Effect of Potassium Ions on the Formation of Crystalline Manganese

Article pubs.acs.org/IECR

Effect of Potassium Ions on the Formation of Crystalline Manganese Oxide Nanorods via Acidic Reduction of Potassium Permanganate Trung-Dung Dang,†,‡ Arghya Narayan Banerjee,*,† Sang Woo Joo,*,† and Bong-Ki Min§ †

School of Mechanical Engineering and §Center for Research Facilities, Yeungnam University, Gyeongsan 712-749, South Korea ‡ School of Chemical Engineering, Hanoi University of Science and Technology, 1st DaiCoViet, Hanoi, Vietnam S Supporting Information *

ABSTRACT: Uniformly grown manganese oxides nanorods are synthesized via a simple and rapid process through an acidic reduction of potassium permanganate followed by heat treatment. The as-produced manganese oxides nanorods are characterized with X-ray diffraction, energy dispersive X-ray analysis, scanning electron microscopy, and transmission electron microscopy. The crystallinity of the nanostructure is found to have a dominant Mn2O3 phase. The compositional and microstructural analyses reveal that the potassium concentration within the as-prepared material has a profound effect on the morphological conversion of manganese oxide nanoparticles to nanorods. Potassium ions are believed to act as a template for the formation of nanorods and the conversion efficiency increases with the increasing content of potassium. Also the morphology of products is found to depend strongly on the annealing time. The mechanism of nanorod growth with respect to different potassium contents and annealing times is discussed.

1. INTRODUCTION Manganese oxides exhibit various properties in energy storage, catalysis, sorption, and magnetism.1−6 It is an intercalation compound in electrode materials for rechargeable lithium batteries7−12 and has attractive applications in the catalytic degradation of water pollutant.13,14 A recent work on the application of MnO2 nanorods as efficient hydrogen storage media has opened up new opportunities for cost-effective hydrogen storage materials in clean energy applications.4 In addition, it can be used for preparation of soft magnetic materials.1 On the other hand, for several decades, onedimensional (1D) nanostructure materials have attracted increasing interest among scientist for their crucial role in the future technological advance in energy harvesting, sensors, nanoelectronics, optoelectronics, nanobiotechnology, and nanofluidics.15,16 Therefore, several techniques have been developed for the preparation of nanosized manganese oxides with controlled morphologies and crystalline structures, such as, discrete nanowires of α-MnO2,17−20 single-crystalline βMnO2 nanorods2,3,7,8 with urchin-like architectures,21 and hierarchical nanoarchitecture22−26 among others. As far as the formation mechanism of the 1D MnO2 nanostructure is concerned, a rolling mechanism and phase transformation of lamellar structure of MnO2 is proposed by several groups.27,28 In the current study, we have reported the controllable synthesis of manganese oxide nanorods via a fast acidic reaction of potassium permanganate and heat treatment. The effect of K+ ions on the growth of the nanorod is investigated, and a growth mechanism is proposed for the formation of 1D MnO2 nanorods under acidic reduction of permanganate.

HCl (Sigma-Aldrich, Korea) at room temperature.29,30 This step actually starts with mixing 0.001 mol KMnO4 with 10 mL of DI water through a magnetic stirrer (Fisher Scientific, Korea) for 30 min. Then 2 mL of 6 M HCl is added to this mixture dropwise. After the mixture is stirred at room temperature for 4 h, amorphous MnO2 is formed, which is then collected by centrifuging. The content of potassium in the samples can be reduced by rinsing 2 or 5 times with 50 mL of DI water and centrifuging at 7000 rpm. After that, the collected solids are dried in air at room temperature for 48 h. In the second step, amorphous MnO2 is heated to form MONRs. The annealing is done in a furnace (Fisher Scientific, Korea) under ambient atmosphere at 650 °C for 2, 5, 7, and 10 h to observe the effect of heating time on the morphology and phase of the nanomaterials. For a systematic comparison, the products are analyzed with an X-ray powder diffraction unit (XRD), using a PANalytical X’Pert PRO X-ray diffractometer with a Cu Kα radiation (40 KV, 30 mA) and a PIXcell solid state detector. The patterns are recorded at room temperature with step-size of 0.02°. Surface morphology, nanostructure, and composition of the assynthesized samples are recorded with the aid of a field emission scanning electron microscope (FESEM, Hitachi S4200, Japan), high resolution transmission electron microscope (HRTEM, Tecnai G2 F20 S-Twin at 200 KV field emission electron gun in Schottky mode), energy-dispersive X-ray spectroscopy (EDX), inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8300 model, Perkin-Elmer), and differential scanning calorimetry-thermogravimetric analysis (DSC-TGA, SDT Q600). For HRTEM

2. EXPERIMENTAL SECTION The fabrication of manganese oxide nanorods (MONRs) is a two-step process. In the first step, amorphous MnO2 is formed by the reaction of KMnO4 (Sigma-Aldrich, Korea, 99.9%) and

Received: Revised: Accepted: Published:

© 2013 American Chemical Society

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2013 9, 2013 13, 2013 13, 2013

dx.doi.org/10.1021/ie4013826 | Ind. Eng. Chem. Res. 2013, 52, 14154−14159

Industrial & Engineering Chemistry Research

Article

Figure 1. Synthesized amorphous manganese oxide (a) and manganese oxide nanorods (b).

