Fabrication and Characterization of VO2 Nanoparticles: A Simple and Low Cost Combustion Method

Abhishek Kumar, F. Rahman, Afroz Khan*Department of Physics, Aligarh Muslim University, Aligarh, UP-202002, India

* Corresponding author: akhan@myamu.ac.in

Abstract: Present study focuses on the fabrication and structural & optical properties of VO 2 nanoparticles prepared by combusting the least cost precursor solution method comprising of NH4VO3, C2H6O2 and C2H5OH. The X-ray diffraction (XRD) with Fourier transform infrared (FTIR) analysis affirms that the prepared monoclinic VO2 is a solitary crystal without any extra contaminated phases. Optical properties of the sample have been measured using ultraviolet-visible spectroscopy (UV-Vis) and photoluminescence (PL) spectroscopy. The optical band gap of VO 2 sample is found to be 3.77 eV. Further, the existence of oxygen vacancies or defect or emission of radiation from intermediate energy levels are signified the green-yellow emission in the material. This could open a powerful method to synthesize a low cost and large scale VO2 (M) nanoparticles for their utilizations in different fields.

INTRODUCTIONVanadium (IV) oxide is a man-made mineral having the chemical formula VO 2 having superior physio-chemical

properties. VO2 is a very remarkable material having metal-insulator (MIT) transition. At transition temperature (T c

= 340 K), VO2 possesses monoclinic (M) crystal structure [1]. Above Tc, the structure changes to tetragonal which has a configuration like rutile TiO2 and temperature below Tc (340K) is called monoclinic transition temperature. VO2 having a metal to semiconductor transition where metallic nature having the rutile phase and semiconducting nature having the monoclinic phase [2]. The metal-insulator transition in VO2 is considered to be of Peierls type or Mott-Hubbard type, involving the electron-phonon and electron-electron interactions, respectively [3]. Probably it has applications in conversion of residual heat from engines and machines into electricity or windows which kept buildings cool [4]. By changing the substrate materials and reshaping the vanadium oxide coating with the use of doping, stretching and other operations, changing wavelength and temperature which attributes the thermal impacts [5]. There are many important applications viz. thermal properties include passive camouflage, thermal beacons and communication or to purposefully accelerate or hinder the cooling [6]. VO2 also acts as an exceptionally fast optical modulators, infrared modulators for missile guidance systems, cameras, data storage etc. The thermochromic phase can have transition between the transparent semi-conductive and reflective conductive phase, which occurs at (341 K) and it happen in times as little as 100 femtoseconds [7]. Below Tc, VO2 is transparent to Infrared radiation while it completely reflects IR above Tc and attributes its uses in thermochromic windows which brilliantly regulates IR transmittance and thus VO2 in the lists of energy saving substances.

In this present work, we fabricated the pure VO2 nanocrystalline powder by combusting the least-expense antecedent solution of NH4VO3, C2H6O2 and C2H5OH. The structural properties of VO2 examined by utilizing XRD and FTIR and their optical properties are measured using PL and UV-Vis Spectroscopy.

EXPERIMENTAL TECHNIQUES

Synthesis

The nanocrystalline pristine VO2 powder was synthesized by combusting the low cost precursor solution of NH4VO3, C2H6O2 and C2H5OH respectively. Firstly, mixture of 2.0 g powder of ammonium metavanadate (NH4VO3) and 75 mL of ethylene glycol was heated in a beaker at 30-70°C with vigorous stirring and after obtaining a yellow solution at room temperature. The equivalent volume of ethanol was mixed to the above arrangement and was stirred for extra 30 minutes at the room temperature. Hence the acquired solution was put inside evaporating dish and combusted legitimately into the air on heating plate. Final obtained black blue colored crystalline flakes of monoclinic VO2 was observed and grounded it into the powder form as illustrated in Figure.1.

