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Radiation Physics and Chemistry 78 (2009) 939–944

Contents lists available at ScienceDirect

Radiation Physics and Chemistry

0969-80

doi:10.1

� Corr

E-m

(R.T. Ols1 Te2 Te

journal homepage: www.elsevier.com/locate/radphyschem

Synthesis and characterization of MnO2 colloids

Pooja Yadav a,�, Richard T. Olsson b,1, Mats Jonsson a,2

a School of Chemical Science and Engineering, Nuclear Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Swedenb Department of Fiber and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden

a r t i c l e i n f o

Article history:

Received 22 August 2008

Accepted 18 February 2009

Keywords:

Nanostructures

Electron microscopy (TEM and SEM)

Oxidation

Gamma radiolysis

Metal oxide

6X/$ - see front matter & 2009 Published by

016/j.radphyschem.2009.02.006

esponding author. Tel.: +4687908789; fax: +

ail addresses: poojay@kth.se (P. Yadav), richar

son), matsj@kth.se (M. Jonsson).

l.: +4687907640.

l.: +4687909123; fax: +4687908772.

a b s t r a c t

This work addresses the issue of radiation chemical synthesis of MnO2 nanoparticles and also illustrates

the ease of formation of nanorods and sheets by adroit manipulation of experimental conditions. The

radiation chemical yield (G-value) for reduction of Mn (VII) by the hydrated electron was found to be

0.27mmol J�1 and 0.17mmol J�1 respectively, when tert. butanol and isopropanol were used as

scavengers in nitrogen-saturated solutions. The colloids formed upon irradiation of air-saturated

solution and N2-purged solution with tert. butanol as scavenger were found to be most stable.

Irradiation of air-saturated solution containing 4�10�4 M KMnO4 at a dose of 1692 Gy resulted in the

formation of nanorods of the dimension 100–150 nm and nanospheres in the range 10–20 nm.

Irradiation of N2-purged solution containing tert. butanol as scavenger for dOH-produced reticulated

structure of nanorods with length varying from 50 to 100 nm at a dose of 1692 Gy. Elemental analysis

was performed using scanning electron microscope on MnO2 formed by reduction and oxidation and

the purity was found to be 98% of elemental Mn content.

& 2009 Published by Elsevier Ltd.

1. Introduction

Nanomaterials have captured the imagination of researcherslately due to the significant difference in their propertiescompared to their coarse-grained counterpart. The greater surfaceto volume ratio and specific binding sites of nanoparticlesenhance catalytic properties (Hiroki and La Verne, 2005).Manganese dioxide is a fascinating inorganic metal oxide owingto its wide range of applications in catalysis, ion exchange,molecular adsorption and particularly in energy storage and alsobecause of its low cost and environmentally benign nature(Nalwa, 2000; Burda et al., 2005). Amongst other things activatedMnO2 is widely used in lithium batteries as lithium intercalationhost and also as cathode material in primary alkaline batteries(Yuan et al., 2003). One of the challenges facing the chemiststoday is to synthesize well-defined mono disperse nanoparticles.

Various polymorphs of MnO2 exist in nature as the basicoctahedral unit (MnO6) and can be linked in different ways. Theirproperties depend on the crystallographic forms. For example,a-MnO2 phase is very favorable to intercalation and b-MnO2 ispassive to it. Therefore, the controlled synthesis of MnO2 hasalways been the objective of synthetic chemists. It has been

Elsevier Ltd.

4687908772.

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shown by the density functional theory (DFT) calculations thatg-MnO2 is the energetically favored structure (Balachandran et al.,2003; Sayle et al., 2005). Several methods have been developedfor MnO2 synthesis ranging from simple reduction (Kim andPopov, 2003; Jeong and Manthiram, 2002), oxidation (Wang andLi, 2002), co-precipitation (Burda et al., 2005; Toupin et al., 2002),sol-gel (AL-Sagheer and Zaki, 2000), thermal decomposition, etc.(Lee and Goodenough, 1999). Radiation chemistry is an effectivetool for the synthesis of particles of nanometer dimension owingto facile manipulation of dose and experimental conditions toobtain the required size distribution. It was effectively shown byHenglein that the colloids formed by radiation-induced reductionwere smaller than those formed by co-precipitation (Lume-Pereira et al., 1985, Baral et al., 1985, 1986).