Figure 2. SEM images of the effect of potassium concentration on the morphology of amorphous and crystalline manganese oxide nanostructures before and after heat treatment. Reducing potassium content by DI water rinsing and centrifuging: (a) 5 times, (b) 2 times, and (c) no rinsing.

Table 1. Potassium Contents of AMO and MONRs Samples manganese oxide nanorods after heat treatment (5 h at 650 °C)

amorphous manganese oxide elements (atom %)

analysis method

5 times rinsing

2 times rinsing

no rinsing

5 times rinsing

2 times rinsing

no rinsing

manganese (%)

EDX ICP-EOS EDX ICP-EOS

42.12 41.975 1.12 1.03

39.14 38.217 2.13 2.361

31.52 34.416 3.54 3.460

37.65 37.232 1.06 1.01

36.25 35.539 2.07 1.870

34.27 32.893 3.64 3.326

potassium (%)

3. RESULTS AND DISCUSSIONS Figure 1 represents the morphology of as-synthesized amorphous manganese oxide (AMO) and manganese oxide nanorods (MONRs). As mentioned in the Experimental Section, AMO nanoparticles are prepared according to the following reaction between KMnO4 and HCl:

imaging, a small amount of the solid powder is dispersed in alcohol, followed by sonication for 1 min, and then the solution is drop-casted on the commercially available carbon-coated copper grids. Also it should be noted that EDX was used as a qualitative compositional analysis technique, whereas ICP-OES is used to obtain the quantitative elemental concentrations within the samples. In both cases, standard calibration techniques with the use of known compositions were performed. Specifically, for EDX measurements, commercially as-received KMnO4 (Sigma-Aldrich, Korea, 99.9%) was used as a calibration standard (Supporting Information B1). The K:Mn atomic ratio is found to be very close to 1, indicating high accuracy of the measurements. Also during EDX analysis, five different portions on the sample were chosen to obtain the compositional data, and then the average value of these five spots was used as the result. This process actually nullifies the variations in the elemental compositions due to microscopic inhomogeneity within the samples and gives representative data for the as-synthesized samples as a whole.

2KMnO4 + 8HCl → 2KCl + 2MnO2 + 4H 2O + 3Cl 2 (1)

Subsequent heat treatments (650 °C in 5 h) of the semiproducts lead to the formation of uniformly grown MONRs. After reaction 1, the semiproduct (amorphous manganese oxide) was collected by centrifuging to remove aqueous solution. However, a small amount of potassium still exists and the formation and growth of manganese oxide nanorods during the heating process strongly depends on the content of potassium. The upper images of Figure 2 show the worm-like morphology of amorphous manganese oxide at different 14155



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cryptomelane MnO2 (1999 JCPDS-ICDD File Card no. 731826, 72-1982). This indicates that the presence of potassium ions does not have any significant effect on the crystalline phase transformation of the nanomaterial, rather a morphological change from nanoparticles to nanorods.32 Also, the amount of the potassium content is too small to be detected through XRD measurements. This phase transformation of AMO under heat treatement is also corroborated by DSC-TGA analysis. Supporting Information B8 represents the typical DSC-TGA curves of manganese oxide.33,34 The continuous weight loss between 25 and 200 °C corresponds to the release of loosely bound (adsorbed) and lattice water from the sample. Existence of the broad trough around 100 °C also supports the endothermic process of dehydration. Another weight loss around 500 °C corresponds to the phase transformation of amorphous to crystalline manganese oxide. From the TGA data it becomes apparent that at an intermediate temperature between 200 and 500 °C, the samples would remain amorphous without the presence of any hydroxides within the material. XRD data of heat-treated samples at 400 °C (not shown here) reveals an identical nature to that given in Figure 3 curve a and b, indicating that the amorphous material is pure oxide rather than hydroxide. Previously, Wang and coauthors also predicted that the presence of K+ ions is the key to the phase-controlled synthesis of manganese oxide with different nanostructures under hydrothermal conditions.28 This group proposed that the K+ ions act as template for the formation of a one-dimensional nanostructure and observed that the concentration of K+ ions has a profound effect on the crystallinity of the MnO2 nanowires/nanorods under hydrothermal treatement. The influence of alkali metals on the formation of MONRs was also mentioned in some other previous reports. Not only K+ but also Li+ and Na+ can work as the initiator for nanorod formation of manganese oxides.26,33,35 We have also performed several tests with Na and Li as growth initiator to prepare MONRs and found that these ions also have significant effect on the one-dimensional morphological conversion. These results will be shown soon in next manuscript. On the other hand, H+ ions can be formed during the formation of AMO, however H+ ions do not independently exist inside solid structures but bond with other anions, such as Cl− (HCl) or O2‑/OH−(H2O). HCl and H2O can easily be evaporated by heat treatment of manganese oxide. This removal of water content is also corroborated by the TGA analysis discussed earlier. Therefore H+ ions do not affect the one-dimensional morphological change of manganese oxide. This argument is further justified by the fact that there are several reports on the wet-chemical or electrochemical deposition of manganese oxide without the use of any K+ (or other alkali) ions, which never showed any one-dimensional nanostructure formation under heat treatement,34 indicating inconclusive evidence of the role of H+ as one-dimensional growth initiator. In our wet-chemical synthesis process, we have observed that K+ ions basically act as “growth director” during the thermal annealing process to convert amorphous manganese oxide nanoparticles to one-dimensional Mn2O3 (and small amount of MnO2) nanostructures, and the conversion efficiency increases with increase in the K+ contents. We speculate that the intercalated potassium ions inside the empty space between manganese oxide layers play crucial role during the crystalline phase transformation. The crystallization process involves two steps: (i) atomic nucleation, and (ii) crystal growth. During the

