Sprayed vanadium pentoxide thin films: Influence of substrate temperature and role of HNO3 on the structural, optical, morphological and electrical properties

Accepted Manuscript Title: Sprayed vanadium pentoxide thin films: Influence of substrate temperature and role of HNO3 on the structural, optical, morphological and electrical properties Authors: Mudaliar Mahesh Margoni, S. Mathuri, K. Ramamurthi, R. Ramesh Babu, K. Sethuraman PII: DOI: Reference:

S0169-4332(17)30390-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.039 APSUSC 35148

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APSUSC

Received date: Revised date: Accepted date:

15-10-2016 19-1-2017 7-2-2017

Please cite this article as: Mudaliar Mahesh Margoni, S.Mathuri, K.Ramamurthi, R.Ramesh Babu, K.Sethuraman, Sprayed vanadium pentoxide thin films: Influence of substrate temperature and role of HNO3 on the structural, optical, morphological and electrical properties, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.039 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.

Sprayed vanadium pentoxide thin films: Influence of substrate temperature and role of HNO3 on the structural, optical, morphological and electrical properties Mudaliar Mahesh Margonia, S. Mathuria, K. Ramamurthia,*, R. Ramesh Babub, K. Sethuramanc,1 a

Crystal Growth and Thin Film Laboratory, Department of Physics and Nanotechnology, Faculty of Engineering and Technology, SRM University, Kattankulathur – 603 203, Kancheepuram Dt., Tamil Nadu, India.

b

Crystal Growth and Thin Film Laboratory, School of Physics, Bharathidasan University, Tiruchirappalli – 620024, Tamil Nadu, India.

c

School of Physics, Madurai Kamaraj University, Madurai – 625 021, Tamil Nadu, India.

Graphical abstract

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Present address: Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama 35487, United States

Highlights 

V2O5 films are deposited at various temperature from HNO3 added precursor solution

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XRD analysis show growth along (101) plane

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Increase in the deposition temperature increases the transmittance

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Film surface morphology is modified effectively by HNO3 added precursor solution

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Deposition temperature of 375˚C improved carrier concentration and conductivity

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Abstract Spray pyrolysis technique was employed to deposit vanadium pentoxide thin films at different substrate temperature of 300 ˚C, 325 ˚C, 350 ˚C, 375 ˚C and 400 ˚C from nitric acid added ammonium meta vanadate aqua precursor solution. X-ray diffraction analysis of the deposited films showed the formation of orthorhombic vanadium pentoxide phase with growth along (101) plane. Average crystallite size showed variation from 21 to 42 nm as a function of substrate temperature. The film deposited at 400 ˚C acquired average visible transmittance of ~77 % in the 500 - 800 nm range. Direct optical band gap of the vanadium pentoxide thin film was slightly increased from 1.98 to 2.05 eV. Field emission scanning electron microscope and atomic force microscope studies showed that the size and shape of the flakes formed on the surface of the film deposited from the nitric acid added ammonium meta vanadate precursor solution were effectively modified with increase in substrate temperature. The carrier concentration and conductivity of the film deposited at 375 °C respectively is 2.138 × 1013 cm-3 and 1.0 × 10-5 Ω-1cm-1 which are relatively high when compared to other films. Photoluminescence spectra of the vanadium pentoxide films deposited at 375 °C and 400 °C showed intense emission peaks at 475 nm and 552 nm. Keywords: V2O5 thin films, XRD, morphological, optical, electrical properties and FTIR. Corresponding Author Dr. K. Ramamurthi Crystal Growth and Thin Film Laboratory Department of Physics and Nanotechnology Faculty of Engineering and Technology SRM University Kattankulathur – 603 203 3

Kancheepuram Dt., Tamil Nadu, India. Tel.: +91 431 2407057; fax: +91 431 2407045 E-mail address: [email protected]; [email protected]

1. Introduction Vanadium exists in V2+, V3+, V4+ and V5+ states in vanadium oxide compounds and they are categorized as single and mixed oxides based on Magneli (VnO2n-1) and Wadsley (V2nO5n-2) series. Existence of various ionic states of vanadium offers a challenge to prepare stoichiometric single phase vanadium oxide thin films [1]. In vanadium oxide compounds, at a critical temperature, crystallographic transformations are observed along with a reversible transition from semiconductor to metallic nature [2]. Among the various phases of vanadium oxides, the stable phase of vanadium pentoxide (V2O5), belonging to the orthorhombic system, exhibits electrochromic, high electrical and optical properties and finds wide 4

