J. Phys. Chem. C 2008, 112, 17089–17094
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Birnessite-type MnO2 Nanowalls and Their Magnetic Properties H. T. Zhu,†,‡ J. Luo,*,† H. X. Yang,† J. K. Liang,†,§ G. H. Rao,† J. B. Li,† and Z. M. Du‡ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China, Department of Materials Science and Engineering, UniVersity of Science and Technology Beijing, Beijing 100083, China, and International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China ReceiVed: May 27, 2008; ReVised Manuscript ReceiVed: September 1, 2008
Birnessite-type MnO2 nanowalls have been fabricated on Si(111) substrate by a facile solution method. The XRD pattern indicates that the sample has the typical feature of turbostratic disorder and prefers to grow in the ab plane. The nanowalls are composed of thin flakes distributing uniformly over the surface of the Si(111) substrate. A magnetic transition temperature of 9.2 K is determined. Prominent magnetic anisotropy with the easy magnetization direction in the ab plane is observed at 5 K. 1. Introduction Birnessite is a layered manganese oxide consisting of twodimensional (2D) sheets of edge-shared MnO6 octahedra separated from each other by exchangeable cations and water molecules located at the interlayer space.1-4 Due to its distinctive chemical and physical properties,4-9 birnessite is of importance in natural systems and for industrial applications. Natural birnessite minerals, ubiquitous in soils and sediments, play a key role in the transport, speciation, and ultimate fate of metals, natural organic matter, and other pollutants in the natural environment, which could be ascribed to their unique cation exchange capacity and sorptive, catalytic, and oxidative properties.2,10 Taking advantage of its high surface area and small particle size,4 the layer-structured MnO2 has been widely used as cathodes for rechargeable lithium batteries11-15 and as catalysts for oxidation-reduction processes.7 In addition, birnessite is an important precursor for the synthesis of many manganese oxides with tunnel structures.16-18 Birnessite-related nanosturctures were synthesized, normally, through oxidation of Mn2+, reduction of MnO4-, redox reactions between Mn2+ and MnO4-, or direct phase transformation from other manganese oxides.8 Birnessite-type MnO2 nanosheets with a thickness of 0.77 nm and nanobelts of 5-15 nm width were prepared in HCl and NaOH solution,4,5 respectivley. These nanobelts might be a promising electrode material for rechargeable lithium batteries in view of their high capacity and good cycling behavior. K-birnessite nanowires and nanoribbons were successfully fabricated through the redox reactions, and a small magnetoresistance was observed in this paramagnetic Kbirnessite.19 One-dimensional (1D) hierarchical nanostructures of layered KxMnO2 bundles exhibiting ferromagnetic behaviors were synthesized based on the reduction of KMnO4.20 Manganesebased oxide nanowires with a layer structure were synthesized via the soft-chemical oxidation of a solid-state precursor, and improved electrode performance could be expected for these nanowires by cation substitution.21 Birnessite nanosheets less * Corresponding author. Tel: +86-10-82648119. Fax: +86-10-82649531. E-mail:
[email protected]. † Institute of Physics, Chinese Academy of Sciences. ‡ University of Science and Technology Beijing. § International Center for Materials Physics, Chinese Academy of Sciences.
