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Materials Chemistry and Physics 133 (2012) 605– 610

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

ynthesis, characterization and catalytic activity of birnessite type potassiumanganese oxide nanotubes and nanorods

halid Abdelazez Mohamed Ahmeda,b,∗, Kaixun Huanga,∗∗

School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR ChinaDepartment of Chemistry, School of Chemistry and Chemical Technology, Faculty of Science and Technology, Al-Neelain University, P.O. Box 12702, Khartoum, Sudan

r t i c l e i n f o

rticle history:eceived 20 June 2011eceived in revised form 3 January 2012ccepted 3 January 2012

a b s t r a c t

Birnessite-type manganese oxide nanotubes and nanorods were synthesized via a calcination pro-cess using manganese acetate and potassium hydroxide as precursors in presence of polyethyleneglycol–melamine–formaldehyde. As-prepared products were characterized by XRD, FT-IR, FE-SEM, TEM,SA-ED, HR-TEM, Brunauer–Emmett–Teller (BET) and TGA analyses. The influences of reaction temper-

eywords:irnessite-type manganese oxideanotubesanorodsalcinationatalysis

ature and time on the morphology of manganese oxide nanocrystals were investigated. The orientedattachment-thermodynamical (OA-TD) process is suggested to describe the transition from tube to rodstructure. Their capability of catalytic degradation of safranin O was compared. The results indicate thatbirnessite-type manganese oxide nanotube has higher catalytic activity for than nanorod crystal in aque-ous solution, because it has a larger surface area. The decomposition of safranin O follows pseudo-firstorder kinetics and is markedly affected by pH.

. Introduction

Low-dimensional nanostructured materials, such as nanorods,anowires, nanobelts, and nanotubes, are vital components forottom-up fabrication of electronic and photonic nanodevices1–3]. Because morphology and dimensions exert a significantnfluence on the physical and chemical properties of nanomaterials,he controlled growth or fabrication of nanomaterial with desiredhapes has attracted a great deal of interest [4].

Due to the nanotubes have cylindrical hollow geometry with aow mass density, a high porosity and a large surface to weight ratio5,6], inorganic nanotube materials have attracted great interest for

wide range of applications [7]. In the past few years, various meth-ds have been developed for the synthesis of inorganic nanotubes,n particular, the template-based methods of nanotube formationre useful, which enable good control over the nanotube dimensionnd can be used to deposit a wide range of materials [8–18].

The crystal structures are generally believed to be responsibleor their properties [19]. Much effort is being made to synthe-ize low-dimensional nanostructure with different crystalloid

∗ Corresponding author at: School of Chemistry and Chemical Engineering,uazhong University of Science and Technology, Wuhan 430074, PR China.el.: +86 27 8754 3532; fax: +86 27 8754 3632.∗∗ Corresponding author. Tel.: +86 27 8754 3532; fax: +86 27 8754 3632.

E-mail addresses: khalidgnad@hotmail.com (K.A.M. Ahmed),xxzrf@mail.hust.edu.cn (K. Huang).

254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2012.01.009

© 2012 Elsevier B.V. All rights reserved.

manganese oxide nanomaterials [20]. In natural, manganese oxidematerials with different crystal structures have been found, such as�-, �-, �-, and �-MnO2. Recently, several techniques routs, such ashydrothermal and solvothermal [21–25], electrochemical [26,27]and micelle-template methods [28], were developed to prepare1D MnO2 nanotubes and nanorods with different crystallographicforms.

Manganese oxide nanomaterials have received extensive atten-tion due to their outstanding structural flexibility combined withnovel chemical and physical properties, and now are widely used ascatalysts [29]. On the other hand, birnessite is one of common com-ponents of manganese nodules, which are abundant on the oceanfloor and are to be as effective as commercial catalysts and adsor-bents because of their relatively high surface area and transitionmetal oxide content [30]. Birnessite-type potassium manganeseoxides have a layered structure which contains two-dimensionalsheets of edge-shared MnO6 octahedra, and water molecules andpotassium ions between the sheets of edge-shared MnO6 octahe-dra [31–34]. The crude manganese nodule samples were reportedto be used for a wide range of catalytic reactions including oxida-tion of alcohols and CO, reduction of NO, hydrogenation of alkenes,and decomposition of organic sulfur compounds [35–38].