applications in electrochromic devices, gas sensor, lithium batteries, thermochromic windows, optical switching devices, solar cell and supercapacitor [3-9]. Vanadium oxide thin films were deposited by various methods such as spray pyrolysis [1012], thermal evaporation [13], electron beam evaporation [14] and ion beam sputter deposition [15] methods. The thin film deposition techniques such as thermal evaporation, electron beam evaporation, ion beam sputter, reactive sputtering and pulsed laser deposition are carried out in vacuum environment which are quite expensive for the large-scale production. Chemical vapour deposition technique required large amount of high purity precursors for large area coatings and hence these techniques are expensive. Spray pyrolysis is a simple and low cost experimental arrangement that has advantages like ease of adding doping material, reproducibility, high growth rate and mass production capability for uniform large area coatings, which are desirable for industrial and electrochromic applications [16]. Abbasi et al. [11] reported that the morphology of nano-crystalline V2O5 films is influenced by the substrate temperature (300-500 ˚C) and the film deposited at 300 °C shows high response to ethanol gas sensing. Ramana et al. [14] studied the influence of substrate temperature (423-603 K) on the properties of V2O5 thin films deposited by electron beam evaporation technique and reported that films deposited at higher temperature are substoichiometric with oxygen deficiency. Irani et al. [17] observed that sprayed nanostructured V2O5 thin films at substrate temperature >450 °C show blue shift due to the formation of chemical bond between glass and V2O5 film. Kumar et al. [18] prepared V2O5 thin films in the temperature range of 300-673 K by spray pyrolysis technique and concluded that microstructural parameters critically vary as a function of substrate temperature. Mane et al. [19] reported that nucleation and growth of the sprayed V2O5 film is influenced by the substrate temperature. Vijayakumar et al. [20] reported that optical band gap of sprayed V2O5 5

film is decreased with increase in substrate temperature and the films deposited at 300 °C showed good sensing properties towards xylene. Abyazisani et al. [21] sprayed fluorine doped nanostructured V2O5 thin films at 450 °C from different mole ratios of F/VO2 (10, 20, 30, 40, 50 and 70 %) precursor solution and reported that F doping decreased the average grain diameter from 57 to 47 nm and the optical band gap increased with F concentration. Li et al. [22] reported that tin doped V2O5 thin films prepared by sol-gel method showed enhanced performance in lithium ion batteries. Jia Chu et al. [23] obtained V2O5 nanorods on the FTO glass from the precursor solution containing ammonium meta vanadate (NH4VO3), oxalic acid, deionized water and 10 ml of hydrochloric acid (HCl) employing hydrothermal method and reported that they are suitable candidates for the smart window application and secondary lithium batteries. Margoni et al. [24] deposited fluorine doped vanadium oxide films by spray pyrolysis technique from the precursor solution containing ammonium meta vanadate, HCl and ammonium fluoride and observed that surface morphology is effectively modified by incorporation of F at various levels. Parida et al. [25] reported the addition of nitric acid (HNO3) in NH4VO3 precursor solution produced nanopetals and nanoflowers like structures. Among them nanoflower structures exhibit large non-linear optical absorption leading to optical limiting. Xiaochuan Ren et al. [26] reported that PEG-400 and HNO3 influenced the morphology of hydrothermally synthesized hierarchical nanostructured V2O5. They observed various morphology of V2O5 such as hollow microsphere, yolk shell structure, double shell structure, triple shell structure and hierarchical hollow superstructures when the concentration of HNO3 was varied. They also concluded that hierarchical V2O5 hollow microsphere exhibits better electrochemical performance than multishell hollow V2O5 microsphere. Thus the reported investigations evidently show that addition of HNO3 effects a large variation in the morphology and 6

properties of the vanadium pentoxide. Hence in the present work, ammonium meta vanadate precursor solution was prepared by adding a few ml of nitric acid and the effect of substrate temperature on the structural, optical, morphological and electrical properties of V2O5 films is reported. 2. Experimental Ammonium meta vanadate and a few ml of concentrated nitric acid was added in a beaker and it was heated at 60 ˚C for 20 min. After natural cooling to room temperature, 30 ml of double distilled water was added which resulted in the 0.1 M clear precursor solution (pH~2). Vanadium oxide thin films were deposited on the micro-slide glass substrates by spray pyrolysis technique at different substrate temperatures of 300 ˚C, 325˚C, 350 ˚C, 375 ˚C and 400 ˚C. Schematic diagram of spray pyrolysis experimental setup employed in this work is shown in Fig. 1. The distance between the spray nozzle and glass substrate was set at 35 cm. Spray time and spray interval was 3 sec and 20 sec respectively and the compressed gas (purified air) pressure was 3 kg/cm2. Structural properties of the deposited films were studied using X-pert powder XRD system with CuKα radiation (λ=1.5405 Å) in BraggBrentano (θ/2θ coupled) geometry. Morphology and elemental analysis of the films were studied by Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Analysis (EDAX) using FEI Quanta FEG200. Optical transmittance of the deposited films was recorded by UV-Vis-NIR spectrophotometer (Specord-200) in the 300-1100 nm wavelength range. Electrical properties of the films were carried out at room temperature using EcopiaHMS 3000 with magnetic field of 0.57 Tesla by van der Pauw configuration. Photoluminescence spectra properties were measured using Fluro Log-Horiba equipment with Xenon ion laser for excitation. The Fourier transform infrared (FTIR) spectra were recorded using Agilent carry 660 FTIR spectrometer. 7

