Vanadium oxides (V₂O₅) nanoparticles have been prepared using a simple chemical method by sodium metavanadate as precursor and Cetyl trimethylammonium bromide (CTAB) as surfactant. The samples were characterized by high resolution transmission electron microscopy (HRTEM), field effect scanning electron microscopy (FESEM) and X-ray diffraction (XRD). As there are many forms of vanadium oxides produced during this process, x-ray diffraction (XRD) technique was used to identify V₂O₅ phases. The size of as-prepared nanoparticles was around 5 nm and average diameter of annealed one was around 10 nm. The effect of CTAB surfactant on the particle morphology showed that the size of particles reduce to 10 nm in presence of CTAB surfactant. FTIR spectrum showed the presence of V-O and V-O-V stretching mode.

Keywords: V₂O₅, Nanocrystals, CTAB, Chemical synthesis, Surfactant

Nano-materials have unique physical properties that have attracted more and more attention as a cathode in rechargeable ion batteries and selective gas sensors such as ammonia because of their high surface area and redox activity.^1-3 Biological activity of vanadium pentoxide nanomaterial depends on factors such as the type of the derivative, manner of its administration, dose, length of treatment, and also individual- and species-specific sensitivity to the administered compound.⁴ V₂O₅ nanomaterial is amphoteric in nature. Vanadium is correlated to its degree of oxidation (vanadyl\vanadate ion) and chemical form (organic\inorganic ligand).^5-7 The existence of the various vanadate species depends on the pH and on the total concentration of vanadium. Their occurrence can be accounted for condensation equilibrium; it is evident that only in very dilute solutions are monomeric vanadium ions found, and increases in concentration, particularly if the solution is acidic, lead to polymerization.^8-10 Vanadium oxygen systems (V₂O₅, VO₂) are prototype strongly-correlated materials that have been widely-studied by theoretical and experimental condensed-matter and materials community for more than half a century .¹¹ Vanadium oxide is a well-known catalyst among various metal oxides, and so many fundamental studies have been developed wide-spreadingly centering on catalytic oxidation.¹² They show metal-semiconductor transition, which implies an abrupt change in optical and electrical properties .¹³ That is why this oxide is used in thermal sensing and switching. Vanadium pentoxide based materials are known to display several types of chromogenic effects, as a window for solar cells and for transmittance modulation in smart windows with potential applications in architecture, automotives and nanomedicine.¹⁴ It shows an atypical behaviour because it cannot be defined exactly either as a cathodically or as anodically colouring material. V₂O₅ exhibit multi-colored electrochromism allowing the use in electrochromic (EC) displays color filters and other optical devices .¹⁵

Transition metal oxides have been a subject of research in recent years in view of their fundamental and technological aspects. Among these, vanadium creates many compounds with oxygen; these have different structural, optical and chemical properties. Meaningful differences between the properties of different phases of vanadium oxides like VO, VO₂, V2O3 and V₂O₅ depend on their structure, which determines other properties.^16,17 Different forms of vanadium oxides can be obtained by changing the deposition process parameters, or by post-process treatment, e.g., additional annealing.¹⁸ From the application point of view, the most interesting vanadium oxides are VO₂ and V₂O₅. Vanadium dioxide is a very good candidate for thermo chromic coatings due to the change of properties from semiconducting to semimetal at 68°C. Vanadium pentoxide (V₂O₅) is a thermodynamically stable form which exhibits electrochromic properties. V₂O₅ thin films can also be used in optical filters, reflectance mirrors, smart windows and surfaces with tunable emittance for temperature control of space vehicles ¹⁹. It can be received by selecting deposition parameters or by the annealing of VO₂ above 350°C ²⁰. In this article, vanadium oxide nanoparticles are fabricated by using sol-gel method. Structural and surface morphological properties have been studied.

