Determination of optical parameters of zinc oxide nanofibre deposited by electrospinning technique Accepted Manuscript Title Determination of optical parameters of zinc oxide nanofibre deposited by el[.]
Accepted Manuscript Title: Determination of optical parameters of zinc oxide nanofibre deposited by electrospinning technique Authors: H.S Bolarinwa, M.U Onuu, A.Y Fasasi, S.O Alayande, L.O Animasahun, I.O Abdulsalami, O.G Fadodun, I.A Egunjobi PII: DOI: Reference: S1658-3655(17)30007-9 http://dx.doi.org/doi:10.1016/j.jtusci.2017.01.004 JTUSCI 352 To appear in: Received date: Revised date: Accepted date: 10-11-2016 2-1-2017 3-1-2017 Please cite this article as: H.S.Bolarinwa, M.U.Onuu, A.Y.Fasasi, S.O.Alayande, L.O.Animasahun, I.O.Abdulsalami, O.G.Fadodun, I.A.Egunjobi, Determination of optical parameters of zinc oxide nanofibre deposited by electrospinning technique., http://dx.doi.org/10.1016/j.jtusci.2017.01.004 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Determination of optical parameters of zinc oxide nanofibre deposited by electrospinning technique 1,2 Bolarinwa, H S., 2,3Onuu, M U., 4Fasasi, A Y., 4Alayande, S O, 1Animasahun L O., Abdulsalami, I O, 4Fadodun O G., 6Egunjobi, I A Department of Physics, Electronics and Earth Sciences, Fountain University, Osogbo Department of Physics, University of Calabar, Calabar Department of Physics/Geology/Geophysics, Federal University Ndufu, Aliko, Ikwo Centre for Energy Research and Development, Obafemi Awolowo University Department of Chemical Sciences, Fountain University, Osogbo Department of Science Laboratory Technology, MAPOLY, Abeokuta, Nigeria Abstract Electrospun ZnO was deposited on a glass substrate from zinc acetate dihydrate (Zn(CH3COO)2.2H2O) with polyvinyl acetate (PVAc) polymer dissolved in N, N, dimethyl formamide (DMF) and annealed in the presence of oxygen until organic molecules were decomposed The resultant fibre was characterized Scanning Electron Microscope attached with Energy dispersive spectrophotometry (SEMEDS), Fourier transform infra-red (FTIR), Rutherford backscattering spectroscopy (RBS) SEMEDS and FTIR showed total decomposition of the organic precursor The mean fibre width was found to be 260 nm while fibre thickness was 460 nm XRD patterns show ZnO was corundum with the hexagonal wurtzite structure The crystallite size obtained from the Debye formula was 54 nm The optical analysis showed that the percentage transmittance increased after calcination The material band gap for this electrospun ZnO fibre was found to be 3.28 eV The material optical parameter such as dispersion energy, average oscillator strength, single oscillator strength were also calculated The optical conductivity and dielectric plot showed that the material conductivity and dielectric properties increase with increasing photon energy and increases sharply around the material energy bandgap The Urbach tail analysis of the materials shows that the materials obey the Urbach rule Therefore, n-type electrospun ZnO fibre high refractive index could be attributed to excess oxygen Keywords: Electrospinning, Calcined, Semiconductor, Nanofibre, Zinc Oxide 1.0.Introduction Zinc oxide semiconductor materials has been widely used owing to it is low cost, outstanding chemical and physical properties [1] ZnO is an n-type material having a wide band gap (3.21– 3.42 eV) at room temperature It is a group II-VI compound semiconductor having a stable wurtzite structure and large exciton bindng energy of 60meV with lattice spacing a = 0.325 nm and c = 0.521 nm It is a unique material that exhibits optical, semiconducting, pyroelectric and piezoelectric properties [2, 3] It has attracted serious research attention because of its wide applications such as light emitting diode [4, 5], solar cells [6, 7], chemical and gas sensors [8], ultraviolent (UV) light detector [9], stimulated emission with low loss and high gain [10], and transparent conducting oxide [11] Due to its wide applications, several methods including thin films and polymeric approaches had been used to deposit ZnO Some of the thin film methods used include; chemical vapour deposition [12, 13, 14], chemical bath deposition [15], laser deposition [16, 17], spray pyrolysis [18], magnetic sputtering [19], thermal oxidation [20], molecular beam epitaxy [21] and electrospraying [8] ZnO fibre on the other hand had been synthesized using template [22] and electrospinning [23, 24] methods Electrospinning method which is dated back to the work of Zeleny in 1914, was previously termed “electrostatic spinning” He found the technique to be possible for spinning polymer fibres having a small diameter [25] This method, which uses the principle of electrostatics depends on electromotive force to form fibre This method has also been described as indispensable in the scientific and economic resurgence for the developing nations [26] Electrospinnig has continued to gain serious research attention owing to its unique properties of the resultant nano/micro fibres (large surface area to the volume ratio and cost effectiveness) [27, 28] ZnO nanofibres belong to the one-dimensional group of