In this paper, two shapes of birnessite potassium manganeseoxides, nanotube and nanorod, were synthesized through calci-

nations process. The effects of reaction temperature and time oncrystal morphologies of products were investigated. Their catalyticactivities for degradation of safranin-O by air oxygen in aqueoussolution were compared.

6 hemistry and Physics 133 (2012) 605– 610

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Fig. 1. XRD pattern and EDX spectra of materials. (a) and (d) Birnessite-typepotassium manganese oxide nanotubes prepared at 700 ◦C for 75 min; (b) and(e) birnessite-type potassium manganese oxide nanorods prepared at 700 ◦C for

06 K.A.M. Ahmed, K. Huang / Materials C

. Experimental

.1. Synthesis of nanocrystals

All chemicals are of analytical grade and purchased fromhanghai Chemical Reagent Company and used without furtherurification. The synthesis of nanocrystals was carried out in alass crucible. 1.4 mmol manganese acetate tetrahydrate, 1 mmolelamine, 0.4 g polyethylene glycol (PEG10000), 1.3 mmol KOH

nd 0.4 mL formaldehyde were added to crucible and then driedn oven at 80 ◦C for 2 h. The resultant powder was transferred to

uffle furnace and heated at different temperature (400–700 ◦C)or different times. The muffle furnace was cooled to room temper-ture naturally when the reaction was finished. The product wasashed with distill water and absolute ethanol by using centrifu-

ation at 8000 rpm for 10 min several times to remove the excessiveeactants and byproduct, followed by drying in a vacuum at 60 ◦Cor 14 h.

.2. Characterization

The XRD analysis was performed using PaNalytical �’Pert Pro,etherlands, X-ray powder diffractometer equipped with Cu K�

adiation (� = 1.5418 A). Fourier transform infrared (FTIR) analy-es were performed with a Bruker Equinox 55 spectrometer in theange of 400–4000 cm−1. The morphology and size of as-preparedanocrystals were evaluated by a field emission scanning electronicroscopy (FE-SEM) on a FEI Sirion 200 with energy-dispersive-ray (EDX) analysis. Transmission electron micrographs (TEM)

mages were taken on a Tecnai G220 and high-resolution trans-ission electron microscopy (HRTEM) images were carried out on

JEM 2010 FEF TEM at an acceleration voltage of 200 kV. Thermalravimetric analysis (TGA) was conducted by a Setarm Setsys 16G/DTA/DSC integration thermal analyzer (N2 stream 50 mL min−1;eating rate 10 ◦C min−1). The surface area and pore diameterere measured by N2 adsorption–desorption technique at −196 ◦C

n an automated surface area and porosity analyzer (ASAP 2020,icromeritics, USA).

.3. Degradation experiments

The catalytic degradation of safranin O was carried out in ahree-neck glass reactor (250 mL) equipped with a reflux condensernder magnetic stirring and air bubbling at 50 ◦C. 0.04 mmol ofatalyst powders was suspended in an aqueous solution contain-ng 0.5 g L−1 of safranin O. The pH of the solution was adjustedo 3.5 by using phosphoric acid or sodium hydroxide. Samplesere taken from the three-neck glass reactor at regular inter-

als during the experiment that lasted 80 min. At the end of eachxperiment, the catalytic powder was removed by filtration or cen-rifuging. The decolorization of dye was analyzed by using a UV-vispectrophotometer (Shimadzu, Model No. 2450). The degradationate of safranin O was calculated based on the following equationD% = (1 − At/A0)/100].