Figure.1: Schematic representation of the formation of VO2 (M) nanoparticles

Characterization

XRD spectrum of unadulterated VO2 (M) was recorded on Rigaku X-ray diffractometer utilizing radiation (Cu-Kα, λ= 1.54 Å) with the 10° to 80° having the step-size of 0.02 ° and rate of examining at 4° per minute. FT-IR spectrum was obtained using the Perkin Elmer Spectrum-2 FT-IR spectrophotometer having the wave number range 400-4000 cm-1 by utilizing KBr pellets. UV-Vis absorption spectrum has been performed on Perkin Elmer Lambda-36 UV-Vis Spectrophotometer and Perkin Elmer LS-55 fluorescence spectrophotometer was utilized for the emission spectrum estimation.

RESULTS AND DISCUSSION

Structural Analysis

XRD pattern of pristine VO2 nanocrystalline powder is shown in Figure 2(a). The characteristic diffraction peaks exhibit the monoclinic unit cell structure of pure VO2 (M) as reported in JCPDS card no. 82-0661 with space group p21/c [8]. Lattice parameter refined by PowderX software of the unit cell are found to be a = 4.5968 Å, b = 5.6844 Å and c = 4.1933 Å respectively. The average crystallite size of the sample that was obtained to be about ~34nm

was calculated using Debye-Scherrer formula (D= 0.9 λβcosӨ

) [9,10], where, crystallite size is expressed as D,

wavelength as λ, β is the FWHM of the diffracted peaks and θ is the diffraction angle.

The results from FTIR spectrophotometer as shown in Figure 2(b) confirm the composition of the VO2 (M) nanocrystalline material. In the typical IR spectrum of monoclinic VO2 sample, at 3432 and 1592 cm-1 are the characteristic peaks which is related to stretching and bending mode of water molecule. Peak positioned at 1375 cm -

1 indicated the vibration mode of C=O. Peak positioned at 1005 cm-1 is identified to be intrinsic for VO2 (M) which are attributed to the V-O-V octahedral bending, deformation modes, the coupled vibration of V=O and V-O-V as well as the stretching of short V = O respectively. Absorption bands focused at 451 and 546 cm -1 can be credited to the long range stretching vibration of V-O-V bond [11].

Figure. 2: XRD spectrum (a) and FTIR spectrum (b) of nanocrystalline VO2 (M) powder

Figure. 3: UV-Vis absorption spectra (a); inset: Tauc plot and PL spectrum (b) of nanocrystalline VO2 (M) powder

Optical Analysis

The absorption spectra of pure VO2 (M) is shown in Figure 3(a). The absorption peak is found between 200 to 300 nm. The keen characteristic absorption peak is observed around 250 nm indicating the presence of good crystalline and contamination stifled VO2 nanostructure. As in equation 1 of Tauc plot we can calculate the optical band gap of the pure sample as demonstrated in the inset of Figure 3(a) [12].αhυ = A(hυ-Eg)1/2 (1)

Here, α=2.303 × At

refers to the absorbance coefficient (A is the absorbance and t is the thickness of the

cuvette), hν is the energy of incident radiation in eV and the optical band gap is denoted as Eg. Thus, linear extrapolation of the intercept on x-axis of the graphical representation between (αhυ) 2 and hυ provides us the optical band gap of the given sample which is seen as 3.77 eV. This wide band of the sample might be because to the less moderate energy levels and average crystallite size of the sample.

Photoluminescence (PL) is an extremely viable and valuable method to consider optical properties of the samples. It manages the transition from the excitation state (conduction band) to the ground state (valence band) and examining the electronic structure of the sample. The PL spectrum of the pristine VO2 nanocrystalline material which unequivocally depicts the imperfection induced emission in Figure 3 (b). The emission peak is found at 600 nm with excitation peak at 300 nm which reveals the green-yellow emission. The peaks observed in the emission spectra are due to the emission of radiation from intermediate energy level or defects or oxygen vacancies i.e. present between the conduction band and the valence band [9].