nanomaterials which are flexible in nature, this group also includes nanotube and nanorod [29] ZnO nanofibres have demonstrated improved properties in photoconducting, semi-conducting and piezoelectric properties [30, 31] It had been reported that in the modern day optoelectronics and optoelectronics design, a good knowledge of refractive index as a function of wavelength is important in predicting the photoelectric characteristics of a device [32] Hence, an accurate knowledge of the optical and structural properties of electrospun ZnO fibres is important in view of its very wide applications in analysis and design of optical and optoelectronics devices Unfortunately, there are very large discrepancies in the optical properties of electrospun ZnO fibre reported from various studies In view of this, reliable determination of the optical properties of the electrospun ZnO fibre is still paramount In this study, we have synthesised ZnO nanofibers on glass substrate by electrospinning technique The fibres have been characterised using x-ray diffraction analysis (XRD) for structural determination, scanning electron microscope (SEM-EDX) for the morphology and the composition UV-Visible spectrophotometer and Fourier Transform Infrared Spectroscopy (FTIR) were also employed for optical parameter determination as well as vibrational and stretching mode of the chemical bonds in the samples 2.0 Materials and method Poly(vinyl acetate) (PVAc) (Mw = 500,000g (Zn(CH3COO)2.2H2O) salt ≥ 99% assay by GPC), zinc acetate dihydrate from Sigma-Aldrich and anhydrous N-N- dimethylformamide ≥ 99.8% assay from Scharlau were used as precursor materials 1g of zinc acetate dihydrate salt with 1.6g of polyvinyl acetate (PVAc) polymer were dissolved in 10 ml of anhydrous N, N, dimethyl formamide (DMF) The solution was stirred till it was homogenous The glass substrate were pre-cleaned using dilute hydrochloric (HCl) acid, ethanol and distilled water The prepared solution was fed into the spinneret The distance between the tip of the spinneret and the substrate was kept at 20 cm 12 kV voltage was applied to the solution and the substrate attached to an aluminium foil was grounded The solution was electrospun between to hours in order to have a thick fibre deposit The deposited fibres on glass substrates were calcined at 700oC in tubular furnace in the presence of oxygen The as-spun and calcined fibre were later analysed for their chemical, structural, optical and electrical properties The scanning electron microscope TESCAN model equipped with Oxford instrument X-Max (EDS) was used to study the surface morphology and elemental composition of the electrospun and annealed fibres Rutherford Backscattering analysis was carried out using the 1.7 MeV Ion Beam Accelerator at the Centre for Energy Research and Development, Obafemi Awolowo University, Ile-Ife, Nigeria to determine the elemental concentration the thickness of the deposited fibres The Fourier transform infrared (FTIR) was done using NICOLET S5 from Thermo Scientific The optical characterisation was carried out using the Stellanet UV-visible Spectrophotometer model EP2000 (UV-VIS-NIR) ImageJ was used to determine the fibre diameter The crystal structure and symmetry were analysed using Brucker D8 high resolution X-ray diffractometre (XRD) 3.0 Result and Discussion 3.1 Morpholgy Figure shows the SEM image of the ZnO nanofibre The average fibre diameter was found to reduce from 480 nm to 260 nm after calcination The deposited nanofibre were non-woven in nature and are bead free The difference in the measured mean fibre diameter observed in the asspun and calcined material could be attributed to the removal of PVAc which served as carrier for the metal oxide and other volatile inorganic constituents like acetate during the calcination process a b Figure 1.The SEM images of the ZnO nanofibre (a) as-spun (b) calcined 3.2 Elemental and chemical analysis The energy dispersive x-ray (EDX) analyses provide a way of analysing the chemical composition of materials The EDX analysis of the calcined ZnO nanofibre is shown in figure and table It is evident from the spectra that the fibres contained the desired elements The presence of silicon in the calcined samples of figure can be attributed to the glass substrate Figure Energy dispersive X-ray (EDX) for the calcined ZnO nanofibres Table Elemental composition of the calcine ZnO nanofibres determined by EDX Elements Weight% Atomic% Compound Formula Si 44.65 32.58 95.51 SiO2 Zn 3.60 1.13 4.49 ZnO O 51.75 66.29 The RBS analysis gives the specific amount of each element present and most importantly the films thickness that is necessary for some optical data analysis Previous findings have shown that compounds composition in semiconductor plays an important role in their properties, hence the need to accurately determine and control their elemental concentration [33] The result of the simulations of the RBS spectra using the SIMNRA software gave the amounts of specific elements and the thickness (in atoms/cm2 ) which was then converted to nanometre The RBS spectra for ZnO is shown in figure and the analysis is presented in