The samples were synthesized by chemical synthesis according to the following manner. At First, 0.1g sodium metavanadate was completely dissolved in 100 mL pure water with stirring at room temperature. Ammonium chloride (1.2g) was then added to the solution until dissolve completely. The color of solution changed from muddy color to transparent color which after a few minutes changed to fuggy color. After 10 minutes, 0.5g, Cetyl trimethylammonium bromide (CTAB) was added to the solution and synthesis temperature was increased to 80oC. The color of solution changed from orange color to dark brown color. The pH was between 6 and 8 during the synthesis. After one hour, the color of solution changed to transparent yellow color. The product were evaporated for 2 hours, cooled to room temperature and finally calcined at 600oC for 4 hours. All analyses were done for samples without any washing and more purification. The specification of the size, structure and optical properties of the as-synthesis and annealed vanadium oxide nanoparticles were carried out. X-ray diffractometer (XRD) was used to identify the crystalline phase and to estimate the crystalline size. The XRD pattern were recorded with 2θ in the range of 4-85o with type X-Pert Pro MPD, Cu-Kα: λ = 1.54 Å. The morphology was characterized by field emission scanning electron microscopy (SEM) with type KYKY-EM3200, 25 kV and transmission electron microscopy (TEM) with type Zeiss EM-900, 80 kV. All the measurements were carried out at room temperature.

Figure 1a shows the XRD pattern of aluminium oxide before annealing. Figure 1b shows the X-ray diffraction patterns of the powder after heat treatment at 600oC for 4 hours. The XRD patterns showed this sample have sharp peaks with (101), (400), (011), (301), (411) and (002) diffraction planes, are in accordance rhombohedral structure of the V₂O₅ phase. The mean size of the ordered V₂O₅ nanoparticles has been estimated from full width at half maximum (FWHM) and Debye-Sherrer formula according to equation the following:

$D=\frac{0.89\lambda }{B\cos \theta }$ (1)

where, 0.89 is the shape factor, λ is the x-ray wavelength, B is the line broadening at half the maximum intensity (FWHM) in radians, and θ is the Bragg angle. The mean size annealed V₂O₅ nanoparticles was around 10 nm from this Debye-Sherrer equation

Figure 1 XRD analyses of (a) as-synthesized (b) annealed V₂O₅ nanoparticles at 600^oC.

In the next step, SEM analysis was used for the morphological study of nanoparticles of V₂O₅ samples. These analyses show the nanoparticles are appeared in the samples by increasing annealing temperature. Figure 2a shows the SEM image of the as-prepared V₂O₅ grains with formation of clusters. Figure 2b shows the SEM image of the annealed V₂O₅ nanoparticle at 600oC for 4 hours. The smallest diameter of V₂O₅ nanoparticles formed was about 10 nm.

TEM analysis was carried out to confirm the actual size of the particles, their growth pattern and the distribution of the crystallites. Figure 3 shows the as-synthesized TEM image of V₂O₅ nanoparticles prepared by sol-gel route. The uniform structure of the vanadium oxide was formed in the range size of 5-10 nm.

Figure 2 SEM images of the (a) as-prepared V₂O₅ (b) annealed one at 600^oC.

Figure 3 TEM image of the as-prepared V₂O₅ sample.

FTIR spectra of the samples were analyzed in the range of 400-4000 cm^-1 wave number which identifies the chemical bonds as well as functional groups in the compound. The large broad band at 3135 cm^-1 and 3037 are ascribed to the O-H and C-H groups. The absorption picks around 1401 cm^-1is due to the bending vibration of C=C vibration. FTIR spectra of V₂O₅ nanoparticles exhibited three characteristic vibration modes: V=O vibrations at 967 cm^-1, the V-O-V symmetric stretch around 531 cm^-1 and the V-O-V asymmetric stretch at 730 cm^-1 (Figure 4). As clearly seen, the bands appearing, between 950 and 1020 cm^-1 were assigned to a vanadly stretching modes (δ V-O). Bands between 700 and 900 cm^-1 were ascribed to the bridging V-O-V stretching.

Figure 4 FTIR spectrum of as-growth V₂O₅ sample.

Vanadium oxide nanoparticles were successfully prepared using simple chemical method by sodium metavanadate as precursor and CTAB as surfactant. XRD spectrum shows rhombohedral structure of V₂O₅ annealed at 600oC. From SEM images, it is clear that with increasing temperature the morphology of the particles changes to nanoparticle shaped and the size of particles decrees to 10 nm. TEM image exhibits that the uniform as-synthesized V₂O₅ nanoparticles in the range size of 5-10 nm. Finally FTIR spectrum shows the presence of V-O and V-O-V stretching mode of V₂O₅.

