Synthesis and Characterization of Hexagonal and Truncated

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Synthesis and Characterization of Hexagonal and Truncated Hexagonal Shaped MoO3 Nanoplates Xiaohui Chen,† Weiwei Lei,† Dan Liu, Jian Hao, Qiliang Cui,* and Guangtian Zou State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: August 24, 2009; ReVised Manuscript ReceiVed: October 31, 2009

Hexagonal and truncated hexagonal shaped MoO3 nanoplates (MoO3 HNP) were synthesized through a simple vapor-deposition method in Ar atmosphere under ambient pressure without the assistant of any catalysts. The structure and morphology of MoO3 HNP were investigated by XRD, EDX, SEM, TEM, and HRTEM. The results reveal that the HNP are R-MoO3 and have a large area surface. The Raman spectrum shows a significant size effect on the vibrational property of MoO3 HNP. The photoluminescence (PL) spectrum was carried out, and two peaks at 351 and 410 nm were observed in the spectrum. In addition, a possible growth mechanism proposed as VS is discussed in detail. Introduction Because of the increasing attention on nanostructured metal oxides with reduced dimensions and high surface area (nanosheets,1 nanowires,2 nanorods,3,4 and nanobelts5), which are expected to have excellent optical, electrical, magnetic, and ionic transport properties, fabrication of such crystalline metal oxides was very popular during the past decade. Molybdenum trioxide (MoO3) as one of the well-known n-type semiconductors is attractive due to its various potential applications in many fields, such as photochromic and electrochromic devices,6,7 gas sensors,8-11 and catalysts.12,13 For these applications, large aspect and surface-to-volume ratio are found to be important factors that affect the efficient parameters, especially when the size of the material is reduced to nanoscale.14 For nanobelts and nanoplates, the single crystalline nature and, in particular, their large aspect ratio as well as the faceting nature make them ideal candidates for probing size- and dimensionality-dependent physical or chemical phenomena as well as for applications in nanodevices. Therefore, MoO3 nanomaterials with a large surface-to-volume ratio are attracting increasing attention. Generally, MoO3 is found to have mainly three crystalline polymorphs, orthorhombic (R) MoO3, monoclinic (β) MoO3, and hexagonal (h) MoO3. R-MoO3 and β-MoO3 are two wellknown modifications, but β-MoO3 is metastable, which can be converted to R-MoO3 at a temperature of about 400 °C.15 Orthorhombic R-MoO3 is a thermodynamically stable phase with layered structure. The structure of bulk R-MoO3 has been theoretically described as possessing a crystallographic anisotropy structure in many articles. Its anisotropy is mainly considered as distorted MoO6 octahedra sharing corners in the direction of [100] (a-axis) and zigzag edges in the direction of [001] (c-axis) to form a single layer. Two single layers stack together by van der Waals force along the b-axis to construct a bilayer.16,17 This unique structure makes the material become one of the most pronounced candidates to form large surface area and high surface-to-volume ratio nanostructures. Despite the importance of MoO3 nanostructures, successful synthesis of such structures is still limited. A variety of techniques have been developed to explore novel architectures * Corresponding author. E-mail: [email protected]. † These authors contributed equally to this work.

