Synthesis of hand α-MoO 3 by Refluxing and Calcination Combination : Phase and Morphology Transformation , Photocatalysis , and Photosensitization

1 Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 2Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 3Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 4 Science and Technology Research Institute, Chiang Mai University, Chiang Mai 50200, Thailand


Introduction
MoO 3 is one of the transition metal oxide materials. It has a wide range of applications such as imaging devices, smart windows, electrodes of rechargeable batteries, gas sensors, and supported catalysts [1]. Since MoO 3 is a 3.15 eV wide band gap n-type semiconductor, it is well known as one of the most widely used photocatalysts. Its catalytic efficiency has long been known, for example, in alcohol and methane [2,3]. Its crystalline structures are known to have three common polymorphs: one thermodynamically stable orthorhombic  [1,4]. Moreover, h-MoO 3 exhibits phase stability up to 436 ∘ C at which the irreversible phase transition occurs to form orthorhombic -MoO 3 [5]. Various molybdenum oxide structures and morphologies have been synthesized by different methods: hexagonal rod-like h-MoO 3 by liquid phase deposition [1], nanospheres by ultrasonic-assisted synthesis [3], hexagonal rod-like h-MoO 3 by precipitation [4,6], MoO 3 nanorods by ultrasonic synthesis [5,7], -MoO 3 nanobelts by hydrothermal synthesis [8,9], h-MoO 3 rods by solution precipitation and solvothermal treatment [10], MoO 3 nanostructures by solution combustion [11], -MoO 3 polycrystalline thin films by spray pyrolysis [12], MoO 3 nanobelts by hydrothermal method [13,14], h-MoO 3 and -MoO 3 nanoparticles by hydrothermal synthesis [  nanorods by precipitation [16,17], -MoO 3 microplates by microwave plasma process [18], and -MoO 3 nanocrystals by oil bath heating and sintering combination [19]. In this research, both hexagonal and orthorhombic molybdenum oxide phases were synthesized by refluxing process to form the first and followed by high temperature calcination to form the second. Different phases, phase and morphology transformation (PMT), degradation of methylene blue dye, and mechanisms of photocatalysis and photosensitization are also discussed.

Experimental Procedures
To synthesize h-MoO 3 and -MoO 3 with different morphologies, 0.005 mole ammonium heptamolybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 ⋅4H 2 O) was dissolved in 40 mL of deionized water with continuous stirring at room temperature for 30 min. Subsequently, 2 M HNO 3 was added to the solution until achieving the pH 1 and forming of the clear solution. The solution was processed by a refluxing method at 90 ∘ C for 1, 3, 5, and 7 h. In the end, light-blue precipitates were synthesized, separated by filtration, washed, and dried in an electric oven at 80 ∘ C for 24 h. The products were further calcined at 450 ∘ C for 6 h to form powders.
Crystalline phases, morphologies, and vibration modes were characterized by a Philips X'Pert MPD X-ray diffractometer (XRD) at 45 kV and 35 mA with Cu K radiation in the 2 range of 20-80 deg with a scanning rate of 0.04 deg per step, a JEOL JSM-6335F scanning electron microscope (SEM) with an accelerating voltage of 15   equilibrium. Thus, the MB dye molecules have a chance to adsorb on the MoO 3 surfaces. Degradation was initiated by a 35 W Xe lamp for 0-180 min. The absorbance peak (strongest intensity) at 664 nm wavelength, determined by UV-visible spectrometer, was assumed to be linearly dependent on the concentration of MB solution. Decolorization efficiency (%) was calculated by (( − )/ ) × 100, where and were the absorbance intensities of the solutions before and after degradation, respectively.

Results and Discussion
3.1. XRD. XRD patterns of the products synthesized by a refluxing method for 1, 3, 5, and 7 h are shown in Figure 1. All the diffraction peaks can be indexed to be pure hexagonal (h) MoO 3 phase (JCPDS no. 21-0569) [20]. Their crystalline degrees were improved by increasing the length of reaction time. Upon calcination of the products at 450 ∘ C for 6 h in air, they were transformed into the orthorhombic ( ) MoO 3 (JCPDS no. 05-0508) [20] (Figure 2) with no impurity detection.