The authors are thankful for the financial support of varamin pishva branch at Islamic Azad University for analysis and the discussions on the results.

None.

1. WS Huang, BD Humphrey AG MacDiarmid et al.Polyaniline, a novel conducting polymer. Morphology and chemistry of its oxidation and reduction in aqueous electrolytes. Journal of the Chemical Society. 1986;82(8):2385‒2400.
2. R Sahay, PS Kumar, R Sridhar Electrospun composite nanofibers and their multifaceted applications. Journal of Materials Chemistry. 2012;22:12953‒12971.
3. M Sethupathy, S Ravichandran, P Manisankar Preparation of PVdF‒PAN‒V2O5 hybrid composite membrane by electrospinning and fabrication of dye‒sensitized solar cells. International Journal of Electrochemical Science. 2014;9:3166‒3180.
4. P Gill, TT Moghadam, B Ranjbar Differential scanning calorimetry techniques: applications in biology and nanoscience. J Biomol Tech. 2010;21(4):167‒193.
5. J Livage Hydrothermal synthesis of nanostructured vanadium oxides. Materials. 2010;3(8):4175‒4195.
6. GY Nagesh, KM Raj, BHM Mruthyunjayaswamy Synthesis, characterization, thermal study and biological evaluation of Cu(II), Co(II), Ni(II) and Zn(II) complexes of Schiff base ligand containing thiazole moiety. Journal of Molecular Structure. 2015;1079:423‒432.
7. GY Nagesh, BHM Mruthyunjayaswamy Synthesis, characterization and biological relevance of some metal (II) complexes with oxygen, nitrogen and oxygen (ONO) donor Schiff base ligand derived from thiazole and 2‒hydroxy‒1‒naphthaldehyde. Journal of Molecular Structure. 2015;1085:198‒206.
8. AM Evangelou Vanadium in cancer treatment. Critical Reviews in Oncology/Hematology. 2002;42(3):249‒265.
9. JL Domingo Vanadium and tungsten derivatives as antidiabetic agents: a review of their toxic effects. Biological Trace Element Research. 2002;88(2):97‒112.
10. NN Greenwood, A Earnshaw Chemistry of the Elements. (1st edn), Pergamon Press, Oxford, UK, p. 1984;1542.
11. CG Granqvist Electrochromic oxides: A unified view. Solid State Ionics. 1994;70:678 ‒685.
12. EA Meulenkamp, W van Klinken, AR Schlatmann In‒situ X‒ray diffraction of Li intercalation in sol‒gel V2O5 films. Solid State Ionics. 1999;126:235‒244.
13. J Haber, M Witko, R Tokarz Vanadium pentoxide I. Structures and properties. Applied Catalysis A: General. 1997;157(1‒2):3‒22.
14. MSR Khan, KA Khan, W Estrada Electrochromism and thermochromism of Lix VO2 thin films. Journal of Applied Physics. 1991;69(5):3231‒3234.
15. Y Fujita, K Miyazaki, T Tatsuyama On the Electrochromism of Evaporated V2O5 Films. Japanese Journal of Applied Physics. 1985;24(1):1082‒1087.
16. K Sieradzka, D Wojcieszak, D Kaczmarek et al. Structural and optical properties of vanadium oxides prepared by microwave‒assisted reactive magnetron sputtering. Optica Applicata. 2011;41(2):463‒469.
17. VS Reddy Channu, R Holze, B Rambabu Soft‒Chemical Synthesis of Vanadium Oxide Nanostructures Using 3,3’,3”‒Nitrilotripropionic Acid (NTP) as a Carrier. Soft Nanoscience Letters. 2011;1(3):66‒70. 
18. M Mousavi, A Kompany, N Shahtahmasebi et al. Study of structural, electrical and optical properties of vanadium oxide condensed films deposited by spray pyrolysis technique. Advanced Manufacture. 2013;1(4):320‒328.
19. B Vijaykumar, K Sangshetty, G Sharanappa Surface Morphology Studies and Thermal analysis of V2O5 doped polyaniline composites. International Journal of Engineering Research and Applications. 2012;2:611‒616.
20. NN Dinh, TT Thao, VN Thuc, NT Thuy Thermochromic properties of VO2 films made by RF‒sputtering. Journal of Science, Mathematics‒Physics. 2010;26:201‒206.