Henglein et al. reported the synthesis of MnO2 colloids byradiolytic reduction of KMnO4 in air-saturated solution at pH 10(Lume-Pereira et al., 1985). They further studied the reaction ofcolloids with various radicals. A size range 3–5 nm was reportedfor a dose of 700 Gy and the radiolytically produced colloids weresmaller in size compared to those prepared by co-precipitation.However, there was no information about the nature and shape ofthe particles. Henglein also described the formation of colloids byoxidation of Mn2+ by dOH, formation of colloids in the pH range3.5–9 was observed (Baral et al., 1985). Size of the particles werenot reported, however, based on the UV-absorption spectra alarger size was predicted compared to that formed from Mn (VII)reduction. Further, they studied the reaction of sols with1-hydroxy-1-methylethyl radical and suggested the formation of

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P. Yadav et al. / Radiation Physics and Chemistry 78 (2009) 939–944940

Mn3+ centers. More recently the MnO2 colloids were synthesizedby radiolytic reduction of KMnO4 with isopropanol as scavengerand using polyvinyl alcohol (PVA) polymer and sodium dodecylsulphate (SDS) as surfactant (Liu et al., 1997). In this study,particles of 6 nm for a dose of 2.01 kGy were obtained. XPSstudies by the same group showed a valency of +4 for the MnO2

colloids.The aim of this work is to identify the optimal conditions for

generation of MnO2 nanoparticles by radiolysis. An effort has beenmade to synthesize colloids by reduction of KMnO4 by variousreducing radicals and also to characterize the MnO2 particles.Alternatively, the synthesis of MnO2 by radiation-induced oxida-tion of Mn (II) has also been attempted.

Table 1The G-values for reduction of 4�10�4 M KMnO4 by various radicals, since KMnO4

oxidizes most alcohols and organic reagents the absorbance was normalized to

correct the background reaction.

Reaction conditions Reactive

species

Ga value at

l540 nm

(mmol J�1)

Percent

background

reaction

Air saturated (dOH, e�aq and

Hd)

0.08 –

N2-purged solution with

(CH3)3COH asdOH scavenger

eaq� 0.27 –

N2 purged solution with

(CH3)2CHOH

as dOH scavenger

eaq� ,

(CH3)2dCOH

0.17 20

N2O-purged solution

with tert. butanol

Tert. butyl

radical

No reduction –

N2O-purged solution

with isopropanol

(CH3)2dCOH 0.14 29

N2O-purged solution

with 4 mM sodium formate

CO2d� 0.17 38

O2-purged solution

with 4 mM sodium formate

O2d� 0.15 37

a The G value was corrected for background reaction.

2. Experimental

KMnO4 and MnSO4 �H2O were obtained from Kebo chemicalswith a purity of 99%. The rest of the chemicals were purchasedfrom SDS, Fluka, Sigma–Aldrich and Merck. The N2O, N2 and O2

gases were procured from Air Liquide and Strandmollen. Allsolutions were freshly prepared using deionised water purified bya Millipore-Milli-Q system having a resistivity of 18 MO cm�1 andthe experiments were carried out at room temperature (�22 1C).The pH of the solution was adjusted by using the NaOH(1�10�4 M) or HClO4.

The mean particle size and size distribution was measuredusing photon correlation spectroscopy with laser of 488 nmwavelength and a fixed scattering angle of 901 (BI–90 particlesize, Brookhaven instruments co., USA). The detection range of PCSis between 10 nm and 3mm and while calculating the mean sizethe software assumes globular particles. However, formation ofnonsperical particles can account for the loss of intensity of thesignal for PCS measurements as the PCS is valid for globularsystems. The geometry affects the translation motion and also thescattering angle will be different for rods and spheres. The countrate (photon counts per second) is proportional to the concentra-tion of a specific size, when the size distribution is nearlyconstant. The refractive index and geometry of the particle caninfluence the intensity of the signal. Wide size distributionpresents greater difficulty as the scattered intensity is a functionof size. Hence, larger particles contribute more to the measuredsignal.

The zeta potential was measured by means of a ZetaPALS zetapotential analyzer (Brookhaven instruments co., USA), where the zpotential was deduced from the particle velocity using Smolu-chowski’s equation (Ledin et al., 1993).

Irradiations were carried out using a gamma cell (Elite 1000)137Cs source. The dose rate was 9.4 Gy/min as determined usingthe Fricke dosimeter. A brief description of generation of radicalfollows.

The major products of water radiolysis are free radicals eaq� , Hd,

dOH, and molecular products like H3O+, H2 and H2O2. The G-valueis defined as moles of species formed or consumed per joule ofabsorbed energy. The G-values are given in parentheses (mmol J�1)(Spinks and Woods, 1990).

