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Structural and morphological evolution of β-MnO2 nanorods during hydrothermal synthesis

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2009 Nanotechnology 20 055610

(http://iopscience.iop.org/0957-4484/20/5/055610)

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Nanotechnology 20 (2009) 055610 (7pp) doi:10.1088/0957-4484/20/5/055610

Structural and morphological evolution ofβ-MnO2 nanorods during hydrothermalsynthesisTao Gao1, Helmer Fjellvag and Poul Norby

Centre for Materials Science and Nanotechnology and Department of Chemistry, University ofOslo, PO Box 1033, Blindern, N-0315 Oslo, Norway

E-mail: tao.gao@kjemi.uio.no

Received 11 July 2008, in final form 20 November 2008Published 12 January 2009Online at stacks.iop.org/Nano/20/055610

Abstractβ-MnO2 nanorods were synthesized via a redox reaction of (NH4)2S2O8 and MnSO4 underhydrothermal conditions. In situ and ex situ x-ray diffraction and scanning electron microscopywere employed to follow the structural and morphological evolution during growth. It wasfound that the crystallization of β-MnO2 nanorods proceeds through two steps: γ -MnO2

nanorods form first via a dissolution–recrystallization process and then transform topologicallyinto β-MnO2 with increasing temperature. The phase transformation was associated with ashort-range rearrangement of MnO6 octahedra. Vibrational spectroscopic studies showed thatthe β-MnO2 nanorods had four infrared absorptions at 726, 552, 462 and 418 cm−1 and fourRaman scattering bands at 759 (B2g), 662 (A1g), 576 (Ramsdellite impurity) and 537(Eg) cm−1, which are in agreement with Mn–O lattice vibrations within a rutile-type MnO6

octahedral matrix.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Manganese dioxide (MnO2) has many important applications,such as ionic and molecular sieves [1, 2], catalysts [3, 4]and cathode materials for rechargeable batteries [5–7]. MnO2

shows also a huge structural flexibility and appears in a numberof crystallographic polymorphs, such as α-, β-, γ -, δ-and ε-MnO2, where the main structural motif is edge- or corner-sharing MnO6 octahedra in different connectivity schemes [8].The large number of structure types that exist in the MnO2

system is attractive from the point of view that it offers awide selection of different materials based on one compoundonly. Consequently, rational synthesis of MnO2 materials withcontrollable crystal structures, morphologies and sizes is veryimportant [6, 9–12].

In recent years, the synthesis of MnO2 nanostructureshas attracted a great deal of interest due to the fact that thephysical properties of these materials can be modified whenthey have a nanometre dimension [9–12]. Among various

1 Author to whom any correspondence should be addressed.

synthetic approaches established for MnO2 nanomaterials,hydrothermal synthesis represents the most common route andoffers advantages such as mild conditions, simple manipulationand good possibilities for scale-up [1, 10–15]. It is knownthat different polymorphs of MnO2 nanomaterials can besuccessfully synthesized by tuning experimental parametersof a hydrothermal reaction system [14–16]. Consequently,understanding the nucleation and crystallization mechanismsof MnO2 under hydrothermal conditions is very importantfor improved control of size, crystallinity, stoichiometry anddimensional anisotropy. However, it is a challenging task:under hydrothermal conditions, there might be numerouschemical and physical processes occurring simultaneouslyand competing. Therefore, in order to understand theprocesses involved, many experimental techniques must beemployed. Both ex situ and in situ studies play importantroles in assembling sufficient information for a detailedunderstanding [17].

Traditionally, products from hydrothermal synthesis haveto be isolated and analysed in an ex situ way. However,the existence and role of metastable intermediates can easily

0957-4484/09/055610+07$30.00 © 2009 IOP Publishing Ltd Printed in the UK1

Nanotechnology 20 (2009) 055610 T Gao et al

be missed. In this context, in situ studies of the structuralevolution as a function of time, pressure or temperatureare highly desired [17]. So far, however, very limitedinformation on such processes for MnO2 nanomaterials isat hand [13]. In this work, we report the hydrothermalsynthesis of β-MnO2 nanorods via a redox reaction between(NH4)2S2O8 and MnSO4. The literature shows that differentpolymorphs may be achieved. Wang and Li reported β-MnO2 nanorods as the final product [15], whereas Yanget al obtained α-MnO2 nanowires [18]. This indicatesthat certain thermodynamical and/or kinetic parameters mustplay important roles in this reaction scheme. To achievesuch details, an in situ hydrothermal synthesis monitored bysynchrotron x-ray diffraction was performed. It is worthpointing out that the high intensity and high energy ofsynchrotron x-ray radiation makes it possible to follow fastchemical reactions with adequate time resolution, and therebywill gain a deeper insight into the hydrothermal synthesisprocesses [13, 17]. Moreover, vibrational spectroscopicproperties of the obtained products are discussed.