and morphological patterns of this important functional material. For instance, X. F. Duan et al. achieved MoO3 and MoO2 nanowires using a thermal evaporation method under vacuum conditions.2 R. Q. Song et al. reported the hydrothermal technique to synthesize MoO3 nanofibers and nanobelts.14 G. C. Li et al. proposed novel nanosheets and crosslike nanoflowers of MoO3 synthesized by chemical route.18 A sol-gel chemistry method was used by G. Wang et al. to yield the a-MoO3 nanoplatelets.19 Although these methods do work, their shortcomings are obvious: (a) many of them require either templates or vacuum or low-pressure conditions; (b) the soft chemical and hydrothermal methods are complex and more or less produce contamination. What’s more, two-dimensional (2D) MoO3 nanostructures such as nanoplates and nanosheets are rarely found to be obtained using environmentally friendly and convenient methods. Therefore, convenient and less toxic methods, such as vapor deposition and oxidation processes, are preferred to grow MoO3 2D nanostructures. Herein, we obtained promising completed hexagonal and truncated hexagonal shaped MoO3 nanoplates (MoO3 NHP) using a simple vapor deposition method without the use of any catalyst or template. This is the first time that MoO3 nanoplates with regular shapes and high surface area have been synthesized using this convenient method. Experimental section The MoO3 microstructure was synthesized by simply oxidizing Mo plates. In this experiment, In2O3 and active carbon powder were thoroughly mixed in a quartz boat and then placed in the center of a quartz tube (diameter 36 mm, length 120 cm). Mo plates were placed in the region 18-24 cm from the boat. Several pieces of Si wafers were placed near the opposite opening of the furnace as substrates. The tube was tightly sealed and purged by introducing high-purity argon flow. Then the center where quartz boat was placed was gradually heated up to 1000 °C. The temperature of the Mo plates and Si substrates were about 800 °C and 300 °C, respectively. These conditions were maintained for 2 h in an atmosphere of flowing Ar and then allowed to cool down naturally to room temperature. Layers of white product were observed on the Si substrates as well as on the inner wall of the quartz tube near the substrates.

10.1021/jp908155m  2009 American Chemical Society Published on Web 11/17/2009

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Figure 1. (a) XRD and (b) EDX spectrum of the as-synthesized MoO3 HNP.

The structure of the MoO3 HNP was measured by X-ray diffraction (XRD, Rigaku D/Max-A, using Cu KR radiation), and its chemical composition was determined by energydispersive X-ray spectroscopy (EDX). The morphology of the synthesized MoO3 HNP was observed by using scanning electron microscopy (SEM, XL 30 ESEM FEG), transmission electron microscopy (TEM, Hitachi-8100), and selected area electron diffraction (SAED). More insight characterization was performed on high-resolution transmission electron microscopy (HRTEM, JEM-2100f). Micro-Raman spectroscopy was measured by Renishaw inVia (excited with an Ar+ line at 514 nm). The photoluminescence spectrum was measured on a Renishaw inVia Raman Spectrometer (excited with a He-Cd line at 325 nm). All measurements were performed at room temperature.

Figure 2. (a) SEM image of as-synthesized MoO3 HNP at low magnification. (b) High-magnification SEM image of MoO3 HNP. (c) TEM image of a typical layered HNP. (d) Lateral section of a typical layered MoO3 HNP.

Results and Discussion X-ray diffraction of the as-synthesized MoO3 HNP is displayed in Figure 1. As can be seen, the well-indexed patterns indicate that the structures consist of phase R-MoO3, according to the standard powder diffraction files JCPDS 76-1003/5-508. The instinct diffraction pattern, in which only (0l0) can be observed, where l is even, indicates that the plates lie along the b-axis and are very consistent with results reported previously.20 The peak at about 69° is attributed to the Si substrate, as denoted in the picture. The lattice parameters are a ) 0.40473 nm, b ) 1.38918 nm, and c ) 0.3723 nm, slightly bigger than those presented in JCPDS. But the atomic ratio O/Mo is 2.93, according to the EDX spectrum shown in Figure 1b, less than the stoichiometric ratio 3. It does not agree with the result provided in previous work, in which the author assumed that the smaller unit cell correlated to the deficiency of oxygen.20 Results of the SEM images of the samples are shown in Figure 2 to demonstrate the structure and morphology of MoO3 HNP. A lower magnification image, Figure 2a, shows the overview of the layered assemblies, of which most are truncated, hexagonally shaped plates, with a few hexagonal-like ones. The truncated hexagonal-like plates are fairly uniform, with an average edge length of less than 10 µm and a width of 5 µm, whereas the hexagonal-like ones are about half that size (Figure 2b). All of them are near-regular with good symmetry at both ends. Shown in Figure 2c is the corresponding TEM image of one assembly of the MoO3 HNP, which is very consistent with the SEM result. Detailed inspection of the lateral side of a single assembly at higher magnification is shown in Figure 2d, which reveals that the assembly is made up of several thinner plates. The thicknesses of the plates are 100, 90.8, 52.5, and 47.8 nm in an assembly with thickness about 500 nm. According to measurement results, we confirm that the plates are nanoscale in thickness and range from about 50 to 100 nm. The surfaceto-volume ratio is at least 10, showing a high surface-to-volume