H2O dOHð0:28Þ; eaq�ð0:28Þ;Hdð0:047Þ;H2O2ð0:073Þ;H2ð0:047Þ (1)

The reactions of the hydrated electron were studied in N2-saturated aqueous solutions containing 0.2 M tert. butanol toeffectively scavenge dOH.

�OHðH�Þ þ ðCH3Þ3COH! ðCH3Þ2�CH2COHþH2OðH2Þ (2)

The reaction of the 2-hydroxy-2-propyl radical was studiedin N2O-saturated aqueous solutions containing 0.2 M

iso-propyl alcohol.

�OHðH�Þ þ ðCH3Þ2CHOH! ðCH3Þ2�COHþH2OðH2Þ (3)

The reactions of dOH radical were studied in N2O-saturatedsolutions. The solubility of N2O in water is �2.5�10�2 M at 25 1Cand at this concentration, eaq

� is quantitatively converted into dOH.

N2Oþ e�aq�!H2O �OHþ OH� þN2 (4)

3. Results and discussion

3.1. Reduction of permanganate

The aqueous solution of 4�10�4 M KMnO4 at pH 10 was girradiated under various conditions to generate the radical ofinterest. The MnO4

� concentration was measured by UV–Visspectroscopy at 540 nm, which is the absorption maximum forpermanganate. Table 1 lists the G-values obtained for reduction ofKMnO4 by various reducing radicals. In case of air-saturatedsolution the G-value for reduction of KMnO4 is 0.08mmol J�1 andin N2-purged solutions containing tert. butanol the G-value is0.27mmol J�1, i.e. almost identical to the G-value for the solvatedelectron. However, the G-value is lowered when isopropanol(0.17mmol J�1) was used as a scavenger despite the increase inyield of reducing radicals. Permanganate is a strong oxidantcapable of oxidizing alcohols and other organic reagents. In caseswhere 2-propanol and HCO2

� were used a significant amount ofMnO4

� was consumed in background reactions. The backgroundreaction was also measured and corrected for. However, theradiolytical reduction of MnO4

� is probably not completelyindependent of the background reaction and the G-valuesobtained in the systems where background reactions occur arenot completely reliable. Furthermore, the 2-hydroxy-2-propylradical has been shown to react with MnO2 colloids (dose ¼ 700Gy, size ¼ 3–5 nm) at a rate of 8�106 M�1 s�1 giving rise to Mn3+,this competing reaction can account for reduced G-value forreduction of MnO4

� (Lume-Pereira et al., 1985). Further Mn3+ arereduced from an organic radical, the resulting Mn2+ ions under-goes a rapid conproportionation with Mn4+ (Lume-Pereira et al.,1985). Also Mulvaney et al., have shown using both thermal- andradiation-induced dissolution of metal oxides that the Mn3+

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Table 2A comparison of the mean particle size and zeta potential values. 4�10�4 M

KMnO4 at pH 10 was irradiated.

Radicals and reaction conditions Dose/Gy Mean

diameter/nm

Zeta

potential/mV

Air saturated (dOH, eaq� and Hd) 846 9071 �38.471.8

1692 8971 �46.770.5

2256 9771 �36.270.9

eaq� With (CH3)3COH as dOH

scavenger and N2 saturation

846 2874 �8.671.4

1692 2474 �3.471.0

2256 2979 �17.471.9

P. Yadav et al. / Radiation Physics and Chemistry 78 (2009) 939–944 941

centre present in colloids are continually depleted to produceMn2+ (Mulvaney et al., 1990). The G-values for solution containingformate (CO2

d�) and formate with O2 (O2d�) were comparable.

3.2. PCS and zeta potential measurements

The mean particle sizes and size distributions were measuredfor all the colloids and are listed in Table 2 along with zetapotential values. Colloidal MnO2 is formed when permanganate isreduced by a multitude of organic reagents (Perez-Benito andArias, 1992). Therefore, a more controlled reaction and smallersize was obtained when there was no background reaction. Thiswas observed in case of air-saturated solutions and N2-purgedsolutions containing tert. butanol. In rest of the cases permanga-nate was reduced giving rise to MnO2 nanoparticles which actedas seeds for further growth rendering it difficult to control the sizeand also this background reaction was difficult to monitor. As canbe expected, aggregation of MnO2 was much faster in cases wheresalts were added leading to precipitation at lower doses.

