Characterization of Donut-Like SrMoO 4 Produced by Microwave-Hydrothermal Process

SrMoO 4 hierarchical nanostructures were successfully produced by a one step of 270Wmicrowave-hydrothermal process of one of the solutions containing three strontium salts [Sr(NO 3 ) 2 , Sr(CH 3 CO 2 ) 2 , and SrCl 2 ⋅6H 2 O] and (NH 4 ) 6 Mo 7 O 24 ⋅4H 2 O for different lengths of time. The as-produced products were characterized by X-ray diffraction, electron microscopy, and spectroscopy. In this research, they were primitive tetragonal structured donut-like SrMoO 4 , with the main 881 cm ] 1 (Ag) symmetric stretching vibration mode of [MoO 4 ] 2− units and 3.92 eV energy gap.


Introduction
Alkaline earth scheelite structured molybdate has been very attractive material for a wide variety of applications such as scintillating materials, laser-host materials, cryogenic detectors, heterogeneous catalysts, photoluminescence, optical fibers, solid-state optical masers, and electrochromic materials [1,2].One of them is SrMoO 4 which is very attractive for using as optoelectronic and electrochromic materials.It was produced by different methods: irregular aggregates of particles and microdisks by an electrochemical process [1], films of micrograins by a nonreversible galvanic cell method [2], hierarchical crystallites by simple precipitation [3], round nanoparticles with uniform sizes by coprecipitation at room temperature [4], spheres and dumbbells by simple aqueous mineralization [5], nanocrystals by microwave-assisted synthesis [6], nanostructured material by solvothermal-mediated microemulsion [7], and powders by coprecipitation and microwave-hydrothermal combination [8].
In this research, hierarchical nanostructures of SrMoO 4 with donut shape were produced by microwave-hydrothermal method for different lengths of time without using surfactants, complexing agents, and other additives.The process is very simple, attractive, and novel for large scale synthesis.O] were separately dissolved in 25 mL distilled water to form strontium and molybdenum solutions, which were mixed, stirred for 10 min at room temperature, and processed by a 270 W microwave-hydrothermal method for 5, 15, 30, and 90 min (encoded as 1, 2, 3, and 4 in sequence) to form precipitates.In this research, the products were encoded as A1, A2, A3, A4, B1, B2, B3, C1, C2, and C3.The A2 product implied that it was produced from Sr(NO The products were characterized by an X-ray diffractometer (XRD, SIEMENS D500) operating at 20 kV and 15 mA  to create Cu-K  line for the analysis; a scanning electron microscope (SEM, JEOL JSM-6335F) operating at 15 kV; a transmission electron microscope (TEM, JEOL JEM-2010), high resolution transmission electron microscope (HRTEM), and selected area electron diffractometer (SAED) operating at 200 kV; a Raman spectrometer (HORIBA Jobin Yvon T64000) using a 50 mW and 514.5 nm wavelength Ar green laser; and a UV-visible spectrometer (PerkinElmer Lambda 25) using a UV lamp with the resolution of 1 nm.

Results and Discussion
Comparing XRD patterns (Figure 1) to the JCPDS no.85-0586 [9], they were specified as primitive tetragonal scheelite structured SrMoO 4 [1][2][3]5].No other characteristic peaks of impurities were detected.The Sr 2+ cations were mixed with [Mo 7 O 24 ] 6− anions to form intermediate complexes at room temperature.Upon processing the complexes by the microwave-hydrothermal combination, they were gradually transformed for a few steps into SrMoO 4 precipitates.
The XRD peaks became sharpened with the increase in the length of time, including the crystalline degree being much improved and the crystals being enlarged.The longer processing time was used, the larger crystallite size and the better crystalline degree would be.Calculated crystallite sizes of the A3, B3, and C3 products [10] were 34.7, 64.7, and 83.2 nm, respectively.They seemed to be influenced by different intermediates, which led to form crystals with different sizes.XRD peaks of the purified SrMoO 4 produced in the solution containing Sr(NO 3 ) 2 and (NH 4 ) 6 Mo 7 O 24 ⋅4H 2 O by the microwave-hydrothermal reaction for 30 min were compared with that obtained by simulation [11] (Figure 2).The 2 Bragg angles and peak intensities obtained from the experiment, simulation, and JCPDS database were in good accordance.Crystal growth rates along the -, -, and -directions could be different.The simulated scheelite-type tetragonal structured SrMoO 4 (Figure 2) belongs to I4 1 /a space group with two SrMoO  This interpreted pattern was in good accordance with the simulated one (Figure 4(d)), although some spots of the simulated pattern did not appear on the interpreted one.To simulate the pattern, intensity and size of the spots (planes) were mutually related.The stronger intensity was used, the larger size was achieved.The intensity and size of the spots were limited by a saturated intensity used for simulation.Thus the spots of the simulated pattern with low intensity were absent from the interpreted one.When Sr and Mo solutions were mixed, the intermediate complexes formed.Subsequently, they were processed by the microwave-hydrothermal reaction and gradually transformed for a few steps into hierarchical nanostructures of SrMoO 4 with donut-like or flower-like shape:

More complete and larger flowers
SrMoO 4 molecules nucleated and grew to form nanoparticles. Furthermore, these nanoparticles selectively grew to form nanosheet petals for the A1 to A4 and C1 to C3 products and nanorod petals for the B1 to B3 products on top.As the processing time passed, the petals were enlarged and squeezed each other.Some petals were bent and some were broken to release stress energy.The flowers (donuts) became more complete as well.In the end, the particles became completely donut-like shape (Figure 5).
Several different vibrations were detected on Raman spectra of the SrMoO 4 crystals (Figure 6).The Raman peaks at 881 cm −1 were specified as the ] 1 (A g ) symmetric stretching vibration mode of [MoO 4 ] 2− units.Those at 838-841 and 788-790 cm −1 corresponded to the ] 3 (B g ) and ] 3 (E g ) antisymmetric stretching vibration modes, respectively.The peaks at 368 and 328-330 cm −1 , respectively, corresponded to the ] 4 (B g ) antisymmetric and ] 2 (A g ) symmetric bending modes, including the 181-183 cm −1 to the ] f.r.(F 1 ) free rotation modes.Those at 118, 141, and 163 cm −1 were specified as the external vibration modes of Sr 2+ cations and [MoO 4 ] 2− units.These vibration modes were very close to those reported by other researchers [3,5,8] and provided the evidence of scheelite structure.UV-visible absorption (Figure 7) of the hierarchical SrMoO 4 architecture of the C3 product synthesized by the 270 W and 30 min microwave-hydrothermal process indicated an exponential decreasing of the UV-visible energy attenuated through the crystalline C3 product.During attenuation, the absorption was controlled by two photon energy (h]) ranges.For h] < E g , the absorption was linearly increased with the increasing of photon energy.The steep inclination of the linear portion of the curve was caused by the UV absorption for charged transition from the topmost occupied state of valence band to the bottommost unoccupied state of the conduction band.For h] < E g , the absorption curve became different from linearity, caused by the UV absorption for charged transition relating to defects.Band gap of the product is related to its absorbance and photonic energy.Thus the combination of absorbance and photonic energy was used to determine the photonic band gap.By extrapolating the linear portion curve (tail of the curve) of the (h]) 2 versus h] plot to zero absorption, its direct energy gap was determined to be 3.92 eV for the hierarchical architecture of the C3 product.This energy gap was very close to the 3.98 eV of SrMoO 4 powder processed by the microwavehydrothermal reaction at 413 K for 5 h reported by Sczancoski et al. [8].

Conclusions
SrMoO 4 hierarchical nanostructures were successfully produced by the one-step microwave-hydrothermal process.In this research, the products were tetragonal scheelite crystal with donut-like or flower-like SrMoO 4 .Their main vibration modes were detected at 881 cm −1 and the energy gap of 3.92 eV.

Figure 5 :Figure 6 :
Figure 5: Schematic illustration for the formation of hierarchical architecture of SrMoO 4 .

Figure 7 :
Figure 7: UV-visible absorption and the (h]) 2 versus h] plot of the C3 product.