table Energy [keV] 200 400 600 800 1000 1200 1400 1600 1800 130 ZnO Simulated O Na Mg Al Si K Ca Fe Zn 125 120 115 110 105 100 95 90 85 Counts 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400 410 420 Channel Figure3 Rutherford backscattering (RBS) spectrum of the calcined ZnO electrospun nanofibres Table Concentration and thickness of the calcined ZnO from RBS after simulation Elements Concentration Zn 0.136863 Bulk thickness Bulk (atm/cm2) (nm) 4.14E+17 O thickness 460.215 0.863137 The distinct nature of the Zn peak from the spectra suggests that the electrospun nanofibre adheres firmly to the substrate The RBS spectra suggest the decomposition of the starting precursor to form crystalline ZnO nanofibres.The spectra also show little or no trace of impurities from the organic constituent like carbon and hydrogen or acetate The FTIR analysis of the studied ZnO electrospun nanofibres for the as-spun and calcined fibre is given in figure The broad peak observed at 3513cm-1 signify the bonded O-H stretching frequencies of the alcohol for the as-spun nanofibre samples These O-H stretching could arise from the water molecule of the crystalline system of the Zn(CH3OO)2.2H2O The strong O-H stretching peaks observed for the calcine samples at 3686cm-1 probably account for the nonbonded free O-H stretching frequencies in the calcine samples The associated hypochromic shift (blue shift) observed for the calcine samples compared with the as-spun samples might be due to the effect of calcination on the sample The double bond of CH, CH2 or CH3 observed at 2920cm1 in the as-spun fibres is attributed to the long chain of the PVAc polymer used The absence of this bond in the calcined sample showed that the PVAc polymer had been removed from the sample as a result of the calcination This is further attestation of likely change of phase from the amorphous to a crystalline phase Figure FTIR spectra of ZnO nanofibres for as-spun and calcined sample 3.3 Structural analysis The X-ray diffraction patterns of the calcined sample was studied to confirm the crystallinity of the samples and investigate its crystal structure The ZnO spectra presented in figure exhibit polycrystalline nature consisting of several diffraction peaks of hexagonal wurtzite structure with major diffraction peaks at [100], [101], and [200] corresponding to 2 = 32.58o, 36.98o and 67.20o respectively with the most pronounce peak indicating a preferred growth along [100] direction The observed peak matches the database of COD-Inorg REV140301 2015.07.06 corresponding to the entry 96-230-0451 [34] The unit cell parameters of the ZnO as given by the reference database is a= 3.2493 Å c= 5.2057 Å and having a density of 5.677g/cm3 [35] The matching phase and Rietveld refinement for the ZnO using the Match software are carried out and the refinement shows there is convergence with a reduced chi-square of 9.6 and weight average of 104.6 The average crystallite grain size of the fibre calculated using the Scherer formula given in equation is 54 nm with lattice strain of 0.0024 = 100 101 200 Figure The XRD spectra of the calcined ZnO 3.4 Optical characterisation Calcined As-spun 4.5 4.0 Refractive index 3.5 3.0 2.5 2.0 1.5 1.0 300 400 500 600 700 800 900 1000 1100 Wavelength(nm) Figure7 Refractive index spectra for as-spun and calcined ZnO electrospun nanofibre The optical bandgap of materials is known as the minimum energy required by semiconductor material to excite a phonon The difference in the optical bandgap to the electrical bandgap lies majorly in the exciton binding energy For material with very small exciton binding energy the optical bandgap and electrical bandgap are similar To determine the optical bandgap of the materials deposited, we used the Tauc plot given in equation 4and take n = since ZnO exhibit direct bandgap [42, 43] = A plot of ( ℎ ) ℎ ( ) gives the Tauc plot for the determination of the bandgap The intersection of the linear fit with the hv axis at (αhv)n = gives the optical band gap The results from the Tauc plot are presented in figure The obtained band gap for the ZnO is found to be 3.28eV, this value is within the range that had been reported by several researchers Bandgap of 3.26eV was reported by [42] ZnO 6.00E+009 5.00E+009 (h) 4.00E+009 3.00E+009 2.00E+009 1.00E+009 0.00E+000 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 hv (ev) Figure Energy bandgap plot for the calcined ZnO nanofibre Oscillator’s strength is one of the several parameters that are widely used in describing the atomic strength and optical molecular transition in materials [44] Among the molecular and atomic parameters that are related to the optical strength are transition dipole moment [44] oscillator strength of a material is a dimensionless quantity and it describes the probability of emission or absorption of electromagnetic radiation in the energy levels transitions of an atom or molecule [45] [46] The spectroscopic data for the electrospun nanofibre were employed to derive their complex dielectric function Dielectric imaginary and the real parts of the function ε” and ε’ respectively are related to the extinction