than 10 nm in thickness and their oriented thin films were generated in a mixed solution of MnCl2 and ethylenediaminetetraacetate (EDTA) at room temperature.22 Sheet-shaped nanoarchitectures of birnessite exhibiting good power capability were obtained by cyclic voltammetry cycling in aqueous Na2SO4 using the high-temperature treated Mn3O4 film as the electrode.9 High-quality nanobelts of K-birnessite were hydrothermally synthesized in a KMnO4-MnCl2-KOH system, and their swelling and delamination behaviors were investigated.8 Birnessite-type MnO2 honeycomb and hollow nanospheres, which showed high catalytic activities for oxidative decomposition of formaldehyde at low temperatures, were prepared facilely at room temperature based on the reduction of KMnO4 in oleic acid.7 Nanowalls, a pretty novel 2D nanostructure, exhibit a unique surface morphology, especially their two-dimensionality and high surface area. Since the discovery of carbon nanowalls in 2002,23 much effort has been devoted to the controllable synthesis of nanowalls in different compositions.24-38 First, the nanowalls may provide an excellent opportunity to explore new physics of this kind of novel nanostructures. For example, according to previous investigations, most of the nanowalls including carbon, ZnO,30 Co3O4,32,33 CuS,34 and Cu-TCNAQ37 exhibit promising field emission (FE) properties, which are ascribed to their nanometer scale wall thickness and open edge geometry. Due to their high surface-to-volume ratio, ZnO nanowalls show also high hydrogen-sensing properties.39 Both Co3O433 and NiO35 nanowalls display not only excellent FE properties but also superior electrochemical performance. The ε-MnO2 nanowalls demonstrate excellent Li+ ion intercalation property and improved cyclic stability.38 The enhanced electrochemical performance has been attributed to their large surface area and shorter diffusion length for mass and charge transport. Mn3O4 nanowalls exhibit a large magnetic anisotropy field at 5 K, which can be ascribed to the prominent shape anisotropy.31 Second, the unique surface feature of the MnO2 nanowalls makes it an ideal template for synthesizing mesoporous materials with high surface areas. For instance, with wellaligned carbon nanowalls as the template, magnetic particles including Fe, Ni, NiFe, and NiCoFe and different types of mesoporous materials such as TiO2, SiOx, SiNx, and AlOx have been successfully deposited on the carbon nanowalls.40,41 Last
10.1021/jp804673n CCC: $40.75 2008 American Chemical Society Published on Web 10/11/2008
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but not least, the 2D nanowalls are important components for nanoscale devices, which is essential in the assembly of functional nanodevices.28,34 In this contribution, we report the well-controlled growth of 2D MnO2 nanowalls by a simple and one-step hydrothermal method. The uniform birnessite-type MnO2 nanowalls may find applications in many areas, such as electrode materials for lithium batteries, generic templates to synthesize mesoporous materials with high surface areas, field emission displays, energy storage, catalysts, and sensors. 2. Experimental Section 2.1. Fabrication of Birnessite-type MnO2 Nanowalls and Powders. Birnessite-type MnO2 nanowalls grown on a Si(111) surface were hydrothermally synthesized by the reduction of KMnO4 in acidified water. All the reagents used in the experiment were of analytical purity and used without further purification. In a typical procedure, 2.5 mmol of KMnO4 and 0.4 mL of concentrated HCl (10 mmol) were added to 45 mL of deionized water to form the precursor solution, which was then transferred into a Teflon-lined stainless steel autoclave of 65 mL capacity after a clean silicon(111) substrate was preplaced in the autoclave. The autoclave was sealed and maintained at 140 °C for 100 min. After the reaction completed, the autoclave was cooled to room temperature naturally. The thin film sample deposited on the Si(111) substrate was collected and washed several times with deionized water to remove any impurities and excess ions and then dried at 60 °C in air overnight. For comparison with the nanowalls, the corresponding powder sample was prepared with the same experimental settings and conditions except that the substrate was not used. 2.2. Sample Characterization. The chemical composition (contents of K and Mn) of the sample was cross-checked by energy-dispersive X-ray spectroscopy (EDS) and chemical analysis with inductively coupled plasma atomic emission spectrometry (ICP-AES). The interlayer water was examined by thermogravimetric analysis (TGA) and differential thermal analysis (DTA), which was performed on a TA-Q600 analyzer in dry air with a heating rate of 10 °C/min. Phase identification and structure analysis were carried out by X-ray powder diffraction (XRD) using a Rigaku D/max 2500 diffractometer with Cu KR radiation (50 kV × 250 mA) and a graphitic monochromator. The overview morphologies and sizes of the samples were obtained by field emission scanning electron microscopy (FESEM), performed on a FEI-Sirion scanning electron microanalyzer with an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and electron diffraction (ED) patterns were recorded on a JEM-2010 and Tecnai F20 TEM using an accelerating voltage of 200 kV. 2.3. Measurements of Magnetic Properties. Magnetic properties of the sample were measured by a commercial superconducting quantum interference device (SQUID) magnetometer. The zero-field-cooling (ZFC) and field-cooling (FC) magnetization of the birnessite-type MnO2 powder was measured from 5 to 300 K in an applied field of 100 Oe. Hysteresis loops of the birnessite-type MnO2 nanowalls and powders were measured at 5 K. 3. Results and Discussion Figure 1a shows the XRD pattern of the birnessite-type MnO2 nanostructures, and the schematic illustration of the layered structure is presented in the inset of Figure 1b. The XRD profile clearly reflects the typical feature of layered compounds with a
Figure 1. (a) XRD patterns of the birnessite-type MnO2 powder (bottom) and nanowalls (top) and (b) EDS spectrum of the birnessitetype MnO2 nanowalls; the inset is a schematic illustration of its crystal structure.