contents of potassium. The amounts of potassium in these samples are controlled by adjusting the rinsing process of the as-prepared samples. Top image of Figure 2a corresponds to 5times rinsing of the samples, whereas, the same images for Figure 2b and 2c correspond to 2 times rinsing and no-rinsing, respectively. When these samples are heat-treated at 650 °C for 5 h, depending on the potassium contents, these samples are converted into either crystalline MnO2 nanoparticles (5 times rinsing, Figure 2a, bottom image) or a mixture of crystalline MnO2 nanoparticles and nanorods (2 times rinsing, Figure 2b, bottom image) or pure crystalline MnO2 nanorods (no rinsing, Figure 2c, bottom image), respectively. The content of potassium in these samples, determined by EDX and ICPEOS analyses, is shown in Table 1. Both the data are found to be fairly close to each other, indicating very good accuracy of the compositional analysis. Without rinsing with DI water, followed by centrifuging, the semiproduct is found to contain approximately 3.5 atom % potassium. Similarly, after rinsing 2 and 5 times, the contents of potassium are found to be around 2.13−2.36% and 1.03−1.12%, respectively. As mentioned in some previous reports,28,31 potassium ions inside amorphous manganese oxide may not stay as free elements or as byproducts, rather they are intercalated inside the empty space between the manganese oxide layers to form some complex like KxMnO2. Therefore, it becomes difficult to remove potassium ions during the rinsing process. Hence, from Table 1 and Figure 2, it becomes apparent that the contents of potassium ions have profound effect on the morphological conversion of amorphous manganese oxide nanoparticles to crystalline manganese oxide nanorods during annealing. To further verify the effect of potassium ions on the phase transformation of the samples, XRD analysis is performed. The XRD patterns of amorphous manganese oxide with 1.03 atom % K-content (5 times rinsing, cf. Figure 3, graph a) and 3.5% K-

Figure 3. XRD patterns of amorphous (a, b) and crystalline (c, d) manganese oxide with (a, c) or without (b, d) a reduction of potassium content.

content (no rinsing, cf. Figure 3, graph b), are found to be nearly identical. On the other hand, the corresponding heattreated samples (@ 650 °C, 5 h) reveal the conversion of the amorphous materials into crystalline manganese oxide (cf. Figure 3, graphs c,d). But in both cases (no rinsing and 5-times rinsing), the XRD data analyses depict identical crystallinity with a dominant of Mn2O3 phase along with a small fraction of 14156