Figure 3. (a) TEM image of a single MoO3 HNP. (b) SAED pattern of the MoO3 HNP. (c) HRTEM of the corresponding area indexed in panel a; inset is the FFT pattern.

ratio property of MoO3 HNP. The nanoplates aggregated with each other so slightly that they can be easily separated under ultrasonic treatment in their dilute suspension. Two dimensional sheets and plate-like shapes have been reported to be the preferences of the instinctive structure of R-MoO3; however, a layered structure with sheets or plates attaching to each other is rarely found. We assume the forming of layered plates relates to the growth mechanism and process we will discuss later. A typical TEM image of the HNP is shown in Figure 3. From the picture, we can see that the nanoplates are well-formed in shape and nearly transparent (Figure 3a). It indicates that the MoO3 HNP possess a large surface area and may be a better candidate for applications such as secondary batteries, display devices, and gas sensor devices.14,17,21,22 A SAED photograph obtained by focusing the electron beams perpendicular to the flat surface of a single MoO3 HNP is shown in Figure 3b. The regular rectangular diffraction dot lattice of the SAED pattern reveals that the HNP is a perfect single-crystal R-MoO3 with a zone axis of [010]. Along with the SAED to further confirm the structure of the HNP is the information provided by HRTEM, as shown in Figure 3c. The interplanar spacings of

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Figure 4. Raman spectrum of one of the layered MoO3 HNP.

TABLE 1: Raman Spectrum Comparison of MoO3 Single Crystal Bulk Materiala and MoO3 HNP.b MoO3 single crystal (cm-1) 998 822 473 366 381 338 219 160 668 285 200 247 131 a

Ag

Bg

a

layered MoO3 HNP (cm-1)b 993 816 468 366 375 334 214 156 662 281 195 241 125

Figure 5. The room temperature photoluminescence spectrum of the as-synthesized MoO3 HNP exited with wavenumber 325 nm.

Ag

Bg

See ref 23. b MoO3 HNP in this experiment.

0.387 and 0.372 nm are assigned to the spacings between the (100) and (001) planes, respectively. Raman scattering spectrum was obtained by focusing the laser dot on one layered plate, and the peak positions are shown in Figure 4. The irreducible representation of MoO3 with space group D2h16 (Pbnm) is given as follows:22

Γ)8Ag+8B1g+4B2g+4B3g+4Au + 3B1u+7B2u+7B3u, in which Ag, B1g, B2g, and B3g are Raman-active modes. the Raman spectrum of a MoO3 single crystal has been studied by Py et al. using Kihlborg’s structural picture.23 Table 1 is the assignments and comparison of the single crystal and our MoO3 HNP. It is obvious that the frequencies of the spectrum display a red shift of about 4-6 cm-1. The decrease in the Raman frequencies may be attributed to the nanosize of our single crystals,24 since according to the valence force field calculations of Py et al., downward frequency shifts point to a reduction of interaction between layers and chains, which contributes to the perturbation in the layered structure.25 The photoluminescence (PL) property of the synthesized sample was measured at room temperature. Figure 5 shows the photoluminescence spectrum of the MoO3 HNP with an excitation wavenumber of 325 nm. As shown in the picture, a broad peak at about 410 nm is observed in the range of 370-460 nm, and another shoulder peak is at 355 nm in the range of 340-370 nm, corresponding to values of 3.5 and 3.0 eV, respectively. Since the forbidden band gap value of a single-crystal MoO3 is reported to be 2.9-3.15 eV,26,27 we suppose that the band at 410 nm is attributed to the intrinsic nature of the MoO3 single crystal, whereas the 355 nm band might be due to an imperfec-

