Synthesis of Crystallized BaWO4 Nanorods in a Microemulsion System

BaWO4 nanorods have been successfully synthesized in w/o microemulsion system containing barium ions via a simple reaction between Ba and 4 WO − . The BaWO4 Nanorods were characterized by XRD, TEM, and SEM, respectively. Results showed that the solvents composition—volume ratio of 4-dioxane and distilled water—played the key role in the formation of BaWO4 Nanorods. Furthermore, the strong vibration at 925 cm on its Raman spectrum indicated that the BaWO4 nanorods is good at stimulating Raman scattering in transient and steady-state, making it as a promising candidate material for laser with self-raman conversion of radiation inside the active medium.


Introduction
Nowadays, tungstate materials, BaSO 4 , have attracted much attention in view of its luminescent behavior and structural properties [1] [2].As compared to other materials, the narrow line width of its stimulated Raman scattering (SRS)-active mode in BaSO 4 crystal (1.6 cm −1 ) leads to high peak intensity (63%).In particular, Raman gain has been measured to be 8.5 cm/GW at 1.06 μm wavelength [2].Furthermore, the material is not hydroscopic and transparent in visible and near-infrared spectral range.It is a promising material for crystalline nano-and picoseconds Raman lasers.A number of methods, including hydrothermal method, flux method and solid-state reaction [3]- [8], have been developed to generate tungstate materials.However, tough reaction conditions, such as highreaction temperature, long-reaction time or complex equipment, were applied in most this approaches [9].Thus, seeking efficient but low-cost techniques for synthesizing BaSO 4 is required for the development of electro-optical materials.
It is well known that different surfactants can form micelles with variable morphologies.This can be utilized for the modification of crystal growth [10] [11] [12].For example, Zhang et al. [12] reported that the penniform super structures of BaWO 4 nanowires have been successfully synthesized in reverse micelles by using a block copolymer as the directing agent.The effects of the mixing ratio between the anionic and cationic surfactants on the crystal growth of BaWO 4 nanowires have been further studied.As proposed, the different morphologies and sizes of BaWO 4 crystals could be synthesized by the employment of super-molecule templates composed of biomembrane and organic reagents at room temperature [13].However, the BaWO 4 nanoparticles with high crystalline and various regular shapes are required in order to enhance physiccal properties.It has thus been indicated that the BaWO 4 microparticles with well crystallinity should be obtained through simple method.Whereas, the composition of the microemulsion influences the structures of the surfactant aggregation as well as the size and shape of the final nanocrystals [14] [15].
Therefore, the present study was conceived to develop BaWO 4 nanoparticles in a water-in-oil (W/O) microemulsion system composed of 1,4-dioxane and water.The phases, morphologies, and luminescent properties have been investigated.

Results and Discussion
The obtained BaWO 4 particle in water-in-oil (W/O) microemulsion system was characterized by XRD, and the typical XRD pattern was shown in Figure 1, in which all the peaks could be indexed to the pure BaWO 4 particle with a tetragonal unit cell (a = b = 0.5626 nm, c = 1.2744 nm).This can be indexed to the JCPDS Card No. 82,457 [12].No other peaks were detected in the pattern, indicating the high purity of the BaWO 4 particle.
The typical TEM images of products synthesized with different ratio of 4dioxane and distilled water were displayed in Figure 2.  SEM image of BaWO 4 nanorods prepared in the solvents with the ration value of 4:6, was showed in Figure 3.The side length was approximately 600 nm, and the diameter was about 50 nm.The Raman spectra of the produced BaWO 4 nanorods with a ration value of 4:6 was shown in Figure 4(a).The peaks at 926.5, 830.7, 794.6 and 330.6 cm −1 belongs to vibration mode of ν 1 (A g ), ν 3 (B g ), ν 3 (E g ) and ν 2 (A g ), respectively.Furthermore, a strong vibration at 925 cm −1 was observed, indicating the BaWO 4 nanorods is good at stimulating Raman scattering in transient and steady-state [2].This made it as a promising material for laser with self-raman conversion of radiation inside the active medium.The blue emission from BaWO 4 materials has been reported at low temperatures in the literature [18] [19] [20].Interestingly, as shown in Figure 4(b), a broad emission band extended from 396 nm to 498 nm (peaked at approximately 425 nm) was observed when the BaWO 4 nanorods excited at 270 nm.The blue emission from BaWO 4 nanorods is known to be due to radiative transitions within ( )

Conclusion
In the present study, BaWO tion of 4-dioxane played a crucial role in the formation of BaWO 4 nanorods with different morphologies.Thus, the addition of 1,4-dioxane should be controlled.Furthermore, the Raman spectra of the produced BaWO 4 nanorods indicated that it is a promising material for laser with self-raman conversion of radiation inside the active medium.

BaCl 2 •
2H 2 O and Na 2 WO 4 •2H 2 O with analytical grade purity were used as, and all other chemical reagents were analytical grade.BaWO 4 nanoparticles were obtained from BaCl 2 and Na 2 WO 4 microemulsion system according to the following reaction:

Firstly, 4 -
dioxane and distilled water were mixed together to prepare a microemulsion system with different volume ratios.Secondly, BaCl 2 •2H 2 O (0.061 g) and Na 2 WO 4 •2H 2 O (0.0821 g) were dissolved in an aliquot of 10 mL of microemulsion system, respectively.The mixture containing Na 2 WO 4 were then added dropwise into the flask containing BaCl 2 at 35˚C and 150 rpm.After mixing for 10 min, the sample was centrifuged at 4024 ×g for 5 min and the white precipitates were collected.Finally, the white precipitates were rinsed three times with distilled water and dried at 80˚C for 6 h.The X-ray diffraction (XRD) patterns were recorded using a Janpan Ridaku D/Max-γA X-ray diffractometer.The particles size and morphology were characterized by scanning electron microscopy (SEM, JEM-200CX) and transmission electron microscopy (TEM, JEOL-2010).Raman spectra were measured at the 514.5 nm line of an Ar laser (Labram HR800).

Figure 3 .Figure 4 .
Figure 3.The TEM (a) and SEM (b) images of the produced BaWO 4 with a ratio of 1,4dioxane and water for 4:6.