. Results and discussion

The XRD patterns of as-prepared birnessite nanocrystals arehown in Fig. 1(a) and (b). All the diffraction peaks can be indexednto monoclinic phase of birnessite potassium manganese oxide

ith a lattice constant a = 5.149, b = 2.834 and c = 7.176 A which aren good agreement with the standard data from JCPDS card (no. 87-

497) (Fig. 1(c)). No impurity phase can be detected. [1 0 0] peak

s much stronger than the standard, implying that the preferredrowth direction of the monoclinic phase of birnessite nanocrys-als is the [1 0 0] direction. The EDX spectrum from as-prepared

120 min; and (c) the standard data from JCPDS card no. 87-1497.

product is shown in Fig. 2(d) and (e), showing that K/Mn/O molarratio is about 0.25:1:2.17, but because EDX is a semi-quantitativetechnique, the exact molecular formula needs to be determined byother method.

The morphology and size of as-prepared nanocrystals were eval-uated by FE-SEM. The FE-SEM images in Fig. 2(a) and (b) showas-prepared products at 700 ◦C for 75 min are nanotubes withouter diameter of approximately 200–360 nm, wall thickness of30–50 nm and inner diameter of 40–90 nm. The morphology of nan-otube was also investigated by TEM images, as shown in Fig. 2(c),which is in agreement with the SEM observation. The electrondiffraction pattern of selected area inserted in Fig. 2(c) shows thatas-prepared product is single crystalline structure. The HRTEMcrystallographic analysis in Fig. 2(d) shows interplanar spacing ofthe lattice fringes is 0.674 nm, corresponding to the (0 0 1) plane ofa birnessite potassium manganese oxide crystal.

Fig. 3(a) and (b) shows SEM images of the as-prepared nanocrys-tal by calcination route at 700 ◦C for 120 min. The low magnificationSEM image (Fig. 3(a)) shows that birnessite potassium manganeseoxide samples are nanorod crystals. High-magnification SEM imagein Fig. 3(b) and TEM image in Fig. 3(c) reveal that the as-preparedsample is nanorod with diameters less than 80 nm. The correspond-ing SAED pattern of an individual birnessite potassium manganeseoxide nanorod, as inserted in Fig. 3(c), shows it is single-crystallinewith dislocation defects (Fig. 3(d)). The lattice fringes of interplanar

spacing is about 0.361 nm, which is close to the lattice spacing ofthe (0 0 2) plane.

K.A.M. Ahmed, K. Huang / Materials Chemistry and Physics 133 (2012) 605– 610 607

Fig. 2. (a) Low- and (b) high-magnification of FE-SEM images, (c) TEM images and inserted SA-ED pattern, and (d) HR-TEM images of birnessite-type manganese oxidenanotubes prepared at 700 ◦C for 75 min.

Fig. 3. (a) Low- and (b) high-magnification of FE-SEM images, (c) TEM images and inserted SA-ED pattern, and (d) HR-TEM images of birnessite-type manganese oxidenanorods prepared at 700 ◦C for 120 min.

608 K.A.M. Ahmed, K. Huang / Materials Chemis

Fig. 4. FT-IR spectrum and TGA curves of synthesized-products. (a) and (curvei) Birnessite-type potassium manganese oxide nanotubes prepared at 700 ◦C for75 min; (b) and (curve ii) birnessite-type potassium manganese oxide nanorodsprepared at 700 ◦C for 120 min.

Fig. 5. FE-SEM images of samples prepared at (a)–(c) 400 ◦C

try and Physics 133 (2012) 605– 610

Fig. 4(a) depicts the FT-IR spectrum of as-synthesized birnessitepotassium manganese oxide nanotubes. The peak at 438.2 cm−1 isattributed to the Mn O Mn bending mode. The high frequencybands at 490.4 and 539.2 cm−1 corresponds to the anti-symmetricstretching mode of the MnO6 octahedra [39,40]. Other peaksmight come from absorbed water, ethanol (wash solvents). The FT-IR spectrum of birnessite potassium manganese oxide nanorods(Fig. 4(b)) is almost the same as that of the as-synthesized nano-tubes. The TGA data of the as-prepared materials as shown in curves(i) and (ii) of Fig. 4(c) show that the weight loss of birnessite potas-sium manganese oxide nanotubes is about 9.8% and nanorods isaround 9.4%, corresponding to absorbed water content of 0.54 and0.52 per unit formula, respectively.

