Characterization of V 2 O 5 nanorods grown by spray pyrolysis technique

Characterization of V 2 O 5 nanorods grown by spray pyrolysis technique
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  Characterization of V 2 O 5  nanorods grown by spray pyrolysistechnique N. M. Abd-Alghafour 1,2 • Naser. M. Ahmed 2 • Z. Hassan 2 • Sabah M. Mohammad 2 • M. Bououdina 3 • M. K. M. Ali 4 Received: 27 October 2015/Accepted: 12 January 2016   Springer Science+Business Media New York 2016 Abstract  V 2 O 5  nanorods (NRs) were prepared by achemical spray pyrolysis (CPS) onto heated glass sub-strates. The effects of vanadium solution concentration,deposition spray rate and substrate temperature on thestructural, morphological and optical properties preparedthin films were investigated. X-ray diffraction (XRD)results indicated that the thin films have orthorhombicstructures with preferential orientations along the (110)direction. Field emission scanning electron microscopy(FESEM) observations showed that rod-like shaped mor-phology is in the 100–150 nm diameter range and in the1–3  l m length range. Optical transmittance in the visiblerange changes to reach a maximum value of approximately80 % and the band gap energy has been found in the rangeof 2.13–2.6 eV. Photoluminescence (PL) spectra revealedintensive and sharp green light emission at about 534 nmwith high intensity. V 2 O 5  thin films can be used as materialin visible electronic and optoelectronic applications. 1 Introduction Transition metal oxide thin films have been widely studiedfor their interesting optical and structural performance [1].V 2 O 5  thin film has attracted significant interest over theyears owing to their wide range of applications. Theexcellent properties, such as multi-valance, wide opticalband gap (2.5–2.8 eV), good chemical and thermal stabil-ity, excellent thermoelectric property make V 2 O 5  thin filma promising material for rechargeable lithium batteries [2],infrared detectors [3], sensors [4], optical switching devices [5] and electrochemical supercapacitors [6, 7]. It is good to know that properties of powders depend on their particlesize and morphology [8]. Deposition parameters play animportant role in the structural and optical properties whenfabricating thin films of the material. Therefore, it is veryimportant to choose the appropriate deposition parametersto prepare the vanadium oxide thin films for a certainapplication [9].V 2 O 5  thin films have been prepared by different tech-niques including pulsed laser deposition [10], sol–gel [11, 12], vacuum evaporation [13], magnetron sputtering [14], electron-beam evaporation [15], and spray pyrolysis [16]. Among the various deposition techniques for the prepara-tion of vanadium oxide thin films, chemical spray pyrolysistechnique is a relatively simple and inexpensive techniquefor large-area coatings. It also offers several advantagesover conventional deposition techniques for the control of stoichiometry and thin film structure. Rozati et al. [17]investigated the influence of substrate temperature on thestructural and optical properties of nanostructural V 2 O 5 thin films. XRD was showed that by increasing the tem-perature, the intensity increased with an orthorhombicstructure. The crystallite size was observed that it increasedwith elevating the substrate temperature to 450   C. In &  N. M. Abd-Alghafourna2013bil@gmail.com 1 Iraqi Ministry of Education, Anbar, Iraq 2 Institute of Nano Optoelectronics Research and Technology(INOR), Universiti Sains Malaysia, 11800 Penang, Malaysia 3 Department of Physics, College of Science, University of Bahrain, PO Box 32038, Zallaq, Kingdom of Bahrain 4 Physics Department, College of Science, Al ImamMohammad Ibn Saud Islamic University, Riyadh, KSA  1 3 J Mater Sci: Mater ElectronDOI 10.1007/s10854-016-4338-3  addition, Akl et al. [18] studied the effect of thermalannealing on optical properties of the sprayed thin films of V 2 O 5 . The influence of thermal annealing time on thedispersion properties has been investigated. The refractiveindex is depending on the growth temperature and filmthickness. The higher value of refractive index at very lowwavelength,  k  =  400 nm was observed in all samples.In the present work, we have systematically studied theinfluence of the deposition conditions such as solutionconcentration and the solution spray rate and substratetemperature on the physical properties of the vanadiumoxide thin films prepared using spray pyrolysis technique.To the best of our knowledge, no previous research work has been reported in the literature using this technique tosynthesize V 2 O 5  NRs. The paper presents studies related togrowth nanorods, lengths, diameters and surface mor-phology in relation with different conditions. Furthermore,the impact of structural and morphological and opticalproperties has been investigated. 