3. Results and Discussion 3.1 X-ray diffraction analysis X-ray diffraction (XRD) pattern recorded for the films at different substrate temperatures (Ts) are shown in Fig. 2. The deposited films exhibit orthorhombic structure of V2O5 phase (JCPDS card no. 85-2422) with the growth orientation along (101) plane positioned at (2θ) 12.48°, 12.89°, 12.80°, 12.87° and 12.45° respectively for the films deposited at 300 ˚C, 325 ˚C, 350 ˚C, 375 ˚C and 400 ˚C. The position of 2θ of the (101) XRD peak of film deposited at 325 ˚C, 350 ˚C and 375 ˚C is shifted towards the higher angle side when compared to that of standard value of (2θ) 12.51°, thus indicating compressive stress in the films. The (101) peak intensity increases with increase in substrate temperature up to 375 ˚C thus revealing the improvement in the crystallinity of the films. Further the film deposited at 375 °C shows another less intense XRD peak of (201) plane at (2θ) 20.16° which belong to V2O5 phase. Zhang et al. [27] prepared vanadium oxide films at different substrate temperature (room temperature to 400 ˚C) by DC magnetron sputtering and reported that 2θ of the (001) plane is shifted towards lower angle side when compared to that of the standard value due to tensile stress. The lattice parameters a and c were calculated using the relation d(hkl) = nλ/2sinθ(hkl) = 1/(h2/a2 + k2/b2 + l2/c2)1/2 where d(hkl) is the interplannar distance corresponding to (hkl) Miller indices [28]. The obtained value of a and c for the film deposited at 375 °C from (101) and (201) XRD peaks is 9.7266 Å and 10.0423 Å respectively which agreed well with the corresponding bulk values a = 9.9461 Å and c = 10.3307 Å (JCPDS card no. 85-2422). The relatively less value of a and c obtained in this work is due to the shift in 2θ = 12.87° of (101) plane and 2θ = 20.16° of (201) peak respectively towards higher angle side when compared to that of standard values of 2θ = 12.51° and 2θ =19.90° of the respective peaks (JCPDS card no. 85-2422). 8

Average crystallite size (D) was calculated using Debye-Scherrer formula [29] D = kλ/ (βcosθ) where k = 0.9, λ is the wavelength of X-rays (1.5406 Å) used, β is the full width at half maximum in radian and θ is Braggs angle in degree. The strain (ԑ) and dislocation density (δ) were derived using the relation ԑ = (βcos θ)/4 [30] and using Williamson and Smallman relation δ = 1/D2 [31] respectively. Crystallite size, strain and dislocation density calculated from the XRD peak of (101) plane of films deposited at different Ts are presented in Table 1. It is evident that the crystallite size of V2O5 film increases systematically from ~24 nm to ~42 nm as a function of the substrate temperature upto 375 °C. However the crystallite size is decreased to 21 nm for the film deposited at 400 °C. The increase in crystallite size upto 375 ˚C may be due to coalescence process in which smaller grains join together to form larger grains. Further film deposited at 400 ˚C shows relatively larger values of strain and dislocation density which may be attributed to the relatively less crystallite size. Strain and dislocation density calculated from (101) peak of V2O5 phase vary from 0.80×10-3 to 1.64×10-3 lin.-2m-4 and 5.59×1014 to 22.52×1014 lin.m-2 respectively and film deposited at 375 ˚C acquires relatively low microstructural defects compared to other films (Table 1). Strain and dislocation density decreases with increase in crystallite size as the crystallite size is inversely proportional to strain and dislocation density [32]. Kaid et al. [12] deposited V2O5 film from different concentration (0.1 to 0.5 M) of NH4VO3 precursor solution in which the peak orientation is along (001) plane. Vijayakumar et al. [20] deposited V2O5 films using NH4VO3 precursor solution by spray pyrolysis technique in which the growth orientation is along (001) plane. Irani et al. [17] coated vanadium oxide films using VCl3 precursor solution which showed predominant peak of (101) plane of V2O5 upto 450 ˚C along with other vanadium oxide phases; whereas the film deposited at 500 ˚C formed only V2O5 phase with predominant growth of (001) plane. Mane 9

et al. [19] reported that V2O5 thin films deposited at various Ts (350-500 °C) using VCl3 precursor solution shows that the intensity of the (011) plane increases upto 400 ˚C but the intensity of (100) plane constantly increases with increase in substrate temperature. From the above reports, it is observed that the films deposited using NH4VO3 precursor solution show predominant growth along (001) [12,20]; whereas the film deposited using VCl3 precursor solution shows predominant growth along (100), (101) and (011) plane [17,19]. In the present work, formation of the single phase V2O5 film and the growth along (101) plane is due to the addition of a few ml of HNO3 in the NH4VO3 precursor solution. The reason for the formation of V2O5 phase and the preferential growth orientation along specific plane can be explained as following. Addition of HNO3 in NH4VO3 and preparing the aqua solution of pH~2 produce [V10O26(OH)2]-4 [33,34] complexes. The growth of the specific phase of the film and orientation depends on the nature of the precursor solution and the energy provided at the substrate temperature to decompose the precursor solution pyrolytically. As the precursor solution of the present work contains [V10O26(OH)2]-4 complexes, their dissociation at the various substrate temperature would be different from that of the other precursor solutions NH4VO3 and VCl3 as evidenced from the previous reports [12,17,19,20]. This may be the reason for the formation of V2O5 film with growth along (101) plane. The growth mechanism along with growth orientation of V2O5 films at different Ts is presented in Fig. 3. 3.2 Optical properties Optical transmittance spectrum recorded for vanadium pentoxide thin films prepared at various Ts in the wavelength range of 350-1100 nm are shown in Fig. 4. Average visible transmittance (AVT) obtained in the visible region (500-800 nm) and the wavelength at which maximum transmittance (Tmax. %) occurred are presented in Table 2. It is observed