4�10�4 M KMnO4 air-saturated solution was reduced radi-olytically and the mean colloid size was found to be 90 nm for adose of 846 Gy and only a marginal increase in size was observedeven after increasing the dose by nearly a factor of three. Theparticle size upon reduction of Mn (VII) by the hydrated electronwas much smaller for the same dose and the corresponding zetapotential absolute values were lower than those obtained for air-saturated reduction. As a consequence, the colloids were lessstable compared to colloids produced in air-saturated solution. Itshould be noted that PCS assumes globular particles which neednot necessarily mean that other shapes may not be present. Therewould be certain discrepancy in readings if the shape is other thanglobular for example nanorods with greater aspect ratios.

3.3. Oxidation of Mn (II)

Colloids were also synthesized by oxidation of Mn (II). Pick-Kaplan, Rabani (1976) and Baral et al. (1986) reported theformation of colloids by oxidation of Mn(ClO4)2 �6H2O. Rabanimeasured the half life of MnO2 nucleation at various [Mn (II)] andconcluded that on increasing [Mn (II)] concentration at constantdose slows down the nucleation (Pick-Kaplan and Rabani, 1976).Hengelein carried out a more detailed study on formation ofcolloids and observed formation of colloidal solutions at pH 3.5–9.Since manganese (II) sulphate monohydrate is colourless theformation of colloid can be followed by change in colour, wherethe colloidal solution is yellowish brown. Of about 2�10�4 M ofMn(SO4)2 �H2O was irradiated at pH 10, for a dose of 564 Gy aparticle size of 51779 nm was recorded on increasing the dose to846 Gy the size increased to 7317123 nm. The zeta potential valuefor these particles at a dose of 564 and 846 Gy was �29.673 mV

and �17.371, respectively. However, these colloids were notstable and precipitated after half an hour. Therefore, theconcentration was reduced to 1�10�4 M accordingly the particlesize reduced to 266740 and 571714 nm for a dose of 564 and846 Gy, respectively, and subsequently there was an increase inabsolute zeta potential value. These colloids were some whatmore stable and precipitated after few hours.

When the pH was varied from 2.7 to 11 for 1�10�4 M ofMn(SO4)2 �H2O pale brown-coloured colloids were formed only atpH 11. Henglein reported formation of colloids at pH 5, 7 and 9(Baral et al., 1986). PCS measurements showed a particle size of354 nm for a dose of 846 Gy with a corresponding zeta potential of�58.973 mV.

3.4. Characterization

3.4.1. X-ray diffraction

The colloid formed by reduction in air-saturated solution wasprecipitated by addition of 0.5 M NaCl and was then filtered andrepeatedly washed with deionised water and later dried in oven at60 1C. The black powder was then analyzed by X-ray powderdiffraction. The diffraction pattern is shown in Fig. 1. As shownfrom figure the diffraction pattern of the solid conforms to thelines for MnO2 and was amorphous. Amorphous phases studiedby solution calorimetry (zirconia and silica) have significantlylower surface enthalpies than their dense crystalline counterparts,indicating that amorphous phases may be thermodynamically aswell as kinetically preferred under constraint of small particle size(Pitcher et al., 2004; Piccione et al., 2000). However, due to poorsignal to noise ratio the exact polymorph could not be identified.Manganese (IV) oxides and manganese are divided into twostructural families: ramsedellite a-MnO2 and pyrolusite b-MnO2.Other forms of manganese dioxies are a random intergrowth oframsedellite and pyrolusite (Kohler et al., 1997). The imperfec-tions are characterized as microtwinning and the de Wolffdisorder and are generally believed to be responsible for poorX-ray diffraction pattern of manganese oxides and oxyhydroxides(Maclean and Tye, 1996). The d-spacings of 0.775, 0.374, 0.242 and0.141 nm were calculated using the Bragg equation (nl ¼ 2d sin y)and matched well with the data published for birnessite (Matochaet al., 2001). The corresponding indices are (10 0), (2 0 0), (310)and (5 2 1). XRD pattern for colloid formed by oxidation of Mn (II)was also studied and powder was identified as MnO2 and thepolymorph could not be determined.