turbostratic disorder,42,43 that is, the sample consists of layers arranged parallel and equidistant but the well-defined displacement/rotation between successive layers is absent.44 The XRD pattern of the powder sample (bottom) shows five diffraction peaks, which can be indexed to the monoclinic potassium birnessite (JCPDS 80-1098). Apart from the turbostratic MnO2, no other phases are observed in the spectra. The two sharp diffraction peaks correspond to (001) and (002) basal reflections, while the three asymmetric broad peaks around d ) 2.4, 1.4, and 1.2 Å are indexed as the (20l; 11l), (02l; 31l), and (22l; 40l) diffraction bands, respectively.42,43 The d001 value of 0.73 nm is related to the interlayer spacing and is characteristic of the birnessite intercalated with K+ ions.1 The XRD pattern of the thin film deposited on Si(111) substrate (top) is a little different from that of corresponding powder sample, that is, the (001) and (002) reflections become much weaker, which implies that the preferred growth direction of the layered MnO2 is in the ab plane. According to the EDS and ICP-AES analysis, almost identical K/Mn atomic ratio around 12% ( 1.0% has been detected for both thin film (Figure 1b) and powder samples (Figure 2a), indicating that the K+ concentration is closely related to the synthesis process but independent of the morphology and size of the sample. The interlayer water content has been detected by TGA measurement (inset of Figure 2a). According to the weight loss of 6.4% below 250 °C (before the TGA-DTA measurement, the physically adsorbed water has been removed by heating the sample at 100 °C), the calculated interlayer water is around 0.35 H2O per chemical formula (K0.12MnO2 · 0.35H2O). Above 300 °C, the oxygen release is evolved due to the partial reduction of the Mn4+ cations to the trivalent state.45 The exothermic peak around 473 °C on DTA curve is related to the phase transformation from the layered structure to the R-MnO2 phase (see also Supporting Information),11 and the weight increase on the corresponding TGA curve indicates that the released oxygen is partly compensated during this phase
MnO2 Nanowalls and Their Magnetic Properties
Figure 2. (a) EDS spectrum and TGA and DTA curves (inset) of the birnessite-type MnO2 powder and (b) XRD pattern of the sample after heated to 1100 °C.
Figure 3. (a) Low- and (b) high-magnification SEM images of the birnessite-type MnO2 powder.