2. Experimental details

2.1. Chemicals

Reagent grade (NH4)2S2O8 and MnSO4 were purchased fromSigma-Aldrich Co. and used without further purification.Double distilled water was used throughout the experiments.

2.2. Hydrothermal synthesis of β-MnO2 nanorods

The in situ hydrothermal synthesis was performed within amicro-reaction cell as described previously [19]. For a typicalsynthesis, equal volumes of (NH4)2S2O8 (0.1 M) and MnSO4

(0.1 M) aqueous solutions were mixed under stirring at roomtemperature. A small amount of the obtained transparentsolution was injected into a quartz glass capillary (innerdiameter: 0.7 mm) and thereafter mounted in a swagelokfitting. In order to maintain hydrothermal conditions duringthe in situ synthesis, a nitrogen pressure of 10 atm was appliedinside the capillary cell. This reaction cell was heated up to160 ◦C at a rate of 2 ◦C min−1 and held at this temperature for1 h.

In addition a corresponding hydrothermal synthesis wasperformed in conventional autoclaves to validate the findingsof the in situ studies and to monitor the morphologicalevolution of the materials. In brief, equal volumes of thereactants (see above) were mixed under vigorous stirring atroom temperature. The obtained solution was transferred intoTeflon-lined stainless steel autoclaves. The autoclaves werefilled up to 80% of the total volume (40 ml), sealed and heatedat controlled conditions. Afterwards, the autoclaves werecooled rapidly by tap water. The solid precipitates were thenfiltered, washed three times with distilled water, and finallydried at 80 ◦C overnight.

2.3. Characterization

In situ synchrotron x-ray powder diffraction data werecollected at the Swiss–Norwegian Beam Line BM01A, at theEuropean Synchrotron Radiation Facility (ESRF), Grenoble,France (1◦ � 2θ � 40◦, λ = 0.07106 nm). XRDpatterns were recorded every 1.8 min. The solid productswere further characterized by conventional XRD (SiemensD5000; Cu Kα1 radiation; λ = 0.15406 nm) for scatteringangles 10◦ � 2θ � 70◦, in steps of 2θ = 0.015◦. Themorphology of the obtained materials was studied by field-emission scanning electron microscopy (SEM, FEI Quanta200F). Raman scattering spectra of the as-prepared materialswere collected in a backscattering configuration at roomtemperature. The samples were illuminated by using a785 nm laser with a 20× objective on a Renishaw In-ViaRaman microscope. A laser power of about 1 mW wasapplied on the sample surface, which was low enough toprevent photodecomposition or denaturation of the samples butsufficient to produce good-quality spectra in a reasonable time.

3. Results and discussion

3.1. In situ synchrotron XRD study

Figure 1(a) shows a three-dimensional (3D) representation ofthe time-resolved synchrotron XRD patterns collected duringhydrothermal synthesis. The starting solution of (NH4)2S2O8

and MnSO4 gives rise to considerable background scattering.Note that the redox reaction is kinetically slow, although astrong thermodynamic driving force has been suggested [15].Upon gradual increase of the reaction temperature to about90 ◦C, an intermediate phase with three broad diffraction peaksat 2θ = 16.98◦ (d = 0.2425 nm), 19.36◦ (d = 0.2124 nm)and 25.26◦ (d = 0.1632 nm) is observed. This crystallinephase can be indexed to orthorhombic γ -MnO2 (JCPDS 14-0644, a = 0.636 nm, b = 1.015 nm and c = 0.409 nm)according to subsequent characterizations. As the reactiontemperature increases further to 135 ◦C, tetragonal β-MnO2

(JCPDS 24-0735, a = 0.440 nm and c = 0.287 nm) startsto form (figure 1(b)). At 160 ◦C, only the β-MnO2 phase ispresent. Since the β-MnO2 is known as the most stable MnO2

phase [8], the in situ XRD data do not support the formation ofan α-MnO2 phase as reported previously [18].

