Synthesis of Birnessite Structure Layers at the Solution–Air Interface

Article pubs.acs.org/Langmuir

Synthesis of Birnessite Structure Layers at the Solution−Air Interface and the Formation of Microtubules from Them Valeri P. Tolstoy and Larisa B. Gulina* Institute of Chemistry, St. Petersburg State University, Universitetskiy pr. 26 St. Peterhof, St. Petersburg, Russia 198504 S Supporting Information *

ABSTRACT: HxMnO2·nH2O layers have been successfully produced through a facile low-temperature process at the solution−air interface without using any templates. The crystalline structures of layers can be tuned by the compositions and the pH of the growth solutions. The analysis of birnessite-like layers indicates that they are formed by nanosheets approximately 4−6 nm thick that are oriented for the most part normally to the interface. Our results demonstrated that 1−3-μm-thick layers can roll up into microtubules 20 to 100 μm in diameter and up to 10 mm long. The hypothesis explaining the formation of the microtubular structures is assumed. up into nanotubes,26,33 also called34 nanoscrolls or nanorolls. Also, tubular structures were synthesized by the thermal evaporation method,35 by the template method,36−38 and by the method offered by Prinz.39,40 In this article, we propose a facile new method to obtain the birnessite structure in manganese oxide synthesis under “soft chemistry” conditions. A distinctive feature of the technique is the formation of an initial layer at the aqueous solution−gas interface. Films formed at the air−water interface in both inorganic and polymeric systems may have advantages in certain applications over films formed by other methods. In the inorganic case, air−water interface films can be thicker than those formed by dip coating. They can also be less synthetically demanding since they will form under a wide range of ambient temperatures, humidities, and concentrations, although currently these films are not for the most part being actively developed by many scientific groups.41−46 For the first time it is shown that microtubules of HxMnO2· nH2O can be obtained from the birnessite layers, which grow in the aqueous solution of a manganese salt−air interface during ozone treatment. The developed method of synthesis is suitable for the microtubule formations of other inorganic compounds.

1. INTRODUCTION Birnessite-type manganese oxides MxMnO2·nH2O (M = H+, Na+, K+ and other ions) have a layered structure in which water molecules or metal ions are placed in the interlayer space of MnO6 octahedral layers formed by manganese−oxygen polyhedra containing Mn3+ and Mn4+ cations.1,2 The metal ions in the interlayer of the birnessite are ion-exchangeable. Manganese oxides with layered structures are attractive materials due to their potential applications as ion exchangers,3,4 electrode materials for secondary ion batteries,5−7 magnetic materials,8 and precursors to the various tunnel structure materials, for example, octahedral molecular sieves.9−11 Therefore, birnessite is a promising material for chemical sensors production,12 superhydrophobic coating,13 heterogeneous catalysts,13,14 effective scavengers,15 and oxidizers16 of materials. Birnessite-type manganese oxides were synthesized by oxidizing Mn2+ salts, for example, solution reaction with H2O23 or O217 and reducing KMnO4 in acid18 or alkaline19 media. Also, birnessite-type manganese oxides were synthesized under hydrothermal conditions.20−22 The single crystals of the birnessite have been obtained by a flux method.23 The various 1D structures for Mn oxides and hydroxides were obtained earlier as a product of hydrothermal synthesis: nanorods,24 tubes,25−27 and cuniform-like structures.28 The interaction of manganese salt with oxygen in a bubbled solution was investigated in ref 29. Much attention has been paid lately to the synthesis of 1D structures of inorganic compounds due to the high potential of their practical application. The results of research work in this direction have been reported in a number of reviews, e.g., in refs 30−32, where conditions of the synthesis and some properties of the nanotubules are considered. According to the reports, in most cases nanotubular structures can be produced under conditions of solvo- or hydrothermal synthesis using compounds with a layered crystal structure. Being subjected to these conditions, separate nanosheets of these compounds roll © 2014 American Chemical Society