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atomic nucleation stage, K+ ions serve as the template for the formation sites of the manganese oxide crystals.36 During the second stage of crystallization, the crystals grow along these sites to form one-dimensional nanostructures. Obviously, under low content of potassium (∼1%), the formation of nanorods is small and dominated by the collapse of amorphous worm-like nanostructure into crystalline nanoparticles. Therefore, in this case, the products of heat treatment are basically crystalline manganese oxide nanoparticles which are shown in the bottom image of Figure 2a. On the other hand, when the content of potassium is higher, the nanorod formation increases, thus increasing the percentage of nanorods within the manganese oxide nanostructures, as observed in the bottom images of Figure 2b and 2c. Since the content of potassium (and also other alkali) ions strongly affect the formation of MONRs, it is pertinent to effectively control these ion concentrations during experiment. There are several ways to control this factor: either by changing the input materials before manganese oxide formation or by adding more alkali ions before heat treatment.26,31,32 But in both cases, several experiments need to be performed for different alkali ion concentrations to get different percentages of MONRs, which is time-consuming and cumbersome. But in our case, for the first time, we have used the rinsing process to control the potassium contents within our samples. Therefore, the as-synthesized AMO (produced only one time by acidic reduction of KMnO4), is separated into several intermediate products containing different K+ concentrations by changing the rinsing steps, without performing repeated chemical synthesis. Thus the process to control the formation of MONRs becomes very fast and simple. A HRTEM micrograph of the conversion of nanoparticle-tonanorod is shown in Figure 4. The nanoparticle is showing a dominant [404]/[440] growth of Mn2O3, whereas the nanorods are having a growth along the (222) plane of the Mn2O3 phase. Interestingly, at the interface, a mixture of both [404]/[440] and [004]/[400] growths of Mn2O3, are observed.

We believe that the presence of potassium, {having bcc crystal structure with lattice parameters roughly half of the orthorhombic crystal structure of Mn2O3 (JCPDS ICDD 1999 File Card No. 73-1826, 01-0500)}, at the interface favors the [222] growth of Mn2O3 during annealing. Although, during the high temperature annealing process, the potassium ions dissolute out, the Mn2O3 crystals continue to grow along this direction to form one-dimensional nanostructures. The selected area electron diffraction (SAED) pattern, shown in the inset of Figure 4, depicts the growth of the characteristics crystal planes of Mn2O3 and matches the XRD data (Figure 3). After the samples are collected from the acidic reaction of permanganate (reaction 1) and allowed to dry without reducing the potassium residues (i.e., no-rinsing condition, having highest K+ content, cf. Table 1), they are heat-treated at 650 °C for 2, 5, 7, and 10 h, respectively, to observe the effect of annealing time on the conversion efficiency of the nanoparticleto-nanorod process. Figure S1 panels a to d of the Supporting Information represent the heat-treated samples at four different annealing times, and the strong dependency of the annealing time on the nanoparticle-to-nanorod conversion efficiency is clearly visible. The cubic-shaped particles which are consumed to grow the rod-shaped morphology are seen in Supporting Information, Figure S1b,c.32 The energy provided for the nucleation and crystal growth of manganese oxide to form the nanorods is proportional to the heating time. Therefore, the density of MONRs is higher when heating time is longer. This argument is corroborated by TEM images (Figure S2 of Supporting Information) of the nanorods synthesized at two different annealing times of 2 (cf. Figure S2a) and 10 h (cf. Figure S2c), respectively. Density of the nanorods are higher at longer annealing time, indicating better conversion in the later case. Figure S2 panels b and d represent the corresponding HRTEM images of single nanorods, indicating very high crystallinity of the nanorods. To confirm the influence of potassium content on the formation of MONRs, the extensively rinsed AMO (5 times rinsing) was treated under long annealing times (15 and 24 h) at 650 °C. Supporting Information, Figure S3 shows these samples which are cubic shaped Mn2O3 crystals without any nanorod formation, indicating that the nanorods formation is indeed mediated by the potassium ions and can be controlled by the rinsing process.

4. CONCLUSIONS A rapid, simple, cost-effective and controllable synthesis of manganese oxide nanorods is presented. The effects of the content of potassium and heating time on the morphologies and phase transformation of amorphous manganese oxide nanoparticles to crystalline manganese oxide nanorods are studied. Potassium ions are believed to act as “growth director” for the morphological conversion of nanoparticle-to-nanorod, and the conversion efficiency increases with increasing potassium content. XRD and HRTEM analyses depict very high crystallinity of the nanorods with a dominant Mn2O3 phase. The successful fabrication of crystalline manganese oxide nanorods will offer a very high specific surface area for promising applications in energy storage, solar cell, supercapacitor, and giant magnetoresistance devices, among others.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 4. HRTEM micrograph showing the interface between the nanoparticle and nanorod. Inset shows the corresponding SAED pattern of the nanostructure.

SEM images of heat-treated samples at four different annealing times (2, 5, 7, 10 h), TEM images and corresponding HRTEM 14157



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images of the nanorods synthesized at two different annealing times of 2 and 10 h, SEM images of the manganese oxide (5 times rinsed sample) at two different annealing times of 15 and 24 h, analysis data include EDX and TGA. This information is availbe free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(A.N.B.) Fax: +82 53 810 2062. Tel: +82 53 810 2453. E-mail: [email protected], [email protected]. *(S.W.J.) Fax: +82 53 810 2062. Tel: +82 53 810 2568. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by the World Class University Grant R322008-000-20082-0 of the National Research Foundation of Korea. We also thank Professor M. A. Cheney of University of Maryland for insightful discussions.

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Effect of Potassium Ions on the Formation of Crystalline Manganese

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