FTIR.
The functional groups of the products with the best crystalline degree were identified by FTIR (Figure 3 the previous report [16]. The 1000-400 cm −1 peaks correspond to the stretching and bending vibrations of metaloxygen characteristic bonds. The peaks at 980 cm −1 and 914 cm −1 are the characteristic of Mo=O stretching vibrations, including those between 600 and 500 cm −1 corresponding to the vibration of Mo-O bonds [15,17]. But for -MoO 3 , the spectrum shows three strong peaks: 994 cm −1 attributed to the terminal M=O stretching vibration with an indicator of the layered orthorhombic MoO 3 phase, 860 cm −1 to the stretching mode of oxygen in Mo-O-Mo bonds, and a broad band at 558 cm −1 to the bending vibration of oxygen atom linked to three metal atoms [14,15,18]. No water was detected in this orthorhombic-structured product.

EM and PMT. Figures 4(a)-4(e) show SEM and TEM
images of h-MoO 3 synthesized by refluxing method for different lengths of reaction time. The products shaped like clusters of hexagonal rods with the most complete at 7 h processing. The rods were 83.82, 143.57, 182.58, and 220.84 nm in diameter for 1, 3, 5, and 7 h processing, respectively. They became enlarged, by transforming from nanosized to microsized rods, with increasing the processing time. In this research, the rods grew out of a center [21], appearing as clusters/flowers of hexagonal rod-like petals and becoming the most complete flowers for 7 h processing. A SAED pattern of h-MoO 3 (Figure 4  During 450 ∘ C calcination, flowers of h-MoO 3 rodlike petals were separated into a number of rods. All atoms of these individual rods were at high energy, leading to the vibration and diffusion process. The strength of vibration and diffusion of the solid was controlled by the calcination temperature, bond strength, type of bonds, and others. Thus, the rods were no longerable to retain their original morphologies. When the calcination process was complete at 6 h, the solid was cooled down to room temperature. During cooling, the vibration and diffusion of all atoms slowed down. The atoms returned to their new lattice sites which were at the lowest energy. In the end, the atoms formed a orthorhombic crystalline structure ( -MoO 3 ) of which the unit cells arranged themselves into assemblies of microplates. SAED pattern (Figure 5 In this research, h-MoO 3 flowers with 5 and 7 h refluxing were more perfect than those synthesized by ultrasonic synthesis [7] and chemical precipitation [16,17], including the corresponding -MoO 3 microplates which were more systematic than those synthesized by microwave plasma [18]. refluxing was studied and became colorless by 30 min stirring during establishing adsorption-desorption equilibrium. Possibly, the solution containing h-MoO 3 was exposed to UV radiation. Photocalytic properties of -MoO 3 containing methylene blue (MB) solution irradiated by xenon light for 0-180 min ( Figure 6) were investigated. For cationic MB dye aqueous solution, there are two absorption bands at 293 nm or 4.23 eV ( - * ) and 664 nm or 1.87 eV ( - * ) [19]. In this research, intensities of absorption peaks at 664 nm were decreased with the increase in the lengths of irradiation time. Comparing to the catalyst-free solution, the 180 min degradation of MB solution (Figure 7) containing -MoO 3 catalyst synthesized by 90 ∘ C refluxing for 7 h in combination with 450 ∘ C calcination for 6 h was 88%. During photocatalysis or changing of a chemical reaction rate by photons, electrons in valence band of -MoO 3 were excited and transferred to its conduction band under xenon light radiation, leaving holes in valence band behind. Holes combined with H 2 O to form •H and •OH radicals. Concurrently, the electrons in conduction band diffused to the adsorbed O 2 to form activated •O 2 − with subsequent transforming of H 2 O molecules into •OH radicals. These oxidative species could mineralize MB dye back into original chemical forms of CO 2 and H 2 O creating a cleaner and safer environment [22]. If the processes were not possible, electron-hole pairs would recombine together to generate heat on the materials. Photocatalytic activity was controlled by various factors, including structure, particle size, surface area, crystalline degree, surface-adsorbed water molecules, and hydroxyl groups [23]. The present results show that catalytic activity of the metastable h-MoO 3 phase has higher efficiency than that of the thermodynamically stable -MoO 3 one.
Photosensitization or the process of initiating reaction by a photonic absorber and transfer of energy to reactants was also possible. Upon irradiation of the solutions by xenon light, the MB dye absorbed photon energy and induced the - * transition (4.23 eV [19]) [14], including - * transition (1.87 eV [19]). The excited electrons of the MB dye diffused to conduction band (d-orbital) of MoO 3 and reacted with adsorbed oxygen to form •O 2 − oxidants which further mineralized the MB dye back into CO 2 and H 2 O. Photolysis or chemical decomposition of dye induced by photon relates to its structural stability [14], which leads to the mineralization process.
In summary, decolorization of MB dye was able to proceed by the photocatalysis and photosensitization processes or either of the two, influenced by photonic energy, energy gap of MoO 3 catalyst, gaps of - * and - * states of MB dye, and others. Figure 8 shows photoluminescence of h-MoO 3 and -MoO 3 excited by 337 nm wavelength at room temperature. Photons with energy exceeding their energy gaps reflected on the products and generated photoexcited electrons, which are not stable. Thus, they jumped back to a basic state and emitted fluorescence photons with lower energy. They both show strong emission peaks at 436 nm in accordance with other reports [9,10,18,19], due to the electron-hole recombination. Moreover, h-MoO 3 has another extra peak at 606 nm specified as the presence of adsorbed oxygen on the material [10].