2. EXPERIMENTAL SECTION Mn(CH3COO)2·4H2O, MnCl2·4H2O, MnSO4, Mn(NO3)2·6H2O, and CH3COONa·3H2O salts (chemically pure) provided by Vekton were used as reagents. Aqueous solutions were made using Milli-Q highpurity water with a resistivity of more than 18 MΩ/cm. The synthesis procedure was as follows. To synthesize a layer at the interface, 4 mL of the 0.02 M Mn(II) salt solution was poured into a flat vessel 1 cm high, 1.5 cm wide, and 3 cm long and put into a glass-lined chemical reactor 2.5 cm in diameter and 20 cm long. For the synthesis of the tubular structures, a blend of Mn(CH3COO)2·nH2O and CH3COONa with concentrations of 0.02 and 0.04 M, respectively, was obtained. Received: October 9, 2013 Revised: June 17, 2014 Published: June 26, 2014 8366

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Figure 1. Reaction scheme for the formation of the HxMnO2·nH2O layer at the solution−air interface and its evolution toward microtubular structures. CH3COONa was supposed to be a background electrolyte in the mixture of these reagents. Then a mixture of air with ozone was fed from one side into the reactor at a rate of 100 mL/min. Ozone was produced by barrier-type pulse generator OZ-1 M with an ozone output of 1 g/h. The treatment lasted from 1 to 15 min. Figure 1 sketches the synthesis process. The experiments have shown that a reddish-brown layer was formed on the solution surface during ozone treatment. After that, the layer was next transferred carefully to the surface of pure distilled water in order to remove excess solution. After 2−15 min of exposure, the film was transferred to the surface of a single-crystal silicon wafer and then dried at 40 °C and analyzed by X-ray diffraction, IR and XPS spectroscopy, SEM, electron probe microanalysis, and optical microscopy. X-ray powder diffraction was performed on a Rigaku Miniflex II diffractometer. The measurement conditions were Cu Kα radiation, 30 kV voltage, and 10 mA current. The sample morphologies were determined using scanning electron microscopy (Zeiss EVO-40EP or Supra VP-40). The chemical composition of the samples (Na/Mn ratio) was determined by EDX analysis using a scanning electron microscope equipped with an INCA 350 Energy EDX analyzer (Oxford Instruments). Infrared spectra were obtained on a PerkinElmer infrared spectrometer (1760x Series FT-IR). To have a high signal-to-noise ratio, each IR spectrum was taken as the average of 50 successive scans obtained at a spectral resolution of 4 cm−1. The content of Na was determined by atomic absorption spectroscopy (AAS) on a Shimadzu AA-7000 spectrometer. The oxide samples were dried at 250 °C to remove the molecular water before AAS analysis. For the analysis, about 10 mg of the oxide samples was carefully weighed, dissolved in 20 mL of concentrated HCl, and then diluted to 100 mL by deionized water. The obtained clear solutions were analyzed by AAS. Rolling processes were recorded with a Penscope video camera at ×50 magnification.

Figure 2. X-ray diffraction pattern of an HxMnO2·nH2O layer synthesized from (a) Mn(CH3COO)2 + CH3COONa solutions and (b) Mn(CH3COO)2 solutions.

missing while the peaks at 21.34 and 23.57° emerge. The XRD pattern of the sample obtained at a different composition of the anions (from Mn(CH3COO)2 solution) is shown in Figure 2b. These peaks can be indexed as the reflections of ε-MnO2 structure-type manganese oxides (JCPDS card 00-012-0141). When the surface of the aqueous solution of Mn(CH3COO)2 is exposed to ozone-containing air, oxidation of the Mn2+aq cations by ozone molecules apparently takes place. As a result of this reaction, according to conclusions of the work,48 manganese changes its oxidation state to Mn(III) and then partially disproportionates to Mn(IV), forming poorly soluble crystals of HxMnO2·nH2O and Mn(II) cations in the solution:

3. RESULTS AND DISCUSSION 3.1. Formation and Characterization of As-Deposited Layer. When the manganese salt solution is treated with ozone, a reddish-brown film is formed on its surface layer. Figure 2a shows XRD patterns of the films obtained from Mn(CH3COO)2 + CH3COONa solutions. In Figure 2a, one can see a series of diffraction peaks at 2θ = 12.16, 36.75, 54.90, and 65.95°. The significant XRD peaks can be well assigned to the (002), (006), (301), and (119) crystal planes of manganese oxide with a birnessite structure, respectively (JCPDS card 00018-0802). As can be seen, the X-ray diffraction pattern is characterized by the absence of reflections from the (004) crystal plane. This effect is observed for the layered crystal modification of birnessite and it is thoroughly discussed in ref 47. The absence of reflection may be attributed to 2D crystals attaining a thickness of only a few atomic layers. In this case, the intensity of reflection from the (004) plane is much lower than that from the (002) and (006) facets. At the same time, when the films are grown in solutions having no excess CH3COONa, the peak at 2θ = 12.2° in their XRD patterns is

Mn 2 +aq + O3 → MnOOH

(1)

MnOOH → HxMnO2 ·nH 2O + Mn 2 +aq

(2)

Since the reaction proceeds in a solution of acetate ions, these ions are probably adsorbed on the HxMnO2·nH2O surface and inhibit the crystal growth along one of the axes, and this results in the formation of nanosheets instead of bulk 3D birnessite crystals. In order to explore the impact of the acetate ion concentration and pH of the solution on the crystalline structure of the film formed at the interface, we carried out the synthesis using solutions with varying acetate ion concentration and constant Mn(II) ion concentration (Table 1). As a result of the experiments performed, it can be concluded that the concentration of acetate ions has a crucial effect on the crystalline structure of the final product and specifically that birnessite is formed only in solutions with an acetate concentration of more than approximately 4 times that of Mn(II) ions. These results are in agreement with the data49 concerning the preferential contribution of acetate ions to birnessite formation. In this article, we postulate that acetate 8367

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Table 1. Crystalline Structures of the Films Synthesized at the Manganese(II) Salt Solution−Air (Ozone) Interface no. 1 2 3 4 5 6 7 8

reagents C(Mn2+) = 0.02 M

C(CH3COO−), M

pH of solution

crystalline structure

Mn(CH3COO)2 Mn(CH3COO)2 + CH3COONa Mn(CH3COO)2 + NaOH MnSO4 MnCl2 Mn(NO3)2 MnSO4 + CH3COONa MnSO4 + CH3COONa + CH3COOH

0.04 0.08

7.7 7.9

ε-MnO2 Birnessite

0.04

7.9

ε-MnO2

0.00 0.00 0.00 0.08 0.08

6.5 6.3 6.3 7.9 6.5

ε-MnO2 ε-MnO2 ε-MnO2 Birnessite Birnessite

Figure 4. FT-IR spectra of an HxMnO2·nH2O layer on a singlecrystalline silicon substrate synthesized from different solutions: (a) Mn(CH3COO)2 + CH3COONa, (b) MnSO4, (c) MnNO3, and (d) MnCl2.

promotes Mn(CH3COO)+ oxidation50 and the formation of the birnessite-type manganese oxide under interfacial synthesis conditions. This phenomenon becomes clear if the results of computations (Figure 3) of mole fractions of different species

designated as the solid−solution or S−S interface). The Mn 2p3/2 XPS spectra of the samples are reproduced in Figure 5a. When the manganese oxidation state is higher, then the peak is shifted toward higher BE values. For example, according to ref 55, Mn4+ has a maximum at 642.0 eV; Mn3+, at 641 eV; and Mn2+, at 640 eV. The maxima of the bands in the experimental spectra of the two surfaces differ only slightly (642.0 eV for S− S and 642.3 eV for S−A), but their mutual 0.3 eV displacement may be evidence of different ratios of Mn3+ and Mn4+ in these two samples, i.e., indicating different average oxidation states (AOSs) of manganese atoms. One may assume that in the case of the S−S (solution side) surface a greater contribution to the band shape is made by Mn in the 2+ and 3+ oxidation states, whereas the S−A (air side) surface is more abundant in manganese in the 4+ oxidation state. Obviously, the number of ions of opposite sign required to attain overall electroneutrality is different at different AOSs. This fact is confirmed by XPS O 1s spectra. It is known56 that the shape and position of the bands of the O 1s spectrum depend on the correlation of O2− structural (529.6 eV), OH− structural (530.9 eV), and H2O sorbed (532.3 eV). Figure 5b demonstrates that for the S−A surface the intensity of peaks at 529.9 eV (refers to the share of O2− oxygen) is practically equal to that at 532.6 eV (refers to oxygen in H2O). Furthermore, for the S−S surface the observed intensity of the 529.7 eV band is lower compared to that of the 533.0 eV band. These data definitely confirm the higher manganese AOS for the S−A surface compared to that of the S−S surface. 3.2. Formation and Characterization of BirnessiteType MnO2 Microtubules. The SEM images of the layers formed at the interface of manganese acetate and air with an ozone admixture and then placed on the surface of singlecrystalline silicon and dried in air (Figure 6a) show that a number of sample microtubules 20−100 μm in diameter and up to 10 mm long can be seen on the surface. For a sample prepared by 8 min of ozone treatment, the wall thickness of a microtubule is approximately 3 μm. An extension of the treatment results in layer thickness growth of up to 10 μm. However, for samples with a wall thickness of less than 1 μm or greater than 4 μm, the formation of microtubules no longer occurs in the course of drying the synthesized layer. Most likely, when the thickness is less than 1 μm microtubules are not formed due to the comparatively low mechanical strength of the synthesized layers; on the contrary, for thicker layers, excessive stiffness hampers their “rolling up”.

