Molybdenum Trioxide Nanostructures: The Evolution from Helical

Nov 15, 2006 - Computational Investigation of Electron Small Polarons in α-MoO3. Hong Ding , Hao Lin , Babak Sadigh , Fei Zhou , Vidvuds Ozoliņš , ...
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J. Phys. Chem. B 2006, 110, 24472-24475

Molybdenum Trioxide Nanostructures: The Evolution from Helical Nanosheets to Crosslike Nanoflowers to Nanobelts Guicun Li,* Li Jiang, Shuping Pang, Hongrui Peng, and Zhikun Zhang Key Laboratory of Nanostructured Materials, Qingdao UniVersity of Science and Technology, Qingdao 266042, People’s Republic of China ReceiVed: July 29, 2006; In Final Form: September 29, 2006

MoO3 nanostructures with different morphologies, such as helical nanosheets, crosslike nanoflowers, and nanobelts, have been synthesized on a large scale by an environmentally friendly chemical route. The evolution process from helical nanosheets to crosslike nanoflowers to nanobelts is observed for the first time. The influences of reaction time and the molar ratio of molybdenum and H2O2 on the morphologies of MoO3 nanostructures have been investigated. The synthetic process is environmentally friendly and may be extended to synthesize nanostructures of other metal (W, Ti, and Cr) oxides.

Introduction Low-dimensional inorganic nanostructures have received considerable interest because of their size- and shape-dependent properties.1 Helical structures are observed in nature and usually associated with biological polymers. Recently, the synthesis of helical inorganic nanostructures has been researched intensively due to their attractive morphology. Despite this, only a few inorganic helical nanostructures composed of nanowires or nanobelts, such as ZnO,2,3 SiC,4 SiO2,5 ZnGa2O4,6 CdS,7 and carbon,8,9 have been fabricated. Nanobelts with a rectangular cross-section are interesting and expected to represent important building blocks for nanodevices.10 Orthorhombic molybdenum trioxide (R-MoO3), a wide-gap n-type semiconductive material, is attractive due to its layered crystal structures. The asymmetrical MoO6 octahedra are interconnected through cornerlinking along [100] and edge-sharing along [001] to form double-layer sheets parallel to the (010) plane. The weak interactions between the double-layer sheets are mostly van der Waals forces.11 MoO3 nanostructures have potential applications in nanocrystal lubricants,12,13 sensors,14,15 rechargeable lithiumion batteries,16 and field emission nanodevices.17-19 Various MoO3 nanostructures, such as nanorods, nanowires, nanobelts, and nanoplatelets, have already been synthesized by a variety of methods such as physical vapor deposition,17-19 vaportransportation,16 and hydro/solvothermal treatment.11,20-24 However, the evolution process of MoO3 nanostructures remains challenging to material researchers. Herein, we report the synthesis of MoO3 nanostructures with different morphologies such as helical nanosheets, crosslike nanoflowers, and nanobelts, on a large scale by an environmentally friendly chemical route. The evolution process from helical nanosheets to crosslike nanoflowers to nanobelts is observed for the first time. Experimental Section Synthesis of MoO3 Nanostructures. In a typical synthesis, 0.72 g of metallic molybdenum powder was added into 60 mL of distilled water with shaking. Then, 5 mL of 30% H2O2 was added dropwise and the reaction stirred for 20 min to become * Corresponding author. Tel: 86-532-84022869. Fax: 86-532-84022869. E-mail: [email protected].

a dark blue slurry. The mixture was placed in a 100-mL autoclave with a Teflon liner. The autoclave was maintained at 180 °C for 2-12 h and then air-cooled to room temperature. The resulting precipitates were collected and washed with distilled water several times and then dried in vacuum at 60 °C for 10 h. The color of the resulting products changed from blue white to white as the reaction time increased. Characterization. The morphologies of the resulting products were characterized by field-emission scanning electron microscopy (FE-SEM, JSM 6700F) and high-resolution transmission electron microscopy (HRTEM, JEOL 2010). The crystal structures of the resulting products were identified by X-ray diffractometer (XRD, Rigaku D-max-γA XRD with Cu KR radiation, λ ) 1.54178 Å). X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300W Al KR radiation. The base pressure was about 3 × 10-9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. Results and Discussion It is known that metal or metal oxides powders, such as molybdenum,25,26 tungsten,27 titanium,28 and vanadium oxides,29 can be dissolved in the hydrogen peroxide (H2O2) aqueous solution, which has inspired us to find an environmentally friendly route to nanostructures of metal oxides. In the present work, it is found that the synthesis of MoO3 nanostructures is affected by the reaction parameters, such as reaction time and the molar ratio of molybdenum and H2O2. When the molar ratio of molybdenum and H2O2 is 0.17, a small amount of metallic molybdenum remains in the slurry, so the color of the mixture is dark blue. In order to investigate the evolution process of MoO3 nanostructures, the time-dependent experiments were carried out at 180 °C (the molar ratio of molybdenum and H2O2 ) 0.17). When the reaction is carried out at 180 °C for 2 h, only colloidal aggregates with sizes of a few micrometers are obtained in the products (Figure S1, see Supporting Information). After 4 h, in addition to colloidal aggregates, circular nanosheets are formed (Figure S2, see Supporting Information). The sizes are similar to that of colloidal aggregates, indicating that the nanosheets may be formed in-situ from the colloidal

