Abstract
Orthorhombic α-MoO3 microplates were produced from (NH4)6Mo7O244H2O solid powder by a 900 W microwave plasma for 40, 50, and 60 min. Phase, morphologies, and vibration modes were characterized by X-ray diffraction (XRD), selected area electron diffraction (SAED), scanning electron microscopy (SEM), and Raman and Fourier transform infrared (FTIR) spectroscopy. Sixty min processing resulted in the best crystallization of the α-MoO3 phase, with photoluminescence (PL) in a wavelength range of 430–440 nm.
1. Introduction
Basically, molybdenum oxides are classified into two types: the thermodynamically stable orthorhombic α-MoO3 phase, and the metastable monoclinic β-MoO3 phase with ReO3-type structure. Orthorhombic α-MoO3 phase is a promising oxide, with structural anisotropy [1]. It is a wide bandgap n-type semiconductor, which is very attractive for different technological applications such as photochromic materials (changing from colorless to blue by UV irradiation) [2–4], smart windows [5], self-developing photography [2], conductive gas sensors [3], lubricants [6], and catalysts [7]. Orthorhombic α-MoO3 was composed of MoO6 octahedral corner-sharing chains, with edge sharing of two similar chains to form layers bonded by the weak van der Waals attraction [2]. Different methods were used to produce the oxide, which led to achieving products with different properties: evaporation of Mo foil by IR in 1 atm synthetic air to produce a uniformly semitransparent film on alumina substrate [3], direct oxidation of a Mo spiral coil in ambient atmosphere to produce film on Si (001) substrate [8], flash evaporation of molybdenum oxide powder on silica glass substrate, and (111)-oriented silicon wafer in vacuum [9], precipitation [10], and hydrothermal method [11].
In the present research, α-MoO3 microplates were produced by exposing a solid powder to microwave plasma. This very simple and rapid process, which is also benign to the environment, may lead to large-scale industrial production.
2. Experiment
To produce MoO3, (NH4)6Mo7O244H2O powder was used as a starting material without further purification. Each 0.5 g powder was loaded into three 14 mm I.D. × 100 mm long silica boats. Each was placed in a horizontal quart tube, which was tightly closed and evacuated until its absolute pressure was kPa. The powder was heated in batches by a 900 W microwave plasma; each batch was irradiated for 5 min. After the processing of each batch, the powder was thoroughly mixed and repeatedly heated for a total of 40, 50, or 60 min. During processing, the horizontal quart tube was continuously evacuated to drain the evolved gases out of the system.
The products were characterized using X-ray diffractometer (XRD, SIEMENS D500) operating at 20 kV, 15 mA, and using Cu-Kα line, in combination with the database of the Joint Committee on Powder Diffraction Standards (JCPDS) [12]; scanning electron microscope (SEM, JEOL JSM-6335F) operating at 15 kV, transmission electron microscope (TEM, JEOL JEM-2010), and selected area electron diffractometer (SAED) operating at 200 kV; Fourier transform infrared spectrometer (FTIR, Bruker Tensor 27) with KBr as a diluting agent and operated in the range of 2000–400 cm−1, Raman spectrometer (T64000 HORIBA Jobin Yvon) using a 50 mW and 514.5 nm wavelength Ar green laser, and photoluminescence (PL) spectrometer (LS 50B PerkinElmer) using a 380 nm excitation wavelength at room temperature.
3. Results and Discussion
3.1. XRD, SAED, and HRTEM
XRD patterns of the products processed for 40, 50, and 60 min are shown in Figure 1(a). Their peaks were specified as orthorhombic MoO3 of JCPDS database number 05–0508 [12], with no impurity detection. The (020) peaks at 2θ of 12.8° were clearly detected, and they indicated the presence of orthorhombic phase instead of monoclinic [13]. It should be noted that their intensities were slightly increased with the increase of processing time. The XRD peaks for 60 min processing time were the strongest, reflecting the product with the best degree of crystallinity. During processing, (NH4)6Mo7O24·4H2O decomposed as follows: MoO3(s) was left as the final solid products. Two gases (NH3 and H2O) diffused out of the system, and evacuated out of the horizontal quart tube. It should be noted that some reactant could remain, and was mixed with the final product if the processing time was less than 40 min. Longer processing times resulted in greater purification of the final product.