3.2. Structural evolution of MnO2 materials

On the basis of the in situ synthesis (figure 1), experimentswere performed at selected conditions in order to expand theunderstanding of the hydrothermal reaction system.

Figures 2 and 3 report the SEM images and XRD patternsof MnO2 materials prepared by autoclaving the (NH4)2S2O8

and MnSO4 solutions at 140 ◦C for different reaction times.The early stage of the reaction yields spherical particles ofγ -MnO2 with almost uniform diameters of about 500 nm(figures 2(a) and 3(a)). These particles grow in size asthe reaction time increases (figure 2(b)). The existence ofthese spherical aggregates suggests a homogeneous nucleationprocess of γ -MnO2 under the present hydrothermal conditions.

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Nanotechnology 20 (2009) 055610 T Gao et al

Figure 1. (a) In situ synchrotron XRD patterns for hydrothermal synthesis of β-MnO2 nanorods. The final β-MnO2 phase is indicated bysolid circles. The presence of metastable γ -MnO2 at intermediate temperatures is illustrated by arrows. (b) Selected XRD patterns near thephase transformation temperature. The XRD patterns are shifted vertically for clarity. The wavelength is 0.07106 nm.

Figure 2. SEM images of MnO2 materials prepared at 140 ◦C for (a) 30 min, (b) 60 min, (c) 120 min and (d) 240 min. Insets of panel (a) and(b) show enlargements.

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Nanotechnology 20 (2009) 055610 T Gao et al

Figure 3. Powder XRD patterns of MnO2 materials prepared at140 ◦C for (a) 30 min, (b) 60 min, (c) 120 min and (d) 240 min.The wavelength is 0.15406 nm.

It is worth pointing out that the surface of these sphericalaggregations is usually protuberant and some initial short rod-like structures are formed on the edge of the particles (seethe inset of figure 2(a)). These short rod-like protuberances

grow in size as the reaction time is increased (see the insetof figure 2(b)). Consequently, urchin-like three-dimensionalnanostructures of γ -MnO2 are obtained for prolonged holdingtimes, i.e. 120 min (figure 2(c)). This reveals clearly ananisotropic growth of small rod-like γ -MnO2 crystallites.After 240 min, the hydrothermal reaction yields β-MnO2

nanorods as the product (figures 2(d) and 3(d)).As can be seen from figure 3, the reflections of γ -MnO2

become more sharp and intense with increasing reaction time,implying a gradual crystallization and growth of the γ -MnO2

crystallites. Note that the (hk0) reflections, such as (120),(230), (300) and (160), are usually broad, which indicates apreferred growth direction along 〈001〉 of orthorhombic γ -MnO2 [13, 20]. Consequently, the formed γ -MnO2 crystalliteswill possess an anisotropic morphology, which is in agreementwith the SEM data (figures 2(a) and (c)).

Figure 4 shows SEM images of MnO2 materials preparedby autoclaving the (NH4)2S2O8 and MnSO4 solutions atdifferent temperatures with a constant holding time of 20 h;their corresponding XRD patterns are reported in figure 5. Itcan be seen that at room temperature the products tend toassemble into spherical γ -MnO2 aggregates with diametersof about 1–3 μm (figure 4(a)). The weak and broad (120)reflection (figure 5(a)) suggests that the (1×2) tunnel structurein the orthorhombic γ -MnO2 was poorly developed. Incomparison with the in situ synthesis (figure 1), it can be seenthat γ -MnO2 forms even at room temperature once the reactiontime is prolonged. Again, some protuberances on the edge ofthe γ -MnO2 particles are observed, as shown in the inset offigure 4(a). These protuberances grow in size with increasingtemperature (see the inset of figure 4(b)). At 100 ◦C, urchin-like γ -MnO2 nanostructures are obtained (figure 4(c)). Theindividual γ -MnO2 nanorods have typical diameters of 20–100 nm and lengths up to 2 μm (see the inset to figure 4(c)).

Figure 4. SEM images of MnO2 materials prepared at different temperatures for 20 h: (a) RT, (b) 60 ◦C, (c) 100 ◦C and (d) 140 ◦C. Insets ofpanel (a), (b) and (c) show enlargements.