Figure 3. Relative equilibrium concentration of Mn2+aq species ([Mn2+] = 0.02 M) as a function of the pH of the Mn(CH3COO)2 + CH3COONa solution.

of complexes in the solutions of manganese salts are considered. Calculations were carried out with the Hydra− Medusa program (http://www.kth.se/che/medusa) using a hydrochemical equilibrium constant database. According to these computations, the greater part of the cations in the manganese acetate solution is arranged into complexes with acetate ions, in contrast to solutions of other manganese(II) salts in which the Mn2+aq species prevails provided the pH of the solution is the same. When the acetate ion concentration exceeds the manganese ion concentration by more than 8-fold, practically all manganese cations in the solution are bound in complexes with acetate. These results were confirmed by the FT-IR spectroscopy data (Figure 4), i.e., by the spectra in which the absorption bands in the 450−800 cm−1 region can be related to Mn−O stretching vibrations.51,52 In the spectrum of the sample synthesized with the use of sodium acetate solution (Figure 4a), there is an absorption band at 635 cm−1 that might be related to the content of Mn(III) in birnessite layered structures.53 The absorption bands with maxima at 3400 and 1630 cm−1 in this spectrum should be attributed, respectively, to the valence and deformational oscillations of the O−H bonds of the water molecules.54 Figure 5 shows XPS spectra of the synthesized layer referring to its outer side (adjacent to air and designated as the solid−air or S−A interface) and inner side (adjacent to the solution and 8368

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Figure 5. XPS spectra of an HxMnO2·nH2O sample synthesized from Mn(CH3COO)2 + CH3COONa solutions: (a) manganese area; (b) oxygen area. S−A, solid−air interface side; S−S, solid−solution interface side.

drying is available in mpg format. The playback speed of the video is set 3 times slower. An examination of the microtubules walls by electron probe microanalysis (EPMA) indicated (Figure 8) that the walls contains Mn and O atoms, but in some of the examined points on the sample, Na atoms could also be detected in concentrations comparable to the measurement error.

Figure 6. SEM images of HxMnO2·nH2O samples: (a) general view of microtubules, (b) side view of a microtubule wall, (c) view of the outer side of microtubules (S−A) at different magnifications, (d) view of the inner side of microtubules (S−S) at different magnifications.

Figure 8. EDX spectrum of HxMnO2·nH2O microtubules on a silicon substrate.

The study of the microtubule oxide product by atomic absorption spectroscopy revealed that the Na content in the test solution is below the detection limit (∼0.05 ppm) and is found to range to 0.05 wt % in birnessite. Aggregated results of the composition study prove that the synthesized tubular product is the birnessite containing only manganese. Consequently, its formula can be written as HxMnO2.nH2O. To explain the observed phenomenon of microtubule formation, one can assume that in course of drying of the film with the varying distribution of birnessite nanosheets along the film thickness, in the layers of the film having a lower density strong attraction forces would emerge due to hydrogen bonds between separate nanoparticles resulting in a distortion of the plane geometry of the film and its transformation into a tubule (Figure 9). The density gradient mentioned above is obviously a result of inhibited diffusion in the surface layer of the solution occurring when gaseous ozone interacts with the manganese cations of the solution.

