso kept constant. Each run was repeated three times, and mean Di

values are taken. The Di data were found to fit in equation [25–27],Di ¼ A exp

E�aRT

� �where Di is a time of ignition (ignition delay), A is

constant, E�a is the energy of activation (ignition) and T is the absolutetemperature. This equation has been found to be obeyed by a largenumber of explosives [28–32]. E�a assessed by above equation alongwith the correlation coefficient (r).

4. Results and discussion

Fig. 1(a)–(c) shows the XRD patterns of the calcined productsand shows that all are crystalline. The average size of MnF, CoF

Fig. 3. EDS of (a) MnF nanorods, (b) CoF nanospheres, and (c) NiF nanocubes.

S. Singh et al. / Materials Research Bulletin 48 (2013) 739–746742

and NiF, estimated from the Scherrer equation, are 26.4, 27.4 and31.4 nm respectively (Table 1). The XRD patterns are wellindexed with the MnFe2O4, CoFe2O4 and NiFe2O4 patternsincluding all relative peak intensities (JCPDS file number,respectively 38-0430, 02-1045 and 44-1485), The morphologyof these three ferrites obtained under different syntheticconditions are presented in Fig. 2(a)–(c). From the FE-SEMimages, it is found that the morphologies of nanoferrites areextremely different shapes (with dimension 10–50 nm) of rod,sphere and cube respectively.

Fig. 4. TEM image of (a) MnF nanorods, (b) CoF nanospheres, and (c) NiF nanocubes; HR-

Fig. 3(a)–(c) shows the EDS spectrum of as obtained ferritenanorods, nanospheres and nanocubes revealing that the ferriteproducts are composed of elements of M (Mn, Co and Ni), Fe and O(other peaks arises from CaCO3, SiO2, albite and KCl grid). Theatomic ratio of M:Fe:O is approximately 1:2:4 with the uniformdistribution of the elements suggests that the prepared productsare indeed in the form of MnFe2O4, CoFe2O4 and NiFe2O4. The TEMimages of nanoferrites are shown in Fig. 4. The micrograph ofnanoferrite (Fig. 4(a)) shows the typical one-dimensional nanorodswith lengths up to 150 and 15–20 nm in width. While for the

TEM and SAED pattern of (d) MnF nanorods, (e) CoF nanospheres, (f) NiF nanocubes.

Table 2Burning rate of HTPB-AP/CSPs with and without nanoferrites.

Additive Burning rate (mm/s) rc/r*

Nil 1.19 1.00

MnF 2.84 2.38

CoF 2.00 1.68

NiF 1.71 1.43

r and r* = Burning rate of propellant with and without nanoferrites.

Table 1Crystal size of nanoferrites.

Nanoferrites Particle size/nm

XRD TEM

MnF 27.4 20

CoF 26.4 24

NiF 31.4 27

S. Singh et al. / Materials Research Bulletin 48 (2013) 739–746 743

samples (Fig. 4(b) and (c)) nanospheres and nanocubes areobserved. The result indicates that average size (20–30 nm) andmorphologies (rod, sphere and cube) are matched well with theestimated size and shape obtained from the XRD and FE-SEMrespectively. Fig. 4(d)–(f) displays the HRTEM images of anindividual nanorod, nanosphere and nanocube. Lattice spacingof 0.39 nm indicated that MnF nanorods predominantly expose thewell-defined and less stable (0 0 1) and (1 1 0) planes, whereas inCoF nanosphere and NiF nanocubes (0.31 nm lattice spacing) themost stable (1 1 1) planes are predominantly exposed. The SAEDpattern (inset images in Fig. 4(d)–(f)) demonstrates that productsare single crystal.

Results reported in Table 2 clearly shows that burning rate isenhanced when nanoferrites are used as catalysts. Increase ofburning rate might be due to the enhanced decomposition of HTPBor AP or CSP. In order to find out which of the above processes is

Fig. 5. TG curve of HTPB with and without nanoferrites.

Fig. 6. TG curves of AP with and without nanoferrites.

accelerated, TG studies on HTPB + nanoferrites, AP + nanoferritesand CSP + nanoferrites were undertaken. From Fig. 5, it is clear thatthe rate of HTPB decomposition is enhanced when catalysts wereadded, which may be due to increase in oxidative degradation [17].It is also known that traces of nanoferrites, catalyze the homolyticdecomposition of the hydroperoxides which are formed duringoxidative degradation of the HTPB [12]. TG curve (Fig. 6) for APwith nanoferrites confirm beyond doubt that nanoferrites affectboth low temperature decomposition (LTD) and high temperaturedecomposition (HTD) of AP, to form gaseous products.