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that film deposited at 300 ˚C shows low AVT and low Tmax. % when compared to the values of other films. The absorption coefficient (α) was calculated from the relation [10] α = ln (1/T)/t where T is the transmittance and t is the film thickness. The absorption coefficient calculated for the films is in the order of 106 m-1. Fig. 5 shows that the value of α of the films is influenced by Ts. Further increase in the Ts improves the transmittance. V2O5 films deposited by Irani et al. [17] at 550 °C showed maximum transmittance of ~45 % at 800 nm. Mousavi et al. [35] reported that transmittance of vanadium oxide thin films prepared by spray pyrolysis technique is increased from 20 to 60 % as a function of deposition temperature. Mane et al. [19] reported that transmittance of the V2O5 film sprayed from VCl3 precursor solution increases with increasing substrate temperature. Relatively higher transmittance observed in the present work may be attributed to the influence of HNO3 added precursor solution. The direct band gap (Eg) of the films was calculated from the relation (αhʋ) = A(hʋ - Eg)1/2 [36] where A is a constant, ʋ is the frequency of the radiation and α is the absorption co-efficient. The direct band gap value of the films estimated from the extrapolation of the straight line portion of the curve to α = 0 is 1.98 eV (Ts = 300 ˚C), 2.01 eV (Ts = 325 ˚C), 2.03 eV (Ts = 350 ˚C), 2.04 eV (Ts = 375 ˚C) and 2.05 eV (Ts = 400 ˚C) (Fig. 6). The bang gap of the present work is low when compared to that of the bulk V2O5 value of Eg~2.2 eV [2,37]. Thus the results show a systematic slight increase in the band gap with increase in Ts. The slight variation in the optical band gap with varying substrate temperature may be due to changes in the growth behaviour and morphology of the sprayed V2O5 films which can be explained on the basis of the electronic band structure of V2O5. The SEM and AFM images of V2O5 films (Figs. 7 and 9) show that the morphology of the film contains layered structure of different size. The band gap corresponds to the energy between the top of the valence O 2p band and the bottom of the conduction V 3d band [38]. Thus the

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layered structure of V2O5 may lead to many bands with low dispersion which may be attributed to a slight change in the optical band gap observed in the present work. 3.3 Surface morphology Scanning Electron Microscope images presented in Fig. 7 evidently show the influence of Ts on the surface morphology of vanadium pentoxide thin films. Randomly oriented rod like structures are formed on the surface of the film deposited at 300 °C. Surface of the film deposited at 325 ˚C shows irregular platelet shaped structures with varying dimension. When the Ts is raised to 350 ˚C, relatively long rod like structures and some platelet structures are formed on the surface of the film. Formation of flakes are observed on the surface of the film deposited at 375 ˚C. The surface morphology of vanadium pentoxide film deposited at 400 ˚C exhibits larger flake structures due to the elevated temperature. The increase in the size of grains observed on the surface of the film at higher temperature is due to the surface diffusion of the smaller grains to form larger grains. The surface morphology of the films deposited at various growth temperature is effectively modified by the addition of HNO3 in the precursor solution. The surface morphology of the films contains nano rods and flake like structures. Mane et al. [19] reported that the dimensions of the rods formed on the surface of the sprayed V2O5 film increase with increasing substrate temperature. Ramana et al. [38] reported that V2O5 film deposited at 200 ˚C shows amorphous nature whereas film coated at 500 ˚C possesses large grains due to enhanced surface diffusion. Vijayakumar et al. [20] reported nanostructured V2O5 films deposited at different substrate temperature by spray pyrolysis technique using NH4VO3 precursor solution formed fibre like structures at 250 ˚C whereas nanoflower structures were formed at 300 ˚C and 350 ˚C. In the present work, it is also evident that the HNO3 added NH4VO3 precursor solution plays an important role in modifying the surface morphology of the vanadium oxide thin films. Energy Dispersive X12

ray Analysis carried out on the vanadium oxide thin films shows the presence of V and O (Fig. 8). 3.4 Atomic Force Microscopy Atomic Force Microscopy (AFM) images of surface topology of deposited are presented in Fig. 9. The Root Mean Square (RMS) roughness value is 1.75 nm, 6.50 nm, 2.30 nm, 7.13 nm and 3.34 nm respectively for the films deposited at 300 ˚C, 325 ˚C, 350 ˚C, 375 ˚C and 400 ˚C. The average roughness of the vanadium pentoxide film deposited at 300 ˚C, 325 ˚C, 350 ˚C, 375 ˚C and 400 ˚C is 1.38 nm, 5.28 nm, 1.65 nm, 5.70 nm and 2.52 nm respectively. The films deposited at 325 ˚C and 375 ˚C shows relatively high RMS and average roughness values compared to that of the other films due to agglomeration of the grains into larger size. Mane at el. [19] reported that the average roughness and grain size increases with increase in substrate temperature upto 450 °C where as it decreases for 500 °C which is due to high reaction rate and decomposition of precursor solution before reaching the substrate which results in non-uniform growth of the film with decreasing grain size and the film thickness. Abbasi et al. [11] reported that the AFM image of the V2O5 film sprayed at 300 ˚C showed densely packed grains with average roughness and RMS of 5.53 nm and 6.95 nm respectively whereas the average roughness and RMS is 6.67 nm and 7.61 nm respectively for the films deposited at 500 °C. Glynn et al. [39] reported polymer assisted fast rate dip coating of vanadium oxide thin films and reported that the surface roughness takes place during crystallization and densification of the thin film coupled with the formation of crystallites on their surface during the thermal treatment. The present work reveals that HNO3 added precursor solution effectively modified the growth, shape and surface morphology of

