Synthesis and Optical Properties of Mn3O4 Nanowires by

shifted 13 cm. -1 compared with that of bulk Mn3O4 crystals.35. Analogy results have been observed preciously and attributed to the effect of phonons ...
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CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 1 358–362

ReViews Synthesis and Optical Properties of Mn3O4 Nanowires by Decomposing MnCO3 Nanoparticles in Flux Wenzhong Wang* and Ling Ao School of Science, Central UniVersity for Nationalities, Beijing 100081, China ReceiVed June 2, 2007; ReVised Manuscript ReceiVed September 21, 2007

ABSTRACT: Mn3O4 nanowires with diameters of 30–60 nm and lengths of up to more than 100 µm were synthesized by decomposing the precursor MnCO3 nanoparticles in NaCl flux. The X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high-resolution TEM (HRTEM) techniques were used to study structural features and chemical compositions of the prepared nanowires. The Raman optical property of the Mn3O4 nanowires was investigated by Raman spectroscopy, the result indicated that the Raman peak is asymmetric and broadened and is red-shifted 13 cm-1 compared with that of bulk Mn3O4 crystals. A rational explanation for the red shift and broadening of the Raman peak is discussed in detail according to phonon confinement model of an infinite crystal.

1. Introduction The synthesis and study of properties of Mn3O4 nanoscale materials have stimulated great interest due to their special electric configuration and distorted spinel structure and their unique ion-exchange, molecular adsorption, catalysis, electrochemical, and magnetic properties.1–3 It has been reported that Mn3O4 material is a promising electrochromic material of anodic coloration since it has a reversible color change from brown (colored state) to yellow (bleached state).4 Mn3O4 is also known to be an active catalyst in several oxidations or reductions. For example, Mn3O4 can be used as catalyst for the oxidation of methane and carbon monoxide5 or the selective reduction of nitrobenzene.6 Moreover, Mn3O4 (hausmannite)7 with different polymorphs has been found to be an active and stable catalyst for the combustion of organic compounds in the temperature range of 373–773 K. These combustion-related catalytic technologies are of interest in relation to several air-pollution problems, allowing limitation of the emission of NOx and volatile organic compounds from waste gases of different origins.8 One-dimensional (1D) nanoscale materials have attracted considerable attention because of their unique electronic, optical, and mechanical properties and their importance in understanding the fundamental properties of low dimensionality, as well as their potential applications in nanodevices. Many methods including vapor–liquid–solid (VLS),9 vapor-solid (VS),10 and solution-based methods have been developed to synthesize 1D nanoscale materials, such as nanowires,11–14 nanobelts,15–17 nanorods,18,19 nanotubes,20–23 and nanoneedles.24,25 In our * Corresponding author. E-mail: [email protected].

previous work,26–28 we have developed a strategy for the synthesis of oxide nanowires by combining a general molten salt process,29 which is usually used to synthesize micrometer ceramic powder, with a solid-state process. The key point of this approach is to prepare nanometer-diameter precursor particles by exploiting a one-step, solid-state reaction at ambient temperature and subsequently decompose precursor nanoparticles in NaCl flux to fabricate oxide nanowires. In this paper, we report on the synthesis and Raman optical properties of Mn3O4 nanowires. The Mn3O4 nanowires were synthesized by decomposing MnCO3 nanoparticles in NaCl flux; the synthesis process is similar to our previous approach used to prepare oxide nanowires. The Raman spectrum of the as-prepared Mn3O4 nanowires indicates that the peak is asymmetric and broadened and is red-shifted compared with that of bulk Mn3O4 crystals. The 1D Mn3O4 nanostructures may provide us another kind of manganese oxides with different characteristics.