The TG–DSC curve (Fig. 7) also verifies the results obtainedfrom TG. The DSC curve for AP has apparently three stages. In thefirst stage, the endothermic peak appears at 242 8C, represents the

Fig. 7. TG–DSC curve of AP and AP with nanoferrites (1 wt%).

Fig. 9. Isothermal TG of AP with and without nanoferrites on 300 8C by varying the

percentage of nanoferrites.

Fig. 8. TG curves of CSPs with and without nanoferrites.

S. Singh et al. / Materials Research Bulletin 48 (2013) 739–746744

transition from orthorhombic to cubic form [33]. In the secondstage, the exothermic peak at 310 8C is attributed to the partialdecomposition of AP. The third main exothermic peak appears atrelatively higher temperature of 430 8C, indicating completedecomposition of AP. When MnF, CoF and Nif were added to AP,the first exothermic peak shifted from 310 8C to 302 8C, 301 8C and294 8C respectively and second exothermic peak significantlyshifted from 430 8C to 335 8C, 344 8C and 357 8C respectively. MnFrods are expected to posses better catalytic properties onthermolysis of AP by predominantly exposing the reactive planesand greater number of well defined active sites.

The data reported in Fig. 8 clearly show that the rate ofdecomposition of CSPs is also enhanced in presence of nanoferriteshaving finer particle size, large number of defect and dislocationsand reactive planes. Thus, a large number of active site would beavailable for the adsorption of reactants, as a consequence, the rateof reaction would be increased. Thus nanoferrites affect all the threeprocess, viz. decomposition of HTPB, oxodizer (AP) and propellants(CSPs). The effects of nanoferrites on the thermolysis of AP byvarying their amount (Fig. 9) clearly shows that increase of catalystmay increase the thermal decomposition of AP. Thus the catalyticactivity was found to depend on the percentage of nanoferrites in AP.

The set of reaction model [21] were used to analyze isothermalTG data (Fig. 10) to calculate the Ea values for thermaldecomposition of AP and AP + nanoferrites (1 wt%). In the modelfitting method, the kinetics is analyzed by choosing a best fitmodel based on the values of correlation coefficient ‘r’ close to 1.Among the various values of r, calculated for different models, thehighest value of r for AP corresponds to model 5 while for AP withMnF, CoF and NiF corresponds to model 1, 5 and 11. Isothermal TGdata best fitted for model 1, 5 and 11 proves the contention thatthe rate controlling step in the thermal decomposition may bepower law, one dimensional diffusion and three dimensionaldiffusion respectively. Kinetic analysis performed by the iso-conversional method (Fig. 11) on isothermal TG data also showslower value of Ea at each a for AP + nanoferrites than AP. Theresults of kinetic analysis clearly show that, in presenceof nanoferrites, noticeable decrease in the activation energy ofthermal decomposition of AP occurred (Table 3). The energies ofactivation for ignition Ea (Table 4) are lower for AP + nanoferrites(24–30 kJ mol�1) and CSP + nanoferrites (21–31 kJ mol�1) com-pared to AP (40.2 kJ mol�1) and CSP (37.7 kJ mol�1). Plots of Di vs.1/T are presented in Fig. 12.

It is also reported [27,34] that overall decomposition process ofAP takes place by the transfer of proton (N–H bond cleavage) fromthe ammonium ion to ClO4

� to form the NH3 and HClO4 molecule

in the condensed phase before ignition. HClO4 involves inoxidation–reduction with ammonia to form gaseous products.Following is the reaction scheme for AP thermolysis, suggested bymany workers [35–40].

When nanoferrites are used, the tremendous interfacesbetween the AP and nanoferrites are formed in the composites.Therefore, the corresponding active sites in ferrites are highlyassociated with the gaseous reactive molecules, which are formed

Fig. 10. Isothermal TG of AP and AP with and without nanoferrites (1 wt%) at

different temperatures.

Table 3Activation energy (Ea), Arrhenius parameters and correlation coefficient (r) for the isothermal decomposition of AP and AP with nanoferrites.

