Materials Science and Engineering B 174 (2010) 174–176
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Synthesis of molybdenum oxide microsheets via close-spaced vapor transport a ˜ O. Goiz a,∗ , F. Chávez b , C. Felipe c , N. Morales b , R. Pena-Sierra a
Department of Electrical Engineering, CINVESTAV-IPN, 07360 México, D.F., Mexico Department of Physical-Chemical Materials, ICUAP-BUAP, 72050 Puebla, Pue., Mexico c Department of Biosciences and Engineering, CIIEMAD-IPN, 07340 México, D.F., Mexico b
a r t i c l e
i n f o
Article history: Received 1 September 2009 Received in revised form 5 February 2010 Accepted 9 March 2010 Keywords: Molybdenum oxide Microsheets Microplatelets
a b s t r a c t Growth of molybdenum oxide microsheets on silicon (1 0 0) substrates using the close-spaced vapor transport (CSVT) technique is proposed. Molybdenum oxide powder is employed as source, the synthesis is carried out at atmospheric pressure with a nitrogen ambient by employing short times (a few minutes), water as reactant and moderate temperatures. The growth process is efficient, fast, and without the use of catalysts. Changes in morphology and structure of products when temperature varies are reported. The produced molybdenum oxide microsheets are analyzed with SEM, XRD and micro-Raman techniques. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nanostructures from different materials have received much more attention since is known that its properties are different from the bulk form. In this way, molybdenum oxide (MoO3 ) has been investigated to apply it in devices such as gas sensors, light emitting diodes, catalyst and lithium batteries [1–5]. Molybdenum oxide nanostructures have been synthesized using several methods, for example; nanobelts have been obtained via hydrothermal synthesis [6], structures assembled by nanoplates/microprisms/nanorods have been synthesized via template-free hydrothermal approach [7], thermal oxidation of molybdenum has been employed to obtain nanobelts and microballs [8], and thermal evaporation technique to prepare nanobelts [9]. Among techniques to synthesize MoO3 products, the CSVT is a simple and cheap technique that can be utilized to synthesize nanostructures. Thus, in this work we show that MoO3 microsheets (also called microflakes or microplatelets) can be obtained on silicon substrates using MoO3 powder as starting material and water as reactant. 2. Experimental Previously, we have reported the preparation of nanostructured materials by CSVT technique employing a horizontal quartz tube
∗ Corresponding author at: Departamento de Fisicoquímica de Materiales, ICUAP, Benemérita Universidad Autónoma, de Puebla, Privada 17 Norte 3417, Col. San Miguel Hueyotlipan, C.P. 72050, Puebla, Pue., Mexico. Tel.: +52 222 2421072; fax: +52 222 2421072. E-mail address:
[email protected] (O. Goiz). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.03.021
[10]. In present work, an analogous system is used to synthesize MoO3 microsheets. MoO3 powder (from Alfa® ÆSAR A Johnson Matthey Company, purity of 99.998%) is used as starting material and silicon wafers (from Epitaxy, INC.) as substrates; these wafers were previously cleaned with xylene, acetone, propanol and dried by nitrogen. The distance between source and substrate is ∼700 m. First, a MoO3 pellet is made compressing MoO3 powder. Then, the MoO3 pellet is placed on a graphite boat. A flux of nitrogen (purity of 99.999%) is employed as carrier gas. Four cases are considered in this report, by maintaining constant nitrogen flux, separation distance of source–substrate and process time, and only by varying the growth temperature at 450, 500, 525 and 550 ◦ C. The growth process was 15 min for all experiments. The products were analyzed with a scanning electron microscope (TESCAN Vega TS-5136SB), X-ray diffraction (X-ray minidiffractometer MD-10), and micro-Raman scattering (HORIBA Jobin Yvon HR800). 3. Results and discussion Figs. 1 and 2 show the SEM images of the MoO3 microsheets. Synthesis at 450 ◦ C is shown in Fig. 1A. In this condition a poor growth of sheets can be seen on the silicon substrate. Sheets appear coming up from the substrate forming certain angle between them. It can be observed that products have hexagonal shape. In its larger side, a sheet have not more than 2 m in length while the shorter side not exceeds 1 m, the thickness of such structures is less than 100 nm. Thickness as well as length of the sheets increases according to growth temperature as shown in Table 1. At low magnification wire-like can be seen among the sheets but with a higher amplification these wire-like are actually sheets viewed vertically
O. Goiz et al. / Materials Science and Engineering B 174 (2010) 174–176
Fig. 1. SEM images of the sub-microsheets grown on silicon substrates held at temperatures: (A) 450 ◦ C and (B) 500 ◦ C.