2. Experimental Section All of the chemical reagents used in this experiment were analytical grade. In a typical Mn3O4 nanowires synthesis, 1.40 g of MnCl2 · 4H2O and 1.50 g of Na2CO3 were ground for 5 min each before mixing together. After 15 min of grinding, the product was washed several times with distilled water to remove unreacted reactants and byproduct. Finally the product was dried in an oven at 60 °C for 5 h. The obtained product was collected for the preparation of Mn3O4 nanowires. The as-prepared precursor (0.2 g) was mixed with 1 g of NaCl and 3 mL of nonyl pheyl ether (9) (NP-9) with an agate mortar, and then the mixture was ground for 5 min. The mixed sample was heated in a porcelain crucible that was placed in the middle of an alumina tube with a horizontal tube electric furnace at 850 °C for 2 h; the heat treated sample was cooled gradually to room temperature in air, washed several

10.1021/cg070502p CCC: $40.75  2008 American Chemical Society Published on Web 12/07/2007

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Crystal Growth & Design, Vol. 8, No. 1, 2008 359

Figure 2. XRD pattern of the as-prepared MnCO3 nanoparticles.

Figure 1. (a) TEM images of the as-prepared precursor MnCO3 nanoparticles and (b) histogram showing the diameter distribution of the MnCO3 nanoparticles. times with distilled water to remove NaCl flux, filtered, and then dried in an oven at 80 °C for 5 h. X-ray powder diffraction (XRD) pattern was obtained on a Rigaku (Japan) DmaxγA rotation anode X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å), employing a scanning rate of 0.02° s-1 in the 2θ range from 10° to 75°. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were taken with a JEM-200 CX transmission electron microscope, using an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) was carried out on a JEOL-2010 transmission electron microscope, using an accelerating voltage of 200 kV. The composition and oxidation state of the prepared Mn3O4 nanowires were further analyzed by X-ray photoelectron spectra (XPS), which were collected on an ESCALAB M K X-ray photoelectron spectrometer, using Mg KR X-ray as the excitation source. The binding energies obtained in the XPS analysis were corrected with reference to C1s (284.6 eV). Laser Raman spectroscopy was obtained using a LABRAM-HR Confocal Laser MicroRaman spectrometer from 1000 to 0 cm-1 at room temperature. The 514.5 nm line of the laser was used as the excitation source, with the capability of supplying 250 mW.

3. Results and Discussion

Figure 3. DTA (a) and TGA (b) curves of the MnCO3 nanoparticles.

Figure 1a shows the TEM images of the precursor sample prepared via one-step, solid-state reaction, indicating that the nanoparticles have spherical morphology with an average diameter of 14 nm. Histogram as shown in Figure 1b reveals that the diameter distribution of the nanoparticles is in the range of 3-20 nm and that the largest percentage (80%) is in the range of 9–18 nm. Figure 2 shows the XRD pattern of the as-prepared precursor nanoparticles. All of the peaks can be easily indexed to that of a pure hexagonal phase of MnCO3 with lattice constants a ) 4.677 and c ) 15.67 Å (JCPDS No 7-268). The particle diameter calculated from the half-width of diffraction peaks by using the

Debye–Scherrer formula, allowing for the experimental error, is 15 nm, which is in good agreement with the result of the TEM observations. The XRD result indicates that the asprepared nanoparticles are composed of pure hexagonal phase MnCO3. Figure 3 shows differential thermal analysis (DTA) and thermogravimetric analysis (TGA) curves of the MnCO3 nanoparticles. Figure 3a indicates that there are two adsorption peaks in DTA curve. The peak at 74.8 °C is related to the evaporation of water adsorbed on the nanoparticles. The peak at 388 °C corresponds to the decomposition of MnCO3 nanoparticles and

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Figure 6. HRTEM image of a 35 nm Mn3O4 nanowire. Figure 4. XRD pattern of the as-prepared Mn3O4 nanowires.

Figure 7. XPS spectrum of the as-prepared Mn3O4 nanowires for Mn2p core level.