CONCLUSIONVO2 nanoparticles were successfully synthesized using low cost combustion method. The XRD patterns confirm

the single phase monoclinic structure of VO2 nanoparticles with average crystallite size ~34 nm. FTIR spectrum indicated the symmetric, asymmetric and bending vibrations of the VO2 nanoparticles. The emission peak found at 600 nm of PL measurement confirm the presence of the defects or oxygen vacancies between the conduction band (CB) and valance band (VB), also attributes the signal of green-yellow emission.

REFERENCES[1] R. Heckingbottom, J.W. Linnett, Structure of vanadium dioxide, Nature. 194 (1962) 678. doi:10.1038/194678a0.[2] T.D. Manning, I.P. Parkin, M.E. Pemble, D. Sheel, D. Vernardou, Intelligent Window Coatings: Atmospheric Pressure

Chemical Vapor Deposition of Tungsten-Doped Vanadium Dioxide, Chemistry of Materials. 16 (2004) 744–749. doi:10.1021/cm034905y.

[3] A. Dey, M.K. Nayak, A.C.M. Esther, M.S. Pradeepkumar, D. Porwal, A.K. Gupta, P. Bera, H.C. Barshilia, A.K. Mukhopadhyay, A.K. Pandey, K. Khan, M. Bhattacharya, D.R. Kumar, N. Sridhara, A.K. Sharma, Nanocolumnar Crystalline Vanadium Oxide-Molybdenum Oxide Antireflective Smart Thin Films with Superior Nanomechanical Properties, Scientific Reports. 6 (2016) 1–18. doi:10.1038/srep36811.

[4] J. Laverock, A.R.H. Preston, D. Newby, K.E. Smith, S. Sallis, L.F.J. Piper, S. Kittiwatanakul, J.W. Lu, S.A. Wolf, M. Leandersson, T. Balasubramanian, Photoemission evidence for crossover from Peierls-like to Mott-like transition in highly strained VO 2, Physical Review B - Condensed Matter and Materials Physics. 86 (2012) 1–5. doi:10.1103/PhysRevB.86.195124.

[5] M.A. Kats, R. Blanchard, S. Zhang, P. Genevet, C. Ko, S. Ramanathan, F. Capasso, Vanadium dioxide as a natural disordered metamaterial: Perfect thermal emission and large broadband negative differential thermal emittance, Physical Review X. 3 (2014) 1–7. doi:10.1103/PhysRevX.3.041004.

[6] M. Chemistry, I. Parkin, Intelligent Window Coatings that Allow Light In but Keep Heat Out – News Item, (2004) 1–3. http://www.azom.com/details.asp?ArticleID=2587.

[7] O. Ridge, Timing natures fastest optical shutter, (2005) 1–3.[8] F.J. Morin, Oxides which show a metal-to-insulator transition at the neel temperature, Physical Review Letters. 3 (1959)

34–36. doi:10.1103/PhysRevLett.3.34.[9] A. Khan, F. Rahman, A. Ahad, P.A. Alvi, Investigation of transport phenomenon and magnetic behavior of Fe doped

In2O3, Physica B: Condensed Matter. 592 (2020) 412282. doi:10.1016/j.physb.2020.412282.[10] A. Khan, F. Rahman, R. Nongjai, K. Asokan, Structural, Optical and Electrical Transport Properties of Sn Doped In

2O3, Solid State Sciences. 109 (2020) 106436. doi:10.1016/j.solidstatesciences.2020.106436.[11] A. Lindgreen, A. Lindgreen, FTIR study of vanadium oxide nanotubes from lamellar structure, Journal of Business

Ethics. 51 (2004) 31–39. doi:10.1023/B.[12] A. Khan, F. Ameen, F. Khan, A. Al-Arfaj, B. Ahmed, Fabrication and antibacterial activity of nanoenhanced conjugate

of silver (I) oxide with graphene oxide, Materials Today Communications. 25 (2020) 101667. doi:10.1016/j.mtcomm.2020.101667.