To shed light on the formation process of birnessite potassiummanganese oxide nanostructures, time-dependent and tempera-tures experiments were carried out and the intermediate productswere inspected by FE-SEM image. As shown in Fig. 5, particles wereformed at 400 ◦C for 120 min (Fig. 5(a)). When the reaction time wasprolonged to 270 min in the same temperature, a mixture of rodsand particles could be observed (Fig. 5(b)). When the reaction timewas prolonged to 420 min, the nanorod was formed, although therewere a few particles (Fig. 5(c)).

Furthermore, when the reaction was performed at 600 ◦C for120 min, the product is rod-like structure with a range of diame-ter of 200–360 nm (Fig. 5(d)). When the reaction was carried outat 700 ◦C for 120 min, the product is nanorod (Fig. 3). Only whenthe reaction temperature was at 700 ◦C for 75 min, the product isnanotube.

Recently, the effects of polyethylene glycol (PEG) on theformation of nanotubes, such as, tellurium, ZnO and Y(OH)3nanotubes, were reported [41–43]. However, in this study, it isa solid-phase reaction to obtain birnessite potassium manganeseoxide nanotubes. We speculate that the mechanism is related todecomposition or carbonization of PEG packed in rod core because

the reaction was conduced in a crucible without sealing in air at ahigh temperature of 700 ◦C. However, when the reaction time was

for 120, 270 and 420 min; and (d) 600 ◦C for 120 min.

K.A.M. Ahmed, K. Huang / Materials Chemistry and Physics 133 (2012) 605– 610 609

0.0 0.2 0.4 0. 6 0.8 1.0

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60

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100

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Fig. 6. N2 adsorption–desorption curves birnessite-type

rolonged to 120 min at 700 ◦C, a transition from hollow tubes intoods occurred.

On the basis of time-temperature dependents experiments, weostulated that the formation of nanotubes may accompany an ori-nted attachment (OA) with PEG as template and the transitionrom hollow tubes into rods may be followed a TD process. The sim-lar hypotheses were proposed to explain the growth mechanism of-AlOOH nanocrystal and titanium oxides nanostructures [45,46].hen this reaction was at a low temperature (≤400 ◦C), it was hard

o obtain the tubular crystals even through a long reaction time. Inhis case, the crystal shapes transferred from particles to nanorods.t high temperature, the nucleation and growth were all acceler-ted and tubular structures were formed. With increasing of times,he tubular structures become less stable than the rod structureecause the hollow tube possesses higher internal energy, which isavorable for transforming hollow tubes into rods by an OA-TD pro-

ess. Generally, the oriented attachment growth process has someypical characteristics, such as dislocations, twins, stacking faultsnd reduction of interlayer space [47,48]. Thus, from as-preparedanocrystal (Fig. 3d), we can see that dislocations and twin do exist.

ig. 7. (a) UV–vis spectra of degradation products of safranin O with O2 catalyzed by birnimes; (b) comparison of the degradation percentages changes of safranin O with (I) O2

anganese oxide nanotubes; (c) comparison of the degradation rate of safranin O with timnd (d) the effect of initial pH on the degradation efficiency for safranin O dye by birnessi

o

anese oxide materials (a) nanotubes and (b) nanorods.

It is indicated that the transition from hollow tubes into rods is anOA-TD process.

N2 adsorption–desorption isotherm was performed to deter-mine the specific surface area of birnessite potassium manganeseoxide nanostructures, which shows that the both materials havea typical type IV adsorption–desorption isotherm with a hys-teresis loop characteristic of mesoporous materials based on theIUPAC [49]. The BET specific surface area was estimated by plotting(P/P0)/[V(1 − (P/P0))] versus (P/P0) in the range from 0.05 to 0.25.Fig. 6(a) and (b) shows that the specific surface area of the birnes-site potassium manganese oxide nanotubes and nanorods is about28.13 and 19.5 m2 g−1, respectively.