Figure 5. TEM images of the as-prepared Mn3O4 nanowires at (a) low and (b) high magnification.

the liberation of carbon dioxide. After 700 °C, no weight loss can be observed as shown in Figure 3b, indicating the completeness of decomposition. Based on the DTA-TGA analysis and the melting point of NaCl, the precursor MnCO3 nanoparticles were heated at 850 °C to fabricate Mn3O4 nanowires. Figure 4 shows the XRD pattern of the as-synthesized Mn3O4 nanowires. All diffraction peaks can be perfectly indexed to the tetragonal phase Mn3O4 structure. The Mn3O4 lattice constants obtained by refinement of the XRD data of as-prepared nanowires are a ) 5.735 and c ) 9.416 Å, which are consistent with those of bulk Mn3O4 (JCPDS No. 24-734). The TEM was used to observe the morphology of the Mn3O4 nanocrystals. Figure 5 shows the TEM images of the as-prepared Mn3O4 nanocrystals at different magnification, indicating that Mn3O4 nanocrystals display wire-like morphology with diameters of 30–60 nm, and lengths of up to more than 100 µm. The nanowires are relatively straight and their surfaces are smooth. A typical SAED pattern (inset in Figure 5), taken from

a single nanowire with a diameter of about 60 nm, can be indexed based on a tetragonal cell with lattice parameters of a ) 5.762 and c ) 9.469 Å, consistent with the above X-ray results. The SAED pattern also confirms that the nanowire is a single crystal tetragonal Mn3O4. Figure 6 shows a typical HRTEM image of a single nanowire with a diameter of about 35 nm. The clear lattice fringes illustrate that the nanowire is single crystalline. The interplanar spacing is about 0.25 nm, which corresponds to the {211} plane of tetragonal Mn3O4, revealing that the growth plane of the nanowires is one of the {211} planes. The composition and oxidation state of the as-prepared product were further analyzed by the XPS spectrum. Figure 7 shows the photoelectron spectrum of Mn2p. The peak at 641 eV corresponds to the binding energy of Mn2p3/2, which is in good agreement with that observed in Mn3O4,30 while for the cases of MnO2 and Mn2O3, the binding energies of Mn2p3/2 are at 642 and 641.8 eV, respectively. Thus the XPS studies further confirm that the as-prepared Mn3O4 nanowires are composed of pure Mn3O4 phase. Raman spectroscopy, which is a sensitive probe to the local atomic arrangements and vibrations of the materials, has been widely used to investigate the microstructure of the nanosized materials.31–34 Figure 8 shows the Raman scattering spectrum of the as-prepared Mn3O4 nanowires by decomposing precursor MnCO3 nanoparticles in NaCl flux. One can find that the Raman peak of the synthesized Mn3O4 nanowires is asymmetric and broadened. The peak position is at 642 cm-1, which is redshifted 13 cm-1 compared with that of bulk Mn3O4 crystals.35 Analogy results have been observed preciously and attributed to the effect of phonons confinements.36

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Crystal Growth & Design, Vol. 8, No. 1, 2008 361

Figure 8. Raman scattering spectrum of the as-prepared Mn3O4 nanowires by decomposing precursor MnCO3 nanoparticles in NaCl flux.

According to the phonon confinement model for an infinite crystal, it is well-known that only phonons near the center of the Brillouin zone (q ≈ 0) contribute to the Raman spectrum because of momentum conservation between phonons and incident light. In an infinite crystal, it also has been reported that phonons can be confined in space by crystal boundaries or defects, resulting in uncertainty in the phonon momentum, and thus allowing phonons with q * 0 to contribute to the Raman spectrum. This uncertainty is larger for smaller grain sizes, thus the shifting and broadening of the Raman peak increase as the size of crystals decreases. Based on the above analysis, a rational explanation for the red shift and broadening for the Raman band of the as-prepared Mn3O4 nanowires can be concluded as follows. It is well-known that the first-order Raman spectrum, I(ω), can be described by the following equation:31 I(ω) )

3

2

q|C(0, q)| ∫ [ω -dω(q)] + (Γ ⁄ 2) 2

(1)