transformation due to the oxidation of the Mn3+ to Mn4+ state.45,46 The weight losses (or the endothermic effects on DTA curve) at 890 and 980 °C are related to the structure transformation from MnO2 to Mn2O3 and from Mn2O3 to Mn3O4,4 respectively. In order to further confirm our TGA-DTA results, the sample has been examined by XRD after the TGA-DTA measurement. As shown in Figure 2b, the resultant sample is the tetragonal R-Mn3O4 compound with a space group I41/amd (the XRD data is not very prominent due to the small amount of the sample, and the broad peak at lower angle originated from the reflections of the glass sample holder). Figure 3 shows the SEM images of the birnessite-type MnO2 powder. The powder sample has the so-called sea urchin nanostructures47-50 with a diameter of 2-3 µm. The sea urchinlike structure is composed of many flakes radiating from its center. Despite the difference in the dimensionality, the shape, thickness, and connected network of the flakes are very similar to those of the nanowalls deposited on Si(111) substrate. Figure 4 shows the typical SEM images of the birnessite-type MnO2
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Figure 4. (a) Low- and (b) high-magnification SEM images of the birnessite-type MnO2 nanowalls grown on Si(111) substrate.
nanowalls grown on Si(111) substrate. The distribution of the nanowalls is uniform over the whole surface area of the substrate, which is typically 15 mm × 10 mm. Therefore, as shown in Figure 4a, the sample has a pretty smooth surface. From the magnified view (Figure 4b), the nanowall sheets are very thin flakes with a thickness of several to ten nanometers. These flakes connect together to form networks and spread over the whole substrate surface. Some tubular-like structures are found, but normally these flakes just curve and connect together randomly. Most flakes of the layered MnO2 are well aligned and approximately vertical to the surface of the silicon substrate, though a deviation from the vertical alignment is also observed for part of the flakes. Assuming that the nanowall sheet grows preferentially in the ab plane to form the nanowalls, this is consistent with the XRD pattern showing weak (00l) diffraction peaks. Figure 5a displays the thin film peeled off from the substrate, which is quite uniform and flat on the whole view. From the side view in Figure 5b, the length of the nanowalls or the thickness of the thin film deposited on Si(111) substrate is around 900 nm. Figure 6 shows the TEM image of the birnessite-type MnO2 nanowalls. As shown in Figure 6a, the flakes are typically several to tens of micrometers in thickness, which are connected to form the network-like structure. Figure 6b shows the HRTEM image of a flake, indicating the birnessite-type MnO2 is highly crystallized. The interplanar spacing measured from the HRTEM image is 0.67 nm on average, which is smaller than the interlayer distance of 0.73 nm detected from the XRD pattern. Furthermore, the collapse of the layered MnO2 structure is observed from the HRTEM image. Similar experimental phenomena have been reported for the hydrous birnessite-type MnO2, and the result could be ascribed to the loss of the interlayer water in high vacuum of the microscope (see also Supporting Information).1,19 The inset of Figure 6a shows the ED pattern of the nanowalls. The spotted diffraction ring indicates that multiple single crystals are present in the incident electron beam. Indeed, both microcrystal and turbostratic disorder are the typical features of the birnessitetype MnO2 prepared by solution method.1,5,8,42,51
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Figure 5. (a) SEM image of the birnessite-type MnO2 film peeled off from the Si(111) substrate and (b) its magnified view.
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Figure 7. (a) M vs T and (b) 1/χ vs T curves of the birnessite-type MnO2 powder in an applied field of 100 Oe. The high-temperature part of the 1/χ vs T curve is fitted using the Curie-Weiss law.
this magnetic transition. Figure 7b shows the fitting result of the ZFC inverse susceptibility according to the Curie-Weiss law
χ)
C T-θ
(1)
where θ and C are the Curie-Weiss temperature and the Curie constant, respectively. As shown in Figure 7b, the paramagnetic behavior at high temperature (above 100 K) is well-described by the Curie-Weiss law. According to the fitting result, a negative Curie-Weiss temperature of 98 K is obtained, indicating strong antiferromagnetic interactions between the Mn moments. The Curie constant C is
C)
Figure 6. (a) TEM image of the birnessite-type MnO2 nanowalls and (b) typical HRTEM image of a flake of the MnO2 nanowalls.