Figure 6. Schematic representation of proposed growth process of MoO3 HNP.

tion of the lattice; namely, the oxygen vacancy caused by the deficiency of oxygen. The result agreed well with a previous report.16 Generally, vapor-solid (VS) and vapor-liquid-solid (VLS) are two main growth mechanisms for the formation of nanostructures grown by evaporation methods. Because of the absence of the droplet that is necessary for the VLS growth mechanism and no impurity or catalyst particles are tested from EDX or XRD, the growth mechanism of our nanoplates is VS, rather than VLS. To prove the necessity of the In2O3 in the forming of MoO3 HNP, we tried to merely heat the Mo plates in air. We got thick, huge plates instead of nanoplates, indicating that the In2O3 must be the key to controlling the size of the MoO3 HNP. The whole process is presented schematically in Figure 6. We assume that in the first step, the reaction described by eq 1 happened to provide the oxygen needed to react with the Mo plates.28 In the second step (step 2), the Mo plates reacted with O2, which was carried by the flowing Ar, as described by eq 2. Because of the low melting point of MoO3 (795 °C) and huge heat released during the oxidation of the Mo plates,29 MoO3 is in the vapor and liquid states after the oxidation. With the help of the Ar current, MoO3 molecules spread to the temperature suitable region for nucleation, where Si substrates were placed, as shown in the picture in the third step. Due to the increase in the supersaturation degree of MoO3 together with the decrease in the temperature, MoO3 molecules prefer to condense on MoO3 nanoparticles on the substrates, which could serve as nuclei.30 Subsequently, the MoO3 nanoparticles would preferentially grow along two directions ([100] and [001]) into 2D hexagonal nanoplates. From the crystal growth point of view, the shape of the crystal highly depends on the relative growth rates of various crystal planes.31 R-MoO3 is distinctive for its layered structure, which is seldom found in metal oxides. The

MoO3 Nanoplates corner along [100] and zigzag edges along the [001] sharing of MoO6 octahedron results in different chemical reactivates because one Mo-O bond will form if one MoO6 is added along the [100] direction, whereas two bonds will form if added along [001]. From an energy point of view, more energy will be released if the MoO6 octahedron is added along [001] than if added along [100],5 which results in the higher growth rate in the [001] direction than that along the [100] direction. So the nanoplates look much like stretched hexagonal plates along [001]. The forming of the triangular tips on both ends of the nanoplates might be attributed to the different close-packing rates along the a- and c-axes.20 The coexistence of hexagonally and truncated hexagonally shaped MoO3 nanoplates might be attributed to the temperature gradient of the growth region. From the morphological observation of the nanoplates, we found that they were grown one over another. The formation of a layered structure could be explained from the viewpoint of thermodynamics, according to which the nanoplates with two main exposed planes have quite high surface energy, so aggregating together with nanoplates nearby is allowed to reduce surface energy by reducing the exposed area.32 Conclusions In summary, we have synthesized layered MoO3 HNP through vapor-deposition, a simple, environmentally friendly method, in an Ar atmosphere. The structure and morphology confirmed by various measurements indicates that the plates are nanoscale with a large surface area. The Raman study of MoO3 HNP compared with the spectrum of MoO3 single crystal bulk materials previously reported reveals that the size effect of the nanoplates plays an important role in the lattice vibrational property of this material. In addition, two peaks were observed in the PL spectrum, indicating that the MoO3 HNP might be photonic material for technical applications. Acknowledgment. This work was supported by the National Science Foundation of China (No. 50772043) and The Graduate Innovative Fund of Jilin University (20092003, 20091011, and MS20080217) and National Basic Research Program of China (Grant Nos. 2005CB724400 and 2001CB711201). References and Notes (1) Liang, Z. H.; Zhu, Y. J.; Cheng, G. F. J. Mater. Sci. 2007, 42, 477.

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