2

where ω(q) represents the phonon dispersion curve, Γ is the natural line width, and C(0, q) is the Fourier coefficient that describes the phonon confinement. Because the Mn3O4 nanowires are column-shaped crystals, the Fourier coefficient can be written as follows:31

|

(√32π)|

|C(0, q1, q2)|2 = e-q1 L1 ⁄16 π2 e-q2 L2 ⁄16 π2 1 - erf 2

2

2

2

iq2L2

2

(2) where L1 and L2 are the diameter and the length of the Mn3O4 nanowires, respectively. The TEM images indicate that the asprepared Mn3O4 nanowires have diameters of 30–60 nm, with an average of about 50 nm, and lengths of up to more than 100 µm, so the length is much larger than the diameter, which means that the phonon confinement effect mainly occurs along the diameter direction of the Mn3O4 nanowires. As the Mn3O4 crystallite is reduced to nanoscale, the most important effect on its Raman spectrum is that the crystal momentum conservation rules will be relaxed.37 This allows phonons with wave vector |k| ) |k′| ( 2π/L to participate in the first-order Raman scattering, where k′ is the wave vector of the incident light and L is the size of the crystal. Here L is the average diameter of Mn3O4 nanowires. The phonon scattering will not be limited to the center of the Brillouin zone, and the phonon dispersion near the zone center must also be considered. As a result, the Raman

spectra may be broadened and down-shifted. Another possible reason for broadening and down shifts of the Raman peaks may be the defects of the Mn3O4 nanowires. The TEM studies indicate that there are no spherical liquid droplets at the tips of the nanowires, which is known to be good evidence for the vapor–liquid–solid (VLS)9 and solution-liquid– solid (SLS)12 mechanism. The results suggest that the Mn3O4 nanowires fabricated via the present method may not grow by the VLS or SLS mechanism. Comparative experiments were carried out to further reveal the growth mechanism and the effects of NaCl and surfactant NP-9 on the formation of the Mn3O4 nanowires. The results indicated that the Mn3O4 nanowires could be formed only in the presence of both molten salt NaCl and surfactant NP-9. Furthermore, the results indicated that the average length and diameter of the nanowires did not show a clear dependence on the cooling rate, indicating that the growth mechanism of the nanowires in our route is different from that in the usual “flux method” in which the product crystals are obtained during the cooling process.38 That is, the nanowires formed via the present method did not grow during the cooling process but grew during the soaking of the melt, suggesting that the nanowires may mainly grow by an Ostwald ripening mechanism, that is, the dissolving of fine particles and the depositing of components on larger particles. When Ostwald ripening is the dominant mechanism of the nanowire growth, the formation of the nanowires must be affected by the character of the starting material, such as, the particle size or chemical activity, because the dissolution rate of the material depends on such characters. The viscosity of the flux during heating of the system also affects the formation of nanowires. Since adding NaCl can significantly decrease the viscosity of the melt, this increases the mobility of components in the flux. Therefore NaCl can provide a favorable liquid environment for the growth of nanowires. When the precursor MnCO3 nanoparticles are ground in the presence of surfactant NP-9, the ground precursor powders have high-specific surfaces and are metastable in the oxidizing process, which is also favorable for the formation of nanowires.

4. Conclusions In summary, single-crystal Mn3O4 nanowires with diameters of 30–60 nm and lengths of up to more than 100 µm were synthesized by decomposing the precursor MnCO3 nanoparticles in NaCl flux. Structure features and chemical compositions of the nanowires have been systematically studied by using XRD, TEM, SAED, and HRTEM. The Raman study of the as-prepared Mn3O4 nanowires indicates that the peak is asymmetric and broadened and is red-shifted 13 cm-1 compared with that of bulk Mn3O4 crystals. The reasons for the red shift and broadening of the Raman peak were discussed in detail according to the phonon confinement model of an infinite crystal. This 1D Mn3O4 nanostructure may provide us another kind of manganese oxide with different characteristics. Acknowledgment. This work was supported by Research Foundation of The State Ethnic Affairs Commission of PRC (Grant No. 07ZY02).

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