V2O5 nanowires: Hydrothermal synthesis and characterization

Abstract. V2O5 nanowires were synthesized via controlled hydrothermal method. The morphology and nano-structure of synthesized samples were characterized by Field-Emission Scanning Electron Microscopy (FE-SEM), Raman spectroscopy and X-ray diffraction (XRD). The results showed that a pure crystalline phase of V2O5 was received. The average size of the obtained V2O5 nanowires was defined as approximately 100 nm in diameter and 4 µm in length. In addition, the absorption spectrum of the sample exhibited a distinctive band edge at about 590 nm (2.14 eV). The V2O5 nanowires exhibited the high photocatalytic activity of 64 % for the photodegradation of methylene blue after 90 min irradiation under ultraviolet light.

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74 HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2017-0057 Chemical and Biological Science 2017, Vol. 62, Issue 10, pp. 74-80 This paper is available online at V2O5 NANOWIRES: HYDROTHERMAL SYNTHESIS AND CHARACTERIZATION Doan Tuan Anh, Vu Thi Thai Ha, Do Thi Anh Thu and Nguyen Trong Thanh Institute of Materials Science, Vietnam Academy of Science and Technology Abstract. V2O5 nanowires were synthesized via controlled hydrothermal method. The morphology and nano-structure of synthesized samples were characterized by Field-Emission Scanning Electron Microscopy (FE-SEM), Raman spectroscopy and X-ray diffraction (XRD). The results showed that a pure crystalline phase of V2O5 was received. The average size of the obtained V2O5 nanowires was defined as approximately 100 nm in diameter and 4 µm in length. In addition, the absorption spectrum of the sample exhibited a distinctive band edge at about 590 nm (2.14 eV). The V2O5 nanowires exhibited the high photocatalytic activity of 64 % for the photodegradation of methylene blue after 90 min irradiation under ultraviolet light. Keywords: V2O5, nanowires, hydrothermal, photocatalysis. 1. Introduction Advanced applications for chemical gas sensors require a development of novel materials with extremely sensitivity and selectivity. It has been proved that, with the large ratio of surface area to volume, the nanostructured metal oxide-based semiconductors, such as nanoparticles, nanorods, nanofibers and nanotubes are absolutely suitable for these purposes [1]. Among these materials, one-dimensional (1D) nanostructures have attracted considerable interest over the years owing to its wide range of applications such as chemical gas sensors and photocatalysis, water-splitting [2-4]. V2O5 nanowires are n-type semiconductors with a very high electric conductivity at room temperature especially, charge transport proceeds via electron hopping between V 4+ and V 5+ centers. This makes one-dimensional vanadium pentoxide nanostructures are suitable for construction of functional materials or novel devices. For the synthesis of these materials, the hydrothermal route seems to be one of the most suitable methods, as in the case of V2O5 nanowires. A benefit of this method is that one obtaining very pure products at low temperature condition. In this work, we report the Received November 28, 2017. Revised December 10, 2017. Accepted December 17, 2017. Contact Doan Tuan Anh, e-mail address: anhdt@ims.vast.ac.vn V2O5 nanowires: Hydrothermal synthesis and characterization 75 structure and photocatalysis characteristics of V2O5 nanowires, which were prepared under hydrothermal conditions. 2. Content 2.1. Experiments 2.1.1. Preparation of V2O5 nanowires by hydrothermal method Vanadium pentoxide nanowires were synthesized by hydrothermal process using V2O5, 25 wt.% NH4OH solution, 65 wt.% HNO3 solution and KMnO4 as precursors. A general flowchart for hydrothermal process is shown in Figure 1. Figure 1.Flowchart of hydrothermal process of V2O5 nanowires Recently, hydrothermal method is used by many groups to prepare V2O5 material with nanometer-sized. However, there are many different types of reactions in the hydrothermal process such as V2O5 with “L” shape were formed in reaction of VOSO4.xH2O and KBrO3 in HNO3 [5], V2O5 nanowire were formed from the reaction of commercial NH4VO3 and HNO3 [6] In our case, V2O5 nanowire were formed by reaction of V2O5.xH2O with KMnO4 and HNO3. In three cases above, the formation mechanism of nanowire is essentially the same, but in our case the layer structures of V2O5.xH2O will be formed when it reacts with KMnO4 oxidizing agent. This formed structure is advantage conditions for the formation of nanowires in hydrothermal process. As shown in Fig. 1: first, commercial V2O5 powders were dispersed in distilled water and stirring for 10 minutes at room temperature. Then, 5 mL of 25wt.% NH4OH solution was added and stirred for 30 minutes to form NH4VO3. Then, 5ml of 65wt.