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Figure 5. SEM images of MnO2 materials prepared at differenttemperatures for 20 h: (a) RT, (b) 60 ◦C, (c) 100 ◦C and (d) 140 ◦C.The wavelength is 0.15406 nm.

β-MnO2 nanorods with diameters 40–200 nm and lengths upto 2 μm are obtained at 140 ◦C (figure 4(d)).

3.3. Growth and phase transformation

As demonstrated by the in situ and ex situ studies, it isclear that the formation of β-MnO2 nanorods from the redoxreaction of (NH4)2S2O8 and MnSO4 involves two differentprocesses, i.e. nucleation and growth of γ -MnO2 crystallitesand subsequent phase transformation from γ -MnO2 to β-MnO2.

The chemical reaction involved in the present study can beformulated as

(NH4)2S2O8 + MnSO4 + 2H2O → MnO2

+ (NH4)2SO4 + 2H2SO4. (1)

Note that the reaction system becomes acidic as the formationof MnO2 proceeds. As shown in figures 2(a) and 4(a), thepresence of the spherical aggregations of γ -MnO2 crystallitesat the early stage of the hydrothermal reaction indicates that thenucleation of MnO2 is homogeneous. The (120) reflection ofthese γ -MnO2 crystallites is usually weak and broad, revealingthat the 1 ×2 tunnel structure in these γ -MnO2 aggregations ispoorly developed at lower reaction temperatures (figures 5(a)and (b)) or shorter reaction times (figure 3(a)). The growthof γ -MnO2 crystallites can be promoted by either increasingthe reaction temperature (see, for example, figure 4(c)) orprolonging the reaction time (figure 2(c)), in which the formerprocess is dominant. The γ -MnO2 crystallites usually have

Figure 6. Polyhedral representation of crystal structure of(a) β-MnO2 and (b) Ramsdellite.

a preferred growth direction along the tunnels [13, 15, 20];therefore, an anisotropic morphology of γ -MnO2 crystallitesat higher reaction temperatures or longer reaction times can beexpected and is actually observed in the present study (see, forexample, figures 2(c) and 4(c)). These observations supporta nucleation–dissolution–anisotropic growth mechanism [21]rather than a rolling mechanism [15]. That is to say, duringthe hydrothermal process, the shorter γ -MnO2 nanorods mayredissolve into the solution phase due to an acid-etchingeffect [13], and the longer ones will grow much longer owing totheir anisotropic growth behaviour [13, 15, 20]. The formationof the urchin-like three-dimensional γ -MnO2 nanostructures(figures 2(c) and 4(c)) seems in harmony with this assumption.

The phase transformation from γ -MnO2 to β-MnO2 canbe understood by considering their structural similarity. Asshown in figure 6(a), in β-MnO2, single chains of edge-sharing MnO6 octahedra share corners with neighbouringchains to form a framework with empty channels with sizescorresponding to the width of a 1×1 MnO6 octahedron. On theother hand, γ -MnO2 can be considered a random intergrowthof β-MnO2 blocks within a Ramsdellite matrix [22], ofwhich the crystal structure is illustrated in figure 6(b). TheRamsdellite is metastable and will transform into β-MnO2 withincreasing temperature [13]. Thus, the phase transformationfrom γ -MnO2 to β-MnO2 can be considered as the growth ofβ-MnO2 domains within a Ramsdellite matrix, which involvesa collapse of the 1 × 2 framework and a subsequent short-range rearrangement of the MnO6 octahedra. Note that theconnectivity of the MnO6 octahedra along [001] is identical inorthorhombic Ramsdellite and tetragonal β-MnO2 (figure 6);therefore, the transformation from γ -MnO2 to β-MnO2 istopologically convenient and consistent with their similar rod-like morphology.

3.4. Vibrational spectroscopic study

The vibrational spectroscopic methods, which include IR andRaman scattering spectroscopy at low wavenumbers, afforduseful alternatives and/or supplements to x-ray diffraction forstructural characterizations of materials [23–25]. Since theyare sensitive to local structures of materials, IR and Ramanscattering spectroscopy can yield a more complete and reliabledescription of materials such as MnO2-related compounds,where crystalline disorders as well as different local structuralproperties can be expected [16].