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the deposited vanadium oxide films at different substrate temperatures when compared with the literature works. 3.5 Electrical properties Hall measurements of vanadium pentoxide thin films performed at room temperature are shown in Fig. 10. The negative sign of the charge carriers indicates that films are of n-type semiconductor. The electrical parameters of the sprayed V2O5 film of the present work are compared with the earlier reports of V2O5 film and the bulk V2O5 [40-45] in Table 3. The conductivity, carrier concentration, resistivity and mobility of the films deposited at substrate temperature of 300 ˚C, 325 ˚C, 350 ˚C, 375 ˚C and 400 ˚C respectively varies in the range from 10.0×10-4-5.13×10-7 Ω-1cm-1, 1.2×1012-2.1×1013 cm-3, 9.9×104-3.7×106 Ωcm and 0.547.79 cm2V-1s-1. The carrier concentration and conductivity of the vanadium pentoxide films are increased with increasing Ts up to 375 ˚C and then decreased for the film prepared at 400 ˚C. Improved values of carrier concentration, resistivity and conductivity of the films deposited upto Ts 375 °C may be due to increase in the crystallite size and decrease in strain and dislocation density. However these electrical parameters are decreased for the film deposited at Ts 400 °C which may be due to relatively less crystalline size and increase in the strain and dislocation density values. The mobility of the film deposited at Ts 350 °C shows relatively higher value when compared with that of the other films. The conductivity of the bulk V2O5 is 6.5× 10-6 Ω-1cm-1 [40] which is lower when compared with the corresponding value of (10.04×10-6 Ω-1cm-1) obtained for the film deposited at 375 °C in the present work. Kovendhan et al. [41] reported the values of carrier concentration, mobility and conductivity for sprayed V2O5 film at 450 °C is respectively 2.4×1013 cm-3, 1.5×10-6 Ω-1cm-1and 4.0×10-1 cm2/Vs. Sanchez et al. [40] grown bulk V2O5 using melt quench technique in which the

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carrier concentration, conductivity and mobility are obtained as 1.9 × 1020 cm-3, 6.5 × 10-6 Ω1

cm-1 and 2 × 10-7 cm2/Vs. Chakrabarty et al. [44] studied the electrical conductivity for

polycrystalline V2O5 and reported that the carrier concentration is 1.04 × 1016 cm-3, conductivity is 5 × 10-5 Ω-1cm-1 and mobility is 3 × 10-2 cm2/Vs. Grygiel et al. [45] deposited V2O3 thin films at a constant temperature by varying the film thickness using pulsed laser technique and obtained carrier concentration, resistivity and mobility value respectively is (4.5-4.9) × 1022 cm-3, (0.19-0.27) × 10-3 Ωcm and (0.15-1.21) cm2/Vs. In this work, carrier concentration, mobility and conductivity of the V2O5 film deposited at 375 ˚C respectively is 2.1×1013 cm-3, 2.93 cm2V-1s-1 and 1.0×10-5 Ω-1cm-1 which shows better electrical properties when compared to the reported values. 3.6 Photoluminescence studies Photoluminescence (PL) spectra recorded at room temperature for vanadium pentoxide films deposited at different Ts are shown in Fig. 11. Excitation wavelength was 285 nm. The emission spectra were recorded in the range of 460 – 560 nm. PL spectrum of vanadium oxide film deposited at various Ts shows emission band at 475 nm (2.61 eV), 513 nm (2.42 eV), 532 nm (2.33 eV) and 552 nm (2.25 eV). When the Ts is increased the PL intensity of the film decreases up to Ts 350 ˚C whereas at 375 ˚C and 400 ˚C the peak intensity increases. The PL peak observed at 475 nm and 532 nm is due to the band transition. The emission peak observed at 552 nm is due to the recombination of electron – hole pair. The PL peak intensity increases for the film deposited at 375 °C as compared to other films which may be due to decrease in the structural defects like strain and dislocation density and improved crystallite size as evident from Table 1. Li-Chia Tien and Yu-Jyun Chen [46] observed the blue (∼400 nm), green (∼550 nm) and red (∼730 nm) emission for V2O5 nanowires and concluded that the peak at 550 nm is responsible for recombined emission from the lowest split-off V 3d 15

