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VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 3 (2019) 62-68 Original Article Synthesis of Micro/nano Urchin-like VO2 Particles and Its Decolorization of Methylene Blue Nguyen The Manh1,2, Duong Hong Quan1,2, Vu Thi Ngoc Minh3, Vuong-Hung Pham1, * 1 Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), No 01, Dai Co Viet, Hanoi, Vietnam 2 School of Engineering Physics, Hanoi University of Science and Technology (HUST), 01 Dai Co Viet, Hanoi, Vietnam 3 School of Chemical Engineering, Hanoi University of Science and Technology (HUST), 01 Dai Co Viet, Hanoi, Vietnam Received 01 April 2019 Revised 21 May 2019; Accepted 21 May 2019 Abstract: Micro/nano urchin-like VO2 particles were synthesized successfully by hydrothermal method. Vanadium pentoxide (V2O5), oxalic acid (C2H2O4) and sodium dodecyl sulfate (SDS) surfactant were used as reagents for the synthesis of VO2. In this article, we have reported the synthesis procedure of VO2 nanorods and micro/nano urchin-like VO2 structure and evaluating the methylene blue (MB) adsorption properties. Morphology and particle size of VO 2 were observed by FE-SEM. The phase formation of VO2 was studied by XRD. Raman spectroscopy was also used for characterization of VO2. Micro/nano urchin-like VO2 structure was showed good MB adsorption properties that have potential applications in dye-contaminated water treatments. Keywords: Micro-/nano-scale; nanoparticles;VO2; methylene Blue 1. Introduction In recent years, waters containing organic dyes coming from textile, leather, paper, and printing have become concerns in the environment and human health [1]. Many technologies are being applied for treatment of contaminated water such as chemical oxidation [2], ion exchange [3], biological ________ Corresponding author. Email address: vuong.phamhung@hust.edu.vn https//doi.org/ 10.25073/2588-1124/vnumap.4343 62 N.T. Manh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 3 (2019) 62-68 63 treatment [4], and adsorption [5] to eliminate residual dyes in contaminated water. Among them, especially the adsorption method is regarded as the most effective method because of the simple treatment process, fast decolourization and low cost [6]. Many materials such as wheat shells [7], activated carbon [8], and biochar [9] have been widely used to adsorb organic dyes in water. Nanostructured transition oxide materials have attracted a lot of attention in dye contaminated water treatments because of the large surface contact area with high absorption capacity [6]. Vanadium oxide has attracted a lot of attention in engineerings such as electrodes for batteries [10], adsorption [11], sensors [12], and smart windows [13] because of its stable chemical and physical properties. VO2 has several polymorphs: VO2 (R), VO2 (D), VO2 (M), VO2 (B), VO2 (A) and VO2 (C) [13,14]. Nevertheless, there are only a few reports on the adsorption of dye by using nanostructure VO 2 (D) [14] and VOx Nanosheets [15]. In particular, in our knowledge, there are no reports on methylene blue (MB) decolorization using micro/nano urchin-like VO2 particles. Therefore, this study proposes the attempt to synthesize micro/nano urchin-like VO2 particles using the hydrothermal method in presence of sodium dodecyl sulfate (SDS) surfactants for potential treatment of MB dye in contaminated water. The microstructures of the micro/nano urchin-like VO2 was characterized by field emission scanning electron microscopy (FE-SEM). Dye decolorization was determined by UV-Vis spectroscopy. 2. Experimental procedure 0.91 g of V2O5 (99.99 % purity, Aldrich) was put into 25 ml of distilled water, and then 25 ml of oxalic acid (C2H2O4, 0.48 M, 99.99 % purity, Aldrich) was added under magnetic stirrer. At this, stage, the color of solution was changed from yellow to blue color. Then, 5 mL of 0.2 M SDS (C12H25SO4Na, 99.99 % purity, Aldrich) solution was added to the above solution for 5 hours. The mixture solution was transferred into 200 ml Teflon-lined autoclave, after that the autoclave was sealed and maintained at 200 oC for 12 h. The resulting particles were washed twice times and then dried at 70 oC for 24h. The crystalline structures of the micro/nano urchin like VO2 particles were characterized by X-ray diffraction (XRD, D8 Advance, Bruker, Germany). The microstructure was determined by field emission scanning electron microscopy (JEOL, JSM-6700F, JEOL Techniques, Tokyo, Japan). Raman spectrometers of the particles were measured by Raman scattering (Renishaw) using 633 nm laser and 15mW power. Methylene blue (MB) decolorization test, 0.003 g of VO 2 nanowires or micro/nano urchin-like VO2 particles were added into 30mL methylene blue solution (20 ppm) which was at pH value of 7 under 20 minutes. The degradation of methylene blue was determined by UV-Vis (Cary 500 spectroscopy). 3. Results and discussions The microstructures of the VO2 nanorods and micro/nano urchin-like VO2 particles synthesized without and with the application SDS surfactants are shown in Figs. 1 {(a)–(d)}. The VO2 synthesized without SDS surfactant showed a nanorods structure, Fig. 1{(a), (c)} with the width of 150 nm and the length of  800 nm, as is often the case with VO2 particles synthesized by hydrothermal [16]. However, when an SDS surfactant was used, a number of nanorods with the width of 100 nm and the length of  1,5 µm were uniformly formed within the grains of VO2 with a diameter of 5 µm, Fig. 1 {(b), (d)}. The micro/nano urchin-like VO2 particles formation mechanism can be explained as following: SDS surfactant (C12H25SO4Na) will be decomposed into C12H25SO4 – and Na+ in solution. The C12H25SO4– will create spherical micelles, with negative charge on the surface. 