% HNO3 solution was added and stirred for 30 minutes. The obtained mixture was aged for 24 hours at room temperature. The V2O5.nH2O precipitate was separated out of the mixture. The mixture of V2O5.nH2O and KMnO4 in a molar ratio of 1.5:1 was stirred for 15 min. Amount of HNO3 solution appropriately was then added droppings to the slurry solution under stirring. This final mixture was poured into a Teflon-lined autoclave and maintained under hydrothermal conditions at 190 o C for 48 hour. After autoclaving, the product was Doan Tuan Anh, Vu Thi Thai Ha, Do Thi Anh Thu and Nguyen Trong Thanh 76 collected and washed several times with distilled water and ethanol and finally dried at 60 o C for 48 hour. The V2O5 with film form were used for measuring optical absorption. These films on glass substrate were prepared by drop-coating technique using solution of dispersed-V2O5 in toluene. 2.1.2. Characterization The characterization of synthesized samples were performed by X-Ray diffraction (XRD) using Bruker D8 analyzer and Raman spectroscopy using XploRA™PLUS Raman microscope. A Field-emission scanning electron microscope (FE-SEM, S4800- Hitachi) is used to study the surface morphology. The optical absorption spectra were recorded using Jasco V670 spectrometer with scanning from 400 to 800 nm, which allows to definite band gap of V2O5. The photocatalytic performance of the samples was tested by photodegradation process of Methylene blue (MB) under ultraviolet irradiation (365 nm) from Hg long arc lamp - 100W. Briefly, 5 mg of synthesized V2O5 was suspended in 150 mL of MB (10 -4 g/L) and stirred in dark for 30 min to establish the adsorption/desorption equilibrium and then the solution was illuminated by UV radiation with power density of 10 mW/cm 2 . At each regular interval (30, 60, 90 min), 5 mL of the solution was withdrawn to measure optical absorption spectra after removing the nanoparticles by centrifugation technique. The photocatalytic activity of sample was evaluated base on the decreasing of MB concentration in solution after each illumination period through its absorption spectra. 2.2. Results and discussion 2.2.1. Structure and morphology characterization The XRD patterns (Fig. 2) showed that the V2O5 sample was well crystallized. These diffraction peaks could be well indexed as the Shcherbinaite (orthorhombic) V2O5 phase (ICSD No. 1#41043) and the trace of other phases was not observed. Figure 2. XRD diagram of V2O5 nanowires obtained by hydrothermal method To further investigate the microstructure of synthesized V2O5 sample, the Raman spectrum was measured and illustrated in Fig. 3. The peaks located at 143, 282, 700 and 993 cm −1 could be indexed to the energy of characteristic vibrations of the V2O5 crystals. V2O5 nanowires: Hydrothermal synthesis and characterization 77 Figure 3. Raman spectra of V2O5 nanowires synthesized by hydrothermal method The peak at 700 cm -1 is assigned to the stretching energy of doubly coordinated oxygen group (V2-O). The peak at 526 cm -1 is assigned to the triply coordinated oxygen (V3-O) stretching mode. The two peaks located at 402 and 282 cm -1 are assigned to the bending vibration of the V=O bonds. The peaks located at 482 and 302 cm -1 are assigned to the bending vibration of the bridging V-O-V (doubly coordinated oxygen), and the triply coordinated oxygen (V3-O) bonds, respectively. Two low frequency peaks at 195 and 143 cm -1 are assigned to the stretching mode of (V2O2)n. On the other hand, the peak at 993 cm -1 is corresponding to the stretching of vanadium atoms connected to oxygen atoms through double bonds (V=O) [5]. This value is also characteristic vibration energy for the layer-type structure of V2O5 nanowires. Moreover, the FE-SEM images indicated the average diameter and length of the V2O5 nanowires was about 100 nm and 4µm, respectively (Fig. 4). Figure 4. FE-SEM image of V2O5 nanowires synthesized by hydrothermal method at 190 o C for 48 h The formation mechanism of V2O5 nanowires can be explained as follows: The V2O5 commercial powder is used as a precursor. Under the condition pH ~ 8, which was adjusted by NH4OH, NH4VO3 were formed. When HNO3 was added into the solution, NH4VO3 reacted with acid molecule to form V2O5.nH2O. The reaction scheme can be expressed as follow: Doan Tuan Anh, Vu Thi Thai Ha, Do Thi Anh Thu and Nguyen Trong Thanh 78 OHnNHOnHOVVONHOV HOH 242523452 )1(2.2   It is indicated that V2O5 can be regarded as a layered structure in which VO5 square pyramids are connected by sharing corners and edges [6]. The interactions between these layers are rather weak, as indicated by the exceptionally long V – O distance of 0.279 nm [7]. In particular, this structure permits H2O molecules to be embedded between the layers without a far - reaching restructuring, which