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Calculated lattice parameters (Å) using the plane spacing equation for orthorhombic phase [14] were a = 3.96, b = 13.86, and c = 3.70, in accordance with those of the JCPDS database [12]. Figure 1(b) shows the SAED pattern of a single crystal processed for 60 min. It was indexed [15] to correspond with the (002), (202), and (200) crystallographic planes, which were specified as orthorhombic α-MoO3 [2, 12, 16]. In the present analysis, an electron beam was sent to the crystal along the [010] direction. The (021) crystallographic plane with 0.33 nm spacing was detected by HRTEM (Figure 1(c)), implying that the product was crystalline in nature. These last two analyses were in accordance with that of the above XRD.
3.2. SEM
SEM images of MoO3 crystals processed for 40, 50, and 60 min are shown in Figures 2(a)–2(d). Clusters of spheres ranging from 100 nm to a few hundred nm, as well as a small fraction of plates, were produced by 40 min processing. When the processing time was 50 min, more plates—about 100 nm thick and a few μm long—were produced, growing perpendicular to the cluster surface. Sixty min processing resulted in a further increase in the number of plates produced, as well as their sizes: 100–200 nm thick and a few μm long. During processing, some plates could be broken due to the internal stress developed inside.
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3.3. Raman and FTIR Analyses
Raman spectra (Figure 3(a)) of MoO3 crystals processed for 40, 50 and 60 min were studied in the range of 150–1050 cm−1. During the analysis, a low-intensity laser was used to avoid crystallization. The product of 60 min processing was a highly ordered crystalline structure, and its Raman peaks were the highest. The heights were reduced when the processing time was shortened. In the present research, 12 typical Raman peaks were detected. The peaks at 990 cm−1 were specified as the Mo=O asymmetric stretching modes of terminal (unshared) oxygen [16]. The strongest peaks were at 813 cm−1, and were specified as the doubly connected bridge-oxygen Mo2–O stretching modes [2] of doubly coordinated oxygen, caused by corner-shared oxygen atoms in common to two MoO6 octahedrons [16]. The peaks at 666 cm−1 were the Mo3–O stretching modes of triply coordinated bridge-oxygen, caused by edge-shared oxygen atoms in common to three octahedrons [2, 16]. Their remains were the O–Mo–O asymmetric stretching/bending modes at 470 cm−1, O–Mo–O scissoring modes at 378 and 364 cm−1, O–Mo–O bending modes at 337 cm−1, O=Mo=O wagging modes at 287 cm−1, O=Mo=O twisting modes at 244 cm−1, Rc modes at 217 cm−1, O=Mo=O twisting modes at 197 cm−1, and Tb modes at 158 cm−1 [16]. Sometimes the Raman peaks were positively/negatively shifted, due to the increase or decrease in the vibration constant of the products [2]. In the present research, the vibrations were the same values, although the processing time and degree of crystallinity were different.
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Figure 3(b) shows the FTIR spectrum of α-MoO3 over the 400–2000 cm−1 range. Three strong vibrations were detected at 621, 874 and 993 cm−1, associated respectively with the stretching mode of oxygen linked with three metal atoms, the stretching mode of oxygen in the Mo–O–Mo units, and the Mo=O stretching mode—the specification of a layered orthorhombic α-MoO3 phase [17]. Two weak vibrations were also detected at 1384 and 1643 cm−1, associated with the vibration mode of the Mo–OH bond and the bending mode of adsorbed water, respectively [17, 18].
3.4. PL Emission
PL emission of orthorhombic α-MoO3 processed for 40, 50, and 60 min was studied using 380 nm excitation wavelength at room temperature. The PL spectra (Figure 2(e)) presented broad peaks over the 400–600 nm range with a strong indigo emission centered at 430–440 nm—in accordance with the report of Song et al. [4]. These emissions were caused by the band-to-band transition. In the present research, very weak shoulders, caused by the electron-hole recombination between the conduction band and the sublevel of adsorbed oxygen acceptors, were also detected; these were able to be reduced by calcination at high temperatures [4]. The luminescence intensity increased with the increase of processing times, in accordance with the improvement of the degree of crystallinity characterized by the above XRD analysis.
4. Conclusions
Orthorhombic α-MoO3 was successfully produced by a 900 W microwave plasma process for 40, 50, and 60 min. The product processed for 60 min was α-MoO3 microplates with three main Raman peaks (666, 813, and 990 cm−1), three main FTIR vibration modes (621, 874, and 993 cm−1), and 430–440 nm indigo emission—a promising material for different applications.
Acknowledgments
The authors wish to thank the National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, Thailand, for providing financial support through the project code: P-10-11345, the Thailand’s Office of the Higher Education Commission through the National Research University Project, and the Scholarships for Thai Ph.D. Program, and the Thailand Research Fund (TRF) through the TRF Research Grant, including the Graduate School of Chiang Mai University through the general support.