The microphotographs in Figure 1b indicate that the layer is formed by an aggregation of nanosheets having an orientation for the most part perpendicular to the surface of the interface while their “packing” density is higher on the side that was in contact with air during preparation and lower on the side in contact with salt solution. The thickness of these nanosheets determined from the electron images is about 4−6 nm. The tendency of nanosheets toward such an orientation may be explained as follows: in the initial stages of the reaction, HxMnO2·nH2O nanosheets are formed at the surface parallel to the solution−air interface. But as the formation of nanosheets goes on and their number increases, the effect of forces of mutual repulsion increases and nanosheets forming next tend to be oriented normally to the solution−air interface. Selected frames during rolling are shown in Figure 7. The frames are numbered in sequence with each integer separated by 1/1.0 s. A video of the spontaneous rolling process during

Figure 7. Series of successive images during the spontaneous rolling process. Images 1−4 were taken at 0.0, 1.0, 2.0, and 3.0 s, respectively. 8369

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XRD nanosheets of the samples obtained from solutions of MnCl2, MnSO4, and Mn(NO3)2. XPS spectrum of an HxMnO2· nH 2 O sample synthesized from Mn(CH 3 COO) 2 + CH3COONa solutions. SEM images of HxMnO2·nH2O samples. EPMA results. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. Hypothetical scheme of microtubule formation in the course of drying. The density gradient in the direction normal to the layer plane is drawn as different shades of gray (S−A, solid−air interface; S− S, solid−solution interface).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

Finally, it should be mentioned that we believe it possible to synthesize by the suggested facile method not only the oriented layers formed by nanosheets of HxMnO2·nH2O birnessite and microtubes produced from them but also a wide variety of its derivatives with the general formula MxMnO2·nH2O (where M stands for a singly or doubly charged metal cation). This problem is a subject for further study.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The reported study was partially supported by RFBR, research project 12-03-00805-a. XRD, FT-IR, and AAS research was carried out in the X-ray Diffraction and Analytical Centers of St. Petersburg State University. The SEM study was carried out by the Nanotechnology Centre of SPbSU. We are grateful to consultation center ECCS (SPbSU) and A. Kh. Abdulmanova for help in writing the article.

4. CONCLUSIONS We have developed a facile and efficient interface-mediated synthesis method for the fabrication of Mn oxide nanosheets and microtubules from them. As a result of the interaction of the Mn(II) aqueous solution with ozone delivered by air flow to the solution−air interface, an HxMnO2·nH2O layer is formed on the surface of the solution. Examinations of such layers have shown that they have an ε-MnO2 or birnessite-type crystal structure. Birnessitetype crystal structure layers are constituted of nanosheets approximately 4−6 nm thick, with their preferable orientation being normal to the interface. The density of sheet packing and manganese AOS decrease with the depth of the layer toward the side in contact with the solution. For the first time, it has been demonstrated that subsequent drying under air for layers having a 1−3 μm thickness leads to the formation of microtubules. We can offer the following working hypothesis of the observed phenomenon. During the synthesis, a layer with nonuniform composition and irregular density is formed. Moreover, the internal stress in the birnessite layer can originate from the crystal defects of nanosheets. Obviously the solution-side layer contains more water molecules between separated sheets than does the air-side layer. During drying, the layer is deformed due to the removal of water from the porous structure of the layer. As a consequence, it can be assumed that the main forces determining the distortion of the plane configuration of the initial layer are the hydrogen bonds between separate nanosheets. Our experiments demonstrated that this interfacial method may be easily extended to the synthesis of other inorganic tubular structures with controllable parameters and shapes, for example.57 We believe that the present method of synthesis is an exclusive feature of the microtube and nanotube formation of many inorganic compounds.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

A video of the process of formation of the tubular structures from the birnessite structure layer. The playback speed of the video is set at a 3-fold slower value. The process was recorded at 50× magnification. Diagram of states of manganese ions in Mn(CH3COO)2 solution with no admixture of CH3COONa. 8370

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