10.1021/jp064855v CCC: $33.50 © 2006 American Chemical Society Published on Web 11/15/2006

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Figure 1. SEM images of helical MoO3 nanostructures synthesized at 180 °C for 6 h (the molar ratio of molybdenum and H2O2 ) 0.17): (A) left-handed single helical structure, (B) right-handed single helical structure, (C) a tilted SEM image of a right-handed single helical structure, and (D) three helical structure.

aggregates. After 6 h, a large quantity of nanosheet aggregates is obtained (Figure S3, see Supporting Information for lowmagnification SEM image). Most of the nanosheet aggregates have a rectangular base and are about several hundreds of nanometers in size. In high-magnification SEM images (Figure 1A-D), it is interesting that some nanosheets tend to be twisted to form helical structures. Figure 1, parts A and B, shows SEM images of left-handed and right-handed single helical nanosheets, respectively. The helical growth behavior becomes clearly visible on top of the nanosheets. In a tilted SEM image (Figure 1C), the layers of single helical nanosheets can be seen clearly. The thickness of the nanosheets in the helical structures is in the range of 10-20 nm. Moreover, helical aggregates composed of three nanosheets (Figure 1D) can also be found in the products. When the reaction is performed at 180 °C for 7 h, interestingly, four fibrillar bundles with lengths up to several tens of micrometers grow from the lateral sides of a nanosheet aggregate to form a cross (Figure S4, see Supporting Information). One crosslike MoO3 nanoflower is made up of four fibrillar bundles and one nanosheet aggregate core. Moreover, a large quantity of isolated nanosheet aggregates coexists with the crosslike nanoflowers. After 8 h, as shown in Figure 2A, crosslike MoO3 nanoflowers are predominant in the products and isolated nanosheet aggregates disappear. The highmagnification SEM image (the inset in Figure 2A) shows that the fibrillar bundles are composed of nanobelts. The sizes of nanosheet aggregates existing in the core of crosslike nanoflowers are less than that in Figure 1. After 12 h, pure MoO3 nanobelts (Figure 2B) are fabricated and nanosheet aggregates in crosslike nanoflowers disappear, indicating that all the nanosheet aggregates are changed to MoO3 nanobelts. The highmagnification SEM image (the inset in Figure 2B) reveals the beltlike morphology. The width, thickness, and length of the nanobelts are about 100-200 nm, 10-20 nm, and several tens of micrometers, respectively. As shown in Figure 2C, the beltlike morphology is further confirmed by TEM observation. The selected area electron diffraction pattern (SAED, Figure 2D) taken from an individual nanobelt is indexed to orthorhombic MoO3, indicating that a nanobelt is a single crystal, with a

Figure 2. SEM (A, B) and TEM (C-E) images of MoO3 nanostructures synthesized at 180 °C for different reaction times (the molar ratio of molybdenum and H2O2 ) 0.17). (A) 8 h, (B-E) 12 h, (C) TEM image of MoO3 nanobelts, (D) SAED pattern taken from an individual MoO3 nanobelt, (E) HRTEM image of MoO3 nanobelt. The insets in parts A and B show high-magnification SEM images of MoO3 nanostructures (the scale bar ) 100 nm).

preferential growth direction along the [001] direction. The typical HRTEM image of MoO3 nanobelts (Figure 2E) also reveals that the nanobelts can grow along the [001] direction. XRD patterns of the products synthesized at 180 °C at different reaction time are shown in Figure 3 (the molar ratio

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Figure 3. XRD patterns of the products synthesized at 180 °C at different reaction time (the molar ratio of molybdenum and H2O2 ) 0.17): (A) 6 h, (B) 8 h, and (C) 12 h.