2 Experimental details V 2 O 5  thin films were deposited by the using chemical spraypyrolysis (CSP) technique onto glass substrates. Prior tothe preparation, the substrates were cleaned with a normaldetergent with, ethanol and deionized distilled water. Fig-ure 1 show a scanning spraying nozzle positioned 35 cmapart from the substrates with a flow rate of 5 ml/min.Compressed air of pressure 6 N/cm 2 is used as a carriergas. The initial spraying solution contains a certain amountof VCl 3  powder (purity 99.9 %, purchased from Sigma-Aldrich) dissolved in 50 ml solution of deionized water(Millipore, USA). The thin films were prepared withvarious solution concentrations at constant substrate tem-perature  T  s  =  350   C and deposition spray rate 10 ml/min.The second group of samples were prepared with differentspray rate, from 5 to 10 ml/min at a constant concentrationof 0.2 M and  T  s  =  350   C. In the final stage, samples wereprepared at different substrate temperature with constantsolution concentration and deposition spray rate of 0.2 and10 ml/min, respectively. The reaction in the synthesis of V 2 O 5  thin films by the CSP technique is presented as fol-lows [19]:4VCl 3 ð s)  þ  5O 2 ð g) ! D 2V 2 O 5 ð s)  þ  6Cl 2 ð g)  "  :  ð 1 Þ The obtained V 2 O 5  thin films were uniform, pinhole freeand adequate adhesion to glass substrate. The shiny visiblehoney with a pale yellow color on the glass substrate is aremarkable color of incorporation of vanadium as V 5 ? inV 2 O 5  lattice. The totally mechanical spray coater was usedfor thin film deposition. The elevated deposition parame-ters are represented in Table 1. 3 Results and discussions 3.1 Structural studies The structural characterization of the prepared V 2 O 5  thinfilms have been investigated, using A PANalytical X , PertPro MRD diffractometer system equipped with Cu K  a radiation ( k  =  1.54056 A˚) at 40 kV and 30 mA. Figure 2shows the development of XRD diffraction patterns of theas-deposited V 2 O 5  thin films at different solution concen-tration. The peaks observed at 26.24  ,27.83  ,29.05  ,41.22  and 50.56   were corresponds to (110), (101), (011), (301),and (002) planes, respectively. These patterns confirm the 350C Air CompressorHeating plateSpray Nozzle Precursor SolutionSpray Chamber 35 cm Fig. 1  Schematic diagram of spray pyrolysis system for thepreparation of V 2 O 5  thin filmsJ Mater Sci: Mater Electron  1 3  formation of V 2 O 5  thin films with orthorhombic symmetry[20] and well-determined layered structure and an agree-ment with the JCPDS Card No. 00-001-0359. Figure 2shows the XRD diffraction patterns of the thin filmsdeposited with various solution concentrations. The typicalpeaks of the polycrystalline phase in V 2 O 5  thin filmsappear in XRD diffraction spectra. It is observed that theincrease in solution concentration increases the crys-tallinity of the thin film. The increase in the intensity of thepeaks due to grain growth associated with an increaseddegree of crystallinity by increasing molality. It is clearthat the thin films appear preferred growth along (110)direction when increasing solution concentration. The dif-ference of the intensity ratio of the (002) and (110) planesas a function of solution concentration are revealed inFig. 2. It is observed that the variation in the intensity of diffraction peaks is quite prominent for the concentrationof 0.2 mol/L. Therefore, it can be concluded that, for thissolution concentration, the preferred orientation along both(110) and (002) planes is observed (Fig. 2).Figure 3 shows the XRD diffraction patterns of V 2 O 5 thin films prepared with different deposition spray rates.All the samples have polycrystalline phase of the V 2 O 5  thinfilms and with increasing the solution spray rate, theintensity of the preferred orientation along (110), (011),(301), and (111) are increased. Furthermore, the intensityof (002) peak was decreased with an increase depositionspray rate, and this is due to the recrystallization processresulting from increased deposition spray rate. It isobserved that the intensity of the diffraction peak