Figure 7a shows the temperature dependence of the magnetization of the birnessite-type MnO2 sample in an applied field of 100 Oe. Both ZFC and FC magnetization vs temperature curves indicate the same magnetic transition temperature of 9.2 K (the inset of Figure 7a). The ZFC and FC curves become identical above the magnetic transition temperature and the sample shows the feature of paramagnetic order, but a bifurcation is observed below 9.2 K. It is notable that there is only one magnetic transition in the range of 5 to 300 K. Based on the bifurcation of the ZFC and FC M vs T curves below 9.2 K, a ferrimagnetism to paramagnetism or a canted antiferromagnetism to paramagnetism transition should be responsible for
nµeff2µB2 3kB
(2)
where n is the number of magnetic ions in the sample of a molar, µB is the Bohr magneton, kB is the Boltzmann constant, and µeff is the effective magnetic moment. With the fitted value of 1.7575 emu · K · Oe-1 · mol-1 for the Curie constant C, an effective magnetic moment of µeff ) 3.75µB is obtained for the mixed-valent Mn (Mn3+ and Mn4+). This effective moment of 3.75µB is between those of Mn4+ (3.87µB) and Mn3+ in lowspin state (2.83µB) but much smaller than that of Mn3+ in highspin state (4.90µB). Therefore, Mn3+ is in the low-spin state in our sample. The presence of the low-spin state Mn3+ under octahedral fields has been explained based on the suppression of the Jahn-Teller distortion.52 In addition, the magnetic measurements are extremely sensitive to the Mn3+/Mn4+ ratio.52,53 With the effective magnetic moment of 3.75µB, atomic fractions of 0.115 and 0.885 are obtained for the Mn3+ and Mn4+ ions, respectively. This agrees well with the chemical composition determined by EDS and ICP-AES. Figure 8a shows the hysteresis loop of the birnessite-type MnO2 powder at 5 K. The magnetization is not prominent since the measuring temperature is very close to its magnetic transition temperature (9.2 K). The hysteresis loops of the birnessite-type MnO2 nanowalls grown on the Si(111) substrate have been
MnO2 Nanowalls and Their Magnetic Properties
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Figure 8. (a) Hystereis loop of the birnessite-type MnO2 at 5 K: (a) powder sample; (b) nanowalls on the Si(111) substrate. The inset shows the orientation of the sample to the applied field, in which the arrows denote the direction of the applied field.
measured parallel and perpendicular to the substrate. As shown in Figure 8b, a clear magnetic anisotropy is exhibited. For the sample with the applied field in plane, the magnetization at higher field is much smaller than that of the sample with the external field out of plane. This indicates that the easy magnetization direction of the sample is out of plane, that is, the easy magnetic direction is in the ab plane. The presence of the magnetic anisotropy agrees with the low-temperature ferrimagnetic or canted antiferromagnetic state determined by the temperature-dependent magnetization. 4. Conclusions In summary, with a one-step hydrothermal method, birnessitetype MnO2 nanowalls were deposited on the Si(111) substrate. The nanowalls are composed of very thin flakes with a thickness of several to ten nanometers. The nanowalls in the length of 900 nm distribute uniformly over the whole surface area of the Si(111) substrate. According to the XRD pattern, the layered MnO2 nanowalls prefer to grow along the direction perpendicular to the c axis and show the typical feature of turbostratic disorder. Magnetic measurements indicate the birnessite-type MnO2 has a magnetic transition temperature of 9.2 K. At 5 K, the nanowalls exhibit prominent magnetic anisotropy. Based on its preferred growth direction, the easy magnetization direction of the nanowall is in the ab plane. These layered MnO2 nanowalls may find application in magnetic nanodevices, batteries, catalysts, and other fields requiring high surface area. Acknowledgment. The work was supported by National Basic Research Program of China (Grant No. 2007CB925003) and Chinese Academy of Sciences. We thank Dr. H. M. Fan for the fruitful discussion. Supporting Information Available: Temperature dependent XRD study of the birnessite-type MnO2 powder sample and SEM images of the birnessite-type MnO2 nanowalls with a reaction time of 60 min. This material is available free of charge via the Internet at http://pubs.acs.org.
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