PL.
3.6. UV-Visible Absorption. UV-visible absorption spectroscopy has been used to study photonic properties of materials. Experimentally, the following equation has usually been used to estimate their energy gaps: where , ℎ, ], and are the photonic absorbance, the Planck constant, photon frequency, and energy gap, respectively. The parameter is a constant associated with different types of electronic transition: = 1/2, 2, 3/2, or 3 for direct allowed, indirect allowed, direct forbidden, and indirect forbidden transitions, respectively, [9,12,21,24]. UVvisible spectra (Figure 9) of -MoO 3 and h-MoO 3 show a photonic energy attenuated through the solids. The ( ℎ]) 2 versus ℎ] plots were used to estimate energy gaps of -MoO 3 and h-MoO 3 to be 3.18 and 3.05 eV, respectively. These energy gaps are in accordance with those of 3.35 eV for polycrystalline -MoO 3 thin film [12], 3.75 eV -MoO 3 nanobelts [9], 3.15 eV for -MoO 3 layered structure and 3.01 eV for h-MoO 3 hexagonal rods [15], and 2.99 eV for h-MoO 3 hexagonal nanorods [16]. Shape, size, size distribution, phase, crystalline degree, and defects can play a role in the energy gaps of materials. Generally, becomes wider by using smaller particles but narrower by the presence of defects.

Conclusions
In this research, h-MoO 3 rods with the shape of flowers were successfully synthesized by refluxing process and further calcination of the flower-like product to form -MoO 3 microplates. Their phases, morphologies, and vibration modes were characterized by XRD, SEM, TEM, SAED, and FTIR. Degradation of MB dye under xenon light was proceeding by the photocatalysis and photosensitization processes. Catalytic activity of the metastable h-MoO 3 phase has higher efficiency than that of the thermodynamically stable -MoO 3 one. PL emissions were determined to be 436 nm for -MoO 3 microplates, and 436 and 606 nm for h-MoO 3 rods, including of 3.18 eV for -MoO 3 microplates, and 3.05 eV for h-MoO 3 rods. Their phase and morphology transformation was also explained according to the experimental results.