along(301) plane is increased for the solution concentration of 10 ml/min which is attributed to an increase in the crys-tallinity of the sample (Fig. 3).Figure 4 shows the XRD diffraction patterns of thesamples prepared at different substrate temperatures withconstant solution concentration 0.2 mol/L and spray rate10 ml/min. An increase of the substrate temperature, theintensity of (300) plane increases clearly up to 350   C andthen decreases for higher substrate temperatures butincrease the intensity of (110) with increasing substratetemperature, exhibiting crystal reorientation effect [21].The average grain size of the as-deposited film wasdetermined based on Scherer’s formula [22]:  D  ¼  0 : 9 kb cos h  ð 2 Þ where  b  is the full-width at half maximum (FWHM) inradians along the (110) of the corresponding XRD peak inradians,  k  is X-ray wavelength ( k cuk  a  =  1.5406A˚),  h  is theBragg diffraction angle. The information obtained from theXRD diffraction analysis is summarized in Table 2.The lattice parameters have been calculated by formbelow [23]: d  hkl  ¼  1  ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ð h 2 = a 2 Þ þ ð k  2 = b 2 Þ þ ð l 2 = c 2 Þ p   ð 3 Þ with  a = b = c  for orthorhombic lattice with ( hkl ) theMiller indices of reflector planes appearing on the XRDdiffraction spectrum and  d  hkl  their inter-reticular distances.The results are illustrated in Table 3. These results areconsistent with those of previous studies. 3.2 FESEM observations and formation mechanismof V 2 O 5  NRs The growth mechanisms of V 2 O 5  NRs using the chemicalspray pyrolysis technique have been studied. When thespray deposition was carried out for 5–10 min, the initial Fig. 2  XRD patterns of V 2 O 5  thin films deposited at various solution concentrations Table 1  Enhanced deposition parametersParameters ValuesCarrier gas AirCompressed air pressure 6 N/cm 2 Flow rate 5 ml/minSpray nozzle diameter 0.7 mmSubstrate nozzle distance 35 cmSpray angle 90  J Mater Sci: Mater Electron  1 3  steady reaction favored the longitudinal growth. However,when the precursor flow was done for 5–10 min, the pro-duction of the new V 2 O 5  species was reduced, whichresults in a reduction in the dimensions, giving nano rods of smaller height. Meanwhile, when the spray was continuedfor a sufficient time (12–15 min), the longitudinal growthwas continued to form the V 2 O 5  NRs arrays. For a longertime (20–25 min), a large quantity of V 2 O 5  nanoparticleswas obtained at the third zone. These V 2 O 5  nanoparticlescould not be absorbed by the V 2 O 5  NRs growth front due tothe unavailability of any further V 2 O 5  nucleation site onthe substrate. Instead, these nanoparticles were depositedon the top of nano rods. As can be seen in the FESEMimage, there is a closed network of nanoparticles over nanorods growth after 20 min. Thus, the above discussionsrevealed that the deposition time, the morphology, size and Fig. 3  XRD patterns of V 2 O 5  thin films deposited at different deposition spray rates Fig. 4  XRD patterns of V 2 O 5  thin films prepared at different substrate temperatures Table 2  Summary of XRD parameters and mean grain size of all samples for (110) orientationSample 2 h  /(  ) d-spacing FWHM/(  ) Mean grain sizeThe effect of different substrate temperature ( T  sb ) 250   C 25.26 0.352 0.59 13.796300   C 25.5 0.349 0.59 13.797350   C 25.52 0.348 0.59 13.8The effect of various solution concentration 0.05 mol/L 26.35 0.338 0.59 13.8260.1 mol/L 26.37 0.337 0.59 13.8270.2 mol/L 26.24 0.339 0.49 16.148The effect of deposition spray rate 5 ml/min 26.23 0.339 0.59 13.82337 ml/min 26.35 0.338 0.59 13.826810 ml/min 26.38 0.337 0.59 13.8275J Mater Sci: Mater Electron  1 3  density of V 2 O 5  thin film are controlled over nano rodsgrowth after 20 min.Surface morphology of the V 2 O 5  thin films was exam-ined, using field emission scanning electron microscopy(FEI Nova NanoSEM 450). Figure 5a–c shows FESEMimages of V 2 O 5  NRs grown at different solution concen-tration. The variation of solution concentration has a sig-nificant influence on the morphology and alignment of theV 2 O 5  NRs. At low solution concentration of 0.05 mol/L,the V 2 O 5  surfaces of nano rods are very thin and randomalignment along the substrate surface as shown in Fig. 5a.Increasing the solution concentration from 0.05 to 0.2 mol/ L induces the formation of crystals in the form of well-aligned nano rods perpendicular to the substrate [24]. As itis clear, V 2 O 5  thin