Model Reaction model AP AP + MnF AP + CoF AP + NiF

Ea (kJ mol�1) r Ea (kJ mol�1) r Ea (kJ mol�1) r Ea (kJ mol�1) r

1 Power law 95.9 0.9609 49.0 0.9991 61.6 0.9233 63.5 0.9488

2 Power law 96.3 0.9612 49.3 0.9990 62.1 0.9258 63.5 0.9496

3 Power law 97.2 0.9598 49.9 0.9619 63.5 0.8776 63.7 0.9296

4 Power law 100.1 0.9634 52.4 0.9985 68.5 0.9523 63.9 0.9598

5 One dimensional diffusion 101.1 0.9636 53.5 0.9982 60.6 0.9593 63.8 0.9632

6 Mampel 99.5 0.9630 52.0 0.9985 67.7 0.9494 63.7 0.9587

7 Avarami-Erofeev 98.5 0.9633 50.8 0.9988 64.6 0.9387 63.6 0.9536

8 Avarami-Erofeev 97.0 0.9616 49.8 0.9989 63.3 0.9316 63.6 0.9518

9 Avarami-Erofeev 97.7 0.9620 50.4 0.9988 64.4 0.9367 63.6 0.9536

10 Contracting sphere 99.2 0.9629 51.7 0.9986 67.1 0.9473 63.7 0.9578

11 Three dimensional diffusion 101.7 0.9634 54.1 0.9978 72.7 0.9631 63.5 0.9657

12 Contrcting cylinder 95.1 0.9629 51.6 0.9986 66.8 0.9436 63.8 0.9573

13 Prout-tompkins 95.9 0.9607 49.1 0.9989 61.9 0.9244 63.4 0.9494

14 Ginstling-Brounshtein 101.5 0.9635 53.9 0.9980 67.3 0.9619 63.6 0.9649

0

1

2

3

4

5

6

0.0014 0.0015 0.0016

1/T

lnDi

AP

AP+MnF

AP+CoF

AP+NiF

0

1

2

3

4

5

0.0014 0.0015 0.0016

1/T

lnDi

HTPB-AP

HTPB-AP+MnF

HTPB-AP+CoF

HTPB-AP+NiF

Fig. 12. Graph of ln Di vs. 1/T (a) AP and (b) CSP with and without nanoferrites.

0

50

100

150

200

0 0.2 0.4 0.6

Alpha

Ea k

J/m

ol

AP

AP+MnF

AP+CoF

AP+NiF

Fig. 11. Dependencies of activation energy on the extent of conversion a.

S. Singh et al. / Materials Research Bulletin 48 (2013) 739–746 745

Table 4Ignition delay (Di) activation energy for ignition (Ea) and correlation coefficient of AP and AP + nanoferrites.

Sample Di (s) Ea (kJ mol�1) r (correlation coefficient)

345 � 1 8C 360 � 1 8C 375 � 1 8C 390 � 1 8C 405 � 1 8C 420 � 1 8C

AP DNI 125 112 95 84 63 40.16 0.9785

AP + MnF (1%) DNI 55 47 43 36 32 24.90 0.9958

AP + CoF(1%) DNI 69 55 52 49 41 26.19 0.9693

AP + NiF(1%) DNI 73 65 55 52 49 30.82 0.9817

HTPB-AP DNI 92 86 67 64 49 37.73 0.9724

HTPB-AP + MnF (1%) DNI 50 42 35 32 27 21.43 0.9860

HTPB-AP + CoF (1%) DNI 52 45 38 35 31 26.56 0.9945

HTPB-AP + NiF(1%) DNI 59 55 49 45 42 31.30 0.9967

DNI = did not ignite.

S. Singh et al. / Materials Research Bulletin 48 (2013) 739–746746

below 350 8C and during the second decomposition step in the gasphase causing complete decomposition of AP. This exhibits HTD atlower temperature with increase in catalytic efficiency. It seems thatthe rod-shaped nanoferrite shows significantly enhanced catalyticactivity by predominantly exposing the reactive planes and welldefined active sites in comparison to nanospheres and nanocubes. Inthe case of MnF nanorods, the more reactive planes are exposed. Onthe other hand, because of the predominant (1 1 1) planes, thecatalytic property of the CoF nanosphere and NiF nanocubes ispoorer than that of the nanorods resulting in a lower activity.

5. Conclusions

In summary, ferrite nanorods, nanospheres and nanocubes havebeen successfully synthesized via a simple wet-chemical methodand characterized by XRD, FE-SEM, EDS, TEM and HR-TEM pattern.These nanoferrites showed good catalytic activity in thermaldecomposition of AP and CSPs. The burning rate of CSPs was alsofound to be enhanced. The catalytic activity of nanoferritesdepends on the calcination temperatures of products as well asthe catalyst addition. The order of catalytic activity of differentcatalysts in thermal decomposition of AP is MnF > CoF > NiF. Thepresent result suggests that catalysts may be designed withpredictable morphologies and explored the optimization of thethermal decomposition of AP, HTPB and CSPs. This paper providesalternative choice of preparing mixed oxides for the thermaldecomposition of AP and AP based propellants.

Acknowledgements

The authors are grateful to Head, Chemistry Department of DDUGorakhpur University for laboratory facility. IIT Roorkee for TG–DSC, STIC Cochin for XRD. Thanks are also due to financialassistance by UGC for providing Emeritus Fellow to G. Singh andDST for INSPIRE fellowship to Supriya Singh.

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