coefficient ‘‘k’’and refractive index ‘‘n’’ by equation and respectively ε’ = n2 – k2 ε” = 2nk where, εi, εr, k and n are imaginary dielectric constant, real dielectric constant, extinction coefficient, and refractive index respectively For the value of n far greater than k, ε’ is approximately equal to n2, the dependence of ε’̍ on λ can be examined using the relation [47] given by equation ′ = = − ∗ λ Where c is the speed of light, m* is the effective mass of the carrier, Nc is the carrier density, e is the electronic charge, and is the high-frequency dielectric constant To obtain the high frequency dielectric constant we plot a graph n2 as a function of λ2, as shown in figure and by extrapolating the linear part of the curve at higher wavelength to the n2 axis at intersection where λ2 = we get the value for the In order to evaluate various oscillator parameters, the long wavelength approximation of the single-term Sellmier relation [47]) is given as (λ) − = Where λ the average oscillator parameter and So is the average oscillator strength plotting of ( (λ) − 1)-1 against λ-2 will produce straight line fit that gives the values of 1/ λ and 1/So from intercept and the slope respectively We can also calculated the dispersion parameter (Eo/So) for each sample using = ℏ The plot is given in figure 10 The relationship between the refractive index and the photon energy is given by [47] (ℎ ) − = ( 10 ) where Eso is the single oscillator strength and Ed is the dissipation energy By linearizing the expression and plotting (n2-1)-1 against (hv)2 and using the dielectric constant = =1+ 11 Frequency refractive index (no) were determined using the values of the slope ( / parameters constants ) and the intercept (1/ obtained the zero- and determined from ) The plot is given in figure 11 The results optical from the analysis are listed in table Figure Plot for determining the high frequency dielectric constant (εα) and density of state effective mass ratio ∗ for the calcined ZnO nanofibre using equation Figure 10 Plot for determining the average oscillator parameters (λ ) and average oscillator Strength (So), dispersion parameter ((Eo/So) for the calcined ZnO nanofibre using equation Fig 11 Plot for determining the dispersion energy (Ed) dielectric constant (εo), single oscillator strength (Eso), and zero frequency refractive index (no) for the calcined nanofibre using equation 10 ZnO Table Optical constants for the calcined ZnO electrospun nanofibre Optical parameters Symbol ZnO Average oscillator strength So 6.37x 1012m-2 Average oscillator parameter λo 0.37 (µm) Dispersion parameter Eo/So 1.23Ex 10-13 eVm2 Single oscillator strength Eso 3.54eV Dispersion energy Ed 3.168eV High-frequency dielectric constant εα 3.29 Zero frequency dielectric constant εo 1.895367 Zero frequency refractive index no 1.38 Density of state effective mass ratio Nc/m* 1.1 x 1050 m/s2C-2 Extinction coefficient k, given by equation 12 is a material parameter that defines how strongly it absorb light at any given wavelength = 12 where k is the extinction coefficient, α= absorption coefficient and λ is the wavelength The value of the extinction coefficient varies from 0.002 to 0.04 for the ZnO as shown fig.14 This agrees with the result of [43] Knowing the Figure 12 Extinction coefficient Spectra for the calcined ZnO nanofibre value of the extinction coefficient and the refractive index of the material will afford us the opportunity of determining the real and imaginary part of the dielectric constant of the nanofibre materials The relationship used is given in equation 12 The optical conductivity of the material was also estimated using equation 13 The relation between the conductivity and photon energy is given in figure 13 and that of the dielectric constant of photon energy is given figure 14 = 13 where α, n and c are absorption coefficient, refractive index and speed of light respectively The conductivity increases towards the high photon energy and increases sharply around the value of the bandgap energy The graph of the dielectric shows the same trend as that of the conductivity The sharp increase in the conductivity and dielectric constant around the bandgap can be attributed to the strong interaction between the photon and electron It could also be seen that the dielectric characteristic for both the real and imaginary follow the same pattern with the value of real higher than that of the imaginary The results obtained is similar to that obtained by [43], [48] for the dielectric property of ZnO Figure 13 Optical conductivity spectra for calcined ZnO nanofibre .. .Determination of optical parameters of zinc oxide nanofibre deposited by electrospinning technique 1,2 Bolarinwa, H S., 2,3Onuu, M U., 4Fasasi,... large discrepancies in the optical properties of electrospun ZnO fibre reported from various studies In view of this, reliable determination of the optical properties of the electrospun ZnO fibre... frequency refractive index (no) for the calcined nanofibre using equation 10 ZnO Table Optical constants for the calcined ZnO electrospun nanofibre Optical parameters Symbol ZnO Average oscillator strength