films of orthorhombic nano rods andV 2 O 5  thin films become denser with increasing solutionconcentration. The diameter and length of the rods for0.2 mol/L are increased as compared to 0.1 mol/L solutionconcentration. The diameter of rods is around 60–90 nmfor 0.2 mol/L and the length of rods is about 200–600 nm.Moreover, the number of grown rods increases byincreasing the solution concentration (Fig. 5c). Theseresults are in agreement with those obtained using the XRDas described above. Figure 6a–c shows FESEM images of V 2 O 5  thin films synthesized at different deposition sprayrate. The FESEM images show that deposition spray ratesignificantly influences the nano rods. This images exhibitfairly uniform structure and orientation. Figure 6a showsfairly smooth and uniform surface because of small crys-tallite size and surface diffusion at higher deposition tem-perature with minor traces of nanostructure formation. TheFESEM analysis of V 2 O 5  thin film for 7 ml/min (Fig. 6b)shows uniform growth of regularly aligned nano rods of V 2 O 5  perpendicular to substrate. The nano rods are typi-cally around 300 nm in length and 80–90 nm in diameter,as shown in Fig. 6c. FESEM images of V 2 O 5  thin filmsprepared at various substrate temperatures are shown inFig. 7a–c. The effects of reaction temperature on themorphology and shape of the as-synthesized products [25].From figure, it can be observed that the NRs are theproducts obtained in the presence of all surfactants. Withchange at the surfactant, morphology of the samplesremains nearly constant, and only size and agglomerationof the nano rods change a little [26]. At glass substratetemperature of 250   C, rod-like shaped morphology isdisplayed in Fig. 7a. It is shown that the sample consists of V 2 O 5  NRs, and most of them are categorized into branchedand urchin-like morphologies. The nano rods are typicallyaround 500 nm in length and 80–100 nm in diameter. Asthe glass substrate temperature increased to 300   C, thediameter of the nano rods shown in Fig. 7b is moreapparent in the 1–2  l m length range. At higher substratetemperature of 350   C illustrated in Fig. 7c, the nano rodshave an average diameter of 100–150 nm and the length of 3  l m due to a very large average of evaporation. Since thesolution drops could reach the glass substrate, the rod-likemorphology was explained. At a relatively higher glasssubstrate temperature of 350   C, the atmospheric air intakeregulated while reaching the glass substrate that led to theformation of thin film with large-sized and rod-shapednano structured morphology [27]. When the synthesisprocess was carried out at lower temperature, thick V 2 O 5 NRs and thick branched rods were obtained, as shown inFig. 7a. Thus, it can be concluded that the temperature of the reactor plays significant role in the formation of theV 2 O 5  nano/microrods. 3.3 Optical characterizations The optical measurements were determined in the range of 200–2000 nm, using Unicorn 4802 spectrophotometersystem. The transmission spectra of the as-prepared thinfilms shown in Fig. 8 indicated that when the substratetemperature increases to 350   C, the transmittance in therange of 300–800 nm increases from 30 to 80 %. This isassociated with increasing the number of defects andimproving crystallinity phase after the glass substratetemperature increases (with O 2 - ions occupying interstitialand substitution sites within V 2 O 5  host lattice [28, 29]. However, due to the glass substrate temperature increase,the transparency of thin film increases as shown in Fig. 8,and with the substrate temperature enhance themicrostructure of the thin film has becomes better-qualityand consequences in a lower reflection. The essentialabsorption edge of semiconductors occurs at wavelengthswhere the energy of an absorbed photon corresponds to anelectronic transition. When the photon energy of the inci-dent radiation becomes smaller than the work required toeject an electron from one or other quantum states in theconstituent absorbing atom the incident radiation ceases tobe absorbed by that state. The value of   E  g  corresponding toindirect energy band gap transition was found by the ( a hv )via  hv  schemes by the formula below [30]: Table 3  Lattice parameters of V 2 O 5  thin filmsSubstrate temperature (  C) Lattice parameters a  (A˚)  b  (A˚)  c  (A˚)250 6.998 5.8 3.532300 6.95 6.48 3.5350 6.954 6.56 3.48J Mater Sci: Mater Electron  1 3
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