band to the O 2p valence band. Kang et al. [47] reported that the PL peaks centered at 480 nm (~2.58 eV) and 525 nm (~2.36 eV) of V2O5 are caused by band transitions. 3.7 FTIR analysis FTIR spectra of V2O5 films were recorded in the wave numbers ranging from 600 to 1200 cm-1 (Fig. 12). The assignment of vibrational frequency of V2O5 films is given in Table 4. The vibrational band observed at 1067 cm-1, 1088 cm-1, 1088 cm-1, 1078 cm-1 and 1096 cm-1 for the V2O5 films deposited at 300 °C, 325 °C, 350 °C, 375 °C and 400 °C are attributed to υs (V = O). The υs (V = O) vibrational band shows a slight variation in the intensity and the frequency varies in the range of 1067-1096 cm-1 due to the effect of substrate temperature on the υs (V = O) bond length. From Table 4 one can observe that vibrational frequency of ~ 670 nm, ~ 729 nm and ~ 820 nm are not much influenced by the substrate temperature. However υs (V = O) frequency is relatively higher for the V2O5 film deposited at 400 °C which may be due to relatively less bond length. Culea et al. [48] recorded infrared absorption spectrum of crystalline V2O5 and observed vibrational peaks at 825 cm-1 and 1020 cm-1 and assigned to (V - O) and (V = O) respectively. These frequencies compare well with the corresponding values of the present work (Table 4). 4. Conclusions XRD results show that the films deposited belongs to the orthorhombic structure of V2O5 due to addition of HNO3 in the precursor solution. Increase in the crystallite size is observed increased for Ts 300 ˚C, 325 ˚C, 350 ˚C and 375 ˚C. However the V2O5 film formed at 400 °C shows a broad (101) peak which reduced the crystallite size to ~21 nm. Optical band gap of the films deposited at various Ts is varied from 1.98 to 2.05 eV which shows that the optical bandgap is not influenced much by the substrate temperature or HNO3. Variation in the deposition temperature influences the average visible transmittance, wavelength at 16

which the maximum transmittance is occurred along with morphology of the films. The high transmittance of V2O5 films achieved at higher Ts may be useful in optoelectronic, electrochromic and thermochromic devices. The carrier concentration, mobility and conductivity of the film deposited at 375 ˚C showed improved values. The high intensity emission band observed at 550 nm represents the recombination of electron-hole pair. Film deposited at 375 ˚C shows relatively improved electrical and PL properties. Thus the results of this work show that substrate temperature along with the addition HNO3 in the solution (pH~2) effectively modified the structural, optical, morphological and electrical properties of the deposited vanadium oxide films.

Acknowledgement One of the authors (M.M.M) sincerely thanks SRM University, Kattankulathur, for the award of SRM University fellowship to carry out the research work. The authors thank Prof. D. John Thiruvadigal, Dean of Sciences and Dr. Preferential Kala, Head, Department of Physics and Nanotechnology, SRM University for extending the DST-FIST(SR/FST/PSI155/2010) facilities. The authors also thank Dr. C. Gopalakrishnan and Dr. Helen Annal Therese, Nano Research Center, SRM University for extending the NRC facilities to record SEM images. The authors thank Dr. B. Neppolian, Research Institute for extending UV-VisIR Spectrophotometer facility. The authors thank Dr. S. Venkataprasad Bhat, Research Institute for extending Photoluminescence spectrophotometer facility.

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[12] M. A. Kaid., Characterization of Electrochromic Vanadium Pentoxide Thin Films Prepared by spray Pyrolysis, Egypt. J. Solids 29 (2006) 273-299. [13] R. Santos, J. Loureiro, A. Nogueira, E. Elangovan, J.V. Pinto, J.P. Veiga, T. Busani, E. Fortunato, R. Martins, I. Ferreira, Thermoelectric properties of V2O5 thin films deposited by thermal evaporation, Appl. Surf. Sci. 282 (2013) 590-594. [14] C.V. Ramana, O.M. Hussain, B. Srinivasulu, C. Julien, M. Balkanski, Physical investigation on electron-beam evaporated vanadium pentoxide films, Mater. Sci. Eng. B 52 (1998) 32-39. [15] T. Gallasch, T. Stockhoff, D. Baither, G. Schmitz, Ion Beam Sputter deposition of V2O5 thin films, J. Power Sources 196 (2011) 428- 435. [16] R. M. Pasquarelli, D.S. Ginley, R. O'Hayre, Solution processing of transparent conductors: from flask to film, Chem. Soc. Rev. 40 (2011) 5406–5441. [17] .R. Irani, S. M. Rozati, S. Beke, Structural and optical properties of nanostructural V2O5 thin films deposited by spray pyrolysis technique: Effect of the substrate temperature, Mater. Chem. Phys. 139 (2013) 489-493. [18] R.T. Rajendra Kumar, B. Karunagaran, V. Senthil Kumar, Y.L. jeyachandran, D. Mangalaraj, Sa. K. Narayandass, Structural properties of V2O5 thin films prepared by vacuum evaporation, Mater. Sci. Semicond. Process. 6 (2003) 543-546. [19] A. A. Mane, V. V. Ganbavle, M. A. Gaikwad, S. S. Nikam, K.Y. Rajpure, A. V. Moholkar, Physicochemical properties of sprayed V2O5 thin films: Effect of substrate temperature, J. Anal. Appl. Pyrolysis 115 (2015) 57–65. [20] Y. Vijayakumar, Ganesh Kumar Mani, M.V. Ramana Reddy, John Bosco Balaguru Rayappan, Nanostructured Flower like V2O5 Thin Films and Its Room Temperature Sensing Characteristics, Ceram. Int. 41 (2015) 2221-2227. 20