64 N.T. Manh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 3 (2019) 62-68 Figure 1: FESEM image of VO2 nanorods and micro/nano urchin-like VO2 particles synthesized without {(a), (c)} and with {(b), (d)} SDS surfactant. (c) and (d) high magnification image. Next, the positive charge VO2+ will be settled down on the negative charge C12H25SO4 –, creating crystal seeds [17-21]. Then, the nucleation of VOC2O4 will take place and VOC2O4 seed will grow into nanowires on the spherical micelles template. Finally, VOC2O4 nanowires will be converted to VO2 under high temperature and pressure hydrothermal condition. The reaction formation process for micro/nano urchin-like VO2 can be illustrated as follows [21]. (1) V2O5 + 3H2C2O4 ↔ 2VOC2O4 +3H2O +2CO2 (2) V2O5 + H2C2O4 ↔ (VO)2C2O4 + H2O (3) (VO)2C2O4 + 2H2C2O4 ↔ 2VOC2O4 +2H2O +2CO2 (4) 2VOC2O4 ↔ VO2 + 3CO2 + C Figure 2 (a) and (b) shows the typical XRD patterns of the VO2 particles processed with and without the use of SDS during hydrothermal, respectively. The VO2 particles synthesized with SDS surfactants showed a relatively strong peak at 2θ = ~ 15,6o 25,4o 29,1o 45,1o 49,4o 59,2o corresponding to the (200) (110) (002) (601) (020) (711) plane. All of the peak can be indexed to the crystalline VO2 (B) structure (JCPDS 81-2392), Fig. 2 (a). On the other hand, the VO2 particles synthesis without SDS surfactants, the peak at 25,4o and 49,4o was shifted to longer angle and their intensity was decreased, Fig. 2 (b). These results indicate that the VO2 particles synthesized with the use of SDS surfactants display an improving the crystallinity due to the preferential nuclear growth in the hydrothermal process. On the basis of these findings, the micro/nano urchin-like VO2 particle synthesized with the use of SDS surfactant was used for further characterizations. N.T. Manh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 3 (2019) 62-68 65 Figure 2. XRD patterns of the VO2 particles (a) with and (b) without the use of SDS surfactant. Figure 3 shows the Raman spectrum of micro/nano urchin-like VO2 particles synthesized by hydrothermal with the use of SDS surfactant. As shown in Fig. 3, two spectrums, peaks at 283 cm-1 and 405 cm -1 correspond to flexural modes of V2-O and V-O. The spectrum is in the range of 400-600 cm-1 is related to bridging modes of V2-O and V3-O. The Raman peak at 692 cm -1 corresponds to the stretching vibration mode of V2-O. The peak appears at  1000 cm-1 is assigned to stretching mode of V = O [16]. All the Raman peaks correspond to the characterization mode of VO2 (B) without any evidence of impurities, indicating that VO2 (B) has been synthesized successfully. Figure 3. Raman spectra of micro/nano urchin-like VO2 particles. The typical UV-Vis absorption spectra of (the bare MB solution, VO2 nanorods and micro/nano urchin-like VO2 particles synthesized without and with the use of SDS are shown in Figs. 4. Bare MB has a strong absorption peak at  660 nm and one weak peak at  630 nm. Compared to the bare MB, lower adsorption intensity was observed for the VO2 particles at the same MB concentration, demonstrating the effective the decolorization of MB. However, it should be noted that the adsorption intensity of micro/nano urchin like VO2 particles synthesized by the use of SDS surfactants was much 66 N.T. Manh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 35, No. 3 (2019) 62-68 lower than that of the VO2 nanowires synthesized without SDS surfactants by a factor of ~2.1. This improvement of MB decolorization was mainly attributed to the achievement of micro/nano urchinlike VO2 structure which is possessed highly contacting area for MB adsorption. Figure 4. UV-Vis spectra showing the MB decolorization of the bare MB, VO 2 nanorods and micro/nano urchinlike VO2 particles. In general, when the particles size of material is enough small, the specific surface area increases and adsorption efficiency increases. However, small particles size of materials will be suspended in the solution after the adsorption process which making the recovering of materials is difficult. Hybridization between adsorbent and iron oxide (GO – Fe3O4 nanohybrid) can recover material by external magnetic field after the adsorption process [22]. This technique can be eliminated ~ 100% of adsorbent materials, but synthesis of nanohybrid materials is quite complicated which limiting its application. Therefore, the micro/nano material has a special structure, which does not require strict processing procedures, high efficiency of adsorption, easy recovery of materials after adsorption is currently an interesting research area. In this work, the micro/nano urchin-like VO2 particles can be deposited at the bottom of the adsorption vessels after the adsorption processes, Fig. 5b. The micro/nano urchin-like VO2 particles can be collected completely after centrifuging and transparent solution is observed, Fig. 5d. This is considered as one of the advantages of micro/nano urchin-like VO2 material compared to the other material. Therefore, micro/nano urchin-like VO2 particle is a promising material for MB contaminated water treatments. Figure 5. Methylene blue adsorption process of VO2. 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