The UV–visible absorption spectra of safranin O solution beforeand after degradation catalyzed by birnessite potassium man-ganese oxide nanotubes are presented in Fig. 7(a). As can be seenfrom the spectra, before the treatment, the UV–visible spectrum

of safranin O consists of two main characteristic absorption bands.One is in visible region (521 nm) and another in UV region (274 nm).The absorption in the visible region can be attributed to chro-mophore containing azine linkage, whereas the bands observed in

essite-type manganese oxide nanotubes in aqueous solutions at 50 ◦C in different, (II) O2 + birnessite-type manganese oxide nanorods and (III) O2 + birnessite-typees catalyzed by birnessite-type manganese oxide nanorods (II) and nanotube (III);

te-type manganese oxide nanorods (I) and nanotube (II).

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10 K.A.M. Ahmed, K. Huang / Materials C

V region can be assigned to aromatic rings. When safranin O solu-ion was treated by birnessite potassium manganese oxide nan-tube in presence of O2 at pH 3.5 and 50 ◦C, the absorption intensityf both peaks is decreased with times, suggesting safranin O is grad-ally degraded, including degradation of the aromatic structure.

In order to compare the catalytic activities of as-prepared prod-cts, the degradation percentage (D%) of safranin O catalyzed byithout catalyst (I) and with birnessite potassium manganese oxideanorods (II) or nanotubes (III) as catalyst under the same condi-ion with times is showed in Fig. 7(b). The degradation percentagef safranin O by Birnessite potassium manganese oxide nanorodII) is 72.2% after 80 min; by nanotubes (III) is about 93.8% at theame time; however, if without catalyst (I), safranin O is difficultlyegraded, which is consistent with the result that manganese oxideanotubes have a larger specific surface area.

Because the oxygen is excess and the reaction rate only dependsn the concentration of safranin O dye, the reaction can be consid-red pseudo first order. As shown in Fig. 7(c), the plots of log(A0/At)s. t (minutes) are straight lines. The rate constant may be calcu-ated as:

= 2.303/t log(A0/At)

From the plots, the rate constants for the degradation ofafranin O catalyzed by birnessite potassium manganese oxideanotubes and nanorods are about 4.62 × 10−2 min−1 (II) and.65 × 10−2 min−1 (III), respectively.

The influence of initial pH on the efficiency of catalytic degra-ation of safranin O in the range of pH 3–7 by birnessite potassiumanganese oxide nanotubes (I) and nanorods (II) were examined

s shown in Fig. 7(d). The results suggest that the degradation effi-iency of safranin O is markedly increased with the pH decrease.

. Conclusion

In summary, birnessite-type potassium manganese oxide nano-ubes and nanorods were synthesized by calcination route using

anganese acetate and potassium hydroxide as precursors inresence of polyethylene glycol–melamine–formaldehyde. Theeaction temperature and time play an important role in the for-ation of final product nanotube and nanorod. The OA-TD process

s proposed to describe the transition from tube to rod. The cat-lytic degradation efficiency of safranin O by as-prepared productsas compared. The results indicate that birnessite-type potassiumanganese oxide nanotube, because of its larger the specific sur-

ace area, has a higher activity of catalytic oxidative degradation ofafranin O than nanorod in aqueous solution. The decompositioneaction of safranin O follows first order kinetics. The degrada-ion efficiency is markedly affected by pH. All results suggesthat birnessite-type potassium manganese oxide nanotubes showromising applications in the oxidative degradation of dye, likeafranin O.

cknowledgments

This research was supported by MOST 973 program (Project No.006CB705606a). We thank the faculty from the Analysis and Testenter of Huazhong University of Science and Technology for theechnical assistance on characterization. [4

try and Physics 133 (2012) 605– 610

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Further reading

4] P.T. Zhao, K.X. Huang, Cryst. Growth Des. 8 (2008) 720.