[21] M. Abyazisani, Mohammad Mehdi Bagheri-Mohagheghi, Mohammad Reza Benam, Study of structural and optical properties of nanostructured V2O5 thin films doped with fluorine, Mater. Sci. Semicond. Process. 31 (2015) 693-699. [22] Y. Li, Jinhuan Yao, Evan Uchaker, Ming Zhang, Jianjun Tian, Xiaoyan Liu, Guozhong Cao, Sn-Doped V2O5 Film with Enhanced Lithium-Ion Storage Performance, J. Phys. Chem. C 117 (2013) 23507−23514. [23] Jia Chu, Zhenzhen Kong, Dengyu Lu, Wenlong Zhang, Xianshan Wang, Yifan Yu, Sai Li, Xiaoqin Wang, Shanxin Xiong, Jing Ma, Hydrothermal synthesis of vanadium oxide nanorods and their electrochromic performance, Mater. Lett. 166 (2016) 179–182 [24] M. M. Margoni, S. Mathuri, K. Ramamurthi, R. Ramesh Babu, K. Sethuraman, Investigation on the pure and fluorine doped vanadium oxide thin films deposited by spray pyrolysis method, Thin Solid Films 606 (2016) 51-56. [25] M. R. Parida, C. Vijayan, C. S. Rout, C. S. Suchand Sandeep, Reji Philip, P. C. Deshmukh, Room temperature ferromagnetism and optical limiting in V2O5 nanoflowers synthesized by a novel method, J. Phys. Chem. C 115 (2011) 112–117. [26] Xiaochuan Ren, Yanjun Zhai, Lin Zhu, Yanyan He, Aihua Li, Chunli Guo, Liqiang Xu, Fabrication of various V2O5 hollow microspheres as excellent cathode for lithium storage and the application in full cells, ACS Appl. Mater. Interfaces 8 (2016) 17205– 17211. [27] D. Zhang, R.i Huang, T. Zhang, Y. Li, Youtong Chen, Yonglin Zhong, Ping Fan Jianjun Huang, Effect of substrate temperature on the microstructure, optical, and electrical properties of reactive DC magnetron suttering vanadium oxide films, Phys. Status Solidi A, 1–6 (2012) doi: 10.1002/pssa.201228211 [28] A. Goswami, Thin Film Fundamentals, First ad., New Age International, New Delhi,

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1996 69. [29] B. D. Cullity, Elements of X-ray Diffraction, Addison-wesley, MA, (1956) 99. [30] G. K. Williamson, W. H. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Metallurgica. 1 (1953) 22-31. [31] G. K. Williamson, R.C. Smallman, Dislocation densities in some annealed and cold – worked metals from measurements on the X-ray debye-scherrer spectrum, Philos. Mag. 1 (1956) 34-46. [32] A. S. Edelestein, R. C. Camarata, Nanomaterials: Synthesis, Properties and Application, Insitute of Physics Publ., Bristol, UK, 1996. [33] D. E. Keller, Frank M. F. de Groot, D. C. Koningsberger, B. M. Weckhuysen, ΛO4 Upside Down: A New Molecular Structure for Supported VO4 Catalysts, J. Phys. Chem. B 109 (2005) 10223-10233. [34] C. F. Baes, Jr. Robert E Mesmer, The hydrolysis of cations, John Wiley & Sons, New York, 1976; p 210. [35] M. Mousavi, A. Kompany, N. Shahtahmasebi, M.M. Bagheri-Mohagheghi, Study of structural, electrical and optical properties of vanadium oxide condensed films deposited by spray pyrolysis technique, Adv. Manuf. 1 (2013) 320-328. [36] J. I. Pankove., Optical Processes in Semiconductors, Prentice-Hall Inc., Englewood Cliffs, New Jersey, (1971) 36. [37] Ming-Cheng Wu, Chi-Shen Lee, Field emission of vertically aligned V2O5 nanowires on an ITO surface prepared with gaseous transport, J. Solid State Chem. 182 (2009) 2285-2289.

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[38] C. V. Ramana, R. J. Smith, O. M. Hussain, C. C. Chusuei, C. M. Julien, Correlation between growth conditions, microstructure and optical properties in pulsed-laserdeposited V2O5 thin films, Chem. Mater. 17 (2005) 1213-1219. [39] C. Glynn, D. Creedon, H. Geaney Eileen Armstrong, Timothy Collins, Michael A. Morris, Colm O’Dwyer, Linking Precursor Alterationsto Nanoscale Structure and Optical Transparency in Polymer Assisted Fast Rate Dip-Coating of Vanadium Oxide Thin Films, (2015) doi:10.1038/srep11574 [40] C. Sanchez, R. Morineau, J. Livage, Electrical Conductivity of Amorphous V2O5, Phys. Status Solidi a 76 (1983) 661. [41] M. Kovendhan, D. Paul Joseph, P. Manimuthu, A. Sendilkumar, S.N. Karthick, S. Sambasivam, K. Vijayarangamuthu, Hee Je Kim, Byung Chun Choi, K. Asokan, C. Venkateswaran, R. Mohan, Prototype electrochromic device and dye sensitized solar cell using spray deposited undoped and ‘Li’ doped V2O5 thin film electrodes, Curr. Appl. Phys. 15 (2015) 622-631. [42] M. Kang, J. Jung, sung-Young Lee, Ji-Wook Ryu, Sok Won Kim Conductivity, carrier density, mobility, seebeck coefficient, and power factor in V2O5. Thermochim. Acta. 576 (2014) 71-74. [43] H. A. Jerome Perlstein, Dislocation model for two-level electron-hopping conductivity in V2O5: Implication for catalysis. J Solid State Chem 3 (1971) 217-226. [44] D. K. Chakrabarty, Dipak Guha, A. B. Biswas, Electrical properties of vanadium pentoxide doped with lithium and sodium in the α-phase range, J. Mater. Sci. 11 (1976) 1347-1353. [45] C. Grygiel, Ch. Simon, B. Mercey, W. Prellier, R. Fresard, Thickness dependence of the electrical properties in V2O3 thin films, Appl. Phys. Lett. 91 (2007) 262103.