of molybdenum and H2O2 ) 0.17). As shown in Figure 3A, when the reaction is carried out for 6 h, most of the diffraction peaks of the products can be indexed to orthorhombic MoO3 (JCPDS 05-0508) with calculated lattice contents a ) 3.946 Å, b ) 13.726 Å, c ) 3.687 Å. The diffraction peaks of MoO3 are very broader, indicating that the nanosheets have low crystallinity. In addition, a small amount of monoclinic MoO2 (JCPDS 32-0671) also exists in the products, because the residual metallic molybdenum is oxidized to MoO2 and then MoO3 in the hydrothermal process. XPS spectra (Figure S5, see Supporting Information) show that two peaks at 232.8 and 236.0 eV are assigned to the Mo 3d binding energy of Mo(VI)O3, indicating that the main products are MoO3.24,30 After 8 h, monoclinic MoO2 disappears and only pure orthorhombic MoO3 exists in the products (Figure 3B), indicating that nanosheet aggregates (Figure 1) are not monoclinic MoO2 but orthorhombic MoO3. With the reaction time increased to 12 h, the intensity of the diffraction peaks of orthorhombic MoO3 (Figure 3C) increases, indicating that the nanobelts have high crystallinity. Before hydrothermal reaction, the residual molybdenum can be removed and discarded by filtration. When the obtained transparent dark blue solution is treated hydrothermally at 180 °C for 6 h (Figure 4A) and 12 h (Figure 4B), respectively, only MoO3 nanobelts are formed, but no nanosheets are found in the products. The width, thickness, and length of the nanobelts are similar to those in Figure 2B. It is clear that the surfaces of the nanobelts become smooth as the reaction time increased. In comparison with Figure 1, it is indicated that the residual molybdenum plays an important role in the formation of MoO3 nanosheets. When the molar ratio of molybdenum and H2O2 decreases to 0.08, metallic molybdenum is dissolved completely in the H2O2 aqueous solution to form a yellow solution. Figure 4C shows a typical SEM image of MoO3 nanobelts synthesized at 180 °C for 12 h. Compared with Figures 2C and 4B, it is found that the widths of the nanobelts increase to 200-800 nm, but

Figure 4. SEM images of MoO3 nanostructures synthesized by hydrothermal treatment of transparent solution under different reaction conditions. (A) the transparent dark blue solution obtained after filtration (the molar ratio of molybdenum and H2O2 ) 0.17), 6 h; (B) the transparent dark blue solution obtained after filtration (the molar ratio of molybdenum and H2O2 ) 0.17), 12 h; (C) transparent yellow solution (the molar ratio of molybdenum and H2O2 ) 0.08), 12 h.

the thickness and lengths change little. The ED pattern of an individual nanobelt (Figure S6, see Supporting Information) indicates the single-crystalline nature of MoO3 nanobelts, which is consistent with Figure 2D. The XRD pattern (not shown here) is similar to that in Figure 3C. Compared with other methods reported in the literature,20-24 the synthetic process is environmentally friendly, and the resulting products are very pure, because no other ions and solvents are used. Before hydrothermal reaction, metallic molybdenum is dissolved partly to form a H2[Mo2O3(O2)4] (or

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Figure 5. Schematic illustration of the evolution process from helical nanosheets to crosslike nanoflowers to nanobelts (bar ) 100 nm).

its hydrated species) solution.26,31 The reaction may be expressed as follows:

References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933.

2Mo + 10H2O2 ) [Mo2O3(O2)4]

2-

+

+ 2H + 9H2O

Under hydrothermal conditions, H2[Mo2O3(O2)4] will be decomposed to produce MoO3. It is supposed that the evolution from helical nanosheet aggregates to crosslike nanoflowers to nanobelts may be related to the orthorhombic crystal structures of MoO3 and undergo a solid-solution-solid (SLS) process. On the basis of the experimental results, the morphologies of MoO3 nanostructures are affected by the synthetic parameters, such as reaction time and the molar ratio of molybdenum and H2O2. The evolution process of MoO3 nanostructures in the reaction process (the molar ratio of molybdenum and H2O2 ) 0.17) is presented in Figure 5. At the early stage of hydrothermal reaction, the residual metallic molybdenum undergoes a hydrothermal oxidation reaction and plays an important role in the formation of MoO3 nanosheets with low crystallinity (6 h). With the reaction proceeding, the MoO3 nanosheets can serve as the heterogeneous nucleation sites for the epitaxial growth of the MoO3 nanobelts, so crosslike MoO3 nanoflowers are obtained (8 h). The formation of crosslike nanoflowers may be associated with the orthorhombic crystal structures of MoO3. Finally (12 h), MoO3 nanosheets are consumed completely and only MoO3 nanobelts with high crystallinity are obtained. However, when no metallic molybdenum exists in the system, the intermediate step of MoO3 nanosheets and crosslike nanoflowers is not observed, so the formation of MoO3 nanobelts may be derived from a solution-solid (SL) process. Conclusion In summary, we have developed an environmentally friendly route to the synthesis of MoO3 nanostructures with different morphologies on a large scale in H2O2 aqueous solution. The evolution process from helical nanosheets to crosslike nanoflowers to nanobelts is observed for the first time. The influences of reaction time and the molar ratio of molybdenum and H2O2 on the morphologies of MoO3 nanostructures have been investigated. The synthetic method may be applicable to synthesize nanostructures of other metal other metal (W, Ti, and Cr) oxides. Acknowledgment. This project is partly supported by National Center for Nanoscience and Technology, People’s Republic of China.

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Supporting Information Available: Figures S1-4, which show the evolution of the aggregates with time, Figure S5, XPS spectra; and Figure S6, which is the ED pattern of an individual nanobelt. This material is available free of charge via the Internet at http://pubs.acs.org.

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