Fig. 2. SEM images of the microsheets grown on silicon substrates held at temperatures: (A) 525 ◦ C and (B) 550 ◦ C.
as shown in Figs. 1B and 2A, furthermore, it allows to determine the thickness of the as-synthesized products. SEM micrographs show that sheets are composed by a layered structure and this could be associated with the intense (0 4 0) and weak (0 6 0) reflection peak of the XRD pattern (Fig. 3) suggesting that sheets are made by stacking layers parallel to the (0 1 0) plane [11]. The XRD patterns of the products synthesized at different temperatures are shown in Fig. 3. First spectrum corresponds to sheets grown at 450 ◦ C, this trace is notably different to the others, and only a weak peak could be indexed to (0 4 0) plane of MoO3 orthorhombic phase according to JCPDS 76-1003. When growth temperature was 500 ◦ C, two intense peaks are identified and indexed to the (1 1 0) and (0 4 0) planes of the ␣-MoO3
Table 1 Dimensions of the sheets according to the growth temperature. Temperature (◦ C) 450 500 525 550
Thickness (m) <0.1 0.1–0.3 0.4–0.5 1–3
Shorter side length (m)
Larger side length (m)
<1 <2 <5 <60
<2 <3 <10 <100
Fig. 3. XRD spectra of ␣-MoO3 microsheets synthesized on silicon substrates.
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material, in this sense, growth of sheets on silicon substrates can be promoted by the residual water absorbed by the MoO3 pellet. Maintaining the experimental conditions, additional experiments (results not shown here) were realized introducing water intentionally into the CSVT system in order to confirm such hypothesis. The yield and dimensions (principally in thickness) of microsheets were notably higher than when no water was introduced demonstrating that water acts as reactant agent. Since MoO3 in wet ambient can form a molybdic acid the formation of sheets is attributed to the Vapor–Solid mechanism [16]. From 300 ◦ C this molybdic acid volatilizes and is transported from the high temperature to the low temperature zone [17] where condenses and crystallizes in ␣-MoO3 products. 4. Conclusion Microsheets of MoO3 were obtained via CSVT technique utilizing MoO3 powder as starting material and residual water as reactant to form a volatile acid. Thickness as well as lengths of the microsheets was directly affected by growth temperature. The wide range in dimensions was from less than 0.1 to 3 m in thickness and from 1 to 100 m in length, from 450 to 550 ◦ C respectively. XRD and micro-Raman confirm that products crystallize in the orthorhombic phase of MoO3 . Acknowledgement Fig. 4. Micro-Raman spectra of ␣-MoO3 microsheets.
phase (orthorhombic phase) [12]. These preferential orientations are present when synthesis was carried out at 500, 525 and 550 ◦ C. Micro-Raman spectroscopy plays an important role especially in the characterization of sample grown at 450 ◦ C where no intense peaks were detectable by XRD because of their small particle size and high dispersion or limitations of the instrument [13]. Due to laser spot can be focused on a single sheet, micro-Raman confirms that products observed on silicon substrates (specifically in Fig. 1A) correspond to the ␣-phase of MoO3 . Raman spectroscopy can give structural information principally on the vibrational properties of the molybdenum oxide. For the present characterization, microRaman spectra were taken at room temperature using a 632.8 nm line of a He–Ne laser as source. Analysis of the sheets was carried out focusing the laser spot on a small area of ∼2 m in diameter. Fig. 4 shows the micro-Raman spectra of MoO3 microsheets. Three main regions are visible; below 200 cm−1 which corresponds to lattice modes (peaks at 82, 98, 115, 128, 157 and 197 cm−1 ), from 200 to 400 cm−1 which corresponds to deformation modes (peaks at 217, 245, 283, 337, 365 and 377 cm−1 ), and from 600 to 1000 cm−1 corresponding to stretching modes (peaks at 666, 819 and 995 cm−1 ), all the peaks in the spectra correspond to the orthorhombic crystal structure [13,14] and are in good agreement with previous reports [8,15]. The sharpness of the peaks indicates that the corresponding vibrational modes are due to a highly ordered structure [9]. The synthesis of the ␣-MoO3 microsheets was carried out without the use of any catalyst and utilizing MoO3 powder as starting
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