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Figure caption Fig. 1. Schematic diagram of spray pyrolysis experimental setup. Fig. 2. XRD peaks of V2O5 films deposited at different Ts. Fig. 3. Growth mechanism along with growth orientation of V2O5 films deposited at different Ts. Fig.4. Transmittance spectrum of V2O5 films deposited at different Ts. Fig.5. Variation of absorption coefficient as a function of wavelength in V2O5 films deposited at different Ts.

24

Fig.6. Direct optical band gap of V2O5 films deposited at different Ts. Fig.7. SEM images of V2O5 thin films deposited at different Ts. Fig.8. Elemental composition of V2O5 films deposited at different Ts. Fig.9. Surface topology of V2O5 films deposited at different Ts. Fig.10. Electrical properties of V2O5 films deposited at different Ts. Fig.11. Photoluminescence spectra of V2O5 films deposited at different Ts. Fig.12. FTIR spectra of V2O5 films deposited at different Ts. Table caption Table 1 Microstructural parameters of V2O5 films deposited at various Ts. Table 2 Optical parameters of V2O5 films deposited at various Ts. Table 3 Comparison of electrical properties of V2O5 films deposited at various Ts. Table 4 FTIR frequency of V2O5 thin films deposited at different Ts and the frequency assignment.

25

Fig. 1

26

Fig. 2

27

Fig. 3

28

Fig. 4

29

Fig. 5

30

Fig. 6

31

300˚C

325˚C

500 nm

500 nm

375˚C

350˚C

500 nm

500 nm

400˚C

32

500 nm

Fig. 7

33

300˚C

325˚C

350˚C

375˚C

400˚C

Fig. 8

34

300°C

325°C

350°C

375°C

400°C

Fig. 9

35

Fig. 10

36

Fig. 11

37

Fig. 12

38

Table 1

Ts

Strain

2θ

Crystallite Size (10-4 lin.-2m-4)

(nm)

(˚)

d- spacing Density

(hkl) (°C)

Dislocation

(Å) (1014 lin.m-2)

300

12.48

(101)

24.2

1.43

17.06

7.08

325

12.89

(101)

36.2

0.95

7.64

6.86

350

12.80

(101)

36.8

0.94

7.38

6.91

375

12.87

(101)

42.3

0.80

5.59

6.87

400

12.45

(101)

21.0

1.64

22.52

7.10

39

Table 2 Ts

AVT (%)

Tmax.%

Band gap

(˚C)

(500-800 nm)

(wavelength)

(eV)

300

58.59

62.23 (618 nm)

1.98

325

72.17

74.98 (702 nm)

2.01

350

72.16

73.82 (620 nm)

2.03

375

65.58

67.59 (654 nm)

2.04

400

76.79

77.79 (722 nm)

2.05

40

Table 3 Technique

Film/crystal

Ts

Carrier concentration (cm-3)

Spray

Pyrolysis rf sputtering

(Ωcm)

(Ω cm )

(cm2V-1s1

)

3.7×106

2.7×10-7

1.13

PW

325°C

3.2×1012

3.4×106

2.8×10-7

0.54

PW

350°C

5.2×1012

1.5×105

6.5×10-6

7.79

PW

375°C

2.1×1013

9.9×104

10.0×10-4

2.93

PW

400°C

1.2×1012

1.9×106

5.1×10-7

2.61

PW

V2O5 film

450°C

2.4×1013

6.2 x103

1.5 x10-6

4.0 x10-1

[41]

V2O5 film

300-350K

0.5-7.4

-

0.43-15×10-4

4.75-1.28

[42]

V2O5 film

V2O5

technique Melt-quench

-1

Ref.

1.4×1012

×10 Melt-quench

-1

Mobility

300°C

Pyrolysis

Spray

Resistivity Conductivity

V2O5

technique

15

-

1.9 × 1020

-

6.5 × 10-6

2 × 10-7

[40]

-

-

-

8.3 × 10-4

-

[43]

450°C

1.04 × 1016

-

5 × 10-5

3× 10-2

[44] -

V2O5

41

Temperature Pulsed laser deposition

on substrate V2O3

heater 600-

(0.19(4.5-4.9) × 1022

0.27) × 10-3

650°C

Table 4 Temperature (˚C)

Frequency (cm-1)

300

672

Stretching (V-O-V)

729

υ (V-O-V)

Assignment

s

826

Stretching (V-O)

1067

υ (V = O) s

325

648

Stretching (V-O-V)

726

υ (V-O-V) s

822

stretching (V-O)

1088

υ (V = O) s

350

663

Stretching (V-O-V)

729

υ (V-O-V) s

822

stretching (V-O)

1088

υ (V = O) s

375

662

Stretching (V-O-V)

729

υ (V-O-V) s

823

stretching (V-O)

1078

υ (V = O) s

400

661

Stretching (V-O-V) 42

-

0.15-0.21

[45]

υ (V-O-V)

732

s

820

stretching (V-O)

1096

υ (V = O) s

43