3.4.2. Transmission electron microscopy

Since PCS gives ambiguous picture about the size of theparticles and none whatsoever what about the shape of theparticles, colloids that were further characterized by transmissionelectron microscopy. 4�10�4 M KMnO4 was reduced in air-saturated solution and for a dose of 1692 Gy nanorods or needlesof dimension 100–150 nm were formed and also visible were afew 20 nm rods that were thicker than the others and nanospheresor dots in the range 10–20 nm were also seen. This is pictoriallyshown in Fig. 2A. Similar results in terms of appearance ofnanospheres and nanodots were obtained on increasing the doseto 2256 Gy (Fig. 2B).

On moving to a cleaner reducing system of hydrated electronwith tert. butanol as scavenger a reticulated structure of nanorodswas formed with length varying from 50 to 100 nm for a dose of1692 Gy. The corresponding TEM image is shown in Fig. 3A. In thesame system increasing the dose to 2256 Gy gave clearly definednanorods of 25–100 nm with 2–3 nm thickness as shown fromFig. 3B. The formation of reticulated rods can be due to the factthat at lower doses reduction of adsorbed ions at the surface of

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Fig. 2. TEM image of nanoparticles formed from reduction of air-saturated

4�10–4 M KMnO4 at pH 10. (A) Dose ¼ 1692 Gy and (B) dose ¼ 2256 Gy.

Fig. 1. X-ray powder diffraction pattern for the product formed by air-saturated

reduction of 4�10�4 M KMnO4 compared to the expected lines for MnO2.

Dose ¼ 1692 Gy.

Fig. 3. TEM image of nanoparticles formed from reduction of 4�10–4 M KMnO4 by

hydrated electron with tert. butanol as scavenger at pH 10. (A) Dose ¼ 1692 Gy and

(B) dose ¼ 2256 Gy.

P. Yadav et al. / Radiation Physics and Chemistry 78 (2009) 939–944942

clusters is predominant that results in less growth centers andlarger clusters.

In radiolytic reduction the samples were radiolysed for 3–4 h.Formation of nanorods and nanospheres in air-saturated reduc-tion of Mn (VII) suggest that oriented aggregation is taking place.As on moving to a cleaner reducing system nanorods were formedexclusively. The occurrence of nanospheres in the former casecould be explained by oxidative termination of growing nanorods.For reduction of Mn (VII) by hydrated electron for a lower dose(Fig. 3A) reticulated structure was obtained and on increasing thedose to 2256 Gy more well-defined rods were obtained.

Oxidation of 1�10�4 M MnSO4 �H2O at pH 10 gave a mixtureof reticulated structure of 20-nm thick rods with the lengthranging from 60 to 100 nm, also seen were sheets of the 200 to300 nm length for a dose of 282 Gy. The TEM pictures are shown inFig. 4A and B, respectively. With further increase in dose (846 Gy)only sheets were seen and the TEM image is reproduced in Fig. 4C.The sheets had folded thickness of 10 nm and length was greaterthan 400 nm. When the concentration was increased to 2�10�4 Msheets were obtained. This could be due to sorption of Mn2+ on thecolloids. 2�10�4 M sodium hexametaphosphate was added to1�10�4 M Mn(ClO4)2 solution before irradiation by Henglein et al

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Fig. 4. TEM image of nanoparticles formed in the reaction of dOH radical with 1�10–4 M Mn(SO4)2. (A), (B) Dose ¼ 282 Gy and (C) dose ¼ 846 Gy.

P. Yadav et al. / Radiation Physics and Chemistry 78 (2009) 939–944 943

and they reported adsorption of 50% of Mn2+ ions on polypho-sphate anions (Baral et al., 1986). Also d and l MnO2 polymorphshave been reported to assume a layer structure, with sheets madefrom MnO6 octahedra, separated by alkali or other ions, and watermolecules (Burns and Burns, 1975a,b).

3.4.3. SEM measurements

Elemental analysis was performed with scanning electronmicroscope and the purity was found to be 98% of elemental Mncontent in MnO2 formed from both by reduction and oxidation.Trace amount of Fe and silica was also found and the latter can befrom the glass used.

4. Conclusions

This work helped in identifying the conditions for MnO2

nanoparticle generation by radiolysis. Colloids formed inN2-purged solutions of KMnO4 with tert. butanol as scavengerproduced homogenous nanorods, whereas in case of air-saturatedreduction additional nanospheres were produced also thesecolloids were stable and devoid of any background reactions.Nanosheets were produced upon oxidation of Mn (II) and the doserequired was much less.

Acknowledgements

The financial support from the Carl Tryggers Stiftelse forVetenskaplig Forskning is gratefully acknowledged. The authorswould like to thank Dr. Susanna Wold for the useful discussions.

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