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Figure 7. (a) FT-IR and (b) Raman scattering spectra of β-MnO2

nanorods.

Structurally, β-MnO2 is tetragonal, space groupP42/mnm. The lattice contains two formula groups per unitcell and 11 optical phonons of symmetry A1g + A2g + A2u +B1g + B2g + Eg + 2B1u + 3Eu, of which A1g + B1g + B2g + Eg

are Raman-active, A2u + 3Eu are IR-active and A2g + 2B2u

are inactive in both Raman and IR spectra. As shown infigure 7(a), the FT-IR spectrum of β-MnO2 nanorods featuresfour absorptions at 726, 552, 462 and 418 cm−1, in goodagreement with the published values [26]. Note that there areno IR bands related to the Ramsdellite impurity (which will bediscussed below). The FT-IR data reported here (figure 7(a))can be ascribed to the Mn–O vibrations of the rutile-type MnO6

octahedra framework, which presents a clear signature that canbe used for probing the β-MnO2 type materials.

The Raman spectrum of β-MnO2 nanorods in figure 7(b)has four bands at 759, 662, 576 and 537 cm−1. This agreeswell with the previous reports on the β-MnO2 phase [16, 23]except for the additional band at 576 cm−1. The Ramanband at 759 cm−1 is attributed to the B2g mode and involvesantisymmetric Mn–O vibrations. The Raman band at 662 cm−1

can be assigned to the A1g mode and is indicative of awell-developed rutile-type framework. Here, the Eg mode isassigned to the band at 537 cm−1. The Raman scatteringband assignment is consistent with those reported for rutile-type compounds [24, 25].

Figure 8. Raman scattering spectrum of γ -MnO2 nanorods.

In order to understand the origin of the Raman bandat 576 cm−1, the γ -MnO2 nanorod precursor, i.e. the 3Durchin-like γ -MnO2 nanorod assemblies (figure 4(c)), wasinvestigated. As shown in figure 8, the Raman spectrum ofthe γ -MnO2 nanorod assemblies displays four main bandsat 280, 491, 576 and 651 cm−1, which are fairly consistentwith previous studies [20, 23]. The most intense band at576 cm−1 belongs to Ramsdellite domains of γ -MnO2 andis indicative of a well-developed orthorhombic structure with1 × 2 tunnels. Since β-MnO2 nanorods are formed from theγ -MnO2 precursors, it is easily possible that the product maycontain some Ramsdellite impurities, although not detectableby XRD (figure 5(d)).

It is worth pointing out that Widjaja and Sampantharmeasured previously Raman scattering data of β-MnO2

nanorods, in which several well-defined Raman bandsat 192, 330, 390, 518, 582, 630 and 737 cm−1 arerecorded [27]. However, these Raman data are much differentfrom our findings (figure 7(b)) and those reported by otherresearchers [16, 23]. Generally, it is impossible to observeso many Raman lines in the rutile-type phase since onlyfour optical phonons A1g + B1g + B2g + Eg are Raman-active. Moreover, due to the low Raman activity of MnO2

materials [23], some of the Raman modes usually have toolittle intensity to be observed [16]. Further studies arestill necessary for an improved understanding of the Ramanscattering properties of the β-MnO2 materials. This work isunder way and the results will be reported elsewhere.

4. Conclusions

The hydrothermal synthesis of β-MnO2 nanorods wasfollowed by in situ and ex situ diffraction studies by reacting(NH4)2S2O8 and MnSO4 solutions. The formation of β-MnO2

nanorods involves homogeneous nucleation and anisotropicgrowth process of γ -MnO2 nanorods and a subsequentisotopological transformation from γ -MnO2 to β-MnO2.Vibrational spectroscopic studies showed that the β-MnO2

nanorods had four infrared absorptions at 726, 552, 462 and

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Nanotechnology 20 (2009) 055610 T Gao et al

418 cm−1, and four Raman bands at 759 (B2g), 662 (A1g),576 (Ramsdellite impurity) and 537(Eg) cm−1, which are inagreement with Mn–O lattice vibrations within a rutile-typeMnO6 octahedral matrix.

Acknowledgments

We thank Dr Juan F Cardenas for collecting the Ramandata. We are grateful to the valuable comments from thereviewers. The authors acknowledge the financial assistancefrom the Research Council of Norway through the NANOMATprogramme (163565-431).

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