leads to the formation of the V2O5.nH2O phase. In the study of Kumar et al. [8] the interlayer spaces in the layered structure of V2O5.nH2O are occupied by H3O + ions, and thus the interactions between the layers are weaken and after the heat treatment, the layered structure of V2O5.xH2O gradually splits to form anhydrous V2O5 nanowires. In our synthetic route, the strong oxidant KMnO4 oxidized the VO 2+ cations in the solution into small V2O5 nano-particles. Since the layered structure of V2O5 was favorable for the intercalation of H2O molecules, the initially formed V2O5 nanoparticles had a strong inclination to have H2O molecules embedded between their layers and transformed into the V2O5.nH2O phase. Meanwhile, the layered structure of vanadium oxides and their derivatives was favorable for the development of 1D nano-structures [9, 10]. In the prolonged hydrothermal treatment, the H2O molecules were all removed from the interplanar regions of the layers in V2O5.nH2O, leading to the formation of the orthorhombic V2O5 phase [11]. 2.2.2. Optical band gap of V2O5 nanowires In general, the optical band gap of V2O5 can be determined by using Tauc’s relation between the absorption coefficient (α) and the incident photon energy (hν) in the high absorption region of semiconductor, as follows: αhν = A(hν - Eg) n where A is a constant and sometimes called the band tailing parameter and it is an energy independent constant, Eg is the optical energy gap, which situated between the localized states near the mobility edges. n is the power factor of the transition mode. The n values for direct and indirect band gap are n = 1/2, 2, respectively. UV-Vis absorption spectra of V2O5 nanowires were measured and illustrated in Fig. 5. The spectra exhibit a strong absorption band with a steep edge in visible range less than 575 nm, this can be attributed by band-to-band transitions. Figure 5. The absorption spectra of V2O5 nanowires film V2O5 nanowires: Hydrothermal synthesis and characterization 79 Inset demonstrates the absorption curve plotting the (αhυ)2 versus hυ The band structure of V2O5 can be characterized as oxygen 2s and 2p like bands separated by a gap from the vanadium 3d like conduction bands. The band gap between them is indirect. Plot of (αhν)2 versus hν from the spectra data are presented in inset of Fig. 5. The estimated Eg from the intercept of the tangents to the plots is 2.14 eV, which is consistent with reference [8]. 2.2.3. Photocatalyic activities V2O5 materials with nano-structure is very commonly used in catalysis because of its’ ability to “give” and “take” oxygen very easily. This process can occur by stimulation of appropriate photon energy. In order to test the photocatalytic activities of the V2O5 nanowires, methylene blue is chosen as a probe solution for this property. Details of experiment are described in experimental part. UV–Vis absorption spectra of methylene blue aqueous solution at 0, 30 and 90 min in the presence of V2O5 nanowires photocatalysts and commercial V2O5 are shown in Fig. 6. It revealed the strong photocatalytic activity of synthesized sample towards MB, with the degradation efficiency of MB is about 64% in 90 min. while that of commercial V2O5 is less than 10 % for 120 min. It can be suggested that the morphology of the catalyst plays a vital role in the photocatalystic process. In details, microstructure of studied sample like specific surface area and complicated structure might facilitate the diffusion and mass transportation of MB molecules and defects in photochemical reaction. Moreover, surface defects or acceptor centers also play an important role in promoting the photocatalytic activities, which are is prevented and subsequent redox reactions may occur [12]. Figure 6. UV–Vis absorption spectra of MB aqueous solution at different times in the presence of V2O5 nanowires photocatalysts and (lower) commercial V2O5 Inset demonstrates the dependence on exposed-UV time of absorption intensity of methylene blue 3. Conclusion V2O5 nanowires with about 100 nm in diameter and 4µm in length were successfully synthesized by hydrothermal method. The X-ray diffraction result showed that the V2O5 nanowires existed in the orthorhombic phase. The V2O5 nanowires exhibited a high photocatalytic activity with the methylene blue photodegradion rate of 64% after 90 min Doan Tuan Anh, Vu Thi Thai Ha, Do Thi Anh Thu and Nguyen Trong Thanh 80 irradiation under UV light. The band gap energy of 2.14 eV of these V2O5 nanowires can be also suggested that photocatalytic potential with photons in the visible region. Acknowledgement. This work was financially supported by the Institute of Materials Science, Vietnamese Academy Science and Technology. REFERENCES [1] Y. Joseph, I. 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